Petrogenesis and tectonic implications of the Early Paleozoic granites in the western segment of the North Qilian orogenic belt, China

NASA Astrophysics Data System (ADS)

Wang, Nan; Wu, Cai-Lai; Lei, Min; Chen, Hong-Jie

2018-07-01

Early Paleozoic granitic magmatism in the North Qilian orogenic belt records a complete Wilson cycle and provides critical geological clues for unraveling the regional tectonic history. In this study, we report the results of zircon U-Pb ages, Hf isotopic analysis and systematic whole-rock geochemical data for the Late Ordovician Hongliuhe granite and Early Silurian Qingshan monzogranite in the western segment of the North Qilian orogenic belt to constrain their emplacement ages, petrogenesis, and regional evolution history. U-Pb dating reveals that the Hongliuhe granite was emplaced around 453-452 Ma, and the Qingshan monzogranite was emplaced about 440-438 Ma. A geochemical study shows that the two granites belong to the calc-alkaline to high-K calc-alkaline series. The Hongliuhe granite shows adakitic and peraluminous features, while the Qingshan monzogranite belongs to metaluminous to weak peraluminous granites. Zircons in the Hongliuhe granite show εHf(t) values ranging from -15.1 to +11.7 with two-stage Hf model ages (tDM2) of 687-2398 Ma, whereas zircons in the Qingshan monzogranite show εHf(t) values ranging from +5.7 to +11.0 with two-stage Hf model ages from 814 to 1057 Ma. The geochemical characteristics indicate that the Hongliuhe granite was a transitional I/S-type granite and was generated from a thickened lower crust with the addition of minor Paleo- to Mesoproterozoic crustal materials, which left a rutile + garnet + pyroxene ± plagioclase residue. The Qingshan monzogranite formed from the partial melting of mafic crust with minor mantle-derived materials, and the fractionation of Ti-bearing phases, apatite and pyroxene occurred during the magma's evolution, which left an amphibole and plagioclase residue. We infer that the Hongliuhe granite formed during the northward subduction of the North Qilian Ocean, while the Qingshan monzogranite was generated during the post-collision stage between the Qilian and Alxa blocks. This observation indicates that syn-collision stage of the North Qilian orogenic belt began before 453 Ma, and that the extension regime began prior to 444 Ma.

Magmatism in the Shapinggou district of the Dabie orogen, China: Implications for the formation of porphyry Mo deposits in a collisional orogenic belt

NASA Astrophysics Data System (ADS)

Ren, Zhi; Zhou, Taofa; Hollings, Pete; White, Noel C.

2018-05-01

The Shapinggou molybdenum deposit is located in the Qinling-Dabie Orogen, which hosts the world's largest molybdenum belt. The igneous rocks at Shapinggou can be divided into two stages (136-127 Ma and 118-114 Ma), the early suite of felsic (136-127 Ma, SiO2 = 58.0 to 72.9 wt%) and mafic rocks (133-128 Ma, SiO2 = 45.2 to 57.0 wt%), and a later suite comprising syenite (117 Ma, SiO2 = 64.2 to 65.0 wt%), quartz syenite porphyry (116 Ma, 62.5 to 70.0 wt%), granite porphyry (112 Ma, SiO2 = 75.5 to 77.6 wt%) and diorite porphyry (111 Ma, SiO2 = 56.6 to 59.7 wt%). The early-stage felsic rocks display high SiO2, Al2O3, Na2O, K2O, Sr, LREE contents, and Sr/Y, (La/Yb)N ratios, initial Sr isotope ratios of 0.7076 to 0.7089, but low MgO, FeOT, Y, Yb contents and negative εNd(t) values, consistent with partial melting of the lower continental crust. The early-stage mafic rocks exhibit low SiO2, high MgO, Ni and Cr contents, consistent with an upper mantle source, but trace element and isotope data suggest a role for crustal contamination. The late-stage syenite and quartz syenite porphyry show high abundances of Na2O, K2O, Al2O3, HFSEs (e.g., Th, U, Zr, Hf) and significant negative Eu anomalies. The late-stage granite porphyry displays high SiO2 contents, and depletions in Ba, Sr, Eu and Ti. The geochemical features of the late-stage intrusions are similar to A-type granites. Crystal fractionation of plagioclase, K-feldspar, biotite/ muscovite, amphibole/ garnet and Fe-Ti oxides controlled the evolution of the magma. The geochemical and isotopic data suggest that the rocks at Shapinggou were likely derived from a mixed source of lithospheric mantle, subducted continental crust of the Yangtze Block (Kongling Group) and partial melts of the Dabie Complex. Early stage rocks represent melts of the source with a lower proportion of Dabie Complex materials, whereas late stage rocks were derived from a source with a higher proportion Dabie Complex component. The geochemical and isotopic variations of the intrusions at Shapinggou were controlled by both source characteristics and fractional crystallization. Although the Shapinggou deposit is located in a continental collision orogen, the magmas formed in an intraplate extension setting, with an increase in the amount of extension from the early to late stages. As well, both stages intrusions at Shapinggou were generated by the addition of heat, due to lithospheric delamination, mantle upwelling and rapid mantle convection, related to the far-field effects of the westward subduction of the paleo-Pacific Plate beneath the Asian continent. The geochemistry and setting suggest that the formation of a giant Mo deposit does not require a Mo-rich magma source, but rather an efficient convection mechanism for the transport of volatiles and Mo in a granitic magma system. The fluids derived from the granite porphyry at Shapinggou were more oxidised than that from the barren intrusions.

Contribution of Columbia and Gondwana Supercontinent assembly- and growth-related magmatism in the evolution of the Meghalaya Plateau and the Mikir Hills, Northeast India: Constraints from U-Pb SHRIMP zircon geochronology and geochemistry

NASA Astrophysics Data System (ADS)

Kumar, Santosh; Rino, Vikoleno; Hayasaka, Yasutaka; Kimura, Kosuke; Raju, Shunmugam; Terada, Kentaro; Pathak, Manjari

2017-04-01

The Meghalaya Plateau and the Mikir Hills constitute a northeastern extension of the Precambrian Indian Shield. They are dominantly composed of Proterozoic basement granite gneisses, granites, migmatites, granulites, the Shillong Group metasedimentary cover sequence, and Mesozoic-Tertiary igneous and sedimentary rocks. Medium to coarse grained, equigranular to porphyritic Cambrian granite plutons intrude the basement granite gneisses and the Shillong Group. U-Pb SHRIMP zircon geochronology and geochemistry of the granite gneisses and granites have been carried out in order to understand the nature and timing of granite magmatism, supercontinent cycles, and crustal growth of the Meghalaya Plateau and Mikir Hills. Zircons from the Rongjeng granite gneiss record the oldest magmatism at 1778 ± 37 Ma. An inherited zircon core has an age of 2566.4 ± 26.9 Ma, indicating the presence of recycled Neoarchaean crust in the basement granite gneisses. Zircons from the Sonsak granite have two ages: 523.4 ± 7.9 Ma and 1620.8 ± 9.2 Ma, which indicate partial assimilation of an older granite gneiss by a younger granite melt. Zircons from the Longavalli granite gneiss of the Mikir Hills has a crystallization age of 1430.4 ± 9.6 Ma and a metamorphic age of 514 ± 18.6 Ma. An inherited core of a zircon from Longavalli granite gneiss has an age of 1617.1 ± 14.5 Ma. Zircons from younger granite plutons have Cambrian mean ages of 528.7 ± 5.5 Ma (Kaziranga), 516 ± 9.0 Ma (South Khasi), 512.5 ± 8.7 Ma (Kyrdem), and 506.7 ± 7.1 Ma and 535 ± 11 Ma (Nongpoh). These plutons are products of the global Pan-African tectonothermal event, and their formation markedly coincides with the later stages of East Gondwana assembly (570-500 Ma, Kuunga orogen). The older inherited zircon cores (2566.4 ± 26.9 Ma, 1758.1 ± 54.3 Ma, 1617.1 ± 14 Ma) imply a significant role for recycled ancient crust in the generation of Cambrian granites. Thus the Meghalaya Plateau and Mikir Hills experienced major felsic magmatic episodes at 1800 Ma, 1600 Ma, 1400 Ma, and 500 Ma with recycling of Neoarchaean crust, and later contributions from Paleo-Mesoproterozoic granite gneiss sources. A 258 ± 20 Ma lower intercept age of the Rongjeng granite gneiss perhaps indicates a Permo-Triassic thermal imprint on the Meghalaya Plateau. The granite gneisses and granites have peraluminous to metaluminous compositions, and syn-orogenic to post-collisional affinities. We conclude that the orogenic history of the Meghalaya Plateau and the Mikir Hills records crustal growth of the Columbia and Gondwana supercontinents as noted in other Pan-African-Indian-Prydz-Brasiliano orogens.

Cumberland batholith, Trans-Hudson Orogen, Canada: Petrogenesis and implications for Paleoproterozoic crustal and orogenic processes

NASA Astrophysics Data System (ADS)

Whalen, Joseph B.; Wodicka, Natasha; Taylor, Bruce E.; Jackson, Garth D.

2010-06-01

Large volume, plutonic belts, such as the ˜ 221,000 km 2, ca. 1.865-1.845 Ga Cumberland batholith (CB) of the Trans-Hudson Orogen in Canada, are major components of Paleoproterozoic orogenic belts. In many cases, they have been interpreted as continental arc batholiths. The petrogenesis and tectonic context of the CB and implications for crustal growth and recycling are interpreted herein based on a 900 km geochemical-isotopic (Nd-O) transect across it and into granitoid plutons within bounding Archean cratons in central and southern Baffin Island. The mainly granulite grade CB, emplaced over an age span of between 14 and 24 Ma, consists mainly of high-K to shoshonitic monzogranite and granodiorite, but also includes low- and medium-K granitoid rocks. Metaluminous to slightly peraluminous compositions and δ 18O (VSMOW) values (+ 6 to + 10‰) indicate derivation from infracrustal (I-type) sources. ɛ Nd 1.85 Ga signatures (- 12 to - 2) of both mafic and felsic units suggest a dominance of evolved sources. Isotopic signatures in the interior of the CB (- 2 to - 7) are more radiogenic than those within Archean domains in central (- 8 to - 15) and southern (- 5 to - 19) Baffin Island. The isotopic transect is interpreted as 'imaging' an accreted microcontinental block (Meta Incognita) and bounding Archean cratons. The CB includes granites of arc, within-plate (A-type) and post-collisional affinity and volumetrically minor mafic rocks with both arc and non-arc features. (La/Yb) CN and Sr/Y values range from < 1 to 225 and < 1 to 611, respectively. In these respects, some CB granitoid rocks resemble Paleozoic adakitic granites, interpreted as partial melts of greatly thickened crust within post-collisional settings, such as Tibet. Thus, the CB likely encompasses various non-consanguineous magmatic suites generated at deep- to mid-crustal depths. Although CB granitoid rocks undoubtedly had important crustal sources, it is hard to assess the relative contribution of mantle-derived magmas. The CB is best interpreted as a post-accretion batholith resulting from large-scale lithospheric mantle delamination followed by the upwelling of hot asthenospheric mantle leading to voluminous crustal partial melting. Contributors to crustal instability which may have facilitated such delamination included: (a) a collage of recently assembled small cratons underlain by hot, weak lithosphere with mantle-depth structural breaks within this segment of the Trans-Hudson Orogen; (b) the gabbro-eclogite phase transformation, and (c) a greatly thickened crustal section (> 60 km), as evidenced by adakitic granites.

Brother is high Sr/Y two-mica granite and sister is leucogranite: twin granites in the Northern Himalayan Gneiss Domes, southern Tibet

NASA Astrophysics Data System (ADS)

Zeng, L.; Gao, L.; Xie, K.

2011-12-01

Leucogranites in the Himalayan orogen is widely considered as the type example of crustal melts, which provides a probe to investigate the interplay among high-grade metamorphism, crustal anatexis, and tectonic transition in large-scale collisional belts. Whether the leucogranite was a daughter product from a more primitive granitic melt is an interesting question that deserves careful examination to address the above issue. We report a new suite of two-mica granite (TMG) and leucogranite (LG) in the Yardoi gneiss dome (YGD) in the easternmost of the Northern Himalayan Gneiss Domes (NHGD), south of the Yarlung-Tsangpo suture. SHRIMP and LA-ICP-MS zircon U/Pb dating show that TMG and LG formed at ~17.7 Ma to ~20.0 Ma and at ~17.1 Ma, respectively. Both suites of granite have high Na/K (>1.30) ratios. The TMGs are characterized by (1) high Sr (>450 ppm), low Rb (<95 ppm) and Y (<6 ppm), and high Sr/Y (>86) ratios; (2) no Eu anomalies; and (3) low initial 87Sr/86Sr ratios (<0.7098) and higher ɛNd (>-8.5) values. In contrast, the LGs have (1) lower Sr (<130 ppm) and higher Rb (92-130 ppm); (2) pronounced negative Eu anomalies with Eu/Eu*<0.55; and (3) relatively higher Sr (87Sr/86Sr(t) =0.7136-0.7148) and unradiogenic Nd (ɛNd(t)=-7.7~-11.1). These data demonstrate that these Mid-Miocene granites have major and trace element and radiogenic isotope compositions similar to those of >35 Ma granites, but significantly different from those granites of similar ages in the High Himalaya as well as in the NHGD. High Sr/Y and relatively unradiogenic Sr isotope compositions in the TMGs could be derived from partial melting of mafic materials formed during previous compressional thickening event which was triggered by the input of juvenile heat and material associated with the Miocene E-W extension. An AFC process (plagioclase fractional crystallization and contamination by crustal materials) could be a primary factor leading to the formation of these LGs. Concurrence of high Sr/Y granites and leucogranites in NHGD indicates that the Miocene rifting could have played an important but previously unrecognized role in producing the Himalayan leucogranite. Similarly, the ~43-44 Ma high Sr/Y two-mica granites were also accompanied by the formation of typical leucogranite. However, the Mid-Eocene suite was derived from melting of lower crustal mafic materials at overthickened crustal conditions. Concurrence of high Sr/Y granites and typical leucogranites in the NHGD demonstrates that leucogranites could be derived from more primitive granitic melts by AFC process.

The main features of the Uralian Paleozoic magmatism and the epioceanic nature of the orogen

NASA Astrophysics Data System (ADS)

Fershtater, G. B.

2013-02-01

The 2000 km Uralian Paleozoic orogen is situated on the western flank of the Uralo-Mongolian folded belt. It is characterized by an abundant variety of magmatic rocks and related ore deposits. Uralian Paleozoic magmatism is entirely subduction-related. It is proposed that the Uralian orogen represents a cold mobile belt in which the mantle temperature was 200 to 500 °C cooler than in the adjacent areas; a situation which is similar to the modern West Pacific Triangle Zone including Indonesia, the Philippine Islands, and southern Asia. During the course of the geological evolution of the Uralian orogen, the nature of the magmatism has changed from basic rocks of indisputable mantle origin (460-390 Ma) to mantle-crust gabbro-granitic complexes (370-315 Ma) followed by pure crustal granite magmatism (290-250 Ma). This order in rock type and age reflects the evolution of Paleozoic magmatic complexes from the beginning of subduction to the final stages of the orogen development.

The Wulonggou metaluminous A2-type granites in the Eastern Kunlun Orogenic Belt, NW China: Rejuvenation of subduction-related felsic crust and implications for post-collision extension

NASA Astrophysics Data System (ADS)

Xin, Wei; Sun, Feng-Yue; Li, Liang; Yan, Jia-Ming; Zhang, Yu-Ting; Wang, Ying-Chao; Shen, Ting-Shuo; Yang, Yi-Jun

2018-07-01

The Wulonggou Pluton is located in Wulonggou area, eastern segment of the Eastern Kunlun Orogenic Belt, NW China, and consists of mainly alkali-feldspar granites covering an area of about 150 km2. Petrogenesis of these granitoids has been investigated through an integrated study of petrography, zircon Usbnd Pb ages, whole-rock geochemistry, and Hfsbnd Nd isotopic compositions. Usbnd Pb dating of magmatic zircons indicated these granites crystallized during 426-424 Ma in the middle Silurian. The granites display high SiO2 (75.26-77.55 wt%), K2O + Na2O (7.98-9.03 wt%), extremely low MgO (0.04-0.19 wt%), CaO (0.28-0.61 wt%), and TiO2 (0.05-0.09 wt%) contents showing metaluminous, calcic-alkali and ferroan features; enrichment in Rb and some HFSEs (Zr, U, Nb, Ta, and Y), depletion in Sr, Ba, P, and Ti, mostly right-inclined REE curve, flat HREE patterns, high 10,000 ∗ Ga/Al and intensively negative Eu anomalies, exhibiting an A2-type granite affinity with Y/Nb > 1.2 mostly. The primitive magma of these large quantities of granites was generated under a high temperature, low pressure, reduced and anhydrous environment indicating intense upwelling of asthenosphere. Combining with the positive uniform zircon εHf(t) values of -0.2 to +3.8 and decoupled εNd(t) values of -4.9 to -2.1 at t = 424 Ma, it can be concluded that subduction-related juvenile materials, probably calc-alkaline granitoids, are the source of these A-type granites. Geochemical studies of Wulonggou granites, spatial and temporal distributions of regional magmatism, metamorphism, and sedimentary records throughout the Eastern Kunlun Orogen Belt jointly indicate that the whole orogenic belt was in a typical post-collision extension setting and experienced an isostatic uplift during the middle Silurian triggered by delamination after the convergence of the northeastern margin of Gondwana.

Origin of the Bashierxi monzogranite, Qiman Tagh, East Kunlun Orogen, NW China: A magmatic response to the evolution of the Proto-Tethys Ocean

NASA Astrophysics Data System (ADS)

Zheng, Zhen; Chen, Yan-Jing; Deng, Xiao-Hua; Yue, Su-Wei; Chen, Hong-Jin; Wang, Qing-Fei

2018-01-01

The Qiman Tagh of the East Kunlun Orogen, NW China, lies within the Tethysides and hosts a large W-Sn belt associated with the Bashierxi monzogranite. To constrain the origin of the granitic magmatism and its relationship with W-Sn mineralization and the tectonic evolution of the East Kunlun Orogen and the Tethys, we present zircon U-Pb ages and Hf isotopic data, and whole-rock compositional and Sr-Nd-Pb isotopic data of the Bashierxi monzogranite. The granite comprises quartz, K-feldspar, plagioclase, and minor muscovite, tourmaline, biotite, and garnet. It contains high concentrations of SiO2, K2O, and Al2O3, and low concentrations of TiO2 and MgO, indicating a peraluminous high-K calc-alkaline affinity. The rocks are enriched in Rb, U, Pb, and light rare earth elements, and relatively depleted in Eu, Ba, Nb, Sr, P, and Ti, and are classified as S-type granites. Twenty zircon grains yield a weighted mean 238U/206Pb age of 432 ± 2.6 Ma (mean square weighted deviation = 1.3), indicating the occurrence of a middle Silurian magmatic event in the region. Magmatic zircons yield εHf(t) values of -6.7 to 0.7 and corresponding two-stage Hf model ages of 1663-1250 Ma, suggesting that the granite was derived from Mesoproterozoic crust, as also indicated by 207Pb/206Pb ages of 1621-1609 Ma obtained from inherited zircon cores. The inherited zircon cores yield εHf(t) values of 8.3-9.6, which indicate the generation of juvenile crust in the late Paleoproterozoic. Samples of the Bashierxi granite yield high initial 87Sr/86Sr ratios and radiogenic Pb concentrations, and negative εNd(t) values. Isotopic data from the Bashierxi granite indicate that it was derived from partial melting of ancient (early Paleozoic to Mesoproterozoic) sediments, possibly representing recycled Proterozoic juvenile crust. Middle Silurian granitic magmatism resulted from continental collision following closure of the Proto-Tethys Ocean. The Qiman Tagh represents a Caledonian orogenic belt containing S-type granites and associated W-Sn deposits.

Igneous petrogenesis and tectonic setting of granitic rocks from the eastern Blue Ridge, Alabama Appalachians

DOE Office of Scientific and Technical Information (OSTI.GOV)

Drummond, M.S.; Allison, D.T.; Tull, J.F.

1994-03-01

A span of 150 my of orogenic activity is recorded within the granitic rocks of the eastern Blue Ridge of Alabama (EBR). Four discrete episodes of plutonism can be differentiated, each event exhibiting distinct field relations and geochemical signatures. (1) Penobscotian stage: this initial stage of plutonic activity is represented by the Elkahatchee Quartz Diorite (EQD), a premetamorphic (495 Ma) batholith and the largest intrusive complex (880 km[sup 2]) exposed in the Blue Ridge. Calc-alkaline I-type tonalite-granodiorite are the principal lithologies, with subordinate cumulate hbl-bt diorite, metadacite, granite and trondhjemite. The parental tonalitic magmas are interpreted to have been derivedmore » from a subducted MORB source under eclogite to get amphibolite conditions. (2) Taconic stage: the Kowaliga augen gneiss (KAG) and the Zana granite gneiss (ZG) are 460 Ma granitic bodies that reside in the SE extremity and structurally highest portion of the EBR. Both of these bodies are pre-metamorphic with strongly elongate sill- and pod-like shapes concordant with S[sub 1] foliation. Granite and granodiorite comprise the bulk of the KAG. (3) Acadian stage: Rockford Granite (RG), Bluff springs Granite (BSG, 366 Ma), and Almond Trondhjemite represent a suite of pre- to syn-metamorphic granitic intrusions. (4) late-Acadian stage: The Blakes Ferry pluton (BFP) is a post-kinematic pluton displaying spectacular by schlieren igneous flow structures, but no metamorphic fabric. The pluton's age can be bracketed between a 366 Ma age on the BSG and a 324 Ma K-Ar muscovite age on the BFP. BFP's petrogenesis has involved partial melting a MORB source followed by assimilation of metasedimentary host rock.« less

In situ U-Pb and Lu-Hf isotopic studies of zircons from the Sancheong-Hadong AMCG suite, Yeongnam Massif, Korea: Implications for the petrogenesis of ∼1.86 Ga massif-type anorthosite

NASA Astrophysics Data System (ADS)

Lee, Yuyoung; Cho, Moonsup; Yi, Keewook

2017-05-01

Isotopic and geochemical characteristics of Proterozoic anorthosite-mangerite-charnockite-granite (AMCG) suite have long been used for tracing the mantle-crustal source and magmatic evolution. We analyzed Lu-Hf isotopic compositions of zircon from the Sancheong-Hadong AMCG complex, Yeongnam Massif, Korea, in order to understand tectonomagmatic evolution of the Paleoproterozoic AMCG suite occurring at the southeastern margin of the North China Craton (NCC). The anorthositic rocks in this complex, associated with charnockitic and granitic gneisses, were recrystallized to eradicate magmatic features. In situ SHRIMP (sensitive high-resolution ion microprobe) U-Pb analyses of zircon from a leuconorite and an oxide-bearing gabbroic dyke yielded weighted mean 207Pb/206Pb ages of 1870 ± 2 Ma and 1861 ± 6 Ma, respectively. Charnockitic, granitic, and porphyroblastic gneisses yielded weighted mean 207Pb/206Pb zircon ages of 1861 ± 6 Ma, 1872 ± 6 Ma, and 1873 ± 4 Ma, respectively. These crystallization ages, together with our previous geochronological data for anorthosites (1862 ± 2 Ma), are indicative of episodic AMCG magmatism over an ∼10 Ma interval. Initial εHf(t) values of zircon analyzed from five anorthositic rocks and four felsic gneisses range from +2.1 to -6.1 and -0.3 to -5.4, respectively. Zircon Hf isotopic data in combination with available whole rock Sr-Nd isotopic data suggest that anorthositic parental magma was most likely derived from a mantle source and variably affected by crustal contamination. This crustal component is also reflected in charnockitic-granitic magmas produced primarily by the melting of lower crust. Taken together, the AMCG magmatism at 1.87-1.86 Ga in the Yeongnam Massif is most likely a late orogenic product of Paleoproterozoic NCC amalgamation tectonically linked to assembly of the Columbia supercontinent.

Contemporaneous migmitization and granite emplacement during regional metamorphism: Evidence for mid-crustal contribution to the batholiths of the Arabian-Nubian Shield

NASA Astrophysics Data System (ADS)

Elisha, B.; Katzir, Y.; Kylander-Clark, A. R.

2017-12-01

Collision-related granitoid batholiths, like those of the Hercynian and Himalayan orogens, are mostly fed by magma derived from meta-sedimentary sources. However, in the late Neoproterozoic calc-alkaline batholiths of the Arabian Nubian Shield (ANS), which constitutes the northern half of the East African orogen, sedimentary contribution is obscured by the juvenile character of the crust and the scarcity of migmatites. Here we use paired in-situ measurements of U-Th-Pb isotope ratios and REE contents of monazite and xenotime by LASS to demonstrate direct linkage between granites and migmatites in the northernmost ANS. Our results indicate a single prolonged period of monazite growth, 640-600 Ma, in metapelites, migmatites and peraluminous granites of the Abu-Barqa (SW Jordan), Roded (S Israel) and Taba-Nuweiba (Sinai, Egypt) metamorphic suites. Distribution of monazite dates and age zoning in single monazite grains in migmatites suggest that peak thermal conditions and partial melting prevailed for 10 Myr, from 620 to 610 Ma. REE patterns of monazite are well correlated with age, recording garnet growth and garnet breakdown in association with the prograde and retrograde stages of the melting reactions, respectively. Xenotime dates (n=40) cluster at 600-580 Ma recording retrogression to greenschist-facies conditions as garnet continues to destabilize. Phase equilibrium modelling and mineral thermobarometry illustrate that melting occurred either by dehydration of muscovite or by water-fluxed melting at 650-680° and 5-7 kbar. The expected melt production is 8-14%, allowing melt connectivity network to form and eventually melt extraction and segregation. The crystallization time of peritectic melt retained in dia- and metataxites overlaps the emplacement time of a vast calc-alkaline granitic flux throughout the northern ANS, which was previously considered post-collisional. Similar monazite ages ( 620 Ma) of the amphilolite-facies non-anatectic Elat schist indicate that migmatites are the result of widespread regional, rather than local contact metamorphism, representing the climax of East African orogenesis.

Zircon morphology and U-Pb geochronology of seven metaluminous and peralkaline post-orogenic granite complexes of the Arabian Shield, Kingdom of Saudi Arabia

USGS Publications Warehouse

Aleinikoff, John Nicholas; Stoeser, D.B.

1988-01-01

The U-Pb zircon method was used to determine the ages of seven metaluminous-to-peralkaline post-orogenic granites located throughout the Late Proterozoic Arabian Shield of Saudi Arabia. Zircons from the metaluminous rocks are prismatic, with length-to-width ratios of about 2-4:1 and small pyramidal terminations. In contrast, zircons from three of the four peralkaline complexes either lack well developed prismatic faces (are pseudo-octahedral) or are anhedral. Some of the zircons from the peralkaline granites contain inherited radiogenic lead. This complicates interpretation of the isotopic data and. in many cases, may make the U-Pb method unsuitable for determining the age of a peralkaline granite. Zircons in the metaluminous granites do not contain inheritance and thus, best-fit chords calculated through the data have upper concordia intercepts that indicate the age of intrusion, and lower intercepts that indicate simple episodic lead loss. The results show that these granites were emplaced during multiple intrusive episodes from 670 to 510 Ma (Late Proterozoic to Cambrian).

Highly differentiated magmas linked with polymetallic mineralization: A case study from the Cuihongshan granitic intrusions, Lesser Xing'an Range, NE China

NASA Astrophysics Data System (ADS)

Fei, Xianghui; Zhang, Zhaochong; Cheng, Zhiguo; Santosh, M.; Jin, Ziliang; Wen, Bingbing; Li, Zixi; Xu, Lijuan

2018-03-01

The genetic link between granitoids and polymetallic skarn mineralization has remained equivocal. The Cuihongshan skarn-porphyry W-Mo-Pb-Zn-(Fe-Cu) deposit in the eastern part of the Central Asian Orogenic Belt provides a unique example to address this issue. The major rock types in the mine area are Early Paleozoic intrusions composed of biotite syenogranite and biotite porphyritic granite and Early Mesozoic intrusions represented by porphyritic quartz monzonite, biotite monzogranite, and porphyritic granite. The diagnostic mineralogical and geochemical features indicate that the rocks belong to A2-type granites. The Early Paleozoic suite shows zircon U-Pb ages of 501 Ma, and εHf(t) values of - 4.4 to + 2.7 and + 2.4 to + 7.6, respectively. In combination with their coherent geochemical trends, these rocks are inferred to be products of in-situ differentiation. Although the Mesozoic suite shows crystallization ages of 194-196 Ma, εHf(t) values are in the range of - 2.5 to + 7.5 for the porphyritic quartz monzonite, the - 1.8 to + 4.5 values for the monzogranite and the + 2.3 to + 8.0 range for the porphyritic granite. The porphyritic quartz monzonite displays distinct mineral assemblage and shows significant compositional gap with the other two lithofacies. In contrast, the monzogranite and porphyritic granite have similar geochemical features, and are thus inferred to be co-magmatic. Considering the high SiO2 contents, variable εHf(t) (- 4.4 to + 8.0) and εNd(t) values (- 8.4 to + 0.28) for the two suites, we infer that both episodes of granitoid magmatism resulted from partial melting of crustal materials with a mixed source containing varying proportions of juvenile and Precambrian crustal components. The Early Mesozoic porphyritic granite shows a highly evolved F-rich geochemical affinity, and experienced magma-fluid interaction. Cassiterite from the calcic skarn and the magnesian skarn that coexists with magnetite orebodies shows a mean U-Pb age of 195.4 ± 1.9 Ma and 191.2 ± 7.5 Ma respectively, indicating that the mineralization is genetically related to the Mesozoic granitic rocks. We infer that halogen-rich magmatic-hydrothermal fluids were derived from the Early Mesozoic highly fractionated magma, and the spatial association of the metallic elements with different geochemical behavior in the Cuihongshan deposit is correlated to multiple stages of evolution of halogen-rich magmatic-hydrothermal fluids.

Cambro-Ordovician post-collisional granites of the Ribeira belt, SE-Brazil: A case of terminal magmatism of a hot orogen

NASA Astrophysics Data System (ADS)

Valeriano, Claudio de Morisson; Mendes, Julio Cezar; Tupinambá, Miguel; Bongiolo, Everton; Heilbron, Monica; Junho, Maria do Carmo Bustamante

2016-07-01

This work presents an overview of the geology and chemical composition of the Cambrian-Ordovician post-collisional (COPC) granites and associated rocks of Ribeira belt, SE-Brazil. These COPC granites make up some of the most picturesque and highest (>2000 m) rocky peaks and cliffs of Rio de Janeiro state, an accessible case of post-orogenic granitic magmatism associated with the terminal stages of a hot Ediacaran-Cambrian (Brasiliano-Panafrican) orogen. The COPC magmatism intruded tonalitic to granitic orthogneisses of the Rio Negro arc (∼790-600 Ma) and associated paragneisses of the São Fidelis Group. Post-collisional magmatism started ∼10 m.y. after the latest collisional event, the Buzios Orogeny, lasting discontinuously from ∼510 Ma until ∼470 Ma. The 15 largest intrusive bodies in Rio de Janeiro State are referred to in the literature as the Parati/Mangaratiba, Vila Dois Rios, Pedra Branca, Suruí, Silva Jardim, Favela, Andorinha, Teresópolis, Frade, Nova Friburgo, Conselheiro Paulino, São José do Ribeirão, Sana and Itaoca granites. They crop out as rounded/elliptical stocks or gently-dipping sheets, always with sharp contacts with the country rocks, along with pegmatite and aplitic veins and dykes. COPC granites are grey and pink undeformed medium-grained biotite monzogranites with (K-feldspar) porphyritic, mega-crystic, equigranular and serial textures. Magmatic flow foliation is frequently observed. Peripheric xenolith zones are common as well as isolated xenoliths from the country rocks. In a compilation of more than 100 chemical compositions, SiO2 contents display a major mode at 71wt%. The COPC magmatism generated high-K calc-alkaline granites and quartz monzonites with predominantly metaluminous granites. Meso to melanocratic gabbroic and dioritic enclaves also have calc-alkaline affinity and likely represent more resistant mafic xenoliths from the Rio Negro Arc.

87Sr/86Sr ratios in some eugeosynclinal sedimentary rocks and their bearing on the origin of granitic magma in orogenic belts

USGS Publications Warehouse

Peterman, Z.E.; Hedge, C.E.; Coleman, R.G.; Snavely, P.D.

1967-01-01

Rb and Sr contents and 87Sr/86Sr values were determined for samples of eugeosynclinal sedimentary rocks, mostly graywackes, from Oregon and California. These data are compatible with the theory of anataxis of eugeosynclinal sedimentary rocks in orogenic belts to produce granitic magmas provided that the melting occurs within several hundreds of m.y. after sedimentation. The low (87Sr/86Sr)0 values of the eugeosynclinal sedimentary rocks are related to the significant amounts of volcanogenic detritus present which probably were originally derived from the mantle. ?? 1967.

The global age distribution of granitic pegmatites

USGS Publications Warehouse

McCauley, Andrew; Bradley, Dwight C.

2014-01-01

An updated global compilation of 377 new and previously published ages indicates that granitic pegmatites range in age from Mesoarchean to Neogene and have a semi-periodic age distribution. Undivided granitic pegmatites show twelve age maxima: 2913, 2687, 2501, 1853, 1379, 1174, 988, 525, 483, 391, 319, and 72 Ma. These peaks correspond broadly with various proxy records of supercontinent assembly, including the age distributions of granites, detrital zircon grains, and passive margins. Lithium-cesium-tantalum (LCT) pegmatites have a similar age distribution to the undivided granitic pegmatites, with maxima at 2638, 1800, 962, 529, 485, 371, 309, and 274 Ma. Lithium and Ta resources in LCT pegmatites are concentrated in the Archean and Phanerozoic. While there are some Li resources from the Proterozoic, the dominantly bimodal distribution of resources is particularly evident for Ta. This distribution is similar to that of orogenic gold deposits, and has been interpreted to reflect the preservation potential of the orogenic belts where these deposits are formed. Niobium-yttrium-fluorine (NYF) pegmatites show similar age distributions to LCT pegmatites, but with a strong maximum at ca. 1000 Ma.

Petrogenesis of Sveconorwegian magmatism in southwest Norway; constraints from zircon U-Pb-Hf-O and whole-rock geochemistry

NASA Astrophysics Data System (ADS)

Roberts, Nick M. W.; Slagstad, Trond; Parrish, Randall R.; Norry, Michael J.; Marker, Mogens; Horstwood, Matthew S. A.; Røhr, Torkil

2013-04-01

The Sveconorwegian orogen is traditionally interpreted as a Himalayan-scale continental collision, and the eastward continuation of the Grenville Province of Laurentia; however, it has recently been proposed that it represents an accretionary orogen without full-scale continental collision (Slagstad et al., in press). We suggest that magmatism is one of the key constraints to differentiate between different types of orogens; thus, detailed investigation of the timing and petrogenesis of the magmatic record is a requirement for better understanding of the Sveconorwegian orogen as a whole. Here, we present new U-Pb geochronology, zircon Hf-O isotope, and whole-rock geochemical data to constrain the petrogenesis of the early -Sveconorwegian Sirdal Magmatic Belt (SMB). The SMB is a batholithic-scale complex of intrusions that intrudes into most of the Rogaland-Hardangervidda Block in southwest Norway. Current age constraints put emplacement between ~1050 to 1020 Ma. New ages from the Suldal region indicate that the onset of SMB magmatism can be put back to 1070 Ma, which is some 30-50 Myrs prior to high-grade metamorphism. Average initial ɛHf signatures range from ~0 to 4; these overlap with later post-Sveconorwegian granites and with early-/pre-Sveconorwegian ferroan (A-type) granites. Average δ18O signatures range from ~5.7 to 8.7, except for one anomalous granite at ~11.6. The Hf-O signatures are compatible with a mixed mantle-crustal source. Crustal sources may include ~1500 Ma Telemarkian or ~1200 Ma juvenile crust. Hf-O bulk-mixing modelling using a 1500 Ma crustal source indicates >50 % mantle input. Although much further mapping and geochronological work is required, granitic magmatism appears to have persisted throughout much of the ~1100 to 900 Ma period that spans the Sveconorwegian orogen. This magmatism is consistently ferroan (i.e. dry); however, the SMB marks a clear transition to magnesian (i.e. wet) magmatism, with a return to ferroan magmatism at >990 Ma. We propose that this transition corresponds to subduction-driven dehydration-melting of the mantle, producing the SMB in a traditional continental volcanic arc environment. A large lower-crustal input is typical of continental arc batholiths (DeCelles et al., 2009). The interpretation of the SMB as a continental arc is key, but not exclusive, to an accretionary model for the Sveconorwegian orogen. The exact timing and setting of syn-/late-Sveconorwegian 990 to 940 Ma ferroan magmatism thus remains a critical link in the understanding of this orogen. Slagstad et al. (in press) A Non-Collisional, Accretionary Sveconorwegian Orogen. Terra Nova, DOI:10.1111/ter.120012 DeCelles et al. (2009) Cyclicity in Cordilleran orogenic systems. Nature Geoscience 2, 251-257.

Peering into the deep: Illuminating the crustal evolution of the Eucla basement and its relationship to the Albany-Fraser Orogen of southwest Australia.

NASA Astrophysics Data System (ADS)

Hartnady, Michael; Kirkland, Chris; Clark, Chris; Spaggiari, Catherine; Smithies, Hugh

2017-04-01

The Albany-Fraser Orogen is a 1200 km long east to northeasterly trending Palaeoproterozoic to Mesoproterozoic orogenic belt that defines the southern to southeastern margin of the West Australian Craton (WAC). The belt records a long and complex geological history spanning the break-up of Nuna between 2000 and 1700 Ma and amalgamation of Rodinia between 1300 and 1000 Ma. Recent geochronological, geochemical and isotopic work has shown that the Albany-Fraser Orogen formed through a protracted period of reworking of the margin of the Archean Yilgarn Craton (part of the WAC) with various additions of mantle-derived material. The Cretaceous Bight and Cenozoic Eucla Basins partially overlie the northeastern part of the Albany-Fraser Orogen and completely cover 1000 km of crystalline basement (the Eucla basement) that separates the belt from the South Australian Craton. This basement constitutes the glue between the major building blocks of Proterozoic Australia, yet, its geological history is poorly understood. New drill cores penetrating the basement have intersected interlayered granitic and gabbroic rocks that yield U-Pb zircon dates that are dissimilar to any magmatic ages from units within the adjoining Albany-Fraser Orogen, with the exception of the youngest, 1190-1125 Ma magmatic suite. In addition, mantle-like hafnium and neodymium isotopic signatures indicate that the rocks of the Eucla basement are dominated by new juvenile addition, and may represent an allochthonous terrane of oceanic heritage. New ɛHf contour maps for the Albany-Fraser Orogen and Eucla basement highlight this difference. Time-slicing the isotopic dataset reveals a pattern of Palaeoproterozoic juvenile magmatism sub-perpendicular to the present day structural grain in the belt. If this marks the presence of an older lithospheric structure then it demonstrates the power that time-constrained isotopic mapping provides for illuminating lithospheric architecture through time. This may be particularly useful for unravelling crustal evolution in regions with complex tectonic histories.

Temporal evolution of granitic magmas in the Luanchuan metallogenic belt, east Qinling Orogen, central China: Implications for Mo metallogenesis

NASA Astrophysics Data System (ADS)

Li, Dong; Han, Jiangwei; Zhang, Shouting; Yan, Changhai; Cao, Huawen; Song, Yaowu

2015-11-01

The Luanchuan metallogenic belt, located within the eastern part of the Qinling Orogen, central China, hosts a number of world-class Mo deposits that are closely related to small late Mesozoic granitic plutons. Zircon U-Pb dating of distinct plutons in the Luanchuan metallogenic belt has yielded ages of 153 ± 1, 154 ± 2, 152 ± 2, and 148 ± 1 Ma. Molybdenite Re-Os isotopic compositions of Yuku ore district in the southern part of Luanchuan metallogenic belt has yielded an isochron age of 146 ± 1 Ma, which is consistent with the large-scale mineralization ages in the northern part of the Luanchuan metallogenic belt. A combination of previous studies and new geochronological and isotopic data show a concordant temporal and genetic link between granitic magmatism and Mo mineralization in the Luanchuan metallogenic belt, suggesting that this mineralization episode formed the most extensive Mo mineralization belt in the east Qinling Orogen. Zircon grains from Mo-related granitic plutons show similar trace element distributions. High-precision Multi Collector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICP-MS) Pb isotope analysis of K-feldspar megacrysts from mineralization-related granites suggest that they were derived from the lower crust. Similarly, the Pb isotopic compositions of pyrite coprecipitated with molybdenite also suggest that the metals were derived form the lower crust, with probably minor mantle contribution. A continuum mineralization model that describes the sourcing of Mo from an evolving granitic magma over successive differentiation events, possibly in separate but connected magma chambers, could explain the remarkable Mo enrichment in the Luanchuan metallogenic belt. The volatile- and Mo-bearing granitic magmas ascended as diapirs from the deep crust, and were emplaced as dikes in the upper crust. Lithological differences between these Mo-bearing granites may relate to different stages in the evolution of individual magmas. Finally, ore-forming fluids were exsolved from the granitic melts in shallow-crustal magma chambers, resulting in the formation of the Mo deposits of the Luanchuan metallogenic belt. This model may account for the widespread late Mesozoic Mo deposits of eastern China.

Neoproterozoic, Paleozoic, and Mesozoic granitoid magmatism in the Qinling Orogen, China: Constraints on orogenic process

NASA Astrophysics Data System (ADS)

Wang, Xiaoxia; Wang, Tao; Zhang, Chengli

2013-08-01

The Qinling Orogen is one of the main orogenic belts in Asia and is characterized by multi-stage orogenic processes and the development of voluminous magmatic intrusions. The results of zircon U-Pb dating indicate that granitoid magmatism in the Qinling Orogen mainly occurred in four distinct periods: the Neoproterozoic (979-711 Ma), Paleozoic (507-400 Ma), and Early (252-185 Ma) and Late (158-100 Ma) Mesozoic. The Neoproterozoic granitic magmatism in the Qinling Orogen is represented by strongly deformed S-type granites emplaced at 979-911 Ma, weakly deformed I-type granites at 894-815 Ma, and A-type granites at 759-711 Ma. They can be interpreted as the products of respectively syn-collisional, post-collisional and extensional setting, in response to the assembly and breakup of the Rodinia supercontinent. The Paleozoic magmatism can be temporally classified into three stages of 507-470 Ma, 460-422 Ma and ˜415-400 Ma. They were genetically related to the subduction of the Shangdan Ocean and subsequent collision of the southern North China Block and the South Qinling Belt. The 507-470 Ma magmatism is spatially and temporally related to ultrahigh-pressure metamorphism in the studied area. The 460-422 Ma magmatism with an extensive development in the North Qinling Belt is characterized by I-type granitoids and originated from the lower crust with the involvement of mantle-derived magma in a collisional setting. The magmatism with the formation age of ˜415-400 Ma only occurred in the middle part of the North Qinling Belt and is dominated by I-type granitoid intrusions, and probably formed in the late-stage of a collisional setting. Early Mesozoic magmatism in the study area occurred between 252 and 185 Ma, with the cluster in 225-200 Ma. It took place predominantly in the western part of the South Qinling Belt. The 250-240 Ma I-type granitoids are of small volume and show high Sr/Y ratios, and may have been formed in a continental arc setting related to subduction of the Mianlue Ocean between the South Qinling Belt and the South China Block. Voluminous late-stage (225-185 Ma) magmatism evolved from early I-type to later I-A-type granitoids associated with contemporaneous lamprophyres, representative of a transition from syn- to post-collisional setting in response to the collision between the North China and the South China blocks. Late Mesozoic (158-100 Ma) granitoids, located in the southern margin of the North China Block and the eastern part of the North Qinling Belt, are characterized by I-type, I- to A-type, and A-type granitoids that were emplaced in a post-orogenic or intraplate setting. The first three of the four periods of magmatism were associated with three important orogenic processes and the last one with intracontinental process. These suggest that the tectonic evolution of the Qinling Orogen is very complicated.

Age distribution of lithium-cesium-tantalum enriched pegmatites and relationships to orogeny

NASA Astrophysics Data System (ADS)

McCauley, A.; Bradley, D. C.

2011-12-01

Pegmatites account for about one third of the world's lithium production, most of the tantalum, and all of the cesium. Pegmatites enriched in these elements (LCT pegmatites) are widely interpreted as extreme fractionation products of orogenic granitic melts, although it is not always possible to tie a particular pegmatite to a known granite of the same age. The global age distribution of LCT pegmatites is similar to the age distributions of common pegmatites, of orogenic granites, and of detrital zircons. Our geochronological synthesis expands on, and generally confirms, the recent study by Tkachev (2011, Geol. Soc. Spec. Publ. 350, 7). The LCT pegmatite maxima at ca. 2650, 1800, 525, 350, and 100 Ma correspond to times of collisional orogeny and, except for the comparatively minor peak at 100 Ma, to times of supercontinent assembly. Between these pulses are long intervals of few or no LCT pegmatites. Global minima in LCT pegmatite abundance overlap with supercontinent tenures at ca. 2450-2225, 1625-1000, 875-725, and 250-200 Ma, as established, for the Precambrian, from global minima in the abundances of passive margins and detrital zircons. A key question that bears on both metallogenesis and exploration strategies is why are some orogenic belts well endowed with LCT pegmatites, whereas other, seemingly similar orogens are barren? For the present study, LCT pegmatites from the Appalachian, Variscan, Damara, and Argentine Precordilleran orogens are being dated by the U-Pb method to relate pegmatite emplacement to other igneous events, shortening, metamorphism, foreland-basin sedimentation, and, on the broadest scale, to supercontinent assembly. Anecdotal evidence suggests that LCT pegmatites typically are emplaced late in orogenic cycles. In the Inland Branch of the Damaride orogen, about 45 m.y. elapsed between initial arc-passive margin collision at ca. 550 Ma and LCT pegmatite emplacement at ca. 505 Ma, very late in the assembly of this part of Gondwana. In the Appalachian orogen, LCT pegmatites evidently were emplaced at ca. 345 and ca. 275 Ma-long after initial arc-passive margin collision. Neither time is particularly remarkable in the long sequence of Appalachian orogenic events. The ca. 275 Ma event was coeval with the last increment of Appalachian plate convergence during the final assembly of Pangea. Possible triggers for melt generation in various pegmatite provinces include late collisional crustal thickening, shear heating, mantle plumes, slab break-off, and lower lithospheric delamination.

Synchronous partial melting, deformation, and magmatism: evidence from in an exhumed Proterozoic orogen

NASA Astrophysics Data System (ADS)

Levine, J. S. F.; Mosher, S.

2017-12-01

Older orogenic belts that now expose the middle and lower crust record interaction between partial melting, magmatism, and deformation. A field- and microstructural-based case study from the Wet Mountains of central Colorado, an exhumed section of Proterozoic rock, shows structures associated with anatexis and magmatism, from the grain- to the kilometer-scale, that indicate the interconnection between deformation, partial melting, and magmatism, and allow reconstructions of the processes occurring in hot active orogens. Metamorphic grade, along with the degree of deformation, partial melting, and magmatism increase from northwest to southeast. Deformation synchronous with this high-grade metamorphic event is localized into areas with greater quantities of former melt, and preferential melting occurs within high-strain locations. In the less deformed northwest, partial melting occurs dominantly via muscovite-dehydration melting, with a low abundance of partial melting, and an absence of granitic magmatism. The central Wet Mountains are characterized by biotite dehydration melting, abundant former melt and foliation-parallel inferred melt channels along grain boundaries, and the presence of a nearby granitic pluton. Rocks in the southern portion of the Wet Mountains are characterized by partial melting via both biotite dehydration and granitic wet melting, with widespread partial melting as evidenced by well-preserved former melt microstructures and evidence for back reaction between melt and the host rocks. The southern Wet Mountains has more intense deformation and widespread plutonism than other locations and two generations of dikes and sills. Recognition of textures and fabrics associated with partial melting in older orogens is paramount for interpreting the complex interplay of processes occurring in the cores of orogenic systems.

Paleozoic intrusive rocks from the Dunhuang tectonic belt, NW China: Constraints on the tectonic evolution of the southernmost Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Zhao, Yan; Sun, Yong; Diwu, Chunrong; Zhu, Tao; Ao, Wenhao; Zhang, Hong; Yan, Jianghao

2017-05-01

The Dunhuang tectonic belt (DTB) is of great importance for understanding the tectonic evolution of the southernmost Central Asian Orogenic Belt (CAOB). In this study, the temporal-spatial distribution, petrogenesis and tectonic setting of the Paleozoic representative intrusive rocks from the DTB were systematically investigated to discuss crustal evolution history and tectonic regime of the DTB during Paleozoic. Our results reveal that the Paleozoic magmatism within the DTB can be broadly divided into two distinct episodes of early Paleozoic and late Paleozoic. The early Paleozoic intrusive rocks, represented by a suite metaluminous-slight peraluminous and medium- to high-K calc-alkaline I-type granitoids crystallized at Silurian (ca. 430-410 Ma), are predominantly distributed along the northern part of the DTB. They were probably produced with mineral assemblage of eclogite or garnet + amphibole + rutile in the residue, and were derived from magma mixing source of depleted mantle materials with various proportions of Archean-Mesoproterozoic continental crust. The late Paleozoic intrusive rocks can be further subdivided into two stages of late Devonian stage (ca. 370-360 Ma) and middle Carboniferous stage (ca. 335-315 Ma). The former stage is predominated by metaluminous to slight peraluminous and low-K tholeiite to high-K calc-alkaline I-type granitic rocks distributed in the central part of the DTB. They were also generated with mineral assemblage of amphibolite- to eclogite-facies in the residue, and originated from magma source of depleted mantle materials mixed with different degrees of old continental crust. The later stage is represented by adakite and alkali-rich granite exposed in the southern part of the DTB. The alkali-rich granites studied in this paper were possibly produced with mineral assemblage of granulite-facies in the residue and were generated by partial melting of thickened lower continental crust. Zircon Hf isotopes and field distribution of those Paleozoic intrusive rocks reveal that both the Silurian and the late Devonian magmatic activities predominantly represent crustal growth processes in the DTB, accompanied by different degrees of reworking of pre-existing continental crust. However, the middle Carboniferous (ca. 335-315 Ma) magmatic activity reflects a crustal reworking process. The Silurian and late Devonian intrusive rocks were most likely formed in the arc-related subduction zones, whereas, the middle Carboniferous intrusive rocks were possibly formed in a transitional tectonic setting from compression to extension, representing the final stage of Paleozoic orogeny in the DTB. These Paleozoic magmatic rocks further suggest that the DTB has reactivated from a stable block to an orogen and undergone two episodes (the early Paleozoic and the late Paleozoic) of orogeny during Paleozoic. It represents a Paleozoic accretionary orogen of the southernmost margin of the CAOB between the Tarim Craton and North China Craton, and tectonically extends northward to the Beishan orogen and westward to the eastern South Tianshan Belt.

Possible genetic link between I-type granite and orogenic gold deposits in Egypt (metamorphic-magmatic interaction?)

NASA Astrophysics Data System (ADS)

Abd El Monsef, Mohamed

2015-04-01

The orogenic gold deposits are a distinctive type of deposits that revealed unique temporal and spatial association with an orogeny. Where, the system of gold veins and related ore minerals was confined to hydrothermal solutions formed during compressional to transpressional deformation processes at convergent plate margins in accretionary and collisional orogens, with the respect to ongoing deep-crustal, subduction-related thermal processes. In Egypt, most of vein-type and dyke-type gold mineralization are restricted to granitic rocks or at least near of granitic intrusion that seems to have had an important influence on gold mineralization. Shear zone-related, mesothermal gold deposits of Fatira and Gidami mines in the northern Eastern Desert of Egypt are found within granitic bodies or at the contact between granites and metavolcanic rocks. The hosting-granitic rocks in Fatira and Gidami areas are mainly of granodioritic composition (I-Type granite) which is related to calc-alkaline magmatic series. However, Fatira granitoids were developed within island arc tectonic settings related to mature island arc system (Late-orogenic stage), at relatively low temperature (around 660° C) and medium pressure between (5 - 10 Kbar). On the other hand, Gidami granitoids were developed during the collision stage in continental arc regime related to active continental margin (Syn-orogeny), which were crystallized at relatively high temperature (700-720° C) and low pressure (around 0.1 Kbar). The ore mineralogy includes pyrite, chalcopyrite, sphalerite, covellite, ilmenite, goethite ± pyrrhotite ± pentlandite ± galena ± molybdenite. Native gold is detected only in Gidami mineralization as small inclusions within pyrite and goethite or as tiny grains scattered within quartz vein (in close proximity to the sulfides). In Fatira deposits, it is detected only by microprobe analysis within the crystal lattice of pyrite and jarosite. Fluid inclusions study for the mineralized ores revealed two main groups of fluid inclusions in both areas: A) Aqueous inclusions (H2O-NaCl±KCl system), and B) Carbonic inclusions (H2O-CO2-NaCl±CH4). A drop of pressure during the migration of these fluids to shallower depths along the shear zones was the main reason for phase separation. Isochores calculation from microthermometric results proved that, the P-T boundary conditions outlined for Fatira gold deposits are of 275° to 297° C and between (0.2 - 1.2 Kbar); and of 277° to 300° C and between (0.2 - 1 Kbar) for Gidami gold deposits. The normalization Chondrite patterns of rare earth elements (REEs) for the gold-ore deposits with the surrounding I-type granitic rocks exhibit an obvious similarity and positive correlation. The geological, mineralogical, geochemical and fluid inclusions studies revealed a genetic link between gold mineralization and intrusion of calc-alkaine granitic magma. Whereas, The granitic magma acts as a supplier for the ore-bearing fluid and as a heat source for metamorphic processes, leading to hydrothermal convection currents.

A-type and I-type granitoids and mylonitic granites of Hassan Salaran area of SE Saqqez, Kurdistan, Iran

NASA Astrophysics Data System (ADS)

Abdullah, Fakhraddin Mohammad; Saeed Ahmad, Sheler

2014-05-01

The Hassan Salarn area is located 20km to southeast of Saqqez city in Kurdistan Province, western Iran. In this area there are two distinct granitic rock suites consisting A-type and I-type granites and also mylonitic granites. These A-type and I-type granites have various petrological and geochemical characteristics. They also have different origins and petrogenesis. A-type granitoids comprise alkali feldspar granite, syenogranite and quartz alkali feldspar syenite, whereas I-type granitoids are composed of monzogranite, granodiorite and tonalite. Geochemically, A-type granitoids are peralkaline and acmite-normative but I-type granitoids are subalkaline (calc-alkaline), metaluminous and diopside-normative. A-type granitoids are also ferroan alkali and ferroan alkali-calcic whereas I-type granitoids are magnesian and calcic. A-type granitoids resemble to within plate granites and post-orogenic granites whereas I-type granitoids resemble to volcanic arc granites. A-type granitoids contain higher concentrations of alkalies, Zr, Rb, Nb, Y, Th, Ce, high FeO/MgO ratios and lower concentrations of Mg, Ca and Sr, resembling post-orogenic A-type granites. It is possible that heat from a mantle-derived magma which intruded into the lower crust, and/or rapid crustal extension have been essential generation of approriate melts producing A-type granitoids. Thus we can conclude that A-type granitoids were generated from a mixed mantle-crust source. Negative Nb anomalies and low contents of Ti and P probably indicate a subduction-related origin for protolith of I-type granitoids. Negative Nb anomalies and enrichment in Ce relative to its adjacent elements can be related to involvement of continental crust in magmatic processes. I-type granitoids are also enriched in Rb, Ba, K, Th, Ce and depleted in Nb, Zr and Y, indicating that they have had interacted with crust. I-type granitoids may result from contamination of mantle-derived magmas by continental crust during a subduction event. The mylonitic granites are elongated masses with a NE-SW trend and their contacts with the A-type and I-type granitoids are fault contact. Hand specimens have a layered appearance with green bands made from chlorite and epidote and grey to white bands with quartz and feldspar. These rocks contain plagioclase, quartz and orthoclase under the microscope. Also fine-grained minerals such as quartz, sericite, epidote, chlorite and opaque minerals make the groundmass wrapping the porphyroclasts. Pressure shadows around porphyroclasts of plagioclase and quartz and crystallization of fine-grained quartz and sericite in these places along with intense alteration of plagioclase to epidote and sericite, existence of quartz with different sizes, andaluse extinction in quartz crystals, and elongation of chlorites, resulted from dynamic recrystallisation of biotites all indicate effect of stresses on the rocks. Considering the similar mineralogical composition of the mylonitic rocks with I-type granitoid, it could be concluded that the granodioritic magma, after intrusion and solidification, is changed to mylonite in a shear zone due to tectonical forces.

Zircon and cassiterite U-Pb ages, petrogeochemistry and metallogenesis of Sn deposits in the Sibao area, northern Guangxi: constraints on the neoproterozoic granitic magmatism and related Sn mineralization in the western Jiangnan Orogen, South China

NASA Astrophysics Data System (ADS)

Chen, Lei; Wang, Zongqi; Yan, Zhen; Gong, Jianghua; Ma, Shouxian

2018-01-01

A number of Sn deposits associated with Neoproterozoic granites are located in the western Jiangnan Orogen of northern Guangxi. The distribution of Sn mineralization is controlled by faults occurring within and around the Neoproterozoic granites. The hydrothermal alteration and mineralization of these Sn deposits exhibit zoning from the granite to the wall rock. The laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb ages of the cassiterite and zircon from ore-bearing granite in the Menggongshan Sn deposit are 829 ± 19 Ma and 822 ± 4 Ma, respectively, indicating that the Sn mineralization and granites formed in the Neoproterozoic and can considered to be products of coeval magmatic and hydrothermal activities. The ore-bearing granite and Neoproterozoic granites in northern Guangxi are high-K, calc-alkaline, peraluminous, S-type granites that are depleted in Nb, Ti, Sr and Ba and highly enriched in Rb, U and Pb. All the granites show steep fractionated light rare earth element (LREE) and flat heavy rare earth element (HREE) patterns, with strongly negative Eu anomalies. The ɛHf(t) values of the ore-bearing granite vary from - 9.0 to - 1.7, with an average value of - 4.1. Additionally, the ore-bearing granite exhibits low oxygen fugacity values. The magmatic source experienced partial melting during their evolution, and the source was dominated by recycled heterogeneous continental crustal materials. Our evidence confirms that the Neoproterozoic granites in northern Guangxi formed in a collisional tectonic setting. The collision between the Cathaysia and Yangtze blocks or between the Sibao arc (Jiangnan arc) and the Yangtze Block caused asthenospheric upwelling, leading to partial melting and recycling of the crust, forming the peraluminous S-type granites in the Neoproterozoic. The Sn mineralization has a close genetic relationship with the Neoproterozoic granite. The highly differentiated, peraluminous, B-enriched, crustally derived Neoproterozoic granites provided the heat source and ore-forming material for the Sn mineralization. Furthermore, the low oxygen fugacity of these Neoproterozoic granites also favoured the Sn mineralization.

Early Paleozoic dioritic and granitic plutons in the Eastern Tianshan Orogenic Belt, NW China: Constraints on the initiation of a magmatic arc in the southern Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Du, Long; Long, Xiaoping; Yuan, Chao; Zhang, Yunying; Huang, Zongying; Sun, Min; Zhao, Guochun; Xiao, Wenjiao

2018-03-01

Early Paleozoic dioritic and granitic plutons in the Eastern Tianshan Orogenic Belt (ETOB) have been studied in order to constraint the initiation of a magmatic arc formed in this region. Zircon U-Pb dating indicates that two dioritic plutons in the northern ETOB were generated in the Late Ordovician (452 ± 4 Ma) and the Early Silurian (442 ± 3 Ma), respectively. Diorites from the two plutons are characterized by enrichments in large ion lithophile elements (LILE) and highly incompatible elements, with depletions in high field strength elements (HSFE) displaying typical geochemical features of a subduction-related origin. They have positive εNd(t) values (+5.08-+6.58), relatively young Nd model ages (TDM = 0.71-1.08 Ga), with Ta/Yb (0.05-0.09) and Nb/Ta ratios (12.06-15.19) similar to those of depleted mantle, suggesting a juvenile mantle origin. Their high Ba/La (13.3-35.9), low Th/Yb (0.72-2.02), and relatively low Ce/Th (4.57-14.7) and Ba/Th (47.8-235) ratios indicate that these diorites were probably produced by partial melting of a depleted mantle wedge metasomatized by both subducted sediment-derived melts and slab-derived aqueous fluids. Zircon U-Pb dating of a granitic pluton in the northern ETOB yielded a Late Ordovician intrusion age of 447 ± 5 Ma. Granites from this pluton show calc-alkaline compositions with geochemical characteristics of I-type granites. They also show positive εNd(t) values (+6.49-+6.95) and young Nd model ages (TDM = 0.69-0.87 Ga), indicating that the granites were most likely derived from juvenile lower crust. Our new dating results on the dioritic and granitic plutons suggest that arc-type magmatism in the northern ETOB began prior to or at the Late Ordovician (452-442 Ma). In addition, north-dipping subduction of the Kangguertage oceanic lithosphere may account for the arc-type magmatism and the geodynamic process of the ETOB in the Early Paleozoic.

Geochemistry, mineralogy, and zircon U-Pb-Hf isotopes in peraluminous A-type granite xenoliths in Pliocene-Pleistocene basalts of northern Pannonian Basin (Slovakia)

NASA Astrophysics Data System (ADS)

Huraiová, Monika; Paquette, Jean-Louis; Konečný, Patrik; Gannoun, Abdel-Mouhcine; Hurai, Vratislav

2017-08-01

Anorogenic granite xenoliths occur in alkali basalts coeval with the Pliocene-Pleistocene continental rifting of the Pannonian Basin. Observed granite varieties include peraluminous, calcic to peralkalic, magnesian to ferroan types. Quartz and feldspars are dominant rock-forming minerals, accompanied by minor early ilmenite and late magnetite-ulvöspinel. Zircon and Nb-U-REE minerals (oxycalciopyrochlore, fergusonite, columbite) are locally abundant accessory phases in calc-alkalic types. Absence of OH-bearing Fe, Mg-silicates and presence of single homogeneous feldspars (plagioclase in calcic types, anorthoclase in calc-alkalic types, ferrian Na-sanidine to anorthoclase in alkalic types) indicate water-deficient, hypersolvus crystallization conditions. Variable volumes of interstitial glass, absence of exsolutions, and lacking deuteric hydrothermal alteration and/or metamorphic/metasomatic overprint are diagnostic of rapid quenching from hypersolidus temperatures. U-Pb zircon ages determined in calcic and calc-alkalic granite xenoliths correspond to a time interval between 5.7 and 5.2 Ma. Positive ɛHf values (14.2 ± 3.9) in zircons from a 5.2-Ma-old calc-alkalic granite xenolith indicate mantle-derived magmas largely unaffected by the assimilation of crustal material. This is in accordance with abundances of diagnostic trace elements (Rb, Y, Nb, Ta), indicating A1-type, OIB-like source magmas. Increased accumulations of Nb-U-REE minerals in these granites indicate higher degree of the magmatic differentiation reflected in Rb-enrichment, contrasting with Ba-enrichment in barren xenoliths. Incipient charnockitization, i.e. orthopyroxene and ilmenite crystallization from interstitial silicate melt, was observed in many granite xenoliths. Thermodynamic modeling using pseudosections showed that the orthopyroxene growth may have been triggered by water exsolution from the melt during ascent of xenoliths in basaltic magma. Euhedral-to-skeletal orthopyroxene growth probably reflects contrasting ascent rates of basaltic magma with xenoliths, intermitted by the stagnation in various crustal levels at a <3 kbar pressure. The Tertiary suite of intra-plate, mantle-derived A1-type granites and syenites is geochemically distinct from pre-Tertiary, post-orogenic A2-type granites of the Carpatho-Pannonian region, which exhibit geochemical features diagnostic of crustal melting along continental margins.

Miocene rapakivi granites in the southern Death Valley region, California, USA

USGS Publications Warehouse

Calzia, J.P.; Ramo, O.T.

2005-01-01

Rapakivi granites in the southern Death Valley region, California, include the 12.4-Ma granite of Kingston Peak, the ca. 10.6-Ma Little Chief stock, and the 9.8-Ma Shoshone pluton. All of these granitic rocks are texturally zoned from a porphyritic rim facies, characterized by rapakivi textures and miarolitic cavities, to an equigranular aplite core. These granites crystallized from anhydrous and peraluminous to metaluminous magmas that were more oxidized and less alkalic than type rapakivi granites from southern Finland. Chemical and isotope (Nd-Sr-Pb) data suggest that rapakivi granites of the southern Death Valley region were derived by partial melting of lower crustal rocks (possibly including Mesozoic plutonic component) with some mantle input as well; they were emplaced at shallow crustal levels (4 km) in an actively extending orogen.

Miocene rapakivi granites in the southern Death Valley region, California, USA

USGS Publications Warehouse

Calzia, James P.; Ramo, O.T.

2005-01-01

Rapakivi granites in the southern Death Valley region, California, include the 12.4-Ma granite of Kingston Peak, the ca. 10.6-Ma Little Chief stock, and the 9.8-Ma Shoshone pluton. All of these granitic rocks are texturally zoned from a porphyritic rim facies, characterized by rapakivi textures and miarolitic cavities, to an equigranular aplite core. These granites crystallized from anhydrous and peraluminous to metaluminous magmas that were more oxidized and less alkalic than type rapakivi granites from southern Finland. Chemical and isotope (Nd–Sr–Pb) data suggest that rapakivi granites of the southern Death Valley region were derived by partial melting of lower crustal rocks (possibly including Mesozoic plutonic component) with some mantle input as well; they were emplaced at shallow crustal levels (4 km) in an actively extending orogen.

Molybdenum mineralization related to the Yangtze's lower crust and differentiation in the Dabie Orogen: Evidence from the geochemical features of the Yaochong porphyry Mo deposit

NASA Astrophysics Data System (ADS)

Liu, Qing-Quan; Li, Bin; Shao, Yong-Jun; Lu, An-Huai; Lai, Jian-Qing; Li, Yong-Feng; Luo, Zheng-Zhuan

2017-06-01

The Dabie Orogen is a world-class case for large amounts of Mo mineralization in that it contains at least 10 porphyry Mo deposits with Mo metal reserves over 3 Mt from the time period of 156-110 Ma. However, the principal mechanism for the Mo mineralization remains controversial due to the lack of a precise definition of its source and shallow ore-forming process, which is essential to understand these rare large Mo deposits. Detailed geochronology, geochemistry, and isotopic data for ore-related granites and minerals were analyzed in order to place constraints on the massive Mo mineralization in the Dabie Orogen in eastern China. The Yaochong molybdenum orebodies were hosted in the transition belt and alteration zone between the granitic stocks and the Dabie Complex and were characterized as numerous veinlets with potassic, phyllic and propylitic alterations. The buried Yaochong granitic intrusions and associated molybdenum mineralization yield Early Cretaceous ages of magmatic activities at ca. 138 Ma and extremely similar Re-Os isotope ages for the corresponding Mo metallogenic event at ca. 137 Ma. The Yaochong monzogranite and granite porphyry belong to the highly fractionated I-type granites, which are believed to be derived from the dominantly Yangtze's lower crust mixed with the Northern Dabie Complex due to their geochemical and isotope features. The elemental diversity and isotopic homogeneity suggest that the formation of the Yaochong monzogranite involved the fractionation of biotite, garnet and minor feldspar and accessory minerals combined with a weak crustal assimilation process. In contrast, the granite porphyry was possibly generated by the partial melting of the same mixed lower continental crust via the differentiation process involving the fractionation of feldspar, apatite, and/or titanite. Fractional crystallization processes can significantly elevate the molybdenum concentration in the residual melts. The biotite fractional crystallization results in removal of molybdenum from the vestigial magma since molybdenum is compatible with it, which may account for the barren monzogranite with a low-grade molybdenum mineralization. The ore-bearing granite porphyry has more source materials from the Yangtze's lower crust, which may have contributed most of the molybdenum for the porphyry-related molybdenum deposits in the Dabie metallogenic zone. This porphyry molybdenum aggregation may have been deposited in a post-collision or intracontinental extensional setting.

Zircon dating and mineralogy of the Mokong Pan-African magmatic epidote-bearing granite (North Cameroon)

NASA Astrophysics Data System (ADS)

Tchameni, R.; Sun, F.; Dawaï, D.; Danra, G.; Tékoum, L.; Nomo Negue, E.; Vanderhaeghe, O.; Nzolang, C.; Dagwaï, Nguihdama

2016-09-01

We present the mineralogy and age of the magmatic epidote-bearing granite composing most of the Mokong pluton, in the Central Africa orogenic belt (North Cameroon). This pluton intrudes Neoproterozoic (~830 to 700 Ma) low- to high-grade schists and gneisses (Poli-Maroua group), and is crosscut or interleaved with bodies of biotite granite of various sizes. The pluton is weakly deformed in its interior, but solid-state deformation increases toward its margins marked by narrow mylonitic bands trending NNE-SSW. The magmatic epidote granitic rocks are classified as quartz monzodiorite, granodiorite, monzogranite, and syenogranite. They are medium- to coarse-grained and composed of K-feldspar + plagioclase + biotite + amphibole + epidote + magnetite + titanite + zircon + apatite. In these granites, the pistacite component [atomic Fe+3/(Fe3+ + Al)] in epidote ranges from 16 to 29 %. High oxygen fugacity (log ƒO2 - 14 to -11) and the preservation of epidote suggest that the magma was oxidized. Al-in hornblende barometry and hornblende-plagioclase thermometry indicate hornblende crystallization between 0.53 and 0.78 GPa at a temperature ranging from 633 to 779 °C. Zircon saturation thermometry gives temperature estimates ranging from 504 to 916 °C, the latter being obtained on samples containing inherited zircons. U/Pb geochronology by LA-ICP-MS on zircon grains characterized by magmatic zoning yields a concordia age of 668 ± 11 Ma (2 σ). The Mokong granite is the only known occurrence magmatic epidote in Cameroon, and is an important milestone for the comparison of the Central Africa orogenic belt with the Brasiliano Fold Belt, where such granites are much more abundant.

Continental origin of the Gubaoquan eclogite and implications for evolution of the Beishan Orogen, Central Asian Orogenic Belt, NW China

NASA Astrophysics Data System (ADS)

Saktura, Wanchese M.; Buckman, Solomon; Nutman, Allen P.; Belousova, Elena A.; Yan, Zhen; Aitchison, Jonathan C.

2017-12-01

The Gubaoquan eclogite occurs in the Paleozoic Beishan Orogen of NW China. Previously it has been interpreted as a fragment of subducted oceanic crust that was emplaced as a mélange within continental rocks. Contrary to this, we demonstrate that the Gubaoquan eclogite protolith was a Neoproterozoic basic dyke/sill which intruded into Proterozoic continental rocks. The SHRIMP Usbnd Pb zircon dating of the metamorphic rims of the Gubaoquan eclogite yields an age 466 ± 27 Ma. Subdued heavy rare earth element abundances and lack of negative Eu anomalies of the metamorphic zircon domains confirm that this age represents eclogite facies metamorphism. The host augen orthogneiss has a Usbnd Pb zircon age of 920 ± 14 Ma, representing the timing of crystallization of the granitic protolith. A leucogranitic vein which intrudes the eclogite has a Usbnd Pb zircon age of 424 ± 8.6 Ma. This granitic vein marks the end of high-grade metamorphism in this area. The overcomplication of tectonic history of the Beishan Orogen is partially caused by inconsistent classifications and nomenclature of the same rock units and arbitrary subdivisions of Precambrian blocks as individual microcontinents. In an attempt to resolve this, we propose a simpler model that involves the partial subduction of the northern passive margin of the Dunhuang Block beneath the active continental margin developing on the Mazongshan-Hanshan Block to the north. Ocean closure and continental collision during the Late Ordovician resulted in continental thickening and eclogite facies metamorphism recorded by the mafic dykes/sills (now the Gubaoquan eclogite). In the light of the new data, the tectonothermal evolution of the Beishan Orogen is reviewed and integrated with the evolution of the Central Asian Orogenic Belt.

Formation of Cretaceous Cordilleran and post-orogenic granites and their microgranular enclaves from the Dalat zone, southern Vietnam: Tectonic implications for the evolution of Southeast Asia

NASA Astrophysics Data System (ADS)

Shellnutt, J. Gregory; Lan, Ching-Ying; Van Long, Trinh; Usuki, Tadashi; Yang, Huai-Jen; Mertzman, Stanley A.; Iizuka, Yoshi; Chung, Sun-Lin; Wang, Kuo-Lung; Hsu, Wen-Yu

2013-12-01

Cordilleran-type batholiths are useful in understanding the duration, cyclicity and tectonic evolution of continental margins. The Dalat zone of southern Vietnam preserves evidence of Late Mesozoic convergent zone magmatism superimposed on Precambrian rocks of the Indochina Block. The Dinhquan, Deoca and Ankroet plutons and their enclaves indicate that the Dalat zone transitioned from an active continental margin producing Cordilleran-type batholiths to highly extended crust producing within-plate plutons. The Deoca and Dinhquan plutons are compositionally similar to Cordilleran I-type granitic rocks and yield mean zircon U/Pb ages between 118 ± 1.4 Ma and 115 ± 1.2 Ma. Their Sr-Nd whole rock isotopes (ISr = 0.7044 to 0.7062; εNd(T) = - 2.4 to + 0.2) and zircon Hf isotopes (εHf(T) = + 8.2 ± 1.2 and + 6.4 ± 0.9) indicate that they were derived by mixing between a mantle component and an enriched component (i.e. GLOSS). The Ankroet pluton is chemically similar to post-orogenic/within-plate granitic rocks and has a zircon U/Pb age of 87 ± 1.6 Ma. Geobarometric calculations indicate that amphibole within the Ankroet pluton crystallized at a depth of ~ 6 kbar which is consistent with the somewhat more depleted Sr-Nd isotope (ISr = 0.7017 to 0.7111; εNd(T) = - 2.8 to + 0.6) and variable εHf(T) compositions suggesting a stronger influence of crustal material in the parental magma. The compositional change of the Dalat zone granitic rocks during the middle to late Cretaceous indicates that the tectonic regime evolved from a continental arc environment to one of post-orogenic extension. The appearance of sporadic post-90 Ma magmatism in the Dalat zone and along the eastern margin of Eurasian indicates that there was no subsequent orogenic event and the region was likely one of highly extended crust that facilitated the opening of the South China Sea during the latter half of the Cenozoic.

Late Triassic granitic rocks of the Central Qiangtang Orogenic Belt, northern Tibet: tracing crustal thickening through post-collisional silicic magmatism

NASA Astrophysics Data System (ADS)

Wu, H.; Chen, J.

2017-12-01

The Central Qiangtang Orogenic Belt (CQOB) was formed through Triassic continental collision between the Southern and Northern Qiangtang terranes. Numerous granitic intrusions occur along the CQOB, forming a Late Triassic granitic belt that stretches 1000 km from west to east. This Central Qiangtang granitic belt was believed to constitute most of the CQOB. Therefore, the CQOB thus provides a typical composite orogen for the study of relationships between granitoid magmatism and orogenic processes. Recently, many studies have been carried out, and the close relationship of the magmatic belt with the evolutionary history of the CQOB is well established. Late Triassic intrusive rocks are widely exposed in the Riwanchaka area of Central Qiangtang, northern Tibet. In this study, new U-Pb zircon ages reveal that Late Triassic magmatism in Riwanchaka took place at ca 225-205 Ma, coeval with exhumation of the metamorphic rocks in Central Qiangtang. Our new and previously published data enable us to correlate the subduction-related volcanic arc rocks in the Riwanchaka area to a post-collisional extension setting related to slab break-off during northward subduction of the Paleo-Tethys Ocean seafloor. Geochemical characteristics suggested that the samples from CQOB can be divided into low-Sr/Y granitoids (LSG) and high-Sr/Y granitoids (HSG). The LSG are normal calc-alkaline I-type granitoids, characterized by varying major and trace element contents indicative of partial melting of ancient mafic lower crust. The HSG are characterized by high Sr/Y ratios and (La/Yb)N (chondrite-normalized) ratios. These signatures indicate that the HSG were derived by partial melting of garnet-bearing thickened lower crust. The crustal structure and evolution of the CQOB are considered on the basis of available data and variations in Sr/Y, La/Yb, and Hf isotopic ratios. Temporal geochemical and Hf isotopic changes, diagnostic of crustal thickening, indicate that the CQOB was greatly thickened (>50 km) within a short timespan ( 20 Myr). Our new data, together with recently published data, lead us to propose that basaltic underplating caused by slab break-off, contributed significantly to crustal thickening of the CQOB.

40 Ma of hydrothermal W mineralization during the Variscan orogenic evolution of the French Massif Central revealed by U-Pb dating of wolframite

NASA Astrophysics Data System (ADS)

Harlaux, Matthieu; Romer, Rolf L.; Mercadier, Julien; Morlot, Christophe; Marignac, Christian; Cuney, Michel

2018-01-01

We present U-Pb thermal ionization mass spectrometer (TIMS) ages of wolframite from several granite-related hydrothermal W±Sn deposits in the French Massif Central (FMC) located in the internal zone of the Variscan belt. The studied wolframite samples are characterized by variable U and Pb contents (typically <10 ppm) and show significant variations in their radiogenic Pb isotopic compositions. The obtained U-Pb ages define three distinct geochronological groups related to three contrasting geodynamic settings: (i) Visean to Namurian mineralization (333-327 Ma) coeval with syn-orogenic compression and emplacement of large peraluminous leucogranites (ca. 335-325 Ma), (ii) Namurian to Westphalian mineralization (317-315 Ma) synchronous with the onset of late-orogenic extension and emplacement of syn-tectonic granites (ca. 315-310 Ma) and (iii) Stephanian to Permian mineralization (298-274 Ma) formed during post-orogenic extension contemporaneous with the Permian volcanism in the entire Variscan belt. The youngest ages (276-274 Ma) likely reflect the reopening of the U-Pb isotopic system after wolframite crystallization and may correspond to late hydrothermal alteration (e.g. ferberitization). Our results demonstrate that W(±Sn) mineralization in the FMC formed during at least three distinct hydrothermal events in different tectono-metamorphic settings over a time range of 40 Ma.

Link between the granitic and volcanic rocks of the Bushveld Complex, South Africa

NASA Astrophysics Data System (ADS)

Schweitzer, J. K.; Hatton, C. J.; De Waal, S. A.

1997-02-01

Until recently, it was proposed that the Bushveld Complex, consisting of the extrusive Rooiberg Group and the intrusive Rashoop Granophyre, Rustenburg Layered and Lebowa Granite Suites, evolved over a long period of time, possibly exceeding 100 Ma. Most workers therefore considered that the various intrusive and extrusive episodes were unrelated. Recent findings suggest that the intrusive, mafic Rustenburg Layered Suite, siliceous Rashoop Granophyre Suite and the volcanic Rooiberg Group were synchronous, implying that the Bushveld igneous event was short-lived. Accepting the short-lived nature of the complex, the hypothesis that the granites are genetically unrelated to the other events of the Bushveld Complex can be reconsidered. Re-examination of the potential Rooiberg Group/Lebowa Granite Suite relationship suggests that the granites form part of the Bushveld event. Rhyolite lava, granite and granophyre melts originated from a source similar in composition to upper crustal rocks. This source is interpreted to have been melted by a thermal input associated with a mantle plume. Granite intruded after extrusion of the last Rooiberg rhyolite, or possibly overlapped in time with the formation of the youngest volcanic flows.

Evolution of the Mount Woods Inlier, northern Gawler Craton, Southern Australia: an integrated structural and aeromagnetic analysis

NASA Astrophysics Data System (ADS)

Betts, Peter G.; Valenta, Rick K.; Finlay, Jim

2003-05-01

Structural mapping integrated with interpretation and forward modelling of aeromagnetic data form complimentary and powerful tools for regional structural analysis because both techniques focus on architecture and overprinting relationships. This approach is used to constrain the geometry and evolution of the sparsely exposed Mount Woods Inlier in the northern Gawler Craton. The Mount Woods Inlier records a history of poly-phase deformation, high-temperature metamorphism, and syn- and post-orogenic magmatism between ca. 1736 and 1584 Ma. The earliest deformation involved isoclinal folding, and the development of bedding parallel and axial planar gneissic foliation (S 1). This was accompanied by high-temperature, upper amphibolite to granulite facies metamorphism at ca. 1736 Ma. During subsequent north-south shortening (D 2), open to isoclinal south-southeast-oriented F 2 folds developed as the Palaeoproterozoic successions of the inlier were thrust over the Archaean nuclei of the Gawler Craton. The syn-D 2 Engenina Adamellite was emplaced at ca. 1692 Ma. The post-D 2 history involved shear zone development and localised folding, exhumation of metamorphic rocks, and deposition of clastic sediments prior to the emplacement of the ca. 1584 Ma Granite Balta Suite. The Mount Woods Inlier is interpreted as the northern continuation of the Kimban Orogen.

A Silurian-early Devonian slab window in the southern Central Asian Orogenic Belt: Evidence from high-Mg diorites, adakites and granitoids in the western Central Beishan region, NW China

NASA Astrophysics Data System (ADS)

Zheng, Rongguo; Xiao, Wenjiao; Li, Jinyi; Wu, Tairan; Zhang, Wen

2018-03-01

The Beishan orogenic belt is a key region for deciphering the accretionary processes of the southern Central Asian Orogenic Belt. Here in this paper we present new zircon U-Pb ages, bulk-rock major and trace element, and zircon Hf isotopic data for the Baitoushan, and Bagelengtai plutons in the western Central Beishan region to address the accretionary processes. The Baitoushan pluton consists of quartz diorites, monzonites and K-feldspar granites, with zircon LA-ICP-MS U-Pb ages of 435 Ma, 421 Ma and 401 Ma, respectively. The Baitoushan quartz diorites and quartz monzonites exhibit relatively high MgO contents and Mg# values (63-72), display enrichments in LILEs and LREEs, and exhibit high Ba (585-1415 ppm), Sr (416-570 ppm) and compatible element (such as Cr and Ni) abundances, which make them akin to typical high-Mg andesites. The Baitoushan quartz diorites and quartz monzonites were probably generated by the interaction of subducted oceanic sediment-derived melts and mantle peridotites. The Baitoushan K-feldspar granites are ascribed to fractionated I-type granites with peraluminous and high-K calc-alkaline characteristics. They exhibit positive εHf(t) values (2.43-7.63) and Mesoproterozoic-Neoproterozoic zircon Hf model ages (0.92-1.60 Ga). Those early Devonian granites, including Baitoushan K-feldspar granite and Gongpoquan leucogranites (402 Ma), are derived from melting of the mafic lower crust and/or sediments by upwelling of hot asthenospheric mantle. The Bagelengtai granodiorites exhibit similar geochemical signatures with that of typical adakites, with a zircon SHRIMP U-Pb age of 435 Ma. They exhibit relatively high Sr (502-628 ppm) and Al2O3 (16.40-17.40 wt.%) contents, and low MgO (1.02-1.29 wt.%), Y (3.37-6.94 ppm) and HREEs contents, with relatively high Sr/Y and (La/Yb)N ratios. The Bagelengtai granodiorites were derived from partial melting of subducted young oceanic crust, with significant contributions of subducted sediments, subsequently hybridized by the mantle peridotite. On the basis of our data and combined with previous data, we conclude that a Silurian-early Devonian slab window model accounts for all geological records in the western segment of the Central Beishan. We further propose that magmatic events associated with a slab window played an important role in crustal growth of the Central Asian Orogenic Belt.

Rhyacian A-type tholeiitic granites in southern Brazil: Geochemistry, U-Pb zircon ages and Nd model ages

NASA Astrophysics Data System (ADS)

Mesquita, Maria José; Bitencourt, Maria de Fátima; Nardi, Lauro Stoll; Picanço, Jefferson; Chemale, Farid, Jr.; Pimenta, Vanessa de Almeida

2017-04-01

In the southern South American platform, 2.5 to 2.0 Ga terranes, probably related to the Atlantica supercontinent, occur mainly as minor reworked inliers within Neoproterozoic, Brasiliano/Pan-African orogenic belts, as the Ribeira Belt in southern Brazil. The dispersion of such fragments has generated uncertainties about their geotectonic reconstruction, and their study has been supported mainly by elemental and isotope geochemistry. The southern Ribeira Belt lies between the Paranapanema and Luiz Alves cratons and contains reworked Neoarquean and Paleoproterozoic terranes which outcrop as basement nuclei in supracrustal sequences, as the Setuva Complex. The Água Comprida Suite, situated in the northern part of the Setuva Complex, consists of Amphibole-Biotite Syenogranite (ABS), Porphyritic Biotite Syenogranite (PBS), and Equigranular Biotite Syenogranite (EBS). All granites are foliated and intensively deformed. The oldest foliation (Sn) is marked by augen feldspars set in a recrystallized matrix, followed by a crenulation cleavage (Sn + 1) which evolves to discrete shear zones. ABS is a metaluminous, reduced A-type granite with FeOt / (FeOt + MgO) > 0.9, with high HFSE and REE contents, corresponding to magmas related to continental medium to high-K tholeiitic series. PBS and specially EBS are highly differentiated, metaluminous to peraluminous (EBS), oxidized granites. The increase of Al2O3 and Rb, and decrease of HFS and RE elements relative to ABS indicate their evolution from tholeiitic magmas. The Água Comprida Suite granites are cogenetic rocks evolved from a within-plate mantle source, marked by high Nb, Ta, and Y. The influence of previously metasomatised mantle sources is evidenced by negative Nb, Ti, and P anomalies. The age of ABS is 2187 ± 26 Ma, and that of PBS is between 2180 ± 13 to 2186 ± 22 Ma. The Nd model age of 2.4 Ga, and εNd(2.18 Ga) between - 0.23 and - 0.27 support the interpretation of ABS being formed from juvenile material with a minor crustal component. The deviation of εNd(2.18 Ga) for PBS and EBS (- 0.69 to - 2.65), plus the higher Nd model ages (2.4 to 2.5 Ga) suggest crustal contamination. A post-collisional environment for the interaction of the mafic tholeiitic and felsic crustal magmas is suggested for the formation of the Água Comprida Suite, based mainly on geochemical features of these magmas. Post-collisional environments are still not described in the Rhyacian orogenies of southern Brazil, and this proposition may be an important contribution for the study of the final period of amalgamation of the Paleoproterozoic supercontinent in southern South America.

Origins of igneous microgranular enclaves in granites: the example of Central Victoria, Australia

NASA Astrophysics Data System (ADS)

Clemens, J. D.; Elburg, M. A.; Harris, C.

2017-10-01

To investigate their genesis and relations with their host rocks, we study igneous microgranular enclaves (IMEs) in the c. 370 Ma, post-orogenic, high-level, felsic plutons and volcanic rocks of Central Victoria, Australia. The IMEs are thermally quenched magma globules but are not autoliths, and they do not form mixing series with their host magmas. These IMEs generally represent hybrids between mantle-derived magmas and very high- T crust-derived melts, modified by fractionation, ingestion of host-derived crystals and, to a lesser extent, by chemical interactions with their hosts. Isotopic and elemental evidence suggests that their likely mafic progenitors formed by partial melting of subcontinental mantle, but that the IME suites from different felsic host bodies did not share a common initial composition. We infer that melts of heterogeneous mantle underwent high- T hybridisation with melts from a variety of crustal rocks, which led to a high degree of primary variability in the IME magmas. Our model for the formation of the Central Victorian IMEs is likely to be applicable to other occurrences, especially in suites of postorogenic granitic magmas emplaced in the shallow crust. However, there are many different origins for the mingled magma globules that we call IMEs, and different phenomena seem to occur in differing tectonic settings. The complexity of IME formation means that it is difficult to unravel the petrogenesis of these products of chaotic magma processes. Nevertheless, the survival of fine-grained, non-equilibrium mineralogy and texture in the IMEs suggests that their tenure in the host magmas must have been geologically brief.

Age, tectonic setting, and metallogenic implication of Phanerozoic granitic magmatism at the eastern margin of the Xing'an-Mongolian Orogenic Belt, NE China

NASA Astrophysics Data System (ADS)

Chen, Cong; Ren, Yunsheng; Zhao, Hualei; Yang, Qun; Shang, Qingqing

2017-08-01

The eastern margin of the Xing'an-Mongolian Orogenic Belt is characterised by widespread Phanerozoic granitic magmatism, some of which is closely related to significant ore mineralisation. This paper presents new geochronological, petrogenetic, and tectonic data for selected intrusions. Zircon U-Pb geochronology for five granitoid plutons indicates they were emplaced during the middle-late Permian (264-255 Ma) and Cretaceous (106-94 Ma), and thus granitic magmatism occurred throughout the Phanerozoic, Permian (268-252 Ma), Early-Middle Triassic (248-240 Ma), Early Jurassic (183 Ma), and Cretaceous (112-94 Ma). The Permian granitoids consist of monzogranite, granodiorite, tonalite, and quartz diorite, characterised by enrichment in Na2O (3.60-4.72 wt.%), depletion in K2O (0.97-2.66 wt.%), and a negative correlation between P2O5 and SiO2. Together with the presence of hornblende, these geochemical features are indicative of an I-type affinity. The Permian granitic magmatism is associated with quartz-vein-type tungsten deposits (252 Ma; unpublished Sm-Nd isochron age), which formed in an active continental margin setting related to subduction of the Palaeo-Asian Ocean. The Cretaceous quartz diorites have an adakitic affinity, having relatively high Sr (374-502 ppm), low Yb (0.51-0.67 ppm) and Y (8.7-10.7 ppm), and high Sr/Y (39.4-46.8) and (La/Yb)N values (16.2-34.7), suggesting that they were related to the partial melting of subducted oceanic crust. In addition, they are associated with porphyry Au-Cu deposits. We conclude that the Cretaceous granitic rocks and associated porphyry Au-Cu mineralisation occurred in an extensional tectonic setting related to the subduction of the Palaeo-Pacific Plate beneath the Eurasian Plate. In addition, the large-scale Early-Middle Triassic syn-collisional granite belt at the eastern margin of the Xing'an-Mongolian Orogenic Belt extends from the middle of Jilin Province to the Wangqing-Hunchun region, constraining the timing of the final collision between the North China Craton and the Jiamusi-Khanka Massif, and suggesting that the Xra Moron River-Changchun Suture likely extends eastward into the eastern Hunchun region. This collision caused the Middle Triassic mesothermal lode gold mineralisation.

Disclosing the Paleoarchean to Ediacaran history of the São Francisco craton basement: The Porteirinha domain (northern Araçuaí orogen, Brazil)

NASA Astrophysics Data System (ADS)

Silva, Luiz Carlos da; Pedrosa-Soares, Antonio Carlos; Armstrong, Richard; Pinto, Claiton Piva; Magalhães, Joana Tiago Reis; Pinheiro, Marco Aurélio Piacentini; Santos, Gabriella Galliac

2016-07-01

This geochronological and isotopic study focuses on one of the Archean-Paleoproterozoic basement domains of the São Francisco craton reworked in the Araçuaí orogen, the Porteirinha domain, Brazil. It also includes a thorough compilation of the U-Pb geochronological data related to the adjacent Archean and Rhyacian terranes from the São Francisco craton and Araçuaí orogen. The main target of this study is the TTG gneisses of the Porteirinha complex (Sample 1). The gneiss dated at 3371 ± 6 Ma unraveled a polycyclic evolution characterized by two metamorphic overprinting episodes, dated at 3146 ± 24 Ma (M1) and ca. 600 Ma (M2). The former (M1) is so far the most reliable evidence of the oldest metamorphic episode ever dated in Brazil. The latter (M2), in turn, is endemic in most of the exposed eastern cratonic margin within the Araçuaí orogen. Whole-rock Sm-Nd analysis from the gneiss provided a slightly negative εNd(t3370) = - 0.78 value, and a depleted mantle model (TDM) age of 3.5 Ga, indicating derivation mainly from the melting of a ca. 3.5 Ga tholeiitic source. Sample 2, a K-rich leuco-orthogneiss from the Rio Itacambiriçu Complex, was dated at 2657 ± 25 Ma and also presents a ca. 600 Ma M2 overprinting M2 age. The other two analyses were obtained from Rhyacian granitoids. Sample 3 is syn-collisional, peraluminous leucogranite from the Tingui granitic complex, showing a crystallization age of 2140 ± 14 Ma and strong post-crystallization Pb*-loss, also ascribed to the Ediacaran overprinting. Accordingly, it is interpreted as a correlative of the late Rhyacian (ca. 2150-2050 Ma) collisional stage of the Mantiqueira orogenic system/belt (ca. 2220-2000 Ma), overprinted by the Ediacaran collage. Sample 4 is a Rhyacian post-orogenic (post-collisional), mixed-source, peralkaline, A1-type suite, with a crystallization age of 2050 ± 10 Ma, presenting an important post-crystallization Pb*-loss related to Ediacaran collision. The focused region records some of the oldest magmatic and metamorphic events ever found in the São Francisco craton basement, a ca. 3.4 Ga TTG generation and ca. 3.1 Ga metamorphism, as well as two younger orogenic events: the Rhyacian (2.15-2.05 Ga), accretionary event, correlated to the Mineiro belt, and the Ediacaran Brasiliano overprinting, event imposed by the Araçuaí orogen.

The formation and rejuvenation of continental crust in the central North China Craton: Evidence from zircon U-Pb geochronology and Hf isotope

NASA Astrophysics Data System (ADS)

Li, Qing; Santosh, M.; Li, Sheng-Rong; Guo, Pu

2014-12-01

The Trans-North China Orogen (TNCO) along the central part of the North China Craton (NCC) is considered as a Paleoproterozoic suture along which the Eastern and Western Blocks of the NCC were amalgamated. Here we investigate the Precambrian crustal evolution history in the Fuping segment of the TNCO and the subsequent reactivation associated with extensive craton destruction during Mesozoic. We present zircon LA-ICP-MS U-Pb and Lu-Hf data on TTG (tonalite-trondhjemite-granodiorite) gneiss, felsic orthogneiss, amphibolite and granite from the Paleoproterozoic suite which show magmatic ages in the range of 2450-1900 Ma suggesting a long-lived convergent margin. The εHf(t) values of these zircons range from -11.9 to 12 and their model ages suggest magma derivation from both juvenile components and reworked Archean crust. The Mesozoic magmatic units in the Fuping area includes granite, diorite and mafic microgranular enclaves, the zircons from which define a tight range of 120-130 Ma ages suggesting a prominent Early Cretaceous magmatic event. However, the εHf(t) values of these zircons show wide a range from -30.3 to 0.2, indicating that the magmatic activity involved extensive rejuvenation of the older continental crust.

Testing ore deposit models using in situ U-Pb geochronology of hydrothermal monazite: Paleoproterozoic gold mineralization in northern Australia

NASA Astrophysics Data System (ADS)

Rasmussen, Birger; Sheppard, Stephen; Fletcher, Ian R.

2006-02-01

The inability to establish absolute ages for gold deposition in the Pine Creek orogen of northern Australia has led to conflicting ore deposit models, ranging from intrusion related, which predict that gold mineralization was synchronous with granite magmatism (ca. 1835 1820 Ma), to orogenic, which place ore deposition nearly 100 m.y. later. Here we present ion microprobe U-Pb geochronology for a mineralized quartz reef from Tom's Gully mine, Mount Bundey, Northern Territory, Australia, and nearby granitic rocks and associated contact aureoles. Isotopic dating of zircon and monazite indicates that intrusion and contact metamorphism occurred ca. 1825 Ma, whereas hydrothermal monazite from the auriferous quartz reef gives a mean 207Pb/206Pb age of 1780 ± 10 Ma, interpreted as the time of gold mineralization. Mineralization therefore postdated intrusion by ˜45 m.y. and preceded a postulated ca. 1740 1730 Ma cratonwide orogenic gold event by ˜50 m.y. Hence, neither the intrusion-related model nor the recently proposed orogenic model is applicable. Combined with a reevaluation of age data from the nearby Goodall gold deposit, our data suggest that mineralization coincides with, and may be related to, an episode of regional low-grade metamorphism, deformation, and fluid circulation (Shoobridge event). Our results demonstrate the importance of high-precision in situ geochronology and detailed petrography for deciphering age relationships in ore deposits, and of testing the veracity of models for ore formation.

La Escalerilla pluton, San Luis Argentina: The orogenic and post-orogenic magmatic evolution of the famatinian cycle at Sierras de San Luis

NASA Astrophysics Data System (ADS)

Morosini, Augusto Francisco; Ortiz Suárez, Ariel Emilio; Otamendi, Juan Enrique; Pagano, Diego Sebastián; Ramos, Gabriel Alejandro

2017-01-01

Field relationships, geochemical analysis and two new absolute ages (LA-MC-ICP-MS U/Pb-zircon) allow the division of the La Escalerilla pluton (previously considered to be a single granitic body) into two different plutons: a new La Escalerilla pluton (s.s.), dated at 476.7 ± 9.6 Ma, that represents the northern portion, and the El Volcán pluton, dated at 404.5 ± 8.5 Ma, located in the southern sector. The La Escalerilla pluton is composed of three facies: (1) biotite-bearing granodiorite, (2) porphyritic biotite-bearing granite, and (3) porphyritic two micas-bearing leucogranite, being the presence of late-magmatic dykes in these facies common. The El Volcán pluton is composed of two main facies: 1) porphyritic biotite-bearing granite, and 2) two micas-bearing leucogranite, but amphibole-bearing monzodioritic and tonalititic mega-enclaves are also common, as well as some dykes of amphibole and clinopyroxene-bearing syenites. A peculiarity between the two plutons is that their most representative facies (porphyritic biotite-bearing granites) have, apart from different absolute ages, distinctive geochemical characteristics in their concentrations of trace elements; the La Escalerilla granite is comparatively poorer in Ba, Sr, Nb, La, Ce, P, and richer in Rb, Tb, Y, Tm and Yb. The El Volcán granite is notably enriched in Sr and depleted in Y, resulting in high Sr/Y ratios (12.67-39.08) compared to the La Escalerilla granite (1.11-2.41). These contrasts indicate that the separation from their sources occurred at different depths: below 25 km for the La Escalerilla, and above 30 km for the El Volcán. Moreover, the contrasts allow us to interpret a thin crust linked to an environment of pre-collisional subduction for the first case, and a thickened crust of post-collisional environment for the second, respectively.

Geothermal energy resources of wadi Al-Lith, Saudi Arabia

NASA Astrophysics Data System (ADS)

Lashin, A.; Chandrasekharam, D.; Al Arifi, N.; Al Bassam, A.; Varun, C.

2014-09-01

The entire western Arabian shield is the domain of both hydrothermal and enhanced geothermal systems associated with volcanic centres (Harrats) and high heat generating granites. The most prominent sites of hydrothermal systems are located around Al-Lith and Jizan. The hydrothermal system in Al Lith is controlled by high heat generating (∼11 μW/m3) post orogenic granites. The high heat flow value of >80 mW/m2 across Al-Lith coast is due to such granite intrusives, presence of dike swarms that intrude into the granites as well as position of Moho at shallow level. Although the thermal waters are chloride rich, Red Sea involvement is not observed. Long residence time and water rock interaction with granites are the main processes responsible for chloride enrichment in the thermal waters. Oxygen isotope shift indicates presence of high temperature geothermal system in the area. The tritium values indicate that the circulating waters are >75 years old.

From the plutonic root to the volcanic roof of a continental magmatic arc: a review of the Neoproterozoic Araçuaí orogen, southeastern Brazil

NASA Astrophysics Data System (ADS)

Gonçalves, Leonardo; Alkmim, Fernando F.; Pedrosa-Soares, Antônio; Gonçalves, Cristiane C.; Vieira, Valter

2018-01-01

The Araçuaí-West Congo orogen (AWCO) is one of the various components of the Brasiliano/Pan-African orogenic network generated during the amalgamation of West Gondwana. In the reconstructions of Gondwana, the AWCO, encompassing the Araçuaí orogen of South America and the West Congo belt of Southwestern Africa, appears as a tongue-shaped orogenic zone embraced by the São Francisco-Congo craton. Differing from the vast majority of the known orogens owing to its singular confined setting, the AWCO contains a large amount of orogenic igneous rocks emplaced in all stages of its tectonic evolution. We present new and revised information about the oldest Ediacaran granitic assemblage, the G1 Supersuite, which together with the Rio Doce Group defines the Rio Doce magmatic arc, and then we propose a new tectonic setting for the arc. Field relationships and mineralogical compositions of the G1 Supersuite allow us to characterize three lithofacies associations, Opx-bearing rocks, enclave-rich Tonalite-Granodiorite and enclave-poor Granite-Tonalite, suggesting different crustal levels are exposed in the central part of the Araçuaí orogen. The region is interpreted to represent a tilted crustal section, with deep arc roots now exposed along its western border. Chemically, these plutonic associations consist mostly of magnesian, metaluminous to slightly peraluminous, calc-alkaline to alkali-calcic and medium- to high-K acidic rocks. The dacitic and rhyolitic rocks of the Rio Doce Group are mainly magnesian, peraluminous, calcic to calc-alkaline, and medium- to high-K acidic rocks. Zircon U-Pb data constrain the crystallization of the granitoids between ca. 625 and 574 Ma, while the age of the metamorphosed volcanic rocks is around ca. 585 Ma. Thus, within errors, these rock associations likely belong to the same magmatic event and might represent the subduction-related, pre-collisional, evolution of the Araçuaí orogen. In addition, whole-rock Sm-Nd isotopic compositions show variable negative ɛNd(t) values between -6.7 and -13.8, and TDM model ages varying from 1.39 to 2.26 Ga, while ɛHf(t) vary between -5.2 and -11.7, with TDM ages from 1.5 to 2.0 Ga. Thus, predominantly constructed upon Paleoproterozoic (Rhyacian) basement, the Rio Doce arc shows crustal sources largely prevailing over mantle sources, providing a well-studied example to be compared with similar orogenic settings around the world.

The dilemma of the Jiaodong gold deposits: Are they unique?

USGS Publications Warehouse

Goldfarb, Richard J.; Santosh, M.

2013-01-01

The ca. 126–120 Ma Au deposits of the Jiaodong Peninsula, eastern China, define the country's largest gold province with an overall endowment estimated as >3000 t Au. The vein and disseminated ores are hosted by NE- to NNE-trending brittle normal faults that parallel the margins of ca. 165–150 Ma, deeply emplaced, lower crustal melt granites. The deposits are sited along the faults for many tens of kilometers and the larger orebodies are associated with dilatational jogs. Country rocks to the granites are Precambrian high-grade metamorphic rocks located on both sides of a Triassic suture between the North and South China blocks. During early Mesozoic convergent deformation, the ore-hosting structures developed as ductile thrust faults that were subsequently reactivated during Early Cretaceous “Yanshanian” intracontinental extensional deformation and associated gold formation.Classification of the gold deposits remains problematic. Many features resemble those typical of orogenic Au including the linear structural distribution of the deposits, mineralization style, ore and alteration assemblages, and ore fluid chemistry. However, Phanerozoic orogenic Au deposits are formed by prograde metamorphism of accreted oceanic rocks in Cordilleran-style orogens. The Jiaodong deposits, in contrast, formed within two Precambrian blocks approximately 2 billion years after devolatilization of the country rocks, and thus require a model that involves alternative fluid and metal sources for the ores. A widespread suite of ca. 130–123 Ma granodiorites overlaps temporally with the ores, but shows a poor spatial association with the deposits. Furthermore, the deposit distribution and mineralization style is atypical of ores formed from nearby magmas. The ore concentration requires fluid focusing during some type of sub-crustal thermal event, which could be broadly related to a combination of coeval lithospheric thinning, asthenospheric upwelling, paleo-Pacific plate subduction, and seismicity along the continental-scale Tan-Lu fault. Possible ore genesis scenarios include those where ore fluids were produced directly by the metamorphism of oceanic lithosphere and overlying sediment on the subducting paleo-Pacific slab, or by devolatilization of an enriched mantle wedge above the slab. Both the sulfur and gold could be sourced from either the oceanic sediments or the serpentinized mantle. A better understanding of the architecture of the paleo-Pacific slab during Early Cretaceous below the eastern margin of China is essential to determination of the validity of possible models.

Syn-orogenic magmatism over 100 m.y. in high crustal levels of the central Grenville Province: Characteristics, age and tectonic significance

NASA Astrophysics Data System (ADS)

Groulier, Pierre-Arthur; Indares, Aphrodite; Dunning, Gregory; Moukhsil, Abdelali; Jenner, George

2018-07-01

The Escoumins Supracrustal Belt (ESB) represents higher levels of the infrastructure of a large hot orogen, exposed in a broadly dome and basin pattern. It consists of remnants of a Pinwarian-age (1.52-1.46 Ga) oceanic arc and arc-rift sequence, preserved in the low-P Belt of the central Grenville Province, and was intruded by diverse Grenvillian-age plutons. The plutonic rocks range from quartz monzodiorite to granite and have intrusion ages covering a time interval of 100 My, that represents the entire range of the Grenvillian orogeny. Moreover, the ages, field relations and geochemical signatures of the different intrusions can be matched with different documented stages of the orogeny. The oldest pluton, the magnesian, biotite-bearing Bon-Désir granite (1086 ± 2 Ma), has positive εNd (+0.6), TDM = 1.52 Ga, and is attributed to melting of a juvenile Pinwarian crust as a result of slab break-off, at the onset of continental collision. The ferroan and Ba-Sr enriched, biotite-, amphibole- and clinopyroxene-bearing Michaud plutonic suite (1063 ± 3 Ma) and biotite-rich felsic sill (1045 ± 3 Ma) have εNd (-0.01 - +0.8) and TDM = 1.45-1.48 Ga. Their geochemistry is consistent with fractionation of a mafic magma derived from melting of a Geon 14 subduction-modified subcontinental lithospheric mantle. This magmatism is consistent with convective thinning of subcontinental lithosphere, potentially linked to tectonic extrusion and orogenic collapse. This collapse ultimately led to the juxtaposition of the low-P Belt with the high-T mid-P Belt in the hinterland of the Grenville Province and to amphibolite-facies metamorphism in the former, producing metamorphic zircon overgrowths at 1037 ± 10 Ma. Finally, 988 ± 5 Ma to 983 ± 5 Ma syn-kinematic peraluminous two-mica garnetiferous leucogranite bodies and pegmatites with inherited 1055 ± 2 Ma metamorphic monazite were derived from melting of previously metamorphosed deeper levels of the low-P Belt. This is consistent with a high geothermal gradient linked to thinning of the crust in a Basin and Range setting. The geochemical and age pattern of Grenvillian-age magmatism in the ESB, in conjunction with the overall architecture of the Province, suggests that Laurentia was the upper plate during the Grenvillian orogeny.

The post-collisional late Variscan ferroan granites of southern Sardinia (Italy): Inferences for inhomogeneity of lower crust

NASA Astrophysics Data System (ADS)

Conte, Aida Maria; Cuccuru, Stefano; D'Antonio, Massimo; Naitza, Stefano; Oggiano, Giacomo; Secchi, Francesco; Casini, Leonardo; Cifelli, Francesca

2017-12-01

The post-collisional late Variscan magmatism of Sardinia-Corsica batholith attained a peak at about 290 Ma. In southern Sardinia, in the frontal part of the Variscan orogenic wedge, this magmatism is represented by three suites of granitoids, here defined as GS1, GS2 and GS3. GS1, GS2 and GS3 are slightly peraluminous and F-bearing granitoids; GS1 and GS3 granites show in addition a ferroan character, whereas GS2 rocks range from magnesian to ferroan, from granodiorites to leucogranites. From magnetic susceptibility data, GS1 and GS2 belong to the ilmenite series, whereas GS3 is a slightly oxidized rock-suite plotting on the ilmenite/magnetite series boundary. Each rock-suite shows distinctive characters, in terms of petrography, petrochemistry, rock associations, as well as metallogenic signature of the related fluids. The distinction among rock-suite types is made on the basis of both mafic and characteristic accessory minerals. Siderophyllitic dark mica as the only mafic phase, and accessory xenotime (Y) characterize the GS1 rocks; GS2 mineral associations include biotite ± hornblende + allanite + magnetite; GS3 rocks show an association of hastingsite + annite + allanite + magnetite. Chemical variations in the studied samples suggest different magmatic evolution of independent magmas. Pb, Sr and Nd isotopic data constrain the origin of magmas to lower crustal sources. Chemical composition of rocks and dark micas meet those of liquids experimentally obtained by low degrees of partial melting of different meta-igneous deep crustal sources, felsic for GS1 rock-types and more mafic for GS3 rock-types. GS1 intrusions show granophile-type (Sn-W-Mo) metallogenic signatures, very low magnetic susceptibility, and Nd model ages (referred to the Depleted Mantle - TDM) of 2.3 Ga, coherent with a possible derivation from an old (early Proterozoic-Neoarchean), reduced and weathered basement, tectonically buried under Variscan covers. A definite deep crustal inhomogeneity is mirrored by GS3 granites, whose compositional and isotopic features indicate a younger (Nd model age: 1.6 Ga) tonalitic amphibolite source. Overall, the peculiarities of the studied granitoids suggest further compositional differences in the deep crust between southern and northern portion of the Sardinia-Corsica Variscan transect. Late Variscan lithospheric delamination appears as the most reliable mechanism that may have determined the high thermal regime that triggered partial melting of the crust. The close field association, at 290 Ma, of tholeiitic dike swarms and ferroan granitoids, supports this inference.

Cretaceous and Paleogene granitoid suites of the Sikhote-Alin area (Far East Russia): Geochemistry and tectonic implications

NASA Astrophysics Data System (ADS)

Grebennikov, Andrei V.; Khanchuk, Alexander I.; Gonevchuk, Valeriy G.; Kovalenko, Sergey V.

2016-09-01

The Mesozoic and Cenozoic geological history of NE Asia comprises alternating episodes of subduction or transform strike-slip movement of the oceanic plate along the continental margin of Eurasia. This sequence resulted in the regular generation of granitoid suites that are characterized by different ages, compositions, and tectonic settings. The Hauterivian-Aptian orogenic stage of the Sikhote-Alin, associated with the strike-slip displacement of the early Paleozoic continental blocks, the successive deformation of the Jurassic and Early Cretaceous terranes, and the injection of the earliest S-type granitoids. During late Albian, the area underwent syn-strike-slip compression caused by collision with the Aptian island arc and resulted in the injection of voluminous magmas of calc-alkaline magnesian (S- and I-type) and alkali-calcic ferroan (A-type) granitoids into syn-faulting compressional and extensional basins, respectively. Northwestward to westward movement of the Izanagi Plate resulted in the initiation of frontal subduction of the Paleo-Pacific Plate during the Cenomanian-Maastrichtian. In turn, this resulted in the generation of plateau-forming ignimbrites and their intrusive analogs formed from metaluminous I-type felsic magmas. Paleocene-Eocene magmatism in the Sikhote-Alin area commenced after the termination of subduction in a rifting regime related to strike-slip movement of the oceanic plate relative to the continent. The break-off of the subducted plate and the injection of oceanic asthenospheric material into the subcontinental lithosphere resulted in the eruption of lamproites and fayalite rhyolites, and coeval intrusions of gabbro and alkali feldspar granites (A-type). The A-type granitic-rocks and coeval gabbro-monzonites are considered to be reliable indicators of the transform continental margin geodynamic settings.

Petrography, geochemistry, and U-Pb geochronology of pegmatites and aplites associated with the Alvand intrusive complex in the Hamedan region, Sanandaj-Sirjan zone, Zagros orogen (Iran)

NASA Astrophysics Data System (ADS)

Sepahi, Ali Asghar; Salami, Sedigheh; Lentz, David; McFarlane, Christopher; Maanijou, Mohammad

2018-04-01

The Alvand intrusive complex in the Hamedan area in Iran is in the Sanandaj-Sirjan zone of the Zagros orogen. It consists of a wide range of plutonic rocks, mainly gabbro, diorite, granodiorite, granite, and leucogranites that were intruded by aplitic and pegmatitic dykes. At least three successive magmatic episodes generated an older gabbro-diorite-tonalite assemblage, followed by a voluminous granodiorite-granite association, which was then followed by minor leucocratic granitoids. Aplitic and pegmatitic dykes and bodies have truncated both plutonic rocks of the Alvand intrusive complex and its metamorphic aureole. Chemically they belong to peraluminous LCT (Li-, Cs-, and Ta-bearing) family of pegmatites. Mineralogically, they resemble Muscovite (MS) and Muscovite Rare Element (MSREL) classes of pegmatites. High amounts of some elements, such as Sn (up to 10,000 ppm), Rb (up to 936 ppm), Ba (up to 706 ppm), and LREE (up to 404 ppm) indicate the highly fractionated nature of some of these aplites and pegmatites. U-Pb dating of monazite, zircon, and allanite by LA-ICPMS indicate the following ages: monazite-bearing aplites of Heydareh-e-Poshteshahr and Barfejin areas, southwest of Hamedan, give an age range of 162-172 Ma; zircon in Heydareh-e-Poshteshar gives an average age of 165 Ma and for allanite-bearing pegmatites of Artiman area, north of Tuyserkan, an age of 154.1 ± 3.7 Ma was determined. These overlap with previously reported ages (ca. 167-153 Ma) for the plutonic rocks of the Alvand complex. Therefore, these data reveal that the Jurassic was a period of magmatism in the Hamedan region and adjacent areas in the Sanandaj-Sirjan zone, which was situated at the southern edge of the central Iranian micro-plate (southern Eurasian plate) at this time. Our results also suggest that advective heating in a continental arc setting has caused melting of fertile supracrustal lithologies, such as meta-pelites. These partial melts were then emplaced at much higher crustal levels, but within a thermally anomalous environment, which, therefore, leads to formation of evolved felsic rocks, such as the studied LCT aplite-pegmatite suite and their parental granitic rocks. This is a new result that indicates the role of syn-subduction crustal partial melting in the region as part of Zagros orogeny.

Two cryptic anatectic events within a syn-collisional granitoid from the Araçuaí orogen (southeastern Brazil): Evidence from the polymetamorphic Carlos Chagas batholith

NASA Astrophysics Data System (ADS)

Melo, M. G.; Stevens, G.; Lana, C.; Pedrosa-Soares, A. C.; Frei, D.; Alkmim, F. F.; Alkmin, L. A.

2017-04-01

From the earliest (ca. 630 Ma) pre-collisional plutons to the latest (ca. 480 Ma) post-collisional intrusions, the Araçuaí orogen (SE Brazil) records an outstanding succession of granite production events in space and time. The Carlos Chagas batholith (CCB) is the largest ( 14,000 km2) granitic body ascribed to the collisional plutonism (G2 supersuite) in the back-arc region of the Araçuaí orogen, to the east of the Rio Doce magmatic arc. A wide range of monazite and zircon ages (> 725 Ma to ca. 490 Ma) have been found in CCB granites, recording a rich history of crustal recycling and inheritance, magmatic crystallization and anatexis. The CCB includes a dominant granite richer in garnet than in biotite, in which three mineral assemblages can be identified: 1) Qz + Pl + Kfs + Bt + Grt + Ilm ± Rt; 2) Qz + Pl + Kfs + Bt + Grt + Ilm + Sil; and 3) Qz + Pl + Kfs + Bt + Grt + Ilm + Sil + Spl. Rocks which contain mineral assemblage 2 and 3 all contain two generations of garnet. Textural evidence for the presence of former melt, recognized in all studied CCB samples, includes: silicate melt inclusions in poikiloblastic garnet, pseudomorphed thin films of melt surrounding both generations of garnet, pseudomorphed melt pools adjacent to garnet and biotite, and plagioclase and quartz with cuspate-lobate shapes occurring among matrix grains. Both generations of garnet crystals (Grt1 and Grt2) are unzoned in terms of major element concentration, contain small rounded inclusions of Ti-rich biotite and, in addition, the Grt2 crystals also contain inclusions of remnant sillimanite needles. Microstructural evidence, in combination with mineral chemistry, indicates that the garnet crystals grew during two distinct metamorphic-anatectic events, as the peritectic products of fluid-absent melting reactions which consumed biotite, quartz and plagioclase, in the case of Grt1, and which consumed biotite, quartz, plagioclase and sillimanite in the case of Grt2. P-T pseudosections calculated via Theriak-Domino, in combination with in situ U-Pb monazite and zircon dating, provide new constraints on the thermal evolution of the back-arc region of the Araçuaí orogen. Data from assemblage 1 suggests P-T conditions for the first granulite-facies metamorphic event (M1) at 790-820 °C and 9.5-10.5 kbar, while the assemblage 2 records P-T conditions for a second granulite-facies metamorphism (M2) of around 770 °C and 6.6 kbar. Monazite and zircon within garnets from the different assemblages give age peaks at 570-550 Ma (M1) and 535-515 Ma (M2), recording two anatectic events in the CCB during a single orogenic cycle. The PT conditions for these metamorphic events can be related to: i) M1, striking crustal thickening, probably involving thrusting of the magmatic arc onto the back-arc region; and ii) M2, decompression related to the gravitational collapse of the Araçuaí orogen.

Origin and geodynamic significance of the early Mesozoic Weiya LP and HT granulites from the Chinese Eastern Tianshan

NASA Astrophysics Data System (ADS)

Mao, Ling-Juan; He, Zhen-Yu; Zhang, Ze-Ming; Klemd, Reiner; Xiang, Hua; Tian, Zuo-Lin; Zong, Ke-Qing

2015-12-01

The Chinese Tianshan in the southwestern part of the Central Asian Orogenic Belt (CAOB) is characterized by a variety of high-grade metamorphic rocks, which provide critical constraints for understanding the geodynamic evolution of the CAOB. In this paper, we present detailed petrological and zircon U-Pb geochronological studies of the Weiya low-pressure and high-temperature (LP-HT) granulites of the Chinese Eastern Tianshan. These granulites were previously considered to be a product of a regional metamorphic orogenic event. Due to different bulk-rock chemistries the Weiya granulites, which occur as lenses within the contact metamorphic aureole of the Weiya granitic ring complex, have a variety of felsic-pelitic and mafic granulites with different textural equilibrium mineral assemblages including garnet-cordierite-sillimanite-bearing granulites, cordierite-sillimanite-bearing granulites, cordierite-orthopyroxene-bearing granulites, and orthopyroxene-clinopyroxene-bearing granulites. Average P-T thermobarometric calculations and conventional geothermobarometry indicates that the Weiya granulites underwent early prograde metamorphism under conditions of 600-650 °C at 3.2-4.2 kbar and peak metamorphism of 750-840 °C at 2.9-6.3 kbar, indicating a rather high geothermal gradient of ca. 60 °C/km. Zircon U-Pb LA-ICP-MS dating revealed metamorphic ages between 244 ± 1 to 237 ± 3 Ma, which are in accordance with the crystallization age of the Weiya granitic ring complex. We suggest that the formation of the Weiya granulites was related to contemporaneous granitic magmatism instead of a regional metamorphic orogenic event. In addition, a Late Devonian metamorphic age of ca. 380 Ma was recorded in zircon mantle domains from two pelitic samples which is consistent with the metamorphic age of the Xingxingxia metamorphic complex in the Chinese Eastern Tianshan. This suggests that the mantle domains of the zircon grains of the Weiya granulites probably formed during the Late Devonian regional metamorphism and were overprinted by the Early Triassic contact metamorphism. Therefore, Early Triassic geodynamic models for the southwestern part of the CAOB, which are based on a previously suggested regional metamorphic orogenic event of the Weiya granulites, need to be viewed with caution.

Geochemistry, geochronology and Nd isotopes of the Gogó da Onça Granite: A new Paleoproterozoic A-type granite of Carajás Province, Brazil

NASA Astrophysics Data System (ADS)

Teixeira, Mayara Fraeda Barbosa; Dall'Agnol, Roberto; Santos, João Orestes Schneider; de Sousa, Luan Alexandre Martins; Lafon, Jean-Michel

2017-12-01

The Gogó da Onça Granite (GOG) comprise a stock located in the Carajás Province in the southeastern part of Amazonian Craton near its border with the Araguaia Belt. Three facies were identified in the pluton: biotite-amphibole granodiorite, biotite-amphibole monzogranite and amphibole-biotite syenogranite. The GGO crosscut discordantly the Archean country rocks and are not foliated. All Gogó da Onça Granite varieties are metaluminous, ferroan A2-subtype granites with reduced character. The major and trace element behavior suggests that its different facies are related by fractional crystallization. Zircon and titanite U-Pb SHRIMP ages show that the pluton crystallized at ∼1880-1870 Ma and is related to the remarkable Paleoproterozoic magmatic event identified in the Carajás Province. Whole-rock Nd isotope data (TDM ages 2.78 to 2.81, εNd values of -9.07 to -9.48) indicate that the GOG magmas derived from an Archaean source compatible with that of some other Paleoproterozoic suites from Carajás Province. The GOG show significant contrasts with the Jamon and Velho Guilherme Paleoproterozoic suites from Carajás Province and the inclusion of the Gogó da Onça granite in any of these suites is not justified. The GOG is more akin to the Serra dos Carajás Suite and to the Seringa and São João granites of Carajás and to the Mesoproterozoic Sherman granite of USA and the Paleoproterozoic Suomenniemi Batholith of Finland. This study puts in evidence the relevance of precise geochronological data and estimation of magma oxidation state in the characterization and correlation of A-type granites.

The Rondonian-San Ignacio Province in the SW Amazonian Craton: An overview

NASA Astrophysics Data System (ADS)

Bettencourt, Jorge Silva; Leite, Washington Barbosa; Ruiz, Amarildo Salina; Matos, Ramiro; Payolla, Bruno Leonelo; Tosdal, Richard M.

2010-01-01

The Rondonian-San Ignacio Province (1.56-1.30 Ga) is a composite orogen created through successive accretion of arcs, ocean basin closure and final oblique microcontinent-continent collision. The effects of the collision are well preserved mostly in the Paraguá Terrane (Bolivia and Mato Grosso regions) and in the Alto Guaporé Belt and the Rio Negro-Juruena Province (Rondônia region), considering that the province was affected by later collision-related deformation and metamorphism during the Sunsás Orogeny (1.25-1.00 Ga). The Rondonian-San Ignacio Province comprises: (1) the Jauru Terrane (1.78-1.42 Ga) that hosts Paleoproterozoic basement (1.78-1.72 Ga), and the Cachoeirinha (1.56-1.52 Ga) and the Santa Helena (1.48-1.42 Ga) accretionary orogens, both developed in an Andean-type magmatic arc; (2) the Paraguá Terrane (1.74-1.32 Ga) that hosts pre-San Ignacio units (>1640 Ma: Chiquitania Gneiss Complex, San Ignacio Schist Group and Lomas Manechis Granulitic Complex) and the Pensamiento Granitoid Complex (1.37-1.34 Ga) developed in an Andean-type magmatic arc; (3) the Rio Alegre Terrane (1.51-1.38 Ga) that includes units generated in a mid-ocean ridge and an intra-oceanic magmatic arc environments; and (4) the Alto Guaporé Belt (<1.42-1.34 Ga) that hosts units developed in passive marginal basin and intra-oceanic arc settings. The collisional stage (1.34-1.32 Ga) is characterized by deformation, high-grade metamorphism, and partial melting during the metamorphic peak, which affected primarily the Chiquitania Gneiss Complex and Lomas Manechis Granulitic Complex in the Paraguá Terrane, and the Colorado Complex and the Nova Mamoré Metamorphic Suite in the Alto Guaporé Belt. The Paraguá Block is here considered as a crustal fragment probably displaced from its Rio Negro-Juruena crustal counterpart between 1.50 and 1.40 Ga. This period is characterized by extensive A-type and intra-plate granite magmatism represented by the Rio Crespo Intrusive Suite (ca. 1.50 Ga), Santo Antonio Intrusive Suite (1.40-1.36 Ga), and the Teotônio Intrusive Suite (1.38 Ga). Magmatism of these types also occur at the end of the Rondonian-San Ignacio Orogeny, and are represented by the Alto Candeias Intrusive Suite (1.34-1.36 Ga), and the São Lourenço-Caripunas Intrusive Suite (1.31-1.30 Ga). The cratonization of the province occurred between 1.30 and 1.25 Ga.

Mineral chemistry and geochemistry of the Late Neoproterozoic Gabal Abu Diab granitoids, Central Eastern Dessert, Egypt: Implications for the origin of rare metal post-orogenic A-type granites

NASA Astrophysics Data System (ADS)

Sami, Mabrouk; Ntaflos, Theodoros; Farahat, Esam S.; Ahmed, Awaad F.; Mohamed, Haroun A.

2015-04-01

The Neoproterozoic Gabal Abu Diab pluton is a part of the Arabian Nubian shield (ANS) continental crust and located in the Central Eastern Desert (CED) of Egypt. It constitutes multiphase granitic pluton intruded into granodiorite and metagabbro-diorite rocks with sharp and nonreactive contacts. Based on field observations, colors, structural variations and petrographic investigations, this granitic outcrop consists of an inner core of two-mica granite (TMG) followed outward by garnet bearing muscovite granite (GBMG) and albite granite (AG). Petrographical study indicated that medium to coarse-grained TMG is dominated by K-feldspar (Or88-98), quartz, plagioclase (albite, An0-7), muscovite and biotite with hypidiomorphic texture. With exception the appearance of garnet and the disappearance of biotite the GBMG resembles the TGM, while AG is leucocratic without any mafic mineral. The main accessories are zircon, Nb and Ta-bearing rutile, columbite, ilmenorutile, ilmenite, magnetite and apatite. This mineralogical similarity and the existence of columbite group minerals (CGM) in all granitoids, indicates a cogenetic relationship. Microprobe analyses reveal that, besides the CGM, rutile and ilmenite are the main repository phases for Nb-Ta-Ti. Columbite-(Mn) exists as individual subhedral crystals (up to 100μm in size) or intimate intergrowth with Nb-bearing rutile and/or ilmenite. The CGM are represented mostly by columbite-(Mn) with Ta/(Ta+Nb) and Mn/(Mn+Fe) ratio ranging from 0.02-0.08 and 0.4-0.9, respectively suggesting extreme degree of magmatic fractionation. Rutile contains significant amounts of Ta (up to 4 wt.% Ta2O5) and Nb (up to 22 wt.% Nb2O5). Biotites are phlogopite-annite in composition (Ann47-60Phlog40-53,on average) and are enriched with AlIV that characterize peraluminous granites. Garnets contain 60-69 mol.% spessartine and 28-36 mol.% almandine where, the ratio of spessartine and almandine together exceeds 95 mole percent, similar to garnet occur within A-type granite worldwide. According to Zhang et al., 2012, the garnet crystallized at the expense of biotite from the MnO-rich evolved melt after fractionation of biotite, plagioclase, K-feldspar, zircon, apatite, and ilmenite. The granitoids are alkali feldspar granites showing distinct geochemical features and most likely, belong to the post-orogenic younger Egyptian granitoids. They are peraluminous A-type alkaline rocks but they have lower Fe2O3, MgO, MnO, CaO, TiO2, P2O5, Sr, Ba, V, and higher SiO2, Na2O, K2O, Nb, Ta, U, Zr, Th, Ga/Al and Rb than the typical rocks of this type. The positive correlation between Ba and Sr, and the negative correlation between Rb and K/Rb reveal fractional crystallization of alkali feldspar. The similarity in most geochemical characteristics suggests that Abu Diab granitoids are genetically related to each other and extremely enrichment in incompatible elements such as Nb and Ta, indicating that they crystallized from extremely differentiated magmas. References: Zhang, J., Ma, C. and She, Z., 2012. An Early Cretaceous garnet-bearing metaluminous A-type granite intrusion in the East Qinling Orogen, central China: Petrological, mineralogical and geochemical constraints. Geoscience Frontiers 3 (5), 635-646.

Geological, geochronological, geochemical, and Sr-Nd-O-Hf isotopic constraints on origins of intrusions associated with the Baishan porphyry Mo deposit in eastern Tianshan, NW China

NASA Astrophysics Data System (ADS)

Wang, Yinhong; Xue, Chunji; Liu, Jiajun; Zhang, Fangfang

2016-10-01

The Baishan porphyry Mo deposit (0.72 Mt; 0.06 % Mo) is located in the interior of the eastern Tianshan orogenic belt in Xinjiang, NW China. The deposit comprises 15 orebodies that are associated with monzogranite and granite porphyry stocks and are structurally controlled by roughly EW-trending faults. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating of the monzogranite and granite porphyry yielded the Middle Triassic age (228 ± 2 to 227 ± 2 Ma), which coincide with the molybdenite Re-Os model ages ranging from 226 ± 3 to 228 ± 3 Ma. The Triassic monzogranite and granite porphyry belong to high-K calc-alkaline series and are characterized by high SiO2 and Al2O3 and low MgO, TiO2, and P2O5 concentrations, with negative Eu anomalies (δEu = 0.55-0.91). The least-altered monzogranite and granite porphyry yield uniform ɛ Nd( t) values from +1.6 to +3.6, and wide (87Sr/86Sr) i ratios ranging between 0.7035 and 0.7071, indicating that they were derived from the lower crust. In situ O-Hf isotopic analyses on zircon using SIMS and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) indicate that the δ18O and ɛ Hf( t) values of zircon from a monzogranite sample vary from 6.1 to 7.3 ‰ and +8.0 to +11.7, respectively, whereas zircon from a granite porphyry sample vary from 6.2 to 6.9 ‰ and +7.3 to +11.2, respectively. The geochemical and isotopic data imply that the primary magmas of the Baishan granite were likely derived from partial melts from the lower crust involving some mantle components. The Baishan Mo deposit and granitic emplacement were proposed to be most likely related to post-orogenic lithospheric extension and magmatic underplating. An extensional event coupled with the rising of hot mantle-derived melts triggered partial melting of the lower crust, as well as provided metals (Mo).

