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

    NASA Technical Reports Server (NTRS)

    Mustard, John F.; Pieters, Carle M.

    1988-01-01

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

  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. Tectonic setting for ophiolite obduction in Oman

    NASA Astrophysics Data System (ADS)

    Coleman, Robert G.

    1981-04-01

    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 northern 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 10°. 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 northeast dipping thrust faults. Amphibolites developed at the base of the detached hot peridotite as it was thrust southward over oceanic volcanic and sedimentary rocks. 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 after 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. Laterites and conglomerates with reworked laterites on the eroded upper surface of the ophiolite indicate a period of emergence prior to the deposition of shallow water Maestrichtian carbonates. Following emplacement (Eocene) of the Samail ophiolite, the Tethyan oceanic crust began northward subduction, and active arc volcanism started just north of the present Jaz Murian depression in Iran.

  5. 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.

  6. Cordilleran- Versus Tethyan-Type Ophiolites and Global Tectonics

    NASA Astrophysics Data System (ADS)

    Dilek, Y.

    2005-12-01

    Distinguishing between Cordilleran- vs. Tethyan-type ophiolites is a first-order tectonic problem in studying orogenic belts and their geodynamic evolution. Cordilleran ophiolites (CO) in accretionary-type orogenic belts structurally overlie subduction-accretion complexes and are incorporated into active continental margins via progressive underthrusting of oceanic material and/or through ridge-trench interactions. Tethyan-type ophiolites (TO) structurally overlie passive continental margins, microcontinents or island arcs, and are emplaced via collision of these buoyant crustal entities with trenches and their attempted subduction. This process may lead to induced subduction initiation in the region (via subduction jump and/or flip) and precedes terminal continental collisions, which create intra-continental mountain belts (Alps, Himalayas). Cordilleran-type ophiolites are commonly polygenetic, developed on and across a deformed, heterogeneous oceanic basement and may include fully developed island arc sequences having island arc tholeiite (IAT) to calcalkaline affinities, pyroclastic rocks, and felsic differentiates. Prolonged history of subduction with variable polarity and kinematics may generate nested Cordilleran ophiolites with different ages and chemical compositions that may have been affected by orogen-parallel wrench faulting due to oblique convergence. Ophiolites in the Caribbean region are composed of dismembered mafic-ultramafic rock assemblages, including LIP-generated lithologies, and represent fragments of vertically thickened (via plume activities) oceanic crust with polygenetic histories; these kinds of LIP-related ophiolites may also exist in other accretionary-type orogenic belts and in the Precambrian record. Tethyan-type ophiolites include the Ligurian ophiolites with Hess-type oceanic crust and Eastern Mediterranean ophiolites with Penrose-type oceanic crust, and may contain well-developed sheeted dyke complexes developed due to robust magmatic extension beneath narrow rift zones during their seafloor spreading history. Igneous accretion of typical Tethyan-type ophiolites may have been facilitated by slab rollback and mantle flow that resulted in upper plate extension and further melting of previously depleted asthenosphere, showing a progressive evolution from MORB-like to IAT to boninitic proto-arc assemblages. Rift-drift sequences recording early phases of basin opening may be preserved as melange units beneath Tethyan ophiolites. Thus, Tethyan-type ophiolites may reflect the Wilson cycle evolution of ancient oceans and the timing of rifting-related dismantling and collision-driven assembly of supercontinents. Some of these global events may have been linked to the production of plumes and accelerated LIP formation. Recognition of Cordilleran- vs. Tethyan-type ophiolites in ancient mountain belts and in the Precambrian record can be a useful tool to distinguish between accretionary and collisional orogens and their mode of continental growth.

  7. 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.

  8. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

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

    SciTech Connect

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

    1985-01-01

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

  11. 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.

  12. 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.

  13. Chunky Gal Melange and its tectonic significance

    SciTech Connect

    Lacazette, A. Jr.; Rast, N.

    1985-01-01

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

  14. Why melange doesn't matter (and how it might)

    NASA Astrophysics Data System (ADS)

    O'leary, M.; Bassis, J. N.; Sundal, A. V.; Shepherd, A.

    2012-12-01

    It has been widely noted that the behaviour of many ocean-terminating glaciers appears to be controlled by pro-glacial melange, a mixture of icebergs and sea ice that seasonally fills fjords. Changes in both glacier flow velocity and calving regime have been linked to qualitative variations in the melange. However, the properties of the melange itself which lead to these changes have received little attention, and no predictive model exists for these effects. We use SAR imagery of the melange at Jakobshavn Isbræ, West Greenland, obtained during the ERS-2 3-day campaign from March to July 2011 to derive velocities via intensity tracking. By combining these velocities with contemporaneous thickness measurements from Operation IceBridge, we are able to analyse the flow of the melange, and the consequences for its mechanical behaviour. We find that the velocity of the melange is spatially uniform, with near-zero strain rates, indicating plug-like flow. Any shearing at the margins occurs over distance scales too small to be resolved by our imagery. These observations are inconsistent with the existence of significant viscous stresses within the melange, and thus with drag at the fjord walls acting as a source of mechanical resistance. At the time of major calving events, we observe translation of the melange by up to a kilometre over a period of less than 3 days, with some accompanying extension of the melange. In the aftermath, melange movement is slowed, and compression is observed as the melange consolidates in its new position. As time passes, this compression lessens, and the melange flow accelerates until the speed of the melange matches the speed of the glacier front . This cycle is repeated several times prior to the eventual breakup of the melange. We also observe clear evidence of compression of the melange as it is forced into the fjord wall by a former tributary of the glacier, which now exists as a separate ice front. We hypothesise that resistance to compression is the source of any force exerted by the melange on the glacier front. If this is true, then we suggest that the detailed geometry of the fjord may play a significant role in determining the effects of melange on tidewater glaciers.

  15. DEFORMATION OF THE HURRICANE MOUNTAIN FORMATION MELANGE ALONG TOMHEGAN AND

    E-print Network

    Beane, Rachel J.

    DEFORMATION OF THE HURRICANE MOUNTAIN FORMATION MELANGE ALONG TOMHEGAN AND COLD STREAMS, WEST through Acadian deformation recorded in foliated pelites of the Hurricane Mountain Formation in west central Maine. The Hurricane Mountain Formation is a melange with a grey sulfidic slate- to gneiss- matrix

  16. 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.

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

    USGS Publications Warehouse

    Donato, M.M.

    1987-01-01

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

  19. Melanges, intrusive and extrusive sediments, and hydraulic arcs

    NASA Astrophysics Data System (ADS)

    Talbot, Christopher J.; von Brunn, Victor

    1989-05-01

    Water-carrying sediment may intrude (with or without associated extrusion) wherever the fluid pressure exceeds the intrusion strength of the surrounding materials. It has recently been suggested that thrust sheets moving toward continental interiors act as giant squeegees driving fluids through their substrates ahead of them. Active ice sheets and sedimentary prisms can also squeeze fluids through rocks. Such fluids potentially inject hydraulic melanges along their plumbing systems, but these are difficult to distinguish from other melanges and from broken formations or olistostromes because of surficial slumping or extrusion. Miniature examples of melanges pumped through a marine slope by the tidal grounding of an ice sheet of Permian-Carboniferous Dwyka age in South Africa are used to illustrate that there are even more categories of melange than previously suspected. Instead of being clarified by recent work, the problem of deducing the origin of individual melanges becomes more difficult. This is because melanges are formed by the mixing of thick hydrous slurries in any confined conditions and are not diagnostic of subduction. Nonetheless, the results of hydraulic activity within a subglacial marine slope suggest that the sedimentary volcanoes developed on many accretionary prisms overlie relatively cold hydraulic arcs oceanward of the hot magmatic arcs behind subduction zones.

  20. 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, Kahramanmara? Region, SE Turkey)

    NASA Astrophysics Data System (ADS)

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

    2015-07-01

    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.

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. Ophiolites as Analogs to Habitats on Mars

    NASA Technical Reports Server (NTRS)

    Schulte, M.; Blake, D. F.

    2003-01-01

    Ophiolite sequences that are located in northern and central California provide easily accessible areas that serve as good analogs for martian crustal rocks. The rock types found in a typical ophiolite sequence compare well with those found in the Mars meteorites, and those expected from spectrophotometric analysis. We have begun investigating and characterizing these sites in order to understand better the processes that may be responsible for the groundwater chemistry, mineralogy and biology of similar environments on Mars.

  7. 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.

  8. 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.

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

    PubMed

    Hall, R

    1980-06-13

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

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

    USGS Publications Warehouse

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

    1997-01-01

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

  11. The Oman Ophiolite from Detachment to Obduction

    NASA Astrophysics Data System (ADS)

    Boudier, F. I.

    2014-12-01

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

  12. 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.

  13. A vestige of Earth's oldest ophiolite.

    PubMed

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

    2007-03-23

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

  14. 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.

  15. Actualistic Ophiolite Provenance: The Cyprus Case.

    PubMed

    Garzanti; Andò; Scutellà

    2000-03-01

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

  16. Dike complexes in ophiolites of the Urals

    NASA Astrophysics Data System (ADS)

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

    2015-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2004-01-01

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

  18. 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. Commonly the dunite contains disseminated sulfide minerals and at the Acoje Mines, platinum-group elements. A compositional layering within the gabbro is in places cumulate in the lower part of the unit but may have formed by nucleation higher up on the relatively steep sides of the magma chamber. A widespread gneissic banding in the gabbro forms large mappable structures which are many times more complex than is the disposition of the major rock units. These structures are believed to be the result of extensive slumping in the magma chamber. The structure produced by the cumulate layering merges with the gneissic banding, commonly without discernible change in attitude. This tectonic layered structure crosses the gabbro-peridotite boundary at any angle without seeming to disturb the original rock distribution. At greater depths below the boundary (ca. 800 m), the harzburgite contains low dipping banding, which probably reflects the result of differential movement within the mantle. Chromite occurs almost exclusively in a zone that generally lies no more than 200-300 m below the gabbro-peridotite boundary. Refractory-grade chromite is found in this zone below the olivine gabbro in the Goto block and as low-grade metallurgical grade chromite below norite in the Acoje block. At Acoje Mines the chromite is present in layers in dunite, which the writer interprets as being distributed in a zone along the gently dipping (ca. 25??) gabbro-peridotite boundary. The steeply dipping (ca. 60-80 ?? ) individual layers lie en echelon along the boundary at an angle (ca. 50 ?? ) to the contact. At Coto the chromite forms large discontinuous masses in the lowest dunite and in the uppermost harzburgite. Except for the chromite present as layers at Acoje, the regional tectonic layering crosses the chromite deposits without structural deviation. The chromite deposits and associated peridotite may be cumulate in origin, but have been modified to such an extent that cumulate textures are gener

  19. 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 setting of the motif formation and assemblage is uncertain at this time. A model involving the obduction of several separate sheets onto the ancestral North American continental margin is appealing in that it involves the closing and destruction of a marginal basin, a relatively simple concept. This model fails, however, to supply a source for the sediment necessary to form the precursory melanges. A trench-slope origin would supply the vast amount of needed sediment by accretion. The precursory melangeallochthon motifs would then be stacked near the base of the trench slope. This model is appealing and requires a rather complicated assemblage of continental, arc, and oceanic fragments, such as those that occur on many modern continental margins.

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

    E-print Network

    Meyers, Stephen R.

    online 13 August 2013 Keywords: Oceanic plagiogranite Dike-gabbro transition Oxygen isotopes Ophiolite the dike-gabbro transition zone of the northern Oman Ophiolite yield the lowest 18 O(Zrn), with rock

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

    E-print Network

    Cattin, Rodolphe

    Subsidence in magma chamber and the development of magmatic foliation in Oman ophiolite gabbros Keywords: Oman ophiolite fast spreading ridges magma chamber gabbro subsidence In the Oman ophiolite to the upper gabbros unit have been mapped in great detail, in selected areas of the southern massifs

  2. Ophiolite and Tectonic Development of the East Pacific Margin

    NASA Astrophysics Data System (ADS)

    Moores, E. M.

    2001-12-01

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

  3. 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.

  4. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

  6. Ophiolites in the Non-volcanic Banda Outer Arc of East Indonesia: Field Occurrence and Petrological Variety of the World's Youngest Ophiolite

    E-print Network

    Ikegami, Takashi

    understanding of Timor-Tanimbar peridotite masses by age-dating studies employing several radiogenic isotope systematics. Key wordswestern Pacific region, ophiolite, peridotite, mantle melting degree, forearc

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

    USGS Publications Warehouse

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

    1998-01-01

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

  8. Precambrian Ophiolites and Related Rocks Edited by Timothy M. Kusky

    E-print Network

    Stern, Robert J.

    Geology, Vol. 13 (K.C. Condie, Series Editor) 95 © 2004 Elsevier B.V. All rights reserved. Chapter 3 NEOPROTEROZOIC OPHIOLITES OF THE ARABIAN-NUBIAN SHIELD ROBERT J. STERNa, PETER R. JOHNSONb, ALFRED KRÖNERc AND BISRAT YIBASd aGeosciences Department, University of Texas at Dallas, Box 830688, Richardson, TX 57083

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

    USGS Publications Warehouse

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

    1991-01-01

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

  10. 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.

  11. 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.

  12. 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.

  13. Albanian ophiolites as probes of a mantle heterogeneity study

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

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

  14. Western Baie Verte Peninsula revisited: from ophiolite obduction onto Laurentia, the Notre Dame continental arc, to post-arc continental volcanism and the Salinic Orogeny.

    E-print Network

    Kidd, William S. F.

    Complex contains dismembered ophiolite including mantle, boninitic cumulates, gabbro and sheeted dykes of boninitic cumulates, 488 Ma gabbro, sheeted dykes and IAT basalts, whereas the ophiolite section

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

    NASA Astrophysics Data System (ADS)

    Dygert, Nick; Liang, Yan

    2015-06-01

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

  16. 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-level change. Terrigenous sediment gradually encroached from neighbouring landmasses as the ophiolite was faulted or eroded. An Aptian transgression was followed by regression, creating a local unconformity (e.g. at Boboshtica). A Turonian marine transgression initiated widespread Upper Cretaceous shelf carbonate deposition. In the regional context, the southern Albania ophiolites appear to have been rapidly emplaced onto a continental margin in a subaqueous setting during the Late Jurassic (Late Oxfordian-Late Tithonian). This was followed by gradual emergence, probably in response to thinning of the ophiolite by erosion and/or exhumation. The sedimentary cover of the south Albanian ophiolites is consistent with rapid, relatively short-distance emplacement of a regional-scale ophiolite over a local Pelagonian-Korabi microcontinent.

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

    USGS Publications Warehouse

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

    2004-01-01

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

  18. 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.

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

    NASA Astrophysics Data System (ADS)

    Acharyya, S. K.

    2015-07-01

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

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

    SciTech Connect

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

    1992-01-01

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

  1. Pictorial Article Late Cretaceous forearc ophiolites of Iraniar_745 1..4

    E-print Network

    Stern, Robert J.

    , * AND ROBERT J. STERN2 1 School of Earth Sciences, Damghan University, Damghan, Iran (email: hadishafaii@du.ac.ir), and 2 Geosciences Department, University of Texas at Dallas, Richardson, TX75083-0688, USA Ophiolites of ophiolite. Sheeted dikes and pillow lavas are common in IB (Fig. 3a,c). The typical IB (Fig. 2a) is overlain

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

    USGS Publications Warehouse

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

    1999-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  4. The Habitability of Mars: Lessons From Ophiolites on Earth

    NASA Astrophysics Data System (ADS)

    Schulte, M.

    2003-12-01

    Ophiolite sequences, sections of lower oceanic crust and upper mantle that have been thrust onto continental craton, are located in northern and central California. These rock suites provide easily accessible outcrops that likely parallel the rock types on Mars. We have begun investigating and characterizing these sites in order to understand better the processes that may be responsible for the water chemistry, mineralogy and biology on Mars. The ophiolites found in northern and central California include the Trinity, Josephine, Coast Range and Point Sal, and all are approximately 160 million years old. These ophiolite bodies are actively serpentinizing. Fluids from serpentinizing springs are generally alkaline, with high pH and H2 contents, indicating that the mafic rock compositions control the fluid chemistry through water-rock reactions during relatively low-grade hydrothermal processes. The general reaction describing the serpentinization of olivine is given by: olivine + H2O = serpentine + brucite + magnetite + H2. We have analyzed the petrography and mineralogical composition of a number of rock samples collected from the Coast Range Ophiolite near Clear Lake, CA by electron microprobe. The remnant primary mineralogy is fairly uniform in composition, with an olivine composition of Fo90, and with pyroxene compositions of En90 for orthopyroxene and En49Wo48Fs03 for the clinopyroxene. There are several generations of alteration products, comprised mostly of serpentines that are magnesium rich, with magnetite, brucite and carbonates observed as accessory minerals. The oxidation of iron in the ferrous component of olivine (and pyroxene) results in the generation of H2, as indicated by the presence of magnetite. The formation of carbonates can be taken to indicate the presence of CO2 in the altering fluids. The H2 generated through water-rock reactions in these systems may provide an energy source for chemolithoautotrophic ecosystems, while the CO2 serves as a carbon source. The identification of several species of Archaea from these rocks, including an alkalophile, indicates that these geochemical environments do serve as habitats. We suggest that serpentinizing springs in mafic to ultramafic terranes may be the most likely areas on Mars to be habitable.

  5. 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.

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

    NASA Astrophysics Data System (ADS)

    Palandzhyan, S. A.

    2015-03-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2006-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

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

  10. 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.

  11. The Late Cretaceous Dehshir ophiolite is an important element within the Inner

    E-print Network

    Stern, Robert J.

    tectonized harzburgites, gabbros, sheeted dike com- plexes, pillowed basalts, and rare ultra- mafic, leucogabbro, and pegmatite gabbro. All the massifs in the Inner Zagros ophiolite belt are overlain by Turonian

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

    NASA Astrophysics Data System (ADS)

    Chavez Garcia, Maria Graciela

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

  13. 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.

  14. 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.

  15. Scientific Drilling on the Diamond-Bearing Luobusa Ophiolite, Tibet

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

  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 ?13C/?18O, 87Sr/86Sr and 14C analysis of all major carbonate lithofacies identified. Our analyses of modern carbonate crusts forming on the surface of stagnant hyperalkaline (pH >11) waters show highly depleted ?13C and ?18O values (-25.5‰ ×0.5 PDB and -16.8‰ ×0.5 PDB respectively). This depletion has been attributed to a kinetic isotope effect occurring during atmospheric CO2 exchange with Ca(OH)2 hyperalkaline waters [1]. By comparison, inactive travertine deposits show a large range in ?13C (-10.5 to -21.8‰ PDB) which lies on a trajectory from the composition of modern crusts towards bicarbonate fluids in equilibrium with soil CO2. We interpret this trend as being produced by the mixing of different carbon sources, either at the time of formation or during later alteration. Modern carbonates and inactive travertines also have 87Sr/86Sr ratios and Sr concentrations similar to Cretaceous and Tertiary limestones which surround the ophiolite, whilst subsurface veins also display 87Sr/86Sr ratios similar to these Cretaceous limestones. Carbon recycling can also be determined with 14C. Modern atmospheric CO2 has a global average of 105-106% modern 14C (pMC), therefore freshwater carbonates forming solely from atmospheric CO2 would be expected to contain >100 pMC. However, modern carbonates display varied results from 94.5-101.4 pMC. Low values could be caused by meteoric waters incorporating 14C 'dead' carbon through the dissolution of limestones and/or uptake of soil CO2. This 'dead' carbon would then be assimilated into veins and surface deposits, offsetting pMC values. Inactive travertines show significant fluctuations in 14C values within a single hand sample, where stratigraphically younger samples give older radiocarbon 'ages' outside of error. These fluctuations may have been caused by the presence of limestone sourced 'dead' carbon in waters at time of formation, surface runoff containing soil CO2 or by later recrystallisation. Isotopic evidence indicates that mixing of contemporary atmospheric carbon and recycled older carbon has taken place during the on-going carbonation of the Oman-UAE ophiolite sequence. Failure to account for this recycled carbon could lead to inaccurate estimates of natural CO2 sequestration rates. References [1] Clark, I.D. and Fontes, J. (1990) Palaeoclimatic reconstruction in Northern Oman based on carbonates from hyperalkaline groundwaters. Quaternary Res, 33, 320-336

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

    USGS Publications Warehouse

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

    1988-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Falk, Elisabeth S.; Kelemen, Peter B.

    2015-07-01

    Extensive outcrops of listvenite-fully carbonated peridotite, with all Mg in carbonate minerals and all Si in quartz-occur along the basal thrust of the Samail ophiolite in Oman. These rocks can provide insight into processes including (a) carbon fluxes at the "leading edge of the mantle wedge" in subduction zones and (b) enhanced mineral carbonation of peridotite as a means of carbon storage. Here we examine mineralogical, chemical and isotopic evidence on the temperatures, timing, and fluid compositions involved in the formation of this listvenite. The listvenites are composed primarily of magnesite and/or dolomite + quartz + relict Cr-spinel. In some instances the conversion of peridotite to listvenite is nearly isochemical except for the addition of CO2, while other samples have also seen significant calcium addition and/or variable, minor addition of K and Mn. Along margins where listvenite bodies are in contact with serpentinized peridotite, talc and antigorite are present in addition to carbonate and quartz. The presence of antigorite + quartz + talc in these samples implies temperatures of 80-130 °C. This range of temperature is consistent with dolomite and magnesite clumped isotope thermometry in listvenite (average T = 90 ± 15 °C) and with conventional mineral-water oxygen isotope exchange thermometry (assuming fluid ?18O near zero). CO2-bearing fluids responsible for the formation of listvenite were likely derived from underlying calcite-bearing metasediment during emplacement of the ophiolite. An internal Rb-Sr isochron from one listvenite sample yields an age of 97 ± 29 Ma, consistent with the timing of emplacement of the ophiolite over allochthonous sediments of the Hawasina group, and autochthonous sediments of the Arabian continental margin. Most of the initial 87Sr/86Sr values in the listvenite, ranging from 0.7085 to 0.7135, are significantly higher than seawater values and consistent with values measured in the underlying metasediments. While constraints on the pressure of listvenite formation are lacking, the moderate temperatures suggest that listvenites formed at relatively shallow depths in the subduction zone, making release of carbonate-saturated pore-water due to compaction of subducted sediment or low-pressure phase transitions of hydrous minerals, such as clays, probable sources of the CO2-bearing fluid. Carbonate dissolution from subducted sediments and transfer of CO2 to the mantle wedge to form listvenites may be an important process in forearc hydrothermal systems. Additionally, the presence of listvenites demonstrate that peridotite carbonation reactions can proceed to completion on large scales, suggesting that in situ mineral carbonation of peridotite may offer a viable solution for carbon storage.

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

    PubMed

    Economou-Eliopoulos, M; Eliopoulos, D G

    1998-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    1988-10-01

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

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

    NASA Astrophysics Data System (ADS)

    García-Casco, A.

    2012-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Anma, Ryo

    2013-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

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

  5. 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.

  6. 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.

  7. 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.

  8. Osmium isotope systematics of the Proterozoic and Phanerozoic ophiolitic chromitites: In situ ion probe analysis of primary

    E-print Network

    Helwan, Cairo, Egypt b Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA c Department chromitites both in the Proterozoic ophiolite, Eastern Desert, Egypt, and in the Phanerozoic Oman ophiolite a significantly higher average ratio. The Moho transition zone (MTZ) chromitites are highly variable in the 187 Os

  9. 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.

  10. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  12. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

  14. 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 followed by rapid slab rollback in ancient ocean basins.

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

    NASA Astrophysics Data System (ADS)

    Wright, James E.; Wyld, Sandra J.

    1986-08-01

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

  16. Half a Century of Oman Ophiolite Studies: SSZ or MOR, the Arc Disposal Problem

    NASA Astrophysics Data System (ADS)

    Gregory, R. T.; Gray, D.

    2014-12-01

    The Samail Ophiolite, one of the largest and best exposed ophiolite complexes, is a Tethyan ophiolite obducted in the Late Cretaceous onto the formerly passive Arabian platform as Arabia began its most recent >1000 km northward migration towards a Miocene collision with Eurasia. The Oman Mountains, northeastern Arabian Peninsula have yet to collide with Eurasia; present uplift and form of the mountains also date to the Miocene. In addition to the scientific scrutiny of the ophiolite complex, the geologic constraints on the timing and emplacement of the ophiolite are abundant with no consensus on the obduction mechanism or its original tectonic setting. The crustal thickness of the ophiolite is comparable to thicknesses observed for "normal" mid-ocean ridges. Largely on the basis of structural and paleomagnetic arguments, some workers have attributed its origin to Pacific-type fast spreading ridges and complex micro plate geometries. Indeed the lower pillow lava sequences and much of the gabbroic crust have isotope and geochemical signatures consistent with a MORB source. However, because of the geochemistry of the upper pillow lavas, the ophiolite is most often characterized as a supra-subduction zone (SSZ) ophiolite, i.e. it sits in the hanging wall of some large tectonic structure for part of its history. In the absence of a preserved arc, the SSZ designation has little explanatory power only being a declaration of allochthony or about chemical properties of the mantle source. That associated continental shelf and oceanic crustal sections have suffered either clockwise or counterclockwise PT time trajectories requires some type of nascent subduction and hanging wall thrust transport of the young ridge crest. The widespread Late Cretaceous obduction of Tethyan oceanic crust and mantle over thousands of kilometers strike length is a problem for SSZ models (arc, forearc, back arc etc.) because arc initiation results in thick crust on short time scales, none of which is preserved. Similarly, fast spreading models require the subduction of >106 km2 without the initiation and preservation of a magmatic arc. Some Late Cretaceous major plate boundary reorganization that resulted in the current plate motion regime with NNE convergence between Afro-Arabia and Eurasia will be part of any final solution to the Oman problem.

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

    NASA Astrophysics Data System (ADS)

    Stern, R.

    2003-04-01

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

  18. Faulting and Serpentinisation of Peridotites in the Leka Ophiolite

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

  19. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  3. Forearc hyperextension by detachment faulting and ophiolite dismemberment: examples from the Yarlung Tsangpo Suture Zone (Southern Tibet)

    NASA Astrophysics Data System (ADS)

    Maffione, M.; Van Hinsbergen, D. J. J.; Huang, W.; Koornneef, L.; Guilmette, C.; Borneman, N.; Hodges, K. V.; Kapp, P. A.; Lin, D.

    2014-12-01

    The broad deformation zone of the Himalayan belt and Tibetan plateau is largely the product of continent-continent collision between India and Eurasia plates since the Early Eocene. Ophiolites exposed along the Yarlung Tsangpo Suture Zone (southern Tibet) demonstrate that a long-lasting intra-oceanic subduction zone must have played a significant role in accommodating closure of the >7500 km wide Neotethyan Ocean before continental collision. Their study can provide key constraints on the initial history of the Neotethyan subduction systems, and the following formation and emplacement of the ophiolite. Paleomagnetic analyses of the sheeted dykes complex of ophiolites have been successfully applied in the past to reconstruct the initial geometry of the spreading system associated to the ophiolite formation. Furthermore, oceanic detachment faults, structures widely occurring in modern magma-poor (slow-spreading) mid-ocean ridges, have been recently recognized also in ophiolites (i.e., Mirdita ophiolite of Albania), and (if present) their study may provide unique insights into the geodynamics and geometry of the associated spreading system. The YZSZ ophiolites form a 2500 km long belt mainly composed of dismembered ultramafic massifs locally covered by a crustal sequence and oceanic sediments, underlying a regionally continuous clastic Xigaze sedimentary basin interpreted as the Tibetan forearc. Our study focused along a ~250 km transect within the eastern sector of the YZSZ between the Sangsang and Xigatze ophiolite. Paleomagnetic, structural geological, and geochemical analyses evidenced the presence of fossil oceanic detachment faults that locally (Sangsang) exposed the lowermost units (mantle) directly at the seafloor. Based on this evidence, and the reconstructed rotation pattern of the region we propose a tectonic evolutionary model characterized by the interplay between magmatic crustal accretion and trench-parallel and trench-perpendicular tectonic extension, which resulted in thinning and dismemberment of the ophiolite.

  4. Root zone of the sheeted dike complex in the Oman ophiolite Adolphe Nicolas and Francoise Boudier

    E-print Network

    Demouchy, Sylvie

    . The Oman ophiolite is derived from a fast spreading ridge which had a melt lens located between the main the sheeted dike complex, up to the seafloor. The isotropic gabbros near the base of the root zone: 13,629 words, 15 figures, 2 tables. Keywords: fast spreading ridges; lid/gabbros transition; Oman

  5. Ophiolites are on-land remnants of oceanic lithosphere, and most of the more extensive

    E-print Network

    Basu, Asish R.

    of subduction beneath young and hot oceanic lithosphere. Disagreement exists as to whether suprasubduction-zone extensive ophiolites apparently formed above a sub- duction zone, a tectonic setting known as a suprasubduction-zone setting. Thin sheets of high-grade metamorphic rocks, known as metamorphic soles, crop out

  6. Birth, death, and resurrection: The life cycle of suprasubduction zone ophiolites

    E-print Network

    Shervais, John W.

    Birth, death, and resurrection: The life cycle of suprasubduction zone ophiolites John W. Shervais of quartz diorite and eruption of silicic lavas; and (4) death, which is the sudden demise of active 13, 2000; Accepted November 1, 2000; Published January 31, 2001. Shervais, J. W., 2001. Birth, death

  7. Deep and High-temperature Hydrothermal Circulation in the Oman Ophiolite-------

    E-print Network

    Cattin, Rodolphe

    search for evidence of high-temperature hydrous alteration within the gabbros of the Samail ophiolite (Oman) shows that most of the gabbros have been affected by successive stages of alteration, starting and origin of the fluids associated with these alteration events. Massive gabbros, dykes and veins

  8. Significance of the concentration gradients associated with dunite bodies in the Josephine and Trinity ophiolites

    E-print Network

    bodies in the Josephine and Trinity ophiolites. The major peridotite lithologies sampled in the Josephine to explain compositional variations across the peridotite sequences. Melt flow from the host harzburgite the Josephine transect. In contrast, melt flow from the dunite into the surrounding peridotite lithologies can

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

    E-print Network

    Royal Holloway, University of London

    Palaeomagnetic data from a Mesozoic Philippine Sea Plate ophiolite on Obi Island, Eastern Indonesia exposed on Obi Island, eastern Indonesia. Until the late Neogene, Obi formed part of the southern tectonic history of eastern Indonesia and northern New Guinea has been dominated by the punctuated

  10. The oceanic crust in 3D: Paleomagnetic reconstruction in the Troodos ophiolite gabbro

    E-print Network

    Granot, Roi

    The oceanic crust in 3D: Paleomagnetic reconstruction in the Troodos ophiolite gabbro Roi Granot a by new paleomagnetic data from the gabbro suite. The gabbro suite is exposed at the extinct spreading-transform intersection. Remanence directions from gabbros (23 sites) were used as indicators for rigid body rotation

  11. 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 ?13C values are present in both sets of diamonds. The Tibetan diamonds (n = 3) show an exceptionally large range in ?15N (-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 ?15N = 0 ‰ in {111} sectors and + 30 ‰ in {100} sectors, as reported in the literature. This comparison is limited by the small sample set combined with the fact the diamonds probably grew by different processes. However, the Tibetan diamonds do have generally higher concentrations and different ratios of trace elements; most inclusions are a NiMnCo alloy, but there are also some small REE-rich phases never seen in HPHT synthetics. These characteristics indicate that the Tibetan diamonds grew in contact with a C-saturated Ni-Mn-Co-rich melt in a highly reduced environment. The stable isotopes indicate a major subduction-related contribution to the chemical environment. The unaggregated nitrogen, combined with the lack of evidence for resorption or plastic deformation, suggests a short (geologically speaking) residence in the mantle. Previously published models to explain the occurrence of the diamonds, and other phases indicative of highly reduced conditions and very high pressures, have failed to take into account the characteristics of the diamonds and the implications for their formation. For these diamonds to be seriously considered as the result of a natural growth environment requires a new understanding of mantle conditions that could produce them.

