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1

Petrological and geochemical characterization of Proterozoic ophiolitic mélange, Nellore-Khammam schist belt, SE India  

NASA Astrophysics Data System (ADS)

The ophiolitic mélange occurs close to a major fault within the metavolcanics of Kanigiri area in the western margin of the Khammam-Nellore schist belt, Andhra Pradesh, India. The unique feature of Kanigiri mélange is the occurrence of ophiolitic bodies of various sizes, most of which display petrological and geochemical characteristics typical of supra-subduction zone oceanic crust. The ophiolitic mélange is composed mainly of blocks of variably deformed and metamorphosed pillow basalts, serpentinized ultramafic rocks, meta-gabbros, cherts and pelagic sediments in a pervasive scaly argillite matrix. The mélange has a tectonic contact with a granite intrusion and numerous thrust faults cut across the mélange exposures. Primary magmatic features such as drainage cavities in pillow basalts, magmatic layering in gabbros, and clinopyroxene cumulates in ultramafic rocks are well preserved in low-strain domains. Mafic components enclosed in the mélange are characterized by low pressure secondary mineral assemblages retrograded from amphibolite down to greenschist facies as commonly observed in subducted and exhumed orogens. All amphiboles in the studied fragments are calcic and cluster in the range from winchite, actinolite to hornblende. Amphibole compositions are characterized by Al 2O 3 (2.4-13.8%), FeO (5-13%), K 2O (0.04-0.7%) and X Mg (0.67-0.77). The metabasalt and wherlite samples exhibit light rare earth element (LREE) - enriched patterns and negative high field strength element (HFSE: Zr and Ti) consistent with a suprasubduction zone geodynamic setting. A notable feature of both metabasalt and wherlite samples is their pronounced negative Ce anomaly and a positive Eu anomaly as observed in a number of modern island arc suites. As the petrological features and the geochemical characteristics of the remnant fragments we analyzed in this study are reminiscent of their Phanerozoic ophiolite counterparts, we suggest that these rocks were derived from a Proterozoic oceanic plate belonging to a suprasubduction ophiolite assemblage. Subduction zone geochemical signatures and ophiolite-like crustal lithologies in the study region are consistent with the accretion of juvenile oceanic fragments to the pre-existing continental blocks through a forearc-continent collision.

Dharma Rao, C. V.; Reddy, U. V. B.

2009-10-01

2

Relict blue-schist in Himalayan Ophiolitic Melange: does it emplaced through subduction channel?  

NASA Astrophysics Data System (ADS)

Among the planets, the Earth face retains signature of emplacement mechanisms which can be viewed through plate tectonics to reveal the paleo-earth history. And this fact has been manifested through the occurrence of high-pressure, low temperature rock of Zildat Ophiolitic Melange in North West Himalaya (India). The Ophiolitic Melange units form thin (~100s metres) layer of thrust sheets along suture zone, sandwiched between the upper mantle dunite- peridotites rock of Nidar ophiolite and ultrahigh-pressure gneissic dome Tso Morari Crystalline (TMC). Our data shows that the mélange matrix represents overprinting of various metamorphisms from granulite to green-schist facies, relics of Na- amphibole point to the occurrence of an older blueschist facies, oceanic basalt clast, pelagic sediment, chert, sheared carbonates, hydrated peridotite(?) , serpentinite melange. Detail of mineral fabric study and chemical analysis suggest the occurrence of heterogeneous metamorphism in Zildat ophiolitic melange likely to be decoupled with tectonic evolution. It has been interpreted that the melange is thought to preserve shearing related to the emplacement mechanism. And the final emplacement path of melange material from different depths of eclogite-blueschist transition possibly returned through fossil subduction channel which is assisted by serpentinised matrix. The serpentine and melange material is now juxtaposed with continental TMC gneissic dome. To validate the final emplacement history combined isotopic (?18O) data has been deployed; results fluid mobility exchange between continental TMC gneiss and Zildat ophiolitic melange is synchronous.

Sen, K.; Mukherjee, B. K.

2012-12-01

3

Petrology and geochemistry of paleoproterozoic A-type granite at Kanigiri in the Nellore Khammam schist belt, Andhra Pradesh, India  

NASA Astrophysics Data System (ADS)

The Kanigiri Pluton (6.2 × 2.1 km 2; 995 Ma) occurs close to a major fault and is intrusive into the meta-volcanics in the western margin of the Nellore-Khammam schist belt in Andhra Pradesh, India. The dominantly metaluminous pluton displays geochemical characteristics of A-type granites, including high abundances of Nb, Zr, Y, Ta, and REE (except Eu), and low concentrations of Sc, Ba, Sr, and Eu, displaying distinct variation trends with increasing silica content. It has less fractionated REE patterns and large negative Eu anomalies. The cluster analyses of major and trace elements indicate three main associations: (a) A primary magmatic association characterized by the presence of La, Ce, Fe, Ti, Mn, K, Ba, and Sr, (b) Ore related association consisting of Zr, Hf, Nb, Y, and (c) a volatile association with Rb, Ta, Li, and Cs. Systematic variation in Sr, Rb/Sr, Rb/Ba ratios along with and low K/Rb ratios, reflects fractionation of feldspars. Variations in Sc, and Ta result from fractionation of ferromagnesian silicates. Removal of zircon and allanite affected the Hf/Ta and Ce/Yb ratios. Limited variation in source-sensitive Y/Nb and Yb/Ta ratios is consistent with the results of melting experiments and indicates that metaluminous granitoids of this pluton were probably derived through melting of lower crustal sources. Geochemical, field and petrological data for the Kanigiri granite suggest that the granite is a partial melting product of mafic rocks like gabbros that are also present in the study region.

Dharma Rao, C. V.; Reddy, U. V. B.

2007-04-01

4

Geochemistry and tectonic significance of the Koçali ophiolite and the related Koçali melange, Adiyaman region, SE Turkey  

NASA Astrophysics Data System (ADS)

The Koçali complex includes the Koçali Ophiolite and the volcanic-sedimentary Koçali melange. The ophiolitic rocks form E-W trending thrust sheets in which all of the components of a complete ophiolite sequence are present, although mainly separated by tectonic contacts; i.e. (from bottom to top), serpentinised harzburgite, layered cumulates, isotropic gabbros, sheeted dykes, pillow lavas, radiolarian and metalliferous sediments. Microgabbro-diabase dykes intrude various levels of the ophiolite pseudostratigraphy. The sheeted dykes and pillow lavas locally exhibit Cyprus-type hydrothermal mineralisation. The Koçali melange is a tectonic slice complex that structurally underlies the ophiolite on a regional scale, although ophiolite and melange are structurally intercalated in places related to emplacement onto the Arabian continental margin. The geochemistry of the crustal rocks of the Koçali ophiolite and the Koçali melange, taken together, shows that are all tholeiitic in composition except for the volcanic rocks which show both tholeiitic and alkaline features. New geochemical data from the ophiolitic isotropic gabbros, sheeted dikes, isolated dikes and volcanics indicate that there are three main types of parental basic magmas: (i) IAT series, comprising isotropic gabbros, sheeted dikes and the Group I isolated dikes; (ii) E-MORB series, characterized by the Group I volcanics and Group II isolated dikes; (iii) OIB series represented by the Group II volcanic rocks. The presence of highly magnesian olivines (Fo84-74), clinopyroxenes (Mg#92-65), orthopyroxenes (Mg#86-75), together with Ca-rich plagioclases (An95-86) in the cumulate rocks suggests that the plutonic suite was derived from an island arc tholeiitic (IAT) source rather than a mid-ocean ridge-type magmatic source. The Koçali Complex is interpreted to have formed during opening of the southern Neotethys, whereas the the Koçali ophiolite formed above a north-dipping intraoceanic subduction zone during ocean basin closure. Taking account of evidence from comparable ophiolites and melanges in Baer-Bassit (N-Syria) and the Mamonia Complex (SW Cyprus), the southern Neotethyan oceanic basin is interpreted to have rifted during the Late Triassic. The Koçali melange restores as the north-facing passive margin of the Arabian plate. The South Neotethyan ocean oceanic floor was covered by Jurassic-Lower Cretaceous continental margin-derived and pelagic/hemipelagic deep-sea sediment. Northward subduction began during the Late Cretaceous and the ophiolite formed above a northward-dipping subduction zone. The subduction trench collided with the Arabian margin during latest Cretaceous, driving southward emplacement of the Koçali, Hatay and Baer-Bassit ophiolites, finally during Maastrichtian time.

Yildirim, N.; Parlak, O.; Robertson, A.

2012-04-01

5

Dynamic melting of the Precambrian mantle: evidence from rare earth elements of the amphibolites from the Nellore-Khammam Schist Belt, South India  

NASA Astrophysics Data System (ADS)

The Nellore-Khammam Schist Belt (NKSB) in South India is a Precambrian greenstone belt sited between the Eastern Ghats Mobile Belt (EGMB) to the east and the Cratonic region to the west. The belt contains amphibolites, granite gneisses and metasediments including banded iron formations. Amphibolites occurring as dykes, sills and lenses—in and around an Archaean layered complex—form the focus of the present study. The amphibolites are tholeiitic in composition and are compositionally similar to Fe-rich mafic rocks of greenstone belts elsewhere. The NKSB tholeiites show highly variable incompatible trace element abundances for similar Mg#s, relatively constant compatible element concentrations, and uniform incompatible element ratios. Chondrite-normalized REE patterns of the tholeiites range from strongly LREE depleted ((La/Yb) N = 0.19) to LREE enriched ((La/Yb) N = 6.95). Constant (La/Ce) N ratios but variable (La/Yb) N values are characteristic geochemical traits of the tholeiites; the latter has resulted in crossing REE patterns especially at the HREE segment. Even for the most LREE depleted samples, the (La/Ce) N ratios are > 1 and are similar to those of the LREE enriched samples. There is a systematic decrease in FeOt, K2O and P2O5, as well as Ce and other incompatible elements from the LREE enriched to the depleted samples without any variation in the incompatible element ratios and Mg#s. Neither batch and fractional melting, nor magma chamber processes can account for the non-correlation between the LREE enrichment and HREE concentrations. We suggest that dynamic melting of the upper mantle is responsible for these geochemical peculiarities of the NKSB tholeiites. Polybaric dynamic melting within a single mantle column with variable mineralogy is the likely mechanism for the derivation of NKSB tholeiitic melts. It is possible that the NKSB tholeiites are derived from a source with higher FeO/MgO than that of present day ridge basalts.

Vijaya Kumar, K.; Narsimha Reddy, M.; Leelanandam, C.

2006-08-01

6

Petrology and geochemistry of late Cretaceous lamprophyric rocks from North Anatolian Ophiolitic Melange-Turkey  

NASA Astrophysics Data System (ADS)

The late Cretaceous lamprophyric rocks from Amasya and Kalecik region occur as dykes, stocks and lava flows intruding volcanoclastic sequence of North Anatolian Ophiolitic Melange. Their major and trace element compositions are very similar and it is not possible to discriminate them as two subgroups according to their geochemical features. The Al2O3 contents are variable, but relatively high (10.66-18.77 wt.%) typical for the ultrapotassic rocks from active orogenic belts. K2O contents (wt.% 1.22-8.39) are variable and Mg numbers (41-60) indicate that they were crystallized from evolved melts. LILE enrichments relative to HFSE, depletions of Nb-Ta and Ti elements are the characteristic features on N-MORB-normalized spider diagrams and this pattern represent the addition of subducted sediment/melt to the source area . The main mineralogic composition of samples from each two regions is clinopyroxene + mica ± feldspar ± olivine ± amphibole ± leucite + opaque minerals. The significant difference is the presence of plagioclase (An47-65) and K-feldspar as matrix in Amasya samples. Also some of the samples from Kalecik contain minor leucite. Based on the mineral paragenesis, Amasya lampropyres are classified as minette-vogesite and Kalecik samples are classified as kersantite-vogesite. Clinopyroxenes are mainly diopsite, salite and fassaitic in composition (Wo 45-50En 26-43 Fs 10-16) for the Kalecik region and displays diopsitic-salitic composition (Wo 44-48En 38-47 Fs 6-16) in Amasya region. The pressure-temperature calculations reveal significant differences for the lamprophyres from Amasya and Kalecik regions. The pressure conditions of the clinopyroxene crystallization for Amasya samples are between 16-24 kbar corresponds to 48-72 Km depth while the crystallization depth of the clinopyroxene from Kalecik lamprophyres is restricted between 12-36 Km. Although ultrapotassic rocks in Turkéy are the products of extension related volcanism in a post-collisional setting, Cretaceous lamprophyric rocks from Amasya and Kalecik regions formed during the closure of Neo-Tethys that means they are either subduction related or generated during incipient phases of postcollisional relaxation.

Gülmez, Fatma; Genç, Can; Prelevic, Dejan

2014-05-01

7

Strongly foliated garnetiferous amphibolite clasts in ophiolitic melanges, Yarlung Zangbo Suture Zone, Tibet; Early Cretaceous disruption of a back-arc basin?  

NASA Astrophysics Data System (ADS)

Metre to decameter-size clasts of amphibolite are found embedded in ophiolitic melanges underlying the Yarlung Zangbo Suture Zone Ophiolites, South Tibet, China. These ophiolites and melanges occur at the limit between Indian and Tibetan-derived rocks and represent remnants of an Early Cretaceous intraoceanic supra-subduction zone domain, the Neo-Tethys. In the Saga-Dazuka segment (500 km along-strike), we discovered new occurrences of strongly foliated amphibolites found as clasts in the ophiolitic melange. In garnet-free samples, hornblende is green-blue magnesio-hornblende and cpx is low-Al diopside. In garnet- bearing samples, garnet is almandine with a strong pyrope component (up to 30 mol%) whereas coexisting hornblende is brown Ti-rich tschermakite and clinopyroxene is Al-diopside. Plagioclase composition was ubiquitously shifted to albite during a late metasomatic event. Geochemistry of these rocks indicates that their igneous protoliths crystallized from a slightly differentiated tholeiitic basaltic liquid that did not undergo major fractionation. Trace element patterns reveal geochemical characteristics identical to those of the overlying ophiolitic crust. These are 1) trace element abundances similar to that of N-MORBs or BABBs, 2) a slight depletion of LREE and 3) a moderate to strong Ta-Nb negative anomaly and a slight Ti anomaly. Such characteristics suggest genesis over a spreading center close to a subduction zone, possibly a back-arc basin. Step-heating Ar/Ar plateau ages were obtained from hornblende separates. All ages fall in the range of 123-128 Ma, overlapping the crystallization ages from the overlying ophiolite (126-131 Ma). Pseudosections were built with the THERMOCALC software in the system NCFMASH. Results indicate that the observed assemblage Hb+Pl+Gt+Cpx is stable over a wide range of P-T conditions, between 10-18 kbars and at more than 800°C. Measured mineral modes and solid solution compositions were successfully modeled, indicating equilibrium between 11-13 kbars and 825-850°C, corresponding to high-P granulite facies conditions. In a general way, the geochemistry of the strongly foliated amphibolite clasts suggests that their igneous protolith probably crystallized within the same supra-subduction zone as the crustal rocks from the overlying ophiolite. Then some of these rocks were entrained to mantle depth and were rapidly exhumed, most likely along a lithospheric scale thrust fault underneath the ophiolite. This event corresponds with the end of magmatic activity within the ophiolitic crust and mantle and could be regarded as the inception of a subduction plane at the spreading ridge of a back-arc basin. The whole package was later on obducted over the Indian passive margin, at about 70 Ma. Such a model suggests that closure of the oceanic domain separating India from Eurasia implied disruption of at least one arc-back-arc system, thus requiring at least one early intraoceanic collision or major plate movement reorganization prior to the Late Cretaceous obduction.

Guilmette, C.; Hebert, R.; Wang, C.; Indares, A. D.; Ullrich, T. D.; Dostal, J.; Bedard, E.

2007-12-01

8

Oceanization of the northern Neotethys: Geochemical evidence from ophiolitic melange basalts within the ?zmir-Ankara suture belt, NW Turkey  

NASA Astrophysics Data System (ADS)

The remnants of the Neotethyan Izmir-Ankara Ocean, the main branch of Neotethys in the eastern Mediterranean are represented by the Dagküplü Melange Complex in Central Sakarya, NW Turkey. It comprises several blocks or tectonic slices of pillow lavas, some of which include mudstones and radiolarian cherts as intra-pillow-fillings or interlayers. In the Igdecik area, a huge basaltic block has been studied in detail. Geochemical data reveal three distinct basalt types separated by sheared contacts. The first of these groups is an enriched mid-oceanic ridge basalt (E-MORB) type which is enriched in the most incompatible trace elements relative to normal MORB (N-NORB) in addition to having heavy rare earth elements (HREE) depletion, suggesting the influence of residual garnet in their mantle source region. The second is back-arc basin basalt (BABB) type with relatively depleted trace element compositions with respect to N-MORB together with a negative Nb anomaly, suggesting generation above an intra-oceanic subduction zone where partial melts are derived from a depleted (MORB-like) mantle. The final group is island-arc tholeiite (IAT) type, displaying the most depleted trace element abundances among the studied groups in addition to marked Nb depletion, reflecting intra-oceanic supra-subduction zone (SSZ) signatures similar to the BABB-type but requiring a depleted mantle source which has experienced a previous melt extraction. Combined with a previously ascribed Late Triassic age of Tekin et al. (2002) (221 Ma, Late Carnian; based on the radiolarian fauna found in a chert layer alternating with mudstones), the associated basalts with E-MORB-type geochemical signatures, suggest formation of oceanic crust as early as Late Carnian. This age is the oldest thus far obtained from the basalts of the Izmir-Ankara Ocean. This new data provides constraints on tectonic models for the opening the Izmir-Ankara Ocean and its relationship to other branches of the Neotethyan ocean in the Eastern Mediterranean area.

Göncüoglu, M. Cemal; Sayit, Kaan; Tekin, U. Kagan

2010-04-01

9

Petrology and Tectonic Environment of the Dehshir Ophiolite, Central Iran  

NASA Astrophysics Data System (ADS)

The Dehshir ophiolite as a tectonic slice of the Nain-Baft ophiolitic melange is distributed around the Nain-Dehshir ophiolite at the northwestern part of the Lut block. This ophiolitic massif is composed of several tectonic slices, consisting of mantle tectonites, pegmatite gabbros, isotropic gabbros, basaltic flows, pillow lavas, metamorphic rocks and upper Cretaceous sediments. Petrographically the mafic rocks are characterized by crystallization of plagioclase and then clinopyroxene typical of MORB materials. Amphibole in these rocks is pale-green fibers of actinolites as a secondary, last-stage crystallized phase. Geochemically the some of mafic rocks are represented by enrichment in heavily mobile large ion lithophile elements and depletion in least mobile high field strength elements. On the other hand some rocks are geochemically similar to MORB materials. These geochemical properties are consistent with derivation of these rocks from an arc-related environment. We can suggest an intra-oceanic arc environment between the Sanandaj-Sirjan zone and the Lut block for the genesis and evolution of the Dehshir ophiolite during lower to upper Cretaceous. This arc-related setting can explain the occurrence of mafic rocks with depletion in HFSE and/or MORB-like basaltic rocks. The presence of Upper Cretaceous limy sediments is evidence suggesting the initiation of intra-oceanic subduction in the Dehshir basin at least during middle Cretaceous. Keywords: Dehshir ophiolite; Nain-Baft ophiolitic melange; Lut block; intra-oceanic arc; Iran

Moharami Gargari, F.; Monsef, I.; Meshkin, M.

2009-04-01

10

Tectonic setting for ophiolite obduction in Oman.  

USGS Publications Warehouse

The Samail ophiolite is part of an elongate belt in the Middle East that forms an integral part of the Alpine mountain chains that make up the N boundary of the Arabian-African plate. The Samail ophiolite represents a portion of the Tethyan ocean crust formed at a spreading center of Middle Cretaceous age (Cenomanian). During the Cretaceous spreading of the Tethyan Sea, Gondwana Land continued its dispersal, and the Arabian-African plate drifted northward about 10o. These events, combined with the opposite rotation of Eurasia and Africa, initiated the closing of the Tethyan during the Late Cretaceous. At the early stages of closure, downwarping of the Arabian continental margin, combined with the compressional forces of closure from the Eurasian plate, initiated obduction of the Tethyan oceanic crust along preexisting transform faults and still-hot oceanic crust was detached along oblique NE dipping thrust faults. Plate configurations combined with palinspastic reconstructions show that subduction and attendant large-scale island arc volcanism did not commence until after the Tethyan sea began to close and the Samail ophiolite was emplaced southward across the Arabian continental margin. The Samail ophiolite nappe now rests upon a melange consisting mainly of pelagic sediments, volcanics and detached fragments of the basal amphibolites, which in turn rest on autochthonous shelf carbonates of the Arabian platform. Following emplacement (Eocene) of the Samail ophiolite, the Tethyan oceanic crust began northward subduction, and active arc volcanism started just N of the present Jaz Murian depression in Iran.-Author

Coleman, R. G.

1981-01-01

11

Geochemistry and tectonomagmatic affinity of the Yungbwa ophiolite, SW Tibet  

Microsoft Academic Search

The Yungbwa ophiolite is a thrust sheet of about 800 km2 tectonically overlying an Upper Cretaceous melange south of the Indus–Tsangpo Suture Zone in SW Tibet. Both units have been thrust over the sedimentary series of the Tibetan Tethys zone in the course of the India–Eurasia collision. Harzburgite and clinopyroxene-poor lherzolite are the dominant lithologies. Occasional gabbronoritic and basaltic dikes

Christine Miller; Martin Thöni; Wolfgang Frank; Ralf Schuster; Frank Melcher; Thomas Meisel; Alberto Zanetti

2003-01-01

12

High-pressure rocks in the serpentinite melange of the Chara zone, Eastern Kazakhstan: Geochemistry, petrology, and age  

Microsoft Academic Search

The nature of the protolith of high-pressure rocks occurring as exotic blocks in serpentinite melange in the Chara ophiolite\\u000a belt, Eastern Kazakhstan, is identified based on petrological and geochemical evidence. The distribution of incompatible HFSE\\u000a in these rocks suggests that their protoliths were similar to E- and N-MORB tholeiites and, more rarely, to OIB alkaline basalts.\\u000a This testifies to the

N. I. Volkova; E. N. Tarasova; N. V. Polyanskii; A. G. Vladimirov; V. D. Khomyakov

2008-01-01

13

Hurricane Mountain Formation melange: history of Cambro-Ordovician accretion of the Boundary Mountains terrane within the northern Appalachian orthotectonic zone  

SciTech Connect

The Hurricane Mountain Formation (HMF) melange and associated ophiolitic and volcanogenic formations of Cambrian and lowermost Ordovician age bound the SE margin of the Precambrian Y (Helikian) Chain Lakes Massif in western Maine. HMF melange matrix, though weakly metamorphosed, contains a wide variety of exotic greenschist to amphibolite facies blocks as components of its polymictic assemblage, but blocks of high-grade cratonal rocks such as those of Chain Lakes or Grenville affinity are lacking. Formations of melange exposed in structural culminations of Cambrian and Ordovician rocks NE of the HMF in Maine and in the Fournier Group in New Brunswick are lithologically similar and probably tectonically correlative with the HMF; taken together, they may delineate a common pre-Middle Ordovician tectonic boundary. The authors infer that the Hurricane Mountain and St. Daniel melange belts define the SE and NW margins of the Boundary Mountains accreted terrane (BMT), which may consist of cratonal basement of Chain Lakes affinity extending from eastern Gaspe (deBroucker and St. Julien, 1985) to north-central New Hampshire. The Laurentian continental margin, underlain by Grenville basement, underplated the NW margin of this terrane, marked by the SDF suture zone, in late Cambrian to early Ordovician time, while terranes marked by Cambrian to Tremadocian (.) lithologies dissimilar to the Boundary Mountains terrane were accreted to its outboard margin penecontemporaneously. The docking of the Boundary Mountains terrane and the initiation of its peripheral melanges are equated to the Penobscottian disturbance.

Boone, G.M.; Boudette, E.L.

1985-01-01

14

Tectonics of the ophiolite belt from Naga Hills and Andaman Islands, India  

NASA Astrophysics Data System (ADS)

The ophiolitic rocks of Naga Hills-Andaman belt occur as rootless slices, gently dipping over the Paleogene flyschoid sediments, the presence of blue-schists in ophiolite melange indicates an involvement of the subduction process. Subduction was initiated prior to mid-Eocene as proved by the contemporaneous lower age limit of ophiolite-derived cover sediment as against the accreted ophiolites and olistostromal trench sediment. During the late Oligocene terminal collision between the Indian and Sino-Burmese blocks, basement slivers from the Sino-Burmese block, accreted ophiolites and trench sediments from the subduction zone were thrust westward as nappe and emplaced over the down-going Indian plate. The geometry of the ophiolites and the presence of a narrow negative gravity anomaly flanking their map extent, run counter to the conventional view that the Naga-Andaman belt marks the location of the suture. The root-zone of the ophiolite nappe representing the suture is marked by a partially-exposed eastern ophiolite belt of the same age and gravity-high zone, passing through central Burma-Sumatra-Java. The ophiolites of the Andaman and Naga Hills are also conventionally linked with the subduction activity, west of Andaman islands. This activity began only in late Miocene, much later than onland emplacement of the ophiolites; it further developed west of the suture in its southern part. Post-collisional northward movement of the Indian plate subparallel to the suture, also developed leaky dextral transcurrent faults close to the suture and caused Neogene-Quatemary volcanism in central Burma and elsewhere.

Acharyya, S. K.; Ray, K. K.; Sengupta, S.

1990-06-01

15

A new genetic model for the East Taiwan Ophiolite and its implications for Dupal domains in the Northern Hemisphere  

Microsoft Academic Search

The Miocene East Taiwan Ophiolite (ETO) has been suggested to have been generated at an ``abnormal'' mid-ocean ridge along a transform fault in the South China Sea [1]. As a result of arc-continent collision, the ETO was fragmented and incorporated into the Pliocene Lichi Melange of the Coastal Range, eastern Taiwan. Based on the structure of the slow-spreading Mid-Atlantic Ridge

Sun-Lin Chung; Shen-Su Sun

1992-01-01

16

In-situ pressure - temperature condition of tectonic melange : constraints from fluid inclusion analysis of syn-melange veins-  

NASA Astrophysics Data System (ADS)

One of the most reliable settings for the tectonic melange with composite planar fabric is along the decollement in the subduction zone (e.g. Cowan, 1985; Moore and Byrne, 1987; Kimura and Mukai, 1991). The decollement is developed from aseismic shallow part to seismogenic zone of several to tens of kilometer in depth. The decollement-related-melange, now exposed on land, might therefore record the deformation process along the aseismic to seismogenic decollement. Trapped pressure and temperature of fluid inclusion is estimated from "syn-melange" veins developed around the necks of boudin of sandstone blocks in the melange of the Shimanto Belt, SW Japan. The melange was decollement-related tectonic melange. Fluid inclusion analysis from "syn-melange" veins might be only tool to reveal the P-T condition during melange formation. The result of P-T estimation ranges from about 150 (}25)° Cto 220 (}31)° C for temperature and from 81 (+15) MPa to 235 (_}18) MPa for pressure. Geothermal gradient estimated from the result is between about 10.0 (+0.2/-1.5)° C/km (lithostatic) and 4.2 (+0.1/-0.9)° C/km (hydrostatic) if the fluid pressure ratio to lithostatic pressure is constant in each datum. The estimated lithostatic thermal gradient is much closer to that calculated from the oldness of the subducting oceanic plate. P-T range suggests that melange was formed enough within the seismogenic zone hypothesized by thermal model although the deformation mechanisms of melange, dominant diffusive mass transfer with minor brittle breakage, do not show the seismic deformation. One possible explanation for this discrepancy is that the melange was formed during the interseismic period. Alternative explanation is that melange formation was accomplished before subducting into the seismogenic zone but around the seismic front. Lithification front hypothesis for the seismic front suggests such frontal setting for the melange.

Hashimoto, Y.; Enjoji, M.; Sakaguchi, A.; Kimura, G.

2001-12-01

17

Origin of tonalites from the Boil Mountain ophiolitic complex, west-central Maine  

SciTech Connect

The Boil Mountain ophiolitic complex, west-central Maine, marks the suture between the Boundary Mountain and Gander terranes that became amalgamated in the late Cambrian during a pre-Taconic collisional event known as the Penobscottian orogeny. This even formed a composite terrane that is believed to have collided with the proto-North American margin during the Ordovician Taconic orogeny. The ophiolite is unusual in that there is no associated tectonized ultramafic section; there is a lack of a sheeted dike sequence; and an intrusive tonalite layer comprises a significant volume of the complex. Preliminary major and trace element geochemical analysis of the tonalites indicates that this unit is derived by partial melting of the associated mafic volcanics. This melting may have been induced by stopping water-rich hydrothermally altered basalts into subcrustal magma chambers. The association of arc-like volcanics and plutonics of the Boil Mountain ophiolite with the adjacent Hurricane melange suggests that the Boil Mountain may be a fragment of a forearc supra-subduction zone complex. Similar interpretations have been made recently for the Late Proterozoic Bou Azzer ophiolite in Morocco and the Coast Range ophiolite.

Chow, J.S. (Boston Univ., MA (United States). Geology Dept.)

1993-03-01

18

Evolution of an ophiolitic tectonic melange, Marble Mountains, northern California Klamath Mountains ( USA).  

USGS Publications Warehouse

Describes multiply deformed amphibolite facies metamorphic rocks of the Marble Mountain and western Hayfork terranes in the western Paleozoic and Triassic belt of the northern Klamath Mountains, California.-from Author

Donato, M. M.

1987-01-01

19

Gravity profiles across the Samail Ophiolite, Oman  

Microsoft Academic Search

Four hundred and sixty gravity stations have been established over the Samail ophiolite and adjacent units exposed in the Oman Mountains. The Bouguer anomaly pattern follows more or less parallel to ophiolite exposures. Along the coastal areas the Bouguer anomaly values range from +20 to +140 mGal and increase northward. Four gravity profiles across the ophiolite are correlated with known

Murli H. Manghnani; Robert G. Coleman

1981-01-01

20

Gravity Profiles Across the Samail Ophiolite, Oman  

Microsoft Academic Search

Four hundred and sixty gravity stations have been established over the Samail ophiolite and adjacent units exposed in the Oman Mountains. The Bouguer anomaly pattern follows more or less parallel to ophiolite exposures. Along the coastal areas the Bouguer anomaly values range from +20 to + 140 mGal and increase northward. Four gravity profiles across the ophiolite are correlated with

Murli H. Manghnani; Robert G. Coleman

1981-01-01

21

The Naga Hills and Andaman ophiolite belt, their setting, nature and collisional emplacement history  

NASA Astrophysics Data System (ADS)

The Indo-Burmese Range and the Andaman-Nicobar Island Arc, form a continuous arcuate trend along which several ophiolite occurrences have been reported. In Naga Hills (NHO) and Andaman (ANO), these ophiolites are represented by dismembered mafic and ultramafic rocks with closely associated oceanic pelagic sediments. They occur as folded thrust slices occupying the highest tectonic levels and are brought to lie over distal shelf sediments of Eocene to Oligocene age. Ophiolites are unconformably overlain by ophiolite-derived clastics of Middle to Late Eocene age. The ophiolites preserved along this belt are remnants of a continuous, narrow, one or several intra-continental ocean basin(s) of broadly comparable age, created during the Late Mesozoic rifting of the Greater India Gondwana continent. Rifting and creation of oceanic crust date between Cretaceous and Early Eocene. In the initial stages, the ocean floor had been deeper than Carbonate Compensation Depth (CCD). Subsequently it had become uneven, when oceanic crust was being added through several seamounts or seamount chains and on top of which calcareous pelagic sediments were deposited. Both tholeiitic and alkaline volcanic rocks are present in these ophiolites. In NHO, the two groups of lavas have generated from different sources in different tectonic settings. The alkalic and some tholeiitic lavas in NHO are similar to off-axis seamount basalts. Tholeiitic lavas from ANO and some NHO resemble MORB or backarc basin basalts and on the basis of certain chemical characters these are suggested to have generated in marginal basin setting. Significant volume of acid differentiates are associated in ANO which also support the marginal basin character of the basalts. The suite of rocks in ANO indicates fractionation in a shallow level magma chamber. Closure of the small ocean basin(s) and emplacement of ophiolites took place in two stages. In the initial stage, the seamount chain brought to the subduction zone collided with the Burmese block prior to Middle Eocene. Part of the ophiolites represent clipped seamounts which got accreted to the leading edge of the eastern continental block. With continued closure, this eastern block with accreted ophiolite slices was brought in juxtaposition with distal shelf sediments of the western block marking the terminal continent-continent collision. The thrust front of ophiolitic rocks apparently advanced further westward in Andaman to the south compared to the northern sector, and thus an imbricated zone and melange involving the Eocene floor sediments (Lipa Fm) has been created, whereas in the Naga Hills the floor sediments (Disang Fm) remained virtually passive. The time of terminal continental collision is represented as the regional Late Oligocene unconformity. The entire thrust stack got deformed and folded into upright geometry after being blocked. The present subduction of oceanic crust beneath the Andaman island arc appears to be a westward jump of subduction zone due to sustained post-collisional NE drive of the Indian plate.

Acharyya, S. K.; Ray, K. K.; Sengupta, Subhasis

22

The mode and nature of faulting and deformation and the emplacement history of the Mineoka ophiolite, NW Pacific Rim: From mid-ocean ridge spreading to tectonic accretion on land  

NASA Astrophysics Data System (ADS)

The Boso Peninsula (Japan), currently located north of the oblique subduction boundary between the Philippine Sea and North American Plates, occurs in a unique area where the Pacific Plate is subducting westwards under the Philippine Sea Plate near the Boso TTT-triple junction. In the southern part of the peninsula, Tertiary chert and limestone and mafic-ultramafic rocks are distributed together with island arc volcaniclastics and continentally-derived detrital rocks, forming an ophiolitic melange (Mineoka belt). The Mineoka ophiolitic melange has developed at the boundary between three different realms of oceanic, continental and island arc settings. Although its stratigraphy and lithologic units are well studied, the structural relations of each rock body in the Mineoka belt and their tectonic implications are not well constrained. Our detailed structural studies show that the fault systems within the basaltic rocks record two episodes of deformation during the evolutionary history of the melange belt. The normal fault system characterized by calcite and zeolite (laumontite, analcime, natrolite, etc.) veining and cataclasis along extensional faults and shear zones of various types and directions marks the first-stage of deformation. These veins were developed either as hybrid extension/shear fractures or cataclastic shear fractures, suggesting high pore-fluid pressures associated with hydrothermal event(s). The second stage of deformation is characterized by thrust-fault related shear zones with a significant strike-slip component. These shear zones are parallel to the general structural trend of the Mineoka belt and are spatially associated with igneous rocks showing MORB, IAT, and/or BABB affinities. We infer that the first stage of deformation, as displayed by the mafic rocks in the melange, is related to extensional tectonics and associated normal faulting at a mid-ocean ridge, and the second stage of deformation related to the final emplacement of the fossil oceanic crust on land by oblique subduction-obduction processes. The mid-ocean ridge component (MORB rocks) of the Mineoka belt formed during the Eocene as part of an oceanic plate, which we interpret as the counterpart of the North New Guinea plate in the south. This oceanic plate (Mineoka Plate) subsequently moved northwards towards the NW Pacific Rim, and part of it was tectonically accreted into the corner of the TTT-type triple junction setting during the Miocene to form the Mineoka ophiolitic melange belt.

Takahashi, A.; Ogawa, Y.; Ogawa, Y.; Ohta, Y.; Hirano, N.

2001-12-01

23

Arc/Forearc Lengthening at Plate Triple Junctions and the Formation of Ophiolitic Soles  

NASA Astrophysics Data System (ADS)

The principal enigma of large obducted ophiolite slabs is that they clearly must have been generated by some form of organized sea-floor spreading/plate-accretion, such as may be envisioned for the oceanic ridges, yet the volcanics commonly have arc affinity (Miyashiro) with boninites (high-temperature/low-pressure, high Mg and Si andesites), which are suggestive of a forearc origin. PT conditions under which boninites and metamorphic soles form and observations of modern forearc systems lead us to the conclusion that ophiolite formation is associated with overidding plate spreading centers that intersect the trench to form ridge-trench-trench of ridge-trench-tranform triple junctions. The spreading centers extend and lengthen the forearc parallel to the trench and by definition are in supra-subduction zone (SSZ) settings. Many ophiolites likewise have complexly-deformed associated mafic-ultramafic assemblages that suggest fracture zone/transform t along their frontal edges, which in turn has led to models involving the nucleation of subduction zones on fracture zones or transpressional transforms. Hitherto, arc-related sea-floor-spreading has been considered to be either pre-arc (fore-arc boninites) or post-arc (classic Karig-style back arc basins that trench-parallell split arcs). Syn-arc boninites and forearc oceanic spreading centers that involve a stable ridge/trench/trench triple or a ridge-trench-transform triple junction, the ridge being between the two upper plates, are consistent with large slab ophiolite formation in a readied obduction settting. The direction of subduction must be oblique with a different sense in the two subduction zones and the oblique subduction cannot be partitioned into trench orthogonal and parallel strike-slip components. As the ridge spreads, new oceanic lithosphere is created within the forearc, the arc and fore-arc lengthen significantly, and a syn-arc ophiolite forearc complex is generated by this mechanism. The ophiolite ages along arc-strike; a distinctive diachronous MORB-like to boninitic to arc volcanic stratigraphy develops vertically in the forearc and eruption centers progressively migrate from the forearc back to the main arc massif with time. Dikes in the ophiolite are highly oblique to the trench (as are back-arc magnetic anomalies. Boninites and high-mg andesites are generated in the fore-arc under the aqueous, low pressure/high temperature, regime at the ridge above the instantaneously developed subducting and dehydrating slab. Subducted slab refrigeration of the hanging wall ensues and accretion of MORB metabasites to the hanging wall of the subduction channel initiates. Mafic protolith garnet/two pyroxene granulites to greenschists accrete and form the inverted P and T metamorphic sole prior to obduction. Sole accretion of lithosphere begins at about 1000°C and the full retrogressive sole may be fully formed within ten to fifteen million years of accretion, at which time low grade subduction melanges accrete. Obduction of the SSZ forearc ophiolite with its subjacent metamorphic sole occurs whenever the oceanic arc attempts subduction of a stable buoyant continental or back arc margin.

Casey, John; Dewey, John

2013-04-01

24

Orogenic, Ophiolitic, and Abyssal Peridotites  

NASA Astrophysics Data System (ADS)

"Tectonically emplaced" mantle rocks include subcontinental, suboceanic, and subarc mantle rocks that were tectonically exhumed from the upper mantle and occur:(i) as dispersed ultramafic bodies, a few meters to kilometers in size, in suture zones and mountain belts (i.e., the "alpine," or "orogenic" peridotite massifs - De Roever (1957), Thayer (1960), Den Tex (1969));(ii) as the lower ultramafic section of large (tens of kilometers) ophiolite or island arc complexes, obducted on continental margins (e.g., the Oman Ophiolite and the Kohistan Arc Complex - Coleman (1971), Boudier and Coleman (1981), Burg et al. (1998));(iii) exhumed above the sea level in ocean basins (e.g., Zabargad Island in the Red Sea, St. Paul's islets in the Atlantic and Macquarie Island in the southwestern Pacific - Tilley (1947), Melson et al. (1967), Varne and Rubenach (1972), Bonatti et al. (1981)).The "abyssal peridotites" are samples from the oceanic mantle that were dredged on the ocean floor, or recovered from drill cores (e.g., Bonatti et al., 1974; Prinz et al., 1976; Hamlyn and Bonatti, 1980).Altogether, tectonically emplaced and abyssal mantle rocks provide insights into upper mantle compositions and processes that are complementary to the information conveyed by mantle xenoliths (See Chapter 2.05). They provide coverage to vast regions of the Earth's upper mantle that are sparsely sampled by mantle xenoliths, particularly in the ocean basins and beneath passive continental margins, back-arc basins, and oceanic island arcs.Compared with mantle xenoliths, a disadvantage of some tectonically emplaced mantle rocks for representing mantle compositions is that their original geodynamic setting is not exactly known and their significance is sometimes a subject of speculation. For instance, the provenance of orogenic lherzolite massifs (subcontinental lithosphere versus upwelling asthenosphere) is still debated (Menzies and Dupuy, 1991, and references herein), as is the original setting of ophiolites (mid-ocean ridges versus supra-subduction settings - e.g., Nicolas, 1989). In addition, the mantle structures and mineralogical compositions of tectonically emplaced mantle rocks may be obscured by deformation and metamorphic recrystallization during shallow upwelling, exhumation, and tectonic emplacement. Metamorphic processes range from high-temperature recrystallization in the stability field of plagioclase peridotites ( Rampone et al., 1993) to complete serpentinization (e.g., Burkhard and O'Neill, 1988). Some garnet peridotites record even more complex evolutions. They were first buried to, at least, the stability field of garnet peridotites, and, in some cases to greater than 150 km depths ( Dobrzhinetskaya et al., 1996; Green et al., 1997; Liou, 1999). Then, they were exhumed to the surface, dragged by buoyant crustal rocks ( Brueckner and Medaris, 2000).Alternatively, several peridotite massifs are sufficiently well preserved to allow the observation of structural relationships between mantle lithologies that are larger than the sampling scale of mantle xenoliths. It is possible in these massifs to evaluate the scale of mantle heterogeneities and the relative timing of mantle processes such as vein injection, melt-rock reaction, deformation, etc… Detailed studies of orogenic and ophiolitic peridotites on centimeter- to kilometer-scale provide invaluable insights into melt transfer mechanisms, such as melt flow in lithospheric vein conduits and wall-rock reactions (Bodinier et al., 1990), melt extraction from mantle sources via channeled porous flow ( Kelemen et al., 1995) or propagation of kilometer-scale melting fronts associated with thermalerosion of lithospheric mantle ( Lenoir et al., 2001). In contrast, mantle xenoliths may be used to infer either much smaller- or much larger-scale mantle heterogeneities, such as micro-inclusions in minerals ( Schiano and Clocchiatti, 1994) or lateral variations between lithospheric provinces ( O'Reilly et al., 2001).The abyssal peridotites are generally stro

Bodinier, J.-L.; Godard, M.

2003-12-01

25

Controls on accretion of flysch and me??lange belts at convergent margins: Evidence from the Chugach Bay thrust and Iceworm me??lange, Chugach accretionary wedge, Alaska  

USGS Publications Warehouse

Controls on accretion of flysch and me??lange terranes at convergent margins are poorly understood. Southern Alaska's Chugach terrane forms the outboard accretionary margin of the Wrangellia composite terrane, and consists of two major lithotectonic units, including Triassic-Cretaceous me??lange of the McHugh Complex and Late Cretaceous flysch of the Valdez Group. The contact between the McHugh Complex and the Valdez Group on the Kenai Peninsula is a tectonic boundary between chaotically deformed me??lange of argillite, chert, greenstone, and graywacke of the McHugh Complex and a less chaotically deformed me??lange of argillite and graywacke of the Valdez Group. We assign the latter to a new, informal unit of formational rank, the Iceworm me??lange, and interpret it as a contractional fault zone (Chugach Bay thrust) along which the Valdez Group was emplaced beneath the McHugh Complex. The McHugh Complex had already been deformed and metamorphosed to prehnite-pumpellyite facies prior to formation of the Iceworm me??lange. The Chugach Bay thrust formed between 75 and 55 Ma, as shown by Campanian-Maastrichtian depositional ages of the Valdez Group, and fault-related fabrics in the Iceworm me??lange that are cut by Paleocene dikes. Motion along the Chugach Bay thrust thus followed Middle to Late Cretaceous collision (circa 90-100 Ma) of the Wrangellia composite terrane with North America. Collision related uplift and erosion of mountains in British Columbia formed a submarine fan on the Farallon plate, and we suggest that attempted subduction of this fan dramatically changed the subduction/accretion style within the Chugach accretionary wedge. We propose a model in which subduction of thinly sedimented plates concentrates shear strains in a narrow zone, generating me??langes like the McHugh in accretionary complexes. Subduction of thickly sedimented plates allows wider distribution of shear strains to accommodate plate convergence, generating a more coherent accretionary style including the fold-thrust structures that dominate the outcrop pattern in the Valdez belt. Rapid underplating and frontal accretion of the Valdez Group caused a critical taper adjustment of the accretionary wedge, including exhumation of the metamorphosed McHugh Complex, and its emplacement over the Valdez Group. The Iceworm me??lange formed in a zone of focused fluid flow at the boundary between the McHugh Complex and Valdez Group during this critical taper adjustment of the wedge to these changing boundary conditions.

Kusky, T. M.; Bradley, D. C.; Haeussler, P. J.; Karl, S.

1997-01-01

26

Nature of the Akamas ophiolite, W Cyprus  

NASA Astrophysics Data System (ADS)

Cyprus has been an ideal place to study the evolution of oceanic lithosphere. The complete preservation of the ophiolite in the Troodos Complex has allowed better understanding on the ocean closure process and the emplacement of oceanic lithospheric fragments on land. These previous studies have put the development of plate tectonic theory a great leap forward. Compared to the famous Troodos ophiolite, the ophiolitic suite lying on Akamas Peninsula at the western end of the island has received far less attention. Different from the updomed Troodos ophiolite, the Akamas ophiolite formed a linear NNW-SSE ridge in Akamas Peninsula, and it was isolated from the Troodos ophiolite by the Polis Graben and Mamonia Complex. The ultramafic section made up the backbone of the ridge. It was composed of intensively serpentinized harzburgite and dunite with extensive magnesite deposit associated. Brecciation and multiphase shears were observed within the ophiolite. The Sheeted Dyke Complex (SDC) and lava sequences were underthrust on the western flank of the ridge. The SDC were generally striking parallel to the orientation of the ridge and dipping east, while the lavas were usually rotated or even overturned. All these observations showed that the Akamas ophiolite was severely tectonized after its formation. The Akamas ophiolite has been regarded as part of the Troodos Complex. Since the 1960s, extensive volcanic geochemical data of the Troodos Complex have been collected for tectonic modelling. As part of the Complex, several Akamas lava samples were also included in these researches. They have shown a geochemical affinity with the Arakapas-type lavas, which were commonly found in the Southern Troodos Transform Fault Zone (STTFZ). However, the result did not lead to any further study on the emplacement of this isolated ophiolitic suite. In this project, over 40 lava and diabase dyke samples have been collected from the Akamas ophiolite. Their major and trace element content were analyzed and compared with the Troodos lavas. The results were consistent with the previous studies, hinting that the ophiolite is closely related with the STTFZ. However, the trend of the SDC in the Akamas ophiolite was nearly perpendicular to that observed in the STTFZ. By tracing the volcanics with similar geochemical affinity, it was found that the transform may have been swung from E-W in Limassol Forest to NNW near Akamas Peninsula. It had branched off into an array of transform fault system in the western extension. The Akamas ophiolite could be emplaced through the activation of this system. Another evidence came from the deposition of the Kannaviou Formation in the area, which sits on the Akamas lavas and brecciated serpentinite within a graben structure. Similar setting were also found in Kannaviou and Agia Varvara, where the Arakapas-type lavas were also found. The association of magnesite within the Akamas serpentinites has showed a carbonate origin from the Mamonia Complex through carbon isotopic analysis. With the help of existing paleomagnetic data, emplacement model for the Akamas ophiolite was provided, relating the paleorotation of the Troodos Microplate and the juxtaposition of the Mamonia Complex.

Wai-Pan Ng, Samuel

2010-05-01

27

Ophiolites of the Kunlun Mountains, China and their tectonic implications  

Microsoft Academic Search

Three ophiolite belts, ranging in age from Cambrian to Triassic, provide valuable data on the tectonic evolution of the Kunlun Mountains which lie along the northern margin of the Tibetan Plateau. All of the ophiolites contain dismembered but nearly complete assemblages of peridotite, mafic and ultramafic cumulates, pillow and massive lavas and radiolarian cherts, (1) The oldest belt contains ophiolites

J.-S. Yang; P. T. Robinson; C.-F. Jiang; Z.-Q. Xu

1996-01-01

28

The Ophiolite Problem, Is It Really a Problem?  

Microsoft Academic Search

Ophiolites and ophiolite complexes have been recognized as having an oceanic affinity or origin since the classic work of Ian Gass in the 1950's on the Troodos Complex. A problem has been that the term ophiolite has included a very diverse range of meanings from obscure slivers of mafic and ultramafic rocks of doubtful origin in orogenic belts to large

J. F. Casey; J. F. Dewey

2009-01-01

29

Mapping the Oman Ophiolite using TM data  

NASA Technical Reports Server (NTRS)

Ophiolite terrains, considered to be the onland occurrences of oceanic crust, host a number of types of mineral deposits: volcanogenic massive sulfides, podiform chromite, and asbestos. Thematic Mapper data for the Semail Ophiolite in Oman were used to separate and map ultramafic lithologies hosting these deposits, including identification of the components of the extrusive volcanic sequence, mapping of serpentinization due to various tectonic processes, and direct identification of gossans. Thematic Mapper data were found to be extremely effective for mapping in this terrain due to the excellent spatial resolution and the presence of spectral bands which allow separation of the pertinent mineralogically caused spectral features associated with the rock types of interest.

Abrams, Michael

1987-01-01

30

Nature of the Akamas ophiolite, W Cyprus  

Microsoft Academic Search

Cyprus has been an ideal place to study the evolution of oceanic lithosphere. The complete preservation of the ophiolite in the Troodos Complex has allowed better understanding on the ocean closure process and the emplacement of oceanic lithospheric fragments on land. These previous studies have put the development of plate tectonic theory a great leap forward. Compared to the famous

Samuel Wai-Pan Ng

2010-01-01

31

Ophiolite Tectonics, Rock Magnetism and Palaeomagnetism, Cyprus  

NASA Astrophysics Data System (ADS)

Magnetic properties of minerals may be sensitive indicators of provenance. Remanence-bearing minerals (RBM) such as iron-titanium oxides, and matrix-forming minerals such as paramagnetic phyllosilicate or diamagnetic calcite yield different clues to provenance, strain history and tectonics, and are essential supplements for the full interpretation of palaeomagnetic data. Moreover, mineral magnetic properties provide magnetic-petrofabric indicators of tectonic strain, determine the suitability of sites for palaeomagnetism, and permit the restoration of palaeomagnetic vectors in some strained rocks. In the Cretaceous Troodos ophiolite (~88 Ma) magnetic properties are dictated by the relative importance of mafic silicates and largely primary, ophiolite-derived RBM. In its cover of deformed pelagic sedimentary rock, magnetic properties are dictated by the balance of clastic RBM versus matrix calcite and in some cases clay. The two larger Cretaceous ophiolite outcrops (Troodos & Akamas) share a common orientation of their plutonic flow fabrics, determined by magnetic methods. The dike complex shows fabrics indicating plume-like feeders spaced along and perpendicular to the spreading axis, with longevities >0.5 Ma. South of the ophiolite, its Cretaceous-Miocene limestone cover possesses ubiquitous tectonic petrofabrics inferred from anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent susceptibility (AARM). Its foliation and maximum extension dip and plunge gently northward, sub-parallel to a common but previously unreported North-dipping stylolitic cleavage. In well-known localized areas, there are S-vergent thrusts and overturned folds. The S-vergent deformation fabrics are due to Late Miocene (pre-Messinian ~8 Ma) deformation. The structures are geometrically consistent with overthrusting of the Cretaceous Troodos-Akamas ophiolite, and its sedimentary cover, onto the underlying Triassic Mamonia terrane. The northern limit of pre-Messinian tectonic fabrics, the Troodos-Mamonia terrane boundary and the Arakapas-Transform fault form an approximate E-W composite boundary that we term the Troodos Tectonic Front. Miocene deformation remagnetized the ophiolite and its sedimentary cover in many places and also affects the Mamonia terrane to the SW, with which the Troodos terrane docked in the late Cretaceous. Magnetic mineralogy, particularly of the RBM traces the progressive un-roofing of the ophiolite during the deposition of its sedimentary cover. During the submarine exposure and erosion of the ophiolite, the contribution of RBM clasts to the overlying sedimentary cover changed qualitatively and quantitatively. Thus, magnetic mineralogy of the sedimentary rock cover records the progressive denudation of the ophiolite from lavas, down through dikes, to gabbros and deeper mantle rocks. Palaeomagnetic studies previously revealed the anticlockwise rotation of the Troodos terrane and its northwards migration. Characteristic remanent magnetism (ChRM) is most reliable for lavas and dikes although it is usually carried by recrystallized RBM. These correspond to the age of greenschist facies ocean-floor metamorphism, perhaps 7-15 Ma after igneous crystallization with an extent and depth dependent on depth and degree of hydrothermal circulation. The gabbros and mantle rocks commonly bear young (<12 Ma) remanences probably acquired (or re-acquired) during uplift of the Troodos terrane. In the cover of pre-Messinian deformed limestone (>8 Ma), the remagnetizing effects of penetrative strain have been under-estimated. Where strain has occurred, un-tilting procedures produce erroneous restorations for the remanence vectors, and thus for the associated paleopoles. We find that de-straining of limestone sites most appropriately restores ChRM vectors to their original orientation. The best-determined and restored ChRMs define an apparent polar wander path (APWP). Since the APWP terminates at the present N-pole, we inverted it to determine the true plate-motion of the Troodos-terrane. Thus, in prese

Borradaile, G. J.; Lagroix, F.; Hamilton, T. D.; Trebilcock, D.-A.

2010-06-01

32

Magma in forearcs: implication for ophiolite generation  

NASA Astrophysics Data System (ADS)

Forearc areas ("non-volcanic" arcs) of contemporary island arcs at convergent plate boundaries contain magmatic rocks. Geological evidence, seismic profiles, heat flow data, density considerations and petrological and geochemical arguments suggest that a forearc tholeiitic association (FAT) (containing high-Mg calc-alkaline andesites) is present in "non-volcanic" arcs at some stage of island-arc development. The fractionated, as well as primitive magma, is unable to penetrate low-density sediments and underplates thick piles of unconsolidated accreting rocks. The underplating causes upwelling. The occurrence of magma in forearcs provides an alternative interpretation for the tectonic setting of some ophiolitic masses. Rather than "ocean-ridge formation" and later "obduction" it offers an autochthonous (island-arc bound and geologically-substantiated) interpretation for the ophiolite suite.

Jake?, Petr; Miyake, Yasuyuki

1984-07-01

33

Actualistic Ophiolite Provenance: The Cyprus Case.  

PubMed

The island of Cyprus represents an excellent site to assess quantitatively petrologic clastic response to actively obducting oceanic sources in order to define an actualistic reference for ophiolite provenance, in terms of framework composition and heavy mineral suites. An improved methodology, an extension of the classic ternary QFL logic to include a wider spectrum of key indexes and ratios, provides an accurate synthesis of modal data and allows differentiation of three main petrographic provinces and at least seven subprovinces. Diagnostic signatures of detritus from various levels of an oceanic lithospheric source, and criteria for distinguishing provenance from suprasubduction versus mid-oceanic ophiolites are also outlined. Modern sands derived from the Troodos Ophiolite contain variable proportions of largely pelagic carbonate to chert, boninite to basalt, diabase to metabasite, plagiogranite to gabbroic, and cumulate grains supplied from progressively deeper-seated levels of the multilayered oceanic crust. Dense minerals are mainly clinopyroxenes (diopside), prevailing over orthopyroxenes (enstatite, hypersthene, clinoenstatite), hornblende, tremolite/actinolite, and epidote. Where serpentinized mantle harzburgites have been unroofed, detritus is markedly enriched in cellular serpentinite grains and enstatite, with still negligible olivine and spinel. Sedimentaclastic sands dominated by chert (Mamonia Province) or carbonate grains (Kyrenia Province) are deposited along the southern and northern shores of the island, respectively. Compositions of Cyprus sands are virtually unaffected by climatic, sedimentary, or anthropic processes; recycling of sandstones from foreign sources is a major process only in the Karpaz Peninsula. Petrographic analysis also provides an independent mean to identify prevalent directions of longshore sand transport. PMID:10736270

Garzanti; Andò; Scutellà

2000-03-01

34

Blueschist Blocks in the Shale-matrix Melange of the Franciscan Complex of California: Metamorphic Aureoles and Subduction Channel Upwelling  

NASA Astrophysics Data System (ADS)

The subduction channel concept postulates that where oceanic lithosphere converges faster than ~2 cm/yr, the plate boundary is best approximated as a viscous shear zone. Channel capacity can abruptly decrease where there is a sharp increase in the pressure gradient along the top of the shear zone. Where this occurs, subducted sediment can upwell and flow back towards the inlet. The Central Belt of the Franciscan Complex is up to tens of km wide and extends from where subduction continues in Oregon southwards to the Nacimiento Block, west of the San Andreas fault. The scaly shale matrix outcrops poorly along the 1500+ km belt because it is weakly metamorphosed (<200°C, pumpellyite±lawsonite-bearing). The best exposures, as pointed out by Ken Hsu in 1969, are found near San Simeon. The Central Belt mélange is renowned for the "high-grade" garnet-bearing blueschists (e.g., Tiburon, Jenner Beach). How these high-T blocks, the oldest metamorphics in the Franciscan and the more voluminous lower-T blueschists became rounded fragments encased in shale-matrix has been the subject of much discussion. Uplift in serpentine diapirs is widely invoked, but the scarcity of these blocks in serpentinite where exposures are good is problematic. Explaining the blocks as sedimentary olistoliths requires both a phantom source terrane and exhumation mechanism. The simplest explanation is that the coarse blueschists are fragments of a metamorphic aureole formed during subduction initiation that were later detached from the base of the ophiolitic leading edge of the North American plate. Later, tectonic melange was generated by subduction-driven shearing that caused upwelling of shale-rich sediment. Slabs of blueschist were boudinaged and mixed with fragments of greenstone and chert detached from seamounts. Blueschist block incorporation into upwelling mélange is a kind of subduction erosion akin to plucking by glaciers. The exposure of the blueschist along the roof of the subduction channel may have been mostly localized to near where normal faults formed in the overriding block as Farallon plate dip flattened from 80 to 40 Ma. Only blocks of the appropriate size and density can upwell to shallower depths where they are accreted by underplating or offscraping. Blocks that are large and dense enough to sink faster than the rate of upwelling, bottom out and become subducted.

Cloos, Mark; Ukar, Estibalitz

2014-05-01

35

Oceanic Detachment Faults Observed in the Troodos Ophiolite, Cyprus  

Microsoft Academic Search

In the Limassol Forest area south of the Arakapas Transform Fault in the Troodos Ophiolite, structural relationships are much more complex than north of the transform fault. To the north, the ophiolite has a classic layer-cake structure of volcanics above sheeted dykes above gabbros above peridotites. To the south the same units are present, but are juxtaposed in a much

J. Cann; A. McCaig

2005-01-01

36

Late Mesozoic and possible early Tertiary accretion in western Washington State: the Helena-Haystack melange and the Darrington- Devils Mountain fault zone  

USGS Publications Warehouse

The Helena-Haystack melange (HH melange) and coincident Darrington-Devils Mountain fault zone (DDMFZ) in northwestern Washington separate two terranes, the northwest Cascade System (NWCS) and the western and eastern melange belts (WEMB). The two terranes of Paleozoic and Mesozoic rocks superficially resemble each other but record considerable differences in structural and metamorphic history. The HH melange is a serpentinite-matrix melange containing blocks of adjacent terranes but also exotic blocks. The HH melange must have formed between early Cretaceous and late middle Eocene time, because it contains tectonic clasts of early Cretaceous Shuksan Greenschist and is overlain by late middle Eocene sedimentary and volcanic rocks. The possible continuation of the DDMFZ to the northwest as the San Juan and the West Coast faults on Vancouver Island suggests that the structure has had a major role in the emplacement of all the westernmost terranes in the Pacific Northwest. -from Author

Tabor, R. W.

1994-01-01

37

Melange: A viscoelastic lattice-particle model applicable to the lithosphere  

NASA Astrophysics Data System (ADS)

This article introduces the software Melange, a 3D lattice-particle hybrid model. The software was specifically designed in order to simulate ductile visco-elasto-plastic deformation and can be used to study tectonic processes in the lithosphere from the micro to the macro scale. Melange is under an open source license. The code takes both relevant yield mechanisms for the deformation of lithospheric material into account: dynamic brittle failure and ductile creep, where ductile creep is modeled as viscoelasticity. The software considers effects of the local geology, of the inherent disorder of geomaterials, of rheological layering of the lithosphere and applies repulsion when the material fractures. Driving forces are the externally applied strain and the gravitational load. Melange applies an elastically isotropic regular 3D lattice with HCP geometry and next-nearest neighbor interactions. Young's modulus, viscosity, material density and system size can be freely chosen. Poisson's ratio is restricted to values <0.25. The most important innovation of the software is a physically consistent scheme to model viscoelasticity in a lattice-particle model. The scheme couples the local lattice-geometry to the time-dependent volume-conservative viscoelastic deformation of particles. The procedure achieves the typical viscoelastic stress-strain relationship. The article incorporates a number of benchmark simulations, where the model output is being tested and evaluated. The results obtained with Melange show strong similarities with the benchmark values, e.g., stress fields in crack vicinity. Geologically relevant examples include structures associated with the brittle-ductile transition, growth of wing cracks and brittle graben formation.

Sachau, Till; Koehn, Daniel

2012-12-01

38

Angat Ophiolitic Complex, Luzon, Philippines: a cretaceous dismembered marginal basin ophiolitic complex  

NASA Astrophysics Data System (ADS)

The dismembered Angat Ophiolitic Complex (DAOC) is made up of pillow basalts with associated sediments, sheeted diabase dike complex and massive/layered gabbros. Available bulk chemistry and petrogenetic modeling suggests that olivine, clinopyroxene, plagioclase ± magnetite are the main fractionating phases of the rocks of this complex. These theoretical predictions are borne out by petrographic observations. The source material of this dismembered ophiolitic suite, assuming primary mantle composition and normal parent-daughter melting relationship, underwent at least 15-30% degrees of partial melting. Available trace element geochemistry, although manifesting MORB-island arc characteristics, do not unequivocally indicate a supra-subduction zone environment of formation. Preliminary evidence, however, suggests formation in a marginal basin environment for the DAOC.

Yumul, Graciano P.

39

Melange a Quatre Ondes Degenere dans les Absorbants Saturables EN Milieu Mince  

NASA Astrophysics Data System (ADS)

Le melange a quatre ondes est utilise notamment pour determiner la duree des temps de reorientation moleculaire et mesurer la grandeur des susceptibilites nonlineaires du troisieme ordre. Nous avons mis en evidence les particularites du melange a quatre ondes degenere en milieu absorbant mince a l'aide du formalisme de l'optique nonlineaire et de l'approche holographique. Des experiences realisees avec des impulsions de 33 ps ont permis de caracteriser la cinetique de la nonlinearite en regime transitoire des molecules de rhodamine 6G incorporees dans des matrices d'alcool polyvinylique; un milieu aussi compose d'absorbants anisotropes, les molecules de rhodamine 6G en solution aqueuse et un autre forme d'absorbants isotropes, les films minces de cristaux de semiconducteur CdS_ {x}Se_{1-x} ont ete etudies. Nous avons observe pour ces trois milieux les signaux generes aux ordres superieurs par melange a quatre ondes degenere en milieu mince pour differentes conditions de polarisation.

O'Neill, Claire

1995-01-01

40

Geology of the Zambales ophiolite, Luzon, Philippines  

NASA Astrophysics Data System (ADS)

The Zambales ophiolite of western Luzon, Philippines, exposes a typical succession of basalt flows, diabasic dikes, gabbro and tectonized harzburgite. The age established by limiting strata is late Eocene. Lack of evidence of thrust faulting and the general domal disposition of the lithologie units indicate that the ophiolitic rocks are exposed by uplift. Highly complex internal layered structures within the complex are related to processes developed during formation of the ophiolite and the Zambales ophiolite may be one of the least disturbed (by emplacement) ophiolitic masses known. The exposed mass trends north and the upper surface plunges at low angles (a few degrees) to the north and south. The chemistry and composition of the rocks in the northwest part of the Zambales area (Acoje block) is distinct from that in the southeastern segment (Coto block). The Acoje block, according to Evans (1983) and Hawkins and Evans (1983), resembles (on a chemical basis) arc-tholeiite series rocks from intra-island arcs and the rocks in the Coto block are typical back-arc basin rock series. The present writer believes that the ophiolite composes a single genetic unit and that the changes in composition are the result of changes that took place during the initial formation. The gabbro probably formed below a spreading center in an elongate, in cross section, V-shaped, magma chamber. The gabbro is estimated by the writer to be less than 2 km thick and may be less than 1 km in places. Numerous erosional windows through the gabbro in the northern and eastern side of the Zambales area show that the gabbro remaining in those areas is likely to be only a few hundred meters thick. Harzburgite is exposed to a depth of about 800 m in the Bagsit River area and this may be the deepest part of the ophiolite accessible for study on which there is any control on depth. A transitional zone, about 200 m thick lying between the gabbro and harzburgite, is composed of serpentinized dunite. Commonly the dunite contains disseminated sulfide minerals and at the Acoje Mines, platinum-group elements. A compositional layering within the gabbro is in places cumulate in the lower part of the unit but may have formed by nucleation higher up on the relatively steep sides of the magma chamber. A widespread gneissic banding in the gabbro forms large mappable structures which are many times more complex than is the disposition of the major rock units. These structures are believed to be the result of extensive slumping in the magma chamber. The structure produced by the cumulate layering merges with the gneissic banding, commonly without discernible change in attitude. This tectonic layered structure crosses the gabbro-peridotite boundary at any angle without seeming to disturb the original rock distribution. At greater depths below the boundary (ca. 800 m), the harzburgite contains low dipping banding, which probably reflects the result of differential movement within the mantle. Chromite occurs almost exclusively in a zone that generally lies no more than 200-300 m below the gabbro-peridotite boundary. Refractory-grade chromite is found in this zone below the olivine gabbro in the Goto block and as low-grade metallurgical grade chromite below norite in the Acoje block. At Acoje Mines the chromite is present in layers in dunite, which the writer interprets as being distributed in a zone along the gently dipping (ca. 25°) gabbro-peridotite boundary. The steeply dipping (ca. 60-80 ° ) individual layers lie en echelon along the boundary at an angle (ca. 50 ° ) to the contact. At Coto the chromite forms large discontinuous masses in the lowest dunite and in the uppermost harzburgite. Except for the chromite present as layers at Acoje, the regional tectonic layering crosses the chromite deposits without structural deviation. The chromite deposits and associated peridotite may be cumulate in origin, but have been modified to such an extent that cumulate textures are generally obliterated. The angle of repose of cumulate layers in the Acoje area and in the Coto block

Rossman, D. L.; Castañada, G. C.; Bacuta, G. C.

1989-10-01

41

Geology of the Zambales ophiolite, Luzon, Philippines  

USGS Publications Warehouse

The Zambales ophiolite of western Luzon, Philippines, exposes a typical succession of basalt flows, diabasic dikes, gabbro and tectonized harzburgite. The age established by limiting strata is late Eocene. Lack of evidence of thrust faulting and the general domal disposition of the lithologie units indicate that the ophiolitic rocks are exposed by uplift. Highly complex internal layered structures within the complex are related to processes developed during formation of the ophiolite and the Zambales ophiolite may be one of the least disturbed (by emplacement) ophiolitic masses known. The exposed mass trends north and the upper surface plunges at low angles (a few degrees) to the north and south. The chemistry and composition of the rocks in the northwest part of the Zambales area (Acoje block) is distinct from that in the southeastern segment (Coto block). The Acoje block, according to Evans (1983) and Hawkins and Evans (1983), resembles (on a chemical basis) arc-tholeiite series rocks from intra-island arcs and the rocks in the Coto block are typical back-arc basin rock series. The present writer believes that the ophiolite composes a single genetic unit and that the changes in composition are the result of changes that took place during the initial formation. The gabbro probably formed below a spreading center in an elongate, in cross section, V-shaped, magma chamber. The gabbro is estimated by the writer to be less than 2 km thick and may be less than 1 km in places. Numerous erosional windows through the gabbro in the northern and eastern side of the Zambales area show that the gabbro remaining in those areas is likely to be only a few hundred meters thick. Harzburgite is exposed to a depth of about 800 m in the Bagsit River area and this may be the deepest part of the ophiolite accessible for study on which there is any control on depth. A transitional zone, about 200 m thick lying between the gabbro and harzburgite, is composed of serpentinized dunite. Commonly the dunite contains disseminated sulfide minerals and at the Acoje Mines, platinum-group elements. A compositional layering within the gabbro is in places cumulate in the lower part of the unit but may have formed by nucleation higher up on the relatively steep sides of the magma chamber. A widespread gneissic banding in the gabbro forms large mappable structures which are many times more complex than is the disposition of the major rock units. These structures are believed to be the result of extensive slumping in the magma chamber. The structure produced by the cumulate layering merges with the gneissic banding, commonly without discernible change in attitude. This tectonic layered structure crosses the gabbro-peridotite boundary at any angle without seeming to disturb the original rock distribution. At greater depths below the boundary (ca. 800 m), the harzburgite contains low dipping banding, which probably reflects the result of differential movement within the mantle. Chromite occurs almost exclusively in a zone that generally lies no more than 200-300 m below the gabbro-peridotite boundary. Refractory-grade chromite is found in this zone below the olivine gabbro in the Goto block and as low-grade metallurgical grade chromite below norite in the Acoje block. At Acoje Mines the chromite is present in layers in dunite, which the writer interprets as being distributed in a zone along the gently dipping (ca. 25??) gabbro-peridotite boundary. The steeply dipping (ca. 60-80 ?? ) individual layers lie en echelon along the boundary at an angle (ca. 50 ?? ) to the contact. At Coto the chromite forms large discontinuous masses in the lowest dunite and in the uppermost harzburgite. Except for the chromite present as layers at Acoje, the regional tectonic layering crosses the chromite deposits without structural deviation. The chromite deposits and associated peridotite may be cumulate in origin, but have been modified to such an extent that cumulate textures are gener

Rossman, D. L.; Castanada, G. C.; Bacuta, G. C.

