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Sample records for ophiolitic melange nellore-khammam

  1. The Riviere des Plante ophiolitic Melange; tectonic setting and melange formation in the Quebec Appalachians

    SciTech Connect

    Cousineau, P.A. )

    1991-01-01

    The Riviere des Plante ophiolitic Melange (RPOM) is the largest and best exposed of the three known ophiolitic melanges that contain blocks of Chain Lakes Massif (CLM). All three lie along the Baie Verte-Brompton line, which marks the suture between the continental rocks of the Humber zone and the oceanic rocks of the Dunnage zone. The ophiolitic melange is composed of: serpentinized ultramafic rocks, some of which are sheared and/or carbonatized; blocks of amphibolitized gabbro; basalt; volcanogenic breccia; and conglomerates. It also contains continental K-rich granitoid rocks and high-grade metamorphic (upper amphibolite facies) rocks. The RPOM is part of the Saint-Daniel Melange, an accretionary prism onto which the RPOM has been tectonically emplaced. The CLM was part of a terrane accreted to the Laurentian margin during the Taconian orogeny. Blocks of the CLM along the Baie Verte-Brompton line are interpreted as fragments of this terrane caught within the suture zone. It is proposed that the CLM could be the equivalent of Grenville-derived greywacke originally laid down during the phase of continental rifting that led to the formation of the Iapetus Ocean and was later tectonized and metamorphosed during the Taconian and Acadian orogenies. The RPOM would represent the relic of a serpentinite diapir that rose within a deep oceanic fault. The presence of continental rocks like the CLM suggest that a continental magmatic arc was put in contact with an oceanic crust along this fault.

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

    NASA Astrophysics Data System (ADS)

    Sen, K.; Mukherjee, B. K.

    2012-12-01

    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.

  3. Application of imaging spectrometer data to the Kings-Kaweah ophiolite melange

    NASA Technical Reports Server (NTRS)

    Mustard, John F.; Pieters, Carle M.

    1988-01-01

    The Kings-Kaweah ophiolite melange in east-central California is thought to be an obducted oceanic fracture zone and provides the rare opportunity to examine in detail the complex nature of this type of terrain. It is anticipated that the distribution and abundance of components in the melange can be used to determine the relative importance of geologic processes responsible for the formation of fracture zone crust. Laboratory reflectance spectra of field samples indicate that the melange components have distinct, diagnostic absorptions at visible to near-infrared wavelengths. The spatial and spectral resolution of AVIRIS is ideally suited for addressing important scientific questions concerning the Kings-Kaweah ophiolite melange and fracture zones in general.

  4. Ophiolites and melange terranes in Iran: A geochronological study and its paleotectonic implications

    NASA Astrophysics Data System (ADS)

    Delaloye, Michel; Desmons, Jacqueline

    1980-09-01

    In Iran, units mapped as "coloured melange zones" consist of either melange terranes, ophiolite complexes, or ultramafic—mafic units, which are not complete ophiolitic sequences. These zones represent block margins that were active in Alpine times. Crystallization of the ophiolitic rocks, their emplacement at continental margins and the mixing of various rocks in melange are post-Paleozoic events that were considered from stratigraphical data to be pre-Upper Cretaceous to Cenozoic in age. Thirteen whole-rock and 30 mineral K-Ar ages were obtained from the ophiolitic complexes and from igneous and metamorphic rocks in the melange zones. Ophiolite crystallization, ocean-floor metamorphism and blueschist metamorphism in the south Baluchestan rocks appear to be of Campanian age. Ocean crust generation occurred at the same time in the Zagros Belt, but is perhaps slightly older in the East Iranian Ranges (north Baluchestan). Middle-grade metamorphic alteration of the exotic rocks of south Baluchestan and south Iran occurred at some time between (?) Late Jurassic to Late Cretaceous times (south Iran) and in the Campanian (south Baluchestan). There is evidence for an Early Eocene low-grade metamorphism in Baluchestan, perhaps of the same age as that in the Zagros Belt. Alkaline to calc-alkaline magmatism in the melange domain of north Baluchestan is Middle—Late Eocene, and Aquitanian. The formation of the melange, stratigraphically of Upper Cretaceous age in the Zagros Belt, may have occurred at the Cretaceous-Paleocene boundary in Baluchestan and continued possibly into post-Eocene times.

  5. West Point Melange, remnants of a Lower Paleozoic ophiolitic, eclogite-bearing melange in the Southern Appalachians, Alabama, Georgia, and North Carolina

    SciTech Connect

    Crawford, R.F. III; Higgins, M.W.; Crawford, T.

    1985-01-01

    The lower Paleozoic West Point melange in the West Point thrust sheet is locally preserved beneath and commonly folded with ocean-floor Ropes Creek Metabasalt of the Ropes Creek thrust sheet in the crystalline terrane of Alabama, Georgia, and North Carolina. The melange overlies a calc-alkaline island arc assemblage known as the Paulding volcanic-plutonic complex in the Paulding thrust sheet. The matrix of the melange is generally highly sheared talc-actinolite schist and (or) highly deformed amphibolite, and less commonly sheared scaly pelitic schist. Most clasts in the melange are of a wide variety of mafic and ultramafic rocks, including dunite, coronite troctolite, olivine gabbro, wehrlite, cortlandite, pyroxenite, and olivine-pyroxenite. Eclogite clasts are found at several localities in northern Georgia, southern North Carolina, and in Alabama. The mostly mafic ophiolitic nature of the West Point melange, the clasts of eclogite, and its location between an island-arc assemblage and ocean-ridge basalts suggests that it is the remnants of a subduction melange formed between the mid-Iapetus ridge and the oceanic Paulding island arc. The melange also forms part of the Hillabee Greenstone in Alabama and the Lake Chatuge sill and Shooting Creek complex in northern Georgia and southern North Carolina.

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

    NASA Astrophysics Data System (ADS)

    Gülmez, Fatma; Genç, Can; Prelevic, Dejan

    2014-05-01

    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.

  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)

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

    2007-12-01

    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.

  8. Ophiolite emplacement by collision between the Sula Platform and the Sulawesi Island Arc, Indonesia

    NASA Astrophysics Data System (ADS)

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

    1983-11-01

    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 geophysical expeditions to the SW Molucca Sea and the NW Banda Sea, plus gravity and geology on the east arms of Sulawesi. The Batui thrust separates the ophiolite from sedimentary rocks deformed along the leading edge of the Sula platform. We mapped this thrust eastward from Sulawesi along the southern margin of the Gorontalo basin. The latter is floored by oceanic crust, and its south edge is uplifted against the thrust. Thus the Sulawesi ophiolite can be traced offshore to its origin as basement of the Gorontalo basin. The ophiolite is composed of harzburgite in the Southeast Arm and passes upward through a complex of gabbros and diabase dikes in the East Arm. Ophiolite melange underlies the harzburgites on the Southeast Arm beneath low-angle thrust contacts where seen. Our local observations show the melange to be composed of thrust packets of both serpentine and red shale matrix varieties. The packets are several hundred meters thick, and the melange, where studied, has a moderate north to northeast dipping foliation. This orientation, if regionally representative of the melange fabric, is consistent with a significant northward component of movement of the lower plate, probably the Sula platform or its margin. Where the ophiolite is in contact with rocks of the central schist belt, it dips under the schist, but where it encounters melange, or Mesozoic or Paleogene sedimentary rocks, the ophiolite is thrust over them. The tectonic overlap sequence, from west to east, is schist over ophiolite over older sediments or melange. The ophiolite appears to have been emplaced by oblique convergence of the Sula platform along the southern edge of the Gorontalo basin. We suggest that the Gorontalo basin represents a forearc basin and the ophiolite is its basement, analogous to a number of other forearc settings. Deformation of the ophiolite may have occurred in part on the seafloor prior to emplacement, but we feel that much of the deformation occurred during emplacement. The Sulawesi ophiolite is only one of a number of ophiolites in the Indonesian region, each of which has a very different origin and tectonic history.

  9. Diverse sources for igneous blocks in Franciscan melanges, California Coast Ranges

    SciTech Connect

    MacPherson, G.J. ); Phipps, S.P. ); Grossman, J.N. )

    1990-11-01

    Igneous blocks in Franciscan melanges are of three chemical-petrologic types: (1) tholeiitic basalts of both arc and spreading center origin, with depletions in light relative to heavy rare-earth elements, 3% > TiO{sub 2} > 1%, high Y/Zr and Y/Ti ratios, and relict augites that generally have low Al and Ti and well-defined iron-enrichment trends; (2) basalts of probable seamount origin with marked enrichments in light relative to heavy rare-earth elements, 5% > TiO{sub 2} > 1%, lower Y/Zr and Y/Ti than (1), and Ti-Al-rich augites showing little if any iron-enrichment trends; and (3) hypabyssal intrusives having SiO{sub 2} > 52%, TiO{sub 2} < 1%, flat or only slightly fractionated rare-earth-abundance patterns, and diopsidic augites that are very low in Ti and Al and show no iron-enrichment trends. All of the blocks are metamorphosed; most are undeformed pumpellyite-bearing greenstones, and a few contain sodic amphibole {plus minus} lawsonite {plus minus} sodic pyroxene. The melanges are probably olistostromal in origin, deriving their igneous block detritus both from the downgoing Pacific plate (ocean floor basalts and seamounts) and from the hanging wall of the Franciscan trench (basalts and arc-related silic intrusive rocks). The silicic intrusive rocks and some of the basalts are eroded fragments of the fore-arc crust that ultimately become the Coast Range Ophiolite. These fragments were incorporated into the Franciscan trench fill and subducted. Results suggest that the igneous blocks in ophiolitic melanges provide important information about melange formation and about the tectonics and paleogeography of the regions in which the melanges are found.

  10. Overview of ophiolites and related units in the Late Palaeozoic-Early Cenozoic magmatic and tectonic development of Tethys in the northern part of the Balkan region

    NASA Astrophysics Data System (ADS)

    Robertson, Alastair; Karamata, Stevan; Šari?, Kristina

    2009-03-01

    The northern Balkan Peninsula, including Serbia, Montenegro, Bosnia, Croatia and the Former Yugoslavian Republic of Macedonia, represents an excellent region for the study of tectonic processes related to Mesozoic Tethyan ophiolite genesis and emplacement. We first summarise the main tectonic units of the northern Balkan Peninsula and then use this information to discuss tectonic processes, including rifting, sea-floor spreading, ophiolite genesis and emplacement, melange accretion, ocean-basin closure and collision. We then discuss alternative models of ophiolite genesis and emplacement for the region and suggest that multi-ocean-basin interpretations fit the data better than single-ocean-basin interpretations. Rifting of Adria (Gondwana) during the Triassic created a rift in the south (Budva zone) and opened a Triassic oceanic basin further north (Dinaride ocean). Occurrences of inferred sub-continental mantle lithosphere in the Dinaride ophiolite belt (e.g. Zlatibor) may record extensional exhumation within an ocean-continent transition zone bordering the Adria/Dinaride continent. This was followed by emplacement together with ophiolites and melange during Upper Jurassic-Early Cretaceous time. Upper Triassic radiolarites and mid-ocean ridge-type basalts formed at a spreading ridge after continental break-up. The oceanic lithosphere of the Dinaride ophiolite belt was partly generated above a subduction zone. The metamorphic soles of the Dinaride ophiolites formed during Mid-Late Jurassic mainly based on K/Ar dating. Widespread melange that is associated with the ophiolites represents a subduction complex, controlled by tectonic accretion and sedimentary reworking in trench and fore-arc basin settings. A possible cause of Jurassic Dinaride ophiolite emplacement was collision of a subduction trench with a continental margin. Further north, Mesozoic oceanic lithosphere subducted northeastwards (present coordinates) opening a Late Jurassic marginal basin in the Main Vardar zone. The Dinaride ocean in the south closed during Late Jurassic-Early Cretaceous time (Tithonian-Berriasian). Deformed oceanic crust, melange and magmatic arc rocks further north (Main Vardar zone) were transgressed by mainly clastic sediments during the Early Cretaceous. However, part of the Vardar ocean (Vardar zone western belt, or Sava zone) remained partially open until latest Cretaceous time. Generally northward subduction within this remnant ocean triggered further supra-subduction zone ophiolite genesis during the Late Cretaceous. The ocean closed by the Maastrichtian, followed by Early Cenozoic regional-scale southward thrusting that locally intercalated older and younger Mesozoic ophiolites and melanges. Future progress particularly depends on determining the crystallisation ages of the ophiolites, obtaining better structural data on the direction of initial ophiolite emplacement and unravelling the Palaeozoic tectonic development of the Eurasian continental margin.

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

    SciTech Connect

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

    1985-01-01

    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.

  12. Ophiolites and oceanic crust

    USGS Publications Warehouse

    Moores, E.M.; Jackson, E.D.

    1974-01-01

    OPHIOLITES consist of a pseudostratiform sequence, of harzburgite, tectonite, ultramafic and mafic cumulates sometimes including gabbro and quartz diorite (plagiogranite) intrusions, dolerite dyke swarms, pillow lava 1, and deep-sea sediments2-4. This assemblage occurs in all Phanerozoic mountain systems and is interpreted as fossil oceanic crust and uppermost mantle5-10. Outstanding problems include differences between the chemical properties of Ophiolites and rocks thought to represent present-day oceanic crust11,12, the lack in some complexes of recognised dyke swarms or cumulates, and the relative thinness of ophiolite mafic rocks compared with standard oceanic crustal sections5,8,13. ?? 1974 Nature Publishing Group.

  13. Macon melange, an ancient subduction melange complex in the southern Appalachians, AL-NC

    SciTech Connect

    Higgins, M.W.; Crawford, R.F. III; Atkins, R.L.; Crawford, T.J.

    1985-01-01

    The Macon Melange is a subduction melange complex comparable in size with the Franciscan melange of California and Oregon. The melange is a structural, sedimentary, and metamorphic chaos, in which clasts of vastly different lithologies, sizes, metamorphic grades, and degrees of roundness exist side by side in a matrix that locally appears to be at only garnet grade. It is interpreted to have formed between a trench and the Little River island arc at the oceanward edge of an African continent during late Precambrian-Middle Cambrian time. The Macon melange is divisible into (structurally lowest to highest) the Po Biddy, Juliette, and Potato Creek slices. The Po Biddy slice is composed of metavolcanic rocks, manganiferous and graphitic rocks (pelagic sediments), laminated pyritic, graphitic, and chloritic marble (pelagic carbonate), and a wide variety of mineral deposits. The Potato Creek slice is composed of metamorphosed pebbly mudstones, semischists with thin tuffaceous metacherts, and semischists with thin broken metagraywacke beds, all with numerous clasts of metagabbro, amphibolite, garnetiferous metadiabase, metavolcanic rocks, and ultramafic rocks. The Juliette slice is identical with the Potato Creek slice except that it is packed with mafic and ultramafic clasts of all sizes that are probably Iapetus Ocean crust and mantle scraped off the downgoing slab and imbricated into the melange wedge. Younger mafic plutons have intruded the melange.

  14. The Kanuti ophiolite, Alaska

    USGS Publications Warehouse

    Loney, R.A.; Himmelberg, G.R.

    1989-01-01

    The Kanuti ophiolite is a mafic-ultramafic thrust sheet of probable Jurassic age, formerly considered to be the upper part of the Yukon-Koyukuk ophiolite (Angayucham terrane). It is here called the Kanuti ophiolite after the Kanuti River region on the southeastern flank of the Yukon-Koyukuk Basin. The thrust sheet crops out discontinuously for a distance of more than 900 km along the northern and southeastern margins of the basin. The Kanuti consistently overlies another extensive thrust sheet, consisting mostly of pillow basalt and radiolarian chert of Devonian to Jurassic age (Narvak thrust panel). This sheet is thrust over a third sheet consisting of probable Devonian phyllite and metagraywacke, which is in turn thrust over older metamorphic rocks (Slate Creek thrust panel). The Kanuti ophiolite is a partial ophiolite that consists of a lower residual mantle suite and an upper magmatic suite, but dikes, extrusives, and sediments are absent. Because of the limited range in rock types in the ophiolite, the tectonic environment cannot be interpreted unambiguously. However, the structural and petrological data are best reconciled with an origin in a volcanic arc tectonic setting. -from Authors

  15. Ophiolite suture in Central Anatolia: New insights from the Sivas Basin (Turkey)

    NASA Astrophysics Data System (ADS)

    Legeay, Etienne; Ringenbach, Jean-Claude; Mohn, Geoffroy; Kergaravat, Charlie; Callot, Jean-Paul

    2015-04-01

    The closure of the Neotethys is classically associated with the obduction of ophiolitic rocks, defining successive suture zones. Theses Alpine-Himalayan ophiolites reflect a complex and still poorly understood paleogeographic framework. In Turkey, various types of ophiolite have been described, involving supra subduction zone and ophiolitic melanges as well. Hence reconstructions of the Anatolian continent assumed the amalgamation of one or more continental fragments during the Mesozoic-Early Cenozoic time. The Sivas Basin is located in a key position at the junction of three crustal domains: the Pontides to the North, the Anatolide - Tauride platforms to the South, and the Central Anatolian Crystalline Complex to the West. These blocks are separated to the North by the Izmir-Ankara-Erzican suture zone (IAESZ), and by the Inner Tauride suture zone (ITSZ) to the South. Ophiolitic outcrops are common in this area, mainly on the basin borders, and sometimes within the central part. These green rocks have been previously related to the ophiolitic melange from the IAESZ in Northern part and to the ITSZ for the southern parts. Recent fieldwork on the southern edge of the Sivas Basin allows a proper description of the ophiolitic complex, including from bottom to top: (1) a large volume of intensely serpentinized peridotites, strongly veined with chrysotile, with minor gabbroic intrusions; (2) upward, serpentinized mantle rocks affected by a cataclastic deformation associated with tectonic breccias and ophicalcites ; and eventually, (3) on the top of the mantle, silicates deposits similar to radiolarian cherts cover by sedimentary breccias with mantle clasts. New geochemical analysis and subsurface data confirm the existence of a southward obducted slice of ophiolite over more than 100km from North to South, forming the basement of the Sivas Basin since the Campanian. This southward obduction related to the IAESZ appears similar to slow spreading ridge or hyper-extended domains derived ophiolites. We interpret the parent body as related to the exhumation of mantle rocks along low angle detachment faults either during the rifting or the oceanic accretion. The peridotites record a high serpentization degree, increasing toward the surface. Outcropping of the mantle rocks at the sea floor is responsible for the reworking of materials as tectonosedimentary breccias (ophicalcites). The emplacement of the gabbro will be constrained by U-Pb zircon geochronology.

  16. Ophiolitic terranes of northern and central Alaska and their correlatives in Canada and northeastern Russia

    SciTech Connect

    Patton, W.W. Jr. )

    1993-04-01

    All of the major ophiolitic terranes (Angayucham, Tozitna, Innoko, Seventymile, and Goodnews terranes) in the northern and central Alaska belong to the Tethyan-type' of Moores (1982) and were obducted onto Paleozoic and Proterozoic continental and continental margin terranes in Mesozoic time. Tethyan-type' ophiolitic assemblages also occur in the Slide Mountain terrane in the Canadian Cordillera and extend from western Alaska into northeastern Russia. Although investigators have suggested widely different ages from their times of abduction onto the continent, these ophiolitic terranes display some remarkably similar features: (1) they consist of a stack of imbricated thrust slices dominated by ocean floor sediments, basalt, and high-level gabbro of late Paleozoic and Triassic age; (2) their mafic-ultramafic complexes generally are confined to the uppermost thrust sheets; (3) they lack the large tectonic melanges zones and younger accretionary flysch deposits associated with the ophiolitic terranes of southern Alaska and the Koryak region of northeastern Russia; (4) blueschist mineral assemblages occur in the lower part of these ophiolite terranes and (or) in the underlying continental terranes; and (5) they are bordered on their outboard' side by Mesozoic intraoceanic volcanic arc terranes. Recent geochemical and geologic studies of the mafic-ultramafic complexes in the Anagayucham and Tozitna terranes strongly suggest they were generated in a supra-subduction zone (SSZ) and that they are directly overlain by volcanic rocks of the Koyukuk terrane.

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

    SciTech Connect

    Chow, J.S. . Geology Dept.)

    1993-03-01

    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.

  18. Chunky Gal Melange and its tectonic significance

    SciTech Connect

    Lacazette, A. Jr.; Rast, N.

    1985-01-01

    Detailed mapping of the Chunky Gal Mafic/Ultramafic Complex of SW North Carolina indicates that the complex is involved in a set of ductile shear zones separating pods of less deformed material. The product is a tectonic mixture of fragments of amphibolites, granites, calc-silicates and ultramafic rocks in the Tallulah Falls Fm. Some inclusions are in different states of metamorphism. Foliation within the amphibolite, migmatite and granite lumps is truncated at their edges. The metasediments show a transposed pervasive late foliation (S/sub 2/.) although the sense of intersection of earlier fabrics is locally determinable. Linear fabric elements plunge east. In the amphibolites, the foliation is folded but not transposed. The zone, therefore, is termed the Chunky Gal Melange. Although syntectonic porphyroblasts suggest that the movement began during the metamorphism, the melange postdates the peak of the main regional metamorphism and separates superposed granulite facies rocks from those of amphibolite facies. A weaker lower amphibolite metamorphism postdates the melange. In the vicinity of Chunky Gal Mountain the melange coincides with the premetamorphic Hayesville Thrust of R. Hatcher. A tectonic model suggested here is that the eastern Blue Ridge represents a poorly to fully developed accretionary prism that was built above an east dipping subduction zone during the closure of a marginal sea. Mafic and ultramafic rocks were incorporated with the metasediments at this time. The Hayesville Thrust itself reflects an early stage of suturing of this terrane with North America.

  19. Glomar Challenger finds ophiolite

    NASA Astrophysics Data System (ADS)

    Deep Sea Drilling Project (DSDP) hole 504B located in the eastern equatorial Pacific, 201 km south of the Costa Rica Rift (CRR), is unique in several ways. Hole 504B extends 1.35 km below the seafloor, which is at a depth of 3.46 km; it has penetrated seismic layers 2A, 2B, and part of 2C and for the first time has provided a very complete section; and it has bottomed in what has been interpreted as a true ophiolite complex. These results constitute for the first time long awaited confirmation of accepted models upon which most oceanic crust theory has been based.A recent report of DSDP Hole 504B noted that ‘… one is led to question the validity of using ophiolite as models of ocean crust’ (Nature, December 16, 1982). That statement was representative of the state of uncertainty that existed before the hole was drilled. The drill site is located on magnetic anomaly 3?, which has an age estimate of 6.2 m.y. The location, south of the CRR, lies on the easternmost arm of the Galapagos Spreading Center, ideally situated over ophiolite lithostratigraphy, if most current models of the oceanic crust are correct. Hesitation with the acceptance of these models has arisen in recent years from the rarity of dredged and drilled rocks that can be positively identified as being samples of an ophiolite complex. Hole 504B, being along the southern flank of the CRR, should be a good test case.

  20. Petrology and structure of greenstone blocks encased in mud-matrix melange of the Franciscan complex near San Simeon, California

    SciTech Connect

    Davidsen, R.K.; Cloos, M.

    1985-01-01

    Greenstones comprise about 20% of all mappable (>1 m) blocks encased in blueschist-block-bearing mud-matrix melange exposed in a 10 km-length of sea cliffs near San Simeon. Field and petrographic analysis of 25 blocks show they vary from finely crystalline (<1 mm) locally porphyritic or amygdaloidal, volcanics to coarsely crystalline (1 to 5 mm) diabase. Some are in contact with bedded chert and two have relict pillows. However, most blocks are intensely deformed. Pinch-and-swell and boundinage are recognized on scales from cm to about 10 m. Distortion was accommodated by cataclasis to an aggregate of pieces from mm to m across. Generally, m-sized blocks are pervasively cataclastic whereas larger blocks are crosscut by cataclastic zones that emanate from pervasively cataclastic margins or necked regions of boudins. Discontinuous, cm-thick veins and cavities that are lined by quartz and clacite and rarely, laumontite, prehnite and aragonite locally crosscut all other structures. Relict igneous textures show the primary minerals are plagioclase and clinopyroxene. Abundant secondary minerals, particularly in cataclastic zones, are albite, chlorite, pumpellyite (some have high Al), and calcite. The metamorphic parageneses indicate relatively minor greenschist-facies, sea-floor-type alterations under static conditions followed by lower-temperature alterations synchronous with cataclasis and the development of boudinage. If the blocks are fragments of disrupted ophiolites, only the uppermost section of the suite are present within the mud-matrix melange near San Simeon. The simplest explanation for their crystallization, metamorphism and incorporation into the melange is that they are fragments of seamounts dismembered during subduction.

  1. Structure and tectonics of a Lower Ordovician forearc ophiolite in central western Maine

    SciTech Connect

    Stetzer, L.M.; Dilek, Y. . Dept. of Geology and Geography)

    1993-03-01

    The Lower Ordovician Boil Mountain ophiolite complex (BMO) in central western Maine occurs in the Gander tectonic zone, nearly 100 km SE of the main Appalachian ophiolite belt, and represents part of the Iapetus oceanic domain. It is exposed in an ENE trending narrow zone immediately south of the Precambrian Chain Lakes massif (CLM). The contact between the CLM and the BMO is characterized by a steeply to vertically south-dipping shear zone composed of several fault planes, which display subhorizontal slickenside lineations with sinistral sense of shearing and counterclockwise rotated porphyroclasts. The BMO consists mainly of pyroxenite, gabbro, diorite, plagiogranite, autobreccia, mafic to felsic volcanic, volcaniclastic, and hemipelagic sedimentary rocks, and contacts between these lithologic units are commonly vertical and faulted. Autobreccia outcrops containing clasts and blocks of serpentinite, diabase, pillowed basalt, and radioalarian chert in a medium-grained hemipelagic matrix indicates deposition penecontemporaneous with ocean floor tectonism during evolution of the ophiolite. Extrusive rocks include basaltic, massive to pillow-lava flows, and andesites, dacites, and rhyolites and are commonly metamorphosed up to a lower-greenschist facies. The BMO is overlain to the SE by a melange-flysch sequence composed mainly of metapelite, metagraywacke, phyllite, and slate with abundant volcanic material suggesting alternated shallow- and deep-water sedimentation in a forearc basin. These relations and the observed structures in the ophiolite indicate its development in an oceanic environment with a low magma budget and active vertical tectonism. The available geochemical data show low Ti, Zr, Y, Cr, and REE contents of volcanic rocks suggesting a depleted magma source in a suprasubduction zone tectonic setting for the ophiolite.

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

    USGS Publications Warehouse

    Donato, M.M.

    1987-01-01

    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

  3. Geochemistry and tectonomagmatic affinity of the Yungbwa ophiolite, SW Tibet

    NASA Astrophysics Data System (ADS)

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

    2003-02-01

    The Yungbwa ophiolite is a thrust sheet of about 800 km 2 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 crosscut the mantle tectonites, but plutonic and volcanic sections are notably absent. The majority of basaltic dikes are tholeiitic and similar to N-MORB with respect to LREE depletion, ratios of diagnostic trace elements and Sr and Nd isotope systematics. Sm-Nd isotope data of three tholeiitic samples yielded an isochron age of 147±25 Ma (MSWD=0.59), and an initial ?( t)Nd value of +8.8. In addition, magnesio-hornblende in a tholeiitic basaltic dike yielded a 40Ar/ 39Ar age of 152±33 Ma. The mineral chemistry of the spinel-peridotites is consistent with an origin in a tectonic setting similar to abyssal peridotites. The nearly chondritic initial 187Os/ 188Os value of a Re-poor orthopyroxene concentrate matches the composition of abyssal peridotites. The strongly LREE depleted trace element signature of the peridotite clinopyroxenes resembles that of clinopyroxenes from ocean floor peridotites. The Nd isotopic composition of a clinopyroxene concentrate is extremely depleted ( 143Nd/ 144Nd=0.514420), indicating an early Jurassic time of depletion ( TDM=187 Ma) and suggesting that this clinopyroxene represents residual mantle material. In contrast, the bulk peridotites are characterized by convex-downward REE patterns, coupled with low 143Nd/ 144Nd ratios and elevated 87Sr/ 86Sr ratios. We suggest that these geochemical signatures resulted from secondary processes during or after emplacement of the ophiolite complex.

  4. Collisional melange development: Geologic associations of active melange-forming processes with exhumed melange facies in the western Banda orogen, Indonesia

    NASA Astrophysics Data System (ADS)

    Harris, R. A.; Sawyer, R. K.; Audley-Charles, M. G.

    1998-06-01

    Analysis of block assemblages, matrix clay mineral composition and microfauna, and offshore seismic reflection profiles reveal that the Bobonaro melange in the Timor region is sourced from mud-rich Australian continental margin sequences that are remobilized during accretion and form various facies depending on the structural conditions of emplacement. Melange facies include broken formation, matrix-rich mud injections, and classic mixed block-in-clay facies. Each of these are distinguished by varying degrees of remobilization, mixing, and dispersion at different structural positions across the orogenic wedge. The most important structural control is whether melange was generated beneath or in front of upper plate Banda forearc basement (Banda Terrane). At the present collisional deformation front in the Timor trough, seismic reflection profiles show that melange forms mostly by stratal detachment and fluid-assisted remobilization above a basal decollement propagating laterally along overpressured Jurassic to Cretaceous clay-rich sequences of the distal Australian continental margin. The broken, clay-rich material injects upward through faults to form intrusive bodies at the base of slope cover sediment to form mud ridges at the surface. Similar patterns of stratal disruption are exposed onshore in the Pliocene Kolbano fold and thrust wedge of southern Timor, which is structurally contiguous with the Timor trough deformation front. Melange in the Kolbano Mountains is mostly broken formation and matrix-rich injections of mud from Jurassic and Cretaceous units. Deformation mechanisms include intense layer-parallel extension associated with emplacement by mud diapirs that rise from near the decollement upward to the surface along fault conduits. In the hinterland of the orogenic wedge (East Timor and northern West Timor), melange is dominantly of mixed block-in-clay facies with large blocks derived from roof thrust sheets of intermixed Banda Terrane and Maubisse Formation units. At the structural base of these thrust sheets is the Sonnebait Disruption Zone (SDZ), which formed the initial suture between the Banda Terrane and structurally underthrust Australian continental margin sequences during the late Miocene to early Pliocene. The thickest accumulations of mixed block-in-clay melange in the Timor region are found at the southern edge (outlet) of the SDZ, near the Central and Viqueque synorogenic basins. The extent of block dispersion and mixing in the SDZ is indicative of intense shear strains perhaps induced by an oversupply of accretable material when the suture zone was clogged by underthrusting of the Australian continental margin.

  5. Implications of Late Cretaceous U-Pb zircon ages of granitic intrusions cutting ophiolitic and volcanogenic rocks for the assembly of the Tauride allochthon in SE Anatolia (Helete area, Kahramanmaraş Region, SE Turkey)

    NASA Astrophysics Data System (ADS)

    Nurlu, Nusret; Parlak, Osman; Robertson, Alastair; von Quadt, Albrecht

    2016-01-01

    An assemblage of NE-SW-trending, imbricate thrust slices (c. 26 km E-W long × 6.3 km N-S) of granitic rocks, basic-felsic volcanogenic rocks (Helete volcanics), ophiolitic rocks (Meydan ophiolite) and melange (Meydan melange) is exposed near the Tauride thrust front in SE Anatolia. The volcanogenic rocks were previously assumed to be Eocene because of associated Nummulitic limestones. However, ion probe U-Pb dating of zircons extracted from the intrusive granitic rocks yielded ages of 92.9 ± 2.2-83.1 ± 1.5 Ma (Cenomanian-Campanian). The Helete volcanic unit and the overlying Meydan ophiolitic rocks both are intruded by granitic rocks of similar age and composition. Structurally underlying ophiolite-related melange includes similar-aged, but fragmented granitic intrusions. Major, trace element and rare earth element analyses coupled with electron microprobe analysis of the granitic rocks show that they are metaluminus to peraluminus and calc-alkaline in composition. A magmatic arc setting is inferred from a combination of tectonomagmatic discrimination, ocean ridge granite-normalized multi-element patterns and biotite geochemistry. Sr-Nd-Pb isotope data further suggest that the granitoid rocks were derived from variably mixed mantle and crustal sources. Granitic rocks cutting the intrusive rocks are inferred to have crystallized at ~5-16 km depth. The volcanogenic rocks and granitic rocks originated in a supra-subduction zone setting that was widely developed throughout SE Anatolia. Initial tectonic assembly took place during the Late Cretaceous probably related to northward subduction and accretion beneath the Tauride continent (Keban and Malatya platforms). Initial tectonic assembly was followed by exhumation and then transgression by shelf-depth Nummulitic limestones during Mid-Eocene, as documented in several key outcrops. Final emplacement onto the Arabian continental margin took place during the Early Miocene.

  6. New structural and petrological data on the Amasia ophiolites (NW Sevan-Akera suture zone, Lesser Caucasus): Insights for a large-scale obduction in Armenia and NE Turkey

    NASA Astrophysics Data System (ADS)

    Hässig, M.; Rolland, Y.; Sosson, M.; Galoyan, G.; Müller, C.; Avagyan, A.; Sahakyan, L.

    2013-03-01

    The ophiolites of Amasia in the northwestern part of the Sevan-Akera suture zone (Lesser Caucasus, NW Armenia) correspond to a well-preserved example of a major obduction of oceanic lithosphere over the South Armenian continental block. Our mapping evidenced a series of (1) un-metamorphosed gabbroic oceanic crust, (2) serpentinites and a greenschist grade tectonic melange composed of deformed pillow-basalts, radiolarites and cherts, and (3) a basal slice of garnet amphibolites bearing similar compositional features as the ophiolite. These units are sliced and deformed by post-Eocene thrusting related to the shortening of the suture zone after the collision of the South Armenian Block with Eurasia. 40Ar/39Ar dating on gabbro amphiboles yielded ages of 169.0 ± 4.6 to 175.8 ± 3.9 Ma. This age and geochemical composition of ophiolite rocks are similar to those of other ophiolite outcrops in Armenia and NE Turkey. Structural and geochemical analyses undertaken on the garnet amphibolites suggest it to represent the obducted ophiolite metamorphic sole. All these data are in agreement with the presence of a unique ophiolite nappe at the scale of NE Turkey-Armenia originating from a Jurassic intra-oceanic back-arc basin, obducted onto the Armenian-Taurides-Anatolides microblocks in the early Late Cretaceous (c. 90 Ma).

  7. Reinterpretation of Mesozoic ophiolite arc, and blueschist terranes in southwestern Baja California

    SciTech Connect

    Sedlock, R.L. . Geology Dept.)

    1993-04-01

    The nature and significance of disrupted Mesozoic oceanic rocks on Isla Santa Margarita and Isla Magdalena, western Baja California Sur, have been reinterpreted on the basis of detailed mapping and petrologic studies. Three structural units are recognized. (1) The upper plate consists of ophiolitic, arc, and forearc basin rocks. Ophiolitic rocks, including metamorphosed ultramafic rocks, gabbro, dikes, volcanic rocks, and chert, underwent strong contractional deformation and penetrative greenschist-facies metamorphism. Arc rocks, including gabbro, a dike and sill complex, compositionally diverse volcanic rocks, lahars, and volcaniclastic strata, lack a penetrative fabric and are weakly metamorphosed. Forearc basin rocks consist of unmetamorphosed conglomerated and rhythmically bedded siliciclastic turbidites. (2) The lower plate is a subduction complex consisting of weakly to moderately foliated and metamorphosed pillow and massive lavas, breccia, and tuff( ), interbedded red and green siliceous argillite, and rare radiolarian ribbon chert and limestone. Blueschist-facies metamorphism is indicated by lawsonite, aragonite, sodic amphibole, and sodic clino-pyroxene. (3) Serpentine-matrix melange crops out in shallowly dipping fault zones between the upper and lower plates. The structural and petrologic characteristics of the Mesozoic units, the geometry of contacts between them, and the age of extension are similar to those in the Isla Cedros-Vizcalno Peninsula region, 400 km to the northwest. The author infers that syn-subduction extension was a regional event that affected much of the western Baja forearc during the Late Cretaceous and Paleogene.

  8. Geochemical characteristics, 40Ar- 39Ar ages and original tectonic setting of the Band-e-Zeyarat/Dar Anar ophiolite, Makran accretionary prism, S.E. Iran

    NASA Astrophysics Data System (ADS)

    Ghazi, A. M.; Hassanipak, A. A.; Mahoney, J. J.; Duncan, R. A.

    2004-11-01

    The Makran accretionary prism in southeastern Iran contains extensive Mesozoic zones of melange and large intact ophiolites, representing remnants of the Tethys oceanic crust that was subducted beneath Eurasia. To the north of the Makran accretionary prism lies the Jaz Murian depression which is a subduction-related back-arc basin. The Band-e-Zeyarat/Dar Anar ophiolite is one of the ophiolite complexes; it is located on the west side of the Makran accretionary prism and Jaz Murian depression, and is bounded by two major fault systems. The principal rock units of this complex are a gabbro sequence which includes low- and high-level gabbros, an extensive sheeted diabase dike sequence, late intrusive rocks which consist largely of trondhjemites and diorites, and volcanic rocks which are largely pillow basalts interbedded with pelagic sedimentary rocks, including radiolarian chert. Chondrite- and primitive-mantle-normalized incompatible trace element data and age-corrected Nd, Pb, and Sr isotopic data indicate that the Band-e-Zeyarat/Dar Anar ophiolite was derived from a midocean ridge basalt-like mantle source. The isotopic data also reveal that the source for basalts was Indian-Ocean-type mantle. Based on the rare earth element (REE) data and small isotopic range, all the rocks from the Band-e-Zeyarat/Dar Anar ophiolite are cogenetic and were derived by fractionation from melts with a composition similar to average E-MORB; fractionation was controlled by the removal of clinopyroxene, hornblende and plagioclase. Three 40Ar- 39Ar plateau ages of 140.7±2.2, 142.9±3.5 and 141.7±1.0 Ma, and five previously published K-Ar ages ranging from 121±4 to 146±5 Ma for the hornblende gabbros suggest that rocks from this ophiolite were formed during the Late Jurassic-Early Cretaceous. Plate reconstructions suggest that the rocks of this complex appear to be approximately contemporaneous with the Masirah ophiolite which has crystallization age of (˜150 Ma). Like Masirah, the rocks from the Band-e-Zeyarat/Dar Anar ophiolite complex represent southern Tethyan ocean crust that was formed distinctly earlier than crust preserved in the 90-100 Ma Bitlis-Zagros ophiolites (including the Samail ophiolite).

  9. Early Proterozoic ophiolite, central Arizona

    SciTech Connect

    Dann, J.C. )

    1991-06-01

    The 1.73 Ga Payson ophiolite is a pseudostratigraphic sequence of mafic plutons, dike swarms, sheeted dikes, and submarine basalts that intruded and erupted upon a 1.75-1.76 Ga magmatic-arc complex. The composition of the sheeted dikes is tholeiitic basalt (minor andesite) with island-arc affinities. The submarine basalts are overlain by dacitic breccias and a thick section of turbidites with ca. 1.72 Ga ash beds. The entire sequence was deformed, intruded by ca. 1.70 Ga granites, and unconformably overlain by fluvial to shallow-shelf sediments. Although most of the 1.8-1.6 Ga juvenile crust of Arizona consists of magmatic-arc rocks, the Payson ophiolite is unique and is interpreted to have formed the floor of an intra-arc basin. The ophiolite developed in situ on the older arc basement, as opposed to being thrust over it. The basin was accreted to the continent by ca. 1.70 Ga.

  10. Ophiolites of Iran: Keys to understanding the tectonic evolution of SW Asia: (II) Mesozoic ophiolites

    NASA Astrophysics Data System (ADS)

    Moghadam, Hadi Shafaii; Stern, Robert J.

    2015-03-01

    Iran is a mosaic of continental terranes of Cadomian (520-600 Ma) age, stitched together along sutures decorated by Paleozoic and Mesozoic ophiolites. Here we present the current understanding of the Mesozoic (and rare Cenozoic) ophiolites of Iran for the international geoscientific audience. We summarize field, chemical and geochronological data from the literature and our own unpublished data. Mesozoic ophiolites of Iran are mostly Cretaceous in age and are related to the Neotethys and associated backarc basins on the S flank of Eurasia. These ophiolites can be subdivided into five belts: 1. Late Cretaceous Zagros outer belt ophiolites (ZOB) along the Main Zagros Thrust including Late Cretaceous-Early Paleocene Maku-Khoy-Salmas ophiolites in NW Iran as well as Kermanshah-Kurdistan, Neyriz and Esfandagheh (Haji Abad) ophiolites, also Late Cretaceous-Eocene ophiolites along the Iraq-Iran border; 2. Late Cretaceous Zagros inner belt ophiolites (ZIB) including Nain, Dehshir, Shahr-e-Babak and Balvard-Baft ophiolites along the southern periphery of the Central Iranian block and bending north into it; 3. Late Cretaceous-Early Paleocene Sabzevar-Torbat-e-Heydarieh ophiolites of NE Iran; 4. Early to Late Cretaceous Birjand-Nehbandan-Tchehel-Kureh ophiolites in eastern Iran between the Lut and Afghan blocks; and 5. Late Jurassic-Cretaceous Makran ophiolites of SE Iran including Kahnuj ophiolites. Most Mesozoic ophiolites of Iran show supra-subduction zone (SSZ) geochemical signatures, indicating that SW Asia was a site of plate convergence during Late Mesozoic time, but also include a significant proportion showing ocean-island basalt affinities, perhaps indicating the involvement of subcontinental lithospheric mantle.

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

    NASA Astrophysics Data System (ADS)

    Casey, John; Dewey, John

    2013-04-01

    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.

  12. From Subduction to Obduction: new insights on the obduction process through the study of the metamorphic sole of the Amasia ophiolites (Lesser Caucasus, Armenia)

    NASA Astrophysics Data System (ADS)

    Hässig, Marc; Rolland, Yann; Sosson, Marc; Galoyan, Ghazar; Müller, Carla; Sahakyan, Lilit; Avagyan, Ara

    2013-04-01

    Outcrops of preserved oceanic lithosphere overthrusted onto the continental South Armenian Block (SAB) from the north are found throughout the Lesser Caucasus. Previous works using geochemical whole-rock analyses, 40Ar/39Ar and paleontological dating have shown that the ophiolite outcrops throughout this area were emplaced during the Upper Cretaceous as one non-metamorphic preserved ophiolitic nappe of back-arc origin that formed during Middle to Late Jurassic. This domain is limited to the North along the Sevan-Akera suture zone (SAS) by a fossil subduction under the southern Eurasian Margin (active from Middle Jurassic to Late Cretaceous times). Two thrust fronts, to the South, are found near Vedi (Armenia) and Khoy (Iran). The Armenian ophiolites show a variety of visible basal contacts considering structural and metamorphic settings. In the locality of Stepanavan (50 km east of Amasia along the SAS) blueschists evidence a fossil subduction zone active prior to or contemporary to obduction. In Amasia, 40Ar/39Ar dating, microprobe mineral characterization and thermodynamic modeling using PerpleX of a basal sliver of garnet amphibolites bring evidence of anticlockwise PT path. Two metamorphic steps are identified: a HT-LP peak of P = 6-7 kbar and T > 630°C followed by a clearly marked greenschist facies overprint (M2: MP-MT peak at P = 8-10 kbar and T = 600°C). The greenschist facies conditions are analog to those of the ophiolitic "melange", in which is found the garnet amphibolite sliver, separating the ophiolite from the underthrusted continental domain (SAB). Considering this unit to be part of the metamorphic sole of the Armenian ophiolitic nappe, we bring crucial P-T-t precisions to the obduction process in the Lesser Caucasus. We emphasize the constrained time span for ophiolite emplacement argued by the Ar/Ar ages (91-90 Ma, Cenomanian-Turonian) for metamorphism and paleontological ages (Coniacian-Santonian) determined from series directly underlying and overlying the ophiolite obduction front. The westward extension of the Lesser Caucasus ophiolitic nappe is believed to be the NE Anatolian ophiolites. The study of the Refahiye ophiolite near Erzincan has identified a well foliated low metamorphic grade volcano-sedimentary unit directly under the ophiolite to the north of the North Anatolian Fault pull-apart basin. This further emphasizes the variability of the material which may be incorporated to form the metamorphic sole, whether it is an ophiolite with a MORB signature overlain by (1) supra-subduction volcanics with a TAB signature or (2) seamounts with an OIB signature. The variable geochemical compositions of the basal metamorphic rocks which endured the obduction of the ophiolite is thus interpreted as resulting from the disappearance of corresponding domains through underthrusting, tectonic erosion, scaling and mixing.

  13. Origin of Zircon in Ophiolitic Mantle Rocks

    NASA Astrophysics Data System (ADS)

    Robinson, P. T.; Yang, J.; Schmitt, A. K.; Li, J.; Ma, C.

    2011-12-01

    Zircon xenocrysts in ultramafic rocks are typically interpreted as grains picked up by intrusion of mantle rocks into crustal sequences and are taken as evidence of underlying continental crust. However, ultramafic rocks of the Luobusa and Dongqiao ophiolites of Tibet and the Semail ophiolite of Oman contain rounded zircon grains that are much older than the ophiolites themselves. In the Tibetan ophiolites the zircon is accompanied by diamonds, moissanite, corundum, coesite, kyanite, garnet and rutile and numerous highly reduced phases, including PGE and base-metal alloys and native elements. The zircon grains range from 20 to 300 ?m across, and are mostly well rounded with very complex internal structures. A few grains are euhedral to subhedral and have concentric zoning suggesting an igneous origin. Many of the grains contain low-pressure inclusions of quartz, rutile, orthoclase, mica, ilmenite and apatite. 206Pb/238U SIMS dates for the Luobusa zircons range from 549±19 to 1657±58 Ma, whereas those for the Dongqiao ophiolites range from 484±49 to 2515±276 Ma. These ages are much older than the host ophiolites (~126 Ma and 147 Ma, respectively). Sixteen dates on zircons from the Semail ophiolite range in age from 84±4 to 1386±48 Ma. Four of these grains are euhedral to subhedral and have late Cretaceous ages essentially the same as the host ophiolite (92±4 to 99±5 Ma), but most are much older. The one younger age of 84 Ma probably reflects slight Pb loss. Many grains in all three ophiolites have distinct cores surrounded by much younger rims, which are still older than the host ophiolites. All of the studied zircons have REE and trace element compositions characteristic of continental crustal grains. Because ophiolites are tectonically emplaced slices of oceanic lithosphere, the possibility of crustal contamination during formation is highly unlikely. Thus, the old zircons in these ophiolites are interpreted as crustal xenocrysts, introduced into the mantle by earlier subduction of continental crust in orogenic belts.

  14. Thematic mapper study of Alaskan ophiolites

    NASA Technical Reports Server (NTRS)

    Bird, John M.

    1988-01-01

    The two principle objectives of the project Thematic Mapper Study of Alaskan Ophiolites were to further develop techniques for producing geologic maps, and to study the tectonics of the ophiolite terrains of the Brooks Range and Ruby Geanticline of northern Alaska. Ophiolites, sections of oceanic lithosphere emplaced along island arcs and continental margins, are important to the understanding of mountain belt evolution. Ophiolites also provide an opportunity to study the structural, lithologic, and geochemical characteristics of ocean lithosphere, yielding a better understanding of the processes forming lithosphere. The first part of the report is a description of the methods and results of the TM mapping and gravity modeling. The second part includes papers being prepared for publication. These papers are the following: (1) an analysis of basalt spectral variations; (2) a study of basalt geochemical variations; (3) an examination of the cooling history of the ophiolites using radiometric data; (4) an analysis of shortening produced by thrusting during the Brooks Range orogeny; and (5) a study of an ophiolite using digital aeromagnetic and topographic data. Additional papers are in preparation.

  15. Ophiolites of Iran: Keys to understanding the tectonic evolution of SW Asia: (I) Paleozoic ophiolites

    NASA Astrophysics Data System (ADS)

    Shafaii Moghadam, Hadi; Stern, Robert J.

    2014-09-01

    Iran is a mosaic of Ediacaran-Cambrian (Cadomian; 520-600 Ma) blocks, stitched together by Paleozoic and Mesozoic ophiolites. In this paper we summarize the Paleozoic ophiolites of Iran for the international geoscientific audience including field, chemical and geochronological data from the literature and our own unpublished data. We focus on the five best known examples of Middle to Late Paleozoic ophiolites which are remnants of Paleotethys, aligned in two main zones in northern Iran: Aghdarband, Mashhad and Rasht in the north and Jandagh-Anarak and Takab ophiolites to the south. Paleozoic ophiolites were emplaced when N-directed subduction resulted in collision of Gondwana fragment “Cimmeria” with Eurasia in Permo-Triassic time. Paleozoic ophiolites show both SSZ- and MORB-type mineralogical and geochemical signatures, perhaps reflecting formation in a marginal basin. Paleozoic ophiolites of Iran suggest a progression from oceanic crust formation above a subduction zone in Devonian time to accretionary convergence in Permian time. The Iranian Paleozoic ophiolites along with those of the Caucausus and Turkey in the west and Afghanistan, Turkmenistan and Tibet to the east, define a series of diachronous subduction-related marginal basins active from at least Early Devonian to Late Permian time.

  16. Controls on accretion of flysch and melange belts at convergent margins: evidence from the Chugach Bay thrust and Iceworm melange, Chugach accretionary wedge, Alaska

    USGS Publications Warehouse

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

    1997-01-01

    Controls on accretion of flysch and melange 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 melange 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 melange of argillite, chert, greenstone, and graywacke of the McHugh Complex and a less chaotically deformed melange of argillite and graywacke of the Valdez Group. We assign the latter to a new, informal unit of formational rank, the Iceworm melange, 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 melange. 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 melange 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 melanges 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 melange 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.

  17. The Tectonic Evolution of Caribbean Ophiolites

    NASA Astrophysics Data System (ADS)

    Draper, G.

    2001-12-01

    Ophiolitic rocks (associated basalts, gabbros and ultramafic rocks) occur in many areas in the circum-Caribbean and Central America. These ophiolites are derived principally from two oceanic provinces: (1) the Atlantic realm, proto-Caribbean sea that formed when North America separated from South America during the opening of the North Atlantic during Jurassic and Early Cretaceous time. These ophiolites were emplaced during a series of late Cretaceous to early Tertiary arc-continent collisions as the east facing Antilles arc migrated into the Caribbean realm. These occurrences include Guatamala, northern Cuba, northern Hispaniola, and northern Venezuela. (2) the Pacific realm, late Cretaceous Caribbean-Colombian plateau that now occupies the central Caribbean, but outcrops on land in Costa Rica, SW Hispaniola, the Netherlands Antilles and western Colombia. Accretion, emplacement and uplift was aided by their buoyancy and took place at various times during the Caribbean plateau's insertion into the middle American continental gap. A third set of occurrences are more uncertain in origin. The central Hispaniola and SE Puerto Rico ophiolites seem to be Jurassic age oceanic plateau rocks that were emplaced in mid-Cretaceous time during an, subduction polarity reversal episode. The emplacement of the metamorphosed ophiolitic rocks of eastern Jamaica may also be associated with this event, but the adjacent, upper Cretaceous, Bath-Dunrobin complex seems to be more related to the Campanian Yucatan-Antillean arc collision. The ultramafic rocks of Tobago are not oceanic, but represent Alaksan-type, assemblages.

  18. Contact metamorphism by an ophiolite peridotite from neyriz, iran.

    PubMed

    Hall, R

    1980-06-13

    Ophiolites are conventionally regarded as fragments of former oceanic lithosphere. Mineralogical and field evidence indicates that peridotite of the Neyriz ophiolite was intruded at high temperature into folded crystalline limestones, forming skarns. This excludes the formation of the ophiolite at a mid-ocean ridge but is consistent with its origin by intrusion during continental rifting. PMID:17830810

  19. Occurrence of Upper Cretaceous pelagic carbonates within ophiolite-related pillow basalts in the Mt. Kozara area of the Vardar zone western belt, northern Bosnia

    NASA Astrophysics Data System (ADS)

    Grubi?, Aleksandar; Radoi?i?, Rajka; Kneževi?, Mirjana; Cviji?, Ranko

    2009-03-01

    Deformed lenses of red silty micrite interstratified with basaltic pillow lavas outcrop at the confluence of the River Crna Reka and Krvavac Creek on Mt. Kozara in northern Bosnia. Interbedded pelagic carbonates contain microfossils, notably several species of Globotruncana, of Upper Santonian-Lower Campanian age. The pillow lavas and sediments form part of ~ 50 km 2 tectonically emplaced body of mafic igneous rocks, composed of pillow lava, sheeted diabase and minor gabbro. This body is tectonically emplaced within melange of the Vardar zone western belt. The basic igneous body has been interpreted as the upper part of an ophiolite that was tectonically emplaced when Neotethys finally closed during latest Cretaceous time. Accordingly, the Late Cretaceous age of the interbedded pelagic sediments, as reported here, indicates that oceanic crust still existed in this region of the Tethyan orogen during Late Cretaceous time.

  20. Obduction-related ophiolitic polymict breccias covering the ophiolites of Antalya (southwestern Turkey)

    SciTech Connect

    Lagabrielle, Y.; Whitechurch, H.; Marcoux, J.; Juteau, T.; Reuber, I.; Guillocheau, F.; Capan, U.

    1986-09-01

    The ophiolites of the Antalya nappes, cropping out in the area if Kemer (southwestern Turkey), are capped by a spectacular detrital sequence composed of various ophiolitic breccias whose total thickness reaches 1000 m. Four main formations have been distinguished within the breccias. The basal formations are in depositional contact over the previously tectonized and eroded deepest part of the ophiolites. The upper breccias are dated as middle Maastrichtian, which is the age of the deposition of synchronous serpentinitic olistostromes onto the continental margin. These data suggest that the breccias formed as debris slides from bathymetric highs on the ocean crust as a result of renewed tectonic activity accompanying obduction onto the continental margin.

  1. Ophiolite Perspectives on Oceanic Mantle Heterogeneity

    NASA Astrophysics Data System (ADS)

    Walker, R. J.; O'Driscoll, B.; Day, J. M.; Ash, R. D.; Daly, J. S.

    2014-12-01

    The mantle sections of ophiolites offer a useful approach to studying compositional heterogeneities in the oceanic mantle. A potential caveat is that the tectonic provenance of ophiolites is often not easy to decipher, although many have undergone at least supra-subduction zone (SSZ) processing. Significant outstanding questions include the degree to which ophiolite peridotites preserve evidence of pre-SSZ events and the way that SSZ melt extraction modifies the character of these peridotites. A suite of Caledonian ophiolites associated with the closure of the Iapetus Ocean offers an opportunity to shed light on these issues, in particular to assess the degree to which long (regional) wavelength compositional heterogeneities survive SSZ melting. Observations on the combined highly-siderophile element (HSE) and 187Os/188Os systematics of the broadly coetaneous (490-500 Ma) Shetland Ophiolite Complex (Scotland) and Leka Ophiolite Complex (LOC; Norway) are presented here. Generally, the lithological composition of each locality is harzburgitic, and hosts lenses and layers of dunite, chromitite and pyroxenite that are interpreted as representing SSZ-related (channelised) melt migration and melt-rock interaction. Although the bulk of the harzburgitic rocks have approximately chondritic initial 187Os/188Os and HSE abundances, ancient (Proterozoic) melt depletion (TRD =1.4 to 1 Ga) is recorded in ~10% of samples from each locality. This is also commonly observed in abyssal peridotites. One important implication of the data is that SSZ melt generation/migration has had no discernible impact on the bulk Os isotopic composition of the Iapetus oceanic mantle. By contrast, non-harzburgitic lithologies consistently exhibit more radiogenic initial 187Os/188Os and more variable HSE abundances. The dunites, chromitites and pyroxenites of the LOC can be separated into two groups on the basis of isochrons that they define; yielding ages of 481±22 Ma and 589±15 Ma, respectively. The former age corresponds, within error, to the accepted age of the ophiolite (497 ± 2 Ma). The meaning of the latter age is uncertain, but may correspond to the early stages of Iapetus opening. The data imply that the oceanic mantle represented by both ophiolites resembles a 'patchwork' of peridotites of different ages and compositions.

  2. Mapping the Oman Ophiolite using TM data

    NASA Technical Reports Server (NTRS)

    Abrams, Michael

    1987-01-01

    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.

  3. The Oman Ophiolite from Detachment to Obduction

    NASA Astrophysics Data System (ADS)

    Boudier, F. I.

    2014-12-01

    An overview model is presented accounting for older and up-dated published data. Detachment of the Oman ophiolite exhumed a 20 km thick fragment of oceanic lithosphere 500 x 100 km2 in extension. This detachment occurred at margin of a fast spreading NeoTethyan Ocean, at P/T conditions ~900°C-200 MPa, 95-95.5 Ma ago. The Hawasinah nappes underlying the ophiolite at present, represent the stacking of the sedimentary cover deposited on the Arabian continental margin, thinned and rifted since Permian time, and extending more than 300 km north from the present shore. The sedimentary record points to the collapse of the continental basement at Jurassic-Cretaceous boundary, 140 Ma ago. Subduction of the rifted continental margin is inferred, as recorded in the Saih Hatat high-pressure rocks whose metamorphic age 80-140 Ma is discussed, as well as the vergence of related subduction. Late Cretaceous time 85-70 Ma marks the emplacement of the Oman ophiolite in the Muti Basin, to its present position inland the Permo-Triassic continental margin. These episodes are not similarly recorded in the northern part of the Oman Mountains, that do not expose any HP metamorphic belt, but granulitic crustal rocks and large development of syn-obduction non-MORB magmatism in the ophiolite crustal section. Collision is achieved at the northern tip, the Musandam area, linked to the opening of the Gulf of Aden 25 Ma ago, and northward drift of the Arabian Plate.

  4. Numerical Models of Ophiolite Genesis and Obduction

    NASA Astrophysics Data System (ADS)

    Guilmette, C.; Beaumont, C.; Jamieson, R.

    2013-12-01

    Ophiolites are relics of oceanic lithosphere tectonically emplaced in continental settings. They are diagnostic features of continental suture zones, where they mark past plate boundaries. Even after having been studied for more than 40 years, the mechanisms involved in the genesis and subsequent obduction of ophiolites over continental margins are still debated. We present the results of 2D thermal-mechanical numerical models that successfully reproduce characteristics of natural examples like the Semail, Bay of Islands, Yarlung-Zangbo, and Coast Range ophiolites. The numerical models are upper mantle scale and use pressure-, temperature- and strain-dependent viscous-plastic rheologies. Both divergent and convergent velocity boundary conditions are used and tectonic boundary forces are monitored. The models start with the rifting of a stable continent, followed by development of an ocean ridge and accretion of oceanic lithosphere at a total rate of 3 cm/y. Once a specified ocean size/age is achieved, the velocity boundary conditions are reversed leading to convergence and the spontaneous inception of a suduction zone at the mid-ocean ridge. We present results for models including different ages of oceans (40 to 90 Ma) and different convergence velocities (5 to 15 cm/y). The interaction between the lower plate passive margin and the oceanic upper plate results in 5 different tectonic styles. These differ mainly by the presence or absence of oceanic spreading in the upper plate (back-arc basin), leading to supra-subduction zone ophiolites vs. MORB-type, and by the behaviour of the oceanic slab, e.g., slab rollback vs. breakoff. The evolution of effective slab pull is interpreted to be the major control on the resulting tectonic style. Low effective slab pull models (young oceans and fast convergence rates) fail to obduct an ophiolite. Strong effective slab pull models (old oceans and lower convergence rates) result in subduction zone retreat and spontaneous oceanic spreading in the upper plate. The collision between the trench and the continental passive margin causes either obduction of a supra-subduction zone spreading center, fore-arc block subduction, or subduction polarity flip, reminiscent of the Semail, Yarlung-Zangbo, and Coast Range ophiolites, respectively. In Semail-style models, there is lithospheric-scale thrusting in the supra-subduction zone region. In Yarlung-Zangbo-style models, a second subduction zone of the same polarity initiates at the back-arc spreading center and the fore-arc block is engulfed. In Coast Range-style models, a second subduction zone of opposite polarity initiates at the back-arc spreading center, preserving the fore-arc block and turning the upper-plate margin into an active margin with a double ophiolite belt, one emplaced on the continental margin (cf. Great Valley Ophiolite) and one on an ocean-facing subduction complex (cf. Coast Range Ophiolite).

  5. Ophiolite Tectonics, Rock Magnetism and Palaeomagnetism, Cyprus

    NASA Astrophysics Data System (ADS)

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

    2010-06-01

    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

  6. Osmium Isotopes from Appalachian Ophiolite Chromites

    NASA Astrophysics Data System (ADS)

    Minarik, W. G.; Pagé, P.; Bécu, V.; Bédard, J. H.; Walker, R. J.

    2002-12-01

    A band of tectonically-emplaced ophiolites stretches along the length of the Appalachian orogen, and represent preserved oceanic lithosphere subducted and accreted to the Laurentian margin during the Taconic orogeny. They may have formed in a mid-ocean ridge, a back-arc, or fore-arc spreading environment. The association of voluminous boninitic magmas with a number of these ophiolites suggests a fore-arc setting based on comparison with modern arcs. Chrome-rich spinels from these rocks are resistant to alteration and contain high concentrations of osmium and low Re/Os ratios, hence preserve information about the initial 187Os/188Os at crystallization. Massive chromites additionally concentrate and average chromium and osmium from large volumes of mantle. The Thetford Mines, Quebec chromites contain 8-80 ppb osmium and 0.1 ppb Re, resulting in little age correction to 480 Ma. The 187Os/188Os ratios range from 0.1246 to 0.1263, corresponding to initial ?Os of +0.7 to +2.0 (2? s.d.<0.2%). These values bracket the fit curve (0.126 at 500 Ma) to ophiolite chromites reported by Walker et al. (2002) for a variety of mostly supra-subduction-zone ophiolites. Os concentrations in slab-derived fluids and melts are probably low relative to fertile mantle, so arc ophiolites are dominated by the mantle Os. A massive chromite sample from Soldier's Delight serpentine barrens, part of the Baltimore Mafic Complex (MD,USA 489 Ma) yields a 187Os/188Os ratio of 0.1284, or ?Os of +3.8. This more radiogenic ratio may be related to the continental arc setting suggested by Nd and Pb isotopes. Analysis of additional ophiolite chromites from the Appalachian chain will constrain the average Os isotopic composition and isotopic variability of asthenospheric mantle at 500 Ma. This variability may provide additional information on contributions to the arc osmium budget from 1) ancient non-radiogenic lithospheric mantle, and 2) subducted slab-derived radiogenic mafic crust delivered in fore-arc systems in melt conduits within highly-depleted mantle.

  7. Detrital zircon geochronology overlying the Naga Hills ophiolite

    NASA Astrophysics Data System (ADS)

    Roeder, T.; Aitchison, J.; Stojanovic, D.; Agarwal, A.; Ao, A.; Bhowmik, S.

    2013-12-01

    The Nagaland ophiolite in NE India represents the easternmost section of the ophiolitic belt running along the India-Asia suture. Outcrops near the border between Nagaland and Myanmar include not only a full suite of ophiolitic rocks but also high P/T blueschist rocks within a serpentinite-matrix mélange. Although Upper Jurassic radiolarians have been reported from the ophiolite itself (Baxter et al., 2011), few constraints have been placed on the timing of its emplacement onto India. Terrestrial sediments of the Phokphur Formation unconformably overlie the ophiolite. Similar to other sediments from along the ophiolite belt such as the Luiqu conglomerates in Tibet (Davis et al., 2002), they contain detritus derived from both the ophiolite and the continental margin onto which the ophiolite was emplaced. The clastic sediments of the Phokphur Formation potentially record not only the timing of ophiolite generation but also the ages of source terranes and can be used to place a minimum age constraint on the timing of ophiolite emplacement. As a contribution towards extending knowledge of the ophiolite belt and the India/Asia collision, we report preliminary results of an investigation into the sedimentology and detrital zircon geochronology of the Phokphur Formation in areas near Salumi and Zephu. Baxter, A.T., Aitchison, J.C., Zyabrev, S.V., Ali, J.R., 2011. Upper Jurassic radiolarians from the Naga Ophiolite, Nagaland, northeast India. Gondwana Research 20, 638-644. Davis, A.M., Aitchison, J.C., Badengzhu, Luo, H., Zyabrev, S., 2002. Paleogene island arc collision-related conglomerates, Yarlung-Tsangpo suture zone, Tibet. Sedimentary Geology 150, 247-273.

  8. Thematic mapper study of Alaskan ophiolites

    NASA Technical Reports Server (NTRS)

    Bird, John M.

    1987-01-01

    LANDSAT Thematic Mapper (TM) images were used to produce improved geologic maps of the ophiolites of the Brooks Range, and to recognize regional-scale structures that might affect the spatial distribution of the ophiolites. From the TM data, significant information was obtained concerning the distribution of rock types and structures that could not easily be acquired with conventional geologic studies. The information obtained from the TM data, in combination with other geologic data, is being used to further the understanding of the tectonic evolution of the Brooks Range. Results of the work in the Maiyumerak Mountains area are used to illustrate the information being obtained with the LANDSAT TM data, and the way that the information is being integrated with other geologic data.