  12. Active compressive intraoceanic deformation: early stages of ophiolites emplacement?

    NASA Astrophysics Data System (ADS)

    Chamot-Rooke, Nicolas; Delescluse, Matthias; Montési, Laurent

    2010-05-01

    Oceanic lithosphere is strong and continental lithosphere is weak. As a result, there is relatively little deformation in the oceanic domain away from plate boundaries. However, the interior of oceanic lithosphere does deform when highly stressed. We review here places where intraoceanic compression is at work. In the more than 30 years since the first observations of active compressive intraplate deformation in the Central Indian Ocean through seismic profiling (Eittreim et al., 1972), compressive deformation has been identified in a variety of other oceanic tectonic settings: as a result of small differential motion between large plates (between North America and South America in the Central Atlantic; between Eurasia and Nubia offshore Gibraltar; between Macquarie and Australia plates in the Southern Ocean), within back-arcs (northwest Celebes Sea, Okushiri Ridge in the Japan Sea, on the eastern border of the Caroline plate), and ahead of subduction (Zenisu Ridge off Nankai Trough). Deformation appears to be more diffuse when larger plates are involved, and more localized for younger plates, perhaps in relation with the increasing rigidity of oceanic plates with age. The best example of diffuse deformation studied so far remains the Central Indian Ocean. Numerous marine data have been collected in this area, including shallow and deep seismic, heat flow measurements, multibeam bathymetry. The present-day deformation field has been modeled using GPS and earthquakes as far field and near field constraints respectively. Reactivation of the oceanic fabric (including for portions of the Indo-Australian plate which are now in subduction as evidenced by the September 2009 Padang earthquake), selective fault abandonment (Delescluse et al., 2008) and serpentinization (Delescluse and Chamot-Rooke, 2008) are some of the important processes that shape the present-day pattern of deformation. These rare intraplate deformation areas constitute excellent natural laboratories to investigate the very early stages of formation of faulted oceanic bodies that may further be incorporated into mountain belts as ophiolites. They allow to discuss rates and duration of deformation, diffuse vs localized deformation, re-activation vs neo-formed faults, serpentinization and thermal regime, spacing of minor and major thrust faults. Delescluse, M., L. G. J. Montesi, and N. Chamot-Rooke (2008) Fault reactivation and selective abandonment in the oceanic lithosphere. Geophys. Res. Lett., v. 35. Delescluse, M., and N. Chamot-Rooke (2008) Serpentinization pulse in the actively deforming Central Indian Basin. Earth Planet. Sci. Lett., v. 276, p. 140-151. Eittreim, S. L., and J. Ewing (1972), Mid-Plate Tectonics in the Indian Ocean, J. Geophys. Res., 77(32), 6413-6421.

  13. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

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

  15. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-10-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

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

    PubMed

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

    2004-11-11

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

  1. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

    We present high-precision Fe and Mg isotopic data for the Purang ophiolite, southwestern Tibet, representing the first combined Fe and Mg isotopic study of the oceanic lithosphere hitherto. The ?56Fe and ?26Mg values of the ophiolitic peridotite, dunite and gabbro vary from -0.209 to 0.187‰ and from -0.28 to - 0.14 ‰, respectively. The average ?56Fe of the peridotites is - 0.030 ± 0.143 ‰ (2SD, n = 17), a value indistinguishable from abyssal peridotites and chondrites, and lower than oceanic basalts. The average ?26Mg value of the peridotites is - 0.20 ± 0.10 ‰, a value slightly higher than both chondrites and oceanic basalts. Correlations between ?56Fe and indices of partial melting indicate fractionation of 0.323‰ in ?56Fe between the oceanic lithospheric mantle and the overlying mafic crust during an early episode of partial melting, presumably beneath a spreading centre. Subsequent metasomatism in a supra-subduction zone caused elevated oxygen fugacity and heavy Fe isotopic compositions in the oceanic lithospheric mantle. The dunite with high Ba/La, a proxy for oxygen fugacity, and high ?56Fe values was likely formed during this process of sub-arc mantle-melt interaction. The negatively coupled Fe-Mg isotopic variations of the Purang ophiolite indicate that Mg isotope fractionation may also occur during high-temperature mantle processes. The observed isotopic variations among different lithologies in the ophiolite may satisfactorily account for the isotopic differences between arc lavas and mantle peridotites with respect to oceanic basalts, thus providing implications for crust-mantle differentiation.

  3. Insights on deep, accretionary subduction processes from the Sistan ophiolitic "mélange" (Eastern Iran)

    NASA Astrophysics Data System (ADS)

    Angiboust, S.; Agard, P.; De Hoog, J. C. M.; Omrani, J.; Plunder, A.

    2013-01-01

    The Sistan ophiolitic belt, formed by the closure of the N-S trending Sistan Ocean during late Cretaceous times, comprises several branches and basins across a 100 × 700 km area along the Iran-Afghanistan border. One of these, the Ratuk complex, exposes disrupted HP ophiolitic blocks from a paleo-subduction complex generally interpreted as a tectonic "mélange". In order to better understand its overall structure and evaluate the degree of mixing within this mélange, an extensive set of serpentinized peridotites, mafic rocks and metasediments was collected in the Sulabest area (Ratuk complex). A detailed geological and structural map of the Sulabest area is herein provided, in which three main units (the Western, Upper and Eclogitic Units) separated by relatively sharp tectonic contacts were identified. The latter two of these slices exhibit metamorphic evidence for burial along the same HP-LT gradient (up to blueschist and eclogite facies, respectively). Sharp differences in peak metamorphic conditions and retrograde parageneses nevertheless suggest that they followed two distinct P-T trajectories. Geochemical signatures of ultramafic rocks indicate an abyssal origin for the non-metamorphic Western Unit while the presence of mantle wedge serpentinites is inferred for some samples from the high-pressure units. The differences in peak temperatures (between 520 and 650 °C) and the geochemical heterogeneity of mafic rocks suggest that tectonic mixing occurred (only) within the high-pressure units, possibly within the hydrated mantle wedge. Our results show that this portion of the Sistan ophiolitic belt did not form, as earlier proposed, by chaotic tectonic "mélange" (i.e. where small tectonic blocks with distinct P-T histories are mixed in a mechanically weak matrix). We instead propose that this segment of the ophiolitic belt formed via accretionary processes deep in the subduction zone, whereby distinct slices with different P-T histories were tectonically juxtaposed at ca. 20 km depth during exhumation.

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

    NASA Astrophysics Data System (ADS)

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

    2010-06-01

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

  5. Subduction initiation at oceanic detachment faults: a mechanism to generate extensive ophiolite belts

    NASA Astrophysics Data System (ADS)

    Maffione, Marco; Thieulot, Cedric; van Hinsbergen, Douwe; Morris, Antony; Plumper, oliver; Spakman, Wim

    2015-04-01

    One of the least understood processes of plate tectonics is the nucleation of new subduction zones and the formation of ophiolites by subsequent upper plate extension. Subduction initiation within ocean basins is thought to occur along weakness zones such as transform faults, fracture zones, and mid-ocean ridges. Detachment faults, which cut across oceanic lithosphere immediately adjacent to slow-spreading mid-ocean ridges may yields ideal rheological conditions for subduction initiation due to their pervasive serpentinization. We numerically test this hypothesis by modeling the inversion of an ocean basin cut by a serpentinized detachment fault adjacent to an active spreading center. The results of our models consistently show that the serpentinized fault effectively localizes deformation, assisting subduction initiation upon compression. Subsequent reactivation of the pre-existing spreading center preserved in the forearc above the nascent subduction zone provides an efficient mechanism for the formation of supra-subduction zone ophiolites. Application of our model of subduction initiation to the ~700 km-long ophiolite belt spanning from Albania to Greece is then discussed.

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

    USGS Publications Warehouse

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

    1999-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Rollinson, H. R.

    2014-12-01

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

  9. 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.

  10. 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.

  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. Composition and geodynamic setting of the volcanic rocks from ophiolites of the Ust'-Belaya Mountains, Chukchi Peninsula

    NASA Astrophysics Data System (ADS)

    Moiseev, A. V.; Sokolov, S. D.; Hyasaka, Y.

    2011-03-01

    In addition to ophiolites in the structure of the Otrozhnaya sheet, the igneous rocks were established within the Middle Devonian-Lower Carboniferous tuffaceous-terrigenous complex earlier considered to be the cover of the ophiolite association. In order to establish their geodynamic formation setting, the geochemical study of igneous rocks was conducted. The volcanic rocks from the ophiolite complex are similar to MORB; subvolcanic rocks of tuffaceous-terrigenous complex have a suprasubduction origin. An abundance of pyroclastic rocks and the type of sediments allow us to conclude about their formation in an island arc setting. The existence of the Middle Devonian-Lower Carboniferous island arc complex within the Ust'-Belaya Mountains gives rise to continue the Koni-Taigonos arc inside the region and testifies to its subsistence in the Devonian.

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

    PubMed

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

    2001-05-11

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

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

    NASA Astrophysics Data System (ADS)

    Koutsovitis, P.; Magganas, A.

    2012-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-05-01

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

  16. The Olivos Ophiolite (Chihuahua, Mexico) Marks the Suture Zone Along the Southwestern Margin of Laurentia

    NASA Astrophysics Data System (ADS)

    Franco-Rubio, M.; Comaduran-Ahumada, O.; Alva-Valdivia, L.; Urrutia-Fucugauchi, J.

    2006-05-01

    We report on the finding of obducted fragments of oceanic crust of Late Permian age within Jurassic sediments (Upper Bajocian-Middle Callovian) on the southwestern margin of the North American craton (Laurentia). Pangea paleoreconstructions for the Gulf of Mexico and Central Atlantic Ocean result in a significant overlap of Mexico, Central America and South America, implying relative motion and different paleopositions for most of Mexico and nuclear Central America. Studies of the basement and major tectonic features have identified several terranes with distinct tectonostratigraphic records, but their origin, nature and evolution are not yet fully understood. This is partly due to the scarcity of Precambrian and Paleozoic basement outcrops. Largest Paleozoic outcrops occur in northern Mexico in the Chihuahua, Caborca and Cortez terranes. Study of the Sierra de Olivos in Chihuahua, northern Mexico offers key elements to decipher the Late Paleozoic and Mesozoic evolution of the southern North American craton. The obducted oceanic crust sequence of Los Olivos ophiolite lies at the southern margin of the North American craton and south of the inferred trace of the Mojave-Sonora megashear. The ophiolitic sequence was tectonically emplaced within the Middle Jurassic sedimentary sequence, which is covered by Cretaceous sedimentary facies and volcanic rocks of the Sierra Madre Occidental magmatic arc. Our study provides evidence on the existence of the Mezcalera oceanic plate, which was subducted during plate interactions and collision of the Guerrero superterrane with Laurentia. The characteristics of the markedly epiclastic units of Middle Jurassic age that host the ophiolite supports the assumption of an active margin setting for the sedimentary deposition at this southern extreme of the Laurentia craton.

  17. 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.

  18. 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.

  19. Upper mantle isotopic heterogeneities - global overview with new results from Alpine-Apennine ophiolites

    NASA Astrophysics Data System (ADS)

    Rampone, E.; Borghini, G.; Hofmann, A. W.; Class, C.; Cipriani, A.; Zanetti, A.; Goldstein, S. L.

    2011-12-01

    Studies on modern oceanic lithosphere and ophiolites show that the petrogenesis of the oceanic mantle is complex. Its chemistry does not reflect simple trace element depletion by partial melting, and this complexity is highlighted by chemical and isotopic heterogeneities in the mantle, and by isotopic contrasts between mantle and crust. We present an overview of the present knowledge on isotopic heterogeneities of Sr, Nd and Os in present-day oceanic peridotites and Alpine-Apennine ophiolites. Compared to modern oceanic lithosphere, these ophiolites represent a fossil analogue of ocean-continent transitions and slow spreading settings. Previous studies show that mantle peridotites record significant isotopic heterogeneity, detectable on a wide range of length scales, much larger than observed in associated MORB. This reflects two major aspects. First, the MORB mantle source region is larger than peridotite sampling scales, and MORB represent aggregated melts that smooth and average mantle source heterogeneities, as supported by melt extraction models (Stracke and Bourdon, 2009). Second, isotopic heterogeneities in oceanic peridotites may result from a combination of processes, reflecting a long-lived history: i) old depletion events, ii) pyroxenite components in the mantle source, iii) recent pre- and/or post-melting metasomatism. In this context, we focus on two major issues: 1) What is the significance of extremely depleted isotopic signature found in oceanic peridotites - do they reflect a subcontinental origin, or preserved isotopic heterogeneities in the asthenospheric MORB source, inherited from old (1-2 Ga) depletion events, not sufficiently stirred by convection; 2) What is the length scale, amplitude and distribution of isotopic heterogeneities in the oceanic mantle, and their relation to lithologic heterogeneities like pyroxenites? Specifically, the effect of a pyroxenite component in creating small-scale chemical and isotopic heterogeneities in the mantle has often been advocated but not adequately investigated. New isotope studies of pyroxenite bands (clinopyroxenites and spinel websterites) and host peridotites from the External Liguride ophiolites (Northern Apennines, Italy) show that pyroxenites define a large Nd isotope range (143Nd/144Nd =0.512936-0.513401). Chemical and isotopic profiles through the pyroxenite-peridotite boundaries indicate that the interaction of the pyroxenites with the peridotite wall rock creates Nd isotopic variations resulting in an overall Nd isotopic range in the host mantle (143Nd/144Nd=0.512799-0.513544, initial EpsilonNd=3-16) that covers nearly the entire abyssal peridotite field. This suggests that pyroxenite components can induce extreme small scale isotopic changes, and strongly calls for detailed field-based studies in spatially-controlled settings in order to unravel the size and distribution of upper mantle isotopic heterogeneity. Stracke A., and Bourdon, B. (2009). GCA 73, 218-238.

  20. Magmatic Diversity of the Wehrlitic Intrusions in the Oceanic Lower Crust of the Northern Oman Ophiolite

    NASA Astrophysics Data System (ADS)

    Kaneko, R.; Adachi, Y.; Miyashita, S.

    2014-12-01

    The Oman ophiolite extends along the east coast of Oman, and is the world's largest and best-preserved slice of obducted oceanic lithosphere. The magmatic history of this ophiolite is complex and is generally regarded as having occurred in three stages (MOR magmatism, subduction magmatism and intraplate magmatism). Wehrlitic intrusions constitute an important element of oceanic lower crust of the ophiolite, and numerous intrusions cut gabbro units in the northern Salahi block of this ophiolite. In this study area, we identified two different types of wehrlitic intrusions. One type of the intrusions mainly consists of dunite, plagioclase (Pl) wehrlite and mela-olivine (Ol) gabbro, in which the crystallization sequence is Ol followed by the contemporaneous crystallization of Pl and clinopyroxene (Cpx). This type is called "ordinary" wehrlitic intrusions and has similar mineral compositions to host gabbros (Adachi and Miyashita 2003; Kaneko et al. 2014). Another type of the intrusions is a single intrusion that crops out in an area 250 m × 150 m along Wadi Salahi. This intrusion consists of Pl-free "true" wehrlite, in which the crystallization sequence is Ol and then Cpx. The forsterite contents (Fo%) of Ol from the "ordinary" wehrlitic intrusions and "true" wehrlitic intrusions have ranges of 90.8-87.0 (NiO = 0.36-0.13 wt%) and 84.7 (NiO = 0.31 wt%), respectively. Cr numbers (Cr#) of Cr-spinel from the "true" wehrlitic intrusions show higher Cr# value of 0.85 than those of the "ordinary" wehrlitic intrusions (0.48-0.64). But the former is characterized by very high Fe3+ values (YFe3+ = 0.49-0.68). Kaneko et al. (2014) showed that the "ordinary" ubiquitous type has similar features to MOR magmatism and the depleted type in the Fizh block (Adachi and Miyashita 2003) links to subduction magmatism. These types are distinguished by their mineral chemistries (TiO2 and Na2O contents of Cpx). The TiO2 and Na2O contents of Cpx from the "true" wehrlitic intrusions have 0.38 wt% and 0.26 wt%, respectively, and plot on the field of MOR magmatism. The most-evolved Ol (Fo% = 84.7) from the wehrlitic intrusions has high NiO (0.31 wt%) and plots on the olivine mantle array (Takahashi 1986). It is suggested that heterogeneity of source mantle influences the magmatic diversity of the wehrlitic intrusions.

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

    NASA Astrophysics Data System (ADS)

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

    2003-12-01

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

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

    USGS Publications Warehouse

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

    1995-01-01

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

  3. 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.

  4. 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 difference in remanence direction between the footwall gabbros and the hanging wall upper crustal rocks is consistent with relative rotation across the intervening extensional structure. These data provide the first quantitative example of OCC-related footwall rotation in an ancient ophiolite.

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

  7. Geol. Mag. 127 (3), 1990, pp. 209-224. Printed in Great Britain 209 The Solund-Stavfjord Ophiolite Complex and associated

    E-print Network

    Andersen, Torgeir Bjørge

    of sedimentary and volcanic rocks, the Stavenes Group, of which the Heggoy Formation of metasandstones associated with volcanic and sedimentary cover rocks, and sedimentary as well as tectonic melanges, can Complex and associated rocks, west Norwegian Caledonides: geology, geochemistry and tectonic environment H

  8. Shervais, J.W., 2008, Tonalites, trondhjemites, and diorites of the Elder Creek ophiolite, California: Low-pressure slab melting and reaction with the mantle wedge, in Wright, J.E., and Shervais, J.W., eds., Ophiolites, Arcs, and Batholiths: A Tribute to

    E-print Network

    Shervais, John W.

    113 Shervais, J.W., 2008, Tonalites, trondhjemites, and diorites of the Elder Creek ophiolite Paper 438 2008 Tonalites, trondhjemites, and diorites of the Elder Creek ophiolite, California: Low with the other lithologies. The felsic lithologies include hornblende diorite, quartz-diorite, tonalite

  9. Burst of high-temperature seawater injection throughout accreting oceanic crust: a case study in Oman ophiolite

    E-print Network

    Cattin, Rodolphe

    of the Hess Deep ODP leg 147 has been the discovery, in gabbros accreted at the East Pacific Rise, of a high events by Man- ning et al. (1996). Subsequently, this alteration was described in one gabbro section of the Oman ophiolite where gabbros comparable with those of Hess Deep display a similar alteration (Manning

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

    E-print Network

    Mcdonough, William F.

    Chemical heterogeneity in the upper mantle recorded by peridotites and chromitites from associated harzburgites. As is common for ophiolitic peridotites, the harzburgites (gOs492Ma of À5.3 to þ2, of the bulk convecting upper mantle. The Shetland peridotite compositions reflect protracted melt depletion

  11. Melt Extraction and Melt Refertilization in Mantle Peridotite of the Coast Range Ophiolite: An LA-ICP-MS Study

    E-print Network

    Shervais, John W.

    1 Melt Extraction and Melt Refertilization in Mantle Peridotite of the Coast Range Ophiolite: An LA, we can use the underlying mantle peridotites to elucidate melt processes in the mantle wedge above) to study pyroxenes in peridotites from four mantle sections in the CRO. Trace element signatures

  12. Nitrogen isotopes in ophiolitic metagabbros: A re-evaluation of modern nitrogen fluxes in subduction zones and implication

    E-print Network

    Cartigny, Pierre

    Nitrogen isotopes in ophiolitic metagabbros: A re-evaluation of modern nitrogen fluxes; available online 6 October 2011 Abstract Nitrogen contents and isotope compositions together with major, nitrogen (2.6­55 ppm) occurs as bound ammonium (NH4 þ ) substituting for K or Na­Ca in minerals. Cu

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

    NASA Astrophysics Data System (ADS)

    Robinson, P. T.; Yang, J.

    2014-12-01

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

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

    NASA Astrophysics Data System (ADS)

    MacLeod, Christopher J.; Lissenberg, C. Johan

    2015-04-01

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

  15. 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.

  16. 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.

  17. 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 may lead to erroneous tectonic interpretation.

  18. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

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

    NASA Astrophysics Data System (ADS)

    Belic, Maximilian; Hauzenberger, Christoph; Dong, Yunpeng

    2013-04-01

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

  1. Preservation of ancient Os isotope signatures in the Yungbwa ophiolite (southwestern Tibet) after subduction modification

    NASA Astrophysics Data System (ADS)

    Liu, Chuan-Zhou; Wu, Fu-Yuan; Chu, Zhu-Yin; Ji, Wei-Qiang; Yu, Liang-Jun; Li, Ji-Liang

    2012-07-01

    Both spinel and plagioclase harzburgites from the Yungbwa ophiolite, southwestern Tibet, have been studied for highly siderophile elements (HSEs) and Re-Os isotopes. The spinel harzburgites can be subdivided into low-Cr# and high-Cr# groups, according to the spinel Cr#. The low-Cr# harzburgites have been estimated to experience with ca. 13% degrees of partial melting, whereas the high-Cr# harzburgites have been subjected to 16-18% degrees of melting. Clinopyroxenes in the low-Cr# harzburgites show depleted REE patterns, indicating they have been negligibly modified by melts. In contrast, both high-Cr# and plagioclase harzburgites have been metasomatized to different extents, as evidenced by the variable enrichment of LREE in their clinopyroxenes. Compositions of the metasomatic agents are of affinities to the subduction-related magmas. The low-Cr# harzburgites have equilibrium temperatures higher than both high-Cr# and plagioclase harzburgites. We explain that the low-Cr# harzburgites represent mantle residues after melt extraction at mid-ocean ridge (MOR), whereas both high-Cr# and plagioclase harzburgites were originated from the supra-subduction zone (SSZ) settings. These different types of mantle peridotites were tectonically juxtaposed during the emplacement of the Yungbwa ophiolite. The low-Cr# harzburgites display consistent highly siderophile element (HSE) patterns with suprachondritic Ru/Ir and Pd/Ir ratios, whereas both high-Cr# and plagioclase harzburgites show fractionated HSE patterns. A high-Cr# harzburgite, GHP-86, is strongly depleted in both Pd and Re. The Yungbwa harzburgites have variable 187Os/188Os ratios ranging from 0.12228 to 0.12876. Sample GHP-86 has the less radiogenic 187Os/188Os ratio, giving a depletion age (TRD) of ˜1 Ga. The Re-Os isotope compositions of different types of the Yungbwa harzburgites suggest that their 187Os/188Os ratios have not been remarkably modified by subduction-related melts. Furthermore, subduction modification has not erased the ancient Os signatures in the Yungbwa peridotites. The reason is probably because the primary unradiogenic Os in the Yungbwa harzburgites had not been significantly reduced during the metasomatism. The asthenospheric mantle beneath the Neo-Tethys Ocean, from which the Yungbwa ophiolite was derived, had heterogeneous Os isotope compositions and contained some ancient mantle domains.

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

    NASA Astrophysics Data System (ADS)

    Shchegolikhina, Anastasia; Guadagnini, Laura; Guadagnini, Alberto

    2015-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-06-01

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

  4. Significance of the concentration gradients associated with dunite bodies in the Josephine and Trinity ophiolites

    NASA Astrophysics Data System (ADS)

    Morgan, Zachary; Liang, Yan; Kelemen, Peter

    2008-07-01

    Detailed transects were sampled across dunite bodies in the Josephine and Trinity ophiolites. The major peridotite lithologies sampled in the Josephine transect are a sequence of dunite and harzburgite and in the Trinity transect a sequence of dunite, harzburgite, lherzolite, and plagioclase lherzolite (DHL-PL). Major, minor, and selected trace element abundances in olivine, orthopyroxene, clinopyroxene, and spinel were measured. The composition profile from the Josephine transect has revealed a concentration gradient near the dunite-harzburgite contact. The composition profile from the Trinity transect has revealed several concentration gradients: two within the dunite, one in the harzburgite, and at least two in the plagioclase lherzolite. The composition profiles record complex histories of melt transport, melt-rock reaction, and subsequent subsolidus reequilibration. Analyses of closure distance suggest that compositional variation trends for a majority of major and minor elements in olivine, clinopyroxene, orthopyroxene, and spinel reported in this study were magmatic in origin. Subsolidus reequilibration may reduce the range or magnitude of variations for the 2+ cations such as Fe and Mg in olivine and spinel and significantly redistribute Ca and Li in coexisting minerals. Numerical simulations exploring the coupling of diffusion and advection in a porous matrix were used to explain compositional variations across the peridotite sequences. Melt flow from the host harzburgite into the dunite produces composition gradients near the dunite-harzburgite contacts similar to those from the Josephine transect. In contrast, melt flow from the dunite into the surrounding peridotite lithologies can produce concentration gradients similar to those observed in the Trinity transect. At least two chemically distinct episodes of melt flow within the same dunite channel system are proposed. Results from this study show that concentration gradients developed around the dunite-harzburgite and DHL-PL sequences can be used to infer part of the melt flow history of the dunite channel systems in the mantle. Results from this study and those from other ophiolite studies also demonstrate that concentration profiles in dunite and the surrounding peridotite lithologies are highly variable even among differing peridotite sequences within the same ophiolite, suggesting that the composition of instantaneous melt flowing through individual dunite channels is quite variable and the mantle source regions are heterogeneous.

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

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

    E-print Network

    Rioux, Matthew

    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 ...

  7. Rapid crustal accretion and magma assimilation in the Oman-U.A.E. ophiolite: High precision U-Pb zircon geochronology of the gabbroic crust

    E-print Network

    Rioux, Matthew

    New high-precision U/Pb zircon geochronology from the Oman-United Arab Emirates (U.A.E.) ophiolite provides insight into the timing and duration of magmatism and the tectonic setting during formation of the lower crust. ...

  8. 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.

  9. 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

  10. 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.

  11. Petrographic Investigation of Microcrack Initiation in Mafic Ophiolitic Rocks Under Uniaxial Compression

    NASA Astrophysics Data System (ADS)

    Rigopoulos, Ioannis; Tsikouras, Basilios; Pomonis, Panagiotis; Hatzipanagiotou, Konstantin

    2013-09-01

    This paper investigates the influence of the petrographic characteristics of mafic ophiolitic rocks on the initiation and propagation of microcracks during uniaxial compression. The microcrack patterns of a troctolite and a diorite, collected from the Pindos and Othrys ophiolites (Greece), respectively, were analysed. Thorough observation and quantification of microcracks before and after the uniaxial compression test were conducted. Combined fluorescent and polarised microscopy of polished thin sections, together with digital image analysis, indicated that the intragranular microcracks are the dominating crack type in both loaded and unloaded specimens, only in terms of their total number and length. On the other hand, the intergranular and transgranular cracks seem to grow more readily compared to the intragranular cracks, implying that the longer microcracks grow more extensively under stress. The orientation of most of the newly formed intragranular and transgranular microcracks is nearly parallel to the loading direction; however, some of the randomly oriented transgranular cracks have probably been formed during the propagation of intergranular cracks. In the troctolite, the frequency of the intragranular microcracks decreases in the olivine crystals after the uniaxial compression test due to their partial serpentinisation, which increases their resistance to brittle deformation. In the plagioclase crystals of the troctolite, microcracks are often oriented parallel to the cleavage planes, implying that such crystallographic orientations act as planes of weakness. On the contrary, the plagioclase crystals of the diorite are mainly crossed by randomly oriented microcracks, presumably due to their high degree of alteration. In the diorite, the evolution of microcracks is substantially controlled by the two perfect cleavages of amphibole. The failure of a rock occurs as a result of the growth, interaction and coalescence of a great number of pre-existing and newly formed intragranular and transgranular microcracks. The petrographic and microcrack analysis may assist in the selection of the most suitable rock type for various construction applications.

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-07-01

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

  14. Melanges, No. 23.

    ERIC Educational Resources Information Center

    Melanges, 1997

    1997-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-08-01

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

  16. 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.

  17. 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).

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

    NASA Astrophysics Data System (ADS)

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

    2015-06-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-06-01

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

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

    PubMed

    Abelson, M; Baer, G; Agnon, A

    2001-01-01

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

  1. 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.

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

    NASA Astrophysics Data System (ADS)

    Dilek, Y.

    2001-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-08-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

  8. Petrology and geochemistry of the Xiugugabu ophiolitic massif, western Yarlung Zangbo suture zone, Tibet

    NASA Astrophysics Data System (ADS)

    Bezard, Rachel; Hébert, Réjean; Wang, Chengshan; Dostal, Jaroslav; Dai, Jingen; Zhong, Hanting

    2011-07-01

    The Yarlung Zangbo Suture Zone (YZSZ), southern Tibet, is a discontinuous belt that is more than 2000 km long, composed of the remnants of Neo-Tethyan Mesozoic ocean. One of these relicts is the Xiugugabu ophiolitic massif which is a mantle thrust sheet of more than 260 km2 overlying the Cretaceous tectonic mélange south of the YZSZ in SW Tibet. The massif is composed of harzburgites and clinopyroxene-harzburgites with porphyroclastic and porphyromylonitic textures. In the southern part of the massif, peridotites were intruded by amphibole-bearing microgabbro and microgabbronorite sills. A diabase unit which is overlaid by a sedimentary sequence crops out on the NE flank of the massif. Mineral chemistry in harzburgites and clinopyroxene-harzburgites indicates compositions similar to abyssal and forearc peridotites. Peridotites are slightly LREE depleted to enriched with [La/Yb]CN 0.06-2.8 and [La/Sm]CN 0.34-2.64. These ultramafic rocks are inferred to be the residues of 5-25% of partial melting of a depleted mantle that has been enriched by percolating metasomatic melts in a suprasubduction environment. Amphibole-microgabbro and amphibole-microgabbronorite sills are mostly composed of brown to green amphibole, calcic plagioclase, clinopyroxene, ilmenite and orthopyroxene in gabbronorite. Textures and compositions of the brown amphiboles indicate a near-solidus high temperature hydrothermal origin (> 800 °C). These intrusive rocks are tholeiitic and show N-MORB type REE patterns ([La/Yb]NC 0.35-0.90), a LILE (mainly Th) enrichment and noticeable Nb, Ta and Ti negative anomalies. They have a suprasubduction affinity and were formed in a back-arc basin setting. The diabase unit outcropping to the NE of the massif is not directly related to the ultramafic and mafic ophiolitic rocks. The diabase shows LREE enriched patterns ([La/Yb]NC 8-8.9) and slight Nb, Ta and Ti negative anomalies. The diabase has an intraplate affinity and could have been derived from a mantle source enriched by subduction-related fluids. The absence of continental crustal assimilation indicates that these rocks were probably emplaced in the Jurassic, in an oceanic environment after the Triassic disaggregation of the Indian plate. The data are consistent with the recent geodynamic model proposed for the central part of the suture for the closure of the Neo-Tethys and suggest that the geodynamic evolution of the western part of the basin was comparable to the central part.

