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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 (δ18O) data has been deployed; results fluid mobility exchange between continental TMC gneiss and Zildat ophiolitic melange is synchronous.

  3. 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.

  4. Sediment melting during subduction initiation: Geochronological and geochemical evidence from the Darutso high-Mg andesites within ophiolite melange, central Tibet

    NASA Astrophysics Data System (ADS)

    Zeng, Yun-Chuan; Chen, Jian-Lin; Xu, Ji-Feng; Wang, Bao-Di; Huang, Feng

    2016-12-01

    In addition to fluids, the concept of sediment-derived melts infiltrating the fore-arc mantle during subduction initiation has been proposed based on studies of modern subduction zones and ophiolite mélange. However, outcrops that contain the products of such melts are rare, especially in conjunction with boninite. New U-Pb zircon dating reveals that the Darutso volcanic rocks (DVRs) within ophiolitic mélange in the Beila area, central Tibet, crystallized at ˜164-162 Ma. This is the first recognition of Jurassic volcanic rocks in the middle section of the Bangong-Nujiang Suture Zone. Geochemically, the DVRs are high-Mg andesites with moderate SiO2 (59.03-63.62 wt %) and high MgO (3.74-6.53 wt %), Cr (up to 395 ppm), and Mg# (50.3-67.9). They also have high Th contents, (La/Sm)N ratios, and (87Sr/86Sr)i values (0.7085-0.7147); low Ba/Th, U/Th, and Sr/Y ratios; and negative values of ɛNd(t) (-8.7 to -9.8) and zircon ɛHf(t) (-7.4 to -9.9). The ɛNd(t) values of the DVRs overlap those of regional sediments. Detailed analyses of these geochemical characteristics indicate that the DVRs were derived from partial melting of subducted sediments and subsequent interaction with overlying mantle peridotite in a shallow and hot setting. In combination with the regional geology, in particular adjacent ophiolites that contain MORB-like and boninite mafic lavas, these rocks collectively recorded the evolution of a fore-arc setting during the initiation of the northward subduction of the south branch of the Bangong-Nujiang Ocean. Therefore, the results provide direct evidence for sediment melting during subduction initiation and constrain the Jurassic tectonic evolution of the Lhasa terrane.

  5. 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

  6. Tectonic setting for ophiolite obduction in Oman.

    USGS Publications Warehouse

    Coleman, R.G.

    1981-01-01

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

  7. 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.

  8. 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.

  9. 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.

  10. Ophiolites in Earth History

    NASA Astrophysics Data System (ADS)

    Selverstone, Jane

    2004-11-01

    I began my undergraduate studies in geology in 1974, shortly after the release of the now famous ``Anonymous'' report on the first Penrose Conference on Ophiolites (Geotimes, 1972). My early geology courses were filled with information on the nature of the seafloor, with details supplied largely from on-land studies of ophiolites. Pillow basalts and sheeted dike complexes, gabbros, peridotites, and cherts-all of these caught my attention and seemed so elegantly explained by formation at oceanic spreading centers. Over the next couple of decades, I simply assumed that ophiolites were direct analogues for material produced at mid-ocean ridges, and I missed the nuances of the ongoing debate about whether or not ophiolites really are representative of this tectonic setting.

  11. 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

  12. 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

  13. 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.

  14. 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.

  15. The Resurrection Peninsula ophiolite

    USGS Publications Warehouse

    Nelson, Steven W.; Miller, M.L.; Dumoulin, Julie A.; Nelson, Steven W.; Hamilton, Thomas D.

    1989-01-01

    The Resurrection Peninsula forms the east side of Resurrection Bay (fig. 3). Relief ranges from 437 m (1,434 ft) at the southern end of the peninsula to more than 1,463 m (4,800 ft) opposite the head of the bay. All rock units composing the informally named Resurrection Peninsula ophiolite of Nelson and others (1987) are visible or accessible by boat."Ophiolite" has been a geologic term since 1827 (Coleman, 1977). The term "ophiolite" initially referred to the rock serpentinite; the Greek root "ophi" (meaning snake or serpent) alluded to the greenish, mottled, and shiny appearance of serpentinites. In 1927, Steinmann described a rock association in the Alps, sometimes known as the "Steinmann Trinity', consisting of serpentine, diabase and spilitic lavas, and chert. Recognition of this suite led to the idea that ophiolites represent submarine magmatism that took place early in the development of a eugeosyncline. In the early 1970s the Steinmann Trinity was reconsidered in light of the plate tectonic theory, new petrologic studies, and the recognition of abducted oceanic lithosphere in orogenic belts of the world. In 1972 at a Geological Society of America Penrose Conference (Anonymous, 1972) the term "ophiolite" was defined as a distinctive assemblage of mafic to ultramafic rocks, with no emphasis on their origin. A complete ophiolite should contain, from bottom to top:1) Tectonized ultramafic rocks (more or less serpentinized)2) Gabbro complex containing cumulus textures and commonly cumulus peridotites3) Mafic sheeted-dike complex, grading upward into;4) Submarine pillow lavas of basaltic composition. Common associated rock types include plagiogranite (Na-rich) and an overlying sedimentary section typically dominated by chert.

  16. The Piney Branch Complex - a metamorphosed fragment of the central Appalachian ophiolite in northern Virginia.

    USGS Publications Warehouse

    Drake, A.A.; Morgan, B.A.

    1981-01-01

    The Piney Branch Complex consists of subequal parts of highly altered peridotite, pyroxenite, and gabbro and crops out over an area of slightly more than 9km2 in Fairfax County, Va. The lack of discernible order to the distribution of these rock types within the Piney Branch Complex suggests it is a melange resulting from the deformation of a layered complex that contained repetitive cycles of ultramafic and mafic layers. Severely metamorphosed relict cumulate textures can be seen in some specimens of both serpentinite and metagabbro. Small dikes and sheets of plagiogranite occur throughout the complex. The Piney Branch closely resembles the ophiolite material that crops out in southeastern Pennsylvania and Maryland, in the kind, and relative abundance of rock types and in the chemical composition of the rocks, and it represents a continuation of the pattern of southwesterly increasing regional metamorphism of ophiolite material in the central Appalachians. Other bodies of ophiolitic material within the central Appalachians, although at different tectonic levels, also evidence of multiple movement. These bodies and the Piney Branch are believed to be fragments of a large dismembered 'Central Appalachian ophiolite' which records part of the complicated tectonic history of the eastern continental margin of N America.-Authors

  17. 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.

  18. 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

  19. 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.

  20. Metamorphosed melange terrane in the eastern Piedmont of North Carolina

    NASA Astrophysics Data System (ADS)

    Wright Morton, J., Jr.; Blake, David E.; Wylie, Albert S., Jr.; Stoddard, Edward F.

    1986-07-01

    The Falls Lake melange crops out in the eastern Piedmont of North Carolina between the Carolina slate belt and the Raleigh belt. The melange is composed of mafic and ultramafic blocks and pods of diverse shapes and sizes, dispersed without apparent stratigraphic continuity, in a matrix of pelitic schist and biotite-muscovite-plagioclase-quartz gneiss. Textures and structural relationships suggest formation by a combination of sedimentary and tectonic processes, perhaps in the accretionary wedge of a convergent plate margin. The Falls Lake melange and the overlying late Proterozoic to Early Cambrian volcanic-arc terrane of the Carolina slate belt were thrust upon a probable continental terrane of the Raleigh belt before overprinting by late Paleozoic folding and metamorphism. *Present address: Chevron USA, P.O. Box 1150, Midland, Texas 79701

  1. Metamorphosed melange terrane in the eastern Piedmont of North Carolina.

    USGS Publications Warehouse

    Horton, J.W.; Blake, D.E.; Wylie, A.S.; Stoddard, E.F.

    1986-01-01

    The Falls Lake melange is a metamorphosed terrain composed of mafic and ultramafic blocks and pods of diverse shapes and sizes, dispersed without apparent stratigraphic continuity in a matrix of pelitic schist and biotite-muscovite-plagioclase-quartz-gneiss. Textural and structural relationships suggest formation by a combination of sedimentary and tectonic processes, perhaps in the accretionary wedge of a convergent plate margin. The Falls Lake melange and the overlying late Proterozoic to early Cambrian volcanic-arc terrain of the Carolina slate belt, were thrust upon a probable continental terrain of the Raleigh belt before overprinting by late Palaeozoic folding and metamorphism.-L.C.H.

  2. Metamorphosed melange in the central Piedmont of South Carolina

    SciTech Connect

    Mittwede, S.K.; Maybin, A.H. III )

    1989-09-01

    The Enoree melange is exposed in the central Piedmont of South Carolina near the boundary between the Piedmont and Carolina terranes. The melange is composed of ultramafic and mafic blocks in a highly deformed matrix of biotite-feldspar-quartz gneiss which has a composition consistent with a felsic-to-intermediate volcanic precursor. The mafic and ultramafic blocks are separated chemically and petrographically into four compositional groups: metagabbro-clinopyroxenite, websterite, orthopyroxenite, and metasomatized (steatitized) orthopyroxenite. Based on their chemistry and mineralogy, the blocks are clearly exotic relative to their metavolcaniclastic( ) matrix and likely originated as parts of a plutonic suite from the basal or forward part of the Carolina arc terrane. If the Piedmont terrane-Carolina terrane boundary is a continent-arc suture, then the Enoree melange probably formed in the accretionary prism at this convergent margin. The matrix gneisses are interpreted as reworked volcanic debris shed by the Carolina arc terrane edifice and accumulated as graywacke in the accretionary deposits. West-vergent structures in the matrix suggest that the melange was emplaced to its present tectonostratigraphic position either during docking of the Carolina terrane or during widespread Alleghenian thrusting.

  3. 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.

  4. Multistage melange formation in the Franciscan Complex, northernmost California

    NASA Astrophysics Data System (ADS)

    Aalto, K. R.

    1981-12-01

    Matrix induration and deformation style denote relative time and process of melange origin related to subduction. Mappable melange units contain chiefly blocks of greenstone, chert, and graywacke dispersed in weakly indurated argillite. Emplacement of these units as olistostromes is indicated by depositional contacts and lateral inter-tonguing with turbidites. Some blocks in sheared olistostromes are in themselves complexly mixed. These include breccias of gabbro, plagiogranite, diabase, basalt, tuff, chert, graywacke, and argillite variously combined and brittlely deformed. Blocks of intercalated tuff and argillite and of diamictite containing rounded clasts of tuff, chert, and graywacke dispersed in highly indurated, foliated argillite exhibit primary ductile and secondary brittle deformation. All block material was indurated, brecciated, and/or foliated during underplating at the base of a trench slope and later uplifted to provide olistostrome detritus. The Elk Head and Klamath coast terranes are highly indurated and compositionally similar to the mixed-lithology blocks in olistostromes. They are interpreted to form an accretionary prism consisting of tectonic melange slabs separating slope-basin rocks.

  5. 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.

  6. 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.

  7. 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.

  8. Structural Analysis of the Khoy Ophiolite, NW Iran from ASTER Imagery

    NASA Astrophysics Data System (ADS)

    Thurmond, A. K.; Abdel-Salam, M.; Yin, Z.; Hassanipak, A.; Ghazi, A. M.

    2002-05-01

    of a tectonic melange that mixes different lithologies of the region. Two NW-trending basaltic belts that have distinctively different spectral signature dominate the southwestern nappe and perhaps structurally lower sub-complex. This nappe is interpreted as a para-authochthon below the central allochthon. Structural evolution of the Khoy ophiolite complex is attributed to an early tectonic event that caused the emplacement of nappes from northeast to southwest, and a late tectonic pulse that resulted in an overall N-S directed shortening manifested by the development of ENE-trending folds, shear zones, and mega kink bands .

  9. Spatial and temporal relations of the ophiolites and the metamorphic soles along the Tauride belt, Turkey

    NASA Astrophysics Data System (ADS)

    Parlak, Osman; Simsek, Emrah; Ezgi Ozturk, Selena; Simsek, Gokce; Simsek, Tugce; Robertson, Alastair; von Quadt, Albrecht; Köpke, Jürgen; Karaoglan, Fatih

    2016-04-01

    The Tauride belt ophiolites were generated above an intra-oceanic subduction zone and emplaced in the Late Cretaceous over the Tauride carbonate platform. The Tauride ophiolites are underlain by well-preserved metamorphic soles that have a constant structural position between the ophiolitic mélange, below and harzburgitic mantle tec- tonites, above. The dynamothermal metamorphic soles display a typical inverted metamorphic sequence, grading from amphibolite facies directly beneath the highly sheared harzburgitic tectonite to greenschist facies close to the melange contact. They display variable structural thickness (up to 500 m). The metamorphic soles beneath the Tauride ophiolites are interpreted to relate to the initiation of subduction and emplacement processes. The metamorphic soles are intruded by isolated post-metamorphic diabase dikes, derived from island arc tholeiitic magmas. In some places along the Tauride belt (Koycegiz and Pozanti-Karsanti regions), the contact between the metamorphic sole and the overlying serpentinized harzburgites is characterized by a 1.5-2 m thick zone of sheared serpentinized harzburgitic mantle tectonites, intercalated with amphibolites. These lithologies are cut by thick mafic dikes (7-8 m thick, individually) which postdate intraoceanic metamorphism and high-temperature ductile deformation. This contact is interpreted as an intra-oceanic decoupling surface along which volcanics in the upper levels of the down-going plate were metamorphosed to amphibolite facies and accreted to the base of the hanging wall plate. The geochemistry of the metamorphic sole amphibolites suggests their derivation from different geochemical environments; i.e. seamount-type alkaline basalts, mid-ocean ridge basalt (MORB) and island arc basalts. Zircon and rutile separates from the crustal rocks (gabbro and diabase) and from the metamorphic soles of the Tauride ophiolites have been dated by U-Pb SIMS (Edinburgh University) and LA-MC-ICP-MS (ETH Zurich

  10. 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

    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.

  11. Estimating ice-melange properties with repeat UAV surveys over Store Glacier, West Greenland

    NASA Astrophysics Data System (ADS)

    Toberg, Nick; Ryan, Johnny; Christoffersen, Poul; Snooke, Neal; Todd, Joe; Hubbard, Alun

    2016-04-01

    In the past decade, tidewater outlet glaciers of the Greenland ice sheet (GrIS) have thinned and retreated when compared to the 1980s when the ice sheet was in a state of dynamic balance. With a growing amount of ice discharged into the sea by tidewater glaciers as well as more ice melting on the surface, the Greenland Ice Sheet has become the single largest cryospheric source of global sea level rise. Today, the ice sheet causes sea level rise of 1 mm per year, highlighting the need to understand the ice sheet's response to climate change. Atmospheric warming will inevitably continue to increase surface meltwater production, but the dynamic response, which includes hundreds of fast-flowing tidewater glaciers, is largely unknown. To develop new understanding of ice sheet dynamics, we investigated the mechanism whereby icebergs break off tidewater glaciers and form a proglacial ice melange. This melange is rigid in winter when sea ice and friction along the sidewalls of the fjord, or even at the sea floor, hold it together. The result is a resistive force, which reduces the rate of iceberg calving when the ice melange is rigid and is lost when the melange disappears in the summer. From early May to late July 2014, we launched unmanned aerial vehicles (UAVs) from a basecamp on a bluff overlooking the calving front of Store Glacier, a 5 km wide tidewater glacier flowing into Uummannaq Fjord in West Greenland. The Skywalker X8 UAVs had a wing-span of 2.1m and a payload containing a high resolution camera, an autopilot system and a GPS data logger. We generated almost 70,000 georeferenced images during 63 sorties over the glacier during a 10 week field season starting 13 May 2014. The images were used to construct orhorectified mosaics and digital elevation models of the proglacial melange with Photoscan structure-from-motion software. The imagery and the DEMs were analysed statistically to understand the spatial characteristics of the ice melange. By combining the

  12. 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, Timothy M.; Bradley, Dwight C.; Haeussler, Peter J.; Karl, Susan M.

    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

  13. 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.

  14. Nature of the Akamas ophiolite, W Cyprus

    NASA Astrophysics Data System (ADS)

    Wai-Pan Ng, Samuel

    2010-05-01

    Cyprus has been an ideal place to study the evolution of oceanic lithosphere. The complete preservation of the ophiolite in the Troodos Complex has allowed better understanding on the ocean closure process and the emplacement of oceanic lithospheric fragments on land. These previous studies have put the development of plate tectonic theory a great leap forward. Compared to the famous Troodos ophiolite, the ophiolitic suite lying on Akamas Peninsula at the western end of the island has received far less attention. Different from the updomed Troodos ophiolite, the Akamas ophiolite formed a linear NNW-SSE ridge in Akamas Peninsula, and it was isolated from the Troodos ophiolite by the Polis Graben and Mamonia Complex. The ultramafic section made up the backbone of the ridge. It was composed of intensively serpentinized harzburgite and dunite with extensive magnesite deposit associated. Brecciation and multiphase shears were observed within the ophiolite. The Sheeted Dyke Complex (SDC) and lava sequences were underthrust on the western flank of the ridge. The SDC were generally striking parallel to the orientation of the ridge and dipping east, while the lavas were usually rotated or even overturned. All these observations showed that the Akamas ophiolite was severely tectonized after its formation. The Akamas ophiolite has been regarded as part of the Troodos Complex. Since the 1960s, extensive volcanic geochemical data of the Troodos Complex have been collected for tectonic modelling. As part of the Complex, several Akamas lava samples were also included in these researches. They have shown a geochemical affinity with the Arakapas-type lavas, which were commonly found in the Southern Troodos Transform Fault Zone (STTFZ). However, the result did not lead to any further study on the emplacement of this isolated ophiolitic suite. In this project, over 40 lava and diabase dyke samples have been collected from the Akamas ophiolite. Their major and trace element content

  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. Field evidence for fault controlled intrusion of blue-schist-bearing melange into an accretionary wedge, Island Mountain, California

    SciTech Connect

    Lamons, R.

    1985-01-01

    Two lithologic units of the Franciscan are well exposed along a loop of the Eel River at Island Mountain. They are 1) zeolite or lower grade lithic graywackes, and 2) a 0.5 km wide band of black shaly melange containing blueschist, chert, greenstone, metagraywacke, and a graywacke-hosted copper deposit. Sedimentary features were not observed in the melange. The graywacke was subdivided on the basis of presence or absence of sodium-cobaltonitrate stained K-spar. Field relationships suggest that the blueschist-bearing melange was emplaced along steep NW-dipping faults in an accretionary wedge. Mapping of S. Jewett Rock and SW Lake Mountain quadrangles show narrow anastomosing bands of the melange following NW-trending faults. East of this band, graywackes without K-spar are folded along NW/SE axes. No folds were found to the west. Other Melange bands pinch out into faults which juxtapose graywackes of different facies. The sheared melange bands are not folded and shale beds in the graywacke show little shear so the melange bands are unlikely to be sheared olistostromes. The areal extent of graywacke is about ten times that of melange shales. Assuming this pattern continues laterally and at depth, the amount of ductile material in the melange is far less than that assumed by Cloos (1982) in his flow model for melange. The ductile melange may have been forced upward by metamorphically produced volatiles, or as a result of relative plate motion. It originated at depth, moved up along the top of a subducting slab, plucking clasts, then splayed upward into pre-existing faults in the accretionary wedge.

  17. Orogenic, Ophiolitic, and Abyssal Peridotites

    NASA Astrophysics Data System (ADS)

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

    2003-12-01

    "Tectonically emplaced" mantle rocks include subcontinental, suboceanic, and subarc mantle rocks that were tectonically exhumed from the upper mantle and occur:(i) as dispersed ultramafic bodies, a few meters to kilometers in size, in suture zones and mountain belts (i.e., the "alpine," or "orogenic" peridotite massifs - De Roever (1957), Thayer (1960), Den Tex (1969));(ii) as the lower ultramafic section of large (tens of kilometers) ophiolite or island arc complexes, obducted on continental margins (e.g., the Oman Ophiolite and the Kohistan Arc Complex - Coleman (1971), Boudier and Coleman (1981), Burg et al. (1998));(iii) exhumed above the sea level in ocean basins (e.g., Zabargad Island in the Red Sea, St. Paul's islets in the Atlantic and Macquarie Island in the southwestern Pacific - Tilley (1947), Melson et al. (1967), Varne and Rubenach (1972), Bonatti et al. (1981)).The "abyssal peridotites" are samples from the oceanic mantle that were dredged on the ocean floor, or recovered from drill cores (e.g., Bonatti et al., 1974; Prinz et al., 1976; Hamlyn and Bonatti, 1980).Altogether, tectonically emplaced and abyssal mantle rocks provide insights into upper mantle compositions and processes that are complementary to the information conveyed by mantle xenoliths (See Chapter 2.05). They provide coverage to vast regions of the Earth's upper mantle that are sparsely sampled by mantle xenoliths, particularly in the ocean basins and beneath passive continental margins, back-arc basins, and oceanic island arcs.Compared with mantle xenoliths, a disadvantage of some tectonically emplaced mantle rocks for representing mantle compositions is that their original geodynamic setting is not exactly known and their significance is sometimes a subject of speculation. For instance, the provenance of orogenic lherzolite massifs (subcontinental lithosphere versus upwelling asthenosphere) is still debated (Menzies and Dupuy, 1991, and references herein), as is the original setting

  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.

  19. 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.

  20. 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.

  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

  2. Comparison of evolution and tectonic significance of the Pindos and Vourinos ophiolite suites, northern Greece

    NASA Astrophysics Data System (ADS)

    Ross, John V.; Zimmerman, Jay

    1996-05-01

    . The composition, internal structure, environments of equilibration, stress history, and inferred direction of obduction of the Pindos ultramafites are strikingly similar to those of the Vourinos Complex located 40 km to the northeast on the eastern margin of the Sub-Pelagonian zone. It is probable that the two ophiolites shared a common origin and an identical early obduction history. During Late Jurassic times, the Vourinos and its metamorphic sole were thrust over a thick sequence of shelf carbonates and a thin, diamictitic melange, both units being metamorphosed to lower greenschist facies. Comparable units beneath the Pindos thrust sheet (the Loumnitsa Unit and the Avdella Melange) are thicker, somewhat more lithologically diverse, and comprise amphibolite, greenschist, radiolarite, pillow lava, garnetite, exotic marble, flysch, and poorly metamorphosed, fossiliferous limestone, typically occurring in tightly folded, thin thrust slices. Differences in the timing and direction of post-emplacement thrust transport of the two complexes are marked and may have begun late in the obduction phase.

  3. Underplating process from melange formation to duplexing: Example from the Cretaceous Shimanto Belt, Kii Peninsula, southwest Japan

    NASA Astrophysics Data System (ADS)

    Hashimoto, Y.; Kimura, G.

    1999-02-01

    Accretionary complexes are considered to form through off scraping and underplating, which results in the lateral and vertical growth of an accretionary prism. Accretion of the upper part of the oceanic crust and the formation of melange are important components of the accretionary process, although the origin of melange is considered controversial. In order to better understand the origin of melange and its role in accretion, in particular during underplating, we have studied the relationship between the formation of melange and duplexing in the Miyama Assemblage. Here the melange is composed dominantly of shale, basalt, chert, and sandstone. This study stresses the following features: (1) the structure of the Miyama Assemblage appears to be a composite suite of landward and seaward dipping duplexes; (2) the melange fabric displays a systematic asymmetry, which may have resulted through shearing along the decollement; and (3) facing of horses within duplexes changes from north to south, coinciding with a change in the sense of shear inferred from the melange fabric, which suggests that it formed prior to duplexing.

  4. 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

  5. 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

  6. Magma in forearcs: implication for ophiolite generation

    NASA Astrophysics Data System (ADS)

    Jakeŝ, Petr; Miyake, Yasuyuki

    1984-07-01

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

  7. 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.

  8. 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.

  9. Interpreting anomalous magnetic fabrics in ophiolite dikes

    NASA Astrophysics Data System (ADS)

    Borradaile, G. J.; Gauthier, D.

    2003-02-01

    Anisotropy of magnetic susceptibility (AMS) may reveal mineral orientation-distributions defining magmatic flow-axes in igneous dikes. The mafic silicates are the best indication of magmatic flow but Fe-Ti accessories may contribute more to the bulk susceptibility. If the orientation-distributions of the two subfabrics are incongruent, anomalous fabrics will occur that do not reflect magma-flow axes. For ophiolite dikes, ocean-floor metamorphism changes the mineralogy producing new Fe-oxides by retrogression and exsolution from mafic silicates and by the oxidation of primary oxides. Incompatibly oriented 'ferro'-magnetic subfabrics may be isolated by anisotropy of anhysteretic remanence (AARM). Anomalous AMS fabrics in ophiolites elsewhere have been attributed to inverse-fabric contributions from single-domain magnetite in varying combinations. However, in ophiolite dikes from the Troodos ophiolite of Cyprus, anomalous fabrics arise from ocean-floor metamorphism extensively or completely replacing the original magnetite and titanomagnetite accessory phases with titanomagnetite (˜Fe 2.4Ti 0.6O 4=TM60) and its oxidised versions, titanomaghemite, to varying degrees according to depth beneath the ocean-floor, distance from spreading axis and proximity to transform-faults. At best, the oxide orientation-distribution defined by AARM could only be indirectly related to magma-flow if its nucleation-orientation controlled by a host-lattice. However, more commonly the topotactic lattice reorganization produces weaker ARM fabric anisotropies. Although 'recrystallized', oxidised TM60 dominates the bulk low-field susceptibility, its anisotropy is generally too feeble to compete with the flow-fabric defined by the AMS contribution from paramagnetic mafic silicates.

  10. 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.

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

    USGS Publications Warehouse

    Tabor, R.W.

    1994-01-01

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

  12. Tectonic implications of the Indian Run Formation; a newly recognized sedimentary melange in the northern Virginia Piedmont

    USGS Publications Warehouse

    Drake, Avery Ala

    1985-01-01

    Sedimentary melange in the northeastern part of Fairfax County, Virginia, contains both mesoscopic and mappable fragments of Accotink Schist, Lake Barcroft Metasandstone, metagabbro, and ultramafic rocks as well as smaller fragments of other rock types. This melange was originally mapped as the Sykesville Formation, a major precursory sedimentary melange in northern Virginia and Maryland. The fragments of Accotink Schist and Lake Barcroft Metasandstone within the Sykesville were considered to be rip-ups of these units over which the Sykesville slid when finally emplaced. More recent study has shown that fragments of Accotink and Lake Barcroft are restricted to a certain area of sedimentary melange originally defined as Sykesville, and this part of the melange is now considered to be a separate mappable unit, here named the Indian Run Formation. The Indian Run underlies the sequence Accotink Schist and Lake Barcroft Metasandstone which is here formally named the Annandale Group. The Indian Run is intruded by the Occoquan Granite of Cambrian age, so it is of Cambrian or Late Proterozoic age. The Sykesville Formation (restricted) is a much more extensive unit than the Indian Run Formation and is characterized by its contained olistoliths of the Peters Creek Schist, the unit that tectonically overlies it. The Sykesville and Peters Creek constitute a precursory melange-allochthon pair which is here termed a 'tectonic motif.' The Indian Run-Annandale pair then forms a tectonically lower motif, and the overlying pair, the Yorkshire Formation-Piney Branch Complex, forms a tectonically higher motif. The Chopawamsic Formation and underlying sedimentary melange in the area south of Fairfax County may form a tectonic motif beneath the Indian Run-Annandale tectonic motif. Thus, three and perhaps four repetitions of precursory melange-allochthon pairs occur in northern Virginia. Other percursory melanges and motifs may occur in the Maryland Piedmont to the north. The tectonic

  13. Solonker ophiolite in Inner Mongolia, China: A late Permian continental margin-type ophiolite

    NASA Astrophysics Data System (ADS)

    Luo, Zhi-wen; Xu, Bei; Shi, Guan-zhong; Zhao, Pan; Faure, M.; Chen, Yan

    2016-09-01

    The Solonker ophiolite is exposed along the border between Mongolia and China within the Solonker zone, the southeastern Central Asian Orogenic Belt (CAOB), and it is composed dominantly of serpentinized peridotite with subordinate gabbro, basaltic lava, radiolarian-bearing siliceous rocks, and minor plagiogranite. Meanwhile, layered mafic-ultramafic cumulates are not ubiquitous. In this study, zircon grains from two gabbros and a plagiogranite yield 206Pb/238U ages of 259 ± 6 Ma, 257 ± 3 Ma and 263 ± 1 Ma. These data were interpreted to represent the formation age of the Solonker ophiolite. The studied gabbros and basalts have a tholeiitic composition, showing a MORB affinity. They are also characterized by enrichment of Pb and depletion of Nb relative to La and Th. Furthermore, the studied gabbros contain inherited zircon grains and display a large range of zircon Hf isotopes (εHf(t) = - 5.27 to + 10.19). These features imply that crustal contamination played an important role in the generation of these mafic rocks. Major elements derived from the radiolarian-bearing siliceous rocks suggest a continental margin setting. This is confirmed by rock association. Terrigenous rocks (sandstones and siltstones) interstratified with siliceous rocks. U-Pb dating of detrital zircon grains in sandstones from both the northern and southern sides of the Solonker ophiolite belt, along with published data, reveals that the Late Carboniferous-Early Permian strata in fault contact with the Solonker ophiolite was deposited above Early Paleozoic orogens. The lines of petrological, geochemical, geochronological, and isotopic evidence led us to propose that the Solonker ophiolite is a Late Permian continental margin-type body formed during the early stages of opening of an ocean basin, following rifting and break-up of the Early Paleozoic orogens. Accordingly, the Permian Solonker zone is characterized by an intra-continental extensional setting.

  14. 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

  15. Geology of the Zambales ophiolite, Luzon, Philippines

    USGS Publications Warehouse

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

    1989-01-01

    The Zambales ophiolite of western Luzon, Philippines, exposes a typical succession of basalt flows, diabasic dikes, gabbro and tectonized harzburgite. The age established by limiting strata is late Eocene. Lack of evidence of thrust faulting and the general domal disposition of the lithologie units indicate that the ophiolitic rocks are exposed by uplift. Highly complex internal layered structures within the complex are related to processes developed during formation of the ophiolite and the Zambales ophiolite may be one of the least disturbed (by emplacement) ophiolitic masses known. The exposed mass trends north and the upper surface plunges at low angles (a few degrees) to the north and south. The chemistry and composition of the rocks in the northwest part of the Zambales area (Acoje block) is distinct from that in the southeastern segment (Coto block). The Acoje block, according to Evans (1983) and Hawkins and Evans (1983), resembles (on a chemical basis) arc-tholeiite series rocks from intra-island arcs and the rocks in the Coto block are typical back-arc basin rock series. The present writer believes that the ophiolite composes a single genetic unit and that the changes in composition are the result of changes that took place during the initial formation. The gabbro probably formed below a spreading center in an elongate, in cross section, V-shaped, magma chamber. The gabbro is estimated by the writer to be less than 2 km thick and may be less than 1 km in places. Numerous erosional windows through the gabbro in the northern and eastern side of the Zambales area show that the gabbro remaining in those areas is likely to be only a few hundred meters thick. Harzburgite is exposed to a depth of about 800 m in the Bagsit River area and this may be the deepest part of the ophiolite accessible for study on which there is any control on depth. A transitional zone, about 200 m thick lying between the gabbro and harzburgite, is composed of serpentinized dunite

  16. Circum-Pacific accretion of oceanic terranes to continental blocks: accretion of the Early Permian Dun Mountain ophiolite to the E Gondwana continental margin, South Island, New Zealand

    NASA Astrophysics Data System (ADS)

    Robertson, Alastair

    2016-04-01

    Accretionary orogens, in part, grow as a result of the accretion of oceanic terranes to pre-existing continental blocks, as in the circum-Pacific and central Asian regions. However, the accretionary processes involved remain poorly understood. Here, we consider settings in which oceanic crust formed in a supra-subduction zone setting and later accreted to continental terranes (some, themselves of accretionary origin). Good examples include some Late Cretaceous ophiolites in SE Turkey, the Jurassic Coast Range ophiolite, W USA and the Early Permian Dun Mountain ophiolite of South Island, New Zealand. In the last two cases, the ophiolites are depositionally overlain by coarse clastic sedimentary rocks (e.g. Permian Upukerora Formation of South Island, NZ) that then pass upwards into very thick continental margin fore-arc basin sequences (Great Valley sequence, California; Matai sequence, South Island, NZ). Field observations, together with petrographical and geochemical studies in South Island, NZ, summarised here, provide evidence of terrane accretion processes. In a proposed tectonic model, the Early Permian Dun Mountain ophiolite was created by supra-subduction zone spreading above a W-dipping subduction zone (comparable to the present-day Izu-Bonin arc and fore arc, W Pacific). The SSZ oceanic crust in the New Zealand example is inferred to have included an intra-oceanic magmatic arc, which is no longer exposed (other than within a melange unit in Southland), but which is documented by petrographic and geochemical evidence. An additional subduction zone is likely to have dipped westwards beneath the E Gondwana margin during the Permian. As a result, relatively buoyant Early Permian supra-subduction zone oceanic crust was able to dock with the E Gondwana continental margin, terminating intra-oceanic subduction (although the exact timing is debatable). The amalgamation ('soft collision') was accompanied by crustal extension of the newly accreted oceanic slab, and

  17. 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

  18. Magnetic signatures of serpentinization at ophiolite complexes

    NASA Astrophysics Data System (ADS)

    Bonnemains, D.; Carlut, J.; Escartín, J.; Mével, C.; Andreani, M.; Debret, B.

    2016-08-01

    We compare magnetic properties of 58 variably serpentinized peridotites from three ophiolite complexes (Pindos, Greece; Oman; Chenaillet, France) and the mid-Atlantic Ridge near the Kane fracture zone (MARK). The Pindos and Oman sites show low susceptibility and remanence (K < 0.02 SI; Ms < 0.4 Am2/kg), while the Chenaillet and MARK sites show instead high susceptibility and remanence (K up to 0.15 SI; Ms up to 6 Am2/kg), regardless of serpentinization degree. Petrographic observations confirm that Pindos and Oman samples contain serpentine with very little magnetite, while Chenaillet and MARK samples display abundant magnetite in serpentine mesh cells. Bulk rock analyses show similar amounts of ferric iron at a given serpentinization degree, suggesting that iron is oxidized during the serpentinization reaction in both cases, but that its distribution among phases differs. Microprobe analyses show iron-rich serpentine minerals (5-7 wt % FeO) in low-susceptibility samples, while iron-poor serpentine minerals (2-4 wt % FeO) occur in high susceptibility samples. The contrasted magnetic properties between the two groups of sites thus reflect different iron partitioning during serpentinization, that must be related to distinct conditions at which the serpentinization reaction takes place. We propose that magnetic properties of ophiolitic serpentinites can be used as a proxy to differentiate between high temperature serpentinization (>˜250-300°C) occurring at the axis (i.e., Chenaillet, similar to serpentinites from magmatically poor mid-ocean ridges), from lower temperature serpentinization (<˜200-250°C), likely occurring off axis and possibly during obduction (i.e., Pindos and Oman). At both settings, serpentinization can result in significant hydrogen release.

  19. 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.

  20. 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.

  1. 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.

  2. Rapid emplacement of young oceanic lithosphere: argon geochronology of the oman ophiolite.

    PubMed

    Hacker, B R

    1994-09-09

    (40)Ar/(39)Ar dates of emplacement-related metamorphic rocks beneath the Samail ophiolite in Oman show that cooling to <525 degrees C occurred within approximately 1 million years of igneous crystallization of the ophiolite. This unexpectedly short time span and rapid cooling means that old, cold continental or oceanic lithosphere must have been adjacent to the ophiolite during spreading and then been thrust beneath the ophiolite almost immediately afterward.

  3. 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

  4. Palaeomagnetism of the Balantak ophiolite, Sulawesi

    NASA Astrophysics Data System (ADS)

    Mubroto, B.; Briden, J. C.; McClelland, E.; Hall, R.

    1994-07-01

    Paleomagnetic data from the East Arm of Sulawesi provide constraints on the Cretaceous and subsequent history of a fragment of the tectonic mosaic of a complex region close to the junction between SE Asia, the Indo-Australian plate and the western Pacific microplates. Primary thermoremanence has been demonstrated in samples from 23 sites in a tilted Cretaceous-Palaeogene lava sequence in the Balantak ophiolite at Batusimpang on the eastern tip of the East Arm of Sulawesi. Its direction (D = 60.5 deg, I = -32.1 deg, alpha(sub 95) = 5.5 deg) indicates formation at 17 +/- 4 deg S and approximately 60 deg of subsequent clockwise rotation. The primary mode of remanence is deduced from NRM/TRM comparisons; low-temperature cycling and other rock magnetic tests point to SD magnetite as the principal remanence carrier. Supporting evidence for the paleolatitude and northward movement of the East Arm is derived from other lavas at Binsil, and from the Boba Cherts, but lack of tectonic control limits the interpretation. The contrast between these results and the subequatorial origin of contemporary rocks on nearly Halmahera is consistent with subduction of Indian Ocean lithosphere beneath the Sunda margin in the late Mesozoic and early Tertiary. Large differences in declination of remanence in the E Sulawesi rocks indicate large clockwise and anticlockwise rotations of tectonic blocks only tens of kilometers across.

  5. 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.

  6. A mid-Archaean ophiolite complex, Barberton Mountain land

    NASA Technical Reports Server (NTRS)

    Dewit, M. J.; Hart, R.; Hart, R.

    1986-01-01

    New field observations and structurally restored geologic sections through the southern part of 3.5-3.6 Ga Barberton greenstone belt show that its mafic to ultramafic rocks form a pseudostratigraphy comparable to that of Phanerozoic ophiolites; this ancient ophiolite is referred to as the Jamestown ophiolite complex. It consists of an intrusive-extrusive mafic-ultramafic section, underlain by a high-temperature tectono-metamorphic residual peridotitic base, and is capped by a chert-shale sequence which it locally intrudes. Geochemical data support an ophiolitic comparison. Fraction of high temperature melting PGE's 2500 C in the residual rocks suggest a lower mantle origin for the precursors of this crust. An oceanic rather than arc-related crustal section can be inferred from the absence of contemporaneous andesites. The entire simatic section has also been chemically altered during its formation by hyrothermal interaction with the Archean hydrosphere. The most primitive parent liquids, from which the extrusive sequence evolved, may have been picritic in character. Rocks with a komatiitic chemistry may have been derived during crystal accumulation from picrite-crystal mushes (predominantly olivine-clinopyroxene) and/or by metasomatism during one or more subsequent episodes of hydration-dehydration. The Jamestown ophiolite complex provides the oldest record with evidence for the formation of oceanic lithosphere at constructive tectonic boundaries.

  7. A mid-Archaean ophiolite complex, Barberton Mountain land

    NASA Astrophysics Data System (ADS)

    Dewit, M. J.; Hart, R.; Hart, R.

    New field observations and structurally restored geologic sections through the southern part of 3.5-3.6 Ga Barberton greenstone belt show that its mafic to ultramafic rocks form a pseudostratigraphy comparable to that of Phanerozoic ophiolites; this ancient ophiolite is referred to as the Jamestown ophiolite complex. It consists of an intrusive-extrusive mafic-ultramafic section, underlain by a high-temperature tectono-metamorphic residual peridotitic base, and is capped by a chert-shale sequence which it locally intrudes. Geochemical data support an ophiolitic comparison. Fraction of high temperature melting PGE's 2500 C in the residual rocks suggest a lower mantle origin for the precursors of this crust. An oceanic rather than arc-related crustal section can be inferred from the absence of contemporaneous andesites. The entire simatic section has also been chemically altered during its formation by hyrothermal interaction with the Archean hydrosphere. The most primitive parent liquids, from which the extrusive sequence evolved, may have been picritic in character. Rocks with a komatiitic chemistry may have been derived during crystal accumulation from picrite-crystal mushes (predominantly olivine-clinopyroxene) and/or by metasomatism during one or more subsequent episodes of hydration-dehydration. The Jamestown ophiolite complex provides the oldest record with evidence for the formation of oceanic lithosphere at constructive tectonic boundaries.

  8. 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.

  9. 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.

  10. 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.

  11. 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

  12. Emplacement of the Western Tethyan Ophiolites and Atlantic Spreading

    NASA Astrophysics Data System (ADS)

    Smith, A. G.

    2005-12-01

    This contribution shows that the cessation of emplacement of the 'Tethyan' supra-suduction zone (SSZ) ophiolites of the circum-Mediterranean region, and Oman, is contemporaneous with a marked drop in the relative velocities between Africa and Eurasia. It is argued that this contemporaneity is to be expected from plate theory. The Hellenic-Dinaric SSZ ophiolites in Greece, Albania and former Yugoslavia, range in age from ~173-168 Ma (mid-Jurassic). They were emplaced, eroded and covered by younger sediments by ~140 Ma, or possibly earlier, on the plate margin between northwest Gondwana and southwest Laurasia (=Africa/Eurasia). During emplacement, a passive continental margin was partially subducted, but because large-scale subduction of a continent appears to be precluded by the low density of continental crust, the subduction zone eventually ceased to exist and moved elsewhere. The SSZ ophiolites of Turkey, Cyprus, Syria, Iran and Oman are of mid-Cretaceous age. The initial setting of the mid-Cretaceous ophiolites is clearest in Oman. The Oman ophiolite appears to have formed during roll-back of Triassic (or possibly Permian) ocean-floor from the Iranian margin at ~95 Ma and to have been emplaced by early Maastrichtian time, ~70 Ma, after which the Arabian passive margin could not undergo further subduction. Despite the complexities and uncertainties in the Atlantic spreading data, the relative motion of Africa to stable Europe in the region of the Tethyan ophiolites is quite simple. A major slowing of the relative motion from ~40-50 mm yr-1 to less than 5 mm yr--1 took place at ~140 Ma, broadly synchronous with the cessation of Hellenic-Dinaric ophiolite emplacement. Slow motion lasted from ~140-125 Ma and is attributed to the failure to subduct the passive continental margin; to the destruction of the Africa-Eurasian margin of that time; and to its eventual reestablishment elsewhere, ~15 m.y. later, after which the relative motion speeded up to ~20 mm yr-1. A

  13. 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.

  14. 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.

  15. 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.

  16. 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

  17. Microfabrics and deformation mechanisms in a jadeite-blueschist from the Franciscan melange, California

    NASA Astrophysics Data System (ADS)

    Wassmann, S.; Krohe, A.; Stoeckhert, B.; Trepmann, C.