  9. A vestige of Earth's oldest ophiolite.

    PubMed

    Furnes, Harald; de Wit, Maarten; Staudigel, Hubert; Rosing, Minik; Muehlenbachs, Karlis

    2007-03-23

    A sheeted-dike complex within the approximately 3.8-billion-year-old Isua supracrustal belt (ISB) in southwest Greenland provides the oldest evidence of oceanic crustal accretion by spreading. The geochemistry of the dikes and associated pillow lavas demonstrates an intraoceanic island arc and mid-ocean ridge-like setting, and their oxygen isotopes suggest a hydrothermal ocean-floor-type metamorphism. The pillows and dikes are associated with gabbroic and ultramafic rocks that together make up an ophiolitic association: the Paleoarchean Isua ophiolite complex. These sheeted dikes offer evidence for remnants of oceanic crust formed by sea-floor spreading of the earliest intact rocks on Earth. PMID:17379806

  10. The Ophiolite - Oceanic Fore-Arc Connection

    NASA Astrophysics Data System (ADS)

    Reagan, M. K.; Pearce, J. A.; Stern, R. J.; Ishizuka, O.; Petronotis, K. E.

    2014-12-01

    Miyashiro (1973, EPSL) put forward the hypothesis that many ophiolites are generated in subduction zone settings. More recently, ophiolitic sequences including MORB-like basalts underlying boninites or other subduction-related rock types have been linked to near-trench spreading during subduction infancy (e.g., Stern and Bloomer, 1992, GSA Bull.; Shervais, 2001, G-cubed; Stern et al., 2012, Lithos.). These contentions were given strong support by the results of Shinkai 6500 diving in the Izu-Bonin-Mariana (IBM) fore-arc (e.g., Reagan et al., 2010, G-cubed; Ishizuka et al., 2011, EPSL; Reagan et al., 2013, EPSL). Based on widely spaced dives and grab sampling at disbursed dive stops, these studies concluded that the most abundant and most submerged volcanic rocks in the IBM fore-arc are MORB-like basalts (fore-arc basalts or FAB), and that these basalts appear to be part of a crustal sequence of gabbro, dolerite, FAB, boninite, and normal arc lavas overlying depleted peridotite. This ophiolitic sequence was further postulated to make up most or all of the IBM fore-arc from Guam to Japan, with similar magmatic ages (52 Ma FAB to 45 Ma arc) north to south, reflecting a western-Pacific wide subduction initiation event. At the time of this writing, IODP Expedition 352 is about to set sail, with a principal goal of drilling the entire volcanic sequence in the Bonin fore-arc. This drilling will define the compositional gradients through the volcanic sequence associated with subduction initiation and arc infancy, and test the hypothesized oceanic fore-arc - ophiolite genetic relationship. A primary goal of this expedition is to illustrate how mantle compositions and melting processes evolved during decompression melting of asthenosphere during subduction initiation to later flux melting of depleted mantle. These insights will provide important empirical constraints for geodynamic models of subduction initiation and early arc development.

  11. Actualistic Ophiolite Provenance: The Cyprus Case.

    PubMed

    Garzanti; Andò; Scutellà

    2000-03-01

    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

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

    NASA Astrophysics Data System (ADS)

    Cloos, Mark; Ukar, Estibalitz

    2014-05-01

    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.

  13. The Misis-And?r?n Complex: a Mid-Tertiary melange related to late-stage subduction of the Southern Neotethys in S Turkey

    NASA Astrophysics Data System (ADS)

    Robertson, Alastair; Unlügenç, Ülvi Can; ?nan, Nurdan; Ta ?li, Kemal

    2004-01-01

    The Mid-Tertiary (Mid-Eocene to earliest Miocene) Misis-And?r?n Complex documents tectonic-sedimentary processes affecting the northerly, active margin of the South Tethys (Neotethys) in the easternmost Mediterranean region. Each of three orogenic segments, Misis (in the SW), And?r?n (central) and Engizek (in the NE) represent parts of an originally continuous active continental margin. A structurally lower Volcanic-Sedimentary Unit includes Late Cretaceous arc-related extrusives and their Lower Tertiary pelagic cover. This unit is interpreted as an Early Tertiary remnant of the Mesozoic South Tethys. The overlying melange unit is dominated by tectonically brecciated blocks (>100 m across) of Mesozoic neritic limestone that were derived from the Tauride carbonate platform to the north, together with accreted ophiolitic material. The melange matrix comprises polymict debris flows, high- to low-density turbidites and minor hemipelagic sediments. The Misis-And?r?n Complex is interpreted as an accretionary prism related to the latest stages of northward subduction of the South Tethys and diachronous continental collision of the Tauride (Eurasian) and Arabian (African) plates during Mid-Eocene to earliest Miocene time. Slivers of Upper Cretaceous oceanic crust and its Early Tertiary pelagic cover were accreted, while blocks of Mesozoic platform carbonates slid from the overriding plate. Tectonic mixing and sedimentary recycling took place within a trench. Subduction culminated in large-scale collapse of the overriding (northern) margin and foundering of vast blocks of neritic carbonate into the trench. A possible cause was rapid roll back of dense downgoing Mesozoic oceanic crust, such that the accretionary wedge taper was extended leading to gravity collapse. Melange formation was terminated by underthrusting of the Arabian plate from the south during earliest Miocene time. Collision was diachronous. In the east (Engizek Range and SE Anatolia) collision generated a Lower Miocene flexural basin infilled with turbidites and a flexural bulge to the south. Miocene turbiditic sediments also covered the former accretionary prism. Further west (Misis Range) the easternmost Mediterranean remained in a pre-collisional setting with northward underthrusting (incipient subduction) along the Cyprus arc. The Lower Miocene basins to the north (Misis and Adana) indicate an extensional (to transtensional) setting. The NE-SW linking segment (And?r?n) probably originated as a Mesozoic palaeogeographic offset of the Tauride margin. This was reactivated by strike-slip (and transtension) during Later Tertiary diachronous collision. Related to on-going plate convergence the former accretionary wedge (upper plate) was thrust over the Lower Miocene turbiditic basins in Mid-Late Miocene time. The Plio-Quaternary was dominated by left-lateral strike-slip along the East Anatolian transform fault and also along fault strands cutting the Misis-And?r?n Complex.

  14. Dike complexes in ophiolites of the Urals

    NASA Astrophysics Data System (ADS)

    Ryazantsev, A. V.; Savelieva, G. N.; Razumovsky, A. A.

    2015-05-01

    The structure of dike complexes and their relationships with country rocks are considered for ophiolitic sections of the Polar and the Southern Urals. It has been established that composition of country rocks intruded by dikes is related to geodynamic settings of spreading. The Ordovician dike complexes of the Polar Urals and the Early Devonian complexes in the Southern Urals formed in the prearc setting contain screens of mantle, lower crustal, and upper crustal rocks pertaining to the ophiolitic section. The Late Silurian-Early Devonian complex of dikes and volcanic rocks in the Aktau-Tanalyk Zone of the Southern Urals was formed in the course of rifting in an island-arc structure. Rocks intruded by dikes are represented by island-arc gabbro, plagiogranite, and calc-alkaline volcanics. The Early-Middle Devonian dike complex of the West Mugodzhary Zone and the Early Devonian complex of the Dombarovsky Zone in the Southern Urals were formed in spreading zones of backarc basins, where dikes intruded the upper crustal gabbro and volcanic rocks.

  15. The Jocotán Ophiolite: A new ophiolite along the Jocotán fault, eastern Guatemala

    NASA Astrophysics Data System (ADS)

    Harlow, G. E.; Flores-Reyes, K.; Sisson, V. B.; Nelson, C.; Cacao, A.

    2011-12-01

    The North American - Caribbean plate boundary traverses central Guatemala and northern Honduras, dispersed along three left lateral faults systems, which from north to south are the Chixoy-Polochic, the Motagua, and the Jocotán-Camelecón faults, with the Motagua as the present active strand. The Motagua Suture Zone (MSZ), which encompasses this area, consists of multiple paleo-convergent boundaries. It includes slices of ultramafic-mafic complexes including both antigorite (Atg) serpentinite mélanges containing high-pressure / low-temperature (HP/LT) blocks, and lizardite-chrysotile (Lzd-Ctl) serpentinites with associated pillow lavas, radiolarian chert, and marine sediments, typically labeled as ophiolites. Guatemala Suture Zone would be a preferable term to MSZ because the area extends over all three faults, not just the Motagua. The MSZ includes the Sierra de Santa Cruz ophiolite north of the east end of the Polochic fault, the Baja Verapaz ultramafic complex (considered an ophiolite in most of the literature) lies just south of the western portion of the Polochic fault and a series of Atg-serpentinite-dominant mélanges (with HP/LT blocks) that decorate both sides of the Motagua fault. In addition, there is the El Tambor Formation, south of the Motagua fault (but west of the known limit of the Jocotán fault), which contains mafic & sedimentary units and has been called an ophiolite. However, no mafic-ultramafic bodies appear on maps that cover the Jocotán fault in eastern Guatemala. Geologic mapping by one of the co-authors located a small suite of ultramafic rocks sandwiched between the Jocotán and Camotán faults in eastern Guatemala, a short distance from the town of Camotán. Outcrops exposed for 3 km along a road and in a small river consist of sheared Lzd-Ctl serpentinite, metagabbro, overturned altered pillow lavas, listwaenite and rodingite dikes, cherts and pelagic metasediments. These units represent fault slivers subparallel to the steeply dipping local faults sandwiched between mostly phyllites, schists, limestones and metabasites. The latter are similar to the Las Ovejas Complex and/or the San Diego Phyllite which bound the El Tambor Formation and mélanges further west. The newly observed lithologic package, although small in areal extent, has clear affinities with an ophiolite. No HP/LT metamorphic blocks, or even true amphibolites were observed, so consistent with the presence of Lzd-Ctl in the serpentinite, the unit is not a subduction related mélange. The potential relationship with the El Tambor Formation to the west requires further analysis and comparison.

  16. Major chemical characteristics of Mesozoic Coast Range ophiolite in California

    USGS Publications Warehouse

    Bailey, E.H.; Blake, Jr., M.C.

    1974-01-01

    Sixty-four major element analyses of rocks representative of the Coast Range ophiolite in California were compared with analyses of other onland ophiolite sequences and those of rocks from oceanic ridges. The rocks can be classed in five groups harzburgite-dunite, clinopyroxenite-wehrlite, gabbro, basalt-spilite, and keratophyre-quartz keratophyre which on various diagrams occupy nonoverlapping fields. The harzburgite-dunite from onland ophiolite and ocean ridges are comparable and very low in alkalies. Possible differentiation trends defined on AFM diagrams by other rocks from onland ophiolites and ocean ridges suggest two lines of descent: (1) A trend much like the calc-alkalic trend, though shifted somewhat toward higher iron, and (2) an iron-enrichment trend defined chiefly by the more iron-rich gabbros and amphibolite. MgO-variation diagrams for rocks from the Coast Range ophiolite further distinguish the iron-rich gabbros and amphibolite from the other rock groups and indicate that the iron enrichment, unlike that of the Skaergaard trend, is related to the formation of amphibole. Ophiolite sequences that include the most silicic rock types, such as quartz keratophyre, also exhibit the most pronounced dual lines of descent, suggesting that the silicic rocks and the amphibole-rich gabbros are somehow related. Although the major element chemistry of the Coast Range ophiolite is clearly like that of rocks dredged from oceanic ridges, it is not sufficiently diagnostic to discriminate among the choices of a spreading ridge, an interarc basin, or perhaps even the root zone of an island arc as the site of ophiolite formation.

  17. Chromites from the early Proterozoic Outokumpu-Jormua Ophiolite Belt: a comparison with chromites from Mesozoic ophiolites

    NASA Astrophysics Data System (ADS)

    Liipo, J.; Vuollo, J.; Nykänen, V.; Piirainen, T.; Pekkarinen, L.; Tuokko, I.

    1995-08-01

    Chromites from the ultramafic rocks of the early Proterozoic Outokumpu-Jormua Ophiolite Belt were found to possess textural and {Cr}/{(Cr + Al)} and {Mg}/{(Mg + Fe 2+) } ratios that resemble those described in Mesozoic ophiolite complexes. Chromites of cumulus texture were found at all locations along the Belt, whereas residual and podiform chromites were found only in the Outokumpu complex. The chromites in the Outokumpu complex originated in a supra-subduction zone environment, whereas those in the Jormua ophiolite complex originated in a mid-ocean ridge environment. The main difference between the early Proterozoic ophiolitic chromites and those of Mesozoic age is the relatively high Zn content in the former, which is a metasomatic feature related to the presence of large amounts of Cu-Co-Zn-Au ore. The data and observations presented provide evidence contributing to the concept of Proterozoic plate tectonics.

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

    NASA Astrophysics Data System (ADS)

    Sachau, Till; Koehn, Daniel

    2012-12-01

    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.

  19. Origin and tectonic significance of the Aldrich Mountains serpentinite matrix melange, northeastern Oregon

    NASA Astrophysics Data System (ADS)

    Carpenter, Paul S.; Walker, Nicholas W.

    1992-06-01

    The Permian - Triassic Aldrich Mountains serpentinite melange consists of a matrix of massive and foliated serpentinite (lizardite + chrysotile) that encases a variety of metamorphic, igneous, and sedimentary block types. Serpentinite matrix foliation trends NE-SW, dips steeply (> 75°), and is parallel to long dimensions of blocks. At block-matrix contacts, the foliation traces block margins. The volumetrically most abundant block types are greenstone, chert, and metagabbro; subordinate block types are amphibolite tectonites, epiclastic sediments (breccia, conglomerate, sandstone, and argillite), silicic volcanic rocks, silicic plutonic rocks, and ultramafic rocks. Three types of metamorphism are evident from textures and mineral assemblages of blocks and matrix: (1) serpentinization and related hydrous alteration confined to matrix, ultramafic blocks, and margins of mafic blocks; (2) lower to middle greenschist and prehnite-actinolite facies metamorphism characterized by static, non penetrative fabrics of some blocks, and; (3) dynamothermal epidote amphibolite to upper amphibolite/lower granulite metamorphism exhibited by strongly schistose amphibolite tectonites. Mineral assemblages and mineral chemistry indicate amphibolite tectonites formed at moderate to high temperatures (550°-780°C) and low to moderate pressures (3-5 kbar). Most blocks were affected by variable degrees of post metamorphic brittle deformation prior to immersion in serpentinite matrix. The melange is interpreted to have formed within a Permian to Late Triassic sediment-starved, tectonically active forearc. Fragments of abyssal oceanic crust and Early Permian ensimatic arc crust were mixed by tectonic and sedimentary processes within the forearc. Highly mobile serpentinite, generated beneath the forearc, was protrusively emplaced into the tectonized forearc and mixed with the blocks, forming the melange. Melange generation probably began in the late Early Permian and ceased by the Late Triassic.

  20. Formation and emplacement ages of the Masirah ophiolite, Sultanate of Oman

    SciTech Connect

    Smewing, J.D. ); Abbotts, I.L. ); Dunne, L.A. ); Rex, D.C. )

    1991-05-01

    Hornblende separates from gabbros and biotite separates from crosscutting potassic granites of the Masirah ophiolite have been analyzed by conventional K-Ar techniques. The gabbros have Late Jurassic-Early Cretaceous ages (126-158 Ma), and the granites have broadly similar ages (124-146 Ma). Because the gabbros are likely to give ophiolite formation ages and the granites ophiolite emplacement ages, it can be concluded that the Masirah ophiolite was emplaced very soon after formation. The Late Jurassic-Early Cretaceous ages contrast with the middle Cretaceous ages (90-95 Ma) of the Semail ophiolite, confirming earlier suggestions that the Masirah ophiolite cannot simply be a right-laterally displaced fragment of the Semail ophiolite, but is instead an uplifted block of Indian Ocean crust. The formation and emplacement of the Masirah ophiolite are discussed in the context of the late Mesozoic evolution of the western Indian Ocean.

  1. Ophiolite and Tectonic Development of the East Pacific Margin

    NASA Astrophysics Data System (ADS)

    Moores, E. M.

    2001-12-01

    Well-preserved ophiolites represent oceanic crust and mantle formed at a spreading center and emplaced by collision of a mantle-rooted thrust fault (subduction zone) with a continental margin or island arc. Ophiolite nappes thus represent remnants of lithospheric plates; their basal thrusts (fossil subduction zones) intrinscally cannot be balanced; their displacements are unknown but very large. Many environments of formation are possible for ophiolites: mid-ocean ridge, back-arc, forearc, or intra-arc spreading vrnyrtd, but geochemistry alone is inadequate to differentiate between the possibilities; geologic field evidence is needed, as well. Mesozoic ophiolites in western North America are associated either with the Stikine-Intermontane superterrane (e.g. Sierra Nevada, Klamath Mountains, California. Guerrero terrane, Mexico?), or lie west of it (e.g. Great Valley/Coast Range ophiolite and correlatives to north and south.). The "Great Arc" of the Caribbean (Burke, 1988), including ophiolitic rocks in Cuba, Hispaniola, Puerto Rico, Venezuela, and Colombia, may also correlate with the Great Valley/Coast Range ophiolite and/or with ophiolites in the Sierra Nevada. The Wrangellia/Insular superterrane may have extended to the south and at times may have included parts of the Chortis-Choco blocks of Central America, as well as the Cordillera Occidental of Colombia and Ecuador). These relations suggest the hypothesis that in mid-late Mesozoic time, a separate intra-oceanic plate similar to the present Philippine plate, herein informally called "Americordilleria" was separated by active island arc complexes from the American andFarallon/Kula plates to the east and west, respectively. Basement rocks of the Colombian, Venezuelan, and Yucatan basins, as well as the Great Valley/Coast Range ophiolite, may represent remnants of "Americordilleria". Convergence and collision of "Americordilleria" and its island arc margins with the American continents were major factors in western American and Caribbean orogenic development. Direct contact between the Kula/Farallon plates and North America may not have occurred until late Cretaceous time.

  2. Thematic mapper study of Alaskan ophiolites

    NASA Technical Reports Server (NTRS)

    Bird, J. M.

    1986-01-01

    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.

  3. A dismembered ophiolite from the Mozambique Belt, West Pokot, Kenya

    NASA Astrophysics Data System (ADS)

    Vearncombe, J. R.

    The role of plate tectonics in the Proterozoic is a controversial subject. here I describe a dismembered ophiolite sequence, including podiform chromites, the root zone of a possible sheeted dyke complex and manganiferous metacherts, from the Marich-Ortum region of West Pokot, Kenya. Associated with the ophiolite are andesitic agglomerates, crystalline limestones, mica schists and psammites. The sequence is dismembered with both upward and downward facing units, interpreted as the consequence of two phases of thrusting and ductile shear. The second deformation, synchronous with or after the intrusion of a sheeted granite complex, involved isoclinal folding. Structurally, beneath the ophiolite is a migmatite complex. Granite from the migmatite locally intruded as dykes into the ophiolite. The lithological association suggests that oceanic crust and a possible island arc existed to the east of the Tanzanian-Congo craton (continent). The ocean closed during the late Proterozoic when the ophiolite was emplaced towards the west, to its present position sub-parallel to the craton boundary. Plate tectonics appears to have operated in this region during the late Precambrian.

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    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.

  5. Magnetism of the oceanic crust: Evidence from ophiolite complexes

    SciTech Connect

    Banerjee, S.K.

    1980-07-10

    The magnetic properties of six ophiolite complexes from around the world, ranging in age from Jurassic to Miocene, are presented. An emphasis is placed in our study on the petrologic and isotopic data from these ophiolite complexes in order to determine first whether the rock samples presently available represent the pristine ocean crust or whether they have been altered subaerially since their formation. Five of the ophiolites are found to be acceptable, and the conclusion is overwhelmingly in favor of a marine magnetic source layer that includes not only the pillow lavas but also the underlying dikes and gabbro. At the moment, however, our observations do not suggest that the magnetic contributions of the basaltic dikes should be overlooked in favor of gabbro. A second important conclusion is that nearly pure magnetite could indeed be a magnetic carrier which contributes to marine magnetic anomanies. It only awaits discovery by deeper ocean crustal penetration by future Deep Sea Drilling Project legs.

  6. Ophiolitic association of Cape Fiolent area, southwestern Crimea

    NASA Astrophysics Data System (ADS)

    Promyslova, M. Yu.; Demina, L. I.; Bychkov, A. Yu.; Gushchin, A. I.; Koronovsky, N. V.; Tsarev, V. V.

    2016-01-01

    An ophiolitic association consisting of serpentinized ultramafic rocks and serpentinite, layered mafic-ultramafic complex, gabbro and gabbrodolerite, fragments of parallel dike complex, pillow lava, black bedded chert, and jasper has been identified for the first time by authors in the Cape Fiolent area. The chemistry of pillow lavas and dolerites, including REE patterns and a wide set of other microelements, indicates suprasubduction nature of the ophiolites and their belonging to a backarc basin that has reached the stage of spreading in its evolution.

  7. Fluid-rock reactions in an evaporitic melange, Permian Haselgebirge, Austrian Alps

    USGS Publications Warehouse

    Spotl, C.; Longstaffe, F.J.; Ramseyer, K.; Kunk, M.J.; Wiesheu, R.

    1998-01-01

    Tectonically isolated blocks of carbonate rocks present within the anhydritic Haselgebirge melange of the Northern Calcareous Alps record a complex history of deformation and associated deep-burial diagenetic to very low-grade metamorphic reactions. Fluids were hot (up to ~ 250 ??C) and reducing brines charged with carbon dioxide. Individual carbonate outcrops within the melange record different regimes of brine-rock reactions, ranging from pervasive dolomite recrystallization to dedolomitization. Early diagenetic features in these carbonates were almost entirely obliterated. Matrix dolomite alteration was related to thermochemical sulphate reduction (TSR) recognized by the replacement of anhydrite by calcite + pyrite ?? native sulphur. Pyrite associated with TSR is coarsely crystalline and characterized by a small sulphur isotope fractionation relative to the precursor Permian anhydrite. Carbonates associated with TSR show low Fe/Mn ratios reflecting rapid reaction of ferrous iron during sulphide precipitation. As a result, TSR-related dolomite and calcite typically show bright Mn(II)-activated cathodoluminescence in contrast to the dull cathodoluminescence of many (ferroan) carbonate cements in other deep-burial settings. In addition to carbonates and sulphides, silicates formed closely related to TSR, including quartz, K-feldspar, albite and K-mica. 40Ar/39Ar analysis of authigenic K-feldspar yielded mostly disturbed step-heating spectra which suggest variable cooling through the argon retention interval for microcline during the Late Jurassic. This timing coincides with the recently recognized subduction and closure of the Meliata-Hallstatt ocean to the south of the Northern Calcareous Alps and strongly suggests that the observed deep-burial fluid-rock reactions were related to Jurassic deformation and melange formation of these Permian evaporites.

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

    USGS Publications Warehouse

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

    1991-01-01

    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

  9. Ophiolite emplacement in NE Oman: constraints from magnetotelluric sounding

    NASA Astrophysics Data System (ADS)

    Thiel, S.; Heinson, G.; Gray, D. R.; Gregory, R. T.

    2009-03-01

    Magnetotelluric (MT) data have been acquired across the Samail Ophiolite, Oman mountains, along a 115km transect in January 2005. Twenty-five MT stations were deployed approximately every 4km along a profile perpendicular to the coastline near Muscat. The profile extends from the Huw'l/Meeh subwindow in the northwestern part of the profile and crosses the Saih Hatat window, a Tertiary domal culmination that folds a major NE-facing recumbent fold nappe, and ends along the southern boundary between the Dasir and Ibra ophiolite blocks. The survey aimed to investigate the tectonic evolution of the emplacement of the Samail Ophiolite by delineating major faults and geological boundaries on a crustal scale. The data shows complex behaviour with the influence of static shift effects and anomalously high phases exceeding 90°. Phase tensor analysis and 3-D forward modelling hint that the upper plate-lower plate (UP-LP) shear zone has a 3-D effect on the data in the northern part of the profile. Rotated MT impedances were inverted using a 2-D code. Major resistivity interfaces coincide with the location of the UP-LP shear zone near the surface, and dip towards the Arabian margin suggesting a southwest orientated underthrusting prior to the ophiolite emplacement. The para-autochthonous and less deformed upper plate is a resistor, whereas the strongly deformed lower plate is more conductive.

  10. The Ophiolite Problem, Is It Really a Problem?

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

    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.

  11. Structural problems of the Brooks Range ophiolite, Alaska

    SciTech Connect

    Harris, R.A.; Bickerstaff, D. . Dept. of Geology); Stone, D.B. . Geophysical Inst.)

    1993-04-01

    Structural and paleogeographic restorations of the Brooks Range ophiolite (hereafter BRO) and other associated mafic and ultramafic bodies of N. Alaska are difficult because of ambiguous relations between sheeted dikes, cover sediments, and steep NW and SE dipping magmatic flow fabrics. Paleomagnetically enhanced structural studies at Misheguk, Avan, and Siniktanneyak Mountains provide new constraints for the initial dip and sequence of deformation for various structural features of the BRO. The angle between magmatic layers near the petrologic moho and the paleomagnetic inclination of these layers is 50--63[degree] at Misheguk. High level gabbro layers that are disrupted by syn- and post-cooling intrusions display a greater variation. Assuming that the characteristic magnetization is primary, and that the primary inclination was > 80[degree], magmatic layers and the moho had initial dips from 17--40[degree]. These layers now dip 40--70[degree]SE suggesting some post-magmatic tilt. The variation of inclinations with depth in the ophiolite suggest that high level gabbro has tilted most. Sheeted dikes are documented at the Maiyumerak and Siniktanneyak ophiolite bodies. At both locations the dikes dip steeply and strike NE-SW. Sedimentary and volcanic flow layers associated with the dikes have the same strike and dip 0--30[degree]. Parallelism between various planar features throughout the BRO indicates that rotations about a vertical axis are either uniform throughout the ophiolite belt or negligible. Assuming the later, the BRO may represent a linear zone of SSZ magmatism that was oriented NE-SW prior to collision. Post-emplacement long wavelength folding of the ophiolite lid can account for its variation in facing direction and some steepening of magmatic layers.

  12. Tectonic setting of the 1. 73 Ga Payson ophiolite

    SciTech Connect

    Dann, J.C.

    1993-04-01

    The Early Proterozoic orogenic belt of central Arizona is divided by north- to northeast-trending shear zones into a collage of crustal blocks assembled during three periods of convergent tectonism. Whether any of the crustal blocks are allochthonous with respect to one another is an outstanding question. The sheeted dike complex of the Payson ophiolite forms a pseudostratigraphic layer that connects underlying gabbro to an overlying sequence of submarine volcanic rocks. The ophiolite intruded and erupted upon an older basement complex that includes 1.75 Ga granitoids, the oldest rocks known in central Arizona. The 1.73 Ga ophiolite is overlain by dacitic and andesitic volcanic and volcaniclastic rocks and a sequence of turbidites with 1.72 Ga ash beds and is intruded by 1.71 Ga granodiorite. In addition to a distinct arc geochemical signature, the fact that the Payson ophiolite intrudes older arc crust, is intruded by arc plutons, and is overlain by arc-derived rocks in consistent with in situ development within an arc. The northwest boundary of the Mazatzal block, the Moore Gulch fault, separates the Mazatzal block from the 1.74--1.735 Ga arc volcanic and plutonic rocks of the Ash Creek block. If the northeast-trending 1.70 Ga deformational structures are parallel to the paleo-convergent plate boundary, then the northwest-trending dikes of the ophiolite suggest arc-parallel extension. This geometry is more consistent with an intra-arc basin formed as a pull-apart structure along an arc-parallel, strike-slip fault than with a back-arc basin.

  13. Lead isotopic studies of the Samail ophiolite, Oman

    SciTech Connect

    Chen, J.H.; Pallister, J.S.

    1981-04-10

    The isotopic composition of Pb and the concentrations of U, Th, and Pb have been determined for samples from various lithologic units and massive sulfides of the Samail ophiolite. The observed /sup 206/Pb//sup 204/Pb ratios range from 17.90 to 19.06, /sup 207/Pb//sup 204/Pb ratios from 15.43 to 15.63, and /sup 208/Pb//sup 204/Pb from 37.66 to 38.78. In Pb isotopic evolution diagrams, the initial Pb isotopic compositions of most of the samples from the Samail ophiolite plot within the field of oceanic basalt, clearly distinct from island arc data, and define some of the least radiogenic Pb observed from oceanic rocks. Lead data from the Samail are compatible with a model involving magma generation from an oceanic mantle source and formation of the ophiolite at an oceanic spreading center. U--Th--Pb isotopic systematics demonstrate that vertical heterogeneity in the oceanic crust can be created through differential concentration of U, Th, and Pb during crystal fractionation and alteration at, or near, the spreading ridge. Calcite form amygdules in the ophiolite basalt has similar Pb isotopic composition to the igneous rocks, suggesting precipitation of the calcite from seawater which contained Pb derived mostly from the oceanic crust. Lead isotopic data on Fe--Cu sulfides are also similar to the results from the igneous suite suggesting that the source of the sulfides is predominently from the oceanic crust. Lead data from serpentinized peridotite and a galena sample from below the ophiolite suggest that part of the serpentinization process and the formation of galena could involve addition of radiogenic Pb from either a continental source or from oceanic sediments.

  14. Alteration of mélange-hosted chromitites from Korydallos, Pindos ophiolite complex, Greece: evidence for modification by a residual high-T post-magmatic fluid

    NASA Astrophysics Data System (ADS)

    Kapsiotis, Argyrios N.

    2014-12-01

    The peridotites from the area of Korydallos, in the Pindos ophiolitic massif, crop out as deformed slices of a rather dismembered sub-oceanic, lithospheric mantle section and are tectonically enclosed within the Avdella melange. The most sizeable block is a chromitite-bearing serpentinite showing a mesh texture. Accessory, subhedral to euhedral Cr-spinels in the serpentinite display Cr# [Cr/(Cr + Al)] values that range from 0.36 to 0.42 and Mg# [Mg/(Mg+ Fe2+ )] values that vary between 0.57 and 0.62, whereas the TiO2 content may be up to 0.47 wt.%. The serpentinite fragment is characterized by low abundances of magmaphile elements (Al2 O3 : 0.66 wt.%, CaO: 0.12 wt.%, Na2 O: 0.08 wt.%, TiO2 : 0.007 wt.%, Sc: 4 ppm) and enrichment in compatible elements (Cr: 2780 ppm and Ni: 2110 ppm). Overall data are in accordance with derivation of the serpentinite exotic block from a dunite that was formed in the mantle region underneath a back-arc basin before tectonic incorporation in the Korydallos melange. Two compositionally different chromitite pods are recognized in the studied serpentinite fragment, a Cr-rich chromitite and a high-Al chromitite, which have been ascribed to crystallization from a single, progressively differentiating MORB/IAT melt. Although both pods are fully serpentinized only the Al-rich one shows signs of limited Cr-spinel replacement by an opaque spinel phase and clinochlore across grain boundaries and fractures. Modification of the ore-making Cr-spinel is uneven among the Al-rich chromitite specimens. Textural features such as olivine replacement by clinochlore and clinochlore disruption by serpentine indicate that Cr-spinel alteration is not apparently related to serpentinization. From the unaltered Cr-spinel cores to their reworked boundaries the Al2 O3 and MgO abundances decrease, being mainly compensated by FeOt and Cr2 O3 increases. Such compositional variations are suggestive of restricted ferrian chromite (and minor magnetite) substitution for Cr-spinel during a short-lived but relatively intense, low amphibolite facies metamorphic episode (temperature: 400-700 °C). The presence of tremolite and clinochlore in the interstitial groundmass of the high-Al chromitite and their absence from the Cr-rich chromitite matrix imply that after chromitite formation a small volume of a high temperature, post-magmatic fluid reacted with Cr-spinel, triggering its alteration.

  15. Obduction initiation: evidence from the base of New Caledonia ophiolite

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

    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.

  16. Temperatures and cooling rates recorded in REE in coexisting pyroxenes in ophiolitic and abyssal peridotites

    NASA Astrophysics Data System (ADS)

    Dygert, Nick; Liang, Yan

    2015-06-01

    Mantle peridotites from ophiolites are commonly interpreted as having mid-ocean ridge (MOR) or supra-subduction zone (SSZ) affinity. Recently, an REE-in-two-pyroxene thermometer was developed (Liang et al., 2013) that has higher closure temperatures (designated as TREE) than major element based two-pyroxene thermometers for mafic and ultramafic rocks that experienced cooling. The REE-in-two-pyroxene thermometer has the potential to extract meaningful cooling rates from ophiolitic peridotites and thus shed new light on the thermal history of the different tectonic regimes. We calculated TREE for available literature data from abyssal peridotites, subcontinental (SC) peridotites, and ophiolites around the world (Alps, Coast Range, Corsica, New Caledonia, Oman, Othris, Puerto Rico, Russia, and Turkey), and augmented the data with new measurements for peridotites from the Trinity and Josephine ophiolites and the Mariana trench. TREE are compared to major element based thermometers, including the two-pyroxene thermometer of Brey and Köhler (1990) (TBKN). Samples with SC affinity have TREE and TBKN in good agreement. Samples with MOR and SSZ affinity have near-solidus TREE but TBKN hundreds of degrees lower. Closure temperatures for REE and Fe-Mg in pyroxenes were calculated to compare cooling rates among abyssal peridotites, MOR ophiolites, and SSZ ophiolites. Abyssal peridotites appear to cool more rapidly than peridotites from most ophiolites. On average, SSZ ophiolites have lower closure temperatures than abyssal peridotites and many ophiolites with MOR affinity. We propose that these lower temperatures can be attributed to the residence time in the cooling oceanic lithosphere prior to obduction. MOR ophiolites define a continuum spanning cooling rates from SSZ ophiolites to abyssal peridotites. Consistent high closure temperatures for abyssal peridotites and the Oman and Corsica ophiolites suggests hydrothermal circulation and/or rapid cooling events (e.g., normal faulting, unroofing) control the late thermal histories of peridotites from transform faults and slow and fast spreading centers with or without a crustal section.

  17. Foliated breccias in the active Portuguese Bend landslide complex, California: bearing on melange genesis

    SciTech Connect

    Larue, D.K.; Hudleston, P.J.

    1987-05-01

    The active portion of the Portuguese Bend landslide complex is approximately 3 km/sup 2/ in area and 30-50 m thick. Measured displacement rates range from less than one to greater than 30 mm/day on different parts of the landslide, with total displacements over the last 30 yrs ranging from about 10 to greater than 150 m. Six types of breccia, each locally with a foliated matrix, were recognized in the active landslide complex and are absent outside the landslide complex. Slide-body breccias are of two types, the first formed by extensional fracturing during bulk pure shear at the top of the landslide (slide-top breccia) and the second by flow of tuffaceous shales and fracture of embedded siliceous shales during simple shear deep in the landslide to the basal decollement (slide-bottom breccias). Slide-margin breccias, also in simple shear, are produced on the lateral margins of individual slide blocks accompanying wrench-fault motion. Other breccias (fault-ramp breccias) are formed during motion over ramps. Colluvial deposits within tension gashes (crack-fill breccias) and at the toe of the slide (slide-toe breccias) represent a fifth breccia type. Diapirs originating from over-pressured zones at the slide base also contain breccia. Recognition of different breccia types in ancient rocks would be difficult, because fabrics in the different types are similar. Foliations are defined by: scaly cleavage, compositional banding and color banding (in shear zones), stretched mud clasts, and aligned hard grains. Foliated breccias are synonymous with melanges. The authors regard the six breccia types described herein as representing the principal types of melange that occur in ancient accretionary settings.

  18. Kinematic analysis of melange fabrics: Examples and applications from the McHugh Complex, Kenai Peninsula, Alaska

    USGS Publications Warehouse

    Kusky, T.M.; Bradley, D.C.

    1999-01-01

    Permian to Cretaceous melange of the McHugh Complex on the Kenai Peninsula, south-central Alaska includes blocks and belts of graywacke, argillite, limestone, chert, basalt, gabbro, and ultramafic rocks, intruded by a variety of igneous rocks. An oceanic plate stratigraphy is repeated hundreds of times across the map area, but most structures at the outcrop scale extend lithological layering. Strong rheological units occur as blocks within a matrix that flowed around the competent blocks during deformation, forming broken formation and melange. Deformation was noncoaxial, and disruption of primary layering was a consequence of general strain driven by plate convergence in a relatively narrow zone between the overriding accretionary wedge and the downgoing, generally thinly sedimented oceanic plate. Soft-sediment deformation processes do not appear to have played a major role in the formation of the melange. A model for deformation at the toe of the wedge is proposed in which layers oriented at low angles to ??1 are contracted in both the brittle and ductile regimes, layers at 30-45??to ??1 are extended in the brittle regime and contracted in the ductile regime, and layers at angles greater than 45??to ??1 are extended in both the brittle and ductile regimes. Imbrication in thrust duplexes occurs at deeper levels within the wedge. Many structures within melange of the McHugh Complex are asymmetric and record kinematic information consistent with the inferred structural setting in an accretionary wedge. A displacement field for the McHugh Complex on the lower Kenai Peninsula includes three belts: an inboard belt of Late Triassic rocks records west-to-east-directed slip of hanging walls, a central belt of predominantly Early Jurassic rocks records north-south directed displacements, and Early Cretaceous rocks in an outboard belt preserve southwest-northeast directed slip vectors. Although precise ages of accretion are unknown, slip directions are compatible with inferred plate motions during the general time frame of accretion of the McHugh Complex. The slip vectors are interpreted to preserve the convergence directions between the overriding and underriding plates, which became more oblique with time. They are not considered indicative of strain partitioning into belts of orogen-parallel and orogen-perpendicular displacements, because the kinematic data are derived from the earliest preserved structures, whereas fabrics related to strain partitioning would be expected to be superimposed on earlier accretion-related fabrics.Permian to Cretaceous melange of the McHugh Complex on the Kenai Peninsula, south-central Alaska includes blocks and belts of graywacke, argillite, limestone, chert, basalt, gabbro, and ultramafic rocks, intruded by a variety of igneous rocks. An oceanic plate stratigraphy is repeated hundreds of times across the map area, but most structures at the outcrop scale extend lithological layering. Strong rheological units occur as blocks within a matrix that flowed around the competent blocks during deformation, forming broken formation and melange. Deformation was noncoaxial, and disruption of primary layering was a consequence of general strain driven by plate convergence in a relatively narrow zone between the overriding accretionary wedge and the downgoing, generally thinly sedimented oceanic plate. Soft-sediment deformation processes do not appear to have played a major role in the formation of the melange. A model for deformation at the toe of the wedge is proposed in which layers oriented at low angles to ??1 are contracted in both the brittle and ductile regimes, layers at 30-45?? to ??1 are extended in the brittle regime and contracted in the ductile regime, and layers at angles greater than 45?? to ??1 are extended in both the brittle and ductile regimes. Imbrication in thrust duplexes occurs at deeper levels within the wedge. Many structures within melange of the McHugh Complex are asymmetric and record

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

    USGS Publications Warehouse

    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

    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.

  20. Indo-Burma Range: a belt of accreted microcontinents, ophiolites and Mesozoic-Paleogene flyschoid sediments

    NASA Astrophysics Data System (ADS)

    Acharyya, S. K.

    2015-07-01

    This study provides an insight into the lithotectonic evolution of the N-S trending Indo-Burma Range (IBR), constituting the southern flank of the Himalayan syntaxis. Paleogene flyschoid sediments (Disang-Barail) that represent a shallow marine to deltaic environment mainly comprise the west-central sector of IBR, possibly resting upon a continental base. On the east, these sequences are tectonically flanked by the Eocene olistostromal facies of the Disang, which developed through accretion of trench sediments during the subduction. The shelf and trench facies sequences of the Disang underwent overthrusting from the east, giving rise to two ophiolite suites ( Naga Hills Lower Ophiolite ( NHLO) and Victoria Hills Upper Ophiolite ( VHUO), but with different accretion history. The ophiolite and ophiolite cover rock package were subsequently overthrusted by the Proterozoic metamorphic sequence, originated from the Burmese continent. The NHLO suite of Late Jurassic to Early Eocene age is unconformably overlain by mid-Eocene shallow marine ophiolite-derived clastics. On the south, the VHUO of Mesozoic age is structurally underlain by continental metamorphic rocks. The entire package in Victoria Hills is unconformably overlain by shallow marine Late Albian sediments. Both the ophiolite suites and the sandwiched continental metamorphic rocks are thrust westward over the Paleogene shelf sediments. These dismembered ophiolites and continental metamorphic rocks suggest thin-skinned tectonic detachment processes in IBR, as reflected from the presence of klippe of continental metamorphic rocks over the NHLO and the flyschoid Disang floor sediments and half windows exposing the Disang beneath the NHLO.

  1. Chronology of ophiolite crystallization, detachment, and emplacement: Evidence from the Brooks Range, Alaska

    SciTech Connect

    Wirth, K.R.; Bird, J.M. )

    1992-01-01

    {sup 40}Ar/{sup 39}Ar data from early Middle Jurassic ophiolites (187-184 Ma) in the western Brooks Range, Alaska, indicate that detachment - related metamorphism occurred {approximately}20 m.y. after crystallization and {approximately}20 m.y. before emplacement onto the Arctic Alaska margin. High-temperature metamorphic rocks along the basal surfaces of many ophiolites have ages that are contemporaneous with ophiolite crystallization, suggesting that detachment and thrust faulting occur while the lithosphere is young ({lt}10 Ma) and relatively hot. From these relations it has been generally assumed that detachment and initial overthrusting of oceanic lithosphere occur near the site of generation, such as a marginal basin or mid-ocean ridge. The new data from the Brooks Range ophiolites confirm previous indications that some ophiolites have much longer intervals between crystallization, thrust-related metamorphism, and emplacement. On the basis of these new results and data from other ophiolites, the authors propose that ophiolites originating within large ocean basins will generally have longer crystallization-to-emplacement intervals than the more commonly recognized marginal basin-type ophiolites.

  2. The `subduction initiation rule': a key for linking ophiolites, intra-oceanic forearcs, and subduction initiation

    NASA Astrophysics Data System (ADS)

    Whattam, Scott A.; Stern, Robert J.

    2011-11-01

    We establish the `subduction initiation rule' (SIR) which predicts that most ophiolites form during subduction initiation (SI) and that the diagnostic magmatic chemostratigraphic progression for SIR ophiolites is from less to more HFSE-depleted and LILE-enriched compositions. This chemostratigraphic evolution reflects formation of what ultimately becomes forearc lithosphere as a result of mantle melting that is progressively influenced by subduction zone enrichment during SI. The magmatic chemostratigraphic progression for the Izu-Bonin-Mariana (IBM) forearc and most Tethyan ophiolites is specifically from MORB-like to arc-like (volcanic arc basalts or VAB ± boninites or BON) because SI progressed until establishment of a mature subduction zone. MORB-like lavas result from decompression melting of upwelling asthenosphere and are the first magmatic expression of SI. The contribution of fluids from dehydrating oceanic crust and sediments on the sinking slab is negligible in early SI, but continued melting results in a depleted, harzburgitic residue that is progressively metasomatized by fluids from the sinking slab; subsequent partial melting of this residue yields `typical' SSZ-like lavas in the latter stages of SI. If SI is arrested early, e.g., as a result of collision, `MORB-only' ophiolites might be expected. Consequently, MORB- and SSZ-only ophiolites may represent end-members of the SI ophiolite spectrum. The chemostratigraphic similarity of the Mariana forearc with that of ophiolites that follow the SIR intimates that a model linking such ophiolites, oceanic forearcs, and SI is globally applicable.

  3. Tectonic Emplacement of the Ophiolitic Mélange in the West Junggar, NW China: Comment on the Plate Boundary Significance of Ophiolitic Mélange Belt

    NASA Astrophysics Data System (ADS)

    Wang, G.; Xu, Y.; Xiao, L.; Chen, C.

    2014-12-01

    Many ophiolitic mélanges distribute in the West Junggar, NW China. They are fault-contacted with Carboniferous turbidites with mostly NE trend and some NS trend with ages mostly Ordovician-Silurian and some Late Devonian. The boundary faults and the foliation inside the mélanges are of high-angle or nearly vertical. The NE trend ophiolitic mélange belts were structurally emplaced into the Carboniferous strata mainly by dextral transpressive deformation, but the NS trend ophiolitic mélange belts mainly by lateral extrusion deformation or pure shearing, suggesting a uniform stress field of nearly EW compression controlled the emplacements. The tectonic relationship between the ophiolitic mélanges and the Carboniferous turbidites imply that the ophiolitic mélanges are the main components of the basement of the Carboniferous strata. The geophysical data also reveal that high gravity, high magnetic and medium resistivity exist under the Carboniferous strata, matching well to the distribution of the ophiolitic mélanges on the surface. The neodymium model ages (TDM) of widely distributed Late Carboniferous-Permian granites are mostly between 0.352-0.923Ga and concentrate in 0.45-0.6Ga with positive eNd(t) mostly between 5~10, suggesting the Early Paleozoic rocks as the main magma source, consistent with the age of the ophiolitic mélanges, also coinciding with the conclusion of the ophiolitic mélanges as the basement of the Carboniferous strata. The Carboniferous turbidites primarily formed in residual basin. Early Permian terrestrial coarse molasses deposits unconformitily cover on the Carboniferous turbidites, suggesting the residual basin closed in Late Carboniferous. The accretionary complex or residual oceanic crust emplaced into the overlying Carboniferous turbidites through the dextral transpression or lateral extrusion due to EW convergent when the residual basin closed. The tectonic juxtaposition relationship between the ophiolitic mélanges and the younger lateral strata with same stratigraphic system suggests that the ophiolitic mélange belts do not separate different tectonic palaeogeographic or stratigraphic divisions. The traditional understanding of the ophiolitic mélange belt as plate or terrane boundary should be carefully to apply to the West Jungar.

  4. Characteristics of ophiolite-related metamorphic rocks in the Beysehir ophiolitic mélange (Central Taurides, Turkey), deduced from whole rock and mineral chemistry

    NASA Astrophysics Data System (ADS)

    Çelik, Ömer Faruk; Delaloye, Michel F.

    2006-04-01

    Small outcrops of the metamorphic rocks of the Beysehir ophiolite appear to the west of Gencek and to the south of Durak (South of Beysehir Lake) in the Central Tauride Belt in Turkey. Amphibolitic rocks in the ophiolitic mélange have an igneous origin. Protoliths of these rocks were probably alkali basalts, gabbros or some ultramafic cumulates, such as pyroxenite. The amphibolites of the Beysehir Ophiolite can be divided into four groups: (1) amphibole+garnet+plagioclase±epidote (as secondary minerals)±opaque such as ilmenite±accessory minerals such as sphene and apatite; (2) amphibole+pyroxene+plagioclase±epidote±accessory minerals such as sphene, apatite±chlorite, calcite (as secondary mineral); (3) amphibole±plagioclase±opaque±accessory minerals; (4) amphibole+plagioclase±epidote±biotite and muscovite±opaque±accessory minerals. These metamorphic rocks show mainly granoblastic, grano-nematoblastic, porphyroblastic and/or poikiloblastic textures. All amphiboles in the amphibolites are calcic and cluster in the range from magnesio-hastingsite, pargasite to actinolite. Amphibole compositions are characterized by SiO 2=(38.02-54.3%), Al 2O 3=(1.5-12.8), FeO=(10.03-14.67%), K 2O=(0.2-1.8%), MgO=(5.5-15.7), Mg*=(0.3-0.8). The amphibolites show an alkaline to subalkaline character. However, the primitive mantle normalized incompatible trace element diagram shows close similarity with the typical ocean island basalt (OIB) pattern. The Rock/Chondrite normalized REE diagram of the amphibolites also confirms their OIB signature. Tectonomagmatic discrimination diagrams based on the immobile trace elements suggest a mostly within-plate alkali basalt (WPB) environment. Beysehir ophiolitic mélange contains amphibolites from ophiolite-related metamorphic rocks, but the matrix of the Beysehir ophiolitic mélange is not metamorphosed. Blocks of metamorphic rocks and the ophiolitic rocks may have been incorporated into the ophiolitic mélange in an oceanic environment during the Late Cretaceous by tectonic forces.

  5. Supersilicic clinopyroxene from Nidar Ophiolite ultramafics, Ladakh, India

    NASA Astrophysics Data System (ADS)

    Das, S.; Mukherjee, B. K.

    2011-12-01

    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.

  6. Ocean Floor Metamorphism Recorded in the Taitao Ophiolite

    NASA Astrophysics Data System (ADS)

    Shibuya, T.; Komiya, T.; Anma, R.; Maruyama, S.

    2004-12-01

    Ocean floor metamorphism is important process of interaction between sea-water and oceanic crust, namely surface environment and solid earth. We reconstructed a complete section of oceanic ridge emplaced near the Chile Triple Junction based on the geological investigation of the Miocene-Pliocene Taitao ophiolite. The Taitao ophilolite consists of harzburgite, massive and layered gabbro, sheeted dike complex and sequence of pillow lavas and sedimentary rocks in ascending order from the south. These sequences generally strike east-west and dip to the north. Our results revealed that a major shear zone is present between harzburgite and 4 Ma arc-derived granitoid in the south. The thickness of harzburgite is about 1 km. The harzburgite and overlying gabbros are folded into complex pattern together, and the boundary between them is mainly tectonic. The gabbro is about 3 km thick. Sheeted-dike complex has two types of intrusions with different directions; NNE-SSW trending in the northern block, and NW-SE trending in the southern blocks. Piles of pillowed basalts and terrigenous sediments with westward younging were inferred to be associated with sheeted-dike complex of the northern block. Observation of thin sections and analysis by electronprobe microanalyser revealed that metamorphic grade ranges from zeolite facies, throuth the greenschist facies, to amphibolite facies, and increases progressively downwards. The zone boundaris are subparallel to the lithostratigraphy. Metamorphic facies series of the Taitao ophiolite corresponds to the low-pressure type. According to our results, ocean floor metamorphism is well-preserved in the Taitao ophiolite.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    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.

  8. The proterozoic ophiolite problem, continental emergence, and the venus connection.

    PubMed

    Moores, E M

    1986-10-01

    To account for the lack of preservation of ophiolites (fragments of oceanic crust and mantle) in old orogenic belts (age 1000 to 2500 million years), a hypothesis proposes that the magmatic oceanic crust formed during sea-floor spreading was thicker during the cited time interval. This thickening led to reduced contrast between the elevation of continental and oceanic regions and to greater average flooding of the continents. The resultant distribution of elevation may have resembled modern Venus more than modern Earth. PMID:17742635

  9. Methane-Hydrogen Generation in the Zambales Ophiolite (Philippines) Revisited

    NASA Astrophysics Data System (ADS)

    Abrajano, J.; Telling, J.; Sherwood-Lollar, B.; Villiones, R.

    2006-05-01

    The so-called Zambales Ophiolite Methane (ZOM) is one of the earliest reported occurrences of reduced gas in ultramafic terranes. The ZOM also holds the distinction of having the most 13C-enriched carbon of naturally occurring methane seeps on Earth. This attribute, along with evidence that shows strong "mantle-like" noble gas components, led to the general acknowledgement that ZOM represents abiotically generated methane. In this presentation, the geologic setting, host rocks, apparent gas flux and composition and other field attributes of ZOM will be described, based on a fieldwork and sampling that we recently conducted. In addition to the original gas occurrence in Los Fuegos Eternos, LFE (e.g., Abrajano et al., 1988), a newly discovered major gas seep occurrence on Nagsaza, San Antonio, Zambales will also be described. It is noteworthy that the new site occurs in a separate ophiolitic block, and is over 70 km away from the LFE site. Analyses of molecular composition and compound-specific carbon and hydrogen isotope composition of methane and minor hydrocarbons are currently on-going. We will conclude this presentation with a re-assessment of the generation mechanism(s) previously considered for the ZOM and other similar occurrences worldwide.

  10. Diversity of ophiolites and obduction processes: examples from Eastern Tethyan regions and New Caledonia.

    NASA Astrophysics Data System (ADS)

    Whitechurch, Hubert; Agard, Philippe; Ulrich, Marc

    2015-04-01

    Diversity of ophiolites and obduction processes: examples from Eastern Tethyan regions and New Caledonia. Whitechurch H.(1) Agard P.(2), Ulrich M.(1) (1) EOST - University of Strasbourg (France) (2) ISTeP - University Pierre et Marie Curie, Paris (France) Ophiolites are considered as pieces of oceanic lithosphere that escaped subduction to be obducted on continental margins. After the Penrose Conference in 1972, they have all been regarded as issued from mid-ocean ridges of large oceans. Subsequently, most of ophiolites have been considered as generated in supra-subduction zone (SSZ) environment, mainly on the basis of geochemical arguments. However, this characterization encompasses very different geological situations, somewhat in contradiction with a univocal geochemical interpretation, both in terms of where ophiolite formed (i.e., ocean-continent transition zones, ocean ridges, marginal basins) and were obducted (contrasting nature of the margins). Examples from eastern Mesozoic Tethyan ophiolites (Cyprus, Turkey, Syria, Iran, Oman) and tertiary New Caledonia ophiolites all show this diversity, both in their internal structures and geological setting of obduction. Several questions will be addressed in this debate: the relationships and paradoxes between the nature of ophiolites, their geodynamic environment of formation, their geochemistry, their modality of obduction and ultimately the mountain range style where they are found.

  11. The Ust-Belaya ophiolite terrane, West Koryak Orogen: Isotopic dating and paleotectonic interpretation

    NASA Astrophysics Data System (ADS)

    Palandzhyan, S. A.

    2015-03-01

    The Ust-Belaya ophiolite terrane in the West Koryak Orogen, which is the largest in northeastern Asia, consists of three nappe complexes. The upper Ust-Belaya Nappe is composed of a thick (>5 km) sheet of fertile peridotites and mafic rocks (remnants of the proto-Pacific lithosphere); its upper age boundary is marked by Late Neoproterozoic plagiogranites. In the middle Tolovka-Otrozhny Nappe, the Late Precambrian lherzolite-type ophiolites are supplemented by fragments of tectonically delaminated harzburgite-type ophiolites, which make up the Tolovka rock association. The isotopic age of metadacite (K-Ar method, whole-rock sample) and zircons from plagiogranite porphyry (U-Pb method, SHRIMP) determines the upper chronological limit of the Tolovka ophiolites as 262-265 Ma ago. It is suggested that igneous rocks of these ophiolites were generated in a backarc basin during the Early Carboniferous and then incorporated into the fold-nappe structure in the Mid-Permian. This was the future basement of the Koni-Taigonos arc, where the Early Carboniferous ophiolites together with Late Neoproterozoic precursors were subject to low-temperature metamorphism and intruded by plagiogranite porphyry dikes in Permian-Triassic. The polymicte serpentinite mélange, which was formed in the accretionary complex of the Koni-Taigonos arc comprises rock blocks of the upper units of Late Precambrian ophiolites (in particular, plagiogranite), the overlying Middle to Upper Devonian and Early Carboniferous deposits, as well as Early Carboniferous (?) Tolovka ophiolites and meta-ophiolites. Mélange of this type with inclusions of Late Precambrian "oceanic" granitoids also developed in the lower Utyosiki Nappe composed of Middle Jurassic-Lower Cretaceous sedimentary and volcanic sequences, the formation of which was related to the next Uda-Murgal island-arc systems.

  12. Using Zircon Geochronology to Unravel the History of the Naga Hills Ophiolite

    NASA Astrophysics Data System (ADS)

    Roeder, T.; Aitchison, J. C.; Clarke, G. L.; Ireland, T. R.; Ao, A.; Bhowmik, S. K.

    2014-12-01

    Outcrops of the Naga Hills Ophiolite (NHO), a possible eastern extension of the ophiolitic belt running along the India-Asia suture, in Northeast India include a full suite of ophiolitic rocks. The ophiolite has been dated Upper Jurassic based on radiolarian studies of the unit (Baxter et al., 2011) but details of its emplacement onto the Indian margin have not been the subject of detailed investigation. Conglomerates of the Phokphur Formation unconformably overlie an eroded surface on top of dismembered ophiolite fragments and include sediments sourced from both the ophiolite and the margin of the Indian subcontinent. Notably no Asian margin-derived detritus is recognised (similar to the Liuqu conglomerates of Tibet (Davis et al., 2002)). Thus, a detailed study of the Phokphur sediments can produce valuable details of the NHO history, including constraining the timing of ophiolite emplacement. Studies of detrital sandstone petrography confirm a recycled orogen provenance for the Phokphur Formation and thus serve as validation of the methods of Dickinson and Suczek (1979) and Garzanti et al. (2007). Detrital zircon data provides further insight as to the age of source rocks of Phokphur sediments and help to further constrain the timing of ophiolite emplacement. We present results of sedimentary and detrital zircon geochronology analyses of Phokphur sediments from outcrops near the villages of Salumi and Wazeho as a contribution to furthering research on aspects of the India-Asia collision. Baxter, A.T., et al. 2011. Upper Jurassic radiolarians from the Naga Ophiolite, Nagaland, northeast India. Gondwana Research, 20: 638-644. Davis, A.M., et al. 2002. Paleogene island arc collision-related conglomerates, Yarlung-Tsangpo suture zone, Tibet. Sedimentary Geology, 150: 247-273. Dickinson, W.R. and Suczek, C.A., 1979. Plate tectonics and sandstone compositions. Am. Assoc. Pet. Geol. Bull., 63, 2164-2182, (1979). Garzanti, E., et al., 2007. Orogenic belts and orogenic sediment provenance. The Journal of Geology, 115: 315-334.

  13. Geochemistry and tectonic evolution of the Late Cretaceous Gogher-Baft ophiolite, central Iran

    NASA Astrophysics Data System (ADS)

    Moghadam, Hadi Shafaii; Stern, Robert J.; Chiaradia, Massimo; Rahgoshay, Mohamad

    2013-05-01

    The Late Cretaceous Gogher-Baft ophiolite is one of the best preserved remnants of Neo-Tethyan oceanic lithospheric within the inner Zagros ophiolite belt. The ophiolite comprises from bottom to top, harzburgites, pegmatite and isotropic gabbroic lenses within the mantle sequence, pillowed to massive basalts to dacites and pyroclastic rocks associated with blocks of pelagic limestone and radiolarite. Basaltic to dacitic sills crosscut the pyroclastic rocks. The ophiolite sequence is overlain by Turonian-Maastrichtian pelagic limestones (93.5-65.5 Ma). Mineral compositions of harzburgites are similar to those of fore-arc peridotites and overlap with abyssal peridotites. Most Gogher-Baft ophiolite magmatic rocks show supra-subduction zone affinities, except for some E-MORB type lavas. The geochemical characteristics suggest that Gogher-Baft ophiolite magmatic rocks were generated during subduction initiation. These show progressive source depletion leading to the formation of MORB to boninitic magmas. Early E-MORB-type pillow lavas may have originated by melting mantle that was not affected by subduction components as the Tethyan oceanic plate began to sink beneath Eurasia as subduction began in the Late Cretaceous. Initial ?Nd (t) values range from + 2.6 to + 9 for Gogher-Baft magmatic rocks. Samples with radiogenic Nd overlap with least radiogenic MORBs and with Oman and other Late Cretaceous Tethyan ophiolitic rocks. The initial 87Sr/86Sr ratios range from 0.7048 to 0.7057, indicating modification due to seafloor alteration. Radiogenic 207Pb/204Pb isotopic compositions (systematically above the NHRL) and less radiogenic Nd isotopic compositions suggest the involvement of sediments in the mantle source in some magmatic rocks. Our results for Gogher-Baft ophiolite and the similarity of these to other Iranian Zagros ophiolites suggest a subduction initiation setting for the generation of these magmatic rocks.

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

    NASA Astrophysics Data System (ADS)

    Chavez Garcia, Maria Graciela

    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.

  15. Formation of a cold ophiolitic sole at the base of the Devonian Balkan Carpathian Ophiolite (Romania, Serbia, Bulgaria)

    NASA Astrophysics Data System (ADS)

    Plissart, Gaëlle; Diot, Hervé; Monnier, Christophe; Maruntiu, Marcel; Debaille, Vinciane; Neubauer, Franz

    2013-04-01

    Our study concerns deformed gabbroic rocks from the Balkan Carpathian Ophiolite (BCO - Romania, Serbia, Bulgaria). The BCO consists of four ophiolitic massifs dismembered during Alpine tectonic and displaying together a complete classical oceanic lithosphere. Our new Sm-Nd dating on fresh lower gabbroic rocks give an accretion age for the BCO crust at 409 ± 38 Ma, in agreement with a previous age of 405 ± 3 Ma (Zakariadze et al. 2012). After removing the Alpine tectonic, the BCO appears as an elongated E-W body tilted to the south. At the base of the ophiolitic complex occurs a thin deformed zone (< 800m) of metagabbroic rocks underlined by Cambro-Ordovician metasediments. Petrostructural observations on metagabbroic rocks coupled with mineralogical and geochemical data indicate that their protoliths were mainly upper gabbros statically metamorphosed in the Greenschist/Amphibolite facies (event 1 = ocean-floor metamorphism at the ridge axis). These rocks have been affected by a second circulation of fluids (event 2), contemporaneous to a deformation and inducing local K-enrichment (formation of Cr-muscovite). Temperature estimates for this event indicate a range of 450°C - 280°C, with the lower values observed for the more intensively metasomatized rocks. 40Ar - 39Ar dating on two Cr-muscovites from slightly and highly deformed metagabbros gives plateau ages of 372.6 ± 1.3 Ma and 360.6 ± 1.2 Ma respectively. We interpret the first age as a mimimum age for the beginning of the event 2, observed into preserved rocks, and the second one as linked to (neo-/)recrystallisation due to localisation of the metasomatism/deformation. The interval of 30 Ma between oceanic crust accretion and initiation of metasomatism/deformation involves that the upper oceanic crust had cooled down to temperatures close to 100°C before the beginning of event 2. Consequently, a temperature increase is required to observe the greenschist facies assemblage. We have tested by tectono-thermal modelling the hypothesis that these rocks could correspond to a slice of upper crust dragged down during intra-oceanic subduction: temperatures of 450°C are reached at a depth of 17 km (5 kbar). These PT conditions are in agreement with the mineralogical assemblage formed during event 2 meanwhile the intense fluids circulation and the K-rich metasomatism (up to 5% K2O for bulk rock analyse) could be explained by the destabilization of deep oceanic sediments. To initiate the subduction of a 30 Ma old oceanic lithosphere, we propose a zone of weakness alongside a transform fault that juxtaposes oceanic lithospheres of different ages and thicknesses, supported by microstructural criteria that evidence a highly oblique tectonic obduction. Our study emphasizes that ophiolitic soles could develop away from the ridge in a context of intra-oceanic subduction along a transform fault. In that case, the hanging wall will not be hot enough to produce classical high-grade metamorphic sole and the resulting rocks could be referred as "cold ophiolitic soles".

  16. Analysis of Environmental Forcing and Melange Fluctuation in Asynchronous Retreat of Ocean Terminating Glaciers in Greenland's Sermilik Fjord

    NASA Astrophysics Data System (ADS)

    Seifert, F.; Galey, C. E.; Bassis, J. N.

    2014-12-01

    Widespread near synchronous retreat of marine terminating outlet glaciers has been observed across wide swaths of the Greenland Ice Sheet. However, despite large-scale patterns of retreat, there is considerable variability in the timing and retreat patterns of individual glaciers with geographically adjacent glaciers that experience similar climate and meteorological forcing displaying markedly different behavior. Here we applied an automated identification algorithm that we developed to track the terminus and melange in order better understand the complex dynamics and varying drivers of glacier retreat. The algorithm was applied to three major glaciers (Helheim Glacier, Fenris Glacier and Midgard Glacier) that terminate in Greenland's Sermilik Fjord over the period of 2000- 2014. The terminus position and the percentage of the fjord filled with melange or sea ice from 2001 to present was determined. Since these glaciers exist within the same fjord system, they should experience comparable environmental forcing conditions, but appear to respond to these conditions differently causing them to have varying patterns of retreat. Helheim Glacier and Fenris Glacier have terminus locations closely spaced in the fjord but Helheim Glacier's terminus retreated over 7 km before advancing to stabilize at a 5 km retreat over the observation period and Fenris Glacier's terminus has stayed in roughly the same place. Midgard Glacier is located across the fjord from Helheim Glacier and its terminus has continuously retreated with a retreat of approximately 8 km. This asynchronous retreat shows that proximity alone cannot determine retreat behavior, and a more complex interaction between internal variability and external forcing must be taking place. To better understand the variability within the system and the cause of asynchronous retreat, ocean and air temperature datasets, in conjunction with the fjord geometry, were compared with our derived melange/sea ice and terminus position data.

  17. Ophiolites of the deep-sea trenches of the western Pacific

    SciTech Connect

    Chudaev, O.