1989-01-01

42

Geochemistry and 40Ar-39Ar ages of the Mashhad Ophiolite, NE Iran: A Rare Occurrence of a 300 Ma (Paleo-Tethys) Oceanic Crust  

NASA Astrophysics Data System (ADS)

The Alborz mountains of northern Iran marks a zone of collision in the Alpine-Himalayan orogenic belt, and it forms a bridge between the of Hindu-Kush and Himalayan in the Middle and Far East and the Lesser Caucasus and Alpine in the Mediterranean and Europe. The tectonic evolution of the Alborz orogenic belt is considered to be related to northward sunduction of the Paleo-Tethys under Eurasia which led to the collision between the Iranian microcontinent and Eurasia. The remnant of the Paleo-Tethys oceanic crust are very scarce along Alborz, and except the 300 Ma Mashhad ophiolite, the rest are in the form of small patches of serpentinized ultramafic rocks. The remnants of the Paleo-Tethys oceanic crust in the proximity of the Mashhad include three distinct rock assemblages, which are the ophiolite complexes (Mashhad and Virani), deep-sea turbidites and a sequence of layered pyroclastic. The ophiolite-coloured melange complexes consists of a mixture of ophiolitic and sedimentary rocks. Although the ophiolite stratigraphy in this area is largely dismembered, with the exception of sheeted dike complex it contains all the rock units that are representative of an ophiolite suite. The rocks from these complexes include 1) ultramafic rocks (lherzolites and wherlites and pyroxinite) which are highly serpentinized and sheared, 2) gabbros, some of which are pegmatitic,3) pillow basalts, and 4) uniformly bedded pelagic fossiliferous limestone and chert. In using different tectonomagmatic classification diagrams (e.g., Zr-Nb-Y and Zr/Y-Zr) we have found that there are at least two different types of basalts (i.e., A and B for the purpose of this preliminary work) in Mashhad ophiolite. Type-A basalts show volcanic arc-like chemical characteristics, whereas the type-B basalts have E-MORB-like chemical affinity. These geochemical differences among the two types of basalts are also clearly evidenced in terms of the REE chondrite-normalized diagrams. The type-A basalts have LREE enriched patters [(La/Yb)ave= 6.7 and (La/Sm)ave= 3.4] in comparison to the type-B (E-MORB-like) basalts with [(La/Yb)ave= 2.3 and (La/Sm)ave= 1.05]. Furthermore type-A basalts have a distinctive negative Eu anomalies. A similar type of comparison using primitive mantle-normalize patterns also show clear differences between the two types of basalts. Two 40Ar-39Ar plateau ages of 281.4 Ma and 277.4 Ma for hornblende gabbros corresponding to isochrone ages of 287.6 and 281.7 Ma. In a separate K/Ar analysis, the same two samples gave values of 273 and 265 Ma, suggesting late Pennsylvanian-early Permian ages (i.e., Paleo-Tethys age for the oceanic crust).

Ghazi, A. M.; Hassanipak, A. A.; Tucker, P. J.; Mobasher, K.; Duncan, R. A.

2001-12-01

43

Effect of melt–rock interaction on geochemistry in the Kudi ophiolite (western Kunlun Mountains, northwestern China): implication for ophiolite origin  

Microsoft Academic Search

Major, trace and rare earth elements (REE), and platinum group elements (PGE) geochemistry have been studied for the Kudi ophiolite in the northwestern part of the western Kunlun Mountains, northwestern China, in order to determine the relationship between the mantle peridotites and mafic lavas and the origin of the ophiolite. The ophiolite consists of harzburgites, dunites, cumulate dunites, pyroxenites and

Zhihong Wang; Shu Sun; Quanlin Hou; Jiliang Li

2001-01-01

44

The Mt. Ochi melange (South Evvia Island, Greece): a case study for HP metamorphism and syn-convergent exhumation.  

NASA Astrophysics Data System (ADS)

The Attic-Cycladic complex (central Aegean Sea, Greece) experienced profound extension since at least the Oligo-Miocene boundary during which the previously thickened crust was reworked by a series of detachments forming the NE directed North Cycladic Detachment System (NCDS) and the SSW directed West Cycladic Detachment System (WCDS). South Evvia Island is located at the northwestern part of the Attic Cycladic complex linking the highly thinned and polymetamorphosed central part of the complex with mainland Greece. Furthermore, greenschists-facies retrograde metamorphism has only partially overprinted the HP mineral assemblages. Consequently, it is an ideal area to study tectonic processes associated with subduction, HP metamorphism and subsequent exhumation from eclogitic depths to the surface. Geological mapping in 1:2:000 scale revealed that the tectonostratigraphy of Mt. Ochi includes three distinct units all metamorphosed in HP conditions followed by greenschist facies overprint. These units are from top to bottom a) the Ochi Unit, a thick metavolcanosedimentary sequence with some intensely folded cipoline marble intercalations and isolated occurrences of metabasic rocks b) the ophiolitic mélange (metagabbros, metawherlites, peridotites, metabasites within a metasedimentary+serpentinite matrix) and c) the lowermost Styra Unit, a cipoline marble-dominated unit with thin mica schists and rare quartzitic layers often boudinaged. The thrust fault that was responsible for the juxtaposition of these three units acted in an early stage during HP metamorphism and it was isoclinally folded and sheared by the following syn-metamorphic deformation events. Detailed structural study in meso- and microscopic scale combined with petrological and geochemical analyses of the Mt Ochi rocks led to the distinction of at least three syn-metamorphic and two post-metamorphic deformation episodes that affected all units. The oldest structure identified is a relic foliation formed by the mineral assemblage Na-amphibole + lawsonite seen as inclusion in epidote porphyroblasts within the melange. It could represent a structure of the prograde path but it could also have formed during the peak HP event. This is followed by successive folding episodes that are related to axial plane foliations and a ~E-W intersection/stretching lineation formed by typical blueschist- to epidote-blueschist facies mineral assemblages. The main foliation that can be observed in all three units is a greenschist-facies axial plane foliation accompanied by a ~ENE-WSW stretching lineation. The shear sense during the prograde path is constantly towards the WSW. In the greenschists-facies an unambiguous top-to ENE can be observed mostly in mylonitic rocks. The following deformation episodes include semi-brittle to brittle structures (shear bands brittle open folds, crenulation cleavage, and faults with increasingly higher-angle) that are not as penetrative and record the passage of the units through the brittle-ductile transitions and to higher structural levels. The kinematics of these late episodes is also towards the NE. Based on the above, the Mt Ochi HP units exhibit a common tectonometamorphic evolution since at least the early stages of the prograde path. The Ochi Unit/Styra Unit contact is a structure that formed prior to or during peak HP metamorphism and therefore it couldn't have served as the normal fault to an extrusion wedge.

Moustaka, Eleni; Soukis, Konstantinos; Huet, Benjamin; Lozios, Stylianos; Magganas, Andreas

2014-05-01

45

Incipient oceanic accretion at the foot of the Gondwanian passive margin : the Neyriz ophiolites (Iran)  

Microsoft Academic Search

The Neyriz ophiolite is part of a 3000 km obduction belt that was thrust over the edge of the Arabian continent during the Late Cretaceous. 40Ar\\/39Ar ages from Neyriz ophiolite range from 83Ma to 98Ma, similar to the age of the Oman ophiolite. The Neyriz ophiolite is remarkable by the presence of marbles lenses in direct contact with the mantle

M. R. Jannessary; H. Whitechurch; L. E. Ricou; O. Muntener; B. Ildefonse; G. Manatschal

2003-01-01

46

Evolution of a Permo-Triassic sedimentary melange, Grindstone terrane, east-central Oregon  

USGS Publications Warehouse

Perceives the Grindstone rocks to be a sedimentary melange composed of Paleozoic limestone slide and slump blocks that became detached from a carbonate shelf fringing a volcanic knoll or edifice in Late Permian to Middle Triassic time and were intermixed with Permian and Triassic slope to basinal clastic and volcaniclastic rocks in a forearc basin setting. Paleogeographic affinities of the Grindstone limestone faunas and volcaniclastic debris in the limestone and clastic rocks all indicate deposition in promixity to an island-arc system near the North American craton. -from Authors

Blome, C. D.; Nestell, M. K.

1991-01-01

47

Thematic mapper study of Alaskan ophiolites  

NASA Technical Reports Server (NTRS)

The combinations of Thematic Mapper (TM) bands that best distinguish basalts of the Brooks Range ophiolites were determined. Geochemical analyses, including major, trace, and rare earth elements (REE), are being done in order to study the significance of TM spectral variations that were observed within some of the sampled rock units. An image of the topography of the western Brooks Range and Colville Basin was constructed. Elevation data for the rest of Northern Alaska are being acquired to expand the area covered by the topography image. Two balanced cross sections (one along the eastern margin, the other along the western margin of the Brooks Range) are being constructed, using the techniques of fault-bend and fault-propagation folding. These are being used to obtain regional shortening estimates for the Brooks Range in an attempt to constrain tectonic models for the evolution of Northern Alaska. The TM data are being used to confirm reconnaissance maps and to obtain structural data where no maps exist. Along with the TM data, digital topography, seismic reflection profiles, and magnetic and gravity surveys are examined to better understand the evolution of the Colville Basin, north of the Brooks Range.

Bird, J. M.

1986-01-01

48

Slow spreading accretion and mantle denudation in the Mirdita ophiolite (Albania)  

Microsoft Academic Search

The Mirdita ophiolite in Albania occupies a N-S corridor which escaped most Alpine and Cenozoic deformation, possibly due to a thick ophiolitic basement. The sheeted dike complex strikes NS and dips steeply, indicating that the ridge was oriented parallel to the NS corridor and that the ophiolite has not been significantly tilted, although differential motion between individual massifs cannot be

Adolphe Nicolas; Françoise Boudier; Avni Meshi

1999-01-01

49

Ocean floor metamorphism in the Betts Cove ophiolite, Newfoundland  

Microsoft Academic Search

The mineralogical and geochemical features of the lower Ordovician Betts Cove ophiolite of northeastern Newfoundland indicate that hydrothermal circulation of seawater near a mid-ocean ridge has been involved in the metamorphism of the complex. The degree of greenschist facies metamorphism increases with stratigraphie depth in the ophioli te. Calcite, hematite and epidote distributions show that the metamorphosing fluid penetrated downward

R. A. Coish

1977-01-01

50

Chromite deposits in the northern Oman ophiolite: Mineralogical constraints  

Microsoft Academic Search

Chromite deposits in the northern Oman ophiolitic complex occur in three structural contexts, i.e., (1) at the base of the cumulate series, (2) in the top kilometer of the mantle sequence, and (3) in the deeper parts of the mantle. Types 1 and 2 are characterized by the diversity of interstitial silicates where in decreasing order of abundance olivine, clinopyroxene,

T. Augé

1987-01-01

51

The Ophiolite Problem, Is It Really a Problem?  

NASA Astrophysics Data System (ADS)

Ophiolites and ophiolite complexes have been recognized as having an oceanic affinity or origin since the classic work of Ian Gass in the 1950’s on the Troodos Complex. A problem has been that the term ophiolite has included a very diverse range of meanings from obscure slivers of mafic and ultramafic rocks of doubtful origin in orogenic belts to large obducted slabs with the full range (Coleman, 1972), from base to top, of lherzolite/ariegite, harzburgite, dunite, gabbro, sheeted dyke complex, pillow basalts, and sediments, commonly with a two-pyroxene mafic granulite as a thin aureole attached to the base of the complex. Large obducted ophiolite slabs are mainly early Ordovician and mid-Cretaceous. The principal enigma of these obducted slabs is that they clearly must have been generated by some form of organized sea-floor spreading/plate-accretion, such as may be envisioned for the oceanic ridges, yet the volcanics commonly have arc affinity (Miyashiro) with boninites (high-temperature/low-pressure, high Mg and Si andesites), which suggest a forearc origin. Our model hinges on the PT conditions under which boninites form. Many ophiolites have complexly-deformed associated assemblages that suggest fracture zone/transform geology, which in turn has led to models involving the nucleation of subduction zones on fracture zones/transforms. Hitherto, arc-related sea-floor-spreading has been considered to be either pre-arc (fore-arc boninites) or post-arc (classic Karig-style back arc basins that split arcs). We propose a new model with syn-arc boninites that involves a stable ridge/trench/trench triple junction, the ridge being between the two upper plates. The direction of subduction must be oblique with a different sense in the two subduction zones and the oblique subduction cannot be partitioned into trench orthogonal and parallel strike-slip components. As the ridge spreads, new oceanic lithosphere is created within the forearc, the arc and fore-arc lengthen significantly, and a syn-arc ophiolite complex is generated that ages along arc-strike; a distinctive diachronous boninite/arc volcanic stratigraphy develops. Dikes in the ophiolite are oblique to the trench as are magnetic anomalies in the “back-arc” basin. Boninites and high-mg andesites are generated in the fore-arc under the aqueous, low pressure/high temperature, regime at the ridge above the dehydrating slab or where a ridge subducts beneath the forearc. The mafic protolith, garnet/two pyroxene, aureole is generated in and sliced from the subducting slab and attached to the base of the overriding lithosphere at about 1000°C, ten to twelve million years from the ridge axis, where the SSZ ophiolite is about ten to twelve kilometers thick, at which thickness of the ophiolite is buffered by the subducting slab. Obduction of the SSZ ophiolite with its subjacent aureole occurs whenever the oceanic arc attempts subduction of a stable continental margin.

Casey, J. F.; Dewey, J. F.

2009-12-01

52

The Angat ophiolite, Luzon, Philippines: lithology, structure, and problems in age interpretation  

NASA Astrophysics Data System (ADS)

The Angat ophiolite is an incompletely unroofed and structurally dissected ophiolitic mass consisting of layered and massive gabbros, diabase sheeted dikes, tonalites, and pillow basalts. Mapping in the Montalban quadrangle has revealed complex structural relationships between pillow basalts and overlying sediments which hinder an accurate determination of ophiolite age. Sparse radiolarian data suggest a Cretaceous age, but undifferentiated as to Early or Late Cretaceous. The structural deformation of the ophiolite is probably due in part to strike-slip faulting during Early to Middle Miocene time, this date being constrained by deformed sediments in fault contact with the ophiolite. Initial geochemical data from sheeted dikes and pillow lavas clearly related to the ophiolite show unusually high silica contents, and trace-element discrimination suggests that different magma types may have been associated with the ophiolite. Pillow basalts that are probably genetically unrelated to ophiolite were also mapped close to the Angat ophiolite. Ages and tectonic settings determined from sedimentary rocks overlying these basalts should not be used to constrain age and paleotectonic interpretations of the ophiolite.

Arcilla, Carlo A.; Ruelo, Hernulfo B.; Umbal, Jesse

1989-10-01

53

Obduction initiation: evidence from the base of New Caledonia ophiolite  

NASA Astrophysics Data System (ADS)

Obduction, whereby fragments of dense, oceanic lithosphere (ophiolites) are presumably 'thrust' on top of light continental ones, remains a poorly understood geodynamic process, in particular with respect to 1) obduction initiation and 2) effective ophiolite emplacement. Most of our knowledge on obduction initiation comes from the amphibolite to granulite facies high-temperature metamorphic soles welded to the base of non-metamorphic large-scale ophiolite thrusts (e.g., Oman, N.Caledonia, Balkans,...), which are interpreted as witnesses of the subduction inception stages preluding to obduction. We herein report for the first time the existence of deformed amphibolites near the base of the ophiolite (~ 50-100 m above), yet within the mantle peridotites proper, from the classic New Caledonia ophiolite (Plum beach, SE of Noumea). These amphibolites correspond to several cm thick thin bands of sheared mafic rocks showing highly deformed to mylonitic textures, which are embedded within well-preserved to strongly serpentinized peridotites. Large-scale shear bands (>100m long in places) show impressive, very consistent deformation patterns and shear senses on the outcrop scale in favor of reverse displacements. Amphibolites and peridotites are both strongly sheared with the common observation of strongly boudinaged peridotites in the core of the largest shear bands. Some gabbroic pods and plagioclase-rich veinlets are spatially associated to the amphibolites, but appear to have formed slightly later based on cross-cutting relationships. Preliminary mineralogical observations indicate complex, successive recrystallisations stages, with olivine-bearing clasts, at least three generations of amphiboles and late stage talc-chlorite associations. Although still preliminary, several interpretations can be proposed for the origin of such structures. They could correspond to (1) very-high temperature sole "amphibolites" deformed (tectonically mixed with the mantle; and possibly partly melted) during strong shearing at the base of the obduction thrust, (2) mafic melts intruding (and later sheared; possibly almost coevally) at the base of the ophiolite sequence, (3) ingression of hydrous fluids producing local melting of the peridotite. Based on several field observations, and on the need for unrealistically high temperature for hydrous peridotite melting (i.e., > ~1100°C), the last hypothesis seems less likely. Whatever the hypothesis, we stress that it is yet unclear if this deformation relates to subduction processes accompanying early obduction stages or to structures inherited from oceanic stages (e.g., deformation structures at the vicinity of major transforms and/or oceanic detachments). In the latter case, this discovery would indicate that the localisation of deformation during subduction/obduction initiation reworks ancient zones of deformation.

Agard, Philippe; Vitale-Brovarone, Alberto; Soret, Mathieu; Chauvet, Alain; Dubacq, Benoit; Monié, Patrick

2013-04-01

54

Albanian ophiolites as probes of a mantle heterogeneity study  

NASA Astrophysics Data System (ADS)

Most ophiolites are believed to be tectonically obducted slivers of oceanic lithosphere. As such they can provide information not only about the history of crust formation, but also about the composition of the chemical composition of the recent and ancient mantle composition. The occurrence of the well preserved Albanian Ophiolite Complex covers the length of Albania (ca. 150 km) is an ideal object not only for the study of the history of Jurassic tectonic event, but also for the study of the heterogeneity of the upper oceanic mantle from a millimeter to a 100 km scale. The occurrence of two almost parallel ophiolite chains, which have been described to be of different petrography presenting different parts of the upper mantle (MOR vs. SSZ type), allows the investigation of additional aspects of mantle heterogeneity. In this study we want to take advantage of the geochemical characteristics of platinum group elements (PGE) and of lithophile elements to estimate the extant of mantle melting, metasomatic and mixing events of a large portion of mantle obducted contemporaneously. In a first step only peridotites from the mantle sections of the ophiolite complexes are studied for the PGE content and the osmium isotopic composition. Together with major and trace element compositional data, following tasks will be addressed: development of a strategy for field and lab sampling, identification of processes that happened before and after obduction such as melt depletion, metasomatism, serpentinisation etc. and the determination of the size of modified and "pristine" domains. Samples from the western Albanian Ophiolite belt have been studied so far. Although the locations spread over the entire belt a remarkable similarity of PGE abundances is observed. In detail deviations from a correlation of Lu and TiO2 concentration data are also reflected in aberrant mantle normalized PGE patterns. Interestingly enough, this behavior is not manifested in a trend in the 187Os/188Os distribution. As a result the Os isotopic compositions of the entire belt represent the range to be expected from a post Archean upper mantle. The observed heterogeneous distribution of osmium isotopic compositions is most likely an image of the long depletion and incomplete remixing history of the upper Earth's mantle which was not significantly modified through event leading to the formation of ophiolite belts.

Meisel, Thomas; Ginley, Stephen; Koller, Friedrich; Walker, Richard J.

2013-04-01

55

Ophiolites and oceanic plateau remnants (greenstones) in Japan and Far East Russia  

Microsoft Academic Search

In Japan, an older ophiolite thrust onto younger ophiolite with tectonic intercalation of accreted oceanic sediments (chert, limestone, shale and sandstone forming _gocean plate stratigraphy_h deposited on the basaltic basement) or their high-P\\/T metamorphosed varieties. For example, the Yakuno ophiolite (SW Japan) of early Permian igneous age and supra-subduction zone (SSZ) origin (Ichiyama & Ishiwatari, Island Arc, 13, 157-) is

A. Ishiwatari; Y. Ichiyama; K. Koizumi

2005-01-01

56

Architecture of the Oman–UAE ophiolite: evidence for a multi-phase magmatic history  

Microsoft Academic Search

The Oman–United Arab Emirates ophiolite is the world’s largest ophiolite. It is divided into 12 separate fault-bounded blocks,\\u000a of which the northern three lie wholly or partly in the United Arab Emirates. Extensive mapping has shown that the United\\u000a Arab Emirates blocks contain mantle and crustal sections which correspond to the classic ‘Penrose conference’ ophiolite definition\\u000a but which are cut

K. M. Goodenough; M. T. Styles; D. Schofield; R. J. Thomas; Q. C. Crowley; R. M. Lilly; J. McKervey; D. Stephenson; J. N. Carney

2010-01-01

57

Mirdita Zone ophiolites and associated sediments in Albania reveal Neotethys Ocean origin  

Microsoft Academic Search

The Mirdita Ophiolite Zone in Albania is associated with widespread mélanges containing components of up to nappe-size. We\\u000a dated matrix and components of the mélange by radiolarians, conodonts, and other taxa. The components consist of radiolarites,\\u000a pelagic limestones and shallow-water limestones, all of Triassic age, as well as ophiolites. Triassic radiolarite as a primary\\u000a cover of ophiolite material proves Middle

Hans-Jürgen Gawlick; Wolfgang Frisch; Lirim Hoxha; Paulian Dumitrica; Leopold Krystyn; Richard Lein; Sigrid Missoni; Felix Schlagintweit

2008-01-01

58

White K-mica evolution from phengite to muscovite in shales and shale matrix melange, Diablo Range, California  

Microsoft Academic Search

Eleven samples from high-pressure\\/low-temperature (HP\\/LT) shales and shale-matrix melange from four areas in the Diablo Range were studied using electron microprobe (EMP), transmission\\u000a electron microscopy (TEM), and analytical electron microscopy (AEM) to provide information about white K-mica evolution and\\u000a about the controls on illite “crystallinity” (IC) in these areas. The data indicate that: (1) compositional gradients from\\u000a phengitic to muscovitic

M. Dalla Torre; Kenneth J. T. Livi; David R. Veblen; M. Frey

1996-01-01

59

Mantle petrology and mineralogy of the Thetford Mines Ophiolite Complex  

Microsoft Academic Search

The Ordovician Thetford Mines ophiolite complex (TMOC) formed by boninite-fed seafloor-spreading, probably in a fore-arc environment. The mantle section is dominated by foliated harzburgite (?5–6% clinopyroxene), cut by dunitic (±chromitite cores) and orthopyroxenitic veins and dykes. Contrasting structures, textures and mineral compositions allow us to subdivide the mantle. The granular-textured rocks of the Duck Lake Block (DLB) have two steeply-dipping

Philippe Pagé; Jean H. Bédard; Jean-Michel Schroetter; Alain Tremblay

2008-01-01

60

Emplacement of the Jurassic Mirdita ophiolites (southern Albania): evidence from associated clastic and carbonate sediments  

NASA Astrophysics Data System (ADS)

Sedimentology can shed light on the emplacement of oceanic lithosphere (i.e. ophiolites) onto continental crust and post-emplacement settings. An example chosen here is the well-exposed Jurassic Mirdita ophiolite in southern Albania. Successions studied in five different ophiolitic massifs (Voskopoja, Luniku, Shpati, Rehove and Morava) document variable depositional processes and palaeoenvironments in the light of evidence from comparable settings elsewhere (e.g. N Albania; N Greece). Ophiolitic extrusive rocks (pillow basalts and lava breccias) locally retain an intact cover of oceanic radiolarian chert (in the Shpati massif). Elsewhere, ophiolite-derived clastics typically overlie basaltic extrusives or ultramafic rocks directly. The oldest dated sediments are calpionellid- and ammonite-bearing pelagic carbonates of latest (?) Jurassic-Berrasian age. Similar calpionellid limestones elsewhere (N Albania; N Greece) post-date the regional ophiolite emplacement. At one locality in S Albania (Voskopoja), calpionellid limestones are gradationally underlain by thick ophiolite-derived breccias (containing both ultramafic and mafic clasts) that were derived by mass wasting of subaqueous fault scarps during or soon after the latest stages of ophiolite emplacement. An intercalation of serpentinite-rich debris flows at this locality is indicative of mobilisation of hydrated oceanic ultramafic rocks. Some of the ophiolite-derived conglomerates (e.g. Shpati massif) include well-rounded serpentinite and basalt clasts suggestive of a high-energy, shallow-water origin. The Berriasian pelagic limestones (at Voskopoja) experienced reworking and slumping probably related to shallowing and a switch to neritic deposition. Mixed ophiolite-derived clastic and neritic carbonate sediments accumulated later, during the Early Cretaceous (mainly Barremian-Aptian) in variable deltaic, lagoonal and shallow-marine settings. These sediments were influenced by local tectonics or eustatic sea-level change. Terrigenous sediment gradually encroached from neighbouring landmasses as the ophiolite was faulted or eroded. An Aptian transgression was followed by regression, creating a local unconformity (e.g. at Boboshtica). A Turonian marine transgression initiated widespread Upper Cretaceous shelf carbonate deposition. In the regional context, the southern Albania ophiolites appear to have been rapidly emplaced onto a continental margin in a subaqueous setting during the Late Jurassic (Late Oxfordian-Late Tithonian). This was followed by gradual emergence, probably in response to thinning of the ophiolite by erosion and/or exhumation. The sedimentary cover of the south Albanian ophiolites is consistent with rapid, relatively short-distance emplacement of a regional-scale ophiolite over a local Pelagonian-Korabi microcontinent.

Robertson, Alastair H. F.; Ionescu, Corina; Hoeck, Volker; Koller, Friedrich; Onuzi, Kujtim; Bucur, Ioan I.; Ghega, Dashamir

2012-09-01

61

Multi-stage origin of the Coast Range ophiolite, California: Implications for the life cycle of supra-subduction zone ophiolites  

USGS Publications Warehouse

The Coast Range ophiolite of California is one of the most extensive ophiolite terranes in North America, extending over 700 km from the northernmost Sacramento Valley to the southern Transverse Ranges in central California. This ophiolite, and other ophiolite remnants with similar mid-Jurassic ages, represent a major but short-lived episode of oceanic crust formation that affected much of western North America. The history of this ophiolite is important for models of the tectonic evolution of western North America during the Mesozoic, and a range of conflicting interpretations have arisen. Current petrologic, geochemical, stratigraphic, and radiometric age data all favor the interpretation that the Coast Range ophiolite formed to a large extent by rapid extension in the forearc region of a nascent subduction zone. Closer inspection of these data, however, along with detailed studies of field relationships at several locales, show that formation of the ophiolite was more complex, and requires several stages of formation. Our work shows that exposures of the Coast Range ophiolite preserve evidence for four stages of magmatic development. The first three stages represent formation of the ophiolite above a nascent subduction zone. Rocks associated with the first stage include ophiolite layered gabbros, a sheeted complex, and volcanic rocks vith arc tholeiitic or (roore rarely) low-K calc-alkaline affinities. The second stage is characterized by intrusive wehrlite-clinopyroxenite complexes, intrusive gabbros, Cr-rich diorites, and volcanic rocks with high-Ca boninitic or tholeiitic ankaramite affinities. The third stage includes diorite and quartz diorite plutons, felsic dike and sill complexes, and calc-alkaline volcanic rocks. The first three stages of ophiolite formation were terminated by the intrusion of mid-ocean ridge basalt dikes, and the eruption of mid-ocean ridge basalt or ocean-island basalt volcanic suites. We interpret this final magmatic event (MORB dikes) to represent the collision of an active spreading ridge. Subsequent reorganization of relative plate motions led to sinistral transpression, along with renewed subduction and accretion of the Franciscan Complex. The latter event resulted in uplift and exhumation of the ophiolite by the process of accretionary uplift. ?? 2004 by V. H. Winston and Son, Inc. All rights reserved.

Shervais, J. W.; Kimbrough, D. L.; Renne, P.; Hanan, B. B.; Murchey, B.; Snow, C. A.; Zoglman, Schuman, M. M.; Beaman, J.

2004-01-01

62

Geochemistry of the Jurassic Mirdita Ophiolite (Albania) and the MORB to SSZ evolution of a marginal basin oceanic crust  

Microsoft Academic Search

The Middle Jurassic Mirdita Ophiolite in northern Albania is part of an ophiolite belt occurring between the Apulian and Pelagonian subcontinents in the Balkan Peninsula. The upper mantle and crustal units of the Mirdita Ophiolite show major changes in thickness, rock types, and chemical compositions from west to east as a result of its complex evolution in a suprasubduction zone

Yildirim Dilek; Harald Furnes; Minella Shallo

2008-01-01

63

Geodynamic Implications of Jurassic Ophiolites Associated with Island-Arc Volcanics, South Apuseni Mountains, Western Romania  

Microsoft Academic Search

The South Apuseni Mountains are located in the inner zone of the Carpathian belt. This area is characterized by a complex assemblage of nappes, in which Jurassic igneous associations are well represented. New geological and geochemical data on these igneous associations document the occurrence of Middle Jurassic ophiolites overlain by Late Jurassic calc-alkaline volcanic rocks.The ophiolite sequence is characterized by:

Valerio Bortolotti; Michele Marroni; Ionel Nicolae; Luca Pandolfi; Gianfranco Principi; Emilio Saccani

2002-01-01

64

Time Relations and Structural-Stratigraphic Patterns in Ophiolite Accretion, West Central Klamath Mountains, California  

Microsoft Academic Search

New geochronological data and published structural and stratigraphic data show that two distinctly different ophiolitic assemblages formed in general proximity to one another at nearly the same time and were subsequently imbricated along a regional thrust zone. The Josephine ophiolite constitutes a complete oceanic crust and upper mantle sequence which lies within the western Jurassic belt of the Klamath province.

Jason B. Saleeby; Gregory D. Harper; Arthur W. Snoke; Warren D. Sharp

1982-01-01

65

A geotraverse across a deformed Ordovician ophiolite and its Silurian cover, Northern New Brunswick, Canada  

NASA Astrophysics Data System (ADS)

The Ordovician ophiolite, pillow lavas and turbidites found in northern New Brunswick represent a complex interpreted as oceanic crust and trench sediments. The ophiolite is exposed mainly in two inliers, the Belledune and the Elmtree, surrounded by Silurian rocks. In the former the ophiolitic rocks consist essentially of a sheeted dyke complex and a gabbro-diorite, the latter having been injected by veins of trondhjemite. The ophiolite in the Belledune inlier although brecciated on a small scale is not chaotically disrupted. The ophiolite of the Elmtree inlier consists of gabbro and minor ultrabasics, heavily deformed and brecciated with the breccia matrix metamorphosed into amphibolites. Later, low-grade metamorphic effects are superposed over the whole ophiolite complex. Overlying the ophiolite in the Elmtree inlier is a mélange of greywackes and pillow lavas. This mélange shows three periods of deformation all of which are considered pre-Silurian. The same deformation affects the ophiolite. The mélange is separated by an unconformity from the overlying, less deformed, Ordovician mafic lavas and volcanics, which are known as the Pointe Verte formation. The unconformable Silurian strata cut across the Ordovician formations, and have a single cleavage, which rarely penetrates the Ordovician rocks. These relationships are used to develop a plate tectonic model which is applied to Gaspé and New Brunswick. The model involves the subduction of a mid-ocean ridge and results in polyphase Taconian deformation events and a single phase Acadian episode. The lapetus ocean in the northern Appalachians essentially closed by Middle Ordovician times.

Rast, N.; Stringer, P.

1980-11-01

66

Back-arc basin origin for the East Sulawesi ophiolite (eastern Indonesia)  

Microsoft Academic Search

The East Sulawesi ophiolite is one of the three largest ophiolites in the world. It displays all the components of a typical sequence, from residual mantle peridotites to cumulate gabbros, sheeted dolerites, and lavas of normal mid-oceanic-ridge basalt (MORB) composition. Trace element data on the lavas and dolerites, and particularly their depletion in Nb compared to neighboring incompatible elements, suggest

Christophe Monnier; Jacques Girardeau; René C. Maury; Joseph Cotten

1995-01-01

67

The osmium isotopic composition of convecting upper mantle deduced from ophiolite chromites  

Microsoft Academic Search

Chromites separated from the upper mantle or lower crustal portions of 18 ophiolites ranging in age from 900 Ma to 50 Ma are examined for Re-Os isotopic systematics. The ophiolites include both MORB and back arc types, although most are from supra-subduction zone (SSZ) settings. The chromites are robust indicators of the initial Os isotopic compositions of the systems sampled.

Richard J. Walker; Hazel M. Prichard; Akira Ishiwatari; Márcio Pimentel

2002-01-01

68

Sr-Nd-Pb Isotope Geochemistry of Melange Formation: Implications for Identification of Fluid Sources in the Mantle Wedge and the Arc  

NASA Astrophysics Data System (ADS)

Paramount to our ability to decipher the behavior of fluids and melts within the mantle wedge and the overall subduction system are the chemical compositions of rocks adjacent to the slab-mantle interface. Profound metamorphic and metasomatic alteration of pre-subduction lithologies to form melange along the slab-mantle interface may yield rock types inheriting mixed chemical compositions of diverse pre-subduction lithologies. Early work on melange geochemistry indicates competitive effects between mechanical mixing, metasomatism by fluids or melts, and mineral stabilities imposed by the resulting bulk composition. We have explored the Sr-Nd-Pb isotope geochemistry of low- to high-grade melange zones in the Catalina Schist, CA, to address this crucial missing component in studies of subduction-zone mass flux. The Catalina Schist contains lawsonite-albite (LA), lawsonite-blueschist (LB), and amphibolite (AM) facies melange zones, all with mineralogy dominated by talc, chlorite, and Na-Ca amphiboles, with additional minerals such as micas, rutile, zircon, and apatite stabilized based on bulk sample chemistry. Major element compositions vary, from strongly ultramafic in the AM melange, to more crustal-like compositions (i.e., more reminiscent of basaltic to sedimentary protoliths) for LA and LB melange. However, initial Sr and Nd isotope ratios for all grades of melange are essentially indistinguishable, displaying a wide variation from 87Sr/86Sr=0.703-0.709 and ? Nd= +15 to -15. Covariations are generally negative, similar to that of the mantle array, but with some samples extending to higher Sr ratios at constant ? Nd that probably reflect inheritance of seawater Sr. No clear mixing relationships between 87Sr/86Sr and 1/Sr exist, suggesting either localized buffering of Sr isotope ratios or that mixing relations are obscured by secondary devolatilization. However, a clear mixing trend for Nd indicates two end-members, one a high-concentration, positive ? Nd source (AOC?), the other with low-concentration and negative ? Nd (devolatilized sediments?). Likewise, initial Pb isotope ratios for all grades of melange form a single array independent of rock type or inferred protolith. Melange matrix of the Catalina Schist preserves initial 206Pb/204Pb of 18.95-19.59, 207Pb/204Pb of 15.61-15.68, and 208Pb/204Pb of 37.85-39.05. Such elevated Pb ratios are typical of subducting oceanic sediments, but not of MORB-like oceanic crust or peridotites of the depleted mantle. The similarity of these initial ratios suggests pervasive alteration of Pb isotope signatures within diverse rock types by fluids during subduction. As Pb concentrations decline from LA/LB to AM melange, this suggests devolatilization of Pb from the ultramafic AM melange will transfer crustal-like Pb isotope ratios. Sr-Nd-Pb isotope systematics for arc volcanic rocks are commonly used as indicators of fluid sources from the subducting slab to the arc magma source region. Our results suggest such an assumption is extremely dangerous, as hybridization processes common to melange zones are more likely to occur along the slab-mantle interface than is preservation of a pre-subduction section. Such metamorphic mediation and buffering of "slab" compositions is essentially unknown, yet our data support an interpretation where these processes impart a fundamental control on the chemistry of fluids passed to the mantle wedge.

Bebout, G. E.; King, R. L.; Moriguti, T.; Nakamura, E.

2004-12-01

69

Supersilicic clinopyroxene from Nidar Ophiolite ultramafics, Ladakh, India  

NASA Astrophysics Data System (ADS)

We discovered silica exsolution lamellae in clinopyroxenes from pyroxenites of Nidar ophiolite ultramafics, Indus Suture Zone in Eastern Ladakh, India. These signify former existence of supersilicic clinopyroxene, an indicator of deep mantle condition. The ophiolite ultramafic suit consists of varieties of peridotites (dunite, chromite, harzburgite etc.) and intrusive pyroxenites. The pyroxenites are heterogeneously altered and dominated in clinopyroxene (>70%) with minor carbonates. Recently numerous silica inclusions and lamellas are found in clinopyroxenes of pyroxenites. The lamellas have been classified on the basis of their various modes of occurrences, size and shape from different clinopyroxenes - (1) rectilinear, strongly oriented rods (2) straight needles of greater size and weaker orientation with respect to type 1 (3) curved lamellas with wider middle part and tapering ends restricted in outer margin of clinopyroxene. All the lamellas display diffused grain boundaries with the host clinopyroxenes. On the basis of higher silica content of the host clinopyroxene compared to the silica free matrix it is interpreted that silica lamellas are exsolution products from break down of primary supersilicic Ca - Eskola end member. The exsolved silica bearing clinopyroxenes are showing increase in concentration of Si along with K which is potential function of pressure in clinopyroxene (Shimizu 1971). Besides these, a quartz inclusion in clinopyroxene show palisade texture with radiating fractures in rim and a polycrystalline core, which may indicate phase transition of higher silica polymorphs during decompression. With the given line of evidences, the Himalayan Ophiolite may become a potential window for deep mantle study and could be equivocal for ultra-high pressure origin. Reference: Shimizu N, EPSL, 11, 374-380; 1971.

Das, S.; Mukherjee, B. K.

2011-12-01

70

Fully carbonated peridotite (listvenite) from the Samail ophiolite, Oman  

NASA Astrophysics Data System (ADS)

Extensive outcrops of listvenite--fully carbonated peridotite, with all Mg in carbonate minerals and all Si in quartz--occur along the basal thrust of the Samail Ophiolite in Oman. The presence of these listvenites demonstrates that peridotite carbonation reactions can proceed to completion on a large scale under natural conditions. Thus, understanding the conditions of listvenite formation can provide insights into the feasibility of achieving complete carbonation of peridotite through engineered approaches for carbon capture and storage. The Oman listvenites likely formed during emplacement of the ophiolite, as CO2-bearing fluids derived from underlying metasediments reacted with peridotite in the hanging wall. Listvenite outcrops occur within 500 meters of the basal thrust, where peridotite overlies carbonate-bearing metasediments. 87Sr/86Sr values in listvenite are higher than seawater values and consistent with values in these underlying metasediments. Carbon and oxygen stable isotope data are also consistent with values in some of the metasediments. An internal Rb-Sr isochron from one listvenite sample yields an age of 97 × 29 Ma, consistent with the timing of emplacement of the ophiolite. Carbonate clumped isotope thermometry in listvenites yields temperatures around 100°C, and thermodynamically stable coexistence of antigorite, talc, and quartz in serpentinite along the margins of the listvenite would require reaction temperatures around 80°C, as calculated in THERMOCALC. While constraints on the pressure of listvenite formation are lacking, these moderate temperatures suggest that listvenites probably formed at relatively shallow depths, making release of carbonate-saturated pore-water due to compaction of subducted sediment or low-pressure phase transitions of hydrous minerals more probable sources of the CO2-bearing fluid than deeper metamorphic reactions without significant transport of fluids along the slab interface. Through EQ3/6 geochemical reaction path models of these processes over a range of pressures, temperatures, and water-rock ratios we explore the conditions under which listvenite could have formed.

Falk, E. S.; Kelemen, P. B.

2013-12-01

71

Spring and surface water quality of the Cyprus ophiolites  

Microsoft Academic Search

A survey of surface, spring and borehole waters associated with the ophiolite rocks of Cyprus shows five broad water types (1) Mg-HCO3, (2) Na-SO4-Cl-HCO3, (3) Na-Ca-Cl-SO4-OH-CO3, (4) Na-Cl-SO4 and (5) Ca-SO4. The waters represent a progression in chemical reactivity from surface waters that evolve within a groundwater setting due to hydrolysis of the basic\\/ultrabasic rock as modified by CO2-weathering. An

Colin Neal; Paul Shand

2002-01-01

72

Etude des melanges co-continus d'acide polylactique et d'amidon thermoplastique (PLA/TPS)  

NASA Astrophysics Data System (ADS)

Les melanges co-continus sont des melanges polymeriques ou chaque composant se trouve dans une phase continue. Pour cette raison, les caracteristiques de chacun des composants se combinent et il en resulte un materiau avec une morphologie et des proprietes particulieres. L'acide polylactique (PLA) et l'amidon thermoplastique (TPS) sont des biopolymeres qui proviennent de ressources renouvelables et qui sont biodegradables. Dans ce projet, differents melanges de PLA et TPS a une haute concentration de TPS ont ete prepares dans une extrudeuse bi-vis afin de generer des structures co-continues. Grace a la technique de lixiviation selective, le TPS est enleve pour creer une structure poreuse de PLA qui a pu etre analysee au moyen de la microtomographie R-X et de la microscopie electronique a balayage MEB. L'analyse des images 2D et 3D confirme la presence de la structure co-continue dans les melanges dont la concentration en TPS. se situe entre 66% et 80%. L'effet de deux plastifiants, le glycerol seul et le melange de glycerol et de sorbitol, dans la formulation de TPS est etudie dans ce travail. De plus, nous avons evalue l'effet du PLA greffe a l'anhydride maleique (PLAg) en tant que compatibilisant. On a trouve que la phase de TPS obtenue avec le glycerol est plus grande. L'effet de recuit sur la taille de phases est aussi analyse. Grace aux memes techniques d'analyse, on a etudie l'effet du procede de moulage par injection sur la morphologie. On a constate que les pieces injectees presentent une microstructure heterogene et differente entre la surface et le centre de la piece. Pres de la surface, une peau plus riche en PLA est presente et les phases de TPS y sont allongees sous forme de lamelles. Plus au centre de la piece, une morphologie plus cellulaire est observee pour chaque phase continue. L'effet des formulations sur les proprietes mecaniques a aussi ete etudie. Les pieces injectees dont la concentration de TPS est plus grande presentent une moindre resistance a la traction. La presence du compatibilisant dans la region co-continue affecte negativement cette resistance. En considerant que l'amidon est un biomateriau abondant, moins cher et plus rapidement biodegradable, son ajout dans le PLA presente l'avantage de reduire le cout tout en augmentant la vitesse de degradation du PLA. De plus, une structure continue poreuse de PLA produit par la technique de lixiviation selective a des applications potentielles soit comme materiau a degradation rapide ou encore, une fois la phase TPS retiree, comme substrat a porosite ouverte pour la fabrication de membranes, de supports cellulaires ou de filtres. Mots-cles : melanges immiscibles, acide polylactique, amidon thermoplastique, morphologie cocontinue, lixiviation selective, microtomographie R-X, materiau rigide poreux biodegradable.

Chavez Garcia, Maria Graciela

73

Varying mantle sources of supra-subduction zone ophiolites: REE evidence from the Zambales Ophiolite Complex, Luzon, Philippines  

NASA Astrophysics Data System (ADS)

The Zambales Ophiolite Complex, a supra-subduction zone ophiolite, is characterized by three volcanic-hypabyssal rock units: the Coto Block volcanic-hypabyssal rocks, the Coto dikes intruded into the Coto residual peridotites and the Acoje Block volcanic-hypabyssal rocks. The first two groups exhibit transitional mid-ocean ridge-island arc characteristics while the latter reveals island arc affinity. Furthermore, these three volcanic rock suites are characterized by differing bulk REE, major-element, trace-element and mineral chemistries. The Coto Block volcanic-hypabyssal rocks [ {(Ce}/{Yb) n} 0.4-1.0; TiO 2 0.50-1.50 wt.%; Zr 31-76 ppm; Y 13-31 ppm] have higher REE, Ti, Zr and Y than the Coto dikes [ {(Ce}/{Yb) n} 0.3-0.8; TiO 2 0.52-0.94 wt.%; Zr 15-55 ppm; Y 10-27 ppm] and the Acoje Block volcanic-hypabyssal rocks [ {(Ce}/{Yb) n} 0.2-0.3; TiO 2 0.26-0.86 wt.%; Zr 11-45 ppm; Y 10-23 ppm] arguing for an increasing degree of melting of the sources from the former to the latter. These data suggest the involvement of several mantle sources which have undergone different degrees of partial melting and LREE addition as evidenced by the presence of LREE-enriched basalts [ {(Ce}/{Yb) n} 3-5] among the Coto Block volcanic-hypabyssal rocks. These results further illustrate the complexities involved in the generation and evolution of supra-subduction zone ophiolites.

Yumul, G. P.

1996-09-01

74

Cretaceous radiolarians from Baliojong ophiolite sequence, Sabah, Malaysia  

NASA Astrophysics Data System (ADS)

The Baliojong ophiolite sequence exposed along the Baliojong River in Northern Sabah consists of volcanic rocks, mostly basalts, overlain by sedimentary layers consisting of well-bedded cherts, mudstones and sandstones. The ophiolite sequence occurs as steeply-dipping overturned thrust slices oriented approximately north-south. A total of 42 chert samples were collected from the sedimentary layers. However, most of the samples contain poorly preserved radiolarians. Only nine samples yielded moderately well-preserved radiolarians from three selected thrust slices. A total of 32 taxa were identified. Based on the stratigraphic distribution of selected taxa, the radiolarians can be divided into two assemblage zones. The first assemblage zone is Dictyomitra communis Zone characterized by the occurrence of Dictyomitra communis, Archaeodictyomitra (?) lacrimula, Sethocapsa (?) orca, Dictyomitra pseudoscalaris, and Pantanellium squinaboli. The assemblage indicates Barremian to Aptian in age. The second assemblage zone Pseudodictyomitra pseudomacrocephala Zone contains Pseudodictyomitra pseudomacrocephala, Dictyomitra gracilis, Dictyomitra montesserei, Xitus mclaughlini, and Dictyomitra obesa. This assemblage indicates an age of Albian and the presence of Pseudodictyomitra tiara suggests the age may extend up to Cenomanian. Each thrust slice yielded more or less similar radiolarian assemblages indicating that they all came from the same sedimentary layers.

Jasin, Basir; Tongkul, Felix

2013-10-01

75

Radiolarian biostratigraphic data from the Casiguran Ophiolite, Northern Sierra Madre, Luzon, Philippines: Stratigraphic and tectonic implications  

NASA Astrophysics Data System (ADS)

Results from the first detailed radiolarian biostratigraphic study conducted in Luzon are reported. The data were obtained from cherts associated with the Casiguran Ophiolite, a dismembered ophiolite mass consisting of serpentinized peridotites, gabbros, dolerite dikes and pillow basalts exposed along the eastern coast of the Northern Sierra Madre, Luzon, Philippines. Cherts and limestone interbeds conformably overlie the ophiolite. The radiolarian assemblages from the cherts constrain the stratigraphic range of the cherts to the Lower Cretaceous (upper Barremian-lower Aptian to Albian). This new biostratigraphic result is in contrast with the Upper Cretaceous stratigraphic range previously reported in the region. Radiolarian biostratigraphic results from the Casiguran Ophiolite provide additional evidence for the existence of Mesozoic oceanic substratum upon which Luzon and neighboring regions within the Philippine archipelago were likely built. Interestingly, the result closely resembles those reported for the ophiolite in southeastern Luzon as well as the oceanic crust of the Huatung Basin situated east of Taiwan and the ophiolites in eastern Indonesia. In light of this, along with previously gathered geochemical data from the ophiolites, a common provenance is being looked into for these crust-upper mantle sequences in the western Pacific region.

Queano, Karlo L.; Marquez, Edanjarlo J.; Aitchison, Jonathan C.; Ali, Jason R.

2013-03-01

76

Oceanic core complexes and ancient oceanic lithosphere: Insights from Iapetan and Tethyan ophiolites (Canada and Albania)  

NASA Astrophysics Data System (ADS)

Oceanic ridges are extensional environments characterized by brittle to ductile normal faulting and episodic magma supply. Studies in modern oceanic settings suggest that the interplay of low-angle detachments and high-angle normal faults locally lead to the exhumation of lower crust and upper mantle peridotites to the seafloor as oceanic core complexes. Along the Mid-Atlantic ridge, core complexes typically occur in inside corners of intersections between the ridge axis and major transform faults. The core complexes were exhumed by movement along long-lived detachment faults that extend for 10s of km, and that are crosscut by high-angle normal faults oriented parallel to the rift axis. Ridge segments with abundant detachments would be characterized by lithosphere having little or no upper crust. Similar features in ophiolites may also reflect the dominance of amagmatic extension. This contribution presents two examples of fossilized oceanic core complexes preserved in the Canadian Appalachian and Albanian Dinaride ophiolite belts. In the Canadian Appalachians, the Southern Quebec ophiolites experienced two Paleozoic orogenies after their obduction onto the Laurentian margin. Although locally obscured by tectonic fabrics and structures, the original relationships between the ophiolitic mantle, the overlying plutonic section, and onlapping Ordovician siliciclastic rocks are well-preserved in the Thetford-Mines ophiolite which also shows similarities with lithological and structural features of the Mirdita ophiolite, where conformable contacts between the sedimentary cover sequence (debris flows and flysch deposits) and the mantle and crustal sections are superbly exposed. The Jurassic Mirdita ophiolite in Albania underlies ca. 4000 km 2 and was only slightly affected by the Tertiary Alpine orogeny of Eastern Europe. Well-exposed sections of the ophiolite suggest that the oceanic mantle was exhumed prior to obduction and locally records the formation of an oceanic core complex. Mafic cumulates of the Mirdita ophiolite have been totally excised in places, and exhumation-related shear zones are preserved in the Krabbi-Puka massif. The shear zones are manifested both within the mantle and at the contact with overlying basalts. The ophiolitic sedimentary sequence was deposited on basalts, sheeted dykes or gabbros, and typically consists of debris flows intercalated with, and overlain by slumped oceanic sedimentary rocks. The lithological and structural characteristics of both the Thetford-Mines and the Mirdita ophiolites are consistent with extensional tectonism, crustal excision and lower crust/upper mantle exhumation during formation. Both ophiolites are believed to have originated during asymmetrical oceanic extension in a suprasubduction zone setting, with preservation of the upper-plate/lower-plate segments during obduction.

Tremblay, Alain; Meshi, Avni; Bédard, Jean H.

2009-07-01

77

Oceanic Detachment Faults Observed in the Troodos Ophiolite, Cyprus  

NASA Astrophysics Data System (ADS)

In the Limassol Forest area south of the Arakapas Transform Fault in the Troodos Ophiolite, structural relationships are much more complex than north of the transform fault. To the north, the ophiolite has a classic layer-cake structure of volcanics above sheeted dykes above gabbros above peridotites. To the south the same units are present, but are juxtaposed in a much more complex way. Two domal core complexes of serpentinised peridotites are capped by major low-angle extensional faults separating the core complexes from overlying slices of sheeted dykes, gabbros and volcanics. Stratigraphic arguments show that the extensional faulting can be shown to be contemporary with the construction of the ophiolitic crust. Fragments of serpentinite are found in clastic sediment that lies between lava flows a few kilometres south of the largest of the core complexes, and are also found in coarse clastic sediment overlain by a lava flow within the transform fault trough. In addition the core complexes are cut by small plutons and dyke swarms that were intruded after serpentinisation of the peridotite was complete. As is common with serpentinite, small-scale structures yield ambiguous indicators of movement direction, but larger-scale structures, such as a westerly structural deepening of individual units within the hangingwall blocks, suggest a generally top to the west sense of shear. East-west trending ridges in the outcrop of one of the fault zones on a scale of tens to hundreds of metres may be corrugations similar to those seen on oceanic detachment faults. One of the core complexes was not exposed at the ancient ocean floor, but remained covered by hangingwall blocks, raising the possibility of similar hidden core complexes within the ocean crust. Shear zones associated with the detachment faults contain tremolite-chlorite assemblages replacing peridotite that are texturally and mineralogically similar to detachment fault rocks from the Mid-Atlantic Ridge. Basaltic rocks from the hangingwall are altered to tremolite, chlorite, epidote, quartz and locally hydrogarnet, with plagioclase often completely chloritised. These observations are consistent with fluid flow along the detachment under greenschist facies conditions, with chemical exchange between mafic and ultramafic rocks.

Cann, J.; McCaig, A.

2005-12-01

78

Submarine hydrothermal metamorphism of the Del Puerto ophiolite, California.  

USGS Publications Warehouse

Metamorphic zonation overprinted on the volcanic member and overlying volcanogenic sediments of the ophiolite complex increases downward in grade and is characterized by the sequential appearance with depth of zeolites, ferric pumpellyite and pistacitic epidote. Metamorphic assemblages of the plutonic member of the complex are characterized by the presence of calcic amphibole. The overprinting represents the effects of hydrothermal metamorphism resulting from the massive interaction between hot igneous rocks and convecting sea-water in a submarine environment. A thermal gradient of 100oC/km is postulated to account for the zonal recrystallization effects in the volcanic member. The diversity and sporadic distribution of mineral assemblages in the amphibole zone are considered due to the limited availability of H2O in the deeper part of the complex. Details of the zonation and representative microprobe analyses are tabulated.-M.S.

Evarts, R. C.; Schiffman, P.

1983-01-01

79

Scientific Drilling on the Diamond-Bearing Luobusa Ophiolite, Tibet  

NASA Astrophysics Data System (ADS)

The "Selecting Continental Scientific Drilling Sites and Experimenting with Drilling Technology Project" is the fifth project of the SinoProbe Program (SinoProbe-5). This project will focus on 7 critical tectonic and mineral resource regions, including the Jinchuan Cu-Ni sulfide deposits in Gansu province, the Luobusa ophiolite in Tibet, the Tengchong volcano-thermal tectonic zone in Yunnan, the boundary of the North China and South China blocks in the Laiyang basin of Shandong province, the Yudu-Ganxian polymetallic deposits in Jiangxi province, the Tongling polymetallic deposit and the Luzhong volcanic basin and mineral deposit district in Anhui province. Scientific drilling, along with geological and geophysical investigations, will help to reveal the composition and structure of the continental crust, verify the results of geophysical explorations and establish standards for deep geophysical exploration. On the basis of pilot-hole drilling, surface studies and geophysical investigations, the Luobusa scientific drilling project is focused on the Luobusa ophiolitic diamond-bearing ultramafic massif along the Yarlung-Zangbo suture between the Indian and Eurasia plates. The main purposes of this work are to investigate the distribution and conditions of formation UHP mantle minerals and podiform chromitites in ophiolites and to develop prospecting criteria for ophiolite-type chromite deposits. The Luobusa pilot-hole (LBS-ZK1) was completed in 2010 to a depth of 1478.8 m with an average core recovery of 93.6%. The second hole (LBS-ZK2) reached a depth of 967.5 m on August 1, 2011 with an average core recovery of about 90%. The ultimate target depth for this hole is 2000 m. Hole LBS-ZK2 penetrated 70 m of Triassic sandstone, marble and chlorite schist strata above the ultramafic body. The contact between the two is a fault marked by extensive shearing and serpentinization but lacking evidence of thermal metamorphism. A preliminary profile of the ultramafic rocks in hole LBS-ZK1 shows that the sequence can be subdivided into three main sections. The first 1260 m consist chiefly of harzburgite with minor dunite, representing a depleted mantle sequence. The lower part of this sequence (from 890 m to 1260 m) is highly serpentinized. The second sequence, from 1260 m to 1414 m, consists mainly of dunite with minor harzburgite, probably representing a cumulate ultramafic section. From 1414 m to the base of the hole is a 350-m-thick sequence of cumulate gabbro. The presence of gabbro beneath the ultramafic rocks without a fault contact supports the interpretation that the entire sequence has been overturned, as suggested by earlier field studies. Detailed studies in mineralogy, petrology, geochemistry and isotope dating are undergoing and will be basically presented.

Yang, J.; Li, J.; Xu, X.; Ba, D.; Li, Z.; Li, Y.; Zhao, L.

2011-12-01

80

Age of amphibolites associated with alpine peridotites in the Dinaride ophiolite zone, Yugoslavia  

USGS Publications Warehouse

Amphibolites associated with alpine peridotites in the Central Ophiolite zone in Yugoslavia have K-Ar ages of 160-170 m.y. These amphibolites and associated peridotites underwent deep-seated metamorphism prior to tectonic emplacement into the sedimentary-volcanic assemblage of the Dinarides. The alpine peridotites and associated local rocks of the ophiolite suite are interpreted as Jurassic oceanic crust and upper mantle. ?? 1975.

Lanphere, M. A.; Coleman, R. G.; Karamata, S.; Pamic, J.

1975-01-01

81

Platinum-group elements for the mantle peridotites in the Dazhuka ophiolite, Tibet, China  

Microsoft Academic Search

The total PGE amounts of mantle peridotites in the Dazhuka ophiolite, Tibet, are 28.37–50.67 ng\\/g, slightly higher than those\\u000a of mantle peridotites in the primitive mantle, and typical ophiolites in the world, and the Alps-type mantle peridotites.\\u000a The PGE distribution patterns in the Dazhuka mantle peridotites are also different from those of the mantle peridotites of\\u000a partial melting relict origin.

Hengxiang Yu; Bin Xia; Houjun Mei; Linzhi Guo; Liang Qi; Xianglin Tu

2001-01-01

82

Preservation of biosignatures in metaglassy volcanic rocks from the Jormua ophiolite complex, Finland  

Microsoft Academic Search

Evidence of microbially-mediated alteration of basaltic glass is preserved in originally glassy basalts (rims of pillow lavas, hyaloclastite breccias, and chilled margins of dykes) from the well-preserved 1.95Ga Jormua ophiolite complex (JOC) of Northeastern Finland. Although textural evidence of microbial alteration is commonly observed in relic glass from recent oceanic crust and some ophiolites, these textures have been destroyed during

Harald Furnes; Neil R. Banerjee; Karlis Muehlenbachs; Asko Kontinen

2005-01-01

83

Tectonic significance of the geochemistry and petrology of ophiolites in southeast Anatolia, Turkey  

Microsoft Academic Search

The southeast Anatolian ophiolites form discontinuous linear belts of oceanic fragments immediately north of the Bitlis–Zagros suture, which marks a continental collision zone between the Arabian platform to the south and the Taurides to the north. The Late Cretaceous ophiolites in the southeast Anatolia are represented by the K?z?lda? in the Hatay area, the Göksun and the Berit to the

Osman Parlak; Tamer Rizaoglu; Utku Bagci; Fatih Karaoglan; Volker Höck

2009-01-01

84

Primitive basaltic melts included in podiform chromites from the Oman Ophiolite  

Microsoft Academic Search

In an attempt to characterize the composition of the parental melts of ophiolitic chromitites and their tectonic setting, we have undertaken a study of polymineralic solid inclusions trapped in chromites from the Oman Ophiolite (Sumail nappe). High-temperature experiments performed on inclusions show that they result from post-entrapment crystallization of homogeneous basaltic melts with primitive compositions (Mg# = 63.5–66.8). The primary

Pierre Schiano; Robert Clocchiatti; Jean-Pierre Lorand; Dominique Massare; Etienne Deloule; Marc Chaussidon

1997-01-01

85

Selenium content of sulfide ores related to ophiolites of Greece.  

PubMed

Several deposits of sulfide mineralization have been described in the ophiolites of Greece. Based on their mineralogical and chemical composition and the host rocks, two types can be distinguished: (1) the Fe-Cu-Ni-Co type consisting of pyrrhotite, chalcopyrite, Co-pentlandite, pyrite, magnetite + arsenides, +/- chromite, hosted in serpentinites, gabbros or diabases, which have variable geochemical characteristics, and (2) sulfide mineralization of the Cyprus type containing variable proportions of pyrite, chalcopyrite, bornite, and sphalerite. The spatial association with shear zones and fault systems, which is a common feature in both types of mineralization, provided the necessary permeability for the circulation of the responsible mineralized hydrothermal fluids. The selenium (Se) content in representative samples of both types of mineralization from the ophiolites of Pindos (Kondro, Perivoli, and Neropriona), Othrys (Eretria and A. Theodoroi), Veria (Trilofon), and Argolis (Ermioni) shows a wide variation. The highest values of Se (130 to 1900 ppm) were found in massive Fe-Cu sulfide ores from Kondro, in particular the Cu-rich portions (average 1300 ppm Se). The average values of Se for the Othrys sulfides are low (< 40 ppm Se). The Se content in a diabase breccia pipe (50 x 200 m) with disseminated pyrite mineralization (Neropriona) ranges from < 1 to 35 ppm Se. The highest values were noted in strongly altered samples that also exhibited a significant enrichment in platinum (1 ppm Pt). Sulfide mineralization (irregular to lens-like masses and stringers) associated with magnetite, hosted in gabbros exposed in the Perivoli area (Tsouma hill), shows a content ranging from 40 to 350 ppm Se. The distribution of Se in the studied type of the sulfide mineralization may be of genetic significance, indicating that the Se level, which often is much higher than in typical magmatic sulfides related to mafic-ultramafic rocks (average 90-100 ppm Se), may positively affect the environment. PMID:9726791

Economou-Eliopoulos, M; Eliopoulos, D G

1998-01-01

86

Frictional Strengths of SAFOD Core and Franciscan Melange Samples at Elevated Temperatures  

NASA Astrophysics Data System (ADS)

We report the first frictional strength data at elevated temperatures for core from the San Andreas Fault Observatory at Depth (SAFOD), comparing the behavior of the two, meters-wide foliated gouge zones where fault creep is localized [central deforming zone (CDZ) and southwest deforming zone (SDZ)] with that of the sedimentary/metasedimentary wall rocks. One sample each from the CDZ (3296.6 m) and SDZ (3197.2 m) was tested. The matrix of the CDZ gouge sample is dominated by Mg-rich, saponitic smectite clays and that of the SDZ gouge by corrensitic + saponitic clays. Two quartzofeldspathic rock types, both relatively enriched in phyllosilicates, were selected for comparison. One is a SAFOD core sample (3295.3 m) from the siltstone unit immediately southwest of the CDZ, considered to be part of the Great Valley Group. Phyllosilicate minerals in this siltstone consist largely of illitic, chloritic, and mixed-layer clays. The other is a surface outcrop of the chlorite + phengite-rich matrix of melange from the Franciscan Complex, a major rock unit associated with the San Andreas Fault (SAF) in central and northern California. The starting materials were disaggregated and sieved to produce synthetic gouge samples for testing. Temperatures ranged from 25 to 250°C and sliding velocities along the sawcut from 0.00115 to 1.15 ?m/s. The slowest rate corresponds to a shearing rate of ~36 mm/yr. Fluid pressure and effective normal stress were held constant at 60 and 100 MPa, respectively. The CDZ and SDZ gouges showed very similar behavior, with a coefficient of friction, ? < 0.2 at all experimental conditions, and no temperature-related variations in strength were discerned. Both gouges were velocity strengthening [i.e., (a-b) > 0] at all temperatures. The CDZ shows no trend in (a-b) with temperature or velocity, whereas (a-b) values of the SDZ sample decrease slightly with increasing temperature and decreasing velocity. In contrast, the Great Valley and Franciscan samples showed ? > 0.35 at all tested conditions. No temperature dependence of strength was found between 25 and 150°C, but ? increases markedly at higher temperatures for both rock types, exceeding 0.5 at 250°C. In addition, (a-b) values decrease progressively with increasing temperature and decreasing velocity, with velocity-weakening behavior beginning at 150°C and stick-slip motion at 250°C. Despite their relatively high phyllosilicate contents, the tested Franciscan and Great Valley rocks have the potential for seismic behavior at depth and they contrast markedly with the foliated gouge zones, which remain weak and stably sliding.

Moore, D. E.; Lockner, D. A.; Morrow, C. A.; Hickman, S.

2012-12-01

87

Alpine ophiolites: product of low-degree mantle melting in a Mesozoic transcurrent rift zone  

NASA Astrophysics Data System (ADS)

The Alpine ophiolites and their original sedimentary cover (Upper Jurassic radiolarite-limestone with ophiolite breccia) occur as dismembered tectonic slices (often inverted by Alpine tectonics) or olistoliths among the calc-schists (Cretaceous?). Original petrologic features of the ophiolites are largely masked by the Alpine metamorphism, but are discernible by the relic-mineral chemistry and petrographic reconstruction. The residual peridotite is generally plagioclase lherzolite resembling those from oceanic fracture zones, but spinel lherzolite is present at Davos (Switzerland) and Péas (France). The latter is characterized by higher Na 2O (1.6%) of clinopyroxene and lower Cr 2O 3 (8%) of spinel than the former ( < 0.7% and > 30%, respectively), and resembles subcontinental lherzolites. Geologic evidence indicates that these two lherzolites were together exposed on the Jurassic ocean floor. The mafic-ultramafic cumulates are of plagioclase-type characterized by the scarcity of orthopyroxene and high TiO 2 (avg. 0.8%) of clinopyroxene. The basaltic volcanics are alkalic rather than tholeiitic in respect to their relic clinopyroxene, which is as rich in TiO 2 and Na 7O as that in Hawaiian alkali basalt. The Alpine ophiolite association of lherzolitic residual peridotite, plagioclase-type cumulates, and alkalic basalt is reasonable as cogenetic products of low-degree partial melting in the mantle. Geologic and petrologic features of the Alpine ophiolites are consistent with a geotectonic model postulating the generation of the Alpine ophiolites in an early Mesozoic rift zone cut by many transform faults.

Ishiwatari, Akira

1985-12-01

88

The supra-ophiolitic sedimentary cover of the Asbestos ophiolite, Québec, Canada: First geochemical evidence of transition from oceanic to continental sediment flux  

NASA Astrophysics Data System (ADS)

The 473 + 5/- 3 Myr Asbestos ophiolite complex of the Quebec Appalachians was formed in a forearc basin and obducted on a margin of Laurentia ~ 460 Myr ago. The complex together with its sedimentary cover is well exposed at Burbank Hill (~ 130 km SW of Québec City) where eight distinct lithologies have been identified: 1) pyroxenites and wehrlites with minor dunitic layers, (2) fractured gabbros, (3) breccia containing gabbro and diabase fragments, (4) polygenic conglomerates, (5) red mudstone/chert/sandstone succession, (6) tuffs with intercalated grey chert, (7) greenish grey mudstones and (8) slates and sandstones of the Saint-Daniel Mélange. The gabbroic and mafic volcanic rocks show the characteristics of arc tholeiites (TiO 2 ~ 0.8-0.85 wt.%) and boninites (TiO 2 < wt. 0.35% and Mg# > 45; (La/Sm) N ~ 1.9-3.3). These rock-types also occur as mafic fragments in the polygenic conglomerate. The mafic fragments (crystals and rocks) within the conglomerate and the ophiolitic sandstones also show the same greenschist facies metamorphism as the mafic igneous rocks of the complex. The conglomerate was probably formed after the forearc crust was fragmented by pre-obduction normal faults. These fault scarps would have promoted the erosion of the oceanic crust, leading to the accumulation of diverse detritus in grabens. The sedimentary lithologies which overlie the conglomerate exhibit a gradual increase in continent-derived material ((La/Sm) N ~ 5 and La/Yb ~ 20) and a decrease in ophiolitic material moving upward in the stratigraphic sequence. The sedimentary rocks at the bottom of the sequence (group 3) are mostly composed of ophiolite material with only 20% of crustal material. They also have a high content of Cr and MgO with values of 350-480 ppm and 5.7-8%, respectively. The rocks of the uppermost part of the pile (group 1) which contain up to 80% crustal material have low abundances of Cr and MgO (30-100 ppm and 2.7-4.5 wt.%). Continental detritus on the top of the Asbestos ophiolite suggests the complex formed near the Laurentian continental margin and that the eroded continental material had access to the basin where the ophiolite was formed. This is consistent with the obduction of the Asbestos and Thetford-Mines ophiolites on the Laurentian margin shortly after their formation. The proximity of thick Laurentian continental crust near the trench could explain why the subduction zone blocked-up soon after the ophiolites formation.

Bédard, É.; Hébert, R.; Guilmette, C.; Dostal, J.

2008-10-01

89

Discovery of Middle Jurassic (Bajocian) Radiolaria from the sedimentary cover of the Vedi ophiolite (Lesser Caucasus, Armenia)  

Microsoft Academic Search

The Vedi ophiolite, situated southeast of Yerevan (Armenia), represents part of the Neotethyan oceanic lithosphere preserved in the Lesser Caucasus. This ophiolite unit constitutes a large tectonic klippe, a result of obduction during the Upper Cretaceous (Coniacian–Santonian). Relatively well-preserved Radiolaria extracted from radiolarites overlying ophiolitic lavas along the Vedi River consist of Middle Jurassic (Bajocian, U.A.Z. 3–4) species, typical of

Taniel Danelian; Gayané Asatryan; Marc Sosson; Alain Person; Lilit Sahakyan; Ghazar Galoyan

2008-01-01

90

Talc and Chlorite 'Hybrid' Rocks in Subduction Melanges; Their Role in Fluid and Element Recycling Through Subduction Zones  

NASA Astrophysics Data System (ADS)

The fluxing of H2O, CO2 and trace elements through subduction zones is one of the most important geochemical cycles on Earth. Volatile components released during progressive metamorphism of subducting oceanic crust are thought to cause melting of the overlying mantle wedge, producing arc magmas. The petrological evolution and devolatilisation of the principle components of subducting slabs (MORB, sediments, serpentinite) are now well constrained. However, volatile and trace element flux through subduction zones remains highly contentious. The eclogite-facies rocks of northern New Caledonia represents oceanic crust that was previously subducted to depths of ~60 km. Within the terrane are high-pressure melange zones consisting of a range of mafic, metasedimentary and ultramafic lithologies that have been strongly sheared and intermixed. These melange zones have been observed on sub-metre to kilometre scales and comprise a significant proportion of the terrane. The melanges contain hybrid rock-types, such as carbonate-rich talc and chlorite schists, that are not equivalents of any typical protolith found at the Earth's surface. Using field relations, bulk-rock geochemistry and oxygen isotopes, we suggest that these hybrid rocks formed during subduction by metasomatism and mixing of serpentinites, mafic rocks and metasediments. Subducted mafic rocks and serpentinites may deliver H2O to sub-arc depths, but fluid release occurs at temperatures too low (>650 °C) to dissolve significant amounts of the slab-sourced elements that are found in arc magmas. Hybrid rock-types may comprise a significant proportion of subducting slabs and have the potential to transport large amounts of volatiles to sub-arc depths. Talc and chlorite-rich hybrid rocks will undergo almost complete dehydration at relatively high temperatures (~ 800 °C) allowing for elevated element solubility in fluids or partial melting of adjacent pelitic and mafic rocks. By contrast, carbonates in these rocks are stable to very great depths and may be important for recycling C into the deep mantle. Therefore, hybrid rock-types in subducting slabs may be critical for element and fluid recycling through subduction zones and the evolution of arc magmas.

Arculus, R. J.; Spandler, C.; Hermann, J.; Mavrogenes, J.