    2010-12-01

    The Franciscan melange is a chaotic mixture of metasedimentary, metabasic and hydrated mantle material forming a typical block in matrix structure with extreme strain gradients on length scales between centimeter and kilometer. Mineral assemblages indicate high-pressure low-temperature metamorphism, with assemblages comprising jadeite + quartz, glaucophane, and aragonite. In combination, the structural relations and metamorphic conditions suggest that the Franciscan melange records processes in the deep level of an accretionary complex or in a subduction channel reaching to great depth. Exhumed materials provide insight into typical deformation mechanisms, stress states, and stress history at depth, which cannot be gained by other approaches. Here we analyze the microfabrics of a highly deformed and compositionally heterogenous jadeite-blueschist. Brittle failure is evident on various length scales. Several generations of fractures are widened and sealed to become veins. The shape of the veins, poor fitting of the walls, and the overall low aspect ratio indicate host rock deformation during sealing. The vein minerals indicate sealing at high-pressure metamorphic conditions, with aragonite being predominant in the latest generation. Individual jadeite porphyroblasts are fragmented. The fractures are sealed by quartz and new jadeite epitactically grown on the broken host. Microstructures suggest that the distributed deformation concomitant with vein formation is predominantly by dissolution precipitation creep. Jadeite porphyroblasts show strain caps and strain shadows, indicating progressive deformation after their crystallization. Unequivocal evidence for dislocation creep is restricted to quartz in some of the veins, where the fine-grained microstructure indicates strain induced grain boundary migration and subgrain rotation. In contrast, in other veins quartz shows a foam structure controlled by interfacial free energy. The microfabrics of the jadeite

  18. 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

  19. Cambrian Kherlen ophiolite in northeastern Mongolia and its tectonic implications: SHRIMP zircon dating and geochemical constraints

    NASA Astrophysics Data System (ADS)

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

    2016-09-01

    The Kherlen terrane, which contains the Kherlen ophiolitic complex, is located between two Precambrian continental blocks in the northeastern Mongolia. We present new geochemical and SHRIMP zircon U-Pb data for the Kherlen ophiolitic complex and for granitic plutons intruding the complex, providing constraints on the regional evolution in Early Paleozoic time. The Kherlen ophiolite, which is geochemically similar to SSZ-type ophiolites, was originated from two distinct mantle sources, a N-MORB-like source and an E-MORB-like source. A gabbro and a plagiogranite dike intruding the gabbro from the Kherlen ophiolite yielded similar SHRIMP zircon U-Pb ages of ca. 500 Ma, suggesting that the ophiolite formed in Late Cambrian time. Post- or syn-collisional granites intruding the ophiolitic complex yielded crystallization ages of ca. 440 Ma, which is interpreted to record the minimum age of the tectonic emplacement of the ophiolite. These new data demonstrate that the Kherlen ophiolite belt is an Early Paleozoic suture between the Ereendavaa and the Idermeg continental terranes, which is generally coeval with the Bayankhongor belt in central Mongolia, indicating that they are regionally correlated, and thus they define a major Early Paleozoic suture between two Precambrian continental blocks in the central-northeastern Mongolia.

  20. Peraluminous potassium-rich granitoids in the Semail Ophiolite

    NASA Astrophysics Data System (ADS)

    Peters, Terk; Kamber, Balz S.

    1994-12-01

    Potassic granites are found as dikes and small stocks in the uppermost part of the mantle sequence of the Semail ophiolite, predominantly in the northern and less frequently in the central and southern part. New chemical analyses show that the biotite granites lie on the same evolutionary trend as the quartz diorites and tonalites, occurring as late intrusives in the gabboro and mantle sequence. Based on mineral chemistry, the P-T conditions of andalusite-cordierite bearing pegmatoidal granites were calculated, giving pressures of 1.8 2.2 kbar and a solidus within the stability field of andalusite. Hydrothermal experiments on five natural granite and tonalite compositions gave minimum melting between 650 and 675° C at 2 kbar. At 700° C, excess H2O, there is already 70 80% melt from the granitic starting compositions. Potassic granites in the ophiolite can be the result of extreme fractional crystallization in the late intrusive series. Granitic melts can also be produced by a two-stage process, in which fluids escaping from the underlying metamorphic sole trigger partial melting of the more felsic members in the ophiolite series. Two-mica granites and pegmatites in the underlying metamorphic sole, possibly melting products of amphibolites and biotite gneisses, are contemporaneous. These escaping fluids could also explain the Pb and Sr isotopic compositions of the leucocratic dikes, indicating variable crustal contamination.

  1. Formation and tectonic evolution of the Cretaceous Jurassic Muslim Bagh ophiolitic complex, Pakistan: Implications for the composite tectonic setting of ophiolites

    NASA Astrophysics Data System (ADS)

    Khan, Mehrab; Kerr, Andrew C.; Mahmood, Khalid

    2007-10-01

    The Muslim Bagh ophiolitic complex Balochistan, Pakistan is comprised of an upper and lower nappe and represents one of a number of ophiolites in this region which mark the boundary between the Indian and Eurasian plates. These ophiolites were obducted onto the Indian continental margin around the Late Cretaceous, prior to the main collision between the Indian and Eurasian plates. The upper nappe contains mantle sequence rocks with numerous isolated gabbro plutons which we show are fed by dolerite dykes. Each pluton has a transitional dunite-rich zone at its base, and new geochemical data suggest a similar mantle source region for both the plutons and dykes. In contrast, the lower nappe consists of pillow basalts, deep-marine sediments and a mélange of ophiolitic rocks. The rocks of the upper nappe have a geochemical signature consistent with formation in an island arc environment whereas the basalts of the lower nappe contain no subduction component and are most likely to have formed at a mid-ocean ridge. The basalts and sediments of the lower nappe have been intruded by oceanic alkaline igneous rocks during the northward drift of the Indian plate. The two nappes of the Muslim Bagh ophiolitic complex are thus distinctively different in terms of their age, lithology and tectonic setting. The recognition of composite ophiolites such as this has an important bearing on the identification and interpretation of ophiolites where the plate tectonic setting is less well resolved.

  2. Hydrocarbon generation and migration induced by ophiolite obduction: The carbonate platform under the Semail Ophiolite, Jebel Akhdar, Oman.

    NASA Astrophysics Data System (ADS)

    Grobe, Arne; Littke, Ralf; Urai, Janos L.

    2016-04-01

    Oman's Semail Ophiolite, as largest ophiolite on earth, fascinated geologists for more than hundred years. It spans over 350 km in a NW-SE orientation and is dominating the northern landscape of Oman. After overthrusting of this oceanic crust onto the passive continental margin of Arabia, updoming of the area during Alpine orogeny exposed the margin sediments which are now easily accessible at the surface. Within the Oman Mountains different canyons provide access to 1,400 m of lithology accumulated through the last 300 My, containing hydrocarbon source and reservoir rocks. These sedimentary rocks offer unique possibilities to analyze the temperature and pressure evolution of sedimentary basins influenced by large scale overthrusts. Understanding the evolution of the overthrusted sedimentary basin is the key to entrapped reservoirs. Hence, it is essential to understand the thermal and tectonic history of the Oman Mountains to elaborate the interplay of overthrusting and ophiolite induced burial of the overridden sedimentary basin. Therefore, we linked structural modelling of the northern Oman Mountains with petrographic analysis and basin modelling. Thermal history was reconstructed using various maturity parameters (e.g. vitrinite and solid bitumen reflectance, Raman spectroscopy of carbonaceous material, fluid inclusion measurements), while reconstruction of the structural history was based on field mapping and stress field restorations. Presented results were summarized and integrated in a numerical basin model of the area.

  3. 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

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

    NASA Astrophysics Data System (ADS)

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

    2012-09-01

    Sedimentology can shed light on the emplacement of oceanic lithosphere (i.e. ophiolites) onto continental crust and post-emplacement settings. An example chosen here is the well-exposed Jurassic Mirdita ophiolite in southern Albania. Successions studied in five different ophiolitic massifs (Voskopoja, Luniku, Shpati, Rehove and Morava) document variable depositional processes and palaeoenvironments in the light of evidence from comparable settings elsewhere (e.g. N Albania; N Greece). Ophiolitic extrusive rocks (pillow basalts and lava breccias) locally retain an intact cover of oceanic radiolarian chert (in the Shpati massif). Elsewhere, ophiolite-derived clastics typically overlie basaltic extrusives or ultramafic rocks directly. The oldest dated sediments are calpionellid- and ammonite-bearing pelagic carbonates of latest (?) Jurassic-Berrasian age. Similar calpionellid limestones elsewhere (N Albania; N Greece) post-date the regional ophiolite emplacement. At one locality in S Albania (Voskopoja), calpionellid limestones are gradationally underlain by thick ophiolite-derived breccias (containing both ultramafic and mafic clasts) that were derived by mass wasting of subaqueous fault scarps during or soon after the latest stages of ophiolite emplacement. An intercalation of serpentinite-rich debris flows at this locality is indicative of mobilisation of hydrated oceanic ultramafic rocks. Some of the ophiolite-derived conglomerates (e.g. Shpati massif) include well-rounded serpentinite and basalt clasts suggestive of a high-energy, shallow-water origin. The Berriasian pelagic limestones (at Voskopoja) experienced reworking and slumping probably related to shallowing and a switch to neritic deposition. Mixed ophiolite-derived clastic and neritic carbonate sediments accumulated later, during the Early Cretaceous (mainly Barremian-Aptian) in variable deltaic, lagoonal and shallow-marine settings. These sediments were influenced by local tectonics or eustatic sea

  5. 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

  6. 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

  7. High-silica Rocks from Oceans, Arcs and Ophiolites: What Can They Tell Us About Ophiolite Origins?

    NASA Astrophysics Data System (ADS)

    Perfit, M. R.; Lundstrom, C.; Wanless, V. D.

    2015-12-01

    Although the volumes of high-silica rocks in submarine oceanic and supra-subduction zone environments are not well constrained, their common occurrence, field relations and compositions have led to various hypotheses suggesting that silicic intrusions (plagiogranites) in ophiolites formed by similar processes to high-silica volcanic rocks at mid-ocean ridge (MOR) or island arc environments. Geochemical attributes of andesite-rhyolite suites from MOR (East Pacific Rise, Juan de Fuca Ridge, Galapagos Spreading Center, Pacific-Antarctic Rise) and back-arc basins (Manus Basin, Lau Basin, East Scotia Ridge) show both similarities and differences to plagiogranitic suites (qtz. diorite-tonalite-trondhjemite) from ophiolites (Troodos and Semail). Both suites are commonly attributed to: extreme (>90%) fractional crystallization of basaltic melts; fractional crystallization coupled with assimilation of hydrated oceanic crust (AFC); or partial melting of preexisting crust. Normalized incompatible trace element patterns show either highly elevated, relatively flat patterns with negative Eu and Sr anomalies similar to high silica volcanics or have complimentary patterns with low abundance, more depleted patterns with positive Eu and Sr anomalies. None of the mechanisms, however, provide a consistent explanation for the compositional and isotopic variations that are observed among plagiogranites. In fact, ophiolitic plagiogranites can have at least two petrogenetic signatures - one indicative of a MORB parent and another that has been related to later, off-axis formation associated with supra-subduction zone magmatism. Based on thermal gradient experiments, the systematic changes in Fe and Si stable isotope ratios with differentiation observed in ophiolite and MOR high-silica suites may result from melt-mineral reactions within a temperature gradient near the boundaries of MOR magma lenses. Comparative major element, trace element and isotopic data will be presented from MOR

  8. 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

  9. Middle Jurassic Radiolaria from a siliceous argillite block in a structural melange zone near Viqueque, Timor Leste: Paleogeographic implications

    NASA Astrophysics Data System (ADS)

    Haig, David W.; Bandini, Alexandre Nicolas

    2013-10-01

    Thin-bedded siliceous argillite forming a large block within a structural melange zone at Viqueque, Timor Leste, has yielded a Middle Jurassic (late Bathonian-early Callovian) radiolarian assemblage belonging to Unitary Association Zone 7. Fifty-five species are recognized and illustrated, forming the most diverse radiolarian fauna yet documented from the Jurassic of Timor. The fauna shows little similarity in species content to the few other assemblages previously listed from the Middle or Late Jurassic of Timor, and also has few species in common with faunas known elsewhere in the region from Rotti, Sumatra, South Kalimantan, and Sula. Based on lithofacies similarities and age, the siliceous argillite succession in the melange block at Viqueque is included in the Noni Group originally described as the lower part of the Palelo Series in West Timor. In terms of lithofacies, the Noni Group is distinct from other stratigraphic units known in Timor. It may be associated with volcanic rocks but age relationships are uncertain, although some of the radiolarian cherts in the Noni Group in West Timor have been reported to include tuffaceous sediment. The deep-water character of the siliceous hemipelagite-pelagite facies, the probable volcanic association, and an age close to that of continental breakup in the region suggest deposition in a newly rifted Indian Ocean. In Timor's tectonostratigraphic classification scheme, the Noni Group is here placed in the "Indian Ocean Megasequence".

  10. Geochemistry of Andaman Ophiolite: Evidence for a Mid-Oceanic Ridge origin

    NASA Astrophysics Data System (ADS)

    Bhattacharya, S.; Ray, J. S.

    2015-12-01

    "Ophiolite conundrum" deals with the unresolved puzzle of the origin of ophiolite, whether they have primarily formed in mid-oceanic ridge (MOR) or supra subduction zone (SSZ) settings. This attracts considerable debate because most of the ophiolites are located along the present day suture zones. The Andaman Ophiolite is a Cretaceous complex that occurs on the Andaman accretionary prism of the Indian-Eurasian convergent plate boundary. Here, we present whole rock trace element and isotope (Sr-Nd) data from the crustal section of the ophiolite comprising of pillow basalt, basalt, dolerite and gabbro from Rutland Island, south and middle Andaman Islands. Trace element patterns for a majority of our samples show N-MORB affinity barring enrichments in Rb and Ba, which could be attributed to secondary alteration. Comparison of trace element patterns and isotopic compositions of these rocks (eNd (t = 95 Ma) = 7.5 to 9.4; 87Sr/86Sri = 0.703 to 0.704) with that of the 127- 64 Ma Indian Ocean MORB suggest strong similarities thus, implying that a large section of Andaman Ophiolite represents the subducting Indian Oceanic Plate. If so, then this ophiolite complex possibly represents an obducted Indian oceanic lithosphere that formed at the Indian Ocean MOR.

  11. 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.

  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

  13. Geochemistry and 40Ar/39Ar geochronology of the ophiolite in Northern Xinjiang

    NASA Astrophysics Data System (ADS)

    Qinqin, Xu; Jianqing, Ji

    2010-05-01

    As the remnant of ancient oceanic crust, ophiolite is extremely important to reconstruct the framework of ancient ocean and continent. There are several ophiolite belts in Northern Xinjiang, assuming planar distribution, and they are generally related to different faults since Paleozoic in the outcrops. This paper presents a systematic study of geochemistry and geochronology of the gabbro, diabase and basalt of ophiolite collected from different regions in Northern Xinjiang. The intermediate and basic rocks in ophiolite show similar patterns in primitive mantle-normalized REE and trace elements diagrams with MORB and different from IAT, indicating that the ophiolite may be formed in mid-ocean ridge. Their Sr, Nd and Pb isotopic compositions also show that they have mid-ocean ridge environment affinity. Additionally, their Sr, Nd and Pb isotopic ratios are similar respectively, suggesting the same source area, and they also have positive epsilon Nd values(4.40 ~ 8.04), which indicates they likely originate from the depleted mantle. The previous and the author's researches show that the forming time of ophiolite in Northern Xinjiang is early Paleozoic, but 40Ar/39Ar ages of gabbro, diabase and basalt are in the range of 396 ~ 226Ma (mainly from 350 to 250Ma), and no ages of early Paleozoic are gained. Moreover, this time coincides with the timing of late Paleozoic post-collisional plutonism, indicating the ophiolite in Northern Xinjiang was reworked by the late thermal events. The ophiolite widely exposed in Northern Xinjiang have similar characteristics of occurrence, lithologic association and isotopes in spite of diverse special features, which indicates that a relatively uniform and integrated source region has existed in Northern Xinjiang since Paleozoic. It is likely to infer that this source region is related with the long lasting remnant oceanic basin and the related lithosphere since Paleozoic in North Xinjiang. Key Words: Ophiolite, 40Ar/39Ar age

  14. 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.

  15. 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.

  16. 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 δ13C/δ18O, 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 δ13C and δ18O 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 δ13C (-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 p

  17. 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

  18. Scientific Drilling in the Samail Ophiolite, Sultanate of Oman

    NASA Astrophysics Data System (ADS)

    Matter, J. M.; Kelemen, P. B.; Teagle, D. A. H.

    2015-12-01

    The Samail ophiolite in Oman, a block of oceanic crust and upper mantle that was thrusted onto the Arabian continent ~100 million years ago and subsequently tilted and eroded, is an excellent field laboratory to explore rock forming processes that occurred near the surface down to 20 km depth in the Earth's interior. The exposure of these rocks to surface conditions provides a large reservoir of chemical potential energy that drives rapid reactions, heat generation, expansion and cracking. The Oman Drilling Project will address long-standing questions regarding mantle melting, melt transport and crystallization of lavas at ocean spreading ridges to form ocean crust, determine the nature and extent of chemical interactions between the oceans and newly formed oceanic crust, improve our understanding of CO2 and H2O uptake via weathering to form hydrated minerals and carbonates including reaction-driven cracking mechanisms as well as explore serpentinite-hosted microbial ecosystem. With funding from the International Continental Scientific Drilling Program (ICDP), U.S. NSF, NASA, IODP, Sloan Foundation and Deutsche Forschungsgesellschaft in place, we will address these objectives via observations on core, geophysical logging, fluid and microbiological sampling, and hydrological measurements in a series of newly drilled boreholes. Preliminary surveys showed that active low-T alteration of upper mantle rocks is an ongoing process. Dissolved hydrogen and methane concentrations in fluid samples collected in existing boreholes are up to 1.3 and 8 mmol/l, respectively [1]. Regarding the physical, chemical and biological processes related to near surface alteration of mantle rocks, a multi-borehole test site will be established in the southern massif of the Samail ophiolite. This test site will facilitate in-situ studies of water-rock-microbe interactions. Technical details and potential opportunities will be discussed. [1] Paukert A. PhD Thesis, Columbia University, New York

  19. Scientific Drilling in the Samail Ophiolite, Sultanate of Oman

    NASA Astrophysics Data System (ADS)

    Matter, Juerg; Kelemen, Peter; Teagle, Damon; Coggon, Judith

    2016-04-01

    The Samail ophiolite in Oman, a block of oceanic crust and upper mantle that was thrusted onto the Arabian continent ~100 million years ago and subsequently tilted and eroded, is an excellent field laboratory to explore rock forming processes that occurred near the surface down to 20 km depth in the Earth's interior. The exposure of these rocks to surface conditions provides a large reservoir of chemical potential energy that drives rapid reactions, heat generation, expansion and cracking. The Oman Drilling Project will address long-standing questions regarding mantle melting, melt transport and crystallization of lavas at ocean spreading ridges to form ocean crust, determine the nature and extent of chemical interactions between the oceans and newly formed oceanic crust, improve our understanding of CO2 and H2O uptake via weathering to form hydrated minerals and carbonates including reaction-driven cracking mechanisms as well as explore serpentinite-hosted microbial ecosystem. With funding from the International Continental Scientific Drilling Program (ICDP), U.S. NSF, NASA, IODP, Sloan Foundation and Deutsche Forschungsgesellschaft in place, we will address these objectives via observations on core, geophysical logging, fluid and microbiological sampling, and hydrological measurements in a series of newly drilled boreholes. Preliminary surveys showed that active low-T alteration of upper mantle rocks is an ongoing process. Dissolved hydrogen and methane concentrations in fluid samples collected in existing boreholes are up to 1.3 and 8 mmol/l, respectively [1]. Regarding the physical, chemical and biological processes related to near surface alteration of mantle rocks, a multi-borehole test site will be established in the southern massif of the Samail ophiolite. This test site will facilitate in-situ studies of water-rock-microbe interactions. Technical details and potential opportunities will be discussed.

  20. 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 δ18O 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

  1. 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.

  2. The application of remote sensing techniques to the study of ophiolites

    NASA Astrophysics Data System (ADS)

    Khan, Shuhab D.; Mahmood, Khalid

    2008-08-01

    Satellite remote sensing methods are a powerful tool for detailed geologic analysis, especially in inaccessible regions of the earth's surface. Short-wave infrared (SWIR) bands are shown to provide spectral information bearing on the lithologic, structural, and geochemical character of rock bodies such as ophiolites, allowing for a more comprehensive assessment of the lithologies present, their stratigraphic relationships, and geochemical character. Most remote sensing data are widely available for little or no cost, along with user-friendly software for non-specialists. In this paper we review common remote sensing systems and methods that allow for the discrimination of solid rock (lithologic) components of ophiolite complexes and their structural relationships. Ophiolites are enigmatic rock bodies which associated with most, if not all, plate collision sutures. Ophiolites are ideal for remote sensing given their widely recognized diversity of lithologic types and structural relationships. Accordingly, as a basis for demonstrating the utility of remote sensing techniques, we briefly review typical ophiolites in the Tethyan tectonic belt. As a case study, we apply integrated remote sensing studies of a well-studied example, the Muslim Bagh ophiolite, located in Balochistan, western Pakistan. On this basis, we attempt to demonstrate how remote sensing data can validate and reconcile existing information obtained from field studies. The lithologic and geochemical diversity of Muslim Bagh are representative of Tethyan ophiolites. Despite it's remote location it has been extensively mapped and characterized by structural and geochemical studies, and is virtually free of vegetative cover. Moreover, integrating the remote sensing data with 'ground truth' information thus offers the potential of an improved template for interpreting remote sensing data sets of other ophiolites for which little or no field information is available.

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

    USGS Publications Warehouse

    McLaughlin, R.J.; Blake, M.C.; 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

  4. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

  6. Timing of tectonic emplacement of the ophiolites and terrane paleogeography in the Hellenides

    NASA Astrophysics Data System (ADS)

    Papanikolaou, Dimitrios

    2009-03-01

    The timing of tectonic emplacement of the ophiolites is analyzed in the four oceanic terranes of the Hellenides (H 2, H 4, H 6, H 8). The criteria for this analysis are based on: a) the post-emplacement sedimentary cover or intrusive rocks, b) the syn-emplacement tectonostratigraphic formations and c) the youngest rocks involved in the structure of the autochthon and the allochthon unit in each case. The timing becomes younger towards the more external tectonic units of the Hellenides with: (i) Late Eocene-Oligocene age in the external ophiolite belt of the Pindos-Cyclades oceanic terrane H 2, (ii) Late Jurassic-Early Cretaceous age in the internal ophiolite belt of the Vardar/Axios oceanic terrane H 4 , (iii) Post-Liassic-pre-Late Jurassic age in the ophiolites of Lesvos-Circum Rhodope oceanic terrane H 6 and (iv) Pre-Late Jurassic age in the ophiolites of Volvi-Eastern Rhodope terrane H 8. An ophiolite obduction model can be applied, with the ophiolitic nappes always emplaced on top of pre-Alpine continental terranes with Mesozoic shallow-water carbonate platforms. The geometry of the continental terranes drifting during the Mesozoic within the Tethys Ocean controls the number and dimensions of the Tethyan oceanic basins. Where a continental terrane dies out, the two adjacent oceanic basins merge into one larger basin. This seems to be the case of the Pelagonian terrane (H 3), which is terminated north of Skopje, where the Pindos oceanic basin (H 2) merges with the Vardar/Axios oceanic basin (H 4).

  7. Gravity fields in eastern Halmahera and the Bonin Arc: Implications for ophiolite origin and emplacement

    NASA Astrophysics Data System (ADS)

    Milsom, John; Hall, Robert; Padmawidjaja, Tatang

    1996-02-01

    Classic ophiolites, as exemplified by the Troodos Massif in Cyprus and the Papuan Ultramafic Belt in eastern New Guinea, are large overthrust masses which are generally associated with large positive gravity anomalies. However, similar rocks occurring in extensive fragmented terranes which have also been described as ophiolitic do not produce large gravity effects. The eastern part of the island of Halmahera, in northeastern Indonesia, is an ophiolite of this latter type. On the two eastern arms of the island, a Mesozoic ophiolitic basement is overlain by, and imbricated with, Upper Cretaceous and Paleogene arc volcanic and sedimentary rocks. Bouguer gravity values are generally in the range +50 to +150 mGal and are characterised by steep local gradients indicative of shallow sources. The Bouguer gravity average suggests that the crust is at least 20 km thick, and it must be even thicker if a significant part of the anomalous gravity field is due to the presence of a cold and therefore dense, lithospheric slab within the asthenosphere, associated with the present-day subduction beneath Halmahera. The absence of any exposures of continental basement rocks or of quartzose sediments in eastern Halmahera suggests that these ophiolites have not been overthrust onto continental crust and that the thickening occurred in an intraoceanic island arc. The Paleogene arc was evidently characterised by volcanism occurring over an unusually wide area. In this it resembles the Izu-Bonin volcanic arc, which, like Halmahera, has been situated at the margin of the Philippine Sea Plate throughout its history. The gravity field of the Halmahera ophiolite is comparable with that of the Bonin volcanic arc, but there is no Halmahera parallel to the very high gravity fields recorded over the Bonin Islands forearc ridge. The equivalents of this part of the Paleogene arc may be represented by the ophiolitic complexes now distributed along the northern margin of the orogenic belt in New

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

    NASA Astrophysics Data System (ADS)

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

    2016-10-01

    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

  9. Bulk rheology and simulated episodic tremor and slip within a numerically-modeled block-dominated subduction melange

    NASA Astrophysics Data System (ADS)

    Webber, S.; Ellis, S. M.; Fagereng, A.

    2015-12-01

    We investigate the influence of melange rheology in a subduction thrust interface on stress and slip cycling constrained by observations from an exhumed subduction complex at Chrystalls Beach, New Zealand. A two-phase mélange dominated by large, competent brittle-viscous blocks surrounded by a weak non-linear viscous matrix is numerically modeled, and the evolution of bulk stress are analysed as the domain deforms. The models produce stress cycling behaviour under constant shear strain rate boundary conditions for a wide range of physical conditions that roughly corresponds to depths and strain rates calculated for instrumentally observed episodic tremor and slip (ETS) in presently-deforming subduction thrust interfaces. Stress cycling is accompanied by mixed brittle plastic-viscous deformation, and occurs as a consequence of geometric reorganisation and the progressive development and breakdown of stress bridges as blocks mutually obstruct one another. We argue that periods of low differential stress correspond to periods of rapid mixed-mode deformation and ETS. Stress cycling episodicities are a function of shear strain rate and pressure/temperature conditions at depth. The time period of stress cycling is principally controlled by the geometry (block distribution and density through time) and stress cycling amplitudes are controlled by effective stress. The duration of stress cycling events in the models (months-years) and rapid strain rates are comparable to instrumentally observed ETS. Shear strain rates are 1 - 2 orders of magnitude slower between stress cycling events, suggesting episodic return times within a single model domain are long duration (> centennial timescales), assuming constant flow stress. Finally, we derive a bulk viscous flow law for block dominated subduction mélanges for conditions 300 - 500°C and elevated pore fluid pressures. Bulk flow laws calculated for block-dominated subduction mélanges are non-linear, owing to a combination of

  10. 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

  11. Plate kinematics, origin and tectonic emplacement of supra-subduction ophiolites in SE Asia

    NASA Astrophysics Data System (ADS)

    Pubellier, Manuel; Monnier, Christophe; Maury, René; Tamayo, R.

    2004-11-01

    A unique feature of the Circum Pacific orogenic belts is the occurrence of ophiolitic bodies of various sizes, most of which display petrological and geochemical characteristics typical of supra-subduction zone oceanic crust. In SE Asia, a majority of the ophiolites appear to have originated at convergent margins, and specifically in backarc or island arc settings, which evolved either along the edge of the Sunda (Eurasia) and Australian cratons, or within the Philippine Sea Plate. These ophiolites were later accreted to continental margins during the Tertiary. Because of fast relative plate velocities, tectonic regimes at the active margins of these three plates also changed rapidly. Strain partitioning associated with oblique convergence caused arc-trench systems to move further away from the locus of their accretion. We distinguish "relatively autochthonous ophiolites" resulting from the shortening of marginal basins such as the present-day South China Sea or the Coral Sea, and "highly displaced ophiolites" developed in oblique convergent margins, where they were dismantled, transported and locally severely sheared during final docking. In peri-cratonic mobile belts (i.e. the Philippine Mobile Belt) we find a series of oceanic basins which have been slightly deformed and uplifted. Varying lithologies and geochemical compositions of tectonic units in these basins, as well as their age discrepancies, suggest important displacements along major wrench faults. We have used plate tectonic reconstructions to restore the former backarc basins and island arcs characterized by known petro-geochemical data to their original location and their former tectonic settings. Some of the ophiolites occurring in front of the Sunda plate represent supra-subduction zone basins formed along the Australian Craton margin during the Mesozoic. The Philippine Sea Basin, the Huatung basin south of Taiwan, and composite ophiolitic basements of the Philippines and Halmahera may represent

  12. P-T evolution of slivers of garnet-bearing micaschist in the sole of the Western Vardar Ophiolite Unit at Brezovica, Kosovo

    NASA Astrophysics Data System (ADS)

    Massonne, Hans-Joachim; Koller, Friedrich; Onuzi, Kujtim

    2016-04-01

    Rocks of the metamorphic sole of ophiolite complexes are regarded as an important factor to understand the process of obduction of former oceanic lithosphere on top of continental crust. The metamorphic evolution of these rocks can give, for instance, hints at the thickness of the obducted oceanic lithosphere. We have started to study the sole of the Western Vardar Ophiolitic Unit at the municipality of Bresovica, Kosovo. This unit is regarded as part of the former Vardar Ocean, a branch of the Neotethys, which was obducted onto the margin of the Adriatic microplate in Jurassic times. The sole in our study area, below strongly serpentinized ultramafic rocks, is characterized by a melange of various rock types, which are of medium metamorphic grade only in the vicinity of the ultramafic rocks. Our field work resulted in the recognition of several slivers of garnet-bearing micaschist among these medium-grade rocks which are dominated by amphibolite. In such a medium-grade rock from Bresovica the mineral assemblage talc + phengite was reported (Abraham and Schreyer, 1976, J. Petrol. 17, 421-439), which turned out by experiments in a piston-cylinder apparatus to be a high-pressure (HP: > 10 kbar) assemblage (Massonne and Schreyer, 1989, Eur. J. Mineral. 1, 391-410). We studied a garnet-bearing micaschist in detail. Elemental mapping and spot analyses of garnet obtained with an electron microprobe yielded core compositions of Alm0.695Gross(+Andr)0.11Pyr0.185Spes0.01. The composition of the garnet rim is Alm0.71Gross(+Andr)0.065Pyr0.21Spes0.015. On the basis of the bulk-rock composition of the micaschist, a P-T pseudosection was constructed with PERPLEX in the system K-Na-Ca-Mg-Mn-Fe-Al-Si-Ti-O-H. This pseudosection was contoured by isopleths for various parameters among them were the molar fractions of garnet components. According to such isopleths and the compositional variation of garnet, a more or less isobaric heating is likely. This heating to 650 °C has occurred

  13. 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.

  14. 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.

  15. Rooted Brooks Range ophiolite: Implications for Cordilleran terranes

    USGS Publications Warehouse

    Saltus, R.W.; Morin, R.L.; Hudson, T.L.

    2001-01-01

    Modeling of gravity and magnetic data shows that areally extensive mafic and ultramafic rocks of the western Brooks Range, Alaska, are at least 8 km thick, and that gabbro and ultramafic rocks underlie basalt in several places. The basalt, gabbro, and ultramafic rocks have been considered parts of a far-traveled ophiolite assemblage. These rocks are the highest structural elements in the Brooks Range thrust belt and are thought to be hundreds of kilometers north of their origin. This requires these rocks to be thin klippen without geologic ties to the continental shelf sedimentary rocks that now surround them. The geophysically determined, thick and interleaved subsurface character of the basalt, gabbro, and ultramafic rocks is inconsistent with this interpretation. An origin within an extensional setting on the continental shelf could produce the required subsurface geometries and explain other perplexing characteristics of these rocks. Early Mesozoic Alaska, from the North Slope southward to the interior, may have had many irregular extensional basins on a broad, distal continental shelf. This original tectonic setting may apply elsewhere in Cordilleran-type margins where appropriate mafic and ultramafic analogs are present.

  16. Nature of ophiolite occurrences along the eastern margin of the Indian plate and their tectonic significance

    NASA Astrophysics Data System (ADS)

    Sengupta, S.; Ray, K. K.; Acharyya, S. K.; de Smeth, J. B.

    1990-05-01

    Upper Mesozoic to lower Eocene ophiolitic rocks occur in two parallel belts along the eastern margin of the Indian plate. The eastern belt passes through central Burma, Sumatra, and Java, and coincides with a zone of gravity highs resulting from steeply dipping mafic rocks. It denotes the locus of the subduction where these ophiolites were accreted just prior to middle Eocene time. In contrast, the western belt, which passes through Nagaland, Manipur, western Burma, and Andaman, is flanked to the east by a negative gravity anomaly zone. In it the ophiolites occur as rootless, subhorizontal bodies, tectonically overlying Eocene-Oligocene flysch sedimentary rocks. They are inferred to be nappes that propagated westward from the eastern belt during late Oligocene terminal collision of the Indian and Eurasian continental blocks. Ophiolite occurrences in Andaman and Mentawai islands belonging to the western belt are generally linked with active subduction west of the island arc. This subduction began only in late Miocene time, and thus it could not have produced the ophiolites, which had been emplaced on land much earlier.

  17. 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.

  18. 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.

  19. 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.

  20. Mapping of Muslim Bagh ophiolite complex (Pakistan) using new remote sensing, and field data

    NASA Astrophysics Data System (ADS)

    Khan, Shuhab D.; Mahmood, Khalid; Casey, John F.

    2007-04-01

    TETHYS is a relational GIS database that combines geophysical, remote sensing, geochemical, and geochronological data, developed as a flexible resource for studying magmatic and geodynamic responses to continental collisions. In this paper, we demonstrate utility of our database by integrating field, remote sensing, and structural data, for detailed mapping and tectonic emplacement of Muslim Bagh ophiolite of western Pakistan. This ophiolite forms the uppermost part of a nappe pile which accreted onto the Indian continental margin during the closure of the Neo-Tethys during a pre-terminal collision that predated the final closure of Tethys during the major collision between India and Eurasia. Utilizing the TETHYS, Landsat, ASTER imagery, and a digital elevation model developed from the ASTER data are used to characterize the lithology and structure of the area. Use of image processing techniques improved the geologic map of the area, for a better understanding of the tectonic emplacement of the Muslim Bagh ophiolite. For the first time we report that the dikes in the Muslim Bagh ophiolite are cutting the metamorphic sole. Our preliminary geochemical data for sheeted dike complex suggest chemical affinities with arc-related rocks. This observation suggests that dikes were intruded in an island arc environment soon after the ophiolite was formed.

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

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

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

  2. Subduction Initiation and Forearc Magmatism as Recorded in Suprasubduction Zone Ophiolites

    NASA Astrophysics Data System (ADS)

    Dilek, Yildirim

    2013-04-01

    The internal structure-stratigraphy and geochemical signatures of most suprasubduction zone (SSZ) ophiolites indicate a seafloor spreading origin in forearc-incipient arc settings during the early stages of subduction. In general, there is a well developed magmatic stratigraphy in the extrusive sequences of these ophiolites from older MORB-like lavas at the bottom towards younger island arc tholeiite (IAT) and boninitic lavas in the upper parts. A similar progression of the lava chemistry also occurs in crosscutting dike swarms and sheeted dikes, indicating increased subduction influence in the evolution of ophiolitic magmas through time. Lherzolitic peridotites in structurally lower parts of the upper mantle sequences of these ophiolites represent the residue after MORB melt extraction. Harzburgite and harzburgite-dunite associations higher up in the mantle sequences and below the mafic-ultramafic cumulates (transitional Moho) are crosscut by networks of orthopyroxenite (opxt) veins, which include hydrous minerals (amphibole). These orthopyroxenite veins represent a reaction product between the host harzburgite (depleted, residual peridotite) and the migrating Si-rich (boninitic) melt. The harzburgite-dunite-opxt suites characterize melt-residue relationships and melt migration patterns in the mantle wedge during the initial stages of subduction and incipient arc construction. Thus, the SSZ ophiolites that we have examined display a lateral and vertical progression of melt evolution in their crustal and upper mantle components that traces different stages of subduction initiation-related magmatism, reminiscent of the forearc magmatism in some of the modern arc-trench rollback systems as in the Izu-Bonin-Mariana and Tonga-Kermadec subduction factories. The along-strike continuity for more than 1500 km of this well-documented chemostratigraphy and geochemical progression in different ophiolite belts is strong evidence for contemporaneous subduction initiation

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

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

    USGS Publications Warehouse

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

    1984-01-01

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

  5. 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

  6. 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

  7. 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

  8. 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.

  9. Synchronous formation of the metamorphic sole and igneous crust of the Semail ophiolite: New constraints on the tectonic evolution during ophiolite formation from high-precision U-Pb zircon geochronology

    NASA Astrophysics Data System (ADS)

    Rioux, Matthew; Garber, Joshua; Bauer, Ann; Bowring, Samuel; Searle, Michael; Kelemen, Peter; Hacker, Bradley

    2016-10-01

    The Semail (Oman-United Arab Emirates) and other Tethyan-type ophiolites are underlain by a sole consisting of greenschist- to granulite-facies metamorphic rocks. As preserved remnants of the underthrust plate, sole exposures can be used to better understand the formation and obduction of ophiolites. Early models envisioned that the metamorphic sole of the Semail ophiolite formed as a result of thrusting of the hot ophiolite lithosphere over adjacent oceanic crust during initial emplacement; however, calculated pressures from granulite-facies mineral assemblages in the sole suggest the metamorphic rocks formed at >35 km depth, and are too high to be explained by the currently preserved thickness of ophiolite crust and mantle (up to 15-20 km). We have used high-precision U-Pb zircon dating to study the formation and evolution of the metamorphic sole at two well-studied localities. Our previous research and new results show that the ophiolite crust formed from 96.12-95.50 Ma. Our new dates from the Sumeini and Wadi Tayin sole localities indicate peak metamorphism at 96.16 and 94.82 Ma (±0.022 to 0.035 Ma), respectively. The dates from the Sumeini sole locality show for the first time that the metamorphic rocks formed either prior to or during formation of the ophiolite crust, and were later juxtaposed with the base of the ophiolite. These data, combined with existing geochemical constraints, are best explained by formation of the ophiolite in a supra-subduction zone setting, with metamorphism of the sole rocks occurring in a subducted slab. The 1.3 Ma difference between the Wadi Tayin and Sumeini dates indicates that, in contrast to current models, the highest-grade rocks at different sole localities underwent metamorphism, and may have returned up the subduction channel, at different times.

  10. 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, α95 = 11° and tilt-corrected direction of D = 342°, I = - 27°, k = 15, α95 = 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.

  11. 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.

  12. Gabbro-peridotite Interaction in the Northern Cache Creek Composite Terrane Ophiolite, British Columbia and Yukon

    NASA Astrophysics Data System (ADS)

    Zagorevski, A.

    2015-12-01

    The northern Cache Creek composite terrane comprises a thrust stack of chert, limestone, siltstone, basalt, gabbro and ultramafic complexes ranging in age from Mississippian to Triassic. Fields studies and geochemical investigations indicate that ophiolitic mafic-ultramafic complexes formed in a supra-subduction zone setting. Ophiolitic rocks in the southeast form a structurally disrupted Penrose-type ophiolite; however, northwestern ophiolitic rocks generally lack lower and middle crust in most sections, exhibit a direct contact between supracrustal and mantle sections and locally contain ophicalcites suggesting that supracrustal rocks were structurally emplaced over mantle along extensional detachment(s). Mantle peridotite in the footwall of the detachment is extensively intruded by vari-textured, fine-grained to pegmatitic gabbro sills, dykes and stocks. These gabbro intrusions are locally boudinaged within fresh peridotite suggesting that the host mantle was rapidly exhumed prior to emplacement of the gabbro. Intrusive relationships between gabbro and variably serpentinized mantle peridotite are observed throughout the northern Cache Creek terrane (>300 km) suggesting a presence of a regional-scale Middle Triassic ocean-core complex. Overall, these data indicate that parts of the northern Cache Creek terrane formed in a setting analogous to backarc ocean core complexes such as the Godzilla Megamullion in the Parece Vela backarc basin, western Pacific.

  13. Is the Troodos ophiolite (Cyprus) a complete, transform fault-bounded Neotethyan ridge segment?

    NASA Astrophysics Data System (ADS)

    Morris, Antony; Maffione, Marco

    2016-04-01

    We report new paleomagnetic data from the sheeted dike complex of the Troodos ophiolite (Cyprus) that indicate a hitherto unrecognized oceanic transform fault system marks its northern limit. The style, magnitude and scale of upper crustal fault block rotations in the northwestern Troodos region mirror those observed adjacent to the well-known Southern Troodos Transform Fault Zone along the southern edge of the ophiolite. A pattern of increasing clockwise rotation toward the north, coupled with consistent original dike strikes and inclined net rotation axes across this region, is compatible with distributed deformation adjacent to a dextrally-slipping transform system with a principal displacement zone just to the north of the exposed ophiolite. Combined with existing constraints on the spreading fabric, this implies segmentation of the Troodos ridge system on length scales of ~40 km, and suggests that a coherent strip of Neotethyan lithosphere, bounded by transforms and containing a complete ridge segment, has been uplifted to form the currently exposed Troodos ophiolite. Moreover, the inferred length scale of the ridge segment is consistent with formation at a slow-spreading rate during Tethyan seafloor spreading and with a supra-subduction zone environment, as indicated by geochemical constraints.

  14. 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

  15. Supra-subduction zone (SSZ) ophiolites: the Fore-arc connection (Invited)

    NASA Astrophysics Data System (ADS)

    Shervais, J. W.; Metcalf, R. V.

    2009-12-01

    Ophiolites are distinct assemblages of submarine volcanic rocks and plutonic rocks that include cumulate dunite, wehrlite, and gabbro, as well as isotropic gabbro and diorite, and peridotite tectonite, representing the underlying refractory mantle. They were originally thought to represent oceanic crust formed at mid-oceanic spreading centers, but their connection with island arcs has become increasingly apparent ever since it was proposed by Miyashiro (1973). Recognition that ophiolites are not normal arc assemblages, but form during unique, transient episodes of arc formation, has led to the concept of supra-subduction zone ophiolites (Pearce, 1984). SSZ ophiolites display a consistent development history from birth through death that implies a common origin and evolution in response to systematic, non-random processes (Shervais 2001). A review of modern volcanic rocks formed at mid-ocean ridges and back-arc basins shows that they have a limited range in major element compositions, and trace element systematics that range from depleted (“normal”) to enriched MORB, in which ratios of fluid-mobile LFS elements to fluid-immobile HFS elements are relatively constant. In contrast, volcanic rocks formed within regionally-extended fore-arcs (which may also form the basement of later arc complexes) have wider range in major element compositions and trace element systematics that are depleted in the HFS elements and enriched in fluid-mobile LFS elements (Metcalf and Shervais 2008). Most ophiolite volcanic suites are dominated by major and trace element systematics that are identical to those displayed by fore-arc volcanic suites, including the occurrence of boninites, which are only found within forearc settings. These systematics are consistent with fluid-enrichment of the mantle source region that had seen a prior extreme melt depletion event. Some ophiolites display more complex relations, with both SSZ and MORB or BAB-like compositions, but the SSZ components are

  16. Genesis of the Batinah mélange above the Semail ophiolite, Oman

    NASA Astrophysics Data System (ADS)

    Robertson, A. H. F.; Woodcock, N. H.

    The Batinah mélange which overlies the late Cretaceous Semail ophiolite in the northern Oman Mountains comprises mostly sedimentary rocks of deep-water facies, alkalic lavas and intrusives, all of continental margin affinities, together with smaller volumes of Semail ophiolitic and metamorphic rocks. Four intergradational textural types of mélange can be recognized. Sheet mélange has large (>1 km) intact sheets either with little intervening matrix or set in other mélange types, and with an organised sheet orientation fabric. Slab mélange is finer textured (>100 m) and more disrupted. Block mélange has smaller (> m) blocks with some matrix and a weak to random block fabric. Clast mélange is matrix-supported rudite with a weak depositional clast fabric. Structural relationships, particularly the absence of tectonic fabrics, the decreasing strength of fragment fabrics with increasing fragmentation, and the abundance of brittle fragmentation, suggest that these mélange types formed by either gravity-driven sedimentary processes or superficial sliding or thrusting of individual rock slabs. In the slab mélange, long sequences can be pieced together, passing up from Upper Triassic mafic sub-marine extrusives and sediments into radiolarian cherts, hemipelagic and redeposited limestones, and terminating in non-calcareous radiolarities with Mn-deposits of early Cretaceous age. Mafic sills are numerous. These sequences can be matched with sub-ophiolite rocks now exposed in fault corridors through the Semail. These sequences become progressively disrupted upwards in the corridors and can be traced continuously into overlying mélange, which then thins away from the corridors. We argue that, during late Cretaceous emplacement over the Arabian margin, active fault corridors split the Semail slab and acted as conduits up which sub-ophiolite rocks were supplied to the ophiolite surface. There the rocks were redisributed by superficial processes.