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

    USGS Publications Warehouse

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

    1998-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Alt, Jeffrey C.; Teagle, Damon A. H.; Brewer, Tim; Shanks, Wayne C., III; Halliday, Alex

    1998-06-01

    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 serialization halos (Na-K sericite, quartz, pyrophyllite, K-feldspar, and pyrite) along quartz veins at temperatures of 200°-250°C. High 87Sr/86Sr ratios of chloritized (˜0.7055) and sericitized (˜0.7065) rocks indicate involvement of seawater via mixing with hydrothermal fluids. Low ?34S of sulfide (-2 to -5.5‰) and sulfate (12.5‰) are consistent with input of magmatic SO2 into hydrothermal fluids and disproportionation to sulfide and sulfate. Alteration processes were generally similar to those at MORs, but the arc section is more intensively altered, in part because of the presence of abundant glassy rocks and mafic phases. The increase in alteration grade below 750 m and the mineralization in the basal dikes are analogous to changes that occur near the base of the volcanic section in MOR and the Troodos ophiolite.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. Age, temperature and pressure of metamorphism in the Tasriwine Ophiolite Complex, Sirwa, Morocco

    NASA Astrophysics Data System (ADS)

    Samson, S. D.; Inglis, J.; Hefferan, K. P.; Admou, H.; Saquaque, A.

    2013-12-01

    Sm-Nd garnet-whole rock geochronology and phase equilbria modeling have been used to determine the age and conditions of regional metamorphism within the Tasriwine ophiolite complex,Sirwa, Morocco. Pressure and temperature estimates obtained using a NaCaKFMASHT phase diagram (pseudosection) and garnet core and rim compositions predict that garnet growth began at ~0.72GPa and ~615°C and ended at ~0.8GPa and ~640°C. A bulk garnet Sm-Nd age of 645.6 × 1.6 Ma, calculated from a four point isochron that combines whole rock, garnet full dissolution and two successively more aggressive partial dissolutions, provides a precise date for garnet formation and regional metamorphism. The age is nearly 20 million years younger than a previous age estimate of regional metamorphism of 663 × 14 Ma based upon a SHRIMP U-Pb date from rims on zircon from the Irri migmatite. The new data provide further constraints on the age and nature of regional metamorphism in the Anti-Atlas mountains and emphasizes that garnet growth during regional metamorphism may not necessarily coincide with magmatism/anatexis which predominate the signature witnessed by previous U-Pb studies. The ability to couple PT estimates for garnet formation with high precision Sm- Nd geochronology highlights the utility of garnet studies for uncovering the detailed metamorphic history of the Anti-Atlas mountain belt.

  18. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-10-01

    The Derakht-Senjed magnesite deposit, hosted by Torbat-e-Heydarieh ophiolite in NE Iran, is developed as veins, veinlets and stockwork type mineralization. While the veins and veinlets only contain magnesite, the stockwork mineralization in addition contains sparry dolomite interlayered with magnesite. Magnesite and dolomite are both poor in FeO and SiO2. The carbon and oxygen isotope compositions of magnesite (?13CV-PDB = -3.9 ± 0.1 to -5.0 ± 0.1‰; ?18OV-SMOW = +25.2 ± 0.1 to +26.5 ± 0.1 ‰) can be explained by contribution of atmospheric CO2 and/or an involvement by organic carbon. Dolomite typically shows slightly lower values of ?13C V-PDB -5.2 ± 0.1 to -5.5 ± 0.1‰ and ?18OV-SMOW + 23.8 ± 0.1 to +24.8 ± 0.1‰ compared to the magnesite. The formation of magnesite at Derakht-Senjed was structurally controlled by a fracture network in the ultramafic host rocks, which provided suitable fluid pathways for leaching of Mg from the host rocks and subsequent precipitation of magnesite from carbonated solutions. It is likely that dolomite formed due to precipitation from a fluid having lower XCO2 and higher Ca2+/Mg2+ activity ratio, rather than by replacement of magnesite.

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

    PubMed

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

    2009-11-17

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

  1. 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.

  2. 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.

  3. Assessment of Zambales Ophiolite formation as a viable site for CO2 storage

    NASA Astrophysics Data System (ADS)

    Magbitang, Riza; Lamorena-Lim, Rheo

    2015-04-01

    Studies involving carbon dioxide (CO2) storage in geologic formations has been increasing over the years. Even though the developed countries are the ones pioneering the large scale storage studies, third world country such as the Philippines, which is one of the most vulnerable to the effects of elevated CO2 levels in the atmosphere, should also intensify CO2 storage research. In this study the potential of utilizing Ophiolite formations in Zambales province, Philippines, in CO2 storage was evaluated. The kinetics of the carbonation reaction was studied using batch reactor, at various temperature and pressure. The concentration of metals involved in the carbonation reaction was monitored by inductively-coupled plasma mass spectrometry (ICP-MS). Flow-through column reactors were used to simulate and study the gas storage in rock columns, hence leading the evaluation of rock mechanical properties. Moreover, thermo-gravimetric analysis (TGA) was used to characterize carbonated and non-carbonated rock samples, thereby resulting to the experimental determination of the amount of CO2 sequestered.

  4. 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.

  5. 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.

  6. Accretion of the lower crust at fast-spreading ridges: constraints from the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    MacLeod, C. J.; Thomas, R. M.; Coogan, L. A.; Thompson, G. M.

    2003-04-01

    Two fundamentally different classes of model for the accretion of the lower crust at fast-spreading ridge magma chambers are currently in vogue: (1) 'Gabbro glacier' models, in which all melt is delivered directly from the mantle to the melt lens and then undergoes wholesale crystallisation because of the large thermal gradient that exists at its roof. Subsidence of crystals precipitated at the floor of the melt lens generates the entire lower crust beneath. (2) 'Sheeted sill' models, in which much more extensive crystallisation occurs in the lower crust, with a lesser role envisaged for vertical mass transport of crystals. For sufficient crystallisation to occur in situ the lower crust must be much cooler and/or heat extracted more efficiently than commonly supposed. Because such little direct constraint exists as to the structure and composition of the lower crust at fast-spreading ridges, the Oman ophiolite - which is believed to have formed at a fast spreading ridge - has assumed a critical role in guiding our thinking regarding crustal accretion processes. Proponents of both gabbro glacier and sheeted sill models claim that field relationships in Oman support their models. Here we re-examine critical field observations and comment upon their implications for magma chamber processes, and present petrological and geochemical data from different vertical sections through the Oman lower crust to give insights into the processes of magma transport and crystallisation.

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

    NASA Astrophysics Data System (ADS)

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

    2014-06-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-02-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-06-01

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

  11. Remagnetization of the coast range ophiolite and lower part of the Great Valley Sequence in northern California and southwest Oregon

    NASA Astrophysics Data System (ADS)

    Frei, Leah S.; Blake, M. C., Jr.

    1987-04-01

    Overprinted magnetizations have been found at four localities in the Middle Jurassic Coast Range ophiolite and the overlying Upper Jurassic and Lower Cretaceous Great Valley sequence in northern California and at one locality in the partially correlative Lower Cretaceous Days Creek Formation in southwest Oregon. At Del Puerto Canyon, on the east side of the Diablo Range, a pilot study of the upper Jurassic Lotta Creek Formation gives in situ paleomagnetic directions grouped around the present geomagnetic field, suggesting magnetic overprinting long after deposition and folding. We suggest that the loss of original magnetization could be the result of long burial at about 7 km depth, followed by Late Tertiary uplift; the possibility of chemical remagnetization, however, can not be excluded. Paleomagnetic data from sandstones from the Lower Cretaceous Great Valley sequence in the Wilbur Springs area suggest Cenozoic remagnetization that could be related to Pliocene and Pleistocene volcanic and hydrothermal activity in the area. The results from calcareous concretions in the Wilbur Springs area, from 21 sites from basalts and overlying sedimentary rocks of the Great Valley sequence at Stonyford, and from several sites in various rock types in the Coast Range ophiolite and Great Valley sequence near Paskenta suggest unblocking of magnetization during the long period of burial at depth of 7 km or more and remagnetization during tectonic uplift in the Late Cretaceous or Tertiary. Chemical remagnetization, however, can not be excluded. The paleomagnetic data from six sites in Lower Cretaceous sedimentary rocks of the Days Creek Formation in southwest Oregon, which overlies a dismembered ophiolite, fail the fold test. In situ paleomagnetic directions group near the expected Tertiary field directions. These sedimentary rocks may have been remagnetized during a major Eocene tectonic event known to have occurred in this region. The paleomagnetic results confirm that the Coast Range ophiolite and the seemingly little-deformed Great Valley sequence in northern California, as well as correlative rocks in southwest Oregon have had a complex tectonic history. Once the nature and timing of these events are better understood, the timing of remagnetization may be further constrained, which, in turn, could give further insight into the nature of the tectonic events.

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

    NASA Astrophysics Data System (ADS)

    Bernoulli, Daniel; Jenkyns, Hugh C.

    2009-05-01

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

  13. 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.

  14. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2004-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2009-05-01

    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 ˜12), as well as heat and hydrogen gas that can sustain subsurface, chemosynthetic ecosystems. The recent observation of seeps from pole-facing crater and canyon walls in the higher Martian latitudes supports the hypothesis that even present conditions might allow for a rock-hosted chemosynthetic biosphere in near-surface regions of the Martian crust. The generation of methane within a zone of active serpentinization, through either abiogenic or biogenic processes, could account for the presence of methane detected in the Martian atmosphere. For all of these reasons, studies of terrestrial alkaline springs associated with mafic and ultramafic rocks are particularly timely. This study focuses on the alkaline Adobe Springs, emanating from mafic and ultramafic rocks of the California Coast Range, where a community of novel bacteria is associated with the precipitation of Mg-Ca carbonate cements. The carbonates may serve as a biosignature that could be used in the search for evidence of life on Mars.

  17. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2013-07-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

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

    USGS Publications Warehouse

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

    1988-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-10-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  4. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 88, NO. Bll, PAGES 9419-9435, NOVEMBER 10, 1983 OPHIOLITE EMPLACEMENT BY COLLISION BETWEEN THE SULA PLATFORM

    E-print Network

    McCaffrey, Robert

    EMPLACEMENT BY COLLISION BETWEEN THE SULA PLATFORM AND THE SULA\\~ESI ISLAND ARC, INDONESIA Eli A. Silver and Robert McCaffreyl Earth Sciences Board and Center for Coastal Marine Studies, University of California displayed in eastern Indonesia results fronl a forearc settings. Deformation of the ophiolite may series

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  9. 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 shear, the lineation is developed in type 3 samples; finally, with lower melt content, suspension flow switches to plastic deformation in the olivine-rich layers, leading to plastic olivine fabrics in type 4 samples. We deduce that layering originates from simple shear, which other studies show induces strong segregations in suspended particles, and plastically deformed rocks. To build the isotropic stage, melt must accumulate faster than the deformation, suggesting that melt is extracted by dikes.

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

    NASA Astrophysics Data System (ADS)

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

    2011-06-01

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

  11. 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

    Hydrothermal circulation is an important component of global biogeochemical cycles. Chemical exchange between seawater and the ocean crust affects the composition of the oceans, the ocean crust, and via subduction the composition and heterogeneity of the mantle. Despite 50 years of scientific ocean drilling, the ultimate goal of drilling a continuous in-situ section through the entire ocean crust has not yet been achieved. The absence of complete oceanic crustal sections makes full quantification of the hydrothermal contributions to global geochemical cycles difficult. In particular, our knowledge of the nature and extent of fluid-rock interaction in the lower crust is limited by the absence of accessible submarine exposures or drill core. Macquarie Island, approximately 1500 km south of New Zealand, is the only sub-aerial exposure of a complete section of ocean crust in the ocean basin in which it formed. The crust formed during a phase of slow spreading along a short segment of mid-ocean ridge ~11 Myr ago and was uplifted during recent transpression along the Pacific Australian plate boundary. Hydrothermally altered rocks from Macquarie Island therefore provide a time-integrated record of the chemical changes due to fluid-rock exchange through a complete section of ocean crust. We exploit the immobile behavior of some elements during hydrothermal alteration to determine the precursor compositions to altered Macquarie whole rock samples, and then evaluate the changes in bulk rock chemistry due to fluid-rock interaction throughout the Macquarie crust. We combine these data with stratigraphic reconstructions through the Macquarie crust to determine its net hydrothermal contributions to global geochemical cycles. The Macquarie crust was a net sink for Mn, Mg, Na, K, Cs and Ba and a net source of Fe, Ca, Cu and Sr to the oceans. To assess the role of hydrothermal circulation in global geochemical cycles we compare the calculated Macquarie hydrothermal fluxes to 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.

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

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

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

  13. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2012-06-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-08-01

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

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

    NASA Astrophysics Data System (ADS)

    Rollinson, Hugh

    2015-09-01

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

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

    NASA Astrophysics Data System (ADS)

    Rajendran, Sankaran; Nasir, Sobhi

    2015-08-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

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

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

    NASA Astrophysics Data System (ADS)

    Miyashita, Sumio; Adachi, Yoshiko

    2013-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

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

  1. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

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

    NASA Astrophysics Data System (ADS)

    D'Amico, M.; Previtali, F.

    2009-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

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

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

    NASA Astrophysics Data System (ADS)

    Schnur, S.; Gilbert, L. A.

    2009-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  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-Ti-metagabbros restricted to the lower part of the shear zone, close to the serpentinite sole; (2) numerous brecciated Fe-Ti-metagabbros scattered in the intermediate to upper levels of the LSZ; (3) blocks showing compositional variations and complex structures, with boudins of intact Fe-Ti-metagabbros embedded in highly foliated and folded Mg-rich rocks bounded on one side by Fe-Ti-breccia planes. In some cases the full transition from intact to highly brecciated rock is recorded in the same block. Here, the contacts between intact metagabbros and breccia are characterized by about 1m-wide zones of non rotated clasts with diameter up to 80 cm, almost matrix-absent. The amount of matrix vs clast increases, associated with a reduction in the clast size and increasing clast rotation, over a few meters up to the end of the bodies. These particular blocks give us a unique opportunity to better characterize the brecciation processes. Different kinds of measurements were realized on the brecciated blocks: (1) block size, (2) clasts vs. matrix relative volumetric abundances, (3) dimension and shape ratio of clasts, and angle of misorientation between their elongation axis or internal foliations (for five selected blocks). Preliminary results show that the majority (82%) of mapped blocks have a diameter of less than 10 meters, with only 8% being larger than 20 meters. In the brecciated Fe-Ti gabbros the average content of matrix is 28%, while for blocks showing compositional variation it varies from zero to 30%. The angle of misorientation between clasts' foliation shows, instead, a chaotic distribution. Preliminary field data thus demonstrate that breccia blocks have to be considered as a constant feature along the LSZ rather than as an exception, and that further work is needed to determine whether they formed through pervasive brecciation (and potentially multiple events) or through a localized event and were later disrupted by ductile deformation along the LSZ.

  8. Consequences of off-axis melt delivery at the Moho: Sr and Nd isotopic results from the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Nicolle, M.; Bosch, D.; Reisberg, L. C.; Jousselin, D.; Stephant, A.

    2010-12-01

    Recent tomographic images beneath the East Pacific Ridge (Toomey et al., 2007) indicate that at the Moho level, much of the melt is delivered off-axis (sometimes more than 10 km away from the axis) instead of beneath the ridge axis. In off-axis melt delivery sites, rising melt may enter directly into contact with cold and hydrothermally altered lithosphere, and thus might be subject to contamination by assimilation of this material. Structural mapping of the Oman ophiolite shows the presence of on and off-axis mantle diapirs, which offer the possibility to investigate the consequences of off-axis melt delivery. Off-axis diapirs display several particularities; for example, they are capped by a clinopyroxene-rich Moho Transition Zone (MTZ) whereas on-axis MTZ contain troctolite and gabbro lenses. To explore whether the differences between the on and off-axis MTZ arise from variations in the melt source or instead from interaction with melt-hydrated lithosphere, we are analyzing the strontium and neodymium isotopic compositions of ~20 samples, including pyroxenites, gabbros, diorites, and dunites, from one on-axis and one off-axis diapir in the Oman ophiolite. Our first results, obtained from whole rock powders, show some overlap between the initial (95 Ma) Nd isotopic compositions of the two diapirs (?Nd = 8.5-9.2 on-axis, 3 samples; ?Nd = 5.6-8.8 off-axis, 8 samples, excluding one sample with a value of -0.3). Though some variations may be suggested by these data, they indicate that both diapirs were derived from roughly comparable mantle sources. This similarity is not surprising, since incorporation of hydrothermally altered lithosphere is not expected to strongly influence Nd isotopic ratios. In contrast, our first Sr isotopic results show a marked difference in composition between on and off- axis localities (87Sr/86Sr = 0.703010 - 0.703438 on-axis, 5 samples; 87Sr/86Sr =0.703714 - 0.706192 off-axis, 13 samples). The on-axis samples display a limited range of enriched MORB-like compositions whereas those from the off-axis diapir have more radiogenic and highly heterogeneous compositions. We tentatively interpret this difference to result from contamination by magmatic assimilation of the overlying hydrated lithosphere. The lack of correlation between Sr content and isotopic composition suggests that the radiogenic compositions are not late-stage features acquired during alteration related to ophiolite emplacement. Analyses of separated clinopyroxenes are in progress to verify this interpretation. If confirmed, our results may indicate that off-axis melt delivery at mid-ocean ridges can produce hydrated melts with isotopic compositions that mimic those of magmas from supra-subduction zone settings. Toomey, D.R., Jousselin, D., Dunn, R.A., Wilcock, W.S.D. and Detrick, R.S. Skew of mantle upwelling beneath the East Pacific Rise governs segmentation. Nature, 446: 409-414, 2007.

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

    USGS Publications Warehouse

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

    1990-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-05-01

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

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

    NASA Astrophysics Data System (ADS)

    Angiboust, Samuel; Oncken, Onno; Agard, Philippe

    2014-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2009-04-01

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

  13. 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…

  14. Melanges Pedagogiques (Pedagogical Mixture), 1983.

    ERIC Educational Resources Information Center

    Melanges Pedagogiques, 1983

    1983-01-01

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

  15. 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

  16. 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.

  17. 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.

  18. Tectonic Evolution of the Careón Ophiolite (Northwest Spain): A Remnant of Oceanic Lithosphere in the Variscan Belt.

    PubMed

    Díaz García F; Arenas; Martínez Catalán JR; González del Tánago J; Dunning

    1999-09-01

    Analysis of the Careón Unit in the Ordenes Complex (northwest Iberian Massif) has supplied relevant data concerning the existence of a Paleozoic oceanic lithosphere, probably related to the Rheic realm, and the early subduction-related events that were obscured along much of the Variscan belt by subsequent collision tectonics. The ophiolite consists of serpentinized harzburgite and dunite in the lower section and a crustal section made up of coarse-grained and pegmatitic gabbros. An Early Devonian zircon age (395+/-2 Ma, U-Pb) was obtained in a leucocratic gabbro. The whole section was intruded by numerous diabasic gabbro dikes. Convergence processes took place shortly afterward, giving rise to a mantle-rooted synthetic thrust system, with some coeval igneous activity. Garnet amphibolite, developed in metamorphic soles, was found discontinuously attached to the thrust fault. The soles graded downward to epidote-amphibolite facies metabasite and were partially retrogressed to greenschist facies conditions. Thermobarometric estimations carried out at a metamorphic sole (T approximately 650 degrees C; P approximately 11.5 kbar) suggested that imbrications developed in a subduction setting, and regional geology places this subduction in the context of an early Variscan accretionary wedge. Subduction and imbrication of oceanic lithosphere was followed by underthrusting of the Gondwana continental margin. PMID:10504137

  19. 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.

  20. Emplacement of serpentinites in the Chohar Gonbad-Gugher-Baft ophiolitic mélange, southeast Iran: examination of the mineral-chemical, petrologic, and structural features

    NASA Astrophysics Data System (ADS)

    Mohammadi, N.; Ahmadipour, H.; Lentz, D. R.; Shafaii Moghadam, H.

    2015-05-01

    The Chohar Gonbad-Gugher-Baft ophiolite mélange, located along the major Baft and Shahr-e-Babak fault zones, southeast Iran, represents remnants of Neo-Tethyan oceanic lithosphere. This mélange contains blocks of harzburgite, dunite, lherzolite, basalt, and other ophiolite-related lithologies tectonically mixed with and embedded in a serpentinite matrix. Field, petrographic, and geochemical data show that peridotites in this mélange belong to the upper mantle. They seem to have undergone up to ~20 % partial melting in a supra-subduction zone setting, based on their spinel Cr# values (0.21-0.53). Chemical compositions and textures in the serpentinites indicate that they were partially hydrated during emplacement and further mobilized diapirically to the surface. The different deformation stages occurred in an accretionary wedge environment. Petrographic evidence shows that the first serpentinization event produced mesh-textured serpentinites formed under static conditions in an ocean floor environment (Nain-Baft ocean crust), where the initial lizardite, bastite, and chrysotile veins formed. Plastic deformation occurred due to the subduction of Nain-Baft oceanic lithospheric beneath the central Iranian microcontinent, with antigorite-bearing flare-textured serpentinites produced. During progressive exhumation of the Nain-Baft ophiolite mélange, the serpentinites were affected by ductile, ductile-brittle, and brittle deformation, respectively. Accretion and resultant diapirism are the most important processes in the emplacement of serpentinite, which is a consequence of hydration of the ocean crust. In this example, late-stage emplacement via thrusting occurred along the northern extent of the southern Sanandaj-Sirjan zone (S-SZ).

  1. The structure and geochemistry of the gabbro zone of the Annieopsquotch ophiolite, Newfoundland: implications for lower crustal accretion at spreading ridges

    NASA Astrophysics Data System (ADS)

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

    2004-12-01

    The Annieopsquotch ophiolite exposes a c. 5.5-km-thick section of tholeiitic gabbros, sheeted dykes and pillow basalts. Based on the along-strike consistency in thickness of the major crustal units, and lack of significant throw on spreading-related normal faults, the Annieopsquotch ophiolite is interpreted to have formed at an intermediate- to fast-spreading ridge. The upper c. 400 m of the gabbro zone is composed of massive, texturally heterogeneous gabbros with compositions that approach those of the sheeted dykes and basalts. Below this is c. 1.6 km of 10-30-m-thick planar intrusive sheets or sills. The lowermost part of the gabbro zone is composed of gabbroic rocks with relict troctolite and troctolitic gabbro enclaves, which are veined and partly replaced by gabbro and pyroxenite. Sill contacts within the central, sill-dominated part of the gabbro zone are sub-parallel to boundaries between the major ophiolite lithostratigraphic units. The upper and lower contacts of individual gabbro sills may have finer grain sizes, indicating that the intrusions cooled from the top and bottom. Locally well-preserved comb structures (crescumulates) indicate downward growth, supporting a sill interpretation. The sills are composed of weakly or un-deformed plagioclase+clinopyroxene ±olivine cumulates. Trace element modeling suggests that the parental magmas of these cumulates had compositions very similar to the overlying sheeted dykes and basaltic lavas, as do dykes emplaced within the gabbro zone. Model liquids calculated from the gabbroic sills generally become more evolved up-section, indicating that magma evolution in the Annieopsquotch ophiolite was dominated by fractionation in lower crustal conduits, below the level of a putative axial melt lens (AML). The model liquids, sheeted dykes and basalts preserve a similar, wide range of compositions, which may indicate that aggregation, homogenization and fractionation in an AML was inefficient. Similar intra-conduit fractionation of mantle-derived melts might also contribute to MORB evolution at mid-ocean ridges.

  2. Forearc Basin Location Originating From Tectonic Inversion Along an old Ophiolite Suture : the Gulf of Guayaquil-Tumbes Basin (Ecuador-Peru Border)

    NASA Astrophysics Data System (ADS)

    Bourgois, J.; Witt, C.

    2008-12-01

    The Gulf of Guayaquil-Tumbes basin (GGTB) located along the Andean forearc (Ecuador-Peru border) developed in the tectonic wake of the coastwise, northward migrating North Andean block (NAB). The Industrial multichannel seismic and well data (Witt and Bourgois, in press) document that E-W trending low- angle (10-20°) detachment normal faults accommodated the main basin subsidence steps during the Late Pliocene-Quaternary times (1.8-1.6 Ma to Present). It includes the Posorja Jambeli and the northward dipping Tumbes Zorritos detachment systems (PJDS and TZDS) located respectively along the northern and southern edge of the basin. A major transfer system, the N-S trending Inner Domito Banco Peru fault system bounds the detachment systems to the West. The right lateral transcontinental strike-slip system of the Dolores Guayaquil Megashear bounds the basin to the East. Since the PJDS and TZDS extend 80 to 120 km at seafloor they must penetrate the brittle continental crust, far below the 6-8 km thick sediment accumulation at basin depocenters. We assume that detachments extend deep into the 8-10 km thick brittle crust down to the Nazca-South America plate interface at less than ~20 km beneath sea bottom at site. The active TZDS, which connects landward with the continental structures assumed to be part of the eastern frontier of the NAB is the master detachment fault system controlling the basin evolution through time. Gravimetric and geologic data show that depocenters are located along the 80-60 Ma obduction bounding at depth the Cretaceous ophiolite of northern Andes from the westward underthrusted South America continental basement (Bourgois et al., 1987). Because inference suggests the obduction megathrust to branch upward to the TZDS, we hypothesized that tectonic inversion occurred along the ophiolite suture during the GGTB evolution, at least for the past 1.8-1.6 Myr. The 80-60 Ma ophiolite suture is an old zone of weakness, which reactivation from the NAB northward drift controlled the GGTB location. Bourgois, J., Toussaint, J-F, Gonzales, H., Azema, J., Calle, B., Desmet, A., Murcia L.A., Acevedo, A.P., Parra, E., and Tournon, J., 1987, Geological history of the Cretaceous ophiolitic complexes of Northwestern South America (Colombia Andes): Tectonophysics, v. 143, p. 307-327. Witt, C. and Bourgois, J., Forearc basin formation in the tectonic wake of a collision-driven, coastwise migrating crustal block: the example of the North Andean block and the extensional Gulf of Guayaquil-Tumbes basin (Ecuador-Peru border area): Geological Society of America Bulletin, in press.

  3. The Imbert Formation of northern Hispaniola: a tectono-sedimentary record of arc-continent collision and ophiolite emplacement in the northern Caribbean subduction-accretionary prism

    NASA Astrophysics Data System (ADS)

    Escuder-Viruete, J.; Suárez-Rodríguez, A.; Gabites, J.; Pérez-Estaún, A.

    2015-06-01

    In northern Hispaniola, the Imbert Formation (Fm) has been interpreted as an orogenic "mélange" originally deposited as trench-fill sediments, an accretionary (subduction) complex formed above a SW-dipping subduction zone, or the sedimentary result of the early oblique collision of the Caribbean plate with the Bahama Platform in the middle Eocene. However, new stratigraphical, structural, geochemical and geochronological data from northern Hispaniola indicate that the Imbert Fm constitutes a coarsening-upward stratigraphic sequence that records the transition of the sedimentation from a pre-collisional forearc to a syn-collisional piggy-back basin. This piggy-back basin was transported on top of the Puerto Plata ophiolitic complex slab and structurally underlying accreted units of the Rio San Juan complex, as it was emplaced onto the North America continental margin units. The Imbert Fm unconformably overlies different structural levels of the Caribbean subduction-accretionary prism, including a supra-subduction zone ophiolite, and consists of three laterally discontinuous units that record the exhumation of the underlying basement. The distal turbiditic lower unit includes the latest volcanic activity of the Caribbean island arc; the more proximal turbiditic intermediate unit is moderately affected by syn-sedimentary faulting; and the upper unit is a (caotic) olistostromic unit, composed of serpentinite-rich polymictic breccias, conglomerates and sandstones, strongly deformed by syn-sedimentary faulting, slumping and sliding processes. The Imbert Fm is followed by subsidence and turbiditic deposition of the overlying El Mamey Group. The 40Ar / 39Ar plagioclase plateau ages obtained in gabbroic rocks from the Puerto Plata ophiolitic complex indicate its exhumation at ∼ 45-40 Ma (lower-to-middle Eocene), contemporaneously to the sedimentation of the overlying Imbert Fm. These cooling ages imply the uplift to the surface and submarine erosion of the complex to be the source of the ophiolitic fragments in the Imbert Fm, during of shortly after the emplacement of the intra-oceanic Caribbean island-arc onto the continental margin.

  4. Petrological characteristics of Opx-bearing primitive gabbros from the East Pacific Rise and the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Python, Marie; Akizawa, Norikatsu; Godard, Marguerite; Ildefonse, Benoît; Koepke, Jürgen

    2014-05-01

    The Hess Deep rift is located at the junction between the fast spreading East Pacific Rise and the Cocos-Nazca Ridge. Lower crust is exposed along the southern slope of the intrarift ridge between 4675 and 4800 m depth and was sampled during IODP Expedition 345. Primitive troctolites and olivine-rich gabbros are the dominant recovered lithologies and shipboard data showed a high Mg# whole rock chemistry in concordance with their primitive nature. In a MOR system, olivine is a typical primitive mineral and orthoyroxene (Opx) usually appear late in the crystallisation sequence, when the magma already reached a significant degree of differentiation. In spite Opx is not expected in any primitive lithology, this mineral is commonly present in Hess Deep gabbros and may be associated with olivine. This curious association of cumulate Opx with olivine and other primitive minerals was also observed at a lower extent in some gabbros from ODP/IODP Hole 1256D, in the upper Hess Deep crustal section (ODP Hole 894G), and in the crustal section of the Oman ophiolite (Kahwad massif) where, in particular, Opx-bearing troctolites coexist with clinopyroxene oikocrysts-bearing troctolites and amphibole-bearing primitive olivine gabbros. Three types of Opx textures may be distinguished in Opx-bearing olivine gabbros and troctolites: (1) recrystallised coronæ around olivine, (2) exsolution within clinopyroxene and (3) large prismatic or poikilitic grains. Prismatic or poikilitic Opx are present at all level of the gabbroic crust, while exsolutions and coronæ were observed only in the lower crust. The mineral chemical compositions vary more with the structural level than with the lithological type and (Opx-bearing) olivine gabbros from Holes 894G, 1256D and from the upper crust of the Oman ophiolite show more differentiated characteristics than the same lithology in the Site 1415 and in the Oman lower crust. Pyroxenes in all samples from the lower crust show a relatively narrow range of Mg# (from 84 to 86% for Opx and 86 to 89% for Cpx) with large variation of minor elements (Ti, Al, Cr) suggesting a strong influence of melt-rock reaction during their formation. On the other hand, the upper crust samples show a large variation in their ferro-magnesian Mg# (72-87% for Cpx and 70-85% for Opx) together with a relatively weak scatter in minor elements. Magmatic crystallisation were then the dominant event in the upper crust, so that Opx is likely to be directly crystallised from magma. In contrast, in the lower crust, magmatic processes were dominated by melt-rock reaction, and the chemical composition and habitus of Opx show that they have been probably formed by reaction between previously abundant olivine and melt.