    1990-06-01

    Igneous and metamorphic rocks of ophiolites are widespread in the basement of the Izu-Bonin, Volcano, Mariana, Philippine, Yap, Palau, New Hebrides, West Melanesian, Tonga, and Mussau trenches. Ophiolite in the trenches includes (1) metamorphic rocks ranging from low-T and low P to high-T and moderate P; (2) serpentinites after harzburgite; (3) ultramafic-mafic layered series; (4) tholeiitic basalts, dolerites, and their metamorphosed varieties; and (5) rocks of the boninitic series. Two geodynamic stages could be distinguished in the formation of the ophiolites of the Western Pacific. At the rifting stage, mantle diapir ascended during rifting and magmatic series were formed. In this period, rocks underwent intense low-temperature metasomatic alterations in the zones of heated seawater circulation. At the compression stage (subduction zone) regional pressure metamorphism of the ophiolites superimposed on the low-temperature metasomatic processes.

  18. Carbon recycling in ophiolite-hosted carbonates, Oman-UAE

    NASA Astrophysics Data System (ADS)

    Stephen, A.; Jenkin, G. R.; Smith, D. J.; Styles, M. T.; Naden, J.; Boyce, A. J.; Bryant, C. L.

    2013-12-01

    Large-scale surface and subsurface freshwater carbonate deposits of probable Quaternary age have formed on the Oman-UAE ophiolite. Here, serpentinisation reactions in ultramafic rocks have produced calcite and magnesite. These carbonates are frequently cited as examples of natural atmospheric CO2 sequestration, but the possibility of carbon recycling has not been addressed. The aim of this study is to assess the degree of atmospheric CO2 being incorporated into carbonates versus that which has been recycled from alternative sources such as soil CO2, or limestones that underlie the ophiolite. This has been determined through ?13C/?18O, 87Sr/86Sr and 14C analysis of all major carbonate lithofacies identified. Our analyses of modern carbonate crusts forming on the surface of stagnant hyperalkaline (pH >11) waters show highly depleted ?13C and ?18O values (-25.5‰ ×0.5 PDB and -16.8‰ ×0.5 PDB respectively). This depletion has been attributed to a kinetic isotope effect occurring during atmospheric CO2 exchange with Ca(OH)2 hyperalkaline waters [1]. By comparison, inactive travertine deposits show a large range in ?13C (-10.5 to -21.8‰ PDB) which lies on a trajectory from the composition of modern crusts towards bicarbonate fluids in equilibrium with soil CO2. We interpret this trend as being produced by the mixing of different carbon sources, either at the time of formation or during later alteration. Modern carbonates and inactive travertines also have 87Sr/86Sr ratios and Sr concentrations similar to Cretaceous and Tertiary limestones which surround the ophiolite, whilst subsurface veins also display 87Sr/86Sr ratios similar to these Cretaceous limestones. Carbon recycling can also be determined with 14C. Modern atmospheric CO2 has a global average of 105-106% modern 14C (pMC), therefore freshwater carbonates forming solely from atmospheric CO2 would be expected to contain >100 pMC. However, modern carbonates display varied results from 94.5-101.4 pMC. Low values could be caused by meteoric waters incorporating 14C 'dead' carbon through the dissolution of limestones and/or uptake of soil CO2. This 'dead' carbon would then be assimilated into veins and surface deposits, offsetting pMC values. Inactive travertines show significant fluctuations in 14C values within a single hand sample, where stratigraphically younger samples give older radiocarbon 'ages' outside of error. These fluctuations may have been caused by the presence of limestone sourced 'dead' carbon in waters at time of formation, surface runoff containing soil CO2 or by later recrystallisation. Isotopic evidence indicates that mixing of contemporary atmospheric carbon and recycled older carbon has taken place during the on-going carbonation of the Oman-UAE ophiolite sequence. Failure to account for this recycled carbon could lead to inaccurate estimates of natural CO2 sequestration rates. References [1] Clark, I.D. and Fontes, J. (1990) Palaeoclimatic reconstruction in Northern Oman based on carbonates from hyperalkaline groundwaters. Quaternary Res, 33, 320-336

  19. Submarine hydrothermal metamorphism of the Del Puerto ophiolite, California.

    USGS Publications Warehouse

    Evarts, R.C.; Schiffman, P.

    1983-01-01

    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.

  20. Petrology of plagiogranite from Sjenica, Dinaridic Ophiolite Belt (southwestern Serbia)

    NASA Astrophysics Data System (ADS)

    Milovanovi?, Dragan; Sre?kovi?-Bato?anin, Danica; Savi?, Marija; Popovic, Dana

    2012-04-01

    The Sjenica plagiogranite occurs in the southern part of the Dinaridic Ophiolite Belt, 5 km northwest of Sjenica. The main minerals are albite with strongly altered biotite (replaced with chlorite), with occasional amphibole (magnesio hornblende to tschermakite) and quartz. An enclave of fine-grained granitic rocks with garnet grains was noted too. Secondary minerals are calcite and chlorite (daphnite). Major, trace and REE geochemistry coupled with field observations support a model by which the Sjenica plagiogranite could be formed by fractional crystallization of mantle origin mafic magma in a supra-subduction zone setting. Occurrences of calcite and chlorite nests in the Sjenica plagiogranites revealed that these rocks underwent hydrothermal alteration due to intensive sea water circulation in a sub-sea-floor environment.

  1. Selenium content of sulfide ores related to ophiolites of Greece.

    PubMed

    Economou-Eliopoulos, M; Eliopoulos, D G

    1998-01-01

    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

  2. Geochemistry and petrology of listvenite in the Samail ophiolite, Sultanate of Oman: Complete carbonation of peridotite during ophiolite emplacement

    NASA Astrophysics Data System (ADS)

    Falk, Elisabeth S.; Kelemen, Peter B.

    2015-07-01

    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. These rocks can provide insight into processes including (a) carbon fluxes at the "leading edge of the mantle wedge" in subduction zones and (b) enhanced mineral carbonation of peridotite as a means of carbon storage. Here we examine mineralogical, chemical and isotopic evidence on the temperatures, timing, and fluid compositions involved in the formation of this listvenite. The listvenites are composed primarily of magnesite and/or dolomite + quartz + relict Cr-spinel. In some instances the conversion of peridotite to listvenite is nearly isochemical except for the addition of CO2, while other samples have also seen significant calcium addition and/or variable, minor addition of K and Mn. Along margins where listvenite bodies are in contact with serpentinized peridotite, talc and antigorite are present in addition to carbonate and quartz. The presence of antigorite + quartz + talc in these samples implies temperatures of 80-130 °C. This range of temperature is consistent with dolomite and magnesite clumped isotope thermometry in listvenite (average T = 90 ± 15 °C) and with conventional mineral-water oxygen isotope exchange thermometry (assuming fluid ?18O near zero). CO2-bearing fluids responsible for the formation of listvenite were likely derived from underlying calcite-bearing metasediment during emplacement of the ophiolite. 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 over allochthonous sediments of the Hawasina group, and autochthonous sediments of the Arabian continental margin. Most of the initial 87Sr/86Sr values in the listvenite, ranging from 0.7085 to 0.7135, are significantly higher than seawater values and consistent with values measured in the underlying metasediments. While constraints on the pressure of listvenite formation are lacking, the moderate temperatures suggest that listvenites formed at relatively shallow depths in the subduction zone, making release of carbonate-saturated pore-water due to compaction of subducted sediment or low-pressure phase transitions of hydrous minerals, such as clays, probable sources of the CO2-bearing fluid. Carbonate dissolution from subducted sediments and transfer of CO2 to the mantle wedge to form listvenites may be an important process in forearc hydrothermal systems. Additionally, the presence of listvenites demonstrate that peridotite carbonation reactions can proceed to completion on large scales, suggesting that in situ mineral carbonation of peridotite may offer a viable solution for carbon storage.

  3. Progressive metamorphism of the Taitao ophiolite; evidence for axial and off-axis hydrothermal alterations

    NASA Astrophysics Data System (ADS)

    Shibuya, Takazo; Komiya, Tsuyoshi; Anma, Ryo; Ota, Tsutomu; Omori, Soichi; Kon, Yoshiaki; Yamamoto, Shinji; Maruyama, Shigenori

    2007-10-01

    We estimated metamorphic conditions for the ˜ 6 Ma Taitao ophiolite, associated with the Chile triple junction. The metamorphic grade of the ophiolite, estimated from secondary matrix minerals, changes stratigraphically downwards from the zeolite facies, through the prehnite-actinolite facies, greenschist facies and the greenschist-amphibolite transition, to the amphibolite facies. The metamorphic facies series corresponds to the low-pressure type. The metamorphic zone boundaries are subparallel to the internal lithological boundaries of the ophiolite, indicating that the metamorphism was due to axial hydrothermal alteration at a mid-ocean ridge. Mineral assemblages and the compositions of veins systematically change from quartz-dominated, through epidote-dominated, to prehnite-dominated with increasing depth. Temperatures estimated from the vein assemblages range from ˜ 230 °C in the volcanic unit to ˜ 380 °C at the bottom of the gabbro unit, systematically ˜ 200 °C lower than estimates from the adjoining matrix minerals. The late development of veins and the systematically lower temperatures suggest that the vein-forming alteration was due to off-axis hydrothermal alteration. Comparison between the Taitao ophiolite with its mid-ocean ridge (MOR) affinity, and other ophiolites and MOR crusts, suggests that the Taitao ophiolite has many hydrothermal alteration features similar to those of MOR crusts. This is consistent with the tectonic history that the Taitao ophiolite was formed at the South Chile ridge system near the South American continent (Anma, R., Armstrong, R., Danhara, T., Orihashi, Y. and Iwano, H., 2006. Zircon sensitive high mass-resolution ion microprobe U-Pb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications. The Island Arc, 15(1): 130-142).

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

    USGS Publications Warehouse

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

    1975-01-01

    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.

  5. Tectonics of formation, translation, and dispersal of the Coast Range ophiolite of California

    USGS Publications Warehouse

    McLaughlin, R.J.; Blake, M.C., Jr.; Griscom, A.; Blome, C.D.; Murchey, B.

    1988-01-01

    Data from the Coast Range ophiolite and its tectonic outliers in the northern California Coast Ranges suggest that the lower part of the ophiolite formed 169 to 163 Ma in a forearc or back arc setting at equatorial latitudes. Beginning about 156 Ma and continuing until 145 Ma, arc magmatism was superimposed on the ophiolite, and concurrently, a transform developed along the arc axis or in the back arc area. Rapid northward translation of this rifted active magmatic arc to middle latitudes culminated in its accretion to the California margin of North America at about 145 Ma. This Late Jurassic episode of translation, arc magmatism, and accretion coincided with the Nevadan orogeny and a proposed major plate reorganization in the eastern Pacific basin. Displacement occurred between about 60 and 52 Ma. Ophiolitic rocks in the Decatur terrane of western Washington that have recently been correlated with the Coast Range ophiolite and the Great Valley sequence of California were apparently displaced at least 950 to 1200 km from the west side of the Great Valley between early Tertiary and Early Cretaceous time. Derived rates of northward translation for the ophiolite outliers in California are in the range of 1 to 4 cm/yr. -from Authors

  6. Reconstructing Ophiolites: Reassessing Assumptions From the Oceanic Crust and Related Terranes

    NASA Astrophysics Data System (ADS)

    Karson, J. A.

    2014-12-01

    The internal structure of ophiolite complexes has long been used as a window into the inaccessible parts of the oceanic lithosphere and by inference, processes beneath spreading centers. However, even the best preserved ophiolite complexes have been tilted, folded, faulting and dismembered during post-spreading tectonic events. Some degree of reconstruction is required to restore ophiolite structures to their appropriate relative orientations in order to relate them to processes beneath spreading centers. A number of assumptions about ophiolite structures have been used to guide reconstructions including: lava flows (horizontal, especially sheet-like lavas), dikes in lavas and sheeted dike complexes (vertical and parallel to spreading centers), the contacts between major rock units (horizontal, analogous to the seismic structure of oceanic crust) and the mafic/ultramafic contact representing the geologic expression of the Moho (horizontal). Based in part on these assumptions the internal structure of rock units, metamorphic relationships, and the kinematics of faults and deformation fabrics are also inferred. The spreading direction is seldom constrained in ophiolites making it difficult to assess the geometry of asymmetrical features, such as the dip of dikes, faults, or igneous layering, relative to spreading axes. Observations from exposures of upper crustal rock units (lavas, transition zones, dike and upper gabbroic rocks) along major tectonic escarpments in oceanic crust formed at fast to intermediate rates, as well as the uplifted and glaciated Tertiary basaltic crust of Iceland, raise questions about several of the assumptions used in ophiolite reconstructions. Alternative reconstructions may provide new ideas about spreading processes.

  7. Imbricate structure of the Luobusa Ophiolite and surrounding rock units, southern Tibet

    NASA Astrophysics Data System (ADS)

    Yamamoto, Hiroshi; Yamamoto, Shinji; Kaneko, Yoshiyuki; Terabayashi, Masaru; Komiya, Tsuyoshi; Katayama, Ikuo; Iizuka, Tsuyoshi

    2007-02-01

    The Cretaceous Luobusa Ophiolite is a tectonic slice less than 1.2 km thick. The structurally underlying unit is the Tertiary Luobusa Formation and the overlying unit is composed of feebly metamorphosed Triassic sedimentary rocks. The top and bottom contacts of the ophiolite dip gently to the south. The major part of the Luobusa Ophiolite is mantle peridotite, including podiform chromitite. This chromitite has received much attention because it contains an 'unusual mineral assemblage' which includes diamond and moissanite. A serpentinite mélange zone, including clasts of basaltic and sedimentary rocks, occurs underneath the mantle peridotite. Mesoscopic-microscopic structures were observed in the sheared rocks. Shear-indicating structures (C'-type shear bands and ?-type porphyroclasts) in the rocks near the top and bottom boundaries of the Luobusa Ophiolite show consistent top-to-the-north (or northeast) reverse displacement. The results reveal that the Luobusa Ophiolite was overturned and intercalated into an imbricate structure. The thrust faults on the top and bottom of the ophiolite can be correlated with north-vergent back-thrusting which was associated with crustal shortening along the Main Central Thrust due to the continued northward movement of India, after the welding of India to Asia.

  8. The P-T-t history of blocks in serpentinite-matrix melange, west-central Baja California

    SciTech Connect

    Baldwin, S.L.; Harrison, T.M.

    1992-01-01

    Thermochronologic, petrologic, and geochemical analyses of epidote-amphibolite-, amphilbolite-, eclogite-, and blueschist-facies blocks in serpentinite-matrix melanges from East San Benito Island and Cedros Island, west-central Baja California, provide constraints on the P-T-t history of this disrupted terrane. Results of {sup 40}Ar/{sup 39}Ar step heating experiments on minerals separated from these blocks vary according to metamorphic grade and indicate different P-T-t histories. Mid-Jurassic (160-170 Ma) epidote-amphibolite- and amphibolite-facies blocks were probably derived from oceanic crust and associated sediments that were metamorphosed during limitation of subduction. Coarse-grained blueschist blocks were likely metamorphosed during continued subduction in late Early Cretaceous time (that is, 115-100 Ma). Some epidote-amphibolite blocks are partially overprinted by blueschist-facies mineral assemblages and apparently record both metamorphic events. Fission-track analyses of apatites indicate that the blocks underwent significantly different post-metamorphic cooling histories. Epidote-amphilbolite and amphibolite blocks cooled below {approximately}100 {degrees}C form mid-Jurassic to Paleocene time; blueschist blocks cooled below {approximately}100 {degrees}C in Oligocene-Miocene time. In general, mafic blocks have trace-element concentrations and REE patterns characteristic of ocean-flood basalts. This study demonstrates that samples with very different P-T-t histories can evidently occur over relatively small length scales (<<1 km) in serpentinite-matrix melanges. 67 refs., 16 figs., 4 tabs.

  9. Dynamics of convergent plate boundaries: Insights from subduction-related serpentinite melanges from the northern edge of the Caribbean plate

    NASA Astrophysics Data System (ADS)

    García-Casco, A.

    2012-04-01

    Subduction-related rock complexes, many of them tectonic melanges, occur in the Central America-Caribbean-Andean belt. I review the lithology and P-T-t paths of HP rocks and offer interpretations and generalizations on the thermal estate of the subducting plate(s), the melange forming events, and the exhumation history of rock complexes formed in the northern branch of the Caribbean subduction zone (Cuba and nearby Guatemala and Dominican Republic; ca. 3000 km apart). These complexes contain high pressure rocks formed and exhumed at the convergent (Pacific-Atlantic) leading edge of the Caribbean plate during ca. 100 Ma (early Cretaceous-Oligocene), attesting for long lasting oceanic -followed by continental- subduction/accretion in the region. Lithologic data indicate a complex melange-forming process. In most cases, the HP rocks represent subducted MOR-related lithologies occurring as tectonic blocks within serpentinite-matrix melanges interpreted as exhumed fragments of the subduction channel(s). Most of these melanges, however, contain fragments of arc/forearc-related non metamorphic and metamorphic (low-P and high-P) sedimentary and igneous rocks. While the HP blocks of arc/forearc material indicate subduction erosion at depth, the interpretation of the LP and non-metamorphic blocks is not straight forward. Indeed, tectonic blocks of HP metamafic rocks are surrounded by antigorite-serpentinite which, in turn, is surrounded by a low-P, low-T (chrysotile-lizardite) serpentinite that makes much of the mélange. These relations indicate that the melanges represent, in fact, tectonic stacks of shallow low-T forearc serpentinite that incorporate tectonic blocks/slices of the subduction-channel (high-P, high-T serpentinite and HP metamafic blocks) and of the arc/forearc crust (low-P and non-metamorphic blocks). This picture is similar to that of HP continental margin-derived tectonic stacks containing exotic slices of antigoritite-serpentine melanges (with blocks of MORB-derived eclogite) incorporated late in the convergent history when oceanic subduction was completed. Hence, incorporation of tectonic slices of the subduction channel into the shallow (low-P, low-T) melanges and subducted/accreted continental margins occur when collision-related dynamics imposed by subduction of buoyant continental or oceanic lithosphere affected the plate margin. Aqueous fluid, sourced from both subducted sediment and metamafic/ultramafic material, was available in large quantity in the subduction environment, as indicated by massive antigoritite, rinds of metasomatic rocks around included HP metamafic rocks, retrogressed eclogite, jadeitite and hydrothermal veins within antigoritite. Such a vigorous hydrology (fluid-flow) deep in the subduction environment point to the development of wide subduction channels and explain the abundance of accreted blocks. It can also explain the scarcity of large scale (>km) slices of the subducted oceanic lithosphere in the belt, for these are likely the result of focalized distribution of deformation occurring when forearc peridotite is barely hydrated (Agard et al., Long-term coupling along the subduction plate interface: Insights from exhumed rocks and models. This session, EGU 2012). Alternatively, these large tectonic slices may have been formed by the collision dynamics caused by late-stage subduction/accretion of the continental margin (or buoyant -thick- oceanic crust). Except maturation (cooling) of the subduction zone with time at orogenic belt-scale, no other simple generalization can be reached on the thermal state of the subducting plate and the exhumation process of the subduction channel. P-T-t paths of HP rocks indicate that slab fragments ranging from cold to hot coexisted during relatively short time intervals (ca. 10 Myr), and some fragments of the subduction channel were exhumed shortly after formation while others lasted several tens of Myr to arrive to the near-surface forearc/accretionary environment. A rather variable thermal state and dynamic history of the subduction environme

  10. Ophiolitic detritus in Kimmeridgian resedimented limestones and its provenance from an eroded obducted ophiolitic nappe stack south of the Northern Calcareous Alps (Austria)

    NASA Astrophysics Data System (ADS)

    Gawlick, Hans-Jürgen; Aubrecht, Roman; Schlagintweit, Felix; Missoni, Sigrid; Plašienka, Dušan

    2015-12-01

    The causes for the Middle to Late Jurassic tectonic processes in the Northern Calcareous Alps are still controversially discussed. There are several contrasting models for these processes, formerly designated "Jurassic gravitational tectonics". Whereas in the Dinarides or the Western Carpathians Jurassic ophiolite obduction and a Jurassic mountain building process with nappe thrusting is widely accepted, equivalent processes are still questioned for the Eastern Alps. For the Northern Calcareous Alps, an Early Cretaceous nappe thrusting process is widely favoured instead of a Jurassic one, obviously all other Jurassic features are nearly identical in the Northern Calcareous Alps, the Western Carpathians and the Dinarides. In contrast, the Jurassic basin evolutionary processes, as best documented in the Northern Calcareous Alps, were in recent times adopted to explain the Jurassic tectonic processes in the Carpathians and Dinarides. Whereas in the Western Carpathians Neotethys oceanic material is incorporated in the mélanges and in the Dinarides huge ophiolite nappes are preserved above the Jurassic basin fills and mélanges, Jurassic ophiolites or ophiolitic remains are not clearly documented in the Northern Calcareous Alps. Here we present chrome spinel analyses of ophiolitic detritic material from Kimmeridgian allodapic limestones in the central Northern Calcareous Alps. The Kimmeridgian age is proven by the occurrence of the benthic foraminifera Protopeneroplis striata and Labyrinthina mirabilis, the dasycladalean algae Salpingoporella pygmea, and the alga incertae sedis Pseudolithocodium carpathicum. From the geochemical composition the analysed spinels are pleonastes and show a dominance of Al-chromites (Fe3+-Cr3+-Al3+ diagram). In the Mg/(Mg+ Fe2+) vs. Cr/(Cr+ Al) diagram they can be classified as type II ophiolites and in the TiO2 vs. Al2O3 diagram they plot into the SSZ peridotite field. All together this points to a harzburgite provenance of the analysed spinels as known from the Jurassic suprasubduction ophiolites well preserved in the Dinarides/Albanides. These data clearly indicate Late Jurassic erosion of obducted ophiolites before their final sealing by the Late Jurassic-earliest Cretaceous carbonate platform pattern.

  11. Tectonics of formation, translation, and dispersal of the Copast Range ophiolite of California

    NASA Astrophysics Data System (ADS)

    McLaughlin, R. J.; Blake, M. C., Jr.; Griscom, A.; Blome, C. D.; Murchey, B.

    1988-10-01

    Data from the Coast Range ophiolite and its tectonic outliers in the northern California Coast Ranges suggest that the lower part of the ophiolite formed 169 to 163 Ma in a forearc or back arc setting at equatorial latitudes. Beginning about 156 Ma and continuing until 145 Ma, arc magmatism was superimposed on the ophiolite, and concurrently, a transform developed along the arc axis or in the back arc area. Rapid northward translation of this rifted active magmatic arc to middle latitudes culminated in its accretion to the California margin of North America at about 145 Ma. This Late Jurassic episode of translation, arc magmatism, and accretion coincided with the Nevadan orogeny and a proposed major plate reorganization in the eastern Pacific basin. The high rate of poleward motion necessary to translate the Coast Range ophiolite to middle latitudes during this time implies that the ophiolite traveled north on a fast-moving plate of the eastern Pacific basin, here termed plate X. Plate X probably was driven by a cryptic ridge east-northeast of the Pacific-Farallon-Izanagi ridge triple junction. Structural relations indicate that following Late Jurassic time, parts of the Coast Range ophiolite were displaced from the west side of the Great Valley province and incorporated into the Central belt of the Franciscan Complex along steep-dipping to low-angle reverse faults having dominant components of dextral shear. A northwest trending eastern zone of these right-laterally displaced outliers shows strong affinities to the main Coast Range ophiolite of the northwestern Sacramento Valley (the Elder Creek terrane), in that the outliers include ophiolitic breccias of Oxfordian to Kimmeridgian age. A southwestern zone of outliers lacks ophiolitic breccia and instead includes latest Oxfordian or Kimmeridgian to Tithonian, arc-derived volcanic rocks like those found in the Del Puerto and Stanley Mountain terranes of the main ophiolite. Whereas outliers of the northeastern outlier zone are right-laterally displaced no more than 260 km from the western side of the Sacramento Valley, outliers of the southwestern zone are displaced a minimum of 169 to 249 km. This displacement occurred between about 60 and 52 Ma. Ophiolitic rocks in the Decatur terrane of western Washington that have recently been correlated with the Coast Range ophiolite and the Great Valley sequence of California were apparently displaced at least 950 to 1200 km from the west side of the Great Valley between early Tertiary and Early Cretaceous time. Derived rates of northward translation for the ophiolite outliers in California are in the range of 1 to 4 cm/yr. Rates for the Decatur terrane are in the range of 2.5 to 4 cm/yr if translation was initiated 90 Ma, but as much as 11.9 to 15 cm/yr if it was not initiated until 60 Ma. The lower rates for the Decatur terrane are consistent with the rates derived for the California outliers and with the northward component of relative motion between the Farallon and North American plates from 90 to 50 Ma. The higher rates require northward transport on the Kula plate or on a fast-moving microplate. The higher translation rates derived for the Decatur terrane are also consistent with paleomagnetically determined rates for some limestones of the Central belt. This may indicate that outliers of the Coast Range ophiolite dispersed in the Central belt of the Franciscan Complex record only part of the total displacement which occurred along the Late Cretaceous to early Tertiary western margin of North America.

  12. Concordant paleolatitudes from ophiolite sequences in the northern California Coast Ranges, U.S.A.

    USGS Publications Warehouse

    Mankinen, E.A.; Gromme, C.S.; Williams, K.M.

    1991-01-01

    Paleomagnetic data have been obtained from two ophiolite sequences in the northern California Coast Ranges: from Mount Diablo in the San Francisco Bay area and from Potter Valley, north of Clear Lake. The ophiolite exposed at Mount Diablo is part of the late Middle to Late Jurassic Coast Range ophiolite, and that exposed near Potter Valley is Late Jurassic to perhaps Early Cretaceous in age and occurs within the Franciscan assemblage. Data from the sheeted-dike complex at Mount Diablo show these rocks to be strongly overprinted, probably following uplift and erosion of the ophiolite. Samples whose primary remanent magnetization seems to be recovered yield a mean paleomagnetic pole at 30.7??N, 159.5??E with ??95 = 5.6??. A comparison of this pole with the Jurassic apparent polar wander path for North America indicates that the ophiolite has rotated 45?? ?? 7?? counterclockwise relative to the craton and has not been latitudinally displaced. The diabase and pillow basalt in Potter Valley have not been strongly overprinted and data from those rocks yield a paleomagnetic pole at 79.0??N, 61.5??E with ??95 = 6.4??. This result indicates that the ophiolite at Potter Valley has rotated approximately 29?? ?? 8?? clockwise, and has undergone little or no latitudinal displacement. Because of the predominantly northeastward transport of oceanic plates converging with the western margin of North America since middle Mesozoic time, the absence of appreciable northward displacement of either ophiolite fragment indicates that both formed close to the continental margin. ?? 1991.

  13. The Maliac Ocean: the origin of the Tethyan Hellenic ophiolites

    NASA Astrophysics Data System (ADS)

    Ferriere, Jacky; Baumgartner, Peter O.; Chanier, Frank

    2016-02-01

    The Hellenides, part of the Alpine orogeny in Greece, are rich in ophiolitic units. These ophiolites and associated units emplaced during Jurassic obduction, testify for the existence of one, or several, Tethyan oceanic realms. The paleogeography of these oceanic areas has not been precisely described. However, all the authors now agree on the presence of a main Triassic-Jurassic ocean on the eastern side of the Pelagonian zone (Vardar Domain). We consider that this Maliac Ocean is the most important ocean in Greece and Albania. Here, we limit the detailed description of the Maliac Ocean to the pre-convergence period of approximately 70 Ma between the Middle Triassic rifting to the Middle Jurassic convergence period. A quick overview on the destiny of the different parts of the Maliac Ocean during the convergence period is also proposed. The studied exposures allow to reconstruct: (1) the Middle to Late Triassic Maliac oceanic lithosphere, corresponding to the early spreading activity at a Mid-Oceanic Ridge; (2) the Western Maliac Margin, widely exposed in the Othris and Argolis areas; (3) the Eastern-Maliac Margin in the eastern Vardar domain (Peonias and Paikon zones). We established the following main characteristics of the Maliac Ocean: (1) the Middle Triassic rifting marked by a rapid subsidence and volcanism seems to be short-lived (few My); (2) the Maliac Lithosphere is only represented by Middle to Late Triassic units, especially the Fourka unit, composed of WPB-OIB and MORB pillow-lavas, locally covered by a pelagic Middle Triassic to Middle Jurassic sedimentary cover; (3) the Western Margin is the most complete and our data allow to distinguish a proximal and a deeper distal margin; (4) the evolution of the Eastern Margin (Peonias and Paikon series) is similar to that of the W-Margin, except for its Jurassic terrigenous sediments, while the proximal W-Margin was dominated by calcarenites; (5) we show that the W- and E-margins are not Volcanic Passive Margins; and (6) during the Middle Jurassic convergence period, the Eastern Margin became an active margin and both margins were affected by obduction processes.

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

    NASA Astrophysics Data System (ADS)

    Osipenko, A.; Krylov, K.

    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

  15. Structural, geochronological, magnetic and magmatic constraints of a ridge collision/ridge subduction-related ophiolite

    NASA Astrophysics Data System (ADS)

    Anma, Ryo

    2013-04-01

    A mid-oceanic ridge system subducts underneath South American plate at latitude 46S off Chilean coast, forming a ridge-trench-trench type triple junction. At ~ 6 Ma, a short segment of the Chile ridge system subducted in south of the present triple junction. This ridge subduction event resulted in emplacement of a young ophiolite (5. 6 to 5. 2 Ma) and rapid crustal uplift (partly emerged after 4.9 Ma), and synchronous magmatism. This ophiolite, namely the Taitao ophiolite, provides criteria for the recognition of ridge collision/ridge subduction-related ophiolites. Aiming to establish recognition criteria, we studied distribution of structures, magnetic properties, geochemical characteristics, and radiometric ages of the Taitao ophiolite and related igneous rocks. The Taitao ophiolite exhibits a classic Penrose-type stratigraphy: ultramafic rocks and gabbros (collectively referred as plutonic section hereafter) in the south, and sheeted dike complex (SDC) and volcanic sequences in the north. Composite foliations developed in the plutonic section, which were folded. SDC were exposed in two isolated blocks having orthogonal strikes of dike margins. Geochemically, gabbros have an N-MORB composition whereas basalts of the volcanic sequence have an E-MORB composition. U-Pb ages of zircons separated from gabbros, SDC and sediments interbeded with billow lavas implied that the center of magmatic activities migrated from the plutonic section to volcanic section during ~5.6 Ma and ~5.2 Ma. Zircon fission track ages of gabbros coincide with U-Pb ages within error range, implying rapid cooling. Demagnetization paths for SDC and lavas form a straight line, whereas those from the plutonic section are Z-shaped and divisble into two components: low coercivity and high coercivity. Restored orientation of gabbro structures imply that the magnetization acquired while gabbroic structures were folding. Thus, magma genesis and emplacement of the plutonic section of ophiolite took place almost instantaneously. The ophiolite is surrounded by synchronous (5.7 Ma to 5.2 Ma) granitic intrusions with various compositions. Our data indicates that the granitic melts started forming near the conjunction of the subducting ridge and transform fault. Generation of granitic melts continued as the spreading center of the same segment subducted, due to partial melting of the oceanic crust and subducted sediments at amphibolite-facies conditions. The obduction of the Taitao ophiolite also accompanied volcanism in the Chile Margin that migrates from west (5.2 Ma) to east (4.6 Ma) at a rate of 5 cm/y as a fracture zone subducted. A ridge collision/ridge subduction-related ophiolite has a short-life. The most intrinsic recognition criteria for such ophiolite must be hot emplacement of plutonic rocks, that represent magmatism at the axial magma chamber in the spreading ridge environment, into cold forearc region, which results in rapid cooling of the deep plutonic section (U-Pb ages coincide with cooling ages within an error range), and pervasive high temperature ductile deformation (after magmatic flow) throughout gabbro. Folding continued until the rocks were cooled to Curie T of magnetite (~580C) in the case of the Taitao ophiolite. A ridge collision/ridge subduction-related ophiolite may accompany block rotation of volcanic sequence, because of high viscosity contrast between hot plutonic section and overriding volcanic section that has already cooled and solidified. It may also accompany acidic intrusions with various compositions and basaltic volcanisms due to subduction of fracture zones.

  16. Chromite deposits in the northern Oman ophiolite: Mineralogical constraints

    NASA Astrophysics Data System (ADS)

    Augé, T.

    1987-01-01

    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, orthopyroxene, plagioclase, and amphibole occur, as opposed to type 3 which contains only olivine. They differ however in ore texture. Similar silicates also occur as euhedral inclusions in chromite crystals, but their proportions are reversed. The composition of the interstitial silicates is comparable to that found in early cumulates. Type-1 and type-2 chromite deposits crystallized from a magma similar to that from which the basal cumulates formed (Al2O3, 15.1 16.1 wt%; FeO/MgO, 0.55 0.60). The type-3 chromites were derived from a magma of much lower Al2O3 content (12.5 wt%). It is considered that they belong to an older episode in the magmatic evolution of the complex.

  17. Sr isotopic tracer study of the Samail ophiolite, Oman

    SciTech Connect

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

    1981-04-10

    We have measured Rb and Sr concentrations and Sr isotopic compositions in 41 whole-rock samples and 12 mineral separates from units of the Samail ophiolite, including peridotite, gabbro, plagiogranite diabase dikes, and gabbro and websterite dikes within the metamorphic peridotite. Ten samples of cummulate gabbro from the Wadir Kadir section and nine samples from the Wadi Khafifah section have mean /sup 87/Sr//sup 86/Sr ratios and standard deviations of 0.70314 +- 0.00030 and 0.70306 +- 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 midocean ridge basalt. The /sup 87/Sr//sup 86/Sr ratios of noncumulate gabbro, plagiogranite, and diabase dikes range from 0.7034 to 0.7047, 0.7038 to 0.7046, and 0.7037 to 0.7061, respectively. These higher /sup 87/Sr//sup 86/Sr ratios are due to alteration of initial magmatic compositions by hydrothermal exchange with seawater. Mineral separates from dikes 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 dikes were derived from partial melting of source regions that had similar long-term histories and chemical compositions.

  18. Sr isotopic tracer study of the Samail ophiolite, Oman.

    USGS Publications Warehouse

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

    1981-01-01

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    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)

  20. Middle Jurassic U-Pb crystallization age for Siniktanneyak Mountain ophiolite, Brooks Range, Alaska

    SciTech Connect

    Moore, T.E. ); Aleinikoff, J.N.; Walter, M. )

    1993-04-01

    The authors report here a U-Pb age for the Siniktanneyak Mountain Ophiolite klippe in the west-central Brooks Range, the first U-Pb ophiolite age in northern Alaska. Like klippen of mafic and ultramafic rocks in the Brooks Range, the Siniktanneyak Mountain klippe is composed of a lower allochthon of Devonian and younger( ) diabase and metabasalt with trace-element characteristics of seamount basalts and an upper allochthon of ophiolite. The ophiolite is partial, consisting of (1) abundant dunite and subordinate harzburgite and wehrlite; (2) cumulate clinopyroxene gabbro, and (3) minor noncumulate clinopyroxene gabbro and subordinate plagiogranite; no sheeted dikes or volcanic rocks are known in the ophilitic allochthon. The plagiogranite forms small dikes and stocks that intrude the noncumulate gabbro and consists of zoned Na-rich plagioclase + clinopyroxene with interstial quartz and biotite. Five fractions of subhedral, tan zircon from the plagiogranite yield slightly discordant U-Pb data with an upper intercept age of 170 [+-] 3 Ma. The U-Pb data indicate that the Siniktanneyak Mountain ophiolite crystallized in the Middle Jurassic and was emplaced by thrusting onto mafic accretionary prism rocks within about 10 m.y. of crystallization. The U-Pb data provide an upper limit to the age of initiation of the Brookian orogeny.

  1. K-Ar ages of metamorphic rocks at the base of the Samail ophiolite, Oman

    SciTech Connect

    Lanphere, M.A.

    1981-04-10

    Hornblendes from amphiobolities in the sheet of metamorphic rocks beneath the peridotite member of the Samail ophiolite and phyllites farther from the peridotite contact have weighted mean /sup 40/Ar//sup 39/Ar total fusion ages of 90.0 +- 3.0 m.y. and 79.5 +- 3.0 m.y., respectively. The amphibolities represent the first tectonic slice welded to the base of the Samail ophiolite after it was detached from the Tethyan oceanic crust. Formation of the amphiobolities occurred no more than 3 to 7 m.y. after crystallization of plagiogranite in the ophiolite. The phyllites represent another tectonic slice of ocean floor sediments welded to the ophiolite as it was transported further from the Tethyan spreading axis. The K-Ar ages suggest, assuming a half-spreading rate of 2 to 5 cm/yr, that detachment of the Samail ophiolite and formation of amphibolite facies rocks occurred no more than 60 to 350 km from spreading center. Using the same spreading rate, one can calculate a minimum half width of 300 to 750 km for the Tethyan Ocean during the Late Cretaceous.

  2. The age and origin of felsic intrusions of the Thetford Mines ophiolite, Quebec.

    USGS Publications Warehouse

    Clague, D.A.; Frankel, C.S.; Eaby, J.S.

    1985-01-01

    This ophiolite was obducted in the early Ordovician during the closing of the proto-Atlantic. The tectonized peridotite of the lower unit of the ophiolite is intruded by felsic dykes and pods, including isolated lenses of massive rodingite, small bodies of strongly deformed diorite, and younger, less deformed monzonite. These intrusions are found only near the base of the ophiolite, and are considered to have been emplaced before the ophiolite reached its present position. The young group of intrusions consists of biotite-muscovite quartz monzonite and leuco-quartz monzonite. Analysed samples have high K2O, high (K2O X 100)/Na2O + K2O) ratios, and high initial Sr ratios, indicating that the magma source was continental and that these felsic rocks formed by partial melting of continental sediments. Whole-rock and mineral isochron ages suggest that the felsic intrusions are approx 456 + or - 4 m.y. old and that they were metamorphosed approx 418 + or - 7 m.y. ago. The detachment of the ophiolite occurred approx 491 + or - 3 m.y. ago. The felsic dykes were intruded approx 35 m.y. later, during the Taconic orogeny. The lengthy time between detachment and final nappe emplacement recorded by the felsic dykes may be a requirement for formation of abundant asbestiform chrysotile. Whole-rock analyses (16) and Rb, Sr and 87Sr/86Sr data from the Colline de Granite, King Mts., Vimy Ridge and Black Lake samples are presented.-P.Br.

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

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

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

  4. Early-Middle Ordovician ridge-trench collision during the closure of Iapetus: Evidence from the Dunnage Melange tract, Newfoundland Appalachians

    NASA Astrophysics Data System (ADS)

    Zagorevski, Alexandre; van Staal, Cees R.; McNicoll, Vicki

    2010-05-01

    The Red Indian Line marks the boundary between the peri-Laurentian and peri-Gondwanan realms in the Northern Apppalachians and delimits the main Iapetus suture. The Middle Ordovician Dunnage Melange tract occurs immediately south of the Red Indian Line in north-central Newfoundland. The genesis of the melange belt has always been contentious, because it preserves structures both suggestive of an olistostromal (e.g. pebbly mudstone) and tectonic origin (e.g. cleaved psammitic clasts). Its spatial association with the suture zone and strong lithological linkages with the Middle Ordovician arc-trench gap rocks of the Popelogan-Victoria arc (PVA) system suggest a genetic linkage to subduction processes along the leading edge of Ganderia prior to its collision with Laurentia's leading edge. The melange hosts a variety of Middle Ordovician mafic to felsic intrusions. This investigation is focused on the Coaker quartz-feldspar porphyry that intruded the Dunnage melange. Thin sheets of the flow banded porphyry intrude cleaved black shales and are intricately folded in complex isoclinal and sheath-like geometries. The fold hinges displays a well-preserved folded trachytic texture indicative of laminar flow in partially molten state and show little intra-crystalline deformation. These relationships suggest the porphyry intruded syn-tectonically into partly lithified sediments. The age of deformation and crystallization is constrained by a new U-Pb SHRIMP age of 469 ± 3 Ma. The porphyry contains a large variety of xenoliths and xenocrysts including variably serpentinized harzburgite, orthopyroxenite, hornblendite, garnet granulite, amphibolites, gabbro, garnet and pyroxene. The variety and composition of xenoliths suggest it was sampling lower and middle arc or continental crust that probably formed the forearc basement to the PVA. The variety of inherited zircons (ca. 489, 517, 547, 620, 655, 895, 940, 1305, 1515, 1740, 1970, 2630 Ma) confirms its affinity with the peri-Gondwanan arc system that was built upon the leading edge of Ganderia. The geochemistry of the porphyry and xenoliths, magmatism, tectonism and melange formation in the arc-trench gap and overall enriched E-MORB-like magmatism in the arc support earlier suggestions of a ridge subduction at c. 469 Ma. The Dunnage Melange is thus interpreted as deformed forearc strata near the boundary between the accretionary wedge and the forearc basin formed during ridge-trench collision.

  5. Magma storage and plumbing of adakite-type post-ophiolite intrusions in the Sabzevar ophiolitic zone, NE Iran

    NASA Astrophysics Data System (ADS)

    Jamshidi, K.; Ghasemi, H.; Troll, V. R.; Sadeghian, M.; Dahren, B.

    2014-08-01

    Subduction-related adakite-type intrusive rocks emplaced into the late Cretaceous-Paleocene Sabzevar ophiolite zone, NE Iran, range from Mg-andesite to rhyodacite in composition. Here we investigate the magma supply system to these subvolcanic intrusive rocks by applying thermobarometric mineral and mineral-melt equilibrium models, including amphibole thermobarometry, plagioclase-melt thermobarometry and clinopyroxene-melt barometry. Based on the results of these thermobarometric models, plagioclase crystallized dominantly at pressures of ~ 350 (468-130) MPa, while amphiboles record both low pressures (~ 300 MPa) and very high pressures (> 700 MPa) of crystallization. The latter is supported by the calculated pressures for clinopyroxene crystallization (550 to 730 MPa). The association of amphibole with clinopyroxene and no plagioclase in the most primitive samples (Mg-andesites) is consistent with amphibole fractionation from very hydrous magmas at deep crustal levels of the plumbing system, which may have been a key process to intensify adakite-type affinities in this rock suite. Barometry, combined with frequent disequilibrium features, such as oscillatory-zoned and sieve-textured plagioclase crystals with An-rich overgrowths in more evolved samples, imply final magma differentiation occurred in an open upper crustal magma system that developed progressively stronger compositional modifications during high-level magma storage.

  6. Magma storage and plumbing of adakite-type post-ophiolite intrusions in the Sabzevar ophiolitic zone, northeast Iran

    NASA Astrophysics Data System (ADS)

    Jamshidi, K.; Ghasemi, H.; Troll, V. R.; Sadeghian, M.; Dahren, B.

    2015-01-01

    Subduction-related adakite-type intrusive rocks emplaced into the late Cretaceous-Paleocene Sabzevar ophiolite zone, northeast Iran, range from Mg-andesite to rhyodacite in composition. Here we investigate the magma supply system to these subvolcanic intrusive rocks by applying thermobarometric mineral and mineral-melt equilibrium models, including amphibole thermobarometry, plagioclase-melt thermobarometry and clinopyroxene-melt barometry. Based on the results of these thermobarometric models, plagioclase crystallized dominantly at pressures of ~350 (130 to 468) MPa, while amphiboles record both low pressures (~300 MPa) and very high pressures (>700 MPa) of crystallization. The latter is supported by the calculated pressures for clinopyroxene crystallization (550 to 730 MPa). The association of amphibole with clinopyroxene and no plagioclase in the most primitive samples (Mg-andesites) is consistent with amphibole fractionation from very hydrous magmas at deep crustal levels of the plumbing system, which may have been a key process in intensifying adakite-type affinities in this rock suite. Barometry, combined with frequent disequilibrium features such as oscillatory-zoned and sieve-textured plagioclase crystals with An-rich overgrowths in more evolved samples, implies that final magma differentiation occurred in an open upper crustal magma system that developed progressively stronger compositional modifications during high-level magma storage.

  7. The emplacement time of the Hegenshan ophiolite: Constraints from the unconformably overlying Paleozoic strata

    NASA Astrophysics Data System (ADS)

    Zhou, Jian-Bo; Han, Jie; Zhao, Guo-Chun; Zhang, Xing-Zhou; Cao, Jia-Lin; Wang, Bin; Pei, Sheng-Hui

    2015-11-01

    Controversy has long surrounded the emplacement time of the Hegenshan ophiolite that is considered to mark a suture zone, called the Hegenshan-Heihe suture, resulting from the closure of a back-arc basin in the Paleo-Asian Ocean. The Hegenshan ophiolite in the Xiaobaliang area is unconformably overlain by a sequence of Paleozoic strata, called the Zhesi Formation that consists of conglomerate, sandstone, siltstone and limestone, some of which contain Permian marine fossils of Brachiopods. Therefore, the ages of these Paleozoic strata can be used to constrain the emplacement time of the Hegenshan ophiolite. Four samples of the Zhesi Formation collected in the Xiaobaliang area yield the detrital zircon U-Pb ages of 285-272 Ma (with the peak at 279 Ma), 315-288 Ma (with a peak at 300 Ma), 320-358 Ma (with a peak at 336 Ma), and 406 ± 3 Ma, of which the ~ 280 and ~ 300 Ma age groups are remarkably similar to the ages of latest Carboniferous-Early Permian Gegenaobao/Dashizai Formation, or A-type granites, which formed under a post-collisional setting. However, the age groups of 320 to 358 Ma with a peak at 336 Ma, show the features of mafic-ultramafic zircons in CL image, most likely derived from local mafic-ultramafic rocks of the Hegenshan ophiolite in the Xiaobaliang area, which is supported by the fact of the ophiolite unconformably overlain by the Middle Permian Zhesi Formation. Therefore, we propose that the emplacement time of the Hegenshan ophiolite must have happened at some time before the Middle Permian (~ 280 Ma), most likely between 300 and 335 Ma, not in the Silurian, Devonian or Mesozoic as previously considered.

  8. Lower Cretaceous Xigaze ophiolites formed in the Gangdese forearc: Evidence from paleomagnetism, sediment provenance, and stratigraphy

    NASA Astrophysics Data System (ADS)

    Huang, Wentao; van Hinsbergen, Douwe J. J.; Maffione, Marco; Orme, Devon A.; Dupont-Nivet, Guillaume; Guilmette, Carl; Ding, Lin; Guo, Zhaojie; Kapp, Paul

    2015-04-01

    The India-Asia suture zone of southern Tibet exposes Lower Cretaceous Xigaze ophiolites and radiolarian cherts, and time-equivalent Asian-derived clastic forearc sedimentary rocks (Xigaze Group). These ophiolites have been interpreted to have formed in the forearc of the north-dipping subduction zone below Tibet that produced the Gangdese magmatic arc around 15-20°N, or in the forearc of a sub-equatorial intra-oceanic subduction zone. To better constrain the latitude of the ophiolites, we carried out an integrated paleomagnetic, geochronologic and stratigraphical study on epi-ophiolitic radiolarites (Chongdui and Bainang sections), and Xigaze Group turbiditic sandstones unconformably overlying the ophiolite's mantle units (Sangsang section). Detrital zircon U-Pb geochronology of tuffaceous layers from the Chongdui section and sandstones of the Xigaze Group at the Sangsang section provides maximum depositional ages of 116.5 ± 3.1 Ma and 128.8 ± 3.4 Ma, respectively, for the Chongdui section and an Asian provenance signature for the Xigaze Group. Paleomagnetic analyses, integrated with rock magnetic experiments, indicate significant compaction-related inclination 'shallowing' of the remanence within the studied rocks. Two independent methods are applied for the inclination shallowing correction of the paleomagnetic directions from the Sangsang section, yielding consistent mean paleolatitudes of 16.2°N [13°N, 20.9°N] and 16.8°N [11.1°N, 23.3°N], respectively. These results are indistinguishable from recent paleolatitude estimates for the Gangdese arc in southern Tibet. Radiolarites from the Chongdui and Bainang sections yield low paleomagnetic inclinations that would suggest a sub-equatorial paleolatitude, but the distribution of the paleomagnetic directions in these rocks strongly suggests a low inclination bias by compaction. Our data indicate that spreading of the Xigaze ophiolite occurred in the Gangdese forearc, and formed the basement of the forearc strata.

  9. Significance of xenocrystic Precambrian zircon contained within the southern continuation of the Josephine ophiolite: Devils Elbow ophiolite remnant, Klamath Mountains, northern California

    NASA Astrophysics Data System (ADS)

    Wright, James E.; Wyld, Sandra J.

    1986-08-01

    The Josephine ophiolite of the western Jurassic belt, Klamath Mountain province of California and Oregon, is the expression of a well-documented Late Jurassic suprasubduction zone rift basin that formed between an active Late Jurassic arc to the west and a remnant Middle Jurassic arc to the east. The Devils Elbow ophiolite remnant (DEO) exposed along the South Fork of the Trinity River represents the southernmost continuation of the Josephine ophiolite and provides important new constraints on the early history of this rift basin. The DEO consists of pillow lavas and breccias and a well-developed sheeted dike complex that are depositionally overlain by a clast-supported breccia derived almost exclusively from ophiolitic detritus. Dikes and irregular pods of plagiogranite are conspicuous elements of the DEO and exhibit mutually crosscutting relations with mafic dikes; thus, they are interpreted as genetically related elements of the dike complex. Zircon separates from two widely separated plagiogranite dike localities yielded two distinct zircon populations: a clear, euhedral, magmatic population and a reddish, rounded xenocrystic population. The isotopic systematics of four zircon fractions from these two dike localities indicate a crystallization age for the DEO of 164 ±1 Ma and an age of ˜1.7 Ga for the xenocrystic zircon component. The occurrence of a xenocrystic Precambrian zircon component within the plagiogranites of the DEO provides unequivocal evidence that rifting occurred within preexisting zircon-bearing crustal rocks of the Klamath Mountains. As there is no Precambrian crust within the Klamath Mountains, the xenocrystic zircon population must have been derived from supracrustal sedimentary sequences. The most likely source for the older zircon component is a terrigenous metasedimentary sequence contained within the Rattlesnake Creek terrane, which formed part of the structural basement of the rifted Middle Jurassic arc. The incorporation of xenocrystic Precambrian zircon from the rifted basement of the Middle Jurassic arc could only have occurred during the earliest stages of rifting. Thus, the DEO must represent a fragment of an initial rift that ultimately formed the Late Jurassic basin now floored by the Josephine ophiolite. In support of the zircon data, dikes and lavas of the DEO have geochemical characteristics transitional between island arc and mid-ocean ridge magma series, as would be expected within an initial rift setting of a suprasubduction zone ophiolite.

  10. Faulting and Serpentinisation of Peridotites in the Leka Ophiolite

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    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.

  11. New Ar-Ar age, isotopic, and geochemical data for basalts in the Neyriz ophiolite, Iran

    NASA Astrophysics Data System (ADS)

    Babaie, H.; Ghazi, A.; Babaei, A.; Duncan, R.; Mahony, J.; Hassanipak, A.

    2003-04-01

    The Neyriz ophiolite of Iran occurs along the Zagros suture zone where the Arabian and Eurasian plates have collided. The principal crustal rock units of the Neyriz ophiolite complex include an intrusive sequence that comprises low- and high-level cumulate gabbros, a late intrusive sequence that consists of diorite and plagiogranite, and a volcanic sequence that includes basalt and rhyo-dacite. These rocks are enveloped by extensive exposures of mantle rocks. Geochemical data including REE, indicate the presence of two distinct types of basalt, which formed either in an island arc or a mid ocean ridge environment. Basalt with calc-alkaline island arc composition has LREE-enriched patterns, and is intercalated with arc-related volcaniclastic rocks that are deposited unconformably above the ophiolite (Babaie et al., 2001). The second group of basalt has distinct MORB affinity, and is compositionally related, through differentiation, with underlying gabbro and plagiogranite, and overlying rhyo-dacite. The MORB-like basalt in Neyriz ophiolite is chemically similar to the tholeiite of the Khoy ophiolite in northwestern Iran [Ghazi and Hassanipak (1997)]. Rare earth element (REE) whole-rock data clearly differentiate the classic ophiolitic lithologies for the crustal rocks in this complex. New 40Ar/39Ar incremental heating plateau ages from two hornblende gabbros, in the crustal sequence in Tang-e Hana, are 92.07±1.69 Ma and 93.19±2.48 Ma, which are compatible with the previously dated samples by conventional K/Ar and 40Ar/39Ar total fusion ages (Lanphere and Pamic, 1983). Preliminary isotopic data for five MORB-type basalts yield ɛ{Nd} values of +7.8 and +7.9, and 87Sr/86Sr values of 0.70403-70475. These new results show very distinct similarities, both in terms of age and isotopic composition, between the MORB-type basalt samples from Neyriz and the Samail ophiolite in Oman. The correlation of the isotopic, geochemical, and age data across Neyriz and Oman indicates that the two ophiolites share the same paleogeographic and tectonic history during the Late Cretaceous and Paleogene.

  12. Mantle Wedge formation during Subduction Initiation: evidence from the refertilized base of the Oman ophiolitic mantle

    NASA Astrophysics Data System (ADS)

    Prigent, Cécile; Guillot, Stéphane; Agard, Philippe; Godard, Marguerite

    2015-04-01

    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. The latter one is supported by studies on volcanic sequences, whereas all studies dealing on the mantle section have recognized a first stage of oceanic accretion before subduction initiation. We herein focus on basal peridotites from all along the ophiolite strike in order to decipher and characterize potential fluid/melt transfers due to subduction processes. Samples were taken along hm- to km-long sections across the basal banded unit directly overlying the amphibolitic/granulitic metamorphic sole. We carried out a petrological, structural and geochemical (major, trace elements and boron isotopes) study on these rocks and their constitutive minerals. Results were then interpreted using thermal modelling of the ophiolitic mantle evolution during subduction initiation. Our results show that basal peridotites range from lherzolites to highly depleted harzburgites in composition. The clinopyroxenes (cpx) display melt impregnation textures and co-crystallized with HT/HP amphiboles (amph). The major and trace elements of the constitutive minerals indicate that the different basal lithologies only result from varying degrees of melt extraction. Combined with isotopic data, we demonstrate that the initial melt reacting with these peridotites derives from the mixing of asthenospheric melt and metamorphic sole-derived fluids, and was later extracted in variable proportions. From these observations and thermal modelling of the Fizh ophiolitic mantle evolution after subduction initiation, we interpret the occurrence of these basal lherzolites as representing a freezing front developed by thermal re-equilibration (cooling) of the subduction: the asthenospheric melt mixed with subduction-related fluids was extracted at different degrees until getting ultimately trapped, and crystallized cpx, amph and other associated minerals. The young and still very hot mantle section of today's northern Fizh massif thus contrasted, during subduction initiation, with the colder southernmost massifs of the ophiolite. These different thermal structures reflect different maturity of the oceanic lithosphere at the onset of subduction, which is hardly reconcilable with the ophiolite being formed in an entire supra-subduction zone system. We therefore favor a model in which a preexisting marginal basin acquired its arc-signature during subsequent subduction processes. If our interpretation is correct, the base of the Oman ophiolite could provide the best proxy for the composition of a frozen-in, incipiently forming mantle wedge.

  13. Subduction-related oceanic crust in the Khantaishir ophiolite (western Mongolia).

    NASA Astrophysics Data System (ADS)

    Gianola, O.; Schmidt, M. W.; Jagoutz, O. E.

    2014-12-01

    Most of the oceanic crust is generated at mid oceanic ridges and only a minor portion results from magmatism related to subduction zones (i.e. back-arc basins or in incipient arcs). However it has been observed that several ophiolites preserve an oceanic crust displaying a subduction zone signature. Such a signature is also found in the Khantaishir ophiolite located in western Mongolia. This ~570 m.y. old ophiolite is ~480 km2 in size and displays a complete sequence, tectonically slightly dismembered during the emplacement process. The ophiolite exposes ~130 km2 of highly refractory harzburgitic mantle with local dunite channels and lenses. Towards its top the mantle is replaced by sub-horizontal km-wide discrete zones of pyroxenites situated either in the mantle or forming a crust-mantle transition zone overlain by gabbros. The crust is then composed of various gabbros and minor gabbronorite (both in part replaced by pyroxenites and/or cut by intermediate dykes), by a dyke/sill-complex and by pillow lavas. The entire ophiolite is re-equilibrated at lower greenschist facies conditions. Major and trace elements of the crustal rocks of the Khantaishir ophiolite show trends similar to those observed for the Izu-Bonin-Mariana subduction system. Mafic dykes/sills and pillow lavas of the Khantaishir ophiolite have overall basaltic-andesite compositions, resembling high-Mg andesites with an average SiO2 of 57 wt%. Their low TiO2 (<1.2 wt%) and Ti/V ratios of ~10 contrast with MOR suites, which are basaltic (average SiO2 ~50 wt%) with TiO2 values usually higher than 1.2 wt% and a Ti/V ratio between 20 and 50. This difference is prominent in Th/Yb and Nb/Yb, where the lavas from the Khantaishir ophiolite plot at higher Th/Yb than the typical MORB-OIB array. Moreover, some volcanics with nearly primitive compositions (XMg = 0.69, SiO2 = 56.5 wt%) have trace element patterns comparable with those displayed in high-Mg andesites and boninites from modern island arcs. This evidence suggests that the Kantaishir crust might represent the submarine initial stage of an incipient arc, probably when the preexisting oceanic crust is spread and incipient island arc crust is formed.

  14. Tectonic evolution of the Brooks Range ophiolite, Alaska

    SciTech Connect

    Harris, R.A. . Dept. of Geology)

    1993-04-01

    Detailed studies of the composition, internal structure, and age of the Brooks Range ophiolite (BRO) and its metamorphic sole reveal new constraints for its tectonic evolution. The BRO consists of six separate thrust masses of consanguineous composition, internal organization, structure and age. Subophiolite metamorphic rocks are locally preserved along its structural base, which is well exposed in several places. The metamorphic sole is locally transitional with mafic volcanic sequences, chert, tuffs, and minor clastic sedimentary material of the Copter Peak Complex, which is correlative with the Angayucham terrane. This terrane is much older than, and chemically distinct from the BRO. The internal structure of the BRO is characterized by NE-SW trending igneous layers that expose the transition zone from crust to mantle. Residual mantle material consists of tectonized peridotite in abrupt contact with dunite pods up to 4 km thick. Ductile and brittle structures of the BRO preserve various phases of its dynamic evolution from a magma body to a fragmented thrust sheet. The earliest deformational effects are recorded by ductile lattice and shape fabrics in dunites and the layered series of the BRO. Magmatic flow planes generally parallel the petrologic moho, and dip 40[degree]--70[degree] to the NW and SE. Flow lineations consistently plunge ESE-ENE from 39[degree]--54[degree]. Igneous laminations and compositional layers represent patterns of magmatic flow in, and plastic deformation of, a cumulate sequence -- not the deposition pattern of cumulate layers. In the upper layered series, amphiboles with a shape-preferred orientation yield Ar/Ar plateau ages of 163--169 Ma. These ages overlap with plateau ages of the same kind from amphibolite of the metamorphic sole. This concordance in age indicates that cooling of the BRO coincided with its tectonic emplacement.

  15. Melt viscosity, temperature and transport processes, Troodos ophiolite, Cyprus

    NASA Astrophysics Data System (ADS)

    Schouten, Hans; Kelemen, Peter B.

    2002-07-01

    The lava section in the Troodos ophiolite, Cyprus, is chemically stratified and divided into a shallow lava sequence with low TiO 2 content and a deeper lava sequence with high TiO 2 content. We calculate the viscosity at magmatic temperature based on major element chemistry of lavas in Cyprus Crustal Study Project (CCSP) Holes CY-1 and 1A. We find that typical shallow low-Ti lavas have a magmatic viscosity that is two to three orders of magnitude lower than that of the deeper high-Ti lavas. This implies that, after eruption on-axis, Troodos low-Ti lavas would have been able to flow down the same slope faster and farther than high-Ti lavas. The calculated lava viscosity increases systematically from the lava-sediment interface to the bottom of the composite Hole CY-1/1A. This suggests that an efficient process of lava segregation by viscosity on the upper flanks of the paleo Troodos rise may have been responsible for the chemical stratification in the Troodos lava pile. Calculated magmatic temperature and molar Mg/(Mg+Fe), or Mg#, decrease systematically down-section, while SiO 2 content increases. Correlation of Mg# in the lavas with Mg# in the underlying, lower crustal plutonic rocks sampled by CCSP Hole CY-4 shows that the shallow lavas came from a high-temperature, lower crustal magma reservoir which is now represented by high-Mg# pyroxenite cumulates, while the deeper lavas were erupted from a lower-temperature, mid-crustal reservoir which is now represented by gabbroic cumulates with lower Mg#.

  16. Beginning the Modern Regime of Subduction Tectonics in Neoproterozoic time: Inferences from Ophiolites of the Arabian-Nubian Shield

    NASA Astrophysics Data System (ADS)

    Stern, R.

    2003-04-01

    It is now clear that the motive force for plate tectonics is provided by the sinking of dense lithosphere in subduction zones. Correspondingly, the modern tectonic regime is more aptly called ``subduction tectonics" than plate tectonics, which only describes the way Earth's thermal boundary layer adjusts to subduction. The absence of subduction tectonics on Mars and Venus implies that special circumstances are required for subduction to occur on a silicate planet. This begs the question: When did Earth's oceanic lithosphere cool sufficiently for subduction to began? This must be inferred from indirect lines of evidence; the focus here is on the temporal distribution of ophiolites. Well-preserved ophiolites with ``supra-subduction zone" (SSZ) affinities are increasingly regarded as forming when subduction initiates as a result of lithospheric collapse (± a nudge to get it started), and the formation of ophiolitic lithosphere in evolving forearcs favors their emplacement and preservation. The question now is what percentage of ophiolites with ``supra-subduction zone" (SSZ) chemical signatures formed in forearcs during subduction initiation events? Most of the large, well-preserved ophiolites (e.g., Oman, Cyprus, California, Newfoundland) may have this origin. If so, the distribution in space and time of such ophiolites can be used to identify ``subduction initiation" events, which are important events in the evolution of plate tectonics. Such events first occurred at the end of the Archean (˜2.5Ga) and again in the Paleoproterozoic (˜1.8 Ga), but ophiolites become uncommon after this. Well-preserved ophiolites become abundant in Neoproterozoic time, at about 800±50 Ma. Ophiolites of this age are common and well-preserved in the Arabian-Nubian Shield (ANS) of Egypt, Sudan, Ethiopia, Eritrea, and Saudi Arabia. ANS ophiolites mostly contain spinels with high Cr#, indicating SSZ affinities. Limited trace element data on pillowed lavas supports this interpretation. Boninites are unusual melts of harzburgite that result from asthenospheric upwelling interactng with slab-derived water. This environment is only common during subduction initiation events. Boninites associated with ophiolites have been reported from Egypt, Ethiopia and Eritrea, but most of the geochemical studies of ANS ophiolitic basalts are based on studies that are a decade or more old. The abundance of ANS ophiolites implies an episode of subduction initiation occurred in Neoproterozoic time.

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    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 Pinarbaşi 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 Pinarbaşi ophiolite to define the formation mechanism of the sole in relation to the overlying SSZ ophiolite.

  18. 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)

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

    2013-12-01

    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.

  19. Paleomagnetism of the Samar Ophiolite: Implications for the Cretaceous sub-equatorial position of the Philippine island arc

    NASA Astrophysics Data System (ADS)

    Balmater, Hertz G.; Manalo, Pearlyn C.; Faustino-Eslava, Decibel V.; Queaño, Karlo L.; Dimalanta, Carla B.; Guotana, Juan Miguel R.; Ramos, Noelynna T.; Payot, Betchaida D.; Yumul, Graciano P.

    2015-11-01

    Samar island in the eastern part of Central Philippines is underlain by a complete ophiolite suite, the Samar Ophiolite. We present the first geochronological and paleomagnetic data for the Samar Ophiolite. Whole rock K-Ar dating of two basalt samples yielded an age of 100.2 ± 2.7 Ma and 97.9 ± 2.8 Ma. Thirteen sites in four localities yielded characteristic remanent magnetization with in situ direction of D = 340°, I = - 24°, k = 15, ?95 = 11° and tilt-corrected direction of D = 342°, I = - 27°, k = 15, ?95 = 11°. These values suggest that the ophiolitic basement rocks of Samar formed in the Late Cretaceous at a paleolatitude of 14°S ± 6°. The paleolatitude is several degrees south of the sub-equatorial positions calculated for the three other Mesozoic ophiolites of the Philippine Mobile Belt (PMB) whose paleomagnetism had been previously studied. The PMB ophiolites in eastern and central Philippines share a common age, geochemistry and paleolatitude with the Halmahera Ophiolite, suggesting that they originated from a Mesozoic supra-subduction zone that spanned a few degrees north of the equator to around 15°S.