  17. Tectonics of the Akamas and Mamonia ophiolites, Western Cyprus: magnetic petrofabrics and paleomagnetism

    NASA Astrophysics Data System (ADS)

    Borradaile, G. J.; Lucas, K.

    2003-12-01

    The Akamas ophiolite is shown to be a distal, off-axis extension of the main outcrop of Cretaceous ophiolite in the Troodos complex of Cyprus. Mantle-sequence harzburgites of both ophiolites share similarly oriented mantle-flow fabrics and the same Tertiary magnetizations acquired during exhumation. However, compared with the Troodos mantle sequence rocks, the Akamas ferromagnetic mineralogy is more oxidized and remanences with lower blocking temperatures were acquired chemically. Paleopoles calculated from published vectors and our own new data define an apparent polar wander path (APWP) for the Troodos microplate. The APWP shows that between 88 and ˜50 Ma the Troodos microplate was equatorial and the vertical axis for its 60° anticlockwise rotation was located within the microplate. Subsequently, the microplate drifted northward to 34°N with minor anticlockwise rotation at a reduced rate. That requires microplate-rotation about a vertical axis located to the west of Cyprus in the last ˜50 Ma. The allochthonous Triassic Mamonia terrane docked with the Cretaceous Troodos terrane in SW Cyprus. Within it, disrupted tectonized ophiolite has been regarded as part of a Triassic ocean floor or as sheared fragments of Cretaceous Troodos ophiolite, incorporated into the Mamonia terrane when it docked with the Troodos terrane. Whatever their provenance, their paleomagnetic signals postdate their penetrative deformation and metamorphism and their paleopoles may still be used to track their post-strain motion. Our calculations of paleopoles from published vectors for the Mamonia terrane smear along an extension of the APWP for the Troodos microplate that is, moreover, concentric with the Troodos microplate. This suggests that the paleopole dispersion of the Triassic Mamonia rocks and their post-magnetization disruption occurred during their accretion onto the anticlockwise-spinning Troodos microplate.

  18. Metamorphic sole formation, emplacement and blueschist overprint: early obduction dynamics witnessed by W. Turkey ophiolites

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    Western Turkey, with a >200 km long-belt of unmetamorphosed ophiolite overlying continental lithosphere is one or even the largest obducted ophiolite on Earth and therefore a key example to study obduction and early subduction dynamics. All Western Turkish ophiolite fragments are considered as part of the same Neotethyan branch resulting of a long-lived continental subduction (or underthrusting). Synchronous (ca. ~ 93 Ma) metamorphic sole formation and preservation at the base of most of the Turkish ophiolite fragments support this single event and place a strong constraint on the age of subduction initiation. Metamorphic soles are indeed generally considered to have formed during the early and hot subduction zone at 25 ± 10 km depths and welded to the overriding oceanic lithosphere. In Western Turkey however (as for most places worldwide) a systematic study of the pressure-temperature conditions with modern thermobarometric tools is generally lacking, and fundamental mechanisms of formation or accretion to the upper plate are poorly (if at all) constrained. We herein reappraise Western Turkish metamorphic soles focusing on the following points and issues: (i) detailed structures of metamorphic sole and other subduction derived units, petrological evolution and refined pressure-temperature conditions; peak pressure-temperature conditions of metamorphic sole were estimated using garnet, clinopyroxene, amphibole and plagioclase as the peak paragenesis at 10.5 ± 2 kbar and 800 ± 50°C based on pseudosections using the Theriak/Domino package (ii) the rather unique (and enigmatic) blueschist facies overprint found in places was investigated in terms of structural position and pressure-temperature conditions. Conditions of overprint were estimated around 12 kbar and 425 °C from the presence of glaucophane, lawsonite, jadeite and garnet overgrowing the amphibolite-facies assemblage. This field-based study provides clues to mechanisms of metamorphic sole underplating

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

    NASA Astrophysics Data System (ADS)

    Tartarotti, Paola

    2013-04-01

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

  20. Formation of the Yakuno ophiolite; accretionary subduction under medium-pressure-type metamorphic conditions

    NASA Astrophysics Data System (ADS)

    Osozawa, Soichi; Takeuchi, Hiroshi; Koitabashi, Toru

    2004-11-01

    The notion that the Yakuno ophiolite and overlying Maizuru Group represents an accretionary prism formed during the Permian evolution of Japan on the Yakuno eruptive sequence, association of hemipelagic mudstone with silicic tuff, exotic fossiliferous limestones derived from previously accreted sea-mounts, upward coarsening of sequences terrigenous sandstone and conglomerate, and mildly deformed Permian and Triassic forearc basin formations. The most important indicator, however, is the seaward imbrication and repetition observed in both the Maizuru Group and the ophiolite itself. D1 deformation structures include axial-planar foliations (pressure-solution cleavage for the Maizuru Group and granulite-amphibolite metamorphic layering in the ophiolite), flattening type strain, symmetric pressure shadows and fringes, and isoclinal folds showing axial-planar foliations and thrust faulting at their overturned limb. The exceptional asymmetry observed indicates seaward-directed shearing near the thrust, while D1 structures in the Maizuru zone are explained by off-scraping, above the basal decollement. The later Jurassic D2 kink fold structure includes a first-order asymmetric kink with a brittle thrust at its overturned limb, more-or-less coeval with M2 retrograde metamorphism. Medium-pressure M1 prograde metamorphism in the Yakuno ophiolite produced layering of granulite and amphibolite, and in the Maizuru Group, formation of illite along pressure-solution cleavage of mudstones. The metamorphic grade is controlled by the stratigraphic relationships and appears typical of that in ocean floor regions. However, there was only one episode of M1 prograde metamorphism which occurred contemporaneously with D1 off-scraping. Given that subduction zones are normally characterized by high P/ T metamorphic regimes, the observed P/ T history appears to reflect relatively unusual conditions. Such high thermal gradients may plausibly reflect the approach of a young, hot oceanic plate

  1. Salvaging primary remanence from hydrothermally altered oceanic gabbros in the Oman ophiolite: A selective destructive demagnetization approach

    NASA Astrophysics Data System (ADS)

    Usui, Yoichi; Yamazaki, Shusaku

    2010-07-01

    Widespread hydrothermal alteration and formation of secondary magnetite have been problems for paleomagnetic work on gabbros in the Oman ophiolite. Mechanical removal of hydrothermally altered ferromagnesian minerals from gabbro and gabbronorite in the Wadi Rajmi area revealed a cryptic remanence which could not be detected by stepwise demagnetization of bulk rock core samples. After the mechanical removal, samples consist of plagioclase and clinopyroxene. These samples exhibit remanence directions of southeast declination and shallow inclination. This direction is consistent with previously reported paleomagnetic directions at crystallization of the Oman ophiolite. In contrast, bulk rock core samples yielded north declination, resembling the younger remanence directions associated with the obduction of the ophiolite. Microscopic observation and paleomagnetic directional comparison concluded that the cryptic remanence is a primary magnetization carried by exsolved magnetite in plagioclase and clinopyroxene. Our results suggest that previous paleomagnetic data from whole rock gabbros in the Oman ophiolite as well as tectonically active ocean floor should be taken with care.

  2. 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.

  3. 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

  4. Petrological constraints upon the provenance and genesis of the East Halmahera ophiolite

    NASA Astrophysics Data System (ADS)

    Ballantyne, Paul

    This petrological and geochemical study of the ophiolitic rocks of the island of Halmahera (eastern Indonesia) has resulted in the first detailed interpretation of their tectonomagmatic provenance and suggested modern analogues around the western Pacific margin. Rocks of ophiolitic affinity are common in the eastern part of Halmahera, but structural dismemberment means that an intact ophiolite stratigraphy is not preserved. However, samples representative of each level of a "complete" ophiolite (with the possible exception of sheeted dykes) have been collected. A "mantle sequence" dominated by depleted harzburgite (spinel cr # = 62, olivine Fo 90.4, bulk (Al 2O 3 + CaO) = 1.2 wt%) suggests it is a mantle residue which has undergone a high degree of partial melt extraction. Subordinate lherzolite of relatively enriched chemistry (spinel cr # = 17, olivine Fo 90.4, bulk (Al 2O 3 + CaO) + 4.2 wt%) is interpreted as locally "fertile" upper mantle material. Cumulate rocks are well represented, particularly by olivine-free gabbronorite in which orthopyroxene and clinopyroxene occur in approximately equal modal proportions, and contain clinopyroxene with low TiO 2 (av. 0.29 wt%). Both pyroxenes appear before plagioclase in the crystallisation sequence, and therefore the Halmahera cumulate rocks are distinct from gabbroic rocks formed at mid-oceanic spreading ridges. The cumulus mineralogy is generally comparable with cumulates of the Papuan and Marum ophiolites of New Guinea and with cumulates dredged from the Mariana Trench; it is consistent with open-system crystalisation from a relatively high-Si, high-Mg, low-Ti magma derived from a high degree of partial melting of a lherzolitic mantle source. This correlates with the evidence from the harzburgites and suggests that the ophiolitic rocks were formed in a supra-subduction zone environment. The plutonic rocks are interpreted as resulting from approximately 20% melting of depleted oceanic upper mantle, triggered by

  5. The tectono-sedimentary evolution of the Sivas ophiolite: Implications for pre to post-obduction processes in Anatolia

    NASA Astrophysics Data System (ADS)

    Legeay, Etienne; Mohn, Geoffroy; Callot, Jean-Paul; Ringenbach, Jean-Claude; Müntener, Othmar; Kavak, Kaan

    2016-04-01

    The Eastern Mediterranean in general and Turkey in particular preserve the remnants of several Neo-Tethysian oceanic basins consumed by north-dipping subductions during the Late Cretaceous prior to collision in the Paleogene. The Sivas basin, belonging to the Late Mesozoic to Cenozoic Central Anatolian basins, is located in a key position at the junction between 1) To the North, the Izmir-Ankara-Erzincan suture zone (IAESZ), 2) To the West, the Kırsehir block, 3) To the South, the Inner-Tauride suture zone (ITSZ). The obduction of ophiolite thrust sheets occurred during Campanian along the IAESZ, and ITSZ. We focus our study on the southern boundary of the Sivas basin, where an ophiolite sequence is capped by Late Cretaceous to Paleocene post-obduction sediments. We present new field observations, new U-Pb zircon dating on magmatic rocks and geochemistry analyses to unravel the pre-obduction nature and origin of the ophiolitic basement and to describe the post-obduction tectono-sedimentary evolution. The pre-obduction evolution show that: (i) the Southern Sivas ophiolite is characterized by highly serpentinized peridotites, with minor magmatic intrusions, (ii) the top of the ophiolite is marked by detachment faulting with ophicalcites, (iii) the U-Pb zircon ages of the magmatic intrusions are constrained at ˜90 Ma, (iv) geochemical data suggest a 'subduction signature' for the magmatic rocks. The, post-obduction evolution is characterized by the emplacement of Maastrichtian and Paleocene sediments carbonate platforms located on ophiolitic highs, associated to volcanoclastics turbidites in the trench northward in the Sivas Basin. These results show that the southern Sivas ophiolite represents magma starved system sharing similarities with present-day (ultra-)slow-spreading systems. This ophiolite belongs to the ITSZ, in contrast to ophiolites located 40km northward from the IAESZ. To resolve the complex paleogeographic framework of East-Anatolia during the

  6. 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

  7. Insight in Ridge Axial Melt Lens in the Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Boudier, F.; Nicolas, A.; Daignieres, M.

    2008-12-01

    As in fast spreading ridges, the Oman ophiolite had a melt lens perched on top of the magma chamber where the gabbro unit was crystallizing. This melt lens is now reduced to an horizon where its roof and floor are coinciding and this horizon is now identified in the field. It is generally marked by a sharp discordance between the isotropic gabbros from the root zone of sheeted dike complex (RZSDC) and steeply dipping foliated gabbros. These gabbros are issued from the mush settled on the floor of the melt lens, after subsidence inside the magma chamber. After stretching, compaction and rotation in the chamber, the mush has drifted through the wall of the chamber with, as a result, the observed steep foliated gabbros. Depending on its vertical distance beneath the lens horizon, a given gabbro derives from increasing distances inside the melt lens. Insights in the active melt lens are possible in three ways. 1) Looking at gabbros from the lens horizon, which virtually have not subsided. 2) Considering uncommon areas which display flat-lying foliated gabbros, below the contact with RZSDC and which grade down section into the steep foliated gabbros. Such situations are ascribed to a retreat of the melt lens, exposing gabbros which crystallized on its floor. Their good foliation points to a dynamic deposition on the floor, presumably by convection currents. 3) Considering the ubiquitous occurrence of anorthosites which are interlayered with the foliated gabbros. The anorthosites carry several important messages such as: - compaction of the mush at early stage of subsidence; - chemical nature of the rising melt which drops plagioclase first, followed by either olivine or clinopyroxene; - frequency and volume of melt intrusions, each one coming as short and massive melt surge; - spacing of areas of melt delivery on the lens floor. These results are essentially derived from anorthosites description and distribution in the field. It is concluded that melt lens activity is

  8. 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

  9. 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

  10. 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

  11. Ophiolitic Chromitites from the Andriamena Greenstone Belt, Madagascar: Possible Evidence for mid-Archean Plate Tectonics

    NASA Astrophysics Data System (ADS)

    Reisberg, L. C.; Ohnenstetter, M.; Zimmermann, C.; Ratefiarimino, A.; Levy, D.

    2015-12-01

    Determining the time of the onset of plate tectonics is critical to understanding the geodynamic processes that controlled the evolution of the early Earth. The near absence of Archean ophiolites, as defined by the presence of a residual ultramafic section, has been considered to be one of the primary arguments against Archean plate tectonics. The Andriamena greenstone belt of Madagascar contains massive chromitite bodies consisting of about 90% chromite and about 10% gangue minerals, mostly secondary (talc, green amphibole, orthopyroxene, Ca and Mg carbonate). Numerous observations argue in favor of an ophiolitic origin for these chromitites, including the high Cr# (0.67-0.74), coupled with relatively high Mg# (0.6-0.78) of the constituent chromite. In addition, these phases display extremely low TiO2 contents (<0.25%), which are also characteristic of ophiolites and possibly suggestive of an arc environment. Though in many places the chromitite is in tectonic contact with a variety of unrelated igneous lithologies, remnants of apparently cogenetic ultramafic rock types, including dunites, harzburgites, and some pyroxenites are sometimes immediately juxtaposed with the chromitite. The very high Fo content of the olivine in the associated dunite, as high as 0.95, also attests to an ophiolitic provenance. Platinum group element (PGE) and 187Os/188Os analyses were performed on several chromitite samples. Chondrite normalized PGE spectra display marked depletions in PPGE relative to IPGE, with (Pt/Ir)N ranging from ~0 to 0.09, though Pd contents are somewhat less depleted than those of Pt. The observed PPGE depletion is another feature characteristic of ophiolitic chromitites. The IPGE enrichment is consistent with the presence of laurite microinclusions in the chromite revealed by SEM. Os isotopic compositions are tightly clustered, with 187Os/188Os ranging from 0.1057 to 0.1059, corresponding to TRD model ages of ~ 3.2 Ga, assuming primitive upper mantle parameters

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    steady-state basis the subduction interface is apparently efficiently decoupled. In this context, we hypothetize that the liberation of fluid through pulses (or a somewhat increased amount of fluids) is required to locally modify mechanical coupling and induce the slicing of large pieces of oceanic material along the subduction interface (type 1). By contrast, an extreme hydration of the subduction interface and mantle wedge will result in the formation of serpentinite melanges and extensive material mixing (e.g., cold plumes, mafic pods and localized melting; type 2). This latter situation may be promoted by young/fast/wet subduction, such as subduction initiation and/or subduction of young lithosphere or subduction of a particularly hydrated lithosphere section (e.g., at the ridge and/or prior to entering the trench), whereas cold, slow subduction (type 1) will result in irregular hydration and localized coupling able to detach large slices.

  13. 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.

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

    USGS Publications Warehouse

    Hagstrum, Jonathan T.

    1992-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Cousineau, Pierre A.; St-Julien, Pierre

    1992-08-01

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

  16. 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.

  17. Precambrian ophiolites of Arabia; a summary of geologic settings, U-Pb geochronology, lead isotope characteristics, and implications for microplate accretion, Kingdom of Saudi Arabia

    USGS Publications Warehouse

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

    1988-01-01

    Feldspar lead-isotope data are of three types: 1) lead from the ophiolitic rocks and arc tonalites of the northwestern Arabian Shield and ophiolitic rocks of the Nabitah suture zone is similar to lead in present midocean ridge basalt, 2) anomalous radiogenic data from the Thurwah ophiolite are from rocks that contain zircons from pre-late Proterozoic continental crust, and 3) feldspar from the Urd ophiolite shows retarded uranogenic lead growth and is related either to an anomalous oceanic mantle source, or in an unknown manner to ancient continental mantle or lower crust of the eastern Arabian Shield.

  18. 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.

  19. Petrology and Rock Magnetism of the peridotites of Pindos Ophiolite (Greece), insights into the serpentinization process

    NASA Astrophysics Data System (ADS)

    Bonnemains, D.; Carlut, J. H.; Mevel, C.; Andreani, M.; Escartin, J.; Debret, B.

    2015-12-01

    We present a petrological and magnetic study of a suite of serpentinized peridotites from the Pindos ophiolite spanning a wide range in the degree of serpentinization (from ~10 to 100%). The Pindos ophiolite, in Northern Greece, is a portion of Late Triassic oceanic lithosphere obducted during the convergence of the Apulian and Pelagonian micro-continents. This ophiolite is interpreted mainly as the result of a supra-subduction zone spreading process but its complete history remains largely unknown. Therefore, it is not clear when the ultramafic section was exposed to fluid circulation that resulted in its serpentinization. Element partitioning during serpentinization reactions is dependent on parameters such as temperature and water-rock ratio. In particular, they affect the behavior of the iron released by olivine, which can be taken up either by magnetite, serpentine and/or brucite. Analyses of the reaction products are therefore a key to constrain the conditions during the main stage of the alteration. Our study was designed to gain insight on the conditions prevailing during hydration. Our results indicate that even fully serpentinized samples have a very low magnetization and magnetite content. Moreover, microprobe and μXanes results show that serpentine is the main host of iron in the divalent but also trivalent form. These results are compared with a set of data from serpentinized ultramafics sampled from the ocean floors, as well as from various other ophiolites. We suggest that serpentinization at Pindos occurred at relatively low-temperature (less than 200 °C), therefore not at a ridge environment. In addition, we stress that the presence of trivalent iron in serpentine indicates that serpentinization may remain a producer of hydrogen even when very little magnetite is formed.

  20. 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.

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

    USGS Publications Warehouse

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

    1998-01-01

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

  2. 40Ar/ 39Ar dating of the emplacement of the Muslim Bagh ophiolite, Pakistan

    NASA Astrophysics Data System (ADS)

    Mahmood, Khalid; Boudier, Françoise; Gnos, Edwin; Monié, Patrick; Nicolas, Adolphe

    1995-11-01

    The obduction-related basal part of the Muslim Bagh ophiolite (Baluchistan, Pakistan) and the underlying metamorphic sequence were studied structurally which demonstrated a WSW-ENE-trending thrusting sequence for the initial obduction. 40Ar/ 39Ar measurements on amphiboles and plagioclase from the subophiolitic metamorphic rocks, and on plastically deformed and recrystallized dolerite samples from the base of the sheeted dyke complex give apparent ages between 70.7 ± 5.0 and 65.1 ± 4.1 Ma interpreted as cooling ages dating approximately the formation of the plastic deformation and obduction. The results indicate that the Muslim Bagh ophiolite represents a segment of ocean floor from the small and slow-spreading ocean branch of the Neo-Tethys located between the Indo-Pakistani and the Afro-Arabian plates. The WSW-ENE-oriented obduction of the Muslim Bagh ophiolite onto the Indo-Pakistani continental margin occurred with the convergence of the Neo-Tethys branch during the Late Cretaceous and before the Tertiary collision of the Indo-Pakistani plate with the Eurasian plate.

  3. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    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. Fluids involved in the hydration of the oceanic lithosphere and in the presence of a secondary boninitic and andesitic volcanism may provide a way to discriminate between these two interpretations: are they descending near-axis hydrothermal fluxes (first model) or ascending from a subducting slab (second model)? We herein focus on the base of the ophiolitic mantle in order to characterize the origin of fluids and decipher hydration processes. Samples were taken along hecto- to kilometre-long sections across the basal banded unit directly overlying the amphibolitic/granulitic metamorphic sole. We carried out a petrological, structural and geochemical study on these rocks and their constitutive minerals. Our results show that, unlike the generally refractory character of Oman harzburgites, all the basal mantle rocks display secondary crystallization of clinopyroxene and amphibole through metasomatic processes. The microstructures and the chronology of these secondary mineralizations (clinopyroxene, pargasitic amphibole, antigorite and then lizardite/chrysotile) suggest that these basal rocks have been affected by cooling from mantle temperatures (<1200°C) to low-T serpentinisation (<300°C). Furthermore, major elements required to crystallize these minerals and the observed fluid-mobile elements (FMEs) enrichments in the clinopyroxenes and in the amphiboles (B, Pb, Sr), as well as in the serpentines (B, Sr, Rb, Ba, As), are consistent with amphibolite-derived fluids (Ishikawa et al., 2005) and cannot be easily explained by other sources. Based on these observations, we propose a geodynamic model in which intense and continuous metasomatism of the

  5. Structure, metamorphism and timing of an exhumed Cretaceous subduction zone beneath the Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Searle, M. P.; Warren, C. J.; Waters, D. J.; Parrish, R. R.

    2003-12-01

    The Semail ophiolite in Oman was emplaced from NE to SW at least 200 km over the Arabian passive margin, probably over 450 km in total, during the late Cretaceous (95-70 Ma). The first phase of obduction involved NE-directed subduction of Triassic-Jurassic basalt at least 45-50 km beneath the ophiolite, whilst the crustal sequence was forming (U-Pb zircons from plagiogranites, ca. 95 Ma). Amphibolites accreted beneath the mantle sequence peridotites have P-T conditions of 840-870§C and 10-12 kbar with 40Ar/39Ar hornblende cooling ages of 95-92 Ma. During the later stages of obduction the leading edge of the continental margin was subducted to depths where carpholite-bearing rocks (6-8 kbar), blueschist (12-15 kbar) and eclogite (ca. 20 kbar) facies metamorphism formed in a ductile deforming NE-dipping subduction zone. Five concordant U-Pb ages from the As Sifah eclogites constrain the HP metamorphic peak at 79.1ñ0.3 Ma. Detailed structural mapping and restoration of the continental margin, combined with P-T and U-Pb geochronology confirms the model of one protracted phase of ophiolite obduction along a NE-dipping subduction zone, at convergence rates of ca. 17 mm/a-1. NE-directed extensional crenulation schistosity and NNE oriented stretching lineations in the eclogite and blueschist facies rocks are consistent with SW-directed exhumation of footwall HP rocks. NE facing folds and spectacular sheath folds with greatly attenuated limbs in the upper plate sediments are interpreted as antithetic backfolds, with shortening in the upper plate balanced by the subduction of the lower plate, consistent with a NE-directed subduction of the continental margin rocks beneath the SW-obducting ophiolite, Haybi and Hawasina thrust sheets. Recent suggestions of a nascent SW-directed subduction beneath the Oman margin are not consistent with the sedimentary evolution of the shelf and slope carbonates, the geological structure of Saih Hatat, or the U-Pb geochronology of the

  6. 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"…

  7. 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.

  8. A fore-arc setting of the Gerf ophiolite, Eastern Desert, Egypt: Evidence from mineral chemistry and geochemistry of ultramafites

    NASA Astrophysics Data System (ADS)

    Abdel-Karim, Abdel-Aal M.; Ali, Shehata; Helmy, Hassan M.; El-Shafei, Shymaa A.

    2016-10-01

    The Gerf ophiolite is the largest mantle-derived complex in the Arabian-Nubian Shield (ANS). This ophiolitic complex extends for tens of kilometers in the south Eastern Desert (SED) of Egypt as part of the Allaqi-Heiani and Oneib-Sol Hamed suture zones. The ultramafic section of the Gerf ophiolite comprises serpentinites, serpentinized peridotites and minor pyroxenites. All rocks contain relics of original magmatic phases. The elevated Cr# (> 0.84) of Cr-spinels indicates that these rocks represent highly-depleted mantle residues after high degrees of melt extraction. Mineral and bulk-rock chemistry show that the Gerf ophiolite suite represents fragments of oceanic lithosphere that developed in fore-arc setting in a supra-subduction zone (SSZ) environment. The pyroxenites have a LREE-enriched pattern relative to the serpentinites while the serpentinized peridotites display depleted patterns [average (La/Yb)n = 0.56)]. Modeling of LREE suggests that the LREE-enriched pyroxenites and serpentinites could have been produced via contamination of their mantle source by crustal material and/or subduction-related slab fluids during the mantle evolution in a SSZ setting or soon after ophiolite assemblage obduction onto the continental crust. In contrast, the LREE-depleted serpentinized peridotites could have been generated through MORB melt/mantle rock reaction.

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

    SciTech Connect

    Hagstrum, J.T. )

    1992-06-01

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

  10. 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.

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

    NASA Astrophysics Data System (ADS)

    Ni, Dong-Hong

    2016-10-01

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

  12. U-Pb ages of detrital zircon of the Paleozoic sedimentary rocks: New constraints on the emplacement time of the Hegenshan ophiolite, NE China

    NASA Astrophysics Data System (ADS)

    Pei, Sheng-Hui; Zhou, Jian-Bo; Li, Long

    2016-11-01

    The Hegenshan ophiolite in the Solonker-Hegenshan belt is the largest ophiolite in the Central Asian Orogenic Belt (CAOB). Despite its significance in constraining regional tectonic evolution, the emplacement time of the Hegenshan ophiolite is still under debate. In this study, we provide new detrital zircon ages of the Paleozoic sedimentary rocks that unconformably overlie the Wusinihei ophiolite (northeastern part of the Hegenshan ophiolite) to constrain the lower limit emplacement time of the Hegenshan ophiolite. The zircon ages obtained from the Paleozoic sedimentary rocks range from 298 ± 8 to 363 ± 7 Ma, and show bimodal distribution at 300-320 Ma (peak at 308 Ma) and 320-360 Ma (peak at 330 Ma). The age group of 300-320 Ma coincides with the age range of the volcanic rocks of the Late Paleozoic Gegenaobao Formation. The age group of 320-360 Ma with a peak at 330 Ma may be linked to local mafic-ultramafic rocks of the Hegenshan ophiolite. Accordingly, we suggest that the emplacement time of the Hegenshan ophiolite should have occurred earlier than the deposition of the Gegenaobao Formation, most likely during the time between 308 and 330 Ma, instead of the Silurian, Devonian or Mesozoic as previously considered.

  13. 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.

  14. A Paleomagnetic Study of Late Cretaceous Ophiolites in SE Turkey: implications for palaeolatitudes of S Neotethyan spreading centers and emplacement-related tectonic rotations

    NASA Astrophysics Data System (ADS)

    Mualla, Cinku; Timur, Ustaömer; Osman, Parlak; Mumtaz, Hisarli

    2016-04-01

    Two E-W trending ophiolite belts crop out in SE Turkey, The southerly located ophiolites (Hatay, Koçali) were emplaced onto the Arabian Platform in Late Cretaceous whereas the northerly located ophiolites (Göksun, İspendere, Kömürhan, Guleman) were underthrust the S Tauride margin (i.e. Malatya-Keban Platform) in Late Cretaceous. Different tectonic models exist in the literature for the origin of these different ophiolite belts that we test here by a detailed palaomagnetic study: a) all the ophiolites in Turkey, including those in the study area were rooted from a single ocean basin to the N (i.e. the N Neotethyan Ocean Basin); b) all the ophiolites in SE Turkey were derived from the S Neotethyan Ocean Basin; c) the two ophiolite belts in SE Turkey are believed to have rooted from two different ocean basins; the Berit ocean to the north and the S Neotethys to the S. Our palaeomagnetic study from 72 different sites was focused on to the sheeted dyke complex, cumulate gabbros and extrusive sequences where available of each ophiolite from the N and S belts. We also sampled the unconformable cover units to distinguish emplacement related tectonic rotations from post-emplacement tectonic rotations. Here we report our first results obtained from the Göksun Ophiolite of the northern belt and the Hatay Ophiolite of the southern belt. Rock magnetic experiments showed evidence od magnetite/titanomagnetite as the main magnetic carriers at the majority of sites. Progressive thermal and alternating demagnetization revealed that the characteristic remanent component is removed between 500 and 580 ?C or 30-100 mT, respectively. Our new paleomagnetic results from the ophiolitic rocks emplaced in Arabian platform and the SE Anatolia show important implications to the spreading centre of the former ocean (s). Large counterclockwise rotations up to 100° are obtained from the sheeded dykes of the Hatay ophiolite in the Arabian plate with a paleolatitude of ˜16° , in contrast

  15. Petrology and geochemistry of mantle peridotites from the Kalaymyo and Myitkyina ophiolites (Myanmar): Implications for tectonic settings

    NASA Astrophysics Data System (ADS)

    Liu, Chuan-Zhou; Zhang, Chang; Xu, Yang; Wang, Jian-Gang; Chen, Yi; Guo, Shun; Wu, Fu-Yuan; Sein, Kyaing

    2016-11-01

    Ophiolites within Myanmar have been commonly divided into two belts, i.e., the Eastern Belt and Western Belt. The Kalaymyo ophiolite from the Western Belt crops out at the eastern margin of the Indo-Burma Range and was formed during the Early Cretaceous, whereas the Myitkyina ophiolite from the Eastern Belt was formed during the Middle Jurassic. Both ophiolites are characterized by the occurrence of big massifs of mantle peridotites. Mantle peridotites of the Kalaymyo ophiolite are mainly composed of spinel lherzolites and harzburgites, with minor plagioclase peridotites. Mantle peridotites of the Myitkyina ophiolite mainly consist of spinel harzburgites, with minor dunites. Spinel lherzolites from the Kalaymyo ophiolite have relatively fertile compositions, with 40.12-45.19 wt.% MgO, 1.1-2.74 wt.% Al2O3 and 0.67-2.67 wt.% CaO. Their spinels have Cr# values of 0.12-0.4, yielding fractional melting degrees of 3-15%. In comparison, spinel harzburgites from the Kalaymyo ophiolite are more refractory, with 42.08-48.73 wt.% MgO, 0.09-0.99 wt.% Al2O3 and 0.07-0.8 wt.% CaO. Their spinels have Cr# values of 0.3-0.73, giving 12-21% degrees of fractional melting. Plagioclase peridotites from the Kalaymyo ophiolite have compositions intermediate between spinel lherzolites and harzburgites. Compared to the spinel peridotites, spinels in the plagioclase peridotites have relatively higher TiO2 contents. Harzburgites from the Myitkyina ophiolite, containing 40.88-48.16 wt.% MgO, 0.13-1.65 wt.% Al2O3 and 0.1-1.68 wt.% CaO, have refractory compositions similar to the Kalaymyo harzburgites. Spinels in the Myitkyina harzburgites with low TiO2 contents (i.e., < 0.2 wt.%) have variable Cr# values of 0.28-0.72, yielding 11-21% degrees of fractional melting. Clinopyroxenes in all Kalaymyo peridotites display flat patterns in MREE and HREE, but variably LREE-depleted patterns. They also show remarkably negative Sr and Zr anomalies. Plagioclases in the Kalaymyo plagioclase peridotites

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

    USGS Publications Warehouse

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

    1988-01-01

    Disrupted ophiolites occur in linear belts up to 900 km long between microplates that collided during the late Proterozoic to form the Arabian Shield. UPb zircon ages and Pb-isotope data from these ophiolitic rocks help constrain the history of accretion of the Arabian Shield and thereby contribute to the definition of its microplates and terranes. Terranes of the central and western Arabian Shield are generally thought to represent intraoceanic island arcs that range in age from about 900 to 640 Ma; however, a region of the eastern Arabian Shield contains rocks of Early Proterozoic age and may represent an exotic continental fragment entrained between the arc complexes. Ophiolites of the Yanbu suture (northwestern shield), dated by UPb (zircon) and SmNd (mineral isochron) methods, yield model ages of 740-780 Ma. These are among the oldest well-dated rocks in the northwestern Arabian Shield. Ages from the Jabal al Wask complex overlap with ages of adjacent arc rocks. This overlap in age supports geologic and geochemical evidence that the Wask complex represents a fragment of back-arc oceanic lithosphere formed during arc magmatism. Older ages of about 780 Ma for gabbro from the Jabal Ess ophiolite suggest that the ophiolite is either a fragment of fore-arc oceanic crust or oceanic basement on which an arc was built. Gabbro samples from ophiolites of the Bir Umq suture (west-central Arabian Shield) yield zircons with ages of 820-870 Ma and $ ??1250 Ma. The 820-870 Ma dates overlap with ages of the oldest nearby arc rocks; this favors an intra-arc or near-arc paleotectonic setting. The older zircons suggest that middle or early Proterozoic crustal material, possibly derived from the Mozambique belt of Africa, was present during back- or intra-arc magmatism. Plagiogranite from the Bir Tuluhah ophiolitic complex at the nothern end of the 900 km-long Nabitah mobile belt was dated by the zircon UPb method at ??? 830 Ma. This date is in the range of the oldest dated arc

  17. 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.

  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

  19. Zircon U-Pb geochronological constraints on rapid uplifting of the mantle peridotite of the Xigaze ophiolite, southern Tibet

    NASA Astrophysics Data System (ADS)

    Liu, Tong; Wu, Fuyuan; Zhang, Liangliang; Zhai, Qingguo; Liu, Chuanzhou; Ji, Wenbin; Zhang, Chang; Xu, Yang

    2016-04-01

    The Xigaze ophiolite is located in the central segment of the Yarlung Zangbo suture zone, southern Tibet. It is characterized by large amounts of ultramafic units with minor gabbros. The gabbroic rocks of the Xigaze ophiolite occur as layered bodies or fine-grained dikes intruding into mantle sections. Large scale gabbroic bodies are well-preserved in Dazhuqu, Baigang and Jiding. However, their formation time and mechanism are not well understood or interpreted. In this study, nine samples of mafic rocks from the Xigaze ophiolite, including seven gabbros and two rodingites, were selected for in situ zircon U-Pb and Hf isotopic analyses. The U-Pb data yielded identical ages of 124-129 Ma within uncertainties. Positive zircon ɛHf(t) values and young model ages indicated that these samples had an origin of depleted mantle source. Combined with previous studies on mafic dikes and radiolarian cherts, it can be concluded that the Yarlung Zangbo ophiolites were formed over a short period of time from 120 to 130 Ma. Hence, a rapid uplifting of the gabbroic rocks and mantle peridotites of the Xigaze ophiolite may have occurred to get intrusion of the diabase/dolerite dikes and sills. It excludes the existence of a long-term ancient magma chamber or lens. It is more likely that the gabbroic rocks are a series of plutonic intrusions beneath an ancient slow-spreading ridge, rather than products of magma chambers. Therefore, the "Chapman detachment model" may be applied to the generation of the Yarlung Zangbo ophiolites.

  20. Emplacement of Bela and Muslim Bagh Ophiolites and Significance of India-Asia Collision in Western Pakistan

    NASA Astrophysics Data System (ADS)

    Xiong, Yingqian

    The collision of India with Asia is an important geologic event preceded by the formation of now obducted ophiolites of Western Pakistan. Ophiolites along the suture zone between India and Asia can help elucidate the pre-terminal and terminal collisional history of the Himalaya orogen. Along the western boundary of the Indian plate, the Bela Ophiolite (BO) and associated allochthonous Sub-Ophiolitic Volcanic Complexes (BSOVC) represent the largest composite exposures of mafic and ultramafic rocks. The Muslim Bagh Ophiolite (MBO) is another well-known ophiolite. Collectively, they have not been extensively studied because of their remote location. A detailed geological map for the BO-BSOVC was created using remote sensing and field data. False-color images (Landsat ETM+ bands 7-4-2 in RGB), color band-ratio composite images of Landsat ETM+ data (5/7-5/1-5/4 in RGB), ASTER data (4/5-6/7-3/4 in RGB), and Mafic Index images along with reflectance spectroscopy data were used to discriminate different lithologies. Based on the geochemistry, age, and tectonic contact relationships of the samples analyzed, the BO most likely formed in an oceanic supra-subduction forearc environment ˜65 Ma on the upper plate of a westward-dipping subduction zone that consumed Indian plate oceanic crust. The alkali basalts trace elements signatures from the BSOVC show OIB signatures. They are likely to be from seamount complexes that were part of the subducting Indian plate. Analyzed dike samples from the MBO show typical subduction-related trace element signatures on a chondrite-normalized diagram and in a Th-Hf/3-Ta ternary diagram. The island arc affinity for the diabase dikes cutting the mantle section and sheeted dikes indicates that their origin could be analogous to some segments of the Chile ridge. An island arc affinity for dikes cutting the metamorphic sole indicates that emplacement of the MBO was followed by island arc-type magmatism.

  1. Subduction Initiation at Oceanic Detachment Faults and the Origin of Supra-subduction Ophiolites

    NASA Astrophysics Data System (ADS)

    Maffione, M.; Thieulot, C.; Van Hinsbergen, D. J. J.; Morris, A.; Spakman, W.; Plümper, O.

    2015-12-01

    Subduction initiation is a critical link in the plate tectonic cycle. Intra-oceanic 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- 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 supra-subduction 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.

  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

  3. 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.

  4. Multiple melting stages and refertilization as indicators for ridge to subduction formation: The New Caledonia ophiolite

    NASA Astrophysics Data System (ADS)

    Ulrich, Marc; Picard, Christian; Guillot, Stéphane; Chauvel, Catherine; Cluzel, Dominique; Meffre, Sébastien

    2010-03-01

    The origin of the New Caledonia ophiolite (South West Pacific), one of the largest in the world, is controversial. This nappe of ultramafic rocks (300 km long, 50 km wide and 2 km thick) is thrust upon a smaller nappe (Poya terrane) composed of basalts from mid-ocean ridges (MORB), back arc basins (BABB) and ocean islands (OIB). This nappe was tectonically accreted from the subducting plate prior and during the obduction of the ultramafic nappe. The bulk of the ophiolite is composed of highly depleted harzburgites (± dunites) with characteristic U-shaped bulk-rock rare-earth element (REE) patterns that are attributed to their formation in a forearc environment. In contrast, the origin of spoon-shaped REE patterns of lherzolites in the northernmost klippes was unclear. Our new major element and REE data on whole rocks, spinel and clinopyroxene establish the abyssal affinity of these lherzolites. Significant LREE enrichment in the lherzolites is best explained by partial melting in a spreading ridge, followed by near in-situ refertilization from deeper mantle melts. Using equilibrium melting equations, we show that melts extracted from these lherzolites are compositionally similar to the MORB of the Poya terrane. This is used to infer that the ultramafic nappe and the mafic Poya terrane represent oceanic lithosphere of a single marginal basin that formed during the late Cretaceous. In contrast, our spinel data highlights the strong forearc affinities of the most depleted harzburgites whose compositions are best modeled by hydrous melting of a source that had previously experienced depletion in a spreading ridge. The New Caledonian boninites probably formed during this second stage of partial melting. The two melting events in the New Caledonia ophiolite record the rapid transition from oceanic accretion to convergence in the South Loyalty Basin during the Late Paleocene, with initiation of a new subduction zone at or near the ridge axis.

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

    NASA Astrophysics Data System (ADS)

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

    1997-03-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

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

  7. 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.

  8. The Qiqinaer mafic-ultramafic complex: A newly identified ophiolitic suite in the southern Tianshan, China

    NASA Astrophysics Data System (ADS)

    Zhao, L.; Encarncion, J.; Zhang, Z.; Zhang, D.; Huang, H.; Dong, S.

    2010-12-01

    The Central Asian orogenic belt (CAOB) is a major accretionary belt, and is a natural laboratory to study Phanerozoic continental growth. The southern Tianshan orogen is the southern boundary of the CAOB. It resulted from the closure of the south Tianshan ocean following collision of the Tarim and Kazakhstan-Yili blocks. We have identified a new ophiolitic complex in the forearc of the southern Tianshan: The Qiqinaer mafic-ultramafic complex, which is inferred to be thrust on to the northern passive margin of the Tarim continental block. The complex consists of an upper sedimentary sequence of interbedded chert, mudstone, limestone, and mafic volcaniclastics, partly interbedded with pillowed and massive lavas. Diabase dikes, coarse to fine-grained gabbro and some possible silicic volcanics are usually in fault contact with the volcano-sedimentary units. These rocks are distributed over 6 km along an almost N-S-trending valley located about 130 km west of the nearest county named Aheqi (within latitudes 40°54'38″ and 40°56'39″ and longitudes 77°10'44″ to 77°11'02″). There is also an area of serpentinite, which is about 140 m wide and 750 m long in an adjacent valley. This suite of rocks is tentatively interpreted as a relatively complete ophiolite. We name this suite of rocks the “Qiqinaer ophiolite” after the valleys in which it is exposed. The ophiolite is relatively unmetamorphosed; most basalts are spilitic, but some contain fresh olivine. Samples from the Qiqinaer Valley are in the process of being analyzed for biostratigraphy, U-Pb zircon geochronology and geochemistry. Previous samples from chert associated with pillow lava a few 10s of km from Qiqinaer Valley, thought to be part of the same sequence, yielded Devonian to Carboniferous radiolaria. Rb-Sr and Sm-Nd errorchrons from the pillow lavas are broadly consistent with these paleontological ages. These pillow lavas exhibit OIB-like trace element geochemistry. Other ophiolitic complexes

  9. Geological and Geochemical Controls on Subsurface Microbial Life in the Samail Ophiolite, Oman

    PubMed Central

    Rempfert, Kaitlin R.; Miller, Hannah M.; Bompard, Nicolas; Nothaft, Daniel; Matter, Juerg M.; Kelemen, Peter; Fierer, Noah; Templeton, Alexis S.

    2017-01-01

    Microbial abundance and diversity in deep subsurface environments is dependent upon the availability of energy and carbon. However, supplies of oxidants and reductants capable of sustaining life within mafic and ultramafic continental aquifers undergoing low-temperature water-rock reaction are relatively unknown. We conducted an extensive analysis of the geochemistry and microbial communities recovered from fluids sampled from boreholes hosted in peridotite and gabbro in the Tayin block of the Samail Ophiolite in the Sultanate of Oman. The geochemical compositions of subsurface fluids in the ophiolite are highly variable, reflecting differences in host rock composition and the extent of fluid-rock interaction. Principal component analysis of fluid geochemistry and geologic context indicate the presence of at least four fluid types in the Samail Ophiolite (“gabbro,” “alkaline peridotite,” “hyperalkaline peridotite,” and “gabbro/peridotite contact”) that vary strongly in pH and the concentrations of H2, CH4, Ca2+, Mg2+, NO3-, SO42-, trace metals, and DIC. Geochemistry of fluids is strongly correlated with microbial community composition; similar microbial assemblages group according to fluid type. Hyperalkaline fluids exhibit low diversity and are dominated by taxa related to the Deinococcus-Thermus genus Meiothermus, candidate phyla OP1, and the family Thermodesulfovibrionaceae. Gabbro- and alkaline peridotite- aquifers harbor more diverse communities and contain abundant microbial taxa affiliated with Nitrospira, Nitrosospharaceae, OP3, Parvarcheota, and OP1 order Acetothermales. Wells that sit at the contact between gabbro and peridotite host microbial communities distinct from all other fluid types, with an enrichment in betaproteobacterial taxa. Together the taxonomic information and geochemical data suggest that several metabolisms may be operative in subsurface fluids, including methanogenesis, acetogenesis, and fermentation, as well as the

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

    NASA Astrophysics Data System (ADS)

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

    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.