Generation of Late Cretaceous silicic rocks in SE China: Age, major element and numerical simulation constraints

NASA Astrophysics Data System (ADS)

Chen, Cheng-Hong; Lee, Chi-Yu; Lu, Hsueh-Yu; Hsieh, Pei-Shan

2008-01-01

Rhyolite-dominating bimodal volcanic suites (rhyolite/basalt), mafic dikes and A-type granites distribute from N Zhejiang to S Fujian over 800 km in the Southeast Coast Magmatic Belt (SCMB) - the Late Yanshanian (LY) orogenic belt in SE China. Data of 40Ar/ 39Ar and K-Ar whole-rock ages and LA-ICPMS U-Pb zircon ages indicate that rhyolitic volcanism (101-72 Ma) is contemporaneous with the A-type granitic intrusions (100-90 Ma) and mafic dike injections (94-77 Ma). This time span is used to define the upper volcanic series in Zhejiang-Fujian areas. One striking feature of rhyolites in the SCMB is that many are strongly peraluminous (SP) and others, mostly restrict in Fujian, are peralkaline to mildly peraluminous (P-MP) and chemically resemble A-type granites. The SP character is unique among well-known large rhyolite provinces worldwide. Based on experimental works for a common thermal regime and inherited zircon age information, we suggest that SP and P-MP rhyolites represent low pressure melting of the felsic (quartzofeldspathic) granite (±metapelite) and the accompanied granodioritic, tonalitic and trondhjemitic member of the core complex assemblage, respectively, to account for the decreasing aluminosity. They could have also been contaminated by young igneous rocks, and ancient crust to a lesser degree, during ascent to the surface. Plate subduction and lithosphere extension processes, respectively, are numerically simulated for the magma generation of these rhyolites using the mafic underplating model. Results suggest that the most effective controlling factor to generate SP and associated P-MP (A-type) magmas during 95-80 Ma is thinning of the lithosphere thickness with a high exhumation rate. Under this circumstance, the core complex assemblage can be uplifted to lower level of the crust and match the constraint of experimental works.

Geochemical evidence for Paleozoic crustal growth and tectonic conversion in the Northern Beishan Orogenic Belt, southern Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Yuan, Yu; Zong, Keqing; He, Zhenyu; Klemd, Reiner; Jiang, Hongying; Zhang, Wen; Liu, Yongsheng; Hu, Zhaochu; Zhang, Zeming

2018-03-01

The Beishan Orogenic Belt is located in the central southernmost part of the Central Asian Orogenic Belt (CAOB), which plays a key role in understanding the formation and evolution of the CAOB. Granitoids are the documents of crustal and tectonic evolution in orogenic belts. However, little is known regarding the petrogenesis and geodynamic setting of the widely distributed Paleozoic granitoids in the Northern Beishan Orogenic Belt (NBOB). The present study reveals significant differences concerning the petrogenesis and tectonic setting of early and late Paleozoic granitoids from the NBOB. The early Paleozoic granitoids from the 446-430 Ma Hongliuxia granite complex of the Mazongshan unit and the 466-428 Ma Shibanjing complex of the Hanshan unit show classic I-type granite affinities as revealed by the relative enrichment of LILEs and LREEs, pronounced depletions of Nb, Ta and Ti and the abundant presence of hornblende. Furthermore, they are characterized by strongly variable zircon εHf(t) values between - 16.7 and + 12.8 and evolved plagioclase Sr isotopic compositions of 0.7145-0.7253, indicating the involvement of both juvenile and ancient continental crust in the magma source. Thus, we propose that the early Paleozoic granitoids in the NBOB were generated in a subduction-related continental arc setting. In contrast, the late Paleozoic 330-281 Ma granitoids from the Shuangjingzi complex of the Hanshan unit exhibit positive zircon εHf(t) values between + 5.8 and + 13.2 and relatively depleted plagioclase Sr isotopic compositions of 0.7037-0.7072, indicating that they were mainly formed by remelting of juvenile crust. Thus, an intra-plate extensional setting is proposed to have occurred during formation of the late Paleozoic granitoids. Therefore, between the early and late Paleozoic, the magma sources of the NBOB granitoids converted from the reworking of both juvenile and ancient crusts during a subduction-induced compressional setting to the remelting of juvenile crust during an intra-plate extensional setting, respectively. The corresponding crustal growth in the southern CAOB is dominated by early Paleozoic lateral accretion of arc complexes and late Paleozoic vertical addition of juvenile material from the mantle.

Mid-Miocene two-mica granites in the Malashan gneiss dome, south Tibet: Geochemical characteristics and formation mechanism

NASA Astrophysics Data System (ADS)

Gao, L.; Zeng, L.

2011-12-01

Knowledge of the timing of formation and geochemical nature of the Cenozoic granites along the High Himalaya as well as the Tethyan Himalaya is essential to test or formulate models that link high-grade metamorphism, crustal anatexis, and tectonic transition during the evolution of the Himalayan orogen. The Malashan gneiss dome, one of the prominent domes within the Tethyan Himalaya, consists of pelitic schists, calc-silicate metamorphic rocks, and at least two generations of granites. Two mica granites(TMG) occur as large plutons in Cuobu and Malashan, whereas a small leucogranite pluton occurs at the western side of the Paiku Lake. Two-mica granites from the Cuobu and the Malashan share similar characteristics in mineral composition, major and trace element geochemistry and isotope(Sr and Nd) compositions. New LA-ICP-MS zircon U/Pb analyses yielded that the Cuobu and the Malashan TMG formed at 17.6±0.1 Ma and 16.9±0.1 Ma, respectively. Both suits of granites are characterized by:(1)high SiO2(>71.3wt%), Al2O3(>14.8wt%), and relatively high CaO(>1.5wt%); (2)high A/CNK(>1.0) and K/Na ratios; (3)relatively high Sr(>146ppm), low Rb(<228ppm) and Rb/Sr ratios(<1.3); (4) enriched in LREE, depleted in HREE, as well as no or weakly negative Eu anomalies(Eu*=0.7~0.9); (5) as compared to leucogranites of similar ages in other Northern Himalayan Gneiss Domes, lower initial 87Sr/86Sr ratios (0.7390~0.7484) and similarly unradiogenic Nd isotope compositions (ɛNd(t)=-13.7~-14.4). Correlations between Ba and Rb/Sr ratios and between Rb/Sr and initial 87Sr/86Sr ratios imply that these two-mica granites were derived from muscovite H2O-fluxed melting of metasedimentary rocks at T=700-780oC. Such a reaction could be represented by 9Muscovite + 15Plagioclase + 7Quartz + xH2O = 31Melt, in which enhances the involvement of plagioclase, but suppresses the biotite due to relatively low temperature and the presence of water. This reaction not only produces granitic melts with low Rb/Sr ratios, relatively high CaO and weak to no Eu anomalies, but also leads to lower initial 87Sr/86Sr ratios than their potential source rocks.

Progressive magmatism and evolution of the Variscan suture in southern Iberia

NASA Astrophysics Data System (ADS)

Braid, James A.; Murphy, J. Brendan; Quesada, Cecilio; Gladney, Evan R.; Dupuis, Nicolle

2018-04-01

Magmatic activity is an integral component of orogenic processes, from arc magmatism during convergence to post-collisional crustal melting. Southern Iberia exposes a Late Paleozoic suture zone within Pangea and where a crustal fragment of Laurussia (South Portuguese Zone) is juxtaposed with parautochthonous Gondwana (Ossa Morena Zone). Fault-bounded oceanic metasedimentary rocks, mélanges and ophiolite complexes characterize the suture zone and are intruded by plutonic rocks and mafic dykes. The generation and emplacement of these intrusive rocks and their relationship to development of the suture zone and the orogen are undetermined. Field evidence combined with U/Pb (zircon) geochronology reveals three main phases of plutonism, a pre-collisional unfoliated gabbroic phase emplaced at ca 354 Ma, crosscut by a syn-tectonic ca 345 Ma foliated granodiorite phase followed by a ca 335 Ma granitic phase. Geochemical analyses (major, trace, rare earth elements) indicate that the gabbro exhibits a calc-alkaline arc signature whereas the granodiorite and granite are typical of post-collisional slab break-off. Taken together, these data demonstrate a protracted development of the orogen and support a complex late stage evolution broadly similar to the tectonics of the modern eastern Mediterranean. In this scenario, the highly oblique closure of a small tract of oceanic lithosphere postdates the main collision event resulting in escape of parautochthonous and allochthonous terranes toward the re-entrant.

U-Pb zircon constraints on the tectonic evolution of southeastern Tibet, Namche Barwa area

USGS Publications Warehouse

Booth, A.L.; Zeitler, P.K.; Kidd, W.S.F.; Wooden, J.; Liu, Yajing; Idleman, B.; Hren, M.; Chamberlain, C.P.

2004-01-01

The eastern syntaxis of the Himalayas is expressed in the crust as a pronounced southward bend in the orogen. The change in strike of geologic features coincides with the high topography of the Namche Barwa region, the exposure of granulite-grade metamorphic rocks, and a 180-degree bend in the Yalu Tsangpo. We have conducted a geochronologic and geochemical investigation of several suites of granitoids collected from the Namche Barwa massif and subjacent terranes of southeastern Tibet, ranging from cm-scale dikes and sills to larger, outcrop-scale intrusions. U-Pb SHRIMP-RG zircon ages establish at least five magmatic episodes: ???400 to 500 Ma, ???120 Ma, 40 to 70 Ma, 18 to 25 Ma, and 3 to 10 Ma. These episodes broadly correlate to spatial patterns in sample localities, as follows: 400 to 500 Ma ages occur in zircon cores collected from within the massif proper; ???120 Ma granites, related to early Gangdese arc plutonism, are primarily located northeast of Namche Barwa; later (40-70 Ma) Gangdese activity is expressed in granites west of Namche Barwa. 18 to 25 Ma granites occur both along the suture zone west of Gyala Peri, and directly north of Namche Barwa along the area of the Jiali fault zone, and are attributed both to shearing within the Jiali fault zone and to an early Miocene Gangdese Thrust event. Exceptionally young (<10 Ma) zircon ages are clustered near the core of the massif, along the Yalu Tsangpo gorge. Trace-element geochemical data indicates the presence of both fluid-present and fluid absent melts, with a fluid-absent (decompression) melting regime dominating near the core of Namche Barwa.

Charnockites and granites of the western Adirondacks, New York, USA: a differentiated A-type suite

USGS Publications Warehouse

Whitney, P.R.

1992-01-01

Granitic rocks in the west-central Adirondack Highlands of New York State include both relatively homogeneous charnockitic and hornblende granitic gneisses (CG), that occur in thick stratiform bodies and elliptical domes, and heterogeneous leucogneisses (LG), that commonly are interlayered with metasedimentary rocks. Major- and trace-element geochemical analyses were obtained for 115 samples, including both types of granitoids. Data for CG fail to show the presence of more than one distinct group based on composition. Most of the variance within the CG sample population is consistent with magmatic differentiation combined with incomplete separation of early crystals of alkali feldspar, plagioclase, and pyroxenes or amphibole from the residual liquid. Ti, Fe, Mg, Ca, P, Sr, Ba, and Zr decrease with increasing silica, while Rb and K increase. Within CG, the distinction between charnockitic (orthopyroxene-bearing) and granitic gneisses is correlated with bulk chemistry. The charnockites are consistently more mafic than the hornblende granitic gneisses, although forming a continuum with them. The leucogneisses, while generally more felsic than the charnockites and granitic gneisses, are otherwise geochemically similar to them. The data are consistent with the LG suite being an evolved extrusive equivalent of the intrusive CG suite. Both CG and LG suites are metaluminous to mildly peraluminous and display an A-type geochemical signature, enriched in Fe, K, Ce, Y, Nb, Zr, and Ga and depleted in Ca, Mg, and Sr relative to I- and S-type granites. Rare earth element patterns show moderate LREE enrichment and a negative Eu anomaly throughout the suite. The geochemical data suggest an origin by partial melting of biotite- and plagioclase-rich crustal rocks. Emplacement occurred in an anorogenic or post-collisional tectonic setting, probably at relatively shallow depths. Deformation and granulite-facies metamorphism with some partial melting followed during the Ottawan phase of the Grenville Orogeny, yielding the present migmatitic granitic and charnockitic gneisses. ?? 1992.

Factors influencing the composition of detrital heavy mineral suites in Holocene sands of the Apure River drainage basin, Venezuela

USGS Publications Warehouse

Morton, Andrew C.

1993-01-01

Heavy mineral assemblages in rivers in the Apure River drainage basin of Venezuela and Colombia closely reflect the nature of the source regions, which lie in the Andean orogenic terranes to the west and northwest. The Caribbean Mountains, largely composed of greenschist-facies pelites, phyllites, carbonates, and metavolcanics, supply assemblages dominated by epidote and calcic amphibole. Minor amounts of the high-pressure index minerals glaucophane and lawsonite indicate the presence of blueschistfacies rocks, reflecting the origin of the Caribbean Mountains by subduction-related tectonism. The northern Mérida Andes, which comprise basement gneisses and granites overlain by unmetamorphosed to low-grade metamorphosed clastics, supply two types of assemblage reflecting these two lithological types: garnet-sillimanite-staurolite-amphibole suites from the basement rocks, and epidote-amphibole suites from the overlying cover sequence. The southern Mérida Andes supply stable heavy mineral suites reflecting recycling from the extensive unmetamorphosed sandstones that occur at outcrop. By considering suites from different physiographical provinces, the effects of short-term alluvial storage in the Llanos on heavy mineral assemblages have been evaluated. Weathering during alluvial storage appears to be effective in modifying the apatite-tourmaline ratio, which shows a steady, marked decline with distance from the mountain front, resulting from the removal of apatite during weathering. Clinopyroxene and garnet may also show evidence of loss through weathering, although the trends are poorly constrained statistically. Epidote and amphibole proportions remain essentially constant, possibly through a balance between loss through weathering and continual resupply from the breakdown of rock fragments. In general, the heavy mineral assemblages are less affected than the bulk mineralogy by alluvial storage on the Llanos.

Linking magmatism with collision in an accretionary orogen

PubMed Central

Li, Shan; Chung, Sun-Lin; Wilde, Simon A.; Wang, Tao; Xiao, Wen-Jiao; Guo, Qian-Qian

2016-01-01

A compilation of U-Pb age, geochemical and isotopic data for granitoid plutons in the southern Central Asian Orogenic Belt (CAOB), enables evaluation of the interaction between magmatism and orogenesis in the context of Paleo-Asian oceanic closure and continental amalgamation. These constraints, in conjunction with other geological evidence, indicate that following consumption of the ocean, collision-related calc-alkaline granitoid and mafic magmatism occurred from 255 ± 2 Ma to 251 ± 2 Ma along the Solonker-Xar Moron suture zone. The linear or belt distribution of end-Permian magmatism is interpreted to have taken place in a setting of final orogenic contraction and weak crustal thickening, probably as a result of slab break-off. Crustal anatexis slightly post-dated the early phase of collision, producing adakite-like granitoids with some S-type granites during the Early-Middle Triassic (ca. 251–245 Ma). Between 235 and 220 Ma, the local tectonic regime switched from compression to extension, most likely caused by regional lithospheric extension and orogenic collapse. Collision-related magmatism from the southern CAOB is thus a prime example of the minor, yet tell-tale linking of magmatism with orogenic contraction and collision in an archipelago-type accretionary orogen. PMID:27167207

Integration of multi-source and multi-scale datasets for 3D structural modeling for subsurface exploration targeting, Luanchuan Mo-polymetallic district, China

NASA Astrophysics Data System (ADS)

Wang, Gongwen; Ma, Zhenbo; Li, Ruixi; Song, Yaowu; Qu, Jianan; Zhang, Shouting; Yan, Changhai; Han, Jiangwei

2017-04-01

In this paper, multi-source (geophysical, geochemical, geological and remote sensing) datasets were used to construct multi-scale (district-, deposit-, and orebody-scale) 3D geological models and extract 3D exploration criteria for subsurface Mo-polymetallic exploration targeting in the Luanchuan district in China. The results indicate that (i) a series of region-/district-scale NW-trending thrusts controlled main Mo-polymetallic forming, and they were formed by regional Indosinian Qinling orogenic events, the secondary NW-trending district-scale folds and NE-trending faults and the intrusive stock structure are produced based on thrust structure in Caledonian-Indosinian orogenic events; they are ore-bearing zones and ore-forming structures; (ii) the NW-trending district-scale and NE-trending deposit-scale normal faults were crossed and controlled by the Jurassic granite stocks in 3D space, they are associated with the magma-skarn Mo polymetallic mineralization (the 3D buffer distance of ore-forming granite stocks is 600 m) and the NW-trending hydrothermal Pb-Zn deposits which are surrounded by the Jurassic granite stocks and constrained by NW-trending or NE-trending faults (the 3D buffer distance of ore-forming fault is 700 m); and (iii) nine Mo polymetallic and four Pb-Zn targets were identified in the subsurface of the Luanchuan district.

New constraints on the age and conditions of LPHT metamorphism in the southwestern Central Zone of the Damara Belt, Namibia and implications for tectonic setting

NASA Astrophysics Data System (ADS)

Longridge, L.; Gibson, R. L.; Kinnaird, J. A.; Armstrong, R. A.

2017-05-01

Orthopyroxene-bearing pelitic migmatites and associated anatectic leucogranites from the southwestern Central Zone of the Damara Belt provide revised constraints on the age and grade of LPHT metamorphism and its timing relative to deformation. Pseudosection modelling using THERMOCALC 3.33 indicates a single metamorphic event with peak temperatures of ca. 835 °C and pressures of 4.9 kbar for a garnet-cordierite-biotite-orthopyroxene schist. These temperatures confirm the attainment of true granulite facies conditions in the belt and are higher than previous estimates based on cation-exchange thermobarometry, which are likely to have been affected by retrograde re-equilibration and underestimate peak temperatures for the Central Zone by 50-150 °C. The early growth of sillimanite, consumption of sillimanite to produce cordierite, and the late development of garnet, together with modal isopleths and textural constraints on mineral reactions suggest a near-isobaric heating path for the southwestern Central Zone. Field and petrographic relationships indicate that the metamorphic peak was coeval with non-coaxial D2 deformation that produced orogen-normal, south- to SE-verging, km-scale, recumbent folds and late-D2 shear zones linked to NE-SW, orogen-parallel, extension. Weighted mean U-Pb single-grain concordia ages of 520.3 ± 4.6 Ma (zircon) and 514.1 ± 3.1 Ma (monazite) from a syn-D2 anatectic garnet-bearing granite constrain the age of metamorphism and the D2 deformation event in the southwestern Central Zone to 520-510 Ma. It is suggested that two tectonometamorphic episodes are preserved in the Central Zone. NW-verging folding and thrusting coeval with the emplacement of the Salem-type granites and mafic-dioritic Goas Suite took place at 550-530 Ma, and south- to SE-verging folding, shearing and NE-SW extension at 520-510 Ma was coeval with granulite-facies metamorphism and the emplacement of crustal melt granitoids. These events are temporally distinct and should not be considered different rheological responses to a single tectonic episode. We suggest that the 550-530 Ma event records crustal thickening related to collision of the Congo and Kalahari cratons, whilst the 520-510 Ma event reflects orogenic collapse and crustal thinning, with a possible heat contribution as the result of detachment of the subcontinental lithosphere following collision, resulting in addition of heat to the lower crust.

Nature, geochemistry and petrogenesis of the syn-tectonic Amspoort suite (Pan-African Boundary Igneous Complex, Kaoko Belt, NW Namibia)

NASA Astrophysics Data System (ADS)

Janousek, Vojtech; Konopasek, Jiri; Ulrich, Stanislav

2010-05-01

Crucial information on the Neoproterozoic-Cambrian amalgamation of Western Gondwana is provided by studies of the large Pan-African collisional belt in central-northern Namibia. This so-called Damara Orogen (Miller, 1983) can be subdivided into two branches, the SW-NE trending Damara Belt and a roughly perpendicular, NNW-SSE trending Kaoko Belt further north. The Kaoko Belt consists of two principal crustal units. The easterly part has a Congo Craton affinity (a basement built mostly by ≥ 1.5 Ga granitic gneisses with Neoproterozoic metasedimentary cover), whereas the westerly Coastal Terrane consists of Neoproterozoic (c.850-650 Ma) metapsammites and minor metabasic bodies; no exposures of the basement were found. The at least 180 km long, NNW-SSE trending suture between both units was intruded by numerous syn-tectonic magmatic bodies with ages spanning the interval 580-550 Ma (Seth et al., 1998; Kröner et al., 2004) designated as the Boundary Igneous Complex by Konopásek et al. (2008). The most typical representatives of this syn-collision igneous association are c.550 Ma old K-feldspar-phyric, Bt ± Cam granites-granodiorites of the Amspoort suite, with minor Cpx gabbro and rare two-pyroxene dolerite bodies. The petrological character, whole-rock geochemistry and Sr-Nd isotopic signatures of the scarce Opx-Cpx-Bt dolerites indicate an origin from a CHUR-like mantle-derived melts (87Sr/86Sr550 ~ 0.7045, ɛNd550 ~ 0) modified by extensive (?Ol-) Cpx fractionation. The rest of the suite is interpreted as a product of a high-temperature anatexis of a heterogeneous lower crust, built mainly by immature metapsammites - rich in arc-derived detritus - with minor metabasite and intermediate metaigneous bodies. The most likely source appears to be the anatectic Coastal Terrane gneisses. Yet, partial melting of the so far little constrained Congo Craton cover, if formed by immature and youthful detritus unrelated to the basement, cannot be discounted. In any case, the rather primitive Sr-Nd isotopic compositions of the Amspoort suite (apart from dolerites; ɛNd550 = -3.4 to -5.3, 87Sr/86Sr550= 0.7063-0.711), rule out any major role for the Congo Craton-basement derived material. On a much broader scale, the contribution of Congo-derived material in the Pan-African granitoids in the Kaoko Belt seems to increase southwards from nearly nil in the studied suite, through minimal in Hoanib Valley (Seth et al., 2002) to maximum in southern extremity of the Belt (Jung et al., 2009 and, in particular, van de Flierdt et al., 2003). This research was financially supported by GAČR Project 205/07/1409 (to JK). Jung, S., et al., 2009. Lithos, 111: 220-235. Konopásek, J., et al., 2008. Journal of the Geological Society, 165, 153-165. Kröner, S., et al., 2004. South African Journal of Geology, 107, 455-476. Miller, R.M., 1983. In: Miller, R.M. (Ed.), Evolution of the Damara Orogen of South West Africa/Namibia. Geological Society of South Africa Special Publications 11, 431-515. Seth, B., et al., 1998. Precambrian Research, 92, 341-363. Seth, B., et al., 2002. South African Journal of Geology, 105, 179-192. van de Flierdt, T. , et al., 2003. Lithos, 67, 205-226.

Variscan potassic dyke magmatism of durbachitic affinity at the southern end of the Bohemian Massif (Lower Austria)

NASA Astrophysics Data System (ADS)

Zeitlhofer, Helga; Grasemann, Bernhard; Petrakakis, Konstantin

2016-06-01

Dykes in the Strudengau area (SW Moldanubian Zone, Austria) can be mineralogically divided into lamprophyres (spessartites and kersantites) and felsic dykes (granite porphyries, granitic dykes and pegmatoid dykes). Geochemical analyses of 11 lamprophyres and 7 felsic dykes show evidence of fractional crystallization. The lamprophyres are characterized by metaluminous compositions, intermediate SiO2 contents and high amounts of MgO and K2O; these rocks have high Ba (800-3000 ppm) and Sr (250-1000 ppm) contents as well as an enrichment of large-ion lithophile elements over high field strength elements, typical for enriched mantle sources with variable modifications due to fractionation and crustal contamination. This geochemical signature has been reported from durbachites (biotite- and K feldspar-rich mela-syenites particularly characteristic of the Variscan orogen in Central Europe). For most major elements, calculated fractionation trends from crystallization experiments of durbachites give an excellent match with the data from the Strudengau dykes. This suggests that the lamprophyres and felsic dykes were both products of fractional crystallization and subsequent magma mixing of durbachitic and leucogranitic melts. Rb-Sr geochronological data on biotite from five undeformed kersantites and a locally deformed granite porphyry gave cooling ages of c. 334-318 Ma, indicating synchronous intrusion of the dykes with the nearby outcropping Weinsberger granite (part of the South Bohemian Batholith, c. 330-310 Ma). Oriented matrix biotite separated from the locally deformed granite porphyry gave an Rb-Sr age of c. 318 Ma, interpreted as a deformation age during extensional tectonics. We propose a large-scale extensional regime at c. 320 Ma in the Strudengau area, accompanied by plutonism of fractionated magmas of syncollisional mantle-derived sources, mixed with crustal components. This geodynamic setting is comparable to other areas in the Variscan belt documenting an orogenic wide extension by the end of the Carboniferous.

Crystalline rocks of the Strawberry Lake area, Front Range, Colorado

USGS Publications Warehouse

Young, Edward J.

1991-01-01

This report is a petrographic and geochemical study of the bedrock and a petrologic discussion based on felsic-mafic and silica-saturation ratios of the Strawberry Lake area. This volume is published as chapters A and B. These chapters are not available separatelyThe Strawberry lake area lies between the Continental Divide and Granby, Colorado, just north of Tabernash. It is underlain by Proterozoic rocks composed of biotite gneiss and two plutons-Boulder Creek Granodiorite of the Routt Plutonic Suite and Silver Plume Granite of the Berthoud Plutonic Suite. Relict enclaves of biotite gneiss are not uncommon in the Boulder Creek Granodiorite, in the Silver Plume Granite, and in the granitic enclaves in the biotite gneiss. Granitic and mafic enclaves in the Boulder Creek Granodiorite, granitic enclaves in the Silver Plume Granite and in the biotite gneiss, and a Tertiary andesite porphyry dike complete the rock types.

Late Paleozoic magmatism in South China: Oceanic subduction or intracontinental orogeny?

NASA Astrophysics Data System (ADS)

Liu, Q.; Yu, J.; Zhao, G.

2013-12-01

The significant late Paleozoic magmatism has been widely recognized in the East Asian Blocks, which sheds a light on the assembly and break-up of the Pangea supercontinent. As one of major components in East Asia, however, the South China Block (SCB) does not have much late Paleozoic magmatism recognized. Here we report a gneissic granite intrusion in northeastern Fujian Province, eastern SCB. It is a S-type granite characterized by high K2O and Al2O3, and low SiO2 and Na2O with a high A/CNK ratio of 1.22. Zircons with stubby morphology from this gneissic granite yield 206Pb/238U ages ranging from 326 Ma to 301 Ma with a weighted average age of 313×4 Ma, and negative epsilonHf(t) values from -8.35 to -1.74 with two-stage Hf model ages of 1.43 to 1.84 Ga. This S-type granite was probably originated from late Paleoproterozoic crust during an intracontinental orogeny, not under oceanic subduction. Integrated with previous results on the paleogeographic reconstruction of the SCB, the nature of Paleozoic basins, Early Permian volcanism and U-Pb-Hf isotope of detrital zircons from the late Paleozoic to early Mesozoic sedimentary rocks, our data support a late Paleozoic orogeny in the SCB, which may have included Late Carboniferous (340-310 Ma) compressive episode and Early Permian (287-270 Ma) post-orogenic or intraplate extensive episode. Our interpretation is consistent with the late Paleozoic orogenic events recognized in other Pangea microcontinents, and thus provides a window for the reconstruction of Pangea. Acknowledgements: NSFC (41190070, 41190075)

Emplacement ages, geochemical and Sr-Nd-Hf isotopic characterization of Mesozoic to early Cenozoic granitoids of the Sikhote-Alin Orogenic Belt, Russian Far East: Crustal growth and regional tectonic evolution

NASA Astrophysics Data System (ADS)

Jahn, Bor-ming; Valui, Galina; Kruk, Nikolai; Gonevchuk, V.; Usuki, Masako; Wu, Jeremy T. J.

2015-11-01

The Sikhote-Alin Range of the Russian Far East is an important accretionary orogen of the Western Pacific Orogenic Belt. In order to study the formation and tectonic evolution of the orogen, we performed zircon U-Pb dating, as well as geochemical and Sr-Nd-Hf isotopic analyses on 24 granitoid samples from various massifs in the Primorye and Khabarovsk regions. The zircon dating revealed that the granitoids were emplaced from 131 to 56 Ma (Cretaceous to Paleogene). In the Primorye Region, granitoids in the coastal Sikhote-Alin intruded the Cretaceous Taukha Accretionary Terrane from ca. 90 to 56 Ma, whereas those along the Central Sikhote-Alin Fault zone intruded the Jurassic Samarka Accretionary Terrane during ca. 110-75 Ma. The "oldest" monzogranite (131 Ma) was emplaced in the Lermontovka area of the NW Primorye Region. Granitoid massifs along the Central Sikhote-Alin Fault zone in the Khabarovsk Region formed from 109 to 58 Ma. Thus, the most important tectonothermal events in the Sikhote-Alin orogen took place in the Cretaceous. Geochemical analysis indicates that most samples are I-type granitoids. They have initial 87Sr/86Sr ratios ranging from 0.7040 to 0.7083, and initial Nd isotopic ratios, expressed as εNd(t) values, from +3.0 to -5.0 (mostly 0 to -5). The data suggest that the granitoid magmas were generated by partial melting of sources with mixed lithologies, including the subducted accretionary complex ± hidden Paleozoic-Proterozoic basement rocks. Based on whole-rock Nd isotopic data, we estimated variable proportions (36-77%) of juvenile component (=mantle-derived basaltic rocks) in the generation of the granitic magmas. Furthermore, zircon Hf isotopic data (εHf(t) = 0 to +15) indicate that the zircon grains crystallized from melts of mixed sources and that crustal assimilation occurred during magmatic differentiation. The quasi-continuous magmatism in the Sikhote-Alin orogen suggests that the Paleo-Pacific plate subduction was very active in the Late Cretaceous. The apparently regular progression of granitic intrusion ages from 80 to 56 Ma in the Taukha Terrane may reflect oblique underflow of the Paleo-Pacific plate beneath the Eurasian continental margin. Subduction was not only manifested by granitic intrusion, but also by abundant silicic volcanism. The Late Cretaceous Paleo-Pacific plate motion probably changed from parallel or sub-parallel to oblique relative to the continental margin of the Sikhote-Alin, leading to the change of magmatic source region and geochemical characteristics of the derived igneous rocks. Late Cretaceous rapid sea-floor spreading at ca. 100 Ma induced highly active subduction and led to voluminous magmatism in the entire Circum-Pacific realm. Finally, the present age and isotopic study lends support to the hypothesis of geologic and tectonic correlation between Sikhote-Alin and SW Japan.

Magnetotelluric Imaging of the Lithosphere Across the Variscan Orogen (Iberian Autochthonous Domain, NW Iberia)

NASA Astrophysics Data System (ADS)

Alves Ribeiro, J.; Monteiro-Santos, F. A.; Pereira, M. F.; Díez Fernández, R.; Dias da Silva, Í.; Nascimento, C.; Silva, J. B.

2017-12-01

A new magnetotelluric (MT) survey comprising 17 MT soundings throughout a 30 km long N30°W transect in the Iberian autochthons domain of NW Iberia (Central Iberian Zone) is presented. The 2-D inversion model shows the resistivity structure of the continental crust up to 10 km depth, heretofore unavailable for this region of the Variscan Orogen. The MT model reveals a wavy structure separating a conductive upper layer underlain by a resistive layer, thus picturing the two main tectonic blocks of a large-scale D2 extensional shear zone (i.e., Pinhel shear zone). The upper layer represents a lower grade metamorphic domain that includes graphite-rich rocks. The lower layer consists of high-grade metamorphic rocks that experienced partial melting and are associated with granites (more resistive) emplaced during crustal thinning. The wavy structure is the result of superimposed crustal shortening responsible for the development of large-scale D3 folds (e.g., Marofa synform), later deflected and refolded by a D4 strike-slip shear zone (i.e., Juzbado-Penalva do Castelo shear zone). The later contribution to the final structure of the crust is marked by the intrusion of postkinematic granitic rocks and the propagation of steeply dipping brittle fault zones. Our study demonstrates that MT imaging is a powerful tool to understand complex crustal structures of ancient orogens in order to design future prospecting surveys for mineral deposits of economic interest.

A hybrid composite dike suite from the northern Arabian Nubian Shield, southwest Jordan: Implications for magma mixing and partial melting of granite by mafic magma

NASA Astrophysics Data System (ADS)

Jarrar, Ghaleb H.; Yaseen, Najel; Theye, Thomas

2013-03-01

The Arabian Nubian Shield is an exemplary juvenile continental crust of Neoproterozoic age (1000-542 Ma). The post-collisional rift-related stage (~ 610 to 542 Ma) of its formation is characterized among others by the intrusion of several generations of simple and composite dikes. This study documents a suite of hybrid composite dikes and a natural example of partial melting of granite by a mafic magma from the northernmost extremity of Arabian Nubian Shield in southwest Jordan. The petrogenesis of this suite is discussed on the basis of field, petrographic, geochemical, and Rb/Sr isotopic data. These dikes give spectacular examples of the interaction between basaltic magma and the granitic basement. This interaction ranges from brecciation, partial melting of the host alkali feldspar granite to complete assimilation of the granitic material. Field structures range from intrusive breccia (angular partially melted granitic fragments in a mafic groundmass) to the formation of hybrid composite dikes that are up to 14 m in thickness. The rims of these dikes are trachyandesite (latite) with alkali feldspar ovoids (up to 1 cm in diameter); while the central cores are trachydacite to dacite and again with alkali feldspar ovoids and xenoliths from the dike rims. The granitic xenoliths in the intrusive breccia have been subjected to at least 33% partial melting. A seven-point Rb/Sr isochron from one of these composite dikes yields an age of 561 ± 33 Ma and an initial 87Sr/86Sr ratio of 0.70326 ± 0.0003 (2σ) and MSWD of 0.62. Geochemical modeling using major, trace, rare earth elements and isotopes suggests the generation of the hybrid composite dike suite through the assimilation of 30% to 60% granitic crustal material by a basaltic magma, while the latter was undergoing fractional crystallization at different levels in the continental crust.

Elemental and Sr-Nd isotopic geochemistry of Cretaceous to Early Paleogene granites and volcanic rocks in the Sikhote-Alin Orogenic Belt (Russian Far East): implications for the regional tectonic evolution

NASA Astrophysics Data System (ADS)

Zhao, Pan; Jahn, Bor-ming; Xu, Bei

2017-09-01

The Sikhote-Alin Orogenic Belt in Russian Far East is an important Late Mesozoic to Early Cenozoic accretionary orogen related to the subduction of the Paleo-Pacific Plate. This belt was generated by successive accretion of terranes made of accretionary prisms, turbidite basins and island arcs to the continental margin of northeastern Asia (represented by the Bureya-Jiamusi-Khanka Block) from Jurassic to Late Cretaceous. In order to study the tectonic and crustal evolution of this orogenic belt, we carried out zircon U-Pb dating, and whole-rock elemental and Sr-Nd isotopic analyses on granites and volcanic rocks from the Primorye region of southern Sikhote-Alin. Zircon dating revealed three episodes of granitoid emplacement: Permian, Early Cretaceous and Late Cretaceous to Early Paleogene. Felsic volcanic rocks (mainly rhyolite, dacite and ignimbrite) that overlay all tectonostratigraphic terranes were erupted during 80-57 Ma, postdating the accretionary process in the Sikhote-Alin belt. The Cretaceous-Paleogene magmatism represents the most intense tectonothermal event in the Sikhote-Alin belt. Whole-rock major and trace elemental data show arc-like affinity for granitoids and volcanic rocks, indicating that they were likely generated in a supra-subduction setting. Their initial 87Sr/86Sr ratios range from 0.7048 to 0.7114, and εNd(t) values vary from +1.7 to -3.8 (mostly < 0). Thus, the elemental and Sr-Nd isotopic data suggest that the felsic magmas were generated by partial melting of source rocks comprising mantle-derived juvenile component and recycled crustal component. In addition to the occurrence in the Sikhote-Alin orogenic belt, Cretaceous to Early Paleogene magmatic rocks are also widespread in NE China, southern Korean peninsula, Japanese islands and other areas of Russian Far East, particularly along the coastal regions of the Okhotsk and Bering Seas. These rocks constitute an extended magmatic belt along the continental margin of NE Asia. The generation of this belt was ascribed to subduction of the Paleo-Pacific Plate.

Geochemistry of Cretaceous granites from Mianning in the Panxi region, Sichuan Province, southwestern China: Implications for their generation

NASA Astrophysics Data System (ADS)

Xu, Cheng; Huang, Zhilong; Qi, Liang; Fu, Pingqing; Liu, Congqiang; Li, Endong; Guan, Tao

2007-03-01

The Cretaceous granites of Mianning, located in the northern Panxi region, were emplaced after collision of the Tibetan Plateau and Yangtze Block. These granites have very high K 2O + Na 2O, Ga, Zr, Nb, Y, REE (except Eu), and very low MgO, CaO, P 2O 5, and Sr contents relative to M-, I- or S-type granites. Based on the chemical discrimination criteria of Whalen et al . [Whalen, J.B., Currie, K.L., Chappell, B.W., 1987. A-type granites: geochemical characteristics, distribution and petrogenesis. Contributions to Mineralogy and Petrology 95, 407-419], most of them are A-type granites. Moreover, the granites plot in the range of post-collision granites and belong to the A2 type. Elevated initial Sr isotopic ratios (>0.72) suggest their derivation dominantly from a crustal source. These features are consistent with granite formation in a post-orogenic setting, such as after subduction or collision between of the Tibetan Plateau and Yangtze Block. In addition, the granites are characterized by low abundances of Ba, Sr, P, Ti, and Eu, positive correlation between Ba and Eu anomalies, and negative correlation between Rb and K/Rb. Plots of Rb vs. Sr suggest that fractional crystallization affected the final compositions of these granites after melting from a dominantly crustal source. From the late Proterozoic to late Mesozoic, the crustal composition, compared to that of the mantle, appears to have increased in the Panxi region. While the mantle component played an important part in the generation of Cretaceous granites in southeastern China, its influence was relatively minor in the Panxi region. Thus, there was a significant difference in mantle evolution between southeastern China and the Panxi region, which led to different metallogenic processes.

The eye of the field geologist and the mind of the tectonician: one view of dynamic crustal rheology in convergent orogens

NASA Astrophysics Data System (ADS)

Brown, M.

2004-05-01

Orogens record evidence of interaction between converging plates. However, the response of continental crust to tectonic and gravitational loads is dependent on rheology, which is influenced by composition, architecture, thermal profile and strain rate. Crustal rocks undergo melting in deeper parts of orogens. Greywackes and metapelites are the most fertile protoliths, generating 20-50 and 30 vol. % melt respectively at 1 GPa and 1173K; geophysical data suggest >6 but <20 vol. % interconnected melt in deep crust of active orogens. In numerical models of orogens, the transition from coupled doubly-vergent wedge structure to plateau formation and full basal decoupling requires a viscosity drop of 4 orders of magnitude, inferred to be melt weakening. Deformation experiments on granite indicate a dramatic drop in strength (to 100-200 MPa) as the melt wetting transition is approached at 7 vol. % melt, and a more gradual decrease to <1 MPa prior to the drop at the solid-to-liquid transition (RCMP). Important properties of melting systems are viscosity of the melt, rheology of the crystalline framework of grains and permeability of this framework to flow. Permeability is due to an intergranular network of connected pores, compositional layering/fabric and networks of deformation bands; melt distribution is heterogeneous on multiple length scales. The microstructure of anatectic rocks and the magnitude of weakening accompanying melting suggest a limited role for intracrystalline plasticity with increasing vol. % melt and dominance of melt-assisted diffusion creep or diffusion accommodated granular flow. The intrinsic weakness of melt-bearing intervals in the crust makes them ideal detachment horizons. Observations from metasedimentary migmatitic granulites show preservation of (i) early fabrics, suggesting that the strain field emergent under subsolidus conditions controlled initial distribution of melt produced by suprasolidus mica breakdown, and (ii) layering in melt-depleted rocks, implying that they were quasi-continuously drained. Studies of migmatitic granulites demonstrate that melt migrates from grain boundaries to mesoscale networks of structures (mm to m) to steeply-inclined conduits recorded by rod or tabular granite intrusions (m to dm). Melt loss from lower crust yields residual rocks composed of strong minerals (feldspar, pyroxene and garnet) with only minor melt on grain boundaries. Thus, weakening of lower crust due to melting is followed by its strengthening. Around the brittle-to-viscous transition zone granite accumulates in subhorizontal tabular plutons, which implies transient presence of significantly weaker layers in shallow orogenic crust; these are potential detachment horizons. Field studies of exhumed orogens suggest deformation commonly is laterally, transversely and vertically diachronous, reflecting the spatial and temporal variation in the weakening-to-strengthening cycle. There may be important sub-horizontal movement horizons, which allow (partial) decoupling of crustal layers. At upper-to-middle crustal levels rocks are metamorphosed in greenschist-amphibolite facies, with local enhancement by pluton-advected heat to amphibolite-granulite facies and thrust-style brittle-ductile deformation (e.g., Acadian, NH). Rocks from middle crustal levels are in amphibolite facies and have penetrative steep fabrics (e.g., Acadian, western ME) or exhibit a complex network of shallowly- and steeply-dipping fabrics (e.g., St. Malo, France). Rocks from lower crustal levels are in amphibolite-granulite facies and have shallow fabrics due to crustal flow, although these may be steepened by later deformation including core-complex formation (e.g., S. Brittany, France).

The Statherian itabirite-bearing sequence from the Morro Escuro Ridge, Santa Maria de Itabira, Minas Gerais, Brazil

NASA Astrophysics Data System (ADS)

Silveira Braga, Flávia Cristina; Rosière, Carlos Alberto; Queiroga, Gláucia Nascimento; Rolim, Vassily Khoury; Santos, João Orestes Schneider; McNaughton, Neal Jesse

2015-03-01

The itabirite-bearing metasedimentary sequence from Morro Escuro Ridge comprises the basal units of the Espinhaço Supergroup and makes up a small tectonic inlier developed during one of the Brasiliano orogenic events (800-500 Ma), amongst horses of the Archean TTG gneisses, including sheared granites of the anorogenic Borrachudos Suite (˜1700 Ma). The metasedimentary rocks are comprised of low-to intermediate-amphibolite facies schists, quartzites, conglomerates and banded iron formation (itabirite) correlatable with the sequences of the Serro Group, which underlies the metasedimentary rocks of the Espinhaço Supergroup in the Serra da Serpentina Ridge. A maximum Statherian deposition age (1668 Ma) was established using SHRIMP U-Pb isotopic constraints on zircon grains from conglomerate and quartzite units overlying the itabirite. The itabirite is predominantly hematitic and its geochemical characteristics are typical of a Lake Superior-type BIF deposited in a platformal, suboxic to anoxic environment distant from Fe-bearing hydrothermal vents. Close to the contact zone with amphibolites of the Early Neoproterozoic Pedro Lessa mafic suite, an increase of the magnetite content and crystallization of metasomatic Mg-hornblende and Ce-allanite can be observed. These mineralogical changes developed preferentially along the igneous contact zone but are probably co-genetic with the formation of alteration haloes in zircon grains during the Neoproterozoic Brasiliano orogeny (506 ± 6 Ma).

Further geochronological and paleomagnetic constraints on Malani (and pre-Malani) magmatism in NW India

NASA Astrophysics Data System (ADS)

Meert, Joseph G.; Pandit, Manoj K.; Kamenov, George D.

2013-11-01

At 750 Ma India was part of a larger fragment of eastern Gondwana blocks that included the Seychelles-Mauritia, Madagascar, Sri Lanka and the Enderby Land-Prydz Bay region of East Antarctica. Subduction of the Mozambique Ocean beneath Seychelles-Mauritia, northern Madagascar and northwestern India formed a lengthy continental arc that remained active during the formation of Gondwana. Paleomagnetic data from the Malani rhyolites and associated dykes provide a robust paleomagnetic pole constraining India's position at this time. The rhyolitic and granitic rocks associated with the Malani Igneous Suite (MIS) have robust age constraints; however, the ages of the mafic dykes were inferred solely on the basis of similarity in paleomagnetic directions to the rhyolitic units. Here we present new geochronological data from the Malani mafic dykes that yield a minimum age of 704 Ma. The 207Pb/206Pb ages obtained for the dykes are less-likely to be affected by Pb-loss and yield a more reliable estimate for the age of the mafic dykes of ~ 750 Ma. We argue that intrusion of these mafic (and minor felsic) dykes represents the final pulse of MIS magmatism. Many of the granitic rocks in the region are reported as ‘unclassified’ due to limited geochemical data and/or geochronological ages. Some of these ‘unclassified’ granites are intruded by the mafic dykes sampled in this study near the town of Bilara. The granites yielded zircon core ages of ~ 1100 Ma with younger rims averaging ~ 1020 Ma. We argue that this provides further evidence for a significant orogenic event ~ 1000 Ma that may relate to the collision of the Marwar block with the Banded Gneiss Complex/Bundelkhand craton in north-central India. Other ~ 1000 Ma orogenesis is also known along the Central Indian Tectonic Zone (CITZ) and the Eastern Ghats Mobile Belt. Globally, this same time interval is thought to represent the amalgamation of the supercontinent Rodinia and may also have resulted in the closure of the major “Purana” basins in India.

A new indicator mineral methodology based on a generic Bi-Pb-Te-S mineral inclusion signature in detrital gold from porphyry and low/intermediate sulfidation epithermal environments in Yukon Territory, Canada

NASA Astrophysics Data System (ADS)

Chapman, R. J.; Allan, M. M.; Mortensen, J. K.; Wrighton, T. M.; Grimshaw, M. R.

2017-12-01

Porphyry-epithermal and orogenic gold are two of the most important styles of gold-bearing mineralization within orogenic belts. Populations of detrital gold resulting from bulk erosion of such regions may exhibit a compositional continuum wherein Ag, Cu, and Hg in the gold alloy may vary across the full range exhibited by natural gold. This paper describes a new methodology whereby orogenic and porphyry-epithermal gold may be distinguished according to the mineralogy of microscopic inclusions observed within detrital gold particles. A total of 1459 gold grains from hypogene, eluvial, and placer environments around calc-alkaline porphyry deposits in Yukon (Nucleus-Revenue, Casino, Sonora Gulch, and Cyprus-Klaza) have been characterized in terms of their alloy compositions (Au, Ag, Cu, and Hg) and their inclusion mineralogy. Despite differences in the evolution of the different magmatic hydrothermal systems, the gold exhibits a clear Bi-Pb-Te-S mineralogy in the inclusion suite, a signature which is either extremely weak or (most commonly) absent in both Yukon orogenic gold and gold from orogenic settings worldwide. Generic systematic compositional changes in ore mineralogy previously identified across the porphyry-epithermal transition have been identified in the corresponding inclusion suites observed in samples from Yukon. However, the Bi-Te association repeatedly observed in gold from the porphyry mineralization persists into the epithermal environment. Ranges of P-T-X conditions are replicated in the geological environments which define generic styles of mineralization. These parameters influence both gold alloy composition and ore mineralogy, of which inclusion suites are a manifestation. Consequently, we propose that this methodology approach can underpin a widely applicable indicator methodology based on detrital gold.

Magma-assisted strain localization in an orogen-parallel transcurrent shear zone of southern Brazil

NASA Astrophysics Data System (ADS)

Tommasi, AndréA.; Vauchez, Alain; Femandes, Luis A. D.; Porcher, Carla C.

1994-04-01

In a lithospheric-scale, orogen-parallel transcurrent shear zone of the Pan-African Dom Feliciano belt of southern Brazil, two successive generations of magmas, an early calc-alkaline and a late peraluminous, have been emplaced during deformation. Microstructures show that these granitoids experienced a progressive deformation from magmatic to solid state under decreasing temperature conditions. Magmatic deformation is indicated by the coexistence of aligned K-feldspar, plagioclase, micas, and/or tourmaline with undeformed quartz. Submagmatic deformation is characterized by strain features, such as fractures, lattice bending, or replacement reactions affecting only the early crystallized phases. High-temperature solid-state deformation is characterized by extensive grain boundary migration in quartz, myrmekitic K-feldspar replacement, and dynamic recrystallization of both K-feldspar and plagioclase. Decreasing temperature during solid-state deformation is inferred from changes in quartz crystallographic fabrics, decrease in grain size of recrystallized feldspars, and lower Ti amount in recrystallized biotites. Final low-temperature deformation is characterized by feldspar replacement by micas. The geochemical evolution of the synkinematic magmatism, from calc-alkaline metaluminous granodiorites with intermediate 87Sr/86Sr initial ratio to peraluminous granites with very high 87Sr/86Sr initial ratio, suggests an early lower crustal source or a mixed mantle/crustal source, followed by a middle to upper crustal source for the melts. Shearing in lithospheric faults may induce partial melting in the lower crust by shear heating in the upper mantle, but, whatever the process initiating partial melting, lithospheric transcurrent shear zones may collect melt at different depths. Because they enhance the vertical permeability of the crust, these zones may then act as heat conductors (by advection), promoting an upward propagation of partial melting in the crust. Synkinematic granitoids localize most, if not all, deformation in the studied shear zone. The regional continuity and the pervasive character of the magmatic fabric in the various synkinematic granitic bodies, consistently displaying similar plane and direction of flow, argue for accommodation of large amounts of orogen-parallel movement by viscous deformation of these magmas. Moreover, activation of high-temperature deformation mechanisms probably allowed a much easier deformation of the hot synkinematic granites than of the colder country rock and, consequently, contributed significantly to the localization of deformation. Finally, the small extent of the low-temperature deformation suggests that the strike-slip deformation ended approximately synchronously with the final cooling of the peraluminous granites. The evolution of the deformation reflects the strong influence of synkinematic magma emplacement and subsequent cooling on the thermomechanical evolution of the shear zone. Magma intrusion in an orogen-scale transcurrent shear zone deeply modifies the rheological behavior of the continental crust. It triggers an efficient thermomechanical softening localized within the fault that may subsist long enough for large displacements to be accommodated. Therefore the close association of deformation and synkinematic magmatism probably represents an important factor controlling the mechanical response of continental plates in collisional environments.

Geology of the Andover Granite and surrounding rocks, Massachusetts

USGS Publications Warehouse

Castle, Robert O.

1964-01-01

Field and petrographic studies of the Andover Granite and surrounding rocks have afforded an opportunity for an explanation of its emplacement and crystallization. The investigation has contributed secondarily to an understanding of the geologic history of southeastern New England, particularly as it is revealed in the Lawrence, Wilmington, South Groveland, and Reading quadrangles of Massachusetts. The Andover Granite and Sharpners Pond Tonalite together comprise up to 90 percent of the Acadian(?) subalkaline intrusive series cropping out within the area of study. The subalkaline series locally invades a sequence of early to middle Paleozoic and possibly Precambrian metasedimentary and metavolcanic rocks. Much of the subalkaline series and most of the Andover Granite is confined between two prominent east-northeast trending faults or fault systems. The northern fault separates the mildly metamorphosed Middle Silurian(?) Merrimack Group on the north from a highly metamorphosed and thoroughly intruded Ordovician(?) sequence on the south. The southern 'boundary '' fault is a major structural discontinuity characterized by penetrative, diffuse shearing over a zone one-half mile or more in width. The magmatic nature of the Andover Granite is demonstrated by: (1) sharply crosscutting relationships with surrounding rocks; (2) the occurrence of tabular-shaped xenoliths whose long directions parallel the foliation within the granite and whose internal foliation trends at a high angle to that of the granite; (3) continuity with the clearly intrusive Sharpners Pond Tonalite; (4) the compositional uniformity of the granite as contrasted with the compositional diversity of the rocks it invades; (5) its modal and normative correspondence with (a) calculated norms of salic extrusives and (b) that of the ternary (granite) minimum for the system NaAlSi3O8-KAlSi3O8-SiO2. Orogenic granites, as represented by the Andover, contrast with post-orogenic granites, represented locally by the Peabody Granite, in their phase composition and texture. Unlike the Peabody, the Andover Granite is thought to have been thoroughly recrystallized through the unmixing of initially homogeneous phases with the concomitant development of extremely intricate, allotriomorphic textures. Textural relationships between potassium and plagioclase feldspars and among quartz and the two feldspars, suggest that the Andover Granite has evolved through exsolution of a single hypersolvus feldspar (or two coexisting subsolvus feldspars of only slightly disparate compositions) into discrete grains of plagioclase and potassium feldspar, much along the lines proposed by Tuttle (1952). A hypothesis is proposed for the origin of myrmekite whereby it is evolved indirectly through exsolution of a homogeneous, hypersolvus, calcalkali feldspar in the presence of a silica reservoir. Where the An 'molecule' is contained in the primary mix crystal, exsolution into potassium and plagioclase feldspar phases normally requires a paired exchange between Ca-Al and K-Si. Should the silicon requirements of the developing potassium feldspar be met by the matrix silica reservoir, the concomitantly evolving plagioclase may become stoichiometrically enriched in silicon and ultimately develop into myrmekite. Discrete unmixing of pure alkali feldspar proceeds through simple alkali ion exchange; ternary compostions high in An are more apt to fall initially in the two-feldspar field, thereby reducing the unmixing potential. General restriction of myrmekite to plagioclase of calcic albite to oligoclase composition is explained accordingly.

Geology of the Byrd Glacier Discontinuity (Ross Orogen): New survey data from the Britannia Range, Antarctica

USGS Publications Warehouse

Carosi, R.; Giacomini, F.; Talarico, F.; Stump, E.

2007-01-01

Field activities in the Britannia Range (Transantarctic Mountains, Antarctica) highlighted new geological features around the so-called Byrd Glacier discontinuity. Recent field surveys revealed the occurrence of significant amounts of medium- to high-grade metamorphic rocks, intruded by abundant coarse-grained porphyritic granitoids. Most of the granitoids are deformed, with foliation parallel to the regional foliation in the metamorphics. Two main episodes of deformation are observed. Tight to isoclinal folds and penetrative axial plane foliation are related to the D1 phase, open folds to the D2. The main foliation (D1) trends nearly E-W in agreement with the trend in the southern portion of the Byrd Glacier. In most outcrops, granitic dykes are folded and stretched by the D2 deformation, which shows similar characteristics with the D2 deformation south of the Byrd Glacier. This suggests the occurrence in the Ross orogen of an orogen-normal structure south and north of the Byrd Glacier.

Timing of the Acadian Orogeny in Northern New Hampshire.

PubMed

Eusden Jr; Guzofski; Robinson; Tucker

2000-03-01

New U-Pb geochronology constrains the timing of the Acadian orogeny in the Central Maine Terrane of northern New Hampshire. Sixteen fractions of one to six grains each of zircon or monazite have been analyzed from six samples: (1) an early syntectonic diorite that records the onset of the Acadian; (2) a schist, a migmatite, and two granites that together record the peak of the Acadian; and (3) a postkinematic pluton that records the end of the Acadian. Zircon from the syntectonic Wamsutta Diorite gives a 207Pb/206Pb age of circa 408 Ma, the time at which the boundary between the deforming orogenic wedge and the foreland basin was in the vicinity of the Presidential Range. This age agrees well with the Emsian position of the northwest migrating Acadian orogenic front and records the beginning of the Acadian in this part of the Central Maine Terrane. We propose a possible Acadian tectonic model that incorporates the geochronologic, structural, and stratigraphic data. Monazite from the schist, migmatite, Bigelow Lawn Granite, and Slide Peak Granite gives 207Pb/206U ages, suggesting the peak of Acadian metamorphism and intrusion of two-mica granites occurred at circa 402-405 Ma, the main pulse of Acadian orogenesis. Previously reported monazite ages from schists that likely record the peak metamorphism in the Central Maine Terrane of New Hampshire and western Maine range from circa 406-384 Ma, with younger ages in southeastern New Hampshire and progressively older ages to the west, north, and northeast. Acadian orogenesis in the Presidential Range had ended by circa 355 Ma, the 207Pb/235U age of monazite from the Peabody River Granite. From 408 to perhaps at least 394 Ma, Acadian orogenesis in the Presidential Range was typical of the tectonic style, dominated by synkinematic metamorphism, seen in central and southern New Hampshire, Massachusetts, and Connecticut. From no earlier than 394 Ma to as late as 355 Ma, the orogenesis was typical of the style in parts of Maine dominated by postkinematic metamorphism.

Timing of the Acadian Orogeny in northern New Hampshire

USGS Publications Warehouse

Eusden, J.D.; Guzofski, C.A.; Robinson, A.C.; Tucker, R.D.

2000-01-01

New U-Pb geochronology constrains the timing of the Acadian orogeny in the Central Maine Terrane of northern New Hampshire. Sixteen fractions of one to six grains each of zircon or monazite have been analyzed from six samples: (1) an early syntectonic diorite that records the onset of the Acadian, (2) a schist, a migmatite, and two granites that together record the peak of the Acadian; and (3) a postkinematic pluton that records the end of the Acadian. Zircon from the syntectonic Wamsutta Diorite gives a 207Pb/206Pb age of circa 408 Ma, the time at which the boundary between the deforming orogenic wedge and the foreland basin was in the vicinity of the Presidential Range. This age agrees well with the Emsian position of the northwest migrating Acadian orogenic front and records the beginning of the Acadian in this part of the Central Maine Terrane. We propose a possible Acadian tectonic model that incorporates the geochronologic, structural, and stratigraphic data. Monazite from the schist, migmatite, Bigelow Lawn Granite, and Slide Peak Granite gives 207Pb/206U ages, suggesting the peak of Acadian metamorphism and intrusion of two-mica granites occurred at circa 402-405 Ma, the main pulse of Acadian orogenesis. Previously reported monazite ages from schists that likely record the peak metamorphism in the Central Maine Terrane of New Hampshire and western Maine range from circa 406-384 Ma, with younger ages in southeastern New Hampshire and progressively older ages to the west, north, and northeast. Acadian orogenesis in the Presidential Range had ended by circa 355 Ma, the 207Pb/235U age of monazite from the Peabody River Granite. From 408 to perhaps at least 394 Ma, Acadian orogenesis in the Presidential Range was typical of the tectonic style, dominated by synkinematic metamorphism, seen in central and southern New Hampshire, Massachusetts, and Connecticut. From no earlier than 394 Ma to as late as 355 Ma, the orogenesis was typical of the style in parts of Maine dominated by postkinematic metamorphism.

An evolving tectonic environment of Late Carboniferous to Early Permian granitic plutons in the Chinese Altai and Eastern Junggar terranes, Central Asian Orogenic Belt, NW China

NASA Astrophysics Data System (ADS)

Zhang, Chen; Liu, Dongdong; Luo, Qun; Liu, Luofu; Zhang, Yunzhao; Zhu, Deyu; Wang, Pengfei; Dai, Quanqi

2018-06-01

The Central Asian Orogenic Belt (CAOB) represents one of the most important sites of juvenile crustal growth during the Phanerozoic. Located in the central part of the CAOB, the Chinese Altai and Eastern Junggar terranes record the collisional processes between the peri-Siberian and Kazakhstan orogenic systems. However, the precise timing of collision between the two terranes remains controversial. The Wukuli and Kadelat plutons in the Chinese Altai belt are dated at ∼305 and ∼280 Ma respectively, whereas the Aketas pluton in the Eastern Junggar terrane is dated at ∼308 Ma. Granites from the Wukuli and Kadelat plutons are strongly peraluminous (A/CNK > 1.1), and are characterized by low Al2O3, Na2O, MnO, MgO, CaO and heavy rare earth element (HREE) contents, but with high SiO2, K2O and Rb contents as well as high Rb/Sr ratios. Granites from the Wukuli pluton have low εNd(t) and εHf(t) values of -3.7 to -3.4 and -9.7 to +4.9, whereas those from the Kadelat pluton have values of -3.6 to -3.4 and -8.0 to +2.6. These features suggest S-type affinity for the Wukuli and Kadelat plutons with magma derivation through partial melting of Mesoproterozoic metasediments. The Aketas pluton is composed of weakly peraluminous quartz monzonites that have A/CNK values ranging from 0.92 to 1.08, with high Na2O, Sr, and Sr/Y, and low Y, Yb, Nb, and Ta. These rocks display positive εNd(t) (+4.8 to +6.4) and εHf(t) (+9.7 to +14.6) values, and low initial 87Sr/86Sr ratios (0.703357-0.703868), similar to modern adakites, suggesting that the quartz monzonites were derived from the partial melting of lower crustal material. The geochemical characteristics suggest that the Aketas pluton was formed in a subduction-related setting, the Wukuli pluton in a syn-collisional setting, and the Kadelat pluton in the subsequent post-orogenic strike-slip-related setting. In combination with data from other granitoids in these two terranes, the Aketas pluton represents the youngest record of subduction-related environments, suggesting that the final collision between the Chinese Altai and Eastern Junggar terranes might have occurred between 308 and 304 Ma.

Geochronology and geochemistry of late Carboniferous-middle Permian I- and A-type granites and gabbro-diorites in the eastern Jiamusi Massif, NE China: Implications for petrogenesis and tectonic setting

NASA Astrophysics Data System (ADS)

Bi, Jun-Hui; Ge, Wen-Chun; Yang, Hao; Wang, Zhi-Hui; Xu, Wen-Liang; Yang, Jin-Hui; Xing, De-He; Chen, Hui-Jun

2016-12-01

Late Carboniferous-middle Permian magmatism in the Jiamusi Massif of northeast China, in the eastern segment of the Central Asian Orogenic Belt (CAOB), provides critical evidence regarding the tectonic history and geodynamic processes in the region. The gabbro-diorites of the Longtouqiao pluton and two groups of coeval granite in the study area comprise a bimodal magmatic suite. Precise LA-ICP-MS U-Pb zircon ages indicate that the granitoids and gabbro-diorites were emplaced in the late Carboniferous-middle Permian (302-267 Ma). Group I granites have high SiO2 (70.75-77.04 wt.%) and K2O (3.65-5.89 wt.%) contents, are enriched in LILEs (e.g., Rb, Th, and U) relative to HFSEs and LREEs, and have negative Nb, Ta, P, and Ti anomalies, which collectively indicate affinities with subduction-related magmas. Group II granites are weakly peraluminous (A/CNK = 1.03-1.07) and are characterized by enrichment in alkalis (Na2O + K2O = 8.22-8.90 wt.%), low MgO (0.04-0.09 wt.%) and P2O5 (0.01-0.04 wt.%) contents, high Zr and Nb contents, high 10,000 × Ga/Al ratios, and they are geochemically similar to aluminous A-type granites. All the magmatic zircons in these granitoids have great variations of εHf(t) (+ 7.89 to - 5.60) and two-stage Hf model ages (TDM2) of 0.8-1.7 Ga, which suggest that the precursor magmas originated from a heterogeneous source that involved juvenile components derived from a depleted mantle source during magma generation. The aluminous A-type granite magmas were probably derived by high-temperature partial melting of a felsic crustal source, whereas the other granite magmas probably resulted from partial melting of a mafic lower crust. The gabbro-diorites of the Longtouqiao pluton are depleted in Nb, Ta, P, and Ti, and show flat distributions of most LILEs and HFSEs, except for large positive anomalies in Ba, K, and Pb. These features reflect a limited degree of crustal contamination associated with the subduction-related magmatic processes. These data, together with previously reported data and the nature of various rock types along the eastern part of the Jiamusi Massif, suggest that the intrusive rocks were formed in a geodynamic regime that changed from compression to extension during the westwards subduction of the Paleo-Pacific oceanic lithosphere, probably as a result of slab break-off of the subducting plate.

Fabrics and geochronology of the Wushan ductile shear zone: Tectonic implications for the Shangdan suture zone in the Qinling orogen, Central China

NASA Astrophysics Data System (ADS)

Liang, Xiao; Sun, Shengshi; Dong, Yunpeng; Yang, Zhao; Liu, Xiaoming; He, Dengfeng

2017-04-01

The ductile shearing along the Shangdan suture zone during the Paleozoic time is a key to understand the collisional deformation and tectonic regime of amalgamation between the North China Block and the South China Blocks. The Wushan ductile shear zone, a branch of the Shangdan suture, records mylonitic deformation that affected granitic and felsic rocks outcropping in an over 1 km wide belt in the western Qinling Orogenic belt. Shear sense indicators and kinematic vorticity number (0.79-0.99) of the mylonites reveal a dextral shear deformation. The quartz c-axis fabrics indicate activation of combined basal and rhomb slip, prism slip and prism slip. The dynamic recrystallization of quartz is accommodated by combined subgrain rotation and grain boundary migration. These characteristics suggest that the mylonites experienced ductile shear deformation under amphibolite facies conditions at temperatures of 500-650 C. Zircons from granitic mylonite yield a U-Pb age of 910 ± 4.8 Ma, which represents the formation age of the protolith of the mylonite. The ductile shear zone was intruded by a granitic dyke, which yields a zircon U-Pb age of 403 ± 3.5 Ma constraining the minimum age of the ductile shear deformation. Together with regional geology and available geochronological data, these structural characteristics and ages indicate that the Wushan ductile shear zone was formed by dextral shearing following the N-S shortening as a result of collision between the North China and South China blocks along the Shangdan suture.

Fabrics and geochronology of the Wushan ductile shear zone: Tectonic implications for the Shangdan suture zone in the Qinling orogen, Central China

NASA Astrophysics Data System (ADS)

Liang, Xiao; Sun, Shengsi; Dong, Yunpeng; Yang, Zhao; Liu, Xiaoming; He, Dengfeng

2017-05-01

The ductile shearing along the Shangdan suture zone during the Paleozoic time is a key to understand the collisional deformation and tectonic regime of amalgamation between the North China Block and the South China Blocks. The Wushan ductile shear zone, a branch of the Shangdan suture, records mylonitic deformation that affected granitic and felsic rocks outcropping in an over 1 km wide belt in the western Qinling Orogenic belt. Shear sense indicators and kinematic vorticity number (0.79-0.99) of the mylonites reveal a dextral shear deformation. The quartz c-axis fabrics indicate activation of combined basal and rhomb slip, prism slip and prism slip. The dynamic recrystallization of quartz is accommodated by combined subgrain rotation and grain boundary migration. These characteristics suggest that the mylonites experienced ductile shear deformation under amphibolite facies conditions at temperatures of ∼500-650 °C. Zircons from granitic mylonite yield a U-Pb age of 910 ± 4.8 Ma, which represents the formation age of the protolith of the mylonite. The ductile shear zone was intruded by a granitic dyke, which yields a zircon U-Pb age of 403 ± 3.5 Ma constraining the minimum age of the ductile shear deformation. Together with regional geology and available geochronological data, these structural characteristics and ages indicate that the Wushan ductile shear zone was formed by dextral shearing following the N-S shortening as a result of collision between the North China and South China blocks along the Shangdan suture.

New Insights Into the Genesis and Compositional Evolution of I-type Granitic magmas in the Lachlan Fold Belt (SE Australia) by in situ Hf Isotopic Analysis of Zircon

NASA Astrophysics Data System (ADS)

Kemp, T. I.; Hawkesworth, C. J.; Hergt, J. M.; Woodhead, J.

2004-05-01

Isotope studies have proved of enormous benefit in fingerprinting the source rocks of silicic magmas and tracing open system petrogenetic processes, such as crustal assimilation or magma mixing. Quantification of these processes, especially the role of mantle-derived magmas, is essential to formulating realistic models for the thermal regime and compositional evolution of the continental crust. However, this remains problematic, since whole-rock isotopic data registers the final state of the magmatic system but gives no information on the pathways by which this state was attained. For example, the eNd - initial 87Sr/86Sr isotopic array defined by the classic I- and S-type granites of the Lachlan Fold Belt has been variously interpreted to reflect (1) mixing between two end-member magmas, one depleted mantle-like, the other evolved and continental crust-like, (2) mixing between a juvenile magma and a magma sourced from mafic lower crust, accompanied by sediment assimilation, (3) derivation of the granites from mixed source rocks and (4) derivation from a sequence of protoliths of various ages and sedimentary maturity. The implications of these possibilities for crustal architecture, and whether granitic magmatism was associated with the recycling or growth of new continental crust are drastically different. One way to now resolve such ambiguities is by unravelling the isotopic information encoded in the fine-scale growth zoning of minerals such as zircon, which potentially tracks the processes operative during crystallisation. To this end we report the first laser-ablation ICP-MS study into the Hf isotope stratigraphy of zircons hosted by LFB I-type granites and their mafic enclaves. This is integrated with a prior U-Pb isotope study and trace element concentrations measured on the same zircons. Two suites were investigated, the Cobargo and Why Worry Suites of the Bega Batholith. Although the bulk rock isotopic variation within these suites is restricted, this study reveals remarkable fluctuations in Hf isotopic ratios recorded within and between melt-precipitated zircons of granitic and enclave samples. This can only be reconciled by open-system behaviour, though contrasting patterns of Hf isotope variation within zoned zircons demonstrate that this differed significantly between the two suites. The Cobargo Suite was generated by mixing between two contrasting magmas, followed by crustal assimilation. Zircons from the Why Worry Suite have more evolved Hf isotope ratios, consistent with recycling of older crust during granitic generation, though increase in eHf towards zircon rims manifests interaction with primitive magmas. Globules of these are represented by mafic enclaves, the mantle heritage of which is preserved by high eHf values of zircon cores, even though whole-rock isotope contrasts with the host have been erased by equilibration. Analysis of inherited zircons contained by the Why Worry Suite establishes that the 450-600 Ma age population have evolved eHf values, and thus meta-igneous rocks of this age are appropriate protoliths for these granites. The primitive eHf values of the Cobargo Suite preclude derivation from similar sources, instead suggesting formation from mantle-derived materials. Incorporating the existing geochemical and isotope datasets, the Hf-in-zircon data will be coupled with recent thermal simulations to erect a general model for granite formation and the evolution of the continental crust during Lachlan orogenesis.

Geodynamic processes and deformation in orogenic belts

NASA Astrophysics Data System (ADS)

Dennis, John G.; Jacoby, Wolfgang R.

1980-03-01

The development of geosynclines and orogenic belts is related to lithosphere convergence. Initial sediment accumulation implying subsidence, and volcanic activity implying extension and rise of geotherms, are in most cases followed by folding and thrusting suggesting compression and by uplift. In terms of recent analogs, sediment accumulation and crustal extension are characteristic of back-arc spreading; subsequent compression would indicate continent—continent collision; and rise of geotherms most likely requires localized thermal flow (convection) in the asthenosphere. These events are here shown to agree with Andrews and Sleep's (1974) numerical model of asthenosphere flow at converging plate margins. Orthogeosynclinal subsidence appears to be a consequence of subcrustal ablation and lithosphere extension and thinning in active marginal basins. Arc and Andean type magmatism mark the reappearance of ablated and transported, relatively low-density subcrustal material. Collision slows and eventually stops the local convection cell, resulting in local heat accumulation and hence high- T, low- P metamorphism and granitization while marginal basin (orthogeosynclinal) deposits are being compressed into Alpine style orogenic structures. Moreover, closing of the marginal basin leads to subsidiary subduction, which in turn may be responsible for some Alpine style structures. Oceanic trench deposits may become incorporated in orogenic zones, as high- P, low- T metamorphic belts (thalassogeosynclines). Dynamic uplift is a fundamental characteristic of orogeny. Most rising and sinking in orogenic zones can be linked to those asthenosphere processes which are a consequence of Andrews-Sleep convection.

Late Precambrian (740 Ma) charnockite, enderbite, and granite from Jebel Moya, Sudan: A link between the Mozambique Belt and the Arabian-Nubian Shield

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stern, R.J.; Dawoud, A.S.

1991-09-01

New Rb-Sr and whole rock and U-Pb zircon data are reported for deep-seated igneous rocks from Jebel Moya in east-central Sudan. This exposure is important because it may link the high-grade metamorphic and deep-seated igneous rocks of the Mozambique Belt with the greenschist-facies and ophiolitic assemblages of the Arabian-Nubian Shield, both of Pan-African (ca. 900-550 Ma) age. The rocks of Jebel Moya consist of pink granite, green charnockite, and dark enderbite. A twelve-point Rb-Sr whole rock isochron for all three lithologies yields an age of 730 {plus minus} 31 Ma and an initial {sup 87}Sr/{sup 86}Sr of 0.7031 {plus minus}more » 1. Nearly concordant zircon ages for granite, charnockite, and enderbite are 744 {plus minus} 2,742 {plus minus} 2, and 739 {plus minus} 2 Ma, respectively. Initial {epsilon}-Nd for these rocks are indistinguishable at 3.0 {plus minus} 0.4. The data suggest that the charnockite, enderbite, and granite are all part of a deep-seated igneous complex. The initial isotopic compositions of Sr and Nd indicate that Jebel Moya melts were derived from a mantle source that experienced significantly less time-integrated depletion of LRE and LIL elements than the source of Arabian-Nubian Shield melts. The ages for Jebel Moya deep-seated igneous rocks are in accord with data from elsewhere in the Mozambique Belt indicating that peak metamorphism occurred about 700-750 Ma. The northward extension of the Mozambique Belt to the Arabian-Nubian Shield defines a single east Pan-African orogen. The principal difference between the northern and southern sectors of this orogen may be the greater degree of thickening and subsequent erosion experienced in the south during the late Precambrian, perhaps a result of continental collision between East (Australia-India) and West Gondwanaland (S. America-Africa) about 750 Ma.« less

Granitoids of the Ufalei block (South Urals): Sr-Nd isotope systematics, geodynamic position and genetic reconstructions

NASA Astrophysics Data System (ADS)

Ronkin, Yu. L.; Shardakova, G. Yu.; Maslov, A. V.; Shagalov, E. S.; Lepikhina, O. P.

2009-04-01

Petrogeochemical and isotopic-geochronological signatures in granitoids developed in structures with complex geological history represent an important feature for reconstructing paleogeodynamic settings. Granitoids are widespread in the western slope of the Urals, where the Uralian Orogen contacts via a collage of different-age blocks of the east European Platform. The Ufalei block located in the Central Urals megazone at the junction between the South and Middle Urals’ segments represents one such boundary structure with multistage geological evolution. The isotopic ages obtained by different methods for acid igneous rocks range from 1290 to 245 Ma. We determined close Rb-Sr and Sm-Nd ages (317 Ma) for granites of the Nizhnii Ufalei Massif. By their petrochemical parameters, granitoids and host granite-gneisses differ principally from each other: the former are close to subduction-related, while the latter, to continental-riftogenic varieties. The primary ratio (87Sr/86Sr)0 = 0.70428 and ɛNd ≈ +4 values indicate significant contribution of oceanic (island-arc?) material to the substrate, which served as a source for granites of the Nizhnii Ufalei Massif. Model Nd ages of granites vary from 641 to 550 Ma. Distinct oceanic rocks and varieties with such ages are missing from the surrounding structures. New isotopic dates obtained for ultramafic and mafic rocks from different zones of the Urals related to the Cadomian cycle imply development of unexposed Upper Riphean-Vendian “oceanic” rocks in the central part of the Ufalei block, which played a substantial role in the formation of the Nizhnii Ufalei granitoids. Such rocks could be represented, for example, by fragments of the Precambrian Timanide-type ophiolite association. The analysis of original materials combined with published data point to the heterogeneous composition and structure of the Ufalei block and a significant part of the western segment of the Central Uralian Uplift and extremely complex geological history of the region coupling the Uralian Orogen with the East European Platform in the present-day structure.

75 FR 51267 - Proposed Consent Decree, Clean Air Act Citizen Suit

Federal Register 2010, 2011, 2012, 2013, 2014

2010-08-19

.... Steel Corporation's Granite City Works facility. Under the terms of the proposed consent decree, EPA has.... Steel Corporation's Granite City Works facility. Under the terms of the proposed consent decree, EPA has...

The Significance of Atypical High-Silica Igneous Rocks

NASA Astrophysics Data System (ADS)

Frazer, Ryan Edward

The origins of high-silica igneous rocks are debated, as they may be products of high-degree fractional crystallization or low-degree partial melting. They may play a role in the generation of intermediate igneous rocks and are responsible for large, ash-rich volcanic eruptions. High-silica granites and rhyolites in the Sierra Nevada, California, and the Colorado Mineral Belt (CMB) are investigated using isotope geochemistry to better understand how they bear on these questions. Zircon U-Pb geochronology identifies two intrusive suites comprising large volumes of high-silica granites emplaced in the mid-Cretaceous Sierra Nevada batholith: the 106-98 Ma Shaver Intrusive Suite (SIS) in the central part of the batholith, and the 103-100 Ma Kearsarge intrusive suite (KIS) on the Sierra Crest and Owens Valley. High-silica granites in both suites have relatively high concentrations of middle rare earth and high field strength elements. Data for these and other discrete high-silica plutons in the batholith suggest they were derived from titanite-free sources in the deep crust, unlike similarly felsic parts of zoned intrusive suites. Despite similar trace element signatures, SIS and KIS high-silica granites have divergent isotopic compositions. High-silica granites of the SIS have supracrustal O in zircon, crustal Sr and Nd whole rock isotopic compositions, and negative Ce anomalies suggesting the SIS granites may have been derived from oceanic sedimentary sources. In contrast, KIS granites have mantle-like isotopic compositions. The location and geochemistry of the KIS suggests it may have resulted from backarc magmatism in the mid-Cretaceous Sierra. Volcanic and plutonic rocks in the central CMB were emplaced during the Laramide orogeny and subsequent Oligocene-Eocene volcanic flare-up. Strontium and Nd data suggest the 63-39 Ma Twin Lakes pluton and igneous rocks as young as 24 Ma were derived from a persistent mafic lower crust or enriched lithospheric mantle source. In contrast, the ˜35 Ma Grizzly Peak Tuff and resurgent plutons are isotopically dissimilar from each other and the CMB as a whole, suggesting derivation by partial melting of ancient felsic lower crust. This distinct source could account for the lack Mo mineralization in the Grizzly Peak caldera relative to other high-silica parts of the CMB.

Geochemical and Nd-Sr-Pb isotope characteristics of synorogenic lower crust-derived granodiorites (Central Damara orogen, Namibia)

NASA Astrophysics Data System (ADS)

Simon, I.; Jung, S.; Romer, R. L.; Garbe-Schönberg, D.; Berndt, J.

2017-03-01

The 547 ± 7 Ma old Achas intrusion (Damara orogen, Namibia) includes magnesian, metaluminous to slightly peraluminous, calcic to calc-alkalic granodiorites and ferroan, metaluminous to slightly peraluminous, calc-alkalic to alkali-calcic leucogranites. For the granodiorites, major and trace element variations show weak if any evidence for fractional crystallization whereas some leucogranites are highly fractionated. Both, granodiorites and leucogranites are isotopically evolved (granodiorites: εNdinit: - 12.4 to - 20.5; TDM: 2.4-1.9; leucogranites: εNdinit: - 12.1 to - 20.6, TDM: 2.5-2.0), show similar Pb isotopic compositions, and may be derived from late Archean to Paleoproterozoic crustal source rocks. Comparison with melting experiments and simple partial melting modeling indicate that the granodiorites may be derived by extensive melting (> 40%) at 900-950 °C under water-undersaturated conditions (< 5 wt.% H2O) of felsic gneisses. Al-Ti and zircon saturation thermometry of the most primitive granodiorite sample yielded temperatures of ca. 930 °C and ca. 800 °C. In contrast to other lower crust-derived granodiorites and granites of the Central Damara orogen, the composition of the magma source is considered the first-order cause of the compositional diversity of the Achas granite. Second-order processes such as fractional crystallization at least for the granodiorites were minor and evidence for coupled assimilation-fractional crystallization processes is lacking. The most likely petrogenetic model involves high temperature partial melting of a Paleoproterozoic felsic source in the lower crust ca. 10-20 Ma before the first peak of regional high-temperature metamorphism. Underplating of the lower crust by magmas derived from the lithospheric mantle may have provided the heat for melting of the basement to produce anhydrous granodioritic melts.

Hydrothermal alteration of deep fractured granite: Effects of dissolution and precipitation

NASA Astrophysics Data System (ADS)

Nishimoto, Shoji; Yoshida, Hidekazu

2010-03-01

This paper investigates the mineralogical effects of hydrothermal alteration at depth in fractures in granite. A fracture accompanied by an alteration halo and filled with clay was found at a depth of 200 m in a drill core through Toki granite, Gifu, central Japan. Microscopic observation, XRD, XRF, EPMA and SXAM investigations revealed that the microcrystalline clays consist of illite, quartz and pyrite and that the halo round the fracture can be subdivided into a phyllic zone adjacent to the fracture, surrounded by a propylitic zone in which Fe-phyllosilicates are present, and a distinctive outer alteration front characterized by plagioclase breakdown. The processes that result in these changes took place in three successive stages: 1) partial dissolution of plagioclase with partial chloritization of biotite; 2) biotite dissolution and precipitation of Fe-phyllosilicate in the dissolution pores; 3) dissolution of K-feldspar and Fe-phyllosilicate, and illite precipitation associated with development of microcracks. These hydrothermal alterations of the granite proceed mainly by a dissolution-precipitation process resulting from the infiltration of hydrothermal fluid along microcracks. Such infiltration causes locally high mobility of Al and increases the ratio of fluid to rock in the alteration halo. These results contribute to an understanding of how granitic rock becomes altered in orogenic fields such as the Japanese island arc.

Late Jurassic-Early Cretaceous continental convergence and intracontinental orogenesis in East Asia: A synthesis of the Yanshan Revolution

NASA Astrophysics Data System (ADS)

Dong, Shuwen; Zhang, Yueqiao; Zhang, Fuqin; Cui, Jianjun; Chen, Xuanhua; Zhang, Shuanhong; Miao, Laicheng; Li, Jianhua; Shi, Wei; Li, Zhenhong; Huang, Shiqi; Li, Hailong

2015-12-01

The basic tectonic framework of continental East Asia was produced by a series of nearly contemporaneous orogenic events in the late Middle Jurassic to Early Cretaceous. Commonly, the Late Mesozoic orogenic processes were characterized by continent-continent collision, large-scale thrusting, strike-slip faulting and intense crustal shortening, crustal thickening, regional anatexis and metamorphism, followed by large-scale lithospheric extension, rifting and magmatism. To better understand the geological processes, this paper reviews and synthesizes existing multi-disciplinary geologic data related to sedimentation, tectonics, magmatism, metamorphism and geochemistry, and proposes a two-stage tectono-thermal evolutionary history of East Asia during the late Middle Jurassic to Early Cretaceous (ca. 170-120 Ma). In the first stage, three orogenic belts along the continental margins were formed coevally at ca. 170-135 Ma, i.e., the north Mongol-Okhotsk orogen, the east paleo-Pacific coastal orogen, and the west Bangong-Nujiang orogen. Tectonism related to the coastal orogen caused extensive intracontinental folding and thrusting that resulted in a depositional hiatus in the Late Jurassic, as well as crustal anatexis that generated syn-kinematic granites, adakites and migmatites. The lithosphere of the East Asian continent was thickened, reaching a maximum during the latest Jurassic or the earliest Cretaceous. In the second stage (ca. 135-120 Ma), delamination of the thickened lithosphere resulted in a remarkable (>120 km) lithospheric thinning and the development of mantle-derived magmatism, mineralization, metamorphic core complexes and rift basins. The Middle Jurassic-Early Cretaceous subduction of oceanic plates (paleo-Pacific, meso-Tethys, and Mongol-Okhotsk) and continent-continent collision (e.g. Lhasa and Qiangtang) along the East Asian continental margins produced broad coastal and intracontinental orogens. These significant tectonic activities, marked by widespread intracontinental orogeny and continental reconstruction, are commonly termed the Yanshan Revolution (Movement) in the Chinese literature.

Orogenic potassic mafic magmatism, a product of alkaline-peraluminous mixing ? Variscan 'calc-alkaline' rocks from the Central Iberian and Ossa Morena Zones, Central Spain.

NASA Astrophysics Data System (ADS)

Scarrow, Jane H.; Cambeses, Aitor; Bea, Fernando; Montero, Pilar; Molina, José F.; Moreno, Juan Antonio

2013-04-01

Orogenic magmatic rocks provide information about mantle and crust melt-generation and -interaction processes. In this context, minor potassic mafic stocks which are formed of enriched mantle and crustal components and are common as late-orogenic intrusions in granitic plutons give insight into the timing of new crust formation and crustal recycling. Potassic mafic stocks are prevalent, albeit low volume, constituents of granite batholiths all through the European Variscan (350-280 Ma). In the Central Iberia Zone, Spanish Central System, crustal-melt, S-type, granitoid plutons are intruded by minor concomitant ultramafic-intermediate appinitic-vaugneritic stocks. Notwithstanding their whole-rock calc-alkaline composition, the stocks apparently did not have a subduction-related origin. Recent studies have attributed their genesis to mixing of alkaline mantle and peraluminous crustal melts. Their primary alkaline character, as indicated by amphibole and biotite mineral chemistry data, points, rather, towards an extension-related genesis. In the Ossa Morena Zone, south of the Central Iberian Zone, the igneous rocks also have a whole-rock calc-alkaline composition which has been considered to be the result of northward subduction of the South Portuguese Zone. Nevertheless, identification of a 'sill' of significant volume of mafic magma in the middle crust, the ´IBERSEIS reflective body', in a seismic profile across the Ossa Morena and South Portuguese Zones has cast doubt upon the calc-alkaline magmatism-subduction model; leading, instead, to the magmatism being attributed to intra-orogenic extension related to a mantle plume active from 340 Ma to 330 Ma. The aim here, then, is to reinvestigate the petrogenesis and age of the calc-alkaline rocks of the Ossa Morena Zone to determine their tectonomagmatic context be it subduction-, plume- or extension-related, and establish what they may reveal about mantle-crust interactions. Focussing, initially, on the Valencia del Ventoso pluton, preliminary mineral chemistry, whole-rock and isotope data indicate that rather than a single-stage cogenetic calc-alkaline intrusion, the pluton is a multi-stage composite of compositionally diverse stocks. Including the metaluminous calc-alkaline Medina de las Torres gabbro and Cortijo del Pozuelo granite to the concentrically zone alkaline core though to calk-alkaline border of the main pluton. In addition, an associated older peraluminous La Jineta granite body and younger cross-cutting tholeiitic dykes crop out in the same region. Here we present new U-Pb single zircon IBERSIMS SHRIMP data which indicate that the compositionally diverse main pluton and associated stocks are contemporaneous at 334 ± 2 Ma. So, rather than reflecting reactivation of a zone of lithospheric weakness by successive magmatic events it appears that at Valencia del Ventoso diverse mantle and crustal sources were being tapped simultaneously. We suggest that this is linked to the generation of and thermal anomaly associated with the coeval ´IBERSEIS reflective body'. The question is, then, if other 'calc-alkaline' plutons have similar compositional, and so, possible tectonomagmatic complexity. To test this hypothesis studies are currently underway of the Ossa Morena Zone Burguillos del Cerro and Brovales plutons.

Mineral-deposit model for lithium-cesium-tantalum pegmatites

USGS Publications Warehouse

Bradley, Dwight C.; McCauley, Andrew D.; Stillings, Lisa L.

2017-06-20

Lithium-cesium-tantalum (LCT) pegmatites comprise a compositionally defined subset of granitic pegmatites. The major minerals are quartz, potassium feldspar, albite, and muscovite; typical accessory minerals include biotite, garnet, tourmaline, and apatite. The principal lithium ore minerals are spodumene, petalite, and lepidolite; cesium mostly comes from pollucite; and tantalum mostly comes from columbite-tantalite. Tin ore as cassiterite and beryllium ore as beryl also occur in LCT pegmatites, as do a number of gemstones and high-value museum specimens of rare minerals. Individual crystals in LCT pegmatites can be enormous: the largest spodumene was 14 meters long, the largest beryl was 18 meters long, and the largest potassium feldspar was 49 meters long.Lithium-cesium-tantalum pegmatites account for about one-fourth of the world’s lithium production, most of the tantalum production, and all of the cesium production. Giant deposits include Tanco in Canada, Greenbushes in Australia, and Bikita in Zimbabwe. The largest lithium pegmatite in the United States, at King’s Mountain, North Carolina, is no longer being mined although large reserves of lithium remain. Depending on size and attitude of the pegmatite, a variety of mining techniques are used, including artisanal surface mining, open-pit surface mining, small underground workings, and large underground operations using room-and-pillar design. In favorable circumstances, what would otherwise be gangue minerals (quartz, potassium feldspar, albite, and muscovite) can be mined along with lithium and (or) tantalum as coproducts.Most LCT pegmatites are hosted in metamorphosed supracrustal rocks in the upper greenschist to lower amphibolite facies. Lithium-cesium-tantalum pegmatite intrusions generally are emplaced late during orogeny, with emplacement being controlled by pre-existing structures. Typically, they crop out near evolved, peraluminous granites and leucogranites from which they are inferred to be derived by fractional crystallization. In cases where a parental granite pluton is not exposed, one is inferred to lie at depth. Lithium-cesium-tantalum LCT pegmatite melts are enriched in fluxing components including H2O, F, P, and B, which depress the solidus temperature, lower the density, and increase rates of ionic diffusion. This, in turn, enables pegmatites to form thin dikes and massive crystals despite having a felsic composition and temperatures that are significantly lower than ordinary granitic melts. Lithium-cesium-tantalum pegmatites crystallized at remarkably low temperatures (about 350–550 °C) in a remarkably short time (days to years).Lithium-cesium-tantalum pegmatites form in orogenic hinterlands as products of plate convergence. Most formed during collisional orogeny (for example, Kings Mountain district, North Carolina). Specific causes of LCT pegmatite-related magmatism could include: ordinary arc processes; over thickening of continental crust during collision or subduction; slab breakoff during or after collision; slab delamination before, during, or after collision; and late collisional extensional collapse and consequent decompression melting. Lithium-cesium-tantalum pegmatite deposits are present in all continents including Antarctica and in rocks spanning 3 billion years of Earth history. The global age distribution of LCT pegmatites is similar to those of common pegmatites, orogenic granites, and detrital zircons. Peak times of LCT pegmatite genesis at about 2640, 1800, 960, 485, and 310 Ma (million years before present) correspond to times of collisional orogeny and supercontinent assembly. Between these pulses were long intervals when few or no LCT pegmatites formed. These minima overlap with supercontinent tenures at ca. 2450–2225, 1625–1000, 875–725, and 250–200 Ma.Exploration and assessment for LCT pegmatites are guided by a number of observations. In frontier areas where exploration has been minimal at best, the key first-order criteria are an orogenic hinterland setting, appropriate regional metamorphic grades, and the presence of evolved granites and common granitic pegmatites. New LCT pegmatites are most likely to be found near known deposits. Pegmatites tend to show a regional mineralogical and geochemical zoning pattern with respect to the inferred parental granite, with the greatest enrichment in the more distal pegmatites. Mineral-chemical trends in common pegmatites that can point toward an evolved LCT pegmatite include: increasing rubidium in potassium feldspar, increasing lithium in white mica, increasing manganese in garnet, and increasing tantalum and manganese in columbite-tantalite. Most LCT pegmatite bodies show a distinctive internal zonation featuring four zones: border, wall, intermediate (where lithium, cesium, and tantalum are generally concentrated), and core. This zonation is expressed both in cross section and map view; thus, what may appear to be a common pegmatite may instead be the edge of a mineralized body.Neither lithium-cesium-tantalum pegmatites nor their parental granites are likely to cause serious environmental concerns. Soils and country rock surrounding a LCT pegmatite, as well as waste from mining operations, may be enriched in characteristic elements relative to global average soil and bedrock values. These elements may include lithium, cesium, tantalum, beryllium, boron, fluorine, phosphorus, manganese, gallium, rubidium, niobium, tin, and hafnium. Among this suite of elements, however, the only ones that might present a concern for environmental health are beryllium and fluorine, which are included in the U.S. Environmental Protection Agency drinking-water regulations with maximum contaminant levels of 4 micrograms per liter and 4 milligrams per liter, respectively.

About the age of the Neoproterozoic Lainici-Paius terrane (South Carpathians, Romania)

NASA Astrophysics Data System (ADS)

Balica, C.; Balintoni, I.; Ducea, M. N.; Berza, T.; Stremtan, C.

2009-12-01

The pre-Alpine basement of the Danubian domain nappes from South Carpathians consists of high grade metamorphic groups and late Neoproterozoic plutons, underlying low grade metamorphosed Ordovician to early Carboniferous formations (e.g. Seghedi et al., 2005). Two types of pre-Ordovician metamorphic complexes with contrasting protoliths petrology, metamorphism and associated igneous activity, involved in a pre-Permian nappe structure are separated: Lainici-Paius group, dominated by HT-LP metasediments and Dragsan group, dominated by medium grade metabasites. Based on their distinct lithologic compositions, geologic histories and clear boundaries, we consider these two groups as parts of two different terranes (i.e. Lainici-Paius and Dragsan terranes). The southern part of Lainici-Paius terrane is intruded by elongated plutons up to 100 km long and 15 km wide. Based on the geochemical composition, the plutons are assigned to two distinct suites, (i) medium K, calc-alkaline, mostly granodioritic-tonalitic suite (i.e. Susita type) and (ii) very high K, calc-alkaline and mostly granitic (i.e. Tismana type). The first suite comprises Susita and Oltet granitoid bodies and the second suite consists of Tismana and Novaci granitic plutons. Previous age dating was carried out only on Tismana (567±3 Ma upper intercept, Liégeois et al., 1996) and Novaci (588±5 Ma, Grünenfelder et al., 1983 recalculated by Liégeois et al., 1996) granites. In situ zircon U/Pb LA-ICP-MS analyses performed on all four granitoid plutons yielded 596.3±5.7 Ma for Tismana granite, 592.0±5.1 Ma for Novaci granite, 591.0±3.5 Ma for Susita granite and 588.7±3 Ma for Oltet granite. The same method has been additionally applied for detrital zircons from a metasandstone sequence comprised by the Lainici-Paius complex. Fifty-five ages out of 78 dated grains are ranging between 690.1±5.5 Ma and 811.4±12,7 Ma. Therefore, considering the protolith ages of the four dated granites and the youngest age within the mentioned detrital age distribution we can constrain the formation of the sedimentary protoliths of the Lainici-Paius group to 690-600 Ma span.

Petrogenesis and tectonic setting of the Devonian Xiqin A-type granite in the northeastern Cathaysia Block, SE China

NASA Astrophysics Data System (ADS)

Cai, Da-wei; Tang, Yong; Zhang, Hui; Lv, Zheng-Hang; Liu, Yun-long

2017-06-01

Most Silurian-Devonian granites in South China are S- or I-type granites, which are suggested to be petrogenetically related to the Wuyi-Yunkai orogeny. In this paper, we present the detailed LA-ICP-MS zircon U-Pb dating, major and trace element geochemical, and Nd-Hf isotopic data for Xiqin A-type granites in the northeastern Cathaysia Block, SE China. Zircon U-Pb dating results show that the Xiqin granites were emplaced at about 410 Ma, indicating that they were generated at the end of Wuyi-Yunkai orogeny. These granites are high in K2O + Na2O (6.31-8.79 wt%), high field strength elements (Zr + Nb + Ce + Y = 427-699 ppm), rare earth elements (total REE = 221-361 ppm) as well as high Ga/Al ratios (10,000 Ga/Al = 2.50-3.10), and show characteristics typical of A-type granites. εHf(t) values of the Xiqin granites mainly vary from -0.4 to -3.1 and yield Mesoproterozoic T2DM(Hf) (mainly ranging from 1.29 to 1.45 Ga). The εNd(t) values are from -1.23 to -2.11 and T2DM(Nd) vary from 1.25 to 1.32 Ga. These isotopic data suggest that the Xiqin granites were generated by partial melting of metavolcanic rocks with minor metasedimentary rocks in the lower crust. Our data on the Xiqin granites, coupled with previous studies of Silurian-Devonian magmatism, suggest that the tectonic regime had changed to a strongly post-collisional extension environment in the Wuyi-Yunkai orogen at least since 410 Ma, and that delamination, which accounts for the change in stress from the compression to extension and asthenospheric upwelling during the early Paleozoic, plays a significant role in the generation of Xiqin A-type granites.

Extensional unroofing of the metamorphic core of the southern Appalachian orogen prior to the breakup of Pangea: Insights from 40Ar/39Ar thermochronology

NASA Astrophysics Data System (ADS)

Ma, C.; Foster, D. A.; Hames, W. E.; Mueller, P. A.

2017-12-01

Orogenic collapse commonly occurs following the collisional phase of an orogeny and often leads to exhumation of deep crustal metamorphic rocks. The Alleghanian orogeny in the southern Appalachian orogen (SAO) occurred during final assembly of Pangea. 40Ar/39Ar data of hornblende, muscovite, and biotite from Alleghanian granitic plutons in Georgia, Alabama, and Florida of the SAO give cooling ages that progressively young toward the south-southeast prior to ca. 280 Ma and young locally toward the north-northwest after ca. 280 Ma. These cooling-age gradients, along with geometry of the Suwannee suture zone and timing/structures of the South Georgia basin, suggest that metamorphic rocks north of the Suwannee suture in the study area formed the lower plate of a metamorphic core complex. The faults of the Suwannee suture zone were reactivated to form a master extensional detachment fault with the Suwannee terrane comprising the upper plate. Thermochronologic data show that rapid extension of the metamorphic core complex footwall started at ca. 300-295 Ma and the extension continued to at least ca. 240 Ma. The maximum average extension rate is estimated to be 10.3 km/m.y. during ca. 300-280 Ma along the master detachment fault and 2.4 km/m.y. during ca. 280-240 Ma along a secondary detachment fault, reflecting differential extension over time. Main cooling rates of 10‒85˚C/m.y. and exhumation rates of 0.3‒2.8 km/m.y. are calculated for the Alleghanian granitic plutons studied. This work shows that, in the southernmost Appalachians, orogenic collapse resulted in metamorphic core complex-style extension between about 300 and 240 Ma. The horst-and-graben systems of the South Georgia basin formed within the upper plate in this tectonic setting. Metamorphic core complex-style extension, therefore, played a critical role in initial rifting that led to the eventual breakup of Pangea and formation of the Atlantic Ocean and the Gulf of Mexico.

Crustal growth and episodic reworking over one billion years in the Capricorn Orogen, Western Australia: evidence from Lu-Hf and O isotope data

NASA Astrophysics Data System (ADS)

Jahn, Inalee; Clark, Chris; Reddy, Steve; Taylor, Rich

2017-04-01

Fundamental to understanding the generation and evolution of a crustal block is knowledge of the relationship between additions of new material from the mantle, and the extent of crustal recycling [1]. Hafnium isotope ratios can be used to characterise relative contributions from mantle, crustal and recycled reservoirs within magmas. Oxygen isotopes can be used to constrain the extent of crustal interaction during magma emplacement. When used in conjunction, they can help unravel multiple crystallisation histories of a crustal block, and follow the source composition through magma evolution. The Capricorn Orogen records the Paleoproterozoic collision of the Yilgarn and Pilbara Cratons to form the West Australian Craton, and over one billion years of subsequent intracontinental crustal reworking. U-Pb zircon geochronology records three discrete tectono-magmatic events which resulted in voluminous granitic magmatism: the 2005-1975 Ma Glenburgh Orogeny, the 1820-1770 Ma Capricorn Orogeny, and the 1680-1620 Ma Durlacher Orogeny [2]. We present U-Pb, Lu-Hf and δ18O isotopic data from zircon from 50 samples of granites and granitoids from the Capricorn Orogen to provide constraints on the crustal evolution of the Paleoproterozoic crust. Our results confirm crustal growth by juvenile mantle input was limited to the Glenburgh Orogeny associated with the amalgamation of the West Australian Craton, while all subsequent Paleoproterozoic magmatism was primarily derived from significant reworking of the pre-existing crustal components. Time-sliced maps showing the variation in Hf and O isotopes can be used to image crustal evolution in space and time, and are particularly useful in constraining the spatial and temporal extent of juvenile magmatic additions to the crust. These maps suggest that crustal growth was concentrated along, or in the terranes adjacent to, the Yilgarn Craton margin. Our results are in agreement with previous isotopic studies [3], and provide additional constraints for the evolution of the Paleoproterozoic crust within the Capricorn Orogen. [1] Cawood et al. 2013. Geological Society of America Bulletin, 125(1-2), 14-32 [2] Sheppard et al. 2010. Geological Survey of Western Australia, Perth, Western Australia, 336 [3] Johnson et al. 2017. Lithos, 268, 76-86

Diffusion-driven magnesium and iron isotope fractionation at a gabbro-granite boundary

NASA Astrophysics Data System (ADS)

Wu, Hongjie; He, Yongsheng; Teng, Fang-Zhen; Ke, Shan; Hou, Zhenhui; Li, Shuguang

2018-02-01

Significant magnesium and iron isotope fractionations were observed in an adjacent gabbro and granite profile from the Dabie Orogen, China. Chilled margin and granitic veins at the gabbro side and gabbro xenoliths in the granite indicate the two intrusions were emplaced simultaneously. The δ26Mg decreases from -0.28 ± 0.04‰ to -0.63 ± 0.08‰ and δ56Fe increases from -0.07 ± 0.03‰ to +0.25 ± 0.03‰ along a ∼16 cm traverse from the contact to the granite. Concentrations of major elements such as Al, Na, Ti and most trace elements also systematically change with distance to the contact. All the observations suggest that weathering, magma mixing, fluid exsolution, fractional crystallization and thermal diffusion are not the major processes responsible for the observed elemental and isotopic variations. Rather, the negatively correlated Mg and Fe isotopic compositions as well as co-variations of Mg and Fe isotopes with Mg# reflect Mg-Fe inter-diffusion driven isotope fractionation, with Mg diffusing from the chilled gabbro into the granitic melt and Fe oppositely. The diffusion modeling yields a characteristic diffusive transport distance of ∼6 cm. Consequently, the diffusion duration, during which the granite may have maintained a molten state, can be constrained to ∼2 My. The cooling rate of the granite is calculated to be 52-107 °C/My. Our study suggests diffusion profiles can be a powerful geospeedometry. The observed isotope fractionations also indicate that Mg-Fe inter-diffusion can produce large stable isotope fractionations at least on a decimeter scale, with implications for Mg and Fe isotope study of mantle xenoliths, mafic dikes, and inter-bedded lavas.

S-type granite from the Gongpoquan arc in the Beishan Orogenic Collage, southern Altaids: Implications for the tectonic transition

NASA Astrophysics Data System (ADS)

Wang, Xinyu; Yuan, Chao; Zhang, Yunying; Long, Xiaoping; Sun, Min; Wang, Lixing; Soldner, Jeremie; Lin, Zhengfan

2018-03-01

Voluminous Paleozoic intrusions occur in the Beishan Orogenic Collage (BOC) and their genesis and tectonic background are important to reconstruct the accretion-collision processes in the southernmost Altaids. Paleozoic is an important period for arc development in the BOC, where the Gongpoquan and Huaniushan arcs are located. There are two pulses of magmatism in the Huaniushan and Gongpoquan arcs, i.e., the ca. 470-423 Ma I-type and ca. 424-395 Ma S- and A-type granitoids. In this study, we focus on two peraluminous granitic plutons in the Gongpoquan arc, i.e., the Baitoushan muscovite granite and Haergen two-mica granite, aiming at unraveling their petrogenesis and tectonic background. Zircon LA-ICP-MS U-Pb dating yields emplacement ages of ca. 409-395 Ma and ca. 409 Ma for the Baitoushan and Haergen plutons, respectively. Both the granitic plutons are strongly peraluminous with A/CNK ratios of 1.10-1.20, indicative of S-type affinities. The rocks are characterized by high SiO2 and K2O contents with high CaO/Na2O ratios. Moreover, the rocks possess low MgO contents, Rb/Sr and Rb/Ba ratios, together with their relatively high initial 87Sr/86Sr ratios (0.7139-0.7152) and less radiogenic εNd(t) values (-3.15 to -5.17), implying a clay-poor and plagioclase-rich crustal source. Compared with earlier pulse of arc-related magmatism (ca. 470-423 Ma), the latter pulse of magmatism (ca. 424-395 Ma) consists mainly of "normal granite" characterized by higher SiO2 (>66%) and K2O contents, weaker fractionated REE patterns and lower δEu values, and gabbroic to dioritic intrusions are only sporadic. Moreover, the granitoids of the latter pulse show variable but more crust-like Sr-Nd isotopic compositions ((87Sr/86Sr)0 = 0.7038-0.7327; εNd(t) = -6.70 to +0.33) than the earlier ones ((87Sr/86Sr)0 = 0.7024-0.7080; εNd(t) = -2.56 to +8.86), indicating that the Early Devonian (ca. 424-395 Ma) experienced extensive crustal melting with minor involvement of mantle materials. Considering Early Devonian geological evidence, the transition from I- to S- and A-type magmatism probably reflects the Early Devonian amalgamation between the Gongpoquan and Huaniushan arcs, which caused not only regional unconformity and strong deformation-metamorphism, but also extensive melting of the accreted crustal materials in the BOC.

Geochronological and geochemical constraints on the petrogenesis of Middle Paleozoic (Kwangsian) massive granites in the eastern South China Block

NASA Astrophysics Data System (ADS)

Zhang, Feifei; Wang, Yuejun; Zhang, Aimei; Fan, Weiming; Zhang, Yuzhi; Zi, Jianwei

2012-10-01

To achieve a better understanding of the Kwangsian orogenic event of the eastern South China Block, this paper documents a set of new zircon U-Pb geochronological and Hf isotopic data and whole-rock elemental and Sr-Nd isotopic analytical results for the representative massive granite intrusions across the Jiangshan-Shaoxing fault. The studied samples are classified into two groups, representing the rocks from the Cathaysia Block to the east of the Jiangshan-Shaoxing Fault (Group 1) and those from the eastern Yangtze Block between the Anhua-Luocheng and Jiangshan-Shaoxing faults (Group 2). The Group 1 samples gave the zircon U-Pb ages of 405-454 Ma and ɛHf(t) values of - 3.6 to - 15.2 with Hf model ages of 1.6-2.4 Ga. Group 2 yielded the zircon U-Pb ages of 400-432 Ma and ɛHf(t) values of - 0.2 to - 12.7 with Hf model ages of 1.3-2.2 Ga. Geochemically, the Group 1 samples (A/CNK = 1.02-1.43) have relatively lower Al2O3, MgO, CaO, P2O5 and ɛNd(t) but higher K2O + Na2O than those of Group 2 (A/CNK = 0.93-1.44). Both groups show similar chondrite-normalized patterns of rare-earth elements with Eu/Eu* values of 0.15-0.92 and strongly negative Ba, Sr, Nb, P and Ti anomalies in primitive mantle-normalized spider diagrams. Their ɛNd(t) values range from - 11.1 to - 8.0 for Group 1, and - 8.9 to - 5.0 for Group 2, generally similar to those of Precambrian paragneiss and contemporaneous gneissoid granites in the eastern South China Block. Our geochronological results indicate that the Kwangsian massive granites in the eastern South China Block were crystallized between 400 Ma and 454 Ma, synchronous to the Kwangsian gneissoid granites along the Wugong and Wuyi-Baiyun-Yunkai domains in the eastern South China Block. The synthesis of these whole-rock geochemical and in-situ zircon Hf isotopic data suggests that both the Group 1 and 2 granites across the Jiangshan-Shaoxing Fault were predominantly derived from a crustal source with some proportional metapelitic and metaigneous components from the Precambrian basement. The input of juvenile mantle-derived magma is insignificant. Taking into account other available evidences, it is proposed that the Jiangshan-Shaoxing Fault might be a pre-existing boundary between the Yangtze and Cathaysia Blocks, which was reactivated during the middle Paleozoic (Kwangsian) orogenic event. The massive granitic magmatism was probably resulted from the doubly crustal thickening and subsequent isostatic readjustment in an intracontinental tectonic regime.

Evolution and tectonic setting of the Malani - Nagarparkar Igneous Suite: A Neoproterozoic Silicic-dominated Large Igneous Province in NW India-SE Pakistan

NASA Astrophysics Data System (ADS)

de Wall, Helga; Pandit, Manoj K.; Donhauser, Ines; Schöbel, Stefan; Wang, Wei; Sharma, Kamal K.

2018-07-01

The Neoproterozoic Malani Igneous Suite (MIS) in NW India, along with analogous magmatic rocks from the adjoining Nagarparkar region in SE Pakistan can be collectively classified as a Silicic-dominated Large Igneous Province (SLIP). This magmatic event includes bimodal (predominantly felsic) volcanism, granite emplacement and felsic and mafic dyke intrusions. Felsic rocks have typical A-type affinity as indicated by high abundance of silica, alkali, high field strength and large ion lithophile element concentrations and low CaO and MgO contents. Their Nb negative anomalies and Zr-saturation parameters indicate significant crustal input and high temperature melting. Mafic volcanics and dykes show geochemical homogeneity and derivation from a depleted continental mantle source without any significant crustal contamination (low U and Th contents and no visible Nb anomaly). The region extending from the Mount Abu batholith in the east to Jaswantpura in the west (2700 km2), representing a transition from the metamorphic Sirohi terrane to the undeformed MIS, was evaluated through an integrated structural (including satellite image analysis), geochemical and geochronological study. During the initial stage (prior to 760 Ma) the granitic basement (Erinpura granites) and overlying Sirohi metasediments behaved in a brittle manner that led to development of linear fractures and NNE trending rift structures, and bimodal volcanic activity. Emplacement of voluminous granitic bodies in response to progressive extension of the crust is inferred during the more evolved second stage (younger than 760 Ma). Mirpur Granite, a representative of this younger granitic suite (Jalor type pink granite) has yielded 753 ± 9 Ma zircon, U-Pb, crystallization age. Granitic plutons mark regions of crustal extension, as seen in parallel alignment of plutonic bodies (Jaswantpura granitic belt) and parallel mafic dyke swarms (340°) transecting the granites. Structural analysis further identified an episode of crustal convergence which is documented in folding and faulting of the Sindreth Basin sequence and in tectonic overprint of early stage mafic rocks. Rifting and bimodal magmatic activity in MIS is coeval with similar rock types in Nagarparkar in SE Pakistan, further traceable into the Seychelles microplate and Central Madagascar. Considering the Neoproterozoic paleogeography and our observations, an extensional setting and an active continental margin position for MIS is inferred.

Tectonics of Antarctica

USGS Publications Warehouse

Hamilton, W.

1967-01-01

Antarctica consists of large and wholly continental east Antarctica and smaller west Antarctica which would form large and small islands, even after isostatic rebound, if its ice cap were melted. Most of east Antarctica is a Precambrian Shield, in much of which charnockites are characteristic. The high Transantarctic Mountains, along the Ross and Weddell Seas, largely follow a geosyncline of Upper Precambrian sedimentary rocks that were deformed, metamorphosed and intruded by granitic rocks during Late Cambrian or Early Ordovician time. The rocks of the orogen were peneplained, then covered by thin and mostly continental Devonian-Jurassic sediments, which were intruded by Jurassic diabase sheets and overlain by plateau-forming tholeiites. Late Cenozoic doming and block-faulting have raised the present high mountains. Northeastern Victoria Land, the end of the Transantarctic Mountains south of New Zealand, preserves part of a Middle Paleozoic orogen. Clastic strata laid unconformably upon the Lower Paleozoic plutonic complex were metamorphosed at low grade, highly deformed and intruded by Late Devonian or Early Carboniferous granodiorites. The overlying Triassic continental sedimentary rocks have been broadly folded and normal-faulted. Interior west Antarctica is composed of miogeosynclinal clastic and subordinate carbonate rocks which span the Paleozoic Era and which were deformed, metamorphosed at generally low grade, and intruded by granitic rocks during Early Mesozoic time and possibly during other times also. Patterns of orogenic belts, if systematic, cannot yet be defined; but fragmentation and rotation of crustal blocks by oroclinal folding and strike-slip faulting can be suggested. The Ellsworth Mountains, for example, consist of Cambrian-Permian metasedimentary rocks that strike northward toward the noncorrelative and latitudinally striking Mesozoic terrane of the Antarctic Peninsula in one direction and southward toward that of the Lower Paleozoic: terrane of the Transantarctic Mountains in the other; the three regions may be separated by great strike-slip faults. The Antarctic Peninsula in west Antarctica, south of South America, consists of metavolcanic and metasedimentary rocks intruded by Late Cretaceous quartz diorite. The pre-granitic rocks are of Jurassic and Early Cretaceous ages wherever they have been dated by fossils, although some crystalline complexes may be older. The S-shape of the peninsula may represent oroclinal bending within Cenozoic time as part of a motion system in which a narrow continental bridge between South America and Antarctica was deformed and ruptured. Perhaps this bridge lagged behind as the larger continental plates drifted into the Pacific Ocean Basin. ?? 1967.

The distribution of uranium over Europe: Geological and environmental significance

USGS Publications Warehouse

Plant, J.A.; Reeder, S.; Salminen, R.; Smith, D.B.; Tarvainen, T.; de Vivo, B.; Petterson, M.G.

2003-01-01

The variation of baseline levels of uranium in soil and stream sediments over Europe is described, based on new data prepared by the Forum of European Geological Surveys (FOREGS). The samples have been collected and analysed according to the protocols established for the International Union of Geological Sciences/International Association of Geochemistry and Cosmochemistry (IUGS/IAGC) Working Group on Global Geochemical Baselines. The baseline levels of U vary between 0??21 to 53 mg kg-1 in topsoils, 0??19 to 30 mg kg-1 in subsoils and < 1 to 59 mg kg-1 in stream sediments. There is generally good agreement between the levels of U in the three sample types, and the median concentration in all three media is approximately 2 mg kg-1. The most anomalous baseline levels occur over the Variscan orogen, especially areas into which late post-orogenic radiothermal high heat production (HHP) granites were emplaced. Spiderdiagrams based on trace element levels and rare earth element (REE) plots, confirm the association between the highest U anomalies and evolved radiothermal granites. High values are also associated with parts of the Alpine terrain especially in Slovenia, where there are historical U workings, and Southern Italy, where high values of U reflect contemporary volcanism. In contrast, much of the Caledonides of North West Europe and the Precambrian of the Baltic Shield and East European craton and its overlying sedimentary cover have very low values, generally < 4 mg kg-1. The results suggest that the main concern for the environment and human health from U, and the Th and K with which it is generally associated, is the naturally occurring total gamma radiation and radon potential associated with radiothermal granites. This is likely to be especially important where the granites are mineralised and have been worked historically, for example in the North West of the Iberian Peninsula where U and its decay products are likely to be more dispersed in the surface environment. The study also indicates the value of multi-element data in distinguishing between anthropogenic and naturally occurring anomalies.

Geochemical characteristics and origin of the Lebowa Granite Suite, Bushveld Complex

USGS Publications Warehouse

Hill, M.; Barker, F.; Hunter, D.; Knight, R.

1996-01-01

The ??? 2052-Ma Lebowa Granite Suite (LGS) represents the culminating phase of an Early Proterozoic magmatic cycle in the Central Transvaal area of the Kaapvaal Province. Following extrusion of at least 200,000 km3 of intermediate to acid volcanics (Rooiberg Felsite), mafic and ultramafic magmas intruded at 2065 Ma to form the Rustenburg Layered Suite (RLS). The LGS includes the Nebo, Makhutso, Bobbejaankop, Lease, and Klipkloof granites. The Nebo Granite intruded the Rooiberg Felsite as sheets up to 4 km thick above the RLS. Smaller stocks of the other granites crosscut the Nebo. We determined major- and trace-element compositions and oxygen, Rb-Sr, and Sm-Nd isotope ratios for samples of: Nebo Granite; Rooiberg Felsite; granophyre and granophyric granite; Makhutso, Bobbejaankop, and Lease granites; and feldspar porphyry from areas throughout the exposed area of the LGS (Dennilton, Verena Balmoral, Enkeldoorn, Sekhukhune Plateau, Zaaiplaats-Potgeitersrus, and Western Transvaal). Coherent floor-to-roof geochemical trends exist in some areas, although it is not possible to model them convincingly. Regional variations in geochemistry exist and likely are related to source variations in the estimated 200,000 km3 of the Nebo Granite sheets. ??18O for the LGS range from +5.9??? to +9.5???; if these are approximate primary magmatic values, pelitic sediments cannot have been an important source for the LGS. The Rb-Sr isotope system has been altered, a finding consistent with previous studies. A mineral isochron for Nebo Granite near Dennilton yields a York regression age of 1995 ?? 99 Ma, with initial 143Nd/144Nd = 0.50978??8 and ???CHUR=-5.12. Samples from the Sekhukhune Plateau have higher 143Nd/144Nd ratios than do Dennilton-area samples, suggesting that the former originated from older or less LREE-enriched sources. We suggest that intrusion of mafic to ultramafic magmas at depth in the continental crust triggered melting of Archean quartzofeldspathic crystalline rocks and formed siliceous melting-precipitating cells (SMPCs) (see, e.g., Huppert and Sparks, 1988). This mass of siliceous magma blocked ascent of denser mafic magmas to higher levels in the crust; hence the RLS is confined to a series of circumferential lobes around the periphery of the Bushveld Complex. Diapirs rose from the SMPCs to form sheets of Nebo Granite, which ascended in the center of the Bushveld Complex and spread laterally along the upper contacts of the RLS lenses.

Chronology, geochemistry and Sr-Nd isotope studies of Jurassic intrusions in the Diyanqinamu porphyry Mo mine, central Inner Mongolia, China

NASA Astrophysics Data System (ADS)

Sun, Hairui; Huang, Zhilong; Li, Wenbo; Leng, Chengbiao; Ma, Deyun; Zhang, Xingchun

2014-07-01

Available cores of porphyritic granite and aplitic granite from the Diyanqinamu porphyry Mo deposit in the north central Great Xing’an Range presented an opportunity to examine and analyze Mesozoic igneous rocks far from the Paleo-Pacific subduction zone. The Diyanqinamu granites are highly fractionated I-type, distinguished from the M-, A- or S-type granite by: high SiO2, and Rb; low Zr, Nb, Y, and Ce; low Fe2O3total/MgO and (K2O + Na2O)/CaO ratios; low alumina saturation index (<1.1); low initial ISr ratios (0.70137-0.70451); positive εNd(t) values (2.37-3.77); and negative correlation between P2O5 and SiO2. The aplitic granites were generated by fractional crystallization of the porphyritic granite, as evidenced by: spatial proximity; consistent zircon U-Pb ages (156 Ma) within error; correlations between other oxides and SiO2 in Haker diagrams; low Ba, Sr, Nb, P, Ti, Eu; linear relationship in both (La/Yb)N vs. La and Sr vs. Ba diagrams; and, decreasing LREE and ∑REE with increasing SiO2. The Diyanqinamu granites have young depleted-mantle two-stage model ages (avg. TDM2 = 660 Ma) similar to those of most Mesozoic voluminous felsic magmas in northeastern China, and were likely sourced from pre-existent crustal components both “old” and juvenile that had been juxtaposed during the tectonic evolution of the Paleo-Asian Ocean. These granites project in the transitional field from syn-collision to post-collision tectonic settings on tectonic discrimination diagrams, implying emplacement in an extensional environment. Extensional volcanism and basin formation in the Great Xing’an Range region in Late Jurassic is coeval with the Diyanqinamu granites, demonstrating that post-orogenic lithospheric extension related to the closure of the Mongol-Okhotsk Ocean was the main driving force for Late Jurassic magmatism in this region.

Petrographic and geochemical characterization of the granitic rocks of the Araguainha impact crater, Brazil

NASA Astrophysics Data System (ADS)

Silva, Dailto; Lana, Cristiano; Souza Filho, Carlos Roberto

2016-03-01

Petrographic and geochemical data obtained on the Araguainha impact crater (Goiás/Mato Grosso States, Brazil) indicate the existence of several molten products that originated during impact-induced congruent melting of an alkali-granite exposed in the inner part of the central uplift of the structure. Although previous studies have described these melts to some extent, there is no detailed discussion on the petrographic and geochemical variability in the granite and its impactogenic derivatives, and therefore, little is known about the geochemical behavior and mobility of trace elements during its fusion in the central part of the Araguainha crater. This paper demonstrates that the preserved granitoid exposed in the core of the structure is a magnesium-rich granite, similar to postcollisional, A-type granites, also found in terrains outside the Araguainha crater, in the Brasília orogenic belt. The molten products are texturally distinct and different from the original rock, but have very similar geochemical composition, making it difficult to separate these lithotypes based on concentrations of major and minor elements. This also applies for trace and rare earth elements (REE), thus indicating a high degree of homogenization during impact-induced congruent melting under high pressure and postshock temperature conditions. Petrographic observations, along with geochemical data, indicate that melting occurs selectively, where some of the elements are transported with the melt. Simultaneously, there is an effective dissolution of the rock (granite), which leads to entrainment of the most resistant solid phases (intact or partially molten minerals) into the melt. Minerals more resistant to melting, such as quartz and oxides, contribute substantially to a chemical balance between the preserved granite and the fusion products generated during the meteoritic impact.

A 3D Magnetotelluric Perspective on the Galway Granite, Western Ireland

NASA Astrophysics Data System (ADS)

Farrell, Thomas; Muller, Mark; Vozar, Jan; Feely, Martin; Hogg, Colin

2017-04-01

Magnetotelluric (MT) and audi-magnetotelluric (AMT) data were acquired at 75 locations across the exposed calc-alkaline Caledonian Galway granite batholith and surrounding country rocks into which the granite intruded. The Galway granite is located in western Ireland on the north shore of Galway bay, and has an ESE-WNW long axis. The granite is cut by trans-batholith faults, the Shannawona Fault Zone (SFZ) in the western part of the batholith, which has a NE-SW trend, and the Bearna Fault Zone (BFZ) in the eastern sector that has a NW-SE trend. Geobarometry data indicate that the central granite block between these fault zones has been uplifted, with the interpretation being that the granite in this central block is thinned. To the west of the SFZ, much of the Galway granite is below sea level, with the majority of the southern granite contact also beneath the sea in Galway bay. To the east of the batholith, the Carboniferous successions, consisting of mainly limestone with shale, overlie the basement rocks. The country rock to the north includes the metagabbro-gneiss suite, which itself intruded the deformed Dalradian successions that were deposited on the Laurentian margin of the Iapetus Ocean. The deformation of the Dalradian rocks, the intrusion of the metagabbro-gneiss suite and the intrusion of the Galway granite were major events in the protracted closure of the Iapetus Ocean. It is clear from geological mapping, from geobarometry and from the present submergence by the sea of a large part of the Galway granite, that inversion of MT data in this structurally complex geology is likely to require a 3D approach. We present a summary of 3D inversion of the Galway MT and AMT data. The study shows that the structure of the Galway granite is quite different from the pre-existing perspective. The central block, thought by its uplifting to be thinned, is shown to be the thickest part of the batholith. A geological model of granite intrusion is offered to explain this structure.

Geothermal potential of Caledonian granites underlying Upper Palaeozoic sedimentary basins astride the Iapetus Suture Zone in Ireland

NASA Astrophysics Data System (ADS)

Fritschle, Tobias; Daly, J. Stephen; Whitehouse, Martin J.; McConnell, Brian; Buhre, Stephan

2014-05-01

Upper Palaeozoic sedimentary basins in Ireland overlie crystalline rocks within the Caledonian Iapetus Suture Zone. Beneath these basins, Lower Palaeozoic rocks, formed and deformed during the Caledonian orogenic cycle, were intruded by c. 420-390 Ma late-tectonic granites at various tectonic levels. These include the subsurface Kentstown and Glenamaddy granites discovered by mineral exploration drilling. While these granites comprise actual targets for Enhanced Geothermal System (EGS) exploration, several others likely exist based on geophysical considerations. In order to test the regional geothermal potential, the buried granites as well as analogue exposed rocks are being investigated geochemically. The geothermal potential of the intrusives depends on their heat production rate (HPR), which is calculated using rock density and concentrations of the heat producing elements (HPE) uranium, thorium and potassium. In spite of their close spacing and similar ages, the whole-rock geochemistry of the granites varies significantly, but with no obvious geographical control (Fritschle et al., 2013; 2014). The granite HPR values range from 1.4 μW/m3 for the Dhoon Granite (Isle of Man) to 4.9 μW/m3 for the Drogheda Granite (Ireland). This compares with the average HPR for a 'typical' granite of 2.7 μW/m3 (Goldstein et al., 2009). It is demonstrated that an elevated HPR of a granite can be related to enrichment in one of the HPE alone (e.g., uranium-enrichment in the Foxdale Granite (Isle of Man), or thorium-enrichment in the Drogheda Granite). Enrichment in HPE in a granite may occur due to different reasons including hydrothermal (re-) distribution of uranium, or the assimilation of thorium-rich wall-rocks. Hence, the distribution of the HPE in particular minerals, veins and source lithologies, along with the petrophysical characteristics of the sedimentary basins and the granites' petrogenesis, are currently being investigated as possible mechanisms controlling their heat production budget. Fritschle, T., Daly, J.S., Whitehouse, M.J., McConnell, B., Buhre, S., 2013. U-Pb Zircon Ages from Granites in the Iapetus Suture Zone in Ireland and the Isle of Man. Mineralogical Magazine, 77(5): 1115. Fritschle, T., Daly, J.S., Whitehouse, M.J., McConnell, B., Buhre, S., 2014. Zircon geochronology and Hf-O isotope geochemistry from granites in the Iapetus Suture Zone in Ireland and the Isle of Man. This issue. Goldstein, B.A., Hill, A.J., Long, A., Budd, A.R., Ayling, B., Malavazos, M., 2009. Hot rocks down under - evolution of a new energy industry. Geothermal Resources Council Transactions, 33: 185-198.

Magmatic and hydrothermal R.E.E. fractionation in the Xihuashan granites (SE China)

NASA Astrophysics Data System (ADS)

Maruéjol, Patricia; Cuney, Michel; Turpin, Laurent

1990-11-01

The Xihuashan stock (South Jiangxi, China) is composed of cogenetic granitic units (granites Xe, γa, γc, γd and γb) and emplaced during the Yanshanian orogeny (153±0.2 Ma). They are two feldspars, Fe-rich biotite±garnet and slightly peraluminous granites. Primary accessory minerals are apatite 1, monazite, zircon, uranothorite±xenotime in granites Xe and γa, zircon, uranothorite, uraninite, betafite, xenotime 1; hydrothermal minerals are monazite altered into parisite and apatite 2, Y-rich parisite, yttroparisite, Y-rich fluorite and xenotime 2 in granites γc and γb. Petrographic observations, major element, REE, Y and Rb-Sr isotropic data point to a magmatic suite (granites Xe and γa → granites γc and γd → granite γb) distinct from hydrothermal Na-or K-alteration of γb. From granite Xe to granite γb, LREE, Eu, Th and Zr content are strongly depleted, while HREE, Y and U content increase. During K-alteration of γb, these variations are of minor importance. Major and accessory mineral evidences, geochemical and fluid inclusion results indicate two successive alteration fluids interacting with γb, (1) a late-magmatic F- and CO2-rich fluid and (2) a post-magmatic, aqueous and slightly saline fluid. The depletion of LREE and Th content and the increase in HREE, Y and U content correspond, in the magmatic suite to the early fractionation of monazite in the granites where there is no hydrothermal alteration (granites Xe and γe) and to the hydrothermal alteration of monazite into parisite and secondary apatite, intense new formation of yttroparisite, Y enrichment and U loss in the uranothorite and late crystallization of uraninite in the granites γc and γb. Moreover, simulated crystallization of monazite and temperature of monazite saturation show early fractionation of monazite from the magma in the less evolved granites (Xe and γe) and prevailing hydrothermal leaching of monazite in the most evolved granites (γc-γd and γb) related to a late-magmetic event. The slight variations of REE, Y, Th and U content in the K-altered granites compared to granite γb emphazes the distinct chemical nature of the successive hydrothermal fluids. Rb-Sr and Sm-Nd isotopic results point to a 30 Ma period of time between the late-magmatic and the post-magmatic fluid circulation.

Petrology of the Western Highland Province: Ancient crust formation at the Apollo 14 site

NASA Astrophysics Data System (ADS)

Shervais, John W.; McGee, James J.

1999-03-01

Plutonic rocks found at the Apollo 14 site comprise four lithologic suites: the magnesian suite, the alkali suite, evolved lithologies, and the ferroan anorthosite suite (FAN). Rocks of the magnesian suite include troctolite, anorthosite, norite, dunite, and harzburgite; they are characterized by plagioclase ~An95 and mafic minerals with mg#s 82-92. Alkali suite rocks and evolved rocks generally have plagioclase ~An90 to ~An40, and mafic minerals with mg#s 82-40. Lithologies include anorthosite, norite, quartz monzodiorite, granite, and felsite. Ferroan anorthosites have plagioclase ~An96 and mafic minerals with mg#s 45-70. Whole rock geochemical data show that most magnesian suite samples and all alkali anorthosites are cumulates with little or no trapped liquid component. Norites may contain significant trapped liquid component, and some alkali norites may represent cumulate-enriched, near-liquid compositions, similar to KREEP basalt 15386. Evolved lithologies include evolved partial cumulates related to alkali suite fractionation (quartz monzodiorite), immiscible melts derived from these evolved magmas (granites), and impact melts of preexisting granite (felsite). Plots of whole rock mg# versus whole rock Ca/(Ca+Na+K) show a distinct gap between rocks of the magnesian suite and rocks of the alkali suite, suggesting either distinct parent magmas or distinct physical processes of formation. Chondrite-normalized rare earth element (REE) patterns show that rocks of both the magnesian suite and alkali suite have similar ranges, despite the large difference in major element chemistry. Current models for the origin of the magnesian suite call for a komatiitic parent magma derived from early magma ocean cumulates; these melts must assimilate plagiophile elements to form troctolites at low pressures and must assimilate a highly enriched KREEP component so that the resulting mixture has REE concentrations similar to high-K KREEP. There are as yet no plausible scenarios that can explain these unusual requirements. We propose that partial melting of a primitive lunar interior and buffering of these melts by ultramagnesian early magma ocean cumulates provides a more reasonable pathway to form magnesian troctolites. Alkali anorthosites and norites formed by crystallization of a parent magma with major element compositions similar to KREEP basalt 15386. If the parent magma of the alkali suite and evolved rocks is related to the magnesian suite, then that magma must have evolved through combined assimilation-fractional crystallization processes to form the alkali suite cumulates.