  20. Age of Alpine Corsica ophiolites revisited: Insights from in situ zircon U-Pb age and O-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Li, Xian-Hua; Faure, Michel; Rossi, Philippe; Lin, Wei; Lahondère, Didier

    2015-04-01

    Knowledge of the age and timing of ophiolite sequences is essential for understanding the mechanisms of plate tectonics. The ophiolites in the Schistes Lustrés and the Upper nappes of Alpine Corsica represent remnants of the Liguria-Piemonte ocean basin that formed as a branch of the Central Atlantic basin during the opening of the Mesozoic Western Alpine Tethys. Despite numerous isotopic and paleontological studies, the age and timing of the ophiolites in the Schistes Lustrés nappe are still controversial. This study presents integrated in situ analyses of zircon U-Pb age and O-Hf isotopic data for ophiolitic gabbros and plagiogranites from three localities in the Schistes Lustrés nappe of Eastern Corsica. Our new results demonstrate that these rocks crystallized synchronously at ~ 159 Ma, approximately 10 m.y. younger than the ophiolites in the Balagne Upper nappe. Zircons from the gabbros and plagiogranites are characterized by highly positive ?Hf(t) (+ 15.0 to + 15.9) and mantle-like ?18O (5.2-5.4‰) values. Thus, these ophiolitic rocks were cogenetic, and crystallized from magmas produced by partial melting of a depleted, N-MORB type mantle. By contrast, in the Balagne Upper nappe, the ~ 169 Ma ophiolites contain numerous xenocrystic zircons inherited from a continental crust. Our current knowledge of isotopic geochronology and geochemistry supports a paleogeographic reconstruction, in which the earliest ophiolites in the Balagne nappe were emplaced close to a continental margin at ~ 169 Ma, while the N-MORB type ophiolites in the Schistes Lustrés nappe were likely formed approximately 10 m.y. later in the central part of the Liguria-Piemonte oceanic basin. The relative location of the Schistes Lustrés and Balagne Upper nappes with respect to continental margins is discussed.

  1. A Magnetotelluric Survey of Ophiolites in the Neyriz area of southwestern Iran

    NASA Astrophysics Data System (ADS)

    Oskooi, B.; Mansoori, I.; Pedersen, L. B.; Koyi, H. A.

    2014-09-01

    A wide band magnetotelluric study of the ophiolitic zone of the Zagros orogenic belt was conducted in the Neyriz area of southwestern Iran. The purpose of the study was to image subsurface structures electrically and relocate the main Zagros thrust fault in the region. The thrust fault has a complex structure with obscure behavior and is believed to be located within a zone of ongoing continental plate convergence. The fault zone with a NW-SE geological trend is parallel to the Zagros orogenic belt and separates the Neyriz ophiolite assemblage from the adjacent Sanandaj-Sirjan metamorphic zone. Magnetotelluric data were collected along a SW-NE profile across the geologic strike; the study included 18 stations and modeling was performed using a 2-D inversion scheme. Analysis of both modes of magnetotelluric data (TE and TM) clarifies the signatures of large resistivity variation in the study area. Due to the presence of a high contrast in resistivity between the ophiolites and neighboring rocks, we are able to discern two sharp boundaries as faulting planes and borders of the ophiolite-radiolarite zone in the north-eastern and southwestern parts of the 2-D resistivity models, respectively.

  2. A Magnetotelluric Survey of Ophiolites in the Neyriz area of southwestern Iran

    NASA Astrophysics Data System (ADS)

    Oskooi, B.; Mansoori, I.; Pedersen, L. B.; Koyi, H. A.

    2015-02-01

    A wide band magnetotelluric study of the ophiolitic zone of the Zagros orogenic belt was conducted in the Neyriz area of southwestern Iran. The purpose of the study was to image subsurface structures electrically and relocate the main Zagros thrust fault in the region. The thrust fault has a complex structure with obscure behavior and is believed to be located within a zone of ongoing continental plate convergence. The fault zone with a NW-SE geological trend is parallel to the Zagros orogenic belt and separates the Neyriz ophiolite assemblage from the adjacent Sanandaj-Sirjan metamorphic zone. Magnetotelluric data were collected along a SW-NE profile across the geologic strike; the study included 18 stations and modeling was performed using a 2-D inversion scheme. Analysis of both modes of magnetotelluric data (TE and TM) clarifies the signatures of large resistivity variation in the study area. Due to the presence of a high contrast in resistivity between the ophiolites and neighboring rocks, we are able to discern two sharp boundaries as faulting planes and borders of the ophiolite-radiolarite zone in the north-eastern and southwestern parts of the 2-D resistivity models, respectively.

  3. 3D Geometry of Fault Intersections in Central California: The Importance of Ultramafic Ophiolitic Rocks

    NASA Astrophysics Data System (ADS)

    Watt, J. T.; Ponce, D. A.; Johnson, S. Y.

    2014-12-01

    Integrated geological and geophysical studies of three fault intersections in central California reveal an abundance of serpentinite and related ultramafic ophiolitic rocks in the subsurface; this is a significant discovery that not only constrains the geometry of structures, but also helps to explain fault development and fault behavior. We present and compare fault framework models from the Hayward-Calaveras, Calaveras-San Andreas, and Hosgri-Shoreline fault intersections and discuss the importance of ultramafic ophiolitic rocks and the implications for seismic hazard. Fault framework models were created based on integrated analysis of geology, potential-field, seismic-reflection, multibeam bathymetry, and seismicity data. In each case, ultramafic ophiolitic rocks have very limited surface expression, but are prevalent in the subsurface from ~1 to ~8 km depth. High-resolution aero- and marine-magnetic data are particularly important for determining the geometry of these strongly magnetic rock bodies and the faults along which they occur. At each fault intersection, the ultramafic ophiolitic rocks characterize a pre-existing structure that either promotes or represents a barrier to faulting, and significantly influences local deformation patterns. Although the faults at each intersection do not connect directly at the surface, combined geologic and geophysical data suggest that these faults connect in the subsurface either along a single through-going fault plane or along multiple splay faults. The continuity of these fault intersections at seismogenic depths suggests the possibility of a combined rupture that should be evaluated in future seismic hazard studies.

  4. Mantle petrology and mineralogy of the Thetford Mines Ophiolite Complex

    NASA Astrophysics Data System (ADS)

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

    2008-01-01

    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.

  5. Diamonds in ophiolites: Contamination or a new diamond growth environment?

    NASA Astrophysics Data System (ADS)

    Howell, D.; Griffin, W. L.; Yang, J.; Gain, S.; Stern, R. A.; Huang, J.-X.; Jacob, D. E.; Xu, X.; Stokes, A. J.; O'Reilly, S. Y.; Pearson, N. J.

    2015-11-01

    For more than 20 years, the reported occurrence of diamonds in the chromites and peridotites of the Luobusa massif in Tibet (a complex described as an ophiolite) has been widely ignored by the diamond research community. This skepticism has persisted because the diamonds are similar in many respects to high-pressure high-temperature (HPHT) synthetic/industrial diamonds (grown from metal solvents), and the finding previously has not been independently replicated. We present a detailed examination of the Luobusa diamonds (recovered from both peridotites and chromitites), including morphology, size, color, impurity characteristics (by infrared spectroscopy), internal growth structures, trace-element patterns, and C and N isotopes. A detailed comparison with synthetic industrial diamonds shows many similarities. Cubo-octahedral morphology, yellow color due to unaggregated nitrogen (C centres only, Type Ib), metal-alloy inclusions and highly negative δ13C values are present in both sets of diamonds. The Tibetan diamonds (n = 3) show an exceptionally large range in δ15N (-5.6 to + 28.7 ‰) within individual crystals, and inconsistent fractionation between {111} and {100} growth sectors. This in contrast to large synthetic HPHT diamonds grown by the temperature gradient method, which have with δ15N = 0 ‰ in {111} sectors and + 30 ‰ in {100} sectors, as reported in the literature. This comparison is limited by the small sample set combined with the fact the diamonds probably grew by different processes. However, the Tibetan diamonds do have generally higher concentrations and different ratios of trace elements; most inclusions are a NiMnCo alloy, but there are also some small REE-rich phases never seen in HPHT synthetics. These characteristics indicate that the Tibetan diamonds grew in contact with a C-saturated Ni-Mn-Co-rich melt in a highly reduced environment. The stable isotopes indicate a major subduction-related contribution to the chemical environment. The unaggregated nitrogen, combined with the lack of evidence for resorption or plastic deformation, suggests a short (geologically speaking) residence in the mantle. Previously published models to explain the occurrence of the diamonds, and other phases indicative of highly reduced conditions and very high pressures, have failed to take into account the characteristics of the diamonds and the implications for their formation. For these diamonds to be seriously considered as the result of a natural growth environment requires a new understanding of mantle conditions that could produce them.

  6. Microbiology of Ultrabasic Groundwaters of the Coast Range Ophiolite, California

    NASA Astrophysics Data System (ADS)

    Schrenk, M. O.; Brazelton, W. J.; Twing, K. I.; Kubo, M.; Cardace, D.; Hoehler, T. M.; McCollom, T. M.

    2013-12-01

    Upon exposure to water, ultramafic rocks characteristic of the Earth's mantle undergo a process known as serpentinization. These water-rock reactions lead to highly reducing conditions and some of the highest pH values reported in nature. In contrast to alkaline soda lakes, actively serpentinizing environments exposed on land are commonly associated with low salinity freshwaters, imparting unique challenges upon their resident microbial communities. These environments are especially prevalent along continental margins, and cover extensive portions of the west coast of North America. Most studies of serpentinizing environments have focused upon springs that emanate from fractures in the subsurface. Here, we present microbiological data from a series of groundwater wells associated with active serpentinization in the California Coast Range, an ophiolite complex near Lower Lake, California. Waters from ultrabasic wells had lower microbial cell concentrations and diversity than were found in moderate pH wells in the same area. Bacteria consistently made up a higher proportion of the microbial communities compared to Archaea as determined by qPCR. High pH wells were dominated by taxa within the Betaproteobacteria and Clostridia, whereas moderate pH wells predominantly contained common soil taxa related to Gammaproteobacteria and Bacilli. Multivariate statistical analyses incorporating key environmental parameters supported these observations and also highlighted correlations between the high-pH taxa and the abundance of hydrogen and methane gas. Similarly, colony forming units of alkaliphilic microorganisms were consistently 1-2 orders of magnitude higher in the ultrabasic wells and were taxonomically distinct from the moderate pH groundwaters. Together, these results show that distinct populations inhabit subsurface environments associated with active serpentinization, consistent with previous observations, and suggest that Betaproteobacteria and Clostridia probably play significant roles in the microbiology of these ecosystems. The low diversity microbial communities of serpentinizing subsurface habitats are likely sustained by the high hydrogen and methane fluxes that emanate from such systems and further investigations will directly test their roles in mediating biogeochemical cycles in these environments.

  7. Timing of ophiolite obduction and regional metamorphism in the Grampian orogen

    NASA Astrophysics Data System (ADS)

    Chew, D. M.; Daly, J. S.; Page, L. M.; Whitehouse, M. J.

    2012-04-01

    The Grampian terrane in the Caledonides of Scotland and NW Ireland is the type locality for Barrovian (regional) metamorphism, recognized in most mountain belts. It is thought to have resulted from the collision of the Laurentian margin with an oceanic arc and associated suprasubduction ophiolite during the Early Ordovician (van Staal et al., 1998). Here we address the timing and P-T conditions of Grampian ophiolite obduction and re-evaluate the link with regional metamorphism. Magmatic zircons from the Highland Border Ophiolite, Scotland define a 499 ± 8 Ma U-Pb Concordia age. Its metamorphism is dated by a 490 ± 4 Ma 40Ar-39Ar hornblende age, and a 488 ± 1 Ma 40Ar-39Ar muscovite age from a metasedimentary xenolith within it, from which P-T estimates of 5.3 kbar and 580 ° C relate to ophiolite obduction. Metamorphism of the Irish correlative of the Highland Border Ophiolite is constrained by a 514 ± 3 Ma 40Ar-39Ar hornblende age, while mica schist slivers within it yield detrital zircon U-Pb ages consistent with Laurentian provenance and Rb-Sr and 40Ar-39Ar muscovite ages of ca. 482 Ma. P-T values of 3.3 kbar and 580 ° C for these rocks constrain the conditions of ophiolite obduction (Chew et al., 2010). Peak Grampian metamorphism on the Laurentian margin (Dalradian Supergroup) is constrained to c. 475 - 465 Ma. This includes U-Pb zircon ages from Grampian syn-orogenic intrusives (a probable Andean-type arc which developed following subduction polarity reversal) and metamorphic mineral ages from Dalradian regions devoid of syn-orogenic intrusive rocks (Flowerdew et al., 2000; Chew et al., 2003). There is therefore a pronounced time gap between c. 470 Ma mineral ages in the Laurentian margin (Dalradian Supergroup) and c. 490 Ma mineral ages in the Grampian ophiolitic rocks. P-T conditions also differ markedly, with high T - low P metamorphism in the Grampian ophiolitic rocks and high P - low T (blueschist-facies) metamorphic conditions in the subducted Laurentian margin sediments of the Dalradian Supergroup. It is envisaged that subduction of the leading edge of the Laurentian plate initiated at c. 490 Ma, contemporaneous with the start of ophiolite obduction and resulted in high-pressure metamorphism of the Laurentian margin. The high-pressure rocks were transferred to the hanging-wall plate and thrust back onto the margin, and exhumed shortly afterwards by extensional collapse preserving mineral cooling ages as old as ca. 475 Ma close to the margin. Away from the Laurentian margin, collisional thickening created the thick Dalradian nappe stack and associated Barrovian metamorphism, with possibly minimal involvement of obducted oceanic lithosphere. If conductive heat transfer in over-thickened crust is the major heating mechanism, then collisional thickening may have initiated shortly after ophiolite obduction started at ca. 490 Ma in order to generate the ca. 470 Ma Grampian peak metamorphism in the Dalradian.

  8. Middle Jurassic age of basalts and the post-obduction sedimentary sequence in the Guevgueli Ophiolite Complex (Republic of Macedonia)

    NASA Astrophysics Data System (ADS)

    Kuko?, Duje; Gori?an, Špela; Košir, Adrijan; Belak, Mirko; Halami?, Josip; Hrvatovi?, Hazim

    2015-03-01

    The Guevgueli Ophiolite Complex near Demir Kapija (Eastern Vardar Ophiolitic Unit) was studied for the age and facies of the overlying sediments. Cherts in direct contact with basalts are dated to late Bathonian-early Callovian with radiolarians. The post-obduction sequence, here informally named the Demir Kapija group, is composed of polymictic conglomerate, probably Kimmeridgian in age, and a more than 350-m thick carbonate succession. The carbonate succession consists of hemipelagic, slope and platform margin facies and contains algae and benthic foraminifers indicative of the Tithonian age. These new data support the previously proposed palaeogeographical connection between the Guevgueli and South Apuseni ophiolite complexes.

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

    USGS Publications Warehouse

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

    1983-01-01

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

    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.

  11. Exploring the deep biosphere through ophiolite-associated surface springs

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    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.

  12. Structure of Submarine Large Lobate Sheet from the Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Umino, S.

    2009-12-01

    Coalescence and inflation of flow lobes are common to fluidal basaltic lava emplaced on a gentle slope and a flat field, which are fundamental mechanisms to form vast sheet-like lava flows. Flow-lobe coalescence and inflation are also known from submarine sheet flows from mid-ocean ridges and submarine extensions of Hawaiian rift zones. The V3 extrusive unit (Salahi Volcanics) of the Oman Ophiolite has an extensive sheet flow of alkali basalt attaining 12 km in length and as thick as 100 m. We propose that this unusually thick sheet flow was formed by complex flow-lobe coalescence and inflation of subaqueous lava lobes extruded at low supply rates of lava. V3 mainly consists of 3 sheet flows separated by red shale beds associated with pillow and pahoehoe flows. An alkali dolerite dyke >30 m in thickness to the southern end of V3 distribution is assumed to be the source of V3 lavas, intruding into the Alley Volcanics (V2) beneath V3. Ropy wrinkles are commonly observed on the top and bottom of the sheets, indicating north to north-westerly flow directions. Sheet flows occasionally grade into pillows and pahoehoe lobes both laterally and downward. Pillows and pahoehoe lobes directly broke out from the base or peripherals of sheet flows are observed. Red shale fills interstices between pillows and fractures along the cooling joints in the base of sheet flows. Because pillows are formed on slopes >5 degrees, the above occurrence indicates that the slowly advancing lava formed pillows as it flowed down into a depression filled with unconsolidated mud. When the depression was filled with the pillows, the lava form changed into pahoehoe lobes which were coalesced and inflated to a thick sheet flow. The lowest sheet flow (SF-1) has the largest extension and thickness among the three flows. It has columnar jointed upper and lower crusts, and massive cores, among which the upper crust is thickest. Such joint structures also develop in subaerial flood basalts, but are more complex in the Salahi SF-1. Most part of SF-1 has only one core between the upper and lower crusts, while in places double cores are present separated by a columnar jointed layer, or no core appears in other places. The core lacks dendritic clinopyroxene, showing a typical doleritic texture. In contrast, the crust contains thin and elongated clinopyroxene, suggestive of crystallization under a large degree of supercooling.The roof of the sheet develops domed structures several metres to a few hundred metres across. Finely jointed zones beneath such domed roofs sometimes continue into and thin out within the crust below the neighbouring roof. Such finely jointed layers and lenses are most plausibly seal zones of coalesced flow lobes. Hyaloclastite veins and lenses are found along vertical joints in the lower part of the upper crust. Repeated fragmentation of chilled margins along the joints indicates that molten lava was in contact with water, which entered through deep cracks into the upper crust. All these observations led us to conclude that the crusts were formed by coalescence of partially overlapped or stacked flow lobes, while the core developed endogenously as the sheet inflated. Meanwhile inflation cracks opened and penetrated deep into the crust.

  13. The Neoproterozoic Abu Dahr ophiolite, South Eastern Desert, Egypt: petrological characteristics and tectonomagmatic evolution

    NASA Astrophysics Data System (ADS)

    Gahlan, Hisham A.; Azer, Mokhles K.; Khalil, Ahmed E. S.

    2015-10-01

    The Neoproterozoic Abu Dahr ophiolite, South Eastern Desert, Egypt, is one of the best preserved and least dismembered ophiolite successions in the Arabian-Nubian Shield. It contains a Penrose-type ophiolite sequence from mantle section below mafic crust upward to oceanic sedimentary cover overlying mafic volcanics, although the original magmatic (stratigraphic) contact between the mantle and crustal sections is disrupted by tectonism. The Abu Dahr ophiolite is metamorphosed under greenschist facies conditions, and low-temperature alteration is widespread. Petrography reveals that: (i) the mantle is homogenous, serpentinized, and dominated by harzburgite and less abundant dunite; (ii) the cumulate ultramafics are represented by wehrlite and pyroxenite; and (iii) the crustal section is represented by metagabbros, meta-anorthosites and metabasalts. The Abu Dahr serpentinized peridotites show high Mg# (0.92-0.93), with enrichment of Ni, Cr and Co, and depletion of Al2O3 and CaO, and nearly flat and unfractionated REE chondrite-normalized pattern. Major and trace element characteristics of the Abu Dahr metagabbro and metabasalt (crustal section) indicate a tholeiitic to calc-alkaline affinity. Units of the crustal section have low-Nb and Zr concentrations, low Dy/Yb and relatively elevated La/Yb ratios, high U/Yb and Th/Yb ratios, and LREE enriched chondrite-normalized pattern. All of the Abu Dahr ophiolite units have trace-element signatures characterized by enrichment of LILE over HFSE. Rare and trace element patterns indicate a genetic link between the Abu Dahr mantle, cumulate ultramafics, and crust. Chromian spinel has survived metamorphism and is used as a petrogenetic indicator in the Abu Dahr serpentinized peridotites. The spinel is homogeneous with a limited composition, and shows high-Cr# (>0.6) combined with low-TiO2 character (mostly <0.1 wt.%). The Abu Dahr ophiolite is interpreted as a fragment of depleted oceanic lithosphere that experienced high degrees of partial melting (up to 35 %) and originated in a fore-arc setting. Such interpretation contributes to the body of evidence suggesting that tectonomagmatic processes of the Neoproterozoic were largely similar to those of the Phanerozoic, implying little, or no significant, change in the geothermal regime of Earth since the Neoproterozoic.

  14. Obduction of western Anatolian ophiolites: from birth to steady state of a subduction zone

    NASA Astrophysics Data System (ADS)

    Plunder, Alexis; Agard, Philippe; Chopin, Christian; Whitechurch, Hubert; Okay, Aral

    2015-04-01

    During Cretaceous times, the convergence between the Anatolide Tauride block (following the movement of Africa) and Eurasia lead to the closure of a branch of the Neotethyan ocean and to ophiolite obduction. Obducted ophiolite and their sub-ophiolitic units can be found along a 400 kilometre-long north to south transect in western Anatolia. The aim of this contribution is twofold: (1) (re)-appraise the metamorphic pressure-temperature (PT) conditions and evolution of the sub-ophiolitic units of western Anatolia, by constraining the formation of the metamorphic sole during the first stages of subduction and the unusual accretion of ocean-derived units along a subduction interface in an evolving, cooling thermal regime, and (2) understand the dynamics of a large-scale and long-lived obduction. Directly below the ophiolite (mostly made of mantle-derived rocks) lies a metamorphic sole. The upper part is this sole is made of garnet and garnet clinopyroxene amphibolites, the lower part consisting in amphibolite or green-schist facies metapelites and metabasite suggestive of discrete accretion steps. In the northern part of the section the metamorphic sole is characterised by an important blueschist-facies overprint destabilizing the amphibolite paragenesis. This high-pressure overprint is lacking in the southern area. Using field and petrological observations, three units (namely and from top to bottom, OC1, OC2 and OC3) were distinguished in the accretionary complex with PT conditions ranging from incipient metamorphism to blueschist facies conditions. OC1 represents most of the outcropping unit, is found all along the section and shows only low-grade metamorphism. Metamorphic conditions remains hard to establish in this unit made of a stack of hm-thick tectonic slices showing subtle differences in their metamorphic grade (from pristine pillow basalts and hydrothermalized lavas to lawsonite pumpellyite-lawsonite bearing basalts). In OC2, Fe-Mg carpholite-bearing layers were found and attest to high-pressure and low-temperature conditions. As OC2, OC3 exhibit a clear blueschist facies metamorphism, but slightly higher PT conditions. Both OC2 and 3 were only found in the northern area close to the suture zone. Combining these data, available radiometric and palaeogeographic data and recent themomechanical modelling a tentative reconstruction of the subduction-zone evolution through time during the emplacement of a large-scale ophiolite is presented. We show that the cooling of the subduction must occur very quickly (~<15 My) after subduction inception and investigate the implications for early subduction and obduction dynamics.

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    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.

  16. The oldest island arc and ophiolite complexes of the Russian Arctic (Taimyr Peninsula)

    NASA Astrophysics Data System (ADS)

    Vernikovskaya, Antonina E.; Vernikovsky, Valery A.; Metelkin, Dmitriy V.; Matushkin, Nikolay Y.; Romanova, Irina V.

    2015-04-01

    Knowing the age of indicator complexes such as island arc, ophiolite, collisional, subductional etc. is extremely important for paleogeodynamic reconstructions. The age along with other geological and geophysical data enables the reestablishing of the positions of terranes of various origins in relation to continental margins and to each other. When studying the issues concerning the ancient Arctida paleocontinent, the nature of terranes and continental plates that compose the present day arctic shelf and submerged ridges it is important to determine the main stages of tectonic events. At the same time it is particularly important to establish the earliest stages of tectonic transformations. The Taimyr-Severnaya Zemlya orogenic belt is one of the large accretionary-collisional key structures in the Arctic. The Central Taimyr accretionary belt includes two granite-metamorphic terranes: Faddey and Mamont-Shrenk that include the oldest igneous formations of Taimyr. Those are granitoids with U/Pb zircons age of 850-830 Ma (Faddey) and 940-885 Ma (Mamont-Shrenk). Presently we have determined fragments of paleo-island arcs and ophiolites in the framing of these terranes. Moreover, in addition to already identified Neoproterozoic (755-730 Ma) ophiolites and island arc rocks (plagiogranites, gabbro, volcanics) we found more ancient rock complexes in the framings of both terranes closer in age to the Meso-Neoproterozoic boundary. In the region of the Tree Sisters Lake a paleo-island arc complex was found including plagiogranites and plagiorhyodacites with U-Pb isotopic zircon age of 969-961 Ma. Sm-Nd isotopic data for these rocks showed a Mesoproterozoic model age: TNd(DM) varies from 1170 to 1219 Ma. These data as well as Rb-Sr isotopic investigations indicate a predominance of a mantle component in the magmatic sources of these rocks: ?Nd (967-961) = 5.1-5.2 and (87Sr/86Sr)0 =0.70258-0.70391. In the framing of the Mamont-Shrenk terrane we determined ophiolite fragments in the mouths of Krasnaya River and Kabachkovaya Hill. The Kabachkovaya ophiolites form near E-W elongated narrow zones of ultramafic rocks and small plutons of fine and medium grained gabbros and diabases among flows of tholeitic basalts forming pillow lavas and tuffs. Ar/Ar dating of amphiboles from metagabbros in the Krasnaya R. mouth yielded an age of 1029 Ma. In conclusion, these data indicate the existence of Meso-Neoproterozoic ophiolites and island arcs in the Russian Arctic, which, with available paleomagnetic data, allows composing more correct plate tectonic reconstructions for the early stages of the evolution of this region.

  17. 40Ar 39Ar Ages and tectonic setting of ophiolite from the Neyriz area, southeast Zagros Range, Iran

    USGS Publications Warehouse

    Lanphere, M.A.; Pamic, J.

    1983-01-01

    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.

  18. Mantle segmentation along the Oman ophiolite fossil mid-ocean ridge.

    PubMed

    Le Mée, Laurent; Girardeau, Jacques; Monnier, Christophe

    2004-11-11

    It has been difficult to relate the segmentation of mid-ocean ridges to processes occurring in the Earth's underlying mantle, as the mantle is rarely sampled directly and chemical variations observed in lavas at the surface are heavily influenced by details of their production as melt extracted from the mantle. Our understanding of such mantle processes has therefore relied on the analysis of pieces of fossil oceanic lithosphere now exposed at the Earth's surface, known as ophiolites. Here we present the phase chemistry and whole-rock major- and trace-element contents of 174 samples of the mantle collected along over 400 km of the Oman Sultanate ophiolite. We show that, when analysed along the fossil ridge, variations of elemental ratios sensitive to the melting process define a three-dimensional geometry of mantle upwellings, which can be related to the segmentation observed in modern mid-ocean ridge environments. PMID:15538358

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

    NASA Astrophysics Data System (ADS)

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

    2008-01-01

    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.

  20. Geochemistry of reduced gas related to serpentinization of the Zambales ophiolite, Philippines

    USGS Publications Warehouse

    Abrajano, T.A.; Sturchio, N.C.; Kennedy, B.M.; Lyon, G.L.; Muehlenbachs, K.; Böhlke, J.K.

    1990-01-01

    Methane-hydrogen gas seeps with mantle-like C and noble gas isotopic characteristics issue from partially serpentinized ultramafic rocks in the Zambales ophiolite, Philippines. New measurements of noble gas and 14C isotope abundances, rock/mixed-volatile equilibrium calculations, and previous chemical and isotopic data suggest that these reduced gases are products of periodotite hydration. The gas seeps are produced in rock-dominated zones of serpentinization, and similar gases may be ubiquitous in ultramafic terranes undergoing serpentinization. ?? 1990.

  1. Iron and magnesium isotope fractionation in oceanic lithosphere and sub-arc mantle: Perspectives from ophiolites

    NASA Astrophysics Data System (ADS)

    Su, Ben-Xun; Teng, Fang-Zhen; Hu, Yan; Shi, Ren-Deng; Zhou, Mei-Fu; Zhu, Bin; Liu, Fan; Gong, Xiao-Han; Huang, Qi-Shuai; Xiao, Yan; Chen, Chen; He, Yong-Sheng

    2015-11-01

    We present high-precision Fe and Mg isotopic data for the Purang ophiolite, southwestern Tibet, representing the first combined Fe and Mg isotopic study of the oceanic lithosphere hitherto. The δ56Fe and δ26Mg values of the ophiolitic peridotite, dunite and gabbro vary from -0.209 to 0.187‰ and from -0.28 to - 0.14 ‰, respectively. The average δ56Fe of the peridotites is - 0.030 ± 0.143 ‰ (2SD, n = 17), a value indistinguishable from abyssal peridotites and chondrites, and lower than oceanic basalts. The average δ26Mg value of the peridotites is - 0.20 ± 0.10 ‰, a value slightly higher than both chondrites and oceanic basalts. Correlations between δ56Fe and indices of partial melting indicate fractionation of 0.323‰ in δ56Fe between the oceanic lithospheric mantle and the overlying mafic crust during an early episode of partial melting, presumably beneath a spreading centre. Subsequent metasomatism in a supra-subduction zone caused elevated oxygen fugacity and heavy Fe isotopic compositions in the oceanic lithospheric mantle. The dunite with high Ba/La, a proxy for oxygen fugacity, and high δ56Fe values was likely formed during this process of sub-arc mantle-melt interaction. The negatively coupled Fe-Mg isotopic variations of the Purang ophiolite indicate that Mg isotope fractionation may also occur during high-temperature mantle processes. The observed isotopic variations among different lithologies in the ophiolite may satisfactorily account for the isotopic differences between arc lavas and mantle peridotites with respect to oceanic basalts, thus providing implications for crust-mantle differentiation.

  2. Metamorphic sole genesis at the base of ophiolite nappes: Insights from numerical models

    NASA Astrophysics Data System (ADS)

    Yamato, Philippe; Agard, Philippe; Duretz, Thibault

    2015-04-01

    Obduction emplaces oceanic lithosphere on top of continental lithosphere. Although a number of studies have focused on this enigmatic process, the initial stages of obduction remain poorly understood. Field, petrological, and geochronological data reveal that during the first stages of the obduction (i.e., during the first 1-2 Myrs) a HT-LP metamorphic sole (~700-800 ° C and ~1 GPa) is systematically welded at the base of ophiolite nappes. However, the reason why such welding of the ophiolite soles occurs at these particular P-T conditions, and only at the onset of obduction, is still an open issue. The aim of this study is to explore the conditions required to explain the genesis of metamorphic soles. For this, we employ two-dimensional numerical modelling, constrained by the wealth of available data from the Oman ophiolite. We first present a thermo-kinematic model in which the velocity field is prescribed in order to simulate obduction initiation. The heat advection-diffusion equation is solved at each time step. The model is intentionally kept simple in order to control each parameter (e.g., convergence rate, dip angle, thermal age) and to test its influence on the resulting P-T conditions obtained through time along the obduction interface. Results show that the key factor allowing the formation of metamorphic soles is the age of the oceanic lithosphere involved. Moreover, we speculate that the reason why metamorphic soles are always welded at the same P-T conditions is due to the fact that, at these particular conditions, strength jumps occur within the oceanic lithosphere. These jumps lead to changes in strain localisation and allow the spalling of oceanic crust and its juxtaposition to the ophiolite nappe. This hypothesis is further tested using thermo-mechanical models in which the obduction initiates dynamically (only initial and boundary conditions are prescribed). The interplay between the temperature evolution and the mechanical behaviour is then discussed.

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

    NASA Astrophysics Data System (ADS)

    Hock, M.; Friedrich, G.

    1985-10-01

    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.

  4. Complete preservation of ophiolite suite from south Andaman, India: A mineral-chemical perspective

    NASA Astrophysics Data System (ADS)

    Saha, Abhishek; Dhang, Avik; Ray, Jyotisankar; Chakraborty, Suvankar; Moecher, David

    2010-06-01

    Field studies supplemented by petrographic analyses clearly reveal complete preservation of ophiolite suite from Port Blair (11°39'N: 92°45'E) to Chiriyatapu (11°30'24?N: 92°42'30?E) stretch of South Andaman. The ophiolite suite reveals serpentinite at the base which is overlain unconformably by cumulate ultramafic-mafic members with discernible cumulus texture and igneous layering. Basaltic dykes are found to cut across the cumulate ultramafic-mafic members. The succession is capped by well exposed pillow basalts interlayered with arkosic sediments. Olivine from the basal serpentinite unit are highly magnesian (Fo80.1-86.2). All clinopyroxene analyses from cumulate pyroxenite, cumulate gabbro and basaltic dyke are discriminated to be ‘Quad’ and are uniformly restricted to the diopside field. Composition of plagioclase in different lithomembers is systematically varying from calcic to sodic endmembers progressively from cumulate pyroxenite to pillow basalt through cumulate gabbro and basaltic dyke. Plagioclase phenocrysts from basaltic dyke are found to be distinctly zoned (An60.7-An35.3) whereas groundmass plagioclase are relatively sodic (An33-An23.5). Deduced thermobarometric data from different lithomembers clearly correspond to the observed preservation of complete ophiolite suite.

  5. Supra-subduction and abyssal mantle peridotites of the Coast Range ophiolite, California

    NASA Astrophysics Data System (ADS)

    Choi, Sung Hi; Shervais, John W.; Mukasa, Samuel B.

    2008-11-01

    The Coast Range ophiolite (CRO) of California is one of the most extensive tracts of oceanic crust preserved in the North American Cordillera, but its origin has long remained controversial. We present here new data on mineral compositions in mantle peridotites that underlie crustal sections of the ophiolite, and show that these are dominantly refractory harzburgites related to high apparent melting in a supra-subduction zone (SSZ) setting. Abyssal peridotite (characterized by high-Al spinels and relatively high Ti, Na, Nd, Sm, Lu, and Hf in pyroxene) occurs at one location where it is associated with SSZ mantle peridotite and volcanic rocks with both oceanic and arc-like geochemistry. SSZ mantle peridotites (characterized by intermediate-Cr/Al or high-Cr spinels, and by extremely low Ti, Na, Nd, Sm, Lu, and Hf in pyroxenes) are associated with crustal sections containing arc-related volcanic rocks, including boninites. This convergence between conclusions based on crustal lithologies and their underlying mantle sections confirms previous proposals that link the CRO to SSZ processes, and seriously undermines hypotheses that invoke formation of the ophiolite at a mid-ocean ridge spreading center.

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

    USGS Publications Warehouse

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

    1999-01-01

    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.

  7. Gridino melange zone of the Belomorian eclogite province: Succession of tectonic events and structural position of mafic dyke swarms

    NASA Astrophysics Data System (ADS)

    Babarina, I. I.; Sibelev, O. S.; Stepanova, A. V.

    2014-07-01

    Based on relationships between Paleoproterozoic mafic dykes, lithotectonic complexes, and tectonic structures of the Gridino Zone in the Belomorian eclogite province of the Fennoscandian Shield, deformations have been divided into groups differing in age and the succession of tectonic events has been reconstructed. The formation of Neoarchean eclogite-bearing melange was related to disintegration of large eclogite sheets in the course of near-horizontal ductile flow accompanied by syntectonic granitoid magmatism, multiple migmatization, and granulite-to amphibolite-facies metamorphism. The exotic blocks, including eclogites, were incorporated into TTG gneisses as sheets and lenses up to a few hundreds of meters in thickness and oriented conformably with gneissic banding. As a result of ductile flow, the lithotectonic complexes were transported at the level of discrete brittle-ductile deformations expressed as strike-slip faults and associated folds. Under conditions of a relatively rigid medium, individual structural elements underwent rotation approximately through 90° in plan view. Under the extension regime in the Early Paleoproterozoic, several swarms of mafic dykes were injected into the already cold framework rocks, as is evident from dyke morphology. The dykes crosscut all predated structures, included turned blocks, and are therefore important reference points for subdivision of Neoarchean and Paleoproterozoic processes. The Svecofennian postdyke tectonic activity was accompanied by local shearing and boudinage of metabasic rocks, development of quartz and pegmatite veins along tension cracks, disharmonic folding, and discrete retrograde metamorphism up to amphibolite-facies conditions. The postdyke deformations did not exert a substantial effect on the previously formed regional structure.

  8. Deep origin of the Luobusa ophiolitic peridotites and chromitites in Tibet

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    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.

  9. Geochemistry and tectonic evolution of the Neoproterozoic Wadi Ghadir ophiolite, Eastern Desert, Egypt

    NASA Astrophysics Data System (ADS)

    El-Rahman, Yasser Abd; Polat, Ali; Dilek, Yildirim; Fryer, Brian; El-Sharkawy, Mohamed; Sakran, Shawki

    2009-11-01

    We report new geochemical data from the Neoproterozoic ophiolite in the Wadi Ghadir area, Eastern Desert, Egypt. The Wadi Ghadir ophiolite (WGO) is composed of layered and isotropic gabbros and amygdaloidal to porphyritic pillow lavas. Both the gabbroic rocks and the pillow lavas are intruded by dike swarms with different chemical affinities and spatial orientations. The WGO occurs in an ophiolitic mélange (Wadi Ghadir mélange, WGM), and both the WGO and WGM are intruded by granitic rocks to the west. On the basis of Zr and Y variations, units of the WGO are classified as tholeiites (e.g., gabbros, amygdaloidal pillow lavas, D1, and D3 dikes). The late-stage D4 dikes show a calc-alkaline affinity, whereas porphyritic pillow lavas and D2 dikes have a transitional character. All ophiolitic units display subduction zone trace element signatures characterized by the enrichment of LILE over HFSE and negative Nb-Ta anomalies. Tholeiitic rocks are further divided into LREE-depleted (gabbros, D3 dikes) and LREE-enriched (amygdaloidal pillow lavas, D1 dikes) groups. Light REE-depleted tholeiitic rocks were derived from melting of a slightly depleted mantle source, whereas the LREE-enriched tholeiitic rocks were derived from a fertile N-MORB source. Calc-alkaline D4 dikes are characterized by steep REE patterns and have low Zr/Nb ratios (23-29), indicating melt contribution from an enriched mantle source, such as sub-continental lithospheric mantle and/or garnet peridotite. The transitional group has LREE-enriched patterns, but its degree of REE enrichment and Zr/Nb ratios are intermediate between the tholeiitic and calc-alkaline groups. The WGO is interpreted to have formed in a back-arc setting behind the Nugrus volcanic arc developed above a NE-dipping subduction zone. The collision of this arc-back-arc system with the passive margin of the Nubian shield (Hafafit dome) resulted in the accretion of the Nugrus arc and the WGO onto the Nubian continental margin and in the initiation of a new subduction zone dipping SW beneath the newly accreted arc-back-arc crust and Hafafit dome. The establishment of this new Andean-type continental margin produced the calc-alkaline D4 dikes, leucogabbros and granitoid plutons intrude the Hafafit dome, the WGO, and the ophiolitic mélange.

  10. Slab and Sediment Melting during Subduction Initiation: Mantle Plagiogranites from the Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Rollinson, H. R.

    2014-12-01

    Granitoid dykes up to several hundred metres wide and 2 km long are found in depleted harzburgites in the mantle section of the Oman ophiolite. They vary in composition from tonalite to potassic granite and are generally more potassic than the crustal plagiogranites found within the sheeted dyke complex higher up within the ophiolite stratigraphy. Some granites are strongly peraluminous and contain garnet and andalusite. They are geochemically variable, some with REE that are relatively unfractionated ((La/Yb)n= 3.5-6.0, flat middle to heavy REE, steep light REE) to those which are highly fractionated ((La/Yb)n= 28-220). On primitive-mantle normalised plots some have very high concentrations of fluid-mobile elements - Cs, Rb, Th, U and Pb. Few have significant Ta-Nb anomalies. On the Ca-Fe-Mg-Ti discrimination diagram of Patino Douce (J. Petrol., 1999) whole-rock compositions define a spectrum between felsic-pelite derived melts and amphibolite-derived melts. There is a chemical similarity between the least REE fractionated plagiogranites (generally tonalites and granodiorites) and melts of an amphibolitic parent. This is supported by the occurrence of mafic xenoliths in some dykes, the presence of hornblende and highly calcic cores (up to An85) in some plagioclase grains. Trace element modelling using Oman Geotimes lavas as the starting composition indicates that melting took place in the garnet stability field, although enrichment in the melt in Cs, Rb, Ba and Pb suggests that there was another component present in addition to the mafic parent. Other plagiogranites (trondhjemites and granites) have a strongly peraluminous chemistry and mineralogy and geochemical similarities with the Himalayan leucogranites implying that they were derived from a sedimentary protolith. These mantle plagiogranites are more prevalent in the northern outcrops of the ophiolite. The volume of granitoid melt and the depth of melting preclude their derivation from the sole of the ophiolite, rather they were derived during subduction by the partial melting of the slab and associated sediment and emplaced into the overlying mantle wedge. Current subduction-initiation models for supra-subduction ophiolites should integrate this process into their thinking.

  11. Geochemistry and geochronology of the metamorphic sole underlying the Xigaze Ophiolite, Yarlung Zangbo Suture Zone, South Tibet

    NASA Astrophysics Data System (ADS)

    Guilmette, Carl; Hébert, Réjean; Wang, Chengshan; Villeneuve, Mike

    2009-09-01

    Strongly foliated amphibolite clasts are found embedded within the ophiolitic mélange underlying the Xigaze Ophiolite near Bainang and Angren, Yarlung Zangbo Suture Zone, Southern Tibet. These high-grade amphibolites are interpreted as remnants of a dismembered subophiolitic metamorphic sole that would have formed during the inception of a subduction. They include garnet-clinopyroxene amphibolites, clinopyroxene amphibolites and common amphibolites. Petrographic descriptions, mineral chemistry and thermobarometry of these rocks can be found in a companion paper [Guilmette, C., Hébert, R., Dupuis, C., Wang, C.S., Li, Z.J., 2008. Metamorphic history and geodynamic significance of high-grade metabasites from the ophiolitic mélange beneath the Yarlung Zangbo Ophiolites, Xigaze area, Tibet. Journal of Asian Earth Sciences, 32, 423-437.]. The geochemistry of the amphibolites confirms that their protoliths were igneous mafic rocks of basaltic to pyroxenitic composition that were likely part of an upper oceanic crust. Rare Earth Elements contents are suggestive of an N-MORB origin. However, enriched LILEs and depleted Nb-Ta-Ti when compared to N-MORBs rather suggest a suprasubduction zone influence. A large proportion of the overlying ophiolitic mafic rocks share the same geochemical characteristics, suggesting the protoliths of the amphibolites might have crystallized in the same environment as the Xigaze ophiolitic crust, likely in a back-arc basin. 40Ar/ 39Ar step-heating dating of hornblende concentrates from three samples yielded ages of 123.6 ± 2.9 Ma, 127.7 ± 2.2 Ma and 127.4 ± 2.3 Ma. These cooling ages are slightly younger or overlapping magmatic and sedimentary ages obtained from the overlying ophiolite. All these new data support a model in which the ophiolitic crust and the protolith of the amphibolites were formed along the same spreading center above a subduction zone. The demise of the early subduction circa 130 Ma forced the inception of a new subduction zone at the SSZ spreading axis, burying SSZ mafic rocks underneath a SSZ ophiolitic mantle.

  12. Spectral transformation of ASTER and Landsat TM bands for lithological mapping of Soghan ophiolite complex, south Iran

    NASA Astrophysics Data System (ADS)

    Pournamdari, Mohsen; Hashim, Mazlan; Pour, Amin Beiranvand

    2014-08-01

    Spectral transformation methods, including correlation coefficient (CC) and Optimum Index Factor (OIF), band ratio (BR) and principal component analysis (PCA) were applied to ASTER and Landsat TM bands for lithological mapping of Soghan ophiolitic complex in south of Iran. The results indicated that the methods used evidently showed superior outputs for detecting lithological units in ophiolitic complexes. CC and OIF methods were used to establish enhanced Red-Green-Blue (RGB) color combination bands for discriminating lithological units. A specialized band ratio (4/1, 4/5, 4/7 in RGB) was developed using ASTER bands to differentiate lithological units in ophiolitic complexes. The band ratio effectively detected serpentinite dunite as host rock of chromite ore deposits from surrounding lithological units in the study area. Principal component images derived from first three bands of ASTER and Landsat TM produced well results for lithological mapping applications. ASTER bands contain improved spectral characteristics and higher spatial resolution for detecting serpentinite dunite in ophiolitic complexes. The developed approach used in this study offers great potential for lithological mapping using ASTER and Landsat TM bands, which contributes in economic geology for prospecting chromite ore deposits associated with ophiolitic complexes.

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

    USGS Publications Warehouse

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

    1984-01-01

    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.

  14. 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)

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

    2014-06-01

    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.

  15. The Archean Dongwanzi ophiolite complex, North China craton: 2.505-billion-year-old oceanic crust and mantle.

    PubMed

    Kusky, T M; Li, J H; Tucker, R D

    2001-05-11

    We report a thick, laterally extensive 2505 +/- 2.2-million-year-old (uranium-lead ratio in zircon) Archean ophiolite complex in the North China craton. Basal harzburgite tectonite is overlain by cumulate ultramafic rocks, a mafic-ultramafic transition zone of interlayered gabbro and ultramafic cumulates, compositionally layered olivine-gabbro and pyroxenite, and isotropic gabbro. A sheeted dike complex is rooted in the gabbro and overlain by a mixed dike-pillow lava section, chert, and banded iron formation. The documentation of a complete Archean ophiolite implies that mechanisms of oceanic crustal accretion similar to those of today were in operation by 2.5 billion years ago at divergent plate margins and that the temperature of the early mantle was not extremely elevated, as compared to the present-day temperature. Plate tectonic processes similar to those of the present must also have emplaced the ophiolite in a convergent margin setting. PMID:11349144

  16. Late Stage MORB Volcanism at the Cuesta Ridge Ophiolite Remnant: Evidence for Ridge Collision or Back-arc Basin Spreading?

    NASA Astrophysics Data System (ADS)

    Snow, C. A.; Shervais, J. W.

    2002-12-01

    The origin and significance of ophiolites has been a major focus of research over the past three decades, with most attention focusing on whether they form at mid-ocean ridges or above subduction zones. The termination of igneous activity in ophiolites has received far less attention, since it is assumed that igneous activity ends when the ophiolite is obducted. The middle Jurassic Coast Range Ophiolite (CRO) of California provides an excellent opportunity to study not only the origin of ophiolites, but also the termination of igneous activity related to ophiolite formation. Geologic mapping of the Cuesta Ridge ophiolite remnant of the CRO reveals an ophiolite sequence in thrust contact with the underlying Franciscan assemblage. The section consists of a 1.5 km thick mantle section of serpentinized harzburgite and a MTZ consisting of dunite, wehrlite, and pyroxenite. The mantle section is overlain by isotropic gabbro, with sills of wehrlite and pyroxenite, a sheeted sill complex of quartz-hornblende diorite, and a 1.3 km thick volcanic section comprising massive flows, pillow lava, and volcanic breccias with calc-alkaline and boninitic affinities, all capped by tuffaceous radiolarian chert. Late stage dikes of tholeiitic basalt cross-cut the quartz diorite sheeted sill complex, and similar basalts occur as flows at the top of the volcanic section, below the tuffaceous cherts. The majority of the volcanic rocks at Cuesta Ridge have low Ti/V ratios (11-21) and other trace element characteristics (e.g., low HFSE, low Zr/Y) that are atypical of MORB and suggest formation in an SSZ setting. High MgO and SiO2-Cr plots show that about 40 percent of the volcanics have strong boninitic affinities, consistent with formation in the fore arc region of an island arc. The quartz-hornblende diorites, which cut the lower volcanics, have SiO2 ranges of 52-75 percent, much too high for rocks formed at mid-ocean spreading centers. In contrast, the late stage dikes and uppermost flows, which cut the sheeted sill complex and overlie the main volcanic section, have a MORB-like affinity with enriched HFSE and TiO2, and high Zr/Y and Ti/V ratios (20-27). These relations are similar to those observed in other CRO remnants such as Elder Creek, Del Puerto, and Mount Diablo, where late dikes with MORB geochemistry cross-cut older plutonic and volcanic rocks with calc-alkaline or arc tholeiite geochemistry. The field and chemical data from Cuesta Ridge and other CRO remnants point toward formation in a SSZ setting above an east-dipping proto-Franciscan subduction zone. This is supported by the abundance of calc-alkaline volcanics and intrusive quartz diorites, the widespread occurrence of lavas with boninitic affinities, and the lack of an emergent arc complex west of the ophiolite. Emplacement of these late stage MORB dikes and flows signify the end of ophiolite formation and require a significant change in tectonic setting that occurred shortly before deposition of the overlying Great Valley series. There are two possibilities that can explain these dikes: back-arc basin spreading or collision of a spreading ridge. Propagation of a back-arc basin spreading center through the ophiolite in response to prolonged arc rifting could explain the shift in geochemistry from arc-like to MORB-like, but seems unlikely in a fore-arc setting. We suggest that collision with a spreading center circa 160 Ma can account for the shift observed in geochemistry and the termination of ophiolite-related igneous activity.

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    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

  18. Petrogenetic implications from ultramafic rocks and pyroxenites in ophiolitic occurrences of East Othris, Greece

    NASA Astrophysics Data System (ADS)

    Koutsovitis, P.; Magganas, A.

    2012-04-01

    Ultramafic rocks and pyroxenites in east Othris are included within ophiolitic units near the villages of Vrinena, Karavomilos, Pelasgia, Eretria, Agios Georgios, Aerino and Velestino. The first five ophiolitic occurrences are estimated to have been emplaced between the Oxfordian and Tithonian-Berriasian[1,2,3], while the latter two have been emplaced during the Eocene[4]. Ultramafic rocks include variably serpentinized harzburgites and lherzolites. Pyroxenites are usually found in the form of crosscutting veins within the harzburgites. Ultramafic rocks include depleted lherzolites, with Al2O3 ranging from 1.12 to 1.80 wt% and Cr from 3250 to 3290 ppm, as well as moderate to highly depleted serpentinized harzburgites, with Al2O3 ranging from 0.69 to 1.98 wt% and Cr from 2663 to 5582 ppm. Pyroxenites have generally higher Al2O3 ranging from 1.91 to 3.08 wt% and variable Cr ranging from 1798 to 3611 ppm. Lherzolites mostly include olivines (Fo=87.07-89.23) and clinopyroxenes (Mg#=85.71-90.12). Spinels from Eretria lherzolite (TiO2=0.02-0.08 wt%, Al2O3=36.06-42.45 wt%, Cr#=31.67-36.33) are compositionally similar with those of MORB peridotites[5], while those from Vrinena lherzolite (TiO2=0.16-0.43 wt%, Al2O3=6.90-22.12 wt%, Cr#=57.69-76.88) are similar to SSZ peridotites[5]. Serpentinized harzburgites include few olivines (Fo=90.51-91.15), enstatite porphyroclasts (Mg#=87.42-88.91), as well as fine grained enstatites of similar composition. Harzburgites from Pelasgia, Eretria and Agios Georgios include spinels (TiO2=0.03-0.08 wt%, Al2O3=23.21-31.58 wt%, Cr#=45.21-56.85) which do not clearly show if they are related with MORB or SSZ peridotites[5]. Spinels from Karavomilos harzburgite (TiO2=0.02-0.05 wt%, Al2O3=45.71-50.85 wt%, Cr#=16.84-22.32) are compositionally similar with MORB peridotites[5], whereas spinels from Vrinena harzburgite (TiO2=0.15-0.19 wt%, Al2O3=1.42-1.86 wt% Cr#=91.64-93.47) with SSZ peridotites[5]. Pyroxenites include clinopyroxenes (Mg#=84.25-91.78) but also enstatites (Mg#=88.37-91.47). Spinels have been analysed in pyroxenites from Aerino and Velestino (TiO2=0.79-1.07 wt%, Al2O3=10.88-18.46 wt% Cr#=60.74-70.78), indicating SSZ settings. Application of the olivine-spinel[6], olivine-augite[7], Cpx-Opx[8,9] geothermometers, yield equilibration temperatures of 961-1075 oC for lherzolites, 895-1084 oC for harzburgites and 990-1011 oC for pyroxenites. Our data indicate that the ophiolitic occurrences of Vrinena, Aerino and Velestino include ultramafic rocks and pyroxenites related to SSZ processes, while the other ophiolitic occurrences embrace ultramafic rocks which originated from a MORB-like setting, similar to west Othris ophiolites. It should be noted that even lherzolites have Cr and Y values similar to those of a highly depleted mantle source. A supra-subduction zone origin of the east Othris ophiolites, possibly with a slab rollback in the Pindos oceanic basin, may explain the different geotectonic environment affinities of the studied rocks.

  19. FROM THE EARLY SIMS TO RECENT LAM DATA: THE ALPINE-APENNINE OPHIOLITIC PERIDOTITES REVISITED

    NASA Astrophysics Data System (ADS)

    Piccardo, G. B.; Vannucci, R.; Tribuzio, R.; Guarnieri, L.

    2009-12-01

    Early ion-microprobe data on minerals of the Alpine-Apennine (AA) mantle peridotites from the ophiolite sequences exposed in the Central and Western Alps, Ligurian Alps, Northern Apennine and Corsica were obtained about twenty years ago. These data provided basic chemical information that highlighted their evolution, with special emphasis to melt extraction processes and sub-solidus re-equilibration that preceded and accompanied the exposure of mantle peridotites at the sea-floor of the Jurassic Ligurian-Piedmontese (or Western Tethys) oceanic basin. In more recent years a wealth of field, structural-petrographic, petrological and geochemical studies allowed to track a more comprehensive petrogenetic scenario of the Ligurian-Piedmontese upper mantle, in the frame of the geodynamic evolution of the Europe-Adria lithosphere during Mesozoic continental extension and rifting. These investigations evidenced that sectors of the sub-continental lithospheric mantle were exposed on the sea-floor at marginal settings of the basin (e.g. North Lanzo and External Ligurides ophiolites) Isotope data and related model ages revealed that they became isolated by the convective mantle and accreted to the sub-continental lithosphere long time before Mesozoic extension. Conversely, peridotites exposed at more distal settings show extreme compositional heterogeneity recognized as effects of melt-peridotite interaction during reactive percolation of melt though the lithospheric mantle during continental extension and rifting. Isotope data pointed to MORB-type signatures and Jurassic ages for gabbroic intrusions and melt percolated peridotites from both marginal and distal settings. Structural and compositional characteristics suggested that peridotite-melt reactions had a crucial role in modifying texture, mode and chemistry of mantle assemblages and highlighted the need of a re-consideration of the evolutionary processes recorded by the ophiolitic peridotites from the Alpine-Apennine orogenic belt. With this aim, in the last ten years early ion-microprobe data were progressively integrated by extensive texture-related LA-ICPMS studies that provided a huge dataset for a complete series of trace elements in AA ophiolitic peridotite minerals and, particularly, clinopyroxene. The whole dataset confirm that clinopyroxenes from most AA ophiolitic peridotites have REE signatures resulting from other processes than batch or fractional melting. These features result from a complex history of depletion and melt-peridotite reaction events, with the latter largely overprinting pristine signatures recorded by the lithospheric sub-continental mantle before melt percolation. Although old melting episodes under garnet-facies conditions are recorded in places, most REE signatures of AA clinopyroxenes can be explained by reaction of variously depleted melting residua with MORB-like infiltrating melts (either single fractional increments or aggregate melts) and, eventually, later chemical modification due to interaction with trapped melts. The whole dataset is re-considered here with the aim of providing an updated scenario for the evolution of the AA ophiolitic mantle peridotites.

  20. The Zermatt-Saas Ophiolite: a Continuous Slice of Oceanic Lithosphere Detached at 80 km Depth in the Subduction Zone

    NASA Astrophysics Data System (ADS)

    Angiboust, S.; Agard, P.; Jolivet, L.; Burov, E.

    2008-12-01

    The western Alps is a classic subduction-related collisional orogen, where large fragments of low-density continental crust (e.g. Dora Maira, Grand Paradis) were deeply subducted and then exhumed together with ophiolitic remnants of the Mesozoic Tethyan oceanic lithosphere (e.g. Monviso, Zermatt-Saas). Whereas the Monviso ophiolitic complex has been recognized as a paleao-subduction channel with tectonic blocks showing a wide range of pressure-temperature conditions, no comprehensive study has yet attempted to evaluate the metamorphic homogeneity of the extensive Zermatt-Saas ophiolite. Zermatt-Saas eclogitic assemblages are represented by omphacite-garnet-epidote-rutile +-lawsonite pseudomorphs +- glaucophane in "classical" MORB-derived metabasalts. Sea-floor hydrothermalized metabasalts are characterized by an unusual peak paragenesis characterized by garnet-chloritoid-talc +- lawsonite pseudomorphs +- glaucophane. Thermobarometric estimates with THERMOCALC and Raman Spectroscopy of carbonaceous material reveal homogeneous peak burial conditions at around 540 +- 20 °C and 23 +- 1kbar. These estimates are slightly lower (c.a. 50 °C - 4 kbar) than those from the rare, adjacent coesite-bearing metasediments, suggesting that most of the ophiolite detached from the slab at depths around 80 km. Our data indicate that the whole of the ophiolite, at least 50 km across, strikingly underwent similar tectonic patterns from burial to early exhumation. During exhumation, pervasive glaucophane recrystallization and later greenschist facies assemblages replaced the earlier eclogitic paragenesis. Early exhumation paths are homogeneous and characterized by nearly isothermal decompression between c.a. 23 and 10 kbar. The Zermatt-Saas ophiolite thus appears to be one of the world's largest oceanic lithosphere fragment exhumed from such depths. These results provide critical constraints on the migration of oceanic crustal slices along the subduction channel and on interplate coupling mechanisms.

  1. Hydrogen Gas from Serpentinite, Ophiolites and the Modern Ocean Floor as a Source of Green Energy

    NASA Astrophysics Data System (ADS)

    Coveney, R. M.

    2008-12-01

    Hydrogen gas is emitted by springs associated with serpentinites and extensive carbonate deposits in Oman, The Philippines, the USA and other continental locations. The hydrogen springs contain unusually alkaline fluids with pH values between 11 and 12.5. Other workers have described off-ridge submarine springs with comparably alkaline fluid compositions, serpentinite, abundant free hydrogen gas, and associated carbonate edifices such as Lost City on the Atlantis Massif 15 km west of the Mid-Atlantic Ridge (D.S. Kelley and associates, Science 2005). The association of hydrogen gas with ultramafites is a consistent one that has been attributed to a redox couple involving oxidation of divalent iron to the trivalent state during serpentinization, although other possibilities exist. Some of the hydrogen springs on land are widespread. For example in Oman dozens of alkaline springs (Neal and Stanger, EPSL 1983) can be found over thousands of sq km of outcropping ophiolite. While the deposits in Oman and the Philippines are well-known to much of the geochemical community, little interest seems to have been displayed toward either the ophiolitic occurrences or the submarine deposits for energy production. This may be a mistake as the showings because they could lead to an important source of green energy. Widespread skepticism currently exists about hydrogen as a primary energy source. It is commonly said that free hydrogen does not occur on earth and that it is therefore necessary to use other sources of energy to produce hydrogen, obviating the general environmental benefit. However the existence of numerous occurrences of hydrogen gas associated with ophiolites and submarine occurrences of hydrogen suggests the likelihood that natural hydrogen gas may be an important source of clean energy for modern society remaining to be tapped. Calculations in progress should establish whether or not this is likely to be the case.

  2. Incidence of Metamorphism on Magnetic Mineralogy in the Sarmiento Ophiolitic Complex, Southern Chile

    NASA Astrophysics Data System (ADS)

    Singer, S.; Rapalini, A.; Calderon, M.; Herve, F.

    2005-05-01

    The Sarmiento Ophiolitic Complex (SOC), located in the southern Andes of Chile, represents the mafic portion of the Late Jurassic to Early Cretaceous oceanic floor of a back-arc basin closed and uplifted in the Mid Cretaceous. Its igneous pseudostratigraphy consists of mainly mafic pillow lavas, dikes, and gabbros. These rocks underwent a non - deformative "ocean floor metamorphism" which developed secondary mineral assemblages in a vertical steep metamorphic gradient passing from zeolite to actinolite facies, followed by a transition to fresh gabbros Syntectonic greenschist facies assemblages in mylonitic rocks bordering the thrust slices that expose the SOC, represent a different metamorphic event, that probably occured before the latest Cretaceous. A paleomagnetic study of these rocks has shown that a post-tectonic widespread remagnetization affected the whole ophiolite. Incidence of the mentioned metamophic processes on the natural remanent magnetization of the ophiolite was evaluated thru a susceptibility survey and a study of ferromagnetic (s.l ) minerals. These were identified by microscope observations under reflected and transmitted light. A very good agreement between magnetic susceptibility values and optical observations was found. Metamorphic processes have produced strong effects on ferromagnetic minerals. These metabasites are disctintly poor in titanomagnetites, except the metagabbros and dikes crosscutting gabbros, which show high susceptibilities due to magnetite from serpentinisation of olivines and uralitization of pyroxenes. Sheeted dikes and pillow lavas show low susceptibilities controlled by paramagnetic minerals with titanite as witness of a primary titanium-bearing mineral. Pure microcrystalline magnetite, main carrier of natural remanent magnetization, likely formed during the greenschist post-tectonic overprint. Sulfide mineralization in the Complex requires further studies. Finally, equilibria between Fe3+ in oxides and Fe3+ in silicates seem to have played an important role in determining the ocurrence and abundance of magnetite during the metamorphism.

  3. The Cretan ophiolite-bearing mélange (Greece): A remnant of Alpine accretionary wedge

    NASA Astrophysics Data System (ADS)

    Tortorici, L.; Catalano, S.; Cirrincione, R.; Tortorici, G.

    2012-09-01

    New data collected in central Crete on the ophiolite-bearing units, result in a better understanding of the role and tectonic significance of these units in the construction of this segment of the Hellenides. These nappes are composed of three main tectonic units that, characterized by different metamorphic facies conditions, are represented by an un-metamorphosed lower unit, a greenschist to HP greenschist-facies intermediate unit and a blueschist-facies upper unit. These chaotic thrust-nappes include blocks of oceanic and continental deriving rocks and can be considered as a remnant of an accretionary complex. The lower unit represents the toe of the wedge whereas the intermediate and upper units refer to the innermost and deeper subducted portions exhumed and superimposed on top of each other during the early stages of continental collision. The structural evolution of the accretionary wedge was controlled by four main contractional deformation events that, including distinct groups of structures developed at different crustal levels, were driven by SSE and SSW directions of tectonic transport. Our data strongly suggest that the ophiolite-bearing tectonic wedge was accreted during the Paleogene subduction of a Late Jurassic-Cretaceous oceanic realm beneath the continental margin of the Pelagonian domain and it was successively involved in the continental collision with the Adria Block. The greenschist to blueschist facies metamorphism and the subsequent exhumation and emplacement of the intermediate and upper units above the frontal portion of the wedge may mainly be due to deep duplexing marking the onset of continental collision. We thus suggest that the ophiolite-bearing units of Crete represent a single suture zone related to the closure of a unique oceanic domain (Pindos-Cycladic Ocean) subducted beneath the Internal Hellenides Platform continental domains thus assuming the significance of a southern oceanic seaway of the largest eastern Neotethys developed since the Triassic between Eurasia and Africa.

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

    NASA Astrophysics Data System (ADS)

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

    2008-12-01

    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.

  5. Mineral chemical composition and geodynamic significance of peridotites from Nain ophiolite, central Iran

    NASA Astrophysics Data System (ADS)

    Ghazi, Javad Mehdipour; Moazzen, Mohssen; Rahgoshay, Mohammad; Moghadam, Hadi Shafaii

    2010-05-01

    The peridotites from north of the town of Nain in central Iran consist of clinopyroxene-bearing harzburgite and lherzolite with small lenses of dunite and chromitite pods. The lherzolite contains aluminous spinel with a Cr number (Cr # = Cr/[Cr + Al]) of 0.17. The Cr number of spinels in harzburgite and chromitite is 0.38-0.42 and 0.62, respectively. This shows that the lherzolite and harzburgite resulted from <18% of partial melting of the source materials. The estimated temperature is 1100 ± 200 °C for peridotites, the estimated pressure is <15 ± 2.3 kbar for harzburgites and >16 ± 2.3 kbar for lherzolites and estimated fo2 is 10 -1±0.5 for peridotites. Discriminant geochemical diagrams based on mineral chemistry of harzburgites indicate a supra-subduction zone (SSZ) to mid-oceanic ridge (MOR) setting for these rocks. On the basis of their Cr #, the harzburgite and lherzolite spinels are analogous to those from abyssal peridotites and oceanic ophiolites, whereas the chromites in the chromitite (on the basis of Cr # and boninitic nature of parental melts) resemble those from SSZ ophiolitic sequences. Therefore, the Nain ophiolite complex most likely originated in an oceanic crust related to supra-subduction zone, i.e. back arc basin. Field observations and mineral chemistry of the Nain peridotites, indicating the suture between the central Iran micro-continent (CIM) block and the Sanandaj-Sirjan zone, show that these peridotites mark the site of the Nain-Baft seaway, which opened with a slow rate of ocean-floor spreading behind the Mesozoic arc of the Sanandaj-Sirjan zone as a result of change of Neo Tethyan subduction régime during middle Cretaceous.