  11. Geological and Geochemical Controls on Subsurface Microbial Life in the Samail Ophiolite, Oman.

    PubMed

    Rempfert, Kaitlin R; Miller, Hannah M; Bompard, Nicolas; Nothaft, Daniel; Matter, Juerg M; Kelemen, Peter; Fierer, Noah; Templeton, Alexis S

    2017-01-01

    Microbial abundance and diversity in deep subsurface environments is dependent upon the availability of energy and carbon. However, supplies of oxidants and reductants capable of sustaining life within mafic and ultramafic continental aquifers undergoing low-temperature water-rock reaction are relatively unknown. We conducted an extensive analysis of the geochemistry and microbial communities recovered from fluids sampled from boreholes hosted in peridotite and gabbro in the Tayin block of the Samail Ophiolite in the Sultanate of Oman. The geochemical compositions of subsurface fluids in the ophiolite are highly variable, reflecting differences in host rock composition and the extent of fluid-rock interaction. Principal component analysis of fluid geochemistry and geologic context indicate the presence of at least four fluid types in the Samail Ophiolite ("gabbro," "alkaline peridotite," "hyperalkaline peridotite," and "gabbro/peridotite contact") that vary strongly in pH and the concentrations of H2, CH4, Ca(2+), Mg(2+), [Formula: see text], [Formula: see text], trace metals, and DIC. Geochemistry of fluids is strongly correlated with microbial community composition; similar microbial assemblages group according to fluid type. Hyperalkaline fluids exhibit low diversity and are dominated by taxa related to the Deinococcus-Thermus genus Meiothermus, candidate phyla OP1, and the family Thermodesulfovibrionaceae. Gabbro- and alkaline peridotite- aquifers harbor more diverse communities and contain abundant microbial taxa affiliated with Nitrospira, Nitrosospharaceae, OP3, Parvarcheota, and OP1 order Acetothermales. Wells that sit at the contact between gabbro and peridotite host microbial communities distinct from all other fluid types, with an enrichment in betaproteobacterial taxa. Together the taxonomic information and geochemical data suggest that several metabolisms may be operative in subsurface fluids, including methanogenesis, acetogenesis, and fermentation

  12. Emplacement of the Irian Ophiolite and unroofing of the Ruffaer Metamorphic Belt of Irian Jaya, Indonesia

    NASA Astrophysics Data System (ADS)

    Weiland, Richard John, Jr.

    The distribution of Irian Ophiolite metabasites near the Gauttier Offset indicate exhumation from a northeast dipping subduction zone. Amphibolites, were metamorphosed at <5 kb and ˜700 °C, blueschists at ˜7 kb and ˜400 °C, and eclogites at ˜450 °C but not necessarily higher pressures. Basaltic compositions, REE concentrations, and radiogenic isotope ratios are characteristic of seawater altered ORB. Isotopic ages record metamorphism between 65 to 70 Ma and 50 to 45 Ma. Northern Ruffaer Metamorphic Belt metapelites have illite "crystallinity" values <0.25 °2theta, paragonite and quartz, rare biotite and spessartine indicating peak temperatures of 300 to 350 °C. Whole-rock K-Ar ages range from 17 to 490 Ma, clustering between 35 and 20 Ma. Ages between 28 and 20 Ma correspond to samples containing lattice-oriented phyllosilicates and destruction of chlorite - white mica aggregates, recording metamorphism of passive margin strata. Intrusive rocks near the Irian Ophiolite are generally dioritic, medium- to low-K, subalkaline rocks. Trace element concentrations and radiogenic isotope ratios are characteristic of volcanic arc rocks. Isotopic ages range from ˜35 to 24 Ma and ˜12 to 10 Ma. The older suite is part of an allochthonous Oligocene - Early Miocene oceanic arc. The younger suite is coeval with the autochthonous Middle Miocene Maramuni Arc. Subduction of Australian passive margin strata and continental lithosphere led to uplift and of the Irian Ophiolite. Exhumation of metamorphic rocks primarily occurred by normal faulting near the ophiolite - metamorphic belt contact. Amphibolites were exhumed from <15 km depth, slate slivers from 15 to 20 km and phyllites from 25 to 30 km. Blueschists and eclogite were exhumed from 25 to 35 km depth along the Gauttier Offset. The forearc was unroofed at ≤ 0.5 km/m.y. Unroofing in the eastern metamorphic belt increased from ˜0.6 km/m.y. between 23 to 12 Ma, to ˜1.2 km/m.y. between 12 to 2 Ma and ˜3.8 km

  13. 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.

  14. Sub-seafloor epidosite alteration: Timing, depth and stratigraphic distribution in the Semail ophiolite, Oman

    NASA Astrophysics Data System (ADS)

    Gilgen, Samuel A.; Diamond, Larryn W.; Mercolli, Ivan

    2016-09-01

    Pervasive epidotization of igneous rocks is a common feature in the ophiolite record of hydrothermally altered oceanic crust. Current genetic models view epidosites as markers of focussed upflow of hydrothermal fluid beneath oceanic spreading ridges. The epidosites are envisaged to form at the base of the sheeted dike complex (SDC) during active plate spreading. Our mapping of the Semail ophiolite in Oman has revealed abundant epidosites in the volcanic sequence, some exceeding 1 km2 in extent. They are more frequent and far larger than the mineralogically identical epidosites in the SDC. We have also found epidosites that traverse the entire SDC from bottom to top. Thus, rather than being restricted to the base of the SDC, as implied by current models, epidosites in fact occur throughout the SDC and dominantly within the overlying volcanic pile. We report the occurrence of 19 epidosite bodies and their crosscutting relations with respect to host lava units, dikes, intrusive stocks and also seafloor umbers. The volcanostratigraphic affiliation of the dikes is identified by their whole-rock and clinopyroxene compositions. The relations set constraints on the timing of epidotization with respect to igneous activity in the ophiolite. At least one of the epidosites in the SDC formed during Lasail off-axis volcanism. Another epidosite in the SDC and many in the volcanic units formed later during post-spreading, Alley and Boninitic Alley supra-subduction zone volcanism. Only permissive, not compelling, evidence allows just two of the epidosites to have formed within the main-stage SDC during or shortly after its emplacement. We conclude that epidotization of the oceanic crust is not necessarily coupled to spreading ridges and that it can occur during fore-arc volcanism. This finding is consistent with evidence from the modern seafloor and it requires a different hydrothermal environment to that traditionally associated with alteration beneath spreading axes. The timing

  15. 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 γ Os of -1 to +9, where γ 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; γ Os +4 to +7) are the most radiogenic. The least radiogenic are chromites from the Staten Island serpentinite and Mont Albert (γ 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

  16. Geological and geochemical controls on subsurface microbial life in the Samail Ophiolite, Oman

    DOE PAGES

    Rempfert, Kaitlin R.; Miller, Hannah M.; Bompard, Nicolas; ...

    2017-02-07

    Microbial abundance and diversity in deep subsurface environments is dependent upon the availability of energy and carbon. However, supplies of oxidants and reductants capable of sustaining life within mafic and ultramafic continental aquifers undergoing low-temperature water-rock reaction are relatively unknown. We conducted an extensive analysis of the geochemistry and microbial communities recovered from fluids sampled from boreholes hosted in peridotite and gabbro in the Tayin block of the Samail Ophiolite in the Sultanate of Oman. The geochemical compositions of subsurface fluids in the ophiolite are highly variable, reflecting differences in host rock composition and the extent of fluid-rock interaction. Principal component analysis of fluid geochemistry and geologic context indicate the presence of at least four fluid types in the Samail Ophiolite (“gabbro,” “alkaline peridotite,” “hyperalkaline peridotite,” and “gabbro/peridotite contact”) that vary strongly in pH and the concentrations of H2, CH4, Ca2+, Mg2+, NO3more » $-$, SO$$2-\\atop{4}$$, trace metals, and DIC. Geochemistry of fluids is strongly correlated with microbial community composition; similar microbial assemblages group according to fluid type. Hyperalkaline fluids exhibit low diversity and are dominated by taxa related to the Deinococcus-Thermus genus Meiothermus, candidate phyla OP1, and the family Thermodesulfovibrionaceae. Gabbro- and alkaline peridotite- aquifers harbor more diverse communities and contain abundant microbial taxa affiliated with Nitrospira, Nitrosospharaceae, OP3, Parvarcheota, and OP1 order Acetothermales. Wells that sit at the contact between gabbro and peridotite host microbial communities distinct from all other fluid types, with an enrichment in betaproteobacterial taxa. Together the taxonomic information and geochemical data suggest that several metabolisms may be operative in subsurface fluids, including methanogenesis, acetogenesis, and

  17. 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.

  18. 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

  19. 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

  20. 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.

  1. 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 δ13C 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.

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

    NASA Astrophysics Data System (ADS)

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

    1987-12-01

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

  3. Palaeomagnetic data from a Mesozoic Philippine Sea Plate ophiolite on Obi Island, Eastern Indonesia

    NASA Astrophysics Data System (ADS)

    Ali, J. R.; Hall, R.; Baker, S. J.

    2001-06-01

    Palaeomagnetic data are presented from part of the Halmahera ophiolite exposed on Obi Island, eastern Indonesia. Until the late Neogene, Obi formed part of the southern Philippine Sea Plate; it is now isolated from that plate and is located between fault strands in the left-lateral Sorong Fault Zone. Two areas were sampled: the first area comprised two sites from a microgabbro and a third site in a thin intruding dyke, and the second area yielded one site from a sheeted dyke suite. The mean in situ direction for the two areas is D=216.1°, I=23.3° , where the angular separation is 34.7°. Rotating the mean directions back to the palaeo-vertical clusters the vectors, so that D=219.4°, I=12.1°, where the angular separation is 20.1°. This clustering, together with other lines of palaeomagnetic evidence, suggests that the magnetisation is primary. The ophiolite is Mesozoic, and most likely formed in the Jurassic. This information, together with recently published palaeomagnetic data from nearby Upper Cretaceous Philippine Sea Plate formations, suggest that the oldest parts of the Philippine Sea Plate were situated close to the equator in the western Pacific in the middle Mesozoic.

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

    NASA Astrophysics Data System (ADS)

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

    2013-07-01

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

  5. 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.

  6. 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.

  7. New paleomagnetic data from the Wadi Abyad crustal section and their implications for the rotation history of the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Meyer, Matthew; Morris, Antony; Anderson, Mark; MacLeod, Chris

    2015-04-01

    The Oman ophiolite is an important natural laboratory for understanding the construction of oceanic crust at fast spreading axes and its subsequent tectonic evolution. Previous paleomagnetic research in lavas of the northern ophiolitic blocks (Perrin et al., 2000) has demonstrated substantial clockwise intraoceanic tectonic rotations. Paleomagnetic data from lower crustal sequences in the southern blocks, however, have been more equivocal due to complications arising from remagnetization, and have been used to infer that clockwise rotations seen in the north are internal to the ophiolite rather than regionally significant (Weiler, 2000). Here we demonstrate the importance and advantages of sampling crustal transects in the ophiolite in order to understand the nature and variability in magnetization directions. By systematically sampling the lower crustal sequence exposed in Wadi Abyad (Rustaq block) we resolve for the first time in a single section a pattern of remagnetized lowermost gabbros and retention of earlier magnetizations by uppermost gabbros and the overlying dyke-rooting zone. Results are supported by a positive fold test that shows that remagnetization of lower gabbros occurred prior to the Campanian structural disruption of the Moho. NW-directed remagnetized remanences in the lower units are consistent with those used by Weiler (2000) to infer lack of significant rotation of the southern blocks and to argue, therefore, that rotation of the northern blocks was internal to the ophiolite. In contrast, E/ENE-directed remanences in the uppermost levels of Wadi Abyad imply large, clockwise rotation of the Rustaq block, of a sense and magnitude consistent with intraoceanic rotations inferred from extrusive sections in the northern blocks. We conclude that without the control provided by systematic crustal sampling, the potential for different remanence directions being acquired at different times may lead to erroneous tectonic interpretation.

  8. New Paleomagnetic Data from the Wadi Abyad Crustal Section and their Implications for the Rotation History of the Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Meyer, M.; Morris, A.; Anderson, M.; MacLeod, C. J.

    2014-12-01

    The Oman ophiolite is an important natural laboratory for understanding the construction of oceanic crust at fast spreading axes and its subsequent tectonic evolution. Previous paleomagnetic research in lavas of the northern ophiolitic blocks (Perrin et al., 2000, Mar. Geophys. Res.) has demonstrated substantial clockwise intraoceanic tectonic rotations. Paleomagnetic data from lower crustal sequences in the southern blocks, however, have been more equivocal due to complications arising from remagnetization, and have been used to infer that clockwise rotations seen in the north are internal to the ophiolite rather than regionally significant (Weiler, 2000, Mar. Geophys. Res.). Here we demonstrate the importance and advantages of sampling crustal transects in the ophiolite in order to understand the nature and variability in magnetization directions. By systematically sampling the lower crustal sequence exposed in Wadi Abyad (Rustaq block) we resolve for the first time in a single section a pattern of remagnetized lowermost gabbros and retention of earlier magnetizations by uppermost gabbros and the overlying dyke-rooting zone. Results are supported by a positive fold test that shows that remagnetization of lower gabbros occurred prior to the Campanian structural disruption of the Moho. NW-directed remagnetized remanences in the lower units are consistent with those used by Weiler (2000, Mar. Geophys. Res.) to infer lack of significant rotation of the southern blocks and to argue, therefore, that rotation of the northern blocks was internal to the ophiolite. In contrast, E/ENE-directed remanences in the uppermost levels of Wadi Abyad imply a large, clockwise rotation of the Rustaq block, of a sense and magnitude consistent with intraoceanic rotations inferred from extrusive sections in the northern blocks. We conclude that without the control provided by systematic crustal sampling, the potential for different remanence directions being acquired at different times

  9. 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.

  10. 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.

  11. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2004-11-01

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

  13. Recycling of ancient subduction-modified mantle domains in the Purang ophiolite (southwestern Tibet)

    NASA Astrophysics Data System (ADS)

    Gong, Xiao-Han; Shi, Ren-Deng; Griffin, W. L.; Huang, Qi-Shuai; Xiong, Qing; Chen, Sheng-Sheng; Zhang, Ming; O'Reilly, Suzanne Y.

    2016-10-01

    Ophiolites in the Indus-Yarlung Zangbo (IYZ) suture (southern Tibet) have been interpreted as remnants of the Neo-Tethyan lithosphere. However, the discovery of diamonds and super-reducing, ultra-high pressure (SuR-UHP) mineral assemblages (e.g., coesite after stishovite, olivine after wadsleyite, native metals, alloys, and moissanite) in some of these massifs and associated chromitites requires a re-evaluation of their origin and evolution. A new petrological and geochemical study of the Purang ophiolite in the western IYZ suture sheds new lights on these issues. The depleted harzburgites of the Purang massif have low modal contents of clinopyroxene (< 2%), and high Cr# [100*Cr3 +/(Cr3 + + Al3 +)] in spinel (> 40 70) and pyroxenes (> 16 in orthopyroxene, and > 20 in clinopyroxene), suggesting high degrees of melt extraction (> 20%). These features are not consistent with formation in a (ultra-) slow-spreading mid-ocean ridge. These peridotites have high modal contents of orthopyroxene; this, and the extremely high Cr# of spinels in these peridotites, suggests modification in a subduction zone. The clinopyroxene-rich harzburgites and lherzolites contain rare spinel-pyroxene symplectites after garnet. Their clinopyroxenes have low MREE-to-HREE ratios ((Sm/Yb)N < 0.1) at relatively high HREE concentrations, and are Na-rich but Nd-poor. The relatively enrichment of Na but depletion of Nd in clinopyroxene cannot be explained by refertilization with MORB melts but are consistent with an origin from Na-rich subcontinental lithospheric mantle (SCLM). All lines of evidence suggest that these peridotites underwent initial melting in the stability field of garnet-facies peridotites, followed by additional melting in the spinel-facies mantle. Whole-rock Os isotopic compositions of the Purang peridotites give ancient TRD model ages (up to 1.3 Ga), indicating that the formation of these ancient depletion residues predated the opening of Neo-Tethyan Ocean. These observations

  14. 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 Ni

  15. From isotropic to layered gabbro: evolution record in the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Jousselin, D.; Morales, L. G.; Stephant, A.; Nicolle, M.

    2010-12-01

    The origin of gabbro layering in ophiolites is widely debated because it is linked to the processes of melt circulation beneath spreading ridges. The Moho Transition Zone (MTZ) of the Oman ophiolite contains layered gabbro lenses that are tens of meters wide. At meter scale, these rocks are not distinguishable from crustal layered gabbros. We describe the first known occurrence of an outcrop of isotropic gabbro in the Oman ophiolite MTZ; the outcrop extends over three hundreds meters and grades into poorly to nicely layered gabbros towards the periphery of the outcrop. When they are present, layering, and magmatic lineation are parallel to the host peridotite plastic foliation and lineation respectively, with microstructures indicative of simple shear deformation. Dunite heterogeneities within the isotropic gabbro, and diffuse limits, suggest that the isotropic gabbro results from melt impregnation of the host dunite, and that olivine within the gabbro is of mantelic origin. Crystallographic preferred orientations (CPO) measurements of olivine and plagioclase show a progressive evolution from a random fabric in the isotropic gabbro, to a well defined fabric in the roughly to nicely layered gabbros. Olivine show [001] (010) B-TYPE fabrics that we interpret as a magmatic flow fabric. In the most deformed samples, the plagioclase texture remains magmatic but olivine CPO is indicative of plastic deformation with a classic A-TYPE pattern. We argue that as a critical threshold of olivine connectivity is reached in the well defined olivine-rich layers, olivine deformation rapidly switches from magmatic to plastic. This last stage of deformation probably contributes to the layering sharpening, as deformation in olivine and in plagioclase rich layers must tend to be decoupled at this stage. We conclude that our observations illustrate rarely preserved transitional stages for the layered gabbros formation, showing that layering can result from the progressive tectonic

  16. 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 δ13C values of ~- 5 indicative of a mantle source for the carbon required for the carbonation. Oxygen isotope values δSMOW18O 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

  17. 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

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

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

    initiation. Alternatively, subduction could have begun spontaneously, facilitated by the density contrast between the arc-bearing Mesozoic Asian crust and the old oceanic Pacific crust to its west. This volcanic stratigraphy and their time-progressive development in the IBM system are analogous to those documented from suprasubduction (SSZ) ophiolites. Most SSZ ophiolites are on-land fragments of forearc oceanic crust, produced at subduction initiation and during the early stages of island arc development (Dilek and Furnes, 2009, 2011). Similarities between the oceanic lithosphere of both forearc settings and SSZ ophiolites also extend to the upper mantle units, which are composed of extremely depleted peridotites.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    Potential-field modeling, surface geologic mapping, and relocated seismicity are used to investigate the three-dimensional structure of the San Andreas-Calaveras fault junction to gain insight into regional tectonics, fault kinematics, and seismic hazard. South of the San Francisco Bay area, the San Andreas and Hayward-Calaveras fault zones join to become a single San Andreas Fault. The fault junction, as defined in this study, represents a three-dimensional volume of crust extending from San Juan Bautista in the north to Bitterwater Valley in the south, bounded by the San Andreas Fault on the southwest and the Calaveras fault zone on the northeast. South of Hollister, the Calaveras fault zone includes the Paicines, San Benito, and Pine Rock faults. Within the junction, the San Andreas and Calaveras faults are both creeping at the surface, and strike parallel to each other for about 50 km, separated by only 2 to 6 km, but never actually merge at the surface. Geophysical evidence suggests that the San Andreas and Calaveras faults dip away from each other within the northern portion of the fault junction, bounding a triangular wedge of crust. This wedge changes shape to the south as the dips of both the San Andreas and Calaveras faults vary along strike. The main trace of the San Andreas Fault is clearly visible in cross-sections of relocated seismicity as a vertical to steeply southwest-dipping structure between 5 and 10 km depth throughout the junction. The Calaveras fault dips steeply to the northeast in the northern part of the junction. Near the intersection with the Vallecitos syncline, the dip of the Calaveras fault, as identified in relocated seismicity, shallows to 60 degrees. Northeast of the Calaveras fault, we identify a laterally extensive magnetic body 1 to 8 km below the surface that we interpret as a folded 1 to 3 km-thick tabular body of Coast Range Ophiolite at the base of the Vallecitos syncline. Potential-field modeling and relocated seismicity

  20. 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.

  1. 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.

  2. 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

  3. AMS-detection of inverse fabrics without AARM, in ophiolite dikes

    NASA Astrophysics Data System (ADS)

    Borradaile, Graham J.; Gauthier, David

    “Inverse” fabric contributions from single-domain (SD) magnetite complicate interpretation of magmatic flow from anisotropy of magnetic susceptibility (AMS) in the S. Troodos ophiolite dike swarm. Flow directions and magma chamber dimensions may only be inferred correctly if magnetic fabrics correspond to the orientation distribution (OD) of minerals. Magnetite shape-fabrics were isolated by anisotropy of anhysteretic remanence (AARM) but AMS ambiguously merges magnetite and silicate fabrics. However, with the retrospective benefit of AARM, in this area, we recommend avoiding SD complications by excluding magnetite-rich samples for which the bulk low-field susceptibility >27,500 µSI. Magma-flow was steeply upwards to the SW proximal to an axial magma chamber.

  4. Ophiolites and Gas Seeps as Terrestrial Analogs for Methane Origin and Degassing on Mars

    NASA Astrophysics Data System (ADS)

    Schoell, M.; Etiope, G.

    2010-12-01

    If confirmed, the recently-discovered methane (CH4) plume on Mars, in the Northern Summer of 2003, would reflect an emission of ~ 19 x103 tonnes y-1 and possibly even ~ 57 x104 tonnes y-1. Serpentinization in ophiolitic rocks is one of the main processes that are inferred for the origin of methane on Mars. Ophiolites or, hydrated mineral-bearing rocks in general on Earth could serve as analogs. So far, however, most of these “analog” studies focused on mineralogical and microbiological processes associated to ophiolitic environments. Analog studies specifically dealing with methane emissions to the Earth’s surface are missing. One of the observations of Mars methane is the transient release of large amounts of methane in a relatively short period, probably a few months. This would imply the existence of a mechanism of gas accumulations in the subsurface and episodic release to the surface. Such release mechanisms may be similar to certain weak and intermittent gas seeps or small mud volcanoes on Earth, rather than to steady, continuous degassing of methane from mineral reactions. Currently, it is not clear whether low-temperature serpentinization can be an abiogenic methane “kitchen” where methane might be generated fast enough to sustain vigorous and long-lasting seeps. In cases of fluxes of the order of several tonnes per year, a pressurized accumulation must exist. In case of lower fluxes, probably gas accumulations are not necessary and low temperature serpentinization can be fast enough to charge episodic seeps. These concepts are fundamental to our understanding of potential sources for the martian methane, and they need to be studied with the support of analog seepage data on Earth. Two examples are presented: (1) a case of terrestrial abiogenic CH4 seepage from ophiolitic rocks at the “eternal fires of Chimaera” in Turkey. Estimated flux data from the abiogenic gas seep of Chimaera in Turkey (>20 tonnes of CH4 per year) suggest a great

  5. Petrological and tectono-magmatic significance of ophiolitic basalts from the Elba Island within the Alpine Corsica-Northern Apennine system

    NASA Astrophysics Data System (ADS)

    Saccani, Emilio; Principi, G.

    2016-12-01

    Two distinct ophiolitic units, which represent remnants of the Jurassic Ligurian-Piedmont Ocean, crop out in the Elba Island. They are the Monte Strega unit in central-eastern Elba and the Punta Polveraia-Fetovaia unit in western Elba. Ophiolitic rocks from the Monte Strega unit are commonly affected by ocean floor metamorphism, whereas those from the Punta Polveraia-Fetovaia unit are affected to various extent by thermal metamorphism associated with the Late Miocene Monte Capanne monzogranitic intrusion. Both ophiolitic units include pillow lavas and dykes with compositions ranging from basalt to basaltic andesite, Fe-basalt, and Fe-basaltic andesite. Basaltic rocks from these distinct ophiolitic units show no chemical differences, apart those due to fractional crystallization processes. They display a clear tholeiitic nature with low Nb/Y ratios and relatively high TiO2, P2O5, Zr, and Y contents. They generally display flat N-MORB normalized high field strength element patterns, which are similar to those of N-MORB. Chondrite-normalized rare earth element patterns show light REE / middle REE (LREE/MREE) depletion and marked heavy (H-) REE fractionation with respect to MREE. This HREE/MREE depletion indicates a garnet signature of their mantle sources. Accordingly, they can be classified as garnet-influenced MORB (G-MORB), based on Th, Nb, Ce, Dy, and Yb systematics. We suggest that the Elba Island ophiolitic basalts were generated at a magma starved, slow-spreading mid-ocean ridge. REE, Th, and Nb partial melting modelling shows that the compositions of the relatively primitive Elba Island ophiolitic basalts are compatible with partial melting of a depleted MORB mantle (DMM) source bearing garnet-pyroxenite relics. Hygromagmatophile element ratios suggest that basalts from both ophiolitic units were originated from chemically very similar mantle sources. A comparison with basalts and metabasalts from Alpine Corsica and northern Apennine ophiolitic units shows

  6. Fractionation of Mantle-Derived Melts in the Annieopsquotch Ophiolite, Newfoundland.

    NASA Astrophysics Data System (ADS)

    Lissenberg, C.; Bédard, J. H.; van Staal, C. R.

    2004-12-01

    The Annieopsquotch ophiolite exposes a tectonically bounded section through c. 5.5 km tholeiitic gabbros, sheeted dykes and pillow basalts. The gabbro zone is divided into three parts. The lower 500 m comprises massive cumulate gabbros with enclaves (<50 m) of partly-reacted and digested layered troctolite/leucotroctolite. These are interpreted as relics of the substrate into which the gabbro-sheeted dyke-basalt sequence was emplaced. Overlying this is 1500 m of cumulate olivine gabbros and gabbros which form sills c. 30 m thick that are oriented parallel to the ophiolite pseudostratigraphy. Finer grain sizes at contacts and inward-growing crescumulates indicate cooling from both top and bottom. Gabbros in the sill complex are characterized by cumulate textures with minor intercumulus amphibole and oxides, and rarely show shape-preferred orientations. The upper 500 m of the gabbro zone is dominated by massive gabbros with more abundant interstitial Fe-Ti-oxides, and diabasic pods that grade up into sheeted dykes, suggesting it represents a level of frozen melt. Incompatible element contents of cumulate gabbros in the sill complex generally increase upwards, and modeling indicates that the cumulate sills crystallized from melts with compositions similar to those of the overlying sheeted dykes and basalts. Trapped melt fractions are estimated to be c. 20%, consistent with the absence of compaction structures in these gabbros. Models indicate that the parental magmas of the gabbros, as well as lavas and dykes, can be produced by an average of c. 40-45% fractionation of mantle-derived melts. Both field- and geochemical data thus suggests the Annieopsquotch lower crust records repeated in-situ intrusion and fractionation during upward migration of mantle-derived melts towards the surface, with localized ponding in an axial melt lens at the base of the dyke complex. The similarity in composition and degree of fractionation between the lower and upper crust suggest that

  7. Metabolic Potential and Activity in Fluids of the Coast Range Ophiolite Microbial Observatory, California, USA

    NASA Technical Reports Server (NTRS)

    Hoehler, T.; Som, S.; Schrenk, M.; McCollom, T.; Cardace, D.

    2016-01-01

    Metabolic potential and activity associated with hydrogen and carbon monoxide were characterized in fluids sampled from the the Coast Range Ophiolite Microbial Observatory (CROMO). CROMO consists of two clusters of science-dedicated wells drilled to varying depths up to 35m in the actively serpentinizing, Jurassic-age Coast Range Ophiolite of Northern California, along with a suite of pre-existing monitoring wells at the same site. Consistent with the fluid chemistry observed in other serpentinizing systems, CROMO fluids are highly alkaline, with pH up to 12.5, high in methane, with concentrations up 1600 micromolar, and low in dissolved inorganic carbon (DIC), with concentrations of 10's to 100's of micromolar. CROMO is conspicuous for fluid H2 concentrations that are consistently sub-micromolar, orders of magnitude lower than is typical of other systems. However, higher H2 concentrations (10's -100's of micromolar) at an earlier stage of fluid chemical evolution are predicted by, or consistent with: thermodynamic models for fluid chemistry based on parent rock composition equivalent to local peridotite and with water:rock ratio constrained by observed pH; the presence of magnetite at several wt% in CROMO drill cores; and concentrations of formate and carbon monoxide that would require elevated H2 if formed in equilibrium with H2 and DIC. Calculated Gibbs energy changes for reaction of H2 and CO in each of several metabolisms, across the range of fluid composition encompassed by the CROMO wells, range from bioenergetically feasible (capable of driving ATP synthesis) to thermodynamically unfavorable. Active consumption relative to killed controls was observed for both CO and H2 during incubation of fluids from the pre-existing monitoring wells; in incubations of freshly cored solids, consumption was only observed in one sample set (corresponding to the lowest pH) out of three. The specific metabolisms by which H2 and CO are consumed remain to be determined.

  8. 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.

  9. Microbial life associated with low-temperature alteration of ultramafic rocks in the Leka ophiolite complex.

    PubMed

    Daae, F L; Økland, I; Dahle, H; Jørgensen, S L; Thorseth, I H; Pedersen, R B

    2013-07-01

    Water-rock interactions in ultramafic lithosphere generate reduced chemical species such as hydrogen that can fuel subsurface microbial communities. Sampling of this environment is expensive and technically demanding. However, highly accessible, uplifted oceanic lithospheres emplaced onto continental margins (ophiolites) are potential model systems for studies of the subsurface biosphere in ultramafic rocks. Here, we describe a microbiological investigation of partially serpentinized dunite from the Leka ophiolite (Norway). We analysed samples of mineral coatings on subsurface fracture surfaces from different depths (10-160 cm) and groundwater from a 50-m-deep borehole that penetrates several major fracture zones in the rock. The samples are suggested to represent subsurface habitats ranging from highly anaerobic to aerobic conditions. Water from a surface pond was analysed for comparison. To explore the microbial diversity and to make assessments about potential metabolisms, the samples were analysed by microscopy, construction of small subunit ribosomal RNA gene clone libraries, culturing and quantitative-PCR. Different microbial communities were observed in the groundwater, the fracture-coating material and the surface water, indicating that distinct microbial ecosystems exist in the rock. Close relatives of hydrogen-oxidizing Hydrogenophaga dominated (30% of the bacterial clones) in the oxic groundwater, indicating that microbial communities in ultramafic rocks at Leka could partially be driven by H2 produced by low-temperature water-rock reactions. Heterotrophic organisms, including close relatives of hydrocarbon degraders possibly feeding on products from Fischer-Tropsch-type reactions, dominated in the fracture-coating material. Putative hydrogen-, ammonia-, manganese- and iron-oxidizers were also detected in fracture coatings and the groundwater. The microbial communities reflect the existence of different subsurface redox conditions generated by

  10. 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.

  11. Carboniferous and Permian evolutionary records for the Paleo-Tethys Ocean constrained by newly discovered Xiangtaohu ophiolites from central Qiangtang, central Tibet

    NASA Astrophysics Data System (ADS)

    Zhang, Xiu-Zheng; Dong, Yong-Sheng; Wang, Qiang; Dan, Wei; Zhang, Chunfu; Deng, Ming-Rong; Xu, Wang; Xia, Xiao-Ping; Zeng, Ji-Peng; Liang, He

    2016-07-01

    Reconstructing the evolutionary history of the Paleo-Tethys Ocean remains at the center of debates over the linkage between Gondwana dispersion and Asian accretion. Identifying the remnants of oceanic lithosphere (ophiolites) has very important implications for identifying suture zones, unveiling the evolutionary history of fossil oceans, and reconstructing the amalgamation history between different blocks. Here we report newly documented ophiolite suites from the Longmu Co-Shuanghu Suture zone (LSSZ) in the Xiangtaohu area, central Qiangtang block, Tibet. Detailed geological investigations and zircon U-Pb dating reveal that the Xiangtaohu ophiolites are composed of a suite of Permian (281-275 Ma) ophiolites with a nearly complete Penrose sequence and a suite of Early Carboniferous (circa 350 Ma) ophiolite remnants containing only part of the lower oceanic crust. Geochemical and Sr-Nd-O isotopic data show that the Permian and Carboniferous ophiolites in this study were derived from an N-mid-ocean ridge basalts-like mantle source with varied suprasubduction-zone (SSZ) signatures and were characterized by crystallization sequences from wet magmas, suggesting typical SSZ-affinity ophiolites. Permian and Carboniferous SSZ ophiolites in the central Qiangtang provide robust evidence for the existence and evolution of an ancient ocean basin. Combining with previous studies on high-pressure metamorphic rocks and pelagic radiolarian cherts, and with tectonostratigraphic and paleontological data, we support the LSSZ as representing the main suture of the Paleo-Tethys Ocean which probably existed and evolved from Devonian to Triassic. The opening and demise of the Paleo-Tethys Ocean dominated the formation of the major framework for the East and/or Southeast Asia.

  12. 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

  13. 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

  14. 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

  15. 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

  16. Counterclockwise P- T evolution of the Aghil Range: Metamorphic record of an accretionary melange between Kunlun and Karakorum (SW Sinkiang, China)

    NASA Astrophysics Data System (ADS)

    Groppo, Chiara; Rolfo, Franco

    2008-10-01

    This paper describes the metamorphic evolution and the tectonic significance of the Aghil Range, a poorly known terrane located between Kunlun and Karakorum north of K2 in the framework of western Tibet. The Aghil Range consists of different units separated by syn- to late-metamorphic thrusts and post-metamorphic faults of similar attitude; among other units, the Surukwat Complex is a composite sequence of thrust sheets trending WNW-ESE and steeply dipping SSW showing a general increase of metamorphic grade from lower to higher structural levels. P- T pseudosections and conventional thermobarometry of metapelites at the top of the Surukwat Complex tightly constrain the P- T path of this highest-grade metamorphic portion of the Aghil Range. The prograde path is characterized by an early increase in both P and T and by a later, nearly isothermal, increase in P, from 500 < T < 530 °C and 0.25 < P < 0.40 GPa to 580 < T < 600 °C and 0.80 < P < 0.90 GPa. The peak metamorphic event is constrained at 550 < T < 590 °C and 0.77 < P < 0.91 GPa. The retrograde path is characterized by decompression associated to a slight cooling to T ~ 500 °C and P < 0.5 GPa. Altogether, the petrology of the studied rocks suggests a P- T path with a narrow counterclockwise shape. The studied sequence could represent the result of the early subduction of an accretionary complex, interpreted further eastward according to a melange underthrusting model. As concerning the tectonic setting of this terrane, a number of geologic and petrologic similarities link the Aghil Range and the central Qiangtang metamorphic belt, suggesting that the Aghil Range is the possible NW extension of the Qiangtang microplate separating Kunlun from Karakorum.

  17. Ca-Mg Carbonate Cements in Ophiolite-Hosted Creek Waters of the Del Puerto Ophiolite, CA, and their Potential Significance as a Planetary Biosignature

    NASA Astrophysics Data System (ADS)

    Blank, J. G.

    2015-12-01

    Serpentinization, the reaction at moderate pressure and temperature of water with olivine and pyroxene that are common in basalts and ultramafic rocks, results in the formation of alkaline fluids and the precipitation of a variety of secondary minerals. Terrestrial localities where active serpentinization is occurring are ideal Mars analogs for examining the characteristics of an environment that possesses two of the key features that we assume necessary to host life: water and an internally generated energy source. This study focuses on a related but different feature present where active serpentinization is occurring - namely, carbonate cements forming under plain air in the vicinity of Adobe Springs, CA. This site is located in the Del Puerto ophiolite about 150 km ESE of San Francisco, in the Coast Range of California. Two alkaline spring water compositions have been described at the site, a Ca-OH water (which is not currently being emitted by the active springs), and a Mg-CO3 water. Abundant dolomitic and calcitic carbonate cements are found in the creek drainages near the springs, associated with a diverse microbial community. We conducted a systematic study of the carbonate cements using SEM, EMP, XRD, TEM, and SIMS, focusing on sub-mm variations in texture, mineral chemistry and stable isotope (COH) composition. We compared our measurements with thermodynamic modeling results constrained by chemical analysis of water chemistry from the site and known partition coefficients and stable isotope fractionation factors. The wide range of carbonate compositions and textures observed at the Adobe Springs site suggests that more than one process is involved in their precipitation, including the possibility of microbially mediated dolomite mineralization. These carbonate cements could be a mineralogic biomarker of serpentinization and microbiological processes on Mars and other rocky planets and, therefore, prime targets for future astrobiological investigations.

  18. 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.

  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. Rapid forearc spreading between 130 and 120 Ma: Evidence from geochronology and geochemistry of the Xigaze ophiolite, southern Tibet

    NASA Astrophysics Data System (ADS)

    Dai, Jingen; Wang, Chengshan; Polat, Ali; Santosh, M.; Li, Yalin; Ge, Yukui

    2013-07-01

    The Cretaceous Xigaze ophiolite is best exposed at the central Yarlung Zangbo Suture Zone (YZSZ, Tibet) which also includes the Gangdese arc and the Xigaze forearc basin. This study reports new geochronological and geochemical data for this ophiolite to revisit its geodynamic and petrogenetic evolution. The Xigaze peridotites have low CaO and Al2O3 contents and U-shaped Rare Earth Element (REE) patterns, suggesting that they are residues after moderate to high degrees of partial melting and were modified by infiltration of Light Rare Earth Element (LREE)-enriched boninitic melts. The Xigaze crustal rocks belong to two groups: Mid-Ocean Ridge Basalt (MORB)-like rocks and boninitic rocks showing a uniform LREE depletion and flat to LREE enrichment on chondrite-normalized patterns, respectively. Geochemically, both groups show the influence of subducting oceanic slab-derived fluids. LA-ICPMS zircon U-Pb and Lu-Hf analyses from dolerite and quartz diorite dikes, which intruded into the mantle peridotite, and dolerite sheeted sills show that they were generated between 127 and 124 Ma. The zircons possess positive εHf(t) values ranging from + 7.5 to + 17.3. Taking into account the geological and geochronological characteristics of the central-western YZSZ, we propose that ophiolites in this region formed in a forearc spreading setting through rapid slab rollback during subduction initiation between 130 and 120 Ma. Following this stage of spreading, the forearc was stabilized and the zone of melting migrated beneath the Gangdese arc producing the voluminous Late Cretaceous granitoids displaying depleted mantle-type Hf isotopic compositions. Our model provides a new explanation for the generation and evolution of forearc-type ophiolites.

  1. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2014-06-01

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

  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. 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

    2001-01-04

    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.

  5. The southern margin of the Caribbean Plate in Venezuela: tectono-magmatic setting of the ophiolitic units and kinematic evolution

    NASA Astrophysics Data System (ADS)

    Giunta, Giuseppe; Beccaluva, Luigi; Coltorti, Massimo; Siena, Franca; Vaccaro, Carmela

    2002-07-01

    The southern Caribbean Plate margin in Venezuela consists of a W-E elongated deformed belt, composed of several tectonic units dismembered along the northern part of the South America continental Plate since the Late Cretaceous. The present review, based on petrology and tectono-magmatic significance of each unit, makes it possible to define the main geotectonic elements and to reconstruct the paleogeographic domains from Late Jurassic to Tertiary: (a) Mid-Ocean Ridge Basalt (MORB) proto-Caribbean oceanic basin (Loma de Hierro Unit); (b) oceanic plateau (Dutch and Venezuelan Islands basement); (c) rifted continental margin (Cordillera de La Costa and Caucagua-El Tinaco Units) with Within Plate Tholeiitic (WPTh) magmatism; (d) an intra-oceanic subduction zone represented by Island Arc Tholeiitic (IAT) magmatism (Villa de Cura and Dos Hermanas Units) of Early Cretaceous age; (e) an Early Cretaceous ocean-continent subduction trench filled by melange (Franja Costera); (f) a new intra-oceanic subduction zone, represented by the tonalitic arc magmatism of Late Cretaceous age (Dutch and Venezuelan Islands). Regional tectonic constraints and coherent kinematic reconstruction suggest an original "near-Mid America" location of the Jurassic-Cretaceous "proto-Caribbean" oceanic realm. From Early to Late Cretaceous one sub-continental subduction with melanges (Franja Costera Unit) and two main stages of intra-oceanic arc magmatism are recorded in the so-called "eo-Caribbean" phases. The first consists of generally metamorphosed and deformed volcano-plutonic sequences with IAT affinity (Villa de Cura and Dos Hermanas Units), probably in relation to a southeastward-dipping subduction. The second is mainly represented by generally unmetamorphosed tonalitic intrusives cutting the oceanic plateau in the Dutch and Venezuelan Islands, and related to the new intra-oceanic subduction with reverse lithospheric sinking. The latter probably marked the onset of the Aves/Lesser Antilles arc

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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.

  12. Monitoring Natural Occurring Asbestos in ophiolite sequences and derived soils: implication with human activities

    NASA Astrophysics Data System (ADS)

    Punturo, Rosalda; Bloise, Andrea; Cirrincione, Rosolino

    2016-04-01

    The present contribution focuses on soils that developed on serpentinite-metabasite bedrocks, which could potentially be rich in asbestos minerals and, as a consequence, have a negative impact on agricultural activity and on environmental quality. In order to investigate the natural occurrences of asbestos (NOA) on the surface of the soil formed from serpentinites and metabasite, we selected a study area located in Sila Piccola (Calabrian Peloritani Orogen, southern Italy), where previous studies highlighted the presence of asbestiform minerals within the large ophiolitic sequences that crop out (Punturo et al., 2015; Bloise et al., 2015). Agricultural soil samples have been collected mainly close to urban centres and characterized by using different analytical techniques such as X-ray powder diffraction (XRPD), transmission electron microscopy combined with energy dispersive spectrometry (TEM-EDS), thermogravimetry (TG) and differential scanning calorimetry (DSC) Results pointed out as all the collected soil samples contain serpentine minerals (e.g., chrysotile), asbestos amphiboles, clays, chlorite, muscovite, plagioclase and iron oxides in various amounts. Electron microscope images of the soils show that their contain a variety of aggregating agents such as organic matter and clay in which individual fibres of chrysotile and tremolite-actinolite are trapped. The investigation showed that both serpentinite and metabasite rocks act as a perennial source of contamination for the agriculture lands because of the high amount of tremolite-actinolite found in the studied soil samples developed on such lithotypes. Even if asbestiform minerals usually occur in aggregates which cannot be suspended in the air, agricultural activities such as plowing can destroy these soil aggregates with the creation of dust containing inhalable asbestos fibres that evolve into airborne increasing the exposure of population to them. Since the dispersion of fibres could be associated with

  13. 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

  14. 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.; 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

  15. Metaultramafic schists and dismembered ophiolites of the Ashe Metamorphic Suite of northwestern North Carolina, USA

    USGS Publications Warehouse

    Raymond, Loren A.; Merschat, Arthur J.; Vance, R. Kelly

    2016-01-01

    both chemical composition and metamorphic history. Spot sampling of heterolithic MUR bodies does not reveal MUR body character or history or allow ‘type’ designations. We recommend that the subdivision of MUR bodies into ‘types’ be abandoned and that the metasomatic hypothesis be carefully applied. AMS MURs and associated metamafic rocks likely represent fragments of dismembered ophiolites from various ophiolite types.