  5. Geological mapping strategy using visible near-infrared-shortwave infrared hyperspectral remote sensing: Application to the Oman ophiolite (Sumail Massif)

    NASA Astrophysics Data System (ADS)

    Roy, R.; Launeau, P.; Carrère, V.; Pinet, P.; Ceuleneer, G.; Clénet, H.; Daydou, Y.; Girardeau, J.; Amri, I.

    2009-02-01

    An airborne hyperspectral survey of the Oman ophiolite (Sumail Massif) has been conducted using the HyMap airborne imaging spectrometer with associated field measurements (GER 3700). An ASD FieldSpec3 spectrometer was also used in order to constrain the spectral signatures of the principal lithologies cropping out in the surveyed area. Our objective was to identify and map the various igneous lithologies by a direct comparison at high spectral resolution between field and airborne spectra despite strong variations in outcropping conditions such as (1) lighting, (2) surface roughness geometry, (3) blocks coated with red/brown patina and exfoliation products, or (4) deep hydrothermal weathering. On the basis of spectral signatures, we are able to distinguish three end-members of olivine-orthopyroxene bearing assemblages in the mantle sequence: (1) harzburgites, (2) dunites, and (3) a harzburgite with interstitial carbonate. Because plagioclase is spectrally featureless in the wavelength range studied it cannot be detected. In the crustal sequence, we therefore identified four end-members with variable abundance of clinopyroxene: (1) massive gabbros, (2) amphibolized (upper) gabbros associated with intrusive dykes, (3) wehrlite with high serpentine content, and (4) gabbronorite (a lithology not previously recognized in the studied area). With the exception of wehrlite, spectra of olivine-rich end-members display characteristic Mg-OH narrow absorption features caused by their high serpentine content. We take advantage of this observation to split the data into two subsets, corresponding to the mantle and crustal sequences, respectively. Pixels of an image often correspond to heterogeneous areas in the field and a direct comparison between airborne and in situ spectra is not straightforward. However, comparing spectra of pixels associated with the most homogeneous areas in the field with the spectra acquired in situ at the same location, we found a systematic change both in mean intensity and overall spectral shape. Dividing each spectrum by its low-pass trend removes the effects caused by surface light scattering associated with each scale of analysis and results in an exceptional match between field and airborne spectra. However, the albedo information is lost and as a consequence, rock types only characterized by albedo change cannot be discriminated. A spectrum of a mixture of powdered minerals is usually seen as a linear combination of mineral spectra proportional to their abundance. However, this is no longer the case when minerals occur in complex arrangements in rock types. We thus develop a synthetic spectral library of all possible combinations of rock types covering the surface area of a pixel and use a simple distance calculation to identify the best match between each pixel and modeled spectra. This procedure allows the determination of the fractional cover of each rock type in a given pixel and to establish maps for each spectral end-member. The final product is a geological map, derived from the combination of end-member fractional cover maps, and is broadly consistent with the existing geological maps. Beyond this general agreement which demonstrates the potential of this new approach for geological mapping, imaging spectrometry allows (1) to map in detail the outline of the Moho north of Maqsad and (2) to identify a new crustal sequence enriched in silica south of Muqzah, revealing the presence of orthopyroxene, the nature and distribution of which are of relevance to the petrological and tectonic understanding of the Oman ophiolite evolution.

  6. 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.

  7. 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 (?13CCO2 =-27 to -14‰; ?18OCO2 =-17 to - 6‰) also provide evidence for active serpentinization and carbonation. Isotope ratios of the alkaline fluids (?18O = -7.9‰, ?2H = -51.4‰) and groundwater (?18O = -7.6‰; ?2H = -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.

  8. 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.

  9. 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.

    2015-07-01

    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.

  10. The Drawdown of Atmospheric CO2 by Hyperalkaline Spring Waters Emanating from Cascade Spring, Dun Mountain Ophiolite Belt, New Zealand

    NASA Astrophysics Data System (ADS)

    Menzies, C. D.; Teagle, D. A. H.; Cox, S.; Boyce, A.; Hathorne, E. C.

    2014-12-01

    The Permian Dun Mountain Ophiolite Belt (DMOB) is an important marker terrane in New Zealand geology and is displaced by ~460 km by right lateral offset on the Alpine Fault that forms the Pacific-Australian plate boundary through the South Island. The DMOB contains a number of ultramafic massifs of partially serpentinized mantle peridotite and notwithstanding that much of this terrane is extremely remote, peridotite-hosted hyperalkaline springs are rare. The Cascade Spring issues ~17°C waters at ~13 L/min from the western slope of a steep harzburgite ridge close to the DMOB's southern intersection with the Alpine Fault. High pH (11.1-11.6), Ca-OH type fluids with low concentrations of HCO3-, Mg and SiO2 continue to form a steep >500 m2 patch of hummocky calcium carbonate travertine. The spring fluids flow more than 50 m across the surface of a ~2 m-thick travertine blanket before it abruptly terminates. This gives the opportunity to study the evolution of the fluids and their precipitates as the waters flow down the travertine terrace. The spring waters have meteoric oxygen and hydrogen isotope ratios similar to local surface waters. 87Sr/86Sr of vent fluids are ~0.7042, higher than DMOB primary mantle values (0.7030-0.7035), indicating exchange with either hydrothermally altered ocean crustal rocks or mixing with fluids that have interacted with nearby tectonically juxtaposed metasediments. As waters flow over this steep terrace their chemistry changes; pH decreases from 11.5 to 9.7, and Ca concentrations decrease from 20 ?g/g to 9.1 ?g/g, corresponding to precipitation of 0.271 mmoles/L calcite and dissolution of 0.012 g/L CO2 across terrace. Considering spring flow rates this equates to precipitation of ~188 kg of calcite and drawdown of 83 kg of atmospheric CO2 from this vent per year. The fluid chemistry changes most in the first 17 m from the vent where pH decreases from 11.5 to 10.8 and Ca concentration almost halves from 20 to 10.9 ?g/g, indicating calcite precipitation and CO2 uptake are most rapid when Ca concentration and pH are highest. Despite the clearly defined extent of the terrace, waters dripping from its termination remain saturated in calcite and under-saturated CO2 meaning a further 0.021 mmoles/L of calcite may be deposited beyond the end of the terrace.

  11. Permeability of sheeted dykes beneath oceanic ridges: Strain experiments coupled with 3D numerical modeling of the Troodos Ophiolite, Cyprus

    NASA Astrophysics Data System (ADS)

    Coelho, Gabriel; Branquet, Yannick; Sizaret, Stanislas; Arbaret, Laurent; Champallier, Rémi; Rozenbaum, Olivier

    2015-03-01

    Permeability laboratory measurements under in situ pressures, temperature and strain have been performed on three different diabase alteration facies (metadiabase, chloritized diabase, epidosite) from the Troodos Ophiolite, Cyprus. This aims to study the relations between hydrodynamics, deformation and hydrothermal reaction in the sheeted dyke complex beneath oceanic ridges. The use of water as pore fluid in these experiments favors hydrothermal fluid-rock interactions. All experiments, performed with a Paterson gas-medium apparatus, were achieved at 400 °C, 100 MPa of confining pressure and 50 MPa of pore fluid pressure, conditions at the base of sheeted dykes. Permeability was measured by injection of water and argon before, during and after coaxial deformation. Resulting textures and mineralogy were studied by microscopy and X-ray microtomography in order to identify mineral reactions and to calculate the permeability by numerical simulation after decompression. During stress loading, a compaction/dilatant evolution is observed only in experiments on epidosite. Failure tends to increase permeability by one order of magnitude. For example, using water as pore fluid, permeability measurements after macroscopic failure give the following variations from 4 × 10- 20 m2 to 2.9 × 10- 19 m2 for metadiabase, 1 × 10- 20 m2 to 2.6 × 10- 19 m2 for chloritized diabase and 6.5 × 10- 19 m2 to 3 × 10- 18 m2 for epidosite. Textures suggestive of self-healing and sealing explain permeability reduction by hydrothermal reaction after macroscopic failure. Paradoxically, even using argon as pore fluid, hydrothermal reaction is possible in metadiabases due to dehydration of chlorite. Moreover, fractures appear much finer in rocks enriched in chlorite because of formation of gouge structures. After the experiments, 3D images of fracture networks enable calculation of permeability by numerical simulations which values are 4-6 orders of magnitude higher than the experimental measured values. Such results demonstrate that the geometry and the textures observed in unloaded samples are not suitable to estimate permeability and have to be very carefully interpreted.

  12. Application of airborne LiDAR data and airborne multispectral imagery to structural mapping of the upper section of the Troodos ophiolite, Cyprus

    NASA Astrophysics Data System (ADS)

    Grebby, Stephen; Cunningham, Dickson; Naden, Jonathan; Tansey, Kevin

    2012-09-01

    Structural maps are traditionally produced by mapping features such as faults, folds, fabrics, fractures and joints in the field. However, large map areas and the spatially limited ground perspective of the field geologist can potentially increase the likelihood that not all structural features will be identified within a given area. The ability to recognise and map both local and regional structural features using high-resolution remote sensing data provides an opportunity to complement field-based mapping to help generate more comprehensive structural maps. Nonetheless, vegetation cover can adversely affect the extraction of structural information from remotely sensed data as it can mask the appearance of subtle spectral and geomorphological features that correspond to geological structures. This study investigates the utility of airborne Light Detection And Ranging (LiDAR) data and airborne multispectral imagery for detailed structural mapping in vegetated ophiolitic rocks and sedimentary cover of a section of the northern Troodos ophiolite, Cyprus. Visual enhancement techniques were applied to a 4-m airborne LiDAR digital terrain model and 4-m airborne multispectral imagery to assist the generation of structural lineament maps. Despite widespread vegetation cover, dykes and faults were recognisable as lineaments in both data sets, and the predominant strike trends of lineaments in all resulting maps were found to be in agreement with field-based structural data. Interestingly, prior to fieldwork, most lineaments were assumed to be faults, but were ground-verified as dykes instead, emphasising the importance of ground-truthing. Dyke and fault trends documented in this study define a pervasive structural fabric in the upper Troodos ophiolite that reflects the original sea-floor spreading history in the Larnaca graben. This structural fabric has not previously been observed in such detail and is likely to be continuous in adjacent regions under sedimentary cover. This information may be useful to future exploration efforts in the region focused on identification of structurally controlled mineral and groundwater resources. Overall, our case study highlights the efficacy of airborne LiDAR data and airborne multispectral imagery for extracting detailed and accurate structural information in hard-rock terrain to help complement field-based mapping.

  13. Multiple refertilisation of oceanic mantle: new insights into the evolution of the southern sector of the Ligurian Tethys from Mt. Pollino ophiolites (Basilicata, Southern Italy)

    NASA Astrophysics Data System (ADS)

    Carmine Mazzeo, Fabio; D'Antonio, Massimo; Zanetti, Alberto; Petrosino, Paola; Aulinas, Meritxell

    2015-04-01

    Southern Apennine ophiolites consist of a serpentinized peridotite basement and a reduced crustal sequence characterized by lack of sheeted-dyke complexes, relatively small volumes of intruded gabbros, and a discontinuous basaltic and pelagic sediments cover. These ophiolites are believed to represent fragments of the Ligurian branch of Tethys oceanic crust that were obducted on continental crust during its closure. A thorough petrological investigation has been carried out on ophiolites that crop out widely along the boundary between Basilicata and Calabria, close to Mt. Pollino (Southern Italy). All peridotite samples contain large amount of serpentine, and are characterized by millimeter-sized porphyroclasts of olivine and orthopyroxene, varying from anhedral to subhedral and showing internal deformation. Clinopyroxene is present as large crystals or as exsolution lamellae in orthopyroxene. Spinels are typically anhedral. The protoliths of all samples were likely depleted harzburgites and/or cpx-poor lherzolites. Three samples (named Type-1) have MgO = 40.9-41.3 wt.%, while the other samples (named Type-2) have higher concentrations of MgO = 43.3-44.6 wt.%. The Type-1 peridotites have the highest values of Al2O3, CaO, SiO2, Sc and V, but lower Ni and Co contents. Peridotites show chondrite-normalized REE patterns with strong, but variable depletions in LREE. Type-1 peridotites are less depleted, whereas Type-2 peridotites are strongly depleted HREE pattern regions are poorly variable, showing chondritic values. The geochemical variations displayed by major oxides and trace elements, and the positive relationship between Fo content of olivine and Cr# of spinel suggest high degrees of partial melting (~20%). However, the degree of partial melting inferred on the basis of LREE concentrations of clinopyroxenes is much lower than that recorded by the spinel-olivine equilibrium (maximum ~6% near-fractional melting of a spinel-facies depleted mantle for both peridotites). However, the geochemical data indicate a different petrological stories for two type of peridotites, because, unlike the Type-2, the Type-1 peridotites have been affected by a multistage refertilization, at least two events. We interpret the apparent discrepancy of the depleted spinel peridotites of Mt. Pollino as the result of a reactive melt/rock interaction with depleted melts of MORB affinity, occurred at spinel-facies conditions after an earlier melt extraction. The geochemical characteristics of these serpentinized peridotites suggest that both types represent a very depleted mantle similar to that typical of intra-basinal settings. For this reason we propose that Mt. Pollino ophiolites may have formed an accretionary wedge (as part of the so-called Ligurian Accretionary Complex) during the early stages of the subduction of oceanic lithosphere below the European margin. This accretionary wedge, however, was dismantled and its remains were deposited in the adjacent Liguride sedimentation basin. Later these terrains were buried during upper Oligocene, reaching HP/LT conditions.

  14. 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 ?13C -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 focused in the frontal part of the Early Paleocene alpine accretionary wedge.

  15. 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 pressure-temperature conditions of their production. This will allow a better understanding of the rheological behaviour of subducting slabs in subduction zones and of amphibolites in the lower continental crust.

  16. 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-rich hydrothermal fluids prior to extensive mixing with seawater in the buoyant plume. Particles that formed later in the highly diluted nonbuoyant plume formed relatively As and Sb poor distal jasper. The large particle sizes and accordingly high settling rates of the particles, together with mass-balance calculations based on modern vent field data, suggest that individual meter-thick jasper beds formed within a plume lifetime of 200 years or less. The lack of thick jasper beds near the L??kken VMS orebody, which is larger than the H??ydal orebody by more than two orders of magnitude, probably reflects a shift to anoxic conditions during L??kken mineralization. This environment limited oxidation of iron in the hydrothermal plume and formation of the ferric oxyhydroxides necessary for the flocculation of silica and sea-floor deposition of the gel precursor of the jasper beds. Distal pyritic and iron-poor cherts are more common than jasper in ancient VMS-hosting sequences. The origin of these other types of siliceous exhalite is enigmatic but at least in some cases involved sulfidation, reduction to magnetite, or dissolution of the original ferric iron in precursor Si-rich gels, either by hydrothermal or diagenetic processes. ?? 2005 Society of Economic Geologists, Inc.

  17. Distribution and PGE mineralization in the formation of chromitite in ophiolite complexes (Ospina-Kitoi Kharanur and ultrabasic massifs of Eastern Sayan, Sousern Siberia)

    NASA Astrophysics Data System (ADS)

    Kiseleva, Olga; Zhmodik, Sergei

    2015-04-01

    New study of PGE in restitic ultrabasic (Kharanur and Ospin-Kitoi) massifs from North and South branches (Dobretsov et al., 1985) of the ophiolite complexes in south-eastern part of the Eastern Sayan show their presence in chromitites of both branches belonging to the different geodynamic settings. Modern concepts model includes several mechanisms of podiform chromitite origin reflected in the chemistry of Cr-spinels (Arai, Yurimoto, 1994; Ballhaus, 1998; Uysal et al., 2009 et al.): 1) partial melting of upper mantle rocks, 2) mixing of primitive melts with melts enriched in SiO2, 3) melt-rock interaction. We estimated the types of interaction of mafic melts with mantle peridotites, with the formation of chromite bodies. For ore chrome spinelides from northern branch (Al2O3) melt = 8 - 14 wt%, (TiO2) melt = 0 - 0,4 wt%, (Fe/Mg) melt = 0,5 - 2,4; Southern branch (Al2O3) melt = 10 - 13 wt%, (TiO2) melt = 0,1 wt%, (Fe/Mg) melt = 0,3 - 1 (Kiseleva, 2014). There are two types of PGE distribution Os-Ir-Ru (I) and Pt-Pd (II). Type I chromitites (mid-Al#Cr-spinels) revealed only Os-Ir-Ru distributions; type II (low-Al#Cr spinelides) show both Os-Ir-Ru and (Pt-Pd) distributions (Kiseleva et al., 2012, 2014). PGE distribution in ultramafic peridotites and chromitites reflects PGE fractionation during partial melting (Barnes et al., 1985; Rehkämper et al., 1997). Processes bringing to extreme fractionation of PGE, may be associated with fluid-saturated supra subduction environment where melting degree near 20% and above is sufficient for the release of PGE from the mantle source (Dick, Bullen, 1984; Naldrett, 2010). Enrichment in PPGE together with a high content of IPGE in same chromite bodies is attributed to the second step of melting, and formation of S-enriched and saturated in PGE melts (Hamlyn, Keays, 1986; Prichard et al., 1996). For type I chromitites platinum group minerals (PGM) are presented by Os-Ir-Ru system. In type II chromitites PGM are represented by Os-Ir-Ru-Rh-Pt system. Solid solutions Os-Ir-Ru and formed in the upper mantle RuS2 conditions together with chromite. The (Os-Ir-Ru)AsS minerals are forming on postmagmatic stage under the influence of S, As-containing fluids Under the influence of mantle reduced fluids the remobilization of PGE during desulfurization and deserpentinization early of "primary" PGM takes place. Changes of the redox environment from reducing to oxidizing condition is followed by creation of PGE together with As, Sb, Sn, and nickel arsenides, ferrichromie, chrommagnetite. The latter association reflects the redistribution of chromite and platinum group metals and formation of new mineral associations within the ultramafic substrate in crustal conditions (Kiseleva, 2014). Kiseleva O.N. Chromitite and PGE mineralization in ophiolites south-eastern part of the East Sayan (Ospina-Kitoi and Kharanur massifs), Thesis of PHD dissertation, Novosibirsk, 2014 IPGG SB RAS, 15p. Kiseleva O.N., Zhmodik SM, Damdinov BB, Agafonov LV, Belyanin D.K. 2014 The composition and evolution of platinum group mineralization in chromite ores Ilchir ophiolite complex (Ospin-Kitoi and Kharanur massifs, Eastern Sayan). Geology and Geophysics 55, 333 - 349.

  18. Osmium isotope systematics of Os-rich alloys and Ru-Os sulfides from oceanic mantle: evidence from Proterozoic and Paleozoic ophiolite-type complexes

    NASA Astrophysics Data System (ADS)

    Badanina, Inna Yu.; Malitch, Kreshimir N.; Belousova, Elena A.; Lord, Richard A.; Meisel, Thomas C.; Murzin, Valery V.; Pearson, Norman J.

    2014-05-01

    This study presents a substantial data set of Os-isotope compositions of Os-rich alloys and Ru-Os sulfides from deep portions of ophiolite sections from oceanic mantle. These are represented by samples from different in age ophiolite-type massifs (i.e., Neoproterozoic Kunar in Northern Taimyr, Russia, and Hochgrossen in Eastern Alps, Austria, Paleozoic Verkh-Neivinsk in Middle Urals, Russia, and Shetland in northern Scotland). The investigation employed a number of analytical techniques, including electron microprobe analysis, ID ICP-MS after high pressure acid digestion, and laser ablation attached to multiple collector-inductively coupled plasma mass-spectrometry (LA MC-ICP-MS). Two distinct platinum-group mineral (PGM) assemblages have been recognized at the Shetland and Verkh-Neivinsk localities: a 'primary' euhedral PGM assemblage, which occur as inclusions in chromite, and a modified 'secondary' subeuhedral to anhedral PGM assemblage observed in cracks filled by chlorite or serpentine, interstitially to chromite grains [1]. A 'primary' PGM assemblage at Shetland is represented by solitary grains of laurite or iridian osmium and composite grains of laurite + osmian iridium ± iridian osmium that display well defined phase boundaries between two or three distinct PGMs. A 'primary' PGM assemblage at Verkh-Neivinsk is represented by Ru-Os-Ir alloy grains that frequently mantled by 'secondary' Ru-Os sulfide and/or Ru-Os sulfarsenide overgrowths. The osmium isotope results identify (1) a restricted range of 'unradiogenic' 187Os/188Os values for coexisting laurite and Os-rich alloy pairs that form 'primary' PGM assemblages at Hochgrossen and Shetland (0.11860-0.11866 and 0.12473-0.12488, respectively); (2) similar 'unradiogenic' 187Os/188Os values for both 'primary' and 'secondary' PGM assemblages at Shetland (with mean 187Os/188Os 0.12419 and 0.12464, respectively) and Verkh-Neivinsk (with distinct mean 187Os/188Os values), and (3) a wide scatter of subchondritic 187Os/188Os values for 'primary' PGM assemblages at Kunar (i.e., 187Os/188Os 0.11848-0.11239), Verkh-Neivinsk (0.11619-0.12565), and Hochgrossen (0.11860-0.12450). The whole-rock Os-isotope budget of chromitite at Shetland (0.1240±0.0006) is largely controlled by laurite-dominant assemblages. In this case, the 'secondary' PGM assemblage inherited the 'unradiogenic' Os-isotope signature of the 'primary' PGMs. No evidence for other source contributions during later thermal events has been observed here. However, the wide range of subchondritic 187Os/188Os values has been found in the 'primary' PGM assemblages (e.g., laurite and Os-rich alloys) from the ophiolite-type complexes worldwide [2 and references cited therein]. This wide range would be consistent with a model, in which a prolonged history of melting events of parent ultramafic source rocks took place in the mantle. This variability is in agreement with the conclusion that the Os-isotope system of PGMs records multiple events during the chemical differentiation history of the mantle [3] and could have been controlled by deep-geodynamic processes [4]. On the other hand, the observed Os-isotope heterogeneity may be also attributed to the presence of subcontinental lithospheric mantle (SCLM), characterized by highly unradiogenic 187Os/188Os values (i.e.,

  19. Petrogenesis of carbonated meta-ultramafic lenses from the Neoproterozoic Heiani ophiolite, South Eastern Desert, Egypt: A natural analogue to CO2 sequestration

    NASA Astrophysics Data System (ADS)

    Gahlan, Hisham A.; Arai, Shoji; Almadani, Sattam A.

    2015-02-01

    Among a set of peculiar meta-ultramafics, carbonate-orthopyroxenites are observed for the first time in the Heiani ophiolite belt, South Eastern Desert, Egypt. They form massive lensoidal masses up to 50 m long and 20 m wide. The lenses show a marked structural concordance with their neighboring country rocks. The typical country rocks are represented by the following high-grade metamorphic rocks: kyanite-muscovite schists, amphibolites, kyanite-bearing biotite gneisses, migmatites, granite gneisses and mobilizates. The studied carbonate-orthopyroxenites consist mainly of metamorphic orthopyroxene + magnesite, among other metamorphic, relict primary and retrograde secondary minerals. According to primary chromian spinel (Cr#, 0.7-0.84) chemistry and morphology, absence of clinopyroxene and presence of primary mantle olivine (Fo89-91) as relicts in the metamorphic orthopyroxene, the Heiani carbonate-orthopyroxenites seem to have formed from a highly depleted mantle peridotite precursor. At a late collisional stage during the Pan-Africa terrane accretion and the E-W crustal shortening (ca. 650-620 Ma), high-grade (upper amphibolite facies) low-P/high-T regional metamorphism (ca. 660 Ma) accompanied by CO2-metasomatism resulted in formation of the Heiani carbonate-orthopyroxenites. Mostly the carbonate-bearing shelf sediments beneath and/or in juxtaposition with the Heiani ophiolite are considered to be the proven source of the CO2-rich fluids. Although, a mixed sedimentary-mantle C source is not unlikely. A mineral paragenetic correlation with experimental data for the system MgO-SiO2-H2O-CO2 suggests metamorphic conditions consistent with those of the high-grade country rocks; i.e. 630-650 °C, 6-7 kbar (20-23 km depth) and high-XCO2 (0.6-0.7). The CO2-bearing fluids discharging along faults gave rise to regionally widespread carbonate-bearing assemblages. Accordingly, the Heiani carbonate-orthopyroxenites are considered the by-products of natural carbon sequestration by an ultramafic rock.

  20. Composition, melting and evolution of the upper mantle beneath the Jurassic Pindos ocean inferred by ophiolitic ultramafic rocks in East Othris, Greece

    NASA Astrophysics Data System (ADS)

    Magganas, Andreas; Koutsovitis, Petros

    2015-07-01

    Large ultramafic bodies of the East Othris ophiolite in Central Greece consist of serpentinites, of harzburgite precursors, as well as serpentinized lherzolites, which have been intruded by thin dykes of olivine-rich and olivine-poor pyroxenites. They represent parts of partially altered upper mantle wedge rocks in a Mid-Late Jurassic intraoceanic subduction setting of the Pindos microocean, a western strand of the Tethyan oceanic realm. Serpentinization and rodingitization occurred during their exhumation toward the fore-arc oceanic region and accretionary prism through a subduction channel. Petrography and geochemistry show that protoliths of most serpentinites and serpentinized peridotites are harzburgites, while few are more fertile lherzolites. Petrogenetic modeling reveals that the former harzburgites correspond to highly depleted residual mantle peridotites, which formed after moderate degrees (~13-20 %) of hydrous partial melting, whereas lherzolites, being closely related to the ophiolitic mantle peridotites of West Othris, resulted after lower partial melting degrees (~7-10 %). Mineral chemistry and geochemical data from pyroxenites imply that they have been derived after crystallization of a subduction-related IAT hydrous magma that formed after moderate partial melting degrees (~14-19 %), quite similar to those that produced the harzburgites. Melting processes for the East Othris mantle peridotites occurred in the spinel-stability field, at estimated equilibrium temperatures ranging between 900 and 1,050 °C and pressures between 1.4 and 1.7 GPa, in a rather highly oxidized environment. It is estimated that the primary IAT magma, formed under relatively high temperatures with liquidus temperature at ~1,260 °C and mantle potential temperature at ~1,372 °C. Cooling rates of the shallow mantle beneath the Pindos oceanic basin, from its Mid-Triassic rift/drift phase and the subsequently developed Mid-Late Triassic short-lived intraoceanic subduction, to the Mid-Late Jurassic main subduction phase, are estimated at ~0.7 and ~1.6 °C/Ma, respectively, with the latter being considered as unusually high.

  1. An eclogite-bearing continental tectonic slice in the Zermatt-Saas high-pressure ophiolites at Trockener Steg (Zermatt, Swiss Western Alps)

    NASA Astrophysics Data System (ADS)

    Weber, Sebastian; Bucher, Kurt

    2015-09-01

    The Theodul Glacier Unit (TGU) at "Trockener Steg" represents a continental slice, embedded within the ophiolitic Zermatt-Saas Zone. The Zermatt-Saas Zone is the remnant of the Piemonte-Liguria oceanic lithosphere, formed in the middle Jurassic and subducted up to eclogite facies conditions in the Early Tertiary. The close spatial association of the TGU to the Zermatt-Saas Zone permits a comparison of the metamorphic evolution of the units by detailed field mapping and a petrological investigation of eclogites. The eclogites from both tectono-metamorphic units can be clearly distinguished by their textures, mineral assemblages and by mineral and bulk-rock composition. Geothermobarometry and computed assemblage stability diagrams for the TGU eclogites indicate P-T conditions of 2.2 ± 0.1 GPa and 580 ± 50 °C. These derived P-T conditions must be considered as minimum peak metamorphic conditions the rocks achieved during subduction. The P-T data are different from those derived for eclogites of Zermatt-Saas Zone adjacent to the Theodul Glacier Unit, that reached maximal burial depths at 2.3-2.4 GPa and 500 ± 50 °C. While the estimates of the eclogites of Zermatt-Saas Zone are in good agreement with some of the previous studies, the contrasting P-T estimates for the TGU eclogites suggest that the Zermatt-Saas complex must be subdivided into several tectonic subunits. The non-uniform peak conditions over the "Trockener Steg" area and the maximum pressures conditions reported from ultra-high pressure localities within Zermatt-Saas Zone suggest, that individual tectonic slices have been assembled after detachment from the slab at the return-point, i.e. along the exhumation path. Detached packages of rocks may range from small tectonic slices up to several kilometer-sized fragments. The TGU is separated from the surrounding rocks of the ophiolite unit by two major tectonic contacts. In addition, the formation of biotite-rich crusts along the basal contact of the TGU is evidence of prolonged fluid channeling along the basal thrust. The presence of hydrous decompression assemblages replacing earlier formed high-pressure mineral assemblages within the studied eclogite suggests that fluids were present throughout most of the TGU exhumation history.

  2. Geodynamic interpretation of the 40Ar/39Ar dating of ophiolitic and arc-related mafics and metamafics of the northern part of the Anadyr-Koryak region

    USGS Publications Warehouse

    Palandzhyan, S.A.; Layer, P.W.; Patton, W.W., Jr.; Khanchuk, A.I.

    2011-01-01

    Isotope datings of amphibole-bearing mafics and metamafics in the northern part of the Anadyr-Koryak region allow clarification of the time of magmatic and metamorphic processes, which are synchronous with certain stages of the geodynamic development of the northwest segment of the Pacific mobile belt in the Phanerozoic. To define the 40Ar/39Ar age of amphiboles, eight samples of amphibole gabbroids and metamafics were selected during field work from five massifs representing ophiolites and mafic plutons of the island arc. Rocks from terranes of three foldbelts: 1) Pekulnei (Chukotka region), 2) Ust-Belaya (West Koryak region), and 3) the Tamvatnei and El'gevayam subterranes of the Mainits terrane (Koryak-Kamchatka region), were studied. The isotope investigations enabled us to divide the studied amphiboles into two groups varying in rock petrographic features. The first was represented by gabbroids of the Svetlorechensk massif of the Pekulnei Range and by ophiolites of the Tamvatnei Mts.; their magmatic amphiboles show the distribution of argon isotopes in the form of clearly distinguished plateau with an age ranging within 120-129 Ma. The second group includes metamorphic amphiboles of metagabbroids and apogabbro amphibolites of the Ust-Belaya Mts., Pekulnei and Kenkeren ranges (El'gevayam subterranes). Their age spectra show loss of argon and do not provide well defined plateaus the datings obtained for them are interpreted as minimum ages. Dates of amphiboles from the metagabbro of the upper tectonic plate of the Ust-Belaya allochthon points to metamorphism in the suprasubduction environment in the fragment of Late Neoproterozoic oceanic lithosphere in Middle-Late Devonian time, long before the Uda-Murgal island arc system was formed. The amphibolite metamorphism in the dunite-clinopyroxenite-metagabbro Pekulnei sequence was dated to occur at the Permian-Triassic boundary. The age of amphiboles from gabbrodiorites of the Kenkeren Range was dated to be Early Jurassic that confirmed their assignment to the El'gevayam volcanic-plutonic assemblage. These data are consistent with geological concepts and make more precise the available age dates. Neocomian-Aptian 40Ar/39Ar age of amphibolites from the Pekulnei and Tamvatnei gabbroids make evident that mafics of these terranes (varying in geodynamic formation settings and in petrogenesis) were generated in later stages of the development of the West Pekulnei and Mainits-Algan Middle-Late Jurassic-Early Cretaceous island arc systems, presumably due to breakup of island arcs in the Neocomian. ?? 2011 Pleiades Publishing, Ltd.