Molybdenite Re-Os, zircon U-Pb dating and Lu-Hf isotopic analysis of the Xiaerchulu Au deposit, Inner Mongolia Province, China

NASA Astrophysics Data System (ADS)

Wang, Jia-xin; Nie, Feng-Jun; Zhang, Xue-ni; Jiang, Si-hong

2016-09-01

The Xiaerchulu Au deposit, located in the Southern Orogenic Belt (SOB) of Western Inner Mongolia (WIM), is hosted in an Early Permian (271-261 Ma) volcanic-plutonic sequence. Mineralization took place in silicified biotite granites or along the contact zone between the Neoproterozoic Baiyinbaolage Group and the biotite granite. In order to constrain the timing of the Xiaerchulu mineralization and discuss the petrogenesis of the hosting granites, molybdenite Re-Os, and zircon U-Pb and, Lu-Hf, and REE, geochemical, and Sr-Nd isotopic studies were completed in this study. We measured Re-Os isotopes of six molybdenite samples from the main ore body, which yielded a weighted average model age of 261.7 ± 1.5 Ma with a MSWD of 0.55, indicating that the time of mineralization was at ca. 262 Ma. High precision U-Pb dating for the studied granites yields Permian 206Pb/238U ages ranging from 271 to 269 Ma. These age data confirm that both the intrusion and related mineralization were initiated in Early Permian period. These granites are strongly peraluminous with A/CNK = 1.11-1.12, high SiO2-K2O contents, as well as containing biotite and muscovite, indicating a petrogenesis of typical S-type granites, the above consideration is also consistent with the result of discrimination diagrams. The Re contents of molybdenite, εNd(t), and zircon εHf(t), as well as the 176Hf/177Hf values of the granites, fall into the ranges from 1.153 to 2.740 μg/g, - 11.1 to - 9.3, - 8.8 to - 0.9, and 0.282358 to 0.282688, respectively. All of this evidence suggests that the metals were derived from a predominantly crustal source, the granites originated from crust in an extensional setting, and the rejuvenation of the continent may have play an important role during the ore-forming processes of the Early Permian epoch.

Jurassic high heat production granites associated with the Weddell Sea rift system, Antarctica

NASA Astrophysics Data System (ADS)

Leat, Philip T.; Jordan, Tom A.; Flowerdew, Michael J.; Riley, Teal R.; Ferraccioli, Fausto; Whitehouse, Martin J.

2018-01-01

The distribution of heat flow in Antarctic continental crust is critical to understanding continental tectonics, ice sheet growth and subglacial hydrology. We identify a group of High Heat Production granites, intruded into upper crustal Palaeozoic metasedimentary sequences, which may contribute to locally high heat flow beneath the West Antarctic Ice Sheet. Four of the granite plutons are exposed above ice sheet level at Pagano Nunatak, Pirrit Hills, Nash Hills and Whitmore Mountains. A new Usbnd Pb zircon age from Pirrit Hills of 178.0 ± 3.5 Ma confirms earlier Rbsbnd Sr and Usbnd Pb dating and that the granites were emplaced approximately coincident with the first stage of Gondwana break-up and the developing Weddell rift, and 5 m.y. after eruption of the Karoo-Ferrar large igneous province. Aerogeophysical data indicate that the plutons are distributed unevenly over 40,000 km2 with one intruded into the transtensional Pagano Shear Zone, while the others were emplaced within the more stable Ellsworth-Whitmore mountains continental block. The granites are weakly peraluminous A-types and have Th and U abundances up to 60.7 and 28.6 ppm respectively. Measured heat production of the granite samples is 2.96-9.06 μW/m3 (mean 5.35 W/m3), significantly higher than average upper continental crust and contemporaneous silicic rocks in the Antarctic Peninsula. Heat flow associated with the granite intrusions is predicted to be in the range 70-95 mW/m2 depending on the thickness of the high heat production granite layer and the regional heat flow value. Analysis of detrital zircon compositions and ages indicates that the high Th and U abundances are related to enrichment of the lower-mid crust that dates back to 200-299 Ma at the time of the formation of the Gondwanide fold belt and its post-orogenic collapse and extension.

Early tectonic evolution of the Thomson Orogen in Queensland inferred from constrained magnetic and gravity data

NASA Astrophysics Data System (ADS)

Spampinato, Giovanni P. T.; Betts, Peter G.; Ailleres, Laurent; Armit, Robin J.

2015-05-01

The crustal architecture as well as the kinematic evolution of the Thomson Orogen in Queensland is poorly resolved because the region is concealed under thick Phanerozoic sedimentary basins and the basement geology is known from limited drill holes. Combined potential field and seismic interpretation indicates that the Thomson Orogen is characterized by prominent regional NE- and NW-trending structural grain defined by long wavelength and low amplitude geophysical anomalies. The 'smooth' magnetic signature is interpreted to reflect deeply buried source bodies in the mid- to lower crust. Short wavelength positive magnetic features that correlate with negative gravity anomalies are interpreted to represent shallower granitic intrusions. They appear to be focused along major fault zones that might have controlled the locus for magmatism. The eastern Thomson Orogen is characterized by a prominent NE structural grain and orthogonal faults and fold interference patterns resulting in a series of troughs and highs. The western Thomson Orogen consists of a series of NW-trending structures interpreted to reflect reverse faults. Sedimentation and basin development are interpreted to have initiated in the Neoproterozoic to Early Cambrian during E-W- to ENE-WSW extension, possibly related to the Rodinia break-up. This extensional event was followed by Late Cambrian shortening recorded in the Maneroo Platform and the Diamantina River Domain which possibly correlates with the Delamerian Orogeny. Renewed deposition and volcanism occurred during the Ordovician and may have continued until Late Silurian, resulting in thinned Proterozoic basement crust and extensive basin systems that formed in a distal continental back-arc environment. Our interpretation places the Thomson Orogen to the west of the Neoproterozoic passive margin preserved in the Anakie Inlier. The region is likely to represent the internal extensional architecture during the Rodinia break-up that has been subsequently extensively modified by multiple extensional basin forming events and transient episodes of crustal shortening and basin inversion.

Detrital fingerprints of fossil continental-subduction zones (Axial Belt Provenance, European Alps)

NASA Astrophysics Data System (ADS)

Resentini, Alberto; Garzanti, Eduardo; Vezzoli, Giovanni; Andò, Sergio; Malusà, Marco G.; Padoan, Marta; Paparella, Paolo

2010-05-01

Alpine-type collision orogens are generated by attempted subduction of thinned continental margins. Because of complex tectonic structure, orogenic detritus is characterized by a range of detrital signatures, making its recognition an arduous task (Dickinson and Suczek, 1979). Among the various orogenic sub-provenances, Axial Belt Provenance, derived from the erosion of the neometamorphic axial pile, can be regarded as the most typifying signature of collision orogens (Garzanti et al., 2007). In the Austroalpine Cretaceous and Penninic Eocene axial belts of the Alps, we ideally distinguish three structural levels, each characterized by diagnostic detrital fingerprints. The shallow level chiefly consists of offscraped remnant-ocean turbidites and unmetamorphosed continental-margin sediments, and mostly produces lithic to lithoquartzose sedimentaclastic sands yielding very-poor heavy-mineral suites including ultrastable minerals. The intermediate level includes low-grade metasediments and polymetamorphic basements, and sheds lithoquartzose to quartzolithofeldspathic metamorphiclastic sands yielding moderately-rich epidote- amphibole suites with chloritoid or garnet. The deep level contains eclogitic remnants of continent- ocean transitions, and supplies quartzofeldspathic to quartzolithic high-rank metamorphiclastic to lithic ultramaficlastic sands yielding rich to extremely-rich suites dominated by garnet, hornblende, or epidote depending on protoliths (continental vs. oceanic) and pressure/temperature paths followed during exhumation. Although widely overprinted under greenschist-facies or amphibolite-facies conditions, occurrence of ultradense eclogite in source areas is readily revealed by the Heavy Mineral Concentration (HMC) index, which mirrors the average density of source rocks in the absence of hydraulic-sorting effects (Garzanti and Andò 2007). The Metamorphic Index (MI, Garzanti and Vezzoli, 2003) and Hornblende Colour Index (HCI) reflect peak temperatures reached at later stages, when subduction is throttled by arrival of thicker continental crust and geothermal gradients increase. Experience gained from modern sediments provides fundamental help to decrypt the innumerable pieces of information stored in the sedimentary record, and thus to identify and reconstruct subduction events of the past. Dickinson, W., R., Suczek, C.A., 1979.Plate tectonics and sandstone compositions. AAPG Bull. 63, 2164-2182. Garzanti, E. and Andò, S. 2007. Heavy-mineral concentration in modern sands: implications for provenance interpretation. In Mange, M., and Wright, D., eds. Heavy Minerals in Use. Developments in Sedimentology Series 58. Amsterdam, Elsevier, p. 517-545. Garzanti, E., and Vezzoli, G. 2003. A classification of metamorphic grains in sands based on their composition and grade. J. Sediment. Res. 73:830-837. Garzanti, E., Doglioni, C., Vezzoli, G., Andò, S., 2007. Orogenic belts and orogenic sediment provenante. J. Geology, 115:315-334.

Zircon U-Pb ages and geochemistry of migmatites and granites in the Foping dome: Evidence for Late Triassic crustal evolution in South Qinling, China

NASA Astrophysics Data System (ADS)

Zhang, He; Li, Shuang-Qing; Fang, Bo-Wen; He, Jian-Feng; Xue, Ying-Yu; Siebel, Wolfgang; Chen, Fukun

2018-01-01

Migmatites provide a record of melt formation and crustal rheology. In this study we present zircon U-Pb ages and geochemical composition of migmatites from the Foping dome and granites from the Wulong pluton. U-Pb results from migmatite zircons indicate two episodes of partial melting. Rim domains from a leucosome in the Longcaoping area yield an age of ca. 209 Ma. Migmatites collected from the Foping dome yield U-Pb zircon ages of 2910 to 190 Ma, suggesting the involvement of meta-sedimentary source components. Rim domains of the zircons with low Th/U ratios (< 0.1) give ages of 225-190 Ma and the youngest age domains (ca. 195 Ma) are characterized by low contents of heavy rare earth elements, which is related to crystallization of garnet. Magmatic rocks from the Wulong pluton can be subdivided into high Sr/Y and low Sr/Y granites. U-Pb zircon ages vary from 219 to 214 Ma for the high Sr/Y granites and from 214 to 192 Ma for the low Sr/Y granites. High Sr/Y granites have higher Na2O and Sr contents than the low Sr/Y granites. They also lack negative Eu anomalies and are depleted in HREE compared to the low Sr/Y granites. Initial 87Sr/86Sr ratios and εNd values of all the samples roughly overlap with those of Neoproterozoic basement rocks exposed in South Qinling. Including previous studies, we propose that the high and low Sr/Y granites formed by melting of thickened and normal crust, respectively. Close temporal-spatial relationship of the high and low Sr/Y granites with the two-stage migmatization events implies variation of crustal thickness and thermal overprints of the orogenic crust in post-collisional collapse. Following the collision of South Qinling and the Yangtze block prior to 219 Ma, partial melting of the deep crust occurred. The melts migrated upwards to form the high Sr/Y granites. This process occurred rapidly and caused collapse of the thickened crust and carried heat upwards, leading to further partial melting within the shallower crust and formation of the low Sr/Y granites.

Continental crust melting induced by subduction initiation of the South Tianshan Ocean: Insight from the Latest Devonian granitic magmatism in the southern Yili Block, NW China

NASA Astrophysics Data System (ADS)

Bao, Zihe; Cai, Keda; Sun, Min; Xiao, Wenjiao; Wan, Bo; Wang, Yannan; Wang, Xiangsong; Xia, Xiaoping

2018-03-01

The Tianshan belt of the southwestern Central Asian Orogenic Belt was generated by Paleozoic multi-stage subduction and final closure of several extinct oceans, including the South Tianshan Ocean between the Kazakhstan-Yili and Tarim blocks. However, the subduction initiation and polarity of the South Tianshan Ocean remain issues of highly debated. This study presents new zircon U-Pb ages, geochemical compositions and Sr-Nd isotopes, as well as zircon Hf isotopic data of the Latest Devonian to Early Carboniferous granitic rocks in the Wusun Mountain of the Yili Paleozoic convergent margin, which, together with the spatial-temporal distributions of regional magmatic rocks, are applied to elucidate their petrogenesis and tectonic linkage to the northward subduction initiation of the South Tianshan Ocean. Our zircon U-Pb dating results reveal that these granites were emplaced at the time interval of 362.0 ± 1.2-360.3 ± 1.9 Ma, suggesting a marked partial melting event of the continental crust in the Latest Devonian to Early Carboniferous. These granites, based on their mineral compositions and textures, can be categorized as monzogranites and K-feldspar granites. Geochemically, both monzogranites and K-feldspar granites have characters of I-type granites with high K2O contents (4.64-4.83 wt.%), and the K-feldspar granites are highly fractionated I-type granites, while the monzogranites have features of unfractionated I-type granites. Whole-rock Sr-Nd isotopic modeling results suggest that ca. 20-40% mantle-derived magmas may be involved in magma mixing with continental crust partial melts to generate the parental magmas of the granites. The mantle-derived basaltic magmas was inferred not only to be a major component of magma mixture but also as an important heat source to fuse the continental crust in an extensional setting, which is evidenced by the high zircon saturation temperatures (713-727 °C and 760-782 °C) of the studied granites. The Latest Devonian to Early Carboniferous extensional setting in the Wusun Mountain region of the Yili Paleozoic convergent margin is addressed by the subduction initiation of the South Tianshan Ocean and constituted a late Paleozoic nascent arc- back-arc system in the southwestern CAOB.

A simple tectonic model for crustal accretion in the Slave Province: A 2.7-2.5 Ga granite greenstone terrane

NASA Technical Reports Server (NTRS)

Hoffman, P. F.

1986-01-01

A prograding (direction unspecified) trench-arc system is favored as a simple yet comprehensive model for crustal generation in a 250,000 sq km granite-greenstone terrain. The model accounts for the evolutionary sequence of volcanism, sedimentation, deformation, metamorphism and plutonism, observed througout the Slave province. Both unconformable (trench inner slope) and subconformable (trench outer slope) relations between the volcanics and overlying turbidities; and the existence of relatively minor amounts of pre-greenstone basement (microcontinents) and syn-greenstone plutons (accreted arc roots) are explained. Predictions include: a varaiable gap between greenstone volcanism and trench turbidite sedimentation (accompanied by minor volcanism) and systematic regional variations in age span of volcanism and plutonism. Implications of the model will be illustrated with reference to a 1:1 million scale geological map of the Slave Province (and its bounding 1.0 Ga orogens).

Geochemistry and chronology of the early Paleozoic diorites and granites in the Huangtupo volcanogenic massive sulfide (VMS) deposit, Eastern Tianshan, NW China: Implications for petrogenesis and geodynamic setting

NASA Astrophysics Data System (ADS)

Zheng, Jiahao; Chai, Fengmei; Feng, Wanyi; Yang, Fuquan; Shen, Ping

2018-03-01

The Eastern Tianshan orogen contains many late Paleozoic porphyry Cu and magmatic Cu-Ni deposits. Recent studies demonstrate that several early Paleozoic volcanogenic massive sulfide (VMS) Cu-polymetallic and porphyry Cu deposits were discovered in the northern part of Eastern Tianshan. This study presents zircon U-Pb, whole-rock geochemical, and Sr-Nd isotopic data for granites and diorites from the Huangtupo VMS Cu-Zn deposit, northern part of the Eastern Tianshan. Our results can provide constraints on the genesis of intermediate and felsic intrusions as well as early Paleozoic geodynamic setting of the northern part of Eastern Tianshan. LA-ICP-MS zircon U-Pb analyses suggest that the granites and diorites were formed at 435 ± 2 Ma and 440 ± 2 Ma, respectively. Geochemical characteristics suggest that the Huangtupo granites and diorites are metaluminous rocks, exhibiting typical subduction-related features such as enrichment in LILE and LREE and depletion in HFSE. The diorites have moderate Mg#, positive εNd(t) values (+6.4 to +7.3), and young Nd model ages, indicative of a depleted mantle origin. The granites exhibit mineral assemblages and geochemical characteristics of I-type granites, and they have positive εNd(t) values (+6.7 to +10.2) and young Nd model ages, suggesting a juvenile crust origin. The early Paleozoic VMS Cu-polymetallic and porphyry Cu deposits in the northern part of Eastern Tianshan were genetically related. The formation of the early Paleozoic magmatic rocks as well as VMS and porphyry Cu deposits in the northern part of Eastern Tianshan was due to a southward subduction of the Junggar oceanic plate.

Heterogeneous Flow of an Extruded granitic dome in the Bronson Hill Terrane, Massachusetts, USA: Evidence for Oblique Convergence and Indentation, and the Alleghanian Orogeny

NASA Astrophysics Data System (ADS)

Massey, M. A.; Moecher, D. P.

2006-12-01

One widely cited model for Appalachian orogenesis in New England invokes the tripartite Alpine sequence of nappe folding/thrusting, back-folding, and doming to explain regional and outcrop-scale structural relationships. Recent work suggests lateral extrusion driven by oblique convergence as an important mechanism responsible for structures, fabrics, and mineral assemblages in the Bronson Hill terrane (BHT) of Connecticut and Massachusetts. Just as the Alpine model has evolved to incorporate elements of lateral extrusion, and syn- to post-orogenic collapse, we propose similar revisions for southern New England. Detailed mapping and structural analysis of the W- to WNW-dipping BHT in south-central MA reveals: (1) a sub-vertical, transpressional dextral thrust high strain zone (Bonemill/Conant Brook shear zone) bounding the eastern margin of the Monson granitic gneiss dome (MG) with two modes of Sil+Qtz+Fs lineations plunging WNW and SSW; (2) a moderate to steeply-dipping sinistral high strain zone bounding the western margin of the MG with WNW- and SSW-plunging Ms+Qtz+Grt lineations; (3) an apparently random arrangement of gneiss, s and s-l tectonites, protomylonites, and mylonites composing the body of the MG, also containing WNW and SSW Qtz+Fs lineations. Extrapolation to a regional scale from central CT to northern MA indicates: (1) a gradual increase in s-l and l-s tectonites to the north from predominantly s-tectonites in central CT; (2) transition of lineation plunge from NW in central CT to bimodal WNW and SSW distribution to the north; (3) amphibolite facies metamorphism was pre- to synkinematic with respect to deformation. We propose that these observations may be accounted for by transpression and extrusion, rather than discreet phases of deformation invoked by the traditional three-stage model. Synchronous operation of high strain zones bounding the MG accommodated northward orogen-parallel extrusion in addition to a component of orogen-normal shortening and sub-vertical extrusion, thus constituting bulk heterogeneous flow. Existing geochronology/thermochronology constrains deformation to the late Paleozoic Alleghanian orogeny. The consistency in timing and similarity in style with deformation associated with the Pelham dome demonstrate the significance of orogen-parallel flow in the BHT. We go further by presenting a working late Paleozoic tectonic model incorporating data from this study with existing contributions from other workers in southern New England. This model involves oblique convergence and underthrusting of Avalon in the late Mississippian/early Pennsylvanian continuing into and throughout most of the Permian. Synorogenic compressional and extensional structures from upper amphibolite to greenschist facies are explained by progressive deformation, including extrusion, orogenic collapse, and wedging, throughout an evolving metamorphic gradient.

A weathering-related origin of widespread monazite in S-type granites

NASA Astrophysics Data System (ADS)

Sawka, Wayne N.; Banfield, Jillian F.; Chappell, Bruch W.

1986-01-01

The S-type granite suites comprising more than a quarter of the extensively developed granites in the Lachlan Fold Belt, Australia, contain monazite which may be related to the chemical weathering of the sedimentary source rocks. We report a process whereby chemical weathering fixes mobile rare-earth elements (REE) in hydrous phosphate phases such as florencite and rhabdophane. This material contains up to 50 wt% LREE and occurs as very small particles (~3μm). Dehydration of these hydrous REE phases during anatexis directly yields monazite. The low solubility of phosphorus in S-type granite melts inhibits dissolution of both monazite and apatite. Refractory monazite may be thus entrained and transported in S-type granites in a manner similar to processes resulting in inherited zircon. Since both Th and the light REE are major components in monazite, materials containing this minute phase may be of widespread geochemical significance in both granites and metamorphic rocks.

Geochemistry and petrogenesis of the Mesoarchean granites from the Canaã dos Carajás area, Carajás Province, Brazil: Implications for the origin of Archean granites

NASA Astrophysics Data System (ADS)

Feio, G. R. L.; Dall'Agnol, R.

2012-12-01

Four Mesoarchean (2.93 to 2.83 Ga) granite units, which encompass the Canaã dos Carajás, Bom Jesus, Cruzadão and Serra Dourada granites, were recognized in the Canaã dos Carajás area of the Archean Carajás Province. The Mesoarchean units are composed dominantly of biotite leucomonzogranites. They are compared with the Neoarchean Planalto suite (2.73 Ga) which encompasses biotite-hornblende monzogranites to syenogranites. The Canaã dos Carajás, Bom Jesus and the variety of the Cruzadão granite with higher (La/Yb)N are geochemically more akin to the calc-alkaline granites, whereas the other varieties of the Cruzadão granite are transitional between calc-alkaline and alkaline granites. The Serra Dourada granite has an ambiguous geochemical character with some features similar to those of calc-alkaline granites and other peraluminous granites. The Planalto granites have ferroan character, are similar geochemically to reduced A-type granites and show a strong geochemical contrast with the Mesoarchean studied granites. The Mesoarchean granites described in the Canaã dos Carajás area are geochemically distinct to those of the Rio Maria domain of the Carajás Province. The Canaã dos Carajás and Bom Jesus granites are similar to the high-Ca granites, whereas the Cruzadão and Serra Dourada are more akin to the low-CaO granites of the Yilgarn craton. The geochemical characteristics of the Mesoarchean studied granites approach those of the biotite granite group of Dharwar but the latter are enriched in HFSE and HREE compared to the Canaã dos Carajás granites. The Neoarchean Planalto suite granite has no counterpart in the Mesoarchean Rio Maria domain of the Carajás Province, neither in the Yilgarn and Dharwar cratons. Geochemical modeling suggests that partial melting of a source similar in composition to an Archean basaltic andesite of the Carajás Province could give origin to the Bom Jesus and Cruzadão granites. In the case of the Bom Jesus granite the residue of melting contained, in similar proportions, plagioclase, clinopyroxene, amphibole, and garnet, with subordinate ilmenite. The variations in (La/Yb)N and Sr/Y of the Cruzadão granite are controlled by changes in the residual melting phases. Garnet and amphibole are abundant in the residue of the variety with higher (La/Yb)N, whereas in the rocks with moderate to lower (La/Yb)N garnet is absent, magnetite appears in the residue and amphibole initially increases but is replaced by orthopyroxene in the rocks with the lowest (La/Yb)N. In the Canaã dos Carajás and Serra Dourada granites garnet was probably an absent phase in the residue of melting and the influence of amphibole was also apparently limited. A crustal environment and a variable pressure from 10 to 7-8 kbar are estimated for the generation of the granite magmas. The Bom Jesus and Cruzadão granites of Carajás derived from basaltic andesite sources and not of TTG and their geochemical contrasts can be explained by variation in the pressure of melting. The sources of Archean granite magmas are more diversified and could be more mafic than generally admitted.

Geochronology and geochemistry of the Borohoro pluton in the northern Yili Block, NW China: Implication for the tectonic evolution of the northern West Tianshan orogen

NASA Astrophysics Data System (ADS)

Wang, Meng; Zhang, Jinjiang; Zhang, Bo; Liu, Kai; Chen, Youxin; Zheng, Yanrong

2018-03-01

The closure of the North Tianshan Ocean between the Junggar Terrane and the Yili Block is a longtime debated issue in literature, because of the different understanding of the Carboniferous volcanic rocks in the northern margin of the Yili Block. This study presents new geochronological and whole-rock geochemical data for the granitic rocks from the Borohoro pluton to provide constraints on the tectonic regime for the northern West Tianshan during the Carboniferous. LA-ICP-MS U-Pb dating results reveal two magmatic phases for the Borohoro pluton. The former magmatic activity in the Early Carboniferous formed the fine-grained granodiorite (332 Ma). The later magmatic activity occurred during the Late Carboniferous (305-300 Ma), forming a diversity of granitic rocks, involving quartz diorite, granodiorite and granite. Geochemical and mineralogical studies reveal that the studied granitic rocks from the Borohoro pluton all belong to metaluminous to weakly peraluminous, calc-alkaline I-type granites. They are characterized by enrichment in LILEs relative to HFSEs, and depletion of Nb, Ti and P, typical of continental arc-type granites. The intermediate SiO2, high Al2O3, and relatively low Fe2O3T, MgO and TiO2 contents reflect that these granitic rocks are mainly crust-derived. But the high Mg# values for most samples and the occurrence of microgranular mafic enclaves indicate that their magma sources were mixed by mantle-derived components. Especially, the Late Carboniferous rocks define an elegant mixing trend in both the Rb-Rb/V and the 1/V-Rb/V diagrams, consistent with mixing between magmas from subcontinental lithospheric mantle and mafic lower crust. Taking into consideration of the facts that all the Devonian to Carboniferous granitoids belong to calc-alkaline I-type granites, and granitoids of A-type didn't appear until the Early Permian, we suggest that the subduction of the North Tianshan Ocean continued to the Late Carboniferous, generating the granitic rocks of the Borohoro pluton.

Geochemical constraints on genesis of Paleoproterozoic A-type granite in the south margin of North China Craton

NASA Astrophysics Data System (ADS)

Xue, Shuo; Xu, Yang; Ling, Ming-Xing; Kang, Qing-Qing; Jiang, Xiao-Yan; Sun, Sai-Jun; Wu, Kai; Zhang, Zhe-Kun; Luo, Ze-Bin; Liu, Yu-Long; Sun, Weidong

2018-04-01

Paleoproterozoic A-type granites are widely outcropped in the North China Craton (NCC), particularly in the Trans-North China Orogen. However, their genesis and tectonic significance remain obscure. Here we report systematic studies on geochronology and geochemical characteristics of A-type granite in Huayangchuan, south margin of the NCC. The samples are enriched in total alkali (K2O + Na2O > 8.97 wt%), and depleted in MgO (0.84-0.93 wt%), CaO (1.28-1.90 wt%) and P2O5 (0.18-0.20 wt%), with high FeOT/MgO (5.69-6.67). They are characterized by high Zr + Y + Nb + Ce values (1293-1392 ppm) and 10,000 × Ga/Al ratios (3.14-3.35), which are typical characteristics of A-type granite. The Huayangchuan A-type granite can be further classified as A1-type subgroup based on particular geochemical features, e.g., low Y/Nb (0.87-1.00) and Yb/Ta (0.88-1.10). High precision zircon U-Pb dating of the A-type granite by secondary ion mass spectrometry (SIMS) yields Paleoproterozoic 207Pb/206Pb ages of 1829.5 ± 2.5 Ma. The low zircon ɛHf(t) values (-6.97 to -10.45), along with zircon Hf model age of 2.7-2.9 Ga, indicate that the Huayangchuan A-type granite was derived from partial melting of the ancient continental crust with contribution of enriched mantle components. The low zircon δ18O composition (4.00 to 6.78‰) indicates that the zircons were crystallized from low δ18O magmas, which derived from the crust metasomatized by low δ18O mantle fluids or melts. The E-W trend A1-type granitic plutons in the NCC are generally outcropped in a rift tectonic regime, which is consistent with the development of the mantle plume in the Xiong'er district. The large volume of basaltic magmas, generated by mantle plume head, underplated the lower continental crust and formed the Huayangchuan A-type granite.

Geologic Map of the Sheep Hole Mountains 30' x 60' Quadrangle, San Bernardino and Riverside Counties, California

USGS Publications Warehouse

Howard, Keith A.

2002-01-01

This data set describes and maps the geology of the Sheep Hole Mountains 30' x 60' quadrangle in southern California. The quadrangle covers an area of the Mojave Desert characterized by desert ranges separated by broad basins. Ranges include parts of the Old Woman, Ship, Iron, Coxcomb, Pinto, Bullion, and Calumet mountains as well as Lead Mountain and the Kilbeck Hills. Basins include part of Ward Valley, part of Cadiz Valley including Cadiz Lake playa, and broad valleys occupied by the Bristol Lake and Dale Lake playas. Bedrock geologic units in the ranges range in age from Proterozoic to Quaternary. The valleys expose Neogene and Quaternary deposits. Proterozoic granitoids in the quadrangle include the Early Proterozoic Fenner Gneiss, Kilbeck Gneiss, Dog Wash Gneiss, granite of Joshua Tree, the (highly peraluminous granite) gneiss of Dry Lakes valley, and a Middle Proterozoic granite. Proterozoic supracrustal rocks include the Pinto Gneiss of Miller (1938) and the quartzite of Pinto Mountain. Early Proterozoic orogeny left an imprint of metamorphic mineral assemblages and fabrics in the older rocks. A Cambrian to Triassic sequence deposited on the continental shelf lies above a profound nonconformity developed on the Proterozoic rocks. Small metamorphosed remnants of this sequence in the quadrangle include rocks correlated to the Tapeats, Bright Angel, Bonanza King, Redwall, Bird Spring, Hermit, Coconino, Kaibab, and Moenkopi formations. The Dale Lake Volcanics (Jurassic), and the McCoy Mountains Formation of Miller (1944)(Cretaceous and Jurassic?) are younger Mesozoic synorogenic supracrustal rocks in the quadrangle. Mesozoic intrusions form much of the bedrock in the quadrangle, and represent a succession of magmatic arcs. The oldest rock is the Early Triassic quartz monzonite of Twentynine Palms. Extensive Jurassic magmatism is represented by large expanses of granitoids that range in composition from gabbro to syenogranite. They include the Virginia May Quartz Monzonite and other members of the Bullion Intrusive Suite, the Chubbock Porphyry, and rocks that form the Goat Basin pluton, Music Valley pluton, and Ship Mountains pluton. The Jurassic plutons range in emplacement depths from mid-crustal to hypabysasal. Mafic and felsic dikes that probably are part of the Late Jurassic Independence dike swarm intrude the Jurassic batholithic rocks. A Mesozoic ductile fault (tectonic slide), the Scanlon thrust, places an inverted sequence of lower Paleozoic rocks and their Proterozoic basement over a lower plate of younger Paleozoic and Triassic rocks. The lower- plate rocks are internally sliced and folded. They in turn are superposed along an attenuation tectonic slide, the Kilbeck fault, over highly strained tectonic schist. The major tectonic slides and associated fabrics are cut by Late Cretaceous batholithic rocks. Widespread Late Cretaceous granitoids assigned to the Cadiz Valley batholith and the Old-Woman Piute Range batholith together form a contiguous super-unit of granite and granodiorite compositions. The Old- Woman Piute Range batholith includes the granite of Sweetwater Wash in the Painted Rock pluton and the Old Woman Mountains Granodiorite forming the Old Woman pluton. The large Cadiz Valley batholith is divided into the Iron Mountains Intrusive Suite and the Coxcomb Intrusive Suite. The Iron Mountains Intrusive Suite includes the Granite Pass Granite (which forms the Granite Pass pluton), the Danby Lake Granite Gneiss, and the Iron Granodiorite Gneiss. The Coxcomb Intrusive Suite consists of many units including the Clarks Pass Granodiorite, the Sheep Hole Mountains Granodiorite (forms the Sheep Hole Mountains pluton), and the Sheep Hole Pass Granite (forms the Sheep Hole Pass pluton). The Cretaceous rocks were emplaced at a range of deep to shallow depths, and their intrusion resulted in an aureole 2-3 km wide in older rocks. Mylonitic fabrics developed through a thickness of >1.3 km, together

Early Carboniferous adakite-like and I-type granites in central Qiangtang, northern Tibet: Implications for intra-oceanic subduction and back-arc basin formation within the Paleo-Tethys Ocean

NASA Astrophysics Data System (ADS)

Liu, Jin-Heng; Xie, Chao-Ming; Li, Cai; Wang, Ming; Wu, Hao; Li, Xing-Kui; Liu, Yi-Ming; Zhang, Tian-Yu

2018-01-01

Recent studies have proposed that the Late Devonian ophiolites in the central Qiangtang region of northern Tibet were formed in an oceanic back-arc basin setting, which has led to controversy over the subduction setting of the Longmucuo-Shuanghu-Lancangjiang Suture Zone (LSLSZ) during the Late Devonian to Early Carboniferous. In this paper we present new data about a suite of granite plutons that intrude into ophiolite in central Qiangtang. Our aim was to identify the type of subduction and to clarify the existence of an intra-oceanic back-arc basin in the LSLSZ during the Late Devonian to Early Carboniferous. The suite of granites consists of monzogranites, syenogranites, and granodiorites. Our laser ablation-inductively coupled plasma-mass spectrometry zircon U-Pb data yielded Early Carboniferous crystallization ages of 357.2 Ma, 357.4 Ma and 351.1 Ma. We subsequently investigated the petrogenesis and tectonic setting of these granites based on their geochemical and Hf isotopic characteristics. First, we divided the granites into high Sr/Y (HSG) and low Sr/Y granites (LSG). The HSG group contains monzogranites and granodiorites that have similar geochemical characteristics to adakites (i.e., high Sr/Y and La/Yb ratios, low MgO, Y, and Yb contents, and no pronounced negative Eu anomaly), although they have slightly lower Sr and Al2O3 contents, caused by crystal fractionation during late magmatic evolution. Therefore, we define the HSG group as adakite-like granites. The study of the HSG shows that they are adakite-like granites formed by partial melting of oceanic crust and experience fractional crystallization process during late evolution. However, some differences between the monzogranites and granodiorites indicate that there are varying degree contributions of subducted sediments during diagenesis. The LSG group contains syenogranites that have distinct negative correlations between their P2O5 and SiO2 contents, and Y and Th contents have significant positive correlations with Rb. The above characteristics indicate that the syenogranites are typical I-type granites. The results of this study also show that the LSG were produced by magma mixing between the mantle and juvenile oceanic crust. The field study found that the Early Carboniferous suite of granites intruded into contemporaneous ophiolites that formed in an intra-oceanic back-arc basin, and were associated with coeval A-type granites in this region. Based on the geochemical and isotopic data presented in this paper and regional geological data, we consider that the HSG were formed during intra-oceanic subduction of the Paleo-Tethys Ocean in the Early Carboniferous. The LSG and A-type granites were formed in an intra-oceanic back-arc basin setting caused by roll-back of the Paleo-Tethys Ocean slab. This confirms that the subduction of the Paleo-Tethys Ocean in the Early Carboniferous was intra-oceanic subduction, and provides important evidence for the existence of an intra-oceanic back-arc basin during the Late Devonian to Early Carboniferous.

Lithium and boron in late-orogenic granites - Isotopic fingerprints for the source of crustal melts?

NASA Astrophysics Data System (ADS)

Romer, Rolf L.; Meixner, Anette; Förster, Hans-Jürgen

2014-04-01

Geochemically diverse late- and post-Variscan granites of the Erzgebirge-Vogtland, the Saxon Granulite Massif, and Thuringia (Germany) formed by anatectic melting of Palaeozoic sedimentary successions and associated mafic to felsic volcanic rocks. The compositional diversity of the least evolved of these granites is largely inherited from the protoliths. We present Li and B-isotopic data of these granites and compare them with the isotopic composition of their protoliths, to investigate whether (i) there exist systematic differences in the Li and B-isotopic composition among different granite types and (ii) Li and B-isotopic compositions provide information on the granite sources complementary to information from the isotopic composition of Sr, Nd, and Pb and the trace-element signatures. Low-F biotite and two-mica granite types have flat upper continental crust (UCC)-normalized trace-element pattern with variable enrichments in Li, Rb, Cs, Sn, and W and depletions in Sr, Ba, and Eu. These signatures are least pronounced for the Niederbobritzsch biotite granite, which has the largest contribution of mafic material, and most pronounced for the two-mica granites. The granites show a relatively narrow range of δ7Li values (-3.0 to -0.5) and a broad range of δ11B values (-13.4 to +20.1). The δ11B values are lower in rocks with distinctly higher contents of Li, Rb, Cs, and Sn. The high δ11B of the Niederbobritzsch granite may be explained by the melting of former altered oceanic crust in its source. Relative to UCC, intermediate-F to high-F low-P granites show strong depletions in Sr, Ba, Eu as well as Zr and Hf, strong enrichments in Li, Rb, and Cs as well as Nb, Sn, Ta, and W, and REE pattern with stronger enrichments for HREE than for LREE. These granites show narrow ranges of δ7Li (-2.0 to +1.6) and δ11B values (-14.7 to -9.1), reflecting the smaller variability of the Li and B-isotopic composition in their source lithologies. The anomalously high δ7Li value (14.7) of one granite sample (Burgberg), which is similar to δ7Li values of its wall rocks (up to 14.5), may indicate late-magmatic fluid-rock interaction with external, wall rock-derived fluids. Because of the small compositional range of most source lithologies, the Li and B-isotopic variation in the granites is also small indicating that the isotopic composition of Li and B does not represent a particularly sensitive source tracer, with the exception of source lithologies characterized by extreme δ7Li or δ11B values.

Th-REE- and Nb-Ta-accessory minerals in post-collisional Ediacaran felsic rocks from the Katerina Ring Complex (S. Sinai, Egypt): An assessment for the fractionation of Y/Nb, Th/Nb, La/Nb and Ce/Pb in highly evolved A-type granites

NASA Astrophysics Data System (ADS)

Moreno, J. A.; Molina, J. F.; Bea, F.; Abu Anbar, M.; Montero, P.

2016-08-01

The relationships of Y/Nb, Th/Nb, La/Nb and Ce/Pb ratios in A-type felsic rocks from the Ediacaran Katerina Ring Complex, northernmost Arabian-Nubian Shield (ANS; S. Sinai, Egypt), are investigated in this work to understand their behavior during generation of highly evolved granitic magmas and to explore the nature of magma sources. Textural and compositional relationships of cognate Th-REE- and Nb-Ta-accessory minerals in Katerina felsic rocks show that chevkinite-group minerals (CGM), monazite, thorite, allanite and xenotime formed from residual liquids in quartz syenite porphyries, quartz monzonites and peralkaline granites, whereas in aluminous granites, allanite and monazite crystallized early, and thorite and columbite formed from residual liquids. Relationships of Y/Nb, Th/Nb, La/Nb and Ce/Pb ratios with Zr/Hf ratios in the aluminous granites and with Be abundances in the peralkaline granites suggest a decrease in La/Nb and Ce/Pb ratios in the former, and in Y/Nb and La/Nb ratios in the latter with crystallization progress. This contrasts with absence of systematic variations of Th/Nb and Ce/Pb ratios in the peralkaline compositions and of Y/Nb ratio in the aluminous ones. In this latter, Th/Nb ratio can present a significant decrease only in highly evolved compositions. An analysis of Y/Nb, Th/Nb, La/Nb and Ce/Pb relationships in worldwide OIB and subduction-related magmatic suites reveals that A-type felsic rocks with (Th/Nb)N < 1.3, (La/Nb)N < 1.3, and (Ce/Pb)N > 1 may have A1-type affinity, and those with (Th/Nb)N > 2, (La/Nb)N > 2, and (Ce/Pb)N < 1 tend to present A2-type affinity. The crystal fractionation of Th-LREE- and Nb-Ta-accessory minerals and mixing of components derived from the two granite groups may cause deviations from these compositional limits that can be evaluated using constraints imposed by Th/Nb-La/Nb, Ce/Pb-Th/Nb and Ce/Pb-La/Nb relationships in OIB and subduction-related magmatic suites. Three mantle sources might have been involved in the Ediacaran alkaline magmatism from northernmost ANS that are chemically similar to those of (1) OIB suites, (2) subduction-related magmatic suites, and (3) carbonatite-metasomatized peridotites. In addition, compositions characterized by (Y/Nb)N > 0.18, (Th/Nb)N > 2, (La/Nb)N > 2, (Ce/Pb)N < 1, and crustal-like Nb/Ta ratios may point to a significant contribution of crustal sources.

Zircon (U-Th)/He evidence for pre-Eocene orogenic exhumation of eastern North Pyrenean massifs, France

NASA Astrophysics Data System (ADS)

Ternois, Sébastien; Vacherat, Arnaud; Pik, Raphaël; Ford, Mary; Tibari, Bouchaïb

2017-04-01

Orogens and their associated foreland basins are considered as part of a single dynamic system evolving from an early, non equilibrated, growth stage to a late, mature, steady-state stage. Most of our understanding in foreland basins, in particular early convergence-stage deposition, comes from the subducting plate, so that the classic paradigm for foreland basins is the pro-wedge. Models that clearly depict the relationship between erosion of the orogenic wedge and sedimentation into its associated foreland basin only focus on the late post-orogenic phase. Relatively little is known and understood about the very long phase of initiation of orogenesis. In the doubly wedged Pyrenean orogen, where we know and understand relatively little about how the early retro-wedge developed, the record of the onset of orogenic denudation from massifs is quite limited, not only in time but also in space. As part of the OROGEN project funded by TOTAL and the BRGM, this study presents first single-grain zircon (U-Th)/He data from two Palaeozoic massifs of the external Northern Pyrenean Zone, the Agly and Salvezines massifs. It aims at constraining the exhumation history of eastern Pyrenean massifs and understanding what is their significance for early orogenic wedge growth. The Pyrenean orogeny was generated from end Santonian (84 Ma) to Oligocene-Miocene due to convergence of the Iberian and European plates. Aquitaine foreland basin history (Ariège region) indicates that convergence took place in two phases, Campanian to Maastrichian and Eocene, separated by a quiet Paleocene phase. Yet, only Eocene cooling events are recorded by low-temperature thermochronometers in the central Pyrenean massifs (Arize and Trois-Seigneurs). Nine bedrock samples were collected along a WNW-ESE traverse (Salvezines and Saint-Arnac granites, Belesta-Caramany gneisses) and analysed for ZHe dating. Zircon (U-Th)/He data for the Agly and Salvezines massifs, together with forward modelling of data for two end-members, Late Cretaceous (Campanian, 75 Ma) and Eocene (50 Ma), show that the easternmost external basement massifs record a first phase of cooling from ˜200°C, between ca. 75 Ma and ca. 60 Ma, which we correlate with the early Pyrenean orogenic phase. Our data provide important new constraints on the timing of early Pyrenean exhumation and temperatures associated with pre-orogenic HT-LP metamorphism but have limited resolution in quantifying the amount of exhumation and shortening at the onset of the convergence in this region. These results are integrated into a tectonic reconstruction of the eastern Pyrenees from an early Cretaceous extensional template to present day

Petrochemical evolution of the White Mfolozi Granite pluton: Evidence for a late Palaeoarchaean A-type granite from the SE Kaapvaal Craton, South Africa

NASA Astrophysics Data System (ADS)

Misra, Saumitra; Reinhardt, Jürgen; Wilson, Allan H.

2017-08-01

One of the major limitations in understanding the geochemical evolution of the Kaapvaal Craton, South Africa, is the scarcity of whole rock trace element data of the granitoid and other rocks compared to the vastness of this cratonic block. Here we present new XRF major oxide and ICP-MS trace element analyses of the White Mfolozi Granitoid (WMG) pluton, SE Kaapvaal Craton, which suggest that the 3.25 Ga (U-Pb zircon age) old WMG pluton is a peraluminous A-type granite and could be equivalent to the intrusive potassic granite phase of the Anhalt Granitoid suite, occurring to the North of the WMG pluton. The pluton was generated by batch partial melting of a pre-existing TTG source in two major phases under relatively anhydrous conditions, and the heat of partial melting could have been provided by a voluminous mantle-derived mafic magma, which intruded into mid-crustal levels (c. 17 km), perhaps during a period of crustal extension. The estimated pressure and temperature of generation of the WMG parent magma with average molar [or/(or + ab)] 0.48 could be 500 MPa and close to 1000 °C, respectively, when compared with the results of experimental petrology. Interstitial occurrence of relatively iron-rich biotite [Mg/(Mg + Fe) 0.41-0.45] suggests that the final temperature of crystallization of the pluton was close to 800 °C. An important magmatic event following the main phase of partial melting was limited mixing between the intrusive mafic magma and co-existing newly generated granitic melt. This magma mixing resulted in distinct variations in SiO2 and a low initial Sr isotopic ratio (0.7013) of the WMG pluton. Although both the models of partial melting of quartzo-feldspathic sources and fractional crystallization of basaltic magmas with or without crustal assimilation have been proposed for the origin of A-type granites, the model of magmatic evolution of the WMG pluton presented here can also be an alternative model for the generation of A-type granites. In this model, post-partial melting magma mixing is perhaps critical in explaining the Daly gap in composition and extreme variations in chemical (e.g., SiO2) and isotopic compositions observed in many bimodal A-type granite suites. The emplacement of the oldest known A-type granitoid suite in the Kaapvaal Craton, the WMG pluton, marks a period of stabilization of the craton before erosion and deposition of the overlying volcano-sedimentary succession of the Pongola Supergroup.

Magmatic Enclaves in Granitic Rocks: Paragons or Parasites?

NASA Astrophysics Data System (ADS)

Clemens, John; Stevens, Gary; Elburg, Marlina

2017-04-01

Granitic rocks form the fundamental building blocks of Earth's continents and provide us with a wide range of resources, so their formation is worth trying to understand. Fine-grained, igneous-textured microgranular enclaves of tonalitic to monzogranitic composition (ME) are common in granitic rocks and their origins have been hotly debated, with some workers suggesting that ME are not igneous. These ME have been studied intensively enough that we are now certain that they are of igneous origin - globules of mingled and quenched magma. Although a mantle connection is evident in many cases, their ultimate origin (including where in the lithosphere they originate) is still debated. This contribution explores the systematics of chemical variation in ME and their host granites, with the aim of uncovering any systematics in their behaviour and modelling the processes that have led to the variations that we measure, comparing host-rock series to their respective ME series. As always, the hope is that the study of ME may lead to improved understanding and modelling of the processes that are responsible for the formation of the host granitic magmas. Using variations between the molecular quantities Ti and M (Fe+Mn+Mg), we demonstrate that the petrogenetic processes that operated within a diverse group of S- and I-type granitic host magmas and their ME suites are dissimilar. Variations within the granitic series result from a variety of what might be called 'orderly' processes, resulting in linear or curvilinear trends in chemical variation diagrams. In contrast, processes that affected the ME series commonly resulted in scattered, chaotic variations. Even in cases in which an ME series displays more orderly variation, it can be shown that the hypothesis of simple mixing between a parent enclave magma and its host granitic magma, to produce the overall variations, cannot be supported. ME magmas had vastly smaller volumes compared with their host granitic magmas. Thus, they have commonly undergone hybridisation through mixing with deep crustal melts and both chemical and mechanical interactions with wall rocks and their host granitic magmas. As a result of this complex and chaotic set of processes, it remains extremely difficult to unravel the precise mechanisms that produced a given suite of ME magmas. Due to the similarities between the studied granites and their ME with occurrences worldwide, we suggest that our findings are likely to be generally applicable.

The petrogenesis of sodic granites in the Niujuanzi area and constraints on the Paleozoic tectonic evolution of the Beishan region, NW China

NASA Astrophysics Data System (ADS)

Yu, Jiyuan; Guo, Lin; Li, Jianxing; Li, Yanguang; Smithies, Robert H.; Wingate, Michael T. D.; Meng, Yong; Chen, Shefa

2016-07-01

Ordovician to Devonian sodic granites dominate the newly recognized Luotuojuan composite granite in the Lebaquan-Luotuojuan-Niujuanzi region of Beishan, along the southern margin of the Central Asian Orogenic Belt in NW China. The granites include sodic (K2O/Na2O > 0.5) tonalites with low Y (< 7 ppm), Yb (< 0.7 ppm), high Sr/Y (> 68) that formed during at least two events at c. 435 and c. 370-360 Ma. Their compositions are consistent with high-pressure melting of basaltic crust, although relatively non-radiogenic Nd isotope compositions (εNd(t) + 0.9) require some crustal assimilation. The interpretation that these granites reflect melts of a subducted slab (i.e. adakite) is supported by independent local and regional geological evidence for an oceanic subduction-accretion setting, including a long history of calc-alkaline magmatism and the identification of a series of early Paleozoic ophiolite belts. Other sodic granites forming the Luotuojuan composite granite are mainly quartz-diorite and granodiorite formed between c. 391 and c. 360 Ma. These rocks are not adakites, having Sr concentrations and Sr/Y ratios too low and Y and Yb concentrations too high. They are low- to medium-K calc-alkaline rocks more typical of magmas derived through melting in a subduction modified mantle wedge. Compositional changes from sodic to potassic granites, over time frames consistent with subduction processes, suggest at least two separate cycles, or pulses, of hot subduction in the Lebaquan-Luotuojuan-Niujuanzi region. Although early Paleozoic adakites have been inferred to exist elsewhere in the Beishan region, many of the reported adakitic rocks have compositions inconsistent with melting of subducted oceanic lithosphere and so tectonic interpretation of hot subduction might not be valid in these cases. A study of regional granite data also shows not only that adakite magmatism does not extend into the Permian but that if subduction-accretion processes extended into the late Paleozoic, no typical subduction-related magmatism was preserved. New and published Nd isotope data from regional granites also requires at least the local presence of Proterozoic basement, or microcontinental slivers, in the evolution of the Beishan region.

Gamma-ray spectrometry of granitic suites of the Paranaguá Terrane, Southern Brazil

NASA Astrophysics Data System (ADS)

Weihermann, Jessica Derkacz; Ferreira, Francisco José Fonseca; Cury, Leonardo Fadel; da Silveira, Claudinei Taborda

2016-09-01

The Paranaguá Terrane, located in the coastal portion of the states of Santa Catarina, Paraná and São Paulo in Southern Brazil is a crustal segment constituted mainly by an igneous complex, with a variety of granitic rocks inserted into the Serra do Mar ridge. The average altitude is approximately 1200 m above sea level, with peaks of up to 1800 m. Due to the difficulty of accessing the area, a shortage of outcrops and the thick weathering mantle, this terrane is understudied. This research aims to evaluate the gamma-ray spectrometry data of the granitic suites of the Paranaguá Terrane, in correspondence with the geological, petrographical, lithogeochemical, relief and mass movement information available in the literature. Aerogeophysical data were acquired along north-south lines spaced at 500 m, with a mean terrain clearance of 100 m. These data cover potassium (K, %), equivalent in thorium (eTh, ppm) and equivalent in uranium (eU, ppm). After performing a critical analysis of the data, basic (K, eU, eTh) and ternary (R-K/G-eTh/B-eU) maps were generated and then superimposed on the digital elevation model (DEM). The investigation of the radionuclide mobility across the relief and weathering mantle consisted of an analysis of the schematic profiles of elevation related with each radionuclide; a comparison of the K, eU and eTh maps with their 3D correspondents; and the study of mass movements registered in the region. A statistical comparison of lithogeochemical (K, U, Th) and geophysical (K, eU, eTh) data showed consistency in all the granitic suites studied (Morro Inglês, Rio do Poço and Canavieiras-Estrela). Through gamma-ray spectrometry, it was possible to establish relationships between scars (from mass movements) and the gamma-ray responses as well as the radionuclide mobility and the relief and to map the granitic bodies.

Mineral inclusions and SHRIMP U-Pb dating of zircons from the Alamas nephrite and granodiorite: Implications for the genesis of a magnesian skarn deposit

NASA Astrophysics Data System (ADS)

Liu, Yan; Zhang, Rongqing; Zhang, Zhiyu; Shi, Guanghai; Zhang, Qichao; Abuduwayiti, Maituohuti; Liu, Jianhui

2015-01-01

Extending approximately 1300 km and located in the Western Kunlun Mountains, the Hetian nephrite belt is the largest nephrite belt in the world and contains approximately 11 major deposits and more than 20 orebodies including the Alamas deposit. Hetian nephrite deposits can be classified as Mg-skarn deposits with Precambrian dolomitic marble host rock and green, green-white and white nephrite zones are distributed gradually in the zone of a granodiorite pluton. The green nephrite is mainly predominately composed of tremolite with generally minor to trace constituents of diopside, grossularitic garnet, actinolite and other minerals. Also green nephrite has higher content of TFe2O3, than green-white and white nephrites have. We subdivided the zircons from the green nephrites into four types, depending on their internal textures, mineral inclusions, and SHRIMP U-Pb ages. Type I zircons are round instead of idiomorphic in shape and lack obvious zoning. Type II and IV zircons have broad, clear oscillatory zoning and are hypidiomorphic or idiomorphic in shape; they contain inclusions of diopside, tremolite, chlorite and calcite. Most Type III zircons are narrow rims (< 10 μm) surrounding Type II and Type I zircons with highly luminous brightness and no zoning. Both Type I and Type II zircons have individual ages of 411 to 445 Ma and Type IV zircons have younger ages (388 to 406 Ma). Among the concordant ages, 425.7 ± 5.8 Ma and 420.0 ± 9.9 Ma for the QYZr1 and QYZr2 are consistent within error, with the 418.5 ± 2.8 Ma of the Alamas granodiorite formation age and the maximum age of the Alamas nephrite deposit. The partially recrystallization of zircons during skarn formation possibly lead to some younger individual ages (406.5 to 308 Ma). In the Western Kunlun Mountain, both Buya granite and Alamas grandiorite are high Ba-Sr granites and crystallized in Western Kunlun Orogen. The Buya granite formed at about 430 Ma in a post-orogenic tectonic environment. Considering Alamas granodiorite formed at about 12 Ma younger than that of Buya granite and it is convincible that Alamas granodiorite also formed at a post-orogenic tectonic environment. Together with the evolution of Western Kunlun Mountain, it is also possible that high Ba-Sr Alamas granodiorite and the nephrite deposit formed in the post-orogenic stage. Most zircons in the Alamas granodiorite and green nephrite have high Th/U ratios (> 0.1), similar REE and trace element patterns, a Ce anomaly (Ce/Ce* > 5), and ΣREE contents of 454 to 922 ppm and 102 to 3182 ppm with averages of 627 ppm and 855 ppm, respectively. The similar geochemical signatures, morphologies, and ages indicate that most zircons (or fragments of zircon) in the nephrite came from the granodiorite and some experience partially recrystallized during skarnization. This is consistent with the field observation that original granodiorite-dolomitic marble boundary is now represented within a nephrite sequence, with the green nephrite close to the granodiorite and the white/white-green nephrites adjoining the dolomitic marble. Typical skarn deposits experience prograde and retrograde metasomatism stages. According to the field observations and petrographic studies, both prograde metasomatism and the early retrograde altered stages are two main stages for the formation of Alamas nephrite deposits. The replacements of coarse-grained tremolite by fine-grained tremolite (nephrite) lead to the formation of nephrite. Based on petrographic studies, the main formation processes of the nephrite are 1) diopside ← dolomite; 2) tremolite (nephrite) ← diopside; and 3) chlorite ← tremolite (nephrite). Thus, the timing of the formation of nephrite is later than that of Mg-skarn.

An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: From subduction magmatism to cratonization

DOE Office of Scientific and Technical Information (OSTI.GOV)

Johnson, Simon P.; Korhonen, Fawna J.; Kirkland, Christopher L.

The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as ‘cratonization’, is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons frommore » several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons.« less

Origin and age of the Eisenkappel gabbro to granite suite (Carinthia, SE Austrian Alps)

PubMed Central

Miller, C.; Thöni, M.; Goessler, W.; Tessadri, R.

2011-01-01

The northern part of the Karawanken plutonic belt is a gabbro–granite complex located just north of the Periadriatic lineament near the Slovenian–Austrian border. Petrographic and geochemical studies of the Eisenkappel intrusive complex indicate that this multiphase plutonic suite developed by a combination of crystal accumulation, fractional crystallization and assimilation processes, magma mixing and mingling. The mafic rocks are alkaline and have within-plate geochemical characteristics, indicating anorogenic magmatism in an extensional setting and derivation from an enriched mantle source. The mafic melts triggered partial melting of the crust and the formation of granite. The granitic rocks are alkalic, metaluminous and have the high Fe/Fe + Mg characteristics of within-plate plutons. Temperature and pressure conditions, derived from amphibole-plagioclase and different amphibole thermobarometers, suggest that the analysed Eisenkappel gabbros crystallized at around 1000 ± 20 °C and 380–470 MPa, whereas the granitic rock crystallized at T ≤ 800 ± 20 °C and ≤ 350 MPa. Mineral-whole rock Sm–Nd analyses of two cumulate gabbros yielded 249 ± 8.4 Ma and 250 ± 26 Ma (εNd: + 3.6), garnet-whole rock Sm–Nd analyses of two silicic samples yielded well-constrained ages of 238.4 ± 1.9 Ma and 242.1 ± 2.1 Ma (εNd: − 2.6). PMID:26525511

Regional metamorphism at extreme conditions: Implications for orogeny at convergent plate margins

NASA Astrophysics Data System (ADS)

Zheng, Yong-Fei; Chen, Ren-Xu

2017-09-01

Regional metamorphism at extreme conditions refers either to Alpine-type metamorphism at low geothermal gradients of <10 °C/km, or to Buchan-type metamorphism at high geothermal gradients of >30 °C/km. Extreme pressures refer to those above the polymorphic transition of quartz to coesite, so that ultrahigh-pressure (UHP) eclogite-facies metamorphism occurs at mantle depths of >80 km. Extreme temperatures refer to those higher than 900 °C at crustal depths of ≤80 km, so that ultrahigh-temperature (UHT) granulite-facies metamorphism occurs at medium to high pressures. While crustal subduction at the low geothermal gradients results in blueschist-eclogite facies series without arc volcanism, heating of the thinned orogenic lithosphere brings about the high geothermal gradients for amphibolite-granulite facies series with abundant magmatism. Therefore, UHP metamorphic rocks result from cold lithospheric subduction to the mantle depths, whereas UHT metamorphic rocks are produced by hot underplating of the asthenospheric mantle at the crustal depths. Active continental rifting is developed on the thinned lithosphere in response to asthenospheric upwelling, and this tectonism is suggested as a feasible mechanism for regional granulite-facies metamorphism, with the maximum temperature depending on the extent to which the mantle lithosphere is thinned prior to the rifting. While lithospheric compression is associated with subduction metamorphism in accretionary and collisional orogens, the thinned orogenic lithosphere undergoes extension due to the asthenospheric upwelling to result in orogen-parallel rifting metamorphism and magmatism. Thus, the rifting metamorphism provides a complement to the subduction metamorphism and its operation marks the asthenospheric heating of the orogenic lithosphere. Because of the partial melting and melt extraction of the lower continental crust, contemporaneous granite-migmatite-granulite associations may serve as a petrological indicator of rifting orogeny that is superimposed on precedingly accretionary and collisional orogens. The UHT metamorphic rocks have occurred since the Archean, suggesting that the hot underplating has operated very early in the Earth's history. In contrast, the UHP metamorphic rocks primarily occur in the Phanerozoic, indicating that the thermal regime of many subduction zones has changed since the Neoproterozoic for the cold subduction.

The Pikes Peak batholith, Colorado front range, and a model for the origin of the gabbro-anorthosite-syenite-potassic granite suite

USGS Publications Warehouse

Barker, F.; Wones, D.R.; Sharp, W.N.; Desborough, G.A.

1975-01-01

This study of the Pikes Peak batholith includes the mineralogy and petrology of quartz syenite at West Creek and of fayalite-bearing and fayalite-free biotite granite near Mount Rosa; major element chemistry of the batholith; comparisons with similar postorogenic, intracratonic, sodic to potassic intrusives; and genesis of the batholith. The batholith is elongate in plan, 50 by 100 km, composite, and generally subalkalic. It was emplaced at shallow depth 1,040 m. y. ago, sharply transects its walls and may have breached its roof. Biotite granite and biotite-hornblende granite are predominant; quartz syenite, fayalite granite and riebeckite granite are present in minor amounts. Fayalite-bearing and fayalite-free quartz syenite, fayalite-biotite granite and riebeckite granite show a well-defined sodic differentiation trend; the less sodic fayalite-free granites exhibit a broader compositional range and no sharp trends. Crystallization was largely at PH2O < Ptotal; PH2O approached Ptotal only at late stages. Aplite residual to fayalite-free biotite granite in the north formed at about 1,500 bars, or 5 km depth. Feldspar assemblages indicate late stages of crystallization at about 720??C. In the south ilmenite and manganian fayalite indicate fO2 of 10-17 or 10-18 bars. Biotite and fayalite compositions and the 'granite minimum' imply completion of crystallization at about 700??C and 1,500 bars. Nearby fayalite-free biotite granite crystallized at higher water fugacity. All types of syenite and granite contain 5-6% K2O through a range of SiO2 of 63-76%. Average Na2O percentages in quartz syenite are 6.2, fayalite granite 4.2, and fayalite-free granite 3.3 MgO contents are low, 0.03-0.4%; FeO averages 1.9-2.5%. FeO/Fe2O3 ratios are high. Fluorine ranges from 0.3 to 0.6%. The Pikes Peak intrusives are similar in mode of emplacement, composition, and probably genesis to rapakivi intrusives of Finland, the Younger Granites of Nigeria, Cape Ann Granite and Beverly Syenite, Mass., and syenite of Kungnat, Greenland, among others - allowing for different levels of erosion. A suite that includes gabbro or basalt, anorthosite, quartz syenite, fayalite granite, riebeckite granite, and biotite and/or hornblende granites is of worldwide occurrence. A model is proposed in which mantle-derived, convecting alkali olivine basaltic magma first reacts with K2O-poor lower crust of granulite facies to produce magma of quartz syenitic composition. The syenitic liquid in turn reacts with granodioritic to granitic intermediate crust of amphibolite facies to produce the predominant fayalite-free biotite and biotite-hornblende granites of the batholith. This reaction of magma and roof involves both partial melting and the reconstitution and precipitation of refractory phases, as Bowen proposed. Intermediate liquids include MgO-depleted and Na2O-enriched gabbro, which precipitated anorthosite, and alkali diorite. The heat source is the basaltic magma; the heat required for partial melting of the roof is supplied largely by heats of crystallization of phases that settle out of the liquid - mostly olivine, clinopyroxene and plagioclase. ?? 1975.

Riparian Systems and Forest Management—Changes in Harvesting Techniques and their Effects on Decomposed Granitic Soils

Treesearch

John W. Bramhall

1989-01-01

In the 1950s, timber on steep granitic terrain in Trinity County, California was harvested by using the logging techniques of the time. After Trinity Dam was built in the 1960s, it became evident these techniques were not suited to quality riparian habitat and healthy anadromous fisheries. Since adoption of the Z'berg-Nejedly Forest Practice Act in 1973, efforts...

A-type granites and related rocks: Evolution of a concept, problems and prospects

NASA Astrophysics Data System (ADS)

Bonin, Bernard

2007-08-01

Although A-type granites have long been recognized as a distinct group of granites, the term A-type was coined first less than thirty years ago. A-type suites occur in geodynamic contexts ranging from within-plate settings to plate boundaries, locations and times of emplacement are not random. Rare in the lower crust, as some charnockite suites, they are fairly common at shallower depths, especially at the subvolcanic level where they form ring complexes rooting caldera volcanoes. Characteristic features include hypersolvus to transsolvus to subsolvus alkali feldspar textures, iron-rich mafic mineralogy, bulk-rock compositions yielding ferroan, alkali-calcic to alkaline affinities, high LILE+HFSE abundances, and pronounced anomalies due to high degrees of mineral fractionation. Isotopic features evidence sources containing a large mantle input. Experimental data show that A-type magmas contain dissolved OH F-bearing fluids, crystallised under reduced and oxidized conditions, and yield high-temperature liquidus, favouring early crystallisation of anhydrous iron minerals, such as fayalite. Though many petrogenetic models imply solely crustal derivation, no convincing A-type liquids were produced experimentally from crustal materials, nor have any leucosomes of A-type composition been detected within migmatitic terranes. As it occurs in association with mafic igneous rocks in continents as well as on the ocean floor, A-type granite is likely to come from mantle-derived transitional to alkaline mafic to intermediate magmas. Rare felsic materials found in the meteoritic and lunar record yield dominantly A-type features. Contrary to the more common types of granite, A-type granite is, therefore, not typical of Earth and was produced in planetary environments differing from those prevailing on Earth.

The origin of Late Ediacaran post-collisional granites near the Chad Lineament, Saharan Metacraton, South-Central Chad

NASA Astrophysics Data System (ADS)

Shellnutt, J. Gregory; Yeh, Meng-Wan; Lee, Tung-Yi; Iizuka, Yoshiyuki; Pham, Ngoc Ha T.; Yang, Chih-Cheng

2018-04-01

The southern Saharan Metacraton is one of the least geologically constrained regions in the world as bedrock exposures are rare. To the west of Lake Fitri near the communities of Ngoura and Moyto in south-central Chad there are two granitic inliers that form a series of lenticular to ellipsoidal low laying hills. Very little is known about the Lake Fitri inliers and their regional correlation to larger massifs in Chad is undetermined. The granites yielded weighted-mean zircon 206Pb/238U ages of 554 ± 8 Ma and 546 ± 8 Ma indicating they were emplaced 45 million years after the cessation of arc-related magmatism and the subsequent collision between the Congo-São Francisco Craton and the Saharan Metacraton. The rocks have distinct groupings of inherited zircons with ages of 580 Ma and 635 Ma suggesting they are at least in part derived by recycling of older crustal rocks. The biotite mineral chemistry, whole rock compositions and petrological modeling indicate the granites were derived by melting of crustal lithologies but the whole rock Nd isotopes (εNd(t) = +1.3-+2.9) are characteristic of a mantle source. The contrasting inheritance-rich nature of the granites with a juvenile Nd isotopic signature is likely due to mixing between magmas derived from juvenile (Neoproterozoic) arc-related crust and asthenospheric magmas. Asthenospheric upwelling was probably a response to post-orogenic lithospheric delamination related to fault movement along the Chad Lineament, a possible extension of the Tcholliré-Banyo shear zone that extended to the interior of the Saharan Metacraton. The implications are that lithospheric delamination may not have occurred immediately after collision but rather propagated along a narrow belt that extended well into the central regions of the Saharan Metacraton.

Zircon geochronology and Hf-O isotope geochemistry from granites in the Iapetus Suture Zone in Ireland and the Isle of Man

NASA Astrophysics Data System (ADS)

Fritschle, Tobias; Daly, J. Stephen; Whitehouse, Martin J.; McConnell, Brian; Buhre, Stephan

2014-05-01

Late Caledonian syn- to post-orogenic granites located in the Iapetus Suture Zone (ISZ) in Ireland and Britain have been related to A-type subduction and possible slab breakoff [1] following the Laurentia-Avalonian collision. Lack of reliable age data (especially in Ireland) has inhibited petrogenetic investigations of these rocks. Hence, ion microprobe U-Pb and oxygen isotope analyses as well as LA-MC-ICPMS Lu-Hf isotopic measurements on zircons from Irish and Isle of Man granites have been undertaken to provide better constraints on this enigmatic episode of the Caledonian Orogeny. Four stages of Late Caledonian granitic magmatism (c. 435, 417, 410 and 394 Ma) are indicated by U-Pb dating of oscillatory-zoned magmatic zircons. The Crossdoney, Kentstown, Drogheda and Ballynamuddagh granites together with a rhyolite from Glenamaddy have yielded U-Pb concordia ages, interpreted as intrusion-ages, between 419.9 ± 4.3 Ma (Glenamaddy) and 415.8 ± 2.0 Ma (Crossdoney) with a weighted average of 417.5 ± 0.9 Ma (MSWD = 1.3). The Glenamaddy Granite - which intruded the Glenamaddy Rhyolite - yielded an age of 410 ± 2.1 Ma. In addition, the Rockabill Granite yielded a younger age of 393.9 ± 1.9 Ma, whereas the Carnsore Granite yielded an older age of 434.6 ± 1.9 Ma. Inherited zircons (487 to 453 Ma) occur in several of the granites, and are interpreted to have been derived from Ordovician arc magmatic rocks accreted within the ISZ. A younger group of c. 440 Ma inherited zircons occurs in the c. 417 Ma Crossdoney and Ballynamuddagh granites. These grains could be related to continued or renewed Silurian arc magmatism. Hf-O isotopic measurements on the dated zircon grains range between -2 and +7 ɛHfi units and 5.5 to 8.5 o δ18O. These are interpreted to indicate the contribution of juvenile mantle melts - possibly derived from the Ordovician arc - to some of the granites. Significant heterogeneities in zircon oxygen isotopes in at least four of the granites further suggest the involvement of isotopically distinct protoliths. The Dhoon and Foxdale granites in the Isle of Man were previously regarded to be of Late Caledonian age. Surprisingly, zircons from these granites yielded concordant U-Pb ages of 455.6 ± 2.1 Ma (Dhoon) and 455.9 ± 2.1 Ma (Foxdale), respectively. Inherited cores are of Meso- and Palaeoproterozoic age. Hafnium isotopic analyses of the dated magmatic zircon rims from both intrusions are slightly more radiogenic than those from the Late Caledonian granites. Their δ18O values range between 5.5 to 7.5 o. The Isle of Man granites are now suggested to be the plutonic equivalents of a Late Ordovician (Caradocian) volcanic arc generated in the Iapetus Ocean. [1] Atherton & Ghani (2002), Lithos 62, 65-85.

Metamorphozed Hercynian granitoids in the Alpine structures of the Central Rhodope, Bulgaria: geotectonic position and geochemistry

NASA Astrophysics Data System (ADS)

Cherneva, Zlatka; Georgieva, Milena

2005-05-01

Orthogneisses of late-Hercynian protolith age crop out in the Central Rhodope high-grade metamorphic complex, which is part of the Alpine orogen in south-eastern Europe. They compose a tectonic unit bordered by late-Alpine extensional shear zones. These rocks reflect Eocene amphibolite facies migmatization (<750 °C/0.9-0.5 GPa). The low-temperature melting favored zircon inheritance and disturbed mainly the LILE protolith compositions. Despite the intense Alpine metamorphic overprint, the major elements, HFSE and REE reflect the initial composition of the Hercynian protolith. A geochemical data set summarizing 200 whole rock analyses testifies to a calc-alkaline magma differentiation producing a compositional range of tonalite and/or granodiorite to granite and leucocratic granite. Geochemical compositions combined with published isotope and age data suggest dominant I-type protoliths and mixed magma sources including crustal and mantle material, and distinguish between older granitoids of volcanic-arc affinity and probably younger ones of late or post-collision origin.

S-type granite generation and emplacement during a regional switch from extensional to contractional deformation (Central Iberian Zone, Iberian autochthonous domain, Variscan Orogeny)

NASA Astrophysics Data System (ADS)

Pereira, M. F.; Díez Fernández, R.; Gama, C.; Hofmann, M.; Gärtner, A.; Linnemann, U.

2018-01-01

Zircon grains extracted from S-type granites of the Mêda-Escalhão-Penedono Massif (Central Iberian Zone, Variscan Orogen) constrain the timing of emplacement and provide information about potential magma sources. Simple and composite zircon grains from three samples of S-type granite were analyzed by LA-ICP-MS. New U-Pb data indicate that granites crystallized in the Bashkirian (318.7 ± 4.8 Ma) overlapping the proposed age range of ca. 321-317 Ma of the nearby S-type granitic rocks of the Carrazeda de Anciães, Lamego and Ucanha-Vilar massifs. The timing of emplacement of such S-type granites seems to coincide with the waning stages of activity of a D2 extensional shear zone (i.e. Pinhel shear zone) developed in metamorphic conditions that reached partial melting and anatexis (ca. 321-317 Ma). Dykes of two-mica granites (resembling diatexite migmatite) are concordant and discordant to the compositional layering and S2 (main) foliation of the high-grade metamorphic rocks of the Pinhel shear zone. Much of the planar fabric in these dykes was formed during magmatic crystallization and subsequent solid-state deformation. Field relationships suggest contemporaneity between the ca. 319-317 Ma old magmatism of the study area and the switch from late D2 extensional deformation to early D3 contractional deformation. Inherited zircon cores are well preserved in these late D2-early D3 S-type granite plutons. U-Pb ages of inherited zircon cores range from ca. 2576 to ca. 421 Ma. The spectra of inherited cores overlap closely the range of detrital and magmatic zircon grains displayed by the Ediacaran to Silurian metasedimentary and metaigneous rocks of the Iberian autochthonous and parautochthonous domains. This is evidence of a genetic relationship between S-type granites and the host metamorphic rocks. There is no substantial evidence for the addition of mantle-derived material in the genesis of these late D2-early D3 S-type granitic rocks. The ɛNd arrays of heterogeneous crustal anatectic melts may be just inherited from the source, probably reflecting mixing of a range of crustal materials with different ages and primary isotopic signatures. The generation of the Bashkirian S-type granites has been dominated by continental crust recycling, rather than the addition of new material from mantle sources.

From Compression to Extension: Cretaceous A-type Granite as Indicator of Geodynamic Changes in the Adria Part of the European Neotethys Suture Zone

NASA Astrophysics Data System (ADS)

Balen, D.; Schneider, P.; Massonne, H. J.; Opitz, J.; Petrinec, Z.

2017-12-01

The Cretaceous suture zone between the colliding plates of European and Adria (Gondwana) marks the closure of the W Neotethys branch. This zone, partly located in the northern Croatia, comprises reddish alkali granite which is mainly composed of alkali feldspar and quartz, with small amounts of albite, white mica and hematite with ilmenite exsolutions. Accessory minerals include zircon, apatite and Fe-(Ti)-oxides. This granite shows a geochemical signature typical for A2-type granite characterized by a highly siliceous composition and an enrichment in alkalies (high-K calc-alkaline series) and Al (strongly peraluminous, ASI>1.1). The rock belongs to the group of oxidized and ferroan granites with low CaO, MgO, MnO and FeO* contents. Characteristic trace element ratios, primitive mantle and OIB normalized spider-diagrams show significant positive anomalies of Rb, Th, U, K, Zr and Pb accompanied with clear negative anomalies of Ba, Nb, Sr, P, Eu and Ti. The negative anomalies suggest fractionation of plagioclase, apatite and Fe-Ti oxide. Based on the geochemical characteristics the magma originated mainly from melting of lower continental crust (granulite facies metasediments) although a mantle contribution cannot be excluded. The melting process could have been triggered by a heat from the upwelling upper mantle as inferred from zircon typology (D and J5 types prevail), as well from the zircon and whole-rock chemistry accompanied with high zircon saturation temperatures (T=860-950°C). Subsequent ascent of granitic magma was localized along the Europe-Adria suture i.e. the Sava Zone segment of the Late Cretaceous collisional zone where granite was emplaced at ca. 20 km depth. The emplacement followed a long period of Mesozoic orogenic compressional activity. Typical for A-type granites, although in our case related to the subduction of the Adria plate underneath the European plate, is their formation in an extensional tectonic regime. Thus, the studied A-type granite indicates the onset of transition from compression to extension at the European margin. This event occurred in the interval between 87.7-85.8 Ma as shown by 207Pb/235U, 206Pb/238U and 208Pb/232Th ratios measured with LA-ICP-MS on zircon. Support by the Croatian Science Foundation (IP-2014-09-9541) is acknowledged.

U-Pb zircon geochronology of the Paleoproterozoic Tagragra de Tata inlier and its Neoproterozoic cover, western Anti-Atlas, Morocco

USGS Publications Warehouse

Walsh, G.J.; Aleinikoff, J.N.; Benziane, F.; Yazidi, A.; Armstrong, T.R.

2002-01-01

New U-Pb zircon data obtained by sensitive high resolution ion microprobe (SHRIMP) from the Tagragra de Tata inlier in the western Anti-Atlas, Morocco establish Paleoproterozoic ages for the basement schists, granites, and metadolerites, and a Neoproterozoic age for an ignimbrite of the Ouarzazate Series in the cover sequence. The age of interbedded felsic metatuff in the metasedimentary and metavolcanic sequence of the basement schists is 2072 ?? 8 Ma. This date represents: (1) the first reliable age from the metasedimentary and metavolcanic sequence; (2) the oldest reliable age for the basement of the Anti-Atlas; (3) the first date on the timing of deposition of the sediments on the northern edge of the Paleoproterozoic West African craton; (4) a lower age limit on deformation during the Eburnean orogeny; and (5) the first date obtained from the non-granitic Paleoproterozoic basement of Morocco. Ages of 2046 ?? 7 Ma (Targant granite) and 2041 ?? 6 Ma (Oudad granite) support earlier interpretations of a Paleoproterozoic Eburnean igneous event in the Anti-Atlas. The granites post-date the Eburnean D1 deformation event in the Paleoproterozoic schist sequence, and place a ???2046 Ma limit on short-lived Eburnean deformation in the area. Cross-cutting metadolerite is 2040 ?? 6 Ma; this is the first date from a metadolerite in the western Anti-Atlas. All of the dolerites in the area post-date emplacement of the two granites and the new age constrains the onset of late- or post-Eburnean extension. Ignimbrite of the Ouarzazate Series, immediately above the Paleoproterozoic basement is 565 ?? 7 Ma. This Neoproterozoic age agrees with ages of similar volcanic rocks elsewhere from the Ouarzazate Series. The date also agrees with the ages of associated hypabyssal intrusions, and marks the second and final stage of Pan-African orogenic activity in the western Anti-Atlas. ?? 2002 Elsevier Science B.V. All rights reserved.

Late Carboniferous to Early Permian magmatic pulses in the Uliastai continental margin linked to slab rollback: Implications for evolution of the Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Chai, Hui; Wang, Qingfei; Tao, Jixiong; Santosh, M.; Ma, Tengfei; Zhao, Rui

2018-05-01

The Paleo Asian Ocean underwent a protracted closure history during Late Paleozoic. Here we investigate the magmatic evolution during this process based on a detailed study in the Baiyinwula region along the Uliastai continental margin. The major rock types in this area are Late Carboniferous-Early Permian volcanic sequences and coeval intrusions. We identified four stages of magmatic evolution based on the diverse assemblages and their precise isotopic ages. The first stage is represented by andesites with a zircon 206Pb/238U age of ca. 326 ± 12 Ma. These rocks are metaluminous to weakly peraluminous, high-K calc-alkaline, and possess high Na2O/K2O ratios in the range of 1.23 to 2.45. They also display enrichment of large ion lithophile elements (LILE) and depletion of high field strength elements (HFSE), with markedly positive zircon εHf (t) varying from 8.1 to 15.6.The geochemical features of these andesites are similar to those of typical arc volcanic rocks. The second stage includes granodiorites emplaced at 318.6 + 1.8 Ma. The rocks are high-K calc-alkaline with A/CNK values ranging from 0.95 to 1.06, and show enrichment in LILE and depletion in HFSE. They show geochemical affinities to adakites, with high Sr and low Y and Yb contents, indicating magma derivation from thickened lower crust. Zircon grains from these rocks display positive initial εHf (t) values ranging from 11.1 to 14.6 with corresponding two-stage Hf model ages (TDM2) of 394-622 Ma. The third stage consists of syenogranite together with a volcanic suite ranging in composition from rhyolite todacite, which formed during 303.4 ± 1.2 to 285.1 ± 2.2 Ma. They possess elevated silica and alkali contents, high FeOt/MgO and Ga/Al ratios, low Al2O3, MgO and CaO contents, and high Rb, Y, Nb, Ce, Zr, Y, and Ga contents, strong negative Ba, Sr and Eu anomalies, showing I- to A-type granitic affinities. Zircons in these rocks show elevated Hf isotopic compositions (εHf (t) = 9.9 to 14.6) with TDM2 varying from 324 to 673 Ma. The fourth magmatic pulse is represented by K-feldspar granite with zircon U-Pb ages from 283.2 ± 1.9 Ma to 280.0 ± 1.4 Ma, and typical alkalic A-type granite geochemistry. These rocks possess positive εHf (t) values in the range of 9.7 to15.2, and a restricted range of Hf model age from 327 to 684 Ma. The magmatic rocks from the four stages show comparable εHf (t) and T2DM, suggesting that the magmas were derived from the same evolving mantle-derived source. We propose a tectonic model linking the evolution of the magmatism with the closure of the Paleo Asian Ocean that involved the following stages. The andesites were formed during the initial oceanic subduction stage with magma sourced from the metasomatized lithospheric mantle. Stage 2 adakite-like rocks were derived from subduction-induced thickened crust. Subsequent slab rollback resulted in asthenospheric upwelling and melting of residual juvenile crust to generate the I- and A- type syenogranite, rhyolite and dacite suite, finally followed by the A-type K-feldspar granite.

Magnetic fabric in granitoid plutons emplaced during Variscan orogeny and timing of their intrusions: Eastern Variscan front

NASA Astrophysics Data System (ADS)

Hrouda, F.; Schulmann, K.; Chlupacova, M.; Aichler, J.; Mixa, P.; Pecina, V.; Zacek, V.; Kroener, A.

2003-04-01

The eastern Variscan front at the Czech and Polish border is characterised by oblique underthrusting of Neo-Proterozoic continental margin below thickened crustal root. The underthrust plate is subsequently imbricated and forms obliquely convergent crustal wedge which was further thrust over the foreland. Several granitic plutons of arc geochemical affinity are intruded during different stages of crustal thickening and exhumation. Analysis of anisotropy of magnetic susceptibility was carried out to study the relationships between host rock deformation and magma emplacement fabrics in different crustal levels and geographical positions with respect to crustal wedge and westerly orogenic root. Deep seated granodiorite sheets (Javornik intrusion 348 Ma, and Stare Mesto sill 340 Ma) are emplaced in the deepest and more internal high grade parts of the orogen along the margin of thickened crustal root. They show AMS fabrics entirely concordant with surrounding high grade gneisses and were emplaced during contractional (transpressive) regime.The Sumperk granodiorite is a more shallow intrusion emplaced in the central part of the crustal wedge. This sheet-like intrusion shows its AMS fabrics conformable to transpressional fabrics of surrounding mylonitised barovian schists and gneisses. The Zulova Pluton 330 Ma, representing the shallowest intrusion, intrudes the most external part of the crustal wedge. It shows the magnetic fabrics virtually perpendicular to compressional structures in the neighbouring areas. In addition, these fabrics are clearly concordant with large-scale detachment zone along which the Devonian meta-sedimentary cover slided to the west. The AMS fabrics of granitoids thus testify the progressive oblique convergence prograding to the east followed by collapse of external part of orogenic wedge. The AMS fabric data allow us to evaluate the mechanical role of arc magmas syntectonically emplaced during oblique convergence and finally during normal shearing perpendicular to the orogen.

Timing, petrogenesis and tectonic setting of the Late Paleozoic gabbro-granodiorite-granite intrusions in the Shalazhashan of northern Alxa: Constraints on the southernmost boundary of the Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Shi, Xingjun; Wang, Tao; Zhang, Lei; Castro, Antonio; Xiao, XuChang; Tong, Ying; Zhang, Jianjun; Guo, Lei; Yang, Qidi

2014-11-01

The Late Paleozoic tectonic setting and location of the southernmost boundary of the Central Asian Orogenic Belt (CAOB) with respect to the Alxa Block or Alxa-North China Craton (ANCC) are debated. This paper presents new geochronological, petrological, geochemical and zircon Hf isotopic data of the Late Paleozoic intrusions from the Shalazhashan in northern Alxa and discusses the tectonic setting and boundary between the CAOB and ANCC. Using zircon U-Pb dating, intrusions can be broadly grouped as Late Carboniferous granodiorites (~ 301 Ma), Middle Permian gabbros (~ 264 Ma) and granites (~ 266 Ma) and Late Permian granodiorites, monzogranites and quartz monzodiorites (254-250 Ma). The Late Carboniferous granodiorites are slightly peraluminous and calcic. The remarkably high zircon Hf isotopes (εHf(t) = + 6-+ 10) and characteristics of high silica adakites suggest that these granodiorites were mainly derived from "hot" basaltic slab-melts of the subducted oceanic crust. The Middle Permian gabbros exhibited typical cumulate textures and were derived from the partial melting of depleted mantle. The Middle Permian granites are slightly peraluminous with high-K calc-alkaline and low εHf(t) values from - 0.9 to + 2.9. These granites were most likely derived from juvenile materials mixed with old crustal materials. The Late Permian granodiorites, monzogranites and quartz monzodiorites are characterized as metaluminous to slightly peraluminous, with variable Peacock alkali-lime index values from calc-alkalic to alkali-calcic. These rocks were mainly derived from juvenile crustal materials, as evidenced by their high εHf(t) values (+ 3.3 to + 8.9). The juvenile sources of the above intrusions in the Shalazhashan are similar to those of the granitoids from the CAOB but distinct from the granitoids within the Alxa Block. These findings suggest that the Shalazhashan Zone belongs to the CAOB rather than the Alxa Block and that its boundary with the Alxa block can be regarded as the southernmost boundary of the CAOB. The recognition of Late Carboniferous typical adakite magmatism in the region provides evidence for the subduction of the oceanic crust of the CAOB. The Middle-Late Permian magmatisms (266-250 Ma) display a bimodal association with high-K calc-alkaline features and are interpreted as forming in a post-collision setting. These studies, by interaction of regional geology, provide new constraints on the tectonic evolution of the southern CAOB during the Late Paleozoic and the location of the southernmost margin of the CAOB.

The Volga-Don orocline stitching Volgo-Sarmatia

NASA Astrophysics Data System (ADS)

Bogdanova, S. V.; Postnikov, A. V.; Bibikova, E. V.

2012-04-01

The crustal segments of Volgo-Uralia and Sarmatia occupy about half of the territory of the East European Craton. They differ from its Fennoscandian part by featuring large Early Archaean blocks and 2.1-2.0 Ga orogenic belts. The Volga-Don belt, which separates Archaean Volgo-Uralia from likewise Archaean eastern Sarmatia (the Oskol-Azov megablock) is one of the intracratonic collisional orogens that stitched together various Sarmatian terranes and Volgo-Uralia during the assembly of megacontinent Volgo-Sarmatia. The Volga-Don orogen is an orocline, NS-trending in the south, but bending and wedging out in the northwest where Sarmatia and Volgo-Uralia were brought into close contact caused by their oblique collision. It extends for more than 600 km and is very wide in the southeast, embracing several tectonic terranes, bounded by strike slip- and thrust faults. There, the Volga-Don orogen comprises the following terranes from the east to the west: (1) The wide South Volga province made up of metasedimentary migmatites and S-type garnet-bearing granitoids of granulite and amphibolite facies having NdTDM isotopic ages between 2.4 and 2.1 Ga. These overlie the Archaean basement of Volgo-Uralia, (2) The Tersa continental- marginal igneous belt, where granitoid intrusions of shoshonitic affinity were emplaced into South Volga metasedimentary rocks and their basement at 2.04 Ga. Their NdTDM model ages vary between 2.6 and 2.1 Ga, (3) The Balashov block consisting of the East Vorontsovka turbiditic rocks metamorphosed in the greenschist- to amphibolite facies of a LP/HT series, and in places migmatized and intruded by 2.02 Ga S-type granites, (4) the East Voronezh block, where accretionary-type turbidites of the West Vorontsovka Group have been penetrated by a number of small mafic-ultramafic and gabbro-dioritic plus granitic intrusions with ages of 2.08-2.05 and 2.06-2.05 Ga, respectively, (5) the 2.1-2.08 Ga Lipetsk-Losevo volcanic arc extending along the continental margin of the Archaean Oskol-Azov (Kursk) block of Sarmatia, and (6) the Oskol-Azov block with tectonic belts of Palaeoproterozoic intensively deformed BIF (banded iron formation) metasediments. Terranes 4, 5 and 6 characterize the East Sarmatian accretionary orogen (Shchipansky et al., 2007) developed shortly before the Volga-Don collision. The Volgo-Uralian terranes (1-3) appear to represent an array of intracratonic basin, active continental margin and mature island arcs. The internal structure of the Volga-Don orogen is bilateral and symmetric, complicated by strike-slip faulting and normal faults mostly related to the formation of the Mesoproterozoic Pachelma aulacogen. Recent seismic reflection profiling revealed typical collisional interfingering of tectonic layers/nappes belonging to the Sarmatian as well as Volgo-Uralian crust, and a mantle reflector dipping beneath Volgo-Uralia (Gusev et al., 2010). On the whole, the deep crustal geometry suggests that the Sarmatia-Volgo-Uralia intersegment suture is situated in the central part of the orocline along the western fault boundary of the Balashov block. Gusev, G.S., Mezhelovsky, N.V. and Fedorchuk, V.P. (Eds.), 2010. Essays for Regional Geology of Russia, 2. GEOKART, GEOS, Moscow, 400 pp. (in Russian). Shchipansky, A.A., Samsonov, A.V., Petrova, A.Y. and Larionova, Y.O., 2007. Geotectonics (Geotektonika), 41(1): 38-62.

Comment on "Timing and nature of the Xinlin-Xiguitu Ocean: constraints from ophiolitic gabbros in the northern Great Xing'an Range, eastern Central Asian Orogenic Belt" by Feng et al. (2016)

NASA Astrophysics Data System (ADS)

Ni, Dong-Hong

2017-09-01

We disagree the transitional supra-subduction zone model of Feng et al. (Int J Earth Sci (Geol Rundsch) 105:491-505, 2016) for the tectonic setting of Jifeng ophiolite suite in NE China. Existence of the komatiite in the Jifeng ophiolite indicates an oceanic plateau environment for this ophiolite suite within the so-called Xinlin-Xiguitu ocean.

Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

USGS Publications Warehouse

Bryant, Bruce; Wooden, J.L.

2008-01-01

In west-central Arizona near the northeast margin of the Basin and Range Province, the Rawhide detachment fault separates Tertiary and older rocks lacking significant effects of Tertiary metamorphism from Precambrian, Paleozoic, and Mesozoic rocks in the Harcuvar metamorphic core complex below. Much of the northern part of the Harcuvar complex in the Buckskin and eastern Harcuvar Mountains is layered granitic gneiss, biotite gneiss, amphibolite, and minor pelitic schist that was probably deformed and metamorphosed in Early Proterozoic time. In the eastern Buckskin Mountains, Early and Middle Proterozoic plutons having U-Pb zircon ages of 1,683?6.4 mega-annum (Ma) and 1,388?2.3 Ma, respectively, intruded the layered gneiss. Small plutons of alkaline gabbro and diorite intruded in Late Jurassic time. A sample of mylonitized diorite from this unit has a U-Pb zircon age of 149?2.8 Ma. In the Early Cretaceous, amphibolite facies regional metamorphism was accompanied by partial melting and formation of migmatite. Zircon from a granitic layer in migmatitic gneiss in the eastern Harcuvar Mountains has a U-Pb age of 110?3.7 Ma. In the Late Cretaceous, sills and plutons of the granite of Tank Pass were emplaced in both the Buckskin and eastern Harcuvar Mountains. In the Buckskin Mountains those intrusions are locally numerous enough to form an injection migmatite. A pluton of this granite crops out over almost half the area of the eastern Harcuvar Mountains. Paleozoic and Mesozoic sedimentary rocks were caught as slices along south-vergent Cretaceous thrusts related to the Maria fold and thrust belt and were metamorphosed beneath a thick sheet of Proterozoic crustal rocks. Inception of volcanism and basin formation in upper-plate rocks indicates that regional extension started at about 26 Ma, in late Oligocene. The Swansea Plutonic Suite, composed of rocks ranging from gabbro to granite, intruded the lower-plate rocks in the Miocene and Oligocene(?). Granite and a gabbro from the suite have a U-Pb zircon age of 21.86?0.60 Ma. Previously published 40Ar/39Ar ages of hornblende suggest that some of the Swansea Suite is Oligocene. The felsic rocks contain numerous inclusions ranging from porphyritic granite to porphyritic granodiorite. A sample from one inclusion has a U-Pb zircon age of 1,409?6.3 Ma. A discordia line for the U-Pb zircon data from the Swansea Plutonic Suite has an upper intercept at 1,408?3.4 Ma. The Swansea Plutonic Suite probably formed by interaction between mantle material and plutonic rocks at least as old as Middle Proterozoic. An irregular layer in the middle crust, which is thickest under and adjacent to the Buckskin Mountains, may be the level where that interaction took place. During extensional deformation these rocks and all the older rocks were displaced southwest from beneath the rocks of the Colorado Plateau transition zone below an area extending 50?80 kilometers northeast of the Buckskin Mountains as far as Bagdad, Arizona, or beyond. At that time the rocks were variably mylonitized, and a northeast-trending lineation formed. Much of the evidence for the complex sequence of structural events preserved in these rocks in the western Harcuvar Mountains has been obliterated in the northern Harcuvar complex by Miocene deformation.

New geological model of the Lagoa Real uraniferous albitites from Bahia (Brazil)

NASA Astrophysics Data System (ADS)

de Oliveira Chaves, Alexandre

2013-09-01

New evidence supported by petrography (including mineral chemistry), lithogeochemistry, U-Pb geochronology by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), and physicochemical study of fluid and melt inclusions by LA-ICP-MS and microthermometry, point to an orogenic setting of Lagoa Real (Bahia-Brazil) involving uraniferous mineralization. Unlike the previous models in which uraniferous albitites represent Na-metasomatised 1.75 Ga anorogenic granitic rocks, it is understood here that they correspond to metamorphosed sodium-rich and quartz-free 1.9 Ga late-orogenic syenitic rocks (Na-metasyenites). These syenitic rocks are rich not only in albite, but also in U-rich titanite (source of uranium). The interpretation of geochemical data points to a petrogenetic connection between alkali-diorite (local amphibolite protolith) and sodic syenite by fractional crystallization through a transalkaline series. This magmatic differentiation occurred either before or during shear processes, which in turn led to albitite and amphibolite formation. The metamorphic reactions, which include intense recrystallization of magmatic minerals, led uraninite to precipitate at 1.87 Ga under Oxidation/Reduction control. A second population of uraninites was also generated by the reactivation of shear zones during the 0.6 Ga Brasiliano Orogeny. The geotectonic implications include the importance of the Orosirian event in the Paramirim Block during paleoproterozoic Săo Francisco Craton edification and the influence of the Brasiliano event in the Paramirim Block during the West-Gondwana assembly processes. The regional microcline-gneiss, whose protolith is a 2.0 Ga syn-collisional potassic granite, represents the albitite host rock. The microcilne-gneiss has no petrogenetic association to the syenite (albitite protolith) in magmatic evolutionary terms.

Petrogenesis of the postcollisional Middle Devonian monzonitic to granitic magmatism of the Sierra de San Luis, Argentina

NASA Astrophysics Data System (ADS)

López de Luchi, Mónica G.; Siegesmund, Siegfried; Wemmer, Klaus; Nolte, Nicole

2017-09-01

Middle Devonian granitoids intruded the Eastern Sierras Pampeanas basement ca. 600 km east of the inferred proto-Pacific margin of Gondwana along which a ca. 390 Ma collisional event developed. In the Sierra de San Luis, voluminous Middle Devonian (393-382 Ma) batholiths are composed of I- to A-type hybrid Monzonite and Granite suites. Shoshonite and subordinated high-K series, stocks, synplutonic dikes and enclaves make up the Monzonite Suite; rocks are metaluminous alkali-calcic magnesian porphyritic or equigranular monzonite, quartz monzonite, monzodiorite and scarce monzogabbro. High-K and subordinated shoshonite series metaluminous to mildly peraluminous magnesian alkali-calcic to calc-alkalic porphyritic or equigranular quartz monzonite, granodiorite, monzogranite and equigranular leucomonzogranites make up the Granite Suite plutons and batholiths. Only a small group of highly evolved granites are ferroan. SiO2 (46-62%), Cr, Ni, V, Sc, LILE, LREE, Th, Zr and variable, Sr/Y, (La/Yb)N and (Tb/Yb)N, smooth Eu/Eu*, moderate Na2O (ca 3.5), and troughs at Nb and Ta for Monzonite Suite rocks suggest an subduction-related enriched lithospheric mantle source. Sm-Nd data (TDM 0.98-1.08 Ga, εNd(380 Ma) 0.66-1.47) and 87Sr/86Sri (0.703520-0.704203) are compatible with an enriched mantle source. The metaluminous porphyritic quartz monzonite-monzogranite and the mildly peraluminous equigranular biotite monzogranites of the Granite Suite are characterized by relatively moderate Al2O3, CaO, and 87Sr/86Sri, high LILE, Cr, variable Sr/Y, (La/Yb)N and Eu/Eu* and low Rb/Sr (< 1.2) suggest a mafic source. The porphyritic monzogranite (TDM 1.20-1.28 Ga, εNd(380Ma) - 3.02 to - 3.3, 87Sr/86Sri 0.706578-0.707027) and the biotite monzogranites (TDM 1.31 Ga, εNd(380Ma) - 3.3, 87Sr/86Sri 0.707782) would share a common source. The equigranular alkali-calcic leucomonzogranites are characterized by Rb/Sr > 1.5, ASI 1.05-1.18, and Ga/Al 2.6-3.9, εNd(380 Ma) - 3.74 to - 3.95 and (87Sr/86Sr)i 0.710743-0.712955 which would point to metasedimentary or felsic igneous crustal sources. Nevertheless their TDM 1.36-1.38 Ga is considerably younger than the mean 1.8-1.6 Ga Eastern Sierras Pampeanas crustal residence age and less radiogenic. Middle Devonian magmatism would record an episode of crustal growth by enriched mantle derived magma input and variable degrees of partial melting of a lower crustal source at the waning stages of the Achalian orogeny.

Geology and geochemistry of the Middle Proterozoic Eastern Ghat mobile belt and its comparison with the lower crust of the Southern Peninsular shield

NASA Technical Reports Server (NTRS)

Rao, M. V. Subba

1988-01-01

Two prominent rock suites constitute the lithology of the Eastern Ghat mobile belt: (1) the khondalite suite - the metapelites, and (2) the charnockite suite. Later intrusives include ultramafic sequences, anorthosites and granitic gneisses. The chief structural element in the rocks of the Eastern Ghats is a planar fabric (gneissosity), defined by the alignment of platy minerals like flattened quartz, garnet, sillimanite, graphite, etc. The parallelism between the foliation and the lithological layering is related to isoclinal folding. The major structural trend (axial plane foliation trend) observed in the belt is NE-SW. Five major tectonic events have been delineated in the belt. A boundary fault along the western margin of the Eastern Ghats, bordering the low grade terrain has been substantiated by recent gravity and the deep seismic sounding studies. Field evidence shows that the pyroxene granulites (basic granulites) post-date the khondalite suite, but are older than the charnockites as well as the granitic gneisses. Polyphase metamorphism, probably correlatable with different periods of deformation is recorded. The field relations in the Eastern Ghats point to the intense deformation of the terrain, apparently both before, during and after metamorphism.

Petrogenesis of Mesozoic granites in the Xitian, South China: Evidence from whole-rock geochemistry and zircon isotopes

NASA Astrophysics Data System (ADS)

Liu, Q.; Sun, J.; He, M.; Hou, Q.; Niu, R.

2017-12-01

Mesozoic granitoids are widespread in southeastern China, which accompanied with lots of world-famous polymetallic deposits. The mineralization is believed to be related to the Mesozoic granitic magmatism. However, the petrogenesis of these granites and their relation to the mineralization are still debated. As a typical granitic pluton, Xitian granites from the eastern Hunan Province are formed during this period and associated with tungsten-tin deposit. Whole-rock geochemical, SIMS zircon geochronology and oxygen isotopes, as well as LA-ICPMS zircon Lu-Hf isotopic analyses, were carried out on a suite of rocks from Xitian granitic pluton to constrain their magmatic sources and petrogenesis. Xitian granitic pluton is mainly composed of biotite adamellite, biotite granite, fine-grained granite. SIMS and LA-ICPMS U-Pb dating of zircons indicate that there are two episodes of these rocks, i.e., Late Triassic granites (227-233Ma) and Late Jurassic granites (150-154Ma). The Xitian granites are silica-rich, potassic and weakly peraluminous. Petrographic and geochemical features show that they are highly fractionated I-type granites. The combined elemental and isotopic results indicated that the Late Triassic granite in Xitian area experienced a process of crystal fractionation of crustal-derived magmas coupled with strong assimilation of the surrounding rocks. The occurrence of Jurassic granitoids in Xitian area is attributed to ascending of mantle-derived magmas, which provide heat for partial melting of crustal materials. The Late Jurassic granite may be derived from juvenile crust or partial melting of ancient crustal rocks, whereas high degrees of crystal fractionation further enriched tungsten-tin in the evolved granitic rocks. This work was financially supported by the Research Cooperation between Institute and University of Chinese Academy of Sciences grant (Y552012Y00), Public Welfare Project of the Ministry of land and Resources of China (201211024-04), National Key R&D Program of China(2016YFC0600401) and National Science Foundation of China (NSFC grant 41273046).

The vernon supersuite: Mesoproterozoic A-type granitoid rocks in the New Jersey highlands

USGS Publications Warehouse

Volkert, R.A.; Drake, Avery A.

1998-01-01

Abundant Mesoproterozoic A-type granitoid rocks of two intrusive suites underlie approximately 50 percent of the New Jersey Highlands. These rocks, the Byram Intrusive and Lake Hopatcong Intrusive Suites, consist of granite, alaskite, quartz monzonite, monzonite, and minor pegmatite. Byram and Lake Hopatcong rocks, although different mineralogically, are similar geochemically and contain overlapping abundances of most major and trace elements. Petrographic relationships, geochronology, field relationships, and geochemical similarities support a comagmatic origin for both suites. They constitute the here named Vernon Supersuite.

Fe isotopes and the contrasting petrogenesis of A-, I- and S-type granite

NASA Astrophysics Data System (ADS)

Foden, John; Sossi, Paolo A.; Wawryk, Christine M.

2015-01-01

We present new Fe isotope data of 42 S-, I- and A-type (ferroan) granites from the Cambrian Delamerian orogen in South Australia, the Palaeozoic Lachlan Fold Belt and Western USA. Interpretation of these data, together with modelling suggests that magmatic processes do result in quite complex Fe-isotopic differentiation trends and can lead to granites with isotopically heavy iron with δ57Fe > 0.35‰. By comparison Mid-Ocean Ridge Basalts (MORBs) have δ57Fe = 0.15‰ (Teng et al., 2013). These variations are similar to those previously reported (Poitrasson and Freydier, 2005; Heimann et al., 2008; Telus et al., 2012), but, contrary to some interpretations (Beard and Johnson, 2006; Heimann et al., 2008), heavy values are not necessarily the product of late-stage hydrothermal fluid loss, though this process is undoubtedly also an important factor in some circumstances. A-type (ferroan) granites reach very heavy δ57Fe values (0.4-0.5‰) whereas I-types are systematically lighter (δ57Fe = 0.2‰). S-type granites show a range of intermediate values, but also tend to be isotopically heavy (δ57Fe ≈ 0.2-0.4‰). Our results show that the iron isotopic values and trends are signatures that reflect granite generation processes. A modelling using the Rhyolite-MELTS software suggests that contrasting trajectories and end-points in Fe isotope evolution towards granite depend on: oxidation state of the evolving magma and, whether or not the system is oxygen-buffered. Iron isotopic evolution supports an origin of ferroan A-type granite from protracted, closed magma chamber fractionation of moderately reduced mafic magmas. In these systems magnetite saturation is delayed and the ferric iron budget is finite. I-type systems originate with the supply of relatively oxidised, hydrous, subduction-related magmas from the mantle wedge to the upper plate crust. These then experience oxygen-buffered open-system AFC processes in lower crustal hot-zones. S-type magmas are crustal melts that crystallise under reduced conditions initially imposed at source by sulphidic or graphitic sedimentary protoliths. The composition of the resulting melts reflects the domination of partial melting where conditions are hence buffered (open system) followed by subsequent late-stage, closed system fractionation of these extracted, reduced magmas.

Palæo- and Neoproterozoic granitoids and rhyolites from the West Congolian Belt (Gabon, Congo, Cabinda, north Angola): chemical composition and geotectonic implications

NASA Astrophysics Data System (ADS)

Vicat, J.-P.; Pouclet, A.

2000-11-01

Various Palæo- and Neoproterozoic granitoid bodies and related rhyolites are located in the West Congolian Belt. The Palæoproterozoic granitoids, dated around 2 Ga, exhibit an Archæan-type tonalite-trondhjemite-granodiorite suite chemical signature and are related to the Eburnean tectnno-magmatic event. In contrast, Neoproterozoic granitoids and rhyolites, dated around 1 Ga, have chemical geotectonic signatures that range from orogenic to intraplate (Nb-negative anomaly, large ion lithophile element enrichment and high field strength element depletion). However, a late Kibaran orogenic event (1.35-1.00 Ga) is unknown in this area. The Neoproterozoic magmatism is interpreted as a consequence of the initiation of pre-Pan-African rifting, which implies the formation or the reactivation of major crustal strike-slip faults with asthenospheric upwelling and the generation of a thermal anomaly. This thermal anomaly could have been responsible for magmatic processes involving the lower crust, as encountered in post-orogenic environments.

Magmatic-hydrothermal origin of the early Triassic Laodou lode gold deposit in the Xiahe-Hezuo district, West Qinling orogen, China: implications for gold metallogeny

NASA Astrophysics Data System (ADS)

Jin, Xiao-ye; Li, Jian-wei; Hofstra, Albert H.; Sui, Ji-xiang

2017-08-01

The Xiahe-Hezuo district in the West Qinling orogen contains numerous Au-(As-Sb) and Cu-Au-(W) deposits. The district is divided into eastern and western zones by the Xiahe-Hezuo Fault. The western zone is exposed at a shallow level and contains sediment-hosted disseminated Au-(As-Sb) deposits, whereas the eastern zone is exposed at a deeper level and contains Cu-Au-(W) skarn and lode gold deposits within or close to granitic intrusions. The Laodou gold deposit in the eastern zone consists of auriferous quartz-sulfide-tourmaline and minor quartz-stibnite veins that are structurally controlled by fault zones transecting the Laodou quartz diorite porphyry stock and enveloped by potassic and phyllic alteration. Both the veins and alteration halos commonly contain quartz, sericite, tourmaline, pyrite, and arsenopyrite, with minor galena, sphalerite, chalcopyrite, tetrahedrite, and enargite. Gold occurs mainly as invisible gold in pyrite or arsenopyrite and locally as inclusions less than 50 μm in diameter. The zircon U-Pb age of 247.6 ± 1.3 Ma (2 σ) on the host quartz diorite porphyry and the sericite 40Ar/39Ar plateau ages of 249.1 ± 1.6 and 249.0 ± 1.5 Ma (2 σ) on two ore-related hydrothermal sericite samples are within analytical errors of one another. At the formation temperature (275 °C) inferred from microthermometric measurements of fluid inclusion, sericite and tourmaline yield calculated δDH2O values of -70 to -45‰ and δ 18OH2O of 5.8 to 9.7‰, while quartz yields calculated δ 18OH2O values of 5.1˜5.7‰. Hydrothermal tourmaline in quartz-sulfide-tourmaline veins has δ 11B of -11.2 to -0.9‰ (mean of -6.3‰) that are similar to the values of magmatic tourmaline (-8.9 to -5.5‰ with a mean of -6.8‰) in the host quartz diorite porphyry. The δ 34S values of sulfide minerals range from -5.9 to +5.8‰ with a mean of -0.6‰ that is typical of magmatic sulfur. Pyrite from hydrothermally altered quartz diorite porphyry and quartz-sulfide-tourmaline veins have relatively homogeneous lead isotopic compositions, compatible with granitic intrusions in the district. The geochronological and isotopic data combined support a magmatic origin for the Laodou gold deposit, most likely formed from fluids exsolved from the Laodou quartz diorite porphyry or associated intrusive phases at deeper levels beneath the stock. Orogenic and Carlin-like gold deposits in the West Qinling orogen have been commonly thought to have formed from metamorphic fluids. This study, however, highlights the role of magmatic-derived fluids in the formation of lode gold deposits. Synthesis of geochronological, geological, and geochemical data on magmatic rocks and ore deposits in and surrounding the Xiahe-Hezuo district indicates that gold mineralization predominantly occurred within a subduction-related magmatic arc prior to collision between the Yangtze and North China cratons that produced the West Qinling orogen.

Mid-Neoproterozoic intraplate magmatism in the northern margin of the Southern Granulite Terrane, India: Constraints from geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes

NASA Astrophysics Data System (ADS)

Deeju, T. R.; Santosh, M.; Yang, Qiong-Yan; Pradeepkumar, A. P.; Shaji, E.

2016-11-01

The northern margin of the Southern Granulite Terrane in India hosts a number of mafic, felsic and alkaline magmatic suites proximal to major shear/paleo-suture zones and mostly represents magmatism in rift-settings. Here we investigate a suite of gabbros and granite together with intermediate (dioritic) units generated through mixing and mingling of a bimodal magmatic suite. The massive gabbro exposures represent the cumulate fraction of a basic magma whereas the granitoids represent the product of crystallization in felsic magma chambers generated through crustal melting. Diorites and dioritic gabbros mostly occur as enclaves and lenses within host granitoids resembling mafic magmatic enclaves. Geochemistry of the felsic units shows volcanic arc granite and syn-collisional granite affinity. The gabbro samples show mixed E-MORB signature and the magma might have been generated in a rift setting. The trace and REE features of the rocks show variable features of subduction zone enrichment, crustal contamination and within plate enrichment, typical of intraplate magmatism involving the melting of source components derived from both depleted mantle sources and crustal components derived from older subduction events. The zircons in all the rock types show magmatic crystallization features and high Th/U values. Their U-Pb data are concordant with no major Pb loss. The gabbroic suite yields 206Pb/238U weighted mean ages in the range of 715 ± 4-832.5 ± 5 Ma marking a major phase of mid Neoproterozoic magmatism. The diorites crystallized during 206Pb/238U weighted mean age of 724 ± 6-830 ± 2 Ma. Zircons in the granite yield 206Pb/238U weighted mean age of 823 ± 4 Ma. The age data show broadly similar age ranges for the mafic, intermediate and felsic rocks and indicate a major phase of bi-modal magmatism during mid Neoproterozoic. The zircons studied show both positive and negative εHf(t) values for the gabbros (-6.4 to 12.4), and negative values for the diorites (-7.8 to -16.7) and granite (-16.6 to -6.7). Together with the Hf depleted model ages and crustal model ages, we infer that the magma sources involved both juvenile depleted mantle and reworked Mesoproterozoic, Paleoproterozoic and Neoarchean components. The mid Neoproterozoic intraplate magmatism is considered to be a response to mantle upwelling in an aborted rift setting.

Two-mica granites of northeastern Nevada.

USGS Publications Warehouse

Lee, D.E.; Kistler, R.W.; Friedman, I.; Van Loenen, R. E.

1981-01-01

The field settings are described and analytical data are presented for six two-mica granites from NE Nevada. High delta 18O and 87Sr/86Sr values indicate that all are S-type granite, derived from continental crust. The major element chemistry and accessory mineral contents of these rocks also are characteristic of S-type granites. Chemical, X ray, and other data are presented for the micas recovered from these granites. The muscovites are notably high in Fe2O3, FeO, and MgO. Except for one hydrobiotite, each of the biotites has an MgO content near 6.0 wt%. Two different types of two-mica granites are recognized in the area of this study. One type is distinguished by the presence of many biotite euhedra within muscovite phenocrysts and by an unusual suite of accessory minerals completely devoid of opaque oxides. This type probably resulted from anatexis of late Precambrian argillites under conditions of relatively low oxygen fugacity, along a line that roughly coincides with the westward disappearance of continental basement. In the other textural type of two-mica granite the micas are equigranular and there is a greater variety of accessory minerals. The magmatic evolution of this type also appears to reflect the influence of late Precambrian argillites; there may be age differences between the two types of two-mica granites.-Author

Sm-Nd and U-Pb isotopic constraints for crustal evolution during Late Neoproterozic from rocks of the Schirmacher Oasis, East Antarctica: geodynamic development coeval with the East African Orogeny

USGS Publications Warehouse

Ravikant, V.; Laux, J.H.; Pimentel, M.M.

2007-01-01

Recent post-750 Ma continental reconstructions constrain models for East African Orogeny formation and also the scattered remnants of ~640 Ma granulites, whose genesis is controversial. One such Neoproterozoic granulite belt is the Schirmacher Oasis in East Antarctica, isolated from the distinctly younger Pan-African orogen to the south in the central Droning Maud Land. To ascertain the duration of granulite-facies events in these remnants, garnet Sm-Nd and monazite and titanite U-Pb IDTIMS geochronology was carried out on a range of metamorphic rocks. Garnet formation ages from a websterite enclave and gabbro were 660±48 Ma and 587±9 Ma respectively, and those from Stype granites were 598±4 Ma and 577±4 Ma. Monazites from metapelite and metaquartzite yielded lower intercept UPb ages of 629±3 Ma and 639±5 Ma, respectively. U-Pb titanite age from calcsilicate gneiss was 580±5 Ma. These indicate peak metamorphism to have occurred between 640 and 630 Ma, followed by near isobaric cooling to ~580 Ma. Though an origin as an exotic terrane from the East African Orogen cannot be discounted, from the present data there is a greater likelihood that Mesoproterozoic microplate collision between Maud orogen and a northerly Lurio-Nampula block resulted in formation of these granulite belt(s).

Foreland-forearc collisional granitoid and mafic magmatism caused by lower-plate lithospheric slab breakoff: The Acadian of Maine, and other orogens

USGS Publications Warehouse

Schoonmaker, A.; Kidd, W.S.F.; Bradley, D.C.

2005-01-01

During collisional convergence, failure in extension of the lithosphere of the lower plate due to slab pull will reduce the thickness or completely remove lower-plate lithosphere and cause decompression melting of the asthenospheric mantle; magmas from this source may subsequently provide enough heat for substantial partial melting of crustal rocks under or beyond the toe of the collisional accretionary system. In central Maine, United States, this type of magmatism is first apparent in the Early Devonian West Branch Volcanics and equivalent mafic volcanics, in the slightly younger voluminous mafic/silicic magmatic event of the Moxie Gabbro-Katahdin batholith and related ignimbrite volcanism, and in other Early Devonian granitic plutons. Similar lower-plate collisional sequences with mafic and related silicic magmatism probably caused by slab breakoff are seen in the Miocene-Holocene Papuan orogen, and the Hercynian-Alleghenian belt. Magmatism of this type is significant because it gives evidence in those examples of whole-lithosphere extension. We infer that normal fault systems in outer trench slopes of collisional orogens in general, and possibly those of oceanic subduction zones, may not be primarily due to flexural bending, but are also driven by whole-lithosphere extension due to slab pull. The Maine Acadian example suggests that slab failure and this type of magmatism may be promoted by pre-existing large margin-parallel faults in the lower plate. ?? 2005 Geological Society of America.

The Hulan Group: Its role in the evolution of the Central Asian Orogenic Belt of NE China

NASA Astrophysics Data System (ADS)

Wu, Fu-Yuan; Zhao, Guo-Chun; Sun, De-You; Wilde, Simon A.; Yang, Jin-Hui

2007-05-01

The Hulan Group of central Jilin Province, northeastern (NE) China, consists of felsic volcanic and sedimentary rocks that underwent sub-greenschist to amphibolite facies metamorphism and multiple phases of deformation during formation of the Central Asian Orogenic Belt. Young Nd model ages of ˜1100 Ma obtained in this study do not support earlier models that the Hulan Group was either rifted from the North China Craton to the south or from the Jiamusi Block to the north, since both these blocks have much older Nd model ages. Detrital zircon U-Pb ages from a meta-sedimentary rock of the Hulan Group define a maximum age of deposition of 287 ± 6 Ma, not Early Palaeozoic as previously believed. Rb-Sr mineral isochron data indicate that metamorphism of the Hulan Group occurred at ˜250 Ma. This conclusion is further supported by an age of 248 ± 4 Ma for the Dayushan granite, which intruded the Hulan Group. These new isotopic data, together with evaluation of the regional geology, indicate that final oceanic closure in the Chinese segment of the Central Asian Orogenic Belt took place in the Late Permian. The Hulan area is located along the Solonker-Xra Moron-Changchun suture that extends from Inner Mongolia across to central Jilin Province. It is this zone that marks the final ocean closure, resulting in the amalgamation of the North China and Siberian cratons.

Paleoproterozoic magmatic and metamorphic events link Yangtze to northwest Laurentia in the Nuna supercontinent

NASA Astrophysics Data System (ADS)

Wang, Wei; Cawood, Peter A.; Zhou, Mei-Fu; Zhao, Jun-Hong

2016-01-01

Zircons from granitic gneisses in North Vietnam have magmatic cores dated at 2.28-2.19 Ga, and constitute the first reported evidence of continental crust with these ages in the Yangtze Block of the South China Craton. Overgrowths on zircon rims indicate two periods of metamorphism at 1.97-1.95 Ga and ∼1.83 Ga. These events, along with a previously reported ∼2.36 Ga metamorphic overgrowth on ∼2.9 Ga crystallized zircons from the same region, suggest a sequence of events similar to that recorded for the northwestern region of Laurentia and possibly Siberia, which are associated with assembly of the Nuna supercontinent. These include the 2.4-2.3 Ga Arrowsmith Orogen and a range of events in the interval 2.32-1.80 Ga, including accretionary magmatism in northwestern Laurentia and Siberia (2.32-2.07 Ga), the Thelon orogeny (2.02-1.96 Ga) and the 1.85-1.80 Ga collision between the Superior and Hearne-Rae cratons during the Trans-Hudson Orogen in Laurentia, and the Akitkan Orogen in Siberia (2.03-1.86 Ga). Subsequent attempted breakup of Nuna may be represented by ca. 1.80 to 1.59 Ga consanguineous extension related sedimentation and magmatism in the southwestern Yangtze Block and northwestern Laurentia. These correlations favor location of the Yangtze Block adjacent to northwest Laurentia, and possibly Siberia, within the Nuna supercontinent.

Melting of subducted continental crust: Geochemical evidence from Mesozoic granitoids in the Dabie-Sulu orogenic belt, east-central China

NASA Astrophysics Data System (ADS)

Zhao, Zi-Fu; Liu, Zhi-Bin; Chen, Qi

2017-09-01

Syn-collisional and postcollisional granitoids are common in collisional orogens, and they were primarily produced by partial melting of subducted continental crust. This is exemplified by Mesozoic granitoids from the Dabie-Sulu orogenic belt in east-central China. These granitoids were emplaced in small volumes in the Late Triassic (200-206 Ma) and the Late Jurassic (146-167 Ma) but massively in the Early Cretaceous (111-143 Ma). Nevertheless, all of them exhibit arc-like trace element distribution patterns and are enriched in Sr-Nd-Hf isotope compositions, indicating their origination from the ancient continental crust. They commonly contain relict zircons with Neoproterozoic and Triassic U-Pb ages, respectively, consistent with the protolith and metamorphic ages for ultrahigh-pressure (UHP) metaigneous rocks in the Dabie-Sulu orogenic belt. Some granitoids show low zircon δ18O values, and SIMS in-situ O isotope analysis reveals that the relict zircons with Neoproterozoic and Triassic U-Pb ages also commonly exhibit low δ18O values. Neoproterozoic U-Pb ages and low δ18O values are the two diagnostic features that distinguish the subducted South China Block from the obducted North China Block. Thus, the magma source of these Mesozoic granitoids has a genetic link to the subducted continental crust of the South China Block. On the other hand, these granitoids contain relict zircons with Paleoproterozoic and Archean U-Pb ages, which are present in both the South and North China Blocks. Taken together, the Mesozoic granitoids in the Dabie-Sulu orogenic belt and its hanging wall have their magma sources that are predominated by the continental crust of the South China Block with minor contributions from the continental crust of the North China Block. The Triassic continental collision between the South and North China Blocks brought the continental crust into the thickened orogen, where they underwent the three episodes of partial melting in the Late Triassic, Late Jurassic and Early Cretaceous, respectively, for granitic magmatism. While partial melting in the Late Triassic is responsible for syn-exhumation magmatism, the Late Jurassic and Early Cretaceous granitoids are independent of the continental collision and thus belong to postcollisional magmatism.

Economic potential of the Rooiberg Group: volcanic rocks in the floor and roof of the Bushveld Complex

NASA Astrophysics Data System (ADS)

Schweitzer, J. K.; Hatton, C. J.; de Waal, S. A.

1995-04-01

Volcanic rocks of the Rooiberg Group are preserved in the floor and roof of the mafic Rustenburg Layered Suite of the Bushveld Complex. Field and geochemical characteristics of these volcanic rocks imply that they are genetically related to the Rustenburg Layered Suite. Four major ore-forming events are identified in the Rooiberg Group. The first phase was accompanied by volcanic hosted, fault controlled, hydrothermal copper mineralisation, which is found in the lowermost portion of the Rooiberg Group, underlying the Rustenburg Layered Suite. This type of mineralisation is tentatively linked to initial Rustenburg Layered Suite intrusions. Stratabound arsenic mineralisation that possibly formed in response to contact metamorphism, characterises the second phase, and occurred after extrusion of the Damwal Formation, possibly due to shallow granophyric intrusion. The third mineralising event occurred in response to contact metamorphism during the final stages of the Rustenburg Layered Suite, where especially Pb and Zn were introduced into the felsite roof rocks. This type of mineralisation affected the majority of the Rooiberg Group, but is most pronounced towards the contact with the Rustenburg Layered Suite. The fourth phase is restricted to the Rooiberg Group in the Nylstroom area and is linked to the granite intrusions of the Lebowa Granite Suite, from which Sn and F were introduced into the uppermost felsite succession. Mineralisation in the Rooiberg Group appears to be controlled by the character and intrusion level of the associated Bushveld magmas. Different styles of mineralisation in Rooiberg Group volcanic rocks are encountered at various stratigraphic levels. Major primary volcanogenic ore deposits appear to be absent.

Kyanite-bearing migmatites in the central Adirondack Mountains: Implications for late to post-orogenic metamorphism and melting in a collisional orogen

NASA Astrophysics Data System (ADS)

Reeder, J.; Metzger, E. P.; Bickford, M. E.; Leech, M. L.

2016-12-01

Sillimanite-rich felsic migmatites exposed at Ledge Mountain in the Central Adirondack Highlands (AH) represent the only location in the AH where kyanite is found. The texturally young kyanite is overprinted on sillimanite in largely undeformed pegmatitic leucosomes, suggesting a late episode of melting taking place deeper than previously thought, and requiring a counter-clockwise P-T path. A final phase of anatexis ca. 1050 Ma in the Eastern AH is consistent with an influx of fluid or decompression from extension in sillimanite-bearing migmatites. Temperatures both from this study and previous work are consistent with granulite-facies metamorphism; however, the presence of kyanite requires higher pressure conditions corresponding to deeper burial of rocks exposed in the central Adirondacks. The Adirondacks are associated with the Grenville Province of eastern North America, that formed during four orogenic events. The most recent (Grenville) orogeny consisted of two stages: crustal thickening and granulite facies metamorphism during the Ottawan phase (ca 1090-1020) then metamorphism and melting in the kyanite field during the much shorter Rigolet pulse (ca 1005-980 Ma). Preliminary U-Pb SHRIMP zircon ages from Ledge Mountain kyanite-bearing migmatites suggest that melting in the Central AH persisted into the Rigolet phase. On the basis of mineral composition and chemistry and the presence of distinctive quartz-sillimanite nodules, the Ledge Mountain migmatites closely resemble the K-rich phase of the Ottawan-age Lyon Mountain granite (LMG) and may represent LMG that was metamorphosed to sillimanite grade and then overprinted by a higher pressure, lower temperature assemblage. Kyanite-bearing felsic anatectites of Rigolet age have previously been observed only in the western portion of the Grenville Province. Documentation of a counterclockwise P-T path and post-Ottawan melting in the Ledge Mountain migmatites requires re-evaluation of current tectonic models for the Grenville Province and its Adirondacks outlier. Further analysis of age, geochemical, and petrographic data will help develop a better-defined P-T-t path and may lead to the development of a new tectonic model to be compared with other collisional orogens such as Himalaya or the Bohemian Massif of the Variscan orogenic belt.

The Tintina Gold Belt - A global perspective

USGS Publications Warehouse

Goldfarb, Richard J.; Hart, Craig J.R.; Miller, Marti L.; Miller, Lance D.; Farmer, G. Lang; Groves, David I.; Tucker, Terry L.; Smith, Moira T.

2000-01-01

The so-called Tintina Gold Belt extends for more than 1000 km along the length of the northern North American Cordillera. Middle to Late Cretaceous Au deposits within the belt have various similar characteristics, among which are a spatial and temporal association with magmatism; Bi-W-Te signatures in deposits hosted by granitod stocks and As-Sb signatures where hosted by sedimentary rocks and dyke systems; and δ180 values consistently > 12 per mil for Au-bearing quartz. Nevertheless significant differences in structural styles, levels of deposit emplacement, ore-fluid chemistry, and Au grades suggest that the characteristics represent a broad range of deposit types. Many of these are best classified as orogenic Au deposits in the Yukon-Tanana terrane, as epithermal and porphyry-style Au deposits in the Kuskokwim region, and as Au-bearing, granite-related veins and stockworks, replacements, and skarns, as well as associated polymetallic lodes, in central Yukon. The diverse types of Au deposits and associated plutons of the Tintina Gold Belt collectively define a 45-m.y.-long period of arc magmatism that migrated northwesterly, for about 1000 km, across the active collisional margin of Cretaceous northwestern North America. The initiation of fluid flow and plutonism in Albian time seems to correlate with the onset of oblique subduction and dextral strike-slip on the Denali-Farewell, Tintina-Kaltag, and related fault systems. Initial Au-vein formation and subduction-related magmatism at about 115-110 Ma (e.g., including the Goodpaster and Fortymile districts), within the seaward side of the Yukon-Tanana terrane, correlate with the arrival of the Wrangellia superterrane off the continental margin. Dextral translation of the allochthonous Wrangellia block was associated with the migration of the thermal pulse to the northwest at about 95-90 Ma. Orogenic (or so­ called mesotherrnal) and granitoid-related Au deposits formed across the width of the Yukon-Tanana terrane (e.g., Fort Knox, True North, Ryan Lode, Kantishna district) and inland into the passive-margin rocks of the Selwyn basin ( e.g., Scheelite Dome, Brewery Creek, Dublin Gulch), respectively. By 70 Ma, the arc had migrated to the vicinity of present-day southwestern Alaska, where it was associated with the formation of additional orogenic Au deposits (e.g., Willow Creek district) and, within still-preserved shallow crustal levels, epithermal Au systems (e.g., Donlin Creek). The Au-bearing deposits of the Tintina Gold Belt are typical of those found in most well-preserved, moderate- to high-temperature Phanerozoic collisional orogens. Around the circum-Pacific region, these would include large areas of Mesozoic tectonism along the Cordilleran orogen, throughout the Russian Far East, and along the margins of the North China craton. Favorable terrain for such Au belts of Paleozoic age worldwide include the active Gondwana margins (e.g., Tasman orogenic system, northern Africa, Telfer district), and the northern margins ( e.g., Caledonian Kazakhstania, Uralian orogen, Baikal orogen, Tian Shan orogenic system) and western margins ( e.g., southern European massifs) to the Paleo-Tethys Ocean. Gold lodes in all of the Phanerozoic belts are dominated by orogenic Au-deposit types; other deposit types are concentrated where relatively shallow levels to the orogens are locally preserved. A significant percentage of the lode-gold resource in many areas was lost to placer accumulation that began forming approximately 100 m.y. after hypogene ore formation, except where continent-continent collision "cratonized" highly mineralized terranes in central Asia.

