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Sample records for monazite geochronology geochemistry

  1. Thickening and propagation of the Pamir plateau: insights from monazite and titanite geochronology and trace-element geochemistry, eastern Tajikistan

    NASA Astrophysics Data System (ADS)

    Stearns, M. A.; Hacker, B. R.; Kylander-Clark, A. R.; Ratschbacher, L.; Seward, G.

    2011-12-01

    The Pamir plateau contains extensive exposures of mid-lower crust, depths not widely exposed in the Tibetan plateau. Determining the spatial and temporal patterns of crustal thickening of the Pamir plateau is critical for understanding how orogenic plateaux are constructed and propagate. The Pamir dome cores consist of upper-amphibolite facies para- and orthogneisses and schists, with the characteristic peak mineral assemblage of kyanite + biotite + garnet ± muscovite in pelites. Thermobarometry indicates peak metamorphism at 600-750°C and 6-10 kbar, representing exhumation depths of 20-35 km (McGraw et al., in review). U-Th-Pb ages of monazite and titanite from the Pamir domes were determined to investigate the timing of thickening and cooling of the mid to lower crust. The closure temperatures of monazite (≥700 °C) and titanite (~650-700 °C) make them especially useful geochronometers for dating high-grade metamorphism. Additionally, maps of yttrium (Y) in monazite and garnet and Zr-in-titanite thermometry allow more robust interpretations of isotopic age information. Ages were obtained using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) following the acquisition of backscattered electron and x-ray element maps to characterize grain zoning and guide LA-ICP-MS spot placement. The monazites in the rock matrix and as garnet inclusions are chiefly Cenozoic and range from 30 to 18 Ma. They record crystallization during prograde and peak metamorphism, based on textures and compositions. The dated titanites are exclusively polycrystalline aggregates recrystallized during metamorphism. They range from 40 to 10 Ma and define two populations of ~40-32 Ma and ~19-10 Ma. The older titanite ages are restricted to the central Pamir where metamorphic temperatures were <650°C, and the youngest ages come from the southern Pamir, where metamorphic temperatures were above titanite closure to Pb. The oldest ages are therefore

  2. Origin of an unusual monazite-xenotime gneiss, Hudson Highlands, New York: SHRIMP U-Pb geochronology and trace element geochemistry

    USGS Publications Warehouse

    Aleinikoff, John N.; Grauch, Richard I.; Mazdab, Frank K.; Kwak, Loretta; Fanning, C. Mark; Kamo, Sandra L.

    2012-01-01

    A pod of monazite-xenotime gneiss (MXG) occurs within Mesoproterozoic paragneiss, Hudson Highlands, New York. This outcrop also contains granite of the Crystal Lake pluton, which migmatized the paragneiss. Previously, monazite, xenotime, and zircon from MXG, plus detrital zircon from the paragneiss, and igneous zircon from the granite, were dated using multi-grain thermal ionization mass spectrometry (TIMS). New SEM imagery of dated samples reveals that all minerals contain cores and rims. Thus TIMS analyses comprise mixtures of age components and are geologically meaningless. New spot analyses by sensitive high resolution ion microprobe (SHRIMP) of small homogeneous areas on individual grains allows deconvolution of ages within complexly zoned grains. Xenotime cores from MXG formed during two episodes (1034 ± 10 and 1014 ± 3 Ma), whereas three episodes of rim formation are recorded (999 ± 7, 961 ± 11, and 874 ± 11 Ma). Monazite cores from MXG mostly formed at 1004 ± 4 Ma; rims formed at 994 ± 4, 913 ± 7, and 890 ± 7 Ma. Zircon from MXG is composed of oscillatory-zoned detrital cores (2000-1170 Ma), plus metamorphic rims (1008 ± 7, 985 ± 5, and ∼950 Ma). In addition, MXG contains an unusual zircon population composed of irregularly-zoned elongate cores dated at 1036 ± 5 Ma, considered to be the time of formation of MXG. The time of granite emplacement is dated by oscillatory-zoned igneous cores at 1058 ± 4 Ma, which provides a minimum age constraint for the time of deposition of the paragneiss. Selected trace elements, including all REE plus U and Th, provide geochemical evidence for the origin of MXG. MREE-enriched xenotime from MXG are dissimilar from typical HREE-enriched patterns of igneous xenotime. The presence of large negative Eu anomalies and high U and Th in monazite and xenotime are uncharacteristic of typical ore-forming hydrothermal processes. We conclude that MXG is the result of unusual metasomatic processes during high grade

  3. LASS U-Th-Pb monazite and rutile geochronology of felsic high-pressure granulites (Rhodope, N Greece): Effects of fluid, deformation and metamorphic reactions in local subsystems

    NASA Astrophysics Data System (ADS)

    Wawrzenitz, Nicole; Krohe, Alexander; Baziotis, Ioannis; Mposkos, Evripidis; Kylander-Clark, Andrew R. C.; Romer, Rolf L.

    2015-09-01

    The specific chemical composition of monazite in shear zones is controlled by the syndeformation dissolution-precipitation reactions of the rock-forming minerals. This relation can be used for dating deformation, even when microfabric characteristics like shape preferred orientation or intracrystalline deformation of monazite itself are missing. Monazite contemporaneously formed in and around the shear zones may have different compositions. These depend on the local chemical context rather than reflecting successive crystallization episodes of monazite. This is demonstrated in polymetamorphic, mylonitic high-pressure (HP) garnet-kyanite granulites of the Alpine Sidironero Complex (Rhodope UHP terrain, Northern Greece). The studied mylonitic rocks escaped from regional migmatization at 40-36 Ma and from subsequent shearing through cooling until 36 Ma. In-situ laser-ablation split-stream inductively-coupled plasma mass spectrometry (LASS) analyses have been carried out on monazite from micro-scale shear zones, from pre-mylonitic microlithons as well as of monazite inclusions in relictic minerals complimented by U-Pb data on rutile and Rb-Sr data of biotite. Two major metamorphic episodes, Mesozoic and Cenozoic, are constrained. Chemical compositions, isotopic characteristics and apparent ages systematically vary among monazite of four different microfabric domains (I-IV). Within three pre-mylonitic domains (inclusions in (I) pre-mylonitic kyanite and (II) garnet porphyroclasts, and (III) in pre-mylonitic microlithons) monazite yields ages of ca. 130-150 Ma for HP-granulite metamorphism, in line with previous geochronological results in the area. Patchy alteration of the pre-mylonitic monazite by intra-grain dissolution-precipitation processes variably increased negative Eu anomaly and reduced the HREE contents. The apparent age of this altered monazite is reduced. Monazite in the syn-mylonitic shear bands (IV) differs in chemical composition from unaltered and

  4. High-spatial-resolution isotope geochemistry of monazite (U-Pb & Sm-Nd) and zircon (U-Pb & Lu-Hf) in the Old Woman and North Piute Mountains, Mojave Desert, California

    NASA Astrophysics Data System (ADS)

    Phillips, Stacy E.; Hanchar, John M.; Miller, Calvin F.; Fisher, Christopher M.; Lancaster, Penny J.; Darling, James R.

    2014-05-01

    Recent improvements in analytical capabilities allow us to reveal details of magmatic processes at an increasingly finer spatial and temporal scale. In situ analyses of the isotopic and trace element composition of accessory minerals at the sub-grain scale have proven to be effective tools for solving a wide range of geological problems. This study presents new data on accessory minerals including monazite & zircon, examined by in situ LA-ICP-MS and Laser Ablation Split Stream (LASS) techniques, analyzing multiple isotopic systems (U-Pb + Sm-Nd, and U-Pb + Lu-Hf in monazite and zircon, respectively) in order to track geochemical changes over time through a magmatic system. The late Cretaceous granitoids of the Old Woman Mountains in the Mojave Desert, California, provide an excellent opportunity to apply these analytical techniques. The peraluminous granites of the Sweetwater Wash, Painted Rock, and North Piute plutons represent different depths of the magmatic system, and are well understood in terms of field relations and whole-rock geochemistry. A preliminary study on the Sweetwater Wash monazites (Fisher et al., in preparation) has revealed significant inter-grain isotopic heterogeneity in the ɛNd composition of the source region (~1700 Ma); however, the U-Pb ages show an isotopic resetting during emplacement at ~75 Ma. This decoupling of U-Pb and Sm-Nd isotopic systems is suggested by Fisher et al. to be due to recrystallisation and/or dissolution-reprecipitation of monazite. If grain boundary diffusion of Pb overrides the more kinetically limited volume diffusion, then the U-Pb systematics will be reset while Sm and Nd remain immobile in the monazite structure as essential structural components of the lattice. This new data will allow the further investigation of these preliminary results, providing new insights into the observed isotopic disequilibrium, with the LASS technique accurately linking the multiple isotopic systems. This will provide important

  5. Geochronology of polygenetic monazites constrained by in situ electron microprobe Th-U-total lead determination: implications for lead behaviour in monazite

    NASA Astrophysics Data System (ADS)

    Cocherie, A.; Legendre, O.; Peucat, J. J.; Kouamelan, A. N.

    1998-07-01

    Monazite grains are generally concordant and, with an electron microprobe, give Th-U-total Pb isochron ages in agreement with conventional U-Pb data. Monazites from four samples of post-Archean migmatite from the Ivory Coast record two main ages: an Archean age at 2.80 Ga partly overprinted by a late migmatization event at 2.03 Ga. The in situ electron microprobe determination has also shown a third event that systematically appears at 2.72 Ga for all the monazites from two of the four studied rocks. The three events at 2.80, 2.72, and 2.03 Ga recorded by the Th-U-total Pb system in monazite were each obtained on independent homogeneous parts of a single grain with no sign of Pb diffusion. Thus the oldest monazite remained closed during a high-grade resetting (migmatization) at a temperature estimated at around 700°C, and also remained closed during the growth of secondary monazite. Conventional U-Pb data (isotope dilution) on monazite from the same rocks did not allow precise age determinations because the representative points were highly (up to 83%) discordant and did not fit well along a chord in the Concordia diagram. The U-Pb isotope dilution method gave a lower intercept age of 2029 ± 25 Ma in agreement with the electron microprobe age, although with an unusually high uncertainty due to a poor fit of the data. The Pb-evaporation method gave an old Archean age at 2830 ± 7 Ma and Proterozoic ages ranging from 2417 ± 10 to 2074 ± 7 Ma. Thus, although the oldest Archean age obtained with the Pb-evaporation and electron microprobe methods is similar within the analytical error, the Proterozoic ages obtained by the Pb-evaporation method are significantly older than the Th-U-total Pb Proterozoic age obtained with the electron microprobe. This discrepancy is presumed to be due to the influence, in the Pb-evaporation method, of radiogenic lead contained within Archean zones of the monazite grains; in other words, it means that migmatization took place later than

  6. Provenance of Permian Malužiná Formation sandstones (Hronicum, Western Carpathians): evidence from monazite geochronology

    NASA Astrophysics Data System (ADS)

    Vozárová, Anna; Konečný, Patrik; Vďačný, Marek; Vozár, Jozef; Šarinová, Katarína

    2014-10-01

    The Permian Malužiná Formation and the Pennsylvanian Nižná Boca Formation are Upper Paleozoic volcano- sedimentary complexes in the Hronicum nappe system. Sandstones, shales and conglomerates are the dominant lithological members of the Malužiná Formation sequence. Detrital monazites were analysed by electron microprobe, to obtain Th-U-Pb ages of the source areas. The majority of detrital monazites showed Devonian-Mississippian ages, ranging from 330 to 380 Ma with a weighted average of 351 ± 3.3 (2σ), that correspond well with the main phase of arcrelated magmatic activity in the Western Carpathians. Only a small portion of detrital monazites displayed Permian ages in the range of 250-280 Ma, with a significant maximum around 255 Ma. The weighted average corresponds to 255 ± 6.2 Ma. These monazites may have been partially derived from the synsedimentary acid volcanism that was situated on the margins of the original depositional basin. However, some of the Triassic ages (230-240 Ma), reflect, most likely, the genetic relationship with the overheating connected with Permian and subsequent Triassic extensional regime. Detrital monazite ages document the Variscan age of the source area and also reflect a gradual development of the Hronicum terrestrial rift, accompanied by the heterogeneous cooling of the lithosphere.

  7. Coupling thermodynamic modeling and high-resolution in situ LA-ICP-MS monazite geochronology: evidence for Barrovian metamorphism late in the Grenvillian history of southeastern Ontario

    NASA Astrophysics Data System (ADS)

    McCarron, Travis; Gaidies, Fred; McFarlane, Christopher R. M.; Easton, R. Michael; Jones, Peter

    2014-12-01

    The Flinton Group is a greenschist to upper amphibolite facies package of metasediments in southeastern Ontario that was metamorphosed during the Ottawan Orogeny. Thermodynamic modeling of metapelitic mineral assemblages suggests an increase in peak conditions of metamorphism across the 40 km wide study area from 3.5 to 7.9 kbar and 540 to 715 °C. Garnet isopleth thermobarometry applied to the cores of compositionally zoned porphyroblasts reveals remarkably similar P-T conditions of initial crystallization at approximately 3.7-4.0 kbar and 512-520 °C, corresponding to a relatively high geothermal gradient of ca. 34-45 °C km-1. It is inferred from modeling and reaction textures that metamorphism was along Barrovian P-T paths. Major and trace element zoning in garnet from one sample records a complex growth history as evidenced by major and trace element zoning and the distribution of xenotime, allanite and monazite inclusions. High-resolution (6 μm) LA-ICP-MS U-Pb geochronology performed on monazite in the rock matrix and included in the outer 150 μm of garnet rim-ward of a Y annulus revealed an age of 976 ± 4 Ma. The age is interpreted to reflect monazite growth at the expense of allanite and apatite late in garnet's growth history over the P-T interval 4.5-6.8 kbar and 540-640 °C. This new age estimate for near peak metamorphism fits well into the regional framework but is significantly younger than previously reported ages for Ottawan metamorphism. Based on microstructures this new age suggests that compressional tectonics were operating much later in the history of the Grenville of southeastern Ontario than previously thought.

  8. EPMA monazite geochronology of the basement and supracrustal rocks within the Pur-Banera basin, Rajasthan: Evidence of Columbia breakup in Northwestern India

    NASA Astrophysics Data System (ADS)

    Ozha, Manoj Kumar; Mishra, Biswajit; Hazarika, Pranjit; Jeyagopal, A. V.; Yadav, G. S.

    2016-03-01

    We deduce the timing of the rift-induced collisional event between two orogenies in the central Aravalli-Delhi Fold Belt using monazite chemical dating and metamorphic P-T estimates from metapelites of the Mangalwar Complex (MC), and the overlying Pur-Banera (PB) supracrustals. The MC rocks preserve evidence of three regional metamorphic events, while the PB rocks record the last event. The M1 metamorphism attained its peak P-T at ∼5.5 kbar and 520-550 °C in the MC rocks at ∼1.82 Ga, followed by the M2 event with peak P-T of ∼7.5 kbar and 580-660 °C at ∼1.35 Ga. The youngest high-pressure M3 metamorphism attained a peak P-T of ∼8.0 kbar and 590-640 °C at ∼0.99 Ga. Thermobarometry coupled with ages of included monazites in chemically zoned garnet from the MC metapelites indicate preservation of ages spanning between ∼1.82 Ga and ∼0.99 Ga from different zones (i.e., core to rim), implying episodic garnet growth during supercontinent cycle. The PB metapelites constitute two prominent ages of ∼1.37 Ga and ∼1.05 Ga. The youngest high-pressure metamorphism (M3) in the PB rocks with maximum P-T of ∼8.0 kbar, and 580-670 °C during the Neoproterozoic has overprinted their earlier metamorphic records. Based on monazite geochronology, we assign the ∼1.82 Ga and ∼1.37-1.35 Ga ages to the amalgamation and breakup of the Columbia supercontinent respectively. The youngest age record of ∼1.05-0.99 Ga indicates evidence of Rodinia formation in and around the Pur-Banera basin.

  9. Monazite geochronology and geothermobarometry in polymetamorphic host rocks of volcanic-hosted massive sulphide mineralizations in the Mesoproterozoic Areachap Terrane, South Africa

    NASA Astrophysics Data System (ADS)

    Bachmann, Kai; Schulz, Bernhard; Bailie, Russell; Gutzmer, Jens

    2015-11-01

    The Areachap Terrane represents the medium-to high-grade metamorphic and deformed remnants of a Mesoproterozoic (ca. 1240-1300 Ma) volcanic arc bound to the margin of the Archean Kaapvaal Craton in the east, and the polydeformed and metamorphosed Proterozoic Namaqua Province in the west. There has been protracted debate as to the exact nature, origin, age and tectonic evolution of this terrane, adjacent to an important Mesoproterozoic crustal suture between the Archean Kaapvaal Craton and the Namaqua Province, which developed during the ∼1200-1000 Ma Namaquan Orogeny. The Areachap Terrane comprises highly deformed bimodal volcanic and volcano sedimentary successions that host a number of massive sulphide base metal orebodies. Samples from three of these orebodies at different locations were analysed to determine the age and P-T conditions of metamorphism along the Areachap Terrane. Metamorphic ages were determined by electron microprobe chemical dating of monazite. Garnet- and amphibole-bearing mineral assemblages were used for geothermobarometry at the Areachap Mine, located in the northern sector of the Areachap Terrane, monazite geochronology yields evidence for two populations of Th-U-Pb-ages at 1432 ± 30 Ma - a possible protolith age - and a metamorphic age of 1153 ± 21 Ma. Kantienpan and Copperton, representing the central and southern sector of the Areachap Terrane respectively, yield monazite ages for a younger metamorphic event with U-Th-Pb-ages of 1108 ± 19 Ma and 1104 ± 17 Ma, respectively. Geothermobarometric data give a differentiated view on the metamorphic evolution of the Areachap Terrane. An arc consistent clockwise P-T evolution path and upper amphibolite-facies peak metamorphic conditions are consistent at the three locations. The Areachap site shows a short prograde development with 8.0 kbar maximum pressure at circa 700 °C maximum temperature and a subsequent retrograde metamorphism. At Kantienpan, on the other hand, maximum metamorphic

  10. Application of Geochronology and Geochemistry of Speleothems to Hydrologic Change

    NASA Astrophysics Data System (ADS)

    Banner, J. L.; Musgrove, M.; Mickler, P. J.; Mack, L. E.; James, E. W.

    2001-12-01

    in spite of the many complexities of regional and local variability both within and between aquifer systems, climate signals are discernable. Coupled studies of speleothems and the modern groundwater system help constrain mechanisms that link hydrologic and climatic processes over multiple timescales. The calibration of modern speleothem geochemistry and growth rates with aquifer and climatic measurements is a new research avenue toward this end.

  11. Linking metamorphic textures to U-Pb monazite in-situ geochronology to determine the age and nature of aluminosilicate-forming reactions in the northern Monashee Mountains, British Columbia

    NASA Astrophysics Data System (ADS)

    Gervais, Félix; Hynes, Andrew

    2013-02-01

    The Monashee Mountains of the Canadian Cordillera are thought to expose a classic Barrovian-facies series of isograds. The timing of aluminosilicate growth in the region was determined for four pelitic schist samples by combining textural relationships with monazite compositional zoning and monazite U-Pb geochronology conducted directly on thin-sections by the laser ablation method. Three distinct phases of kyanite growth are recorded in the kyanite zone: at c. 153 Ma, between 122 and 94 Ma and between 76 and 58 Ma. For each phase, monazite and garnet grew synchronously with kyanite, probably by a reaction involving the breakdown of staurolite. In contrast, sillimanite growth by muscovite dehydration melting occurred at or before c. 104 Ma in the sillimanite zone, and retrograde sillimanite grew in schists previously metamorphosed at the kyanite grade during the first two phases by the influx of hot, acidic fluids during top-to-the-east shearing at ca. 71 Ma. These results indicate that rocks metamorphosed at different places and different times in the orogen were juxtaposed prior to being overprinted at the sillimanite grade in the Late Cretaceous-Early Paleocene during the influx of hot fluids in a structurally coherent body deforming by easterly directed shearing. This study also provides new insight into monazite petrogenesis and suggests that, at least in some circumstances, monazite formation is linked to the staurolite-out reaction that produces kyanite.

  12. Paragneiss zircon geochronology and trace element geochemistry, North Qaidam HP/UHP terrane, western China

    USGS Publications Warehouse

    Mattinson, C.G.; Wooden, J.L.; Zhang, J.X.; Bird, D.K.

    2009-01-01

    In the southeastern part of the North Qaidam terrane, near Dulan, paragneiss hosts minor peridotite and UHP eclogite. Zircon geochronology and trace element geochemistry of three paragneiss samples (located within a ???3 km transect) indicates that eclogite-facies metamorphism resulted in variable degrees of zircon growth and recrystallization in the three samples. Inherited zircon core age groups at 1.8 and 2.5 Ga suggest that the protoliths of these rocks may have received sediments from the Yangtze or North China cratons. Mineral inclusions, depletion in HREE, and absence of negative Eu anomalies indicate that zircon U-Pb ages of 431 ?? 5 Ma and 426 ?? 4 Ma reflect eclogite-facies zircon growth in two of the samples. Ti-in-zircon thermometry results are tightly grouped at ???660 and ???600 ??C, respectively. Inclusions of metamorphic minerals, scarcity of inherited cores, and lack of isotopic or trace element inheritance demonstrate that significant new metamorphic zircon growth must have occurred. In contrast, zircon in the third sample is dominated by inherited grains, and rims show isotopic and trace element inheritance, suggesting solid-state recrystallization of detrital zircon with only minor new growth. ?? 2009 Elsevier Ltd.

  13. Geochronology and geochemistry of volcanic glasses associated with early Homo sapiens in Ethiopia.

    NASA Astrophysics Data System (ADS)

    Morgan, L. E.; Renne, P. R.; Woldegabriel, G.; White, T. D.

    2005-12-01

    In past work at hominid sites in Ethiopia, 40Ar/39Ar dating was used to constrain obsidian from the base of the Upper Herto Member of the Bouri Formation to 160 ± 2 ka. An overlying vitric tuff was then geochemically correlated to one from the Konso region of Ethiopia, which is constrained to be older than 154 ± 7 ka, thus leaving only 6 ± 7 ky between eruption and deposition of the fossils and artifacts at Herto. To continue these studies, we have collected and are currently analyzing obsidian and associated volcanic ashes from Middle Stone Age (MSA) archaeological and paleontological sites in the Middle Awash, Ethiopia. Distinctive geochemical signatures among most obsidian fragments collected (n=20 per site) suggest that obsidian was being derived from a variety of sources. By comparing our geochemical data with that from known obsidian deposits in Ethiopia and elsewhere in East Africa, we hope to determine the source localities for the obsidian and thus gauge the extent of trade networks during the MSA. Thus, by characterizing obsidian using both 40Ar/39Ar geochronology and trace element geochemistry, will make it possible to temporally refine the stratigraphy and prehistory at hominid sites, which in turn improves understanding of hominid behavior and evolution.

  14. Cold seep carbonates along the Norwegian margin, insights into U-Th geochronology and S geochemistry

    NASA Astrophysics Data System (ADS)

    Cremiere, A.; Lepland, A.; Wing, B. A.; Sahy, D.; Condon, D. J.; Chand, S.; Noble, S. R.; Bui, T. H.; Thorsnes, T.; Brunstad, H.

    2015-12-01

    Cold seep carbonates along the Norwegian margin, insights into U-Th geochronology and S geochemistryAuthigenic carbonate crusts form in shallow subsurface of marine sediments due to the microbial anaerobic oxidation of methane (AOM). As a result they are unique archives of the locus and intensity of past methane seepage that can be dated by using U-daughter decay affording the unique opportunity to constrain the absolute timing of methane release events. Because AOM is mainly driven by the microbial reduction of seawater sulfate, multiple sulfur isotope compositions of paired carbonate-associated sulfate (CAS) and pyrite in seep carbonates taken as proxies for porewater sulfate and sulfide, respectively, have the potential to reconstruct the biogeochemical conditions under which seep carbonates precipitate. Methane-derived carbonate crusts were collected from several seepage sites on the Norwegian continental shelf, including sites in the North Sea, the Norwegian Sea and the Barents Sea. The U-Th dating results constrain the main episode of carbonate crust formation in the Barents and Norwegian seas during the time interval between 14 and 7 ka. Such ages suggest that the methane seepage along the northern Norwegian margin was most active after the collapse of the Scandinavian ice sheet and deglaciation of the area that took place at about 15 ka. The methane flux for the carbonate crust formation was likely provided by the dissociation of methane hydrates that extensively formed in underlying sediments during the last glacial period, but became unstable due to depressuring effects of retreating ice sheet. The precipitation of studied North Sea carbonate crusts occurred more recently, from 6 to 1 ka, suggesting that their formation is unrelated to the glacial history of the area. The paired sulfur stable isotope compositions of pyrite-CAS record a large range of fractionation factors (from 30 to 70 ‰) reflecting change of sulfate-reduction rates possibly controlled

  15. Geochronology of granulite, charnockite and gneiss in the poly-metamorphosed Gaozhou Complex (Yunkai massif), South China: Emphasis on the in-situ EMP monazite dating

    NASA Astrophysics Data System (ADS)

    Chen, Cheng-Hong; Liu, Yung-Hsin; Lee, Chi-Yu; Xiang, Hua; Zhou, Han-Wen

    2012-07-01

    The in-situ EMP (electron microprobe) monazite age dating performed directly in the polished sections, in addition to the conventional U-Pb zircon and EMP monazite age dating on grains from heavy mineral concentrates, has been applied to the granulite, charnockite and gneiss in the Gaozhou Complex of the Yunkai massif in South China. While the conventional dating systems all give Caledonian ages, the in-situ EMP monazite ages provide more information to reveal not only detailed age groups pertaining to the Caledonian orogeny but also traces of later thermal events overprinting these rocks. For granulites, although some monazites present zoning (concentric, patchy and complex) in the BSE images, no discernable age differences are observed. Resetting of the Th-U-Pb monazite dating system under the high temperature condition could be the reason. Ages of homogeneous monazite in garnet porphyroblast (ca. 440 Ma) of the garnet-cordierite granulite that match nicely with the U-Pb zircon ages are systematically older than those in the matrix (ca. 430 Ma). The same case of two age groups is also present in the orthopyroxene-biotite granulite as revealed by monazite inclusions in plagioclase and orthopyroxene and those in quartz, respectively. For charnockites, despite similar ages of ca. 430 Ma are given by monazite in biotite and zircon in the rock, significant younger ages are obtained from monazites with particular features. Relict monazites with a breakdown texture to form successive layers of apatite and allanite in the rim as well as those which are close to the biotite-chlorite microvein always show a similar age of ca. 230 Ma. Moreover, tiny monazites in close association with the garnetiferous corona mainly surrounding orthopyroxene give rise to another age group around 370 Ma. For gneissic rocks, monazites enclosed by quartz give 434 Ma and those setting in the chlorite-epidote microvein of a paragneiss yield 237 Ma, consistent with the U-Pb zircon core-rim age

  16. Deciphering igneous and metamorphic events in high-grade rocks of the Wilmington complex, Delaware: Morphology, cathodoluminescence and backscattered electron zoning, and SHRIMP U-Pb geochronology of zircon and monazite

    USGS Publications Warehouse

    Aleinikoff, J.N.; Schenck, W.S.; Plank, M.O.; Srogi, L.A.; Fanning, C.M.; Kamo, S.L.; Bosbyshell, H.

    2006-01-01

    High-grade rocks of the Wilmington Complex, northern Delaware and adjacent Maryland and Pennsylvania, contain morphologically complex zircons that formed through both igneous and metamorphic processes during the development of an island-arc complex and suturing of the arc to Laurentia. The arc complex has been divided into several members, the protoliths of which include both intrusive and extrusive rocks. Metasedimentary rocks are interlayered with the complex and are believed to be the infrastructure upon which the arc was built. In the Wilmingto n Complex rocks, both igneous and metamorphic zircons occur as elongate and equant forms. Chemical zoning, shown by cathodoluminescence (CL), includes both concentric, oscillatory patterns, indicative of igneous origin, and patchwork and sector patterns, suggestive of metamorphic growth. Metamorphic monazites are chemically homogeneous, or show oscillatory or spotted chemical zoning in backscattered electron images. U-Pb geochronology by sensitive high resolution ion microprobe (SHRIMP) was used to date complexly zoned zircon and monazite. All but one member of the Wilmington Complex crystallized in the Ordovician between ca. 475 and 485 Ma; these rocks were intruded by a suite of gabbro-to-granite plutonic rocks at 434 ?? Ma. Detrital zircons in metavolcanic and metasedimentary units were derived predominantly from 0.9 to 1.4 Ga (Grenvillian) basement, presumably of Laurentian origin. Amphibolite to granulite facies metamorphism of the Wilmington Complex, recorded by ages of metamorphic zircon (428 ?? 4 and 432 ?? 6 Ma) and monazite (429 ?? 2 and 426 ?? 3 Ma), occurred contemporaneously with emplacement of the younger plutonic rocks. On the basis of varying CL zoning patterns and external morphologies, metamorphic zircons formed by different processes (presumably controlled by rock chemistry) at slightly different times and temperatures during prograde metamorphism. In addition, at least three other thermal episodes are

  17. Progress integrating ID-TIMS U-Pb geochronology with accessory mineral geochemistry: towards better accuracy and higher precision time

    NASA Astrophysics Data System (ADS)

    Schoene, B.; Samperton, K. M.; Crowley, J. L.; Cottle, J. M.

    2012-12-01

    It is increasingly common that hand samples of plutonic and volcanic rocks contain zircon with dates that span between zero and >100 ka. This recognition comes from the increased application of U-series geochronology on young volcanic rocks and the increased precision to better than 0.1% on single zircons by the U-Pb ID-TIMS method. It has thus become more difficult to interpret such complicated datasets in terms of ashbed eruption or magma emplacement, which are critical constraints for geochronologic applications ranging from biotic evolution and the stratigraphic record to magmatic and metamorphic processes in orogenic belts. It is important, therefore, to develop methods that aid in interpreting which minerals, if any, date the targeted process. One promising tactic is to better integrate accessory mineral geochemistry with high-precision ID-TIMS U-Pb geochronology. These dual constraints can 1) identify cogenetic populations of minerals, and 2) record magmatic or metamorphic fluid evolution through time. Goal (1) has been widely sought with in situ geochronology and geochemical analysis but is limited by low-precision dates. Recent work has attempted to bridge this gap by retrieving the typically discarded elution from ion exchange chemistry that precedes ID-TIMS U-Pb geochronology and analyzing it by ICP-MS (U-Pb TIMS-TEA). The result integrates geochemistry and high-precision geochronology from the exact same volume of material. The limitation of this method is the relatively coarse spatial resolution compared to in situ techniques, and thus averages potentially complicated trace element profiles through single minerals or mineral fragments. In continued work, we test the effect of this on zircon by beginning with CL imaging to reveal internal zonation and growth histories. This is followed by in situ LA-ICPMS trace element transects of imaged grains to reveal internal geochemical zonation. The same grains are then removed from grain-mount, fragmented, and

  18. Stratigraphy, geochronology, geochemistry and tectonic setting of the Mesozoic Nazas Formation, north-central Mexico

    NASA Astrophysics Data System (ADS)

    Bartolini, Claudio

    Late Triassic to Middle Jurassic volcanic-sedimentary sequences that were part of the Mesozoic continental-margin of western North America are exposed in northern and central Mexico. These sequences have been grouped into the Nazas Formation and crop out in the states of Durango, Coahuila, Zacatecas, and San Luis Potosi. The Nazas Formation consists of 2,500 m or more of volcanic and pyroclastic rocks and interbedded clastic sedimentary rocks that were deposited in alluvial fan and fluvial depositional systems that developed in intra-arc basins, mainly fault-bound grabens and topographic depressions within an extending Mesozoic volcanic arc. Major and trace element geochemistry of volcanic rocks suggests that the volcanic suite is calc-alkaline and includes rhyolite, dacite, rhyodacite, andesite, trachyandesite and rare basalt. Pyroclastic rocks are basically air-fall tuffs and volcanic breccias. The sedimentary strata include conglomerate, sandstone, shale, and siltstone, locally red in color. Geochronology (Ar-Ar, K-Ar and Rb-Sr) and field evidence indicate that the age of the Nazas Formation ranges from Late Triassic to Middle Jurassic, but the peak of arc volcanism appears to be Early and Middle Jurassic. The Mesozoic magmatic arc in Mexico has a northwest trend and extends from Sonora to Chiapas. The arc structure is more than 2,000 km long, and possibly up to 150 km wide. The width of the arc is uncertain due to the limited number of surface outcrops, however, it did not extend east into the Gulf of Mexico. Arc-related magmatism began in latest Triassic time, but the peak of arc evolution occurred during the Early and Middle Jurassic. By Oxfordian time, the arc was deeply dissected and eroded, and magmatic activity had ceased. A marine transgression from the Gulf of Mexico covered most of the Nazas arc, depositing the initial sediments of the Oxfordian Zuloaga Limestone in the Mexican Geosyncline. Jurassic crustal extension in the Gulf of Mexico was

  19. Geochemistry and Rb-sr geochronology of associated proterozoic peralkaline and subalkaline anorogenic granites from Labrador

    NASA Astrophysics Data System (ADS)

    Collerson, Kenneth D.

    1982-12-01

    Anorogenic granites of middle to late Proterozoic age in the Davis Inlet — Flowers Bay area of Labrador are subdivided on the basis of petrology and geochemistry into three coeval suites. Two of these are high-temperature anhydrous hypersolvus granites: a peralkaline aegirine-sodic-calcic to sodic amphibole-bearing suite and a non-alkaline fayalite-pyroxene-bearing suite. The third is a group of non-alkaline subsolvus hornblende-biotite-bearing granites. Associated with the hypersolvus peralkaline suite is a group of genetically related syenites and quartz syenites. The granites cut ca. 3,000 Ma old Archaean gneisses as well as Elsonian layered basic intrusions of the Nain Complex. One of these, a crudely layered mass which ranges in composition from gabbro to diorite and monzonite, appears to be related to the syenites. The peralkaline granites and some of the syenites are extremely enriched in the high field-strength elements such as Y, Zr, Nd, as well as Rb, Ga and Zn, and have low abundances of Ba, Sr and most of the transition elements. In contrast, the non-alkaline hypersolvus and subsolvus granites do not show the same degree of enrichment. Concentration of the highly charged cations in the peralkaline suite is believed to be the result of halogen-rich fluid activity during fractionation of the magma. The sodic evolution trend in the peralkaline suite is reflected mineralogically by the development of aegirine and aegirine-hedenbergite solid solutions, and by a spectacular amphibole compositional range from katophorite through winchite, richterite, riebeckite to arfvedsonite and ferro eckermannite. Accessory phases which are ubiquitous in these rocks include aenigmatite, astrophyllite, fluorite, monazite and zircon. The non-alkaline hypersolvus granites typically contain iron-rich phases such as fayalite, eulite, ferrosilite-hedenbergite, and annite rich biotite. In the subsolvus granites, amphiboles range in composition from edenite through common

  20. The Geochronology and Geochemistry of Highly Fractionated I-type Granite within Gangdese batholith in Sangri Area, South Tibet

    NASA Astrophysics Data System (ADS)

    Wang, Z.; Zhao, Z.; Mo, X.; Zhu, D. C.; Wei, Y.; Liu, D.

    2015-12-01

    Long-term subduction of Neo-Tethyan oceanic lithosphere beneath an active continental margin arise prolonged plutonic activity, which is illustrated by the vast chain of Gangdese batholith in the south margin of Lhasa Terrane, Southern Tibet. Although extensive studies have performed on the source regions, spatial-temporal associations and geotectonic implications of the granitic rocks within the batholith, the magmatic evolution and petrogenesis of plutons in Sangri area have remained poorly studied. Here we present zircon U-Pb geochronology, Hf isotope and bulk-rock geochemistry of the late Cretaceous Sangri biotite granites (SBG) in order to shed light on this issue. Zircon U-Pb geochronology demonstrates the plutonic activity emplaced at 67~65Ma. The SBG belong to high-K calc-alkaline series, displaying highly fractionated I-type signature with high content of SiO2 (74.26~76.93%), K2O+Na2O (7.87~8.56%), but low content of CaO (0.28~1%) and P2O5 (0.02~0.04%) and pronounced depletion in Sr, Ba, Eu, Nb and Zr. The affinity of mantle component in the SBG that was elucidated by positive zircon Epsilon Hf(t) of 4.6~10.9, which makes it indistinguishable from other granitic rocks within Gangdese batholith. Tectonic models of evolution of the Neo-Tethyan Ocean suggest that the break-off of the oceanic slab is required in order to generate enormous magmatism in Gangdese batholith during 66~50Ma. Associated with the coeval gabbroic and dioritic intrusions occurred in Sangri area, we suggest the magma of SBG had been derived from juvenile lower crust mingled with mantle component upwelling through the window of Neo-Tethyan oceanic slab, and were subjected to high degree of fractionation in the magma chamber.

  1. A new method fingerprinting magmatic processes using combined U/Pb ID-TIMS geochronology and accessory mineral geochemistry

    NASA Astrophysics Data System (ADS)

    Schoene, B.; Schaltegger, U.; Latkoczy, C.; Günther, D.

    2009-12-01

    Zircon is commonly used as a recorder of magmatic processes because of its utility in geochronology and ability to retain primary growth information at prolonged magmatic conditions. Recent applications of U-Pb ID-TIMS geochronology have shown that precision on dates of single zircons are often far smaller than the time-scales of magmatic systems. The result is that dates on such grains record various processes occurring over an interval of time during the production, mobilisation, and emplacement of magmas. Combining age information with geochemical data from the same zircons has proved a useful tool, but thus far has been restricted to low-precision in situ dating techniques. Furthermore, these techniques have been unable to combine age data with trace element geochemistry on the same volume of zircon. We have developed a new technique that combines high-precision ID-TIMS dating with geochemical characterization on the exact same material. U and Pb are commonly separated from the other chemical constituents by ion exchange chemistry during sample preparation. We retain this fraction and analyse it using solution nebulization ICP-SFMS with matrix-matched external liquid calibration. We measured elements such as Zr, Hf, Y, Sc, and the REE, which are present in solution at between 10 and >105 ppt; Hf isotopes are subsequently measured on the same solution. Data obtained using this approach allow us to distinguish between models for melt generation, transport, and assembly of the ca. 40 Ma composite mafic to felsic Re di Castello pluton, Adamello batholith, northern Italy. Coupled with age uncertainties on single zircons as low as 10,000 years, zircon geochemistry preserves a rich record of fractional crystallization, crustal assimilation and magma mixing over timescales of <20,000 to >400,000 years. These data can also be used to distinguish between auto-, xeno-, and antecrystic zircon. Combined with field observation and U-Pb sphene thermochronology, we show that

  2. The Anari and Tapirapua Jurassic formations, western Brazil: Paleomagnetism, geochemistry and geochronology

    NASA Astrophysics Data System (ADS)

    Montes-Lauar, C. R.; Pacca, I. G.; Melfi, A. J.; Piccirillo, E. M.; Bellieni, G.; Petrini, R.; Rizzieri, R.

    1994-12-01

    The Anari and Tapirapua formations are very similar from the point of view of paleomagnetic, geochronological and geochemical results. They date from around 197 Ma and the flows are mainly tholeiitic basalts with a low TiO2 and incompatible element content. The magnetic carriers in rocks from these two formations were highly oxidized titanomagnetites, maghemites and, probably, titanomaghemites. Paleomagnetic analysis has shown that the magnetizations are all normal in polarity and virtual geomagnetic poles obtained for both formations are indistinguishable at the 95% confidence level. The calculated mean for these poles is 250.3 deg E, 65.5 deg S (N = 15; A95 = 3.6 deg; KSC = 1578). This pole is compatible with Jurassic poles which have been determined for South America or transposed from Africa.

  3. Geochronology and geochemistry of tuff beds from the Shicaohe Formation of Shennongjia Group and tectonic evolution in the northern Yangtze Block, South China

    NASA Astrophysics Data System (ADS)

    Du, Qiuding; Wang, Zhengjiang; Wang, Jian; Deng, Qi; Yang, Fei

    2016-03-01

    Meso- to Neoproterozoic magmatic events are widespread in the Yangtze Block. The geochronology and tectonic significance of the Shennongjia Group in the Yangtze Block are still highly controversial. An integrated geochronology and geochemistry approach provides new insights into the geochronological framework, tectonic setting, magmatic events, and basin evolution of the northern Yangtze Block. Our new precise sensitive high-resolution ion microprobe U-Pb data indicate a deposition age of 1180 ± 15 Ma for the Shicaohe Formation subalkaline basaltic tuff that is geochemically similar to modern intracontinental rift volcanic rocks. The integration of available geochemical data together with our new U-Pb ages indicates the Shicaohe Formation subalkaline basaltic tuff formed ca. 1180 in a continental rift-related setting on a passive continental margin. The Shennongjia Group is topped by the Zhengjiaya Formation volcanic sequence, indicating arc-related igneous events at 1103 Ma. The transition of the late Mesoproterozoic tectonic regime from intracontinental extension to convergence occurred between ca. 1180 and 1103 Ma in the northern Yangtze Block. Tectonic evolution in the Neoproterozoic led to accretion along the northern margin of the Yangtze Block. These results provide geochronological evidence, which is of utmost importance for reconfiguration of the chronostratigraphic framework and for promoting research on Mesoproterozoic strata in China, thereby increasing understanding of magmatic events and basin evolutionary history in the northern Yangtze Block.

  4. Geochemistry and Geochronology of Middle Tertiary Volcanic Rocks of the Central Chiricahua Mountains, Southeast Arizona

    USGS Publications Warehouse

    du Bray, Edward A.; Pallister, John S.; Snee, Lawrence W.

    2004-01-01

    Middle Tertiary volcanic rocks of the central Chiricahua Mountains in southeast Arizona are the westernmost constituents of the Eocene-Oligocene Boot Heel volcanic field of southwestern New Mexico and southeastern Arizona. About two dozen volumetric ally and stratigraphically significant volcanic units are present in this area. These include large-volume, regionally distributed ash-flow tuffs and smaller volume, locally distributed lava flows. The most voluminous of these units is the Rhyolite Canyon Tuff, which erupted 26.9 million years ago from the Turkey Creek caldera in the central Chiricahua Mountains. The Rhyolite Canyon Tuff consists of 500-1,000 cubic kilometers of rhyolite that was erupted from a normally zoned reservoir. The tuff represents sequential eruptions, which became systematically less geochemically evolved with time, from progressively deeper levels of the source reservoir. Like the Rhyolite Canyon Tuff, other ashflow tuffs preserved in the central Chiricahua Mountains have equivalents in nearby, though isolated mountain ranges. However, correlation of these other tuffs, from range to range, has been hindered by stratigraphic discontinuity, structural complexity, and various lithologic similarities and ambiguities. New geochemical and geochronologic data presented here enable correlation of these units between their occurrences in the central Chiricahua Mountains and the remainder of the Boot Heel volcanic field. Volcanic rocks in the central Chiricahua Mountains are composed dominantly of weakly peraluminous, high-silica rhyolite welded tuff and rhyolite lavas of the high-potassium and shoshonitic series. Trace-element, and to a lesser extent, major-oxide abundances are distinct for most of the units studied. Geochemical and geochronologic data depict a time and spatial transgression from subduction to within-plate and extensional tectonic settings. Compositions of the lavas tend to be relatively homogeneous within particular units. In

  5. Peri-Amazonian provenance of the Proto-Pelagonian basement (Greece), from zircon U-Pb geochronology and Lu-Hf isotopic geochemistry

    NASA Astrophysics Data System (ADS)

    Zlatkin, Olga; Avigad, Dov; Gerdes, Axel

    2014-01-01

    The basement of the Pelagonian zone of the Hellenides, in the Eastern Mediterranean realm, has been shaped by mid-Neoproterozoic (700 Ma) and Variscan (300 Ma) igneous activities. In the present study, detrital zircon U-Pb geochronology and Lu-Hf isotope geochemistry of ca. 700 Ma-aged granites and of pre-700 Ma metasediments from the Pelagonian zone allow a genuine perspective into the provenance and origin of this terrane which hosts one of the oldest sedimentary sequences known in SE Europe. Pelagonian crustal vestiges comprising 700 Ma granitoids and their hosting metasediments are termed here "Proto-Pelagonian".

  6. Geochemistry and geochronology of HP mélanges from Tinos and Andros, cycladic blueschist belt, Greece

    NASA Astrophysics Data System (ADS)

    Bulle, Florian; Bröcker, Michael; Gärtner, Claudia; Keasling, Alan

    2010-06-01

    U-Pb zircon geochronology, Sr-Nd isotope and bulk-rock geochemistry have been applied to meta-igneous and meta-sedimentary rocks from high-pressure metamorphic mélanges exposed on the Cycladic islands of Tinos, Syros and Andros. Ion microprobe (SHRIMP) U-Pb zircon dating of 7 samples representing meta-igneous blocks (Tinos), a blackwall zone (Tinos) and chlorite-talc schists from block-matrix contacts (Syros and Tinos) yielded Cretaceous ages of c. 80 Ma. Many of the criteria commonly used to distinguish between magmatic or metamorphic zircon genesis (internal structure, Th/U ratio, REE characteristics, Ti-in zircon thermometry, enclosed mineral phases) do not provide unambiguous constraints for the mode of formation. However, a magmatic origin for Cretaceous zircon of meta-gabbros and eclogites is considered likely. Supporting evidence for a previously suggested metamorphic origin for c. 80 Ma zircon in eclogite has not been found. Zircon of the same age occurring in chlorite-talc schists is presumably related to non-magmatic processes. Well-defined Cretaceous age groups clustering at c. 79 Ma also occur in the detrital zircon populations of 2 quartz mica schists representing the mélange matrix on Tinos, and suggest a much later time for sediment accumulation than previously assumed. The importance of c. 57 Ma zircon ages remains unclear, but may record either HP metamorphic processes or a post-57 Ma depositional age. The youngest age group in a third quartz mica schist from Tinos, collected outside the main mélange occurrences, clusters at c. 226-238 Ma. In all clastic metasediments from Tinos, most data points plot along the concordia between c. 300 and 900 Ma; single data points indicate concordant ages of c. 2.5 Ga, 2.3 Ga and 1 Ga, respectively. The youngest 206Pb/ 238U age group that has been recognized in a felsic paragneiss from Andros indicates an age of 163.1 ± 3.9 Ma, and mostly represents overgrowths around zircon with ages in the range from ˜ 272

  7. Paleozoic development of the Qilian orogen: Insights from a ductile right-slip shear zone and monazite geochronology in the Central Qilian Shan, northeastern Tibetan Plateau

    NASA Astrophysics Data System (ADS)

    Zuza, A. V.; Yin, A.; Dong, S.; Liu, W.; Reith, R. C.; Zhang, J.; Wu, C.; Wu, L.; Gong, J.

    2012-12-01

    schist ± garnet, foliated granitoids, and garnet amphibolite, which make up a broadly defined ductile right-slip strike-slip shear zone. Consistent metamorphic fabric strikes NW-SE, possesses subhorizontal stretching lineations, and has dextral shear kinematic indicators. From samples with synkinematic garnets, we present new in-situ ion-microprobe Th-Pb ages of matrix and included monazite. Based on textural patterns within the synkinematic garnet inclusions, these ages are interpreted to represent the age of ductile shearing. Preliminary analysis of monazite grains yield ages of ~420 Ma, possibly indicating a metamorphic event in the Silurian. Further P-T-t analysis will reveal the temporal evolution of this shear zone. A detailed study of the relationship between the high-grade metamorphic rocks hosting the shear zone and the Ordovician mélange and ophiolite complex will constrain the initial geometry of this shear zone. The Ordovician complex may represent either an obducted ophiolite segment and/or a volcanic arc. Specifically, we seek to understand if this shear zone was a large-scale strike-slip fault within a growing accretionary wedge or if it represents a suture resulting from an oblique subduction and/or ocean closure.

  8. The Vanda Dike Swarm, Dry Valleys, Antarctica II: Geochemistry and Geochronology

    NASA Astrophysics Data System (ADS)

    Harpp, K. S.; Bray, B.; Geist, D.; Garcia, M. O.

    2014-12-01

    One of the most spectacular features of the Dry Valleys of Antarctica is the Vanda Dikes, an exceptionally well exposed swarm of >600 Cambro-Ordovician dikes emplaced near the end of the Ross Orogeny. Whereas their compositions range from mafic to felsic, they are primarily bimodal, with few intermediate compositions between 56 and 66 wt.% SiO2; mafic dikes vary from calc-alkaline to shoshonitic. The suite of dikes exhibits 87Sr/86Sr from 0.704-0.711 and 143Nd/144Nd from 0.51217-0.51242; the wide range of Nd isotopic ratios may reflect variable degrees of assimilation of the subcontinental lithospheric mantle inherited during Proterozoic crustal generation and the Ross Orogeny. The mafic dikes have a subduction-related trace element signature and we propose they originated as melts of the subcontinental lithospheric mantle. The siliceous suite is characterized by elevated 87Sr/86Sri and an average Nd model age (τDM) of 1444 Ma, suggesting that the silica-rich dikes were generated by melting of or contamination by a Neoproterozoic crustal source, followed by variable fractional crystallization and further continental crust contamination. Analysis of zircons by CA-TIMS U-Pb geochronology reveals that the dikes were emplaced in a narrow time window, from 491 to 495 ± 0.3 Ma, which falls between emplacement of a synorogenic pluton (DV1) and extension-related plutons (DV2). Thus, we conclude that the dikes were intruded during the transition of the Ross Orogeny from collision to extension. During orogenic uplift and collapse, decompression melting of the subcontinental lithospheric mantle was initiated, resulting in the emplacement of the Vanda Dike Swarm. Because there is a strong correlation between dike age and latitude, the dikes can be used to map the progress of the collision-extension transition during the Ross Orogeny across the Dry Valleys.

  9. Geochronology and geochemistry of pre-Jurassic superterranes in Marie Byrd Land, Antarctica

    NASA Astrophysics Data System (ADS)

    Pankhurst, R. J.; Weaver, S. D.; Bradshaw, J. D.; Storey, B. C.; Ireland, T. R.

    1998-02-01

    Marie Byrd Land, Antarctica, is a major part of the proto-Pacific supercontinental margin. On the basis of new geochronological and geochemical data relating to its pre-Jurassic evolution, Marie Byrd Land is subdivided into western or interior ("Ross") and eastern or exterior ("Amundsen") provinces, equivalent to two superterranes in New Zealand. The Ross province is characterized by Cambrian? metagraywackes and I-type orthogneiss dated at 505±5 Ma by U-Pb SHRIMP (Sensitive High Resolution Ion Microprobe). Its magmatic record consists of Devonian-Carboniferous (375±5 Ma and circa 339±6 Ma), predominantly I-type granitoids, and further minor granitic magmatism in Permo-Triassic times. This Paleozoic history is comparable to that of the Gondwana margin in northern Victoria Land, western New Zealand, and SE Australia. The Amundsen province has no observed Paleozoic graywacke succession; evidence from Rb-Sr and U-Pb SHRIMP dating supports calc-alkaline granitoid events in Ordovician/Silurian (450-420 Ma) and Permian (276±2 Ma) times. The latter may be the previously unknown source of Permian volcanic detritus in the Ellsworth and Transantarctic mountains. The Amundsen province is considered to be the equivalent of the Median Tectonic Zone of New Zealand, and arc magmatism of comparable ages is found in the Antarctic Peninsula and Thurston Island. The underlying lithosphere of the two provinces may be distinguished by Nd isotope data; granitoids and metasedimentary rocks of the Ross province have Meso-Proterozoic Nd model ages, generally 1300-1500 Ma, compared to 1000-1300 Ma for the Amundsen province. On the basis of published palaeomagnetic data, the two provinces amalgamated to form Marie Byrd Land in mid-Cretaceous times, only shortly before rifting of the New Zealand continental block away from Antarctica.

  10. Igneous rocks of Arctic Ocean deep sea ridges: new data on petrology, geochemistry and geochronology

    NASA Astrophysics Data System (ADS)

    Petrov, Oleg; Morozov, Andrey; Shokalsky, Sergey; Sobolev, Nikolay; Kashubin, Sergey; Shevchenko, Sergey; Sergeev, Sergey; Belyatsky, Boris; Shatov, Vitaly; Petrov, Eugeny

    2015-04-01

    The aggregate results of studies of igneous rocks, collected from the central part of the Arctic Ocean during scientific marine expeditions «Arctic-2000, 2005, 2007 and 2012» are presented and discussed in the frame of modern understanding of High Polar Arctic tectonic constraint. Petrological, geochemical and isotope-geochronological studies of more than 500 samples have shown that the sedimentary rocks are of dominated population among the rock fragments dredged from deep-sea bottom, and represented by metamorphosed dolomite and quartz sandstone, limestone, sometimes with the Devonian - Permian fauna. Igneous rocks are 10-15% only (Archean and Paleoproterozoic gneissouse granites and gabbro, Neoproterozoic dolerite) and metamorphic rocks (green shales, metabasites, gneisses). Apparently, these rocks are part of the acoustic basement underlying the Late Mesozoic - Cenozoic layered loose sediments. In addition to the dredged fragments of the ancient mafic rocks, some samples were taken as a core during deep-water drilling in the northern and southern slopes of the Mendeleev Ridge and represented by trachybasalts, marking the border of Late-Cenozoic deposit cover and acoustic basement and quite similar in composition to those of Early-Late Cretaceous basalts form northward of the Chukchi Plateau seamounts, Alpha Ridge, Franz Josef Land, De Long islands and other parts of the large igneous province of the High Arctic (HALIP). Video-filming of Mendeleev Ridge escarps proofs the existing of rock outcrops and supports local origin of most of the rock fragments found in the sampling areas. Thus the continental type of the earth's crust of the Central Arctic Ridges basement is based on all obtained results of our study of sea-bottom excavated rock material.