2005-05-01

91

Boron contents and isotopic compositions of the hydrothermally altered oceanic crust from the Troodos ophiolite, Cyprus  

NASA Astrophysics Data System (ADS)

The boron contents and isotopic compositions were determined for the hydrothermally altered oceanic crust through the Troodos ophiolite. The samples were represented by the International Crustal Research Drilling Group (ICRDG) drill-Holes CY1 (479m), CY2A (689m), CY4 (2263m), and selected outcrops along the Akaki river. Hole CY1 was composed upper and lower pillow lava, CY4 constituted sheeted dike complex and gabbro section, and the samples along Akaki river formed from pillow lava to sheeted dike complex. Hole CY2A was composed pillow lava and sheeted dike, drilled near Agrokipia ‘B’ deposit a stockwork type which completely enclosed within the lower pillow lava. The goal of this study is to understand the Boron geochemistry during hydrothermal alteration of the oceanic crust including hydrothermal ore deposit as Agrokipia ‘B’. The average boron contents of each sequence from Troodos ophiolite were pillow lava (63.2ppm), sheeted dike complex (4.5ppm), gabbro section (1.6ppm). But then, those of Oman ophiolite were 7.9ppm, 5.3ppm, 1.7ppm (Yamaoka et al., 2010 submitted). Thus, both of these ophiolites, the vertical profile of boron content decreased with depth, also the boron contents were much richer than fresh-MORB (0.5ppm) (Spivack and Edmond, 1987; Chaussidon and Jambon, 1994). This indicates boron rich of the altered oceanic crust were derived from seawater. And sheeted dike complex and gabbro section were similar value relatively, but pillow lava differed widely. These results may represent the difference of length being submarine, because these ophiolites were generated in deep water of the Tethys sea about 90Ma (Late Cretaceous) (Tilton et al., 1981; Mukasa and Ludden, 1987), and Oman ophiolite was obducted about 70Ma (Lanphere, 1981) but Troodos ophiolite uplifted about 10Ma (Middle Miocene) (Robertson and Woodcock, 1979).

Matsukura, S.; Yamaoka, K.; Ishikawa, T.; Kawahata, H.

2010-12-01

92

Subduction-related metamorphism beneath ophiolites (Oman) and during early stages of continental collision (Himalaya)  

NASA Astrophysics Data System (ADS)

Subduction-related metamorphism occurs beneath ophiolites (Oman), beneath island arcs (Kohistan) and during the early stages of continental collision (Kaghan, Tso Morari; Himalaya). Ophiolite obduction necessarily involves subduction of first oceanic, then continental crust to mantle depths beneath the ophiolite. In Oman an inverted pressure and temperature profile is exposed beneath the Semail ophiolite from garnet+clinopyroxene-bearing granulite to hornblende+plagioclase amphibolite down through epidote amphibolite and a variety of greenschist facies meta-sediments, dominantly cherts, marbles and quartzites. Thermobarometry on Grt+Cpx-bearing amphibolites immediately beneath the contact with mantle sequence harzburgites shows that the upper sole rocks formed at PT conditions of 770-900°C and 11-13 kbar, equivalent to depths of 30-40 km in oceanic lithosphere. Heat for metamorphism can only have been derived from the overlying mantle peridotites. Pressures are higher than can be accounted for by the thickness of the preserved ophiolite (15-20 km). Timing of peak metamorphism was synchronous with formation of the ophiolite gabbroic - trondhjemite crustal sequence and eruption of the pillow lavas (Cenomanian; 96-95 Ma). During the later stages of obduction the continental margin was dragged down to depths of nearly 100 km and basaltic sills within calc-schists were converted to eclogites (20-25 kbar; 500-560oC; 79.1 Ma), then exhumed back up the same subduction channel. Apparent 'extensional' fabrics throughout the HP units are related to upward flow of deeply buried rocks in a wholly compressional environment. Eclogites in a similar structural position occur along the Himalaya in the northernmost exposures of Indian plate rocks. These eclogites formed either during the latest stage of ophiolite obduction or the earliest stage of continental collision.

Searle, Mike; Waters, David; Cowan, Robert; Cherry, Alan; Cooper, Charles

2014-05-01

93

High-T Detachment Shear Zone in Mirdita Ophiolite (albania)  

NASA Astrophysics Data System (ADS)

Two oceanic core complex (OCC) extending over 50km have been mapped in the northern part of the Mirdita ophiolite. Despite the fact that the ophiolite is encased between major Dinaric thrusts, a late Jurassic marine topography is still preserved, as demonstrated by the nearly horizontal sedimentary cover. The study area exposes two peridotite domes, aligned on a N-S trend, separated by a talweg 1km wide, occupied by gabbros. This alignment is parallel to the paleoridge indicated by the sheeted dike complex, exposed at the eastern margin, and trending N-S. The two mantle domes composed of clinopyroxene bearing harzburgite with high-T porphyroclastic textures are roofed by a ~1km thick mylonitic shell, particularly well exposed at the limit with the gabbros, and interpreted as an oceanic high-T detachment shear zone. Six mylonite samples are studied for textures and crystal preferred orientation (CPO) aiming to improve the kinematics of the oceanic detachment. The mylonitic peridotite are exceptionally fresh, serpentine minerals being restricted to the bordering porphyroclastic harzburgites. They exhibit a tight millimetric layering formed by olivine / olivine+orthopyroxene / olivine+clinopyroxene or pargasitic amphibole, with grain-size 100-200?m in olivine bands vs 20-50?m in polyphase bands; plagioclase is ubiquitous. Orthopyroxene porphyroclasts show both body rotation and slip with boudinage in the flowing matrix. Electron back scattering diffraction (EBSD) maps provide precise modal composition and phase distribution. Although olivine CPO is not strong, it consistently records solid state flow on the [100](0kl)(010) slip system; the slight obliquity of [100] slip line on the mineral lineation marks the sense of shear. Pargasitic amphibole having grown in the mylonitic development has a strong CPO with [001]pg parallel to [100]ol. The most surprising result is a weak but constant orientation of [001]opx, known as the unique slip direction in orthopyroxene, at high angle to foliation, possibly resulting from body-rotation of porphyroclasts, then stretched passively in the flowing matrix. Clinopyroxene mimics orthopyroxene; plagioclase is minor and random. These mylonites are interpreted as marking the deep detachment fault which exhumed the peridotite massifs. They have been flowing in solid state by intracrystalline gliding, at very high strain rate, recording very high strain developed in the pargasite stability field 700-900°C. The detachment fault was localized at the mantle-crust boundary.

Jousselin, D.; Nicolas, A. A.; Boudier, F. I.; Meshi, A.

2013-12-01

94

Mid-ocean ridge and supra-subduction affinities in the Pindos ophiolites (Greece): implications for magma genesis in a forearc setting  

Microsoft Academic Search

The Pindos ophiolitic massif is considered an important key area within the Albanide–Hellenide ophiolitic belt and is represented by two tectonically distinct ophiolitic units: (1) a lower unit, including an intrusive and a volcanic section; and (2) an Upper Ophiolitic Unit, mainly including mantle harzburgites. Both units share similar metamorphic soles and tectono-sedimentary mélanges at their bases.The intrusive section of

Emilio Saccani; Adonis Photiades

2004-01-01

95

Compositional Heterogeneity and Spatial Segmentation of Suprasubduction (ssz-type) Ophiolites: Evidence From The Kamchatka Arc  

NASA Astrophysics Data System (ADS)

In ophiolite complexes from the Eastern Asian accretion belts the spatial heterogeneity of geochemical parameters for different components of an ophiolite sequence is estab- lished: restite mantle-derived peridotites, cumulative layered complex and volcanics. This heterogeneity is displayed as at a regional level (tens - hundred km), and at a level of local structures (hundred i - first tens km). As a rule, distinction is observed on a complex of geochemical parameters (concentration and form of REE spectra, EPG distribution, isotope characteristics, Cr-spinel and pyroxene composition etc.). Revealed at once in several suprasubduction-type ophiolite belts (Kamuikotan, Philip- pines New Guinea etc.), the spatial variations of geochemical parameters have not gradual, and discrete character. For an explanation of the reasons of ophiolite com- positional heterogeneity several mechanisms are offered: (1) tectonical overlapping of various fragments of lithosphere; (2) different specify of deep processes, resulting to compositional heterogeneity of rocks from the same lithosphere level; 3) hetero- geneity of the upper mantle and/or mantle metasomatism; 4) evolution of ophiolites (Shervais, 2001) and/or center of magma generation (mixture of continuous series of melt portions, separated during different stages of progressive mantle source melting (Bazylev et al., 2001)); 5) preservations of relict blocks of low lithosphere and upper mantle from the previous stage in suprasubduction conditions. The authors consider regional geochemical heterogeneity and segmentation of suprasubduction ophiolites (SSZ-type) on an example of peridotites from the Eastern Kamchatka ophiolite belt (EKOB), where sublongitude zones, crossed the basic geological structures of a penin- sula (including EROB) were allocated earlier. For each of zones the complex of geo- chemical attributes, steady is established within the limits of a zone, but distinct from of the characteristics of other zones. Among the factors causing an unequal degree of partial melting of peridotites, a main role play a geothermal regime and composition of fluid phase (first of all, the role of water fluid is great). These parameters, in turn, are supervised by a geodynamic regime of magma generation (such characteristics as speed of subduction and geometry of a subducted plate) and finally determine speed of uplift from the diapir in mantle, depth of the termination of partial melting, amount of 1 extracted melt, form and capacity of the magma chamber etc. The local heterogeneity in SSZ-ophiolites is considered on an example of a complex of the Kamchatka Cape Peninsula - the largest ophiolite complex in EKOB. Isotope, geochemical and miner- alogical study have shown, that a part, prevailing on volume, of this complex consist suprasubduction-type magmatic rocks (restite high-depleted harzburgites and related layered cumulative complex), whereas peridotites of harzburgite-lherzolite series and high-grade metabasites (retrograde eclogites and garnet amphibolites) composition- ally correspond to series of N-MORB and Ò-MORB-type. The presence in ophiolite of the Kamchatka Cape Peninsula alongside with high-depleted harzburgites as well moderately- and low-depleted peridotites of harzburgite-lherzolite series allows to as- sume, that Late Mesozoic suprasubduction ophiolites were formed on peridotitic basis of abyssal type. Thus the transformation of "oceanic" substrate was not complete, that has allowed to be kept relict peridotites of lherzolitic type and high-pressure metamor- phics. Probably it reflects pulsing character of geodynamics of suprasubduction-type ophiolite formation, it is possible is connected with "jumping" of spreading axes in suprasubduction conditions. During followed multistage napping in a northeast direc- tion in the Upper Cretaceous time disintegrated fragments of both mantle complexes were tectonically concurrent. In the report the alternative versions of tectonic models of development are also discussed for the Eastern Kamchatka ophiolites. 2

Osipenko, A.; Krylov, K.

96

Neoproterozoic ophiolitic peridotites along the Allaqi-Heiani suture, South Eastern Desert, Egypt  

NASA Astrophysics Data System (ADS)

The Wadi Allaqi ophiolite along the Egyptian-Sudanese border defines the southernmost ophiolitic assemblage and suture zone in the Eastern Desert. Ophiolite assemblages comprise nappes composed mainly of mafic and ultramafic rocks that were tectonically emplaced and replaced by serpentine and carbonates along shear zones probably due to CO2-metasomatism. Serpentinites, altered slices of the upper mantle, represent a distinctive lithology of dismembered ophiolites of the western YOSHGAH suture. Microscopically, they are composed of more than 90 % serpentine minerals with minor opaque minerals, carbonate, brucite and talc. The mineral chemistry and whole-rock chemical data reported here indicate that the serpentinized peridotites formed as highly-depleted mantle residues. They show compositions consistent with formation in a suprasubduction zone environment. They are depleted in Al2O3 and CaO similar to those in fore-arc peridotites. Also, high Cr# (Cr/ (Cr+Al)) in the relict chrome spinels (average ~0.72) indicates that these are residual after extensive partial melting, similar to spinels in modern fore-arc peridotites. Therefore, the studied serpentinites represent fragments of an oceanic lithosphere that formed in a fore-arc environment, which belongs to an ophiolitic mantle sequence formed in a suprasubduction zone.

Azer, M. K.; Samuel, M. D.; Ali, K. A.; Gahlan, H. A.; Stern, R. J.; Ren, M.; Moussa, H. E.

2013-10-01

97

Crystallization and age of zircon from Corsican ophiolitic albitites: consequences for oceanic expansion in Jurassic times  

NASA Astrophysics Data System (ADS)

In Corsica, albitites and albite-granites are considered to be the latest products of an ophiolitic magma. Two albitites have been selected for a study of the morphology, typology and age of their zircons, applying for the first time the U-Pb method to Western Tethyan ophiolites. Both samples come from the Rospigliani series, possibly formed in a transform fault zone. The first one shows a cumulate texture, whereas the second has features indicative of more rapid cooling. Zircons have precipitated at the end of the fractional crystallization process after a strong enrichment in alkali and incompatible elements. In a typological diagram they plot at high temperature and high agpaicity indexes. These abundant large, euhedral to subhedral zircons, sometimes with fine zoning, may also contain inclusions of granitic composition which testifies to K fractionation in ophiolitic magmas. The U-Pb age of 161±3Myr is in the range of the ages obtained for Alpine and Apennines ophiolites. All these ophiolitic remnants belonging to the Liguro-Piedmont Basin may represent a portion of the Central Atlantic Ocean created during Middle and Upper Jurassic times.

Ohnenstetter, M.; Ohnenstetter, D.; Vidal, Ph.; Cornichet, J.; Hermitte, D.; Mace, J.

1981-08-01

98

Spring and surface water quality of the Cyprus ophiolites  

NASA Astrophysics Data System (ADS)

A survey of surface, spring and borehole waters associated with the ophiolite rocks of Cyprus shows five broad water types (1) Mg-HCO3, (2) Na-SO4-Cl-HCO3, (3) Na-Ca-Cl-SO4-OH-CO3, (4) Na-Cl-SO4 and (5) Ca-SO4. The waters represent a progression in chemical reactivity from surface waters that evolve within a groundwater setting due to hydrolysis of the basic/ultrabasic rock as modified by CO2-weathering. An increase in salinity is also observed which is due to mixing with a saline end-member (modified sea-water) and dissolution of gypsum/anhydrite. In some cases, the waters have pH values greater than 11. Such high values are associated with low temperature serpentinisation reactions. The system is a net sink for CO2. This feature is related not only to the hydrolysis of the primary minerals in the rock, but also to CaCO3 or Ca-Mg-CO3 solubility controls. Under hyperalkaline conditions, virtually all the carbon dioxide is lost from the water due to the sufficiently high calcium levels and carbonate buffering is then insignificant. Calcium sulphate solubility controls may also be operative when calcium and sulphate concentrations are particularly high.

Neal, C.; Shand, P.

99

Sr isotopic tracer study of the Samail ophiolite, Oman.  

USGS Publications Warehouse

Rb and Sr concentrations and Sr-isotopic compositions were measured in 41 whole-rock samples and 12 mineral separates from units of the Samail ophiolite, including peridotite, gabbro, plagiogranite, diabase dykes, and gabbro and websterite dykes within the metamorphic peridotite. Ten samples of cumulate gabbro from the Wadir Kadir section and nine samples from the Wadi Khafifah section have 87Sr/86Sr ratios of 0.70314 + or - 0.00030 and 0.70306 + or - 0.00034, respectively. The dispersion in Sr- isotopic composition may reflect real heterogeneities in the magma source region. The average Sr-isotopic composition of cumulate gabbro falls in the range of isotopic compositions of modern MORB. The 87Sr/86Sr ratios of noncumulate gabbro, plagiogranite, and diabase dykes range 0.7034-0.7047, 0.7038-0.7046 and 0.7037- 0.7061, respectively. These higher 87Sr/86Sr ratios are due to alteration of initial magmatic compositions by hydrothermal exchange with sea-water. Mineral separates from dykes that cut harzburgite tectonite have Sr-isotopic compositions which agree with that of cumulate gabbro. These data indicate that the cumulate gabbro and the different dykes were derived from partial melting of source regions that had similar long-term histories and chemical compositions.-T.R.

Lanphere, M. A.; Coleman, R. G.; Hopson, C. A.

1981-01-01

100

Ophiolite accretion in the Lachlan Orogen, Southeastern Australia  

NASA Astrophysics Data System (ADS)

Slivers of dismembered Cambrian ophiolites are preserved in major fault zones of the turbidite-dominated, western and central Lachlan Orogen of southeastern Australia. Geometrical, chronological, and metamorphic constraints indicate that these slivers did not undergo classic Tethyan-style obduction but were incorporated into evolving turbidite wedges as offscraped slices, imbricated fault slivers or duplexes, and as blocks in mélange. The processes involved include decoupling of the oceanic crustal stratigraphy predominantly above the moho, formation and metamorphism of serpentinite/talc matrix mélange up to blueschist conditions, and accretion via underplating or offscraping during underthrusting. These processes have produced major, large-scale fault zones within the chevron folded turbidites that show marked along-strike variation in structural complexity that relate to magnitude of displacement. The fault zones may have formed preferentially where topographic highs occurred in the oceanic crust, promoting fold and fault nucleation within irregularities in the layering. These fault zones have predominantly brittle character due to their formation under a low geothermal gradient. Duplex-like structures in the fault zones and the inferred development through buckling of, then faulting of, the oceanic lithosphere, have analogues in modern oceanic environments (e.g. Indian Ocean) and young subduction systems (e.g. Sulu Sea, southeast Asia). They are interpreted to have formed due to convergence in a non-collisional, intra-oceanic setting during closure of a Cambrian backarc basin.

Spaggiari, Catherine V.; Gray, David R.; Foster, David A.

2004-01-01

101

Potential Hydrogen Yields from Ultramafic Rocks of the Coast Range Ophiolite and Zambales Ophiolite: Inferences from Mössbauer Spectroscopy  

NASA Astrophysics Data System (ADS)

The reduced status of mantle rocks is a possible controller and indicator of deep life habitat, due to interactions between water and ultramafic (Fe, Mg-rich) minerals, which, under reducing conditions, can yield copious free hydrogen, which is an energy source for rock-hosted chemosynthetic life. In this work, Mössbauer spectroscopy was used to parameterize the redox status of Fe in altering peridotites of the Coast Range Ophiolite (CRO) in California, USA and Zambales Ophiolite (ZO) in the Philippines. Fe-bearing minerals were identified and data were collected for the percentages of Fe(III)and Fe(II)and bulk Fe concentration. Thin section analysis shows that relict primary olivines and spinels generally constitute a small percentage of the ZO and CRO rock, and given satisfactory estimates of the volume of the ultramafic units of the ZO and CRO, a stoichiometric H2 production can be estimated. In addition, ZO serpentinites are ~63,000 ppm Fe in bulk samples; they contain ~41-58% Fe(III)and ~23-34% Fe(II) in serpentine and relict minerals along with ~8-30% of the total Fe as magnetite. CRO serpentinites are ~42,000 ppm Fe in bulk samples; they contain ~15-50% Fe(III), ~22-88% Fe(II) in serpentine and relict minerals, and ~0-52% of total Fe is in magnetite (Fe(II)Fe(III)2O4). Assuming stoichiometric production of H2, and given the following representation of serpentinization 2(FeO)rock + H2O ? (Fe2O3)rock +H2, we calculated the maximum quantity of hydrogen released and yet to be released through the oxidation of Fe(II). Given that relatively high Fe(III)/Fetotal values can imply higher water:rock ratios during rock alteration (Andreani et al., 2013), we can deduce that ZO ultramafics in this study have experienced a net higher water:rock ratio than CRO ultramafics. We compare possible H2 yields and contrast the tectonic and alteration histories of the selected ultramafic units. (M. Andreani, M. Muñoz, C. Marcaillou, A. Delacour, 2013, ?XANES study of iron redox state in serpentine during oceanic serpentinization, Lithos, Available online 20 April 2013)

Stander, A.; Nelms, M.; Wilkinson, K.; Dyar, M. D.; Cardace, D.

2013-12-01

102

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)

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 ophiolites show the maximum of arc component, whereas the Lower Cretaceous ones show little to strong mixing. In addition, most ophiolites were created in short lived (30 m.y.) basins and generated close to the continental margin of Eurasia. We propose that the Ladakh-Tibet ophiolites were generated in a suprasubduction setting similar to the Mariana arc, interarc and back-arc or to the Tonga-Lau system. The variable arc signature of these ophiolites is directly related to their initial position in the suprasubduction system.

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

2013-04-01

103

The seismic velocity structure of a traverse through the Bay of Islands ophiolite complex, Newfoundland, an exposure of oceanic crust and upper mantle  

Microsoft Academic Search

Although the ophiolites are widely recognized as segments of oceanic crust emplaced on land, direct correlation between the ophiolites and the the oceanic crust has proven difficult owing to the near absence of common criteria on which to base a comparison; the ophiolites are defined petrologically, while the oceanic crust is defined largely in terms of seismic structure. To bridge

Matthew H. Salisbury; Nikolas I. Christensen

1978-01-01

104

Geodynamic evolution of ophiolites from Albania and Greece (Dinaric-Hellenic belt): one, two, or more oceanic basins?  

NASA Astrophysics Data System (ADS)

All the geological constraints for an exhaustive reconstruction of the Triassic to Tertiary tectonic history of the southern Dinaric-Hellenic belt can be found in Albania and Greece. This article aims to schematically reconstruct this long tectonic evolution primarily based on a detailed analysis of the tectonic setting, the stratigraphy, the geochemistry, and the age of the ophiolites. In contrast to what was previously reported in the literature, we propose a new subdivision on a regional scale of the ophiolite complexes cropping out in Albania and Greece. This new subdivision includes six types of ophiolite occurrences, each corresponding to different tectonic units derived from a single obducted sheet. These units are represented by: (1) sub-ophiolite mélange, (2) Triassic ocean-floor ophiolites, (3) metamorphic soles, (4) Jurassic fore-arc ophiolites, (5) Jurassic intra-oceanic-arc ophiolites, and (6) Jurassic back-arc basin ophiolites. The overall features of these ophiolites are coherent with the existence of a single, though composite, oceanic basin located east of the Adria/Pelagonian continental margin. This oceanic basin was originated during the Middle Triassic and was subsequently (Early Jurassic) affected by an east-dipping intra-oceanic subduction. This subduction was responsible for the birth of intra-oceanic-arc and back-arc oceanic basins separated by a continental volcanic arc during the Early to Middle Jurassic. From the uppermost Middle Jurassic to the Early Cretaceous, an obduction developed, during which the ophiolites were thrust westwards firstly onto the neighboring oceanic lithosphere and then onto the Adria margin.

Bortolotti, Valerio; Chiari, Marco; Marroni, Michele; Pandolfi, Luca; Principi, Gianfranco; Saccani, Emilio

2013-04-01

105

Age and evolution of Western Brooks Range ophiolites, Alaska: Results from 40Ar/39Ar thermochronometry  

NASA Astrophysics Data System (ADS)

Basalt, gabbro, and peridotite in the western Brooks Range structurally overlie Mississippian through Late Jurassic platform sediments that were deformed during Late Jurassic - Cretaceous time. 40Ar/39Ar step-heating experiments on hornblende, biotite, plagioclase, and potassium feldspar from gabbro, diorite, plagiogranite, and granite yielded complex release spectra. On isotope correlation diagrams, the incremental-heating data indicate that gabbro, diorite, and plagiogranite cooled below the closure-temperature for argon in amphibole between 187 and 184 Ma. Because the closure temperature of argon retention in amphibole is relatively high, these ages are interpreted to be the time since crystallization. Biotite and hornblende from metamorphic rock exposed along faults separating peridotite and gabbro from underlying basalt yielded plateau and isochron dates of 169 - 163 Ma; these dates are interpreted to record postmetamorphic cooling following intraoceanic detachment and transport of "hot" peridotite and gabbro over "cold" basalt and sediment. Detachment and faulting of the gabbros and peridotites is interpreted to have occurred approximately 20 Ma after crystallization, and is longer than the 5-10 Ma interval commonly observed in studies of other ophiolites. Fossils in flysch underlying the ophiolites indicate that the ophiolites were emplaced onto the continental margin during the Tithonian, approximately 15-20 Ma after initial detachment. The ages of feldspars (149 - 137 Ma) in the ophiolites and associated volcanic rocks are similar to the fossil ages of the underlying flysch and might record uplift during emplacement of the ophiolites onto the continental margin. A fission-track age of apatite implies that the ophiolitic rocks were overlain by >4 km of rock until ˜59 Ma ago, the time of regional uplift in the Brooks Range. Supplement Appendix 1 is available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. document T92-005; $2.50. Payment must accompany order.

Wirth, Karl R.; Bird, John M.; Blythe, Ann E.; Harding, David J.; Heizler, Matthew T.

1993-01-01

106

Oxygen isotope evidence for submarine hydrothermal alteration of the Del Puerto ophiolite, California  

USGS Publications Warehouse

The oxygen isotope compositions and metamorphic mineral assemblages of hydrothermally altered rocks from the Del Puerto ophiolite and overlying volcaniclastic sedimentary rocks at the base of the Great Valley sequence indicate that their alteration occurred in a submarine hydrothermal system. Whole rock ??18O compositions decrease progressively down section (with increasing metamorphic grade): +22.4??? (SMOW) to +13.8 for zeolite-bearing volcaniclastic sedimentary rocks overlying the ophiolite; +19.6 to +11.6 for pumpellyite-bearing metavolcanic rocks in the upper part of the ophiolite's volcanic member; +12.3 to +8.1 for epidote-bearing metavolcanic rocks in the lower part of the volcanic member; +8.5 to +5.7 for greenschist facies rocks from the ophiolite's plutonic member; +7.6 to +5.8 for amphibolite facies or unmetamorphosed rocks from the plutonic member. Modelling of fluid-rock interaction in the Del Puerto ophiolite indicates that the observed pattern of upward enrichment in whole rock ??18O can be best explained by isotopic exchange with discharging 18O-shifted seawater at fluid/rock mass ratios near 2 and temperatures below 500??C. 18O-depleted plutonic rocks necessarily produced during hydrothermal circulation were later removed as a result of tectonism. Submarine weathering and later burial metamorphism at the base of the Great Valley sequence cannot by itself have produced the zonation of hydrothermal minerals and the corresponding variations in oxygen isotope compositions. The pervasive zeolite and prehnite-pumpellyite facies mineral assemblages found in the Del Puerto ophiolite may reflect its origin near an island arc rather than deep ocean spreading center. ?? 1984.

Schiffman, P.; Williams, A. E.; Evarts, R. C.

1984-01-01

107

COOL: Crust of the Oman Ophiolite and its Lithosphere - a passive seismic experiment  

NASA Astrophysics Data System (ADS)

Plate tectonics has established a framework for geoscientists to understand most geologic/tectonic processes that shaped our present-day Earth. 'Obduction', the emplacement of young, dense oceanic lithosphere (ophiolites) on top of older lighter continental lithosphere remains, however, a rather odd phenomenon. Some ophiolites are fundamentally similar to young oceanic crust and it is hence assumed that they were obducted as thrust sheets at the onset of continental subduction in a previously intra-oceanic subduction setting. The Peri-Arabic obduction corresponded to a spectacular, almost synchronous thrust movement along thousands of km from Turkey to Oman. At the eastern margin of the Arabian plate, the world's largest and best preserved ophiolite was emplaced in only a few My during Upper Cretaceous and is exposed today atop the Oman Mountain range. Although being the best studied ophiolite in the world, rather little is still known about the internal structure of the ophiolite and the Oman Mountains. The dimension of the ophiolite is large enough (~700 km) to be studied with seismological methods, providing thus a rare setting to investigate oceanic crust on land without ocean bottom installations. We have deployed a network of 40 broadband seismometers across the Oman Mountains in Oct/Nov 2013 for passive seismic registration for a duration of ca. 15 months. The network is complemented by 10 permanent stations in the area operated by the Earthquake Monitoring Center in Oman. Aims of the project include: - Seismological imaging of the geometry and internal properties of obducted oceanic, and its underlying continental lithosphere. - Regional tomographic velocity models will provide constraints on geodynamic processes that led to large scale obduction. - Investigating the "quiet" Makran subduction zone for local seismicity will improve understanding of seismic hazard on the eastern Arabian plate.

Weidle, Christian; Agard, Philippe; Ducassou, Céline; El-Hussain, Issa; Prigent, Cécile; Meier, Thomas

2014-05-01

108

Palaeogeographical evolution of the Arabian platform during the obduction of the Samail ophiolite  

NASA Astrophysics Data System (ADS)

Obduction of a dense oceanic lithosphere on top of a light continental lithosphere remains one of the oddest phenomenons in plate tectonics. In northern Oman, the emplacement of the Samail (or Oman) ophiolite took place during Upper Cretaceous. The Samail ophiolite is probably the best studied ophiolite in the world because of its large size and because it has been relatively well preserved from deformation after its emplacement. If many studies have been carried out on the ophiolite itself in order to better understand the structure of an oceanic crust, little is known on the processes leading to its emplacement on the top of the Arabian platform. The sedimentary series deposited on the Arabian platform during the emplacement of the ophiolite could help us to understand the processes involved during the obduction by providing tectono-stratigraphic constraints on the evolution of the platform affected by the obduction. Moreover the reconstitution of the palaeoenvironments and the identification of source areas would allow discussing the evolution of reliefs formed in such a unique context. However, these series, being located just below the allochthonous units, have been affected by deformation. They are grouped in one formation, the Muti Formation, consisting of various lithologies of which the age is uncertain. In consequence, sedimentological analyses require careful mapping of the sedimentary series and biostratigraphical determinations remain difficult. The first results of our sedimentological and stratigraphical investigations show (i) a diachronism of the onset of syn-tectonic sedimentation (chemostratigraphic and biostratigraphic data) and (ii) a high variation of the source areas within the basin. These data are used in order to present a new model of evolution of the basin formed during the emplacement of the Samail ophiolite on top of the Arabian Platform. Implications for the processes involved during the obduction are also discussed.

Ducassou, Céline; Robin, Cécile; Wohlwend, Stephan; Piuz, André; Gorican, Spela

2014-05-01

109

The Significance of Paired Ophiolite Belts - Examples From the Western Pacific and Caribbean  

NASA Astrophysics Data System (ADS)

Both the Zambales Ophiolite Complex (ZOC), Philippines, and the Mayari-Baracoa Ophiolite Belt (MBOB), Cuba, comprise two geochemically distinct ophiolitic assemblages. In the ZOC, the Coto Block contains a mantle sequence characterised by depleted residual harzburgites with a thin dunitic transition zone. Chromitites are of the high-Al variety (Cr2O3=30-44 wt%; Al2O3=20-30 wt%). The adjacent Acoje Block has a less depleted lherzolite-harzburgite mantle sequence, a well developed dunitic transition zone and high-Cr type chromitites (Cr2O3=45-53 wt%; Al2O3=12-18 wt%). The MBOB is made up of two massifs, the Moa-Baracoa Massif (MBM) and the Mayari-Cristal Massif (MCM). The MBM consists of depleted harzburgites with abundant high-Al podiform chromitites, whereas the MCM comprises peridotites with high-Cr podiform chromitites. The occurrence of two adjacent mantle sequences in each of these ophiolites makes them excellent examples of "paired ophiolite belts". The distinct mantle sequences have different REE and PGE patterns that demonstrate that they have different origins in terms of their source regions, partial melting and melt-rock interaction processes. High-Cr chromitites crystallised from refractory (boninitic) melts whereas high-Al chromitites were associated with tholeiitic melts. These differences can best be accommodated in a model in which the more depleted sections are formed as part of a volcanic arc sequence, whereas the less depleted sections formed in a back arc basin. As such, paired ophiolite belts may provide evidence of subduction polarity and the nature of mantle dynamics in a suprasubduction zone environment.

Malpas, J.; Zhou, M.

2001-12-01

110

Coesite After Stishovite in a Tibetan Ophiolite: Primary Evidence of Deep Mantle Plume?  

NASA Astrophysics Data System (ADS)

The Luobusa massif is situated within the Yarlung-Zangbo suture zone, the largest ophiolite belt in Asia, which is of great interest because of its geodynamic location along the great Himalayan suture defining the geological boundary between Asia and India The massif marks the closure of the Neo-Tethyan ocean in the Early Tertiary (~65Ma). Podiform chromitites within the Luobusa ophiolite of Tibet have been reported to contain diamonds and other unusual minerals, some of which suggest very great depths of formation (Yang et al., 2003, Progress in Natural Science, 13, 528-531). However, the unusual setting (an ophiolite terrane lacking significant metamorphic overprint) has resulted in some skepticism concerning these claims. On a larger scale, similar claims have been made over the last 30 years for other rocks of similar tectonic setting "Bird and Weathers, 1975, Earth Planet. Sci. Lett. 28, 51-64). Here we report the `smoking gun' that establishes that parts of the mantle section of the Luobusa ophiolite originated at a minimum depth of ~300 km and possibly much more. The critical observation is coesite pseudomorphic after stishovite in contact with Fe-Ti alloy included in chromitite. This is the first observation of coesite in an ophiolite and the occurrence of coesite with the morphology of stishovite is unique. Coexisting titanium oxide containing >30 atom% SiO2 suggests formation under lower mantle conditions, reopening the dialog concerning possible mantle plume origin of some peridotite massifs incorporated in ophiolites worldwide "Bird et al., 1999 Earth Planet. Sci. Lett. 170, 83-92 ).

Yang, J.; Dobrzhinetskaya, L.; Bai, W.; Fang, Q.; Robinson, P. T.; Green, H.

2005-12-01

111

Oxygen isotope evidence for submarine hydrothermal alteration of the Del Puerto ophiolite, California  

NASA Astrophysics Data System (ADS)

The oxygen isotope compositions and metamorphic mineral assemblages of hydrothermally altered rocks from the Del Puerto ophiolite and overlying volcaniclastic sedimentary rocks at the base of the Great Valley sequence indicate that their alteration occurred in a submarine hydrothermal system. Whole rock ? 18O compositions decrease progressively down section (with increasing metamorphic grade): +22.4‰ (SMOW) to +13.8 for zeolite-bearing volcaniclastic sedimentary rocks overlying the ophiolite; +19.6 to +11.6 for pumpellyite-bearing metavolcanic rocks in the upper part of the ophiolite's volcanic member; +12.3 to +8.1 for epidote-bearing metavolcanic rocks in the lower part of the volcanic member; +8.5 to +5.7 for greenschist facies rocks from the ophiolite's plutonic member; +7.6 to +5.8 for amphibolite facies or unmetamorphosed rocks from the plutonic member. Modelling of fluid-rock interaction in the Del Puerto ophiolite indicates that the observed pattern of upward enrichment in whole rock ? 18O can be best explained by isotopic exchange with discharging 18O-shifted seawater at fluid/rock mass ratios near 2 and temperatures below 500°C. 18O-depleted plutonic rocks necessarily produced during hydrothermal circulation were later removed as a result of tectonism. Submarine weathering and later burial metamorphism at the base of the Great Valley sequence cannot by itself have produced the zonation of hydrothermal minerals and the corresponding variations in oxygen isotope compositions. The pervasive zeolite and prehnite-pumpellyite facies mineral assemblages found in the Del Puerto ophiolite may reflect its origin near an island arc rather than deep ocean spreading center.

Schiffman, Peter; Williams, Alan E.; Evarts, Russell C.

1984-10-01

112

Himalayan detrital chromian spinels and timing of Indus-Yarlung ophiolite erosion  

NASA Astrophysics Data System (ADS)

The geochemistry of detrital chromian spinels is commonly used to discriminate provenance from different tectonic settings of mafic and ultramafic igneous rocks. Detrital spinels in Cenozoic foreland-basin successions fed from the Himalaya Orogen were assertively interpreted as sourced from the ophiolitic rocks of the Indus-Yarlung suture zone. This study compares the geochemistry of detrital Cr-spinels from the Tethys Himalaya passive margin and Cretaceous Xigaze forearc successions with those from the Indus-Yarlung ophiolites. Cr-spinels in the Indus-Yarlung ophiolites have low TiO2 (mostly < 0.2%) and high Al2O3 (10-48%). Detrital Cr-spinels from the Tethyan Himalaya have instead high TiO2 (mostly > 0.2%) and low Al2O3 (mainly 6-23%), indicating a rift-related basaltic origin. Detrital Cr-spinels from the Xigaze forearc basin have either low TiO2 (mostly < 0.2%) and low Al2O3 (4-34%), suggesting provenance from a supra-subduction-zone peridotite, or high TiO2 (> 1.0%), indicating intra-plate basaltic origin. Compositional fingerprints of detrital Cr-spinels from Lower Eocene foreland-basin strata in the central-eastern Himalaya indicate provenance from the Lhasa Block without input from the Indus-Yarlung ophiolites. Only Cr-spinels from the Lower Eocene foreland-basin strata in the north-western Himalaya and the Upper Eocene-Lower Miocene remnant-ocean turbidites of the Bengal basin are mostly ophiolite-derived. The Indus-Yarlung ophiolites were thus emplaced and exposed to erosion since the Early Eocene (> 50 Ma) in the NW Himalaya, but only subsequently (50-38 Ma) in the eastern Himalaya.

Hu, Xiumian; An, Wei; Wang, Jiangang; Garzanti, Eduardo; Guo, Ronghua

2014-05-01

113

Recycling of crustal minerals into the upper mantle: evidence from ophiolites  

NASA Astrophysics Data System (ADS)

A wide variety of ultrahigh pressure and crustal minerals has been recovered from podiform chromitites of the Donqiao and Luobusa ophiolites of Tibet, the Ray-Iz ophiolite of the Polar Urals and the Semail ophiolite of Oman. Microdiamonds are abundant in the Luobusa, Donqiao and Ray-Iz ophiolites, coesite and kyanite occur in Luobusa and moissanite is present in all four ophiolites. Numerous crustal minerals, including zircon, corundum, quartz, almandine garnet, rutile, and feldspar are also present. The diamonds are mostly euhedral grains,100-200 µm across, commonly containing metallic and Mg-Fe silicate inclusions. One small grain occurs as an inclusion in an Os-Ir alloy. Coesite and kyanite are intergrown with each other on the rim of a grain of Ti-Fe alloy, and the coesite has a prismatic form suggesting it may be pseudomorphic after stishovite. Moissanite is common in all four ophiolites and occurs as small colorless, green or blue, vitreous fragments. Zircon grains range from 20 to 300 µm, and are mostly well rounded with very complex internal structures. They commonly contain low-pressure inclusions of quartz, rutile, orthoclase, mica, ilmenite and apatite. 206Pb/238U SIMS and SHRIMP dates for the zircons are mostly Paleozoic and Precambrian, far older than the ophiolites. The grains of quartz, almandine garnet, corundum and feldspar range up to about 0.5 mm and are moderately to well rounded. Smaller, angular fragments of such grains are also present. The rounded morphology of these grains, as well as the zircons, strongly suggests derivation from sedimentary material, presumably transported into the mantle by subduction. The microdiamonds and moissanite could also have been derived from crustal materials recycled into the mantle. We suggest that the various minerals were picked up by melts from which the chromitites precipitated and carried to shallow crustal levels. The preservation of such minerals, particularly quartz and coesite, in the mantle implies isolation from the enclosing rock, perhaps in xenoliths. The recovery of essentially the same minerals from four widely separated ophiolites, processed in different laboratories, argues strongly against natural or anthropogenic contamination.

Robinson, P. T.; Yang, J.; Trumbull, R.

2009-05-01

114

Faulting and Serpentinisation of Peridotites in the Leka Ophiolite  

NASA Astrophysics Data System (ADS)

The ocean floor is strongly affected by seismic activity along mid ocean ridges and transform faults, where the upper mantle may undergo extensive alteration and serpentinisation. While the spatial link between faulting and serpentinisation is generally accepted, the causal connection between these two processes is not well understood. The cumulate section of the Leka Ophiolite, north-central Norway, is transected by kilometre long sets of parallel faults and shear zones with a dextral shear. The discrete faults have spacings from centimetres to decimetres and displacements in the same range. They alternate with breccia zones and metre thick shear zones with displacements up to 30 m. The extent of the faulted areas and the consistent dextral displacements indicate a regional deformation process, possibly related to a transform. The observation of progressive peridotite alteration in the Leka Ophiolite gives new insights into the interplay between serpentinisation and deformation. In the least altered peridotite, propagating fractures produce a texture reminiscent of cleaved olivine. Serpentinisation initiates along the cleavage planes. In more deformed samples, the centres of the discrete faults contain relatively large coherent olivine fragments alternating with trails of small spinel grains parallel to the fault. Most of the spinel is ferrichromite or magnetite, but some contain cores of primary chromite. Towards the margins of the faults, the amount of (fibrous) serpentine and fracturing of olivine increases significantly. The fault margins appear dark in hand specimens, which is due to finely dispersed magnetite grains in olivine and serpentine. The surrounding damage zones contain moderately serpentinised olivine grains exhibiting different deformation indicators such as undulous extinction, deformation lamellae and subgrain boundaries. In the breccia zones, where peridotite clasts are enclosed by deformation zones with the same buildup as the faults, this texture is, to different degrees, overprinted by late-stage antigorite serpentinisation. The faults themselves appear unaffected by this last stage of alteration. This challenges the common assumption that fractures enhance the progress of serpentinisation by providing fluid pathways. During the first stage of serpentinisation, the faults have been preferential reaction sites, as suggested by the abundance of magnetite along these zones and the serpentine along the cleavage planes in olivine. During the later antigorite formation, however, they remained unchanged. We envisage a process similar to the permeability reduction caused by deformation bands in sandstones: The grain size reduction and compaction during shearing apparently resulted in impermeable bands in the peridotites. Nevertheless, there must have been a significant mass flow from the antigorite domains, as they hardly contain any magnetite and brucite, both of which should have formed during serpentinisation. Consequently, the permeability of the deformed peridotite complex as a whole persisted despite the sealing of the fault cores. It remains to explore if the fault barriers played a role in seismic pumping and to couple the tectonic deformation to possible volume changes due to reaction.

Dunkel, Kristina G.; Drivdal, Kerstin; Austrheim, Håkon; Andersen, Torgeir B.; Jamtveit, Bjørn

2014-05-01

115

First pressure- and temperature estimates of the metamorphic sole of the Pinarbasi ophiolite, central Turkey  

NASA Astrophysics Data System (ADS)

Ophiolites are interpreted as remnants of oceanic lithosphere. Many have a so-called supra-subduction zone (SSZ) geochemical signature, suggestive of formation at a spreading ridge overlying a subduction zone. Supra-subduction zone ophiolites frequently have a several-hundred-meter thick sequence of metamorphic rocks below their mantle section: the metamorphic sole. These dominantly mafic and generally heavily sheared metamorphic rocks have been shown to preserve an inverted metamorphic gradient with the highest pressures and temperatures at the top of the sole, decreasing downwards. Pressure estimates from rocks found at the top of metamorphic soles may be as much as 10-15 kbar with temperature estimates up to 875°C. The metamorphic grade varies from greenschist near the base, up to granulite facies at the top, with the bulk comprising of amphibolite facies rocks. At some locations a blueschist overprint of the amphibolite facies mineral assemblages has been described. The relative high pressures preserved in the metamorphic sole cannot simply result from overburden pressure of the currently overlying ophiolite, which is a long-standing problem. This raises the question of what process(es) can explain pressures up to 10-15 kbar in the top of metamorphic soles, in relation to the approximately synchronous formation of the SSZ oceanic lithosphere above the sole. One of the places to study the formation of SSZ ophiolites and their metamorphic soles is the Neotethyan Suture zone. Remnants of Neotethyan lithosphere are preserved as ophiolites that are discontinuously exposed from the Mediterranean region through the Himalaya to SE Asia. Supra-subduction zone ophiolites are particularly widespread in Turkey. The Pinarba?i ophiolite is located in the SE of Central Anatolia, and overlies the Tauride fold-and-thrust belt that formed since the Late Cretaceous. It comprises mantle tectonites consisting of serpentinized harzburgite and dunite with remnants of gabbro to the top, cross-cut by felsic dikes. The mantle tectonites are highly brecciated at the bottom, near the contact with the metamorphic sole. The ophiolite is less than 3 km thick. A well-preserved metamorphic sole structurally underlies the mantle tectonites and consists in ascending order of non-metamorphic rocks, sheared and foliated greenschists, amphibolites, and garnet-amphibolites. The greenschist facies rocks are foliated and folded. The amphibolite is well foliated and has asymmetric isoclinal folds and a lineation defined by amphibole crystals. It is fine-grained at the bottom and coarser at structurally higher levels with up to 8 mm garnet porphyroblasts near the contact with the mantle tectonite. The lower-grade rocks at the base of the sole are predominantly metasediments, whilst the (garnet-) amphibolites mostly appear to be of magmatic origin. The sole is between 200 and 300 meters thick and is laterally discontinuous, at least in part due to late brittle faulting. The metamorphic sole overlies a tectonic mélange that separates the ophiolite and the sole from Tauride carbonate platform rocks. In this presentation we will show preliminary results constraining the PT-conditions for the different levels in the metamorphic sole underneath the Pinarba?i ophiolite to define the formation mechanism of the sole in relation to the overlying SSZ ophiolite.

Peters, Kalijn; van Hinsbergen, Douwe; van Roermund, Herman; Brouwer, Fraukje; Drury, Martyn

2014-05-01

116

Evaluation of Heterotrophy in in Serpentinite-Associated Waters from the Coast Range Ophiolite, Northern California, USA and the Zambales Ophiolite, Philippines  

NASA Astrophysics Data System (ADS)

The deep biosphere in cold, dark sub-seafloor ultramafic rocks (i.e., those rocks rich in Fe and Mg) is stressed by exceedingly high pH, transient, if any, inorganic carbon availability, and little known organic carbon inventories. As a test of heterotrophic carbon use, serpentinite-associated waters (from groundwater sampling wells and associated surface seepages in tectonically uplifted mantle units in ophiolites) were tested for differences with respect to aqueous geochemistry and performance in EcoPlates™ - Biolog Inc. .. This work focuses on two field locations for water sampling: the Coast Range Ophiolite, CA, USA, and the Zambales Ophiolite, Philippines. Characteristics of each sampling site are presented (pH, mineral substrate, Ca2+/Mg2+ ratio, aqueous metal loads, etc.). Complementary EcoPlate™ results [prefabricated 96-well plates, seeded with triplicate experiments for determining microbiological community response to difference organic carbon sources; a triplicate control experiment with just water is built in to the plate also] are also presented. We found that waters from selected California [groundwater wells (7 discrete wells) and related surface seeps (5 hydrologically connected sites)] and Philippines [4 Zambales Ophiolite springs/seepages] sourced in serpentinites were analyzed. EcoPlate™ average well-color development (AWCD), which demonstrates microbial activities averaged per plate (as in Garland and Mills, 1991), differs across sites. Correlations of AWCD with environmental data (such as pH, oxidation-reduction potential or ORP, Ca2+/Mg2+ ratio, and Fe contents) are evaluated. Clarifying the geochemical-biological relationships that bear out in these analyses informs discourse on the energetic limits of life in serpentinizing systems, with relevance to ultramafic-hosted life on continents and in the seabed.

Scott, T. J.; Arcilla, C. A.; Cardace, D.; Hoehler, T. M.; McCollom, T. M.; Meyer-Dombard, D. R.; Schrenk, M. O.

2013-12-01

117

Carbonate-orthopyroxenite lenses from the Neoproterozoic Gerf ophiolite, South Eastern Desert, Egypt: The first record in the Arabian Nubian Shield ophiolites  

NASA Astrophysics Data System (ADS)

Carbonate-orthopyroxenites (classic sagvandites) are reported in the Gerf ophiolite, South Eastern Desert, Egypt: the first finding from the Arabian Nubian Shield (ANS) ophiolites. They form massive lenses at the southern tip of the Gerf ophiolite, along the contact between the Shinai granite and Gerf serpentinized peridotites. The lenses show structural concordance with the neighboring country rocks and the granite contact. They consist mainly of metamorphic orthopyroxene + magnesite, among other metamorphic, relict primary and retrograde secondary minerals. Based only on chemistry, two types of carbonate-orthopyroxenites can be recognized, Types I (higher-Mg) and II (lower-Mg and higher-Fe). Field constraints, petrography and mineral chemistry indicate a metamorphic origin for the Gerf carbonate-orthopyroxenites. The euhedral form of relict primary chromian spinels combined with their high Cr#/low-TiO 2 character, and absence of clinopyroxene suggest that the protolith for the Gerf carbonate-orthopyroxenites is a highly depleted mantle peridotite derived from a sub-arc setting. Contact metamorphism accompanied by CO 2-metasomatism resulted in formation of the Gerf carbonate-orthopyroxenites during intrusion of the Shinai granite. The source of CO 2-rich fluids is most likely the neighboring impure carbonate layers. Correlation of the carbonate-orthopyroxenite mineral assemblages with experimental data for the system MgO-SiO 2-H 2O-CO 2 suggests metamorphic/metasomatic conditions of 520-560 °C, Pfluid = 2 kbar and extremely high X values (0.87-1).

Gahlan, Hisham A.; Arai, Shoji

2009-01-01

118

The geochemical history of the mantle section of the Bay of Islands Ophiolite  

Microsoft Academic Search

Geochemical mapping of the mantle sections in the four massifs of the Bay of Islands Ophiolite has nearly been completed. Pending some remaining data from the North Arm Massif, the following picture emerges: The mantle sections show an extreme range of apparent degrees of depletion. This variation is non-random and occurs with depth beneath the Moho. It is roughly correlated

G. Suhr; H. Seck

2003-01-01

119

Early Paleozoic Ophiolite Complexes of the Newfoundland Appalachians as Mantle-Oceanic Crust Sequences  

Microsoft Academic Search

The layered ultramarie-marie complexes of the Northwest platform and the Fleur de Lys orthotectonic zones of the Newfoundland Appalaehians are comparable to layered Tethyan ophiolite suites of the Piedmont Alps, the Dinarides, and the Oman. They can be interpreted as oceanic crust and mantle: the peridotires, with high-pressure mineral assemblages, kaersutire, and titaniferous phlogopite, represent the upper mantle; the overlying

W. R. Church; R. K. Stevens

1971-01-01

120

Petrofabric Investigation of Gabbros from the Oman Ophiolite: Comparison between AMS and Rock Fabric  

Microsoft Academic Search

The anisotropy of magnetic susceptibility was measured on 42 gabbros sampled across a complete plutonic sequence from the Oman ophiolite. The rock fabrics, investigated in the field and through plagioclase crystallographic fabric measurements, were compared to the magnetic fabrics. This comparative study reveals that from the paleo-Moho to the top of the foliated gabbros level, 73% of the rocks display

G. Yaouancq; C. J. MacLeod

2000-01-01

121

Subsidence in magma chamber and the development of magmatic foliation in Oman ophiolite gabbros  

Microsoft Academic Search

In the Oman ophiolite, the horizon where the melt lens pinched during drifting away from the ocean ridge axis has been identified. Starting in the Root Zone of the Sheeted Dike Complex (RZSDC) located above this horizon, 18 sections down to the upper gabbros unit have been mapped in great detail, in selected areas of the southern massifs of this

Adolphe Nicolas; Françoise Boudier

2009-01-01

122

Geology and evolution of the magmatic rocks of the Vourinos Ophiolite, northern Greece  

Microsoft Academic Search

Magmatic stratigraphy and structure in an ophiolite complex is described. The integration of these features with chemical studies of the magmatic rocks resulted in a model detailing formation of the magmatic complex. This model requires that one or two parental magmas mix with fractionated liquids from older, partly solidified intrusions or with volatiles introduced by hydrothermal circulation during crystallization. One

A. E. Rassios

1981-01-01

123

Ophiolite emplacement by collision between the sula platform and the sulawesi island arc, Indonesia  

Microsoft Academic Search

Much of the tectonic complexity displayed in eastern Indonesia results from a series of Neogene collision events between island arcs, continental fragments, and the Australian continent. Here we examine the emplacement of a large ophiolite belt, resulting from the Miocene collision between the Sulawesi island arc and a continental fragment, the Sula platform. We present the results of several marine

Eli A. Silver; Robert McCaffrey; Yoko Joyodiwiryo; Scott Stevens

1983-01-01

124

Emplacement of the East Sulawesi Ophiolite: evidence from subophiolite metamorphic rocks  

Microsoft Academic Search

A newly discovered high-temperature metamorphic sole at the base of the extensive East Sulawesi Ophiolite in Sulawesi, Indonesia yields new structural and petrological data with which to constrain the emplacement history. The metamorphic sole is composed of a thin sequence of garnet and epidote amphibolite and basal greenschist metaclastics. East-dipping tectonite fabrics in the metamorphic sole amphibolite are coincident with

Chris Parkinson

1998-01-01

125

Remagnetization of the Coast Range ophiolite at Stanley Mountain, California, during accretion near 10??N paleolatitude  

USGS Publications Warehouse

Paleomagnetic data are presented for a 50-m-thick sequence of Oxfordian to Tithonian sedimentary rocks conformably overlying Upper Jurassic pillow basalt. These new data are similar in direction and polarity to previously published paleomagnetic data for the pillow basalt. The Jurassic sedimentary rocks were deposited during a mixed-polarity interval of the geomagnetic field, and uniformity of the remanent magnetization within the entire section of pillow basalt and sedimentary rocks indicates later remagnetization. Remagnetization is interpreted to have occurred during accretion to the continental margin. Similar paleolatitudes calculated for the ophiolite (11?? ?? 3??) and for mid-Cretaceous sedimentary rocks of the Stanley Mountain terrane at Figueroa Mountains (6?? ?? 5??) are consistent with remagnetization of the ophiolite at low paleolatitudes. Comparison with uniform-polarity directions for other remnants of ophiolite in southern California and elsewhere along the Pacific coast suggests that the Coast Range ophiolite at Stanley Mountain has been remagnetized along the North American margin near 10??N paleolatitude between earliest and mid-Cretaceous time and subsequently transported northward by strike-slip faulting related to relative motions between the Farallon, Kula, Pacific, and North American plates. -from Author

Hagstrum, J. T.

1992-01-01

126

Rare earth and trace element geochemistry of metabasalts from the Point Sal ophiolite, California  

Microsoft Academic Search

The Point Sal ophiolite, California, contains a volcanic pile of pillowed and massive basalts and keratophyres that exhibit a downward change in metamorphic facies, i.e., zeolite to greenschist. Analyses of 14 stratigraphically sequential lavas reveals the existence of three lava types: (1) highly fractionated plagioclase phyric lavas at the base of the lava pile; (2) clinopyroxene-plagioclase phyric lavas at different

Martin Menzies; Douglas Blanchard; Jeff Jacobs

1977-01-01

127

Mantle petrology and mineralogy of the Thetford Mines Ophiolite Complex  

NASA Astrophysics Data System (ADS)

The Ordovician Thetford Mines ophiolite complex (TMOC) formed by boninite-fed seafloor-spreading, probably in a fore-arc environment. The mantle section is dominated by foliated harzburgite (? 5-6% clinopyroxene), cut by dunitic (± chromitite cores) and orthopyroxenitic veins and dykes. Contrasting structures, textures and mineral compositions allow us to subdivide the mantle. The granular-textured rocks of the Duck Lake Block (DLB) have two steeply-dipping foliations. The older foliation strikes NW, is sub-perpendicular to the Moho, and is interpreted to have resulted from upflow of the asthenosphere beneath the spreading ridge. This fabric is overprinted by a 2nd ductile foliation striking ENE, oriented sub-parallel to the Moho, which we interpreted as having formed by crust-mantle shear as the lithosphere migrated away from the spreading ridge. The DLB mantle has a limited range of spinel Cr# (100Cr / (Cr + Al) = 51-71). Comparison with experimentally determined residual spinel compositions (equilibrium melting) implies a maximum loss of 27-38% melt if the protolith had a fertile MORB mantle composition. However, interstitial-textured clinopyroxene may have high TiO 2 (< 0.04wt.%) and Na 2O (< 0.27wt.%), and some interstitial spinel has higher TiO 2 (< 0.09wt.%), suggesting interaction with (or crystallization from) an "impregnating" melt. Interstitial tremolitic amphibole also indicates the passage of late hydrous fluids. The harzburgite in the Caribou Mountain Block (CMB) has a porphyroclastic texture, with a strong, locally mylonitic foliation striking roughly N-S, parallel to the orientation of seafloor-spreading related paleo-normal faults in the crust. These fabrics and textures imply a colder, lithospheric deformation, possibly related to tectonic denudation (oceanic core complex). This would explain problematic lava/mantle contacts, favour infiltration of seawater, serpentinization, and reduced fO 2 conditions. The CMB mantle shows a wider range of mineral compositions than the DLB, with spinel Cr# (28-86) implying ? 15-45% of equilibrium melting. Locally higher TiO 2 in spinel (< 0.05wt.%) and clinopyroxene (< 0.11wt.%), a local rimward decrease in spinel Cr#, clinopyroxene Cr#, and olivine Fo-content, and traces of interstitial amphibole, are attributed to the circulation of an evolved hydrous melt during peridotite deformation. This suggests that the lower limit to the extent of melting inferred for the CMB (15%), established on the basis of Al-rich spinel rims and neoblasts, is probably too low. On the other hand, the higher inferred degree of depletion of the CMB is probably unaffected by the metasomatic overprint and is a more robust conclusion.

Pagé, Philippe; Bédard, Jean H.; Schroetter, Jean-Michel; Tremblay, Alain

2008-01-01

128

The fossil hydrothermal rootzone from the Northern Apennine ophiolites (Italy)  

NASA Astrophysics Data System (ADS)

The Northern Apennine ophiolites are lithosphere remnants of the Late Jurassic -- Early Cretaceous Ligurian Tethys, which is considered to have developed in conjunction with the opening of the Central Atlantic Ocean. In the Bonassola area, a km-scale gabbroic body permits the study of the magmatic-hydrothermal transition. The body mostly consists of coarse-grained gabbros of cumulus origin that exhibit granulite-facies recrystallization along ductile shear zones, which most likely occurred in the absence of seawater-derived fluids. These shear zones are crosscut at high angle by parallel swarms of hornblende (± plagioclase) veins. The development of these veins is correlated with coronal hornblende growth at the expenses of igneous clinopyroxene in the host gabbro. Scattered, elongated bodies of hornblende-bearing albitites also crop out. In particular, two different generations of albitite bodies have been recognized. The albitite (1) bodies show irregular contacts against the host gabbro, which are characterized by hornblende-rich reaction zones. These albitites are inferred to have developed when the gabbro was not completely solidified. The albitite (2) bodies has sharp contacts, post-date the granulite-facies foliation in the host gabbros, and show the same elongation direction of hornblende veins. The albitite (2) bodies are therefore related to the same brittle deformation event that gave rise to the hornblende veins. Major, trace, halogen and oxygen isotope analyses of hornblende from both veins and albitite bodies have been carried out. The geochemical signature of hornblende from albitite (1) bodies and related contact reaction zones is similar to that of accessory titanian pargasite of igneous origin in the host gabbro, thus indicating that these albitites were derived by extreme differentiation of basaltic liquid. Two different chemical fingerprints have been recognized for the vein hornblendes. The first type indicates a formation by local reaction between migrating seawater-derived fluids and the gabbros. The development of these veins can be ascribed to a high-temperature, amphibolite-facies hydrothermal event. On the other hand, the hornblende from the less diffuse, thickest and fibrous veins has intermediate geochemical features, similar to the hornblende from albitite (2) bodies. These hornblendes provide evidence for interaction between magmatic and hydrothermal systems.

Tribuzio, R.; Zanetti, A.; Dallai, L.

2003-04-01

129

Geochemical complexities preserved in the volcanic rocks of the Zambales Ophiolite, Philippines  

NASA Astrophysics Data System (ADS)

The geochemical characteristics of the volcanic rocks from the Zambales Ophiolite, Luzon, Philippines, indicate that the oceanic crust now preserved in the ophiolite must have been generated in an oceanic subduction zone environment. The volcanic rocks and dikes range in composition from magnesian basalt (including mid-ocean ridge basalt (MORB), island arc tholeiite, and boninite-like lavas) to andesite and dacite and have trace element abundances which suggest derivation from a mantle source which was heterogeneous on both regional and local scales. the sources are mixtures of components variably depleted with respect to a MORB source. Differences exist between respective exposures of the ophiolite. From north to south the following geochemical patterns in the basaltic rocks emerge: Sm/Nd decreases with corresponding decreases in 143Nd/144Nd. Rare earth element (REE) patterns range from extremely light REE (LREE) depleted with generally low abundances of all REE (1-5X chondritic) to patterns which are slightly LREE depleted, similar to MORB patterns. Throughout the range, Sr isotopic abundances are relatively high (87Sr/86Sr = 0.7035-0.7040). Perhaps more significant than the regional geochemical trends, however, is that all volcanic provinces display ranges of rock compositions, and the spatial distribution of basalts with such variable geochemical affinities indicate superposition, interfingering, and possible mixing of different lavas. We suggest that the array of geochemical data from the Zambales ophiolite can be explained in terms of processes observed in present-day convergent plate margins, such as the Marianas or Lau Basin in the western Pacific. Complicated plate boundaries which have existed for long periods of time, including closely opposing and changing subduction systems, the rifting of arcs, and the formation of backarc basins may result in the superposition of one lava type on another or may produce many small domains in the upper mantle sources for subduction-related lavas, some of which become extremely depleted or secondarily enriched. Magmas derived from such a heterogeneous mantle will display ranges in geochemical characteristics, possibly similar to those observed in the Zambales ophiolite. Indeed, recent studies from present-day backarc systems suggest that young arc volcanism is superimposed on MORB-like crust and that basalt compositions of certain backarc basins are zoned inward from arc-like to MORB-like. These studies indicate that relationships between arc and MORB-like crust in young subduction complexes is extremely complex (Hawkins and Melchior, 1985; Volpe et al. 1987, 1990). We extrapolate arguments from studies of zoned backarc systems to the Zambales Ophiolite and, in turn, use data from the Zambales Ophiolite as a ruler for defining the scale of variation in these complex supra-subduction zone systems.

Evans, Cynthia A.; Casteneda, Gerry; Franco, Helen

1991-09-01

130

U-Pb zircon geochronology of the Internal Liguride ophiolite (Northern Apennine, Italy)  

NASA Astrophysics Data System (ADS)

The ophiolitic bodies from the Alpine-Apennine belt are lithosphere remnants of the Middle to Late Jurassic Ligure-Piemontese basin. Most of these ophiolites preserve primary features recalling the magma-poor continental margin of Western Iberia. On the other hand, the Internal Liguride ophiolites from Northern Apennine display close structural and compositional similarities to modern slow and ultraslow spreading ridges. The Internal Liguride ophiolites preserve evidence for the presence of morphological highs made of gabbroic plutons within mantle peridotites (Cortesogno et al. 1987). The gabbroic plutons consist mostly of coarse-grained gabbros to olivine-gabbros interlayered with sill-like hectometre-scale lenses of olivine-rich troctolites (e.g. Tribuzio et al. 2000). The gabbro-peridotite association records a high-temperature deformation event in ductile shear zones and is locally crosscut by later dolerite dykes with chilled margins. Waning of the magma supply and onset of hydrothermalism was followed by the exposure of the gabbro-peridotite association at the seafloor by amagmatic tectonic extension. The morphological depressions from the Internal Liguride domain are recognised on the basis of massive and pillow basalt flows interlayered with breccias and cherts (Cortesogno et al. 1987). Whereas the geological and petrological evolution of the Internal Liguride ophiolites was extensively investigated, little is know about the timing of the magmatic events recorded by the Internal Liguride ophiolites. Previous geochronological studies yielded a discordant U-Pb zircon age of 153 +/- 1 Ma for a plagiogranite emplaced within the mantle peridotites (Borsi et al. 1996), and a Sm-Nd mineral isochron of 164 +/- 14 Ma for the gabbros (e.g. Rampone et al. 1998). On the basis of new single-grain U-Pb zircon datings by laser ablation ICP-MS, we therefore wish to constrain the timing of the evolution recorded by the Internal Liguride ophiolites. The preliminary U-Pb zircon data indicate that the formation of Internal Liguride igneous rocks cover a relatively long time span, thus gaining insights into absolute timing, duration and patterns of magma emplacement. Cortesogno L, Galbiati B, Principi G (1987) Ofioliti 12: 261-342 Borsi L, Schaerer U, Gaggero L, Crispini L (1996) Earth Planet Sci Lett 140:227-241 Rampone E, Hofmann AW, Raczek I (1998). Earth Planet Sci Lett 163: 175-89 Tribuzio R, Tiepolo M, Vannucci R (2000) Geol Soc Am Spec Paper 349: 129-138

Garzetti, F.; Tribuzio, R.; Tiepolo, M.

2009-04-01

131

Eclogite nappe-stack in the Grivola-Urtier Ophiolites (Southern Aosta Valley, Western Alps)  

NASA Astrophysics Data System (ADS)

In the Western Alpine chain, ophiolites represent a section of the Mesozoic Tethys oceanic lithosphere, involved in subduction during the convergence between the paleo-Africa and paelo-Europe continents during the Cretaceous - Eocene. The Western Alpine ophiolites consist of several tectonic units, the most famous being the Zermatt-Saas and Combin nappes, and other major ophiolite bodies as the Voltri, Monviso, and Rocciavrè that show different rock assemblages and contrasting metamorphic imprints. The Grivola-Urtier (GU) unit is exposed in the southern Aosta Valley, covering an area of about 100 km2; it is tectonically sandwiched between the continentally-derived Pennidic Gran Paradiso Nappe below, and the Austroalpine Mount Emilius klippe above. This unit has been so far considered as part of the Zermatt-Saas nappe extending from the Saas-Fee area (Switzerland) to the Aosta Valley (Italy). The GU unit consists of serpentinized peridotites that include pods and boudinaged layers of eclogitic Fe-metagabbro and trondhjemite, rodingites and chloriteschists transposed in the main foliation together with calcschists and micaschists. All rocks preserve particularly fresh eclogitic mineral assemblages. The contact between the serpentinites and calcshists is marked by a tectonic mélange consisting of mylonitic marble and calcschist with stretched and boudinaged serpentinite blocks. Continentally-derived allochthonous blocks ranging in size from100 meters to meters are also included within the ophiolites. New field, petrographic and geochemical data reveal the complex nature of the fossil Tethyan oceanic lithosphere exposed in the southern Aosta Valley, as well as the extent and size of the continental-oceanic tectonic mélange. The geological setting of the GU unit is here inferred as a key tool for understanding the complex architecture of the ophiolites in the Western Alps.

Tartarotti, Paola

2013-04-01

132

Accessory minerals and subduction zone metasomatism: a geochemical comparison of two me??langes (Washington and California, U.S.A.)  

USGS Publications Warehouse

The ability of a subducted slab or subducted sediment to contribute many incompatible trace elements to arc source regions may depend on the stabilities of accessory minerals within these rocks, which can only be studied indirectly. In contrast, the role of accessory minerals in lower-T and -P metasomatic processes within paleo-subduction zones can be studied directly in subduction-zone metamorphic terranes. The Gee Point-Iron Mountain locality of the Shuksan Metamorphic Suite, North Cascades, Washington State, is a high-T me??lange of metamafic blocks in a matrix of meta-ultramafic rocks. This me??lange is similar in geologic setting and petrology to the upper part of an unnamed amphibolite unit of the Catalina Schist, Santa Catalina Island, southern California. Both are interpreted as shear zones between mantle and slab rocks that formed during the early stages of subduction. Some garnet amphibolite blocks from the Gee Point-Iron Mountain locality display trace-element enrichments similar to those in counterparts from the Catalina Schist. Some Catalina blocks are highly enriched in Th, rare-earth elements (REE), the high-field-strength elements Ti, Nb, Ta, Zr and Hf (HFSE), U and Sr compared to mid-ocean ridge basalt (MORB), and to other garnet amphibolite blocks in the same unit. Textural and geochemical data indicate that accessory minerals of metamorphic origin control the enrichment of Th, REE and HFSE in blocks from both areas. The Mg-rich rinds around blocks and the meta-ultramafic matrix from both me??langes are highly enriched in a large number of trace elements compared to harzburgites, dunites and serpentinites. Evidence for recrystallization or formation of accessory minerals in the former rocks suggests that these minerals control some of the trace-element enrichments. Data from the Gee Point and Catalina me??langes suggest that the accessory minerals titanite, rutile, apatite, zircon and REE-rich epidote play a significant role in the enrichment of trace elements in both mafic and ultramafic rocks during subduction-related fluid-rock interaction. Mobilization of incompatible elements, and deposition of such elements in the accessory minerals of mafic and ultramafic rocks may be fairly common in fluid-rich metamorphic environments in subduction zones. ?? 1993.

Sorensen, S. S.; Grossman, J. N.

1993-01-01

133

Platinum-group elements in rocks from the voikar-syninsky ophiolite complex, Polar Urals, U.S.S.R.  

USGS Publications Warehouse

Analyses of platinum-group elements (PGE) in rocks collected from the Voikar-Syninsky ophiolite in the Polar Urals suggest that the distribution and geochemistry of PGE in this Paleozoic ophiolite are similar to those in Mesozoic ophiolites from elsewhere. Chondrite-normalized PGE patterns for chromitite, the tectonite unit, and ultramafic and mafic cumulate unit have negative slopes. These results are similar to those found for chromitites from other ophiolites; stratiform chromities show positive slopes. If the magmas that form both types of chromitite originate from similar mantle source material with respect to PGE content, the processes involved must be quite different. However, the distinct chondrite-normalized PGE patterns may reflect differing source materials. ?? 1983 Springer-Verlag.

Page, N. J.; Aruscavage, P. J.; Haffty, J.

1983-01-01

134

Mechanisms of formation of mantle section pyroxenites of Voykar Ophiolite, Polar Urals, Russia  

NASA Astrophysics Data System (ADS)

Ural Mountains mark a major mid Paleozoic collision event, which resulted in the closure of an ocean basin separating the Siberian and European plates. Voykar Ophiolite is located in the Northern part of Uralian Ophiolite Belt. Ophiolitic sequence rocks of Polar Urals are usually considered as giant fragments of mantle and MORB-type crust formed in back-arc settings (e.g. Savelieva et al., 1987). Mantle section of Voykar Ophiolite comprises most of the ophiolitic sequence. It is up to 8 km thick and consists mostly of spinel harzburgites with multiple dunitic bodies and pyroxenitic veins representing pathways for different melts/fluids. While it is generally accepted that dunites in mantle sections are formed by melt-rock reaction and mark melt pathways (e.g. Kelemen et al., 1995), formation of pyroxenites is a subject of debate. Often pyroxenites from mantle sections of ophiolites (Varfalvy et al., 1997), as well as pyroxenites from mantle wedge xenoliths (Arai et al., 2006, Bali et al., 2007, Gregoire et al., 2008) are interpreted as interaction products between high-SiO2 melts and mantle peridotites. Such melts are believed to be widespread in SSZ mantle: boninites, high-MgO andesites and adakites. However, some researchers (e.g. Berly et al., 2006, Halama et al., 2009) propose pyroxenite formation in metasomatic reaction with fluid from subducting plate. Moreover, some pyroxenites could be formed by the melt crystallization in hydrous conditions (Muntener et al., 2001). We present comprehensive study of mineral major and trace element compositions from the mantle section rocks of Voykar Ophiolite in order to determine mechanism of formation of pyroxenites in ophiolitic mantle sections. Compositions of clinopyroxene and olivine from pyroxenites were compared to their compositions in harzburgites and dunites. Furthermore, compositions of clinopyroxene and magmatic amphibole from mantle section pyroxenites were used to calculate equilibrium melts. Geochemical data (enrichment of clinopyroxenes and amphiboles in LREE and LILE (Rb, Ba, Sr) relative to HFSE (Nb, Zr)) together with structural data suggests that pyroxenites display clear suprasubduction signatures and refer to the last stage of Voykar Ophiolite mantle section formation. All minerals from pyroxenites tend to have lower Mg-numbers and, therefore, high-Si meltsfluids have played major role in their formation. Depletion of clinopyroxenes in HREE and enrichment in Sr and LREE across the harzburgite-pyroxenite contacts suggests that this high-Si melts most probably refer to depleted SSZ melts, such as boninites. One group of magmatic amphibole in pyroxenites refers to such melts. However, another group of magmatic amphiboles probably refers to high-Si fluid depleted in REE and enriched in LILE (Rb, Ba, Sr) and Pb. Therefore, the variety of pyroxenite segregations, veins, and dikes reflects progressive stages of melt/fluid migration through harzburgite and dunite at various P-T conditions. For some pyroxenites (such as zoned websterite dikes) formation by fractional crystallization of hydrous magmas couldn't be excluded.