  16. Isotopic equilibrium between mantle peridotite and melt: Evidence from the Corsica ophiolite

    NASA Astrophysics Data System (ADS)

    Rampone, Elisabetta; Hofmann, Albrecht W.; Raczek, Ingrid

    2009-11-01

    A widely used assumption of mantle geochemistry and the theory of partial melting at oceanic settings is the existence of isotopic equilibrium between mantle source and melt. Yet, recent diffusion studies and isotopic investigations of ophiolites, abyssal peridotites and associated MORBs have cast doubts on this assumption, by providing evidence for isotopic disequilibrium between residual peridotites and MORBs. Here we present Sr and Sm-Nd isotope data on mantle peridotites and gabbroic intrusions from the Mt. Maggiore (Alpine Corsica, France) Tethyan ophiolite, which document Nd isotopic homogeneity, implying isotopic equilibrium, on a 1-kilometer scale. The peridotites record multi-stage melt-rock interaction and melt intrusion occurring at different lithospheric depths. Samples studied are residual cpx-poor spinel lherzolites, reactive spinel harzburgites, impregnated plagioclase peridotites and related gabbronoritic veinlets, later gabbroic dykes. Strontium isotopes in peridotites and gabbros are highly variable, due to interaction with sea-water derived fluids, and cannot be used to test melt-residue isotopic equilibrium. In contrast, Nd isotopes are unaffected by sea-water alteration. Peridotites display present-day high 147Sm/ 144Nd (0.49-0.59) and 143Nd/ 144Nd (0.513367-0.513551) ratios, with no appreciable differences between residual and reactive spinel peridotites, and between spinel and plagioclase peridotites. Gabbroic dykes have present-day Nd isotopic compositions typical of MORB ( 143Nd/ 144Nd = 0.513122-0.513138). Internal (plag-whole rock-cpx) Sm-Nd isochrons for olivine gabbro dykes and a gabbronoritic veinlet yield Jurassic ages (162 ± 10 and 159 ± 15 Ma in ol-gabbros, 155 ± 6 Ma in gabbronorite), and initial ɛNd = 8.9-9.7 indicative of a MORB-type source. Sm-Nd isotopic compositions of peridotites conform to the linear array defined by the gabbroic rocks, and yield initial (160 Ma) ɛNd values of 7.6-8.9, again consistent with a MORB

  17. 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

  18. Impacts and Ophiolites: A Way to Recognize Large Terrestrial Impact Basins?

    NASA Astrophysics Data System (ADS)

    Olds, E. P.

    2015-12-01

    That Chicxulub Crater is located on ~35 km thick continental crust is apparently inconsistent with oceanic crustal/upper mantle geochemical signatures detected globally in the KT boundary impact layer [1-5 and unpublished Cr isotope data from the Yin lab at UC Davis] since introduction of the Alvarez hypothesis [6]. Apparent excavation and ejection of mafic/ultramafic target rock by the KT boundary impact might imply an additional KT impact site involving oceanic lithosphere. We speculate: 1) The Greater Antilles island chain ophiolite belt marks the rim of a ~700 km diameter impact basin, deformed and dismembered from an originally circular form by at least 50 million years of left lateral shear on the North American-Caribbean transform plate boundary; 2) Other ophiolite segments may similarly mark rims of large impact basins deformed to greater or lesser extent by, and serving as strain markers for, relative plate motions over geologic time; 3) The Greater Antilles/Chicxulub and Sulu Sea Basin/Spratly Island cases may constitute doublet craters of similar size ratio and separation distance; 4) Plate boundaries may be formed or modified by such impacts. Problems include: 1) The KT fireball layer should be tens of cm thick rather than a few mm thick [8-9]; 2) Impact basins of this size/scale are not expected in the Phanerozoic/Proterozoic [10]; References: [1] DePaolo D. J. et al. 1983. EPSL 64:356-373. [2] Hildebrand A. R. and Boynton W. V. 1988, LPI Contributions 673:78-79. [3] Hildebrand A. R. and Boynton W. V.. 1990. Science 248:843-847. [4] Montanari A. et al. 1983. Geology 11:668. [5] Bohor B. F. et al. 1989. Meteoritics 24:253. [6] Alvarez L. W. et al. 1980 Science 208:1095-1108. [7][8] Grieve R.A.F. and Cintala M.J. 1992 Meteoritics 27: 526-538. [9] Pierazzo E. et al. 1997 Icarus 127/2:408-423. [10] Ivanov B.A. et al. 2002 Asteroids III 89-101

  19. 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

  20. Partial melting and refertilization of mantle peridotites in the Xigaze ophiolite: constraints from whole-rock and mineral geochemistry

    NASA Astrophysics Data System (ADS)

    Zhang, Chang; Liu, Chuan-Zhou; Wu, Fu-Yuan

    2016-04-01

    Ophiolites along the E-W trending Yarlung-Tsangpo Suture (YTS), which separates the Indian plate from the Eurasian plate, have been regarded as relics of the Neo-Tethyan Ocean. The Xigaze ophiolite in the central YTS has been extensively studied. One of the most intact crust-mantle sequences is preserved in the Luqu (or Beimarang) ophiolite. Mantle peridotites of the Luqu ophiolite are dominated by harzburgites, with 55-65% olivine, 30-40% orthopyroxene, 1-5% clinopyroxene and 1-3% spinel. Minor lherzolites and dunites are also outcropped, and the mode contents of clinopyroxene in lherzolite can be locally up to 10%. This contribution presented whole-rock major element and mineral chemistry including EMPA (Electronic MicroProbe Analysis) and clinopyroxene in situ trace elements. Whole rock Al2O3 (0.23-2.05%) and CaO (0.41-1.7%) contents are very low but show obviously inverse correlation with MgO (39.7-47.0%), indicating that the Luqu peridotites are residues of variable degrees of partial melting. This is supported by the Cr# (=molar Cr/(Cr+Al)) values of spinels which vary from 0.36 to 0.69. Meanwhile, the high Cr# values of spinels and homogenously high Mg# (= molar Mg/(Mg+Fe2+)) values of olivines, clustering at 0.91, indicate high degrees of partial melting. The low REE (rare earth elements) concentrations and chondrite-normalized distribution partterns of clinopyroxenes reflect ultra-depleted natures, with most showing LREE (light REEs) and MREE (medium REEs) depleted patterns and strong fractionations between MREEs and HREE (heavy REEs) ((Sm/Yb)N: 0.021-0.184). Based on the observations and analyses, a model of two-stage melting process was proposed that the primitive mantle underwent 2-8% melting in garnet stability field which was followed by 10-15% melting in spinel stability field. The clinopyroxenes in some peridotites exhibit obvious enrichment of somestrongly incompatible elements (such as sodium and LREE) that reveal later refertilization process for

  1. 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

  2. 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

  3. 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

  4. 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.

  5. Microstructures and crystallographic preferred orientation of anorthosites from Oman ophiolite and the dynamics of melt lenses

    NASA Astrophysics Data System (ADS)

    Morales, Luiz F. G.; Boudier, FrançOise; Nicolas, Adolphe

    2011-04-01

    Microstructures and crystallographic preferred orientation (CPO) of anorthosite samples interlayered in the upper and lower gabbro sections in the Oman ophiolite were analyzed in this paper. In the anorthosites registering the dynamics of the melt lenses, foliation is flat lying and starts to develop a few meters below the root zone of the sheeted dike complex (RZSDC). Microstructures and CPO of these rocks were developed in response to four different mechanisms: (1) density-controlled settling of plagioclase on the lens floor, (2) deposition of anorthosites related to convection currents, (3) melt compaction, and (4) uncompacted melt accumulation. In these anorthosites, the poles to (010) of plagioclase are parallel to the flow plane of convection, whereas the [100] axes and poles to (001) express the convection flow direction and the axis of convection rolls, respectively. The effect of subsidence of melt lens floor is recorded immediately below the RZSDC and is characterized by the rapid (but progressive) development of dipping foliation and lineation, reflecting the increase of deformation downsection. The degree of foliation and CPO development in the anorthosites is directly related to the distance of the center of the melt lenses before the subsidence starts. Despite the uncertain origin of the anorthosites from the lower gabbro section, all the samples lost the magmatic microstructural characteristics and presently are reequilibrated aggregates. However, they still preserve plagioclase CPO, where some of these patterns present similarities with the anorthosites from the upper gabbro section, but no evidence of intracrystalline deformation under high temperatures.

  6. 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.

  7. 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

  8. 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.

  9. Geology and origin of the late Proterozoic Darb Zubaydah ophiolite, Kingdom of Saudi Arabia

    USGS Publications Warehouse

    Quick, J.E.

    1990-01-01

    The Darb Zubaydah ophiolite, north-central Arabian Shield, preserves a largely intact section consisting of ultramafic rocks, gabbro, diabase, granodiorite, and interbedded volcanic and sedimentary rocks. Formation of these rocks within or near an island arc is indicated by the absence of pelagic sediments and the abundance of pillow basalt, turbiditic sediments, lahar deposits, and basaltic to rhyolitic tuff. The oldest extrusive rocks formed in a young, relatively unevolved island arc or in a back-arc basin sufficiently close to an arc to receive calc-alkaline lava flows and coarse-grained, arc-derived detritus. Overlying turbidites and lahar deposits of the Kaffan sandstone point to the initiation of a rifting event. High-Ti basalts, which erupted above the Kaffan sandstone, and related diabase are interpreted to be magmatic products of incipient intra-arc rifting. Renewed arc volcanism produced calc-alkaline volcanic rocks that interfingered with the high-Ti basalt and later dominated the section as the volcanic apron of the arc prograded basinward. Extrusion of voluminous calc-alkaline tuff may have been contemporaneous with intrusion of granodiorite and gravity-driven landsliding. -from Author

  10. 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.

  11. 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.

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

    NASA Astrophysics Data System (ADS)

    Tsikouras, Basilios; Karipi, Sofia; Hatzipanagiotou, Konstantin

    2013-08-01

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

  13. Localized slip controlled by dehydration embrittlement of partly serpentinized dunites, Leka Ophiolite Complex, Norway

    NASA Astrophysics Data System (ADS)

    Dunkel, Kristina G.; Austrheim, Håkon; Renard, François; Cordonnier, Benoit; Jamtveit, Bjørn

    2017-04-01

    Dehydration of partly or completely serpentinized ultramafic rocks can increase the pore fluid pressure and induce brittle failure, a process referred to as dehydration embrittlement. However the extents of strain localization and unstable frictional sliding during deserpentinization are still under debate. In the layered ultramafic sections of the Leka Ophiolite Complex in the Central Norwegian Caledonides, prograde metamorphism of serpentinite veins led to local fluid production and to the growth of Mg-rich and coarse-grained olivine with abundant magnetite inclusions and δ18O values 1.0- 1.5 ‰ below the host rock. Embrittlement associated with the dehydration caused faulting along highly localized (<10 μm-wide) slip planes near the centers of the original serpentinite veins and pulverization of wall rock olivine. These features along with an earthquake-like size distribution of fault offsets suggest unstable frictional sliding rather than slower creep. Structural heterogeneities in the form of serpentinite veins clearly have first-order controls on strain localization and frictional sliding during dehydration. As most of the oceanic lithosphere is incompletely serpentinized, heterogeneities represented by a non-uniform distribution of serpentinite are common and may increase the likelihood that dehydration embrittlement triggers earthquakes.

  14. 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.

  15. 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.

  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. Deep carbon cycle recorded by calcium-silicate rocks (rodingites) in a subduction-related ophiolite

    NASA Astrophysics Data System (ADS)

    Dai, J. G.; Wang, C. S.; Liu, S. A.; Qian, X. Y.; Zhu, D. C.; Ke, S.

    2016-11-01

    Carbon cycling in subduction zones remains poorly constrained due to the lack of relevant geological records. Here we report magnesium isotope data (δ26MgDSM3) from calcium-silicate rocks (rodingites) from the Xigaze ophiolite, southern Tibet, which is thought to represent remnants of Neo-Tethyan oceanic lithosphere. Behaviors of immobile trace elements in rodingites resemble those of their mafic dike protoliths, showing subduction-related signatures. The majority of rodingites exhibits low δ26Mg values of -0.72‰ to -0.33‰ with a weighted average of -0.47 ± 0.11‰ (2 SD), significantly lighter than that of their protoliths (-0.31 ± 0.03‰). This difference likely reflects the interaction of the protolith with isotopically light carbonate fluids. Modeling indicates that this hypothesis requires the input of 5 to 15 wt % carbonates during rodingitization. Our study suggests that rodingite may represent a previously unrecognized reservoir of dissolved Ca from subducted carbonates.

  18. 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.

  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. 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.

  1. Paleomagnetic and Rock Magnetic Results From the Chenaillet Ophiolite (French/Italian Alps) and its Interpretation as a Former Oceanic Core Complex

    NASA Astrophysics Data System (ADS)

    Carlut, J. H.; Bonnemains, D.; Mevel, C.; Debret, B.; Cannat, M.; Escartin, J.; Manatschal, G.

    2014-12-01

    The Chenaillet ophiolite lies at the Franco-Italian border in the Alps and consists of a rather well-exposed, undeformed and complete ocean-floor sequence. This sequence was formed within the Alpine Tethys oceanic domain at about 160 Ma and was subsequently obducted during Europe-Adria convergence. Well preserved pillow lavas, gabbros and serpentinized mantle rocks can be found following a stratigraphic order. It has been recently suggested, based on stratigraphic observations, that Chenaillet could be a remnant Oceanic Core Complex (Manatschal et al., 2011). In order to have a better understanding of the nature of this ophiolite, a paleomagnetic and rock magnetic study was undertaken. Results are compared with a set of data from a recent (less than 1 Ma) serpentinized ultramafic complex at the Atlantic ridge (ODP leg 153) and from various ophiolites. Our results show that the magnetic behavior of the serpentinized section from Chenaillet is rather different than in most other ophiolites but shows strong similarities with that observed for the Atlantic set. This suggests that serpentinization at Chenaillet occurs in an environment which share similarities with slow spreading ridges. In addition preliminary paleomagnetic results show that the mantle section from the Chenaillet ophiolite may have been rotated during unroofing as observed in recent Oceanic Core Complexes.

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

    NASA Astrophysics Data System (ADS)

    Kryza, Ryszard; Beqiraj, Arjan

    2014-04-01

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

  3. 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.

  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. Geochemistry and origin of plagiogranites from the Eldivan Ophiolite, Çankırı (Central Anatolia, Turkey)

    NASA Astrophysics Data System (ADS)

    Üner, Tijen; Çakir, Üner; Özdemir, Yavuz; Arat, Irem

    2014-06-01

    The Eldivan Ophiolite, exposed around Ankara and Çankırı cities, is located at the central part of the Izmir-Ankara-Erzincan Suture Zone (IAESZ). It represents fragments of the Neotethyan Oceanic Lithosphere emplaced towards the south over the Gondwanian continent during the Albian time. It forms nearly complete series by including tectonites (harzburgites and rare dunites), cumulates (dunites, wherlites, pyroxenites, gabbro and plagiogranites) and sheeted dykes from bottom to top. Imbricated slices of volcanic-sedimentary series and discontinuous tectonic slices of ophiolitic metamorphic rocks are located at the base of tectonites. Plagiogranitic rocks of the Eldivan Ophiolite are mainly exposed at upper levels of cumulates. They are in the form of conformable layers within layered diorites and also dikes with variable thicknesses. Plagiogranites have granular texture and are mainly composed of quartz and plagioclases. The occurrences of chlorite and epidote revealed that these rocks underwent a low grade metamorphism. Eldivan plagiogranites have high SiO2 content (70-75 %) and low K2O content (0.5-1 %) and display flat patterns of REE with variable negative Eu anomalies. LREE/HREE ratio of these rocks varies between 0.2-0.99. All members of the Eldivan rocks have high LILE/HFSE ratios with depletion of Nb, Ti and P similar to subduction related tectonic settings. Geochemical modelling indicates that the Eldivan plagiogranites could have been generated by 50-90 % fractional crystallization and/or 5-25 % partial melting of a hydrous basaltic magma

  6. 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.

  7. U Pb zircon age constraints on the Dongwanzi ultramafic mafic body, North China, confirm it is not an Archean ophiolite

    NASA Astrophysics Data System (ADS)

    Zhao, Guochun; Wilde, Simon A.; Li, Sanzhong; Sun, Min; Grant, Matthew L.; Li, Xuping

    2007-03-01

    In Science May 11 2001 (p.1142-1145), Kusky et al. reported an ophiolite complex of late Archean age in the Dongwanzi area of the North China Craton, which has subsequently been considered a hallmark for Archean plate tectonics. However, this interpretation has been questioned because no convincing pillow lavas or sheeted dikes have been found in the Dongwanzi complex and, more importantly, as a major part of the ophiolite complex, the Dongwanzi ultramafic-mafic body has not previously been dated and it therefore remains unknown whether it is of Archean age. In this study, we carried out detailed field investigations and SHRIMP U-Pb zircon dating on the rocks of the Dongwanzi ultramafic-mafic body, as recognized by Kusky et al. Our field observations reveal that in several places along the boundaries of the Dongwanzi body, mafic dikes emanating from the main body intrude the country rocks of the unmetamorphosed Mesoproterozoic sedimentary successions of the Changcheng-Jixian 'System'. One of these dikes yielded a SHRIMP U-Pb zircon age of 306 ± 6 Ma, similar to the SHRIMP U-Pb zircon age of 308 ± 4 Ma obtained from a plagioclase-bearing pyroxenite from the Dongwanzi ultramafic-mafic body. Four other samples collected from various parts of the ultramafic-mafic body (gabbro, leucogabbro or pyroxenite) record similar SHRIMP U-Pb zircon ages around ˜ 300 Ma. Therefore, both field relationships and SHRIMP U-Pb zircon dating results unequivocally demonstrate that the Dongwanzi ultramafic-mafic body is not part of an Archean ophiolite.

  8. Sulphide mineralization and wall-rock alteration in ophiolites and modern oceanic spreading centres

    USGS Publications Warehouse

    Koski, R.A.

    1983-01-01

    Massive and stockwork Fe-Cu-Zn (Cyprus type) sulphide deposits in the upper parts of ophiolite complexes represent hydrothermal mineralization at ancient accretionary plate boundaries. These deposits are probable metallogenic analogues of the polymetallic sulphide deposits recently discovered along modern oceanic spreading centres. Genetic models for these deposits suggest that mineralization results from large-scale circulation of sea-water through basaltic basement along the tectonically active axis of spreading, a zone of high heat flow. The high geothermal gradient above 1 to 2 km deep magma chambers emplaced below the ridge axis drives the convective circulation cell. Cold oxidizing sea-water penetrating the crust on the ridge flanks becomes heated and evolves into a highly reduced somewhat acidic hydrothermal solvent during interaction with basaltic wall-rock. Depending on the temperature and water/rock ratio, this fluid is capable of leaching and transporting iron, manganese, and base metals; dissolved sea-water sulphate is reduced to sulphide. At the ridge axis, the buoyant hydrothermal fluid rises through permeable wall-rocks, and fluid flow may be focussed along deep-seated fractures related to extensional tectonic processes. Metal sulphides are precipitated along channelways as the ascending fluid undergoes adiabatic expansion and then further cooling during mixing with ambient sub-sea-floor water. Vigorous fluid flow results in venting of reduced fluid at the sea-floor/sea-water interface and deposition of massive sulphide. A comparison of sulphide mineralization and wall-rock alteration in ancient and modern spreading centre environments supports this genetic concept. Massive sulphide deposits in ophiolites generally occur in clusters of closely spaced (< 1-5 km) deposits. Individual deposits are a composite of syngenetic massive sulphide and underlying epigenetic stockwork-vein mineralization. The massive sulphide occurs as concordant tabular

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

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

    The southern Apennine chain is a fold-and thrust belt formed between the upper Oligocene and Quaternary as a result of the convergence between the African and European plates and a simultaneous SE-directed rollback of the Ionian subducting lithospere. The ophiolitic sequences, which are part of the Southern Apennines, are remnants of the Ligurian oceanic lithosphere pertaining to the Jurassic western Tethys. The Liguride Units of the Southern Apennines include sequences characterized by an HP/LT metamorphic overprint in the Frido Unit and sequences lacking orogenic metamorphism North-Calabria Unit. The ophiolitic rocks occurring in the Frido Unit include serpentinite derived from a lherzolitic to harzburgitic mantle, as suggested by microstructural and petrographical features. The serpentinites are frequently associated to tectonic slices and dykes composed of diabase and medium to high-grade metamorphic rocks such as amphibolites, gneiss, granofels as well as gabbros and basalts with a pillow structure. The studied serpentinites of the Frido Unit show mesh, xenomorphic and mylonitic texture. Primary mantle minerals are represented by olivine, orthopyroxene, clinopyroxene and spinel. Pseudomorphic minerals are serpentine, magnetite and tremolite. Olivine is replaced by serpentine forming a mesh texture; orthopyroxene is mostly altered to bastite and in some cases shows exsolution lamellae of clinopyroxene and kink bands. Clinopyroxene is armoured by a tremolite rim. Spinel shows a holly-leaf habit and is often armoured by a corona of Cr-chlorite. The core of the analysed spinel has a Cr-Al spinel composition corresponding to chromite (Al2O3=29-31 wt %; Cr2O3= 28-37 wt%), whereas the rim has a Fe-Cr spinel composition corresponding to ferritchromite (Al2O3= 1-2% wt; Cr2O3=28-30 wt %). The Cr-Al spinel/ferritchromite ratio may be various in different spinel porphyroclasts. Serpentine has a fibrous stretched subidiomorphic habit, it is colourless or pale green

  10. Magnetic studies of magma-supply and sea-floor metamorphism: Troodos ophiolite dikes

    NASA Astrophysics Data System (ADS)

    Borradaile, G. J.; Gauthier, D.

    2006-05-01

    Dikes of the eastern Troodos ophiolite of Cyprus intruded at slow ocean-spreading axes with dips ranging up to 15° from vertical and with bimodal strikes (now NE-SW and N-S due to post-88 Ma sinistral microplate rotation). Varied dike orientations may represent local stress fields during dike-crack propagation but do not influence the spatial-distributions or orientation-distributions of dikes' magnetic fabrics, nor of their palaeomagnetic signals. Anisotropy of magnetic susceptibility (AMS) integrates mineral orientation-distributions from each of 1289 specimens sampled from dikes at 356 sites over ˜400 km 2 in the eastern Troodos ophiolite of Cyprus. In 90% of dikes, AMS fabrics define a foliation ( kMAX- kINT) parallel to dike walls and a lineation ( kMAX) that varies regionally and systematically. Magma-flow alignment of accessory magnetite controls the AMS with a subordinate contribution from the mafic silicate matrix that is reduced in anisotropy by sea-floor metamorphism. Titanomagnetite has less influence on anisotropy. Occasionally, intermediate and minimum susceptibility axes are switched so as to be incompatible with the kinematically reasonable flow plane but maximum susceptibility ( kMAX) still defines the magmatic flow axis. Such blended subfabrics of kinematically compatible mafic-silicate and misaligned multidomain magnetite subfabrics; are rare. Areas of steep magma flow ( kMAX plunge ≥ 70°) and of shallow magma-flow alternate in a systematic and gradual spatial pattern. Foci of steep flow were spaced ˜4 km parallel to the spreading axes and ˜6 km perpendicular to the spreading axes. Ridge-parallel separation of steep flow suggest the spacing of magma-feeders to the dikes whereas ridge-perpendicular spacing of 6 km at a spreading rate of 50 mm/a implies the magma sources may have been active for ˜240 Ka. The magma feeders feeding dikes may have been ≤ 2 km in diameter. Stable paleomagnetic vectors, in some cases verified by reversal tests

  11. 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

    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.

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

    NASA Astrophysics Data System (ADS)

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

    2011-10-01

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

  13. Extensional to compressive Mesozoic magmatism at the SE Eurasia margin as recorded from the Meratus ophiolite (SE Borneo, Indonesia)

    NASA Astrophysics Data System (ADS)

    Monnier, Christophe; Polvé, Mireille; Girardeau, Jacques; Pubellier, Manuel; Maury, René C.; Bellon, Hervé; Permana, Haryadi

    1999-01-01

    The Meratus ophiolitic series (SE Borneo) present a specific assemblage that have recorded (1) a continental extensional episode mostly seen within the peridotites and 2) later subduction-related magmatic events marked by the emplacement of calc-alkaline magmas. These events relate the magmatic activity and geodynamic evolution of the SE Eurasia margin in Mesozoic times. The ophiolitic series comprise ultramafic rocks with minor metavolcanic rocks. The ultramafic rocks include dominant lherzolites and pyroxenites with rather scarce harzburgites and dunites. Spinel peridotite, mineral chemistry data and bulk rock Rare Earth Element (REE) abundances show that most rocks underwent a low degree of partial melting. However, a few samples display significant depletions in Light REE (LREE), which are interpreted as the result of fractional melting under shallow conditions. Plagioclase-bearing peridotites are characterized by high REE abundances which also point to a very low degree of melting followed by reequilibration in the plagioclase facies, as seen from phase chemistry data. These peridotites are locally crosscut by dikelets containing high-temperature K-and Cr-rich amphiboles. Lavas closely associated with the Meratus peridotites have REE compositions ranging from the ones typical of enriched MORB (E-MORB) to normal MORB (N-MORB) types. We believe that the Meratus peridotites represent a fragment of subcontinental lithospheric mantle that locally suffered a low degree of fractional melting during the last stages of a continental rifting phase, in agreement with the presence of metamorphic K- and Cr-rich amphiboles in the peridotites. The E-MORB basalts might result from the melting of an enriched subcontinental lithosphere thermally eroded during the rifting phase by rising asthenosphere which might have produced N-MORB volcanic rocks. Back-arc basin basalts (BABB) now associated with E-MORB and N-MORB have also been found in the metamorphic soles of the

  14. 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.

  15. 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.

  16. Subduction initiation for the formation of high-Cr chromitites in the Kop ophiolite, NE Turkey

    NASA Astrophysics Data System (ADS)

    Zhang, Peng-Fei; Uysal, Ibrahim; Zhou, Mei-Fu; Su, Ben-Xun; Avcı, Erdi

    2016-09-01

    The Kop ophiolite in NE Turkey is a forearc fragment of Neo-Tethys ocean, consisting mainly of a paleo-Moho transition zone (MTZ) and a harzburgitic upper mantle unit. Locally, the Kop MTZ contains cumulate dunites and high-Cr chromitites (Cr# up to ca. 79), which are cut by pyroxenites. Dunites and chromitites in the MTZ have REE concentrations that are 1-2 orders of magnitude lower than those of chondrite; they are either depleted in LREE or have concave REE shapes. The LREE depleted patterns are interpreted to reflect production of cumulate rocks by magmas derived from a depleted mantle, the concave patterns the modification of these rocks by LREE-enriched fluids. Clinopyroxenes from pyroxenites are diopsidic and characterized by high Mg#s (ca. 92-96) and high CaO contents (ca. 24-25 wt.%); their Al2O3 contents (1.0-3.0 wt.%) fall between those of clinopyroxenes in N-MORB and komatiite/boninite, suggesting that the parental melts originated from more refractory mantle than abyssal lherzolites. However, these clinopyroxenes display LREE depleted patterns consistent with those of clinopyroxenes in abyssal lherzolites, indicating their genetic connection with decompression melting of asthenosphere. The cross-cutting relationship between pyroxenite veins and chromitiferous rocks suggests that depleted mantle remained beneath the proto-forearc after chromitite formation; it had not been significantly modified by slab-derived components and continued interacting with the upwelling asthenosphere until pyroxenite crystallization. This study provides a temporal constraint on the formation of high-Cr chromitites; they possibly began to be produced during the transition between early and late proto-forearc spreading, during which subduction dehydration had not well developed.

  17. Tectonic discrimination of chromian spinels, olivines and pyroxenes in the Northeastern Jiangxi Province ophiolite, South China

    NASA Astrophysics Data System (ADS)

    Guo, Guolin; Liu, Xiaodong; Yang, Jingsui; Pan, Jiayong; Fan, Xiujun; Zhou, Wenting; Duan, Gehong

    2016-11-01

    The peridotites of the Northeastern Jiangxi Province Ophiolite (NJO), including dunite, harzburgite and clinopyroxene-bearing harzburgite, are strongly altered under serpentinization, except for minor aggregations of partially-altered olivines, chromian spinels and pyroxenes. The forsterite content of olivines in dunites (Fo 93.6) is slightly higher than in harzburgites (Fo 91.4). Chromian spinels in harzburgites and dunites are very refractory, with restricted chemical compositions of high-Cr varieties. The unaltered cores of chromian spinels have low Al2O3 and TiO2 content, and display a large range of Mg# (100× [Mg/(Mg + Fe)], 41-64) and Cr# (100× [Cr/(Cr + Al)], 53-83) values, suggesting that the peridotites originated from a highly-depleted mantle. The spinels plotted in "olivine-spinel mantle array" (OSMA) diagram and Cr# versus. Mg# diagram both indicate the peridotite of NJO experienced a >25 % partial melting. The positive correlation between the Cr# and the TiO2 content probably resulted from the reaction between boninitic melt and mantle peridotite, as a consequence of melt-mantle interaction within the arc setting. The oxygen fugacity (ƒO2), calculated using chromian spinel-olivine pairs, indicates that the peridotites in the NJO were formed under relatively low oxidizing conditions quite different from those commonly found in supra-subductions zones (SSZ). This might be explained by the reaction between fore-arc magmas and residual mantle in a back-arc oceanic basin during a rapid subduction process. The Neoproterozoic subduction between the Yangtze and Cathaysia blocks was therefore probably rapid, and the addition of water and other volatiles to the mantle wedge beneath the island arc would have enhanced melting, leading to the production of highly depleted boninitic melts.

  18. Vestiges of a continental margin ophiolite type in the Novo Oriente region, Borborema Province, NE Brazil

    NASA Astrophysics Data System (ADS)

    Pitombeira, João Paulo Araújo; Amaral, Wagner da Silva; Uchôa Filho, Evilarde Carvalho; Fuck, Reinhardt Adolfo; Dantas, Elton Luiz; Parente, Clóvis Vaz; da Costa, Felipe Grandjean; Veríssimo, César Ulisses Vieira

    2017-01-01

    Margin Ophiolite type.

  19. 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.

  20. 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.

  1. 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…

  2. 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…

  3. 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…

  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…

  5. Neoproterozoic ensialic back-arc spreading in the eastern Arabian shield: geochemical evidence from the Halaban Ophiolite

    NASA Astrophysics Data System (ADS)

    Al-Saleh, A. M.; Boyle, A. P.

    2001-07-01

    Two continental terranes (Afif and Ar Rayn) of the eastern Arabian Shield are separated by the Al-Amar Suture Zone, the western boundary of which is marked by the Halaban Belt, the largest ophiolite sheet in the eastern shield. REE and other immobile trace element concentrations suggest the Halaban Ophiolite formed in an ensialic back-arc basin in a supra-subduction zone setting. Based on this, a new tectonic model for the eastern Arabian Shield was constructed. This model assumes the existence of a westerly-dipping subduction zone at about 700 Ma above which back-arc spreading resulted in the break up of the continental margin of the Afif Terrane and the formation of a short-lived marginal basin. Basin closure took place at around 680 Ma, but a westerly-dipping subduction zone continued to exist further east beneath the Ar Rayn Terrane for at least 60 Ma. In the period 620—600 Ma, collision with a continental mass (now concealed by Phanerozoic sedimentary rocks) gave rise to widespread granitic magmatism in the eastern shield, as well as crustal-scale strike-slip faulting.

  6. Origin of primary PGM assemblage in сhromitite 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 μm 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.

  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. Collisional orogenesis in the northern Canadian Cordillera: Implications for Cordilleran crustal structure, ophiolite emplacement, continental growth, and the terrane hypothesis

    NASA Astrophysics Data System (ADS)

    English, Joseph M.; Johnston, Stephen T.

    2005-04-01

    During Upper Triassic to Lower Jurassic time, arc magmatic rocks of the Stikine terrane, arc-marginal sediments of the Whitehorse Trough, igneous and mantle rocks of the Cache Creek 'ophiolite' and Kutcho assemblage, and oceanic sedimentary and volcanic rocks of the Cache Creek terrane represented a magmatic arc, forearc basin, forearc basement, and subduction complex, respectively. The Cache Creek subduction complex was thrust to the southwest over the Whitehorse Trough forearc basin and the Stikine terrane in the Middle Jurassic during collision with an inboard continental domain. Interpretation of the various lithotectonic assemblages of the northern Intermontane belt in terms of a new plate tectonic model has a number of important implications for the Canadian Cordillera: (a) the model allows comparisons to be drawn with available seismic reflection interpretations of Cordilleran crustal structure; (b) 'ophiolite' emplacement was achieved by ramping of forearc oceanic lithosphere onto thick crustal parts of a subducting plate during collisional orogenesis; (c) island-arc collision and accretion were the principal mechanisms for continental growth with relatively minor contributions from 'sliced-off' oceanic seamounts and/or plateaux; and (d) some terrane-bounding faults such as the Nahlin Fault do not represent major lithospheric-scale boundaries and their importance in tectonic reconstructions has been overemphasised.

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

    USGS Publications Warehouse

    Zierenberg, R.A.; Shanks, Wayne C.; Seyfried, W.E.; 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

  10. The Generation of Oceanic Lithosphere in an Embryonic Oceanic Crust : the Example of the Chenaillet Ophiolite in the Western Alps

    NASA Astrophysics Data System (ADS)

    Masini, E.; Manatschal, G.; Muntener, O.

    2007-12-01

    The Chenaillet Ophiolite exposed in the Franco-Italian Alps represents a well-preserved ocean-floor sequence that was only weakly affected by later Alpine convergence. Based on the similarity between rock types and structures reported from ultraslow spreading ridges and those observed in the Chenaillet Ophiolite, it may represent a field analogue for slow to ultraslow spreading ridges such as the Gakkel Ridge or the Southwest Indian Ridge. Mapping of the Chenaillet Ophiolite enabled to identify an oceanic detachment fault that extends over a surface of about 16 km2 capping exhumed mantle and gabbros onto which clastic sediments have been deposited. The footwall of the detachment is formed by mafic and ultramafic rocks. The mantle rocks are strongly serpentinized lherzolites and subordinate harzburgites and dunites. Microstructures reminiscent of impregnation, and cpx major and trace element chemistry indicate that spinel peridotite is (locally) replaced by plagioclase-bearing assemblages. Pyroxene thermometry on primary minerals indicates high temperatures of equilibration ( max 1200°C) for the mantle rocks. Gabbros range from troctolite and olivine-gabbros to Fe-Ti gabbros and show clear evidence of syn-magmatic deformation, partially obliterated by retrograde amphibolite and low-grade metamorphic conditions. In sections perpendicular to the detachment within the footwall, syn-tectonic gabbros and serpentinized peridotites grade over some tens of meters into cataclasites that are capped by fault gouges. Petro-structural investigations of the fault rocks reveal a syn-tectonic retrograde metamorphic evolution. Clasts of dolerite within the fault zone suggest that detachment faulting was accompanied by magmatic activity. Hydrothermal alteration is indicated by strong mineralogical and chemical modifications. Gabbro and serpentinized peridotite, together with serpentinite cataclasites occur as clasts in tectono-sedimentary breccias overlying directly the detachment

  11. 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

  12. 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

  13. Resurrection Peninsula and Knight Island ophiolites and recent faulting on Montague Island, southern Alaska

    USGS Publications Warehouse

    Nelson, Steven W.; Miller, Marti L.; Dumoulin, Julie A.

    1987-01-01

    The Resurrection Peninsula forms the east side of Resurrection Bay (Fig. 1). The city of Seward is located at the head of the bay and can be reached from Anchorage by highway (127 mi;204 km). Relief ranges from 1,434 ft (437 m) at the southern end of the peninsula to more than 4,800 ft (1,463 m) 17 mi (28 km) to the north. All rock units composing the informally named Resurrection Peninsula ophiolite are visible and (or) accessible by boat.The eastern half of the peninsula is located within the Chugach National Forest; the western half is mainly state land, but there is some private land with recreational cabins. The Seward A6 and A7 and Blying Sound D6 and D7 maps at 1:63,360 scale (mile-to-the-inch) cover the entire Resurrection Peninsula.Knight Island is located 53 mi (85 km) east of Seward (Fig. 1). Numerous fiords indent the 31-mi-long (50 km) by 7.4-mi-wide (12 km) island and offer excellent bedrock exposures. The island is rugged and has a maximum elevation of 3,000 ft (914 m). It has numerous mineral prospects (Tysdal, 1978; Nelson and others, 1984; Jansons and others, 1984; Koski and others, 1985), and several abandoned canneries are located on the island. Knight Island lies entirely within the Chugach National Forest—state and private inholdings constitute less than five percent of its total land area. The Seward A2, A3, B2, B3, and C2, 1:63,360-scale U.S. Geological Survey topographic maps cover the entire island.Montague Island, 50 mi (80 km) long and up to 11 mi (18 km) wide, lies 10.6 mi (17 km) southeast of Knight Island. It belongs to an island group that forms the southern margin of Prince William Sound (Fig. 1). Montague Island is less rugged and less heavily vegetated than either the Resurrection Peninsula or Knight Island. Rock exposures are excellent along the beaches, and ground disruption due to recent fault movements is clearly visible. The Seward Al and A2 and Blying Sound Dl, D2, and D3 maps cover the areas of interest on Montague Island

  14. 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

  15. Gabbro layering induced by simple shear in the Oman ophiolite Moho transition zone

    NASA Astrophysics Data System (ADS)

    Jousselin, David; Morales, Luiz F. G.; Nicolle, Marie; Stephant, Aurore

    2012-05-01

    We investigate the origin of modal layering in gabbro lenses of the Moho transition zone in the Oman ophiolite with a microstructural study. Gabbro lenses exhibit a shape preferred orientation of plagioclase crystals, that are euhedral and devoid of any intracrystalline deformation. This texture and field kinematic indicators show a strong simple shear deformation in magmatic conditions. The parallelism of the lineation in gabbros and the plastic lineation of the host dunite indicate that their development is contemporaneous, and that the magmatic features are passively coupled to the solid-plastic flow of the host mantle. We also found undeformed-isotropic and weakly deformed-roughly layered gabbros. The host rock is plagioclase and clinopyroxene impregnated dunite; it contains euhedral plagioclase suggesting that some grains crystallized in suspension. There is no clear boundary between the gabbro and the host rock; on the contrary, the limit is diffuse, with increasing plagioclase and clinopyroxene content from the dunite to the gabbro over tens of meters. This suggests that the gabbro corresponds to melt impregnated dunite. We defined 4 gabbro types, from unlayered (type 1) to well layered (type 4) with progressively more continuous and distinct layers. We characterized deformation with crystal shape and crystallographic preferred orientation (CPO) measurements. Unlayered samples have a random fabric; type 2 have a very weak shape fabric and planar CPO, defined by olivine and plagioclase (010) planes; type 3 have a weak fabric, with a lineation defined by the [001] olivine axis and the [100] plagioclase axis, which reflects the shape fabric; type 4 have a strong shape fabric and CPO, with a lineation defined by the [100] olivine and plagioclase axis. We interpret the progression from isotropic to clear S-L fabrics as a result of increasing deformation imposed by the flowing host mantle. The type 2 gabbros must result from compaction; with increasing simple

  16. Anorthosites in Oman ophiolite crust, a clue to crust origin at a fast spreading ridge

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    A first requirement to obtain anorthosites in the gabbro unit and Moho transition zone (MTZ) in the Oman ophiolite is that anorthite be on the liquidus of the basaltic melt issued from the rising mantle beneath the ridge axis. The primitive melt having olivine+spinel on the liquidus near the Moho at ~2Kb pressure evolves to having plagioclase (+olivine) at pressure ~0.5Kb of the perched axial magma chamber (AMC). A second requirement is that some physical process segregates plagioclase from the olivine appearing along the cotectic line and clinopyroxene appearing at the solidus. Within the melt lens, this physical process is fractional crystallization from a melt intrusion in conditions ascribed in Oman and also modeled at 9°N EPR. Pure anorthosite mounds are formed within a time shorter than a few years. Mounds are relayed by swarms of thin layers of anorthosites within gabbros, pointing to voluminous melt surge on average every ~100 years. From there, the anorthosite layers are involved in the gabbro subsidence and preserved throughout the gabbro magma chamber down to the Moho. In deeper horizons, the anorhosite layers may have been contaminated by percolating melts, introducing clinopyroxene. Some anorthosites interlayered with ultramafics in the lower layered gabbros or in MTZ sills have formed in situ as a result of a very active segregation process at depth. The MTZ acts as a filter, crystallizing a limited fraction of olivine+spinel followed by clinopyroxene, from a large volume of primitive melt. Within the MTZ, ascending melt accumulated in spongy impregnated dunites is sporadically expelled through hydrofractures to feed the perched AMC. Alternatively, the fraction of melt may exceed the melt percolation threshold and a wehrlitic mush (olivine + melt) is injected in the lower gabbros as sills and local intrusions. Wehrlites injected within the magma chamber are involved in the large horizontal magmatic flow of the lower gabbros. This flow may contribute

  17. Assessing Hydrothermal Contributions to Global Biogeochemial Cycles; Insights From the Macquarie Island Ophiolite

    NASA Astrophysics Data System (ADS)

    Coggon, R. M.; Teagle, D. A. H.; Davidson, G.; Alt, J.; Brewer, T. S.; Harris, M.

    2014-12-01

    published estimates of the hydrothermal contributions from (i) drilled sections of in-situ upper ocean crust produced at slow, intermediate and fast spreading rates and of differing crustal ages; and (ii) supra-subduction zone ophiolites, which consistently record a greater extent of fluid-rock exchange than crust from mid-ocean ridges.

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

    NASA Astrophysics Data System (ADS)

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

    2014-04-01

    The gabbroic bodies from the Jurassic Ligurian ophiolites are structurally and compositionally similar to the gabbroic sequences from the oceanic core complexes of the Mid Atlantic Ridge. Initial cooling of the Ligurian gabbros is associated with local development of hornblende-bearing felsic dykes and hornblende vein networks. The hornblende veining is correlated with the widespread development of hornblende as coronas/pseudomorphs after the clinopyroxene in the host gabbros. In addition, the studied gabbroic body includes a mantle sliver locally containing hornblende gabbros and hornblendite veins. The hornblendes from the felsic dykes and the hornblende-rich rocks within the mantle sliver show a similar geochemical signature, characterized by low Mg#, CaO and Al2O3, negligible Cl, and high TiO2, K2O, REE, Y, Zr and Nb concentrations. The whole-rock Sm-Nd isotopic compositions of the felsic dykes and the hornblende-rich rocks define a Sm-Nd isochron corresponding to an age of 154 ± 20 Ma and an initial ɛNd of 9.2 ± 0.5. The δ18O of the hornblendes and coexisting zircons from these rocks (about +4.5‰ and +5.8‰, respectively) do not indicate the presence of a seawater component in these melts. The formation of the felsic dykes and of the hornblende-rich rocks within the mantle sliver involved SiO2-rich silicate melts with negligible seawater component, which presumably were derived from high degree fractional crystallization of MOR-type basalts. The vein and the coronitic/pseudomorphic hornblendes show high Mg# and CaO, significant Cl (0.02-0.17 wt%) and low TiO2 and K2O concentrations. The coronitic/pseudomorphic hornblendes have trace element compositions similar to those of the clinopyroxenes from the gabbros and δ18O values (+1.0‰ to 0.7‰) close to seawater, suggesting an origin by reaction between migrating seawater-derived fluids and the host gabbros. The vein hornblendes commonly show slight LREE enrichment, relatively high concentrations of Nb

  19. 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

  20. 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 (δ2H, δ18O). 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 δ18O. 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

  1. 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

  2. Structure and Tectonics of Subophiolitic Mélanges in the Western Hellenides (Greece) and Implications for Ophiolitic Root Zones in the Balkans

    NASA Astrophysics Data System (ADS)

    Ghikas, Constandina; Dilek, Yildirim; Rassios, Anne E.