  11. Resolving arc processes through detrital zircon U-Pb geochronology and geochemistry: a case study from the southern California Mesozoic convergent margin

    NASA Astrophysics Data System (ADS)

    Johnston, S. M.; Kylander-Clark, A. R.

    2015-12-01

    Detrital zircon geochronology has been widely exploited to establish temporal characteristics in sedimentary source terranes. Detrital zircon geochemistry, however, has been largely overlooked given results from continentally derived igneous zircon that show subtle intersample variation in trace-element concentrations, and which make correlation between detrital zircon and their host terrane difficult. Nevertheless, recent studies suggest systematically variable geochemistry in McCoy Mountain detrital zircons derived from the southern California Mesozoic arc, and our preliminary data from the Peninsular Ranges batholith indicates strong correlations between whole-rock and zircon geochemistry. Here, we present coupled U-Pb geochronology and geochemistry measured by laser ablation split stream ICPMS on detrital zircons from Nacimiento block forearc sediments in Central California to characterize temporal and geochemical trends in the adjacent Mesozoic arc terrane. 1098 grains of Mesozoic age analyzed from 22 samples in the Nacimiento block define three periods of high magmatic flux in the Permian (270-250 Ma), Jurassic (170-140 Ma), and late Cretaceous (115-90 Ma). Zircon from the Permian arc is the least abundant of the three magmatic pulses, although they consistently display elevated Yb/Gd and U/Yb. Jurassic zircons display consistently low U/Yb, variably elevated Yb/Gd, abruptly higher Th/U and LREE from 155-145, and abruptly lower REE concentrations from 145-140 Ma. Zircon from the Cretaceous arc displays gradually increasing U/Yb, Th/U and LREE, with abruptly decreasing Yb/Gd at 95 Ma. The geochemical trends observed in the Nacimiento block detrital zircons of Cretaceous age are strikingly similar to temporal changes in geochemistry known from Cretaceous arc rocks of the Mojave and Peninsular Ranges, and strongly suggest a southern California provenance for Nacimiento block sediments. Furthermore, the similarity of geochemical trends between Cretaceous detrital

  12. Geochemistry and zircon geochronology of Late Proterozoic leucogranites north of Boston, eastern Massachusetts

    SciTech Connect

    Markus, R.; Hon, R. . Geology and Geophysics); Dunning, G. . Dept. of Earth Sciences)

    1993-03-01

    An igneous sequence that includes Late Precambrian volcanics (Lynn Volcanics) and granites, granodiorites, tonalites, plus diorites of the Dedham North suite, is located in a tectonic block bounded by the Walden Pond and Northern Boundary Faults north of the Boston Basin. Within the block between the rhyolites and granodiorites is a several hundred foot wide zone of leucogranites that contains frequent roof pendants in various stages of partial melting. The migmatitic nature of the pendants suggests that the leucogranites were locally derived by melt extraction from the partially melted pendant xenoliths. U-Pb zircon ages were obtained from samples of the leucogranite, granodiorite and diorite. Their crystallization ages are: leucogranite 609 [+-] 4 Ma, granodiorite 607 [+-] 4 Ma, and diorite 606 [+-] 3 Ma. All three samples yield ages that are identical within their statistical error limits and all three samples contain inherited component with average mid-Proterozoic ages. Major and trace element geochemistry of 43 samples of all representative types show that the predominantly pelitic'' layers underwent extensive partial melting and that the leucogranites represent a minimum granite melt at 0.5 to 2.0 kb of P(H2O). Geochemical modeling also supports the origin by partial melting of the pendant inclusions. Once formed, the leucogranite melts were then mixed with mafic magmas which must have also been the provider of the necessary heat to sustain the partial melting process. The data indicate that the Dedham North plutonic suite was formed at shallow crustal levels and that its compositional range is a result of magma mixing of varying proportions between the leucogranite and mafic melts.

  13. Geochemistry and Geochronology of Oceanic Rock Assemblages in the Izmir-Ankara-Erzincan Suture Zone, Turkey

    NASA Astrophysics Data System (ADS)

    Sarifakioglu, Ender; Dilek, Yildirim; Sevin, Mustafa

    2013-04-01

    ophiolitic thrust sheets within the entire IAESZ are hence nearly coeval and are contemporaneous with the ophiolites in the Vardar Suture Zone in the Balkan Peninsula. The 40Ar-39Ar dates from the volcanic rocks of the seamount fragments give 99.6±1.8 to 72.8±3 Ma ages and of the oceanic plateau fragments reveal an age of 96.6±1.8Ma. These geochemical and geochronological data, combined with the structural field observations, indicate that the Tethyan oceanic lithosphere, the fragments of which are now exposed within the IAESZ, evolved in the Triassic-Jurassic (likely even earlier in the late Paleozoic) through the late Cretaceous between Sakarya and Anatolide-Tauride continental blocks, and that it is structurally and geochemically much more heterogeneous than previously thought.

  14. Geochemistry and geochronology of Tobago Island: a preliminary re-appraisal

    NASA Astrophysics Data System (ADS)

    Neill, I.; Kerr, A. C.; Snoke, A. W.; Hastie, A.; Pindell, J. L.; Chamberlain, K.; Millar, I. L.

    2009-12-01

    A unifying model to constrain the sources, polarity and geometry of the Great Arc of the Caribbean during the Jurassic-Cretaceous remains elusive. The arc was the leading edge of the Caribbean plate prior to, and during, its late Cretaceous tectonic emplacement between the Americas1,2. Tobago Island in the SE Caribbean has long been considered a partial cross-section through at least two generations of mid-Cretaceous Great Arc magmatism3. ICP-OES/MS and Nd-Hf-Pb radiogenic isotope whole rock determinations are combined with three new U-Pb zircon LA-ICP-MS and several existing mineral ages to present a geochemical and geochronological re-appraisal of the origin of the igneous suites on Tobago. The North Coast Schist (>115 Ma3) is a suite of tholeiitic mafic-felsic tuffs and volcanic breccias with variable Nb-Ta, Ti and slight Th depletions indicative of an arc or back-arc origin. This suite was deformed and metamorphosed to greenschist facies prior to the formation of the Tobago pluton and volcanic suite. The pluton (104±1 Ma3) comprises peridotite cumulates, gabbro-diorites and hornblende pegmatites. It is cogenetic with the volcanic suite, which consists of mafic volcanic breccias, tuffs and lavas (~104 Ma3), and a suite of mafic dykes (~105-91 Ma3). The volcano-plutonic suite has a tholeiitic island arc composition and appears geochemically similar to the North Coast Schist. A 6 km-long tonalite body cross-cuts the pluton and has a trondhjemitic composition, with high Si, Al, La/Yb and Sr/Y, low MgO, Y and Nb. It may be derived from partial melting of an enriched, garnet-bearing basaltic or amphibolitic source region. Furthermore, several mafic-to-granitic dykes have non-arc highly enriched signatures consistent with a plume source and may be related to the ~90 Ma Caribbean Oceanic Plateau2. The data indicate a more complex magmatic history for the igneous rocks of Tobago than suggested by previous studies3 and thus they require a more detailed tectonomagmatic

  15. El Ventorrillo, a paleostructure of Popocatépetl volcano: insights from geochronology and geochemistry

    NASA Astrophysics Data System (ADS)

    Sosa-Ceballos, G.; Macías, J. L.; García-Tenorio, F.; Layer, P.; Schaaf, P.; Solís-Pichardo, G.; Arce, J. L.

    2015-10-01

    Volcán Popocatépetl (México) was constructed over the remains of a volcanic paleostructure. Based on fieldwork, 40Ar/39Ar dating, U-Pb dating, and geochemistry, we have determined the age, chemistry, and location of this paleostructure and named it El Ventorrillo. Most remnants of El Ventorrillo are covered by deposits from subsequent activity of Popocatépetl, except for the El Abanico scarp and the Barranca de Nexpayantla, where the stratigraphy of El Ventorrillo eruptive products can be investigated. Inception of volcanism at El Ventorrillo occurred at 331 ± 10 ka with emission of the Nexpayantla andesitic lavas, and continued with extrusion of the Yoloxochitl (267 ± 31 ka) and microwave (227 ± 6 ka) domes. Intrusion of dikes occurred at 298 ± 94 and 230 ± 3 ka. Activity at El Ventorrillo continued with the emission of lavas that built the El Abanico scarp (193 ± 29 to 96 ± 8 ka) and continued until the Tutti Frutti eruption destroyed the cone 14.1 kyr ago. El Ventorrillo magmas produced rocks divided into two mineralogical groups. The first group contains biotite-amphibole-rich rocks and the second group consists of biotite-amphibole-free lavas. The rocks that contain biotite and amphibole are older than 198 ± 13 ka, whereas the rocks with no hydrous phases are younger than 227 ± 6 ka and contain skarn and granodiorite xenoliths. We interpret the change to an anhydrous mineral assemblage and the occurrence of skarn and granodiorite xenoliths as evidence for the formation of a new, shallower reservoir. A granodiorite xenolith was chosen for 40Ar/39Ar dating and U-Pb zircon analyses. The U-Pb method yielded an age of 540 ± 110 ka and the 40Ar/39Ar an age of 109 ± 24 ka. These ages are interpreted to indicate granodiorite crystallization (540 ± 110 ka), which metamorphosed the calcareous basement beneath Popocatépetl into skarn and an influx of magma (109 ± 24 ka) that reheated the granodiorite. Major and trace elements, Sr, Nd, and Pb isotopic

  16. Geochronology, geochemistry, and petrology of the Precambrian Sandia granite, Albuquerque, New Mexico

    SciTech Connect

    Majumdar, A.

    1985-01-01

    The Precambrian Sandia granite of north-central New Mexico belongs to the 1.2-1.6 Ga crustal province of the western USA. The granite shows an intrusive contact with the metamorphic country rocks. The Rb-Sr whole rock isochron age of the country rocks is 1.61 +/- 0.06 Ga, (/sup 87/Sr//sup 86/Sr)/sub 0/ = 0.705 +/- 0.001. The culmination of the intrusion of the Sandia granite took place at 1.44 +/- 0.04 GA. (/sup 87/Sr//sup 86/)/sub 0/ = 0.7054 +/- 0.0005. Rb-Sr ages on biotite-whole rock pairs and an /sup 40/Ar//sup 39/Ar dating of a biotite from the granite indicate final cooling to 300-350/sup 0/C at 1.33 Ga. This suggests slow cooling of the granite at rates which averaged 4/sup 0/C/Ma for about 100 Ma after its emplacement; during this period the Rb-Sr isotope system perhaps remained partially open. The Sandia granite shows compositional variation from granodiorite to quartz monzonite in both the northern and southern blocks. The field geology, petrology, and geochemistry of the two blocks suggest that they form a single pluton. Both the Sandia granite and mesocratic, two pyroxene granulite xenoliths therein give an ..delta../sup 18/O value of +8.0 +/- 0.5% indicating (meta)igneous source ricks for each of them. These values tend to rule out Condie's (1978) favored hypothesis for magma generation of 30-50% partial melting of siliceous gradulites. Rather, they favor an alternative hypothesis, equally satisfactory from the geochemical viewpoint, involving 5-10% melt of a gabbroic or two-pyroxene granulite parent rock in the lower crust. The Sandia granite, and by implication, the other high-Ca granitic rocks of the western USA thus do not seem to represent addition of new mantle-derived materials to the middle-late Proterozoic crust of this section of the continent.

  17. Geochemistry.

    ERIC Educational Resources Information Center

    Fyfe, William S.

    1979-01-01

    Techniques in geochemistry continue to improve in sensitivity and scope. The exciting areas of geochemistry still include the classical fields of the origin of the elements and objects in space, but environmental crisis problems are important as well. (Author/BB)

  18. Elucidating the construction of the Austurhorn Intrusion, SE Iceland, using zircon elemental and isotopic geochemistry and geochronology

    NASA Astrophysics Data System (ADS)

    Padilla, A. J.; Miller, C. F.; Carley, T. L.; Economos, R. C.; Schmitt, A. K.; Fisher, C. M.; Hanchar, J. M.; Bindeman, I. N.; Wooden, J. L.; Sigmarsson, O.

    2013-12-01

    The Austurhorn Intrusive Complex (AIC) in SE Iceland comprises large bodies of granophyre, gabbro, and a mafic-felsic composite zone (MFCZ) that exemplifies mafic-felsic interactions common in Icelandic silicic systems. However, despite being one of Iceland's best-studied intrusions (Blake 1966; Furman et al 1992a,b; Thorarinsson & Tegner 2009), few studies have included detailed analyses of zircon, a mineral widely recognized as a valuable tracer of the history and evolution of its parental magma(s). In this study, we employ in-situ zircon elemental and isotopic (hafnium and oxygen) geochemistry, as well as U-Pb geochronology, as tools for elucidating the complex construction and magmatic evolution of Austurhorn's MFCZ. The elemental compositions of AIC zircons form a broad but coherent array partly overlapping with the zircon geochemical signature for Icelandic silicic volcanic rocks (Carley et al 2011). With some exceptions (see below), Hf concentrations are low (less than 10,000 ppm), typical of Icelandic zircon, and Ti concentrations range from 6 to 25 ppm (Ti-in-zircon temps. 730-870°C). Their δ18O values are generally well-constrained at +2.5 to +4 ‰, consistent with other Icelandic magmatic zircon (Bindeman et al 2012) and preserving evidence for partial melting of hydrothermally-altered crust as the source of silicic magmas within the Austurhorn system. Epsilon-Hf values cluster tightly at +13×1 ɛ-units, suggesting a single source for the different units of the MFCZ. The notable exceptions to the trends described above are zircons from a high-silica granophyre displaying CL-dark zones and convoluted zoning. These fall well outside the AIC geochemical arrays, primarily distinguished by high Hf (up to 24,000 ppm) and lower Ti (down to 2 ppm), far higher Hf and lower Ti than any other analyzed Icelandic zircon, and extremely low δ18O values (down to -6 ‰). We interpret these to reflect multiple episodes of partial melting and melt extraction of the

  19. Geochemistry

    ERIC Educational Resources Information Center

    Ailin-Pyzik, Iris B.; Sommer, Sheldon E.

    1977-01-01

    Enumerates some of the research findings in geochemistry during the last year, including X-ray analysis of the Mars surface, trace analysis of fresh and esterarine waters, and analysis of marine sedements. (MLH)

  20. Geochemistry

    SciTech Connect

    Foster, N.H.; Beaumont, E.A.

    1988-01-01

    Papers in this text are part of a reprint series designed to be useful to those involved in petroleum exploration and development. The authors divide the topic of geochemistry into four sections: (1) petroleum formation,(2) source rock evaluation, (3) migration, and (4) surface geochemistry. Petroleum formation contains papers that discuss accumulation and preservation of organic matter, conversion to kerogen, conversion to petroleum, and effects that different types of kerogen have on the types of petroleum generated. The section on source rocks contain papers that review methods for estimating total organic content of source rocks, potential amount of petroleum they can expel, and quantity of petroleum expelled from these source rocks.Migration deals with mechanisms for expulsion and migration of petroleum from source rocks and in carrier beds. The section on surface geochemistry discusses the problems associated with using geochemical methods in exploration.

  1. Black monazite from Taiwan

    USGS Publications Warehouse

    Matzko, J.J.; Overstreet, W.C.

    1977-01-01

    Two forms of detrital monazite are known in offshore bars in southwestern Taiwan: a yellow-green to colorless form and an unusual but abundant pelletlike form, generally black but also colored gray or brown. These black pellets, which are about 160 by 200 microns in size, are composed of fine-grained monazite crystals from 2 to 20 microns in size. The pellets are associated with highly variable amounts of discrete grains of detrital quartz, rutile, amphibole, tourmaline, and other minerals. Intergrown with the monazite are quartz, a cerium oxide mineral, chlorite, sulfides, and other minerals. Opaqueness of the pelletlike monazite is due principally to the cryptocrystalline nature of part of the monazite; only a small part of the opaqueness can be attributed to opaque inclusions. The black pelletlike monazite lacks thorium and has a high content of europium. In this respect, as in color, shape, size, and inclusions, the pelletlike monazite differs from the yellow-green detrital monazite. Despite the fact that they occur together in the littoral placers, they appear to have had different origins. The yellow-green monazite originated as an accessory mineral in plutonic rocks and has accumulated at the coast through erosion and transport. The origin of the pelletlike monazite is as yet unknown, but it is here inferred that it originated in unconsolidated coastal plain sediments through migration of cerium from the detrital monazite during weathering, and of the intermediate weight mobile rare earths from clay minerals during diagenesis. Possibly these pelletlike grains are detrital particles formed through erosion and transport from originally larger aggregates cemented by diagenetic monazite.

  2. Origin and time-space distribution of hydrothermal systems in east-central Australian sedimentary basins: Constraints from illite geochronology and isotope geochemistry.

    NASA Astrophysics Data System (ADS)

    Uysal, I. Tonguç

    2016-04-01

    Some well-known precious mineral deposits and hydrocarbon resources occur extensively in east-central Australian sedimentary Basins. The metal occurrences are abundant in northwestern and eastern part of Queensland, whereas no significant deposits are known in large areas further south, which may, however, be hidden beneath the Jurassic-Cretaceous sedimentary basins. Important hydrocarbon resources exist within the Jurassic-Cretaceous sedimentary rocks at relatively shallow depths, of which the distribution represent zones of high paleo-geothermal gradients. This study examines the time-space distribution in relation to the regional tectonic history of concealed metal deposits and areas of high paleo-geothermal gradient leading to hydrocarbon maturation. To this end, authigenic illitic clay minerals representing various locations and stratigraphic depths in east-central Australia were investigated, of which the Rb-Sr and Ar-Ar geochronology and stable isotope geochemistry assist in delineating zones of hydrothermal systems responsible for hydro-carbon maturation/migration and potentially ore deposition. The Late Carboniferous - Early Permian crustal extension that affected large areas of eastern Australia and led to the epithermal mineralisations (e.g., the Drummond Basin) is also recorded in northern South Australia and southwest Queensland. A Late Triassic - Early Jurassic tectonic event being responsible for coal maturation and gas generation in the Bowen Basin and the epithermal mineralisation in the North Arm goldfield in SE Queensland likewise affected the areas much further west in Queensland. Some illites from the basement in outback Queensland and fault gouges from the Demon Fault in NE New South Wales yield younger Rb-Sr and Ar-Ar ages indicating the effect of hydrothermal processes as a result of a Middle-Upper Jurassic tectonic event. The majority of illite samples from the crystalline basement rocks, Permian Cooper Basin, and Jurassic

  3. Decoding polyphase migmatites using monazite petrochronology

    NASA Astrophysics Data System (ADS)

    Yakymchuk, C.; Brown, M.; Korhonen, F. J.; Piccoli, P. M.; Siddoway, C. S.

    2014-12-01

    Unraveling the P-T-t evolution of deep crustal rocks requires the use of multiple high-temperature geochronometers integrated with careful petrography and quantitative phase equilibria modeling. As an example, in situ U-Pb monazite ages and Lu-Hf garnet geochronology are used to distinguish mineral parageneses associated with overprinting suprasolidus metamorphic events in migmatitic paragneisses and orthogneisses from the Fosdick migmatite-granite complex in West Antarctica. Then phase equilibria modeling is used to quantify the P-T conditions for each event. In the Fosdick complex, U-Pb monazite ages define two populations at 365-300 Ma (minor population; cores of polychronic grains) and 120-96 Ma (dominant population; monochronic grains and rims of polychronic grains). For seven samples, Lu-Hf ages of garnet range from 116 to 111 Ma, which are interpreted to record the approximate timing of peak metamorphism during the overprinting Cretaceous metamorphic event. Phase equilibria modeling constrains peak P-T conditions to 720-800°C at 0.45-1.0 GPa for the older (Devonian-Carboniferous) metamorphic event and 850-880°C at 0.65-0.80 GPa for the overprinting Cretaceous event. This younger metamorphic event is dominant throughout the Fosdick complex; it has extensively reworked evidence of the older metamorphic event as indicated by resorbed Devonian-Carboniferous cores of polychronic monazite grains that are always surrounded by Cretaceous overgrowths. Within the Cretaceous monazite population, the paucity of ages predating peak metamorphism suggests that prograde monazite growth was limited or prograde monazite was obliterated. Y-enriched overgrowths on monazite spatially associated with cordierite and biotite yield ages of 106-97 Ma, which are interpreted to record growth during breakdown of garnet in the presence of melt in the course of exhumation and cooling of the complex. Most monazite in the Cretaceous population yields ages that range from 106 to 96 Ma with

  4. Thorium Diffusion in Monazite

    NASA Astrophysics Data System (ADS)

    Cherniak, D. J.

    2006-05-01

    Diffusion of thorium has been characterized in synthetic monazite under dry conditions. The synthetic monazites (either pure CePO4, NdPO4, or a mixed LREE phosphate containing Ce, Nd, and Sm) were grown via a Na2CO3-MoO3 flux method. The source of diffusant for the experiments were either synthesized ThSiO4 or CaTh(PO4)2 powders. Experiments were performed by placing source and monazite in Pt capsules and annealing capsules in 1 atm furnaces for times ranging from 10 days to a few hours, at temperatures from 1400 to 1550C. The Th distributions in the monazite were profiled by Rutherford Backscattering Spectrometry (RBS). The following Arrhenius relation was obtained for diffusion in monazite: DSm = 7.2x103 exp(-814 kJ mol-1/RT) m2sec-1 The diffusivity of Th was similar for monazites containing a single REE and the mixed LREE phosphates. Th diffusion was also similar for experiments run using the Th silicate and Ca-Th phosphate sources, suggesting that the substitutional mechanism for Th in monazite, i.e, Th+4 + Si+4 for REE+3 + P+5 with the ThSiO4 source, and Th+4 + Ca+2 for 2REE+3 with the CaTh(PO4)2 source, does not significantly affect Th diffusivities, and that Th is likely the rate-limiting species. Th diffusion in monazite is about 4 orders of magnitude slower than Pb diffusion (Cherniak et al., 2004). This contrasts with findings of Gardes et al. (2005) who determined that Pb, Th and REE diffusivities in monazite are similar. Th diffusion in zircon (Cherniak et al., 1997) is about an order of magnitude slower than in monazite, but with similar activation energy for diffusion. The smaller diffusivities in zircon may be a consequence of the larger disparity in size between Th and the Zr site in zircon as compared with Th and the REE site in monazite. Nonetheless, Th is essentially immobile in monazite with respect to exchange by volume diffusion under most geologic conditions; these findings may have implications for containment of high- level actinide

  5. Constraints on the collision and the pre-collision tectonic configuration between India and Asia from detrital geochronology, thermochronology, and geochemistry studies in the lower Indus basin, Pakistan

    NASA Astrophysics Data System (ADS)

    Zhuang, Guangsheng; Najman, Yani; Guillot, Stephane; Roddaz, Martin; Antoine, Pierre-Olivier; Metais, Gregoire; Carter, Andrew; Marivaux, Laurent; Solangi, Sarfraz

    2016-04-01

    The timing of India-Asia suturing in the Western Himalaya is complex, with the relative timings of collision between the Indian plate and Asian plate with the Kohistan Island arc and a proposed Tethyan Himalayan microcontinent, debated. Here we present an integrated provenance study of geochronology, thermochronology, and geochemistry on the late Cretaceous-Pleistocene sediments from the lower Indus basin on the Indian plate. The detrital zircon U-Pb and fission track data show a reversal in sediment source from a pure Indian signature to increasing inputs from the suture zone and the Asian plate between the middle Paleocene and early Oligocene. The Nd and Sr isotopes narrow down this change to 50 Ma by revealing input of Asian detritus and the establishment of a Nd & Sr isotopic pattern similar to that of the present-day Indus Fan by 50 Ma, with no significant variations up section, contrary to what might be expected if later major collisions had occurred. Our isotopic data indicate that since 50 Ma, Greater India was occupied by a fluvial-deltaic system, analogous to the present-day Indus and named as the Paleo-Indus, which has been transporting Asian detritus southward across the suture zone and Kohistan-Ladakh arc. This suggests that no other ocean basins were located between India and Asia after this time in this region. Our data require that in the west, the India-Asia collision was accomplished by ˜50 Ma.

  6. Paleomagnetism, Geochronology, and Geochemistry of the Type Section of the Stanislaus Group: Reference Parameters from the Stable Sierra Nevada Microplate, CA

    NASA Astrophysics Data System (ADS)

    Farner, M. J.; Pluhar, C. J.; Asami, R.; Putirka, K. D.; Busby, C.; Renne, P. R.

    2012-12-01

    The Late Miocene Stanislaus Group, of California and Nevada is composed of Table Mountain Formation, Eureka Valley Tuff, and Dardanelles Formation. This ~9.0-~10.2 million year old unit interrupted Miocene andesitic arc volcanism in the Sierra Nevada, providing a regional lithostratigraphic marker that has been used extensively to reconstruct tilt and uplift of the range, Neogene tectonics of the Walker Lane Belt, magmagenetic processes beneath the Sierra Nevada, and lithospheric evolution of the Sierra Nevada and Eastern California. A recent study (Koerner et al, 2009) produced a measured section and geologic map of the Stanislaus Group type section, but until now this locality has never seen comprehensive multidisciplinary study of the geochronology, geochemistry, and magnetostratigraphy of the site and to integrate this into the overall understanding of the Stanislaus Group. Stratigraphy, geochemistry, and paleomagnetism from the type section suggest addition of a basal trachyte lava flow member to the Eureka Valley tuff and adds an additional intermediate-polarity lava flow to Table Mountain Formation magnetostratigraphy. This study dates the youngest member of the Stanislaus Group, the Dardanelles Formation, by 40Ar/39Ar radioisotopic dating for the first time, yielding an age of 9.048 ± 0.017 Ma. Paleomagnetic results verify the previous paleomagnetic reference direction from the Sierra Nevada microplate for the Tollhouse Flat Member of the Eureka Valley Tuff (King et al., 2007). However, our work revises the By-Day Member reference direction to D = 349.6°, I = 51.9° n = 8, α95 = 3.0°, k = 346. This difference is because the prior work analyzed By-Day localities within the tectonically-active Walker Lane Belt. The revised reference direction is critical for measurements of relative vertical-axis rotation studies in the Walker Lane. Our study also demonstrates that little to no vertical-axis rotation of the Sierra Nevada microplate has occurred since

  7. CO2 outburst events in relation to seismicity: Constraints from microscale geochronology, geochemistry of late Quaternary vein carbonates, SW Turkey

    NASA Astrophysics Data System (ADS)

    Ünal-İmer, Ezgi; Uysal, I. Tonguç; Zhao, Jian-Xin; Işık, Veysel; Shulmeister, James; İmer, Ali; Feng, Yue-Xing

    2016-08-01

    Vein and breccia carbonates precipitated in a highly fractured/faulted carbonate bedrock in SW Turkey were investigated through high-resolution U-series geochronology, microstructural and geochemical studies including C-O-Sr isotope and rare-earth element and yttrium (REY) analyses. Petrographical observations and geochronological data are interpreted as evidence that the calcite veins formed through a crack-seal mechanism, mostly accompanied/initiated by intensive hydraulic fracturing of the host limestone in response to high-pressure fluids, which is manifested by multi-stage breccia deposits. Microscale U-series dates (272.6-20.5 kyr) and geochemical compositions of the vein/breccia samples provide information on the timing and mechanism of the vein formation and identify the source of CO2-bearing fluids responsible for the carbonate precipitation. δ18OVPDB and δ13CVPDB values of the calcite veins range between -5.9 and -1.7‰, and -10.6 and -4.6‰, respectively. The isotopic compositions of the veins show highly fluctuating values as calcite grew successively perpendicular to vein walls, which, in combination with microstructural and geochronological constraints, are interpreted to reflect episodic CO2 degassing events associated with seismic and aseismic deformation. Oxygen and Sr isotope compositions (δ18OVPDB: -5.9 to -1.7‰; 87Sr/86Sr: 0.7082 to 0.7085) together with REY concentrations indicate deep infiltration of meteoric waters with various degrees of interactions mostly with the host limestone and siliciclastic parts of the basement rocks. Oxygen and carbon isotope compositions suggest CO2 degassing through intensive limestone dissolution. While majority of the veins display similar Post-Archaean Australian Shale (PAAS)-normalised REY variations, some of the veins show positive EuPAAS anomalies, which could be indicative of contributions from a deeply derived, heated, and reduced fluid component, giving rise to multiple fluid sources for the

  8. Th-U-total Pb geochronology of authigenic monazite in the Adelaide rift complex, South Australia, and implications for the age of the type Sturtian and Marinoan glacial deposits

    NASA Astrophysics Data System (ADS)

    Mahan, K. H.; Wernicke, B. P.; Jercinovic, M. J.

    2010-01-01

    The Adelaide rift complex in South Australia contains the type sections for Sturtian and Marinoan glacial deposits. The litho- and chemo-stratigraphy of these deposits play a central role in evaluating global Neoproterozoic ice age hypotheses and Rodinia supercontinent reconstructions, but reliable depositional age constraints have been extremely limited. We report results of in situ Th-U-total Pb (electron microprobe) dating of detrital and authigenic monazite in two samples from the Umberatana Group (Sturtian Holowilena Ironstone and pre-Marinoan Enorama Shale) in the Central Flinders Ranges. Several texturally and chemically distinct detrital and authigenic populations are recognized. Detrital dates range from 1600 Ma to 760 Ma and most relate to well-known orogenic or igneous events in surrounding cratonic regions. Authigenic monazite grew in three or more pulses ranging from 680 Ma to 500 Ma. The date of 680 ± 23 Ma (2 σ) for the earliest generation of authigenic monazite in sandstone from the Enorama Shale (1) provides an estimate for the age of the base of the Trezona carbon isotopic anomaly just beneath the Marinoan glacial deposits, (2) provides an absolute minimum age constraint on the underlying Sturtian glacial deposits, and (3) supports proposed correlations between type Marinoan deposits and precisely dated glacial deposits in Namibia and China, which bracket the presumed Marinoan equivalents between 655 and 635 Ma. This age is inconsistent with a Re-Os isochron age of 643 ± 2.4 Ma (2 σ) on shales near the bottom of the Sturtian-Marinoan interglacial succession, stratigraphically > 3000 m below the Enorama Shale sample, and militate against the hypothesis that the type Marinoan is correlative with the 580 Ma Gaskiers glaciation. Monazite growth near 600 Ma and again at about 500 Ma probably represent hydrothermal fluid-flow events, the latter of which also corresponds to the well-known Delamerian Orogeny during which the Adelaide sediments were

  9. U-Pb-Th geochronology of monazite and zircon in albitite metasomatites of the Rožňava-Nadabula ore field (Western Carpathians, Slovakia): implications for the origin of hydrothermal polymetallic siderite veins

    NASA Astrophysics Data System (ADS)

    Hurai, V.; Paquette, J.-L.; Lexa, O.; Konečný, P.; Dianiška, I.

    2015-10-01

    Sodic metasomatites (albitites) occur around and within siderite veins in the southern part of the Gemeric tectonic unit of the Western Carpathians. Accessory minerals of the metasomatites represented by monazite, zircon, apatite, rutile, tourmaline and siderite are basically identical with the quartz-tourmaline stage of other siderite and stibnite veins of the tectonic unit. Statistical analysis of chemical Th-U(total)-Pb isochron method (CHIME) of monazite dating yielded Jurassic-Cretaceous ages subdivided into 3-4 modes, spreading over time interval between 78 and 185 Ma. In contrast, LA-ICPMS 206Pb/238U dating carried out on the same monazite grains revealed a narrow crystallization interval, showing ages of Th-poor cores with phengite inclusions identical within the error limit with Th-rich rims with cauliflower-like structure. The determined lower intercept at 139 ± 1 Ma overlapped the Vallanginian-Berriasian boundary, thus corroborating the model of formation of hydrothermal vein structures within an arcuate deformation front built up in the Variscan basement as a response to Early Cretaceous compression, folding and thrusting. In contrast, associated zircons are considerably older than the surrounding Early-Palaeozoic volcano-sedimentary rocks, showing Neoproterozoic ages. The zircon grains in albitite metasomatites are thus interpreted as fragments of Pan-African magmatic detritus incorporated in the vein structures by buoyant hydrothermal fluids.

  10. Geochemistry and geochronology of granitoids in the Kibi-Asamankese area of the Kibi-Winneba volcanic belt, southern Ghana

    NASA Astrophysics Data System (ADS)

    Anum, Solomon; Sakyi, Patrick Asamoah; Su, Ben-Xun; Nude, Prosper M.; Nyame, Frank; Asiedu, Daniel; Kwayisi, Daniel

    2015-02-01

    In Ghana the West African Craton is represented by Birimian and Tarkwaian rocks with extensive granitoid bodies. Granitoids from Asamankese area of the Kibi-Winneba volcanic belt, southern Ghana were analysed for major and trace element contents and found to be characterised by highly-fractionated REE, enrichments, in LILE, and depletion in Nb, Ta and Sr. The LILE enrichment relative to strong Nb-Ta depression, indicates that these granitoids were emplaced in an active margin. Based on field relations, geochemical composition and geochronological data, the granitoids from the Kibi-Asamankese area can be divided into three types, namely; the Eburnean biotite granodiorite (2133-2127 Ma) and hornblende granodiorite (2147 Ma), and the Pre-Eburnean gneissic biotite granite (2193 Ma). The geochemical data of the studied rocks plot in the tholeiitic field, whereas on the A/CNK-A/NK diagram, they generally fall within the metaluminous field, with A/CNK values between 0.69 and 0.88. U-Pb dating of zircons in the granitoids yielded ages ranging from 2193 to 2127 Ma, which are among the oldest ages obtained from the granitoid plutons in Ghana. Such high-precision geochronological data indicate that magmatism occurred over a time-span of about 70 Ma. This provides further evidence that the period 2.1-2.2 Ga was one of the important stages of Birimian magmatism that led to the generation of the granitoids. From the above-mentioned ages, it is possible to link the geological activities to crustal processes and establish the cyclic geotectonic evolution in the West African Craton over time as part of an arc-back-arc basin system.

  11. Geochronology and geochemistry of zircon from the northern Western Gneiss Region: Insights into the Caledonian tectonic history of western Norway

    NASA Astrophysics Data System (ADS)

    Gordon, Stacia M.; Whitney, Donna L.; Teyssier, Christian; Fossen, Haakon; Kylander-Clark, Andrew

    2016-03-01

    The Western Gneiss Region (WGR) of Norway is divided by the Møre-Trøndelag shear zone (MTSZ) into a southern region that contains domains of Caledonian ultrahigh-pressure (UHP) metamorphic rocks (> 2.5 GPa) and a northern area of similar Caledonian-aged rocks that record a maximum pressure reported thus far of ~1.5 GPa. Although both regions contain similar lithologies (primarily migmatitic quartzofeldspathic gneiss containing mafic lenses) and structural relationship of basement rocks to infolded nappes, this difference in maximum pressure implies a difference in tectonic history (continental subduction south of the shear zone, none to the north) and raises questions about the role of the MTSZ in the metamorphic history (including exhumation) of the WGR. Previous geochronology results indicated a difference in timing of peak metamorphism (older in north, younger in south). In order to better understand the tectonic history of the northern WGR and the MTSZ, and in particular the late- to post-Caledonian tectonic history, U-Pb zircon geochronology and trace-element abundances were obtained using the split-stream, laser-ablation ICPMS technique from metabasaltic lenses and migmatitic quartzofeldspathic host rocks from the structurally lowest exposed region of the northern WGR (Roan Peninsula basement), as well as leucosomes from an intercalated portion of the Seve Nappe Complex and a pegmatite in the MTSZ. Zircon from Roan gneiss and metabasite yield metamorphic ages of ca. 410-406 Ma, and zircon from a variety of migmatite samples (foliation-parallel leucosome to dikes) indicate melt crystallization at ca. 410 to 405 Ma. The Seve Nappe leucosomes yield only early Caledonian dates that cluster at ca. 437 Ma and ca. 465 Ma, suggesting that the allochthons in this region did not experience (or record) the same Scandian tectonic history as the basement rocks. Zircon from a weakly deformed pegmatite dike within the MTSZ crystallized at ca. 404 Ma, indicating that this

  12. Geochemistry and Re-Os geochronology of the organic-rich sedimentary rocks in the Jingtieshan Fe-Cu deposit, North Qilian Mountains, NW China

    NASA Astrophysics Data System (ADS)

    Yang, Xiuqing; Zhang, Zuoheng; Li, Chao; Duan, Shigang; Jiang, Zongsheng

    2016-04-01

    The Jingtieshan Group in the North Qilian Mountains, NW China, is dominantly composed of banded iron formations (BIFs), copper deposits and organic-rich sedimentary rocks (ORS, carbonaceous phyllite). X-ray diffraction analysis of the ORS shows the mineral assemblage to be quartz + clay minerals. The total organic carbon contents show a range of 0.44-1.72%. Here we present the results of the geochemistry and Re-Os geochronology of the ORS from the Jingtieshan Group. The high values of Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Plagioclase Index of Alteration (PIA), and Th/U ratio, indicate intense weathering. The Al2O3/TiO2, Zr/Sc, Th/Sc, La/Th ratios, high rare earth elements abundances, light rare earth elements enrichment (normalized to chondrite), and distinctly negative Eu anomalies, suggest that the Jingtieshan Group ORS were derived mainly from felsic volcanic units. The new Re-Os isochron age of 1308 ± 100 Ma (2σ, n = 6, MSWD = 23) broadly overlap with the previous published ages determined using Sm-Nd and U-Pb isotope systems. The new age represents the depositional age of the Jingtieshan Group, as well that of BIF in the Jingtieshan area. Furthermore, the initial 187Os/188Os ratios (0.44 ± 0.07) indicate that the Os in the seawater was dominantly derived from hydrothermal fluids (∼75%). The Ce anomaly (Ce/Ce∗ = 0.95-1.00) and V/(V + Ni) ratios (0.71-0.86), as well as the lack of enrichment in redox-sensitive trace elements such as U, V, Zn, Pb, Cu, Ni, Cr, Co and Mn, together with the presence of overlying BIF, suggest that the Jingtieshan area represents a ferruginous deep-water succession. This, and intense submarine hydrothermal activities contributed to the deposition of the Jingtieshan BIF.

  13. Geology, geochronology, and geochemistry of basaltic flows of the Cat Hills, Cat Mesa, Wind Mesa, Cerro Verde, and Mesita Negra, central New Mexico

    USGS Publications Warehouse

    Maldonado, F.; Budahn, J.R.; Peters, L.; Unruh, D.M.

    2006-01-01

    The geochronology, geochemistry, and isotopic compositions of basaltic flows erupted from the Cat Hills, Cat Mesa, Wind Mesa, Cerro Verde, and Mesita Negra volcanic centres in central New Mexico indicate that each of these lavas had unique origins and that the predominant mantle involved in their production was an ocean-island basalt type. The basalts from Cat Hills (0.11 Ma) and Cat Mesa (3.0 Ma) are similar in major and trace element composition, but differences in MgO contents and Pb isotopic values are attributed to a small involvement of a lower crustal component in the genesis of the Cat Mesa rocks. The Cerro Verde rock is comparable in age (0.32 Ma) to the Cat Hills lavas, but it is more radiogenic in Sr and Nd, has higher MgO contents, and has a lower La/Yb ratio. This composition is explained by the melting of an enriched mantle source, but the involvement of another crustal component cannot be disregarded. The Wind Mesa rock is characterized by similar age (4.01 Ma) and MgO contents, but it has enriched rare-earth element contents compared with the Cat Mesa samples. These are attributed to a difference in the degree of partial melting of the Cat Mesa source. The Mesita Negra rock (8.11 Ma) has distinctive geochemical and isotopic compositions that suggest a different enriched mantle and that large amounts of a crustal component were involved in generating this magma. These data imply a temporal shift in magma source regions and crustal involvement, and have been previously proposed for Rio Grande rift lavas. ?? 2006 NRC Canada.

  14. Neogene source-to-sink relations between the Pamir and Tarim Basin: insights from stratigraphy, detrital zircon geochronology, and whole-rock geochemistry

    NASA Astrophysics Data System (ADS)

    Cao, K.; Xu, Y.; Wang, G.; Zhang, K.; Van Der Beek, P.; Jiao, Y.; Wang, C.; Jiang, S.; Bershaw, J. T.

    2013-12-01

    The Tarim Basin, as the largest inland basin on planet, contains large amounts of Cenozoic sediments, providing valuable opportunity to understand the mountain building of the Northern Tibetan Plateau and its effects on basin development. Here, we present a synthesis of sedimentology, zircon U-Pb geochronology, and bulk-rock geochemistry of Neogene sediments in the Qimugan section, southwest Tarim Basin. Our geochemical analyses of fine-grained sediments show a decreasing amount of chemical weathering in sediment source regions by the Middle Miocene. Thereafter, a stable pattern of chemical weathering suggests a cooler and drier climate regime, a possible result of tectonic uplift. Our zircon U-Pb analyses show ~20 Ma and ~107 Ma igneous rocks exhumed by the Middle Miocene, likely originating in the Central to Southeast Pamir. It indicates significant headwater erosion of the Tashkurgan River related to tectonic uplift of Pamir interiors, which can also account for a change of depositional environment from delta plain to shallow lake at Qimugan, and synchronous disappearance of Eocene igneous zircons and dominance of Triassic zircons at Oytag basin. The provenance of sedimentary deposits, temporal stability of depocenters, and timing of Muztagata doming and deformation along the Main Pamir Thrust (MPT) and Kashgar-Yecheng transfer system (KYTS) suggest the present tectonic-sedimentary state has been established since the Middle-Late Miocene. Our results suggest that tectonic deformation had advanced to all margins of the Northern Tibetan Plateau by the Middle Miocene. Moreover, an observed increase in Triassic igneous zircons, change of sedimentary facies from relatively distal to proximal, and intensification of chemical weathering, corroborate rapid exhumation of crystalline domes in the eastern Pamir since the Pliocene.

  15. First evidence for Permian-Triassic boundary volcanism in the Northern Gemericum: geochemistry and U-Pb zircon geochronology

    NASA Astrophysics Data System (ADS)

    Vozárová, Anna; Presnyakov, Sergey; Šarinová, Katarína; Šmelko, Miloš

    2015-10-01

    Several magmatic events based on U-Pb zircon geochronology were recognized in the Permian sedimentary succession of the Northern Gemeric Unit (NGU). The Kungurian magmatic event is dominant. The later magmatism stage was documented at the Permian-Triassic boundary. The detrital zircon assemblages from surrounding sediments documented the Sakmarian magmatic age. The post-orogenic extensional/transtensional faulting controlled the magma ascent and its emplacement. The magmatic products are represented by the calc-alkaline volcanic rocks, ranging from basaltic metaandesite to metarhyolite, associated with subordinate metabasalt. The whole group of the studied NGU Permian metavolcanics has values for the Nb/La ratio at (0.44-0.27) and for the Nb/U ratio at (9.55-4.18), which suggests that they represent mainly crustal melts. Magma derivation from continental crust or underplated crust is also indicated by high values of Y/Nb ratios, ranging from 1.63 to 4.01. The new 206U-238Pb zircon ages (concordia age at 269 ± 7 Ma) confirm the dominant Kungurian volcanic event in the NGU Permian sedimentary basin. Simultaneously, the Permian-Triassic boundary volcanism at 251 ± 4 Ma has been found for the first time. The NGU Permian volcanic activity was related to a polyphase extensional tectonic regime. Based on the new and previous U-Pb zircon ages, the bulk of the NGU Permian magmatic activity occurred during the Sakmarian and Kungurian. It was linked to the post-orogenic transpression/transtension tectonic movements that reflected the consolidation of the Variscan orogenic belt. The Permian-Triassic boundary magmatism was accompanied by extension, connected with the beginning of the Alpine Wilson cycle.

  16. Geochemistry and geochronology of the volcano-plutonic rocks associated with the Glojeh epithermal gold mineralization, NW Iran

    NASA Astrophysics Data System (ADS)

    Siani, Majid Ghasemi; Mehrabi, Behzad; Azizi, Hossein; Wilkinson, Camilla Maya; Ganerød, Morgan

    2015-08-01

    Eocene to Oligocene volcano-plutonic rocks are widespread throughout NW Iran. The Tarom-Hashtjin metallogenic province is one of the most promising epithermal-porphyry ore mineralized districts in NW Iran. The Glojeh gold deposit, located in the center of this province, is a typical high to intermediate sulfidation epithermal system, spatially and temporally associated with a granite intrusion and associated high-K calc-alkaline to shoshonitic volcano-plutonic rocks. The intrusive complexes of the Glojeh district are characterized by: SiO2 contents of 60.9 to 70.7 wt.%, K2O+Na2O of 7.60 to 8.92 wt.%, and K2O/Na2O ratios of 0.9 to 1.8. They are enriched in light rare earth elements (LREEs), and large ion lithophile elements (LILEs), depleted in high field strength elements (HFSEs), and have weak negative Eu anomalies (Eu/Eu*= 0.5 to 0.9). 40Ar/39Ar geochronology applied to biotite and feldspar, separated from two intrusives (Goljin and Varmarziar), and two feldspar aliquots separated from hydrothermal veins at North Glojeh and South Glojeh, was carried out to constrain magmatic and hydrothermal events. Plagioclase (± sericite), from North Glojeh and South Glojeh produced ages (42.20±0.34 Ma, and 42.56±1.47 Ma respectively) that overlap with the age of the Goljin intrusion (41.87±1.58 Ma). Geochemical data for the volcano-plutonic rocks in the Glojeh district, that have87Sr/86Sr isotopic compositions that range from 0.706344 to 0.708331, suggest an origin involving partial melting of a depleted mantle source during Neo-Tethyan subduction.

  17. Evolution of Pleistocene to Holocene eruptions in the Lesser Caucasus Mts:Insights from geology, petrology, geochemistry and geochronology

    NASA Astrophysics Data System (ADS)

    Savov, Ivan; Meliksetian, Khachatur; Connor, Charles; Karakhanian, Arkadi; Sugden, Patrick; Navasardyan, Gevorg; Halama, Ralf; Ishizuka, Osamu; Connor, Laura; Karapetian, Sergei

    2016-04-01

    Both effusive and highly explosive (VEI>5) and often voluminous caldera volcanism has developed atop the collision zone between the Arabian and the Eurasian plates. Currently what is exposed on the Anatolian-Armenian-Iranian active orogenic plateau is post-Mesozoic felsic to intermediate collision-related plutons, and mostly collision or post-collision related Quaternary volcanic structures. We have studied in detail the volcanism, tectonics and geophysics on the territory of E.Turkey and Armenia, where several large stratovolcanoes (Ararat, Lesser Ararat, Aragats, Tsghuk, Ishkhanasar) are surrounded by distinct monogenetic volcanic fields (distributed volcanism). These large in volume stratovolcanoes and the associated low volume monogenetic cones range from normal calk-alkaline to high-K shoshonitic in affinity, with their products ranging from basanites to high K trachytes and rhyolites. Several volcanic provinces, namely Kechut/Javakheti, Aragats, Gegham, Vardenis and Syunik are recognized in Armenia and each of them has > 100 mapped volcanoes. These have distinct geochemical (mineral chemistry, trace element and Sr-Nd-B isotope systematics) and petrological (melt eruption temperatures and volatile contents) fingerprints that may or may not vary over time. Age determinations and volcano-stratigraphy sections for each of the case studies we aim to present shows that the volcanism includes a continuous record from Pleistocene to Holocene, or even historical eruptions. The excellent volcano exposures and the now complete high resolution database (GIS), geological mapping, and new and improved K-Ar and Ar-Ar geochronology, uniquely allows us to evaluate the driving forces behind the volcanism in this continent-continent collision setting that is uniquely associated with long lasting eruption episodes. We shall compare the now well studied historical/Holocene eruptions with those pre-dating them, with the aim to identify possible geochemical or petrological

  18. Detrital zircon U-Pb geochronology, Hf isotopes and geochemistry constraints on crustal growth and Mesozoic tectonics of southeastern China

    NASA Astrophysics Data System (ADS)

    Meng, Lifeng; Li, Zheng-Xiang; Chen, Hanlin; Li, Xian-Hua; Zhu, Chen

    2015-06-01

    In situ U-Pb geochronological, Hf isotopic and REE geochemical analyses of detrital zircons from Mesozoic sandstones in central Jiangxi and east Hunan of the South China Block (SCB) are used to provide not only information about crustal evolution process, but more importantly new constraints on sedimentary provenances as well as Mesozoic basin evolutions in central SCB. A total of 417 concordant zircon analyses define five U-Pb age populations at ca. 2.6-2.3 Ga, ca. 2.0-1.7 Ga, ca. 1.0-0.7 Ga, ca. 500-400 Ma and ca. 300-200 Ma. Integrated analyses of zircon U-Pb ages and Hf isotopes of detrital zircons reveal five episodes of juvenile continental crust growth: ca. 2.5 Ga, ca. 1.7 Ga, ca. 850 Ma, ca. 440 Ma and ca. 230 Ma, with all but the ca. 2.5 Ga episode likely represent that of the SCB. None of the three samples from T3-J1 strata showed strong ca. 1850 Ma and ca. 230 Ma peaks suggesting that the T3-J1 sediments probably sourced from the Yangtze Block. In contrast, stronger peaks of ca. 1850 Ma and ca. 250-230 Ma in post-J1 strata relative to that of T3-J1 strata suggest a dominantly local Cathaysian provenance. In addition, the distinct low εHf(T) values for zircons of ca. 430 Ma from T3-J1 strata in comparison with higher εHf(T) values for that from post-J1 strata also support aforementioned viewpoint. Such sharp changes between the pre-J1 and post-J1 strata coincide with the remarkable change in regional palaeogeography from a broad shallow marine basin in the Late Triassic-Early Jurassic time to a basin-and-range-style province in the Middle Jurassic. The characteristics and time evolution of detrital zircons from the studied area are consistent with the flat-slab subduction model which involves the development of a broad sag basin during the T3-J1 time, and a rapid regional uplift in the Cathaysia Block caused by the reinitiating of normal subduction along the coastal region at ca. 190 Ma.

  19. Geochronology, geochemistry and origins of the Paleozoic-Triassic plutons in the Langshan area, western Inner Mongolia, China

    NASA Astrophysics Data System (ADS)

    Wang, Zeng-Zhen; Han, Bao-Fu; Feng, Li-Xia; Liu, Bo

    2015-01-01

    The Langshan area is the northeastern part of the Alxa block and adjacent to the Inner Mongolia-Da Hinggan Orogenic Belt (IMDOB) to the north, and geochronological and geochemical studies of the Langshan plutons would be helpful for unraveling the relationship of the magmatism in the Langshan area with that in the IMDOB. Based on zircon U-Pb ages presented in this study and in published papers, five magmatic stages are recognized in Langshan area: Late Silurian (∼418 Ma), Carboniferous (328-304 Ma), Early Permian (294-272 Ma), Late Permian (260-254 Ma) and Middle-Late Triassic (245-227 Ma). The Late Silurian two-mica granite is peraluminous and has a source from ancient continental crust, indicating a collision event. The late Early Carboniferous mafic plutons are subalkaline, show appinitic affinities, and may originate from the subduction-modified continental lithospheric mantle. The late Early Permian granodiorite is weakly peraluminous and high-K calc-alkaline and may be derived from ancient continental crust, while the late Early Permian alkaline gabbro-diorite displays EM I-like Sr-Nd isotopic signatures, indicating an enriched subcontinental lithospheric mantle source. The Late Permian granodiorite is calc-alkaline and peraluminous and may result from the partial melting of ancient lower continental crust, whereas the Late Permian leuconorite belongs to low-K tholeiitic series and has a cumulate origin from an enriched mantle source with the input of juvenile mantle materials. The Paleozoic-Triassic magmatic stages in the Langshan area are consistent with the main magmatic periods in the northern NCC, which are the Late Silurian to Middle Devonian (∼418-386 Ma) and Early Carboniferous to Late Triassic (∼342-211 Ma, with three magmatic stages of Carboniferous, Permian and Triassic). Especially, the Carboniferous to Late Triassic magmatic period in the northern NCC are closely comparable with that in the IMDOB, indicating the two domains might be

  20. Metallogenic epoch of the Jiapigou gold belt, Jilin Province, China: Constrains from rare earth element, fluid inclusion geochemistry and geochronology

    NASA Astrophysics Data System (ADS)

    Huang, Zhixin; Yuan, Wanming; Wang, Changming; Liu, Xiangwei; Xu, Xiaotong; Yang, Liya

    2012-12-01

    The Jiapigou gold belt is located on the northern margin of the North China Craton, and is one of the most important gold-mining and production regions in the circum-Pacific metallogenic zone. Research has been conducted in this area since the 1960s, however, the timing of the gold mineralisation is still unresolved, and an ideal metallogenic model has not been well established. To address these questions, a systematic geological, geochemical and geochronological investigation was conducted. The study revealed that (i) the gold-bearing quartz veins can be divided into two groups, earlier and later gold-bearing quartz veins according to their occurrence and the geochemical characteristics, (ii) the geochemical characteristics of the ore bodies, while similar to granite, are clearly different from the altered rock, and (iii) the geochemical characteristics of the later gold-bearing quartz veins have more similarity to the altered rock than the earlier gold-bearing quartz veins do. Therefore, we conclude that two independent stages of metallogenesis within the Jiapigou gold deposit area are related to magmatic activity in the Palaeoproterozoic and the Yanshanian stage of the Mesozoic, that the ore-forming fluids are mainly of magmatic origin, and that magma contamination by the altered rock was stronger in the Mesozoic. Zircon LA-ICP-MS U-Pb data show that the age of the Palaeoproterozoic granite is ~2426.0 Ma and that of the Mesozoic granite is ~166.2 Ma; these ages can be interpreted as the maximum ages of the two periods of gold mineralisation. In addition to investigating the geotectonic and regional structure of the Jiapigou gold belt, this study also proposes that the WNW-trending zone of gold mineralisation is a result of a magmatic event within the basement in the early Palaeoproterozoic, and that large-scale sinistral strike-slip displacements of the Huifahe and Liangjiang Faults in the late Middle Triassic (Yanshanian epoch) controlled the later tectono

  1. Geochemistry and geochronology of Hangay Dome volcanic rocks: exploring the source of high topography and volcanism in an intracontinental setting

    NASA Astrophysics Data System (ADS)

    Ancuta, L. D.; Carlson, R. W.; Idleman, B. D.; Zeitler, P. K.