Belousov, Ivan; Batanova, Valentina; Sobolev, Alexander; Savelieva, Galina

2010-05-01

135

Exploring the deep biosphere through ophiolite-associated surface springs  

NASA Astrophysics Data System (ADS)

The process of serpentinization in the subsurface produces highly reduced, high pH fluids that provide habitats for microbial communities. At depth, these fluids likely contain appreciable hydrogen and methane gas, very little inorganic carbon, and limited electron acceptors. As fluids affected by serpentinization reach the oxygenated surface environment, microbial biomes may shift to include organisms capable of metabolizing oxygen. Our study of serpentinizing and hydrothermal fluids in the Zambales and Palawan regions of the Philippines includes a suite of geochemical analyses to define microbial habitats, targeted culturing efforts, and community analysis (the latter presented in a companion abstract). We measured temperature, pH, conductivity, ORP, dissolved oxygen, nitrate, ammonia, iron, sulfide, and silica in the fluids on site, and have analyzed dissolved gases, major ions, and trace elements. These analyses provide a description of the near subsurface and surface ecosystems of the study areas. Carbon and nitrogen isotopic ratios in solid biofilm and biominerals from the springs allow inference of metabolic cycling. Fluids expressed from the Zambales and Palawan ophiolites range from pH 9-11.5, 0.06-2 mg/L dissolved oxygen, 0-3.7 mg/L sulfide, and 30-800 mg/L silica in the source pools. Temperatures vary from 34-50C. Analysis of dissolved gases reveal concentrations of H2 and CH4 > 10uM, CO2 > 1 mM, and trace amounts of CO. This variability in fluid and gas composition indicates the potential for different mixing ratios with local meteoric water, local hydrothermal fluids, and deep-seated sediments. Results will be compared with other serpentinizing fluids to place these habitats in context of the ecology of serpentinizing springs globally. Anaerobic and aerobic growth media were designed to target heterotrophy, sulfate reduction, and iron reduction (the latter two with, and without organic carbon input). We have calculated that these processes have potential to yield energy for metabolism in these ecosystems. Media were diluted with filtered spring fluid to the desired concentration on location, and inoculated immediately. We found positive growth in cultures from all sample locations (seven in all), at temperatures ranging from 28-48C under both aerobic and anaerobic conditions. Growth on complex organic media was successful in five samples, and media including sugars or organic acids in two and four samples, respectively. Heterotrophic sulfate reduction was seen in six samples, and autotrophic sulfate reduction in only three samples. Four locations yielded heterotrophic iron reducers, and five locations host organisms capable of autotrophic iron reduction. This variety of positive growth indicates a metabolically flexible community, complementing data obtained from previously reported communities in serpentinizing systems. We begin to obtain a picture of community dynamics and functional diversity in these ecosystems that bridge the subsurface and surface biospheres.

Meyer-Dombard, D. R.; Cardace, D.; Woycheese, K. M.; Vallalar, B.; Arcilla, C. A.

2013-12-01

136

Samarium-neodymium data on two late Proterozoic ophiolites of Saudi Arabia and implications for crustal and mantle evolution  

Microsoft Academic Search

Whole-rock and mineral samples from the Jabal al Wask and Jabal Ess ophiolites, northwestern Saudi Arabia, yield Sm-Nd isochron ages of 743+24 Ma and 782±38 Ma, respectively. These formation ages, which provide maximum limits for possible obduction ages, are in broad but not precise agreement with the previously known geologic history of the Arabian Shield. They indicate that the ophiolitic

Stefan Claesson; John S. Pallister; Mitsunobu Tatsumoto

1984-01-01

137

Unusual mantle mineral group from chromitite orebody Cr11 in Luobusa ophiolite of Yarlung-Zangbo suture zone, Tibet  

Microsoft Academic Search

A wide variety of unusual mantle has been reported from podiform chromitite orebodies Cr-31 and Cr-74 in the Luobusa (???)\\u000a ophiolite, Tibet. A detailed investigation of chromitite orebody Cr-11, located in the Kangjinla (???) district at the eastern\\u000a end of the ophiolite, has revealed many of the same minerals, including diamond, moissanite, and some native elements, alloys,\\u000a oxides, sulphides, silicates,

Xiangzhen Xu; Jingsui Yang; Songyong Chen; Qingsong Fang; Wenji Bai; Dengzhu Ba

2009-01-01

138

Microbial Community Diversity in Fault-Associated and Ophiolite-Hosted Springs  

NASA Astrophysics Data System (ADS)

Deep biosphere habitats and hydrothermal systems are ideal candidates for analog ecosystems to life on Early Earth and Astrobiological targets. They also likely harbor vast repositories of novel biological and genetic diversity. This study compares the biological and genetic diversity of microbial communities in terrestrial hydrothermal and cool fluid seeps and springs, occuring in both ophiolite-hosted and non-ophiolite sequences. Fluids and solids (biofilms and sediment) with variable fractions of ultramafic-sourced and serpentinizing reaction fluids and mineral fragments were collected from surface seeps and deeply-sourced springs associated with the Northern Anatolian Fault Zone (NAFZ, Turkey) and the Anita Shear Zone (ASZ, New Zealand). Comparisons are drawn between three regimes: 1) cool fluid systems in ophiolite-hosted sequences in NAFZ vs. ASZ, 2) cool fluid vs. hydrothermal fluid systems in the NAFZ, and 3) hydrothermal systems in NAFZ ophiolite-hosted vs. non-ophiolite sequences. These comparisons help differentiate microbial community structure and metabolic strategies between hydrothermal and serpentinizing input to these ecosystems. The integration of geobiological data from these sites clarifies how microbial systems respond to even subtle shifts in geochemistry of the water-rock system, and our consideration of mafic/ultramafic rocks as habitable formations brings new astrobiological relevance to this work. Microbial communities were examined using a suite of culture-dependant and independent methods, co-registered with a network of geochemical contextual samples. Geochemical datasets allow prediction of available sources of energy in these nutrient-limited ecosystems. Sample locations varied in temperature 30-90C and pH 6.5-9.0 Potential sources of energy and carbon include dissolved organic carbon, CO2, sulfide, sulfate, and ferrous iron, depending on the sample location. Enrichments were obtained using a variety of carbon and energy sources, in growth media that are designed to mimick each natural system. Heterotrophic cultures, in both aerobic and anaerobic media, were obtained for all cool and hydrothermal fluid locations. In addition, evidence for microbial sulfate reduction and iron oxidation were seen in the NAFZ hydrothermal ophiolite-hosted samples. Nucleic acid-based analysis of sediments and biofilms revealed wide variation in diversity of 16S rRNA and nitrogen cycle related genes in each system. We present field observations and interpret new data, establishing a geobiological baseline for previously undescribed sites of active serpentinization.

Meyer-Dombard, D. R.; Cardace, D.; Uzunlar, N.; Gulecal, Y.; Yargicoglu, E. N.; Carbone, J. N.

2010-12-01

139

40Ar 39Ar Ages and tectonic setting of ophiolite from the Neyriz area, southeast Zagros Range, Iran  

USGS Publications Warehouse

An ophiolite, considered to be an allochthonous fragment of Tethyan oceanic crust and mantle, crops out near Neyriz in the Zagros Range, Iran. 40Ar 39Ar ages ranging from 76.8 ?? 23.8 Ma to 105 ?? 23.3 Ma were measured on hornblende from five samples of plagiogranite and diabase from the ophiolite. The most precise ages are 85.9 ?? 3.8 Ma for a diabase and 83.6 ?? 8.4 Ma for a plagiogranite. The weighted mean age of hornblende from the five samples is 87.5 ?? 7.2 Ma which indicates that the igneous part of the Neyriz ophiolite formed during the early part of the Late Cretaceous. Pargasite from amphibolite below peridotite of the Neyriz ophiolite has a 40Ar 39Ar age of 94.9 ?? 7.6 Ma. The pargasite age agrees within analytical uncertainty with the ages measured on diabase and plagiogranite. Comparable ages have been measured on igneous rocks from the Samail ophiolite of Oman and on amphibolite below peridotite of the Samail ophiolite. ?? 1983.

Lanphere, M. A.; Pamic, J.

1983-01-01

140

Petrology of Lasail plutonic complex, northern Oman ophiolite, Oman: An example of arc-like magmatism associated with ophiolite detachment  

NASA Astrophysics Data System (ADS)

Lasail plutonic complex (4.7 × 3.8 km), as a typical example of late stage intrusive rocks, is located to the south of Wadi Jizi, and intrudes into the base of V1 volcanic rocks and sheeted dike complex. The Lasail plutonic complex consists of various rock types ranging from ultramafic cumulates to tonalite, and is associated with minor amounts of axis stage gabbro to quartz diorite. These rocks are classified into the following eleven rock types: massive gabbro 1, quartz diorite 1 (axis stage intrusive rocks), olivine websterite, olivine gabbronorite, gabbronorite, hornblende gabbronorite, leucogabbronorite (layered gabbros), massive gabbro 2, diorite, quartz diorite 2, and tonalite (late stage intrusive rocks). The layered gabbros are intruded by the massive gabbro 2, and often occur as large irregular blocks in the massive gabbro 2. The massive gabbro 2 intrudes into the layered gabbros, and sometimes grades into hornblende gabbronorite layer of the layered gabbros. In some places, anastomosing veins of the hornblende quartz gabbronorite injected into the gabbronorite, which continue to the layer of leucogabbronorite. These gabbroic rocks are intruded by small intrusions of diorite, quartz diorite 2, and tonalite. The quartz diorite 2 forms rather larger intrusive bodies that intrude into the gabbroic rocks in the higher level. The tonalite occurs as thin dike and sheet mainly in the layered gabbros. N-MORB normalized trace element patterns for the massive gabbro 2 are characterized by enrichment of LILEs relative to REEs, and resemble to 'subduction component' from the island arc tholeiite except a weak enrichment for middle to light REE and P. In contrast, the massive gabbro 1 (axis stage gabbro) shows rather flat pattern similar to MORB with no remarkable 'subduction component'. From the examination of Sr and Nd isotopic signature, the Lasail plutonic rocks are characterized by higher ?-Sr values than those of MORB. This suggests that the primitive magma of Lasail complex was influenced by the rocks with higher ?-Sr values than those of MORB, e.g., the axis stage rocks interacted with seawater. These lines of evidence suggests that the massive gabbro 2 was formed by the partial melting of residual MORB mantle which is contaminated with slab melt derived from the axis stage rocks interacted with seawater. In addition, petrogenesis of felsic rocks in the Lasail complex can be explained by the partial melting model of pre-existing layered gabbro. U-Pb zircon ages analyzed by LA-ICPMS are 100 ± 2 and 99 ± 2 Ma for late stage tonalite and 100 ± 1 Ma for axis stage quartz diorite. These ages are slightly older than the ages reported for felsic rocks in the Oman ophiolite (ca., 95 Ma), and suggest that the conversion from ridge stage to detachment stage took place rapidly.

Tsuchiya, Nobutaka; Shibata, Tomoyuki; Yoshikawa, Masako; Adachi, Yoshiko; Miyashita, Sumio; Adachi, Tatsurou; Nakano, Nobuhiko; Osanai, Yasuhito

2013-01-01

141

Petrology and geochemistry of the Late Jurassic calc-alkaline series associated to Middle Jurassic ophiolites in the South Apuseni Mountains (Romania)  

Microsoft Academic Search

Abstract In the South Apuseni Mountains (SAM), Romania, Jurassic calc-alkaline magmatic series occur in association with Jurassic ophiolites within a narrow belt that marks the boundary,between,the Eurasian and Adria Paleozoic conti- nental margins. This association of magmatic series has been previously reported,as a single ophiolitic sequence,by many authors. Calc-alkaline rocks include volcanic and intrusive rocks and, along with associated ophiolites,

Ionel Nicolae; Emilio Saccani

2003-01-01

142

Zircon U-Pb geochronology of the Zambales and Angat ophiolites, Luzon, Philippines: Evidence for an Eocene arc-back arc pair  

NASA Astrophysics Data System (ADS)

Two basement terranes, the Zambales ophiolite in the west, and the Angat ophiolite in the east, are exposed on the island of Luzon, separated by a circa 10 km thich and circa 100 km wide sedimentary basin. The structural and age relationships between the two ophiolitic blocks are center to understanding the geologic and tectonic development of the northern Philippines and evaluating models of terrane evolution proposed for this area of the western Pacific. We analyzed zircons from the Zambales and Angat terranes to better constrain their origin. Two zircon fractions from tonalite in the Acoje block of the Zambales ophiolite give concordant U-Pb ages at 44.2 (+/- 0.9) Ma. Two zircon fractions from plagiogranite and one fraction from diorite in the Coto block of the Zambales ophiolite give concordant U-Pb ages of 45.1 (+/- 0.6) Ma. These results provide a Middle Eocene age for the Zambales ophiolite, in agreement with the minimum Late Eocene age of the overlying Akistero Formation. No age difference is discerned between the arc-like Acoje block and MORB-like Coto block of the Zambales ophiolite. Four zircon fractions from two sample sites in the Angat ophiolite give concordant ages of 48.1 (+/- 0.5) Ma. This age is considerably younger than the Late Cretaceous age based on radiolarian fauna derived from a sheeted dike-pillow lava-sediment sequence south-southeast of the main ophiolite. The small age difference between the Zambales and Angat ophiolites suggests a common origin and obviates the need for a major structural discontinuity west of the Southern Sierra Mandre beneath the Central Valley of Luzon. The Cretaceous biostratigraphic ages of the ophiolitic rocks southeast of the Eocene Angat ophilolite implies that there are two ophiolitic basements exposed in the Southern Sierra Mandre. The relationship between the two ophiolites is constrainted by the overlying stratigraphic relations wich indicate that an Eocene volcanic arc and associated volcaniclastic apron was built on both the Eocene and Cretaceous ophiolitic basement. This suugests that the Zambales-Angat ophiolite represents a preserved Eocene back-arc basin that opened behind an Eocene arc that developed within Cretaceous oceanic basement. In this model, the Zambales-Angat ophiolites are therefore not allochthonous terranes but part of a single plate, generated in situ, forming part of the autochthonous basement of Luzon.

Encarnacion, John P.; Mukasa, Samuel B.; Obille, Eligio C., Jr.

1993-11-01

143

Semial ophiolite veins of United Arab Emirates—A fluid inclusions study  

Microsoft Academic Search

Calcite and quartz veins in the gabbroic and dunite rocks of the Semial ophiolite (UAE) were selected for fluid inclusion\\u000a analysis. The inclusions contain both aqueous low-salinity and hydrocarbon-dominated fluids. Microthermometry data indicate\\u000a that the aqueous fluids contain 0.22 to 1.45 equivalent wt% NaCl and occasionally contain traces of hydrocarbons. Homogenization\\u000a to liquid occurred between 91 and 152°C. Modeling based

Mohamed El Tokhi; Abdulla Musallam

2011-01-01

144

Stratigraphy and radiolarians of upper cretaceous sedimentary cover of the Arakapas ophiolite massif (Cyprus)  

Microsoft Academic Search

In the basal interval, sedimentary cover of the Arakapas ophiolite massif (southern Cyprus) is composed of metalliferous sediments\\u000a of the Perapedhi Formation that is divided into three sequences based on diverse radiolarian assemblages. These are basal\\u000a umbers of the Cenomanian age presumably (2–20 m), interlayering cherts and umbers of the Turonian-Coniacian (6–10 m), and\\u000a opoka-like cherts of the Coniacian-Santonian. Higher

L. G. Bragina; N. Yu. Bragin

2006-01-01

145

Platinum group minerals in podiform chromitites of the Bou Azzer ophiolite, Anti Atlas, Central Morocco  

NASA Astrophysics Data System (ADS)

The Neoproterozoic Bou Azzer ophiolite complex hosts numerous, small lenticular bodies of massive and disseminated chromite. Metallurgical-grade high-Mg and high-Cr spinels (cores with 48-62 wt% Cr2O3) reveal complex alteration patterns of successive Cr and Mn enrichment and loss of Al towards the rims, while the Mg# ratios [(Mg/(Mg + Fe2+)] remain almost constant. Concentration patterns of platinum-group elements are typical for ophiolitic chromitite poor in sulfides, with predominance of the IPGE, variable Rh, and low Pt and Pd. The most abundant platinum-group mineral is Rh-bearing laurite that occurs either included in spinel or in silicate matrix, whereas Os-Ir-Ru alloy is always included in spinel. Laurite inclusions reveal complex intergrowth textures with Rh-Ru-Pt rich alloy, and with Rh-rich sulfide. Most laurites display trends to sulfur-poor compositions leading to local formation of very fine-grained Ru-Os-Ir alloy phases. Ni-Co-Fe sulfides, arsenides and sulfarsenides devoid of PGE are associated with the alteration of chromite. Textural position and chemical composition of the base metal inclusions, as well as comparison of alteration features between chromite and accessory chromian spinel in the Co-Ni-As ores of the Bou Azzer ophiolite indicate a close connection. It is suggested that hydrothermal fluids percolated through the marginal zones of the ophiolite belt during greenschist facies metamorphism and deposited Ni-Co-Fe arsenides, sulfarsenides and minor sulfides as accessories within altered chromitites, and also in structurally favourable zones as Ni-Co-As ores.

El Ghorfi, M.; Melcher, F.; Oberthür, T.; Boukhari, A. E.; Maacha, L.; Maddi, A.; Mhaili, M.

2008-01-01

146

Hydrous magmatism triggered by assimilation of hydrothermally altered rocks in fossil oceanic crust (northern Oman ophiolite)  

NASA Astrophysics Data System (ADS)

Mid-ocean ridges magmatism is, by and large, considered to be mostly dry. Nevertheless, numerous works in the last decade have shown that a hydrous component is likely to be involved in ocean ridges magmas genesis and/or evolution. The petrology and geochemistry of peculiar coarse grained gabbros sampled in the upper part of the gabbroic sequence from the northern Oman ophiolite (Wadi Rajmi) provide information on the origin and fate of hydrous melts in fast-spreading oceanic settings. Uncommon crystallization sequences for oceanic settings (clinopyroxene crystallizing before plagioclase), extreme mineral compositions (plagioclase An% up to 99, and clinopyroxene Mg # up to 96), and the presence of magmatic amphibole, imply the presence of a high water activity during crystallization. Various petrological and geochemical constraints point to hydration, resulting from the recycling of hydrothermal fluids. This recycling event may have occurred at the top of the axial magma chamber where assimilation of anatectic hydrous melts is recurrent along mid-ocean ridges or close to segments ends where fresh magma intrudes previously hydrothermally altered crust. In ophiolitic settings, hydration and remelting of hydrothermally altered rocks producing hydrous melts may also occur during the obduction process. Although dry magmatism dominates oceanic magmatism, the dynamic behavior of fast-spreading ocean ridge magma chambers has the potential to produce the observed hydrous melts (either in ophiolites or at spreading centers), which are thus part of the general mid-ocean ridges lineage.

France, Lydéric; Ildefonse, Benoit; Koepke, Juergen

2013-08-01

147

Structural features of ophiolitic chromitites in the Zambales Range, Luzon, Philippines  

NASA Astrophysics Data System (ADS)

The chromitite-bearing peridotites of the Zambales mafic-ultramafic complex form the lowermost level of the Zambales ophiolite, which exposes a complete ophiolitic sequence. The chromitites occur close to the peridotite/gabbro transition zone. The chromite orebodies are structurally classified into three major types: (1) concordant tabular deposits, (2) strings of pods and (3) pocketlike deposits. Concordant tabular deposits show a gradational transition from chromitite to host rock (modal grading) and are characterized by the parallelism of ore and host-rock structures. Primary magmatic features like inch-scale layering, size grading, glomeroporphyric chromite aggregates, skeletal chromite growth and adcumulus growth (cumulus textures) are common. The concordant chromite bodies are often tectonically disrupted and boudined forming strings of pods or fault-controlled pocketlike deposits. With increasing tectonization chromite shows pull-apart textures and lineations (plastic deformation), shearing, prismatic jointing, brecciation and mylonitization (brittle deformation). Recrystallization of cataclastic chromite occurs on a microscopic scale. Plastic deformation is caused by mantle flow and/or the volume increase of the peridotites during serpentinization. The influence of mantle flow is indicated by the orientation of the pod strings and lineations in chromitite perpendicular to the ridge axis. Brittle deformation of chromite (cataclasis) and disruption by faults is related to the emplacement of the ophiolite.

Hock, M.; Friedrich, G.

1985-10-01

148

Paleomagnetism, paleogeographic origins, and uplift history of the Coast Range ophiolite at Mount Diablo, California  

USGS Publications Warehouse

Divergent paleogeographic origins have been proposed for the Coast Range ophiolite of western California which are testable using paleomagnetic methods. Paleomagnetic data for Middle Jurassic pillow lavas and diabase sills of the Coast Range ophiolite at Mount Diablo, northern California, indicate that they contain two components of remanent magnetization. The characteristic directions have normal and reversed polarities and apparently are carried by Ti-poor magnetite. This magnetization is inferred to have been acquired during emplacement and seafloor alteration at an ancient spreading ridge. The paleolatitude calculated from its structurally corrected mean direction is 20??N ?? 9?? and agrees with the expected direction for stable North America; this result is also consistent with the concordant paleolatitude (32??N ?? 8??) recently determined for Upper Jurassic Coast Range ophiolite at Stanley Mountain in southern California. In addition, clockwise vertical axis rotation of Mount Diablo (143?? ?? 11??) is indicated by the characteristic magnetization direction. An overprint component is inferred to have been acquired during uplift of Mount Diablo since the Miocene.

Hagstrum, J. T.; Jones, D. L.

1998-01-01

149

Petrogenesis of ultramafic rocks and associated chromitites in the Nan Uttaradit ophiolite, Northern Thailand  

NASA Astrophysics Data System (ADS)

The ultramafic sequence and associated chromitites of the Nan-Uttaradit ophiolite in the northeastern part of Thailand have been studied in the field and by applying petrography and geochemistry to whole rock samples and minerals. The ultramafic rocks comprise irregulary shaped bodies of dunite, harzburgite, orthopyroxene-rich lherzolite and orthopyroxene-rich harzburgite, clinopyroxene-rich dunite and intrusive clinopyroxenite-websterite bodies. Three types of chromitite were distinguished. Type I chromitite lenses and type II layers which are hosted in orthopyroxenite in the northern part and in dunite in the central part of the ophiolite. Type III chromitite forms lenses or layers in clinopyroxenites in the central and southern parts of the belt. According to the modal and chemical composition the peridotites and orthopyroxenites are strongly refractory. They originated during different stages of interaction between percolating melts and peridotite. The chromitites of types I and II, which are very rich in Cr (up to 68 wt.% Cr 20 3), crystallized from a boninitic parental magma under highly reducing conditions in the northern part and moderate oxygen fugacities (FMQ) in the central part of the ophiolite. The chromitite of type III which are characterized by the highest {Fe 3+}/{(Fe 3+ + Cr + Al) } -ratios, and hosted in intrusive clinopyroxenite-websterite-rocks, cumulated from a CaO-rich transitional boninitic melt under fO 2 conditions around FMQ.

Orberger, B.; Lorand, J. P.; Girardeau, J.; Mercier, J. C. C.; Pitragool, S.

1995-06-01

150

Ophiolites as indicators of the geodynamic evolution of the Tethyan ocean  

NASA Astrophysics Data System (ADS)

Reviewing the ophiolites of the Alpine chain from the Western Mediterranean to the Indian Ocean, we subdivide them into ten groups of outcrops. The geological environment through time, the internal composition and history are given for each. Emphasis is put on the variegated and successive geodynamic processes which can be reconstructed for the different ophiolitic belts since their birth as an ocean-like crust to their incorporation through accretion to a continent. One can distinguish: low versus high tholeiitic partial melting; fairly or poorly established ridges; simple ridge-type crust versus complex ridge + ensimatic arc (and even more complex) type; long-lived versus short-lived oceanic crust stages; abrupt, large-scale, pure obduction versus progressive collision-related obduction or simple progressive juxtaposition to continent. The major events through time in the Tethyan ophiolitic belts can be listed as follows: spreading ridge activity during the Jurassic (Liguria, Dinaro-Hellenides, Lesser Caucasus) and the Cretaceous (Peri-Arabic, Nain Sabzewar) with a jump of ridge in between; repeated change from ridge to ensimatic arc (Dinaro-Hellenic, Lesser Caucasus, Peri-Arabic, ? West Indian) during the latest Jurassic and Cretaceous; Upper Cretaceous (+Tertiary) collision-related obductions (Pontic-Lesser Caucasus, Liguria) squeezing out the elevated portions of these basins; lack of oceanic magmatism during the Cenozoic but progressive accretion along accretionary wedges of the elevated portions of the remaining Mesozoic crust (Van, Naïn, Sabzewar, Zahedan, MeKran).

Knipper, A.; Ricou, L.-E.; Dercourt, J.

1986-03-01

151

Mineralogical and geochemical investigation on podiform chromitites from the Guleman Ophiolite, eastern Turkey  

NASA Astrophysics Data System (ADS)

The chromitites of the Guleman ophiolites are mostly found in the mantle peridotites of harzburgite and dunite, in a banded, massive and disseminated texture. The chromite crystals of chromitites have a wide range of chemical compositions (Cr2O3 = 47-62 wt.% and Al2O3 = 5-19 wt.%). Having TiO2 contents lower than 0.33 wt.%, the chromite crystals have their Cr# and Mg# values in the range of 61-89 and 55-73, respectively, and these values support that those chromitites are podiform and ophiolitic in origin. The chromitites from the Guleman ophiolite have low whole-rock PGE (Platinum Group Element) contents (PGE = 44-255 ppb, 168 ppb average). On the other hand, the Cr-rich chromitites (Cr# >70; Mg# = 56-72) have lower TiO2 contents (?0.2 wt.%) and contain relatively higher content of total PGE (mean 187 ppb) contents, whereas Cr-poor chromitites (Cr#

Melih Akmaz, Recep; Uysal, Ibrahim; Saka, Samet

2014-05-01

152

Continental back-arc basin origin of some ophiolites from the Eastern Desert of Egypt  

NASA Astrophysics Data System (ADS)

Geochemical and petrographical data of three ophiolitic pillow metavolcanic occurrences from the central Eastern Desert of Egypt are presented. The investigated rocks show a subalkaline, tholeiitic affinity. Chemical data indicate that the metavolcanics have transitional within-plate basalt to island-arc basalt features, which are characteristics of basalts formed in ensialic back-arc basins. The association of the investigated ophiolites with volcanoclastic metasedimentary rocks of marine to continental facies is a further confirmation of their ensialic evolution. This suggestion, along with the geochronologic, isotopic and crustal growth rate evidences, revives interest in models that involve contribution from a pre-Pan-African continental crust at least in the southern part of the Egyptian Shield. Mixing between a depleted mantle-derived magma and an enriched crustal melt, a process similar to AFC (assimilation and fractional crystallization), is suggested for the evolution of the investigated rocks. This study provides evidence for formation of some ophiolites in the Eastern Desert of Egypt in continental (ensialic) back arc basins.

Farahat, E. S.; El Mahalawi, M. M.; Hoinkes, G.; Abdel Aal, A. Y.

2004-09-01

153

Geochemical constraints on the origin of the Hegenshan Ophiolite, Inner Mongolia, China  

NASA Astrophysics Data System (ADS)

The Hegenshan ophiolite in Inner Mongolia is a remnant of oceanic lithosphere of probable Devonian age. The ophiolite consists of several blocks composed chiefly of serpentinized ultramafic rocks with lesser amounts of troctolite and gabbro, and sparse lavas and dikes. The ultramafic rocks consist chiefly of depleted harzburgite and minor dunite and are interpreted as mantle tectonites. In the Hegenshan block dunite is relatively abundant and is typically associated with podiform chromitite. Both the chromite ore and the residual chromites in this body are relatively aluminous with average Cr numbers of 44-54. A few small chromite bodies and some of the residual chromites have much higher Cr numbers (72-76). Several blocks have well-layered cumulate sequences of gabbro and troctolite. Sheeted dikes are absent but small mafic dikes are common in some of the ultramafic sections. Most of the mafic dikes have flat chondrite-normalized REE patterns and are strongly depleted in incompatible elements, similar to depleted tholeiites from immature island arcs. The basaltic lavas of the Hegenshan ophiolite have two distinctly different chemical signatures—one similar to the mafic dikes and one similar to ocean island basalts. The entire complex was probably formed within an island arc-marginal basin system that was later accreted to the southern margin of the Siberian Altaids.

Robinson, P. T.; Zhou, Mei-fu; Hu, Xu-Feng; Reynolds, P.; Wenji, Bai; Yang, Jingsui

1999-08-01

154

Evolution of composition of major mineral phases in layered complex of ophiolite assemblage: Evidence for the Voykar ophiolites (Polar urals, Russia)  

USGS Publications Warehouse

We present a detailed study of compositional variation of major minerals through a cross section of the layered complex of the Late Devonian Voykar ophiolite assemblage (Polar Urals). The principal characteristics of this layered complex suggest crystallization from a periodically replenished open magma system in a tectonically dynamic, oceanic environment. The complex may be described in terms of two sequences of cumulus rocks, or megarhythms, that each display an upward progression from ultramafic to gabbroic composition. A transitional zone between the megarhythms is characterized by an upwardly reverse lithologic progression from gabbroic to ultramafic composition. Broad cryptic variation in mineral composition over intervals >100 m parallel changes in the lithologic abundances and suggest changes in the rate of magma supply relative to crystallization and(or) tapping of different mantle sources that had been previously depleted to different degrees. The mineralogy, mineral compositions and isotopic composition of the layered complex coupled with the association of the Voykar ophiolite with island-arc complexes suggest that it most likely formed in a back-arc basin.

Sharkov, E. V.; Chistyakov, A. V.; Laz'ko, E. E.; Quick, J. E.

1999-01-01

155

The basal part of the Oman ophiolitic mantle: a fossil Mantle Wedge?  

NASA Astrophysics Data System (ADS)

Although the Oman ophiolite is classically regarded as being the direct analog of oceanic lithosphere created at fast spreading ridges, the geodynamic context of its formation is still highly debated. The other alternative end-member model suggests that this ophiolite entirely formed in a supra-subduction zone setting. Fluids involved in the hydration of the oceanic lithosphere and in the presence of a secondary boninitic and andesitic volcanism may provide a way to discriminate between these two interpretations: are they descending near-axis hydrothermal fluxes (first model) or ascending from a subducting slab (second model)? We herein focus on the base of the ophiolitic mantle in order to characterize the origin of fluids and decipher hydration processes. Samples were taken along hecto- to kilometre-long sections across the basal banded unit directly overlying the amphibolitic/granulitic metamorphic sole. We carried out a petrological, structural and geochemical study on these rocks and their constitutive minerals. Our results show that, unlike the generally refractory character of Oman harzburgites, all the basal mantle rocks display secondary crystallization of clinopyroxene and amphibole through metasomatic processes. The microstructures and the chronology of these secondary mineralizations (clinopyroxene, pargasitic amphibole, antigorite and then lizardite/chrysotile) suggest that these basal rocks have been affected by cooling from mantle temperatures (<1200°C) to low-T serpentinisation (<300°C). Furthermore, major elements required to crystallize these minerals and the observed fluid-mobile elements (FMEs) enrichments in the clinopyroxenes and in the amphiboles (B, Pb, Sr), as well as in the serpentines (B, Sr, Rb, Ba, As), are consistent with amphibolite-derived fluids (Ishikawa et al., 2005) and cannot be easily explained by other sources. Based on these observations, we propose a geodynamic model in which intense and continuous metasomatism of the cooling base of the ophiolitic mantle is due to the release of fluids coming from the progressive dehydration of underlying amphibolitic rocks. This process is compatible with the progressive subduction of the Arabian margin during the Upper Cretaceous (e.g., HP-LT units history, and tectonic structures observed on top of it). The basal part of the Oman ophiolite would thus represent a fossil incipient mantle wedge.

Prigent, Cécile; Guillot, Stéphane; Agard, Philippe; Godard, Marguerite; Chauvet, Alain; Dubacq, Benoit; Monié, Patrick; Yamato, Philippe

2014-05-01

156

Deep origin of the Luobusa ophiolitic peridotites and chromitites in Tibet  

NASA Astrophysics Data System (ADS)

The Luobusa ophiolite in the 1400-km-long Yarlung-Zangbo suture zone, marking the boundary between India and Asia, hosts significant podiform chromitites. Abundant diamonds have been recovered from peridotites and podiform chromitites by heavy mineral separation and in-situ grains in the chromitites confirm that the diamonds are natural. The diamonds are associated with other ultrahigh pressure (UHP) and highly reduced minerals, such as coesite, kyanite, moissanite, base-metal alloys and native elements. In-situ grains of diamond and moissanite occur in both podiform chromitite and mantle peridotite of the Luobusa ophiolite. SIMS analysis shows that the diamonds have delta13CPDB = -18 to -28, distinctly different from kimberlitic diamonds (delta13C near -5‰). It is suggested that the diamonds formed from C-rich fluids with highly reduced phases and low delta13CPDB values derived from previously subducted slabs. A 1400-km-deep borehole was recently drilled by Luobusa Scientific Drilling Project (LSD) to investigate the distribution and conditions of formation of the podiform chromitites. Borehole LSD-1 penetrated 1100 m of mantle peridotite separated from a lower 300-m-thick sequence of dunite and pyroxenite by a fault. Detailed mineralogical studies indicate at least two stages of mineral formation of Cpx, Opx, Ol and Sp, suggesting late-stage modification of a MORB-type ophiolite in a SSZ environment. We propose here a new model for the formation of ophiolitic peridotites and chromitites. Previously subducted slabs of continental and oceanic lithosphere, lying in the lower part of the mantle transition zone, undergo partial melting at high temperatures in a highly reduced environment. H2O, CO2 and other fluids are released from the rocks and some of them are reduced to single element material, such as C and H. The melts then rise through the mantle to the top of, or above, the transition zone (> 300 km depth), where diamond, moissanite, ultrahigh-pressure chromite (with dissolved Si) and stishovite may crystallize from the melts or fluids. With continued upwelling, coesite exsolution lamellae form in the chromite grains and stishovite is replaced by coesite, but the diamonds are preserved as inclusions in chromite grains. This new model challenges the prevailing view that ophiolites and their podiform chromitites originated at shallow depths in the mantle.

Yang, J.; Robinson, P. T.; Li, J.; Xu, X.; Xiong, F.; Li, Y.; Liu, Z.; Liu, F.

2012-12-01

157

Exhumation and Coupling at the Plate Interface: Large Tectonic Slices V. Melange Formation? Key Contexts and Possible Controlling Parameters  

NASA Astrophysics Data System (ADS)

Fragments of subducted oceanic lithosphere returned along the plate interface convey crucial information regarding the thermal and rheological conditions of convergent plate boundaries. Geological evidence indicate that, unlike subduction, exhumation is non-steady (Agard et al., Earth Sci. Rev. 2009). We herein focus on deep processes along the plate interface (40-80 km depth), for which there is no counterflow (unlike in accretionary prisms) and no other known mechanisms to return eclogites than interplate friction or buoyancy. These eclogites are of two major types: large scale (>km) slices with coherent PT estimates (W. Alps) versus isolated fragments (frequently m-hm) in a serpentinite- or sedimentary-rich matrix showing contrasting equilibration depths (with hints of punctuated exhumation and even reburial; Franciscan, Cuba, Sistan; e.g., Garcia-Casco et al., Geol. Acta 2006). This latter type tends to show warmer equilibration paths, whereas the larger tectonic slices from the former type remain systematically cold. Serpentinites are crucial for both in permitting decoupling and acting as a buoy, and fluid budget is important too in enhancing floatability and allowing large slices to survive (Angiboust and Agard, Lithos 2010). Numerical models implementing free migration of fluids in the subduction zone also show that the plate interface is strongly localized in the absence of fluids: mechanical decoupling efficiently occurs along the sediment veneer and/or at the top of the highly hydrothermalized crust. Whenever fluids are released in greater amounts (depending on initial fluid content and/or thermal structure), deformation becomes much more distributed and affects both the mantle wedge and the top of the downgoing lithosphere (crust and hydrated mantle top), thereby increasing mechanical coupling between the two plates. Based on natural data and numerical modelling we herein propose that rheological contrast chiefly controls mechanical decoupling. On a steady-state basis the subduction interface is apparently efficiently decoupled. In this context, we hypothetize that the liberation of fluid through pulses (or a somewhat increased amount of fluids) is required to locally modify mechanical coupling and induce the slicing of large pieces of oceanic material along the subduction interface (type 1). By contrast, an extreme hydration of the subduction interface and mantle wedge will result in the formation of serpentinite melanges and extensive material mixing (e.g., cold plumes, mafic pods and localized melting; type 2). This latter situation may be promoted by young/fast/wet subduction, such as subduction initiation and/or subduction of young lithosphere or subduction of a particularly hydrated lithosphere section (e.g., at the ridge and/or prior to entering the trench), whereas cold, slow subduction (type 1) will result in irregular hydration and localized coupling able to detach large slices.

Agard, P.; Angiboust, S.; Guillot, S.; Garcia-Casco, A.

2011-12-01

158

The Saint-Daniel Melange: Evolution of an accretionary complex in the Dunnage Terrane of the Quebec Appalachians  

NASA Astrophysics Data System (ADS)

The Saint-Daniel Mélange is part of a series of mélanges located along the Baie Verte-Brompton line in the Northern Appalachians. This line marks the suture between rocks of oceanic affinities and those of the ancient passive margin of North America with which they collided during the Taconian (Middle to Late Ordovician) orogeny. The Saint-Daniel Mélange contains a wide variety of lithologies including well-bedded to dismembered sedimentary sequences, pebbly mudstone, olistostromes, and slivers of igneous and metamorphic rocks. Black shales with interbeds of green shale, calcareous siltstone, or sandstone are the dominant units. They exhibit various stages of mélange formation such as those present in shallow parts of an accretionary complex. Units of oceanic origin include sediments derived from the forearc basin and slivers of an ophiolite and of a magmatic arc. Units derived from sediments of the passive margin of North America are also present. The ratio between these various lithologies changes greatly within the mélange on a kilometric scale along strike. The Saint-Daniel Mélange is a structural complex in which the various units were assigned a sequential order mimicking a stratigraphic order. The Saint-Daniel Mélange is interpreted as the relict of an accretionary complex because of its actual structural position within the Northern Appalachians and because all its lithologies and their structural fabric can be found in modern accretionary complexes.

Cousineau, Pierre A.; St-Julien, Pierre

1992-08-01

159

Remagnetization of the Coast Range ophiolite at Stanley Mountain, California, during accretion near 10 degree N paleolatitude  

SciTech Connect

Paleomagnetic data are presented for a 50-m-thick sequence of Oxfordian to Tithonian sedimentary rocks conformably overlying Upper Jurassic pillow basalt within the Coast Range ophiolite at Stanley Mountain, California. These new data are similar in direction and polarity to previously published paleomagnetic data for the pillow basalt. The Jurassic sedimentary rocks were deposited during a mixed-polarity interval of the geomagnetic field, and uniformity of the remanent magnetization within the entire section of pillow basalt and sedimentary rocks indicates later remagnetization. Remagnetization of the Coast Range ophiolite is interpreted to have occurred during accretion to the continental margin, possibly by burial and low-temperature alteration related to this event. Similar paleolatitudes calculated for the ophiolite (11{degree} {plus minus} 3{degree}) and for mid-Cretaceous sedimentary rocks of the Stanley Mountain terrane at Figueroa Mountain (6{degree} {plus minus} 5{degree}) are consistent with remagnetization of the ophiolite in southern California and elsewhere along the Pacific coast imply that these rocks were also overprinted, and their magnetic inclinations suggest remagnetization at low paleolatitudes as well. The Coast Range ophiolite at Stanley Mountain is thus inferred to have been remagnetized along the North American margin near 10{degree}N paleolatitude between earliest and mid-Cretaceous time and subsequently transported northward by strike-slip faulting related to relative motions between the Farallon, Kula, Pacific, and North American plates.

Hagstrum, J.T. (Geological Survey, Menlo Park, CA (United States))

1992-06-01

160

Collisional emplacement history of the Naga-Andaman ophiolites and the position of the eastern Indian suture  

NASA Astrophysics Data System (ADS)

Dismembered late Mesozoic ophiolites occur in two parallel belts along the eastern margin of the Indian Plate. The Eastern Belt, closely following the magmatic arc of the Central Burma Basin, coincides with a zone of high gravity. It is considered to mark a zone of steeply dipping mafic-ultramafic rocks and continental metamorphic rocks, which are the locus of two closely juxtaposed sutures. In contrast, the Western Belt, which follows the eastern margin of the Indo-Burma Range and the Andaman outer-island-arc, broadly follows a zone of negative gravity anomalies. Here the ophiolites occur mainly as rootless subhorizontal bodies overlying Eocene-Oligocene flyschoid sediments. Two sets of ophiolites that were accreted during the Early Cretaceous and mid-Eocene are juxtaposed in this belt. These are inferred to be westward propagated nappes from the Eastern Belt, emplaced during the late Oligocene collision between the Burmese and Indo-Burma-Andaman microcontinents. Ophiolite occurrences in the Andaman Islands belong to the Western Belt and are generally interpreted as upthrust oceanic crust, accreted due to prolonged subduction activity to the west of the island arc. This phase of subduction began only in the late Miocene and thus could not have produced the ophiolitic rocks, which were accreted in the late Early Eocene.

Acharyya, S. K.

2007-02-01

161

Samarium-neodymium data on two late Proterozoic ophiolites of Saudi Arabia and implications for crustal and mantle evolution  

USGS Publications Warehouse

Whole-rock and mineral samples from the Jabal al Wask and Jabal Ess ophiolites, northwestern Saudi Arabia, yield Sm-Nd isochron ages of 743+24 Ma and 782??38 Ma, respectively. These formation ages, which provide maximum limits for possible obduction ages, are in broad but not precise agreement with the previously known geologic history of the Arabian Shield. They indicate that the ophiolitic rocks are roughly coeval with nearby volcanic and plutonic rocks, supporting a back-arc origin for the two ophiolites. We suggest that the Jabal al Wask and Jabal Ess ophiolites were parts of the same northeast-southwest trending ophiolite belt, now offset along the Najd fault system. Initial e{open}Nd values range from +6.6 to+ 7.6, indicating derivation from a mantle source that has been LIL-depleted for at least 2 Ga. Reported e{open}Nd values from the Arabian Shield that are lower than this suggest the presence of older, reworked continental crust. ?? 1984 Springer-Verlag.

Claesson, S.; Pallister, J. S.; Tatsumoto, M.

1984-01-01

162

Geochemistry of metabasites from the Nevado-Filabride complex, betic cordilleras, Spain: Relics of a dismembered ophiolitic sequence  

NASA Astrophysics Data System (ADS)

Major- and trace-element distribution, including REE data, in the metabasites of the Nevado-Filabride Complex (Betic Cordilleras, Spain) suggests that the whole mafic-ultramafic association represents a dismembered ophiolitic sequence. All the analyzed samples have been affected by sea-floor alteration. Yet only the fine-grained rocks have suffered some significant mobilization of the LREE. Life the other Western Mediterranean ophiolites, this sequence is related to the opening of the Liguro-Piemontese basin during Mesozoic times. However, the metabasites from Betic Cordilleras differ from the ophiolitic basalts by higher values of the more incompatible over less incompatible element ratios. The entire variation of these ratios in the Liguro-Piemontese basin is related to a dynamic melting process coupled with a diapiric uprise of the mantle source from the stability field of garnet peridotites. possibly up to the spinel-plagioclase facies transition. In this process, the ophiolites of the Betic Cordilleras would record the deepest stages and the lowest degrees of melting. This feature is related to the peculiar location of these ophiolites at the western end of the Liguro-Piemontese basin.

Bodinier, Jean-Louis; Morten, Lauro; Puga, Encarnacion; Diaz de Federico, Antonio

1987-06-01

163

Petrology and geochemistry of the Cyclops ophiolites (Irian Jaya, East Indonesia): Consequences for the Cenozoic evolution of the north Australian margin  

Microsoft Academic Search

Summary The Cyclops massif (Irian Jaya - Western Indonesia) displays all components of an ophiolitic sequence including residual mantle peridotites (harzburgites and dunites), cumulate gabbros, dolerites, normal mid-oceanic ridge basalts (N-MORB) and minor amounts of boninitic lavas. This ophiolitic series tectonically overlies high temperature (HT)-high pressure (HP) mafic rocks metamorphosed during the Miocene.

C. Monnier; J. Girardeau; M. Pubellier; M. Polvé; H. Permana; H. Bellon

1999-01-01

164

Significance of E. Paleozoic Paleo-Tethyan Ophiolites in the Balkan Terrane and the Greater Caucasus for the Cadomian-Hercynian Continental Growth of Southern Europe  

Microsoft Academic Search

New geochronological & geochemical data from dismembered E. Paleozoic ophiolites in the Balkan Terrane (Serbia) & the Greater Caucasus (Russian Federation) link these remnants of the Paleo-Tethyan oceanic crust to a series of events associated with the Cadomian and Hercynian evolution of S. Europe. The Balkan-South Carpathian ophiolite belt (BSCOB) WSW of the Moesian platform is part of the South

G. S. Zakariadze; S. O. Karamata; Y. Dilek

2007-01-01

165

Geochemistry and Ar–Ar muscovite ages of the Daraban Leucogranite, Mawat Ophiolite, northeastern Iraq: Implications for Arabia–Eurasia continental collision  

NASA Astrophysics Data System (ADS)

Daraban Leucogranite dykes intruded discordantly into the basal serpentinized harzburgite of the Mawat Ophiolite, Kurdistan region, NE Iraq. These coarse grained muscovite-tourmaline leucogranites are the first leucogranite dykes identified within the Mawat Ophiolite. They are mainly composed of quartz, K-feldspar, plagioclase, tourmaline, muscovite, and secondary phologopite, while zircon, xenotime, corundum, mangano-ilemnite and cassiterite occur as accessories.

Mohammad, Yousif O.; Cornell, David H.; Qaradaghi, Jabbar H.; Mohammad, Fahmy O.

2014-06-01

166

Tectonic significance of 400 Ma zircon ages for ophiolitic rocks from the Lachlan fold belt, eastern Australia  

NASA Astrophysics Data System (ADS)

New SHRIMP ion microprobe data provide constraints on the age of formation of ophiolitic rocks from the Tumut region of the Lachlan fold belt, southeastern Australia. Magmatic zircons from plagiogranites within both the Coolac serpentinite belt and Wambidgee serpentinite belt give 206Pb/238U ages of about 400 Ma. This Devonian age is considerably younger than those previously proposed for these ophiolitic rocks (Cambrian-Ordovician to Early Silurian) and provides important tectonic data for this region of the Lachlan fold belt. We suggest that these rocks formed within a narrow marginal basin akin to those of Cretaceous age within the Chilean Andes of South America. The dates presented are within the 420 390 Ma range assigned to most of the granitic bodies of the Lachlan fold belt. We therefore suggest that the generation of both the ophiolitic rocks and granitic magmas of the Lachlan fold belt were part of the same major tectonothermal event.

Graham, I. T.; Franklin, B. J.; Marshall, B.; Leitch, E. C.; Fanning, M.

1996-12-01

167

Precambrian ophiolites of arabia: geologic settings, UPb geochronology, Pb-isotope characteristics, and implications for continental accretion  

USGS Publications Warehouse

Disrupted ophiolites occur in linear belts up to 900 km long between microplates that collided during the late Proterozoic to form the Arabian Shield. UPb zircon ages and Pb-isotope data from these ophiolitic rocks help constrain the history of accretion of the Arabian Shield and thereby contribute to the definition of its microplates and terranes. Terranes of the central and western Arabian Shield are generally thought to represent intraoceanic island arcs that range in age from about 900 to 640 Ma; however, a region of the eastern Arabian Shield contains rocks of Early Proterozoic age and may represent an exotic continental fragment entrained between the arc complexes. Ophiolites of the Yanbu suture (northwestern shield), dated by UPb (zircon) and SmNd (mineral isochron) methods, yield model ages of 740-780 Ma. These are among the oldest well-dated rocks in the northwestern Arabian Shield. Ages from the Jabal al Wask complex overlap with ages of adjacent arc rocks. This overlap in age supports geologic and geochemical evidence that the Wask complex represents a fragment of back-arc oceanic lithosphere formed during arc magmatism. Older ages of about 780 Ma for gabbro from the Jabal Ess ophiolite suggest that the ophiolite is either a fragment of fore-arc oceanic crust or oceanic basement on which an arc was built. Gabbro samples from ophiolites of the Bir Umq suture (west-central Arabian Shield) yield zircons with ages of 820-870 Ma and $ ??1250 Ma. The 820-870 Ma dates overlap with ages of the oldest nearby arc rocks; this favors an intra-arc or near-arc paleotectonic setting. The older zircons suggest that middle or early Proterozoic crustal material, possibly derived from the Mozambique belt of Africa, was present during back- or intra-arc magmatism. Plagiogranite from the Bir Tuluhah ophiolitic complex at the nothern end of the 900 km-long Nabitah mobile belt was dated by the zircon UPb method at ??? 830 Ma. This date is in the range of the oldest dated arc rocks along the northern and central parts of the Nabitah suture, but is ??? 100 Ma older than the oldest arc plutons (tonalites) associated with the southern part of the belt. These age relations suggest that the northern part of the Nabitah belt contains an extension of the Bir Umq suture that was transposed parallel to the Nabitah trend during collision of the arc terranes of the northwest Arabian Shield with the Afif plate to the east. Feldspar lead-isotope data from the ophiolites are of three types: (1) lead from the ophiolitic rocks and arc tonalites of the northwestern Shield and ophiolitic rocks of the Nabitah suture is similar to lead in modern mid-ocean ridge basalt, (2) anomalous radiogenic data from the Thurwah ophiolite are from rocks that contain zircons from pre-late Proterozoic continental crust, and (3) feldspar from the Urd ophiolite shows retarded uranogenic lead growth and is related either to an anomalous and perhaps primitive oceanic mantle source, or in an unknown manner to ancient continental mantle or lower crust of the eastern Arabian Shield. ?? 1988.

Pallister, J. S.; Stacey, J. S.; Fischer, L. B.; Premo, W. R.

1988-01-01

168

Prospects for Enhancing In Situ CO2 Mineralization in the Peridotite Aquifer of the Samail Ophiolite  

NASA Astrophysics Data System (ADS)

The mantle peridotite section of the Samail Ophiolite in the Sultanate of Oman is a site of exceptionally well-developed, naturally occurring in situ CO2 mineralization and serves as a natural analog to an enhanced process. The evolution of groundwater along the CO2 mineralization pathway in ultramafic rocks is generally thought to follow a progression from surface water to shallow Mg-HCO3 groundwater to deep, alkaline Ca-OH groundwater [e.g., 1-3], but the timescale for this evolution is not known. In order to assess the prospects for an enhanced CO2 mineralization process, we must first have a better understanding of the time necessary to attain natural CO2 mineralization, as well as the rate-limiting factors for the natural process. To that end, a reactive transport model was developed to simulate water-rock interaction during the natural CO2 mineralization process in the peridotite of the Samail Ophiolite aquifer. The model was created using the geochemical code EQ3/6 v.8.0 4, and it tracks a two stage process in which surface water first interacts with peridotite in a shallow aquifer open to atmosphere, and then progresses to a closed system in which the water interacts with peridotite isolated from the atmosphere. The incorporation of dissolution kinetics for the primary minerals in peridotite allowed for an estimate of the time required for water to evolve to the extent seen in the field. Model results suggest that it may take less than 50 years to develop the shallow Mg-HCO3 water, but up to 5,600 years to form the deeper, alkaline Ca-OH water. Rock and water chemistry collected from the Samail Ophiolite and its aquifer were used to calibrate the model. The modeled water chemistry is in agreement with that seen in the field, suggesting that the model offers a fair representation of the natural CO2 mineralization process. The natural system model indicates that CO2 availability is the limiting factor for mineralization in the subsurface, so the model was expanded to include CO2 injection scenarios to determine if increasing the supply could enhance the rate of CO2 mineralization. Model results show that CO2 injection at 100 bar pCO2 and ambient temperature (30oC) would result in a 40x increase in CO2 mineralization over a 30 year period, while injection at 90oC would result in a 3,600x increase in mineralization. Thus far, these model results do not include hydrogeological parameters for the system. Porosity and permeability, and their change with secondary mineralization, may affect the injectivity of CO2 into the aquifer, so they should be included when modeling CO2 injection. However, permeability and porosity in fractured rock aquifers are notoriously complex and remain poorly constrained for the peridotite of the Samail Ophiolite; these parameters warrant further study prior to their inclusion in a model. Results from permeability tests on peridotite cores from the Samail Ophiolite will be presented, with emphasis on how these measurements contribute to our understanding of the potential for enhanced CO2 mineralization in the peridotite of the Samail Ophiolite aquifer. 1Barnes and O'Neil, 1969; 2Stanger, 1986; 3Bruni et al., 2002; 4Wolery and Jarek, 2003

Paukert, A. N.; Matter, J. M.; Kelemen, P. B.; Shock, E.; Havig, J. R.

2011-12-01

169

Quantifying Strain Across a Paleotransform Fault in the Mantle Section of an Ophiolite, New Caledonia  

NASA Astrophysics Data System (ADS)

The Massif du Sud in New Caledonia exposes the mantle section of a Cretaceous-age ophiolite generated by spreading in a small basin northeast of the island. Most of the ophiolite has shallow SW-dipping foliations and subhorizontal NS lineations interpreted as reflecting fabric formed at a mid-ocean spreading ridge. Fabric intensity increases and foliation steepens to NS-striking and subvertical in the Bogota Peninsula shear zone while lineations remain shallowly S-plunging. The rotation of field foliation and pyroxenite dike orientations across a 50-km wide region is symmetric about a central high-strain zone and shear sense indicators suggest that relative displacement was dextral. The symmetry of fabrics is consistent with formation in the mantle section of a true paleotransform fault zone as opposed to a fracture zone, where fabric rotation would only be expected on one side of the shear zone. Thus, the New Caledonia ophiolite includes rocks that record both ridge- and transform-related deformation. To better understand the evolution of the transform fault, the fabrics within the Bogota Peninsula shear zone were subdivided into three regions (farfield, nearfield, and central high strain zone) of approximately homogeneous deformation. We used fabrics from these regions together with those from the Massif du Sud to model how kinematics change from ridge-dominated deformation outside the shear zone to transform- dominated inside the shear zone. Shear zone localization was modeled as superimposed increments of deformation using a grid search method that varied parameters including the orientation of the shear plane, shear sense, and transpressional versus transtensional kinematics. The best models were determined by comparing (1) the orientation of the strain ellipse with field foliation and lineation, and (2) the predicted and observed rotations of pyroxenite dikes across the shear zone. This approach allows quantitative estimation of the total deformation accumulated across this ridge-transform system.

Titus, S. J.; Davis, J. R.; Ferre, E. C.; Tikoff, B.

2008-12-01

170

Mesozoic ophiolites, sutures, and arge-scale tectonic movements in Afghanistan  

NASA Astrophysics Data System (ADS)

The tectonic history of Afghanistan appears to be the result of successive accretion of fragments of Gondwana to the active margin of Laurasia since the end of the Paleozoic. The margin, in Afghanistan, lies along the present Herat and Panjshir faults, south of Hindu Kush, swings around the central Pamirs and can presumably be traced along the present western Altyn Tagh and Kunlun faults in Tibet. North of this boundary, Paleozoic rocks have been deformed in the Upper Paleozoic, whereas south of it, there is no trace of the Hercynian orogeny. The first collision of Gondwanian fragments with Laurasia probably occurred in the early Mesozoic along the Hindu Kush and Kunlun. To the south, ophiolites along the Panjao and Pangong-Nu Chiang sutures (respectively in central Afghanistan and central Tibet) testify for another suturing event in the Upper Jurassic or Lower Cretaceous. The Indus-Tsangpo suture between India and Tibet corresponds, in eastern Afghanistan, to two ophiolite subbelts, near Kabul and Khost. Both ophiolite complexes have been emplaced between Maestrichtian and Lower Eocene by choking of two northward-dipping subduction zones. After complete contact between the Indian and Asian continents was achieved, presumably in the end of Eocene, the penetration of India into Asia caused large-scale intra-continental shortening. A large part of the shortening was accommodated by strike-slip faulting along Mesozoic and more ancient sutures. Central Afghanistan, in particular, was extruded to the west along the Herat suture by the protrusion of the Pamir wedge. It subsequently collided with the Lut block.

Tapponnier, Paul; Mattauer, Maurice; Proust, François; Cassaigneau, Christian

1981-02-01

171

Late Cretaceous/early Tertiary convergence between the Indian and Arabian plates recorded in ophiolites and related sediments  

NASA Astrophysics Data System (ADS)

Remnants of ocean floor forming the Eastern Ophiolite Belt in Oman and the Western Ophiolite Belt in Pakistan have a common plate-tectonic history culminating in emplacement at the Cretaceous/Tertiary boundary. Fragments of ocean floor in these two belts have ages between 150 and 65 Ma and recorded tectonic events in the early Indian Ocean at 150 Ma, 130-120 Ma, 110-100 Ma and 70-65 Ma. New radiometric and chronostratigraphic ages and paleomagnetic and sedimentary information are used to relocate these ophiolites in the frame of the evolving Indian Ocean, which was characterised by the stepwise breakup of Gondwana at 158 Ma (East and West Gondwana), 130 Ma (Southern Atlantic, breakup of East Gondwana), 95-84 Ma (Madagascar and India/Seychelles), 65 Ma (India and Seychelles) and, finally, at 40 Ma, rifting between Africa and Arabia. The 150-Ma-old oceanic rocks of Masirah Island in Oman originally formed the extension of the basins now preserved along the eastern edge of the Afro-Arabian plate. Masirah drifted together with India-Seychelles when a new ridge formed at approximately 130 Ma separating microplates (e.g., Kabul Block) from the northern edge of Greater India. Parts of this new ocean were later emplaced to form the western ophiolite belt in Pakistan. Consumption of oceanic crust and emplacement of ophiolites during the closure of the Neotethys occurred not only north of India (± Seychelles), but also along two or three subduction zones between the Afro-Arabian plate and India/Seychelles, documented by the formation of metamorphic soles beneath ophiolites. Relics of ocean floor formed approximately 65-70 Ma ago, now form the upper units in the Western Ophiolite belt of Pakistan where they overlie an accretionary prism of pillow lavas of dominantly (Aptian-) Albian age and sedimentary rocks. The breakup of India and the Seychelles at 65 Ma was associated with the eruption of flood basalts (Deccan and Seychelles) and enhanced counter-clockwise movement of India. This breakup possibly caused the Late Jurassic/Early Cretaceous fragments of ocean floor (e.g., Masirah) to be thrust onto the eastern edge of Oman. The ophiolites, which were emplaced onto the western edge of India, drifted as part of this plate farther north where collision with Eurasia and the accreted microplates occurred in the Eocene (˜ 55 Ma).

Gnos, E.; Immenhauser, A.; Peters, Tj.

1997-03-01

172

Discovery of Wadsleyite (?-phase olivine) in Peridotites of the Nidar Ophiolite, Indus Suture Zone, Indian Himalaya  

NASA Astrophysics Data System (ADS)

In two presentations at this meeting (this and Das et al.) we report the discovery of ultra high-pressure mineral assemblages in peridotites of the Nidar Ophiolite in NW Indian Himalaya. The peridotite hosts of the ultra-deep origin minerals occur as 'xenoliths' within mapped dunite channels at the base of the ophiolite. Here we report the discovery of wadsleyite (?-phase olivine) within a Cr-spinel grain of the peridotite. The wadsleyite measures 120 ?m x 30 ?m included in the 300 ?m x 500 ?m Cr-spinel grain of the peridotite. The euhedral opaque wadsleyite grain displays Raman spectra of forsteritic olivine, doublet at 823 cm-1, 854 cm-1 with a 961 cm-1 peak. The peaks at 542 cm-1 and 918 cm-1 are also seen in experimentally shocked dunite. The 918 cm-1 is the principal wadsleyite peak; one at 699 cm-1 may also belong to it. The 372 cm-1 peak along with 605 cm-1 peak may be assigned to ringwoodite. These spectra were obtained at the Wadia Institute using a LabRAM HR-Horibe Jovin Yovon instrument, and confirmed at Cornell University's CCMR with a Renishaw IN Via Conofocal Raman Spectroscope. Thus the grain is primarily a forsteritic olivine but signatures of high pressure Mg2SiO4 spinelloid are present. Based on the P-T stability of wadsleyite, we infer this grain began its journey at depths between 410 km - 660 km in the mantle transition zone, and reflecting the scale and dynamics of convective flow of heat and matter arrived within the peridotites beneath the Nidar Ophiolite's volcanic crust. Laser Raman spectra of the wadsleyite grain within a Cr-spinel of a peridotite body at the base of the Nidar Ophiolite Complex, Indus Suture Zone , Indian Himalaya. The spectra is one of several confirmed by two different Laser Raman facilities at the Wadia Institute , India and at the CCMR, Cornell University , USA. See text for detailed analysis of the spectra.

Basu, A. R.; Das, S.; Mukherjee, B. K.

2013-12-01

173

Palaeomagnetic evidence for an oceanic core complex in the Mirdita ophiolite of Albania  

NASA Astrophysics Data System (ADS)

Oceanic core complexes (OCCs) are the uplifted footwalls of oceanic detachment faults that unroof upper mantle and lower crustal lithologies and expose them at the seafloor. Their common occurrence in slow and ultra-slow spreading rate oceanic crust suggests they accommodate a significant component of plate divergence, representing a newly recognised class of seafloor spreading. Numerical modelling and palaeomagnetic results from the Integrated Ocean Drilling Program (IODP) have shown that the footwalls beneath oceanic detachment faults rotate during their evolution, initiating at steep angles at depth and then "rolling-over" to their present day low angle orientations as a result of flexural isostasy during unroofing. This footwall rotation provides a means of testing whether extensional structures separating upper mantle/lower crustal rocks from upper crustal rocks in ophiolites potentially represent fossil OCCs. Here we present the results of an extensive paleomagnetic study of an inferred OCC in the Mirditata ophiolite of the Albanian Dinarides, first proposed by Tremblay et al. (2009). The western part of Mirdita ophiolite is composed of mantle sequence overlain by a thin gabbro/troctolitic sequence and MORB-like pillow lavas. The sheeted dyke complex and gabbroic sequence are missing locally, and the upper crustal volcanic sequence then rests directly on the mantle sequence in tectonic contact. This anomalous situation is directly comparable to lithostratigraphic relationships in oceanic detachment fault settings. In order to understand this tectonic contact and describe its kinematics we sampled 73 sites from ultramafic rocks, gabbros, lava flows, pillow lavas and dykes, around the Puka and Krabbi massifs between the villages of Puka and Rreshen in northern Albania. Results demonstrate that gabbroic bodies in the mantle sequence preserve a highly stable remanence that differs in direction and polarity to the serpentinized peridotite host rock. A significant difference in remanence direction between the footwall gabbros and the hanging wall upper crustal rocks is consistent with relative rotation across the intervening extensional structure. These data provide the first quantitative example of OCC-related footwall rotation in an ancient ophiolite.

Maffione, M.; Morris, A.; Anderson, M. W.

2012-04-01

174

Fossils of hydrothermal vent worms from Cretaceous sulfide ores of the Samail ophiolite, Oman  

USGS Publications Warehouse

Fossil worm tubes of Cretaceous age preserved in the Bayda massive sulfide deposit of the Samail ophiolite, Oman, are apparently the first documented examples of fossils embedded in massive sulfide deposits from the geologic record. The geologic setting of the Bayda deposit and the distinctive mineralogic and textural features of the fossiliferous samples suggest that the Bayda sulfide deposit and fossil fauna are remnants of a Cretaceous sea-floor hydrothermal vent similar to modern hot springs on the East Pacific Rise and the Juan de Fuca Ridge.

Haymon, R. M.; Koski, R. A.; Sinclair, C.

1984-01-01

175

Ore petrology of chromite-PGE mineralization in the Kempirsai ophiolite complex  

Microsoft Academic Search

Summary  The platinum group minerals (PGM) in chromite ores of the Kempirsai ophiolite massif, located south of the Ural Mountains,\\u000a are extremely varied in composition and represented predominantly by alloys, sulfides, arsenides, and sulfosalts of the iridium-group\\u000a PGE (IPGE). The earlier Ir-Os-Ru alloys prevail over the later Cu-Os-Ru, Cu-Ir, Ni-Ir, Ni-Os-Ir-Ru, and Ni-Ru-Os-Fe alloys\\u000a rich in base metals (BM). The earlier

V. V. Distler; V. V. Kryachko; M. A. Yudovskaya

2008-01-01

176

Kinematic evolution of the Yarlung Zangpo Suture Zone ophiolites (Southern Tibet): Early Cretaceous saloon-door spreading?  

NASA Astrophysics Data System (ADS)

The broad deformation zone of the Himalayan belt and Tibetan plateau is largely the product of continent-continent collision between India and Eurasia plates since the Early Eocene. Continental collision, however, is only the ultimate effect of long-lasting plate convergence and subduction below the Lhasa block since at least early Cretaceous times. Supra-subduction zone ophiolites exposed along the Yarlung Zangbo Suture Zone (YZSZ) between Himalayan (Indian) and Tibetan (Eurasiatic) terranes demonstrate that a long-lasting intra-oceanic subduction zone, close to Lhasa or far outboard, must have played a significant role in accommodating closure of the >7500 km wide Neotethyan Ocean. These ophiolites are the best-preserved, yet still highly incomplete record of the vast Neotethys. However, their study can provide key constraints on the plate kinematic history of the Neotethyan subduction systems, in particular their early stages. Paleomagnetic analyses of the upper crustal sequence (pillow basalt and sheeted dykes, sills) of ophiolites have been successfully applied in the past to reconstruct the initial geometry of the spreading system associated to the ophiolite formation. Furthermore, oceanic detachment faults, structures widely occurring in modern magma-poor (slow-spreading) mid-ocean ridges, have been recently recognized also in ophiolites (i.e., Mirdita ophiolite of Albania), and (if present) their study may provide unique insights into the geodynamics of the associated spreading system. The YZSZ ophiolites form a 2500 km long belt mainly composed of dismembered ultramafic massifs locally covered by a crustal sequence and oceanic sediments, underlying a regionally continuous clastic Xigaze sedimentary basin interpreted as the Tibetan forearc. Our study focused along a ~250 km transect within the eastern sector of the YZSZ between the Sangsang and Xigatze ophiolite. More than 500 cores were paleomagnetically sampled at 22 localities within sheeted dykes/sills, pillow lavas, mantle peridotites and mantle-hosted gabbros. Paleomagnetic sampling was coupled with a structural geological analysis in the field. The geochemistry of the crustal units, and its geodynamic setting inferred from that (back-arc vs. forearc) was constrained through specific geochemical analyses. Since the YZSZ ophiolite likely suffered of multiple stages of vertical axis rotation and tilt associated to (i) the Mesozoic ocean spreading and subduction dynamics, (ii) the Eocene thrusting of the ophiolites over the Greater Indian continental margin, and (iii) the subsequent continent-continent collision, we adopted a Net Tectonic Rotation (NTR) approach for our kinematic analysis based on paleomagnetic data. This technique, already tested in other ophiolitic belts, describes the total deformation through inclined axes that can then be decomposed into simple vertical and horizontal axis components of rotation. Relying on the calculated rotation and paleo-spreading direction pattern, and the possible occurrence of oceanic detachment faults, we propose a tectonic evolutionary model characterized by the interplay between subduction-related back-arc spreading generating magmatic ocean floor, and detachment faulting associated to trench-parallel stretching upon saloon-door back arc basin opening.

Maffione, Marco; van Hinsbergen, Douwe; Huang, Wentao; Koornneef, Louise; Guilmette, Carl; Borneman, Nathaniel; Hodges, Kip; Li, Shun; Kapp, Paul; Ding, Lin

2014-05-01

177

Melanges, No. 23.  

ERIC Educational Resources Information Center

Articles on second language teaching and learning include: "Ethnographie comparee de la salle de classe en France et en grande-Bretagne" (Comparative Ethnography of the Classroom Environment in France and Great Britain) (F. Carton); "Pour une approche micro et macro du francais parle dans la formation des enseignants de FLE" (Toward a Micro and…

Melanges, 1997

1997-01-01

178

K/Ar and Rb/Sr ages of celadonites from the Troodos ophiolite, Cyprus  

SciTech Connect

K/Ar dating and Rb/Sr isochrons of two celadonite veins from the Troodos ophiolite, Cyprus, consistently yield precipitation ages of 77-80 and 87-92 Ma, respectively. Their K/Ar bulk-rock age data (84.7 +/- 2.3 Ma), combined with the oldest celadonite age and recently published radiolarian ages, suggest that the Troodos ophiolite was formed between 85 and 92 Ma, providing an estimate for the duration of celadonite formation of at least 7 m.y., but possibly up to 15 m.y. These data are consistent with data from Deep Sea Drilling Project holes, suggesting that precipitation of the most significant portion of vein minerals in the oceanic crust occurs within about 12 m.y., but possibly up to 19 m.y. after crustal formation. Leaching experiments suggest that K, Ar, Rb, and Sr in exchangeable and nonexchangeable sites of the structure of stoichiometric celadonite maintain their isotope characteristics and are resistant to change from diagenetic processes or surface alteration.

Staudigel, H.; Gillis, K.; Duncan, R.

1986-01-01

179

Geological history of the Cretaceous ophiolitic complexes of northwestern South America (Colombian Andes)  

NASA Astrophysics Data System (ADS)

The Western Cordillera of Colombia was formed by intense alpine-type nappe-forming folding and thrusting. The Cretaceous (80-120 Ma B.P.) tholeiitic material of the Western Cordilleran nappes has been obducted onto the Paleozoic and Precambrian polymetamorphic micaschists and gneiss of the Central Cordillera. Near Yarumal, the Antioquia batholith (60-80 Ma B.P.) intrudes both obducted Cretaceous oceanic material and the polymetamorphic basement rock of the Central Cordillera. Therefore, nappe emplacement and obduction onto the Central Cordillera occurred during Late Senonian to Early Paleocene. The nappes travelled from northwest to southeast so that the highest unit, the Rio Calima nappe therefore has the most northwestern source, whereas the lowest units originated from a more southeastward direction. Sedimentological analysis of the volcanoclastic and sandy turbidite material from each unit suggests a marginal marine environment. During Cretaceous times the opening of this marginal sea, from now on called the "Colombia marginal basin", probably originated by detachment of a block from the South American continent related to the Farallon-South America plate convergence. In the Popayan area (southern Colombia), the Central Cordilleran basement exhibits glaucophane schist facies metamorphism. This high pressure low temperature metamorphism is of Early Cretaceous (125 Ma B.P.) age and is related to an undated metaophiolitic complex. The ophiolitic material originating from the Western Cordilleran is thrust over both the blueschist belt and the metaophiolitic complex. These data suggest that the "Occidente Colombiano" suffered at least two phases of ophiolitic obduction during Mesozoic time.