    2010-05-01

    The Jurassic Vourinos ophiolite is part of the Western Hellenide ophiolite belt in Greece and rests tectonically on the Pelagonian ribbon continent. The Vourinos and coeval Pindos ophiolite to the west display suprasubduction-zone geochemical affinities, and represent remnants of oceanic lithosphere formed in a rifted incipient arc-forearc setting within the Pindos Basin. In structurally descending order, and from west to east, the subophiolitic mélange beneath the Vourinos ophiolite contains the Agios Nikolaos Formation (ANF) and a rift assemblage, both of which display ENE-vergent thrust faults, shear zones, and folds. The ANF comprises schistose mudstone with pebbles, cobbles, and boulders of arenite and wacke derived from the crystalline basement of Pelagonia. Imbricated along ENE-directed thrust faults and metamorphosed up to lower amphibolite facies, the ANF represents continental rise deposits of the rifted Pelagonian margin. The rift assemblage includes blocks of basaltic lavas, ribbon chert, micritic cherty limestone, metagabbro, dolerite dikes, and serpentinite breccia that are commonly in thrust contact with each other and are tectonically imbricated with the Pelagonian carbonates; however, primary intrusive and depositional contacts are locally well preserved. Gabbro and dolerite dikes are locally intrusive into the recrystallized carbonates and metapelitic rocks of Pelagonia. Lavas display mid-ocean ridge basalt-within plate basalt affinities and represent Upper Triassic rift units that erupted during the separation of Pelagonia from Apulia. Gabbro, dolerite, and serpentinite breccia are the products of a magmatic rifting episode prior to the onset of seafloor spreading in the Pindos Basin. The Vourinos subophiolitic mélange thus consists of passive margin and rift assemblages that were tectonically overridden by the Vourinos ophiolite in the middle Jurassic. Its internal structure and evolutionary history represent a tectonic mélange character of

  3. 40Ar/39Ar geochronology of Andaman Ophiolite: Evidence for a Pleistocene mega thrusting event within the Andaman-Nicobar Accretionary Wedge

    NASA Astrophysics Data System (ADS)

    Pande, K.; Ray, J. S.

    2015-12-01

    A geochronological study the Andaman ophiolite was performed along a ~120 km corridor on the eastern margin of the Andaman Islands, which forms part of the Andaman-Nicobar accretionary wedge. This Cretaceous ophiolite sequence occurs as imbricate thrust wedges overlying the Paleogene flysch and Neogene pelagic sediments. Incremental heating 40Ar/39Ar dating of three pillow basalts and a peridotite samples reveals that the ophiolite has a composite thermal history. Apparent age spectra of all the samples suggest a clear two stage evolution, with the high temperature steps (1000 to 1400oC) and the lower temperature steps (400-950oC) showing a staircase pattern and a plateau-like pattern, respectively. The apparent ages of the highest temperature steps vary from 135 Ma to 97 Ma, which appear to suggest multiple crystallization ages. Interestingly, the plateau-like spectra for the lower temperature steps yield indistinguishable ages, although with very high errors (as high as 67% at 1s), across all the samples. The isochron ages too overlap with the plateau-like ages. Thus the weighted average of the plateau-like ages of 0.9±0.3 (2s) Ma most likely represents the timing of the latest thermal resetting. Based on the above information we conclude that while different units of the Andaman ophiolite may have been formed at different times, all had seen a major thermal resetting. Considering the position of these rocks vis-à-vis the structural configuration of the Andaman-Nicobar accretionary wedge we infer that there was a mega thrusting event in the region at ~0.9 Ma which was possibly responsible for exhumation of the current outcrops of the ophiolite sequence along the east coast of the Andman Islands.

  4. The Queensborough mafic-ultramafic complex: a fragment of a Meso-Proterozoic ophiolite? Grenville Province, Canada

    NASA Astrophysics Data System (ADS)

    Smith, T. E.; Harris, M. J.

    1996-11-01

    The Queensborough mafic-ultramafic complex occurs as a fault-bounded block, up to 10 km wide and having an area > 220 km 2. It lies in the Grimsthorpe Domain of the Bancroft-Elzevir-Mazinaw-Sharbot Lake Terrane in the Central Metasedimentary Belt of the Grenville Province. It has been suggested, without adequate supporting data, that the complex may represent oceanic crust, a fragment of an ophiolite, or even a metavolcanic sequence made up of basaltic and komatiitic flows. The geological and tectonic significance of the complex is assessed using field relationships, petrography and geochemistry. Structurally the lowest part of the complex comprises a series of ultramafic rocks characterized by metre-scale compositional layering, represented by several different metamorphic assemblages of talc, chlorite, carbonate, anthophyllite, and actinolite-tremolite. These assemblages indicate that the original rocks were cumulate peridotites and pyroxenites. The ultramafic rocks are overlain structurally by a series of mafic rocks, predominantly massive to highly sheared gabbros. The gabbros are penetrated by a series of mafic dykes and include a few small enclaves of pillowed mafic volcanics. Major- and trace-element chemistry shows that the mafic rocks represent a fractionally crystallized sequence of tholeiitic gabbros, lavas, and mafic dykes and that the ultramafic cumulates are co-genetic. The regional geological setting, and the trace-element signatures of the mafic rocks suggest that they were formed in a back-arc basin. Comparison of the Queensborough Complex with Proterozoic and Phanerozoic igneous complexes suggests that it represents a partially preserved crustal section of a Mesoproterozoic ophiolite. In addition, the rocks of the Queensborough Complex are petrographically and geochemically similar to those of the Vavilov Basin which occurs in the deepest part of the Tyrrhenian Sea. By analogy with this Neogene back-arc basin we suggest that the Central

  5. 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

  6. High- & Low-δ18O magma: Comparative study of crustal and mantle plagiogranites from the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Alberts, R. C.; Grimes, C. B.; Koepke, J.; Erdmann, M.; Kitajima, K.; Spicuzza, M. J.; Valley, J. W.

    2015-12-01

    Plagiogranite (PLGT) from the crust and mantle sections of the Oman ophiolite preserve widely varied δ18O values that monitor different processes occurring during ophiolite construction. Mantle-like δ18O values are expected if MORB fractionation played a dominant role in PLGT genesis. Magmatic values (monitored here by zircon) shifted away from the mantle-like range indicate open system processes which include partial melting of hydrothermally-altered crust or influx of subduction-related, sediment-derived melt. Zircon (zrn) and quartz (qtz) from twenty-four new samples of PLGT from the crustal and mantle sections of the Oman ophiolite were analyzed for δ18O. Rock-averaged δ18O from the sheeted dikes (zrn: 4.3-4.5‰, qtz: 6.7-6.9‰) and dike-gabbro transition (zrn: 3.9-4.8‰, qtz: 4.7-7.7‰) are mostly below values in magmatic equilibrium with MORB (zrn = 5.2±0.5‰, qtz = 7.0-7.5‰). δ18O for PLGT in the gabbro section (zrn: 4.8-5.1‰, qtz: 7.7-8.3‰) are mostly mantle-like. Quartz is generally found to be more variable than coexisting zrn and likely experienced some sub-solidus exchange. When organized into a relative structural position, δ18Ozrn values typically increase with depth. The lowest δ18Ozrn are observed near the dike-gabbro transition and are consistent with petrogenesis involving hydrous partial melting of mafic crust previously hydrothermally-altered at high-T. The return to nominally mantle-equilibrated δ18Ozrn deeper in the gabbro section may reflect decreasing seawater-signatures of fluids penetrating to depth, lower water/rock ratios, or extreme fractional crystallization. Crustal PLGT thus predate the development of high δ18O signatures in the upper oceanic crust as it cools and experiences low temperature hydrothermal alteration. Mantle PLGT intrusions (1-3 m thick) from the Haylayn block extend to considerably higher rock-averaged δ18O values (zrn: 5.1-15.4‰, qtz: 7.0-18.5‰). Individual rocks (5 samples) were uniform in

  7. 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

  8. Strontium alteration in the Troodos ophiolite: implications for fluid fluxes and geochemical transport in mid-ocean ridge hydrothermal systems

    NASA Astrophysics Data System (ADS)

    Bickle, Mike J.; Teagle, Damon A. H.

    1992-09-01

    New and published strontium isotope analyses from the Troodos ophiolite constrain fluid-solid exchange processes, and the magnitude and circulation paths of the hydrothermal fluids. The 87Sr/ 86Sr profile reflects alteration in the recharge zone of an evolving hydrothermal system. Fluid-rock strontium isotope exchange in the upper ˜ 1.5 km of extrusive lavas was kinetically limited and seawater-derived fluids emitted from the base of this zone were buffered to 87Sr/ 86Sr ratios between ˜ 0.7047 and 0.7059. In contrast, over the next ˜ 1 km depth interval of sheeted dykes and the uppermost plutonics, 87Sr/ 86Sr values cluster about0.7054 ± 7 (2σ) and fluid flow is inferred to have been pervasive with near-equilibrium fluid-rock exchange. Quartz-chlorite and epidosite zones, the probable pathways of the concentrated, high-temperature upwelling fluids, have identical 87Sr/ 86Sr ratios to adjacent diabase dykes. On Troodos a time-integrated fluid flux in excess of2.9 × 10 7 kg m -2 is required to transport the strontium isotope composition of ˜ 0.7054, set in the kinetically controlled exchange zone, through the ˜ 1 km of sheeted dykes and into the zones of concentrated upwelling. The uniformity of the 87Sr/ 86Sr ratios in the diabase sheeted dykes and high-temperature epidosite and quartz-chlorite rocks indicate that the strontium isotopic alteration took place during the high temperature phase of hydrothermal circulation. The inferred minimum time-integrated fluid flux of2.9 × 10 7 kg m -2 substantially exceeds that of˜ 5 × 10 6 kg m -2 inferred from thermal models of high temperature circulation, but is comparable with estimates of the hydrothermal flux from oceanic budgets of 3He, Mg and 87Sr. The high flux estimate for Troodos is consistent with the ophiolite venting fluids, with 87Sr/ 86Sr elevated significantly above rock values, which contrasts with the near-MORB 87Sr/ 86Sr ratios of fluids from active high-temperature vents at mid-ocean ridges and

  9. 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

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    Oceanic core complexes (OCCs) are dome-shaped massifs commonly associated with the inside corners of the intersection of transform faults and slow (and ultra-slow) spreading centres. They represent the uplifted footwalls of large-slip oceanic detachment faults (e.g. Cann et al., 1997; Blackman et al., 1998) and are composed of mantle and lower crustal rocks exhumed during fault displacement (Smith et al., 2006, 2008). Recent paleomagnetic studies of core samples from OCCs in the Atlantic Ocean (Morris et al., 2009; MacLeod et al., in prep) have confirmed that footwall sections undergo substantial rotation around (sub-) horizontal axes. These studies, therefore, support “rolling hinge” models for the evolution of OCCs, whereby oceanic detachment faults initiate at a steep angle at depth and then “roll-over” to their present day low angle orientations during unroofing (Buck, 1988; Wernicke & Axen, 1988; Lavier et al., 1999). However, a fully integrated paleomagnetic and structural analysis of this process is hampered by the one-dimensional sampling provided by ocean drilling of OCC footwalls. Therefore, ancient analogues for OCCs in ophiolites are of great interest, as these potentially provide 3-D exposures of these important structures and hence a more complete understanding of footwall strain and kinematics (providing that emplacement-related phases of deformation can be accounted for). Recently, the relationship between outcropping crustal and upper mantle rocks led Tremblay et al. (2009) to propose that an OCC is preserved within the Mirdita ophiolite of the Albanian Dinarides (northern Albania). This is a slice of Jurassic oceanic lithosphere exposed along a N-S corridor which escaped the main late Cenozoic Alpine deformation (Robertson, 2002, 2004; Dilek et al., 2007). Though in the eastern portion of the Mirdita ophiolite a Penrose-type sequence is present, in the western portion mantle rocks are in tectonic contact with upper crustal lithologies

  11. 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.

  12. 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

  13. Suprasubduction volcanic rocks of the Char ophiolite belt, East Kazakhstan: new geochemical and first geochronological data

    NASA Astrophysics Data System (ADS)

    Safonova, Inna; Simonov, Vladimir; Seltmann, Reimar; Yamamoto, Shinji; Xiao, Wenjiao

    2016-04-01

    The Char ophiolite belt is located in the western Central Asian Orogenic Belt, a world largest accretionary orogen, which has evolved during more than 800 Ma. The Char belt formed during Kazakhstan - Siberia collision. It has been known for hosting fragments of Late Devonian-Early Carboniferous oceanic crust, MORB, OPB and OIB, of the Paleo-Asian Ocean (Safonova et al., 2012). The Char is surrounded by two Paleozoic island-arc terranes: Zharma-Saur in the west and Rudny Altai in the east, however, until recent times, no island-arc units have been found within it. We were the first to find island-arc units as tectonic sheets occurring adjacent to those consisting of oceanic rocks. In places, island-arc andesites cut oceanic basalts. The Char volcanic and subvolcanic rocks of a probable suprasubduction origin are basalt, microgabbro, dolerite, andesite, tonalite and dacite. The mafic to andesitic volcanics possessing low TiO2 (0.85 wt.%av.) and show MgO vs. major elements crystallization trends suggesting two magma series: tholeiitic and calc-alkaline. The tholeiitic varieties are less enriched in incompatible elements then the calc-alkaline ones. Two samples are high-Mg and low-Ti andesibasalts similar to boninites. The rocks possess moderately LREE enriched rare-earth element patterns and are characterized by negative Nb anomalies present on the multi-element spectra (Nb/Lapm = 0.14-0.47; Nb/Thpm = 0.7-1.6).The distribution of rare-earth elements (La/Smn = 0.8-2.3, Gd/Ybn = 0.7-1.9) and the results of geochemical modeling in the Nb-Yb system suggest high degrees of melting of a depleted harzburgite-bearing mantle source at spinel facies depths. Fractional crystallization of clinopyroxene, plagioclase and opaque minerals also affected the final composition of the volcanic rocks. Clinopyroxene monomineral thermometry indicates crystallization of melts at 1020-1180°C. Melt inclusion composition based numerical calculations show that primary melts were derived at 1350

  14. Tracking Melt-Rock Reaction Using Os Isotopes: Maqsad Diapir (Oman Ophiolite)

    NASA Astrophysics Data System (ADS)

    Godard, M.; Alard, O.; Lorand, J.; Burton, K. W.

    2001-12-01

    The exceptional exposure of the Oman ophiolite allows to study in detail the mantle processes occurring beneath a spreading centre. We have carried out a Re-Os pilot study focussed on the Maqsad diapir in the western part of the Sumail massif. The Maqsad diapir is roughly constituted of 3 petrographic units: 1, the main harzburgitic domain (MHD); 2, the diapir harzburgite (DH) showing plunging lineation; 3, a thick dunitic, mantle-crust transition zone (MTZ) topping the DH section. The MHD samples are characterised by Os content about 4 ppb and 187Os/188Os down to 0.1210. These features are in agreement with a >15% melt extraction. These unradiogenic Os compositions yield a model Re-depletion age ca. 1 Ga, similar to other Re-Os model age obtained for other oceanic related mantle samples worldwide. Samples from the DH domain despite similar range in Al2O3 content show more radiogenic Os composition (0.1240<187Os/188Os <0.1298). The MTZ dunites show even more radiogenic Os composition (0.1352<187Os/188Os <0.1399). The MTZ dunite are further characterised by low Os content (<1.5 ppb) not in agreement with the well know high compatible nature of Os. These observations are inconsistent with a residual origin after melting for these dunites. In contrast, Os systematic indicate than the MTZ dunites as well as the diapir harzburgites are not a mere product of melt depletion but rather result from extensive melt-rock reaction as previously demonstrated (e.g., Kelemen et al., 1995; Godard et al., 2000). Those authors have postulated that dunites are the product of melt-rock reaction leading to the crystallisation of olivine and Fe-enrichment (leading to low Mg-number) similar but less extensive process has been envisaged for the diapir harzburgite. 187Os/188Os is inversely correlated with the Mg-number of the MTZ and DH samples. Suggesting that indeed the petrographic and geochemical characteristics of both domains are due to the same process occurring at various extend

  15. 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

  16. Initiating intermediate-depth earthquakes: Insights from a HP-LT ophiolite from Corsica

    NASA Astrophysics Data System (ADS)

    Deseta, N.; Ashwal, L. D.; Andersen, T. B.

    2014-10-01

    The hypocentres of intermediate-depth earthquakes have been shown to overlap with the regions in subducting slabs that contain high abundances of hydrous minerals. This relationship was initially revealed using geophysical and numerical modelling and until recently has lacked corroboration from direct field-based research. We evaluated the relationship of the coincidence of intermediate-depth earthquakes with hydrous minerals in the slab by undertaking detailed geochemical analyses of blueschist to lawsonite to eclogite facies pseudotachylytes and their hostrocks located within an exhumed ophiolite, the Eocene Schistes Lustres Complex in Corsica. These units comprise incompletely metamorphosed metagabbro and peridotite. The wallrocks of the pseudotachylytes contain variable amounts of hydrous minerals: tremolite, Mg-hornblende, glaucophane in the metagabbro, and serpentine, tremolite and chlorite in the peridotite. Back-scatter-electron images show that the hydrous minerals entrained in the melt undergo fusion rather than dehydration. Vesicular and H2O-rich melt veins are observed cross-cutting partially molten pseudotachylyte fault veins and show evidence of H2O exsolution during melt solidification. The crystallisation products of these melts indicate formation under high temperature, high pressure conditions (1400-1700 °C; 1.5 GPa). The peridotite-hosted pseudotachylytes crystallised olivine, orthopyroxene and diopside, which are surrounded by interstitial Al- and H2O-rich glass. The metagabbro pseudotachylyte is dominated by Al-rich omphacite, ilmenite and high-Fe anorthite. XRF bulk analyses of the wallrock of the pseudotachylyte and electron microprobe analyses of the pseudotachylyte matrix, entrained survivor clasts and the crystallisation products show that near-total disequilibrium melting took place. The peridotite-hosted pseudotachylyte composition is skewed strongly towards chlorite; however, the preservation of delicate dendritic diopside and olivine

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

    USGS Publications Warehouse

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

    1996-01-01

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

  18. Uplifted ophiolitic rocks on Isla Gordon, southernmost Chile: implications for the closure history of the Rocas Verdes marginal basin and the tectonic evolution of the Beagle Channel region

    NASA Astrophysics Data System (ADS)

    Cunningham, W. D.

    1994-04-01

    A succession of mafic rocks that includes gabbro, sheeted dikes and deformed pillow basalts has been mapped in detail on Isla Gordon, southernmost Chile and is identified as an upper ophiolitic complex representing the uplifted floor of the Late Jurassic-Early Cretaceous Rocas Verdes marginal basin. The complex was uplifted, deformed, and regionally metamorphosed prior to the intrusion of an undeformed 90 Ma granodiorite that cuts the complex. The complex appears para-autochthonous, is gently tilted to the northeast and is internally sheared by near-vertical foliation zones. No evidence for obduction was observed although the base of the complex is not exposed. The ophiolitic rocks have been regionally metamorphosed to mid-upper greenschist levels. Isla Gordon is bounded by the northwest and southwest arms of the Beagle Channel, two important structural boundaries in the southernmost Andes that are interpreted to have accommodated north-side-up and left-lateral displacements. Directly north of Isla Gordon is the Cordillera Darwin metamorphic complex that exposes the highest grade metamorphic rocks in the Andes south of Peru. On the north coast of Isla Gordon a volcaniclastic turbidite sequence that is interpreted to have been deposited above the mafic floor is metamorphosed to lower greenschist levels in strong metamorphic contrast to amphibolite-grade othogneisses exposed in Cordillera Darwin only 2 km away across the northwest arm of the Beagle Channel. The profound metamorphic break across the northwest arm of the Beagle Channel and the regional northeast tilt of the ophiolitic complex are consistent with the previously proposed hypothesis that Isla Gordon represents the upper plate to an extensional fault that accommodated tectonic unroofing of Cordillera Darwin. However, limited structural evidence for extension was identified in this study to support the model and further work is needed to determine the relative importance of contractional, extensional and

  19. Subduction zone decoupling/retreat modeling explains south Tibet (Xigaze) and other supra-subduction zone ophiolites and their UHP mineral phases

    NASA Astrophysics Data System (ADS)

    Butler, Jared P.; Beaumont, Christopher

    2017-04-01

    The plate tectonic setting in which proto-ophiolite 'oceanic' lithosphere is created remains controversial with a number of environments suggested. Recent opinions tend to coalesce around supra-subduction zone (SSZ) forearc extension, with a popular conceptual model in which the proto-ophiolite forms during foundering of oceanic lithosphere at the time of spontaneous or induced onset of subduction. This mechanism is favored in intra-oceanic settings where the subducting lithosphere is old and the upper plate is young and thin. We investigate an alternative mechanism; namely, decoupling of the subducting oceanic lithosphere in the forearc of an active continental margin, followed by subduction zone (trench) retreat and creation of a forearc oceanic rift basin, containing proto-ophiolite lithosphere, between the continental margin and the retreating subduction zone. A template of 2D numerical model experiments examines the trade-off between strength of viscous coupling in the lithospheric subduction channel and net slab pull of the subducting lithosphere. Three tectonic styles are observed: 1) C, continuous subduction without forearc decoupling; 2) R, forearc decoupling followed by rapid subduction zone retreat; 3) B, breakoff of subducting lithosphere followed by re-initiation of subduction and in some cases, forearc decoupling (B-R). In one case (BA-B-R; where BA denotes backarc) subduction zone retreat follows backarc rifting. Subduction zone decoupling is analyzed using frictional-plastic yield theory and the Stefan solution for the separation of plates containing a viscous fluid. The numerical model results are used to explain the formation of Xigaze group ophiolites, southern Tibet, which formed in the Lhasa terrane forearc, likely following earlier subduction and not necessarily during subduction initiation. Either there was normal coupled subduction before subduction zone decoupling, or precursor slab breakoff, subduction re-initiation and then decoupling

  20. 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

  1. 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

  2. U-Pb dating and composition of inclusions in zircon from ophiolitic gabbro of the Klyuchevsk massif (Middle Urals): Results and geological interpretation

    NASA Astrophysics Data System (ADS)

    Smirnov, V. N.; Ivanov, K. S.; Koroteev, V. A.; Erokhin, Yu. V.; Khiller, V. V.

    2016-06-01

    The U-Pb (SHRIMP) dating of zircon from the layered complex of ophiolitic gabbro in the Klyuchevsk massif yielded an age of 456 ± 6 Ma corresponding within the limits of error to zircon dates obtained for other petrographic varieties from this massif. The investigation of the composition of silicate inclusions in dated zircon grains revealed that they are represented by typical metamorphic minerals: albite, zoisite, and secondary amphiboles. The data indicate that zircon was crystallized during metamorphic transformations of gabbroids and its U-Pb age (Late Ordovician-Silurian) is characteristic of all rocks in the ophiolite association of the Klyuchevsk massif indicating the age of metamorphism, not their formation time.

  3. The internal structure of eclogite-facies ophiolite complexes: Implications from the Austroalpine outliers within the Zermatt-Saas Zone, Western Alps

    NASA Astrophysics Data System (ADS)

    Weber, Sebastian; Martinez, Raul

    2016-04-01

    The Western Alpine Penninic domain is a classical accretionary prism that formed after the closure of the Penninic oceans in the Paleogene. Continental and oceanic nappes were telescoped into the Western Alpine stack associated with continent-continent collision. Within the Western Alpine geologic framework, the ophiolite nappes of the Zermatt-Saas Zone and the Tsate Unit are the remnants of the southern branch of the Piemonte-Liguria ocean basin. In addition, a series of continental basement slices reported as lower Austroalpine outliers have preserved an eclogitic high-pressure imprint, and are tectonically sandwiched between these oceanic nappes. Since the outliers occur at an unusual intra-ophiolitic setting and show a polymetamorphic character, this group of continental slices is of special importance for understanding the tectono-metamorphic evolution of Western Alps. Recently, more geochronological data from the Austroalpine outliers have become available that make it possible to establish a more complete picture of their complex geological history. The Lu-Hf garnet-whole rock ages for prograde growth of garnet fall into the time interval of 52 to 62 Ma (Weber et al., 2015, Fassmer et al. 2015), but are consistently higher than the Lu-Hf garnet-whole rock ages from several other locations throughout the Zermatt-Saas zone that range from 52 to 38 Ma (Skora et al., 2015). This discrepancy suggests that the Austroalpine outliers may have been subducted earlier than the ophiolites of the Zermatt-Saas Zone and therefore have been tectonically emplaced into their present intra-ophiolite position. This points to the possibility that the Zermatt-Saas Zone consists of tectonic subunits, which reached their respective pressure peaks over a prolonged time period, approximately 10-20 Ma. The pressure-temperature estimates from several members of the Austroalpine outliers indicate a complex distribution of metamorphic peak conditions, without ultrahigh

  4. 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.

  5. High-temperature metamorphism of the Yushugou ophiolitic slice: Late Devonian subduction of seamount and mid-oceanic ridge in the South Tianshan orogen

    NASA Astrophysics Data System (ADS)

    Zhang, Li; Jin, Zhenmin

    2016-12-01

    The South Tianshan Orogenic Belt (STOB), representing the southern segment of the Central Asian Orogenic Belt (CAOB), underwent a long-lived and subduction-related accretionary orogenic process. Revealing the petrogenesis of high-pressure (HP) metamorphic ophiolitic slices within this orogen is of crucial importance to understanding the geodynamic evolution of the STOB. In this study, we carry out a petrological, geochemical and geochronological study of HP mafic granulites from the Yushugou ophiolitic slice within the South Tianshan Accretionary Complex. Our results combined with previously published data suggest that the Yushugou mafic granulites, including garnet-clinopyroxene granulite, garnet two-pyroxene granulite and garnet-orthopyroxene granulite, are generally subalkaline to alkaline basalts, and show geochemical characteristics of MORB and OIB. The nominally anhydrous minerals of the mafic granulites contain certain but trace amounts of water in the manner of structural OH and sub-microscopic fluid inclusions. The granulites have a possible protolith age of ca. 400 Ma and metamorphic age of 390-360 Ma, and underwent HP and high-temperature (HT) granulite-facies metamorphism under conditions of 12-14 kbar and 840-950 °C and low H2O activity. Our study indicates that the Yushugou ophiolitic slice was probably derived from seamount that formed at mid-oceanic ridge closing to the oceanic trench and subduction zone during the Early Devonian, and then underwent metamorphism and deformation as a result of the subduction of the seamount and associated spreading ridge during the Middle to Late Devonian. Therefore, the Yushugou HP ophiolitic slice provides an important information of the Paleozoic tectonic evolutionary of the STOB.

  6. 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

  7. 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

  8. Structural rejuvenation of the eastern Arabian Shield during continental collision: 40Ar/ 39Ar evidence from the Ar Ridayniyah ophiolitic mélange

    NASA Astrophysics Data System (ADS)

    Al-Saleh, A. M.; Boyle, A. P.

    2001-07-01

    The Ar Ridayniyah ophiolitic mélange is one of a number of such complexes found within or at the peripheries of the Neoproterozoic Al-Amar Suture of the eastern Arabian Shield. This suture is sandwiched between the Ar Rayn island-arc terrane on the east and the much larger Afif continental block to the west, and is thought to represent the site of a 695—680 Ma back-arc basin that separated the two terranes. A thick and monotonous unit of metagraywacke (Abt Schist) underlies most of the suture along with scattered outcrops of metavolcanics and ophiolitic mélange. One of these bodies is the Ar Ridayniyah mélange, which occurs as a longitudinal belt of sheared ultramafic schists enclosing abundant blocks of oceanic serpentinites, as well as subordinate gabbros and basalts. The western boundary of this mélange is defined by the Ar Ridayniyah thrust fault. The 610—600 Ma ages obtained from the metagabbros of this complex are considered to record the reactivation of the Ar Ridayniyah Fault during continental collision, 60 Ma after ophiolite emplacement.

  9. 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.

  10. Mineral chemistry of isotropic gabbros from the Manamedu Ophiolite Complex, Cauvery Suture Zone, southern India: Evidence for neoproterozoic suprasubduction zone tectonics

    NASA Astrophysics Data System (ADS)

    Yellappa, T.; Tsunogae, T.; Chetty, T. R. K.; Santosh, M.

    2016-11-01

    The dismembered units of the Neoproterozoic Manamedu Ophiolite Complex (MOC) in the Cauvery Suture Zone, southern India comprises a well preserved ophiolitic sequence of ultramafic cumulates of altered dunites, pyroxenites, mafic cumulates of gabbros, gabbro-norites and anorthosites in association with plagiogranites, isotropic gabbros, metadolerites, metabasalts/amphibolites and thin layers of ferruginous chert bands. The isotropic gabbros occur as intrusions in association with gabbroic anorthosites, plagiogranite and metabasalts/amphibolites. The gabbros are medium to fine grained with euhedral to subhedral orthopyroxenes, clinopyroxenes and subhedral plagioclase, together with rare amphiboles. Mineral chemistry of isotropic gabbros reveal that the clinopyroxenes are diopsidic to augitic in composition within the compositional ranges of En(42-59), Fs(5-12), Wo(31-50). They are Ca-rich and Na poor (Na2O < 0.77 wt%) characterized by high-Mg (Mg# 79-86) and low-Ti (TiO2 < 0.35 wt%) contents. The tectonic discrimination plots of clinopyroxene data indicate island arc signature of the source magma. Our study further confirms the suprasubduction zone origin of the Manamedu ophiolitic suite, associated with the subduction-collision history of the Neoproterozoic Mozambique ocean during the assembly of Gondwana supercontinent.

  11. Geochemistry and tectonic significance of the Gongzhu peridotites in the northern branch of the western Yarlung Zangbo ophiolitic belt, western Tibet

    NASA Astrophysics Data System (ADS)

    Lian, Dongyang; Yang, Jingsui; Liu, Fei; Wu, Weiwei; Zhang, Li; Zhao, Hui; Huang, Jian

    2017-01-01

    The Gongzhu ophiolite is situated in the northern branch of the western Yarlung Zangbo ophiolitic belt. This massif consists of a strongly dismembered ophiolitic sequence dominated by mantle peridotites. The peridotites comprise lherzolite with low- to moderately-depleted mineral and bulk rock compositions. The degree of partial melting deduced from Cr# values of the Gongzhu peridotites varies between 7% and 10%. The mineral and whole rock compositions of the Gongzhu peridotites are comparable to those of abyssal peridotites. The chondrite normalized REE compositions of the peridotites typically display U-shaped or spoon-shaped patterns, and primitive mantle-normalized PGEs patterns show Ir depletion relative to Os and Ru, and Pt enrichment relative to Rh and Pd. On the basis of the petrological, mineralogical and geochemical data, we concluded that the Gongzhu peridotites either formed in the back-arc setting of an intra-oceanic subduction system or the Gongzhu and Dajiweng peridotites both formed in the in the same incipient forearc/proto-forearc environment of an intra-oceanic subduction zone.

  12. Formation, obduction and provenance of the Støren-Bymarka-Løkken ophiolite, Central Norwegian Caledonides; constraints from geochronology, geochemistry, Sm-Nd and Lu-Hf data

    NASA Astrophysics Data System (ADS)

    Slagstad, T.; Pin, C.; Roberts, D.; Kirkland, C. L.; Grenne, T.; Dunning, G.; Sauer, S.; Andersen, T.

    2012-04-01

    Suprasubduction-zone ophiolites are a characteristic feature of the Palaeozoic Caledonian-Appalachian orogenic belt, and mark the onset of convergence and closure of one or more oceans separating the Baltican, Laurentian and Avalonian continents, ending with continent-continent collision in the Mid Palaeozoic. The Bymarka-Løkken ophiolite in the Central Norwegian Caledonides is a variably tectonised ophiolite fragment, locally with an incomplete, but nevertheless well-preserved ophiolite pseudostratigraphy. Previous work has concluded that the ophiolite formed in an Early Ordovician suprasubduction-zone environment, most likely in an oceanic arc/back-arc basin system, but little is know about the evolution of the ophiolitic crust. There has also been some debate as to the whether obduction of the ophiolite upon the subjacent Gula Complex was onto Laurentia, Baltica, or a microcontinent of Baltican affinity. Here we present new, high-precision TIMS and SHRIMP zircon analyses from felsic rocks in the ophiolite. Combined with geochemical and Sm-Nd whole-rock and Lu-Hf zircon analyses from the same rocks, these data allow us to elucidate the timing of various stages in the evolution of the ophiolite. Plagiogranite bodies range in age from 493 to 480 Ma and have relatively juvenile isotopic compositions. Geochemical data suggest subduction-zone influence and we interpret this stage to represent formation of the ophiolite in an oceanic back-arc setting. At 480 Ma, a large granitoid body with an unradiogenic isotopic composition and strong subduction-zone geochemical signature intruded the ophiolite. We interpret this stage to reflect convergence in the back-arc basin and formation or migration of an oceanic arc. The unradiogenic isotopic composition probably reflects subduction of back-arc basin crust with sediments derived from the nearby continent or microcontinent. At 480-470 Ma, a greenstone-dominated conglomerate and an overlying volcaniclastic sequence was

  13. Major Element Geochemistry of Peridotites from Santa Elena Ophiolite Complex, NW Costa Rica and Their Tectonic Implications

    NASA Astrophysics Data System (ADS)

    Wright, S.; Snow, J. E.; Gazel, E.; Sisson, V.

    2010-12-01

    The Santa Elena Ophiolite Complex (SEOC) is located on the west coast of Northern Costa Rica, near the Nicaraguan border. It consists primarily of preserved oceanic crustal rocks and underlying upper mantle thrust onto an accretionary complex. The petrogenesis and tectonic origin of this complex have widely been interpreted to be either a preserved mantle portion of the Caribbean Large Igneous Province (CLIP) as it drifted between North and South America from the Galapagos hotpot into the present day Caribbean Ocean around 80 Ma or as the mantle section to the nearby Nicoya complex. Previous structural work suggests that SEOC is a supra-subduction complex, not related to the CLIP or Nicoya. Our preliminary results agree. Mantle peridotites collected from the Santa Elena Ophiolite Complex consist primarily of spinel lherzolite (61 %) with minor amounts of harzburgite and dunite (22 % and 16 % respectively). Spinel Cr# [molar Cr / (Cr+Al)*100] is widely accepted to constrain mantle partial melting and lithospheric melt stagnation. Cr# of spinels within Santa Elena lherzolites fall between 12 and 35, suggesting an extent of 3 % to 13 % partial melting. Cr# of harzburgites range from 35 to 39, suggesting 13 % to 14 % partial melting. This range of partial melting suggests only modest depletion of this exposed portion of the ancient uppermost mantle. TiO2 concentrations of the lherzolite and harzburgite range from 0.004% to 0.128%, with the exception of one sample, SE10 - 17 (0.258%), and fall within the normal melting trend for mantle peridotites. The presence of dunite indicates that melt flow and associated melt - rock reaction with the surrounding peridotite took place within this portion of the mantle. A Cr# of 84.5 from one of these dunite samples indicate that significant melt rock reaction with refractory melts took place. Such results are rarely found in mid-ocean ridge abyssal peridotite settings, and are currently found primarily in forearc tectonic settings

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

    NASA Astrophysics Data System (ADS)

    Nomoto, Y.; Takazawa, E.

    2013-12-01

    The boninites in the Oman ophiolite occur as lavas and dikes of the Alley volcanic sequence (Ishikawa et al., 2002). Moreover, Yamazaki and Miyashita (2008) reported about boninitic dike swarms in the Fizh crustal section. The boninitic melt generation requires hydrous melting of refractory mantle peridotite under an extremely high temperature and low pressure condition. This condition is generally explained by the addition of slab-derived fluids into a hot young oceanic lithosphere, which previously experienced MORB melt extraction. In this study, we report an ultramafic complex mainly composed of dunite which is in equilibrium with chemical composition of boninites in the southwestern part of the Salahi mantle section in the northern Oman ophiolite. Based on the study by Nomoto and Takazawa (2013) the complex consists mainly of massive dunite associated with minor amounts of harzburgite, pyroxenites and wehrlite. We use spinel Cr# (=Cr/[Cr+Al] atomic ratio) as an indicator of extent of melt extraction in harzburgites. For dunites spinel Cr# varies as a function of extent of reaction and of melt composition (Dick and Bullen, 1984; Arai, 1994; Ozawa, 2008). The spinels in the dunites from the complex have Cr# greater than 0.7 indicating highly refractory signature. The range of spinel Cr# is similar to those of spinels in boninites reported worldwide (Umino, 1986; van der Laan et al., 1992; Sobolev and Danyushevsky, 1994; Ishikawa et al., 2002). The complex might be a section of dunite channel that formed by flux melting of harzburgites as a result of infiltration of a voluminous fluid from the basal thrust. We determined the abundances of rare earth elements (REE) in the peridotite clinopyroxenes (cpxs) by LA-ICP-MS to estimate the compositions of the melts in equilibrium with these clinopyroxenes. The chondrite-normalized patterns for clinopyroxenes in the dunites are characterized by enrichments in light REE (LREE) relative to those of the harzburgite

  15. 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

  16. Os Isotopic Composition and Highly Siderophile Elements: Tracers of Mantle Melting and Melt Percolation Processes (Voykar Complex, Polar Ural Ophiolites)

    NASA Astrophysics Data System (ADS)

    Batanova, V.; Bruegmann, G.; Savelieva, G.

    2006-12-01

    The Voykar complex is located in the Northern part of the Uralian ophiolite belt and represents Early Devonian lithosphere formed in a suprasubduction environment (e. g. (1)). It is unique in that the mantle peridotites are very well preserved and virtually free of serpentine, and it provides excellent exposures. The mantle section of Voykar complex is composed of harzburgite which is intruded by numerous dunite and pyroxenite channels and contains chromitite ore deposits. The distribution of highly siderophile elements (HSE) in harzburgite and dunite channels can be explained by melt-peridotite reaction processes. The harzburgites (Cr/(Cr+Al) of cr-spinel = 0.3-0.4) show two types of HSE patterns. One is depleted in Pd, Pt and Re relatively to Os, Ir, Ru with (Pt/Ir)N=0.3, the second pattern is flat with (Pt/Ir)N=0.9. The first type of harzburgite has lower ratios of 187Os/188Os (0.1149) compared to the second (0.1236). Dunite and associated orthopyroxenite are significantly enriched in 187Os (187Os/188Os=0.1279 - 0.1327), hence shifted towards lava compositions. Their HSE patterns also show the influence of mantle melts, because they are enriched in Pd ((Pt/Ir)N=5-6). The clinopyroxenite has the highest ratio of 187Os/188Os up to 32.38 and shows even stronger depletion of Os, Ir, Ru relative to Pt, Pd, Re ((Pt/Ir)N=20). They could represent crystallization products of percolating melts. Chromitites have variable 187Os/188Os=0.1244-0.1352, and their HSE concentrations overlap with the range observed in chromitites world-wide. Osmium isotope data show evidence for at least two significant events in the magmatic history of the Voykar ophiolite mantle section: an ancient (2.1-1.9 Ga) melting event formed the depleted spinel harzburgite; a younger melt percolation event (0.6-0.5 Ga) led to the formation of pyroxenite veins, dunite channels and chromitites. The age of melt percolation events based on Os isotopic data coincides with the U-Pb age of zircons found in

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

    New geochemical and microstructural data from the Marum ophiolite in Papua New Guinea describe a piece of most depleted mantle made essentially of dunite and harzburgite showing compositions of supra-subduction zone (SSZ) peridotite. Strong olivine crystallographic preferred orientations (CPO) in dunite and harzburgite inferred the activation of both (001)[100] and (010)[100] slip systems. Clinopyroxene and orthopyroxene CPOs inferred the activation of (100)[001] and (010)[001] slip systems. This plastic deformation is interpreted to have developed at high temperature 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 polarization direction parallel to the arc. Marum depleted mantle has been fertilised by diffuse crystallisation of a low proportion of clinopyroxene (1-2%) in the dunite and formation of cm-scale ol-clinopyroxenite and ol-websterite veins cross-cutting the foliation. 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; these are interpreted to form after the boninitic event. The small proportion of newly crystallized pyroxene distributed 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 to 60˚ to olivine <100> axes. For low melt proportion, such as crystallization of pyroxenes in the dunite, the crystallization is governed by epitaxial growth, and when the proportion of melt is higher the newly formed

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

    USGS Publications Warehouse

    Bacuta, G.C.; 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

  19. Lithological, Chemical and Chronological Constraints on Melt Extraction from the Mantle Section of the ~492 Ma Shetland Ophiolite Complex, Scotland

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

    The mantle sections of ophiolites offer a means of studying the composition and structure of the oceanic mantle. In particular, the relations between different lithologies can be established in the field, permitting an assessment of the relative timing of processes such as melt extraction and melt-rock reaction. The Shetland Ophiolite Complex (SOC) contains a well-preserved mantle section that is dominated by harzburgite (≥70 vol.%), with dominantly chondritic present-day 187Os/188Os compositions1. Melt extraction and melt-rock reaction is evident in the form of dunite and chromitite layers and lenses, with thicknesses ranging from millimetres-to-metres. These lithologies are characteristic of supra-subduction zone processing and are considered to relate to closure of the Iapetus Ocean at ~492 Ma1. However, evidence of much earlier melt extraction has been suggested for some SOC harzburgites, which have relatively unradiogenic 187Os/188Os compositions that yield TRD model ages as old as ~1.4 Ga1. In order to assess the scales at which such compositional heterogeneities are preserved in the mantle, a small (45 m2) area of the SOC mantle section was selected for detailed lithological mapping and sampling. A selection of harzburgites (n=8), dunites (n=6) and pyroxenites (n=2) from this area has been analysed for their Os isotope and highly-siderophile element (HSE) compositions. Six of the harzburgites and four of the dunites have relative HSE abundances and gOs values that are approximately chondritic, with gOs ranging only from -0.6 to +2.7 (n=10). Two dunites have more radiogenic gOs (up to +7.5), that is correlated with enhanced concentrations of accessory base-metal sulphides, suggesting formation via melt percolation and melt-rock reaction. The two remaining harzburgites have less radiogenic gOs (-3.5 and -4), yielding Mesoproterozoic TRD ages. The new data indicate that a comparable range of Os isotope compositions to that previously measured across the

  20. Late Cretaceous to Late Eocene Hekimhan Basin (Central Eastern Turkey) as a supra-ophiolite sedimentary/magmatic basin related to the later stages of closure of Neotethys

    NASA Astrophysics Data System (ADS)

    Booth, Matthew G.; Robertson, Alastair H. F.; Tasli, Kemal; İnan, Nurdan

    2014-11-01

    The Hekimhan Basin is here put forward as a type example of a globally important class of basin, known as a supra-ophiolite basin. Such basins form after the emplacement of ophiolitic (i.e. oceanic) rocks onto a passive continental margin, but long prior to continental collision. The Hekimhan Basin developed as part of the northern margin of the Tauride microcontinent during the collision and suturing of two Neotethyan oceans to the north, namely the Inner Tauride Ocean and the İzmir-Ankara-Erzincan ocean. The basin records two main stages of tectonic development, during latest Cretaceous to Late Eocene time. The first phase of basin development during the Late Cretaceous (Maastrichtian) began with the erosion of emplaced ophiolitic rocks, resulting in non-marine clastic sedimentation. Subsequently, the basin rapidly subsided, in response to inferred regional crustal extension, resulting in the deposition of hemipelagic marls and local sapropelic mudstones. The axial parts of the basin experienced alkaline, within-plate-type, basaltic volcanism. The Late Maastrichtian culminated in deposition of shallow-marine carbonates. Overlying Paleocene sediments are restricted to thin, localised, marine evaporates, associated with a low-angle unconformity. The second stage of basin development began during the Early Eocene with deposition of shallow-marine carbonates, coupled with localised basaltic volcanism, again of extensional type. The basin emerged during the Mid-Late Eocene in a late-stage collisional to post-collisional setting. Compressional deformation largely reflects post-suture tightening. A short-lived marine transgression occurred during the Mid-Miocene. The basin was later deformed by both left-lateral and right-lateral strike-slip. Several different tectonic models are considered, notably extension related to the northward pull of a still-subducting oceanic slab, and back-arc extension related to northward subduction of Neotethys (to the south). The first

  1. Sr-Nd-Hf isotopes of the intrusive rocks in the Cretaceous Xigaze ophiolite, southern Tibet: Constraints on its formation setting

    NASA Astrophysics Data System (ADS)

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

    2016-08-01

    The Cretaceous Xigaze ophiolite is best exposed at the central part of the Yarlung-Zangbo Suture Zone, Tibet Plateau. It consists of a thick section of mantle peridotites, but a relatively thin mafic sequence. This study presents geochronological and geochemical data for intrusive dykes (both mafic and felsic) and basalts to revisit the formation setting of the Xigaze ophiolite. The rodingites are characterized by high CaO and low Na2O contents relative to mafic dykes and show big variations in trace element compositions. Both gabbros and diabases have similar geochemical compositions, with MgO contents of 6.42-11.48 wt% and Mg# of 0.56-0.71. They display REE patterns similar to N-MORB and are variably enriched in large ion lithophile elements. Basalts have fractionated compositions and display LREE-depleted patterns very similar to N-MORB. They do not show obvious enrichment in LILE and depletion in high-field-strength elements, but a negative Nb anomaly is present. The studied plagiogranites have compositions of trondhjemite to tonalite, with high Na2O and low K2O contents. They have low TiO2 contents less than 1 wt%, consistent with melts formed by anatexis of gabbros rather than by differentiation of basalts. Zircons from seven samples, including three rodingites, three plagiogranites, and one gabbro, have been dated and yielded U-Pb ages of 124.6 ~ 130.5 Ma, indicating the Xigaze ophiolite was formed during the Early Cretaceous. They have mantle-like δ18O values of + 4.92 ~ + 5.26‰ and very positive εHf(t) values of + 16 ~ + 13.3. Ages of the rodingites and less altered gabbros indicate that serpentinization was occurred at ~ 125 Ma. Occurrence of both gabbroic and diabase dykes within the serpentinites suggests that the mantle lithosphere of the Xigaze ophiolite was rapidly exhumed. Both mafic and felsic dykes have slightly more radiogenic 87Sr/86Sr ratios relative to MORB, but depleted Hf-Nd isotpe compositions. They have a limited range of ε

  2. 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.