Two Late Cretaceous A-type granites related to the Yingwuling W-Sn polymetallic mineralization in Guangdong province, South China: Implications for petrogenesis, geodynamic setting, and mineralization

NASA Astrophysics Data System (ADS)

Zheng, Wei; Mao, Jingwen; Zhao, Haijie; Zhao, Caisheng; Yu, Xiaofei

2017-03-01

Major and trace elements, whole rock Sr-Nd-Pb isotopes, LA-ICP-MS U-Pb zircon dating, zircon trace elements and Hf isotope data are reported for a suite of A-type granites from Yingwuling pluton in western Guangdong province, South China. Zircon U-Pb ages obtained by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) show that biotite granite and alkali feldspar granite were emplaced in 81.3 ± 0.6 Ma and 80.6 ± 0.5 Ma, respectively. Both of the two suites have the petrographic and geochemical characteristics of A-type granite. These granitic rocks are metaluminous to weakly peraluminous and have pronounced contents of total alkalis (Na2O + K2O = 7.80-8.84%), Fe2O3T/MgO and Ga/Al ratios. They exhibit low MgO, CaO and TiO2 contents, enrichment in some LILEs and HFSEs (except for Zr, Eu and Y), depletion in Ba, Sr, P and Ti. They show A2 subtype affinity and were probably formed a temperature of 800 °C. The Yingwuling biotite granite has relatively high (87Sr/86Sr)i ratios of 0.70655 to 0.70928, low εNd(t) values of - 5.8 to - 4.2 and zircon εHf(t) values (- 5.70-1.37). Whole-rock Nd isotopic and zircon Hf isotopic two-stages model ages mostly vary from 1057 to 1506 Ma. The alkali feldspar granite display bulk rock εNd(t) values and (87Sr/86Sr)i ratios in the range of - 6.6 to - 6.1 and 0.70640 to 0.71077, respectively, and zircon εHf(t) values from - 5.44 to 0.54, with Mesoproterozoic T2DM for both Nd and Hf isotopes. Geochemical and isotopic data indicate the Yingwuling A-type granitic magmas were drived from mantle-crust interaction. Zircon grains of Yingwuling granites have relatively low Eu/Eu* and Ce4 +/Ce3 + ratios, indicating low oxygen fugacity. The visible tetrad effect in the Yingwuling granites indicates that it experienced strong fractionation and is close relationship to the W-Sn mineralization. Our new data together with previous published data indicate that Late Mesozoic A-type granitiods or alkaline intrusive rocks in South China can be grouped into four pulses: 195-180 Ma, 180-149 Ma, 136-116 Ma and 110-79 Ma. After 135 Ma, with a change in the polarity of the subduction of paleo-Pacific plate, the crust and lithospheric mantle in eastern China became progressively thinned causing the upwelling of asthenosphere, which would have triggered crustal rocks to partially melt and generate Yingwuling A-type granitic magmas.

Geochronology and geochemistry of early Paleozoic igneous rocks of the Lesser Xing'an Range, NE China: Implications for the tectonic evolution of the eastern Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Wang, Zhi-wei; Xu, Wen-liang; Pei, Fu-ping; Wang, Feng; Guo, Peng

2016-09-01

This paper presents new zircon U-Pb, Hf isotope, and whole-rock major and trace element data for early Paleozoic igneous rocks of the Lesser Xing'an Range, NE China, in order to constrain the early Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt (CAOB). Zircon U-Pb dating indicates that early Paleozoic magmatic events within the northern Songnen-Zhangguangcai Range Massif (SZM) can be subdivided into four stages: Middle Cambrian ( 505 Ma), Late Cambrian ( 490 Ma), Early-Middle Ordovician ( 470 Ma), and Late Ordovician (460-450 Ma). The Middle Cambrian monzogranites are K-rich, weakly to strongly peraluminous, and characterized by pronounced heavy rare earth element (HREE) depletions, high Sr/Y ratios, low Y concentrations, low primary zircon εHf(t) values (- 6.79 to - 1.09), and ancient two-stage model (TDM2) ages (1901-1534 Ma). These results indicate derivation from partial melting of thickened ancient crustal materials that formed during the amalgamation of the northern SZM and the northern Jiamusi Massif (JM). The Late Cambrian monzonite, quartz monzonite, and monzogranite units are chemically similar to A-type granites, and contain zircons with εHf(t) values of - 2.59 to + 1.78 and TDM2 ages of 1625-1348 Ma. We infer that these rocks formed from primary magmas generated by partial melting of Mesoproterozoic accreted lower crustal materials in a post-collisional extensional environment. The Early-Middle Ordovician quartz monzodiorite, quartz monzonite, monzogranite, and rhyolite units are calc-alkaline, relatively enriched in light REEs (LREEs) and large ion lithophile elements (LILEs; e.g., Rb, Th, and U), depleted in HREEs and high field strength elements (HFSEs; e.g., Nb, Ta, and Ti), and contain zircons with εHf(t) values of - 7.33 to + 4.98, indicative of formation in an active continental margin setting. The Late Ordovician alkali-feldspar granite and rhyolite units have A-type granite affinities that suggest they formed in an extensional environment. A comparison of early Paleozoic magmatic events and Hf isotopic model ages between the northern SZM and the JM indicates that these two massifs have similar histories of Mesoproterozoic and early Paleozoic crustal accretion and reworking, although the SZM contains much older crustal materials than the JM.

Geochemistry and stratigraphic relations of middle Proterozoic rocks of the New Jersey Highlands

USGS Publications Warehouse

Volkert, Richard A.; Drake, Avery Ala

1999-01-01

Middle Proterozoic rocks of the New Jersey Highlands consist of a basement of dacitic, tonalitic, trondhjemitic, and charnockitic rocks that constitute the Losee metamorphic suite. These rocks are unconformably overlain by a layered supracrustal sequence of quartzo-feldspathic and calcareous rocks. Abundant sheets of hornblende- and biotite-bearing rocks of the Byram intrusive suite and clinopyroxene-bearing rocks of the Lake Hopatcong intrusive suite were synkinematically emplaced at about 1,090 Ma. These intrusive suites constitute the Vernon Supersuite. The postorogenic Mount Eve Granite has been dated at 1,020?4 Ma and is confined to the extreme northern Highlands.

Lateral variations in lithospheric and landscape evolution at both ends of the Himalaya-Tibet orogen

NASA Astrophysics Data System (ADS)

Zeitler, P. K.; Schmidt, J. L.; Meltzer, A.

2015-12-01

At the broadest scale, like many orogens the Himalaya encompass a range of orogenic features that are remarkably similar along much of the length of the mountain belt and its neighboring terranes. At one scale of consideration, these similarities appear to be a signal that fundamental processes associated with lithospheric collision have been active. However, the vast size of the Himalaya and Tibet, the different climate regimes experienced by the orogen across time and space, and the along-strike variations in the continental and arc margins that faced one another before collision, make it at once remarkable that any similarities exist, and important to more critically evaluate their nature. The eastern and western Himalayan syntaxes confound any attempt to generalize too much about the Himalaya-Tibet orogen. By area these features occupy at least 25% of the orogenic belt, and compared to the "main" portions of the arc they show clear differences in their lithospheric structures, landscapes, and evolution. The boundary and initial conditions that shaped the eastern and western indentor corners were and are different, as is the nature and timing of erosional exhumation. Some of the most active geologic processes on Earth have recently been in play within the syntaxes, and the evolution of landscapes and fluvial systems, important in developing the sedimentary record of the Himalaya-Tibet system, has been complex and variable in space and time. Southeasternmost Tibet and the Lhasa Block in particular exemplify this complexity both in its complex topographic evolution linked to surface processes and climate, and in lateral variability in lithospheric structure. Taking a system viewpoint, an important question to debate is the degree to which there are features in the Himalaya-Tibet system that are robustly emergent, given the broad boundary conditions of the continental collision plus the suite of local and regional geodynamical processes that have operated during orogenesis. A related question is the degree to which the variability seen within the orogen represents important information about process that is exportable to other orogens, or is in effect tectonic noise contingent on local geologic details and secular changes.

Accretionary history of the Altai-Mongolian terrane: perspectives from granitic zircon U-Pb and Hf-isotope data

NASA Astrophysics Data System (ADS)

Cai, Keda; Sun, Min; Xiao, Wenjiao

2014-05-01

The Central Asian Orogenic Belt (CAOB) consists of many tectonic terranes with distinct origin and complicated evolutionary history. Understanding of individual block is crucial to reconstruct the geodynamic history of the gigantic accetionary collage. This study presents zircon U-Pb ages and Hf isotopes for the granitoid rocks in the Russian Altai mountain range (including Gorny Altai, Altai-Mongolian terrane and CTUS suture zone between them), in order to clarify the timing of granitic magmatism, source nature, continental crustal growth and tectonic evolution. Our dating results suggest that granitic magmatism of the Russian Altai mountain range occurred in three major episodes including 445~429 Ma, 410~360 Ma and ~241 Ma. Most of the zircons within the Paleozoic granitoids present comparable positive ɛHf(t) values and Neoproterozoic crustal model ages, which favor the interpretation that the juvenile crustal materials produced in the early stage of CAOB were probably dominant sources for the Paleozoic magmatism in the region. The inference is also supported by widespread occurrence of short-lived juvenile materials including ophiolites, seamount relics and arc assemblages in the north CAOB. Consequently, the Paleozoic massive granitic rocks maybe not represent continental crustal growth at the time when they were emplaced, but rather record reworking of relatively juvenile Proterozoic crustal rocks although mantle-derived mafic magma was possibly involved to sever as heat engine during granitic magma generation. The Early Triassic granitic intrusion may be product in an intra-plate environment, as the case of same type rocks in the adjacent areas. The positive ɛHf(t) values (1.81~7.47) and corresponding Hf model ages (0.80~1.16 Ga) together with evidence of petrology are consistent with the interpretation that the parental magma of the Triassic granitic intrusion was produced from enriched mantle-derived sources under an usually high temperature condition which is likely due to basaltic magma that underplated the lower crust. Our data combined with evidence of the regional geology enable us to conclude that the Gorny Altai and Altai-Mongolian terranes possibly have similar tectonic natures, but represent two separate accretionary systems before Devonian collision. The accretion and amalgamation processes resulted in the Paleozoic granitoid magmatism and caused the two terranes to merge as a composite tectonic domain at the Siberian continental margin.

Origin of the Wunugetushan porphyry Cu-Mo deposit, Inner Mongolia, NE China: Constraints from geology, geochronology, geochemistry, and isotopic compositions

NASA Astrophysics Data System (ADS)

Zhang, Fang-Fang; Wang, Yin-Hong; Liu, Jia-Jun; Wang, Jian-Ping; Zhao, Chun-Bo; Song, Zhi-Wei

2016-03-01

The Wunugetushan porphyry Cu-Mo deposit is located in the southeastern margin of the Mongol-Okhotsk Orogenic Belt and in the northwestern segment of the Great Xing'an Range, NE China. The orebodies of this deposit are mainly hosted in the monzogranitic porphyry stock and in contact with the granitic porphyry dyke and biotite granite batholith. The SHRIMP zircon U-Pb dating of the granitic porphyry dyke yielded ages of 201.4 ± 3.1 Ma (2σ, MSWD = 1.5). These results indicate that the magmatism in the Wunugetushan area might have occurred at ca. 201 Ma in the early Jurassic, and that the mineralization age (ca. 181 Ma) of this deposit is later than the age of intrusive granitic porphyry in the area. Geochemically, the Wunugetushan granitoids belong to high-K calc-alkaline and shoshonitic series, enriched in K, Rb, Nd, and Pb, and depleted in Sr, Nb, Ti and P, with negative Eu anomalies. In situ Hf isotopic analyses of zircons using LA-MC-ICP-MS indicate that the εHf(t) values for zircons from a granitic porphyry sample vary from +2.4 to +11.8 and that the corresponding crustal model ages (TDMC) vary from 483 to 1088 Ma. The least-altered monzogranitic porphyry, granitic porphyry and biotite granite yielded relatively uniform εNd(t) values from -1.0 to +0.6 and low (87Sr/86Sr)i ratios ranging from 0.704387 to 0.708385. The geochemical and Sr-Nd-Hf isotopic data for the granitoids indicate that the source magma for these rocks could be derived from a juvenile lower crust. The δ34S values of sulfides show a narrow range (+0.76‰ to +3.20‰) similar to those of magmatic sulfur, further implying a lower crust origin. Based on the results of this study and the regional geodynamic evolution, it is proposed that the formation of the Wunugetushan deposit and associated granitoids should be linked to the southeastward subduction of the Mongol-Okhotsk oceanic plate beneath the Erguna Massif during the early Jurassic, and that the monzogranitic porphyry intrusions in Wunugetushan area probably provided important ore metals responsible for the large-scale Cu-Mo mineralization.

Continental crustal growth and the supercontinental cycle: evidence from the Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Hong, Dawei; Zhang, Jisheng; Wang, Tao; Wang, Shiguang; Xie, Xilin

2004-09-01

Studies of supercontinental cycle are mainly concentrated on the assembly, breakup and dispersal of supercontinents, and studies of continental crustal growth largely on the growth and loss (recycling) of the crust. These two problems have long been studied separately from each other. The Paleozoic-Mesozoic granites in the Central Asian Orogenic Belt have commonly positive ɛNd values, implying large-scale continental crustal growth in the Phanerozoic. They coincided temporally and spatially with the Phanerozoic Pangea supercontinental cycle, and overlapped in space with the P-wave high- V anomalies and calculated positions of subducted slabs for the last 180 Ma, all this suggests that the Phanerozoic Laurasia supercontinental assembly was accompanied by large-scale continental crustal growth in central Asia. Based on these observations, this paper proposes that there may be close and original correlations between a supercontinental cycle, continental crustal growth and catastrophic slab avalanches in the mantle. In this model we suggest that rapid continental crustal growth occurred during supercontinent assembly, whereas during supercontinental breakup and dispersal new additions of the crust were balanced by losses, resulting in a steady state system. Supercontinental cycle and continental crustal growth are both governed by changing patterns of mantle convection.

Early Cretaceous adakitic magmatism in central eastern China controlled by ridge subduction

NASA Astrophysics Data System (ADS)

Ling, M.; Luo, Z.; Sun, W.

2017-12-01

Early Cretaceous adakites are widely distributed in central eastern China, e.g., Lower Yangtze River Belt (LYRB), Dabie orogen and south Tan-Lu Fault (STLF) area. Adakite from the LYRB is closely associated with mineralization, while adakites from Dabie orogen and STLF are ore barren. Their origins, however, remain controversial. Detailed geochemical comparison between these adakites indicates that the LYRB adakite are formed by partial melting of oceanic crust, i.e., slab melting, whereas those from Dabie orogen and STLF (e.g., Guandian pluton) have origin of lower continental crust (LCC) 1,2. Base on the distribution of igneous rocks, e.g., adakite, A-type granite and Nb-enriched basalts, as well as other lines of evidence, ridge subduction of the Pacific and Izanagi plates was proposed to explain the genesis of Cretaceous magmatism and associated mineralization in the LYRB 1. Ridge subduction is a special plate tectonic process that can provide both physical erosion and thermal erosion 3. Flat subduction of a spreading ridge will result in strong physical subduction-related erosion, and trigger destruction (e.g., in the Dabie orogen) or delamination (e.g., in the STLF) of the thickened LCC. Subsequently, ridge subduction, accompanied by opening of a slab window, will facilitate partial melting of the LCC by thermal erosion. References: 1. Ling, M. X. et al. Cretaceous ridge subduction along the Lower Yangtze river belt, eastern China. Econ. Geol. 104, 303-321, doi:10.2113/gsecongeo.104.2.303 (2009). 2. Ling, M. X., Wang, F. Y., Ding, X., Zhou, J. B. & Sun, W. D. Different origins of adakites from the Dabie Mountains and the Lower Yangtze River Belt, eastern China: Geochemical constraints. International Geology Review 53, 727-740 (2011). 3. Ling, M. X. et al. Destruction of the North China Craton Induced by Ridge Subductions. Journal of Geology 121, 197-213 (2013).

Restoring the supercontinent Columbia and tracing its fragments after its breakup: A new configuration and a Super-Horde hypothesis

NASA Astrophysics Data System (ADS)

Yakubchuk, Alexander

2010-09-01

Paleoproterozoic collisional (internal) and accretionary (external) orogens, additionally constrained by the matches between the Archaean granulite-gneiss and granite-greenstone terranes, are used to reconstruct the Mesoproterozoic supercontinent Columbia. The Archaean granulite-gneiss terranes occupy an axial position, forming the Archaean Super-Horde, traceable through almost all present cratons. Restored Columbia is a 30,000 km long supercontinent, assembled by ca 1.85 Ga. There is no evidence of its breakup during the Mesoproterozoic, and it subsequently grew via external accretion until ca. 1.25 Ga. After 1.25 Ga, the Atlantica group of cratons was split from Columbia and rotated to collide with the remaining intact part of Columbia to produce the 1.0 Ga Grenville orogen, hence assembling the supercontinent Rodinia. At 1000-720 Ma, penetration of oceanic spreading centres into Rodinia between Siberia and the Australian cratons split the remaining part of Columbia into the Ur and Nena cratonic groups. Nena was then quickly rifted apart into Laurentia, Eastern Europe, and Siberia. Siberia started its drift from the present western edge of Laurentia towards Eastern Europe. This drift might have caused the separation from Nena of parts of the Palaeoproterozoic external orogen to form the Great Steppe superterrane, which later was assimilated into the basement of Neoproterozoic to Palaeozoic magmatic arcs with adjacent backarc oceanic basins, whose fragments are at present found inside the Central Asia supercollage. Simultaneously with Siberia, the remaining intact Ur began moving in the opposite direction around Atlantica. During this translation, Atlantica was fragmented into Congo-Tanzania, West Africa, Amazonia and Rio-de-la-Plata with opening of the internal Brasiliano oceanic basin and its subsequent suturing. This closure might have happened due to the arrival of Ur, whose Kalahari and India portions collided with Congo-Tanzania to produce the Damara and Mozambique orogens, welding Ur and Atlantica into Gondwana at 540-500 Ma.

Mineral potential tracts for orogenic, Carlin-like, and epithermal gold deposits in the Islamic Republic of Mauritania, (phase V, deliverable 69): Chapter H in Second projet de renforcement institutionnel du secteur minier de la République Islamique de Mauritanie (PRISM-II)

USGS Publications Warehouse

Goldfarb, Richard J.; Marsh, Erin; Anderson, Eric D.; Horton, John D.; Finn, Carol A.; Beaudoin, Georges

2015-01-01

A preliminary evaluation of these gold data can be used to develop broad, firstorder tracts defining favorable and permissive areas for gold resources; detailed metamorphic and structural maps are required for more detailed future tract definition. Such a first-order assessment can, nonetheless, broadly identify four tracts of gold resource potential. Three of these are favorable for discovery of new orogenic gold deposits. One tract, although not favorable, is nevertheless permissive for discovery of epithermal gold deposits. Tract 1 is defined by favorable medium metamorphic grade greenstone belts within vast areas of unfavorable high metamorphic grade, Mesoarchean and Paleoproterozoic granite-gneiss basement of the Rgueïbat Shield. Faults >200 km in length following the general strike of the greenstone belts; lineament intersections with both exposed and buried parts of greenstone belts within 500 m of the surface, as defined by aeromagnetic data (Finn and Anderson, 2015); and areas of banded iron formation (BIF) in the belts are particularly favorable areas for hosting gold resources in orogenic gold deposits within and along the margins of the greenstone belts. Tracts 2 and 3, also for orogenic gold, reflect the favorable Proterozoic-Cambrian metamorphic rocks of the Northern and Southern Mauritanides, with >200-km-long faults following the general strike of the range, and areas underlain by ultramafic and BIF rocks being particularly favorable. Outcrops of Triassic-Jurassic igneous rocks along the margins of the Taoudeni Basin define tract 4, which is permissive for epithermal gold deposits. Although extensive data are lacking for the area, carbonate units along the northern side of the Taoudeni Basin could be considered permissive host rocks for Carlin-type mineralization, but the deep-water carbonate lithologies are typically not favorable for such. 

Zircon U-Pb age, Hf isotopic compositions and geochemistry of the Silurian Fengdingshan I-type granite Pluton and Taoyuan mafic-felsic Complex at the southeastern margin of the Yangtze Block

NASA Astrophysics Data System (ADS)

Zhong, Yufang; Ma, Changqian; Zhang, Chao; Wang, Shiming; She, Zhenbing; Liu, Lei; Xu, Haijin

2013-09-01

This work presents an integrated study of zircon U-Pb ages and Hf isotope along with whole-rock geochemistry on Silurian Fengdingshan I-type granites and Taoyuan mafic-felsic intrusive Complex located at the southeastern margin of the Yangtze Block, filling in a gap in understanding of Paleozoic I-type granites and mafic-intermediate igneous rocks in the eastern South China Craton (SCC). The Fengdingshan granite and Taoyuan hornblende gabbro are dated at 436 ± 5 Ma and 409 ± 2 Ma, respectively. The Fengdingshan granites display characteristics of calc-alkaline I-type granite with high initial 87Sr/86Sr ratios of 0.7093-0.7127, low ɛNd(t) values ranging from -5.6 to -5.4 and corresponding Nd model ages (T2DM) of 1.6 Ga. Their zircon grains have ɛHf(t) values ranging from -2.7 to 2.6 and model ages of 951-1164 Ma. The Taoyuan mafic rocks exhibit typical arc-like geochemistry, with enrichment in Rb, Th, U and Pb and depletion in Nb, Ta. They have initial 87Sr/86Sr ratios of 0.7053-0.7058, ɛNd(t) values of 0.2-1.6 and corresponding T2DM of 1.0-1.1 Ga. Their zircon grains have ɛHf(t) values ranging from 3.2 to 6.1 and model ages of 774-911 Ma. Diorite and granodiorite from the Taoyuan Complex have initial 87Sr/86Sr ratios of 0.7065-0.7117, ɛNd(t) values from -5.7 to -1.9 and Nd model ages of 1.3-1.6 Ga. The petrographic and geochemical characteristics indicate that the Fengdingshan granites probably formed by reworking of Neoproterozoic basalts with very little of juvenile mantle-derived magma. The Taoyuan Complex formed by magma mixing and mingling, in which the mafic member originated from a metasomatized lithospheric mantle. Both the Fengdingshan and Taoyuan Plutons formed in a post-orogenic collapse stage in an intracontinental tectonic regime. Besides the Paleozoic Fengdingshan granites and Taoyuan hornblende gabbro, other Neoproterozoic and Indosinian igneous rocks located along the southeastern and western margin of the Yangtze Block also exhibit decoupled Nd-Hf isotopic systemics, which may be a fingerprint of a previous late Mesoproterozoic to early Neoproterozoic oceanic subduction.

Uranium in granites from the Southwestern United States: actinide parent-daughter systems, sites and mobilization. First year report

DOE Office of Scientific and Technical Information (OSTI.GOV)

Silver, L T; Williams, I S; Woodhead, J A

1980-10-01

Some of the principal findings of the study on the Lawler Peak Granite are: the granite is dated precisely by this work at 1411 +- 3 m.y., confirming its synchroneity with a great regional terrane of granites. Uranium is presently 8-10 times crustal abundance and thorium 2-3 times in this granite. Uranium is found to be enriched in at least eight, possibly ten, primary igneous mineral species over the whole-rock values. Individual mineral species show distinct levels in, and characteristics ranges of, uranium concentration. It appears that in a uraniferous granite such as this, conventional accuracy mineral suites probably cannotmore » account for most of the uranium in the rock, and more rare, high U-concentration phases also are present and are significant uranium hosts. It appears that at least two different geological episodes have contributed to the disturbance of the U-Th-Pb isotope systems. Studies of various sites for transient dispersal of uranium, thorium, and radiogenic lead isotopes indicate a non-uniform dispersal of these components. It appears that the bulk rock has lost at least 24 percent of its original uranium endowment, accepting limited or no radiogenic lead or thorium migration from the sample.« less

Application of multi-dimensional discrimination diagrams and probability calculations to Paleoproterozoic acid rocks from Brazilian cratons and provinces to infer tectonic settings

NASA Astrophysics Data System (ADS)

Verma, Sanjeet K.; Oliveira, Elson P.

2013-08-01

In present work, we applied two sets of new multi-dimensional geochemical diagrams (Verma et al., 2013) obtained from linear discriminant analysis (LDA) of natural logarithm-transformed ratios of major elements and immobile major and trace elements in acid magmas to decipher plate tectonic settings and corresponding probability estimates for Paleoproterozoic rocks from Amazonian craton, São Francisco craton, São Luís craton, and Borborema province of Brazil. The robustness of LDA minimizes the effects of petrogenetic processes and maximizes the separation among the different tectonic groups. The probability based boundaries further provide a better objective statistical method in comparison to the commonly used subjective method of determining the boundaries by eye judgment. The use of readjusted major element data to 100% on an anhydrous basis from SINCLAS computer program, also helps to minimize the effects of post-emplacement compositional changes and analytical errors on these tectonic discrimination diagrams. Fifteen case studies of acid suites highlighted the application of these diagrams and probability calculations. The first case study on Jamon and Musa granites, Carajás area (Central Amazonian Province, Amazonian craton) shows a collision setting (previously thought anorogenic). A collision setting was clearly inferred for Bom Jardim granite, Xingú area (Central Amazonian Province, Amazonian craton) The third case study on Older São Jorge, Younger São Jorge and Maloquinha granites Tapajós area (Ventuari-Tapajós Province, Amazonian craton) indicated a within-plate setting (previously transitional between volcanic arc and within-plate). We also recognized a within-plate setting for the next three case studies on Aripuanã and Teles Pires granites (SW Amazonian craton), and Pitinga area granites (Mapuera Suite, NW Amazonian craton), which were all previously suggested to have been emplaced in post-collision to within-plate settings. The seventh case studies on Cassiterita-Tabuões, Ritápolis, São Tiago-Rezende Costa (south of São Francisco craton, Minas Gerais) showed a collision setting, which agrees fairly reasonably with a syn-collision tectonic setting indicated in the literature. A within-plate setting is suggested for the Serrinha magmatic suite, Mineiro belt (south of São Francisco craton, Minas Gerais), contrasting markedly with the arc setting suggested in the literature. The ninth case study on Rio Itapicuru granites and Rio Capim dacites (north of São Francisco craton, Serrinha block, Bahia) showed a continental arc setting. The tenth case study indicated within-plate setting for Rio dos Remédios volcanic rocks (São Francisco craton, Bahia), which is compatible with these rocks being the initial, rift-related igneous activity associated with the Chapada Diamantina cratonic cover. The eleventh, twelfth and thirteenth case studies on Bom Jesus-Areal granites, Rio Diamante-Rosilha dacite-rhyolite and Timbozal-Cantão granites (São Luís craton) showed continental arc, within-plate and collision settings, respectively. Finally, the last two case studies, fourteenth and fifteenth showed a collision setting for Caicó Complex and continental arc setting for Algodões (Borborema province).

An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: From subduction magmatism to cratonization

NASA Astrophysics Data System (ADS)

Johnson, Simon P.; Korhonen, Fawna J.; Kirkland, Christopher L.; Cliff, John B.; Belousova, Elena A.; Sheppard, Stephen

2017-01-01

The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as 'cratonization', is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons from several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons. The majority of magmatic zircons from the main magmatic cycles have Hf isotopic compositions that are generally more evolved than CHUR, forming vertical arrays that extend to moderately radiogenic compositions. Complimentary O isotope data, also show a significant variation in composition. However, combined, these data define not only the source components from which the magmas were derived, but also a range of physio-chemical processes that operated during magma transport and emplacement. These data also identify a previously unknown crustal reservoir in the Capricorn Orogen.

Sr and Nd isotopic compositions, age and petrogenesis of A-type granitoids of the Vernon Supersuite, New Jersey Highlands, USA

USGS Publications Warehouse

Volkert, R.A.; Feigenson, M.D.; Patino, L.C.; Delaney, J.S.; Drake, Avery A.

2000-01-01

Voluminous late Mesoproterozoic monzonite through granite of the Vernon Supersuite underlies an area of approximately 1300 km2 in the Highlands of northern New Jersey. The Vernon Supersuite consists of hastingsite ?? biotite-bearing granitoids of the Byram Intrusive Suite (BIS) and hedenbergite-bearing granitoids of the Lake Hopatcong Intrusive Suite (LHIS). These rocks have similar major and trace element abundances over a range of SiO2 from 58 to 75 wt.%, are metaluminous to weakly peraluminous, and have a distinctive A-type chemistry characterized by high contents of Y, Nb, Zr, LREE, and Ga/Al ratios, and low MgO, CaO, Sr and HREE. Whole-rock Rb-Sr isochrons of BIS granite yield an age of 1116 ?? 41 Ma and initial 87Sr/86Sr ratio of 0.70389, and of LHIS granite an age of 1095 ?? 9 Ma and initial 87Sr/86Sr ratio of 0.70520. Both suites have similar initial 143Nd/144Nd ratios of 0.511267 to 0.511345 (BIS) and 0.511359 to 0.511395 (LHIS). Values of ??(Nd) are moderately high and range from +1.21 to +2.74 in the BIS and +2.24 +2.95 in the LHIS. Petrographic evidence, field relationships, geochemistry, and isotopic data support an interpretation of comagmatism and the derivation of both suites from a mantle-derived or a juvenile lower crustal parent with little crustal assimilation. Both suites crystallized under overlapping conditions controlled by P-T-f(H(2)O). Lake Hopatcong magma crystallized at a liquidus temperature that approached 900??C and a pressure of about 6 kbar, and remained relatively anhydrous throughout its evolution. Initial P-T conditions of the Byram magma were ??? 850??C and about 5.5 kbar. BIS magma was emplaced contemporaneous with, or slightly preceding LHIS magma, and both magmas were emplaced during a compressional tectonic event prior to granulite facies metamorphism that occurred in the Highlands between 1080 and 1030 Ma. (C) 2000 Elsevier Science B.V. All rights reserved.

Geochronology and geochemistry of the Niujuanzi ophiolitic mélange, Gansu Province, NW China: implications for tectonic evolution of the Beishan Orogenic Collage

NASA Astrophysics Data System (ADS)

Wang, Shengdong; Zhang, Kexin; Song, Bowen; Li, Shucai; Li, Ming; Zhou, Jie

2018-01-01

The Niujuanzi ophiolitic mélange (NOM), located in the Beishan Orogenic Collage, marks the termination between the Huaniushan arc and Mingshui-Hanshan Massifs. The NOM is mainly composed of gabbros, diabases, plagiogranites, basalts, and greywacke. Two gabbros have ages of 433.8 ± 3.1 and 354.0 ± 3.3 Ma, two plagiogranites have ages of 429.8 ± 2 and 448.7 ± 2.0 Ma, and a diabase has an age of 433.4 ± 3.2 Ma. The gabbros and diabases are calc-alkaline and tholeiitic, with high Al2O3, CaO, and TiO2 contents and low FeOT contents. The gabbros have high Mg# values (49-82), while the diabases have relatively low Mg# values (46-61). The plagiogranites are calc-alkaline and metaluminous, with high SiO2 and Na2O contents and low Al2O3 and K2O contents. The gabbros and diabases are enriched in large iron lithophile elements and slightly depleted in high field strength elements relative to N-MORB and their trace element characteristics are similar to E-MORB. With respect to rare earth element (REE), they have slightly enriched LREEs relative to HREEs. The majority of the plagiogranite trace elements approximate those of the volcanic arc granite. The plagiogranites have obviously enriched LREEs relative to HREEs, with a slightly to strongly negative Eu anomaly, which is similar to ORG but distinct from volcanic arc and within plate granite. The NOM was formed from the Ordovician to the Carboniferous, representing the expansion period of the Niujuanzi Ocean. The gabbros, diabases, and plagiogranites were formed in a mid-ocean ridge environment. The gabbros and diabases were generated by different degrees of partial melting of the mantle, and the plagiogranites derived from both the crystallization differentiation of basaltic magma and the partial melting of amphibolites in the crust.

Constraints of C-O-S isotope compositions and the origin of the Ünlüpınar volcanic-hosted epithermal Pb-Zn ± Au deposit, Gümüşhane, NE Turkey

NASA Astrophysics Data System (ADS)

Akaryali, Enver; Akbulut, Kübra

2016-03-01

The Eastern Pontide Orogenic Belt (EPOB) constitutes one of the best examples of the metallogenic provinces in on the Alpine-Himalayan belt. This study focuses on the genesis of the Ünlüpınar Pb-Zn ± Au deposit in the southern part of the Eastern Pontide Orogenic Belt. The main lithological units in the study area are the Early Carboniferous Kurtoğlu Metamorphic Complex the Late Carboniferous Köse Granitoid and the Early-Middle Jurassic Şenköy Formation. The studied deposit is hosted by the Şenköy Formation, which consists predominantly of basaltic-andesitic rocks and associated pyroclastic rocks that are calc-alkaline in composition. Silicic, sulfidic, argillic, chloritic, hematitic, carbonate and limonite are the most obvious alteration types observed in the deposit site. Ore microscopy studies exhibit that the mineral paragenesis in deposits includes pyrite, chalcopyrite, sphalerite, galena, gold, quartz and calcite. Electron microprobe analyses conducted on sphalerite indicate that the Zn/Cd ratio varies between 84 and 204, and these ratios point at a hydrothermal deposit related to granitic magmas. Fluid inclusion studies in calcite and quartz show that the homogenization temperature of the studied deposit ranges between 90-160 °C and 120-330 °C respectively. The values of sulfur isotope analysis of pyrite, sphalerite and galena minerals vary between 1.6‰ and 5.7‰, and the results of oxygen and carbon isotope analysis range between 8.4‰ and 18‰ and -5‰ and -3.6‰, respectively. The average formation temperature of the ore was calculated as 264 °C with a sulfur isotope geothermometer. All of the data indicate that the Ünlüpınar deposit is an epithermal vein-type mineralization that was formed depending on the granitic magmatism.

LA-SF-ICP-MS zircon U-Pb geochronology of granitic rocks from the central Bundelkhand greenstone complex, Bundelkhand craton, India

NASA Astrophysics Data System (ADS)

Verma, Sanjeet K.; Verma, Surendra P.; Oliveira, Elson P.; Singh, Vinod K.; Moreno, Juan A.

2016-03-01

The central Bundelkhand greenstone complex in Bundelkhand craton, northern India is one of the well exposed Archaean supracrustal amphibolite, banded iron formation (BIF) and felsic volcanic rocks (FV) and associated with grey and pink porphyritic granite, tonalite-trondhjemite-granodiorite (TTG). Here we present high precision zircon U-Pb geochronological data for the pinkish porphyritic granites and TTG. The zircons from the grey-pinkish porphyritic granite show three different concordia ages of 2531 ± 21 Ma, 2516 ± 38 Ma, and 2514 ± 13 Ma, which are interpreted as the best estimate of the magmatic crystallization age for the studied granites. We also report the concordia age of 2669 ± 7.4 Ma for a trondhjemite gneiss sample, which is so far the youngest U-Pb geochronological data for a TTG rock suite in the Bundelkhand craton. This TTG formation at 2669 Ma is also more similar to Precambrian basement TTG gneisses of the Aravalli Craton of north western India and suggests that crust formation in the Bundelkhand Craton occurred in a similar time-frame to that recorded from the Aravalli craton of the North-western India.

Transformation from Paleo-Asian Ocean closure to Paleo-Pacific subduction: New constraints from granitoids in the eastern Jilin-Heilongjiang Belt, NE China

NASA Astrophysics Data System (ADS)

Ma, Xing-Hua; Zhu, Wen-Ping; Zhou, Zhen-Hua; Qiao, Shi-Lei

2017-08-01

The eastern Jilin-Heilongjiang Belt (EJHB) of NE China is a unique orogen that underwent two stages of evolution within the tectonic regimes of the Paleo-Asian and Paleo-Pacific oceans. 158 available zircon U-Pb ages, including 26 ages obtained during the present study and 132 ages from the literature, were compiled and analyzed for the Mesozoic and Cenozoic granitoids from the EJHB and the adjacent Russian Sikhote-Alin Orogenic Belt (SAOB), to examine the temporal-spatial distribution of the granitoids and to constrain the tectonic evolution of the East Asian continental margin. Five stages of granitic magmatism can be identified: Early Triassic (251-240 Ma), Late Triassic (228-215 Ma), latest Triassic to Middle Jurassic (213-158 Ma), Early Cretaceous (131-105 Ma), and Late Cretaceous to Paleocene (95-56 Ma). The Early Triassic granitoids are restricted to the Yanbian region along the Changchun-Yanji Suture, and show geochemical characteristics of magmas from a thickened lower crust source, probably due to the final collision of the combined NE China blocks with the North China Craton. The Late Triassic granitoids, with features of A-type granites, represent post-collisional magmatic activities that were related to post-orogenic extension, marking the end of the tectonic evolution of the Paleo-Asian Ocean. The latest Triassic to Paleocene granitoids with calc-alkaline characteristics were NE-trending emplaced along the EJHB and SAOB and young towards the coastal region, and represent continental marginal arc magmas that were associated with the northwestwards subduction of the Paleo-Pacific Plate. Two periods of magmatic quiescence (158-131 and 105-95 Ma) correspond to changes in the subduction direction of the Paleo-Pacific Plate from oblique relative to the continental margin to subparallel. Taking all this into account, we conclude that: (1) the final closure of the Paleo-Asian Ocean occurred along the Changchun-Yanji Suture during the Early Triassic; (2) the onset of the subduction of the Paleo-Pacific Plate beneath the East Asian continental margin probably took place during the latest Triassic (ca. 215 Ma); (3) changes in the drifting direction of the Paleo-Pacific Plate were responsible for the intermittent magmatic activities; and (4) roll-back of the subducted plate resulted in the oceanwards migration of the magmatic arc and large-scale back-arc extension throughout NE China during the Early Cretaceous.

Evolving Mantle Sources in Postcollisional Early Permian-Triassic Magmatic Rocks in the Heart of Tianshan Orogen (Western China)

NASA Astrophysics Data System (ADS)

Tang, Gong-Jian; Cawood, Peter A.; Wyman, Derek A.; Wang, Qiang; Zhao, Zhen-Hua

2017-11-01

Magmatism postdating the initiation of continental collision provides insight into the late stage evolution of orogenic belts including the composition of the contemporaneous underlying subcontinental mantle. The Awulale Mountains, in the heart of the Tianshan Orogen, display three types of postcollisional mafic magmatic rocks. (1) A medium to high K calc-alkaline mafic volcanic suite (˜280 Ma), which display low La/Yb ratios (2.2-11.8) and a wide range of ɛNd(t) values from +1.9 to +7.4. This suite of rocks was derived from melting of depleted metasomatized asthenospheric mantle followed by upper crustal contamination. (2) Mafic shoshonitic basalts (˜272 Ma), characterized by high La/Yb ratios (14.4-20.5) and more enriched isotope compositions (ɛNd(t) = +0.2 - +0.8). These rocks are considered to have been generated by melting of lithospheric mantle enriched by melts from the Tarim continental crust that was subducted beneath the Tianshan during final collisional suturing. (3) Mafic dikes (˜240 Ma), with geochemical and isotope compositions similiar to the ˜280 Ma basaltic rocks. This succession of postcollision mafic rock types suggests there were two stages of magma generation involving the sampling of different mantle sources. The first stage, which occurred in the early Permian, involved a shift from depleted asthenospheric sources to enriched lithospheric mantle. It was most likely triggered by the subduction of Tarim continental crust and thickening of the Tianshan lithospheric mantle. During the second stage, in the middle Triassic, there was a reversion to more asthenospheric sources, related to postcollision lithospheric thinning.

The evolution of a Gondwanan collisional orogen: A structural and geochronological appraisal from the Southern Granulite Terrane, South India

NASA Astrophysics Data System (ADS)

Plavsa, Diana; Collins, Alan S.; Foden, John D.; Clark, Chris

2015-05-01

Gondwana amalgamated along a suite of Himalayan-scale collisional orogens, the roots of which lace the continents of Africa, South America, and Antarctica. The Southern Granulite Terrane of India is a generally well-exposed, exhumed, Gondwana-forming orogen that preserves a record of the tectonic evolution of the eastern margin of the East African Orogen during the Ediacaran-Cambrian (circa 600-500 Ma) as central Gondwana formed. The deformation associated with the closure of the Mozambique Ocean and collision of the Indian and East African/Madagascan cratonic domains is believed to have taken place along the southern margin of the Salem Block (the Palghat-Cauvery Shear System, PCSS) in the Southern Granulite Terrane. Investigation of the structural fabrics and the geochronology of the high-grade shear zones within the PCSS system shows that the Moyar-Salem-Attur shear zone to the north of the PCSS system is early Paleoproterozoic in age and associated with dextral strike-slip motion, while the Cauvery shear zone (CSZ) to the south of the PCSS system can be loosely constrained to circa 740-550 Ma and is associated with dip-slip dextral transpression and north side-up motion. To the south of the proposed suture zone (the Cauvery shear zone), the structural fabrics of the Northern Madurai Block suggest four deformational events (D1-D4), some of which are likely to be contemporaneous. The timing of high pressure-ultrahigh temperature metamorphism and deformation (D1-D3) in the Madurai Block (here interpreted as the southern extension of Azania) is constrained to circa 550-500 Ma and interpreted as representing collisional orogeny and subsequent orogenic collapse of the eastern margin of the East African Orogen. The disparity in the nature of the structural fabrics and the timing of the deformation in the Salem and the Madurai Blocks suggest that the two experienced distinct tectonothermal events prior to their amalgamation along the Cauvery shear zone during the Ediacaran/Cambrian.

S-type granitic magmas—petrogenetic issues, models and evidence

NASA Astrophysics Data System (ADS)

Clemens, J. D.

2003-04-01

Despite a perception that it represents a perverse divergence, it is perfectly possible to believe in the existence of S- and I-type granites (and the implications for the nature of their protoliths), and to disbelieve in the applicability of the restite-unmixing model for chemical variation in granitic magmas. White and Chappell erected the S-I classification with impeccable validity. The isotopic evidence demands contrasting source reservoirs for S- and I-type granitic magmas. However, the major advance was not the classification, but the recognition that highly contrasting parental materials must be involved in the genesis of granitic magmas. The restite-unmixing model is commonly seen as a companion to the S-I classification, but it is really a separate issue. This model implies that the compositions of granites 'image' those of their source rocks in a simple way. However, there are other equally valid models that can explain the data, and none of them represents a unique solution. The most cogent explanation for the high-grade metasedimentary enclaves in most S-type granites is that they represent mid-crustal xenoliths; restitic enclaves are either rare or absent. Inherited zircons in S-type rocks are certainly restitic. However, the occurrence of a substantial restitic zircon population does not imply an equally substantial restitic component in the rest of the rock. Zircon and zirconium behaviours are controlled by disequilibrium and kinetics, and Zr contents of granitic rocks can rarely be used to infer magma temperatures. Since the dominant ages among inherited zircons in Lachlan Fold Belt (LFB) S-type granites are Ordovician and Proterozoic, it seems likely that crust of this age, but geochemically different from the exposed rocks, not only underlies much of the LFB but also forms a component in the granite magma sources. The evidence is overwhelming that the dark, microgranular enclaves that occur in both S- and I-type granites are igneous in origin. They represent globules of quenched, more mafic magma mingled and modified by exchange with the host granitic magma. However, magma mixing does not appear to be a significant process affecting the chemical evolution of the host magmas. Likewise, the multicomponent mixing models erected for some granitic rock suites are mathematically nonunique and, in some cases, violate constraints from isotopic studies. S- and I-type magmas commonly retain their distinct identities. This suggests limited source mixing, limited magma mixing and limited wall-rock assimilation. Though intermediate types certainly exist, they are probably relatively minor in volume. Crystal fractionation probably plays the major role in the differentiation of very many granitic magmas, including most S-types, especially those emplaced at high crustal levels or in the volcanic environment. Minor mechanisms include magma mixing, wall-rock assimilation and restite unmixing. Isotopic variations within plutons and in granite suites could be caused by source heterogeneities, magma mixing, assimilation and even by isotopic disequilibrium. However, source heterogeneity, coupled with the inefficiency of magma mixing is probably the major cause of observed heterogeneity. Normal geothermal gradients are seldom sufficient to provide the necessary heat for partial melting of the crust, and crustal thickening likewise fails to provide sufficient heat. Generally, the mantle must be the major heat source. This might be provided through mantle upwelling and crustal thinning, and possibly through the intra- and underplating of mafic magmas. Upper crustal extension seems to have been common in regions undergoing granitic magmatism. Migmatites probably provide poor analogues of granite source regions because they are mostly formed by fluid-present reactions. Granitic magmas are mostly formed by fluid-absent processes. Where we do see rare evidence for arrested fluid-absent partial melting, the melt fraction is invariably concentrated into small shear zones, veinlets and small dykes. Thus, it seems likely that dyking is important in transporting granitic magma on a variety of scales and at many crustal levels. However, one major missing link in the chain is the mechanism by which melt fractions, in small-scale segregations occurring over a wide area, can be gathered and focused to efficiently feed much wider-spaced major magma conduits. Answers may lie in the geometry of the melting zones and in the tendency of younger propagating fractures to curve toward and merge with older ones. Self-organization almost certainly plays a role.

Kilbuck terrane: oldest known rocks in Alaska

USGS Publications Warehouse

Box, S.E.; Moll-Stalcup, E. J.; Wooden, J.L.; Bradshaw, J.Y.

1990-01-01

The Kilbuck terrane in southwestern Alaska is a narrow, thin crustal sliver or flake of amphibolite facies orthogneiss. The igneous protolith of this gneiss was a suite of subduction-related plutonic rocks. U-Pb data on zircons from trondhjemitic and granitic samples yield upper-intercept (igneous) ages of 2070 ?? 16 and 2040 ?? 74 Ma, respectively. Nd isotope data from these rocks suggest that a diorite-tonalite-trondhjemite suite (??Nd[T] = +2.1 to +2.7; T is time of crystallization) evolved from partial melts of depleted mantle with no discernible contamination by older crust, whereas a coeval granitic pluton (??Nd[T] = -5.7) contains a significant component derived from Archean crust. Orthogneisses with similar age and Nd isotope characteristics are found in the Idono complex 250 km to the north. Early Proterozoic rocks are unknown elsewhere in Alaska. The possibility that the Kilbuck terrane was displaced from provinces of similar age in other cratons (e.g., Australian, Baltic, Guiana, and west African shields), or from the poorly dated Siberian craton, cannot be excluded. -from Authors

The susceptibility of large river basins to orogenic and climatic drivers

NASA Astrophysics Data System (ADS)

Haedke, Hanna; Wittmann, Hella; von Blanckenburg, Friedhelm

2017-04-01

Large rivers are known to buffer pulses in sediment production driven by changes in climate as sediment is transported through lowlands. Our new dataset of in situ cosmogenic nuclide concentration and chemical composition of 62 sandy bedload samples from the world largest rivers integrates over 25% of Earth's terrestrial surface, distributed over a variety of climatic zones across all continents, and represents the millennial-scale denudation rate of the sediment's source area. We can show that these denudation rates do not respond to climatic forcing, but faithfully record orogenic forcing, when analyzed with respective variables representing orogeny (strain rate, relief, bouguer anomaly, free-air anomaly), and climate (runoff, temperature, precipitation) and basin properties (floodplain response time, drainage area). In contrast to this orogenic forcing of denudation rates, elemental bedload chemistry from the fine-grained portion of the same samples correlates with climate-related variables (precipitation, runoff) and floodplain response times. It is also well-known from previous compilations of river-gauged sediment loads that the short-term basin-integrated sediment export is also climatically controlled. The chemical composition of detrital sediment shows a climate control that can originate in the rivers source area, but this signal is likely overprinted during transfer through the lowlands because we also find correlation with floodplain response times. At the same time, cosmogenic nuclides robustly preserve the orogenic forcing of the source area denudation signal through of the floodplain buffer. Conversely, previous global compilations of cosmogenic nuclides in small river basins show the preservation of climate drivers in their analysis, but these are buffered in large lowland rivers. Hence, we can confirm the assumption that cosmogenic nuclides in large rivers are poorly susceptible to climate changes, but are at the same time highly suited to detect changes in orogenic forcing in their paleo sedimentary records.

Early Silurian to Early Carboniferous ridge subduction in NW Junggar: Evidence from geochronological, geochemical, and Sr-Nd-Hf isotopic data on alkali granites and adakites

NASA Astrophysics Data System (ADS)

Zhang, Chen; Santosh, M.; Liu, Luofu; Luo, Qun; Zhang, Xin; Liu, Dongdong

2018-02-01

The Central Asian Orogenic Belt (CAOB) evolved through a long-lived orogeny involving multiple episodes of subduction and accretion marking a major phase of continental growth during the Paleozoic. The northern part of the Western Junggar region (NW Junggar) offers a window into these processes, particularly to constrain the timing of closure of the Paleo-Asian Ocean. Here we report geochemical, geochronological, and isotopic data from K-feldspar granites and adakitic rocks from the NW Junggar region. Zircon U-Pb ages suggest that the granites were emplaced during Early Silurian to the Early Carboniferous (434-328 Ma). The granites show geochemical characteristics similar to those of A-type granites, with high SiO2 (71.13-76.72 wt%), Na2O + K2O (8.00-9.59 wt%), and Al2O3 (12.28-14.08 wt%), but depleted Sr, Nb, Ta and Eu. They display moderate to high positive εNd(t) and εHf(t) values (4.26-8.21 and 7.69-14.60, respectively) and young Nd and Hf model ages (T2DM-Nd = 489-740 Ma and T2DM-Hf = 471-845 Ma), suggesting magma derivation through partial melting of lower crust in the Boshchekul-Chingiz and Zharma-Saur arcs. The adakites are characterized by high Sr content (406.5-751.6 ppm), and low Y (13.8-16.4 ppm) and Yb (1.5-1.8 ppm) content, yielding relatively high Sr/Y ratios (25.38-49.41) similar to those of modern adakites. They have high positive εNd(t) and εHf(t) values (7.85-8.25 and 13.23-15.97, respectively) and young Nd and Hf model ages (T2DM-Nd = 429-535 Ma and T2DM-Hf = 355-550 Ma), indicating that their source magmas were likely derived from partial melting of the oceanic crust beneath the Boshchekul-Chingiz arc. The petrogenesis and distribution of the A-type granites and adakites, as well as the tectonic architecture of the region, suggest that a ridge subduction event might have occurred during the Early Silurian to Early Carboniferous. In combination with previous studies in the Chinese Altai, we suggest a two-sided ridge subduction model for the Junggar-Altai region.

A-type granites from the Guéra Massif, Central Chad: Petrology, geochemistry, geochronology, and petrogenesis.

NASA Astrophysics Data System (ADS)

Pham, Ngoc Ha T.; Shellnutt, J. Gregory; Yeh, Meng-Wan; Lee, Tung-Yi

2017-04-01

The poorly studied Saharan Metacraton of North-Central Africa is located between the Arabian-Nubian Shield in the east, the Tuareg Shield in the west and the Central African Orogenic Belt in the south. The Saharan Metacraton is composed of Neoproterozoic juvenile crust and the relics of pre-Neoproterozoic components reactivated during the Pan-African Orogeny. The Republic of Chad, constrained within the Saharan Metacraton, comprises a Phanerozoic cover overlying Precambrian basement outcroppings in four distinct massifs: the Mayo Kebbi, Tibesti, Ouaddaï, and the Guéra. The Guéra massif is the least studied of the four massifs but it likely preserves structures that were formed during the collision between Congo Craton and Saharan Metacraton. The Guéra Massif is composed of mostly granitic rocks. The granitoids have petrologic features that are consistent with A-type granite, such as micrographic intergrowth of sodic and potassic feldspar, the presence of sodic- and iron-rich amphibole, and iron-rich biotite. Compositionally, the granitic rocks of the Guéra Massif have high silica (SiO2 ≥ 68.9 wt.%) content and are metaluminous to marginally peraluminous. The rocks are classified as ferroan calc-alkalic to alkali-calcic with moderately high to very high Fe* ratios. The first zircon U/Pb geochronology of the silicic rocks from the Guéra Massif yielded three main age groups: 590 Ma, 570 Ma, 560 Ma, while a single gabbro yielded an intermediate age ( 580 Ma). A weakly foliated biotite granite yielded two populations, in which the emplacement age is interpreted to be 590 ± 10 Ma, whereas the younger age (550 ± 11 Ma) is considered to be a deformation age. Furthermore, inherited Meso- to Paleoproterozoic zircons are found in this sample. The geochemical and geochronology data indicate that there is a temporal evolution in the composition of rocks with the old, high Mg# granitoids shifting to young, low Mg# granitoids. This reveals that the A-type granites in the Guéra Massif were probably derived by the repeated melting of a single source region, from an initial fertile source to a less fertile source at distinct intervals. Finally, the oldest inherited zircons indicate that the Guéra Massif is either built upon Paleoproterozoic continental crust or that pre-Neoproterozoic rocks were the source of the silicic Ediacaran rocks.

Origin and evolution of multi-stage felsic melts in eastern Gangdese belt: Constraints from U-Pb zircon dating and Hf isotopic composition

NASA Astrophysics Data System (ADS)

Guo, Liang; Zhang, Hong-Fei; Harris, Nigel; Pan, Fa-Bin; Xu, Wang-Chun

2011-11-01

This integrated study of whole rock geochemistry, zircon U-Pb dating and Hf isotope composition for seven felsic rocks from the Nyingchi Complex in eastern Himalayan syntaxis has revealed a complex magmatic history for the eastern Gangdese belt. This involves multiple melt sources and mechanisms that uniquely identify the tectonic evolution of this part of the Himalayan orogen. Our U-Pb zircon dating reveals five stages of magmatic or anatectic events: 165, 81, 61, 50 and 25 Ma. The Jurassic granitic gneiss (165 Ma) exhibits εHf(t) values of + 1.4 to + 3.5. The late Cretaceous granite (81 Ma) shows variable εHf(t) values from - 0.9 to + 6.2, indicating a binary mixing between juvenile and old crustal materials. The Paleocene granodioritic gneiss (61 Ma) has εHf(t) values of + 5.4 to + 8.0, suggesting that it originated from partial melting of a juvenile crustal material. The Eocene anatexis is recorded in the leucosome, which has Hf isotopic composition similar to that of the Jurassic granite, indicating that the leucosome could be derived from partial melting of the Jurassic granite. The late Oligocene biotite granite (25 Ma) shows adakitic geochemical characteristics, with Sr/Y = 49.3-56.6. The presence of a large number of inherited zircons and negative εHf(t) values suggest that it sourced from anatexis of crustal materials. In contrast to the Gangdese batholiths that are mainly derived from juvenile crustal source in central Tibet, the old crustal materials play an important role for the magma generation of the felsic rocks, suggesting the existence of a crustal basement in the eastern Gangdese belt. These correspond to specific magmatic evolution stages during the convergence between India and Asia. The middle Jurassic granitic gneiss resulted from the northward subduction of the Neo-Tethyan oceanic slab. The late Cretaceous magmatism is probably related to the ocean ridge subduction. The Paleocene-Eocene magmatism, metamorphism and anatexis are interpreted to result from roll-back and break-off of the subducted Neo-Tethyan slab that occurred in the early stage of the India-Asian collision, respectively. The late Oligocene adakitic rocks resulted from the break-off of the subducted Indian continental crust starting at ~ 25 Ma.

Selected physical properties of rocks from the Baid al Jimalah West tungsten deposit, Kingdom of Saudi Arabia, and recommendations for geophysical surveys

USGS Publications Warehouse

Gettings, M.E.

1983-01-01

Bulk density and magnetic susceptibility of 11 outcrop samples representing the Proterozoic lithologic units at the Baid al Jimalah West tungsten deposit, Kingdom of Saudi Arabia were measured. Induced polarization response, apparent resistivity, and electromagnetic conductivity were determined for four specimens of the sample suite. Measurements show that there is a negative density contrast of about -0.17 g-cm^-3 between metasedimentary rocks of the Murdama group and the Bald al Jimalah graaite and that this contrast decreases with increasing mineralization of the granite. Similarly, the bulk magnetic susceptibility of the granite is about one-third that of the Murdama rocks for this sample suite; however, magnetic susceptibility increases with increasing mineralization in the granite specimens. Electromagnetic conductivities are uniformly low, in part because the specimens are weathered, but probably also because intense silicification accompanies the mineralization. Induced polarization chargeability increases in the granitic specimens with increasing mineralization and reflects higher percentages of sulfide minerals. Chargeability for the mineralized rocks is about four times higher than for the Murdama host rocks, and apparent resistivity values are about one-fifth the values of host rocks. Based on these results, it is recommended that during reconnaissance exploration of the area 15 detailed high-precision gravity profiles at 10 m to 50 m station spacing and eight induced polarization dipole-dipole profiles at 25 m dipole spacing and maximum 'n' of 6 be measured. To help define subsurface structure, a high-precision, ground-magnetic survey (map at 2-gamma contour interval) and a four-channel gamma ray spectrometric survey on a 25x50 m grid covering the area of the profiles are recommended.

Onset of the Sveconorwegian orogeny: 1220-1130 Ma bimodal magmatism, sedimentation and granulite-facies metamorphism

NASA Astrophysics Data System (ADS)

Bingen, Bernard; Viola, Giulio; Engvik, Ane K.; Solli, Arne

2013-04-01

The Grenville orogen of Laurentia and the Sveconorwegian orogen of Baltica are generally interpreted as long-lived, hot, collisional orogens resulting from collision of a possibly joined Laurentia-Baltica margin with another major plate, possibly Amazonia. Here we report new mapping, petrologic and SIMS U-Pb geochronological data from S Norway, to address the pre- to early-Sveconorwegian evolution between 1220 and 1130 Ma. The Sveconorwegian belt includes from west to east the Telemarkia terrane characterized by 1520-1480 Ma magmatism and the Idefjorden terrane characterized by Gothian active margin 1660-1520 Ma magmatism. The Idefjorden terrane is thrusted eastwards onto the parauthochthonous Eastern Segment. The Kongsberg and Bamble are two small terranes between the Idefjorden and Telemarkia terranes. They have a strong N-S and NE-SW structural grain, respectively, and are thrust westwards on top of the Telemarkia terrane. Basement metavolcanic and metaplutonic rocks in the Kongsberg terrane range from c. 1534 to 1500 Ma (5 new samples) and in Bamble from c. 1572 to 1460 Ma, overlapping with both the Telemarkia and Idefjorden terranes. New and published data show the following: (1) In Telemark, a c. 1200 Ma granitoid from the Flåvatn complex and a c. 1195 Ma granite sheet in the bimodal Nissedal supracrustals demonstrate that 1220-1180 Ma comparatively juvenile magmatism is the dominant rock type over much of southern part of Telemark. (2) A rhyolite dated at 1155 Ma complement available data showing low grade bimodal mafic-felsic volcanism interlayered with immature clastic sediments in central Telemark between 1169 and 1145 Ma (the ex-Bandak group). These supracrustals are intruded by c. 1153-1144 Ma A-type granite plutons. (3) Ten samples of foliated commonly porphyritic ganitoid and one granite dyke in gabbro collected in Kongsberg and along the Kongsberg-Telemark boundary demonstrate that c. 1171-1147 Ma bimodal plutonism occurred in Kongsberg. This indicates that Kongsberg was linked to Telemarkia, before 1147 Ma and before their final tectonic juxtaposition. A similar pattern is known between the Bamble and Telemarkia terranes, indicating similar relations. (4) The classical medium pressure granulite-facies metamorphism in Tromøy-Arendal, Bamble, was redated. Three granulite samples show metamorphic zircon at 1147 +/-18 and 1132 +/-7 Ma. Protolith ages between c. 1553 and 1544 Ma demonstrate a Gothian low-K calc-alkaline orthogneiss protolith and question recent interpretations representing the Tromøy complex as an early Sveconorwegian oceanic volcanic arc accreted to the Bamble terrane. (5) A granulite-facies domain was discovered north of Kragerø in Bamble, in an area generally assigned to amphibolites-facies metamorphism. Geothermobarometry and pseudosection calculation using the Grt +Opx +/-Cpx +Pl +Qtz assemblage yield an estimate of about 1.15 GPa and 800°C for peak granulite facies metamorphism. Late clinopyroxene and garnet zoning are consistent with an anticklockwise P-T path and suggest magma loading and heating of the crust. Soccer ball zircon dates this metamorphism at 1144 ±6 Ma. (6) C. 1193-1183 Ma A-type granite plutonism is reported in the Caledonian Middle-Allochthon Risberget Nappe and c. 1221-1204 Ma syenite plutons are known along the Sveconorwegian Frontal Deformation Zone. C. 1220-1130 Ma magmatism is however entirely lacking in the Idefjorden terrane. Using these constraints, we envisage the 1220-1130 Ma pre- to early-Sveconorwegian event in a trans(?)-tensional continental setting at the margin of Baltica, before final continental collision. The Telemarkia terrane was possibly located in a back arc position above an east dipping subduction system. Abundant magmatism is possibly a consequence of subduction of an oceanic ridge. Inversion took place after 1130 Ma leading to westwards thrusting of the Bamble and Kongsberg terranes.

The Freyenstein Shear Zone - Implications for exhumation of the South Bohemian Batholith (Moldanubian Superunit, Strudengau, Austria)

NASA Astrophysics Data System (ADS)

Griesmeier, Gerit; Iglseder, Christoph; Konstantin, Petrakakis

2016-04-01

The Moldanubian superunit is part of the internal zone of the Variscan Orogen in Europe and borders on the Saxothuringian and Sudetes zones in the north. In the south, it is blanketed by the Alpine foreland molasse. Tectonically it is subdivided into the Moldanubian Nappes (MN), the South Bohemian Batholith (SBB) and the Bavarian Nappes. This work describes the ~ 500 m thick Freyenstein shear zone, which is located at the southern border of the Bohemian Massif north and south of the Danube near Freyenstein (Strudengau, Lower Austria). The area is built up by granites of Weinsberg-type, which are interlayered by numerous dikes and paragneisses of the Ostrong nappe system. These dikes include medium grained granites and finegrained granites (Mauthausen-type granites), which form huge intrusions. In addition, smaller intrusions of dark, finegrained diorites und aplitic dikes are observed. These rocks are affected by the Freyenstein shear zone und ductily deformed. Highly deformed pegmatoides containing white mica crystals up to one cm cut through the deformed rocks and form the last dike generation. The Freyenstein shear zone is a NE-SW striking shear zone at the eastern edge of the SBB. The mylonitic foliation is dipping to the SE with angles around 60°. Shear-sense criteria like clast geometries, SĆ structures as well as microstructures show normal faulting top to S/SW with steep (ca. 50°) angles. The Freyenstein shear zone records a polyphase history of deformation and crystallization: In a first phase, mylonitized mineral assemblages in deformed granitoides can be observed, which consist of pre- to syntectonic muscovite-porphyroclasts and biotite as well as dynamically recrystallized potassium feldspar, plagioclase and quartz. The muscovite porphyroclasts often form mica fishes and show top to S/SW directed shear-sense. The lack of syntectonic chlorite crystals points to metamorphic conditions of lower amphibolite-facies > than 450° C. In a later stage fluid infiltration under lower greenschist-facies conditions locally lead to sericitization of feldspar and development of pseudomorphs after it. In addition, syn-mylonitic biotite has been chloritized mimetically. Chlorite growth across the mylonitic foliation occurs rarely. Brittle faulting, overprinting the shear zone features, is documented by the occurrence of numerous harnish planes. They show normal faulting to the N with angles around 30° and locally sinistral shear-sense. The Freyenstein shear zone belongs to a system of NE-SW striking shear zones and faults in the Moldanubian superunit and is located at the border between the SBB and MN ductily deforming both. Therefore, it plays an important role in exhumation processes of last stage SBB (synkinematic) intrusions during Late Variscan orogenic extension. According to cooling ages in other shear zones and (synkinematic) intrusions an age of ca. 320-290 Ma for the ductile deformation can be assumed.

Leucogranites in Lhozag, southern Tibet: Implications for the tectonic evolution of the eastern Himalaya

NASA Astrophysics Data System (ADS)

Huang, Chunmei; Zhao, Zhidan; Li, Guangming; Zhu, Di-Cheng; Liu, Dong; Shi, Qingshang

2017-12-01

Petrogenesis of the Himalayan leucogranite is strongly influenced by conditions which are associated with the tectonic evolution of Himalayan orogen. In this article, we present petrological, geochronological and geochemical results of the Lhozag leucogranites that crop out alongside the South Tibetan Detachment System (STDS) in the east of Himalaya. Zircon U-Pb dating revealed three episodes of leucogranitic magmatism in Lhozag at 17.8 ± 0.1 Ma, 15.1 ± 0.1 Ma, and 12.0 ± 0.1 Ma, respectively. The Lhozag leucogranites show relatively low εNd(t), low zircon εHf(t) and high initial 87Sr/86Sr ratios, which are similar to the High Himalayan Crystalline Series (HHCS), indicating that they were derived from the HHCS. The characteristics of relatively high Na2O and Rb contents, high Rb/Sr ratios and low CaO, MgO, TFe2O3, TiO2, and Sr contents indicate that both the ca. 18 Ma Lhozag tourmaline leucogranites and the ca. 15 Ma Lhozag two-mica granites were derived from fluid-absent muscovite-dehydration melting of metasediments. The opposite geochemistry characteristics of the ca. 12 Ma Khula Kangri two-mica granites imply that these granites are derived from fluid-present melting of metasediments. Four Khula Kangri two-mica granite samples with relatively lower TiO2, TFe2O3, MgO, and CaO contents, higher Rb concentrations and Rb/Sr ratios could be evolved from the Khula Kangri two-mica granites with relatively lower Rb/Sr ratios. The melting behaviors of the Lhozag leucogranites varied from fluid-absent melting to fluid-present melting, implying that there were P-T-XH2O variations in the deep crust. The tectonic evolution would give rise to variation of P-T-XH2O variation, and subsequent transformation of melting behavior. Our new results display the transformation of melting behavior of the Lhozag leucogranites, which implies the tectonic evolution from earlier N-S extension to later E-W extension in the eastern Himalaya at ca. 12 Ma.

O, Sr and Nd isotopic constraints on Cenozoic granitoids of Northwestern Anatolia, Turkey: Enrichment by subduction zone fluids

NASA Astrophysics Data System (ADS)

Yücel-Öztürk, Yeşim

2016-05-01

The oxygen and strontium isotope compositions of Cenozoic granitoids cropping out in the İzmir-Ankara-Erzincan suture zone help constrain the petrological evolution of magmatism in northwest Anatolia. The magmatism was mostly widespread between late Eocene (∼37 Ma) and the middle Miocene (∼14-15 Ma), and is represented by volcanic and plutonic rocks of orogenic affinity, of which Ezine, Eğrigöz, Çataldağ and Kozak are the largest Tertiary granitic plutons exposed in northwest Anatolia. They vary from granite to granodiorite, and are subalkaline, belonging to the high-K calc-alkaline I-type granite series. All these characteristics, combined with major, trace element geochemical data as well as mineralogical and textural evidence, reveal that the Oligocene-Miocene granitoids of NW Anatolia are comparable with volcanic arc granites, formed in a transitional oceanic to continental collisional tectonic setting, from a hybrid source, having crustal and mantle components that underwent further interaction with the upper crust. These plutons have initial 87Sr/86Sr ratios of 0.7072-0.7094, and εNd(t) values ranging from -3.48 to -1.20. These characteristics also indicate that a crustal component played an important role in the petrogenesis of NW Anatolian Oligocene-Miocene granitoids. The moderately evolved Ezine, Eğrigöz, Çataldağ and Kozak granitoids, have δ18O values that are consistent with those of normal I-type granites (6-10‰), but the δ18O relationships among minerals of samples collected from the intrusive contacts which are closest to mineralized zones, indicate a major influence of hydrothermal processes under subsolidus conditions. The oxygen isotope systematics of the samples from these plutons result from the activity of high-δ18O fluids (magmatic water), with major involvement of low-δ18O fluids (meteoric water) evident, near the edge zone of these plutons. This is most evident in δ18O quartz-feldspar pairs from these granitoids, which commonly have values characteristic of open-system hydrothermal conditions, and is consistent with the presence of large scale base-metal mineralization around the NW Anatolian granitoids.

Along-strike continuity of structure, stratigraphy, and kinematic history in the Himalayan thrust belt: The view from Northeastern India

NASA Astrophysics Data System (ADS)

DeCelles, P. G.; Carrapa, B.; Gehrels, G. E.; Chakraborty, T.; Ghosh, P.

2016-12-01

The Himalaya consists of thrust sheets tectonically shingled together since 58 Ma as India collided with and slid beneath Asia. Major Himalayan structures, including the South Tibetan Detachment (STD), Main Central Thrust (MCT), Lesser Himalayan Duplex (LHD), Main Boundary Thrust (MBT), and Main Frontal Thrust (MFT), persist along strike from northwestern India to Arunachal Pradesh near the eastern end of the orogenic belt. Previous work suggests significant basement involvement and a kinematic history unique to the Arunachal Himalaya. We present new geologic and geochronologic data to support a regional structural cross section and kinematic restoration of the Arunachal Himalaya. Large Paleoproterozoic orthogneiss bodies (Bomdila Gneiss) previously interpreted as Indian basement have ages of 1774-1810 Ma, approximately 50 Ma younger than Lesser Himalayan strata into which their granitic protoliths intruded. Bomdila Gneiss is therefore part of the Lesser Himalayan cover sequence, and no evidence exists for basement involvement in the Arunachal Himalaya. Minimum shortening in rocks structurally beneath the STD is 421 km. The MCT was active during the early Miocene; STD extension overlapped MCT shortening and continued until approximately 15-12 Ma; and growth of the LHD began 11 Ma, followed by slip along the MBT (post-7.5 Ma) and MFT (post-1 Ma) systems. Earlier thrusting events involved long-distance transport of strong, low-taper thrust sheets, whereas events after 12-10 Ma stacked smaller, weaker thrust sheets into a steeply tapered orogenic wedge dominated by duplexing. A coeval kinematic transition is observed in other Himalayan regions, suggesting that orogenic wedge behavior was controlled by rock strength and erodibility.

Moho Depth and Bulk Crustal Properties in Northern Quebec and Labrador

NASA Astrophysics Data System (ADS)

Vervaet, F.; Darbyshire, F. A.

2016-12-01

Northern Quebec and Labrador lie at the heart of the Laurentian landmass and preserve over 3 billion years of continental evolution. In this region the Archean Superior and Nain cratons are surrounded by Paleoproterozoic orogens such as New-Quebec, Trans-Hudson and Torngat, as well as the younger Grenville orogen to the SE. Study of crustal structure in this region provides valuable information on the assembly of the North American continent. We use data from 8 seismic stations installed in summer 2011 as part of the QUiLLE (Quebec-Labrador Lithospheric Experiment) project to investigate crustal structure, using receiver function analysis. The data set covers 5 years (2011-2016) for most of the stations, comprising several hundred events of magnitude ≥5 and epicentral distance 30-90°. After initial data processing and quality control, several tens of events per station were used in an H-κ stacking analysis to estimate Moho depth and bulk crustal properties. Some stations show significant complexity in their receiver functions, leading to inconclusive H-κ results, but the majority show a consistent Moho signal from which crustal parameters are successfully extracted. Crustal thickness varies from 33 to 49 km, with the thickest crust associated with the Trans-Hudson orogen in the Ungava region of northernmost Quebec and the thinnest beneath the central Labrador coast. Vp/Vs ratios (κ) lie in the range 1.71-1.86, with the majority of values consistent with granite-gneiss-tonalite bulk crustal compositions. The receiver functions are combined with surface-wave group velocity data to model the crustal structures in more detail beneath each station, allowing us to investigate crustal layering, Moho complexity and lateral heterogeneity.

Geological and Geophysical Integration Regarding a Structural Evolution Modelling of a Suture Zone Controlled by a Cratonic Buttress - The Case of Dom Feliciano Orogenic Belt, SSE Brazil, Implications for Western Gondwana Assembly

NASA Astrophysics Data System (ADS)

Bruno, H.; Almeida, J.; Heilbron, M. C. P. L.; Salomão, M.

2017-12-01

The matters surrounding the amalgamation of tectonic blocks during the Brasiliano / Pan-African orogeny have been the main subject of study of several works in recent years. The main objective of this work is the hierarchy and discrimination of the boundaries between the known tectonic blocks, integrating geological and geophysical data. The geology of the study area is dominated by Precambrian terranes; Luís Alves Terrane, the vulcanosedimentary sequences of the Itajaí and Campo Alegre Basins, the metasedimentary sequences of the Brusque and Paranaguá Terranes and their granitic suites besides the granitoids of the Florianópolis Terrane. The shear zones and faults that separate these crustal blocks were developed during the Brasiliano / Pan-African orogenic cycle that led to the formation of the supercontinent Gondwana. These tectonic boundaries generally separate blocks of different rheology and crustal thickness. The integration of geological and geophysical data allowed the identification of important structural lineaments and crustal boundaries. The presented geodynamic model suggests that the suture between the block composed of the Brusque, Paranaguá and Florianópolis Terranes and the block composed by the Luís Alves Terrane is the Itajaí Perimbó Shear Zone, and not the Major Gercino Shear Zone as previously suggested. Considering the Itajaí Perimbó Shear Zone as the suture zone, the metassediments of the Brusque Terrane were deposited on the basement of the Florianópolis Terrane, hereby declared as part of the Angola Craton, and are correlated to the metassediments of the Paranaguá Terrane as a passive margin that in approximately ca. 650 My became active margin, functioning as a forearc basin. The oblique collision between the blocks would have occurred with the development of a dextral transpression in the Itajaí Perimbó Shear Zone, separating the Luís Alves Terrane from the Brusque Terrane, a sinistral transcurrence represented by the Palmital Shear Zone separating the Luís Alves Terrane from the Paranaguá Terrane and a frontal thrust, represented by the Icapara and Serra Negra Shear Zones, separating the already amalgamated block from the Luís Alves and Curitiba Terranes of the Paranaguá Terrane.

Crustal structure of the alaska range orogen and denali fault along the richardson highway

USGS Publications Warehouse

Fisher, M.A.; Pellerin, L.; Nokleberg, W.J.; Ratchkovski, N.A.; Glen, J.M.G.

2007-01-01

A suite of geophysical data obtained along the Richardson Highway crosses the eastern Alaska Range and Denali fault and reveals the crustal structure of the orogen. Strong seismic reflections from within the orogen north of the Denali fault dip as steeply as 25?? north and extend downward to depths between 20 and 25 km. These reflections reveal what is probably a shear zone that transects most of the crust and is part of a crustal-scale duplex structure that probably formed during the Late Cretaceous. These structures, however, appear to be relict because over the past 20 years, they have produced little or no seismicity despite the nearby Mw = 7.9 Denali fault earthquake that struck in 2002. The Denali fault is nonreflective, but we interpret modeled magnetotelluric (MT), gravity, and magnetic data to propose that the fault dips steeply to vertically. Modeling of MT data shows that aftershocks of the 2002 Denali fault earthquake occurred above a rock body that has low electrical resistivity (>10 ohm-m), which might signify the presence of fluids in the middle and lower crust. Copyright ?? 2007 The Geological Society of America.

Anatexis of mafic and felsic lower crust: Geochemistry and Nd, Sr and Pb isotopes of late-orogenic granodiorites and leucogranites (Damara orogen, Namibia)

NASA Astrophysics Data System (ADS)

Osterhus, Lennart; Jung, Stefan

2010-05-01

The Damara orogen (Namibia) represents a well-exposed and deeply eroded orogenic mobile belt consisting of the north-south trending Kaoko belt and the northeast-southwest trending intracontinental Damara belt. The latter has been subdivided into a Northern, a Central and a Southern Zone based on stratigraphy, metamorphic grade, structure and geochronology. The late-orogenic granodioritic to leucogranitic Gawib pluton is a cross-cutting, pear-shaped post-tectonic stock within the southern Central Zone which is elsewhere dominated by basement rocks, high-grade metasedimentary rocks of the Tinkas Formation and syn-orogenic granites (Salem-type). The non-foliated granodiorites consist of plagioclase, quartz, microcline, hornblende and biotite whereas the leucogranites consist of microcline, quartz, plagioclase and biotite. Major element variation of the granodiorites show two distinct magma types were some samples have high TiO2, MgO and Fe2O3 and low Al2O3 and others have low TiO2, MgO and Fe2O3 and high Al2O3. Based on high REE, Nb, Zr and Y concentrations some granodiorites can be classified as A-type granitoids. Strontium concentrations are high in the granodiorites (up to 939 ppm) and decrease to < 200 ppm in the leucogranites. Rb/Sr ratios are low (1) in the leucogranites. Granodiorites have moderately radiogenic initial 87Sr/86Sr ratios (0.7088-0.7132), strongly negative initial ɛ Nd values (ca. -12) and comparatively unradiogenic Pb isotope data, the latter obtained on acid-leached feldspar separates. Leucogranites have more radiogenic initial 87Sr/86Sr ratios (0.7223-0.7336) and more negative initial ɛ Nd values (ca. -18). Pb isotopes tend to be less radiogenic than in the granodiorites. The mean crustal residence ages of the granodiorites, expressed as depleted mantle Nd model ages, are ca. 2.0 Ga but the leucogranites tend to have older Nd model ages (2.5 Ga). Therefore, a likely source for the granodiorites and leucogranites is a sequence of mafic to intermediate or felsic lower crust. In a plate-tectonic context, a correlation between lower crustal magmatism and changes in the direction of micro-plate movements between the central and the southern part of the Damara orogen can be suggested, provoking re-activation of lithospheric shear zones. If such reactivation caused a reversal in the sense of movement, the associated faults opened and propagated as tensional faults. This would have allowed fracturing through the continental crust causing pressure release, channeling of volatiles, partial melting and generation of magmas from the lower crust. This suggestion is confirmed by the intrusion of the pluton along a major crustal shear zone, the Okahandja Lineament. The intrusion was probably accompanied by a change in the stress field which renewed transcurrent movements along this lithospheric shear zone.

Les minéraux accessoires des granitoïdes de la suite taourirt, Hoggar (Algérie): conséquences pétrogénétiques

NASA Astrophysics Data System (ADS)

Azzouni-Sekkal, Abla; Bonin, Bernard

1998-01-01

The post-Pan-African 'taourirt' suite of Hoggar (Algeria) is made up by themagmatic suite: G I monzogranite, G IIa monzogranite+syenogranite, G IIb subsolvus alkali feldspar granite (alaskite), GIII hypersolvus alkali feldspar syenite + granite. Silicates (zircon, thorite, allanite, chevkinite, titanite), oxides (magnetite, ilmenite, fergusonite) and phosphates (apatite, monazite), often abundant, constitute various accessory mineral associations. Crystal morphology, place in the sequence of crystallisation and mineral chemistry change as a function of parageneses. The constant titanite + magnetite + ilmenite assemblage in G I and G II rocks illustrates more oxidising conditions than in G III group, where hedenbergite + magnetite + ilmenite coexist. Two groups of accessory minerals are distinguished: (1) LREE rich (3×10 4 to 3×10 5 × chondrites) and Y rich (50 to 10 4 × chondrites), such as allanite, chevkinite and monazite, (2) LREE poor (100 to 10 4 × chondrites) and Y rich (1000 to 10 5 × chondrites), such as zircon, thorite, titanite and fergusonite. Shapes of chondrite-normalised patterns and evolutionary trends of REE, as well as of other HFSE, such as Nb, Zr, U and Th, in bulk rocks are dependent on relative abundances of the different accessory minerals.

Lead isotope systematics of some igneous rocks from the Egyptian Shield

NASA Technical Reports Server (NTRS)

Gillespie, J. G.; Dixon, T. H.

1983-01-01

Lead isotope data on whole-rock samples and two feldspar separates for a variety of Pan-African (late Precambrian) igneous rocks for the Egyptian Shield are presented. It is pointed out that the eastern desert of Egypt is a Late Precambrian shield characterized by the widespread occurrence of granitic plutons. The lead isotope ratios may be used to delineate boundaries between Late Precambrian oceanic and continental environments in northeastern Africa. The samples belong to three groups. These groups are related to a younger plutonic sequence of granites and adamellites, a plutonic group consisting of older tonalites to granodiorites, and the Dokhan volcanic suite.

The shear zone-related gold mineralization at the Turmalina deposit, Quadrilátero Ferrífero, Brazil: structural evolution and the two stages of mineralization

NASA Astrophysics Data System (ADS)

Fabricio-Silva, Wendell; Rosière, Carlos Alberto; Bühn, Bernhard

2018-05-01

Turmalina is an important orogenic gold deposit located in the NW region of the Quadrilátero Ferrífero. The deposit is hosted in an Archean greenstone belt composed of ortho-amphibolites and pelites with interleaved tuffs metamorphosed under amphibolite facies conditions and intruded by a granite stock. The orebodies are controlled by WNW-ESE-trending shear zones, associated with hydrothermal alteration. Three deformation events are recognized in the Turmalina gold deposit: D1 and D2 are the result of a progressive Archean deformation under ductile conditions between 2749 ± 7 and 2664 ± 35 Ma; D3 is characterized by a transpressional event under ductile-brittle conditions with the age still unclear. The three generations of garnet observed show that Grt1 blastesis is pre- to syn-D1 and Grt2 growth during the late to post-deformation stages of the D2 event. The initial temperature (Grt1 core) is around 548-600 °C, whereas during late D2, the temperatures reached 633 °C (metamorphic peak-Grt2 rim), likely as a result of granite intrusion. The Grt3 resulted from re-equilibration under retrograde conditions. Two gold-bearing sulfide stages were identified: pyrrhotite-arsenopyrite ± löllingite ± chalcopyrite ± gold stage I precipitated below a metamorphic peak temperature of 598 ± 19 °C associated with S1 foliation (D1), and pyrrhotite-pyrite-arsenopyrite ± chalcopyrite ± gold stage II is located commonly along V3 quartz-carbonate veinlets with a temperature range between 442 ± 9 and 510 ± 30 °C. We suggest that the granite intrusion imposed an additional thermal effect that promoted further dehydration of country rocks. The Au derived mainly from a metamorphic fluid source but potentially mixed with magmatic fluids from the granite.

Petrography and geochemistry of the primary ore zone of the Kenticha rare metal granite-pegmatite field, Adola Belt, Southern Ethiopia: Implications for ore genesis and tectonic setting

NASA Astrophysics Data System (ADS)

Mohammedyasin, Mohammed Seid; Desta, Zerihun; Getaneh, Worash

2017-10-01

The aim of this work is to evaluate the genesis and tectonic setting of the Kenticha rare metal granite-pegmatite deposit using petrography and whole-rock geochemical analysis. The samples were analysed for major elements, and trace and rare earth elements by ICP-AES and ICP-MS, respectively. The Kenticha rare metal granite-pegmatite deposit is controlled by the N-S deep-seated normal fault that allow the emplacement of the granite-pegmatite in the study area. Six main mineral assemblages have been identified: (a) alaskitic granite (quartz + microcline + albite with subordinate muscovite), (b) aplitic layer (quartz + albite), (c) muscovite-quartz-microcline-albite pegmatite, (d) spodumene-microcline-albite pegmatite, partly albitized or greisenized, (e) microcline-albite-green and pink spodumene pegmatite with quartz-microcline block, which is partly albitized and greisenized, and (f) quartz core. This mineralogical zonation is also accompanied by variation in Ta ore concentration and trace and rare earth elements content. The Kenticha granite-pegmatite is strongly differentiated with high SiO2 (72-84 wt %) and enriched with Rb (∼689 ppm), Be (∼196 ppm), Nb (∼129 ppm), Ta (∼92 ppm) and Cs (∼150 ppm) and depleted in Ba and Sr. The rare earth element (REE) patterns of the primary ore zone (below 60 m depth) shows moderate enrichment in light REE ((La/Yb)N = ∼8, and LREE/HREE = ∼9.96) and negative Eu-anomaly (Eu/Eu* = ∼0.4). The whole-rock geochemical data display the Within Plate Granite (WPG) and syn-Collisional Granite (syn-COLG) suites and interpret as its formation is crustal related melting. The mineralogical assemblage, tectonic setting and geochemical signatures implies that the Kenticha rare metal bearing granite pegmatite is formed by partial melting of metasedimentary rocks during post-Gondwana assembly and further tantalite enrichment through later hydrothermal-metasomatic processes.

Petrogenesis and tectonic implications of Late Carboniferous A-type granites and gabbronorites in NW Iran: Geochronological and geochemical constraints

NASA Astrophysics Data System (ADS)

Moghadam, Hadi Shafaii; Li, Xian-Hua; Ling, Xiao-Xiao; Stern, Robert J.; Santos, Jose F.; Meinhold, Guido; Ghorbani, Ghasem; Shahabi, Shirin

2015-01-01

Carboniferous igneous rocks constitute volumetrically minor components of Iranian crust but preserve important information about the magmatic and tectonic history of SW Asia. Ghushchi granites and gabbronorites in NW Iran comprise a bimodal magmatic suite that intruded Ediacaran-Cambrian gneiss and are good representatives of carboniferous igneous activity. Precise SIMS U-Pb zircon ages indicate that the gabbronorites and granites were emplaced synchronously at ~ 320 Ma. Ghushchi granites show A-type magmatic affinities, with typical enrichments in alkalis, Ga, Zr, Nb and Y, depletion in Sr and P and fractionated REE patterns showing strong negative Eu anomalies. The gabbronorites are enriched in LREEs, Nb, Ta and other incompatible trace elements, and are similar in geochemistry to OIB-type rocks. Granites and gabbronorites have similar εNd(t) (+ 1.3 to + 3.4 and - 0.1 to + 4.4, respectively) and zircon εHf(t) (+ 1.7 to + 6.2 and + 0.94 to + 6.5, respectively). The similar variation in bulk rock εNd(t) and zircon εHf(t) values and radiometric ages for the granites and gabbronorites indicate a genetic relationship between mafic and felsic magmas, either a crystal fractionation or silicate liquid immiscibility process; further work is needed to resolve petrogenetic details. The compositional characteristics of the bimodal Ghushchi complex are most consistent with magmatic activity in an extensional tectonic environment. This extension may have occurred during rifting of Cadomian fragments away from northern Gondwana during early phases of Neotethys opening.

Kinematics of post-orogenic extension and exhumation of the Taku Schist, NE Peninsular Malaysia

NASA Astrophysics Data System (ADS)

Md Ali, M. A.; Willingshofer, E.; Matenco, L.; Francois, T.; Daanen, T. P.; Ng, T. F.; Taib, N. I.; Shuib, M. K.

2016-09-01

Recent studies imply that the formation and evolution of many SE Asian basins was driven by extensional detachments or systems of low-angle normal faults that created significant crustal exhumation in their footwalls. In this context, the architecture of the Triassic Indosinian orogen presently exposed in Peninsular Malaysia is compatible with significant extension post-dating the orogenic event. In this study we performed a kinematic analysis based on fieldwork and microstructural observations in the Taku Schist, Kemahang granite and the surrounding Gua Musang sediments of northern Peninsular Malaysia in order to shed light on processes related to the build-up and subsequent demise of the Indosinian orogen. The first three phases of deformation were related to an overall period of E-W oriented contraction and burial metamorphism. These phases of deformation are characterized by isoclinal folding with flat lying axial plane cleavages (D1), asymmetrical folding, top-to-the-W-SW shearing (D2) and upright folding (D3). All are in general agreement with observations of the previously inferred Permo-Triassic Indosinian orogeny. During these times, the Taku Schist, a sequence of Paleozoic clastic sediments with mafic intercalations was metamorphosed to amphibolite facies. These rocks are most likely equivalent to the ones exposed in the Bentong-Raub suture zone. Structural relations suggest that the Triassic Kemahang pluton is syn-kinematic, which provides important constraints for the timing of these contractional events. We demonstrate that the overall shortening was followed by a hitherto undescribed extension in NW-SE direction resulting in the formation of a large-scale detachment, the Taku detachment, in northern Peninsular Malaysia. Extension probably reactivated the former subduction plane as a detachment and exhumed previously buried and metamorphosed rocks of similar lithological composition to the neighboring Bentong-Raub suture zone. Such a mechanism is similar to that observed in other regions, such as the Aegean, Apennines, Dinarides or the Betics-Rif system, where exhumation of (high-pressure) metamorphic rocks is largely controlled by detachments or low angle normal shear/fault systems.

Late Mesozoic deformations of the Verkhoyansk-Kolyma orogenic belt, Northeast Russia

NASA Astrophysics Data System (ADS)

Fridovsky, Valery

2016-04-01

The Verkhoyansk-Kolyma orogenic belt marks the boundary between the Kolyma-Omolon superterrane (microcontinent) and the submerged eastern margin of the North Asian craton. The orogenic system is remark able for its large number of economically viable gold deposits (Natalka, Pavlik, Rodionovskoe, Drazhnoe, Bazovskoe, Badran, Malo-Tarynskoe, etc.). The Verkhoyansk - Kolyma orogenic belt is subdivided into Kular-Nera and the Polousny-Debin terranes. The Kular-Nera terrane is mainly composed of the Upper Permian, Triassic, and Lower Jurassic black shales that are metamorphosed at lower greenschist facies conditions. The Charky-Indigirka and the Chai-Yureya faults separate the Kular-Nera from the Polousny-Debin terrane that is predominantly composed of the Jurassic flyschoi dturbidites. The deformation structure of the region evolved in association with several late Mesozoic tectonic events that took place in the north-eastern part ofthe Paleo-Pacific. In Late Jurassic-Early Cretaceous several generations of fold and thrust systems were formed due to frontal accretion of the Kolyma-Omolon superterrane to the eastern margin of the North Asian craton.Thrusting and folding was accompanied by granitic magmatism, metamorphic reworking of the Late Paleozoic and the Early Mesozoic sedimentary rocks, and formation of Au-Sn-W mineralization. Three stages of deformation related to frontal accretion can be distinguished. First stage D1 has developed in the north-eastern part of the Verkhoyansk - Kolyma orogenic belt. Early tight and isoclinal folds F1 and assosiated thrusts are characteristic of D1. Major thrusts, linear concentric folds F2 and cleavage were formed during D2. The main ore-controlling structures are thrust faults forming imbricate fan systems. Frontal and oblique ramps and systems of bedding and cross thrusts forming duplexes are common. It is notable that mineralized tectonized zones commonly develop along thrusts at the contacts of rocks of contrasting competence. The superimposed structures are recognized from the early cleavage deformations. Folds F3 are often chevron type, open or tight. D1, D2 and D3 deformations are coaxial. In the Late-Neocomian-Aptian the Kolyma-Omolon superterrane started moving to the west. As a result, the thrust faults were reactivated with sinistral strike-slip motions along fault planes. At that time, granitoids of the North and Transverse belts were emplaced in the northwestern part of the Kolyma-Omolon superterrane. The strike slip faults were associated with cross open folds. The postacrettionary stage is associated with the development of the Albian-Late Cretaceous Okhotsk-Chukotka subduction zone. During this stage strike-slip faults and associated deformation structures were superimposed upon accretion-related tectonic structures of the Verkhoyansk - Kolyma orogenic belt.

Is the Ventersdorp rift system of southern Africa related to a continental collision between the Kaapvaal and Zimbabwe Cratons at 2.64 Ga AGO?

NASA Technical Reports Server (NTRS)

Burke, K.; Kidd, W. S. F.; Kusky, T.

1985-01-01

Rocks of the Ventersdorp Supergroup were deposited in a system of northeast trending grabens on the Kaapvaal Craton approximately 2.64 Ga ago contemporary with a continental collision between the Kaapvaal and Zimbabwe Cratons. It is suggested that it was this collision that initiated the Ventersdorp rifting. Individual grabens strike at high angles toward the continental collision zone now exposed in the Limpopo Province where late orogenic left-lateral strike-slip faulting and anatectic granites are recognized. The Ventersdorp rift province is related to extension in the Kaapvaal Craton associated with the collision, and some analogy is seen with such rifts as the Shansi and Baikal Systems associated with the current India-Asia continental collision.

U-Pb isotopic evidence for accretion of a continental microplate in the Zalm region of the Saudi Arabian Shield

USGS Publications Warehouse

Stacey, J.S.; Agar, R.A.

1986-01-01

U-Pb zircon age determinations show that this "Andean" continental margin developed before about 720 Ma, and emplacement of calc-alkaline plutonic rocks continued until about 690 Ma. During the period 690-640 Ma, the continental Afif microplate collided with the Asir terrane as part of the Nabitah orogeny. At approximately 640 Ma ago, the Najd strike-slip orogen commenced with a dextral phase that controlled the emplacement of granitic plutons as well as the development of a series of large pull-apart grabens. Some of these grabens were floored by new oceanic crust and were filled with volcano-sedimentary rocks of the Bani Ghayy group. Subsequently, the Najd fault system changed to sinistral strike slip motion at about 620 Ma ago.

Ar/Ar geochronology in the western Tianshan (northwestern China): from Carboniferous (ultra)high-pressure metamorphism and thrusting to Permian strike-slip deformation and fluid ingress

NASA Astrophysics Data System (ADS)

de Jong, K.; Wang, B.; Ruffet, G.; Shu, L. S.; Faure, M.

2012-04-01

The Tianshan belt (northwestern China) is a major tectonic element of the southern Central Asian Orogenic Belt that contains a number of ophiolitic mélanges and (ultra)high-pressure metamorphic belts formed after closure of oceanic and back-arc basins that resulted in terrane collisions. Deciphering its tectonic evolution is thus crucial for understanding the amalgamation of Central Asia. We produce robust 40Ar/39Ar laser-probe evidence that the Tianshan is a Late Palaeozoic (ultra)high-pressure metamorphic collision belt, not a Triassic one, as suggested by some SHRIMP zircon ages in recent literature. Instead of trying to date the peak pressure conditions we focused on 40Ar/39Ar analysis of white mica formed during retrograde recrystallisation when the (ultra)high-pressure metamorphic rocks of the Changawuzi-Kekesu complex were exhumed. Exhumation was coeval with their northward thrusting over the southern margin of the Yili terrane, the easternmost element of the Kazakhstan composite super-terrane, which produced main phase tectonic structures. The Yili terrane comprises a Proterozoic basement covered by metasediments, intruded by Early Carboniferous granites when it formed part of a continental margin arc. During the Permian deformation was partitioned in vertical brittle-ductile strike-slip fault zones that reactivated these suture zones and in which bimodal magmatism was concentrated. We also investigate the effects of these events on the isotopic ages of mica. 40Ar/39Ar laser-probe dating of white mica reveals that the strongest retrogressed blueschists immediately above the basal thrust fault of the Changawuzi-Kekesu belt gave the youngest plateau age of 316 ± 2 Ma (1σ). White mica in greenschist-facies metamorphic quartzite from the ductilely deformed metasedimentary cover of the Yili terrane's crystalline basement, taken at about 1 km below the thrust contact with the overlying Changawuzi-Kekesu belt, yielded a plateau age of 323 ± 1 Ma (1σ). Elsewhere, such metasediments yielded plateau ages (1σ) of 253 ± 1 (muscovite) and 252 ± 1 (biotite) Ma, whereas biotite from an undeformed ca. 340 Ma-old granite intruding the Yili terrane's southern margin gave a 263 ± 1 Ma plateau age (1σ). The 263-252-Ma-old samples were taken between 2 and 5 km across strike from the Permian Qingbulak-Nalati strike-slip fault, and within the 15-20 km wide zone with steeply dipping tectonic fabrics used by intruding Permian granites, and associated mineralisations. We interpret these Permian ages by recrystallisation of the mica by (late magmatic?) fluid flow channeled into these steep zones. Laser-probe dating of mylonite whole-rock samples from the North Tianshan - Main Tianshan strike-slip fault zone yielded 40Ar/39Ar spectra with step ages in the 255-285 Ma range, which date the movement on this ductile shear zone. The picture is emerging that a convective fluid system partly driven by magmatic heat, existed in a strongly fractured and weakened crust with an elevated heat flow, leading to regional-scale isotope resetting. We suggest that surprisingly young isotopic ages for early orogenic (ultra)high-pressure metamorphism are similarly due to fluid-mediated recrystallisation, leading to the erroneous view that the Tianshan is a Triassic orogenic belt.

Magmatic zircon Lu-Hf isotopic record of juvenile addition and crustal reworking in the Gawler Craton, Australia

NASA Astrophysics Data System (ADS)

Reid, Anthony J.; Payne, Justin L.

2017-11-01

New in situ zircon Lu-Hf isotopic data are presented from magmatic rocks distributed across the Gawler Craton, Australia. These rocks range in composition from granite to gabbro, with the majority being granite or granodiorite and moderately peraluminous in composition. The new Lu-Hf isotopic data, together with previously published data, provide insight into the magmatic evolution of the craton and crust and mantle interaction through time. Increased juvenile content of magmatic rocks correlate with periods of extensional tectonism, in particular basin formation and associated magmatism during the Neoarchean to earliest Paleoproterozoic (c. 2555-2480 Ma), Middle Paleoproterozoic (c. 2020-1710 Ma) and Late Paleoproterozoic (c. 1630-160 Ma). In contrast, magmatic rocks associated with periods of orogenic activity show greater proportions of crustal derivation, particularly the magmatic rocks generated during the c. 1730-1690 Ma Kimban Orogeny. The final two major magmatic events of the Gawler Craton at c. 1630-1604 Ma and c. 1595-1575 Ma both represent periods of juvenile input into the Gawler Craton, with εHf(t) values extending to as positive as + 8. However, widespread crustal melting at this time is also indicated by the presence of more evolved εHf(t) values to - 6.5. The mixing between crust and mantle sources during these two youngest magmatic events is also indicated by the range in two stage depleted mantle model ages (TDMc) between 1.76 Ga and 2.51 Ga. Significant mantle input into the crust, particularly during formation of the c. 1595-1575 Ma Hiltaba Suite and Gawler Range Volcanics, likely facilitated the widespread crustal magmatism of this time period. Viewed spatially, average εHf(t) and TDMc values highlight three of the major shear zones within the Gawler Craton as potentially being isotopic as well as structural boundaries. Differences in isotopic composition across the Coorabbie Shear Zone in the western Gawler Craton, the Middle Bore Fault in the northern Gawler Craton and, to a lesser extent, the Kalinjala Shear Zone in the southern Gawler Craton, broadly correspond to crustal and even lithospheric-scale discontinuities evident in geophysical studies. Therefore, these shear zones may approximate some of the first order crustal domains within the Gawler Craton.

East African and Kuunga Orogenies in Tanzania - South Kenya

NASA Astrophysics Data System (ADS)

Fritz, H.; Hauzenberger, C. A.; Tenczer, V.

2012-04-01

Tanzania and southern Kenya hold a key position for reconstructing Gondwana consolidation because here different orogen belts with different tectonic styles interfere. The older, ca. 650-620 Ma East African Orogeny resulted from the amalgamation of arc terranes in the northern Arabian-Nubian Shield (ANS) and continental collision between East African pieces and parts of the Azania terrane in the south (Collins and Pisarevsky, 2005). The change form arc suturing to continental collision settings is found in southern Kenya where southernmost arcs of the ANS conjoin with thickened continental margin suites of the Eastern Granulite Belt. The younger ca. 570-530 Ma Kuunga orogeny heads from the Damara - Zambesi - Irumide Belts (De Waele et al., 2006) over Tanzania - Mozambique to southern India and clashes with the East African orogen in southern-central Tanzania. Two transitional orogen settings may be defined, (1) that between island arcs and inverted passive continental margin within the East African Orogen and, (2) that between N-S trending East African and W-E trending Kuungan orogenies. The Neoproterozoic island arc suites of SE-Kenya are exposed as a narrow stripe between western Azania and the Eastern Granulite belt. This suture is a steep, NNW stretched belt that aligns roughly with the prominent southern ANS shear zones that converge at the southern tip of the ANS (Athi and Aswa shear zones). Oblique convergence resulted in low-vorticity sinstral shear during early phases of deformation. Syn-magmatic and syn-tectonic textures are compatible with deformation at granulite metamorphic conditions and rocks exhumed quickly during ongoing transcurrent motion. The belt is typified as wrench tectonic belt with horizontal northwards flow of rocks within deeper portions of an island arc. The adjacent Eastern Granulite Nappe experienced westward directed, subhorizontal, low-vorticity, high temperature flow at partly extreme metamorphic conditions (900°C, 1.2 to 1.4 GPa) (Fritz et al., 2009). Majority of data suggest an anticlockwise P-T loop and prolonged, slow cooling at deep crustal levels without significant exhumation. Isobaric cooling is explained by horizontal flow with rates faster than thermal equilibration of the lower crust. Those settings are found in domains of previously thinned lithosphere such as extended passive margins. Such rheolgically weak plate boundaries do not produce self-sustaining one-sided subduction but large areas of magmatic underplating that enable melt enhanced lateral flow of the lower crust. Western Granulites deformed by high-vorticity westwards thrusting at c. 550 Ma (Kuunga orogeny). Rocks exhibit clockwise P-T paths and experienced significant exhumation during isothermal decompression. Overprint between Kuungan structures and 620 Ma East African fabrics resulted in complex interference pattern within the Eastern Granulites. The three orogen portions that converge in Tanzania / Southern Kenya have different orogen styles. The southern ANS formed by transcurrent deformation of an island arc root; the Eastern Granulites by lower crustal channelized flow of a hot inverted passive margin; the Western Granulites by lower to mid crustal stacking of old and cold crustal fragments. Collins, A.S., Pisarevsky, S.A. (2005). Amalgamating eastern Gondwana: The evolution of the Circum-Indian Orogens. Earth-Science Reviews, 71, 229-270. De Waele, B., Kampunzu, A.B., Mapani, B.S.E., Tembo, F. (2006). The Mesoproterozoic Irumide belt of Zambia. Journal of African Earth Sciences, 46, 36-70 Fritz, H., Tenczer, V., Hauzenberger, C., Wallbrecher, E., Muhongo, S. (2009). Hot granulite nappes — Tectonic styles and thermal evolution of the Proterozoic granulite belts in East Africa. Tectonophysics, 477, 160-173.

Late Proterozoic charnockites in Orissa, India: A U-Pb and Rb-Sr isotopic study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aftalion, M.; Bowes, D.R.; Dash, B.

1988-11-01

Charnockite formation in the Angul district of Orissa took place between 1088 + 26/ -17 Ma, the U-Pb zircon upper intercept crystallization age of a leptynite neosome, and 957 +8/ -4-956 {plus minus} 4 Ma, the U-Pb zircon-monazite upper intercept and U-Pb monazite crystallization ages of a granite. Confirmation of the Proterozoic age of the charnockites is given by (1) a U-Pb zircon upper intercept 1159 + 59/ -30 Ma age and a Rb-Sr whole-rock 1080 {plus minus} 65 Ma age for an augen gneiss which pre-dates the leptynite, and (2) U-Pb monazite ages of 973 {plus minus} 5,964 {plusmore » minus} 4, and 953 {plus minus} 4 Ma for a gray quartzofeldspathic gneiss, the augen gneiss, and the leptynite, respectively: these late Proterozoic dates are interpreted as representing ages recorded during charnockitization. The ca. 950-980 Ma charnockite- and granite-forming events are related to the evolution of mantle-derived, CO{sub 2}-bearing basic magma emplaced into the deeper levels of an extensional tectonic-transcurrent fault regime. The ca. 1100-1150 Ma tectonothermal and igneous events represent compressional tectonism in reactivated crystalline basement in the late mid-Proterozoic Eastern Ghats orogenic belt.« less

The kinematic history of the Khlong Marui and Ranong Faults, southern Thailand

NASA Astrophysics Data System (ADS)

Watkinson, Ian; Elders, Chris; Hall, Robert

2008-12-01

The Khlong Marui Fault (KMF) and Ranong Fault (RF) are major NNE-trending strike-slip faults which dissect peninsular Thailand. They have been assumed to be conjugate to the NW-trending Three Pagodas Fault (TPF) and Mae Ping Fault (MPF) in Northern Thailand, which experienced a diachronous reversal in shear sense during India-Eurasia collision. It follows that the KMF and RF are expected to show the opposite shear sense and a slip sense reversal at a similar time to the TPF and MPF. New field data from the KMF and RF reveal two phases of ductile dextral shear separated by Campanian magmatism. Paleocene to Eocene post-kinematic granites date the end of this phase, while a brittle sinistral phase deforms the granites, and has exhumed the ductile fault rocks. The timing of these movements precludes formation of the faults in response to Himalayan extrusion tectonics. Instead, they formed near the southern margin of a Late Cretaceous-Paleocene orogen, and may have been influenced by variations in the rate of subduction ahead of India and Australia. North-south compression prior to reactivation of the subduction zone around southern Sundaland in the Eocene caused widespread deformation in the over-riding plate, including sinistral transpression on the KMF and RF.

Ring complexes and related rocks in Africa

NASA Astrophysics Data System (ADS)

Vail, J. R.

Over 625 igneous complexes throughout Africa and Arabia have been selected and classified on the basis of petrographic association and chronology into six broad age groups forming 29 provinces. The groups range from Mid-Proterozoic to Tertiary and include gabbro, granite, syenite, foid syenite and carbonatite plutonic rocks, the majority in the form of ring-dykes, cone-sheets, plugs, circular intrusions, and their associated extrusive phases. Pan-African late or post-orogenic complexes (720-490 Ma) are common in the Arabian-Nubian and Tuareg shields of north Africa originating from subduction zone derived magmatism. Anorogenic complexes in Egypt, NE and central Sudan, Niger, Nigeria, Cameroon, Zaïre-Burundi, Malawi, Mozambique, Zimbabwe, Namibia and Angola span 550 to 50 Ma and are dominantly alkali granites and foid syenites. Many groups occur as en-echelon bands within linear arrays, and show migrating centres of intrusion in variable directions. In W. Africa there was a progressive shift of emplacement southwards during early Ordovician to Mid-Cretaceous times. Distribution patterns suggest thatdeep seated features, such as shear zones associated with lithospheric plate movements,controlled melting, and the resultant location of the complexes. Economic mineralization is not widespread in the rocks of the African ring complexes and is mainly restricted to small deposits of Sn, W, F, U and Nb.

Bedrock geologic map of the Nashua South quadrangle, Hillsborough County, New Hampshire, and Middlesex County, Massachusetts

USGS Publications Warehouse

Walsh, Gregory J.; Jahns, Richard H.; Aleinikoff, John N.

2013-01-01

The bedrock geology of the 7.5-minute Nashua South quadrangle consists primarily of deformed Silurian metasedimentary rocks of the Berwick Formation. The metasedimentary rocks are intruded by a Late Silurian to Early Devonian diorite-gabbro suite, Devonian rocks of the Ayer Granodiorite, Devonian granitic rocks of the New Hampshire Plutonic Suite including pegmatite and the Chelmsford Granite, and Jurassic diabase dikes. The bedrock geology was mapped to study the tectonic history of the area and to provide a framework for ongoing hydrogeologic characterization of the fractured bedrock of Massachusetts and New Hampshire. This report presents mapping by G.J. Walsh and R.H. Jahns and zircon U-Pb geochronology by J.N. Aleinikoff. The complete report consists of a map, text pamphlet, and GIS database. The map and text pamphlet are only available as downloadable files (see frame at right). The GIS database is available for download in ESRITM shapefile and Google EarthTM formats, and includes contacts of bedrock geologic units, faults, outcrops, structural geologic information, photographs, and a three-dimensional model.

Eastern Indian 3800-million-year-old crust and early mantle differentiation

USGS Publications Warehouse

Basu, A.R.; Ray, S.L.; Saha, A.K.; Sarkar, S.N.

1981-01-01

Samarium-neodymium data for nine granitic and tonalite gneisses occurring as remnants within the Singhbhum granite batholith in eastern India define an isochron of age 3775 ?? 89 ?? 106 years with an initial 143Nd/144Nd ratio of 0.50798 ?? 0.00007. This age contrasts with the rubidium-strontium age of 3200 ?? 106 years for the same suite of rocks. On the basis of the new samarium-neodynium data, field data, and petrologic data, a scheme of evolution is proposed for the Archean crust in eastern India. The isotopic data provide evidence that parts of the earth's mantle were already differentiated with respect to the chondritic samarium-neodymium ratio 3800 ?? 106 years ago.

Petrography and geochemistry of granitoids from the Samphire Pluton, South Australia: Implications for uranium mineralisation in overlying sediments

NASA Astrophysics Data System (ADS)

Domnick, Urs; Cook, Nigel J.; Bluck, Russel; Brown, Callan; Ciobanu, Cristiana L.

2018-02-01

The Blackbush uranium deposit (JORC Inferred Resource: 12,580 tonnes U), located on the north-eastern Eyre Peninsula, is currently the only sediment-hosted U deposit investigated in detail in the Gawler Craton. Uranium is hosted within Eocene sandstone of the Kanaka Beds, overlying Mesoproterozoic granites of the Samphire pluton, affiliated with the Hiltaba Intrusive Suite ( 1.6 Ga). These are considered the most probable source rocks for uranium mineralisation. By constraining the petrography and mineralogy of the granites, insights into the post-emplacement evolution can be gained, which may provide an exploration indicator for other sediment-hosted uranium systems. Three geochemically distinct granite types were identified in the Samphire Pluton and correspond to domains interpreted from geophysical data. All granites show complex alteration overprints and textures with increasing intensity closer to the deposit, as well as crosscutting veining. Alkali feldspar has been replaced by porous K-feldspar and albite, and plagioclase is overprinted by an assemblage of porous albite + sericite ± calc-silicates (prehnite, pumpellyite and epidote). This style of feldspar alteration is regionally widespread and known from Hiltaba-aged granites associated with iron-oxide copper-gold mineralisation at Olympic Dam and in the Moonta-Wallaroo region. In two granite types biotite is replaced by calcic garnet. Calc-silicates are indicative of Ca-metasomatism, sourced from the anorthite component of altered plagioclase. Minor clay alteration of feldspars is present in all samples. Mineral assemblages in veins include quartz + hematite, hematite + coffinite, fluorite + quartz, and clay minerals. Minor chlorite and sericite are found in all vein types. All granite types are anomalously rich in U (concentrations between 10 and 81 ppm). Highly variable Th/U ratios, as well as hydrothermal U minerals (mostly coffinite) in granites and veins, are clear evidence for U mobility. Uranium may have been preconcentrated in veins in the upper parts of the pluton, and was subsequently leached after deposition of the sediment.

Late Cambrian - Early Ordovician turbidites of Gorny Altai (Russia): Compositions, sources, deposition settings, and tectonic implications

NASA Astrophysics Data System (ADS)

Kruk, Nikolai N.; Kuibida, Yana V.; Shokalsky, Sergey P.; Kiselev, Vladimir I.; Gusev, Nikolay I.

2018-06-01

The Cambrian-Ordovician transition was the time of several key events in the history of Central Asia. They were the accretion of Mariana-type island arc systems to the Siberian continent, the related large-scale orogeny and intrusions of basaltic and granitic magma and the formation of a huge turbidite basin commensurate with the Bengal Gulf basin in the western part of the Central Asian orogenic belt (CAOB). The structure of the basin, as well as the sources and environments of deposition remain open to discussion. This paper presents new major- and trace-element data on Late-Cambrian-Early Ordovician turbidites from different parts of the Russian Altai and a synthesis of Nd isotope composition and ages of detrital zircons. The turbidites share chemical similarity with material shed from weathered continental arcs. Broad variations of CIA (39-73) and ICV (0.63-1.66) signatures in sandstones suggest origin from diverse sources and absence of significant sorting. Trace elements vary considerably and have generally similar patterns in rocks from different terranes. On the other hand, there are at least two provinces according to Nd isotope composition and age of detrital zircons. Samples from eastern Russian Altai contain only Phanerozoic zircons and have Nd isotope ratios similar to those in Early Cambrian island arcs (εNdt + 4.4… + 5.4; TNd(DM)-2-st = 0.8-0.9 Ga). Samples from central, western, and southern parts of Russian Altai contain Precambrian zircons (some as old as Late Archean) and have a less radiogenic Nd composition (εNdt up to -3.6; TNd(DM)-2-st up to 1.5 Ga). The chemical signatures of Late Cambrian to Early Ordovician turbidites indicate a provenance chemically more mature than the island arc rocks, and the presence of zircons with 510-490 Ma ages disproves their genetic relation with island arcs. The turbidite basin formed simultaneously with peaks of granitic and alkali-basaltic magmatism in the western Central Asian orogen and resulted from interplay of plate tectonic and plume tectonic processes.

Petrogenesis of Mesoproterozoic granitic plutons, eastern Llano Uplift, central Texas, USA

NASA Astrophysics Data System (ADS)

Smith, R. K.; Gray, Walt; Gibbs, Tyson; Gallegos, M. A.

2010-08-01

The Llano Uplift of central Texas is a gentle structural dome exposing ˜ 1370 to 1230 Ma metaigneous and metasedimentary rocks of Grenville affinity along the southern margin of Laurentia. The metamorphic rocks were subsequently intruded by ˜ 1119 to 1070 Ma late syn- to post-tectonic granites collectively known as the Town Mountain Granite (TMG). The eastern most of the TMG, the Marble Falls (MF), Kingsland (KL), and Lone Grove (LG) plutons, are metaluminous to marginally peraluminous, high-K, calc-alkaline, ferroan, biotite-calcic amphibole granites [Fe/(Fe + Mg) = 0.71-0.92 and 0.78-0.91 for biotite and calcic amphibole, respectively] displaying distinct variation trends with increasing silica content. They are chemically and texturally zoned and have mineralogical and chemical characteristics similar to A-type granites; i.e., 1) Fe-rich biotites, calcic amphiboles, accessory fluorite, and sporadic rapakivi texture, 2) high K 2O (> 4 wt.%), 3) low Al 2O 3 (< 16 wt.%) and CaO (< 3 wt.%), 4) high Fe/(Fe + Mg), 5) enrichments in Zr, Nb, REE, Ga/Al, and 6) depleted Eu. However, in contrast to typical A-type granites (having low Sr and Ba) the MF, KL,and LG plutons are enriched in Sr and Ba; i.e., up to 229 ppm and 1090 ppm, respectively. On granite discrimination diagrams [(K 2O + Na 2O)/CaO vs. Zr + Nb + Ce + Y (ppm) and Zr (ppm) vs. Ga/Al*10,000] the KL and MF plutons plot within the A-type field, whereas the LG pluton compositions are divided between A-type and fractionated granite fields (I-, S- and M-types). On tectonic discrimination diagrams (Y vs. Nb ) the MF and KL granites plot in the "within-plate" granite field, but the LG pluton plots across several fields including "within-plate" and "volcanic arc plus syn-collisional" fields. Consequently the tectonic classification on a geochemical basis for the LG pluton is unclear. Based on thermal metamorphic mineral assemblages, normative Q-Ab-Or plots, and Q-Ab-Or-H 2O experimental data (Johannes and Holtz, 1996), crystallization temperatures and pressures are estimated to range from 750 to 850 °C and 200 to 500 MPa, respectively. The assemblage of titanite + magnetite + quartz suggests crystallization at low fO2 [confirmed by Fe/(Fe + Mg) vs. [4] Al microprobe analyses of calcic amphibole] and a water content of less than 1.5 wt.% (Wones, 1989). Like other Town Mountain-type plutons, the MF, KL, and LG granites display comparable iron contents at similar alkali and silica enrichments. Melting models (Ba vs. Sr) suggest the MF, KL, and LG plutons may have evolved from the partial melting (anatexis) of juvenile, tonalitic, lower crustal rocks, followed by plagioclase and pyroxene dominated fractionation. Nd isotopic data for the MF pluton ( ɛNd = + 3.4 at 1.06 Ga; Patchett and Ruiz, 1989) and whole-rock δ18O values for the MF, KL, and LG plutons (+ 7.0 < δ 18O >+10.1‰; Rangel et al., 2008) suggest that the magmas in the eastern Llano Uplift may contain a significant mantle component, whereas relatively high δ18O values (+ 9.3 to + 9.7‰; Bebout and Carlson, 1986) for other coeval TMG rocks suggest that a significant crustal component is involved. Whole-rock and trace-element chemistry indicate that the MF and KL plutons, along with the coarser grained textures of the LG pluton, are 'A-type' granites. However, with no coeval mafic dikes, syenitic compositions, or volcanic rocks it is clear that the TMG plutons do not represent anorogenic granites. The available evidence is most compatible with emplacement of the TMG plutons in a post-orogenic (Grenville), relaxation and extensional (i.e., slab breakoff) setting.

Zircon U-Pb ages and Sr-Nd-Hf isotopes of the highly fractionated granite with tetrad REE patterns in the Shamai tungsten deposit in eastern Inner Mongolia, China: Implications for the timing of mineralization and ore genesis

NASA Astrophysics Data System (ADS)

Jiang, Si-Hong; Bagas, Leon; Hu, Peng; Han, Ning; Chen, Chun-Liang; Liu, Yuan; Kang, Huan

2016-09-01

The Shamai tungsten deposit is located in the eastern part of the Central Asian Orogenic Belt (CAOB). Tungsten mineralization is closely related to the emplacement of fine- to medium-grained biotite monzogranite (G1) and porphyritic biotite monzogranite (G2) in the Shamai Granite. NW-trending joints and faults host orebodies in the Shamai Granite and Devonian hornfels. The mineralization is characterized by a basal veinlet zone progressing upwards to a thick vein zone followed by a mixed zone, a veinlet zone, and a thread vein zone at the top. The ore-related alteration typically consists of muscovite, greisen, and hornfels. In order to constrain the timing of the Shamai mineralization and discuss the ore genesis, muscovite Ar-Ar, molybdenite Re-Os, and zircon U-Pb geochronological, geochemical, and Sr-Nd-Hf isotopic studies were completed on the deposit. The U-Pb zircon dating yielded weighted mean ages of 153 ± 1 Ma for G1 and 146 ± 1 Ma for G2. Muscovite from a wolframite-bearing quartz vein yielded an Ar-Ar plateau age of 140 ± 1 Ma, whereas two molybdenite samples yielded identical Re-Os model ages of 137 ± 2 Ma. These two ages are younger than the two monzogranites, suggesting a prolonged magmatic-hydrothermal interaction during tungsten mineralization. Major and trace element geochemistry shows that both G1 and G2 are characterized by high SiO2 and K2O contents, high A/CNK values (1.08-1.40), a spectacular tetrad effect in their REE distribution patterns, and non-CHARAC (charge-and-radius-controlled) trace element behavior. This suggests that both G1 and G2 are highly differentiated peraluminous rocks with strong hydrothermal interaction. The Nd-Hf isotope data for the Shamai Granite (εNd(t) between - 1.9 and + 7.4, ɛHf(t) from 5.2 to 12.8) are largely compatible with the general scenario for much of the Phanerozoic granite emplaced in the CAOB. It is here suggested that the Shamai Granite originated from partial melting of a juvenile lower crust with minor input of upper crustal material caused by the underplating of mafic magma in an extensional setting. It can also be concluded that the prolonged fractional crystallization and magmatic-hydrothermal interactions have contributed to the formation of the Shamai tungsten deposit.

Emplacement and reworking of the Marampa Group allothchon, northwestern Sierra Leone, West Africa

NASA Astrophysics Data System (ADS)

Latiff, R. S. A.; Andrews, J. R.; Wright, L. I.

1997-10-01

The structural evolution and relative age of the Precambrian Marampa Group, a 60 km wide north-northwest trending fold thrust belt is described in detail. The Marampa Group is shown to be unconformably overlain by the Rokel River Group which lies immediately to the east and is separated by a major crustal shear zone from gneisses and amphibolites of the Kasila Group to the west. Previous workers have interpreted the fold thrust belt as a klippe of the adjacent Kasila Group derived from the west or as an autochthonous volcano-sedimentary deposit engulfed by granitic. basement. Ages ranging from 500 to > 2700 Ma have been suggested. Evidence is presented to show that the important deformation of the Marampa Group clearly predates the deposition of the Rokel River Group and must represent a significant earlier orogenic event. Constraints on the relationship of this older deformation to the 2700-2750 Ma deformation of the Kasila Group are discussed. The earliest structures consist of flat lying thrusts which transported Marampa Group metasediments, with or without their basal metavolcanic formation, eastward from their source basin over the basin margin and onto a flanking heterogeneously deformed older granitic gneiss basement. Subsequent intrusion of megacrystic, now porphiyroclastic granites was followed by a major period of crustal extension during which sediments and volcanics of the Rokel River Group were deposited in rift basins. Renewed east-west crustal shortening ascribed to the Pan-African event inverted earlier extensional structures thrusting the Rokel River Group westward over -the Marampa Group and leading to local facing confrontations where east dipping faults were reactivated. The relationship of the Marampa Group to the greenstone belts of Guinea, Liberia and Sierra Leone remains unresolved.

Geochemistry and petrology of the indium-bearing polymetallic skarn ores at Pitkäranta, Ladoga Karelia, Russia

NASA Astrophysics Data System (ADS)

Valkama, M.; Sundblad, K.; Cook, N. J.; Ivashchenko, V. I.

2016-08-01

The historic mining district of Pitkäranta in the Ladoga region, Fennoscandian Shield, was exploited for Fe, Cu, Zn, Pb, Sn and Ag in the nineteenth to twentieth centuries. The Pitkäranta region is dominated by Palaeoproterozoic supracrustal rocks, which, together with gneissic Archaean dome structures, constitute an allochthonous terrane complex that amalgamated to the Archaean continent during the Svecokarelian orogeny at 1.9-1.8 Ga. This crustal complex was intruded by 1.8 Ga Late orogenic granites, 1.54 Ga anorogenic rapakivi granites and 1.45 Ga dolerites. The polymetallic skarn ores of Pitkäranta extend over a 25-km-long zone in Palaeoproterozoic supracrustal rocks and formed from hydrothermal solutions, which emanated from the anorogenic rapakivi granites and reacted with marble layers. Four major ore types are recognised after the dominating metal: Fe, Cu, Sn and Zn, respectively. These types are not restricted to individual mines or mine fields but represent end members in zonation patterns within each ore body. Pitkäranta was the second discovery site in the world for indium but has been without modern documentation for more than 75 years. The indium contents in the ores are up to 600 ppm, in most cases sphalerite-hosted. The only roquesite-bearing sample in this study had an indium grade of 291 ppm and an In/Zn ratio of 51 (close to the criteria for the limiting conditions for creating an In-rich mineral). The Pitkäranta ores have a potential for future small-scale exploitation, but all such plans are hampered by high contents if Bi, Cd and As.

Structure of the Castillo granite, Southwest Spain: Variscan deformation of a late Cadomian pluton

NASA Astrophysics Data System (ADS)

EguíLuz, L.; Apraiz, A.; ÁBalos, B.

1999-12-01

A geometrical reconstruction of the 500 Ma old Castillo granite pluton (SW Iberia) is completed on the basis of structural and geophysical (rock magnetism) techniques. The pluton is intrusive into latest Proterozoic-earliest Cambrian metasediments and conforms a tabular intrusion 6 km in diameter and 1.7 km thick that was emplaced at a depth of 10 km. Its magnetic fabric reveals that the strike of moderately to steeply dipping magmatic flow planes forms a high angle to the regional tectonic trends. Magnetic foliations and associated moderately to gently plunging magnetic lineations represent magmatic flow planes and directions. The internal anisotropy of the granite together with the structure shown by the country rocks attest the lateral propagation of the pluton and its latter inflation. The pluton's root zone would correspond to a likely thin, subvertical feeder structure initiated near the orientation of regional σ1 at the time of emplacement. During the Variscan orogeny the pluton was tilted and underwent localized brittle-ductile strain in relation to shear zone deformation in the footwall of a major ductile thrust. Tilting permits the observation and study of a vertical profile of the intrusion. Localized deformation caused superposition of tectonic zonations on the magmatic ones, a reactivation of the basal contact of the pluton, and dismemberment from its root. This and other granitoid plutons of similar age emplaced at a similar depth constrained the creation of crustal mechanical heterogeneity and anisotropy. This controlled the site of pluton emplacement, the nucleation of a major ductile thrust, and localization of deformation and tectonic displacements along the pluton margins during later orogenic reactivation.

Polyphase exhumation in the western Qinling Mountains, China: Rapid Early Cretaceous cooling along a lithospheric-scale tear fault and pulsed Cenozoic uplift

PubMed Central

Heberer, Bianca; Anzenbacher, Thomas; Neubauer, Franz; Genser, Johann; Dong, Yunpeng; Dunkl, István

2014-01-01

The western sector of the Qinling–Dabie orogenic belt plays a key role in both Late Jurassic to Early Cretaceous “Yanshanian” intracontinental tectonics and Cenozoic lateral escape triggered by India–Asia collision. The Taibai granite in the northern Qinling Mountains is located at the westernmost tip of a Yanshanian granite belt. It consists of multiple intrusions, constrained by new Late Jurassic and Early Cretaceous U–Pb zircon ages (156 ± 3 Ma and 124 ± 1 Ma). Applying various geochronometers (40Ar/39Ar on hornblende, biotite and K-feldspar, apatite fission-track, apatite [U–Th–Sm]/He) along a vertical profile of the Taibai Mountain refines the cooling and exhumation history. The new age constraints record the prolonged pre-Cenozoic intracontinental deformation as well as the cooling history mostly related to India–Asia collision. We detected rapid cooling for the Taibai granite from ca. 800 to 100 °C during Early Cretaceous (ca. 123 to 100 Ma) followed by a period of slow cooling from ca. 100 Ma to ca. 25 Ma, and pulsed exhumation of the low-relief Cretaceous peneplain during Cenozoic times. We interpret the Early Cretaceous rapid cooling and exhumation as a result from activity along the southern sinistral lithospheric scale tear fault of the recently postulated intracontinental subduction of the Archean/Palaeoproterozoic North China Block beneath the Alashan Block. A Late Oligocene to Early Miocene cooling phase might be triggered either by the lateral motion during India–Asia collision and/or the Pacific subduction zone. Late Miocene intensified cooling is ascribed to uplift of the Tibetan Plateau. PMID:27065503

Age and provenance constraints on seismically-determined crustal layers beneath the Paleozoic southern Central Asian Orogen, Inner Mongolia, China

NASA Astrophysics Data System (ADS)

Jian, Ping; Kröner, Alfred; Shi, Yuruo; Zhang, Wei; Liu, Yaran; Windley, Brian F.; Jahn, Bor-ming; Zhang, Liqao; Liu, Dunyi

2016-06-01

We present 110 ages and 51 in-situ δ18O values for zircon xenocrysts from a post-99 Ma intraplate basaltic rock suite hosted in a subduction-accretion complex of the southern Central Asian Orogenic Belt in order to constrain a seismic profile across the Paleozoic Southern Orogen of Inner Mongolia and the northern margin of the North China Craton. Two zircon populations are recognized, namely a Phanerozoic group of 70 zircons comprising granitoid-derived (ca. 431-99 Ma; n = 31; peak at 256 Ma), meta-granitoid-derived (ca. 449-113 Ma; n = 24; peak at 251 Ma) and gabbro-derived (436-242 Ma; n = 15; peaks at 264 and 244 Ma) grains. Each textural type is characterized by a distinct zircon oxygen isotope composition and is thus endowed with a genetic connotation. The Precambrian population (2605-741 Ma; n = 40) exhibits a prominent age peak at 2520 Ma (granulite-facies metamorphism) and four small peaks at ca. 1900, 1600, and 800 Ma. Our new data, together with literature zircon ages, significantly constrain models of three seismically-determined deep crustal layers beneath the fossil subduction zone-forearc along the active northern margin of the North China Craton, namely: (1) an upper arc crust of early to mid-Paleozoic age, intruded by a major Permian-Triassic composite granitoid-gabbroic pluton (8-20 km depth); (2) a middle crust, predominantly consisting of mid-Meso- to Neoproterozoic felsic and mafic gneisses; and (3) a lower crust composed predominantly of late Archean granulite-facies rocks. We conclude that the Paleozoic orogenic crust is limited to the upper crustal level, and the middle to lower crust has a North China Craton affinity. Furthermore, integrating our data with surface geological, petrological and geochronological constraints, we present a new conceptual model of orogenic uplift, lithospheric delamination and crustal underthrusting for this key ocean-continent convergent margin.

Fluid fractionation of tungsten during granite-pegmatite differentiation and the metal source of peribatholitic W quartz veins: Evidence from the Karagwe-Ankole Belt (Rwanda)

NASA Astrophysics Data System (ADS)

Hulsbosch, Niels; Boiron, Marie-Christine; Dewaele, Stijn; Muchez, Philippe

2016-02-01

The identification of a magmatic source for granite-associated rare metal (W, Nb, Ta and Sn) mineralisation in metasediment-hosted quartz veins is often obscured by intense fluid-rock interactions which metamorphically overprinted most source signatures in the vein system. In order to address this recurrent metal sourcing problem, we have studied the metasediment-hosted tungsten-bearing quartz veins of the Nyakabingo deposit of the Karagwe-Ankole belt in Central Rwanda. The vein system (992 ± 2 Ma) is spatiotemporal related to the well-characterised B-rich, F-poor G4 leucogranite-pegmatite suite (986 ± 10 Ma to 975 ± 8 Ma) of the Gatumba-Gitarama area which culminated in Nb-Ta-Sn mineralisation. Muscovite in the Nyakabingo veins is significantly enriched in granitophile elements (Rb, Cs, W and Sn) and show alkali metal signatures equivalent to muscovite of less-differentiated pegmatite zones of the Gatumba-Gitarama area. Pegmatitic muscovite records a decrease in W content with increasing differentiation proxies (Rb and Cs), in contrast to the continuous enrichment of other high field strength elements (Nb and Ta) and Sn. This is an indication of a selective redistribution for W by fluid exsolution and fluid fractionation. Primary fluid inclusions in tourmaline of these less-differentiated pegmatites demonstrate the presence of medium to low saline, H2O-NaCl-KCl-MgCl2-complex salt (e.g. Rb, Cs) fluids which started to exsolve at the G4 granite-pegmatite transition stage. Laser ablation inductively coupled plasma mass-spectrometry shows significant tungsten enrichment in these fluid phases (∼5-500 ppm). Fractional crystallisation has been identified previously as the driving mechanism for the transition from G4 granites, less-differentiated biotite, biotite-muscovite towards muscovite pegmatites and eventually columbite-tantalite mineralised pegmatites. The general absence of tungsten mineralisation in this magmatic suite, including the most differentiated columbite-tantalite mineralised pegmatites of the Gatumba-Gitarama area, emphasises the efficiency of fluid saturation to extract crystal-melt incompatible tungsten from the differentiating melt phase. Fluid-melt-crystal partitioning calculations support the concept of a magmatic-hydrothermal fluid source for tungsten and constrain the range of permissible crystal-melt and fluid-melt partition coefficients together with realistic values for water solubility in the parental G4 granitic melt. Consequently, we propose that for highly-differentiated B-rich, F-poor granite systems fluid saturation started prior to or at the granite-pegmatite transition stage resulting in apical to peribatholitic tungsten veins systems that are paragenetically older than the final pegmatite stage.