Bourgois, Jacques; Toussaint, Jean-François; Gonzalez, Humberto; Azema, Jacques; Calle, Bernardo; Desmet, Alain; Murcia, Luis A.; Acevedo, Alvaro P.; Parra, Eduardo; Tournon, Jean

1987-12-01

180

Geodynamics of ophiolites and formation of hydrocarbon fields on the shelf of eastern Sakhalin  

NASA Astrophysics Data System (ADS)

A model is proposed showing the formation of hydrocarbon fields on the shelf of eastern Sakhalin as being caused by sustained (from the Late Cretaceous to the present) extension in the adjacent deepwater Deryugin Basin with exposure of the upper mantle rocks at the bottom of the sedimentary basin. The thrust faults and detachments formed through this process facilitated the penetration of seawater into ultramafic rocks, thus providing large-scale serpentinization accompanied by generation of hydrocarbons. Extension in the Deryugin Basin was compensated by horizontal shortening at its margins, and as a result, by the formation of ophiolitic allochthons as constituents of the accretionary prism of eastern Sakhalin. Hydrocarbons were injected and pumped in the root zones of the allochthons, giving rise to their westward migration and the formation of petroleum pools in fault-line and underthrust traps on the shelf of Sakhalin Island. The Deryugin Basin is a petroleum-collecting area for oil and gas fields localized in the upper part of its western margin. More broadly, the work considers interrelations between hydrocarbon generation and the geodynamics of tectonic couples of ophiolitic allochthons and adjacent deepwater basins of marginal seas, in particular, in the western Pacific.

Raznitsin, Yu. N.

2012-01-01

181

Ophiolites and oceanic plateau remnants (greenstones) in Japan and Far East Russia  

NASA Astrophysics Data System (ADS)

In Japan, an older ophiolite thrust onto younger ophiolite with tectonic intercalation of accreted oceanic sediments (chert, limestone, shale and sandstone forming _gocean plate stratigraphy_h deposited on the basaltic basement) or their high-P/T metamorphosed varieties. For example, the Yakuno ophiolite (SW Japan) of early Permian igneous age and supra-subduction zone (SSZ) origin (Ichiyama & Ishiwatari, Island Arc, 13, 157-) is tectonically underlain by the Ultra-Tamba nappe (chert, shale, sandstone) accreted in Late Permian, which is further underlain by the Tamba nappes (greenstone, chert, limestone, shale and sandstone) accreted in Jurassic. Major occurrence of the greenstones (mainly Permian) in the Upper Tamba nappe (consisting of 3 sub-nappes) is more than 1 km thick intact sheet of >200 km extension forming the structurally basal part of each sub-nappe, originated in an oceanic edifice composed of pillow lava, massive lava, hyaloclastite and dikes (_gBasal TypEh). Another minor occurrence is greenstone fragments of a few cm to 100 m size in the muddy matrix (_gMixed TypEh), constituting structurally upper part of each sub-nappe. The Basal Type greenstones show uniform E-MORB affinity, but the Mixed Type ones show diverse features such as N-MORB, OIT and OIA. This clear correlation between the occurrence of greenstones and their chemistries suggests the accretion of thick crust of oceanic plateau (E-MORB) to make Basal Type greenstones and the accretion of thin normal oceanic crust (N-MORB) with disseminated small seamounts (OIT and OIA) to make Mixed Type greenstones (Koizumi & Ishiwatari, Island Arc, in submission.). We discovered HFSE-rich picrite (meimechite) sills and hyaloclastites as well as ferropicrite and picritic ferrobasalt dikes emplaced in the Basal Type greenstones and its chert-dolomite cover of Late Permian age. Zr/Y and Ti/Al signatures of these ultramafic volcanic rocks are intermediate between Polynesian picrites and Siberian meimechites, suggesting their origin by deep (4-5 GPa) partial melting of a superplume (Ichiyama & Ishiwatari, 2005; CMP, 149, 373-; Lithos, in submission.). The Yakuno ophiolite and the Tamba greenstone thus represent coeval but unrelated SSZ and oceanic magmatisms, respectively, in the same Permian time. The Jurassic superplume-related volcanic rocks are also reported from Hokkaido (Japan) and Sakhalin (Russia), and Jurassic meimechite lavas and related Alaskan-type zoned ultramafic plutons are reported from Primorye (Russia), but their age of accretion is also Late Jurassic. The Jurassic ophiolites in Hokkaido and Sakhalin are characterized by unusually depleted harzburgite (Spinel Cr#80-90), indicating very high degree of melting (Ishiwatari et al. GSL Spec. Publ. 218, 597-). These facts suggests that the superplume was placed beneath the subduction zone (Ishiwatari & Ichiyama, 2004; Int. Geol. Rev., 46, 316-), and facilitated plate convergence and SSZ magmatism. This study provides a new evidence for Permian oceanic superplume magmatism that is coeval with Siberian and Emeishan LIPs, and postulates possibility of superplume-SSZ interaction in Jurassic NW Pacific margin.

Ishiwatari, A.; Ichiyama, Y.; Koizumi, K.

2005-12-01

182

Chromitites in ophiolites: How, where, when, why? Part II. The crystallization of chromitites  

NASA Astrophysics Data System (ADS)

A review of previous work relevant to the formation of concentrations of chromite in peridotites from ophiolitic (s.l.) sequences highlights some of the key problems in understanding the complex processes involved. This review forms the basis for chromitite-genesis models that integrate new geochemical data with petrologic, field and microstructural observations, and for a re-interpretation of previous data and concepts. The geochemical data include major- and trace-element contents of chromite and coexisting phases and especially the nature and Os-isotope compositions of platinum-group minerals (PGM) and base-metal sulfides (BMS); the PGM data in particular provide new insights into chromitite formation. Differences in the morphology, structural relationships, and geochemical signatures of chromitites allow the recognition of three distinct types. Type I is the most abundant and is distinguished by bulk-rock enrichment in Os, Ir and Ru relative to Rh, Pt, and Pd; it shows no consistent spatial location within the ophiolite "stratigraphy". The second type (Type IIA) is generally confined to the shallower zones of the oceanic lithosphere (mainly as concordant layers, bands and seams, but also as discordant pods or irregular bodies), and is significantly enriched in the incompatible platinum-group elements (PGE) with generally higher total PGE contents than Type I. The third type (Type IIB) shows the same spatial distributions and PGE patterns as Type IIA but has a more limited range of Cr# and a wider range of Mg# that overlap with the compositional range of chromites from layered mafic intrusions. Reaction of melts with peridotite wall-rocks results in the extraction of pyroxene into the melts, forming anastomosing dunitic melt channels in the mantle sections of ophiolites. The Os-isotope heterogeneity in PGMs within single chromitite samples, as described in Part I, provides clear evidence that melt mingling take place on very small scales. This suggests that ophiolitic chromitites are generated through the disequilibrium precipitation of chromite, forced by small-scale mingling of melts that had different SiO2 contents, reflecting derivation from different source rocks, different degrees of partial melting and/or wall-rock reaction. Progressive reaction, crystallization and mixing of melts within the channel system assures the presence of a spectrum of melts at any one time, making the system self-sustaining; each new injection of mafic melt would find more evolved melts with which to react, producing more chromite. Chromite is carried to its final deposition by migration of the chromite-bearing melts, or fluids derived from them. This explains the general association of chromitite with the dunitic portions of ophiolitic mantle; dunite margins around chromite segregations represent the original host rock intruded by chromitite-forming fluids.

González-Jiménez, José María; Griffin, William L.; Proenza, Joaquín A.; Gervilla, Fernando; O'Reilly, Suzanne Y.; Akbulut, Mehmet; Pearson, Norman J.; Arai, Shoji

2014-02-01

183

Identification of an early cretaceous ophiolite in the Camarines Norte-Calaguas Islands basement complex, eastern Luzon, Philippines  

NASA Astrophysics Data System (ADS)

New geological studies of the Camarines Norte-Calaguas Islands (CNCI) basement complex have resulted in the identification of a pre-Late Cretaceous ophiolite assemblage underlying much of the eastern part of the complex. Dominant ophiolite lithologies include: harzburgites, gabbros, diabasic and basaltic dikes, and pillow lavas. A stratigraphically complete igneous section is lacking due to extensive structural disruption of the ophiolite by westward directed thrusting and displacement along high angle faults. Despite this disruption, geochemical ([ La/ Yb] n ? 1, La/ Ta ? 15) and mineralogical data, as well as field relationships, support a cogenetic (N-MORB) origin for the basalt and gabbro units. A minimum crustal age of 100 Ma is inferred from recent {40Ar }/{39Ar } analyses of amphibolites associated with the gabbros. Other lithologies occurring within the basement complex include: medium to high-grade garnet-hornblende-mica schists, low-grade metaigneous rocks, and dioritic and granodioritic intrusives. Geological and geochemical data on these units indicate that they are not genetically related to the ophiolite but have been in close spatial proximity since the Middle Miocene. Together these studies raise new questions concerning the nature of Philippine basement complexes and extent of fundamental, pre-Tertiary basement in the eastern Philippines.

Geary, E. E.; Kay, R. W.

1989-10-01

184

La Desirade Island (Lesser Antilles) revisited: Lower Cretaceous radiolarian cherts and arguments against an ophiolitic origin for the basal complex  

Microsoft Academic Search

La Desirade Island in the Guadeloupe archipelago is a keystone of the structural framework of the Lesser Antilles island arc. Hypothetical ophiolitic series and wide discrepancy in the radiometric datings of its igneous basement have raised considerable debate concerning the structural significance of this island and the geodynamic constraints for the Caribbean plate. Recent detailed field work leads to a

Philippe Bouysse; Reinhard Schmidt-Effing; Denis Westercamp

1983-01-01

185

Copper Mineralizations in the Ophiolite of Oman: The Genesis and Emplacement Relationship with the Orogenic Movements of Serpentinized Peridotite  

Microsoft Academic Search

The hydrothermal process that led to the deposition of epigenetic sulfide mineralizations (mostly Fe-Cu sulfides) in the ophiolite of Oman appears to be closely related to successive orogenic movements. Vertical (diapiric) uplift of peridotite, along the axial trend of the mountains of Oman, resulted from pressure exerted during the counterclockwise movement and subduction of the Arabian plate during the Miocene

Robert F. Mahfoud; James N. Beck

1997-01-01

186

Tertiary age and paleostructural inferences of the eclogitic imprint in the Austroalpine outliers and Zermatt-Saas ophiolite, western Alps  

NASA Astrophysics Data System (ADS)

The Austroalpine Sesia-Lanzo inlier and upper Austroalpine Dent Blanche, Mt. Mary and Pillonet outliers occur on top of the western-Alpine orogenic wedge and, as a whole, override the structurally composite ophiolitic Piemonte zone. Instead, the Mt. Emilius, Glacier-Rafray, Etirol-Levaz and other lower Austroalpine eclogitic outliers are inserted within the Piemonte zone, between its upper (Combin) and lower (Zermatt-Saas) tectonic elements, or within the latter. Rb-Sr dating on phengitic micas show that the eclogitic imprint in the lower Austroalpine outliers, conventionally regarded as Late Cretaceous by comparison with the Sesia-Lanzo inlier, is of Eocene age (49-40 Ma), like the underlying Zermatt-Saas ophiolite (45-42 Ma) between the Aosta valley and Gran Paradiso massif. 40Ar-39Ar plateau ages on the same mica concentrates of the ophiolitic Zermatt-Saas nappe (46-43 Ma) are consistent with Rb-Sr dating, whereas that on the Austroalpine Glacier-Rafray klippe (92 Ma) is influenced by argon excess. The lower Austroalpine outliers underwent the subduction metamorphism concurrently with the Zermatt-Saas nappe, 20-25 Ma later than the eclogitic Sesia-Lanzo inlier and blueschist Pillonet klippe. The temporal gap and present intra-ophiolitic position mean that the lower Austroalpine outliers were probably derived from an intraoceanic extensional allochthon (Mt. Emilius domain) stranded inside the Piemonte-Ligurian ocean far from the Dent Blanche-Sesia domain and Adriatic margin.

Dal Piaz, G. V.; Cortiana, G.; Del Moro, A.; Martin, S.; Pennacchioni, G.; Tartarotti, P.

2001-03-01

187

Remagnetization of the coast range ophiolite and lower part of the great valley sequence in northern California and southwest Oregon  

Microsoft Academic Search

Overprinted magnetizations have been found at four localities in the Middle Jurassic Coast Range ophiolite and the overlying Upper Jurassic and Lower Cretaceous Great Valley sequence in northern California and at one locality in the partially correlative Lower Cretaceous Days Creek Formation in southwest Oregon. At Del Puerto Canyon, on the east side of the Diablo Range, a pilot study

Leah S. Frei; M. C. Blake

1987-01-01

188

Jurassic back-arc and Cretaceous hot-spot series In the Armenian ophiolites — Implications for the obduction process  

Microsoft Academic Search

The identification of a large OIB-type volcanic sequence on top of an obducted nappe in the Lesser Caucaus of Armenia helps us explain the obduction processes in the Caucasus region that are related to dramatic change in the global tectonics of the Tethyan region in the late Lower Cretaceous. The ophiolitic nappe preserves three distinct magmatic series, obducted in a

Yann Rolland; Ghazar Galoyan; Delphine Bosch; Marc Sosson; Michel Corsini; Michel Fornari; Chrystèle Verati

2009-01-01

189

Hydration, 18 O enrichment and oxidation during ocean floor hydrothermal metamorphism of ophiolitic metabasic rocks from E. Liguria, Italy  

Microsoft Academic Search

During ocean-floor hydrothermal metamorphism of a 225 m thick allochthonous Jurassic sequence of ophiolitic pillow lavas and underlying material in E. Liguria, Italy, the rocks were hydrated, enriched in 18 O and oxidised. H 2 O + contents increased from ~0.3 to 3.8 wt.%, 18 O values increased from ~ +6 to values as high as +13.2 , and (

E. T. C. Spooner; R. D. Beckinsale; P. C. England; A. Senior

1977-01-01

190

35. OPHIOLITIC ROCKS OF THE MIDDLE AMERICA TRENCH LANDWARD SLOPE OFF GUATEMALA: DEFORMATIONAL CHARACTERISTICS AND TECTONIC SIGNIFICANCE1  

Microsoft Academic Search

Dismembered ophiolitic rocks including abundant sheared, serpentinized peridotite (mostly harzburgite) and minor basalts, dolerites, gabbros, and altered metabasites (mainly altered amphibolite) were drilled at most of the sites on the upper to lower Middle America Trench landward slope off Guatemala during Leg 84 of the Deep Sea Drilling Project. These rocks show characteristic Cataclastic deformation with zeolite facies metamorphism and

Yujiro Ogawa; Kantaro Fujioka; Tadao Nishiyama; Seiichiro Uehara; Masaharu Nakagawa

191

Seismic waves velocities and anisotropy in serpentinized peridotites from Xigaze ophiolite: Abundance of serpentine in slow spreading ridge  

Microsoft Academic Search

The effect of serpentinization on seismic wave velocity and anisotropy has been analyzed in 6 peridotite samples of the Xigaze ophiolite, having harzburgitic composition and a degree of serpentinization ranging from 3% to 70%. We found: i) P- and S-wave velocities are linearly correlated with serpentine content; ii) anisotropy of P- and S-waves decreases with increasing serpentinization (while in fresh

H. Horen; M. Zamora; G. Dubuisson

1996-01-01

192

Fault-related oceanic serpentinization in the Troodos ophiolite, Cyprus: Implications for a fossil oceanic core complex  

Microsoft Academic Search

Ultramafic rocks are exposed at the core of a domal structure near a fossil ridge–transform intersection (RTI) in the Troodos ophiolite. A sequence of heavily serpentinized rocks occurs adjacent to a major axis–parallel fault, the Amiandos Fault (AF), which juxtaposes them against gabbro. Previously, serpentinization and faulting were not considered to be related to the Cretaceous ocean spreading history of

Perach Nuriel; Yaron Katzir; Meir Abelson; John W. Valley; Alan Matthews; Michael J. Spicuzza; Avner Ayalon

2009-01-01

193

Structural anatomy of a dismembered ophiolite suite from Gondwana: The Manamedu complex, Cauvery suture zone, southern India  

NASA Astrophysics Data System (ADS)

Detailed geological and structural mapping of the Manamedu ophiolite complex (MOC), from the south-eastern part of the Cauvery suture zone (CSZ) within the Gondwana collisional suture in southern India reveals the anatomy of a dismembered ophiolite succession comprising pyroxenite actinolite-hornblendite, hornblendite, gabbro-norite, gabbro, anorthosite, amphibolite, plagiogranite, mafic dykes, and associated pelagic sediments such as chert-magnetite bands and carbonate horizons. The magmatic foliation trajectory map shows inward dipping foliations and a variety of fold structures. Structural cross-sections of the MOC reveal gentle inward dips with repetition and omission of different lithologies often marked by curvilinear hinge lines. The succession displays imbricate thrust sheets and slices of dismembered ophiolite suites distributed along several localities within the CSZ. The MOC can be interpreted as a deformed large duplex structure associated with south-verging back thrust system, consistent with crustal-scale 'flower structure'. The nature and distribution of ophiolitic rocks in the CSZ suggest supra-subduction zone setting associated with the lithospheric subduction of the Neoproterozoic Mozambique Ocean, followed by collision and obduction during the final stage of amalgamation of the Gondwana supercontinent in the end Precambrian.

Chetty, T. R. K.; Yellappa, T.; Nagesh, P.; Mohanty, D. P.; Venkatasivappa, V.; Santosh, M.; Tsunogae, T.

2011-08-01

194

Characterization of hyperalkaline fluids produced by low-temperature serpentinization of mantle peridotites in the Oman and Ligurian ophiolites  

NASA Astrophysics Data System (ADS)

A regional survey of alkaline springs in Oman and Ligurian ophiolites shows that the alkaline water compositions significantly vary from one ophiolite to the other and within the same ophiolite. The first-order correlation between the Na (and K) and Cl concentrations points to fluid compositions only partly due to evaporation. The scatter around the evaporation line implies that Na and Cl may not be conservative during the alteration of the ultramafic rocks. Mg is almost entirely depleted at pH > 10.5 as a result of serpentine formation within the ultramafic body and of brucite (and minor hydrotalcite) precipitation at the springs. Ca accumulates in the high-pH fluids and is consumed by Ca-carbonate formation at the springs, by mixing with river waters or by the CO2 supply from the atmosphere. Thermodynamic calculations show that brucite saturation is reached at pH values around 10.5 which triggers major changes in the water composition. The waters evolve from a quartz-saturated low-pH continental environment to a brucite-dominated high-pH serpentinizing system at low temperature. The highest water salinities are found in springs located along the basal thrust plane of the ophiolite. The highest Al concentrations are found in some springs located on the crustal side of the mantle/crust boundary. This poses the question of the hydrologic pathways and of the role of the mineralogical composition of the altered formations.

Chavagnac, ValéRie; Monnin, Christophe; Ceuleneer, Georges; Boulart, CéDric; Hoareau, Guilhem

2013-07-01

195

The carbon isotope composition of natural SiC (moissanite) from the Earth's mantle: New discoveries from ophiolites  

NASA Astrophysics Data System (ADS)

Moissanite (natural SiC) has been recovered from podiform chromitites of several ophiolite complexes, including the Luobusa and Donqiao ophiolites in Tibet, the Semail ophiolite in Oman and the United Arab Emirates, and the Ray-Iz ophiolite of the Polar Urals, Russia. Taking these new occurrences with the numerous earlier reports of moissanite in diamondiferous kimberlites leads to the conclusion that natural SiC is a widespread mineral in the Earth's mantle, which implies at least locally extremely low redox conditions. The ophiolite moissanite grains are mostly fragments (20 to 150 µm) with one or more crystal faces, but some euhedral hexagonal grains have also been recovered. Twinned crystals are common in chromitites from the Luobusa ophiolite. The moissanite is rarely colorless, more commonly light bluish-gray to blue or green. Many grains contain inclusions of native Si and Fe-Si alloys (FeSi 2, Fe 3Si 7). Secondary ion mass spectrometric (SIMS) analysis shows that the ophiolite-hosted moissanite has a distinctive 13C-depleted isotopic composition (? 13C from - 18 to - 35‰, n = 36), much lighter than the main carbon reservoir in the upper mantle (? 13C near - 5‰). The compiled data from moissanite from kimberlites and other mantle settings share the characteristic of strongly 13C-depleted isotopic composition. This suggests that moissanite originates from a separate carbon reservoir in the mantle or that its formation involved strong isotopic fractionation. The degree of fractionation needed to produce the observed moissanite compositions from the main C-reservoir would be unrealistically large at the high temperatures required for moissanite formation. Subduction of biogenic carbonaceous material could potentially satisfy both the unusual isotopic and redox constraints on moissanite formation, but this material would need to stay chemically isolated from the upper mantle until it reached the high- T stability field of moissanite. The origin of moissanite in the mantle is still unsolved, but all evidence from the upper mantle indicates that it cannot have formed there, barring special and local redox conditions. We suggest, alternatively, that moissanite may have formed in the lower mantle, where the existence of 13C-depleted carbon is strongly supported by studies of extraterrestrial carbon (Mars, Moon, meteorites).

Trumbull, Robert B.; Yang, Jing-Sui; Robinson, Paul T.; Di Pierro, Simonpietro; Vennemann, Torsten; Wiedenbeck, Michael

2009-12-01

196

Chromites from the Nidar ophiolite and Karzok complex, Transhimalaya, eastern Ladakh: their magmatic evolution  

NASA Astrophysics Data System (ADS)

In the Indus Suture Zone, Ladakh Transhimalaya, chromites occur in different modes, viz. (a) as disseminations (chromite N1) and massive chromitite (chromite NIC) in dunite of the Nidar ophiolite and (b) as disseminations (chromite NIC2) in metamorphosed gabbro of the Karzok complex. Chromites occur both as cumulus and intercumulus phases and show a resorbed boundary with olivine. XRD study shows their uniform lattice parameters ( a0˜8.33 Å). EPMA scan shows variable Cr 2O 3 content of chromite N1 ranging from 52.7 to 60.4% and restricted Cr 2O 3 content for chromite NIC as 61.0-61.5%. The chromite NIC2 also shows a narrow range of Cr 2O 3 content (56.0-58.2%). A 57Fe Mössbauer spectroscopic investigation shows that chromites have similar iron distribution as: Fe 3+(A), Fe 2+(A) and Fe 2+(B). The iron distribution have been linked to oxidation; the higher Fe 3+/Fe 2+ ratio of chromite NIC (0.67) than that of chromite NIC2 (0.30) suggests higher oxidation of the former than the latter. High Mg-olivine (˜Fo 95), high Cr-chromite (Cr#=80) and Al 2O 3 wt% (12.01-12.22) of chromites from the Nidar ophiolite are typical for a boninitic parentage. The Cr# vs Mg# plot and FeO/MgO in melt (˜0.5) also suggest derivation from the boninitic melt. On the other hand, the chemical signatures of chromite NIC2, Al 2O 3 wt% (14.91-15.21) and FeO/MgO (1.17-1.32) for corresponding melt are very different from a boninitic parentage and may be indicative of a depleted peridotite/MORB type-source. The partitioning of Mg and Fe between coexisting olivine and Cr-spinel in chromite NIC indicates sub-solidus temperatures of around 1000 °C. The differences in chemical composition of chromites as well as parental melts suggest different sources for the Nidar ophiolite and the Karzok complex.

Ravikant, V.; Pal, Tapan; Das, Dipankar

2004-11-01

197

Perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon  

NASA Astrophysics Data System (ADS)

The formation of oceanic plagiogranite has been attributed primarily to either 1) extreme fractional crystallization of a mantle melt, or 2) partial melting of hydrated mafic crust, with support for the latter from field evidence and recent melting experiments. Remelting of hydrothermally-altered ocean crust could yield rocks (and minerals) with diverse primary magmatic ?18O values forming proximal to the magmatic center where crustal growth is occurring. To constrain the magmatic ?18O of a wide range of silicic rocks in oceanic crust and evaluate their petrogenesis, we characterized the ?18O of zircons in 22 plagiogranite samples (tonalite and trondhjemite) from 8 different ophiolites and one dacite sampled along the East Pacific Rise using Secondary Ion Mass Spectrometry (SIMS). The ?18O values of 202 magmatic zircons from ophiolites range from 3.9 to 5.6‰ (n = 244 spots; average 4.9 ± 0.6‰; 2SD), extending ~ 1‰ below typical zircon in equilibrium with mantle and from gabbroic massifs along slow-spreading mid-ocean ridges (4.7-5.9‰). East Pacific Rise dacite zircons range from 4.6 to 5.0‰ (n = 12 spots). Plagiogranite from the dike-gabbro transition zone of the northern Oman Ophiolite yield the lowest ?18O(Zrn), with rock-average values of 4.3-5.0‰. The low-?18O values are best explained by remelting of crust altered by hydrothermal fluids with seawater-like isotopic compositions at high temperatures, possibly due to vertical migration of the boundary between an active magma chamber and a vigorous high-temperature hydrothermal system in the overlying crust. If the partial melt was assimilated into a fractionating melt lens with MORB-like ?18O, as envisioned for km-scale plagiogranite bodies in Oman, up to 20% contamination by a protolith with ?18O = 2‰ would be required. Previous oxygen isotope constraints from quartz in Oman plagiogranite suggested melting of both high and low ?18O crust had occurred; comparison of quartz-zircon pairs indicates that quartz has been modified in most samples, and we find no evidence for the involvement of high-?18O rocks during plagiogranite formation.

Grimes, Craig B.; Ushikubo, Takayuki; Kozdon, Reinhard; Valley, John W.

2013-10-01

198

Possible polyphase metamorphic evolution of high grade metabasic rocks from the Songshugou ophiolite, Qinling orogen, China  

NASA Astrophysics Data System (ADS)

The Proterozoic Songshugou ophiolite consists of a series of ultrabasic and tholeitic metabasic rocks. They were emplaced as a lense shaped body into the southern margin of the Qinling Group. Isotope composition and trace element geochemistry display an E-MORB and T-MORB signature for the mafic rocks (Dong et al., 2008). Within the ophiolite sequence some rudimental fresh peridotites (dunites and harzburgites) within serpentines display low CaO (<0.39 wt.%) and Al2O3 (<0.51 wt.%) as well as high MgO (41-48 wt.%) contents, which can be classified as depleted non-fertile mantle rocks. The metabasic rocks comprise the mineral assemblage garnet, amphibole, symplectitic pyroxenes, ilmenite, apatite, ±zoisite, ±sphene and show a strong retrograde metamorphic overprint. Garnet typically contains many inclusions within the core but are nearly inclusion free at the rim. The cores have sometimes snowball textures indicating initially syndeformative growth. Albite and prehnite were found in central parts of garnet. In the outer portions, pargasitic amphibole, rutile and a bluish amphibole, probably glaukophane were found. Garnet zoning pattern clearly show a discontinous growth seen in an sudden increase in grossular and decrease in almandine components. The symplectitic pyroxenes are of diopsidic composition which enclose typically prehnite and not albite, as common in retrograde eclogitic rocks. Different stages of garnet breakdown to plagioclase and amphibole, from thin plagioclase rims surrounding the garnets to plagioclase rich pseudomorphs, can be observed in different samples. Based on symplectitic pyroxenes a high pressure metamorphic event can be concluded (Zhang, 1999). The garnet breakdown to plagioclase and the symplectites clearly indicate a rapid exhumation phase. The age of the metamorphic event is probably related to the closure of the Shangdan ocean during the early Paleozoic. It is unclear if the garnet rims grew during a later stage of the metamorphic cycle or developed during a separate event. The financial support by Eurasia-Pacific Uninet is gratefully acknowledged. Dong, Y.P., Zhou, M.F., Zhang, G.W., Zhou, D.W., Liu, L., Zhang, Q., 2008. The Grenvillian Songshugou ophiolite in the Qinling Mountains, Central China: implications for the tectonic evolution of the Qinling orogenic belt. Journal of Asian Earth Science 32 (5-6), 325-335. Zhang, Z.J., 1999. Metamorphic evolution of garnet-clinopyroxene-amphibole rocks from the Proterozoic Songshugou mafic-ultramafic complex, Qinling Mountains, central China. The Island Arc, 8, 259-280.

Belic, Maximilian; Hauzenberger, Christoph; Dong, Yunpeng; Chen, Danling

2014-05-01

199

Petrology of metabasic and peridotitic rocks of the Songshugou ophiolite, Qinling orogen, China  

NASA Astrophysics Data System (ADS)

The Proterozoic Songshugou ophiolite outcrops as a rootless nappe which was emplaced into the southern margin of the Qinling Group. It consists mainly of amphibolite facies metamafic and -ultramafic rocks. Trace element geochemistry and isotope composition show that the mafic rocks are mainly E-MORB and T-MORB metabasalts (Dong et al., 2008b). Within the ophiolite sequence, ultramafic rocks consist mainly of peridotites and serpentinites. Particularly, extremely fresh dunites and harzburgites, are found which do not display a conspicuous metamorphic overprint. The low CaO (<0.39 wt.%) and Al2O3 (<0.51 wt.%) as well as high MgO (41-48 wt.%) contents classify them as depleted non-fertile mantle rocks. Chromite is found as disseminated phase but can sometimes form massive chromite bands. The platinumgroup mineral Laurite (RuS2) could be identified as inclusion in chromites. Usually part of Ru is substituted by Os and Ir. The metamafic rocks consist of garnet, amphibole, symplectitic pyroxenes, ilmenite, apatite, ±zoisite, ±sphene and show a strong metamorphic overprint. Garnet contains numerous inclusions in the core but are nearly inclusion free at the rim. The cores have sometimes snowball textures indicating initially syndeformative growth. Pure albite and prehnite were found in the central parts of the garnets. In the outer portions, pargasitic amphibole, rutile and rarely glaukophane were found. The symplectitic pyroxenes are of diopsidic composition which enclose prehnite and not albite, as common in retrograde eclogitic rocks. Different stages of garnet breakdown to plagioclase and amphibole, from thin plagioclase rims surrounding the garnets to plagioclase rich pseudomorphs, can be observed in different samples. Based on the glaukophane inclusions and symplectitic pyroxenes a high pressure metamorphic event can be concluded. The garnet breakdown to plagioclase and the symplectites clearly indicate a rapid exhumation phase. The age of the metamorphic event is unclear but probably related to the closure of the Shangdan ocean during the early Paleozoic. The financial support by Eurasia-Pacific Uninet is gratefully acknowledged. Dong, Y.P., Zhou, M.F., Zhang, G.W., Zhou, D.W., Liu, L., Zhang, Q., 2008. The Grenvillian Songshugou ophiolite in the Qinling Mountains, Central China: implications for the tectonic evolution of the Qinling orogenic belt. Journal of Asian Earth Science 32 (5-6), 325-335.

Belic, Maximilian; Hauzenberger, Christoph; Dong, Yunpeng

2013-04-01

200

Izu-Bonin-Mariana forearc crust as a modern ophiolite analogue  

NASA Astrophysics Data System (ADS)

Recent geological and geophysical surveys in the Izu-Bonin-Mariana (IBM) fore-arc have revealed the occurrence on the seafloor of oceanic crust generated in the initial stages of subduction and embryonic island arc formation. The observed forearc section is composed of (from bottom to top): (1) mantle peridotite, (2) gabbroic rocks, (3) a sheeted dyke complex, (4) basaltic pillow lavas, (5) boninites and magnesian andesites, and (6) tholeiites and calc-alkaline arc lavas. The oldest magmatism after subduction initiation generated forearc basalts (FAB) between 52 and 48 Ma, and then boninitic and calc-alkaline lavas that collectively make up the extrusive sequence of the forearc oceanic crust. The change from FAB magmatism to flux melting and boninitic volcanism took 2-4 m.y., and the change to flux melting in counter-flowing mantle and "normal" arc magmatism took 7-8 m.y. This evolution from subduction initiation to true subduction occurred nearly simultaneously along the entire length of the IBM subduction system. One important characteristic feature of the common forearc stratigraphy in the IBM forearc is the association of sheeted dykes with basaltic pillow lavas, which strongly implies that the eruption of FAB was associated with seafloor spreading. This is supported by the seismic velocity structure of the Bonin Ridge area (Kodaira et al., 2010), showing it to have a thin ocean-ridge-like crust (< 10km). It appears that the FAB was produced by sea-floor spreading associated with subduction initiation along the length of the IBM forearc. A potential location of subduction nucleation along the Mesozoic-aged crust has been found along the margins of the West Philippine Basin. One possible scenario for subduction initiation at the IBM arc was that it was induced by overthrusting of the Mesozoic arc and backarc or forearc terranes bounding the east side of the Asian Plate over the Pacific Plate, followed by failure of the Pacific plate lithosphere and subduction initiation. Alternatively, subduction could have begun spontaneously, facilitated by the density contrast between the arc-bearing Mesozoic Asian crust and the old oceanic Pacific crust to its west. This volcanic stratigraphy and their time-progressive development in the IBM system are analogous to those documented from suprasubduction (SSZ) ophiolites. Most SSZ ophiolites are on-land fragments of forearc oceanic crust, produced at subduction initiation and during the early stages of island arc development (Dilek and Furnes, 2009, 2011). Similarities between the oceanic lithosphere of both forearc settings and SSZ ophiolites also extend to the upper mantle units, which are composed of extremely depleted peridotites.

Ishizuka, Osamu; Tani, Kenichiro; Reagan, Mark; Kanayama, Kyoko; Umino, Susumu; Harigane, Yumiko

2013-04-01

201

Biomarker insights into microbial activity in the serpentinite-hosted ecosystem of the Semail Ophiolite, Oman  

NASA Astrophysics Data System (ADS)

Serpentinization is a process in which ultramafic and mafic rocks undergo exothermic reactions when exposed to water. The products of these reactions, including methane, hydrogen, and hydrogen sulfide, can sustain microbially dominated ecosystems [1,2,3]. Here, we report the lipid biomarker record of microbial activity in carbonate veins of the Semail Ophiolite, a site currently undergoing serpentinization [4]. The ophiolite, located in the Oman Mountains in the Sultanate of Oman, was obducted onto the Arabian continental margin during the closure of the southern Tethys Ocean (~70 Ma) [5]. We detected bacterial and archaeal glycerol dialkyl glycerol tetraether (GDGT) lipids in Semail carbonates. In addition to archaeal isoprenoidal GDGTs with 0-3 cyclopentane moieties, we detected crenarchaeol, an iGDGT containing 4 cyclopentane and 1 cyclohexane moiety. Crenarchaeol biosynthesis is currently understood to be limited to thaumarchaea, representatives of which have been found to fix inorganic carbon in culture. We also analyzed isoprenoidal diether lipids, potentially derived from methanogenic euryarchaea, as well as non-isoprenoidal diether and monoether lipids that may be indicative of methane cycling bacteria. The stable carbon isotopic composition of these compounds is potentially useful in determining both their origin and the origin of methane detected in ophiolite fluids. We compare our results to those found at the Lost City Hydrothermal Field, a similar microbially-dominated ecosystem fueled by serpentinization processes [3]. Modern serpentinite-hosted ecosystems such as this can serve as analogs for environments in which ultramafic and mafic rocks were prevalent (e.g. early Earth and other early terrestrial planets). Additionally, an analysis of modern serpentinite systems can help assess conditions promoting active carbon sequestration in ultramafic rocks [6]. References [1] Russell et al. (2010). Geobiology 8: 355-371. [2] Kelley et al. (2005). Science 307: 1428-1434. [3] Bradley et al. (2009). Geochimica Et Cosmochimica Acta 73 (1): 102-118. [4] Barnes et al. (1978). Geochimica Et Cosmochimica Acta 42: 144-145. [5] Nasir et al. (2007). Chemie Der Erde-Geochemistry 67(3): 213-228. [6] Kelemen et al. (2008). Proceedings of the National Academy of Sciences of the United States of America 45: 17295-17300.

Newman, S. A.; Lincoln, S. A.; Shock, E.; Kelemen, P. B.; Summons, R. E.

2012-12-01

202

From Aptian Onset to Danian Demise of Subduction along the Northern Margin of the Caribbean Plate (Sierra del Convento Melange, Eastern Cuba)  

NASA Astrophysics Data System (ADS)

The serpentinite-matrix melange of the Sierra del Convento, eastern Cuba, represents an oceanic subduction channel related to Mesozoic subduction in the Caribbean realm which provides evidence for a long-lasting history of subduction, accretion, melange formation, and uplift, and for Aptian onset of subduction in the region. Exotic blocks of MORB-derived plagioclase-free epidote±garnet amphibolite followed a hot subduction-related prograde P-T path, reaching ca. 750 °C, and 14-16 kbar at peak conditions. Fluid flux at this stage triggered melting of the amphibolites to yield peraluminous tonalitic-trondhjemitic melts, which appear intimately associated with the amphibolites. Trondhjemitic-granitic varieties richer in K2O suggest the local participation of a sedimentary source, likely diluted through the infiltrating fluid. Calculated conditions for the magmatic assemblages (plagioclase, quartz, epidote, ±paragonite, ±pargasite, ±muscovite) of the siliceous rocks yield pressures of ca. 15 kbar, indicating crystallization at depth in the subduction environment. SHRIMP U-Pb zircon dating of two granitoid samples gives crystallization ages of 113-114 Ma. Partial melting of subducted oceanic crust in eastern Cuba is unique in the Caribbean realm and is interpreted as the result of onset of subduction of young oceanic lithosphere during the Aptian (ca. 120 Ma), in agreement with regional geological data. Calculated P-T conditions for the retrograde blueschist-facies overprints present in all rocks indicate counterclockwise P-T paths during exhumation in a colder, syn-subduction scenario. Ar-Ar amphibole dating yielded two groups of cooling ages of 106-97 Ma (interpreted as cooling of metamorphic/magmatic pargasite) and 87-83 Ma (interpreted as growth/cooling of retrograde overprints). The above P-T-t data and additional ages of other rocks from the area suggest the following stages of evolution:(a) hot subduction during 120-115 Ma with heating and burial rates of 150 °C/Ma and 11 km/Ma, respectively, developed shortly after onset of subduction of young oceanic lithosphere; (b) relatively fast near-isobaric cooling (25 °C/Ma) during 115-107 Ma, developed after accretion of the blocks to the upper plate mantle; (c) slow syn-subduction cooling (4 °C/Ma) and exhumation (0.7 km/Ma) in the subduction channel in a colder (mature) subduction environment during 107-70 Ma, and d) fast cooling (70 °C/Ma) and exhumation (5 km/Ma) during 70-65 Ma, when arc-continent collision occurred and subduction terminated in the region.

Garcia-Casco, A.; Lazaro, C.; Rojas Agramonte, Y.; Kroner, A.; Neubauer, F.

2007-12-01

203

Subsurface geometry of the San Andreas-Calaveras fault junction: influence of the Coast Range Ophiolite  

NASA Astrophysics Data System (ADS)

Potential-field modeling, surface geologic mapping, and relocated seismicity are used to investigate the three-dimensional structure of the San Andreas-Calaveras fault junction to gain insight into regional tectonics, fault kinematics, and seismic hazard. South of the San Francisco Bay area, the San Andreas and Hayward-Calaveras fault zones join to become a single San Andreas Fault. The fault junction, as defined in this study, represents a three-dimensional volume of crust extending from San Juan Bautista in the north to Bitterwater Valley in the south, bounded by the San Andreas Fault on the southwest and the Calaveras fault zone on the northeast. South of Hollister, the Calaveras fault zone includes the Paicines, San Benito, and Pine Rock faults. Within the junction, the San Andreas and Calaveras faults are both creeping at the surface, and strike parallel to each other for about 50 km, separated by only 2 to 6 km, but never actually merge at the surface. Geophysical evidence suggests that the San Andreas and Calaveras faults dip away from each other within the northern portion of the fault junction, bounding a triangular wedge of crust. This wedge changes shape to the south as the dips of both the San Andreas and Calaveras faults vary along strike. The main trace of the San Andreas Fault is clearly visible in cross-sections of relocated seismicity as a vertical to steeply southwest-dipping structure between 5 and 10 km depth throughout the junction. The Calaveras fault dips steeply to the northeast in the northern part of the junction. Near the intersection with the Vallecitos syncline, the dip of the Calaveras fault, as identified in relocated seismicity, shallows to 60 degrees. Northeast of the Calaveras fault, we identify a laterally extensive magnetic body 1 to 8 km below the surface that we interpret as a folded 1 to 3 km-thick tabular body of Coast Range Ophiolite at the base of the Vallecitos syncline. Potential-field modeling and relocated seismicity indicate that the southwestern edge of this magnetic body is defined by a northeast-dipping structure that we interpret as part of the Calaveras fault. The base of this magnetic slab, which is folded up along the Calaveras fault, may represent a roof thrust formed by an eastward-migrating wedge of Franciscan Complex. Fragments of Coast Range Ophiolite caught up within the San Andreas-Calaveras junction may facilitate creep and slip transfer between structures that have no apparent connection at the surface. Combined geological and geophysical results suggest that during development of the junction, the Calaveras fault preferentially followed a zone of weakness represented by the roof thrust and associated Coast Range Ophiolite. The Hayward fault occupies a similar position with respect to the Coast Range Ophiolite near San Leandro to the north.

Watt, J. T.; Ponce, D. A.; Graymer, R. W.; Jachens, R. C.; Simpson, R. W.

2013-12-01

204

Diamonds in ophiolitic mantle rocks and podiform chromitites: An unsolved mystery  

NASA Astrophysics Data System (ADS)

In recent years ultrahigh pressure minerals, such as diamond and coesite, and other unusual minerals were discovered in chromitites of the Luobusa ophiolite in Tibet, and 4 new minerals have been approved by the CNMMN. These results have raised many questionsWhat are the occurrences of the diamonds, what is the source of their carbon and how were they formed? What is the origin of the chromites hosting the diamonds and at what depth did they form? What is the genetic relationship between the diamonds and the host chromitites? In what geological, geophysical and geochemical environments can the diamonds be formed and how are they preserved? The UHP minerals from Luobusa are controversial because they have not been found in situ and because ophiolites are currently believed to form at shallow levels above oceanic spreading centers in suprasubduction zone environments. More detailed study and experimental work are needed to understand the origin and significance of these unusual minerals and investigations of other ophiolites are needed to determine if such minerals occur elsewhere To approach these problems, we have collected two one-ton samples of harzburgite hosting chromitite orebodies in the Luobusa ophiolite in Tibet. The harzburgite samples were taken close to chromitite orebody 31, from which the diamonds, coesite and other unusual minerals were recovered. We processed these two samples in the same manner as the chromitites and discovered numerous diamonds and more than 50 other mineral species. These preliminary results show that the minerals in the harzburgites are similar to those in the chromitites, suggesting a genetic relationship between them. To determine if such UHP and unusual minerals occur elsewhere, we collected about 1.5 t of chromitite from two orebodies in an ultramafic body in the Polar Urals. Thus far, more than 60 different mineral species have been separated from these ores. The most exciting discovery is the common occurrence of diamond, a typical UHP mineral in the Luobusa chromitites. Other minerals include: (1) native elements: Cr, W, Ni, Co, Si, Al and Ta; (2) carbides: SiC and WC; (3) alloys: Cr-Fe, Si-Al-Fe, Ni-Cu, Ag-Au, Ag-Sn, Fe-Si, Fe-P, and Ag-Zn-Sn; (4) oxides: NiCrFe, PbSn, REE, rutile and Si-bearing rutile, ilmenite, corundum, chromite, MgO, and SnO2; (5) silicates: kyanite, pseudomorphs of octahedral olivine, zircon, garnet, feldspar, and quartz,; (6) sulfides of Fe, Ni, Cu, Mo, Pb, Ab, AsFe, FeNi, CuZn, and CoFeNi; and (7) iron groups: native Fe, FeO, and Fe2O3. These minerals are very similar in composition and structure to those reported from the Luobusa chromitites.

Yang, J.; Zhang, Z.; Xu, X.; Ba, D.; Bai, W.; Fabg, Q.; Meng, F.; Chen, S.; Robinson, P. T.; Dobrzhinetskaya, L.

2009-05-01

205

Acadian remobilization of a Taconian ophiolite, Hare Bay allochthon, northwestern Newfoundland  

SciTech Connect

The Hare Bay fault is a major subhorizontal detachment at the base of the ophiolitic St. Anthony Complex in the Hare Bay allochthon, northwestern Newfounland. The fault is a postmetamorphic brittle detachment that truncates footwall structures related to both initial Ordovician (Taconian) assembly of the allochthon and subsequent Silurian-Devonian (Acadian) deformation. Although previously mapped as a thrust, the fault has an extensional rather than a contractional geometry; it cuts downsection to the west in the direction of transport, and it juxtaposes a hanging-wall sequence that contains little or no Acadian deformation against a footwall sequence that was pervasively deformed during the Acadian orogeny. The St. Anthony Complex lies on the western margin of the Acadian deformed zone. Its final emplacement, through movement on the Hare Bay fault, probably occurred through extensional faulting during gravitational collapse of the Acadian mountain front.

Cadwood, P.A. (Memorial Univ. of Newfoundland, St. Johns's (Canada))

1989-03-01

206

PALLADIUM, PLATINUM, RHODIUM, RUTHENIUM AND IRIDIUM IN PERIDOTITES AND CHROMITITES FROM OPHIOLITE COMPLEXES IN NEWFOUNDLAND.  

USGS Publications Warehouse

Samples of spinel lherzolite, harzburgite, dunite, and chromitite from the Bay of Islands, Lewis Hills, Table Mountain, Advocate, North Arm Mountain, White Hills Periodite Point Rousse, Great Bend and Betts Cove ophiolite complexes in Newfoundland were analyzed for the platinum-group elements (PGE) Pd, Pt, Rh, Ru and Ir. The ranges of concentration (in ppb) observed for all rocks are: less than 0. 5 to 77 (Pd), less than 1 to 120 (Pt), less than 0. 5 to 20 (Rh), less than 100 to 250 (Ru) and less than 20 to 83 (Ir). Chondrite-normalized PGE ratios suggest differences between rock types and between complexes. Samples of chromitite and dunite show relative enrichment in Ru and Ir and relative depletion in Pt and Pd.

Page, Norman, J.; Talkington, Raymond, W.

1984-01-01

207

Mineralogy and geochemistry of listwanite occurrences from the Othris ophiolite, Greece.  

NASA Astrophysics Data System (ADS)

Three small occurrences of extensively carbonate-altered serpentinite (listwanite) have been identified in East Othris at the ophiolitic formation of Vrinena, and in South Othris at the ophiolitic mélange formations of Agios Georgios and Paleokerasia. Their mineral assemblage includes calcite + quartz + chlorite + spinel ± clinopyroxene, as well as accessory Fe-oxides, titanite and apatite. Based upon their mineralogical composition they belong to the Type IB listwanite, characterized by the predominance of calcite and the presence of Mg-rich chlorite, mainly clinochlore and diabantite. In the ophiolitic mélange formation of Agios Georgios listwanite was found in proximity with serpentinite (former harzburgite), which could be considered as the precursor protolith. Changes of major and minor elements between the listwanite and the protolith have been calculated based on the method for mass-balance analyses[1]. Results show that this listwanite resulted after metasomatic processes dominated by Ca enrichment. Sr, Y and Pb contents were also significantly increased, whereas rather moderate enrichments of Al, Mn, Cr and Cu also took place. Small reductions were observed for Mg and Ni. Si, Ti and Fe remained relatively immobile. The chondrite normalized REE patterns reveal significant enrichment of all analysed REE, and especially of the LREE [(La/Yb)CN=20.4], and also with a negative Eu anomaly (EuCN/Eu*=0.79). Spinel grains from the Agios Georgios listwanite and the adjacent serpentinite are in most elements compositionally similar. The listwanites from Agios Georgios, Paleokerasia and Vrinena all contain spinel grains. Their mineral chemistry is respectively: TiO2=0.18-0.25; 0.04-0.10; 0.22-0.54 wt%, Al2O3=23.13-25.03; 27.69-29.70; 5.69-7.35 wt%, FeO=18.24-22.98; 16.44-19.49; 21.47-24.61 wt%, CaO= 0.01-0.07; 0.03-0.15; 0.01-0.28 wt%, Cr#=52.28-54.93; 45.57-48.85; 83.58-87.59, Mg#=51.07-65.39; 56.68-65.62; 46.77-55.35. Their rims exhibit slightly higher FeO and CaO contents compared to cores. Relict clinopyroxenes have been found in listwanite from Vrinena, classified as augites (Mg#=84.55-85.91; Wo=42.50-44.34; TiO2=0.50-0.70 wt%). The abundance of calcite and of REE enrichment indicate that the listwanite-forming metasomatic event occurred with hydrothermal circulation of a CO2-rich fluid phase in a high water/rock ratio. REE mobilized mainly as REE-carbonate complexes under mildly alkaline conditions. Based upon the reciprocal slopes of the isocon method[2] the total mass gains are restricted, indicating that this hydrothermal alteration event occurred isochemically under mass preservation. Their formation is most likely associated with shallow level ocean-floor metasomatism, observed also in listwanites from the Iti ophiolitic mélange formation[3]. References: [1] Gresens 1967: Chemical Geol., 2, 47-65; [2] Grant 1986: Econ. Geol., 81, 1976-1982; [3] Tsikouras et al., 2006: Eur. J. Mineral. 18, 243-255.

Koutsovitis, Petros; Magganas, Andreas

2013-04-01

208

Platinum-group elemental geochemistry of mafic and ultramafic rocks from the Xigaze ophiolite, southern Tibet  

NASA Astrophysics Data System (ADS)

The Xigaze ophiolite in the central part of the Yarlung-Zangbo suture zone, southern Tibet, has a well-preserved sequence of sheeted dykes, basalts, cumulates and mantle peridotites at Jiding and Luqu. Both the basalts and diabases at Jiding have similar compositions with SiO 2 ranging from 45.9 to 53.5 wt%, MgO from 3.1 to 6.8 wt% and TiO 2 from 0.87 to 1.21 wt%. Their Mg #s [100Mg/(Mg + Fe)] range from 40 to 60, indicating crystallization from relatively evolved magmas. They have LREE-depleted, chondrite-normalized REE diagrams, suggesting a depleted mantle source. These basaltic rocks have slightly negative Nb- and Ti-anomalies, suggesting that the Xigaze ophiolite represents a fragment of mature MORB lithosphere modified in a suprasubduction zone environment. The mantle peridotites at Luqu are high depleted with low CaO (0.3-1.2 wt%) and Al 2O 3 (0.04-0.42 wt%). They display V-shaped, chondrite-normalized REE patterns with (La/Gd) N ratios ranging from 3.17 to 64.6 and (Gd/Yb) N from 0.02 to 0.20, features reflecting secondary metasomatism by melts derived from the underlying subducted slab. Thus, the geochemistry of both the basaltic rocks and mantle peridotites suggests that the Xigaze ophiolite formed in a suprasubduction zone. Both the diabases and basalts have Pd/Ir ratios ranging from 7 to 77, similar to MORB. However, they have very low PGE abundances, closely approximating the predicted concentration in a silicate melt that has fully equilibrated with a fractionated immiscible sulfide melt, indicating that the rocks originated from magmas that were S-saturated before eruption. Moderate degrees of partial melting and early precipitation of PGE alloys explain their high Pd/Ir ratios and negative Pt-anomalies. The mantle peridotites contain variable amounts of Pd (5.99-13.5 ppb) and Pt (7.92-20.5 ppb), and have a relatively narrow range of Ir (3.47-5.01 ppb). In the mantle-normalized Ni, PGE, Au and Cu diagram, they are relatively rich in Pd and depleted in Cu. There is a positive correlation between CaO and Pd. The Pd enrichment is possibly due to secondary enrichment by metasomatism. Al 2O 3 and Hf do not correlate with Ir, but show positive variations with Pt, Pd and Au, indicating that some noble metals can be enriched by metasomatic fluids or melts carrying a little Al and Hf. We propose a model in which the low PGE contents and high Pd/Ir ratios of the basaltic rocks reflect precipitation of sulfides and moderate degrees of partial melting. The high Pd mantle peridotites of Xigaze ophiolites were formed by secondary metasomatism by a boninitic melt above a subduction zone.

Chen, Genwen; Xia, Bin

2008-04-01

209

An oceanic core complex (OCC) in the Mirdita ophiolite of the Albanian Dinarides?  

NASA Astrophysics Data System (ADS)

The basic premise of seafloor spreading is that magmatic activity at mid-ocean ridges is responsible for creation of new crust; however only several years after the formulation of plate tectonic theory we have recognised the importance of tectonic, rather than magmatic, processes in accommodating the divergence of the lithospheric plates in slow-spreading oceans. In these settings, magma supply is spatially and temporally discontinuous and the crust freezes solid between delivery of magma batches from the underlying mantle. Large-scale extensional detachment faults develop during these periods of reduced magma supply, and play a fundamental role in accommodating plate separation. Extensional faulting occurs at the corner between ridge and transform fault and is responsible for the generation of the Oceanic Core Complexes (OCCs). OCCs are the uplifted footwalls of oceanic detachment faults forming dome-shaped massifs consisting of mantle and lower crustal lithologies exposed at the seafloor. Numerical modelling and results from the Integrate Ocean Drilling Program (IODP) has shown that the footwalls beneath oceanic detachment faults rotate during their evolution, proving that they initiate at a steep angle at depth and then "roll-over" to their present day low angle orientations as a result of flexural isostasy during unroofing. Accordingly, information from paleomagnetic analyses provided strong constraints to the detachment kinematics, since a difference of 45-65° in paleomagnetic inclination across the fault has been documented by previous studies, confirming the rolling-hinge model for OCCs formation. Here we present the results of an extensive paleomagnetic study of an OCC preserved in the Mirditata ophiolite of the Albanian Dinarides, first recognized by Trembley et al. (2009). Ophiolites are slices of oceanic lithosphere which have been emplaced onto continental margins during the closure of ocean basins, and provide opportunities for the study of oceanic structures in three-dimensions from kilometric to millimetric spatial scales.The western part of Mirdita ophiolite is composed of a lherzolitic mantle sequence overlain by a thin gabbro/troctolitic sequence and MORB-like pillow lavas. The sheeted dyke complex and gabbroic sequence are missing locally, and the upper crustal volcanic sequence then rests directly on the mantle sequence in tectonic contact. This anomalous situation is directly comparable to lithostratigraphic relationships in oceanic detachment fault settings. In order to understand this tectonic contact and describe its kinematics we sampled 73 sites from ultramafics, lava flows, pillow lavas, and dykes, at the Puka and Krabbi massifs between the villages of Puka and Rreshen in northern Albania. Preliminary results showed a ca. 20° difference in paleomagnetic inclination between ultramafics and the volcanic sequence, quite minor than the 45-60° expected for detachment faults at modern OCCs. Now we discuss the results from all the samples sites and will provide a complete interpretation of the relationship between ultramafic and volcanics rocks from the northwestern part of the Mirdita ophiolite, in the framework of the OCC hypothesis.

Maffione, M.; Morris, A.; Anderson, M.

2011-12-01

210

Stable isotope geochemistry of Alpine ophiolites: a window to ocean-floor hydrothermal alteration and constraints on fluid–rock interaction during high-pressure metamorphism  

Microsoft Academic Search

The subduction of hydrated oceanic lithosphere potentially transports large volumes of water into the upper mantle; however,\\u000a despite its potential importance, fluid–rock interaction during high-pressure metamorphism is relatively poorly understood.\\u000a The stable isotope and major element geochemistry of Pennine ophiolite rocks from Italy and Switzerland that were metamorphosed\\u000a at high pressures are similar to that of unmetamorphosed ophiolites, suggesting that

I. Cartwright; A. C. Barnicoat

1999-01-01

211

Geochemistry, petrology and tectonomagmatic significance of basaltic rocks from the ophiolite mélange at the NW External-Internal Dinarides junction (Croatia)  

Microsoft Academic Search

At the NW inflexion of the Sava-Vardar Suture Zone ophiolite mélanges, known as the Kalnik Unit, form the surface of the slopes of several Pannonian inselbergs in the SW Zagorje-Mid-Transdanubian Zone. The Mt Samoborska Gora ophiolite mélange, thought to be a part of the Kalnik Unit, forms a separate sector obducted directly onto Dinaric Triassic carbonate sediments. Basaltic rocks, the

Damir Slovenec; Bosko Lugovic; Irena Vlahovic

2010-01-01

212

The eroded Late Jurassic Kurbnesh carbonate platform in the Mirdita Ophiolite Zone of Albania and its bearing on the Jurassic orogeny of the Neotethys realm  

Microsoft Academic Search

Several Late Jurassic (Kimmeridgian?-Tithonian) to Early Cretaceous (Late Berriasian-Valanginian) shallow-water carbonate\\u000a clasts of different facies are contained in mass-flow deposits in a pelagic sequence in the Kurbnesh area of central Albania.\\u000a These clasts are used to reconstruct shallow-water carbonate platforms, which formed on top of the radiolaritic-ophiolitic\\u000a wildflysch (ophiolitic mélange) of the Mirdita Zone. Stratigraphic interpretation of the platform carbonates

Felix Schlagintweit; Hans-Jürgen Gawlick; Sigrid Missoni; Lirim Hoxha; Richard Lein; Wolfgang Frisch

2008-01-01

213

Geochemical processes and petrogenetic evolution of rodingite dykes in the ophiolite complex of Othrys (Central Greece)  

NASA Astrophysics Data System (ADS)

Rodingite dykes occur in serpentinised harzburgites in the Othrys ophiolite. Hydrogrossular and diopside along with chlorite appeared first and mark the initiation of rodingitisation of the dolerite protolith. Subsequently calcite formed after diopside and consequently reacted with chlorite to enrich the rodingites in diopside and garnet. Apatite also forms at the late stages of alteration. Several geochemical exchanges were triggered by variably alkaline and continuously evolving fluids passing through the serpentinising peridotites. A possible T-X CO2 path along with a reaction series are proposed for the development of the rodingitic assemblages that entail a fluid phase. Leaching of the peridotite was triggered by fluids very rich in CO 2 and carbonate complexes are thought to be responsible for mobilization of Cr and Ni. Under such conditions, REE also were mobile and the presence of CO 32- and PO 43- ions that complexed them further assisted their transportation. Zr mobilised at increased pH conditions with the aid of the OH - and PO 43- during the late stage of alteration, and due to the incorporation of more saline fluids. After formation of calcite and apatite in the two metasomatic stages, respectively, the REE complexes were destabilised due to reduced activity of CO 32- and PO 43- and free REEs were able to precipitate onto garnet. Cr and Ni hydroxides were precipitated after formation of calcite in the rodingite. The saline fluids were also reducing, hence Eu remained in its divalent state and was highly absorbed on garnet surfaces. The rodingitisation at Othrys ophiolite occurred in a subduction regime as it is implicated by the very rich in CO 2 fluid phase which is readily explained by involvement of subducted carbonate sediments and is favoured by the development of structural zones that acted as conduits to the metasomatising fluid phase.

Tsikouras, Basilios; Karipi, Sofia; Rigopoulos, Ioannis; Perraki, Maria; Pomonis, Panagiotis; Hatzipanagiotou, Konstantin

2009-12-01

214

Mineral Relicts of the Earth's Mantle Transition Zone in Peridotites of the Nidar Ophiolite, Himalaya, India  

NASA Astrophysics Data System (ADS)

In this paper, we report on the discovery of mineral relicts of the Earth's Mantle Transition Zone recovered from the peridotite section of the Nidar Ophiolite Complex (NOC), Indus Suture Zone, NW Himalaya. Field relations indicate the Nidar ophiolite as a large, continuous body at least 300 sq. km in size with the basal section comprising of ~ 8 km thick ultramafics. Using mineral chemical data and mineral phase identification through Laser micro Raman spectroscopy, we have discovered high- pressure mineral assemblages in relict grains within orthopyroxenes of the peridotites. The peridotite, host of the high-pressure minerals occur as a 'xenolith' in a well-mapped dunite channel of NOC. Here we focus on a ~100 micron size mineral relict found within an orthopyroxene grain. The orthopyroxene has been characterized as mixed phases of ortho and clino-enstatite. Near the core of the orthopyroxene grain, the semi translucent relict exhibits fine infuse cracks sharp edges and complex mineralogy. We have identified high-pressure clinoenstatite, square shaped coesite, disordered ? - hematite, carbonaceous matter and a mixture of pyroxene -akimotoite glass in this grain. Based on the above observations, and the high-pressure phase boundaries of the identified minerals, we have constructed a tentative P- T trajectory of the relict grain along the mantle adiabat starting from the base of the mantle transition zone at ~25 GPa and ~ 1500°C at the 660 km discontinuity at the top of the lower mantle. This study will have implications for constraints for the nature and extent of convective flow in the Earth's mantle and on the origin of MORB.

Das, S.; Mukherjee, B. K.; Basu, A. R.

2013-12-01

215

Os Isotope Heterogeneity of the Convecting Upper Mantle: The Mayari-Baracoa Ophiolitic Belt (Eastern Cuba)  

NASA Astrophysics Data System (ADS)

Chromite separates from a set of historically important chromite deposits from the 90 Ma old Mayarí-Baracoa Ophiolitic Belt in eastern Cuba were inspected for Re-Os isotopic systematics in an attempt to quantify the extent of Os isotopic heterogeneities within a restricted upper mantle portion represented by a single ophiolite. Compositional variations of chromites indicate their crystallization from hydrous melts varying in composition from back-arc basin basalts (Al-rich chromites; Cr# = 0.43-0.55; low Pd/Ir) to boninites (Cr-rich chromites; Cr# = 0.60-0.83; high Pd/Ir) in a supra-subduction zone setting. Initial Os isotopic compositions of the studied chromites can be grouped according to their distribution in 3 regional districts. Results indicate systematically negative calculated initial ?Os values varying from -1.06 ± 0.79 (Moa-Baracoa district), -1.77 ± 0.80 (Sagua de Tanamo district) and -2.79 ± 0.31 (Mayari district). These suprachondritic values are distinctly (3.5-5.2%) less radiogenic than the estimated minimum 187Os/188Os composition of the primitive upper mantle of 0.1296 ± 8 and can be explained by Re depletion during ancient partial melting and melt percolation events. Old Os isotope model ages (<2100 Ma)of some of the chromites (or platinum-group minerals included in them) show and confirm previous findings that ancient Os isotopic signatures can survive in the Earth's upper mantle. Our systematically negative initial ?Os values do not improve the definition of an already statistically poorly defined present-day Os isotopic composition of the convecting upper mantle, but instead indicate a complex history for the convecting upper mantle which precludes the calculation of a uniform regional Os isotopic signature for this reservoir.

Frei, R.; Gervilla, F.; Meibom, A.; Proenza, J. A.

2005-12-01

216

Diamond and Coesite After Former Stishovite in Tibet Ophiolites: Is That Myth or Reality?  

NASA Astrophysics Data System (ADS)

An extremely controversial theme in the Earth Sciences over the last 30 years has been the origin of chromitite deposits found worldwide within depleted peridotites (harzburgites) of the uppermost mantle in ¨C rock sequences representing former oceanic lithosphere. The majority of workers support a model in which massive chromitites containing Os- and Ir-rich platinum group element (PGE) alloys form by interaction of H2O-saturated boninite melt with harzburgites of the mantle wedge above a subduction zone at comparatively shallow levels in the mantle, from depths of 30-50 km to a maximum pressure of 4.5 GPa (ca.150 km). A second model suggests that the Os/Ir-rich PGE alloys associated with podiform chromitites formed in the very deep mantle, either before formation of Earth's core or, if later, at the core-mantle boundary, and that they have been transported to the surface as xenoliths in deep-rooted mantle plumes. We report the presence of diamond and prismatic coesite associated with metallic grains from chromitite of a Tibetan ophiolite, implying 4 GPa minimum pressure. The microstructure of coesite strongly suggests replacement of stishovite, implying P over10 GPa. This is the first confirmed observation of coesite or diamond in an ophiolite and the occurrence of coesite with the morphology of stishovite is unique. Diamonds have been recovered previously from the Luobusa chromitites, but only as individual grains. Here we describe a composite microdiamond-silicate inclusion in a grain of Os/Ir alloy. Our study also involves examination of grains of Os-Ir, Ni-Fe-Cr-C and CrC alloys, native Si, Fe, Ti and Cr, and chromite separated from massive chromitite ore. Two explanations appear possible: derivation from the deep mantle or meteorite impact, but no direct evidence of shock has been observed. However, at this stage the interpretation should leave open.

Yang, J.; Dobrzhinetskaya, L. F.; Bai, W.; Fang, Q.; Robinson, P. T.; Zhang, J.; Green, H. W.; Wirth, R.

2006-12-01

217

Geochemical evidence for Late Cretaceous marginal arc-to-backarc transition in the Sabzevar ophiolitic extrusive sequence, northeast Iran  

NASA Astrophysics Data System (ADS)

The ophiolitic extrusive sequence, exposed in an area north of Sabzevar, has three major parts: a lower part, with abundant breccia, hyaloclastic tuff, and sheet flow, a middle part with vesicular, aphyric pillow lava, and an upper part with a sequence of lava and volcanic-sedimentary rocks. Pelagic limestone interlayers contain Late Cretaceous (Maastrichtian-Late Maastrichtian) microfauna. The supra-ophiolitic series includes a sequence of turbidititic and volcanic-sedimentary rocks with lava flow, aphyric and phyric lava, and interlayers of pelagic limestone and radiolarian chert. Paleontological investigation of the pelagic limestone and radiolarite interlayers in this series gives a Late Cretaceous age, supporting the idea that the supra-ophiolitic series formed in a trough, synchronous with the Sabzevar oceanic crust during the Late Cretaceous. Geochemical data indicate a relationship between lava in the upper part of the extrusive sequence and lava in the supra-ophiolitic series. These lavas have a calc-alkaline to almost alkaline characteristic, and show a clear depletion in Nb and definite depletions in Zr and Ti in spider diagrams. Data from these rocks plot in the subduction zone field in tectonomagmatic diagrams. The concentration and position of the heavy rare earth elements in the spider diagrams, and their slight variation, can be attributed to partial melting of the depleted mantle wedge above the subducted slab, and enrichment in the LILE can be attributed to subduction components (fluid, melt) released from the subducting slab. In comparison, the sheet flow and pillow lava of the lower and middle parts of the extrusive sequence show OIB characteristics and high potassium magmatic and shoshonitic trends, and their spider diagram patterns show Nb, Zr, and Ti depletions. The enrichment in the LILE in the spider diagram patterns suggest a low rate of partial melting of an enriched, garnet-bearing mantle. It seems that the marginal arc basin, in which the Sabzevar ophiolite was forming, experienced lithospheric extension in response to slab rollback. This process, which formed a backarc basin, may have aborted the embryonic arc, stopped arc magmatism, and led to the rise of mantle diapirs. The extrusive ophiolite sequence, north of Sabzevar probably formed during the transition from a marginal arc basin to a backarc basin during the Late Cretaceous.

Khalatbari Jafari, Morteza; Babaie, Hassan A.; Gani, Moslem

2013-07-01

218

U-Pb dating of large zircons in low-temperature jadeitite from the Osayama serpentinite me??lange, Southwest Japan: Insights into the timing of serpentinization  

USGS Publications Warehouse

Crystals of zircon up to 3 mm in length occur in jadeitite veins in the Osayama serpentinite me??lange, Southwest Japan. The zircon porphyroblasts show pronounced zoning, and are characterized by both low Th/U ratios (0.2-0.8) and low Th and U abundances (Th = 1-81 ppm; U = 6-149 ppm). They contain inclusions of high-pressure minerals, including jadeite and rutile; such an occurrence indicates that the zircon crystallized during subduction-zone metamorphism. Phase equilibria and the existing fluid-inclusion data constrain P-T conditions to P> 1.2 GPa at T < 350??C for formation of the jadeitite. Most U/Pb ages obtained by SHRIMP-RG are concordant, with a weighted mean 206Pb/238U age of 472 ?? 8.5 Ma (MSWD = 2.7, n = 25). Because zircon porphyroblasts contain inclusions of high-pressure minerals, the SHRIMP U-Pb age represents the timing of jadeitite formation, i.e., the timing of interaction between alkaline fluid and ultramafic rocks in a subduction zone. Although this dating does not provide a direct time constraint for serpentinization, U-Pb ages of zircon in jadeitite associated with serpentinite result in new insights into the timing of fluid-rock interaction of ultramafic rocks at a subduction zone and the minimum age for serpentinization. Copyright ?? 2005 by V.H. Winston & Son, Inc. All rights reserved.

Tsujimori, T.; Liou, J. G.; Wooden, J.; Miyamoto, T.

2005-01-01

219

Plagiogranites as late-stage immiscible liquids in ophiolite and mid-ocean ridge suites - An experimental study  

NASA Technical Reports Server (NTRS)

A study of relationships between basic and acidic rocks was made by fractionating primitive basalt at low pressure anhydrous conditions at various fugacities. Fractionally crystallized basalt became increasingly enriched in iron which became silicate liquid immiscible, producing Fe-enriched basaltic and granitic liquids. The latter is similar to plagiogranites found in mid-oceanic rift (MOR) regions, showing that silicate liquid immiscibility could be the petrogenic process which produces plagiogranites in some MOR regions and ophiolites. The major problem in considering plagiogranites as products of silicate liquid immiscibility is absence of any description of the Fe-enriched conjugate liquid in the ophiolite or MOR literature, and the identification of this magma is essential for a definite case of silicate liquid immiscibility.

Dixon, S.; Rutherford, M. J.