  6. Mapping in the Oman ophiolite using enhanced Landsat Thematic Mapper images

    NASA Technical Reports Server (NTRS)

    Abrams, M. J.; Rothery, D. A.; Pontual, A.

    1988-01-01

    The level of apparent lithological discrimination possible with Landsat TM images in the Oman are discussed. It is found that by using parts of the short-wavelength IR spectrum, the discrimination revealed by the TM data is sufficiently uniform throughout the Oman ophiolite to produce lithological maps at 1:100,000 scale. Decorrelation stretching of the data produces images in which allows for the recognition of variations in gabbro composition, the identification of small acidic, gabbroic, and ultramafic intrusions, the discrimation of the uppermost mantle from the deeper mantle, the precise location of the Moho, and the delineation of gossans and areas subject to choritic-epidotic alteration.

  7. Mont Albert to Buck Mountain: Provenance of Appalachian Ophiolite Chromites Using Osmium Isotopes

    NASA Astrophysics Data System (ADS)

    Minarik, W. G.; Gale, A.; Booker, C.

    2003-12-01

    Osmium 187Os/188Os isotopic ratios have been determined for chrome-rich spinels from a suite of Appalachian ophiolites thought to represent Iapetus margin mantle formed and emplaced during the Ordovician. Because Re is incompatible during mantle melting while Os is compatible, non-radiogenic initial 187Os/188Os can constrain the average source and the timing of melt extraction, especially as Os is concentrated in chromite. Radiogenic ratios indicate contamination from aged sources with high Re/Os, such as mafic or continental crust. In rocks where spinel is the only remaining primary mineral, these properties can constrain the tectonic environment of formation as well as active-margin Os transport. There is little correction for 187Os in-growth since the Ordovician due to very low sample Re. Each ultramafic unit (from Mont Albert on the Gaspé Peninsula of Québec down to the Blue Ridge of North Carolina) forms a unique cluster of 187Os/188Os ratios, spanning 1 to 3%, but the whole range is about 10%. This corresponds to a range of initial ? Os of -1 to +9, where ? Os is the percent deviation from a chondritic source at the age of formation (roughly 500 Ma). Within ophiolites where detailed mapping and other geochemical information are available, there is a correlation between mantle-like Os and tholeiitic basalts; radiogenic Os and boninites (Thetford Mines). Continental arc-related mantle chromites (Baltimore Mafic Complex; ? Os +4 to +7) are the most radiogenic. The least radiogenic are chromites from the Staten Island serpentinite and Mont Albert (? Os -1 and 0, respectively), either indicating formation from a previously depleted source or that they predate the other Taconic ophiolites. The restricted range of each ophiolite, compared to the whole of the data set, allow provenance links to be made between isolated bodies. For example, the Buck Creek, NC ultramafic complex, which has undergone granulite facies metamorphism, (Tenthorey et al., 1996) has a similar 187Os/188Os to other Ordovician NC Blue Ridge dunites (? Os = +6.5 to +8.5), but distinctly different from a NC Piedmont chromitite (? Os = +2, Falls Lake mélange, Stoddard et al, 1989) that is inferred to be Neoproterozoic in age.

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

    USGS Publications Warehouse

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

    1984-01-01

    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.

  9. The Jamestown Ophiolite Complex, Barberton mountain belt - A section through 3.5 Ga oceanic crust

    NASA Technical Reports Server (NTRS)

    De Wit, Maarten J.; Hart, Roger A.; Hart, Rodger J.

    1987-01-01

    The Jamestown Ophiolite Complex of the Barberton greenstone belt, South Africa, is investigated, and the intrusive nature of mafic-ultramafic units from the Komati and Kromberg formations into overlying pillow lavas and sediments is documented. Evidence is presented for multiple intrusive events within the igneous sections, including crosscutting intrusives, multiple injection of magma in the Komati section, and sheeted intrusions in the Kromberg section. The thinness of the Jamestown complex suggests that, locally at least, the ca 3.5 Ga oceanic crust was also thin, consistent with the regionally extensive metasomatic alteration.

  10. A mechanism for decoupling within the oceanic lithosphere revealed in the Troodos ophiolite

    USGS Publications Warehouse

    Agar, S.M.; Klitgord, Kim D.

    1995-01-01

    Contrasting kinematic histories recorded in the sheeted dykes and underlying plutonic rocks of the Troodos ophiolite provide a new perspective on the mechanical evolution of oceanic spreading centres. The kinematic framework of the decoupling zone that partitions deformation between the sheeted dykes and plutonics contrasts with low-angle detachment models for slow-spreading ridges based on continental-rift analogues. A model for the generation of multiple, horizontal decoupling horizons, linked by planar normal faults, demonstrates new possibilities for the kinematic and rheological significance of seismic reflectors in oceanic lithosphere.

  11. Metsovo lung outside Metsovo. Endemic pleural calcifications in the ophiolite belts of Greece

    SciTech Connect

    Constantopoulos, S.H.; Theodoracopoulos, P.; Dascalopoulos, G.; Saratzis, N.; Sideris, K. )

    1991-05-01

    Endemic PCs and high incidence of malignant mesothelioma from household use of asbestos have been reported in Metsovo in northwestern Greece ('Metsovo lung'). In the present study, we present similar findings in six more areas of Greece. Like Metsovo, all these areas are located within ophiolite belts. Like Metsovo, material similar to 'Metsovo whitewash' has been used for various domestic uses. Asbestos fibers (chrysotile, antigorite and tremolite) were found in three of the six areas. Also, in two, MPM has been diagnosed. These findings suggest that 'Metsovo lung' occurs in several areas of Greece and has similar etiology and epidemiology.

  12. Sabzevar Ophiolite, NE Iran: Progress from embryonic oceanic lithosphere into magmatic arc constrained by new isotopic and geochemical data

    NASA Astrophysics Data System (ADS)

    Moghadam, Hadi Shafaii; Corfu, Fernando; Chiaradia, Massimo; Stern, Robert J.; Ghorbani, Ghasem

    2014-12-01

    The poorly known Sabzevar-Torbat-e-Heydarieh ophiolite belt (STOB) covers a large region in NE Iran, over 400 km E-W and almost 200 km N-S. The Sabzevar mantle sequence includes harzburgite, lherzolite, dunite and chromitite. Spinel Cr# (100Cr/(Cr + Al)) in harzburgites and lherzolites ranges from 44 to 47 and 24 to 26 respectively. The crustal sequence of the Sabzevar ophiolite is dominated by supra-subduction zone (SSZ)-type volcanic as well as plutonic rocks with minor Oceanic Island Basalt (OIB)-like pillowed and massive lavas. The ophiolite is covered by Late Campanian to Early Maastrichtian (~ 75-68 Ma) pelagic sediments and four plagiogranites yield zircon U-Pb ages of 99.9, 98.4, 90.2 and 77.8 Ma, indicating that the sequence evolved over a considerable period of time. Most Sabzevar ophiolitic magmatic rocks are enriched in Large Ion Lithophile Elements (LILEs) and depleted in High Field Strength Elements (HFSEs), similar to SSZ-type magmatic rocks. They (except OIB-type lavas) have higher Th/Yb and plot far away from mantle array and are similar to arc-related rocks. Subordinate OIB-type lavas show Nb-Ta enrichment with high Light Rare Earth Elements (LREE)/Heavy Rare Earth Elements (HREE) ratio, suggesting a plume or subcontinental lithosphere signature in their source. The ophiolitic rocks have positive ?Nd (t) values (+ 5.4 to + 8.3) and most have high 207Pb/204Pb, indicating a significant contribution of subducted sediments to their mantle source. The geochemical and Sr-Nd-Pb isotope characteristics suggest that the Sabzevar magmatic rocks originated from a Mid-Ocean Ridge Basalt (MORB)-type mantle source metasomatized by fluids or melts from subducted sediments, implying an SSZ environment. We suggest that the Sabzevar ophiolites formed in an embryonic oceanic arc basin between the Lut Block to the south and east and the Binalud mountains (Turan block) to the north, and that this small oceanic arc basin existed from at least mid-Cretaceous times. Intraoceanic subduction began before the Albian (100-113 Ma) and was responsible for generating Sabzevar SSZ-related magmas, ultimately forming a magmatic arc between the Sabzevar ophiolites to the north and the Cheshmeshir and Torbat-e-Heydarieh ophiolites to the south-southeast.

  13. Post-emplacement history of the Zambales Ophiolite Complex: Insights from petrography, geochronology and geochemistry of Neogene clastic rocks

    NASA Astrophysics Data System (ADS)

    Dimalanta, C. B.; Salapare, R. C.; Faustino-Eslava, D. V.; Ramos, N. T.; Queaño, K. L.; Yumul, G. P.; Yang, T. F.

    2015-05-01

    The Zambales Ophiolite Complex in Luzon, Philippines is made up of two blocks with differing geochemical signatures and ages - the Middle Jurassic to Early Cretaceous Acoje Block-San Antonio Massif that is of island arc tholeiite composition and the Eocene Coto Block-Cabangan Massif which is of transitional mid-ocean ridge basalt-island arc tholeiite affinity. These ophiolitic bodies are overlain by Miocene to Pliocene sedimentary units whose petrochemistry are reported here for the first time. Varying degrees of influences from ophiolitic detritus and from arc volcanic materials, as shown by petrography and indicator elements including Cr, Co and Ni, are observed in these sedimentary formations from north to south and from the oldest to the youngest. The Early to Middle Miocene Cabaluan Formation, whose outcrops are found to overlie only the Acoje Block, registers a more dominant ophiolitic signature as compared to the Late Miocene to Pliocene Santa Cruz Formation. The Santa Cruz Formation is generally characterized by fewer ophiolitic clasts and higher amounts of felsic components. Additionally, within this formation itself, a pronounced compositional change is observed relative to its spatial distribution. From the south to the north, an increase in ophiolitic components and a relative decrease in felsic signature is noted in units of the Santa Cruz Formation. It is therefore inferred that changes in the petrochemistry of rocks from the older Cabaluan to the younger Santa Cruz sedimentary formations record a decline in the influx of ophiolitic detritus or, conversely, the introduction of more diverse sediment sources as the deposition progressed. Detrital zircon U-Pb ages from the Santa Cruz Formation, with peaks at 46.73 ± 0.94 and 5.78 ± 0.13 Ma, reflects this change in provenance from the unroofing of an Early Eocene oceanic crust to fresh contributions from an active volcanic arc during the Late Miocene. The contrast in compositions of the southern and northern Santa Cruz Formation also indicates a closer proximity of the southern units to the source of these non-ophiolitic sources, which most likely corresponds to the Pliocene volcanoes of the West Luzon Arc.

  14. New constraints for the tectonic development of the western Pacific margin since the Mesozoic: comprehensive SHRIMP zircon U-Pb dating of the Philippine ophiolite belts

    NASA Astrophysics Data System (ADS)

    Tani, K.; Gabo, J. A. S.; Horie, K.; Ishizuka, O.; Padrones, J.; Payot, B. D.; Tejada, M. L. G.; Faustino-Eslava, D. V.; Imai, A.; Arai, S.; Yumul, G. P., Jr.; Dimalanta, C. B.

    2014-12-01

    The post-Mesozoic tectonic history of the western Pacific margin is critical in understanding the major global tectonic events that occurred in the Eocene, such as the westward change in Pacific Plate motion and the simultaneous subduction initiation of the Izu-Bonin-Mariana (IBM) Arc (Ishizuka et al., 2011 EPSL). The present location of the Philippine Islands continuously serves as a major tectonic boundary between the southeastern Eurasian and the Pacific Plates, accommodating the large left-lateral movement associated with the clockwise rotation of the Philippine Sea Plate (PSP) from Eocene to Miocene and now the subduction of the PSP from the east at the Philippine Trench - East Luzon Trough. The basement rocks of the Philippine Islands are characterized by the presence of ophiolitic complexes exposed among the islands. Yumul (2007, Island Arc) defined four belts in the Philippine ophiolites and proposed that they progressively become younger towards west, from Early - Late Cretaceous at the easternmost belt to Eocene - Oligocene in the west. However, most of the ophiolitic complexes have been dated by radiolarians and foraminifera in the overlying sediments. To precisely determine the igneous ages of the Philippine ophiolites, we have conducted SHRIMP zircon U-Pb dating of the gabbroic and leucocratic rocks collected from the ophiolitic complexes in the Philippine Islands, including those from Luzon (Zambales and Isabela ophiolites), Masbate (Balud ophiolite), Tablas (Sibuyan Ophiolite), and Cebu. New zircon ages show that all of the ages obtained so far from the eastern ophiolite belts are Eocene in age, from 52 Ma to 41 Ma. These ages coincide well with the opening of the West Philippine Basin (49 - 33 Ma, Taylor and Goodliffe, 2004 JGR), which is a backarc basin formed behind the incipient IBM Arc. Furthermore, geochemical data available from the igneous rocks in the eastern ophiolite belts show backarc basin basalt-like geochemical affinities (e.g. Yumul, 2007), suggesting that these ophiolites are genetically associated with the West Philippine Basin. On the contrary, gabbroic and leucocratic rocks that are associated with the ophiolitic complex in Cebu are Late Cretaceous (~90 Ma) in age, significantly older than those in the eastern ophiolite belts, suggesting a different origin.

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    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.

  16. Constraints on the formation of geochemically variable plagiogranite intrusions in the Troodos Ophiolite, Cyprus

    NASA Astrophysics Data System (ADS)

    Freund, Sarah; Haase, Karsten M.; Keith, Manuel; Beier, Christoph; Garbe-Schönberg, Dieter

    2014-02-01

    The geochemistry and petrology of tonalitic to trondhjemitic samples ( n = 85) from eight different plagiogranite intrusions at the gabbro/sheeted dyke transition of the Troodos Ophiolite were studied in order to determine their petrogenetic relationship to the mafic plutonic section and the lava pile. The plagiogranitic rocks have higher SiO2 contents than the majority of the glasses of the Troodos lava pile, but lie on a continuation of the chemical trends defined by the extrusive rocks, indicating that the shallow intrusions generally represent crystallised magmas. We define three different groups of plagiogranites in the Troodos Ophiolite based on different incompatible element contents and ratios. The first and most common plagiogranite group has geochemical similarities to the tholeiitic lavas forming the lavas and sheeted dyke complex in the Troodos crust, implying that these magmas formed at a spreading axis. The second plagiogranite group occurs in one intrusion that is chemically related to late-stage and off-axis boninitic lavas and dykes. One intrusion next to the Arakapas fault zone consists of incompatible element-enriched plagiogranites which are unrelated to any known mafic crustal rocks. The similarities of incompatible element ratios between plagiogranites, lavas and mafic plutonic rocks, the continuous chemical trends defined by plagiogranites and mafic rocks, as well as incompatible element modelling results, all suggest that shallow fractional crystallisation is the dominant process responsible for formation of the felsic magmas.

  17. A Deep Seismic Study of the United Arab Emirates: Implications for Collision Tectonics and Ophiolite Emplacement

    NASA Astrophysics Data System (ADS)

    Ali, M. Y.; Watts, A. B.

    2014-12-01

    We recently carried out the first integrated deep seismic experiment in the United Arabic Emirates (UAE) from the Arabian Gulf to the Gulf of Oman. Reflection data were acquired along 925 line km in the Arabian Gulf and Gulf of Oman using a large-volume airgun source (7060 cubic inches) and up to 5 km long streamer. Refraction data were acquired along selected reflection lines using 25 land recording stations in the UAE. In addition, gravity and magnetic anomaly data were acquired along all the seismic lines. The objective of the seismic experiment is to determine the crust and mantle structure associated with UAE orogenic belt, the Gulf of Oman and the Arabian Gulf. In addition, the experiment aims to determine mechanisms of ophiolite emplacement and the velocity structure of the crust and uppermost mantle beneath the orogenic belt and the flanking UAE foreland basin and offshore Furairah/Sharjah. Preliminary results of the experiment will be presented including initial findings of seismic reflection, refraction and potential field data. Seismic reflection data in the Gulf of Oman, for example, show evidence for post-rift and syn-rift sedimentation, tilted fault blocks and re-activated faults that appear to have offset the seafloor. Other findings include delineation, using gravity and magnetic data, of the offshore extent of the ophiolite, its thickness and the nature of its bounding faults.

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

    SciTech Connect

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

    1986-01-01

    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.

  19. Discovery of New Methane-bearing Hyperalkaline Springs in the Serpentinized Dun Mountain Ophiolite, New Zealand

    NASA Astrophysics Data System (ADS)

    Pawson, J. F.; Oze, C.; Etiope, G.; Horton, T. W.

    2014-12-01

    The production of H2 and CH4 following serpentinization is considered a fundamental process for the origin/sustenance of microbial life, with several implications in astrobiology and petroleum geology. In recent years an increasing number of gas seeps or springs, with dominantly abiotic CH4, have been discovered in land-based serpentinized peridotites worldwide. We report the discovery of a new site in the Dun Mountain Ophiolite (DMO), New Zealand. This is the second gas-bearing peridotite discovered so far in NZ, after Poison Bay which was documented in the 1990s. The DMO area is characterized by active and present-day low temperature serpentinization driven by meteoric water, a common process in ophiolites. A localised area of hyperalkaline (pH >11.6) and reduced (-243.4 mV) waters contain relatively high concentrations of dissolved calcium (43.2 ppm) aiding in carbonate precipitation. Directly above these waters flux measurements by closed chamber methods reveal CH4 emission rates exceeding 2800 mg m-2day-1. The ?13C values of CH4 suggest the presence of a dominant abiotic gas component (e.g. -32.7 ‰ VPDB). Additionally, an unusual clear to black gel is released from less alkaline springs elsewhere in the DMO. The origin and nature of this gel-like material is currently unknown.

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

    NASA Astrophysics Data System (ADS)

    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

    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.

  1. U-Pb zircon geochronology of the Ligurian ophiolites (Northern Apennine, Italy): Implications for continental breakup to slow seafloor spreading

    NASA Astrophysics Data System (ADS)

    Tribuzio, Riccardo; Garzetti, Fabio; Corfu, Fernando; Tiepolo, Massimo; Renna, Maria Rosaria

    2016-01-01

    Fragments of Jurassic oceanic crust exposed in the Northern Apennine (Italy) are either associated with continental lithosphere material (External Ligurian ophiolites), or bear structural and compositional resemblances to slow spreading ridge crust (Internal Ligurian ophiolites). To acquire new information about the transition from continental breakup to slow seafloor spreading, we carried out a U-Pb geochronological study of zircons from gabbro bodies of both External and Internal Ligurian ophiolites. Zircons were separated from seven samples and analyzed for U-Pb isotopes by laser ablation ICPMS and isotope dilution TIMS. The zircons were also investigated for morphology, internal structures, inclusions and chemistry. These characteristics reveal remarkable similarities to zircons collected from modern oceanic crust. Taken as a whole, the new U-Pb zircon dates obtained for the Ligurian ophiolites range from ~ 165 to ~ 161 Ma, thereby arguing against previous geochronological investigations suggesting a period of ~ 26 Ma for the formation of the Ligurian gabbroic crust. The time interval intervened from onset of gabbroic crust formation to configuration of a "slow spreading ridge type" crust was most likely ≤ 5 Ma. New insights into the opening mechanisms of the fossil, slow seafloor spreading basin are provided.

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

    NASA Astrophysics Data System (ADS)

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

    2001-08-01

    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.

  3. New Perspectives on Ophiolite Formation: Evidence from Ultrahigh Pressure (UHP), Highly Reduced and Crustal-type Minerals in Podiform Chromitites

    NASA Astrophysics Data System (ADS)

    Robinson, P. T.; Yang, J.

    2014-12-01

    Separated and in situ ultrahigh pressure (UHP), highly reduced and crustal-type minerals are common in podiform chromitites of the Luobusa and Dongqiao ophiolites, Tibet and the Ray-Iz ophiolite of the Polar Urals, Russia. Highly reduced and crustal-type minerals have also been recovered from the Oman ophiolite. UHP minerals include diamond, coesite-stishovite and kyanite, whereas highly reduced minerals are mainly moissanite (SiC), native elements (e.g., Si, Fe, Cr, Al, Ti, Mn, W, Ta) and a wide variety of metallic alloys. Crustal-type minerals are represented by various combinations of zircon, corundum, almandine garnet, kyanite, andalusite, quartz, K-feldspar, plagioclase, apatite, amphibole, rutile, and titanite. Most in-situ grains are hosted in small, circular to irregular patches of amorphous carbon within grains of magnesiochromite, indicating the former presence of a C-rich fluid, either during or after crystallization of the chromite. The recovered zircons are typically rounded to sub-rounded grains with complex internal structures indicating polyphase growth. Their trace element contents and low-pressure inclusion assemblages (quartz, muscovite, K-feldspar, apatite, ilmenite, rutile) indicate a continental crustal origin. The zircons have SIMS U-Pb ages that are generally much older than the host ophiolite (total range: 90 to 2500 Ma). The presence of numerous crustal minerals, particularly zircon, suggests derivation from metasedimentary rocks subducted into the mantle. The preservation of UHP, highly reduced and crustal-type minerals in chromitites implies effective isolation from the mafic melts that formed the ophiolites and chromitites. Clearly, the formation of ophiolites and podiform chromitites must be a complex, multistage process involving crystallization of magnesiochromite grains at depth in the upper mantle, upwelling of the host peridotites and chromitites, capture of mantle wedges above suprasubduction zones, further crystallization and modification of magnesiochromite grains, allowing incorporation of crustal-type minerals from the underlying subducting slab. Slab rollback and/or slab tear may facilitate uprise of mantle peridotites leading to concentration of podiform chromitites within individual ophiolitic blocks.

  4. a Plate-Tectonic Model for Late Jurassic Ophiolite Genesis, Nevadan Orogeny and Forearc Initiation, Northern California

    NASA Astrophysics Data System (ADS)

    Ingersoll, Raymond V.; Schweickert, Richard A.

    1986-10-01

    Recently published age and structural data allow the reconciliation of previously conflicting models for Late Jurassic genesis of the Josephine, Smartville and Coast Range ophiolites, and the Nevadan orogeny in the Klamath Mountains and Sierra Nevada. The resulting model is consistent with the mode of initiation, location and geometry of the Great Valley forearc basin, and with the lack of a significant forearc basin west of the Klamath Mountains. The Coast Range ophiolite formed by backarc spreading west of an east-facing intraoceanic arc. Soon thereafter, a remnant arc was calved off the west side of this arc, and the Smartville ophiolite formed by backarc (interarc) spreading. During this time, the Sierran phase of the Nevadan orogeny began as the intraoceanic arc encountered the west-facing continental-margin arc of North America. An east-west-trending calcalkaline dike swarm in the Sierra Nevada foothills may mark the trajectory of the colliding arcs at the initiation of the collision. Simultaneously, a new subduction zone was initiated west of the collision (suture) zone, and this new trench propagated southward, thus trapping the Coast Range ophiolite in the new forearc area south of the Klamath area. Intense deformation in the Sierran region resulted from this collision, and both magmatic arcs became inactive as the last remnant of intervening oceanic crust was subducted. Continued westward relative movement of the North American arc was permitted north of the Sierra Nevada owing to the lack of a colliding intraoceanic arc. The result was the westward rifting of the continental-margin arc by intraarc spreading, which formed the Josephine ophiolite in the Klamath area. The Klamath phase of the Nevadan orogeny resulted from contraction of the west-facing intraoceanic arc and Josephine backarc basin beneath the continental margin. Basal sediments of the Great Valley forearc basin were derived primarily from the sutured arc/ophiolite terranes, and were deposited on top of the Coast Range ophiolite, the southern edge of the Klamaths, and the western side of the Sierra Nevada. A new (late Mesozoic) magmatic arc was superposed across the previously accreted terranes, and formed the primary sediment source for the Cretaceous forearc basin.

  5. On the limits and limitations of the ophiolite - mid-ocean ridge analogy: Oman vs the East Pacific Rise

    NASA Astrophysics Data System (ADS)

    MacLeod, Christopher J.; Lissenberg, C. Johan

    2015-04-01

    Throughout the near half century since Ian Gass proposed that ophiolite complexes formed by a process directly analogous to seafloor spreading, the study of ophiolites has been central to the development of our conceptual understanding of the mechanisms of formation of oceanic lithosphere at mid-ocean ridges. This role has been affected little by the recognition - on the basis of their geochemistry - that most ophiolites must have formed by spreading above subduction zones rather than at 'proper' open-ocean mid-ocean ridges. Why? - because we will never be able to gain access to the internal structure of modern ocean lithosphere to the extent we can by walking over the largest and best-preserved ophiolites (e.g. Cyprus, Oman, Newfoundland etc.). Ophiolites will always provide vital insights into the mechanisms of formation of lithosphere formed at submarine volcanic spreading centres. To what extent, however, can we be confident that what we learn from ophiolite studies is directly applicable to modern open-ocean mid-ocean ridges? Exactly how far can we press the analogy? To a first order it is reasonable to assume that the physical processes of crustal formation at an open-ocean mid-ocean ridge and at a supra-subduction zone spreading centre should be closely comparable: the presence of an organised sheeted dyke complex, representing 100% extension accompanied by magmatism, is convincing evidence for seafloor spreading. But does this mean the processes of crustal formation are identical in these different geodynamic environments? In this presentation we compare the 'crown jewel' of ophiolites, Oman, with the East Pacific Rise to explore the veracity of the widely-held belief that Oman represents a direct analogue for lithosphere formed at a fast-spreading (open-ocean) mid-ocean ridge. Whereas the mantle source of the axial volcanic suite in Oman is very similar to that of mid-ocean ridge basalt, we have recently shown (MacLeod et al. 2013, Geology v.41, p.459-462) that the magmas were generated in the presence of water, from which we deduce that the ophiolite was very probably formed by a short-lived spreading episode immediately following subduction initiation. We here compare and contrast more than 25 years of our own observations and data from Oman with a unique suite of samples we recently collected from the Hess Deep rift valley (the first ever complete crustal section recovered from a modern fast-spreading ridge), examining the extent to which the presence of water and the unstable geodynamic environment during the generation of the Oman ophiolite have had a material effect on the internal structure and composition of the crust thus produced. We here demonstrate that, although the physical processes of crustal generation at the Oman and East Pacific Rise spreading ridges are similar overall, systematic differences in crustal structure exist. We discuss the significant implications these have for our models of crustal accretion based solely on ophiolite observations.

  6. Mantle Recycling of Crustal Materials through Study of Ultrahigh-Pressure Minerals in Collisional Orogens, Ophiolites, and Xenoliths

    NASA Astrophysics Data System (ADS)

    Liou, J. G.; Tsujimori, T.; Yang, J.; Zhang, R. Y.; Ernst, W. G.

    2014-12-01

    Newly recognized ultrahigh-pressure (UHP) mineral occurrences including diamonds in ultrahigh-temperature (UHT) felsic granulites of orogenic belts, in chromitites associated with ophiolitic complexes, and in mafic/ultramafic xenoliths suggest the recycling of crustal materials through profound subduction, mantle upwelling, and return to the Earth's surface. Recycling is supported by unambiguously crust-derived mineral inclusions in deep-seated zircons, chromites, and diamonds from collision-type orogens, from eclogitic xenoliths, and from ultramafic bodies of several Alpine-Himalayan and Polar Ural ophiolites; some such phases contain low-atomic number elements typified by crustal isotopic signatures. Ophiolite-type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone. In addition to ringwoodite and stishovite, a wide variety of nanometric minerals have been identified as inclusions employing state-of-the-art analysis. Reconstitution of now-exsolved precursor UHP phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier UHP conditions. Some podiform chromitites and associated peridotites contain rare minerals of undoubted crustal origin, including Zrn, corundum, Fls, Grt, Ky, Sil, Qtz, and Rtl; the zircons possess much older U-Pb ages than the formation age of the host ophiolites. These UHP mineral-bearing chromitites had a deep-seated evolution prior to extensional mantle upwelling and its partial melting at shallow depths to form the overlying ophiolite complexes. These new findings plus stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere, recycling of biogenic carbon into the lower mantle, and ascent to the Earth's surface through deep mantle ascent.

  7. Ages and Nd, Sr isotopic systematics in the Sierran foothills ophiolite belt, CA: the Smartville and Feather River complexes

    SciTech Connect

    Shaw, H.F.; Niemeyer, S.

    1985-01-01

    Sm-Nd dating has shown the Kings-Kaweah ophiolite to be approx. 480 My old. Its Nd, Sr, and Pb isotopic compositions require an unusually old depleted mantle source. Samples from the Smartville and Feather River complexes have been analyzed in a search for similar highly depleted, early Paleozoic ophiolites in the northern foothills ophiolite belt. Six whole rocks from Smartville, encompassing representative lithologies, plus plagioclase and pyroxene mineral separates define a 183 +/- 22 My Sm-Nd isochron. This age, interpreted as the igneous age, is older than, but within error of, approx. 160 My U-Pb ages previously obtained from plagiogranite zircon analyses. One diabase with unusually high Rb/Sr yields a depleted mantle Sr model age of 200 +/- 25 My, consistent with the Sm-ND age. These compositions are clearly oceanic in character but do not discriminate among possible tectonic settings for the formation of the Smartville complex. Sm-Nd data for flaser gabbros and related rocks from Feather River scatter about an approx. 230 My errorchron with element of/sub Nd/(T) = +6.3 to +8.7. Initial /sup 87/Sr//sup 86/Sr ranges from 0.7028 to 0.7031. These results indicate a complex history with initial isotopic heterogeneities and/or disturbances of the isotopic systems. If primary, the element of/sub Nd/ (T) values are somewhat low, suggesting a possible arc origin for these rocks. Neither the Smartville nor Feather R. complexes appear to be related to the Kings-Kaweah ophiolite which, so far, is unique among foothill ophiolites in having an early Paleozoic age and a clear MORB, as opposed to arc or marginal basin, isotopic signature.

  8. Tectonic interactions between India and Arabia since the Jurassic reconstructed from marine geophysics, ophiolite geology, and seismic tomography

    NASA Astrophysics Data System (ADS)

    Gaina, Carmen; Hinsbergen, Douwe J. J.; Spakman, Wim

    2015-05-01

    Gondwana breakup since the Jurassic and the northward motion of India toward Eurasia were associated with formation of ocean basins and ophiolite obduction between and onto the Indian and Arabian margins. Here we reconcile marine geophysical data from preserved oceanic basins with the age and location of ophiolites in NW India and SE Arabia and seismic tomography of the mantle below the NW Indian Ocean. The North Somali and proto-Owen basins formed due to 160-133 Ma N-S extension between India and Somalia. Subsequent convergence destroyed part of this crust, simultaneous with the uplift of the Masirah ophiolites. Most of the preserved crust in the Owen Basin may have formed between 84 and 74 Ma, whereas the Mascarene and the Amirante basins accommodated motion between India and Madagascar/East Africa between 85 and circa 60 Ma and 75 and circa 66 Ma, respectively. Between circa 84 and 45 Ma, oblique Arabia-India convergence culminated in ophiolite obduction onto SE Arabia and NW India and formed the Carlsberg slab in the lower mantle below the NW Indian Ocean. The NNE-SSW oriented slab may explain the anomalous bathymetry in the NW Indian Ocean and may be considered a paleolongitudinal constraint for absolute plate motion. NW India-Asia collision occurred at circa 20 Ma deforming the Sulaiman ranges or at 30 Ma if the Hindu Kush slab north of the Afghan block reflects intra-Asian subduction. Our study highlights that the NW India ophiolites have no relationship with India-Asia motion or collision but result from relative India-Africa/Arabia motions instead.

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

    NASA Astrophysics Data System (ADS)

    Nicolas, Adolphe; Boudier, Françoise; France, Lydéric

    2009-06-01

    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 ophiolite. They are complemented by 133 sites, located throughout the entire ophiolite, where the transition from the RZSDC to the uppermost foliated gabbros is well exposed. Altogether, half the sites and 11 cross sections display, within a few tens of meters beneath the RZSDC, a magmatic foliation which is parallel to the overlying sheeted dikes. In the other sites and cross sections, the gabbro foliation is either flat-lying or steep but not parallel to the sheeted dikes. Compared to the RZSDC isotropic ophitic gabbros where clinopyroxene is interstitial between plagioclase laths, in the topmost steeply foliated gabbros, clinopyroxene is idiomorphic, becoming rapidly granular and tabular down section by recrystallization and peripheral alteration to hornblende. Moving down from these uppermost gabbros and over one hundred meters, the steep foliation becomes stronger and the poorly recovered top gabbros grade into the recrystallized, granular gabbros of the gabbro unit. These repeated observations indicate to ascribe these gabbros to subsidence of a compacting mush from the floor of the melt lens into the underlying, main magma chamber. The topmost gabbros beneath RZSDC, which were expelled from the melt lens by drifting very soon after settling on the melt lens floor, display in plagioclase a spectacular zoning pointing to a fast cooling. Moving downwards, stronger foliation, increased compaction and recovery-recrystallization are explained by the time spent by the subsiding mush inside the magma chamber increasing by one order of magnitude (~ 50 to 500 yr) over this vertical distance. This field-based study brings compelling field evidence supporting the former models of subsidence which were based on the assumption that the mush that settled onto the floor of the melt lens is sucked downwards during drifting of the crust away from the ridge axis.

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

    NASA Astrophysics Data System (ADS)

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

    2013-10-01

    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.

  11. Genesis of Diamond-bearing and Diamond-free Podiform Chromitites in the Luobusa Ophiolite, Tibet

    NASA Astrophysics Data System (ADS)

    Yang, J.; Xiong, F.; Xu, X.; Robinson, P. T.; Dilek, Y.; Griffin, W. L.

    2014-12-01

    Micro-diamonds, moissanite and many highly reduced minerals, such as native Fe, Cr, Ni, Si, Al, and metallic alloys, have been reported previously from podiform chromitites and peridotites of the Luobusa ophiolite in the eastern segment of the Yarlung-Zangbo suture of southern Tibet.. Similar mineral associations have now been confirmed in mantle peridotites or chromitites of 11 other ophiolites in 5 orogenic belts, in Tibet, Myanmar, North China and the Polar Urals. However, detailed studies of the Luobusa ophiolite show that not all chromitites contain these UHP and highly reduced minerals. Diamond-bearing chromitites are chiefly massive bodies composed of over 95 modal% magnesiochromite with Cr#s [100Cr/(Cr+Al)] of 77-83 and Mg#s [100Mg/(Mg+Fe)] of 71-82. Most of these bodies have sharp contacts with the host harzburgites and are only rarely enclosed in dunite envelopes. Many magnesiochromite grains in the massive chromitites contain inclusions of forsterite and pyroxene. Forsterite inclusions have Fo numbers of 97-99 and NiO contents of 1.11-1.29 wt%. Mg#s of clinopyroxene inclusions are 96-98 and those of orthopyroxene are 96-97. X-ray studies show that the olivine inclusions have very small unit cells and short cation-oxygen bond distances, suggesting crystallization at high pressure. In contrast, diamond-free chromitites typically occur as layers within thick dunite sequences or as irregular patches surrounded by dunite envelopes. They consist of variable proportions of magnesiochromite (Cr# = 76-78; Mg# = 58-61) and olivine, and have banded, nodular and disseminated textures. The dunite envelopes consist chiefly of granular olivine with a few relatively large, amoeboidal grains of magnesiochromite, and typically grade into the host peridotites with increasing pyroxene. Unlike those in the massive ores, magnesiochromite grains in nodular and disseminated chromitites lack pyroxene inclusions, and their olivine inclusions have relatively low Fo (94-96) and NiO (0.35-0.58 wt%). We propose that the diamond-bearing chromitite ores formed within the deeper parts of the upper mantle and were emplaced at an oceanic spreading ridge, whereas the diamond-free chromitites formed at shallow levels by melt/rock reaction, most likely in a SSZ environment.

  12. Genesis and transport of hexavalent chromium in the system ophiolitic rocks - groundwater

    NASA Astrophysics Data System (ADS)

    Shchegolikhina, Anastasia; Guadagnini, Laura; Guadagnini, Alberto

    2015-04-01

    Our study aims at contributing to the quantification and characterization of chromium transport processes from host rocks and soil matrices to groundwater. We focus on dissolved hexavalent chromium detected in groundwaters of geological regions with ophiolitic rocks (ophiolites and serpentinites) inclusions due to its critical ecological impact. (Oze et al., 2004). Despite the large number of analyses on the occurrence of high concentrations of hazardous hexavalent chromium ions in natural waters, only few studies were performed with the objective of identifying and investigating the geochemical reactions which could occur in the natural system rock - groundwater - dissolved chromium (Fantoni et al., 2002, Stephen and James, 2004, Lelli et al., 2013). In this context, there is a need for integration of results obtained from diverse studies in various regions and settings to improve our knowledge repository. Our theoretical analyses are grounded and driven by practical scenarios detected in subsurface reservoirs exploited for civil and industrial use located in the Emilia-Romagna region (Italy). Available experimental datasets are complemented with data from other international regional-scale settings (Altay mountains region, Russia). Modeling of chromium transformation and migration particularly includes characterization of the multispecies geochemical system. A key aspect of our study is the analysis of the complex competitive sorption processes governing heavy metal evolution in groundwater. The results of the research allow assessing the critical qualitative features of the mechanisms of hexavalent chromium ion mobilization from host rocks and soils and the ensuing transformation and migration to groundwater under the influence of diverse environmental factors. The study is then complemented by the quantification of the main sources of uncertainty associated with prediction of heavy metal contamination levels in the groundwater system explored. Fantoni, D., Brozzo, G., Canepa, M., Cipolli, F., Marini, L., Ottonello, G., Zuccolini, M., 2002. Natural hexavalent chromium in groundwaters interacting with ophiolitic rocks. Environmental Geology 42, 871-882. Lelli, M., Grassi, S., Amadori, M., Franceschini, F., 2013. Natural Cr(VI) contamination of groundwater in the Cecina coastal area and its inner sectors (Tuscany, Italy). Environmental Earth Sciences 71, 3907-3919. Oze, C., Fendorf, S., Bird, D.K., Coleman, R.G., 2004. Chromium geochemistry of serpentine soils. International Geology Review 46, 97-126. Stephen, M.T., James, A.J., 2004. Overview of chromium (VI) in the environment. Chromium (VI) Handbook. CRC Press, pp. 21.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

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

  15. Talc-carbonate alteration of ultramafic rocks within the Leka Ophiolite Complex, Central Norway

    NASA Astrophysics Data System (ADS)

    Bjerga, A.; Konopásek, J.; Pedersen, R. B.

    2015-06-01

    Petrographic and geochemical analyses of partly and completely serpentinized and carbonated peridotites within the ultramafic section of the Leka Ophiolite Complex have been used to elucidate the evolution of alterations and identify possible fluid sources. The alterations show no evidence for any major deformation and are located along low-angle structures that were formed in a late stage of the structural evolution of the ophiolite complex. Modeling of mineral equilibria in the SiO2-MgO-FeO-Fe2O3-CaO-H2O ± CO2 system has been utilized to constrain the conditions during serpentinization and carbonation. The partly altered peridotites consist of the mineral assemblage olivine-clinopyroxene-serpentine-magnetite-brucite and formed at temperatures < 410 °C by infiltration of aqueous fluids. Completely serpentinized rocks with the mineral assemblage serpentine-magnesite-magnetite-dolomite formed at temperatures < 500 °C and low XCO2 (? 0.05) by the breakdown of the minerals in the partly altered peridotites. Talc-carbonate rocks formed by the breakdown of the serpentine in the previously formed serpentinite rock at temperatures < 550 °C and elevated XCO2, which resulted in the stabilization of the assemblage talc-magnesite-magnetite-dolomite. Carbon isotope values determined for dolomite from crosscutting carbonate lenses within the talc-carbonate rock yield ?13C values of ~- 5 indicative of a mantle source for the carbon required for the carbonation. Oxygen isotope values ?SMOW18O of ~ 10.8-11.3‰ together with initial 87Sr/86Sri = 400Ma values of 0.7029 and 0.7063, suggest dehydration of rocks with mantle affinity as a source for the fluids. Based on analytical results and field observations we propose that the formation of the talc- and carbonate-bearing alteration zones is caused by the focused infiltration of fluids that originated at the bottom of already partly serpentinized ophiolite complex during extension-driven burial at the late stage of the Caledonian orogeny.

  16. Plume type ophiolites in Japan, East Russia and Mongolia: Peculiarity of the Late Jurassic examples

    NASA Astrophysics Data System (ADS)

    Ishiwatari, Akira; Ichiyama, Yuji; Ganbat, Erdenesaikhan

    2013-04-01

    Dilek and Furnes (2011; GSAB) provided a new comprehensive classification of ophiolites. In addition to the mid-ocean ridge (MOR) and supra-subduction zone (SSZ) types that are known for decades, they introduced rift-zone (passive margin) type, volcanic arc (active margin) type, and plume type. The last type is thought to be originated in oceanic large igneous provinces (LIPs; oceanic plateaus), and is preserved in the subduction-accretion complexes in the Pacific margins. The LIP-origin greenstones occur in the Middle Paleozoic (Devonian) accretionary complex (AC) in central Mongolia (Ganbat et al. 2012; AGU abst.). The Late Paleozoic and Mesozoic plume-type ophiolites are abundant in Japan. They are Carboniferous greenstones covered by thick limestone in the Akiyoshi belt (Permian AC, SW Japan; Tatsumi et al., 2000; Geology), Permian greenstones in the Mino-Tamba belt (Jurassic AC, SW Japan; Ichiyama et al. 2008; Lithos), and Late Jurassic-Early Cretaceous greenstone in the Sorachi (Hokkaido; Ichiyama et al, 2012; Geology) and Mikabu (SW Japan; this study) belts. The LIP origin of these greenstones is indicated by abundance of picrite (partly komatiite and meimechite), geochemical features resembling HIMU basalts (e.g. high Nb/Y and Zr/Y) and Mg-rich (up to Fo93) picritic olivines following the "mantle array", suggesting very high (>1600oC) temperature of the source mantle plume. The Sorachi-Mikabu greenstones are characterized by the shorter time interval between magmatism and accretion than the previous ones, and are coeval with the meimechite lavas and Alaskan-type ultramafic intrusions in the Jurassic AC in Sikhote-Alin Mountains of Primorye (E. Russia), that suggest a superplume activity in the subduction zone (Ishiwatari and Ichiyama, 2004; IGR). The Mikabu greenstones extend for 800 km along the Pacific coast of SW Japan, and are characterized by the fragmented "olistostrome" occurrence of the basalts, gabbros and ultramafic cumulate rocks (but no mantle peridotite), suggesting tectonism in a sediment-starved subduction zone or a transform fault zone that transected the thick oceanic LIP crust. The Sorachi greenstones are associated with depleted mantle peridotite, and are covered by the thick Cretaceous turbidite formation (Yezo Group), and Takashima et al. (2002; JAES) concluded the marginal basin origin for the "Sorachi ophiolite". We know that some oceanic LIPs were developed into marginal basins (e.g. Caribbean basin). The Late Jurassic-Early Cretaceous greenstone belts of Japan and eastern Russia may represent relics of a 2000 km-size superplume activity that hit the subduction zone and the adjacent ocean floor in NW Pacific.

  17. Melanges, No. 23.

    ERIC Educational Resources Information Center

    Melanges, 1997

    1997-01-01

    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…

  18. Gabbro-norite cumulates from strongly depleted MORB melts in the Alpine-Apennine ophiolites

    NASA Astrophysics Data System (ADS)

    Piccardo, G. B.; Guarnieri, L.

    2011-06-01

    Hundred-meter wide cumulate bodies and decimetric dykelets of gabbro-norites are widespread within the distal ophiolitic peridotites from the Jurassic Ligure-Piemontese oceanic basin, now emplaced in the Alpine-Apennine orogenic system. These peridotites derived from the sub-continental mantle of the pre-Triassic Europe-Adria lithosphere and underwent profound modifications of their structural and compositional characteristics via melt-rock interaction during diffuse percolation by porous flow of upwelling asthenospheric melts. Gabbro-norite cumulates show the peculiar association of high forsteritic olivine, high-Mg# clinopyroxenes and orthopyroxenes and high anorthitic plagioclase with respect to mineral compositions in common ophiolitic and oceanic MORB gabbros. Abundance and early crystallization of magnesian orthopyroxene suggests that parental magmas of the gabbro-noritic cumulates were relatively silica-rich basaltic liquids. Clinopyroxenes and plagioclase have anomalously low Sr and LREE, resulting in highly fractionated C1-normalized LREE patterns in clinopyroxenes and negatively fractionated C1-normalized LREE patterns in plagioclases. Modal mineralogy and mineral major and trace element compositions indicate that these gabbro-norites crystallized from MORB-type basaltic liquids that were strongly depleted in Na, Ti, Zr, Sr and other incompatible trace elements relative to any erupted liquids of MORB-type ophiolites and modern oceanic lithosphere. Computed melt compositions in equilibrium with gabbro-norite clinopyroxenes are closely similar to depleted MORB-type single melt increments after 5-7% of fractional melting of a DM asthenospheric mantle source under spinel-facies conditions. Present knowledge on the ophiolitic peridotites of Monte Maggiore indicate that they were formed by interaction of lithospheric mantle protoliths with depleted, MORB-type single melt increments produced by the ascending asthenosphere. Their composition was progressively modified from olivine-saturated to orthopyroxene-saturated by the early reactive melt-peridotite interaction (i.e., pyroxene dissolution and olivine precipitation). Gabbro-norite cumulates marked the change from diffuse porous flow percolation to intrusion and crystallization when cooling by conducive heat loss became dominant on heating by melt percolation. Progressive upwelling and cooling of the host peridotite during rifting caused transition to more brittle conditions and to hydration and serpentinization. The Monte Maggiore peridotite body was then intruded along fractures by variably evolved, Mg-Al- to Fe-Ti-rich gabbroic dykes. Computed melt compositions in equilibrium with clinopyroxenes from less evolved gabbro dykes are closely similar to aggregated MORBs. The event of gabbro intrusion indicates that aggregated MORB-type liquids: i) migrated through and stagnated in the mantle lithosphere and ii) underwent evolution into shallow ephemeral magma chambers to form the parental magmas of the gabbroic dykes and the basaltic lava flows of the Ligurian oceanic crust.

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

    NASA Astrophysics Data System (ADS)

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

    2009-05-01

    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.

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

    USGS Publications Warehouse

    Page, Norman J; Talkington, Raymond W.

    1984-01-01

    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.

  1. Mineral compositions of plutonic rocks from the Lewis Hills massif, Bay of Islands ophiolite

    NASA Technical Reports Server (NTRS)

    Smith, Susan E.; Elthon, Don

    1988-01-01

    Mineral compositions of residual and cumulate rocks from the Lewis Hills massif of the Bay of Islands ophiolite complex are reported and interpreted in the context of magnetic processes involved in the geochemical evolution of spatially associated diabase dikes. The mineral compositions reflect greater degrees of partial melting than most abyssal peridotites do and appear to represent the most depleted end of abyssal peridotite compositions. Subsolidus equilibration between Cr-Al spinal and olivine generally has occurred at temperatures of 700 to 900 C. The spinel variations agree with the overall fractionation of basaltic magmas producing spinels with progressively lower Cr numbers. The compositions of clinopyroxenes suggest that the fractionation of two different magma series produced the various cumulate rocks.

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

    NASA Astrophysics Data System (ADS)

    Chen, Genwen; Xia, Bin

    2008-04-01

    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.

  3. The Development of Subduction Below the Oman-UAE Ophiolite: Detailed Temporal Constraints from High Precision U-Pb Zircon Geochronology

    NASA Astrophysics Data System (ADS)

    Rioux, M. E.; Bowring, S. A.; Garber, J. M.; Kelemen, P. B.; Searle, M. P.; Miyashita, S.; Adachi, Y.

    2014-12-01

    The tectonic setting during formation of the Oman-UAE ophiolite is debated. High precision 206Pb/238U zircon dates from gabbros indicate that ridge magmatism responsible for formation of most of the ophiolite crust occurred between ~96.0-95.5 Ma. Here we present new high precision 206Pb/238U zircon dates and whole-rock Nd isotopic data from three rock sequences that constrain the development of thrusting/subduction below the ophiolite: 1. The V2 volcanic sequence and associated plutonic rocks in the ophiolite have geochemical patterns that have been interpreted as a subduction signature. V2 plutonic rocks from both Oman and the UAE have dates of ~95.6-95.0 Ma, consistent with rapid development of thrusting/subduction following formation of the ophiolite crust. 2. Felsic plutonic rocks with ?Nd < 0 intrude residual mantle harzburgites below the crust-mantle boundary, consistent with derivation of the magmas from a source with time-integrated LREE enrichment, such as continental crust or terrigenous sedimentary rocks. Dates on these rocks in Oman range from ~95.5-95.0, consistent with synchronous thrusting of continent derived rocks below the ophiolite during V2 magmatism. Similar intrusive rocks from the UAE are younger, ranging from ~94-91 Ma, indicating a more protracted history of thrusting and melting in this area. 3. Amphibolite facies metamorphic rocks in the sole of the ophiolite are interpreted as crust thrust beneath the ophiolite during formation or emplacement. Garnet amphibolites and small leucocratic (?) pods and veins from sole exposures in Oman are 96.0-94.7 Ma. The oldest dates are synchronous with ridge magmatism and suggest some sole amphibolites underwent metamorphism and melting while the ridge was active, favoring a supra-subduction zone model for ophiolite formation. Rocks from sole exposures in the UAE are slightly younger, ranging from 94.5-92.5 Ma. In summary, the data strongly support a supra-subduction zone model for formation of the ophiolite. If, as has been proposed, the ophiolite formed during subduction initiation, the new dates provide an unprecedented record of this process, which can be used to test and constrain geodynamic models.

  4. Hidden tectonics beneath the seafloor at mid-ocean ridges: constraints from the Troodos ophiolite, Cyprus.

    NASA Astrophysics Data System (ADS)

    Young, Ella; MacLeod, Christopher; Morris, Antony; Lissenberg, Johan

    2013-04-01

    Seafloor spreading at mid-ocean ridges (MOR) is accommodated by a combination of magmatism and tectonic stretching of the crust. Recent investigations of mid-ocean ridges show evidence for large offset normal faults or detachments that accommodate a significant fraction of the plate separation. Once thought of as rarities along the global spreading system, detachment faults are now considered to play an integral role in seafloor spreading at slow spreading ridges. Estimates of the proportion and extent of tectonic spreading however vary widely; some authors suggest as much as 50% of the Mid Atlantic Ridge is underlain by active detachment faults. The lack of consensus is in part a consequence of the difficulty in documenting the extent of tectonic stretching and detachment faulting, especially from surface morphology alone. On the modern seafloor tectonic activity may potentially be under-estimated if lava flows blanket and obscure the faulting below. In this contribution we present field observations from the Troodos ophiolite, Cyprus, which is believed to be a slow-spreading ridge analogue. Our interdisciplinary approach utilises structural mapping, palaeomagnetism and geochemistry to investigate estimates of the extent and effects of detachment faulting within the ophiolite. We document significant extensional faulting and rotation within the crust above a detachment fault at the level of the sheeted dyke complex, yet show that the lavas at the surface are sub-horizontal and unaffected by the deformation beneath. Unconformities in the lava section demonstrate progressively greater rotations deeper in the extrusive pile. We show these rotations to be controlled by tectonic stretching and tilting of the underlying dykes rather than rotation by loading and/or subsidence within the lava pile. Our results show that syn-tectonic volcanism fills half-graben on the seafloor such that minimal tectonic stretching is evident at the surface, but blankets tens of per cent tectonic extension at depth. We discuss the implications of these findings for modern slow-spreading mid-ocean ridge systems.

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

    NASA Astrophysics Data System (ADS)

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

    2006-12-01

    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.

  6. Oxygen Isotope Composition of the Oceanic Crust in the Oman ophiolite, Wadi Fizh section

    NASA Astrophysics Data System (ADS)

    Yamaoka, K.; Hiroyasu, Y.; Matsubaya, O.; Ishiyama, D.; Chiba, H.; Kawahata, H.

    2008-12-01

    Oxygen isotope compositions of oceanic crustal rocks of a complete section through the Wadi Fizh area in the Oman ophiolite were determined in order to investigate water-rock interaction at mid-ocean ridges. Bulk rock ?18O values decreased with increasing depth (the mean ?18O values of pillow basalt, sheeted dike and gabbro were 10.9, 7.6 and 4.8 per mil, respectively). Plagiogranites were relatively ?18O-enriched (6.2 to 9.0 per mil), whereas two samples of epidosites showed the lowest ?18O values (1.5 and 2.7 per mil). Average ?18O value (6.1 per mil) of bulk crustal rocks is almost identical to that of MORB basalt (5.7±0.3 per mil). Assuming that the ?18O of hydrothermal solution is 2 per mil and water-rock ratio is 9.4 (calculated from 87Sr/86Sr by the previous study), maximum temperature of alteration in the lower gabbro section is estimated as >500° C, which is consistent with the temperature of alteration estimated from assemblage of secondary minerals. If the reacted solution is more enriched in ?18O, the temperature of alteration might be higher. Although the trend of ?18O depth profile in the Wadi Fizh section is consistent with the previous studies of the other sections (Ibra, Hilti, Shafan and Rajimi) in the Oman ophiolite, the ?18O of the gabbro section in the Wadi Fizh section is highly depleted in comparison with the other sections. It indicates that large volume of high-temperature seawater circulated down to the Moho and the gabbros rarely overprinted by the late stage, low-temperature hydrothermal alteration at off-axis. Thus, the oceanic crust in the Wadi Fizh area is an ideal section to interpret the hydrothermal circulation at fast-spreading ridge system.

  7. Tectonic significance of the Dongqiao ophiolite in the north-central Tibetan plateau: Evidence from zircon dating, petrological, geochemical and Sr-Nd-Hf isotopic characterization

    NASA Astrophysics Data System (ADS)

    Liu, Tong; Zhai, Qing-guo; Wang, Jun; Bao, Pei-sheng; Qiangba, Zhaxi; Tang, Suo-han; Tang, Yue

    2016-02-01

    The Dongqiao ophiolite occurs in the central segment of the Bangong-Nujiang suture zone, in north-central Tibet, China. It is still debated on the tectonic setting of the Dongqiao ophiolite despite after more than 30 years' studies. The Dongqiao ophiolite has a complete section of a typical ophiolite, composed of harzburgite, dunite, layered and isotropic gabbros, pillow and massive basalts, as well as radiolarian chert. Whole-rock geochemical analyses show that harzburgite displays a broad U-shaped REE pattern and has a fore-arc affinity, whereas basalts show affinities of E-MORB, OIB and IAB. The basalts were probably formed in different tectonic settings, that is, mid-ocean ridge, oceanic island and island arc. The gabbros and basalts are characterized by positive εNd(t) (+1.6 to +6.7) and εHf(t) (+8.1 to +13.9) values. Zircon U-Pb dating yielded ages of 188 ± 1 Ma for the layered gabbro and 181 ± 1 Ma for the amphibole gabbro. The new ages and the published age data of the Dingqing and Dong Co ophiolites led us to conclude that the Bangong-Nujiang Ocean existed from the Late Triassic to Early Cretaceous. The new geochemical data also suggested that the Dongqiao ophiolite was a typical SSZ-type ophiolite formed in an initial fore-arc oceanic basin. Fore-arc ophiolites are probably widely distributed along the Bangong-Nujiang suture zone. If so, the Tethys Ocean of the Bangong-Nujiang area probably existed as a fore-arc oceanic basin during the Late Triassic to Early Jurassic.

  8. Geochronology and geochemistry of basaltic rocks from the Sartuohai ophiolitic mélange, NW China: Implications for a Devonian mantle plume within the Junggar Ocean

    NASA Astrophysics Data System (ADS)

    Yang, Gaoxue; Li, Yongjun; Santosh, M.; Yang, Baokai; Yan, Jing; Zhang, Bing; Tong, Lili

    2012-10-01

    The West Junggar domain in NW China is a distinct tectonic unit of the Central Asian Orogenic Belt (CAOB). It is composed of Paleozoic ophiolitic mélanges, arcs and accretionary complexes. The Sartuohai ophiolitic mélange in the eastern West Junggar forms the northeastern part of the Darbut ophiolitic mélange, which contains serpentinized harzburgite, pyroxenite, dunite, cumulate, pillow lava, abyssal radiolarian chert and podiform chromite, overlain by the Early Carboniferous volcano-sedimentary rocks. In this paper we report new geochronological and geochemical data from basaltic and gabbroic blocks embedded within the Sartuohai ophiolitic mélange, to assess the possible presence of a Devonian mantle plume in the West Junggar, and evaluate the petrogenesis and implications for understanding of the Paleozoic continental accretion of CAOB. Zircon U-Pb analyses from the alkali basalt and gabbro by laser ablation inductively coupled plasma mass spectrometry yielded weighted mean ages of 375 ± 2 Ma and 368 ± 11 Ma. Geochemically, the Sartuohai ophiolitic mélange includes at least two distinct magmatic units: (1) a Late Devonian fragmented ophiolite, which were produced by ca. 2-10% spinel lherzolite partial melting in arc-related setting, and (2) contemporary alkali lavas, which were derived from 5% to 10% garnet + minor spinel lherzolite partial melting in an oceanic plateau or a seamount. Based on detailed zircon U-Pb dating and geochemical data for basalts and gabbros from the Sartuohai ophiolitic mélange, in combination with previous work, indicate a complex evolution by subduction-accretion processes from the Devonian to the Carboniferous. Furthermore, the alkali basalts from the Sartuohai ophiolitic mélange might be correlated to a Devonian mantle plume-related magmatism within the Junggar Ocean. If the plume model as proposed here is correct, it would suggest that mantle plume activity significantly contributed to the crustal growth in the CAOB.

  9. The bilateral and nearly simultaneous obduction of Gaize (central Tibet) ophiolites at the Late Jurassic: constraints on the closure of Bangong-Nujiang suture

    NASA Astrophysics Data System (ADS)

    Yuxiu, Z.; Kaijun, Z.

    2009-12-01

    Systematic mapping of the Gaize area (central Tibet) across the middle-western Bangong-Nujiang suture and the Lhasa and Qiangtang blocks was conducted, during which ophiolite overthrust sheets in both northern Lhasa and southern South Qiangtang blocks were documented (Fig. 1A). Geochronological dating shows that these ophiolite sheets could have been transported bilaterally, both northward and southward, nearly simultaneously at the Late Jurassic (151~153 Ma). The Laguocuo ophiolite overthrust sheet is in northern Lhasa block south of Gaize county, and the electron microprobe analysis (EMPA) result shows that the newly-formed syntectonic minerals within the mica-quartz schist are phengites. Thermobaric conditions of those phengites are about 350±50 °C and 7~10 kb, and their 40Ar/39Ar plateau age is about 153 Ma. The counterpart ophiolite overthrust sheet in Cha’erkangcuo area is in southern South Qiangtang north of Gaize county and the EMPA result shows that the newly-formed syntectonic minerals within the mica-quartz schist are biotites with 40Ar/39Ar plateau ages of 151~153 Ma. Comprehensive analyses show that the Laguocuo and Cha’erkangcuo ophiolite mélanges are the north-to-south and south-to-norht obduction of the Early-Middle Jurassic Bangonghu-Nujiang ophiolites at the Late Jurassic respectively. The thermobaric conditions and age study of the Laguocuo and Cha’erkangcuo ophiolitie sheets show that there existed bilateral and nearly simultaneous obdution of Gaize ophiolites in middle-western Bangong-Nujiang ophiolitie belt and the north-to-south obduction remained dominant. The obduction could have been driven by external force, most probably by the flat-lying oceanic subduction of the Yarlung-Zangpo Neo-Tethys (Fig. 1B). Fig. 1. The simplified tectonic map of Tibetan Plateau (A) and schematic cross section showing the bilateral obdution of Gaize ophiolites (B)

  10. Recycling of crustal materials through study of ultrahigh-pressure minerals in collisional orogens, ophiolites, and mantle xenoliths: A review

    NASA Astrophysics Data System (ADS)

    Liou, Juhn G.; Tsujimori, Tatsuki; Yang, Jingsui; Zhang, R. Y.; Ernst, W. G.

    2014-12-01

    Newly recognized occurrences of ultrahigh-pressure (UHP) minerals including diamonds in ultrahigh-temperature (UHT) felsic granulites of orogenic belts, in chromitites associated with ophiolitic complexes, and in mantle xenoliths suggest the recycling of crustal materials through deep subduction, mantle upwelling, and return to the Earth's surface. This circulation process is supported by crust-derived mineral inclusions in deep-seated zircons, chromites, and diamonds from collision-type orogens, from eclogitic xenoliths in kimberlites, and from chromitities of several Alpine-Himalayan and Polar Ural ophiolites; some of these minerals contain low-atomic number elements typified by crustal isotopic signatures. Ophiolite-type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone. In addition to ringwoodite and inferred stishovite, a number of nanometric minerals have been identified as inclusions employing state-of-the-art analytical tools. Reconstitution of now-exsolved precursor UHP phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier UHP conditions. For example, Tibetan chromites containing exsolution lamellae of coesite + diopside suggest that the original chromitites formed at P > 9-10 GPa at depths of >250-300 km. The precursor phase most likely had a Ca-ferrite or a Ca-titanite structure; both are polymorphs of chromite and (at 2000 °C) would have formed at minimum pressures of P > 12.5 or 20 GPa respectively. Some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin, including zircon, feldspars, garnet, kyanite, andalusite, quartz, and rutile; the zircons possess much older U-Pb ages than the time of ophiolite formation. These UHP mineral-bearing chromitite hosts evidently had a deep-seated evolution prior to extensional mantle upwelling and partial melting at shallow depths to form the overlying ophiolite complexes. These new findings together with stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere, recycling of surface 'organic' carbon into the lower mantle, and ascent to the Earth's surface through mantle upwelling. Intensified study of UHP granulite-facies lower crustal basement and ophiolitic chromitites should allow a better understanding of the geodynamics of subduction and crustal cycling.

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

    NASA Astrophysics Data System (ADS)

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

    2013-07-01

    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.

  12. Origin of gabbroic sequences from the Ligurian ophiolites: implications for lower crust generation at slow spreading settings

    NASA Astrophysics Data System (ADS)

    Tribuzio, R.; Renna, M.; Sanfilippo, A.

    2011-12-01

    The Ligurian Jurassic ophiolites (northern Apennines, Italy) are lithospheric remnants of an embryonic slow spreading basin that developed in conjunction with the opening of the Central Atlantic Ocean. These ophiolites are characterized by km-scale gabbroic sequences intruded into mantle peridotites and exposed at the seafloor. These gabbroic sequences mostly consist of troctolites, olivine-gabbros and clinopyroxene-rich gabbros and locally include olivine-rich troctolite bodies (Renna and Tribuzio, 2011). The large-scale gabbroic sequences from the Ligurian ophiolites also enclose a few mantle peridotite bodies and bear striking structural and compositional resemblances to the gabbroic sequences from modern slow spreading ridges (Sanfilippo and Tribuzio, 2011). Field observations and petrological and geochemical data are used to constrain a conceptual model for the formation of the gabbroic sequences from the Alpine ophiolites. The proposed model begins with a hot mantle evolution under plagioclase facies conditions, in which melt transport occurred mainly in the form of grain scale porous flow. In particular, reactive channeling of olivine-saturated melts formed replacive dunitic conduits, whereas residual orthopyroxene-saturated melts led to melt impregnation of the mantle section. The hot lithospheric evolution is followed by an evolution characterized by melt transport through fractures, which started with crystallization of melt into gabbroic dikes. This diking event is likely correlated with the formation of the olivine-rich troctolites. These rocks show a process of infiltration of MORB-type melts saturated in plagioclase + clinopyroxene into an olivine-spinel matrix that is inferred to have formed in mantle melt conduits of replacive origin. As the mantle section cooled significantly, the dip of the melt migration structures evolved from sub-vertical to sub-horizontal. This is shown by the occurrence of sill-shaped gabbroic intrusions, which locally crosscut the gabbroic dikes within the mantle section and the olivine-rich troctolites. The growth of the gabbroic sequences is attributed to series of sill-like separate intrusions. The olivine-rich troctolite and the mantle peridotite bodies present within the gabbroic sequences may be similarly considered as remnants of a mantle section that was dissected by the multiple sill intrusions. Renna M.R., Tribuzio R. (2011) Olivine-rich troctolites from Ligurian ophiolites (Italy): Evidence for impregnation of replacive mantle conduits by MORB-type melts. Journal of Petrology, doi:0.1093/petrology/egr029 Sanfilippo A., Tribuzio R. (2011) Melt transport and deformation history in an "non-volcanic" ophiolitic section (Northern Apennine, Italy): implications for crustal accretion at slow spreading settings. Geochemistry, Geophysics, Geosystems, v. 12, doi:10.1029/2010GC003429

  13. MORB to supra-subduction geochemical transition in the extrusive sequences of major upper Cretaceous ophiolites of Iran

    NASA Astrophysics Data System (ADS)

    Babaie, H. A.; Khalatbari Jafari, M.; Moslempour, M. E.