    2013-12-01

    The Hangay dome in central Mongolia is an anomalous uplifted continental interior that is partially covered by diffuse Cenozoic basaltic rocks. Here we present new data on the geochemistry, stratigraphy, geomorphology and 40Ar/39Ar ages of the basaltic rocks to help elucidate the cause of the uplift and 33 Ma of volcanism in the region. 187Os/188Os ratios for the basaltic rocks range from 0.1363-0.3440. The higher values implicate crustal contamination, but the less radiogenic values limit the amount of contamination to the point where the Sr, Nd and Hf isotopic composition of the lavas are little affected, allowing them to be used as reliable tracers of the initial melt source. 87Sr/86Sr and 143Nd/144Nd ratios for the basaltic rocks from the region range from 0.7039-0.7050 and 0.5120-0.5127 respectively. These values are higher and lower, respectively, than Sr and Nd isotopic composition of the majority of spinel peridotite xenoliths contained in recent Hangay lavas, implicating a sub-lithospheric source for the magmas. The basalts have isotopic compositions approaching the EM-1 enriched mantle end member, similar to a number of other sites of young east Asian magmatism. An EM-1 type mantle source may have been generated regionally across East Asia by incorporation of pelagic sediments into the upper mantle during the protracted history of terrane accretion and subduction associated with the formation of the Central Asian orogenic system. New stratigraphically correlated 40Ar/39Ar ages for basalts from the Hangay region show that multiple episodes of laterally extensive flows occurred between 28.30×0.19 and 4.11×0.11 Ma. This first phase of volcanism was the most voluminous and long-lived. A later stage of valley-filling eruptions occurred between 3.28×0.50 Ma and 5 Ka. Flows across this range of ages occur in a number of locations within the Hangay, with no discernable age progression, indicating that the region has been the site of volcanism for over 30 Ma

  2. Geochronology and geochemistry of the high Mg dioritic dikes in Eastern Tianshan, NW China: Geochemical features, petrogenesis and tectonic implications

    NASA Astrophysics Data System (ADS)

    Li, Deng-Feng; Zhang, Li; Chen, Hua-Yong; Hollings, Pete; Cao, Ming-Jian; Fang, Jing; Wang, Cheng-Ming; Lu, Wan-Jian

    2016-01-01

    Zircon U-Pb ages of high Mg dioritic dikes in the Mesoproterozoic Kawabulake Group in the Eastern Tianshan area, NW China indicate that they were emplaced in the Early Carboniferous at 353-348 Ma. The dikes consist of medium-grained plagioclase and hornblende with minor clinopyroxene and trace quartz. They are characterized by intermediate SiO2 (60-62 wt.%), low TiO2 (0.63-0.71 wt.%), relatively high Al2O3 (15.1-15.8 wt.%) and MgO contents (3.45-4.15 wt.%) with Mg# generally higher than 56 (56-59). The geochemistry of the high Mg diorites suggest they were formed by similar magmatic processes to sanukitoid high Mg Andesites such as those of the Setouchi volcanic belt, Japan. Zircons from the high Mg dioritic dikes have εHf(t) values of -6.8 to +14.5. The dominantly positive values suggest a juvenile source, whereas the small number of negative values suggests mature components were also incorporated into the source. Similarly, the positive εNd(t) values (0 to +2.2) are interpreted to reflect a juvenile source whereas the negative values of (-5.2 to 0) suggest participation of old crustal rocks in the petrogenesis of the diorites. The variable εHf(t) and εNd(t) values suggest that the mature material was assimilated during magma ascent rather than in the mantle wedge which would result in more uniform values. Mass balance calculations suggest that the dioritic dikes were derived from sources composed of approximately 97% juvenile mantle-derived material and 3% sediment. Petrographic, elemental, and isotopic evidence suggest that the dioritic dikes were generated by partial melting of depleted mantle that migrated into the shallow crust where it assimilated older sedimentary rocks of the Mesoproterozoic Kawabulake Group.

  3. Petrology, geochemistry and geochronology of the magmatic suite from the Jianzha Complex, central China: Petrogenesis and geodynamic implications

    NASA Astrophysics Data System (ADS)

    Li, Xiaowei; Mo, Xuanxue; Bader, Thomas; Scheltens, Mark; Yu, Xuehui; Dong, Guochen; Huang, Xiongfei

    2014-12-01

    The intermediate-mafic-ultramafic rocks in the Jianzha Complex (JZC) at the northern margin of the West Qinling Orogenic Belt have been interpreted to be a part of an ophiolite suite. In this study, we present new geochronological, petrological, geochemical and Sr-Nd-Hf isotopic data and provide a different interpretation. The JZC is composed of dunite, wehrlite, olivine clinopyroxenite, olivine gabbro, gabbro, and pyroxene diorite. The suite shows characteristics of Alaskan-type complexes, including (1) the low CaO concentrations in olivine; (2) evidence of crystal accumulation; (3) high calcic composition of clinopyroxene; and (4) negative correlation between FeOtot and Cr2O3 of spinels. Hornblende and phlogopite are ubiquitous in the wehrlites, but minor orthopyroxene is also present. Hornblende and biotite are abundant late crystallized phases in the gabbros and diorites. The two pyroxene-bearing diorite samples from JZC yield zircon U-Pb ages of 245.7 ± 1.3 Ma and 241.8 ± 1.3 Ma. The mafic and ultramafic rocks display slightly enriched LREE patterns. The wehrlites display moderate to weak negative Eu anomalies (0.74-0.94), whereas the olivine gabbros and gabbros have pronounced positive Eu anomalies. Diorites show slight LREE enrichment, with (La/Yb)N ratios ranging from 4.42 to 7.79, and moderate to weak negative Eu anomalies (Eu/Eu∗ = 0.64-0.86). The mafic and ultramafic rocks from this suite are characterized by negative Nb-Ta-Zr anomalies as well as positive Pb anomalies. Diorites show pronounced negative Ba, Nb-Ta and Ti spikes, and typical Th-U, K and Pb peaks. Combined with petrographic observations and chemical variations, we suggest that the magmatism was dominantly controlled by fractional crystallization and crystal accumulation, with limited crustal contamination. The arc-affinity signature and weekly negative to moderately positive εNd(t) values (-2.3 to 1.2) suggest that these rocks may have been generated by partial melting of the juvenile

  4. Constraints on the collision and the pre-collision tectonic configuration between India and Asia from detrital geochronology, thermochronology, and geochemistry studies in the lower Indus basin, Pakistan

    NASA Astrophysics Data System (ADS)

    Zhuang, Guangsheng; Najman, Yani; Guillot, Stéphane; Roddaz, Martin; Antoine, Pierre-Olivier; Métais, Grégoire; Carter, Andrew; Marivaux, Laurent; Solangi, Sarfraz H.

    2015-12-01

    Knowledge of the timing of India-Asia collision is a fundamental prerequisite for understanding the evolution of the Himalayan-Tibetan orogen and its role in global climate, oceanic chemistry, and ecological evolution. Despite much active research, the basic pre-collision tectonic configuration and the timing of terminal India-Asia suturing remain debated. For example, debates regarding when and how the intervening Kohistan-Ladakh arc was sutured with India and Asia still remain elusive; some models propose the arc collided with Asia at about 100 Ma, with India-Asia collision at ca. 55 Ma, whilst a newer model proposed the arc's collision with India at 50 Ma and subsequently with Asia at 40 Ma. Another example is the recent proposition that an oceanic Greater India Basin separated the Tethyan Himalaya microcontinent from the remaining Indian plate until 20- 25 Ma with the consumption of this oceanic basin marking the final collision at this time. These controversies relate to whether the commonly documented 50 Ma contact represents the terminal India-Asia suturing or the amalgamation between various arcs or microcontinents with India or Asia. Here we present an integrated provenance study of geochronology, thermochronology, and geochemistry on the late Cretaceous-Pleistocene sediments from the lower Indus basin on the Indian plate. The detrital zircon U-Pb and fission track data show a reversal in sediment source from a pure Indian signature to increasing inputs from the suture zone and the Asian plate between the middle Paleocene and early Oligocene. The Nd and Sr isotopes narrow down this change to 50 Ma by revealing input of Asian detritus and the establishment of a Nd & Sr isotopic pattern similar to the present-day Indus Fan by 50 Ma, with no significant variations up section, contrary to what might be expected if later major collisions had occurred. Our isotopic data indicate that Greater India was occupied by a fluvial-deltaic system, analogous to the

  5. The syncollisional granitoid magmatism and continental crust growth in the West Kunlun Orogen, China - Evidence from geochronology and geochemistry of the Arkarz pluton

    NASA Astrophysics Data System (ADS)

    Zhang, Yu; Niu, Yaoling; Hu, Yan; Liu, Jinju; Ye, Lei; Kong, Juanjuan; Duan, Meng

    2016-02-01

    The West Kunlun orogenic belt (WKOB) at the northwest margin of the Greater Tibetan Plateau records seafloor subduction, ocean basin closing and continental collision with abundant syncollisional granitoids in response to the evolution of the Proto- and Paleo-Tethys Oceans from the early-Paleozoic to the Triassic. Here we present a combined study of detailed zircon U-Pb geochronology, whole-rock major and trace elements and Sr-Nd-Hf isotopic geochemistry on the syncollisional Arkarz (AKAZ) pluton with mafic magmatic enclaves (MMEs) exposed north of the Mazha-Kangxiwa suture (MKS) zone. The granitoid host rocks and MMEs of the AKAZ pluton give the same late Triassic age of ~ 225 Ma. The granitoid host rocks are metaluminous granodiorite and monzogranite. They have initial 87Sr/86Sr of 0.70818 to 0.70930, εNd(225 Ma) = - 4.61 to - 3.91 and εHf(225 Ma) = - 3.01 to 0.74. The MMEs are more mafic than the host with varying SiO2 (51.00-63.24 wt.%) and relatively low K2O (1.24-3.02 wt.%), but have similar Sr-Nd-Hf isotope compositions to the host ((87Sr/86Sr)i = 0.70830-0.70955, εNd(225 Ma) = - 4.88 to - 4.29, εHf(225 Ma) = - 2.57 to 0.25). Both the host and MMEs have rare earth element (REE) and trace element patterns resembling those of bulk continental crust (BCC). The MMEs most likely represent cumulate formed from common magmas parental to the granitoid host. The granitoid magmatism is best explained as resulting from melting of amphibolite of MORB protolith during continental collision, which produces andesitic melts with a remarkable compositional similarity to the BCC and the inherited mantle-like isotopic compositions. Simple isotopic mixing calculations suggest that ~ 80% ocean crust and ~ 20% continental materials contribute to the source of the AKAZ pluton. Thus, the hypothesis "continental collision zones as primary sites for net continental crust growth" is applicable in the WKOB as shown by studies in southern Tibet, East Kunlun and Qilian orogens. In

  6. Field geology, geochronology and geochemistry of mafic-ultramafic rocks from Alxa, China: Implications for Late Permian accretionary tectonics in the southern Altaids

    NASA Astrophysics Data System (ADS)

    Feng, Jianyun; Xiao, Wenjiao; Windley, Brian; Han, Chunming; Wan, Bo; Zhang, Ji'en; Ao, Songjian; Zhang, Zhiyong; Lin, Lina

    2013-12-01

    The time of termination of orogenesis for the southern Altaids has been controversial. Systematic investigations of field geology, geochronology and geochemistry on newly discriminated mafic-ultramafic rocks from northern Alxa in the southern Altaids were conducted to address the termination problem. The mafic-ultramafic rocks are located in the Bijiertai, Honggueryulin, and Qinggele areas, stretching from west to east for about 100 km. All rocks occur high-grade gneisses as tectonic lenses that are composed of peridotite, pyroxenite, gabbro, and serpentinite, most of which have undergone pronounced alteration, i.e., serpentinization and chloritization. Geochemically, the rocks are characterized by uniform compositional trends, i.e., with low SiO2-contents (42.51-52.21 wt.%) and alkalinity (Na2O + K2O) (0.01-5.45 wt.%, mostly less than 0.8 wt.%), and enrichments in MgO (7.37-43.36 wt.%), with Mg# = 52.75-91.87. As the rocks have been strongly altered and have a wide range of loss-on-ignition (LOI: 0.44-14.07 wt.%) values, they may have been subjected to considerable alteration by either seawater or metamorphic fluids. The REE and trace element patterns show a relatively fractionated trend with LILE enrichment and HFSE depletion, similar to that of T-MORB between N-MORB and E-MORB, indicating that the parental melt resulted from the partial melting of oceanic lithospheric mantle overprinted by fluid alteration of island-arc origin. The ultramafic rocks are relics derived from the magma after a large degree of partial melting of oceanic lithospheric mantle with superposed island arc processes under the influence of mid-ocean-ridge magmatism. LA-ICP MS U-Pb zircon ages of gabbros from three spots are 274 ± 3 Ma (MSWD = 0.35), 306 ± 3 Ma (MSWD = 0.49), 262 ± 5 Ma (MSWD = 1.2), respectively, representing the formation ages of the mafic-ultramafic rocks. Therefore, considering other previously published data, we suggest that the mafic-ultramafic rocks were products of

  7. Field geology, geochronology and geochemistry of mafic-ultramafic rocks from Alxa, China: Implications for Late Permian accretionary tectonics in the southern Altaids

    NASA Astrophysics Data System (ADS)

    Jianyun, Feng; Wenjiao, Xiao

    2013-04-01

    The termination of orogenesis for the southern Altaids has been controversial. Systematical investigations of field geology, geochronology and geochemistry on mafic-ultramafic rocks from the northern Alxa of the southern Altaids were conducted to address the termination controversy. The newly discriminated mafic-ultramafic rocks belt is located at Bijiertai, Honggueryulin, and Qinggele areas, stretching from west to east for about 100 km in length. All of the three rock associations contact tectonically with the adjacent metamorphic and deformed Precambrian rocks as tectonic blocks or lenses, and are composed of peridotite, pyroxenite, gabbro, and serpentinite, most of which have subjected to pronounced alteration, i.e., serpentinization and chloritization. Geochemically, the rocks are characterized by a uniform trend of compositional distribution, e.g., with low SiO2-contents (42.51-52.21 wt.%) and alkalinity (Na2O+K2O) (0.01-5.45 wt.%, mostly less than 0.8 wt.%), and enriched in MgO (7.37-43.36 wt.%), with Mg# = 52.75-91.87. As the rocks have had strong alteration and have a wide range of loss-on-ignition (LOI: 0.44-14.07 wt.%), the rocks may be subjected to considerable alteration by either sea-water or metamorphic fluid. The REE and trace element patterns for the rocks show a relatively fractionated trend with LILE enrichment and HFSE depletion, similar to that of T-MORB between N-MORB and E-MORB, indicating that the parental melt resulted from the partial melting of oceanic lithospheric mantle overprinted by fluid alteration of island-arc subsequently. The ultramafic rocks are relics derived from the magma after large degree of partial melting of the oceanic lithospheric mantle with overprinted by island-arc processes under the influence of mid-ocean-ridge magmatism. LA - ICP MS U - Pb zircon ages of gabbros from the three spots are 274 ± 3 Ma (MSWD = 0.35), 306 ± 3 Ma (MSWD = 0.49), 262 ± 5 Ma (MSWD = 1.2), respectively, representing the formation ages of

  8. The behaviour of monazite from greenschist facies phyllites to anatectic gneisses: An example from the Chugach Metamorphic Complex, southern Alaska

    PubMed Central

    Gasser, Deta; Bruand, Emilie; Rubatto, Daniela; Stüwe, Kurt

    2012-01-01

    Monazite is a common accessory mineral in various metamorphic and magmatic rocks, and is widely used for U–Pb geochronology. However, linking monazite U–Pb ages with the PT evolution of the rock is not always straightforward. We investigated the behaviour of monazite in a metasedimentary sequence ranging from greenschist facies phyllites into upper amphibolites facies anatectic gneisses, which is exposed in the Eocene Chugach Metamorphic Complex of southern Alaska. We investigated textures, chemical compositions and U–Pb dates of monazite grains in samples of differing bulk rock composition and metamorphic grade, with particular focus on the relationship between monazite and other REE-bearing minerals such as allanite and xenotime. In the greenschist facies phyllites, detrital and metamorphic allanite is present, whereas monazite is absent. In lower amphibolites facies schists (~ 550–650 °C and ≥ 3.4 kbar), small, medium-Y monazite is wide-spread (Mnz1), indicating monazite growth prior and/or simultaneous with growth of garnet and andalusite. In anatectic gneisses, new low-Y, high-Th monazite (Mnz2) crystallised from partial melts, and a third, high-Y, low-Th monazite generation (Mnz3) formed during initial cooling and garnet resorption. U–Pb SHRIMP analysis of the second and third monazite generations yields ages of ~ 55–50 Ma. Monazite became unstable and was overgrown by allanite and/or allanite/epidote/apatite coronas within retrograde muscovite- and/or chlorite-bearing shear zones. This study documents polyphase, complex monazite growth and dissolution during a single, relatively short-lived metamorphic cycle. PMID:26525358

  9. The Siderian-Orosirian magmatism in the Gavião Paleoplate, Brazil: U–Pb geochronology, geochemistry and tectonic implications

    NASA Astrophysics Data System (ADS)

    Pereira Cruz, Simone Cerqueira; Figueiredo Barbosa, Johildo Salomão; Pinto, Marilda Santos; Peucat, Jean-Jacques; Paquette, Jean Louis; Santos de Souza, Jailma; de Souza Martins, Violeta; Júnior, Farid Chemale; Carneiro, Mauricio Antonio

    2016-08-01

    The southern portion of the Gavião Paleoplate is composed by Archean orthogneisses, Archean-Paleoproterozoic metavolcano-sedimentary rocks and Siderian-Rhyacian-Orosirian granitoids. Petrographic, geochemical, U-Pb (Laser Ablation, ICPMS) and Sm-Nd data are presented for five Paleoproterozoic granitoids that were recently mapped: Jussiape II, Lagoa das Almas, Humaitá, Belo Campo and Broco granitoids. These granitoids present U-Pb zircon (LA-ICPMS) ages of 2052 ± 43, 2114 ± 24, 2140 ± 9, 2049 ± 23 and 2038 ± 8 Ma, respectively. In addition to these granitoids, another twenty-five ones were identified and studied by several authors, resulting in a total of twenty-nine plutons. Despite the previous petrography, geochemistry and geochronology studies that have been performed, no model had been proposed to explain the tectonic setting of this extensive granitogenesis. Integration of the new data and the literature has been done and corresponds to the second part of the article. Based on U-Pb dating and geochemical data, Siderian-Rhyacian-Orosirian granitoids of the southern Gavião Paleoplate were classified into five groups, or five suites: 1 (2324 ± 6 to 2091 ± 6.6 Ma), 2a (2054 -6/+8 to 2041 ± 23 Ma), 2b (2066 ± 37 to 2019 ± 32 Ma), 2c (2058 ± 8 to 1852 ± 50 Ma) and 2d (2049 ± 12 to 1929 ± 16 Ma). The granitoids of Group 1 present heterogeneous deformation, while the granitoids of groups 2a to 2d are generally not deformed. Usually the rocks are potassic, but sodic granitic rocks can be found in samples of groups 1, 2c and 2d. Several chemical classification parameters are presented and discussed herein, but it is noteworthy that the granitoids of Group 1 are mainly classified as calcic to calc-alkalic, while the rocks of the second group are mostly classified as alkalic ones. In the remaining groups, the samples vary between calc-alkalic and alkali-calcic. The εNd values range between 4.0 and -15.4 and suggest an important and varied share of the

  10. A review of the geochronology and geochemistry of Late Yanshanian (Cretaceous) plutons along the Fujian coastal area of southeastern China: Implications for magma evolution related to slab break-off and rollback in the Cretaceous

    NASA Astrophysics Data System (ADS)

    Li, Zhen; Qiu, Jian-Sheng; Yang, Xue-Mei

    2014-01-01

    The Cretaceous plutonic suites in the Fujian coastal area include abundant I-type and A-type granitoids and lesser gabbroids. They are important components of the Late Yanshanian magmatic belt along the southeastern coast of China, and define a linear NNE-SSW-trending belt of magmatism. Geochronological, geochemical and geological data from thirty intrusions are summarised in this paper, and the data provide distinct magmatic, geochemical and tectonic patterns in the area. A compilation of geochronological data for these intrusive rocks indicates emplacement mainly from around 125 to 90 Ma, with a major peak from 115 to 90 Ma, and a subordinate peak from 125 to 115 Ma. Besides their temporal and spatial coexistence, all these intrusive rocks have similar geochemical patterns which point to involvement of components from a depleted asthenospheric mantle source for the parental magmas, most probably by magma mixing. The first appearance of sparse I-type granitoids with post-collisional extensional granite affinities, and the emplacement of the Baijuhuajian and Suzhou A-type granites, mark the beginning of extension during the Early Cretaceous at ca. 125 to 119 Ma. The subsequent development of bimodal magmatism at 115 to 90 Ma, with numerous arc-related mafic gabbros and I-type granites, together with some A-type granites, suggests that this major igneous event took place as a response to back-arc extension. On the basis of petrology, geochronology, tectonics, and elemental and isotopic geochemistry, we speculate that break-off and rollback of the subducting Palaeo-Pacific Plate during the Cretaceous were responsible for the Late Yanshanian regional tectono-magmatic evolution in the area. We suggest that this process facilitated a strong and rapid linear upwelling of the asthenospheric mantle beneath the coastal area of southeastern China, with consequential extension of the overlying continental lithosphere, and ultimately the large-scale Late Yanshanian magmatism

  11. PROCESSING OF MONAZITE SAND

    DOEpatents

    Calkins, G.D.; Bohlmann, E.G.

    1957-12-01

    A process for the recovery of thorium, uranium, and rare earths from monazite sands is presented. The sands are first digested and dissolved in concentrated NaOH, and the solution is then diluted causing precipitation of uranium, thorium and rare earth hydroxides. The precipitate is collected and dissolved in HCl, and the pH of this solution is adjusted to about 6, precipitating the hydroxides of thorium and uranium but leaving the rare earths in solution. The rare earths are then separated from the solution by precipitation at a still higher pH. The thorium and uranium containing precipitate is redissolved in HNO/sub 3/ and the two elements are separated by extraction into tributyl phosphate and back extraction with a weakly acidic solution to remove the thorium.

  12. Geology, geochemistry, geochronology, and economic potential of Neogene volcanic rocks in the Laguna Pedernal and Salar de Aguas Calientes segments of the Archibarca lineament, northwest Argentina

    NASA Astrophysics Data System (ADS)

    Richards, J. P.; Jourdan, F.; Creaser, R. A.; Maldonado, G.; DuFrane, S. A.

    2013-05-01

    This study presents new geochemical, geochronological, isotopic, and mineralogical data, combined with new geological mapping for a 2400 km2 area of Neogene volcanic rocks in northwestern Argentina near the border with Chile, between 25°10‧S and 25°45‧S. The area covers the zone of intersection between the main axis of the Cordillera Occidental and a set of NW-SE-trending structures that form part of the transverse Archibarca lineament. This lineament has localized major ore deposits in Chile (e.g., the late Eocene La Escondida porphyry Cu deposit) and large volcanic centers such as the active Llullaillaco and Lastarría volcanoes on the border between Chile and Argentina, and the Neogene Archibarca, Antofalla, and Cerro Galán volcanoes in Argentina. Neogene volcanic rocks in the Laguna Pedernal and Salar de Aguas Calientes areas are mostly high-K calc-alkaline in composition, and range from basaltic andesites, through andesites and dacites, to rhyolites. Magmatic temperatures and oxidation states, estimated from mineral compositions, range from ~ 1000 °C and ∆FMQ ≈ 1.0-1.5 in andesites, to ~ 850 °C and ∆FMQ ≈ 1.5-2.0 in dacites and rhyolites. The oldest rocks consist of early-middle Miocene andesite-dacite plagioclase-pyroxene-phyric lava flows and ignimbrites, with 40Ar/39Ar ages ranging from 17.14 ± 0.10 Ma to 11.76 ± 0.27 Ma. Their major and trace element compositions are typical of the Andean Central Volcanic Zone, and show strong crustal contamination trends for highly incompatible elements such as Cs, Rb, Th, and U. These rocks are geochemically grouped as sub-suite 1. This widespread intermediate composition volcanism was followed in the middle-late Miocene by a period of more focused rhyodacitic flow-dome complex formation. These felsic rocks are characterized by less extreme enrichments in highly incompatible elements, and increasing depletion of heavy rare earth elements. These rocks are geochemically grouped as sub-suite 2. The

  13. Geochemistry, petrography, and zircon U-Pb geochronology of Paleozoic metaigneous rocks in the Mount Veta area of east-central Alaska: implications for the evolution of the westernmost part of the Yukon-Tanana terrane

    USGS Publications Warehouse

    Dusel-Bacon, Cynthia; Day, Warren C.; Aleinikoff, John N.

    2013-01-01

    We report the results of new mapping, whole-rock major, minor, and trace-element geochemistry, and petrography for metaigneous rocks from the Mount Veta area in the westernmost part of the allochthonous Yukon–Tanana terrane (YTT) in east-central Alaska. These rocks include tonalitic mylonite gneiss and mafic metaigneous rocks from the Chicken metamorphic complex and the Nasina and Fortymile River assemblages. Whole-rock trace-element data from the tonalitic gneiss, whose igneous protolith was dated by SHRIMP U–Pb zircon geochronology at 332.6 ± 5.6 Ma, indicate derivation from tholeiitic arc basalt. Whole-rock analyses of the mafic rocks suggest that greenschist-facies rocks from the Chicken metamorphic complex, a mafic metavolcanic rock from the Nasina assemblage, and an amphibolite from the Fortymile River assemblage formed as island-arc tholeiite in a back-arc setting; another Nasina assemblage greenschist has MORB geochemical characteristics, and another mafic metaigneous rock from the Fortymile River assemblage has geochemical characteristics of calc-alkaline basalt. Our geochemical results imply derivation in an arc and back-arc spreading region within the allochthonous YTT crustal fragment, as previously proposed for correlative units in other parts of the terrane. We also describe the petrography and geochemistry of a newly discovered tectonic lens of Alpine-type metaharzburgite. The metaharzburgite is interpreted to be a sliver of lithospheric mantle from beneath the Seventymile ocean basin or from sub-continental mantle lithosphere of the allochthonous YTT or the western margin of Laurentia that was tectonically emplaced within crustal rocks during closure of the Seventymile ocean basin and subsequently displaced and fragmented by faults.

  14. Petrogenesis of the Late Triassic volcanic rocks in the Southern Yidun arc, SW China: Constraints from the geochronology, geochemistry, and Sr-Nd-Pb-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Leng, Cheng-Biao; Huang, Qiu-Yue; Zhang, Xing-Chun; Wang, Shou-Xu; Zhong, Hong; Hu, Rui-Zhong; Bi, Xian-Wu; Zhu, Jing-Jing; Wang, Xin-Song

    2014-03-01

    Studies on zircon ages, petrology, major and trace element geochemistry, and Sr-Nd-Hf-Pb isotopic geochemistry of intermediate volcanic rocks from the Southern Yidun arc, Sanjiang-Tethyan Orogenic Belt, SW China have been undertaken in this paper. They are used to discuss the petrogenesis of these rocks and to constrain the tectonic setting and evolution of the Yidun arc. These intermediate volcanic rocks were erupted at ca. 220 Ma (U-Pb zircon ages). Trachyandesite is the dominant lithology among these volcanic rocks, and is mainly composed of hornblende and plagioclase, with minor clinopyroxene and biotite. A hornblende geobarometer suggests that the stagnation of magma in the lower crust, where plagioclase crystallization was suppressed while hornblende crystallized, giving rise to high Sr/Y ratios that are one of the distinguishing features of adakites, after the primary magma originated from the lithospheric mantle wedge. Steeply right-inclined Rare Earth Element (REE) pattern combined with high La/Yb ratios suggests adakitic affinity of these volcanic rocks, implying that slab-melt from the subducting oceanic crust is a necessary component in the primary magma. Besides, trace element geochemistry and isotopic geochemistry also indicate that partial melting of pelagic sediments in the subduction zone and noticeable contamination with the lower crust were involved in the evolution of parental magma of these volcanic rocks. Based on previous work on the Northern Yidun arc and this study, we propose that the subduction was initiated in the Northern Yidun arc and extended to the southern part and that the Northern Yidun arc is an island arc while the Southern Yidun arc represents a continental arc, probably caused by the existence of the Zhongza Massif, that was invoked to be derived from Yangtze Block, as a possible basement of the Southern Yidun arc.

  15. METHOD OF PROCESSING MONAZITE SAND

    DOEpatents

    Welt, M.A.; Smutz, M.

    1958-08-26

    A process is described for recovering thorium, uranium, and rare earth values from monazite sand. The monazite sand is first digested with sulfuric acid and the resulting "monazite sulfate" solution is adjusted to a pH of between 0.4 and 3.0, and oxalate anions are added causing precipitation of the thorium and the rare earths as the oxalates. The oxalate precipitate is separated from the uranium containing supernatant solution, and is dried and calcined to the oxides. The thorium and rare earth oxides are then dissolved in nitric acid and the solution is contacted with tribntyl phosphate whereby an organic extract phase containing the cerium and thorium values is obtained, together with an aqueous raffinate containing the other rare earth values. The organic phase is then separated from the aqueous raffinate and the cerium and thorium are back extracted with an aqueous medium.

  16. U-Pb Geochronology, Geochemistry and Kinematic Analyses of Subduction-Related Late Triassic Basins in Northern Chile (24.5º-26ºS).

    NASA Astrophysics Data System (ADS)

    Espinoza, M. E.

    2015-12-01

    In northern Chile (24.5°-26°S) two Pre-Andean depocenters crop out: the Cifuncho basin in the Coastal Cordillera and the Profeta basin in the Precordillera. These basins have been classically interpreted as a continental rifting unrelated to subduction during the period prior to the Andean orogenic cycle. However, recent petrographic and geochemical data suggest the development of these basins in an active subduction system. In order to test this hypothesis and to establish the geologic evolution of the basins and the strain field during the rifting process, we present preliminary U-Pb geochronological and geochemical data together with structural analyses of synrift structures. The geochronological data along the Cifuncho and Profeta basins, show a main continental sedimentary deposition during the Norian to Raethian. Volcanosedimentary rocks show a main detrital supply of Early Permian age (~297-283 Ma). This input can be associated with the volcanic La Tabla Formation and/or the exhumation of Permian granitoids. A minor supply close to ~478 Ma is related to a source from the Lower Ordovician arc (~480 Ma), suggesting the tectonic exhumation of this source to the east of the Profeta basin during the Late Triassic. On the other hand, structural analysis was carried in third and four order extensional faults (<10 m of slip) along the Profeta basin. Most of the faults show a clear synrift character with the development of fault controlled growing strata. The kinematic analyses evidence a variability in the orientation of the maximum strain axes from a main northwest to a subordinate northeast direction of extension. Thus, the intimate relation between the continental sedimentary deposition and a proximal volcanism of intermediate composition and calk-alkaline affinity, suggests the development of these basins in a supra-subduction setting during the Late Triassic. Structural data probably reflect local variation in the strain field across the basins.

  17. Geochronology, geochemistry, and tectonic characterization of Quaternary large-volume travertine deposits in the southwestern United States and their implications for CO2 sequestration

    NASA Astrophysics Data System (ADS)

    Priewisch, Alexandra

    Travertines are freshwater carbonates that precipitate from carbonic groundwater due to the degassing of CO2. Travertine deposits are often situated along faults that serve as conduits for CO2-charged groundwater and their geochemistry often records mixing of deeply-derived fluids and volatiles with shallow meteoric water. Travertines are surface expressions of dynamic mantle processes related to the tectonic setting. This dissertation includes four chapters that focus on different aspects of travertine formation and their scientific value. They are excellent, although underestimated, diagnostic tools for climatology, hydrology, tectonics, geochemistry, geomicrobiology, and they can inform carbon sequestration models. Quaternary large-volume travertine deposits in New Mexico and Arizona occur in an extensional tectonic stress regime on the southeastern Colorado Plateau and along the Rio Grande rift. They accumulated above fault systems during episodes of high hydraulic head in confined aquifers, increased regional volcanic activity, and high input of mantle-derived volatiles such as CO 2 and He. Stable isotope and trace element geochemistry of travertines is controlled by groundwater geochemistry as well as the degassing of CO 2. The geochemical composition allows for distinguishing different travertine facies and evaluating past groundwater flow. The travertine deposits in New Mexico are interpreted to be extinct CO2 fields due to the large volumes that accumulated and in analogy to the travertine deposits in Arizona that are associated with an active CO2-gas field. Travertines are natural analogues for CO2 leakage along fault systems that bypassed regional cap rocks and they provide important insight into the migration of CO2 from a reservoir to the surface. The volume of travertine can be used to infer the integrated CO2 leakage along a fault system over geologic time. This leakage is estimated as: (1) CO2 that becomes fixed in CaCO3/travertine (tons of carbon

  18. A-type granites from the Pan-African orogenic belt in south-western Chad constrained using geochemistry, Sr-Nd isotopes and U-Pb geochronology

    NASA Astrophysics Data System (ADS)

    Isseini, Moussa; André-Mayer, Anne-Sylvie; Vanderhaeghe, Olivier; Barbey, Pierre; Deloule, Etienne

    2012-11-01

    The Zabili granitic pluton (SW Chad) exposed in the Mayo Kebbi massif is dominated by a coarse-grained hornblende biotite granite grading into a fine-grained biotite granite along its southern margin. Petrologic (micrographic intergrowth of quartz and alkali feldspars, granophyric microstructures, the presence of fluorite and bastnaesite as accessory minerals) and geochemical data (high silica, alkalis and Fe/Mg, depletions in CaO, MgO, TiO2; high Ga, Nb, Zr, Ga/Al, REE, depletions in Ba, Sr, Eu and compatible elements) indicate that this pluton consists of A-type granites crystallized from hot (apatite and zircon saturation temperatures ranging from 744 °C to 923 °C), extremely differentiated magmas. U-Pb zircon geochronology indicates that the magmas crystallized at 567 ± 10 Ma and reveals the presence of older Neoproterozoic xenocrystic zircons at 668 ± 5 Ma in both facies. Within the fine-grained biotite granite, discordant zircons with U-Pb and Pb-Pb ages ranging from Neoproterozoic to Archaean are also reported. The 668 ± 5 Ma old zircons are considered to derive from country-rocks while discordant zircons, characterized by angular shapes, internal fractures and inherited cores, are likely to represent multi-sources detrital crystals that have recorded at least one metamorphic event. Old pre-Neoproterozoic zircons are reported for the first time for rocks of the Mayo Kebbi massif and they attest to the contribution of an old basement (likely to be the Eastern Nigeria basement and/or the Congo craton) involved in a collisional event with a juvenile Neoproterozoic crust prior to the emplacement of the Zabili granitic pluton. Initial ɛNd values calculated for the Zabili pluton range from + 2.6 to + 7.0, the highest value recorded by one sample from the coarse-grained hornblende-biotite granite being close to the one of the depleted mantle at 570 Ma (ɛNd = + 7.4). Combining geochronology, Nd isotopes composition and geochemical modeling, leads us to

  19. Paleoproterozoic crustal evolution in the East Sarmatian Orogen: Petrology, geochemistry, Sr-Nd isotopes and zircon U-Pb geochronology of andesites from the Voronezh massif, Western Russia

    NASA Astrophysics Data System (ADS)

    Terentiev, R. A.; Savko, K. A.; Santosh, M.

    2016-03-01

    Andesites and related plutonic rocks are major contributors to continental growth and provide insights into the interaction between the mantle and crust. Paleoproterozoic volcanic rocks are important components of the East Sarmatian Orogen (ESO) belonging to the East European Craton, although their petrogenesis and tectonic setting remain controversial. Here we present petrology, mineral chemistry, bulk chemistry, Sr-Nd isotopes, and zircon U-Pb geochronological data from andesites and related rocks in the Losevo and Vorontsovka blocks of the ESO. Clinopyroxene phenocrysts in the andesites are depleted in LREE, and enriched in HFSE (Th, Nb, Zr, Hf, Ti) and LILE (Ba, Sr). Based on the chemistry of pyroxenes and whole rocks, as well as Fe-Ti oxides, we estimate a temperature range of 1179 to 1262 °C, pressures of 11.3 to 13.0 kbar, H2O content of 1-5 wt.%, and oxygen fu gacity close to the MH buffer for the melts of the Kalach graben (KG) and the Baygora area (BA) andesites. Our zircon U-Pb geochronological data indicate new zircon growth during the middle Paleoproterozoic as displayed by weighted mean 207Pb/206Pb ages of 2047 ± 17 Ma and 2040 ± 16 Ma for andesite and dacite-porphyry of the BA, and 2050 ± 16 Ma from high-Mg basaltic andesite of the KG. The andesites and related rocks of the KG and BA are characterized by high magnesium contents (Mg # up to 0.68). All these volcanic rocks are depleted in LREE and HFSE, and display negative Nb and Ti anomalies relative to primitive mantle. The high-Mg bulk composition, and the presence of clinopyroxene phenocrysts suggests that the parent melts of the KG and BA suite were in equilibrium with the mantle rocks. The rocks show positive εNd(T) values and low initial 87Sr/86Sr, suggesting that the magmas were mostly derived from metasomatized mantle source. The geochemical differences between the two andesite types are attributed to: the predominance of fractional crystallization, and minor role of contamination in the

  20. Differentiating Detrital and Metamorphic Monazite in Greenschist-Facies Sandstones From the Witwatersrand Supergroup

    NASA Astrophysics Data System (ADS)

    Muhling, J. R.; Rasmussen, B.

    2009-05-01

    , calcite and titanate, and monazite has been partially replaced by fine intergrowths of allanite, apatite and Th-silicate. Careful characterisation of monazite in low-grade metasedimentary rocks can distinguish detrital grains from metamorphic, and open the way for precise geochronology of low-temperature events.

  1. Detrital zircon U-Pb geochronology, Lu-Hf isotopes and REE geochemistry constrains on the provenance and tectonic setting of Indochina Block in the Paleozoic

    NASA Astrophysics Data System (ADS)

    Wang, Ce; Liang, Xinquan; Foster, David A.; Fu, Jiangang; Jiang, Ying; Dong, Chaoge; Zhou, Yun; Wen, Shunv; Van Quynh, Phan

    2016-05-01

    In situ U-Pb geochronology, Lu-Hf isotopes and REE geochemical analyses of detrital zircons from Cambrian-Devonian sandstones in the Truong Son Belt, central Vietnam, are used to provide the information of provenance and tectonic evolution of the Indochina Block. The combined detrital zircon age spectra of all of the samples ranges from 3699 Ma to 443 Ma and shows with dominant age peaks at ca. 445 Ma and 964 Ma, along with a number of age populations at 618-532 Ma, 1160-1076 Ma, 1454 Ma, 1728 Ma and 2516 Ma. The zircon age populations are similar to those from time equivalent sedimentary sequences in continental blocks disintegrated from the East Gondwana during the Phanerozoic. The younger zircon grains with age peaks at ca. 445 Ma were apparently derived from middle Ordovician-Silurian igneous and metamorphic rocks in Indochina. Zircons with ages older than about 600 Ma were derived from other Gondwana terrains or recycled from the Precambrian basement of the Indochina Block. Similarities in the detrital zircon U-Pb ages suggest that Paleozoic strata in the Indochina, Yangtze, Cathaysia and Tethyan Himalayas has similar provenance. This is consistent with other geological constrains indicating that the Indochina Block was located close to Tethyan Himalaya, northern margin of the India, and northwestern Australia in Gondwana.

  2. Geochemistry and Sr-Nd isotopes of the subvolcanic sill complex and sandstone geochronology from María Magdalena island, Nayarit, Mexico

    NASA Astrophysics Data System (ADS)

    Villanueva, D.; Schaaf, P. E.; Hernandez, T.; Solis, G.; Weber, B.; Pompa, V.

    2013-12-01

    María Magdalena island is part of the Islas Marías archipielago, located at the mouth of the Gulf of California. Understanding the nature and origin of the archipelago is very important for reconstructing the paleoposition of Baja California Peninsula prior to the opening of the Gulf of California. We present the first geochemical, isotopic and geochronologic data from María Magdalena, a lithologically different island compared to the rest of the archipelago. María Magdalena island is located southeast of María Madre and northeast of María Cleofas islands and is composed by a sedimentary sequence of sandstones and minor shale, which is intruded by gabbroic sills. The sedimentary sequence dips approximately 20 degrees to the NW. The thickness of sills range from 1 to 3 meters with mineralogical variations of plagioclase +/- orthopyroxene +/- clinopyroxene +/- hornblende and some altered olivine crystals. Textures are mostly porphyritic with plagioclase crystals sizes up to 5 cm and olivines up to 5 mm. The gabbroic sills show SiO2 contents from 42.7 to 47.5 wt. %; TiO2 from 0.8 to 2 wt. %; Fe2O3t from 7.7 to 11.9 wt. %; MgO from 6.2 to 19.8 wt. % and of CaO from 6 to 11.6 wt. %, indicating mafic to ultramafic compositions. A multielement spider diagram as well as REE patterns show compositions very similar to N-MORB or even peridotites, which is confirmed by 87Sr/86Sr values from 0.70273 to 0.70497, and 143Nd/144Nd values from 0.513003 to 0.513100. U-Pb single zircon geochronology of the intruded sandstones display the following age distribution: eight crystals show ages from 80 to 86 Ma, three crystals have ages from 61 to 72 Ma, and three are around 21 Ma which constrains a maximum sandstone deposition age. Consequently, the sills must be younger than 21 Ma. There is not much lithological similarity with neighboring María Madre island to the NW (containing a metamorphic complex, granitoids and acid volcanic roks) and with María Cleofas island to the SE

  3. U-Pb geochronology and geochemistry of Bibi-Maryam pluton, eastern Iran: Implication for the late stage of the tectonic evolution of the Sistan Ocean

    NASA Astrophysics Data System (ADS)

    Delavari, Morteza; Amini, Sadraddin; Schmitt, Axel K.; McKeegan, Kevin D.; Mark Harrison, T.

    2014-07-01

    The Bibi-Maryam pluton crops out in the Sistan suture zone, eastern Iran. This pluton is a 1.5 × 2 km stock composed of leucocratic tonalite, granodiorite and granite. U-Pb zircon geochronology of a leucogranite indicates an emplacement age of 58.6 ± 2.1 Ma (95% confidence). The Bibi-Maryam rock suite is sodic with elevated Na2O/K2O (2.9 to 5.5), Sr/Y (15.6-62.2), La/Yb (13.3-22.2), and low MgO (0.86-1.81) abundances. It lacks significant Eu anomalies. Because of these geochemical characteristics, Bibi-Maryam rocks are similar to high-SiO2 adakites. Trace element modeling indicates that the Bibi-Maryam adakitic rocks could be produced by 5-8% non-modal batch partial melting from a source with composition of 95% N-MORB + 5% sediment in the presence of 35-40% amphibole + 5-10% garnet + 55-60% clinopyroxene + 1% apatite + 1% rutile. This source mineralogy is similar to hornblende eclogite or garnet amphibolites. Collectively, these data provide new constraints for the evolution of the Sistan suture zone and suggest that the Bibi-Maryam pluton formed via slab melting in an oceanic arc and pre-plate collision tectonic setting. This implies that the closure of the Sistan Ocean and Lut-Afghan continental blocks collision happened after the Bibi-Maryam emplacement at 58.6 ± 2.1 Ma.

  4. Geochemistry and geochronology of the blueschist in the Heilongjiang Complex and its implications in the late Paleozoic tectonics of eastern NE China

    NASA Astrophysics Data System (ADS)

    Ge, Mao-hui; Zhang, Jin-jiang; Liu, Kai; Ling, Yi-yun; Wang, Meng; Wang, Jia-min

    2016-09-01

    The Paleozoic to early Mesozoic tectonic framework and evolution of Northeast China, especially the Jiamusi block and its related structural belts, are highly debated. In this paper, geochemical, geochronological and isotopic analyses were carried out on the blueschist in the Heilongjiang Complex to address these issues. The Heilongjiang Complex defines the suture belt between the Jiamusi block and the Songliao block in NE China, and the blueschist is a major composition for this complex, coexisting with mafic-ultramafic rocks, greenschist, quartzite and mica schist. The blueschist has a mineral association of sodic amphibole, epidote, chlorite, phengite, albite and quartz with accessory phases of apatite, titanite, zircon and ilmenite. Together with the lithological association, the major and trace element compositions present that the protoliths of the blueschist can be divided into the alkaline and tholeiitic basalts and have OIB affinities, formed in an ocean island setting, indicated by the (La/Yb) N values of 3.57 - 11.54, and the (La/Sm) N values of 0.69 - 3.64. The high and positive εNd (t) values of + 3.7 to + 9.0, and relative enrichment in Nb (vs. Th) and Ta (vs. U) show that both the alkaline and tholeiitic basalts may be derived from the asthenospheric mantle with insignificant crustal contamination. Magmatic zircons from the blueschist in Yilan area yield a 206Pb/238U age of 281 ± 3 Ma, interpreted as its protolithic age. The youngest ages of ~ 200 Ma of the detrital zircons in the associated mica schist from Mudanjiang area place constraints on the timing of metamorphism for the blueschist. These indicate that a big ocean existed between the Jiamusi and Songliao blocks at least since the early Permian, and the blueschist formed since the late Triassic to late Jurassic by the subduction of this ocean. Such an ocean during the Permian - Jurassic is difficult to be interpreted by the tectonic evolution of the Paleo-Asian Ocean.

  5. Geochronology and geochemistry of the Parashi granitoid, NE Colombia: Tectonic implication of short-lived Early Eocene plutonism along the SE Caribbean margin

    NASA Astrophysics Data System (ADS)

    Cardona, A.; Weber, M.; Valencia, V.; Bustamante, C.; Montes, C.; Cordani, U.; Muñoz, C. M.

    2014-03-01

    The Parashi granitoid of northeasternmost Colombia intrudes the Upper Cretaceous to Lower Paleocene accretionary complex formed by the collision of the Caribbean arc and the continental margin of South America. This granitoid presently separated of the continental margin includes a major quartzdiorite body with andesite to dacite dikes and mafic enclaves. Zircon U-Pb LA-MC-ICP-MS and K-Ar geochronology on the quartzdiorite and the dikes suggest that crystallization extended from ca. 47 to 51 Ma. Major and trace elements are characterized by a medium-K, immature continental arc signature and high Al2O3, Na2O and Ba-Sr contents. Initial 87Sr/86Sr isotopic values range between 0.7050 and 0.7054, with 143Nd/144Nd = 0.51235-0.51253, ɛNd and ɛHf values from -0.81 to -4.40 and -4.4 and -5.2. Major and trace element ratios and isotopic modeling suggest that sedimentary and/or quartzofeldspathic crustal sources were mixed with a mafic melt input. The petrotectonic and geological constraints derived from this granitoid suggest that Parashi plutonism records an immature, oblique subduction-zone setting in which the presence of a high-temperature mantle realm and strong plate coupling associated to upper crust subduction caused the partial fusion of a previously tectonically underplated mafic crust and associated metasediments exposed in the continental margin. The limited temporal expression of this magmatism and the transition to a regional magmatic hiatus are related to a subsequent change to strongly and slow oblique tectonics in the Caribbean-South America plate interactions and the underflow of a relatively thick slab of Caribbean oceanic crust.

  6. METHOD OF PROCESSING MONAZITE SAND

    DOEpatents

    Calkins, G.D.

    1957-10-29

    A method is given for the pretreatment of monazite sand with sodium hydroxide. When momazite sand is reacted with sodium hydroxide, the thorium, uranium, and rare earths are converted to water-insoluble hydrous oxides; but in the case of uranium, the precipitate compound may at least partly consist of a slightly soluble uranate. According to the patent, monazite sand is treated with an excess of aqueous sodium hydroxide solution, and the insoluble compounds of thorium, uranium, and the rare earths are separated from the aqueous solution. This solution is then concentrated causing sodium phosphate to crystallize out. The crystals are removed from the remaining solution, and the solution is recycled for reaction with a mew supply of momazite sand.

  7. U-Pb geochronology and geochemistry of Zahedan and Shah Kuh plutons, southeast Iran: Implication for closure of the South Sistan suture zone

    NASA Astrophysics Data System (ADS)

    Mohammadi, Ali; Burg, Jean-Pierre; Bouilhol, Pierre; Ruh, Jonas

    2016-04-01

    We present U-Pb zircon crystallization ages combined with major and trace element analyses, Sr-Nd isotopes and zircon in-situ Hf isotope analyses of intermediate to granitic intrusions along the southern segment of the N-S trending Sistan suture zone in eastern Iran. The Zahedan and Shah-Kuh Eocene plutons consist in a series of granite-granodiorite-rhyolite with U-Pb zircon ages of 40.5-44.3 Ma and ca. 28.9-30.9 Ma. Isotope geochemistry and modeling suggest that 40.5-44.3 Ma plutons represent melts derived from the turbidites of the surrounding accretionary wedge. Melting of the wedge was induced by the intrusion of mantle magmas, with subsequent interaction between mantle- and turbiditic melts being responsible for the wide range of compositions. Most of the 28.9-30.9 Ma magmas were generated from mantle melting, with assimilation of the surrounding turbidites. The rare setting of within-wedge intrusions is attributed to mantle upwelling reaching wedge sediments at the inception of delamination processes, which signal the end of subduction-related deformational and thermal events in the Sistan suture zone.

  8. The age of oceanic crust and of HP/LT-metamorphism on Syros (Cyclades, Greece) based on Lu-Hf geochronology and geochemistry

    NASA Astrophysics Data System (ADS)

    Lagos, M.; Münker, C.; Tomaschek, F.; Ballhaus, C.; Scherer, E. E.

    2003-04-01

    The ages of protolith formation and high pressure metamorphism of blueschist facies rocks in the Cyclades (Greece) are not completely constrained because they rely largely on U-Pb zircon geochronology plus Rb-Sr, and Ar-Ar cooling ages rather than direct dating of the high pressure assemblage. To adress this question further, we examined a meta-igneous sequence, which occurs in a dismembered meta-ophiolite belt in northern Syros. The sequence was subjected to blueschist-eclogite facies metamorphism at pressures of 16 kbar and temperatures up to 480^oC and forms part of the Cycladic Lower Unit. All examined samples were taken from one coherent mélange block at the Grizzas locality, comprising a complete compositional spectrum from mafic to felsic. Lu-Hf dating of WR-Grt-Omph in two samples yields ages between 47 and 51 Ma, overlapping with U-Pb-SHRIMP ages of ca. 52 Ma that were obtained from metamorphic zircons [1]. Such zircons are frequently found as inclusions in peak metamorphic garnet. Hence, both Lu-Hf and U-Pb ages most likely date garnet growth and peak metamorphism on Syros because the closure temperature of Lu-Hf in Grt is probably above 540^oC [2]. Such temperatures were not reached during HP-metamorphism on Syros. Major and trace element patterns indicate that the protoliths of the examined sequence are cogenetic and related by fractional crystallization. ɛHf(t) at 80 Ma range from +12.4 to +13.8, consistent with inferences from trace element patterns that suggest deposition of the Grizzas metavolcanics in a back-arc environment. The 176Hf/177Hf and 176Lu/177Hf of 11 whole rock samples define an errorchrone of 63±37 Ma (2σ). The errorchrone provides an upper limit of 100 Ma for the protolith age of the Grizzas sequence, implying that the oceanic crust on Syros was formed in the Upper Cretaceous. There is an older zircon generation (ca. 80 Ma) in Grizzas [1] that displays grossly different trace element characteristics than the 50 Ma old population

  9. Importance of LA-ICP-MS Zircon Geochronology and Geochemistry in Determining the History of Magmatic Systems: Insights from the Graciosa A-type Province, Southern Brazil

    NASA Astrophysics Data System (ADS)

    Braun, S.; Gualda, G. A.; Bream, B. R.; Vlach, S. R.