The geology of the northern tip of the Arabian-Nubian Shield

NASA Astrophysics Data System (ADS)

Beyth, M.; Eyal, Y.; Garfunkel, Z.

2014-11-01

Recently, a detailed (1:50,000) geological map of the Elat area, southern Israel was published. Attached to this map is a stratigraphic table of the Neoproterozoic metamorphic-magmatic complex of the study area. The Neoproterozoic basement in the Elat area encapsulates the Arabian Nubian Shield (ANS) geologic evolution. Uranium-Lead and Lead-Lead zircon ages, included in previous studies and referred to in this paper, reveal that these rocks were formed during more than 300 million years of Neoproterozoic time. The major process controlling the formation of the ANS as part of the East African Orogen is the closure of the Mozambique Ocean. The first orogenic phase in the Elat area, represented by the metamorphic rocks, includes the development of an island arc, erosion of the islands and deposition, and metamorphism. This event took place between ∼950 Ma and 780-790 Ma. Elat Schist, the oldest metamorphic rock in the area, was deformed and then intruded by quartz dioritic and granitic plutons that were later deformed and metamorphosed. The amphibolite metamorphic rock facies indicate metamorphic conditions of up to 650 °C and between 4 and 5 kbar. The peak of the metamorphic event was most probably before 750 Ma. A gradual change from compressional to extensional stress regime is evidenced by emplacement andesitic magnesium-rich dykes dated to 705 Ma that were later metamorphosed to schistose dykes at a greenschist metamorphic facies. The second orogenic phase (terrane amalgamation, main shaping of crust) was associated with the emplacement of large volumes (>50% of area) of calc-alkaline intrusions in a post-collision setting. These very last stages of metamorphism and deformation are characterized by intrusion of ∼630 Ma granitoids exhibiting some foliation. Pluton emplacement continued also after the end of deformation. Exhumation and transition to an extensional regime is recorded by the intrusion of shallow alkaline granites in ∼608 Ma which were accompanied in ∼609 Ma by rhyolite, andesite and composite dykes. The last magmatic event in the Elat area is represented by the volcano-conglomeratic series comprising rhyolites, basalts, andesites, hypabyssal intrusions of monzonite and syenite and conglomerates. The conglomerates, dated to about 590 Ma, are the products of a major erosion phase in which about 12,000 m of the section were removed. These conglomerates were intruded by 585 Ma rhyolite, andesite and composite dykes. The Neoproterozoic basement is truncated by a peneplain whose age, post 532 Ma, is constrained by the age of the youngest eroded dolerite dykes. This Early Cambrian peneplain was associated with erosion of 2000 m of the section and by chemical weathering. Three major breaks in Neoproterozoic magmatic activity are recognized: the first, occurred in Cryogenian time, lasted ∼60 million years after the amphibolite facies metamorphism and before emplacement of the calc alkaline plutons, separating the first and the second orogenic phases; the second break between the orogenic and the extensional phases occurred in early Ediacaran time, encompassed ∼20 million years between the emplacement of the calc-alkaline and alkaline plutonic rocks and rhyolite, andesite and the composite dykes; and the third, ∼50 Ma break, occurred between the emplacement of the last felsic intrusions at ∼585 Ma and intrusion of the dolerite dykes in 532 Ma, before the Early Cambrian peneplain developed. The great lateral extension of the Cambrian to Eocene sedimentary rocks and their slow facies and thickness changes suggest a stable flat platform area at the northern tip of the ANS. Early Cambrian sedimentation began with fluviatile subarkoses of the Amudei Shlomo Formation. It was overlain by an Early to Middle Cambrian transgressive-regressive lagoonal cycle of dolostones, sandstones, and siltstones of the Timna Formation. Then Middle Cambrian subarkoses and siltstones of the Shehoret Formation and the quartz arenite of the Netafim Formation were deposited in a coastal, intertidal environment representing the southern transgression of a Cambrian ocean.

A Ta-rich low-P peraluminous granite: the Rechla cupola (Hoggar, Algeria) and associated pegmatites, the result of extreme fractionation of a A2-type magma.

NASA Astrophysics Data System (ADS)

Kesraoui, M.; Marignac, C.; Hamis, A.; Cuney, M.

2012-04-01

In the c. 525 Ma RMG province of the Laouni terrane of the Pan-African Tuareg Shield (Hoggar), the small N20°E elliptic Rechla cupola (200x100 m) is particularized by a rim of Qtz-Kfs-Znw pegmatite. It is a medium-grained Na-Li-F granite, with quartz, albite (An01), rare microcline, topaz, Mn-lepidolite (≤ 8% MnO) and Hf-zircon, and: 71.4 % SiO2, 0.93% FeO+MgO+MnO (Mg # 0.19, Mg/Mg+Fe+Mn 0.09), 9.22% Na2O+K2O (Na # 0.7), Al-Na-K-2Ca from 55 to 85, and low P2O5 (0.05%) and ∑ REE (23 ppm) contents, with a pronounced tetrad effect and <0 Eu anomaly in the REE pattern. Such a composition is typical of a low-P peraluminous RMG deriving from highly potassic calcalkaline suites (A2 type) (Linnen & Cuney 2005), enriched in F (1.6%), Li (1,600 ppm), Zn (300 ppm), Be (7 ppm), Sn (740 ppm), W (40 ppm) and specially Ta (165 ppm, Ta/Nb between 2.4 and 2.6), the latter as columbo-tantalite and Mn-wodginite (Ta # 0.8). The pegmatite rim comprises, towards the intrusion (i) thick Kfs lenses (palissadic crystals ≥ 50 cm), (ii) a laminated quartz-zinnwaldite-(beryl) sequence , and (iii) a discontinuous band of fine-grained granite, with quartz, albite, topaz, Mn-lepidolite and beryl, equally fractionated: 69.4% SiO2, 0.85% FeO+MgO+MnO (Mg# 0.06, Mg/Mg+Fe+Mn 0.02), Al-Na-K-2Ca = 32, F 0.4%, Li 610 ppm, Ta 240 ppm (Ta/Nb = 2.4), Be 500 ppm. The laminated sequence overprints the Kfs lenses. It comprises thick (≤ 20 m) quartz lenses cross-cut by 10 cm-sized alternating bands of euhedral quartz and Mn-zinnwaldite (≤ 6.5% MnO). REE-patterns of the Mn-Znw display a clear inverse tetrad effect, symmetrical of the granite pattern. At the boundary with the fine-grained internal band, euhedral quartz crystals are projecting toward the inner wall. The Rechla body and its surrounding pegmatites are intrusive into a porphyritic biotite-granite representative of the evolved magmas of the A2-type Taourirt suite (Azzouni-Sekkal & Boissonnas 1993), with a classical "seagull" pattern and a pronounced <0 Eu anomaly. Geochemical modelling shows that the main Rechla magma is likely the fractionated product of this already differentiated magma, mainly involving quartz and Kfs. The pegmatite rim is interpreted as the result of the sequential crystallization of a Rechla-type melt, with late individualisation of a Fe-rich magmatic-hydrothermal phase responsible for the quartz-zinnwaldite assemblage, leaving a strongly Be-enriched residual liquid (the fine-grained granite). As demonstrated by the Rechla occurrence, Ta concentration at levels similar to those in Beauvoir-type high-P peraluminous granites may be reached in the low-P low-Ta A2 suites, provided that extreme fractionation processes are established. Azzouni-Sekkal, A., Boissonnas, J. (1993). Une province magmatique de transition du calco-alcalin à l'alcalin : les granitoïdes pan-africains à structure annulaire de la chaîne pharusienne du Hoggar (Algérie). Bulletin Société Géologique France 164, 597-608. Linnen, R.L., Cuney, M. (2005). Granite-related rare-element deposits and experimental constraints on Ta-Nb-W-Sn-Zr-Hf mineralization. In: RL Linnen, IM Samson (eds), Rare-element geochemistry and mineral deposits, Geological Association of Canada (GAC) Short Course Notes 17, pp. 45-67.

The influence of upper-crust lithology on topographic development in the central Coast Ranges of California

USGS Publications Warehouse

Garcia, A.F.; Mahan, S.A.

2012-01-01

A fundamental geological tenet is that as landscapes evolve over graded to geologic time, geologic structures control patterns of topographic distribution in mountainous areas such that terrain underlain by competent rock will be higher than terrain underlain by incompetent rock. This paper shows that in active orogens where markedly weak and markedly strong rocks are juxtaposed along contacts that parallel regional structures, relatively high topography can form where strain is localized in the weak rock. Such a relationship is illustrated by the topography of the central Coast Ranges between the Pacific coastline and the San Andreas fault zone (SAFZ), and along the length of the Gabilan Mesa (the "Gabilan Mesa segment" of the central Coast Ranges). Within the Gabilan Mesa segment, the granitic upper crust of the Salinian terrane is in contact with the accretionary-prism m??lange upper crust of the Nacimiento terrane along the inactive Nacimiento fault zone. A prominent topographic lineament is present along most of this lithologic boundary, approximately 50 to 65. km southwest of the SAFZ, with the higher topography formed in the m??lange on the southwest side of the Nacimiento fault. This paper investigates factors influencing the pattern of topographic development in the Gabilan Mesa segment of the central Coast Ranges by correlating shortening magnitude with the upper-crust compositions of the Salinian and Nacimiento terranes. The fluvial geomorphology of two valleys in the Gabilan Mesa, which is within the Salinian terrane, and alluvial geochronology based on optically-stimulated luminescence (OSL) age estimates, reveal that the magnitude of shortening accommodated by down-to-the-southwest tilting of the mesa since 400ka is less than 1 to 2m. Our results, combined with those of previous studies, indicate that at least 63% to 78% of late-Cenozoic, northeast-southwest directed, upper-crustal shortening across the Gabilan Mesa segment has been accommodated within the Nacimiento terrane. This is significant because perpendicular to orogenic strike the Nacimiento terrane constitutes less than 1/4 of the distance between the coast and the SAFZ, and the other 3/4 (or greater) of the distance between the coast and the SAFZ is underlain by the granitic upper crust of the Salinian terrane. We propose that strain and mountain building are localized within the Nacimiento terrane because it consists predominantly of the relatively weak Franciscan Complex m??lange, and because the upper crust of the Salinian terrane is composed of relatively strong granitic rocks. Our hypothesis is supported by the distribution of post-seismic surface uplift associated with the 2003, 6.5M W San Simeon earthquake, which mimics the topography of the southwestern part of the Gabilan Mesa segment of the central Coast Ranges. ?? 2011 Elsevier B.V.

Geophysical Character and Geochemical Evolution of the Mesoproterozoic Figueira Branca Intrusive Suite, SW Amazon Craton (Brazil)

NASA Astrophysics Data System (ADS)

Louro, Vinicius; Cawood, Peter; Mantovani, Marta

2016-04-01

The Jauru Terrain hosts the Figueira Branca Intrusive Suite (FBS) in the SW of the Amazon Craton (Brazil). The FBS is a series of 1425 Ma layered mafic intrusions, previously interpreted as anorogenic. The FBS area is located in foreland to the Santa Helena orogen, formed by the subduction of the Rio Alegre Terrain under the Jauru Terrain. Potential field methods (magnetic and gravity), gamma-ray spectrometry, geochemical and isotope data were used to characterize and to model the extent of FBS magmatism, the distribution of faults and shear zones in the area, to evaluate affinities of the magmatic activity, and the relation between the FBS and the Santa Helena orogen. The geophysical methods identified three anomalies corresponding with FBS outcrops. A fourth anomaly with significantly higher amplitude was observed to the north of the three anomalies. From south to north, the anomalies were named Indiavaí, Azteca, Figueira Branca and Jauru. These anomalies were modeled and indicated a northwest-southeast trend, parallel to regional shear zones. The gamma-ray data enabled the collection of 50 samples from the FBS rocks, the Alto Jauru group that hosts the FBS, from nearby intrusive suites, and the Rio Alegre Terrain. The 30 freshest samples were analyzed by X-ray fluorescence for oxides and some trace elements, 20 by ICP-MS for Rare-Earth Elements and 10 for Nd-Sr isotope analyses. The FBS samples were gabbros and gabbro-norites with Nb/Yb-Th/Yb and TiO2/Yb-Nb/Yb ratios indicating varying degrees of crustal interaction. The TiO2/Yb-Nb/Yb data suggested a subduction related component and the ɛNd-ɛSr indicated a juvenile source. Samples from coeval adjacent intermediate magma suites displayed similar characteristics, which suggest derivation from a bimodal source probably related with the subduction of the Rio Alegre Terrain. We interpreted the tectonic setting of the FBS as a result of a roll-back of the subducted slab, which resulted in rejuvenation of the mantle under Jauru Terrain to form the FBS and nearby suites, but also under the subducting Rio Alegre Terrain, producing the magma that formed the 1412 Ma to 1380 Ma Santa Rita Suite in the Rio Alegre Terrain, which has a juvenile ɛNd(t) signature (+3.6). Thus in summary out data indicate that the FBS was part of the origin and evolution of the magmatic suites of the Rio Alegre and Jauru Terrains.

Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis

NASA Astrophysics Data System (ADS)

Wang, Xiangsong; Cai, Keda; Sun, Min; Xiao, Wenjiao; Xia, Xiaoping; Wan, Bo; Bao, Zihe; Wang, Yannan

2018-03-01

Late Carboniferous to Early Permian is a critical period for the final amalgamation of the Central Asian Orogenic Belt (CAOB). However, as most of the accreted terranes of the CAOB are unclear in tectonic nature and origin, the timing and processes of their mutual amalgamation have been poorly constrained. To understand assembly of the West Junggar Terrane with the Yili Block, a suite of the late Paleozoic magmatic rocks, including ignimbrite, rhyolite and granite, in northwestern Chinese Tianshan Belt were studied for their petrogenesis and tectonic implications. Our new results of secondary ion mass spectrometry (SIMS) zircon U-Pb dating reveal two separate magmatic episodes, ca. 300 Ma volcanism (ignimbrite and rhyolite) and ca. 288 Ma plutonsim (biotite granite). Geochemically, for the ca. 300 Ma volcanism, the ignimbrites have low SiO2 (65.8-71.5 wt.%) and Mg# (6-13) values, and exhibit arc affinity with significantly enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE) such as Nb, Ta and Ti. The whole-rock εNd(t) and zircon εHf(t) values range from +6.9 to +7.0 and +9.9 to +14.1 respectively, indicating a juvenile basaltic lower crustal origin. Rhyolites have slightly high SiO2 (72.7-74.0 wt.%) and K2O (3.86-4.53 wt.%) contents, high zircon δ18O (11.67-13.23‰) values, and low whole-rock εNd(t) (+2.9 to +3.8) and zircon εHf(t) (+2.8 to +10.0) values, which may suggest sediment involvements during magma generation. In contrast, for the ca. 288 Ma plutonism, the biotite granites have obviously higher SiO2 (74.7-75.5 wt.%) contents and whole-rock εNd(t) (+7.7 to +8.8), zircon εHf(t) (+9.8 to +12.7), and lower zircon δ18O (5.99-6.84‰) values, than those of the ca. 300 Ma volcanic rocks, which are consistent with signatures of juvenile magma source. According to our estimates of zircon saturation temperatures, together with their contrasting genesis, we attribute the formation of ca. 300 Ma high temperature (815-938 °C) volcanism to oceanic slab break-off during assembly of the West Junggar Terrane with the Yili Block, and relate the generation of ca. 288 Ma low temperature (723-735 °C) plutonism to subsequent strike-slipping of North Tianshan Fault that facilitated introduction of water-fluxes triggering hydrous partial melting of juvenile lower crust. The sequential magmatic episodes in the northwestern Chinese Tianshan Belt may provide a crucial clue to a tectonic transition from plate convergence to intra-plate adjustment during the formation of the Kazakhstan Orocline in the late Paleozoic.

A Laurentian margin back-arc: the Ordovician Wedowee-Emuckfaw-Dahlonega basin

USGS Publications Warehouse

Barineau, Clinton I.; Tull, James F.; Holm-Denoma, Christopher S.

2015-01-01

Independent researchers working in the Talladega belt, Ashland-Wedowee-Emuckfaw belt, and Opelika Complex of Alabama, as well as the Dahlonega gold belt and western Inner Piedmont of Alabama, Georgia, and the Carolinas, have mapped stratigraphic sequences unique to each region. Although historically considered distinct terranes of disparate origin, a synthesis of data suggests that each includes lithologic units that formed in an Ordovician back-arc basin (Wedowee-Emuckfaw-Dahlonega basin—WEDB). Rocks in these terranes include varying proportions of metamorphosed mafic and bimodal volcanic rock suites interlayered with deep-water metasedimentary rock sequences. Metavolcanic rocks yield ages that are Early–Middle Ordovician (480–460 Ma) and interlayered metasedimentary units are populated with both Grenville and Early–Middle Ordovician detrital zircons. Metamafic rocks display geochemical trends ranging from mid-oceanic-ridge basalt to arc affinity, similar to modern back-arc basalts. The collective data set limits formation of the WEDB to a suprasubduction system built on and adjacent to upper Neoproterozoic–lower Paleozoic rocks of the passive Laurentian margin at the trailing edge of Iapetus, specifically in a continental margin back-arc setting. Overwhelmingly, the geologic history of the southern Appalachians, including rocks of the WEDB described here, indicates that the Ordovician Taconic orogeny in the southern Appalachians developed in an accretionary orogenic setting instead of the traditional collisional orogenic setting attributed to subduction of the Laurentian margin beneath an exotic or peri-Laurentian arc. Well-studied Cenozoic accretionary orogens provide excellent analogs for Taconic orogenesis, and an accretionary orogenic model for the southern Appalachian Taconic orogeny can account for aspects of Ordovician tectonics not easily explained through collisional orogenesis.

Phase equilibrium modelling of granite magma petrogenesis: B. An evaluation of the magma compositions that result from fractional crystallization

NASA Astrophysics Data System (ADS)

Garcia-Arias, Marcos; Stevens, Gary

2017-04-01

Several fractional crystallization processes (flow segregation, gravitational settling, filter-pressing), as well as batch crystallization, have been investigated in this study using thermodynamic modelling (pseudosections) to test whether they are able to reproduce the compositional trends shown by S-type granites. Three starting compositions comprising a pure melt phase and variable amounts of entrained minerals (0, 20 and 40 wt.% of the total magma) have been used to study a wide range of likely S-type magma compositions. The evolution of these magmas was investigated from the segregation from their sources at 0.8 GPa until emplacement at 0.3 GPa in an adiabatic path, followed by isobaric cooling until the solidus was crossed, in a closed-system scenario. The modelled magmas and the fractionated mineral assemblages are compared to the S-type granites of the Peninsula pluton, Cape Granite Suite, South Africa, which have a composition very similar to most of the S-type granites. The adiabatic ascent of the magmas digests partially the entrained mineral assemblage of the magmas, but unless this entrained assemblage represents less than 1 wt.% of the original magma, part of the mineral fraction survives the ascent up to the chosen pressure of emplacement. At the level of emplacement, batch crystallization produces magmas that only plot within the composition of the granites of the Peninsula pluton if the bulk composition of the original magmas already matched that of the granites. Flow segregation of crystals during the ascent and gravitational settling fractional crystallization produce bodies that are generally more mafic than the most mafic granites of the pluton and the residual melts have an almost haplogranitic composition, producing a bimodal compositional distribution not observed in the granites. Consequently, these two processes are ruled out. Filter-pressing fractional crystallization produces bodies in an onion-layer structure that become more felsic with increasing crystallization, culminating in a haplogranitic melt, and is able to reproduce the compositional trends of the granites, but only if the original magmas already had the composition of the granites. Filter-pressing fractionation produces a mineral assemblage that is 1.5 times more mafic than the magma fraction from which it is derived. However, the mineral assemblages produced by crystallization of an originally pure melt phase are still too felsic to account for the bulk of the granites of the Peninsula pluton. For filter-pressing to produce the most mafic granites of the pluton, the original magmas must already contain an entrained mafic mineral assemblage and have the same composition of the granites, otherwise the modelled trends do not match the maficity (FeO + MgO) or the slope against maficity of the granites. Crystallization of the magma in filter-pressing releases a free water phase, whose amount depends on the amount of water of the original magma, and whose behaviour may be controlled by a water-saturation front. In summary, the main control in the composition of S-type granites is the amount and nature of the entrained mineral assemblage, and filter-pressing fractional crystallization can only modify slightly the compositions of the granitic bodies derived from these magmas.

Proterozoic metamorphism and uplift history of the north-central Laramie Mountains, Wyoming, USA

USGS Publications Warehouse

Patel, S.C.; Frost, B.R.; Chamberlain, K.R.; Snyder, G.L.

1999-01-01

The Laramie Mountains of south-eastern Wyoming contain two metamorphic domains that are separated by the 1.76 Ga. Laramie Peak shear zone (LPSZ). South of the LPSZ lies the Palmer Canyon block, where apatite U-Pb ages are c. 1745 Ma and the rocks have undergone Proterozoic kyanite-grade Barrovian metamorphism. In contrast, in the Laramie Peak block, north of the shear zone, the U-Pb apatite ages are 2.4-2.1 Ga, the granitic rocks are unmetamorphosed and supracrustal rocks record only low-T amphibolite facies metamorphism that is Archean in age. Peak mineral assemblages in the Palmer Canyon block include (a) quartz-biotite-plagioclase-garnet-staurolite-kyanite in the pelitic schists; (b) quartz-biotite-plagioclase-low-Ca amphiboles-kyanite in Mg-Al-rich schists, and locally (c) hornblende-plagioclase-garnet in amphibolites. All rock types show abundant textural evidence of decompression and retrograde re-equilibration. Notable among the texturally late minerals are cordierite and sapphirine, which occur in coronas around kyanite in Mg-Al-rich schists. Thermobarometry from texturally early and late assemblages for samples from different areas within the Palmer Canyon block define decompression from > 7 kbar to < 3 kbar. The high-pressure regional metamorphism is interpreted to be a response to thrusting associated with the Medicine Bow orogeny at c. 1.78-1.76 Ga. At this time, the north-central Laramie Range was tectonically thickened by as much as 12 km. This crustal thickening extended for more than 60 km north of the Cheyenne belt in southern Wyoming. Late in the orogenic cycle, rocks of the Palmer Canyon block were uplifted and unroofed as the result of transpression along the Laramie Peak shear zone to produce the widespread decompression textures. The Proterozoic tectonic history of the central Laramie Range is similar to exhumation that accompanied late-orogenic oblique convergence in many Phanerozoic orogenic belts.

Geochemical Characteristics of Granitoids in southwest Tianshan: Four Stages for Geodynamic Evolution of the Southwest Tianshan Orogenic Belt

NASA Astrophysics Data System (ADS)

Zhu, Y.

2016-12-01

Paleozoic intrusive rocks widely exposed in the west Tianshan orogenic belt provides key to understand the geodynamic evolution of the central Asian orogenic belt. A synthesis involving the data for Chinese Yili-central Tianshan and southwest Tianshan and comparison of Kyrgyz Tianshan with a broader dataset including zircon U-Pb ages, zircon Hf isotopic composition, major and trace elements for Paleozoic intrusions are presented to classify the Paleozoic intrusive rocks in four categories which corresponding to subduction of the Terskey Ocean, initial subduction stage of South Tianshan Ocean (STO), major subduction stage of the STO, and collisional to post-collisional stages. The subduction of the Terskey Oceanic crust finally caused the closure of the Terskey Ocean and the opening of the South Tianshan back-arc basin. The development of the Southwest Tianshan back-arc basin formed the STO, which subducted under the Yili-central Tianshan during early Silurian to early Carboniferous, and consequently formed huge arc magmatic rocks. Both the Silurian and early Carboniferous intrusions showing arc geochemical characteristics were derived from partial melting of juvenile arc-derived rocks with involvement of old continental crust. The STO finally closed by the end of early Carboniferous. Afterwards, geodynamic setting changed from convergence to extensional during late Carboniferous to early Permian periods. There is a significant geodynamic change from convergence to extension during late Carboniferous to early Permian, which may be resulted from breakoff of the subducted slab (Fig. 1). Such processes caused upwelling of asthenosphere and triggered partial melting of continental crust, as evidenced by emplacement of voluminous granitic rocks. References: An F, et al, 2013. Journal of Asian Earth Sciences, 78: 100-113; Zhu YF, 2011. Ore Geology Reviews, 40: 108-121; Zhu YF, et al, 2009. Geological Society, London, 166: 1085-1099; Zhu YF et al, 2016. Journal of Earth Science 27: 491-506

The late Variscan ferroan granite magmatism of southern Sardinia: inferences from Mo metallogenesis

NASA Astrophysics Data System (ADS)

Naitza, Stefano; Conte, Aida Maria; Cuccuru, Stefano; Fadda, Sandro; Fiori, Maddalena; Oggiano, Giacomo; Secchi, Francesco

2017-04-01

Metallogeny is a powerful tool to investigate crustal evolution; a good example is offered by the Variscan basement of Sardinia and its Mo deposits. Mo ores are poorly represented in Variscan metallogenic provinces of Europe: however, in Sardinia, numerous small Mo deposits, often associated to Sn, W and F ores, are present, invariably related to an early Permian intrusive peak bracketed at about 290 Ma (Fadda et al., 2015; Naitza et al., 2017). In Sardinia, two main magmatic peaks have been schematized at pre-300 and 290 Ma. In southern Sardinia, the 290 magmatic peak is made up of several intrusive F-bearing rock-suites (Conte et al., 2016), belonging to ilmenite series, showing a slight peraluminous character and mostly classifiable as ferroan granites (sensu Frost and Frost, 2011). Mo-bearing granites form a distinct suite of relatively small plutons, emplaced at very shallow depth (about 1kb) in an exhumed Variscan low-grade basement. Peculiar characters of Mo-bearing granites are the occurrence of greisenized microgranite and granophyre cupolas, with fayalite-bearing pegmatites, and ilmenite, xenotime-(Y), monazite, fluorite, and local topaz as accessory phases. Recently, Conte et al. (2016) interpreted these granites as originated by partial melting of low crustal felsic metaigneous photoliths enriched in granophiles (Mo, Sn, W). Mo ores occur as: a) endo- and exo- quartz-muscovite greisens with molybdenite±Fe-Cu sulphides, and b) quartz-molybdenite±wolframite±Fe-Cu-Zn sulphides±fluorite±topaz hydrothermal veins and stockworks, hosted in granites or in country rocks. Redox state of magmas exerts a strong control on Mo metallogeny, as in Mo districts worldwide ores are usually hosted by high-fO2 magnetite series intrusions (Ishihara, 1981). The close field association of Sardinian Mo mineralization with ferroan, low-fO2 ilmenite-series granites may be explained in terms of Mo-enriched crustal sources of magmas, and very efficient geochemical concentration processes. Mo concentration occurred during latest intrusive phases in granitic cupolas, as local subsolidus reactions. They involve: 1) leaching of Mo5+ and Mo6+ from their primary hosts (e.g., biotites) by Cl- and F-rich fluids under decreasing pH and increasing fH2O and fO2, and 2) transport of Mo cations towards intrusive contacts, where they quickly reacted with Sulphur from country rocks to be reduced as MoS2. Overall, the late Variscan Mo metallogenic peak in Sardinia is a single event located in a short timespan at around 290 Ma. In the early Permian extensional setting of Corsica-Sardinia batholith, a distinct HT/LP event related to melting of mantle lithosphere (Rossi et al., 2015), triggered partial melting of Mo-bearing deep crustal sources, producing the F-bearing ferroan magmas. The singular concentration of Mo-bearing granites could indicate a peculiar compositional character of the Sardinian lower crust. References: Conte A.M. et al. (2016). Proc. 2nd European Mineralogical Conference, Rimini, Italy, 154. Fadda et al. (2015). Proc. 13th Biennial SGA Meeting, Nancy, France, Vol. 2, 721-724. Frost, C. D. and Frost R. B. (2011). J. Petrol. 52, 39-53. Ishihara S. (1981). Econ. Geol., 75th Anniversary Volume, 458-484. Naitza S. et al. (2017). Ore Geol. Rev. 80, 1259-1278. Rossi et al. (2015). Bull. Soc. Géol. France 186, 171-192.

Estimating the Reactivation Potential of Pre-Existing Fractures in Subsurface Granitoids from Outcrop Analogues and in-Situ Stress Modeling: Implications for EGS Reservoir Stimulation with an Example from Thuringia (Central Germany)

NASA Astrophysics Data System (ADS)

Kasch, N.; Ustaszewski, K. M.; Siegburg, M.; Navabpour, P.; Hesse, G.

2014-12-01

The Mid-German Crystalline Rise (MGCR) in Thuringia (central Germany) is part of the European Variscan orogen and hosts large extents of Visean granites (c. 350 Ma), locally overlain by up to 3 km of Early Permian to Mid-Triassic volcanic and sedimentary rocks. A geothermal gradient of 36°C km-1 suggests that such subsurface granites form an economically viable hot dry rock reservoir at > 4 km depth. In order to assess the likelihood of reactivating any pre-existing fractures during hydraulic reservoir stimulation, slip and dilation tendency analyses (Morris et al. 1996) were carried out. For this purpose, we determined orientations of pre-existing fractures in 14 granite exposures along the southern border fault of an MGCR basement high. Additionally, the strike of 192 Permian magmatic dikes affecting the granite was considered. This analysis revealed a prevalence of NW-SE-striking fractures (mainly joints, extension veins, dikes and subordinately brittle faults) with a maximum at 030/70 (dip azimuth/dip). Borehole data and earthquake focal mechanisms reveal a maximum horizontal stress SHmax trending N150°E and a strike-slip regime. Effective in-situ stress magnitudes at 4.5 km depth, assuming hydrostatic conditions and frictional equilibrium along pre-existing fractures with a friction coefficient of 0.85 yielded 230 and 110 MPa for SHmax and Shmin, respectively. In this stress field, fractures with the prevailing orientations show a high tendency of becoming reactivated as dextral strike-slip faults if stimulated hydraulically. To ensure that a stimulation well creates fluid connectivity on a reservoir volume as large as possible rather than dissipating fluids along existing fractures, it should follow a trajectory at the highest possible angle to the orientation of prevailing fractures, i.e. subhorizontal and NE-SW-oriented. References: Morris, A., D. A. Ferrill, and D. B. Henderson (1996), Slip-tendency analysis and fault reactivation, Geology, 24, 275-278.

Late Quaternary cooling rate constrained by multiple IRSL thermochronometers of potassium feldspars for granites from Kongur Shan, Chinese Pamir

NASA Astrophysics Data System (ADS)

Qin, Jintang; Chen, Jie; Valla, Pierre; Herman, Frédéric

2015-04-01

The Kongur Shan (East Pamir), located at the northwestern Tibetan Plateau, is one of the most active orogens on Earth, where both tectonic processes along major active faults and climatic forcing (extensive glaciers coverage) are contributing to the regional landscape evolution. The exhumation rates since late Miocene was constrained to be ~6.5 - 4.2 mm/yr. However, it is still debated whether the exhumation rate accelerated since the Quaternary, of which the climate was featured by the cyclic glaciations with periods of 100 ka and 40 ka. In this study, we tried to employ luminescence thermochronology, which is a still in developing method, to resolve the impact of glacial cycles on exhumation rate. Our study site is located ~10 km to the east of the active Kongur normal fault, along the major valley of Gez river. We sampled three granite rocks from a sub-horizontal tunnel across the granite massif; one was from the entrance of the tunnel, and other two samples were from inside of the tunnel, where the measured ambient temperature is as high as 60-70 ° C. The distances of these samples are within 2 km. Four types of IRSL signals extracted from potassium feldspars (K-feldspars) were measured for each individual sample, and the results of isothermal decay experiments indicated these signals were of different thermal stabilities. Therefore, they may serve as four thermochronometers with different closure temperature. We employ these multiple thermochronometers together for each single sample to constrain their cooling rates. Our preliminary results, which are based on the simplified luminescence model of K-feldspars, suggest that the averaged cooling rate of the last 200 ka is as high as 1.4 oC/ka, which corresponds to an exhumation rate of ~ 2.3 to 0.9 cm/yr with the geothermal gradient assumed to be 60 to 150 oC/km. It seems to imply that the glacial cycles during the Quaternary substantially accelerated the exhumation rate of granite massif of Kongur Shan.

Mid-Cretaceous oblique rifting of West Antarctica: Emplacement and rapid cooling of the Fosdick Mountains migmatite-cored gneiss dome

USGS Publications Warehouse

McFadden, Rory; Teyssier, Christian; Siddoway, Christine; Cosca, Michael A.; Fanning, C. Mark

2015-01-01

In Marie Byrd Land, West Antarctica, the Fosdick Mountains migmatite-cored gneiss dome was exhumed from mid- to lower middle crustal depths during the incipient stage of the West Antarctic Rift system in the mid-Cretaceous. Prior to and during exhumation, major crustal melting and deformation included transfer and emplacement of voluminous granitic material and numerous intrusions of mantle-derived diorite in dikes. A succession of melt- and magma-related structures formed at temperatures in excess of 665 ± 50 °C based on Ti-in-zircon thermometry. These record a transition from wrench to oblique extensional deformation that culminated in the development of the oblique South Fosdick Detachment zone. Solid-state fabrics within the detachment zone and overprinting brittle structures record translation of the detachment zone and dome to shallow levels.To determine the duration of exhumation and cooling, we sampled granite and gneisses at high spatial resolution for U–Pb zircon geochronology and 40Ar/39Ar hornblende and biotite thermochronology. U–Pb zircon crystallization ages for the youngest granites are 102 Ma. Three hornblende ages are 103 to 100 Ma and 12 biotite ages are 101 to 99 Ma. All overlap within uncertainty. The coincidence of zircon crystallization ages with 40Ar/39Ar cooling ages indicates cooling rates > 100 °C/m.y. that, when considered together with overprinting structures, indicates rapid exhumation of granite and migmatite from deep to shallow crustal levels within a transcurrent setting. Orientations of structures and age-constrained crosscutting relationships indicate counterclockwise rotation of stretching axes from oblique extension into nearly orthogonal extension with respect to the Marie Byrd Land margin. The rotation may be a result of localized extension arising from unroofing and arching of the Fosdick dome, extensional opening within a pull-apart zone, or changes in plate boundary configuration.The rapid tectonic and temperature evolution of the Fosdick Mountains dome lends support to recently developed numerical models of crustal flow and cooling in orogenic crust undergoing extension/transtension, and accords with numerous studies of migmatite-cored gneiss domes in transcurrent settings.

Cenozoic landforms and post-orogenic landscape evolution of the Balkanide orogen: Evidence for alternatives to the tectonic denudation narrative in southern Bulgaria

NASA Astrophysics Data System (ADS)

Gunnell, Y.; Calvet, M.; Meyer, B.; Pinna-Jamme, R.; Bour, I.; Gautheron, C.; Carter, A.; Dimitrov, D.

2017-01-01

Continental denudation is the mass transfer of rock from source areas to sedimentary depocentres, and is typically the result of Earth surface processes. However, a process known as tectonic denudation is also understood to expose deep-seated rocks in short periods of geological time by displacing large masses of continental crust along shallow-angle faults, and without requiring major contributions from surface erosion. Some parts of the world, such as the Basin and Range in the USA or the Aegean province in Europe, have been showcased for their Cenozoic tectonic denudation features, commonly described as metamorphic core-complexes or as supradetachment faults. Based on 22 new apatite fission-track (AFT) and 21 helium (AHe) cooling ages among rock samples collected widely from plateau summits and their adjacent valley floors, and elaborating on inconsistencies between the regional stratigraphic, topographic and denudational records, this study frames a revised perspective on the prevailing tectonic denudation narrative for southern Bulgaria. We conclude that conspicuous landforms in this region, such as erosion surfaces on basement-cored mountain ranges, are not primarily the result of Paleogene to Neogene core-complex formation. They result instead from "ordinary" erosion-driven, subaerial denudation. Rock cooling, each time suggesting at least 2 km of crustal denudation, has exposed shallow Paleogene granitic plutons and documents a 3-stage wave of erosional denudation which progressed from north to south during the Middle Eocene, Oligocene, Early to Middle Miocene, and Late Miocene. Denudation initially prevailed during the Paleogene under a syn-orogenic compressional regime involving piggyback extensional basins (Phase 1), but subsequently migrated southward in response to post-orogenic upper-plate extension driven by trench rollback of the Hellenic subduction slab (Phase 2). Rare insight given by the denudation pattern indicates that trench rollback progressed at a mean velocity of 3 to 4 km/Ma. The Neogene horst-and-graben mosaic that defines the modern landscape (Phase 3) has completely overprinted the earlier fabrics of Phases 1 and 2, and has been the prime focus of tectonic geomorphologists working in the region. The new narrative proposed here for linking the geodynamic evolution of SE Europe with surface landform assemblages raises issues in favour of better documenting the regional sedimentary record of existing Paleogene basins, which constitute a poorly documented missing link to the thermochronological evidence presented here.

Controlling factors of spatial and temporal preservation of the geochronological signal in sediments during an orogenic cycle

NASA Astrophysics Data System (ADS)

Rat, Juliette; Mouthereau, Frédéric; Bernet, Matthias; Brichau, Stéphanie; Balvay, Mélanie; Garzanti, Eduardo; Ando, Sergio

2017-04-01

Detrital content of sediments preserved in basins provide constraints on the nature of source rocks, dynamics of sediment transport, and potentially on tectonics and climate changes. U-Pb dating method on detrital zircon is ideally suited for provenance studies due to the ability of U-Pb age data to resist several orogenic cycles. However, with the aim to track sediment source evolution over a single orogenic cycle and determine characteristic time and parameters controlling the geochronological signal preservation throughout the cycle from rifting, mountain building to post-collision evolution, low-temperature thermochronology combined with sediment petrography are more appropriate than the U-Pb dating approach taken alone. To better understanding processes at play in the long-term geochronological signal preservation we focus on the sediment record associated with the Iberia plate tectonic evolution, which is part of the OROGEN research project, co-financed by BRGM, TOTAL & CNRS. The Iberian plate recorded a period of extension in the Late Jurassic, followed during the Early Cretaceous (Aptian-Albian) by a major thinning event documented by thick syn-rift sediments in intraplate basins and plate-scale heating/cooling of the Iberia crust, as argued by published fission track ages. Paleogeographic reconstructions that are based on stratigraphic and lithofacies analyses in northern Iberia (Iberian Range, Pyrenees and Basque-Cantabrians Range), describe a large domain of continental/fluvial and shallow-marine siliciclastic deposition. The related detrital content was then recycled during the subsequent Pyrenean orogenic phase in the Ebro foreland basin, and eventually transfer to the Mediterranean realm during post-orogenic re-excavation of the Ebro basin. In this study, we complete the published time-temperature paths in the mesozoic syn-rift basins by providing new thermo-chronological analyses of well-dated syn-collision and post-collision stratigraphic sections of the Ebro basin to determine thermal control on preservation through burial and geothermal evolution. We combined this study with sediments petrography analyses to identify relative control of source petrography, hydraulic sorting, alteration and diagenesis processes on the signal preservation during sediment transfer. All these observations will ultimately be incorporated in a geodynamic reconstruction of Iberia, and compared with age predictions from a model coupling surface processes and thermal evolution.

Geochemical signature variation of pre-, syn-, and post-shearing intrusives within the Najd Fault System of western Saudi Arabia

NASA Astrophysics Data System (ADS)

Hassan, M.; Abu-Alam, T. S.; Hauzenberger, C.; Stüwe, K.

2016-10-01

Late Precambrian intrusive rocks in the Arabian-Nubian Shield emplaced within and around the Najd Fault System of Saudi Arabia feature a great compositional diversity and a variety of degrees of deformation (i.e. pre-shearing deformed, sheared mylonitized, and post-shearing undeformed) that allows placing them into a relative time order. It is shown here that the degree of deformation is related to compositional variations where early, usually pre-shearing deformed rocks are of dioritic, tonalitic to granodioritic, and later, mainly post-shearing undeformed rocks are mostly of granitic composition. Correlation of the geochemical signature and time of emplacement is interpreted in terms of changes in the source region of the produced melts due to the change of the stress regime during the tectonic evolution of the Arabian-Nubian Shield. The magma of the pre-shearing rocks has tholeiitic and calc-alkaline affinity indicating island arc or continental arc affinity. In contrast, the syn- and post-shearing rocks are mainly potassium rich peraluminous granites which are typically associated with post-orogenic uplift and collapse. This variation in geochemical signature is interpreted to reflect the change of the tectonic regime from a compressional volcanic arc nature to extensional within-plate setting of the Arabian-Nubian Shield. Within the context of published geochronological data, this change is likely to have occurred around 605-580 Ma.

Evidence from the lamarck granodiorite for rapid late cretaceous crust formation in California

USGS Publications Warehouse

Coleman, D.S.; Frost, T.P.; Glazner, A.F.

1992-01-01

Strontium and neodymium isotopic data for rocks from the voluminous 90-million-year-old Lamarck intrusive suite in the Sierra Nevada batholith, California, show little variation across a compositional range from gabbro to granite. Data for three different gabbro intrusions within the suite are identical within analytical error and are consistent with derivation from an enriched mantle source. Recognition of local involvement of enriched mantle during generation of the Sierran batholith modifies estimates of crustal growth rates in the United States. These data indicate that parts of the Sierra Nevada batholith may consist almost entirely of juvenile crust added during Cretaceous magmatism.

Petrotectonic characteristics, geochemistry, and U-Pb geochronology of Jurassic plutons in the Upper Magdalena Valley-Colombia: Implications on the evolution of magmatic arcs in the NW Andes

NASA Astrophysics Data System (ADS)

Rodríguez, G.; Arango, M. I.; Zapata, G.; Bermúdez, J. G.

2018-01-01

Field, petrographic, and geochemical characterization along with U-Pb zircon geochronology of the Jurassic plutons exposed in the Upper Magdalena Valley (Colombia) allowed recognizing distinct western and eastern suites formed in at least three magmatic pulses. The western plutons crop out between the eastern flank of the Central Cordillera and the Las Minas range, being limited by the Avirama and the Betania-El Agrado faults. The western suite comprises a quartz monzonite - quartz monzodiorite - quartz diorite series and subordinate monzogranites. Chemically, the rocks are high-K calc-alkaline I-type granitoids (some reaching the shoshonitic series) with metaluminous of magnesium affinity. Trace-element tectonic discrimination is consistent with magmatism in a continental arc environment. Most rocks of this suite crystallized between 195 and 186 Ma (Early Jurassic, Pliensbachian), but locally some plutons yielded younger ages between 182 and 179 Ma (Early Jurassic, Toarcian). The eastern suite crops out in the eastern margin of the Upper Magdalena Valley, east of the Betania - El Agrado fault. Plutons of this unit belong to the monzogranite series with rock types ranging between syenogranites and granodiorites. They are high-K calc-alkaline continental granitoids, some metaluminous and some peraluminous, related to I-type granites generated in a volcanic arc. Crystallization of the suite was between 173 and 169 Ma (Middle Jurassic, Aalenian-Bajocian), but locally these rocks contain zircon with earlier inherited ages related to the magmatic pulse of the western suite between 182 and 179 Ma (Early Jurassic, Toarcian). The evolution of the Jurassic plutons in the Upper Magdalena Valley is best explained by onset or increase in subduction erosion of the accretionary prism. This explains the eastward migration of the arc away from the trench. Subduction of prism sediments increased the water flux from the subducting slab, decreasing solidus temperatures, therefore increasing the volume of magma and the amount of crustal melts involved in the magma. This is explains the crystallization of older and more primitive quartz-monzodiorite stocks in the west and the later crystallization of granitic bodies with batholitic dimensions in the east.

Extensional Late Paleozoic deformation on the western margin of Pangea, Patlanoaya area, Acatlán Complex, southern Mexico

NASA Astrophysics Data System (ADS)

Ramos-Arias, M. A.; Keppie, J. D.; Ortega-Rivera, A.; Lee, J. W. K.

2008-02-01

New mapping in the northern part of the Paleozoic Acatlán Complex (Patlanoaya area) records several ductile shear zones and brittle faults with normal kinematics (previously thought to be thrusts). These movement zones separate a variety of units that pass structurally upwards from: (i) blueschist-eclogitic metamorphic rocks (Piaxtla Suite) and mylonitic megacrystic granites (Columpio del Diablo granite ≡ Ordovician granites elsewhere in the complex); (ii) a gently E-dipping, listric, normal shear zone with top to the east kinematic indicators that formed under upper greenschist to lower amphibolite conditions; (iii) the Middle-Late Ordovician Las Minas quartzite (upper greenschist facies psammites with minor interbedded pelites intruded by mafic dikes and a leucogranite dike from the Columpio del Diablo granite) unconformably overlain by the Otate meta-arenite (lower greenschist facies psammites and pelites): roughly temporal equivalents are the Middle-Late Ordovician Mal Paso and Ojo de Agua units (interbedded metasandstone and slate, and metapelite and mafic minor intrusions, respectively) — some of these units are intruded by the massive, 461 ± 2 Ma, Palo Liso megacrystic granite: decussate, contact metamorphic muscovite yielded a 40Ar/ 39Ar plateau age of 440 ± 4 Ma; (iv) a steeply-moderately, E-dipping normal fault; (v) latest Devonian-Middle Permian sedimentary rocks (Patlanoaya Group: here elevated from formation status). The upward decrease in metamorphic grade is paralleled by a decrease in the number of penetrative fabrics, which varies from (i) three in the Piaxtla Suite, through (ii) two in the Las Minas unit (E-trending sheath folds deformed by NE-trending, subhorizontal folds with top to the southeast asymmetry, both associated with a solution cleavage), (iii) one in the Otate, Mal Paso, and Ojo de Agua units (steeply SE-dipping, NE-SW plunging, open-close folds), to (iv) none in the Patlanoaya Group. 40Ar/ 39Ar analyses of muscovite from the earliest cleavage in the Las Minas unit yielded a plateau age of 347 ± 3 Ma and show low temperature ages of ˜ 260 Ma. Post-dating all of these structures and the Patlanoaya Group are NE-plunging, subvertical folds and kink bands. An E-W, vertical normal fault juxtaposes the low-grade rocks against the Anacahuite amphibolite that is cut by megacrystic granite sheets, both of which were deformed by two penetrative fabrics. Amphibole from this unit has yielded a 40Ar/ 39Ar plateau age of 299 ± 6 Ma, which records cooling through ˜ 490 °C and is probably related to a Permo-Carboniferous reheating event during exhumation. The extensional deformation is inferred to have started in the latest Devonian (˜ 360 Ma) during deposition of the basal Patlanoaya Group, lasting through the rapid exhumation of the Piaxtla Suite at ˜ 350-340 Ma synchronous with cleavage development in the Las Minas unit, deposition of the Patlanoaya Group with active fault-related exhumation suggested by Mississippian and Early Permian conglomerates (˜ 340 and 300 Ma, respectively), and continuing at least into the Middle Permian (≡ 260 Ma muscovite ages). The continuity of Mid-Continent Mississippian fauna from the USA to southern Mexico suggests that this extensional deformation occurred on the western margin of Pangea after closure of the Rheic Ocean.

Axial Belt Provenance: modern river sands from the core of collision orogens

NASA Astrophysics Data System (ADS)

Resentini, A.; Vezzoli, G.; Paparella, P.; Padoan, M.; Andò, S.; Malusà, M.; Garzanti, E.

2009-04-01

Collision orogens have a complex structure, including diverse rock units assembled in various ways by geodynamic processes. Consequently, orogenic detritus embraces a varied range of signatures, and unravelling provenance of clastic wedges accumulated in adjacent foreland basins, foredeeps, or remnant-ocean basins is an arduous task. Dickinson and Suczek (1979) and Dickinson (1985) recognized the intrinsically composite nature of orogenic detritus, but did not attempt to establish clear conceptual and operational distinctions within their broad "Recycled Orogenic Provenance". In the Alpine and Himalayan belts, the bulk of the detritus is produced by focused erosion of the central backbone of the orogen, characterized by high topography and exhumation rates (Garzanti et al., 2004; Najman, 2006). Detritus derived from such axial nappe pile, including slivers of thinned continental-margin lithosphere metamorphosed at depth during early collisional stages, has diagnostic general features, which allows us to define an "Axial Belt Provenance" (Garzanti et al., 2007). In detail, "Axial Belt" detrital signatures are influenced by metamorphic grade of source rocks and relative abundance of continental versus oceanic protoliths, typifying distinct subprovenances. Metasedimentary cover nappes shed lithic to quartzolithic detritus, including metapelite, metapsammite, and metacarbonate grains of various ranks; only amphibolite-facies metasediments supply abundant heavy minerals (e.g., almandine garnet, staurolite, kyanite, sillimanite, diopsidic clinopyroxene). Continental-basement nappes shed hornblende-rich quartzofeldspathic detritus. Largely retrogressed blueschist to eclogite-facies metaophiolites supply albite, metabasite and foliated antigorite-serpentinite grains, along with abundant heavy minerals (epidote, zoisite, clinozoisite, lawsonite, actinolitic to barroisitic amphiboles, glaucophane, omphacitic clinopyroxene). Increasing metamorphic grade and deeper tectonostratigraphic level of source rocks are reflected by: a) increasing rank of metamorphic rock fragments (as indicated by progressive development of schistosity and growth of micas and other index minerals; MI index of Garzanti and Vezzoli, 2003); b) increasing feldspars; c) increasing heavy-mineral concentration (HMC index); d) increasing hornblende, changing progressively in color from blue/green to green/brown (HCI index); e) successive appearance of chloritoid, staurolite, kyanite, fibrolitic and prismatic sillimanite (MMI index; Garzanti and Andò, 2007). Dickinson W.R. 1985. Interpreting provenance relations from detrital modes of sandstones. In: Zuffa G.G. (ed.), Reidel, NATO ASI Series 148: 333-361. Dickinson W.R. and C.A. Suczek. 1979. Plate tectonics and sandstone composition. Am. Assoc. Pet. Geol. Bull. 63: 2164-2172. Garzanti E. and S. Andò. 2007, Plate tectonics and heavy-mineral suites of modern sands. In: Mange M. and D. Wright (eds.), Elsevier, Developments in Sedimentology Series 58: 741-763. Garzanti E. and G. Vezzoli. 2003. A classification of metamorphic grains in sands based on their composition and grade. J. Sedimentary Res. 73: 830-837. Garzanti E., C. Doglioni, G. Vezzoli and S. Andò. 2007. Orogenic Belts and Orogenic Sediment Provenances. J. Geology 115: 315-334. Garzanti E., G. Vezzoli, S. Andó, C. France-Lanord, S.K. Singh and G. Foster. 2004. Sediment composition and focused erosion in collision orogens: the Brahmaputra case. Earth Planet. Sci. Lett. 220: 157-174. Najman Y. 2006. The detrital record of orogenesis: a review of approaches and techniques used in the Himalayan sedimentary basins. Earth Sci. Rev. 74: 1-72.

Final « pop-up » structural reactivation of the internal part of an orogenic wedge: west-central Pyrenees

NASA Astrophysics Data System (ADS)

Meresse, F.; Jolivet, M.; Labaume, P.; Teixell, A.

2009-04-01

Université Montpellier 2, INSU-CNRS, Laboratoire Géosciences Montpellier, cc060, 34095 Montpellier Cedex 5, France florian.meresse@gm.univ-montp2.fr Tectonics-sedimentation relationships are often used to describe the tectonic evolution of orogenic wedges. However, does the sedimentary record associated to the build-up of the wedge recall the entire tectonic history? Numerous studies based on tectono-stratigraphic and thermochronological data, as well as numerical modeling, have demonstrated that on the large scale the growth of the Pyrenees is characterized by a southward propagation of the deformation (e.g., Muñoz, 1992; Morris et al., 1998; Fitzgerald et al., 1999; Beaumont et al., 2000). However, in the west-central Pyrenees, recent thermochronological data have suggested that the in-sequence propagation of the basement thrust system was followed by out-of-sequence (re)activation of hinterland structures after the South-Pyrenean Frontal Thrust had been sealed (Jolivet et al., 2007). To better describe the structural evolution of the Pyrenean prism, we focused our work on a NNE-SSW transect from the northern piedmont (Bagnères-de-Bigorre), through the Axial Zone and down to the Jaca basin where tectonics-sedimentation relationships have been extensively described (e.g., Teixell, 1996). A crustal scale cross-section combined with detailed apatite fission track analysis are used as a case study to unravel in detail the deformation history. Apatite fission track data from the Bagnères-de-Bigorre Paleozoic massif (central ages: 41-42 Ma) and the Lesponne Hercynian granite (central age: 31 Ma) located in the North-Pyrenean Zone and in the north of the Axial Zone, respectively, reveal Middle Eocene-Early Oligocene denudation ages of the northern part of the wedge. Immediately to the south, central ages around 24-20 Ma attest to a Latest Oligocene-Early Miocene denudation ages of the Chiroulet granite. According to the structural context, these results suggest a late exhumation stage associated with the tectonic (re)activation of north-vergent thrusts in the northern part of the Axial Zone. Similarly, results from the southern flank of the Axial Zone and the northern part of the Jaca basin suggest a denudation age around 18 Ma (Meresse et al., this volume), which may be linked to out-of-sequence tectonic movements on a south-vergent basement thrust (Bielsa thrust, Jolivet et al., 2007). In conclusion, thermochronological data reveal an Early Miocene "pop-up" exhumation of the internal parts of the Pyrenean wedge, which also shows that the Pyrenean compressional deformation ended later than the generally accepted Aquitanian age deduced from tectonics-sedimentation relationships. This late exhumation was achieved through out-of-sequence (re)activation of hinterland structures linked to a final internal thickening stage in the orogenic prism.

Example of trondhjemite genesis by means of alkali metasomatism: Rockford Granite, Alabama Appalachians

DOE Office of Scientific and Technical Information (OSTI.GOV)

Drummond, M.S.; Wesolowski, D.; Ragland, P.C.

1985-01-01

An alternative model for trondhjemite genesis is proposed where granite is transformed to trondhjemite via infiltration by a Na-rich metamorphic fluid. The Rockford Granite serves as the case example for this process and is characterized as a synmetamorphic, peraluminous trondhjemite-granite suite. The major process operative in the conversion of granite to trondhjemite involves cation exchange of Na for K in the feldspar and mica phases through a volatile fluid medium. Whole-rock delta/sup 18/O values for the trondhjemites are negatively correlated with the atomic prop. K/Na ratio indicating a partial reequilibration of the altered granitoids with a Na- and /sup 18/O-richmore » metamorphically derived fluid. Biotite decomposition to an Al-epidote-paragonitic muscovite-quartz assemblage is also associated with the Na-metasomatism, as are apatite replacement by Al-epidote and secondary zircon crystallization. The replacement of magmatic phases by metasomatic phases exemplifies the chemical changes produced during infiltration metasomatism where the trondhjemites are depleted in all REE's. The timing of the infiltration metasomatism is thought to have occurred during regional metamorphism, producing a discrete fluid phase in the surrounding amphibolite-grade metasediments. Foliation planes in the granitoid apparently served as conduts for fluid flow with reaction enhanced permeability accompanying the 8% molar volume reduction during Na for K exchange in the feldspars. A model is proposed where metamorphic fluids migrated updip and along strike from their source and were buffered by the presence or absence of two feldspars in the metasediments.« less

New contributions to granite characterization by ultrasonic testing.

PubMed

Cerrillo, C; Jiménez, A; Rufo, M; Paniagua, J; Pachón, F T

2014-01-01

Ultrasound evaluation permits the state of rocks to be determined quickly and cheaply, satisfying the demands faced by today's producers of ornamental stone, such as environmental sustainability, durability and safety of use. The basic objective of the present work is to analyse and develop the usefulness of ultrasound testing in estimating the physico-mechanical properties of granite. Various parameters related to Fast Fourier Transform (FFTs) and attenuation have been extracted from some of the studies conducted (parameters which have not previously been considered in work on this topic, unlike the ultrasonic pulse velocity). The experimental study was carried out on cubic specimens of 30 cm edges using longitudinal and shear wave transducers and equipment which extended the normally used natural resonance frequency range up to 500 kHz. Additionally, a validation study of the laboratory data has been conducted and some methodological improvements have been implemented. The main contribution of the work is the analysis of linear statistical correlations between the aforementioned new ultrasound parameters and physico-mechanical properties of the granites that had not previously been studied, i.e., resistance to salt crystallization and breaking load for anchors. Being properties that directly affect the durability and safety of use of granites, these correlations consolidate ultrasonics as a nondestructive method well suited to this type of material. Copyright © 2013 Elsevier B.V. All rights reserved.

Nature and time of emplacement of a pegmatoidal granite within the Delhi Fold Belt near Bayalan, Rajasthan, India

NASA Astrophysics Data System (ADS)

Dasgupta, N.; Sen, J.; Pal, T.; Ghosh, T.

2009-04-01

The study area is situated about 70 km south east of Ajmer, in Rajasthan, India around the village Bayala (26o 02' 19 N''; 74o 21' 01'') within the Ajmer district of Central Rajasthan. The area is along the eastern flank of the central portion of the Precambrian South Delhi Fold Belt (SDFB) and it stratigraphically belongs to the Bhim Group of rocks. Basement rocks of Archaean age, commonly known as the Banded gneissic Complex (BGC), is exposed to the east, where the rocks of the Bhim Group rests unconformably over BGC. To the west gneissic basement rocks of mid-Proterozoic times underlie the Bhim Group and have been referred to as the Beawar gneiss (BG). The Bhim Group of rocks comprises of metamorphosed marls and calc-silicate gneisses with minor amounts of quartzites and pelitic schists, indicative of its shallow marine origin. Within the Bhim Group, a pegmatoidal granite has intruded the calc silicate gneisses of the area. The pegmatoidal granite body is elliptical in outline with the long dimension(20 km) trending N-S and covers an area of 300 sq. km. approximately. This granite have so far been mapped as basement rocks (BG) surrounding the Beawar town (26o 06' 05'' N; 74o 19' 03'' E), 50 km south east of Ajmer. Rafts of calc-silicate gneisses, belonging to the Bhim Group, are seen to be entrapped within granite. Fragments of BG and its equivalents have also been found as caught up blocks within this pegmatoidal granite body near Andheri Devari, a small hamlet east of Beawar. The objective of the study was to map this pegmatoidal body, and decipher the mechanism and time of emplacement of this granite. A detailed structural mapping of the area in a 1:20000 scale spread over a 30 sq. km area in the vicinity of Bayala was carried out to analyse the geometry and the time of emplacement of the pegmatitic granite. The ridges of calc silicates and marbles adjoining the area were studied for the structural analyses of the Delhi fold belt rocks of the area. The calc silicate gneisses of the Bhim Group have been deformed by three major phases of folding, namely D1, D2 and D3. Of these the D1 folds defined by transposed compositional layering are intrafolial and isoclinal in nature. The D2 folds are asymmetric with alternate steeply and gently easterly dipping limbs and are defined by compositional banding and schistosity (S1). A good compositional layering parallel to the S2 fabric has been observed within the calc silicate gneisses. The D2 folds are close to tight, gently plunging with a modal plunge of 20o towards 40o; and has an inclined axial plane which has an easterly vergence. This is the most dominant phase of deformation. The D3 folds have developed on the gentle limbs of the D2 folds with a horizontal axis on a vertical axial plane. Interference of the D1 with D2 and D1 with D3 has produced Type III type of interference pattern. The pegmatitic granite body is a coarse grained rock composed of quartz feldspar (dominantly K-feldspar), muscovite, biotite, and tourmaline. A weak foliation has developed within this rock which is parallel to the D3 axial planar structure found within the calc silicate gneisses. Thus from the structural study it is proposed that the pegmatitic granite was emplaced post-D2 and possibly syn D3. The presence of narrow planar zones of hornfelsic rocks parallel to the D3 axial plane within the calc silicate rocks also attests to the above fact. Therefore the D3 axial planes provided the necessary conduits of the granite fluid movement within the calcsilicate rocks. The intrusions have scaled off the calc silicate gneisses into large continuous pieces along the gneissosity plane and got emplaced along the hinges of the D2 and D3 folds. Though disturbed, it has been seen that the orientation of the structural elements within these ripped off blocks of the calc silicate gneisses were quite similar to those found within the ridges, unaffected by the granite. The granites were thus emplaced lit-par-lit in the country rocks and on their contact with the calc gneisses shows hornfelsic textures within the calcsilicates (Tremolite-actinolite schists). It is thought of that this pegmatoidal granite has a bearing in age with the wide scale granite activity seen within this region. The structural study shows that this pegmatitic granite was emplaced synchronous to the D3 deformation episode which is the last major recognizable deformation within the Delhi Orogeny. Therefore this pegmatitic granite marks the closing stages of the Delhi Orogeny. Combined with the 1 Ga age of the formation of crust, in the western segment of the Delhi basin, a proper dating of this granite could give us the exact time span of the delhi orogenic cycle.

The ~2.4 Ga granitoids in the Quanji Massif，NW China: Petrogenesis and Implication for the early Paleoproterozoic Tectonics

NASA Astrophysics Data System (ADS)

Gong, S.; Chen, N.; Wang, Q.

2014-12-01

The 2.4-2.2 Ga collision-related magmatism had once been considered to be weak or lacking, and were interpreted to represent the stagnation and deceleration of plate subduction. However, large-scale intermediate to acid intrusions at 2.4-2.2 Ga are widely reported in the Trans-North China Orogen and Tarim Craton, NW China. Here we presented geochronological and geochemical and Nd-Hf isotopic studies of the Hudesheng and Delingha granitoids in the Quanji Massif, which could shed new light on the NW China and global tectonics in the Paleoproterozoic. The two granitoids are exposed in the eastern and central parts of the Quanji Massif, showing similar rock types and including mainly the syenogranitic, monzogranitic and adamellitic gneisses, with ~2.41 Ga low P/T-type amphibolite enclaves. They were formed at 2.37-2.39 Ga and underwent medium P/T-type amphibolite-facies metamorphism at ~1.95-1.90 Ga. Both granitoids have relatively high Si, K contents and FeO*/Mg, 104Ga/Al ratios, and are characterized by a relative enrichment in LILEs and LREEs, but depletion in Ba, Sr, Eu and Nb. These signatures are similar to those of the high-K calc-alkaline I-type granites, and with some geochemical features of A-type granites as well, suggesting that their formation might be related to a post-collisional setting with transition from compression to extension regimes at ~2.4 Ga. The whole-rock Nd and zircon Hf isotopes suggest important crustal growth occurred at ~2.5-2.8 Ga, and the precursor magmas of both granitoids were derived from partial melting of the Neo-Archean juvenile and ancient crustal components. Collectively, the ~2.37-2.39 Ga magmatic activities in the Quanji Massif took place right after arc-related metamorphism and arc-continent collision, thus probably making up part of the arc-related ~2.4-2.2 Ga magmatic activities, including those in the Trans-North China Orogen and Tarim Craton, NW China. This implies that a prolonged strong subduction-accretion-collision process occurred in the North China, consistent with those magmatic events having been discovered recently worldwide. This study is supported by National Natural Science Foundation of China (Grant Nos. 40972042, 41172069 and 41372075)

The Santa Izabel Complex, Gavião Block, Brazil: Components, geocronology, regional correlations and tectonic implications

NASA Astrophysics Data System (ADS)

Medeiros, Eder Luis Mathias; Cruz, Simone Cerqueira Pereira; Barbosa, Johildo Salomão Figueiredo; Paquette, Jean Louis; Peucat, Jean Jacques; Jesus, Silvandira dos Santos Góes Pereira de; Barbosa, Rafael Gordilho; Brito, Reinaldo Santana Correia de; Carneiro, Mauricio Antônio

2017-12-01

Cratons, as well as the basement of their marginal orogens, may represent important sites of research regarding the formation and evolution of Archean continental crusts. The Gavião Block is one of the oldest terranes in South America with rocks aged up to 3.6 Ga. Among the Archean units that outcrop in the southern sector of this block is the Santa Izabel Complex, which for the most part is located in the São Francisco Craton, close to its limit with the Araçuaí-West Congo Orogen. This complex has generally been described as comprising ortho- and paraderived rocks that were metamorphosed in high amphibolite facies. Studies in the southern region of this complex have shown the main components: (i) orthogneisses, whose protoliths are the Mesoarchean rocks of the Santa Izabel Magmatic Suite; and (iii) migmatites. and (iv) amphibolitic and metaultramafic enclaves. U-Pb studies (LA-ICPMS and SHRIMP) performed on zircons of the paleosome in metatexites and inherited zircons in migmatites indicate crystallization ages between 3091 ± 24 and 3136 ± 8 Ma for the rocks of the Santa Izabel Magmatic Suite. Inherited zircons aged ca. 3.4 Ga in paleosomes demonstrate the influence of older continental crust in the formation of these rocks. For the Caraguatai Magmatic Suite, the alignment of zircons and monazites suggests a crystallization age around 2.6 Ga. The Rhyacian migmatites were divided into metatexites and diatexites. Diatexites were divided into: (i) discontinuous boudinated early diatexites, which are parallel to stromatic metatexites, composing the gneissic banding. These rocks have diffuse metamorphic banding and features that suggest the action of mylonitization processes; and (ii) late diatexites, forming more continuous bodies, which truncate the gneissic banding. The migmatization occurred in two stages, with time interval between ca. 2.1 Ga and 2.07 Ga. The structural framework reveal the existence of four progressive Rhyacian deformation phases (Dn to Dn+3), and one deformation phase assumed to be Ediacaran (Dn+4). Gneissic banding is the dominant structure and lithofacies of the Santa Izabel and Caraguatai Magmatic suites, in which metatexites and late diatexites alternate between themselves. This progressive deformation occurred in conditions of high amphibolite facies, with stress fields varying between NW-SE and WSW-ENE. The youngest phase, Dn+4, was associated with distal deformations related to the evolution of the intracontinental Araçuaí-West Congo Orogen with a WSW-ENE stress field. The combined data suggest a complex evolution for the Gavião Block, involving juvenile accretion, crustal reworking, deformation and metamorphism/migmatization from the Paleoarchean to the early Rhyacian. Almost all elements of the evolutionary stages of the Gavião Block can be recognized in the study area, setting it as a natural laboratory to unravel the evolutionary history of this sector of the South American continental crust.

Superposed orogenic collision and core-complex formation at the present contact between the Dinarides and the Pannonian basin: The Bukulja and Cer Mountains in central and western Serbia

NASA Astrophysics Data System (ADS)

Matenco, Liviu; Toljic, Marinko; Ducea, Mihai; Stojadinovic, Uros

2010-05-01

Formation of large extensional detachments during orogenic collapse can follow inherited weakness zones such as major asymmetries given by pre-existing subduction zones active during mountain building processes. This is valid in particular in low-topography foreland coupling orogens of Mediterranean type where large amounts of deformation is concentrated in their lower plates, favoring weakness zones activated during a subsequent phase of extensional collapse. One good place to study the orogenic collapse post-dating major collision is the NE margin of the Dinarides in central and western Serbia, where Cretaceous-Eocene shortening and collision was recorded in the Alpine Tethys Sava zone between the European-derived Dacia and Tisza mega-units and the lower Adriatic plate. This is the same place where the Pannonian basin formed as a Miocene back-arc basin in response to a different subduction and roll-back taking place along the external Carpathians. A lineament of Paleogene and Miocene plutons is observed at the northern and eastern margin of the Dinarides, interpreted to be the product of both syn- to post-orogenic subduction magmatism and of decompressional melting during the Pannonian extension. Two of these plutons, Cer and Bukulja, located in western and respectively central Serbia, are intruded in the Jadar-Kopaonik composite thrust sheet, part of the lower Adriatic plate, near the contact with the main suture formed during the Cretaceous-Eocene subduction of the Sava zone. The Lower Miocene age (19-17Ma) Bukulja intrusion is a S-type granite with rare aplitic veins (Cvetkovic et al., 2007). The Cer intrusive complex is a S type two mica granite of around 16Ma in age with an older I-type quartz monzonite component (Koroneos et al. in press). Both granitoids are intruded into the Jadar-Kopaonik metamorphic series, which are in direct contact along the northern, eastern and southern flank with non-metamorphosed, mainly clastic sediments of Cretaceous-Miocene in age and, in the case of Bukulja, with serpentinized ophiolites. The metamorphic sequences are generally characterized by a Paleozoic age meta-sedimentary basement and a meta-sedimentary and meta-volcanic sequence. In the case of Bukulja, a succession of contrasting metamorphosed lithologies has been observed such as sandstones, black limestones, shallow water white limestones, basic volcanic sequences, deep nodular limestones and turbiditic sequences. The lower part of the sequence represents a metamorphosed Triassic sequence similar to what has been defined as the Kopaonik and Studenica series in southern Serbia. This part of the sequence is characterized by at least 3 successive stages of folding, asymmetric folds with WSW-ward vergence and NNE-SSW upright folds being affected by vertical flattening folds associated with extension (see also Marovic et al., 2007). The upper part of the sequence, which is the only part outcropping along the eastern flank of the Cer granitoid, is made up by metamorphosed distal turbidites which have been palinologically dated in Bukulja as Upper Cretaceous in age. This is the metamorphosed equivalent of the Upper Cretaceous - Eocene "flysch"-type of deposits commonly observed elsewhere in the main Sava subduction zone. These rocks are overprinted with a pervasive and strong extensional milonitic foliation indicating top-100 movement of the hanging-wall and are in direct contact with non-metamorphosed, but similar Upper Cretaceous distal turbidites. This suggests a large-scale tectonic omission along the eastern flanks of the Bukulja and Cer detachment. In the case of Bukulja, the extension was associated with the formation of the Early Miocene Morava basin in the detachment hanging-wall, which is an endemic lacustrine precursor of the much larger Middle-Late Miocene Pannonian basin. These finding points towards a bi-modal evolution of the internal Dinarides in central and western Serbia near the present-day contact with the Pannonian basin. An Upper Cretaceous-Eocene phase of top-WSW shortening and metamorphism in the Sava zone and its subducting lower Adriatic plate was subsequently followed by massive core-complex exhumation and top-E directed extension during initiation of the Carpathians back-arc extension. Interestingly, the newly defined extensional detachments accompanying the Pannonian extension closely follow the pre-existing subduction zone and its associated duplications in the lower orogenic plate. This conclusion is compatible with observations in other areas of the Dinarides, such as the Prosara-Motajica in Bosnia/Croatia or Kopaonik-Studenica in southern Serbia (Schefer et al., 2008; Ustaszewski et al., 2009).

Sapphirine-bearing granulites from the Tongbai orogen, China: Petrology, phase equilibria, zircon U-Pb geochronology and implications for Paleozoic ultrahigh temperature metamorphism

NASA Astrophysics Data System (ADS)

Xiang, Hua; Zhong, Zeng-Qiu; Li, Ye; Qi, Min; Zhou, Han-Wen; Zhang, Li; Zhang, Ze-Ming; Santosh, M.

2014-11-01

We report here for the first time the occurrence of sapphirine-bearing granulites within the Qinling Group of the Qinling-Tongbai orogen and provide robust evidence for extreme crustal metamorphism at ultrahigh-temperature (UHT) conditions. We document the UHT indicator of sapphirine and spinel in a mafic granulite consisting of orthopyroxene, biotite, plagioclase, amphibole and rutile/ilmenite. The ferromagnesian minerals in the sapphirine-bearing granulite have high XMg [Mg/(Mg + Fe)] (orthopyroxene XMg = 0.84-0.95; biotite XMg = 0.81; amphibole XMg = 0.87-0.96). The phase equilibria modeling demonstrates that the early spinel-bearing assemblage is stable at 923-950 °C and 6.7-8.9 kbar, and the peak assemblage of Opx + Pl + Spr/Spl + Amp + Bt + Ilm (+ melt) defines a field at 922-947 °C and 8.4-10.2 kbar. Rutiles have variable Zr concentrations but mostly cluster at ca. 1,500 and 3400 ppm. Zr-in-rutile geothermometry yielded high temperatures of up to 890-940 °C. Zircon U-Pb dating of the granulite constrains the timing of the immediate post-peak and retrograde metamorphic stages as 429 ± 7 Ma and 412 ± 4 Ma, respectively. The UHT metamorphism, together with extensive occurrence of coeval magmatic suites suggests that the Tongbai orogen experienced a Paleozoic Andean-type orogeny probably derived from mid-oceanic ridge subduction of the Qinling Ocean.

Usbnd Pb detrital zircon ages from some Neoproterozoic successions of Uruguay: Provenance, stratigraphy and tectonic evolution

NASA Astrophysics Data System (ADS)

Pecoits, Ernesto; Aubet, Natalie R.; Heaman, Larry M.; Philippot, Pascal; Rosière, Carlos A.; Veroslavsky, Gerardo; Konhauser, Kurt O.

2016-11-01

The Neoproterozoic volcano-sedimentary successions of Uruguay have been the subject of several sedimentologic, chrono-stratigraphic and tectonic interpretation studies. Recent studies have shown, however, that the stratigraphy, age and tectonic evolution of these units remain uncertain. Here we use new Usbnd Pb detrital zircon ages, combined with previously published geochronologic and stratigraphic data in order to provide more precise temporal constraints on their depositional age and to establish a more solid framework for the stratigraphic and tectonic evolution of these units. The sequence of events begins with a period of tectonic quiescence and deposition of extensive mixed siliciclastic-carbonate sedimentary successions. This is followed by the development of small fault-bounded siliciclastic and volcaniclastic basins and the emplacement of voluminous granites associated with episodic terrane accretion. According to our model, the Arroyo del Soldado Group and the Piedras de Afilar Formation were deposited sometime between ∼1000 and 650 Ma, and represent passive continental margin deposits of the Nico Pérez and Piedra Alta terranes, respectively. In contrast, the Ediacaran San Carlos (<552 ± 3 Ma) and Barriga Negra (<581 ± 6 Ma) formations, and the Maldonado Group (<580-566 Ma) were deposited in tectonically active basins developed on the Nico Pérez and Cuchilla Dionisio terranes, and the herein defined Edén Terrane. The Edén and the Nico Pérez terranes likely accreted at ∼650-620 Ma (Edén Accretionary Event), followed by their accretion to the Piedra Alta Terrane at ∼620-600 Ma (Piedra Alta Accretionary Event), and culminating with the accretion of the Cuchilla Dionisio Terrane at ∼600-560 Ma (Cuchilla Dionisio Accretionary Event). Although existing models consider all the Ediacaran granites as a result of a single orogenic event, recently published age constraints point to the existence of at least two distinct stages of granite generation, which are spatially and temporally associated with the Edén and Cuchilla Dionisio accretionary events.

Making the case for the Picuris orogeny: Evidence for a 1500 to 1400 Ma orogenic event in the southwestern United States

USGS Publications Warehouse

Daniel, Christopher G.; Jones, James V.; Andronicos, Christopher L.; Gray, Mary Beth; Abbott, Lon D.; Hancock, Gregory S.

2013-01-01

The early Mesoproterozoic (ca. 1400 Ma) is an enigmatic time in the tectonic evolution of southern Laurentia. Circa 1400 Ma granites within Laurentia and multiple other continents have distinctive geochemistry consistent with crustal extension or mantle upwelling. In the southwestern United States, these granites are commonly foliated and are often spatially associated with km-scale ductile shear zones. Deformation is attributed to intracontinental tectonism driven by active convergence along the distal southern margin of Laurentia. The recent discovery of deformed and metamorphosed, ca. 1450 Ma sedimentary rocks in northern New Mexico has strengthened the case for regional deformation and orogenesis. However, important questions remain about the tectonic significance of these events and how to reconcile tectonic models with granite petrology at the regional to global scale. This trip focuses on the protolith age of Proterozoic metasedimentary rocks and the kinematics, timing, and tectonic significance of deformation, magmatism, and metamorphism for the Mesoproterozoic across different crustal levels in the southern Rocky Mountains to highlight the ongoing questions and controversies regarding the Mesoproterozoic tectonic setting of Laurentia.This field trip will examine some of the diverse and most recently discovered evidence for ca. 1400 Ma orogenesis in the southern Rocky Mountains. We hope this trip will promote new interest and discussion about the Mesoproterozoic tectonic evolution of Laurentia. We will visit multiple outcrops in the Wet Mountains of southern Colorado and the Picuris Mountains of northern New Mexico. Stops in the Wet Mountains are arranged from north to south to examine contrasting styles of ca. 1400 Ma deformation with increasing paleodepth across the tilted Proterozoic crustal section. In the Picuris Mountains, we focus on detrital zircon geochronology and revisions to the lithostratigraphy of Paleoproterozoic and recently documented Mesoproterozoic metasedimentary rocks, the nature of regional metamorphism, and the style of deformation, ca. 1450–1400 Ma.

A Geothermochronologic Investigation of the Coyote Mountains Metamorphic Core Complex (AZ)

NASA Astrophysics Data System (ADS)

Borel, M.; Gottardi, R.; Casale, G.

2017-12-01

The Coyote Mountains metamorphic core complex (CM-MCC) makes up the northern end of the Baboquivari Mountain complex, which is composed of Mesozoic rocks, Tertiary granites, pegmatites, and metasediments. The CM-MCC expose the Pan Tak granite, a 58 Ma intrusive muscovite-biotite-garnet peraluminous granite. The Pan Tak and other intrusions within the Baboquivari Mountains have been interpreted as anatectic melts representing the culmination of a Laramide crustal shortening orogenic event started in the Late Cretaceous ( 70 Ma). Evidence of this magmatic episode includes polysynthetic twinning in plagioclase, myrmekitic texture in alkali feldspars, and garnet, mica and feldspar assemblages. The magmatic fabric is overprinted by a Tertiary tectonic fabric, associated with the exhumation of the CM-MCC along the Ajo road décollement and associated shear zone. In the shear zone, the Pan Tak mylonite display N-dipping foliation defined by gneissic layering and aligned muscovite, and N-trending mineral stretching lineation. Various shear sense indicators are all consistent with a top-to the-N shear sense. Preliminary argon geochronology results suggest that the shear zone was exhumed 29 Ma ago, an age similar to the onset of detachment faulting in other nearby MCCs (Catalina, Rincon, Pinaleño). In the Pan Tak mylonite, quartz grains display regime 2 to 3 microstructures and shows extensive recrystallization by subgrain rotation and grain boundary migration. The recrystallized grain size ranges between 20 and 50 µm in all samples. Quartz crystallographic preferred orientation measured using electron backscatter diffraction (EBSD) shows that recrystallization was accommodated by dominant prism and minor rhomb slip, suggesting deformation temperature ranging from 450°C to 550°C. These preliminary results constrain the timing of uplift and exhumation, and thermomechanical evolution of the CM-MCC, and improve our understanding of recycling of the continental crust in southern Arizona.

Magmatic context of Bou Skour copper deposit (Eastern Anti-Atlas, Morocco): Petrogrography, geochemistry and alterations

NASA Astrophysics Data System (ADS)

EL Azmi, Daoud; Aissa, M.; Ouguir, H.; Mahdoudi, M. L.; El Azmi, M.; Ouadjo, A.; Zouhair, M.

2014-09-01

The Bou Skour copper deposit is located in the western part of the Saghro massif (Eastern Anti-Atlas), about 50 km East of the city of Ouarzazate. It is subdivided into several areas that are, from North to South: “Panthère”, “Chaigne”, “Anne Marie”, “Chapeau de fer” and “Patte d'Oie”. The latter is economically the most important and is the object of this study. The “Patte d'Oie” district consists mainly of extrusive and intrusive igneous rocks. The extrusive rocks are represented by andesites spatially associated with pyroclastic terms (ignimbrites and pyroclastic breccias). This volcanic unit is intruded by a pink granite pluton and a I-type granodiorite with equigranular texture (Bou Skour granodiorite) showing to the border a microgranular facies (microgranodiorite). All these magmatic formations are intersected by rhyolitic dykes (NNE-SSW) and doleritic dykes (WNW-ESE to NW-SE). The granodiorite and andesite have undergone a polyphase hydrothermal alteration: (i) potassic alteration, (ii) phyllitic alteration, (iii) silicification, (iv) argillic alteration and (v) propylitic alteration. The analysis of geochemical data of granodiorite, granite, andesite and dolerite confirmed: (i) their petrographic natures, (ii) the medium-K calc-alkaline affiliation of andesite and granodiorite, which would have been set up into an active geotectonic environment, probably of island arc or collision, during the Pan-African orogeny, (iii) The high-K calc-alkaline character of granite indicating a post-collision development during the Pan-African orogeny and (iv) The alkaline affinity of the dolerite which is linked to an extensive post-orogenic setting (post-Pan-African). The copper mineralization of “Patte d'Oie” area is hosted, exclusively, in the andesitic and granodioritic facies. It is represented, essentially, by chalcopyrite and bornite minerals and is, probably, related to a porphyry system (disseminated and stockwork mineralization) remobilized late, in Hercynian structures (vein mineralization).

High-K granites of the Rum Jungle Complex, N-Australia: Insights into the Late Archean crustal evolution of the North Australian Craton

NASA Astrophysics Data System (ADS)

Drüppel, K.; McCready, A. J.; Stumpfl, E. F.

2009-08-01

The Late Archean (c. 2.54-2.52 Ga) high-K granitoids of the Rum Jungle Complex, Northern Australia, display the igneous mineral assemblage of K-feldspar, quartz, plagioclase, biotite, and magnetite, and accessories such as zircon, monazite, titanite, allanite, apatite, and ilmenite. The granites underwent a variably severe greenschist facies alteration and associated deformation during the Barramundi Orogeny (1.88-1.85 Ga). The K-rich granitoids have variable compositions, mainly comprising syenogranite and quartz-monzonite. They can be subdivided into two major groups, (1) felsic granites and (2) intermediate to felsic granites, quartz-monzonites, and diorite. The felsic group (69-76 wt.% SiO 2) shares many features with typical Late Archean potassic granites. They are K- and LILE-rich and show marked depletion in Sr and Eu and the high field strength elements (HFSE), particularly Nb and Ti, relative to LILE and LREE. Compared to the average upper crust they have anomalously high Th (up to 123 ppm) and U (up to 40 ppm). The intermediate to felsic group (56-69 wt.% SiO 2) differs from the felsic group in having weakly lower Th and U but higher Mg#, Ti, Ba, Sr, Ni, Cr and REE, with a less pronounced negative Eu anomaly. This group displays well-defined trends in Harker diagrams, involving a negative correlation of Si with Sr, Ca, Na, and P whereas K, Rb, and Ba increase in the same direction, suggesting fractional crystallization of feldspar was more prominent than in the felsic suite. The mineralogical and geochemical characteristics of the felsic group are consistent with granite formation by intracrustal melting of plagioclase-rich igneous protoliths, probably of tonaltic to granodioritic composition, at moderate crustal levels. The intermediate to felsic granites, on the other hand, appear to be the products of mantle-crust interaction, possibly by melting of or mixing with more mafic igneous rocks. As evidenced by the presence of older inherited zircons crustal recycling of a pre-greenstone crust of the North Australian Craton of > 3.5 Ga played an important role in the formation of the Late Archean granites of the Rum Jungle Complex.

The Cauaburi magmatic arc: Litho-stratigraphic review and evolution of the Imeri Domain, Rio Negro Province, Amazonian Craton

NASA Astrophysics Data System (ADS)

Carneiro, Marcia C. R.; Nascimento, Rielva S. C.; Almeida, Marcelo E.; Salazar, Carlos A.; Trindade, Ivaldo Rodrigues da; Rodrigues, Vanisse de Oliveira; Passos, Marcel S.

2017-08-01

A lithostratigraphic review of the Cauaburi Complex was carried out by means of field, tectono-metamorphic and geochemical data, which were the basis for the sub-division of the Cauaburi Complex orthogneisses into the Santa Izabel do Rio Negro, Cumati and São Jorge facies. These rocks crop out between São Gabriel da Cachoeira and Santa Izabel do Rio Negro, Amazonas, Brazil. The gneisses of the Santa Izabel do Rio Negro and Cumati facies are metaluminous and of calc-alkaline affinity; in turn, the rocks of the São Jorge facies are peraluminous and of alkaline affinity. They vary from (amphibole)-biotite granodiorites/monzogranites (Cumati and Santa Izabel do Rio Negro facies) to spessartite-bearing biotite monzogranites (São Jorge facies). The Cauaburi Complex geochemical signature is compatible with that of granites generated in collisional settings (magmatic arc?) and its evolution is related to three distinct tectono-metamorphic events: D1, causing foliation S1, which developed during the Cauaburi Complex syn-tectonic emplacement in the Cauaburi Orogeny; D2/M2, causing foliation S2, which was generated under amphibolite facies conditions (717.9 °C and 5.84 kbars), and the emplacement of I- and S-type granite during the Içana Orogen, and low-temperature D3, associated with the K'Mudku Event, which caused foliation S3 and reworking via transcurrent shear zones under greenschist facies conditions.

CHRONOLOGY OF MAJOR METAMORPHIC EVENTS IN THE SOUTHEASTERN UNITED STATES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Long, L.E.; Kulp, J.L.; Eckelmann, F.D.

1959-10-01

Potassium-argon and rubidium-strontium age measurements have been made on a variety of granites, pegmatites, gneiases and schists which comprise the plutonicmetamorphic complex of the Piedmont and Blue Ridge of the southeastern United States. Large portions of the area appear to have been metamorphosed initially approximately at the same time as the Grenville Province, i.e., about 900 to 1100 m.y. ago. Superimposed on this older metamorphic province was a major orogenic event culminating at about 350 m.y. with widespread recrystallization of existing rocks and intrusion of pegmatites in the Spruce Pine, Franklin-Sylva and Bryson City Districts, and granites in the Virginiamore » and North Carolina Piedmont. There is strong evidence of an additional metamorphic epoch between 350 and 1000 m.y., but its effects have been largely obliterated by the 350 m.y. event. In western North Carolina a transition of apparent ages from 355 to 890 m.y. occurs in the same rock unit. (Cranberry gneiss) over a distance of about 10 miles across the strike of the border- of the 350 m.y. event. In the southeastern Piedmont of Georgia and South Carolina a younger metamorphic event or events can be detected producing rocks of apparent age ranging from 230 to 310 m.y. The time of these orogenies is compared with those in the Central and Northern Appalachians. Evidence is accumulating that the Holmes' time scale will have to be considerably lengthened. (auth)« less

Geology and ore deposits of the Casto quadrangle, Idaho

USGS Publications Warehouse

Ross, Clyde P.

1934-01-01

The study of the Casto quadrangle was undertaken as the first item in a project to obtain more thorough knowledge of the general geology of southcentral Idaho on which to base study of the ore deposits of t he region. The quadrangle conta ins fragmentary exposures of Algonkian and Paleozoic sedimentary rocks, extensive deposits of old volcanic strata, presumably Permian, not heretofore recognized in this part of Idaho, and a thick succession of Oligocene(?) lava and pyroclastic rocks. The Idaho batholith and its satellites extend into the quadrangle, and in addition there a re large masses of Tertiary granitic rock, not previously distinguished in Idaho, and many Tertiary dikes, some of which are genetically associated with contact-metamorphic deposits. The area contains injection gneiss of complex origin, largely related to the Idaho batholith but in part resulting from injection by ~he Tertiary granitic rocks under relatively light load. Orogenic movement took place in Algonkian, Paleozoic, and Tertiary time. There is a summit peneplain or par tial peneplain of Tertiary, perhaps Pliocene age, and the erosional history since its elevation has been complex. The ore deposits include lodes and placers. The lodes are related to both the Idaho batholith and the Tert iary intrusive rocks and have yielded gold and copper ore of a total value of about 1,000,000. Placers, largely formed in an interglacial inter val, have yielded about an equal amount. There has been some prospecting but almost no production since 1916.

Sr-Nd-Hf-O isotope geochemistry of the Ertaibei pluton, East Junggar, NW China: Implications for development of a crustal-scale granitoid pluton and crustal growth

NASA Astrophysics Data System (ADS)

Tang, Gong-Jian; Wang, Qiang; Zhang, Chunfu; Wyman, Derek A.; Dan, Wei; Xia, Xiao-Ping; Chen, Hong-Yi; Zhao, Zhen-Hua

2017-09-01

To better understand the compositional diversity of plutonic complexes and crustal growth of the Central Asian Orogenic Belt (CAOB), we conducted an integrated study of the Ertaibei pluton, which obtained geochronological, petrological, geochemical, and isotopic (including whole rock Sr-Nd, in situ zircon Hf-O) data. The pluton (ca. 300 Ma) is composed of granodiorites that contain mafic microgranular enclaves (MMEs), dolerite dikes, and granite dikes containing quartz-tourmaline orbicules. The dolerite dikes were possibly generated by melting of an asthenospheric mantle source, with discrete assimilation of lower crustal components in the MASH (melting, assimilation, storage, and homogenization) zone. The MMEs originated from hybridization between mantle and crust-derived magmas, which spanned a range of melting depths (˜25-30 km) in the MASH zone and were episodically tapped. Melting of the basaltic lower crust in the core of the MASH zone generated magmas to form the granodiorites. The granite dikes originated from melting of an arc-derived volcanogenic sedimentary source with a minor underplated basaltic source in the roof of the MASH zone (˜25 km). The compositional diversity reflects both the magma sources and the degree of maturation of the MASH zone. Although having mantle-like radiogenic isotope compositions, the Ertaibei and other postcollisional granitoids show high zircon δ18O values (mostly between +6 and +9‰), indicating a negligible contribution to the CAOB crustal growth during the postcollisional period.

Eclogites and garnet clinopyroxenites in the Anrakhai complex, Central Asian Orogenic Belt, Southern Kazakhstan: P-T evolution, protoliths and some geodynamic implications

NASA Astrophysics Data System (ADS)

Pilitsyna, Anfisa V.; Tretyakov, Andrey A.; Degtyarev, Kirill E.; Cuthbert, Simon J.; Batanova, Valentina G.; Kovalchuk, Elena V.

2018-03-01

The Anrakhai Metamorphic Complex (AMC), located in the SE part of the Chu-Ili Mountains of Southern Kazakhstan in the western part of Central Asian Orogenic Belt, exhibits occurrences of HP metamorphic rocks in the form of eclogites and garnet clinopyroxenites with peak metamorphic conditions of 750-850° and 15-19 kbar estimated with both conventional geothermobarometric methods and phase diagram modeling. P-T estimates as well as intimate field relations evidently imply a common metamorphic history for eclogites and garnet clinopyroxenites of the AMC. These high-pressure, medium temperature eclogite facies P-T conditions are indicative of a collision or subduction tectonic setting. Major and trace element geochemistry suggests that they probably had a common magmatic origin as part of a suite of differentiated tholeiitic intrusions. Furthermore, distinctive mineral and chemical compositions of these eclogites and garnet clinopyroxenites correspond to the Fe-Ti type of ultramafic rocks suggesting that they may have been derivatives of intraplate tholeiitic melts, introduced into continental crust before HP metamorphism.

Southernmost Andes and South Georgia Island, North Scotia Ridge: Zircon U-Pb and muscovite {40Ar }/{39Ar } age constraints on tectonic evolution of Southwestern Gondwanaland

NASA Astrophysics Data System (ADS)

Mukasa, Samuel B.; Dalziel, Ian W. D.

1996-11-01

Zircon U-Pb and muscovite {40Ar }/{39Ar } isotopic ages have been determined on rocks from the southernmost Andes and South Georgia Island, North Scotia Ridge, to provide absolute time constraints on the kinematic evolution of southwestern Gondwanaland, until now known mainly from stratigraphic relations. The U-Pb systematics of four zircon fractions from one sample show that proto-marginal basin magmatism in the northern Scotia arc, creating the peraluminous Darwin granite suite and submarine rhyolite sequences of the Tobifera Formation, had begun by the Middle Jurassic (164.1 ± 1.7 Ma). Seven zircon fractions from two other Darwin granites are discordant with non-linear patterns, suggesting a complex history of inheritances and Pb loss. Reference lines drawn through these points on concordia diagrams give upper intercept ages of ca. 1500 Ma, interpreted as a minimum age for the inherited zircon component. This component is believed to have been derived from sedimentary rocks in the Gondwanaland margin accretionary wedge that forms the basement of the region, or else directly from the cratonic "back stop" of that wedge. Ophiolitic remnants of the Rocas Verdes marginal basin preserved in the Larsen Harbour complex on South Georgia yield the first clear evidence that Gondwanaland fragmentation had resulted in the formation of oceanic crust in the Weddell Sea region by the Late Jurassic (150 ± 1 Ma). The geographic pattern in the observed age range of 8 to 13 million years in these ophiolitic materials, while not definitive, is in keeping with propagation of the marginal basin floor northwestward from South Georgia Island to the Sarmiento Complex in southern Chile. Rocks of the Beagle granite suite, emplaced post-tectonically within the uplifted marginal basin floor, have complex zircon U-Pb systematics with gross discordances dominated by inheritances in some samples and Pb loss in others. Of eleven samples processed, only two had sufficient amounts of zircon for multiple fractions, and only one yielded colinear points. These points lie close to the lower concordia intercept for which the age is 68.9 ± 1.0 Ma, but their upper intercept is not well known. Inasmuch as this age is similar to the {40Ar }/{39Ar } age of secondary muscovite growing in extensional fractures of pulled-apart feldspar phenocrysts in a Beagle suite granitic pluton (plateau age is 68.1 ± 0.4 Ma), we interpret the two dates as good time constraints for cooling following a period of extensional deformation probably related to the tectonic denudation of the highgrade metamorphic complex of Cordillera Darwin in Tierra del Fuego.

Evolution of U fractionation processes through geologic time : consequences for the variation of U deposit types from Early Earth to Present

NASA Astrophysics Data System (ADS)

Cuney, M.

2009-12-01

U deposits are known at nearly all stages of the geological cycle, but are not known prior to 2.95 Ga. Also, U deposit types vary greatly from Mesoarchean to Present. Most of these changes through time can be attributed to major modifications in the geodynamic evolution of the Earth, in magmatic fractionation processes, in the composition of the Atmosphere and in the nature of life. The first U-rich granites able to crystallize uraninite, appeared at about 3.1 Ga. They correspond to the most fractionated terms of high-K calcalkaline suites, resulting from crystal fractionation of magmas possibly derived from melting of mantle wedges enriched in K, U, Th. Highly fractionated peraluminous leucogranites, able to crystallize uraninite, appeared at about 2.6 Ga. Erosion of these two granite types led to the detrital accumulation of uraninite that formed the first U deposits on Earth: the Quartz Pebble Conglomerates from 2.95 to 2.4 Ga. From 2.3 Ga onwards, uprise of oxygen level in the atmosphere led to the oxidation of U(IV) to U(VI), U transport in solution, and exuberant development of marine algae in epicontinental platform sediments. From 2.3 to 1.8 Ga large amounts of U, previously accumulated as U(IV) minerals, were dissolved and trapped preferentially in passive margin settings, in organic-rich sediments, and which led to the formation of the world’s largest Paleoproterozoic U provinces, e.g. : the Wollaston belt, Canada and the Cahill Formation, Australia. During and after the worldwide 2.1-1.75 Ga orogenic events, responsible for the formation of the Nuna supercontinent, U trapped in these formations was the source for several types of mineralization: (i) metamorphosed U-mineralized graphitic schists, calcsilicates and meta-arkoses, (ii) diagenetic-hydrothermal remobilization with the formation of the first deposits related to redox processes at 2.0 Ga (Oklo, Gabon), (iii) partial melting of U-rich metasediments forming the uraninite disseminations in pegmatoids (Charlebois, Canada), (iv) hydrothermal remobilization in veins (Beaverlodge, Canada) at about 1.75 Ga, and (v) U mineralization related to Na-metasomatism (Lagoa Real, Brazil ; Central Ukraine). After 1.75 Ga, a long period of tectonic quiescence occurred on the Earth, and large intracontinental basins, comprising at their base thick oxidized siliciclastic sequences were formed in many parts of the Nuna. In the Athabasca (Canada) and Kombolgie (Australia) basins, the siliciclastic sediments represented huge aquitards for sodic brines derived from overlying evaporites. The brines became calcic when infiltrated into the basement and leached U dominantly from Paleoproterozoic epicontinental sediments, their anatectic derivatives and high-K-U granites, to form the unconformity related U deposits. By the end of Silurian, with the apparition of land plants, deposits hosted by continental to marginal marine sandstone (roll front, tabular, tectono-lithologic, paleovalleys) became widespread. The largest volcanic related U-deposits are mostly known during the Mesozoic and calcrete are only known during late Caenozoic to Quaternary, but this may by due to the non preservation from erosion of such deposits formed at very shallow levels.

Geophysical Framework of a Rare Earth Element Enriched Terrane, Mountain Pass, California

NASA Astrophysics Data System (ADS)

Denton, K. M.; Ponce, D. A.; Peacock, J.; Miller, D. M.; Miller, J. S.

2016-12-01

Carbonatite ore deposits continue to be the primary source for rare earth elements (REEs), however large viable REE ore deposits are uncommon. The Mountain Pass carbonatite deposit, located in the eastern Mojave Desert of California, is the largest economic deposit of light REEs in North America. A 1.417 Ga ultrapotassic suite (shonkinite, syenite, and granite) and a 1.375 Ga barite-bastnasite-rich carbonatite (sovite) ore deposit comprise the enclave of REE-enriched outcrops and dikes that occupy a narrow ( 3 km) zone of 1.7 Ga gneiss extending at least 10-km to the southeast from southern Clark Mountain. Modeling of gravity, magnetic, and magnetotelluric (MT) data reveals subsurface features that form the structural framework of the REE terrane. The carbonatite and ultrapotassic mafic suite is associated with a local gravity high that is superimposed on a 4 km-wide gravity terrace, likely related to less dense granitic gneiss basement. Although physical property data indicate that the intrusive suite and carbonatite are essentially and nonmagnetic, aeromagnetic data indicate that these rocks occur along the eastern edge of a prominent north-northwest trending aeromagnetic high. This relationship suggests that they may have been preferentially emplaced along a zone of weakness or fault. The source of the magnetic high is 2-3 km below the surface and coincides with a relatively electrically conductive (3 orders of magnitude higher than surrounding rock) feature. MT data indicate that the western edge of the magnetic feature could be connected to a deeper ( 8 km) conductive feature related to possible intrusions and/or hydrothermal systems. The lack of a magnetic signature of the REE terrane can be explained by alteration of magnetite, given that the terrane lies within a broader alteration zone and observed magnetic low. If so, such an alteration event, capable of remobilizing rare earth elements, likely occurred during or after emplacement of the intrusive suite. Furthermore, an alteration event is consistent with local geology, high rare-earth element concentration, and unusual geochemistry of the carbonatite deposit and associated intrusive suite.

Critical elements in Carlin, epithermal, and orogenic gold deposits

USGS Publications Warehouse

Goldfarb, Richard J.; Hofstra, Albert H.; Simmons, Stuart F.

2016-01-01

Carlin, epithermal, and orogenic gold deposits, today mined almost exclusively for their gold content, have similar suites of anomalous trace elements that reflect similar low-salinity ore fluids and thermal conditions of metal transport and deposition. Many of these trace elements are commonly referred to as critical or near-critical elements or metals and have been locally recovered, although typically in small amounts, by historic mining activities. These elements include As, Bi, Hg, In, Sb, Se, Te, Tl, and W. Most of these elements are now solely recovered as by-products from the milling of large-tonnage, base metal-rich ore deposits, such as porphyry and volcanogenic massive sulfide deposits.A combination of dominance of the world market by a single country for a single commodity and a growing demand for many of the critical to near-critical elements could lead to future recovery of such elements from select epithermal, orogenic, or Carlin-type gold deposits. Antimony continues to be recovered from some orogenic gold deposits and tellurium could potentially be a primary commodity from some such deposits. Tellurium and indium in sphalerite-rich ores have been recovered in the past and could be future commodities recovered from epithermal ores. Carlin-type gold deposits in Nevada are enriched in and may be a future source for As, Hg, Sb, and/or Tl. Some of the Devonian carbonaceous host rocks in the Carlin districts are sufficiently enriched in many trace elements, including Hg, Se, and V, such that they also could become resources. Thallium may be locally enriched to economic levels in Carlin-type deposits and it has been produced from Carlin-like deposits elsewhere in the world (e.g., Alsar, southern Macedonia; Lanmuchang, Guizhou province, China). Mercury continues to be recovered from shallow-level epithermal deposits, as well as a by-product of many Carlin-type deposits where refractory ore is roasted to oxidize carbon and pyrite, and mercury is then captured in air pollution control devices.

Origin and structure of major orogen-scale exhumed strike-slip

NASA Astrophysics Data System (ADS)

Cao, Shuyun; Neubauer, Franz

2016-04-01

The formation of major exhumed strike-slip faults represents one of the most important dynamic processes affecting the evolution of the Earth's lithosphere and surface. Detailed models of the potential initiation and properties and architecture of orogen-scale exhumed strike-slip faults and how these relate to exhumation are rare. In this study, we deal with key properties controlling the development of major exhumed strike-slip fault systems, which are equivalent to the deep crustal sections of active across fault zones. We also propose two dominant processes for the initiation of orogen-scale exhumed strike-slip faults: (1) pluton-controlled and (2) metamorphic core complex-controlled strike-slip faults. In these tectonic settings, the initiation of faults occurs by rheological weakening along hot-to-cool contacts and guides the overall displacement and ultimate exhumation. These processes result in a specific thermal and structural architecture of such faults. These types of strike-slip dominated fault zones are often subparallel to mountain ranges and expose a wide variety of mylonitic, cataclastic and non-cohesive fault rocks, which were formed at different structural levels of the crust during various stages of faulting. The high variety of distinctive fault rocks is a potential evidence for recognition of these types of strike-slip faults. Exhumation of mylonitic rocks is, therefore, a common feature of such reverse oblique-slip strike-slip faults, implying major transtensive and/or transpressive processes accompanying pure strike-slip motion during exhumation. Some orogen-scale strike-slip faults nucleate and initiate along rheologically weak zones, e.g. at granite intrusions, zones of low-strength minerals, thermally weakened crust due to ascending fluids, and lateral borders of hot metamorphic core complexes. A further mechanism is the juxtaposition of mechanically strong mantle lithosphere to hot asthenosphere in continental transform faults (e.g., San Andreas Fault, Alpine Fault in New Zealand) and transtensional rift zones such as the East African rift. In many cases, subsequent shortening exhumes such faults from depth to the surface. A major aspect of many exhumed strike-slip faults is its lateral thermal gradient induced by the juxtaposition of hot and cool levels of the crust controlling relevant properties of such fault zones, e.g. the overall fault architecture (e.g., fault core, damage zone, shear lenses, fault rocks) and the thermal structure. These properties and the overall fault architecture include strength of fault rocks, permeability and porosity, the hydrological regime, as well as the nature and origin of circulating hydrothermal fluids.

Mobility of partially molten crust, heat and mass transfer, and the stabilization of continents

NASA Astrophysics Data System (ADS)

Teyssier, Christian; Whitney, Donna L.; Rey, Patrice F.

2017-04-01

The core of orogens typically consists of migmatite terrains and associated crustal-derived granite bodies (typically leucogranite) that represent former partially molten crust. Metamorphic investigations indicate that migmatites crystallize at low pressure (cordierite stability) but also contain inclusions of refractory material (mafic, aluminous) that preserve evidence of crystallization at high pressure (HP), including HP granulite and eclogite (1.0-1.5 GPa), and in some cases ultrahigh pressure (2.5-3.0 GPa) when the continental crust was subducted (i.e. Norwegian Caledonides). These observations indicate that the partially molten crust originates in the deep crust or at mantle depths, traverses the entire orogenic crust, and crystallizes at shallow depth, in some cases at the near-surface ( 2 km depth) based on low-T thermochronology. Metamorphic assemblages generally show that this nearly isothermal decompression is rapid based on disequilibrium textures (symplectites). Therefore, the mobility of partially molten crust results in one of the most significant heat and mass transfer mechanisms in orogens. Field relations also indicate that emplacement of partially molten crust is the youngest major event in orogeny, and tectonic activity essentially ceases after the partially molten crust is exhumed. This suggests that flow and emplacement of partially molten crust stabilize the orogenic crust and signal the end of orogeny. Numerical modeling (open source software Underworld; Moresi et al., 2007, PEPI 163) provides useful insight into the mechanisms of exhumation of partially molten crust. For example, extension of thickened crust with T-dependent viscosity shows that extension of the shallow crust initially drives the mobility of the lowest viscosity crust (T>700°C), which begins to flow in a channel toward the zone of extension. This convergent flow generates channel collision and the formation of a double-dome of foliation (two subdomes separated by a steep high strain zone). In turn, the rapid exhumation of low-viscosity deep crust within and between the two subdomes enhances localization of extension in the shallow crust; the positive feedback between exhumation of low-viscosity crust and localization of shallow crust extension explains the exhuming power of migmatite domes, the rapid isothermal decompression of dome rocks (order of 1.0-1.5 GPa), and the crystallization of melt at shallow depth followed by rapid cooling. Modeling results indicate that the mobility of low-viscosity (partially molten) crust is a major process for transferring heat and mass during the late stages of orogeny.

Ages of the Xinghuadukou Group in the Erguna Block, NE China

NASA Astrophysics Data System (ADS)

Liu, X.; Hou, W.

2016-12-01

The Xinghuadukou group is outcropped in the Erguna block (EB) of NE China, which is an important component of the eastern segment of the Central Asian Orogenic Belt (CAOB). This group was previously classified as Paleoproterozoic in age. However, recent studies reported Paleozoic ages from the meta-volcanic rocks, Paleoproterozoic to Neoarchean detrital zircon ages from the meta-sedimentary rocks and Neoproterozoic ages from the granitoids. The tectonic affinity of the EB is still debated. In order to clarify the aforementioned issues, 19 samples were collected from the Xinghuadukou group from the Mohe region in NE China. All samples underwent gneiss facies metamorphism, including two-mica granitic gneiss and quartz biotite gneiss. Based on the protolith discrimination diagram of Si—(al+fm)-(c+alk) system, 7 samples originated from sedimentary rocks and the other 12 of igneous origin. The orthogneiss samples were plotted as diorite, granodiorite and granite respectively in TAS, showing felsic character (SiO2 57% - 74%). One orthogneiss and one paragneiss samples were chosen to conduct the LA-ICP-MS U-Pb zircon age analysis. Apart from one zircon with the age of 742 Ma shows evident metamorphic rim, all zircons from the orthogneiss show euhedral to subhedral prismatic shape and typical concentric or oscillatory structure indicating the igneous origin. The concordant age of 2478±26 Ma was generated, indicating the existence of the near Archean basement of the EB. The detrital zircons from the paragneiss produced age populations cluster at 0.6, 0.8, 1.9, 2.6 and 2.7 Ga, lacking of the Grenville event age. The youngest zircon age is 395 Ma, taken as the maximum depositional age of the sedimentary protolith. According to the new data obtained, it is suggested that the Xinghuadukou group comprises the early Paleoproterozoic granite-gneiss, which proves the granitic basement of the Erguna block. The sedimentary rocks formed overlying the basement during the early Paleozoic and underwent metamorphism afterward. Compared with the detrital zircon ages profile of S. Siberia, NCC, Tarim, Mongolia and NE Gondwana, the EB is suggested most likely affiliated with NE Gondwana. Therefore, this study prefers a tectonic scenario that the EB drifted from the NE Gondwana and joined the CAOB tectonic domain during the Paleozoic.

Thermal history of low metamorphic grade Paleoproterozoic sedimentary rocks of the Penokean orogen, Lake Superior region: Evidence for a widespread 1786 Ma overprint based on xenotime geochronology

USGS Publications Warehouse

Vallini, D.A.; Cannon, W.F.; Schulz, K.J.; McNaughton, N.J.

2007-01-01

Paleoproterozoic strata in northern Michigan, Wisconsin, and Minnesota were deposited between 2.3 and 1.75 Ga within the rifted margin and subsequent foreland basin of the Penokean orogen. These strata show evidence for multiple regional metamorphic events previously attributed entirely to the Penokean orogeny (1875-1835 Ma). Metasandstones from the Marquette Range Supergroup and the Animikie, Mille Lacs, and North Range Groups were sampled at multiple localities across Minnesota, Wisconsin and Michigan for metamorphic xenotime suitable for in situ SHRIMP U-Pb geochronology. All samples are from the northern Penokean foreland basin where the metamorphic grade is greenschist to sub-greenschist and the strata are virtually undeformed. Xenotime U-Pb ages in these samples have a bimodal population with means of 1786 ?? 4 Ma (n = 32) and 1861 ?? 10 Ma (n = 9). Xenotime of both ages are contained in metasandstones from the basal Chocolay Group in Michigan and Wisconsin and the Mille Lacs Group and North Range Groups in Minnesota. The older age records a regional low-temperature thermal event that is slightly older than the overlying Menominee Group in Michigan and the Animikie Group in Minnesota and Ontario. This 1861 Ma event coincides with regional uplift that led to the formation of the unconformity between the Menominee Group and the overlying Baraga Group in Michigan; hence xenotime growth must have occurred at shallow burial depths. Younger units from the Menominee and Baraga Groups in Michigan and the Animikie Group in Minnesota, record only the 1786 Ma event. A dominant 1800-1790 Ma metamorphic monazite population that overprints Penokean-interval monazite has been documented within amphibolite- to granulite-facies rocks immediately north of the Niagara Fault Zone within the vicinity of gneiss domes and granitic plutons. In contrast, the 1786 Ma xenotime ages are from low-grade, virtually undeformed rocks 50-150 km from the high-grade zones and thus do not appear to reflect a local thermal imprint. Rather, the geographic extent of the 1786 Ma xenotime growth event suggests that it reflects a basin-wide, subtle thermal pulse. It is proposed that the xenotime ages record widespread subtle heating triggered by renewed subduction along the orogen due to Yavapai-interval convergence. The 1800-1700 Ma Yavapai terrane forms an accretionary belt throughout the central and southwestern U.S. and truncates the southern part of the Penokean orogen in central Wisconsin and southeastern Minnesota, about 200 km south of the sample sites. Alternatively, an 1800-1765 Ma interval of gravitational collapse of overthickened crust of the Penokean orogen immediately north of the Niagara Fault Zone may have driven a northward flow of hydrothermal fluids which subtly but pervasively altered the northern parts of the Penokean foreland and resulted in xenotime growth. ?? 2007 Elsevier B.V. All rights reserved.

Late Paleozoic tectonic evolution and concentrated mineralization in Balkhash and West Junggar, western part of the Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Dong, Shuwen; Chen, Xuanhua; Chen, Zhengle

2016-04-01

The Central Asia Orogenic Belt (CAOB) is an important area with significant growth of the crust and metallogeny in the Late Paleozoic. The Balkhash-Junggar tectono-metallogenic belt consists of the Balkhash, the West Junggar, and the East Junggar tectono-metallogenic belts in western part of the Central Asian Orogenic Belt (CAOB). According to the structural geological relationship, the East Junggar, the West Junggar, and the Balkhash belts are considered to be once a continuous E-W-trending tectono-metallogenic belt in Late Carboniferous. The West Junggar belt is featured with NE-trending left-lateral strike-slip faulting tectonic system (WJTS), while the left-lateral strike-slip faults are E-W-trending in the Balkhash belt. The WJTS consists of the Darabut, the Mayile, and the Baerluke faults, and the blocks among them. All these left-lateral strike-slip faults are forming due to the transition of tectonic settings from syn-collisional orogeny to post-collisional extension during the closure of the ocean (the Junggar Sea) in Late Carboniferous, with significant intrusion of batholiths and crust growth occurred in this period. These faults are truncated by the right-lateral strike-slip faults, such as the Chingiz-Junggar fault, and the Central Balkhash fault in Mesozoic. The Balkhash-Junggar tectono-metallogenic belt is important for the occurrence of many well-known super-large and large porphyry Cu-Mo deposits (such as the Kounrad, the Aktogai, the Borly, and the Baogutu deposits), large skarn Cu deposits (in the Sayak ore-filed), large rare metal deposits (such as the East Kounrad, the Zhanet, and the Akshatau deposits), and large gold deposits (such as the Hatu deposit). Zircon U-Pb ages, Re-Os isotopic dating of molybdenites, 40Ar/39Ar thermochronology of hornblendes, muscovites, biotites, and K-feldspars, and zircon and apatite fission track (FT) and (U-Th)/He dating and thermal history modeling, provide a multidisciplinary approach to constrain the whole course thermo-history of the minearl deposits from their formation in the deep to the exhumation in the surface. It reveals the arc-related granitic magmatism and the metallogeneses of skarn Cu, porphyry Cu-Mo, quartz-vein/greisen W-Mo, and orogenic Au in Late Paleozoic, the medium-temperature regional cooling in Late Paleozoic and Early Mesozoic, and the low-temperature exhumation of the deposits in Mesozoic. The timing, combined with geochemistry of granitoids, suggests a transition of tectonic environment from syn-collision and volcanic arc in Late Carboniferous to post-collision extension in Early Permian, and the concentrated mineralization of Cu, Mo, rare metals, and Au during this tectonic transition. The complete metallogenic series for the concentrated mineralization are from skarn and porphyry Cu-Mo deposits to rare metal and gold deposits. Key words: Late Paleozoic; Tectonic evolution; Concentrated mineralization; Balkhash-Junggar tectono-metallogenic belt; Central Asian Orogenic Belt

High precision ages from the Torres del Paine Intrusion, Chile

NASA Astrophysics Data System (ADS)

Michel, J.; Baumgartner, L.; Cosca, M.; Ovtcharova, M.; Putlitz, B.; Schaltegger, U.

2006-12-01

The upper crustal bimodal Torres del Paine Intrusion, southern Chile, consists of the lower Paine-Mafic- Complex and the upper Paine-Granite. Geochronologically this bimodal complex is not well studied except for a few existing data from Halpern (1973) and Sanchez (2006). The aim of this study is to supplement the existing data and to constrain the age relations between the major magmatic pulses by applying high precision U-Pb dating on accessory zircons and 40Ar/39Ar-laser-step-heating-ages on biotites from the Torres del Paine Intrusion. The magmatic rocks from mafic complex are fine to medium-grained and vary in composition from quartz- monzonites to granodiorites and gabbros. Coarse-grained olivine gabbros have intruded these rocks in the west. The granitic body is represented by a peraluminous, biotite-orthoclase-granite and a more evolved leucocratic granite in the outer parts towards the host-rock. Field observations suggest a feeder-zone for the granite in the west and that the granite postdates the mafic complex. Two granite samples of the outermost margins in the Northeast and South were analyzed. The zircons were dated by precise isotope-dilution U-Pb techniques of chemically abraded single grains. The data are concordant within the analytical error and define weighted mean 206/238U ages of 12.59 ± 0.03 Ma and 12.58 ± 0.01 Ma for the two samples respectively. A 40Ar/39Ar-age for the second sample yield a date of 12.37 ± 0.11 Ma. Three 40Ar/39Ar -ages of biotites were obtained for rocks belonging to the mafic complex. A hbl-bio- granodiorite from the central part, approximately 150 m below the subhorizontal contact with the granite, gives an age of 12.81 ± 0.11 Ma. A hbl-bio-granodiorite and an olivine-gabbro west of the feeder-zone date at 12.42 ± 0.14 Ma and 12.49 ± 0.11 Ma, respectively. The obtained older age of 12.81 Ma for the granodiorite in the central part is consistent with structural relationships of brittle fracturing of the mafic complex by the granite and we conclude that some parts of the mafic complex were emplaced before the granite. The well defined 206/238U-age for zircons and the slightly younger 40Ar/39Ar -ages for biotites of both rock suites show that emplacement and cooling of the Torres del Paine Intrusion took place in a relatively short time-frame. Halpern, 1973, Geological Society of America Bulletin, 84/7: 2407-2422. Sanchez et.al., 2006. V SSAGI, Punta del Este, April 2006.

Permian magmatic sequences of the Bilihe gold deposit in central Inner Mongolia, China: Petrogenesis and tectonic significance

NASA Astrophysics Data System (ADS)

Liu, Chunhua; Nie, Fengjun

2015-08-01

The Bilihe gold deposit is located in the eastern section of the Ondor Sum-Yanji Suture at the southern margin of the Xing'an-Mongolian Orogenic Belt (XMOB) and the northern margin of the North China Craton (NCC), central Inner Mongolia. The magmatic rocks in the ore district are generally high-K calc-alkaline, enriched in LREE, Zr, and Hf, and depleted in HREE, Nb, Ta, and P. The magmatic evolution sequences are norite gabbro → granodiorite porphyry → granite or norite gabbro → andesite → dacite porphyry → granodiorite, which show a trend of decreasing TiO2, FeO, MgO, CaO, and P2O5 with increasing SiO2. In the Bilihe ore district, hydrothermal processes were coeval with granitic magmatism for a period of ~ 17 Myr (272-255 Ma). The ages of the granite, granodiorite porphyry, granodiorite, and dacite porphyry are 271.5-264.1 Ma, 269.8-255.8 Ma, 268.3 Ma, and 268.6-259.4 Ma, respectively. The magmatic rocks contain magmatic, hydrothermal, and magmatic-hydrothermal zircons. The magmatic zircons have δCe > 4, La < 3 ppm, and SmN/LaN > 2.5; the hydrothermal zircons have δCe < 4, La > 3 ppm, and SmN/LaN < 2.5. The Nb/Ta and Zr/Hf ratios of granodiorite are 12.7-14.99 and 40.2-46.56, respectively. The Zr/Hf ratios successively increase in the sequence of granite (27.4-29.02) → granodiorite porphyry (29.19-32.18) → dacite porphyry (33.54-38.5) → norite gabbro (36.75-38.37), and their Nb/Ta ratios are 9.09-12.38. Zircons in granodiorite yield ε Hf (t) values of - 0.29 to - 56 (n = 13) and 2.07-7.62 (n = 5), and they give a Hf two-stage model age (tDM2) of 807-4765 Ma. The ε Hf (t) values of the zircons in granite, granodiorite porphyry, and dacite porphyry are - 0.46 to 8.03, 3.17 to 10.32, and - 0.78 to 6.58, respectively, and their Hf tDM2 ages are 787-1324 Ma, 638-1091 Ma, and 868-1343 Ma, respectively. Dehydration partial melting of subducted oceanic crust resulted in the formation of dacite porphyry; partial melting of depleted mantle resulted in the formation of norite gabbro; mixing of depleted mantle and lower crust resulted in the formation of granodiorite porphyry; partial melting of lower crust resulted in the formation of granite; and mixing of lower crust and old upper crust resulted in the formation of granodiorite. Magmatic rocks in the ore district with ages of 272-255 Ma were formed during the late stages of closure of the Paleoasian Ocean; i.e., during the transformation from a collisional to extensional setting.

Detrital zircons from phanerozoic rocks of the Songliao Block, NE China: Evidence and tectonic implications

NASA Astrophysics Data System (ADS)

Zhou, Jian-Bo; Wilde, Simon A.; Zhang, Xing-Zhou; Liu, Fu-Lai; Liu, Jian-Hui

2012-03-01

Rocks that crop out in the northern part of the Songliao Block are mainly consist of high-grade metamorphic gneiss, Paleozoic strata and Mesozoic granites. They are essentially similar to rocks reported from beneath the Songliao Basin that occupies the majority of the Songliao Block. Four samples of Paleozoic metasedimentary rocks from Tieli in the north-eastern part of the Songliao Block yield detrital zircon U-Pb ages ranging from 2690 to 501 Ma, with four age populations at: 2071-2690 Ma, with a peak at 2585 Ma; 1776-1997 Ma, with a peak at 1890 Ma; 719-991 Ma, with a peak at 800 Ma; and 501-592 Ma, with a peak at 518 Ma. These are similar to age populations in other parts of the Central Asian Orogenic Belt (CAOB), although sediments from the Songliao Block contain more abundant Archean and Proterozoic detrital zircons than the neighboring Jiamusi-Khanka Block to the east and Xing'an Block to the west. This may indicate that rocks of this age comprise a minor component of the Songliao Block. The Pan-African zircon ages from the Songliao Block, taken together with ˜500 Ma magmatic and high-grade metamorphic zircons obtained from the nearby Erguna, Xing'an and Jiamusi-Khanka blocks, indicate that Pan-African events affected all blocks of the CAOB in NE China. This suggests that these blocks not only share a common basement, but that they had a common history. An extensive Late Pan-African (˜500 Ma) orogenic terrane thus occupies much of the CAOB in NE China.

U Pb zircon age, geochemical and Sr Nd Pb Hf isotopic constraints on age and origin of alkaline intrusions and associated mafic dikes from Sulu orogenic belt, Eastern China

NASA Astrophysics Data System (ADS)

Liu, Shen; Hu, Ruizhong; Gao, Shan; Feng, Caixia; Qi, Youqiang; Wang, Tao; Feng, Guangying; Coulson, Ian M.

2008-12-01

Post-orogenic alkaline intrusions and associated mafic dikes from the Sulu orogenic belt of eastern China consist of quartz monzonites, A-type granites and associated mafic dikes. We report here U-Pb zircon ages, geochemical data and Sr-Nd-Pb-Hf isotopic data for these rocks. The SHRIMP U-Pb zircon analyses yield consistent ages ranging from 120.3 ± 2.1 Ma to 126.9 ± 1.9 Ma for five samples from the felsic rocks, and two crystallization ages of 119.0 ± 1.7 Ma and 120.2 ± 1.9 Ma for the mafic dikes. The felsic rocks and mafic dikes are characterized by high ( 87Sr/ 86Sr) i ranging from 0.7079 to 0.7089, low ɛNd( t) values from - 15.3 to - 19.2, 206Pb/ 204Pb = 16.54-17.25, 207Pb/ 204Pb = 15.38-15.63, 208Pb/ 204Pb = 37.15-38.45, and relatively uniform ɛHf( t) values of between - 21.6 ± 0.6 and - 23.7 ± 1.0, for the magmatic zircons. The results suggest that they were derived from a common enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials before magma generation. Geochemical and isotopic characteristics imply that the primary magma to these rocks originated through partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite. The mafic dikes may have been generated by subsequent fractionation of clinopyroxene, whereas the felsic rocks resulted from fractionation of potassium feldspar, plagioclase and ilmenite or rutile. Both were not affected by crustal contamination. Combined with previous studies, these findings provide new evidence that the intense lithospheric thinning beneath the Sulu belt of eastern China occurred between 119 and 127 Ma, and that this was caused by the removal of the lower lithosphere (mantle and lower crust).

New Insights into Arctic Tectonics: Uranium-Lead, (Uranium-Thorium)/Helium, and Hafnium Isotopic Data from the Franklinian Basin, Canadian Arctic Islands

NASA Astrophysics Data System (ADS)

Anfinson, Owen Anthony

More than 2300 detrital zircon uranium-lead (U-Pb) ages, 32 176Hf/177Hf (eHf) isotopic values, 37 apatite helium (AHe) ages, and 72 zircon helium (ZHe) ages represent the first in-depth geochronologic and thermochronologic study of Franklinian Basin strata in the Canadian Arctic and provide new insight on >500 M.y. of geologic history along the northern Laurentian margin (modern orientation). Detrital zircon U-Pb age data demonstrate that the Franklinian Basin succession is composed of strata with three distinctly different provenance signatures. Neoproterozoic and Lower Cambrian formations contain detrital zircon populations consistent with derivation from Archean to Paleoproterozoic gneisses and granites of the west Greenland--northeast Canadian Shield. Lower Silurian to Middle Devonian strata are primarily derived from foreland basin strata of the East Greenland Caledonides (Caledonian orogen). Middle Devonian to Upper Devonian strata also contain detrital zircon populations interpreted as being primarily northerly derived from the continental landmass responsible for the Ellesmerian Orogen (often referred to as Crockerland). U-Pb age data from basal turbidites of the Middle to Upper Devonian clastic succession suggest Crockerland contributed sediment to the northern Laurentian margin by early-Middle Devonian time and that prior to the Ellesmerian Orogeny Crockerland had a comparable geologic history to the northern Baltica Craton. Detrital zircon U-Pb ages in Upper Devonian strata suggest Crockerland became the dominant source by the end of Franklinian Basin sedimentation. Mean eHf values from Paleozoic detrital zircon derived from Crockerland suggest the zircons were primarily formed in either an island arc or continental arc built on accreted oceanic crust setting. ZHe cooling ages from Middle and Upper Devonian strata were not buried deeper than 7 km since deposition and suggest Crockerland was partially exhumed during the Caledonian Orogen. AHe cooling ages are partially reset since deposition and experienced varying burial histories depending on stratigraphic and geographic location within the basin. AHe ages from Middle Devonian strata from the western margin of the basin indicate episodes of exhumation associated with clastic influxes of sediment into the Sverdrup Basin during the Late Jurassic-Early Cretaceous and Late Cretaceous.

Accessing The Fourth Dimension In Orogenic Reconstruction Using Granitoid Thermobarometry

NASA Astrophysics Data System (ADS)

Alexander, E.; Wielicki, M. M.; Harrison, M.; Lovera, O. M.; DePaolo, D.

2016-12-01

Tectonic models for the Tibetan-Himalayan orogen predict very different crustal thickness histories, providing a possible test of these various hypotheses. However, reconstructing the evolution of the Tibetan-Himalayan crust is a four-dimensional problem. Knowing the 2D distribution of U-Pb zircon dated samples permits insights into changes in magmatic style, but understanding of N-S thickening history requires depth information. In S. Tibet, voluminous granitoids emplaced between 200-20 Ma provides a spatiotemporal window into the tectonic evolution of the Tibetan crust. A thermoisotopic model utilizing systematic N-S ɛNd variations as a crustal thickness proxy indicates that, at the time of collision, the crust beneath the Indus-Tsangpo suture was relatively thin ( 20 km), increasing to >45 km 100 km to the N. Given evidence of little post-50 Ma upper-crustal shortening, the Tibetan crust appears to have reached its present 85 km thickness via 20 km of tectonic accretion, 15 km of juvenile magma input, as well as the underthrusted Indian crust. Since zircon ɛHf correlates well with whole rock ɛNd, using it together with in situ δ18O, Ti-thermometry and trace element analyses, has provided a refined picture of the relative roles of assimilation and recharge in granitoid formation. To understand the spatiotemporal progression of thickening, we utilize two thermobarometers to reconstruct crystallization depths: Al-in-hornblende and Ti-in-quartz. While Al-in-hornblende has been more widely tested on igneous systems, our data show that it may be sensitive to closure effects during high T storage and hydrothermal alteration. In contrast, Ti-in-quartz preserves magmatic Ti signatures, and quartz's exclusive structure and resistance to alteration provides a more robust proxy for emplacement conditions. Using these thermobarometers in conjunction with Ti-in-zircon thermometry can establish emplacement depth of syn-collisional Lhasa block granites providing the missing dimension in reconstructing orogenic histories. Together, our data show that the southern margin of Asia remained thin (<25 km) until collision began whereas the northern terrane was continuously thick from 200 Ma. This observation reinforces the growing view that Tibet hosted significant topography prior to collision.

What can hafnium isotope ratios arrays tell us about orogenic processes? An insight into geodynamic processes operating in the Alpine/Mediterranean region

NASA Astrophysics Data System (ADS)

Henderson, B.; Murphy, J.; Collins, W. J.; Hand, M. P.

2013-12-01

Over the last decade, technological advances in laser-ablation sampling techniques have resulted in an increase in the number of combined U-Pb-Hf zircon isotope studies used to investigate crustal evolution on a local, regional and global scale. Hafnium isotope arrays over large time scales (>500 myr) have been interpreted to track evolving plate tectonic configurations, and the geological outputs associated with changing plate boundaries. We use the Alpine-Mediterranean region as an example of how hafnium isotope arrays record the geodynamic processes associated with the complex geological evolution of a region. The geology of Alpine-Mediterranean region preserves a complex, semi-continuous tectonic history that extends from the Neoproterozoic to the present day. Major components of the Variscan and Alpine orogens are microcontinental ribbons derived from the northern Gondwanan margin, which were transferred to the Eurasian plate during the opening and closing of the Rheic and Paleo-Tethys Oceans. Convergence of the Eurasian and African plates commenced in the Mid-Late Cretaceous, following the destruction of the Alpine-Tethys Ocean during the terminal breakup of Pangea. In general, convergence occurred slowly and is characterised by northward accretion of Gondwanan fragments, interspersed with subduction of African lithosphere and intermittent roll-back events. A consequence of this geodynamic scenario was periods of granite-dominated magmatism in an arc-backarc setting. New Hf isotope data from the peri-Gondwanan terranes (Iberia, Meguma and Avalonia) and a compilation of existing Phanerozoic data from the Alpine-Mediterranean region, indicate ~500 myr (Cambrian-Recent) of reworking of peri-Gondwanan crust. The eHf array follows a typical crustal evolution pattern (Lu/Hf=0.015) and is considered to reflect reworking of juvenile peri-Gondwanan (Neoproterozoic) crust variably mixed with an older (~1.8-2.0 Ga) source component, probably Eburnian crust from the West Africa Craton. The Phanerozoic Hf isotopic data from Variscan and Alpine Europe suggest that slow translation of continental fragments from one continent to another produces a characteristic, long-term crustal reworking eHf array, which strongly contrasts with the Hf array defined by Phanerozoic circum-Pacific orogens.