    2014-12-01

    We discuss the geochemical patterns and tectonomagmatic setting of the extrusive sequences in the Khoy, Kermanshah, Fannuj, Nosratabad, Dehshir, south and north Fariman, and Sabzevar ophiolite massifs of Iran. These sequences include pillow lava, sheet flow, hyaloclastite, hyaloclastic breccia, and interbeds of chert and pelagic limestone with Late Cretaceous micro fauna. The Khoy, north Fariman, and Sabzevar massifs also include Late Cretaceous-Early Paleocene supra-ophiolitic volcanic and volcano-sedimentary rocks that formed in a trough near the extrusive sequence. The Khoy pillow lava displays T-MORB characteristics but no chemical contribution from the components released from the subducted slab. On the other hand, the diabase dikes that cut the Khoy extrusive sequence show signatures of subduction zone magmatism and contribution from the melt released through the partial melting of the subducted slab. While lava in the Harsin (Kermanshah) extrusive sequence in west Iran displays E-MORB and P-MORB characteristics, the pillows in the Fannuj, north Fariman, Dehshir, and Sabzevar extrusive sequences indicate the contribution of both fluids and melt from the subducted slab. The Nosratabad and south Fariman ophiolites also show evidence for either melt or fluids, respectively. Partial melting of the subducted slab sedimentary cover may have formed the acidic pillow lava and sheet flow in the Fannuj and Nosratabad extrusive sequence, respectively. Some pillows in the Nosratabad, Sabzevar, north Fariman, and to a lesser extent, Dehshir extrusive sequence display the OIB geochemical characteristics. Mantle plumes or asthenospheric flow that probably moved up through weak zones of the subducted slab may have affected the partial melting of the mantle wedge above the slab. The combined OIB and supra-subduction characteristics suggest the role of the roll-back of the subducted slab in the magmatism of the northeast Iranian ophiolites. The clear MORB-like geochemical characteristics in the extrusive sequence of the ophiolites in northwest and west Iran, and supra-subduction zone characteristics in the extrusive sequence of the ophiolites in southeast and east Iran, west of the Lut Block, and northeast Iran may represent a major Late Cretaceous transition from the MORB to a supra-subduction zone setting.

  14. Broken formations, melanges and olistostromes in Puerto Plata area (Northern Dominican Republic) as a record of subduction and collisional processes between the Caribbean and North-American plates

    NASA Astrophysics Data System (ADS)

    Hernaiz Huerta, Pedro Pablo; Valera Fernando, Pérez; de Los Santos Manuel, Abad; Jacques, Monthel; de Neira Alberto, Díaz

    2010-05-01

    The Northern Cordillera of the Dominican Republic records the (oblique) subduction and collisional processes occurred between the Caribbean and North-American plates during Upper Cretaceous to Lower Paleogene times. The boundary between these two plates can be traced within this range disrupted by an Upper Paleogene to present intense left-lateral strike-slip tectonism, onset after collision. In the western part of the range this boundary might be defined by the Camu fault. In the coastal area of Puerto Plata, located on the northern block of the Camu fault, basement rocks belonging to the subducting plate (the Puerto Plata Basement Complex) and several related units probably formed in an accretionary prism, preserve in a large (300 km2) outcrop of chaotic formations presumably formed (and exhumed) during collision. They include from broken formations and tectonic melanges to olistostromes and other coeval sedimentary deposits. The Puerto Plata Basement Complex (PPBC) consists of highly faulted and dismembered blocks formed by discontinuous but sometimes coherent outcrops of serpentinized or massive peridotite, pods of ultramafic cumulates, massive or banded gabbros and Los Caños Fm, a thick sequence of gross bedded volcaniclastic material with interbedded basaltic (sometimes pillowed) or andesitic flows. All these rocks bear low grade metamorphism and lack a general deformation fabric apart from occasional transformation to mylonites due to localized shearing. The PPBC has been interpreted as a fragment of oceanic crust, belonging to the subducting (North-American) slab that has been exhumed as a tectonic melange or a broken complex. The Imbert Fm, of Palaeocene-Eocene age, is formed by a well bedded succession of white very fine grained porcelaneous tuffs, with eventual intercalations of cherts, limestones and marls that, towards the lower part, is interbedded with volcanic-derived graywackes and limolites, and more occasionally, thick beds of conglomerates and debris. These last ones typically incorporate fragments of serpentized peridotites and blocks of the volcanic rocks identified in the PPBC. The Imbert Fm is also internally disrupted and although not a single clear contact can be observed in the field, it is considered to rest unconformably over the complex, so postdating its exhumation. A separate mappable unit of serpentinitic brecchias has recently been identified mainly distributed along the outer limits of the PPBC but also in several scattered outcrops inside it. The unit is dominantly made of fragments and blocks of serpentinized peridotites, embedded in an abundant matrix of the same composition and includes also blocks of Los Caños and Imbert Fm, as well as other exotic blocks of unknown origin. In the southeastern limit of the complex, close to the Camu fault, there is a particular high concentration of exotic blocks derived from metamorphic rocks (greenschists, anfibolites, marbles and even blueschists) not exposed in any neighbouring areas and thus suggesting a deep-sited, subduction-related, feeding. The basal contact of this unit with the rocks of the PPBC is usually faulted and difficult to observe in the field, but cartographic patterns suggest that it is an unconformity. Outcrops of serpentinitic breccias show a wide variety of internal chaotic organization, from pods of tectonic melanges to the most frequent block-in matrix fabric, but most of them also show evidences of sedimentary rework. In the easternmost part of the PPBC, the serpentinitic brecchias are, in turn, the base of an olistostromic complex widely represented in the region, the San Marcos Fm. The olistostrome includes similar exotic blocks than observed in the serpentinitic brecchias and blocks and olistolithes derived from the PPBC but, mainly, from the Imbert Fm. According to the described relationships, the serpentinitic breccias and San Marcos olistostrome are considered partially coeval and laterally equivalent to the Imbert Fm.

  15. The effects of partial melting, melt-mantle interaction and fractionation on ophiolite generation: Constraints from the late Cretaceous Pozant?-Karsant? ophiolite, southern Turkey

    NASA Astrophysics Data System (ADS)

    Saka, Samet; Uysal, Ibrahim; Akmaz, Recep Melih; Kaliwoda, Melanie; Hochleitner, Rupert

    2014-08-01

    The Pozant?-Karsant? ophiolite consists of a mantle unit and overlying crustal rocks. The mantle peridotite is composed of harzburgitic to dunitic rocks that are depleted in Al2O3 and CaO compared to primitive mantle. The low whole-rock Al and Ca values are consistent with the high Cr# [= 100 × Cr/(Cr + Al)] values of spinel that range from 44 to 70. These spinels generally have low TiO2 (< 0.06 wt.%) concentrations, although spinels with high-Cr compositions in certain samples exhibit enrichments of up to 0.16 wt.% TiO2. The concentrations of chondrite-normalized rare earth elements (REEs) indicate depletion toward heavy to middle REEs. However, the entire mantle peridotite samples exhibit marked enrichment in light REEs and large-ion lithophile elements (LILEs) compared to middle REEs. The heavy REE patterns of some peridotite samples are comparable to the calculated melting curves representing various degrees of melting and are modeled by ~ 24% to 30% melting in the spinel stability field. However, some samples are more depleted in the middle REEs than heavy REEs. The patterns of heavy to middle REEs do not follow the melting lines produced by various degrees of melting in the spinel stability field. The heavy REE composition of these peridotite samples suggests that partial melting began in the garnet stability field and continued into the spinel stability field; they represent ~ 22% to 26% melting under various pressure conditions. High anorthite content of plagioclase as well as Nb-depleted composition of mafic cumulates and isotropic gabbros indicate that the crustal rocks of the Pozant?-Karsant? ophiolite formed from a melt that has been produced by melting of depleted source. This melt is thought to have enriched in light REE and fluid mobile elements that were released from the subducting slab during the subduction of Neotethyan ocean. The whole-rock light REE and LILE enrichments and the higher TiO2 concentrations of the high-Cr# spinels in some mantle peridotite samples cannot be explained by simple melt extraction in a mid-ocean ridge (MOR) environment and require melting and enrichment processes in a supra-subduction zone tectonic setting. However, these features of the peridotites can be explained by the interaction of light REEs and TiO2-rich melts and fluids with peridotites that were depleted during melting events in an MOR environment. This interaction, took place in a supra-subduction zone environment, may have increased the light REE concentrations in the peridotites and caused the re-equilibration of the Ti-depleted spinel to produce Ti-richer spinel and therefore explains the geochemical signatures of the Pozant?-Karsant? peridotites.

  16. 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)

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

    2007-12-01

    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.

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

    NASA Technical Reports Server (NTRS)

    Dixon, S.; Rutherford, M. J.

    1979-01-01

    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.

  18. Upper crustal structure beneath the Zambales Ophiolite Complex, Luzon, Philippines inferred from integrated gravity, magnetic and geological data

    NASA Astrophysics Data System (ADS)

    Salapare, Ricky C.; Dimalanta, Carla B.; Ramos, Noelynna T.; Manalo, Pearlyn C.; Faustino-Eslava, Decibel V.; Queaño, Karlo L.; Yumul, Graciano P.

    2015-06-01

    The Zambales Ophiolite Complex (ZOC) in the western portion of Luzon Island, Philippines represents a typical exposure of an emplaced crust-upper mantle section of an ancient lithosphere. The ZOC is divided into the Acoje and Coto Blocks based on petrological, geochemical and age disparities, thus implying diverse and complex origins for the archipelago's lithospheric sources. We used gravity and magnetic data to reveal differences in the subsurface characteristics of the two ophiolite blocks for the first time. Low Bouguer gravity (<135 mGal) and magnetic (<69 nT) anomalies characterize the ophiolitic units in the Acoje Block whereas high Bouguer gravity (>150 mGal) and magnetic (>110 nT) anomalies typify the Coto Block. Such contrasting signatures further extend at greater depths which reflect the disparities in the crustal density, the basement structure, and the depth to Moho. Petrophysical characteristics such as density, magnetic susceptibility and natural remanent magnetization (NRM) intensities correlate well with the interpreted Bouguer gravity and magnetic anomalies. Densities of gabbros and peridotites from the Acoje and Coto Blocks reveal mean values ranging from 2640 to 2810 kg m-3 and 2570 to 2690 kg m-3, respectively. Magnetic susceptibility (>4.43 × 10-3 SI) and NRM (>0.69 A m-1) data are also generally higher over the Coto Block. Both Bouguer gravity and magnetic anomaly maps reveal a prominent steep gradient that potentially marks the structural contact between Acoje and Coto Blocks. We further infer that the steep anomaly gradient validates the presence of the Lawis Fault Zone which separates the two ophiolite blocks in the Masinloc Massif. Recent field evidence from rock exposures in Coto reveals both right lateral and vertical displacements along the fault zone.

  19. A melt-focusing zone in the lithospheric mantle preserved in the Santa Elena Ophiolite, Costa Rica

    NASA Astrophysics Data System (ADS)

    Madrigal, Pilar; Gazel, Esteban; Denyer, Percy; Smith, Ian; Jicha, Brian; Flores, Kennet E.; Coleman, Drew; Snow, Jonathan

    2015-08-01

    The Santa Elena Ophiolite in Costa Rica is composed of a well-preserved fragment of the lithospheric mantle that formed along a paleo-spreading center. Within its exposed architecture, this ophiolite records a deep section of the melt transport system of a slow/ultra-slow spreading environment, featuring a well-developed melt-focusing system of coalescent diabase dikes that intrude the peridotite in a sub-vertical and sub-parallel arrangement. Here we present an integrated analysis of new structural data, 40Ar/39Ar geochronology, major and trace element geochemistry and radiogenic isotope data from the diabase dikes in order to elucidate the tectonic setting of the Santa Elena Ophiolite. The dikes are basaltic and tholeiitic in composition. Petrological models of fractional crystallization suggest deep pressures of crystallization of > 0.4 GPa for most of the samples, which is in good agreement with similar calculations from slow/ultra-slow spreading ridges and require a relatively hydrated (~ 0.5 wt.% H2O) MORB-like source composition. The diabase dikes share geochemical and isotope signatures with both slow/ultra-slow spreading ridges and back-arc basins and indicate mixing of a DMM source and an enriched mantle end-member like EMII. The 40Ar/39Ar geochronology yielded an age of ~ 131 Ma for a previous pegmatitic gabbroic magmatic event that intruded the peridotite when it was hot and plastic and an age of ~ 121 Ma for the diabase intrusions, constraining the cooling from near asthenospheric conditions to lithospheric mantle conditions to ~ 10 Ma. Our findings suggest a complex interplay between oceanic basin and back-arc extension environments during the Santa Elena Ophiolite formation. We propose an alternative hypothesis for the origin of Santa Elena as an obducted fragment of an oceanic core complex (OCC).

  20. Dynamics of intraoceanic subduction initiation: 2. Suprasubduction zone ophiolite formation and metamorphic sole exhumation in context of absolute plate motions

    NASA Astrophysics Data System (ADS)

    van Hinsbergen, Douwe J. J.; Peters, Kalijn; Maffione, Marco; Spakman, Wim; Guilmette, Carl; Thieulot, Cedric; Plümper, Oliver; Gürer, Derya; Brouwer, Fraukje M.; Aldanmaz, Ercan; Kaymakcı, Nuretdin

    2015-06-01

    Analyzing subduction initiation is key for understanding the coupling between plate tectonics and the underlying mantle. Here we focus on suprasubduction zone (SSZ) ophiolites and how their formation links to intraoceanic subduction initiation in an absolute plate motion frame. SSZ ophiolites form the majority of exposed oceanic lithosphere fragments and are widely recognized to have formed during intraoceanic subduction initiation. Structural, petrological, geochemical, and plate kinematic constraints on their kinematic evolution show that SSZ crust forms at fore-arc spreading centers at the expense of a mantle wedge, thereby flattening the nascent slab. This leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during SSZ crust crystallization. Former spreading centers are preserved in forearcs when subduction initiates along transform faults or off-ridge oceanic detachments. We show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone. Upon inception of slab pull due to, e.g., eclogitization, the sole is separated from the slab, remains welded to the thinned overriding plate lithosphere, and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge. We propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust. Our study highlights how far-field geodynamic processes and absolute plate motions may force intraoceanic subduction initiation as key toward advancing our understanding of the entire plate tectonic cycle.

  1. Evidence from gabbro of the Troodos ophiolite for lateral magma transport along a slow-spreading mid-ocean ridge.

    PubMed

    Abelson, M; Baer, G; Agnon, A

    2001-01-01

    The lateral flow of magma and ductile deformation of the lower crust along oceanic spreading axes has been thought to play a significant role in suppressing both mid-ocean ridge segmentation and variations in crustal thickness. Direct investigation of such flow patterns is hampered by the kilometres of water that cover the oceanic crust, but such studies can be made on ophiolites (fragments of oceanic crust accreted to a continent). In the Oman ophiolite, small-scale radial patterns of flow have been mapped along what is thought to be the relict of a fast-spreading mid-ocean ridge. Here we present evidence for broad-scale along-axis flow that has been frozen into the gabbro of the Troodos ophiolite in Cyprus (thought to be representative of a slow-spreading ridge axis). The gabbro suite of Troodos spans nearly 20 km of a segment of a fossil spreading axis, near a ridge-transform intersection. We mapped the pattern of magma flow by analysing the rocks' magnetic fabric at 20 sites widely distributed in the gabbro suite, and by examining the petrographic fabric at 9 sites. We infer an along-axis magma flow for much of the gabbro suite, which indicates that redistribution of melt occurred towards the segment edge in a large depth range of the oceanic crust. Our results support the magma plumbing structure that has been inferred indirectly from a seismic tomography experiment on the slow-spreading Mid-Atlantic Ridge. PMID:11343114

  2. Petrology and geochemistry of the ophiolitic and volcanic suites of the Taitao Peninsula — Chile triple junction area

    NASA Astrophysics Data System (ADS)

    Le Moigne, José; Lagabrielle, Yves; Whitechurch, Hubert; Girardeau, Jacques; Bourgois, Jacques; Maury, RenéC.

    1996-03-01

    This paper presents detailed petrologic and geochemical analysis of mafic and ultramafic rocks from the Taitao Peninsula, South Chile, and contributes to improve the knowledge of the Taitao ophiolite. These data allow us to identify three different units on the Taitao Peninsula, which include an ophiolitic body (peridotites, gabbros and dykes) and two Pliocene to Pleistocene volcanic-sedimentary units which accumulated as subduction of the Chile ridge occurred. The petrologic, mineralogical and geochemical data strongly suggest an oceanic origin of the peridotites and the gabbros, possibly at an active spreading center; harzburgites and CPX-bearing harzburgites can be considered as fragments of obducted mantle, whereas werhlites could represent a different magma source, enriched in K, Ti, Na and water, that impregnated the peridotites in a later stage. The two volcanic-sedimentary units, termed the Main Volcanic Unit (MVU) and the Chile Margin Unit (CMU) are geochemically unconnected with the ophiolite. They range in composition from basaltic to dacitic and can be subdivided into three groups: (i) a group with MORB affinities (ii) a group with calc-alkaline affinities (iii) a group with intermediate characteristics. MVU exhibits greenschist facies mineral assemblages whereas CMU shows assemblages ranging from zeolite facies to very low temperature conditions. We suggest that these different metamorphic overprints are a consequence of the collision of two different ridge segments with the South-America margin in front of the Taitao Peninsula.

  3. The Central Sudetes Rheic Ocean Ophiolites: Quantifying the spatial and temporal extent of the Indian Ocean-Dupal mantle signature

    NASA Astrophysics Data System (ADS)

    band, Ade R.; Barry, Tiffany; Murphy, Brendan; Saunders, Andrew

    2014-05-01

    The Indian Ocean-Dupal (Dupal herein) mantle signature is documented to be an integral component in the formation of the Carboniferous Palaeo-Tethys ophiolites. Spatially associated with the Palaeo-Tethys Ocean, the Rheic Ocean separated Laurussia and Gondwana after its conception during the latest Cambrian until closure during the Devonian-Carboniferous Variscan-Alleghanian orogeny. The age and location of Rheic Ocean ophiolites preserved within the Variscide belt offers an insight into the spatial and temporal extent, and also the origin of the Dupal mantle signature. The Central Sudetes Ophiolites consist of the low metamorphic grade, partially dismembered Nowa Ruda, Braszowice and Ślęża mafic/ultramafic bodies of Lower Silesia, Poland. Geochemical analysis of extrusive and hypabyssal lithologies indicate that despite the majority of samples exhibiting characteristics compatible with formation within a SSZ setting (Th and LILE enrichment, Ta and Nb depletion), a significant MORB suite is also present. MORB offer an opportunity to isotopically fingerprint the underlying mantle source region, thus offering an insight into the chemistry of the southern hemisphere mantle at c. 420-400 Ma. Utilising the robust Hf-Nd systematics this study has succeeded in documenting the mantle domain from which the eastern Rheic Ocean was sourced whilst simultaneously constraining the timing and nature of a previous melt extraction event.

  4. Origin and serpentinization of ultramafic rocks of Manipur Ophiolite Complex in the Indo-Myanmar subduction zone, Northeast India

    NASA Astrophysics Data System (ADS)

    Ningthoujam, P. S.; Dubey, C. S.; Guillot, S.; Fagion, A.-S.; Shukla, D. P.

    2012-05-01

    The Manipur Ophiolite Complex (MOC) is part of the Manipur-Nagaland ophiolite belt (MNOB). The belt is exposed in the eastern margin of the Indo-Myanmar Ranges (IMRs), which formed by the collision between the India and Myanmar continental plates. Several contrasting views were put forward concerning the origin of the MNOB. The complex represents a dismembered ophiolite sequence with serpentinite as the largest litho-unit formed. Petrography and Raman spectroscopy of the serpentinite suggest that they are serpentinized ultramafic cumulate and peridotite. The serpentinization may have occurred at a condition of low pressure and low temperature metamorphism. Geochemical signatures of the rocks and spinel grains revealed that the protolith be an abyssal peridotite, derived from a less depleted fertile mantle melt at a MORB setting after low degree (10-15%) partial melting. The study concluded that the serpentinite may have been created at a slow-spreading ridge, rather than a supra-subduction-zone setting. These rocks were later obducted and incorporated into the IMR of Indo-Myanmar suture zone.

  5. Modeling Enhanced In Situ CO2 Mineralization in the Samail Ophiolite Aquifer

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    The Samail Ophiolite aquifer in the Sultanate of Oman is a site of exceptionally well-developed naturally occurring in situ CO2 mineralization, and serves as a natural analog for an engineered CO2 sequestration process. Natural processes within the aquifer can be described by the following reactions [e.g.1,2]: near the surface, infiltrating rainwater dissolves peridotite, increasing dissolved Mg, Ca, and Si; interaction with soil CO2 and carbonate rocks and dust further increases Ca and dissolved C. At deeper levels, groundwater is cut off from the atmosphere-and hence its CO2 source- but continues to dissolve peridotite, and precipitates serpentine, magnesite, and dolomite. The resulting water has a high Ca-OH concentration, essentially no Mg or dissolved C, and ultrabasic pH. When this alkaline water reaches the shallow subsurface or surface, it mixes with CO2-saturated shallow groundwater or absorbs CO2 directly from the atmosphere. Dissolved C reacts with Ca to precipitate calcite on the surface, lowering the pH to basic. This process forms abundant carbonate minerals, both in the subsurface and in surficial travertine terraces. Water chemistry data can be used to determine the amount of CO2 sequestered. The quantity of CO2 mineralized at the surface as CaCO3 can be calculated from the removal of Ca from alkaline water once it discharges at springs, assuming CaCO3 precipitation is the only surficial Ca sink. Water samples from 22 alkaline spring outlets and 16 surface water bodies were used to calculate the average decrease in Ca and increase in TIC as alkaline spring water discharges and flows along the surface, losing its high pH and converting to basic surface water; the values are 1.26 mmol/L Ca and 3.13 mmol/L TIC, respectively. The increase in TIC can be attributed to absorption of atmospheric CO2. In regions with known flow rates, it is possible to determine the total amount of CO2 mineralized annually. For example, near Masibt where the flow rate of a single spring is 3x107 L/yr, the annual loss of Ca is 3.8x104 moles/yr and the amount of CO2 mineralized as CaCO3 by that spring is 0.85 kg/yr. Over 70 alkaline springs have been mapped throughout the Samail Ophiolite3, and doubtless many more exist. At the surface, Ca availability limits carbonate mineral formation; however, in the subsurface, dissolved CO2 must be the limiting species. TIC decreases from 3.24 mmol/L in shallow groundwater to 0.27 mmol/L in alkaline springs. The loss of 2.96 mmol/L TIC likely occurs by magnesite precipitation, meaning that this amount of CO2 is mineralized in the subsurface. If the availability of dissolved CO2 is the limiting factor in mineralization by the Samail Ophiolite aquifer, it may be possible to engineer the system to increase the rate of sequestration by injecting CO2 into the aquifer. To simulate the outcome of such an engineered system, data from the natural system have been incorporated into a reactive transport model. Results of this simulation will be presented. 1Barnes and O’Neil, 1969; 2Bruni et al., 2002; 3Stanger, 1986

  6. Seismic wave velocity of rocks in the Oman ophiolite: constraints for petrological structure of oceanic crust

    NASA Astrophysics Data System (ADS)

    Saito, S.; Ishikawa, M.; Shibata, S.; Akizuki, R.; Arima, M.; Tatsumi, Y.; Arai, S.

    2010-12-01

    Evaluation of rock velocities and comparison with velocity profiles defined by seismic refraction experiments are a crucial approach for understanding the petrological structure of the crust. In this study, we calculated the seismic wave velocities of various types of rocks from the Oman ophiolite in order to constrain a petrological structure of the oceanic crust. Christensen & Smewing (1981, JGR) have reported experimental elastic velocities of rocks from the Oman ophiolite under oceanic crust-mantle conditions (6-430 MPa). However, in their relatively low-pressure experiments, internal pore-spaces might affect the velocity and resulted in lower values than the intrinsic velocity of sample. In this study we calculated the velocities of samples based on their modal proportions and chemical compositions of mineral constituents. Our calculated velocities represent the ‘pore-space-free’ intrinsic velocities of the sample. We calculated seismic velocities of rocks from the Oman ophiolite including pillow lavas, dolerites, plagiogranites, gabbros and peridotites at high-pressure-temperature conditions with an Excel macro (Hacker & Avers 2004, G-cubed). The minerals used for calculations for pillow lavas, dolerites and plagiogranites were Qtz, Pl, Prh, Pmp, Chl, Ep, Act, Hbl, Cpx and Mag. Pl, Hbl, Cpx, Opx and Ol were used for the calculations for gabbros and peridotites. Assuming thermal gradient of 20° C/km and pressure gradient of 25 MPa/km, the velocities were calculated in the ranges from the atmospheric pressure (0° C) to 200 MPa (160° C). The calculation yielded P-wave velocities (Vp) of 6.5-6.7 km/s for the pillow lavas, 6.6-6.8 km/s for the dolerites, 6.1-6.3 km/s for the plagiogranites, 6.9-7.5 km/s for the gabbros and 8.1-8.2 km/s for the peridotites. On the other hand, experimental results reported by Christensen & Smewing (1981, JGR) were 4.5-5.9 km/s for the pillow lavas, 5.5-6.3 km/s for the dolerites, 6.1-6.3 km/s for the plagiogranites, 6.5-7.7 km/s for the gabbros and 6.3-7.9 km/s for the peridotites. Although the two results are broadly comparable to each other for plagiogranites and gabbros, the calculated velocities are considerably higher than the experimental ones for pillow lavas, dolerites and peridotites. The discrepancy for the pillow lavas and dolerites can be attributed to the presence of pore-spaces in the experimental samples. On the other hand, serpentinization of peridotite samples likely resulted in lower velocities in experiments than in calculation. We compared our results with Vp structure of the oceanic crust and mantle (White et al. 1992, JGR). The calculated Vp of peridotites and gabbros are comparable to those of mantle and layer-3, respectively. The calculated Vp of dolerites is comparable to layer-3 and considerably higher than layer-2 velocities. However, recent deep drilling results (Holes 504B and 1256D) indicate the seismic layer-2 of oceanic crust mainly composed of dolerites, which is consistent with the experimental P-wave velocities of dolerites (Christensen & Smewing, 1981, JGR). These results imply that the velocity structure of seismic layer-2 reflects the distribution of pore-spaces in the upper oceanic crust.

  7. Ophiolites and Continental Margins of the Mesozoic Western U.S. Cordillera

    NASA Astrophysics Data System (ADS)

    Dilek, Y.

    2001-12-01

    The Mesozoic tectonic history of the western U.S. Cordillera records evidence for multiple episodes of accretionary and collisional orogenic events and orogen-parallel strike-slip faulting. Paleozoic-Jurassic volcanic arc complexes and subduction zone assemblages extending from Mexico to Canada represent an East-Pacific magmatic arc system and an accretionary-type orogen evolved along the North American continental margin. Discontinuous exposures of Paleozoic upper mantle rocks and ophiolitic units structurally beneath this magmatic arc system are remnants of the Panthalassan oceanic lithosphere, which was consumed beneath the North American continent. Pieces of this subducted Panthalassan oceanic lithosphere that underwent high-P metamorphism are locally exposed in the Sierra Nevada foothills (e.g. Feather River Peridotite) indicating that they were subsequently (during the Jurassic) educted in an oblique convergent zone along the continental margin. This west-facing continental margin arc evolved in a broad graben system during much of the Jurassic as a result of extension in the upper plate, keeping pace with slab rollback of the east-dipping subduction zone. Lower to Middle Jurassic volcanoplutonic complexes underlain by an Upper Paleozoic-Lower Mesozoic polygenetic ophiolitic basement currently extend from Baja California-western Mexico through the Sierra-Klamath terranes to Stikinia-Intermontane Superterranes in Canada and represent an archipelago of an east-facing ensimatic arc terrane that developed west and outboard of the North American continental margin arc. The Smartville, Great Valley, and Coast Range ophiolites (S-GV-CR) in northern California are part of this ensimatic terrane and represent the island arc, arc basement, and back-arc tectonic settings, respectively. The oceanic Josephine-Rogue-Chetco-Rattlesnake-Hayfork tectonostratigraphic units in the Klamath Mountains constitute a west-facing island arc system in this ensimatic terrane as a counterpart of the east-facing S-GV-CR system to the south. The Guerrero intra-oceanic island arc system in Mexico was also part of the ensimatic arc terrane. Incorporation of this super arc terrane into the North American continent occurred diachronously along the irregular continental margin in the Middle Jurassic (in the north) through Early Cretaceous (in the south) during an arc-continent collision, marking a collisional orogenic episode in the North American Cordilleran history. Rifting of this accreted arc in the Late Jurassic (155-148 Ma) might have resulted from a sinistral transtensional deformation associated with the rapid NW motion of North America. Magmas generated during this rifting event probably migrated through the accreted arc crust and the continental margin units in the tectonic lower plate. The Franciscan subduction zone dipping eastwards beneath the continent was established in the latest Jurassic, following the collisional event and restoring the North American Cordillera back into an accretionary-type, Andean-style orogen. Different episodes of orogen-parallel intra-continental strike-slip faulting facilitated lateral dispersion of accreted terranes and continental margin units during the Early Cretaceous and transpressional deformation and batholithic magmatism in the Sierra Nevada magmatic arc in the Late Cretaceous. A Jurassic-Cretaceous island arc system (Wrangellia-Insular Superterrane) that had developed west of the Jurassic archipelago collapsed into the edge of North America during Late Cretaceous-Tertiary time and underwent northward lateral translation along the continental margin. These observations and interpretations have strong implications for the tectonic evolution of Central America and the Caribbean region.

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

    NASA Astrophysics Data System (ADS)

    Batanova, Valentina; Savelieva, Galina

    2010-05-01

    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.

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

    NASA Astrophysics Data System (ADS)

    Sonzogni, Y.; Treiman, A. H.

    2012-12-01

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    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.

  11. Proprietes des melanges de poudres d'aciers inoxydables 316L/h-BN et 409L/h-BN frittes sous differentes atmospheres (hydrogen-nitrogen)

    NASA Astrophysics Data System (ADS)

    Mercier, Thierry

    L'influence de la composition d'atmospheres de frittage "hydrogene---azote" sur les proprietes de specimens produits par metallurgie des poudres a partir de melanges de poudres d'aciers inoxydables 316L et 409L et de poudre de nitrure de bore hexagonal (h-BN) a ete etudiee. Ces proprietes ont ete comparees a celles de specimens sans h-BN frittes simultanement dans la meme atmosphere. Une mince couche dense s'est formee a la surface des echantillons contenant du h-BN frittes dans une atmosphere contenant au moins 50% en volume d'hydrogene. Cette couche dense ameliore generalement les proprietes de l'alliage 316L. Les proprietes de l'alliage 409L montrent une amelioration beaucoup moins significative qui s'amenuise lorsque l'atmosphere s'appauvrit en azote. L'azote favorise la corrosion de specimens de 409L avec ou sans h-BN. Quant au 316L, il demeure resistant a la corrosion malgre la presence d'un peu d'azote au frittage et l'addition de h-BN lui est benefique particulierement lorsque fritte en presence d'une teneur elevee d'azote (jusqu'a 50%vol. N2) qui permet tout de meme la formation d'une couche dense en surface.

  12. Zircon ages and geochemical compositions of the Manlay ophiolite and coeval island arc: Implications for the tectonic evolution of South Mongolia

    NASA Astrophysics Data System (ADS)

    Zhu, Mingshuai; Baatar, Munkhtsengel; Miao, Laicheng; Anaad, Chimedtseren; Zhang, Fochin; Yang, Shunhu; Li, Yueming

    2014-12-01

    Numerous small dismembered ophiolite fragments occur in South Mongolia, but they are very poorly studied. The lack of age data and geochemical analysis hampers our understanding of the Paleozoic tectonic evolution of the region. We conducted detailed studies on the Manlay ophiolitic complex and Huree volcanic rocks south of the Main Mongolian Lineament (MML) to provide some constraints on these rocks. The Manlay ophiolite consists of dunite, harzburgite, pyroxenite, gabbro, plagiogranite, basalt and chert, locally with chromite mineralization in dunite. The gabbro and plagiogranite yielded SHRIMP zircon weighted mean 206Pb/238U ages of 509 ± 5 Ma and 482 ± 4 Ma, respectively. The basalt and dolerite samples of this complex show enrichment in LREE and LILE and negative Nb, Ta and Ti anomalies, and the chrome spinel from the chromitite lens in the dunite is characterized by high Cr# and low TiO2 contents. These features suggest a supra-subduction zone (SSZ) origin for the ophiolitic complex. The Huree volcanic rocks, ranging from basalt to dacite, display enrichment in LREE and LILE, weak Eu anomalies and distinctly negative Nb, Ta and Ti anomalies, consistent with those of typical magmas in a subduction environment. An andesite sample from this arc yielded a SHRIMP 206Pb/238U zircon age of 487 ± 5 Ma, which is the oldest reliable age for an island arc in South Mongolia. Recognition of an Early Paleozoic ophiolitic complex and a coeval island arc indicates that South Mongolia underwent a period of active volcanism during Late Cambrian to Ordovician. Additionally, the tuff overlying the ophiolitic complex and a granite intruding the ophiolite have SHRIMP zircon U-Pb ages of 391 ± 5 Ma and 304 ± 4 Ma, respectively. Combining the available data, we propose that the Early Paleozoic subduction-accretionary complexes likely constitute the basement of the Late-Paleozoic arc formations and correlate with the Lake Zone in western Mongolia.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    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.

  14. Archaean Greenstone Belt Architecture and Stratigraphy: are Comparisons With Ophiolites and Oceanic Plateaux Valid?

    NASA Astrophysics Data System (ADS)

    Bedard, J. H.; Bleeker, W.; Leclerc, F.

    2005-12-01

    Archaean greenstone belts and coeval plutonic belts (dominated by TTGs, tonalite-tronhjemite-granodiorite), are commonly interpreted to represent assembled fragments of oceanic crust, oceanic plateaux or juvenile arc terranes, variably reworked by Archaean orogenic processes related to the operation of plate tectonics. However, many of the lava successions that have been interpreted to represent accreted oceanic plateaux are demonstrably ensialic, can be correlated over long distances along-strike, have depositional contacts onto older continental crustal rocks, show tholeiitic to calc-alkaline cyclicity, and have isotopic signatures indicating assimilation of older felsic crust. Inferred Archaean ophiolites do not have sheeted dyke complexes or associated mantle rocks, and cannot be proven to be oceanic terranes formed by seafloor-spreading. Archaean supracrustal sequences are typically dominated by tholeiitic to komatiitic lavas, typically interpreted to represent the products of decompression melting of mantle plumes. Subordinate proportions of andesites, dacites and rhyolites also occur, and these, together with the coeval TTGs, are generally interpreted to represent arc magmas. In the context of uniformitarian interpretations, the coeval emplacement of putative arc- and plume-related magmas requires extremely complex geodynamic scenarios. However, the relative rarity of the archetypal convergent margin magma type (andesite) in Archaean sequences, and the absence of Archaean blueschists, ultra-high-pressure terranes, thrust and fold belts, core complexes and ophiolites, along with theoretical arguments against Archaean subduction, together imply that Archaean cratonic crust was not formed through uniformitarian plate-tectonic processes. A simpler interpretation involves soft intraoceanic collisions of thick (30-50km), plume-related, basaltic-komatiitic oceanic plateaux, with ongoing mafic magmatism leading to anatexis of the hydrated plateau base to generate TTGs, and eruption to form the platformal greenstone successions. The negative Nb-Ta-Ti anomalies and steep REE profiles of TTGs would result from the presence of residual garnet and rutile, and do not require the existence of subduction zones. Alternative models involving vertical tectonic processes, lower crustal restite delamination and multi-stage catalytic mantle and crustal melting are preferred.

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

    NASA Astrophysics Data System (ADS)

    Evans, Cynthia; Hawkins, James W.

    1989-10-01

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    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.

  17. The Xigaze ophiolite, southern Tibet: a fossil oceanic forearc generated during subduction initiation

    NASA Astrophysics Data System (ADS)

    Dai, J.; Wang, C.; Zhu, D. C.; Li, Y.

    2014-12-01

    The Xigaze ophiolite is located at the central Yarlung Zangbo Suture Zone, southern Tibet representing the remnants of the Neo-Tethyan oceanic lithosphere. It displays well preserved sections from mantle to crustal rocks, together with overlying marine sediments. The Xigaze peridotites have low CaO and Al2O3 contents and U-shaped REE patterns, suggesting that they are residues after moderate to high degrees of partial melting and were modified by infiltration of LREE-enriched boninitic melts. Two of five massifs from the Xigaze pillow lavas display tholeiitic affinity, while other three massifs show calcalkalic characteristics. The latter lavas show more HFSE-depleted and LILE-enriched compositions than those of the former. The latest diabase dikes possess high SiO2 and low TiO2 contents and show flat to LREE enrichment patterns, indicating they are analogue with boninitic rocks. Tholeiitic lavas were formed via decompression melting of a fertile lherzolitic source, whereas calcalkalic lavas and latest stage dikes were generated by melting of residual harzburgite and were influenced by slab-derived fluids. Both mantle and crustal rocks were invaded by plagiogranite dikes. The first plagiogranite group invaded into the crustal rocks lies on a continuation of the chemical trends defined by the mafic rocks and show similar REE patterns with the lavas, indicating they were generated by shallow fractional crystallisation. The second plagiogranite group invaded into the mantle displays different trends on Harker and REE patterns with diabase dike and enclave, but show uniform LREE depletion similar with mafic rocks, suggesting that they might be the product of subducted mafic slabs melting. LA-ICPMS zircon U-Pb and Lu-Hf analyses from the second plagiogranites reveal that they were generated between 125-122 Ma, indistinguishable from those of mafic rocks. The zircons possess positive ?Hf(t) values ranging from +12.3 to +16.4, suggesting they were derived from depleted source. Taking into account the geological and geochronological characteristics of the Xigaze ophiolite, we propose they were formed in a forearc setting through rapid slab rollback during subduction initiation between 127-122 Ma.

  18. Rare-earth element geochemistry of the Samail Ophiolite near Ibra, Oman

    NASA Astrophysics Data System (ADS)

    Pallister, John S.; Knight, Roy J.

    1981-04-01

    Rare-earth element (REE) analyses of 68 rock and mineral separate samples from the Samail ophiolite clearly differentiate the various units of the ophiolite suite and indicate that the crustal suite is cogenetic, produced by crystal fractionation of basaltic magma in a spreading ridge magma chamber. Mantle peridotites are residual in rare-earth character, but cannot be clearly related to the overlying mafic rocks. Chromian spinel is probably insignificant in its effect on REE distribution during partial melting and crystal fractionation, as indicated by the low REE concentrations in chromitite from the Samail. Layered gabbro REE patterns are dominated by cumulus clinopyroxene and plagioclase. Large positive Eu anomalies demonstrate plagioclase accumulation. Modal mixing (mass balance) calculations reveal that most of these adcumulus gabbros have REE patterns that are the products of the REE concentrations of their constituent cumulus phases in the observed modal proportions; hence no appreciable REE-rich mesostasis is present. Such calculations also allow the prediction of mineral REE concentrations not actually determined by mineral separate analyses. Several high-level (noncumulus?) gabbros yield patterns with positive Eu anomalies suggesting relative plagioclase accumulation, probably due to liquid fractionation (filter pressing). Dike complex REE patterns show light rare-earth element depletions and are similar to, but not necessarily diagnostic of, midocean ridge basalt. They cluster at similar abundance levels; however, the absolute variation in abundance is large, indicative of modification by crystal fractionation. Calculations using partition coefficient data indicate that many dikes represent liquids that could have existed in equilibrium with cumulus minerals of the plutonic suite. Mineral separate REE data from layered cumulus gabbro of the Khafifah stratigraphic section reveal cryptic variation trends that are correlated with major element variation, in support of a long-lived (periodically replenished) magma chamber model (Pallister and Hopson, 1978, 1979, 1981). (Cryptic variation as used herein is defined as the change in mineral composition with respect to stratigraphic position.) The REE cryptic variation shows both direct and inverse correlation to major element variation, indicating that magmatic replenishment was complicated by changes in parent melt REE abundance. A crystal fractionation origin for the small plagiogranite bodies of the Ibra area is favored by REE modeling, although larger bodies (Dasir) may also be related to magmatic inclusion of roofrock with diabase level REE distribution (Gregory and Taylor, 1979).

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

    USGS Publications Warehouse

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

    1998-01-01

    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.

  20. Supra-subduction zone tectonic setting of the Muslim Bagh Ophiolite, northwestern Pakistan: Insights from geochemistry and petrology

    NASA Astrophysics Data System (ADS)

    Kakar, Mohammad Ishaq; Kerr, Andrew C.; Mahmood, Khalid; Collins, Alan S.; Khan, Mehrab; McDonald, Iain

    2014-08-01

    The geology of the Muslim Bagh area comprises the Indian passive continental margin and suture zone, which is overlain by the Muslim Bagh Ophiolite, Bagh Complex and a Flysch Zone of marine-fluvial successions. The Muslim Bagh Ophiolite has a nearly-complete ophiolite stratigraphy. The mantle sequence of foliated peridotite is mainly harzburgite with minor dunite and contains podiform chromite deposits that grade upwards into transition zone dunite. The mantle rocks (harzburgite/dunite) resulted from large degrees of partial melting of lherzolite and have also been affected by melt-peridotite reaction. The Muslim Bagh crustal section has a cyclic succession of ultramafic-mafic cumulate with dunite at the base, that grades into wehrlite/pyroxenite with gabbros (olivine gabbro, norite and hornblende gabbro) at the top. The sheeted dykes are immature in nature and are rooted in crustal gabbros. The dykes are mainly metamorphosed dolerites, with minor intrusions of plagiogranites. The configuration of the crustal section indicates that the crustal rocks were formed over variable time periods, in pulses, by a low magma supply rate. The whole rock geochemistry of the gabbros, sheeted dykes and the mafic dyke swarm suggests that they formed in a supra-subduction zone tectonic setting in Neo-Tethys during the Late Cretaceous. The dykes of the mafic swarm crosscut both the ophiolite and the metamorphic sole rocks and have a less-marked subduction signature than the other mafic rocks. These dykes were possibly emplaced off-axis and can be interpreted to have been generated in the spinel peridotite stability zone i.e., < 50-60 km, and to have risen through a slab window. The Bagh Complex is an assemblage of Triassic-Cretaceous igneous and sedimentary rocks, containing tholeiitic, N-MORB-like basalts and alkali basalts with OIB-type signatures. Nb-Ta depletion in both basalt types suggests possible contamination from continental fragments incorporated into the opening Tethyan oceanic basin during break-up of Gondwana. The lithologies and ages of the Bagh Complex imply that these rocks formed in an area extending from the continental margin over the Neo-Tethyan ocean floor. The Bagh Complex was then juxtaposed with the Muslim Bagh Ophiolite in the final stage of tectonic emplacement.

  1. Relationship between the early Paleozoic SSZ ophiolite complex and North Qaidam UHP metamorphic belt in NW China

    NASA Astrophysics Data System (ADS)

    Menold, C. A.; Walsh, E. O.; Manning, C. E.; Yin, A.

    2009-12-01

    The presence of an supra-subduction zone ophiolite complex in close proximity to ultrahigh-pressure rocks in the North Qaidam of northern Tibet suggests a complex early Paleozoic collisional geometry. This newly described ophiolite complex is found in the hanging wall of a Paleozoic detachment considered to be the structure that emplaced the North Qaidam UHP rocks into the lower crust. In recent years, significant progress has been made in deciphering the Paleozoic tectonic history of northern Tibet. Accretion of the Qaidam and Qilian terranes to the Tarim and Sino-Korean cratons is known to have occurred during Silurian-Devonian time, as terrestrial Devonian and younger strata unconformably overlie all terranes and the adjacent craton. This event resulted in the emplacement of the early Paleozoic North Qilian Franciscan-type oceanic-subduction complex, ~400 km to the northeast. The presence of ophiolitic material emplaced in the North Qaidam, in contact with UHP rocks in the lower crust suggests an additional basin closure. The North Qaidam ophiolite complex was deformed and metamorphosed to epidote-amphibolite facies by a late Ordovician ductile folding event, after juxtaposition with the UHP rocks. Despite the strong fabric development, four mappable units can be readily identified: (1) a serpentinized ultramafic layer containing broad quartz-phlogopite veins, (2) vari-textured metagabbro with rare plagiogranite, (3) a sheeted dike complex which is now mafic schist, and (4) a thin and discontinuous sequence of vesicular metabasalt interlayed with metasediments and marble. Zircons were separated from the plagiogranite and analysed by SIMS to have a U-Pb age of 545.5 + 6.0 Ma (MSWD 1.1), which is interpreted to be the crystallization age. The North Qaidam ophiolite complex was determined to have supra-subduction zone affinity using major and trace element whole rock geochemistry and is envisioned to have formed in a back-arc basin setting prior to the closure of the North Qilian Ocean. This interpretation is supported by thermochronological data which places the UHP rocks in the lower crust by 477 ± 8 Ma (Ar/Ar hornblende). The exhumation timing predates final closure in the North Qilian and requires a second subduction zone in the early Paleozoic, north Tibet.

  2. Crustal Accretion at Subduction Initiation Along Izu-Bonin-Mariana Arc and the Link to SSZ Ophiolites

    NASA Astrophysics Data System (ADS)

    Ishizuka, O.; Tani, K.; Reagan, M. K.; Kanayama, K.; Umino, S.; Harigane, Y.; Sakamoto, I.

    2014-12-01

    The Izu-Bonin-Mariana (IBM) forearc preserves the earliest arc magmatic history from subduction initiation to the establishment of the arc. Recent investigations have established a bottom to top igneous stratigraphy of: 1) mantle peridotite, 2) gabbroic rocks, 3) a sheeted dyke complex, 4) basaltic pillow lavas (forearc basalts: FAB), 5) boninites and magnesian andesites, 6) tholeiites and calcalkaline arc lavas. This stratigraphy has many similarities to supra-subduction zone (SSZ) ophiolites. One of the most important common characteristics between the SSZ ophiolites and the forearc crust is the occurrence of MORB-like basaltic lavas underlying or accompanying boninites and early arc volcanic suites. A key observation from the IBM forearc is that FAB differs from nearby back-arc lavas in chemical characteristics, including a depletion in moderately incompatible elements. This indicates that FAB is not a pre-existing oceanic basement of the arc, but the first magmatic product after subduction initiation. Sheeted dikes of FAB composition imply that this magmatism was associated with seafloor spreading, possibly triggered by onset of slab sinking. Recognition of lavas with transitional geochemical characteristics between the FAB and the boninites strongly implies genetic linkage between these two magma types. The close similarity of the igneous stratigraphy of SSZ ophiolites to the IBM forearc section strongly implies a common magmatic evolutionary path, i.e., decompressional melting of a depleted MORB-type mantle is followed by melting of an even more depleted mantle with the addition of slab-derived fluid/melt to produce boninite magma. Similarity of magmatic process between IBM forearc and Tethyan ophiolites appears to be reflected on common characteristics of upper mantle section. Peridotite from both sections show more depleted characteristics compared to upper mantle rocks from mid-ocean ridges. Age determinations reveal that first magmatism at the IBM arc occurred at c. 52 Ma, and transition from forearc basalt to normal arc magmatism took 7-8 million years. Combined with the age information from SSZ-ophiolites, significant constraints on time scale of subduction initiation and associated crustal accretion might be obtained.

  3. 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)

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

    2013-12-01

    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.

  4. 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)

    Casey, J.; Dewey, J. F.

    2013-12-01

    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.

  5. 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

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

    1997-01-01

    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.

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

    USGS Publications Warehouse

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

    1982-01-01

    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.

  7. Reintrusion of silicic magma chambers by mafic dike complex: evidence from the northern Semail ophiolite

    SciTech Connect

    Stakes, D.S.; Shervais, J.; Ressetar, R.

    1985-01-01

    Late plagiogranite bodies in the Semail ophiolite have been previously suggested to represent late stage fractionates within an episodic spreading center magma chamber or the roots of seamount chains. Field and lab observations suggest that these late silicic magma chambers represent zones of repeated injection by dikes of intermediate to mafic composition. Multiple generations of intrusion, partial resorption and reintrusion are preserved in the plagiogranite as 1) relict phantom xenoliths, 2) angular xenoliths with quartz-rich margins, 3) deformed fine-grained dikes with distinct chilled margins, and 4) planes of rectangular blocks with cuspate margins or ellipsoids of similar fine grained mafic materials. The blocks and ellipsoids are actually dismembered mafic dikes that chilled by intruding a cooler silicic liquid and were either thermally fractured or pinched out. All of the dikes are hydrothermally altered to assemblages including amph., qtz., epi., preh., and chl. and are enriched in delta/sup 18/O. Extremely altered diabase from a copper sulfide-bearing normal fault is isotopically depleted (delta/sup 18/0=2.0 per mil) suggesting that such deep faults are high temperature hydrothermal conduits. Malachite and amphibole bearing veins along the margins of the plagiogranite suggest a genetic relationship between the silicic intrusions, the multiple diking events and copper sulfide deposition.

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

    NASA Astrophysics Data System (ADS)

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

    1986-07-01

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2012-06-01

    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.

  10. Oman Ophiolite Structural Constraints Complement Models of Crustal Accretion at the EAST Pacific RISE

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    This review documents significant similarities between East Pacific Rise (EPR), especially EPR at 9°-10°N and the Oman ophiolites. Both share comparable fast spreading rates, size and their dominant source of information that is mainly geophysical in EPR and structural in Oman. In these respects, they are remarkably complementary. Mantle upwelling zones at the EPR and mantle diapirs in Oman have a similar size and spacing. They punctually introduce basaltic melt and heat in the accreting crust, thus controlling elementary segments structure and activity. A tent-shaped magma chamber fits onto the diapir head, the top of which is a Mantle Transition Zone (MTZ) that stores, modifies, and injects the modified melt into the upper Axial Melt Lens (AML) beneath the lid. This MTZ-AML connection is central in crustal accretion, as documented in Oman. Heat from the diapir is captured above the Moho by the magma chamber and escapes through its walls, into a thin thermal boundary layer that bounds the chamber. Beyond, seawater at lower temperatures feeds smokers on the seafloor.

  11. Olivine fabric evolution in a hydrated ductile shear zone at the Moho Transition Zone, Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Michibayashi, Katsuyoshi; Oohara, Tatsuya

    2013-09-01

    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?°C) outside of the high strain zone compared with inside this zone (˜750?°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.

  12. Geochemical consequences of flow differentiation in a multiple injection dike (Trinity ophiolite, N. California)

    USGS Publications Warehouse

    Brouxel, M.

    1991-01-01

    A clinopyroxene-rich dike of the Trinity ophiolite sheeted-dike complex shows three different magmatic pulses, probably injected in a short period of time (no well developed chilled margin) and important variations of the clinopyroxene and plagioclase percentages between its core (highly porphyritic) and margins (aphyric). This variation, interpreted as related to a flow differentiation phenomenon (mechanical phenocryst redistribution), has important geochemical consequences. It produces increases in the FeO, MgO, CaO, Cr and Ni contents from the margin to the core, together with increases in the clinopyroxene percentage, and decreases in the SiO2, Zr, Y, Nb and REE contents together with a decrease in the percentage of the fine-grained groundmass toward the core of the dike. This mineralogical redistribution, which also affects the incompatible trace element ratios because of the difference in plagioclase and clinopyroxene mineral/liquid partition coefficients, illustrate the importance of fractionation processes outside of a magma chamber. ?? 1991.

  13. Geochemistry and origin of the ophiolite hosted magnesite deposit at Derakht-Senjed, NE Iran

    NASA Astrophysics Data System (ADS)

    Mirnejad, Hassan; Aminzadeh, Mahrokh; Ebner, Fritz; Unterweissacher, Thomas

    2015-12-01

    The Derakht-Senjed magnesite deposit, hosted by Torbat-e-Heydarieh ophiolite in NE Iran, is developed as veins, veinlets and stockwork type mineralization. While the veins and veinlets only contain magnesite, the stockwork mineralization in addition contains sparry dolomite interlayered with magnesite. Magnesite and dolomite are both poor in FeO and SiO2. The carbon and oxygen isotope compositions of magnesite (δ13CV-PDB = -3.9 ± 0.1 to -5.0 ± 0.1‰; δ18OV-SMOW = +25.2 ± 0.1 to +26.5 ± 0.1 ‰) can be explained by contribution of atmospheric CO2 and/or an involvement by organic carbon. Dolomite typically shows slightly lower values of δ13C V-PDB -5.2 ± 0.1 to -5.5 ± 0.1‰ and δ18OV-SMOW + 23.8 ± 0.1 to +24.8 ± 0.1‰ compared to the magnesite. The formation of magnesite at Derakht-Senjed was structurally controlled by a fracture network in the ultramafic host rocks, which provided suitable fluid pathways for leaching of Mg from the host rocks and subsequent precipitation of magnesite from carbonated solutions. It is likely that dolomite formed due to precipitation from a fluid having lower XCO2 and higher Ca2+/Mg2+ activity ratio, rather than by replacement of magnesite.

  14. The crustal section of the Siniktanneyak Mountain ophiolite, Brooks Range, Alaska

    SciTech Connect

    Bickerstaff, D.; Harris, R.A.; Miller, M.A. . Dept. of Geology and Geography)

    1993-04-01

    Fragments of the upper crustal section of the Brooks Range Ophiolite on the west flank of Siniktanneyak Mountain expose important contact relations and paleohorizontal indicators. The nearly complete crustal sequence faces northwest. Based on field observations, the crustal units encountered at Siniktanneyak Mountain from bottom to top are: (1) layered gabbro, (2) isotropic gabbro, (3) high level and late-stage intrusions of diorite and diabase, (4) rare sheeted dikes, (5) basalt, and (6) a bedded volcanic tuff. Potassium feldspar-bearing pegmatites are also found. Of particular interest is the orientation of the layered gabbro, sheeted dikes, and the bedded volcanic tuff. The steeply dipping gabbro layers strike N-S, the adjacent vertical sheeted dikes strike NE-SW. Bedded volcanic tuff and lavas are flat lying. Contacts within the upper crust units are often covered by talus. Contacts between various plutonic rocks are both sharp and gradational, suggesting syn- and post-cooling intrusions. Contacts between plutonic rock and higher volcanic rock appear to be fault contacts.

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

    PubMed Central

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

    2009-01-01

    The deepest rocks known from within Earth are fragments of normal mantle (≈400 km) and metamorphosed sediments (≈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 TiO2, 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 Fe2+ at very high pressure and consequent low oxygen fugacity (fO2) 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-fO2 environment at the very high temperatures of oceanic spreading centers. PMID:19880742

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    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.

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

    USGS Publications Warehouse

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

    2002-01-01

    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.

  18. Establishment of the Coast Range ophiolite microbial observatory (CROMO): drilling objectives and preliminary outcomes

    NASA Astrophysics Data System (ADS)

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

    2013-11-01

    This project aimed to establish a subsurface microbial observatory in ultramafic rocks, by drilling into an actively serpentinizing peridotite body, characterizing cored rocks, and outfitting the boreholes for a program of long-term observation and experimentation to resolve the serpentinite-hosted subsurface biosphere. We completed drilling in August 2011, drilling two boreholes with core recovery and possibility for down-hole experimentation, and six smaller-diameter monitoring wells arrayed around the two primary holes, in the Coast Range ophiolite (CRO) locality in the UC-Davis McLaughlin Natural Reserve, Lower Lake, CA. Every effort was made during drilling to keep the cores and wells as free of drilling-induced contamination as possible: clean, purified water was used as drilling fluid, fluorescent microbead tracers were suspended in that water for quantification of drilling fluid penetration into the cores, and high resolution next generation sequencing approaches were used to characterize the microbial populations in the drill fluids and core materials. In December 2011, we completed installation of well pumps (slow flow bladder pumps) in the monitoring wells, and have deployed a set of in situ incubation experiments in the two uncased boreholes. Preliminary findings illustrate natural variability in actively serpentinizing strata, and confirm distinct groundwater flow regimes and microbial ecosystems in (a) shallow, surface-impacted soil water horizons and (b) deeper, ultramafic bedrock-sourced formation fluids.

  19. One-carbon (bio ?) Geochemistry in Subsurface Waters of the Serpentinizing Coast Range Ophiolite

    NASA Technical Reports Server (NTRS)

    Hoehler, Tori M.; Mccollom, Tom; Schrenk, Matt; Cardace, Dawn

    2011-01-01

    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.

  20. 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)

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

    2014-11-01

    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.

  1. Microbial Metabolic Landscapes Derived from Complementary Mineralogical, Aqueous Geochemical, and Gas Data Associated with High pH, Actively Serpentinizing Springs in the Coast Range Ophiolite (CA,USA) and Zambales and Palawan Ophiolites (Philippines)

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    We applied x-ray diffraction and thin section petrography to profile the mineralogy of serpentinites and relict peridotites pertinent to the Coast Range Ophiolite Microbiological Observatory (CROMO, an array of 8 water monitoring wells installed in serpentinizing ultramafic rocks, sited at the UC-Davis McLaughlin Natural Reserve, Lower Lake, CA) and Zambales and Palawan ophiolites in the Philippines. In general, serpentinization in near surface samples was extensive, obscuring many protolith characteristics, but relict olivine grains are apparent. Upwelling serpentinizing formation fluids react to varying degrees with shallow hydrological regimes impacted by meteoric inputs. In the vicinity of CROMO, modest pH (7 to 8.5) waters form spring deposits. In the Philippines ophiolites, high pH (10.8 to 11. 3) waters form extensive travertines near Manleluag Springs and newly faulted sections of the Poon Bato River. Travertine fabric and chemistry indicate episodic spring flow and suggest that ambient water chemistry shifts over time. A multiprobe meter simultaneously measured pH, temperature, conductivity, oxidation-reduction potential, and dissolved oxygen at selected springs. Filtered water samples from monitoring wells and springs were analyzed for major elements and some ions. Dissolved gases and gas bubbles were captured and transported for analysis of H2, CO, and CH4. Aqueous and gas geochemistry data were transformed into activity data using EQ3: A Computer Program for Geochemical Aqueous Speciation-Solubility Calculations (Wolery, 1992) and the Gibbs Energy values for selected metabolic reactions, given the environmental conditions, were calculated. Metabolisms considered were: methanogenesis, methane oxidation, ferric iron reduction, ferrous iron oxidation, oxidation of S in pyrite, nitrification, denitrification, and N-fixation. At all sites tapping waters sourced in actively serpentinizing systems, regardless of geography, ferrous iron oxidation was the most strongly favored of the modeled reactions, while methanogenesis and nitrification reactions were not thermodynamically feasible, under the modeled conditions. Indeed, the lack of favorable biological methanogenesis is consistent with microbiological studies at CROMO. Also, concentrations of aqueous ferrous iron may be extremely low at high pH, however, causing the amount of energy to be gained from the reaction to be very small. Taken together, data from CROMO formation fluids and deeply sourced springs at the Philippines Ophiolite sites describe a similar landscape of favorable microbial metabolisms that could be extended to other serpentinizing ecosystems. The recurrent problem of a disrupted nitrogen cycle in these ultramafic-hosted springs is discussed.

  2. 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)

    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

    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.

  3. The Vardar zone as a suture for the Mirdita ophiolites, Albania: Constraints from the structural analysis of the Korabi-Pelagonia zone

    NASA Astrophysics Data System (ADS)

    Tremblay, Alain; Meshi, Avni; Deschamps, Thomas; Goulet, François; Goulet, Normand

    2015-02-01

    The Dinarides-Hellenides result from underthrusting of the Adriatic margin during Africa-Europe convergence. In Albania, they consist of (1) a western zone of nappes derived from Adria; (2) a central belt made up of the Mirdita ophiolites; and (3) an eastern zone, the Korabi-Pelagonia zone, of Variscan basement overlain by Permian to Mesozoic rift deposits and carbonates. Some authors interpret the Korabi-Pelagonia zone as a microcontinent between the Mirdita-Pindos oceanic basin to the west and the eastern Vardar oceanic basin to the east; other regard the Korabi-Pelagonia zone as a tectonic window below a single ophiolitic nappe. This contribution argues for a far-traveled thrust sheet. The Mirdita ophiolites are 165-160 Ma. The metamorphic sole yielded 40Ar/39Ar ages of 171 to 162 Ma. The Korabi-Pelagonia zone is subdivided into the Korabi and Gjegjan subzones. The structural analysis of these rocks supports the rooting of the Mirdita ophiolites in the Western Vardar zone. The post-Variscan cover sequence of the Korabi subzone records two phases of deformation: D1 is associated with a SE dipping to flat-lying schistosity axial planar to NW verging folds and thrust faults, related to ophiolite obduction; D2 is a postobduction NNE trending crenulation cleavage. Published zircon fission track analyses yielded 150-125 Ma, suggesting that regional metamorphism is Early Cretaceous or older. K-Ar mica ages from correlative rocks of Macedonia cluster between 148 and 130 Ma, indicating that D1 is Late Jurassic. A west directed obduction is favored, as is a rooting east of the Mirdita ophiolites because of the top-to-the-west structural polarity of obduction-related deformation.

  4. Tracking the Oman Ophiolite to the surface - New fission track and (U-Th)/He data from the Aswad and Khor Fakkan Blocks, United Arab Emirates

    NASA Astrophysics Data System (ADS)

    Jacobs, Joachim; Thomas, Robert J.; Ksienzyk, Anna K.; Dunkl, István

    2015-03-01

    The Oman Ophiolite in the United Arab Emirates (UAE) was formed in a supra-subduction zone environment at about 95 Ma and was almost immediately obducted onto the eastern margin of Arabia. The timing of obduction is well constrained, but the post-obduction tectonic, uplift and exhumation history of the ophiolite and associated rocks are less well understood. We present twenty-one new fission track and (U-Th)/He analyses of apatite and zircon from the Hajar Mountains. The data show that the Oman Ophiolite had a complex exhumation history to present exposure levels in the Khor Fakkan and Aswad Blocks, resulting from at least three distinct exhumation events: 1) initial ophiolite obduction between ca. 93 and 83 Ma is characterised by tectonic exhumation and rapid cooling, as revealed by zircon (U-Th)/He and apatite fission-track data, but it is not associated with major erosional exhumation; 2) data from the lower part of the ophiolite and the metamorphic sole document a second exhumation event at ca. 45-35 Ma, interpreted to represent an early phase of the Zagros orogeny that led to reactivation of pre-existing structures and the differential exhumation of the Khor Fakkan Block along the Wadi Ham Shear Zone. This event led to significant erosional exhumation and deposition of a thick sedimentary succession in the Ras Al Khaimah foreland basin; and 3) Neogene exhumation is recorded by ca. 20-15 Ma apatite (U-Th)/He data and a single apatite fission track date from the lowermost part of the metamorphic sole. This event can be linked to the main phase of the Zagros orogeny, which is manifested in large fans with ophiolite-derived debris (Barzaman Formation conglomerates). During this period, the metamorphic sole of the Masafi window stayed at temperatures in excess of ca. 120 °C, corresponding to ca. 4 km of overburden, only later to be eroded to present day levels.

  5. Ophiolites in the Xing'an-Inner Mongolia accretionary belt of the CAOB: Implications for two cycles of seafloor spreading and accretionary orogenic events

    NASA Astrophysics Data System (ADS)

    Song, Shuguang; Wang, Ming-Ming; Xu, Xin; Wang, Chao; Niu, Yaoling; Allen, Mark B.; Su, Li

    2015-10-01

    The Xing'an-Inner Mongolia accretionary belt in the southeastern segment of the Central Asian Orogenic Belt (CAOB) was produced by the long-lived subduction and eventual closure of the Paleo-Asian Ocean and by the convergence between the North China Craton and the Mongolian microcontinent. Two ophiolite belts have been recognized: the northern Erenhot-Hegenshan-Xi-Ujimqin ophiolite belt and the southern Solonker-Linxi ophiolite belt. Most basalts in the northern ophiolite belt exhibit characteristics of normal-type to enriched-type mid-ocean ridge basalt affinities with depleted Nd isotopic composition (ɛNd(t) > +5), comparable to modern Eastern Pacific mid-ocean ridge basalts. Most basaltic rocks in the southern belt show clear geochemical features of suprasubduction zone-type oceanic crust, probably formed in an arc/back-arc environment. The inferred back-arc extension along the Solonker-Linxi belt started at circa 280 Ma. Statistics of all the available age data for the ophiolites indicates two cycles of seafloor spreading/subduction, which gave rise to two main epochs of magmatic activity at 500-410 Ma and 360-220 Ma, respectively, with a gap of ~50 million years (Myr). The spatial and temporal distribution of the ophiolites and concurrent igneous rocks favor bilateral subduction toward the two continental margins in the convergence history, with final collision at ~230-220 Ma. In the whole belt, signals of continental collision and Himalayan-style mountain building are lacking. We thus conclude that the Xing'an-Inner Mongolia segment of the CAOB experienced two cycles of seafloor subduction, back-arc extension, and final "Appalachian-type" soft collision.