  3. Geochronology and geochemistry of the Eastern Erenhot ophiolitic complex: Implications for the tectonic evolution of the Inner Mongolia-Daxinganling Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Zhang, Zhicheng; Li, Ke; Li, Jianfeng; Tang, Wenhao; Chen, Yan; Luo, Zhiwen

    2015-01-01

    The Eastern Erenhot ophiolitic complex (EOC) is one of the numerous fragments of oceanic lithosphere in southeastern Central Asian Orogenic Belt. It is composed dominantly of serpentinized ultramafic rocks with subordinate gabbros, mafic lavas and minor plagiogranite dikes. Zircons from two gabbros and one plagiogranite yielded weighted mean 206Pb/238U ages of 354.2 ± 4.5 Ma, 353.3 ± 3.7 Ma and 344.8 ± 5.5 Ma. These ages suggest that the oceanic crust of the EOC formed in a maximum time period of 10 Ma, and that the plagiogranite may have formed later than the gabbroic section. An undeformed and unmetamorphosed dioritic porphyry dike intruded in the Carboniferous strata near the EOC has an intrusive age of 313.6 ± 2.9 Ma and provides a possible younger minimum time limit for the formation of the early Carboniferous ophiolitic complex. All the mafic rocks have similar chondrite normalized REE patterns characterized by moderate depletion in LREE with (La/Yb)N (0.20-0.75) similar to normal middle oceanic ridge basalt (N-MORB). The PM-normalized trace element patterns of the gabbros and massive basalts are also reasonably consistent, essentially similar to those of N-MORB except for some enrichment in LILE (e.g. Rb, Ba) and slightly negative Ti anomalies. The plagiogranite samples are characterized by lower K2O (0.45-0.73 wt%) comparable with oceanic plagiogranite. They have LREE-enriched, chondrite-normalized REE patterns with varying Eu anomalies and the trace elements (e.g. Rb, Y, Nb) show similarity to volcanic arc granite. These geochemical features of the EOC show a similar volcanic arc affinity, suggesting that they form in a back-arc-type environment. Their origin is attributed to asthenospheric upwelling and further lithospheric extension during early Carboniferous, formed as a consequence of slab breakoff on collision of the northern early to mid-Paleozoic orogenic terrane and the Hunshandake Block.

  4. systematics of island arc crustal amphibolite migmatites from the Asago body of the Yakuno Ophiolite, Japan: a field evaluation of some model predictions

    NASA Astrophysics Data System (ADS)

    Pu, Xiaofei; Brophy, James G.; Tsujimori, Tatsuki

    2014-09-01

    The two most commonly invoked processes for generating silicic magmas in intra-oceanic arc environments are extended fractional crystallization of hydrous island arc basalt magma or dehydration melting of lower crustal amphibolite. Brophy (Contrib Mineral Petrol 156:337-357, 2008) has proposed on theoretical grounds that, for liquids >~65 wt% SiO2, dehydration melting should yield, among other features, a negative correlation between rare earth element (REE) abundances and increasing SiO2, while fractional crystallization should yield a positive correlation. If correct, the REE-SiO2 systematics of natural systems might be used to distinguish between the two processes. The Permian-age Asago body within the Yakuno Ophiolite, Japan, has amphibolite migmatites that contain felsic veins that are believed to have formed from dehydration melting, thus forming an appropriate location for field verification of the proposed REE-SiO2 systematics for such a process. In addition to a negative correlation between liquid SiO2 and REE abundance for liquids in excess of ~65 % SiO2, another important model feature is that, at very high SiO2 contents (75-76 %), all of the REE should have abundances less than that of the host rock. Assuming an initial source amphibolite that is slightly LREE-enriched relative to the host amphibolites, the observed REE abundances in the felsic veins fully support all theoretical predictions.

  5. The discovery and significance of the northeastern Jiangxi Province ophiolite (NEJXO), its metamorphic peridotite and associated high temperature-high pressure metamorphic rocks

    NASA Astrophysics Data System (ADS)

    Guoqing, Zhou

    The NEJXO with a N.E.-S.W. elongation occurs in the mid-Lower Qigong Group, under which lies the Jiuling Group (1401 Ma) and above which lies the Shangshu Group (817 ± 87 Ma), so that the age of NEJXO is defined to be Proterozoic between 1401 Ma and 817 ± 87 Ma. The sediments of the Jiuling Group show evidence of continental derivation, but the Qigong Group and Shangshu Group are characterised by CA volcanic rocks and probably represent a gradually growing island-arc. Thus, we regard the NEJXO as occurring in a back-island-arc basin between the ancient continent and the island-arc. On the whole, the main members of dismembered ophiolite are all present. The metamorphic periodotite present in them, is considered to be especially important, because it may be the sole representative of the older mantle present and it differs from those younger. The high-T metamorphic rocks associated with the NEJXO are various hornstones and melilite marble, whereas the high-P metamorphic rocks are aragonite-jadeite-glaucophane schist and schistose lawsonite marble. From the fact that high-P metamorphism was superimposed on the high-T metamorphic rocks, it may be suggested that early tension (at opening stage) and late compression (at closing stage) occurred during the development of the basin.

  6. Omphacite-bearing calcite marble and associated coesite-bearing pelitic schist from the meta-ophiolitic belt of Chinese western Tianshan

    NASA Astrophysics Data System (ADS)

    Lü, Zeng; Bucher, Kurt; Zhang, Lifei

    2013-10-01

    In the meta-ophiolitic belt of Chinese western Tianshan, marble (5-50 cm thick) is found interlayered with pelitic schist. The marble is mainly composed of calcite (>90% in volume) and accessory phases include omphacite, quartz, dolomite, albite, phengite, clinozoisite and titanite with or without rutile core. This is the first omphacite (Jd35-50) reported from marble of Chinese western Tianshan. It mainly occurs in the calcite matrix, rarely as inclusion in albite. The presence of omphacite suggests that the layered marble was subjected to eclogite-facies metamorphism, consistent with the occurrence of high-Si phengite (Si a.p.f.u. up to 3.7) and aragonite relic in albite. The associated pelitic schist consists of quartz, white mica (phengite + paragonite), garnet, albite, amphibole (barroisite ± glaucophane) and rutile/titanite, as well as minor amounts of dolomite, tourmaline and graphite. Coesite is optically recognized within porphyroblastic pelitic garnet and is further confirmed via Raman spectroscopy. Thermodynamic models support the UHP metamorphism of calcite marble, similar to the associated pelitic schist. Shared UHP-LT history of calcareous and pelitic rocks in Chinese western Tianshan suggests that the supracrustal carbon-rich sediments have been carried to depths of >90 km during fast subduction and thus are potential sources for carbon recycled into arc crust.

  7. Melanges pedagogiques 1990 (Pedagogical Miscellany 1990).

    ERIC Educational Resources Information Center

    Centre de Recherches et d'Applications Pedagogiques en Langues, Nancy (France).

    This issue of an annual publication on second language teaching contains nine articles. All but one of the these is written in French; each paper, however, is preceded by an English-language abstract. The articles are as follows: "Des cultures, des pubs" ("Cultures, Advertising") (Sophie Bailly, Isabelle Tolle); "Construire son apprentissage:…

  8. Melanges Pedagogiques (Pedagogical Mixture), 1986/87.

    ERIC Educational Resources Information Center

    Melanges Pedagogiques, 1987

    1987-01-01

    The 1986/87 issue of the journal on second language teaching and learning contains seven articles in French and four in English, including: "Learning How To Learn English"; "Socrate est-il un chat? Pratiquer le syllogisme pour apprendre a argumenter (Is Socrates a Cat? Practicing Syllogisms To Learn To Argue)"; "La production orale en francais des…

  9. Granitoids associated with East Vardar ophiolites (Serbia, F.Y.R. of Macedonia and northern Greece): Origin, evolution and geodynamic significance inferred from major and trace element data and Sr-Nd-Pb isotopes

    NASA Astrophysics Data System (ADS)

    Šari?, Kristina; Cvetkovi?, Vladica; Romer, Rolf L.; Christofides, George; Koroneos, Antonis

    2009-03-01

    The paper presents and discusses a new set of major, trace and Sr-Nd-Pb data concerning Upper Jurassic granitoid rocks associated with the East Vardar ophiolites. These rocks form a discontinuous belt from Serbia in the north to northern Greece in the south. Two major styles of granitoid magmatism are recognized: (1) the southern granitoids form large intrusions cutting both the East Vardar ophiolites and the metamorphic rocks of the Serbo-Macedonian massif; (2) the northern granitoids are small petrogenetically variable bodies that are always spatially associated with ophiolites; these are probably multiphase, resulting in numerous low-volume granitoid intrusions within ophiolites. The southern granitoids (except at Fanos, N Greece) form an almost complete suite of decreasing radiogenic neodymium ( ?Nd( T) = 3.3 to - 8.9) and increasing radiogenic strontium (Sr i = 0.70740-0.71588) with increasing silica contents, and assimilation fractional crystallization (AFC) processes seem to have played an important role in their petrogenesis. Their primary magmas most probably originated by melting of a slightly enriched MORB-like mantle. The Fanos granite is a special case as it is characterized by a uniform isotopic composition for differently evolved rocks (Sr i = 0.70516-0.70559, ?Nd( T) = - 1.6 to - 0.7). This granite is interpreted to have derived from lower crustal melts that differentiated mainly through fractionation processes. The northern granitoid group consists of intermediate rocks (Sr i = 0.70557-0.70746, ?Nd( T) = - 4.5 to - 0.8), high-Sr i granites (Sr i = 0.70956-0.71602, ?Nd( T) = insertionofequalsignintheequationof(Nd(T)=-6to-5.1ifappropriate.-->- 6 to - 5.1, HREE- and Y-enriched) and low-Sr i granites (Sr i = 0.70330-0.70767, ?Nd( T) = - 5.1 to 1.5). High-Sr i granites are interpreted to have crystallized from peraluminous magmas generated by fusion of (meta)sedimentary rocks caused by obduction-induced melting. The northern intermediate rocks and the related low-Sr i granites have a wider range of composition and may be explained as genetically different rock groups. Some of them could have originated during obduction-induced melting of a source that was different from the source of high-Sr i granitoids, whereas others could be products of subduction-related volcanic arc magmatism.

  10. The ophiolite of the Eohellenic nappe in the island of Skyros, Greece: Geotectonic environment of formation and metamorphic conditions inferred by mineralogical and geochemical data

    NASA Astrophysics Data System (ADS)

    Karkalis, Christos; Magganas, Andreas; Koutsovitis, Petros

    2014-05-01

    The island of Skyros is located in the Sporades-Aegean region. It includes an ophiolitic mélange sequence consisting of serpentinites, gabbroic and doleritic rocks, and also lavas which mostly appear in massive form, but in rare cases as deformed pillows. The ophiolitic mélange sequence also includes rodingites, ophicalcites, as well as radiolarites. This formation belongs to the Eohellenic tectonic nappe, which encompasses marbles, sandstones and schists and was emplaced onto the Pelagonian Zone during Early Cretaceous [1, 2]. Serpentinites were most likely formed after serpentinization of harzburgitic protoliths and consist of serpentine, bastite, spinel and magnetite. The chemistry of spinels (TiO2=0.14-0.25 wt.%, Al2O3=35.1-35.21 wt.%, Cr#=37.38-38.87), shows that the harzburgitic protoliths plausibly resemble back-arc basin peridotites [3]. Gabbros and dolerites present mostly subophitic textures, between the hornblende/clinopyroxene and plagioclase grains. Based upon their petrography and on their mineral chemistry hornblendes have been distinguished into magmatic and metamorphic hornblendes, with the first occurring mostly in gabbroic rocks. Magmatic hornblendes exhibit relatively high TiO2 (1.42-1.62 wt.%), Al2O3 (5.11-5.86 wt.%) and Na2O (1.01-1.09 wt.%) contents, with their presence implying that the magma was at least to some degree hydrous. Lavas are tholeiitic basalts with relatively high FeOt?12 wt.% and low K2O and Th contents, consisting mostly albite, altered clinopyroxene and devitrified glass. Tectonomagmatic discrimination diagrams [4, 5] illustrate that the studied gabbros and lavas of Skyros are most likely associated with SSZ processes. Gabbroic rocks, subvolcanic dolerites and lavas have been subjected to greenschist/subgreenschist metamorphic processes, as confirmed by the presence of secondary amphiboles (metamorphic hornblende, actinolite/tremolite), epidote, pumpellyite and chlorite in all of the studied samples. On the other hand, the occurrence of rodingites and ophicalcites clearly point to interaction of the gabbroic rocks and serpentinites with hydrothermal fluids, which most probably took place during the stage of exhumation and tectonic emplacement. Ophicalcites contain serpentine, calcite, magnetite, as well as rare pyroxene and spinel. Rodingites on their behalf include hydroandradite (Alm0.00Adr61.33-67.43Grs28.25-35.18Prp0.10-2.49Sps0.00-0.33Uv0.41-2.75), vesuvianite (MgO=2.78-3.33 wt.%; TiO2=0.02-0.59 wt.%) diopside neoblasts (En48.53-49.89Wo47.56-48.10Fs2.32-3.33; Mg#=93.96-96.28), chlorite and also accessory prehnite. Some small-sized Cr-bearing hydrogarnet crystals (Cr2O3=10.34 wt.%) were most likely formed at the expense of spinel. The types of hydrogarnet and vesuvianite crystals are highly indicative for the involvement of subduction-related fluids during the formation of the rodingites [6]. References: [1] Jacobshagen & Wallbrecher 1984: Geol. Soc., London, Sp. Pub. 17, 591-602, [2] Pe-Piper 1991: Ofioliti, 16, 111 - 120, [3] Kamenetsky Sobolev, Joron & Semet 2001: J Petrol 42, 655-671, [4] Agrawal, Guevara & Verma 2008: Intern. Geol. Rev. 50, 1057-1079, [5] Pearce & Cann 1973: Earth Plan. Sci. Lett. 19, 290-300, [6] Koutsovitis, Magganas, Pomonis & Ntaflos 2013. Lithos 172-173, 139-157.

  11. Sierran affinity (?) metasedimentary rocks beneath the Coast Range Ophiolite of the Sierra Azul block east of the San Andreas fault, Santa Clara County, CA

    NASA Astrophysics Data System (ADS)

    McLaughlin, R. J.; Dumitru, T. A.; Ernst, W. G.

    2011-12-01

    The Loma Prieta slate (LPS) is a <100 m thick slice of highly flattened and stretched, pebbly to shaly metasedimentary rocks exposed for a length of 700 m at Loma Prieta Peak, east of the San Andreas fault in the southern Santa Cruz Mountains. The LPS occurs along a low-dipping segment of the NW-trending, dextral-reverse Sargent fault, which places the slate and overlying Middle Jurassic Coast Range Ophiolite in the hanging wall, eastward over lower Eocene strata. The LPS and overlying Coast Range Ophiolite, in turn, form the base of a 60-80 km long fault block east of the San Andreas fault, overlain by Jurassic-lower Miocene marine strata that together define the Sierra Azul structural block (SAB). These rocks overlie terranes of the Franciscan Complex. The Sargent fault bisects the SAB section and is truncated along-strike and at depth, by the San Andreas fault. Reconstituted clastic grains of the LPS have dominant rhyo-dacitic and granitic sources and felsitic to granophyric grains preserve K-feldspar. Newly crystallized phengitic mica, chlorite and speculatively, incipient pumpellyite, are present in the LPS. No new high P/T metamorphic minerals are petrographically discernible, seemingly distinguishing the LPS from known cataclastic Franciscan Complex rocks structurally beneath the SAB. The LPS instead, has been proposed to correlate with the Jurassic arc-derived Mariposa Formation (MFS) in the Sierra Nevada Foothills, metamorphosed during the Nevadan orogeny. The correlation, however, has been problematic due to a lack of age control on the LPS, its limited surface distribution and its wide separation from the MFS. To test the correlation, we dated detrital zircons from the LPS at University of Arizona's LA-ICPMS lab and compared the results with detrital zircon data from the MFS (Snow and Ernst, 2008, GSA Special Paper 438). Weighted mean age calculations indicate a youngest zircon age cluster at about 152.5±2 Ma for the LPS, which indicates its maximum depositional age. The large number of zircons in the youngest LPS age cluster (31 out of 87) may reflect a contemporaneous volcanic source, consistent with the petrography. Zircon data for the MFS indicate earliest possible deposition at about 152 ± 1 Ma. Zircon ages >200 Ma are generally similar in the LPS and MFS, with minor age groupings at roughly 950-1450 and 1750-2100 Ma. As with the MFS, the LPS data suggest a major influence from sources in the Sierra Nevada arc, with minimal influences from sources in the Klamath Mountains and Nevada miogeocline. Available detrital zircon data require Cretaceous or younger maximum depositional ages for metaclastic terranes of the Franciscan Complex. The LPS detrital zircon data thus, are in reasonable agreement with the MFS data and permit interpretation of the LPS as displaced northward by the San Andreas and Hayward-Calaveras faults from the southwestern Great Valley margin.

  12. Trace element abundances in pyroxenes from a dunite-harzburgite-lherzolite sequence at the Trinity ophiolite: Evidence for multiple episodes of melt migration and melt-rock reaction

    NASA Astrophysics Data System (ADS)

    Dygert, N. J.; Liang, Y.; Kelemen, P. B.

    2011-12-01

    Dunite, harzburgite, lherzolite, and plagioclase lherzolite (DHL-PL) sequences have been observed in the mantle sections of the Trinity ophiolite [1]. In an earlier study, detailed microprobe analyses of olivine, orthopyroxene (opx), clinopyroxene (cpx), and spinel were conducted on a densely sampled traverse across a 20-meter DHL-PL sequence [2]. Systematic outcrop-scale compositional gradients in major elements such as Ni in olivine and Ti in cpx suggest basaltic melt migration from the dunite into adjacent plagioclase lherzolite, causing melt-rock reactions that dissolved plagioclase, opx, and cpx and precipitated olivine. Analogous geochemical gradients and lithological sequences have been produced in reactive dissolution experiments involving melt-bearing peridotite and basalt [3]. In an effort to further improve our understanding of melt migration and melt-rock reaction processes at Trinity, we analyzed pyroxene grains in all four lithologies for transition metals and rare earth elements (REE) using LA-ICPMS. Generally, REE patterns fall into two distinct groups, those within and near (<4m from) the dunite body will be referred to as group 1, while those farther from the dunite body (9-17.5m from the dunite-harzburgite contact) will be referred to as group 2. Group 1 cpx are extremely depleted in LREE with (Ce)C1 = 0.08-0.3, (Sm/Ce)C1 = 3-10, and (Sm/Yb)C1 = 0.3 to 0.8. Group 2 cpx have higher REE abundances but a similar extent of LREE depletion with (Ce)C1 = 0.5-1.5, (Sm/Ce)C1 = 5-16, and (Sm/Yb)C1 = 0.7-1.0. In general, REE concentrations in cpx cores and rims are similar across the transect, but two group 2 samples farthest from the dunite-harzburgite contact (15.4m and 17.5m) have lower mean rim concentrations than core concentrations, trending toward the more depleted REE abundances in group 1 cpx. Among the four group 1 samples in which opx was measured, cpx/opx REE partitioning is similar and suggests equilibration at near-magmatic temperatures (~1200° C) [4]. Remarkably, the compositional range of cpx grains measured in this 20m traverse covers nearly all abyssal peridotites sampled globally. This observation suggests that in some cases the scale of significant mantle heterogeneity is on the order of meters. In addition, melts in equilibrium with Trinity cpx nearly cover the entire N-MORB field. Trinity cpx in and near dunite (group 1) are more depleted than those in lherzolite farther from the dunite (group 2), in contrast to cpx from the Wadi Tayin section of the Oman ophiolite which show the opposite relationship [5]. Trinity cpx grains interacted with two distinct, LREE depleted melts of dramatically different REE abundances. Whether these melts were cogenetic but evolved differently or were produced by distinct sources, and how they achieved such steep LREE depletion with relatively high mid-HREE abundances will be a subject of continuing study. [1]. Quick (1981), CMP, 78, 413-422. [2]. Morgan et al. (2008), G3, Q07025. [3]. Morgan and Liang (2005), CMP, 150, 369-385. [4]. Witt-Eickschen and O'Neill (2005), Chem. Geol., 221, 65-101. [5]. Kelemen et al. (1995), Nature, 375, 747-753.

  13. Diverse contributing sources to chromitite petrogenesis in the Shebenik Ophiolitic Complex, Albania: evidence from new PGE- and Os-isotope data

    NASA Astrophysics Data System (ADS)

    Kocks, H.; Melcher, F.; Meisel, T.; Burgath, K.-P.

    2007-11-01

    Chromitites sampled from four different pseudostratigraphic levels of the Mesozoic Shebenik Ophiolite Complex, Albania, have low PGE totals <1 µg/g but show different types of PGE enrichment ( Burgath et al., 2003) as well as differing mineral chemistry, PGM mineralogy and Os isotopic signatures. To circumvent analytical problems with low PGE abundances, representative samples were analyzed using HPA-digestion followed by isotope dilution ICP-MS. Osmium isotopes were determined by ICP-QMS and N-TIMS techniques. Podiform chromitites exposed in the mantle (Group I) and tabular chromitites exposed in the upper mantle (Group II) are Os-Ir-Ru-Rh enriched. In the upper mantle to mantle-crust transition zone, schlieren type chromitites (Group III) are enriched in Ru-Rh with low Os-Ir and low Pt-Pd. Within the mantle-crust transition zone disseminated chromitites in dunite are variably enriched in Ru-Rh-Pt, however, Os, Ir and Pd are low. IPGE rich chromites contain abundant small laurite inclusions whilst Rh and Pt are located in sulfarsenides marginally attached to transition zone chromites (see also Burgath et al., 2003). High Cr/Al ratios (>0.75) and low titanium contents of chromites throughout the sample suite are consistent with chromitite petrogenesis in a SSZ environment. Shebenik mantle chromitites with low 187Re/188Os ratios have an average, slightly suprachondritic initial osmium isotopic composition of 0.1285 ± 0.0022 (2s). Towards higher pseudostratigraphic levels, 187Re/188Os ratios increase and initial Os isotopies are very heterogeneous. Distinctly suprachondritic Os signatures require input of radiogenic source components, whereas subchondritic samples require assimilation of long term Re-depleted PGM.

  14. Sm-Nd, Rb-Sr, and /sup 18/O//sup 16/O isotopic systematics in an oceanic crustal section: Evidence from the Samial ophiolite

    SciTech Connect

    McCulloch, M.T.; Gregory, R.T.; Wasserburg, G.J.; Taylor, H.P. Jr.

    1981-04-10

    The Sm-Nd, Rb-Sr, and /sup 18/O//sup 16/O isotopic systems have been used to distinguish between the effects of seafloor hydrothermal alteration and primary magmatic isotopic variations. The Sm-Nd isotopic system is essentially unaffected by seawater alteration, while the Rb-Sr and /sup 18/O//sup 16/O systems are sensitive to hydrothermal interactions with seawater. Sm-Nd mineral isochrons from the cumulate gabbros of the Samail ophiolite have an initial /sup 143/Nd//sup 144/Nd ratio of e/sub Nd/ = 7.8 +- 0.3, which clearly substantiates the oceanic affinity of this complex. The initial /sup 143/Nd//sup 144/Nd ratios for the harzburgite, plagiogranite, sheeted diabase dikes, and basalt units have a limited range in e/sub Nd/ of from 7.5 to 8.6, indicating that all the lithologies have distinctive oceanic affinities, although there is also some evidence for small isotopic heterogeneities in the magma reservoirs. The Sm-Nd mineral isochrons give crystallization ages of 128 +- 20 m.y. and 150 +- 40 m.y. from Ibra and 100 +- 20 m.y. from Wadi Fizh, which is approximately 300 km NW of Ibra. These crystallization ages are interpreted as the time of formation of the oceanic crust. The /sup 87/Sr//sup 86/Sr initial ratios on the same rocks have an extremely large range of from 0.7030 to 0.7065 and the d/sup 18/O values vary from 2.6 to 12.7. These large variations clearly demonstrate hydrothermal interaction of oceanic crust with seawater.

  15. By-products of the serpentinization process on the Oman ophiolite : chemical and isotopic composition of carbonate deposits in alkaline springs, and associated secondary phases

    NASA Astrophysics Data System (ADS)

    Sissmann, O.; Martinez, I.; Deville, E.; Beaumont, V.; Pillot, D.; Prinzhofer, A.; Vacquand, C.; Chaduteau, C.; Agrinier, P.; Guyot, F. J.

    2014-12-01

    The isotopic compositions (d13C, d18O) of natural carbonates produced by the alteration of basic and ultrabasic rocks on the Oman ophiolite have been measured in order to better understand their formation mechanisms. Fossil carbonates developed on altered peridotitic samples, mostly found in fractures, and contemporary carbonates were studied. The samples bear a large range of d13C. Those collected in veins are magnesian (magnesite, dolomite) and have a carbon signature reflecting mixing of processes and important fractionation (-11‰ to 8‰). Their association with talc and lizardite suggests they are by-products of a serpentinization process, that must have occurred as a carbon-rich fluid was circulating at depth. On the other hand, the carbonates are mostly calcic when formed in alkaline springs, most of which are located in the vicinity of lithological discontinuities such as the peridotite-gabbro contact (Moho). Aragonite forms a few meters below the surface of the ponds in Mg-poor water, and is systematically associated with brucite (Mg(OH)2). This suggests most of the Mg dissolved at depth has reprecipitated during the fluid's ascension through fractures or faults as carbonates and serpentine. Further up, on the surface waters of the ponds (depleted in Mg and D.I.C.), thin calcite films precipitate and reach extremely negative d13C values (-28‰), which could reflect either a biological carbon source, or kinetic fractionation from pumping atmospheric CO2. Their formation represent an efficient and natural process for carbon dioxide mineral sequestration. The d18O signature from all samples confirm the minerals crystallized from a low-temperature fluid. The hyperalkaline conditions (pH between 11 and 12) allowing for these fast precipitation kinetics are generated by the serpentinization process occurring at depth, as indicated by the measured associated H2-rich gas flows (over 50%) seeping out to the surface.

  16. Chronology of the Pueblo Viejo epithermal gold-silver deposit, Dominican Republic: formation in an Early Cretaceous intra-oceanic island arc and burial under ophiolite

    NASA Astrophysics Data System (ADS)

    Mueller, Andreas G.; Hall, Gregory C.; Nemchin, Alexander A.; O'Brien, Darren

    2008-11-01

    The Pueblo Viejo deposit (production to 1996: 166 t Au, 760 t Ag) is located in the Dominican Republic on the Caribbean island of Hispaniola and ranks as one of the largest high-sulfidation/acid-sulfate epithermal deposits (reserves in 2007: 635 t Au, 3,648 t Ag). One of the advanced argillic ore bodies is cut by an inter-mineral andesite porphyry dike, which is altered to a retrograde chlorite-illite assemblage but overprinted by late-stage quartz-pyrite-sphalerite veins and associated low-grade Au, Ag, Zn, Cd, Hg, In, As, Se, and Te mineralization. The precise TIMS U-Pb age (109.6 ± 0.6 Ma) of the youngest zircon population in this dike confirms that the deposit is part of the Early Cretaceous Los Ranchos intra-oceanic island arc. Intrusion-related gold-sulfide mineralization took place during late andesite-dacite volcanism within a thick pile (>200 m) of carbonaceous sand- and siltstones deposited in a restricted marine basin. The high-level deposit was shielded from erosion after burial under a late Albian (109-100 Ma) ophiolite complex (8 km thick), which was in turn covered by the volcano-sedimentary successions (>4 km) of a Late Cretaceous-Early Tertiary calc-akaline magmatic arc. Estimates of stratigraphic thickness and published alunite, illite, and feldspar K-Ar ages and closure temperatures (alunite 270 ± 20°C, illite 260 ± 30°C, K-feldspar 150°C) indicate a burial depth of about 12 km at 80 Ma. During peak burial metamorphism (300°C and 300 MPa), the alteration assemblage kaolinite + quartz in the deposit dehydrated to pyrophyllite. Temperature-time relations imply that the Los Ranchos terrane then cooled at a rate of 3-4°C/Ma during slow uplift and erosion.

  17. First description of a metamorphic sole related to ophiolite obduction in the northern Caribbean: Geochemistry and petrology of the Güira de Jauco Amphibolite complex (eastern Cuba) and tectonic implications

    NASA Astrophysics Data System (ADS)

    Lázaro, C.; Blanco-Quintero, I. F.; Rojas-Agramonte, Y.; Proenza, J. A.; Núñez-Cambra, K.; García-Casco, A.

    2013-10-01

    The Güira de Jauco Amphibolite Complex underlies the Mayarí-Baracoa Ophiolitic Belt (eastern Cuba) and is composed of highly foliated amphibolite, locally with blocks of sheared serpentinitie that enclose concordant layers of amphibolite. These amphibolites are interpreted as remnants of a sub-ophiolitic metamorphic sole likely formed during late Cretaceous times before the initiation of the collision between the Caribbean and the North American plates. The complex includes common amphibolites (Hbl + Pl ± Ep ± Ttn ± Rt ± Qtz ± Ap), garnet amphibolites (Hbl + Grt + Pl + Ep ± Ttn ± Qtz ± Ap) and clinopyroxene amphibolites (Hbl + Pl + Cpx ± Ep ± Ttn ± Qtz ± Ap). Geochemical data indicates that the protoliths are igneous mafic rocks of basaltic composition that likely formed part of the upper and lower(?)crust of an oceanic lithosphere. Rare Earth element contents suggest a Mid-Ocean Ridge Basalt (MORB) origin. However, the enrichment in Large-Ion Lithophile elements and the depletion in NbTaTi as compared to Normal-MORB, suggests a suprasubduction scenario, either a back-arc or a fore-arc setting. The pressure-temperature estimates of metamorphism indicate that the entire amphibolite body underwent similar peak metamorphic conditions at 650-665 °C and 8.5-8.7 kbar (ca. 30 km depth), corresponding to a relatively high apparent geothermal gradient of 23 °C/km.