Lithospheric delamination in post-collisional setting: Evidence from intrusive magmatism from the North Qilian orogen to southern margin of the Alxa block, NW China

NASA Astrophysics Data System (ADS)

Zhang, Liqi; Zhang, Hongfei; Zhang, Shasha; Xiong, Ziliang; Luo, Biji; Yang, He; Pan, Fabin; Zhou, Xiaochun; Xu, Wangchun; Guo, Liang

2017-09-01

Post-collisional granitoids are widespread in the North Qilian and southern margin of the Alxa block and their petrogenesis can provide important insights into the lithospheric processes in a post-collisional setting. This paper carries out an integrated study of U-Pb zircon dating, geochemical and Sr-Nd-Hf isotopic compositions for five early Paleozoic intrusive plutons from the North Qilian to southern margin of the Alxa block. The geochronological and geochemical results show that their magmatism can be divided into three periods with distinct geochemical features. The early-period intrusive rocks ( 440 Ma) include the Lianhuashan (LHS) and Mengjiadawan (MJDW) granodiorites. Both of them display high Sr/Y ratios (52-91), coupled with low Y and HREE contents, implying that they were derived from partial melting of thickened lower crust, with garnet in the residue. The middle-period intrusive rocks ( 430 Ma), including the MJDW quartz diorites and Yangqiandashan (YQDS) granodiorites, are high-K calc-alkaline with low Sr/Y values. The geochemical and isotopic data suggest that they are generated from partial melting of lower crust without garnet in the residue. The late-period intrusive rocks (414-422 Ma), represented by the Shengrongsi (SRS) and Xinkaigou (XKG) plutons, are A-type or alkali-feldspar granites. They are possibly derived from partial melting of felsic crustal material under lower pressure condition. Our data show decreasing magma crystallization ages from MJDW pluton in the north and LHS pluton in the south to the SRS and XKG plutons in the central part of the study area. We suggest that such spatial and temporal variations of magmatic suites were caused by lithospheric delamination after the collision between the Central Qilian and the Alxa block. A more plausible explanation is that the delamination propagated from the margin part of the thickened lithosphere to inward beneath the North Qilian and southern margin of the Alxa block.

New observations on the quartz monzodiorite-granite suite. [in lunar soil

NASA Technical Reports Server (NTRS)

Marvin, U. B.; Holmberg, B. B.; Lindstrom, M. M.; Martinez, R. R.

1991-01-01

Five new fragments of quartz monzodiorite (QMD) were identified in particles from soil 15403, which was collected from the boulder sampled as rock 15405, an impact-melt breccia containing clasts of KREEP basalt, QMD, granite, and a more primitive alkali norite. Petrographic and geochemical studies of the fragments show considerable variation in modal proportions and bulk composition. This heterogeneity is due to unrepresentative sampling in small fragments of coarse-grained rocks. Variations in the proportions of accessory minerals have marked effects on incompatible-trace-element concentrations and ratios. Semiquantitative calculations support the derivation of QMD from 60-percent fractional crystallization of a KREEP basalt magma as suggested by Hess (1989). Apollo 15 KREEP basalt cannot be the actual parent magma because the evolved rocks predate volcanic KREEP basalts. It is suggested that ancient KREEP basalt magmas have crystallized as plutons, with alkali norite clasts offering the only direct evidence of this precursor.

Geology and metallogeny of the Ar Rayn terrane, eastern Arabian shield: Evolution of a Neoproterozoic continental-margin arc during assembly of Gondwana within the East African orogen

USGS Publications Warehouse

Doebrich, J.L.; Al-Jehani, A. M.; Siddiqui, A.A.; Hayes, T.S.; Wooden, J.L.; Johnson, P.R.

2007-01-01

The Neoproterozoic Ar Rayn terrane is exposed along the eastern margin of the Arabian shield. The terrane is bounded on the west by the Ad Dawadimi terrane across the Al Amar fault zone (AAF), and is nonconformably overlain on the east by Phanerozoic sedimentary rocks. The terrane is composed of a magmatic arc complex and syn- to post-orogenic intrusions. The layered rocks of the arc, the Al Amar group (>689 Ma to ???625 Ma), consist of tholeiitic to calc-alkaline basaltic to rhyolitic volcanic and volcaniclastic rocks with subordinate tuffaceous sedimentary rocks and carbonates, and are divided into an eastern and western sequence. Plutonic rocks of the terrane form three distinct lithogeochemical groups: (1) low-Al trondhjemite-tonalite-granodiorite (TTG) of arc affinity (632-616 Ma) in the western part of the terrane, (2) high-Al TTG/adakite of arc affinity (689-617 Ma) in the central and eastern part of the terrane, and (3) syn- to post-orogenic alkali granite (607-583 Ma). West-dipping subduction along a trench east of the terrane is inferred from high-Al TTG/adakite emplaced east of low-Al TTG. The Ar Rayn terrane contains significant resources in epithermal Au-Ag-Zn-Cu-barite, enigmatic stratiform volcanic-hosted Khnaiguiyah-type Zn-Cu-Fe-Mn, and orogenic Au vein deposits, and the potential for significant resources in Fe-oxide Cu-Au (IOCG), and porphyry Cu deposits. Khnaiguiyah-type deposits formed before or during early deformation of the Al Amar group eastern sequence. Epithermal and porphyry deposits formed proximal to volcanic centers in Al Amar group western sequence. IOCG deposits are largely structurally controlled and hosted by group-1 intrusions and Al Amar group volcanic rocks in the western part of the terrane. Orogenic gold veins are largely associated with north-striking faults, particularly in and near the AAF, and are presumably related to amalgamation of the Ar Rayn and Ad Dawadimi terranes. Geologic, structural, and metallogenic characteristics of the Ar Rayn terrane are analogous to the Andean continental margin of Chile, with opposite subduction polarity. The Ar Rayn terrane represents a continental margin arc that lay above a west-dipping subduction zone along a continental block represented by the Afif composite terrane. The concentration of epithermal, porphyry Cu and IOCG mineral systems, of central arc affiliation, along the AAF suggests that the AAF is not an ophiolitic suture zone, but originated as a major intra-arc fault that localized magmatism and mineralization. West-directed oblique subduction and ultimate collision with a land mass from the east (East Gondwana?) resulted in major transcurrent displacement along the AAF, bringing the eastern part of the arc terrane to its present exposed position, juxtaposed across the AAF against a back-arc basin assemblage represented by the Abt schist of the Ad Dawadimi terrane. Our findings indicate that arc formation and accretionary processes in the Arabian shield were still ongoing into the latest Neoproterozoic (Ediacaran), to about 620-600 Ma, and lead us to conclude that evolution of the Ar Rayn terrane (arc formation, accretion, syn- to postorogenic plutonism) defines a final stage of assembly of the Gondwana supercontinent along the northeastern margin of the East African orogen. ?? 2007 Elsevier B.V. All rights reserved.

Geochronology and petrogenesis of the western highlands alkali suite: Radiogenic isotopic evidence from Apollo 14

NASA Astrophysics Data System (ADS)

Snyder, Gregory A.; Taylor, Lawrence A.; Halliday, Alex N.

1993-03-01

Several rocks of alkalic affinity, from the western highlands of the Moon, have been analyzed for their Nd and Sr isotopic compositions. One sample yields a Sm-Nd mineral isochron of 4110 = 41 Ma. This age, in conjunction with U-Pb zircon ages on two other alkalic rocks from the Apollo 14 landing site suggests a distinct western highlands 'event' which was approximately 100 Ma in duration. Since the last dregs of the lunar magma ocean likely crystallized prior to 4.3 Ga, this alkalic 'event' may have included the re-melting of evolved plutons or the remobilization of urKREEP trapped liquid from upper mantle cumulates. Alkalic lithologies such as granites and felsites have been known from the Moon since the earliest days of the Apollo lunar sample returns. However, not until 1977 were alkali-rich rocks recognized from typical highlands suites such as ferroan anorthosites (FAN) and norites and Mg-suite rocks. In the intervening years, several other alkali suite samples have been discovered and characterized, mostly through labor-intesive breccia pull-apart studies of clasts and analyses of coarse-fine fractions of soils. We will speculate on the origins of this suite of lunar highlands rocks.

Geochronology and petrogenesis of the western highlands alkali suite: Radiogenic isotopic evidence from Apollo 14

NASA Technical Reports Server (NTRS)

Snyder, Gregory A.; Taylor, Lawrence A.; Halliday, Alex N.

1993-01-01

Several rocks of alkalic affinity, from the western highlands of the Moon, have been analyzed for their Nd and Sr isotopic compositions. One sample yields a Sm-Nd mineral isochron of 4110 = 41 Ma. This age, in conjunction with U-Pb zircon ages on two other alkalic rocks from the Apollo 14 landing site suggests a distinct western highlands 'event' which was approximately 100 Ma in duration. Since the last dregs of the lunar magma ocean likely crystallized prior to 4.3 Ga, this alkalic 'event' may have included the re-melting of evolved plutons or the remobilization of urKREEP trapped liquid from upper mantle cumulates. Alkalic lithologies such as granites and felsites have been known from the Moon since the earliest days of the Apollo lunar sample returns. However, not until 1977 were alkali-rich rocks recognized from typical highlands suites such as ferroan anorthosites (FAN) and norites and Mg-suite rocks. In the intervening years, several other alkali suite samples have been discovered and characterized, mostly through labor-intesive breccia pull-apart studies of clasts and analyses of coarse-fine fractions of soils. We will speculate on the origins of this suite of lunar highlands rocks.

Localized collision vs regional heating: the paradoxical aspects of 2720-2670 Ma geological evolution in the Kaapvaal craton, southern Africa.

NASA Astrophysics Data System (ADS)

Vezinet, Adrien; Nicoli, Gautier; Moyen, Jean-François; Laurent, Oscar; Stevens, Gary

2014-05-01

The Kaapvaal craton (KC) in South Africa, one of the oldest cratonic nuclei on Earth, is bounded to the North by the Limpopo belt, whose Southern Marginal Zone (SMZ) is regarded as a ca. 2.7Ga collisional orogeny. This is substantiated by structural data, such as the south verging thrust system that bounds the SMZ to the South (HRSZ); metamorphic data, demonstrating that metapelites from the SMZ underwent a very fast (˜40Ma) clockwise PTt loop at ca. 2.7Ga, with a peak of metamorphism dated at 2713±8Ma. The SMZ is therefore interpreted as representing a partially molten orogenic channel behaving like modern Himalaya. However, a review of geochronological data of Limpopo Belt, KC and Zimbabwean Craton shows that geological activity at around 2.7Ga was not restricted to the Limpopo belt, but was instead scattered throughout the KC. That is not in agreement with a Himalayan model but could be a response to a general thermal event, which is recorded by: 1) The massive flood basalts of the Vendersdorp Supergroup in central South Africa. This supergroup crops out from Johannesburg in the North to Prieska in the South and records the most widespread sequence of volcanic rocks of the KC. This massive basaltic extrusion occurs between 2714±8Ma (Klipriviersberg Group) and ˜2650Ma (Transvaal Supergroup lower limit). 2) A series of granitic plutons immediately south of the HRSZ, emplaced at 2670-2680Ma; all of them include a mantle-derived component whose composition is similar to the Ventersdorp basalts. A similar and synchronous pluton, the Mashishimale, emplaces South-East of the HRSZ. 3) Further afield, in the Eastern KC, slightly older plutons (I-type (high-Ca) granitoids) emplace in Swaziland between 2720 and 2700Ma. 4) Granulite-facies metamorphism in Swaziland at ca. 2.7Ga. 5) Finally, in the Zimbabwe Craton intermediate to felsic lavas erupted at the same time as well as gneisses and granitoids from northern Botswawa However, interactions of Zimbabwe craton with Limpopo Complex and KC are still vagueness. Collectively, the data demonstrate that during the evolution of the SMZ, the whole of the KC crust underwent significant heating, permitting partial melting and extraction of granitic melts even far away from the main collision. The crustal heating is linked to mantle activity, as mafic, mantle-derived melts also form and emplace at the same time. The coexistence of both an Himalayan-style belt, and the far-field heating (apparently not related to any plate boundary), can be interpreted in different ways: (i) Coexistence of a mantle hotspot and a collision orogeny in close vicinity; (ii) Small-scale convections in the asthenospheric mantle, driven by differences in thickness of the lithosphere and resulting in heating of the foreland; (iii) Alternatively, the combination of apparently distinct events can actually reflect specificities of neo-Archaean orogenic style and could be the response of a hot, possibly molten or near-melting crust to regional convergence, permitting generation or extraction of melt over a zone much wider than in typical modern orogens.

How fast is the denudation of the Taiwan Mountains? (Invited)

NASA Astrophysics Data System (ADS)

Siame, L. L.; Derrieux, F.; KANG, C.; Bourles, D. L.; Braucher, R.; Léanni, L.; Chen, R.; Lee, J.; Chu, H.; Chang, C.; Byrne, T. B.

2013-12-01

Orogenic settings are particularly well suited to study and quantify the coupling relations between tectonics, topography, climate and erosion since they record tectonic evolution along convergent margins and the connection between deep and surface processes. However, the interaction of deep and shallow processes is still poorly understood and the role they play in the exhumation of rocks, the structural and kinematic evolution of orogenic wedges, and the relation between tectonics and climate-dependent surface processes are still debated. Therefore, quantification of denudation rates in a wide range of climatic and tectonic settings, as well as at various time and space scales, is a critical step in calibrating and validating landscape evolution models. In this study, we focus on the mountains of the arc-continent collision in Taiwan, which serve as one of the best examples in the world to understand and study mountain building processes. We investigate the pattern and magnitude of denudation rates at the scale of the orogenic system, deriving denudation rates from in situ-produced cosmogenic nuclide 10Be concentrations measured in (1) river-borne quartz minerals sampled at major watersheds outlets, and (2) bedrock outcrops along ridge crests and at summits located along the major drainage divide of the belt. We determined a denudation pattern showing a clear discrepancy between the western (1.7×0.2 mm/yr) and eastern (4.1×0.5 mm/yr) sides of the range. Conversely, bedrock denudation determined along ridge crests, summits and flat surfaces preserved at high elevations are characterized by significantly lower denudation rates on the order of 0.24×0.03 mm/yr. Altogether, the cosmogenic-derived denudation pattern at the orogen-scale reflects fundamental mountain building processes from frontal accretion in the Western Foothills to basal accretion and fast exhumation in the Central Range. Applied to the whole orogen, such field-based approach thus provides important input data to validate and calibrate the parameters to be supplied to landscape evolution models. Moreover, the comparison between cosmogenic bedrock-derived and basin-derived denudation rates allows discussing how the topographic relief of Taiwan has evolved through the last thousands of years, and thus documenting whether or not the Taiwan Mountains are in a topographic steady state.

Timing of tectonic evolution of the East Kunlun Orogen, Northern Tibet Plateau

NASA Astrophysics Data System (ADS)

Dong, Yunpeng

2017-04-01

The East Kunlun Orogen, located at the northern Tibet Plateau, represents the western segment of the Central China Orogenic Belt which was formed by amalgamation of the North China blocks and South China blocks. It is a key to understanding the formation of Eastern Asian continent as well as the evolution of the Pangea supercontinent. Based on detailed geological mapping, geochemical and geochronological investigations, the orogen is divided into three main tectonic belts, from north to south, including the Northern Qimantagh, Central Kunlun and Southern Kunlun Belts by the Qimantagh suture, Central Kunlun suture and South Kunlun fault. The Qimantagh suture is marked by the Early Paleozoic ophiolites outcropped in the Yangziquan, Wutumeiren, and Tatuo areas, which consist mainly of peridotites, gabbros, diabases and basalts. Besides, the ophiolite in the Wutumeiren is characterized by occurring anorthosite while the ophiolite in the Tatuo occurring chert. The basalts and diabases from both Yaziquan and Tatuo areas display depletion of Nb, Ta, P and Ti, and enrichment of LILE, suggesting a subduction related tectonic setting. LA-ICP-MS zircon U-Pb age of 421 Ma for the diabase represents the formation age of the Yaziquan ophiolite, while the U-Pb ages of 490 Ma and 505 Ma for gabbro and anorthosite, respectively, constrain the formation age of the Tatuo ophiolite. The basaltic rocks in the Wutumeiren area display flat distribution of HFSEs (such as Nb, Ta, K, La, Ce, Pr, Nd, Zr, Sm, Eu, Ti, Dy, Y, Yb and Lu) and slightly enrichment in LREEs, while the peridotites showing depletion in MREEs. The LA-ICP-MS zircon U-Pb age of 431 Ma for the gabbro represents the formation age of the Wutumeiren ophiolite. Together with regional geology, we suggest herewith a back-arc basin tectonic setting during ca. 505-421 Ma at least for the Qimantagh suture. The Central Kunlun suture is represented by the ophiolite in the Wutuo area, which is characterized by depletion of Nb, Ta, P and Ti, and enrichment of LILEs, LREEs, K, Pb, Sr and Nd, accounting for a subduction relation setting. The gabbro yields a LA-ICP-MS zircon U-Pb age of 243 Ma, representing the formation age of the ophiolite. Taking into account of evidence from the Early Paleozoic ophiolites in the Buqinshan ( Bian Qiantao et al., 2001, 2007; Li Zuochen et al., 2013; Li Ruibao et al., 2014; Liu Zhanqing et al., 2011) and the Derni areas (Chen Liang et al., 2001, 2003), the Central Kunlun ocean might be existed from Early Paleozoic to Middle Triassic time. The Northern Qimantagh tectonic belt, to the north of the Qimantagh suture, exposes a large volume of Early Paleozoic granitic plutons and volcanic rocks. Geochemistry of the granites suggests an arc setting. LA-ICP-MS zircon U-Pb ages ranging from ca. 440 to 402 Ma constrain the time of the subduction and arc setting. The Central Kunlun tectonic belt is characterized by occurring of Paleo-Proterozoic basement which was intruded by large amounts of Triassic granitoids. The basement represented by the Jinshuikou Group including gneisses, amphibolites and marbles, yields a protolith formation age of 2.2 Ga which was overprinted by Neoproterozoic tectono-thermal event. The plutonic intrusions display LA-ICP-MS zircon ages mainly of 260-200 Ma with minor ages of 470-400 Ma, revealing a long-lived subduction from Early Paleozoic to Late Triassic. Taken into together all above evidence, trench-arc-back arc basin tectonics were suggested here accounting for the tectonic evolution of the East Kunlun Orogeny during Early Paleozoic to Triassic time.

Crustal architecture of the oblique-slip conjugate margins of George V Land and southeast Australia

USGS Publications Warehouse

Stagg, H.M.J.; Reading, A.M.

2007-01-01

A conceptual, lithospheric-scale cross-section of the conjugate, oblique-slip margins of George V Land, East Antarctica, and southeast Australia (Otway Basin) has been constructed based on the integration of seismic and sample data. This cross-section is characterised by asymmetry in width and thickness, and depth-dependent crustal extension at breakup in the latest Maastrichtian. The broad Antarctic margin (~360 km apparent rift width) developed on thick crust (~42 km) of the Antarctic craton, whereas the narrow Otway margin (~220 km) developed on the thinner crust (~31 km) of the Ross–Delamerian Orogen. The shallow basement (velocities ~5.5 km.s-1) and the deep continental crust (velocities >6.4 km.s-1) appear to be largely absent across the central rift, while the mid-crustal, probably granitic layer (velocities ~6 km.s-1) is preserved. Comparison with published numerical models suggests that the shallow basement and deep crust may have been removed by simple shear, whereas the mid-crust has been ductilely deformed.

Voluminous low-T granite: fluid present partial melting of the crust?

NASA Astrophysics Data System (ADS)

Hand, Martin; Barovich, Karin; Morrissey, Laura; Bockmann, Kiara; Kelsey, David; Williams, Megan

2017-04-01

Voluminous low-T granite: fluid present partial melting of the crust? Martin Hand(1), Karin Barovich(1), Laura Morrissey(1), Vicki Lau(1), Kiara Bockmann(1), David Kelsey(1), Megan Williams(1) (1) Department of Earth Sciences, University of Adelaide, Adelaide, Australia Two general schools of thought exist for the formation of granites from predominantly crustal sources. One is that large-scale anatexis occurs via fluid-absent partial melting. This essentially thermal argument is based on the reasonable premise that the lower crust is typically fluid depleted, and experimental evidence which indicates that fluid-absent partial melting can produce significant volumes of melt, creating compositionally depleted residua that many believe are recorded by granulite facies terranes. The other school of thought is that large-scale anatexis can occur via fluid-fluxed melting. This essentially compositional-based contention is also supported by experimental evidence which shows that fluid-fluxed melting is efficient, including at temperatures not much above the solidus. However, generating significant volumes of melt at low temperatures requires a large reservoir of fluid. If fluid-fluxed melting is a realistic model, the resultant granites should be comparatively low temperature compared to those derived from predominantly fluid-absent partial melting. Using a voluminous suite of aluminous granites in the Aileron Province in the North Australian Craton together with metasedimentary granulites as models for source behaviour, we evaluate fluid-absent verse fluid-present regimes for generating large volumes of crustally-derived melt. The central Aileron Province granites occupy 32,500km2, and in places are in excess of 8 km thick. They are characterised by abundant zircon inheritance that can be matched with metasedimentary successions in the region, suggesting they were derived in large part from melting of crust similar to that presently exposed. A notable feature of many of the granites is their enriched Th concentrations compared to typical Aileron Province sub solidus metapelitic successions. However, based on continuous transects within metasedimentary rocks from a number of different regions that record transitions from sub-solidus assemblages to supra-solidus rocks petrologically characterised by typical fluid-absent peritectic assemblages (central Aileron Province, Broken Hill Zone, Ivrea-Verbano Zone), fluid-absent partial melting does not deplete Th concentrations in the residuum with respect to their sub-solidus protoliths. If these compositional transects are used as a guide to the general behaviour of Th during fluid-absent partial melting, the voluminous Th-enriched granites in the Aileron Province are unlikely to be the products of fluid-absent partial melting. This contention is supported by phase equilibria modelling of sub-solidus metasedimentary units whose detrital zircons match in age the granite-hosted xenocrysts, which indicate that temperatures in excess of 840°C are required to generate significant volumes (ie ≥ 30%) of melt under fluid-absent conditions. However, zircon saturation temperatures for the granites have a weighted mean of 776 ± 4 °C (n = 220). Because the granites contain abundant inheritance, this is an upper-T limit that also suggests fluid-absent partial melting was not the primary mechanism for granite formation. We suggest that voluminous granite formation in the Aileron Province occurred in a fluid-rich regime that was particularly effective at destabilising monazite and liberating Th into melt. Because of the propensity of monazite to destabilise in the presence of fluid, we suggest that high-grade metasedimentary terrains that are notably depleted in Th may be residuum associated with fluid-fluxed melt loss.

Coeval emplacement and orogen-parallel transport of gold in oblique convergent orogens

NASA Astrophysics Data System (ADS)

Upton, Phaedra; Craw, Dave

2016-12-01

Varying amounts of gold mineralisation is occurring in all young and active collisional mountain belts. Concurrently, these syn-orogenic hydrothermal deposits are being eroded and transported to form placer deposits. Local extension occurs in convergent orogens, especially oblique orogens, and facilitates emplacement of syn-orogenic gold-bearing deposits with or without associated magmatism. Numerical modelling has shown that extension results from directional variations in movement rates along the rock transport trajectory during convergence, and is most pronounced for highly oblique convergence with strong crustal rheology. On-going uplift during orogenesis exposes gold deposits to erosion, transport, and localised placer concentration. Drainage patterns in variably oblique convergent orogenic belts typically have an orogen-parallel or sub-parallel component; the details of which varies with convergence obliquity and the vagaries of underlying geological controls. This leads to lateral transport of eroded syn-orogenic gold on a range of scales, up to > 100 km. The presence of inherited crustal blocks with contrasting rheology in oblique orogenic collision zones can cause perturbations in drainage patterns, but numerical modelling suggests that orogen-parallel drainage is still a persistent and robust feature. The presence of an inherited block of weak crust enhances the orogen-parallel drainage by imposition of localised subsidence zones elongated along a plate boundary. Evolution and reorientation of orogen-parallel drainage can sever links between gold placer deposits and their syn-orogenic sources. Many of these modelled features of syn-orogenic gold emplacement and varying amounts of orogen-parallel detrital gold transport can be recognised in the Miocene to Recent New Zealand oblique convergent orogen. These processes contribute little gold to major placer goldfields, which require more long-term recycling and placer gold concentration. Most eroded syn-orogenic gold becomes diluted by abundant lithic debris in rivers and sedimentary basins except where localised concentration occurs, especially on beaches.

Felsic plutonism in the Al Amar—Idsas area, Kingdom of Saudi Arabia

NASA Astrophysics Data System (ADS)

Le Bel, L.; Laval, M.

A tonalite—trondhjemite suite, calc-alkalic plutons and alkali-feldspar granites dated 670 and 580 Ma, intrude thick volcano-sedimentary rocks of the Al Amar group E of the Al Amar fault and the Abt schist W of the fault. The tonalite—trondhjemite suite (group I) is characterized by low Rb (50 ppm) and Sr (100-400 ppm) and by weakly fractionated rare-earth patterns (La/Yb Nca 2-3) with a weak negative Eu anomaly. Calc-alkalic plutons (group II) are richer in Rb (50-150 ppm), contain variable Sr (50-1000 ppm), and have strongly fractionated rare-earth patterns (La/Yb Nca 6-22) with no Eu anomaly. Alkali-feldspar granite (group III) is characterized by high Rb (150-200 ppm) and shows fractionated rare-earth patterns (La/Yb Nca 6-18) with a well-developed Eu anomaly. Group III includes 'specialized granites' with high Rb (300-400 ppm) and Sn (28-66 ppm), and rare-earth patterns showing a distinctive 'sea gull' profile with a very strong Eu anomaly (Eu*/Eu = 20). Oxygen isotope geochemistry suggests that group I rocks (¯x δ18O ca 7.0) were mantle-derived, and that group II and III rocks intruding the Al Amar group ( δ18O ca 7.9 and 8.8 respectively) were derived by remelting of group I, whereas those intruding Abt schist ( δ18O ca 8.7 and 10.8 respectively) were partially derived by anatexis of the Afif block. Magmatogenesis reflects an island-arc development. Rocks of group I represent the initial subduction phase. Syn- to late-tectonic plutons of group II intruded the arc east of the Al Amar fault and the accretionary prism (Abt schist) to the west, which was in collision with the older Afif block. Post-tectonic group III rocks were emplaced in an already cratonized area.

Temporal evolution of the giant Salobo IOCG deposit, Carajás Province (Brazil): constraints from paragenesis of hydrothermal alteration and U-Pb geochronology

NASA Astrophysics Data System (ADS)

deMelo, Gustavo H. C.; Monteiro, Lena V. S.; Xavier, Roberto P.; Moreto, Carolina P. N.; Santiago, Erika S. B.; Dufrane, S. Andrew; Aires, Benevides; Santos, Antonio F. F.

2017-06-01

The giant Salobo copper-gold deposit is located in the Carajás Province, Amazon Craton. Detailed drill core description, petrographical studies, and U-Pb SHRIMP IIe and LA-ICP-MS geochronology unravel its evolution regarding the host rocks, hydrothermal alteration and mineralization. Within the Cinzento Shear Zone, the deposit is hosted by orthogneisses of the Mesoarchean Xingu Complex (2950 ± 25 and 2857 ± 6.7 Ma) and of the Neoarchean Igarapé Gelado suite (2763 ± 4.4 Ma), which are crosscut by the Old Salobo granite. Remnants of the Igarapé Salobo metavolcanic-sedimentary sequence are represented by a quartz mylonite with detrital zircon populations (ca. 3.1-3.0, 2.95, 2.86, and 2.74 Ga). High-temperature calcic-sodic hydrothermal alteration (hastingsite-actinolite) was followed by silicification, iron-enrichment (almandine-grunerite-magnetite), tourmaline formation, potassic alteration with biotite, copper-gold ore formation, and later Fe-rich hydrated silicate alteration. Myrmekitic bornite-chalcocite and magnetite comprise the bulk of copper-gold ore. All these alteration assemblages have been overprinted by post-ore hematite-bearing potassic and propylitic alteration, which is also recognized in the Old Salobo granite. In the central zone of the deposit the mylonitized Igarapé Gelado suite rocks yield an age of 2701 ± 30 Ma. Zircon ages of 2547 ± 5.3 and 2535 ± 8.4 Ma were obtained for the Old Salobo granite and for the high-grade copper ore, respectively. A U-Pb LA-ICP-MS monazite age (2452 ± 14 Ma) from the copper-gold ore indicates hydrothermal activity and overprinting in the Siderian. Therefore, a protracted tectono-thermal event due to the reactivation of the Cinzento Shear Zone is proposed for the evolution of the Salobo deposit.

Petrogenesis and tectonic implications of the Early Carboniferous to the Late Permian Barleik plutons in the West Junggar (NW China)

NASA Astrophysics Data System (ADS)

Liu, Bo; Han, Bao-Fu; Ren, Rong; Chen, Jia-Fu; Wang, Zeng-Zhen; Zheng, Bo

2017-02-01

The Paleozoic accretionary orogenesis and continental crustal growth in Central Asia are thought to have close relationship with the evolution of the Paleo-Asian Ocean (PAO). The well-exposed plutons in the northern Barleik Mountains of the West Junggar region, NW China, may provide essential insights into the evolution of the Junggar Ocean, a branch of the PAO, and mechanism of continental crustal growth. Our work on the Barleik plutons indicates an early suite of 324-320 Ma diorite and a late suite of 314-259 Ma quartz syenite and granitic porphyry. All the plutons are characterized by high-K calc-alkaline to shoshonitic signatures, varying depletion in Nb, Ta, Sr, P, Eu, and Ti, low initial 87Sr/86Sr ratios (0.70241-0.70585), strongly positive εNd(t) values (+ 5.7-+7.7), and young one-stage Nd model ages (390-761 Ma), suggesting that they resulted from different batches of magma that were produced by fractional crystallization of a metasomatized mantle source with minor crustal contamination. The diorite is coeval with the youngest arc magmatic rocks, indicating a subduction-related origin. By contrast, the quartz syenite and granitic porphyry are geochemically similar to A2-type granites, with high Zr, Ga, and FeOT/[FeOT + MgO], and are coeval with the widespread plutons in the West Junggar. This, together with the occurrence of Late Carboniferous fluvial deposits and the lack of < 320 Ma ophiolitic and subduction-related metamorphic lithologies, definitively indicates a post-collisional setting after the closure of the Junggar Ocean. Slab breakoff accompanied by asthenospheric upwelling and basaltic underplating is a possible geodynamic process that is responsible for the post-collisional magmatism and vertical crustal growth in the region. Thus a tectonic switch from subduction to post-collision started at the end of the Early Carboniferous ( 320 Ma), probably as a result of the final closure of the Junggar Ocean.

A new interpretation of the structure of the Sept Iles Intrusive suite, Canada

NASA Astrophysics Data System (ADS)

Higgins, Michael D.

2005-08-01

The layered mafic intrusion at Sept Iles, Canada, is one of the largest intrusions in the world. A new interpretation of its structure is proposed, based on a review of its geology and a comparison with the Skaergaard intrusion, Greenland. Several different magmatic components are recognized; hence the name Sept Iles Intrusive suite (SIIS) is proposed. Emplacement of the suite may have been preceded by eruption of flood basalts. The first magmas of the suite rose in the crust to accumulate beneath the density filter afforded by the basalts. The largest component is the Sept Iles Mafic intrusion (SIMI). The Lower series of the SIMI is dominated by leucotroctolites and leucogabbros. Above it lie the Layered series, which is largely comprised of gabbro and troctolite. Both these units are unchanged from earlier interpretations. The anorthosites (s.l.), gabbros and monzogabbros, formerly called the Transitional series, are now considered to be the Upper Border series, developed by floatation of plagioclase. Common autoliths in the Layered series are parts of the hydrothermally altered Upper Border series from towards the interior of the intrusion, which have foundered and settled through the magma. The contamination of the magma that accompanied this event oxidised iron in the magma and led to the precipitation of magnetite around the periphery of the intrusion. The subsequent depletion of Fe 3+ and/or increase in SiO 2, CaO and P 2O 5 may have induced apatite saturation and accumulation to form two layers rich in apatite, near the base and at top of the Layered series. Granitic magma was developed by fractional crystallisation and was emplaced along the roof of the chamber, where it acquired large quantities of xenoliths. These were probably derived from the flood basalts, their evolved members and fragments of mafic dykes chilled by the granitic magma. Accumulations of monzonite pillows in this unit testify to another magmatic event and a floor to the granitic magma chamber, indicating lateral transport of magma. Chemically distinct syenites in the upper part of the intrusion are part of the Point du Criade intrusion, a large, late composite sill. Diabase and leucogabbro components show a close link with the SIMI and all the acidic magmas may have originally formed by differentiation of the main magma in cupolas towards the centre of the intrusion. A series of late gabbro intrusions that cut the SIMI may represent a rejuvenation of magmatism. The Border zone is a mass of fine-grained rocks that occurs along the border of the SIMI: it may be another magmatic component, or just the lateral border series of the SIMI.

Identification and characterization of hydrothermally altered zones in granite by combining synthetic clay content logs with magnetic mineralogical investigations of drilled rock cuttings

NASA Astrophysics Data System (ADS)

Meller, Carola; Kontny, Agnes; Kohl, Thomas

2014-10-01

Clay minerals as products of hydrothermal alteration significantly influence the hydraulic and mechanical properties of crystalline rock. Therefore, the localization and characterization of alteration zones by downhole measurements is a great challenge for the development of geothermal reservoirs. The magnetite bearing granite of the geothermal site in Soultz-sous-Forêts (France) experienced hydrothermal alteration during several tectonic events and clay mineral formation is especially observed in alteration halos around fracture zones. During the formation of clay minerals, magnetite was oxidized into hematite, which significantly reduces the magnetic susceptibility of the granite from ferrimagnetic to mostly paramagnetic values. The aim of this study was to find out if there exists a correlation between synthetic clay content logs (SCCLs) and measurements of magnetic susceptibility on cuttings in the granite in order to characterize their alteration mineralogy. Such a correlation has been proven for core samples of the EPS1 reference well. SCCLs were created from gamma ray and fracture density logs using a neural network. These logs can localize altered fracture zones in the GPK1-4 wells, where no core material is available. Mass susceptibility from 261 cutting samples of the wells GPK1-GPK4 was compared with the neural network derived synthetic logs. We applied a combination of temperature dependent magnetic susceptibility measurements with optical and electron microscopy, and energy dispersive X-ray spectroscopy to discriminate different stages of alteration. We found, that also in the granite cuttings an increasing alteration grade is characterized by an advancing oxidation of magnetite into hematite and a reduction of magnetic susceptibility. A challenge to face for the interpretation of magnetic susceptibility data from cuttings material is that extreme alteration grades can also display increased susceptibilities due to the formation of secondary magnetite. Low magnetic susceptibility can also be attributed to primary low magnetite content, if the granite facies changes. In order to interpret magnetic susceptibility from cuttings, contaminations with iron from wear debris of the drilling tools must be eliminated. Provided that the magnetic mineralogy of the granite is known in detail, this method in combination with petrographic investigations is suited to indicate and characterize hydrothermal alteration and the appearance of clay.

The pre-Mesozoic tectonic unit division of the Xing-Meng orogenic belt (XMOB)

NASA Astrophysics Data System (ADS)

Xu, Bei; Zhao, Pan

2014-05-01

According to the viewpoint that the paleo-Asian ocean closed by the end of early Paleozoic and extended during the late Paleozoic, a pre-Mesozoic tectonic unit division has been suggested. Five blocks and four sutures have been recognized in the pre-Devonia stage, the five blocks are called Erguna (EB), Xing'an (XB), Airgin Sum-Xilinhot (AXB), Songliao-Hunshandak (SHB) and Jiamusi (JB) blocks and four sutures, Xinlin-Xiguitu (XXS), Airgin Sum-Xilinhot-Heihe (AXHS), Ondor Sum-Jizhong-Yanji (OJYS) and Mudanjiang (MS) sutures. The EB contains the Precambrian base with the ages of 720-850Ma and ɛHf(T)=+2.5to +8.1. The XB is characterized by the Paleoproterozoic granitic gneiss with ɛHf(T)=-3.9 to -8.9. Several ages from 1150 to 1500 Ma bave been acquired in the AXB, proving presence of old block that links with Hutag Uul block in Mongolia to the west. The Paleoproterozoic (1.8-1.9Ga) and Neoproterozoic (750-850Ma) ages have been reported from southern and eastern parts of the SHB, respectively. As a small block in east margin of the XMOB, the JB outcrops magmatite and granitic gneiss bases with ages of 800-1000Ma. The XXS is marked by blueschists with zircon ages of 490-500Ma in Toudaoqiao village, ophiolites in Xiguitu County and granite with ages of about 500Ma along the northern segment of XXS. The AXHS is characterized by the early Paleozoic arc magmatic rocks with ages from 430Ma to 490Ma, mélange and the late Devonia molass basins, which indicates a northward subduction of the SHB beneath the AXB during the early-middle Paleozoic. The OJYS is composed of the early Paleozoic volcanic rocks, diorites and granites with ages of 425-475Ma, blueschists, ophiolitic mélange, the late Silurian flysch and Early-Middle Devonian molasses in western segment, granites (420-450Ma) in middle segment, and plagiogranites (443Ma) and the late Silurian molasses in eastern segment. This suture was caused by a southward subduction of the SHB beneath the North China block. The MS is between the SHB and JB, marked by the three phase granites of 485, 450 and 425Ma in the SHB. Tectonic units of the middle Devonian-Carboniferous tectonic stage include the middle-late Devonian continental basin, Carboniferious continental and epeiric sea basin, intrusive and irruptive igneous rock belt with ages from 300Ma to 330Ma containing granites, diorites, gabbros and biomodal volcanic rocks, and early Carboniferious ophiolites of 330-350Ma in Hegenshan and Erenhot. The Permian tectonic units can be divided into continental rift belt, ophiolite belt, alkaline rock belt and "red sea"-like ocean basin, which indicates an continuous extension environment during the Permian. The continental rift belt is composed of thick continental sedimentary rocks containing plant fossils, biomodal volcanic rocks (270-290Ma). The alkaline rocks can be divided into north and south belts by their distribution. The Solonker ophiolite is a thrust sheet that is inserted in a thrust stack containing the Upper Carboniferious epeiric sea clastic rocks and carbornates. The "red sea"-like ocean basin is characterized by basalt sequences with ages of 246-260Ma, which shows an affinity to E-MORB and a tendency towards OIB.

Carboniferous continental arc in the Hegenshan accretionary belt: Constrains from plutonic complex in central Inner Mongolia

NASA Astrophysics Data System (ADS)

Wei, Ruihua; Gao, Yongfeng; Xu, Shengchuan; Santosh, M.; Xin, Houtian; Zhang, Zhenmin; Li, Weilong; Liu, Yafang

2018-05-01

The architecture and tectonic evolution of the Hegenshan accretionary belt in the Central Asian Orogenic Belt (CAOB) remains debated. Here we present an integrated study of zircon U-Pb isotopic ages, whole rock major-trace elements, and Sr-Nd-Pb isotopic data from the Hegenshan volcanic-plutonic belt in central Inner Mongolia. Field observations and zircon U-Pb ages allow us to divide the intrusive complex into an early phase at 329-306 Ma and a late phase at 304 to 299 Ma. The intrusive bodies belong to two magma series: calc-alkaline rocks with I-type affinity and A-type granites. The early intrusions are composed of granodiorite, monzogranite and porphyritic granite, and the late calc-alkaline intrusions include gabbro though diorite to granodiorite. The calc-alkaline intrusive rocks exhibit a well-defined compositional trend from gabbro to granite, reflecting continuous fractional crystallization. These rocks show obvious enrichment in LILEs and LREEs and relative depletion of HFSEs, typical of subduction-related magma. They also exhibit isotopic characteristics of mantle-derived magmas such as low initial 87Sr/86Sr (0.7029-0.7053), positive ɛNd(t) values (0.06-4.76) and low radiogenic Pb isotopic compositions ((206Pb/204Pb)I = 17.907-19.198, (207Pb/204Pb)I = 15.474-15.555, (208Pb/204Pb)I = 37.408-38.893). The marked consistency in geochemical and isotopic compositions between the intrusive rocks and the coeval Baoligaomiao volcanic rocks define a Carboniferous continental arc. Together with available regional data, we infer that this east-west trending continental arc was generated by northward subduction of the Hegenshan ocean during Carboniferous. The late alkali-feldspar granites and the high-Si rhyolites of the Baoligaomiao volcanic succession show similar geochemical compositions with high SiO2 and variable total alkali contents, and low TiO2, MgO and CaO. These rocks are characterized by unusually low Sr and Ba, and high abundances of Zr, Th, Nb, HREEs and Y, comparable to the features of typical A2-type granites including their high ratios of FeOT/MgO, Ga/Al and Y/Nb. Our study suggests that the A-type granite was derived from a distinct magma source rather than through fractional crystallization of the coeval calc-alkaline magmas. Their Nd-Pb isotopic compositions are similar to those of calc-alkaline arc rocks and are compatible with partial melting of pre-existing juvenile basaltic crust in the continental arc. Notably, the widespread eruptions of A2-type rhyolitic magmas (305.3 Ma-303.4 Ma) following a short period of magmatic quiescence was temporally and spatially associated with bimodal magmatism with mantle-derived gabbro-diorites and A-type granites (304.3 Ma-299.03 Ma) in the pre-existing arc volcanic-plutonic belt (329 Ma-306 Ma). Such a marked change in the magma affinity likely indicates subducted slab break-off resulting in a change of the regional stress field to an extensional setting within the Carboniferous continental arc that runs E-W for few thousands of kilometers. Thus, the onset of the late magmatism (305-299 Ma) likely represents the maximum age for the cessation of the northward subduction in the Hegenshan ophiolite-arc-accretion belt.

The Ackley City Batholith, southeastern Newfoundland: Evidence for crystal versus liquid-state fractionation

NASA Astrophysics Data System (ADS)

Whalen, J. B.

1983-08-01

The 345 ± 10 Ma old composite Ackley City Batholith of southeastern Newfoundland, consists largely of very felsic K-feldspar megacrystic granite and alaskite. Spatially related to the southeast contact of the alaskite are younger aplites and pegmatite, intrusive phases which are interpreted to be pan of a tilted, high level roof zone complex to the batholith. The compositions of the alaskite and roof zone complex define major and trace element gradients similar to those in voluminous high-silica eruptive suites; i.e., the alaskite is more chemically evolved (higher in Rb, lower in Ca, Fe, Mn, Ti, P, Sr, Ba and LREE) toward the roof. Apparently these chemical gradients in the batholith are restricted to the top 2 to 3 kms of the former magma chamber. Fractional crystallization is a plausible process for generating the chemical dispersion in the granites, although very high feldspar partition coefficients for Ba, Sr and Eu are required to generate the observed chemical gradients by a reasonable degree of fractional crystallization. Restriction of crystal fractionation to near the roof of the batholith may reflect a decreased viscosity which would facilitate crystal-liquid separation by processes such as filter pressing, flow differentiation or convective fractionation. The chemical gradients in these granites closely resemble those attributed in high-silica volcanics to the process of thermogravitational diffusion (TGD). Compositional gradients in the upper portion of a magma chamber are consistent with the TGD model. This model, although still poorly understood, is, like fractional crystallization, a plausible mechanism to generate the chemical features of the Ackley City granites.

Spatially controlled Fe and Si isotope variations: an alternative view on the formation of the Torres del Paine pluton

NASA Astrophysics Data System (ADS)

Gajos, Norbert A.; Lundstrom, Craig C.; Taylor, Alexander H.

2016-11-01

We present new Fe and Si isotope ratio data for the Torres del Paine igneous complex in southern Chile. The multi-composition pluton consists of an approximately 1 km vertical exposure of homogenous granite overlying a contemporaneous 250-m-thick mafic gabbro suite. This first-of-its-kind spatially dependent Fe and Si isotope investigation of a convergent margin-related pluton aims to understand the nature of granite and silicic igneous rock formation. Results collected by MC-ICP-MS show a trend of increasing δ56Fe and δ30Si with increasing silica content as well as a systematic increase in δ56Fe away from the mafic base of the pluton. The marginal Torres del Paine granites have heavier Fe isotope signatures (δ56Fe = +0.25 ± 0.02 2se) compared to granites found in the interior pluton (δ56Fe = +0.17 ± 0.02 2se). Cerro Toro country rock values are isotopically light in both Fe and Si isotopic systems (δ56Fe = +0.05 ± 0.02 ‰; δ30Si = -0.38 ± 0.07 ‰). The variations in the Fe and Si isotopic data cannot be accounted for by local assimilation of the wall rocks, in situ fractional crystallization, late-stage fluid exsolution or some combination of these processes. Instead, we conclude that thermal diffusion or source magma variation is the most likely process producing Fe isotope ratio variations in the Torres del Paine pluton.

Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England

USGS Publications Warehouse

Moench, R.H.; Aleinikoff, J.N.

2002-01-01

The Ammonoosuc Volcanics, Partridge Formation, and the Oliverian and Highlandcroft Plutonic Suites of the Bronson Hill anticlinorium (BHA) in axial New England are widely accepted as a single Middle to Late Ordovician magmatic arc that was active during closure of Iapetus. Mapping and U-Pb dating indicate, however, that the BHA contains two volcano-sedimentary-intrusive sequences of probable opposite subduction polarity, here termed the Ammonoosuc and Quimby sequences. The Ammonoosuc sequence is defined by the Middle Ordovician Ammonoosuc Volcanics near Littleton, N.H., the type area, northeast to Milan, N.H., and Oquossoc, Me.; it also includes black slate of the Partidge Formation (C. bicornis zone graptolites, ???457 Ma). Related metamorphosed intrusive are the tonalitic Joslin Turn pluton (469 ?? 2 Ma), the Cambridge Black granitic pluton (468 ?? 3 Ma), and gabbro, tonalite (467 ?? 4 Ma), and sheeted diabase of the Chickwolnepy instructions. These intrusives cut lowermost Ammonoosuc (therefore>469 Ma). Probable uppermost Ammonoosuc is dated at 465 ?? 6 and 461 ?? 8 Ma. Successively below the Ammonoosuc are the Dead River and Hurricane Mountain Formations (flysch and melange), and the Jim Pond Formation (484 ?? 5 Ma) and Boil Mountain Complex (both ophiolite), which are structurally underlain by the Neoproterozoic(?) Chain Lakes massif. The Quimby sequence is defined by the Lower Silurian(?) to Upper Ordovician Quimby Formation, composed of bimodal volcanics (443 ?? 4 Ma) and sulfidic shale and graywacke that lie conformably to unconformably above the Ammonoosuc Volcanics and Partridge Formation. Also in the Quimby sequence are several granitic to sparsely gabbroic plutons of the Highlandcroft (441-452 Ma) and Oliverian (435-456 Ma) Plutonic Suites, which intrude the Dead River, Ammonoosuc and Partridge, but not the Quimby Formation. Based on faunal, paleolatitude, and isotropic data, the Ammonoosuc sequence and its correlative and underlying sequences formed off the southern Laurentian margin, but northwest of the principal Iapetan suture, or Red Indian line (RIL). The Boil Mountain-Jim Pond-Hurricane Mountain sequence was ramped northwestward over the Chain Lakes massif at ???475 Ma, on the basal Boil Mountain surface. This obduction probably occurred slightly before obduction on the Baie Verte-Brompton surface (BBL), farther NW, over the Laurentian margin, and was followed by Dead River flysch sedimentation, which ended with the abrupt onset of Ammonoosuc-sequence arc magmatism at ???470 Ma. Ammonoosuc eruptions probably ended at ???460 Ma, when Iapetus closed along the Red Indian line. During a following magmatic hiatus of ???3-5 m.y., now represented by portions of the Partridge Formation that overlie the Ammonoosuc Volcanics, subduction polarity reversed, and subduction resumed below the northwest-dipping Brunswick subduction complex (BSC) of New Brunswick, Canada. Quimby-sequence magmatism (???456-435 Ma) on the the newly accreted Laurentian margin occurred above the BSC, whose footwall is now buried to the southeast by mainly Silurian clastic sediments of the Merrimack-Fredericton trough, deposited in the "Fredericton Sea". In Silurian to Early Devonian time, the NW-dipping BSC footwall was paired with a SE-dipping subduction zone that produced arc magmas of the Coastal Volcanic belt, built on the composite Avalon and adjacent peri-Avalonian terranes. Orogen-normal extension produced by rapid rollback of both subduction zones narrowed the Fredericton Sea, produced the Central Maine and Connecticut Valley-Gaspe?? basins, and culminated in the Acadian orogeny when the sea completely closed in Early Devonian time. Published by Elsevier Science Ltd.

Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England

USGS Publications Warehouse

Moench, R.H.; Aleinikoff, J.N.

2003-01-01

The Ammonoosuc Volcanics, Partridge Formation, and the Oliverian and Highlandcroft Plutonic Suites of the Bronson Hill anticlinorium (BHA) in axial New England are widely accepted as a single Middle to Late Ordovician magmatic arc that was active during closure of Iapetus. Mapping and U-Pb dating indicate, however, that the BHA contains two volcano-sedimentary-intrusive sequences of probable opposite subduction polarity, here termed the Ammonoosuc and Quimby sequences. The Ammonoosuc sequence is defined by the Middle Ordovician Ammonoosuc Volcanics near Littleton, NH, the type area, northeast to Milan, NH, and Oquossoc, ME; it also includes black slate of the Partridge Formation ( C. bicornis--zone graptolites, ???457 Ma). Related metamorphosed intrusives are the tonalitic Joslin Turn pluton (469 ?? 2 Ma), the Cambridge Black granitic pluton (468 ?? 3 Ma), and gabbro, tonalite (467 ?? 4 Ma), and sheeted diabase of the Chickwolnepy intrusions. These intrusives cut lowermost Ammonoosuc (therefore >469 Ma). Probable uppermost Ammonoosuc is dated at 465 ?? 6 and 461 ?? 8 Ma. Successively below the Ammonoosuc are the Dead River and Hurricane Mountain Formations (flysch and melange), and the Jim Pond Formation (484 ?? 5 Ma) and Boil Mountain Complex (both ophiolite), which are structurally underlain by the Neoproterozoic(?) Chain Lakes massif. The Quimby sequence is defined by the Lower Silurian(?) to Upper Ordovician Quimby Formation, composed of bimodal volcanics (443 ?? 4 Ma) and sulfidic shale and graywacke that lie conformably to unconformably above the Ammmonoosuc Volcanics and Partridge Formation. Also in the Quimby sequence are several granitic to sparsely gabbroic plutons of the Highlandcroft (441-452 Ma) and Oliverian (435-456 Ma) Plutonic Suites, which intrude the Dead River, Ammonoouc and Partridge, but not the Quimby Formation. Based on faunal, paleolatitude, and isotopic data, the Ammonoosuc sequence and its correlatives and underlying sequences formed off the southern Laurentian margin, but northwest of the principal Iapetan suture, or Red Indian line. The Boil Mountain-Jim Pond-Hurricane Mountain sequence was ramped northwestward over the Chain Lakes massif at ???475 Ma, on the basal Boil Mountain surface. This obduction probably occurred slightly before obduction on the Baie Vert-Brompton surface (BBL), farther NW, over the Laurentian margin, and was followed by Dead River flysch sedimentation, which ended with the abrupt onset of Ammonoosuc-sequence arc magmatism at ???470 Ma. Ammonoosuc eruptions probably ended at ???460 Ma, when Iapetus closed along the Red Indian line. During a following magmatic hiatus of ???3-5 m.y., now represented by portions of the Partridge Formation that overlie the Ammonoosuc Volcanics, subduction polarity reversed, and subduction resumed below the northwest-dipping Brunswick subduction complex (BSC) of New Brunswick, Canada. Quimby-sequence magmatism (???456-435 Ma) on the newly accreted Laurentian margin occurred above the BSC, whose footwall is now buried to the southeast by mainly Silurian clastic sediments of the Merrimack-Fredericton trough, deposited in the "Fredericton Sea". In Silurian to Early Devonian time, the NW-dipping BSC footwall was paired with a SE-dipping subduction zone that produced arc magmas of the Coastal Volcanic belt, built on the composite Avalon and adjacent peri-Avalonian terranes. Orogen-normal extension produced by rapid rollback of both subduction zones narrowed the Fredericton Sea, produced the Central Maine and Connecticut Valley-Gaspe?? basins, and culminated in the Acadian orogeny when the sea completely closed in Early Devonian time. Published by Elsevier Science Ltd.

Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England

NASA Astrophysics Data System (ADS)

Moench, Robert H.; Aleinikoff, John N.

The Ammonoosuc Volcanics, Partridge Formation, and the Oliverian and Highlandcroft Plutonic Suites of the Bronson Hill anticlinorium (BHA) in axial New England are widely accepted as a single Middle to Late Ordovician magmatic arc that was active during closure of Iapetus. Mapping and U-Pb dating indicate, however, that the BHA contains two volcano-sedimentary-intrusive sequences of probable opposite subduction polarity, here termed the Ammonoosuc and Quimby sequences. The Ammonoosuc sequence is defined by the Middle Ordovician Ammonoosuc Volcanics near Littleton, NH, the type area, northeast to Milan, NH, and Oquossoc, ME; it also includes black slate of the Partridge Formation ( C. bicornis--zone graptolites, ∼457 Ma). Related metamorphosed intrusives are the tonalitic Joslin Turn pluton (469 ± 2 Ma), the Cambridge Black granitic pluton (468 ± 3 Ma), and gabbro, tonalite (467 ± 4 Ma), and sheeted diabase of the Chickwolnepy intrusions. These intrusives cut lowermost Ammonoosuc (therefore >469 Ma). Probable uppermost Ammonoosuc is dated at 465 ± 6 and 461 ± 8 Ma. Successively below the Ammonoosuc are the Dead River and Hurricane Mountain Formations (flysch and melange), and the Jim Pond Formation (484 ± 5 Ma) and Boil Mountain Complex (both ophiolite), which are structurally underlain by the Neoproterozoic(?) Chain Lakes massif. The Quimby sequence is defined by the Lower Silurian(?) to Upper Ordovician Quimby Formation, composed of bimodal volcanics (443 ± 4 Ma) and sulfidic shale and graywacke that lie conformably to unconformably above the Ammmonoosuc Volcanics and Partridge Formation. Also in the Quimby sequence are several granitic to sparsely gabbroic plutons of the Highlandcroft (441-452 Ma) and Oliverian (435-456 Ma) Plutonic Suites, which intrude the Dead River, Ammonoouc and Partridge, but not the Quimby Formation. Based on faunal, paleolatitude, and isotopic data, the Ammonoosuc sequence and its correlatives and underlying sequences formed off the southern Laurentian margin, but northwest of the principal Iapetan suture, or Red Indian line. The Boil Mountain-Jim Pond-Hurricane Mountain sequence was ramped northwestward over the Chain Lakes massif at ∼475 Ma, on the basal Boil Mountain surface. This obduction probably occurred slightly before obduction on the Baie Vert-Brompton surface (BBL), farther NW, over the Laurentian margin, and was followed by Dead River flysch sedimentation, which ended with the abrupt onset of Ammonoosuc-sequence arc magmatism at ∼470 Ma. Ammonoosuc eruptions probably ended at ∼460 Ma, when Iapetus closed along the Red Indian line. During a following magmatic hiatus of ∼3-5 m.y., now represented by portions of the Partridge Formation that overlie the Ammonoosuc Volcanics, subduction polarity reversed, and subduction resumed below the northwest-dipping Brunswick subduction complex (BSC) of New Brunswick, Canada. Quimby-sequence magmatism (∼456-435 Ma) on the newly accreted Laurentian margin occurred above the BSC, whose footwall is now buried to the southeast by mainly Silurian clastic sediments of the Merrimack-Fredericton trough, deposited in the “Fredericton Sea”. In Silurian to Early Devonian time, the NW-dipping BSC footwall was paired with a SE-dipping subduction zone that produced arc magmas of the Coastal Volcanic belt, built on the composite Avalon and adjacent peri-Avalonian terranes. Orogen-normal extension produced by rapid rollback of both subduction zones narrowed the Fredericton Sea, produced the Central Maine and Connecticut Valley-Gaspé basins, and culminated in the Acadian orogeny when the sea completely closed in Early Devonian time.

Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England

NASA Astrophysics Data System (ADS)

Moench, Robert H.; Aleinikoff, John N.

2002-01-01

The Ammonoosuc Volcanics, Partridge Formation, and the Oliverian and Highlandcroft Plutonic Suites of the Bronson Hill anticlinorium (BHA) in axial New England are widely accepted as a single Middle to Late Ordovician magmatic arc that was active during closure of Iapetus. Mapping and U-Pb dating indicate, however, that the BHA contains two volcano-sedimentary-intrusive sequences of probable opposite subduction polarity, here termed the Ammonoosuc and Quimby sequences. The Ammonoosuc sequence is defined by the Middle Ordovician Ammonoosuc Volcanics near Littleton, N.H., the type area, northeast to Milan, N.H., and Oquossoc, Me.; it also includes black slate of the Partidge Formation ( C. bicornis zone graptolites, ∼457 Ma). Related metamorphosed intrusive are the tonalitic Joslin Turn pluton (469±2 Ma), the Cambridge Black granitic pluton (468±3 Ma), and gabbro, tonalite (467±4 Ma), and sheeted diabase of the Chickwolnepy instructions. These intrusives cut lowermost Ammonoosuc (therefore >469 Ma). Probable uppermost Ammonoosuc is dated at 465±6 and 461±8 Ma. Successively below the Ammonoosuc are the Dead River and Hurricane Mountain Formations (flysch and melange), and the Jim Pond Formation (484±5 Ma) and Boil Mountain Complex (both ophiolite), which are structurally underlain by the Neoproterozoic(?) Chain Lakes massif. The Quimby sequence is defined by the Lower Silurian(?) to Upper Ordovician Quimby Formation, composed of bimodal volcanics (443±4 Ma) and sulfidic shale and graywacke that lie conformably to unconformably above the Ammonoosuc Volcanics and Partridge Formation. Also in the Quimby sequence are several granitic to sparsely gabbroic plutons of the Highlandcroft (441-452 Ma) and Oliverian (435-456 Ma) Plutonic Suites, which intrude the Dead River, Ammonoosuc and Partridge, but not the Quimby Formation. Based on faunal, paleolatitude, and isotropic data, the Ammonoosuc sequence and its correlative and underlying sequences formed off the southern Laurentian margin, but northwest of the principal Iapetan suture, or Red Indian line (RIL). The Boil Mountain-Jim Pond-Hurricane Mountain sequence was ramped northwestward over the Chain Lakes massif at ∼475 Ma, on the basal Boil Mountain surface. This obduction probably occurred slightly before obduction on the Baie Verte-Brompton surface (BBL), farther NW, over the Laurentian margin, and was followed by Dead River flysch sedimentation, which ended with the abrupt onset of Ammonoosuc-sequence arc magmatism at ∼470 Ma. Ammonoosuc eruptions probably ended at ∼460 Ma, when Iapetus closed along the Red Indian line. During a following magmatic hiatus of ∼3-5 m.y., now represented by portions of the Partridge Formation that overlie the Ammonoosuc Volcanics, subduction polarity reversed, and subduction resumed below the northwest-dipping Brunswick subduction complex (BSC) of New Brunswick, Canada. Quimby-sequence magmatism (∼456-435 Ma) on the the newly accreted Laurentian margin occurred above the BSC, whose footwall is now buried to the southeast by mainly Silurian clastic sediments of the Merrimack-Fredericton trough, deposited in the “Fredericton Sea”. In Silurian to Early Devonian time, the NW-dipping BSC footwall was paired with a SE-dipping subduction zone that produced arc magmas of the Coastal Volcanic belt, built on the composite Avalon and adjacent peri-Avalonian terranes. Orogen-normal extension produced by rapid rollback of both subduction zones narrowed the Fredericton Sea, produced the Central Maine and Connecticut Valley-Gaspé basins, and culminated in the Acadian orogeny when the sea completely closed in Early Devonian time.

Syntectonic Variscan magmatism in the Aguiar da Beira region (Iberian Massif, Portugal)

NASA Astrophysics Data System (ADS)

Mafalda Costa, Maria; Margarida Neiva, Ana; do Rosario Azevedo, Maria; Corfu, Fernando

2014-05-01

The Aguiar da Beira region (Portugal) is located in the core of the Iberian Massif, more precisely in the Central-Iberian Zone, which is dominantly composed by abundant volumes of plutonic rocks, emplaced into Late Proterozoic - Early Cambrian and Palaeozoic metasediments, mainly during or slightly after the third deformation phase of the Variscan Orogeny (D3). A considerable amount of these granites are syntectonic, intruded during the peak of this deformation event (D3). In particular, at the Aguiar da Beira region, two suites of syntectonic granitoids represent distinct magmatic series: a medium- to coarse-grained porphyritic biotite granodiorite-granite (322 Ma), which belongs to the early granodiorite series, and a medium-grained muscovite-biotite granite (317 Ma) that is part of the two-mica peraluminous leucogranites suite. The petrographic, geochemical (whole-rock and mineral compositions) and isotopic (Sr-Nd, δ18O-wr and δ18O-zr) study of the two intrusions reveals their remarkably different character. It is concluded that they correspond to two independent magma pulses, derived from distinct sources and/or petrogenetic processes. The biotite granodiorite-granite is a weakly peraluminous intrusion, characterized by intermediate to felsic SiO2 contents (66 - 68 %), high Ba, Sr and REE, and biotite with high Al and Mg contents, typical of the calc-alkaline associations. The Sr-Nd initial ratios are homogeneous (87Sr-86Sr322: 0.7070 - 0.7074; ɛNd322: -3.9 to -4.6) and overlap the isotopic signatures of lower crustal felsic metaigneous granulites (Villaseca et al. 1999). This similarity, which is further supported by δ18O-wr and δ18O-zr data, may indicate an origin by anatexis of lower felsic metaigneous rocks. Alternatively, the same data, allied to the presence of microgranular enclaves seen in this intrusion, can also be explained by the mixing of lower crustal derived magmas and mantle melts. By contrast, the muscovite-biotite granite has an entirely distinct geochemical signature, typical of S-type granites: a highly evolved and strongly peraluminous character (SiO2 = 72 - 74 %; CaO = 0.3-0.6 %; A/CNK = 1.18 - 1.36, low Mg, Ti, Ba, Sr, ΣREE, HFSE contents, and high Al2O3-TiO2 ratio e Rb-Sr ratios), high (87Sr-86Sr)317 (0.7104 - 0.7146), low ɛNd317 (-7.7 to -8.7), and high δ18O (δ18O-wr = 11.33 %; δ18O-zr = 9.5 ± 0.2 o). The data suggest that this magma was derived from the partial melting of metasedimentary middle crustal protoliths, which has been successfully modeled. The observed variation in major and trace element composition suggests an evolution controlled by fractionational crystallization of a mineral association composed by plagioclase, biotite, apatite, zircon, monazite, ilmenite. The first author benefited from a PhD grant from the Portuguese Science and Technology Foundation (SFRH.BD.2005.21410)and support from PETROCHRON (PTDC.CTE-GIX.112561.2009).

The thermal evolution of Chinese central Tianshan and its implications: Insights from multi-method chronometry

NASA Astrophysics Data System (ADS)

Yin, Jiyuan; Chen, Wen; Hodges, Kip V.; Xiao, Wenjiao; Cai, Keda; Yuan, Chao; Sun, Min; Liu, Li-Ping; van Soest, Matthijs C.

2018-01-01

The Chinese Tianshan is located in the south of the Central Asian Orogenic Belt and formed during final consumption of the Paleo-Asian Ocean in the late Palaeozoic. In order to further elucidate the tectonic evolution of the Chinese Tianshan, we have established the temperature-time history of granitic rocks from the Chinese Tianshan through a multi-chronological approach that includes U/Pb (zircon), 40Ar/39Ar (biotite and K-feldspar), and (U-Th)/He (zircon and apatite) dating. Our data show that the central Tianshan experienced accelerated cooling during the late Carboniferous- to early Permian. Multiple sequences of complex multiple accretionary, subduction and collisional events could have induced the cooling in the Tianshan Orogenic Belt. The new 40Ar/39Ar and (U-Th)/He data, in combination with thermal history modeling results, reveal that several tectonic reactivation and exhumation episodes affected the Chinese central Tianshan during middle Triassic (245-210 Ma), early Cretaceous (140-100 Ma), late Oligocene-early Miocene (35-20 Ma) and late Miocene (12-9 Ma). The middle Triassic cooling dates was only found in the central Tianshan. Strong uplift and deformation in the Chinese Tianshan has been limited and localized. It have been concentrated in around major fault zone and the foreland thrust belt since the early Cretaceous. The middle Triassic and early Cretaceous exhumation is interpreted as distal effects of the Cimmerian collisions (i.e. the Qiangtang and Kunlun-Qaidam collision and Lhasa-Qiangtang collision) at the southern Eurasian margin. The Cenozoic reactivation and exhumation is interpreted as a far field response to the India-Eurasia collision and represents the beginning of modern mountain building and denudation in the Chinese Tianshan.

The tectonic setting and evolution of the 2.7 Ga Kalgoorlie-Kurnalpi Rift, a world-class Archean gold province

NASA Astrophysics Data System (ADS)

Witt, Walter K.; Cassidy, Kevin F.; Lu, Yong-Jun; Hagemann, Steffen G.

2018-01-01

The Yilgarn Craton results from three major mantle input events (at ca 3.0-2.9, 2.8 and 2.7 Ga) that have interacted with > 3.0 Ga continental crust. Zircon geochronology and Sm-Nd isotopic data subdivide the craton into an older Yilgarn proto-craton and the younger, more primitive Eastern Goldfields Superterrane (EGST). Formation of the Kalgoorlie-Kurnalpi Rift (KKR) within the EGST was associated with the 2.7 Ga event, which exploited weakened crust at the eastern margin of the Yilgarn proto-craton where thick sequences of komatiite and basalt were erupted between ca 2710 and 2690 Ma in the Kalgoorlie Terrane. Calc-alkaline volcanism in the Kurnalpi Terrane began at ca 2730 Ma and continued to ca 2690 Ma, overlapping rifting and plume-related volcanism in the Kalgoorlie Terrane. Deposition of siliciclastic sedimentary rocks within basins at ca 2660 resulted from an intra-orogenic extensional event and coincided with the transition from High-Ca to Low-Ca granite magmatism and peak emplacement of intrusions with a metasomatised mantle source component. Most aspects of the KKR are satisfied by broadly coincident plume-related magmatism in the Kalgoorlie Terrane and westward subduction to the east of the Burtville Terrane. Geochemical characteristics of 2730-2700 Ma calc-alkaline volcanism and 2685-2630 Ma low-SiO2 and alkali-rich intrusions support models for a continental margin subduction zone setting. World-class gold deposits formed in reactivated margins of the KKR, which became flux zones for mantle-derived magmas, hydrothermal fluids and heat during 2675-2620 Ma orogenesis. The orogenic gold mineralisation can be subdivided into proximal intrusion-related and distal-source deposits.

Fault distribution in the Precambrian basement of South Norway

NASA Astrophysics Data System (ADS)

Gabrielsen, Roy H.; Nystuen, Johan Petter; Olesen, Odleiv

2018-03-01

Mapping of the structural pattern by remote sensing methods (Landsat, SPOT, aerial photography, potential field data) and field study of selected structural elements shows that the cratonic basement of South Norway is strongly affected by a regular lineament pattern that encompasses fault swarms of different orientation, age, style, attitude and frequency. Albeit counting numerous fault and fracture populations, the faults are not evenly distributed and N-S to NNE-SSW/NNW-SSE and NE-SE/ENE-WSW-systems are spatially dominant. N-S to NNW-SSE structures can be traced underneath the Caledonian nappes to the Western Gneiss Region in western and central South Norway, emphasizing their ancient roots. Dyke swarms of different ages are found within most of these zones. Also, the Østfold, Oslo-Trondheim and the Mandal-Molde lineament zones coincide with trends of Sveconorwegian post-collision granites. We conclude that the N-S-trend includes the most ancient structural elements, and that the trend can be traced back to the Proterozoic (Svecofennian and Sveconorwegian) orogenic events. Some of the faults may have been active in Neoproterozoic times as marginal faults of rift basins at the western margin of Baltica. Remnants of such fault activity have survived in the cores of many of the faults belonging to this system. The ancient systems of lineaments were passively overridden by the Caledonian fold-and-thrust system and remained mostly, but note entirely inactive throughout the Sub-Cambrian peneplanation and the Caledonian orogenic collapse in the Silurian-Devonian. The system was reactivated in extension from Carboniferous times, particularly in the Permian with the formation of the Oslo Rift and parts of it remain active to the Present, albeit by decreasing extension and fault activity.

Provenance and accommodation pathways of late Quaternary sediments in the deep-water northern Ionian Basin, southern Italy

NASA Astrophysics Data System (ADS)

Perri, Francesco; Critelli, Salvatore; Dominici, Rocco; Muto, Francesco; Tripodi, Vincenzo; Ceramicola, Silvia

2012-12-01

The northern Calabria along the southeastern coast of Italy provides a favorable setting in which to study complete transects from continental to deep-marine environments. The present northern Ionian Calabrian Basin is a wedge-top basin within the modern foreland-basin system of southern Italy. The Ionian margin of northern Calabria consists of a moderately developed fluvial systems, the Crati and Neto rivers, and diverse smaller coastal drainages draining both the Calabria continental block (i.e., Sila Massif) and the southern Apennines thrust belt (i.e., Pollino Massif). The main-channel sand of the Crati and Neto rivers is quartzofeldspathic with abundant metamorphic and plutonic lithic fragments (granodiorite, granite, gneiss, phyllite and sedimentary lithic fragments). Sedimentary lithic fragments were derived from Jurassic sedimentary successions of the Longobucco Group. The mud samples contain mostly phyllosilicates, quartz, calcite, feldspars and dolomite. Traces of gypsum are present in some samples. The I-S mixed layers, 10 Å-minerals (illite and micas), chlorite and kaolinite are the most abundant phyllosilicates, whereas smectite and chlorite/smectite mixed layers are in small amounts. The geochemical signatures of the muds reflect a provenance characterized by both felsic and mafic rocks with a significant input from carbonate rocks. Furthermore, the degree of source-area weathering was most probably of low intensity rather than moderately intense because CIA values for the studied mud samples are low. Extrapolation of the mean erosion budget from 1 to 25 Ma suggests that at least 5 to 8 km of crust have been removed from the Calabrian orogenic belt and deposited in the marine basins. The Calabrian microplate played an important role in the dynamic evolution of southern Italian fossil and modern basins, representing the key tectonic element of the entire orogenic belt.

Mantle contribution and tectonic transition in the Aqishan-Yamansu Belt, Eastern Tianshan, NW China: Insights from geochronology and geochemistry of Early Carboniferous to Early Permian felsic intrusions

NASA Astrophysics Data System (ADS)

Du, Long; Long, Xiaoping; Yuan, Chao; Zhang, Yunying; Huang, Zongying; Wang, Xinyu; Yang, Yueheng

2018-04-01

Late Paleozoic is a key period for the accretion and collision of the southern Central Asian Orogenic Belt (CAOB). Here, we present new zircon U-Pb ages, whole-rock geochemistry and Sr-Nd isotopic compositions for four Late Paleozoic felsic plutons in Eastern Tianshan (or Tienshan in some literatures) in order to constrain the tectonic evolution of the southern CAOB. The granodioritic pluton and its dioritic enclaves were synchronously formed in the Early Carboniferous (336 ± 3 Ma and 335 ± 2 Ma, respectively). These rocks are depleted in Nb, Ta and Ti, and enriched in Rb, Ba, Th and U related to the primitive mantle, which show typical features of arc rocks. They both have similar Sr-Nd isotopic ratios to those granitic rocks from the eastern Central Tianshan Block and have the latest Mesoproterozoic two stage Nd model ages (TDM2) (1111-1195 Ma for the granodioritic pluton and 1104-1108 Ma for the enclaves, respectively), indicating that their source magmas may have been derived from the Mesoproterozoic crust. The albitophyric pluton was also emplaced in the Early Carboniferous (333 ± 3 Ma). Rocks of this pluton have similar εNd(t) values (-0.69 to -0.37) and TDM2 ages (1135-1161 Ma) to those of the granodioritic rocks, suggest similar crustal source for both types of rocks. In contrast, the K-feldspar granitic and monzonitic plutons were emplaced in the Early Permian (292 ± 3 Ma and 281 ± 2 Ma, respectively). Samples of the K-feldspar granitic pluton have high K2O + Na2O, FeO/MgO, Ga/Al, HFSE (e.g., Zr and Hf) and low CaO, Sr and Ba, exhibiting characteristics of A2-type granites, which probably emplaced in a post-collisional extension environment. They have higher εNd(t) values (+2.77 to +3.27) and more juvenile TDM2 ages (799-841 Ma) than the Early Carboniferous plutons, suggesting that they were derived from relatively younger crustal sources. The monzonitic granites are metaluminous to weakly peraluminous with A/CNK ranging from 0.93 to 1.05, and have very low P2O5, indicating characteristics of I-type granites. They also have positive εNd(t) values (+2.22 to +2.34) and juvenile TDM2 ages (868-878 Ma), suggesting this pluton was also produced by partial melting of relatively young crustal source. Based on an isotopic mixing simulation, significant mantle contributions were added to the magma source of both the Early Carboniferous and the Early Permian felsic rocks. The mantle contribution changes from 60% in the Early Carboniferous to 75% in the Early Permian. The remarkably increasing of mantle materials in the magma source of the felsic rocks in the Aqishan-Yamansu belt was most likely induced by the tectonic transition from an Early Carboniferous continental arc to an Early Permian post-collisional extension environment.

The Myszkow porphyry copper-molybdenum deposit, Poland

USGS Publications Warehouse

Chaffee, M.A.; Eppinger, R.G.; Lason, K.; Slosarz, J.; Podemski, M.

1994-01-01

The porphyry copper-molybdenum deposit at Myszkow, south-central Poland, lies in the Cracow-Silesian orogenic belt, in the vicinity of a Paleozoic boundary between two tectonic plates. The deposit is hosted in a complex that includes early Paleozoic metasedimentary rocks intruded in the late Paleozoic by a predominantly granodioritic pluton. This deposit exhibits many features that are typical of porphyry copper deposits associated with calc-alkaline intrusive rocks, including ore- and alteration-mineral suites, zoning of ore and alteration minerals, fluid-inclusion chemistry, tectonic setting, and structural style of veining. Unusual features of the Myszkow deposit include high concentrations of tungsten and the late Paleozoic (Variscan) age. -Authors

Deformation processes in orogenic wedges: New methods and application to Northwestern Washington State

NASA Astrophysics Data System (ADS)

Thissen, Christopher J.

Permanent deformation records aspects of how material moves through a tectonic environment. The methods required to measure deformation vary based on rock type, deformation process, and the geological question of interest. In this thesis we develop two new methods for measuring permanent deformation in rocks. The first method uses the autocorrelation function to measure the anisotropy present in two-dimensional photomicrographs and three-dimensional X-ray tomograms of rocks. The method returns very precise estimates for the deformation parameters and works best for materials where the deformation is recorded as a shape change of distinct fabric elements, such as grains. Our method also includes error estimates. Image analysis techniques can focus the method on specific fabric elements, such as quartz grains. The second method develops a statistical technique for measuring the symmetry in a distribution of crystal orientations, called a lattice-preferred orientation (LPO). We show that in many cases the symmetry of the LPO directly constrains the symmetry of the deformation, such axial flattening vs. pure shear vs. simple shear. In addition to quantifying the symmetry, the method uses the full crystal orientation to estimate symmetry rather than pole figures. Pole figure symmetry can often be misleading. This method works best for crystal orientations measured in samples deformed by dislocation creep, but otherwise can be used on any mineral without requiring information about slip systems. In Chapter 4 we show how deformation measurements can be used to inform regional tectonic and orogenic models in the Pacific Northwestern United States. A suite of measurements from the Olympic Mountains shows that uplift and deformation of the range is consistent with an orogenic wedge model driven by subduction of the Juan de Fuca plate, and not northward forearc migration of the Oregon block. The deformation measurements also show that deformation within the Olympic Mountains is essentially two-dimensional. We use this constraint to develop a suite of orogenic deformation models that use slab height and erosion rate data as boundary conditions. We use the models to show that influx of sediments distributed along an accretionary front can greatly reduce deformation required to maintain wedge taper. Due to the two-dimensional nature of deformation in the Olympics, a series of two-dimensional transects across the peninsula provides an approximation for non-elastic deformation across the Peninsula. We show how the shallow slab height and deeper exhumation at the core of peninsula led to the domal structure of the Olympics. This model also explains the counter-clockwise vertical axis rotations north of the peninsula, and clockwise rotations south of the peninsula through horizontal shear, similar to opening a gate. Finally, the horizontal surface velocities predicted by the models suggests that up to 15% of GPS velocities may reflect non-elastic, permanent translation of material towards the rear of the wedge.

Magmatism evolution on the last Neoproterozoic development stage of the western Siberian active continental margin

NASA Astrophysics Data System (ADS)

Vernikovskaya, Antonina E.; Vernikovsky, Valery A.; Matushkin, Nikolay Yu.; Kadilnikov, Pavel I.; Romanova, Irina V.

2017-04-01

Rocks from active continental margin complexes are characterized by a wide variety of chemical compositions from depleted in alkali to alkali differentiates. When addressing issues of geodynamic settings in which such rocks form, it is important to understand the evolution of the host tectonic structure, as well as the chemical affiliation of the various rocks composing it. The Yenisey Ridge orogen located in the south-western framing of Siberia is one of the more studied regions with a long history of Neoproterozoic magmatic events. This orogen was formed during the collision of the Central Angara terrane with Siberia, which took place 761-718 Ma. Subsequent subduction-related events in the orogen have been recorded in the coeval magmatism (711-629 Ma) of two complexes: one is the active continental margin complex (Nb enriched igneous rocks - gabbroids, trachybasalts, A-type granites and carbonatites, including contact metasomatites zones with Nb mineralization), and the other one is an island arc complex (differentiated series volcanics, gabbroids and plagiogranites). The rocks of these complexes are respectively located in two suture zones: the Tatarka-Ishimba zone that formed due to the collision mentioned above, and the Yenisei suture marking the subduction zone [Vernikovsky et al., 2003; 2008]. The final Neoproterozoic stage in the evolution of the active margin of Siberia is manifested as adakite-gabbro-anorthosite magmatism in the 576-546 Ma interval. Our results indicate a genetic relationship between the adakites and their host NEB-type metabasites of the Zimovey massif. These Neoproterozoic adakites could have formed in a setting of transform-strike-slip drift of lithospheric plates after the subduction stopped, both from a crustal and mantle-crustal source, similarly to the Cenozoic magmatic complexes of the transform margin in the eastern framing of Eurasia [Khanchuk et al., 2016]. Vernikovsky V.A., Vernikovskaya A.E., Kotov A.B., Sal'nikova E.B., Kovach V.P. Neoproterozoic accretionary and collisional events on the western margin of the Siberian craton: new geological and geochronological evidence from the Yenisey Ridge // Tectonophysics, 2003, V. 375, P. 147-168. Vernikovsky V.A., Vernikovskaya A.E., Sal'nikova E.B., Berezhnaya N.G., Larionov A.N., Kotov A.B., Kovach V.P., Vernikovskaya I.V., Matushkin N.Yu., Yasenev A.M. Late Riphean alkaline magmatism in the western margin of the Siberian Craton: A result of continental rifting or accretionary events? // Doklady Earth Sciences, 2008, V. 419, Iss. 1, P. 226-230. Khanchuk A.I., Kemkin I.V., Kruk N.N. The Sikhote-Alin orogenic belt, Russian South East: Terranes and the formation of continental lithosphere based on geological and isotopic Data // Journal of Asian Earth Sciences, 2016, V. 120, P. 117-138.

Late Mesoproterozoic to Early Paleozoic history of metamorphic basement from the southeastern Chiapas Massif Complex, Mexico, and implications for the evolution of NW Gondwana

NASA Astrophysics Data System (ADS)

Weber, Bodo; González-Guzmán, Reneé; Manjarrez-Juárez, Román; Cisneros de León, Alejandro; Martens, Uwe; Solari, Luigi; Hecht, Lutz; Valencia, Victor

2018-02-01

In this paper, U-Pb zircon geochronology, Lu-Hf and Sm-Nd isotope systematics, geochemistry and geothermobarometry of metaigneous basement rocks exposed in the southeastern Chiapas Massif Complex are presented. Geologic mapping of the newly defined "El Triunfo Complex" located at the southeastern edge of the Chiapas Massif reveals (1) partial melting of a metamorphic basement mainly constituted by mafic metaigneous rocks (Candelaria unit), (2) an Ediacaran metasedimentary sequence (Jocote unit), and (3) occurrence of massif-type anorthosite. All these units are intruded by undeformed Ordovician plutonic rocks of the Motozintla suite. Pressure and temperature estimates using Ca-amphiboles, plagioclase and phengite revealed prograde metamorphism that reached peak conditions at 650 °C and 6 kbar, sufficient for partial melting under water saturated conditions. Relict rutile in titanite and clinopyroxene in amphibolite further indicate a previous metamorphic event at higher P-T conditions. U-Pb zircon ages from felsic orthogneiss boudins hosted in deformed amphibolite and migmatite yield crystallization ages of 1.0 Ga, indicating that dry granitic protoliths represent remnants of Rodinia-type basement. Additionally, a mid-Tonian ( 920 Ma) metamorphic overprint is suggested by recrystallized zircon from a banded gneiss. Zircon from folded amphibolite samples yield mainly Ordovician ages ranging from 457 to 444 Ma that are indistinguishable from the age of the undeformed Motozintla plutonic suite. Similar ages between igneous- and metamorphic- zircon suggest a coeval formation during a high-grade metamorphic event, in which textural discrepancies are explained in terms of differing zircon formation mechanisms such as sub-solidus recrystallization and precipitation from anatectic melts. In addition, some amphibolite samples contain inherited zircon yielding Stenian-Tonian ages around 1.0 Ga. Lu-Hf and Sm-Nd isotopes and geochemical data indicate that the protoliths of the amphibolite have E-MORB characteristics and were derived from a depleted mantle source younger than the Rodinia-type basement. Inasmuch as similar amphibolites also occur in the Ediacaran metasedimentary rocks as dykes or lenses, Late Neoproterozoic magmatism in a rift setting is suggested. Hence, the geologic record of the El Triunfo Complex includes evidences for Rodinia assemblage, Tonian circum-Rodinia subduction, and breakup during the Late Neoproterozoic. Metamorphism, and partial melting are interpreted in terms of a convergent margin setting during the Ordovician. The results place the southern Chiapas Massif along with Oaxaquia and similar Northern Andes terranes on the NW margin of Gondwana interpreted as the extension of the Famatinian orogen that evolved during the closure of the Iapetus Ocean.

40Ar/39Ar geochronological constraints on the formation of the Dayingezhuang gold deposit: New implications for timing and duration of hydrothermal activity in the Jiaodong gold province, China

USGS Publications Warehouse

Yang, Li-Qiang; Deng, J.; Goldfarb, Richard J.; Zhang, Jiahua; Gao, Bang-Fei; Wang, Zhong-Liang

2014-01-01

China's largest gold resource is located in the highly endowed northwestern part of the Jiaodong gold province. Most gold deposits in this area are associated with the NE- to NNE-trending shear zones on the margins of the 130–126 Ma Guojialing granite. These deposits collectively formed at ca. 120 ± 5 Ma during rapid uplift of the granite. The Dayingezhuang deposit is a large (> 120 t Au) orogenic gold deposit in the same area, but located along the eastern margin of the Late Jurassic Linglong Metamorphic Core Complex. New 40Ar/39Ar geochronology on hydrothermal sericite and muscovite from the Dayingezhuang deposit indicate the gold event is related to evolution of the core complex at 130 ± 4 Ma and is the earliest important gold event that is well-documented in the province. The Dayingezhuang deposit occurs along the Linglong detachment fault, which defines the eastern edge of the ca. 160–150 Ma Linglong granite–granodiorite massif. The anatectic rocks of the massif were rapidly uplifted, at rates of at least 1 km/m.y. from depths of 25–30 km, to form the metamorphic core complex. The detachment fault, with Precambrian metamorphic basement rocks in the hangingwall and the Linglong granitoids and migmatites in the footwall, is characterized by early mylonitization and a local brittle overprinting in the footwall. Gold is associated with quartz–sericite–pyrite–K-feldspar altered footwall cataclasites at the southernmost area of the brittle deformation along the detachment fault. Our results indicate that there were two successive, yet distinct gold-forming tectonic episodes in northwestern Jiaodong. One event first reactivated the detachment fault along the edge of the Linglong massif between 134 and 126 Ma, and then a second reactivated the shears along the margins of the Guojialing granite. Both events may relate to a component of northwest compression after a middle Early Cretaceous shift from regional NW–SE extension to a NE–SW extensional regime.

Metabasalts as sources of metals in orogenic gold deposits

NASA Astrophysics Data System (ADS)

Pitcairn, Iain K.; Craw, Dave; Teagle, Damon A. H.

2015-03-01

Although metabasaltic rocks have been suggested to be important source rocks for orogenic gold deposits, the mobility of Au and related elements (As, Sb, Se, and Hg) from these rocks during alteration and metamorphism is poorly constrained. We investigate the effects of increasing metamorphic grade on the concentrations of Au and related elements in a suite of metabasaltic rocks from the Otago and Alpine Schists, New Zealand. The metabasaltic rocks in the Otago and Alpine Schists are of MORB and WPB affinity and are interpreted to be fragments accreted from subducting oceanic crust. Gold concentrations are systematically lower in the higher metamorphic grade rocks. Average Au concentrations vary little between sub-greenschist (0.9 ± 0.5 ppb) and upper greenschist facies (1.0 ± 0.5 ppb), but decrease significantly in amphibolite facies samples (0.21 ± 0.07 ppb). The amount of Au depleted from metabasaltic rocks during metamorphism is on a similar scale to that removed from metasedimentary rocks in Otago. Arsenic concentrations increase with metamorphic grade with the metabasaltic rocks acting as a sink rather than a source of this element. The concentrations of Sb and Hg decrease between sub-greenschist and amphibolite facies but concentration in amphibolite facies rocks are similar to those in unaltered MORB protoliths and therefore unaltered oceanic crust cannot be a net source of Sb and Hg in a metamorphic environment. The concentrations of Au, As, Sb, and Hg in oceanic basalts that have become integrated into the metamorphic environment may be heavily influenced by the degree of seafloor alteration that occurred prior to metamorphism. We suggest that metasedimentary rocks are much more suitable source rocks for fluids and metals in orogenic gold deposits than metabasaltic rocks as they show mobility during metamorphism of all elements commonly enriched in this style of deposit.

Geochronology of the proterozoic basement of southwesternmost North America, and the origin and evolution of the Mojave crustal province

USGS Publications Warehouse

Barth, Andrew P.; Wooden, Joseph L.; Coleman, Drew S.; Fanning, C. Mark

2000-01-01

The Proterozoic Baldwin gneiss in the central Transverse Ranges of southern California, a part of the Mojave crustal province, is composed of quartzofeldspathic gneiss and schist, augen and granitic gneiss, trondhjemite gneiss, and minor quartzite, amphibolite, metagabbro, and metapyroxenite. Sensitive high resolution ion microprobe (SHRIMP) data indicate that augen and granitic gneisses comprise a magmatic arc intrusive suite emplaced between 1783 ± 12 and 1675 ± 19 Ma, adjacent to or through thinned Archean crust. High U/Th rims on zircons in most samples suggest an early metamorphic event at ∼1741 Ma, but peak amphibolite facies metamorphism and penetrative, west vergent deformation occurred after 1675 Ma. The Baldwin gneiss is part of a regional allochthon emplaced by west vergent deformation over a Proterozoic shelf-slope sequence (Joshua Tree terrane). We hypothesize that emplacement of this regional allochthon occurred during a late Early or Middle Proterozoic arc-continent collision along the western margin of Laurentia.

Geology of Seward Peninsula and Saint Lawrence Island

USGS Publications Warehouse

Till, Alison B.; Dumoulin, Julie A.

1994-01-01

Seward Peninsula (Fig. 1) may be divided into two geologic terranes (Fig. 2) on the basis of stratigraphy, structure, and metamorphic history. The Seward terrane, an area 150 by 150 km in the central and eastern peninsula, is dominated by Precambrian(?) and early Paleozoic blueschist-, greenschist-, and amphibolite-facies schist and marble, and intruded by three suites of granitic rocks. The York terrane, roughly 100 by 75 km, occupies western Seward Peninsula and the Bering Straits region; it is composed of Ordovician, Silurian, Devonian, Mississippian, and possibly older limestone, argillaceous limestone, dolostone, and phyllite, which are cut by a suite of Late Cretaceous tin-bearing granites. The boundary between the Seward and York terranes is poorly exposed but is thought to be a major thrust fault because of its sinuous map trace, a discontinuity in metamorphic grade, and differences in stratigraphy across the boundary (Travis Hudson, oral communication, 1984). The boundary between the Seward terrane and the Yukon-Koyukuk province to the east is complicated by vertical faults (the Kugruk fault Zone of Sainsbury, 1974) and obscured by Cretaceous and Tertiary cover.The Seward Peninsula heretofore was thought to consist largely of rocks of Precambrian age (Sainsbury, 1972, 1974, 1975; Hudson, 1977), Microfossil data, however, indicate that many of the rocks considered to be Precambrian are early Paleozoic in age (Till and others, 1986; Dumoulin and Harris, 1984; Dumoulin and Till, 1985; Till and others, 1983; Wandervoort, 1985). It is likely that Precambrian rocks are a minor part of the stratigraphy of the Seward Peninsula.

Zircon U-Pb geochronology, Sr-Nd-Hf isotopic composition and geological significance of the Late Triassic Baijiazhuang and Lvjing granitic plutons in West Qinling Orogen

NASA Astrophysics Data System (ADS)

Duan, Meng; Niu, Yaoling; Kong, Juanjuan; Sun, Pu; Hu, Yan; Zhang, Yu; Chen, Shuo; Li, Jiyong

2016-09-01

The Qinling Orogen was a consequence of continental collision of the South China Craton with the North China Craton in the Triassic and caused widespread granitoid magmatism. However, the petrogenesis of these granitoids remains controversial. In this paper, we choose the Baijiazhuang (BJZ) and Lvjing (LJ) plutons in the West Qinling Orogen for a combined study of the zircon U-Pb geochronology, whole-rock major and trace element compositions and Sr-Nd-Hf isotopic characteristics. We obtained zircon crystallization ages of 216 Ma and 212 Ma for the BJZ and the LJ plutons, respectively. The granitoid samples from both plutons have high K2O metaluminous to peraluminous compositions. They are enriched in large ion lithophile elements (LILEs), light rare earth elements (LREEs) and depleted in high field-strength elements (HFSEs) with significant negative Eu anomalies. The BJZ samples have initial Sr isotopic ratios of 0.7032 to 0.7078, εNd(t) of - 10.99 to - 8.54 and εHf (t) of - 10.22 to - 6.41. The LJ granitoids have initial Sr isotopic ratios of 0.7070 to 0.7080, εNd(t) of - 5.37 to - 4.58 and εHf(t) of - 3.64 to - 1.78. The enriched isotopic characteristics of the two plutons are consistent with their source being dominated by ancient continental crust. However, two BJZ samples show depleted Sr isotope compositions, which may infer possible involvement of mantle materials. Mantle-derived melt, which formed from partial melting of mantle wedge peridotite facilitated by dehydration of the subducted/subducting Mianlue ocean crust, provide the required heat for the crustal melting while also contributing to the compositions of these granitoids. That is, the two granitic plutons are magmatic responses to the closure of the Mianlue ocean basin and the continental collision between the Yangtze and South Qinling crustal terranes. (143Nd/144Nd)i = (143Nd/144Nd) - (147Sm/144Nd) × (eλt - 1), εNd(t) = [(143Nd/144Nd) / (143Nd/144Nd)CHUR(t) - 1] × 104, (143Nd/144Nd)CHUR(t) = 0.512638-0.1967 × (eλt - 1), TDM = 1/λ × ln{1 + [((143Nd/144Nd) - 0.51315) / ((147Sm/144Nd) - 0.21317)]}, λ Sm-Nd = 6.54 × 10- 12a- 1. (176Hf/177Hf)i = [(176Hf/177Hf) - (176Lu/177Hf) × (eλt - 1), εHf(t) = [(176Hf/177Hf) / (176Hf/177Hf)CHUR(t) - 1] × 104, TDM1 = 1/λ × ln[1 + ((176Hf/177Hf)Sample - (176Hf/177Hf)DM)/((176Lu/177Hf)Sample - (176Lu/177Hf)DM)], TDM2 = TDM1 - (TDM1 - t)((fCC - fSample) / (fCC - fDM)), fLu-Hf = (176Lu/177Hf)Sample / (176Lu/177Hf)CHUR-1, (176Lu/177Hf)CHUR = 0.0332, (176Hf/177Hf)CHUR, 0 = 0.282785, (176Lu/177Hf)DM = 0.0384, (176Hf/177Hf)DM = 0.28325, fCC, fSample, fDM are their own fLu-Hf, λ = 1.867 × 10- 11a- 1.

Temporal evolution of mineralization events in the Bohemian Massif inferred from the Re-Os geochronology of molybdenite

NASA Astrophysics Data System (ADS)

Ackerman, Lukáš; Haluzová, Eva; Creaser, Robert A.; Pašava, Jan; Veselovský, František; Breiter, Karel; Erban, Vojtěch; Drábek, Milan

2017-06-01

Molybdenite is a common mineral accompanying Sn-W, Au, and base metal mineralizations located in different geotectonic units of the Bohemian Massif, but it is also widespread in granitoids and/or related quartz veins/pegmatites forming disseminated Mo mineralization. Thirty Re-Os ages were obtained for molybdenite samples from the Bohemian Massif to provide constraints on the timing and duration of mineralization event(s) within the framework of previously published geochronological data for the host and/or associated rocks. The obtained data for Sn-W-(Li) deposits in the Erzgebirge metallogenetic province indicate the predominance of one and/or multiple short-time mineralization events taking place between ˜319 and 323 Ma, with the exception of the Krupka deposit associated with the Altenberg-Teplice caldera where the data may suggest prolonged activity until ˜315 Ma. The ages of the Pb-Zn-(Au-Mo) Hůrky u Rakovníka and Fe-Cu-As Obří důl mineralizations from the exocontacts of the Čistá pluton and Krkonoše-Jizera Plutonic Complex, respectively, provide evidence for synchronous emplacement of the ore and the associated granitic rocks. In contrast, the Padrť Fe-As-Mo mineralization postdates the age of the associated Padrť granite. Disseminated Mo mineralization in Cadomian and Variscan granitoids and/or related to quartz veins/pegmatites provides Re-Os ages that overlap with the previously published geochronological data for the host rocks, suggesting coeval evolution. Molybdenite samples from the Sázava suite granites of the Central Bohemian Plutonic Complex (CBPC) have resolvable younger ages than their host granites, but similar to the age of spatially related Au mineralization which is associated with the latest evolution of the CBPC.

Mineralization related to Alvand pluton in the Hamadan, western Iran

NASA Astrophysics Data System (ADS)

Salehi, M. H.; Doosti, E. A.; Ahadnejad, V.

2009-04-01

The Alvand (Hamadan) plutonic batholith is one of the largest plutonic bodies in the Sanandaj-Sirjan metamorphic belt. This complex is consist of mafic part (gabbro-diorite-tonalite), intermediate (granite-granodiorite porphyroids), and hololeucocratic granitoids. Previous studies have shown that S-type granite-granodiorites are mostly peraluminous and calc-alkaline; the gabbro-diorite-tonalite suite is mostly metaluminous and tholeiitic to calc-alkaline (Sepahi, 2008). High initial 87Sr-86Sr ratios (0.7081 to 0.7115), low epsilon Nd values (-1.0 to -3.3), and peraluminous character reflects a different origin for the granites, possibly crustal sources (Ghalamghash et al, 2007). Aplite-pegmatite dikes are intruded in granitoide rocks, metamorphic rocks and the contact of Alvand granite with metamorphic rocks. The contact of Alvand granite with metamorphic rocks is sharp. By using heavy mineral studies on the alluvium of Alvand complex, it is recognized 28 minerals amongst Scheelite, Cassiterite, Ilmenite, Zircon and Garnet. Different geostatistical studies such as variant, bivariant and multivariant studies have been done on rough data of heavy minerals. They showed normal concentration of gold in studied rocks and low enrichment of tin and tungsten. The index of the ore elements average, frequeney distribution criteria of elements, the ratio of elements index and multielements show that Alvand granite is barren. Mineralography studies did not recognized any tin and tungsten minerals. The grains of gold was recognized in some of the microscopic thin sections. Calcopyrotite is the most important ore mineral that is accompanied with oxides and iron carbonates. The contacts of aplite-pegmatite dikes with granitoide rocks mostly are not prolific. For recognizing Scheelite, some samples of rocks studied by ultraviolet and few Scheelite is recognized in the samples. Some alteration zone observed in this area but they are not accompany with main mineralization. Although the expanded turmalinization has occurred but it is not seen any mineralization. Unless some small part of Alvand complex (i.e. stibnite in the Faghireh area, Maniijou and Aliani, 2000) and based on the main geochemical index, Alvand granitod pluton is classified as a barren type granite. References: Ghalamghash, J., Mirnejad, H., Rashid, H., Mohammadiha, K., Ghahraei-poor, M., Zakeri, M., 2007. Geochemistry of gabbro, granite and mafic enclaves of Alvand batholithe and their origin. 25th conference of geoscinces, Geological Survey of Iran. Maniijou, M., and Aliani, F., 2000. Stibnite mineralization in the Hamedan, west Iran. Mineral Deposits at the Beginning of the 21st Century: Proceedings of the Joint Sixth Biennial SGA-SEG Meeting, Krakow, Poland, 26-29 August 2001 By Adam Piestrzyński, Society for Geology Applied to Mineral Deposits, Society of Economic Geologists (U.S.) Published by Taylor & Francis. ISBN 9026518463, 9789026518461. 1148 pages. Sepahi, Ali A, 2008. Typology and petrogenesis of granitic rocks in the Sanandaj-Sirjan metamorphic belt, Iran: with emphasis on the Alvand plutonic complex. Neues Jahrbuch für Geologie Und Palaeontologie. Abhandlungen, 2008; 247 (3), 295-312.

1.88 Ga São Gabriel AMCG association in the southernmost Uatumã-Anauá Domain: Petrological implications for post-collisional A-type magmatism in the Amazonian Craton

NASA Astrophysics Data System (ADS)

Valério, Cristóvão da Silva; Macambira, Moacir José Buenano; Souza, Valmir da Silva; Dantas, Elton Luiz; Nardi, Lauro Valentim Stoll

2018-02-01

In the southernmost Uatumã-Anauá Domain, central Amazonian craton (Brazil), crop out 1.98 Ga basement inliers represented by (meta)leucosyenogranites and amphibolites (Igarapé Canoas Suite), 1.90-1.89 Ga high-K calc-alkaline granitoids (Água Branca Suite), a 1.88-1.87 Ga alkali-calcic A-type volcano-plutonic system (Iricoumé-Mapuera), Tonian SiO2-satured alkaline granitoids, 1.45-1.25 Ga orthoderived metamorphic rocks (Jauaperi Complex) and Orosirian-Upper Triassic mafic intrusions. New data on petrography, multielementar geochemistry, in situ zircon U-Pb ages and Nd and Hf isotopes of alkali-calcic A-type granites (São Gabriel Granite, Mapuera Suite) and related rocks are indicative of a 1.89-1.87 Ga volcano-plutonic system integrated to the São Gabriel AMCG association. Its magmatic evolution was controlled by the fractional crystallization combined with magma mixing and cumulation processes. Nd isotope values (εNdt values = - 3.71 to + 0.51 and Nd TDM model age = 2.44 to 2.12 Ga) and U-Pb inherited zircon crystals (2115 ± 22 Ma; 2206 ± 21 Ma; 2377 ± 17 Ma, 2385 ± 17 Ma) of the São Gabriel system indicate a large participation of Siderian-Rhyacian crust (granite-greenstones and granulites) and small contribution of Rhyacian mantelic magma. εHft values (+ 5.2 to - 5.8) and Hf TDM ages (3.27-2.14 Ga) also point to contribution of Paleoarchean-Rhyacian crustal melts and small participation of Siderian-Rhyacian mantle melts. Residual melts from the lower crust have been mixed with basaltic melts generated by partial melting of the subcontinental lithospheric mantle (peridotite) in a post-collisional setting at 1.89-1.87 Ga. The mafic melts of such a mixture could have been originated through partial melting of residual ocean plate fragments (eclogites) which ascended onto a residual mantle wedge (hornblende peridotite) and melted it, resulting in modified basaltic magma which, by underplating, led heat to the anatexis of the lower continental crust. This widespread alkali-calcic A-type magmatism was caused by destabilization and magmatic reactivation in the Ventuari-Tapajós and Central Amazonian Provinces, resulting from the Ventuari-Tapajós collision periods. Younger igneous reactivations have been registered at 1.45 to 1.25 Ga, 1.0 Ga and 100 Ma, culminating with the production of mafic and felsic alkaline magmas, along of the northern border of the Amazon Basin.

Quantitative measure of the variation in fault rheology due to fluid-rock interactions

USGS Publications Warehouse

Blanpied, M.L.; Marone, C.J.; Lockner, D.A.; Byerlee, J.D.; King, D.P.

1998-01-01

We analyze friction data from two published suites of laboratory tests on granite in order to explore and quantify the effects of temperature (T) and pore water pressure (Pp) on the sliding behavior of faults. Rate-stepping sliding tests were performed on laboratory faults in granite containing "gouge" (granite powder), both dry at 23?? to 845??C [Lockner et al., 1986], and wet (Pp = 100 MPa) at 23?? to 600??C [Blanpied et al., 1991, 1995]. Imposed slip velocities (V) ranged from 0.01 to 5.5 ??m/s, and effective normal stresses were near 400 MPa. For dried granite at all temperatures, and wet granite below -300??C, the coefficient of friction (??) shows low sensitivity to V, T, and Pp. For wet granite above -350??, ?? drops rapidly with increasing T and shows a strong, positive rate dependence and protracted strength transients following steps in V, presumably reflecting the activity of a water-aided deformation process. By inverting strength data from velocity stepping tests we determined values for parameters in three formulations of a rate- and state-dependent constitutive law. One or two state variables were used to represent slip history effects. Each velocity step yielded an independent set of values for the nominal friction level, five constitutive parameters (transient parameters a, b1, and b2 and characteristic displacements Dcl and Dc2), and the velocity dependence of steady state friction ?????ss/??? In V = a-b1-b2. Below 250??, data from dry and most wet tests are adequately modeled by using the "slip law" [Ruina, 1983] and one state variable (a = 0.003 to 0.018, b = 0.001 to +0.018, Dc ??? 1 to 20 ??m). Dried tests above 250?? can also be fitted with one state variable. In contrast, wet tests above 350?? require higher direct rate dependence (a = 0.03 to 0.12), plus a second state variable with large, negative amplitude (b2 = -0.03 to -0.14) and large characteristic displacement (Dc2 = 300 to >4000 ??m). Thus the parameters a, b1, and b2 for wet granite show a pronounced change in their temperature dependence in the range 270?? to 350??C, which may reflect a change in underlying deformation mechanism. We quantify the trends in parameter values from 25?? to 600??C by piecewise linear regressions, which provide a straightforward means to incorporate the full constitutive response of granite into numerical models of fault slip. The modeling results suggest that the succeptibility for unstable (stick-slip) sliding is maximized between 90?? and 360??C, in agreement with laboratory observations and consistent with the depth range of earthquakes on mature faults in the continental crust.

Role of mantle dynamics in rebuilding the Tianshan Orogenic Belt in NW China: A seismic tomographic investigation

NASA Astrophysics Data System (ADS)

He, Chuansong; Santosh, M.

2018-05-01

The Tianshan orogenic belt, Junggar terrane and Altai terrane are located at the southwestern part of the Central Asian Orogenic Belt (CAOB). Here, we investigate the velocity structure beneath the Xinjiang region in NW China, which includes the Tarim terrane, Tianshan orogenic belt, Junggar terrane and Altai terrane with a view to evaluate the mantle dynamics based on teleseismic data recorded by 103 seismic stations. Our tomographic results show both high and low velocity perturbations beneath the Tianshan orogenic belt. We suggest that the high velocity perturbations beneath this orogenic belt might represent the northward subducted lithosphere of the Tarim Basin and the southward subducted lithosphere of the Junggar Basin. The low velocity structure beneath the Tianshan orogenic belt might represent asthenosphere upwelling that triggered the extensive magmatism which contributed to rebuilding of the Tianshan orogenic belt.

Petrology and geochemistry of charnockites (felsic ortho-granulites) from the Kerala Khondalite Belt, Southern India: Evidence for intra-crustal melting, magmatic differentiation and episodic crustal growth

NASA Astrophysics Data System (ADS)

Ravindra Kumar, G. R.; Sreejith, C.

2016-10-01

The Kerala Khondalite Belt (KKB) of the southern India encompasses volumetrically significant magmatic components. Among these, orthopyroxene-bearing, felsic ortho-granulites, popularly known as charnockites in Indian context, constitute an important lithology. In contrast to the well-known phenomena of arrested charnockitization, the geochemical characteristics and petrogenesis of these ortho-granulite suites remain poorly studied, leaving geodynamic models envisaged for the KKB highly conjectural. In this paper, we try to bridge this gap with detailed results on orthopyroxene-bearing, felsic ortho-granulites spread over the entire KKB and propose a new petrogenetic and crustal evolution model. Based on geochemical characteristics, the orthopyroxene-bearing, felsic ortho-granulites (charnockites sensu lato) of KKB are classified into (1) tonalitic (TC), (2) granitic (GC), and (3) augen (AC) suites. Members of the TC follow sodic (characterized by decreasing CaO/Na2O), whereas those of the GC and AC follow calc-alkaline trends of differentiation. Geochemical patterns of the TC resemble those of the Archaean tonalite-trondhjemite-granodiorite (TTG) suites, with slightly magnesian character (average Mg# = 33), moderate LREE (average LaN = 154), low HREE (average YbN = 6) and Y (1-53 ppm; average 11 ppm). The TC is also characterized by positive to slightly negative europium anomalies (Eu/Eu* = 0.7 to 1.67). The GC and AC suites, on the other hand, resemble post-Archaean arc-related granites. The GC displays ferroan nature (average Mg# = 22), low to moderate degrees of REE fractionation (average [La/Yb]N = 34.84), high contents of Y (5-128 ppm; average 68), and low Sr/Y (1-98) ratios. Significant negative Eu anomalies (Eu/Eu* = 0.18-0.91; average 0.50) and low Sr (65-690 ppm) are also noted in the GC. Similar chemical characteristics are shown by the AC, with ferroan nature (average Mg# = 21), low to moderate degrees of REE fractionation (average [La/Yb]N = 26), high contents of Y (71-99 ppm; average 87), and low Sr/Y (average 2) ratios with significant negative Eu anomalies (Eu/Eu* = 0.03-0.31; average 0.23) and low Sr (average 160 ppm) contents. The protoliths of the TC are interpreted as being derived from partial melting of thickened oceanic-arc crust composed of Archaean mafic composite source rocks (i.e., eclogite and/or garnet amphibolite) with a garnet amphibolite residue. Geochemical features of the GC, such as high Rb/Sr (average 1.80) and Ba/Sr ratios (average values > 6), are considered as evidence for crustal reworking in their genesis, suggesting remelting of a quartzo-feldspathic (TTG) source, within the plagioclase stability fields. The geochemical features of the felsic ortho-granulite suite, substantiated with published geochronological data on members of the TC, GC, and AC suites, suggest a four-stage crustal evolution of the KKB. The first stage is marked by the formation of an over-thickened oceanic-arc. Zircon Hfc model ages of the TC and GC suites constrain the time of this juvenile magmatic crust-forming event as Meso- to Neoarchaean (2.8 to 2.6 Ga). The second stage corresponds to the production of TTG magmas by melting of the over-thickened oceanic-arc crust, subsequent to basaltic underplating during Palaeoproterozoic (ca. 2.1 Ga). The third stage was initiated by a transition in subduction style from shallow to steep due to continent-arc accretion. This stage is marked by the formation of granitic magmas through partial melting of the TTG crust and their differentiation into GC and TC. The zircon crystallization ages (1.89 and 1.85 Ga) of the GC indicate arc accretion occurred during the Palaeoproterozoic. The fourth stage of crustal evolution is correlated with the Mesoproterozoic ( 1.5 Ga) emplacement of megacrystic K-feldspar granites (protoliths to the AC and augen gneisses). The distinct petrography, geochemistry and crystallization ages of the AC suggests recurrence of megacrystic, high-K calc-alkaline granitoids as the product of final phases of crustal-remelting marking subduction cessation. All these magmatic events are fairly well correlated with the major episodes of crustal growth observed in the once contiguous continental fragments (Sri Lanka and Madagascar) and worldwide events (2.7, 1.9, and 1.2 Ga) implying similar episodic nature in the lower crustal evolution of the KKB.

The "granite pump": LP/HT metamorphism and exhumation in the Montagne Nore (S-France)

NASA Astrophysics Data System (ADS)

Franke, W.; Doublier, M. P.; Doerr, W.; Stein, E.

2003-04-01

The Montagne Noire at the southern margin of the French Massif Central represents an exceptional case of a hot metamorphic core complex evolved from a thrust stack in a foreland position. The core of the structure (Zone Axiale) exposes granites and LP/HT gneisses up to anatectic grade. The hot core is encased by ENE-trending shear zones, which define a dextral pull-apart structure. Ductile extension is documented by top WSW shearing in the W, and ENE shearing in the E part of the Zone Axiale (eg, MATTE et al., 1998). Extension in ENE and reduction of the metamorphic profile are accompanied by NNW-directed contraction ("pinched pull-apart"). Palaeozoic sediments on the southern flank of the Zone Axiale exhibit only greenschist to diagenetic grades of metamorphism. Conodont alteration index (WIEDERER et al., 2002) and illite crystallinity (Doublier, this meeting) reveal a decrease of metamorphic temperature away from the hot core. Metamorphic isograds cut across the axial planes of D1 nappes. These features suggest that metamorphism was imposed by the rising hot core. Accordingly, the palaeozoic sediments show a tectonic evolution which closely resembles that of the gneissic core (extension top ENE, contraction in NNW). Structures relating to stacking (D1) have survived at the southern margin of the Montagne Noire. U-Pb studies (TIMS on single zircon and monazite) reveal peak metamorphism and magmatism already at c. 315 Ma (KLAMA et al., 2001), i.e., only <10 Ma after the end of flysch deposition in latest Visean/Early Namurian time (<= 323 Ma). The coincidence, within error, of the U-Pb ages and earlier Ar/Ar ages (MALUSKI et al., 1991) suggest rapid cooling. Synchronous granite emplacement and metamorphism is best explained by advective heating. Since granites are not generated in foreland settings, we propose derivation of the melts from areas of thickened crust adjacent to the N. Transport and emplacement of granites was essentially driven by the hydraulic gradient between the locus of melt generation in the orogenic root, and the opening pull-apart window. Such a pumping model may also be applied to other thermal anomalies in the Variscan Belt, e.g., in the SW-Bohemian Transverse Zone (FRANKE 2000), or in the Saxonian Granulites (FRANKE and STEIN 2000). Hydraulic expulsion of hot, low viscously materials has played an important role in the transport of heat for the hot Variscan root to higher and more external parts of the crust. FRANKE (2000); Geol. Soc. Spec. Publ. No. 179, 35-63. FRANKE and STEIN (2000); Geol. Soc. Spec. Publ. No. 179, 337-355. KLAMA et al. (2001); J. Conf. Abs.,6, 235. MALUSKI et al. (1991); Lithos, 26: 287-304. MATTE et al. (1998); Geodynamica Acta: 13-22. WIEDERER et al. (2002); Schweiz. Mineral. Petrogr. Mitt. 82, 393-407.

The Phillips pluton, Maine, USA: evidence of heterogeneous crustal sources and implications for granite ascent and emplacement mechanisms in convergent orogens

NASA Astrophysics Data System (ADS)

Pressley, Rachel A.; Brown, Michael

1999-03-01

The Phillips pluton (age of 403.8±1.3 Ma) was assembled at a crustal level below the contemporary brittle-plastic transition during regional dextral-reverse transpressive deformation. The pluton is composed dominantly of medium- to coarse-grained leucogranite sensu lato (s.l.), but within its bounds includes decametric massive outcrop of fine- to medium-grained granodiorite (s.l.). In places, the leucogranite contains centimetric enclaves apparently of the granodiorite. Granodiorite is host to more biotite than muscovite, and more calcic, oscillatory-zoned plagioclase, compared to the leucogranite. Pegmatitic granite and composite pegmatite-aplite occur as metric sheets within the pluton and as larger bodies outside the pluton to the SW. Magmatic fabrics, defined by biotite schlieren, occur locally in the leucogranite; the attitude of these fabrics and layering within the leucogranite are concordant with the NE-striking, steeply-dipping country rock foliation. K 2O contents, Rb/Sr ratios, Rb, Sr and Ba covariations, and chondrite-normalized rare earth element (REE) patterns of leucogranite are consistent with high-to-moderate a(H 2O) muscovite dehydration equilibrium eutectic melting of a predominantly pelite source similar to metasedimentary rocks of the surrounding central Maine belt (CMB). The REE patterns and Rb/Sr ratios of granodiorite also suggest derivation from a metasedimentary source, but more likely by moderate-to-low a(H 2O) (muscovite-) biotite dehydration equilibrium eutectic to non-eutectic (minimum) melting of a protolith dominated by greywacke in which garnet and plagioclase were residual phases. Both granite (s.l.) types have heterogeneous initial Nd isotope compositions. Samples of granodiorite define a range in ɛNd (404 Ma) of -1.8 to +0.1 (±0.3 2 σ uncertainty), and samples of leucogranite define a range in ɛNd (404 Ma) of -8.0 to -5.3 (±0.3 2 σ uncertainty). This bimodal distribution suggests that melts were derived from a minimum of two sources. The data are consistent with these sources being CMB metasedimentary rocks ( ɛNd (404 Ma)<-4) for the leucogranite, and Avalon-like (peri-Gondwanan) metasedimentary crust ( ɛNd (404 Ma)>-4) for the granodiorite. The range of Nd isotope compositions within each granite type most likely reflects isotopic heterogeneity inherited from the source. These data imply that the integrity of individual melt batches was maintained during ascent, and that extensive mixing of melt batches during emplacement at this level in the pluton did not occur, although centimetric enclaves have intermediate Nd isotope compositions consistent with small-scale interactions between magmas. We infer that the Phillips pluton represents the root of a larger pluton, and that what remains of this larger pluton is the feeder constructed from multiple melt batches arrested during waning flow of granite magma through a crustal-scale shear zone system.

Flow of ultra-hot Precambrian orogens and the making of crustal layering in Phanerozoic orogenic plateaux

NASA Astrophysics Data System (ADS)

Chardon, Dominique; Gapais, Denis; Cagnard, Florence; Jayananda, Mudlappa; Peucat, Jean-Jacques

2010-05-01

Reassessment of structural / metamorphic properties of ultra-hot Precambrian orogens and shortening of model weak lithospheres support a syn-convergence flow mode on an orogen scale, with a large component of horizontal finite elongation parallel to the orogen. This orogen-scale flow mode combines distributed shortening, gravity-driven flow, lateral escape, and three-dimensional mass redistribution of buried supracrustal rocks, magmas and migmatites in a thick fluid lower crust. This combination preserves a nearly flat surface and Moho. The upper crust maintains a nearly constant thickness by real-time erosion and near-field clastic sedimentation and by ablation at its base by burial of pop-downs into the lower crust. Steady state regime of these orogens is allowed by activation of an attachment layer that maintains kinematic compatibility between the thin and dominantly plastic upper crust and a thick "water bed" of lower crust. Because very thin lithospheres of orogenic plateaux and Precambrian hot orogens have similar thermomechanical structures, bulk orogenic flow comparable to that governing Precambrian hot orogens should actually operate through today's orogenic plateaux as well. Thus, syn-convergence flow fabrics documented on exposed crustal sections of ancient hot orogens that have not undergone collapse may be used to infer the nature of flow fabrics that are imaged by geophysical techniques beneath orogenic plateaux. We provide a detailed geological perspective on syn-convergence crustal flow in relation to magma emplacement and partial melting on a wide oblique crustal transition of the Neoarchean ultra-hot orogen of Southern India. We document sub-horizontal bulk longitudinal flow of the partially molten lower crust over a protracted period of 60 Ma. Bulk flow results from the interplay of (1) pervasive longitudinal transtensional flow of the partially molten crust, (2) longitudinal coaxial flow on flat fabrics in early plutons, (3) distributed, orogen-normal shortening, (4) emplacement of late prolate shape plutons in the direction of flow, and (5) late, conjugate strike-slip shearing. The macroscopic- to regional scale tectonoplutonic pattern produced by longitudinal flow forms a flat composite anisotropy throughout the lower crust. In the light of GPS data, these results suggest that bulk longitudinal flow accounts for observed deformation of the Tibetan plateau as well as for its seismic structure. This flow mode may be preferred to lateral, east-directed channel flow because it combines both lateral gravity-driven thinning and distributed, orogen-normal shortening of the crust. These results further suggest that lower crustal seismic reflectivity in orogenic belts may not necessarily images fabrics produced by extensional tectonics, as commonly thought, but crustal layering produced by syn-convergence lateral flow.

Mantle evolution in the Variscides of SW England: Geochemical and isotopic constraints from mafic rocks

NASA Astrophysics Data System (ADS)

Dupuis, Nicolle E.; Murphy, J. Brendan; Braid, James A.; Shail, Robin K.; Nance, R. Damian

2016-06-01

The geology of SW England has long been interpreted to reflect Variscan collisional processes associated with the closure of the Rhenohercynian Ocean and the formation of Pangea. The Cornish peninsula is composed largely of Early Devonian to Late Carboniferous volcanosedimentary successions that were deposited in pre- and syn-collisional basins and were subsequently metamorphosed and deformed during the Variscan orogeny. Voluminous Early Permian granitic magmatism (Cornubian Batholith) is broadly coeval with the emplacement of ca. 280-295 Ma lamprophyric dykes and flows. Although these lamprophyres are well mapped and documented, the processes responsible for their genesis and their relationship with regional Variscan tectonic events are less understood. Pre- to syn-collisional basalts have intra-continental alkalic affinities, and have REE profiles consistent with derivation from the spinel-garnet lherzolite boundary. εNd values for the basalts range from + 0.37 to + 5.2 and TDM ages from 595 Ma to 705 Ma. The lamprophyres are extremely enriched in light rare earth elements, large iron lithophile elements, and are depleted in heavy rare earth elements, suggesting a deep, garnet lherzolite source that was previously metasomatised. They display εNd values ranging from - 1.4 to + 1.4, initial Sr values of ca. 0.706, and TDM ages from 671 Ma to 1031 Ma, suggesting that metasomatism occurred in the Neoproterozoic. Lamprophyres and coeval granite batholiths of similar chemistry to those in Cornwall occur in other regions of the Variscan orogen, including Iberia and Bohemia. By using new geochemical and isotopic data to constrain the evolution of the mantle beneath SW England and the processes associated with the formation of these post-collisional rocks, we may be able to gain a more complete understanding of mantle processes during the waning stages of supercontinent formation.

Proterozoic geochronological links between the Farewell, Kilbuck, and Arctic Alaska terranes

USGS Publications Warehouse

Bradley, Dwight C.; McClelland, William C.; Friedman, Richard M.; O'Sullivan, Paul B.; Layer, Paul; Miller, Marti L.; Dumoulin, Julie A.; Till, Alison B.; Abbott, J. Grant; Bradley, Dan B.; Wooden, Joseph L.

2014-01-01

New U-Pb igneous and detrital zircon ages reveal that despite being separated by younger orogens, three of Alaska’s terranes that contain Precambrian rocks—Farewell, Kilbuck, and Arctic Alaska—are related. The Farewell and Kilbuck terranes can be linked by felsic magmatism at ca. 850 Ma and by abundant detrital zircons in the Farewell that overlap the ca. 2010–2085 Ma age range of granitoids in the Kilbuck. The Farewell and Arctic Alaska terranes have already been linked via correlative Neoproterozoic to Devonian carbonate platform deposits that share nearly identical faunas of mixed Siberian and Laurentian affinity. New igneous ages strengthen these ties. Specifically, 988, 979, and 979 Ma metafelsites in the Farewell terrane are close in age to a 971 Ma granitic orthogneiss in the Arctic Alaska terrane. Likewise, 852, 850, 845, and 837 Ma granitic orthogneisses, metafelsite, and rhyolite in the Farewell terrane are similar to the reported 874 to 848 Ma age range of metarhyolites in the Arctic Alaska terrane. The Kilbuck and Arctic Alaska terranes have been previously linked on the basis of provenance: detrital zircons from the Carboniferous Nuka Formation in the Arctic Alaska terrane range from 2013 to 2078 Ma, overlapping the age of Kilbuck granitoids. A new 849 Ma age of a Kilbuck granitoid strengthens the proposed connection. Among the other new results from Kilbuck terrane is a 2085 Ma zircon from a granitoid that now stands as the oldest tightly dated rock in Alaska. We conclude that the Kilbuck, Farewell, and Arctic Alaska terranes were not independent entities with unique geologic histories but instead are related pieces of the circum-Arctic tectonic puzzle.

Petrogenesis of the mineralized granitoids from the Kounrad and Borly porphyry Cu deposits and the East Kounrad porphyry Mo deposit in Kazakhstan: Implication for tectonic evolution and mineralization of the western part of the Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Shen, Ping; Pan, Hongdi; Seitmuratova, Eleonora

2017-08-01

The Kounrad region, located in the western part of the Central Asian Orogenic Belt, hosts the Kounrad porphyry Cu, the Borly porphyry Cu-Mo, and the East Kounrad porphyry Mo deposits. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating indicates that the mineralized granitoids from the Kounrad, the Borly and the East Kounrad deposits emplaced at 331.7 ± 2.2 Ma, 311.6 ± 2.6 Ma, and 295.4 ± 2.9 Ma, respectively. The mineralized granodiorite porphyries at Kounrad show a geochemical affinity to adakitic rocks with high Sr (357-670 ppm), Sr/Y (40-68) and Mg numbers (Mg# = molar Mg/(Mg + Fe2 +)) from 0.43 to 0.51, low Yb (0.97-1.1 ppm) and Y (8.3-11.1 ppm). They have variable Sr-Nd-Hf-O isotopic compositions ((87Sr/86Sr)i = 0.7046 to 0.7051, εNd(t) = - 0.1 to + 1.1, εHf(t) = + 5.2 to + 9.0, δ18O = + 5.7 to + 6.8). These features indicate that the Kounrad adakitic magmas derived from the MASH (melting, assimilation, storage, homogenization) zone at depth of 40 km with 5-15% ancient basement rocks contamination. The mineralized granodiorite porphyries at Borly have a normal arc magma geochemical signature (e.g., enrichment of light rare earth elements (LREE) and depletion of heavy REE, Nb and Ti) and experienced fractional crystallization. They also have variable Sr-Nd-Hf-O isotopic compositions ((87Sr/86Sr)i = 0.7047 to 0.7053, εNd(t) = 0 to - 1.3, εHf(t) = - 0.6 to + 7.4, δ18O = + 5.5 to + 6.7) and Mg# (0.45 to 0.51), indicating that they were generated by melting of juvenile basaltic lower crust with normal thickness, followed by 10-30% ancient crustal contamination. The East Kounrad mineralized intrusions, consisting of granite and leucogranite, have experienced advanced degrees of fractional crystallization and have similar Sr-Nd-Hf-O isotopic compositions ((87Sr/86Sr)i = 0.7048, ɛNd (t) = + 1.0 to + 2.8, εHf(t) = + 4.8 to + 8.9, δ18O = + 5.6 to + 6.4) and low Mg# (0.18-0.37), indicating a juvenile lower crust source at depths of < 27 km with 10-20% ancient crustal contamination. The link between geochemistry and mineralization indicate that the subduction of the Junggar-Balkhash Ocean at 332 Ma resulted in the formation of the Kounrad adakitic rocks and assocatied Cu deposit, and continuing subduction at around 311 Ma resulted in the formation of the Borly granodiorite porphyry and assocatied Cu-Mo deposit, but entered a collisional stage by the Early-Permian ( 295 Ma) and resulted in the formation of the East Kounrad granite and leucogranite and assocatied Mo deposit. It is likely that evolution from porphyry Cu to Mo mineralization in the Kounrad region is owing to tectonic evolution from subduction to collision settings, crust depth variation from thickness to thinness, and magma evolution from adaktic and normal arc magma to highly fractionated I-type granitic magma.

The Wisconsin magmatic terrane: An Early Proterozoic greenstone-granite terrane formed by plate tectonic processes

NASA Technical Reports Server (NTRS)

Schulz, K. J.; Laberge, G. L.

1986-01-01

The Wisconsin magmatic terrane (WMT) is an east trending belt of dominantly volcanic-plutonic complexes of Early Proterozoic age (approx. 1850 m.y.) that lies to the south of the Archean rocks and Early Proterozoic epicratonic sequence (Marquette Range Supergroup) in Michigan. It is separated from the epicratonic Marquette Range Supergroup by the high-angle Niagara fault, is bounded on the south, in central Wisconsin, by Archean gneisses, is truncated on the west by rocks of the Midcontinent rift system, and is intruded on the east by the post-orogenic Wolf river batholith. The overall lithologic, geochemical, metallogenic, metamorphic, and deformational characteristics of the WMT are similar to those observed in recent volcanic arc terranes formed at sites of plate convergence. It is concluded that the WMT represents an evolved oceanic island-arc terrane accreated to the Superior craton in the Early Proterozoic. This conclusion is strengthened by the apparent absence of Archean basement from most of the WMT, and the recent recognition of the passive margin character of the epicratonic Marquette Range Supergroup.

Les intrusions de Wirgane (Haut Atlas occidental, Maroc): témoins d'un magmatisme syn- à tardi-cinématique hercynien? (Intrusions of Wirgane [western High Atlas, Morocco]: evidence for a syn- to late kinematic magmatism of Variscan age?)

NASA Astrophysics Data System (ADS)

Eddif, A.; Gasquet, D.; Hoepffner, C.; Ayad, N. Ait

2000-11-01

The Wirgane intrusives were emplaced into the Late Neoproterozoic to Palæozoic series of the northeast of the Moroccan western High Atlas. The intrusions exhibit a large compositional range from monzogabbro to granite, and they have suffered, together with the country rocks, part of the Variscan tectonic evolution. In the immediate vicinity of the intrusions, thermal metamorphism developed in the country rocks. According to the mineral chemistry of igneous amphibole compositions of diorites and metamorphic minerals, the depth of intrusives was estimated to be less than 11 km. Strain patterns, mapped in both the plutons and the country rocks, and microtectonic data indicate that the intrusions were emplaced in a dextral transcurrent shearing context during the Variscan Orogen. When compared with other intrusions of the western High Atlas (Tichka, Azegour), the Wirgane intrusives are considered to be related to the late stages of the Variscan Belt of Morocco.

Polyphase Neoproterozoic orogenesis within the east Africa- Antarctica orogenic belt in central and northern Madagascar

USGS Publications Warehouse

Key, R.M.; Pitfield, P.E.J.; Thomas, Ronald J.; Goodenough, K.M.; Waele, D.; Schofield, D.I.; Bauer, W.; Horstwood, M.S.A.; Styles, M.T.; Conrad, J.; Encarnacion, J.; Lidke, D.J.; O'connor, E. A.; Potter, C.; Smith, R.A.; Walsh, G.J.; Ralison, A.V.; Randriamananjara, T.; Rafahatelo, J.-M.; Rabarimanana, M.

2011-01-01

Our recent geological survey of the basement of central and northern Madagascar allowed us to re-evaluate the evolution of this part of the East Africa-Antarctica Orogen (EAAO). Five crustal domains are recognized, characterized by distinctive lithologies and histories of sedimentation, magmatism, deformation and metamorphism, and separated by tectonic and/or unconformable contacts. Four consist largely of Archaean metamorphic rocks (Antongil, Masora and Antananarivo Cratons, Tsaratanana Complex). The fifth (Bemarivo Belt) comprises Proterozoic meta-igneous rocks. The older rocks were intruded by plutonic suites at c. 1000 Ma, 820-760 Ma, 630-595 Ma and 560-520 Ma. The evolution of the four Archaean domains and their boundaries remains contentious, with two end-member interpretations evaluated: (1) all five crustal domains are separate tectonic elements, juxtaposed along Neoproterozoic sutures and (2) the four Archaean domains are segments of an older Archaean craton, which was sutured against the Bemarivo Belt in the Neoproterozoic. Rodinia fragmented during the early Neoproterozoic with intracratonic rifts that sometimes developed into oceanic basins. Subsequent Mid- Neoproterozoic collision of smaller cratonic blocks was followed by renewed extension and magmatism. The global 'Terminal Pan-African' event (560-490 Ma) finally stitched together the Mid-Neoproterozoic cratons to form Gondwana. ?? The Geological Society of London 2011.

Erosional Reduction of an Orogenic Wedge: Structural Response to Neogene Climate Change within the St. Elias Orogen, Alaska

NASA Astrophysics Data System (ADS)

Berger, A. L.; Spotila, J. A.; Chapman, J. B.; Pavlis, T. L.; Enkelmann, E.; Buscher, J. T.