  6. Pull-apart formation and strike-slip partitioning in an obliquely divergent setting, Leka Ophiolite, Norway

    NASA Astrophysics Data System (ADS)

    Titus, S. J.; Fossen, H.; Pedersen, R. B.; Vigneresse, J. L.; Tikoff, B.

    2002-08-01

    The Leka Ophiolite Complex (LOC) is located on the island of Leka, Norway, and belongs to the Uppermost Allochthon of the Scandinavian Caledonides. The rocks of the adjacent mainland and most of the surrounding islands are basement gneisses and supracrustal rocks not related to the ophiolite complex. Paleostress analysis, gravity inversion, and regional geology support a fault-bounded rhombochasm geometry for the LOC. The paleostress inversions revealed two types of tensors, interpreted as small strains: (1) horizontal extension, generally E-W to NE-SW, and (2) horizontal extension in the same direction with an added component of perpendicular horizontal contraction. A strong positive gravity anomaly (25 mGal) is centered on Leka, and gravity inversion indicates that the LOC lies directly below its surface exposures with steep-sided walls and a flat bottom located at ˜7 km depth. The faults bounding the LOC probably initiated during postorogenic extension in the Scandinavian Caledonides. The faults are regional in scale and are parallel to other NE-SW trending en echelon faults along the Norwegian coastline and on the adjacent mainland. A pull-apart structure explains the down-dropping and subsequent preservation of the LOC, as it is surrounded by rocks from lower structural positions within the nappe stack. The paleostress directions from Leka support a sinistral component of shear along these faults. The gravity inversion is consistent with a fault-bounded geometry. This pull-apart structure, as uniquely recorded by the dense ophiolitic rocks, suggests that strike-slip partitioning was active in an obliquely divergent setting.

  7. Dynamics of intraoceanic subduction initiation: 1. Oceanic detachment fault inversion and the formation of supra-subduction zone ophiolites

    NASA Astrophysics Data System (ADS)

    Maffione, Marco; Thieulot, Cedric; van Hinsbergen, Douwe J. J.; Morris, Antony; Plümper, Oliver; Spakman, Wim

    2015-06-01

    Subduction initiation is a critical link in the plate tectonic cycle. Intraoceanic subduction zones can form along transform faults and fracture zones, but how subduction nucleates parallel to mid-ocean ridges, as in e.g., the Neotethys Ocean during the Jurassic, remains a matter of debate. In recent years, extensional detachment faults have been widely documented adjacent to slow-spreading and ultraslow-spreading ridges where they cut across the oceanic lithosphere. These structures are extremely weak due to widespread occurrence of serpentine and talc resulting from hydrothermal alteration, and can therefore effectively localize deformation. Here, we show geochemical, tectonic, and paleomagnetic evidence from the Jurassic ophiolites of Albania and Greece for a subduction zone formed in the western Neotethys parallel to a spreading ridge along an oceanic detachment fault. With 2-D numerical modeling exploring the evolution of a detachment-ridge system experiencing compression, we show that serpentinized detachments are always weaker than spreading ridges. We conclude that, owing to their extreme weakness, oceanic detachments can effectively localize deformation under perpendicular far-field forcing, providing ideal conditions to nucleate new subduction zones parallel and close to (or at) spreading ridges. Direct implication of this, is that resumed magmatic activity in the forearc during subduction initiation can yield widespread accretion of suprasubduction zone ophiolites at or close to the paleoridge. Our new model casts the enigmatic origin of regionally extensive ophiolite belts in a novel geodynamic context, and calls for future research on three-dimensional modeling of subduction initiation and how upper plate extension is associated with that.

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

    NASA Astrophysics Data System (ADS)

    Maibam, B.; Foley, S.

    2009-04-01

    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.

  9. Where and what is the Deep Off-Axis Magmatism in the Oman Ophiolite (Invited)

    NASA Astrophysics Data System (ADS)

    Jousselin, D.; Reisberg, L. C.; Nicolas, A. A.; Nicolle, M.; Bosch, D.; Boudier, F. I.

    2013-12-01

    We present a review of structures in the Oman ophiolite that can be related to off-axis magmatism in the lower crust and uppermost mantle at fast spreading ridges. First, we show evidence that a ridge axis is preserved within the ophiolite. As this axis can be used as a reference frame, we distinguish three cases of off-axis magma injection near the Moho or in the lower crust. The first two cases, though partly situated off-axis, are related to excess magmatism caused by mantle diapirs rising beneath the ridge axis, while the third is linked to the emplacement of a diapir far from the axis. (1) The Moho transition zone (MTZ) beneath the ridge axis is normally a few meters thick, but increases up to several hundred meters thick above mantle diapirs, within a ten Km radius. It forms from underplating of successive melt pulses. Once melt ponds at the base of the MTZ, it transforms harzburgite into dunite; then most of the melt is compacted and injected into the crust, but a portion forms plurimetric gabbro lenses within the dunite. Eight to ten Km away from the axis, the MTZ thickness is greatly reduced, thus on-axis diapiric emplacement may not induce off-axis magmatism at this level further than 10 Km away from the axis. (2) Pseudo "wehrlites" that form up to 30% of the lower crust define a second type of off-axis melt body. These intrusions are rooted in the MTZ where dunite is turned into an olivine-rich mush upon melt delivery, so they may be considered as chunks of MTZ intruded into the crust. Though many of these intrusions are tectonically transposed by the magmatic flow that affects lower crustal gabbros, showing that they were injected within the magma chamber, others are undeformed intrusions, showing that it was emplaced in an off-axis melt-poor crust, probably away from the crustal seismic low velocity zone. (3) We mapped an off-axis diapir located 30 km from the ridge axis that may be the root of an incipient seamount. The overlying MTZ is up to a thousand meters thick and contains pyroxenite instead of gabbro lenses in dunite. Above, and at the diapir periphery, melt intrudes a cooled lithosphere and forms amphibole-rich-microgabbro intrusions. Low ?Nd may suggest that off-axis melting occurred because of a compositional anomaly such as enriched veins. However, at the MTZ we find low incompatible element contents and LREE depletion more marked than that of typical MORB. We explain this compositional paradox using geological evidence of assimilation and mixing between incoming melt and the pre-existing depleted and hydrated off-axis lithosphere that drifted from the rise axis.

  10. Ophiolites in ocean-continent transitions: From the Steinmann Trinity to sea-floor spreading

    NASA Astrophysics Data System (ADS)

    Bernoulli, Daniel; Jenkyns, Hugh C.

    2009-05-01

    Before the theory of plate tectonics took hold, there was no coherent model for ocean-continent transitions that included both extant continental margins and fragmentary ancient examples preserved in orogenic belts. Indeed, during the early 1900, two strands of thought developed, one relying on the antiquity and permanence of continents and oceans, advocated by the mainstream of the scientific community and one following mobilist concepts derived from Wegener's hypothesis (1915) of continental drift. As an illustration of the prevailing North-American view, the different composition and thickness of continental and oceanic crust and the resulting isostatic response showed "how improbable it would be to suppose that a continent could founder or go to oceanic depth or that ocean floor at ± 3000 fathoms could ever have been a stable land area since the birth of the oceans" [H.H. Hess, Trans. R. Soc. London, A 222 (1954) 341-348]. Because of the perceived permanence of oceans and continents, mountain chains were thought to originate from narrow, elongated, unstable belts, the geosynclines, circling the continents or following "zones of crustal weakness" within them, from which geanticlines and finally mountain belts would develop. This teleological concept, whereby a geosyncline would inevitably evolve into a mountain chain, dominated geological interpretations of orogenic belts for several decades in the mid-twentieth century. However, the concept of permanence of oceans and continents and the concept of the geosyncline had already met with the critiques of Suess and others. As early as 1905, Steinmann considered the association of peridotite, "diabase" (basalt/dolerite) and radiolarite (a typical ocean-continent transition assemblage), present in the Alps and Apennines, as characteristic of the deep-ocean floor and Bailey (1936) placed Steinmann's interpretation into the context of continental drift and orogeny. Indeed, in both authors' writings, the concept of ophiolites as ocean crust is apparent. Between 1920 and 1930, the stage was thus potentially set for modern mobilist concepts that were, however, to prove attractive to only a small circle of Alpine and peri-Gondwanian geologists. After the Second World War, the 1950s saw the rapid progress of the geophysical and geological exploration of oceans and continental margins that provided the data for a reevaluation of the geosynclinal concept. Actualistic models now equated the former preorogenic miogeosyncline of Stille (1940) and Kay (1951) with passive continental margins [C.L. Drake, M. Ewing, G.H. Sutton, Continental margin and geosynclines: the east coast of North America, north of Cape Hatteras, in: L. Ahrens, et al. (Eds.), Physics and Chemistry of the Earth 3, Pergamon Press, London, 1959, pp. 110-189], the (American version of the) eugeosyncline and its igneous rocks with "collapsing continental rises" [R.S. Dietz, J. Geol. 71 (1963) 314-333] and the ophiolites, the Steinmann Trinity, of the (European) eugeosyncline with fragments of oceanic lithosphere [H.H. Hess, History of ocean basins, in: Petrologic Studies: a Volume to Honor A.F. Buddington, Geol. Soc. Am., New York. 1962, pp. 599-620]. The concept of sea-floor spreading [H.H. Hess, History of ocean basins, in: Petrologic Studies: a Volume to Honor A.F. Buddington, Geol. Soc. Am., New York. 1962, pp. 599-620; H.H. Hess, Mid-oceanic ridges and tectonics of the sea-floor, in: W.F. Whittard, R. Bradshaw (Eds), Submarine Geology and Geophysics, Colston Papers 17, Butterworths, London, 1965, pp. 317-333] finally eliminated the weaknesses in Wegener's hypothesis and, with the coming of the "annus mirabilis" of 1968, the concept of the geosyncline could be laid to rest. Ocean-continent transitions of modern oceans, as revealed by seismology and deep-sea drilling, could now be compared with the remnants of their ancient counterparts preserved in the Alps and elsewhere.

  11. Uranium-lead isotopic ages of the Samail ophiolite, Oman, with applicatons to Tethyan ocean ridge tectonics

    SciTech Connect

    Tilton, G.R.; Hopson, C.A.; Wright, J.E.

    1981-04-10

    Plagiogranites are a minor but widespread component of the Samail ophiolite plutonic member. They crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the Tethyan spreading ocean ridge, and their ages are thought to mark the time of ocean crust formation. Isotopic U--Pb ages of zircons from 13 plagiogranites collected along a 270-km segment of the Samail ophiolite subparallel to the regional trend of the sheeted dike complex (the former spreading ridge axis direction) define a narrow time interval of 93.5--97.9 m.y., with a pronounced clustering about 95 m.y. The zircon ages of the plagiogranites agree remarkably well with the early Cenomanian to early Turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them (Tippit et al., 1981). The agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite U--Pb ages closely date the time span of ocean crust formation. No step changes in age patterns are observed along the ridge axis (sheeted dike) direction, suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse. One possible exception occurs at the southeastern end of the sampled interval (Ibra area), where a 3 m.y. discontinuity might be caused by an unmapped fault. Assuming that the regional trend of the sheeted dikes (N10 /sup 0/--25 /sup 0/W) marks the direction of the former spreading ridge axis, the present array of sample localities spans a distance of 130 to 195 km normal to that axis (i.e., in the spreading direction). The data as a whole do not define a clear-cut age trend normal to the spreading axis, but by eliminating samples that may be aberrant due to faulting, the data array suggests a pattern of increasing ages from east to west.

  12. Timing and nature of the Xinlin-Xiguitu Ocean: constraints from ophiolitic gabbros in the northern Great Xing'an Range, eastern Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Feng, Zhiqiang; Liu, Yongjiang; Liu, Binqiang; Wen, Quanbo; Li, Weimin; Liu, Qing

    2016-03-01

    Jifeng ophiolitic mélange (ultramafic rocks, meta-basalts and gabbros) crops out in the northern segment of the Great Xing'an Range, the eastern segment of the Central Asian Orogenic Belt, which marks the closure of the Xinlin-Xiguitu Ocean associated with the collision between the Erguna block and Xing'an block. In order to investigate the formation age and magma source of the Jifeng ophiolitic mélange, the gabbros from newly discovered the Jifeng ophiolitic mélange are studied with zircon U-Pb ages, whole-rock geochemistry and zircon Hf isotopes. Zircon U-Pb dating from the ophiolitic gabbros yields U-Pb age of 647 ± 5.3 Ma, which may represent the formation age of the ophiolitic mélange. The gabbros display low SiO2, TiO2, K2O contents, high Na2O, LREE contents and indistinctive REE fractionation [(La/Yb)N = 1.97-2.98]. It shows an E-MORB-like affinity, while the element concentrations of the Jifeng samples are lower than that of E-MORB. More importantly, Nb displays negative anomaly in comparison with Th, which shows a transitional SSZ-type ophiolite signature. Moreover, the ɛ Hf ( t) values of ~647 Ma zircons in the gabbros range from +8.4 to +13.4, and the corresponding Hf single-stage ages ( T DM1) are between 687 and 902 Ma, which is obviously older than the crystallization age of 647 Ma. These geochemical features can be explained as melts from the partial melting of a depleted mantle source meta-somatized by fluids derived from a subducted slab. Accordingly, we conclude that the Jifeng ophiolitic mélange is probably related to transitional SSZ-type ophiolite and developed in an intra-oceanic subduction, which indicates that an ocean (the Xinlin-Xiguitu Ocean) existed between the Erguna block and Xing'an block. The Ocean's formation might be no later than the Neoproterozoic (647 Ma), and it was closed in the Late Cambrian because of the collision between the Erguna block and Xing'an block.

  13. Timing and nature of the Xinlin-Xiguitu Ocean: constraints from ophiolitic gabbros in the northern Great Xing'an Range, eastern Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Feng, Zhiqiang; Liu, Yongjiang; Liu, Binqiang; Wen, Quanbo; Li, Weimin; Liu, Qing

    2015-05-01

    Jifeng ophiolitic mélange (ultramafic rocks, meta-basalts and gabbros) crops out in the northern segment of the Great Xing'an Range, the eastern segment of the Central Asian Orogenic Belt, which marks the closure of the Xinlin-Xiguitu Ocean associated with the collision between the Erguna block and Xing'an block. In order to investigate the formation age and magma source of the Jifeng ophiolitic mélange, the gabbros from newly discovered the Jifeng ophiolitic mélange are studied with zircon U-Pb ages, whole-rock geochemistry and zircon Hf isotopes. Zircon U-Pb dating from the ophiolitic gabbros yields U-Pb age of 647 ± 5.3 Ma, which may represent the formation age of the ophiolitic mélange. The gabbros display low SiO2, TiO2, K2O contents, high Na2O, LREE contents and indistinctive REE fractionation [(La/Yb)N = 1.97-2.98]. It shows an E-MORB-like affinity, while the element concentrations of the Jifeng samples are lower than that of E-MORB. More importantly, Nb displays negative anomaly in comparison with Th, which shows a transitional SSZ-type ophiolite signature. Moreover, the ? Hf (t) values of ~647 Ma zircons in the gabbros range from +8.4 to +13.4, and the corresponding Hf single-stage ages (T DM1) are between 687 and 902 Ma, which is obviously older than the crystallization age of 647 Ma. These geochemical features can be explained as melts from the partial melting of a depleted mantle source meta-somatized by fluids derived from a subducted slab. Accordingly, we conclude that the Jifeng ophiolitic mélange is probably related to transitional SSZ-type ophiolite and developed in an intra-oceanic subduction, which indicates that an ocean (the Xinlin-Xiguitu Ocean) existed between the Erguna block and Xing'an block. The Ocean's formation might be no later than the Neoproterozoic (647 Ma), and it was closed in the Late Cambrian because of the collision between the Erguna block and Xing'an block.

  14. Seawater Circulation and Thermal Sink at OCEAN Ridges - FIELD Evidence in Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Nicolas, A. A.; Boudier, F. I.; Cathles, L. M.; Buck, W. R.; Celerier, B. P.

    2014-12-01

    Exceptionally, the lowermost gabbros in the Oman ophiolite are black and totally fresh, except for minute traces of impregnation by seawater fluids at very high temperature (~1000°C). These black gabbros sharply contrast with normal, whitish gabbros altered down to Low-T~500-350°C. These hydrous alterations are ascribed to an unconventional model of seawater circulation and cooling of the permanent magma chambers of fast spreading ocean ridges. In this model, gabbros issued from the magma chamber cross a ~100 m thick thermal boundary layer (TBL) before reaching a narrow, Low-T high permeability channel where the heated return seawater is flowing towards black smokers and the local gabbros are altered. Uprising mantle diapirs in Oman diverge at ~5 km on each side of the palaeo-ridge axis and feed an overlying magma chamber that closes at this distance from axis. Preservation of black gabbros along the Moho implies that the loop of seawater alteration locally does not reach Moho beyond this ~5km distance (otherwise black gabbros would be altered in whitish gabbros). This defines an internal "thermal sink" within ~5 km to the ridge axis. There, the sink is efficiently cooled by the active hydrothermal convection that is ridge transverse. This has been documented near the Galapagos ridge by marine geophysical data, within the same distance. Beyond this critical distance, the cooling system becomes dominantly conductive and ridge-parallel. The TBL and attached return flow channels must be rising into the overcooled, accreted crust. Beyond the thermal sink, the 500°C isotherm rebounds into the crust. It is only after ~ 1My of crustal drift that this isotherm penetrates into the uppermost mantle in a sustained fashion, developing serpentinites at the expense of peridotites.

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

    NASA Astrophysics Data System (ADS)

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

    2008-01-01

    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.

  16. An alkaline spring system within the Del Puerto ophiolite (California USA): A Mars analog site

    SciTech Connect

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

    2008-10-01

    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 {approx}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 rockhosted 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.

  17. Water and fabric in an ophiolitic peridotite from a supra-subduction zone

    NASA Astrophysics Data System (ADS)

    Wang, Yongfeng; Ren, Huaping; Jin, Zhenmin

    2016-03-01

    Mantle peridotites from modern supra-subduction zones (SSZs) are important windows through which we can investigate the geodynamic processes active in the subduction factory, but they are unfortunately rare and hard to access. Most ophiolitic peridotites stem from SSZ settings and are therefore good candidates to explore the water budget and deformation in mantle wedges. We present here an integrated study of the geochemistry, deformation microstructures and water contents of olivine and orthopyroxene from the Dongqiao harzburgites, central Tibet. These peridotites are characterized by an absence of interstitial clinopyroxene, the partial replacement of orthopyroxene by olivine, the highly magnesian olivine and chromium spinel, and remarkable LREE enrichments. These features suggest that the Dongqiao harzburgites are highly depleted and have undergone a high degree (~40 %) of partial melting, followed by infiltration of and interaction with melt while they were part of the mantle wedge. Olivine and orthopyroxene show prominent plastic deformation microstructures and have developed significant crystallographic preferred orientations (CPOs), suggestive of dislocation creep deformation. Fourier transform infrared analyses show that olivine is essentially dry, while orthopyroxene contains an average water content of 70 ± 14 wt ppm. We propose that orthopyroxene largely retains its in- situ water content from the mantle source, while olivine completely loses its water during emplacement. The orthopyroxene water contents fall into the lower end of the range observed in SSZ peridotites. We consider that the high degree of partial melting and the interaction with a water-undersaturated melt contribute to the relatively lower water contents in orthopyroxene from the Dongqiao harzburgites. Based on experimentally determined hydrogen partition coefficients between olivine and orthopyroxene, the water contents of olivine in the mantle source are calculated to be 7-9 wt ppm (or 114-147 ppm H/Si), which is consistent with the observed A-type CPOs in olivine.

  18. VERY High Temperature Hydrothermal Record in Plagioclase of BLACK Gabbros in Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Boudier, F. I.; Mainprice, D.; Nicolas, A. A.

    2014-12-01

    The lower crustal section in Oman ophiolite includes 'black gabbros' that have escaped the common medium-low temperature hydrous alteration. Their plagioclases are totally fresh, but contain in their mass, nebulous inclusions most times below the resolution of optical microscope, or expressed as solid silicate phases clinopyroxene and pargasitic amphibole, up to 10 µm sized, having T equilibrium above 900°C with their host plagioclase. These gabbros have a well-expressed magmatic foliation, relayed by plastic strain marked by stretched olivine crystals, and pinching twins in plagioclase. In addition to major elements analyses, the crystallographic relationships of these Mg silicate inclusions to their host plagioclase are explored by Electron Back Scattering Diffraction (EBSD) processing. - Diopsidic clinopyroxene inclusions are dominant over pargasitic amphibole that tend to locate close to the margins of host plagioclase (Fig 1). Some inclusions are mixed clinopyroxene-amphibole, separated by a non-indexed phase that could represent a pyribole-type structure, suggesting transformation from clinopyroxene to amphibole during cooling. High chlorine content in the amphibole sign the seawater contamination at least during the development of this phase. - Preliminary statistical pole figures (Fig. 2) in the six joined plagioclase grains studied, show that both plagioclase and diopside inclusions have a strong crystal preferred orientation (CPO) connected such that the strong [010]pl maximum coincide with the strong [100]di. In addition, a coincidence appears between three sub-maxima of [100]pl and [001]di. These interesting relationships are refined. It is inferred that clinopyroxene developed through corrosion of the plagioclase by a Mg-bearing hydrous fluid, penetrating possibly via twin interface and diffusing at T~1100°C, upper limit of clinopyroxene stability in hydrous conditions. Development of pargasite implies increasing hydration during cooling.

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    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.

  20. Hydrous partial melting in the lower crust of the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Boudier, F. I.; Koepke, J.; Nicolas, A. J.

    2004-12-01

    Series of water-saturated melting experiments have been performed on natural gabbros between 900° and 1000°C, at crustal pressure up to 200MPa (Koepke et al., 2004), that put new constraints on the composition of melt and residual crystals at increasing temperature and melt fraction produced. In the gabbro section of the Oman ophiolite, the development of high-T secondary parageneses is ubiquist, represented by orthopyroxene+pargasite rims within contact between olivine and plagioclase, while clinopyroxene is replaced by pargasite. The reference to the experimental results, and isotopic tracing (Bosch et al., 2004) lead to interpret these reactions as representing initiation of hydrous partial melting by fluids circulating at grain boundaries. The inferred mechanism allowing supercritical water to penetrate the deep gabbro section is a strong anisotropy of thermal compression inducing microcracking in the cooling lower gabbros (Nicolas et al., 2003). In the Oman gabbros, another important petrologic feature is the local occurrence, in the deeper section, of large amounts of orthopyroxene bearing gabbros either interlayered with olivine gabbros or intrusive as pegmatitic patches, in association with wehrlites, or mixed with pargasitic gabbros. The corresponding upper levels are rich in dioritic or trondjemitic dikes. These occurrences are restricted to localized areas that coincide with tips and segments limits as deduced from the detailed mapping along the NW-SE paleospreading axis. The origin of these parageneses as products of hydrous melting of the gabbros, at various melt fraction, is explored by reference to the experimental data. Koepke, J., Feig, S.T., Snow, J., Freise, M., 2004. Contrib. Mineral. Petrol. 146, 414-432. Bosch, D.et al., 2004, J. Petrology, 45, 1181-1208. Nicolas, A., Mainprice, D., Boudier, F., 2003, J. Geophys. Res. 108 (B8) 2371.

  1. Oman Ophiolite: Petrological and Geochemical Investigation of Fast-Spreading Crust Formation Processes

    NASA Astrophysics Data System (ADS)

    Müller, T.; Koepke, J.; Garbe-Schoenberg, C. D.; Schuth, S.; Wolff, P. E.

    2014-12-01

    We undertook a detailed field campaign in the Wadi Gideah, which is located in the Wadi-Tayin Massif in the southern part of the Oman Ophiolite, to sample a complete section of fast-spreading oceanic crust. Our concept of performing different analytical and structural investigations on the same samples enabled us to create a coherent data set. The thickness of the layered and virtually undeformed oceanic crust, containing pillow lavas and sheeted dikes as well as varitextured, foliated and layered gabbros resting on a relatively thin MOHO transition zone, was recalculated to approximately 6km. Here we present our data focusing on the petrological and geochemical logs obtained. Samples from the layered gabbro sequence show modal compositions of ~50 vol% plagioclase, ~40 vol% clinopyroxene and ~10 vol% olivine in average. The samples from the foliated gabbro sequence display a slightly higher amount of plagioclase. In very few samples up to 20 vol% of orthopyroxene is present. The layered gabbro sequence display Mg# 71-82 for olivine, Mg# 75-83 for clinopyroxene and An% in plagioclase of 71-93 mol%. The foliated gabbro sequence display Mg# 67-79 for olivine, Mg# 76-85 for clinopyroxene and An% in plagioclase of 58-85 mol%. The varitextured gabbro sequence display Mg# 74-80 for clinopyroxene and An% 59-86 in plagioclase mol%. The generally evolving trends in mineral major element composition from bottom to top of the profile are also observed for bulk rock major and trace element data. The average Sr87/Sr86 ratio is 0.7033 ± 0.0002 for the entire foliated and layered gabbro with significantly higher values for samples from fault zones cutting the gabbros at all crustal level which here are interpreted as possible hydrothermal pathways for cooling of the deep crust. We calculated the Wadi Gideah bulk crust composition and modeled possible fractionation paths, implying significant crystallization in the deep crust.

  2. Reactive flow as dominant evolution process in the lowermost oceanic crust: evidence from olivine of the Pineto ophiolite (Corsica)

    NASA Astrophysics Data System (ADS)

    Sanfilippo, Alessio; Tribuzio, Riccardo; Tiepolo, Massimo; Berno, Davide

    2015-10-01

    The Jurassic Pineto ophiolite from Corsica exposes a ~1-km-thick troctolite-olivine-gabbro sequence, interpreted to represent a lowermost sector of the gabbroic oceanic crust from a (ultra-)slow spreading system. To constrain the petrogenesis of the olivine-gabbros, minor and trace element analyses of olivine (forsterite = 84-82 mol%) were carried out. Olivine from the olivine-gabbros is depleted in incompatible trace elements (Sc, V, Ti, Y, Zr and heavy rare earth elements) with respect to olivines from associated troctolites. Depleted incompatible element compositions are also shown by olivine (forsterite = 86 mol%) from a clinopyroxene-rich troctolite. The incompatible element compositions of olivine argue against a petrogenetic process entirely driven by fractional crystallization. We propose that melts migrating through an olivine-plagioclase crystal mush chemically evolved by reaction with the existing minerals, changing in composition as it flowed upward. The melt residual from these interactions led to partial dissolution of preexisting olivine and to crystallization of clinopyroxene, generating olivine-gabbro bodies within a troctolite matrix. Reactive flow was the major evolution process active in the ~1-km crustal transect exposed at the Pineto ophiolite, producing lithological variations classically attributed to fractional crystallization processes.

  3. Mineralogical assemblages forming at hyperalkaline warm springs hosted on ultramafic rocks: A case study of Oman and Ligurian ophiolites

    NASA Astrophysics Data System (ADS)

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

    2013-07-01

    We report on the mineralogical assemblages found in the hyperalkaline springs hosted on Liguria and Oman ophiolites based on exhaustive X-ray diffraction and scanning electron microprobe analyses. In Liguria, hyperalkaline springs produce a thin brownish calcite precipitate that covers the bedrock due to the concomitant atmospheric CO2 uptake and neutralization of the hyperalkaline waters. No brucite and portlandite minerals are observed. The discharge of alkaline waters in Oman ophiolite forms white-orange precipitates. Calcium carbonate minerals (calcite and/or aragonite) are the most abundant and ubiquitous precipitates and are produced by the same mechanism as in Liguria. This process is observed as a thin surface crust made of rhombohedral calcite. Morphological features of aragonite vary from needle-, bouquet-, dumbbell-, spheroidal-like habitus according to the origin of carbon, temperature, and ionic composition of the hyperalkaline springs, and the biochemical and organic compounds. Brucite is observed both at hyperalkaline springs located at the thrust plane and at the paleo-Moho. The varying mixing proportions between the surface runoff waters and the hyperalkaline ones control brucite precipitation. The layered double hydroxide minerals occur solely in the vicinity of hyperalkaline springs emerging within the bedded gabbros. Finally, the dominant mineralogical associations we found in Oman (Ca-bearing carbonates and brucite) in a serpentinizing environment driven by the meteoric waters are surprisingly the same as those observed at the Lost City hydrothermal site in a totally marine environment.

  4. Mineralization, alteration, and hydrothermal metamorphism of the ophiolite-hosted Turner-Albright sulfide deposit, southwestern Oregon

    USGS Publications Warehouse

    Zierenberg, R.A.; Shanks, Wayne C., III; Seyfried, W.E., Jr.; Koski, R.A.; Strickler, M.D.

    1988-01-01

    The Turner-Albright sulfide deposit, part of the Josephine ophiolite, formed on and below the seafloor during Late Jurassic volcanism at a back arc spreading center. Ore fluids were probably localized by faults which were active on the seafloor at the time of sulfide deposition. The uppermost massive sulfide formed on the seafloor at hydrothermal vents. The bulk of the sulfide mineralization formed below the seafloor within olivine basalt hyaloclastite erupted near the time of mineralization. Infiltration of hydrothermal fluid into the hyaloclastite altered the rock. The fluid responsible for the hydrothermal alteration was evolved seawater with low pH and Mg and high Fe. The average value of sulfide and the difference between sulfide and contemporaneous seawater sulfate values are similar to ophiolite-hosted sulfide deposits in Cyprus. Mudstone and clinopyroxene basalt above the sulfide horizons were not altered by the ore-transporting hydrothermal fluid, but these rocks were hydrothermally metamorphosed by altered seawater heated by deep circulation into hot oceanic crust. This subseafloor metamorphism produced a mineral assemblage typical of prehnite-pumpellyite facies metamorphism. Exchange with altered seawater increased the whole-rock ??18O of the basalts to values of 9.4-11.2%. -from Authors

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

    NASA Astrophysics Data System (ADS)

    Ulrich, M.; Munoz, M.; Guillot, S.; Cathelineau, M.; Picard, C.; Quesnel, B.; Boulvais, P.; Couteau, C.

    2014-12-01

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2014-01-01

    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.

  7. Geochronology and geochemistry of the rocks associated with a late proterozoic ophiolite in West Pokot, NW Kenya

    NASA Astrophysics Data System (ADS)

    Ries, A. C.; Vearncombe, J. R.; Price, R. C.; Shackleton, R. M.

    1992-01-01

    Mafic and ultramafic rocks in the W Pokot area, NW Kenya are identified as parts of a dismembered ophiolite. They lie within the late Proterozoic Mozambique Belt and are associated with metasediments and calc-alkaline volcanics and intruded by granitic rocks. All these rocks are allochthonous, thrust westwards towards the Archaean Tanzanian Craton. The calc-alkaline volcanics, which are chemically similar to present-day island-arc volcanic rocks, give a Rb/Sr whole-rock isochron age of 663 ± 49 Ma, and the associated metasediments give an age of 584 ± 25 Ma, both ages interpreted as dating a regional amphibolitefacies metamorphism. These rocks are intruded by the Marich Granite from which a Rb/Sr whole-rock isochron age of 593 ± 50 Ma was obtained with an 87Sr/ 86Sr initial ratio of 0/7072 ± 5 implying some crustal contamination either from the magmatite complex, which structurally underlies the ophiolitic rocks, or from deeper crustal rocks. The age spectrum is broadly similar to that established for similar sequences of rocks northwards along strike in Sudan, Egypt and Saudi Arabia. There is no support for the view that these high-grade metamorphic rocks of this part of the Mozambique Belt are an older series underlying the lower grade Late Prpterozoic rocks of NE Africa and Saudi Arabia.

  8. Origin of primary PGM assemblage in ?hromitite from a mantle tectonite at Harold's Grave (Shetland Ophiolite Complex, Scotland)

    NASA Astrophysics Data System (ADS)

    Badanina, Inna Yu.; Malitch, Kreshimir N.; Lord, Richard A.; Meisel, Thomas C.

    2013-12-01

    In this paper we present textural and mineral chemistry data for a PGM inclusion assemblage and whole-rock platinum-group element (PGE) concentrations of chromitite from Harold's Grave, which occurrs in a dunite pod in a mantle tectonite at Unst in the Shetland Ophiolite Complex (SOC), Scotland. The study utilized a number of analytical techniques, including acid digestion and isotope dilution (ID) ICP-MS, hydroseparation and electron microprobe analysis. The chromitite contains a pronounced enrichment of refractory PGE (IPGE: Os, Ir and Ru) over less refractory PGE (PPGE: Rh, Pt and Pd), typical of mantle hosted `ophiolitic' chromitites. A `primary' magmatic PGM assemblage is represented by euhedrally shaped (up to 60 ?m in size) single and composite inclusions in chromite. Polyphase PGM grains are dominated by laurite and osmian iridium, with subordinate laurite + osmian iridium + iridian osmium and rare laurite + Ir-Rh alloy + Rh-rich sulphide (possibly prassoite). The compositional variability of associated laurite and Os-rich alloys at Harold's Grave fit the predicted compositions of experiment W-1200-0.37 of Andrews and Brenan (Can Mineral 40: 1705-1716, 2002) providing unequivocal information on conditions of their genesis, with the upper thermal stability of laurite in equilibrium with Os-rich alloys estimated at 1200-1250 °C and f(S2) of 10-0.39-10-0.07.

  9. Subsidence and the development of magmatic foliation in Oman ophiolite magma chambers

    NASA Astrophysics Data System (ADS)

    Boudier, F. I.; Nicolas, A. J.

    2009-12-01

    Shifting from mapping at the scale of Oman ophiolite belt to that of a few tens of meters makes it possible to study fine-scale structures, specifically the paleo-melt lens of a fast spreading ridge. Due to spreading, this melt lens has been pinched and transformed in an horizon between the Root Zone of the Sheeted Dike Complex (RZSDC) and the upper gabbros from the gabbro unit. The field study has been conducted through the previous record of 185 sites showing this transition and 18 new sections down to the upper gabbros unit that have been mapped in great detail. Subsidence is demonstrated here by the field data displaying in 11/18 stations in the gabbros beneath the pinched melt lens floor, a discordant and steep magmatic foliation parallel to the sheeted dike complex and, presumably, to the ridge plane. One quarter of the stations have foliations parallel or close to the floor of the RZSDC. They reveal information on the melt lens floor. When the crust issued from a stable melt lens drifts away from the ridge axis, only the points issued from the pinching limit of the lens and drifting through the triple junction carry information on the nature and structure of the interior of the lens floor, because they have subsided and rotated only over meters before crossing and being frozen at the magma chamber wall (Figure). While it is subsiding from the melt lens floor into the magma chamber, the highly viscous gabbroic mush progressively develops a magmatic foliation that rotates to become parallel to the magma chamber walls. The textural evolution of upper gabbros from isotropic with poikilitic clinopyroxene to foliated granular record the two components of deformation of a subsiding layer, simple shear and compaction, together with increasing recovery and recrystallization with subsidence depth. The steep foliation induced by subsidence is traced downward in the upper gabbros over 1 to 1.5 km, before progressively rotating into the flat-lying lower gabbros. From this vertical distance of subsidence, it is possible to estimate the time they spent in the magma chamber to ~500 yrs. Outstanding among the gabbros, anorthosite layers record new melt intrusion in the melt lens and from their spacing estimated in the field between standard gabbros, it is possible to obtain an estimation on the episodicity of melt lens refilling.

  10. Timescales and rates for peridotite-groundwater reactions in the Samail Ophiolite, Sultanate of Oman

    NASA Astrophysics Data System (ADS)

    Paukert, A. N.; Matter, J.; Stute, M.; Kelemen, P. B.

    2014-12-01

    The peridotite section of the Samail Ophiolite, Sultanate of Oman is a site of ongoing low temperature serpentinization and carbonation. We present geochemical data for waters collected from boreholes in the peridotite to help describe subsurface water-rock reactions and rates. We constrained groundwater age using 3H-3He dating, He accumulation, dissolved noble gas thermometry, and stable isotopes (?2H, ?18O). Dissolved gas samples were collected from boreholes and used to estimate ongoing serpentinization rates. Boreholes in peridotite contain two water types: Type I Mg2+-HCO3- water and Type II Ca2+-OH- water. All of our groundwater dating techniques suggest boreholes with Type I water contain modern recharge. Type I boreholes have 3H-3He dates of 0-40 years, 3He/4He ratios similar to the atmosphere, and low levels of He accumulation. Noble gas thermometry shows recharge temperatures of 32oC, the modern mean annual ground temperature. Stable isotopes in Type I waters fall between the northern and southern local meteoric water lines, similar to other modern northern Oman groundwaters (Weyhenmeyer et al., Wat. Resour. Res., 2002). In contrast, Type II hyperalkaline groundwaters from boreholes in peridotite appear to be significantly older. Waters have pre-bomb 3H levels, and significant He accumulation from sources such as crustal production from radioactive decay or mantle He. Noble gas temperatures range from 30oC to 25oC, with the cooler samples potentially having recharged during a glacial period. Stable isotopes for Type II waters plot either between the northern and southern meteoric water lines or are enriched in ?18O. The enrichment may be due to evaporation or a change in the source of local water vapor during glacial periods. Dissolved gases from boreholes with Type I water are primarily N2, while gases from boreholes with Type II water include millimolar concentrations of H2, CH4, or both. These dissolved gases suggest the Type II boreholes are in or near areas of active low temperature serpentinization. Serpentinization rates calculated from dissolved gas concentrations and groundwater flow vary from 4x10-8 to 2x10-6 volume fraction peridotite serpentinized per year.

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

    NASA Astrophysics Data System (ADS)

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

    2008-06-01

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    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.

  13. An evolved axial melt lens in the Northern Ibra Valley, Southern Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Loocke, M. P.; Lissenberg, C. J.; MacLeod, C. J.

    2014-12-01

    The axial melt lens (AML) is a common feature lying at the base of the upper crust at fast-spreading mid-ocean ridges. It is thought to play a major role in the evolution of MORB and, potentially, accretion of the plutonic lower crust. In order to better understand the petrological processes that operate in AMLs we have examined the nature and variability of the horizon equivalent to the AML preserved in the Oman ophiolite. We present the results of a detailed investigation of a section east of Fahrah in the Ibra Valley. Here, a suite of 'varitextured' gabbros separates the sheeted dykes above from foliated gabbros below. It comprises 3 distinct units: an ophitic gabbro with pegmatitic patches (patchy gabbro; 70 m thick), overlain by a spotty gabbro (50 m), capped by a quartz-diorite (120 m). The sheeted dykes are observed to root in the quartz-diorite. Contacts between the plutonic units are gradational and subhorizontal. All of the units are isotropic. A total of 110 samples were collected for detailed petrographic and chemical analysis. With the exception of a small number of the diorites, all of the samples have a 'cumulate' component. Primary igneous amphibole is ubiquitous, present even as a minor phase in the foliated gabbros beneath, and indicating extensive differentiation and/or the presence of water in the primary liquid. France et al. (2014, Lithos) report patches of granoblastic material from this horizon in the Fahrah area, and suggest they represent the restites of partially melted pieces of the sheeted dykes. We did not, however, find any such granoblastic material, nor can the quartz-diorites represent partial melt; instead, preliminary geochemical modeling suggests that all of the units can be related by simple progressive fractional crystallization of an Oman axial ('V1' or 'Geotimes') melt. Along with the field relationships, as well as the basaltic andesite to dacite composition of the overlying sheeted dykes, this suggests that the AML was the locus of formation of the highly evolved melts. This contrasts with the more primitive AML and sheeted dyke complex documented in Wadi Abyad. From this we conclude that there is significant lateral variability in AML compositions along the Oman ridge axis.

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

    NASA Astrophysics Data System (ADS)

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

    2012-08-01

    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.

  15. Tracing ancient events in the lithospheric mantle: A case study from ophiolitic chromitites of SW Turkey

    NASA Astrophysics Data System (ADS)

    Akbulut, Mehmet; González-Jiménez, José María; Griffin, William L.; Belousova, Elena; O'Reilly, Suzanne Y.; McGowan, Nicole; Pearson, Norman J.

    2016-04-01

    New major-, minor- and trace-element data on high-Cr chromites from several ophiolitic podiform chromitites from Lycian and Antalya peridotites in southwestern Turkey reveal a polygenetic origin from a range of arc-type melts within forearc and back-arc settings. These forearc and the back-arc related high-Cr chromitites are interpreted to reflect the tectonic juxtaposition of different lithospheric mantle segments during the obduction. The diversity of the γOs(t=0) values (-8.28 to +13.92) in the Antalya and Lycian chromitite PGMs and their good correlations with the sub- to supra-chondritic 187Os/188Os ratios (0.1175-0.1459) suggests a heterogeneous mantle source that incorporated up to 40% recycled crust, probably due to subduction processes of the orogenic events. The few model ages calculated define two significant peaks in TRD model ages at 1.5 and 0.25 Ga, suggesting that the chromitites are younger than 0.25 Ga and include relics of an at least Mesoproterozoic or older (>1.0 Ga) mantle protolith. Eight of the nine zircon grains separated from the chromitites, are interpreted as detrital and/or resorbed xenocrystic relics, whilst a significantly less reworked/resorbed one is considered to be of metasomatic origin. In-situ U-Pb dating of the xenocrystic zircon grains yielded a spread of ages within ca 0.6-2.1 Ga, suggesting recycling of crustal rocks younger than 0.6 Ga (Late Neoproterozoic). The notable coincidence between the lower age limit of the older zircons (ca 1.6 Ga) and the oldest Os model age peak (ca 1.5 Ga) from the PGM may suggest a Mesoproterozoic rifting stage. These findings imply a Paleoproterozoic sub-continental lithospheric mantle (SCLM) protolith for the SW Anatolian mantle which was later converted into an oceanic lithospheric mantle domain possibly following a rifting and continental break-up initiated during Mesoproterozoic (ca 1.5-1.0 Ga). The single metasomatic zircon of ca 0.09 Ga age coinciding with the initiation of the contraction of the Neotethys may be interpreted to date the timing of a metasomatic event beneath the intra-oceanic subduction zone.

  16. Evidence of Melt percolation and Mantle Wedge Deformation in the Marum Ophiolite (Papua New Guinea)

    NASA Astrophysics Data System (ADS)

    Kaczmarek, M. A.; Jonda, L.; Davies, H. L.

    2014-12-01

    The Marum ophiolite in Papua New Guinea described an ultra-depleted mantle made by dunite and harzburgite showing compositions of supra-subduction zone peridotite. This depleted mantle has been fertilised by diffuse crystallisation of a low proportion of clinopyroxene in the dunite and formation of ol-websterite and ol-clinopyroxenite at cm scale cross-cutting the foliation and the primary pyroxenite layering. This percolating melt shows silica-rich magnesian affinities (boninite-like) related to supra-subduction zone in a fore-arc environment. The peridotite has also been percolated by a melt with more tholeiite affinities precipitating plagioclase-rich wehrlite and thin gabbroic veins. Crystallographic preferred orientations of minerals show that clinopyroxene and orthopyroxene in the harzburgite and orthopyroxenite veins crystallized under the same deformation constraints as the depleted dunite. However, for low melt proportion, such as the clinopyroxenes in the dunite, their crystallization can be governed by epitaxial growth. Epitaxial growth is also possible in the ol-websterite where an area with low melt fraction (5% of clinopyroxenes), clinopyroxene and orthopyroxene <001> axes are parallel to olivine <100> axes, while in an area with higher melt fraction (28% of clinopyroxenes) clinopyroxenes record their own orientation with <001> axes at nearly 90? to the olivine <100> axes suggesting a reorientation of the constraints. The same relationship has been observed within the plagioclase-rich wehrlite suggesting the same deformation conditions during percolation of boninitic affinities melt and a more tholeiitic affinities melt. The major inferred slip system in olivine is (001)[100] E-type slip with the possible activation of both (001)[100] E-type and (010)[100] A-type slip systems, which are activated at high-temperature low-stress conditions. Olivine E-type slip is predicted to operate within the mantle wedge in a back-arc position however the Marum peridotite has a fore-arc geodynamic position and chemical composition similar to mantle from SSZ. As a consequence, the Marum peridotite is considered as mantle in front of the arc located near the E-type to B-type transition.

  17. Origin of ophiolite complexes related to intra-oceanic subduction initiation: implications of IODP Expedition 352 (Izu-Bonin fore arc)

    NASA Astrophysics Data System (ADS)

    Robertson, Alastair; Avery, Aaron; Carvallo, Claire; Christeson, Gail; Ferré, Eric; Kurz, Walter; Kutterolf, Steffen; Morgan, Sally; Pearce, Julian; Reagan, Mark; Sager, William; Shervais, John; Whattam, Scott; International Ocean Discovery Program Expedition 352 (Izu-Bonin-Mariana Fore Arc), the Scientific Party of

    2015-04-01

    Ophiolites, representing oceanic crust exposed on land (by whatever means), are central to the interpretation of many orogenic belts (e.g. E Mediterranean). Based mostly on geochemical evidence, ophiolites are widely interpreted, in many but by no means all cases, as having formed within intra-oceanic settings above subduction zones (e.g. Troodos ophiolite, Cyprus). Following land geological, dredging and submersible studies, fore arcs of the SW Pacific region became recognised as likely settings of supra-subduction zone ophiolite genesis. This hypothesis was tested by recent drilling of the Izu-Bonin fore arc. Four sites were drilled, two on the outer fore arc and two on the upper trench slope. Site survey seismic data, combined with borehole data, indicate that three of the sites are located in fault-controlled sediment ponds that formed in response to dominantly down-to the-west extensional faulting (with hints of preceding top-to-the-east compressional thrusting). The sediments overlying the igneous basement, of maximum Late Eocene to Recent age, document ash and aeolian input, together with mass wasting of the fault-bounded sediment ponds. At the two more trenchward sites (U1440 and U1441), mostly tholeiitic basalts were drilled, including massive and pillowed lavas and hyaloclastite. Geochemically, these extrusives are of near mid-oceanic ridge basalt composition (fore arc basalts). Subtle chemical deviation from normal MORB can be explained by weakly fluid-influenced melting during decompression melting in the earliest stages of supra-subduction zone spreading (not as 'trapped' older MORB). The remaining two sites, c. 6 km to the west (U1439 and U1442), penetrated dominantly high-magnesian andesites, known as boninites, largely as fragmental material. Their formation implies the extraction of highly depleted magmas from previously depleted, refractory upper mantle in a supra-subduction zone setting. Following supra-subduction zone spreading, the active modern arc formed c. 200 km westwards of the trench. The new drilling evidence proves that both fore arc-type basalt and boninite formed in a fore arc setting soon after subduction initiation (c.52 Ma). Comparisons with ophiolites reveal many similarities, especially the presence of fore arc-type basalts and low calcium boninites. The relative positions of the fore arc basalts, boninites and arc basalts in the Izu Bonin and Mariana forearc (based on previous studies) can be compared with the positions of comparable units in a range of ophiolite complexes in orogenic belts including the Troodos, Oman, Greek (e.g. Vourinos), Albanian (Mirdita), Coast Range (California) and Bay of Islands (Newfoundland) ophiolites. The comparisons support the interpretation that all of the ophiolites formed during intra-oceanic subduction initiation. There are also some specific differences between the individual ophiolites suggesting that ophiolites should be interpreted individually in their regional tectonic settings.

  18. Ophiolitic mélanges in crustal-scale fault zones: Implications for the Late Palaeozoic tectonic evolution in West Junggar, China

    NASA Astrophysics Data System (ADS)

    Chen, Shi; Pe-Piper, Georgia; Piper, David J. W.; Guo, Zhaojie

    2014-12-01

    The Baijiantan and Darbut ophiolites in West Junggar are exposed in steep fault zones (>70°) containing serpentinite mélange, in contact on either side with regionally distributed Upper Devonian-Lower Carboniferous ocean floor peperitic basalts and overlying sedimentary successions. The ophiolitic mélanges show classic structural features created by strike-slip faulting and consistent shear sense indicators of left-slip kinematics. Sandstone blocks within the mélanges resemble the surrounding sediments in lithology and age, indicating that the ophiolitic mélanges consist of locally derived rocks. The ophiolitic mélanges therefore originated from left-slip fault zones within a remnant basin and are not plate boundaries nor subduction suture zones. Sandstone is the youngest lithology involved in the mélange and provides a maximum age for the mélange of 322 Ma, whereas stitching plutons are younger than 302 Ma. Multiple clusters in zircon ages from single gabbro blocks in the mélange at ~375, ~360, ~354, and ~340 Ma are inconsistent with accretionary incorporation of subducting ocean crust but rather suggest that episodic movement of the faults provided pathways for magma from the mantle into magma chambers. Late Paleozoic tectonic evolution of West Junggar involved Late Devonian to Carboniferous relative motion between the Junggar block and West Junggar ocean basin, which triggered the left-slip fault zones within a remnant ocean basin, along which the oceanic crust was disrupted to form linear ophiolitic mélanges. Final filling of this remnant ocean basin and its dismemberment by strike-slip faulting occurred in the late Carboniferous, followed by crustal thickening by juvenile granites at the Carboniferous-Permian boundary.

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

    NASA Astrophysics Data System (ADS)

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

    2012-06-01

    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.

  20. Slab and sediment melting during subduction initiation: granitoid dykes from the mantle section of the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Rollinson, Hugh

    2015-09-01

    New geochemical data are presented for a suite of tonalites, granodiorites, trondhjemites and granites intrusive into depleted mantle harzburgites of the Oman-UAE ophiolite. A detailed field, petrological and geochemical examination suggests that these `mantle granitoids' are the product of three processes: (a) the mixing of melts derived from both mafic and metasedimentary sources, (b) interaction with the mantle harzburgite host and (c) the fractional crystallisation of plagioclase, hornblende ± accessory phases. Geochemical data are used to characterise the identity of the protolith(s) by first screening the data for those samples which have experienced fractional crystallisation during emplacement. The resultant `reduced' data set has moderately fractionated REE, with small negative Eu anomalies and fractionated primitive mantle-normalised trace element patterns with high concentrations of fluid mobile elements and lower concentrations of HFS elements and with positive peaks for Rb and Pb and negative troughs for Ba, Nb, Sr and Ti. The character of the protolith was quantified using a melting model based upon a MORB-type basalt similar in composition to the Oman Geotimes lavas and a model using the MUQ (MUd from Queensland) global sediment composition (Kamber et al. Geochim Cosmochim Acta 69:1041-1058, 2005) both with an amphibolite/granulite facies mineralogy. The two compositions bracket the mantle granitoid data set with partial melts of the MORB source yielding trace element compositions lower than the granitoids, whereas melts of the MUQ source yield melts with compositions higher than the granitoids. Mixing of the calculated melt compositions indicates that the measured granitoid compositions represent between 10 and 30 % mixing of a metasedimentary melt into the melt of a mafic source. Current petrological, structural and geochronological data suggest a model for the origin of the Oman ophiolite in which it is formed by spreading above a subduction zone. The results of this study support this model in three ways: (a) trace element data show that the granitoids have interacted with the mantle, implying that they originated below the mantle section of the ophiolite and were emplaced into the mantle wedge from below; (b) a most probable setting for the melting of a sediment-basalt mix is in the upper part of a subducting slab; (c) the high temperature/shallow melting of the sediment and basaltic slab mix can only have been driven by the high temperatures of the overlying mantle wedge; this is most likely during subduction initiation. Thus, sediment melting during subduction initiation represents an important means of refertilising what is otherwise highly depleted mantle.

  1. Mapping of Moho and Moho Transition Zone (MTZ) in Samail ophiolites of Sultanate of Oman using remote sensing technique

    NASA Astrophysics Data System (ADS)

    Rajendran, Sankaran; Nasir, Sobhi

    2015-08-01

    Moho and Moho Transition Zone (MTZ) of the Samail ophiolite of Sultanate of Oman are characteristic to potential occurrences of chromite deposit, hydrothermal mineralization and serpentinization. Mapping of Moho and MTZ, and discriminating them in between the mafic and ultramafic rocks in ophiolite sequence are more significant and important. The present study describes the remote sensing spectral characters of minerals and rocks of the Moho and MTZ and discriminates the Moho of Wadi Al Abyad of Nakhl massif, and Wadi Nidab and Wadi Abda regions of Sumail massif in the visible and near infrared (VNIR), and short wavelength infrared (SWIR) spectral regions using low-cost multispectral satellite data of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). Results of this study show that the red-green-blue (RGB) color composite images of ASTER spectral bands 8, 3 and 1, and 8, 7 and 4 are able to delineate the Moho and MTZ of the regions. The RGB images of ASTER band ratios (4/8, 4/1, 3/2 * 4/3 and (1 + 3)/2, (4 + 6)/5, (7 + 9)/8) are capable to discriminate the mantle material (ultramafic harzburgites) and crustal rocks (mafic gabbros). The occurrence of such rock types is demonstrated by detection of their minerals using Spectral Angle Mapper (SAM) image processing method. The presence of Moho and MTZ, and associated lithologies are verified in field at Wadi Al Abyad, Wadi Nidab, Wadi Abda, Wadi Tayin, Wadi Fizh and several locations of Nakhl regions of Samail ophiolites. The laboratory study shows the occurrence of typical minerals namely olivine, orthopyroxene and clinopyroxene in the harzburgite and the minerals such as plagioclase, clinopyroxene, hornblende, orthopyroxene and olivine in the layered gabbro. The spectral properties of the rocks are studied using Portable Infrared Mineral Analyzer (PIMA) spectrometer and the occurrences of minerals are confirmed by X-ray diffraction (XRD) analyses. This study demonstrates the sensor capability of ASTER to map Moho and MTZ, and thus it is recommended to the geoscientists and geophysicists working on Moho and MTZ. The satellite image interpretations of Moho and MTZ, and the associated rocks and minerals are consistent with the field research studies carried out in these regions. Our images have potential to map the chromites mineralized zone and dunites (refractory materials) bearing MTZ.

  2. Hydrothermal Alteration of the Lower Oceanic Crust: Sr Isotopic Constraints from the CCSP CY-4 Drill Hole, Troodos Ophiolite, Cyprus

    NASA Astrophysics Data System (ADS)

    King, L. E.; Teagle, D. A.; Cooper, M. J.; Alt, J. C.

    2003-12-01

    Our understanding of the structure and geometry of mid-ocean ridge hydrothermal systems is severely hindered by a dearth of continuous samples into the plutonic complexes of the ocean crust as it is this region that provides the power to drive hydrothermal circulation and preserves a record of magma chamber processes. Although the Troodos ophiolite most probably formed in a supra-subduction zone environment, its well-preserved outcrops have made an invaluable contribution to our understanding of ocean ridge processes. CY-4 drilled as part of the Cyprus Crustal Study Project provides a unique continuous sample of 2263 m of the lower Troodos crust. The hole was initiated in sheeted dikes before grading into isotropic gabbros around 675 m. An intrusive transition is evident at 1330 m between the isotropic gabbros and lower layered gabbros, pyroxenites and crustal peridotites. Petrographic observations and strontium isotopic analyses presented here investigate the incursion of seawater derived hydrothermal fluids into the lower oceanic crust. Previous studies of the Troodos ophiolite have determined the primary igneous composition of the magmas (87Sr/86Sr =0.7032-0.7040) and the range of hydrothermal fluids (87Sr/86Sr =0.7047-0.7059). Samples from the sheeted dike complex are variably altered with strontium isotopic compositions elevated above the primary igneous signatures of the Troodos magmas but generally lower than the restricted hydrothermal range observed for dikes throughout the rest of the ophiolite. Samples proximal to the sheeted dike - gabbro boundary do yield strontium isotope ratios within the hydrothermal fluid range. Within a few hundred metres below the dike-gabbro transition most samples yield near primary strontium isotopic ratios suggesting only limited pervasive penetration of seawater-derived Sr. Most samples down to the bottom of the hole show little increase in Sr ratios. Rare hydrothermally altered samples with associated veins and secondary halos have 87Sr/86Sr elevated towards hydrothermal values indicating significant channelling of hydrothermal fluids at deeper levels in the Troodos crust.

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    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.

  4. Evidence of melting, melt percolation and deformation in a supra-subduction zone (Marum ophiolite complex, Papua New Guinea)

    NASA Astrophysics Data System (ADS)

    Kaczmarek, Mary-Alix; Jonda, Leo; Davies, Hugh L.

    2015-08-01

    New geochemical and microstructural data are presented for a suite of ultramafic rocks from the Marum ophiolite in Papua New Guinea. Our results describe a piece of most depleted mantle made essentially of dunite and harzburgite showing compositions of supra-subduction zone peridotite. Strong olivine crystallographic preferred orientations (CPOs) in dunite and harzburgite inferred the activation of both (001)[100] and (010)[100] slip systems, which are activated at high-temperature and low-stress conditions. Clinopyroxene and orthopyroxene CPOs inferred the activation of (100)[001] and (010)[001] slip systems, which are common for pyroxenes deformed at high temperature. This plastic deformation is interpreted to have developed during the formation of the Marum ophiolite, prior to melt percolation. The orientation of the foliation and olivine [100] slip directions sub-parallel to the subduction zone indicates that mantle flow was parallel to the trench pointing a fast polarisation direction parallel to the arc. This provides new evidence that fast polarisation direction parallel to the arc could be caused by anisotropic peridotite and not by olivine [001] slip. After its formation, Marum ophiolite has been fertilised by diffuse crystallisation of a low proportion of clinopyroxene (1-2 %) (P1) and formation of cm-scale ol-clinopyroxenite and ol-websterite veins cross-cutting the foliation (P2). This percolating melt shows silica-rich magnesian affinities (boninite-like) related to supra-subduction zone in a young fore-arc environment. The peridotite has also been percolated by a melt with more tholeiite affinities precipitating plagioclase-rich wehrlite and thin gabbroic veins (P3); these are interpreted to form after the boninitic event. The small proportion of newly crystallised pyroxene in the dunite shows similar orientation of crystallographic axes to the host dunite (<100>ol parallel to <001>cpx-opx). In contrast, the pyroxenes in ol-clinopyroxenite, ol-websterite and the thin gabbroic veins in the wehrlite, record their own orientation with <001> axes at 45°-60° to olivine <100> axes. Our results indicate that for low melt proportion the crystallisation is governed by epitaxial growth, and when the proportion of melt is higher the newly formed minerals record syn-kinematic crystallisation. This switch of crystallographic axes orientation of newly formed minerals indicates a reorientation of the constraints during the boninitic and tholeiitic melts event probably due to a variation of lateral mantle flow within the fore-arc area. The variation of the crystallographic axes orientation could be an indicator for the development of a young fore-arc mantle in supra-subduction zone.

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

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

    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.

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

    USGS Publications Warehouse

    Foose, M.P.; Economou, M.; Panayiotou, A.

    1985-01-01

    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. ?? 1985 Springer-Verlag.

  7. Magma system along fast-spreading centers controlled by ridge segmentation: Evidence from the northern Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Miyashita, Sumio; Adachi, Yoshiko

    2013-04-01

    Mid-ocean ridges are segmented at various scales with a hierarchy, from the biggest 1st- order to the smallest 4th-order segments. These segment structures control magmatic processes beneath the mid-ocean ridges such as mantle upwelling, partial melting of the upper mantle, and magma delivery system to form the oceanic crust (Macdonald, 1998). However, systematic studies on the segment control for magmatic processes are rare at modern mid-ocean ridges due to the difficulty of obtaining in-situ samples from different crustal-lithospheric depths. Sampling at ocean floors is generally exclusively limited only to the surface (i.e. the seafloor). Furthermore, the samples obtained from the surface of the ocean floor may likely represent the products of off-axis magmatism (Kusano et al., 2012). Therefore, studies of ocean ridge segmentation in ophiolites provide important constraints for the magmatic processes beneath seafloor spreading centers, because the precise 3-D architecture of the upper mantle and the crust (all the way to the uppermost extrusive layer) and their lateral variations could be observed and investigated in ophiolites. We have studied the northern Oman ophiolite where a complete succession from the upper mantle peridotites to the uppermost extrusive rocks is well exposed. Miyashita et al. (2003), Adachi and Miyashita (2003) and Umino et al. (2003) proposed a segment structure in the northern Oman ophiolite; the Wadi Fizh area is regarded as a northward propagating tip of a mid-ocean ridge based on geological evidence (Adachi and Miyashita. 2003). On the other hand, the Wadi Thuqbah area, about 25 km south of Wadi Fizh, is regarded as a segment center based on the thickest Moho transition zone, well developed EW-trending lineations in the MTZ and layered gabbro, and the comparatively primitive compositions of the layered gabbros. Furthermore, the southern margin of the Hilti block (Salahi block), about 40 km south of Wadi Thuqbah, is inferred to be the segment-end, based on the compositional variations within the sheeted dike complex (Miyashita et al., 2003). In this presentation, we document the occurrence of along-axis variations along the Moho transition zones, the wehrlite intrusions, the extrusive sequence and the upper gabbro unit, and discuss the significance of ridge segmentation as a major controlling factor of magmatic systems beneath mid-ocean ridges. Adachi, Y. and Miyashita, S., 2003, Geochem. Geophys. Geosyst. 4, 8619, DOI 10.1029/2001GC000272. Kusano, Y., Adachi, Y., Miyashita, S. and Umino, S., 2012,Geochem. Geophys. Geosys., 13, 2012 Q05012, doi:10.1029/2011GC004006. Macdonald, K. C., 1998, Geophys. Monograph, 106, 27-58. MacLeod, C. J. and Yaouancq, G., 2001, Earth and Planet. Sci. Lett., 176, 357-373. Miyashita, S., Adachi, Y. and Umino. S., 2003, Geochem. Geophys. Geosyst. 4, 8617, DOI 10.1029/2001GC000235. Umino, S., Miyashita, S., Hotta, F. and Adachi, Y., 2003, Geochem. Geophys. Geosyst. 4, 8618, DOI 10.1029/2001GC000233.

  8. Hydrogen generation during serpentinisation in ophiolite complexes: A comparison of H2-rich gases from Oman, Philippines and Turkey.

    NASA Astrophysics Data System (ADS)

    Beaumont, Valérie; Vacquand, Christèle; Deville, Eric; Prinzhofer, Alain

    2013-04-01

    H2-rich gas seepages in ultrabasic to basic contexts both in marine and continental environment are by-products of serpentinisation. Hydrothermal systems at MOR expose ultrabasic rocks to thermodynamic conditions favouring oxidation of FeII bearing minerals and water reduction. In continental context such thermodynamic conditions do not exist although active serpentinisation occurs in all known ophiolitic complexes (Barnes et al., 1978; Bruni et al., 2002; Cipolli et al., 2004; Boschetti and Toscani, 2008; Marques et al., 2008). Hyperalkaline springs are reported in these contexts as evidence of this active serpentinisation (Barnes et al., 1967) and are often associated with seepages of reduced gases (Neal and Stanger, 1983; Sano et al., 1993). Dry gas seepages are also observed (Abrajano et al., 1988, 1990; Hosgörmez, 2007; Etiope et al., 2011) Such H2-rich gases from ophiolite complexes were sampled in the Sultanate of Oman, the Philippines and Turkey and were analysed for chemical composition, noble gases contents, stable isotopes of carbon, hydrogen and nitrogen. The conditions for present-day serpentinisation in ophiolites were recognised as low temperature processes in Oman with high rock/water ratios (Neal and Stanger, 1985), while the origin of gases is not as univocal for Philippines and Turkey gas seepages. Although, H2 generation is directly linked with FeII oxidation, different reactions can occur during peridotite hydration (McCollom and Bach, 2009; Marcaillou et al., 2011) and serpentine weathering. Produced H2 can react with carbonate species to produce methane via processes that could be biological or abiotic, while carbon availability depends on water recharge chemistry. In the present study, the geochemical properties of gases sampled from three different ophiolite complexes are compared and provide evidence that weathering reactions producing H2 depend on structural, geological, geomorphologic and hydrological local features. REFERENCES Abrajano, T. A., et al. (1988). Chemical Geology, 71(1-3), 211-222. Abrajano, T. A, et al. (1990). Applied Geochemistry, 5(5-6), 625-630. Barnes, I., et al. (1967). Science (New York, N.Y.), 156(3776), 830-2. Barnes, I., et al. (1978). Geochimica et Cosmochimica Acta, 42(1), 144-145. Boschetti, T., & Toscani, L. (2008). Chemical Geology, 257(1-2), 76-91. Bruni, J., et al. (2002). Applied Geochemistry, 17, 455-474. Cipolli, F., et al. (2004). Applied Geochemistry, 19(5), 787-802. Etiope, G., et al. (2011). Earth and Planetary Science Letters, 310(1-2), 96-104. Hosgörmez, H. (2007). Journal of Asian Earth Sciences, 30(1), 131-141. Marcaillou, C., et al. (2011). Earth and Planetary Science Letters, 303(3-4), 281-290. Marques, J. M., et al. (2008). Applied Geochemistry, 23(12), 3278-3289. McCollom, T. M. & Bach, W. (2009). Geochimica et Cosmochimica Acta, 73(3), 856-875. Neal, C. & Stanger, G. (1983). Earth and Planetary Science Letters, 66(66), 315-320. Neal, C. & Stanger, G. (1985). In J. I. Dever (Ed.), The Chemistry of Weathering (pp. 249-275). D. Reidel Publishing Company. Sano, Y., et al. (1993). Applied Geochemistry, 8(1), 1-8.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    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.