    2010-12-01

    textural and geochemical evidence, we interpret these 3 age populations as being representative of inheritance (>620 Ma), magmatism (~581 Ma), and hydrothermal alteration (~540 Ma). In light of these results, it is not surprising that previous age determinations using whole-zircon TIMS U/Pb and whole-rock Rb/Sr found a wide range of ages. Our study highlights the benefits of in-situ LA-ICPMS zircon geochronology as a method to deconvolve the evolution of igneous provinces, and shows that adequate precision (~0.5%) can be achieved in favorable circumstances. 1 GAR Gualda & SRF Vlach (2007) Anais Acad. Bras. Ciências, 79: 405-430

  10. Zircon U-Pb geochronology, geochemistry, and Sr-Nd-Hf isotopes of granitoids in the Yulekenhalasu copper ore district, northern Junggar, China: Petrogenesis and tectonic implications

    NASA Astrophysics Data System (ADS)

    Yang, Fuquan; Chai, Fengmei; Zhang, Zhixin; Geng, Xinxia; Li, Qiang

    2014-03-01

    The Yulekenhalasu porphyry copper deposit is located in the Kalaxiange'er metallogenic belt in northern Junggar, China. We present the results from zircon U-Pb geochronology, and geochemical and Sr-Nd-Hf isotope analyses of the granitoids associated with the ore deposits with a view to constrain their petrogenesis and tectonic setting. The granitoids consist of quartz diorite, diorite porphyry, porphyritic monzonite, and quartz porphyry, emplaced at 382, 379, 375-374, and 348 Ma, respectively, which span Late Devonian to early Carboniferous ages. The ore-bearing intrusion is mainly diorite porphyry, with subordinate porphyritic monzonite. The Late Devonian intrusions are characterized by SiO2 contents of 54.5-64.79 wt.%, Na2O contents of 3.82-8.24 wt.%, enrichment in Na, light rare-earth elements (LREEs), and large ion lithophile elements. They also display relative depletion in Y, Ba, P, Nb, Ta, and Ti, and weak negative Eu anomalies (δEu = 0.6-0.87). The early Carboniferous quartz porphyry is characterized by high SiO2 content (72.26-73.35 wt.%), enrichment in LREEs, K, and Sr, and relative depletion in Y (10.82-12.52 ppm) and Yb (1.06-1.15 ppm). The Late Devonian and early Carboniferous granitoids are characterized by positive ɛNd(t) values (5.2-10.1, one sample at - 1.9), positive ɛHf(t) values (7.46-18.45), low (87Sr/86Sr)i values (0.70363-0.70476), and young crustal residence ages. These data indicate that the sources of the granitoids were mainly mantle-derived juvenile rocks. Geochemical and Nd-Sr-Hf isotopic data demonstrate that the Late Devonian granitoids formed in an oceanic island arc, and they were formed from different sources, among which the mineralized diorite porphyry might have originated from a mixed slab-derived and mantle wedge melt source. The early Carboniferous quartz porphyry was likely emplaced in a mature island arc environment, and was probably derived from juvenile crust.

  11. U-Pb geochronology and geochemistry of Eocene and Oligocene plutons in southeast Iran: Implication for closure of the South Sistan Suture Zone

    NASA Astrophysics Data System (ADS)

    Burg, Jean-Pierre; Mohammadi, Ali; Ruh, Jonas; Bouilhol, Pierre

    2016-04-01

    The N-S trending Sistan Suture Zone in eastern Iran is a splay of the Tethys sutures. It represents an oceanic embayment that separated the Central Iran from the Afghan continental blocks. Structural, tectonic and petrological/geochemical evidence define eastward subduction beneath the Afghan continental block of the Sistan inlet of the Mesozoic Tethys Ocean. Mapping of the area allowed reaching and sampling intermediate to granitic intrusions stretched along the southern segment of this collisional suture zone. U-Pb zircon crystallization ages combined with major and trace element analyses, dated the series of granite-granodiorite-rhyolite at ca 40.5-44.3 Ma and ca 28.9-30.9 Ma. Isotopic geochemistry, including Sr-Nd isotopes and Hf isotope analyses, and petrological modelling suggest that the 40.5-44.3 Ma plutons crystallized from melts largely derived from the turbidites of the host accretionary wedge. Melting of the deep wedge was induced by the intrusion of mantle magmas interacting with the crustal turbiditic melts, which is responsible for the wide range of compositions. Most of the 28.9-30.9 Ma magmas were generated from mantle melting, with assimilation of the surrounding turbidites. The rare setting of within-wedge intrusions is attributed to mantle upwelling reaching wedge sediments at the inception of delamination processes, which sign the end of subduction-related deformational and thermal events in the Sistan Suture Zone. Numerical modelling of subduction - magma production - intrusion and melting of wedge sediments further constrains this collisional to post-collisional scenario.

  12. Geochronology and geochemistry of the Badaguan porphyry Cu-Mo deposit in Derbugan metallogenic belt of the NE China, and their geological significances

    NASA Astrophysics Data System (ADS)

    Gao, Bingyu; Zhang, Lianchang; Jin, Xindi; Li, Wenjun; Chen, Zhiguang; Zhu, Mingtian

    2016-03-01

    The Badaguan porphyry Cu-Mo deposit belongs to the Derbugan metallogenic belt, which is located in the Ergun block, NE China. In the mining area, the Cu-Mo mineralization mainly occurs in quartz diorite porphyry and is hosted within silicified-sericitized and sericite alteration zone. Geochemical results of the host porphyry is characterized by high SiO2, high Al2O3, low MgO, weak positive Eu anomalies and clearly HREE depletion, high Sr, low Y and low Yb, similar to those of adakite. The Sr-Nd isotopic composition of the host porphyry displays an initial (87Sr/86Sr)i ratio of 0.7036-0.7055 and positive Nd( t) values of +0.1 to +0.6, which are similar to the OIB, reflecting the source of the host porphyry may derive from subducted ocean slab, and the new lower crust also had some contribution to the magma sources. The SIMS zircon U-Pb age from the host porphyry is 229 ± 2 Ma. The Re-Os isochron age for the molybdenite in the deposit is 225 ± 2 Ma closed to zircon U-Pb age of the host porphyry, indicating that Cu-Mo mineralization event occurred in Triassic. Combining the geology-geochemistry of the host porphyry and the regional tectonic evolution, we infer that the subduction processes of Mongol-Okhotsk oceanic slab under the Ergun block led to the formation of the Badaguan porphyry Cu-Mo deposit during the Triassic.

  13. Geochronology, geochemistry and Sr-Nd-Hf isotopes of mafic dikes in the Huicheng Basin: Constraints on intracontinental extension of the Qinling orogen

    NASA Astrophysics Data System (ADS)

    Li, Wei; Dong, Yunpeng; Guo, Anlin; Liu, Xiaoming; Wang, Yuejun; Liu, Wenhang; Yang, Yuanzhen

    2015-05-01

    The diabase dikes intruded the Lower Cretaceous sandstone in the Huicheng Basin are key to understanding the Mesozoic tectonic evolution of the Qinling orogen. LA-ICP-MS zircon U-Pb dating yields a crystallization age of 107 ± 1 Ma for them. The dikes have low contents of SiO2 (42.46-50.16 wt.%), MgO (3.47-5.59 wt.%) with low Mg# of 49-59, and TiO2 (1.35-1.63 wt.%), high TFe2O3 (8.15-9.36 wt.%), Al2O3 (14.75-17.23 wt.%) and K2O (0.87-3.61 wt.%). Their Ni and Cr contents are in range of 16.8-111 and 45.7-315 ppm, respectively. They are significantly enriched in light rare earth elements and large ion lithophile elements (e.g., Cs, Pb and Ba), and depleted in Rb, K, P, and Ti. The dikes show relatively high whole-rock initial 87Sr/86Sr ratios (0.7071-0.7079) and negative εNd (t) values (-1.5 to -4.8) with single-stage Nd model ages of 941-1186 Ma. In addition, they have low zircon εHf (t) values (-8.6 to +3.5) with single-stage Hf model ages of 674-1117 Ma. Both elemental and isotopic geochemistry suggests that the magma of these dikes has undergone significant crystallization fractionation of olivine and pyroxene but weak crustal contamination during magma evolution. They were derived from partial melting of an enriched lithospheric mantle source. Together with regional geological data, these results suggest that the Qinling orogen experienced an intracontinental extension during the late Early Cretaceous.

  14. Detrital zircon geochemistry and U-Pb geochronology as an indicator of provenance of the Namakwa Sands heavy mineral deposit, west coast of South Africa

    NASA Astrophysics Data System (ADS)

    Philander, C.; Rozendaal, A.

    2015-10-01

    A representative suite of detrital zircons from the Cenozoic Namakwa Sands heavy mineral placer deposit, which is hosted by mainly unconsolidated clastic sediments of the West Coast Group, has been studied from a provenance perspective. The deposit is located along the west coast of South Africa and is underlain by a metamorphosed Meso- and Neoproterozoic basement. Mineral geochemistry of individual zircon grains indicated that the entire population is continentally derived and is dominated by zircons from a felsic magmatic source, some highly evolved, with minor contributions from a metamorphic and mafic provenance. The U-Pb zircon age distribution is diverse, but is dominated by three distinct age populations. The 1100-1020 Ma interval can be equated with the felsic meta-intrusives of the Namaquan Orogeny of the Namaqualand Metamorphic Province, and the 650-500 Ma period with the meta-volcanosedimentary Pan-African Gariep and Saldania Belts. A particularly dominant group with a narrow time interval (145-130 Ma) is correlated with the proximal intrusives of the anorogenic Koegel Fontein Complex. Equivalent source rocks of the above time intervals occur proximal to the Namakwa Sands deposit. As a result, it was demonstrated that the Namakwa Sands zircon population has mainly a proximal, primary provenance, whereas contributions attributed to secondary, reworked or distal sources proved minor. The high concentration and quality of zircons in this deposit compared to similar placers globally is attributed to the proximity of a primary, zircon-rich provenance to a geomorphologically controlled depositional trap. The results of the study suggest that the contribution of Pan-African (750-500 Ma) sources is a key prerequisite in the successful exploration for similar high-quality zircon placer deposits along the west coast of South Africa.

  15. Space and time evolution of volcanism within Basse-Terre Island (Guadeloupe, Lesser Antilles) based on new geochronology, geochemistry and geomorphology data

    NASA Astrophysics Data System (ADS)

    Ricci, J.; Quidelleur, X.; Lahitte, P.; Pallares, C.

    2013-12-01

    Capesterre volcanoes, display similar geochemical signatures and that their activity appears coeval from our new age dataset, we propose that a single volcanic massif was established there between 659 × 11 and 509 × 10 ka. Following a major erosional phase, having affected this massif, the Sans-Toucher volcano grew between 447 × 6 and 435 × 8 ka in the central part of the southern Axial Chain. Finally, a catastrophic event affecting the southeast flank favored the establishment of the Grande-Decouverte volcanic complex where the younger volcanic activity lies. Having now a more accurate framework of the south Basse-Terre Island volcanic evolution, we can infer the successive geomorphological reconstructions of paleo-surfaces for the different volcanic massifs. Combined with the new geochronological dataset, these numerical reconstructions will allow us to compute the volume of each edifice in order to calculate their erosion rates throughout the last million year at different time and space scales.

  16. Provenance and paleogeography of the Late Cretaceous Mengyejing Formation, Simao Basin, southeastern Tibetan Plateau: Whole-rock geochemistry, U-Pb geochronology, and Hf isotopic constraints

    NASA Astrophysics Data System (ADS)

    Wang, Licheng; Liu, Chenglin; Gao, Xiang; Zhang, Hua

    2014-05-01

    The Late Cretaceous Mengyejing Formation, which contains the only pre-Quaternary potash salt deposit in the Simao Basin, southeastern Tibet, is thought to be genetically related to the Maha Sarakham Formation in the Khorat Basin. The provenance and paleogeography of these two basins have been under debate, although little diagnostic evidence has been previously published. A combined analysis of whole rock geochemistry, zircon U-Pb chronology, and Hf isotopic compositions was performed to characterize the provenance of the Mengyejing Formation. These formation's sandstones are characterized by moderate chemical index of alteration (CIA) values. These values, together with plots of the Th/U-Th ratios, suggest that certain samples have undergone moderate weathering and sedimentary recycling. The major and trace elements (La/Th-Hf, Th/Sc-Zr/Sc, Eu/Eu*-Th/Sc, TiO2-Fe2O3T + MgO, Al2O3/SiO2-Fe2O3T + MgO, K2O/Na2O-SiO2) indicate that the sedimentary sources were felsic rocks from an active continental margin or continental arc with a minor amount of recycled sediment from a passive continental margin. The Mengyejing Formation contains detrital zircons primarily with U-Pb ages of 2.45-2.57 Ga, 1.8-1.9 Ga, 740-880 Ma, 410-470 Ma, and 215-300 Ma. The results reveal that the pre-Devonian zircons are derived from the recycled sediments of the Yangtze block originating in the Qinling Orogenic Belt, and they share this provenance with the coeval sediments in the Khorat Basin. The magmatic rocks of the Ailaoshan and Lincang areas are responsible for supplying the Devonian to Triassic detrital zircons. These provenance data combined with published paleocurrent results suggest that the Simao Basin was situated on the western margin of the Khorat Basin during the Late Cretaceous. The basins were connected when marine incursion occurred. We propose that pre-Devonian materials from the southwestern Sichuan Basin first supplied detritus to the Simao Basin and subsequently to the Khorat

  17. Geochemistry and U-Pb zircon geochronology of the pegmatites in Ede area, southwestern Nigeria: A newly discovered oldest Pan African rock in southwestern Nigeria

    NASA Astrophysics Data System (ADS)

    Adetunji, A.; Olarewaju, V. O.; Ocan, O. O.; Ganev, V. Y.; Macheva, L.

    2016-03-01

    Field and petrographic studies, whole rock geochemistry and in-situ LA-ICP-MS geochemical and isotopic U-Pb measurements on zircons have been performed on granitic pegmatites of Ede area, southwestern Nigeria with a view to characterize them, determining their mineralization potentials, petrogenetic attributes and emplacement age. The pegmatites are hosted by migmatite gneiss complex, biotite-muscovite schist and associated quartzite. The textural and mineralogical characteristics of these pegmatites indicate the occurrence of two main varieties, namely, muscovite pegmatite and garnet pegmatite. Of less importance are inclusions and pods of graphic granite, quartz-microcline aplitic and pegmatitic bodies. At the present level of erosion, the parent igneous rocks of the pegmatites are not exposed. The two dominant pegmatite varieties show slightly different chemical peculiarities but similar peraluminous character. The average K/Rb ratios of 165 and 163, respectively, for muscovite and garnet pegmatites combined with other trace element compositions are indicative of affinity to muscovite class of pegmatite which are generally not promising for rare elements mineralization. However, the unusually high concentration of bismuth in the zircons indicates Bi mineralization in the area which can either be in the pegmatites or host rocks. The Nb/Ta ratios for both muscovite and garnet pegmatites range from 0.7 to 15.2 and 1.0 to 14.8, respectively. These Nb/Ta ratios and Eu anomalies are statistically similar for both pegmatites. These probably indicate the pegmatites crystallized from a common source but separated into crystallization paths that produced different pegmatite varieties through liquid-liquid immiscibity mechanism. In-situ measurements of REE, P, Y, Nb, Hf, Ta, Bi, Th and U of individual zircon grains show the existence of two chemically and texturally different domains which are indicative of alteration that may be due to interface-coupled dissolution

  18. U-Pb zircon geochronology, Sr-Nd geochemistry, petrogenesis and tectonic setting of Mahoor granitoid rocks (Lut Block, Eastern Iran)

    NASA Astrophysics Data System (ADS)

    Beydokhti, Roohollah Miri; Karimpour, Mohammad Hassan; Mazaheri, Seyed Ahmad; Santos, José Francisco; Klötzli, Urs

    2015-11-01

    The Mahoor Cu-Zn-bearing porphyritic granitoid rocks belong to the Lut Block volcanic-plutonic belt (central Eastern Iran). These granitoid rocks occur mainly as dykes and stocks that intrude into Eocene volcanics and pyroclastic rocks. Petrographically, all the studied intrusives display porphyritic textures with mm-sized phenocrysts, most commonly of plagioclase and hornblende, embedded in a fine-grained groundmass with variable amounts of plagioclase, hornblende, clinopyroxene, quartz and opaque minerals. Hydrothermal alteration affected these granitoid rocks, as revealed by the common occurrence of sericite, chlorite, titanite, epidote and calcite. Chemical classification criteria show that the intrusives may be named as gabbrodiorites, diorites, monzodiorites and tonalites. Major elements geochemistry reveals that all the studied lithologies are typically metaluminous (A/CNK ⩽ 0.9). Magnetic susceptibility (1485 × 10-5 SI) together with mineralogical and geochemical features shows that they belong to magnetite granitoid series (I-type). Trace element patterns normalized to chondrite and primitive mantle are very similar to each other and show enrichments in LREE relative to HREE and in LILE relative to HFSE, as well as negative anomalies of Ta, Nb and Ti. Eu/Eu∗ ratios vary from 0.88 (in the most mafic composition) to 0.65, showing that plagioclase played a role in magma differentiation. LA-MC-ICP-MS U-Pb zircon data from a diorite, yielded similar concordia ages of ca. 31.88 ± 0.2 Ma (Error: 2σ), which corresponds to the Oligocene period. These granitoid rocks have (87Sr/86Sr)i values vary between 0.7055 and 0.7063. In terms of isotopic compositions, while εNdi is between -0.6 and -2.5, suggesting that magmas underwent contamination through being exposed to the continental crust. The whole set of geochemical data agree with the emplacement of the studied intrusions in a magmatic belt above a subduction zone. Primitive magmas should have formed by

  19. Zircon geochronology and geochemistry of Dorud-Azna basement and gabbroic rocks, NW Iran: evidence of Panafrican basement and oceanic units in the Zagros orogenic belt

    NASA Astrophysics Data System (ADS)

    Shakerardakani, Farzaneh; Neubauer, Franz; Masoudi, Fariborz; Mehrabi, Behzad; Liu, Xiaomong; Dong, Yunpeng; Mohajjel, Mohammad; Monfaredi, Behzad; Friedl, Gertrude

    2014-05-01

    The Dorud-Azna basement, exposed in the Sanandaj-Sirjan metamorphic zone (SSMZ) in western Iran, is composed of both continental (granitic Galeh-Doz orthogneiss) and oceanic units (various amphibolites and metagabbros) that experienced a complex history of deformation and polyphase greenschist- to amphibolite-facies metamorphism. These units are intruded by the Darijune gabbro. The Sanandaj-Sirjan zone is part of Zagros orogenic belt. The SSMZ formed during continental collision between the African-Arabian continent and the Iranian microcontinent. A combined study of laser-ablation ICP-MS U-Pb zircon ages from granitic orthogneiss and gabbroic rocks and their geochemical characteristics as well as the geochemistry of various amphibolites and metagabbros of the Dorud-Azna basement provides new constraints on the crustal evolution of this part of SSMZ. The U-Pb zircon ages of 608±18 Ma and 588±41 Ma of the granitic Galeh-Doz orthogneiss indicate the presence of a Panafrican basement with Gondwanan affiliation in the central part of the SSMZ close to the Main Zagros thrust, which is similar to Central Iran and the Alborz Mountains. We conclude that this Panafrican basement in the Zagros belt is part of Iranian microcontinent, which was subducted and subsequently exhumed following subduction of the Paleotethys Ocean. Geochemically, undated amphibolites and metagabbros show sub-alkaline to alkaline basaltic compositions of tholeiitic affinity. These metamorphic rocks originated from a MORB mantle source and their age must be older than a first stage of metamorphism in the SSMZ at around 180 Ma. Therefore, these undated amphibolites and metagabbros are interpreted as the oceanic basement and to have its origin likely in an Upper Paleozoic to Mesozoic N-MORB ophiolite. Finally, in the eastern part of the Dorud-Azna region, the post-metamorphic Darijune gabbro emplaced within the metamorphic complex testified by a U-Pb zircon mean age of 170.2 ± 3.1 Ma. The

  20. Paleomagnetism and Monazite Dating of Grenville Rocks, Adirondack Mountains, NY

    NASA Astrophysics Data System (ADS)

    Brown, L.; McEnroe, S.; Jercinovic, M.

    2003-12-01

    Paleomagnetic studies on three rock units from the Adirondack Highlands, New York State yield stable magnetic directions. Electron microprobe monazite geochronology suggests a strong ca. 1050 Ma signature, corresponding to Ottawan granulite-facies metamorphism. Remnants of older (ca. 1130-1190 Ma) monazite, consistent with early-Grenville tectonomagmatic events are also documented. There is no evidence of younger (<1050 Ma) events with the exception of partial alteration (with Ca-enrichment) of some monazite. Sillimanite-microcline gneisses (gms) of the far-western Highlands, associated with negative aeromagnetic anomalies, exhibit strong stable magnetization dominated by titanohematite with abundant exsolutions of ilmenite, pyrophanite, rutile and spinel. Mean magnetic directions for 14 sites are I-62.8, D=289.2 and a-95=7.6. Sampled in the central Highlands is the post-orogenic fayalite ferro-hedenbergite Wanakena Granite. Samples contain magnetite with ilmenite oxy-exsolution, occurring as discrete grains and inclusions in silicates. Directions from the Wanakena are steeply negative with westerly declinations (I=-76.4, D=296.7, a-95=4.4, N=7). The Marcy meta-anorthosite was sampled in the central and eastern Highlands, although many of these sites proved unstable. Stable results were combined with unpublished data from Rob Hargraves for 13 sites (I=-64.4, D=286.2, a-95=9.1). Over half of the anorthosites and one gms site have normal directions; all Wanakena sites are reversed. Combined anorthosites and gms units give a pole position of 20S/151E; the Wanakena pole is at -29S/132E. Both poles fall in the southern extent of the Grenville loop. The thermodynamically constrained equilibrium phase diagram for ilm-hem predicts that very fine exsolution, most likely responsible for the stable magnetization of the gms rocks, starts to form around 390C, well below the conditions of granulite grade metamorphism. The abundant lamellae provide a stable NRM through the

  1. Geochronology, geochemistry, and Sr-Nd-Hf isotopes of the early Paleozoic igneous rocks in the Duobaoshan area, NE China, and their geological significance

    NASA Astrophysics Data System (ADS)

    Wu, Guang; Chen, Yuchuan; Sun, Fengyue; Liu, Jun; Wang, Guorui; Xu, Bei

    2015-01-01

    The Duobaoshan area of northwestern Heilongjiang Province is the most important copper resource concentration region in NE China. To date, the Duobaoshan superlarge Cu-Mo deposit and the Tongshan large Cu-Mo deposit have been discovered in the Duobaoshan area. Both the deposits are hosted by granodiorites and volcanic rocks. Zircon LA-ICP-MS U-Pb dating indicates that these granodiorites emplaced approximately 479 Ma ago and that those volcanic rocks erupted between 447 and 450 Ma. The early Ordovicain granodiorites belong to the high-K to medium-K calc-alkaline series and are characterized by high Al2O3 and Sr contents, low Yb and Y contents, and relatively low Mg# values and Na2O/K2O ratios, with positive Eu or slight negative Eu anomalies (averaging 1.18). All of these geochemical characters are similar to those of the adakites generated by partial melting of a thickened lower crust in the world. Moreover, the granodiorites have low initial 87Sr/86Sr ratios (varying from 0.703474 to 0.704436), very high zircon εHf(t) and whole-rock εNd(t) values (varying from 13.0 to 16.8 and 5.27 to 5.46, respectively), and young zircon Hf and whole-rock Nd single-stage and two-stage model ages. Taking these geochemical characteristics and Sr-Nd-Hf isotope compositions together, we suggest that the early Ordovician granodiorites in the Duobaoshan area occurred in a post-collision environment and were formed by partial melting of a juvenile thickened lower crust dominated by depleted mantle-derived material. These late Ordovician volcanic rocks, which are composed of basalt, basaltic andesite, and andesite, belong to the tholeiitic or calc-alkaline series. They are generally enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, Zr, Hf, P, and Ti), consistent with the geochemistry of igneous rocks from island arcs or active continental margins. Compared with the early Ordovician granodiorites, these volcanic rocks

  2. Provenance and tectonic setting of Proterozoic metasedimentary sequences of the São Roque Domain, Ribeira Fold Belt, Brazil: a combination of whole-rock geochemistry, Sm-Nd isotopic systematics and detrital zircon U-Pb geochronology

    NASA Astrophysics Data System (ADS)

    Henrique Pinto, Renato; Janasi, Valdecir de A.; Barnes, Sarah-Jane; Borges Carvalho, Bruna; Tassinari, Colombo C. G.; Basei, Miguel A. S.

    2014-05-01

    The Proterozoic São Roque Group (Ribeira Fold Belt, southeast Brazil) is a metasedimentary sequence deposited in a marine environment consisting of proximal metasandstones and meta-felspathic wackes with some volcanic layers (Boturuna Formation) and more distal metawackes and metamudstones (Piragibu Formation). A combination of zircon provenance studies in metasandstones (textural and trace-element analysis and U-Pb geochronology) and whole-rock major and trace-element geochemistry and Sm-Nd isotopic systematics in metamudstones was used to understand the provenance and tectonic significance of this sequence, and their implications to the evolution of the Precambrian crust in the region. Whole-rock geochemistry indicates that the sources are largely granitic (as indicated for instance by the LREE-rich moderately fractionated REE patterns and subtle negative Eu anomalies) with some mafic contribution (responsible for higher contents of Fe2O3, MgO, V, and Cr) and were subject to moderate weathering (CIA - 60 to 82). The trace element signatures in detrital zircons indicate that most of them are derived from plagioclase-rich felsic rocks, as indicated by strong positive Ce anomalies, high (Lu/Sm)N ratios, low U/Yb, and a concave-down shape of the intermediate REE in chondrite-normalized plots. A significant proportion of the zircon crystals show rounded cores with growth zoning truncated and overgrown by a thin rim that has dark color in cathodoluminescence images. These overgrowths are chemically distinct, being enriched in trace elements, especially the LREE, and were dated at 584±47 Ma, reflecting the regional Neoproterozoic metamorphism. Sm-Nd isotope data for Piragibu Formation metamudstones show four main groups of Nd TDM ages at ca. 1.9 Ga, 2.1 Ga, 2.4 Ga and 3.0 Ga. The younger ages define an upper limit for the deposition of the unit, and reflect greater contributions from sources younger than the >2.1 Ga basement. The oldest Nd TDM age (3.0 Ga) is similar

  3. The role of electron microprobe mapping and dating in tectonic geochronology

    NASA Astrophysics Data System (ADS)

    Williams, M. L.; Jercinovic, M. J.; Dumond, G.; Mahan, K. H.; Flowers, R. M.

    2007-12-01

    Electron microprobe geochronology occupies a special niche within the spectrum of geochronological techniques and may be particularly relevant to the question, "What are we dating?" The technique was originally envisioned to be a low-cost, reconnaissance dating tool, opening low-resolution geochronology to a large number of researchers. However, more than a decade of research has shown that, when used in a reconnaissance fashion (i.e. using major-element analytical techniques for trace-element analysis) uncertainties are unsuitably large (several 10s of m.y. or more) for solving most tectonic problems. Using trace element analytical techniques (background modeling, interference correction, highly conductive coating, multi-analysis measurement, etc.) precision and accuracy are dramatically increased, but analysis time and cost are also increased, challenging the "quick, cheap, and easy" description. The power of microprobe geochronology comes from the spatial resolution and the natural integration with compositional data. High-resolution compositional mapping is valuable for all in-situ geochronology. Large area maps provide petrologic and textural context for chronometer phases; small scale maps illuminate the history of the chronometers themselves. Compositional maps associated with monazite are particularly informative, but examples from the East Athabasca granulite terrane using zircon, titanite, and rutile will be discussed. Most monazite crystals are 30μ or less and most have several compositional domains. Rim compositions and dates are particularly critical because they can commonly be tied to reactions and to matrix texture and fabric. Commonly, rims and internal sub domains are several microns in width and can only be analyzed by electron probe. Y has been widely used to tie monazite to Grt growth or breakdown, but current studies use a suite of trace and REE (Y, Sm, Nd, Ca, Eu, Gd, etc) to tie monazite into chemical reactions. A rapidly growing

  4. Interpretation of U-Th-Pb in-situ ages of hydrothermal monazite-(Ce) and xenotime-(Y): evidence from a large-scale regional study in clefts from the western alps

    NASA Astrophysics Data System (ADS)

    Grand'Homme, A.; Janots, E.; Bosse, V.; Seydoux-Guillaume, A. M.; De Ascenção Guedes, R.

    2016-06-01

    In eleven Alpine clefts of the western Alps, in-situ dating of monazite-(Ce) and xenotime-(Y) has been attempted to gain insights on possible disturbances of the geochronological U-Th-Pb systems and age interpretations in hydrothermal conditions. In most clefts, monazite-(Ce) in-situ 208Pb/232Th dating using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) yields well-resolved ages (with errors typically <0.3 Ma, quoted at the 2σ level), indicative of a short duration monazite-(Ce) growth. However, monazite-(Ce) dating demonstrates two successive episodes of growth separated by several million years within two single clefts. Only in one cleft, complex age mixture in a porous and complex zoned monazite-(Ce) suggests disturbance of the 208Pb/232Th ages due to replacement by dissolution-precipitation processes. While some U-Pb ages are coherent with the 208Pb/232Th ages, U-Pb ages are generally disturbed by significant 206Pb excess in monazite-(Ce) with high Th/U ratio (>100). Xenotime-(Y) has remarkably high Th/U ratios and U-Pb dating is also disturbed by 206Pb excess, whereas 208Pb/232Th dating gave well-resolved ages (34.9 ± 0.5 Ma), close to but higher than the monazite-(Ce) age obtained in the same cleft (32.3 ± 0.3 Ma). Correlation of the monazite-(Ce) U-Th-Pb age dataset with other geochronological data suggests for monazite-(Ce) precipitation at periods of high tectonic activity. In the external massifs, monazite-(Ce) dating confirms a polyphased transpressive regime with activity periods around 13-11 Ma and 8-6 Ma. Older monazite-(Ce) ages in the Argentera massif (20.6 ± 0.3 Ma) are consistent with the regional diachronism in the western external Alps. In the 2 clefts of the internal massifs, monazite-(Ce) dating provides first ages of hydrothermal activity: the monazite-(Ce) age at 32.3 ± 0.3 Ma coincides with the exhumation along the Penninic front, but the monazite-(Ce) age at 23.3 ± 0.2 Ma is complex to attribute to a

  5. Geochronology and Nd isotope geochemistry of the Gameleira Cu-Au deposit, Serra dos Carajás, Brazil: 1.8-1.7 Ga hydrothermal alteration and mineralization

    NASA Astrophysics Data System (ADS)

    Pimentel, Márcio M.; Lindenmayer, Zara G.; Laux, Jorge H.; Armstrong, Richard; de Araújo, Janice Caldas

    2003-01-01

    . The geochronological data suggest that the Gameleira Cu-Au mineralization is related to a Paleoproterozoic (ca. 1.83 Ga) episode of hydrothermal activity and is not Archaean. The younger ages of ca. 1.70-1.73 might be interpreted as products of the lower blocking temperatures of biotite in relation to the Ar-Ar and Sm-Nd isotopic systems. Combined with previous geochemistry and stable isotope data, the Nd isotopic data suggest that the mineralizing fluids were derived from, or strongly interacted with, a Paleoproterozoic crustal granite, possibly similar in age and composition to the Pojuca granite.

  6. EBSD Imaging of Monazite: a Petrochronological Tool?

    NASA Astrophysics Data System (ADS)

    Mottram, C. M.; Cottle, J. M.

    2014-12-01

    Recent advances in in-situ U-Th/Pb monazite petrochronology allow ages obtained from micron-scale portions of texturally-constrained, individual crystals to be placed directly into a quantitative Pressure-Temperature framework. However, there remain major unresolved challenges in linking monazite ages to specific deformation events and discerning the effects of deformation on the isotopic and elemental tracers in these phases. Few studies have quantitatively investigated monazite microstructure, and these studies have largely focused only on crystals produced experimentally (e.g. Reddy et al., 2010). The dispersion in age data commonly yielded from monazite U-Th/Pb datasets suggest that monazite dynamically recrystallises during deformation. It remains unclear how this continual recrystallisation is reflected in the monazite crystal structure, and how this subsequently impacts the ages (or age ranges) yielded from single crystals. Here, combined laser ablation split-stream analysis of deformed monazite, EBSD imaging and Pressure-Temperature (P-T) phase equilibria modelling is used to quantify the influence of deformation on monazite (re)crystallisation mechanisms and its subsequent effect on the crystallographic structure, ages and trace-element distribution in individual grains. These data provide links between ages and specific deformation events, thus helping further our understanding of the role of dynamic recrystallisation in producing age variation within and between crystals in a deformed rock. These data provide a new dimension to the field of petrochronology, demonstrating the importance of fully integrating the Pressure-Temperature-time-deformation history of accessory phases to better interpret the meaningfulness of ages yielded from deformed rocks. Reddy, S. et al., 2010. Mineralogical Magazine 74: 493-506

  7. Structural interpretation of the Steenkampskraal monazite deposit, Western Cape, South Africa

    NASA Astrophysics Data System (ADS)

    Basson, I. J.; Muntingh, J. A.; Jellicoe, B. C.; Anthonissen, C. J.

    2016-09-01

    The Steenkampskraal Monazite Mine was first established in 1952, to extract monazite ore for the production of thorium and rare earth element (REE) concentrate. Refurbishment of the mine in recent years has required the re-inspection and re-evaluation of the mineralized monazite zone (MMZ). This contribution presents a structural review of the MMZ and its emplacement, based on recent data and its setting at the southern extent of the Bushmanland Sub-province of the Namaqua-Natal Metamorphic Belt. New surface and underground mapping confirm that the MMZ is a moderately-dipping body within gneissic host rocks on the southern limb of a broad F3 antiform. Thickness variations, both down-dip and along-strike, are the result of D2 and D3 deformation. The MMZ has been locally transected and steepened by subsequent late-D3, "steep-structures", which are typical of the Okiep copper district, ∼150 km north of Steenkampskraal. Geochronological data suggest that the MMZ was intruded, emplaced or formed at 1046 ± 7.5 Ma, at the start of the D3 Klondikean Episode (1040-1020 Ma). Unlike the analogous copper-bearing Koperberg Suite in the Okiep Copper district, the MMZ was not intruded into Klondikean-aged steep structures, but was rather transected and steepened by these. Local steepening of the otherwise moderately-dipping to flat-lying MMZ makes it locally amenable to detection by soil sampling and radiometric surveys.

  8. Using multi-scale structural and petrological analysis coupled with zircon and monazite SIMS and in-situ EPMA geochronology to document the evolution of a mid-crustal transpression system: a case study from the Northern Appalachians, U.S.A.

    NASA Astrophysics Data System (ADS)

    Massey, M. A.; Moecher, D. P.; McCulla, J. K.; Draper, K. P. J.; Young, J.; Rohrer, L.; Walker, T. B.; O'Brien, T.

    2015-12-01

    Three-dimensional transpressional strain is commonly associated with zones of oblique convergence, rather than ideal 2D simple shear or pure shear. Consequently, a considerable body of modeling has been aimed at understanding the progressive evolution of transpression, which has been used to explain an assortment of structures observed in natural settings. The basic tenants of most models involve simultaneous strike-slip and shortening, which provide the underlying mechanism for a constantly evolving finite strain geometry and magnitude. Despite the obvious temporal-dependence, very few studies have evaluated timescales of transpression. In the Northern Appalachians, the Bronson Hill arc and Central Maine basin of southern New England largely reflect highly oblique dextral transpression. Fabrics were initially characterized by strong foliations, subhorizontal lineations, and dextral kinematics, all of which are present in 360-354 Ma tonalite, diorite, and granite intrusions, the youngest placing a maximum age on transpression. As strains accumulated, fabrics began to reflect the increasing manipulation of the shortening component, marked by tightening of foliations, closed to isoclinal folding, and reverse high strain zones; stretching lineations changed in orientation to steeply plunging parallel to dip, while older pre-existing subhorizontal lineations were rotated. Syntectonic monazite and metamorphic zircon nucleated episodically throughout this time. Y-enriched monazite nucleated at 330 Ma along with fabric-forming biotite and sillimanite, and place a minimum age on the development of dip-parallel lineations. Mineral assemblages and associated ages document retrograde cooling attending deformation from partial melting at 355-350 Ma, to sillimanite grade at 330 Ma, below the Ar closure temperature for amphibole of 500°C at 326-314 Ma, and into biotite grade deformation as young as 295 Ma. Collectively, structures, fabrics, mineral assemblages, and

  9. Geochemistry, geochronology, mineralogy, and geology suggest sources of and controls on mineral systems in the southern Toquima Range, Nye County, Nevada; with geochemistry maps of gold, silver, mercury, arsenic, antimony, zinc, copper, lead, molybdenum, bismuth, iron, titanium, vanadium, cobalt, beryllium, boron, fluorine, and sulfur; and with a section on lead associations, mineralogy and paragenesis, and isotopes

    USGS Publications Warehouse

    Shawe, Daniel R.; Hoffman, James D.; Doe, Bruce R.; Foord, Eugene E.; Stein, Holly J.; Ayuso, Robert A.

    2003-01-01

    Geochemistry maps showing the distribution and abundance of 18 elements in about 1,400 rock samples, both mineralized and unmineralized, from the southern Toquima Range, Nev., indicate major structural and lithologic controls on mineralization, and suggest sources of the elements. Radiometric age data, lead mineralogy and paragenesis data, and lead-isotope data supplement the geochemical and geologic data, providing further insight into timing, sources, and controls on mineralization. Major zones of mineralization are centered on structural margins of calderas and principal northwest-striking fault zones, as at Round Mountain, Manhattan, and Jefferson mining districts, and on intersections of low-angle and steep structures, as at Belmont mining district. Paleozoic sedimentary rocks, mostly limestones (at Manhattan, Jefferson, and Belmont districts), and porous Oligocene ash-flow tuffs (at Round Mountain district) host the major deposits, although all rock types have been mineralized as evidenced by numerous prospects throughout the area. Principal mineral systems are gold-silver at Round Mountain where about 7 million ounces of gold and more than 4 million ounces of silver has been produced; gold at Gold Hill in the west part of the Manhattan district where about a half million ounces of gold has been produced; gold-mercury-arsenic-antimony in the east (White Caps) part of the Manhattan district where a few hundred thousand ounces of gold has been produced; and silver-lead-antimony at Belmont where more than 150,000 ounces of silver has been produced. Lesser amounts of gold and silver have been produced from the Jefferson district and from scattered mines elsewhere in the southern Toquima Range. A small amount of tungsten was produced from mines in the granite of the Round Mountain pluton exposed east of Round Mountain, and small amounts of arsenic, antimony, and mercury have been produced elsewhere in the southern Toquima Range. All elements show unique

  10. Radiation damage in zircon and monazite

    SciTech Connect

    Meldrum, A.; Boatner, L.A.; Weber, W.J.; Ewing, R.C.

    1998-07-01

    Monazite and zircon respond differently to ion irradiation and to thermal and irradiation-enhanced annealing. The damage process (i.e., elastic interactions leading to amorphization) in radioactive minerals (metamictization) is basically the same as for the ion-beam-irradiated samples with the exception of the dose rate which is much lower in the case of natural samples. The crystalline-to-metamict transition in natural samples with different degrees of damage, from almost fully crystalline to completely metamict, is compared to the sequence of microstructures observed for ion-beam-irradiated monazite and zircon. The damage accumulation process, representing the competing effects of radiation-induced structural disorder and subsequent annealing mechanisms (irradiation-enhanced and thermal) occurs at much higher temperatures for zircon than for monazite. The amorphization dose, expressed as displacements per atom, is considerably higher in the natural samples, and the atomic-scale process leading to metamictization appears to develop differently. Ion-beam-induced amorphization data were used to calculate the {alpha}-decay-event dose required for amorphization in terms of a critical radionuclide concentration, i.e., the concentration above which a sample of a given age will become metamict at a specific temperature. This equation was applied to estimate the reliability of U-Pb ages, to provide a qualitative estimate of the thermal history of high-U natural zircons, and to predict whether actinide-bearing zircon or monazite nuclear waste forms will become amorphous (metamict) over long timescales.

  11. Actinide valences in xenotime and monazite

    NASA Astrophysics Data System (ADS)

    Vance, E. R.; Zhang, Y.; McLeod, T.; Davis, J.

    2011-02-01

    Tetravalent U, Np and Pu can be substituted by ceramic methods into the rare earth site of xenotime and monazite in air atmospheres using Ca ions as charge compensators, while no evidence of penta- or hexavalent actinide ions was found. Some Pu 3+ and Np 3+ can be incorporated in xenotime samples fired in a reducing atmosphere.

  12. Analyses and economic potential of monazite in Liberia

    USGS Publications Warehouse

    Rosenblum, Sam

    1974-01-01

    Eleven monazite samples from Liberia, including seven from beach sands, were analyzed by the X-ray fluorescence method. The monazite samples, containing only one-half percent impurities, were obtained by use of a hot Clerici-solution procedure for purification which was devised by the author. The percentage of the rare-earth elements in Liberian monazite concentrates does not differ greatly from that of monazite sands elsewhere in the world. The average of the 11 samples shows less praseodymium and neodymium than Russian and American monazites, but more cerium. Liberian coastal sands apparently contain sufficient reserves of monazite (and other heavy minerals of commerce) to encourage mining. A feasibility study of production and marketing of monazite from Liberian beach sands is recommended.

  13. Isotopic geochemistry and cosmochemistry

    NASA Astrophysics Data System (ADS)

    Shchukoliukov, Iu. A.

    The book includes recent information on isotope geology, geochemistry, and cosmochemistry, discussed at a recent Soviet-Japanese symposium (at Irkutsk, USSR). Attention is given to numerical modeling of geochronometric systems, a classification of noble-gas components in the earth's interior, the feasibility of using ion microprobe for local isotope analysis of zircons for the purpose of deriving the early history of the earth (on the example of the Novopavlovsk complex from the Ukranian shield), a geological and geochronological study of the Ganalski complex of Kamchatka, and strontium isotopes as a criterion of the nature of acid melts (i.e., mantle- or crust-related). Other papers are on the geochronology and geology of Siberian kimberlites, the nature of sulfur from effusive rocks of the Kamchatka-Kuril-Japan island arc, mass-spectrometric studies of volatile components in exocontact rocks of alkaline-basic intrusions, and an analytical method for stable-isotope analysis in ultrasmall amounts of CO2 and its application to studies of the microscale isotopic zoning in calcite and graphite crystals in marble.

  14. Dithizone method for determination of lead in monazite

    USGS Publications Warehouse

    Powell, R.A.; Kinser, C.A.

    1958-01-01

    In the determination of lead in monazite-to be used as the basis for geologic age measurements-it was necessary to eliminate interferences due to the presence of phosphates of thorium and the rare earth metals. The method involves attacking the monazite samples with hot, concentrated sulfuric acid, then taking them up with dilute nitric acid. Lead is extracted as the dithizonate and determined spectrophotometrically at 520 m??. Rapid determinations were made with good reproducibility on a series of monazite samples.

  15. Eocene Granitic Magmatism in NW Anatolia (Turkey) revisited: New implications from comparative zircon SHRIMP U-Pb and 40Ar-39Ar geochronology and isotope geochemistry on magma genesis and emplacement

    NASA Astrophysics Data System (ADS)

    Altunkaynak, Şafak; Sunal, Gürsel; Aldanmaz, Ercan; Genç, Can Ş.; Dilek, Yıldırım; Furnes, Harald; Foland, Kenneth A.; Yang, Jingsui; Yıldız, Merve

    2012-12-01

    During the Eocene, the northern Neo-Tethys suture zone and immediately adjacent areas in NW Turkey were loci of emplacement of a number of granitoid plutons with age relations and geochemical signatures indicative of magma generation in association with plate convergence. Discrete granitoid plutons of Eocene age in this region are exposed mainly in two distinct belts within and north of the northern Neo-Tethys suture zone. We report new SHRIMP U-Pb zircon and 40Ar/39Ar geochronology along with Sr-Nd isotope data from the granitoids in order to constrain their melt source(s), cooling history, and tectono-magmatic evolution with respect to the Early Cenozoic collisional tectonics of the region. Ranging in composition from diorite to granite, the plutons are largely represented by medium- to high-K calc-alkaline, I-type granites accompanied by minor amounts of shoshonitic rocks with syenite compositions and mafic microgranular enclaves in some of the plutons. In terms of trace element systematics the granitoids and the associated enclaves from the entire Eocene suite display close similarities to magmas from subduction-related or active continental margin settings, characterized by significant enrichment in LILE/HFSE relative to MORB. The rocks display significantly heterogeneous distributions of radiogenic isotopes with 87Sr/86Sr and 143Nd/144Nd ranging from 0.705824 to 0.708363 and from 0.512384 to 0.512718 respectively, suggesting multi-component melt interaction in their genesis. The granitoids from the southern and northern belts yielded zircon SHRIMP U-Pb ages of 52.8 ± 1.8 to 45.41 ± 0.34 Ma and 47.02 ± 0.82 to 36.79 ± 0.67 Ma, respectively, indicating overlapping emplacement ages, with some relatively younger ages from the northern belt. 40Ar/39Ar dating of biotite and hornblende also yielded similarly overlapping cooling ages (51.8 ± 0.1 to 44.9 ± 0.2 Ma and 45.3 ± 0.1 to 36.0 ± 0.1 Ma, respectively). Our combined geochronological data from the co

  16. Constraining metamorphic rates through allanite and monazite petrochronology: a case study from the Miyar Valley (High Himalayan Crystalline of Zanskar, NW India)

    NASA Astrophysics Data System (ADS)

    Robyr, Martin; Goswami-Banerjee, Sriparna

    2014-05-01

    porphyroblasts yield respective ages of 33.6 ± 0.9 Ma and 29.5 ± 0.2 Ma, constraining the time elapsed between allanite crystallization (~ 420 °C) and monazite crystallization (~ 600°C). These data indicate that the rock needed ~ 4 Myr to be subducted from the 420 °C isotherm down to the 600°C isotherm, implying a heating rate of ca. 45°C/m.y. References Robyr, M., Epard, J.-L. & El Korh, A., 2014. Structural, metamorphic and geochronological relations between the Zanskar Shear Zone and the Miyar Shear Zone (NW Indian Himalaya): Evidence for two distinct tectonic structures and implications for the evolution of the High Himalayan Crystalline of Zanskar. Journal of Asian Earth Sciences, 79, 1-15. Robyr, M., Hacker, B. R. & Mattinson, J. M., 2006. Doming in compressional orogenic settings: New geochronological constraints from the NW Himalaya. Tectonics, 25. Robyr, M., Vannay, J. C., Epard, J. L. & Steck, A., 2002. Thrusting, extension, and doming during the polyphase tectonometamorphic evolution of the High Himalayan Crystalline Zone in NW India. Journal of Asian Earth Sciences, 21, 221-239.

  17. Petrology, geochemistry and modelling of the granulitic-ultramafic rocks in Beni Bousera (Rif, Morocco): implications for direct crust-mantle interactions and melt-extraction systems

    NASA Astrophysics Data System (ADS)

    Manthei, C. D.; Álvarez-Valero, A.; Jagoutz, O. E.

    2011-12-01

    The Beni Bousera (N. Morocco) and Ronda (S. Spain) ultramafic massifs of the Betic-Rif orogenic belt are two of the most pristine exposures of upper-mantle/lower crustal material on Earth's surface. Unlike other samples of the mantle, they are relatively unaltered and preserve a record of ultra-high pressure conditions, within the diamond-stability field (e.g. Slodkevich, 1980; Pearson et al., 1989). The process of removing of the massifs from the diamond-stability field, and the ensuing emplacement into the continental crust, is an ongoing area of research in regional tectonics. Here, we focus specifically on Beni Bousera, and note that the up-risen material is of higher density than its host, prompting the development of models that use melt-induced buoyancy forces as the primary driver of exhumation (Jagoutz et al., 2006; Gerya and Burg, 2007). We find evidence for discrete reaction zones in the ultramafic rocks that were formed by pervasive infiltration of melt, which may have channelized, lowered the integrated bulk density of the massif (e.g., Jagoutz et al., 2006), and driven exhumation. Since key questions concerning the emplacement mechanisms are still unanswered, complementary studies of the surrounding crustal material -granulitic rocks, which are mostly metapelitic with local intercalation of mafic composition-, assist in deepening our understanding crust-mantle processes. We will discuss our ongoing research at Beni Bousera, focusing on: (1) the petrological, structural, geochronological and physical relationships between mantle and crust by combining field petrology, petrography and phase diagram modeling, geochemistry, zircons/monazite dating, and numerical modeling; (2) the emplacement mechanisms of ultramafic and granulitic rocks by proposing a new hypothesis of very rapid exhumation of the mantle material. This rapid ascent is currently being constrained/tested by combining geobarometric calculations and high precision U-Pb zircon geochronology on

  18. Zircon geochronology and geochemistry to constrain the youngest eruption events and magma evolution of the Mid-Miocene ignimbrite flare-up in the Pannonian Basin, eastern central Europe

    NASA Astrophysics Data System (ADS)

    Lukács, Réka; Harangi, Szabolcs; Bachmann, Olivier; Guillong, Marcel; Danišík, Martin; Buret, Yannick; von Quadt, Albrecht; Dunkl, István; Fodor, László; Sliwinski, Jakub; Soós, Ildikó; Szepesi, János

    2015-12-01

    A silicic ignimbrite flare-up episode occurred in the Pannonian Basin during the Miocene, coeval with the syn-extensional period in the region. It produced important correlation horizons in the regional stratigraphy; however, they lacked precise and accurate geochronology. Here, we used U-Pb (LA-ICP-MS and ID-TIMS) and (U-Th)/He dating of zircons to determine the eruption ages of the youngest stage of this volcanic activity and constrain the longevity of the magma storage in crustal reservoirs. Reliability of the U-Pb data is supported by (U-Th)/He zircon dating and magnetostratigraphic constraints. We distinguish four eruptive phases from 15.9 ± 0.3 to 14.1 ± 0.3 Ma, each of which possibly includes multiple eruptive events. Among these, at least two large volume eruptions (>10 km3) occurred at 14.8 ± 0.3 Ma (Demjén ignimbrite) and 14.1 ± 0.3 Ma (Harsány ignimbrite). The in situ U-Pb zircon dating shows wide age ranges (up to 700 kyr) in most of the crystal-poor pyroclastic units, containing few to no xenocrysts, which implies efficient recycling of antecrysts. We propose that long-lived silicic magma reservoirs, mostly kept as high-crystallinity mushes, have existed in the Pannonian Basin during the 16-14 Ma period. Small but significant differences in zircon, bulk rock and glass shard composition among units suggest the presence of spatially separated reservoirs, sometimes existing contemporaneously. Our results also better constrain the time frame of the main tectonic events that occurred in the Northern Pannonian Basin: We refined the upper temporal boundary (15 Ma) of the youngest counterclockwise block rotation and the beginning of a new deformation phase, which structurally characterized the onset of the youngest volcanic and sedimentary phase.

  19. Geochemistry and zircon U-Pb geochronology of the ultramafic and mafic rocks emplaced within the anatectic series of the Variscan Pyrenees: The example of the Gavarnie-Heas dome (France)

    NASA Astrophysics Data System (ADS)

    Kilzi, Mohammad A.; Grégoire, Michel; Bosse, Valérie; Benoît, Mathieu; Driouch, Youssef; de Saint Blanquat, Michel; Debat, Pierre

    2016-02-01

    The origin and evolution of ultramafic (UM), mafic (M) and intermediate rocks emplaced within the metamorphic and anatectic series of the middle crust of the Variscan segment of the Pyrenees are defined in the light of new isotopic data U-Pb zircon ages, and Sr, Nd isotopic ratios. In the Gavarnie-Heas dome (central Pyrenees), ultramafic, mafic and intermediate rocks form three massifs several kilometers in size emplaced within the anatectic series: (i) the Gloriettes massif, which mostly consists of norites with enclaves of ultramafic rocks; (ii) the Troumouse massif, which comprises intermediate rocks (gabbro-diorite and diorite) with norite enclaves, and (iii) the Aguila massif, which consists of intermediate rocks with hornblendite enclaves. U-Pb zircon geochronology (first data for these rock types in the Axial Zone of the Pyrenees) revealed an age of 294 Ma for the diorite crystallization and correlatively for the crystallization of the anatectic granite. This new radiometric age allows us to suggest that the Variscan orogeny continued at least until the Early Permian and spread over around 30 Ma. The Nd and Sr isotopic compositions of all UM and M samples plot within the field of the continental crust. Two scenarios can explain the genesis and the emplacement of the components of the magmatic suite: i) evolution of mantle melts and contamination (AFC); ii) evolution of melts originating from a heterogeneous source (mantle + crust) at the local or regional scale. The strong crustal affinity of all UM and M rock types from Gavarnie-Heas leads us to favor the second scenario.

  20. A Monazite-bearing clast in Apollo 17 melt breccia

    NASA Technical Reports Server (NTRS)

    Jolliff, Bradley L.

    1993-01-01

    A phosphate-rich clast in a pigeonite-plagioclase mineral assemblage occurs in Apollo 17 impact-melt breccia 76503,7025. The clast, measuring 0.9 x 0.4 mm in thin section, contains 3.3 percent (volume) apatite (Ca5P3O12(F,Cl)), 0.8 percent whitlockite (Ca16(Mg,Fe)2REE2P14O56), and trace monazite ((LREE)PO4). Major minerals include 26 percent pigeonite, En53-57FS34-35W08-13, and 69 percent plagioclase, An84-92Ab7-15Oro.6-1.1. Troilite, ilmenite, and other accessory minerals constitute less than 1 percent of the assemblage and Fe-metal occurs along fractures. Also present in the melt breccia as a separate clast is a fragment of felsite. Based on the association of these clasts and their assemblages, a parent lithology of alkali-anorthositic monzogabbro is postulated. Monazite occurs in the phosphate-bearing clast as two less than 10 micron grains intergrown with whitlockite. The concentration of combined REE oxides in monazite is 63.5 percent and the chondrite-normalized REE pattern is strongly enriched in LREE, similar to lunar monazite in 10047,68 and terrestrial monazite. Thorium concentration was not measured in monazite, but based on oxide analyses of approximately 100 percent (including interpolated values for REE not measured), substantial Th concentration is not indicated, similar to monazite in 10047,68. Measured monazite/whitlockite REE ratios are La: 11, Ce: 8, Sm: 3.6, Y: 0.9, and Yb: 0.5. Compositions of monazite and coexisting whitlockite and apatite are given.