  18. Petrogenesis and PGE distribution in the Al- and Cr-rich chromitites of the Qalander ophiolite, northeastern Iraq: Implications for the tectonic environment of the Iraqi Zagros Suture Zone

    NASA Astrophysics Data System (ADS)

    Ismail, Sabah A.; Kettanah, Yawooz A.; Chalabi, Sawsan N.; Ahmed, Ahmed H.; Arai, Shoji

    2014-08-01

    The Qalander ophiolite is a small, poorly preserved and incomplete mélange-type complex situated within the Eocene-Oligocene Walash-Naopurdan Group in the Iraqi Zagros Suture Zone (IZSZ). It is one of six fragmented ophiolite complexes emplaced in the IZSZ during the Cretaceous and Tertiary. Within the Qalander ophiolite, serpentinized dunite and harzburgite hosts small lens-shaped podiform high-Al (North Qalander) and high-Cr (North Shitna) chromitite bodies. The average range of Cr-, Mg-, and Fe3-numbers for North Qalander chromitites is 39-53, 75-77, and 3-5, respectively; meanwhile those for North Shitna chromitites are 78-80, 65-73, and 6-8, respectively. The ranges of Al2O3 wt.% and FeO/MgO for the North Qalander and North Shitna chromitites are 15-15.5 and 0.8-1.0, and 9.0-10.5 and 0.4-1.0, respectively. In addition to pyroxene and olivine, inclusions of laurite, millerite and galena were detected within the chromitites of Qalander ophiolite. The matrix minerals between chromite grains are serpentinized olivine and pyroxene, chlorite, and calcite; grains of magnetite, pyrite and ilmenite are also common accessories. The concentrations of platinum-group elements (PGE) in both varieties are typical for the ophiolitic chromitites, but they show two distinct patterns of PGE enrichment. The high-Cr chromitites have relatively uniform mantle-normalized PGE distribution patterns with a steep slope, positive Ru and negative Pt anomalies, and they show enrichment of PGE and depletion in Pt relative to the average upper mantle. The high-Al chromitites show relatively gently sloping patterns with slight positive Ru and negative Pt anomalies, high iridium-group PGE (IPGE) abundances relative to the high platinum-group PGE (PPGE), and are distinctly enriched in Pt and Pd relative to the upper mantle and the average abundances in the high-Cr chromitites. The differences in the PGE content, geochemistry, rare earth element (REE), mineral chemistry and petrographical characteristics of the chromitites suggests two stages of magmatic activity which were responsible for the chromitite genesis in the region. The first stage involved low degree partial melting of S-saturated peridotite melt that produced high-Al chromitites of MORB affinity, which crystallized in the upper mantle close to the Moho. The second stage involved a high degree partial melting of S-undersaturated boninitic melt that produced high-Cr chromitites of arc affinity and crystallized in a deeper mantle section. The presence of low-Cr MORB-type chromitites with high-Cr boninitic-type chromitites in the same mantle section suggests their formation in aback-arc supra-subduction zone tectonic setting.

  19. Early Neoproterozoic multiple arc-back-arc system formation during subduction-accretion processes between the Yangtze and Cathaysia blocks: New constraints from the supra-subduction zone NE Jiangxi ophiolite (South China)

    NASA Astrophysics Data System (ADS)

    Wang, Xin-Shui; Gao, Jun; Klemd, Reiner; Jiang, Tuo; Zhai, Qing-Guo; Xiao, Xu-Chang; Liang, Xin-Quan

    2015-11-01

    The NE Jiangxi ophiolite in the eastern Jiangnan Orogen is a tectonic mélange that mainly consists of individual tectonic blocks comprising pyroxenite, gabbro, basalt, diorite, granite and chert in a matrix of serpentinite or tuffaceous greywacke. A combined geochemical and geochronological study of the NE Jiangxi ophiolite was undertaken to constrain the timing and tectonic setting of its formation. The basalts were geochemically subdivided into three groups with different FeOt and TiO2 contents. Group 1 basalts have the lowest FeOt (12.17-13.07 wt.%) and TiO2 (1.48-1.62 wt.%) contents and the lowest Nb/Yb (0.80-0.88) and Th/Nb (0.02-0.03) ratios. Furthermore, they have normal mid-ocean ridge basalt (N-MORB)-like trace element patterns, suggesting derivation from an N-MORB-type mantle source without subduction input. Group 2 Fe-Ti basalts have the highest FeOt (15.52-16.30 wt.%) and TiO2 (3.06-3.23 wt.%) contents, Nb/Yb and Th/Nb ratios from 1.75 to 1.89 and from 0.11 to 0.15, respectively, and trace element patterns similar to those of back-arc basin basalts. The geochemical characteristics suggest that Group 2 basalts were derived from a slightly enriched MORB-type mantle source with a minor subduction contribution. In contrast, Group 3 Fe-Ti basalts have moderate FeOt (12.98-13.40 wt.%) and TiO2 (2.37-2.71 wt.%) contents, and Nb/Yb and Th/Nb ratios from 1.28 to 1.45 and from 0.27 to 0.30, respectively. These basalts further display markedly negative Nb-Ta anomalies and show a geochemical affinity to island-arc basalts (IAB), indicating a slightly enriched MORB-type mantle source that was significantly influenced by subduction-derived fluids and/or melts. SIMS zircon U-Pb dating on gabbros gave ages of 995 ± 22 Ma and 993 ± 12 Ma, which are interpreted as the formation age of the NE Jiangxi ophiolite. Positive zircon ?Hf(t) (+ 8.8 to + 13.8) values for the gabbros and whole-rock ?Nd(t) (+ 5.5 to + 6.6) values for the basalts indicate that the NE Jiangxi ophiolite originated from an isotopically homogeneous depleted mantle source. The diversity of MORB- to IAB-like basalts and the presence of Fe-Ti basalts favor a formation of the NE Jiangxi ophiolite during the initial rifting phase of an intra-oceanic back-arc basin between an oceanic arc (Huaiyu Terrane) and the continental margin of the Yangtze Block (Jiuling Terrane) at ca. 990 Ma. Both the present and previous studies imply that multiple arc-back-arc systems formed during long-lasting subduction-accretion processes between the Yangtze and Cathaysia blocks during the early Neoproterozoic.

  20. Compositional variations across a dunite - harzburgite - lherzolite - plagioclase lherzolite sequence at the Trinity ophiolite: Evidence for multiple episodes of melt flow and melt-rock reaction in the mantle.

    NASA Astrophysics Data System (ADS)

    Morgan, Z. T.; Liang, Y.; Kelemen, P.

    2004-12-01

    In the preceding report we showed experimentally that the dunite-harzburgite-lherzolite (DHL) sequence found in the mantle sections of ophiolite could be formed by reactive dissolution of lherzolite in a basaltic liquid. The most striking results of our lherzolite dissolution experiments are the sharp mineralogical boundaries between adjacent lithologies and simple monotonic composition variations in minerals across the DHL sequence. Here we present a detailed compositional traverse across a dunite (3.64 m wide) - harzburgite-lherzolite (5.64 m) - plagioclase lherzolite (> 10 m) sequence (referred to as DHL-PL) at the Trinity ophiolite that shows complicated composition trends and melt flow history. With the exception of a small (1 m wide) anomalous region within the dunite, less than 1 m away from the dunite-harzburgite contact, the Mg#s of olivine (90), cpx (92.8), opx (90.4 in harzburgite) and spinel (40), as well as Al2O3 and TiO2 abundance in cpx, opx and spinel are essentially constant from dunite to lherzolite. The CaO content in olivine (0.02%), opx (1%) and cpx (23.5%) are also uniform throughout the harzburgite-plagioclase lherzolite sequence. However, the Mg# of olivine and opx, Al2O3, TiO2 and Cr2O3 in cpx and opx, as well as Na2O in cpx increase 2 meters into the plagioclase lherzolite. In addition, asymmetric concentration gradients are observed for CaO in olivine and Cr2O3, Al2O3, MgO, and FeO in spinel. These asymmetric concentration gradients are mostly in the dunite-side of the dunite-harzburgite contact. And finally, the 1 m wide anomalous region within the dunite is characterized by elevated Mg# and NiO in olivine, Al2O3, TiO2, Cr2O3, and REE in cpx, and very distinct elemental abundance in spinel. The composition variations reported here are substantially different from those of Quick (1981) who measured a smaller (1 m) DHL-PL sequence at the Trinity ophiolite. Together these two Trinity data sets show a large variation in DHL-PL chemistry within the same ophiolite. Concentration gradients across DHL-PL sequences have been observed in the mantle sections of ophiolites around the world. Based on our lherzolite dissolution experiments, preliminary numerical calculations, and previous studies we suggest that the DHL-PL sequence at Trinity was formed by pervasive melt flow and reactive dissolution of a plagioclase lherzolite in basaltic liquids. Although the details are still unknown, the complicated concentration profiles reported in this study can be explained by a model that involves multiple episodes of melt flow and melt-rock reaction in an evolving dunite channel system. Multiple episodes of melt flow in the mantle, each with distinct elemental and isotopic characteristics, have already been documented in olivine-hosted melt inclusions. The spatial distributions of the compositional variations reported here can be used to further constrain the time interval between different episodes of melt flow in the dunite channel. For example, the two (or more) episodes of melt flow that created the anomalous region in the dunite and the DHL-PL sequence could at most be separated by less than a few hundred years.

  1. Fluid evolution during tectonic exhumation of oceanic crust at a slow-spreading paleoridge axis: evidence from the Lizard ophiolite U.K.

    NASA Astrophysics Data System (ADS)

    Hopkinson, L.; Roberts, S.

    1996-06-01

    Zoned quatz crystals within late syntectonic veins from brittle-ductile shear zones situated within the lowermost plutonic rocks of the Lizard ophiolite contain three groups of fluid inclusions. Type I are a phase separated primary and pseudosecondary group. Liquid-rich inclusions contain 3.2-8.0wt% NaCl equivalent with a mode at 5.7wt% NaCl equiv. Vapour-rich inclusions contain 0.0-6.1wt% NaCl equiv. with a mode at 2.6wt% NaCl equiv. Inclusions homogenize either by disappearance of the liquid or vapour phase, with a mode at 380°C. Type II inclusions contain 2.1-5.7wt% NaCl equiv. with a mode at 4.1 wt% NaCl equiv. Inclusions homogenize by disappearance of the vapour phase (L + V ? L) with a mode at 230°C. Type III inclusions are a pseudosecondary and secondary group, with textures which are suggestive of underpressure-induced re-equilibration. They contain methane and a low salinity aqueous solution (0.3-2.2wt% NaCl equiv.). Liquid-rich satellite inclusions homogenize at 310-240°C (L + V ? L). Vapour-rich inclusions homogenize by disappearance of the liquid phase (L + V ? V) at 370-320°C. By assuming that the range in salinity is exclusively the product of phase separation of a 3.2wt% NaCl seawater solution, and by applying microthermometric data to the NaCl?H 2O system, analysis of the data suggests two-phase boundary constrained conditions of crystal growth, initiated within the super-critical field at around ˜410°C, with declining temperature passing into subcritical portions of the system, beyond which the fluid entered single-phase liquid conditions of crystal growth. Microthermometric and petrographic data, in comparison with present day ocean-ridge hydrothermal systems, is consistent with a reaction zone situated within an inferred decollement at or near the petrological moho, from which fluids were derived from and expelled along intersecting listric faults, at temperatures broadly comparable to shallow subsurface levels within present-day systems. This interpretation suggests that high vent temperatures were maintained as the heat flux waned, and that the hydrothermal system migrated downward through the uplifting gabbros, culminating in the reaction zone intersecting the upper mantle during the terminal pulses of tectonic exhumation.

  2. Compositional diversity in peridotites as result of a multi-process history: The Pacific-derived Santa Elena ophiolite, northwest Costa Rica

    NASA Astrophysics Data System (ADS)

    Escuder-Viruete, Javier; Baumgartner, Peter O.; Castillo-Carrión, Mercedes

    2015-08-01

    The Santa Elena ophiolite (SEO) is an ultramafic nappe of more than 270 km2 overlying a tectonic serpentinite-matrix mélange in northwest Costa Rica. It is mainly composed of Cpx-rich and Cpx-poor harzburgites (~ 2.5 km-thick), with minor lherzolite, dunite and chromitite, as well as intrusive mafic sills and subvertical dikes, which coalesce into an upper Isla Negritos gabbroic sill complex. Minerals and whole-rock features of the Cpx-rich and Cpx-poor harzburgites share features of the abyssal and supra-subduction zone (SSZ) peridotites, respectively. To explain these characteristics two-stages of melting and refertilization processes are required. By means of trace element modeling, the composition of Cpx-rich harzburgites may be reproduced by up to ~ 5-10% melting of a primitive mantle source, and the composition of Cpx-poor harzburgites and dunites by ~ 15-18% melting of an already depleted mantle. Therefore, the Cpx-rich harzburgites can be interpreted as product of first-stage melting and low-degrees of melt-rock interaction in a mid-ocean ridge environment, and the Cpx-poor harzburgites and dunites as the product of second-stage melting and refertilization in a SSZ setting. The mafic sills and the Isla Negrito gabbros are genetically related and can be explained as crystallization from the liquids that were extracted from the lower SSZ mantle levels and emplaced at shallow conditions. The Murciélagos Island basalts are not directly related to the ultramafic and mafic rocks of the SEO. Their E-MORB-like composition is similar to most of the CLIP mafic lavas and suggests a common Caribbean plume-related source. The SEO represents a fragment of Pacific-derived, SSZ oceanic lithosphere emplaced onto the southern North America margin during the late Cretaceous. Because of the predominance of rollback-induced extension during its history, only a limited amount of crustal rocks were formed and preserved in the SEO.

  3. Effects of interaction between ultramafic tectonite and mafic magma on Nd-Pb-Sr isotopic systems in the Neoproterozoic Chaya Massif, Baikal-Muya ophiolite belt

    USGS Publications Warehouse

    Amelin, Y.V.; Ritsk, E. Yu; Neymark, L.A.

    1997-01-01

    Sm-Nd, Rb-Sr and U-Pb isotopic systems have been studied in minerals and whole rocks of harzburgites and mafic cumulates from the Chaya Massif, Baikal-Muya ophiolite belt, eastern Siberia, in order to determine the relationship between mantle ultramafic and crustal mafic sections. Geological relations in the Chaya Massif indicate that the mafic magmas were emplaced into, and interacted with older solid peridotite. Hand picked, acid-leached, primary rock-forming and accessory minerals (olivine, orthopyroxene, clinopyroxene and plagioclase) from the two harzburgite samples show coherent behavior and yield 147Sm/144Nd- 143Nd/144Nd and 238U/204Pb-206Pb/204Pb mineral isochrons, corresponding to ages of 640 ?? 58 Ma (95% confidence level) and 620 ?? 71 Ma, respectively. These values are indistinguishable from the crystallization age of the Chaya mafic units of 627 ?? 25 Ma (a weighted average of internal isochron Sm-Nd ages of four mafic cumulates). The Rb-Sr and Sm-Nd isotopic systems in the harzburgite whole-rock samples were disturbed by hydrothermal alteration. These alteration-related isotopic shifts mimic the trend of variations in primary isotopic compositions in the mafic sequence, thus emphasizing that isotopic data for ultramafic rocks should be interpreted with great caution. On the basis of initial Sr and Nd values, ultramafic and mafic rocks of the Chaya Massif can be divided into two groups: (1) harzburgites and the lower mafic unit gabbronorites with ??Nd = +6.6 to +7.1 and ??Sr = -11 to -16; and (2) websterite of the lower unit and gabbronorites of the upper mafic unit: ??Nd = + 4.6 to + 6.1 and ??Sr = - 8 to -9. Initial Pb isotopic ratios are identical in all rocks studied, with mean values of 206Pb/204Pb = 16.994 ?? 0.023 and 207Pb/204Pb = 15.363 ?? 0.015. The similarity of ages and initial isotopic ratios within the first group indicates that the isotopic systems in the pre-existing depleted peridotite were reset by extensive interaction with basaltic magma during formation of the mafic crustal sequence. The isotopic data agree with a hypothesized formation of the Chaya Massif in a suprasubduction-zone environment.

  4. Hydrous lithosphere and diffuse crustal accretion and tectonics in the southern Mariana margin: a possible analog for subduction zone infancy and ophiolites

    NASA Astrophysics Data System (ADS)

    Martinez, F.; Fryer, P. B.; Sleeper, J. D.; Stern, R. J.; Kelley, K. A.; Ohara, Y.; Ribeiro, J. M.

    2014-12-01

    The mode of extension and crustal accretion may vary significantly during subduction margin evolution. Mantle water content likely has a strongly influence on this evolution as it strongly affects the mantle solidus and rheology. Effects of mantle hydration on volcanism and tectonics were examined in the southern Mariana margin in 2012 on a R/V Thompson cruise. The southern Mariana margin is actively rifting sub-parallel to the trench forming new crust and lithosphere directly above the de-watering slab (see Ribeiro et al. session T011). Shallow seismicity shows broadly distributed active deformation in the upper plate. Shallow-towed and near-bottom sidescan sonar data map a highly faulted terrain with rotated crustal blocks and distributed volcanic emplacements. The near-bottom sidescan sonar data also image an apparent corrugated core complex structure, the first such described from a convergent margin setting, indicating low-angle normal faulting during the extension. Water content in sampled volcanics is ~2 %, approaching that of the volcanic arc itself. Volcanic rocks from the eastern margin are mostly ~2-4 m.y. old, but younger basaltic volcaniclastics were recovered farther west suggesting that active volcanism may continue. We hypothesize that the broadly distributed volcanism and tectonic activity is due to high mantle water content that weakens the margin lithosphere. Continual water addition from the subducting slab inhibits melting-related dehydration and strengthening as has been proposed for lithosphere formed at mid-ocean ridges. A consequence of a broadening zone of rifting is that extension-related mantle upwelling rates will decrease with time. Surface cooling will thus progressively depress the mantle solidus, perhaps explaining the paucity of current observed volcanism at the margin. The volcano-tectonic processes active today in the southern Mariana margin may be modern analogs of those inferred at subduction zone infancy where broadly distributed contemporaneous extension and volcanism above the initially subducting and de-watering slab have been proposed. If so, and if ophiolites are relicts of early crust formed at subduction settings, they may reflect crustal accretion processes that differ significantly from those at mid-ocean ridges.

  5. Detailed geological mapping in vegetated terrain using airborne multispectral imagery and LiDAR data: An example from the Troodos ophiolite, Cyprus

    NASA Astrophysics Data System (ADS)

    Grebby, S. R.; Cunningham, W. D.; Naden, J.; Tansey, K.

    2010-12-01

    Practical and financial constraints associated with traditional field-based lithological mapping are often responsible for the generation of maps with insufficient detail and inaccurately located contacts. Remote sensing data, such as aerial photographs and multi- or hyperspectral imagery, offers solutions to many of the limitations associated with field-based surveys. For instance, remotely sensed data can provide more continuous and detailed information for large areas, thus enabling even the most inaccessible terrain to be mapped for only a fraction of the time and cost required for an equivalent field survey. In arid areas with well exposed rocks and soils, high-resolution multi- and hyperspectral imagery is a valuable mapping aid as lithological units can be readily discriminated and mapped by automatically matching image pixel spectra to a set of reference reflectance spectra. However, the use of spectral imagery in all but the most barren terrain is problematic because just small amounts of vegetation cover can obscure or mask the spectra of the underlying geological substrate. Nevertheless, indirect lithological discrimination may be possible if geobotanical relationships with the underlying substrates can be realised. Additionally, airborne Light Detection And Ranging (LiDAR) offers the potential to provide a novel solution to the vegetation problem because accurate and high-resolution topographic data can be acquired in either forested or non-forested terrain, allowing discrimination of individual rock types that typically have distinct topographic characteristics. This study assesses the efficacy of both airborne multispectral imagery and airborne LiDAR for detailed lithological mapping in a vegetated section of the Troodos ophiolite, Cyprus. Mapping algorithms are presented for the individual use of Airborne Thematic Mapper (ATM) imagery and airborne LiDAR data, in addition to their integrated use. The algorithms involve spectral/morphometric characterisation of the four main lithological units—the Basal Group lavas and dykes, pillow lavas, Lefkara Formation chalky marls and alluvium-colluvium—followed by image classification. Despite the problems posed by vegetation, geobotanical relationships and the correlation between lithology and topography were exploited individually to generate detailed lithological maps which define contacts much more accurately than the existing geological maps. Moreover, the mapping accuracy can be significantly enhanced through integration of the two datasets. The results of this study demonstrate the significant potential of airborne multispectral imagery and airborne LiDAR topographic data to aid rapid high-resolution lithological discrimination and the generation of detailed geological maps over large areas of vegetated or non-vegetated terrain.

  6. The evolution of the Bangong-Nujiang Neo-Tethys ocean: Evidence from zircon U-Pb and Lu-Hf isotopic analyses of Early Cretaceous oceanic islands and ophiolites

    NASA Astrophysics Data System (ADS)

    Fan, Jian-Jun; Li, Cai; Xie, Chao-Ming; Wang, Ming; Chen, Jing-Wen

    2015-08-01

    We conducted in situ U-Pb analyses of zircons from three basalts and one gabbro from the Zhonggang oceanic island, one basalt from the Zhaga oceanic island, and one gabbro from the Kangqiong ophiolite (all located in the middle segments of the Bangong-Nujiang suture zone of Tibetan Plateau), as well as in situ Hf isotope analyses of zircons from one gabbro from the Zhonggang oceanic island to constrain the tectonic evolution of the Bangong-Nujiang Neo-Tethys ocean. All samples contain numerous inherited zircons, and all the zircons contain magmatic oscillatory zoning and have Th/U ratios exceeding 0.4. Moreover, the average ?REE content of these zircons is less than 2000 ppm, and they display clear negative Eu and variable positive Ce anomalies, indicating a magmatic origin. LA-ICP-MS U-Pb dating of the zircons revealed three clear peaks in the age distribution, at 248-255, 162-168, and 117-120 Ma; Lu-Hf isotopic analyses of zircons from the gabbro of the Zhonggang oceanic island yielded a 269 Ma crust-mantle separation age. Taking into account the regional geology, previous data, and our new analyses, we infer that the middle and western segments of the Bangong-Nujiang Neo-Tethys ocean had initially opened in the late Permian (254-269 Ma) and that the ocean opened substantially between the late Permian and the Early Triassic (248-255 Ma). In addition, we infer that the initiation of subduction of the Bangong-Nujiang Neo-Tethys ocean took place at ~ 162-168 Ma, which is Middle Jurassic. The 117-120 Ma age is the time when the oceanic islands and ophiolites were formed, indicating that the Bangong-Nujiang Neo-Tethys ocean was, to some extent, still open at that time.

  7. Coexistence of compositionally heterogeneous podiform chromitites in the Antalya-Isparta ophiolitic suite, SW Turkey: a record of sequential magmatic processes in the sub-arc lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Uysal, Ibrahim; Kapsiotis, Argyrios; Melih Akmaz, Recep; Saka, Samet; Avci, Erdi; Müller, Dirk

    2015-04-01

    The Antalya-Isparta region in southwestern Turkey is well known for large, ophiolitic in origin, peridotite exposures hosting various chromite orebodies. These are small-sized, massive to disseminated in texture chromitites that occur in the form of lenses or veinlets and are commonly surrounded by dunite envelopes of variable thickness. Chromitite seams from the Antalya mantle suite belong to both high-Cr and high-Al varieties (Cr#: 0.56-0.83), whereas chromitites in the Isparta mantle sequence are merely Cr-rich (Cr#: 0.75-0.85). In situ minor and trace element abundances obtained by LA-ICP-MS analyses of unaltered Cr-spinel from the Cr-rich chromitites are comparable to those reported in Cr-spinel of chromitites from typical fore-arc peridotite complexes. Nevertheless, minor and trace element concentrations in Cr-spinel from the Al-rich chromitites do not bear resemblance with those acquired from Cr-spinels of chromitites from well-known back-arc basin-derived ultramafic massifs. Calculation of parental magma compositions indicates that both types of chromitites share a common parentage with progressively fractionating arc-related melts. A quite interesting dissimilarity between the unaltered Cr-spinel compositions from both Cr-rich and Al-rich chromitites is that the former display a perceptible positive Ti anomaly in ChromiteMORB-normalized profiles, which signifies the hidden impact of post-magmatic processes in the composition of the high-Cr chromitite bodies that otherwise seem to be unaffected by metamorphism. The studied chromitites are characterized by a systematic enrichment in IPGE [Os, Ir and Ru (41-317 ppb)] with respect to PPGE [Rh, Pt and Pd (3-49 ppb)], resulting to negatively sloping chondrite-normalized PGE patterns that are less fractionated in case of high-Al chromitites. Their noble mineral assemblage is vastly dominated by tiny (up to 10 ?m), euhedral laurite crystals followed by subsidiary irarsite and trivial Os-Ir alloy grains. PGM grains were not encountered in the Al-rich chromitites, plausibly as a result of crystallization from PGE poor melt. Laurite is Os-poor and exhibits a narrow range of Os-for-Ru substitution [Ru/(Ru+Os): 0.75-0.99]. However, the concomitance of laurite and millerite in the Cr-rich chromitites of the mutual Antalya-Isparta mantle suite is in favor of their precipitation from an Os-depleted melt, characterized by local and rapid variations of fS2 prior or coevally to Cr-spinel crystallization. Moreover, the presence of amphibole inclusions in Cr-spinel indicates that the melt triggered chromitite genesis conceivably had a hydrous component. Overall data suggest that the investigated orebodies were produced by a successively fractionating arc-derived melt that had the opportunity to generate compositionally distinct chromitites at two different pseudo-stratigraphic levels within the Antalya-Isparta arc-type mantle suite. This study was supported by TUBITAK #109Y219.

  8. Petrology and geochemistry of mafic magmatic rocks from the Sarve-Abad ophiolites (Kurdistan region, Iran): Evidence for interaction between MORB-type asthenosphere and OIB-type components in the southern Neo-Tethys Ocean

    NASA Astrophysics Data System (ADS)

    Saccani, Emilio; Allahyari, Khalil; Rahimzadeh, Bahman

    2014-05-01

    The Sarve-Abad (Sawlava) ophiolites crop out in the Main Zagros Thrust Zone and represent remnants of the Mesozoic southern Neo-Tethys Ocean that was located between the Arabian shield and Sanandaj-Sirjan continental block. They consist of several incomplete ophiolitic sequences including gabbroic bodies, a dyke complex, and pillow lava sequences. These rocks generally range from sub-alkaline to transitional character. Mineral chemistry and whole-rock geochemistry indicate that they have compositions akin to enriched-type mid-ocean ridge basalts (E-MORB) and plume-type MORB (P-MORB). Nonetheless, the different depletion degrees in heavy rare earth elements (HREE), which can be observed in both E-MORB like and P-MORB like rocks enable two main basic chemical types of rocks to be distinguished as Type-I and Type-II. Type-I rocks are strongly depleted in HREE (YbN < ~ 6), whereas Type-II rocks are moderately depleted in HREE (YbN > 9.0). Petrogenetic modeling shows that Type-I rocks originated from 7 to 16% polybaric partial melting of a MORB-type mantle source, which was significantly enriched by plume-type components. These rocks resulted from the mixing of variable fractions of melts generated in garnet-facies and the spinel-facies mantle. In contrast, Type-II rocks originated from 5 to 8% partial melting in the spinel-facies of a MORB-type source, which was moderately enriched by plume-type components. A possible tectono-magmatic model for the generation of the southern Neo-Tethys oceanic crust implies that the continental rift and subsequent oceanic spreading were associated with uprising of MORB-type asthenospheric mantle featuring plume-type component influences decreasing from deep to shallow mantle levels. These deep plume-type components were most likely inherited from Carboniferous mantle plume activity that was associated with the opening of Paleo-Tethys in the same area.

  9. Correlation of the Applegate Group in the Oregon Klamath Mountains with terranes of the western Paleozoic and Triassic belt in California

    SciTech Connect

    Barnes, C.G.; Tomlinson, S.L. . Dept. of Geosciences); Donato, M.M. )

    1993-04-01

    F.G. Wells' 1955 definition (U.S.G.S map MF-38) of the Applegate Group encompasses all metamorphic rocks in the western Paleozoic and Triassic belt (TrPz) in Oregon. In an attempt to understand the nature and origin of the Applegate Group, the authors have mapped two adjacent areas in southernmost Oregon and have geochemically analyzed representative metavolcanic and metasedimentary rocks. They recognize two lithologic units within the Applegate Group: (1) interbedded volcanogenic metasandstone and argillite, and (2) block-on-block ophiolitic melange that contains metaserpentinite, marble, metabasite, metagabbro, and siliceous metasedimentary rocks. In the Bolan Lake area, the volcanogenic rocks structurally overlie ophiolitic melange along a thrust fault and occupy a N-S trending graben that developed prior to 153 Ma. In both areas the volcanogenic units are characterized by crystal-lithic arenites and conglomerates. Plagioclase, clinopyroxene, and brown hornblende occurs as phenocrysts in lithic clasts and as individual detrital grains. Low-Ca pyroxene is absent. They interpret the volcanogenic units to represent submarine fan deposits on the basis of graded bedding, partial Bouma sequences, rip-up clasts, and channelized conglomerates. Geochemical compositions of the volcanogenic rocks from both areas are identical, with higher Zr/Ti and P/Ti ratios than those of the orphiolitic melange. The volcanogenic rocks are lithologically and compositionally identical to the western Hayfork terrane, which structurally overlies the Rattlesnake Creek terrane in California. The lithology and geochemistry of the ophiolitic melange are identical to those of the Rattlesnake Creek terrane in California. Therefore they correlate the Applegate Group south of approximately 42 [degree] 15[prime]N with these terranes in the TrPz in California.

  10. Relations rheologie-morphologie dans les melanges de polymeres immiscibles

    NASA Astrophysics Data System (ADS)

    Lacroix, Christophe

    1998-12-01

    This work is devoted to the study of the relationships between rheology and morphology in immiscible polymer blends. These systems show, a two phase morphology which can be affected by the flow during the processing. In the first part of this work, the linear viscoelastic properties of several molten blends with immiscible components have been investigated. A comparative study of the Palierne (1990) and Lee and Park (1994) models has been carried out. The Palierne model is shown to predict well the linear behavior of all the blends, and the Lee and Park model is also shown to describe the behavior of the blends relatively well. The characteristic times associated with the interface and derived from the Palierne and Lee and Park models are of the same order of magnitude and compare well with those obtained from the weighted relaxation spectra. An approximate method is then proposed to estimate this adjustable parameter. In the second part, non linear viscoelastic properties of immiscible polymer blends have been investigated via transient shear flow tests. The transient rheological data have been fitted by the Lee and Park model, its modified version (established in the first part of this work), and by a modified version of the Grmela and Ait-Kadi model (1994). These models have been used to predict the morphological evolution of the blends under moderately large deformation flows. The very large morphological changes induced by the flow have been predicted either qualitatively or quantitatively. For transient shear flow experiments with undeformable dispersed phase, the predictions of the different models have been found unsatisfactory. In the third part of this thesis, the morphological evolution of immiscible polymer blends induced by both shear and extensional flows is studied. It is shown that the model predictions are very sensitive to the choice of the mixing rule used to describe the bulk properties of the blend. Limitations of the different relations are underlined. The transition from a spherical to a fibrillar morphology, irrespectively of the zero shear viscosity ratio, shows that large morphological changes can be obtained in elongational flow. (Abstract shortened by UMI.)

  11. Experiments in English for Special Purposes. Melanges Pedagogiques, 1975.

    ERIC Educational Resources Information Center

    Duda, R.; And Others

    This report describes four experiments in English for Special Purposes conducted by or involving the Centre de Recherches et d'Applications Pedagogiques en Langues. English for Special Purposes is discussed in relation to language skills, grammatical and functional progression, "autonomization" and individualization of language acquisition, and…

  12. Viewing Comprehension "L'oeil ecoute." Melanges pedagogiques, No. 10.

    ERIC Educational Resources Information Center

    Riley, P.