  3. Petrology and oxygen isotope geochemistry of a fossil seawater hydrothermal system within the Solea graben, northern Troodos ophiolite, Cyprus

    NASA Astrophysics Data System (ADS)

    Schiffman, Peter; Smith, Brian M.

    1988-05-01

    Hydrothermal mineral zonations and O isotope patterns of the northern Troodos complex do not parallel the ophiolite pseudostratigraphy, but reflect the convective geometry of an Upper Cretaceous seawater hydrothermal system. Large areas of the sheeted intrusive complex (SIC), including the subaxial region of the Solea graben, are composed of 18O-rich, subgreenschist mineral assemblages and may represent regions of diffuse seawater recharge. Other areas of the SIC are recrystallized to distinctive epidosite rocks: granular, high-variance assemblages of epidote + quartz ± chlorite that are depleted in 18O, Al2O3, Na2O, K2O, Zr, Cu, and Zn and are enriched in CaO and Sr compared with other mafic volcanic and dike rocks of the Solea graben. Epidosite alteration occurred at temperatures of ˜310-370°C and involved fluids with δ18O values and salinities similar to those of Upper Cretaceous seawater. The epidosite zones are discordant with earlier, mineral/O isotope zonations and with the axis of spreading in the Solea graben, suggesting a postspreading, off-axis origin. The seawater hydrothermal system responsible for Solea graben massive sulfide deposits was probably driven by hypabyssal intrusions (not exposed), emplaced in a terminal, failed spreading episode. The geometries of O isotope surfaces within the Solea graben imply that the epidosites formed in fossil upflow and deep recharge conduits. Depletions in base metals show that epidosite alteration liberated Cu and Zn to mineralizing fluids within the fossil upflow zone.

  4. Relation between ore-forming hydrothermal systems and extensional deformation in the Solea graben spreading center, Troodos ophiolite, Cyprus

    NASA Astrophysics Data System (ADS)

    Bettison-Varga, Lori; Varga, Robert J.; Schiffman, Peter

    1992-11-01

    Field relations indicate that high-temperature hydrothermal circulation and accumulation of massive sulfide deposits within the Solea graben of the Troodos ophiolite, Cyprus, followed extreme crustal attenuation. Zones of pervasive, massive epidosite strike parallel to the axis of the Solea graben and to the strike of extensional normal faults. Initial fluid flow, evidenced by preferential epidotization in weakly altered areas surrounding massively altered regions, was focused along joints, microfractures, and (now) low-angle normal-fault zones related to graben formation. Permeability within the sheeted-dike section was enhanced by brittle deformation related to extensional structures as well as through volume reduction inherent in the diabase to epidosite mineralogic phase transformations. Intrusion of high-level gabbros into epidosite zones occurred both before and after significant amagmatic tectonic extension. Structural control on epidotization suggests that intrusion of late stocks into attenuated and highly deformed crust is necessary to drive the vigorous hydrothermal circulation that produced the epidosites and ore bodies of the Solea graben. A similar sequence of events is more likely to occur in the modern oceans along ridge crests with ephemeral magmatism, especially at intermediate- to slow-spreading ridges near transform faults.

  5. 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

  6. High-pressure experiments provide insights into the Mantle Transition Zone history of chromitite in Tibetan ophiolites

    NASA Astrophysics Data System (ADS)

    Zhang, Yanfei; Jin, Zhenmin; Griffin, William L.; Wang, Chao; Wu, Yao

    2017-04-01

    The chromitites in the Luobusha ophiolite (Tibet) have been proposed as messengers from the deep mantle. The exsolution of diopside, MgSiO3 and coesite in chromite from these bodies has been used to argue that the chromite previously existed as a phase with the CaFe2O4 structure (idealized composition (Mg, Fe)(Cr, Al)2O4), an indicator of Mantle Transition Zone (MTZ) conditions. However, evidence about the stability of the CaFe2O4-structured phase and its formation mechanism are limited. Here we present experimental evidence for the depth of metamorphism and subsequent petrogenesis of the podiform chromitite. The CaFe2O4-structured phase was observed at ∼14-18 GPa, and can contain several weight percent of CaO and SiO2. Partial-melting experiments show that chromite cannot be formed through direct crystallization in the MTZ. Therefore, we suggest that the Tibetan chromitites formed under shallow conditions, were then subducted, and were metamorphosed near the top of MTZ. During the tectonics/buoyancy-driven ascent of the enclosing peridotites, the CaFe2O4-phase transformed to chromite at depths of ∼400 km, accompanied by the simultaneous exsolution of diopside and other phases, which were then preserved during transportation to shallow depths. These observations provide a new window into the processes of mantle geodynamics, and constraints on mantle convection in major collision zones.

  7. 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.

  8. The Chenaillet Ophiolite in the French/Italian Alps: An ancient analogue for an Oceanic Core Complex?

    NASA Astrophysics Data System (ADS)

    Manatschal, Gianreto; Sauter, Daniel; Karpoff, Anne Marie; Masini, Emmanuel; Mohn, Geoffroy; Lagabrielle, Yves

    2011-06-01

    The Chenaillet Ophiolite in the Franco-Italian Alps represents a well-exposed ocean-floor sequence that was only weakly overprinted by Alpine metamorphism during its emplacement in the Alpine nappe stack. Pillow lavas showing a Mid Ocean Ridge Basalt (MORB) signature overlie tectonically exhumed gabbros and serpentinized mantle rocks similar to oceanic core complexes (OCC) described from present-day slow- to ultraslow-spreading ridges. Based on detailed mapping it can be shown that: 1) syn-magmatic high-temperature shear zones are truncated by oceanic detachment faults; 2) sediments consisting exclusively of clasts derived from the footwall of the detachment are overlain by voluminous lavas, and 3) emplacement of lavas is linked with high-angle faulting that overprint the oceanic detachment fault. Based on stratigraphic and cross cutting relationships we conclude that: 1) a complex relation between high- and low-temperature shear zones exists, 2) oceanic detachment faults may be obliterated by later emplaced of MOR basalts, and 3) high-angle faults may serve at shallow levels as feeder channels for the overlying pillow basalts. These observations raise questions about the relative relations in time and space between magmatic, hydrothermal and tectonic processes at spreading systems. We propose that the observed change from exhumation along an oceanic detachment fault to high-angle faulting associated with extrusion of volcanic rocks may represent an evolution within a life cycle of a spreading system that may be comparable to present-day slow- to ultraslow-spreading ridges.

  9. Mineral chemistry and petrology of mantle peridotites from the Guleman ophiolite (SE Anatolia, Turkey): Evidence of a forearc setting

    NASA Astrophysics Data System (ADS)

    Rizeli, Mustafa Eren; Beyarslan, Melahat; Wang, Kuo-Lung; Bingöl, A. Feyzi

    2016-11-01

    The mantle section of Guleman ophiolite, southeast (SE) Turkey consists mainly of harzburgites and dunite lenses and large chromitite pods. The average Cr ratio = [100 × Cr/(Cr + Al) atomic ratio] of Cr-spinels in harzburgites and dunites is remarkably high (>63). The forsterite (Fo) content of olivine is between 90.9 and 92.3 in harzburgites and dunites. These features indicate that the harzburgites and dunites resulted from >35% of partial melting of a depleted mantle source. Discriminant geochemical diagrams based on the mineral chemistry of harzburgites indicate a supra-subduction zone (SSZ) origin. Orthopyroxene and clinopyroxene from the Guleman harzburgites have low CaO, Al2O3 and TiO2 contents, resembling those of depleted harzburgites from modern forearcs and contrasting with moderately depleted abyssal peridotites. Consequently, we propose that the Guleman peridotites formed in a forearc setting during the subduction initiation that developed as a result of northward subduction of the southern branch of the Neo-Tethys in response to the convergence between the Arabian and Anatolian plates.

  10. 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.

  11. Iron isotopic fractionation and origin of chromitites in the paleo-Moho transition zone of the Kop ophiolite, NE Turkey

    NASA Astrophysics Data System (ADS)

    Zhang, Peng-Fei; Zhou, Mei-Fu; Su, Ben-Xun; Uysal, Ibrahim; Robinson, Paul T.; Avcı, Erdi; He, Yong-Sheng

    2017-01-01

    The paleo-Moho transition zone (MTZ) of the Kop ophiolite in NE Turkey is mainly composed of dunites, which are locally interlayered with chromitites and contain minor relics of harzburgites. Large Fe isotopic variations were observed for magnesiochromite (- 0.14 to 0.06‰) and olivine (- 0.12 to 0.14‰) from the MTZ rocks. In individual samples, magnesiochromite has lighter Fe isotopic compositions than olivine, which was probably caused by subsolidus Mg-Fe exchange between them. Both magnesiochromite and olivine display an increasing trend of δ56Fe along a profile from chromitite to dunite. This trend reflects continuous fractional crystallization in a magma chamber, which resulted in heavier Fe isotopes concentrated in the evolved magmas. In each cumulate cycle of chromitite and dunite, dunite was formed from relatively evolved melts after massive precipitation of magnesiochromite. Mixing of more primitive and evolved melts in the magma chamber was a potential mechanism for triggering the crystallization of magnesiochromite, generating chromitite layers in the cumulate pile. Before mixing happened, the primitive melts had reacted with mantle harzburgites during their ascendance; whereas the evolved melts may lie on the olivine-chromite cotectic near the liquidus field of pyroxene. Variable degrees of magma mixing and differentiation are expected to generate melts with different δ56Fe values, accounting for the Fe isotopic variations of the Kop MTZ.

  12. 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).

  13. A Review on Forearc Ophiolite Obduction, Adakite-Like Generation, and Slab Window Development at the Chile Triple Junction Area: Uniformitarian Framework for Spreading-Ridge Subduction

    NASA Astrophysics Data System (ADS)

    Bourgois, Jacques; Lagabrielle, Yves; Martin, Hervé; Dyment, Jérôme; Frutos, Jose; Cisternas, Maria Eugenia

    2016-10-01

    This paper aggregates the main basic data acquired along the Chile Triple Junction (CTJ) area (45°-48°S), where an active spreading center is presently subducting beneath the Andean continental margin. Updated sea-floor kinematics associated with a comprehensive review of geologic, geochemical, and geophysical data provide new constraints on the geodynamics of this puzzling area. We discuss: (1) the emplacement mode for the Pleistocene Taitao Ridge and the Pliocene Taitao Peninsula ophiolite bodies. (2) The occurrence of these ophiolitic complexes in association with five adakite-like plutonic and volcanic centers of similar ages at the same restricted locations. (3) The inferences from the co-occurrence of these sub-coeval rocks originating from the same subducting oceanic lithosphere evolving through drastically different temperature-pressure ( P- T) path: low-grade greenschist facies overprint and amphibolite-eclogite transition, respectively. (4) The evidences that document ridge-jump events and associated microplate individualization during subduction of the SCR1 and SCR-1 segments: the Chonos and Cabo Elena microplates, respectively. The ridge-jump process associated with the occurrence of several closely spaced transform faults entering subduction is controlling slab fragmentation, ophiolite emplacement, and adakite-like production and location in the CTJ area. Kinematic inconsistencies in the development of the Patagonia slab window document an 11- km westward jump for the SCR-1 spreading segment at ~6.5-to-6.8 Ma. The SCR-1 spreading center is relocated beneath the North Patagonia Icefield (NPI). We argue that the deep-seated difference in the dynamically sustained origin of the high reliefs of the North and South Patagonia Icefield (NPI and SPI) is asthenospheric convection and slab melting, respectively. The Chile Triple Junction area provides the basic constraints to define the basic signatures for spreading-ridge subduction beneath an Andean

  14. Peridotites of the Izu-Bonin-Mariana forearc and the Eastern Mirdita ophiolite (Albania) : implications for igneous activity during subduction initiation

    NASA Astrophysics Data System (ADS)

    Morishita, T.; Tani, K.; Dilek, Y.

    2011-12-01

    There have been few studies of the mantle evolution related to igneous activity in the earliest stages of subduction initiation. We examined peridotites recovered from an exhumed crust/mantle section exposed along the landward slopes of the northern Izu-Bonin Trench (Morishita et al., Geology, 2011) and peridotite bodies in the Eastern Mirdita ophiolite, Albania (Morishita et al., Lithos, 2011). Based on the Cr# (=Cr/(Cr+Al) atomic ratio) of spinel in the IBM, two distinctive groups, (1) High-Cr# (> 0.8) dunite and (2) Medium-Cr# (0.4-0.6) dunite, occur close to each other and are associated with refractory harzburgite. Two distinctive melts were in equilibrium with these dunites. In the case of the Eastern Mirdita ophiolite, cpx porphyroclast-bearing harzburgite (Cpx-harzburgite) occurs structurally in the lower parts of the peridotite massifs, whereas harzburgite and dunite are more abundant towards the upper parts. The Cpx-harzburgite was formed as the residue of less-flux partial melting, which are similar to those in abyssal peridotites from MOR systems. On the other hand, harzburgite was produced as a result of enhanced partial melting of depleted peridotites due to infiltration of hydrous LREE-enriched fluids/melts. The wide range of variation in dunites from the IBM forearc and the uppermost section of the EMO probably reflects changing melt compositions from MORB-like melts to boninitic melts in the forearc setting due to an increase of slab-derived hydrous fluids/melts during subduction initiation. This scenario is consistent with the temporal and spatial variation of volcanic rocks in the Izu-Bonin-Mariana (IBM) arc (Reagan et al., G-cubed, 2010). If the "MORB-like" FAB is a ubiquitous phenomenon during the initiation of subduction, we should reconsider our interpretation of the ophiolites.

  15. 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.

  16. Subduction-related prograde metamorphism of the ultramafic members of the Central-Sudetic Ophiolite (SW Poland)

    NASA Astrophysics Data System (ADS)

    Wojtulek, Piotr; Puziewicz, Jacek; Ntaflos, Theodoros

    2016-04-01

    The Central-Sudetic Ophiolite (CSO) consists of Ślęża (SM), Braszowice-Brzeźnica (BBM), Szklary (SZM) and Nowa Ruda massifs. Ultramafic rocks occurring in ŚM, BBM and SM have MgO/SiO2 (0.82-1.20) and Al2O3/SiO2 (~0.01) ratios typical for serpentinized mantle peridotites. They are enriched in Cs, Pb and Sb and depleted in Rb, Ba, Nb, La, Ce, Sr, Zr, Er and Y relative to primitive mantle. The serpentinites are antigorite ones, pseudomorphic chrysotile varieties occur sparsely. Serpentinites from each massif contain specific non-serpentine phases. Ślęża serpentinites contain primary olivine-chromite aggregates, olivine and clinopyroxene aggregates interpreted as basaltic melt percolation phases, secondary olivine with magnetite inclusions (locally with cleavage) and secondary microcrystalline olivine-clinopyroxene-magnetite aggregates ("brownish aggregates") with bastite and mesh textures. The BBM serpentinites contain primary olivine-chromite aggregates, primary diopside grains, secondary magnetite-bearing olivine and tremolite. The SZM serpentinites contain olivine, tremolite and enstatite grains. Enstatite (Mg# = 92.8-93.0) contains >0.2 wt.% Cr2O3 and >0.7 Al2O3. All secondary non-serpentine phases are intergrown by antigorite. Very low overall trace element contents, Cs and high Pb-Sb anomalies of the CSO serpentinites are similar to subduction zone related serpentinites not affected by later fluid refertilization. Mineral assemblages shows prograde alteration of the rocks: (1) low-T serpentinization I forming pseudomorphic lizardite-chrysotile serpentinites; (2) antigorite recrystallization; (3) deserpentinization forming secondary olivine with magnetite inclusions, "brownish structures", tremolite and/or enstatite; (4) high-T serpentinization II forming antigorite intergrowths. Alteration degree is different in each massif: rocks from the SM are the most altered, they contain antigorite-olivine-enstatite-tremolite assemblage typical for temperatures

  17. 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

  18. Tectonic development of the Samail ophiolite: High-precision U-Pb zircon geochronology and Sm-Nd isotopic constraints on crustal growth and emplacement

    NASA Astrophysics Data System (ADS)

    Rioux, Matthew; Bowring, Samuel; Kelemen, Peter; Gordon, Stacia; Miller, Robert; DudáS, Frank

    2013-05-01

    New high-precision single grain U-Pb zircon geochronology and whole rock Nd isotopic data provide insight into the magmatic and tectonic development of the Samail ophiolite. The analyzed rocks can be broadly divided into two groups based on their structural position, dates, and isotopic composition: an older group related to on-axis magmatism and a younger group of post-ridge dikes, sills, and stocks. On-axis gabbros, tonalites and trondhjemites yielded Th-corrected 206Pb/238U dates from 96.441 ± 0.062 to 95.478 ± 0.056 Ma. These dates, combined with dates from Rioux et al. (2012), suggest that most of the ophiolite crust formed at an oceanic spreading center in <1 Ma. The post-ridge intrusions come from all depths in the crust, the upper mantle, and the metamorphic sole. Post-ridge gabbros, tonalites, and trondhjemites from the crust and mantle yielded Th-corrected 206Pb/238U dates of 95.405 ± 0.062 to 95.077 ± 0.062 Ma. A small trondhjemitic pod from the metamorphic sole yielded younger Th-corrected 206Pb/238U dates of 94.90 ± 0.38 to 94.69 ± 0.12 Ma. Isotopic data suggest two distinct sources for the post-ridge magmas: five of the gabbros and tonalites from the crust have ɛNd(96 Ma) = 6.90 ± 0.12 to 7.88 ± 0.16, and two trondhjemites from the upper mantle and metamorphic sole have ɛNd(96 Ma) = -7.77 ± 0.08 and -7.01 ± 0.16. The negative ɛNd(t) and U-Pb dates from the mantle dike require that subduction or thrusting was established below the ophiolite ≤0.25-0.5 Ma after formation of the crust. The bimodal isotopic composition of post-ridge magmas may reflect coeval decompression and/or fluid fluxed melting of the mantle and melting, dehydration, or assimilation of sediment in the down going plate at this time. The new data place temporal constraints on mid-ocean ridge and supra-subduction zone models for ophiolite formation.

  19. 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

  20. Constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the Samail ophiolite

    NASA Astrophysics Data System (ADS)

    VanTongeren, J. A.; Hirth, G.; Kelemen, P. B.

    2015-10-01

    Oceanic crust represents more than 60% of the earth's surface and despite a large body of knowledge regarding the formation and chemistry of the extrusive upper oceanic crust, there still remains significant debate over how the intrusive gabbroic lower oceanic crust is accreted at the ridge axis. The two proposed end-member models, the Gabbro Glacier and the Sheeted Sills, predict radically different strain accumulation in the lower crust during accretion. In order to determine which of these two hypotheses is most applicable to a well-studied lower crustal section, we present data on plagioclase lattice preferred orientations (LPO) in the Wadi Khafifah section of the Samail ophiolite. We observe no systematic change in the strength of the plagioclase LPO with height above the crust-mantle transition, no dominant orientation of the plagioclase a-axis lineation, and no systematic change in the obliquity of the plagioclase LPO with respect to the modal layering and macroscopic foliation evident in outcrop. These observations are most consistent with the Sheeted Sills hypothesis, in which gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with increasing depth in a Gabbro Glacier. Our data support the hypothesis of MacLeod and Yaouancq (2000) that the rotation of the outcrop-scale layering from sub-horizontal in the layered gabbros to sub-vertical near the sheeted dikes is due to rapid vertical melt migration through upper gabbros close to the axial magma chamber. Additionally, our results support the hypothesis that the majority of extensional strain in fast spreading ridges is accommodated in partially molten regions at the ridge axis, whereas in slow and ultra-slow ridges large shear strains are accommodated by plastic deformation.

  1. Multi-stage melt-rock interaction in the Mt. Maggiore (Corsica, France) ophiolitic peridotites: microstructural and geochemical evidence

    NASA Astrophysics Data System (ADS)

    Rampone, Elisabetta; Piccardo, Giovanni B.; Hofmann, Albrecht W.

    2008-10-01

    Spinel and plagioclase peridotites from the Mt.Maggiore (Corsica, France) ophiolitic massif record a composite asthenosphere-lithosphere history of partial melting and subsequent multi-stage melt-rock interaction. Cpx-poor spinel lherzolites are consistent with mantle residues after low-degree fractional melting ( F = 5-10%). Opx + spinel symplectites at the rims of orthopyroxene porphyroclasts indicate post-melting lithospheric cooling ( T = 970-1,100°C); this was followed by formation of olivine embayments within pyroxene porphyroclasts by melt-rock interaction. Enrichment in modal olivine (up to 85 wt%) at constant bulk Mg values, and variable absolute REE contents (at constant LREE/HREE) indicate olivine precipitation and pyroxene dissolution during reactive porous melt flow. This stage occurred at spinel-facies depths, after incorporation of the peridotites in the thermal lithosphere. Plagioclase-enriched peridotites show melt impregnation microtextures, like opx + plag intergrowths replacing exsolved cpx porphyroclasts and interstitial gabbronoritic veinlets. This second melt-rock interaction stage caused systematic chemical changes in clinopyroxene (e.g. Ti, REE, Zr, Y increase), related to the concomitant effects of local melt-rock interaction at decreasing melt mass, and crystallization of small (<3%) trapped melt fractions. LREE depletion in minerals of the gabbronoritic veinlets indicates that the impregnating melts were more depleted than normal MORB. Preserved microtextural evidence of previous melt-rock interaction in the impregnated peridotites suggests that they were progressively uplifted in response to lithosphere extension and thinning. Migrating melts were likely produced by mantle upwelling and melting related to extension; they were modified from olivine-saturated to opx-saturated compositions, and caused different styles of melt-rock interaction (reactive spinel harzburgites, vs. impregnated plagioclase peridotites) depending on the

  2. Geochemical evidence for active tropical serpentinization in the Santa Elena Ophiolite, Costa Rica: An analog of a humid early Earth?

    NASA Astrophysics Data System (ADS)

    Sánchez-Murillo, Ricardo; Gazel, Esteban; Schwarzenbach, Esther M.; Crespo-Medina, Melitza; Schrenk, Matthew O.; Boll, Jan; Gill, Ben C.

    2014-05-01

    is a planetary process that has important consequences on geochemical cycles, supporting microbial activity through the formation of H2 and CH4 and having the potential to sequester atmospheric CO2. We present geochemical evidence of active serpentinization in the Santa Elena Ophiolite, Costa Rica which is sustained by peridotites with a degree of serpentinization less than 50% with no evidence of an internal heat source. Average spring water temperatures are 29.1°C. Two hyperalkaline spring systems were discovered, with a spring fluid pH up to 11.18. The fluids are characterized by low Mg (1.0-5.9 mg/L) and K (1.0-5.5 mg/L) and relative high Ca (29-167 mg/L), Na (16-27 mg/L), Cl (26-29 mg/L), hydroxide (41-63 mg/L), and carbonate (31-49 mg/L). Active CH4 (24.3% v/v) vents coupled with carbonate deposits (δ13CCO2 =-27 to -14‰; δ18OCO2 =-17 to - 6‰) also provide evidence for active serpentinization and carbonation. Isotope ratios of the alkaline fluids (δ18O = -7.9‰, δ2H = -51.4‰) and groundwater (δ18O = -7.6‰; δ2H = -48.0‰) suggests that, during base flow recession, springs are fed by groundwater circulation. Methanogenic Archaea, which comprises a relatively high percentage of the 16S rRNA gene tag sequences, suggests that biological methanogenesis may play a significant role in the system. Santa Elena's extreme varying weather results in a scenario that could be of significant importance for (a) improving the knowledge of conditions on a humid early Earth or Mars that had periodic changes in water supply, (b) revealing new insights on serpentinizing solute transport, and (c) modeling hydrogeochemical responses as a function of recharge.

  3. Lava accretion system around mid-ocean ridges: Volcanic stratigraphy in the Wadi Fizh area, northern Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Kusano, Yuki; Adachi, Yoshiko; Miyashita, Sumio; Umino, Susumu

    2012-05-01

    Detailed lithological study combined with geochemical variations of lavas reveals the across-axis accretionary process at Wadi Fizh in the northern Oman ophiolite. The >900 m thick V1 sequence is divided into the lower V1 (LV1), middle V1 (MV1) and upper V1 (UV1) sequence by 0.4 m and 0.8 m thick umbers at 410 mab (meters above the base of the extrusive rocks) and 670 mab, respectively. The lowest part of the LV1 (LV1a) consists of lobate sheet and pillow lava flows extruded on the relatively flat ridge crest. Elongate pillows at 230 mab are flows draping downslope from the ridge crest and characterize the lithofacies on the ridge flank. Just above a jasper layer at 270 mab, 130 m thick evolved lavas were transported from the crest and emplaced on the ridge flank (LV1b). Off-axial accretionary processes recorded in the MV1 resulted in alternating flows of less evolved, depleted lava and evolved lava, suggesting that the MV1 off-axial lava sequence comprises flows emanated from both on- and off-axis source vents. The less evolved and depleted UV1 flows suggest independent sources distinct from the axial lavas. The Lasail Unit is regarded as a subunit of the V1 because it is comparable to the UV1 in the geological, petrological, and geochemical characteristics. The broad compositional range of the V1 sequence endorses a view that the Wadi Fizh area corresponds to a segment end of the Oman paleospreading system accompanied by off-axis volcanism as in segment boundaries of the present East Pacific Rise.

  4. Subsurface geometry of the San Andreas-Calaveras fault junction: influence of serpentinite and the Coast Range Ophiolite

    USGS Publications Warehouse

    Watt, Janet Tilden; Ponce, David A.; Graymer, Russell W.; Jachens, Robert C.; Simpson, Robert W.

    2014-01-01

    While an enormous amount of research has been focused on trying to understand the geologic history and neotectonics of the San Andreas-Calaveras fault (SAF-CF) junction, fundamental questions concerning fault geometry and mechanisms for slip transfer through the junction remain. We use potential-field, geologic, geodetic, and seismicity data to investigate the 3-D geologic framework of the SAF-CF junction and identify potential slip-transferring structures within the junction. Geophysical evidence suggests that the San Andreas and Calaveras fault zones dip away from each other within the northern portion of the junction, bounding a triangular-shaped wedge of crust in cross section. This wedge changes shape to the south as fault geometries change and fault activity shifts between fault strands, particularly along the Calaveras fault zone (CFZ). Potential-field modeling and relocated seismicity suggest that the Paicines and San Benito strands of the CFZ dip 65° to 70° NE and form the southwest boundary of a folded 1 to 3 km thick tabular body of Coast Range Ophiolite (CRO) within the Vallecitos syncline. We identify and characterize two steeply dipping, seismically active cross structures within the junction that are associated with serpentinite in the subsurface. The architecture of the SAF-CF junction presented in this study may help explain fault-normal motions currently observed in geodetic data and help constrain the seismic hazard. The abundance of serpentinite and related CRO in the subsurface is a significant discovery that not only helps constrain the geometry of structures but may also help explain fault behavior and the tectonic evolution of the SAF-CF junction.

  5. Geochemistry of a continental site of serpentinization, the Tablelands Ophiolite, Gros Morne National Park: A Mars analogue

    NASA Astrophysics Data System (ADS)

    Szponar, Natalie; Brazelton, William J.; Schrenk, Matthew O.; Bower, Dina M.; Steele, Andrew; Morrill, Penny L.

    2013-06-01

    The presence of aqueously altered, olivine-rich rocks along with carbonate on Mars suggest that serpentinization may have occurred in the past and may be occurring presently in the subsurface, and possibly contributing methane (CH4) to the martian atmosphere. Serpentinization, the hydration of olivine in ultramafic rocks, yields ultra-basic fluids (pH ⩾ 10) with unique chemistry (i.e. Ca2+-OH- waters) and hydrogen gas, which can support abiogenic production of hydrocarbons (i.e. Fischer-Tropsch Type synthesis) and subsurface chemosynthetic metabolisms. Mars analogue sites of present-day serpentinization can be used to determine what geochemical measurements are required for determining the source methane at sites of serpentinization on Earth and possibly on Mars. The Tablelands Ophiolite is a continental site of present-day serpentinization and a Mars analogue due to the presence of altered olivine-rich ultramafic rocks with both carbonate and serpentine signatures. This study describes the geochemical indicators of present-day serpentinization as evidenced by meteoric ultra-basic reducing groundwater discharging from ultramafic rocks, and travertine and calcium carbonate sediment, which form at the discharge points of the springs. Dissolved hydrogen concentrations (0.06-1.20 mg/L) and methane (0.04-0.30 mg/L) with δ13CCH4 values (-28.5‰ to -15.6‰) were measured in the spring fluids. Molecular and isotopic analyses of CH4, ethane, propane, butane, pentane and hexane suggest a non-microbial source of methane, and attribute the origin of methane and higher hydrocarbon gases to either thermogenic or abiogenic pathways.

  6. In situ Re-Os isotopic analysis of platinum-group minerals from the Mayarí-Cristal ophiolitic massif (Mayarí-Baracoa Ophiolitic Belt, eastern Cuba): implications for the origin of Os-isotope heterogeneities in podiform chromitites

    NASA Astrophysics Data System (ADS)

    Marchesi, Claudio; González-Jiménez, José María; Gervilla, Fernando; Garrido, Carlos J.; Griffin, William L.; O'Reilly, Suzanne Y.; Proenza, Joaquín A.; Pearson, Norman J.

    2011-06-01

    Chromitite pods in the Mayarí-Cristal ophiolitic massif (eastern Cuba) were formed in the Late Cretaceous when island arc tholeiites and MORB-like back-arc basin basalts reacted with residual mantle peridotites and generated chromite-rich bodies enclosed in dunite envelopes. Platinum-group minerals (PGM) in the podiform chromitites exhibit important Os-isotope heterogeneities at the kilometric, hand sample and thin section scales. 187Os/188Os calculated at the time of chromitite crystallization (~90 Ma) ranges between 0.1185 and 0.1295 (γOs = -7.1 to +1.6, relative to enstatite chondrite), and all but one PGM have subchondritic 187Os/188Os. Grains in a single hand sample have initial 187Os/188Os that spans from 0.1185 to 0.1274, and in one thin section it varies between 0.1185 and 0.1232 in two PGM included in chromite which are only several millimeters apart. As the Os budget of a single micrometric grain derives from a mantle region that was at least several m3 in size, the variable Os isotopic composition of PGM in the Mayarí-Cristal chromitites probably reflects the heterogeneity of their mantle sources on the 10-100 m scale. Our results show that this heterogeneity was not erased by pooling and mingling of individual melt batches during chromitite crystallization but was transferred to the ore deposits on mineral scale. The distribution of the Os model ages calculated for PGM shows four main peaks, at ~100, 500, 750 and 1,000 Ma. These variable Os model ages reflect the presence of different depleted domains in the oceanic (Pacific-related) upper mantle of the Greater Antilles paleo-subduction zone. The concordance between the age of crystallization of the Mayarí-Cristal chromitites and the most recent peak of the Os model age distribution in PGM supports that Os in several grains was derived from fertile domains of the upper mantle, whose bulk Os isotopic composition is best approximated by that of enstatite chondrites; on the other hand, most PGM are

  7. Stratal disruption and development of mélange, Western Newfoundland: effect of high fluid pressure in an accretionary terrain during ophiolite emplacement

    NASA Astrophysics Data System (ADS)

    Waldron, J. W. F.; Turner, D.; Stevens, K. M.

    The Bay of Islands Ophiolite was emplaced onto the continental margin of North America during the mid-Ordovician Taconic orogeny, when tectonic slices of continental margin sediments were accreted to the moving allochthon. Tectonic slices grade into and are surrounded by mélange. Early fracture in sandstones formed without grain breakage and allowed penetration of liquid petroleum along fracture planes. Other fractures involved cataclastic flow and were sometimes re-activated during formation of later pressure solution cleavage. Shear-fracture and extension-fracture boudinage affect competent strata; extensional veins cut cement in limestone beds and are filled by shale, quartz, calcite and bitumen. Folds also formed, at a time when siltstone and sandstone were at least partially lithified. Mélange matrix shows abundant shear and extension fractures in a variety of orientations. Coaxial extension responsible for disruption of bedding can be explained by a brittle accretionary wedge model in which high fluid pressures resulted from tectonic dewatering of shales. Surface slope decreased as fluid pressure rose beneath the ophiolite, causing horizontal extension of the wedge. After escape of excess water the surface slope steepened again as renewed stacking occurred.

  8. Geochemical and isotopic constraints on the age and origin of the Nidar Ophiolitic Complex, Ladakh, India: Implications for the Neo-Tethyan subduction along the Indus suture zone

    NASA Astrophysics Data System (ADS)

    Ahmad, T.; Tanaka, T.; Sachan, H. K.; Asahara, Y.; Islam, R.; Khanna, P. P.

    2008-04-01

    The Nidar ophiolite complex is exposed within the Indus suture zone in eastern Ladakh, India. The suture zone is considered to represent remnant Neo-Tethyan Ocean that closed via subduction as the Indian plate moved northward with respect to the Asian plate. The two plates ultimately collided during the Middle Eocene. The Nidar ophiolite complex comprises a sequence of ultra-mafic rocks at the base, gabbroic rocks in the middle and volcano-sedimentary assemblage on the top. Earlier studies considered the Nidar ophiolite complex to represent an oceanic floor sequence based on lithological assemblage. However, present study, based on new mineral and whole rock geochemical and isotopic data (on bulk rocks and mineral separates) indicate their generation and emplacement in an intra-oceanic subduction environment. The plutonic and volcanic rocks have nearly flat to slightly depleted rare earth element (REE) patterns. The gabbroic rocks, in particular, show strong positive Sr and Eu anomalies in their REE and spidergram patterns, probably indicating plagioclase accumulation. Depletion in high field strength elements (HFSE) in the spidergram patterns may be related to stabilization of phases retaining the HFSE in the subducting slab and / or fractional crystallization of titano-magnetite phases. The high radiogenic Nd- and low radiogenic Sr-isotopic ratios for these rocks exclude any influence of continental material in their genesis, implying an intra-oceanic environment. Nine point mineral-whole rock Sm-Nd isochron corresponds to an age of 140 ± 32 Ma with an initial 143Nd/ 144Nd of 0.513835 ± 0.000053 ( ENdt = + 7.4). This age is consistent with the precise Early Cretaceous age of Hauterivian (132 ± 2 to 127 ± 1.6 Ma) to Aptian (121 ± 1.4 to 112 ±1.1 Ma) for the overlying volcano-sedimentary (radiolarian bearing chert) sequences based on well-preserved radiolarian fossils (Kojima, S., Ahmad, T., Tanaka, T., Bagati, T.N., Mishra, M., Kumar, R. Islam, R., Khanna, P

  9. Clinopyroxenite dikes crosscutting banded peridotites just above the metamorphic sole in the Oman ophiolite: early cumulates from the primary V3 lava

    NASA Astrophysics Data System (ADS)

    Ishimaru, Satoko; Arai, Shoji; Tamura, Akihiro

    2013-04-01

    Oman ophiolite is one of the well-known ophiolites for excellent exposures not only of the mantle section but also of the crustal section including effusive rocks and the underlying metamorphic rocks. In the Oman ophiolite, three types of effusive rocks (V1, V2 and V3 from the lower sequences) are recognized: i.e., V1, MORB-like magma, V2, island-arc type lava, and V3, intra-plate lava (Godard et al., 2003 and references there in). V1 and V2 lavas are dominant (> 95 %) as effusive rocks and have been observed in almost all the blocks of northern part of the Oman ophiolite (Godard et al., 2003), but V3 lava has been reported only from Salahi area (Alabaster et al., 1982). It is clear that there was a time gap of lava eruption between V1-2 and V3 based on the presence of pelagic sediments in between (Godard et al., 2003). In addition, V3 lavas are fed by a series of doleritic dikes crosscutting V2 lava (Alley unit) (Alabaster et al., 1982). We found clinopyroxenite (CPXITE) dikes crosscutting deformation structure of basal peridotites just above the metamorphic sole in Wadi Ash Shiyah. The sole metamorphic rock is garnet amphibolite, which overlies the banded and deformed harzburgite and dunite. The CPXITE is composed of coarse clinopyroxene (CPX) with minor amount of chlorite, garnet (hydrous/anhydrous grossular-andradite) with inclusions of titanite, and serpentine formed at a later low-temperature stage. The width of the CPXITE dikes is 2-5 cm (10 cm at maximum) and the dikes contain small blocks of wall harzburgite. Almost all the silicates are serpentinized in the harzburgite blocks except for some CPX. The Mg# (= Mg/(Mg + Fe) atomic ratio) of the CPX is almost constant (= 0.94-0.95) in the serpentinite blocks but varies within the dikes, highest at the contact with the block (0.94) and decreasing with the distance from the contact to 0.81 (0.85 on average). The contents of Al2O3, Cr2O3, and TiO2 in the CPX of the dikes are 0.5-2.0, 0.2-0.6, and 0

  10. Late Jurassic-Early Cretaceous radiolarian age constraints from the sedimentary cover of the Amasia ophiolite (NW Armenia), at the junction between the Izmir-Ankara-Erzinçan and Sevan-Hakari suture zones

    NASA Astrophysics Data System (ADS)

    Danelian, T.; Asatryan, G.; Galoyan, Gh.; Sahakyan, L.; Stepanyan, J.

    2016-01-01

    The Amasia ophiolite, situated at the northernmost corner of Armenia, is part of the Sevan-Hakari suture zone which links with the Izmir-Ankara-Erzinçan suture zone in northern Turkey. Three new radiolarian assemblages have been extracted from siliceous sedimentary rocks that accumulated on the Amasia ophiolite in an oceanic setting. Two of these assemblages were extracted from red-brownish bedded cherts overlying basaltic lavas; one of these is likely to be middle Oxfordian to early Kimmeridgian in age, while the second correlates with the Berriasian. Similar time-equivalent lava-chert sequences have been dated recently using radiolarians from the Stepanavan, Vedi and Sevan ophiolite units, where they are considered to relate to submarine volcanic activity in the back-arc marginal basin in which the Armenian ophiolites were formed. The third radiolarian assemblage, of late Barremian age, was extracted from a more than 15-m-thick volcaniclastic-chert sequence. The related volcanic activity is likely to have been subaerial and probably relates to the formation of an oceanic volcanic plateau; no Cretaceous subaerial volcanism has been previously recorded in the Lesser Caucasus area.

  11. High-temperature hydrothermal activities around suboceanic Moho: An example from diopsidite and anorthosite in Wadi Fizh, Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Akizawa, Norikatsu; Tamura, Akihiro; Fukushi, Keisuke; Yamamoto, Junji; Mizukami, Tomoyuki; Python, Marie; Arai, Shoji

    2016-10-01

    Reaction products between hydrothermal fluids and uppermost mantle harzburgite-lowermost crustal gabbro have been reported along Wadi Fizh, northern Oman ophiolite. They are named mantle diopsidite (MD) or crustal diopsidite (CD) depending on the stratigraphic level. They construct network-like dikes crosscutting structures of the surrounding harzburgite or gabbro. The MD is mainly composed of diopsidic clinopyroxene, whereas the CD is of diopsidic clinopyroxene and anorthitic plagioclase. Here, we report a new reaction product, crustal anorthosite (CA), from the lowermost crustal section. The CA is always placed in the center of the CD network, and mainly consists of anorthitic plagioclase with minor titanite and chromian minerals such as chromite and uvarovite. Aqueous fluid inclusions forming negative crystals are evenly distributed in minerals of the CA. The fluid inclusions contain angular-shaped or rounded daughter minerals as calcite or calcite-anhydrite composite, which were identified by Raman spectroscopic analysis. We estimated their captured temperature at 530 °C at least by conducting microthermometric analysis of the fluid inclusions. Furthermore, we examined their chemical characteristics by direct laser-shot sampling conducted by laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS). The results indicate that the trapped aqueous fluids contain an appreciable amount of Na, but no K and Cr. Hydrothermal fluids involved in the CA formation transported Cr, which was probably taken up from chromite seams in the uppermost mantle section. Cr got soluble by forming complexes with anions as SO42-, CO32- and Cl-. In addition, these hydrothermal fluids transported Fe, Mg and trace elements (Ti, Sr, Y, Zr and rare-earth elements) governing whole-rock chemical compositions of the MDs, CDs and CAs. Our estimation for the condition of CA formation yielded rather low temperatures (530-600 °C), which indicates a later stage production of the CA

  12. Geochemistry and petrology of altered peridotite overlain by Maastrichtian to Miocene sediments in the SE Samail Ophiolite, Oman

    NASA Astrophysics Data System (ADS)

    de Obeso, J. C.; Kelemen, P. B.