  1. A Critical Look at NORM In The Monazite Cycle

    NASA Astrophysics Data System (ADS)

    Paschoa, Anselmo S.; da Cunha, Kenya Dias

    2008-08-01

    Thorium, cerium, lanthanum and other rare earths have been extracted from monazite sands for a long time due to several reasons. At the end of the XIX century monazite sands from Brazilian beaches were concentrated gravimetrically to be clandestinely transported to Europe to be used in the manufacture of fabric bags to be burned. Those bags when burning had their brightness enhanced by a mixture of thorium, lanthanum and cerium. The thorium oxide (ThO2) percentages found in monazite concentrates from several parts of the world range from 1 to 11%. While not burned or processed chemically monazite concentrate can be considered hazardous only due to gamma radiation and 220Rn (thoron) inhalation by those who stay very close to storage piles. In old monazite plants the thoron concentration in air reached levels higher than 20 kBqṡm-3. Fortunately, the industrial hygiene improved with time and today's monazite separation plants do not present such high thoron concentrations. Old and recent data from rare earth processing plants suggest that occupational annual doses indoors may be up to two orders of magnitude higher than the worldwide average effective dose reported by the United Nations Scientific Committee on the Effects of the Atomic Radiation (UNSCEAR) due to the inhalation of thoron. Consumer products, like fabric bags used in gas or kerosene lamps have 232Th concentrations of the order of 150 Bqṡg-1. Remnants of the monazite industrial cycle can present 228Ra activity concentrations as high as 5.0 kBqṡg-1. Piles of monazite concentrates were kept for strategic reasons during cold war times in several places of the territories of the United States and the former Soviet Union. Cleanup at those sites may be hazardous mostly due to high concentrations of thoron daughters that may be inhaled by workers. The paper will take a critical look at the naturally occurring radioactive materials (NORM) extant in the monazite cycle and its remnants.

  2. Geochemistry, U–Pb geochronology, Sm–Nd and O isotopes of ca. 50 Ma long Ediacaran High-K Syn-Collisional Magmatism in the Pernambuco Alagoas Domain, Borborema Province, NE Brazil

    NASA Astrophysics Data System (ADS)

    Francisco da Silva Filho, Adejardo; de Pinho Guimarães, Ignez; Santos, Lucilene; Armstrong, Richard; Van Schmus, William Randall

    2016-07-01

    The Pernambuco Alagoas (PEAL) domain shows the major occurrence of granitic batholiths of the Borborema Province, NE Brazil, with Archean to Neoproterozoic range of Nd TDM model ages, giving clues on the role of granites during the Brasiliano orogeny. SHRIMP U/Pb zircon geochronological data for seven granitic intrusions of the PEAL domain divide the studied granitoids into three groups: 1) early-to syn-collision granitoids with crystallization ages ca. 635 Ma (Serra do Catú pluton), 2) syn-collision granitoids with crystallization ages 610-618 Ma (Santana do Ipanema, Água Branca, Mata Grande and Correntes plutons) and 3) late-to post-collision granitoids with ages of ca. 590 Ma (Águas Belas, and Cachoeirinha plutons). The intrusions of group 1 and 2, except the Mata Grande and Correntes plutons, show Nd TDM model ages ranging from 1.2 to 1.5 Ga, while the granitoids from group 3, and Mata Grande Pluton and Correntes plutons have Nd TDM model ages ranging from 1.7 to 2.2 Ga. The studied granitoids with ages <600 Ma are high-K, calc-alkaline, shoshonitic and those with ages <600 Ma are transitional high-K calc-alkaline to alkaline. The volcanic arc signatures associated with the Paleoproterozoic Nd TDM model ages are interpreted as inherited from the source rocks. The oldest ages and lower Nd TDM model ages are recorded from granitoids intruded in the southwest part of the PEAL domain, suggesting that these intrusions are associated with slab-tearing during convergence between the PEAL and the Sergipano domains. Zircon oxygen isotopic data in some of the studied plutons, together with the available Nd isotopic data suggest that the Brasiliano orogeny strongly reworked older crust, of either Paleoproterozoic or Tonian ages. The studied granitoids are coeval with calc-alkaline granitoids of the Transversal Zone and Sergipano domains and rare high-K calc-alkaline granitoids from the Transversal Zone domain. Such large volumes of high-K granitoids with

  3. Petrogenesis of late Paleozoic volcanic rocks from the Daheshen Formation in central Jilin Province, NE China, and its tectonic implications: Constraints from geochronology, geochemistry and Sr-Nd-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Yu, Qian; Ge, Wen-Chun; Yang, Hao; Zhao, Guo-Chun; Zhang, Yan-Long; Su, Li

    2014-04-01

    We present geochronological, geochemical, whole-rock Sr-Nd and zircon Hf-isotopic data for late Paleozoic volcanic rocks from the Daheshen Formation in central Jilin Province, northeastern China, and constrain the petrogenesis of the volcanic rocks and late Paleozoic tectonic evolution of the northern margin of the Northern China Craton, which is regarded as the eastern segment of the Central Asian Orogenic Belt (CAOB). Lithologically, the Daheshen Formation is composed mainly of rhyolite, rhyolitic tuff, dacite and andesite, with minor basalt. The zircons from three rhyolites, two dacites, one rhyolitic tuff and one basalt are euhedral-subhedral, display oscillatory zoning and have high Th/U ratios (0.50-2.28), implying a magmatic origin. LA-ICP-MS zircon U-Pb age data indicate that the volcanic rocks from the Daheshen Formation formed during Late Carboniferous-Early Permian time (302-299 Ma). Geochemically, late Paleozoic volcanic rocks have SiO2 = 52.13-81.77 wt.% and K2O = 0.86-6.88 wt.%, belonging to mid-K to high-K calc-alkaline series. These rocks are characterized by enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs, such as Nb, Ta, and Ti) and heavy rare earth elements (HREEs), with affinities to igneous rocks forming in an active continental margin setting. All volcanic rocks have depleted Nd isotopic compositions (ɛNd(t) = + 2.4 to + 2.5 for the basalts and + 5.8 to + 7.1 for the andesites and dacites, respectively). In situ Hf isotopic results of zircon from the rhyolites show that they have ɛHf(t) = - 1.1 to + 10.6. All these geochemical features indicate that the andesites, dacites, and rhyolites likely originated from the partial melting of Meso-Neoproterozoic accreted lower crust (Hf and Nd model ages (TDM2) of 1384-662 Ma and 1061-800 Ma, respectively). In contrast, the basalts were derived from the partial fusion of a depleted lithospheric mantle that

  4. The Indus-Yarlung Zangbo (IYZ) ophiolites from Nanga Parbat to Namche Barwa syntaxes, Southern Tibet: First synthesis of the petrology, geochemistry and geochronology of the IYZ ophiolites, and implications for geodynamic reconstructions of Neo-Tethys

    NASA Astrophysics Data System (ADS)

    Hébert, Réjean; Bezard, Rachel; Guilmette, Carl; Dostal, Jaroslav; Wang, Chengshan; Liu, Zf

    2013-04-01

    The purpose of this first synthesis is to summarize findings on the Yarlung Zangbo Suture Zone (YZSZ) ophiolites in Southern Tibet, and to discuss some of thel remaining scientific problems. The YZSZ ophiolites have been studied for almost 30 years and constitute the youngest of the sutures recognized on the Tibet Plateau. It is now acknowledged the YZSZ is a complex assemblage of sedimentary, metamorphic and igneous rocks produced during and shortly after the collision between India and Eurasia. The ages of the various lithological units span a time interval from the Jurassic to the Middle Miocene, with some Permian and Devonian exotic blocks in the mélange zone. The YZSZ is characterized by ophiolitic complexes and an ophiolitic mélange. The ophiolites are of two types with complete and incomplete pseudostratigraphies. The complete sections, although tectonically reworked, are observed along the segment from Dazhuqu to Jiding in the Xigaze area and the Spontang ophiolite. The incomplete sequences are found in various locations, including Nidar, Kiogar, Jungbwa, Saga, Sangsang, Xigugabu, Luobusa. The incomplete nature of these ophiolites could be related to intraoceanic or orogenic/collisional origins. The YZSZ ophiolites are also distributed into two groups of ages: the Luobusa, Zedang and Kiogar sequences are Jurassic-Lower Cretaceous in age, whereas all other sequences are of a Lower Cretaceous age. Compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70 m.y. from their inital igneous genesis to obduction, which occurred around 70-90 Ma. Although the YZSZ ophiolites differ in terms of their petrological and geochemical characters, they were all generated in a suprasubduction zone setting, and more specifically in arc (few fore-arc) and back-arc environments. Our synthesis of ~500 geochemical analyses show variable mixing of components from N-MORB-type to IAT-CAB and to OIB end-members. The Jurassic

  5. Orosirian (ca. 1.96 Ga) mafic crust of the northwestern São Francisco Craton margin: Petrography, geochemistry and geochronology of amphibolites from the Rio Preto fold belt basement, NE Brazil

    NASA Astrophysics Data System (ADS)

    Caxito, Fabrício de Andrade; Uhlein, Alexandre; Dantas, Elton Luiz; Stevenson, Ross; Pedrosa-Soares, Antônio Carlos

    2015-04-01

    The bulk of the Atlantic shield basement in the eastern South American platform is made of Rhyacian calc-alkaline magmatic rocks (˜2.2-2.15 Ga) affected by granulite-facies metamorphism at around 2.06 Ga, as part of a Paleoproterozoic orogenic system which joined Archean crustal fragments together. Orosirian (2050-1800 Ma) rocks, on the other hand, are scarce in all four major provinces of eastern South America (the Borborema, São Francisco, Tocantins and Mantiqueira provinces). Recently, however, sparse occurrences of Orosirian-aged rocks have been described, particularly in the northern São Francisco Craton margin and adjoining fold belts that compose the Borborema Province further north. In this paper, we present new geochemical, geochronological and isotopic data of amphibolites which compose the basement of the Rio Preto fold belt, in the northwestern São Francisco Craton margin. Geochemical data suggest tholeiitic gabbro protoliths intruded in a subduction-related setting, as suggested by tectonic discrimination diagrams and differentiation trends. U-Pb analyses of magmatic zircon crystals yielded a 1958.3 ± 16 Ma Concordia age. Slightly negative to positive ɛNd(1.96) = (-0.3 to +1.0) and ɛHf(1.96) from -0.76 to -4.55 suggest variable mixing of a depleted mantle source and older continental crust. We interpret the amphibolite body as a remnant of a subduction-related crustal fragment developed in the Orosirian, around 1.96 Ga ago, possibly in a continental back-arc setting. This crustal fragment was further reworked as part of the basement of the Rio Preto fold belt during the Brasiliano Orogeny (˜600-550 Ma). The Cristalândia do Piauí Complex gneisses further north might represent fragments of a coeval continental magmatic arc. These rocks could be the first well documented representatives of an Orosirian tectono-magmatic event which has been, up to now, broadly ignored within this portion of West Gondwana, and thus present major implications for

  6. 40Ar/39Ar Geochronology, Isotope Geochemistry (Sr, Nd, Pb), and petrology of alkaline lavas near Yampa, Colorado: migration of alkaline volcanism and evolution of the northern Rio Grande rift

    USGS Publications Warehouse

    Cosca, Michael A.; Thompson, Ren A.; Lee, John P.; Turner, Kenzie J.; Neymark, Leonid A.; Premo, Wayne R.

    2014-01-01

    Volcanic rocks near Yampa, Colorado (USA), represent one of several small late Miocene to Quaternary alkaline volcanic fields along the northeast margin of the Colorado Plateau. Basanite, trachybasalt, and basalt collected from six sites within the Yampa volcanic field were investigated to assess correlations with late Cenozoic extension and Rio Grande rifting. In this paper we report major and trace element rock and mineral compositions and Ar, Sr, Nd, and Pb isotope data for these volcanic rocks. High-precision 40Ar/39Ar geochronology indicates westward migration of volcanism within the Yampa volcanic field between 6 and 4.5 Ma, and the Sr, Nd, and Pb isotope values are consistent with a primary source in the Proterozoic subcontinental lithospheric mantle. Relict olivine phenocrysts have Mg- and Ni-rich cores, whereas unmelted clinopyroxene cores are Na and Si enriched with finely banded Ca-, Mg-, Al-, and Ti-enriched rims, thus tracing their crystallization history from a lithospheric mantle source region to one in contact with melt prior to eruption. A regional synthesis of Neogene and younger volcanism within the Rio Grande rift corridor, from northern New Mexico to southern Wyoming, supports a systematic overall southwest migration of alkaline volcanism. We interpret this Neogene to Quaternary migration of volcanism toward the northeast margin of the Colorado Plateau to record passage of melt through subvertical zones within the lithosphere weakened by late Cenozoic extension. If the locus of Quaternary alkaline magmatism defines the current location of the Rio Grande rift, it includes the Leucite Hills, Wyoming. We suggest that alkaline volcanism in the incipient northern Rio Grande rift, north of Leadville, Colorado, represents melting of the subcontinental lithospheric mantle in response to transient infiltration of asthenospheric mantle into deep, subvertical zones of dilational crustal weakness developed during late Cenozoic extension that have been

  7. Early Cretaceous arc volcanic suite in Cebu Island, Central Philippines and its implications on paleo-Pacific plate subduction: Constraints from geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Deng, Jianghong; Yang, Xiaoyong; Zhang, Zhao-Feng; Santosh, M.

    2015-08-01

    The Philippine island arc system is a collage of amalgamated terranes of oceanic, continental and island arc affinities. Here we investigate a volcanic suite in Cebu Island of central Philippines, including basalt, diabase dike, basaltic pyroclastic rock and porphyritic andesite. LA-ICP-MS U-Pb geochronology of zircon grains from the porphyritic andesite and pyroclastic rock yielded ages of 126 ± 3 Ma and 119 ± 2 Ma, respectively, indicating an Early Cretaceous age. The age distribution of the detrital zircons from river sand in the area displays a peak at ca. 118 Ma, close to the age of the pyroclastic rock. The early Cretaceous volcanic rocks in the central Philippines were previously regarded as parts of ophiolite complexes by most investigators, whereas the Cebu volcanics are distinct from these, and display calc-alkaline affinity and island arc setting, characterized by high LREE/HREE ratios and low HFSE contents. These features are similar to the Early Cretaceous arc basalts in the Amami Plateau and east Halmahera in the northernmost and southernmost West Philippine Basin respectively. Zircon Hf isotopes of the pyroclastic rocks show depleted nature similar to those of the Amami Plateau basalts, implying the subducted Pacific-type MORB as probable source. Zircon Hf isotopes of the porphyritic andesite show slight enrichment relative to that of the pyroclastic rocks and MORB, indicating subducted sediments as a minor end-member in the source. The Hf isotopic compositions of the volcanic rocks are also reflected in the detrital zircons from the river sands. We propose that the volcanic rocks of Cebu Island were derived from partial melting of sub-arc mantle wedge which was metasomatized by dehydration of subducted oceanic crust together with minor pelagic sediments. Within the tectonic environment of Southeast Asia during Early Cretaceous, the volcanic rocks in Cebu Island can be correlated to the subduction of paleo-Pacific plate. The Early Cretaceous

  8. A dithizone method for the determination of lead in monazite

    USGS Publications Warehouse

    Powell, R.A.; Kinser, C.A.

    1956-01-01

    In determining lead in monazite [(Ce,La,Th)PO4]--to be used as the basis for geologic age measurements--it was necessary to eliminate interferences due to the presences of phosphates of thorium and the rare-earth metals. The method, in which monazite samples are attacked with hot concentrated sulfuric acid, taken up with dilute nitric acid, lead extracted as the dithizonate and then determined spectrophotometrically at 520 mμ, was successfully applied to a series of monazite samples. Rapid determinations were made with good reproducibility.

  9. Evolution of the Mazatzal province and the timing of the Mazatzal orogeny: Insights from U-Pb geochronology and geochemistry of igneous and metasedimentary rocks in southern New Mexico

    USGS Publications Warehouse

    Amato, J.M.; Boullion, A.O.; Serna, A.M.; Sanders, A.E.; Farmer, G.L.; Gehrels, G.E.; Wooden, J.L.

    2008-01-01

    New U-Pb zircon ages, geochemistry, and Nd isotopic data are presented from three localities in the Paleoproterozoic Mazatzal province of southern New Mexico, United States. These data help in understanding the source regions and tectonic setting of magmatism from 1680 to 1620 Ma, the timing of the Mazatzal orogeny, the nature of postorogenic maginatism, Proterozoic plate tectonics, and provide a link between Mazatzal subblocks in Arizona and northern New Mexico. The data indicate a period from 1680 to 1650 Ma in which juvenile felsic granitoids were formed, and a later event between 1646 and 1633 Ma, when these rocks were deformed together with sedimentary rocks. No evidence of pre-1680 Ma rocks or inherited zircons was observed. The igneous rocks have ENd(t) from -1.2 to +4.3 with most between +2 and +4, suggesting a mantle source or derivation from similar-aged crust. Nd isotope and trace element concentrations are consistent with models for typical are magmatism. Detrital zircon ages from metasedimentary rocks indicate that sedimentation occurred until at least 1646 Ma. Both local and Yavapai province sources contributed to the detritus. All of the samples older than ca. 1650 Ma are deformed, whereas undeformed porphyroblasts were found in the contact aureole of a previously dated 1633 Ma gabbro. Regionally, the Mlazatzal orogeny occurred mainly between 1654 and 1643 Ma, during final accretion of a series of island arcs and intervening basins that may have amalgamated offshore. Rhyolite magmatism in the southern Mazatzal province was coeval with gabbro intrusions at 1633 Ma and this bimodal magmatism may have been related to extensional processes following arc accretion. ?? 2007 Geological Society of America.

  10. Geochronology, geochemistry, and Hf isotopes of Jurassic intermediate-acidic intrusions in the Xing'an Block, northeastern China: Petrogenesis and implications for subduction of the Paleo-Pacific oceanic plate

    NASA Astrophysics Data System (ADS)

    Dong, Yu; Ge, Wen-chun; Yang, Hao; Xu, Wen-liang; Zhang, Yan-long; Bi, Jun-hui; Liu, Xi-wen

    2016-03-01

    Zircon U-Pb dating, whole-rock geochemistry, Hf isotopic compositions, and regional geological observations of Jurassic intermediate-acidic intrusions in the Xing'an Block, northeastern China, are presented to constrain their petrogenesis and the tectonic evolution of the Paleo-Pacific Ocean. Zircon U-Pb age dating indicates that the intrusions were emplaced in three stages: during the Early Jurassic (180-177 Ma), Middle Jurassic (171-170 Ma), and Late Jurassic (∼151 Ma). Despite the wide range in ages of the intrusions, the magmas of Jurassic acidic intrusions were likely derived from a similar or common source and experienced different degrees of magmatic differentiation, as inferred from their geochemical and Hf isotopic characteristics. The Jurassic acidic intrusions are characterized by high SiO2 and total Na2O + K2O, low MgO, and I-type affinities, suggesting that the primary magmas were derived from partial melting of lower crustal material. These findings, combined with their εHf(t) values and two-stage model ages, indicate the primary magmas originated from partial melting of juvenile crustal material accreted during the Neoproterozoic to Phanerozoic. The Middle Jurassic intermediate-acidic rocks (diorites and granodiorites of the TJ pluton) have SiO2 contents of 57.96-69.10 wt.%, MgO contents of 4.48-1.81 wt.%, and high Mg numbers (45-54). They are enriched in large ion lithophile elements (e.g., Rb, Ba, Th, U, and K) and light rare earth elements, depleted in high field strength elements (e.g., Nb, Ta, Zr, Hf, and Ti) and heavy rare earth elements, and have εHf(t) values of +6.5 to +9.1. These data suggest that the magma was derived from partial melting of a depleted mantle wedge that had been metasomatized by subduction-related fluids. According to these findings and previous studies that focused on contemporaneous magmatic-tectonic activity in northeastern China, we conclude that the generation of Jurassic intermediate-acidic intrusions in the Xing

  11. Geochemistry, Sr-Nd-Pb isotopes and geochronology of amphibole- and mica-bearing lamprophyres in northwestern Iran: Implications for mantle wedge heterogeneity in a palaeo-subduction zone

    NASA Astrophysics Data System (ADS)

    Aghazadeh, Mehraj; Prelević, Dejan; Badrzadeh, Zahra; Braschi, Eleonora; van den Bogaard, Paul; Conticelli, Sandro

    2015-02-01

    Lamprophyres of different age showing distinctive mineralogy, geochemistry and isotopic ratios are exposed in northwestern Iran. They can be divided into Late Cretaceous sannaite, Late Oligocene-Early Miocene camptonite (amphibole-bearing) and Late Miocene minette (mica-bearing) and spessartite (amphibole-bearing) lamprophyres. Sannaites have high-Ti amphibole along with high-Ti and Al clinopyroxene, and they are characterised by homogeneous enrichment in incompatible trace elements with troughs at Pb. Spessartites have hornblende and low-Al and Ti clinopyroxene, and they are characterised by enriched incompatible trace element pattern with depletions of Nb, Ta, Pb, and Ti with respect to large ion lithophile elements. Minettes have high-Ti and Al brown mica and low-Al and Ti clinopyroxene, and similar to spessartite, are characterised by fractionation of high field strength elements with respect to large ion lithophile elements, with troughs at Nb, Ta, and Ti and a peak at Pb. Minettes show high initial 87Sr/86Sr values up to 0.70760 and low initial 143Nd/144Nd down to 0.512463 with a negative correlation, consistent with the trace element distribution related with an enriched mantle source modified after sediment recycling during subduction and continental collision. Cretaceous sannaites and Early Miocene spessartites show low initial 87Sr/86Sr approaching 0.70447 and high 143Nd/144Nd values up to 0.512667, which are consistent with a depleted within-plate mantle source. Minette and spessartite lamprophyres show high initial 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values, whereas sannaites have lower, but variable, initial 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values with respect to those of calc-alkaline lamprophyres. Minettes originated by partial melting of a metasomatised lithospheric mantle following siliciclastic sediment recycling by subduction. In contrast, sannaites were generated from the partial melting of a similar lithospheric mantle that was

  12. The Oldest Known Caldera Associated with the Yellowstone Hotspot: New Geologic Mapping, Geochemistry, and 40Ar/39Ar Geochronology for the Northern McDermitt Volcanic Field, Northern Nevada and Southeastern Oregon

    NASA Astrophysics Data System (ADS)

    Benson, T. R.; Mahood, G. A.

    2015-12-01

    McDermitt Volcanic Field (MVF) of Nevada and Oregon is one of three major caldera centers associated with Mid-Miocene Steens/Columbia River flood basalts. Pioneering geologic mapping of MVF by Rytuba and McKee (1984) and subsequent work established four main ignimbrites within the field. Our new 40Ar/39Ar ages (FCT=28.02 Ma) are 16.41±0.02 (±2σ) Ma for Tuff of Oregon Canyon, 16.35±0.04 Ma for Tuff of Trout Creek Mountains, 16.30±0.04 Ma for Tuff of Long Ridge, and 15.56±0.08 Ma for Tuff of Whitehorse Creek. We have mapped two previously unrecognized overlapping calderas that we interpret as sources for Tuff of Oregon Canyon and Tuff of Trout Creek. These ~20-km diameter calderas lie north of the well-known McDermitt Caldera; a smaller 7-km caldera that formed on eruption of the Tuff of Whitehorse Creek is nested within them. Argon ages and geochemistry of alkali rhyolite lava domes in the northern MVF define two populations: ~16.6-16.3 Ma associated with the newly recognized calderas, and ~15.5-15.3 Ma outlining the margins of the younger Whitehorse Caldera. Consistent with both ignimbrites erupting from the same evolving magma system, the high-silica alkali rhyolite Tuff of Oregon Canyon lies on compositional trends defined by the Tuff of Trout Creek, which is zoned from a moderately crystal-rich high-silica alkali rhyolite to a strongly porphyritic low-silica alkali rhyolite. They both are distinguished from the Tuff of Long Ridge from McDermitt Caldera by their higher Zr/Rb, and relatively high FeO* concentrations distinguish all MVF ignimbrites from ignimbrites from the nearby High Rock Caldera Complex, where the oldest caldera formed on eruption of the Idaho Canyon Tuff at 16.38±0.02 Ma (Coble and Mahood, in review). The Tuff of Trout Creek rests conformably on the Tuff of Oregon Canyon west and southwest of the calderas, where they overlie a thick stack of Steens Basalt lavas. To the east and southeast the two ignimbrites are separated by as much as

  13. Composition of monazites from pegmatites in eastern Minas Gerais, Brazil

    USGS Publications Warehouse

    Murata, K.J.; Dutra, C.V.; da Costa, M.T.; Branco, J.J.R.

    1959-01-01

    Two zoned pegmatites in south-eastern Minas Gerais were sampled in detail for their content of monazite and xenotime and the monazite was analysed for certain of the rare-earth elements and thorium. The ratio of xenotime to monazite increases in both pegmatites from the wall toward the quartz core. The content of the less basic rare-earth elements and of thorium in monazite rises in the same direction. These variation trends suggest that during the crystallization of these pegmatites there was a fractionation of the elements leading to a more or less steady enrichment of the less basic rare-earth elements and of thorium in the residual fluids. One mode of explaining these observed effects postulates that the rare-earth elements and thorium were present in pegmatitic fluids as co-ordination complexes rather than as simple cations. ?? 1959.

  14. Exploration Geochemistry.

    ERIC Educational Resources Information Center

    Closs, L. Graham

    1983-01-01

    Contributions in mineral-deposit model formulation, geochemical exploration in glaciated and arid environments, analytical and sampling problems, and bibliographic research were made in symposia held and proceedings volumes published during 1982. Highlights of these symposia and proceedings and comments on trends in exploration geochemistry are…

  15. Mineral chemistry and U-series geochronology reveal timescales of differentiation for late Pleistocene peraluminous rhyolite erupted from Hayes Volcano, Alaska

    NASA Astrophysics Data System (ADS)

    Coombs, M. L.; Vazquez, J. A.; Hayden, L. A.; Calvert, A. T.

    2014-12-01

    The Hayes River ignimbrite is a recently recognized deposit from Hayes volcano, the northernmost and easternmost volcano in the Aleutian-Alaskan arc, with unusual whole-rock composition (peraluminous rhyolite; 74.2‒75.5 wt% SiO2, 1.14 to 1.18 ASI) and phenocryst mineralogy (biotite-sanidine-plagioclase-quartz) compared to the Quaternary arc. The accessory minerals zircon, monazite [(LREE)PO4], and xenotime [(Y,HREE)PO4] are also present. We use ion microprobe 238U-230Th ages and trace-element geochemistry of unpolished rims and sectioned interiors of individual zircon and monazite grains to track differentiation of the silicic magma body. Core-to-rim zoning in zircon indicates that the parent melt became progressively enriched with U, HREEs, P, and Sc, and depleted in Th and LREEs due to monazite crystallization. Zircon (238U/232Th) values reach as high as 110 in the most differentiated rims. Monazite rims exhibit similar differentiation trends with lower LREE, higher M-HREEs, and higher U than crystal interiors, which eventually led to co-precipitation of monazite and xenotime. Monazite grains form a curved array on an activity ratio plot, with unpolished rims at the higher end. The unusual abundance of monazite, which can accommodate up to several weight percent Th, in the crystallizing assemblage significantly affected the U-Th ratio of the magma as differentiation progressed. 238U/232Th values ranges from 2.6 for early melt, represented by the whole-rock value, to 7.4 for groundmass glass. Assuming monazite fractionation alone is responsible for this change, it would take ~0.12 wt% monazite crystallization, using partition coefficients of 120 and 1000 for U and Th, respectively. This amount of monazite is consistent with that observed in the samples. An isochron for early melt and low-238U/232Th monazites yields an age of 67.0±2.8 ka, whereas one for late melt and high-238U/232Th monazites yields 42.5±0.9 ka. This younger age is indistinguishable from the

  16. Bioleaching of rare earth elements from monazite sand.

    PubMed

    Brisson, Vanessa L; Zhuang, Wei-Qin; Alvarez-Cohen, Lisa

    2016-02-01

    Three fungal strains were found to be capable of bioleaching rare earth elements from monazite, a rare earth phosphate mineral, utilizing the monazite as a phosphate source and releasing rare earth cations into solution. These organisms include one known phosphate solubilizing fungus, Aspergillus niger ATCC 1015, as well as two newly isolated fungi: an Aspergillus terreus strain ML3-1 and a Paecilomyces spp. strain WE3-F. Although monazite also contains the radioactive element Thorium, bioleaching by these fungi preferentially solubilized rare earth elements over Thorium, leaving the Thorium in the solid residual. Adjustments in growth media composition improved bioleaching performance measured as rare earth release. Cell-free spent medium generated during growth of A. terreus strain ML3-1 and Paecilomyces spp. strain WE3-F in the presence of monazite leached rare earths to concentrations 1.7-3.8 times those of HCl solutions of comparable pH, indicating that compounds exogenously released by these organisms contribute substantially to leaching. Organic acids released by the organisms included acetic, citric, gluconic, itaconic, oxalic, and succinic acids. Abiotic leaching with laboratory prepared solutions of these acids was not as effective as bioleaching or leaching with cell-free spent medium at releasing rare earths from monazite, indicating that compounds other than the identified organic acids contribute to leaching performance. PMID:26332985

  17. Geochemistry for Chemists.

    ERIC Educational Resources Information Center

    Hostettler, John D.

    1985-01-01

    A geochemistry course for chemists is described. Includes: (1) general course information; (2) subject matter covered; and (3) a consideration of the uses of geochemistry in a chemistry curriculum, including geochemical "real world" examples, geochemistry in general chemistry, and geochemistry as an elective. (JN)

  18. CHEMICAL AND CERAMIC METHODS TOWARD SAFE STORAGE OF ACTINIDES USING MONAZITE

    EPA Science Inventory

    The program will address more particularly the section, "Plutonium behaviorin mixed matrices - specialized waste forms", with the concept that monazite ceramic will provide the most safe, most secure, geologically tested, very long term, containment for actinides. That monazites...

  19. Geochronology, geochemistry and geophysics of Mahukona Volcano, Hawai`i

    NASA Astrophysics Data System (ADS)

    Hanano, D.; Garcia, M. O.; Weis, D. A.; Flinders, A. F.; Ito, G.; Kurz, M. D.

    2009-12-01

    Mahukona is an extinct submarine volcano that fills a gap in the Loa-trend of paired Hawaiian volcanoes between Hualalai and Kaho`olawe. A new marine survey of the seamount was undertaken in an attempt to resolve the location of the volcano’s summit. The multibeam bathymetry showed no clear summit. The gravity data reveals a central oval-shaped residual gravity anomaly with a maximum density 85 kg/m3 greater than the surrounding edifice, which could be the frozen magmatic center of Mahukona. Eighteen weakly to strongly olivine-phyric samples were collected by submersible from the shallower parts (>2 km) of the volcano to supplement previous dredged samples. These fresh, mostly glassy samples vary from low-silica tholeiites to weakly alkali basalts. Ar-Ar weighted plateau ages range from 653 ka for a tholeiite to 479 and 351 ka for transitional basalts. These ages straddle the predicted age for the end of shield building (435 ka) and are older than previous ages for transitional basalts (310-298 ka; Clague and Calvert, 2008). Trace elements show a moderate range of variability (33% for Ba and Nb) and parallel primitive mantle normalized patterns suggesting variable degrees of melting of a similar source. Zr/Nb ratios for this Loa chain volcano (11-14) span the Loa-Kea boundary. Pb, Sr, Nd and Hf isotope ratios for 12 samples are distinct from adjacent Kohala volcano with Loihi-like values, although they are slightly higher in Hf and Nd at a given Pb isotope value. Most samples have Loa-like Pb isotope ratios, although two tholeiites have Kea-like ratios but high, Loa-like Zr/Nb. Sr isotopes are well correlated with the other isotopic systems indicating no ancient carbonate-rich sediment source component is needed. Mahukona He isotope ratios overlap with those found at Lo`ihi Seamount. Higher values are found in transitional basalts and lower in the tholeiites (16-21 vs. 12-14 Ra), which is opposite to other Hawaiian volcanoes. With high-precision data sets for more volcanoes along the Hawaiian chain in multiple stages of growth, we will be able to resolve the fine structure and evolution of the Hawaiian plume. Clague, D. and Calvert, A. 2008. Bull. Volcanol.

  20. Late Devonian - Early Carboniferous polyphase metamorphic evolution of the Orlica-Śnieżnik Dome (NE Bohemian Massif, Poland): evidence from Th-U-total Pb monazite dating

    NASA Astrophysics Data System (ADS)

    Budzyń, Bartosz; Jastrzębski, Mirosław; Stawikowski, Wojciech

    2014-05-01

    younger age domains of 371-356 Ma and 336-331 Ma are defined by monazite from four K-feldspar free light quartzites (SF), and two K-feldspar bearing light quartzite (SF). Similar two age domains of 372-363 Ma and 342-332 Ma yields monazite from six mica schists (SF), with a faint record of ca. 406 Ma in one of these samples. The geochronological results suggest polyphase Devono-Carboniferous metamorphic evolution that embraced at least two tectonometamorphic episodes. Microstructures indicate that the record of 370-360 Ma ages presumably defines a progressive metamorphism, whereas pervasive record of 340-330 Ma ages presumably reflects the superimposed penetrative shearing connected with exhumation. There is no evidence of pre-Variscan regional or contact metamorphism of the Młynowiec-Stronie Group. Cambrian to Ordovician monazites developed only in K-feldspar bearing rocks, i.e. orthogneisses and light quartzites (SF), which suggests growth of the Early Palaeozoic monazites during formation of their, respectively, magmatic and partially volcanic protoliths. Acknowledgements. The project was funded by the National Science Center of Poland, grant number DEC 2011/03/B/ST10/05638.

  1. The Argon Geochronology Experiment (AGE)

    NASA Technical Reports Server (NTRS)

    Swindle, T. D.; Bode, R.; Fennema, A.; Chutjian, A.; MacAskill, J. A.; Darrach, M. R.; Clegg, S. M.; Wiens, R. C.; Cremers, D.

    2006-01-01

    This viewgraph presentation reviews the Argon Geochronology Experiment (AGE). Potassium-Argon dating is shown along with cosmic ray dating exposure. The contents include a flow diagram of the Argon Geochronology Experiment, and schematic diagrams of the mass spectrometer vacuum system, sample manipulation mechanism, mineral heater oven, and the quadrupole ion trap mass spectrometer. The Laser-Induced Breakdown Spectroscopy (LIBS) Operation with elemental abundances is also described.

  2. Microstructural observation and chemical dating on monazite from the Shilu Group, Hainan Province of South China: Implications for origin and evolution of the Shilu Fe-Co-Cu ore district

    NASA Astrophysics Data System (ADS)

    Xu, Deru; Kusiak, Monika A.; Wang, Zhilin; Chen, Huayong; Bakun-Czubarow, Nonna; Wu, Chuanjun; Konečný, Patrik; Hollings, Peter

    2015-02-01

    New monazite chemical U-Th-total-Pb (CHIME) ages, combined with microstructural observations, mineral compositions, and whole-rock geochemistry, indicate that the large-scale, banded iron formation (BIF)-type Shilu Fe-Co-Cu ore district in Hainan Province, South China is a multistage product of sedimentation, metamorphism, and hydrothermal-metasomatic alteration associated with multiple orogenies. Two types of monazite, i.e. "polygenetic" and "metamorphic", were identified. The "polygenetic monazite" comprises a magmatic and/or metamorphic core surrounded by a metamorphic rim, and shows complex zoning. Breakdown corona structure, with a core of monazite surrounded by a mantle of fluorapatite, allanite, and/or epidote as concentric growth rings, is commonly observed. This type of monazite yielded three main CHIME-age peaks at ca. 980 Ma, ca. 880 Ma and ca. 450 Ma. The ages which range up to ca. 880 Ma for detrital cores, record a pre-deformational magmatic and/or metamorphic event(s), and is considered to be the depositional time-interval of the Shilu Group and interbedded BIFs in a marine, back-arc foreland basin likely due to the Grenvillian or South China Sibao orogeny. After deposition, the Shilu district was subjected to an orogenic event, which is recorded by the syndeformational metamorphic monazite with ca. 560-450 Ma population. Probably this event not only caused amphibolite facies metamorphism and associated regional foliation S1 but also enriched the original BIFs, and most likely corresponds to the "Pan-African" and/or the South China Caledonian orogeny. The post-deformational "metamorphic" monazite occurs mostly as inclusions in garnet and shows ca. 260 Ma age. It likely represents the Late Permian post-magmatic hydrothermal and related retrograde event(s) initiated by the Indosinian orogeny due to the closure of the Paleo-Tethys. The breakdown of monazite to secondary coronal mineral phases as well as the Fe-remobilization and associated skarnization

  3. Neutron activation analysis of an Egyptian monazite ore sample

    NASA Astrophysics Data System (ADS)

    Eissa, E. A.; Rofail, N. B.; Ashmawy, L. S.; Hassan, A. M.

    1999-01-01

    The absolute sensitivity of a gamma-ray line following thermal or epithermal neutron activation is expressed as a product of four terms, namely, the intrinsic, irradiation, decay and measurement factors. The total absolute sensitivity is the sum of the absolute sensitivities due to thermal and epithermal activation. A. FORTRAN computer program was prepared on the TANDY 3000 NL PCA to calculate the total absolute sensitivity of the intensive gamma-ray lines from the thermal and epithermal neutron activation of all the naturally occurring nuclides. Another program was prepared for the determination of the total absolute sensitivity for gammarays emitted by radioactive daughters such as233Pa from233Th and239Np from239U. Long time irradiation periods (about 48 hours) for specimens of the monazite ore sample were carried out at the (ET-RR-1) reactor core periphery. The monazite ore sample was separated from the associating minerals in the sand of Abou-Khashaba of Rashied (Rosetta) area on the Mediterranean (North of Egypt). The separated monazite ore sample was provided by the Nuclear Material Authority of Egypt. The cadmium difference method was applied to thin gold foils for absolute thermal and epithermal neutron flux determination. The gamma-ray spectra were measured using a spectrometer with a HPGe coaxial detector. The evaluated absolute sensitivity tables were helpful in identifying the radioisotopes contributing to the gamma- ray spectra and in evaluating the elemental concentration of the monazite constitutents. Most of the rare earth elements were observed and their concentrations are reported. La, Ce, Nd, Sd and Th were found as major elements, U, Tb, Hf and Eu as minor elements.

  4. A Tale of Two Plutons: Using Monazite to Reconstruct the Fluid History of Contact Metamorphic Aureoles

    NASA Astrophysics Data System (ADS)

    Ayers, J. C.; Crombie, S.; Miller, C.; Luo, Y.; Loflin, M.

    2006-12-01

    The rare earth phosphate monazite can be a powerful tool for characterizing the timing and spatial extent of fluid infiltration during contact metamorphism. We used an ion microprobe to investigate how wallrock monazite responded to the intrusion of two different granitic plutons, the Cretaceous Birch Creek Pluton (BCP) in the White Mountains of eastern California and the Miocene Searchlight pluton in southern Nevada. The contact metamorphic aureoles of both plutons contain monazite and display evidence of alteration by acidic magmatic fluids (sericitization). Contact metamorphism occurred at conditions under which monazite in granitic systems has been shown to be susceptible to hydrothermal alteration: mildly acidic fluids at temperatures of ~250-400C and pressures of ~0.15-0.4 GPa. Monazite from the hydrothermal aureole of the BCP records the infiltration of magmatic fluids into the Early Cambrian Deep Spring Formation (DSF) containing metaquartzites and metasandstones. Monazites in the DSF < 0.6 km from the contact show patchy zoning and have Th-Pb ages and oxygen isotope compositions similar to monazites in the Birch Creek granite but different from monazites > 0.6 km from the contact, suggesting that they dissolved and reprecipitated in infiltrating magmatic fluids. In contrast, the stable isotope compositions of monazites and host rocks (Proterozoic gneisses and the Cretaceous Ireteba granite) on the flanks of the Searchlight pluton do not show evidence of hydrothermal alteration, even though many of the monazites display patchy zoning and ages corresponding to the Searchlight intrusion. The Searchlight gold-silver-copper mining district is primarily located in the roof of the Searchlight intrusion, which contains rocks that were intensely hydrothermally altered but contain no monazite. Searchlight magmatic fluids were strongly focused into the roof zone, with little or no fluid escaping out the sides of the Searchlight pluton during crystallization. Monazite

  5. National Geochronological Database

    USGS Publications Warehouse

    Revised by Sloan, Jan; Henry, Christopher D.; Hopkins, Melanie; Ludington, Steve; Original database by Zartman, Robert E.; Bush, Charles A.; Abston, Carl

    2003-01-01

    The National Geochronological Data Base (NGDB) was established by the United States Geological Survey (USGS) to collect and organize published isotopic (also known as radiometric) ages of rocks in the United States. The NGDB (originally known as the Radioactive Age Data Base, RADB) was started in 1974. A committee appointed by the Director of the USGS was given the mission to investigate the feasibility of compiling the published radiometric ages for the United States into a computerized data bank for ready access by the user community. A successful pilot program, which was conducted in 1975 and 1976 for the State of Wyoming, led to a decision to proceed with the compilation of the entire United States. For each dated rock sample reported in published literature, a record containing information on sample location, rock description, analytical data, age, interpretation, and literature citation was constructed and included in the NGDB. The NGDB was originally constructed and maintained on a mainframe computer, and later converted to a Helix Express relational database maintained on an Apple Macintosh desktop computer. The NGDB and a program to search the data files were published and distributed on Compact Disc-Read Only Memory (CD-ROM) in standard ISO 9660 format as USGS Digital Data Series DDS-14 (Zartman and others, 1995). As of May 1994, the NGDB consisted of more than 18,000 records containing over 30,000 individual ages, which is believed to represent approximately one-half the number of ages published for the United States through 1991. Because the organizational unit responsible for maintaining the database was abolished in 1996, and because we wanted to provide the data in more usable formats, we have reformatted the data, checked and edited the information in some records, and provided this online version of the NGDB. This report describes the changes made to the data and formats, and provides instructions for the use of the database in geographic

  6. Monazite U-Th-Pb EPMA and zircon U-Pb SIMS chronological constraints on the tectonic, metamorphic, and thermal events in the inner part of the Variscan orogen, example from the Sioule series, French Massif Central

    NASA Astrophysics Data System (ADS)

    Do Couto, Damien; Faure, Michel; Augier, Romain; Cocherie, Alain; Rossi, Philippe; Li, Xian-Hua; Lin, Wei

    2016-03-01

    In the northern Variscan French Massif Central, the Sioule metamorphic series exposes from top to bottom the tectonic superposition of the Upper Gneiss Unit (UGU), Lower Gneiss Unit (LGU), and Para-autochthonous Unit (PAU). The nappe stacking developed throughout two prograde syn-metamorphic events: D1 is a top-to-the-SW shearing coeval with a probable Devonian migmatization and D2 is a top-to-the-NW shearing event. Both events were completed before the unconformable deposition of the undeformed and unmetamorphosed "Tufs anthracifères" formation, dated at ca 330 Ma (Late Visean). Furthermore, the UGU experienced a high-pressure metamorphism ascribed to a D0 event during which eclogite or granulite crystallized in several parts of the UGU. Monazite U-Th-Pb and zircon U-Pb SIMS datings were carried out in order to constrain the ages of these D0, D1, and D2 tectono-metamorphic events. These new geochronological results are placed in a P-T-t diagram constructed for the UGU, LGU, and PAU. Monazite sampled in UGU, LGU, and PAU rocks yields similar 365-350 Ma ages consistent with the D2 event dated in other places of the French Massif Central. A zoned monazite grain from a granulitic paragneiss yields 416 ± 15 and 362 ± 14 Ma ages interpreted as those of the D0 and D2 events, respectively. Zircon from the same granulitic paragneiss yields SIMS ages at 343 ± 2 and 328 ± 2 Ma that are interpreted as recrystallization processes associated with post-thickening thermal events, possibly recording the onset of orogenic collapse of the Northern Massif Central. It is worth to note that neither monazite nor zircon recorded the D1 event.

  7. Monazite chemical age and composition correlations, an insight in the Palaeozoic evolution of the Leaota Massif, South Carpathians

    NASA Astrophysics Data System (ADS)

    Săbău, Gavril; Negulescu, Elena

    2015-04-01

    Notwithstanding remarkable advantages of monazite microprobe U-Th-PbT geochronology of metamorphic formations, such as the direct investigation of a metamorphic mineral in a truly in situ setting, unequalled spatial resolution, and cost-effective analyses, it essentially remains affected by indeterminations as regards the accuracy and the representativity of the results. Besides the experimental hurdles related to trace element analyses with the microprobe (sensitivity, background and overlap effects) the method faces two main biases, firstly its inherently blind status emerging from the aprioric assumption of isotopic equilibrium, and secondly the marked susceptibility of monazite to fluid-stimulated chemical recrystallization and compositional resetting (e. g. Kelly et al. 2012). Age spectra obtained from individual sampled habitually display a significant scatter of calculated age data, in such a way that the separation of coherent and geologically relevant populations may often represent a substantial challenge. The interpretation of the results greatly benefits from the qualitative analysis of the textural and paragenetic setting or a trial-and error quantitative statistical assessment of distinct age clusters (Montel et al., 1996), though still maintaining a variable degree of subjectivity, as in any interpretative process not fully sustained by quantitative analysis. Additional dependable support can be gained from further qualitative parameters characterizing, besides the distribution of individual age data, also the global chemical composition of the analysed monazite grains, as well as the relationship to the corresponding metamorphic assemblages (Săbău & Negulescu, 2013). The quantitative assessment of the age patterns of individual samples can be achieved by plotting the normalized age gradient from the sorted age pattern, allowing distinction of quasi-gaussian distribution domains likely to correspond to coherent age clusters of geologic significance

  8. U-Th-PbT Monazite Gechronology in the South Carpathian Basement: Variscan Syn-Metamorphic Tectonic Stacking and Long-Lasting Post-Peak Decompressional Overprints

    NASA Astrophysics Data System (ADS)

    Săbău, G.; Negulescu, E.

    2012-12-01

    Dating metamorphic events appears to be unsatisfactorily addressed by most of the widely-employed and otherwise accurate and productive isotopic techniques, because the phases and systems investigated do not directly relate to the metamorphic events themselves. An adequate answer to this challenge is instead provided by microprobe-assisted chemical U-Th-PbT monazite geochronology, by its spatial resolution, truly in situ character and the possibility to reference analyses against well-defined textural environments and features, as well as a qualitative timeframe derived therefrom. Though chemical U-Th-PbT monazite geochronology is increasingly applied to seek answers ranging form a general characterization to fine details of the thermotectonic evolution of magmatic and metamorphic rocks, there are so far, unlike in the case of isotopic geochronological methods, no clearly defined standard analytical and data processing protocols. Two main reasons for this have to be mentioned, namely that chemical U-Th-PbT chronology is actually a proxy for isotopic geochronology, and the quantification of the errors and their propagation cannot be directly assessed because apparent ages are related to the measured element concentrations by an implicit function, the law of radioactive decay. Current approaches rely on treating calculated individual age values as primary data, a priori grouping of analyses supposed (and subsequently tested) to be coeval, and their statistical processing in order to obtain age values. An alternative approach we applied in basement units of the South Carpathians consists in an explicit approximation of the age formula and associated errors propagated from element concentrations to age values, and individual treatment of each age datum. The separation of the age clusters from the overall age spectrum of each sample was operated by tracing the variations of the normalized age gradient on the age spectrum sorted by increasing age values, and fine

  9. Assembling and disassembling california: A zircon and monazite geochronologic framework for proterozoic crustal evolution in southern California

    USGS Publications Warehouse

    Barth, A.P.; Wooden, J.L.; Coleman, D.S.; Vogel, M.B.

    2009-01-01

    The Mojave province in southern California preserves a comparatively complete record of assembly, postorogenic sedimentation, and rifting along the southwestern North American continental margin. The oldest exposed rocks are metasedimentary gneisses and amphibolite, enclosing intrusive suites that range from tonalite and quartz mon-zodiorite to granite with minor trondhjemite. Discrete magmatic episodes occurred at approximately 1790-1730 and 1690-1640 Ma. Evidence from detrital and premagmatic zircons indicates that recycling of 1900-1790 Ma Paleopro-terozoic crust formed the unique isotopic character of the Mojave province. Peak metamorphic conditions in the Mojave province reached middle amphibolite to granulite facies; metamorphism occurred locally from 1795 to 1640 Ma, with widespread evidence for metamorphism at 1711-1689 and 1670-1650 Ma. Structures record early, tight to isoclinal folding and penetrative west-vergent shear during the final metamorphic event in the west Mojave province. Proterozoic basement rocks are overlain by siliciclastic-carbonate sequences of Mesoproterozoic, Neoproterozoic, and Cambrian age, recording environmental change over the course of the transition from stable Mojave crust to the rifted Cordilleran margin. Neoproterozoic quartzites have diverse zircon populations inconsistent with a southwest North American source, which we infer were derived from the western conjugate rift pair within Rodinia, before establishment of the miogeocline. Neoproterozoic-Cambrian miogeoclinal clastic rocks record an end to rifting and establishment of the Cordilleran miogeocline in southern California by latest Neoproterozoic to Early Cambrian time. ?? 2009 by The University of Chicago.

  10. Zoning and sectoriality of monazite-(Ce) from granite pegmatites of the central and South Urals

    NASA Astrophysics Data System (ADS)

    Popova, V. I.; Churin, E. I.

    2010-12-01

    Zonal and sectorial monazite-(Ce) crystals from ceramic granite pegmatites of the Adui pluton are enriched in Ce and La, whereas monazite from the miarolitic gemstone pegmatites at the western contact of the pluton are enriched in Nd, Sm, Gd, and Y. This difference is caused by earlier crystallization of ceramic pegmatites and higher temperatures of their formation (650-600°C for ceramic pegmatites and 550-300°C for miarolitic pegmatites). Monazites from ceramic and miarolitic pegmatites of the Adui pluton differ in La and Nd contents, but their compositional trend in La-Nd coordinates is similar to the variation in monazite composition from the early to late granite pegmatites in the Ilmeny Mountains. It is suggested that decrease in temperature is a factor controlling REE contents in monazite. Heterovalent P ↔ Si and REE ↔ Th, Ca isomorphism in the consecutive zones of growth pyramids in monazite is both unidirectional and wave-like. Monazite from granite pegmatites of the Adui pluton and Ilmeny Mountains crystallized mainly under slightly alkaline conditions.

  11. Sulfate incorporation in monazite lattice: potential for dating the cycle of sulfur in metamorphic belts

    NASA Astrophysics Data System (ADS)

    Laurent, Antonin; Seydoux-Guillaume, Anne-Magali; Duchene, Stéphanie; Bingen, Bernard; Bosse, Valérie

    2016-04-01

    Monazite is a common accessory mineral in magmatic and metamorphic rocks that often shows complex chemical zoning at the μm- to nm-scale. The large number of cations that may be accommodated in its lattice, makes monazite particularly responsive to changes in the rock-forming minerals and fluid composition. Chemical zoning resulting from replacement or overgrowth may coincide, or not, with age zoning derived from U-Th-Pb isotopes. In this study, we focus on the potential for monazite to record both the redox condition of its crystalizing medium and an absolute U-Th-Pb isotopic age, during polyphase metamorphism in the Proterozoic province of Rogaland, S. Norway. The metamorphic evolution of several samples is derived from phase diagrams and the oxygen fugacity estimated from the FeO/Fe2O3 ratio measured by titration. Monazite grains were mapped at high spatial resolution for minor elements with electron microprobe, revealing convolute chemical zoning. Some of these zones yield appreciable content of S (up to 7000 ppm), accommodated following the Ca2+ + S6+ = REE3+ + P5+ substitution vector. The incorporation of sulfate in monazite has been subsequently investigated by TEM thanks to site specific FIB preparations. Besides, LA-ICP-MS U‑Pb isotopic ages of monazite grains show a remarkable correlation with the sulfate content. It is therefore possible to distinguish different generations of monazite based on their S-content. From our petrological study we conclude that sulfate-bearing monazite reflects incongruent melting of Fe-Cu-As sulfides under oxidizing conditions, coeval with biotite dehydration melting. Monazite may therefore be used to probe the presence of sulfur in anatectic melts from high-grade terrains at a specific point in time. This property can be used to investigate the mineralization potential of a given geological event within a larger orogenic framework.

  12. Siluro-Carboniferous Transpression in Central-Coastal Maine: Constraints From New Chemical Monazite Ages

    NASA Astrophysics Data System (ADS)

    Short, H.; Yates, M.; Johnson, S.