2007-12-01

The kinematics and architecture of orogenic systems may be heavily influenced by climate, but little research has focused on the long term effects of glacial erosion on orogenesis. Apatite and zircon (U-Th)/He thermochronometry on >75 bedrock samples across the St. Elias orogen, one of the best examples of a glaciated orogenic wedge, is the basis for a new kinematic model and demonstrates an association between glacial denudation and orogenic architecture. The spatial pattern of low temperature cooling indicates that exhumation and deformation are focused within a thin-skinned fold and thrust belt on the windward flank, whereas the leeward flank of the orogen functions as a deformational backstop. A previously unrecognized structure beneath the Bagley ice field must separate these domains with south-side-up motion. We propose this structure is a backthrust making the orogen doubly-vergent. Suggestive of accelerated backthrust motion in response to climate change, cooling rates within the hanging wall block and across the entire windward flank of the orogen accelerated ten-fold coeval with enhanced glaciation. As backthrust motion increased, glacial unroofing also coincided with a regional shift in deformation away from prominent forethrusts including the North American-Yakutat terrane suture (Chugach St. Elias fault) and the seaward deformation front (Pamplona zone). Across the windward flank of the orogen, exhumation, at rates of up to 5 mm/yr, is focused within a narrow zone, where the glacial equilibrium line altitude (ELA) intersects the orogenic wedge. This zone of rapid exhumation, not present prior to the onset of enhanced glaciation, cuts across the structural trend of the orogen and is more narrowly focused than orographic precipitation. Accelerated denudation at the ELA thus appears to have redistributed strain along a series of forethrusts that lie at the zone of heaviest glacial flux, while the backthrust progressively truncates the southward-vergent forethrusts. In a cause and effect response, the expansion of glaciers therefore appears to have resulted in a narrowing of the orogenic wedge due to increased backthrust motion and a landward propagation of deformation in order to preserve topographic slope. This focusing of long- term glacial erosion and deformation at the ELA matches predictions from analytical models of orogenic wedges (i.e. Tomkin and Braun, 2002) and implies a high degree of coupling between climate and tectonics in this glacially-dominated orogen.

The Chinese North Tianshan Orogen was a rear-arc (or back-arc) environment in the Late Carboniferous: constraint from the volcanic rocks in the Bogda Mountains

NASA Astrophysics Data System (ADS)

Xie, W.

2017-12-01

The Tianshan Orogen is a key area for understanding the Paleozoic tectonics and long-lasting evolution of the Central Asian Orogenic Belt (CAOB). However, considerable debate persists as to its tectonic setting during the late Paleozoic, with active subduction system and intraplate large igneous provinces as two dominant schools (Ma et al., 1997; Gu et al., 2000; Xiao et al., 2004; Han et al., 2010; Shu et al., 2011; Chen et al., 2011; Xia et al., 2012). With aims of providing constraints on this issue, petrology, mineralogy, geochronological and geochemistry for the Late Carboniferous volcanics from the Bogda Mountains have been carried out. We find two suits of high-Al basalt (HAB, 315-319 Ma) and a suit of submarine pillow basalt ( 311 Ma) in this region. Both of the two basalts belong to the tholeiitic magma (the tholeiitic index THI > 1) and contain low pre-eruptive magmatic H2O (< 2%). High Al content of the Bogda HABs is due to high crystallization pressure rather than water content. It is different from the pillow lavas that are formed in a shallower and more stable magma chamber (Xie et al., 2016a, b). The felsic volcanism coexisted with the Bogda HABs is I-type intermediate ignimbrites and rhyolite lavas. The rhyolites are formed by partial melting of a hydrated and juvenile arc crust and the ignimbrites are affected by magma mingling and feldspar fractionation (Xie et al., 2016c). The two basalts both have the MORB-like Sr-Nd-Hf-Pb isotopes and arc-like trace element compositions. We discuss that they may have been generated from a dry and depleted mantle source metasomatized by <1% sediment-derived melts. Compared with basalts from the Permian large igneous provinces (e.g., the Siberia, Emeishan and Tarim), they are different from the mantle plume-related basalts in many aspects. Meanwhile, we also compare the Bogda basalts with the Izu-Bonin fore-arc and rear-arc/back-arc basalts. Our samples show great resemblance to the Izu-Bonin rear-arc basalt (including the arc-like back-arc basalt). These lines of evidence indicate that these basalts and coexisted felsic volcanics were likely formed in a rear-arc or back-arc environment, probably related to southward subduction of the Paleo-Tianshan Ocean (Xie et al., 2016a, b, c).

Age and geochemistry of Neoproterozoic granitoids in the Songnen-Zhangguangcai Range Massif, NE China: Petrogenesis and tectonic implications

NASA Astrophysics Data System (ADS)

Luan, Jin-Peng; Xu, Wen-Liang; Wang, Feng; Wang, Zhi-Wei; Guo, Peng

2017-10-01

This study presents new zircon U-Pb ages and geochemical data for Neoproterozoic granitoids in the Songnen-Zhangguangcai Range Massif (SZRM) of NE China. This dataset provides insights into the Neoproterozoic tectonic setting of the SZRM and the links between this magmatism and the evolution of the Rodinia supercontinent. The zircon U-Pb dating indicates that the Neoproterozoic magmatism within the SZRM can be subdivided into two stages: (1) a ∼917-911 Ma suite of syenogranites and monzogranites, and (2) an ∼841 Ma suite of granodiorites. The 917-911 Ma granitoids contain high concentrations of SiO2 (67.89-71.18 wt.%), K2O (4.24-6.91 wt.%), and Al2O3 (14.89-16.14 wt.%), and low concentrations of TFe2O3 (1.63-3.70 wt.%) and MgO (0.53-0.88 wt.%). They are enriched in the light rare earth elements (LREE) and the large ion lithophile elements (LILE), are depleted in the heavy REE (HREE) and the high field strength elements (HFSE; e.g., Nb, Ta, and Ti), and have slightly positive Eu anomalies, indicating that they are geochemically similar to high-K adakitic rocks. They have zircon εHf (t) values and TDM2 ages from -4.4 to +1.5 and 1915 Ma to 1592 Ma, respectively, suggesting that they were derived from a primary magma generated by the partial melting of ancient thickened lower crustal material. In comparison, the 841 Ma granodiorites contain relatively low concentrations of Al2O3 (14.50-14.58 wt.%) and K2O (3.27-3.29 wt.%), relatively high concentrations of TFe2O3 (3.78-3.81 wt.%) and the HREE, have negative Eu anomalies, and have zircon εHf (t) values and TDM2 ages from -4.7 to +1.0 and 1875 to 1559 Ma, respectively. These granodiorites formed from a primary magma generated by the partial melting of ancient crustal material. The ∼917-911 Ma magmatism within the SZRM is inferred to have formed in an orogenic setting, whereas the ∼841 Ma magmatism formed in an anorogenic setting related to either a post-orogenic tectonic event or the onset of Neoproterozoic continental rifting. It is proposed that the microcontinental massifs within the eastern central Asian orogenic belt of NE China formed during or following the final stage of assembly of Rodinia before rifting away from the Tarim Craton in response to the Rodinia breakup.

Assessing the isotopic evolution of S-type granites of the Carlos Chagas Batholith, SE Brazil: Clues from U-Pb, Hf isotopes, Ti geothermometry and trace element composition of zircon

NASA Astrophysics Data System (ADS)

Melo, Marilane G.; Lana, Cristiano; Stevens, Gary; Pedrosa-Soares, Antônio C.; Gerdes, Axel; Alkmin, Leonardo A.; Nalini, Hermínio A.; Alkmim, Fernando F.

2017-07-01

The Carlos Chagas batholith (CCB) is a very large ( 14,000 km2) S-type granitic body formed during the syn-collisional stage of the Araçuaí orogen (southeastern Brazil). Zircons extracted from the CCB record a wide range of U-Pb ages (from 825 to 490 Ma), indicating a complex history of inheritance, magmatic crystallization and partial melting during the evolution of the orogeny. Magmatic zircons (ca. 578-588 Ma) are marked by similar Hf isotope compositions and REE patterns to those of inherited cores (ca. 825-600 Ma), indicating that these aspects of the chemical signature of the magmatic zircons have likely been inherited from the source. The U-Pb ages and initial 176Hf/177Hf ratios from anatectic and metamorphic zircon domains are consistent with a two-stage metamorphic evolution marked by contrasting mechanisms of zircon growth and recrystallization during the orogeny. Ti-in-zircon thermometry is consistent with the findings of previous metamorphic work and indicates that the two metamorphic events in the batholith reached granulite facies conditions (> 800 °C) producing two generations of garnet via fluid-absent partial melting reactions. The oldest metamorphic episode (ca. 570-550 Ma) is recorded by development of thin anatectic overgrowths on older cores and by growth of new anatectic zircon crystals. Both domains have higher initial 176Hf/177Hf values compared to relict cores and display REE patterns typical of zircon that grew contemporaneously with peritectic garnet through biotite-absent fluid partial melting reactions. Hf isotopic and chemical evidences indicate that a second anatectic episode (ca. 535-500 Ma) is only recorded in parts from the CCB. In these rocks, the growth of new anatectic zircon and/or overgrowths is marked by high initial 176Hf/177Hf values and also by formation of second generation of garnet, as indicated by petrographic observations and REE patterns. In addition, some rocks contain zircon crystals formed by solid-state recrystallization of pre-existing zircon, which exhibit similar Hf isotope composition to those of inherited/magmatic core domains. The first anatectic event is interpreted as result of crustal thickening after the intrusion of the batholith. This introduced the batholith to a depth in excess of 30 km and produced widespread anatexis throughout the batholith. The second event was associated with asthenospheric upwelling during extensional thinning and gravitational collapse of the orogen, this produced anatexis in parts from the CCB that had been re-fertilized for anatexis by retrogression along shear zones following the first granulite facies event.

A Tale of Two Orogens: Comparing Crustal Processes in the Proterozoic Trans-Hudson and Grenville Orogens, Eastern Canada

NASA Astrophysics Data System (ADS)

Darbyshire, F. A.; Bastow, I. D.; Gilligan, A.; Petrescu, L.

2016-12-01

The Precambrian core of North America is an assemblage of Archean cratons and Proterozoic orogenic belts, preserving over 3 billion years of Earth history. Here we focus on two of the largest collisional orogens, using recent and ongoing seismological studies to probe their present-day structure and tectonic history. The 1.8 Ga collision between the Western Churchill and Superior cratons, along with microcontinental and island arc terranes, formed the Trans-Hudson Orogen (THO), a collisional belt similar in scale and shape to the present-day Himalaya-Karakoram-Tibet Orogen (HKTO). In the Mesoproterozoic, a series of collisions reworked the SE margin of the Superior craton and added new material over a period of several hundred Ma, culminating in the Grenvillian orogeny and the assembly of the supercontinent Rodinia. The Grenville Orogen is thought to have been a large, hot, long-lived plateau which subsequently underwent orogenic collapse. While similar in spatial scale, the Trans-Hudson and Grenville Orogens have significantly different tectonic histories, notably in terms of longevity and tectonic evolution. Comparison of these collisional belts with each other, and with the HTKO, provide valuable insights into plate-tectonic history. Recently a number of broadband seismograph installations have allowed a detailed study of present-day crustal structure beneath the THO and the Grenville. Receiver-function and surface wave studies provide information on crustal thickness variations, bulk crustal composition and crustal heterogeneity. The crust beneath the orogens is generally thicker, more mafic and more heterogeneous than that beneath neighbouring Archean and Phanerozoic domains, with significant along-strike variability and Moho complexity. We review and interpret the new crustal structure information in the context of the tectonic processes affecting the two contrasting orogens.

Architecture, kinematics, and exhumation of a convergent orogenic wedge: A thermochronological investigation of tectonic-climatic interactions within the central St. Elias orogen, Alaska

NASA Astrophysics Data System (ADS)

Berger, Aaron L.; Spotila, James A.; Chapman, James B.; Pavlis, Terry L.; Enkelmann, Eva; Ruppert, Natalia A.; Buscher, Jamie T.

2008-06-01

The kinematics and architecture of orogenic systems along the leading edges of accreting terranes may be heavily influenced by climate, but little research has been devoted to the long-term effects of glacial erosion on orogenesis. Here we use low-temperature apatite and zircon (U-Th)/He and fission-track thermochronometry, along with subsidiary structural relationships and seismicity, to develop a new architectural model of the St. Elias orogen in southern Alaska, which is one of the best examples of a glaciated orogenic wedge worldwide. These data illustrate that the orogen consists of a deformational backstop on the leeward flank and a rapidly deforming and eroding, thin-skinned fold and thrust belt on the windward flank. A structure beneath the Bagley ice field separates these distinct deformational domains, which we propose is a backthrust that makes the orogen doubly-vergent. Thermochronometry within the orogenic wedge suggests that denudation and deformation are strongly influenced by glacial erosion. Long-term exhumation, at rates of up to 4 mm/yr, is concentrated within a narrow zone along the windward flank, where glacier equilibrium lines intersect the orogenic wedge. The onset of enhanced glaciation also coincided with a marked acceleration in exhumation across the orogenic wedge, accelerated backthrust motion, and a major shift in deformation away from the North American-Yakutat terrane suture (Chugach St. Elias fault). We propose that accelerated glacial erosion forced the redistribution of strain along the backthrust and an en echelon array of forethrusts that lie beneath the zone of heaviest glaciation, which in turn are systematically truncated by the backthrust. This focusing of deformation matches predictions from analytical models of orogenic wedges and implies a high degree of coupling between climate and tectonics in this glacially-dominated orogen.

Archean crustal evolution of the Narryer Gneiss Terrane, Western Australia, as revealed by the U-Pb age and Hf-isotope compositions of zircon from the granitic gneisses

NASA Astrophysics Data System (ADS)

Sylvester, P.; Souders, K.; Crowley, J. L.; Myers, J.

2011-12-01

The Narryer Gneiss Terrane of the Yilgarn Craton, Western Australia, is an important area for studies of early crustal evolution because of the preservation of (1) detrital zircons of Hadean to Archean age in the Jack Hills and Mt. Narryer metasedimentary belts, and (2) several widespread units of granitic gneisses emplaced between ca. 3.7 and 2.6 Ga. We have analyzed the U-Pb geochronology and Hf-isotope geochemistry of magmatic zircons from 38 samples of the granitic gneisses using laser ablation - (multicollector) - ICPMS. The sample suite is dominated by the Meeberrie gneiss, a banded quartz-microcline-oligoclase-biotite gneiss of monzogranite to granodiorite composition, and the Dugel gneiss, a leucocratic, pegmatite-layered syenogranite gneiss, but gneisses of dioritic to tonalitic composition, as well as less deformed granite sheets, are also represented. Magmatic zircons were identified on the basis of the preservation of oscillatory zoning in BSE and CL images, igneous Th/U ratios (>0.2), and concordant U-Pb isotopic systematics with low common Pb contents. The results indicate many of the gneisses are composed of the products of multiple magmatic events, as has been reported previously for samples of the Meeberrie gneiss (Kinny & Nutman, 1996, Precambrian Res. 78, 165-178). Major ages of magmatism preserved in the gneisses occurred at ca. 3685-3665 Ma, 3620-3565 Ma, 3495-3440 Ma, 3375-3330 Ma, and 3300-3260 Ma. The late granite sheets crystallized at 2710-2645 Ma. Hf-isotope compositions of the zircons trend to less radiogenic values with decreasing age, with ɛHf values of ca. 0 to -5 for 3.7-3.4 Ga gneisses, ca. -1 to -9 for 3.4-3.2 Ga gneisses and ca. -5 to -20 for the late granite sheets. The array of the Hf isotopic compositions with time for the entire sample set are fit well by a regression indicating a source reservoir with a 176Lu/177Hf of 0.022 extracted from the depleted mantle at 3.9 Ga. This suggests that the Narryer gneisses and late granite sheets were derived by repeated partial melting of an Eoarchean mafic to intermediate crustal source that persisted throughout the Archean. The ɛHf evolution trend for the Narryer gneisses plots distinctly above the ɛHf trend of the Hadean mafic crustal reservoir inferred for the source of the Hadean detrital zircons from the Jack Hills. The implication is that the inferred Hadean crustal reservoir was not an important source of the Narryer gneisses, either because it was largely isolated from zones of melting in the crust of the Narryer Gneiss Terrane during the Archean, or because it was simply of limited volume.

Tectonic mode switches and the nature of orogenesis

NASA Astrophysics Data System (ADS)

Lister, Gordon; Forster, Marnie

2009-12-01

The birth and death of many mountain belts occurs in lithosphere that over-rides major subduction zones. Here the tectonic mode (shortening versus extension) can abruptly switch, even during continuous and otherwise smooth convergence. If the hinge line of the foundering slab rapidly retreats (i.e. rolls back), the foundering slab creates a gravitational potential well into which the orogen collapses. This motion, coupled with stress guides, can "pull" the orogen apart. A slowing of roll-back (or of hinge retreat) means that the subduction flexure may subsequently begin to be "pushed back" or be "pushed over" by the advancing orogen. The consequence of such changes in relative motion is that orogenic belts are affected by abrupt tectonic mode switches. The change from "push" to "pull" leads to a sudden change from horizontal extension to horizontal shortening, potentially throughout the entire mass of the orogenic lithosphere that over-rides the subducting slab. The sequencing of these tectonic mode switches affects the thermal evolution of the orogen, and thus fundamentally determines the nature of orogenesis. This insight led to us to our quite different views as to how orogens work. It is evident that orogens affected by abrupt "push-pull" mode switches are characterized by high-pressure metamorphism, whereas orogens affected by abrupt "pull-push" mode switches are characterized by high-temperature metamorphism, magmatism and anatexis.

Two types of gneisses associated with eclogite at Shuanghe in the Dabie terrane: carbon isotope, zircon U-Pb dating and oxygen isotope

NASA Astrophysics Data System (ADS)

Zheng, Yong-Fei; Gong, Bing; Zhao, Zi-Fu; Fu, Bin; Li, Yi-Liang

2003-10-01

There are two types of gneisses, biotite paragneiss and granitic orthogneiss, to be closely associated with UHP eclogite at Shuanghe in the Dabie terrane. Both concentration and isotope composition of bulk carbon in apatite and host gneisses were determined by the EA-MS online technique. Structural carbonate within the apatite was detected by the XRD and FTIR techniques. Significant 13C-depletion was observed in the apatite with δ13C values of -28.6‰ to -22.3‰ and the carbon concentrations of 0.70-4.98 wt.% CO 2 despite a large variation in δ18O from -4.3‰ to +10.6‰ for these gneisses. There is significant heterogeneity in both δ13C and δ18O within the gneisses on the scale of several tens meters, pointing to the presence of secondary processes after the UHP metamorphism. Considerable amounts of carbonate carbon occur in some of the gneisses that were also depleted in 13C primarily, but subjected to overprint of 13C-rich CO 2-bearing fluid after the UHP metamorphism. The 13C-depleted carbon in the gneisses is interpreted to be inherited from their precursors that suffered meteoric-hydrothermal alteration before plate subduction. Both low δ13C values and structural carbonate in the apatite suggest the presence of 13C-poor CO 2 in the UHP metamorphic fluid. The 13C-poor CO 2 is undoubtedly derived from oxidation of organic matter in the subsurface fluid during the prograde UHP metamorphism. Zircons from two samples of the granitic orthogneiss exhibit low δ18O values of -4.1‰ to -1.1‰, demonstrating that its protolith was significantly depleted in 18O prior to magma crystallization. U-Pb discordia datings for the 18O-depleted zircons yield Neoproterozoic ages of 724-768 Ma for the protolith of the granitic orthogneiss, consistent with protolith ages of most eclogites and orthogneisses from the other regions in the Dabie-Sulu orogen. Therefore, the meteoric-hydrothermal alteration is directly dated to occur at mid-Neoproterozoic, and may be correlated with the Rodinia supercontinental breakup and the snowball Earth event. It is thus deduced that the igneous protolith of the granitic orthogneiss and some eclogites would intrude into the older sequences composing the sedimentary protoliths of the biotite paragneiss and some eclogites along the northern margin of the Yangtze plate at mid-Neoproterozoic, and drove local meteoric-hydrothermal circulation systems in which both 13C- and 18O-depleted fluid interacted with the protoliths of these UHP rocks now exposed in the Dabie terrane.

Petrogenesis of Permian A-type granitoids in the Cihai iron ore district, Eastern Tianshan, NW China: Constraints on the timing of iron mineralization and implications for a non-plume tectonic setting

NASA Astrophysics Data System (ADS)

Zheng, Jiahao; Mao, Jingwen; Chai, Fengmei; Yang, Fuquan

2016-09-01

The geochronology and geochemistry of granitoids in the Eastern Tianshan, NW China provide important constraints on the timing of iron mineralization, as well as in understanding evolution history of the southern Central Asian Orogenic Belt (CAOB). Here we present results from a detailed study on granitoid rocks from the Cihai iron ore district in the Beishan region, southern part of the Eastern Tianshan. The granitoid rocks are composed of granodiorite, quartz monzonite, granite, and monzonite. Zircon U-Pb analyses yielded the ages of 294.1 ± 2.2 Ma, 286.5 ± 0.7 Ma, 284.3 ± 3.3 Ma, and 265.6 ± 3.0 Ma, respectively, suggesting they were formed in Early-Middle Permian. Among these granitoid rocks, the ages of quartz monzonite and granite are close to the timing of iron mineralization ( 282 Ma), indicating they may provide a source of iron in the Cihai ore district. Geochemically, the granodiorite, granite, and quartz monzonite samples are characterized by high FeOt/(FeOt + MgO) and Ga/Al ratios (0.84-0.94 and 2.28-3.27, respectively), as well as high zircon saturation temperatures (781-908 °C), similar to those of typical A-type granitoids. Isotopically, they display consistently depleted Hf isotopic compositions (εHf(t) = + 1.18 to + 15.37). Geological, geochemical, and isotopic data suggest that the Cihai A-type granitoids were derived from melting of juvenile lower crust. Some Early Permian A-type granitoids were recently identified in the Tarim and Eastern Tianshan with the ages between 294 and 269 Ma. The A-type granitoids in the Eastern Tianshan formed earlier between 294-284 Ma and exhibit characteristics of A2 type granitoids, whereas the A-type granitoids in the Tarim formed later between 277-269 Ma and show A1 granitoids affinity. We suggest that the Permian Tarim mantle plume does not account for the formation of the A-type granitoids in the Eastern Tianshan area, and the Eastern Tianshan was in a non-plume tectonic setting during Early Permian time. A slab break-off event has been suggested as the possible tectonic mechanism that triggered the formation of Early Permian A-type granitoids in the Eastern Tianshan.

Crustal thinning and exhumation along a fossil magma-poor distal margin preserved in Corsica: A hot rift to drift transition?

NASA Astrophysics Data System (ADS)

Beltrando, Marco; Zibra, Ivan; Montanini, Alessandra; Tribuzio, Riccardo

2013-05-01

Rift-related thinning of continental basement along distal margins is likely achieved through the combined activity of ductile shear zones and brittle faults. While extensional detachments responsible for the latest stages of exhumation are being increasingly recognized, rift-related shear zones have never been sampled in ODP sites and have only rarely been identified in fossil distal margins preserved in orogenic belts. Here we report evidence of the Jurassic multi-stage crustal thinning preserved in the Santa Lucia nappe (Alpine Corsica), where amphibolite facies shearing persisted into the rift to drift transition. In this nappe, Lower Permian meta-gabbros to meta-gabbro-norites of the Mafic Complex are separated from Lower Permian granitoids of the Diorite-Granite Complex by a 100-250 m wide shear zone. Fine-grained syn-kinematic andesine + Mg-hornblende assemblages in meta-tonalites of the Diorite-Granite Complex indicate shearing at T = 710 ± 40 °C at P < 0.5 GPa, followed by deformation at greenschist facies conditions. 40Ar/39Ar step-heating analyses on amphiboles reveal that shearing at amphibolite facies conditions possibly began at the Triassic-Jurassic boundary and persisted until t < 188 Ma, with the Mafic Complex cooling rapidly at the footwall of the Diorite-Granite Complex at ca. 165.4 ± 1.7 Ma. Final exhumation to the basin floor was accommodated by low-angle detachment faulting, responsible for the 1-10 m thick damage zone locally capping the Mafic Complex. The top basement surface is onlapped at a low angle by undeformed Mesozoic sandstone, locally containing clasts of footwall rocks. Existing constraints from the neighboring Corsica ophiolites suggest an age of ca. 165-160 Ma for these final stages of exhumation of the Santa Lucia basement. These results imply that middle to lower crustal rocks can be cooled and exhumed rapidly in the last stages of rifting, when significant crustal thinning is accommodated in less than 5 Myr through the consecutive activity of extensional shear zones and detachment faults. High thermal gradients may delay the switch from ductile shear zone- to detachment-dominated crustal thinning, thus preventing the exhumation of middle and lower crustal rocks until the final stages of rifting.

Using Zircon Geochronology to Unravel the History of the Naga Hills Ophiolite

NASA Astrophysics Data System (ADS)

Roeder, T.; Aitchison, J. C.; Clarke, G. L.; Ireland, T. R.; Ao, A.; Bhowmik, S. K.

2014-12-01

Outcrops of the Naga Hills Ophiolite (NHO), a possible eastern extension of the ophiolitic belt running along the India-Asia suture, in Northeast India include a full suite of ophiolitic rocks. The ophiolite has been dated Upper Jurassic based on radiolarian studies of the unit (Baxter et al., 2011) but details of its emplacement onto the Indian margin have not been the subject of detailed investigation. Conglomerates of the Phokphur Formation unconformably overlie an eroded surface on top of dismembered ophiolite fragments and include sediments sourced from both the ophiolite and the margin of the Indian subcontinent. Notably no Asian margin-derived detritus is recognised (similar to the Liuqu conglomerates of Tibet (Davis et al., 2002)). Thus, a detailed study of the Phokphur sediments can produce valuable details of the NHO history, including constraining the timing of ophiolite emplacement. Studies of detrital sandstone petrography confirm a recycled orogen provenance for the Phokphur Formation and thus serve as validation of the methods of Dickinson and Suczek (1979) and Garzanti et al. (2007). Detrital zircon data provides further insight as to the age of source rocks of Phokphur sediments and help to further constrain the timing of ophiolite emplacement. We present results of sedimentary and detrital zircon geochronology analyses of Phokphur sediments from outcrops near the villages of Salumi and Wazeho as a contribution to furthering research on aspects of the India-Asia collision. Baxter, A.T., et al. 2011. Upper Jurassic radiolarians from the Naga Ophiolite, Nagaland, northeast India. Gondwana Research, 20: 638-644. Davis, A.M., et al. 2002. Paleogene island arc collision-related conglomerates, Yarlung-Tsangpo suture zone, Tibet. Sedimentary Geology, 150: 247-273. Dickinson, W.R. and Suczek, C.A., 1979. Plate tectonics and sandstone compositions. Am. Assoc. Pet. Geol. Bull., 63, 2164-2182, (1979). Garzanti, E., et al., 2007. Orogenic belts and orogenic sediment provenance. The Journal of Geology, 115: 315-334.

Reworked crustal of early Paleozoic WuYi Orogen revealed by receiver function data

NASA Astrophysics Data System (ADS)

Wei, Y.; Duan, Y.; Tian, X.; Zhao, Y.

2017-12-01

Intraplate orogenic belt, which occurs at the rigid and undeformable plate interiors, is a distinct new type of orogen rather than an interplate or plate marginal orogenic belt, whose deformation occurs exclusively at plate margins. Therefore, intraplate orogenic belts are the most obvious exception to the plate-tectonic paradigm, they are uncommon in Earth's history. The early Paleozoic Wuyi orogen in South China is one of the few examples of intraplate orogen, and is a key to understanding the process of intraplate orogenesis and global early Paleozoic geodynamics. In this study, we select teleseismic records from 45 mobile linear seismic stations deployed in Wuyi Mountain and 58 permanent stations setting in Jiangxi and Fujian provinces, from January 2011 to December 2012, and calculate the crustal thickness and average crustal Vp/Vs ratio using the H-κ stacking method. The main results include the following: 1) the crustal average Poission's ratio shows an increase tendency from land to sea, the interior of Wuyi orogen belt with an low ration less than 0.23, and the coastline with high ration which is up to 0.28, which indicate a very heterogeneous crustal structure and composition in Wuyi orogen and coast belt. 2) the crustal thickness ranges 28-34 km and shows a tendency of thinning from inland to coast in the region of SE China margin, which maight mean the eastern Eurasia lithospheric is extension and thinning induced by the subducted paleo-Pacific slab. To conclusion, we assume that Wuyi orogen experienced upper crustal thickening, lower crust and lithosphere delamination during the early Paleozoic orogeny, and lithosphere extension in Mesozoic. This research is founded by the Natural Science Foundation of China (41174052 and 41604048).

Continuation of the New England Orogen, Australia, beneath the Queensland Plateau and Lord Howe rise

USGS Publications Warehouse

Mortimer, N.; Hauff, F.; Calvert, A.T.

2008-01-01

Greywacke, argillite, greyschist and hypabyssal igneous rocks have been obtained from an Ocean Drilling Program core on the Queensland Plateau and from xenoliths in a volcanic breccia dredged from the crest of the Lord Howe Rise. Low to intermediate detrital quartz contents, 260-240 Ma K-Ar ages, and only moderately radiogenic Sr and Nd isotope compositions, suggest a correlation with the New England Orogen of eastern Australia, rather than with Australia's Lachlan Orogen or other adjacent geological provinces. Our results indicate that the New England Orogen terranes continue towards New Zealand at least as far as the southern Lord Howe Rise. The projected offshore boundaries of the major east Australian orogens are now known with more confidence, and do not appear to require any major cross-orogen offsets.

Tectonomagmatic evolution of the proto Andean Margin: Geochemical characterization and zircon U-Pb geochronologic constraints from the Ecuadorian Eastern Cordilleran granitoids

NASA Astrophysics Data System (ADS)

Buchwaldt, R.; Toulkeridis, T.

2013-05-01

The timing of pan-Pacific Gondwanide Orogeny in the proto-Andes, and its driving mechanisms are still highly debated and relies predominantly upon whole-rock Rb-Sr and K-Ar chronology and rudimentary mineralogy and geochemistry. In order to decipher these uncertainties we have studied the composition, age and provenance of granitoids along the strike of the Eastern Cordillera of Ecuador and related these deep-seated and surface tectonic processes attending the Late Cretaceous-Palaeogene history of the northern Andes. The plutonic rocks constitute a metaluminous to peraluminous (A/CNK ~ 0.8-1.2), calc-alkaline suite. A unimodal and wide compositional range of the intrusives (49-78 wt. % SiO2) is characteristic of this I-type orogenic suites. Mantle-normalized trace element patterns reveal typical subduction-related signature. Chondrite-normalized REE patterns do not show significant HREE fractionation suggesting the absence of high-pressure residual mineralogy in the source and formation in a "normal thickness", garnet-free crust. Slight Eu anomalies, lowering Sr contents, and concave-up REE patterns of samples dioritic in composition indicate a model involving fractionation of plagioclase, amphibole and pyroxene from a basaltic parent. The analyzed zircon crystals are colorless - transparent ranging in size from 50 to 250 μm. In CL images, 95% of the zircons exhibit oscillatory zonation, characteristic of a magmatic origin. This observation is consistent with the REE zircon composition showing a are characteristic steep positive slope from La to Lu with a significantly positive Ce-anomaly and slight negative Eu-anomaly. There is very little variation in Hf isotopic composition with most of the crust maintains near chondritic Zr/Hf ratios of around 35-40. Our results indicate the development of two tectonic episodes; with the first varying between 210-250 Ma and the second approximate 170-180 Ma. These results are consistent with similar events observed throughout the northern Andes from Peru to Columbia. The occurrence of numerous granite intrusions during Triassic times along the South American margin indicates that western South America underwent a widespread thermal anomaly between 250 and 210 Ma, which produced large amounts of granitoids. We argue that this occurrence is associated with the closure of the Panthalassan Gondwana sutur between 200 and 300 Ma and the correlated magmatism is related to the development of strike-slip tectonics subparallel to the proto-Andean margin. Contrasting to the older episode the younger magmatics appears to be the ultimate dispersal of Gondwana in the Cretaceous, which has been followed by the establishment of a continental arc that has intermittently characterized the modern Andean orogeny for the past 180 Ma.

SHRIMP U-Pb in zircon geochronology of granitoids from Myanmar: temporal constraints on the tectonic evolution of Southeast Asia

NASA Astrophysics Data System (ADS)

Barley, M. E.; Zaw, Khin

2009-04-01

The Mesozoic to Tertiary tectonic evolution of Southeast Asia is the result of the convergence and collision of fragments of Gondwanaland with Eurasia culminating in the collision of India. A rapidly growing geochronological database is placing tight constraints on the timing and duration of magmatic episodes, metallogenic and tectonic events in the Himalayas, Tibet and eastern Indochina. However, there is little comparable geochronology for Myanmar. This SHRIMP U-Pb in zircon geochronology focuses on granitoids from the Mogok Metamorphic Belt (MMB, a belt of high grade metamorphic rocks at the edge of the Shan-Thai Terrane), the Myeik Archipelago (Shan-Thai Terrane) and the west Myanmar Terrane. Strongly deformed granitic orthogneisses in the MMB near Mandalay contain Jurassic (~170 Ma) zircons that have partly recrystallised during ~43 Ma high-grade metamorphism. A hornblende syenite from Mandalay also contains Jurassic zircons with evidence of Eocene metamorphism rimmed by thin zones of 30.9 ±0.7 Ma magmatic zircon. The relative abundance of Jurassic zircons in these rocks is consistent with suggestions that southern Eurasia had an Andean-type margin at that time. Mid-Cretaceous to earliest Eocene (120 to 50 Ma). I-type granitoids in the MMB, Myeik Archipelago and west Myanmar confirm that prior to the collision of India, an up to 200km wide magmatic belt extended along the Eurasian margin. The primitive I-type Khanza Chaung granodiorite in the Wuntho batholith in the west Myanmar terrane hosts porphyry-style mineralisation and has a magmatic age of 94  1 Ma. Triassic (~240 Ma), Jurassic (~170 Ma) and Early Cretaceous xenocryst zircons in this granitoid correspond with peaks of granitoid magmatism in the Shan-Thai terrane and establish that west Myanmar was part of the margin of Eurasia during the Mesozoic. A suite of highly fractionated metaluminous to peraluminous I-type granitoids with associated Sn-W-Ta mineralisation emplaced in the Myeik Archipelago of southern Myanmar (Shan-Thai terrane) have magmatic ages of 82  1.4 Ma (Kawthoung), 62  1.2 Ma (Hermyngi) and 50  0.5 Ma (Auk Bok). Xenocryst zircons in these granitoids are either Proterozoic or derived from older members of the suite. This suite which extends into adjacent peninsular Thailand and was emplaced into thickened continental crust well inboard of the subduction zone during rapid convergence and subduction of the India-Australia plate. The primitive I-type Shangalon granodiorite in the Wuntho Batholith of west Myanmar has a magmatic age of 38.5  0.6 Ma indicating subduction continued until ~40 Ma. Metamorphic overgrowths to zircons in the MMB orthogneiss near Mandalay date a period of Eocene (~43 Ma) high-grade metamorphism possibly during crustal thickening related to the initial collision between India and Eurasia (65 to 55 Ma). This was followed by emplacement of syn-tectonic hornblende syenites and leucogranites between 35 and 23 Ma. Comparison of the geochronology of Myanmar granitoids with the Hymalayas, Tibet and other parts of Southeast Asia indicates that Myanmar played a key role linking the Hymalayan Orogen to the tectonic evolution of Southeast Asia.

Petrogenesis of granodiorite in the Balong region, eastern Kunlun Orogen, China: Constraints from zircon U-Pb geochronology, geochemistry and Sr-Nd-Hf isotopes

NASA Astrophysics Data System (ADS)

Huang, X.; Li, H.; Wang, Y.; Liu, Y.

2017-12-01

Numerous granitoid intrusions that close to the Balong region have great scientific significance to reveal tectonic evolution and geodynamic background of eastern Kunlun Orogen (EKO). Balong granodiorite (BLG) is located at the northern of the EKO. It generally emplaced into the Proterozoic to Lower Palaeozoic rocks and contains abundant mafic microgranular enclaves. LA-ICP-MS zircon U-Pb dating of the BLG gives a 206Pb/238U age of 230.7±1.9 Ma, indicating that it was emplaced in the Late Triassic. The BLG is high-K calc-alkaline series and metaluminous, with SiO2 of 59.86 61.83%, K2O+Na2O of 5.98 6.40%, CaO of 4.95 5.77% and P2O5 of 0.14% 0.18%. The granodioritic rocks are enriched in LILE (Ba, Rb, Sr), but depleted in HFSE (Nb, Ta, P, Ti), with weak negative Eu anomalies (δEu=0.70 0.82). Mineralogy and geochemistry of the rocks show an affinity to I-type granite. The BLG, having (87Sr/86Sr)i ratios of 0.70819 to 0.70832, ɛNd(t) values of -5.27 to -5.75, and zircon ɛHf(t) values ranging from -3.86 to -1.34. The whole-rock Nd isotopic model ages (1432 1471 Ma) and zircon Hf isotopic model ages (1205 1357 Ma) indicate that the BLG is generated by partial melting of lower crust (Precambrian metabasaltic basement rocks) with different degree of involvement of mantle material. Combined with regional geological data, the granodiorite was derived from dehydration melting of mafic lower crustal rocks during the subduction of the Anyemaqen ocean lithosphere at Late Permian-Triassic in a subduction setting. Basaltic magma underplating and crust-mantle mixing are main mechanisms for the origin of large-scale I-type granitoid in Balong.

Leucogranites of the Teton Range, Wyoming: A record of Archean collisional orogeny

NASA Astrophysics Data System (ADS)

Frost, Carol D.; Swapp, Susan M.; Frost, B. Ronald; Finley-Blasi, Lee; Fitz-Gerald, D. Braden

2016-07-01

Leucogranitic rocks formed by crustal melting are a prominent feature of collisional orogens of all ages. This study describes leucogranitic gneisses associated with an Archean collisional orogeny preserved in the Teton Range of northwestern Wyoming, USA. These leucogneisses formed at 2.68 Ga, and initial Nd isotopic compositions suggest they are derived from relatively juvenile sources. Two distinct groups of leucogneisses, both trondhjemitic, are identified on the basis of field relations, petrology, and geochemistry. The Webb Canyon gneiss forms large, sheet-like bodies of hornblende biotite trondhjemite and granodiorite. This gneiss is silica-rich (SiO2 = 70-80%), strongly ferroan, comparatively low in alumina, and is characterized by high Zr and Y, low Sr, and high REE contents that define ;seagull;-shaped REE patterns. The Bitch Creek gneiss forms small sills, dikes, and plutons of biotite trondhjemite. Silica, Zr, Y, and REE are lower and alumina and Sr are higher than in the Webb Canyon gneiss. These differences reflect different melting conditions: the Webb Canyon gneiss formed by dehydration melting in which amphibole and quartz breaks down, accounting for the low alumina, high FeO, high silica content and observed trace element characteristics. The Bitch Creek gneiss formed by H2O-excess melting in which plagioclase breaks down leaving an amphibole-rich restite, producing magmas higher in alumina and Sr and lower in FeO and HREE. Both melt mechanisms are expected in collisional environments: dehydration melting accompanies gravitational collapse and tectonic extension of dramatically thickened crust, and water-excess melting may occur when collision places a relatively cool, hydrous lower plate beneath a hotter upper plate. The Archean leucogranitic gneisses of the Teton Range are calcic trondhjemites and granodiorites whereas younger collisional leucogranites typically are true granites. The difference in leucogranite composition reflects the geochemically immature Archean crust that partially melted in the Teton Range compared to the more geochemically evolved rocks typically involved in younger collisional orogens.

The Maritsa strike-slip shear zone between Kostenets and Krichim towns, South Bulgaria — Structural, petrographic and isotope geochronology study

NASA Astrophysics Data System (ADS)

Naydenov, Kalin; Peytcheva, Irena; von Quadt, Albrecht; Sarov, Stoyan; Kolcheva, Krastina; Dimov, Dimo

2013-06-01

The present study describes the characteristics of the Maritsa Shear Zone (MSZ), a major tectonic element in the Balkanides in South Central Bulgaria. Metamorphic rocks of four lithotectonic units — Madan, Chepinska, Asenitsa and Thrace units crop out in the study area. Strike-slip ductile deformation in MSZ affects the Thrace Lithotectonic Unit (TLU) for up to 15 km. The stratigraphy of this unit is divided in two: Parvenets succession and variegated succession. U-Pb zircon dating reveals Late Jurassic protolith age for metagranitoids and metagabbros of the variegated succession. For its metasedimentary part Triassic to Upper Jurassic age is suggested based on the strontium isotope signature of the marbles. The Parvenets succession affiliates to the Variscan metamorphic basement of Europe. The metamorphic evolution of the zone is subdivided into synmetamorphic strike-slip deformations and annealing stages. The ductile shearing occurred in greenschist to lower amphibolite facies between 130 Ma (discordant U-Pb ages) and 82-78 Ma (late-syntectonic granites). This stage is connected with the oblique collision of the Rhodope Late Jurassic arc with the European platform. With the docking of the arc and the triggering of the strike-slip movements, MSZ represents an orogen-scale border between the Rhodope south-vergent thrust complex and the north-vergent deformations in the Srednogorie and Sakar-Strandzha zones. During the Late Cretaceous MSZ is the contact between the Srednogorie magmatic arc (part of the Apuseni-Banat-Timok-Srednogorie Belt) and the Rhodopean metamorphic core complexes. NW-SE dextral faulting characterized the brittle tectonics along the zone. Strike-slip faults of the southern border of the TLU are transferred into reverse faults, along which the TLU overthrusted Oligocene sediments. MSZ is an orogen-scale transpressional shear zone and an important border in the structure of the Balkanides. This multidisciplinary research emphasizes its role as a major tectonic element by presenting new structural, petrographic and isotope geochronology data.

The Rae craton of Laurentia/Nuna: a tectonically unique entity providing critical insights into the concept of Precambrian supercontinental cyclicity

NASA Astrophysics Data System (ADS)

Bethune, K. M.

2015-12-01

Forming the nucleus of Laurentia/Nuna, the Rae craton contains rocks and structures ranging from Paleo/Mesoarchean to Mesoproterozoic in age and has long been known for a high degree of tectonic complexity. Recent work strongly supports the notion that the Rae developed independently from the Hearne; however, while the Hearne appears to have been affiliated with the Superior craton and related blocks of 'Superia', the genealogy of Rae is far less clear. A diagnostic feature of the Rae, setting it apart from both Hearne and Slave, is the high degree of late Neoarchean to early Paleoproterozoic reworking. Indeed, following a widespread 2.62-2.58 Ga granite bloom, the margins of Rae were subjected to seemingly continuous tectonism, with 2.55-2.50 Ga MacQuoid orogenesis in the east superseded by 2.50 to 2.28 Ga Arrowsmith orogenesis in the west. A recent wide-ranging survey of Hf isotopic ratios in detrital and magmatic zircons across Rae has demonstrated significant juvenile, subduction-related crustal production in this period. Following break-up at ca. 2.1 Ga, the Rae later became a tectonic aggregation point as the western and eastern margins transitioned back to convergent plate boundaries (Thelon-Taltson and Snowbird orogens) marking onset of the 2.0-1.8 Ga assembly of Nuna. The distinctive features of Rae, including orogenic imprints of MacQuoid and Arrowsmith vintage have now been identified in about two dozen cratonic blocks world-wide, substantiating the idea that the Rae cratonic family spawned from an independent earliest Paleoproterozoic landmass before its incorportation in Nuna. While critical tests remain to be made, including more reliable ground-truthing of proposed global correlations, these relationships strongly support the notion of supercontinental cyclicity in the Precambrian, including the Archean. They also challenge the idea of a globally quiescent period in the early Paleoproterozoic (2.45-2.2 Ga) in which plate tectonics slowed or shut down.

Enhanced provenance interpretation using combined U-Pb and (U-Th)/He double dating of detrital zircon grains from lower Miocene strata, proximal Gulf of Mexico Basin, North America

NASA Astrophysics Data System (ADS)

Xu, Jie; Stockli, Daniel F.; Snedden, John W.

2017-10-01

Detrital zircon U-Pb analysis is an effective approach for investigating sediment provenance by relating crystallization age to potential crystalline source terranes. Studies of large passive margin basins, such as the Gulf of Mexico Basin, that have received sediment from multiple terranes with non-unique crystallization ages or sedimentary strata, benefit from additional constraints to better elucidate provenance interpretation. In this study, U-Pb and (U-Th)/He double dating analyses on single zircons from the lower Miocene sandstones in the northern Gulf of Mexico Basin reveal a detailed history of sediment source evolution. U-Pb age data indicate that most zircon originated from five major crystalline provinces, including the Western Cordillera Arc (<250 Ma), the Appalachian-Ouachita orogen (500-260 Ma), the Grenville (1300-950 Ma) orogen, the Mid-Continent Granite-Rhyolite (1500-1300 Ma), and the Yavapai-Mazatzal (1800-1600 Ma) terranes as well as sparse Pan-African (700-500 Ma) and Canadian Shield (>1800 Ma) terranes. Zircon (U-Th)/He ages record tectonic cooling and exhumation in the U.S. since the Mesoproterozoic related to the Grenville to Laramide Orogenies. The combined crystallization and cooling information from single zircon double dating can differentiate volcanic and plutonic zircons. Importantly, the U-Pb-He double dating approach allows for the differentiation between multiple possible crystallization-age sources on the basis of their subsequent tectonic evolution. In particular, for Grenville zircons that are present in all of lower Miocene samples, four distinct zircon U-Pb-He age combinations are recognizable that can be traced back to four different possible sources. The integrated U-Pb and (U-Th)/He data eliminate some ambiguities and improves the provenance interpretation for the lower Miocene strata in the northern Gulf of Mexico Basin and illustrate the applicability of this approach for other large-scale basins to reconstruct sediment provenance and dispersal patterns.

An evaporite-bearing accretionary complex in the northern front of the Betic-Rif orogen

NASA Astrophysics Data System (ADS)

Pérez-Valera, Fernando; Sánchez-Gómez, Mario; Pérez-López, Alberto; Pérez-Valera, Luis Alfonso

2017-06-01

The Guadalquivir Accretionary Complex forms a largely oblique prism at the northern edge of the Betic-Rif orogen, where Miocene sediments plus allochthonous evaporite-bearing units were accreted during the displacement of the Alborán Domain toward the west. Traditional interpretations end the tectonic structuring of the Betic Cordillera at the present topographic front, beyond which gravitational and/or diapiric processes would predominate. However, this study shows pervasive tectonic deformation in the outer prism with coherent oblique shortening kinematics, which is achieved through an alternation of roughly N-S arcuate thrust systems connected by E-W transfer fault zones. These structures accord well with the geophysical models that propose westward rollback subduction. The main stage of tectonic activity occurred in the early-middle Miocene, but deformation lasted until the Quaternary with the same kinematics. Evaporite rocks played a leading role in the deformation as evidenced by the suite of ductile structures in gypsum distributed throughout the area. S- and L- gypsum tectonites, scaly clay fabrics, and brittle fabrics coexist and consistently indicate westward motion (top to 290°), with subordinate N-S contraction almost perpendicular to the transfer zones. This work reveals ductile tectonic fabrics in gypsum as a valuable tool to elucidate the structure and deformational history of complex tectonic mélanges involving evaporites above the décollement level of accretionary wedges.

The Kharapeh orogenic gold deposit: Geological, structural, and geochemical controls on epizonal ore formation in West Azerbaijan Province, Northwestern Iran

USGS Publications Warehouse

Niroomand, Shojaeddin; Goldfarb, Richard J.; Moore, Farib; Mohajjel, Mohammad; Marsh, Erin E.

2011-01-01

The Kharapeh gold deposit is located along the northwestern margin of the Sanandaj–Sirjan Zone (SSZ) in the West Azerbaijan province, Iran. It is an epizonal orogenic gold deposit formed within the deformed zone between central Iran and the Arabian plate during the Cretaceous–Tertiary Zagros orogeny. The deposit area is underlain by Cretaceous schist and marble, as well as altered andesite and dacite dikes. Structural analysis indicates that the rocks underwent tight to isoclinal recumbent folding and were subsequently co-axially refolded to upright open folds during a second deformation. Late- to post-tectonic Cenozoic granites and granodiorites occur northeast of the deposit area. Mineralization mainly is recognized within NW-trending extensional structures as veins and breccia zones. Normal faults, intermediate dikes, and quartz veins, oriented subparallel to the axial surface of the Kharapeh antiform, indicate synchronous extension perpendicular to the fold axis during the second folding event. The gold-bearing quartz veins are >1 km in length and average about 6 m in width; breccia zones are 10–50 m in length and ≤1 m in width. Hydrothermal alteration mainly consists of silicification, sulfidation, chloritization, sericitization, and carbonatization. Paragenetic relationships indicate three distinct stages—replacement and silicification, brecciation and fracture filling, and cataclastic brecciation—with the latter two being gold-rich. Fluid inclusion data suggest mineral deposition at temperatures of at least 220–255°C and depths of at least 1.4–1.8 km, from a H2O–CO2±CH4 fluid of relatively high salinity (12–14 wt.% NaCl equiv.), which may reflect metamorphism of passive margin carbonate sequences. Ore fluid δ18O values between about 7‰ and 9‰ suggest no significant meteoric water input, despite gold deposition in a relatively shallow epizonal environment. Similarities to other deposits in the SSZ suggest that the deposit formed as part of a diachronous gold event during the middle to late Tertiary throughout the SSZ and during the final stages of the Zagros orogeny. The proximity of Kharapeh to the main tectonic suture of the orogen, well-developed regional fold systems with superimposed complex fracture geometries, and recognition of nearby volcanogenic massive sulfide systems that suggest a region characterized by sulfur- and metal-rich crustal rocks, collectively indicate an area of the SSZ with high favorability for undiscovered gold resources.

Continental Subduction: Mass Fluxes and Interactions with the Wider Earth System

NASA Astrophysics Data System (ADS)

Cuthbert, S. J.

2011-12-01

Substantial parts of ultra-high pressure (UHP) terrains probably represent subducted passive continental margins (PCM). This contribution reviews and synthesises research on processes operating in such systems and their implication for the wider Earth system. PCM sediments are large repositories of volatiles including hydrates, nitrogen species, carbonates and hydrocarbons. Sediments and upper/ mid-crustal basement are rich in incompatible elements and are fertile for melting. Lower crust may be more mafic and refractory. Juvenile rift-related mafic rocks also have the potential to generate substantial volumes of granitoid melts, especially if they have been hydrated. Exposed UHP terrains demonstrate the return of continental crust from mantle depths, show evidence for substantial fluxes of aqueous fluid, anatexis and, in entrained orogenic peridotites, metasomatism of mantle rocks by crust- derived C-O-H fluids. However, substantial bodies of continental material may never return to the surface as coherent masses of rock, but remain sequestered in the mantle where they melt or become entrained in the deeper mantle circulation. Hence during subduction, PCM's become partitioned by a range of mechanisms. Mechanical partitioning strips away weaker sediment and middle/upper crust, which circulate back up the subduction channel, while denser, stronger transitional pro-crust and lower crust may "stall" near the base of the lithosphere or be irreversibly subducted to join the global mantle circulation. Under certain conditions sediment and upper crustal basement may reach depths for UHPM. Further partitioning takes place by anatexis, which either aids stripping and exhumation of the more melt-prone rock-masses through mechanical softening, or separates melt from residuum so that melt escapes and is accreted to the upper plate leading to "undercrusting", late-orogenic magmatism and further refinement of the crust. Melt that traverses sections of mantle will interact with it causing metasomatism and refertilisation. Partitioning also takes place by solid-fluid and melt-fluid partitioning. Dehydration may take place both during subduction and exhumation, and fluxes between dehydrating and hydrating rock masses influence the internal fluid budget of the orogen (essential for eclogitisation and densification of mafic lithologies). Ascending granitic melts advect dissolved water to shallow levels, or even the atmosphere. Irreversible subduction of PCM sediment carries water plus nitrogen species to the deeper mantle. Decarbonation of voluminous PCM carbonates depends on thermal regime and may release a pulse of CO2 to the atmosphere, but is limited in colder subduction zones hence transferring large volumes of carbon to the deep mantle. This may ultimately be mobilised by melting or dissolution to form fluid media for diamond formation.

Petrogenesis and tectonic significance of the late Triassic mafic dikes and felsic volcanic rocks in the East Kunlun Orogenic Belt, Northern Tibet Plateau

NASA Astrophysics Data System (ADS)

Hu, Yan; Niu, Yaoling; Li, Jiyong; Ye, Lei; Kong, Juanjuan; Chen, Shuo; Zhang, Yu; Zhang, Guorui

2016-02-01

We present zircon U-Pb ages and geochemical data on the late Triassic mafic dikes (diabase) and felsic volcanic rocks (rhyolite and rhyolitic tuffs) in the East Kunlun Orogenic Belt (EKOB). These rocks give a small age window of 228-218 Ma. The mafic dikes represent evolved alkaline basaltic melts intruding ~ 8-9 Myrs older and volumetrically more abundant A-type granite batholith. Their rare earth element (REE) and multi-element patterns are similar to those of the present-day ocean island basalts (OIBs) except for a weak continental crustal signature (i.e., enrichment of Rb and Pb and weak depletion of Nb, Ta and Ti). Their trace element characteristics together with the high 87Sr/86Sr (0.7076-0.7104), low εNd(t) (- 2.18 to - 3.46), low εHf(t) (- 2.85 to - 4.59) and variable Pb isotopic ratios are consistent with melts derived from metasomatized subcontinental lithospheric mantle with crustal contamination. The felsic volcanic rocks are characterized by high LREE/HREE (e.g., [La/Yb]N of 5.71-17.00) with a negative Eu anomaly and strong depletion in Sr and P, resembling the model upper continental crust (UCC). Given the high 87Sr/86Sr (0.7213-0.7550) and less negative εNd(t) (- 3.83 to - 5.09) and εHf(t) (- 3.06 to - 3.83) than the UCC plus the overlapping isotopes with the mafic dikes and high Nb-Ta rhyolites, the felsic volcanic rocks are best interpreted as resulting from melting-induced mixing with 45-50% crustal materials and 50-55% mantle-derived mafic melts probably parental to the mafic dikes. Such mantle-derived melts underplated and intruded the deep crust as juvenile crustal materials. Partial melting of such juvenile crust produced felsic melts parental to the felsic volcanic rocks in the EKOB. We hypothesize that the late Triassic mafic dikes and felsic volcanic rocks are associated with post-collisional extension and related orogenic collapse. Such processes are probably significant in causing asthenospheric upwelling, decompression melting, induced melting of the prior metasomatized mantle lithosphere and the existing crust. This work represents our ongoing effort in understanding the origin of the juvenile crust and continental crustal accretion through magmatism in the broad context of orogenesis from seafloor subduction to continental collision and to post-collisional processes.

Kilbuck terrane: Oldest known rocks in Alaska

DOE Office of Scientific and Technical Information (OSTI.GOV)

Box, S.E.; Moll-Stalcup, E.J.; Wooden, J.L.

1990-12-01

The Kilbuck terrane in southwestern Alaska is a narrow, thin crustal sliver or flake of amphibolite facies orthogneiss. The igneous protolith of this gneiss was a suite of subduction-related plutonic rocks. U-Pb data on zircons from trondhjemitic and granitic samples yield upper-intercept (igneous) ages of 2,070 {plus minus}16 and 2,040 {plus minus}74 Ma, respectively. Nd isotope data from these rocks suggest that a diorite-tonalite-trondhjemite suite ({epsilon}{sub Nd}(T) = +2.1 to +2.7; T is time of crystallization) evolved from partial melts of depleted mantle with no discernible contamination by older crust, whereas a coeval granitic pluton ({epsilon}{sub Nd}(T) = {minus}5.7) containsmore » a significant component derived from Archean crust. Orthogneisses with similar age and Nd isotope characteristics are found in the Idono complex 250 km to the north. Early Proterozoic rocks are unknown elsewhere in Alaska. However, Phanerozoic plutons cutting several continental terranes in Alaska (southern Brooks Range and Ruby, Seward, and Yukon-Tanana terranes) have Nd isotope compositions indicative of Early Proterozoic (or older) crustal components that could be correlative with rocks of the Kilbuck terrane. Rocks with similar igneous ages in cratonal North America are rare, and those few that are known have Nd isotope compositions distinct from those of the Kilbuck terrane. Conversely, provinces with Nd model ages of 2.0-2.1 Ga are characterized by extensive 1.8 Ga or younger plutonism, which is unknown in the Kilbuck terrane. At present the case for a North American parentage of the Kilbuck terrane is not compelling. The possibility that the Kilbuck terrane was displaced from provinces of similar age in other cratons (e.g., Australian, Baltic, Guiana, and west African shields), or from the poorly dated Siberian craton, cannot be excluded.« less

Partial melting of TTG gneisses: crustal contamination and the production of granitic melts

NASA Astrophysics Data System (ADS)

Meade, F. C.; Masotta, M.; Troll, V. R.; Freda, C.; Johnson, T. E.; Dahren, B.

2011-12-01

Understanding partial melting of ancient TTG gneiss terranes is crucial when considering crustal contamination in volcanic systems, as these rocks are unlikely to melt completely at magmatic temperatures (1000-1200 °C) and crustal pressures (<500 MPa). Variations in the bulk composition of the gneiss, magma temperature, pressure (depth) and the composition and abundance of any fluids present will produce a variety of melt compositions, from partial melts enriched in incompatible elements to more complete melts, nearing the bulk chemistry of the parent gneiss. We have used piston cylinder experiments to simulate partial melting in a suite of 12 gneisses from NW Scotland (Lewisian) and Eastern Greenland (Ammassalik, Liverpool Land) under magma chamber temperature and pressure conditions (P=200 MPa, T=975 °C). These gneisses form the basement to much of the North Atlantic Igneous Province, where crustal contamination of magmas was commonplace but the composition of the crustal partial melts are poorly constrained [1]. The experiments produced partial melts in all samples (e.g. Fig 1). Electron microprobe analyses of glasses indicate they are compositionally heterogeneous and are significantly different from the whole rock chemistry of the parent gneisses. The melts have variably evolved compositions but are typically trachy-dacitic to rhyolitic (granitic). This integrated petrological, experimental and in-situ geochemical approach allows quantification of the processes of partial melting of TTG gneiss in a volcanic context, providing accurate major/trace element and isotopic (Sr, Pb) end-members for modeling crustal contamination. The experimental melts and restites will be compared geochemically with a suite of natural TTG gneisses, providing constraints on the extent to which the gneisses have produced and subsequently lost melt. [1] Geldmacher et al. (2002) Scottish Journal of Geology, v.38, p.55-61.

Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution.

PubMed

Fritz, H; Abdelsalam, M; Ali, K A; Bingen, B; Collins, A S; Fowler, A R; Ghebreab, W; Hauzenberger, C A; Johnson, P R; Kusky, T M; Macey, P; Muhongo, S; Stern, R J; Viola, G

2013-10-01

The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world́s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara-Congo-Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian-Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite-Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ∼650-620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo-Tanzania-Bangweulu Cratons and the Zimbabwe-Kalahari Craton. They closed during the ∼600-500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600-550 Ma extension is recorded in the Arabian-Nubian Shield and the Eastern Granulite-Cabo Delgado Nappe Complex. Later ∼550-480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings resulted in the evolution of distinctly different orogen styles. The Arabian-Nubian Shield is an accretion-type orogen comprising a stack of thin-skinned nappes resulting from the oblique convergence of bounding plates. The Eastern Granulite-Cabo Delgado Nappe Complex is interpreted as a hot- to ultra-hot orogen that evolved from a formerly extended crust. Low viscosity lower crust resisted one-sided subduction, instead a sagduction-type orogen developed. The regions of Tanzania and Madagascar affected by the Kuungan Orogeny are considered a Himalayan-type orogen composed of partly doubly thickened crust.

Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution☆

PubMed Central

Fritz, H.; Abdelsalam, M.; Ali, K.A.; Bingen, B.; Collins, A.S.; Fowler, A.R.; Ghebreab, W.; Hauzenberger, C.A.; Johnson, P.R.; Kusky, T.M.; Macey, P.; Muhongo, S.; Stern, R.J.; Viola, G.

2013-01-01

The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world́s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara–Congo–Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian–Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite–Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ∼650–620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo–Tanzania–Bangweulu Cratons and the Zimbabwe–Kalahari Craton. They closed during the ∼600–500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600–550 Ma extension is recorded in the Arabian–Nubian Shield and the Eastern Granulite–Cabo Delgado Nappe Complex. Later ∼550–480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings resulted in the evolution of distinctly different orogen styles. The Arabian–Nubian Shield is an accretion-type orogen comprising a stack of thin-skinned nappes resulting from the oblique convergence of bounding plates. The Eastern Granulite–Cabo Delgado Nappe Complex is interpreted as a hot- to ultra-hot orogen that evolved from a formerly extended crust. Low viscosity lower crust resisted one-sided subduction, instead a sagduction-type orogen developed. The regions of Tanzania and Madagascar affected by the Kuungan Orogeny are considered a Himalayan-type orogen composed of partly doubly thickened crust. PMID:27065752

Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution

NASA Astrophysics Data System (ADS)

Fritz, H.; Abdelsalam, M.; Ali, K. A.; Bingen, B.; Collins, A. S.; Fowler, A. R.; Ghebreab, W.; Hauzenberger, C. A.; Johnson, P. R.; Kusky, T. M.; Macey, P.; Muhongo, S.; Stern, R. J.; Viola, G.

2013-10-01

The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world´s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara-Congo-Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian-Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite-Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ∼650-620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo-Tanzania-Bangweulu Cratons and the Zimbabwe-Kalahari Craton. They closed during the ∼600-500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600-550 Ma extension is recorded in the Arabian-Nubian Shield and the Eastern Granulite-Cabo Delgado Nappe Complex. Later ∼550-480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings resulted in the evolution of distinctly different orogen styles. The Arabian-Nubian Shield is an accretion-type orogen comprising a stack of thin-skinned nappes resulting from the oblique convergence of bounding plates. The Eastern Granulite-Cabo Delgado Nappe Complex is interpreted as a hot- to ultra-hot orogen that evolved from a formerly extended crust. Low viscosity lower crust resisted one-sided subduction, instead a sagduction-type orogen developed. The regions of Tanzania and Madagascar affected by the Kuungan Orogeny are considered a Himalayan-type orogen composed of partly doubly thickened crust.

Evidence for multiple metamorphic events in the Adirondack Mountains, N. Y

DOE Office of Scientific and Technical Information (OSTI.GOV)

McLelland, J.; Lochhead, A.; Vyhnal, C.

1988-05-01

Field evidence consisting of: (1) rotated, foliated xenoliths, (2) country rock foliation truncated by isoclinally folded igneous intrusions bearing granulite facies assemblages document one, or more, early dynamothermal event(s) of regional scale and high grade. Early metamorphism resulted in pronounced linear and planar fabric throughout the Adirondacks and preceded the emplacement of the anorthosite-mangerite-charnockite-granite-alaskite (AMCA) suite which contains xenoliths of the metamorphosed rocks. Olivine metagabbros, believed to be approximately contemporaneous with the AMCA-suite, also crosscut and contain xenoliths of, strongly foliated metasediments. These intrusive rocks caused contact metamorphism in the metasediments which locally exhibit both anatectite and restite assemblages. Subsequently,more » this already complex framework underwent three phases of folding, including an early recumbent isoclinical event, and was metamorphosed to granulite facies P,T conditions. The age of the early metamorphism cannot yet be narrowly constrained, but isotopic results suggest that it may be as young as approx. 1200 Ma or older than approx. 1420 Ma. U-Pb zircon ages indicate emplacement of the AMCA-(metagabbro)-suite in the interval 1160-1130 Ma and place the peak of granulite facies metamorphism between 1070-1025 Ma. The anorogenic character of the AMCA-suite, and the occurrence of metadiabase dike swarms within it, are further evidence of the separate nature of the metamorphic events that precede and postdate AMCA emplacement.« less

Lithospheric controls on the formation of provinces hosting giant orogenic gold deposits

USGS Publications Warehouse

Bierlein, F.P.; Groves, D.I.; Goldfarb, R.J.; Dube, B.

2006-01-01

Ages of giant gold systems (>500 t gold) cluster within well-defined periods of lithospheric growth at continental margins, and it is the orogen-scale processes during these mainly Late Archaean, Palaeoproterozoic and Phanerozoic times that ultimately determine gold endowment of a province in an orogen. A critical factor for giant orogenic gold provinces appears to be thickness of the subcontinental lithospheric mantle (SCLM) beneath a province at the time of gold mineralisation, as giant gold deposits are much more likely to develop in orogens with subducted oceanic or thin continental lithosphere. A proxy for the latter is a short pre-mineralisation crustal history such that thick SCLM was not developed before gold deposition. In constrast, orogens with protracted pre-mineralisation crustal histories are more likely to be characterised by a thick SCLM that is difficult to delaminate, and hence, such provinces will normally be poorly endowed. The nature of the lithosphere also influences the intrinsic gold concentrations of potential source rocks, with back-arc basalts, transitional basalts and basanites enriched in gold relative to other rock sequences. Thus, segments of orogens with thin lithosphere may enjoy the conjunction of giant-scale fluid flux through gold-enriched sequences. Although the nature of the lithosphere plays the crucial role in dictating which orogenic gold provinces will contain one or more giant deposits, the precise siting of those giants depends on the critical conjunction of a number of province-scale factors. Such features control plumbing systems, traps and seals in tectonically and lithospherically suitable terranes within orogens. ?? Springer-Verlag 2006.

Crustal structure and evolution of the Trans-Hudson orogen: Results from seismic reflection profiling

NASA Astrophysics Data System (ADS)

Baird, D. J.; Nelson, K. D.; Knapp, J. H.; Walters, J. J.; Brown, L. D.

1996-04-01

A 400-km-long deep seismic reflection transect across northeastern Montana and northern North Dakota reveals the crustal-scale structural fabric of the Early Proterozoic Trans-Hudson orogen beneath the Williston basin. Comparison with deep seismic reflection data across the Canadian portion of the same orogen ˜700 km to the north reveals first-order similarities in crustal architecture but documents significant along-strike variation in orogenic evolution. Both transects display a broad crustal-scale antiform axial to the orogen. In the north, geologic data suggest that this antiform is cored by an Archean microcontinent. In the south, west dipping reflections on the western flank of the antiform extend from the upper crust to the uppermost mantle and truncate prominent subhorizontal lower crustal reflections of the Archean Wyoming craton. Within the Wyoming craton, the eastern limit of east dipping midcrustal reflections coincides with the subsurface age boundary between the craton and the Early Proterozoic Trans-Hudson orogen as interpreted from potential field and drill core data. On the basis of subsurface geochronologic data from the crystalline basement and by analogy with the Glennie domain within the exposed Trans-Hudson orogen in Canada, we suggest that the southern antiform is cored by an Archean crustal fragment that was caught up in the terminal collision of the Wyoming and Superior cratons during Hudsonian orogeny. The eastern side of the Trans-Hudson orogen is characterized on both seismic transects by predominantly east dipping crustal penetrating reflections. We interpret the easterly dip of these reflections as evidence that the Superior province was thrust westward over the interludes of the orogen during terminal collision. Although juvenile Early Proterozoic terranes characterize the exposed segment of the Trans-Hudson orogen in Canada, limited drill core information within the Dakota segment of the orogen shows a predominance of granulitic Archean age crust. This difference in basement lithologies along strike within the orogen may indicate the following: either juvenile crust comparable to that exposed in the northern Trans-Hudson was never present in the south, or it was removed by progressive over thickening, erosion, and/or faulting. Postorogenic extensional collapse may be responsible for preservation of juvenile terranes in the north.

Orogenic gold deposits: a proposed classification in the context of their crustal distribution and relationship to other gold deposit types

USGS Publications Warehouse

Groves, D.I.; Goldfarb, R.J.; Gebre-Mariam, M.; Hagemann, S.G.; Robert, F.

1998-01-01

The so-called 'mesothermal' gold deposits are associated with reginally metamorphosed terranes of all ages. Ores were formed during compressional to transpressional deformation processes at convergent plate margins in accretionary and collisional orogens. In both types of orogen, hydrated marine sedimentary and volcanic rocks have been added to continental margins during tens to some 100 million years of collision. Subduction-related thermal events, episodically raising geothermal gradients within the hydrated accretionary sequences, initiate and drive long-distance hydrothermal fluid migration. The resulting gold-bearing quartz veins are emplaced over a unique depth range for hydrothermal ore deposits, with gold deposition from 15-20 km to the near surface environment. On the basis of this broad depth range of formation, the term 'mesothermal' is not applicable to this deposit types as a whole. Instead, the unique temporal and spatial association of this deposit type with orogeny means that the vein systems are best termed orogenic gold deposits. Most ores are post-orogenic with respect to to tectonism of their immediate host rocks, but are simultaneously syn-orogenic with respect to ongoing deep-crustal, subduction-related thermal processes and the prefix orogenic satisfies both these conditions. On the basis of their depth of formation, the orogenic deposits are best subdivided into epizonal (12 km) classes.

Evidence for large-scale imbrication during Eocene syn-orogenic exhumation of the Hellenic subduction channel (Cyclades, Greece)

NASA Astrophysics Data System (ADS)

Grasemann, Bernhard; Huet, Benjamin; Schneider, David; Rice, Hugh; Lemonnier, Nicolas; Tschegg, Cornelius

2017-04-01

In the Cyclades, Miocene post-orogenic back-arc extension overprinted the exhumed syn- orogenic Eocene subduction channel. Whereas the exact geometry and kinematics of the syn-orogenic exhumation are still controversial, but must have involved a floor thrust and an apparent normal fault at the roof, the post-orogenic extension, leading to the exhumation of Cordilleran-type metamorphic core complexes, is well constrained by several major detachment systems. On the island of Milos, which is part of the South Aegean Volcanic Arc, minor outcrops of schist occur. New data indicate that these witnessed Eocene blueschist facies metamorphism at 8.5 kbar and 400°C, but escaped the Miocene extensional overprint, as they lie in the hanging wall of the West Cycladic Detachment System. In contrast, eclogite pebbles in "Green Lahars" on Milos yield metamorphic conditions of 19.5 kbar at 550°C. Both high-pressure units belong to the Cycladic Blueschist Unit and can only have been juxtaposed by thrusting. This indicates that two nappes, the newly defined Cycladic Blueschist Nappe and the overlying Cycladic Eclogite Nappe, both comprising rocks of the Cycladic Blueschist Unit, exist on Milos. These nappes probably also form the other Cycladic islands, separated by a syn-orogenic thrust, which we name the Trans Cycladic Thrust. The Trans Cycladic Thrust, which traces the orientation of the syn-orogenic exhumation channel, is partly offset by the post-orogenic Miocene extensional detachment systems. As a result of the Mid- to Late Miocene clockwise crustal block rotation, the syn-orogenic channel, and hence the Trans Cycladic Thrust, bends through 90° at Milos, changing from a W-E trending to a N-S trending extrusion-related stretching lineation. Restoration of the Miocene block-rotation and extension results in syn-orogenic thrusting kinematics (top-SSW) in the Cycladic Blueschist Nappe and along the Trans Cycladic Thrust and syn-orogenic apparent normal faulting kinematics (top-NNE) at the roof of the Cycladic Eclogite Nappe, consistent with the Eocene extrusion of the high-pressure rocks in the Cyclades.

Stratigraphic variation in petrographic composition of Nanushuk Group sandstones at Slope Mountain, North Slope, Alaska: A section in Geologic studies in Alaska by the U.S. Geological Survey, 1998

USGS Publications Warehouse

Johnsson, Mark J.; Sokol, Nikolas K.

2000-01-01

Fluvial, deltaic, and marine sediments of the Nanushuk Group (Albian to Cenomanian), North Slope, Alaska, record Early Cretaceous orogenic events in the Brooks Range to the south. The 1,060-m section at Slope Mountain is part of the Lower Cretaceous Umiat Delta, shed from the Endicott and De Long Mountains subterranes in the central Brooks Range. These sandstones are litharenites dominated by metasedimentary lithic fragments. Subtle and previously unrecognized stratigraphic variations in composition (up-section increases in metasedimentary lithic fragments, volcanic lithic fragments, and quartz interpreted to be of metamorphic origin) reflect a combination of facies migration and changes in provenance associated with unroofing of the ancestral Brooks Range. We recognize stratigraphic variation in sandstone composition at Slope Mountain whereas previous workers have not, probably because of our use of finely subdivided point-counting categories. The source of the volcanic lithic fragments in the Nanushuk Group remains enigmatic; the most likely candidate is a now-eroded volcanic arc, perhaps a volcanic superstructure to granitic rocks of the Ruby terrane to the south.

Isotopic variation in the Tuolumne Intrusive Suite, central Sierra Nevada, California

USGS Publications Warehouse

Kistler, R.W.; Chappell, B.W.; Peck, D.L.; Bateman, P.C.

1986-01-01

Granitoid rocks of the compositionally zoned Late Cretaceous Toulumne Intrusive Suite in the central Sierra Nevada, California, have initial87Sr/86Sr values (Sri) and143Nd/144Nd values (Ndi) that vary from 0.7057 to 0.7067 and from 0.51239 to 0.51211 respectively. The observed variation of both Sri and Ndi and of chemical composition in rocks of the suite cannot be due to crystal fractionation of magma solely under closed system conditons. The largest variation in chemistry, Ndi, and Sri is present in the outer-most equigranular units of the Tuolumne Intrusive Suite. Sri varies positively with SiO2, Na2O, K2O, and Rb concentrations, and negatively with Ndi, Al2O3, Fe2O3, MgO, FeO, CaO, MnO, P2O5, TiO2, and Sr concentrations. This covariation of Sri, Ndi and chemistry can be modeled by a process of simple mixing of basaltic and granitic magmas having weight percent SiO2 of 48.0 and 73.3 respectively. Isotopic characteristic of the mafic magma are Sri=0.7047, Ndi=0.51269 and ??18O=6.0, and of the felsic magma are Sri=0.7068, Ndi=0.51212 and ??18O=8.9. The rocks sampled contain from 50 to 80% of the felsic component. An aplite in the outer equigranular unit of the Tuolumne Intrusive Suite apparently was derived by fractional crystallization of plagioclase and hornblende from magma with granudiorite composition that was a product of mixing of the magmas described above. Siliceous magmas derived from the lower crust, having a maximum of 15 percent mantle-derived mafic component, are represented by the inner prophyritic units of the Tuolumne Intrusive Suite. ?? 1986 Springer-Verlag.

When the CA-TIMS therapy fails: the over-enthusiastic, the mixed-up, and the stubborn zircon

NASA Astrophysics Data System (ADS)

Corfu, F.

2009-12-01

Mattinson’s CA-TIMS technique has proven to be highly successful in efficiently removing disturbed domains of zircon, thus enabling the determination of precise and accurate ages in a variety of geological situations. The method generally produces equal or better results than mechanical abrasion. There are, however, cases where CA-TIMS fails to achieve perfect concordance, and occasionally actually increases the degree of discordance. Such a behaviour (the over-reaction) is observed for example in U-rich (>1000 ppm) but texturally homogeneous zircon, a type quite common in highly differentiated portions of mafic intrusions, in granitic pegmatites, and in some metamorphic rocks. Because of their internal uniformity, such zircons do not exhibit large contrasts in crystallinity and solubility and after the baking stage of the CA-TIMS procedure they will either totally dissolve in the first HF attack, or produce discordant results. By contrast, mechanical abrasion of this type of zircon can isolate alteration-free zircon domains that yield concordant and reproducible data. This is due to the homogeneity of the crystals, the lack of zoning limiting contrasts in the degrees of metamictization and brittleness and preventing fracturing. The absence of fractures helps to confine alteration of zircon to the marginal domains, and these can be removed very efficiently by mechanical abrasion. This behaviour is exemplified by zircon populations from 252.0 Ma and 182.5 Ma mafic sills in Siberia and the Karoo basin. Another class of zircons that in general does not, or only partially comply with CA-TIMS includes populations from metamorphic and poly-orogenic rocks, typical of many Proterozoic orogens. The metamorphic reprocessing of zircon into low-U domains can freeze-in old Pb-loss patterns that cannot be undone by any technique. New metamorphic zircon growth can also create mixtures of different aged domains that cannot be resolved by CA-TIMS. In favourable circumstances, however, CA-TIMS could provide elegant ways to isolate the metamorphic components. A third category includes zircon populations that fail to achieve consistent ages, or concordant plateaus in multi-step partial dissolutions, even though they may have all the attributes of ideal CA-TIMS objects. The category is exemplified by a 62 Ma magmatic suite with a simple geological history and good quality zircon populations. The zircons have moderate U contents, regular growth zoning and few visible imperfections, yet the data reveal very extensive Pb loss requiring the almost total dissolution of the zircon before reaching the concordant residues. Baking of some of these zircon populations turns the colourless crystals brick-red (but still transparent), the red colour persisting long into the partial dissolution procedure. These features suggest that Pb loss may be related to a submicroscopic network of impurities and primary defects in the crystal structures which form pathways for the diffusion of Pb. Subsequent annealing appears to make the discordant domains impervious to partial dissolution. In conclusion, it is evident that CA-TIMS is a very helpful new technique for U-Pb geochronology, but some limitations must be considered that require case-by-case judgment and good Krogh-type abraders in reserve.

Episodicity of Orogeny Revisited

NASA Astrophysics Data System (ADS)

Condie, K. C.; Aster, R. C.

2008-12-01

Although it is well established that orogeny is episodic, the duration, correlation and geographic distribution of orogenic episodes is not well constrained. Using large numbers of concordant U/Pb zircon ages from subduction-related granitoids (> 7000), it is now possible to better constrain these variables. Monte Carlo simulation probabilistic histograms of zircon age spectra remove questionable and spurious age peaks, yet allow resolution of peaks with >10 My duration with the data sets. Orogenic episodes with durations < 20 My, herein called pulses, are generally of regional geographic extent, whereas long-lived events (100-250 My), herein called periods, may be of regional or global extent. Orogenic periods comprise several to many pulses. Most orogenic pulses reflect geographic variations in intensity of subduction or/and plate collisions as for instance recorded around the perimeter of the Pacific basin in the last 100 My. Neither of the widely recognized pulses at 2.7 nor 1.9 Ga is global in extent. Orogenic pulses at 2700 and 2680 Ma occur on four continents each (2700: Superior, Hearne-Rae, Nain, North China; 2680: Yilgarn, Africa, Slave, Wyoming). Likewise, an orogenic pulse at 1880 is found on four continents (Laurentia, Baltica, East Asia, South America), and another pulse at 1860 Ma occurs on three continents (Africa, Siberia, Australia). Some orogenic pulses track lateral continental growth, such as 2730, 2715, and 2700 Ma pulses in the Abitibi greenstone belt, and 850, 800 and 750 Ma pulses in the Arabian-Nubian shield. Major orogenic periods are recognized at 2750-2650, 1900-1650, and 1250-1000 Ma and each of these is associated with supercontinent formation. Orogenic periods at 2600-2500 (China and India) and 2150-2050 Ma (West Africa, Amazonia, Rio de la Plata) may be associated with the formation of small supercontinents. Our results suggest that orogenic periods with intervening gaps may not require sudden and short-lived changes in mantle behavior, but may be associated primarily with the supercontinent cycle, and thus be a characteristic feature of planets with plate tectonics.

New insights into the origin of ladder dikes: Implications for punctuated growth and crystal accumulation in the Cathedral Peak granodiorite

NASA Astrophysics Data System (ADS)

Wiebe, R. A.; Jellinek, A. M.; Hodge, K. F.

2017-04-01

Ladder dikes are steep tabular bodies, typically a meter or less thick, composed of moderately dipping, concave upward, alternating dark (i.e. schlieren) and light bands oriented roughly perpendicular to the ladder dike margins. These structures occur widely but sparsely in granitic rocks and are found prominently in the Cathedral Peak granodiorite (CPG) of the Tuolumne Intrusive suite. Previous studies have interpreted that ladder dikes form as a result of processes including the downward flow of crystal mush in cracks within strong crystal mush or by upward flow in steep tubes that migrate within a strong crystal mush. Our new observations indicate that ladder dikes formed by downward flow of crystal mush in troughs or valleys, in a manner potentially comparable to trough bands in mafic layered intrusions. Extensions of the schlieren outward and upward away from the ladder dike margins into the host granite demonstrate that the host granite was deposited as mounds on both sides at the same time as the ladder dikes. Ladder dikes, therefore, record lateral flows of crystal mush on a magma chamber floor. Vertical exposures suggest these flows are on the order of ten meters thick. Some steep exposures on granite domes indicate multiple ladder dikes (and flows) over a stratigraphic height of 80-100 m. Later (stratigraphically higher) flows commonly deform and erode the top of an earlier flow, and granitic material rich in K-feldspar megacrysts has locally engulfed large blocks of ladder dikes, demonstrating that the megacrysts were also transported in flows. Flows in the CPG are directed away from the center of the pluton toward the western and eastern margins and apparently spread along a strong crystal mush floor and into a rheologically complex CPG magma. Whereas established dynamical models for spreading (single phase) gravity currents with simple and complex rheologies explain the elongate geometry, spacing and orientation of the tabular bodies, the origin and character of the downward flows required to explain the trough band schlieren structures is challenging. However, an intermittent and progressive deposition of trough bands, consistent with field observations, is potentially explained if the two-phase (crystals and melt) dynamics governing the response of the CPG magma to a new injection are considered.

The Effect of Experimental Weathering on the Multiscale Pore Structure of Granites: A (U)SANS, Imaging Analysis

NASA Astrophysics Data System (ADS)

Anovitz, L. M.; Sheets, J.; Gu, X.; Brantley, S.; Cole, D. R.; Ilton, E. S.; Mildner, D. F. R.; Littrell, K. C.; Gagnon, C.

2017-12-01

The microstructure and evolution of pore space is a critically important factor controlling fluid flow properties in geological formations, including the migration and retention of water, gas and hydrocarbons, sequestration of wastes, the formation of ore deposits, and the evolution of metamorphic terranes. As noted by Navarre-Sitchler et al. (2013), pristine igneous bedrock does not usually contain significant water. However, infiltration of meteoric water causes the rock to begin to disaggregate, increasing the porosity and surface area. In many igneous rocks this process is enhanced by oxidation. However, the mechanisms of porosity formation during weathering are poorly understood, and we cannot accurately extrapolate laboratory reaction rates to the field in predictive numerical models. While there are many methods for interrogating pore structure, it is difficult to satisfactorily describe textural and porosity changes in rock samples using direct imaging techniques because of the wide variation in length scales involved. A combination of SANS, USANS and imaging is, however, well suited to this task. The weathering process in granite is especially complex. While more mafic rocks tend to alter diffusively, forming altered rims, stresses caused by oxidation of ferromagnesian minerals—particularly biotites—tend to cause granites to spall. In order to better understand the effects of oxidation on the weathering process we have performed a series of experiments on granite cores approximately 5/8" in diameter by 5/8" long. These have been reacted in a solution of Se6+ at 200°C for periods of 1, 2, 4, 8, 16, 32, 64, 256 and 438 days. The reaction of Se6+ to Se4+ is very oxidizing relative to that of the Fe2+ in the granite, the solution contained enough to buffer twice the ferrous iron expected in the rock sample, and no secondary phases are expected to form. Because of the geometry these are expected to form oxidized rims that grow with time prior to failure, and the reactive porosity and micro-stress-fractures are easily discernable and quantifiable by (U)SANS. Annular Cd masks were used to isolate core and rim processes during the (U)SANS analyses. These were supplemented by SEM analyses. Results suggest that reactive stresses play a significant role in the evolution of porosity during weathering.

Glaciation as a destructive and constructive control on mountain building.

PubMed

Thomson, Stuart N; Brandon, Mark T; Tomkin, Jonathan H; Reiners, Peter W; Vásquez, Cristián; Wilson, Nathaniel J

2010-09-16

Theoretical analysis predicts that enhanced erosion related to late Cenozoic global cooling can act as a first-order influence on the internal dynamics of mountain building, leading to a reduction in orogen width and height. The strongest response is predicted in orogens dominated by highly efficient alpine glacial erosion, producing a characteristic pattern of enhanced erosion on the windward flank of the orogen and maximum elevation controlled by glacier equilibrium line altitude, where long-term glacier mass gain equals mass loss. However, acquiring definitive field evidence of an active tectonic response to global climate cooling has been elusive. Here we present an extensive new low-temperature thermochronologic data set from the Patagonian Andes, a high-latitude active orogen with a well-documented late Cenozoic tectonic, climatic and glacial history. Data from 38° S to 49° S record a marked acceleration in erosion 7 to 5 Myr ago coeval with the onset of major Patagonian glaciation and retreat of deformation from the easternmost thrust front. The highest rates and magnitudes of erosion are restricted to the glacial equilibrium line altitude on the windward western flank of the orogen, as predicted in models of glaciated critical taper orogens where erosion rate is a function of ice sliding velocity. In contrast, towards higher latitudes (49° S to 56° S) a transition to older bedrock cooling ages signifies much reduced late Cenozoic erosion despite dominantly glacial conditions here since the latest Miocene. The increased height of the orogenic divide at these latitudes (well above the equilibrium line altitude) leads us to conclude that the southernmost Patagonian Andes represent the first recognized example of regional glacial protection of an active orogen from erosion, leading to constructive growth in orogen height and width.

The Ross Orogen and Lachlan Fold Belt in Marie Byrd Land, Northern Victoria Land and New Zealand: implication for the tectonic setting of the Lachlan Fold Belt in Antarctica

USGS Publications Warehouse

Bradshaw, J.D.

2007-01-01

Correlation of the Cambrian Delamerian Orogen of Australia and Ross Orogen of the Transantarctic Mountains widely accepted but the extension of the adjacent Lachlan Orogen into Antarctica is controversial. Outside the main Ross-Delamerian belt, evidence of this orogeny is preserved at Mt Murphy in Marie Byrd Land and the in Takaka Terrane of New Zealand. In all pre-break- configurations of the SW Pacific, these two areas are far removed from the Ross-Delamerian belt. Evidence from conglomerates in the Takaka Terrane, however, shows that in Late Cambrian times it was adjacent to the Ross Orogen. This indicates major tectonic displacements within Gondwana after the Cambrian and before break-up. The Lachlan Orogen formed in an extensional belt in a supra-subduction zone setting and the Cambrian rocks of Marie Byrd Land and New Zealand are interpreted as parts of a rifted continental ribbon on the outboard side of the Lachlan belt.

Geophysical Characterization of a Rare Earth Element Enriched Carbonatite Terrane at Mountain Pass, California Eastern Mojave Desert

NASA Astrophysics Data System (ADS)

Denton, Kevin M.

Mountain Pass, California, located in the eastern Mojave Desert, hosts one of the world's richest rare earth element (REE) deposits. The REE-rich rocks occur in a 2.5 km- wide, north-northwest trending zone of Mesoproterozoic (1.4-1.42 Ga) stocks and dikes, which intrude a larger Paleoproterozoic (1.7 Ga) schist-gneiss terrane that extends 10 km southward from Clark Mountain to the Mescal Range. Several REE-enriched bodies make up the Mountain Pass intrusive suite including shonkinite, syenite, and granite comprising an ultrapotassic intrusive suite and the Sulphide Queen carbonatite body. Two-dimensional modeling of gravity, magnetic, and electrical resistivity data reveals that the Mountain Pass intrusive suite is associated with a local gravity high that is superimposed on a 4-km wide gravity terrace. Rock property data indicate that the Mountain Pass intrusive suite is unusually nonmagnetic at the surface (2.0 x 10-3 SI, n = 67). However, aeromagnetic data indicate that these rocks occur along the eastern edge of a prominent north-northwest trending aeromagnetic high of unknown origin. The source of this unknown magnetic anomaly is 2-3 km below the surface and coincides with a body of rock having high electrical conductivity. Electrical resistivity models indicate that this unknown magnetic anomaly is several orders of magnitude more conductive (103 O•m) than the surrounding rock. Combined geophysical data suggest that the carbonatite and its associated ultrapotassic intrusive suite were preferentially emplaced along a northwest zone of weakness and/or a fault.

Aluminum enrichment in silicate melts by fractional crystallization: some mineralogic and petrographic constraints.

USGS Publications Warehouse

Zen, E.

1986-01-01

The degree of Al saturation of an igneous rock may be given by its aluminium saturation index (ASI), defined as the molar ratio Al2O3/(CaO+K2O+Na2O). One suggested origin for mildly peraluminous granites (ASI 1-1.1) is fractional crystallization of subaluminous magmas (ASI 1. For hornblende to effectively cause a melt to evolve into a peraluminous composition, it must be able to coexist with peraluminous magmas; e.g. at = or <5 kbar hornblende can coexist with strongly peraluminous melts (ASI approx 1.5). Potentials and problems of using coarse-grained granitic rocks to prove courses of magmatic evolution are illustrated by a suite of samples from the Grayling Lake pluton, SW Montana. Such rocks generally contain a large cumulate component and should not be used as a primary test for the occurrence or efficacy of a fractionation process that might lead to peraluminous melts. The process is unlikely to give rise to peraluminous plutons of batholithic dimensions. A differential equation is presented which allows the direct use of mineral chemistry and modal abundance to predict the path of incremental evolution of a given magma.-R.A.H.

Neoproterozoic chromite-bearing high-Mg diorites in the western part of the Jiangnan orogen, southern China: Geochemistry, petrogenesis and tectonic implications

NASA Astrophysics Data System (ADS)

Chen, Xin; Wang, Di; Wang, Xiao-Lei; Gao, Jian-Feng; Shu, Xu-Jie; Zhou, Jin-Cheng; Qi, Liang

2014-07-01

High-Mg diorites were discovered in the southern part of the ca. 830 Ma Dongma Pluton, northern Guangxi Province of southern China. The diorites (SiO2 = 59-65 wt%) are characterized by high MgO (6.7-8.9 wt%) contents and Mg-number [Mg# = 100 × Mg/(Mg + Fe)] (69-73), in contrary to the associated medium-Mg (MgO = 3.4-3.8 wt%, Mg# = 59-63) granodiorites in the Dongma main body and the low-Mg (MgO = 1.4-1.9 wt%, Mg# = 46-51) granodiorites in the Bendong Pluton to the north. Moreover, the high-Mg diorites show surprisingly high Cr (595-640 ppm) and Ni (171-194 ppm) concentrations, which are beyond the ranges of most coeval mafic rocks in the study area. Correspondingly, chromite crystals were separated from the high-Mg diorites and some of the medium-Mg granodiorites, and they show high Cr# [100 × Cr/(Cr + Al)] (average of 75), but low Mg# (0.34-2.51) and low Fe3 +. The decoupling of Cr# and Mg# and the existence of quartz + apatite mineral inclusion in chromites suggest Mg-Fe exchange that may be facilitated by the disequilibrium resulted from magma mixing. The high-Mg diorites show low La/Yb (6.8-8.5) and Sr/Y (2.1-3.1) ratios, significant negative anomalies of Nb and Ti and positive anomaly of Pb, resembling the Setouchi high-Mg andesites, despite of their relatively low Sr (71-100 ppm). All of the studied diorites and granodiorites show enriched Nd isotope compositions, with εNd(t) values (- 3.2 to - 5.9) a bit higher than some of the associated mafic rocks. Some of the high-Mg diorites show whole-rock εHf(t) (- 6.0 to - 6.2) coupled with Nd isotopes, similar to the associated mafic-ultramafic rocks in northern Guangxi, suggesting the metasomatism by melts of subducting sediments in the mantle source. Whereas, others show decoupled Nd-Hf isotopes that are similar to the medium- and low-Mg granodiorites [εHf(t) = - 1.8 to + 0.05], probably indicating the late magma mixing with granitic magmas at a crustal level for the dioritic magmas. We propose a two-stage model for the petrogenesis of the high-Mg diorites: 1) the mantle source was firstly metasomatized by melts from partial melting of subducting terrigenous sediments to form the enriched Nd-Hf isotopic characteristics; and then 2) the mantle-derived high-Mg mafic melts mixed with the crust-derived low-Mg granitic melts to form the high-Mg diorites and medium-Mg granodiorites. The occurrence of high-Mg diorites implies the existence of Neoproterozoic subduction-related metasomatism in the western part of the Jiangnan orogen.

The Penokean orogeny in the Lake Superior region

USGS Publications Warehouse

Schulz, K.J.; Cannon, W.F.

2007-01-01

The Penokean orogeny began at about 1880 Ma when an oceanic arc, now the Pembine-Wausau terrane, collided with the southern margin of the Archean Superior craton marking the end of a period of south-directed subduction. The docking of the buoyant craton to the arc resulted in a subduction jump to the south and development of back-arc extension both in the initial arc and adjacent craton margin to the north. A belt of volcanogenic massive sulfide deposits formed in the extending back-arc rift within the arc. Synchronous extension and subsidence of the Superior craton resulted in a broad shallow sea characterized by volcanic grabens (Menominee Group in northern Michigan). The classic Lake Superior banded iron-formations, including those in the Marquette, Gogebic, Mesabi and Gunflint Iron Ranges, formed in that sea. The newly established subduction zone caused continued arc volcanism until about 1850 Ma when a fragment of Archean crust, now the basement of the Marshfield terrane, arrived at the subduction zone. The convergence of Archean blocks of the Superior and Marshfield cratons resulted in the major contractional phase of the Penokean orogeny. Rocks of the Pembine-Wausau arc were thrust northward onto the Superior craton causing subsidence of a foreland basin in which sedimentation began at about 1850 Ma in the south (Baraga Group rocks) and 1835 Ma in the north (Rove and Virginia Formations). A thick succession of arc-derived turbidites constitutes most of the foreland basin-fill along with lesser volcanic rocks. In the southern fold and thrust belt tectonic thickening resulted in high-grade metamorphism of the sediments by 1830 Ma. At this same time, a suite of post-tectonic plutons intruded the deformed sedimentary sequence and accreted arc terranes marking the end of the Penokean orogeny. The Penokean orogen was strongly overprinted by younger tectonic and thermal events, some of which were previously ascribed to the Penokean. Principal among these was a period of vertical faulting in the Archean basement and overlying Paleoproterozoic strata. This deformation is now known to have post-dated the terminal Penokean plutons by at least several tens of millions of years. Evidence of the Penokean orogen is now largely confined to the Lake Superior region. Comparisons with more recent orogens formed by similar plate tectonic processes implies that significant parts of a once more extensive Penokean orogen have been removed or overprinted by younger tectonic events. ?? 2007 Elsevier B.V. All rights reserved.

Geochonology and Tectonic Significance of post-collisional porphyry in Qulong area, southeast segment of the Gangdese belt, Tibet, China

NASA Astrophysics Data System (ADS)

Liu, D.; Zeren, Z.; Du, C.; Feng, L.; Nima, C.; Zhang, L.

2012-12-01

With the collision of Indian plate and Eura-Asian plate, there developed complicated tectono-magmanism during Meso-Cenozoic in Gangdese belt, Xizang Tibet. Therefore, it resulted that plentiful tectonism and post-collision high-K calc-alkaline magmatism related to mineralization distrubted from east to west in the belt. And this is quite significant for us to do some rearch on large-scale metallogenetics, uplifting epoch and EW-striking extension during the post-collision in Xizang-Qinghai Plateau. Zircon samples from Cangrila Granodiorite-porphyry in Qulong area, Southeast of Gangdese porphyry copper belt, Xiangbeishan diorite-porphyrite and from Jiama Granite-porphyry give LA-ICPMS U-Pb ages of 16.3Ma, 14. 4Ma, and 15.4Ma, respectively, and all these ages represent the porphyries' forming ages. From barren Xiangbeishan diorite-porphyrite through intermediately mineralized Jiama Granite-porphyry to stongly mineralized Cangrila Granodiorite-porphyry, the LA-ICPMS U-Pb ages of zircon become younger and younger. According to the regional data and geochemical characteristics, these porphyries were mainly derived directly from the thickened mafic lower-crust formed in south Tibet during collision and epeirogeny. During post-collisional extension stage, for inter-earth thermal flowing, SN-striking normal faulting systems across the Tibetan orogen caused rapid rising and localization of porphyry magmas and adequately separating of massive ore-bearing fluids from the magmatic hydrothermal systems. All these data indicate that Gandese belt has experienced from post-collisional extrusion changed into intra-plate extension since Miocene. During Miocene, Gangdese belt undergone violent intraplate extension, post-collisional porphyry intrusion and paroxysmal massive mineralization of porphyry-type, and all these was controlled by deep dynamics. Key Words: Qulong area; Gangdese tectonic belt; Tibet;Zircon La-ICP-MS U-Pb dating; intra-plate extension; Image Information For CL images of zircon grains from porphyries

A review of structural patterns and melting processes in the Archean craton of West Greenland: Evidence for crustal growth at convergent plate margins as opposed to non-uniformitarian models

NASA Astrophysics Data System (ADS)

Polat, Ali; Wang, Lu; Appel, Peter W. U.

2015-11-01

The Archean craton of West Greenland consists of many fault-bounded Eoarchean to Neoarchean tectonic terranes (crustal blocks). These tectonic terranes are composed mainly of tonalite-trondhjemite-granodiorite (TTG) gneisses, granitic gneisses, metavolcanic-dominated supracrustal belts, layered anorthositic complexes, and late- to post-tectonic granites. Rock assemblages and geochemical signatures in these terranes suggest that they represent fragments of dismembered oceanic island arcs, consisting mainly of TTG plutons, tholeiitic to calc-alkaline basalts, boninites, picrites, and cumulate layers of ultramafic rocks, gabbros, leucogabbros and anorthosites, with minor sedimentary rocks. The structural characteristics of the terrane boundaries are consistent with the assembly of these island arcs through modern style of horizontal tectonics, suggesting that the Archean craton of West Greenland grew at convergent plate margins. Several supracrustal belts that occur at or near the terrane boundaries are interpreted as relict accretionary prisms. The terranes display fold and thrust structures and contain numerous 10 cm to 20 m wide bifurcating, ductile shear zones that are characterized by a variety of structures including transposed and redistributed isoclinal folds. Geometrically these structures are similar to those occurring on regional scales, suggesting that the Archean craton of West Greenland can be interpreted as a continental scale accretionary complex, such as the Paleozoic Altaids. Melting of metavolcanic rocks during tectonic thickening in the arcs played an important role in the generation of TTGs. Non-uniformitarian models proposed for the origin of Archean terranes have no analogs in the geologic record and are inconsistent with structural, lithological, petrological and geochemical data collected from Archean terranes over the last four decades. The style of deformation and generation of felsic rocks on outcrop scales in the Archean craton of West Greenland and the Mesozoic Sulu orogenic belt of eastern China are similar, consistent with the formation of Archean continental crust by subduction zone processes.

Granite-related Yangjiashan tungsten deposit, southern China

NASA Astrophysics Data System (ADS)

Xie, Guiqing; Mao, Jingwen; Li, Wei; Fu, Bin; Zhang, Zhiyuan

2018-04-01

The Yangjiashan scheelite-bearing deposit (38,663 metric tons of WO3 with an average ore grade of 0.70% WO3) is hosted in quartz veins in a biotite monzogranite intrusion and surrounding slate in the Xiangzhong Metallogenic Province of southern China. The monzogranite has a zircon SHRIMP U-Pb age of 406.6 ± 2.8 Ma (2σ, n = 20, MSWD = 1.4). Cassiterite coexisting with scheelite yields a weighted mean 206Pb/238U age of 409.8 ± 5.9 Ma (2σ, n = 30, MSWD = 0.20), and molybdenite intergrown with scheelite yields a weighted mean Re-Os age of 404.2 ± 3.2 Ma (2σ, n = 3, MSWD = 0.10). These results suggest that the Yangjiashan tungsten deposit is temporally related to the Devonian intrusion. The δD and calculated δ18OH2O values of quartz intergrown with scheelite range from - 87 to - 68‰, and - 1.2 to 3.4‰, respectively. Sulfides have a narrow range of δ34S values of - 2.9 to - 0.7‰ with an average value of - 1.6‰ (n = 16). The integration of geological, stable isotope, and geochronological data, combined with the quartz-muscovite greisen style of ore, supports a magmatic-hydrothermal origin for the tungsten mineralization. Compared to the more common tungsten skarn, quartz-wolframite vein, and porphyry tungsten deposits, as well as orogenic gold deposits worldwide, the Yangjiashan tungsten deposit is an unusual example of a granite-related, gold-poor, scheelite-bearing quartz vein type of deposit. The calcium needed for the formation of scheelite is derived from the sericitization of calcic plagioclase in the monzogranite and Ca-bearing psammitic country rocks, and the relatively high pH, reduced and Ca-rich mineralizing fluid may be the main reasons for the formation of scheelite rather than wolframite at Yangjiashan.

What controls deformation in a bent three-dimensional orogen? An example from the Bolivian Andes

NASA Astrophysics Data System (ADS)

Kaislaniemi, L.; Whipp, D. M., Jr.

2017-12-01

The width of orogens is thought to be affected by both erosional intensity and strength of the rocks. Along-strike variation of the orogen width can be expected to reflect shifts in these factors. An example of such variation can be found around the Bolivian orocline, which is a change in the orientation of the central Andes, in central Bolivia, from N-S south of 18°S to roughly NW-SE in the north. This bend coincides with 50% reduction in the width of the orogen east of the Altiplano, an approximately eight-fold increase in the annual precipitation, and the presence of a basement arch that reduces the thickness of relatively weak Paleozoic sediments upon which the orogen detaches. This has led to uncertainty about whether the growth of the orogen is controlled primarily by climate (erosion) or tectonics (strength of the basal detachment). We study deformation in a segmented orogen using 3D geodynamic models to understand how along-strike variations in rainfall and basal detachment strength affect orogen deformation and growth of the frontal part of the Andean fold-and-thrust belt (FTB). We calculate the visco-plastic deformation in the retro-wedge of an Andean-style orogen using the finite element software DOUAR (Braun et al. 2008) coupled to the surface process model FastScape (Braun & Willett 2013). The model design includes the basement, the Altiplano, and the FTB east of the plateau. A weak basal detachment zone is prescribed. Strain softening allows development of new faults and free evolution of the detachment zone. The effects of varying rock strength and varying precipitation are considered to determine the primary control(s) on the geometry and evolution of curved orogens. Results show that both increased precipitation and stronger detachment zone can explain differences in the width of the FTB, as reflected in the topography. These factors, however, lead to different structural evolution of the orogen: Weak basal detachment zone promotes growth of the FTB towards the foreland, whereas strong basal detachment keeps the deformation nearer to the plateau. Increased precipitation causes strong localization of the frontal thrust and no internal deformation in the foreland or near the plateau. Strike-slip faults are produced by variation in detachment zone strength, but not by shifts in precipitation rates.

Timing of crust formation and recycling in accretionary orogens: Insights learned from the western margin of South America

NASA Astrophysics Data System (ADS)

Bahlburg, Heinrich; Vervoort, Jeffrey D.; Du Frane, S. Andrew; Bock, Barbara; Augustsson, Carita; Reimann, Cornelia

2009-12-01

Accretionary orogens are considered major sites of formation of juvenile continental crust. In the central and southern Andes this is contradicted by two observations: siliciclastic fills of Paleozoic basins in the central Andean segment of the accretionary Terra Australis Orogen consist almost exclusively of shales and mature sandstones; and magmatic rocks connected to the Famatinian (Ordovician) and Late Paleozoic magmatic arcs are predominantly felsic and characterized by significant crustal contamination and strongly unradiogenic Nd isotope compositions. Evidence of juvenile crustal additions is scarce. We present laser ablation (LA)-ICPMS U-Pb ages and LA-MC-ICPMS Hf isotope data of detrital zircons from seven Devonian to Permian turbidite sandstones incorporated into a Late Paleozoic accretionary wedge at the western margin of Gondwana in northern Chile. The combination with Nd whole-rock isotope data permits us to trace the evolution of the South American continental crust through several Proterozoic and Paleozoic orogenic cycles. The analyzed detrital zircon spectra reflect all Proterozoic orogenic cycles representing the step-wise evolution of the accretionary SW Amazonia Orogenic System between 2.0 and 0.9 Ga, followed by the Terra Australis Orogen between 0.9 and 0.25 Ga. The zircon populations are characterized by two prominent maxima reflecting input from Sunsas (Grenville) age magmatic rocks (1.2-0.9 Ga) and from the Ordovician to Silurian Famatinian magmatic arc (0.52-0.42 Ga). Grains of Devonian age are scarce or absent from the analyzed zircon populations. The Hf isotopic compositions of selected dated zircons at the time of their crystallization ( ɛHf ( T) ; T = 3.3-0.25 Ga) vary between - 18 and + 11. All sandstones have a significant juvenile component; between 20 and 50% of the zircons from each sedimentary rock have positive ɛHf ( T) and can be considered juvenile. The majority of the juvenile grains have Hf-depleted mantle model ages (Hf TDM) between 1.55 and 0.8 Ga, the time of the Rondonia-San Ignacio and Sunsas orogenic events on the Amazonia craton. The corresponding whole-rock ɛNd ( T) values fot these same rocks are between - 8 and - 3 indicating a mixture of older evolved and juvenile sources. Nd-depleted mantle model ages (Nd TDM*) are between 1.5 and 1.2 Ga and coincide broadly with the zircon Hf model ages. Our data indicate that the Paleo- and Mesoproterozoic SW Amazonia Orogenic System, and the subsequent Neoproterozoic and Paleozoic Terra Australis Orogen in the region of the central and southern Andes, developed following two markedly different patterns of accretionary orogenic crustal evolution. The SW Amazonia Orogenic System developed by southwestward growth over approximately 1.1 Ga through a combination of accretion of juvenile material and crustal recycling typical of the extensional or retreating mode of accretionary orogens. In contrast, the central Andean segment of the Terra Australis Orogen evolved from 0.9 to 0.25 Ga in the compressional or advancing mode in a relatively fixed position without the accretion of oceanic crustal units or large scale input of juvenile material to the orogenic crust. Here, recycling mainly of Mesoproterozoic continental crust has been the dominant process of crustal evolution.

Linkages and feedbacks in orogenic systems: An introduction

USGS Publications Warehouse

Thigpen, J. Ryan; Law, Richard D.; Merschat, Arthur J.; Stowell, Harold

2017-01-01

Orogenic processes operate at scales ranging from the lithosphere to grain-scale, and are inexorably linked. For example, in many orogens, fault and shear zone architecture controls distribution of heat advection along faults and also acts as the primary mechanism for redistribution of heat-producing material. This sets up the thermal structure of the orogen, which in turn controls lithospheric rheology, the nature and distribution of deformation and strain localization, and ultimately, through localized mechanical strengthening and weakening, the fundamental shape of the developing orogenic wedge (Fig. 1). Strain localization establishes shear zone and fault geometry, and it is the motion on these structures, in conjunction with climate, that often focuses erosional and exhumational processes. This climatic focusing effect can even drive development of asymmetry at the scale of the entire wedge (Willett et al., 1993).

First-order control of syntectonic sedimentation on crustal-scale structure of mountain belts

NASA Astrophysics Data System (ADS)

Erdős, Zoltán.; Huismans, Ritske S.; van der Beek, Peter

2015-07-01

The first-order characteristics of collisional mountain belts and the potential feedback with surface processes are predicted by critical taper theory. While the feedback between erosion and mountain belt structure has been fairly extensively studied, less attention has been given to the potential role of synorogenic deposition. For thin-skinned fold-and-thrust belts, recent studies indicate a strong control of syntectonic deposition on structure, as sedimentation tends to stabilize the thin-skinned wedge. However, the factors controlling basement deformation below fold-and-thrust belts, as evident, for example, in the Zagros Mountains or in the Swiss Alps, remain largely unknown. Previous work has suggested that such variations in orogenic structure may be explained by the thermotectonic "age" of the deforming lithosphere and hence its rheology. Here we demonstrate that sediment loading of the foreland basin area provides an additional control and may explain the variable basement involvement in orogenic belts. When examining the role of sedimentation, we identify two end-members: (1) sediment-starved orogenic systems with thick-skinned basement deformation in an axial orogenic core and thin-skinned deformation in the bordering forelands and (2) sediment-loaded orogens with thick packages of synorogenic deposits, derived from the axial basement zone, deposited on the surrounding foreland fold-and-thrust belts, and characterized by basement deformation below the foreland. Using high-resolution thermomechanical models, we demonstrate a strong feedback between deposition and crustal-scale thick-skinned deformation. Our results show that the loading effects of syntectonic sediments lead to long crustal-scale thrust sheets beneath the orogenic foreland and explain the contrasting characteristics of sediment-starved and sediment-loaded orogens, showing for the first time how both thin- and thick-skinned crustal deformations are linked to sediment deposition in these orogenic systems. We show that the observed model behavior is consistent with observations from a number of natural orogenic systems.

On protolith-, metamorphic overprint, microstructure and rheology of mineral assemblages in orogenic peridotites of the central Scandinavian Caledonides

NASA Astrophysics Data System (ADS)

Gilio, Mattia; Clos, Frediano; Van Roermund, Herman L. M.

2013-04-01

The Scandinavian Caledonides (SC) are a deeply eroded Alpine-type orogenic belt formed by closure of the Iapetus ocean and collision between Baltica and Laurentia (500-380 Ma). The SC consists of a stack of Nappe Complexes (from bottom to top called Lower, Middle, Upper and Uppermost Allochthons) thrusted to the east over the Baltic Shield (Brueckner and Van Roermund, 2004; Gee et al., 2008). Fossil lithospheric mantle fragments, called orogenic peridotites, have been found within the (upper part of) middle, upper and uppermost Allochthons, as well as in the reworked basement gneisses (a.o Western Gneiss Complex (WGC)) along the Norwegian west coast. They occur as isolated lenses that contain diverse mineral parageneses and/or bulk rock compositions. Crustal incorporation of orogenic peridotite is classically interpreted to be the result of plate collisional processes related to orogeny (Brueckner and Medaris, 2000). The WGC and parts of the upper part of the Middle Allochthon (a.o. Seve Nappe Complex (SNC) in N Jämtland/S Västerbotten, central Sweden), are well known for the occurrence of high (HP) and ultrahigh pressure (UHP) metamorphic terranes (of Caledonian age). The (U)HPM evidence clearly demonstrates the deep metamorphic origin of these rocks interpreted to be caused by continental subduction and/or collision. Other metamorphic rocks (of Caledonian age) exposed in allochthonous nappes are solely characterised by greenschist-, amphibolite- and/or MP granulite "facies" mineral assemblages that can be interpreted, in the absence of retrogression, to have formed in less deeply subducted (and/or metamorphic) environments. This duality in metamorphic "facies" allows for a discrimination (at least theoretically) between "deep" versus "shallow" rooted nappes (in central parts of the Scandinavian Caledonides). Conform this reasoning, this duality should also be present within the Caledonian mineral assemblages (= metamorphic overprint) of orogenic peridotites (in central parts of the orogen), which, at least in the allochtonous nappes, have been interpreted to be "isofacial" with their host country rocks (Bucher, 1991). The latter strongly contrast to the interpretation of their "primary" (="protolith"- related) mineral assemblage(s) which clearly suggest a bimodal origin: here called thick (>80 km) versus thin (< 70 km) rooted lithospheric mantle protoliths. Distinction can be made on the basis of the presence of the stable (minimal Proterozoic) garnet-olivine assemblages in the protolith (i.e. much older than the Scandian collision event (Brueckner et al., 2010). For this reason orogenic garnet peridotite was first called "relict" garnet peridotite (Brueckner and Medaris, 2000), later rephrased into mantle wedge garnet peridotite (MWgp) by Van Roermund (2009). MWgp occurs in the WGC and in the SNC of the Upper Allochthon in central Sweden (Zhang et al., 2009). Most (All?) other protolith assemblages of orogenic peridotite in the CSC belong to the thin-rooted protolith subtype. No examples are known to us in which thin rooted prototypes became overprinted (during the Caledonian orogeny) by (U)HP metamorphic minerals, except for the subduction zone garnet peridotites (SZgp) in the WGC (Van Roermund, 2009). The latter can thus savely be interpreted as being enclosed within normal "MP" (or lower pressure) nappe sequences. As such it will be clear that this duality in protolith (and/or metamorphic) mineral assemblages of orogenic peridotite can be used to identify former, but now strongly retrogressed, (U)HP metamorphic terranes in other parts of the CSC (Gee et al, 2012). For this reason a comparative study has been made concerning field, (micro-)structural, mineral-chemical and/or geochemical aspects of two major orogenic peridotites from the SNC, central Sweden; here called the Friningen Garnet Peridotite (FGP) and the Kittelfjäll Spinel Peridotite (KSP), both exposed within the central belt of the SNC in central Sweden. The ultimate aim was to investigate whether the MWgp sub-type can be extended towards (Al-poor) spinel-bearing protolith assemblages or not. Results, including some hitherto unexpected mechanical effects, will be presented. References: Brueckner, H.K., Carswell, D.A., Griffin, W.L., Medaris, L.G., Van Roermund, H.L.M., Cuthbert, S.J. (2010). The mantle and crustal evolution of two garnet peridotite suites from the Western Gneiss Region, Norwegian Caledonides: An isotopic investigation. Lithos, 117, 1-19. doi:10.1016/j. Lithos.2010.01.011 Brueckner, H.K.and Medaris, L.G. (2000). A general model for the intrusion and evolution of "mantle" garnet peridotites in high-pressure and ultra-high-pressure metamorphic terranes. J. Metamorphic Geol., 18, 123-133. Brueckner H.K. and Van Roermund,H.L.M. (2004). Dunk tectonics: A multiple subduction//eduction model for the evolution of the Scandinavian Caledonides. Tectonics, 23, TC2004, doi:10.1029/2003tc001502. Bucher, K. (1991). Mantle fragments in the Scandinavian Caledonides. Tectonophysics, 190, 173-192. Gee, D.G., Fossen, H., Henriksen, N., Higgins, K. (2008). From the Early Paleozoic Platforms of Baltica and Laurentia to the Caledonide Orogen of Scandinavia and Greenland. Episodes, 31, 44-51. Gee, D.G., Janak, M., Majka, J., Robinson, P., Van Roermund, H.L.M (2012). UHP metamorphism along the Baltoscandian outer margin: evidence from the Seve Nappe Complex of the Swedish Caledonides. Lithosphere, in press. Janak, M., Van Roermund, H., Majka, J., Gee, D. (2012). UHP metamorphism recorded by kyanite-bearing eclogite in the Seve Nappe Complex of northern Jämtland, Swedish Caledonides. Gondwana Research, in press. Van Roermund, H.L.M. (2009). Mantle-wedge garnet peridotites from the northernmost ultra-high pressure domain of the Western Gneiss Region, SW Norway. Eur. J. Mineralogy, 21, 1085-1096. Zhang, C., Van Roermund, H.L.M., Zhang, L.F (2011). 16 - Orogenic Garnet Peridotites: Tools to Reconstruct Paleo-Geodynamic Settings of Fossil Continental Collision Zones. In: Ultrahigh Pressure Metamorphism, 25 Years After The Discovery Of Coesite And Diamond. London. Doi:10.1016/B978-0-12-385144-4.00015-1

Orogenic delamination - dynamics, effects, and geological expression

NASA Astrophysics Data System (ADS)

Ueda, Kosuke; Gerya, Taras

2010-05-01

Unbundling of continental lithosphere and removal of its mantle portion have been described by two mutually rather exclusive models, convective thinning and integral delamination. Either disburdens the remaining lithosphere, weakens the remainder, and causes uplift and extension. Increased heat flux is likely to promote high-degree crustal melting, and has been viewed as a source for voluminous granitic intrusions in late or collapsing orogenic settings. Collapse may be driven by any of gravitational potential differences from orogen to foreland, by stress inversion in the unburdened domain, or by suction of a retreating trench. In this study, we investigate prerequisites, mechanism, and development paths for orogeny-related mantle lithosphere removal. Our experiments numerically reproduce delamination which self-consistently results from the dynamics of a decoupling collision zone. In particular, it succeeds without a seed facilitating initial separation of layers. External shortening of a continent - ocean - continent assembly, such as to initiate oceanic subduction, is lifted before the whole oceanic part is consumed, leaving slab pull to govern further convergence. Once buoyant continental crust enters, the collision zone locks, and convergence diminishes. Under favourable conditions, delamination then initiates close to the edge of the mantle wedge and at deep crustal levels. While it initially separates upper crust from lower crust according to the weakness minimum in the lithospheric strength profile, the lower crust is eventually also delaminated from the subducting lithospheric mantle, owing to buoyancy differences. The level of delamination within the lithosphere seems thus first rheology-controlled, then density-controlled. Subduction-coupled delamination is contingent on retreat and decoupling of the subducting slab, which in turn is dependent on effective rheological weakening of the plate contact. Weakening is a function of shear-heating and hereby of collision rate, melting and hydration, the latter two incorporating the effects of sediment subduction and phase changes. The drag available for slab retreat scales with the age of the descending oceanic lithosphere; integrated strength of the lithosphere and activation volume for mantle creep additionally control angle and depth of the descent. Fully developed delamination is observed from between 10 to 15 Ma after collision ceases, with following trenchward migration of the delamination front. Consequently, the main maximum extension migrates, while local, partly intermittent compression can be observed on smaller scale. Across the orogen, extension thus has a strongly diachronous main component. We track common surface observables such as heat flow, partially melted rocks (domal migmatites), and predicted geo-/thermochronological ages over the evolving plate boundary. Geochemical projections of our observations confirm potential contamination of reservoirs - although the net delamination level follows the Moho, some crustal remnants along the old slab still sink through the 660-discontinuity. On the other hand, the base of the delaminated domain is not as plain a contact as in concept. Where the contact of asthenosphere with delaminated crust is the location of high-degree melting, also traces of original lithospheric mantle can be entangled. Our results do not fully support the conceptual distinction between convective thinning and blockwise delamination. While the foundering portion initially retains a fairly coherent, slab-like perimeter, the actual separation of layers in a limited process-zone occurs in smaller -scale eddies. Also, convection of the whole uprising asthenosphere wedge is dynamically not discernible from the latter and crucial for the removal of lithospheric mantle. The removed lithosphere does initially not convect, but subsequently shows an increasing tendency to drip down. In the presented case, extension in the axial zone of the orogen is not (only) caused by unsupported gravitational potential of the core domain itself, but actively driven by slab retreat with a shallow mantle dynamic contribution.

Pre-Alpine evolution of the Seckau Complex (Austroalpine basement/Eastern Alps): Constraints from in-situ LA-ICP-MS Usbnd Pb zircon geochronology

NASA Astrophysics Data System (ADS)

Mandl, Magdalena; Kurz, Walter; Hauzenberger, Christoph; Fritz, Harald; Klötzli, Urs; Schuster, Ralf

2018-01-01

The Variscan European Belt is a complex orogen with its southern margin partly obscured by Alpine tectonics and metamorphism. We present a study of one of the units, the Seckau Complex, that constitute the southern part of the Variscan European Belt in the Eastern Alps in order to clarify its origin, age and lithostratigraphy. The magmatic and geochronological evolution of this Complex in the northwestern part of the Seckau Nappe (as part of the Austroalpine Silvretta-Seckau Nappe System) was investigated by zircon Usbnd Pb dating of paragneisses and metagranitoids coupled with petrological and geochemical data. This reveals the distinction of three newly defined lithostratigraphic/lithodemic sub-units: (1) Glaneck Metamorphic Suite, (2) Hochreichart Plutonic Suite and (3) Hintertal Plutonic Suite. The Glaneck Metamorphic Suite is mainly composed of fine-grained paragneisses that yield Usbnd Pb zircon ages in the range between 2.7 Ga and 2.0 Ga, as well as concordia ages from 572 ± 7 Ma to 559 ± 11 Ma. All of these ages are interpreted as detrital zircon ages originating from an igneous source. The paragneisses are the host rock for the large volumes of metagranitoids of the Hochreichart Plutonic Suite and the Hintertal Plutonic Suite. The Hochreichart Plutonic Suite comprises highly fractionated melts with mainly S-type characteristics and late Cambrian to Early Ordovician Usbnd Pb zircon ages (508 ± 9 Ma to 486 ± 9 Ma), interpreted as magmatic protolith ages. The Hintertal Plutonic Suite is composed of metagranitoids with Late Devonian to early Carboniferous (365 ± 11 Ma and 331 ± 10 Ma) protolith ages, that intruded during an early phase of the Variscan tectonometamorphic event. The metagranitoids of the Hintertal Plutonic Suites define a magmatic fractionation trend, seen in variable Rb/Sr ratios. On this base they can be further subdivided into (a) the Griessstein Pluton characterized by S-type metagranitoids and (b) the Pletzen Pluton distinguished by intermediate to acidic metagranitoids with I-type affinity. The detrital zircon age spectra suggest a Neoproterozoic ancestry of the Glaneck Metamorphic Suite, which was located west of the Arabian Nubian Shield, probably next to the Trans-Saharan Belt. The early Paleozoic evolution of the recent Seckau Complex shows similarities to basement units of the Southalpine Unit, parts of the Austroalpine Unit and the Tatric and Veporic units of the Central Western Carpathians.

Unusual ruby-sapphire transition in alluvial megacrysts, Cenozoic basaltic gem field, New England, New South Wales, Australia

NASA Astrophysics Data System (ADS)

Sutherland, Frederick L.; Graham, Ian T.; Harris, Stephen J.; Coldham, Terry; Powell, William; Belousova, Elena A.; Martin, Laure

2017-05-01

Rare ruby crystals appear among prevailing sapphire crystals mined from placers within basaltic areas in the New England gem-field, New South Wales, Australia. New England ruby (NER) has distinctive trace element features compared to those from ruby elsewhere in Australia and indeed most ruby from across the world. The NER suite includes ruby (up to 3370 ppm Cr), pink sapphire (up to 1520 ppm Cr), white sapphire (up to 910 ppm) and violet, mauve, purple, or bluish sapphire (up to 1410 ppm Cr). Some crystals show outward growth banding in this respective colour sequence. All four colour zones are notably high in Ga (up to 310 ppm) and Si (up to 1820 ppm). High Ga and Ga/Mg values are unusual in ruby and its trace element plots (laser ablation-inductively coupled plasma-mass spectrometry) and suggests that magmatic-metasomatic inputs were involved in the NER suite genesis. In situ oxygen isotope analyses (secondary ion mass spectrometry) across the NER suite colour range showed little variation (n = 22; δ18O = 4.4 ± 0.4, 2σ error), and are values typical for corundum associated with ultramafic/mafic rocks. The isolated NER xenocryst suite, corroded by basalt transport and with few internal inclusions, presents a challenge in deciphering its exact origin. Detailed consideration of its high Ga chemistry in relation to the known geology of the surrounding region was used to narrow down potential sources. These include Late Palaeozoic-Triassic fractionated I-type granitoid magmas or Mesozoic-Cenozoic felsic fractionates from basaltic magmas that interacted with early Palaeozoic Cr-bearing ophiolite bodies in the New England Orogen. Other potential sources may lie deeper within lower crust-mantle metamorphic assemblages, but need to match the anomalous high-Ga geochemistry of the New England ruby suite.

Tectonic evolution of the Black Sea orogene belt and the history of opening of the Black Sea basin

DOE Office of Scientific and Technical Information (OSTI.GOV)

Uesuemezsoy, S.

1988-08-01

The Black Sea basin is surrounded by successive orogenic belts of Hercynian, Cimmerian, and Alpine ages. The Rhodope, Thracian, western Pontian, and Transcaucasian (RTPT) blocks of Precambrian age were involved by the circum-Black Sea orogene belts. The Hercynian orogene was documented in the Balkanide, Great Caucasian, Kriastide, southern Pontian, and Transcaucasian belts. The Cimmerian orogene extended north and south of the Black Sea. The southern Cimmerian orogene was represented by the circum-Rhodope and East Thracian-Strandja-Kuere belts. The northern Cimmerian orogene belt extended along the Dobruca-Crimean and southern slope belts. Following the demise of the Black Sea Cimmerian basin, the northernmostmore » oceanic branch extending from Nish-Trajan through the present Black Sea to the intra-Transcaucasian basin, was opened within the Hercynian and Cimmerian consolidated terrain in the Late Jurassic. The other oceanic branch, extending from Izmir-Ankara through circum Kirsehir to various basins, was opened within the Paleotethyan collision belt, considered to be eastern extension of the Pindus basin. The Nish-Trajan sector of the northernmost basin was closed in the middle Cretaceous, and the Moesian platform re-fused to the Getic-Serbo-Macedonian-Rhodope belt. The easternmost extension of the intra-Transcaucasian basin disappeared in the Late Cretaceous. Consequently, the northernmost oceanic branch was reduced to the present Black Sea basin.« less

Syn-extensional emplacement of the 1. 42 Ga Sandia Granite, N. M

DOE Office of Scientific and Technical Information (OSTI.GOV)

Karlstrom, K.E.; Kirby, E.; Andronicos, C.

1993-02-01

The 1.42 Ga Sandia pluton is one of a suite of Middle Proterozoic granitoid intrusions exposed in northern New Mexico. It crops out over 420 km[sup 2] in the Sandia Mountains, just east of Albuquerque. Recent structural work indicates that the pluton was emplaced syntectonically with respect to a transtensional ductile shear zone on its southeastern side. The shear zone is 1-2 km wide, dips 60[degree] NW under the pluton, and is thus inferred to be a deformed base or lower side of the pluton. From NW to SE, a transect across the zone consists of (1) essentially undeformed, locallymore » flow-foliated Sandia pluton (megacrystic monzogranite); (2) mylonitic, augen orthogneiss that is clearly sheared Sandia pluton; its matrix is depleted in quartz and K-feldspar and enriched in biotite relative to the undeformed pluton; (3) Cibola granite - a 1 km wide zone of fine to coarse grained, equigranular, leucocratic granite; (4) the Tijeras Fault - a Phanerozoic brittle fault; (5) the Tijeras Greenstone - unsheared amphibolite and interlayered pelitic schist and quartzite. Field and microstructural relationships indicate that pluton crystallization was synchronous with shear zone movements. Shear zone movement is interpreted to have punctuated segregation of evolved melts during fractional crystallization of the magma. Geochemical data show linear trends in major and trace elements, with compositional gaps between the main pluton and leucocratic phases. Strain studies suggest that the biotite-rich mylonitic augen orthogneiss records significant volume loss in the matrix. Melts presumably were drawn down pressure gradients into the active shear zone after the main unit was 50--70% crystallized. Further work is required to constrain whether extension was related to regional deformation or only to pluton emplacement.« less

Bokan Mountain peralkaline granitic complex, Alexander terrane (southeastern Alaska): evidence for Early Jurassic rifting prior to accretion with North America

USGS Publications Warehouse

Dostal, Jaroslav; Karl, Susan M.; Keppie, J. Duncan; Kontak, Daniel J.; Shellnutt, J. Gregory

2013-01-01

The circular Bokan Mountain complex (BMC) on southern Prince of Wales Island, southernmost Alaska, is a Jurassic peralkaline granitic intrusion about 3 km in diameter that crosscuts igneous and metasedimentary rocks of the Alexander terrane. The BMC hosts significant rare metal (rare earth elements, Y, U, Th, Zr, and Nb) mineralization related to the last stage of BMC emplacement. U–Pb (zircon) and 40Ar/39Ar (amphibole and whole-rock) geochronology indicates the following sequence of intrusive activity: (i) a Paleozoic basement composed mainly of 469 ± 4 Ma granitic rocks; (ii) intrusion of the BMC at 177 ± 1 Ma followed by rapid cooling through ca. 550 °C at 176 ± 1 Ma that was synchronous with mineralization associated with vertical, WNW-trending pegmatites, felsic dikes, and aegirine–fluorite veins and late-stage, sinistral shear deformation; and (iii) intrusion of crosscutting lamprophyre dikes at >150 Ma and again at ca. 105 Ma. The peralkaline nature of the BMC and the WNW trend of associated dikes suggest intrusion during NE–SW rifting that was followed by NE–SW shortening during the waning stages of BMC emplacement. The 177 Ma BMC was synchronous with other magmatic centres in the Alexander terrane, such as (1) the Dora Bay peralkaline stock and (2) the bimodal Moffatt volcanic suite located ∼30 km north and ∼100 km SE of the BMC, respectively. This regional magmatism is interpreted to represent a regional extensional event that precedes deposition of the Late Jurassic – Cretaceous Gravina sequence that oversteps the Wrangellia and Alexander exotic accreted terranes and the Taku and Yukon–Tanana pericratonic terranes of the Canadian–Alaskan Cordillera.

Bokan Mountain peralkaline granitic complex, Alexander terrane (southeastern Alaska): evidence for Early Jurassic rifting prior to accretion with North America

USGS Publications Warehouse

Dostal, Jaroslav; Karl, Susan M.; Keppie, J. Duncan; Kontak, Daniel J.; Shellnutt, J. Gregory

2013-01-01

The circular Bokan Mountain complex (BMC) on southern Prince of Wales Island, southernmost Alaska, is a Jurassic peralkaline granitic intrusion about 3 km in diameter that crosscuts igneous and metasedimentary rocks of the Alexander terrane. The BMC hosts significant rare metal (rare earth elements, Y, U, Th, Zr, and Nb) mineralization related to the last stage of BMC emplacement. U–Pb (zircon) and 40Ar/39Ar (amphibole and whole-rock) geochronology indicates the following sequence of intrusive activity: (i) a Paleozoic basement composed mainly of 469 ± 4 Ma granitic rocks; (ii) intrusion of the BMC at 177 ± 1 Ma followed by rapid cooling through ca. 550 °C at 176 ± 1 Ma that was synchronous with mineralization associated with vertical, WNW-trending pegmatites, felsic dikes, and aegirine–fluorite veins and late-stage, sinistral shear deformation; and (iii) intrusion of crosscutting lamprophyre dikes at >150 Ma and again at ca. 105 Ma. The peralkaline nature of the BMC and the WNW trend of associated dikes suggest intrusion during NE–SW rifting that was followed by NE–SW shortening during the waning stages of BMC emplacement. The 177 Ma BMC was synchronous with other magmatic centres in the Alexander terrane, such as (1) the Dora Bay peralkaline stock and (2) the bimodal Moffatt volcanic suite located ~30 km north and ~100 km SE of the BMC, respectively. This regional magmatism is interpreted to represent a regional extensional event that precedes deposition of the Late Jurassic – Cretaceous Gravina sequence that oversteps the Wrangellia and Alexander exotic accreted terranes and the Taku and Yukon–Tanana pericratonic terranes of the Canadian–Alaskan Cordillera.