1979-01-01

220

Oceanic and orogenic fluid–rock interaction in 18O\\/ 16O-enriched metagabbros of an ophiolite (Tinos, Cyclades)  

Microsoft Academic Search

Two spatial scales of fluid–rock interaction in an ophiolite suite are revealed by oxygen isotope and hydrogen isotope studies of metagabbros on the island of Tinos (Cyclades, Greece). Sequentially formed mineral generations in the metagabbros include relict igneous augite, hornblende of sub-seafloor hydrothermal origin, and actinolite and albite formed by regional greenschist-facies metamorphism during orogenesis. With the exception of augite

Benita Putlitz; Yaron Katzir; Alan Matthews; John W. Valley

2001-01-01

221

Petrogenetic implications of ophiolitic chromite from Rutland Island, Andaman—a boninitic parentage in supra-subduction setting  

Microsoft Academic Search

Chromites occurring in different modes have been characterized from ophiolites of Rutland Island, a part of Burma-Andaman-Java\\u000a subduction complex in the Bay of Bengal. Chromite mainly occurs as massive chromitite pods in mantle ultramafic tectonite\\u000a and as thin massive chromitite bands together with minor disseminations in crustal ultramafic cumulate. Other than pods chromite\\u000a also appears as: (a) anhedral restitic grains,

Biswajit Ghosh; Tapan Pal; Anindya Bhattacharya; Dipankar Das

2009-01-01

222

Platinum-group minerals and other solid inclusions in chromite of ophiolitic complexes: Occurrence and petrological significance  

Microsoft Academic Search

Summary Platinum-group mineral, silicate and other solid and fluid inclusions occur in disseminated and massive chromite in a variety of lithologies from ophiolitic and other mafic-ultramafic complex-types. The inclusions are small (<250 microns) and randomly distributed throughout their host. Silicate inclusions are modally more abundant than the other inclusion types. Platinum-group mineral phases are ruthenium-rich sulphides and PGE alloys are

R. W. Talkington; D. H. Watkinson; P. J. Whittaker; P. C. Jones

1984-01-01

223

Moho transition zone in the Cretaceous Andaman ophiolite, India: A passage from the mantle to the crust  

NASA Astrophysics Data System (ADS)

We examined the composition and lithological variability from a road section in south-Andaman which represents a pathway to the crustal section of the Cretaceous Andaman ophiolite. Like other well-studied ophiolites worldwide this transition zone is marked by association of olivine-rich troctolite, wehrlite, pyroxenite and gabbroic rocks. The mineral chemical variations document the evolution of this zone by melt-mantle interaction and fractional crystallization. Petrographic evidence suggests that water was introduced during the evolution of this transition zone. The olivine-rich troctolites record impregnation of MORB melt into a residual olivine-rich lithology (replacive dunite) that formed by an earlier episode of melt-peridotite interaction at a slow spreading ridge. The clinopyroxenites indicate formation from an extreme clinopyroxene saturated melt that might be genetically linked with the formation of olivine-rich protolith of the troctolitic rocks prior to melt impregnation. The wehrlite crystallized from the melt residual after the formation of clinopyroxenite. The composition of the impregnating melt that transformed the replacive dunite to olivine-rich troctolite is identical to the gabbroic rocks. We conclude that the association of these rock types from south-Andaman provides us with a snapshot of the switch over of geodynamic setting of the Andaman ophiolite (MOR to arc) as preserved presently between north-Andaman in the north and Rutland Island in the south.

Ghosh, Biswajit; Morishita, Tomoaki; Gupta, Bidisa Sen; Tamura, Akihiro; Arai, Shoji; Bandyopadhyay, Debaditya

2014-06-01

224

Origin of Neoproterozoic ophiolitic peridotites in south Eastern Desert, Egypt, constrained from primary mantle mineral chemistry  

NASA Astrophysics Data System (ADS)

The ophiolitic peridotites in the Wadi Arais area, south Eastern Desert of Egypt, represent a part of Neoproterozoic ophiolites of the Arabian-Nubian Shield (ANS). We found relics of fresh dunites enveloped by serpentinites that show abundances of bastite after orthopyroxene, reflecting harzburgite protoliths. The bulk-rock chemistry confirmed the harzburgites as the main protoliths. The primary mantle minerals such as orthopyroxene, olivine and chromian spinel in Arais serpentinites are still preserved. The orthopyroxene has high Mg# [=Mg/(Mg + Fe2+)], ~0.923 on average. It shows intra-grain chemical homogeneity and contains, on average, 2.28 wt.% A12O3, 0.88 wt.% Cr2O3 and 0.53 wt.% CaO, similar to primary orthopyroxenes in modern forearc peridotites. The olivine in harzburgites has lower Fo (93-94.5) than that in dunites (Fo94.3-Fo95.9). The Arais olivine is similar in NiO (0.47 wt.% on average) and MnO (0.08 wt.% on average) contents to the mantle olivine in primary peridotites. This olivine is high in Fo content, similar to Mg-rich olivines in ANS ophiolitic harzburgites, because of its residual origin. The chromian spinel, found in harzburgites, shows wide ranges of Cr#s [=Cr/(Cr + Al)], 0.46-0.81 and Mg#s, 0.34-0.67. The chromian spinel in dunites shows an intra-grain chemical homogeneity with high Cr#s (0.82-0.86). The chromian spinels in Arais peridotites are low in TiO2, 0.05 wt.% and YFe [= Fe3+/(Cr + Al + Fe3+)], ~0.06 on average. They are similar in chemistry to spinels in forearc peridotites. Their compositions associated with olivine’s Fo suggest that the harzburgites are refractory residues after high-degree partial melting (mainly ~25-30 % partial melting) and dunites are more depleted, similar to highly refractory peridotites recovered from forearcs. This is in accordance with the partial melting (>20 % melt) obtained by the whole-rock Al2O3 composition. The Arais peridotites have been possibly formed in a sub-arc setting (mantle wedge), where high degrees of partial melting were available during subduction and closing of the Mozambique Ocean, and emplaced in a forearc basin. Their equilibrium temperature based on olivine-spinel thermometry ranges from 650 to 780 °C, and their oxygen fugacity is high (?log ƒO2 = 2.3 to 2.8), which is characteristic of mantle-wedge peridotites. The Arais peridotites are affected by secondary processes forming microinclusions inside the dunitic olivine, abundances of carbonates and talc flakes in serpentinites. These microinclusions have been formed by reaction between trapped fluids and host olivine in a closed system. Lizardite and chrysotile, based on Raman analyses, are the main serpentine minerals with lesser antigorite, indicating that serpentines were possibly formed under retrograde metamorphism during exhumation and near the surface at low T (<400 °C).

Khedr, Mohamed Zaki; Arai, Shoji

2013-10-01

225

Serpentinization reactions in peridotite from the Josephine ophiolite: implications for life on Mars  

NASA Astrophysics Data System (ADS)

Serpentinization of ocean crustal peridotite, both beneath the seafloor and as ophiolites on land, has been identified as a source of hydrogen that can support microbial activity. The similarity of Mars' crust to terrestrial ocean lithosphere thus suggests that ophiolites may be good analogs to some martian environments where life might have existed and may persist today. However, peridotite-water reactions are poorly understood in detail, and serpentinization is commonly idealized as isovolumetric or isochemical hydration of olivine to form serpentine, brucite, magnetite, and H2 gas. Here, a net-veined serpentinite from the Josephine ophiolite, California, was studied in order to characterize in detail the physical-chemical nature of its serpentinization. The extent of serpentinization in the studied sample is ~60%. Remnants of the original harzburgite include ~30% olivine, ~10% orthopyroxene, and accessory augite and chromite. Two generations of serpentinite veins are present, the distinction between them being in their textures (in SEM imagery); type 1 veins appear striated, while type 2 veins are massive. Both types of veins consist almost entirely of serpentine. Both types contain <5% magnetite, which occurs as one or more distinct bands near the veins' centers, and as rare isolated grains elsewhere. No brucite has been identified by optical petrography, BSE/SEI/EDS imagery, or by electron microprobe point analyses. The serpentine mineral in both types of vein was identified as lizardite based on its foliate texture (as shown in SEM images), suggesting that serpentinization occurred at T<300°C. The lizardite in type 1 veins is more magnesian (Mg# 96) than the lizardite in type 2 veins (Mg# 93). Based on the mineral proportions in the serpentinite and original harzburgite and their mineral compositions, this reaction approximates the formation of type 1 serpentine veins: 22.5 Mg1.80Fe0.20SiO4 + 7.5 Mg0.91Fe0.09SiO3 + 31.15 H2O ? 15 Mg2.88Fe0.12Si2O5(OH)4 + 1.13 Fe3O4 + 4.13 MgO(aq) + 31.15 H2. This reaction conserves Si and Fe, but is not isovolumetric nor isochemical. Considering that half of the serpentine in our sample is represented by type 1 veins, this reaction results in a 10% volume increase and removal of 10 wt% MgO from the solids by aqueous fluids. A similar reaction may be proposed for type 2 veins, with only a small change in the stoichiometric coefficients. Considering the bulk chemistry of the original and serpentinized harzburgite, it appears that both types of veins contains less Mn and Ni and more Al and Na than expected if the hydrating fluid was pure water. Because brucite is absent from the serpentinite veins, all the iron that cannot be incorporated in lizardite forms magnetite, thus enhancing the production of hydrogen. According to the above reaction, 1 kg of rock may produce 7.6 moles of H2. At a water-rock mass ratio of 10, this corresponds to ~0.8 mol H2. This is below the solubility of H2(aq) at 300°C but ~50 times higher than H2(aq) concentrations measured in fluids from sea floor hydrothermal vents. We calculated that each cubic meter of rock could potentially support significant biomass.

Sonzogni, Y.; Treiman, A. H.

2012-12-01

226

Nitrogen cycling in Ophiolite-hosted and Fault-associated Hydrothermal Systems; Spacial and Temporal Variations  

NASA Astrophysics Data System (ADS)

Gene-targeted analyses have revealed that microbial communities in hydrothermal environments can be surprisingly diverse. However, we know shockingly little about basic ecological functions such as carbon and nitrogen cycling, or community shifts over time or with seasons. Previous work has shown that nitrogen cycling in a Yellowstone National Park hot spring, "Bison Pool", and its associated runoff channel functions as a complex system. Analysis of nitrogen isotopes and sequencing of metagenomes from multiple locations at "Bison Pool" revealed that nitrogen fixation and denitrification varies across the chemosynthetic/photosynthetic ecotone [1-3], and high temperature activity of nifH genes has been shown for another nearby feature [4]. Other recent studies have explored the diversity of nifH and archaeal amoA genes in various terrestrial hydrothermal systems [5-8]. Despite these advances, we are still ignorant of changes in N-cycling over time in the same sample location, and in particular during seasonal climate changes. Further, the role of N-cycling in deeply-sourced fluids associated with ophiolites, which represent deep biosphere processes, is unknown. This study will compare evidence of N-cycling in terrestrial hydrothermal and ophiolite-hosted springs, focusing on the role of microbes as environmental conditions vary over space and time. Here, we explore the diversity of nifH [nitrogen fixation], amoA [ammonia oxidation], narG, nirKS, and nosZ [denitrification] genes in a variety of hydrothermal environments in Yellowstone and Turkey, representing fault-associated and deeply-sourced fluids. Environmental nucleic acids were extracted, and PCR-directed screens reveal the presence or absence of functional genes, indicating genetic capacity for N-cycling. We have examined the transition of genetic diversity and genetic capacity within sediments and biofilms at the chemosynthetic/photosynthetic ecotone in several hot springs spanning ranges of pH and geochemical conditions. By sampling across this ecotone, changes in the genetic capacity for nitrogen fixation as a function of changing community structure become apparent. Fault-associated and ophiolite-hosted springs in Turkey were sampled over several seasons, and the presence and diversity of N-cycle related genes appears to shift temporally. Our results provide insight into shifts in genomic function in the context of niches within hot spring environments, and the effect of availability of fixed nitrogen on the growth habit of microbial communities in these ecosystems. [1]Havig et al., 2010. JGR-Biogeo (in press). [2]Raymond et al., 2008. EOS Trans AGU. Abstract B14A-03. [3]Swingley et al., submitted to PLoS One. [4]Loiacono et al., submitted. FEMS Microbiol Ecol [5]Hall et al., 2008. AEM 74:4910-4922. [6]Steunou et al., 2006. PNAS 103:2398-2403. [7]Hamilton et al., 2011. Env. Micro. DOI 10.1007/s00248-011-9824-9. [8]Zhang et al. 2008, AEM 74:6417-6426.

Meyer-Dombard, D. R.; Gulecal, Y.; Loiacono, S. T.; Cardace, D.; Uzunlar, N.; Temel, M.

2011-12-01

227

Melt transport and compositional heterogeneities of oceanic mantle: evidence from ophiolites  

NASA Astrophysics Data System (ADS)

Numerous studies of ophiolitic and abyssal peridotites published in last two decades convincingly demonstrate that compositional heterogeneities observed in mantle rocks have been largely produced by two main processes: partial melting and melt migration (e.g. [1]). While the effects and degrees of partial melting are more or less easy to decipher by use of petrologic indicators, the compositional changes of mantle peridotites resulting from the magma migration are highly variable and depend on various factors, among which the more important are the mechanisms of melt transport and composition of migrating magmas. Thermo-mechanical modeling suggests that porous flow of melt is the dominant mode of melt migration in the mantle (e.g. [2]). On the other hand, it is widely accepted that melt extraction from the mantle beneath mid-ocean ridges occurs as a focused flow via chemically isolated channels. In their pioneering works P. Kelemen and co-authors have shown, that mantle dunites mark such highly permeable channels and were formed by complete dissolution of pyroxene in peridotite during reactive melt flow (e.g. [3]). It is assumed that focusing of diffuse porous melt flow into the channel flow may occur as a result of reactive infiltration instability or/and under the influence of stress (e.g. [3, 4]). Focused magma ascent does not rule out a diffuse porous flow of small amount of melts in shallow mantle, resulted in refertilization of mantle peridotites [5]. For the spreading in supra-subduction zone (SSZ) settings (fore-arc, immature island arc or back-arc setting), where many of ophiolites were formed [6] one should consider that melt transport processes should have specific features due to thermal structures of SSZ and influence of fluid and/or melt flux derived from the subducted slab and induced the melting in the wedge. We illustrate the current state of issues discussed above by the examples of the mantle section of Voykar ophiolite, Polar Urals, where several stages of plastic deformation and melt migration event are documented [7]. Numerous dunite channels were formed as a result of focused melt flow driven by stress. The latest stage of melt migration is marked by pyroxenite veins formed by SiO2- rich slab-derived melts or fluids, which predominantly moved by cracks under conductive cooling in lithospheric mantle. Following from mineral chemistry, these melts also migrated by diffuse porous flow around crack-channels, impregnating and refertilizing harzburgites. References: 1. Bodinier & Godard, 2003,Treatise Geochem., Elsevier, 2, 103-107; 2. McKenzie, 1984, J. Petrol., 25, 713-765; 3. Kelemen et al., 1997, Phil. Trans. R. Soc. London; 355, 283-318; 4. Batanova & Savelieva, 2009, Rus. Geol. Geoph, 50, 763-778; 5. Brunelli et al, 2006, J. Petrol., 47, 745-771 6. Dilek, 2003, Geol. Soc. Amer. Spec. paper, 373, 1-16; 7. Savelieva et al, 2008, Geotectonics,42, 430-447.

Batanova, Valentina; Savelieva, Galina

2010-05-01

228

Metamorphic evolution of metadolerites from the Frido Unit ophiolites (Southern Apennine-Italy)  

NASA Astrophysics Data System (ADS)

The Southern Apennines chain is a fold-and-thrust belt resulting from the convergence of the African and European plates and simultaneous roll-back of SE-directed Ionian subduction (Upper Oligocene-Quaternary). Ophiolites in the Southern Apennines are related to northwest subduction of the oceanic lithosphere pertaining to the Ligurian sector of the Jurassic western Tethys. The ophiolitic sequences are enclosed within remnants of the Liguride accretionary wedge now incorporated in the Southern Apennine chain and they crop out in the north-eastern slope of the Pollino Ridge (Calabria-Lucania border zone). Mafic and ultramafic rocks, with garnet-bearing felses, amphibolites, gneiss and granitoides occur as tectonic slices within a matrix mainly composed of calcschists and phyllites. Metadolerites occur as dikes cutting through serpentinized peridotites. Metadolerites have different kinds of texture reflecting various degree of crystallinity and strain: porphyritic or aphyric, intersertal/intergranular, blastophitic, cataclastic to mylonitic. In all metadolerites primary plagioclase and clinopyroxene can be observed. The metamorphic mineral assemblage consists of brown amphibole, green amphibole, chlorite, blue amphibole, pumpellyite, prehnite, quartz, epidote, white mica, lawsonite and plagioclase (Pl2 and Pl3). Accessory phases are opaque minerals, Fe-hydroxides and zircon. Metadolerites are cross- cut by veins filled with pumpellyite, chlorite, prehnite, tremolite, plagioclase, white, mica, quartz, lawsonite, epidote and zircon. The veins are straight, a few millimetres in thickness and occur isolated or in closely spaced sets. The vein morphology ranges from planar to sinuous and irregular. On the basis of metamorphic mineral phases three different types of metadolerite can be distinguished: i) rocks with a high content of prehnite crystals in cataclastic-mylonitic bands, exhibiting an intersertal or a blastophitic texture or a mylonitic fabric and in some cases a seriate texture; ii) rocks with brown horneblende showing an intersertal or a blastophitic texture or a partially blastophitic and foliate texture in one specimen; iii) rocks with brown horneblende and blue amphibole with an intersertal or a blastophitic texture. Primary clinopyroxene is replaced by brown and green amphiboles interpreted as being of oceanic origin; brown amphiboles show Mg-hastingsite, edenite, pargasite, Fe-hastingsite, Mg-horneblende and tschermakite compositions, whereas green amphiboles show Mg-hastingsite, hastingsite, edenite, Mg-horneblende, tschermakite and Fe-tschermakite compositions. Other minerals developed in the amphibolite facies conditions are: oligoclase, titanite and apatite. The blue amphiboles have a winchite and barrowisite composition and are interpreted as being originated during the early stages of the orogenic metamorphism, since they rim the oceanic brown and green amphiboles. The mineral assemblage of orogenic metamorphism is typical of the LT-blueschist facies conditions and consists of glaucophane, Mg-riebekite, lawsonite, phengite, pumpellyite and aegerin-augite. Bulk-rock chemistry of metadolerites suggests that protoliths of the mafic rock have a N-MORB-type affinity. Ca-rich metadolerites are affected by ocean-floor rodingitic alteration, whereas Na-rich metadolerites show a spilitic alteration. The study of metadolerites from Frido Unit show evidence of the entire evolution from their origin in the ocean floor to their emplacement in the accretionary wedge. Textural and mineralogical observations suggest that the metadolerites of the Frido Unit have been affected by both ocean-floor metamorphism in the amphibolite to greenschist facies- and subsequent orogenic metamorphism under relatively HP/LT conditions. The HP/LT orogenic metamorphism reflecting underplating of the ophiolitic suite at the base of the Liguride accretionary wedge during subduction produced mineral assemblages typical of the lawsonite-glaucophane facies. Such polyphase metamorphic evolution has been entirely preserved in the metadolerites pro

Cristi Sansone, Maria T.; Prosser, Giacomo; Rizzo, Giovanna; Tartarotti, Paola

2010-05-01

229

Alteration and mineralization of an oceanic forearc and the ophiolite-ocean crust analogy  

USGS Publications Warehouse

Mineralogical, chemical, and isotopic (O, C, S, and Sr) analyses were performed on minerals and bulk rocks from a forearc basement section to understand alteration processes and compare with mid-ocean ridges (MOR) and ophiolites. Ocean Drilling Program Hole 786B in the Izu-Bonin forearc penetrates 103 m of sediment and 725 m into volcanic flows, breccias, and basal dikes. The rocks comprise boninites and andesites to rhyolites. Most of the section was affected by low-temperature (<100??C) seawater alteration, with temperatures increasing downward. The rocks are partly (5-25%) altered to smectite, Fe-oxyhydroxide, calcite, and phillipsite, and exhibit gains of K, Rb, and P, loss of Ca, variable changes in Si, Na, Mg, Fe, Sr, and Y, and elevated ??18O and 87Sr/86Sr. Higher temperatures (???150??C) in the basal dikes below 750 m led to more intense alteration and formation of chlorite-smectite, corrensite, albite, K-feldspar, and quartz (??chlorite). A 5 m thick hydrothermally altered and pyritized zone at 815 m in the basal dikes reacted with mixtures of seawater and hydrothermal fluids to Mg-chlorite, albite, and pyrite, and gained Mg and S and lost Si and Ca. Focused flow of hydrothermal fluids produced sericitization halos (Na-K sericite, quartz, pyrophyllite, K-feldspar, and pyrite) along quartz veins at temperatures of 200??-250??C. High 87Sr/86Sr ratios of chloritized (???0.7055) and sericitized (???0.7065) rocks indicate involvement of seawater via mixing with hydrothermal fluids. Low ??34S of sulfide (???2 to -5.5???) and sulfate (12.5???) are consistent with input of magmatic SO2 into hydrothermal fluids and disproportionation to sulfide and sulfate. Alteration processes were generally similar to those at MORs, but the arc section is more intensively altered, in part because of the presence of abundant glassy rocks and mafic phases. The increase in alteration grade below 750 m and the mineralization in the basal dikes are analogous to changes that occur near the base of the volcanic section in MOR and the Troodos ophiolite.

Alt, J. C.; Teagle, D. A. H.; Brewer, T.; Shanks, III, W. C.; Halliday, A.

1998-01-01

230

Compositional heterogeneities in upper mantle peridotites from the Zambales Range Ophiolite, Luzon, Philippines  

NASA Astrophysics Data System (ADS)

The Zambales Range of western Luzon, Philippines, is formed largely of mafic and ultramafic rocks that constitute a classic ophiolite assemblage. We recognize two distinct blocks, Acoje and Coto, on the basis of mineralogy, chemistry and the thickness of crustal sections. We interpret these ophiolite units as remnants of crust and mantle from supra-subduction zone settings—the Coto block from a backarc basin and the Acoje block from a nascent island arc (Hawkins and Evans, 1983; Evans, 1983). We present data to contrast the peridotite sections of the two different blocks and discuss their origin. Both peridotite units are primarily harzburgite with minor dunite; in both units there are large concentrations of chromite that form ore deposits. There are subtle changes in lithology and mineralogy in each unit that are correlated with depth below the crustal section. Dunite, orthopyroxene-poor harzburgite and chromite increase in abundance upward toward the "geological Moho" and the base of cumulate ultramafic rocks of the lower crustal section. The upward changes in lithology are matched by changes in modal abundance and composition of the minerals that document increasing (upward) depletion of the peridotite units in "basaltic" components. Mineral show an increase in Mg and Cr and a decrease in Fe, Ca and Al. These changes are best developed in the uppermost 1000 m of each peridotite unit. We attribute these compositional changes mainly to different extents of partial melting. To a lesser extent the changes reflect local exchange between mafic melts, in transit to the surface, and depleted mantle, and exchanges during metamorphic recrystallization. The crustal rocks of the Coto block resemble rocks from backarc basins; the uppermost levels of the peridotite comprise rocks similar to many abyssal peridotites. The Acoje block crustal rocks are best explained as remnants of a nascent island arc; the upper mantle peridotite from Acoje is more depleted than that from Coto having higher {Mg}/{(Mg + Fe)} in silicates, higher {Cr}/{(Cr + Al)} in chromites, a greater proportion of dunite and less clinopyroxene in the harzburgite. The more extensive depletion of the Acoje block (island arc) peridotite probably is due to the multiple melting events that were imposed on these rocks during their evolution in, first, generation of ocean crust and then, later, the generation of island arc crust.

Evans, Cynthia; Hawkins, James W.

1989-10-01

231

Serpentinization Of The Leka Ophiolite Complex, Norway: Geochemical And Physical Implications  

NASA Astrophysics Data System (ADS)

The Leka Ophiolite Complex (LOC) contains all the principle components of an ophiolite and is a part of the Upper Allochthon of the Scandinavian Caledonides. The ultramafic lithologies (harzburgites, dunites, wehrlites and orthopyroxenite dykes) of the LOC undergo hydration dependent on the constituent primary mineral assemblages thus preserving the hydration history over a wide range of temperatures, where the orthopyroxenite dykes record high temperature reactions (~650°C) while the dunites and harzburgites undergo serpentinization reactions at lower temperatures (<400°C). The ubiquity of bastites and mesh textures in the lithologies suggests that hydration occurred under static conditions. One of the important implications of hydration of ultramafic rocks is the effect on element mobility during the process. Although concentrations of some major elements (Mg and Si) in the lithologies of the LOC do not change with hydration, there is ample evidence for transfer of Fe, Mn and Ca within the rocks at the grain- scale. Fe and Mn are mobilized during the alteration of orthopyroxene and results in the formation of secondary olivine with high Fe and Mn contents which later forms serpentine and ferroan-brucite with high Fe and Mn contents. However, Mn is lost during the subsequent oxidation of ferroan-brucite to magnetite. Ca is also mobilized during the serpentinization of primary clinopyroxene, some of which forms secondary diopside after olivine. The release and uptake of elements during hydration processes in ultramafic rocks, therefore, plays an important role in modifying fluid properties and may have a significant impact on vent-fluid chemistry. Another point of interest is the density change occurring during serpentinization and its impact on deformation. Observations suggest that hydration of the orthopyroxenite dykes took place at temperatures higher than serpentinization of the surrounding dunites. The serpentinization of dunites results in a volume change of 25% which 'squeezes' the embedded orthopyroxenite dykes resulting in fracturing. The geometrical and statistical characteristics of the 2-D fracture networks in the dykes are typical of patterns generated during hierarchical fracturing where the layer is progressively broken up into smaller domains. The mechanism of fluid-migration, stress-inducing reactions and fracturing is an important process which creates fresh, reactive surfaces and new pathways for fluid infiltration.

Iyer, K.; Austrheim, H.; John, T.; Jamtveit, B.

2008-12-01

232

Mapping Microbial Populations Relative to Sites of Ongoing Serpentinization: Results from the Tablelands Ophiolite Complex, Canada  

NASA Astrophysics Data System (ADS)

The aqueous alteration of ultramafic rocks (serpentinization) has been suggested to be a favorable process for the habitability of astrobodies in our solar system including subsurface environments of Mars and Europa. Serpentinization produces copious quantities of hydrogen and small organic molecules, and leads to highly reducing, highly alkaline conditions (up to pH 12) and a lack of dissolved inorganic carbon, which both stimulates and challenges microbial activities. Several environments on Earth provide insight into the relationships between serpentinization and microbial life including slow-spreading mid-ocean ridges, subduction zones, and ophiolite materials emplaced along continental margins. The Tablelands, an ophiolite in western Newfoundland, Canada provides an opportunity to carefully document and map the relationships between geochemical energy, microbial growth, and physiology. Alkaline fluids at the Tablelands originate from 500-million year old oceanic crust and accumulate in shallow pools or seep from beneath serpentinized talus. Fluids, rocks, and gases were collected from the Tablelands during a series of field excursions in 2009 and 2010, and geochemical, microscopic, molecular, and cultivation-based approaches were used to study the serpentinite microbial ecosystem. These samples provide an opportunity to generate a comprehensive map of microbial communities and their activities in space and time. Data indicate that a low but detectable stock of microorganisms inhabit high pH pools associated with end-member serpentinite fluids. Enrichment cultures yielded brightly pigmented colonies related to Alphaproteobacteria, presumably carrying out anoxygenic photosynthesis, and Firmicutes, presumably catalyzing the fermentation of organic matter. Culture-independent analyses of SSU rRNA using T-RFLP indicated low diversity communities of Firmicutes and Archaea in standing alkaline pools, communities of Beta- and Gammaproteobacteria at high pH seeps, and assemblages consisting of diverse taxa at neutral pH background sites. Terrestrial serpentinite-hosted microbial ecosystems with their accessibility, their low phylogenetic diversity, and limited range of energetic resources provide an excellent opportunity to explore the interplay between geochemical energy and life and to elucidate the native serpentinite subsurface biosphere. From the perspective of Mars exploration, studies of serpentinite ecosystems provide the opportunity to pinpoint the organisms and physiological adaptations specifically associated with serpentinization and to directly measure their geochemical impacts. Both of these results will inform modeling and life detection efforts of the Martian subsurface environment.

Schrenk, M. O.; Brazelton, W. J.; Woodruff, Q.; Szponar, N.; Morrill, P. L.

2010-12-01

233

Tectonic Implications of Ultra-High Pressure Minerals in the Loubusa Ophiolite, Tibet  

NASA Astrophysics Data System (ADS)

An unusual collection of ultrahigh pressure (UHP) and associated minerals has been recovered from podiform chromitites of the Luobusa ophiolite, southern Tibet. The minerals were hand picked from mineral separates but many of the minerals are enclosed in, or attached to, chromite grains leaving now doubt as to their provenance. The mineral collection includes diamond, graphite, moissonite, coesite, CrC, SiFe, silicates, w\\x81stite, PGE and base metal alloys, and a wide variety of native elements (Si, Fe, Ti, Ni, Cr, W, Au, Ag, Zn, Cu, Pb, Sn). Diamonds from Luobusa are clear, colorless octahedra with high Ni aggregation states confirming their natural origin and indicating a long residence time in the mantle. A few have dark inclusions of a Mg-Fe silicate, probably enstatite. Graphite occurs as grey, tabular prisms and irregular grains, many of which preserve a hexagonal morphology. Abundant moissonite forms small, euhedral or broken crystals ranging from dark blue to green to colorless. Some grains of moissonite and Fe-silicides contain inclusions of native Si. Coesite occurs with kyanite as rims on native Ti. Primary Os-Ir and Pt-Fe alloys, interpreted to be of UHP origin, are intergrown with chromite grains, whereas secondary PGE minerals and alloys occur along cracks where they are associated with a variety of sulfide minerals. Numerous octahedral Mg-Fe silicate grains have been pseudomorphed by serpentine. There is no evidence that the Luobusa ophiolite itself was formed at great depth, thus the UHP minerals are interpreted as xenocrysts incorporated into the chromitites during crystallization. Their preservation in this high-temperature, relatively oxidizing environment is difficult to explain. Our preferred model calls for rapid rise of deep mantle rocks to relatively shallow levels where they were picked up by boninitic melts and incorporated into the chromitites upon cooling and crystallization. Preservation of the UHP minerals in this environment may have been facilitated by inclusion in xenolithic blocks and by relatively rapid crystallization and cooling of the host chromitites.

Robinson, P.; Bai, W.; Yang, J.; Malpas, J.; Fang, Q.

2003-12-01

234

Informing geobiology through GIS site suitability analysis: locating springs in mantle units of ophiolites  

NASA Astrophysics Data System (ADS)

Springs sourced in the mantle units of ophiolites serve as windows to the deep biosphere, and thus hold promise in elucidating survival strategies of extremophiles, and may also inform discourse on the origin of life on Earth. Understanding how organisms can survive in extreme environments provides clues to how microbial life responds to gradients in pH, temperature, and oxidation-reduction potential. Spring locations associated with serpentinites have traditionally been located using a variety of field techniques. The aqueous alteration of ultramafic rocks to serpentinites is accompanied by the production of very unusual formation fluids, accessed by drilling into subsurface flow regimes or by sampling at related surface springs. The chemical properties of these springs are unique to water associated with actively serpentinizing rocks; they reflect a reducing subsurface environment reacting at low temperatures producing high pH, Ca-rich formation fluids with high dissolved hydrogen and methane. This study applies GIS site suitability analysis to locate high pH springs upwelling from Coast Range Ophiolite serpentinites in Northern California. We used available geospatial data (e.g., geologic maps, topography, fault locations, known spring locations, etc.) and ArcGIS software to predict new spring localities. Important variables in the suitability model were: (a) bedrock geology (i.e., unit boundaries and contacts for peridotite, serpentinite, possibly pyroxenite, or chromite), (b) fault locations, (c) regional data for groundwater characteristics such as pH, Ca2+, and Mg2+, and (d) slope-aspect ratio. The GIS model derived from these geological and environmental data sets predicts the latitude/longitude points for novel and known high pH springs sourced in serpentinite outcrops in California. Field work confirms the success of the model, and map output can be merged with published environmental microbiology data (e.g., occurrence of hydrogen-oxidizers) to showcase patterns in microbial community structure. Discrepancies between predicted and actual spring locations are then used to tune GIS suitability analysis, re-running the model with corrected geo-referenced data. This presentation highlights a powerful GIS-based technique for accelerating field exploration in this area of ongoing research.

Bowman, A.; Cardace, D.; August, P.

2012-12-01

235

A Geophysical Study Of The Ophiolite Complex And The Sedimentary Basins In The Northwest Part Of The Chalkidiki Peninsula (N. Greece)  

NASA Astrophysics Data System (ADS)

The present work focuses on the study of the main ophiolite complex ofNorthern Greece, which is one of the dominant geological features in thebroader Aegean area, by the use of geophysical (gravity and magnetic)data. This ophiolite complex, which trends in a NW-SE direction, startsat the eastern part of the borders of Greece with F.Y.R.O.M. and continuesup to the southern part of the Chalkidiki Peninsula. The ophiolites mainlyconsist of dense, high-susceptibility peridotitic and gabbroic rocks. As aresult, the southwestern part of the ophiolitic complex, which crosses thenorthwestern part of the Chalkidiki-Peninsula, gives rise to both highamplitude aeromagnetic and Bouguer anomaly values. On the other hand,the Axios-Thermaikos basin, which is situated at the western border of theophiolitic complex, exhibits a deep sedimentary cover that results in lowBouguer anomaly values. The corresponding Bouguer anomaly decreasesto the southwest, indicating an increase of the sedimentary layer thicknessin that direction.2.5-D inversion was applied to both the aeromagnetic and the Bouguergravity data along several profiles. All the profiles were oriented normalto the main trend of the ophiolitic complex. Information from two deepboreholes, as well as the surface occurrence of the ophiolites was used asconstrains to the inversion scheme. The produced model shows an averagesedimentary thickness of 2.5 km along the coastline. From the joint inversionof the Bouguer and aeromagnetic anomaly data the existence of two ophioliticstripes is revealed. The first ``external'' one is located in the southwest part, while the other ``internal'' one to the northeast part of the belt. In the internal one, the depth extent of the ophiolites was estimated to range between 1 to 4 km. Moreover, the ophiolites were found to dip towards the northeast, but their dip varies from 20-45° in the northern part to 10-15° in the southern part of the stripe. For the ``external'' stripe the extent of the ophiolitic bodies varies from northwest to southeast, reaching its highest depth of 5 km to the south. This stripe is also dipping towards the northeast with a dip of 10-15°.

Savvaidis, A. S.; Tsokas, G. N.; Papazachos, C. B.; et al.

236

Ophiolitic basement to the Great Valley forearc basin, California, from seismic and gravity data: Implications for crustal growth at the North American continental margin  

USGS Publications Warehouse

The nature of the Great Valley basement, whether oceanic or continental, has long been a source of controversy. A velocity model (derived from a 200-km-long east-west reflection-refraction profile collected south of the Mendocino triple junction, northern California, in 1993), further constrained by density and magnetic models, reveals an ophiolite underlying the Great Valley (Great Valley ophiolite), which in turn is underlain by a westward extension of lower-density continental crust (Sierran affinity material). We used an integrated modeling philosophy, first modeling the seismic-refraction data to obtain a final velocity model, and then modeling the long-wavelength features of the gravity data to obtain a final density model that is constrained in the upper crust by our velocity model. The crustal section of Great Valley ophiolite is 7-8 km thick, and the Great Valley ophiolite relict oceanic Moho is at 11-16 km depth. The Great Valley ophiolite does not extend west beneath the Coast Ranges, but only as far as the western margin of the Great Valley, where the 5-7-km-thick Great Valley ophiolite mantle section dips west into the present-day mantle. There are 16-18 km of lower-density Sierran affinity material beneath the Great Valley ophiolite mantle section, such that a second, deeper, "present-day" continental Moho is at about 34 km depth. At mid-crustal depths, the boundary between the eastern extent of the Great Valley ophiolite and the western extent of Sierran affinity material is a near-vertical velocity and density discontinuity about 80 km east of the western margin of the Great Valley. Our model has important implications for crustal growth at the North American continental margin. We suggest that a thick ophiolite sequence was obducted onto continental material, probably during the Jurassic Nevadan orogeny, so that the Great Valley basement is oceanic crust above oceanic mantle vertically stacked above continental crust and continental mantle.

Godfrey, N. J.; Beaudoin, B. C.; Klemperer, S. L.; Levander, A.; Luetgert, J.; Meltzer, A.; Mooney, W.; Trehu, A.

1997-01-01

237

A possible difference in cooling rates recorded in REE in coexisting pyroxenes in peridotites from supra-subduction ophiolites and mid-ocean ridges  

NASA Astrophysics Data System (ADS)

Recently a REE-in-two-pyroxene thermometer was developed for mafic and ultramafic rocks [1]. This new thermometer is based on temperature sensitive REE partition coefficients between coexisting pyroxenes and calibrated against laboratory partitioning data. Because REE diffusion rates in pyroxene are relatively slow, the thermometer reads a higher temperature than major element based pyroxene thermometers. The difference between major and trace element derived temperatures depends primarily on cooling rate. Here we report new trace element data for peridotites from Trinity and Josephine ophiolites and a modern supra-subduction zone (SSZ) ophiolite analogue (the Mariana trench) determined by laser ablation ICP-MS. We inverted temperatures from the new data and globally distributed ophiolitic peridotite from eight literature studies (Figure 1). Data quality was carefully monitored leaving temperatures from 65 samples. Individual ophiolites usually have temperatures clustered within a range of a few hundred degrees, but the temperature range for the global dataset is greater than 700°C (688-1401°C). Temperatures calculated for the same samples using the two pyroxene thermometer of Brey and Köhler [2] are considerably lower (564-1049°C). REE temperatures are plotted against the major element temperatures [2] in Figure 1. Abyssal peridotites reported in [1] are shown by the peach field. Much of the ophiolite data plots farther from the blue 1:1 line than the abyssal peridotites, suggesting SSZ lithospheric mantle may cool more rapidly at those ophiolites. Fast cooling can be attributed to one or more dynamic differences between mid-ocean ridge (MOR) environments and supra-subduction environments, such as enhanced hydrothermal circulation, thinner oceanic crust, or rapid cooling due to basin closure and obduction. We note that several ophiolites appear to cool more slowly than the abyssal peridotites, however in those samples geochemical evidence suggests secondary overprinting by melt-rock reaction. By defining the thermometric extent of the global dataset we demonstrate surprising diversity among ophiolites and the potential utility of REE geothermometry to illuminate the dynamics of ophiolite formation and emplacement. [1] Liang et al. (2013), GCA 102, 246-260. [2] Brey and Köhler (1990), J. Pet. 31, 1353-1378.

Dygert, N. J.; Liang, Y.; Kelley, K. A.

2013-12-01

238

The Relationships of Upper Plate Ridge-Trench-Trench and Ridge-Trench-Transform Triple Junction Evolution to Arc Lengthening, Subduction Zone initiation and Ophiolitic Forearc Obduction  

NASA Astrophysics Data System (ADS)

The principal enigma of large obducted ophiolite slabs is that they clearly must have been generated by some form of organized sea-floor spreading/plate-accretion, such as may be envisioned for the oceanic ridges, yet the volcanics commonly have arc affinity (Miyashiro) with boninites (high-temperature/low-pressure, high Mg and Si andesites), which are suggestive of a forearc origin. PT conditions under which boninites and metamorphic soles form and observations of modern forearc systems lead us to the conclusion that ophiolite formation is associated with overriding plate spreading centers that intersect the trench to form ridge-trench-trench of ridge-trench-tranform triple junctions. The spreading centers extend and lengthen the forearc parallel to the trench and by definition are in supra-subduction zone (SSZ) settings. Many ophiolites likewise have complexly-deformed associated mafic-ultramafic assemblages that suggest fracture zone/transform along their frontal edges, which in turn has led to models involving the nucleation of subduction zones on fracture zones or transpressional transforms. Hitherto, arc-related sea-floor-spreading has been considered to be either pre-arc (fore-arc boninites) or post-arc (classic Karig-style back arc basins that trench-parallel split arcs). Syn-arc boninites and forearc oceanic spreading centers that involve a stable ridge/trench/trench triple or a ridge-trench-transform triple junction, the ridge being between the two upper plates, are consistent with large slab ophiolite formation in an obduction-ready settting. The direction of subduction must be oblique with a different sense in the two subduction zones and the oblique subduction cannot be partitioned into trench orthogonal and parallel strike-slip components. As the ridge spreads, new oceanic lithosphere is created within the forearc, the arc and fore-arc lengthen significantly, and a syn-arc ophiolite forearc complex is generated by this mechanism. The ophiolite ages along arc-strike; a distinctive diachronous MORB-like to boninitic to arc volcanic stratigraphy develops vertically in the forearc and eruption centers progressively migrate from the forearc back to the main arc massif with time. Dikes in the ophiolite are commonly highly oblique to the trench (as are back-arc magnetic anomalies in modern environments). Boninites and high-mg andesites are generated in the fore-arc under the aqueous, low pressure/high temperature, regime at the ridge above the instantaneously developed subducting and dehydrating slab. We review both modern subduction environments and ancient obducted ophiolite analogues that illustrate this tectonic model for subduction initiation and the creation and rapid divergent-convergent plate tectonic transitions to ophiolitic forearcs.

Casey, J.; Dewey, J. F.

2013-12-01

239

Palaeoproterozoic (1.83 Ga) zircons in a Bajocian (169 Ma) granite within a Middle Jurassic ophiolite (Rubiku, central Albania): a challenge for geodynamic models  

NASA Astrophysics Data System (ADS)

Two distinct zircon populations, 1,827 ± 17 and 169 ± 2 Ma in age, have been found in the Rubiku granite dyke in the Middle Jurassic Mirdita ophiolite in central Albania. The old inherited zircons represent a homogeneous population formed during a discrete Palaeoproterozoic, likely magmatic, zircon crystallization event. These older zircons were likely incorporated, in large part, into the granite magma that crystallized broadly at the time of the ophiolite emplacement (around 169 Ma). The limited data available do not allow for the construction of an unequivocal petrogenetic model, though several palaeotectonic scenarios are discussed as possible settings for the granite formation. The models refer to recent findings of old inherited zircons in rocks at recent mid-ocean ridge settings, but also consider likely contributions of crustal materials to primary basic ophiolitic magmas within supra-subduction settings and subsequent accretion/collision circumstances. The presence of old zircons in much younger rocks within ophiolite successions runs counter to geodynamic models of interaction between the oceanic lithosphere and continental crust, but constraining their genesis would require further systematic studies on these old inherited zircons, both in mafic (if present) and in felsic rocks of the ophiolites.

Kryza, Ryszard; Beqiraj, Arjan

2014-04-01

240

Concordant paleolatitudes for Neoproterozoic ophiolitic rocks of the Trinity Complex, Klamath Mountains, California  

USGS Publications Warehouse

New paleomagnetic results from the eastern Klamath Mountains of northern California show that Neoproterozoic rocks of the Trinity ophiolitic complex and overlying Middle Devonian volcanic rocks are latitudinally concordant with cratonal North America. Combining paleomagnetic data with regional geologic and faunal evidence suggests that the Trinity Complex and related terranes of the eastern Klamath plate were linked in some fashion to the North American craton throughout that time, but that distance between them may have varied considerably. A possible model that is consistent with our paleomagnetic results and the geologic evidence is that the Trinity Complex formed and migrated parallel to paleolatitude in the basin between Laurasia and Australia-East Antarctica as the Rodinian supercontinent began to break up. It then continued to move parallel to paleolatitude at least through Middle Devonian time. Although the eastern Klamath plate served as a nucleus against which more western components of the Klamath Mountains province amalgamated, the Klamath superterrane was not accreted to North America until Early Cretaceous time.

Mankinen, E. A.; Lindsley-Griffin, N.; Griffin, J. R.

2002-01-01

241

Refractory- and metallurgical-type chromite ores, Zambales ophiolite, Luzon, Philippines  

NASA Astrophysics Data System (ADS)

The Zambales ophiolite is the major source of chromite ore in the Philippines. The chromitites are concordant cumulates and are associated with distinct chromitite-bearing sequences within the mantle peridotites. Refractory and metallurgical chromite deposits are spatially separated and related to different lithologic associations, which crystallized from different parental magmas. — Refractory chromite ores (30 44 wt% Cr2O3; 20 30 wt% Al2O3) are linked with the peridotite-troctolite-olivine gabbro lineage. Two main types were found: (1) Al-rich refractory ores associated with harzburgites and feldspathic periodotites and (2) more Cr-rich varieties associated with lherzolites. — Metallurgical chromite ores (45 53 wt% Cr2O3; 12 18 wt% Al2O3) are linked with the peridotite-pyroxenite-norite lineage. Two main types were also found: (1) Cr-rich metallurgical ores associated with orthopyroxenites and (2) more Al-rich varieties related to clinopyroxenites. — The chemical composition of chromite within the deposits varies depending on the chromite/silicate ratios of the ore types and grades continuously into accessory chrome spinels in the wall-rock peridotites. — The geochemistry of accessory chrome spinels in various peridotites and mafic cumulates depends on the mineralogical composition and the stratigraphic position of their host rocks.

Hock, M.; Friedrich, G.; Plüger, W. L.; Wichowski, A.

1986-07-01

242

Pumpellyite from zeolite facies metabasites of the Horokanai ophiolite in the Kamuikotan zone, Hokkaido, Japan  

NASA Astrophysics Data System (ADS)

Pumpellyite occures in zeolite facies metabasites of the Horokanai ophiolite in the Kamuikotan zone, Hokkaido, Japan, filling veins or amygdules, replacing igneous plagioclase or clinopyroxene or olivine, and occupying the matrix. Its composition and pleochroism vary greatly even within a single sample, but appear to be related to its mode of occurrence. Thus, the most Al-rich pumpellyite with pale green to green pleochroism develops in pseudomorphs after plagioclase, whereas the most Fe*-rich variety with deep green to brown pleochroism occurs in the matrix. In low-grade metamorphic rocks which commonly contain relict minerals, chemical equilibrium is attained only locally. This results in the correlation of the composition of pumpellyite with its mode of occurrence, such as the precursor phases which are replaced by pumpellyite. On the other hand, among pumpellyites occurring in similar mode and coexisting with Ca-zeolite (laumontite or wairakite), epidote, chlorite and quartz, the Al content tends to be enriched in the wairakite-bearing metabasites over the laumontite-bearing metabasites. It follows that the composition of pumpellyite is also dependent upon the temperature of metamorphism.

Ishizuka, Hideo

1991-03-01

243

High-pressure highly reduced nitrides and oxides from chromitite of a Tibetan ophiolite.  

PubMed

The deepest rocks known from within Earth are fragments of normal mantle ( approximately 400 km) and metamorphosed sediments ( approximately 350 km), both found exhumed in continental collision terranes. Here, we report fragments of a highly reduced deep mantle environment from at least 300 km, perhaps very much more, extracted from chromite of a Tibetan ophiolite. The sample consists, in part, of diamond, coesite-after-stishovite, the high-pressure form of TiO(2), native iron, high-pressure nitrides with a deep mantle isotopic signature, and associated SiC. This appears to be a natural example of the recently discovered disproportionation of Fe(2+) at very high pressure and consequent low oxygen fugacity (fO(2)) in deep Earth. Encapsulation within chromitite enclosed within upwelling solid mantle rock appears to be the only vehicle capable of transporting these phases and preserving their low-fO(2) environment at the very high temperatures of oceanic spreading centers. PMID:19880742

Dobrzhinetskaya, Larissa F; Wirth, Richard; Yang, Jingsui; Hutcheon, Ian D; Weber, Peter K; Green, Harry W

2009-11-17

244

Subducted seamounts in an eclogite-facies ophiolite sequence: slab dismembering due to local stress amplification?  

NASA Astrophysics Data System (ADS)

The metamorphic Raspas Complex of southwest Ecuador comprises blueschists, eclogites, and high-pressure ultramafic and sedimentary rocks. The Lu-Hf garnet-whole rock ages of a blueschist, a metapelite, and an eclogite cluster at ca. 130 Ma. Peak metamorphic conditions of the high-pressure rocks occurred at 1.8 GPa and 600 °C, which corresponds to a maximum burial depth of about 60 km. According to their geochemical signatures, the eclogites formed from typical mid-ocean ridge basalts (MORB), whereas the blueschists had seamount-like protoliths and the eclogite-facies peridotites apparently stem from depleted, MORB-source mantle. These rocks were contemporaneously subjected to similar peak PT conditions, suggesting that they were subducted together as a coherent section within the downgoing slab. We hypothesize that this section became dismembered from the slab during burial at great depth as the seamounts were being scraped off. The spatially close association of MORB-type eclogite, seamount-type blueschist, serpentinized peridotite, and metasediments is suggestive of an exhumed high-pressure ophiolite sequence.

John, Timm; Scherer, Erik; Schenk, Volker; Herms, Petra; Halama, Ralf; Garbe-Schönberg, Dieter

2010-05-01

245

Subducted seamounts in an eclogite-facies ophiolite sequence: the Andean Raspas Complex, SW Ecuador  

NASA Astrophysics Data System (ADS)

The metamorphic Raspas Complex of southwest Ecuador consists of high-pressure mafic, ultramafic, and sedimentary rocks. The Lu-Hf ages of a blueschist, a metapelite, and an eclogite overlap at around 130 Ma and date high-pressure garnet growth. Peak metamorphic conditions in the eclogites reached 1.8 GPa at 600°C, corresponding to a maximum burial depth of ~60 km. The geochemical signatures of the eclogites suggest that their protoliths were typical mid-ocean ridge basalts (MORB), whereas the blueschists exhibit seamount-like characteristics, and the eclogite-facies peridotites seem to represent depleted, MORB-source mantle. That these rocks were subjected to similar peak PT conditions contemporaneously suggests that they were subducted together as an essentially complete section within the slab. We suggest that this section became dismembered from the slab during burial at great depth—perhaps as a consequence of scraping off the seamounts. The spatially close association of MORB-type eclogite, seamount-type blueschist, serpentinized peridotite, and metasediments points to an exhumed high-pressure ophiolite sequence.

John, Timm; Scherer, Erik E.; Schenk, Volker; Herms, Petra; Halama, Ralf; Garbe-Schönberg, Dieter

2010-02-01

246

Evolution of rodingites along stratigraphic depth in the Iti and Kallidromon ophiolites (Central Greece)  

NASA Astrophysics Data System (ADS)

Rodingitised rocks were collected from the neighbouring Iti and Kallidromon ophiolites. They comprise metasomatic assemblages after serpentinised lherzolite and harzburgite, gabbro and dolerite dykes. The main mineral phases in the metasomatised mantle and gabbroic rocks include grossularitic garnet, chlorite and diopside whereas epidote group minerals were mainly developed at the expense of the dolerite dykes. The fluid phase involved in the metasomatic reactions was highly alkaline in the metasomatising peridotites and less alkaline in the altered gabbro and dolerite. Under such conditions, Ti is assumed to have remained immobile in the whole process while Zr remained constant in the stratigraphically upper rodingitised gabbro and dolerite. Transportation of rare earth elements, Zr, Cr and Ni from the altered mantle segment to the upper gabbro level was assisted by the presence of carbonate and hydroxyl ligands. After consequent breakdown of the carbonate complexes, these elements were deposited in the rodingitised gabbro and dolerite. Rare earths were mainly integrated in the neoblastic diopside whereas Cr and Ni likely formed insoluble hydroxides in that less reducing environment. The metasomatic event happened in the mantle wedge close to the subduction of the Pelagonian carbonates that strongly enriched the fluid phase in CO2. The entrance of an externally derived hydrothermal fluid was responsible for the reduction of pH at the higher levels and the calculated influx of Si in the system.

Tsikouras, Basilios; Karipi, Sofia; Hatzipanagiotou, Konstantin

2013-08-01

247

The chemical characterization and tectonic significance of ophiolite terrains in southeastern Papua New Guinea  

NASA Astrophysics Data System (ADS)

The evolution of the southeastern Gondwana margin in the Papua New Guinea segment is manifested by ophiolites signifying plate collision, volcanic arcs marking subduction, and metamorphic core complexes that have developed during extensional events and exhumation associated with sea floor spreading in the Woodlark Basin to the east. In detail the Papuan Ultramafic Belt marks a well-documented collision between continental crust and a subduction system. However, to the southeast, an extensive sequence of basaltic rocks known as the Milne Terrain is more problematic. Geochemical data indicate that these upper Cretaceous and Eocene rocks have MORB-type affinities, and their most likely tectonic association is with the opening of the Coral Sea Basin. Milne Terrain rocks represent the lower plate in the obduction system along which the Papuan Ultramafic Belt was emplaced, and thus they are the structural equivalent of the continental crust which was separated from the Australian continental block by the opening of the Coral Sea. Spectacular uplift (>4 km) of the oceanic basaltic crust of the Milne Terrain may be due to the underlying presence of underplated material associated with a Late Miocene-Pliocene episode of subduction immediately prior to the encroachment of the Woodlark spreading center into the Papuan area.

Smith, Ian E. M.

2013-03-01

248

The role of ophiolite in metallogeny of the Sikhote-Alin region  

NASA Astrophysics Data System (ADS)

Metalliferous sediments of the Triassic siliceous formation of the Sikhote-Alin (manganese-silicate rocks and cherts with dispersed rhodochrosite, silicate-magnetite ores, and jasper) and skarns of the Dalnegorsk and Olginsk ore districts were initially the wash away products (Late Anisian-end of the Triassic) of the lateritic weathering crust on ophiolite in the islands. Manganese, iron, and other metals were deposited in the sediments of both lagoons (present-day, skarns) and island water areas (manganese-silicate and siliceousrhodochrosite rocks, silicate-magnetite ores, and jasper). Skarns contain boric and polymetallic ores thus indicating the occurrence of both shallow (periodically drying up) and quite deep (with hydrogen sulfide contamination zones) lagoons. Lead was deposited in protoliths of the skarn deposits in lagoons from the beginning of the Carboniferous to the beginning of the Late Anisian (initial island submergence). Tin, tin-leadzinc (with Ag), and silver-lead-zinc (with Sn and Au) vein deposits (Late Cretaceous-Paleogene) of the Taukha and Zhuravlevka Terrains contain lead deposited in the sediments flanking the islands of water areas with the hydrogen sulfide contamination zones, in the Carboniferous-Permian and Triassic metalliferous sediments.

Kazachenko, V. T.; Perevoznikova, E. V.; Lavrik, S. N.; Skosareva, N. V.

2012-06-01

249

Paleomagnetic Properties of Volcanic and Ophiolitic Units From Northeastern Cuba: A Pilot Study  

NASA Astrophysics Data System (ADS)

We sampled 19 sites (135 oriented cores) in Upper Jurassic to Eocene ophiolitic and volcanic units from northeastern Cuba. NRM vectorial composition and magnetic stability of samples were analyzed by stepwise thermal and alternating field (AF) demagnetization. AF demagnetization worked generally better than thermal demagnetization in isolating magnetization components, except in the gabbros that are characterized by one to two magnetization components with mean unblocking temperatures around 350°C. Strong viscous magnetizations appear between 10-30 mT and 500-550°C and are destroyed between 35-80 mT or 575°C, suggesting magnetite as major carrier of remanence. 'Pot bellied' and 'wasp waisted' behavior curves of hysteresis experiments suggests ferromagnetic phases. Saturation magnetization and coercivity have, in general, medium to high values. Characteristic remanent magnetization (ChRM) was defined in 102 samples, and site mean directions and associated Fisher statistics were calculated. Coercivity and unblocking spectra and magnetic property experiments indicate magnetite, titanomagnetite and pyrrotite as main magnetic carriers, with dominantly pseudo-single-domain magnetic states. ChRM mean directions of 14 sites were well defined and stable, showing NE to NW directions, valuable for tectonic and geologic analyses. Mean paleodirections and pole positions are calculated for the different age intervals and compared with reference pole positions, which provide initial insight on possible tectonic and stratigraphic implications, and permit planning of following phase of the project.

Batista, J. A.; Alva-Valdivia, L. M.; Blanco, J.; Urrutia-Fucugauchi, J.

2008-05-01

250

One-carbon (bio?)geochemistry in subsurface waters of the serpentinizing Coast Range Ophiolite  

NASA Astrophysics Data System (ADS)

Serpentinization - the aqueous alteration of ultramafic rocks - typically imparts a highly reducing and alkaline character to the reacting fluids. In turn, these can influence the speciation and potential for metabolism of one-carbon compounds in the system. We examined the aqueous geochemistry and assessed the biological potential of one-carbon compounds in the subsurface of the McLaughlin Natural Reserve (Coast Range Ophiolite, California, USA). Fluids from wells sunk at depths of 25-90 meters have pH values ranging from 9.7 to 11.5 and dissolved inorganic carbon (DIC concentrations) generally below 60 micromolar. Methane is present at concentrations up to 1.3 millimolar (approximately one-atmosphere saturation), and hydrogen concentrations are below 15 nanomolar, suggesting active consumption of H2 and production of CH4. However, methane production from CO2 is thermodynamically unfavorable under these conditions. Additionally, the speciation of DIC predominantly into carbonate at these high pH values creates a problem of carbon availability for any organisms that require CO2 (or bicarbonate) for catabolism or anabolism. A potential alternative is carbon monoxide, which is present in these waters at concentrations 2000-fold higher than equilibrium with atmospheric CO. CO is utilized in a variety of metabolisms, including methanogenesis, and bioavailability is not adversely affected by pH-dependent speciation (as for DIC). Methanogenesis from CO under in situ conditions is thermodynamically favorable and would satisfy biological energy requirements with respect to both Gibbs Energy yield and power.

Hoehler, T. M.; McCollom, T.; Schrenk, M. O.; Kubo, M.; Cardace, D.

2011-12-01

251

Palladium, platinum, rhodium, iridium and ruthenium in chromite- rich rocks from the Samail ophiolite, Oman.  

USGS Publications Warehouse

30 samples of chromitite and chromite-rich rocks from two stratigraphic sections, 250 km apart, through the basal ultramafic member of the Samail ophiolite were spectrographically analysed for platinum-group elements (PGE) and for Co, Cu, Ni and V. These data are reported as are Cr/(Cr + Al), Mg/(Mg + Fe) and wt.% TiO2 for most samples. The chromitite occurs as pods or lenses in rocks of mantle origin or as discontinuous layers at the base of the overlying cumulus sequence. PGE abundances in both sections are similar, with average contents in chromite-rich rocks: Pd 8 ppb, Pt 14 ppb, Rh 6 ppb, Ir 48 ppb and Ru 135 ppb. The PGE data, combined with major-element and petrographic data on the chromitite, suggest: 1) relatively larger Ir and Ru contents and highest total PGE in the middle part of each section; 2) PGE concentrations and ratios do not correlate with coexisting silicate and chromite abundances or chromite compositions; 3) Pd/PGE, on average, increases upward in each section; 4) Samail PGE concentrations, particularly Rh, Pt and Pd, are lower than the average values for chromite-rich rocks in stratiform intrusions. 2) suggests that PGEs occur in discrete alloy or sulphide phases rather than in the major oxides or silicates, and 4) suggests that chromite-rich rocks from the oceanic upper mantle are depleted in PGE with respect to chondrites. L.C.C.

Page, N. J.; Pallister, J. S.; Brown, M. A.; Smewing, J. D.; Haffty, J.

1982-01-01

252

Stratigraphy and tectonics of a time-transgressive ophiolite obduction onto the eastern margin of the Pelagonian platform from Late Bathonian until Valanginian time, exemplified in northern Evvoia, Greece  

NASA Astrophysics Data System (ADS)

The obduction of an ophiolite sheet onto the eastern Pelagonian carbonate platform complex of the Hellenides began during the Late Bathonian and ended with the final emplacement of the ophiolite during Valanginian time. The early stages of obduction caused subaerial exposure of the platform, recorded by an unconformity of Callovian age, which is marked by laterites overlying folded and faulted, karstic substrates. The laterites have distinct ophiolitic geochemical signatures, indicating that emergent ophiolite had been undergoing lateritic weathering. This unconformity coincides with widespread western Tethyan, Callovian gaps, indicating that the obduction in the Hellenides was probably related to far-reaching plate tectonic processes. Resumed gravitational pull and rollback of the subducted, oceanic leading edge of the temporarily exposed ophiolite. Platform drowning continued into Tithonian-Valanginian time, documented initially by reefal carbonates and then by below-CCD, carbonate-free radiolarian cherts and shales. Subsequently, siliciclastic turbidites, which apparently originated from uplifted Variscan basement, were deposited together with and over the radiolarite as the ophiolite nappe sheet advanced. The nappe substrate underwent tectonic deformations of varying intensity, while polymictic mélange and syntectonic sedimentary debris accreted beneath the ophiolite and at the nappe front. The provenience of the ophiolite nappe complexes of northern Evvoia most probably has to be looked for in the Vardar ocean.

Scherreiks, Rudolph; Meléndez, Guillermo; BouDagher-Fadel, Marcelle; Fermeli, Georgia; Bosence, Dan

2014-06-01

253

Zircon dating of Neoproterozoic and Cambrian ophiolites in West Mongolia and implications for the timing of orogenic processes in the central part of the Central Asian Orogenic Belt  

NASA Astrophysics Data System (ADS)

We present new isotopic and trace element data to review the geochronological/geochemical/geological evolution of the central part of the Central Asian Orogenic Belt (CAOB), and find a fundamental geological problem in West Mongolia, which has traditionally been subdivided into northwestern early Paleozoic (formerly Caledonian) and southerly late Paleozoic (formerly Hercynian) belts by the Main Mongolian Lineament (MML). We resolve this problem with SHRIMP zircon dating of ophiolites and re-evaluation of much published literature. In Northwest Mongolia the Dariv-Khantaishir ophiolite marks the boundary between the Lake arc in the west and the Dzabkhan-Baydrag microcontinent in the east. Zircons from a microgabbro and four plagiogranites yielded weighted mean 206Pb/238U ages of 568 ± 5 Ma, 567 ± 4 Ma, 560 ± 8 Ma (Dariv), 573 ± 8 Ma and 566 ± 7 Ma (Khantaishir) that we interpret as reflecting the time of ophiolite formation (ca. 573-560 Ma). Metamorphic zircons from an amphibolite on a thrust boundary between the Khantaishir ophiolite and the Dzabkhan-Baydrag microcontinent formed at 514 ± 8 Ma, which we interpret as the time of overthrusting. In South Mongolia the Gobi Altai ophiolite and the Trans-Altai Gurvan Sayhan-Zoolen forearc with an ophiolite basement were investigated. Zircons of a layered gabbro (lower ophiolite crust) and a leucogabbro (mid-upper crust) of the Gobi Altai ophiolite yielded crystallization ages of 523 ± 5 Ma and 518 ± 6 Ma. The age data constrain the formation time of ophiolite within ca. 523-518 Ma. Zircons from four samples of the Gurvan Sayhan-Zoolen forearc, with similar hybrid adakite-boninite affinities, yielded 519 ± 4 Ma for an anorthosite, ? 512 ± 4 Ma for a hornblendite and 520 ± 5 and 511 ± 5 Ma for two diorites. The ophiolite basement has an upper age limit of 494 ± 6 Ma, determined by dating a tonalite dike cutting the Zoolen ophiolite. Integrating available zircon ages as well as geochemical and geological data, we re-subdivide West Mongolia into: a latest Neoproterozoic-early Cambrian, arc-microcontinent collision zone north of the MML; a Cambrian Gobi Altai ophiolite-microcontinent collision zone and a Cambrian Trans-Altai forearc complex south of the MML. The central CAOB evolved in five phases: subduction initiation and arc formation (ca. 573 to > ca. 540 Ma); arc-microcontinent collision (ca. 535-524 Ma); a continuum of slab delamination, overthrusting, crustal thickening and surface uplift (ca. 519-482 Ma) in Northwest Mongolia; initiation of new subduction zones in South Mongolia (ca. 523-511 Ma); and continuing orogeny with local surface uplift. Overall, the current, documented timing of orogenic development in the central CAOB is largely consistent with a W/SW-Pacific style of evolution in terms of subduction initiation, short timescales of individual orogenies, and episodic subduction-collision during a continuing migration of subduction zones.

Jian, Ping; Kröner, Alfred; Jahn, Bor-ming; Windley, Brian F.; Shi, Yuruo; Zhang, Wei; Zhang, Fuqin; Miao, Laicheng; Tomurhuu, Dondov; Liu, Dunyi

2014-06-01

254

The Dupal isotope anomaly in the Paleo-Asian sub-oceanic mantle: Nd-Sr-Pb isotope evidence from ophiolites in Northern China  

NASA Astrophysics Data System (ADS)

It has been suggested that the Dupal isotope anomaly in the mantle (Hart, Nature 309, 1984) can be traced in the Paleozoic ophiolites from the Paleo-Asian Ocean (PAO) that were formed during the break up of the Rodinia Supercontinent between 1000 and 300 Ma (Dobrestove et al., Int. Geo. Rev. 37, 1995). New Sr, Nd and high-precision Pb isotope data for mafic rocks from the ophiolites in the East (370Ma) and West (300Ma) Junggar blocks of the Altaids terrane in China, which belongs to the southern branch of the PAO, combined with published data suggest that the PAO crust and related arc magmas indeed have a Dupal-like isotope signature. In general, the samples have high 208Pb/206Pb for given 206Pb/204Pb ratios (i.e., positive ?8/4 values), similar to Indian Ocean MORB. Data also seem to indicate that the basalts and gabbros from West Junggar ophiolites have an enriched-MORB trace element signature whereas those from East Junggar ophiolites have a signature transitional between MORB-like and arc-like compositions. The new samples have higher ?8/4 than those from the early-Paleozoic (570Ma) Paleo-Asian ophiolites from Agardagh Tes-Chem, which belongs to the northern branch of the PAO (Pfänder et al., Contrib. Mineral. Petrol. 143, 2002). Thus, the combined results indicate that the Dupal isotope signature of the Paleo-Asian ophiolites became stronger with time and from north to south. Moreover, the results lend support to the idea that the Dupal isotope anomaly existed prior to the opening of the Indian Ocean and was probably present throughout the whole PAO. Such a long period of existence and large-scale distribution indicate that the Dupal isotope anomaly may not simply be an attribute of certain mantle domain, but a reflection of a mantle process as well.

Liu, X.; Castillo, P. R.; Xu, J.; Hou, Q.

2009-12-01

255

Petrological significance of the abyssal peridotites from the ophiolite belt of Indo-Myanmar Range, northeastern India  

NASA Astrophysics Data System (ADS)

The ophiolite belt of the Indo-Myanmar Range of northeastern India is regarded as the continental extension of the Indonesia island arc. The ophiolite sequence forms a belt extending about 200 km from Pukhpur (Nagaland state) in the north to Moreh (Manipur state) in the south. Here we report preliminary mineralogical and phase geochemical data of peridotites collected from the ophiolite sequences of the Ukhrul district in Manipur. Primary minerals are clinopyroxene, orthopyroxene and spinel with scarce relicts of olivine. Andraditic garnet present in a few samples is of secondary origin. The clinopyroxene and orthopyroxene show limited compositional range. Well's (1977) single clinopyroxene thermometry yielded variable equilibrium temperature between 780-1250oC. The lower temperature value suggests post-crystallisation re-equilibration. Equilibrium pressure estimates based on Nimis and Ulmer's single clinopyroxene (1998) and Köhler and Brey's (1990) olivine-spinel geobarometries range from 19 to 25 kbars. Oxygen fugacity expressed in terms of quartz-fayalite-magnetite (Delta QFM) value is calculated as -1.32. Spinel Cr# (Cr/(Cr+Al)) of the peridotites reflects mantle residues after low degree of melting (F = 0.5 - 11%) if derived from primitive mantle. Mineral composition tectonomagmatic discrimination diagrams confirm the studied samples to be abyssal peridotites. Laser Ablation ICPMS analyses of the rare earth element patterns of clinopyroxene in the peridotites show depleted light rare earth element with flat middle and heavy rare earth element patterns, similar to the reported abyssal peridotites. References Wells P.R.A. (1977) CMP, 62, 129-139. Nimis P., Ulmer P. (1998) CMP, 133, 122-135. Köhler T., Brey G.P. (1990) GCA, 54, 2375-2388.

Maibam, B.; Foley, S.

2009-04-01

256

The petrogenesis of leucogranitic dykes intruding the northern Semail ophiolite, United Arab Emirates: field relationships, geochemistry and Sr\\/Nd isotope systematics  

Microsoft Academic Search

The Khawr Fakkan block of the Semail ophiolite (United Arab Emirates) exhibits a suite of 10–100?m scale metaluminous to\\u000a peraluminous granitic intrusions, ranging from cordierite-andalusite-biotite monzogranites to garnet-tourmaline leucogranites,\\u000a which intrude mantle sequence harzburgites and lower crustal sequence cumulate gabbros. Structural constraints suggest that\\u000a the subduction of continental sedimentary material beneath the hot proto-ophiolite in an intra-oceanic arc environment led

Jon Cox; Mike Searle; Rolf Pedersen

1999-01-01

257

Supra-ophiolitic formations from the Thessaloniki nappe (Greece), and associated magmatism: An intra-oceanic subduction predates the Vardar obduction  

NASA Astrophysics Data System (ADS)

The Vardar ophiolites from Chalkidiki (Thessaloniki nappe) show autochthonous cover formations. These formations include reefal limestones of Kimmeridgian-Tithonian age, and clastic deposits with spilite, andesite and granodiorite elements, suggesting an intra-oceanic island arc setting. The earliest continental clasts are found in post-Tithonian layers. A granodioritic intrusion associated with granitoid dykes crosscutting the ophiolite corresponds to deep parts of the arc. The Chaikidiki arc, 150-140 My old, can be followed northward for at least 150 km (Guevgueli). The Vardarian obduction would have occurred during the Early Cretaceous owing to the arc-continent collision.

Michard, André; Feinberg, Hugues; Montigny, Raymond

1998-10-01

258

Ultramafic rocks of a fracture-zone ophiolite, North Cascades, Washington  

NASA Astrophysics Data System (ADS)

The Ingalls Complex was deformed in a Late Jurassic oceanic fracture zone. An unusually diverse group of ultramafic tectonites comprise three units in this ophiolite. Unit 1 consists mostly of poorly to moderately foliated harzburgite and dunite characterized by porphyroclastic textures. Irregular-shaped and tabular dunite bodies probably represent intrusive bodies or residues of partial melting. Voluminous Unit 2 consists mostly of poorly to strongly foliated Iherzolite and clinopyroxene-bearing harzburgite, plagioclase peridotite is present locally. Olivine and enstatite generally define equigranular mosaics or weakly porphyroclastic textures. Clinopyroxene, however, in some samples displays only weak deformation, compositional zoning, simple (growth?) twins and interstitial, commonly poikilitic texture. Clinopyroxene and plagioclase in these samples probably formed from a melt after recrystallization of olivine and enstatite, indicating that these Iherzolites are impregnated peridotites. Other Iherzolites and clinopyroxene-bearing harzburgites may represent weakly depleted mantle. Pods of metagabbroic gneiss within Unit 2 probably are small intrusions that were deformed as they cooled. Unit 3 represents a major high-temperature ( 700 °? 900 ° C) shear zone that separates Units 1 and 2, and consists of strongly foliated, commonly mylonitic Iherzolites and hornblende peridotites. The latter are the most strongly foliated ultramafites, and olivine in them records stresses as high as 275 MPa. The abundance of hornblende implies a genetic relationship between mylonitization and the hydration and metasomatism necessary to form such rocks from Iherzolites. Mineral chemistry and geothermometry are typical of mantle tectonites in many ophiolites and oceanic fracture zones. There is a particularly strong similarity between the spinels in the Ingalls Complex and the spinels from the Owen and Vema fracture zones. Hornblende in Unit 3 ranges from edenite to edenitic hornblende. Calculated temperatures from CPX-OPX pairs in Unit 2 range from 950 ° to 1000 ° C. These data are best explained by horizontal and vertical displacements in oceanic fracture zones which may juxtapose lithosphère formed at different structural levels and with different petrologic histories. Unit 1 represents a block of upper mantle which probably experienced convective upwelling and partial melting beneath a spreading ridge near its intersection with a fracture zone. During convective upwelling at least parts of the upper mantle represented by Unit 2 were impregnated by a melt. The impregnated Iherzolite may be accounted for by the edge effect at a fracture zone or reduced melt production at a fracture zone due to ribbon spreading. The mylonite zone of Unit 3 reworks Iherzolites and harzburgites which were originally part of the block represented by Unit 2 and probably records deformation in the active segment of the fracture zone. These observations document the complexities that may occur in ultramafic rocks within oceanic fracture zones. They also suggest that impregnation, metasomatism, and high stresses are important in these zones.