    2004-05-01

    New in-situ chemical age determinations of monazite from several different lithologic units across central and eastern Maine suggest a protracted regional history of dextral transpression, terrane exhumation, fault reactivation, and punctuated metamorphism from the Silurian through the Carboniferous. Acadian-age orogenesis in Maine is considered to be a high-temperature, low-pressure series of events during which the dominant direction of accommodation of convergence switched from orogen-perpendicular to orogen-parallel near the end of the Devonian. New chemical monazite ages suggest this orogenesis involved a significant and long-lived component of transpression at pressures high enough to produce kyanite. A kyanite-bearing schist that occurs along the boundary of the higher-strain, higher-metamorphic-grade Liberty-Orrington belt in the middle of the orogen returns preliminary Late Silurian-Early Devonian ages from both matrix monazite and monazite included in kyanite. These data suggest that the kyanite is contemporaneous with established `Acadian-age' tectonism, and that the Liberty-Orrington belt represents a higher-pressure terrane that was exhumed during Siluro-Devonian time. Late Devonian-Carboniferous monazite ages for grains included in garnet and andalusite from the Liberty-Orrington belt suggest continued metamorphism and fault reactivation. To the east of the Liberty-Orrington belt, preliminary monazite ages of 400 Ma within inclusion trails in cores of staurolite porphyroblasts, 373 Ma from monazite included in andalusite, and 358 Ma for monazite in the dextral matrix emphasizes the protracted transpressive nature of Devonian tectonism in the area. This new monazite work suggests that a geodynamic model incorporating localized exhumation during transpression is a more appropriate model for Acadian-age orogenesis in central-eastern Maine. All analyses were carried out in-situ using the Cameca SX100 electron microprobe at the Department of Earth Sciences

  13. Polymetamorphic evolution of the upper part of the Iezer Complex (Leaota Massif, South Carpathians) constrained by petrological data and monazite ages

    NASA Astrophysics Data System (ADS)

    Negulescu, Elena; Săbău, Gavril; Massonne, Hans-Joachim

    2015-04-01

    The Leaota Massif in Romania consists of a flat-lying sequence of five structurally concordant units displaying mutual and partly internal lithologic and metamorphic contrasts. The lower part of the lithologic sequence is the Iezer Complex, a medium-grade psammopelitic unit with a structurally concordant thin granite sill located at its upper part. The lower limit of the granite is marked discontinuously by hornfels, also present as enclaves, which experienced intense strain and a subsequent low-pressure thermal overprint. Both granite and hornfels were affected by a medium-temperature, medium- to high-pressure event (Săbău, 2000). This event was also identified in gneisses below the hornfels. These rocks contain the assemblage garnet-phengite-chloritoid-kyanite which had overprinted an older garnet-kyanite-staurolite-biotite-muscovite assemblage. Available U-Th zircon ages indicate 472.7 ± 7.3 Ma (Balintoni et al. 2009) for the granite. Monazite geochronology (Săbău & Negulescu, 2013) reveals for the associated hornfels (1) inherited ages of 528 ± 17.86 Ma overprinted by pervasive Ordovician contact metamorphism (462 ± 4.54 Ma), slightly postdating the age of magmatic zircon in the granite, (2) Silurian to Early Devonian recrystallization episodes, and (3) a Variscan medium- to high-pressure metamorphic overprint responsible for the garnet-phengite-kyanite assemblage. New petrological and geochronological data constraining the polymetamorphic evolution of the upper part of the Iezer Complex were acquired from kyanite-garnet mylonitic gneisses made up of large garnet porphyroclasts embedded in a strongly deformed matrix. Large garnets are rich in quartz, phengite, epidote, kyanite, rutile, and ilmenite inclusions. Biotite, chlorite, apatite, monazite, and Al-cerite inclusions are also present. Garnet porphyroclasts are wrapped by laminae of small garnet - white mica - biotite - quartz or zoisite - kyanite - plagioclase alternating with bands made up of fine

  14. Constraints on the timing of Co-Cu ± Au mineralization in the Blackbird district, Idaho, using SHRIMP U-Pb ages of monazite and xenotime plus zircon ages of related Mesoproterozoic orthogneisses and metasedimentary rocks

    USGS Publications Warehouse

    Aleinikoff, John N.; Slack, John F.; Lund, Karen; Evans, Karl V.; Fanning, C. Mark; Mazdab, Frank K.; Wooden, Joseph L.; Pillers, Renee M.

    2012-01-01

    The Blackbird district, east-central Idaho, contains the largest known Co reserves in the United States. The origin of strata-hosted Co-Cu ± Au mineralization at Blackbird has been a matter of controversy for decades. In order to differentiate among possible genetic models for the deposits, including various combinations of volcanic, sedimentary, magmatic, and metamorphic processes, we used U-Pb geochronology of xenotime, monazite, and zircon to establish time constraints for ore formation. New age data reported here were obtained using sensitive high resolution ion microprobe (SHRIMP) microanalysis of (1) detrital zircons from a sample of Mesoproterozoic siliciclastic metasedimentary country rock in the Blackbird district, (2) igneous zircons from Mesoproterozoic intrusions, and (3) xenotime and monazite from the Merle and Sunshine prospects at Blackbird. Detrital zircon from metasandstone of the biotite phyllite-schist unit has ages mostly in the range of 1900 to 1600 Ma, plus a few Neoarchean and Paleoproterozoic grains. Age data for the six youngest grains form a coherent group at 1409 ± 10 Ma, regarded as the maximum age of deposition of metasedimentary country rocks of the central structural domain. Igneous zircons from nine samples of megacrystic granite, granite augen gneiss, and granodiorite augen gneiss that crop out north and east of the Blackbird district yield ages between 1383 ± 4 and 1359 ± 7 Ma. Emplacement of the Big Deer Creek megacrystic granite (1377 ± 4 Ma), structurally juxtaposed with host rocks in the Late Cretaceous ca. 5 km north of Blackbird, may have been involved in initial deposition of rare earth elements (REE) minerals and, possibly, sulfides. In situ SHRIMP ages of xenotime and monazite in Co-rich samples from the Merle and Sunshine prospects, plus backscattered electron imagery and SHRIMP analyses of trace elements, indicate a complex sequence of Mesoproterozoic and Cretaceous events. On the basis of textural relationships

  15. Timing of Deformation in the Central Metasedimentary Belt Boundary Thrust Zone (CMBbtz), southern Ontario, Canada, from Electron Microprobe Dating of Monazite

    NASA Astrophysics Data System (ADS)

    Markley, M. J.; Dunn, S. R.; Peck, W. H.; Jercinovic, M. J.; Williams, M. L.

    2015-12-01

    In the Grenville Province of Southern Ontario, the Central Metasedimentary Belt boundary thrust zone (CMBbtz) is a crustal-scale tectonic boundary between the older, granulite-facies Central Gneiss Belt to the NW and the younger, amphibolite-facies Central Metasedimentary Belt to the SE. Although there are a range of tectonic models for the CMBbtz, most workers agree it is a major tectonic boundary that accommodated ductile thrusting and crustal shortening during the Ottawan phase of the Grenville Orogeny (~1080-1020 Ma). Some studies suggest that ductile thrusting in the CMBbtz was roughly synchronous with synorogenic extensional collapse below an orogenic lid. Previous geochronological studies also provide evidence of earlier deformation and/or metamorphic events in the CMBbtz, although the relation between deformation in the CMBbtz to the Elzeviran (~1230 Ma) and Shawinigan (~1180 Ma) orogenies is unclear. Our study is the first to report in situ electron microprobe monazite (mnz) dates from amphibolite-grade ortho- and para-gneisses of the CMBbtz. Our results are broadly consistent with other chronometers. We present dates from 132 age-domains within 83 mnz grains in 14 samples. Although our data provide strong evidence for deformation and metamorphism along the length of the CMBbtz during the Ottawan (1080-1020 Ma), we also report two other clusters of ages: 1140-1110 Ma and 1230-1170 Ma. The latter cluster falls between the widely accepted ranges for the Elzeviran and Shawinigan orogenies. In addition, some individual outcrops, particularly those in Killaloe and Minden, show mnz ages spanning over 200 m.y., and the setting and compositions of individual monazite domains allow us to link mnz growth to episodes of garnet growth during multiple events. Together these data indicate an unexpectedly continuous and long-lived period of deformation and metamorphism in the CMBbtz.

  16. Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia

    NASA Astrophysics Data System (ADS)

    Rubatto, Daniela; Williams, Ian S.; Buick, Ian S.

    2001-01-01

    We report an extensive field-based study of zircon and monazite in the metamorphic sequence of the Reynolds Range (central Australia), where greenschist- to granulite-facies metamorphism is recorded over a continuous crustal section. Detailed cathodoluminescence and back-scattered electron imaging, supported by SHRIMP U-Pb dating, has revealed the different behaviours of zircon and monazite during metamorphism. Monazite first recorded regional metamorphic ages (1576 ± 5 Ma), at amphibolite-facies grade, at ˜600 °C. Abundant monazite yielding similar ages (1557 ± 2 to 1585 ± 3 Ma) is found at granulite-facies conditions in both partial melt segregations and restites. New zircon growth occurred between 1562 ± 4 and 1587 ± 4 Ma, but, in contrast to monazite, is only recorded in granulite-facies rocks where melt was present (≥700 °C). New zircon appears to form at the expense of pre-existing detrital and inherited cores, which are partly resorbed. The amount of metamorphic growth in both accessory minerals increases with temperature and metamorphic grade. However, new zircon growth is influenced by rock composition and driven by partial melting, factors that appear to have little effect on the formation of metamorphic monazite. The growth of these accessory phases in response to metamorphism extends over the 30 Ma period of melt crystallisation (1557-1587 Ma) in a stable high geothermal regime. Rare earth element patterns of zircon overgrowths in leucosome and restite indicate that, during the protracted metamorphism, melt-restite equilibrium was reached. Even in the extreme conditions of long-lasting high temperature (750-800 °C) metamorphism, Pb inheritance is widely preserved in the detrital zircon cores. A trace of inheritance is found in monazite, indicating that the closure temperature of the U-Pb system in relatively large monazite crystals can exceed 750-800 °C.

  17. U-Pb Geochronology: Taking or Creating the Pulse of Magmatic Systems?

    NASA Astrophysics Data System (ADS)

    Schoene, B.; Barboni, M.; Samperton, K. M.

    2015-12-01

    A combination of field, laboratory, and theoretical investigations has led to models for constructing upper crustal batholiths through pulsed emplacement of smaller magma batches. Whether melt accumulates or freezes at the emplacement level depends in part on the magma flux and the size of individual injections. Calibrating these variables in real systems has benefited greatly from the application of U-Pb zircon geochronology; however, as sampling density and number of analyses increases, it is commonly observed that zircon dates are continuous, not pulsed, on the pluton scale. Accurately interpreting such datasets to calibrate magmatic tempos thus requires an improved set of tools that link zircon crystallization histories to magmatic processes in dynamic systems. This contribution evaluates recent progress and challenges in using U-Pb geochronology to building models for magma transport and residence, as driven by the following questions: What are the timescales and sites of magmatic differentiation? What are the supersolidus temperature-time paths of magmas? What controls magma accumulation and eruptibility? Do zircon or other accessory minerals actually record these processes uniquely? To answer these questions, our recent work has emphasized integrating zircon geochronology and geochemistry with petrologic techniques, numerical modeling, and field mapping. Using this approach on arc systems from the pluton to batholith scale, we can now better characterize the pulsed nature of upper crustal magmatism and track the presence and crystallization history of melts. However, uncertainties persist in regard to our understanding of, e.g., zircon trace element partition coefficients, controls on magma zircon saturation, and how sampling bias at the handsample and regional scales affects our models of crustal magmatism. Addressing these unknowns will only further augment geochronology's role in reconstructing the formation, evolution and emplacement of magmas.

  18. Radioactive Dating: A Method for Geochronology.

    ERIC Educational Resources Information Center

    Rowe, M. W.

    1985-01-01

    Gives historical background on the discovery of natural radiation and discusses various techniques for using knowledge of radiochemistry in geochronological studies. Indicates that of these radioactive techniques, Potassium-40/Argon-40 dating is used most often. (JN)

  19. Community-Based Development of Standards for Geochemical and Geochronological Data

    NASA Astrophysics Data System (ADS)

    Lehnert, K. A.; Walker, D.; Vinay, S.; Djapic, B.; Ash, J.; Falk, B.

    2007-12-01

    The Geoinformatics for Geochemistry (GfG) Program (www.geoinfogeochem.org) and the EarthChem project (www.earthchem.org) aim to maximize the application of geochemical data in Geoscience research and education by building a new advanced data infrastructure for geochemistry that facilitates the compilation, communication, serving, and visualization of geochemical data and their integration with the broad Geoscience data set. Building this new data infrastructure poses substantial challenges that are primarily cultural in nature, and require broad community involvement in the development and implementation of standards for data reporting (e.g., metadata for analytical procedures, data quality, and analyzed samples), data publication, and data citation to achieve broad acceptance and use. Working closely with the science community, with professional societies, and with editors and publishers, recommendations for standards for the reporting of geochemical and geochronological data in publications and to data repositories have been established, which are now under consideration for adoption in journal and agency policies. The recommended standards are aligned with the GfG and EarthChem data models as well as the EarthChem XML schema for geochemical data. Through partnerships with other national and international data management efforts in geochemistry and in the broader marine and terrestrial geosciences, GfG and EarthChem seek to integrate their development of geochemical metadata standards, data format, and semantics with relevant existing and emerging standards and ensure compatibility and compliance.

  20. Microporosity linked fluid focusing and monazite instability in greenschist facies para-conglomerates, western India

    NASA Astrophysics Data System (ADS)

    Rekha, S.; Bhattacharya, A.; Viswanath, T. A.

    2013-03-01

    In a deformed para-conglomerate (western India), monazites occur as discrete grains (50-250 μm long axis) in tonalite clasts, in the warping chlorite-biotite-calcite schist layers, and as disseminated grains (<5 μm) along with xenotime and huttonite forming veins in schist layers. The veins are absent in clasts. Chlorite and biotite defining schistosity in schist layers and in synchronously-formed extensional veins/fractures in clasts are chemically identical. δ18O (VSMOW) and δ13C (VPDB) in calcite in schistosity, in clasts, and in "beards" in pressure shadow zones around clasts are tightly bracketed, 11.4-12.2‰ and -3.1 to -3.6‰ respectively. Margins of clast-hosted monazites are partly altered to fluorapatite - huttonite ± allanite ± Y-xenotime. By contrast, monazite alteration in schist layers is extensive. In schist layers, rafts of embayed monazite occur in fluorapatite-dominated corona studded with idiomorphic huttonite, Y-xenotime ± allanite. Mass balance calculation in a well-defined alteration zone around monazite in schist layer with P conserved indicate large gain in mass fraction of lower-abundance, less soluble HREEs, Y and Th in the alteration zone was compensated by minor loss in mass fraction of higher abundance elements (LREEs, MREEs) resulting in negligible net mass change. Monazite decomposition was associated with ˜50% dilation strain. Yb, Th and U abundances in alteration zones around decomposed monazites in schist layer are computed to be manifold higher than average schists. The contemporaneous formation of pore microstructures, the chemical homogeneity among ferromagnesian minerals and C-O isotope data in calcite in different textural domains, and results of mass balance calculations are compatible with deformation-aided porosity generation, synchronous fluid-mediated advection transport of elements, and dissolution-re-precipitation of mica-calcite aggregates at upper crustal conditions, ˜500 °C, ˜4 kbar. Monazite instability

  1. Appendix G: Geochemistry

    SciTech Connect

    Cantrell, Kirk J.; Serne, R. Jeffrey; Zachara, John M.; Krupka, Kenneth M.; Dresel, P. Evan; Brown, Christopher F.; Freshley, Mark D.

    2008-01-17

    This appendix discusses the geology of the Hanford Site and singe-shell tank (SST) waste management areas (WMAs). The purpose is to provide the most recent geochemical information available for the SST WMAs and the Integrated Disposal Facility. This appendix summarizes the information in the geochemistry data package for the SST WMAs.

  2. Leaching of uranium and thorium from monazite: I. Initial leaching

    NASA Astrophysics Data System (ADS)

    Eyal, Yehuda; Olander, Donald R.

    1990-07-01

    Interaction of three natural monazite specimens with a bicarbonate-carbonate solution was investigated for times up to 6.8 years. Dissolution was observed to be incongruent with respect to 238U and 232Th as well as their radiogenic daughters 234U, 230Th, and 228Th. Leaching was divided into a very rapid initial stage lasting a few hours and a slower process active for the remaining time. The initial stage was modeled as the sum of a contribution from a mechanico-chemically damaged portion of the specimen, which did not exhibit isotopic selectivity in leach properties, and a contribution from the selective removal of recoil daughter products from their recoil tracks in the surface of the otherwise undamaged bulk mineral. The latter effect is greater for short-lived 228Th compared to long-lived 234U. A correlation between the magnitude of the effect and the half-life of the radiogenic nuclide suggests an upper limit of ~ 10 6 years for the timescale of natural track annealing. After the initial dissolution stage, insoluble precipitates of the intermediate product in the Th chain, 228Ra, provide a supplementary source of 228Th by radioactive decay. Contributions to these precipitates come from dissolution-released and recoil-released 228Ra. This source is manifest as large apparent release rates of 228Th which begin after several weeks of leaching. Preannealing of a specimen at 800°C depresses the elemental Th leach rate but enhances the amount of 228Th /232Th fractionation. This enhancement is associated with rejection of Ra from the mineral during annealing of α-recoil damage.

  3. First-principles elastic stiffness of LaPO4 monazite

    NASA Astrophysics Data System (ADS)

    Wang, Jingyang; Zhou, Yanchun; Lin, Zhijun

    2005-08-01

    In this letter, the full set of elastic coefficients of LaPO4 monazite is presented based on the first-principles plane-wave pseudopotential total energy method. Mechanical parameters (bulk modulus, shear modulus, Young's moduli, and Poisson's ratio) are also presented and compared with experimental results for polycrystalline monazite. The responses of electronic structure and chemical bonds to a series of {010}⟨001⟩ shear strains are examined in order to study the mechanism of low shear strain resistance. The results show that small shear moduli originate from the inhomogeneous strengths of atomic bonds. For example, the weak La-O bonds accommodate the shear strain locally, while the PO4 tetrahedra are almost rigid. The theoretical elastic stiffness may be useful to understand the deformation mechanisms of LaPO4 monazite.

  4. New geochronological history of the Mbuji-Mayi Supergroup (Proterozoic, DRC) through U-Pb and Sm-Nd dating

    NASA Astrophysics Data System (ADS)

    François, Camille; Baludikay, Blaise K.; Storme, Jean-Yves; Baudet, Daniel; Paquette, Jean-Louis; Fialin, Michel; Debaille, Vinciane; Javaux, Emmanuelle J.

    2016-04-01

    The Mbuji-Mayi Supergroup, DRC is located between the Archean-Paleoproterozoic Kasai Craton and the Mesoproterozoic Kibaran Belt. This sedimentary sequence, unaffected by regional metamorphism, preserves a large diversity of well-preserved acritarchs (organic-walled microfossils), evidencing the diversification of complex life (early eukaryotes) for the first time in mid-Proterozoic redox stratified oceans of Central Africa (Baludikay et al., in review). This Supergroup is composed of two distinct lithostratigraphic successions (i) BI Group: a lower siliciclastic sequence (ca. 1175 Myr to ca. 882 Myr or ca. 1050 Myr (Cahen, 1954; Holmes & Cahen, 1955; Delpomdor et al., 2013) unconformably overlying the ca. 2.82-2.56 Gyr granitoid Dibaya Complex to the North (Cahen & Snelling; recent notice on DRC geological map); and (ii) BII Group: a poorly age-constrained upper carbonate sequence with sparse shales . Basaltic lavas (including pillow lavas) overlying the Mbuji-Mayi Supergroup were dated around 950 Myr (Cahen et al., 1974; Cahen et al., 1984). To better constraint the age of this Supergroup in the Meso-Neoproterozoic limit, we combine different geochronological methods, in particular on diagenetic minerals such as monazite (Montel et al., 1996; Rasmussen & Muhling, 2007) and xenotime (McNaughton et al., 1999) but also on detrital zircons. For the BI Group, results of in situ U-Pb dating with LA-ICP-MS on monazite, xenotime and zircon (Laboratoire Magmas et Volcans, Clermont-Ferrand) provide ages between 2.9 and 1.2 Gyr for zircons and between 1.4 and 1.03 Gyr for monazites and xenotimes. New results of in situ U-Th-Pb dating of well-crystallized monazites and xenotimes with Electron MicroProbe (Camparis, UPMC, Paris), highlight that some crystals display zonations with an inherited core older than 1125 Myr and diagenetic rims around 1050-1075 Myr. This suggests that the diagenesis of BI Group is younger than 1175 Myr (Delpomdor et al., 2013) and probably around

  5. Direct Comparison of Monazite Ages Obtained By in situ Techniques: Ion-Probe Isotopic Ages Versus Electron Microprobe Chemical Ages

    NASA Astrophysics Data System (ADS)

    Tracy, R. J.; Loehn, C. W.; Dahl, P. S.; Aleinikoff, J. N.; Wooden, J. L.; Hamilton, M.; Mazdab, F.; Jones, C.

    2005-12-01

    In situ analytical techniques for geochronology are rapidly becoming the method of choice for characterizing compositionally and chronologically complex minerals, including monazite (mnz) and zircon (zrc). Two such techniques include Ion Microprobe (IMP) (esp. SHRIMP) for both mnz and zrc, and electron microprobe (EMP) for mnz. Debate remains concerning comparability of ages obtained by the two different techniques: U-Pb isotopic dating (IMP) versus Th-total U-total Pb (EMP). The IMP has an advantage in analytical precision whereas the EMP has an advantage in spatial resolution. We report 6 examples of individual monazite grains that have been dated by both techniques, covering a range of ages from 300 to 2850 Ma. Three of our examples are grains that have been used as IMP standards at the Geological Survey of Canada (GSC) or at the USGS-Stanford SHRIMP lab; the other grains are from research samples. Most grains display complex zoning in Y, Th and U, both in BSE images and in compositional maps, reflecting complex growth and recrystallization histories. In all cases, the ages obtained by the two techniques agree within their 2-sigma associated error, except where IMP ablation pits cross age boundaries and resolve a mixed age of the two domains, or where EMP spots fall near cracks or pits in the grain surface. One prominent example is a 100-micron mnz from the Tobacco Root Mountains, Montana. It contains a low-Th older core (ca. 2.85 Ga), a higher-Th mantle domain of about 2.45 Ga, and a low-Th rim of 1.78 Ga. This grain has 6 IMP spots that range in age from 1880 Ma (near-rim) to 2785 Ma (core). Only two IMP pits fall totally within a single chemical and age zone delineated by EMP analyses or compositional maps (the medial age zone): 2451 (+/-4) and 2432 (+/-10). The weighted mean EMP age of this domain is 2452 (+/-6). IMP spots aimed at the older core are 2619 (+/-11) and 2785 (+/-9); the weighted mean core age from EMP analyses is 2859 (+/-14). This suggests

  6. Stratigraphy, regional distribution, and reconnaisance geochemistry of Oligocene and Miocene volcanic rocks in the Paradise Range and northern Pactolus Hills, Nye County, Nevada

    SciTech Connect

    Not Available

    1992-01-01

    In this paper, stratigraphy, geochronology, and geochemistry of about 40 units of Oligocene and Miocene silicic ash-flow tuff, intermediate to silicic lava, and minor sedimentary rock exposed in three stratigraphic sections are described. The distribution and sources of five regionally widespread ash-flow tuff units were delineated. The late Cenozoic volcanic and tectonic history of the Paradise Range and northern Pactolus Hills was compared to other areas in the Great Basin.

  7. An application of Petrochronology: U/Th-Pb geochronologic rates of burial and exhumation in the Cordilleran hinterland, northern Nevada, USA

    NASA Astrophysics Data System (ADS)

    Hallett, B. W.

    2014-12-01

    Monazite and zircon U/Th-Pb dating and yttrium thermometry results from metapelitic rocks of the northern East Humboldt Range, Nevada provide insight into the tectonic evolution of the North American Cordillera. Combining a petrologic approach with in situ U/Th-Pb SHRIMP geochronology gives constraints on the timing and rates of burial and exhumation. When integrated with the results from studies of the intrusive igneous and structural evolution of a complexly deformed crustal block, a broad picture of the behavior of the thickened middle crust in an orogenic wedge begins to emerge. In samples from the upper limb of the south verging Winchell Lake nappe, monazite cores giving moderate YPO4 components yield a mean U-Pb age of 82.8 ± 1.3 Ma representing initial growth near garnet zone conditions. A cycle of burial and heating giving way to decompression and melting, followed by zircon growth accompanied by low YPO4 monazite growth giving a wide range of ages (77.1-62.4 Ma) apparently documents cooling and melt crystallization. Monazite core and zircon ages from these samples bracket the time frame for this cycle through peak pressure and then temperature. Rates of burial therefore are ~1.0­-4.5 mm/year for the Winchell Lake nappe. Migmatitic pelites from the Lizzies Basin area, structurally beneath the Winchell Lake nappe give U-Pb monazite age brackets of 96.5-79.4 Ma, yielding a narrower range of burial rates, ~0.5-2.0 mm/year. Calculated exhumation rates for the same rocks from both the upper limb of the Winchell Lake nappe and the Lizzies Basin area fall between ~0.4 and 1.7 mm/year. Rapid burial followed by rapid exhumation is consistent with thickening by an overthrust mechanism followed by vertical ductile thinning, underplating, erosion, and/or normal faulting. Exhumation is generally slower than estimates for exhumation of metamorphic core complexes north of the Snake River Plain calculated by Bendick and Baldwin (2009). This may represent a difference in

  8. Mafic dykes intrusive into Pre-Cambrian rocks of the São Luís cratonic fragment and Gurupi Belt (Parnaíba Province), north-northeastern Brazil: Geochemistry, Sr-Nd-Pb-O isotopes, 40Ar/39Ar geochronology, and relationships to CAMP magmatism

    NASA Astrophysics Data System (ADS)

    Klein, Evandro L.; Angélica, Rômulo S.; Harris, Chris; Jourdan, Fred; Babinski, Marly

    2013-07-01

    Dykes of diabase and microgabbro intruded into Pre-Cambrian rocks of the São Luís cratonic fragment and Gurupi Belt, which are tectonic and erosive windows of the Parnaíba Basin in north-northeastern Brazil. Ar-Ar ages were determined, and major, trace element, and Nd-Sr-Pb-O isotopic compositions of these dykes were measured to provide insights into their age, and into the nature of their mantle sources and petrogenetic processes. The data have also been used to compare the chemical and isotopic signatures of the dykes with those of the Central Atlantic Magmatic Province (CAMP). Four chemical groups of mafic dykes have been identified. These comprise two subtypes of high-Ti rocks (i) HTi-1 (TiO2 < 2.3 wt.%; SiO2 > 47 wt.%), (ii) HTi-2 (TiO2 > 2.7 wt.%; SiO2 > 47 wt.%), in addition to (iii) evolved high-Ti (TiO2 > 4 wt.%; SiO2 of ~ 46 wt.%) and (iv) low-Si (TiO2 > 2.2 wt.%; SiO2 < 45 wt.%) rocks. 40Ar/39Ar geochronology of plagioclase returned ages of 201 ± 4 Ma and 193 ± 10 Ma for the HTi-2 subtype, and of 201 ± 2 Ma and 207 ± 9 Ma for the evolved high-Ti group. The HTi-1 and low-Si groups presented highly disturbed age spectra, and did not allow the definition of their emplacement ages. The Argon data indicate an age > 200 Ma for the low-Si group and are dubious with respect to the age of theHTi-1 subtype, if coeval with (i.e., ~ 200 Ma), or older than, the HTi-2 and evolved high-Ti types. All groups present δ18O values of pyroxene that are compatible with uncontaminated mantle-derived magmas. The HTi-1 subtype (average 143Nd/144Nd200 = 0.512644; 87Sr/86Sr200 = 0.7035; 206Pb/204Pb of 17.86) shows the less enriched and less fractionated (more primitive) trace element distribution of all groups. The HTi-2 subtype shows enriched trace element pattern and depleted Nd-Sr signature (143Nd/144Nd200 = 0.512610; 87Sr/86Sr200 = 0.7037) and average 206Pb/204Pb ratios of 17.23. The evolved high-Ti chemical group shows average ratios of 143Nd/144Nd200 = 0.512558, 87Sr

  9. Metamorphic and structural evolution of the Straits Schist, W. Connecticut: U-PB monazite constraints

    SciTech Connect

    Lanzirotti, A.; Hanson, G.N. . Dept. of Earth and Space Sciences)

    1993-03-01

    U-Pb dating of monazites from two outcrops of The Straits Schist south of Waterbury Dome precisely constrains the timing of Acadian metamorphism and deformation. Synkinematic pegmatites which parallel schistosity are common in The Straits Schist south of Waterbury. A pegmatite at the type-section of The Straits Schist east of Straitsville yields U-Pb monazite ages of 385 [+-] 2.5 Ma. At both outcrops the axial planar schistosity is defined by muscovites and biotites which wrap around axes of folds but are not themselves deformed, suggesting they represent a recrystallized fabric. Monazites in the schists at both outcrops display petrographic evidence of grain boundary pinning against mica surfaces and growth across muscovite-biotite grain boundaries, consistent with growth during or after biotite and muscovite crystallization. Monazites from the schist at Straitsville and Naugatuck yield U-Pb ages of 383 [+-] 3 Ma. The U-Pb data, coupled with field and petrographic observations, suggest that pegmatite intrusion, deformation, and metamorphic recrystallization occurred within 3 Ma of about 383 Ma.

  10. The Influence of Interfacial Roughness on Fiber Sliding in Oxide Composites with La-Monazite Interphases

    NASA Technical Reports Server (NTRS)

    Davis, J. B.; Hay, R. S.; Marshall, D. B.; Morgan, P. E. D.; Sayir, A.; Gray, Hugh R. (Technical Monitor); Farmer, Serene C. (Technical Monitor)

    2002-01-01

    Room temperature debonding and sliding of La-Monazite coated fibers is assessed using a composite with a polycrystalline alumina matrix and fibers of several different single crystal (mullite, sapphire) and directionally solidified eutectic (Al2O3/Y3Al5O12 and Al2O3/Y-ZrO2) compositions. These fibers provide a range of residual stresses and interfacial roughnesses. Sliding occurred over a debond crack at the fiber-coating interface when the sliding displacement and surface roughness were relatively small. At large sliding displacements with relatively rough interfaces, the monazite coatings were deformed extensively by fracture, dislocations and occasional twinning, whereas the fibers were undamaged. Dense, fine-grained (10 nm) microstructures suggestive of dynamic recrystallization were also observed in the coatings. Frictional heating during sliding is assessed. The possibility of low temperature recrystallization is discussed in the light of the known resistance of monazite to radiation damage. The ability of La-Monazite to undergo plastic deformation relatively easily at low temperatures may be enabling for its use as a composite interface.

  11. CHIME monazite dating using FE-EPMA equipped with R=100 mm spectrometers

    NASA Astrophysics Data System (ADS)

    Shibata, K.; Shimizu, M.; Suzuki, K.; Sueoka, S.; Niwa, M.

    2015-12-01

    The age spectrum of detrital monazite grains is used to unravel the tectono-thermal history of the pre-Neogene terranes, which is required for geological disposal of high-level radioactive waste on tectonically active Japanese Islands. The CHIME (Chemical Th-U-total Pb isochron method, Suzuki and Adachi, 1991) is best suited for dating of detrital monazite whose grains are not chronologically uniform. In the previous studies (eg, Suzuki, 2011), EPMA equipped with conventional R=140 mm spectrometers was used for measurement of U, Th and Pb. However the spectrometers have low count rate of measurement of Pb. The JEOL JXA-8530F FE-EPMA equipped with R=100 mm spectrometers has been applied for the CHIME monazite dating. The intrinsic responses of each of the R=100 mm spectrometers for PbMβ are around ten times higher than that of the R=140 mm spectrometer. The R=100 mm spectrometers permits obtaining high count rate, which enables us to shorten measurement time than before. As these spectrometers have peculiar spectral interference, the method reported by Amli and Griffin (1978) is applied for correction of the interference. In order to verify the dating using the FE-EPMA and the interference correction method, two distinct age groups of monazite were measured. The ages were 425±25 Ma for monazite from Cooma granite in southeastern Australia, which had dated by SHRIMP as 432.8 ± 3.5 Ma (Williams, 2001) and 67 ± 7 Ma for monazite of the Kojaku granite in southwestern Japan, which is corresponding to the LA-ICP-MS U-Pb zircon ages of 68.5 ± 0.7 Ma. These results indicate that the FE-EPMA and the interference correction method are useful for the CHIME monazite dating and for revealing the tectono-thermal history of the terranes. This study was carried out under a contract with Agency of Natural Resources and Energy(ANRE), part of Ministry of Economy, Trade and Industry (METI) of Japan as part of its R&D supporting program for developing geological disposal technology.

  12. Swelling induced by alpha decay in monazite and zirconolite ceramics: A XRD and TEM comparative study

    NASA Astrophysics Data System (ADS)

    Deschanels, X.; Seydoux-Guillaume, A. M.; Magnin, V.; Mesbah, A.; Tribet, M.; Moloney, M. P.; Serruys, Y.; Peuget, S.

    2014-05-01

    Zirconolite and monazite matrices are potential ceramics for the containment of actinides (Np, Cm, Am, Pu) which are produced over the reprocessing of spent nuclear fuel. Actinides decay mainly through the emission of alpha particles, which in turn causes most ceramics to undergo structural and textural changes (amorphization and/or swelling). In order to study the effects of alpha decays on the above mentioned ceramics two parallel approaches were set up. The first involved the use of an external irradiation source, Au, which allowed the deposited recoil energy to be simulated. The second was based on short-lived actinide doping with 238Pu, (i.e. an internal source), via the incorporation of plutonium oxide into both the monazite and zirconolite structures during synthesis. In both types of irradiation experiments, the zirconolite samples became amorphous at room temperature with damage close to 0.3 dpa; corresponding to a critical dose of 4 × 1018 α g-1 (i.e. ∼1.3 × 1021 keV cm-3). Both zirconolite samples also showed the same degree of macroscopic swelling at saturation (∼6%), with ballistic processes being the predominant damaging effect. In the case of the monazite however, the macroscopic swelling and amorphization were dependent on the nature of the irradiation. Externally, (Au), irradiated samples became amorphous while also demonstrating a saturation swelling of up to 8%. In contrast to this, the swelling of the 238Pu doped samples was much smaller at ∼1%. Also, unlike the externally (Au) irradiated monazite these 238Pu doped samples remained crystalline up to 7.5 × 1018 α g-1 (0.8 dpa). XRD, TEM and swelling measurements were used to fully characterize and interpret this behavior. The low swelling and the conservation of the crystalline state of 238Pu doped monazite samples indicates that alpha annealing took place within this material.

  13. Surface chemistry and flotation behavior of monazite, apatite, ilmenite, quartz, rutile, and zircon using octanohydroxamic acid collector

    NASA Astrophysics Data System (ADS)

    Nduwa Mushidi, Josue

    Global increase in rare earth demand and consumption has led to further understanding their beneficiation and recovery. Monazite is the second most important rare earth mineral that can be further exploited. In this study, the surface chemistry of monazite in terms of zeta potential, adsorption density, and flotation response by microflotation using octanohydroxamic acid is determined. Apatite, ilmenite, quartz, rutile, and zircon are minerals that frequently occur with monazite among other minerals. Hence they were chosen as gangue minerals in this study. The Iso Electric Point (IEP) of monazite, apatite, ilmenite, quartz, rutile, and zircon are 5.3, 8.7, 3.8, 3.4, 6.3, and 5.1 respectively. The thermodynamic parameters of adsorption were also evaluated. Ilmenite, rutile and zircon have high driving forces for adsorption with DeltaGads. = 20.48, 22.10, and 22.4 kJ/mol respectively. The free energy of adsorption is 14.87 kJ/mol for monazite. Adsorption density testing shows that octanohydroxamic acid adsorbs on negatively charged surfaces of monazite and its gangue minerals which indicates chemisorption. This observation was further confirmed by microflotation experiments. Increasing the temperature to 80°C raises the adsorption and flotability of monazite and gangue minerals. This does not allow for effective separation. Sodium silicate appeared to be most effective to depress associated gangue minerals. Finally, the fundamentals learned were applied to the flotation of monazite ore from Mt. Weld. However, these results showed no selectivity due to the presence of goethite as fine particles and due to a low degree of liberation of monazite in the ore sample.

  14. Geochronologic constraints on syntaxial development in the Nanga Parbat region, Pakistan

    NASA Astrophysics Data System (ADS)

    Winslow, David M.; Zeitler, Peter K.; Chamberlain, C. Page; Williams, Ian S.

    1996-12-01

    40Ar/39Ar data (hornblende, biotite, muscovite, and K-feldspar) and U/Pb data (zircons) were obtained from the Nanga Parbat-Haramosh Massif (NPHM), NW Pakistan, along three transects in the southern regions of the NPHM. We have based our interpretations on our new data as well as geochronologic dates from previous studies in the northern regions of the massif. Geochronologic data show that the NPHM has experienced exceptionally high denudation and cooling rates over the past 10 m.y. U/Pb ages determined through sensitive high-resolution ion microprobe (SHRIMP) "depth-profiling" experiments on metamorphic zircons and conventional U/Pb monazite dates suggest that the timing of metamorphism varied across the massif. In addition, we have documented that the massif has experienced postmetamorphic, differential cooling both along and across strike. Thermochronologic data on currently exposed surface rocks suggest that cooling occurred more recently and at greater rates in the south-central regions of the massif (representing deeper crustal levels) than along the margins and northern regions of the massif. Within the Tato region, cooling following peak metamorphic temperatures of 600°-700 °C was as high as 140 °C/m.y. following partial melting of pelitic units. Biotites from this area record plateau ages of 0.9 ± 0.1 Ma. Along the Astor and Indus gorges, cooling was less rapid (approximately 70°-80°C/m.y.) following peak metamorphism as indicated by U/Pb monazite ages of 6-8 Ma and 40Ar/39Ar muscovite cooling ages of 2.2-3.4 Ma. Cooling over the last 3 m.y. occurred at rates of 100°-140 °C/m.y. The overall cooling age pattern within the massif is interpreted syntaxial growth through the development of north plunging antiforms prior to 3 Ma, followed by reverse faulting along east dipping fault zones. Along the Raikot River transect the biotite cooling age pattern is consistent with the folding of isotherms during folding of the foliation surfaces. The age pattern

  15. Heterogeneous extrusion and exhumation of deep-crustal Variscan assembly: Geochronology of the Western Tatra Mountains, northern Slovakia

    NASA Astrophysics Data System (ADS)

    Moussallam, Y.; Schneider, D. A.; Janák, M.; Thöni, M.; Holm, D. K.

    2012-07-01

    The nature and style of mid-crustal assembly and exhumation during continental collision has been investigated in the Tatra Mountains of the Western Carpathians. The pre-Alpine basement of the Western Carpathians represents the easternmost exposure of the Variscan orogen in Europe, which marks the collision of Laurasia with Gondwanian-affiliated terranes during the Palaeozoic. The Tatric crystalline unit of the Western Tatra in northern Slovakia displays an inverted metamorphic sequence where a high-grade unit comprising migmatites with relicts of eclogite has been thrust over a lower-grade mica schist unit. New geochronological and thermochronological data together with published thermobarometry illuminate the metamorphic history of the Western Tatra. The Upper Unit eclogites with occasionally preserved omphacite record near isothermal decompression from 1.6 GPa to 1.0-1.2 GPa at 750-800 °C which lead to intensive re-equilibration at high-pressure granulite facies conditions, comparable to the peak metamorphic conditions of the host migmatite. Both eclogite and migmatite shared a retrograde P-T path following the insertion of the eclogite assemblage into the migmatites. The metamorphic evolution of the Lower Unit mica schist is constrained to peak P-T conditions of 0.6-0.8 GPa and 640 and 660 °C followed by retrogression. This suggests that different rock types of the Western Tatra metamorphic core shared only their exhumation path from mid-crustal levels. ID-TIMS Sm-Nd dating of garnet from eclogite yields a whole rock-garnet isochron age of 337 ± 10 Ma, with an initial ɛNd isotopic composition of + 8.3. In situ U-Pb dating of monazite from a migmatite surrounding the eclogite shows one age population of c. 380 Ma whereas monazite from a migmatite away from the eclogite preserves a robust 340 ± 11 Ma age which is indistinguishable from Sm-Nd garnet age and U-Pb age of zircons in the anatectic leucosome of the migmatite (347 ± 7 Ma). A younger monazite age

  16. Experimental and geologic evaluation of monazite (U-Th)/He thermochronometry: Catnip Sill, Catalina Core Complex, Tucson, AZ

    NASA Astrophysics Data System (ADS)

    Peterman, E. M.; Hourigan, J. K.; Grove, M.

    2014-10-01

    Monazite is a petrologically important and analytically promising target for (U-Th)/He thermochronology. Previous studies have reported highly variable He diffusion results from monazite from a single sample and demonstrated that composition can significantly affect He diffusion parameters. In this study, we performed incremental heating of single monazite grains to experimentally determine the 4He diffusion properties of reference monazite ‘554’ that occurs within a peraluminous two-mica granite from the Catnip Sill within the Catalina Core complex, Arizona. Assuming that the grain size defines the diffusion geometry, the six experiments yielded Ea values of 212 to 238±5 kJ mol (1σ) and Do values of 15.7 to 103 cm s with one value of 784 cm s. Monazite (U-Th)/He data from five grains yielded closure temperatures of 291 to 262 °C (± c. 15 °C) and ages of 23.8-20.3 (±∼1.2;2σ) Ma; the weighted mean age is 21.8±0.73 (MSWD=1.83, n=5) and the weighted mean closure temperature is 282±6 °C (MSWD=0.96, n=5;1σ). We tested the accuracy of these results by comparing our monazite thermochronology data with monazite Th/Pb depth profiling results, the 40Ar/39Ar thermal history for the Catnip Sill constrained using coexisting muscovite, biotite, and K-feldspar, and published regional zircon and apatite fission track results. The monazite Th/Pb data indicate emplacement of the Catnip Sill at ∼45 Ma. The 40Ar/39Ar muscovite and biotite data indicate cooling from 460 to 350 °C from 27 to 26 Ma. K-feldspar MDD modeling suggests cooling from 360 to 240 °C from 26 to 24 Ma. Zircon fission track data indicate cooling through 250 °C between 29 and 20 Ma. Additional cooling through 110 °C is recorded by apatite fission track ages of 19-16 Ma. Because the monazite thermochronology results are reproducible and consistent with the thermal history constrained by the other chronometers, our results 1) confirm the accuracy of the 4He diffusion kinetics from monazite

  17. Kyanite-garnet gneisses of the Kåfjord Nappe - North Norwegian Caledonides: P-T conditions and monazite Th-U-Pb dating

    NASA Astrophysics Data System (ADS)

    Ziemniak, Grzegorz; Kośmińska, Karolina; Majka, Jarosław; Janák, Marian; Manecki, Maciej

    2016-04-01

    The Kåfjord Nappe is the part of the Skibotn Nappe Complex traditionally ascribed to the Upper Allochthon of the North Norwegian Caledonides. Pressure-temperature (P-T) conditions and metamorphic age of the Kåfjord Nappe are not well constrained, geochronological data are limited to a single Rb-Sr age of c. 440 Ma (Dangla et al. 1978). Metamorphic evolution of kyanite-garnet gneisses of the Kåfjord Nappe is presented here. The kyanite-garnet gneisses are associated with a few meters thick amphibolite lenses. The gneisses mainly consist of quartz, plagioclase, biotite, muscovite, garnet, kyanite, and rutile. Retrograde minerals are represented by sillimanite and chlorite. Garnet occurs as two textural types. Garnet-I forms euhedral porphyroblasts with multiple small inclusions. Profiles through garnet-I show chemical zonation in all components. The composition varies from Alm64-68Prp11-16Grs13-18Sps2-8 in the core to Alm68-70Prp17-18Grs10-13Sps1-3 in the rim. Garnet-II is subhedral to anhedral, its core is inclusion-rich, whereas rim contains only single inclusions. Chemical composition of garnet-II is similar to that of the garnet-I rim. P-T conditions have been estimated using the garnet-biotite-muscovite-plagioclase (GBPM) geothermobarometer (Holdaway, 2001; Wu, 2014). Calculated peak P-T metamorphic conditions are 610-625 °C and 7.6-8.2 kbar corresponding to the amphibolite facies conditions. Phase equilibrium modelling in the NCKFMMnASH system yields peak metamorphic conditions of c. 620 °C at 8 kbar. Growth conditions of garnet-I core modelled in the NCKFMMnASH system are c. 570 °C at 9.7 kbar. Chemical Th-U-total Pb monazite dating has been performed. Preliminary dating results from the kyanite-garnet gneiss of the Kåfjord Nappe yield an array of dates from 468 Ma to 404 Ma. There is a correlation between an increase of yttrium content and decrease of monazite single dates. Compositional maps confirm an increase of yttrium towards the rim of the

  18. Monazite in Mafic Dykes from the Bastar Craton, India: Implications for LREE Depletion in Martian Basalts

    NASA Astrophysics Data System (ADS)

    Herd, C. D.; French, J. E.; Chacko, T.; Heaman, L. M.; Borg, L. E.

    2004-05-01

    Martian basalts (shergottite meteorites) display very large variations in their incompatible element systematics, as evidenced by their highly fractionated and variable whole-rock REE patterns. Some samples show a marked depletion in LREE, which is apparent in CI-normalized bulk rock La/Sm ratios: some samples, such as Shergotty and Zagami, have La/Sm~0.9, i.e., relatively flat REE patterns; others, such as QUE 94201, have La/Sm~0.1, reflecting a depletion in LREE relative to the M-HREE. Furthermore, Sm-Nd isotopes in these samples indicate that their source regions are more enriched in LREE relative to M-HREE than the basalts themselves: the 147Sm/144Nd ratios estimated for martian basalt sources (from initial ɛ -Nd143) are 50 to 80% less than the whole-rock 147Sm/144Nd ratios of the basalts (Borg et al. 2001, 2003). This suggests that fractionation of LREE from M-HREE was a relatively recent phenomenon and occurred either in the source region, during magma generation, or during subsequent magma differentiation. This fractionation could reflect the involvement of a LREE-enriched phase in the petrogenesis of the martian basalts. One possibility is that this phase remained in the restite during partial melting, so that the melting process preferentially excluded LREE (Borg et al. 2003). This is supported by modeling of REE partitioning between liquid (=parental melt) and orthopyroxene, olivine and majoritic garnet (Borg and Draper 2003; Draper et al. 2003), which is unable to reproduce the LREE depletions. The phase (or process) responsible for the LREE depletion in the basalts must be able to strongly fractionate La-Nd from Sm-Lu. Monazite is a LREE-phosphate, generally: (La, Ce, Nd, Th)PO4; it is poor in HREE relative to LREE (Spear and Pyle 2002). Although a common accessory mineral in granites and metapelites (Rapp et al. 1987; Spear and Pyle 2002), to our knowledge, none has previously been reported in basalts. The Bastar craton of India is transected by

  19. Chemical and ceramic methods toward safe storage of actinides using monazite. 1998 annual progress report

    SciTech Connect

    Morgan, P.E.D.; Boatner, L.A.

    1998-06-01

    'The use of ceramic monazite, (La,Ce)PO{sub 4}, for sequestering actinides, especially plutonium, and some other radioactive waste elements (rare earths e.g.) and thus isolating them from the environment has been championed by Lynn Boatner of ORNL. It may be used alone or, as it is compatible with many other minerals in nature, can be used in composite combinations. Natural monazite, which almost invariably contains Th and U, is often formed in hydrothermal pegmatites and is extremely water resistant--examples are known where the mineral has been washed out of rocks (becoming a placer mineral as on the beach sands of India, Australia, Brazil etc.) then reincorporated into new rocks with new crystal overgrowths and then washed out again--being 2.5--3 billion years old. During this demanding water treatment it has retained Th and U. Where very low levels of water attack have been seen (in more siliceous waters), the Th is tied up as new ThSiO{sub 4} and remains immobile. Lest it be thought that rare-earths are rare or expensive, this is not so. In fact, the less common lanthanides such as gadolinium, samarium, europium, and terbium, are necessarily extracted and much used by, e.g., the electronics industry, leaving La and Ce as not-sufficiently-used by-products. The recent development of large scale use of Nd in Nd-B-Fe magnets has further exaggerated this. Large deposits of the parent mineral bastnaesite are present in the USA and in China. (Mineral monazite itself is not preferred due to its thorium content.) In the last 5 years it has become apparent show that monazite (more specifically La-monazite) is an unrecognized/becoming-interesting ceramic material. PuPO4 itself has the monazite structure; the PO{sub 4} 3-unit strongly stabilizes actinides and rare earths in their trivalent state. Monazite melts without decomposition (in a closed system) at 2,074 C and, being compatible with common ceramic oxides such as alumina, mullite, zirconia and YAG, is useful in

  20. Characterization of a site contaminated by waste from a monazite ore processing plant

    SciTech Connect

    Lauria, D.C.; Reis, V.R.; Nouailhetas, Y.; Godoy, J.M.; Agudo, E.G.

    1993-12-31

    A radiological survey of an area of 60,000 m{sup 2}, previously occupied by the Usina de Interlagos (USIN), a branch of the Brazilian State Monazite Company was conducted. External exposure gamma rates, surface soil, subsurface soil and groundwater concentration of the long-life radionuclides from the uranium and thorium decay chain were determined. Two areas, one of 4,800 m{sup 2} and other of 1,750 m{sup 2}, were found to be contaminated with different radioactive materials, originating from the chemical and physical processing of the monazite sand. {sup 228}Ra is present up to 2.2 {times} 10{sup 4} Bq/kg in soil and 93 Bq/l in groundwater. Based on future scenarios, an allowable residual contamination level of {sup 232}Th and {sup 226}Ra of around 200 Bq/kg was derived. Clean-up actions are suggested.

  1. Resolving Bias in Laser Ablation Geochronology

    NASA Astrophysics Data System (ADS)

    Bowring, James; Horstwood, Matthew; Gehrels, George

    2013-06-01

    Increasingly, scientific investigations requiring geochronology utilize laser ablation (LA)-inductively coupled plasma mass spectrometry (ICPMS), taking advantage of the efficiency and throughput possible for uranium-thorium-lead (U-Th-Pb) dating. A number of biases exist when comparing data among laboratories and an ongoing community-based effort is working to resolve and eliminate these biases to improve the accuracy of scientific interpretation based on these data.

  2. Sulfide Mineralogy and Geochemistry

    NASA Astrophysics Data System (ADS)

    Dilles, John

    2007-02-01

    Reviews in Mineralogy and Geochemistry Series, Volume 61 David J. Vaughan, Editor Geochemical Society and Mineralogical Society of America; ISBN 0-939950-73-1 xiii + 714 pp.; 2006; $40. Sulfide minerals as a class represent important minor rock-forming minerals, but they are generally known as the chief sources of many economic metallic ores. In the past two decades, sulfide research has been extended to include important roles in environmental geology of sulfide weathering and resultant acid mine drainage, as well as in geomicrobiology in which bacteria make use of sulfides for metabolic energy sources. In the latter respect, sulfides played an important role in early evolution of life on Earth and in geochemical cycling of elements in the Earth's crust and hydrosphere.

  3. Improving Consistency in Laser Ablation Geochronology

    NASA Astrophysics Data System (ADS)

    Horstwood, Matt; Gehrels, George; Bowring, James

    2010-07-01

    Workshop on Data Handling in LA-ICP-MS U-Th-Pb Geochronology; San Francisco, California, 12-13 December 2009; The use of uranium-thorium-lead (U-Th-Pb) laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) geochronology involves rapid analysis of U-and Th-rich accessory minerals. It routinely achieves 1-2% precision for U-Th-Pb dates constituting detrital mineral age spectra and for dating igneous and metamorphic events. The speed and low setup and analysis cost of LA-ICP-MS U-Th-Pb geochronology has led to a proliferation of active laboratories. Tens of thousands of analyses are produced per month, but there is little agreement on how to transform these data into accurate U-Th-Pb dates. Recent interlaboratory blind comparisons of zircon samples indicate that resolvable biases exist among laboratories and the sources of bias are not fully understood. Common protocols of data reduction and reporting are essential for scientists to be able to compare and interpret these data accurately.

  4. In-situ Rb-Sr geochronology

    NASA Astrophysics Data System (ADS)

    Anderson, F. S.; Nowicki, K.; Whitaker, T.

    This paper reports on the first rubidium-strontium (Rb-Sr) radiometric dates using a Laser Desorption Resonance Ionization Mass Spectrometry (LDRIMS) instrument capable of being miniaturized for flight to another planet. The LDRIMS instrument produces dates in under 24 hours, requires minimal sample preparation, and avoids the interference and mass resolution issues associated with other geochronology measurements. We have begun testing the bench-top prototype on the Boulder Creek Granite (BCG), from Colorado, comprised primarily of a gneissic quartz monzonite and granodiorite; whole rock Rb-Sr TIMS measurements result in dates of 1700± 40 Ma [1]. Data reduction of the LDRIMS Rb-Sr measurements on calibrated repeat runs result in a date for the BCG of 1.727± 0.087 Ga (n=288, MSWD=1). Most geochronology applications are willing to accept an MSWD up to ~2.7; at MSWD=2, the precision improves to ± 0.062 Ga. This technology is moving from lab prototype to field deployable instrument, and provides an opportunity to directly address the science goals of Mars Sample Return (MSR) within the bounds posed by current scientific, fiscal, and political pressures on the Mars program. Additionally, LDRIMS could potentially be flown to the Moon under the Discovery or New Frontiers program. We posit that in-situ geochronology missions to Mars to triage and validate samples for Mars Sample Return (MSR) are technically feasible in the 2018-2022 time frame.

  5. Synthesis and sintering of a monazite brabantite solid solution ceramic for nuclear waste storage

    NASA Astrophysics Data System (ADS)

    Montel, Jean-Marc; Glorieux, Benoit; Seydoux-Guillaume, Anne-Magali; Wirth, Richard

    2006-12-01

    Various geological arguments suggest that monazite can be an interesting waste-form for actinides such as Np, Pu, Cm and Am. We set up a simple procedure for making dense pellets of monazite brabantite solid solution ceramics with composition Ca0.092Th0.092Ce0.089La0.727PO4. It consists of co-milling CaCO3, ThO2, CeO2, La2O3, and NH4H2PO4, 1250 °C calcination, milling, cold-pressing, and sintering at 1450 °C for 4 h. X-ray investigations showed that the reaction scheme from oxides to monazite is complex and involves various P+La-based intermediate compounds. The final density of the the product is around 95% of the theoretical density. The texture is homogeneous with a typical grain of size 5 20 μm. This process is designed to be adapted to hot cells and telemanipulators.