    2015-12-01

    In the southeastern Oman Mountains the mantle section of the Samail ophiolite is unconformably capped by large units of Maastrichtian to Miocene, shallow marine limestones. Oceanic crust and upper mantle were thrust onto the Arabian continental margin from ca 96 to 80 Ma. Subaerial erosion locally removed the crustal section and exposed peridotite to weathering. A Maastrichtian transgression led to deposition of Late Cretaceous to Miocene, shallow marine limestones and shales above the unconformity. Near the town of Fins, a deep canyon exposes the unconformity and underlying, altered mantle peridotite. A striking series of clastic dikes of grey, micritic limestone extends across the unconformity, up to 10 meters down into the peridotite. Tips of these dikes are choked with angular peridotite blocks. Deeper, the peridotite is cut by a spectacular grid of carbonate-serpentine veins with abundant vugs. The peridotite matrix has high concentrations of calcium and small enrichments of silica compared to the Oman protolith, resulting from reaction with a hydrous fluid derived from seawater equilibrated with the overlying sediments. δ13C from -1.3-0.61 per mil (VPDB) and δ18O between 22.2 to 28 per mil (SMOW) are similar to values in the overlying sediments (Schluter et al., Facies 2008). Clumped isotope thermometry on calcite crystals in veins cutting the peridotite outcrops indicate crystallization at 25-60 °C. Sediment thickness reconstructions place the pressure of alteration between 300-600 bars. 87Sr/86Sr values of 0.7078 and 0.7079 correspond to seawater Sr isotope ratios at the Cretaceous-Tertiary boundary, which is present in the overlying limestones about 50 meters above the unconformity. This suggests that the veins in the peridotite formed at about 60 Ma, or that their parental fluid was in Sr isotope exchange equilibrium with 60 Ma sediments. Reaction path modeling of carbonated saturated seawater and peridotite reproduces the observed vein mineral

  13. Constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the Samail ophiolite

    NASA Astrophysics Data System (ADS)

    VanTongeren, J. A.; Hirth, G.; Kelemen, P. B.

    2015-12-01

    The debate over the processes of igneous accretion of gabbroic lower crust at submarine spreading centers is centered on two end-member hypotheses: Gabbro Glaciers and Sheeted Sills. In order to determine which of these two hypotheses is most applicable to a well-studied lower crustal section, we present newly published data (VanTongeren et al., 2015 EPSL v. 427, p. 249-261) on plagioclase lattice preferred orientations (LPO) in the Wadi Khafifah section of the Samail ophiolite, Oman. Based on our results we provide five critical observations that any model for the accretion of the lower oceanic crust must satisfy: (1) There is a distinctive change in the orientation of the outcrop-scale layering from near-vertical to sub-horizontal that is also reflected in the plagioclase fabrics in the uppermost ~1000-1500 m of the gabbroic crust; (2) The distinction between the upper gabbros and lower gabbros is not a geochemical boundary. Rather, the change in outcrop-scale orientation from near-vertical to sub-horizontal occurs stratigraphically lower in the crust than a change in whole-rock geochemistry; (3) There is no systematic difference in plagioclase fabric strength in any crystallographic axis between the upper gabbros and the lower gabbros; (4) Beneath the abrupt transition from sub-vertical to sub-horizontal fabric, there is no systematic change in the geographic orientation of the plagioclase fabric, or in the development of a dominant lineation direction within the upper gabbros or the lower gabbros; (5) In the lower gabbros, the obliquity between the (010) and the modal layering remains approximately constant and indicates a consistent top to the right sense of shear throughout the stratigraphy. Our observations are most consistent with the Sheeted Sills hypothesis, in which the majority of lower crustal gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with

  14. Platinum group and chalcophile element systematics of serpentinized peridotites from the St. Elena ophiolite in Costa Rica

    NASA Astrophysics Data System (ADS)

    Holm, J.; Bizimis, M.; Schwarzenbach, E. M.; Foustoukos, D.; Frisby, C. P.; Brandon, A. D.; Gazel, E.

    2015-12-01

    We present in situ LA-ICPMS data on platinum group element (PGE) and chalcophile element (namely Cu, Ag, Te, Au, Pb) systematics in sulfides from partially serpentinized peridotites of the St. Elena ophiolite, Costa Rica. PGE are strong indicators of primary mantle processes, though their behavior during low temperature alteration processes such as serpentinization is not well understood. St. Elena sulfides are dominantly pentlandites that coexist with Fe-Ni alloys and native Cu. This indicates extremely low fO2 and fS2 conditions likely established during the early stages of serpentinization. We observe extremely variable PGE-Re concentrations in the sulfides, (e.g. [Os] = 2 - 100,000 times primitive mantle, PM). Low [Os] sulfides have high Pd/Os, which in turn correlates positively with Cu concentrations, suggesting Pd enrichment through Cu-rich fluids (e.g. Schwarzenbach et al., 2014, CMP) as opposed to melt-rock reaction. Sulfide PM-normalized PGE-Re patterns are dominated by strong Pt depletions (e.g., Pt/Pd = 0.80-0.0009). Occasional Pt enrichments over Pd and Ru (or Rh) in a PM-normalized pattern (~5% of the sulfide population) often correlate with Te and/or Au enrichments. Pt enrichment was also observed in a composite pentlandite-awaruite, suggesting possible exsolution of Pt from sulfides under extremely low fS2 conditions. Pb concentrations do not correlate with other chalcophiles or PGE. Pb ranges from 0.01-31.64 ppm with the majority of sulfides <5 ppm, and an average concentration of 2.77 ppm (n=64). Assuming that this Pb concentration is representative of mantle sulfides, this implies that Pb is not dominantly held in sulfides in the upper mantle. Combination of in situ and bulk rock PGE analyses will be used to distinguish the effects of primary magmatic signatures (e.g., melt depletion, melt-rock interaction) and secondary processes such as serpentinization on the PGE-Re and chalcophile element systematics of these sulfides.

  15. Sedimentary record of the obduction of the Samail ophiolite in northern Oman: the Muti Formation in the Sail Hatat window

    NASA Astrophysics Data System (ADS)

    Ducassou, Céline; Robin, Cecile; Poujol, Marc; Al-Rahbi, Basim; Estournes, Guilhem

    2016-04-01

    The obduction of the Samail Ophiolite in Oman took place during the Upper Cretaceous. Since then, the northern part of Oman has been relatively preserved from deformation and is therefore one of the best places to study obduction processes. In addition, radiometric data provide good constraints on the timing of obduction from the formation of the metamorphic sole until the exhumation of the high-pressure / low-temperature metamorphic rocks involved in the subduction zone below the oceanic lithosphere. However, the response of the continental margin during the obduction is still poorly constrained. If most of the models suggest the development of a flexural basin and an associated forebulge such as in continental collision, their recognition within the syn-tectonic deposits led to different interpretations. The geometry of the youngest syn-tectonic deposits (Fiqa Formation) is relatively well constrained by sub-surface data that suggest a southward migration of the depocenter and progressive onlaps on the southern margin of the basin. The context of sedimentation of the oldest syn-tectonic deposits (Muti Formation) preserved below the nappes in the Oman Mountains is, however, still poorly understood. The location of the sedimentation area with respect of the forebulge, for instance, remains unclear. In order to acquire better constraints on the record, on the Arabian platform, of first steps of the obduction, the analysis of several sections of the Muti Formation has been performed. We present here our main results for the north-eastern part of the Oman Mountains, in the Sail Hatat window, where the thickest successions have been described in Quryat and Bidbid area, respectively located in the eastern and western part of the Sail Hatat window. Sedimentological and structural analysis have been combined to reconstitute the evolution of depositional environments in areas strongly affected by deformation. In addition, systematic measurements and restoration of

  16. Fingerprinting fluid sources in Troodos ophiolite complex orbicular glasses using high spatial resolution isotope and trace element geochemistry

    NASA Astrophysics Data System (ADS)

    Fonseca, Raúl O. C.; Kirchenbaur, Maria; Ballhaus, Chris; Münker, Carsten; Zirner, Aurelia; Gerdes, Axel; Heuser, Alexander; Botcharnikov, Roman; Lenting, Christoph

    2017-03-01

    The Troodos igneous complex (Cyprus) is a ca. 90 Ma old, well preserved supra-subduction zone ophiolite. Troodos is unique in that it shows evidence of fluid-saturation throughout the complex, from its base (i.e. podiform chromitites) to its uppermost units - the upper pillow lavas (UPL). However, it is unclear what the source of dissolved water in UPL tholeiites is, with possibilities including shallow seawater infiltration, assimilation of altered Troodos oceanic crust, recycled serpentinized oceanic crust, or subducted pelagic sediments. In order to identify and characterize these components we have carried out a detailed high-resolution study on tholeiitic lavas on orbicular structures and glasses from the UPL in Troodos. Basaltic orbicules were measured for their Sr-Nd-Hf-Pb isotope compositions, and in situ for their B isotopes using LA-MC-ICP-MS. UPL orbicules display a very narrow range in ɛ Nd and ɛ Hf (+7 to +8 and +13 to +15, respectively) indicating melting of a depleted mantle source. Lead isotopes, specifically 207Pb/204Pb vs. 206Pb/204Pb, form a mixing array with pelagic sediments. Furthermore, high-resolution characterization of individual orbicules revealed that UPL tholeiites display strong variability in 87Sr/86Sr (0.7039-0.7060) at the outcrop scale. Samples display δ11 B between -8.2 (± 0.5)‰ and +5.9 (± 1.1)‰ with an average B content of ca. 5 μg/g. Contrary to expectation, altered orbicules and their associated hyaloclastite matrixes display lower δ11 B (down to -10‰) and higher B contents (max. 200 μg/g) when compared to fresh glass. Furthermore, the orbicules studied here show little or no evidence of interaction with seawater, which is supported by their trace element contents and isotope compositions. When all isotope systems are taken into account, UPL lavas reflect melting of a depleted mantle source that was overprinted by hydrous sediment melts, and potentially, fluid-like subduction components that in part originate

  17. Microbially-influenced Fe-Cycling within high pH serpentinizing springs of the Zambales Ophiolite, Philippines

    NASA Astrophysics Data System (ADS)

    Casar, C.; Meyer-Dombard, D. R.; Simon, A.; Cardace, D.; Arcilla, C. A.

    2014-12-01

    The Zambales ophiolite region in the Philippines contains high pH springs associated with serpentinization. At the surface where calcium-saturated ­fluids mix with air, fluid becomes aerobic and diffusion of CO2 occurs. At depth, there are low concentrations of dissolved inorganic carbon and O2, and high concentrations of CH4 and H2. Redox potential of iron in the fluids is largely dependent on pH. Fe2+ is unstable at a high pH, and spontaneously reacts with atmospheric O2 to form Fe3+, which is then hydrolysed to ferrihydrite. The reaction kinetics may be too rapid for microbes to harness energy for growth, however cells have been documented to act as nucleation sites for ferrihydrite precipitation in natural environments. Precipitates that sink to the subsurface act as substrates for microbes where they may carry out Fe3+ reduction in the presence of H2. Predictions made about Gibbs energy of reaction for iron metabolisms in serpentinizing systems show that Fe3+ reduction in the subsurface is energetically favorable (Fig. 1A) (Cardace, et al., 2013). Spring fluid and rock samples from the Zambales region were collected in September 2013. Time series microcosms including sample rock, spring fluid, and gas simulating the spring surface and subsurface (Fig. 1B) will investigate microbial growth rates and microbial reaction products over one year. Microcosms will undergo cell counts via fluorescence microscopy, SEM, and XRD to examine cell growth rates, microbial action on mineral surfaces, minerals forming around cells, and changes in mineralogy. After one year, microbial community structure and iron metabolizers will be identified via DNA sequencing.­­ Surface microcosms are expected to show abiotic oxidation of Fe2+ and formation of Fe3+ precipitates preferentially around cells acting as nucleation sites (except in abiotic control microcosms). Subsurface microcosms are expected to show biotic reduction of Fe3+ and signs of microbial action on mineral surfaces

  18. Anatomy of a frozen axial melt lens from a fast-spreading paleo-ridge (Wadi Gideah, Oman ophiolite)

    NASA Astrophysics Data System (ADS)

    Müller, T.; Koepke, J.; Garbe-Schönberg, C.-D.; Dietrich, M.; Bauer, U.; Wolff, P. E.

    2017-02-01

    At fast-spreading mid-ocean ridges, axial melt lenses (AMLs) sandwiched between the sheeted dyke section and the uppermost gabbros are assumed to be the major magma source of crust formation. Here, we present our results from a field study based on a single outcrop of a frozen AML in the Samail ophiolite in the Sultanate of Oman which presents a whole suite of different lithologies and complex cutting relationships: varitextured gabbro with relics of primitive poikilitic clinopyroxene is intruded by massive quartz diorites and tonalites bearing relics of assimilated sheeted dykes, which in turn are cut by trondhjemite dykes. The whole is cut by basaltic dykes with chilled margins. The geochemical evolutionary trend of the varitextured gabbros, including some of the quartz diorites and tonalites, can be best modelled by fractional crystallisation of an experimental MORB parental melt composition containing 0.4 to 0.8 wt.% H2O. Patchy varitextured gabbros containing domains of primitive poikilitic clinopyroxene and evolved granular networks represent the record of in situ crystallisation. Some quartz diorites, often with xenoliths of sheeted dykes and exceptionally high Al2O3 contents, show a bulk trace element pattern more in accord with melts generated by experimental partial melting of dyke material. Highly evolved, crosscutting trondhjemite dykes show characteristic trace element patterns implying a formation by partial melting of sheeted dykes under lower water activity which is indicated by relatively low Al2O3 contents. The late basaltic dykes with chilled margins crosscutting all other lithologies show a relatively depleted geochemical character with pronounced negative Nb-Ta anomalies implying a genetic relationship to the second phase of magmatic Oman paleo-ridge activity (V2). The field relationships in combination with the petrological/geochemical trends reveal multiple sequences of MORB-type magma cooling (resulting in fractional crystallisation) and re

  19. Petrology, geochemistry and Sm-Nd analyses on the Balkan-Carpathian Ophiolite (BCO - Romania, Serbia, Bulgaria): Remnants of a Devonian back-arc basin in the easternmost part of the Variscan domain

    NASA Astrophysics Data System (ADS)

    Plissart, Gaëlle; Monnier, Christophe; Diot, Hervé; Mărunţiu, Marcel; Berger, Julien; Triantafyllou, Antoine

    2017-04-01

    The pre-Alpine basement of the Southern Carpathians/Western Balkans contains four ophiolitic massifs dismembered by Alpine tectonics, which define the ;Balkan-Carpathian Ophiolite; (BCO) for which the tectonic setting and age of formation are still debated (Precambrian or Early Devonian). In this contribution, we demonstrate that, in light of a Pre-Alpine restoration, the four massifs belonged to a unique slice of very complete, obducted oceanic lithosphere and we re-evaluate its tectonic setting. Large chromitite volumes with Al-rich spinel compositions (Cr# = 0.39-0.48), as well as major and trace geochemical results on basalts (slightly enriched N-MORBs with low negative Nb anomaly associated with calk-alkaline BABBs), point to a formation in a back-arc basin. Mantle spinel composition (Cr# = 0.49-0.51) and melting modeling indicate mean melting extents of 8.5-11% favouring intermediate spreading rate. New Sm-Nd dating on lower gabbroic rocks give a whole rock isochron, interpreted as the age of formation of the BCO crust at 409 ± 38 Ma, thus confirming an Early Devonian oceanic crust. The previous ∼563 Ma U-Pb zircon age can be interpreted as casual inheritance indicating the proximity of an old continental lithosphere. Taking into account the lithological evidences and paleocontinental affinities of the two recognized terranes separated by the BC oceanic basin (Balkans and Sredna Gora) and by analogy with other Variscan ophiolites in Western/Central Europe, we suggest that the BC ophiolite belong to the ∼400 Ma ophiolites group obducted between West and East Galatia and belonging to the southern Variscan suture. However, the BC ophiolite is the only one of this group obducted to the north and not involved in the Lower Allochthon/ophiolite/Upper Allochthon thrust pile, likely explaining its exceptional preservation. Finally, we tentatively propose a new unifying tectonic model where different terrane drift rates and highly oblique displacements create two

  20. Geochemistry of amphibolites and related graphitic gneisses from the Suchý and Malá Magura Mountains (central Western Carpathians) - evidence for relics of the Variscan ophiolite complex

    NASA Astrophysics Data System (ADS)

    Ivan, Peter; Méres, Štefan

    2015-10-01

    Three small bodies of amphibolites and associated graphitic gneisses from the Suchý and Malá Magura Mountains (Tatric Megaunit, central Western Carpathians) have been studied by petrographic and geochemical methods. Isolated, fault-bounded bodies first hundreds of meters in size are located in the complex of the Early Paleozoic paragneisses and migmatites intruded by the Lower Carboniferous granitoid rocks. Amphibolites (locally actinolite schists) were formed from effusive basalts, dolerites or isotropic gabbros hydrothermally altered and veined before the regional metamorphic transformation. Distribution of the trace elements relatively immobile during the metamorphic alteration (HFSE, REE, Cr, V, Sc) is similar to E-MORB type in the Malá Magura Mountain or to N-MORB/E-MORB types in the Suchý Mountain. Graphitic gneisses to metacherts are rich in silica (up to 88 wt. %) and Ctot, poor in other major element contents and display negative Ce-anomaly, enrichment in HREE, V, Cr and U. They were probably originally deposited as non-carbonate and silica-rich deep-sea sediments in anoxic conditions. The oceanic provenance of amphibolites and related graphitic gneisses clearly indicates their oceanic crust affinity and identity with the uppermost part of the ophiolite sequence. Ophiolite bodies from the Suchý and Malá Magura Mountains are supposed to be relic fault blocks identical with the Upper Devonian Pernek Group which represents a Variscan ophiolite nappe preserved to large extent in the Malé Karpaty Mountains located in the Tatric Megaunit further to the southwest. All these ophiolite relics are vestiges of the original ophiolite suture created by oceanic closure in the Lower Carboniferous.

  1. Strain localization and fluid infiltration in the mantle wedge during subduction initiation: Evidence from the base of the New Caledonia ophiolite

    NASA Astrophysics Data System (ADS)

    Soret, M.; Agard, P.; Dubacq, B.; Vitale-Brovarone, A.; Monié, P.; Chauvet, A.; Whitechurch, H.; Villemant, B.

    2016-02-01

    Despite decades of petrological and geochemical studies, the nature and setting of obducted ophiolites remain controversial: the influence of supra-subduction zone environments on pre-existing oceanic lithosphere is yet to assess, and the processes leading to subduction/obduction initiation are still poorly constrained. Our study documents successive influx of slab-derived fluids and progressive strain localization within the upper mantle in a supra-subduction environment during the first few My of the subduction history. We focus on strongly sheared mafic amphibolites intruding peridotites near the mantle-crust transition of the New Caledonia obducted ophiolite and ~ 50 to 100 m above the basal thrust contact of the ophiolite. These m- to hm-long and several m-thick shear bands are interpreted as inherited small-scale intrusions of mafic melts, probably dikes or sills, which were derived from a moderately refractory mantle source refertilized by supra-subduction zone fluids. 40Ar/39Ar age constraints on pargasite at ca. 90 Ma suggest that they could be inherited from the former Pacific west-dipping subduction. Secondary deformation of these mafic intrusions is intimately associated to three major stages of fluid infiltration: (1) the first stage of deformation and metasomatism is marked by syn-kinematic growth of Ca-amphibole (at 700-800 °C and 3-5 kbar) with a distinctive supra-subduction zone signature, and controlled later channelization of aqueous fluids. 40Ar/39Ar dating on magnesio-hornblende indicates that this deformation episode occurred at ca. 55 Ma, coincident with east-dipping subduction initiation; (2) the main metasomatic stage, characterized by the development of a phlogopite-rich matrix wrapping peridotites and amphibolite boudins, points to the percolation of alkali-rich aqueous fluids at still high temperature (650-750 °C); (3) the last, low temperature (< 600 °C) metasomatic stage results in the formation of deformed veinlets containing talc

  2. Petrogenesis and tectonic implications of Triassic mafic complexes with MORB/OIB affinities from the western Garzê-Litang ophiolitic mélange, central Tibetan Plateau

    NASA Astrophysics Data System (ADS)

    Liu, Bin; Ma, Chang-Qian; Guo, Yu-Heng; Xiong, Fu-Hao; Guo, Pan; Zhang, Xin

    2016-09-01

    Although numerous Paleo-Tethyan ophiolites with mid-oceanic ridge basalts (MORB) and/or oceanic-island basalt (OIB) affinities have been reported in the central Tibetan Plateau (CTP), the origin and tectonic nature of these ophiolites are not well understood. The petrogenesis, mantle sources and geodynamic setting of the mafic rocks from these ophiolites are unclear, which is the main reason for this uncertainty. In this paper, we present new geochronological, mineralogical and Sr-Nd isotopic data for the Chayong and Xiewu mafic complexes in the western Garzê-Litang suture zone (GLS), a typical Paleo-Tethyan suture crossing the CTP. Zircon LA-ICP-MS U-Pb ages of 234 ± 3 Ma and 236 ± 2 Ma can be interpreted as formation times of the Chayong and Xiewu mafic complexes, respectively. The basalts and gabbros of the Chayong complex exhibit enriched MORB (E-MORB) compositional affinities except for a weak depletion of Nb, Ta and Ti relative to the primitive mantle, whereas the basalts and gabbros of the Xiewu complex display distinct E-MORB and OIB affinities. The geochemical features suggest a probable fractionation of olivine ± clinopyroxene ± plagioclase as well as insignificant crustal contamination. The geochemical and Sr-Nd isotopic data reveal that the Chayong mafic rocks may have been derived from depleted MORB-type mantle metasomatized by crustal components and Xiewu mafic rocks from enriched lithospheric mantle metasomatized by OIB-like components. The ratios of Zn/Fet, La/Yb and Sm/Yb indicate that these mafic melts were produced by the partial melting of garnet + minor spinel-bearing peridotite or spinel ± minor garnet-bearing peridotite. We propose that back-arc basin spreading associated with OIB/seamount recycling had occurred in the western GLS at least since the Middle Triassic times, and the decompression melting of the depleted MORB-type asthenosphere mantle and partial melting of sub-continental lithosphere were metasomatized by plume

  3. Multi-stage Development From Ultrahigh-pressure Environment to Low-pressure Magmatic Processes: New Insight From Podiform Chromitites in the Luobusa Ophiolite, Southern Tibet

    NASA Astrophysics Data System (ADS)

    Yamamoto, S.; Komiya, T.; Maruyama, S.

    2007-12-01

    Unusual silicate lamellae were discovered within chromites of podiform chromitites in the Luobusa ophiolite, southern Tibet, especially from massive- and nodular-type chromites. Using analytical transmission electron microscopy (ATEM), we found coesite, clinopyroxene and MgSiO3 phase as exsolution lamellae from the host chromites. There is no evidence that the Luobusa ophiolite itself formed at great depth, and the presence of coesite lamellae in a chromite directly indicate that podiform chromitites originate from deep mantle environment. On the other hand, magmatic structures are still observed in the podiform chromitite ore-body, such as banding chromites and the sharp contact between host peridotite and dunite envelopes. The characteristic features of the disseminated- and banded-type chromites, such as their interstitial distribution, euhedral to subhedral morphology and absence of exsolution lamellae, suggest they formed under low-pressure magmatic conditions. According to petrographic investigations, nodular-type chromites with numerous lamellae seem to be changing into disseminated-type chromite with no lamellae. Therefore, we conclude that the podiform chromitites at Luobusa retain evidence of their multi-stage development from ultrahigh-pressure environment to low-pressure magmatic processes under a ridge. On the basis of our results, we propose that a significant component of the podiform chromitite at Luobusa originate from the ultra-deep environment, and that chromitites were transported with mantle upwelling from a deep mantle to a shallow level under a mid-ocean ridge. Subsequently, the mantle peridotite with its podiform chromitite underwent partial melting, and the chromites without exsolution lamellae were largely recrystallized or newly precipitated by a shallow-level magmatic process under the mid-ocean ridge. As chromite is a highly refractory mineral, the petrological ultrahigh-pressure evidence can be preserved in spite of its long evolved

  4. Pre-Obduction, Syn-Magmatic Extensional Deformation and Unroofing of a Fore-Arc Ophiolite, the Thetford-Mines Complex of Southern Quebec.

    NASA Astrophysics Data System (ADS)

    Schroetter, J.; Page, P.; Bedard, J. H.; Tremblay, A.

    2003-12-01

    The Ordovician Thetford Mines Ophiolite Complex (TMOC) is a peri-continental supra-subduction zone fore-arc terrane obducted onto the Laurentian margin during the Taconic Orogeny. Stratigraphic correlations suggest that the Mont-Orford and Asbestos Ophiolites are correlative, which implies obduction of a 100 km long oceanic slab. The TMOC is affected by syn-obduction (syn-emplacement) deformation, and two post-obduction events: (i) Silurian backthrusting and normal faulting, and (ii) Acadian folding and reverse faulting. The post-obduction deformation tilted the southern part of the TMOC to the vertical, exposing from base to top: cumulate Dunitic, Pyroxenitic and Gabbroic Zones, a hypabyssal unit (either sheeted dykes or a subvolcanic breccia), and an ophiolitic extrusive/sedimentary sequence, upon which were deposited (unconformably) a forearc basin. Our mapping has revealed the presence of numerous pre-obduction faults, spaced c.1 km apart on average. In the plutonic part of the crust, the faults are manifested as sheared or mylonitic dunites and syn-magmatic breccias, and may correspond to along-strike breaks in lithology. Fault breccias are cut by undeformed, 10-m scale, websteritic to lherzolitic intrusions, demonstrating the pre- to syn-magmatic nature of the faulting, and suggesting a role in transfer of melt to the surface. Assuming that rhythmic cumulate layering was originally paleo-horizontal, then kinematic analysis implies that these were originally normal faults separating a series of tilted (30-90 degrees) blocks. Swarms of dykes are oriented parallel to the major faults and locally constitute a sheeted complex, locally removed by syn-volcanic unroofing and erosion. In the upper part of the crust, the faults correspond to marked lateral changes in the thickness and facies assemblages seen in supracrustal rocks, are locally marked by prominent subvolcanic breccias, and have upwardly decreasing throws, which together suggest that they are growth

  5. Formation of ophiolite-bearing tectono-sedimentary mélanges in accretionary wedges by gravity driven submarine erosion: Insights from analogue models and case studies

    NASA Astrophysics Data System (ADS)

    Malavieille, Jacques; Molli, Giancarlo; Genti, Manon; Dominguez, Stephane; Beyssac, Olivier; Taboada, Alfredo; Vitale-Brovarone, Alberto; Lu, Chia-Yu; Chen, Chih-Tung

    2016-10-01

    Orogenic wedges locally present chaotic tectonostratigraphic units that contain exotic blocks of various size, origin, age and lithology, embedded in a sedimentary matrix. The occurrence of ophiolitic blocks, sometimes huge, in such "mélanges" raises questions on (i) the mechanisms responsible for the incorporation of oceanic basement rocks into an accretionary wedge and (ii) the mechanisms allowing exhumation and redeposition of these exotic elements in "mélanges" during wedge growth. To address these questions, we present the results of a series of analogue experiments performed to characterize the processes and parameters responsible for accretion, exhumation and tectonosedimentary reworking of oceanic basement lithospheric fragments in an accretionary wedge. The experimental setup is designed to simulate the interaction between tectonics, erosion and sedimentation. Different configurations are applied to study the impact of various parameters, such as irregular oceanic floor due to structural inheritance, or the presence of layers with contrasted rheology that can affect deformation partitioning in the wedge (frontal accretion vs basal accretion) influencing its growth. Image correlation technique allows extracting instantaneous velocity field, and tracking of passive particles. By retrieving the particle paths determined from models, the pressure-temperature path of mélange units or elementary blocks can be discussed. The experimental results are then compared with observations from ophiolite-bearing mélanges in Taiwan (Lichi and Kenting mélanges) and Raman spectroscopy of carbonaceous material (RSCM) Thermometry data on rocks from the northern Apennines (Casanova mélange). A geological scenario is proposed following basic observations. The tectonic evolution of the retroside of doubly vergent accretionary wedges is mainly controlled by backthrusting and backfolding. The retro wedge is characterized by steep slopes that are prone to gravitational

  6. Chromian spinels in mafic ultramafic mantle dykes: Evidence for a two-stage melt production during the evolution of the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Python, Marie; Ceuleneer, Georges; Arai, Shoji

    2008-11-01

    This paper describes a comprehensive study of the chromian spinels present in mafic-ultramafic dykes cropping out along the mantle section of the Oman ophiolite. We studied about 1100 samples in thin section and with the electron microprobe. Chromian spinel is almost ubiquitous in primitive dykes (high-Mg# troctolites and pyroxenites) and less common in more differentiated ones (olivine gabbros and gabbronorites). The Cr#, TiO 2 content, and other compositional parameters are well correlated to the nature and composition of the co-genetic silicate assemblage. Chromian spinel composition contributes to establish that the mantle dykes of Oman are more or less evolved cumulates that crystallised from two main types of primary melts: tholeitic melt similar to Mid-Ocean Ridge basalts (MORB; 0.45 < Cr# < 0.63; 0.3 < Mg# < 0.6; TiO 2 up to2 wt.%), and more silicic melts issued from a highly depleted mantle source, similar to boninitic-andesitic melts that preferentially form in subduction zone settings (0.35 < Cr# < 0.80; 0.1 < Mg# < 0.7; TiO 2 up to 0.2 wt.%). The chromian spinel composition presents a higher variability than the associated silicates and allows us to further unravel the petrological evolution and segmentation of the Oman ophiolite. The composition of chromian spinel in mantle dykes and in the spatially related residual harzbugites display well correlated variations at the scale of the Oman ophiolite. This shows that these two lithologies share a common magmatic history, even if, strictly speaking, they cannot be related through direct parent-daughter relationships. The Cr# is on average higher, and the TiO 2 lower in the NW than in the SE, consistent with an increasing influence of "marginal" magmatic processes in the NW, while the southeastern area has petrological characteristics closer to those of an "open" ocean. In this southeastern part, compositional variations of the chromian spinel are correlated to structural characteristics related to the

  7. Mineralogy and Geochemistry of Coast Range Ophiolite, CA Rock Cores Keys to Understanding Subsurface Serpentinite Habitability on Mars

    NASA Astrophysics Data System (ADS)

    Carnevale, D.; Cardace, D.; Mccann, A. R.; Hoehler, T. M.

    2011-12-01

    Ophiolite in California, where altered peridotites crop out in serpentine barrens. The same rocks have been detected in certain regions of Mars (Ehlmann et al., 2009), and we seek field, mineral, and geochemical evidence that MCL is an apt point of comparison. Specifically, we will be looking at two cores from MCL, each cored to ~50 m depth and 5 cm in diameter. Assays of host rock and altered horizons using XRD, XRF, and thin-section petrography will be employed to constrain the hydrogen production possible with this particular serpentinite assemblage. We will make the case for MCL as a strong terrestrial analog for serpentinite bodies on Mars, and consider the implications of a similar mineral and geochemical "habitat" for the Martian subsurface. Schulte, M et al (2006) Astrobiology, Vol 6, No 2 Ehlmann et al. (2009) Journal of Geophysical Research, Vol 114 E00D08 Kelley et al, (2005) Science 307 (5714): 1428-1434 Malin and Edgett (2000) Science, 288, 2330-2335 Andrews-Hannah and Phillips (2007) Journal of Geophysical Research Vol 12 E08001

  8. Systematics of Platinum-Group Element Distribution in the Boninitic Thetford Mines Ophiolite Complex, Canada: Melting and Fractional Crystallization Effects

    NASA Astrophysics Data System (ADS)

    Page, P.; Bedard, J.; Tremblay, A.; Minarik, W.

    2004-05-01

    The Ordovician Thetford Mines Ophiolitic Complex of southern Quebec has a 5 km thick mantle section, a 1.5 - 2.5 km thick cumulate crust, divided into Dunitic, Pyroxenitic and Gabbroic Zones; a partly-eroded sheeted dyke complex, subvolcanic breccias, tholeiitic (subordinate) and boninitic (dominant) lavas (up to 3 km) and associated dacitic tuffs, followed by a fore-arc flysch deposit. Mantle rocks are mostly harzburgite tectonites with diffuse compositional layering and occasional pyroxenite dykes. Most rocks exhibit porphyroclastic to weakly deformed, nearly equigranular coarse textures. Dunite forms irregular masses and dykes (< 0.1 - 5 m), the largest of which contain massive chromitite pods and nodules at their centers. The refractory mantle mineralogy, and low whole-rock CaO, Al2O3 and HREE, imply that the mantle peridotites are slightly to highly melt-depleted residues (up to 25% melting). The peridotites have U-shaped REE chondrite-normalized patterns (LREE enrichment) and some show preferential enrichment of Pt-Pd over Ir-Os, which we attribute to metasomatism of previously-depleted peridotite in the mantle wedge by a subducted slab-derived fluid or melt. Primitive mantle-normalized PGE patterns for harzburgite show negative to slightly positive slopes with Pd/Ir ranging from 0.21 to 3.1, and Pt/Ir ranging from 0.20 to 8.60. Dunites show the same patterns, but with lower concentrations of Pd and Pt, with Pd/Ir ranging from 0.01 to 0.60 and Pt/Ir from 0.007 to 2.26. The whole-rock chemical stratigraphy of the Pyroxenitic Zone suggests construction via episodic cycles of fractionation and replenishment in sizeable (> 250m) magmatic reservoirs, with localized intracrustal sill injection. Primitive mantle-normalized PGE patterns vary systematically from stratiform chromitite Pd/Ir < 1, dunite Pd/Ir < 10 - 20, pyroxenite Pd/Ir < 100 - 150, to gabbro Pd/Ir < 180 - > 1000. Modeling suggests that the PGE fractionation results from extraction of common silicates

  9. Geochemistry and jasper beds from the Ordovician Løkken ophiolite, Norway: origin of proximal and distal siliceous exhalites

    USGS Publications Warehouse

    Grenne, Tor; Slack, John F.

    2005-01-01

    Stratiform beds of jasper (hematitic chert), composed essentially of SiO2 (69-95 wt %) and Fe2O3 (3-25 wt %), can be traced several kilometers along strike in the Ordovician L??kken ophiolite, Norway. These siliceous beds are closely associated with volcanogenic massive sulfide (VMS) deposits and are interpreted as sea-floor gels that were deposited by fallout from hydrothermal plumes in silica-rich seawater, in which plume-derived Fe oxyhydroxide particles promoted flocculation and rapid settling of large (???200 ??m) colloidal particles of silica-iron oxyhydroxide. Concentrations of chalcophile elements in the jasper beds are at the parts per million level implying that sulfide particle fallout was insignificant and that the Si-Fe gel-forming plumes were mainly derived from intermediate- (100??-250??C) to high-temperature (>250??) white smoker-type vents with high Fe/S ratios. The interpreted setting is similar to that of the Lau basin, where high-temperature (280??-334??C) white smoker venting alternates or overlaps with sulfide mound-forming black smoker venting. Ratios of Al, Sc, Th, Hf, and REE to iron are very low and show that the detrital input was <0.1 percent of the bulk jasper. Most jasper beds are enriched in U, V, P, and Mo relative to the North American Shale Composite, reflecting a predominantly seawater source, whereas REE distribution patterns (positive Eu and negative Ce anomalies) reflect variable mixing of hydrothermal solutions with oxic seawater at dilution ratios of ???102 to 104. Trace element variations in the gel precursor to the jasper are thought to have been controlled by coprecipitation and/or adsorption by Fe oxyhydroxide particles that formed by the oxidation of hydrothermal Fe2+ within the variably seawater-diluted hydrothermal plume(s). Thick jasper layers near the H??ydal VMS orebody show distinct positive As/Fe and Sb/Fe anomalies that are attributed to near-vent rapid settling of Si-Fe particles derived from As- and Sb

  10. Plagioclase-peridotites recording the incipient stage of oceanic basin formation: new constraints from the Nain ophiolites (central Iran)

    NASA Astrophysics Data System (ADS)

    Pirnia Naeini, Tahmineh; Arai, Shoji; Saccani, Emilio

    2016-04-01

    spinel; (2) Al, Ca decrease and Cr, Ti increase in pyroxenes and pargasite; (3) slight overall increase in the concentration of rare earth elements and most trace elements in pyroxenes and pargasite, except for Eu and Sr for which a slight decrease is observed. The pointed variations all support the subsolidus origin of the plagioclases, since they took place at constant Mg#s of pyroxenes and coexisting olivines. The magmatic and metamorphic events that affected the Nain peridotites correspond well to the expected geodynamics of a short-lived oceanic basin. Melting was a result of peridotites upwelling (up to the plagioclase-facies) in the early stages of the ocean spreading. Cooling and the subsequent recrystallization in the spinel-facies correspond to the early ceasing of spreading and magmatism in the ocean. Reconditioning of the peridotites in the plagioclase-facies is likely related to the ocean closure and Nain ophiolite emplacement. Based on the short-lived and subduction-related character deduced from the ubiquitous occurrence of amphiboles in the peridotites, we strongly suggest a back-arc setting for the Nain Mesozoic Ocean.

  11. Petrology and geochemistry of Abyssal Peridotites from the Manipur Ophiolite Complex, Indo-Myanmar Orogenic Belt, Northeast India: Implication for melt generation in mid-oceanic ridge environment

    NASA Astrophysics Data System (ADS)

    Krishnakanta Singh, A.

    2013-04-01

    The Manipur Ophiolite Complex (MOC) located in the Indo-Myanmar Orogenic Belt (IMOB) of Northeast India forms a section of the Tethyan Ophiolite Belt of the Alpine-Himalayan orogenic system. Whole rock compositions and mineral chemistry of mantle peridotites from the MOC show an affinity to the abyssal peridotites, characterized by high contents of Al2O3 (1.28-3.30 anhydrous wt.%); low Cr# of Cr-spinel (0.11-0.27); low Mg# of olivine (˜Fo90) and high Al2O3 in pyroxenes (3.71-6.35 wt.%). They have very low REE concentrations (∑REE = 0.48-2.14 ppb). Lherzolites display LREE-depleted patterns (LaN/SmN = 0.14-0.45) with a flat to slightly fractionated HREE segments (SmN/YbN = 0.30-0.65) whereas Cpx-harburgites have flat to upward-inflected LREE patterns (LaN/SmN = 0.13-1.23) with more fractionated HREE patterns (SmN/YbN = 0.13-0.65) than the lherzolite samples. Their platinum group elements (PGE) contents (<50 ppb) and distinct mantle-normalised PGE patterns with the Pd/Ir values (1.8-11.9) and Pt/Pt* values (0.2-1.1) show an affinity to the characteristic of the residual mantle material. Evaluation of mineralogical and petrological characteristics of these peridotites suggests that they represent the residues remaining after low degree of partial melting (˜2-12%) in the spinel stability field of a mid-oceanic ridge environment. The well-preserved mid-oceanic ridge characteristics of these peridotites further suggest that the mantle section was subsequently trapped in the forearc region of the subduction zone without undergoing significant modification in their chemistry by later subduction-related tectonic and petrological processes before its emplacement to the present crustal level.

  12. Seep carbonates and chemosynthetic coral communities in the Early Paleocene alpine accretionary wedge: evidences from the Bocco Shale (Internal Liguride ophiolitic sequence, Northern Apennine, Italy)

    NASA Astrophysics Data System (ADS)

    Pandolfi, Luca; Boschi, Chiara; Luvisi, Edoardo; Alessandro, Ellero; Marroni, Michele; Meneghini, Francesca

    2014-05-01

    In Northern Apennines, the Internal Liguride units are characterized by an ophiolite sequence that represents the stratigraphic base of a Late Jurassic-Early Paleocene sedimentary cover. The Bocco Shale represents the youngest deposit recognized in the sedimentary cover of the ophiolite sequence, sedimented just before the inception of subduction-related deformation history. The Bocco Shale has been interpreted as a fossil example of deposits related to the frontal tectonic erosion of the alpine accretionary wedge slope. The frontal tectonic erosion resulted in a large removal of material from the accretionary wedge front reworked as debris flows and slide deposits sedimented on the lower plate above the trench deposits. These trench-slope deposits may have been successively deformed and metamorphosed during the following accretion processes. The frontal tectonic erosion can be envisaged as a common process during the convergence-related evolution of the Ligure-Piemontese oceanic basin in the Late Cretaceous-Early Tertiary time span. In the uppermost Internal Liguride tectonic unit (Portello Unit of Pandolfi and Marroni. 1997), that crops-out in Trebbia Valley, several isolated blocks of authigenic carbonates, unidentificated corals and intrabasinal carbonatic arenites have been recognized inside the fine-grained sediments that dominate the Early Paleocene Lavagnola Fm. (cfr. Bocco Shale Auctt.). The preliminary data on stable isotopes from blocks of authigenic carbonates (up to 1 m thick and 3 m across) and associated corals archive a methane signatures in their depleted carbon isotope pattern (up to δ13C -30‰ PDB) and suggest the presence of chemosynthetic paleocommunities. The seep-carbonates recognized at the top of Internal Liguride succession (cfr. Bocco Shale Auctt.) occur predominantly as blocks in very thick mudstone-dominated deposits and probably developed in an environment dominated by the expulsion of large volume of cold methane-bearing fluids

  13. Slab dehydration and fluid-producing metamorphic reactions in early subduction stages: the record of the metamorphic sole of the Mont Albert ophiolite (Quebec, Canada)

    NASA Astrophysics Data System (ADS)

    Jewison, Ella; Soret, Mathieu; Dubacq, Benoït; Agard, Philippe; Labrousse, Loïc

    2015-04-01

    Metamorphic soles found at the base of obducted ophiolites provide valuable information on the early history of the subduction / obduction system. Metamorphic soles are characterised by rocks originating from the ocean floor (basalts and sediments in variable proportions) metamorphosed up to granulite facies, where the intensity of metamorphism increases to the top of the unit, towards the contact with peridotite. Their mafic and less frequently pelitic lithologies make them sensitive recorders of their pressure-temperature conditions of crystallization and allow radiometric dating. In addition, metamorphic soles have directly witnessed slab dehydration as they underwent similar fluid-producing metamorphic reactions before being accreted to the mantle wedge peridotites (i.e. before "underplating"). The mechanisms of underplating remain uncertain, because of the somewhat obscure link between weakening through fluid production and hardening via garnet crystallization, with direct consequences on the rheology of the plate interface. In this study, we document fluid-producing reactions occurring during the prograde history of the metamorphic sole of the Taconian (ca. 460 Ma) ophiolite from Mont Albert (Quebec, Canada). This metamorphic sole shows variably metamorphosed mafic and pelitic rocks with metamorphic gradients over the scale of 10 metres, with clinopyroxene-garnet-amphibole granulite facies mafic rocks at the contact with the overlying peridotites. Evidences of melting of pelitic lithologies increase towards the contact, and no remains of metapelites have been found within about 20 m from the contact. Fluid channelization and melt migration is evidenced by decimetric dykes and veins. Away from the contact, metamorphism intensity gradually decreases to greenschist facies with abundant hydrated silicates. The aim of the study is to provide constraints (i) on the nature of the fluids produced (aqueous versus melt), (ii) on their composition and (iii) on the

  14. High-pressure subduction-related serpentinites and metarodingites from East Thessaly (Greece): Implications for their metamorphic, geochemical and geodynamic evolution in the Hellenic-Dinaric ophiolite context

    NASA Astrophysics Data System (ADS)

    Koutsovitis, Petros

    2017-04-01

    Metaophiolites that consist mainly of serpentinites or metabasites outcrop in the East Thessaly region, Central Greece. These formations, along with some ophiolite outcrops, have