Miller, Robert B.; Mogk, David W.

1987-11-01

259

Oceanic evolution of Spl-peridotites of the Frido Unit ophiolites (Southern Apennine-Italy)  

NASA Astrophysics Data System (ADS)

The southern Apennine chain is a fold-and thrust belt formed between the upper Oligocene and Quaternary as a result of the convergence between the African and European plates and a simultaneous SE-directed rollback of the Ionian subducting lithospere. The ophiolitic sequences, which are part of the Southern Apennines, are remnants of the Ligurian oceanic lithosphere pertaining to the Jurassic western Tethys. The Liguride Units of the Southern Apennines include sequences characterized by an HP/LT metamorphic overprint in the Frido Unit and sequences lacking orogenic metamorphism North-Calabria Unit. The ophiolitic rocks occurring in the Frido Unit include serpentinite derived from a lherzolitic to harzburgitic mantle, as suggested by microstructural and petrographical features. The serpentinites are frequently associated to tectonic slices and dykes composed of diabase and medium to high-grade metamorphic rocks such as amphibolites, gneiss, granofels as well as gabbros and basalts with a pillow structure. The studied serpentinites of the Frido Unit show mesh, xenomorphic and mylonitic texture. Primary mantle minerals are represented by olivine, orthopyroxene, clinopyroxene and spinel. Pseudomorphic minerals are serpentine, magnetite and tremolite. Olivine is replaced by serpentine forming a mesh texture; orthopyroxene is mostly altered to bastite and in some cases shows exsolution lamellae of clinopyroxene and kink bands. Clinopyroxene is armoured by a tremolite rim. Spinel shows a holly-leaf habit and is often armoured by a corona of Cr-chlorite. The core of the analysed spinel has a Cr-Al spinel composition corresponding to chromite (Al2O3=29-31 wt %; Cr2O3= 28-37 wt%), whereas the rim has a Fe-Cr spinel composition corresponding to ferritchromite (Al2O3= 1-2% wt; Cr2O3=28-30 wt %). The Cr-Al spinel/ferritchromite ratio may be various in different spinel porphyroclasts. Serpentine has a fibrous stretched subidiomorphic habit, it is colourless or pale green. Tremolite is present as nematoblasts associated with orthopyroxene. Magnetite replaces spinel or occurs within the mesh textured serpentine. The metamorphic assemblages in the Frido Unit serpentinites allowed to infer the physical conditions operating during serpentinization. The mineralogical assemblages found are typical of the amphibolite facies, greenschist-amphibolite transition and greenschist facies conditions. Serpentinites are cut by veins filled with mineralogical assemblages typical of prehnite-pumpellyite facies likely related to the later orogenic Apennine evolution. The geochemical features of serpentinites show differences in compositions with respect to the Primitive Upper Mantle (PUM). These are likely related to serpentinization processes, since elements normalised to PUM show different trends, comparable to Residual MORB Mantle and to Primitive Upper Mantle respectively. HP/LT metamorphic conditions can be documented in mafic dykes enclosed in serpentinites, but similar conditions are not recorded in serpentinites. This suggests that P-T conditions during the orogenic event were not able to produce a HP/LT mineral assemblage in serpentinites.

Cristi Sansone, Maria T.; Prosser, Giacomo; Rizzo, Giovanna; Tartarotti, Paola

2010-05-01

260

Fragment of the Coast Range ophiolite and the Great Valley sequence in the San Juan Islands, Washington  

SciTech Connect

The Decatur terrane is a small, allochthonous fragment of Upper Jurassic oceanic crust with an overlying blanket of Upper Jurassic-Lower Cretaceous sedimentary rocks in the San Juan Islands. The stratigraphy and petrotectonic elements are dissimilar to those of coeval sequences in the Pacific Northwest (Oregon, Washington, and southern British Columbia), but are virtually identical to those of the Coast Range ophiolite and overlying Great Valley sequence south of San Francisco, California. This stratigraphic correlation, together with regional considerations, suggests that the Decatur terrane was detached and transported during Cretaceous time. The resultant displacement is in excess of 1000 km.

Garver, J.I. (Univ. of Washington, Seattle (USA))

1988-10-01

261

Geological and geochemical evolution of the Trincheira Complex, a Mesoproterozoic ophiolite in the southwestern Amazon craton, Brazil  

NASA Astrophysics Data System (ADS)

We document the first-known Mesoproterozoic ophiolite from the southwestern part of the Amazon craton, corresponding to the Trincheira Complex of Calymmian age, and propose a tectonic model that explains many previously enigmatic features of the Precambrian history of this key craton, and discuss its role in the reconstruction of the Columbia supercontinent. The complex comprises extrusive rocks (fine-grained amphibolites derived from massive and pillowed basalts), mafic-ultramafic intrusive rocks, chert, banded iron formation (BIFs), pelites, psammitic and a smaller proportion of calc-silicate rocks. This sequence was deformed, metasomatized and metamorphosed during the development of the Alto Guaporé Belt, a Mesoproterozoic accretionary orogen. The rocks were deformed by a single tectonic event, which included isoclinal folding and metamorphism of the granulite-amphibolite facies. Layered magmatic structures were preserved in areas of low strain, including amygdaloidal and cumulate structures. Metamorphism was pervasive and reached temperatures of 780-853 °C in mafic granulites and 680-720 °C in amphibolites under an overall pressure of 6.8 kbar. The geochemical composition of the extrusive and intrusive rocks indicates that all noncumulus mafic-ultramafic rocks are tholeiitic basalts. The mafic-ultramafic rocks display moderate to strong fractionation of light rare earth elements (LREE), near-flat heavy rare earth element (HREE) patterns and moderate to strong negative high field strength element (HFSE) anomalies (especially Nb), a geochemical signature typical of subduction zones. The lowest units of mafic granulites and porphyroblastic amphibolites in the Trincheira ophiolite are similar to the modern mid-ocean ridge basalt (MORB), although they locally display small Ta, Ti and Nb negative anomalies, indicating a small subduction influence. This behavior changes to an island arc tholeiite (IAT) signature in the upper units of fine-grained amphibolites and amphibole rich-amphibolites, characterized by progressive depletion in the incompatible elements and more pronounced negative Ta and Nb anomalies, as well as common Ti and Zr negative anomalies. Tectono-magmatic variation diagrams and chondrite-normalized REE and primitive mantle normalized patterns suggest a back-arc to intra-oceanic island arc tectonic regime for the eruption of these rocks. Therefore, the Trincheira ophiolite appears to have originated in an intraoceanic supra-subduction setting composed of an arc-back-arc system. Accordingly, the Trincheira Complex is a record of oceanic crust relics obducted during the collision of the Amazon craton and the Paraguá block during the Middle Mesoproterozoic. Thus, the recognition of the Trincheira ophiolite and suture significantly changes views on the evolution of the southern margin of the Amazon craton, and how it can influence the global tectonics and the reconstruction of the continents.

Rizzotto, Gilmar José; Hartmann, Léo Afraneo

2012-09-01

262

Variation of magma source in the Oman ophiolite inferred from distribution of magmatic dykes in the mantle  

NASA Astrophysics Data System (ADS)

Magmatic relics are ubiquitous in the mantle of the Oman ophiolite. They include discordant and concordant dykes showing sharp contact with their host. Their lithological nature is extremely variable but can be classified into four lithological types: troctolites, olivine gabbros, pyroxenites and gabbronorites with euhedral Opx (i.e. where Opx appears early in the crystallisation sequence). The distribution map of the magmatic dykes show that troctolites and olivine gabbros crop out in two localized area, the main one is centred on the Maqsad mantle diapir. Pyroxenites are highly dominant in the mantle section of the Oman ophiolite but Opx-rich concordant and discordant dykes are abundant only in the northern massifs. On the other hand, these features are scarce or absent in the southern massifs where troctolite and olivine gabbro dykes are abundant. The dykes chemical composition show that they can be linked to two different magmatic series: one is similar to a typical MOR magma, and the other, richer in Si and poor in incompatible elements, is closer to an andesitic to boninitic magma. The dykes lithological nature is partly related to the nature of the parental magma and troctolites and Opx-free olivine gabbros crystallised exclusively from the MOR magma. However, pyroxenites are ubiquitous and may have crystallised from both types of magma. This is in particular reflected in Cr-spinel chemistry, which Cr# is higher and Ti contents lower, and the higher amount of Opx when the parental magma is andesitic. The dyke distribution all over the mantle section of the Oman ophiolite show a clear magmatic dichotomy between the southern and northern parts of the ophiolite. MOR-type magma was preponderant southward to the Nakhl massif while the northern areas are dominated by andesitic to boninitic magma. Mineral chemistry in the lower crust show characteristics intermediate between the two types of melt that circulated in the mantle, suggesting that magma mixing hardly occur in the mantle but may be an important process of ocean crust building. In the lower crust, the most MOR-like gabbros are to the area close to the Maqsad mantle diapir and the andesitic end-member becomes dominant in the northernmost massif, far from the Maqsad diapir. This North-South chemical gradation shows that mixing occurs along the ridge axis, implying that magma can circulate on significant distances along the ridge axis at crustal level.

Python, Marie; Abily, Bénédicte; Ceuleneer, Georges; Arai, Shoji

2014-05-01

263

Petrogenetic and geotectonic significance of Neoproterozoic suprasubduction mantle as revealed by the Wizer ophiolite complex, Central Eastern Desert, Egypt  

NASA Astrophysics Data System (ADS)

Ophiolite complexes, formed in a suprasubduction zone environment during Neoproterozoic time, are widely distributed in the Eastern Desert of Egypt. Their mantle sections provide important information on the origin and tectonic history of ocean basins these complexes represent. The geochemistry and mineralogy of the mantle section of the Wizer ophiolite complex, represented by serpentinites after harzburgite containing minor dunite bodies, are presented. Presence of antigorite together with the incipient alteration of chromite and absence of chlorite suggests that serpentinization occurred in the mantle wedge above a Neoproterozoic subduction zone. Wizer peridotites have a wide range of spinel compositions. Spinel Cr# [100Cr/(Cr + Al)] decrease gradually from dunite bodies (Cr# = 81-87) and their host highly depleted harzburgites (Cr# = 67-79) to the less depleted harzburgites (Cr# = 57-63). Such decreases in mantle refractory character are accompanied by higher Al and Ti contents in bulk compositions. Estimated parental melt compositions point to an equilibration with melts of boninitic composition for the dunite bodies (TiO2 = ~<0.07-0.22 wt%; Al2O3 = 9.4-10.6 wt%), boninitic-arc tholeiite for the highly depleted harzburgites (TiO2 = <0.09-0.28 wt%; Al2O3 = 11.2-14.1 wt%) and more MORB-like affinities for the less depleted harzburgites (TiO2 = ~<0.38-0.51 wt%; Al2O3 = 14.5-15.3 wt%). Estimated equilibrium melts are found in the overlying volcanic sequence, which shows a transitional MORB-island arc geochemical signature with a few boninitic samples. Enrichment of some chromites in TiO2 and identification of sulfides in highly depleted peridotites imply interaction with an impregnating melt. A two-stage partial melting/melt-rock reaction model is advocated, whereby, melting of a depleted mantle source by reaction with MORB-like melts is followed by a second stage melting by interaction with melts of IAT-boninitic affinities in a suprasubduction zone environment to generate the highly depleted harzburgites and dunite bodies. The shift from MORB to island arc/boninitic affinities within the mantle lithosphere of the Wizer ophiolite sequence suggests generation in a protoarc-forearc environment. This, together with the systematic latitudinal change in composition of ophiolitic lavas in the Central Eastern Desert (CED) of Egypt from IAT-boninitic affinities to more MORB-like signature, implies that the CED could represent a disrupted forearc-arc-backarc system above a southeast-dipping subduction zone.

Farahat, E. S.; Hoinkes, G.; Mogessie, A.

2011-10-01

264

Distribution of platinum-group elements and Os isotopes in chromite ores from Mayarí-Baracoa Ophiolitic Belt (eastern Cuba)  

Microsoft Academic Search

The Mayarí-Baracoa ophiolitic belt in eastern Cuba hosts abundant chromite deposits of historical economic importance. Among\\u000a these deposits, the chemistry of chromite ore is very variable, ranging from high Al (Cr#=0.43–0.55) to high Cr (Cr#=0.60–0.83)\\u000a compositions. Platinum-group element (PGE) contents are also variable (from 33 ppb to 1.88 ppm) and correlate positively with\\u000a the Cr# of the ore. Bulk PGE abundances correlate

F. Gervilla; J. A. Proenza; R. Frei; J. M. González-Jiménez; C. J. Garrido; J. C. Melgarejo; A. Meibom; R. Díaz-Martínez; W. Lavaut

2005-01-01

265

Compositional and mineralogic constraints on the genesis of ophiolite hosted nickel mineralization in the Pevkos area, Limassol Forest, Cyprus  

Microsoft Academic Search

Mineralization composed dominantly of primary troilite, maucherite, pentlandite, and chalcopyrite, and secondary valleriite occurs in serpentinized transition zone rocks of the Limasol Forest segment of the Troodos ophiolite complex, Cyprus. Whole-rock and electron microprobe analyses of this mineralization gives ranges of Cu\\/(Cu+Ni)=0.16 to 0.47, Pt\\/(Pt+Pd)=0.66 to 0.51, Ni\\/Co=6.33 to 13.4, and chondrite normalized plots with low concentrations of Rh, Pt,

Michael P. Foose; Maria Economou; A. Panayiotou

1985-01-01

266

Modeling the Evolution of a Transform Fault in the Mantle Section of the New Caledonia Ophiolite  

NASA Astrophysics Data System (ADS)

The Bogota Peninsula shear zone has been interpreted as a paleotransform fault in the mantle section of the New Caledonia ophiolite. The shear zone was originally identified based on stronger fabric development on the Bogota Peninsula and the rotation of foliation from shallow NW-striking outside the shear zone to subvertical NNE-striking inside the shear zone. This rotation is generally symmetric and occurs across a 50 km-wide region centered on a 3 km-wide high strain core. To better understand the evolution of this dextral transform fault, we develop a kinematic model constrained by several types of field and laboratory data. Field fabrics were used to empirically define three distinct domains within the shear zone: a farfield, nearfield, and central high-strain core. Our model assumes that the shear zone localized during its development so that farfield features preserve the first increment of deformation and the center records the last increment. We specify the orientation of the shear zone boundary based on the lattice preferred orientation of olivine within the center. For each increment of deformation, the orientation of the finite strain ellipsoid is constrained by foliation and lineation while the magnitude is constrained the shape preferred orientation of macroscopic orthopyroxene. Our modeling also incorporates the changing orientation of orthopyroxene dikes that, like field fabrics, rotate systematically across the shear zone. Because many dikes also show increasing boudinage towards the center of the shear zone, we estimate stretches recorded by these dikes using the length, width, and separation between boudins along a variety of outcrop surfaces. This data-rich approach to modeling is challenging, but may ultimately provide more insight into the development of a transform fault at a deeper lithospheric level than is commonly observed.

Titus, S. J.; Davis, J. R.

2010-12-01

267

An alkaline spring system within the Del Puerto Ophiolite (California, USA): A Mars analog site  

NASA Astrophysics Data System (ADS)

Mars appears to have experienced little compositional differentiation of primitive lithosphere, and thus much of the surface of Mars is covered by mafic lavas. On Earth, mafic and ultramafic rocks present in ophiolites, oceanic crust and upper mantle that have been obducted onto land, are therefore good analogs for Mars. The characteristic mineralogy, aqueous geochemistry, and microbial communities of cold-water alkaline springs associated with these mafic and ultramafic rocks represent a particularly compelling analog for potential life-bearing systems. Serpentinization, the reaction of water with mafic minerals such as olivine and pyroxene, yields fluids with unusual chemistry (Mg-OH and Ca-OH waters with pH values up to ˜12), as well as heat and hydrogen gas that can sustain subsurface, chemosynthetic ecosystems. The recent observation of seeps from pole-facing crater and canyon walls in the higher Martian latitudes supports the hypothesis that even present conditions might allow for a rock-hosted chemosynthetic biosphere in near-surface regions of the Martian crust. The generation of methane within a zone of active serpentinization, through either abiogenic or biogenic processes, could account for the presence of methane detected in the Martian atmosphere. For all of these reasons, studies of terrestrial alkaline springs associated with mafic and ultramafic rocks are particularly timely. This study focuses on the alkaline Adobe Springs, emanating from mafic and ultramafic rocks of the California Coast Range, where a community of novel bacteria is associated with the precipitation of Mg-Ca carbonate cements. The carbonates may serve as a biosignature that could be used in the search for evidence of life on Mars.

Blank, J. G.; Green, S. J.; Blake, D.; Valley, J. W.; Kita, N. T.; Treiman, A.; Dobson, P. F.

2009-05-01

268

Ore petrology of chromite-PGE mineralization in the Kempirsai ophiolite complex  

NASA Astrophysics Data System (ADS)

The platinum group minerals (PGM) in chromite ores of the Kempirsai ophiolite massif, located south of the Ural Mountains, are extremely varied in composition and represented predominantly by alloys, sulfides, arsenides, and sulfosalts of the iridium-group PGE (IPGE). The earlier Ir-Os-Ru alloys prevail over the later Cu-Os-Ru, Cu-Ir, Ni-Ir, Ni-Os-Ir-Ru, and Ni-Ru-Os-Fe alloys rich in base metals (BM). The earlier Ru-Os disulfides crystallize coevally with Ir-Os-Ru alloys, whereas the later sulfides are represented by compounds with a variable stoichiometry and a wide miscibility of Ni, Cu, Ir, Rh, Os, and Fe. Phase relations of PGE alloys with PGE-BM alloys, sulfides and sulfoarsenides confirm that deposition of these minerals was defined by a general evolution of PGE fractionation in the mineral-forming system but not by a super-imposed process. The leading mechanism of PGM crystallization is thought to be their dendritic growth during gas-transport reactions from low-density gaseous fluid enriched in PGE. The representative technological sampling of 0.5 million tons of an ore showed that the average PGE content in chromite ore is 0.71 ppm which leads to an evaluation of the PGE resources to be no less than 250 tons. Hence, the Kempirsai deposit is not only a giant chromium deposit, but also a giant deposit of IPGE: Ir, Ru, and Os. The size parameters of PGM and their aggregates suggests that the PGE may be recoverable in separate concentrates.

Distler, V. V.; Kryachko, V. V.; Yudovskaya, M. A.

2008-01-01

269

The Upper Jurassic Monopigadon pluton related to the Vardar-Axios ophiolites and its geotectonic significance  

NASA Astrophysics Data System (ADS)

The ophiolite complex exposed in the NW-SE trending Vardar-Axios Zone is characterized by granitic rocks associated with it. In central Macedonia (Northern Greece), it is intruded by the Upper Jurassic Fanos granite and Monopigadon pluton. The origin, evolution and geotectonic setting of the latter are studied. The pluton is composed of slightly peraluminous to peraluminous high-K calc-alkaline biotite granodiorite (BGrd), biotite granite (BGr), leucogranite (LGr) and aplites (Apl). Enclosed rocks (Enc) are mostly xenoliths, surmicaceous enclaves and biotite clots occurring frequently in BGrd and BGr indicating an extensive incorporation of country rocks in the magma, whereas their variability implies that the magma intruded an inhomogeneous crust. In addition, a serpentinite body as well as amphibolite and calc-silicate hornfelses are exposed as inliers in the pluton. The granitoids are characterized by relatively high-K, low Sr contents (<180 ppm), and low Sr/Y ratio (0.4-6.4). REE are enriched in all granitic rocks (LaCN=89-148, LuCN=6-25. The (La/Lu)CN ratio ranges from 10.5 to 4.9 in BGrd, from 11.1 to 3.8 in BGr, and from 11.3 to 25.7 in LGr. The BGrd and BGr show similar LILE-enriched, and spiked patterns with negative anomalies at Ba, Ta, Nb, Sr and Ti and a positive anomaly at Pb, whereas the patterns of LGr show higher Ta, Nb, Sr and Ti negative anomalies. The Sr initial isotopic ratios, typical of the Earth's crust, vary from 0.7147 to 0.7174 in BGrd, are relatively constant at 0.7105 - 0.7113 in BGr, and range from 0.7213 to 0.7340 in LGr, whereas they are lower in the enclaves (0.7087 to 0.7094). BGrd shows the lowest ?Nd values (-8.31 to -6.43), while it ranges from -6.11 to -4.26 in BGr and from -3.37 to -0.89 in LGr. Late Triassic to Late Jurassic intrusion zircon ages are reported for the Monopigadon pluton which is unconformably overlain by Kimmeridgian - Tithonian limestones and fragments of the plutonic rocks occur in the limestones. The geochemical data imply that the evolution in BGr must be considered as independent of BGrd, and that LGr generate by partial melting of crustal material. The geochemical variability of the BGrd is reproduced by two different AFC models having the same parental magma and assimilation/fractionation ratio but different assimilant. Geochemical modeling suggests that the BGr variability could be reproduced by two different FC models having the same parental magma but with different fractionating assemblages. Comparison of BGrd and BGr with experimental data obtained by melting experiments of crustal protoliths The comparison indicates that the BGrd and BGr have similar sources and they are likely originated by partial melting of middle-lower crustal rocks with intermediate-basaltic compositions, such as amphibolites, andesites and basalts. In both cases (BGrd and BGr) any sedimentary source is precluded. Felsic garnet granulites and metapelites are candidate source rocks for LGr. The geochemical data, used in order to clarify the geotectonic setting of the Monopigadon pluton, the relationship of the latter with the ophiolites, along with the suggested source and evolution process, support magma genesis by melting of an inhomogeneous middle to lower crust due to mantle-derived magmas underplating. The latter which had not mixed/mingled with the crustal melts are related with a volcanic arc environment. The inhomogenous crust explains both the diversity of the sources for BGrd+BGr and LGr as well as the different kinds of xenoliths. The 159?1 Ma age of Monopigadon, similar to Fanos (158 Ma), fits well with the following scenario: emplacement of the East Vardar ophiolites in the Late Jurassic; partial melting of an inhomogenous crust originating the different Monopigadon magmas; high-temperature collision processes; Fanos pluton genesis representing the Late Jurassic closure of the ocean.

Koroneos, Antonios; Poli, Giampiero; Christofides, Georgios

2010-05-01

270

Petrogenesis and tectono-magmatic significance of basalts and mantle peridotites from the Albanian-Greek ophiolites and sub-ophiolitic mélanges. New constraints for the Triassic-Jurassic evolution of the Neo-Tethys in the Dinaride sector  

NASA Astrophysics Data System (ADS)

The Albanide-Hellenide ophiolites and related ophiolitic mélanges include eight different types of volcanic and subvolcanic rocks: 1) Triassic, within-plate alkaline rocks (WPB); 2) Triassic high-Ti mid-ocean ridge basalts showing enriched compositions (E-MORB); 3) Triassic and Jurassic high-Ti mid-ocean ridge basalts showing normal compositions (N-MORB); 4) Jurassic basalts with geochemical features between MORB and island arc tholeiites; hereafter defined as medium-Ti basalts (MTB); 5) Jurassic low-Ti, island arc tholeiitic (IAT) rocks; 6) Jurassic very low-Ti (boninitic) rocks; 7) Jurassic backarc basin basalts and basaltic andesites (BABB); 8) Triassic and Jurassic calc-alkaline rocks (CAB). The geochemical and petrogenetic features of these rock-types, as well as the results from REE modelling of mantle sources, primary melt generation, and mantle residua indicate that they have formed in distinct tectonic settings within an oceanic environment. Both Triassic and Jurassic N-MORBs primary magmas derived from ~ 10 to 20% partial melting of a primitive asthenosphere, whereas Triassic alkaline WPB basalts originated from low degrees of partial melting of an OIB-type mantle source and were most likely erupted in seamounts. Triassic E-MORBs originated from ~ 12% partial melting of a primitive asthenosphere influenced by the OIB-type component. The residual MORB mantle is represented by depleted lherzolites, which are commonly found in the Albanide-Hellenide ophiolites. Mid Jurassic MTB and IAT primary magmas derived from ~ 10% and 10-20% partial melting of the MORB residual mantle, respectively with the variable addition of subduction components and were erupted in an intra-oceanic, supra-subduction zone setting. The residual mantle associated with these magmatic events is represented by harzburgites. Mid Jurassic boninitic primary magmas may have originated either from 10 to 20% partial melting of the MTB and IAT residual mantle or from ~ 30% partial melting of the MORB residual mantle. In both cases, the depleted mantle sources were enriched in light rare earth elements (LREE) by subduction-derived fluids. The extremely depleted harzburgites, which are widespread in the Albanide-Hellenide ophiolites, are interpreted as the residual mantle associated with boninite formation. Mid-Late Jurassic CABs originated from ~ 15 to 20% partial melting of a depleted peridotite mantle significantly enriched in Th and LREE by subduction-derived fluids, whereas BABBs originated from 10 to 20% partial melting of a primitive asthenosphere somewhat enriched in Th and LREE by a nearby subduction. Both these rock-types were erupted in a continental arc-backarc setting. The different rock-types of the Albanide-Hellenide ophiolites record the fundamental stages of the Triassic-Jurassic evolution of the Neo-Tethys in the Dinaride sector: from sea-floor spreading, after continental break-up, to intra-oceanic subduction initiation and supra-subduction zone (SSZ) lithospheric accretion.

Saccani, Emilio; Beccaluva, Luigi; Photiades, Adonis; Zeda, Ottavia

2011-06-01

271

Dissolution-precipitation processes governing the carbonation and silicification of the serpentinite sole of the New Caledonia ophiolite  

NASA Astrophysics Data System (ADS)

The weathering of mantle peridotite tectonically exposed to the atmosphere leads commonly to natural carbonation processes. Extensive cryptocrystalline magnesite veins and stock-work are widespread in the serpentinite sole of the New Caledonia ophiolite. Silica is systematically associated with magnesite. It is commonly admitted that Mg and Si are released during the laterization of overlying peridotites. Thus, the occurrence of these veins is generally attributed to a per descensum mechanism that involves the infiltration of meteoric waters enriched in dissolved atmospheric CO2. In this study, we investigate serpentinite carbonation processes, and related silicification, based on a detailed petrographic and crystal chemical study of serpentinites. The relationships between serpentine and alteration products are described using an original method for the analysis of micro-X-ray fluorescence images performed at the centimeter scale. Our investigations highlight a carbonation mechanism, together with precipitation of amorphous silica and sepiolite, based on a dissolution-precipitation process. In contrast with the per descensum Mg/Si-enrichment model that is mainly concentrated in rock fractures, dissolution-precipitation process is much more pervasive. Thus, although the texture of rocks remains relatively preserved, this process extends more widely into the rock and may represent a major part of total carbonation of the ophiolite.

Ulrich, Marc; Muñoz, Manuel; Guillot, Stéphane; Cathelineau, Michel; Picard, Christian; Quesnel, Benoit; Boulvais, Philippe; Couteau, Clément

2014-01-01

272

New petrological and structural constraints on the origin of the Mayarí-Baracoa ophiolitic belt (eastern Cuba)  

NASA Astrophysics Data System (ADS)

The Jurassic-Cretaceous Mayarí-Baracoa ophiolitic belt (MBOB) is situated in the easternmost part of the east-west trending Cuban ophiolitic belt. The MBOB comprises the Mayarí-Cristal and Moa-Baracoa massifs located, respectively, to the east and the west of the Riito de Miraflores fault. The Mayarí-Cristal massif is essentially made up of mantle tectonites (> 5 km thick) and subordinate gabbro-diabase dikes intruding the mantle section. These mafic dikes cut the peridotites but no diabase sheeted complex has been observed. We differentiate two main domains: 1) the lower domain, which outcrops in the southern and central part of the massif, is composed of porphyroclastic harzburgite tectonites. Dunites are widespread and occur as irregular patches and sub-concordant layers (< 1 m thick) in harzburgites and as the wall-rock of chromite pods. In this domain, peridotites are cut by, at least, three generations of pyroxenitic dikes; 2) the upper domain, which outcrops in the northern part of the massif, is constituted of porphyroclastic harzburgite tectonites locally enriched in clinopyroxene. These harzburgites are cut by a network of discordant zoned mafic dikes (0.1-15 meters) composed of diabase chilled-margins and microgabbro cores, suggesting they intrude in a cold geotherm. The structural and petrological characteristics of the Mayarí-Cristal massif differ from those of ophiolites formed at mid-ocean fast-spreading ridges (e.g., Oman ophiolite) and bear resemblance with those observed in outcrops and rocks from active slow spreading ridges such as MAR. The Moa-Baracoa massif comprises mantle tectonites (> 2.2 km thick) topped by a thin crustal section made up of lower layered gabbros (ca. 300 meters thick) and discordant volcanics. Isotropic gabbros and diabase sheeted-dyke complex are lacking. This massif exhibits an unusual crust-mantle transition zone made up of residual harzburgite tectonites and minor dunites. Some peridotites are enriched in plagioclase and clinopyroxene and may form layers of “impregnated peridotites” and troctolites. In the transition zone concordant and rare sub-concordant bodies of dunite and chromitite and sills of gabbro also occur. Peridotites are cut by dikes of gabbro, pegmatitic gabbro, minor diabase, olivine norite and orthopyroxenite. The layered gabbros are composed of olivine gabbro and gabbronorites. The composition of olivine (Mg# = 85-86), clinopyroxene (Mg# = 78-90), orthopyroxene (Mg# = 73-87), and plagioclase (An = 60-77 wt %) are similar to those from ophiolitic cumulates associated to arc or back-arc settings. Many geochemical features suggest that peridotites from the MBOB were formed in a supra-subduction setting. The Cr# of spinel in peridotites (0.44-0.80) is in equilibrium with melts ranging in composition from MORB to boninites. Harzburgites and dunites are enriched in highly incompatible elements (Cs, Rb, Ba, Th, U and Nb) relative to REE (Th/La = 0.6-0.8) and display trace element normalized patterns with U spikes (U/Th > 1.3) suggesting the involvement of a volatile-rich fluid/melt. Moreover MBOB peridotites show high fO2 values typical of peridotites from subduction settings [delta logfO2 (FMQ) > 1]. We propose that the MBOB was initially part of an ocean basin formed by the rifting and spreading of the “Protocaribbean” ocean. This proto-ocean was subsequently affected by the development of a juvenile island-arc in a Cretaceous supra-subduction setting and by the late opening of a back-arc basin where most of the intrusive plutonic and volcanic crust of the MBOB was accreted.

Marchesi, C.; Proenza, J.; Gervilla, F.; Garrido, C. J.; Melgarejo, J. C.; Díaz-Martínez, R.; Godard, M.

2003-04-01

273

Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization of a dynamic melt lens at fast spreading ridges  

Microsoft Academic Search

The transition between the small melt lens observed on top of fast spreading ridge magma chambers and the overlying sheeted dike complex marks the interface between magma and the hydrothermal convective system. It is therefore critical to our understanding of fast spreading ridge accretion processes. We present maps of two areas of the Oman ophiolite where this transition zone is

Benoit Ildefonse; Juergen Koepke

2009-01-01

274

New 40Ar\\/39Ar Ages, Biostratigraphic and Geochemical Data from the Sabzevar Ophiolite, North Central Iran: Implications for Tectonic of Iranian Plate  

Microsoft Academic Search

The igneous rocks of the Sabzevar ophiolite in north central Iran composed of peridotites, serpentinite, minor pyroxenite, gabbros, and a volcanic sequence that exhibits a wide range of composition from basalts to basaltic andesites to rhyodacite-dacites, rhyolites and basanites. Sedimentary rocks include a variety of Upper Triassic to Lower Cretaceous deep- and shallow-marine rocks. These include pelagic fossiliferous carbonates, which

A. Hassanipak; M. Kariminia; K. Mobasher; M. Ghazi

2003-01-01

275

Tectonic implications of a paleomagnetic study of the Sarmiento Ophiolitic Complex, southern Chile  

NASA Astrophysics Data System (ADS)

A paleomagnetic study was carried out on the Late Jurassic Sarmiento Ophiolitic Complex (SOC) exposed in the Magallanes fold and thrust belt in the southern Patagonian Andes (southern Chile). This complex, mainly consisting of a thick succession of pillow-lavas, sheeted dikes and gabbros, is a seafloor remnant of the Late Jurassic to Early Cretaceous Rocas Verdes basin that developed along the south-western margin of South America. Stepwise thermal and alternating field demagnetization permitted the isolation of a post-folding characteristic remanence, apparently carried by fine grain (SD?) magnetite, both in the pillow-lavas and dikes. The mean "in situ" direction for the SOC is Dec: 286.9°, Inc: - 58.5°, ?95: 6.9°, N: 11 (sites). Rock magnetic properties, petrography and whole-rock K-Ar ages in the same rocks are interpreted as evidence of correlation between remanence acquisition and a greenschist facies metamorphic overprint that must have occurred during latest stages or after closure and tectonic inversion of the basin in the Late Cretaceous. The mean remanence direction is anomalous relative to the expected Late Cretaceous direction from stable South America. Particularly, a declination anomaly over 50° is suggestively similar to paleomagnetically interpreted counter clockwise rotations found in thrust slices of the Jurassic El Quemado Fm. located over 100 km north of the study area in Argentina. Nevertheless, a significant ccw rotation of the whole SOC is difficult to reconcile with geologic evidence and paleogeographic models that suggest a narrow back-arc basin sub-parallel to the continental margin. A rigid-body 30° westward tilting of the SOC block around a horizontal axis trending NNW, is considered a much simpler explanation, being consistent with geologic evidence. This may have occurred as a consequence of inverse reactivation of old normal faults, which limit both the SOC exposures and the Cordillera Sarmiento to the East. The age of tilting is unknown but it must postdate remanence acquisition in the Late Cretaceous. Two major orogenic events of the southern Patagonian Andes, in the Eocene (ca. 42 Ma) and Middle Miocene (ca. 12 Ma), respectively, could have caused the proposed tilting.

Rapalini, A. E.; Calderón, M.; Singer, S.; Hervé, F.; Cordani, U.

2008-06-01

276

The fossilisation of a dynamic melt lens at fast spreading centers: insights from the Oman ophiolite  

NASA Astrophysics Data System (ADS)

Thin and narrow melt lenses are observed on top of crystal-rich magma chambers at fast spreading ridges. This particular horizon, which marks the interface between the magmatic and the hydrothermal system, presents complex petrological and structural features. Its dynamics remain poorly constrained. We present here new detailed mapping of continuous, undisturbed areas of the Oman ophiolite and discuss evidences for a dynamic system with upward and downward vertical movements of the melt lens. These observations are consistent with episodic dikes injections in a steady state model, but also suggest that the root of the sheeted dike complex is generally overprinted by the upward vertical movements of the melt lens. The latter trigger prograde recrystallization in the dikes. However, because of later, low-temperature alteration, the upward extent of such a possible recrystallization front is unconstrained. Dike assimilation is evidenced in the field; it provides a mechanism for the incorporation of hydrated phases in the melt lens during its upward migrations. Recrystallized-dike-enclaves accumulations are observed at the foliated gabbro/isotropic gabbro transition, and inferred to represent fragment of the assimilated dikes, stoped at the bottom of the melt lens during its upward migration. In the isotropic gabbro horizon, some coarse grained gabbros display a crystallisation sequence with the clinopyroxenes crystallizing earlier than the plagioclases, which is uncommon in oceanic gabbros. Plagioclase composition is also abnormal with anorthite content reaching An99. These unusual observations are consistent with crystallization under hydrous conditions. In these samples relict domains (reaching 5mm) display Al-Ti poor clinopyroxenes. They contain small oxide inclusions (1mm), which we interpret as an evidence of prograde metamorphic recrystallization of amphiboles. These domains illustrate the recycling of previously hydrothermalized lithologies in the melt lens. The described melt lens upward migration, which results in reheating and assimilation of hydrothermalized sheeted dikes, is consistent with observations made in IODP Hole 1256D. A general dynamic model for the melt lens evolution at fast spreading ridges is proposed, in which episodic dike injections during steady state regime stages alternate with vertical migrations of the melt lens. Upward and downward movements of the lens result in reheating/assimilation of dikes, and crystallization of varitextured isotropic gabbros, respectively.

France, L.; Ildefonse, B.; Koepke, J.

2008-12-01

277

Mineral Controls on Microbial Niche Space in Subsurface Serpentinites of the Coast Range Ophiolite, Northern California  

NASA Astrophysics Data System (ADS)

Serpentinites on Earth are excellent candidate environments for the origin of life and continue to serve as an outstanding model system for study of modern selection pressures (such as gradients in temperature, pressure, aqueous geochemistry, oxidation-reduction potential) on microbial communities. Recent coring (August 2011) in the Coast Range Ophiolite near Lower Lake, CA resulted in the establishment of two sets of reference continental serpentinite cores (to 30 and 42 m depth, respectively) and the installation of eight monitoring wells drawing from different depths. Six of these wells plumb serpentinizing waters, with characteristic high pH (up to 12.5), high calcium loads (10-200 ppm), and methane concentrations that are at or near one-atmosphere saturation, while hydrogen levels are orders of magnitude lower than is typically observed in other similar systems. Low hydrogen levels are cryptic; they may reflect consumption by microbial communities or possibly a pressure- or temperature-related solubility response. Two reference wells provide access to quasi-parallel, shallower groundwater flow regimes with near neutral pH values and surface-related major element and organic acid inventories. Many environmental parameters pertinent to microbiological communities active in serpentinites (such as dissolved hydrogen, methane, and other gas concentrations, redox status, ambient pH, and ionic strength) are largely controlled by host rock mineralogy and, thus, the temperature and progress of serpentinization. Since iron partitioning between co-existing minerals in serpentinites is strongly linked to the evolution of hydrogen (McCollom and Bach, 2009), we report here on mineralogy, bulk geochemistry, and selected mineral chemistry data for the serpentinite cores in order to constrain the production of hydrogen, the dominant bioenergetic driving force in this land-based deep biosphere setting. Parallel microbiological analyses of the same drill cores have shown the predominance of putative hydrogen-oxidizing bacteria. Together these data seek to link the mineralogy of microbial habitats in the serpentinite subsurface to the communities they support, which may have important implication for the study of both active and ancient serpentinizing ecosystems.

Cardace, D.; Carnevale, D.; Schrenk, M. O.; Twing, K. I.; McCollom, T. M.; Hoehler, T. M.

2012-12-01

278

Petrography and mineralogy study of the ultramafic rocks in Separou peridotites, Nain Ophiolite, Central Iran  

NASA Astrophysics Data System (ADS)

The Nain ophiolite comprise of mantle and crustal sequences.The mantle sequence consist of lherzolite, harzburgite which mark mantle deformation , cut across by plagioclase-bearing harzburgite, wherlite and pyroxenite dikes and dunitic pods. The isolated diabasic dike cut all of the mantle sequence.The plutonic part of crustal sequence contain isotropic gabbro, diabase sheeted dike complex and pegmatite gabbro.The extrusive sequence comprise of pillow lavas and sheet flows, radiolarite, chert and pelagic limestone which have Upper Cretaceous microfunas. Base of the microscopic studies, most of harzburgite have granoblastic and porphyroclastic textures which confirm their mantle deformation. Olivine is partly serpentinized, chrysotile and lizardite and clinopyroxene is fresh but locally altered to tremolite-actinolite, chlorite and talc. The microprobe analysis of minerals indicate that the olivines mostly have forsterite composition (Fo93.76Fa6.23). The orthopyroxene have enstatite (En89.9Fs8.4Wo1.56), bear exsolution lamellas of clinopyroxene with diopside composition (En48.3Fs3.7Wo47.9). The Cr-spinel formed as subhedral to euhedral. The plagioclase-bearing harzburgitic dikes have cumulate texture, and consist of olivine with forsterite composition (Fo94Fa6), orthopyroxene with enstatite composition(En86.43Fs10.5Wo3.0.6). The plagioclase have anortite composition (AN=90). Dunite have granular to granoblastic texture and mostly composed of olivine with forsterite composition (Fo92.15Fa7.33). Pyroxenites have granular texture, and consist of orthopyroxene, clinopyroxene and very low serpentinized olivine. The clinopyroxene have diopside composition from (En50.6Fs7.2Wo42.1) to (En52.7Fs6.2Wo41.41) and orthopyroxene have enstatite composition from (En86.3Fs10.6Wo2.9) to (En89.05Fs7.9Wo2.9). The chondrite- normalized spider diagram of harzburgite show a depleted soure for Separou peridotite but the cross cutting peridotite with cumulative character are moderately enriched.

Foudazi, Mohammad; Alai Mahabadi, Soleiman

2010-05-01

279

Sulfur geochemistry of peridotite-hosted hydrothermal systems: Comparing the Ligurian ophiolites with oceanic serpentinites  

NASA Astrophysics Data System (ADS)

Serpentinization processes at slow- and ultraslow-spreading ridges control the exchange of various elements between seawater and the oceanic lithosphere and play a major role in marine geochemical cycles. We use opaque mineral assemblages and sulfur isotope geochemistry to reconstruct variations in fluid fluxes, redox conditions and microbial activity prevailing during serpentinization and carbonate precipitation of serpentinites and ophicalcites from an ophiolite sequence in the Northern Apennines (Italy). We then compare our results with calcite-veined serpentinites from the Iberian Margin formed during the opening of the North Atlantic, and with serpentinites sampled at the active peridotite-hosted Lost City hydrothermal field on the Atlantis Massif near the Mid-Atlantic Ridge. The sulfide and oxide mineral assemblage of the serpentinites from the Northern Apennines is dominated by pyrite, pentlandite, millerite, siegenite, magnetite, and hematite, reflecting relatively oxidizing conditions, while the sulfur geochemistry reveals a dominance of sulfide sulfur over sulfate sulfur. ?34S values of sulfide and sulfate indicate that microbial sulfate reduction, leaching, and oxidation are the main processes that affected the sulfur isotope signature of these serpentinites. The opaque mineralogy in the serpentinites from the Northern Apennines is similar to that of the Iberian Margin and the southern wall of the Atlantis Massif, and generally represents the late stages of serpentinization, where intense fluid circulation leads to fairly oxidizing conditions and alkaline fluids lead to carbonate precipitation. However, the mineral assemblages also indicate strong fluctuations in oxygen fugacity, likely caused by volume expansion during serpentinization and tectonic activity along a mid-ocean ridge. Additionally, local enrichment in 34S in sulfides suggests that fluids interacted at depth with gabbros and subsequently circulated along shear zones, similar to observations along detachment fault surfaces at the Atlantis Massif. Our results, therefore, give further evidence that the ophicalcites and serpentinites from the Northern Apennines formed in a tectonic setting similar to the Atlantis Massif. We estimate a global annual uptake of seawater sulfate of 5.6 to 12 × 1011 g S by serpentinization of ultramafic rocks exposed to seawater on the ocean floor. We suggest that the thermal structure of the exposed mantle rocks and, in turn, the efficiency of microbes to reduce seawater sulfate to sulfide is a major factor controlling the storage of sulfur in serpentinites. Thus, cycling of sulfur in peridotite-hosted hydrothermal systems during the Jurassic was probably very similar to the processes observed today in systems along the Mid-Atlantic Ridge.

Schwarzenbach, Esther M.; Früh-Green, Gretchen L.; Bernasconi, Stefano M.; Alt, Jeffrey C.; Shanks, Wayne C., III; Gaggero, Laura; Crispini, Laura

2012-08-01

280

Insight into the uppermost mantle section of a maturing arc: The Eastern Mirdita ophiolite, Albania  

NASA Astrophysics Data System (ADS)

We examined peridotite massifs in the eastern part of the Mirdita ophiolite (EMO), Albania, where arc-related magmas are abundant in the upper volcanic sequences. Structurally, clinopyroxene porphyroclast-bearing harzburgites (Cpx-harzburgite hereafter) occur in the lower parts of the peridotite massifs, whereas harzburgites and dunites are more abundant towards the upper parts. Dunite is commonly associated with chromitite layers. Orthopyroxenite occurs as dikes and/or networks at all structural levels, although it is more abundant in the uppermost sections. Orthopyroxenite commonly crosscuts the foliation of peridotites and the lithological boundaries between dunites (chromitite) and harzburgites, suggesting that it was formed in the late stage. Major and trace element compositions of minerals in the Cpx-harzburgites indicate that they were formed as the residue of less-flux partial melting, and are similar to those in abyssal peridotites from mid-ocean ridge systems. Harzburgites have more depleted major element compositions than the Cpx-harzburgites. Light rare earth element (LREE)-enrichment in clinopyroxene coupled with hydrous silicate mineral inclusions in spinels in harzburgites indicate that harzburgites were produced as a result of enhanced partial melting of depleted peridotites due to infiltration of hydrous LREE-enriched fluids/melts. Based on olivine and spinel chemistries, dunites are classified into two types: high-Cr# (= Cr/(Cr + Al) atomic ratio) spinel-bearing dunite and medium-Cr# spinel-bearing dunite. Orthopyroxenites formed at the expense of the pre-existing peridotite by reaction with hydrous orthopyroxene-saturated melts, which were produced by assimilation of dissolved pyroxene during the formation of the dunite. Refractory harzburgite, high-Cr# spinel-bearing dunite, and orthopyroxenite may have a genetic link to the late stage boninitic magmas in the crustal section of the EMO. In contrast, the Cpx-harzburgite was a residue related to mid-ocean ridge basalts (MORBs) or the "MORB-like" fore-arc basalt recently proposed by Reagan et al. (2010) from the Izu-Bonin-Mariana fore-arc. The medium-Cr# spinel-bearing dunite can be caused by interaction with a melt transitional between MORB-like and boninitic melts. The lithological variations and their relationships in the upper mantle section of the EMO were caused by changes in magmatic compositions from mid-ocean ridge signatures to boninitic magmas, due to an increasing contribution of slab-derived fluids in an island arc setting.

Morishita, Tomoaki; Dilek, Yildirim; Shallo, Minella; Tamura, Akihiro; Arai, Shoji

2011-06-01

281

The magmatic-hydrothermal transition in the lower oceanic crust: Clues from the Ligurian ophiolites, Italy  

NASA Astrophysics Data System (ADS)

The gabbroic bodies from the Jurassic Ligurian ophiolites are structurally and compositionally similar to the gabbroic sequences from the oceanic core complexes of the Mid Atlantic Ridge. Initial cooling of the Ligurian gabbros is associated with local development of hornblende-bearing felsic dykes and hornblende vein networks. The hornblende veining is correlated with the widespread development of hornblende as coronas/pseudomorphs after the clinopyroxene in the host gabbros. In addition, the studied gabbroic body includes a mantle sliver locally containing hornblende gabbros and hornblendite veins. The hornblendes from the felsic dykes and the hornblende-rich rocks within the mantle sliver show a similar geochemical signature, characterized by low Mg#, CaO and Al2O3, negligible Cl, and high TiO2, K2O, REE, Y, Zr and Nb concentrations. The whole-rock Sm-Nd isotopic compositions of the felsic dykes and the hornblende-rich rocks define a Sm-Nd isochron corresponding to an age of 154 ± 20 Ma and an initial ?Nd of 9.2 ± 0.5. The ?18O of the hornblendes and coexisting zircons from these rocks (about +4.5‰ and +5.8‰, respectively) do not indicate the presence of a seawater component in these melts. The formation of the felsic dykes and of the hornblende-rich rocks within the mantle sliver involved SiO2-rich silicate melts with negligible seawater component, which presumably were derived from high degree fractional crystallization of MOR-type basalts. The vein and the coronitic/pseudomorphic hornblendes show high Mg# and CaO, significant Cl (0.02-0.17 wt%) and low TiO2 and K2O concentrations. The coronitic/pseudomorphic hornblendes have trace element compositions similar to those of the clinopyroxenes from the gabbros and ?18O values (+1.0‰ to 0.7‰) close to seawater, suggesting an origin by reaction between migrating seawater-derived fluids and the host gabbros. The vein hornblendes commonly show slight LREE enrichment, relatively high concentrations of Nb (up to 2.5 ppm) and ?18O ranging from +3.7‰ to +0.8‰. The crystallization of these hornblendes most likely involved both seawater and magmatic components.

Tribuzio, Riccardo; Renna, Maria Rosaria; Dallai, Luigi; Zanetti, Alberto

2014-04-01

282

Jurassic back-arc and Cretaceous hot-spot series In the Armenian ophiolites — Implications for the obduction process  

NASA Astrophysics Data System (ADS)

The identification of a large OIB-type volcanic sequence on top of an obducted nappe in the Lesser Caucaus of Armenia helps us explain the obduction processes in the Caucasus region that are related to dramatic change in the global tectonics of the Tethyan region in the late Lower Cretaceous. The ophiolitic nappe preserves three distinct magmatic series, obducted in a single tectonic slice over the South Armenian Block during the Coniacian-Santonian (88-83 Ma), the same time as the Oman ophiolite. Similar geological, petrological, geochemical and age features for various Armenian ophiolitic massifs (Sevan, Stepanavan, and Vedi) argue for the presence of a single large obducted ophiolite unit. The ophiolite, shows evidence for a slow-spreading oceanic environment in Lower to Middle Jurassic. Serpentinites, gabbros and plagiogranites were exhumed by normal faults, and covered by radiolarites. Few pillow-lava flows have infilled the rift grabens. The ophiolite lavas have hybrid geochemical composition intermediate between Arc and MORB signatures: (La/Yb) N = 0.6-0.9; (Nb/Th) N = 0.17-0.57; ( 143Nd/ 144Nd) i = 0.51273-0.51291; ( 87Sr/ 86Sr) i = 0.70370-0.70565; ( 207Pb/ 204Pb) i = 15.4587-15.5411; ( 208Pb/ 204Pb) i = 37.4053-38.2336; ( 206Pb/ 204Pb) i = 17.9195-18.4594. These compositions suggest they were probably formed in a back-arc basin by melting of a shallow asthenosphere source contaminated by a deeper mantle source modified by subducted slab-derived products. 87Sr/ 86Sr ratios and petrological evidence show that these lavas have been intensely altered by mid-oceanic hydrothermalism as well as by serpentinites, which are interpreted as exhumed mantle peridotites. The gabbros have almost the same geochemical composition as related pillow-lavas: (La/Yb) N = 0.2-2.3; (Nb/Th) N = 0.1-2.8; ( 143Nd/ 144Nd) i = 0.51264-0.51276; ( 87Sr/ 86Sr) i = 0.70386-0.70557; ( 207Pb/ 204Pb) i = 15.4888-15.5391; ( 208Pb/ 204Pb) i = 37.2729-37.8713; ( 206Pb/ 204Pb) i = 17.6296-17.9683. Plagiogranites show major and trace element features similar to other Neo-Tethyan plagiogranites (La/Yb) N = 1.10-7.92; (Nb/Th) N = 0.10-0.94; but display a less radiogenic Nd isotopic composition than basalts [( 143Nd/ 144Nd) i = 0.51263] and more radiogenic ( 87Sr/ 86Sr) i ratios. This oceanic crust sequence is covered by variable thicknesses of unaltered pillowed OIB alkaline lavas emplaced in marine conditions. 40Ar/ 39Ar dating of a single-grain amphibole phenocryst provides a Lower Cretaceous age of 117.3 ± 0.9 Ma, which confirms a distinct formation age of the OIB lavas. The geochemical composition of these alkaline lavas is similar to plateau-lavas [(La/Yb) N = 6-14; (Nb/Th) N = 0.23-0.76; ( 143Nd/ 144Nd) i = 0.51262-0.51271; ( 87Sr/ 86Sr) i = 0.70338-0.70551; ( 207Pb/ 204Pb) i = 15.5439-15.6158; ( 208Pb/ 204Pb) i = 38.3724-39.3623; ( 206Pb/ 204Pb) i = 18.4024-19.6744]. They have significantly more radiogenic lead isotopic compositions than ophiolitic rocks, and fit the geochemical compositions of hot-spot derived lavas mixed with various proportions of oceanic mantle. In addition, this oceanic + plateau sequence is covered by Upper Cretaceous calc-alkaline lavas: (La/Yb) N = 2.07-2.31; (Nb/Th) N = 0.08-0.15; ( 144Nd/ 143Nd) i = 0.51271-0.51282; ( 87Sr/ 86Sr) i = 0.70452-0.70478), which were likely formed in a supra-subduction zone environment. During the late Lower to early Upper Cretaceous period, hot-spot related magmatism related to plateau events may have led to significant crustal thickening in various zones of the Middle-eastern Neotethys. These processes have likely hindered subduction of some of the hot and thickened oceanic crust segments, and allowed them to be obducted over small continental blocks such as the South Armenian Block.

Rolland, Yann; Galoyan, Ghazar; Bosch, Delphine; Sosson, Marc; Corsini, Michel; Fornari, Michel; Verati, Chrystèle

2009-10-01

283

Chemical heterogeneity in the upper mantle recorded by peridotites and chromitites from the Shetland Ophiolite Complex, Scotland  

NASA Astrophysics Data System (ADS)

The timing, causes and extent of mantle heterogeneity preserved in the ˜492 Ma Shetland Ophiolite Complex (Scotland) are evaluated using Re-Os isotope and whole rock highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance measurements of a suite of eight chromitites and 21 serpentinised harzburgites and dunites. Shetland dunites have more variable initial 187Os/188Os, as well as absolute and relative abundances of the HSE, compared to spatially associated harzburgites. As is common for ophiolitic peridotites, the harzburgites (?Os492Ma of -5.3 to +2.6) preserve evidence for a Mesoproterozoic depletion event, but are dominated by contemporary chondritic, ambient upper mantle compositions. The dunites have ?Os492Ma values ranging between -3.3 and +12.4, reflecting dunite formation by higher degrees of melt interaction with mantle rock than for the spatially associated harzburgites. Chromitite seams from three locations separated by <500 m have a large range in HSE concentrations (e.g., 0.09 to ˜2.9 ?g g-1 Os) with initial ?Os492Ma values ranging only from +0.48 to +3.95. Sulphides, arsenides and platinum-group minerals are the primary hosts for the HSE in the chromitites. Their isotopic variations reflect initial isotopic heterogeneity in their primary magmatic signatures. Coupled with field observations that support chromitite formation in concentrated zones of enhanced melt flow, the isotopic dichotomy between the harzburgites and the chromitites suggests that chromitite 187Os/188Os compositions may better approximate the upper limit, rather than an average value, of the bulk convecting upper mantle. The Shetland peridotite compositions reflect protracted melt depletion (low-Al2O3) and melt percolation events in a supra-subduction zone (SSZ) setting at ˜492 Ma, following an earlier (Mesoproterozoic) melt-depletion event. These results provide further evidence that ancient chemical complexities can be preserved in the upper mantle during ocean plate formation. Chromitites and peridotites from the Shetland Ophiolite Complex also attest to lithological and geochemical heterogeneities generated at scales of less than tens of metres during the formation of ancient oceanic lithosphere by high-degree SSZ melt extraction, percolation and during chromitite formation in the oceanic lithosphere.

O'Driscoll, Brian; Day, James M. D.; Walker, Richard J.; Daly, J. Stephen; McDonough, William F.; Piccoli, Philip M.

2012-06-01

284

Mineralogy of Surface Serpentinite Outcrops in the Coast Range Ophiolite: Implications for the Deep Biosphere and Astrobiology  

NASA Astrophysics Data System (ADS)

California contains a number of ultramafic (Fe- and Mg rich) rock bodies, including the Coast Range Ophiolite, a block of oceanic crust and upper mantle tectonically emplaced onto land. These ultramafic rocks are primarily composed of olivine and pyroxene, both of which are stable at the high temperatures and pressures in the deep subsurface where they crystallize but become unstable at low temperature and low pressure conditions near the surface. They are highly reduced rocks, creating chemical disequilibria, which can theoretically provide energy to chemoautotrophic organisms. Serpentinization (serpentine-forming) reactions between the rocks and water produce hydrogen molecules, which can be metabolized by diverse organisms. Earth and Mars have shown evidence of similar early geologic histories, possibly with widespread reducing habitable environments (Schulte et al., 2006). Recent data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) have shown serpentine-bearing outcrops near Nili Fossae (21 N, 282 W) and elsewhere in Mars' cratered highlands. Serpentine-bearing outcrops are rare, but their presence confirms that such systems involving the aqueous alteration of ultramafic rocks were active in the past (specifically during the Noachian epoch (older than ~3.7 billion years), possibly producing aqueous habitats suitable for chemoautotrophic life (Ehlmann et al., 2010). Remotely sensed data cannot confirm whether there is active serpentinization on Mars, however exposed, presently serpentinizing ultramafics in terrestrial ophiolites such as those of the California Coast Range provide points of comparison for similar Martian rocks. Volume expansion during serpentinization fractures the host rock, exposing new reaction surfaces, allowing further serpentinization. If subsurface liquid water is present on Mars, serpentinization may still be occurring. We will provide x-ray diffraction and petrographic data for surface serpentinites from the Coast Range Ophiolite, along with aerial-view maps, which will be compared with imagery and data for recently confirmed serpentinite exposures in the Nili Fossae region of the Martian surface. A summary table of terrestrial microbes (and their metabolisms) detected in serpentinite groundwaters will be provided, to add specificity to candidate subterranean life forms on Mars, be they active presently or in the planet's history. Ehlmann et al. 2010. GRL 37:1-5 Schulte et al. 2006. Astrobiology 6(2):364-376

Mccann, A. R.; Cardace, D.; Carnevale, D.; Ehlmann, B. L.

2011-12-01

285

Serpentinization and fluid-rock interaction in Jurassic mafic and ultramafic sea-floor: constraints from Ligurian ophiolite sequences  

NASA Astrophysics Data System (ADS)

The Bracco-Levanto ophiolitic complex (Eastern Liguria) represents one of the largest and better-exposed ophiolitic successions in the Northern Apennines. It is considered to be a fragment of heterogeneous Jurassic lithosphere that records tectono-magmatic and alteration histories similar to those documented along the Mid-Atlantic Ridge, such as at the 15°20'N area and the Atlantis Massif at 30°N. Structural and petrological studies on these rocks provide constraints on metamorphic/deformation processes during formation and hydrothermal alteration of the Jurassic oceanic lithosphere. We present a petrological and geochemical study of deformation processes and fluid-rock interaction in the Bracco-Levanto ophiolitic complex and compare these to modern oceanic hydrothermal systems, such as the Lost City Hydrothermal Field hosted in ultramafic rocks on the Atlantis Massif. A focus is on investigating mass transfer and fluid flow paths during high and low temperature hydrothermal activity, and on processes leading to hydrothermal carbonate precipitation and the formation of ophicalcites, which are characteristic of the Bracco-Levanto sequences. Major element and mineral compositional data allow us to distinguish a multiphase history of alteration characterized by: (1) widespread SiO2 metasomatism during progressive serpentinization, and (2) multiple phases of veining and carbonate precipitation associated with circulation of seawater and high fluid-rock ratios in the shallow ultramafic-dominated portions of the Jurassic seafloor. We observe regional variations in MgO, SiO2 and Al2O3, suggesting Si-flux towards stratigraphically higher units. In general, the ophicalcites have higher Si, Al and Fe concentrations and lower Mg than the serpentinite basement rocks or serpentinites with minimal carbonate veins. Bulk rock trace element data and Sr isotope ratios indicate seawater reacting with rocks of more mafic composition, then channeled towards stratigraphically higher units, leading to Si metasomatism in the serpentinites and ophicalcites. Channelling of Si-rich fluids is also indicated by amphibole and talc growth in shear zones and wall rock around the ophicalcites. ?18O-values of the carbonate veins indicate temperatures up to 150°C and document a decrease in temperature with ongoing serpentinization. Comparison with serpentinites from the Atlantis Massif and 15°20'N indicates a similar degree of Si enrichment in the modern seafloor and suggests that Si-metasomatism may be a fundamental process associated with serpentinization at slow-spreading ridge environments.

Vogel, Monica; Früh-Green, Gretchen L.; Boschi, Chiara; Schwarzenbach, Esther M.

2014-05-01

286

Fault-related oceanic serpentinization in the Troodos ophiolite, Cyprus: Implications for a fossil oceanic core complex  

NASA Astrophysics Data System (ADS)

Ultramafic rocks are exposed at the core of a domal structure near a fossil ridge-transform intersection (RTI) in the Troodos ophiolite. A sequence of heavily serpentinized rocks occurs adjacent to a major axis-parallel fault, the Amiandos Fault (AF), which juxtaposes them against gabbro. Previously, serpentinization and faulting were not considered to be related to the Cretaceous ocean spreading history of the Troodos ophiolite, but instead were interpreted as associated with late, emplacement-related tectonics and diapirism. Unusually high ?18O values previously measured in Troodos serpentine (Srp) supported this view. Here, petrographic and isotope tracers ( ?D, ?18O) of water-rock interaction are examined in three profiles across the AF to determine the spatial distribution, temperature, and the type of water involved in serpentinization in the Troodos RTI. Complete serpentinization, widespread chrysotile veining and very high ?18O (Srp), 10.6 to 12.6‰, characterize the rocks along a 2.5 km long profile at the central part of the serpentinite exposure. Much lower ?18O(Srp) values, 4.6 to 6.6‰, were measured at the AF fault zone in the highly serpentinized 300 m-long northern profile. Hydrogen isotope ratios also spatially vary: from - 70 to - 86‰ and - 57 to - 75‰ in the central and northern profiles, respectively. The bimodal distribution of isotope ratios calls for two distinct serpentinization events: localized 'oceanic-type' hydrothermal (100-200 °C) alteration initiated by deep infiltration of seawater during seafloor spreading followed by pervasive 'ophiolite-type' low-temperature hydration and veining along the AF. Superposition of the two serpentinization events is evident at a third, 2 km long southernmost profile, where ?18O (Srp) values decrease gradually from 10.7‰ 2 km away from the fault to 5.0‰ at the AF fault zone. Post-magmatic decrease of ?18O(plagioclase) in gabbros in the footwall of the AF and the apparent lack of amphibolitization suggest high-temperature, off-axis gabbro-water interaction and focused fluid flow to the lower crust through the AF zone. These observations suggest that the Amiandos fault was active during seafloor spreading, possibly operating as a detachment fault in a core complex structure, exhuming progressively deeper levels of the oceanic lithosphere. This scenario is supported by additional observations such as the proximity to RTI and association with highly-rotated blocks in the sheeted dikes.

Nuriel, Perach; Katzir, Yaron; Abelson, Meir; Valley, John W.; Matthews, Alan; Spicuzza, Michael J.; Ayalon, Avner

2009-05-01

287

Melanges Pedagogiques (Pedagogical Mixture), 1985.  

ERIC Educational Resources Information Center

The 1985 issue of the journal on second language teaching and learning contains four articles in French and three in English, including: "Lire et comprendre un texte informatif (Reading and understanding an Informative Text)" (Jacqueline Billant, Pascale Fade); "Les nouveaux diplomes de Francais Language Etrangere: Le D.E.L.F. et le D.A.L.F. (The…

Melanges Pedagogiques, 1985

1985-01-01

288

Melanges Pedagogiques (Pedagogical Mixture), 1983.  

ERIC Educational Resources Information Center

The 1983 issue of the journal on second language teaching and learning contains six articles in French. These include the following: "E.A.O.: Expression avec ordinateur (E.A.O.: Computer-Aided Expression)" (Daniele Abe, Michele Cembalo); "Ou suis-je? De la relation apprenant/environnement (Where Am I? On the Learner/Environment Relationship)"…

Melanges Pedagogiques, 1983

1983-01-01

289

Melanges Pedagogiques (Pedagogical Mixture), 1988.  

ERIC Educational Resources Information Center

The 1988 issue of the journal on second language teaching and learning contains six articles in French and two in English, including: "Production orale: Comment mettre en place des strategies d'enseignment/apprentissage (Oral Production: How To Put Teaching/Learning Strategies in Place)" (Francis Carton, Richard Duda); "Trois jours pour parler…

Melanges Pedagogiques, 1988

1988-01-01

290

Melanges Pedagogiques (Pedagogical Mixture), 1984.  

ERIC Educational Resources Information Center

The 1984 issue of the journal on second language teaching and learning contains five articles in French and two in English. These include the following: "Enseignement individuel vs. enseignement de masse (Individual Instruction vs. Large Group Instruction)" (Jacqueline Billant, Pascal Fade); "Systemes 'autonomisants' d'apprentissage des langues…

Melanges Pedagogiques, 1984

1984-01-01

291

An oceanic core complex (OCC) in the Albanian Dinarides? Preliminary paleomagnetic and structural results from the Mirdita Ophiolite (northern Albania)  

NASA Astrophysics Data System (ADS)

Oceanic core complexes (OCCs) are dome-shaped massifs commonly associated with the inside corners of the intersection of transform faults and slow (and ultra-slow) spreading centres. They represent the uplifted footwalls of large-slip oceanic detachment faults (e.g. Cann et al., 1997; Blackman et al., 1998) and are composed of mantle and lower crustal rocks exhumed during fault displacement (Smith et al., 2006, 2008). Recent paleomagnetic studies of core samples from OCCs in the Atlantic Ocean (Morris et al., 2009; MacLeod et al., in prep) have confirmed that footwall sections undergo substantial rotation around (sub-) horizontal axes. These studies, therefore, support “rolling hinge” models for the evolution of OCCs, whereby oceanic detachment faults initiate at a steep angle at depth and then “roll-over” to their present day low angle orientations during unroofing (Buck, 1988; Wernicke & Axen, 1988; Lavier et al., 1999). However, a fully integrated paleomagnetic and structural analysis of this process is hampered by the one-dimensional sampling provided by ocean drilling of OCC footwalls. Therefore, ancient analogues for OCCs in ophiolites are of great interest, as these potentially provide 3-D exposures of these important structures and hence a more complete understanding of footwall strain and kinematics (providing that emplacement-related phases of deformation can be accounted for). Recently, the relationship between outcropping crustal and upper mantle rocks led Tremblay et al. (2009) to propose that an OCC is preserved within the Mirdita ophiolite of the Albanian Dinarides (northern Albania). This is a slice of Jurassic oceanic lithosphere exposed along a N-S corridor which escaped the main late Cenozoic Alpine deformation (Robertson, 2002, 2004; Dilek et al., 2007). Though in the eastern portion of the Mirdita ophiolite a Penrose-type sequence is present, in the western portion mantle rocks are in tectonic contact with upper crustal lithologies. This main fault has been interpreted by Tremblay et al. (2009) as originally an oceanic detachment fault that exhumed mantle rocks and put them in contact with upper crustal basalts according to the “rolling-hinge” model. In order to test this model and document the kinematics of the proposed detachment fault, we carried out a preliminary paleomagnetic and structural sampling campaign in July 2010. The principal aims were: (i) to determine whether paleomagnetic remanences provide evidence for early relative rotation of footwall and hanging wall sequences either side of the proposed detachment that may be consistent with rolling-hinge models for OCCs; & (ii) to provide insights into the broader tectonic evolution of the Mirdita units. We collected c. 200 oriented samples at 32 localities distributed within a 30 km x 15 km area located between the Puka and Krabbi massifs, near the villages of Puka and Reps. Here we present the preliminary results of this study and discuss their geological implications for the history of the Mirdita ophiolite, including the interpretation of the Puka and Krabbi massifs as a fossil OCC and the primary orientation of the Mirdita spreading axis.

Maffione, M.; Morris, A.; Anderson, M.

2010-12-01

292

Compositional and mineralogic constraints on the genesis of ophiolite hosted nickel mineralization in the Pevkos area, Limassol Forest, Cyprus  

NASA Astrophysics Data System (ADS)

Mineralization composed dominantly of primary troilite, maucherite, pentlandite, and chalcopyrite, and secondary valleriite occurs in serpentinized transition zone rocks of the Limasol Forest segment of the Troodos ophiolite complex, Cyprus. Whole-rock and electron microprobe analyses of this mineralization gives ranges of Cu/(Cu+Ni)=0.16 to 0.47, Pt/(Pt+Pd)=0.66 to 0.51, Ni/Co=6.33 to 13.4, and chondrite normalized plots with low concentrations of Rh, Pt, and Pd, but relatively high Au. Estimated distribution coefficients of nickel and iron between olivine and ore range from 0.5 to 7.4. Most of these data are unlike values from magmatic sulfide deposits and indicate either a complete alteration of a preexisting magmatic sulfide concentration or, more likely, a nonmagmatic origin for this mineralization.

Foose, Michael P.; Economou, Maria; Panayiotou, A.