  6. The timing of gold mineralization across the eastern Yilgarn craton using U-Pb geochronology of hydrothermal phosphate minerals

    NASA Astrophysics Data System (ADS)

    Vielreicher, Noreen; Groves, David; McNaughton, Neal; Fletcher, Ian

    2015-04-01

    The highly mineralized Eastern Goldfields of the eastern Yilgarn craton is an amalgamation of dominantly Neoarchaean granitoid-greenstone terranes and domains that record a history of early rifting, followed by westward directed collision with initial arc formation, collision and clastic basin formation, and final accretion to the western Yilgarn proto-craton between 2.66 and 2.60 billion years ago. The gold deposits that define this region as a world-class gold province are the product of orogenic processes that operated during accretion late in the tectonic history, after initial compressional deformation (D1-D2) and the majority of granitoid magmatism. Minor gold was also deposited throughout the entire tectonic history in magmatic-hydrothermal-related systems. However, such mineralization (mostly < 0.3 g/t gold) is nowhere economic unless it overprints, or is overprinted by, much higher-grade orogenic gold lodes. Robust SHRIMP U-Pb geochronology of gold-related hydrothermal xenotime and monazite supports structural studies that gold mineralization occurred during late transpressional events (D3-D4), shortly before cratonization. However, westward migration of collision and accretion produced a complementary diachroneity in the timing of gold mineralization of 5 to 20 m.y. between c. 2.65 Ma in the east (including Laverton District, Kurnalpi Terrane) to c. 2.63 Ma in the west (including Kalgoorlie Terrane) across the eastern part of the craton. The robust geochronology refutes previous suggestions that significant gold mineralization events extended from DE to D4 in the evolution of the orogen and that the Kalgoorlie gold deposits formed over a period of 45 m.y. The crustal continuum model is applicable within terranes where orogenic gold depositional events were penecontemporaneous, but must be modified to account for diachroneity of orogenic events and gold mineralization across the Eastern Goldfields.

  7. Disequilibrium Textures vs Equilibrium Modelling: Geochronology at the Crossroads

    NASA Astrophysics Data System (ADS)

    Villa, I. M.

    2007-12-01

    Observations made by electron microscopy show the processes affecting minerals at the atomic scale. The majority of reported analyses demonstrate chemical disequilibrium. A classic example are overgrowths of one mineral generation by a secondary one, which may be recognized on textural grounds. Disequilibrium recrystallization is promoted by water, which is everywhere on this planet (granites, contact aureoles, regional metamorphism, faults). It is mostly easier and energetically less costly to recrystallize a mineral at any temperature than to induce genuine volume diffusion in it. However, these observations are only relevant to geochronologists if chemical disequilibria are also accompanied by isotopic disequilibria. If a mineral mixture gives a mixed isotope record, then the interpretation of ages does not come cheap. If, on the contrary, diffusive reequilibration of the isotopic record is faster than that of chemical heterogeneities, then the petrology and microchemistry of a mineral could be ignored and its apparent age termed a "cooling age". First principle arguments and experimental data of the last decade concordantly show that the diffusivity of radiogenic isotopes is never higher than that of major elements forming the mineral structure. And indeed, end- member ages of mineral mixtures can be unravelled if the petrogenesis is understood. This was first shown by CL images of zircon grains (Gebauer et al, Schweiz Min Pet Mitt 68 (1988) 485-490). Similar progress was reported on monazite (Williams et al, Ann Rev Earth Planet Sci 35 (2007) 137-175), amphibole (Belluso et al, Eur J Mineral 12 (2000) 45-62), K-feldspar (Nyfeler et al, Schweiz Min Pet Mitt 78 (1998) 11-21), biotite (Villa et al, Water Rock Interaction 10 (2001) 1589-92). The mechanism for resetting the isotope record in nature thus seems more dependent on the availability of water to enhance disequilibrium recrystallization than on reaching a preset temperature. Intercomparison of laboratory

  8. Geochronologic, Thermochronologic, and Thermobarometric Constraints on the Tectonic Evolution of the Northeastern Pamir

    NASA Astrophysics Data System (ADS)

    Robinson, A. C.; Yin, A.; Manning, C. E.; Harrison, T. M.; Hei, W.; Xiong, M. Y.; Feng, W. X.

    2001-12-01

    exhumation of the Kongur Shan (and Muztaghata) massifs along the Kongur normal fault. High grade metamorphism in the Kongur Shan massif due to tectonic burial is constrained to be ~ 9 Ma from in-situ SIMS monazite geochronology. A 10 Ma biotite 40Ar/39Ar age from two km west of the ductile shear zone is interpreted to roughly indicate the timing of initiation of normal faulting.

  9. Coal Formation and Geochemistry

    NASA Astrophysics Data System (ADS)

    Orem, W. H.; Finkelman, R. B.

    2003-12-01

    Coal is one of the most complex and challenging natural materials to analyze and to understand. Unlike most rocks, which consist predominantly of crystalline mineral grains, coal is largely an assemblage of amorphous, degraded plant remains metamorphosed to various degrees and intermixed with a generous sprinkling of minute syngenetic, diagenetic, epigenetic, and detrital mineral grains, and containing within its structure various amounts of water, oils, and gases. Each coal is unique, having been derived from different plant sources over geologic time, having experienty -45ced different thermal histories, and having been exposed to varying geologic processes. This diversity presents a challenge to constructing a coherent picture of coal geochemistry and the processes that influence the chemical composition of coal.Despite the challenge coal presents to geochemists, a thorough understanding of the chemistry and geology of this complex natural substance is essential because of its importance to our society. Coal is, and will remain for sometime, a crucial source of energy for the US and for many other countries (Figure 1). In the USA, more than half of the electricity is generated by coal-fired power plants, and almost 90% of the coal mined in the USA is sold for electricity generation (Pierce et al., 1996). It is also an important source of coke for steel production, chemicals, pharmaceuticals, and even perfumes ( Schobert, 1987). It may also, in some cases, be an economic source of various mineral commodities. The utilization of coal through mining, transport, storage, combustion, and the disposal of the combustion by-products, also presents a challenge to geochemists because of the wide range of environmental and human health problems arising from these activities. The sound and effective use of coal as a natural resource requires a better understanding of the geochemistry of coal, i.e., the chemical and mineralogical characteristics of the coal that control its

  10. Eocene Kashmar granitoids (NE Iran): Petrogenetic constraints from U-Pb zircon geochronology and isotope geochemistry

    NASA Astrophysics Data System (ADS)

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

    2015-02-01

    Kashmar granitoids outcrop for ~ 100 km along the south flank of the Sabzevar ophiolite (NE Iran) and consist of granodiorite and monzogranite along with subordinate quartz monzonite, syenogranite and aplitic dikes. These granitoids intruded Early to Middle Eocene high-K volcanic rocks and can spatially be grouped into eastern and western granitoids. Five samples of granite have identical zircon U-Pb ages of ca. 40-41 Ma. The granitoids have quite high K2O (~ 1.3-5.3 wt.%) and Na2O (~ 1.1-4.6 wt.%) with SiO2 ranging between ~ 62 and 77 wt.%. They are metaluminous to peraluminous, calc-alkaline and I-type in composition. Their chondrite-normalized REE patterns are characterized by LREE enrichment and show slight negative Eu anomalies. Kashmar granitoids have low whole rock εNd (- 0.43 to - 2.3), zircon εHf values (- 1.9 to + 7.2), and somewhat elevated δ18O (+ 6.1 to + 8.7‰) in the range of I-type granites. The Kashmar granitoids show Early Neoproterozoic zircon second-stage Hf and bulk rock Nd model ages at ca. 500-1000 Ma (associated with ca. 640 Ma old inherited zircons). Bulk rock Nd-Sr isotopic modeling suggests that 10-20% assimilation of Cadomian lower crust by juvenile mantle melts and then fractional crystallization (AFC process) can explain the Sr-Nd isotopic compositions of Kashmar granitoids. Kashmar granitoids are products of crustal assimilation by mantle melts associated with extension above the subducting Neotethyan Ocean slab beneath SW Eurasia. Similar subduction-related extension was responsible for the flare-up of Eocene-Oligocene magmatism across Iran, associated with core complex formation in central Iran.

  11. Geochronology and geochemistry of Paleozoic plutons in the Alxa Terrane: petrogenesis and tectonic implications

    NASA Astrophysics Data System (ADS)

    Liu, Qian; Zhao, Guochun

    2016-04-01

    Situated between the Tarim Craton and the North China Craton (NCC), the Paleozoic magmatic record in the Alxa Terrane places important constraints on the accretionary orogenesis of the southern Paleo-Asian Ocean (PAO) forming the southern section of the Central Asian Orogenic Belt. New results of LA-ICPMS zircon U-Pb ages and whole-rock major- and trace-element compositions reveal two groups of diorites and granitoids in the Alxa Terrane. One group consists of diorites and granitoids that were emplaced at 458-440 Ma, characterized by lower Al2O3/TiO2 ratios and higher TiO2 contents, implying high temperature - low pressure crystallization conditions and a shallow source region. In contrast, the second group consists of granitoids that were formed at 417-407 Ma, displaying low high rare earth elements, very high Sr/Y ratios and mostly positive Eu anomalies, suggesting low temperature - high pressure crystallization conditions and source regions at deep crustal levels where garnet is stable in the residual phase. Both of two groups are mostly calc-alkaline to high-K calc-alkaline, and depleted in Nb, Ta and Ti and enriched in Ba, K and Sr, suggesting an arc affinity related to a PAO oceanic subduction regime since the Late Ordovician. Both zircon ɛHf(t) and whole-rock ɛNd(t) values decrease from 458 Ma to 440 Ma but increase from 417 Ma to 407 Ma, whereas whole-rock initial 87Sr/86Sr ratios display an opposite trend. Such an isotopic change suggests a tectonic switch from an advancing to a retreating subduction setting in the Early Devonian. Compiled with previous studies, we suggest that the early Paleozoic magmatic arc existing in the Alxa Terrane represented the western extension of the super-large early Paleozoic active continental margin on the northern margin of the NCC.

  12. Geochronology, Geochemistry and Petrogenesis of the Intermediate and Acid Dykes in Linzhou Basin, Southern Tibet

    NASA Astrophysics Data System (ADS)

    Dong, M.; Zhao, Z.; Zhu, D. C.; Dong, G.; Mo, X.

    2015-12-01

    The Linzizong volcanic succession (also called Linzizong Group, ~65-45 Ma), which occurred in southern Gangdese magmatic belt in response to the collision processes between India and Eurasia continents, have been well studied in the Linzhou Basin, to the northeast of Lhasa. Our research obtains some new results of zircon U-Pb, Hf isotopic data, and whole rock major and trace elements geochemical data of the intermediate (diorite porphyry) to acid (granite porphyry) dykes intruded into the Linzizong volcanics in Linzhou Basin. These dykes intruded into Dianzhong and Nianbo formations of Linzizong Group. All the samples are sub-alkaline, varying from calc-alkaline to high-K calc-alkaline series. They are peraluminous (A/CNK>1.1). The diorite porphyry, intruded in 62.4Ma with positive zircon Hf isotopes (ɛHf(t)=+5.1~+7.6), have similar composition to the andesitic rocks from Dianzhong Formation. The granite porphyries, intruded between 55.1Ma and 61.1Ma, with ɛHf(t) ranging from -1.1 to +10.4, have comparable composition with the rhyolitic Nianbo Formation. A series of evidences, including: (1) the samples have positive, mantle-like Hf feature (average ɛHf(t)=+5.9 of 86 samples); (2) wide-range variation of ɛHf(t) in samples (3.5~8.8 ɛ units in the four granite porphyries), which implying an inhomogeneous source regions; (3) magma-mixing trend in the plots of FeOT against MgO, suggesting that the magma-mixing processes that have taken place in the southern Gangdese belt, can also be applied to explain the origin of these dykes in Linzhou Basin. The dykes intruded coeval or shortly later than their equivalent volcanic rocks (Dianzhong and Nianbo formations), are in the transitional settings from subduction of Tethyan oceanic crust to the collisional between India and Eurasia continents.

  13. Re-Os geochemistry and geochronology of the Ransko gabbro-peridotite massif, Bohemian Massif

    NASA Astrophysics Data System (ADS)

    Ackerman, Lukáš; Pašava, Jan; Erban, Vojtěch

    2013-10-01

    The Ransko gabbro-peridotite massif in Eastern Bohemia is a strongly differentiated intrusive complex, which hosts low-grade Ni-Cu ores mainly developed close to the contact of olivine-rich rocks with gabbros, in troctolites, and to a much lesser extent in both pyroxene and olivine gabbros and plagioclase-rich peridotites. Gabbro, troctolite, peridotite and Ni-Cu ores from the Jezírka Ni-Cu (PGE) deposit, considered to be a typical example of the liquid segregation style of mineralization, were analyzed for Re-Os concentrations and isotopic ratios. Seven barren and mineralized samples from the Jezírka deposit yielded a Re-Os regression of 341.5 ± 7.9 Ma (MSWD = 69). Strongly mineralized peridotite with mantle-like initial 187Os/188Os ratio of 0.125 suggests that Os as well as other PGE present in the Ni-Cu mineralization are predominantly of mantle origin. On the other hand, barren and low-mineralized samples have radiogenic initial 187Os/188Os ratios of 0.14-0.16 suggesting some import of Re and/or radiogenic 187Os most likely through contamination by continental crust during magma emplacement. The Re-Os age of the Ransko Massif is significantly younger than the previously suggested Lower Cambrian age, but it is similar to and/or younger than the age of metamorphism of the adjacent Kutná Hora crystalline complex and the Moldanubian unit. Therefore, it is likely that the emplacement of the Ransko massif and its Ni-Cu mineralization was closely connected with the late-stage evolution of the Kutná Hora crystalline complex.

  14. Petrology, geochemistry and geochronology of the Chilka Lake igneous complex, Orissa state, India

    NASA Astrophysics Data System (ADS)

    Sarkar, Amitabha; Bhanumathi, L.; Balasubrahmanyan, M. N.

    1981-04-01

    The Chilka Lake igneous complex of Orissa, the largest known anortosite massif of the Indian Shield, occurs in a catazonal environment of high-grade metamorphics of the Eastern Ghats Precambrian Orogenic Province. The syntectonic massif consists of the anorthositic Balugaon dome, leuconoritic Rambha lobe and quartz-mangeritic Kallikota cover. A completely gradational suite comprising anorthosite-leuconorite-norite-minor jotunite (the anorthositic suite) constitutes most of the complex. The subordinate of suite of acid rocks spatially associated with this is of a broad quartz-mangeritic lithology with minor granitic rocks (the acidic suite). Geochemical evolution of the complex in the sequence anorthosite-leuconorite-norite-jotunite-acidic rocks shows moderate iron enrichment in the noritic-jotunitic stage and is marked by an overall decrease in Al 2O 3, CaO, MgO, Ni/Co, Sr/Ba, K/Rb and increase in SiO 2, K 2O, V/Ni, K/Ba and Rb/Sr. Such progressive variation in geochemical parameters appears (i) essentially gradual and frequently overlapping in rock members of the intergradational anorthositic suite and (ii) rather abrupt across transition zones between the anorthositic suite and the acidic suite due to near absence of intervening intermediate lithologies. RbSr whole rock isochron studies indicate that the complex was emplaced ca. 1400 Ma ago. The initial 87Sr/ 68Sr (0.70661) implies limited hybridisation of the parent magma prior to emplacement. A critical appraisal of all the available evidence suggests that (i) the anorthositic suite of rocks form a perfectly consanguinous and comagmatic assemblage and (ii) the spatially associated acidic suite emerged through a convergence of magmatic and metasomatic processes (the latter brought about by contact anatexis of the host rocks). The complex as well as the host metamorphics are intruded by an atectonic suite of noritic dykes emplaced ca 850 Ma ago.

  15. Geochronology and geochemistry of the Yilan blueschists in the Heilongjiang Complex, northeastern China and tectonic implications

    NASA Astrophysics Data System (ADS)

    Zhu, Chloe Yanlin; Zhao, Guochun; Sun, Min; Liu, Qian; Han, Yigui; Hou, Wenzhu; Zhang, Xiaoran; Eizenhofer, Paul R.

    2015-02-01

    The geological attraction of the Heilongjiang Complex, extending along a suture zone between the Jiamusi and Songliao blocks in Northeast China, resides in excellent exposures of blueschists, which occur in association with granitic gneisses, marbles, meta-ultramafic rocks, greenschists, quartzites, muscovite-albite schists and two-mica schists from the complex. However, controversy has long surrounded the ages and tectonic settings of mafic protoliths of the blueschists from the complex. The lithological association and major and trace element compositions indicate that the ultramafic-mafic protoliths of the Yilan blueschists can be subdivided into the tholeiitic and alkali groups, both of which were derived from partial melting of garnet-facies peridotites, but at different degrees of melting. Magmatic zircons from a tholeiitic blueschist sample yield a 206Pb/238U age of 275 ± 2 Ma, interpreted as its protolithic age. The sample also contains large amounts of older inherited zircons up to 1200 Ma, which, together with the geochemical features of the sample, indicate that the tholeiitic basalts were generated in a continental rift. The further development of the rift led to the formation of an ocean between the Jiamusi and Songliao blocks, in which some ocean islands developed, represented by the alkali mafic protoliths of the Yilan blueschists, as supported by geochemical data. Magmatic zircons from an alkaline mafic blueschist sample yield a 206Pb/238U age of 141.8 ± 1 Ma, indicating that the ocean between the Jiamusi and Songliao blocks had not been closed by ~ 141 Ma, not at 210-180 Ma as previously considered.

  16. Petrology, geochemistry, and geochronology of trondhjemites from the Qori Complex, Neyriz, Iran

    NASA Astrophysics Data System (ADS)

    Fazlnia, Abdolnaser; Schenk, Volker; van der Straaten, François; Mirmohammadi, Mirsaleh

    2009-10-01

    Metamorphism, magmatism, and thrusting were the result of subduction of Neotethys beneath the continental-margin arc of the Sanandaj-Sirjan shear zone (SSSZ) during the Mesozoic. The Qori metamorphic complex is a part of the southern SSSZ. Leuco-granitic (trondhjemitic) rocks crop out in the Qori metamorphic complex and are rare rock types in the SSSZ. These rocks have intruded into the marbles and garnet amphibolites, the highest grade metamorphic rocks of the Qori metamorphic complex, and in some outcrops, a transitional boundary between the amphibolites and the granitoids can be distinguished. The granitoids are granular in texture and consist of plagioclase (albite-oligoclase), quartz ± K-feldspar ±muscovite and subordinate garnet, spinel, rutile, and apatite which primarily occur as inclusions in the main phases. The peraluminous trondhjemitic rocks are enriched in Na 2O and SiO 2 and depleted in FeO, MgO, and CaO. Similarities with some trondhjemitic liquids produced through partial melting of amphibolites or hydrous basalts (i.e., low-Al 2O 3 content, less than 15 wt.%; low Ba, Sr, TiO 2, and Eu content, all with negative anomalies; moderately enriched LREEs and Y, and flat HREE patterns) suggest that the evolution of the parental magma was controlled by residual plagioclases during partial melting of a garnet amphibolite source. Concentrations of ferromagnesian elements, Mg, Fe, and Mn, are low, suggesting that the granitic rocks were not produced by high degrees of partial melting. Furthermore, they display low amounts of ferromagnesian components from the protolith (garnet amphibolite). This is supported by consideration of compatible elements, especially Cr, Ni and Ti (and the less robust HREE), which respectively show very high and high bulk partition coefficients for relatively small degrees (< 20%) of partial melting of the source. The partial melting of the garnet amphibolites occurred at pressures and temperatures between 7.5 and 9.5 kbar (at a depth of 25 to 32 km) and 680 and 720 °C, respectively, based on the Grt-Hbl and Hbl-Pl thermometers and a Grt-Hbl-Pl-Qtz barometer. Precise U-Th zircon SHRIMP ages of the trondhjemite show magmatic ages of 147.4 ± 0.76 Ma, Volgian, Late Jurassic, and suggest that Neotethys began to subduct beneath the continental-margin arc of the SSSZ. As a result of this process, an arc-related metamorphism occurred, leading to the development of the garnet amphibolites in the Qori metamorphic complex.

  17. Geochronology and geochemistry of late Cenozoic basalts from the Leiqiong area, southern China

    NASA Astrophysics Data System (ADS)

    Ho, Kung-suan; Chen, Ju-chin; Juang, Wen-shing

    2000-06-01

    The Leiqiong area, which includes the Leizhou Peninsula and the northern part of the Hainan Island, is the largest province of exposed basalts in southern China. Ar-Ar and K-Ar dating indicates that incipient volcanism in the Leiqiong area may have taken place in late Oligocene time and gradually increased in tempo toward the Miocene and Pliocene Epoch. Volcanic activities were most extensive during Pleistocene, and declined and ended in Holocene. Based on radiometric age dating and geographic distribution, Pliocene and Quaternary volcanism in Hainan Island can be grouped into two stages and six eruptive regions. The early volcanism is dominated by flood type fissure eruption of quartz tholeiites and olivine tholeiites whereas the later phase is dominated by central type eruption of alkali olivine basalts and olivine tholeiites. The systematic decrease of MgO, ΣFeO and TiO 2 with increasing SiO 2 content for basalts from Hainan Island indicates that fractional crystallization of olivine, clinopyroxene and Ti-bearing opaques may have occurred during magmatic evolution. From coexisting Fe-Ti oxide minerals, it is estimated that the equilibrium temperatures range from 895-986°C and oxygen fugacities range from 10 -13.4 to 10 -10.7 atmospheres in the basaltic magmas. The incompatible element ratios and the chondrite-normalized REE patterns of basalts from the Leiqiong area are generally similar to OIB. The Nb/U ratios (less than 37) in most of the tholeiitic rocks and the negative Nb anomaly observed in the spidergram of some basalts indicated that the influence of a paleo-subduction zone derived component can not be excluded in considering the genesis of the basalts from the Leiqiong area. The tholeiites in the Leiqiong area may have mixed with a more enriched lithospheric mantle component as well as undergone relatively larger percentages of partial melting than the alkali basalts.

  18. Geochronology, geochemistry and tectonic implications of Late Triassic granites in the Mongolian Altai Mountains

    NASA Astrophysics Data System (ADS)

    Dash, Batulzii; Boldbaatar, Enkhjargal; Zorigtkhuu, Oyun-Erdene; Yin, An

    2016-03-01

    Although the closure of the Paleo-Asian Ocean in western China and western Mongolia occurred in the Late Carboniferous and Early Permian, widespread intra-continental magmatism continued to occur across this region from the Late Permian to the end of the Triassic. In this study we document field relationships and geochemical characterization of a Late Triassic felsic intrusive complex in the western Mongolian Altai. The plutonic complex occurs as sills, dikes, and small stocks and its composition varies from biotite granite, two-mica granite, to leucogranite. Structurally, the plutonic complex occurs in the hanging wall of a segment of the regionally extensively (>1500 km long) Irtysh-Ertix-Bulgan thrust zone. As the plutonic bodies both cut and are deformed by the shear fabrics in this regional thrust shear zone, the duration of felsic magmatism and regional thrusting was temporally overlapping. This suggests that magmatism was coeval with crustal thickening. Major- and trace-element data and isotopic analysis of granitoid samples from our study area indicate that the felsic intrusions were derived from partial melting of meta-sediments, with the biotite and two-mica granite generated through vapor-absent melting and the leucogranite from flux melting. Although the Mongolian Altai intrusions were clearly originated from anatexis, coeval granite in the Chinese Altai directly west of our study area in the hanging wall of the Irtysh-Ertix-Bulgan thrust was derived in part from mantle melting. To reconcile these observations, we propose a Himalayan-style intracontinental-subduction model that predicts two geologic settings for the occurrence of felsic magmatism: (1) along the intracontinental thrust zone where granite was entirely generated by anatexis and (2) in the hanging wall of the intracontinental thrust where convective removal and/or continental subduction induced mantle melting.

  19. Geochemistry and geochronology of mafic rocks from the Luobusa ophiolite, South Tibet

    NASA Astrophysics Data System (ADS)

    Zhang, Chang; Liu, Chuan-Zhou; Wu, Fu-Yuan; Zhang, Liang-Liang; Ji, Wei-Qiang

    2016-02-01

    This study presents geochemical compositions of mafic rocks outcropped in the Luobusa ophiolite that locates at the eastern part of the Yarlung Zangbo Suture Zone (YZSZ). The studied mafic rocks include gabbroic/diabase dykes cutting the peridotites and foliated amphibolites embedded within the subophiolitic mélange. The diabases have low K2O contents and display N-MORB-like geochemical characteristics, i.e., with flat REE patterns and weak enrichment in LILE (e.g., Rb, Ba, Th and U). The gabbros show LREE-depleted patterns and variable enrichment in Rb and Ba. Foliated amphibolites mainly consist of hornblendes and plagioclases, with minor titanites. They commonly show LREE-depleted patterns, with strong enrichment in LILE. Three diabases have depleted Sr-Nd-Hf isotope compositions, with initial 87Sr/86Sr ratios of 0.703009-0.703502, εNd(t) of + 5.0 to + 8.5 and εHf (t) of ca + 14. Two gabbros have similar Nd-Hf isotopes with the diabases, but slightly higher initial 87Sr/86Sr ratios (i.e., 0.704820 and 0.704550). Compared to both diabases and gabbros, the amphibolites have higher initial 87Sr/86Sr ratios (i.e., 0.705131-0.705825), but more depleted Nd-Hf isotope compositions, with εNd(t) of + 9.1 to + 11.6 and εHf(t) of + 18.2 to + 21.9. Geochemical compositions of the diabase dykes indicate that they were formed in a mid-ocean ridge setting. Zircon SIMS U-Pb dating of the gabbroic dyke cutting the serpentinites yields an age of 128.4 ± 0.9 Ma, which is identical within uncertainty to the zircon U-Pb age of the amphibolite (i.e., 131.0 ± 1.2 Ma). Low U and Th contents of zircons in the amphibolite support their metamorphic origin. Titanites in the amphibolites have been dated by LA-ICPMS and give U-Pb ages of ~ 131-134 Ma, which are similar to the zircon U-Pb ages of the dated gabbro and amphibolite. Therefore, we suggest that the Luobusa ophiolite was generated at the Early Cretaceous and underwent the intra-oceanic emplacement immediately after its formation. This implies that ophiolites in the eastern segment of the YZSZ were formed roughly synchronously with those in both central and western segments of the YZSZ.

  20. Detrital zircon geochronology and Nd isotope geochemistry of an early Paleozoic succession in Korea:

    NASA Astrophysics Data System (ADS)

    Lee, Yong Il; Choi, Taejin; Lim, Hyoun Soo; Orihashi, Yuji

    2015-04-01

    This study reports the results of an analysis of U-Pb ages of detrital zircons and Nd isotope compositions from the well-established lower Paleozoic platform succession developed on the Precambrian gneiss and metasedimentary rocks in South Korea. The three stratigraphic units in the basal part of the succession are the Jangsan, Myeonsan, and Myobong Formations. The unfossiliferous Jangsan (white­to­pink quartz sandstone) and Myeonsan (dark-gray ilmenite-rich sandstone/shale) Formations are in fault contact and are generally considered to be coeval (Early Cambrian). Both formations are also generally considered to be conformably overlain by the dark­ gray, fossiliferous, fine-grained Myobong Formation (late Early-early Middle Cambrian). We here report U-Pb ages of detrital zircons and Nd isotopic data from the Jangsan, Myeonsan, and Myobong Formations. The Jangsan and Myeonsan Formations provide Archean-Paleoproterozoic U-Pb ages, but the former is characterized by Archean Sm-Nd model ages and the latter by late Paleoproterozoic Sm-Nd model ages, which is indicative of a significant change in provenance. This suggests that the Jangsan Formation predates the Myeonsan Formation. The Myobong Formation provides dominantly Meso- to Neoproterozoic U-Pb ages and Sm-Nd model ages that are slightly younger than those of the Myeonsan Formation. Contrary to the conventional wisdom, the combined evidence of unconformable contact and marked changes in zircon U-Pb ages and Nd isotopic compositions suggests that the Myobong Formation overlies the Jangsan and Myeonsan Formations unconformably. Considering the metamorphic age of the immediately underlying Precambrian basement metasediments (0.8 to 0.9 Ga), this stratigraphic relationship strongly suggests that the Jangsan Formation may be Neoproterozoic in age and that the Myeonsan Formation may be latest Neoproterozoic to Early Cambrian and calls for reevaluation of Precambrian-Paleozoic history of the Korean Peninsula. The results of this study also recommend application of this technique to unfossiliferous stratigraphic successions elsewhere.

  1. The Jurassic-early Cretaceous Ilo batholith of southern coastal Peru: geology, geochronology and geochemistry

    NASA Astrophysics Data System (ADS)

    Boekhout, Flora; Sempere, Thierry; Spikings, Richard; Schaltegger, Urs

    2010-05-01

    The Ilo batholith (17°00 - 18°30 S) crops out in an area of about 20 by 100 km, along the coast of southern Peru. This batholith is emplaced into the ‘Chocolate‘ Formation of late Permian to middle Jurassic age, which consists of more than 1000 m of basaltic and andesitic lavas, with interbedded volcanic agglomerates and breccias. The Ilo Batholith is considered to be a rarely exposed fragment of the Jurassic arc in Peru. Our aim is to reconstruct the magmatic evolution of this batholith, and place it within the context of long-lasting magma genesis along the active Andean margin since the Paleozoic. Sampling for dating and geochemical analyses was carried out along several cross sections through the batholith that were exposed by post-intrusion eastward tilting of 20-30°. Sparse previous work postulates early to middle Jurassic and partially early Cretaceous emplacement, on the basis of conventional K/Ar and 40Ar/39Ar dating methods in the Ilo area. Twenty new U-Pb zircon ages (LA-ICP-MS and CA-ID-TIMS) accompanied by geochemical data suggests the Ilo batholith formed via the amalgamation of middle Jurassic and early Cretaceous, subduction-related plutons. Preliminary Hf isotope studies reveal a primitive mantle source for middle Jurassic intrusions. Additional Sr, Nd and Hf isotope analyses are planned to further resolve the source regions of different pulses of plutonic activity. We strongly suggest that batholith emplacement was at least partly coeval with the emplacement of the late Permian to middle Jurassic Chocolate Formation, which was deposited in an extensional tectonic regime. Our age results and geochemical signature fit into the scheme of episodic emplacement of huge amounts of subduction related magmatism that is observed throughout the whole Andean event, particularly during the middle Jurassic onset of the first Andean cycle (southern Peru, northern Chile and southern Argentina). Although the exact geodynamic setting remains to be precisely defined, these events can be linked to extensional episodes during the breakup of Pangea, which commenced at 230-220 Ma along the western South American margin, with a period of rifting, and culminated in the Jurassic with arc and back-arc extension.

  2. High-temperature geochronology constraints on the tectonic history and architecture of the ultrahigh-pressure Dabie-Sulu Orogen

    NASA Astrophysics Data System (ADS)

    Hacker, Bradley R.; Wallis, Simon R.; Ratschbacher, Lothar; Grove, Marty; Gehrels, George

    2006-10-01

    New U/Pb zircon and Th/Pb monazite ages are presented from the giant Sulu ultrahigh-pressure (UHP) terrane. Combined with Sm/Nd ages, Rb/Sr ages, inclusion relationships, and geologic relationships, they help define the timing of peak recrystallization, the timing of subsequent amphibolite-facies metamorphism, and the architecture of the Dabie-Sulu suture zone between the collided Sino-Korean and Yangtze cratons. The data indicate a ˜15 Myr record of UHP recrystallization, the first clearly documented for a giant UHP terrane; this requires that continental subduction in the Dabie-Sulu orogen involved multiple UHP tectonic or recrystallization events. A 244-236 Ma "precursor" UHP event, seen only in the Dabie Shan, was followed by a second, ˜230-220 Ma "main" UHP event, which was itself terminated by a 220-205 Ma amphibolite-facies overprint. Older eclogite-facies events seen in the Qinling segment of this orogenic belt raise the possibility that these rocks have undergone (U)HP metamorphism three or four times, but at present, there is no geochronological evidence in the Dabie-Sulu area to support this. The subduction of the lower, Yangtze plate did not proceed in a simple fashion: The ages of inherited zircon cores demonstrate that a ribbon continent of Yangtze affinity escaped subduction and became wedged against the Sino-Korean plate hanging wall.

  3. Low-temperature ceramic radioactive waste form characteriztion of supercalcine-based monazite-cement composites

    SciTech Connect

    Roy, D.M.; Wakeley, L.D.; Atkinson, S.D.

    1980-04-18

    Simulated radioactive waste solidification by a lower temperature ceramic (cement) process is being investigated. The monazite component (simulated by NdPO/sub 4/) of supercalcine-ceramic has been solidified in cement and found to generate a solid form with low leachability. Several types of commercial cements and modifications thereof were used. No detectable release of Nd or P was found through characterizing the products of accelerated hydrothermal leaching at 473/sup 0/K (200/sup 0/C) and 30.4 MPa (300 bars) pressure.

  4. A first principle study of the pressure dependent elastic properties of monazite LaPO4

    NASA Astrophysics Data System (ADS)

    Ali, Kawsar; Arya, A.; Ghosh, P. S.; Dey, G. K.

    2016-05-01

    DFT based ab-initio simulations have been performed to study the effect of pressure on the elastic properties of monazite LaPO4 which is a promising host material for immobilization of high level nuclear waste. The phase is found to be stable up to 30 GPa. The calculated polycrystalline bulk, shear and Young moduli show an increasing trend as a function of pressure. The ductility and anisotropy in shear modulus of the material have been found to increase with pressure; whilethe bulk modulus anisotropy decreases with pressure.

  5. Mechanical, Chemical and Microstructural Characterization of Monazite-Coated Silicon Carbide Fibers

    NASA Technical Reports Server (NTRS)

    Bansal, N. P.; Wheeler, D. R.; Chen, Y. L.

    2000-01-01

    Tensile strengths of as-received Hi-Nicalon and Sylramic fibers and those having monazite surface coatings, deposited by atmospheric pressure chemical vapor deposition, were measured at room temperature and the Weibull statistical parameters determined. The average tensile strengths of uncoated Hi-Nicalon and Sylramic fibers were 3.19 +/- 0.73 and 2.78 +/- 0.53 GPa with a Weibull modulus of 5.41 and 5.52, respectively. The monazite-coated Hi-Nicalon and Sylramic fibers showed strength loss of approx. 10 and 15 percent, respectively, compared with the as-received fibers. The elemental compositions of the fibers and the coatings were analyzed using scanning Auger microprobe and energy dispersive X-ray spectroscopy. The LaPO4 coating on Hi-Nicalon fibers was approximately stoichiometric and about 50 nm thick. The coating on the Sylramic fibers extended to a depth of about 100 to 150 nm. The coating may have been stoichiometric LaPO4 in the first 30 to 40 nm of the layer. However, the surface roughness of Sylramic fiber made this profile somewhat difficult to interpret. Microstructural analyses of the fibers and the coatings were done by scanning electron microscopy, transmission electron microscopy, and selected area electron diffraction. Hi-Nicalon fiber consists of fine beta-SiC nanocrystals ranging in size from 1 to 30 mn embedded in an amorphous matrix. Sylramic is a polycrystalline stoichiometric silicon carbide fiber consisting of submicron beta-SiC crystallites ranging from 100 to 300 nm. Small amount of TiB2 nanocrystallites (approx. 50 nm) are also present. The LaPO4 coating on Hi-Nicalon fibers consisted of a chain of peanut shape particles having monazite-(La) structure. The coating on Sylramic fibers consisted of two layers. The inner layer was a chain of peanut shape particles having monazite-(La) structure. The outer layer was comprised of much smaller particles with a microcrystalline structure.

  6. Investigating Age Resolution in Laser Ablation Geochronology

    NASA Astrophysics Data System (ADS)

    Horstwood, Matt; Kosler, Jan; Jackson, Simon; Pearson, Norman; Sylvester, Paul

    2009-02-01

    Workshop on Data Handling in LA-ICP-MS U-Th-Pb Geochronology; Vancouver, British Columbia, Canada, 12-13 July 2008; Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) uranium-thorium-lead (U-Th-Pb) dating is an increasingly popular approach for determining the age of mineral grains and the timing of geological events. The spatial resolution offered by this technique allows detailed investigations of complex igneous and metamorphic processes, and the speed of data capture allows vast amounts of information to be gathered rapidly. Laser ablation U-Th-Pb dating is therefore becoming an increasingly influential technique to the geochronology community, providing cost-effective and ready access to age data for laboratories and end users worldwide. However, complications in acquiring, processing, and interpreting data can lead to inaccurate age information entering the literature. With the numbers of practitioners expanding rapidly, the need to standardize approaches and resolve difficulties (particularly involving the subjectivity in processing laser ablation U-Th-Pb data) is becoming important.

  7. The Yanaurcu volcano (Western Cordillera, Ecuador): A field, petrographic, geochemical, isotopic and geochronological study

    NASA Astrophysics Data System (ADS)

    Béguelin, Paul; Chiaradia, Massimo; Beate, Bernardo; Spikings, Richard

    2015-03-01

    The Yanaurcu volcanic center in the Ecuadorian frontal arc is characterized by several epochs of activity from the Early Pliocene to approx. 61 ka, with important changes in geochemistry and isotope ratio values throughout its history. Most of its units have high Sr/Y and La/Yb signatures. We present a comprehensive study of this volcano involving morphological and stratigraphical observations, sampling, petrography, whole rock and in situ geochemistry, whole rock radiogenic isotope analysis and 40Ar/39Ar geochronology. We identify three magmatic series: (1) Early Pliocene andesitic and dacitic volcanics, (2) Pliocene andesitic flows (~ 3.6 Ma), and (3) two Pleistocene andesitic domes (~ 172 ka and ~ 61 ka). Radiogenic isotope data suggest an increasing amount of basement assimilation through the evolution of Yanaurcu, as well as lower rates of ascent and increasing recharge at upper crustal levels, with the Pleistocene domes representing a thermally more mature state of the crust (deep and mid-crust becoming warmer with time when fluxed by a continuous magmatic supply). We also investigate the deep and mid-crustal magma evolution of Yanaurcu by modeling REE + Sr + Y with the Monte Carlo approach in order to get constraints on the fractionating assemblage. We highlight the importance of fractionating garnet in the development of high Sr/Y and La/Yb signatures, and propose a changing regional stress to modify the depth of magma evolution, and therefore the stability of garnet in the fractionating assemblage. Switches from transtensional to compressional regimes in Ecuador have been highlighted by various studies based on tectonic and thermochronological evidences. We conclude that (1) Yanaurcu high Sr/Y and La/Yb ratios can be fully explained by crustal processes and do not need an enriched mantle source nor slab melting, and (2) local thermal maturation of the crust is responsible for an enrichment of incompatible elements through time via recharge processes

  8. Medical geochemistry of tropical environments

    NASA Astrophysics Data System (ADS)

    Dissanayake, C. B.; Chandrajith, Rohana

    1999-10-01

    Geochemically, tropical environments are unique. This uniqueness stems from the fact that these terrains are continuously subjected to extreme rainfall and drought with resulting strong geochemical fractionation of elements. This characteristic geochemical partitioning results in either severe depletion of elements or accumulation to toxic levels. In both these situations, the effect on plant, animal and human health is marked. Medical geochemistry involves the study of the relationships between the geochemistry of the environment in which we live and the health of the population living in this particular domain. Interestingly, the relationships between geochemistry and health are most marked in the tropical countries, which coincidentally are among the poorest in the world. The very heavy dependence on the immediate environment for sustainable living in these lands enables the medical geochemist to observe correlations between particular geochemical provinces and the incidence of certain diseases unique to these terrains. The aetiology of diseases such as dental and skeletal fluorosis, iodine deficiency disorders, diseases of humans and animals caused by mineral imbalances among others, lie clearly in the geochemical environment. The study of the chemistry of the soils, water and stream sediments in relation to the incidence of geographically distributed diseases in the tropics has not only opened up new frontiers in multidisciplinary research, but has offered new challenges to the medical profession to seriously focus attention on the emerging field of medical geochemistry with the collaboration of geochemists and epidemiologists.

  9. Multiple generations of monazite growth in metapelites of the Wepawaug Schist, southern Connecticut

    SciTech Connect

    Lanzirotti, A.; Hanson, G.N. . Dept. of Earth and Space Sciences)

    1992-01-01

    The Wepawaug Schist, part of the Orange Milford Belt of southern Connecticut, consists dominantly of micaceous phyllites and schists. A progression in metamorphic zonation from chlorite through kyanite grade occurs within 3 km. There were previously no constraints on either the depositional age of the Wepawaug Schist or the time of metamorphism. Concordant U-Pb sphene ages of 457 [+-] 3 Ma for the Woodbridge granite, which cuts the Wepawaug Schist, mean that the metapelites are older than late Ordovician. Kyanite grade rocks contain two distinct monazite populations within a single outcrop: (1) Crystals from a graphitic schist are clear, round, and xenoblastic. They have [approximately]0.9 wt.% Th, Sm/Nd = 0.12--0.14, and display pronounced chemical zonation. These give normality discordant U-Pb ages that define an upper intercept age of 420 [+-] 12 Ma. (2) In a less graphitic schist they are light yellow, idioblastic, tabular crystals that cut muscovites. They have Sm/Nd = 0.16 and [approximately] 2.6 wt.% Th and give concordant U-Pb ages of 377 [+-] 3 Ma. The authors suggest that the Wepawaug Schist is older than 457 Ma and was affected by two events which resulted in monazite growth, one during the Middle Devonian (377 Ma) and one in the Late Silurian (420 Ma).

  10. Autonomous anorthosites of the Anabar Shield: Age, geochemistry, and formation mechanism

    NASA Astrophysics Data System (ADS)

    Glukhovskii, M. Z.; Kuz'min, M. I.; Bayanova, T. B.; Bazhenova, G. N.; Lyalina, L. M.; Serov, P. A.

    2015-10-01

    The new high-accuracy data on U-Pb zircon geochronology, Sm-Nd systematics, and geochemistry of anorthosites of the Anabar Shield are discussed. It is established that anorthosite massifs are composed of gabbro-anorthosites (1.96 Ga old) and oligoclasites (1.93 Ga old) in association with monzodiorites (1.84-1.90 Ga old) and porphyroblastic granites. These rocks were generated in the Archean (3.2-2.7 Ga ago) in the lower crust from quartz-diorite melts under the plume tectonics regime in line with the filterpressing mechanism. The rocks were successively exhumed to upper levels of the crust owing to the Paleoproterozoic impact-triggered process to form a tectonically juxtaposed complementary magmatic complex.

  11. Petrography and geochemistry evidence for supra-subduction ophiolites in Makran, SE Iran

    NASA Astrophysics Data System (ADS)

    Hunziker, Daniela; Burg, Jean-Pierre; Bouilhol, Pierre; Omrani, Jafar

    2010-05-01

    Ophiolites archive tectonic and chemical processes from crystallization of the oceanic lithosphere to accretion during obduction and/or continental collision. The variety of ophiolites has shown that they form in various geotectonic settings and experience complex petrological and geochemical histories during their evolution. We present new results on the structure, petrography, geochemistry and geochronology of the Remeshk/Mokhtaramabad and Fannuj/Maskutan ophiolitic complexes in Makran (Southeast Iran), which have been very scarcely studied before this work. A detailed map and cross sections illustrate temporal and structural relationships between the different lithologies. The extensive ultramafic complexes comprise a lower, harzburgite-dominated unit with few lherzolites overlain by dunites. Pyroxene-bearing peridotites show typical features of tectonized mantle deformed at sub-solidus conditions. The olivine chemistry (xMg = 0.90-0.92, NiO content of 0.4-0.47wt%) indicates that the ultramafic rocks represent an ophiolitic upper mantle. Most dunites are characterized by cumulate textures in olivine and a slightly lower xMg = 0.87-0.89 and NiO content of 0.25-0.35wt%. Dunites are locally impregnated by plagioclase-rich melts with minor amounts of clinopyroxene. They were intruded by gabbroic dykes marking the transition zone between mantle and crust. The gabbroic sequence displays increasingly differentiated rocks originated from the same magma source in the following order: troctolite - olivine gabbro - gabbro - anorthositic dykes - diabase. These rocks were later intruded by plagiogranites and hornblende-gabbros. Petrography and geochemistry show oceanic features of a shallow environment for some lithologies; other lithologies indicate a supra-subduction environment, in particular the late-Cretaceous calc-alkaline pillow lavas that yield a clear arc signal. Advanced trace element analyses and geochronology will constrain the evolution of the Tethys

  12. History of the recognition of organic geochemistry in geoscience

    USGS Publications Warehouse

    Kvenvolden, K.A.

    2002-01-01

    The discipline of organic geochemistry is an outgrowth of the application of the principles and methods of organic chemistry to sedimentary geology. Its origin goes back to the last part of the nineteenth century and the first part of the twentieth century concurrent with the evolution of the applied discipline of petroleum geochemistry. In fact, organic geochemistry was strongly influenced by developments in petroleum geochemistry. Now, however, organic geochemistry is considered an umbrella geoscience discipline of which petroleum geochemistry is an important component.

  13. The growth of the continental crust: Constraints from radiogenic isotope geochemistry

    NASA Technical Reports Server (NTRS)

    Taylor, Paul N.

    1988-01-01

    Most models for evolution of continental crust are expressed in the form of a diagram illustrating the cumulative crustal mass (normalized relative to the present crustal mass) as a function of time. Thus, geochronological data inevitably play a major role in either constructing or testing crustal growth models. For all models, determining the start-time for effective crustal accretion is of vital importance. To this end, the continuing search for, and reliable characterization of, the most ancient crustal rock-units remains a worthy enterprise. Another important role for geochronology and radiogenic isotope geochemistry is to assess the status of major geological events as period either of new crust generation or of reworking of earlier formed continental crust. For age characterization of major geological provinces, using the critieria outined, the mass (or volume) of crust surviving to the present day should be determinable as a function of crust formation age. More recent developments, however, appear to set severe limitations on recycling of crust, at least by the process of sediment subduction. In modeling crustal growth without recycling, valuable constaints on growth rate variations through time can be provided if variations in the average age of the continental crust can be monitored through geological history. The question of the average age of the exposed continental crust was addressed by determining Sm-Nd crustal residence model ages (T-CR) for fine-grained sediment loads of many of the world's major rivers.

  14. Th-Pb Monazite-in-Garnet Ages From the Greater Himalayan Sequence of Central Nepal

    NASA Astrophysics Data System (ADS)

    Gehrels, G.; Decelles, P.; Martin, A.

    2007-12-01

    431 new Th-Pb ages have been determined from rocks of the Greater Himalayan Sequence (GHS) of central Nepal. Some of these ages are reported by Martin et al. (2007, Chem. Geol.), most have not been reported previously. 42 samples were collected from transects along Kali Gandaki, Modi Khola, Seti Nadi, Madi Nadi, Nayu Ridge, and Marsyangdi Nadi in the Annapurna region, with three transects extending across the GHS and three transects concentrated near the base of the sequence. Garnet crystals were extracted from the samples, and monazite inclusions were identified by BSE imaging and then analyzed by LA-MC-ICPMS with a 10 micron laser beam. Where possible, inclusions were sampled from both cores and rims of the garnet crystals. The resulting ages belong to four groups: (1) 3 ages (all cores) between 801 Ma and 1407 Ma that are inherited from GHS protoliths, (2) 42 ages (all cores) from 550 Ma to 400 Ma (peak age of 487 Ma) that record early Paleozoic prograde metamorphism, (3) 102 ages (nearly all cores) scattered between 400 Ma and ~50 Ma that are interpreted as early Paleozoic grains which have experienced either Pb loss or overgrowth of Tertiary monazite, and (4) 284 ages (2/3 cores, 1/3 rims) between ca. 50 Ma and ca. 10 Ma, with cores only slightly (avg of 1.5 m.y.) older than rims. The Tertiary ages consistently young northward/upsection from ca. 35 to ca. 18 Ma, and in Marsyangdi Nadi define two separate panels that are interpreted to be imbricated along a north-dipping thrust fault. This fault is near the base of sillimanite-bearing rocks, similar to the Langtang thrust (Kohn et al. 2005, JMG), and may be partly responsible for the inverted metamorphic gradient discussed by many previous workers. Our preferred structural scenario is that the Tertiary monazite ages record progressive burial of the GHS by shortening in the Tethyan thrust belt between ca. 35 and ca. 18 Ma, termination of this metamorphism due to onset of motion along the MCT, and ca. 10 Ma

  15. Rethinking early Earth phosphorus geochemistry

    PubMed Central

    Pasek, Matthew A.

    2008-01-01

    Phosphorus is a key biologic element, and a prebiotic pathway leading to its incorporation into biomolecules has been difficult to ascertain. Most potentially prebiotic phosphorylation reactions have relied on orthophosphate as the source of phosphorus. It is suggested here that the geochemistry of phosphorus on the early Earth was instead controlled by reduced oxidation state phosphorus compounds such as phosphite (HPO32−), which are more soluble and reactive than orthophosphates. This reduced oxidation state phosphorus originated from extraterrestrial material that fell during the heavy bombardment period or was produced during impacts, and persisted in the mildly reducing atmosphere. This alternate view of early Earth phosphorus geochemistry provides an unexplored route to the formation of pertinent prebiotic phosphorus compounds, suggests a facile reaction pathway to condensed phosphates, and is consistent with the biochemical usage of reduced oxidation state phosphorus compounds in life today. Possible studies are suggested that may detect reduced oxidation state phosphorus compounds in ancient Archean rocks. PMID:18195373

  16. Monazite and zircon as major carriers of Th, U, and Y in peraluminous granites: examples from the Bohemian Massif

    NASA Astrophysics Data System (ADS)

    Breiter, Karel

    2016-06-01

    The chemical compositions of zircon and monazite and the relationships between the contents of Th, U, Y, and REE in both minerals and in the bulk samples of their parental rocks were studied in three Variscan composite peraluminous granite plutons in the Bohemian Massif. It was established that granites of similar bulk composition contain zircon and monazite of significantly different chemistry. Monazite typically contains 5-13 wt% (rarely up to 28 wt%) ThO2, 0.4-2 wt% (up to 8.2 wt%) UO2, and 0.5-2 wt% (up to 5 wt%) Y2O3, whereas zircon typically contains less than 0.1 wt% (rarely up to 1.7 wt%) ThO2, less than 1 wt% UO2 (in the Plechý/Plockenstein granite, commonly, 1-2 wt% and scarcely up to 4.8 wt% UO2), and less than 1 wt% Y2O3 (in the Nejdek pluton often 2-5, maximally 7 wt% Y2O3). Monazite is an essential carrier of thorium, hosting more than 80 % of Th in all studied granites. Monazite also appears to be an important carrier of Y (typically 14-16 %, and in the Melechov pluton, up to 81 % of the total rock content) and U (typically 18-35 % and occasionally 6-60 % of the total rock budget). The importance of zircon for the rock budget of all the investigated elements in granites is lower: 4-26 % U, 5-17 % Y, and less than 5 % Th.

  17. U-Th-Pb ion microprobe analysis of monazite from the Paleoproterozoic Karrat rare earth element (REE) deposit, western Greenland

    NASA Astrophysics Data System (ADS)

    Mott, A.; Grove, M.; Bird, D. K.

    2012-12-01

    The Karrat rare earth element (REE) deposit is located at 72°N on the Niaqornakavsak peninsula of Qeqertarssuq Island on the western coast of Greenland. Metasomatic alteration of an amphibolite host rock by carbonatite derived fluids resulted in REE mineralization in the Karrat Isfjord area. REE in the mineralization are primarily found in bastnasite, allanite, and monazite. In-situ analysis of monazite was conducted on samples obtained from three sites of mineralization: (1) the primary deposit at Niaqornakavsak consisting of a single distinct ~30m thick unit; (2) at Umiamako Nuna 7 km to the east of Niaqornakavsak where the majority of REE mineralization occurs within the first 20m of the surface; and (3) a 6m thick REE-rich vein 100m below the surface at Umiamako Nuna. Formation ages for monazite at Niaqornakavsak, Umiamako Nuna (surface), and Umiamako Nuna (vein) have been calculated using 207Pb/206Pb, 206Pb/238U, and 208Pb/232Th isotope ratios. Multiple isotope ratios were examined to determine the ideal method of monazite analysis based on the inherent issues of low U content of monazite, difficulties measuring 204Pb, common Pb corrections, and peak interferences resulting from high concentrations of REE. 208Pb/232Th analysis resulted in the best precision and smallest spread of values. Energy filtering was applied to 208Pb/232Th analyses in an effort to reduce interferences at several peaks. Although all three isotope ratio analyses result in a Paleoproterozoic age similar to the timing of convergence of the North Atlantic craton, Rae craton, and Aasiat domain as well as the emplacement of the Prøven Igneous Complex in Greenland (1.95-1.80Ga), the values range between 1.7-1.9Ga depending on the isotope ratio.

  18. Characterization of PAH matrix with monazite stream containing uranium, gadolinium and iron

    NASA Astrophysics Data System (ADS)

    Pal, Sangita; Meena, Sher Singh; Goswami, D.

    2016-05-01

    Uranium (U) gadolinium(Gd) and iron (Fe) containing alkaline waste simulated effluent (relevant to alkaline effluent of monazite ore) has been treated with a novel amphoteric resin viz, Polyamidehydroxamate (PAH) containing amide and hydroxamic acid groups. The resin has been synthesized in an eco-friendly manner by polymerization nad conversion to functional groups characterized by FT-IR spectra and architectural overview by SEM. Coloration of the loaded matrix and de-coloration after extraction of uranium is the special characteristic of the matrix. Effluent streams have been analyzed by ICP-AES, U loaded PAH has been characterized by FT-IR, EXAFS, Gd and Fe by X-ray energy values of EDXRF at 6.053 Kev and 6.405 Kev respectively. The remarkable change has been observed in Mössbauer spectrum of Fe-loaded PAH samples.