    In order to answer the question as to what can be done with video that cannot be done with sound-only recordings, the role of the visual channel of communication in interaction is examined. This examination is based on the experience of foreign language teachers who have been using video in the classroom for five years. The point is made that in…

  13. Coexistence of abyssal and ultra-depleted SSZ type mantle peridotites in a Neo-Tethyan Ophiolite in SW Turkey: Constraints from mineral composition, whole-rock geochemistry (major-trace-REE-PGE), and Re-Os isotope systematics

    NASA Astrophysics Data System (ADS)

    Uysal, ?brahim; Ersoy, E. Yalç?n; Karsl?, Orhan; Dilek, Yildirim; Sad?klar, M. Burhan; Ottley, Chris J.; Tiepolo, Massimo; Meisel, Thomas

    2012-02-01

    We present new, whole-rock major and trace element chemistry, including rare earth elements (REE), platinum-group elements (PGE), and Re-Os isotope data from the upper mantle peridotites of a Cretaceous Neo-Tethyan ophiolite in the Mu?la area in SW Turkey. We also report extensive mineral chemistry data for these peridotites in order to better constrain their petrogenesis and tectonic environment of formation. The Mu?la peridotites consist mainly of cpx-harzburgite, depleted harzburgite, and dunite. Cpx-harzburgites are characterized by their higher average CaO (2.27 wt.%), Al 2O 3 (2.07 wt.%), REE (53 ppb), and 187Os/ 188Os ( i) ratios varying between 0.12497 and 0.12858. They contain Al-rich pyroxene with lower Cr content of coexisting spinel (Cr# = 13-22). In contrast, the depleted harzburgites and dunites are characterized by their lower average CaO (0.58 wt.%), Al 2O 3 (0.42 wt.%), and REE (1.24 ppb) values. Their clinopyroxenes are Al-poor and coexist with high-Cr spinel (Cr# = 33-83). The 187Os/ 188Os ( i) ratios are in the range of 0.12078-0.12588 and are more unradiogenic compared to those of the cpx-harzburgites. Mineral chemistry and whole rock trace and PGE data indicate that formation of the Mu?la peridotites cannot be explained by a single stage melting event; at least two-stages of melting and refertilization processes are needed to explain their geochemical characteristics. Trace element compositions of the cpx-harzburgites can be modeled by up to ~ 10-16% closed-system dynamic melting of a primitive mantle source, whereas those of the depleted harzburgites and dunites can be reproduced by ~ 10-16% open-system melting of an already depleted (~ 16%) mantle. These models indicate that the cpx-harzburgites are the products of first-stage melting and low-degrees of melt-rock interaction that occurred in a mid-ocean ridge (MOR) environment. However, the depleted harzburgites and dunites are the product of second-stage melting and related refertilization which took place in a supra subduction zone (SSZ) environment. The Re-Os isotope systematics of the Mu?la peridotites gives model age clusters of ~ 250 Ma, ~ 400 Ma and ~ 750 Ma that may record major tectonic events associated with the geodynamic evolution of the Neo-Tethyan, Rheic, and Proto-Tethyan oceans, respectively. Furthermore, > 1000 Ma model ages can be interpreted as a result of an ancient melting event before the Proto-Tethys evolution.

  14. Structure of the ophiolite-hosted Outokumpu Cu-Co-Zn-Ni-Ag-Au sulfide ore district revealed by combined 3D modelling and 2D high-resolution seismic reflection data

    NASA Astrophysics Data System (ADS)

    Saalmann, Kerstin; Laine, Eevaliisa

    2015-04-01

    The Outokumpu district within the North Karelia Schist Belt in eastern Finland hosts Cu-Co-Zn-Ni-Ag-Au sulfide deposits which are associated with Palaeoproterozoic ophiolitic metaperidotites that were tectonically interleaved with allochthonous metaturbidites. Extensive metasomatism of the peridotites produced a rim of quartz-carbonate-calc-silicate rocks, grouped as the Outokumpu assemblage (OKA). A tectonic history comprising various phases of folding and shearing followed by several faulting events dismembered the metaperidotites so that ore bodies cannot be easily followed along strike. Future exploration has to expand the search into deeper areas and consequently requires better knowledge of the subsurface geology. In order to unravel the complex structure 3D geologic models of different scales have been built using a variety of information: geological maps, aeromagnetic and gravity maps, digital terrain models, mine cross sections, drill core logs combined with observations from underground mine galleries, structural measurements, and data from seismic survey lines. The latter have been used to detect upper crustal-scale structures and have been reprocessed for our purpose. The models reveal that the ore body has formed during remobilisation of a proto-ore and is closely related to thrust zones that truncate the OKA. Later faults dismembered the ore explaining the variable depth of the different ore bodies along the Outokumpu ore zone. On a larger scale, at least four km-scale thrust sheets separated by major listric shear zones (curved dislocations in the seismic lines) can be recognized, each internally further imbricated by subordinate shear zones containing a number of lens-shape bodies of probably OKA rocks. Thrust stacking was followed by at least 3 stages of faulting that divided the ore belt into fault-bounded blocks with heterogeneous displacements: (i) NW-dipping faults with unresolved kinematics, (ii) reverse faulting along c.50°-60° SE-dipping faults , (iii) SW-NE to SSW-NNE striking faults which may have formed at an earlier stage and have been reactivated. The specific Outokumpu alteration assemblage around metaperidotite bodies combined with shear zones acting as pathways for fluids are the main vectors to mineralization. Seismic reflection data do not provide a simple tool to directly detect the sites of Outokumpu assemblage bodies at depth but they identify strong reflector zones which are characteristic for though not exclusive to the assemblage. Our approach shows that 3D modelling combining surface geology and geophysical data and a good knowledge about the structural evolution substantially improves the interpretation of reflectors and their assignments to rock units of interest. It thus enhances the chances for locating potentially economic bodies at depth and allows delineating target areas for detailed exploration.

  15. Origin and significance of the ophiolitic high-P mélanges in the northern Caribbean convergent margin: Insights from the geochemistry and large-scale structure of the Río San Juan metamorphic complex

    NASA Astrophysics Data System (ADS)

    Escuder-Viruete, Javier; Friedman, Richard; Castillo-Carrión, Mercedes; Jabites, Janet; Pérez-Estaún, Andrés

    2011-12-01

    The Río San Juan metamorphic complex exposes a segment of a high-pressure accretionary wedge, built during Late Cretaceous intra-oceanic subduction below the Caribbean island-arc. Detailed mapping, large-scale structural analysis, U-Pb/Ar-Ar geochronology, in situ trace element analyses of clinopyroxene, and bulk-rock geochemical data show that the northern half of the complex is composed of arc- and oceanic-derived units, whose large-scale internal structure consists of an imbricate stack of mainly high-P rocks. In the lower structural levels, the Gaspar Hernández serpentinized peridotite-tectonite is composed of massive, serpentinized harzburgite that grades into surrounding sheared serpentinite or gouge. Gabbroic rocks and dolerite sills with N-MORB geochemical signatures are sparse and altered to greenschists facies. U-Pb data indicate a Lower Cretaceous magmatic age (136.40 ± 0.32 Ma). These rocks are interpreted as ophiolitic fragments of the proto-Caribbean Ocean. In the intermediate Jagua Clara serpentinite-matrix mélange, the massive serpentinite with relict peridotite textures was overprinted by schistose and sheared serpentinite, and serpentinite gouge, recording various degrees and conditions of internal deformation. Serpentinite schistosity surfaces warp around mélange hard blocks (< 1 m to 2.5 km). Blocks are massive serpentinized peridotite and exotic high-P crustal rocks. The protoliths of the mafic metaigneous rocks originated from arc-like and non-arc-like magmas. The Jagua Clara serpentinite-matrix mélange represents the deep subduction channel, formed during intra-oceanic subduction. In the upper structural levels, the Morrito unit is composed of high-P Puerca Gorda and Guineal Schists. Protoliths are basic-intermediate volcanic rocks with IAT, low-Ti IAT and boninitic geochemical signatures, similar in composition to the Lower Cretaceous Caribbean island-arc. The Morrito basal fault-zone thrusted the Puerca Gorda Schists onto the Jagua Clara mélange. The geochemical characteristics of many mafic metaigneous rocks found within the Jagua Clara mélange, and particularly in the Morrito unit, indicate that they occupied a supra-subduction zone position at some time in their history and, therefore, the mélange reflect the position of the Late Cretaceous intra-oceanic subduction zone. The presence in the mélange of exotic blocks plucked from both the upper plate (Caribbean island-arc) and the lower plate (proto-Caribbean Ocean) suggests that the arc crust was eroded at its base by tectonic processes in the Late Cretaceous. Regional structural data indicate that the Morrito basal fault-zone now forms the suture between the Caribbean island-arc terranes and the accreted units of the paleo-continental margin of North America.

  16. Fore-arc mantle peridotites and back-arc basin basalts from the Izu-Bonin-Mariana subduction factory (ODP LEGs 125 and 195): a modern analogue for Mediterranean ophiolites

    NASA Astrophysics Data System (ADS)

    Zanetti, A.; D'Antonio, M.; Vannucci, R.; Raffone, N.; Spadea, P.

    2009-04-01

    Serpentinites, basaltic lavas and calc-alkaline volcanoclastic sequences sampled during recent Ocean Drilling Program cruises in the western Pacific Ocean allow comparisons with ophiolites from eastern Mediterranean area, which are believed to be related to marginal seas characterised by rapidly propagating back-arc extension and slab rollback (e.g. Albania and Cyprus). Serpentinites recovered at the Torishima, Conical and South Chamorro Seamounts (ODP Legs 125 and 195), located on the Izu-Bonin-Mariana (IBM) forearc, still record complex petrochemical features acquired during their high-T mantle evolution. This latter has been referred to a three-stages-model, involving in chronological sequence: 1) adiabatic mantle upwelling accompanied by 20-25% polybaric partial melting; 2) local depletion in modal orthopyroxene determined by reactive melt migration; 3) late interstitial crystallisation of ultra-depleted to depleted melts. The record of the first stage is preserved in the less-refractory IBM forearc peridotites, which compositions lie on trends describing the decompression melting of uprising asthenospheric mantle. During this stage, the peridotites were actual melt sources. The large average degree of depletion suggests that partial melting events were assisted by particularly hot geotherms. The second stage occurred at relatively lower pressures, according to the large orthopyroxene dissolution, and is guessed to be firmly related to arc volcanism. Nevertheless, the progressive change of oxidation state of the mantle minerals, which decreases from the Torishima (N Izu-Bonin forearc) through the Conical (N Mariana forearc) to the South Chamorro Seamount (S Mariana forearc), highlights a marked gradient in terms of contribution to the uprising melts from slab-derived component. It is argued that the melt compositions changed from boninitic (at Torishima) to depleted-MORB at (South Chamorro). The third stage determined the petrographic and mineralogical features occurring in all IBM forearc peridotites (e.g. crystallisation of late cpx, embayment of opx porphyroclasts), and likely marks the accretion of the mantle sequence to the thermal boundary layer. It was accompanied by the devolopment of transient geochemical gradients in the migrating liquids mainly governed by chromatographic-type chemical exchange with the peridotite. The West Philippine Basin (WPB) is a back-arc basin that opened in the Philippine Sea Plate (PSP) between the current position of the Palau-Kyushu Ridge (PKR) and the margin of East Asia. Spreading occurred at the Central Basin Fault (CBF) from 54 to 30 Ma. The PKR was active since ~48 to 35 Ma constituting a single volcanic arc with the Izu-Bonin-Mariana Arc. ODP Leg 195 Site 1201 is located in the WPB, ~100 km west of the PKR, on 49 Ma basaltic crust formed by NE-SW spreading at the CBF. From ~35 to 30 Ma, pelagic sedimentation at Site 1201 was followed by turbidite sedimentation, fed mostly by early Mariana Arc (PKR)-derived volcanic clasts. These volcanics are calc-alkaline, whereas PKR rocks from literature have mostly boninitic and arc tholeiitic affinity; the WPB basement basalts have MORB to arc-like affinity, as expected for a back-arc basin. Sr, Nd, Pb and Hf isotope data highlight the Indian Ocean MORB-like character of WPB basement basalts, suggesting an upper mantle domain distinct from that underlying the Pacific Plate. The geochemical and isotopic features of PKR volcanics reflect higher amounts of subduction-derived components, added mostly as siliceous melts, in the source of arc magmas relative to that of basement basalts. In that respect, Site 1201 PKR volcanics resemble calc-alkaline volcanics of the currently active Mariana Arc. In addition, their calc-alkaline affinity, unradiogenic neodymium, and inferred Middle Oligocene age, suggest they might represent an evolved stage of arc volcanism at Palau-Kyushu Ridge, perhaps shortly before the end of its activity.

  17. A new integrated tectonic model for the Mesozoic-Early Cenozoic subduction, spreading, accretion and collision history of Tethys adjacent to the southern margin of Eurasia (NE Turkey)

    NASA Astrophysics Data System (ADS)

    Robertson, Alastair; Parlak, Osman; Ustaömer, Timur; Tasl?, Kemal; ?nan, Nurdan; Dumitrica, Paulian; Karao?lan, Fatih

    2014-05-01

    A major Tethyan suture zone (?zmir-Ankara-Erzincan-Kars Suture Zone) borders the southern margin of Eurasia throughout the Pontides. In eastern Turkey the suture zone includes a range of redeposited terrigenous and volcanogenic sedimentary rocks, pelagic sedimentary rocks and also igneous/metamorphic rocks. The igneous rocks are mostly basaltic blocks and thrust sheets within melange, plus relatively intact, to dismembered, ophiolitic rocks (oceanic crust). Two alternative hypotheses have been developed and tested during this work: 1. The suture zone preserves a single Andean-type active continental margin associated with northward subduction, accretion and arc magmatism during Mesozoic-early Cenozoic time; 2. The suture zone preserves the remnants of two different subduction zones, namely a continental margin subduction zone (as above) and an intra-ocean subduction zone (preferred model). To determine the age of the oceanic crust, relevant to both hypotheses, zircons were extracted from basic ophiolitic rocks (both intact and dismembered) and dated by the U/Pb method (U238/U236) using an ion probe at Edinburgh University. This yielded the following results for the intact ophiolites (Ma): plagiogranite cutting sheeted dykes of the Refahiye ophiolite (east of Erzincan), 183.6±1.7 (2?); isotropic gabbro from the Karada? ophiolite (northeast of Erzurum), 179.4±1.7 (2?). In addition, dismembered ophiolites gave the following ages: gabbro cumulate (Bayburt area), 186.2±1.4 (2?), gabbro cumulate (N of Horasan), 178.1±1.8 (2?). Furthermore, two samples from a kilometre-sized (arc-related) tonalite body, mapped as cutting a thrust sheet of ophiolitic isotropic gabbro in the K?rda? area, yielded ages of 182.1±3.2 (2?) and 185.1±3.0 (2?) Ma. We infer that the ophiolitic and related magmatic arc rocks formed by spreading in a supra-subduction zone setting during the late Early Jurassic (Pliensbachian-Toarcian). This amends former assumptions of a Late Cretaceous age for the E Pontide ophiolites, with important implications for alternative tectonic hypotheses. The two-subduction-zone hypothesis is supported by sedimentological and structural studies of the volcanic-sedimentary melange and of the sedimentary thrust sheets within the suture zone. Geochemical studies of oceanic basaltic rocks in the melange and also new biostratigraphic dating of radiolarites and calcareous microfossils within pelagic and redeposited deep-sea/slope sediments add to the picture. Taken together, the evidence suggests the former existence of both an oceanic and a continental margin subduction complex that are now amalgamated within the suture zone. We propose the following tectonic hypothesis: Fragments of oceanic basaltic lithologies and their deep-sea sedimentary cover accreted to form a Jurassic-Cretaceous intra-oceanic subduction complex. Terrigenous and arc-derived volcaniclastic gravity flows and pelagic carbonates accumulated in a continental margin forearc basin, mainly during the Cretaceous. Subduction melange was first emplaced over the distal Eurasian margin during the Late Cretaceous owing to thickening of the accretionary prism. During suturing, the continental margin forearc basin was emplaced southwards over the oceanic-derived accretionary wedge. The Eurasian continental margin was imbricated and thrust northwards as collision proceeded. Final closure of the adjacent Tethys took place prior to late Middle Eocene. This was followed by marine transgression and the accumulation of non-marine to shallow-marine sediments, including Nummulitic limestones. Regional correlations suggest that the double subduction zone hypothesis, notably involving Jurassic intra-oceanic spreading, is applicable to >1000 km of the Eurasian margin, specifically the Lesser Caucasus and possibly also the Central Pontides.

  18. Note-Notis The Trvikbygd Ophiolite -a newly discovered ophiolite fragment

    E-print Network

    Andersen, Torgeir Bjørge

    1984. ISSN 0029-196X. Diabases from a sheeted dyke complex associated with greenstones, gabbros and Vangdalsberget, where the greenstones constitute a sheeted diabase complex. The gab bros in this area occur

  19. Sedimentary development of the Oligocene Karsant? Basin, southern Turkey, in its regional tectonic setting

    NASA Astrophysics Data System (ADS)

    Ünlügenç, Ulvi Can; Ak?nc?, Ahmet Can

    2015-06-01

    Following Late Cretaceous ophiolite and melange emplacement within the Tauride belt several Neogene sedimentary basins of variable size were formed along the southern flank of the Taurus continent in southern Turkey. These include the Pozant? and Karsant? Basins and the regional scale Çukurova Basin Complex, extending southwestwards into the Cilicia-Kyrenia Basin. The Karsant? Basin is bounded by the regional scale sinistral Ecemi? Fault Zone to the west, the East Anatolian Fault Zone to the southeast and the Negoene Adana Basin to the south. Deformed Palaeozoic and Mesozoic rock units that display an irregular palaeotopography form the basement of the Karsant? Basin. These units are overlain by an allochthonous K?z?lda? melange and by thrust slices of basic/ultrabasic ophiolitic rocks (Fara?a ophiolites) that were emplaced in this region during the Late Maastrichtian. The Karsant? Basin was formed during the Oligocene above the thrust sheets. The Karsant? Basin disconformably overlies the ophiolitic nappes and is interpreted as a N-S trending half graben which was probably most active following the deposition of lacustrine sediments during the late Oligocene. The main Karsant? Basin infill is represented by four lithological units: 1. Alluvial fan deposits (A1), 2. shallow-marine deposits (A2), 3. lacustrine deposits (A3), and 4. fluvial deposits (A4). These sediments were deposited during the Oligocene, prior to the initiation of the main Adana Basin, which formed in a separate intermontane setting. The Karsant? Basin fill is unconformably overlain by early Miocene sediments of the Neogene Adana basin.

  20. Early Paleozoic magmatic events in the eastern Klamath Mountains, northern California

    NASA Astrophysics Data System (ADS)

    Wallin, E. Timothy; Mattinson, James M.; Potter, A. W.

    1988-02-01

    New U-Pb zircon ages for nine samples of tonalite and pegmatitic trondhjemite from the Trinity ophiolite and associated melange reveal a complex history of magmatic activity extending back into the earliest Cambrian, much older than previously believed. Earlier investigations, based on limited data, recognized lower Paleozoic crustal elements in the eastern Klamath terrane (EKT) ranging in age from Middle Ordovician to Early to Middle Devonian. The new work in the Yreka-Callahan area of the EKT confirms the Ordovician (440-475 Ma) and younger ages, but reveals for the first time the presence of tonalitic rocks that crystallized during a narrow time interval at about 565-570 Ma. We also recognize younger, Late Silurian magmatism at 412 Ma. In the context of available mapping, these ages indicate that the Trinity ophiolite is broadly polygenetic because parts of it yield crystallization ages that span approximately 150 m.y. Superjacent dismembered units of probable early Paleozoic age may be tectonostratigraphically equivalent to the Sierra City melange in the northern Sierra Nevada.

  1. Virtual Patterson Experiment - A Way to Access the Rheology of Aggregates and Melanges

    NASA Astrophysics Data System (ADS)

    Delannoy, Thomas; Burov, Evgueni; Wolf, Sylvie

    2014-05-01

    Understanding the mechanisms of lithospheric deformation requires bridging the gap between human-scale laboratory experiments and the huge geological objects they represent. Those experiments are limited in spatial and time scale as well as in choice of materials (e.g., mono-phase minerals, exaggerated temperatures and strain rates), which means that the resulting constitutive laws may not fully represent real rocks at geological spatial and temporal scales. We use the thermo-mechanical numerical modelling approach as a tool to link both experiments and nature and hence better understand the rheology of the lithosphere, by enabling us to study the behavior of polymineralic aggregates and their impact on the localization of the deformation. We have adapted the large strain visco-elasto-plastic Flamar code to allow it to operate at all spatial and temporal scales, from sub-grain to geodynamic scale, and from seismic time scales to millions of years. Our first goal was to reproduce real rock mechanics experiments on deformation of mono and polymineralic aggregates in Patterson's load machine in order to deepen our understanding of the rheology of polymineralic rocks. In particular, we studied in detail the deformation of a 15x15 mm mica-quartz sample at 750 °C and 300 MPa. This mixture includes a molten phase and a solid phase in which shear bands develop as a result of interactions between ductile and brittle deformation and stress concentration at the boundaries between weak and strong phases. We used digitized x-ray scans of real samples as initial configuration for the numerical models so the model-predicted deformation and stress-strain behavior can match those observed in the laboratory experiment. Analyzing the numerical experiments providing the best match with the press experiments and making other complementary models by changing different parameters in the initial state (strength contrast between the phases, proportions, microstructure, etc.) provides a number of new elements of understanding of the mechanisms governing the localization of the deformation across the aggregates. We next used stress-strain curves derived from the numerical experiments to study in detail the evolution of the rheological behavior of each mineral phase as well as that of the mixtures in order to formulate constitutive relations for mélanges and polymineralic aggregates. The next step of our approach would be to link the constitutive laws obtained at small scale (laws that govern the rheology of a polymineralic aggregate, the effect of the presence of a molten phase, etc.) to the large-scale behavior of the Earth by implementing them in lithosphere-scale models.

  2. Etude des proprietes de surface et des proprietes rheologiques des melanges polymeres thermotropes

    NASA Astrophysics Data System (ADS)

    Tovar Hernandez, Maria Gabriela

    We studied the surface and rheological properties of thermotropic liquid crystal polymers (TLCP) mixed with thermoplastics. We first investigated acid-base interactions in polymer component as a function of temperature, and could identified the many phase changes in TLCP. We found that acid-base interactions in TLCP decrease significantly with temperature, down to a point Tc where they become negligible. To our knowledge, it is the first time such observation is reported concerning TLCP. Acid-base interactions in the thermoplastics also vary with temperature, but they remain non-negligible, and reach a plateau at high temperature. In theory, one can obtain compatible blends between polymers A and B when their interaction parameters are both small and similar. The negligible acid-base parameter of TLCP at T > Tc should enhance the compatibility with thermoplastics. For that reason, we prepared the TLCP/thermoplastic blends at temperatures superior and inferior to Tc. We restricted our investigation to blends prepared in a Brabender Plasticorder in order to control the temperature in the mixing chamber and reduce the effects of TLCP domains deformation and reorientation. We prepared Vectra/polycarbonate blends, optimizing the drying conditions and the TLCP concentration. We optimized the drying conditions based on our previous results, finding that the transesterification reaction in presence of water happens at high temperature. Transesterification reaction was identified using infrared spectroscopy in the polymer components and in the blends. We found that this reaction occurring mainly between the ester groups in the polymer components. The product of this reaction, concentrated at the interfaces, has a remarkable effect on the blend morphology, similar to the one in compatible blend, and on its mechanical properties. To measure the effect of transesterification at the interfaces, we studied the linear viscoelasticity properties of Vectra/polycarbonate blend and their relation with their morphology. We found from the time variation of the elastic modulus at very low frequencies that the transesterification reaction was still occurring during rheological measurements. We observed coalescence of the dispersed phase droplets in all blends. Size of the droplets changed with the blend composition, the preparation temperature and the rheological characterization temperature. In addition, we compared Palierne (1990, 1991) and Lee-Park models when applied to Vectra/polycarbonate blends. We found that the Palierne model does not predict the rheological behavior of the blend, due to the mixture rule used in that model. Lee-Park model, using a different mixture rule, gives a good prediction of the linear viscoelastic properties. We succeeded in modeling the Vectra/polycarbonate rheological properties combining the characteristic relaxation times of the Palierne and Lee-Park models. Using this approach, we could clearly show that the decrease of the interfacial tension is due to the copolymer produced at the interface by transesterification reaction.

  3. Le phenomene de mouillage partiel dans les melanges de polymeres non miscibles ternaires et quaternaires

    NASA Astrophysics Data System (ADS)

    Virgilio, Nick

    In the initial part of the work, it is shown that PS droplets in a melt processed 10/45/45 PS/PP/HDPE blend can self-organize and form a close-packed array at the PP/HDPE interface. This microstructure results from the partial wetting of the PP/HDPE interface by the PS droplets and reveals the formation of a 3-phase line of contact between the phases, as predicted by the spreading coefficients. Remarkably, even when highly concentrated at the interface, the PS remains in droplet form and does not coalesce into a uniform layer. Furthermore, the affinity of the PS droplets for the PP/HDPE interface has been controlled by the addition of a styrene-(ethylene-butylene) (SEB) diblock copolymer, which selectively migrates at the PS/HDPE interface, reduces the interfacial tension and induces a redistribution of the PS droplets exclusively at the PP/HDPE interface on the HDPE side. In the second part of this work, the FIB-AFM-Neumann triangle method has been used to measure interfacial tension ratios in four systems displaying partial wetting: (1) PS/PP/HDPE, (2) PS/PCL/PP, (3) PLLA/PCL/PS and (4) PMMA/PS/PP. The results compare well with those obtained with the classical breaking thread technique. Subsequently, a variation of the PS/HDPE interfacial tension has been measured in the PS/PP/HDPE blend modified with 1% of an asymmetric SEB diblock copolymer, decreasing from 4.2 +/- 0.6 mN/m to 3.3 +/- 0.4 mN/m for an apparent areal density of 0.19 +/- 0.07 copolymer molecules/nm 2, which is close to previously reported saturation values. Finally, a spectacular microstructure transition has been observed in a quaternary PMMA/PP/PS/HDPE 5/45/5/45 blend modified with a PE-b-PMMA diblock copolymer, going from PS/PMMA composite droplets located on the PP side of the PP/HDPE interface to Janus droplets located symmetrically at the PP/HDPE interface, showing a side-by-side structure with a PS hemisphere in contact with the PP phase, a PMMA hemisphere in contact with the HDPE phase and a 4-phase line between the phases. Using the Neumann method, it has been shown that the PMMA/HDPE interfacial tension decreased from 8.6 +/- 0.9 mN/m to 3.7 +/- 0.9 after the addition of the PE-bPMMA copolymer, for an apparent areal density of 0.30 +/- 0.8 copolymer molecule/nm2. In the third part of this study, the FIB-AFM-Neumann triangle method has been used to measure PS/HDPE modified tensions in ternary PS/PP/HDPE 10/45/45 blends compatibilized with various SEB, SB and SEBS copolymers. The morphological analysis of the blends has confirmed that the microstructure, similar to the one described in the first part, is sensitive to the copolymer concentration, but also to its architecture, molecular weight and presence of insaturation within it. The symmetric SEB diblock copolymer has proven to be the most effective at decreasing the PS/HDPE interfacial tension, as compared to the triblock SEBS which has shown almost no effect at all. Using the FIB-AFM-Neumann triangle method, the PS/HDPE interfacial tension decreases from 4.2 +/- 0.6 mN/m to 3.0 +/- 0.4 mN/m for an apparent areal density of 0.16 copolymer molecule/nm 2 for the symmetric SEB copolymer, which is also near reported saturation values. By varying the concentration of the copolymer, an emulsion curve reporting the value of the PS/HDPE modified interfacial tension as a function of the apparent areal density of the symmetric SEB copolymer at the PS/HDPE interface has been obtained. Finally, this work has shown that quiescent annealing of uncompatibilized or compatibilized multiphase blends after the melt mixing preparation step can result in a significant structuring of the morphology through the action of the capillary forces. The development of a close-packed array of PS droplets at the PP/HDPE interface and the formation of Janus PS/PMMA droplets at the same interface clearly illustrate the potential of this approach. Furthermore, classical capillary theory of multiphase blends can explain the formation of these complex architectures and suggests ways to control morphology development is such sys

  4. L'Information sur la Technologie Educative (Information about Educational Technology). Melanges Pedagogiques, 1976.

    ERIC Educational Resources Information Center

    Cembalo, M.

    This paper deals with the problems of information about educational technology among teachers of foreign languages. It investigates the various definitions of educational technology as they appear explicitly in technological publications or implicitly among language teachers. The stress on the systems approach on the researcher's part and the…

  5. Potentiel de l'utilisation des melanges hydrocarbures/alcools pour les moteurs a allumage commande

    NASA Astrophysics Data System (ADS)

    Broustail, Guillaume

    For the past few years, the oil stock decrease encourages the different countries to increase their energy independence. Moreover, reducing the environmental impact of transportation became one of the priorities of our society. In this way, European emissions standards are stricter while several pollutants have been identified to have a negative impact on health and the environment. To answer this double problem, the use of alcohols biofuels in sparkignition engines is one the promising ways. The European Union have already taken a small step in that direction by allowing a maximum of 10% of ethanol into gasoline. As well as ethanol is already marketed, Biobutanol, a 2nd generation biofuel, appears as a serious candidate with a strong potential for a spark-ignition engines use. The objective of this dissertation is to study the potential of the iso-octane/butanol blends use in spark-ignition engines, in terms of performance and pollutants emissions. Moreover, these results are compared to iso-octane/ethanol blends. The heat release in spark-ignition engine is piloted for a part by laminar burning velocity. This characteristic was studied experimentally and numerically for different initial conditions (pressure and equivalence ratio) in a constant volume bomb. Then, the early flame kernel growth was studied in an spark-ignition single cylinder engine equipped with optical accesses. Those results were correlated with the results on the laminar burning velocity. Finally, regulated and non-regulated pollutants emissions and engine performance were investigated in a spark-ignition single cylinder engine. A decrease of most pollutant emissions was observed with both alcohols addition.