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

    NASA Astrophysics Data System (ADS)

    D'Amico, M.; Previtali, F.

    2009-04-01

    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.

  11. A Model for Seamount Formation Based on Observations of a California Ophiolite

    NASA Astrophysics Data System (ADS)

    Schnur, S.; Gilbert, L. A.

    2009-12-01

    Seamounts are an important feature of the seafloor but relatively little is known about their internal structure. Previous work has examined seamount surface morphology via seafloor mapping and submersible observations, broad geophysical characteristics via remote methods and one-dimensional structure through drilling. However, to model the formation of seamounts we need a better understanding of two- and three-dimensional variations, which are difficult to study in any detail beyond the surface of the seafloor. Ophiolitic seamounts provide a unique window for studying these difficult to reach features. This study proposes a model for seamount formation using field observations and laboratory measurements of a seamount preserved in the subduction-related Cretaceous Franciscan formation of northern California. The preserved seamount in our study site likely formed in water deeper than 1.5 km, reached over 1 km high, and may have been formed at or near a ridge axis, with some similarities to seamounts formed on or near the modern East Pacific Rise. The relative paucity of faulting and fractured shear zones throughout the field area indicates little or no rearrangement of units during emplacement of the seamount onto land. The dominant lava flow morphology is pillow lavas, which constitute roughly three-quarters of observed flows. Massive flows and hyaloclastite and pillow fragment (HPF) flows each represent about one-eighth of the total thickness of flows, and sheet and lobate flows are both relatively minor. Volcanic facies are further classified based on density, porosity, igneous and metamorphic petrography, and major and trace element geochemistry. Based on variations in flow morphology and related physical properties, we generalize seamount formation into three phases. A first ‘basal’ stage is dominated by small, tightly-packed, lower-porosity pillows which form at greater water depths atop existing oceanic crust. A second ‘intermediate’ stage is represented by several individual flow sequences, with the number and thickness of flow sequences present dependent upon rate and duration of eruption at a specific seamount. A typical flow sequence includes massive flows at the base, followed by pillow lavas and capped by HPF flows. The pillow lavas of this stage are more vesicular than the basal stage pillows and have a relatively high fraction of inter-pillow hyaloclastite, in part because they are emplaced above the initial basal flows in shallower water. Hydrothermal alteration of the larger pillows and field relations imply the intermediate stage lavas at this site may have been emplaced within a collapsed caldera structure. Finally, a third ‘cap’ stage occurs on the upper edifice of a seamount, at the end of an eruptive period. This cap stage is not dominated by a single morphology, but instead includes significant variation in flow morphology and the increased presence of sheet and HPF flows. Our model has important implications for understanding the structure and formation of seamounts, and provides context for one-dimensional drill holes, surface observations, and regional-scale geophysical measurements of seamounts.

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

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

    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 μm 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.

  13. 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)

    Etiope, Giuseppe; Schoell, Martin; Hosgörmez, Hakan

    2011-10-01

    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 (δD 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.

  14. Strain localization and fluid infiltration during subduction initiation: the record from sheared mafic amphibolites at the base of the New Caledonian ophiolite

    NASA Astrophysics Data System (ADS)

    Soret, Mathieu; Agard, Philippe; Dubacq, Benoît; Vitale-Brovarone, Alberto; Monié, Patrick; Chauvet, Alain; Whitechurch, Hubert

    2015-04-01

    Most of our knowledge on subduction inception and obduction processes comes from metamorphic soles structurally associated with peridotite tectonites at the base of many ophiolites, and from early-obduction, rarely deformed, magmatic dikes emplaced at different level of the mantle sequence. These dikes record a partial refertilization of obducted ophiolites through subduction-derived fluids. However, these dikes are rarely deformed and/or metamorphosed. Here, we study the base of the New Caledonian ophiolite, using a combination of structural field studies and petrological-geochemical-geochronological analysis, with the aim of linking deformation and metasomatism through fluid infiltration and recrystallization. We report the existence of strongly sheared mafic amphibolites within the base of the New Caledonian obducted ophiolite, ~ 50-100 m above the basal thrust contact and < 1000 m below the mantle-crust transition. These ~ N150-striking, hm-long and m- to several m-thick shear bands correspond to former small-scale intrusions (and surrounding peridotites), highly boudinaged and amphibolitized at high temperatures (750-800 °C), providing evidence that strain localized at the base of the ophiolite. Mafic protoliths of these amphibolites consisted of plagioclase and orthopyroxene (± olivine and calcic amphibole in places). We show that deformation is intimately associated to at least three major stages of fluid infiltration on mafic intrusions. The first stage of deformation and metasomatism coincides with amphibolitization and controlled the later channelization of fluids. The formation of calcic amphiboles records the percolation of Ca and Al-rich aqueous fluids. Amphibole-plagioclase geothermobarometry indicates high temperature and low pressure conditions (i.e. 750-800 °C; 3-5 kbar). Thermochronological data from hornblende (40Ar/39Ar) suggest that this deformation episode occurred at ~ 55 Ma, coinciding with E-dipping subduction initiation and incipient obduction. The main metasomatic stage is evidenced by a phlogopite-rich matrix wrapping peridotite and amphibolite boudins. The formation of phlogopite records the percolation of alkali-rich aqueous fluids at still high temperature (700-750 °C). The last metasomatic stage is characterized by infiltration of aqueous fluids at lower temperature (< 500 °C), in the stability field of talc, chlorite and serpentine, and results in the formation of deformed veinlets wrapping and cross-cutting peridotites boudins. This study documents a valuable record of refertilization (by successive fluid influx) and progressive deformation to better understang the mechanisms controlling subduction initiation and early obduction of the New Caledonian ophiolite.

  15. Southward trench migration at ∼130-120 Ma caused accretion of the Neo-Tethyan forearc lithosphere in Tibetan ophiolites

    NASA Astrophysics Data System (ADS)

    Xiong, Qing; Griffin, William L.; Zheng, Jian-Ping; O'Reilly, Suzanne Y.; Pearson, Norman J.; Xu, Bo; Belousova, Elena A.

    2016-03-01

    The preservation of ultrahigh-pressure and super-reduced phases (diamond, moissanite, etc.) in the harzburgites and chromitites of the Yarlung Zangbo ophiolites (South Tibet, China) has major implications for mantle recycling and lithosphere evolution in the tectonic system related to the closing of the Neo-Tethyan Ocean. However, important aspects of the genesis of these enigmatic ophiolites and the related geodynamic evolution are still unclear. In the Zedang ophiolite of the eastern Yarlung Zangbo Suture, detailed mineral chemical data reveal that the harzburgite domain in the east [spinel Cr# (mole Cr3+/(Cr3+ + Al3+) = 0.62-0.33] is more depleted than the lherzolite domain in the west (spinel Cr# = 0.30-0.17) and shows much lower equilibration temperatures (by ∼250-150 °C) than the lherzolites. Clinopyroxene trace-element compositions indicate that the harzburgites underwent pervasive metasomatism after melt extraction, while the lherzolites did not. New zircon U-Pb ages show that the harzburgites were intruded by dolerite dykes with chilled margins at ∼130-128 Ma, consistent with the widespread mafic magmatism at ∼130-120 Ma in the Yarlung Zangbo ophiolites. Nd-Hf isotopic data indicate that the Zedang lherzolites subcreted the pre-emplaced harzburgites concurrently with the intrusion of the dolerite dykes into the harzburgites, and that the lherzolites and dolerites both were derived from upwelling asthenosphere with minor slab input. Available zircon geochronology and Hf-isotope data show that juvenile magmatism in the adjacent Gangdese Arc was almost completely interrupted from ∼130-120 Ma. We suggest that the extension of the overlying harzburgitic lithosphere, subcretion of lherzolites, intrusion of mafic dykes, and the waning of Gangdese-Arc magmatism all reflect a southward trench migration in the Neo-Tethyan subduction system from the Gangdese Arc to the oceanic forearc lithosphere. This magmatic relocation and tectonic linkage are inferred to be the far-field effects of plate reorganization related to the Lhasa-Qiangtang collision and the breakup of Gondwanaland in the early Cretaceous. This model provides a solution for the global "ophiolite conundrum": supra-subduction-zone type ophiolites with mid-ocean-ridge features.

  16. 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)

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

    2014-01-01

    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.

  17. Ground-based Hyperspectral Remote Sensing for Mapping Rock Alterations and Lithologies: Case Studies from Semail Ophiolite, Oman and Rush Springs Sandstone, Oklahoma

    NASA Astrophysics Data System (ADS)

    Sun, L.; Khan, S.; Hauser, D. L.; Glennie, C. L.; Snyder, C.; Okyay, U.

    2014-12-01

    This study used ground-based hyperspectral remote sensing data to map rock alterations and lithologies at Semail Ophiolite, Oman, as well as hydrocarbon-induced rock alterations at Cement, Oklahoma. The Samail Ophiolite exposed the largest, least-deformed, and the most-studied ophiolite in the world. Hydrocarbon seepages at Cement, Oklahoma brought hydrocarbons to the Rush Springs sandstones at surface, and generated rock alterations including bleaching of red beds, and carbonate cementation. Surficial expressions of rock alterations and different lithofacies are distinct from adjacent rocks, and can be detected by remote sensing techniques. Hyperspectral remote sensing acquires light intensity for hundreds of bands in a continuous electromagnetic spectrum from visible light to short-wave infrared radiation, and holds potential to characterize rocks with great precision. Ground-based hyperspectral study could scan the objects at close ranges thus provide very fine spatial resolutions (millimeters to centimeters). This study mapped all the major iconic outcrops of Semail ophiolite including pillow lava, sheeted dykes, layered gabbros, and peridotites. This study also identified surficial rock alterations induced by hydrocarbons at Cement, Oklahoma. Reddish-brown Rush Spring sandstones are bleached to pink, yellow, and gray colors; pore spaces in the sandstones have been filled with carbonate cementation. Laboratory spectroscopy was used to assist with mineral identification and classification in hyperspectral data. Terrestrial laser scanning (TLS) was used to provide high-accuracy spatial references. Principal component analysis, minimum noise fraction, spectral angle mapper, and band ratios are used in image processing. Combining lithological, remote sensing and geochemical data, this study built a model for petroleum seepage and related rock alterations, and provided a workflow for employing ground-based hyperspectral remote sensing techniques in petrological study as well as resource exploration.

  18. Thermodynamic modelling of Sol Hamed serpentinite, South Eastern Desert of Egypt: Implication for fluid interaction in the Arabian-Nubian Shield ophiolites

    NASA Astrophysics Data System (ADS)

    Abu-Alam, Tamer S.; Hamdy, Mohamed M.

    2014-11-01

    The Arabian-Nubian Shield is the largest tract of juvenile continental crust of the Neoproterozoic. This juvenile crust is composed of intra-oceanic island arc/back arc basin complexes and micro-continents welded together along sutures as the Mozambique Ocean was closed. Some of these sutures are marked by ophiolite decorated linear belts. The Sol Hamed ophiolite (808 ± 14 Ma) in southeastern Egypt at the Allaqi-Heiani-Onib-Sol Hamed-Yanbu arc-arc suture represents an uncommon example of rocks that might be less deformed than other ophiolites in the Arabian-Nubian Shield. In order to understand fluid-rock interactions before and during arc-arc collision, petrological, mineral chemistry, whole-rock chemistry and thermodynamic studies were applied to the Sol Hamed serpentinized ophiolitic mantle fragment. These studies reveal that the protolith had a harzburgite composition that probably originated as forearc mantle in the subducted oceanic slab. We propose that these rocks interacted with Ti-rich melts (boninite) in suprasubduction zone, which latter formed the Sol Hamed cumulates. Spinel's Cr# associated with the whole rock V-MgO composition suggest that the harzburgites are highly refractory residues after partial melting up to 29%. The melt extraction mostly occurred under reducing conditions, similar to peridotites recovered from the subducted lithosphere. Protolith alteration resulted from two stages of fluid-rock interaction. The first stage occurred as a result of infiltration of concentrated CO2-rich fluid released from carbonate-bearing sediments and altered basalt at the subduction zone. The alteration occurred during isobaric cooling at a pressure of 1 kbar. The fluid composition during the isobaric cooling was buffered by the metamorphic reactions. The second stage of fluid-rock interactions took place through prograde metamorphism. The increase in pressure during this stage occurred as a result of thrusting within the oceanic crust. In this process the forearc crust was loaded by roughly 20-30 km of overthrust rocks.

  19. Melanges Pedagogiques (Pedagogical Mixture), 1988.

    ERIC Educational Resources Information Center

    Melanges Pedagogiques, 1988

    1988-01-01

    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…

  20. Melanges Pedagogiques (Pedagogical Mixture), 1985.

    ERIC Educational Resources Information Center

    Melanges Pedagogiques, 1985

    1985-01-01

    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…

  1. Melanges Pedagogiques (Pedagogical Mixture), 1983.

    ERIC Educational Resources Information Center

    Melanges Pedagogiques, 1983

    1983-01-01

    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)"…

  2. Melanges Pedagogiques (Pedagogical Mixture), 1983.

    ERIC Educational Resources Information Center

    Melanges Pedagogiques, 1983

    1983-01-01

    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)"…

  3. Melanges Pedagogiques (Pedagogical Mixture), 1988.

    ERIC Educational Resources Information Center

    Melanges Pedagogiques, 1988

    1988-01-01

    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…

  4. Melanges Pedagogiques (Pedagogical Mixture), 1985.

    ERIC Educational Resources Information Center

    Melanges Pedagogiques, 1985

    1985-01-01

    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…

  5. Melanges Pedagogiques (Pedagogical Mixture), 1984.

    ERIC Educational Resources Information Center

    Melanges Pedagogiques, 1984

    1984-01-01

    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…

  6. Eclogitic breccia from the Monviso ophiolite complex: new field and petrographic data

    NASA Astrophysics Data System (ADS)

    Locatelli, Michele; Verlaguet, Anne; Federico, Laura; Agard, Philippe

    2015-04-01

    The Monviso meta-ophiolite complex (Northern Italy, Western Alps) represents a coherent portion of oceanic lithosphere metamorphosed under eclogite facies conditions during the Alpine orogeny (2.6 GPa - 550°C, Lago Superiore Unit, Angiboust et al., 2011), and exhibits from bottom to top a thick serpentinite sole locally capped by metasediments, Mg-Al-rich metagabbros, then Fe-Ti-metagabbros capped by metabasalts. This section is disrupted by three main shear zones. Our study focusses on the Lower Shear Zone (LSZ), situated between the serpentinite sole (to the East) and the Mg-metagabbro bodies (to the West), and composed of blocks of both Fe-Ti and Mg-Al metagabbros embedded in a talc and tremolite-rich serpentinite matrix. Among these blocks, some were described as eclogitic breccias and interpreted as the result of a seismic rupture plane (Angiboust et al., 2012). These breccias correspond to blocks of Fe-Ti-metagabbros that were brecciated in eclogitic facies conditions (as attested by the omphacite + garnet ± lawsonite cement of the breccia) in a fluid-rich environment, as suggested by the abundance of lawsonite in the cement. Here we present new field data on the distribution and petrographic characterization of these eclogitic blocks in the LSZ. The aim of this work is twofold: (I) detailed mapping of the eclogitic block distribution along the LSZ, in order to determine precisely the extent and representativity of the breccias and (II) characterization of the brecciated blocks, at the outcrop scale, to explore the brecciation processes and structures. Between Pian del Re and Colle di Luca localities, the occurrence of eclogite blocks is uniform along the strike of the shear-zone, resulting in a 16 km-long belt of outcropping eclogitic bodies embedded in serpentinite matrix. The shear-zone width, by contrast, varies from 1.3 km to 0.8 km. Three types of eclogitic blocks can be distinguished: (1) intact (i.e., not brecciated) blocks of Fe-Ti-metagabbros restricted to the lower part of the shear zone, close to the serpentinite sole; (2) numerous brecciated Fe-Ti-metagabbros scattered in the intermediate to upper levels of the LSZ; (3) blocks showing compositional variations and complex structures, with boudins of intact Fe-Ti-metagabbros embedded in highly foliated and folded Mg-rich rocks bounded on one side by Fe-Ti-breccia planes. In some cases the full transition from intact to highly brecciated rock is recorded in the same block. Here, the contacts between intact metagabbros and breccia are characterized by about 1m-wide zones of non rotated clasts with diameter up to 80 cm, almost matrix-absent. The amount of matrix vs clast increases, associated with a reduction in the clast size and increasing clast rotation, over a few meters up to the end of the bodies. These particular blocks give us a unique opportunity to better characterize the brecciation processes. Different kinds of measurements were realized on the brecciated blocks: (1) block size, (2) clasts vs. matrix relative volumetric abundances, (3) dimension and shape ratio of clasts, and angle of misorientation between their elongation axis or internal foliations (for five selected blocks). Preliminary results show that the majority (82%) of mapped blocks have a diameter of less than 10 meters, with only 8% being larger than 20 meters. In the brecciated Fe-Ti gabbros the average content of matrix is 28%, while for blocks showing compositional variation it varies from zero to 30%. The angle of misorientation between clasts' foliation shows, instead, a chaotic distribution. Preliminary field data thus demonstrate that breccia blocks have to be considered as a constant feature along the LSZ rather than as an exception, and that further work is needed to determine whether they formed through pervasive brecciation (and potentially multiple events) or through a localized event and were later disrupted by ductile deformation along the LSZ.

  7. Petrogenesis of chromites from the Manipur ophiolite belt, NE India: evidence for a supra-subduction zone setting prior to Indo-Myanmar collision

    NASA Astrophysics Data System (ADS)

    Pal, Tapan; Bhattacharya, Anindya; Nagendran, G.; Yanthan, N. M.; Singh, R.; Raghumani, N.

    2014-10-01

    The Manipur ophiolite belt within the Western Ophiolite Belt of the Indo-Myanmar Ranges (IMR), consists of tectonised to massive serpentinised peridotite, dunite pods, chromitite pods/lenses, cumulates, dykes, volcanic rocks and pelagic sediments. Chromitite pods and lenses hosted in peridotitic mantle rocks show magmatic textures, post magmatic brecciation and ferritchromitisation. Electron microprobe analyses show two types of massive chromitite, with one group having high-Cr (Cr# 75-76), medium-Al (Al2O3 12.2-12.4 wt%) chromites (Sirohi-type) and the other group (Gamnom-type) having a wide range of compositions with generally lower Cr and higher Al (Cr# 65-71, Al2O3 15.7-19 wt%). Accessory chromites in peridotitic mantle rocks have consistently low Cr (Cr# 38-39) and high Al (Al2O3 34-35 wt%), whereas chromites in dunite pods have intermediate compositions (Cr# ~60; Al2O3 20.7-21.2 wt%). The chromite chemistry suggests moderate (20 %) partial melting of the tectonised mantle harzburgite. The estimated Al2O3melt, (FeO/MgO)melt and TiO2melt for the Sirohi-type chromites indicate boninitic parentage, whereas chromite compositions from the Gamnom area suggest mixed boninitic—island arc tholeiitic magmas. The compositions of magmatic chromites suggest that the Manipur ophiolite was formed in a supra-subduction zone (SSZ) setting.

  8. 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)

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

    2013-03-01

    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.

  9. Oceanic faulting and fault-controlled subseafloor hydrothermal alteration in the sheeted dike complex of the Josephine Ophiolite

    NASA Astrophysics Data System (ADS)

    Alexander, Robert J.; Harper, Gregory D.; Bowman, John R.

    1993-06-01

    Based upon detailed mapping (1:10 and 1:100) of a large water-polished outcrop of the sheeted dike/gabbro transition zone in the Josephine ophiolite of NW California and SW Oregon, the following history of alternating episodes of magmatic, structural, and hydrothermal events has been documented using crosscutting relationships, petrography, geochemistry, and strontium and oxygen isotopic data: (1) crystallization of gabbro and later subvertical mafic dike injection, (2) amphibolite facies metamorphism, (3) extensional faulting and tilting of dikes, (4) continued faulting, tilting, and dike injection associated with retrograde metamorphism at greenschist facies conditions, (5) continued extensional faulting and tilting synchronous with the development of a variety of hydrothermal veins at decreasing temperature and increasing fluid/rock ratios, and (6) subvertical injection of a highly fractionated dike which truncates all previous features (features 1-5). Trace element geochemistry indicates the highly fractionated dike (feature 6) is genetically related to the other dikes and thus was intruded at or near the paleospreading axis. Hence, all previous events (features 1-5) can be constrained to have occurred at the rift axis, including large-scale tilting (˜ 50°) of the sheeted dikes and extensional faulting. The [87Sr/86Sr]initial ratios from recrystallized whole rocks and from hornblende, epidote, and prehnite separates from veins display a systematic increase with relative age, from 0.7033 for altered gabbro screens and mafic dikes to 0.7049 for prehnite in the youngest oceanic fault rock. Calculated oxygen isotope fluid compositions for the same suite of samples range from ?18Ofluid = +5 to -1 with time, indicating a change to a seawater-dominated hydrothermal system with time, consistent with observed increased permeability due to seafloor extensional faulting. The crosscutting relationships, alteration mineral assemblages, and isotopic data suggest (1) an early stage of high-temperature (?450°C) hydrothermal alteration with low permeability (i.e., grain-scale flow), followed by (2) a decrease in temperature (˜350to ?200°C), and an increase in permeability due to faulting and accompanied by tectonic tilting at the rift axis. The consistency of these crosscutting relationships at similar pseudo-stratigraphic levels at different localities in the Josephine ophiolite suggests that alternating magmatic and structural extension with synchronous retrograde alteration is common in crust formed at similar rates of spreading (slow- to intermediate-spreading centers), such as the modern Mid-Atlantic Ridge.

  10. Melt extraction and metasomatism recorded in basal peridotites above the metamorphic sole of the northern Fizh massif, Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Yoshikawa, Masako; Python, Marie; Tamura, Akihiro; Arai, Shoji; Takazawa, Eiichi; Shibata, Tomoyuki; Ueda, Akira; Sato, Tsutomu

    2015-05-01

    The Oman ophiolite is one of the best preserved sections of oceanic crust and upper mantle worldwide, and consists of multiple massifs that lie along more than 400 km of the Arabian coast. In the northernmost massifs, the oceanic crust preserves a record of polygenetic magmatism from mid-ocean ridge to subduction-related stages. The lherzolites and clinopyroxene (Cpx)-rich harzburgites of the Fizh block are located a few tens to a hundred meters above the metamorphic sole of the ophiolite and the geochemistry of these Cpx-rich peridotites provides evidence of a genetic link between oceanic crust and mantle. These Cpx-rich peridotites contain olivine with a restricted range of forsterite contents (90-91), but variable Cr-spinel Cr# (Cr/(Cr + Al) atomic ratio) values (0.12-0.33), suggesting that these Cpx-rich peridotites have undergone variable degrees of melt extraction. Cpxs within the Cpx-rich peridotites have chondrite-normalised trace element variation patterns that slope either gently or steeply between the heavy rare earth elements (REEs) and the middle REEs ((Sm/Yb)N = 0.08-0.55, where N chondrite-normalised) and are enriched in highly incompatible elements such as Rb, Ba and Nb. This Cpx chemistry can be explained by a polygenetic evolution whereby an initial 4-12% of melt was extracted from the depleted mantle source before this mantle was metasomatised by interaction with fluids derived from dehydration of the metamorphic sole during subduction initiation and obduction. A comparison between 143Nd/144Nd versus 147Sm/144Nd for Cpx in the Fizh basal Cpx-rich peridotites and a mineral-whole rock Sm-Nd isochron for a gabbro from the same massif suggests a genetic link between crustal and mantle rocks in this area. In addition, Cpxs within the basal Cpx-rich peridotites have highly variable Sr isotopic compositions that are indicative of a significant contribution of seawater from the metamorphic sole, originally derived from subducted oceanic crustal material.

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

    NASA Astrophysics Data System (ADS)

    Angiboust, Samuel; Oncken, Onno; Agard, Philippe

    2014-05-01

    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.

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

    NASA Astrophysics Data System (ADS)

    Farahat, E. S.

    2011-07-01

    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 (An3-8) in the epidote amphibolites to andesine and labradorite (An36-65) 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 (LaN/YbN = 0.29 to 0.49) to LREE-enriched (LaN/YbN = 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.

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

    NASA Astrophysics Data System (ADS)

    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

    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).

  14. Melt extraction and melt refertilization in mantle peridotite of the Coast Range ophiolite: an LA-ICP-MS study

    NASA Astrophysics Data System (ADS)

    Jean, Marlon M.; Shervais, John W.; Choi, Sung-Hi; Mukasa, Samuel B.

    2010-01-01

    The middle Jurassic Coast Range Ophiolite (CRO) is one of the most important tectonic elements in western California, cropping out as tectonically dismembered elements that extend 700 km from south to north. The volcanic and plutonic sections are commonly interpreted to represent a supra-subduction zone (SSZ) ophiolite, but models specifying a mid-ocean ridge origin have also been proposed. These contrasting interpretations have distinctly different implications for the tectonic evolution of the western Cordillera in the Jurassic. If an SSZ origin is confirmed, we can use the underlying mantle peridotites to elucidate melt processes in the mantle wedge above the subduction zone. This study uses laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to study pyroxenes in peridotites from four mantle sections in the CRO. Trace element signatures of these pyroxenes record magmatic processes characteristic of both mid-ocean ridge and supra-subduction zone settings. Group A clinopyroxene display enriched REE concentrations [e.g., Gd (0.938-1.663 ppm), Dy (1.79-3.24 ppm), Yb (1.216-2.047 ppm), and Lu (0.168-0.290 ppm)], compared to Group B and C clinopyroxenes [e.g., Gd (0.048-0.055 ppm), Dy (0.114-0.225 ppm), Yb (0.128-0.340 ppm), and Lu (0.022-0.05 ppm)]. These patterns are also evident in orthopyroxene. The differences between these geochemical signatures could be a result of a heterogeneous upper mantle or different degrees of partial melting of the upper mantle. It will be shown that CRO peridotites were generated through fractional melting. The shapes of REE patterns are consistent with variable degrees of melting initiated within the garnet stability field. Models call for 3% dry partial melting of MORB-source asthenosphere in the garnet lherzolite field for abyssal peridotites and 15-20% further partial melting in the spinel lherzolite field, possibly by hydrous melting for SSZ peridotites. These geochemical variations and occurrence of both styles of melting regimes within close spatial and temporal association suggest that certain segments of the CRO may represent oceanic lithosphere, attached to a large-offset transform fault and that east-dipping, proto-Franciscan subduction may have been initiated along this transform.

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

    USGS Publications Warehouse

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

    1990-01-01

    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.

  16. U/Pb, Sm/Nd and Rb/Sr geochronological and isotopic study of northern Sierra Nevada ophiolitic assemblages, California

    NASA Astrophysics Data System (ADS)

    Saleeby, J. B.; Shaw, H. F.; Niemeyer, Sidney; Moores, E. M.; Edelman, S. H.

    1989-06-01

    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.

  17. The Armenian and NW Anatolian ophiolites: new insights for the closure of the Tethys domain and obduction onto the South Armenian Block and Anatolian-Tauride Platform before collision through dynamic modeling

    NASA Astrophysics Data System (ADS)

    Hässig, Marc; Rolland, Yann; Sosson, Marc; Hassani, Riad; Topuz, Gultekin; Faruk Çelik, Ömer; Gerbault, Muriel; Galoyan, Ghazar; Müller, Carla; Sahakyan, Lilit; Avagyan, Ara

    2013-04-01

    In the Lesser Caucasus three main domains are distinguished from SW to NE: (1) the South Armenian Block (SAB), a Gondwanian-derived continental terrane; (2) scattered outcrops of ophiolites coming up against the Sevan-Akera suture zone; and (3) the Eurasian plate. The Armenian ophiolites represent remnants of an oceanic domain which disappeared during Eurasia-Arabia convergence. Previous works using geochemical whole-rock analyses, 40Ar/39Ar and paleontological dating have shown that the ophiolite outcrops throughout this area were emplaced during the Late Cretaceous as one non-metamorphic preserved ophiolitic nappe of back-arc origin that formed during Middle to Late Jurassic. From these works, tectonic reconstructions include two clearly identified subductions, one related to the Neotethys subduction beneath the Eurasian margin and another to intra-oceanic subduction responsible for the opening of the back-arc basin corresponding to the ophiolites of the Lesser Caucasus. The analysis of the two stages of metamorphism of the garnet amphibolites of the ophiolite obduction sole at Amasia (M1: HT-LP peak of P = 6-7 kbar and T > 630°C; M2; MP-MT peak at P = 8-10 kbar and T = 600°C) has allowed us to deduce the onset of subduction of the SAB at 90 Ma for this locality, which age coincides with other paleontological ages at the obduction front. A preliminary paleomagnetic survey has also brought quantification to the amount of oceanic domain which disappeared by subduction between the SAB and Eurasia before collision. We propose a dynamic finite element model using ADELI to test the incidence of parameters such as the density of the different domains (or the interval between the densities), closing speed (or speeds if sporadic), the importance and interactions of mantle discontinuities with the subducting lithosphere and set a lithospheric model. Our field observations and analyses are used to validate combinations of factors. The aim is to better qualify the predominant factors and quantify the conditions leading to the onset of obduction, the paradox of dense oceanic lithosphere emplaced on top of a continental domain, after subduction and prior to collision. The results of this modeling are also compared to new observations of the assumed eastward extension of this ophiolitic nappe in NW Anatolia. Analyses of the Refahiye ophiolites show similar geochemical signatures as the Armenian ophiolites, due to a similar setting of formation (back-arc). The impact of the obduction of such a vast oceanic domain is not to be taken for granted when considering the following collision stage.

  18. Tectonic Evolution of the Careón Ophiolite (Northwest Spain): A Remnant of Oceanic Lithosphere in the Variscan Belt.

    PubMed

    Díaz García F; Arenas; Martínez Catalán JR; González del Tánago J; Dunning

    1999-09-01

    Analysis of the Careón Unit in the Ordenes Complex (northwest Iberian Massif) has supplied relevant data concerning the existence of a Paleozoic oceanic lithosphere, probably related to the Rheic realm, and the early subduction-related events that were obscured along much of the Variscan belt by subsequent collision tectonics. The ophiolite consists of serpentinized harzburgite and dunite in the lower section and a crustal section made up of coarse-grained and pegmatitic gabbros. An Early Devonian zircon age (395+/-2 Ma, U-Pb) was obtained in a leucocratic gabbro. The whole section was intruded by numerous diabasic gabbro dikes. Convergence processes took place shortly afterward, giving rise to a mantle-rooted synthetic thrust system, with some coeval igneous activity. Garnet amphibolite, developed in metamorphic soles, was found discontinuously attached to the thrust fault. The soles graded downward to epidote-amphibolite facies metabasite and were partially retrogressed to greenschist facies conditions. Thermobarometric estimations carried out at a metamorphic sole (T approximately 650 degrees C; P approximately 11.5 kbar) suggested that imbrications developed in a subduction setting, and regional geology places this subduction in the context of an early Variscan accretionary wedge. Subduction and imbrication of oceanic lithosphere was followed by underthrusting of the Gondwana continental margin. PMID:10504137

  19. Magnetic properties of the Bay of Islands ophiolite suite and implications for the magnetization of oceanic crust

    USGS Publications Warehouse

    Swift, B. Ann; Johnson, H. Paul

    1984-01-01

    Rock magnetic properties, opaque mineralogy, and degree of metamorphism were determined for 101 unoriented samples from the North Arm and Blow-Me-Down massifs of the Bay of Islands ophiolite complex, Newfoundland. The weathered and metamorphosed extrusive basalt samples have a weak, secondary magnetization arising from oxidation and exsolution of ilmenite of unknown origin. The initial magnetization of the underlying sheeted dike complex appears to have been destroyed by hydrothermal alteration soon after formation. The magnetic intensity of the gabbroic samples increases as the degree of alteration increases, with the highly altered upper metagabbros having an average intensity of 3×10−3 emu/c3. Because magnetization of the metagabbro samples is related to nonpervasive, variable alteration, these crustal units are unlikely to make a significant contribution to lineated magnetic anomalies. A compilation of our results and other studies suggests a model in which oceanic crust magnetization results from an upper extrusive basalt source layer, roughly 600 m thick, with no contribution from a deeper source layer recognizable from these Bay of Islands data.

  20. Shallow intrusive directions of sheeted dikes in the Troodos ophiolite: Anisotropy of magnetic susceptibility and structural data

    SciTech Connect

    Staudigel, H.; Gee, J.; Tauxe, L. ); Varga, R.J. )

    1992-09-01

    Sheeted dikes play a central role in the formation of oceanic crust. It is commonly assumed that sheeted dikes intrude vertically upward, from elongated mid-ocean ridge (MOR) magma chambers, but there are no direct observational data bearing on this hypothesis. This assumption contrasts with the intrusive behavior of subaerial volcanoes where magmas rise into shallow central magma chambers that laterally feed vertically oriented dikes. The authors have studied intrusive directions of sheeted dikes in a structural analogue to oceanic crust, the Troodos ophiolite. Structural and magnetic fabric data of 65 dikes provide consistent results and suggest a broad distribution of shallow (< 20[degree]) to nearly vertical, upward magma-transport directions. These data suggest that horizontal emplacement has to be considered for sheeted dikes at MORs, implying more centralized MOR plumbing systems than previously thought. Such plumbing systems provide ample opportunity for complex mixing, fractionation, and contamination of MOR lavas in magma chambers and tabular magma-storage volumes. Whether the MOR magma supply is linear or centralized also has a fundamental effect on crustal accretion processes and the geometry of hydrothermal convection systems.

  1. Weathering and transport of chromium and nickel from serpentinite in the Coast Range ophiolite to the Sacramento Valley, California, USA

    USGS Publications Warehouse

    Morrison, Jean M.; Goldhaber, Martin B.; Mills, Christopher T.; Breit, George N.; Hooper, Robert L.; Holloway, JoAnn M.; Diehl, Sharon F.; Ranville, James F.

    2015-01-01

    A soil geochemical study in northern California was done to investigate the role that weathering and transport play in the regional distribution and mobility of geogenic Cr and Ni, which are both potentially toxic and carcinogenic. These elements are enriched in ultramafic rocks (primarily serpentinite) and the soils derived from them (1700–10,000 mg Cr per kg soil and 1300–3900 mg Ni per kg soil) in the Coast Range ophiolite. Chromium and Ni have been transported eastward from the Coast Range into the western Sacramento Valley and as a result, valley soil is enriched in Cr (80–1420 mg kg−1) and Ni (65–224 mg kg−1) compared to median values of U.S. soils of 50 and 15 mg kg−1, respectively. Nickel in ultramafic source rocks and soils is present in serpentine minerals (lizardite, antigorite, and chrysotile) and is more easily weathered compared to Cr, which primarily resides in highly refractory chromite ([Mg,Fe2+][Cr3+,Al,Fe3+]2O4). Although the majority of Cr and Ni in soils are in refractory chromite and serpentine minerals, the etching and dissolution of these minerals, presence of Cr- and Ni-enriched clay minerals and development of nanocrystalline Fe (hydr)oxides is evidence that a significant fractions of these elements have been transferred to potentially more labile phases.

  2. Formation of anorthosite-Gabbro rhythmic phase layering: an example at North Arm Mountain, Bay of Isands ophiolite

    USGS Publications Warehouse

    Komor, S.C.; Elthon, D.

    1990-01-01

    Rhythmically layered anorthosite and gabbro are exposed in a 4-10-m thick interval at the base of the layered gabbro unit on North Arm Mountain, one of four massifs that compose the Bay of Islands ophiolite, Newfoundland. The rhythmically layered interval is sandwiched between thick layers of adcumulate to orthocumulate uniform gabbro. Calculated fractional crystallization paths and correlated cryptic variation patterns suggest that uniform and rhythmically layered gabbros represent 20-30% in situ crystallization of two distinct magma batches, one more evolved and the other more primitive. When the more primitive magma entered the crystallization site of the NA300-301 gabbros, it is estimated to have been ~40??C hotter than the resident evolved magma, and may have been chilled by contact with a magma chamber margin composed of uniform gabbro. In this model, chilling caused the liquid to become supercooled with respect to plagioclase nucleation temperatures, resulting in crystallization of gabbro deficient in plagioclase relative to equilibrium cotectic proportions. Subtraction of a plagioclase-poor melagabbro enriched the liquid in normative plagioclase, which in turn led to crystallization of an anorthosite layer. -from Authors

  3. Olivine fabric evolution in a hydrated ductile shear zone at the Moho Transition Zone, Oman Ophiolite (Invited)

    NASA Astrophysics Data System (ADS)

    Michibayashi, K.

    2013-12-01

    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.

  4. 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)

    Kapsiotis, Argirios; Grammatikopoulos, Tassos A.; Tsikouras, Basilios; Hatzipanagiotou, Konstantin; Zaccarini, Federica; Garuti, Giorgio

    2011-01-01

    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.

  5. Emplacement of serpentinites in the Chohar Gonbad-Gugher-Baft ophiolitic mélange, southeast Iran: examination of the mineral-chemical, petrologic, and structural features

    NASA Astrophysics Data System (ADS)

    Mohammadi, N.; Ahmadipour, H.; Lentz, D. R.; Shafaii Moghadam, H.

    2016-03-01

    The Chohar Gonbad-Gugher-Baft ophiolite mélange, located along the major Baft and Shahr-e-Babak fault zones, southeast Iran, represents remnants of Neo-Tethyan oceanic lithosphere. This mélange contains blocks of harzburgite, dunite, lherzolite, basalt, and other ophiolite-related lithologies tectonically mixed with and embedded in a serpentinite matrix. Field, petrographic, and geochemical data show that peridotites in this mélange belong to the upper mantle. They seem to have undergone up to ~20 % partial melting in a supra-subduction zone setting, based on their spinel Cr# values (0.21-0.53). Chemical compositions and textures in the serpentinites indicate that they were partially hydrated during emplacement and further mobilized diapirically to the surface. The different deformation stages occurred in an accretionary wedge environment. Petrographic evidence shows that the first serpentinization event produced mesh-textured serpentinites formed under static conditions in an ocean floor environment (Nain-Baft ocean crust), where the initial lizardite, bastite, and chrysotile veins formed. Plastic deformation occurred due to the subduction of Nain-Baft oceanic lithospheric beneath the central Iranian microcontinent, with antigorite-bearing flare-textured serpentinites produced. During progressive exhumation of the Nain-Baft ophiolite mélange, the serpentinites were affected by ductile, ductile-brittle, and brittle deformation, respectively. Accretion and resultant diapirism are the most important processes in the emplacement of serpentinite, which is a consequence of hydration of the ocean crust. In this example, late-stage emplacement via thrusting occurred along the northern extent of the southern Sanandaj-Sirjan zone (S-SZ).

  6. Emplacement of serpentinites in the Chohar Gonbad-Gugher-Baft ophiolitic mélange, southeast Iran: examination of the mineral-chemical, petrologic, and structural features

    NASA Astrophysics Data System (ADS)

    Mohammadi, N.; Ahmadipour, H.; Lentz, D. R.; Shafaii Moghadam, H.

    2015-05-01

    The Chohar Gonbad-Gugher-Baft ophiolite mélange, located along the major Baft and Shahr-e-Babak fault zones, southeast Iran, represents remnants of Neo-Tethyan oceanic lithosphere. This mélange contains blocks of harzburgite, dunite, lherzolite, basalt, and other ophiolite-related lithologies tectonically mixed with and embedded in a serpentinite matrix. Field, petrographic, and geochemical data show that peridotites in this mélange belong to the upper mantle. They seem to have undergone up to ~20 % partial melting in a supra-subduction zone setting, based on their spinel Cr# values (0.21-0.53). Chemical compositions and textures in the serpentinites indicate that they were partially hydrated during emplacement and further mobilized diapirically to the surface. The different deformation stages occurred in an accretionary wedge environment. Petrographic evidence shows that the first serpentinization event produced mesh-textured serpentinites formed under static conditions in an ocean floor environment (Nain-Baft ocean crust), where the initial lizardite, bastite, and chrysotile veins formed. Plastic deformation occurred due to the subduction of Nain-Baft oceanic lithospheric beneath the central Iranian microcontinent, with antigorite-bearing flare-textured serpentinites produced. During progressive exhumation of the Nain-Baft ophiolite mélange, the serpentinites were affected by ductile, ductile-brittle, and brittle deformation, respectively. Accretion and resultant diapirism are the most important processes in the emplacement of serpentinite, which is a consequence of hydration of the ocean crust. In this example, late-stage emplacement via thrusting occurred along the northern extent of the southern Sanandaj-Sirjan zone (S-SZ).

  7. Geochemical characteristics of basaltic rocks from the Nain ophiolite (Central Iran); constraints on mantle wedge source evolution in an oceanic back arc basin and a geodynamical model

    NASA Astrophysics Data System (ADS)

    Ghazi, Javad Mehdipour; Moazzen, Mohssen; Rahgoshay, Mohammad; Shafaii Moghadam, Hadi

    2012-10-01

    The Nain ophiolitic complex is situated at the north west of the Central Iran Micro-continent (CIM) block. The basaltic rocks of this complex consist of both mantle and crustal suites and include pegmatitic and isotropic gabbros, gabbroic-dibasic dykes, dyke swarm complex and pillow lavas. The chondrite-normalized rare earth element (REE) patterns of most of these rocks show LREE depletion and the primary mantle-normalized incompatible elements indicate depletion in HFSEs (Nb, Ta) and enrichment in LILEs. The rocks show characters of island arc tholeiite/mid-ocean ridge basalt magma types. Whole rock chemistry of the rocks shows that they are originated in an oceanic back arc basin, and subsequently have been enriched by slab-derived fluids. Abundances of HFSE and HREE in most of the basaltic samples, suggest a slow subduction rate. Opening of Nain-Baft Ocean, which was probably a marginal basin, occurred during Lower Jurassic. After generation of an inter-oceanic island arc in the Nain-Baft Ocean during the Late Jurassic, a second rifting phase started within the inter-ocean island arc during Late Cretaceous (Cenomanian-Senonian). The inter-ocean island arc was developed and formed an oceanic back arc basin, the site of generation of most of the Nain ophiolitic rocks. The Nain-Baft Ocean finally closed in Maastrichtian. According to the new tectono-magmatic evolution model proposed here, the arc volcanic-like magmas were produced at the early stage (producing gabbros, gabbroic-diabasic dykes and dyke swarm complex) and then MORB-like basalts (producing pillow lavas) were generated at the later stage of evolution of the Nain ophiolitic complex.

  8. Supra-subduction and abyssal mantle peridotites of the Coast Range ophiolite, California: Initiation of Franciscan subduction along a large-offset fracture zone

    NASA Astrophysics Data System (ADS)

    Choi, S. H.; Shervais, J. W.; Mukasa, S. B.

    2009-04-01

    The Coast Range ophiolite (CRO) of California is one of the most extensive tracts of oceanic crust preserved in the Cordillera, but its origin has long remained controversial. We present here new data on mineral compositions in mantle peridotites that underlie crustal sections of ophiolite, and show that these are dominantly refractory harzburgites related to high apparent melting in a supra-subduction zone (SSZ) setting. Abyssal peridotite occurs at one location, Stonyford, in the CRO where it is associated with SSZ mantle peridotite and volcanic rocks with both oceanic and arc-like geochemistry. SSZ mantle peridotites are associated with crustal sections containing arc-related volcanics, including boninites. This convergence between conclusions based on crustal lithologies and their underlying mantle sections confirms previous proposals that link the CRO to SSZ processes, and seriously undermines hypotheses that call on formation of the ophiolite at a mid-ocean ridge spreading center. We also report on the radiogenic isotope characteristics of peridotite clinopyroxenes. Based on the Sr-Nd-Pb-Hf isotopic compositions and estimated temperatures, we suggest that the abyssal peridotite block represents a remnant of large-offset transform oceanic lithosphere at >172 Ma. East-dipping, proto-Franciscan subduction is likely to have been initiated along this fault zone, which produced the supra-subduction zone (SSZ) peridotites in the CRO. We propose that the remnant abyssal peridotites represent a snapshot of the mantle wedge composition prior to overprinting of large sectors by hydrous melting during the subsequent subduction-controlled SSZ processes.

  9. Petrological characteristics of Opx-bearing primitive gabbros from the East Pacific Rise and the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Python, Marie; Akizawa, Norikatsu; Godard, Marguerite; Ildefonse, Benoît; Koepke, Jürgen

    2014-05-01

    The Hess Deep rift is located at the junction between the fast spreading East Pacific Rise and the Cocos-Nazca Ridge. Lower crust is exposed along the southern slope of the intrarift ridge between 4675 and 4800 m depth and was sampled during IODP Expedition 345. Primitive troctolites and olivine-rich gabbros are the dominant recovered lithologies and shipboard data showed a high Mg# whole rock chemistry in concordance with their primitive nature. In a MOR system, olivine is a typical primitive mineral and orthoyroxene (Opx) usually appear late in the crystallisation sequence, when the magma already reached a significant degree of differentiation. In spite Opx is not expected in any primitive lithology, this mineral is commonly present in Hess Deep gabbros and may be associated with olivine. This curious association of cumulate Opx with olivine and other primitive minerals was also observed at a lower extent in some gabbros from ODP/IODP Hole 1256D, in the upper Hess Deep crustal section (ODP Hole 894G), and in the crustal section of the Oman ophiolite (Kahwad massif) where, in particular, Opx-bearing troctolites coexist with clinopyroxene oikocrysts-bearing troctolites and amphibole-bearing primitive olivine gabbros. Three types of Opx textures may be distinguished in Opx-bearing olivine gabbros and troctolites: (1) recrystallised coronæ around olivine, (2) exsolution within clinopyroxene and (3) large prismatic or poikilitic grains. Prismatic or poikilitic Opx are present at all level of the gabbroic crust, while exsolutions and coronæ were observed only in the lower crust. The mineral chemical compositions vary more with the structural level than with the lithological type and (Opx-bearing) olivine gabbros from Holes 894G, 1256D and from the upper crust of the Oman ophiolite show more differentiated characteristics than the same lithology in the Site 1415 and in the Oman lower crust. Pyroxenes in all samples from the lower crust show a relatively narrow range of Mg# (from 84 to 86% for Opx and 86 to 89% for Cpx) with large variation of minor elements (Ti, Al, Cr) suggesting a strong influence of melt-rock reaction during their formation. On the other hand, the upper crust samples show a large variation in their ferro-magnesian Mg# (72-87% for Cpx and 70-85% for Opx) together with a relatively weak scatter in minor elements. Magmatic crystallisation were then the dominant event in the upper crust, so that Opx is likely to be directly crystallised from magma. In contrast, in the lower crust, magmatic processes were dominated by melt-rock reaction, and the chemical composition and habitus of Opx show that they have been probably formed by reaction between previously abundant olivine and melt.

  10. Geological mapping strategy using visible near-infrared-shortwave infrared hyperspectral remote sensing: Application to the Oman ophiolite (Sumail Massif)

    NASA Astrophysics Data System (ADS)

    Roy, R.; Launeau, P.; Carrère, V.; Pinet, P.; Ceuleneer, G.; Clénet, H.; Daydou, Y.; Girardeau, J.; Amri, I.

    2009-02-01

    An airborne hyperspectral survey of the Oman ophiolite (Sumail Massif) has been conducted using the HyMap airborne imaging spectrometer with associated field measurements (GER 3700). An ASD FieldSpec3 spectrometer was also used in order to constrain the spectral signatures of the principal lithologies cropping out in the surveyed area. Our objective was to identify and map the various igneous lithologies by a direct comparison at high spectral resolution between field and airborne spectra despite strong variations in outcropping conditions such as (1) lighting, (2) surface roughness geometry, (3) blocks coated with red/brown patina and exfoliation products, or (4) deep hydrothermal weathering. On the basis of spectral signatures, we are able to distinguish three end-members of olivine-orthopyroxene bearing assemblages in the mantle sequence: (1) harzburgites, (2) dunites, and (3) a harzburgite with interstitial carbonate. Because plagioclase is spectrally featureless in the wavelength range studied it cannot be detected. In the crustal sequence, we therefore identified four end-members with variable abundance of clinopyroxene: (1) massive gabbros, (2) amphibolized (upper) gabbros associated with intrusive dykes, (3) wehrlite with high serpentine content, and (4) gabbronorite (a lithology not previously recognized in the studied area). With the exception of wehrlite, spectra of olivine-rich end-members display characteristic Mg-OH narrow absorption features caused by their high serpentine content. We take advantage of this observation to split the data into two subsets, corresponding to the mantle and crustal sequences, respectively. Pixels of an image often correspond to heterogeneous areas in the field and a direct comparison between airborne and in situ spectra is not straightforward. However, comparing spectra of pixels associated with the most homogeneous areas in the field with the spectra acquired in situ at the same location, we found a systematic change both in mean intensity and overall spectral shape. Dividing each spectrum by its low-pass trend removes the effects caused by surface light scattering associated with each scale of analysis and results in an exceptional match between field and airborne spectra. However, the albedo information is lost and as a consequence, rock types only characterized by albedo change cannot be discriminated. A spectrum of a mixture of powdered minerals is usually seen as a linear combination of mineral spectra proportional to their abundance. However, this is no longer the case when minerals occur in complex arrangements in rock types. We thus develop a synthetic spectral library of all possible combinations of rock types covering the surface area of a pixel and use a simple distance calculation to identify the best match between each pixel and modeled spectra. This procedure allows the determination of the fractional cover of each rock type in a given pixel and to establish maps for each spectral end-member. The final product is a geological map, derived from the combination of end-member fractional cover maps, and is broadly consistent with the existing geological maps. Beyond this general agreement which demonstrates the potential of this new approach for geological mapping, imaging spectrometry allows (1) to map in detail the outline of the Moho north of Maqsad and (2) to identify a new crustal sequence enriched in silica south of Muqzah, revealing the presence of orthopyroxene, the nature and distribution of which are of relevance to the petrological and tectonic understanding of the Oman ophiolite evolution.

  11. Forearc Basin Location Originating From Tectonic Inversion Along an old Ophiolite Suture : the Gulf of Guayaquil-Tumbes Basin (Ecuador-Peru Border)

    NASA Astrophysics Data System (ADS)

    Bourgois, J.; Witt, C.

    2008-12-01

    The Gulf of Guayaquil-Tumbes basin (GGTB) located along the Andean forearc (Ecuador-Peru border) developed in the tectonic wake of the coastwise, northward migrating North Andean block (NAB). The Industrial multichannel seismic and well data (Witt and Bourgois, in press) document that E-W trending low- angle (10-20°) detachment normal faults accommodated the main basin subsidence steps during the Late Pliocene-Quaternary times (1.8-1.6 Ma to Present). It includes the Posorja Jambeli and the northward dipping Tumbes Zorritos detachment systems (PJDS and TZDS) located respectively along the northern and southern edge of the basin. A major transfer system, the N-S trending Inner Domito Banco Peru fault system bounds the detachment systems to the West. The right lateral transcontinental strike-slip system of the Dolores Guayaquil Megashear bounds the basin to the East. Since the PJDS and TZDS extend 80 to 120 km at seafloor they must penetrate the brittle continental crust, far below the 6-8 km thick sediment accumulation at basin depocenters. We assume that detachments extend deep into the 8-10 km thick brittle crust down to the Nazca-South America plate interface at less than ~20 km beneath sea bottom at site. The active TZDS, which connects landward with the continental structures assumed to be part of the eastern frontier of the NAB is the master detachment fault system controlling the basin evolution through time. Gravimetric and geologic data show that depocenters are located along the 80-60 Ma obduction bounding at depth the Cretaceous ophiolite of northern Andes from the westward underthrusted South America continental basement (Bourgois et al., 1987). Because inference suggests the obduction megathrust to branch upward to the TZDS, we hypothesized that tectonic inversion occurred along the ophiolite suture during the GGTB evolution, at least for the past 1.8-1.6 Myr. The 80-60 Ma ophiolite suture is an old zone of weakness, which reactivation from the NAB northward drift controlled the GGTB location. Bourgois, J., Toussaint, J-F, Gonzales, H., Azema, J., Calle, B., Desmet, A., Murcia L.A., Acevedo, A.P., Parra, E., and Tournon, J., 1987, Geological history of the Cretaceous ophiolitic complexes of Northwestern South America (Colombia Andes): Tectonophysics, v. 143, p. 307-327. Witt, C. and Bourgois, J., Forearc basin formation in the tectonic wake of a collision-driven, coastwise migrating crustal block: the example of the North Andean block and the extensional Gulf of Guayaquil-Tumbes basin (Ecuador-Peru border area): Geological Society of America Bulletin, in press.

  12. The Imbert Formation of northern Hispaniola: a tectono-sedimentary record of arc-continent collision and ophiolite emplacement in the northern Caribbean subduction-accretionary prism

    NASA Astrophysics Data System (ADS)

    Escuder-Viruete, J.; Suárez-Rodríguez, A.; Gabites, J.; Pérez-Estaún, A.

    2015-06-01

    In northern Hispaniola, the Imbert Formation (Fm) has been interpreted as an orogenic "mélange" originally deposited as trench-fill sediments, an accretionary (subduction) complex formed above a SW-dipping subduction zone, or the sedimentary result of the early oblique collision of the Caribbean plate with the Bahama Platform in the middle Eocene. However, new stratigraphical, structural, geochemical and geochronological data from northern Hispaniola indicate that the Imbert Fm constitutes a coarsening-upward stratigraphic sequence that records the transition of the sedimentation from a pre-collisional forearc to a syn-collisional piggy-back basin. This piggy-back basin was transported on top of the Puerto Plata ophiolitic complex slab and structurally underlying accreted units of the Rio San Juan complex, as it was emplaced onto the North America continental margin units. The Imbert Fm unconformably overlies different structural levels of the Caribbean subduction-accretionary prism, including a supra-subduction zone ophiolite, and consists of three laterally discontinuous units that record the exhumation of the underlying basement. The distal turbiditic lower unit includes the latest volcanic activity of the Caribbean island arc; the more proximal turbiditic intermediate unit is moderately affected by syn-sedimentary faulting; and the upper unit is a (caotic) olistostromic unit, composed of serpentinite-rich polymictic breccias, conglomerates and sandstones, strongly deformed by syn-sedimentary faulting, slumping and sliding processes. The Imbert Fm is followed by subsidence and turbiditic deposition of the overlying El Mamey Group. The 40Ar / 39Ar plagioclase plateau ages obtained in gabbroic rocks from the Puerto Plata ophiolitic complex indicate its exhumation at ∼ 45-40 Ma (lower-to-middle Eocene), contemporaneously to the sedimentation of the overlying Imbert Fm. These cooling ages imply the uplift to the surface and submarine erosion of the complex to be the source of the ophiolitic fragments in the Imbert Fm, during of shortly after the emplacement of the intra-oceanic Caribbean island-arc onto the continental margin.

  13. The Imbert Formation of northern Hispaniola: a tectono-sedimentary record of arc-continent collision and ophiolite emplacement in the northern Caribbean subduction-accretionary prism

    NASA Astrophysics Data System (ADS)

    Escuder-Viruete, J.; Suárez-Rodríguez, Á.; Gabites, J.; Pérez-Estaún, A.

    2016-01-01

    In northern Hispaniola, the Imbert Formation (Fm) has been interpreted as an orogenic "mélange" originally deposited as trench-fill sediments, an accretionary (subduction) complex formed above a SW-dipping subduction zone, or the sedimentary result of the early oblique collision of the Caribbean plate with the Bahama Platform in the middle Eocene. However, new stratigraphical, structural, geochemical and geochronological data from northern Hispaniola indicate that the Imbert Fm constitutes a coarsening-upward stratigraphic sequence that records the transition of the sedimentation from a pre-collisional forearc to a syn-collisional basin. This basin was transported on top of the Puerto Plata ophiolitic complex slab and structurally underlying accreted units of the Rio San Juan complex, as it was emplaced onto the North America continental margin units.

    The Imbert Fm unconformably overlies different structural levels of the Caribbean subduction-accretionary prism, including a supra-subduction zone ophiolite, and consists of three laterally discontinuous units that record the exhumation of the underlying basement. The distal turbiditic lower unit includes the latest volcanic activity of the Caribbean island arc; the more proximal turbiditic intermediate unit is moderately affected by syn-sedimentary faulting; and the upper unit is a (chaotic) olistostromic unit, composed of serpentinite-rich polymictic breccias, conglomerates and sandstones, strongly deformed by syn-sedimentary faulting, slumping and sliding processes. The Imbert Fm is followed by subsidence and turbiditic deposition of the overlying El Mamey Group.

    The 40Ar / 39Ar plagioclase plateau ages obtained in gabbroic rocks from the Puerto Plata ophiolitic complex indicate its exhumation at ˜ 45-40 Ma (lower-to-middle Eocene), contemporaneously to the sedimentation of the overlying Imbert Fm. These cooling ages imply the uplift to the surface and submarine erosion of the complex to be the source of the ophiolitic fragments in the Imbert Fm, during or shortly after the emplacement of the intra-oceanic Caribbean island arc onto the continental margin.

  14. The Relationships of Plate Triple-junction Evolution, Trench-Arc Lengthening, Boninite Generation, and SSZ Spreading Centers to Ophiolite Formation, High-Temperature Soles, and Obduction

    NASA Astrophysics Data System (ADS)

    Casey, J.; Dewey, J. F.

    2014-12-01

    A review of modern-day island arcs, the locations of boninite eruptions, the conditions necessary for hot upper plate spreading, potential regions of shallow SSZ flux melting, and formation of high-temperature metamorphic soles along the subduction channels indicates that many future, recent and ancient large slab ophiolite obduction events can be related to triple junctions that link SSZ spreading centers with trenches. These subduction systems leading to large slab ophiolite obduction events typically face stable continental margins. 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 mantle asthenosphere and lithosphere, which previously likely experienced melt extraction. Metamorphic conditions associated with metamorphic soles formation likewise require a hot upper plate lithosphere that, based on sole protolith, geochronologic and thermochronologic data, rapidly heats and then refrigerates and decompresses MORB-OIB type subcreted lithosphere. Numerous examples of present-day and recent SSZ spreading centers that link with two trenches or a trench and transform are considered ideal sites for ophiolite and boninite generation. The SZZ fore-arc spreading centers that link to the trench lines and triple junctions at the front of the arc may also continue towards the arc and back arc, creating no distinction between fore-arc and back-arc spreading episodes or to the transform-linked spreading centers from fore-arc to back arc. These SSZ spreading centers, which may be transiently produced during arc evolution over short or protracted time periods, act to open gaps in the arc massif and lengthen the trench, fore-arc and the arc crustal massif. They lead to an evolving arc magmatic front that begins in the infant fore-arc where ophiolite generation occurs at, near or in the trench, followed then by migration to sites distal from the trench line where a new arc massif is established on newly created oceanic lithosphere. Modern and ancient analogues of these setting are reviewed.

  15. The structure and geochemistry of the gabbro zone of the Annieopsquotch ophiolite, Newfoundland: implications for lower crustal accretion at spreading ridges

    NASA Astrophysics Data System (ADS)

    Lissenberg, C. J.; Bédard, J. H.; van Staal, C. R.

    2004-12-01

    The Annieopsquotch ophiolite exposes a c. 5.5-km-thick section of tholeiitic gabbros, sheeted dykes and pillow basalts. Based on the along-strike consistency in thickness of the major crustal units, and lack of significant throw on spreading-related normal faults, the Annieopsquotch ophiolite is interpreted to have formed at an intermediate- to fast-spreading ridge. The upper c. 400 m of the gabbro zone is composed of massive, texturally heterogeneous gabbros with compositions that approach those of the sheeted dykes and basalts. Below this is c. 1.6 km of 10-30-m-thick planar intrusive sheets or sills. The lowermost part of the gabbro zone is composed of gabbroic rocks with relict troctolite and troctolitic gabbro enclaves, which are veined and partly replaced by gabbro and pyroxenite. Sill contacts within the central, sill-dominated part of the gabbro zone are sub-parallel to boundaries between the major ophiolite lithostratigraphic units. The upper and lower contacts of individual gabbro sills may have finer grain sizes, indicating that the intrusions cooled from the top and bottom. Locally well-preserved comb structures (crescumulates) indicate downward growth, supporting a sill interpretation. The sills are composed of weakly or un-deformed plagioclase+clinopyroxene ±olivine cumulates. Trace element modeling suggests that the parental magmas of these cumulates had compositions very similar to the overlying sheeted dykes and basaltic lavas, as do dykes emplaced within the gabbro zone. Model liquids calculated from the gabbroic sills generally become more evolved up-section, indicating that magma evolution in the Annieopsquotch ophiolite was dominated by fractionation in lower crustal conduits, below the level of a putative axial melt lens (AML). The model liquids, sheeted dykes and basalts preserve a similar, wide range of compositions, which may indicate that aggregation, homogenization and fractionation in an AML was inefficient. Similar intra-conduit fractionation of mantle-derived melts might also contribute to MORB evolution at mid-ocean ridges.

  16. The Dupal isotopic anomaly in the southern Paleo-Asian Ocean: Nd-Pb isotope evidence from ophiolites in Northwest China

    NASA Astrophysics Data System (ADS)

    Liu, Xijun; Xu, Jifeng; Castillo, Paterno R.; Xiao, Wenjiao; Shi, Yu; Feng, Zuohai; Guo, Lin

    2014-02-01

    It has been suggested that the Dupal isotopic anomaly in the mantle can be traced in the Paleozoic ophiolites from the Neo- and Paleo-Tethyan Ocean (275-350 Ma). The Karamaili ophiolite (KO) and Dalabute ophiolite (DO) in the eastern and western corners, respectively, of the Junggar basin in NW China represent remnants of the relatively older (> 350 Ma) Paleo-Asian Ocean (PAO) crust. Thus, these ophiolites can provide additional constraints on the long-term composition and evolution of the Paleozoic suboceanic mantle. We present new major-trace element and Sr, Nd and high-precision Pb isotope data for the basalts, gabbros and a plagioclase separate from the KO and DO. Our results indicate that the PAO crust indeed has a Dupal-like isotopic signature. In detail, all samples have relatively low ?Nd(t) and high 208Pb/204Pb(t) for given 206Pb/204Pb(t) ratios (i.e., positive ?8/4 values), similar to the Dupal isotopic characteristics of Indian Ocean mid-ocean ridge basalts (MORB). The trace element signature of DO mafic rocks is similar to that of normal- and enriched-MORB whereas that of the KO is transitional between MORB and arc basalt. Therefore, the DO mantle domain reflects the PAO asthenosphere and the KO domain additionally shows the influence of the subduction process. Geochemical modeling using Th/Nd as well as Nd and Pb isotopic ratios indicates that up to 2% subduction component had been added to a depleted Indian MORB-type mantle to produce the bulk of KO rocks. The subduction component in the KO rocks consisted of variable proportions of ? 1% partial melt of unradiogenic sediment similar to modern Izu-Bonin trench sediment and hydrous fluid dehydrated from the subducted altered oceanic crust. The Devonian asthenospheric mantle beneath the southern PAO is isotopically heterogeneous, but lends support to the idea that the Dupal isotopic anomaly existed prior to the opening of the Indian Ocean. Finally, plate tectonic reconstruction indicates that the anomaly was present in the Neo- and Paleo-Tethyan oceans in the southern hemisphere and in the southern part of PAO in the northern hemisphere during the late Paleozoic.

  17. Rare earth element evolution and migration in plagiogranites: a record preserved in epidote and allanite of the Troodos ophiolite

    NASA Astrophysics Data System (ADS)

    Anenburg, Michael; Katzir, Yaron; Rhede, Dieter; Jöns, Niels; Bach, Wolfgang

    2015-03-01

    Plagiogranites from the Troodos ophiolite in Cyprus are occasionally epidotised, either partially or completely. Epidotisation phenomena include replacement of pre-existing minerals and filling of miarolitic cavities. In addition to epidote, miarolites in one plagiogranite body (located near the village of Spilia) contain coexisting ferriallanite-(Ce) and allanite-(Y). Textural and geochemical evidence indicates that late-stage REE-enriched granitic melt facilitated crystallisation of magmatic ferriallanite-(Ce). High REE contents persisted after fluid exsolution, causing crystallisation of allanite-(Y) from hydrothermal fluids in the miarolites. The REE pattern of the hydrothermal allanite-(Y) is characterised by LREE and Eu depletion, similar to the parent plagiogranitic magma. As allanite had sequestered most of the REE in the fluid, epidote took over as the principle hydrothermal mineral. Epidote in Troodos plagiogranites records a fluid evolutionary trend beginning with REE-rich-Eu-depleted similar to allanite-(Y) and gradually transforming into the REE-depleted-Eu-enriched pattern prevalent throughout `conventional' sub-seafloor fluids. A comparison of allanite-bearing and allanite-absent plagiogranites from the same locality suggests that REE-bearing fluids mig