  19. Quaternary evolution of the rivers of northeast Hainan Island, China: Tracking the history of avulsion from mineralogy and geochemistry of river and delta sands

    NASA Astrophysics Data System (ADS)

    Pe-Piper, Georgia; Piper, David J. W.; Wang, Ying; Zhang, Yongzhan; Trottier, Corwin; Ge, Chendong; Yin, Yong

    2016-03-01

    The mineralogy and geochemistry of sands were investigated in the Nandu and Wanquan rivers, Hainan Island, China, to determine the history of avulsion in the lower reaches of the Nandu River. The study also provided the opportunity to assess the utility of geochemical analysis of sands as a provenance tool. Much of the heavy mineral fraction in the rivers consists of subangular Fe-Ti oxide and Fe-(hydr)oxide minerals, and less stable minerals such as amphibole, epidote, and andalusite, whereas rounded resistant ilmenite, rutile, tourmaline and zircon predominate on the deltaic coast. Mineral assemblage and chemical composition of individual samples are related to specific source areas and river tributaries. The results demonstrate northwestwards flow of the Nandu River during the mid-Holocene and earlier avulsion of the river to the northeast coast, probably during a Late Pleistocene marine highstand. Minor basement tilting, producing little relief, was sufficient to divert the lower reaches of rivers, and this effect was enhanced where basalt flows dammed former river courses. Bulk sample REE geochemistry is largely controlled by the relative abundance of monazite, allanite, titanite, zircon and epidote, derived principally from granites. Detrital geochemistry alone shows too much variability to interpret provenance. However, a smaller number of heavy mineral analyses provide an understanding of the mineralogical origins of geochemical variation, thus enabling interpretations of provenance.

  20. EARTHTIME: Teaching geochronology to high school students

    NASA Astrophysics Data System (ADS)

    Bookhagen, Britta; Buchwaldt, Robert; McLean, Noah; Rioux, Matthew; Bowring, Samuel

    2010-05-01

    The authors taught an educational module developed as part of the EARTHTIME (www.earth-time.org) outreach initiative to 215 high school students from a Massachusetts (USA) High School as part of an "out-of-school" field trip. The workshop focuses on uranium-lead (U-Pb) dating of zircons and its application to solving a geological problem. The theme of our 2.5-hour module is the timing of the K-T boundary and a discussion of how geochronology can be used to evaluate the two main hypotheses for the cause of the concurrent extinction—the Chicxlub impact and the massive eruption of the Deccan Traps. Activities are divided into three parts: In the first part, the instructors lead hands-on activities demonstrating how rock samples are processed to isolate minerals by their physical properties. Students use different techniques, such as magnetic separation, density separation using non-toxic heavy liquids, and mineral identification with a microscope. We cover all the steps from sampling an outcrop to determining a final age. Students also discuss geologic features relevant to the K-T boundary problem and get the chance to examine basalts, impact melts and meteorites. In the second part, we use a curriculum developed for and available on the EARTHTIME website (http://www.earth-time.org/Lesson_Plan.pdf). The curriculum teaches the science behind uranium-lead dating using tables, graphs, and a geochronology kit. In this module, the students start by exploring the concepts of half-life and exponential decay and graphically solving the isotopic decay equation. Manipulating groups of double-sided chips labeled with U and Pb isotopes reinforces the concept that an age determination depends on the Pb/U ratio, not the absolute number of atoms present. Next, the technique's accuracy despite loss of parent and daughter atoms during analysis, as well as the use of isotopic ratios rather than absolute abundances, is explained with an activity on isotope dilution. Here the students

  1. Rejuvenation effects during plastic deformation of Zircon: geochronological implications

    NASA Astrophysics Data System (ADS)

    Kovaleva, Elizaveta; Klötzli, Urs

    2013-04-01

    -deformation events and hosted by the shear zones? How to evaluate the significance of isotopic-system resetting in deformed domains? How to estimate the possibility of dating the deformation events and what is the reliability of such dating? A complex of methods is used for this research: transmitted and reflected light microscopy, CL (cathode luminescence) imaging for growth history, BSE (backscattered electrons) imaging for orientation and chemical contrast, SE (secondary elements) pictures for surface relief, SHRIMP (Sensitive High Resolution Ion Microprobe) for age dating, EBSD for crystallographic orientations. We suppose that careful combination of all these methods will lead to better understanding of some internal processes which happen with mineral during its evolution in host rocks. References: Moser D.E., Davis W.J., Reddy S.M., Flemming R.L., Hart R.J., 2009. Zircon U-Pb strain chronometry reveals deep impact-triggered flow. Earth and Planetary Science Letters 277, 73-79. Reddy, S.M., Timms, N.E., Pantleon, W., Trimby, T., 2007. Quantitative characterization of plastic deformation of zircon and geological implications. Contributions to Mineralogy and Petrology 153, 625-645. Reddy, S.M., Timms, N.E., 2010. Deformation of zircon and implications for geochemistry and geochronology. Source Abstracts with Programs - Geological Society of America 42 (5), 634.

  2. Building Interactive Visualizations for Geochronological Data

    NASA Astrophysics Data System (ADS)

    Zeringue, J.; Bowring, J. F.; McLean, N. M.; Pastor, F.

    2014-12-01

    Since the early 1990s, Ken Ludwig's Isoplot software has been the tool of choice for visualization and analysis of isotopic data used for geochronology. The software is an add-in to Microsoft Excel that allows users to generate visual representations of data. However, recent changes to Excel have made Isoplot more difficult to use and maintain, and the software is no longer supported. In the last several years, the Cyber Infrastructure Research and Development Lab for the Earth Sciences (CIRDLES), at the College of Charleston, has worked collaboratively with geochronologists to develop U-Pb_Redux, a software product that provides some of Isoplot's functionality for U-Pb geochronology. However, the community needs a full and complete Isoplot replacement that is open source, platform independent, and not dependent on proprietary software. This temporary lapse in tooling also presents a tremendous opportunity for scientific computing in the earth sciences. When Isoplot was written for Excel, it gained much of the platform's flexibility and power but also was burdened with its limitations. For example, Isoplot could not be used outside of Excel, could not be cross-platform (so long as Excel wasn't), could not be embedded in other applications, and only static images could be produced. Nonetheless this software was and still is a powerful tool that has served the community for more than two decades and the trade-offs were more than acceptable. In 2014, we seek to gain flexibility not available with Excel. We propose that the next generation of charting software be reusable, platform-agnostic, and interactive. This new software should allow scientists to easily explore—not just passively view—their data. Beginning in the fall of 2013, researchers at CIRDLES began planning for and prototyping a 21st-century replacement for Isoplot, which we call Topsoil, an anagram of Isoplot. This work is being conducted in the public domain at https://github.com/CIRDLES/topsoil. We

  3. Statistics of large detrital geochronology datasets

    NASA Astrophysics Data System (ADS)

    Saylor, J. E.; Sundell, K. E., II

    2014-12-01

    Implementation of quantitative metrics for inter-sample comparison of detrital geochronological data sets has lagged the increase in data set size, and ability to identify sub-populations and quantify their relative proportions. Visual comparison or application of some statistical approaches, particularly the Kolmogorov-Smirnov (KS) test, that initially appeared to provide a simple way of comparing detrital data sets, may be inadequate to quantify their similarity. We evaluate several proposed metrics by applying them to four large synthetic datasets drawn randomly from a parent dataset, as well as a recently published large empirical dataset consisting of four separate (n = ~1000 each) analyses of the same rock sample. Visual inspection of the cumulative probability density functions (CDF) and relative probability density functions (PDF) confirms an increasingly close correlation between data sets as the number of analyses increases. However, as data set size increases the KS test yields lower mean p-values implying greater confidence that the samples were not drawn from the same parent population and high standard deviations despite minor decreases in the mean difference between sample CDFs. We attribute this to the increasing sensitivity of the KS test when applied to larger data sets, which in turn limits its use for quantitative inter-sample comparison in detrital geochronology. Proposed alternative metrics, including Similarity, Likeness (complement to Mismatch), and the coefficient of determination (R2) of a cross-plot of PDF quantiles, point to an increasingly close correlation between data sets with increasing size, although they are the most sensitive at different ranges of data set sizes. The Similarity test is most sensitive to variation in data sets with n < 100 and is relatively insensitive to further convergence between larger data sets. The Likeness test reaches 90% of its asymptotic maximum at data set sizes of n = 200. The PDF cross-plot R2 value

  4. Relating Major Silicates and Monazite Growth in Metamorphic Rocks: Application to the Upper Granite Gorge (Grand Canyon, USA)

    NASA Astrophysics Data System (ADS)

    Allaz, J.; Williams, M. L.; Jercinovic, M. J.

    2010-12-01

    Relating deformation and metamorphic conditions with time is a major goal in tectonic studies. The challenge is to find suitable minerals to be dated, and to associate them with specific events and with pressure and temperature (PT) conditions. Monazite has proven to be an excellent chronometer for dating stages in a tectonic history. It contains very little common lead, is highly retentive, and it can be dated by electron microprobe at very high spatial resolution. In addition, it commonly preserves different growth stages in distinct compositional domains that can be identified via EPMA mapping. However, its growth history and relation with major silicate assemblages and reactions are usually poorly constrained. The growth of monazite can be related to the petrology of major silicate assemblages in metamorphic rocks if reactions forming specific monazite compositional domains can be identified and geochemically linked to silicate assemblages. This is accomplished here through use of thermobarometric tools, mainly TWQ [1] for determination of PT-conditions and THERIAK-DOMINO for pseudo-sections [2]. The composition of each monazite domain is then obtained, and microprobe ages using the technique described in [3] are obtained. The next step currently being developed is to identify the metamorphic reaction responsible for each monazite growth stage. The area selected for this study is the Upper Granite Gorge of the Grand Canyon (USA). Paleoproterozoic basement crops out in this area, characterized by a segmented, block-type architecture consisting of km-scale blocks of granitic and psammitic/pelitic rocks, separated by shear zones. Previous work [4] reveals constant P but variable T from block to block. At least one block preserves a T-gradient from upper greenschist to granulite facies. Compositional mapping of hundreds of monazite grains in several samples, revealed a complex growth history within and between samples. Analysis is currently in progress. First

  5. Altération hydrothermale des monazites-(Ce) des pegmatites du district de Santa Maria de Itabira (Minas Gerais, Brésil)

    NASA Astrophysics Data System (ADS)

    Bilal, Essaïd; Marciano, Vitoria; Marques^Correia Neves, José; Fuzikawa, Kazuo; Riffel, Bruno Fernando; Nalini, Herminio Arias; Fernandes, Maria Lourdes; Nasraoui, Mohamed

    1998-05-01

    Monazite-(Ce) is found in granitic pegmatites in the Santa Maria de Itabira pegmatite district (Minas Gerais, Brazil). During the magmatic stage, monazite-(Ce) seems to have had higher contents of cheralite and huttonite in the solid solution. The Th content in primary monazite-(Ce) is high and characteristic for each pegmatite body. During the late stage (albitization), the mean LREE content in the altered zone is slightly higher and Th content is very low. The accessory mineral assemblages changed; huttonite and cheralite crystallize together with Th-poor and La-rich monazite-(Ce) at the border of altered crystals. Nd/Sm and U/Pb ratios are also changed during the hydrothermal stage.

  6. Unraveling P-T-Time-Fluids Paths: in-situ Geochronology Combined with Oxygen Isotopes in Metamorphic Rocks

    NASA Astrophysics Data System (ADS)

    Rubatto, D.; Angiboust, S.

    2014-12-01

    Geochronology of metamorphic rocks aims to determine pressure-temperature-time paths. The development of in-situ oxygen isotope analysis by ion microprobe permits to add fluid evolution to metamorphic histories. Accurate oxygen isotope analysis requires matrix-matched standards and, for chemically complex minerals, also appropriate matrix corrections. We will report on recent progresses on the analysis of oxygen isotopes in monazite, allanite and garnet using the SHRIMP ion microprobe. Zircon is a prime candidate for retrieving the oxygen composition of the pre-metamorphic protolith, as inherited zircons are preserved in many high-grade rocks. Metamorphic zircon rims that differ from the cores in δ18O offer insight into the origin of the fluid and the timing as well as degree of fluid-rock interaction. Garnet is another exceptional recorder of multiple metamorphic stages. Highly zoned garnets are found in a variety of eclogite-facies rocks and they can preserve dramatic oxygen isotopic variations (differences of a few to 10 δ18O ‰ within single crystal). Garnet and zircons from eclogite-facies metamorphic veins and shear zones within the metamorphosed Lago Superiore Unit from the Monviso ophiolite (W. Alps) have distinct growth zones with different oxygen isotopes composition. The shift in oxygen isotopes between the magmatic zircon core and the metamorphic zircon rim testifies to metasomatism by fluids equilibrated with serpentinites. The oxygen isotopes zoning in garnet confirms large scale fluid metasomatism concentrated in shear zones. It also indicates that some major shear zones from the Monviso ophiolitic sequence possibly underwent two stages of metasomatism, an event before or during early subduction and a later stage at around 550°C/2.5 GPa at peak pressure metamorphism.

  7. A direct comparison of the ages of detrital monazite versus detrital zircon in Appalachian foreland basin sandstones: Searching for the record of Phanerozoic orogenic events

    NASA Astrophysics Data System (ADS)

    Hietpas, Jack; Samson, Scott; Moecher, David

    2011-10-01

    The provenance potential of detrital monazite was investigated by in situ measurement of 232Th- 208Pb dates of grains isolated from six Middle Carboniferous-Permian sandstones from the Appalachian foreland basin. Provenance assessment of these units was previously investigated by measuring U-Pb crystallization ages of detrital zircon (Thomas et al., 2004; Becker et al., 2005, 2006). Approximately 90% of the detrital zircon ages record Mesoproterozoic or older ages, with only 10% recording the three major pulses of tectonism (Taconian, Acadian and Alleghanian) that are the hallmark of the Appalachian Orogen. 232Th- 208Pb ages of detrital monazite, however, strongly record the complex phases of Paleozoic orogenesis. Nearly 65% of the ages record Paleozoic events, while 35% record Neoproterozoic or older ages. In several of the analyzed sandstones, detrital monazite ages record Paleozoic orogenic events that are completely missed by detrital zircon ages, demonstrating that monazite ages more accurately reflect the character of the sediment source rocks. The inferred maximum age of sediment deposition, as determined by the youngest monazite grains, is ~ 550 Ma younger for two of the analyzed sandstones compared to depositional constraints based on the youngest detrital zircon. The different physical properties and petrogenesis of zircon and monazite are interpreted to be factors for the dramatic differences in sediment provenance information provided by each mineral. The results from this study have important implications for determining sediment provenance, constraining maximum age of sediment deposition, and developing robust regional tectonic models.

  8. Combined Raman and photoluminescence spectroscopic investigation of He-irradiation effects in monazite

    NASA Astrophysics Data System (ADS)

    Lenz, C.; Petautschnig, C.; Akhmadaliev, S.; Hanf, D.; Talla, D.; Nasdala, L.

    2012-04-01

    We present first results of a study addressing effects of the corpuscular self-irradiation on stress/strain and optical properties (in particular the photoluminescence behaviour) in monazite-(Ce). For this, natural samples, and synthetic CePO4 crystals mildly doped with Nd3+, were irradiated with 7.7 MeV He ions, which are analogues of alpha particles generated in the 214Po → 210Pb decay event (238U decay series). Light-ion irradiation was preferred over heavy-ion irradiation (i.e., ~100 keV heavy ions as analogues of alpha recoils) for two reasons. First, MeV He ions penetrate much deeper into mineral targets (tens of micrometres vs. a few hundred angstroms), resulting in irradiated volumes that are "measurable" using micro-spectroscopy techniques. Second, the depth distribution profiles of nuclear and electronic energy losses vary appreciably in the case of MeV He ions. The latter is most helpful in assigning observed effects to either point defects created or target ionisation. The irradiations were done at the 3 MV Tandem accelerator of the Helmholtz-Zentrum Dresden-Rossendorf (Germany). The fluences applied varied in the range 1013-1017 He/cm2. The degree of disturbance of the short-range order in the samples irradiated was evaluated from the broadening of the ν1(PO4) Raman band (symmetric PO4 stretching; A1g mode) near 970 cm-1 [1,2]. The majority of the damage created was observed near 28 μm into the monazite-(Ce) targets, i.e., near the ends of the helium trajectories. Here, the ν1(PO4) Raman band may broaden appreciably, from 2.2 cm-1 (in the case of synthetic CePO4) to well above 20 cm-1. The generation of additional damage was also observed upon He irradiation of naturally radiation-damaged monazite-(Ce). By contrast, there was no indication of any annealing effect of the MeV He ions irradiated in these samples (compare [3]; a similar effect of alpha-assisted annealing was suspected by [4] in the discussion of fission tracks). Sharp luminescence

  9. Constraints on the genesis of gold mineralization at the Homestake Gold Deposit, Black Hills, South Dakota from rhenium-osmium sulfide geochronology

    NASA Astrophysics Data System (ADS)

    Morelli, Ryan M.; Bell, Chris C.; Creaser, Robert A.; Simonetti, Antonio

    2010-06-01

    The Homestake gold deposit, located in the Black Hills, South Dakota, USA, is one of the largest known hydrothermal gold deposits globally, with total mining production exceeding 40 Moz Au. Rhenium-osmium geochronology of ore-associated arsenopyrite and pyrrhotite was performed in an effort to delineate the timing of gold mineralization in relation to known tectonothermal events in the northern Black Hills. Arsenopyrite yields a rhenium-osmium (Re-Os) age of 1,736 ± 8 Ma (mean squared weighted deviation = 1.6), consistent with existing age constraints for gold mineralization, whereas Re-Os pyrrhotite data are highly scattered and do not yield a meaningful mineralization age. This is taken to indicate that the Re-Os arsenopyrite chronometer is robust to at least 400°C, whereas the Re-Os pyrrhotite chronometer is likely disturbed by temperatures of 300-350°C. The Re-Os arsenopyrite age and initial Os ratio (0.28 ± 0.15) are interpreted to indicate that gold was introduced at ca. 1,730 Ma, coincident with the onset of exhumation of crustal blocks and, possibly, the earliest intrusive phases of Harney Peak granite magmatism. New in situ U-Pb monazite analyses from an aplite dike in the east-central Black Hills indicate that granite magmatism was a protracted event, persisting until at least ca. 1,690 Ma.

  10. Tracing the provenance and recrystallization processes of the Earth's oldest detritus at Mt. Narryer and Jack Hills, Western Australia: An in situ Sm-Nd isotopic study of monazite

    NASA Astrophysics Data System (ADS)

    Iizuka, Tsuyoshi; Nebel, Oliver; McCulloch, Malcolm T.

    2011-08-01

    Mount Narryer and Jack Hills metasedimentary rocks in the Narryer Gneiss Complex of the Yilgarn Craton, Western Australia, contain zircons with ages up to 4.4 Ga, the oldest known crustal materials on Earth, and monazites up to 3.6 Ga. In this study, we have investigated 147Sm- 143Nd systematics of detrital and metamorphic monazites from these metasedimentary rocks using laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS). All detrital monazites have negative initial ɛNd(t) values, indicating that their parental magmas formed by remelting of older crustal materials. A comparison between the initial ɛNd(t) values of the detrital monazites and granitoids in the Narryer Gneiss Complex indicates that the Mt. Narryer and Jack Hills sediments were partly derived from the most isotopically enriched surrounding granitoids with ages of ca. 3.6 and 3.3 Ga. The metamorphic monazites generally have lower initial ɛNd(t) values when compared to the detrital monazites. However, the detrital and metamorphic monazites show similar distributions of ɛNd(t) at the ages of sediment deposition (3.28 Ga for Mt. Narryer and 3.05 Ga for Jack Hills). In addition, multiple analyses on single monazite grains having core-rim structures reveal that the cores and the recrystallized rims had identical Nd isotopic compositions at the time of recrystallization. These findings indicate that older monazites are source of light rare earth elements for younger metamorphic monazite formation and, therefore, that monazite can inherit its primary Sm-Nd isotopic signature during the recrystallization processes. We calculated the Nd model ages for all analyzed monazites to estimate crustal residence time of their source materials. We find that no igneous monazites older than 4.0 Ga were recrystallized to form the monazites. This implies that the lack of Hadean monazites is not due to recrystallization of ancient monazites during later metamorphism, but due to high

  11. Magnetic measurements of the transuranium elements and charge state characterization of actinides in monazite. Progress report

    SciTech Connect

    Huray, P. G.

    1980-01-01

    A micromagnetic susceptometer for the purpose of measuring extremely small sample quantities (on the microgram level) was designed, constructed, and calibrated in previous years. (The 1979 progress report gives details of its operation.) This device has operated without significant downtime in this funding period, and much progress has been made in the magnetic characterization of elements beyond Am in the periodic table. This program has roughly doubled man's knowledge of magnetism in Cm, Bk, and Cf, and includes the only Es magnetic measurements to date. The incorporation of an automatic data collection system in this period has made analysis much more accurate, and has allowed quicker turnaround of compounds and metals for study. Results obtained for the compounds and metals studied this year are summarized. The lanthanide orthophosphates are being investigated as an alternate means of primary containment for high-level actinide wastes. Researchers at the Oak Ridge National Laboratory are involved in preparation of actinide-doped compounds for all of the lanthanide transition series (La through Lu) for a study of leaching characteristics and E.S.R. classification. To aid this study the charge state of /sup 237/Np or /sup 57/Fe has been identified, either in the as-prepared compounds or following radioactive decay of /sup 241/Am via the Moessbauer Effect. The final charge state will be an influential variable in the immobilization characteristics of the waste products stored in this synthetic monazite form. 10 figures, 1 table. (RWR)

  12. Geochemistry

    ERIC Educational Resources Information Center

    Brett, Robin; Hanshaw, Bruce B.

    1978-01-01

    The past year has seen the development of certain fields of geochemical research including Nd-Sm isotope studies of meteorites and ancient terrestrial rocks; the use of the consortium approach of assembling a multidisciplined team to tackle a problem; and the handling and analysis of small quantities of materials. (Author/MA)

  13. Electron-microprobe Th-U-Pb monazite dating in Early-Palaeozoic high-grade gneisses as a completion of U-Pb isotopic ages (Wilson Terrane, Antarctica)

    NASA Astrophysics Data System (ADS)

    Schulz, B.; Schüssler, U.

    2013-08-01

    The electron microprobe (EMP) Th-U-Pb monazite bulk chemical dating method was applied to granulite-facies rocks of the Wilson Terrane in Antarctica. A combination of this method to isotopic U-Pb-SHRIMP ages for the evaluation of metamorphic processes required the analysis of reference monazites. These can be subdivided into three groups: a) Monazite with variable total Pb at constant Th (e.g. VK-1) is unsuitable for EMP data evaluation; b) Monazite with highly variable total Pb and Th, but with at least some Th/Pb approximating an apparent isochrone (e.g. MPN) is partly useful; and c) Monazite with constant Th/Pb at high Th (e.g. Madmon monazite) is best suitable for the combined approach and can be additionally used to improve the Th calibration for EMP. Study of monazite in grain mounts and in thin sections led to partly different but complementary results: Older monazites with EMP ages up to 680 Ma occur mainly in a grain mount from diatexite and metatexite and are interpreted as detrital relics. Some of these monazites show structures and mineral-chemical zonation trends resembling metasomatism by alkali-bearing fluids. A marked mobility of Th, P, Ce, Si and U is observed. The age of the metasomatic event can be bracketed between 510 and 450 Ma. Furthermore, in the grain mount and in numerous petrographic thin sections of migmatites and gneisses, the EMP Th-U-Pb and SHRIMP U-Pb monazite data uniformly signal a major metamorphic event with a medium-pressure granulite facies peak between 512 and 496 Ma. Subsequent isothermal uplift and then amphibolite-facies conditions between 488 and 466 Ma led to crystallisation of pristine monazite. The high-grade metamorphic event, related to the Ross Orogeny, can be uniformly traced more than 600 km along strike in the Wilson Terrane.

  14. Evaluation of DNA damage in a population of bats (Chiroptera) residing in an abandoned monazite mine.

    PubMed

    Meehan, Kathleen A; Truter, Ernest J; Slabbert, Jacobus P; Parker, M Iqbal

    2004-02-14

    Ionising radiation has the ability to induce DNA damage. While the effects of high doses of radiation of short duration have been well documented, the biological effects of long-term exposure to low doses are poorly understood. This study evaluated the clastogenic effects of low dose ionising radiation on a population of bats (Chiroptera) residing in an abandoned monazite mine. Bats were sampled from two chambers in the mine, where external radiation levels measured around 20 microSv/h (low dose) and 100 microSv/h (higher dose), respectively. A control group of bats was sampled from a cave with no detectable radiation above normal background levels. The micronucleus assay was used to evaluate residual radiation damage in binucleated lymphocytes and showed that the micronucleus frequency per 500 binucleated lymphocytes was increased in the lower radiation-exposed group (17.7) and the higher radiation-exposed group (27.1) compared to the control group (5.3). This study also showed that bats exposed to radiation presented with an increased number of micronuclei per one thousand reticulocytes (2.88 and 10.75 in the lower and high radiation-exposed groups respectively) when compared to the control group (1.7). The single-cell gel electrophoresis (comet) assay was used as a means of evaluating clastogenecity of exposure to radiation at the level of individual cells. Bats exposed to radiation demonstrated increased DNA damage as shown by the length of the comet tails and showed an increase in cumulative damage. The results of the micronucleus and the comet assays indicated not only a statistically significant difference between test and control groups (P<0.001), but also a dose-dependent increase in DNA damage (P<0.001). These assays may thus be useful in evaluating the potential clastogenecity of exposure to continuous low doses of ionising radiation. PMID:14729373

  15. Revised Geochronology and Magnetostratigraphy of Northwest Iceland

    NASA Astrophysics Data System (ADS)

    Riishuus, M. S.; Duncan, R. A.; Kristjansson, L.

    2013-12-01

    .5-15.0 Ma - and at a significantly higher growth rate. This suggests that the period of volcanic quiescence, during which the lignite-bearing laterites were deposited, was long-lived (1-1.5 Myr). Our results also reveal significant variations in growth rates SW-NE along strike of the lava pile, presumably reflecting differences in the volcanic productivity along the rift zone, as well as higher resolution of temporal growth rate variations from 17 to 8 Ma (~350 m/Myr @ 17-15 Ma, ~2200m/Myr @ 14.5-13.8 Ma, ~700m/Myr @ 13.8-11.6 Ma, ~1500m/Myr @ 11.6-10.6 Ma, ~800m/Myr @ 10.5-8 Ma) than hitherto thought. At several stratigraphic levels the new absolute ages are significantly older than the recalculated age data from earlier studies. This requires reassessment of the correlation of the observed polarity patterns with the Geomagnetic Polarity Time Scale. McDougall, I., Kristjansson, L. and Saemundsson, K., 1984. Magnetostratigraphy and geochronology of Northwest Iceland. Journal of Geophysical Research 89, 7029-7060.

  16. Ordovician appinites in the Wugongshan Domain of the Cathaysia Block, South China: Geochronological and geochemical evidence for intrusion into a local extensional zone within an intracontinental regime

    NASA Astrophysics Data System (ADS)

    Zhong, Yufang; Ma, Changqian; Liu, Lei; Zhao, Junhong; Zheng, Jianping; Nong, Junnian; Zhang, Zejun

    2014-06-01

    Palaeozoic mafic igneous rocks are potentially significant in constraining the tectonic nature and evolution of the Kwangsian Orogeny in the eastern South China Block, yet they have received little attention because of their limited outcrop. Geochemistry and geochronology was carried out on newly identified Ordovician ultramafic-mafic appinites in the Wugongshan Domain of the Cathaysia Block. Seven appinite samples yielded 206Pb/238U crystallisation ages ranging from 452 ± 4 Ma to 473 ± 3 Ma. Abundant 480-500 Ma zircon xenocrysts and/or inherited zircons were found in the appinites, possibly indicating an earlier magmatism episode in the early Palaeozoic period. The Wugongshan appinites are ultramafic to mafic in composition, and the ultramafic rocks display features of cumulates (high concentrations of MgO, Fe2O3t, Cr, Ni, and low concentrations of total alkali and total rare earth elements [REE]). The appinite geochemistry displays: relatively flat chondrite normalised REE patterns with slight enrichment in light REE and weak negative Eu anomalies; enrichment in large-ion lithophile elements (such as Rb, K), and weak depletion in Nb-Ta in primitive mantle normalised trace element patterns. We suggest that the Wugongshan appinites likely originated from an ancient metasomatised mantle, and that crustal assimilation, fractional crystallisation (AFC), magma mingling and hydration were involved in the petrogenetic process, based on the combination of geochemistry, crust-like bulk Sr, Nd and zircon Hf isotopic compositions (εNd (t) = - 8.2 to - 3.2, initial 87Sr/86Sr ratios of 0.7067-0.7144, zircon εHf (t) values peaking at - 9 to - 3) and regional geological data. Further considering the alignment and chronology of the appinites, we suggest that the appinitic magmas probably were emplaced along the Jiangshan-Shaoxing Fault in a local extensional zone in an intracontinental regime in the early Palaeozoic.

  17. Fluid-induced dissolution breakdown of monazite from Tso Morari complex, NW Himalayas: evidence for immobility of trace elements

    NASA Astrophysics Data System (ADS)

    Upadhyay, Dewashish; Pruseth, Kamal Lochan

    2012-08-01

    Primary igneous monazite from the Polokongka La granite of the Tso Morari complex in the western Himalayas has been partially replaced by a three-layered corona of metamorphic fluor-apatite, allanite + U- and Th-bearing phases (huttonite + brabantite), and epidote. The alteration is related to high-pressure amphibolite-facies (10-11 kbar and 587-695 °C) fluid-induced retrogression of the ultra-high-pressure granite during exhumation after India-Asia collision. The corona textures can be explained by pseudomorphic partial replacement of the original monazite to apatite and allanite via a fluid-mediated coupled dissolution-reprecipitation process. Mass balance calculations using the volume proportions and compositions of coronal minerals show that the REE, U, Th, Pb, Ba and P were conserved and not transported outside the alteration corona. The formation of fluor-apatite, allanite, huttonite and coffinite from monazite and the immobility of REE, U and Th require an influx of alkali- and F-bearing, Ca-rich fluid having high Ca/Na into the corona. We are aware of only two other occurrences of such alteration textures, and these have several similarities in terms of geodynamic setting and P-T histories of the host rocks. We suggest that there may be a common mechanism of exhumation style, and source and composition of fluids during retrogression of granitoid rocks in collisional orogens and that such breakdown textures can be used to identify metagranites that have experienced high-P metamorphism in continental collision zones, which is otherwise difficult to constrain due to the high variance of the mineral assemblages in these rocks.

  18. Abundance and distribution of uranium and thorium in zircon, sphene, apatite, epidote, and monazite in granitic rocks

    USGS Publications Warehouse

    Hurley, Patrick M.; Fairbairn, Harold W.

    1956-01-01

    Analyses were made of uranium and thorium in ziircon, sphene, apatite, epidote, and monazite separated as accessory minerals from samples of granitic rock from widely scattered localities to indicate the abundance and distribution of these two elements among the five mineral phases.  For any pair of mineral phases the distribution ratio remains within the same order of magnitude over the different rocks tested, although the variability of the data is such that only wide departures from constancy could be ascertained.  Such gross differences have not been found. 

  19. Nd Isotope and U-Th-Pb Age Mapping of Single Monazite Grains by Laser Ablation Split Stream Analysis

    NASA Astrophysics Data System (ADS)

    Fisher, C. M.; Hanchar, J. M.; Miller, C. F.; Phillips, S.; Vervoort, J. D.; Martin, W.

    2015-12-01

    Monazite is a common accessory mineral that occurs in medium to high grade metamorphic and Ca-poor felsic igneous rocks, and often controls the LREE budget (including Sm and Nd) of the host rock in which it crystallizes. Moreover, it contains appreciable U and Th, making it an ideal mineral for determining U-Th-Pb ages and Sm-Nd isotopic compositions, both of which are readily determined using in situ techniques with very high spatial resolution like LA-MC-ICPMS. Here, we present the results of laser ablation split stream analyses (LASS), which allows for simultaneous determination of the age and initial Nd isotopic composition in a single analysis. Analyses were done using a 20mm laser spot that allowed for detailed Nd isotope mapping of monazite grains (~30 analyses per ~250mm sized grain). Combined with LREE ratios (e.g., Sm/Nd, Ce/Gd, and Eu anomalies) these results yield important petrogenetic constraints on the evolution of peraluminous granites from the Old Woman-Piute batholith in southeastern California. Our findings also allow an improved understanding of the causes of isotope heterogeneity in granitic rocks. U-Th-Pb age mapping across the crystals reveals a single Cretaceous age for all grains with precision and accuracy typical of laser ablation analyses (~2%). In contrast, the concurrent Nd isotope mapping yields homogeneous initial Nd isotope compositions for some grains and large initial intra-grain variations of up to 8 epsilon units in others. The grains that yield homogeneous Nd isotope compositions have REE ratios suggesting that they crystallized in a fractionally crystallizing magma. Conversely, other grains, which also record fractional crystallization of both feldspar and LREE rich minerals, demonstrate a change in the Nd isotope composition of the magma during crystallization of monazite. Comparison of inter- and intra-grain Nd isotope compositions reveals further details on the potential mechanisms responsible for isotope heterogeneity

  20. Divisions of geologic time-major chronostratigraphic and geochronologic units

    USGS Publications Warehouse

    U.S. Geological Survey Geologic Names Committee

    2010-01-01

    Effective communication in the geosciences requires consistent uses of stratigraphic nomenclature, especially divisions of geologic time. A geologic time scale is composed of standard stratigraphic divisions based on rock sequences and is calibrated in years. Over the years, the development of new dating methods and the refinement of previous methods have stimulated revisions to geologic time scales. Advances in stratigraphy and geochronology require that any time scale be periodically updated. Therefore, Divisions of Geologic Time, which shows the major chronostratigraphic (position) and geochronologic (time) units, is intended to be a dynamic resource that will be modified to include accepted changes of unit names and boundary age estimates. This fact sheet is a modification of USGS Fact Sheet 2007-3015 by the U.S. Geological Survey Geologic Names Committee.

  1. Uranium geochemistry of Orca Basin

    NASA Astrophysics Data System (ADS)

    Weber, F. F., Jr.; Sackett, W. M.

    1981-08-01

    Orca Basin, an anoxic, brine-filled depression at a depth of 2200 m in the Northwestern Gulf of Mexico continental slope, has been studied with respect to its uranium geochemistry. Uranium concentration profiles for four cores from within the basin were determined by delayed-neutron counting. Uranium concentrations ranged from 2.1 to 4.1 ppm on a salt-free and carbonate-corrected basis. The highest uranium concentrations were associated with the lowest percentage and δ 13C organic carbon values. For comparison, cores frm the brine-filled Suakin and Atlantis II Deeps, both in the Red Sea, were also analyzed. Uranium concentrations ranged from 1.2 to 2.6 ppm in the Suakin Deep and from 8.0 to 11.0 ppm in the Atlantis II Deep. No significant correlation was found between uranium concentrations and organic carbon concentrations and δ 13C values for these cores. Although anoxic conditions are necessary for significant uranium uptake by non-carbonate marine sediments, other factors such as dilution by rapidly depositing materials and uranium supply via mixing and diffusion across density gradients may be as important in determining uranium concentrations in hypersaline basin sediments.

  2. A precise 232Th-208Pb chronology of fine-grained monazite: Age of the Bayan Obo REE-Fe-Nb ore deposit, China

    USGS Publications Warehouse

    Wang, Jingyuan; Tatsumoto, M.; Li, X.; Premo, W.R.; Chao, E.C.T.

    1994-01-01

    We have obtained precise Th-Pb internal isochron ages on monazite and bastnaesite for the world's largest known rare earth elements (REE)-Fe-Nb ore deposit, the Bayan Obo of Inner Mongolia, China. The monazite samples, collected from the carbonate-hosted ore zone, contain extremely small amounts of uranium (less than 10 ppm) but up to 0.7% ThO2. Previous estimates of the age of mineralization ranged from 1.8 to 0.255 Ga. Magnetic fractions of monazite and bastnaesite samples (<60-??m size) showed large ranges in 232Th 204Pb values (900-400,000) and provided precise Th-Pb internal isochron ages for paragenetic monazite mineralization ranging from 555 to 398 Ma within a few percent error (0.8% for two samples). These results are the first indication that REE mineralization within the giant Bayan Obo ore deposit occurred over a long period of time. The initial lead isotopic compositions (low 206Pb 204Pb and high 208Pb 204Pb) and large negative ??{lunate}Nd values for Bayan Obo ore minerals indicate that the main source(s) for the ores was the lower crust which was depleted in uranium, but enriched in thorium and light rare earth elements for a long period of time. Zircon from a quartz monzonite, located 50 km south of the ore complex and thought to be related to Caledonian subduction, gave an age of 451 Ma, within the range of monazite ages. Textural relations together with the mineral ages favor an epigenetic rather than a syngenetic origin for the orebodies. REE mineralization started around 555 Ma (disseminated monazite in the West, the Main, and south of the East Orebody), but the main mineralization (banded ores) was related to the Caledonian subduction event ca. 474-400 Ma. ?? 1994.

  3. Preliminary Monazite U-Th-Total Pb Absolute Age Constraints on Crustal Thickening and Siluro-Devonian Dextral Transpression: Central Appalachian Piedmont, SE Pennsylvania

    NASA Astrophysics Data System (ADS)

    Bosbyshell, H.; Srogi, L.; Pyle, J. M.; Blackmer, G. C.

    2007-12-01

    High grade metamorphic rocks in the Central Appalachian piedmont of SE Pa. occur in at least three distinct lithotectonic belts that are bounded by steeply dipping, NE-striking transcurrent or transpressional shear zones. From NW to SE these belts consist of rocks of Laurentian-, uncertain-, and arc-affinity. The central belt is characterized by basement gneiss-cored nappes and associated shallowly to moderately SE dipping S2 foliation. This belt followed a clockwise metamorphic P-T path; maximum temperatures (T ~ 640 °C at P ~ 0.7 GPa) are associated with syn- to post-S2 garnet growth. An increase in pressure of ~ 0.1 GPa is inferred from post- kinematic high-Ca garnet rims. This paper reports preliminary EPMA results on monazite from two samples, WG- 216 and U-154, from the Glenarm Wissahickon, the metasedimentary cover sequence in the central belt. A few monazite grains from WG-216 exhibit small low-Th cores which give latest Cambrian ages (492 +/- 10 Ma, n = 10; n = # of analyses, all precision is reported as 2 s.e.). All grains in WG-216 contain cores with irregular, patchy zoning that yield late Ordovician ages (454 +/- 6 Ma, n = 31). High-Th, low-Y rims, which yield early Devonian ages (410 +/- 11 Ma, n = 12), are present on monazite inclusions in the outer portions of garnet and staurolite, but are absent in monazite inclusions within microlithons characterized by S1 foliation. High-Th, low-Y rims grow along 001 cleavage in S2 foliation-forming micas, so S2 can be no younger than monazite rims. Similar EPMA monazite results were obtained from U-154, also from the Glenarm Wissahickon. At least one Ordovician core is present (455 +/- 4 Ma, n = 8); pre-S2 matrix monazite and one inclusion in staurolite yield late Silurian ages (422 +/- 4 Ma, n = 29); and one inclusion within outermost garnet yields a Devonian age (415 +/- 8 Ma, n = 8). These results constrain the timing of S2 formation and peak metamorphism in the central belt to the early Devonian. The SE

  4. Solid-state synthesis of monazite-type compounds containing tetravalent elements.

    PubMed

    Bregiroux, Damien; Terra, Olivier; Audubert, Fabienne; Dacheux, Nicolas; Serin, Virgine; Podor, Renaud; Bernache-Assollant, Didier

    2007-11-26

    On the basis of optimized grinding/heating cycles developed for several phosphate-based ceramics, the preparation of brabantite and then monazite/brabantite solid solutions loaded with tetravalent thorium, uranium, and cerium (as a plutonium surrogate) was examined versus the heating temperature. The chemical reactions and transformations occurring when heating the initial mixtures of AnO2/CeO2, CaHPO(4).2H2O (or CaO), and NH4H2PO4 were identified through X-ray diffraction (XRD) and thermogravimetric/differential thermal analysis experiments. The incorporation of thorium, which presents only one stabilized oxidation state, occurs at 1100 degrees C. At this temperature, all the thorium-brabantite samples appear to be pure and single phase as suggested by XRD, electron probe microanalyses, and micro-Raman spectroscopy. By the same method, tetravalent uranium can be also stabilized in uranium-brabantite, i.e., Ca0.5U0.5PO4, after heating at 1200 degrees C. Both brabantites, Ca0.5Th0.5PO4 and Ca0.5U0.5PO4, begin to decompose when increasing the temperature to 1400 and 1300 degrees C, respectively, leading to a mixture of CaO and AnO2 by the volatilization of P4O10. In contrast to the cases of thorium and uranium, cerium(IV) is not stabilized during the heating treatment at high temperature. Indeed, the formation of Ca0.5Ce0.5PO4 appears impossible, due to the partial reduction of cerium(IV) into cerium(III) above 840 degrees C. Consequently, the systems always appear polyphase, with compositions of CeIII1-2xCeIVxCaxPO4 and Ca2P2O7. The same conclusion can be also given when discussing the incorporation of cerium(IV) into La1-2xCeIIIx-yCeIVyCay(PO4)1-x+y. This incomplete incorporation of cerium(IV) confirms the results obtained when trying to stabilize tetravalent plutonium in Ca0.5PuIV0.5PO4 samples. PMID:17963377

  5. It's About Time: How Accurate Can Geochronology Become?

    NASA Astrophysics Data System (ADS)

    Harrison, M.; Baldwin, S.; Caffee, M. W.; Gehrels, G. E.; Schoene, B.; Shuster, D. L.; Singer, B. S.

    2015-12-01

    As isotope ratio precisions have improved to as low as ±1 ppm, geochronologic precision has remained essentially unchanged. This largely reflects the nature of radioactivity whereby the parent decays into a different chemical species thus putting as much emphasis on the determining inter-element ratios as isotopic. Even the best current accuracy grows into errors of >0.6 m.y. during the Paleozoic - a span of time equal to ¼ of the Pleistocene. If we are to understand the nature of Paleozoic species variation and climate change at anything like the Cenozoic, we need a 10x improvement in accuracy. The good news is that there is no physical impediment to realizing this. There are enough Pb* atoms in the outer few μm's of a Paleozoic zircon grown moments before eruption to permit ±0.01% accuracy in the U-Pb system. What we need are the resources to synthesize the spikes, enhance ionization yields, exploit microscale sampling, and improve knowledge of λ correspondingly. Despite advances in geochronology over the past 40 years (multicollection, multi-isotope spikes, in situ dating), our ability to translate a daughter atom into a detected ion has remained at the level of 1% or so. This means that a ~102 increase in signal can be achieved before we approach a physical limit. Perhaps the most promising approach is use of broad spectrum lasers that can ionize all neutrals. Radical new approaches to providing mass separation of such signals are emerging, including trapped ion cyclotron resonance and multi-turn, sputtered neutral TOF spectrometers capable of mass resolutions in excess of 105. These innovations hold great promise in geochronology but are largely being developed for cosmochemistry. This may make sense at first glance as cosmochemists are classically atom-limited (IDPs, stardust) but can be a misperception as the outer few μm's of a zircon may represent no more mass than a stardust mote. To reach the fundamental limits of geochronologic signals we need to

  6. Performance of DFT+U method for prediction of structural and thermodynamic parameters of monazite-type ceramics.

    PubMed

    Blanca Romero, Ariadna; Kowalski, Piotr M; Beridze, George; Schlenz, Hartmut; Bosbach, Dirk

    2014-07-01

    We performed a density functional theory (DFT) study of the monazite-type ceramics using DFT+U method, where the Hubbard U parameters are derived ab initio, with the main goal in testing the predictive power of this computational method for modeling of f-electron materials that are of interest in nuclear waste management. We show that DFT+U approach with PBEsol as the exchange-correlation functional significantly improves description of structures and thermodynamic parameters of lanthanide-bearing oxides and monazites over commonly used standard DFT (PBE) approach. We found that it is essential to use the Hubbard U parameter derived for a given element and a given structure to reproduce the structural parameters of the measured materials. We obtained exceptionally good description of the structural parameters with U parameter derived using the linear response approach of Cococcioni and de Gironcoli (Phys. Rev. B 2005, 71, 035105). This shows that affordable methods, such as DFT+U with a clever choice of exchange-correlation functional and the Hubbard U parameter can lead to a good description of f-electron materials. PMID:24760757

  7. Phosphate monazite- and NaZr2(PO4)3 (NZP)-like ceramics containing uranium and plutonium

    NASA Astrophysics Data System (ADS)

    Orlova, A. I.; Kitaev, D. B.; Lukinich, A. N.; Tomilin, S. B.; Lizin, A. A.; Kulikov, I. A.; Samsonov, V. E.

    2003-01-01

    The synthesis and results of the X-ray diffraction examination of complex orthophosphates involving the cations Ca2+, Cd2+, Gd3+, Ti4+, Hf4+, Ce4+, U4+, and Pu4+ are described. Crystalline products containing either cerium or plutonium or these two elements together were prepared. Cerium was used as plutonium simulator. Phosphates containing cations Ti4+ and Hf4+ were found to form two-phase products. One of these phases was assumed to be of the NaZr2(PO4)3 (NZP) type, the other one of the CePO4 type (monazite). Formation of single-phase products with a monazite structure was found in the specimens that did not contain Ti and Hf. X-ray diffraction characteristics of synthesised phases were identified. An effect of the cation composition on the phase formation and also the effect of the cation size on the crystal lattice parameters of the formed phases were established.

  8. U-Pb SHRIMP geochronology and trace-element geochemistry of coesite-bearing zircons, North-East Greenland Caledonides

    USGS Publications Warehouse

    McClelland, W.C.; Power, S.E.; Gilotti, J.A.; Mazdab, F.K.; Wopenka, B.

    2006-01-01

    Obtaining reliable estimates for the timing of eclogite-facies metamorphism is critical to establishing models for the formation and exhumation of high-pressure and ultrahigh-pressure (UHP) metamorphic terranes in collisional orogens. The presence of pressure-dependent phases, such as coesite, included in metamorphic zircon is generally regarded as evidence that zircon growth occurred at UHP conditions and, ifdated, should provide the necessary timing information. We report U-Pb sensitive high-resolution ion microprobe (SHRIMP) ages and trace-element SHRIMP data from coesite-bearing zircon suites formed during UHP metamorphism in the North- East Greenland Caledonides. Kyanite eclogite and quartzofeldspathic host gneiss samples from an island in J??kelbugt (78??00'N, 18??04'W) contained subspherical zircons with well-defined domains in cathodoluminescence (CL) images. The presence of coesite is confirmed by Raman spectroscopy in six zircons from four samples. Additional components of the eclogite-facies inclusion suite include kyanite, omphacite, garnet, and rutile. The trace-element signatures in core domains reflect modification of igneous protolith zircon. Rim signatures show flat heavy rare earth element (HREE) patterns that are characteristic of eclogite-facies zircon. The kyanite eclogites generally lack a Eu anomaly, whereas a negative Eu anomaly persists in all domains of the host gneiss. The 207Pb- corrected 206Pb/238U ages range from 330 to 390 Ma for the host gneiss and 330-370 Ma for the kyanite eclogite. Weighted mean 206Pb/238U ages for coesite-bearing domains vary from 364 ?? 8 Ma for the host gneiss to 350 ?? 4 Ma for kyanite eclogite. The combined U-Pb and REE data interpreted in conjunction with observed CL domains and inclusion suites suggest that (1) Caledonian metamorphic zircon formed by both new zircon growth and recrystallization, (2) UHP metamorphism occurred near the end of the Caledonian collision, and (3) the 30-50m.y. span of ages records long residence times at eclogite-facies conditions for the UHProcks of North-East Greenland. This spread in observed ages is interpreted to be characteristic of metamorphic rocks that have experienced relatively long (longer than 10 m.y.) residence times at UHP conditions. ?? 2006 Geological Society of America.

  9. Geochronology and geochemistry of Cretaceous magmatic rocks of Arctic Chukotka: An update of GEOCHRON2.0

    NASA Astrophysics Data System (ADS)

    Akinin, V. V.; Miller, E. L.; Gottlieb, E.; Polzunenkov, G.

    2012-04-01

    Field work near and along the Arctic cost of Chukotka (Pevek to Cape Schmidt) contributes new data on the geology of this remote area which belongs to Arctic Alaska-Chukotka terrane or microcontinent which lies to the south of the vast and unexplored East Siberian Shelf, providing better constraints on basement ages and the magmatic and tectonic evolution of this part of the circum Arctic. U-Pb SHRIMP RG zircon ages from eight largest granitoid plutonic complexes intruded across this region are: Velitkinay (105-100 Ma), Kuvet (104±1 Ma), Pegtymel (108±2 Ma), Lootaypin (107±1 Ma), Inroginay (109-104 Ma), Pevek (108-105 Ma), Severny (88±1 Ma), Pyrkanay (92±1 Ma). Two last plutonic complexes are coeval with calc-alkaline volcanic rocks of the suprasubduction Okhotsk-Chukotka volcanic belt (Arctic Chukotka segment). Earlier plutons have extension-related geochemical signatures (monzonite trend and medium negative Nb-Ta anomalies) and Nd model ages of 1.0-1.8 Ga. The Velitkinay migmatite-granite complex, south of Cape Billings extends 150 km in a NW-SE direction. Along the southwest flank of the Velitkinay complex, poorly fossiliferous, metamorphosed Devonian (?), Carboniferous, Permian and Triassic strata are involved in regional W-NW-E-SE trending folds with steep axial planes. Country rocks to the plutonic complex dip steeply to gently SW and are intruded by variably deformed K-spar megacrystic biotite granites (102-106 Ma) in turn intruded by variably deformed medium- grained sphene and biotite bearing granites (~100 Ma, with zircon-core inheritance of 600-630 Ma) related to the migmatitic core of the complex. Precise U-Pb and Ar-Ar dates such as those above have been collected across North East Russia in the last decade and allow more modern regional synthesis of the age of main magmatic events in order to correlate them with the evolution of the Arctic Ocean basins. Our updated GEOCHRON data base documents important Jura-Cretaceous magmatic provinces and events: (1) 160-145 Ma granitoids of Kolyma batholith belt and coeval Uyandino-Yasachnaya volcanic arc (partly coeval with closure of the Anyui Ocean around c. 160-145 Ma); (2) 142-145 Ma syenites of Egdegkych alkaline complex, and volcanics of subduction related Nutesyn margin continental arc in the South Anyui suture zone; (3) 130-135 Ma Northern belt granites and oldest granitic complexes in Eastern Chukotka (coeval with beginning of HALIP volcanism); (4) 118-122 Ma Tytylveem continental volcano-plutonic belt in Chukotka (5) 100-109 Ma extension-related granite-metamorphic core complexes along Arctic cost of Chukotka (coeval with WPB alkaline basalts of De-Longy archipelago) (6) 90-88 Ma subduction-related Central Chukotka segment of Okhotsk-Chukotka volcanic belt (coeval with opening of the Labrador Sea and Baffin Bay between 90 and 55 Ma, which may have affected the Central Arctic region); (7) 54 to to 37 Ma alkali basalts in the Chersky seismic belt triggered by extension and thinning of the lithosphere combined with adiabatic upwelling of the underlying mantle (coeval with opening of the Eurasia oceanic basin at 55-33 Ma).

  10. Geochronology and geochemistry constraints of the Early Cretaceous Taibudai porphyry Cu deposit, northeast China, and its tectonic significance

    NASA Astrophysics Data System (ADS)

    Zhou, Zhen-Hua; Mao, Jing-Wen; Wu, Xin-Li; Ouyang, Hen-Gen

    2015-05-01

    The southern Great Xing'an Range (SGXR), located in the southeastern part of Inner Mongolia, China, shows intense Mesozoic tectono-magmatic activity and hosts economically important polymetallic (Cu-Pb-Zn-Sn-Fe-Ag-Au-Mo) mineralization. Here, we present new zircon U-Pb ages, whole-rock geochemical data, Nd-Sr-Hf isotopic data and Re-Os ages for the Taibudai deposit in the SGXR. The Taibudai granitoids show high SiO2 (70.62-72.13 wt.%) and alkali (Na2O + K2O = 7.04-8.60 wt.%) concentrations, low MgO (0.89-1.37 wt.%) and Al2O3 (∼14 wt.%), ASI ratios <1.1 (0.94-0.97), LILEs (e.g., Rb) enriched, HFSEs (e.g., Nb, Ta, Ti, and P) depleted, and have low Sr and Yb concentrations, classifying these rocks as fractionated I-type granites. The Taibudai granitoids have negative εNd (t) values ranging from -2.2 to -1.6 and relatively low initial 87Sr/86Sr ratios from 0.70536 to 0.70581. In situ Hf isotopic analyses on zircons using LA-MC-ICP-MS show variable positive εHf (t) values ranging from +0.80 to +13.55, corresponding to relatively young two-stage Hf model ages from 801 to 942 Ma (excluding one spot). These mineralogical, geochemical, and isotopic features strongly suggest that the primary magmas of the Taibudai granitoids were derived mainly from the partial remelting of Neoproterozoic juvenile crustal material, with no remarkable modification through incorporation of continental or subduction-related material. Re-Os isotope analyses of molybdenite from the deposit yield an ore-forming age of 137.1 ± 1.4 Ma. Re contents range from 4.37 to 41.77 ppm, implying ore material components have a mixed crust-mantle origin. SHRIMP analysis of zircons show that the monzogranitic porphyry and biotite granite in the Taibudai deposit were formed at 137.0 ± 0.9 Ma and 138.3 ± 0.9 Ma, respectively, indicating a temporal link between granitic magmatism and Cu mineralization. This result, combined with the regional geology, tectonic evolution, and age data from the literature, suggests that the Early Cretaceous (∼140 Ma) was the peak metallogenic epoch for the Great Xing'an Range, and the mineralization in this period generally takes the form of porphyry, skarn, or hydrothermal polymetallic ore deposits in an active extensional continental margin environment. The Taibudai porphyry and associated mineralization provides a typical example of magmatism and metallogeny associated with a Paleo-Pacific plate subduction, continental margin, back-arc extensional setting.