1985-10-01

293

A segment of oceanic crust, in Macedonian (FYROM) ophiolites, seen through sheeted dyke diabases, keratophyres and adakite-like volcanics  

NASA Astrophysics Data System (ADS)

Macedonian Eastern Vardar Ophiolitic unit represents a part of the Neotethys that was obducted onto the European Margin (Serbian Macedonian Massif). It comprises of well exposed sheeted dykes complex, with pillow lavas as a top unit, which is intruded by intermediate to acid dykes and sills of different geochemistry. We present new geochemical data for both of the units as well as an Ar-Ar age for acid rocks. Investigated samples can be clearly distinguished into the two groups based on geochemical features: i) Samples collected from the sheeted dykes and pillows, show SiO2ranging from 47-56%, relatively high TiO21.5-2.8%, and Al2O3 ranging from 12-15 %. In terms of trace element geochemistry, they show flat trace element patterns ((La/Yb)n = 0.5-2.4, (La/Sm)n =0.5-1.3 and (Sm/Yb)n = 0.8-1.2) consistent with MORB compositions; ii) Intermediate to felsic intrusions (keratophyres and adakite-like volcanic) show large range of SiO2and Al2O3 contents (45-73% and 16-13.5%, respectively), and are markedly depleted in TiO2, Nb and Ta, as well as enriched in LILE, like U, Th; they also show enriched REE patterns ((La/Yb)n = 3.56-14.79, (La/Sm)n =2.30-3.70 and (Sm/Yb)n =1.20-4.04) characteristic for arc volcanic rocks generated in a subduction zone environment. The most mafic members of this group demonstrate many features typical for adakites, like high Al2O3, extreme depletion in HREE with large extent of REE fractionation, high Sr concentrations and high Sr/Y, Sr/Yb, Dy/Yb ratios. Their adakitic features are also seen in trace element compositions of clinopyroxene (cpx) phenocrysts, having high Sr and Dy/Yb for high Mg#, comparable to cpx from Aleutians which is adakitic type locality ..........(Yogodzinski and Kelemen, 1998). On the other hand, the most evolved rocks from this group geochemically resemble upper Jurassic granitic rocks from the area: Fanos (Greece), Furka and Stip granites (FYRO-Macedonia) ..(...)..(Saric et al., 2009). Our Ar-Ar potassium feldspar dating gave 164 ± 0.15 Ma which is in agreement with K-Ar ages available for Macedonian granitic rocks (around 160 Ma). In our view, the rocks which occur within volcanic sequence of Macedonian (FYROM) ophiolites, represent two genetically different magmatic suites. The first one represents a volcanic member of oceanic crust of MORB composition, probably erupted in a fast-spreading environment. The second suite is younger and we interpret it as being produced by simple mixing or AFC, of the adakite-like primary melts and a felsic end-member similar to the melts which crystallized Jurassic granitic rocks from the area. The extent of granite involvement goes up to 45% for the most felsic samples from our intermediate suite. In our contribution, we will combine our geochemical data with available geological data, in order to review possible geodynamic scenarios which enable formation of Macedonian ophiolites. .Saric, K., Cvetkovic, V., Romer, R.L., Christofides, G. and Koroneos, A., 2009. Granitoids associated with East Vardar ophiolites (Serbia, F.Y.R. of Macedonia and northern Greece): Origin, evolution and geodynamic significance inferred from major and trace element data and Sr-Nd-Pb isotopes. Lithos, 108(1-4): 131-150. Yogodzinski, G.M. and Kelemen, P.B., 1998. Slab melting in the Aleutians: implications of an ion probe study of clinopyroxene in primitive adakite and basalt. Earth and Planetary Science Letters, 158(1-2): 53-65.

Bozovic, Milica; Prelevic, Dejan; Romer, Rolf L.; Barth, Matthias; Boev, Blazo; van den Bogaard, Paul

2010-05-01

294

Modeling a CO2 mineralization experiment of fractured peridotite from the Semail ophiolite/ Oman  

NASA Astrophysics Data System (ADS)

Most geologic CO2 sequestration technologies focus on sedimentary rocks, where the carbon dioxide is stored in a fluid phase. A possible alternative is to trap it as a mineral in the subsurface (in-situ) in basaltic or even (ultra)mafic rocks. Carbon dioxide in aqueous solution reacts with Mg-, Ca-, and Fe-bearing silicate minerals, precipitates as (MgCa,Fe)CO3 (carbonate), and can thus be permanently sequestered. The cation donors are silicate minerals such as olivine and pyroxene which are abundant in (ultra)mafic rocks, such as peridotite. Investigations are underway to evaluate the sequestration potential of the Semail Ophiolite in Oman, utilizing the large volumes of partially serpentinized peridotite that are present. Key factors are the rate of mineralization due to dissolution of the peridotite and precipitation of carbonate, the extent of the natural and hydraulic fracture network and the accessibility of the rock to reactive fluids. To quantify the influence of dissolution rates on the overall CO2 mineralization process, small, fractured peridotite samples were exposed to supercritical CO2 and water in laboratory experiments. The samples are cored from a large rock sample in the dimension of small cylinders with 1 cm in height and diameter, with a mass of ~2g. Several experimental conditions were tested with different equipment, from large volume autoclave to small volume cold seal vessel. The 650 ml autoclave contained 400-500g of water and a sample under 10 MPa of partial CO2 pressure up to 150. The small capsules in the cold seal vessel held 1-1.5g of water and the sample under CO2 partial pressure from 15MPa to 70 MPa and temperature from 60 to 200°C. The samples remained for two weeks in the reaction vessels. In addition, bench acid bath experiments in 150 ml vials were performed open to the atmosphere at 50-80°C and pH of ~3. The main observation was that the peridotite dissolved two orders of magnitude slower in the high pressure and temperature cell of the cold seal vessel than comparative experiments in large volume autoclaves and bench acid bath vials under lower and atmospheric pressure conditions. We attributed this observation to the limited water availability in the cold seal vessel, limiting the aqueous reaction of bi-carbonate formation and magnesite precipitation. To test this hypothesis, one of the cold seal vessel experiments at 20 MPa and 100°C was simulated with a reactive transport model, using TOUGHREACT. To simulate the actual experimental conditions, the model used a grid on mm and 100's of ?m scale and a fractured peridotite medium with serpentine filling the fractures. The simulation produced dissolution comparable to the experiment and showed an effective shut down of the bi-carbonation reaction within one day after the start of the experiment. If the conditions of limited water supply seen in our experiments are applicable in a field setting, we could expect dissolution may be limited by the buffering of the pH and shut down of the bi-carbonate formation. Under field conditions water and CO2 will only flow in hydraulic induced fractures and the natural fracture network that is filled with serpentine and some carbonate. The simulation result and potential implication for the field application will require further experimental investigation in the lab or field in the future.

Muller, Nadja; Zhang, Guoxiang; van Noort, Reinier; Spiers, Chris; Ten Grotenhuis, Saskia; Hoedeman, Gerco

2010-05-01

295

Heavy metal fractionation and pedogenesis in subalpine and alpine soils on ophiolitic materials, western Alps  

NASA Astrophysics Data System (ADS)

Soils on ultramafic materials are usually rich in Mg, Fe and heavy metals (particularly Ni, Cr, Mn, Co). These chemical properties could cause toxicity effects on biological communities. Metal fractionation shows the soil phases to which metals are associated (exchangeable, associated with organic matter, with amorphous or crystalline Fe or Mn oxides, in the crystal structure of primary minerals), and thus it is strictly related with metal mobilization and bioavailability. Ni, Cr, Mn, Co and Fe fractionations (6 fractions, analysed by a selective sequential extraction technique) were analysed in 6 subalpine and in 17 alpine soils (i.e., respectively under coniferous forest or above the present-day treeline) in the ophiolitic area of Mont Avic Natural Park (Valle d'Aosta, Italian Alps), on soils formed from metal-rich serpentinite or from metal-poor mafic rocks and calcschists. The results show a tight relationship between vegetation, soil forming processes, metal fractionation and bioavailability: below and above the present-day treeline soil forming processes and metal speciation change dramatically. Serpentinite soils are always extremely rich in metals, but metal speciation in analogous habitats is similar on every substrate. The results show a tight relationship between vegetation, soil forming processes, metal fractionation and bioavailability: below and above the present-day treeline soil forming processes and metal speciation change dramatically. Serpentinite soils are always extremely rich in metals, but metal speciation in analogous habitats is similar on every substrate. Under subalpine forest, the main pedogenic process is podzolization. In the extremely acidic and leached podzolic soils, all metals are mobilized and their lowest concentration is in the bleached E horizon, while there is a higher content in organic matter-rich surface horizon and in the spodic (illuvial) B. Not considering the amount associated with primary minerals (residual fraction), all the fractions of Ni, Co and Mn are strictly correlated with each other: in A and Bs horizons, the greatest amount is associated with organic matter and with crystalline Fe-oxides. The content in easily mobilizable forms associated with Mn and amorphous Fe oxides is only slightly lower. The greatest amount is in the residual fraction, as pedogenic forms are easily removed from the soil profiles by leaching; this is particularly evident in E horizons. Cr is less released by weathering, and the greatest fraction is associated with organic matter and amorphous Fe-oxides. No Cr could be detected associated with Mn oxides. The high mobility of metals in these soils increases their bioavailability. Above the treeline, the situation changes dramatically. Leaching is important only on stable, flat surfaces. Total and pedogenic fractions of Ni, Cr, Co and Fe increase from the bottom to the top of the soil profile, while all forms of Mn are strongly depleted in the upper horizons because of chemical reduction due to waterlogging at snowmelt. In fact, Mn is particularly sensitive to reduction processes. The most important factors involved in metal geochemistry are erosion and cryoturbation, which bring "fresh", metal-rich materials on the top of the profiles; weathering later releases the metals associated with pedogenic materials. The weak leaching due to limited acidification increases the concentration of potentially bioavailable metals (Fe, Co, Cr, and Ni) in the biologically active soil horizons. All metals are mobilized by waterlogging at snowmelt: extremely high contents of "labile" pedogenic forms of metals also in deep horizons of soils developed on metal-poor materials. However, the concentration due to the processes described above is stronger than leaching for Ni, Co, Fe and Cr.

D'Amico, M.; Previtali, F.

2009-04-01

296

Petrogenesis of Franciscan Complex and Coast Range Ophiolite Serpentinites in northern California  

NASA Astrophysics Data System (ADS)

Franciscan Complex serpentinites have been interpreted as eroded pieces of the overriding Coast Range Ophiolite (CRO), off-scraped pieces of the subducting oceanic plate, and as sedimentary serpentinites (e.g., Wakabayashi, 2004); however, most of these interpretations are based on tectonic models and field relationships. Here we present bulk rock major and trace element geochemistry, pyroxene and spinel geochemistry, and stable isotope data (O, H, Cl) for serpentinite samples with the goal of determining protolith origin and subsequent serpentinizing fluid sources of several metasomatized Franciscan and CRO ultramafic rocks in order to decipher the tectonic setting of serpentinization. We focused on serpentinite bodies found in the Franciscan Complex (west of Cuesta Ridge; south of San Francisco; Tiburon Peninsula; Healdsburg) (n = 12). Three samples from Cuesta Ridge (CRO) were also analyzed for comparison. All samples are >~95% serpentinized and consist of lizardite +/- chrysotile. Relict grains are rarely preserved. Franciscan serpentinites (Tiburon Peninsula, west of Cuesta Ridge) show positive-sloped REE patterns. This depletion in LREE is typical of abyssal peridotites. Relict clinopyroxenes from Tiburon Peninsula have high HREE concentrations, also supporting an abyssal origin. 2 of the 3 samples from the Cuesta Ridge show flat REE patterns; whereas, one is U-shaped. This enrichment in LREE is similar to forearc peridotites. Spinels from Cuesta Ridge have Cr# > 0.60 also implying a forearc setting; whereas, Franciscan localities have typically have lower Cr# (0.21 to 0.51). All samples show remarkable positive Ce and Y anomalies. We speculate that these anomalies may be due to interaction with ferromanganese nodules and crusts (also high in Ce and Y) on the seafloor prior to subduction. Cuesta Ridge samples have ?18O values between +6.0 to +6.6‰. Franciscan serpentinites (except those south of San Francisco) have ?18O values of +5.4 to +7.9‰. These ?18O values are similar to typical oceanic serpentinites and likely represent low-T seawater hydration on the seafloor. ?D values of all samples are extremely low (-107 to -90‰) and likely result from post-serpentinization, post-emplacement interaction with meteoric water at low temperature. Samples south of San Francisco lie on the San Andreas fault and have high ?18O values (+7.2 to +9.5‰) and low ?D values (-107 to -104‰) likely due to low-T interaction with meteoric water at high fluid-rock ratios. Most of the serpentinites (12 of the 15) have ?37Cl values between +0.2 and +0.9‰, typical values for serpentinites formed by interaction with seawater. Based on bulk rock geochemistry and pyroxene and spinel compositions, serpentinites located within the Franciscan Complex have geochemical characteristics of abyssal peridotites; whereas, those from Cuesta Ridge are more chemically heterogeneous with most having affinity to forearc peridotites. All stable isotope geochemistry indicates seafloor serpentinization by seawater. Wakabayashi, J., 2004, International Geology Review, 46, 1103-1118.

Eldam, R.; Barnes, J.; Lee, C.; Errico, J. C.; Loewy, S. L.; Cisneros, M.

2012-12-01

297

Tectonic mélanges and the exhumation of HP ophiolites: a case-study from the Ligurian Alps  

NASA Astrophysics Data System (ADS)

Mélanges form in a variety of geodynamic settings and can be related to either sedimentary, tectonic or diapiric processes, or a combination of them. We studied in detail a 100 m-scale tectonic mélange formed in the context of the alpine subduction/collision and we tested if the local-scale pattern could be applicable at larger scale in the Ligurian Western Alps. The studied mélange crops out inside metamorphic serpentinites belonging to the high-pressure (HP), meta-ophiolitic Voltri Massif (southern end of the Western Alps). It is made up of a foliated chlorite-actinolite greenschist matrix enclosing 10m-scale lenses of metabasites and metasediments. These blocks appear to be exotic because similar rocks do not outcrop in the surrounding HP-units. The matrix records three sets of superposed folds from blueschist to greenschist-facies conditions. The metabasite lenses preserve internal HP schistosities forming high angles with the greenschist matrix foliation. The lenses equilibrated at different peak metamorphic conditions (ranging from eclogite- to blueschist-facies). The matrix is widely retrogressed in greenschist facies, but it contains rare relics of Na-amphibole. Individual lenses display different segments of typical subduction PT paths which apparently converge in the blueschist facies. Moreover, geochronological data for the different HP blocks show that two undistinguishable blueschist samples display distinct peak ages of 43 and 40 Ma. One blueschist age is contemporaneous with the eclogitic equilibration of another block (43.2 ± 0.5 Ma) (Federico et al., 2007). The described structural, metamorphic and geochronological features suggest that this mélange formed at depth in a subduction channel and was active at least from blueschist- to greenschist-facies conditions, but possibly also at higher pressures. The subduction channel formed between the overriding and the subducting plates, as a consequence of progressive hydration of the mantle wedge by fluids released from the slab (e.g. Gerya et al., 2002). Here a forced flow inside serpentinites sampled different HP lenses from different depths and forced them to flow towards surface. The pattern of the studied mélange could be applicable at larger scale in the Ligurian Alps, where various HP units may be larger-scale equivalents of blocks, and may have been exhumed in a subduction channel inside serpentinites. Different units of the Voltri massif display metamorphic peaks at conditions ranging from eclogite- to blueschist-facies and available geochronological constraints reveal the heterogeneous timing of eclogite- facies metamorphic re-equilibrations (Federico et al., 2005; Rubatto & Scambelluri, 2003). However, pervasive retrograde - stage tectonics and greenschist imprint (Capponi & Crispini, 2002) mask structural relationships among units acquired at high pressure conditions. As a consequence, we discuss formation of the studied mélange and the feasibility of the subduction channel mode of exhumation at larger scale, in the Voltri massif, as already invoked for the Monviso Massif of the Western Alps (Guillot et al., 2004). Capponi, G., Crispini, L. - 2002 - Eclogae Geologicae Helvetiae, v. 95, 31-42. Federico, L., Capponi, G., Crispini, L., Scambelluri, M., & Villa, I.M. - 2005 - EPSL, 240, 668-680 Federico L., Crispini L., Scambelluri M. & Capponi G. - Geology, 35 (6), 499 - 502. Gerya, T.V., Stockhert, B. & Perchuk, A.L. - 2002 - Tectonics, 21(6), 1056. Guillot, S., Schwartz, S., Hattori, K., Auzende, A.L. & Lardeaux, J.M. - 2004 - in Beltrando, M., Lister, G., Ganne, J., and Boullier, A., eds., Evolution of the western Alps: Insights from metamorphism, structural geology, tectonics and geochronology, Journal of the Virtual Explorer, Electronic Edition, ISSN 1441-8142, v. 16, Paper 3. Rubatto D., Scambelluri M. - 2003 - Contrib. Mineral. Petrol. 146 (3), 341- 355.

Federico, Laura; Crispini, Laura; Scambelluri, Marco; Capponi, Giovanni; Malatesta, Cristina

2010-05-01

298

Paleomagnetism of Jurassic radiolarian chert above the Coast Range ophiolite at Stanley Mountain, California, and implications for its paleogeographic origins  

USGS Publications Warehouse

Upper Jurassic red tuffaceous chert above the Coast Range ophiolite at Stanley Mountain, California (lat 35??N, long 240??E), contains three components of remanent magnetization. The first component (A; removed by ???100-???200 ??C) has a direction near the present-day field for southern California and is probably a recently acquired thermoviscous magnetization. A second component (B; removed between ???100 and ???600 ??C) is identical to that observed by previous workers in samples of underlying pillow basalt and overlying terrigenous sedimentary rocks. This component has constant normal polarity and direction throughout the entire section, although these rocks were deposited during a mixed polarity interval of the geomagnetic field. The B magnetization, therefore, is inferred to be a secondary magnetization acquired during accretion, uplift, or Miocene volcanism prior to regional clockwise rotation. The highest temperature component (C; removed between ???480 and 680 ??C) is of dual polarity and is tentatively interpreted as a primary magnetization, although it fails a reversal test possibly due to contamination by B. Separation of the B and C components is best shown by samples with negative-inclination C directions, and a corrected mean direction using only these samples indicates an initial paleolatitude of 32??N ?? 8??. Paleobiogeographic models relating radiolarian faunal distribution patterns to paleolatitude have apparently been incorrectly calibrated using the overprint B component. Few other paleomagnetic data have been incorporated in these models, and faunal distribution patterns are poorly known and mostly unqualified. The available data, therefore, do not support formation of the Coast Range ophiolite at Stanley Mountain near the paleoequator or accretion at ???10??N paleolatitude, as has been previously suggested based on paleomagnetic data, but indicate deposition near expected paleolatitudes for North America (35??N ?? 4??) during Late Jurassic time.

Hagstrum, J. T.; Murchey, B. L.

1996-01-01

299

Petrogenesis of highly depleted peridotites and gabbroic rocks from the Mayarí-Baracoa Ophiolitic Belt (eastern Cuba)  

NASA Astrophysics Data System (ADS)

The Moa-Baracoa and Mayarí-Cristal massifs (eastern Cuba) are two ophiolitic complexes mainly constituted by harzburgite tectonites and minor dunites, cut by gabbroic dykes. The Moa-Baracoa massif exhibits a well developed Moho transition zone and an incomplete crustal section made up of layered gabbros and tectonically emplaced pillow basalts. A plutonic crustal section is absent in the Mayarí-Cristal massif and mantle tectonites are in tectonic contact with arc-related volcanic rocks. Mantle peridotites are very refractory in terms of modal composition, whole rock major element and HREE contents implying that Moa-Baracoa and Mayarí-Cristal harzburgites are residues after high degrees (20-30%) of partial melting. The relative enrichment of Th, Nb, Ta and LREE in peridotites is due to re-equilibration of melting residues with percolating melts. Peridotites lost on average 6 wt% of relative MgO by intense seafloor weathering. REE contents and Mg# of melts in equilibrium with cumulate gabbros from the Moho transition zone and crustal section of the Moa-Baracoa massif coincide with those of the spatially-related pillow basalts. On the other hand, no geochemical relation has been inferred between melt in equilibrium with Mayarí-Cristal segregate and the spatially-related arc volcanics. Our results indicate that the Mayarí-Baracoa Ophiolitic Belt formed at an original back-arc spreading centre. The Moa-Baracoa massif represents a portion of MORB-like lithosphere located nearby a back-arc mid-ocean spreading ridge, and the Mayarí-Cristal massif represents a piece of transitional (MORB to IAT) mantle located closer to the paleo-volcanic arc than Moa-Baracoa.

Marchesi, Claudio; Garrido, Carlos J.; Godard, Marguerite; Proenza, Joaquín A.; Gervilla, Fernando; Blanco-Moreno, Jesús

2006-06-01

300

Mineralogy and distribution of Platinum-Group Minerals (PGM) and other solid inclusions in the Faryab ophiolitic chromitites, Southern Iran  

NASA Astrophysics Data System (ADS)

High-Cr podiform chromitites hosted by upper mantle depleted harzburgite were investigated for PGM and other solid inclusions from Faryab ophiolitic complex, southern Iran. Chemical composition of the chromian spinels, Cr#[100*Cr/(Cr+Al) = 77-85], Mg# [100*Mg/(Mg+Fe2+) = 56-73], TiO2?0.25wt%, and the presence of abundant primary hydrosilicates included in the chromian spinels indicate that the deposits were formed from aqueous melt generated by high degree of partial melting in a suprasubduction zone setting. Solid phases hosted by chromian spinel grains from the Faryab ophiolitic chromitites can be divided into three categories: PGM, base-metal minerals and silicates. Most of the studied PGM occurred as very small (generally less than 20 ?m in size) primary single or composite inclusions of IPGE-bearing phases with or without silicates and base metal minerals. The PGM were divided into the three subgroups: sulfides, alloys and sulfarsenides. Spinel-olivine geothermometry gives the temperatures 1,131-1,177 °C for the formation of the studied chromitites. At those temperatures, fS2 values ranged from 10-3 to 10-1 and provided a suitable condition for Ru-rich laurite formation in equilibrium with Os-Ir alloys. Progressive crystallization of chromian spinel was accompanied by increase of fS2 in the melt. The formation of Os-rich laurite, erlichmanite and then sulfarsenides occurred by increase of fS2 and slight decrease in temperature of the milieu. The compositional and mineralogical determinations of PGM inclusions respect to their spatial distribution in chromian spinels show that the minerals regularly distributed within the chromitites, reflecting cryptic variation consistent with magmatic evolution during host chromian spinel crystallization.

Rajabzadeh, Mohammad Ali; Moosavinasab, Zohreh

2013-12-01

301

Life detection at a Mars analogue site of present-day serpentinization in the Tablelands Ophiolite of Newfoundland (Invited)  

NASA Astrophysics Data System (ADS)

The Tableland Ophiolite was created during the collision of Laurentia and Gondwana continents ca. 470 million years ago. Ultramafic mantle rocks, from the ancient sea bed that once separated these continents, were thrusted westward onto the old continental margin, which is now Western Newfoundland. Weathering due to recent glaciations has left large areas of ultramafic rock at the surface and created fissures for fluid flow. As a result serpentinization is occurring as fresh water penetrates the unaltered ultramafic rock. Serpentinization is of particular interest because, through hydration of ultramafic rock, this reaction produces H2 and the reducing conditions necessary for abiogenic hydrocarbon synthesis, while also producing conditions amenable for chemolithotrophic life. Therefore sites of active serpentinization can be the source of either abiogenic or biogenic organics, or both. Serpentinization is a suspected (past or present) source of (detected or putative) hydrocarbons on Mars, Titan and Europa, hence these astrobodies may be potentially habitable or once habitable environments. The Tablelands Ophiolite is an analogue site that is ideal for testing methods of life detection in an extreme environment of high pH and low microbial biomass characteristic of sites of serpentinization. Multiple ultrabasic reducing springs characteristic of present-day serpentinization have been identified and characterized based on their geochemistry and microbiology. Field-based instruments were deployed for the detection of microbial activity (ATP), microbial cell wall material, and mineralogy, in yet untested high pH and low biomass environment. In this talk I will give an overview of the in situ measurements of life detection and put these measurements in context of geochemistry, microbiology, carbon source and reaction pathways, and I will discuss what we have learned that will help us plan for future mission measurements.

Morrill, P. L.; Szponar, N.; Brazelton, W. J.; Woodruff, Q.; Schrenk, M. O.; Bower, D. M.; Steele, A.

2010-12-01

302

Abiotic methane flux from the Chimaera seep and Tekirova ophiolites (Turkey): Understanding gas exhalation from low temperature serpentinization and implications for Mars  

NASA Astrophysics Data System (ADS)

The emission of abiotic methane (CH 4) into the atmosphere from low temperature serpentinization in ophiolitic rocks is documented to date only in four countries, the Philippines, Oman, New Zealand, and Turkey. Serpentinization produces large amounts of hydrogen (H 2) which in theory may react with CO 2 or CO to form hydrocarbons (Fischer-Tropsch Type synthesis, FTT). Similar mechanisms have been invoked to explain the CH 4 detected on Mars, so that understanding flux and exhalation modality of ophiolitic gas on Earth may contribute to decipher the potential degassing on Mars. This work reports the first direct measurements of gas (CH 4, CO 2) flux ever done on onshore ophiolites with present-day serpentinization. We investigated the Tekirova ophiolites at Çirali, in Turkey, hosting the Chimaera seep, a system of gas vents issuing from fractures in a 5000 m 2 wide ophiolite outcrop. At this site at least 150-190 t of CH 4 is annually released into the atmosphere. The molecular and isotopic compositions of C 1-C 5 alkanes, CO 2, and N 2 combined with source rock maturity data and thermogenic gas formation modelling suggested a dominant abiotic component (~ 80-90%) mixed with thermogenic gas. Abiotic H 2-rich gas is likely formed at temperatures below 50 °C, suggested by the low deuterium/hydrogen isotopic ratio of H 2 (?D H2: - 720‰), consistent with the low geothermal gradient of the area. Abiotic gas synthesis must be very fast and effective in continuously producing an amount of gas equivalent to the long-lasting (> 2 millennia) emission of > 100 t CH 4 yr - 1 , otherwise pressurised gas accumulation must exist. Over the same ophiolitic formation, 3 km away from Chimaera, we detected an invisible microseepage of abiotic CH 4 with fluxes from 0.07 to 1 g m - 2 d - 1 . On Mars similar fluxes could be able to sustain the CH 4 plume apparently recognised in the Northern Summer 2003 (10 4 or 10 5 t yr - 1 ) over the wide olivine bedrock and outcrops of hydrated silicates in the Syrtis Major and Nili Fossae; just one seep like Chimaera or, more realistically, a weak, spatially sporadic microseepage, would be sufficient to maintain the atmospheric CH 4 level on Mars.

Etiope, Giuseppe; Schoell, Martin; Hosgörmez, Hakan

2011-10-01

303

Large scale obduction of preserved oceanic crust: linking the Lesser Caucasus and NE Anatolian ophiolites and implications for the formation of the Lesser Caucasus-Pontides Arc  

NASA Astrophysics Data System (ADS)

During the Mesozoic, the Southern margin of the Eurasian continent was involved in the closure of the Paleotethys and opening Neotethys Ocean. Later, from the Jurassic to the Eocene, subductions, obductions, micro-plate accretions, and finally continent-continent collision occurred between Eurasia and Arabia, and resulted in the closure of Neotethys. In the Lesser Caucasus and NE Anatolia three main domains are distinguished from South to North: (1) the South Armenian Block (SAB) and the Tauride-Anatolide Platform (TAP), Gondwanian-derived continental terranes; (2) scattered outcrops of ophiolite bodies, coming up against the Sevan-Akera and Ankara-Erzincan suture zones; and (3) the Eurasian plate, represented by the Eastern Pontides margin and the Somkheto-Karabagh Arc. The slivers of ophiolites are preserved non-metamorphic relics of the now disappeared Northern Neotethys oceanic domain overthrusting onto the continental South Armenian Block (SAB) as well as on the Tauride-Anatolide plateform from the north to the south. It is important to point out that the major part of this oceanic lithosphere disappeared by subduction under the Eurasian Margin to the north. In the Lesser Caucasus, works using geochemical whole-rock analyses, 40Ar/39Ar dating of basalts and gabbro amphiboles and paleontological dating have shown that the obducted oceanic domain originates from a back-arc setting formed throughout Middle Jurassic times. The comprehension of the geodynamic evolution of the Lesser Caucasus supports the presence of two north dipping subduction zones: (1) a subduction under the Eurasian margin and to the south by (2) an intra-oceanic subduction allowing the continental domain to subduct under the oceanic lithosphere, thus leading to ophiolite emplacement. To the West, the NE Anatolian ophiolites have been intensely studied with the aim to characterize the type of oceanic crust which they originated from. Geochemical analyses have shown similar rock types as in Armenia, Mid Ocean Ridge Basalt (MORB) to volcanic arc rocks and Intra-Plate Basalts (IPB). Lithostratigraphic comparisons have shown that the relations between the three units, well identified in the Lesser Caucasus, are similar to those found in NE Anatolia, including the emplacement of stratigraphically conform and discordant deposits. New field data has also shed light on an outcrop of low-grade metamorphic rocks of volcanic origin overthrusted by the ophiolites towards the south on the northern side of the Erzincan basin, along the North Anatolian Fault (NAF). We extend our model for the Lesser Caucasus to NE Anatolia and infer that the missing of the volcanic arc formed above the intra-plate subduction may be explained by its dragging under the obducting ophiolite with scaling by faulting and tectonic erosion. In this large scale model the blueschists of Stepanavan, the garnet amphibolites of Amasia and the metamorphic arc complex of Erzincan correspond to this missing volcanic arc. We propose that the ophiolites of these two zones originate from the same oceanic domain and were emplaced during the same obduction event. This reconstructed ophiolitic nappe represents a preserved non-metamorphic oceanic domain over-thrusting up to 200km of continental domain along more than 500km. Distal outcrops of this exceptional object were preserved from latter collision which was concentrated along the suture zones.

Hassig, Marc; Rolland, Yann; Sosson, Marc; Galoyan, Ghazar; Sahakyan, Lilit; Topuz, Gultelin; Farouk Çelik, Omer; Avagyan, Ara; Muller, Carla

2014-05-01

304

Mineral chemistry and petrology of highly magnesian ultramafic cumulates from the Sarve-Abad (Sawlava) ophiolites (Kurdistan, NW Iran): New evidence for boninitic magmatism in intra-oceanic fore-arc setting in the Neo-Tethys between Arabia and Iran  

NASA Astrophysics Data System (ADS)

The Sarve-Abad (Sawlava) ophiolitic complex consists of several tectonically dismembered ophiolitic sequences. They are located along the Main Zagros Thrust Zone, which marks the ophiolitic suture between the Arabian and Sanandaj-Sirjan continental blocks. They represent a portion of the southern Neo-Tethyan oceanic lithosphere, which originally existed between the Arabian (to the south) and Eurasian (to the north) continental margins. The Sarve-Abad ophiolites include cumulitic lherzolites bearing minor dunite and chromitite lenses in places. The main rock-forming minerals in ultramafic cumulates are cumulus olivine and inter-cumulus clinopyroxene and orthopyroxene. Minor (<5%) chromian spinel occurs as both cumulus and inter-cumulus phases.

Allahyari, Khalil; Saccani, Emilio; Rahimzadeh, Bahman; Zeda, Ottavia

2014-01-01

305

The 190Pt186Os Decay System Applied to Dating Platinum-Group Element Mineralization in Layered Intrusions, Ophiolites and Detrital Deposits  

Microsoft Academic Search

Discrete platinum-group minerals (PGM) occur as accessory phases in mafic-ultamafic intrusions and ophiolitic chromitites, as well as numerous detrital deposits globally. The 190Pt-186Os decay system, measured by laser ablation MC-ICPMS (LA-MC-ICPMS) provides a useful geochronometric tool for direct dating of PGM. Here we present two examples that verify the accuracy of the technique in geologically well constrained situations and demonstrate

J. A. Coggon; G. Nowell; G. Pearson; T. Oberthür; J. Lorand; F. Melcher; S. W. Parman

2010-01-01

306

Abiotic methane flux from the Chimaera seep and Tekirova ophiolites (Turkey): Understanding gas exhalation from low temperature serpentinization and implications for Mars  

Microsoft Academic Search

The emission of abiotic methane (CH4) into the atmosphere from low temperature serpentinization in ophiolitic rocks is documented to date only in four countries, the Philippines, Oman, New Zealand, and Turkey. Serpentinization produces large amounts of hydrogen (H2) which in theory may react with CO2 or CO to form hydrocarbons (Fischer–Tropsch Type synthesis, FTT). Similar mechanisms have been invoked to

Giuseppe Etiope; Martin Schoell; Hakan Hosgörmez

2011-01-01

307

Bedded jaspers of the Ordovician Løkken ophiolite, Norway: seafloor deposition and diagenetic maturation of hydrothermal plume-derived silica-iron gels  

Microsoft Academic Search

Sedimentary beds of jasper (red hematitic chert) in the Ordovician Løkken ophiolite of Norway are closely associated with volcanogenic massive sulphide (VMS) deposits. The jaspers occur in the immediate hangingwall and laterally peripheral to the large Løkken (25–30 Mt) and small Høydal (0.1 Mt) VMS deposits, and are exposed discontinuously for several kilometres along strike. Massive or laminated types predominate; jasper-sulphide debris-flow

Tor Grenne; John F. Slack

2002-01-01

308

Petrology, geochemistry and U-Pb geochronology of the Betic Ophiolites: Inferences for Pangaea break-up and birth of the westernmost Tethys Ocean  

NASA Astrophysics Data System (ADS)

The Betic Ophiolitic Unit, part of the Mulhacén Complex in the Betic Cordillera (SE Spain), is composed of several tectonic slices of eclogitized mafic and ultramafic rocks, together with oceanic metasediments. A comprehensive review of the petrological characteristics of the Betic Ophiolites shows notable analogies with other ophiolites from the Alps-Apennines system, indicating that they represent relics of oceanic lithosphere created at the westernmost end of the Western Tethys. U-Pb dating by SHRIMP (Sensitive High Resolution Ion MicroProbe) on zircon grains carefully investigated by cathodoluminescence images has been carried out on an eclogitized metagabbro (Lugros outcrop) and a metadolerite dyke intruded in ultramafic rocks (Almirez outcrop), giving consistent magmatic ages of around 185 ± 3 Ma. This age marks the beginning of the Betic oceanic magmatism (transitional to enriched MORB) that followed Pangaea break-up and the earliest oceanic accretion of the Western Tethys Ocean. Subsequently, ultraslow mid-ocean ridge spreading developed from the Betic oceanic basin northeastward to the Ligurian and Alpine Tethys domains, mainly around 165 to 140 Ma.

Puga, Encarnación; Fanning, Mark; Díaz de Federico, Antonio; Nieto, José Miguel; Beccaluva, Luigi; Bianchini, Gianluca; Díaz Puga, Miguel Angel

2011-06-01

309

Petrological studies of neoproterozoic serpentinized ultramafics of the nubian shield: spinel compositions as evidence of the tectonic evolution of egyptian ophiolites  

NASA Astrophysics Data System (ADS)

The mafic-ultramafic rocks of the Gabal El-Degheimi area, Central Eastern Desert of Egypt, are parts of an ophiolitic section. The ophiolitic rocks are dismembered and tectonically enclosed within, or thrust over, island arc assemblages. Serpentinites, altered slices of the upper mantle, represent a distinctive lithology of the dismembered ophiolites. Some portions of the serpentinized rocks contain fresh relicts of primary minerals such as chromian spinel and olivine. The abundance of bastite and mesh textures suggests harzburgite and dunite protoliths, respectively, for these serpentinites. Some fresh cores of chromian spinel are rimmed by ferritchromite and Cr-magnetite. The development of alteration rims around chromian spinel cores indicates their formation during prograde alteration and under oxidizing conditions during lower amphibolite facies metamorphism. Fresh chromian spinels are characterized by high contents of Cr2O3 (48.92-56.74 wt. %), Al2O3 (10.29-20.08wt. %), FeO (16.24-28.46 wt. %) and MgO (4.89-14.02 wt. %), and very low TiO2 contents (<0.16 wt. %). The analyzed fresh chromian spinels have high Cr# (0.62-0.79) characteristic of spinels in mantle peridotite that has undergone some degree of partial melting. The data presented here suggest that the mantle peridotites of the Gabal El-Degheimi area are similar to forearc peridotites of suprasubduction zone environments.

Azer, Mokhles K.

2014-03-01

310

Compositionally heterogeneous podiform chromitite in the Shetland Ophiolite Complex (Scotland): Implications for chromitite petrogenesis and late-stage alteration in the upper mantle portion of a supra-subduction zone ophiolite  

NASA Astrophysics Data System (ADS)

The mantle sequence of the ~ 492 Ma Shetland Ophiolite Complex (SOC; Scotland) contains abundant compositionally heterogeneous podiform chromitite bodies enclosed in elongate dunite lenses in the vicinity of the petrological Moho. Chromitite petrogenesis and late-stage alteration events recorded in these seams are examined here using petrography, mineral chemistry and crystal structural data. The resistant nature of Cr-spinel to serpentinisation and other late-stage alteration means that primary igneous compositions are preserved in unaltered crystal cores. Chromitite mineralogy and texture from five sampled localities at The Viels, Hagdale, Harold's Grave, Nikka Vord and Cliff reveal significant inter-pod chemical heterogeneity. The Cr-spinel mineral chemistry is consistent with supra-subduction zone melt extraction from the SOC peridotites. The occurrence of chromitite seams in the centres of the dunite lenses combined with variable Cr-spinel compositions at different chromitite seam localities supports a model of chromitite formation from spatially (and temporally?) fluctuating amounts of melt-rock interaction through channelised and/or porous melt flow. Pervasive serpentinisation of the SOC has led to the almost complete replacement of the primary (mantle) silicate mineral assemblages with serpentine (lizardite with minor chrysotile and antigorite). Magmatic sulphide (e.g., pentlandite) in dunite and chromitite is locally converted to reduced Ni-sulphide varieties (e.g., heazlewoodite and millerite). A post-serpentinisation (prograde) oxidisation event is recorded in the extensively altered Cliff chromitite seams in the west of the studied area, where chromitite Cr-spinel is extensively altered to ferritchromit. The ferritchromit may comprise > 50% of the volume of the Cliff Cr-spinels and contain appreciable quantities of 1-2 ?m inclusions of sperrylite (PtAs2) and Ni-arsenide, signifying the coeval formation of these minerals with ferritchromit at temperatures of up to ~ 500 °C. The SOC chromitite Cr-spinels thus not only preserve key insights into the complex melting processes occurring in the upper mantle wedge but can also be utilised to construct a comprehensive alteration history of the lower mantle portions of such supra-subduction zone ophiolites.

Derbyshire, E. J.; O'Driscoll, B.; Lenaz, D.; Gertisser, R.; Kronz, A.

2013-03-01

311

Evidence for the formation of boninitic melt in the base of the Salahi mantle section, the northern Oman ophiolite  

NASA Astrophysics Data System (ADS)

The boninites in the Oman ophiolite occur as lavas and dikes of the Alley volcanic sequence (Ishikawa et al., 2002). Moreover, Yamazaki and Miyashita (2008) reported about boninitic dike swarms in the Fizh crustal section. The boninitic melt generation requires hydrous melting of refractory mantle peridotite under an extremely high temperature and low pressure condition. This condition is generally explained by the addition of slab-derived fluids into a hot young oceanic lithosphere, which previously experienced MORB melt extraction. In this study, we report an ultramafic complex mainly composed of dunite which is in equilibrium with chemical composition of boninites in the southwestern part of the Salahi mantle section in the northern Oman ophiolite. Based on the study by Nomoto and Takazawa (2013) the complex consists mainly of massive dunite associated with minor amounts of harzburgite, pyroxenites and wehrlite. We use spinel Cr# (=Cr/[Cr+Al] atomic ratio) as an indicator of extent of melt extraction in harzburgites. For dunites spinel Cr# varies as a function of extent of reaction and of melt composition (Dick and Bullen, 1984; Arai, 1994; Ozawa, 2008). The spinels in the dunites from the complex have Cr# greater than 0.7 indicating highly refractory signature. The range of spinel Cr# is similar to those of spinels in boninites reported worldwide (Umino, 1986; van der Laan et al., 1992; Sobolev and Danyushevsky, 1994; Ishikawa et al., 2002). The complex might be a section of dunite channel that formed by flux melting of harzburgites as a result of infiltration of a voluminous fluid from the basal thrust. We determined the abundances of rare earth elements (REE) in the peridotite clinopyroxenes (cpxs) by LA-ICP-MS to estimate the compositions of the melts in equilibrium with these clinopyroxenes. The chondrite-normalized patterns for clinopyroxenes in the dunites are characterized by enrichments in light REE (LREE) relative to those of the harzburgite clinopyroxenes. The chondrite-normalized REE patterns for the calculated melts in equilibrium with clinopyroxenes in the dunites do not resemble to the pattern of N-MORB (Sun and McDonought, 1989) but fit very well to the patterns of the boninites (Cameron et al., 1983; Cameron, 1985; Taylor et al., 1994; Ishikawa et al., 2005). With increasing the spinel Cr# from harzburgite to dunite, the Yb content of clinopyroxenes decreases whereas the Ce content increases. Chondrite-normalized REE patterns of clinopyroxenes in dunites indicate that the dunites are not a residue of closed system melting but a product of open system melting with addition of a LREE-enriched fluid. Our results supports a hypothesis that the dunites formed as residue after flux melting of harzburgite accompanied with LREE-enriched fluid infiltrated from the base of the ophiolite. The pyroxenites frequently occur in the periphery of the voluminous dunite mass. These pyroxenites were considered as crystal cumulates from the boninitic melt.

Nomoto, Y.; Takazawa, E.

2013-12-01

312

Platinum-group element abundance and distribution in chromite deposits of the Acoje Block, Zambales Ophiolite Complex, Philippines  

USGS Publications Warehouse

Platinum-group elements (PGE) occur in ore-grade concentration in some of the chromite deposits related to the ultramafic section of the Acoje Block of the Zambales Ophiolite Complex. The deposits are of three types: Type 1 - associated with cumulate peridotites at the base of the crust; Type 2 - in dunite pods from the top 1 km of mantle harzburgite; and Type 3 - like Type 2, but in deeper levels of the harzburgite. Most of the deposites have chromite compositions that are high in Cr with Cr/(Cr + Al) (expressed as chromium index, Cr#) > 0.6; high-Al (Cr# Pd, thought to be characteristic of PGE-barren deposits) and positive slope (Ir < Pd, characteristic of PGE-rich deposits). Iridium, Ru and Os commonly occur as micron-size laurite (sulfide) inclusions in unfractured chromite. Laurite and native Os are also found as inclusions in interstitial sulfides. Platinum and Pd occur as alloy inclusions (and possibly as solid solution) in interstitial Ni-Cu sulfides and as tellurobismuthides in serpentine and altered sulfides. Variability of PGE distribution may be explained by alteration, crystal fractionation or partial melting processes. Alteration and metamorphism were ruled out, because PGE contents do not correlate with degree of serpentinization or the abundance and type (hydroxyl versus non-hydroxyl) of silicate inclusions in chromite. Preliminary Os isotopic data do not support crustal contamination as a source of the PGEs in the Acoje deposits. The anomalous PGE concentrations in Type 1 high-Cr chromite deposits are attributed to two stages of enrichment: an early enrichment of their mantle source from previous melting events and a later stage of sulfide segregation accompanying chromite crystallization. High-Al chromite deposits which crystallized from basalts derived from relatively low degrees of melting owe their low PGE content to partitioning of PGEs in sulfides and alloys that remain in the mantle. High-Cr deposits crystallized from melts that were previously enriched with PGEs during early melting events of their mantle source; Pt and Pd ore concentrations (ppm levels) are attained by segregation of magmatic sulfides. The Acoje deposits indicate that ophiolites are a potential economic source of the PGEs. ?? 1990.

Bacuta, Jr. , G. C.; Kay, R. W.; Gibbs, A. K.; Lipin, B. R.

1990-01-01

313

Intermediate-depth Fracturing of Oceanic Lithosphere in Subduction Zones: Memories from Exhumed High-Pressure Ophiolites  

NASA Astrophysics Data System (ADS)

Understanding processes acting along the subduction interface is crucial to assess lithospheric scale coupling between tectonic plates and mechanisms causing intermediate-depth seismicity. Despite a wealth of geophysical studies aimed at better characterizing/localizing this seismicity, we still critically lack constrains on processes triggering fracturing in regions (40-100km depths; T > 400°C) where deformation is expected to be achieved by plastic flow. We herein attempt to bridge this gap by providing a review of available evidence from brittle deformation patterns in exhumed High Pressure (HP) ophiolites, together with some new, critical observations. Field examples from various ophiolitic terranes (New-Caledonia, W. Alps, Tian Shan…) indicate that brittle deformation under HP conditions generally implies vein filling and precipitation of HP minerals, probably under very high pore fluid pressure conditions. Coalescence of such vein networks could explain some of the seismic events recorded along the fluid-rich subduction interface region. By contrast, HP pseudotachylites (though reported in only few localities so far) are apparently restricted to somehow deeper slab regions where fluid-deficient conditions are prevalent (Corsica, Zambia, Voltri?). The recent discovery of eclogite breccias, found as m-sized dismembered fragments within an eclogite-facies shear zone from the Monviso area (W. Alps), provides a new opportunity to study the genesis of intermediate-depth earthquakes. We herein argue that these eclogite breccias constitute unique remnants from an ancient fault zone associated with intraslab, intermediate-depth seismicity at ca. 80 km depth. The breccia is internally made of 1-10 cm-sized rotated fragments of eclogite mylonite cemented by an eclogite-facies matrix attesting of fracturing and fault sealing under lawsonite-eclogite facies conditions (550°C, 2.5 GPa) during subduction of the Tethyan seafloor. Textural observations and polyphased fracturing-healing events frozen in garnet zoning patterns indicate that brecciation was most likely seismic and was accompanied by the input of externally-derived fluids, thereby highlighting again the role of fluids. Although dehydration embrittlement or hydrofracturing are generally amongst the most preferred candidates for explaining intermediate-depth seismicity, this contribution stresses the need for even more detailed studies to better understand the formation of such events.

Angiboust, Samuel; Oncken, Onno; Agard, Philippe

2014-05-01

314

Garnet clinopyroxenite layers from the mantle sequences of the Northern Apennine ophiolites (Italy): Evidence for recycling of crustal material  

NASA Astrophysics Data System (ADS)

This study aims to define the origin of garnet clinopyroxenite layers from the mantle sequences of the External Ligurian ophiolites. These mantle sequences retain a subcontinental origin and were exposed at a Jurassic ocean-continent transition. The garnet clinopyroxenites are mafic rocks with Mg# values of 66-71. Their chondrite-normalised REE patterns are characterised by severe LREE depletion (CeN/SmN=0.1-0.2) and nearly flat (Type-A pyroxenites) to moderately enriched HREE (Type-B pyroxenites). In addition, Type-A pyroxenites display a small positive Eu anomaly. The whole-rock REE variations are paralleled by the garnet REE compositions. We attribute the major and trace element characteristics of the garnet clinopyroxenites to recycling of gabbroic protoliths that underwent partial melting under eclogite facies conditions. The garnet clinopyroxenites may represent variably evolved garnet+clinopyroxene cumulates formed by eclogite-derived melts. In an alternative hypothesis, Type-A and -B pyroxenites are residual rocks after eclogite melting and cumulates derived from the eclogite melts, respectively. The high pressure fractionation event that gave rise to the garnet clinopyroxenites is considered of Triassic age on the basis of Sm-Nd and Lu-Hf isotope correlations. The Nd-Hf isotopic compositions of the garnet clinopyroxenites in the Triassic (?Nd=+4.7 to +7.6, ?Hf=+4.4 to +12.8) lie below the mantle array, in agreement with recycled ancient MOR-type material. The oxygen isotopic composition of garnet and clinopyroxene from the garnet clinopyroxenites (?18O=+4.9‰ to +5.2‰) may be reconciled with subduction-related recycling of the lowermost oceanic crust, or delamination and foundering of underplated gabbros from the continental lithosphere. The potential involvement of the garnet clinopyroxenites in the melting processes that gave rise to the MOR-type oceanic crust in the Jurassic would account for the moderate Nd isotope variability and the garnet geochemical signature of the ophiolitic basalts.

Montanini, A.; Tribuzio, R.; Thirlwall, M.

2012-10-01

315

The exhumation of an HP ophiolitic massif (Voltri Massif, Western Alps): insight from 3D numerical models  

NASA Astrophysics Data System (ADS)

The high-pressure ophiolitic Voltri Massif outcrops at the eastern sector of the Ligurian Western Alps. Highly deformed serpentinite and metasediments wrap eclogite to blueschist facies metagabbro and metabasalt bodies; these lenses have foliated rims and preserve undeformed textures in their cores revealing a strong strain-partitioning. The mechanism that drove the exhumation of these high-pressure rocks has been already analyzed in detail using 2D numerical simulations (Malatesta et al., 2011). In particular they reproduce the subduction process that in the Mesozoic affected the Alpine branch of the Western Tethys. In the Ligurian area subduction was intraoceanic and involved a non-layered oceanic lithosphere with gabbro as discrete bodies inside serpentinized peridotites that were overlain by a limited basaltic cover. The comparison of field and petrologic evidence with model results showed that the exhumation of the high-pressure Voltri rocks was related to the formation of a serpentinite channel above the downgoing slab. This low-viscosity area formed after the hydration of the mantle-wedge rocks by the uprising fluids that migrate from the slab. Buoyancy of the high-pressure serpentinitic mélange that included slices of the slab finally triggered their exhumation. Alpine subduction was however oblique (Marroni and Treves, 1998; Malusà et al., 2011) thus including a trench-parallel left-lateral motion and not only a trench-normal motion as in 2D simulations. We have studied this particular setting through 3D numerical simulations starting from the setup of 2D models. We therefore designed an oceanic basin (500 km-wide) surrounded by continental margins and floored by a non-layered oceanic lithosphere. Subduction starts at a prescribed weak zone in the mantle; the weak zone defines the plate margins geometry. We test different "lateral" geometries of the weak zone (e.g. continuous, segmented). We designed "continuous" weak zones either parallel or increasingly moving away from the continental margins. Moreover, we tested the effect on subduction/exhumation dynamics of varying values of the trench-parallel component of convergence-rate vector. The comparison among field and petrologic data of the Voltri Massif rocks with 3D numerical models results will finally provide a more detailed description of the subduction dynamics acting in the Ligurian-Piedmontese basin and in particular will shed more light on the mechanism that drove the exhumation of the high-pressure ophiolitic Voltri Massif.

Malatesta, C.; Gerya, T.; Crispini, L.; Federico, L.; Scambelluri, M.; Capponi, G.

2012-04-01

316

Geotectonic significance of Neoproterozoic amphibolites from the Central Eastern Desert of Egypt: A possible dismembered sub-ophiolitic metamorphic sole  

NASA Astrophysics Data System (ADS)

Supra-subduction zone ophiolites in the Egyptian Central Eastern Desert (CED) occur as clusters in its northern (NCEDO) and southern (SCEDO) parts, displaying abundant island arc-boninitic and MORB/island-arc geochemical affinities, respectively. An amphibolite belt, including the investigated massive to slightly foliated Wadi Um Gheig (WUG) amphibolites, is exposed in the southeast most of the NCEDO thrusting over the El Sibai gneissic association and intruded by late- to post-orogenic granitoids and gabbros. The WUG rocks are metamorphosed under epidote amphibolite to common amphibolite facies. The amphiboles are calcic and represented by actinolitic hornblende to magnesio-hornblende in the epidote amphibolites and magnesio- to ferro-hornblende in the amphibolites. Plagioclase composition varies from pure albite (An 3-8) in the epidote amphibolites to andesine and labradorite (An 36-65) in the amphibolites. The estimated P-T conditions are in favor of their metamorphism under epidote amphibolite (c. 550-600 °C and 2-3 ± 1.5 kbar) and amphibolite (c. 618-720 °C and 3-6 ± 1.5 kbar) facies. The peak metamorphic conditions point to a burial depth of c.15-20 km. Geochemically, the WUG amphibolites show basaltic to andesitic compositions of tholeiitic affinity. They display LILE-enriched MORB-normalized patterns with negative Nb anomalies characteristic of the subduction-related rocks. However, their chondrite-normalized rare-earth element (REE) patterns vary from LREE-depleted (La N/Yb N = 0.29 to 0.49) to LREE-enriched (La N/Yb N = 2.97 to 3.74). Few samples show major and trace element contents typical of boninitic rocks, including U-shaped REE pattern. On the standard tectonic discrimination diagrams the WUG amphibolites plot mostly in the island-arc fields with some samples of MORB and boninitic affinities. Greenschist facies metamorphosed NCEDO obviously share these geochemical characteristics, implying formation in the same tectonic environment, i.e. forearc basin. This argues that the WUG amphibolites likely represent remnants of a dismembered metamorphic sole beneath the NCEDO. Their formation possibly involves intra-forearc basin thrusting followed by emplacement of ophiolite as imbricated stack of dismembered thrust slices in an accretionary wedge setting. This revives interests in geotectonic model in which the CED represents a forearc-arc-back-arc system above a southeast-dipping subduction zone.

Farahat, E. S.

2011-07-01

317

Different Scales of os Isotopic Heterogeneity in Ophiolite Chromitites from Sagua de TÁNAMO and MAYARÍ Mining Districts (eastern Cuba)  

NASA Astrophysics Data System (ADS)

We performed in situ laser ablation MC-ICP-MS measurements of Os isotopes in platinum-group minerals (PGM) included in unaltered chromite from ophiolite chromitites of the Sagua de Tánamo mining district (eastern Cuba). The results reveal important heterogeneities at the km, hand sample and thin section scales. Initial 187Os/188Os (calculated at 90Ma, the estimated age of ophiolite formation) spans from 0.1185 to 0.1295 in the whole district. These values correspond to γOs = -8.1-0.4, calculated by comparison with the Os isotopic evolution of the primitive upper mantle (PUM; Meisel et al., 2001, GCA 65), and all but one PGMs have γOs lower than PUM. PGMs in a single hand sample from the Caridad Mine exhibit 187Os/188Os ratios from 0.1185 to 0.1274, which overlap almost the entire range of values measured in the Sagua de Tánamo district. In one thin section from the same mine 187Os/188Os varies between 0.1200 and 0.1263 in two PGMs that are only few millimetres from each other. The few analyzed PGM grains from the Mayarí district have 187Os/188Os = 0.1271-0.1272 (γOs = -1.4) that are generally higher than in Sagua de Tánamo and much more homogeneous. The sub-PUM (i.e. negative) initial γOs values can be explained by Re depletion during a long history of partial melting starting at 1.61 Ga, as indicated by calculated Os model ages. However, the heterogeneous isotopic signature of PGMs in a single hand sample and thin section suggests a more complex magmatic scenario for the formation of PGMs and the host chromite. At such small scales, the formation of PGMs with variable Os isotopic signatures requires a heterogeneous genetic environment where melts with different Os isotopic compositions coexist in space and/or time. This scenario can be achieved during chromite crystallization by mixing in mantle conduits of primitive and differentiated melts. Each new batch of primitive melt (with its own Os isotopic signature inherited from a highly heterogeneous mantle source) mixes turbulently with the pooled, differentiated melt promoting the crystallization of chromite and, as a consequence, the formation of PGMs attached at grain boundaries of chromite. Once PGMs become completely trapped in chromite, they retain their Os isotopic signature because the host chromite prevents any exchange with incoming melts. Thus PGM formed from successive batches of isotopically different melts would be the responsible of the heterogeneity observed at different length scales in the chromitites (Gervilla et al., 2005, CMP 150; Frei et al., 2006, EPSL 241).

Gervilla, F.; Marchesi, C.; González-Jiménez, J. M.; Proenza, J. A.; Garrido, C. J.; Griffin, W. L.; O'Really, S.; Pearson, N. J.

2009-04-01

318

Origin and geodynamic significance of metabasic rocks from the Antrona ophiolites (Western Alps): new insights from SHRIMP-dating  

NASA Astrophysics Data System (ADS)

Subduction/collision and exhumation of continental and oceanic fragments between the European and Adriatic plates resulted in highly complex nappe structures in the Alpine chain. Geotectonic segments bearing metabasic rocks are key domains for unraveling the pre-metamorphic paleogeography. Metabasic rocks in the Alps commonly represent remnants of the Piemont-Ligurian (PL) and the Valais oceanic domains. We dated by SHRIMP (GSC, Ottawa and ANU, Canberra) zircon domains from two metabasites of the Western Alps, areas of Quarata (S of Domodossola) and at Guggilihorn (SE of Simplon). The samples belong to the Antrona ophiolite unit, which was attributed to the Valais domain, based on structural/geological arguments. The Quarata amphibolite (metagabbro) contains exclusively magmatic zircons with weak oscillatory zoning in cathodoluminescence. They show no metamorphic domains, probably due to the relatively low-T and/or limited amounts of fluids during metamorphism. 7 analysed spots yield a weighted mean (crystallisation) age at 155.6 ± 2.1 Ma. The Simplon metagabbro contains magmatic, oscillatory-zoned zircons that also yield a crystallisation age at 155.0 ± 2.0 Ma (10 spots). No age of metamorphism could be obtained from the relatively thin metamorphic domains. The ca. 155 Ma age obtained for both metabasites attributed to the Valais domain is within the time span accepted for the formation of Piemont-Ligurian oceanic crust and older than assumed for Valaisan oceanic crust. This apparent contradiction can be resolved if we invoke Cretaceous opening of the Valais ocean oblique to the NW margin of the Piemont-Ligurian ocean (as suggested earlier by Stampfli 1993). Re-rifting of PL oceanic crust, together with the eastward movement of Iberia caused older PL oceanic crust to be locally positioned to the N of the newly formed Brianconnais peninsula within the younger Valais domain. This scenario can explain the presence of oceanic crust of PL age in a position occupied by the Valais domain. This view is also supported by a 38.5 ± 0.7 Ma age of an eclogite at Passo del Mottone (Antrona ophiolites), attributed to subduction metamorphism in the Valais domain. New data on the protolith age of this eclogite are in agreement with the ca. 155 Ma protolith ages obtained for the Quarata and Simplon metabasites. The age of subduction for the Valais domain can thus be clearly differentiated from that of the PL domain (ca. 44--45 Ma) and that of the European margin (ca. 35 Ma). Stampfli, G., Eclogae Geol. Helv. 86: 1--45 (1993).

Liati, A.; Gebauer, D.; Froitzheim, N.; Fanning, M.

2003-04-01

319

U/Pb, Sm/Nd and Rb/Sr geochronological and isotopic study of northern Sierra Nevada ophiolitic assemblages, California  

NASA Astrophysics Data System (ADS)

Distinct ophiolitic assemblages occur as oceanic basement within three of the four regional tectonic belts of the northern Sierra Nevada. New U/Pb zircon, Sm/Nd and Rb/Sr data are presented for each assemblage, providing critical geochronological and isotopic constraints on the petrogenesis and tectonic evolution of the ophiolitic and associated ensimatic assemblages. Ophiolitic assemblages include from west to east the Smartville complex, Central belt and Feather River belt. The Smartville complex represents an island arc volcanic-plutonic sequence with a major late-stage sheeted dike swarm. The Sm/Nd systems from a wide compositional spectrum of rocks record a 178±21 Ma petrogenetic age and an ? Nd(T)=+9.2±0.6. Zircon U/Pb systems on an uppermost dacite yield a 164±2 Ma age, and on a number of plagiogranite screens and dikes from the sheeted complex 162±1 Ma ages. The Central and Feather River belts are structurally complex polygenetic assemblages. The U/Pb zircon and Sm/Nd systems record major ˜205 Ma and ˜315 Ma petrogenetic events respectively both involving depleted mantle derived magmas. Such magmatism probably occurred in marginal basin/transform systems developed within an older oceanic depleted mantle basement regime. Both Sm/Nd and U/Pb zircon systems show local components of Proterozoic sialic material. The sialic contaminants were probably introduced into the system as craton derived detritus. It is doubtful that any of the ophiolitic assemblages studied represent genetically related crust-upper mantle sequences generated during the development of new oceanic lithosphere. Integration of the geochronological data with geological relations reveals a pattern of petrogenesis and tectonics whereby progressively younger ensimatic terranes were added to the continental margin through time by plate convergence, and were ultimately welded into North American sial by a crosscutting batholithic belt. This accretionary pattern is reflected in both the protolith ages and deformation-metamorphic ages of each of the regional belts which progressively young westward. Crustal components of the accreted ensimatic terranes grew by mainly basaltic igneous activity within island arc, marginal basin and leaky transform systems adjacent to the continent edge prior to final tectonic accretion. Such complexities are suggested to be typical of Cordilleran-type ophiolites and representative of the circum-Pacific erogenic style.

Saleeby, J. B.; Shaw, H. F.; Niemeyer, Sidney; Moores, E. M.; Edelman, S. H.

1989-06-01

320

Direction and intensity of the geomagnetic field in the Middle Devonian and Lower Ordovician: southern Mugodjary ophiolites (Urals)  

NASA Astrophysics Data System (ADS)

A considerable quantity of petromagnetic and palaeomagnetic measurements in the field has allowed us to obtain reliable palaeomagnetic results for such difficult and complex objects as ophiolites. Eifelian baked sediments, diabases (dyke contacts) and gabbros, and Tremadocian baked sediments were studied. Continuous thermocleaning, conglomerate and fold tests were applied. The palaeointensity was investigated by means of several palaeomagnetic methods: Wilson-Burakov, van Zijl-Shaw and Bagina-Petrova. The mean intensities of the geomagnetic fields are 7 and 16.5 ?T, with mean palaeomagnetic pole positions of 9°N, 134°E and 6°S, 83°E, respectively. These data demonstrate that southern Urals (Mugodgary) was located near the northern margin of the East European plate during Ordovician-Devonian time. The allochthonous crustal oceanic plate was thrust upon the edge of the plate and deformed as late as the Permian. Changes in the polarity of the geomagnetic field are recorded in the succession of sheeted dykes, a direct corroboration of the Vine-Mathews hypothesis.

Didenko, A. N.; Pechersky, D. M.

1989-12-01

321

Olivine fabric evolution in a hydrated ductile shear zone at the Moho Transition Zone, Oman Ophiolite (Invited)  

NASA Astrophysics Data System (ADS)

The Fizh massif, Oman ophiolite, contains a ductile shear zone at the Moho Transition Zone. The dunites in the shear zone are classified based on microstructures into coarse granular texture, medium-grained texture, mylonite, and ultramylonites toward a gabbro contact. The average grain size of olivine decreases toward the shear zone, which contains a zone of high strain ( 15 m wide). The proportion of hydrous minerals (amphibole and chlorite) in the shear zone show an increase toward the gabbro contact, suggesting that water infiltrated the shear zone from the gabbro contact. Equilibrium temperatures indicate a higher deformation temperature ( 900 degree C) outside of the high strain zone compared with inside this zone ( 750 degree C). Under these geochemical and temperature conditions, the temporal evolution of olivine crystal-preferred orientations (CPO) indicates the following continuous deformation scenario. First, deformation by dislocation creep under higher temperatures resulted in slip by D-type {0kl}[100] and then weak E-type (001)[100] slip. Next, deformation by dislocation creep under lower temperatures and higher stress conditions produced a C-type (100)[001] CPO. Finally, superplastic deformation by grain boundary sliding resulted in a random CPO.

Michibayashi, K.

2013-12-01

322

Petrology and oxygen isotope geochemistry of a fossil seawater hydrothermal system within the Solea graben, northern Troodos ophiolite, Cyprus  

NASA Astrophysics Data System (ADS)

Hydrothermal mineral zonations and O isotope patterns of the northern Troodos complex do not parallel the ophiolite pseudostratigraphy, but reflect the convective geometry of an Upper Cretaceous seawater hydrothermal system. Large areas of the sheeted intrusive complex (SIC), including the subaxial region of the Solea graben, are composed of 18O-rich, subgreenschist mineral assemblages and may represent regions of diffuse seawater recharge. Other areas of the SIC are recrystallized to distinctive epidosite rocks: granular, high-variance assemblages of epidote + quartz ± chlorite that are depleted in 18O, Al2O3, Na2O, K2O, Zr, Cu, and Zn and are enriched in CaO and Sr compared with other mafic volcanic and dike rocks of the Solea graben. Epidosite alteration occurred at temperatures of ˜310-370°C and involved fluids with ?18O values and salinities similar to those of Upper Cretaceous seawater. The epidosite zones are discordant with earlier, mineral/O isotope zonations and with the axis of spreading in the Solea graben, suggesting a postspreading, off-axis origin. The seawater hydrothermal system responsible for Solea graben massive sulfide deposits was probably driven by hypabyssal intrusions (not exposed), emplaced in a terminal, failed spreading episode. The geometries of O isotope surfaces within the Solea graben imply that the epidosites formed in fossil upflow and deep recharge conduits. Depletions in base metals show that epidosite alteration liberated Cu and Zn to mineralizing fluids within the fossil upflow zone.

Schiffman, Peter; Smith, Brian M.

1988-05-01

323

Relation between ore-forming hydrothermal systems and extensional deformation in the Solea graben spreading center, Troodos ophiolite, Cyprus  

NASA Astrophysics Data System (ADS)

Field relations indicate that high-temperature hydrothermal circulation and accumulation of massive sulfide deposits within the Solea graben of the Troodos ophiolite, Cyprus, followed extreme crustal attenuation. Zones of pervasive, massive epidosite strike parallel to the axis of the Solea graben and to the strike of extensional normal faults. Initial fluid flow, evidenced by preferential epidotization in weakly altered areas surrounding massively altered regions, was focused along joints, microfractures, and (now) low-angle normal-fault zones related to graben formation. Permeability within the sheeted-dike section was enhanced by brittle deformation related to extensional structures as well as through volume reduction inherent in the diabase to epidosite mineralogic phase transformations. Intrusion of high-level gabbros into epidosite zones occurred both before and after significant amagmatic tectonic extension. Structural control on epidotization suggests that intrusion of late stocks into attenuated and highly deformed crust is necessary to drive the vigorous hydrothermal circulation that produced the epidosites and ore bodies of the Solea graben. A similar sequence of events is more likely to occur in the modern oceans along ridge crests with ephemeral magmatism, especially at intermediate- to slow-spreading ridges near transform faults.

Bettison-Varga, Lori; Varga, Robert J.; Schiffman, Peter

1992-11-01

324

Significance of E. Paleozoic Paleo-Tethyan Ophiolites in the Balkan Terrane and the Greater Caucasus for the Cadomian-Hercynian Continental Growth of Southern Europe  

NASA Astrophysics Data System (ADS)

New geochronological & geochemical data from dismembered E. Paleozoic ophiolites in the Balkan Terrane (Serbia) & the Greater Caucasus (Russian Federation) link these remnants of the Paleo-Tethyan oceanic crust to a series of events associated with the Cadomian and Hercynian evolution of S. Europe. The Balkan-South Carpathian ophiolite belt (BSCOB) WSW of the Moesian platform is part of the South European suture zone (SESZ) & includes the Deli Jovan massif (DJM) composed of ultramafic mantle restites, ultrabasic & basic cumulates, gabbros, diabasic dikes and basalts. The analyzed gabbros of DJM are high-Al (19-24.5 wt.%) cumulate gabbros-troctolites (Ol82-89+Cpx79-87+Pl77-91) originated from shallow-depth crystallization of low-K (<0.3 wt.%) tholeiitic basaltic melt (T=1050°-1160°). Dike rocks are composed of fine-grained, Cpx-Pl microphyric, high-Mg tholeiitic microgabbro-diabase showing MORB affinities [(La/Sm)n=0.18-1.31; (La/Yb)n=0.22-1.35; Th/La=0.073-0.19; Zr/Y=1.60-2.39] derived from depleted mantle source(s). New isotopic ages range between 406±24 Ma and 399.7±5.2 Ma [Sm-Nd isochron age of 406±24 Ma, MSWD=0.71; U-Pb isotope dilution zircon age of 399.7±5.2 Ma; U-Pb SHRIMP zircon age of 405.0±2.6 Ma]. Ophiolitic rocks of the BSCOB continue E into the Paleozoic metamorphic basement of the Istanbul-Zonguldak Unit (IZU) in NW Turkey & the Crystalline Core and the Fore Range Zones of the Greater Caucasus. The metaophiolites in the Greater Caucasus include residual peridotites, ultrabasic & basic cumulates, gabbros, dike complexes, and volcanic rocks with limestones, ranging in age from 490 to 416 Ma. The volcanic sequences in these ophiolites include low-silica (SiO2=43.94-48.10 wt.%), high-Ti (TiO2=2.0-3.3 wt.%) and high phosphorus tholeiitic basalts showing slightly enriched T-MORB affinities [(La/Sm)n=2.98±0.57; (La/Yb)n=5.72±2.05; (Tb/Yb)n=1.42±0.62; and (Yb)MORB=0.97±0.35], and high-silica (SiO2=47.54-53.94 wt.%), low-Ti (TiO2=0.46-1.09 wt.%) and low-K tholeiitic basalts, basaltic andesites & andesites, showing suprasubduction zone affinities [(Tb/Yb)n=1.04±0.34; (Yb)MORB=0.62±0.28; and negative Nb and Zr anomalies]. The Early Paleozoic ophiolitic rocks within the SESZ along the southern edge of the European Platform (including the Moesian and Scythian platforms) developed during the evolution of the Paleo-Tethyan back-arc basin via seafloor spreading and arc-trench-subduction rollback processes behind the northward drifting peri-Gondwanan terranes (i.e. Transcaucasian massif in Georgia; IZU, Menderes and Armutlu massifs in Turkey; Florina terrane in Greece). They were accreted to the Eurasian continental margin during the Hercynian orogeny. The Cadomian peri- Gondwanan terranes & the SESZ ophiolites subsequently became part of the basement of a Late Paleozoic magmatic arc above a Paleo-Tethyan subduction zone along the southern margin of Europe.

Zakariadze, G. S.; Karamata, S. O.; Dilek, Y.; Three Others, A.

2007-12-01

325

Mineralogy, composition and PGM of chromitites from Pefki, Pindos ophiolite complex (NW Greece): evidence for progressively elevated fAs conditions in the upper mantle sequence  

NASA Astrophysics Data System (ADS)

The Pindos ophiolite complex, located in the northwestern part of continental Greece, hosts various chromite deposits of both metallurgical (high-Cr) and refractory (high-Al) type. The Pefki chromitites are banded and sub-concordant to the surrounding serpentinized dunites. The Cr# [Cr/(Cr + Al)] of magnesiochromite varies between 0.75 and 0.79. The total PGE grade ranges from 105.9 up to 300.0 ppb. IPGE are higher than PPGE, typical of mantle hosted ophiolitic chromitites. The PGM assemblage in chromitites comprises anduoite, ruarsite, laurite, irarsite, sperrylite, hollingworthite, Os-Ru-Ir alloys including osmium and rutheniridosmine, Ru-bearing oxides, braggite, paolovite, platarsite, cooperite, vysotskite, and palladodymite. Iridarsenite and omeiite were also observed as exsolutions in other PGM. Rare electrum and native Ag are recovered in concentrates. This PGM assemblage is of great petrogenetic importance because it is significantly different from that commonly observed in podiform mantle-hosted and banded crustal-hosted ophiolitic chromitites. PGE chalcogenides of As and S are primary, and possibly crystallized directly from a progressively enriched in As boninitic melt before or during magnesiochromite precipitation. The presence of Ru-bearing oxides implies simultaneous desulfurization and dearsenication processes. Chemically zoned laurite and composite paolovite-electrum intergrowths are indicative of the relatively high mobility of certain PGE at low temperatures under locally oxidizing conditions. The PGM assemblage and chemistry, in conjunction with geological and petrologic data of the studied chromitites, indicate that it is characteristic of chromitites found within or close to the petrologic Moho. Furthermore, the strikingly different PGM assemblages between the high-Cr chromitites within the Pindos massif is suggestive of non-homogeneous group of ores.

Kapsiotis, Argirios; Grammatikopoulos, Tassos A.; Tsikouras, Basilios; Hatzipanagiotou, Konstantin; Zaccarini, Federica; Garuti, Giorgio

2011-01-01

326