Determination of geochemical affinities of granitic rocks from the Aue-Schwarzenberg zone (Erzgebirge, Germany) by multivariate statistics

USGS Publications Warehouse

Forster, H.-J.; Davis, J.C.

2000-01-01

Variscan granites of the Erzgebirge region can be effectively classified into five genetically distinct major groups by canonical analysis of geochemical variables. The same classification procedure, when applied to small plutons in the Aue-Schwarzenberg granite zone (ASGZ), shows that all ASGZ granites have compositional affinities to low-F biotite or low-F two-mica granite groups. This suggests that the ASGZ granites were emplaced during the first, late-collisional stage of silicic magmatism in the region, which occurred between about 325 and 318 Ma. The numerous biotite granite bodies in the zone are geochemically distinct from both the neighboring Kirchberg granite pluton and the spatially displaced Niederbobritzsch biotite granite massif. Instead, these bodies seem to constitute a third sub-group within the low-F biotite granite class. The ASGZ biotite granites represent three or more genetically distinct bodies, thus highlighting the enormous compositional variability within this group of granites. Least evolved samples of two-mica granites from the ASGZ apparently reflect the assimilation of low-grade metamorphic country rocks during emplacement, altering the original composition of the melts by enhancing primary Al content. The same genesis is implied for the rare "cordierite granite" facies of the Bergen massif, the type pluton for the low-F two-mica granite group in the Erzgebirge.

Crustal structure and continental dynamics of Central China: A receiver function study and implications for ultrahigh-pressure metamorphism

NASA Astrophysics Data System (ADS)

He, Chuansong; Dong, Shuwen; Chen, Xuanhua; Santosh, M.; Li, Qiusheng

2014-01-01

The Qinling-Tongbai-Hong'an-Dabie-Sulu orogenic belt records the tectonic history of Paleozoic convergence between the South China and North China Blocks. In this study, the distribution of crustal thickness and P- and S-wave velocity ratio (Vp/Vs) is obtained by using the H-k stacking technique from the Dabie-Sulu belt in central China. Our results show marked differences in the crustal structure between the Dabie and Sulu segments of the ultrahigh-pressure (UHP) orogen. The lower crust in the Dabie orogenic belt is dominantly of felsic-intermediate composition, whereas the crust beneath the Sulu segment is largely intermediate-mafic. The crust of the Dabie orogenic belt is thicker by ca. 3-5 km as compared to that of the surrounding region with the presence of an ‘orogenic root’. The crustal thickness is nearly uniform in the Dabie orogenic belt with a generally smooth crust-mantle boundary. A symmetrically thickened crust in the absence of any deep-structural features similar to that of the Yangtze block suggests no supportive evidence for the proposed northward subduction of the Yangtze continental block beneath the North China Block. We propose that the collision between the Yangtze and North China Blocks and extrusion caused crustal shortening and thickening, as well as delamination of the lower crust, resulting in asthenospheric upwelling and lower crustal UHP metamorphism along the Dabie Orogen. Our results also reveal the presence of a SE to NW dipping Moho in the North China Block (beneath the Tran-North China Orogen and Eastern Block), suggesting the fossil architecture of the northwestward subduction of the Kula plate.

High-pressure metamorphism in the southern New England Orogen: Implications for long-lived accretionary orogenesis in eastern Australia

NASA Astrophysics Data System (ADS)

Phillips, G.; Offler, R.; Rubatto, D.; Phillips, D.

2015-09-01

New geochemical, metamorphic, and isotopic data are presented from high-pressure metamorphic rocks in the southern New England Orogen (eastern Australia). Conventional and optimal thermobarometry are augmented by U-Pb zircon and 40Ar/39Ar phengite dating to define pressure-temperature-time (P-T-t) histories for the rocks. The P-T-t histories are compared with competing geodynamic models for the Tasmanides, which can be summarized as (i) a retreating orogen model, the Tasmanides formed above a continuous, west dipping, and eastward retreating subduction zone, and (ii) a punctuated orogen model, the Tasmanides formed by several arc accretion, subduction flip, and/or transference events. Whereas both scenarios are potentially supported by the new data, an overlap between the timing of metamorphic recrystallization and key stages of Tasmanides evolution favors a relationship between a single, long-lived subduction zone and the formation, exhumation, and exposure of the high-pressure rocks. By comparison with the retreating orogen model, the following links with the P-T-t histories emerge: (i) exhumation and underplating of oceanic eclogite during the Delamerian Orogeny, (ii) recrystallization of underplated and exhuming high-pressure rocks at amphibolite facies conditions coeval with a period of rollback, and (iii) selective recrystallization of high-pressure rocks at blueschist facies conditions, reflecting metamorphism in a cooled subduction zone. The retreating orogen model can also account for the anomalous location of the Cambrian-Ordovician high-pressure rocks in the Devonian-Carboniferous New England Orogen, where sequential rollback cycles detached and translated parts of the leading edge of the overriding plate to the next, younger orogenic cycle.

Maximum depositional ages and evolving provenance of Franciscan metagraywackes, NW California: LA-ICPMS zircon U-Pb data

NASA Astrophysics Data System (ADS)

Ernst, W. G.; Dumitru, T. A.; Tsujimori, T.; McLaughlin, R. J.; Makishima, A.; Nakamura, E.

2012-12-01

In the Cape Mendocino-Garberville-Covelo area, the Franciscan Complex comprises an imbricate stack of east-rooting allochthons. Five structurally higher to lower thrust sheets crop out from east to west: Eastern Belt outliers; Central Belt mélange; Coastal Belt Yager terrane; Coastal Belt Coastal terrane; and Coastal Belt King Range/False Cape terranes. We analyzed detrital zircons from 11 rocks: 2 Eastern Belt; 5 Central Belt; 4 King Range/False Cape terrane. Combined with earlier analyses of 3 Yager terrane and 3 Coastal terrane zircon suites (Dumitru et al., in review), 17 rocks were investigated. Maximum ages of sedimentation and inferred ultimate sources of these units as follows. Eastern Belt (Yolla Bolly): 98-120 Ma Sierran batholith, 140- 230 Ma Andean arc, minor 1300-1400 Ma Mazatzal granites, minor 1800 Ma Yavapai basement, trace >2.5 Ga Archean craton. Central Belt: minor 62-80 Ma Idaho batholith, 85-200 Ma Sierran batholith-Andean arc, 1300-1400 Ma Mazatzal granites, minor 1600-1750 Ma Mazatzal-Yavapai basement. Yager terrane: 50-75 Ma Idaho batholith, 85-120 Ma Sierran batholith, minor 160-200 Ma Andean arc. Coastal terrane: 30-50 Ma, Cascade + Challis volcanics, 55-80 Ma Idaho batholith, 100 Ma Sierran batholith, 1300-1400 Ma, Mazatzal granites. King Range/False Cape terrane: 22-50 Ma Cascade + Challis Idaho batholith, 100-180 Ma Sierran batholith-Andean arc, minor 1400 Ma Mazatzal-Yavapai granites. Depositional ages of Franciscan imbricate thrust sheets young westward from the mid Cretaceous Eastern Belt through the end-of-Cretaceous Central Belt, to the Paleogene Coastal Belt. Over time, the Franciscan received greater proportions of younger clastics derived from more northerly sources. Although mostly arc-derived, some recycled 1400 and 1700-1800 Ma ± 2.5 Ga arc zircons probably were supplied to the Franciscan Complex by erosion and westward transport of detrital grains from Lower Paleozoic miogeoclinal strata covering the cratonal edge. Except for metagraywackes of the Early Cretaceous Skaggs Spring Schist and Picket Peak terrane, Grenville and Appalachian igneous zircons are conspicuously missing from mid-Cretaceous and younger Franciscan rocks, supporting northwestward offset of the trench deposits relative to SW North America.

Kinematics, partitioning and the relationship between velocity and strain in shear zones

NASA Astrophysics Data System (ADS)

Murphy, Justin James

Granite Point, southeast Washington State, captures older distributed deformation deflected by younger localized deformation. This history agrees with mathematical modeling completed by Watkinson and Patton (2005; 2007 in prep). This model suggests that distributed strain occurs at a lower energy threshold than localized strain and predicts deformation histories similar to Granite Point. Ductile shear zones at Granite Point define a zone of deformation where strain is partitioned and localized into at least ten sub parallel shear zones with sinistral, west side down shear sense. Can the relative movement of the boundaries of this partitioned system be reconstructed? Can partitioning be resolved from a distributed style of deformation? The state of strain and kinematics of actively deforming zones was studied by relating the velocity field to strain. The Aleutian Arc, Alaska and central Walker Lane, Nevada were chosen because they have a wealth of geologic data and are recognized examples of obliquely deforming zones. The graphical construction developed by Declan De Paor is ideally suited for this application because it provides a spatially referenced visualization of the relationship between velocity and strain. The construction of De Paor reproduces the observed orientation of strain in the Aleutian Arc, however, the spatial distribution of GPS stations suggest a component of partitioning. Partitioning does not provide a unique solution and cannot be differentiated from a combination of partitioning and distributed strain. In the central Walker Lane, strain trajectories can be reproduced at the domain scale. Furthermore, the effect of anisotropy from Paleozoic through Cenozoic crustal structure, which breaks the regional strain field into pure shear and simple shear dominated transtension can be detected. Without GPS velocities to document strictly coaxial strain, the strain orientation should not be taken as the velocity orientation. The strain recorded at Granite Point should not be used to reconstruct the relative movement of the boundaries because the strain direction may not be parallel to the velocity orientation. Kinematic reconstructions of obliquely deforming zones that assume a palaeo-velocity orientation equal to the measured orientation of finite strain may not accurately reflect the deviation between velocity and strain.

The Proterozoic of NW Mexico revisited: U-Pb geochronology and Hf isotopes of Sonoran rocks and their tectonic implications

NASA Astrophysics Data System (ADS)

Solari, L. A.; González-León, C. M.; Ortega-Obregón, C.; Valencia-Moreno, M.; Rascón-Heimpel, M. A.

2018-04-01

Several Proterozoic basement units crop out in the Sonora State of NW Mexico, and the same can be correlated with crustal provinces of southern Laurentia in the neighboring southwestern USA. Zircon U-Pb and Hf isotopic determinations in more than 300 grains separated from igneous and metaigneous rocks from these units indicate that the crystalline basement in Sonora is made up of different components, which are from west to east: (1) The Caborca-Mojave province to the west, characterized by the so-called Bámori Complex, have U-Pb ages between 1696 and 1772 Ma, with moderately juvenile to slightly evolved ɛHf values, yielding T DM ages of ca. 2.1-2.4 Ga; (2) in the intermediate area, east of Hermosillo, the Palofierral and La Ramada orthogneiss units yield an age of 1640 and 1703 Ma, respectively, both having juvenile ɛHf with the Palofierral overlapping the depleted mantle curve at ca. 1.65 Ga; and (3) in the northeastern Sonora, samples from the southern extension of the Mazatzal province, represented by the Pinal Schist, yielded ages between 1674 and 1694 Ma, with moderately juvenile to juvenile ɛHf values and a T DM age of ca. 1.9 Ga. In addition, a suite of post-tectonic granites was also studied in Caborca (San Luis granite) as well as in northeastern Sonora (Cananea granite), both yielding ages of ca. 1.44 Ga with moderately juvenile ɛHf values ranging from -1 to +8 and T DM dates of ca. 1.8-1.9 Ga and 1.6-1.7 Ga, respectively. These two isotopically contrasting provinces may imply the existence of a Proterozoic paleo-suture. However, if the Palofierral gneiss, of which the Hf signature straddles the depleted mantle array, is taken as the source for the 1.44 Ga Cananea granite, then the location of such a suture zone should lay farther south than the proposed trace of the Mojave-Sonora megashear.

Geochronology, geochemical and Sr-Nd-Hf-Pb isotopic compositions of the granitoids in the Yemaquan orefield, East Kunlun orogenic belt, northern Qinghai-Tibet Plateau: Implications for magmatic fractional crystallization and sub-solidus hydrothermal alteration

NASA Astrophysics Data System (ADS)

Yin, Shuo; Ma, Changqian; Xu, Jiannan

2017-12-01

A general consensus has emerged that high field strength elements (HFSE) can mobile to some extent in a hydrothermal fluid. However, there are hot debates on whether sub-solidus hydrothermal alteration can lower the Nb/Ta ratio in evolved melts. In this study, we present petrography, geochronology and geochemistry of the barren and mineralized rocks in the Yemaquan skarn iron deposit, northern Qinghai-Tibet Plateau, to probe magmatic-hydrothermal transition. The barren rocks consist of diorites, granodiorites, granites and syenogranites, whereas the porphyritic granodiorites are associated with mineralization for an excellent consistency between the magmatic zircon U-Pb age (225 ± 2 Ma) and the hydrothermal phlogopite 40Ar-39Ar age (225 ± 1.5 Ma). The Sr-Nd-Hf-Pb isotopic data demonstrate that the Yemaquan granitoids are originated from a relatively homogenous enriched mantle with different degrees of crust contamination (assimilation fractional crystallization, AFC). Trace elements signatures indicate that the porphyritic granodiorites related to mineralization display amphibole crystallization for high water contents, whereas the barren granites have gone through biotite crystallization due to potassium enrichment by continuous upper crust contamination, both of which are responsible for their Nb/Ta ratios, respectively. Modeling results suggest that a basaltic melt with Nb/Ta ratio of 15.3 can reach a minimum Nb/Ta ratio of 12 in the producing granodioritic melt by amphibole fractional crystallization based on partition coefficients of Nb and Ta between amphibole and melts from previous experiments. This may explain the average Nb/Ta ratio (13.7) of the barren granodiorites, while it cannot account for the average Nb/Ta ratio (8.4) of the mineralized porphyritic granodiorites, and it is even lower than that of the granites (10.3) with biotite fractional crystallization. Exsolution of a magmatic-hydrothermal fluid is inevitable when a water saturated magma emplaced in shallow crust, leading to a transportation of certain chemical components from the magmatic melts to exsolved fluids. Because Nb seems more mobile than Ta in fluorine-bearing fluids, we contend that a preferentially transport Nb over Ta by sub-solidus hydrothermal alteration can further lower the Nb/Ta ratios of the mineralized porphyritic granodiorites, which may also result in a broad range of HFSE contents and their ratios in the altered porphyritic granodiorites formed in a post-magmatic process.

New data on the substantial composition of Kalba rare metal deposits

NASA Astrophysics Data System (ADS)

Oitseva, T. A.; Dyachkov, B. A.; Vladimirov, A. G.; Kuzmina, O. N.; Ageeva, O. V.

2017-12-01

Geotectonic position, features of the geological structure and rare metal specialization of the Kalba-Narym granitoid belt formed in the Hercynian cycle in the postcollision (orogenic) geodynamic situation are considered. A geological-genetic model for the formation of the leading type of rare-metal pegmatite deposits (Ta, Nb, Be, Li, etc.) is presented. They are spatially and genetically related mainly to the granitoids of the 1st phase of the Kalba complex, P1 (Bakennoye, Jubilee, Belaya Gora, etc.). The rhythmically pulsating orientation of the process of pegmatite formation with the introduction of ore-bearing fluids (H2O, F, B, Cl, Ta, Nb, Be, etc.) is emphasized from the intracamera focus of a semi-closed magmatic system. The preferred location of ore pegmatite veins in granitoids of moderate basicity occupying an intermediate position in the petrochemical composition between normal granites and granodiorites geochemically specialized in Li, Rb, Cs, Sn, Nb, Ta. The leading ore-controlling role of the latitudinal deep faults of the ancient site in the distribution of rare-metal ore fields and deposits (Ognevsk-Bakennoye, Asubulak, Belogorsk, etc.) is determined. There is a zonal structure of pegmatite veins, a gradual development of mineral complexes from the graphic and oligoclase-microcline (non-ore) to microcline-albite and color albite-spodumene (ore). The mineralization of pegmatite veins is determined by the degree of intensity of the manifestation in them of metasomatic processes (microclinization, alibitization, greisenization, spodumenization, tourmalinization, etc.) and the identification of the main ore minerals (tantalite-columbite, cassiterite, spodumene and beryl). The diversity of the material composition of rare-metal pegmatites containing many unique minerals (cleavelandite, lepidolite, ambligonite, color tourmaline, spodumene, pollucite, etc.) is reflected, which brings them closer to the pegmatite deposits of foreign countries (Koktogai, Bernik Lake, etc.). New results of the investigation of the material composition of ore-bearing granites, pegmatites and typomorphic minerals using electron microscopy reflecting the distribution of rare-earth, rare-metal, chalcophile and other elements in them are presented. Indicators of rare metal ore formation are rock-forming minerals of granites (quartz, microcline, biotite, muscovite), ore and associated minerals (cleavelandite, lepidolite, cassiterite, etc.). The most informative minerals include mica (muscovite, giltbertite, lepidolite), colored tourmalines and beryls of different composition and color. Identified typomorphic minerals and geochemical elements-indicators of rare metal pegmatite formation are considered as a leading search criterion in assessing the prospects of the territory of East Kazakhstan.

Timescales of orogeny: Jurassic construction of the Klamath Mountains

NASA Astrophysics Data System (ADS)

Hacker, Bradley R.; Donato, Mary M.; Barnes, Calvin G.; McWilliams, M. O.; Ernst, W. G.

1995-06-01

An electronic supplement of this material may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG (LOGIN to AGU's FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 94YCJ2454, Timescales of orogeny: Jurassic construction of the Klamath Mountains, B.R. Hacker, M.M. Donato, C.G. Barnes, M.O. McWilliams, and W.G. Ernst). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00. Payment must accompany order. Classical interpretations of orogeny were based on relatively imprecise biostratigraphic and isotopic age determinations that necessitated grouping apparently related features that may in reality have been greatly diachronous. Isotopic age techniques now have the precision required to resolve the timing of orogenic events on a scale much smaller than that of entire mountain belts. Forty-five new 40Ar/39Ar ages from the Klamath Mountains illuminate the deformation, metamorphism, magmatism, and sedimentation involved in the Jurassic construction of that orogen, leading to a new level of understanding regarding how preserved orogenic features relate to ancient plate tectonic processes. The new geochronologic relationships show that many Jurassic units of the Klamath Mountains had 200 Ma or older volcanoplutonic basement. Subsequent formation of a large ˜170 Ma arc was followed by contractional collapse of the arc. Collision with a spreading ridge may have led to large-scale NW-SE extension in the central and northern Klamaths from 167 to ˜155 Ma, coincident with the crystallization of voluminous plutonic and volcanic suites. Marked cooling of a large region of the central Klamath Mountains to below ˜350°C at ˜150 Ma may have occurred as the igneous belt was extinguished by subduction of colder material at deeper structural levels. These data demonstrate that the Klamath Mountains—and perhaps other similar orogens—were constructed during areally and temporally variant episodes of contraction, extension, and magmatism that do not fit classical definitions of orogeny.

Fault Dating in the US Rockies and Large Regional Extent of Deformation Pulses Along the Sevier Orogen of North America.

NASA Astrophysics Data System (ADS)

van der Pluijm, B.; Lynch, E. A.; Pana, D.; Yonkee, A.

2017-12-01

Recent Ar dating of clay-rich fault rock in the Canadian Rockies identified multiple orogenic pulses: Late Jurassic (163-146 Ma), Mid-Cretaceous (103-99 Ma), Late Cretaceous (76-72 Ma) and Eocene (54-52 Ma; Pana and van der Pluijm, GSAB 2015). New dating in the US Rockies combined with ages in the most frontal section along an Idaho-Wyoming transect show a remarkably similar age pattern: Meade Thrust, 108-102 Ma; (S)Absaroka Thrust, 73 Ma; Darby-Bear Thrust, 56-50 Ma. These radiometric fault ages in the US Rockies match field and tectono-stratigraphic predictions, analogues to those in the Canadian Rockies. Thus, a remarkably long (>1500km) lateral tract along the North American Sevier orogen is characterized by at least three major orogenic pulses that are structurally contiguous. These orogenic pulses are progressively younger in the direction of easterly thrust fault motion (toward cratonic interior) and are separated by long periods of relative tectonic quiescence. We interpret the extensive regional continuity of deformation pulses and tectonic quiescence along the Sevier Orogen as the result of three plate reorganization events in western North America since the Late Jurassic.

Three depositional states and sedimentary processes of the western Taiwan foreland basin system

NASA Astrophysics Data System (ADS)

Lin, Yi-Jung; Wu, Pei-Jen; Yu, Ho-Shing

2010-05-01

The western Taiwan foreland basin formed during the Early Pliocene as the flexural response to the loading of Taiwan orogen on the Eurasian plate. What makes Taiwan interesting is the oblique collision, which allows the foreland basin to be seen at different stages in its evolution at the present day. Due to oblique arc-continent collision from north to south, the western Taiwan foreland basin has evolved into three distinct subbasins: an over-filled basin proximal to the Taiwan orogen, mainly distributed in the Western Foothills and Coastal Plain provinces, a filled basin occupying the shallow Taiwan Strait continental shelf west of the Taiwan orogen and an under-filled basin distal to the Taiwan orogen in the deep marine Kaoping Slope offshore southwest Taiwan, respectively. The over-filled depositional phase is dominated by fluvial environments across the structurally controlled piggy-back basins. The filled depositional state in the Taiwan Strait is characterized by shallow marine environments and is filled by Pliocene-Quaternary sediments up to 4,000 m thick derived from the Taiwan orogen with an asymmetrical and wedge-shaped cross section. The under-filled depositional state is characteristic of deep marine environments in the wedge-top basins accompanied by active structures of thrust faults and mud diapers. Sediments derived from the Taiwan orogen have progressively filled the western Taiwan foreland basin across and along the orogen. Sediment dispersal model suggests that orogenic sediments derived from oblique dischronous collisional highlands are transported in two different ways. Transport of fluvial and shallow marine sediments is perpendicular to hill-slope and across-strike in the fluvial and shallow marine environments proximal to the orogen. Fine-grained sediments mainly longitudinally transported into the deep marine environments distal to the orogen. The present sedimentary processes in the over-filled basin on land are dominated by fluvial processes of small mountainous rivers. Tidal currents are prevalent in the filled basin in Taiwan Strait, transporting shelf sands and forming sand ridges. The deep marine under-filled basin are dominated by down-slope mass wasting processes, eroding slope strata and transporting sediments to the basin floor. In addition, many submarine canyons on the continental slope offshore southwest Taiwan serve as major sediment pathways, delivering shallow marine sediments to the basin floor.

Basin-mountain structures and hydrocarbon exploration potential of west Junggar orogen in China

NASA Astrophysics Data System (ADS)

Wu, X.; Qi, X.; Zheng, M.

2015-12-01

Situated in northern Xinjiang, China, in NE-SW trend, West Junggar Orogen is adjacent to Altai fold belt on the north with the Ertix Fault as the boundary, North Tianshan fold belt on the south with the Ebinur Lake Strike-slip Fault as the boundary, and the Junggar Basin on the southeast with Zaire-Genghis Khan-Hala'alat fold belt as the boundary. Covering an area of about 10×104 km2 in China, there are medium and small intermontane basins, Burqin-Fuhai, Tacheng, Hefeng and Hoxtolgay, distributing inside the orogen. Tectonically West Junggar Orogen lies in the middle section of the Palaeo-Asian tectonic domain where the Siberia, Kazakhstan and Tarim Plates converge, and is the only orogen trending NE-SW in the Palaeo-Asian tectonic domain. Since the Paleozoic, the orogen experienced pre-Permian plate tectonic evolution and post-Permian intra-plate basin evolution. Complex tectonic evolution and multi-stage structural superimposition not only give rise to long term controversial over the basin basement property but also complex basin-mountain coupling relations, structures and basin superimposition modes. According to analysis of several kinds of geological and geophysical data, the orogen was dominated by compressive folding and thrust napping from the Siberia plate in the north since the Late Paleozoic. Compressive stress weakened from north to south, corresponding to subdued vertical movement and enhanced horizontal movement of crustal surface from north to south, and finally faded in the overthrust-nappe belt at the northwest margin of the Junggar Basin. The variation in compressive stress is consistent with the surface relief of the orogen, which is high in the north and low in the south. There are two kinds of basin-mountain coupling relationships, i.e. high angle thrusting and overthrusting and napping, and two kinds of basin superimposition modes, i.e. inherited and progressive, and migrating and convulsionary modes. West Junggar orogen has rich oil and gas shows. Tacheng Basin, north faulted fold belt in the Heshituoluogai basin, and Hongyan fault bench zone in north Ulungur Depression in the Junggar Basin are promising areas for hydrocarbon exploration.

Evolving lithospheric flexure and paleotopography of the Pyrenean Orogen from 3D flexural modeling and basin analysis

NASA Astrophysics Data System (ADS)

Curry, M. E.; van der Beek, P.; Huismans, R. S.; Muñoz, J. A.

2017-12-01

The Pyrenees are an asymmetric, doubly-vergent orogen with retro- and pro- foreland basins that preserve a record of deformation since the Mesozoic. The extensive research and exploration efforts on the mountain belt and flanking foreland basins provide an exceptional dataset for investigating geodynamics and surface processes over large spatial and temporal scales in western Europe. We present the results of a numerical modeling study investigating the spatio-temporal variation in lithospheric flexure in response to the developing orogen. We employ a finite element method to model the 3D flexural deformation of the lithosphere beneath the Pyrenean orogen since the onset of convergence in the late Cretaceous. Using subsurface, geophysical, and structural data, we describe the evolving geometry of both the French Aquitaine and Spanish Ebro foreland basins at the present (post-orogenic), the mid-Eocene (peak orogenic), the Paleocene (early orogenic), and the end of the Cretaceous (pre- to early orogenic). The flexural modeling provides insight into how both the rigidity of the lithosphere and the paleotopographic load have varied over the course of orogenesis to shape the basin geometry. We find that the overriding European plate has higher rigidity than the subducting Iberian plate, with modern Effective Elastic Thickness (EET) values of 20 ± 2 and 12 ± 2 km, respectively. Modeling indicates that the modern rigidity of both plates decreases westward towards the Bay of Biscay. The lithospheric rigidity has increased by 50% since the Mesozoic with early Cenozoic EET values of 13 ± 2 and 8 ± 1 km for the European and Iberian plates, respectively. The topographic load began increasing with convergence in the late Cretaceous, reaching modern levels in the central and eastern Pyrenees by the Eocene. In contrast, the topographic load in the western Pyrenees was 70% of the modern value in the Eocene, and experienced topographic growth through the Oligo-Miocene. The westward propagation of topographic growth and erosion is supported by subsidence analysis and low-temperature thermochronology data. These results have implications for surface processes and foreland basin development of the Pyrenean Orogen, inheritance of Hercynian crustal properties, and the geodynamic evolution of western Europe.

Magma mixing in granite petrogenesis: Insights from biotite inclusions in quartz and feldspar of Mesozoic granites from South China

NASA Astrophysics Data System (ADS)

Gao, Peng; Zhao, Zi-Fu; Zheng, Yong-Fei

2016-06-01

Magma mixing is a common process in granite petrogenesis. The major element composition of biotites in granites is primarily controlled by the composition of magmas from which they crystallized. Biotite grains enclosed in quartz and feldspars of granites are naturally protected by their host minerals, so that their compositions are likely original and can potentially be used to track the magma mixing. This is illustrated by a combined study of matrix and inclusion biotites from Mesozoic granites in the Nanling Range, South China. Three granite samples have been used in this study: one two-mica granite and two biotite granites. The biotites of different occurrences in the two-mica granite have no compositional distinctions. Biotites in the two-mica granite have higher Al2O3 and lower MgO than those in the biotite granites. The former is consistent with biotites from typical S-type granites of metasedimentary origin. In contrast, biotites from the biotite granites can be categorized into different groups based on their paragenetic minerals and geochemical compositions. They have relatively low aluminous saturation indices but higher Mg numbers, falling in the transitional field between typical S- and I-type granites. In addition, there are two contrasting zircon populations with nearly identical U-Pb ages in the biotite granites. One shows clearly oscillatory zonings in CL images, whereas the other is totally dark and often overgrew on the former one. The zircons with oscillatory zonings have higher δ18O values than the dark ones, indicating their growth from two compositionally different magmas, respectively, with different sources. An integrated interpretation of all these data indicates that mixing of two different magmas was responsible for the petrogenesis of biotite granites. Therefore, the study of biotite inclusions provides insights into the magma mixing in granite petrogenesis.

Glimpses of East Antarctica: Aeromagnetic and satellite magnetic view from the central Transantarctic Mountains of East Antarctica

USGS Publications Warehouse

Finn, Carol A.; Goodge, John W.

2010-01-01

Aeromagnetic and satellite magnetic data provide glimpses of the crustal architecture within the Ross Sea sector of the enigmatic, ice-covered East Antarctic shield critical for understanding both global tectonic and climate history. In the central Transantarctic Mountains (CTAM), exposures of Precambrian basement, coupled with new high-resolution magnetic data, other recent aeromagnetic transects, and satellite magnetic and seismic tomography data, show that the shield in this region comprises an Archean craton modified both by Proterozoic magmatism and early Paleozoic orogenic basement reactivation. CTAM basement structures linked to the Ross Orogeny are imaged 50–100 km farther west than previously mapped, bounded by inboard upper crustal Proterozoic granites of the Nimrod igneous province. Magnetic contrasts between craton and rift margin sediments define the Neoproterozoic rift margin, likely reactivated during Ross orogenesis and Jurassic extension. Interpretation of satellite magnetic and aeromagnetic patterns suggests that the Neoproterozoic rift margin of East Antarctica is offset by transfer zones to form a stepwise series of salients tracing from the CTAM northward through the western margin of the Wilkes Subglacial Basin to the coast at Terre Adélie. Thinned Precambrian crust inferred to lie east of the rift margin cannot be imaged magnetically because of modification by Neoproterozoic and younger tectonic events.

Aperçu de precambrien de côte d'Ivoire: geologie-metallogenie

NASA Astrophysics Data System (ADS)

Angoran, Y.; Kadio, E.

The Ivory Coast is situated at the southern limits of the West African Craton and constitute a part of the 'Dorsale de Man'. The precambrian rocks occupy 97% of the superficial area of the country and include rocks of two orogenic episodes: the Liberian (3000-2580 Ma) and the Eburnian of lower Proterozoic (2400-1550 Ma). Liberian Orogeny, which is the most ancient, consists of gneisses, amphibo-pyroxinites, fine-grained itabirites and coarse-grained ferruginous quartzites. The aluminous gneisses, amphibo-pyroxinites and ferruginous quartzites are supracrustals that have been transformed by a high grade Catasonal metamorphism resulting in highly folded rocks. The Liberian plutons are infracrustals consisting of complex basic and ultrabasic rocks, migmatites, charnockites and granites associated with magmatites. This Liberian complex is intruded by some dolerites (2200 Ma), and kimberlites with diamond (2210-2500 Ma) which have been eroded to produce Birrimian placer deposits of Tortiya and Birrim in Ghana. The eburnian geosyncline consists of alternating subparallel intrageosynclines and intrageanticlines. The volcano-sedimentary complexes were intruded by eburnian plutons of 2100-1550 Ma. About 20 different types of mineralisations are common within the Pre-Cambrian rocks of the Ivory Coast and they are of Archaen to lower Proterozoic age. Examples of these mineral concentrations are cited in this paper.

Apollo 16 Evolved Lithology Sodic Ferrogabbro

NASA Technical Reports Server (NTRS)

Zeigler, Ryan; Jolliff, B. L.; Korotev, R. L.

2014-01-01

Evolved lunar igneous lithologies, often referred to as the alkali suite, are a minor but important component of the lunar crust. These evolved samples are incompatible-element rich samples, and are, not surprisingly, most common in the Apollo sites in (or near) the incompatible-element rich region of the Moon known as the Procellarum KREEP Terrane (PKT). The most commonly occurring lithologies are granites (A12, A14, A15, A17), monzogabbro (A14, A15), alkali anorthosites (A12, A14), and KREEP basalts (A15, A17). The Feldspathic Highlands Terrane is not entirely devoid of evolved lithologies, and rare clasts of alkali gabbronorite and sodic ferrogabbro (SFG) have been identified in Apollo 16 station 11 breccias 67915 and 67016. Curiously, nearly all pristine evolved lithologies have been found as small clasts or soil particles, exceptions being KREEP basalts 15382/6 and granitic sample 12013 (which is itself a breccia). Here we reexamine the petrography and geochemistry of two SFG-like particles found in a survey of Apollo 16 2-4 mm particles from the Cayley Plains 62283,7-15 and 62243,10-3 (hereafter 7-15 and 10-3 respectively). We will compare these to previously reported SFG samples, including recent analyses on the type specimen of SFG from lunar breccia 67915.

Growth of early continental crust by partial melting of eclogite.

PubMed

Rapp, Robert P; Shimizu, Nobumichi; Norman, Marc D

2003-10-09

The tectonic setting in which the first continental crust formed, and the extent to which modern processes of arc magmatism at convergent plate margins were operative on the early Earth, are matters of debate. Geochemical studies have shown that felsic rocks in both Archaean high-grade metamorphic ('grey gneiss') and low-grade granite-greenstone terranes are comprised dominantly of sodium-rich granitoids of the tonalite-trondhjemite-granodiorite (TTG) suite of rocks. Here we present direct experimental evidence showing that partial melting of hydrous basalt in the eclogite facies produces granitoid liquids with major- and trace-element compositions equivalent to Archaean TTG, including the low Nb/Ta and high Zr/Sm ratios of 'average' Archaean TTG, but from a source with initially subchondritic Nb/Ta. In modern environments, basalts with low Nb/Ta form by partial melting of subduction-modified depleted mantle, notably in intraoceanic arc settings in the forearc and back-arc regimes. These observations suggest that TTG magmatism may have taken place beneath granite-greenstone complexes developing along Archaean intraoceanic island arcs by imbricate thrust-stacking and tectonic accretion of a diversity of subduction-related terranes. Partial melting accompanying dehydration of these generally basaltic source materials at the base of thickened, 'arc-like' crust would produce compositionally appropriate TTG granitoids in equilibrium with eclogite residues.

Lead isotopic evidence for mixed sources of Proterozoic granites and pegmatites, Black Hills, South Dakota, USA

NASA Astrophysics Data System (ADS)

Krogstad, Eirik J.; Walker, Richard J.; Nabelek, Peter I.; Russ-Nabelek, Carol

1993-10-01

The lead isotopic compositions of K-feldspars separated from the ca. 1700 Ma Harney Peak Granite complex and spatially associated granitic pegmatites indicate that these rocks were derived from at least two sources. It has been reported previously that the core of the Harney Peak Granite complex is dominated by relatively lower/ gd18O (avg. 11.5 %.) granites, whereas higher / gd18O (avg. 13.2%.) granites occur around the periphery of the complex. The higher δ 18O granites and one simple pegmatite have low values of 207Pb /204Pb for their 206Pb /204Pb Thus, they likely were derived from a source with a short crustal residence time. This source may have been the pelitic schists into which the Harney Peak Granite complex and pegmatites were intruded. Feldspars from granites with lower / gd18O values have significantly higher 207Pb /204Pb for their 206Pb /204Pb . The data define a linear array with a slope equivalent to an age of ca. 2.6 Ga with t 2 defined to be 1.7 Ga. Such a slope could represent a mixing array or a secondary isochron for the source. These low δ18O granites could have been derived from a source with a high U/ Pb and with a crustal residence beginning before the Proterozoic. The source (s) of these granites may have been a sediment derived from late Archean continental crust. The highly evolved Tin Mountain pegmatite has lead isotopic systematics intermediate between those of the two granite groups, suggesting either a mixed source or contamination. Two late Archean granites, the Little Elk Granite and the Bear Mountain Granite, had precursors with high U/Pb and low Th/U histories. The Th/U history of the Bear Mountain Granite is too low for this rock to have been an important component of the source of the Proterozoic granites. However, crustal rocks with lead isotopic compositions similar to those of the Little Elk Granite were an important source of lead for some of the Proterozoic granitic rocks.

Reactivation of the Archean-Proterozoic suture along the southern margin of Laurentia during the Mazatzal orogeny: Petrogenesis and tectonic implications of ca. 1.63 Ga granite in southeastern Wyoming

USGS Publications Warehouse

Jones, Daniel S.; Barnes, Calvin G.; Premo, Wayne R.; Snoke, Arthur W.

2013-01-01

The presence of ca. 1.63 Ga monzogranite (the “white quartz monzonite”) in the southern Sierra Madre, southeastern Wyoming, is anomalous given its distance from the nearest documented plutons of similar age (central Colorado) and the nearest contemporaneous tectonic margin (New Mexico). It is located immediately south of the Cheyenne belt—a ca. 1.75 Ga Archean-Proterozoic tectonic suture. New geochronological, isotopic, and geochemical data suggest that emplacement of the white quartz monzonite occurred between ca. 1645 and 1628 Ma (main pulse ca. 1628 Ma) and that the white quartz monzonite originated primarily by partial melting of the Big Creek Gneiss, a modified arc complex. There is no evidence that mafic magmas were involved. Open folds of the ca. 1750 Ma regional foliation are cut by undeformed white quartz monzonite. On a regional scale, rocks intruded by the white quartz monzonite have experienced higher pressure and temperature conditions and are migmatitic as compared to the surrounding rocks, suggesting a genetic relationship between the white quartz monzonite and tectonic exhumation. We propose that regional shortening imbricated the Big Creek Gneiss, uplifting the now-exposed high-grade rocks of the Big Creek Gneiss (hanging wall of the thrust and wall rock to the white quartz monzonite) and burying correlative rocks, which partially melted to form the white quartz monzonite. This tectonism is attributed to the ca. 1.65 Ga Mazatzal orogeny, as foreland shortening spread progressively into the Yavapai Province. Mazatzal foreland effects have also been described in the Great Lakes region and have been inferred in the Black Hills of South Dakota. We suggest that the crustal-scale rheologic contrast across the Archean-Proterozoic suture, originally developed along the southern margin of Laurentia, and including the Cheyenne belt, facilitated widespread reactivation of that boundary during the Mazatzal orogeny. This finding emphasizes the degree to which crustal heterogeneities can localize subsequent deformation in accretionary orogens, producing significant crustal melting in the distal foreland—a region not typically associated with orogenic magmatism.

Geochemical modeling of orogenic gold deposit using PCANN hybrid method in the Alut, Kurdistan province, Iran

NASA Astrophysics Data System (ADS)

Mohammadzadeh, Mohammadjafar; Nasseri, Aynur

2018-03-01

In this paper stream sediments based geochemical exploration program with the aim of delineating potentially promising areas by a comprehensive stepwise optimization approach from univariate statistics, PCA, ANN, and fusion method PCANN were under taken for an orogenic gold deposit located in the Alut, Kurdistan province, NW of Iran. At first the data were preprocessed and then PCA were applied to determine the maximum variability directions of elements in the area. Subsequently the artificial neural network (ANN) was used for quick estimation of elemental concentration, as well as discriminating anomalous populations and intelligent determination of internal structure among the data. However, both the methods revealed constraints for modeling. To overcome the deficiency and shortcoming of each individual method a new methodology is presented by integration of both "PCA & ANN" referred as PCANN method. For integrating purpose, the detected PCs pertinent to ore mineralization selected and intruded to neural network structure, as a result different MLPs with various algorithms and structures were produced. The resulting PCANN maps suggest that the gold mineralization and its pathfinder elements (Au, Mo, W, Bi, Sb, Cu, Pb, Ag & As) are associated with metamorphic host rocks intruded by granite bodies in the Alut area. In addition, more concealed and distinct Au anomalies with higher intensity were detected, confirming the privileges of the method in evaluating susceptibility of the area in delineating new hidden potential zones. The proposed method demonstrates simpler network architecture, easy computational implementation, faster training speed, as well as no need to consider any primary assumption about the behavior of data and their probability distribution type, with more satisfactory predicting performance for generating gold potential map of the area. Comparing the results of three methods (PCA, ANN and PCANN), representing the higher efficiency and more reliability of PCANN with lesser training time, simple structure, and correlate components while avoiding the duplicate entry of data to network. This study also suggests that in many similar cases integrated methods have capability to fix bugs more effectively and successfully in exploration programs.

Cl-rich hydrous mafic mineral assemblages in the Highiș massif, Apuseni Mountains, Romania

NASA Astrophysics Data System (ADS)

Bonin, Bernard; Tatu, Mihai

2016-08-01

The Guadalupian (Mid-Permian) Highiș massif (Apuseni Mountains, Romania) displays a bimodal igneous suite of mafic (gabbro, diorite) and A-type felsic (alkali feldspar granite, albite granite, and hybrid granodiorite) rocks. Amphibole is widespread throughout the suite, and yields markedly high chlorine contents. Three groups are identified: Cl-rich potassic hastingsite (2.60-3.40 wt% Cl) within A-type felsic rocks and diorite, mildly Cl-rich pargasite to hornblende (0.80-1.90 wt% Cl) within gabbro, and low F-Cl hornblende within gabbro and hybrid granodiorite. Coexisting biotite is either Cl-rich within diorite, or F-Cl-poor to F-rich within A-type felsic rocks. Chlorine and fluorine are distributed in both mafic phases, according to the F-Fe and Cl-Mg avoidance rules. The low-Ti contents suggest subsolidus compositions. Cl-rich amphibole within diorite and A-type felsic rocks yields a restricted temperature range - from 575 °C down to 400 °C, whereas mildly Cl-rich amphibole within gabbro displays the highest range - from 675 to 360 °C. Temperatures recorded by Cl-rich biotite within diorite range from 590 to 410 °C. Biotite within A-type felsic rocks yields higher temperatures than amphibole: the highest values- from 640 to 540 °C - are recorded in low-F-Cl varieties, whereas the lowest values- from 535 to 500 °C - are displayed by F-rich varieties. All data point to halogen-rich hydrothermal fluids at upper greenschist facies conditions percolating through fractures and shear zones and pervasively permeating the whole Highiș massif, with F precipitating as interstitial fluorite and Cl incorporating into amphibole, during one, or possibly several, hydrothermal episodes that would have occurred during a ~ 150 My-long period of time extending from the Guadalupian (Mid-Permian) to the Albian (Mid-Cretaceous).

Perogenesis of granites, Sharm El-Sheikh area, South Sinai, Egypt: petrological constrains and tectonic evolution

NASA Astrophysics Data System (ADS)

Sherif, Mahmoud I.; Ghoneim, Mohamed F.; Heikal, Mohamed Th. S.; El Dosuky, Bothina T.

2013-10-01

Precambrian granites of the Sharm El-Sheikh area in south Sinai, Egypt belong to collisional and post-collisional Magmatism (610-580 Ma). The granites are widely distributed in the northern part of the Neoproterozoic Arabian-Nubian Shield. South Sinai includes important components of successive multiple stages of upper crust granitic rocks. The earliest stages include monzogranite and syenogranites while the later stages produced alkali feldspar granites and riebeckite-bearing granites. Numerous felsic, mafic dikes and quartz veins traverse the study granites. Petrographically, the granitic rocks consist mainly of perthite, plagioclase, quartz, biotite and riebeckite. Analysis results portray monzogranites displaying calc-alkaline characteristics and emplaced in island-arc tectonic settings, whereas the syenogranites, alkali-feldspar granites and the riebeckite bearing-granites exhibit an alkaline nature and are enriched in HFSEs similar to granites within an extensional regime. Multi-element variation diagrams and geochemical characteristics reinforce a post-collision tectonic setting. REEs geochemical modeling reveals that the rocks were generated as a result of partial melting and fractionation of lower crust basaltic magma giving rise to A1 and A2 subtype granites. They were subsequently emplaced within an intraplate environment at the end of the Pan-African Orogeny.

Extension and gold mineralisation in the hanging walls of active convergent continental shear zones

NASA Astrophysics Data System (ADS)

Upton, Phaedra; Craw, Dave

2014-07-01

Orogenic gold-bearing quartz veins form in mountain belts adjacent to convergent tectonic boundaries. The vein systems, hosted in extensional structures within compressively deformed rocks, are a widespread feature of these orogens. In many cases the extensional structures that host gold-bearing veins have been superimposed on, and locally controlled by, compressional structures formed within the convergent orogen. Exploring these observations within the context of a three-dimensional mechanical model allows prediction of mechanisms and locations of extensional zones within convergent orogens. Our models explore the effect of convergence angle and mid-crustal strength on stress states and compare them to the Southern Alps and Taiwan. The dilatation zones coincide with the highest mountains, in the hanging walls of major plate boundary faults, and can extend as deep as the brittle-ductile transition. Extensional deformation is favoured in the topographic divide region of oblique orogens with mid-lower crustal rheology that promotes localisation rather than diffuse deformation. In the near surface, topography influences the stress state to a depth approximately equal to the topographic relief, bringing the rock closer to failure and rotating σ1 to near vertical. The distribution of gold-bearing extensional veins may indicate the general position of the topographic divide within exhumed ancient orogens.

Rotund versus skinny orogens: Well-nourished or malnourished gold?

USGS Publications Warehouse

Goldfarb, R.J.; Groves, D.I.; Gardoll, S.

2001-01-01

Orogenic gold vein deposits require a particular conjunction of processes to form and be preserved, and their global distribution can be related to broad-scale, evolving tectonic processes throughout Earth history. A heterogeneous distribution of formation ages for these mineral deposits is marked by two major Precambrian peaks (2800-2555 Ma and 2100-1800 Ma), a singular lack of deposits for 1200 m.y. (1800-600 Ma), and relatively continuous formation since then (after 600 Ma). The older parts of the distribution relate to major episodes of continental growth, perhaps controlled by plume-influenced mantle overturn events, in the hotter early Earth (ca. 1800 Ma or earlier). This worldwide process allowed preservation of gold deposits in cratons, roughly equidimensional, large masses of buoyant continental crust. Evolution to a less episodic, more continuous, modern-style plate tectonic regime led to the accretion of volcano-sedimentary complexes as progressively younger linear orogenic belts sorrounding the margins of the more buoyant cratons. The susceptibility of these linear belts to uplift and erosion can explain the overall lack of orogenic gold deposits at 1800-600 Ma, their exposure in 600-50 Ma orogens, the increasing importance of placer deposits back through the Phanerozoic since ca. 100 Ma, and the absence of gold deposits in orogenic belts younger than ca. 50 Ma.

The tectonothermal evolution of the Venezuelan Caribbean Mountain System: 40Ar/39Ar age insights from a Rodinian-related rock, the Cordillera de la Costa and Margarita Island

NASA Astrophysics Data System (ADS)

Fournier, Herbert W.; Lee, James K. W.; Urbani, Franco; Grande, Sebastián

2017-12-01

The Caribbean Mountain System in Venezuela contains rocks formed at high-pressure/low-temperature (HP/LT) conditions by the Cretaceous-Paleocene oblique collision occurred between the Caribbean and South American plates and involving Rodinian-related blocks. 40Ar/39Ar dating of rocks from the Cordillera de la Costa and Margarita Island has constrained key pre- and syntectonothermal events associated with the emplacement of this system. In a Rodinian marble, two phlogopite crystals of different grain sizes yield plateau ages of 888 ± 4 Ma and 874 ± 4 Ma. These results are interpreted as cooling ages after a major anorthosite-mangerite-charnockite-granite-suite intrusion at 920 Ma related to the break-up of Rodinia along the Amazonian-Baltica collisional zone - the Putumayo Orogen. Current plate reconstructions during the Neoproterozoic and previous age results indicate a correlation between the anorthositic complexes located in northwestern Venezuela (Yumare Complex) and southern Norway (Rogaland Complex), suggesting a similar tectonic setting during orogenic relaxation along the Amazonian and Baltica suture. A temperature-time path based on calculated Ar-closure temperatures of phlogopite indicates rapid cooling of 14 ± 4 °C Ma-1 from 920 Ma to 888 Ma, and a very slow to almost isothermal cooling of 4 ± 2 °C Ma-1 from 888 Ma to 874 Ma. On Margarita Island, magnesiohornblende and (alumino) barroisite from HP/LT rocks and muscovite from a leucocratic rock that was intruded before the HP/LT event yield identical ages within error at c. 54-47 Ma, consistent with previous dating results across the island utilizing different isotopic systems. The close association of these rocks and the Manzanillo Shear Zone indicates a main pathway for Ca-rich, 40Ar-free and hot fluids that locally raised the ambient T of the already exhumed and juxtaposed rock units. These fluids crystallized new hornblende and muscovite and thermally reset barroisite. This fluid activity event is 5 Ma younger than the last magmatic activity in the Aves Arc (c. 59 Ma) along its southern edge and related to the opening of the Grenada Basin. In the Cordillera de la Costa, magnesiohornblende, phengite, magnesian siderophyllite, ferroan phlogopite and K-feldspar from rock units of the Carayaca and Ávila terranes yield a wide range of ages (275-20 Ma). The oldest ages (275 Ma and 120 Ma) obtained from the Caruao Metatonalite suggest at least two thermal events at these times and are in agreement with the amalgamation of Pangaea and fluid infiltration, respectively. The remaining ages (35-20 Ma) are consistent with previous results and reflect short-lived fluid infiltrations related to tectonothermal events. A phengite age of 35 Ma from an omphacitite lens in the Antímano Marble of the Carayaca Terrane indicates a younger HP/LT event than previously stated. The preservation of this phengite age and constant Si-content profile indicate a short-lived HP/LT event followed by a very rapid exhumation. At 35-20 Ma rocks from the Ávila Terrane remained at shallower crustal levels than the ones of the Carayaca Terrane, but were thermally affected by tectonism and the infiltration of relatively cold and 40Ar-free fluids causing widespread chloritization of trioctahedral micas.

Accretionary and collisional orogenesis in the south domain of the western Central Asian Orogenic Belt (CAOB)

NASA Astrophysics Data System (ADS)

Cai, Keda; Long, Xiaoping; Chen, Huayong; Sun, Min; Xiao, Wenjiao

2018-03-01

The Central Asian Orogenic Belt (CAOB) was the result of long-lived multi-stage tectonic evolution, including Proterozoic to Paleozoic accretion and collision, Mesozoic intracontinental modification, and Cenozoic rapid deformation and uplift. The accretionary and collisional orogenesis of its early history generated a huge orogenic collage consisting of diverse tectonic units including island arcs, ophiolites, accretionary prisms, seamounts, oceanic plateaus and micro-continents. These incorporated orogenic components preserved valuable detailed information on orogenic process and continental crust growth, which make the CAOB a key region to understanding of continental evolution, mantle-crust interaction and associated mineralization. The western CAOB refers to the west region in North Xinjiang of China and circum-Balkash of Kazakhstan, with occurrences of the spectacular Kazakhstan orocline and its surrounding mountain belts. Because orogenic fabrics of this part mostly preserve their original features caused by the interactions among the southern Siberian active margin in the north and the Tarim Craton in the south, the western CAOB can be regarded as an ideal region to study the processes of the accretionary and collisional orogenesis and associated mineralization. Since a large number of researchers have been working on this region, research advances bloom strikingly in a short-time period. Therefore, we, in this special issue, focus on these new study advances on the south domain of the western CAOB, including the Kazakhstan collage system, Tianshan orogenic belt and Beishan region, and it is anticipated that this issue can draw more attention from the international research groups to be interested in the studies on orogenesis of the CAOB.

Tibet and Beyond: Magmatic Records from CIA (Caucasus-Iran-Anatolia) and Southern Tibet with Implications for Asian Orogeny and Continental Growth

NASA Astrophysics Data System (ADS)

Chung, Sun-Lin

2016-04-01

This study, based on an ongoing joint research project "Tibet and Beyond", presents a synthesis of principal magmatic records from the CIA (Caucasus-Iran-Anatolia) and Tibet-Himalaya orogens resulting from the continental collisions of Arabia and India, respectively, with Eurasia. In both orogens, through this and other recent studies, the temporal and spatial variations in magmatism pre-, syn- and post-dating the collisions can now be much better defined, thus improving our understanding of collision zone magmatism that appears to have evolved with changes in the lithospheric structures over time and space by collisional processes. The two "collisional" Tethyan orogens were preceded by accretionary orogenic processes, which not only had produced a substantial amount of juvenile continental crust but also fulfill the "orogenic cycle" that evolved from an accretionary into a collisional system. Geochemical data reveal that in contrast to generating vast portions of juvenile crust in the early, accretionary stages of orogenic development, crustal recycling plays a more important role in the later, collisional stages. The latter, as exemplified in SE Turkey and southern Tibet, involves addition of older continental crust material back into the mantle, which subsequently melted and caused compositional transformation of the juvenile crust produced in the accretionary stages. Similar features are observed in young volcanic rocks from eastern Taiwan, the northern Luzon arc complex and part of the active subduction/accretion/collision system in Southeast Asia that may evolve one day to resemble the eastern Tethyan and central Asian orogenic belts by collision with the advancing Australian continent.

Tectonic stress evolution in the Pan-African Lufilian Arc and its foreland (Katanga, DRC): orogenic bending, late orogenic extensional collapse and transition to rifting

NASA Astrophysics Data System (ADS)

Kipata, M. L.; Delvaux, D.; Sebagenzi, M. N.; Cailteux, J.; Sintubin, M.

2012-04-01

Between the paroxysm of the Lufilian orogeny at ~ 550 Ma and the late Neogene to Quaternary development of the south-western branch of the East African rift system, the tectonic evolution of the Lufilian Arc and Kundelungu foreland in the Katanga region of the Democratic Republic of Congo remains poorly unknown although it caused important Cu-dominated mineral remobilizations leading to world-class ore deposits. This long period is essentially characterized by brittle tectonic deformations that have been investigated by field studies in open mines spread over the entire arc and foreland. Paleostress tensors were computed for a database of 1450 fault-slip data by interactive stress tensor inversion and data subset separation, and the relative succession of 8 brittle deformation events established. The oldest brittle structures observed are related to the Lufilian brittle compressional climax (stage 1). They have been re-oriented during the orogenic bending that led to the arcuate shape of the belt. Unfolding the stress directions from the first stage allows to reconstruct a consistent NE-SW direction of compression for this stage. Constrictional deformation occurred in the central part of the arc, probably during orogenic bending (Stage 2). After the orogenic bending, a sequence of 3 deformation stages marks the progressive onset of late-orogenic extension: strike-slip deformations (stages 3-4) and late-orogenic arc-parallel extension (stage 5). It is proposed that these 3 stages correspond to orogenic collapse. In early Mesozoic, NW-SE compression was induced by a transpressional inversion, interpreted as induced by far-field stresses generated at the southern active margin of Gondwana (stage 6). Since then, this region was affected by rift-related extension, successively in a NE-SW direction (stage 7, Tanganyika trend) and NW-SE direction (stage 8, Moero trend).

Non-linear feedbacks drive strain partitioning within an active orogen, southern Alaska

NASA Astrophysics Data System (ADS)

Hooks, B.; Koons, P. O.; Upton, P.

2011-12-01

Temperature plays a very important role in the partitioning of deformation within an active orogen. Local variations in the thermal structure of actively uplifting areas can reinforce focused partitioning of strain locally, whereas regional variations can alter deformation patterns on a much broader scale resulting in the re-organization of an entire orogen. Within southern Alaska, the Yakutat micro-terrane has been subducting beneath North America over the previous ~10 Ma. Early deformation related to this event drove uplift of the Alaska Range, as evidenced by stratigraphic and thermochronologic datasets. This was followed by a southerly discontinuous spatial jump in the deformation front to the coastal St. Elias Range. Here we present 3D numerical models that simulate deformation of Earth materials given assigned applied velocity boundary conditions and mechanical and thermal constitutive relationships on a macro- (plate boundary) and meso-scale (<50-km). The goal is to reproduce first-order strain and uplift patterns within this evolving orogen. The macro-scale model undergoes a spatial and temporal reorganization of deformation as strain is progressively shifted to a trench-ward orogenic wedge, the inlet orogen. Subduction related cooling of the fore-arc (i.e. tectonic refrigeration) provides control on the location of the inlet orogen. This control is based upon the creation of a thin sliver of cold, strong material along the mega-thrust interface. The stronger mega-thrust facilitates more efficient transfer of strain, driving the formation of the inlet orogen and determining the location of its frontal toe. This toe is further stabilized by upward displacement of the upper crust over the refrigerated section. This upward motion causes thermal weakening of the upper crust as a tectonic aneurysm with the location controlled by the thermally strengthened lower crust. The net result is an ever weakening upper crust that focuses strain creating dramatic topography, extreme rates of erosion and uplift, and fast exhumation.

The Capricorn Orogen Passive source Array (COPA) in Western Australia

NASA Astrophysics Data System (ADS)

Gessner, K.; Yuan, H.; Murdie, R.; Dentith, M. C.; Johnson, S.; Brett, J.

2015-12-01

COPA is the passive source component of a multi-method geophysical program aimed at assessing the mineral deposits potential of the Proterozoic Capricorn Orogen. Previous results from the active source surveys, receiver functions and magnetotelluric studies show reworked orogenic crust in the orogen that contrasts with more simple crust in the neighbouring Archean cratons, suggesting progressive and punctuated collisional processes during the final amalgamation of the Western Australian craton. Previous seismic studies are all based on line deployment or single station analyses; therefore it is essential to develop 3D seismic images to test whether these observations are representative for the whole orogen. With a careful design that takes advantage of previous passive source surveys, the current long-term and short-term deployments span an area of approximately 500 x 500 km. The 36-month total deployment can guarantee enough data recording for 3D structure imaging using body wave tomography, ambient noise surface wave tomography and P- and S-wave receiver function Common Conversion Point (CCP) stacking techniques. A successive instrument loan from the ANSIR national instrument pool, provided 34 broadband seismometers that have been deployed in the western half of the orogen since March 2014. We expect approximately 40-km lateral resolution near the surface for the techniques we propose, which due to low frequency nature of earthquake waves will degrade to about 100 km near the base of the cratonic lithosphere, which is expected at depths between 200 to 250 km. Preliminary results from the first half of the COPA deployment will be presented in the light of the hypotheses that 1) distinct crustal blocks can be detected continuously throughout the orogen (using ambient noise/body wave tomography); 2) distinct lithologies are present in the crust and upper mantle across the orogen (using receiver function CCP images); and 3) crustal and lithosphere deformation along craton margins in general follows the "wedge" tectonic model (e.g. subduction of Juvenile blocks under the craton mantle as represented by craton-ward dipping sutures.

The effect of scale on the interpretation of geochemical anomalies

USGS Publications Warehouse

Theobald, P.K.; Eppinger, R.G.; Turner, R.L.; Shiquan, S.

1991-01-01

The purpose of geochemical surveys changes with scale. Regional surveys identify areas where mineral deposits are most likely to occur, whereas intermediate surveys identify and prioritize specific targets. At detailed scales specific deposit models may be applied and deposits delineated. The interpretation of regional geochemical surveys must take into account scale-dependent difference in the nature and objectives of this type of survey. Overinterpretation of regional data should be resisted, as should recommendations to restrict intermediate or detailed follow-up surveys to the search for specific deposit types or to a too limited suite of elements. Regional surveys identify metallogenic provinces within which a variety of deposit types and metals are most likely to be found. At intermediate scale, these regional provinces often dissipate into discrete clusters of anomalous areas. At detailed scale, individual anomalous areas reflect local conditions of mineralization and may seem unrelated to each other. Four examples from arid environments illustrate the dramatic change in patterns of anomalies between regional and more detailed surveys. On the Arabian Shield, a broad regional anomaly reflects the distribution of highly differentiated anorogenic granites. A particularly prominent part of the regional anomaly includes, in addition to the usual elements related to the granites, the assemblage of Mo, W and Sn. Initial interpretation suggested potential for granite-related, stockwork Mo deposits. Detailed work identified three separate sources for the anomaly: a metal-rich granite, a silicified and stockwork-veined area with scheelite and molybdenite, and scheelite/powellite concentrations in skarn deposits adjacent to a ring-dike complex. Regional geochemical, geophysical and remote-sensing data in the Sonoran Desert, Mexico, define a series of linear features interpreted to reflect fundamental, northeast-trending fractures in the crust that served as the prime conduits for mineralizing fluids. At a larger scale, the linear, northeast-trending anomalies can be shown to result from a series of discrete mineralized systems with different ages and mineral assemblages. The linear pattern of anomalies disintegrates. A regional geochemical survey in the Sonoran Desert in southwestern Arizona displays a cluster of samples anomalous in Pb, Mo, Bi and W. In detail, the original regional anomaly separates into four discrete anomalous areas, each with its own distinctive suite of elements, geographic distribution and age of mineralization. A prominent regional gold anomaly in the Gobi Desert, Xinjiang, Peoples Republic of China, extends southeastward for 30 km from known lode gold deposits. Because the anomaly cuts both lithologic units and the structural grain, and because it parallels the prevailing direction of high-velocity winds, it was originally attributed to eolian dispersion. In detail, the regional anomaly consists of several east-west-trending anomalies, parallel to local lithology and structure that most likely reflect independent sources of lode gold. The regional anomaly results from smoothing of an en-echelon set of local anomalies. These examples emphasize that interpretation of regional anomalies must be tempered to consider regional-sized geologic features. Attempts to overinterpret anomalies by assigning deposit-scale attributes to regional anomalies can lead to confusion and incorrect interpretations. Potential targets that can be readily resolved only at intermediate or detailed scales of study may be overlooked. ?? 1991.

Late Triassic granites from Bangka, Indonesia: A continuation of the Main Range granite province of the South-East Asian Tin Belt

NASA Astrophysics Data System (ADS)

Ng, Samuel Wai-Pan; Whitehouse, Martin J.; Roselee, Muhammad H.; Teschner, Claudia; Murtadha, Sayed; Oliver, Grahame J. H.; Ghani, Azman A.; Chang, Su-Chin

2017-05-01

The South-East Asian Tin Belt is one of the most tin-productive regions in the world. It comprises three north-south oriented granite provinces, of which the arc-related Eastern granite province and the collision-related Main Range granite province run across Thailand, Singapore, and Indonesia. These tin-producing granite provinces with different mineral assemblages are separated by Paleo-Tethyan sutures exposed in Thailand and Malaysia. The Eastern Province is usually characterised by granites with biotite ± hornblende. Main Range granites are sometimes characterised by the presence of biotite ± muscovite. However, the physical boundary between the two types of granite is not well-defined on the Indonesian Tin Islands, because the Paleo-Tethyan suture is not exposed on land there. Both hornblende-bearing (previously interpreted as I-type) and hornblende-barren (previously interpreted as S-type) granites are apparently randomly distributed on the Indonesian Tin Islands. Granites exposed on Bangka, the largest and southernmost Tin Island, no matter whether they are hornblende-bearing or hornblende-barren, are geochemically similar to Malaysian Main Range granites. The average ɛNd(t) value obtained from the granites from Bangka (average ɛNd(t) = -8.2) falls within the range of the Main Range Province (-9.6 to -5.4). These granites have SIMS zircon U-Pb ages of ca. 225 Ma and ca. 220 Ma, respectively that are both within the period of Main Range magmatism (∼226-201 Ma) in the Peninsular Malaysia. We suggest that the granites exposed on Bangka represent the continuation of the Main Range Province, and that the Paleo-Tethyan suture lies to the east of the island.

Quaternary tectonic evolution of the Pamir-Tian Shan convergence zone, Northwest China

NASA Astrophysics Data System (ADS)

Thompson Jobe, Jessica Ann; Li, Tao; Chen, Jie; Burbank, Douglas W.; Bufe, Aaron

2017-12-01

The Pamir-Tian Shan collision zone in the western Tarim Basin, northwest China, formed from rapid and ongoing convergence in response to the Indo-Eurasian collision. The arid landscape preserves suites of fluvial terraces crossing structures active since the late Neogene that create fault and fold scarps recording Quaternary deformation. Using geologic and geomorphic mapping, differential GPS surveys of deformed terraces, and optically stimulated luminescence dating, we create a synthesis of the active structures that delineate the timing, rate, and migration of Quaternary deformation during ongoing convergence. New deformation rates on eight faults and folds, when combined with previous studies, highlight the spatial and temporal patterns of deformation within the Pamir-Tian Shan convergence zone during the Quaternary. Terraces spanning 130 to 8 ka record deformation rates between 0.1 and 5.6 mm/yr on individual structures. In the westernmost Tarim Basin, where the Pamir and Tian Shan are already juxtaposed, the fastest rates occur on actively deforming structures at the interface of the Pamir-Tian Shan orogens. Farther east, as the separation between the Pamir-Tian Shan orogens increases, the deformation has not been concentrated on a single structure, but rather has been concurrently distributed across a zone of faults and folds in the Kashi-Atushi fold-and-thrust belt and along the NE Pamir margin, where shortening rates vary on individual structures during the Quaternary. Although numerous structures accommodate the shortening and the locus of deformation shifts during the Quaternary, the total shortening across the western Tarim Basin has remained steady and approximately matches the current geodetic rate of 6-9 mm/yr.

Oxygen isotope studies of early Precambrian granitic rocks from the Giants Range batholith, northeastern Minnesota, U.S.A.

USGS Publications Warehouse

Viswanathan, S.

1974-01-01

Oxygen isotope studies of granitic rocks from the 2.7 b.y.-old composite Giants Range batholith show that: (1) ??(O18)quartz values of 9 to 10 permil characterize relatively uncontaminated Lower Precambrian, magmatic granodiorites and granites; (2) granitic rocks thought to have formed by static granitization have ??(O18)quartz values that are 1 to 2 permil higher than magmatic granitic rocks; (3) satellite leucogranite bodies have values nearly identical to those of the main intrusive phases even where they transect O18-rich metasedimentary wall rocks; (4) oxygen isotopic interaction between the granitic melts and their O18-rich wall rocks was minimal; and (5) O18/O18 ratios of quartz grains in a metasomatic granite are largely inherited from the precursor rock, but during the progression - sedimentary parent ??? partially granitized parent ??? metasomatic granite ??? there is gradual decrease in ??(O18)quartz by 1 to 2 permil. ?? 1974.

Late Cretaceous stratigraphy, deformation and intrusion in the Madison Range of southwestern Montana ( USA).

USGS Publications Warehouse

Tysdal, R.G.; Marvin, R.F.; Dewitt, E.

1986-01-01

Dating of orogenic rock units in the central part of the Madison Range shows that Laramide deformation was virtually completed by the end of the Cretaceous. Early Campanian K-Ar dates of about 79 m.y. were obtained from welded tuffs in the basal part of the Livingston Formation, a volcanic and volcaniclastic assemblage that is conformable with underlying Cretaceous clastic rocks and with the overlying Sphinx Conglomerate. The Sphinx and the Livingston were deformed by the Hilgard fault system which extends along the western side of the southern two-thirds of the range. This north-trending fault system represents the culmination of Laramide shortening within the range. Dating of hornblende indicates an approximate date of 68-69 m.y. B.P. for emplacement of the igneous suite. The dacite postdates movement along faults of the Hilgard fault system, and postdates the synorogenic Sphinx Conglomerate. -from Authors

Petrogenesis of the Late Jurassic peraluminous biotite granites and muscovite-bearing granites in SE China: geochronological, elemental and Sr-Nd-O-Hf isotopic constraints

NASA Astrophysics Data System (ADS)

Jiang, Yao-Hui; Zhu, Shu-Qi

2017-12-01

Biotite granites and muscovite-bearing granites are dominant rock types of the widespread granites in SE China. However, their petrogenesis has been enigmatic. A combined study of zircon U-Pb dating and Lu-Hf isotopes, whole-rock element geochemistry and Sr-Nd-O isotopes was performed for three late Mesozoic granitic plutons (Xinfengjie, Jiangbei and Dabu) in central Jiangxi province, SE China. All the plutons are composed of biotite granites and muscovite-bearing granites that have been poorly investigated previously. The new data not only allow us to assess their sources and magma evolution processes, but also helps us to better understand the genetic link to the large-scale polymetallic mineralization in SE China. LA-ICP-MS zircon U-Pb dating shows that three plutons were emplaced in the Late Jurassic (159-148 Ma) and that the muscovite-bearing granites are almost contemporaneous with the biotite granites. The biotite granites have SiO2 contents of 70.3-74.4 wt% and are weakly to strongly peraluminous with ASI from 1.00 to 1.26, and show a general decrease in ASI with increasing SiO2. They have relatively high zircon saturation temperatures ( T Zr = 707-817 °C, most > 745 °C) and show a general decrease in T Zr with increasing SiO2. They have high initial 87Sr/86Sr ratios (0.7136 to 0.7166) and high δ18O values (9.1-12.8‰, most > 9.5‰) and clearly negative ɛ Nd (T) (- 9.5 to - 11.8) and ɛ Hf (T) (in situ zircon) (- 13.1 to - 13.5). The muscovite-bearing granites have high SiO2 contents (74.7-78.2 wt%). They are also weakly to strongly peraluminous with ASI of 1.04-1.18 but show a general increase in ASI with increasing SiO2. They have relatively low T Zr (671-764 °C, most < 745 °C) and also show a general decrease in T Zr with increasing SiO2. The muscovite-bearing granites have high Rb (up to 810 ppm) and high (K2O + Na2O)/CaO (up to 270), Rb/Sr (up to 42) and Rb/Ba (up to 30) as well as low K/Rb (< 150, down to 50), Zr/Hf (< 24, down to 11) and Nb/Ta (< 6, down to 2). They show similar Nd-O-Hf isotopic compositions to the biotite granites with ɛ Nd (T) of - 8.7 to - 12.0, δ18O of 8.7-13.0‰ (most > 9.5‰) and ɛ Hf (T) (in situ zircon) of - 11.3 to - 13.1. Geochemical data suggest the origin of the biotite granites and muscovite-bearing granites as follows: Partial melting of Precambrian metasedimentary rocks (mainly two-mica schist) in the lower crust at temperatures of ca. 820 °C generated the melts of the less felsic biotite granites. Such primary crustal melts underwent biotite-dominant fractionation crystallization, forming the felsic biotite granites. Progressive plagioclase-dominant fractionation crystallization from the evolved biotite granites produced the more felsic muscovite-bearing granites. Thus, the biotite granites belong to the S-type whereas the muscovite-bearing granites are highly fractionated S-type granites. We further suggest that during the formation of the muscovite-bearing granites the fractional crystallization was accompanied by fluid fractionation and most likely the addition of internally derived mineralizing fluids. That is why the large-scale polymetallic mineralization is closely related to the muscovite-bearing granites rather than biotite granites in SE China. This is important to further understand the source and origin of biotite granites and muscovite-bearing granites in SE China even worldwide.

Petrogenesis of the Zheduoshan Cenozoic granites in the eastern margin of Tibet: Constraints on the initial activity of the Xianshuihe Fault

NASA Astrophysics Data System (ADS)

Lai, Shao-cong; Zhao, Shao-wei

2018-06-01

The Zheduoshan Miocene granitic pluton is exposed at the eastern margin of Tibet and along the strike-slip Xianshuihe Fault, and is the product of syn-tectonic magmatism closely related to this fault. This paper is focused on the petrogenesis of different granitic lithological units in the Zheduoshan composite intrusion, and the results of geochronology and lithology show that the Zheduoshan Miocene granitic pluton is incremental assembly by three stages of granitic magma influx and growth, represented by fine-grain biotite granite at 18.0 Ma, corase-grain and porphyraceous biotite monzogranite at 16.0 Ma and medium-grain two-mica monzogranite at 14.0 Ma. Combining with the geochemical signatures, these granitic rocks have high intial 87Sr/86Sr ratios, enriched Nd and Hf isotopic compositions, revealing that the sources of these granitic rocks are metabasatic rocks for fine-grain biotite granite, greywackes for coarse-grain biotite monzogranite and medium-grain monzogranite. These granites have high Sr/Y ratios, revealing that these granitic magma form at high pressure condition. The Sr/Y ratios and calculated crystallization pressure gradually decreased, implying the pressure gradually decreasing with the formation of these three stages of granites, which is probably caused by the tectonic mechanism transition from compression to strike-slip extension during the generation of these granites at 18.0-14.4 Ma. This tectonic mechanism change implied the initial activity of Xianshuihe Fault at least before 14.4 Ma.

From a collage of microplates to stable continental crust - an example from Precambrian Europe

NASA Astrophysics Data System (ADS)

Korja, Annakaisa

2013-04-01

Svecofennian orogen (2.0-1.7 Ga) comprises the oldest undispersed orogenic belt on Baltica and Eurasian plate. Svecofennian orogenic belt evolved from a series of short-lived terrane accretions around Baltica's Archean nucleus during the formation of the Precambrian Nuna supercontinent. Geological and geophysical datasets indicate W-SW growth of Baltica with NE-ward dipping subduction zones. The data suggest a long-lived retreating subduction system in the southwestern parts whereas in the northern and central parts the northeasterly transport of continental fragments or microplates towards the continental nucleus is also documented. The geotectonic environment resembles that of the early stages of the Alpine-Himalayan or Indonesian orogenic system, in which dispersed continental fragments, arcs and microplates have been attached to the Eurasian plate margin. Thus the Svecofennian orogeny can be viewed as proxy for the initial stages of an internal orogenic system. Svecofennian orogeny is a Paleoproterozoic analogue of an evolved orogenic system where terrane accretion is followed by lateral spreading or collapse induced by change in the plate architecture. The exposed parts are composed of granitoid intrusions as well as highly deformed supracrustal units. Supracrustal rocks have been metamorphosed in LP-HT conditions in either paleo-lower-upper crust or paleo-upper-middle crust. Large scale seismic reflection profiles (BABEL and FIRE) across Baltica image the crust as a collage of terranes suggesting that the bedrock has been formed and thickened in sequential accretions. The profiles also image three fold layering of the thickened crust (>55 km) to transect old terrane boundaries, suggesting that the over-thickened bedrock structures have been rearranged in post-collisional spreading and/or collapse processes. The middle crust displays typical large scale flow structures: herringbone and anticlinal ramps, rooted onto large scale listric surfaces also suggestive of spreading. Close to the original ocean-continent plate boundary, in the core of the Svecofennian orogen, the thickened accretionary crust carries pervasive stretching lineations at surface and seismic vp-velocity anisotropy in the crust. The direction of spreading and crustal flow seems to be diverted by shapes of the pre-existing boundaries. It is concluded that lateral spreading and midcrustal flow not only rearrange the bedrock architecture but also stabilize the young accreted continental crust in emerging internal orogenic systems. Pre-existing microplate/terrane boundaries will affect the final architecture of the orogenic belt.

Structure and Tectonics of the Saint Elias Orogen

NASA Astrophysics Data System (ADS)

Bruhn, R. L.; Pavlis, T. L.; Plafker, G.; Serpa, L.; Picornell, C.

2001-12-01

The Saint Elias orogen of western Canada and southern Alaska is a complex mountain belt formed by transform faulting and subduction between the Pacific and North American plates, and collision of the Yakutat terrane. The orogen is segmented into three regions of different structural style caused by lateral variations in transpression and processes of terrane accretion. Deformation is strain and displacement partitioned throughout the orogen; transcurrent motion is focused along discrete strike-slip faults, and shortening is distributed among reverse faults and folds with sub-horizontal axes. Plunging folds accommodate horizontal shortening and extension in the western part of the orogen. Segment boundaries extend across the Yakutat terrane where they coincide with the courses of huge piedmont glaciers that flow from the topographic backbone of the range onto the coastal plain. The eastern segment is marked by strike-slip faulting along the Fairweather transform fault and by a narrow belt of reverse faulting where the transpression ratio is 0.4:1 shortening to dextral shear. The transpression ratio is 1.7:1 in the central part of the orogen where a broad thin-skinned fold and thrust belt deforms the Yakutat terrane south of the Chugach-Saint Elias (CSE) suture. Dextral shearing is accommodated by strike-slip faulting beneath the Seward and Bagley glaciers in the hanging wall of the CSE suture, and partly by reverse faulting along a structural belt that cuts across the Yakutat terrane along the western edge of the Malaspina Glacier and links to the Pamplona fold and thrust belt offshore. Deformation along this segment boundary is probably also driven by vertical axis bending of the Yakutat microplate during collision. Subduction & accretion in the western segment of the orogen causes re-folding of previously formed structures when they are emplaced into the upper plate of the Alaska-Aleutian mega-thrust. Second phase folds plunge at moderate to steep angles and accretion is marked by only modest amounts of uplift. The structural boundary between the central and western segments of the orogen localizes the course of the Bering piedmont glacier. The structural segments coincide with subdivisions in historical seismicity, particularly ruptures of great to large magnitude earthquakes. The results of this structural study provide the requisite geological framework to design new-generation geophysical monitoring systems to study active deformation within the orogen.

Linking time-Temperature history of the Aquitaine basin with post-orogenic evolution of the Pyrenees : new insights from borehole thermochronology

NASA Astrophysics Data System (ADS)

Fillon, Charlotte; Calassou, Sylvain; Mouthereau, Frédéric; Pik, Raphaël; Bellahsen, Nicolas; Gautheron, Cécile

2017-04-01

Within their sedimentary record, foreland basins document vertical movements of the lithosphere, climatic changes, paleogeograhic evolution but also history of exhumation of the adjacent mountain belt. Comparing vertical movements in a range and in its foreland is key to identify processes involved in growth and destruction of mountain belts. The Aquitaine basin, geomorphologically stable since the early Pyrenean orogenesis has the potential to help understanding the driving mechanisms during the late to post-orogenic phases, but the lack of outcrops makes the studies particularly difficult to achieve. To bring a new point of vue on the processes involved in the Cenozoic exhumation of this range, we present new low-Temperature thermochronology data from boreholes of the Aquitaine basin. With the objectives to study rift-related to post-orogenic processes, numerous low-T thermochronological ages ( 300 across the range) have been published, documenting pre-, syn- , and post-orogenic exhumation in the Pyrenees. Using thermal modeling of a new low-T database in the western Axial Zone, we show that a late Miocene (around 10 Ma) uplift occured in the western Pyrenees, which generalizes the post-orogenic signal already detected in the south central Pyrenees. In previous studies, we linked the post-orogenic exhumation in the Southern Pyrenees to the excavation of the foreland valleys caused by the opening of the endorheic Ebro basin towards the Mediterranean Sea. To the West, the tectonic out-of sequence reactivation of the Gavarnie thrust has been invoked to explain the late Miocene AHe ages in the Bielsa massif. These new data might lead us to re-think the causes for such an exhumation signal during "post-orogenic" times. We thus summarize all evidences for the post-orogenic phase and attempt to provide explanation for it: is exhumation driven by Aquitaine foreland basin evolution? Does it reflect a tectonic reactivation of the Pyrenees? or is the signature of a regional/global climate change conditions ? To answer these questions, we present a new dataset of ZHe and AFT ages from borehole samples in three localities of the Aquitaine basin. We use these new data to link the late Miocene exhumation history with the vertical movements in the Aquitaine basin. This study is part of the Orogen projet, an academic-industrial collaboration (CNRS-Total-BRGM)

Accretionary orogens through Earth history

USGS Publications Warehouse

Cawood, Peter A.; Kroner, A.; Collins, W.J.; Kusky, T.M.; Mooney, W.D.; Windley, B.F.

2009-01-01

Accretionary orogens form at intraoceanic and continental margin convergent plate boundaries. They include the supra-subduction zone forearc, magmatic arc and back-arc components. Accretionary orogens can be grouped into retreating and advancing types, based on their kinematic framework and resulting geological character. Retreating orogens (e.g. modern western Pacific) are undergoing long-term extension in response to the site of subduction of the lower plate retreating with respect to the overriding plate and are characterized by back-arc basins. Advancing orogens (e.g. Andes) develop in an environment in which the overriding plate is advancing towards the downgoing plate, resulting in the development of foreland fold and thrust belts and crustal thickening. Cratonization of accretionary orogens occurs during continuing plate convergence and requires transient coupling across the plate boundary with strain concentrated in zones of mechanical and thermal weakening such as the magmatic arc and back-arc region. Potential driving mechanisms for coupling include accretion of buoyant lithosphere (terrane accretion), flat-slab subduction, and rapid absolute upper plate motion overriding the downgoing plate. Accretionary orogens have been active throughout Earth history, extending back until at least 3.2 Ga, and potentially earlier, and provide an important constraint on the initiation of horizontal motion of lithospheric plates on Earth. They have been responsible for major growth of the continental lithosphere through the addition of juvenile magmatic products but are also major sites of consumption and reworking of continental crust through time, through sediment subduction and subduction erosion. It is probable that the rates of crustal growth and destruction are roughly equal, implying that net growth since the Archaean is effectively zero. ?? The Geological Society of London 2009.

Oblique contractional reactivation of inherited heterogeneities: Cause for arcuate orogens

PubMed Central

Sokoutis, D.; Willingshofer, E.; Brun, J.‐P.; Gueydan, F.; Cloetingh, S.

2017-01-01

Abstract We use lithospheric‐scale analog models to study the reactivation of pre‐existing heterogeneities under oblique shortening and its relation to the origin of arcuate orogens. Reactivation of inherited rheological heterogeneities is an important mechanism for localization of deformation in compressional settings and consequent initiation of contractional structures during orogenesis. However, the presence of an inherited heterogeneity in the lithosphere is in itself not sufficient for its reactivation once the continental lithosphere is shortened. The heterogeneity orientation is important in determining if reactivation occurs and to which extent. This study aims at giving insights on this process by means of analog experiments in which a linear lithospheric heterogeneity trends with various angles to the shortening direction. In particular, the key parameter investigated is the orientation (angle α) of a strong domain (SD) with respect to the shortening direction. Experimental results show that angles α ≥ 75° (high obliquity) allow for reactivation along the entire SD and the development of a linear orogen. For α ≤ 60° (low obliquity) the models are characterized by the development of an arcuate orogen, with the SD remaining partially non‐reactivated. These results provide a new mechanism for the origin of some arcuate orogens, in which orocline formation was not driven by indentation or subduction processes, but by oblique shortening of inherited heterogeneities, as exemplified by the Ouachita orogen of the southern U.S. PMID:28670046

Petrochemistry and zircon U-Pb geochronology of granitic rocks in the Wang Nam Khiao area, Nakhon Ratchasima, Thailand: Implications for petrogenesis and tectonic setting

NASA Astrophysics Data System (ADS)

Fanka, Alongkot; Tsunogae, Toshiaki; Daorerk, Veerote; Tsutsumi, Yukiyasu; Takamura, Yusuke; Sutthirat, Chakkaphan

2018-05-01

Carboniferous biotite granite, Late Permian hornblende granite, and Triassic biotite-hornblende granite, all of which belong to the Eastern Granite Belt, expose in the Wang Nam Khiao area, Nakhon Ratchasima, northeastern Thailand. The Carboniferous biotite granite is dominated by quartz, K-feldspar, plagioclase, and biotite. The Late Permian hornblende granite contains dominant assemblages of plagioclase, quartz, K-feldspar, hornblende, and minor amount of biotite, while the Triassic biotite-hornblende granite consists of quartz, plagioclase, K-feldspar with small amounts of biotite, and hornblende. The REE patterns with steep decrease from light to heavy REE together with the LILE (e.g. K, Sr) enrichment and depletion of some particular HFSE (e.g. Nb, Ti) indicate low degree of partial melting. Mineral chemistry of biotite and hornblende in the granites reflects crystallization from hydrous calc-alkaline arc-derived magmas possibly formed by subduction. Amphibole-plagioclase thermometry and Al-in-hornblende barometry indicate that the Late Permian hornblende granite and the Triassic biotite-hornblende granite may have equilibrated at 3.0-5.8 kbar/700-820 °C and 2.0-3.2 kbar/600-750 °C, respectively, in the middle-upper crust (about 10-15 km depth). Zircon U-Pb geochronology of the Carboniferous biotite granite, Late Permian hornblende granite and Triassic biotite-hornblende granite yielded intrusion ages of 314.6-284.9 Ma, 253.4 Ma, and 237.8 Ma, respectively, which implies multiple episodes of arc-magmatism formed by Palaeo-Tethys subduction beneath Indochina Terrane during Late Carboniferous/Early Permian, Late Permian and Middle Triassic.

The crustal structures from Wuyi-Yunkai orogen to Taiwan orogen: the onshore-offshore wide-angle seismic experiment of TAIGER and ATSEE projects

NASA Astrophysics Data System (ADS)

Kuochen, H.; Kuo, N. Y. W.; Wang, C. Y.; Jin, X.; Cai, H. T.; Lin, J. Y.; Wu, F. T.; Yen, H. Y.; Huang, B. S.; Liang, W. T.; Okaya, D. A.; Brown, L. D.

2015-12-01

The crustal structure is key information for understanding the tectonic framework and geological evolution in the southeastern China and its adjacent area. In this study, we integrated the data sets from the TAIGER and ATSEE projects to resolve onshore-offshore deep crustal seismic profiles from the Wuyi-Yunkai orogen to the Taiwan orogen in southeastern China. Totally, there are three seismic profiles resolved and the longest profile is 850 km. Unlike 2D and 3D first arrival travel-time tomography from previous studies, we used both refracted and reflected phases (Pg, Pn, PcP, and PmP) to model the crustal structures and the crustal reflectors. 40 shots, 2 earthquakes, and about 1,950 stations were used and 15,319 arrivals were picked among three transects. As a result, the complex crustal evolution since Paleozoic era are shown, which involved the closed Paleozoic rifted basin in central Fujian, the Cenozoic extension due to South China sea opening beneath the coastline of southern Fujian, and the on-going collision of the Taiwan orogen.

76 FR 60493 - Settlement Agreements for Recovery of Past Response Costs; Granite Timber Post and Pole Site...

Federal Register 2010, 2011, 2012, 2013, 2014

2011-09-29

... Costs; Granite Timber Post and Pole Site, Philipsburg, Granite County, MT AGENCY: Environmental... Agency (EPA) and Margery Metesh (Settling Party) and Mark Metesh (Settling Party), regarding the Granite... Granite County, Montana. The Settlement Agreements propose to compromise a claim the United States has at...

77 FR 14426 - Petitions for Modification of Application of Existing Mandatory Safety Standards

Federal Register 2010, 2011, 2012, 2013, 2014

2012-03-09

... Granite Company, LLC, 369 North State Street, Concord, New Hampshire 03301. Mine: Swenson Granite gray... Swenson Granite Gray Quarry is an open dimensional granite quarry operating fixed stiffleg derricks to... granite product if it can not be seen. (6) The quarry employees direct all derrick moves and positioning...

Geochronology and geochemistry of the granites from the Zhuxi W-Cu ore deposit in South China: Implication for petrogenesis, geodynamical setting and mineralization

NASA Astrophysics Data System (ADS)

Pan, Xiaofei; Hou, Zengqian; Zhao, Miao; Chen, Guohua; Rao, Jianfeng; Li, Yan; Wei, Jin; Ouyang, Yongpeng

2018-04-01

The giant Zhuxi tungsten deposit is located in the Taqian-Fuchun Ore Belt in northeastern Jiangxi province, and genetically associated with the Zhuxi granitic stocks and dykes. Three mineralization-related granites including granite porphyry dykes (GP), biotite granitic stocks (BG), and white granitic dykes (WG), were identified in the Zhuxi deposit. SHRIMP zircon U-Pb analysis for the three granitic rocks present ages ranging from 153.5 ± 1.0 Ma to 150.4 ± 1.0 Ma. The BG mainly contains quartz, microcline, albite, biotite and muscovite with minor accessory minerals including zircon, apatite, monazite, Ti/Fe oxides, and dolerite. However, the WG is mainly composed of quartz, microcline and albite with minor muscovite and accessory minerals. The GP is a medium-grained porphyritic granite and its phenocrysts include quartz, alkali feldspar, muscovite and plagioclase. All the Zhuxi granites have high SiO2 content (71.97 wt%-81.19 wt%) and total alkali (3.25 wt%-9.42 wt%), and their valid aluminum saturation index (ASI) values show a wide range of 1.03 to 2.49. High Rb/Sr ratios, low Sr content (<50 ppm) and markedly negative Eu anomalies of GP, WG and BG demonstrated that the Zhuxi granites are highly fractioned and intensive crystal differentiated. Because they display the features of both I- and S-types granites, they were confirmed to be I-S transform-type granites. Whole rock εNd(t) and zircon εHf(t) values fall into the ranges of -6.98 to -11.97, and -3.1 to -11.5, and the Nd (TDM2) and Hf two-stage model ages (TDMc) are 1.51-1.92 Ga and 1.42-2.01 Ga, respectively. Geochemical and isotopic data suggest that these highly fractionated I-S transform-type granites were originated from magmas which showed affinity with the Proterozoic continent and the Shuangqiaoshan Group and little mantle contribution was involved during the generation of Zhuxi granitic rocks. Extreme fractional crystallization resulted in further enrichment of tungsten in the evolved granitic magma. New data, presented together with previously published data, suggest that the Zhuxi granitic complex was likely to be formed during lithospheric compression setting during the late Jurassic to early Cretaceous. The biotite granite stock predominately contributed to the production of skarn alteration and mineralization, followed by the white granite dyke; the granite porphyry dykes have little effect.

Sources of granite magmatism in the Embu Terrane (Ribeira Belt, Brazil): Neoproterozoic crust recycling constrained by elemental and isotope (Sr-Nd-Pb) geochemistry

NASA Astrophysics Data System (ADS)

Alves, Adriana; Janasi, Valdecir de Assis; Campos Neto, Mario da Costa

2016-07-01

Whole rock elemental and Sr-Nd isotope geochemistry and in situ K-feldspar Pb isotope geochemistry were used to identify the sources involved in the genesis of Neoproterozoic granites from the Embu Terrane, Ribeira Belt, SE Brazil. Granite magmatism spanned over 200 Ma (810-580 Ma), and is dominated by crust-derived relatively low-T (850-750 °C, zircon saturation) biotite granites to biotite-muscovite granites. Two Cryogenian plutons show the least negative εNdt (-8 to -10) and highest mg# (30-40) of the whole set. Their compositions are strongly contrasted, implying distinct sources for the peraluminous (ASI ∼ 1.2) ∼660 Ma Serra do Quebra-Cangalha batholith (metasedimentary rocks from relatively young upper crust with high Rb/Sr and low Th/U) and the metaluminous (ASI = 0.96-1.00) ∼ 630 Ma Santa Catarina Granite. Although not typical, the geochemical signature of these granites may reflect a continental margin arc environment, and they could be products of a prolonged period of oceanic plate consumption started at ∼810 Ma. The predominant Ediacaran (595-580 Ma) plutons have a spread of compositions from biotite granites with SiO2 as low as ∼65% (e.g., Itapeti, Mauá, Sabaúna and Lagoinha granites) to fractionated muscovite granites (Mogi das Cruzes, Santa Branca and Guacuri granites; up to ∼75% SiO2). εNdT are characteristically negative (-12 to -18), with corresponding Nd TDM indicating sources with Paleoproterozoic mean crustal ages (2.0-2.5 Ga). The Guacuri and Santa Branca muscovite granites have the more negative εNdt, highest 87Sr/86Srt (0.714-0.717) and lowest 208Pb/206Pb and 207Pb/206Pb, consistent with an old metasedimentary source with low time-integrated Rb/Sr. However, a positive Nd-Sr isotope correlation is suggested by data from the other granites, and would be consistent with mixing between an older source predominant in the Mauá granite and a younger, high Rb/Sr source that is more abundant in the Lagoinha granite sample. The Ediacaran granites are coeval with profuse granite magmatism attributed to continental arc magmatism in northern Ribeira and Araçuaí belts. However, their evolved compositions with low mg# and dominantly peraluminous character are unlike those of magmatic arc granites, and they are more likely products of post-collisional magmatism or correspond to an inner belt of crust-derived granites.

Geochemical and Sm-Nd isotope-geochemical patterns of metavolcanic rocks, diabase, and metagabbroids on the northeastern flank of the South Mongolian-Khingan orogenic belt

NASA Astrophysics Data System (ADS)

Smirnov, Yu. V.; Sorokin, A. A.

2017-05-01

The first results of geochemical and Sm-Nd isotope-geochemical studies of metavolcanic rocks, metagabbroids, and diabase of the Nora-Sukhotino terrane, the least studied part of the South Mongolian-Khingan orogenic belt in the system of the Central Asian orogenic belt are reported. It is established that the basic rocks composing this terrane include varieties comparable with E-MORB, tholeiitic, and calc-alkaline basalt of island arc, calc-alkaline gabbro-diabase, and gabbroids of island arcs. Most likely, these formations should be correlated with metabasalt and associated Late Ordovician gabbro-amphibolite of the Sukdulkin "block" of the South Mongolian-Khingan orogenic belt, which are similar to tholeiite of intraplate island arcs by their geochemical characteristics.

Svecofennian orogeny in an evolving convergent margin setting

NASA Astrophysics Data System (ADS)

Korja, Annakaisa

2015-04-01

The dominant tectonic mode changes from extension to convergence at around 1.9 Ga in Fennoscandian. The lithological record suggests short lived subduction-related magmatic events followed by deformation and low-pressure high temperature metamorphism. At around 1.8 Ga the subduction systems seem to have stabilized implying continuous supply of oceanic lithosphere. The evolution of the convergent margin is recorded in the rock record and crustal architecture of the long lived Svecofennian orogeny (1.9-1.7 Ga). A closer look at the internal structure of the Svecofennian orogen reveals distinct regional differences. The northern and central parts of the Svecofennian orogen that have been formed during the initial accretionary phase - or compilation of the nucleus - have a thick three-layer crust and with thick mafic lower crust (10-30 km) and block-like internal architecture. Reflection profiles (FIRE1-3) image listric structures flattening on crustal scale décollement zones at the upper-middle crust and middle-upper crust boundaries. The crustal architecture together with large volumes of exposed granitoid rocks suggests spreading of the orogen and the development of an orogenic plateau west of the continental convergence boundary. The architecture is reminiscent of a large hot orogen. Within the western and southwestern part of the Svecofennian orogen (BABEL B, 1, 2, 3&4), which have been envisioned to have formed during continuous subduction phase, the crust is thinner (45-50 km) and it is hosting crustal blocks having one to two crustal layers. Layering is poorly developed in crustal blocks that are found S-SW of NE-dipping mantle reflections previously interpreted as paleo-subduction zones. Within these blocks, the crustal scale reflective structures dip NE (prowedge) or form pop-up wedges (uplifted plug) above the paleo-subduction zones. Crustal blocks with well-developed two-layer crust are located NE of the paleo-subduction zone. The architecture can be interpreted to image a series of abandoned accretion zones where the orogenic structure has developed from a young and cold orogen (BABEL 2,3&4) to a transitional (BABEL 1,6,B) one as the plate boundary is retreating during SW wards. The fast retreating rate of the subduction zone may not only have formed continental back-arc environment but may have restricted the thickening of the upper plate and the growth rate of the orogen. Altogether the architecture suggests a long-lived southwesterly retreating subduction system, with continental back-arc formation in its rear parts and well developed system of prowedge-retrowedge-uplifted plug close to a subduction conduit. Changes in the relative velocities of the upper and lower plate may have resulted in repetitive extensional and compressional phases of the orogeny as has been previously suggested for the southern part of the Svecofennian orogen.

Venus orogenic belt environments - Architecture and origin

NASA Astrophysics Data System (ADS)

Head, James W.; Vorder Bruegge, Richard W.; Crumpler, L. S.

1990-08-01

Orogenic belt environments (Danu, Akna, Freyja, and Maxwell Montes) in Western Ishtar Terra, Venus, display a range of architectural elements, including (from the center of Western Ishtar outward) an inboard plateau (Lakshmi Planum), the linear orogenic belts themselves, outboard plateaus, steep scarps bounding Ishtar, adjacent linear foredeeps and outboard rises, and outboard low-lying volcanic plains. The main elements of the architecture are interpreted to be due to the convergence, underthrusting, and possible subduction of lowland plains at the margins of a preexisting tessera plateau of thicker crust.

Effect of Anisotropy on the Long-Term Strength of Granite

NASA Astrophysics Data System (ADS)

Nara, Yoshitaka

2015-05-01

Granite rock mass is used for various rock engineering purposes. To ensure long-term stability, information about the subcritical crack growth (SCG) and an estimate of the long-term strength (LTS) of the rock are necessary. The influence of the anisotropy of granite on its LTS has not yet been clarified. In this study, the anisotropy of the long-term rock strength was investigated for two types of granite rocks, Oshima granite and Inada granite. Specifically, the effect of the anisotropy in crack propagation on the LTS was examined. The results showed that the LTS of granite is anisotropic, as are the fracture toughness and Brazilian tensile strength measured in this study. The LTS was lowest when crack propagation occurred parallel to the rift plane, where most of the microcracks occur. For Inada granite, which has an anisotropic SCG index, the degree of anisotropy of the LTS increased as the time-to-failure increased. This suggests that the LTS of granite is anisotropic.

40Ar/39Ar geochronology, elemental and Sr-Nd-Pb isotope geochemistry of the Neogene bimodal volcanism in the Yükselen area, NW Konya (Central Anatolia, Turkey)

NASA Astrophysics Data System (ADS)

Gençoğlu Korkmaz, Gülin; Asan, Kürşad; Kurt, Hüseyin; Morgan, Ganerød

2017-05-01

Bimodal volcanic suites occur in both orogenic and anorogenic geotectonic settings. Although their formation can be attributed to either fractional crystallization from basaltic parents to felsic derivatives or partial melting of different sources, the origin of bimodal suites is still unclear. By reporting mineral chemistry, 40Ar/39Ar geochronology, elemental and Sr-Nd-Pb isotope geochemistry data, this study aims to investigate the genesis of bimodal basalt-dacite association from the Yükselen area located on the northern end of the Sulutas Volcanic Complex (Konya, Central Anatolia). The Yükselen area volcanic rocks are represented by basaltic lava flows, and dacitic dome with enclaves and pyroclastics. Basaltic flows and pyroclastic rocks are interlayered with the Neogene fluvio-lacustrine sedimentary units, while dacitic rocks cut the pre-Neogene basement in the area. A biotite separation from dacites yielded 40Ar/39Ar plateau age of 16.11 ± 0.18 Ma. On the other hand, a whole rock sample from basalts gave two plateau ages of 16.45 ± 0.76 Ma and 22.37 ± 0.65 Ma for the first steps and next steps, respectively. The investigated basalts are sodic alkaline, and characterized by ocean island basalt (OIB)-like anorogenic geochemical signatures. However, dacites are calc-alkaline and metaluminous, and carry geochemical signatures of orogenic adakites. Sr-Nd-Pb isotopic systematics suggest that the basalts were derived from an asthenospheric mantle source enriched by recycled crustal rocks. The dacites show more enriched Sr and Pb ratios and more depleted Nd ones relative to the basalts, which at the first glance might be attributed to crustal contamination of the associated basalts. However, trace element features of the dacites rule out cogenetic relationship between the two rock types, and point to an origin by melting of lower crust. On the other hand, enclaves share several elemental and isotopic characteristics with the dacites, and appear to be fragments of sub-volcanic intrusions closely related to the dacitic host magma. Based on the obtained geochemical data combined with the published geological and geophysical data, the investigated bimodal volcanic activity can be explained by slab break-off process in the convergence system between the African and Anatolian plates.

Lithosphere structure of the west Qinling orogenic belt revealed by deep seismic reflection profile

NASA Astrophysics Data System (ADS)

Wang, H.

2009-12-01

The west Qinling orogen located in the northeastern margin of the Qinghai-Tibet plateau, is transformation zone between the N-S-trending and E-W-trending tectonics in the Chinese continent. Further study of the fine crust structure of the west Qinling orogen and its relationships with surrounding basins have very important significance for understanding tectonic response of the northeastern margin of the plateau about collision convergence of the Indian block and Asian block and learning formation and evolution of the plateau. In 2009, we reprocessed the data of the Tangke-Hezuo deep seismic reflection profiles collected in 2004 across the west Qinling orogen and the northern Songpan block. The new results show the lithosphere fine structure of the west Qinling orogen. Reflection features indicate that an interface at 6.0-7.0s (TWT) divided the crust into the upper and lower crust, whose structural style and deformation are totally different. Integrating geological data, we deduce that the interface at 6.0-7.0s (depth with 18-21 km) was the basement detachment, which made deformation decoupled of the upper and lower crust. The multi-layered reflections in the upper crust reveal the sedimentary covers of the west Qinling orogen, disclose the thickness of the various structure layer and deformation degree, and provide a basis for the prospective evaluation of a multi-metallic mineral and energy exploration. The north dipping strong reflection characteristics of the lower crust in the west Qinling orogen constituted imbricate structure, such imbricate structural features provide seismology evidence for researching the west Qinling thrusting toward the northern Songpan block, and have great significance for studying formation and evolution of the Songpan-Garze structure. Moho reflections are observed around 17.0-17.2s, characterized by nearly horizontal reflections, which implies the west Qinling orogen underwent an intense extension post orogeny caused the lithosphere extensional thinning formed a nearly level Moho reflections. The study was financed by National Natural Science Foundation of china (No. 40830316 and 40604010),the Basic outlay of scientific research work from Ministry of Science and Technology of the People’s Republic of China and SINOPPROBE-02.

The 131-134 Ma A-type granites from northern Zhejiang Province, South China: Implications for partial melting of the Neoproterozoic lower crust

NASA Astrophysics Data System (ADS)

Hu, Qinghai; Yu, Kaizhang; Liu, Yongsheng; Hu, Zhaochu; Zong, Keqing

2017-12-01

Although Mesozoic granites are widely distributed in the Gan-Hang Belt in Southeast China, their petrogenesis and geodynamic settings are still matters of dispute. Here, the major and trace elements, Sr-Nd isotopic compositions, zircon U-Pb dating and Hf isotopes of three late Mesozoic granite plutons from Machebu, Shenzhongwu and Daixi in northern Zhejiang Province were analyzed to investigate their petrogenesis. These granite plutons are featured by an A2-type granite geochemical signature (e.g., high SiO2 (71 to 78 wt.%), total alkalis (Na2O + K2O = 7.57 to 9.12 wt.%), rare earth elements (total REE = 174 to 519 ppm) and HFSE contents, with mostly high FeOT/(FeOT + MgO) (0.82 to 0.93) and Ga/Al ratios (2.49 to 5.07) and low Ce4 +/Ce3 + ratios in zircons (1 to 90)), suggesting they were formed in an extensional tectonic setting. Combining whole rock Zr contents, mineral assemblages and the An content of plagioclase, it was suggested that these granites could have formed at a high temperature (> 850 °C) with a low H2O content ( 2.5 wt.%). Assuming the granite with the lowest SiO2 and high CaO contents as the "primary granite melt", the melting pressure was estimated to be 2.5 kbar based on model calculations using MELTs. Taking into account the effect of plagioclase fractional crystallization during the granite emplacement into shallow crustal levels, the initial melting pressure could be > 2.5 kbar at the stable field of plagioclase. This agrees well with the very low and variable Sr and Eu contents of these granite plutons. Zircon U-Pb dating yields a weighted mean 206Pb/238U age of 130.9 ± 1.0 Ma for the Machebu granite, 134.4 ± 2.1 Ma for the Shenzhongwu granite and 131.9 ± 1.2 Ma for the Daixi granite. The εHf(t) values of the zircons in these plutons gradually increase from southwest to northeast, i.e., from - 14.5 - 3.5 for the Machebu granite to - 1.5 - 0.2 for the Shenzhongwu granite and from 5.1 8.6 for the Daixi granite, suggesting the origin of different parent rocks. These granites have two-stage Nd model (TDM2(Nd)) ages of 1099 Ma-838 Ma, and zircons from these granites and the Neoproterozoic basement in the Gan-Hang Belt plot on the same evolutionary trend in the εHf(t)-age diagram. It is interesting to note that the collection of literature data shows a positive correlation between SiO2 and εNd(t) for the late Mesozoic A-granites in the Gan-Hang Belt, and the Neoproterozoic A-granites in the Gan-Hang Belt cluster in two groups of the high-SiO2-εNd(t) group and low-εNd(t) group. The positive correlation of SiO2-εNd(t) demonstrated by the late Mesozoic A-granites can be well explained by a high-degree of melting of mixtures between the two groups of Neoproterozoic A-granites. We thus suggest that the late Mesozoic A-type granites in the Gan-Hang Belt could have been derived from the rejuvenated Neoproterozoic rocks rather than directly from the Mesoproterozoic metamorphic basement as a result of subduction.

Enigmatic diamonds in Archean calc-alkaline lamprophyres of Wawa, southern Ontario, Canada

NASA Astrophysics Data System (ADS)

de Stefano, Andrea; Lefebvre, Nathalie; Kopylova, Maya

2006-02-01

A suite of 80 macrodiamonds recovered from volcaniclastic breccia of Wawa (southern Ontario) was characterized on the basis of morphology, nitrogen content and aggregation, cathodoluminescence (CL), and mineral inclusions. The host calc-alkaline lamprophyric breccias were emplaced at 2.68-2.74 Ga, contemporaneously with voluminous bimodal volcanism of the Michipicoten greenstone belt. The studied suite of diamonds differs from the vast majority of diamond suites found worldwide. First, the suite is hosted by calc-alkaline lamprophyric volcanics rather than by kimberlite or lamproite. Second, the host volcanic rock is amongst the oldest known diamondiferous rocks on Earth, and has experienced regional metamorphism and deformation. Finally, most diamonds show yellow-orange-red CL and contain mineral inclusions not in equilibrium with each other or their host diamond. The majority of the diamonds in the Wawa suite are colorless, weakly resorbed, octahedral single crystals and aggregates. The diamonds contain 0-740 ppm N and show two modes of N aggregation at 0-30 and 60-95% B-centers suggesting mantle storage at 1,100-1,170°C. Cathodoluminescence and FTIR spectroscopy shows that emission peaks present in orange CL stones do not likely result from irradiation or single substitutional N, in contrast to other diamonds with red CL. The diamonds contain primary inclusions of olivine (Fo92 and Fo89), omphacite, orthopyroxene (En93), pentlandite, albite, and An-rich plagioclase. These peridotitic and eclogitic minerals are commonly found within single diamonds in a mixed paragenesis which also combines shallow and deep phases. This apparent disequilibrium can be explained by effective small-scale mixing of subducted oceanic crust and mantle rocks in fast “cold” plumes ascending from the top of the slabs in convergent margins. Alternatively, the diamonds could have formed in the pre-2.7-2.9 Ga cratonic mantle and experienced subsequent alteration of syngenetic inclusions related to host magmatism and ensuing metamorphism. Neither orogenic nor cratonic model of the diamond origin fully explains all of the observed characteristics of the diamonds and their host rocks.

Revised nomenclature and stratigraphic relationships of the Fredericksburg Complex and Quantico Formation of the Virginia Piedmont

USGS Publications Warehouse

Pavlides, Louis

1980-01-01

The Fredericksburg Complex, in part a migmatitic terrane in northeast Virginia, is subdivided on the basis of lithology, as well as aeromagnetic and aeroradiometric data, into two metamorphic suites. These suites are separated by the northeast-trending Spotsylvania lineament, a rectilinear geophysical feature that is probably the trace of an old fault zone. East of the lineament, the Po River Metamorphic Suite, of Proterozoic Z and (or) early Paleozoic age, consists dominantly of biotite gneiss, generally augen gneiss, and lesser amounts of hornblende gneiss and mica schist. West of the Spotsylvania lineament is the Ta River Metamorphic Suite, composed mostly of amphibolite and amphibole gneiss. However, to the southwest, along its strike belt, the Ta River contains abundant biotite gneiss and mica schist. Both the Ta River and Po River contain abundant foliated granitoid and pegmatoid bodies as concordant tabular masses and as crosscutting dikes; these rocks are considered part of the Ta River and Po River Metamorphic Suites. The amphibolitic Holly Corner Gneiss is interpreted to be a western allochthonous equivalent of the Ta River. Both the Ta River and Holly Corner are considered to be coeval, eastern, distal facies of the Lower Cambrian(?) Chopawamsic Formation. The Paleozoic Falls Run Granite Gneiss intrudes the Ta River Metamorphic Suite and the Holly Corner Gneiss; locally the Falls Run is interpreted to have been transported westward with the Holly Corner after intrusion. The Quantico Formation, in the core of the Quantico-Columbia synclinorium, rests with angular unconformity along its northwest and southeast limbs, respectively, on the Chopawamsic Formation and the Ta River Metamorphic Suite. The Quantico Formation is assigned the same Late Ordovician age and similar stratigraphic position as the Arvonia Slate of the Arvonia syncline. The youngest rocks of the area are the granitoid and pegmatoid bodies of the Falmouth Intrusive Suite. They consist of several generations of chiefly dikes and sills that are intrusive into the Fredericksburg Complex and into the Quantico Formation. Granitoid rocks also form small plutons. The Falmouth is isotopically dated as Carboniferous in age. Some of the metavolcanic rocks of the Evington Group and part of the amphibolite gneiss and amphibolite of the Hatcher Complex, named by W. B. Brown in 1969, are probably coeval with the Chopawamsic Formation and hence equivalents of the Ta River Metamorphic Suite and the Holly Corner Gneiss. The biotitic gneiss and granitoid rocks east of the Spotsylvania lineament in the Dillwyn area are considered to be coeval with the Po River Metamorphic Suite.

A Human Systems Integration Approach to Energy Efficiency in Ground Transportation

DTIC Science & Technology

2015-12-01

Granite Construction Organizational Structure .........................................53 Figure 7. A Comparison of USMC Structure to Granite Construction...Caterpillar Corporation and the implementation and use of their telematics systems within a company called Granite Construction. Granite Construction...profit over 250 million dollars annually. In addition, similar to the USMC, Granite Construction handles both large and small scale projects in a

Late-Eburnean tectonic emplacement of Wayen syenite (Burkina Faso)

NASA Astrophysics Data System (ADS)

Seydoux Traoré, Abraham; Naba, Séta; Lompo, Martin

2010-05-01

The alkali plutonism of the Palaeoproterozoic domain of the West African craton remains very poorly studied. The rare data available are those from the Ninakri syenite and the alkali granites of central Côte d'Ivoire (Bonhomme, 1962; Morel and Alinat, 1993 ; Doumbia et al, 1998) and those of the alkali granites of central Burkina Faso (Wenmenga, 1986). All these studies focused on the petrographical characterization and the determination of the radiometric ages of these plutons. All ages were determined by the Rb/Sr and they vary between 1.8 Ga and 2.1 Ga. The span of emplacement ages suggests that the alkali plutonism of the Palaeoproterozoic of West Africa continued throughout the tectono-magmatic history of the craton. The purpose of the present study is to characterize the late eburnean tectonic processes using the structural data of the Wayen syenite which was dated at 2.1 Ga by Rb/Sr isochron by Vachette and Ouédraogo (1978) Map view and field relationships show that the Wayen syenite intrudes the metavolcanic and early plutonic rocks (tonalites, trondhjemites and granodiorites). The geochemical data show that the syenite has a peraluminous character (Peccerrilo and Taylor, 1976) and belongs to the S-type granitoids (Chappell and White, 1992). The structural data have been obtained by magnetic susceptibility (Km) measurements on 190 samples from 47 sampling stations. The values of susceptibility range between 253 µSI and 595,314 µSI. This means that we have both samples with ferromagnetic character (Km > 500 µSI) and paramagnetic character (Km ≤ 500 µSI). In the case of paramagnetism, the minerals bearing the magnetic susceptibility are ferriferous silicates (amphiboles and pyroxenes) whereas the in the ferromagnetic samples, it is magnetite. The microstructures of the syenite are mostly magmatic and very locally they show both high temperature solid state deformation features and relatively low temperature solid state conditions along narrow shear bands. At the map scale the regular orientation of lineations and foliations strongly suggests that the eburnean tectonic event was active during emplacement, which contradicts the widespread idea that the syenites represent post-orogenic granitoids.

Grenvillian magmatism in the northern Virginia Blue Ridge: Petrologic implications of episodic granitic magma production and the significance of postorogenic A-type charnockite

USGS Publications Warehouse

Tollo, R.P.; Aleinikoff, J.N.; Borduas, E.A.; Dickin, A.P.; McNutt, R.H.; Fanning, C.M.

2006-01-01

Grenvillian (1.2 to 1.0 Ga) plutonic rocks in northern Virginia preserve evidence of episodic, mostly granitic magmatism that spanned more than 150 million years (m.y.) of crustal reworking. Crystallization ages determined by sensitive high resolution ion microprobe (SHRIMP) U-Pb isotopic analyses of zircon and monazite, combined with results from previous studies, define three periods of magmatic activity at 1183-1144 Ma (Magmatic Interval I), 1120-1111 Ma (Magmatic Interval II), and 1078-1028 Ma (Magmatic Interval III). Magmatic activity produced dominantly tholeiitic plutons composed of (1) low-silica charnockite, (2) leucogranite, (3) non-leucocratic granitoid (with or without orthopyroxene (opx)), and (4) intermediate biotite-rich granitoid. Field, petrologic, geochemical, and geochronologic data indicate that charnockite and non-charnockitic granitoids were closely associated in both space and time, indicating that presence of opx is related to magmatic conditions, not metamorphic grade. Geochemical and Nd isotopic data, combined with results from experimental studies, indicate that leucogranites (Magmatic Intervals I and III) and non-leucocratic granitoids (Magmatic Intervals I and II) were derived from parental magmas produced by either a high degree of partial melting of isotopically evolved tonalitic sources or less advanced partial melting of dominantly tonalitic sources that also included a more mafic component. Post-orogenic, circa 1050 Ma low-silica charnockite is characterized by A-type compositional affinity including high FeOt/(FeOt + MgO), Ga/Al, Zr, Nb, Y, and Zn, and was derived from parental magmas produced by partial melting of potassic mafic sources in the lower crust. Linear geochemical trends defined by leucogranites, low-silica charnockite, and biotite-rich monzogranite emplaced during Magmatic Interval III reflect differences in source-related characteristics; these features do not represent an igneous fractionation sequence. A compositional gap between circa 1160 Ma magnesian low-silica charnockite and penecontemporaneous higher silica lithologies likewise precludes a fractionation relationship among plutons intruded during Magmatic Interval I. Correspondence in timing of magmatic activity between the Blue Ridge and neighboring Mesoproterozoic terranes underscores the widespread nature of Grenvillian processes in the region.

Lithologically controlled invisible gold, Yukon, Canada

NASA Astrophysics Data System (ADS)

MacKenzie, Doug; Craw, Dave; Finnigan, Craig

2015-02-01

The newly discovered Cretaceous Coffee orogenic gold deposit (>4 Moz resource) consists of an extensive oxidised zone developed on primary sulphidic rock. The primary mineralised rock is characterised by invisible gold in arsenian pyrite that has replaced biotite in selected host rocks. The deposit has a cryptic surface expression and is an example of an extremely subtle exploration target. Hydrothermal emplacement was controlled by extensional fractures, with breccias, but most mineralisation was focused on biotite-bearing granitic gneiss, metasedimentary gneisses, and younger biotite granite. Fine-grained (<0.1 mm) arsenian pyrite replaced biotite along mineral cleavage planes and followed biotite-rich metamorphic and post-metamorphic structural fabrics. Arsenian pyrite also formed overgrowths on earlier coarse-grained (up to 2 mm) barren hydrothermal pyrite. Arsenian pyrite is concentrically zoned on the 1-10-μm scale with respect to As, Sb, and Au contents and typically contains ˜5 wt% As, ˜500 mg/kg Sb, and ˜500 mg/kg Au, in solid solution. Biotite replacement was accompanied by sericitisation, silicification, and ankerite impregnation. Hydrothermal alteration involved dilution and localised depletion of K, Na, and Al in silicified host rocks, but most Ca, Mg, and Fe concentrations remained broadly constant. Magnesium-rich ultramafic host rocks were only weakly mineralised with auriferous arsenian pyrite and have fuchsite and magnesite alteration. Near-surface oxidation has liberated nanoparticulate and microparticulate supergene gold, which remains essentially invisible. Varying degrees of oxidation extend as deep as 250 m below the present subdued topographic surface, well beyond the present vadose zone, and this deep oxidation may have occurred during post-mineralisation uplift and erosion in the Cretaceous. Oxidation has leached some As from the surficial mineralised rocks, decreasing the geochemical signal, which is also obscured by the localised presence of high background As (up to 100 mg/kg) in metasedimentary quartzites in the region. Antimony provides more reliable soil anomalies than As, but most Sb anomalies are <100 mg/kg. The persistence of invisible gold into the extensive supergene zone, with little gold particle size enhancement, has ensured that no placer deposits have formed in nearby streams, further restricting the surface footprint and Au dispersal halo of this subtle exploration target.

Preface to special issue: Granite magmatism in Brazil

NASA Astrophysics Data System (ADS)

Janasi, Valdecir de Assis; de Pinho Guimarães, Ignez; Nardi, Lauro Valentim Stoll

2016-07-01

Granites are important both to the geologic evolution and to the economy of Brazil. Deposits of precious and rare metals, such as Au, Sn and many others, are directly or indirectly associated with granites, especially in the geologically under-explored Amazon region. On the opposite eastern side of the country, expanding exploitation of natural granite as dimension stone makes Brazil currently the world's second largest exporter of granite blocks. Granites are a major constituent of the Brazilian Archean-Proterozoic cratonic domains (the Amazon and São Francisco cratons) and their surrounding Neoproterozoic fold belts. The granites are thus fundamental markers of the major events of crustal generation and recycling that shaped the South American Platform. As a result, Brazilian granites have received great attention from the national and international community, and a number of influential meetings focused on the study of granites were held in the country in the last three decades. These meetings include the two International Symposia on Granites and Associated Mineralization (Salvador, January 21-31, 1987, and August 24-29, 1997), the Symposium on Rapakivi Granites and Related Rocks (Belém, August 2-5, 1995) and the Symposium on Magmatism, Crustal Evolution, and Metallogenesis of the Amazonian Craton (Belém, August 2006). Special issues dedicated to contributions presented at these meetings in the Journal of South American Earth Sciences (Sial et al., 1998), Lithos (Stephens et al., 1999), Canadian Mineralogist (Dall'Agnol and Ramo, 2006), Precambrian Research (Ramo et al., 2002) and Anais da Academia Brasileira de Ciências (Dall'Agnol and Bettencourt, 1997; Sial et al., 1999a) are still important references on the knowledge of Brazilian granites and granite petrology in general.

Iron isotope behavior during fluid/rock interaction in K-feldspar alteration zone - A model for pyrite in gold deposits from the Jiaodong Peninsula, East China

NASA Astrophysics Data System (ADS)

Zhu, Zhi-Yong; Jiang, Shao-Yong; Mathur, Ryan; Cook, Nigel J.; Yang, Tao; Wang, Meng; Ma, Liang; Ciobanu, Cristiana L.

2018-02-01

Mechanisms for Fe isotope fractionation in hydrothermal mineral deposits and in zones of associated K-feldspar alteration remain poorly constrained. We have analyzed a suite of bulk samples consisting of granite displaying K-feldspar alteration, Precambrian metamorphic rocks, and pyrite from gold deposits of the Jiaodong Peninsula, East China, by multi-collector inductively-coupled plasma mass spectrometry. Pyrites from disseminated (J-type) ores show a δ56Fe variation from +0.01 to +0.64‰, overlapping with the signature of the host granites (+0.08 to +0.39‰). In contrast, pyrites from quartz veins (L-type ores) show a wide range of Fe-isotopic composition from -0.78 to +0.79‰. Negative values are never seen in the J-type pyrites. The Fe isotope signature of the host granite with K-feldspar alteration is significantly heavier than that of the bulk silicate Earth. The Fe isotopic compositions of Precambrian metamorphic rocks across the district display a narrow range between -0.16‰ and +0.19‰, which is similar to most terrestrial rocks. Concentrations of major and trace elements in bulk samples were also determined, so as to evaluate any correlation between Fe isotope composition and degree of alteration. We note that during progressive K-feldspar alteration to rocks containing >70 wt% SiO2, >75 ppm Rb, and <1.2 wt% total Fe2O3, the Fe isotope composition of the granite changes systematically. The Fe isotope signature becomes heavier as the degree of alteration increases. The extremely light Fe isotopic compositions in L-type gold deposits may be explained by Rayleigh fractionation during pyrite precipitation in an open fracture system. We note that the sulfur isotopic compositions of pyrite in the two types of ores are also different. Pyrite from J-type ores has a systematically 3.5‰-higher δ34S value (11.2‰) than those of pyrite from the L-type ores (7.7‰). There is, however, no correlation between Fe and S isotope signatures. The isotopic fractionation of sulfur is used to constrain a change in the fO2 of the hydrothermal fluids from which pyrite precipitated. This work demonstrates that the Fe isotope composition of pyrite displays a significant response to the process of pyrite precipitation in hydrothermal systems, and that systematic fractionation of iron isotopes occurs during fluid/rock reaction in the K-feldspar alteration zone of the Linglong granite. The implications of the results are that processes of mineralization and associated fluid-rock interaction, which are ubiquitously observed in porphyry-style Cu-Au-Mo and other hydrothermal deposits, may be readily traceable using Fe isotopes.

Identification of granite varieties from colour spectrum data.

PubMed

Araújo, María; Martínez, Javier; Ordóñez, Celestino; Vilán, José Antonio

2010-01-01

The granite processing sector of the northwest of Spain handles many varieties of granite with specific technical and aesthetic properties that command different prices in the natural stone market. Hence, correct granite identification and classification from the outset of processing to the end-product stage optimizes the management and control of stocks of granite slabs and tiles and facilitates the operation of traceability systems. We describe a methodology for automatically identifying granite varieties by processing spectral information captured by a spectrophotometer at various stages of processing using functional machine learning techniques.

Identification of Granite Varieties from Colour Spectrum Data

PubMed Central

Araújo, María; Martínez, Javier; Ordóñez, Celestino; Vilán, José Antonio

2010-01-01

The granite processing sector of the northwest of Spain handles many varieties of granite with specific technical and aesthetic properties that command different prices in the natural stone market. Hence, correct granite identification and classification from the outset of processing to the end-product stage optimizes the management and control of stocks of granite slabs and tiles and facilitates the operation of traceability systems. We describe a methodology for automatically identifying granite varieties by processing spectral information captured by a spectrophotometer at various stages of processing using functional machine learning techniques. PMID:22163673

Mesoproterozoic juvenile mafic-ultramafic magmatism in the SW Amazonian Craton (Rio Negro-Juruena province): SHRIMP U-Pb geochronology and Nd-Sr constraints of the Figueira Branca Suite

NASA Astrophysics Data System (ADS)

Teixeira, Wilson; Geraldes, Mauro C.; D'Agrella-Filho, Manoel S.; Santos, João O. S.; Sant'Ana Barros, Márcia A.; Ruiz, Amarildo S.; Corrêa da Costa, Paulo C.

2011-12-01

The Figueira Branca Suite (FBS) comprises a layered mafic-ultramafic complex which together with mafic-felsic plugs makes up a string of NW-trending intrusive bodies that are emplaced into the Jauru domain (Rio Negro-Juruena province; 1.80-1.60 Ga). This domain comprises Orosirian calc-alkaline rocks and coeval metamorphic volcanic-sedimentary associations, intruded by voluminous granitoid plutons resulted from outboard Cachoeirinha (1587-1522 Ma) and Santa Helena (1485-1420 Ma) accretionary orogens that eventually created the Rondonian-San Ignacio province along the SW margin of the proto-Amazonian Craton. SHRIMP U-Pb age in zircon for one cumulatic gabbro from the FBS yielded a concordia intercept age of 1425.5 ± 8.0 Ma (MSWD = 1.11). Another gabbroic plug which crops out to the East gives a similar within error concordia intercept zircon age of 1415.9 ± 6.9 Ma (MSWD = 0.25), whereas a nearby monzogranite yields a concordia intercept zircon age of 1428.9 ± 2.8 Ma (MSWD = 1.30). All these results are crystallization ages and constrain an important intraplate magmatic event within the Orosirian continental crust at the time of outboard Santa Helena orogen. On the other hand, igneous titanite from another gabbro located to the West of the FBS yielded a weighted mean 207Pb/ 206Pb crystallization age of 1541 ± 23 Ma (MSWD = 0.74). Therefore this rock is not genetically associated with the FBS, as previously suggested by the field information. Additional Nd-Sr isotopic analyses of the FBS mafic-ultramafic rocks and coeval gabbro showed comparable ɛNd(1.42Ga) values (+3.0 to +4.7) and variable ɛSr(1.42Ga) ones (-39.1 to -8.1). These data plot in the depleted field quadrant of the Nd-Sr diagram, indicating a significant influence of the MORB end-member reservoir in the magma genesis. This interpretation is similarly supported by comparison of the Nd evolutionary path of the FBS with those that characterize the isotopic evolution of the Jauru crust and the Cachoeirinha and Santa Helena intrusive plutons. The bulk isotopic signature allows the genetic relationship among the Santa Helena Suite and the FBS and coeval rocks. The anorogenic character of the FBS is supported by the geologic framework of the Jauru domain, given that the emplacement took place under an extensional regime, associated with the recognized regional NW-trending structures. 40Ar- 39Ar analyses were carried out in mafic rocks that are assigned to the FBS. Biotite from a gabbronorite yielded an ideogram age of 1222 ± 5 Ma while a nearby troctolite yielded plateau biotite ages of 1275 ± 4 Ma and 1268 ± 4 Ma. These ages are minimum estimates of the regional cooling of the FBS. One gabbro of the Alto Jauru Group yields a 40Ar- 39Ar plateau age of 1781 ± 15 Ma, interpreted as the time of regional cooling that succeeded continental accretion and metamorphism at Orosirian times. Later on heterogenous crustal thickening and uplift took place from West to East, as response from the Cachoeirinha and Santa Helena orogen dynamics, which is marked by the 1539-1510 Ma and 1452-1322 Ma 40Ar- 39Ar age-patterns, respectively. The available apparent ages suggest that Cachoeirinha crust was subjected to a fast exhumation rate (ca. 50 m.y.), as estimated by the zircon-titanite-hornblende-biotite time-path, while the Santa Helena crust seems to display a lower exhumation rate. Particularly, the youngest 40Ar- 39Ar biotite age (1322 Ma) available for the Santa Helena crust signals the post-tectonic phase of the Rondonian-San Ignacio province.

Petrogenesis and magmatic evolution of ∼130 Ma A-type granites in Southeast China

NASA Astrophysics Data System (ADS)

Sun, Fajun; Xu, Xisheng; Zou, Haibo; Xia, Yan

2015-02-01

A number of Late Mesozoic (∼130 Ma) A-type granitic plutons have been identified in Southeast China. Here we investigate the petrogenesis of one of these granitic plutons in Southeast China, the Sanqingshan-Damaoshan (SD) granites in northeastern Jiangxi Province, using zircon U-Pb geochronology, Hf isotopic analyses, and major and trace element analyses. The SD granites are metaluminous to weakly peraluminous and show typical A-type affinity, which is characterized by high SiO2, Na2O + K2O, rare earth element (REE), high field strength element (HFSE) contents, Ga/Al and Fe# [FeOt/(FeOt + MgO)] values. Zircon grains from the SD granites and some other ∼130 Ma A-type granites commonly contain oscillatory zoning ;cores; surrounded by unzoned to weakly zoned ;rims;. Detailed studies of zircons from the SD granites show that ;rims; are enriched in LREE, Th and U compared with ;cores;. Chondrite-normalized REE patterns of the ;cores; increase steeply from La to Lu and show pronounced Ce and Eu anomalies, while REE patterns of the ;rims; display higher REE abundances with flatter LREE patterns and moderate Ce anomalies. Nevertheless, Lu-Hf isotopic analyses and Ti-in zircon thermometer show similar characteristics between ;rims; and ;cores;, indicating that the ;rims; may crystallize under the effect of internal magmatic hydrothermal fluids. U-rich ;rims; are more susceptible to Pb loss caused by self-irradiation, which may lead to significant younger U-Pb ages. As a result, U-Pb ages of zircon ;cores; (∼130 Ma) represent crystallization ages of the SD granites. εHf(t) values of zircon grains from the SD granites are between -6.4 and -0.4 with Mesoproterozoic model ages (T2DM) ranging from 1.22 to 1.59 Ga, suggesting that the granites may be formed by partial melting of Proterozoic basement. Compared with other adjacent ∼130 Ma A-type granitic plutons in SE China, the SD granites have similar geochemical characteristics and Hf isotopic compositions to those of Xiangshan, Daqiaowu, Yangmeiwan, and Tongshan granites, but different from the Baijuhuajian granite. εHf(t) values of the Baijuhuajian granites are higher than other granites, indicating significant participation of juvenile materials. These ∼130 Ma A-type granites indicate a back-arc extension setting due to the roll-back of paleo-Pacific plate, where the crust and lithospheric mantle became progressively thinned. The upwelling of asthenosphere triggered the partial melting of crustal rocks and generated the Sanqingshan-Damaoshan, Tongshan, Daqiaowu and Yangmeiwan granitic plutons. With ongoing back-arc extension and increased subduction angle during the roll-back of subducted paleo-Pacific slab, the back-arc extension gradually intensified, resulting in significant additions of mantle juvenile materials to the crustal magma and the formation of the Baijuhuajian granite.