  6. Extensional Detachment faulting in melange rocks. Plurikilometres migration by W the External Zone (Cordillera Bética, Spain)

    NASA Astrophysics Data System (ADS)

    Roldán, Francisco Javier; Azañon, Jose Miguel; Rodríguez, Jose; Mateos, Rosa Maria

    2014-05-01

    The synthesis and correlation of units carried out in the continuous geological map (Roldán et al., 2012), has revealed a fragmentation of the carbonate outcrops belong to the Subbetic Domain (García-Hernández et al., 1980). Subbetic NW verging thrust and fold axial traces have not lateral continuity and Jurassic carbonate outscrops appear as klippes on the olistotromic unit. These ductile structures that can be observed in the internal structure of these jurassic blocks are unrelated to the brittle-ductile deformation bands observed at the basal pelitic levels. Basal detachments are rooted in: a) the Olistostromic unit, a Upper Langhian-Lower Serravallian breccia constituted by gypsum-bearing clay and marls; b) Cretaceous-Tertiary marly sedimentary rocks (Rodríguez-Fernández, et al., 2013) . In both kind of rocks, cataclastic structures allows to infer a top-to-the WSW displacement. Paleostress measurements, made on these detachments levels, are compatible with a extensional regime (Roldán et al., 2012). At the same time, the analysis and interpretation of subsurface data (seismic surveys and borehole testing) shows that the Subbetic Domain (External Subbetic, Molina 1987) are affected by westward low-angle normal faults. A balanced cross-section, based on morphological and cartographic data in the area between Sierra de Cabra and Sierra de Alta Coloma (Valdepeñas de Jaén), shows plurikilometric displacements which has been produced during Late Serravallian-Early Tortonian times. References: García-Hernández, M., López-Garrido, A.C., Rivas, P., Sanz de Galdeano, C., Vera, J.A. (1980): Mesozoic paleogeographic evolution of the zones of the Betic Cordillera. Geol. Mijnb. 59 (2). 155-168. Molina, J.M. (1987). Análisis de facies del Mesozoico en el Subbético. Tesis Doctoral, Univ. Granada. 518 p. Rodríguez-Fernández, J., Roldán, F. J., Azañón, J.M. y García-Cortés, A. (2013). El colapso gravitacional del frente orogénico a lpino en el Dominio Subbético durante el Mioceno medio-superior: El Complejo Extensional Subbético. Boletín Geológico y Minero, 124 (3): 477-504. Roldán, F.J., Azañón, J.M. y Rodríguez-Fernández, J. (2012): Desplazamiento extensional del Subbético entre las sierras de Cabra y Alta Coloma (Valdepeñas de Jaén. Zonas Externas de la Cordillera Bética). VIII Congreso Geológico de España oviedo. GEOTEMAS, V-13: 484. Roldán, F.J., Rodríguez-Fernández, J., Villalobos, M., Lastra, J., Díaz-Pinto, G., Pérez Rodríguez, A.B. (2012). Zonas: Subbético, Cuenca del Guadalquivir y Campo de Gibraltar. In GEODE. Mapa Geológico Digital Continuo de España. Sistema de Información Geológica Continua: SIGECO. IGME. Editor Navas, J. Disponible en: http://cuarzo.igme.es/sigeco.default.htm

  7. Mud volcanoes, shale diapirs, wrench faults, and melanges in accretionary complexes, eastern Indonesia

    SciTech Connect

    Barber, A.J.; Tjokrosapoetro, S.; Charlton, T.R.

    1986-11-01

    In Timor, eastern Indonesia, where the northern margin of the Australian continent is colliding with the Banda Arc, Australian continental margin sediments are being incorporated into an imbricate wedge, which passes northward into a foreland fold and thrust belt. Field mapping in Timor has shown that scale clays, containing irregularly shaped or phacoidal blocks (up to several meters long) and composed of a wide range of lithologies derived from local stratigraphic units, occur in three environments: along wrench faults, as crosscutting shale diapirs, and associated with mud volcanoes. A model is proposed linking these phenomena. Shales become overpressured as a result of overthrusting; this overpressure is released along vertical wrench faults, which cut through the overthrust units; overpressured shales containing blocks of consolidated units rise along the fault zones as shale diapirs; and escaping water, oil, and gas construct mud volcanoes at the surface. 6 figures, 1 table.

  8. Structure and kinematics of the Suzume fault, Okitsu melange, Shimanto accretionary complex, Japan 

    E-print Network

    Kanaya, Takamasa

    2007-04-25

    underplated to the prism along the subduction plate-boundary at seismogenic depth. An internal, horsebounding thrust of the duplex, referred to as the Suzume fault, juxtaposes basalt in the hanging wall and sedimentary rocks in the footwall. Structure...

  9. Autonomie de l'Apprentissage: Realites et Perspectives (Autoinstruction: Realities and Perspectives). Melanges Pedagogiques, 1974.

    ERIC Educational Resources Information Center

    Cembalo, M.; Gremmo, M.-J.

    This article describes an attempt at organizing a continuing, self-instructional language program designed at the request of a commercial enterprise. The program was to be over a three-year period, and was originally aimed at producing reading comprehension in English, but at the request of the students the program was expanded to add listening…

  10. Melange: Supporting Heterogeneous QoS Requirements in Delay Tolerant Sensor Networks

    E-print Network

    Whitehouse, Kamin

    are particularly useful in mobile networks such as vehicular networks [5], indoor firefighter networks [4 not need to be sent quickly. Another example can be seen in indoor firefighter tracking applications

  11. The oroclinal bend in the South Island, New Zealand

    NASA Astrophysics Data System (ADS)

    Mortimer, N.

    2014-07-01

    Most of the South Island of New Zealand lies within an Eocene-Recent continental shear zone related to Pacific-Australia plate motion. Macroscopic finite strain in this shear zone has, in the past, been tracked through the deformation of the Dun Mountain Ophiolite Belt. This paper identifies additional sub-vertical basement strain markers including: Buller-Takaka Terrane boundary, Darran Suite and Jurassic volcanic belt within the Median Batholith, Taieri-Wakatipu-Goulter Synform axial trace, Esk Head Melange and bedding form surfaces within the Buller, Takaka and Torlesse terranes. An analysis of the oroclinal bend over the entire Zealandia continent shows that it is a composite feature involving pre- as well as post-Eocene bending of basement structures. Satisfactory paleogeographic reconstructions of Zealandia cannot be made without the use of substantial regional scale, non-rigid intracontinental deformation.

  12. Effects of antecedent topography on reefal carbonate deposition: Early-Middle Miocene of the Adana Basin, S Turkey

    NASA Astrophysics Data System (ADS)

    Gül, Murat

    2007-08-01

    Early-Middle Miocene reefal limestones and other shallow marine sedimentary sequences were important components of Neogene Basins (especially the Adana Basin) in Southern Turkey. The most important controlling factors of sedimentation were antecedent topography and substratum type, clastic input and sea level changes of different scales. The effects of these factors can be observed on the southwest extension of the Adana Basin. Miocene sediments unconformably overlie the older limestones, ophiolite and ophiolitic melange. Lithological variations and structural properties of Pre-Miocene rocks caused an irregular sea floor topography development that was initially filled by local terrestrial conglomerates. During the Early Miocene transgression, these terrestrial clastics transitionally passed into shallow marine clastics, and also new pebbly conglomerate-sandstone bearing deposits observed in local depressions of Pre-Miocene topography. Remaining low lying areas were filled by claystones and sandy-clayey limestones. At the same time, reef limestone deposition developed over the higher areas where a hard-substratum was available for red algae and other reef builders. Large shelled organisms in the clastics created a suitable substratum for successive reef level development. The carbonate deposition shifted landwards due to continuous transgression (intermediate sea level changes) causing backstepping reef development. Small sea level changes in the study area controlled the energy of the environment, lithological variations in the clastics and lateral facies changes in each reef body.

  13. The East Falcon Basin: Its Caribbean roots

    SciTech Connect

    Bartok, P.; Boesi, T.

    1996-08-01

    The East Falcon Basin has been described persistently in the context of the Maracaibo Basin tectonic framework. It is the objective of the present study to demonstrate that the Falcon Basin is, in effect, a Caribbean basin juxtaposed on South America and affected by Caribbean tectonics. The oldest rocks outcropping in the region are Late Paleozoic metamorphic and igneous rocks rafted from northcentral Colombia, Middle Jurassic ophiolite complexes, sediments and metasediments and Cretaceous ophiolites transported by a melange of late Cretaceous to early Tertiary sediments. The south vergence of the Caribbean Nappe province has been documented and extends to the present limit of the Andean uplift and to the southern limit of the Coastal Range. The migrating foredeep that developed during the Paleocene-Eocene deposited dominantly basinal shales and thin sandstones. During the Oligocene the Caribbean faults of the Oca system and conjugates began with a dominantly transtensional regime becoming progressively transpressional by Miocene time. The facies development of the Oligocene-Miocene documents the tectonic history. Unique blocks remained as resistant blocks creating ramparts and modifying the basin configuration. During transpression northward-verging thrusting progressively migrated towards the present coastline. The most evident structures of the region are Caribbean in affinity and combined with the sedimentary history of the region can serve to unravel the complex Caribbean-South American plate interaction.

  14. Tectonic evolution of the Intra-Pontide suture zone in the Armutlu Peninsula, NW Turkey

    NASA Astrophysics Data System (ADS)

    Robertson, Alastair H. F.; Ustaömer, Timur

    2004-03-01

    The Armutlu Peninsula and adjacent areas in NW Turkey play a critical role in tectonic reconstructions of the southern margin of Eurasia in NW Turkey. This region includes an inferred Intra-Pontide oceanic basin that rifted from Eurasia in Early Mesozoic time and closed by Late Cretaceous time. The Armutlu Peninsula is divisible into two metamorphic units. The first, the Armutlu Metamorphics, comprises a ?Precambrian high-grade metamorphic basement, unconformably overlain by a ?Palaeozoic low-grade, mixed siliciclastic/carbonate/volcanogenic succession, including bimodal volcanics of inferred extensional origin, with a possibly inherited subduction signature. The second unit, the low-grade ? znik Metamorphics, is interpreted as a Triassic rift infilled with terrigenous, calcareous and volcanogenic lithologies, including basalts of within-plate type. The Triassic rift was unconformably overlain by a subsiding Jurassic-Late Cretaceous (Cenomanian) passive margin including siliciclastic/carbonate turbidites, radiolarian cherts and manganese deposits. The margin later collapsed to form a flexural foredeep associated with the emplacement of ophiolitic rocks in Turonian time. Geochemical evidence from meta-basalt blocks within ophiolite-derived melange suggests a supra-subduction zone origin for the ophiolite. The above major tectonic units of the Armutlu Peninsula were sealed by a Maastrichtian unconformity. Comparative evidence comes from the separate Almac?k Flake further east. Considering alternatives, it is concluded that a Mesozoic Intra-Pontide oceanic basin separated Eurasia from a Sakarya microcontinent, with a wider Northern Neotethys to the south. Lateral displacement of exotic terranes along the south-Eurasian continental margin probably also played a role, e.g. during Late Cretaceous suturing, in addition to overthrusting.

  15. Solubilisation de filtres-UV dans des melanges ternaires de phases apolaires, optimisation par l'utilisation de reseaux centres.

    PubMed

    Marti-Mestres, G; Laget, J P; Maillols, H; Fernandez, C

    1998-02-01

    This paper is concerned with the solubility of UV filters. Emulsions are perfect formulations used for sunscreens, but to give the skin silky feel without being greasy, the oily phase content is low. Considerations of sunscreen solubility can help to solve specific problems of stability in formulations. It is important to determine what solvent dissolves them and what mixture of solvent improves their solubility. Two water insoluble sunscreens were tested at several concentrations in various non-polar solvents. A strategy with simplex centroid design was used to optimize this work. The graphical interpretation of the data assists our understanding of the solubility of UV filters. PMID:18505486

  16. Mesozoic sedimentary and magmatic evolution of the Arabian continental margin, northern Syria: evidence from the Baer–Bassit Melange 

    E-print Network

    Al-Riyami, Khalil; Robertson, Alastair H F

    2002-01-01

    One of the few detailed records of Mesozoic deep-water sedimentation and volcanism preserved along the tectonically emplaced Arabian continental margin is from the Baer–Bassit region of northern Syria. South-Tethyan units ...

  17. Relationships of soil, grass, and bedrock over the Kaweah serpentine melange through spectral mixture analysis of AVIRIS data

    NASA Technical Reports Server (NTRS)

    Mustard, John F.

    1991-01-01

    A linear mixing model is used to model the spectral variability of an AVIRIS scene from the western foothills of the Sierra Nevada and calibrate these radiance data to reflectance. Five spectral endmembers from the AVIRIS data, plus an ideal 'shade' endmember were required to model the continuum reflectance of each pixel in the image. Three of the endmembers were interpreted to model the surface constituents green vegetation, dry grass, and illumination. These are the main transient surface constituents that are expected to change with shifts in land use or climatic influences and viewing conditions ('shade' only). The spectral distinction between the other three endmembers is very small, yet the spatial distributions are coherent and interpretable. These distributions cross anthropogenic and vegetation boundaries and are best interpreted as different soil types. Comparison of the fraction images to the bedrock geology maps indicates that substrate composition must be a factor contributing to the spectral properties of these endmembers. Detailed examination of the reflectance spectra of the three soil endmembers reveals that differences in the amount of ferric and ferrous iron and/or organic constituents in the soils is largely responsible for the differences in spectral properties of these endmembers.

  18. Documents Non Didactiques et Formation en Langues (Non-Didactic Documents and Language Training). Melanges Pedagogiques, 1972.

    ERIC Educational Resources Information Center

    Duda, R.; And Others

    In second language learning, it is often the case that a discrepancy exists between the language of the pedagogical materials and that of the media or of the native speaker. This article discusses the advantages and problems involved in using authentic, non-didactic materials in post-introductory second language instruction, as found in an…

  19. Substrats poreux biodegradables prepares a partir de phases co-continues dans les melanges de polymeres immiscibles

    NASA Astrophysics Data System (ADS)

    Sarazin, Pierre

    2003-06-01

    In this thesis a novel approach to preparing biodegradable materials with highly structured and interconnected porosity is proposed. The method involves the controlled preparation of immiscible co-continuous polymer blends using melt-processing technology followed by a bulk solvent extraction step of one of the phases (the porogen phase). A co-continuous structure is defined as the state when each phase of the blend is fully interconnected through a continuous pathway. This method allows for the preparation of porous materials with highly controlled pore size, pore volume and pore shape which can then be transformed and shaped in various forms useful for biomedical applications. Various properties of the skin of the polymeric articles (closed-cell, open-cell, modification of the pore size) can be controlled. Initially, the study on the immiscible binary and compatibilized poly(L-lactide)/polystyrene blends (PLLA/PS) after extraction of the PS phase demonstrated that highly percolated blends exist from 40--75%PS and 40--60%PS for the binary and compatibilized blends, respectively. It is demonstrated that both the pore size and extent of co-continuity can be controlled through composition and interfacial modification. The subsequent part of our work treats of the preparation of porous PLLA from a blend of two biodegradable polymers and the performance of such porous materials. This portion of the work uses only polymer materials which have been medically approved for internal use. In this case, small amounts of the porogen phase can be tolerated in the final porous substrate. Co-continuous blends comprised of poly(L-lactide)/Poly(epsilon-caprolactone) PLLA/PCL, were prepared via melt processing. A wide range of phase sizes for the co-continuous blend is generated through a combination of concentration control and quiescent annealing. As the PLLA phase can not be dissolved selectively in PLLA/PS blends, the co-continuity range was evaluated indirectly. To precisely assess the formation of the co-continuous morphology, the polylactide was replaced by a poly(epsilon-caprolactone) (PCL) in the following work. PCL possesses a similar biocompatibility, although it exhibits a much slower degradation rate. These results practically allow for a separation of the effects of deformation/disintegration processes and coalescence on continuous and co-continuous morphology development. Coalescence phenomena for systems such as the PS in PCL case is clearly the dominant parameter controlling phase size at higher compositions. These results underline the requirement of co-continuity models to include parameters related to coalescence effects. The data indicate the significant potential of mixing temperature as a tool for the morphology control of co-continuous polymer blends. (Abstract shortened by UMI.)

  20. Preparation a l'Autonomie en Expression Orale. (Preparation for Autonomy in Oral Communication). Melanges Pedagogiques, 1972.

    ERIC Educational Resources Information Center

    Boulanger, C.

    This article deals with the issue of gaining proficiency in oral expression in a second language. A functional approach to teaching oral proficiency is outlined, an approach which focuses on the audience as a determining factor in the speaker's linguistic output. Claims in the article are based on an experiment in the teaching of oral proficiency…

  1. Relationships of soil, grass, and bedrock over the Kaweah Serpentinite Melange through spectral mixture analysis of AVIRIS data

    NASA Technical Reports Server (NTRS)

    Mustard, John F.

    1993-01-01

    A linear mixing model is used to model the spectral variability of an AVIRIS scene from the western foothills of the Sierra Nevada and calibrate these radiance data to reflectance. Five spectral endmembers from the AVIRIS data, plus an ideal 'shade' endmember were required to model the continuum reflectance of each pixel in the image. Three of the endmembers were interpreted to model the surface constituents green vegetation, dry grass, and illumination. Comparison of the fraction images to the bedrock geology maps indicates that substrate composition must be a factor contributing to the spectral properties of these endmembers. Detailed examination of the reflectance spectra of the three soil endmembers reveals that differences in the amount of ferric and ferrous iron and/or organic constituents in the soils is largely responsible for the differences in spectral properties of these endmembers.

  2. Didactique et authentique: Du document a la pedagogie (Didactic and Authentic: From Document to Method). Melanges pedagogiques, No. 10.

    ERIC Educational Resources Information Center

    Abe, D.; And Others

    This discussion shows that "authentic documents" are a basic tool for the acquisition of communicative competence in a second language. An authentic document is a sort of photograph of discourse produced at a given time and in a given place. Like a cliche, it has its own existence. Two reasons for choosing authentic documents in second language…

  3. Tectonic framework of the Parkfield-Cholame area, central San Andreas fault zone, California

    SciTech Connect

    Sims, J.D.; Ross, D.C.; Irwin, W.P.

    1985-01-01

    Recent geologic mapping of the NW-trending San Andreas fault zone (SAFZ) in the southern Diablo Range reveals details of this structurally complex region. Movement on the fault juxtaposes dissimilar tectonic terranes. The region on the NE side is characterized by complexly folded and faulted rocks of the Franciscan assemblage, the Coast Range ophiolite, and sedimentary rocks of the Great Valley sequence and younger formations. The region on the SW side is characterized by crystalline basement rocks of the Salinia terrane overlain by slightly deformed Pliocene and Pleistocene gravel and Miocene and Pliocene sedimentary rocks. The active trace of the SAFZ is along the SW side of a belt of melange that separates the Salinia terrane from the terranes to the NE. The active main trace is notable for a right step over of about 1 km in the southern part of the area and a 5/sup 0/ left bend in the northern part of the area. The melange consists of highly sheared and deformed rocks of late Cenozoic units, and exotic blocks of granite, gabbro, and marble. Deformation of Late Cretaceous and younger rocks east of the SAFZ varies with their age as follows: 1) Late Cretaceous rocks are strongly deformed and overlain by late Cenozoic rocks with angular unconformity, 2) early(.) and middle Miocene rocks are the most complexly folded, 3) late Miocene and early Pliocene strata are less complexly deformed, and 4) Pliocene and Pleistocene rocks the least deformed. Folding resulted from north-south compression across the SAFZ since early (.) Miocene time.

  4. Evidence for the tectonic development and subduction-exhumation of a Palaeozoic-Mesozoic continental margin in the westernmost Anatolides, W Turkey

    NASA Astrophysics Data System (ADS)

    Üçta? Özbey, Zeynep; Ustaömer, Timur; Robertson, Alastair; Ayda Ustaömer, P.; Dixon, John

    2010-05-01

    The Anatolide regional tectonic unit comprising the Tav?anl? Zone and its lower-grade equivalent further south (Afyon Zone) represents one of the world's best examples of a subducted, then rapidly exhumed continental margin. Additional to the well-documented Mesozoic succession, a Palaeozoic "basement" has now been discovered in the westernmost Anatolides (northwest of Dursunbey). The blueschist facies exposure (~50 km2) is characterised by a mainly S-dipping foliation and S-plunging mineral stretching lineation on the main foliation surface. In contrast, further northeast in the Tav?anl? Zone the main foliation dips gently northwards and the mineral lineation trends east-west. The lowest unit in the study area ("basement") is made up of finely banded, dark grey schists, intercalated with metabasic rocks (~600 m-thick). Graphitic layers occur near the base and thin (<25 cm) metaquartzite intercalations above. The "basement" is cut by a foliated metagranite intrusion (~1 km2) associated with metarhyolite flows (< 10 m thick). Ion microprobe U-Pb dating of zircons separated from the metagranite yielded an inferred crystallisation age of 445.8 ± 8.0 Ma. The country rock schists are overlain, above a possible unconformity, by paler coloured schists with metabasite lenses, followed by thick-bedded, to massive meta-carbonates (>1300 m). The uppermost levels of the carbonate platform are relatively thin bedded (5 cm-1 m thick) and chert rich. A HP/LT mineral assemblage in both "basement" and "cover" units includes sodic pyroxene, sodic amphibole, lawsonite, chloritoid, phengite and quartz, also minor tourmaline, apatite, rutile and graphite. The rare co-existence of sodic pyroxene and chloritoid indicates that metamorphic conditions in this region reached P>18 kbar and T<400-580?C. The intercalated metabasites contain lawsonite and sodic amphibole with minor calcite and quartz; the metagranite has sodic pyroxene, chloritoid, phengite and quartz, while the metarhyolites contain sodic pyroxene, feldspar, quartz and white mica. The presence of a common mineral assemblage shows that the entire Palaeozoic-Mesozoic succession underwent similar HP/LT metamorphism. Microstructures seen in orientated thin sections indicate at least three deformation phases. The blueschist minerals crystallised within an already folded cleavage (D1 & D2), and are enveloped by the main foliation (D3) that formed after peak metamorphism (probably exhumation related). A melange follows directly above the platform in the west of the area, whereas in the east the platform is cut out by a low-angle extensional fault (probably also exhumation-related), so that the melange there lies directly on "basement" schists. Blocks in the melange include meta-basalt, meta-gabbro, meta-chert and meta-carbonate rocks. The matrix of the melange locally includes glaucophane and lawsonite, but generally exhibits lower-grade blueschist metamorphism. Supported by new geochronological, geochemical and microprobe data (from the School of GeoSciences, University of Edinburgh), the succession as a whole can be interpreted as a continental basement affected by latest Ordovician magmatism. After a possible unconformity, rift-related (?) siliciclastic sediments with basaltic intercalations were covered by a carbonate platform that can be related to rifting and subsidence of the northern margin of the Anatolide continent. The platform collapsed during Late Cretaceous time related to regional southward emplacement of over-riding accretionary melange and supra-subduction zone-type ophiolites. The continental margin was regionally exhumed by Palaeocene time, followed by further compressional deformation during Mid-Eocene final closure of Izmir-Ankara ocean ("N Neotetethys").

  5. Displacement along the Karakoram fault, NW Himalaya, estimated from LA-ICP-MS U-Pb dating of offset geologic markers

    NASA Astrophysics Data System (ADS)

    Wang, Shifeng; Wang, Chao; Phillips, Richard J.; Murphy, Michael A.; Fang, Xiaomin; Yue, Yahui

    2012-07-01

    The magnitude of fault offset, a key parameter of fault geometry and kinematics, provides critical information on the role of the Karakoram fault (KKF) in accommodating deformation of the Tibetan crust. Geologic markers used for estimating the magnitude of offset along the KKF include: (1) geomorphologic features, (2) correlative stratigraphic sequences, (3) ophiolite melange belts, (4) igneous bodies, and (5) regional fault systems. The debated offsets of the KKF range from 40 km to as much as 1000 km. Conflicting offsets may result from a lack of available offset correlations based on quantitative measurements. The Kunsha granite and the Ayilari granite provide another set of potential markers for estimating the offset along the KKF. U-Pb zircon ages and textural observations provide the basis for correlating these granites. Zircon U-Pb ages show that both the Kunsha granite and the South Ayilari granite crystallized around 50 Ma ago, whereas the North Ayilari granite formed mostly around 20 Ma ago. Reconstruction of batholiths on two sides of the Namru-Menshi basin, as well as the Yalung-Zangbo suture and the trace of the South Kailas thrust, show the offset along the KKF to be about 52±2 km. The chronologic data appear to preclude a 100 km offset, based on the correlation between the North Ayilari granite and the Kunsha granite. Our data further disproves the conjecture that the North Ayilari granite is synkinematic granite of the KKF, and thus contradicts a 280-400 km ophiolite melange belt offset, since it is based on the an incorrect fault initiation time. We calculate a slip rate of 4.5±0.1 mm/yr since 12 Ma, which implies that the average long-term slip rate is low. This does not support the lateral extrusion model, which predicts a high slip rate. It does support the concept that the collision between India and Eurasia proceeded by distributed deformation rather than lateral extrusion along faults that bound a rigid Tibet.

  6. Le mtamorphisme ocanique dans le complexe gabbroque de l'ophiolite Christian Nicollet 1

    E-print Network

    Nicollet, Christian

    base du complexe filonien et dans les gabbros isotropes, la proximité de la lentille magmatique permet lentille magmatique. Dans la partie inférieure du complexe gabbroïque, les textures des gabbros lités plagioclase; il forme, avec l'amphibole, des symplectites dans le clinopyroxène. Dans les gabbros-norites, les

  7. Draft Genome Sequence of Haloalkaliphilic Exiguobacterium sp. AB2 from Manleluag Ophiolitic Spring, Philippines

    PubMed Central

    Argayosa, Vina B.; Lazaro, Jose Enrico H.; Argayosa, Anacleto M.; Arcilla, Carlo A.

    2014-01-01

    Exiguobacterium sp. AB2 is a haloalkaliphilic bacterium isolated from a hyperalkaline spring in Manleluag, Pangasinan, Philippines. Sequencing of bacterial DNA assembled a 2.85 MB draft genome. Analysis suggests the presence of genes for tolerance to stresses such as elevated pH and salt concentrations and toxic metals. PMID:25125654

  8. Determining timescales of natural carbonation of peridotite in the Samail Ophiolite, Sultanate of Oman

    E-print Network

    Mervine, Evelyn Martinique

    2012-01-01

    Determining timescales of the formation and preservation of carbonate alteration products in mantle peridotite is important in order to better understand the role of this potentially important sink in the global carbon ...

  9. Initiating intermediate-depth earthquakes: Insights from a HPLT ophiolite from Corsica

    E-print Network

    Andersen, Torgeir Bjørge

    incompletely metamorphosed metagabbro and peridotite. The wallrocks of the pseudotachylytes contain variable, tremolite and chlorite in the peridotite. Back-scatter-electron images show that the hydrous minerals temperature, high pressure conditions (1400­1700 °C; 1.5 GPa). The peridotite-hosted pseudotachylytes

  10. Collisional orogenesis in the northern Canadian Cordillera: Implications for Cordilleran crustal structure, ophiolite

    E-print Network

    Johnston, Stephen T.

    T. Johnston1 School of Earth and Ocean Sciences, University of Victoria, PO Box 3055, STN CSC, Victoria, British Columbia, Canada VSW 3P6 Received 15 April 2004; received in revised form 28 January 2005 British Columbia Atlin Lake Te

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

    E-print Network

    Meyers, Stephen R.

    of unambiguous evidence of life (cf., Beaty et al., 2005). The presence of microbial life on Earth or an extraterrestrial planet does not ensure our ability to detect it. Evidence of life must be distinctive from, USA d Lunar and Planetary Institute, Houston, TX 77058, USA e Earth Sciences Division, Lawrence

  12. Cooling rates in the lower crust of the Oman ophiolite: Ca in olivine, revisited

    E-print Network

    VanTongeren, Jill A.

    around the gabbro glacier and the sheeted sills models. The thermal profile of the crust, specifically of accretion by gabbro glacier, they do contradict the initial tenet of the gabbro glacier model ­ convective is centered on two end-member models: the gabbro glacier model and the sheeted sills model (Fig. 1

  13. Seismic wave velocity and anisotropy of serpentinized peridotite in the Oman ophiolite

    E-print Network

    Cattin, Rodolphe

    of peridotite and gabbro in the same conditions suggest that P- and S-waves anisotropies are a possible.elsevier.com/locate/tecto Tectonophysics 370 (2003) 77­94 #12;the crustal constituents, gabbro and dolerite. In the recent years, several

  14. Detrital high-pressure/low-temperature minerals in a late Turonian flysch sequence of the eastern Alps (western Austria): Implications for early Alpine tectonics

    NASA Astrophysics Data System (ADS)

    Winkler, Wilfried; Bernoulli, Daniel

    1986-07-01

    Along the northern margin of the eastern Alps, structurally intercalated between the main Austroalpine thrust and the Pennine Rhenodanubic flysch nappe, a thin, tectonically complex zone occurs that comprises a series of tectonic melanges and imbricates of oceanic (South Pennine) and distal continental margin (Austroalpine) origin. This zone is the lateral continuation of the ophiolite-bearing Arosa Zone of the Swiss Alps; we refer to it as the Walsertal Zone. In a broken flysch formation of the Walsertal Zone, turbiditic sandstones yield detrital grains of the high-P/low-T metamorphic minerals lawsonite and glaucophane (crossite, ferroglaucophane). The sandstones are dated as late Turonian to earliest Coniacian, about 87 to 89 Ma. This age falls into the time span of radiometric ages of subduction-related high-P/low-T metamorphism along the South Pennine/Austroalpine convergent margin, and of coeval cooling ages of medium-P/medium-T metamorphism in the middle Austroalpine nappes. If the high-P/low-T minerals are related to mid-Cretaceous high-P/low-T metamorphism as we think, the high-P metamorphic rocks must have been brought up from about 20-km depth within about 10 m.y. or even less. As isostatic uplift alone would have been too slow, alternative tectonic mechanisms such as “corner flow” must be responsible for rapid uplift and erosion in a fore-arc environment.

  15. Mapping of serpentinites in the Eastern Desert of Egypt by using Landsat thematic mapper data

    SciTech Connect

    Sultan, M.; Arvidson, R.E.; Sturchio, N.C.

    1986-12-01

    Serpentinites in the Eastern Desert of Egypt were mapped from Landsat thematic mapper (TM) data by using procedures that take advantage of the distinctive spectral reflectance of these rocks caused by the abundance of antigorite, lizardite, clinochrysotile, and magnetite. The method employs a threshold classifier based on three reflectance ratios: (1) band 5/7 for estimating the abundance of hydroxyl-bearing phases, (2) band 5/1 for magnetite content, and (3) the calculated value of reflectance for band 4, based on a linear interpolation between bands 3 and 5, divided by the observed band-4 reflectance. The third ratio was used to identify rocks in iron-bearing aluminosilicates and thereby to distinguish mafic rocks containing substantial amounts of magnetite and hydroxyl-bearing phases from serpentinites. The method was first successfully tested over the Meatiq dome and Wadi Ghadir areas, where serpentinites and ophiolitic melanges dominated by serpentinites have been mapped during the course of field work. A TM-based map was then generated; the map covered about 60,000 km/sup 2/ in the Eastern Desert. Results demonstrate that TM data can be used with reliability to distinguish serpentinites from surrounding rocks in arid regions and to generate detailed maps over wide regions by using quantitative, reproducible mapping criteria. Possibilities for locating suture zones over the less well known parts of arid continents are clear.

  16. The significance of gneissic rocks and synmagmatic extensional ductile shear zones of the Barud area for the tectonics of the North Eastern Desert, Egypt

    NASA Astrophysics Data System (ADS)

    Fowler, Abdel-Rahman; Ali, Khaled G.; Omar, Sayed M.; Eliwa, Hassan A.

    2006-10-01

    The mainly tonalitic gneissic rocks, amphibolites, schists and rarer migmatites of the Barud area, at the southern margin of the Egyptian North Eastern Desert (NED) have previously been viewed as products of ultrametamorphism or granitization of pre-PanAfrican basement. The Qena-Safaga Line of approximately NW-striking steeply dipping faults was also regarded as marking the boundary between these NED rocks and the low grade metavolcanics and ophiolitic melange of the Central Eastern Desert (CED). Detailed investigation of the Barud area indicates that the amphibolites, schists and migmatites formed by shearing and medium grade metamorphism of similar arc metavolcanics to those of the CED in normal shear sense extensional ductile shear zones heated by numerous syn-kinematic dolerite, gabbro, diorite, granodiorite, tonalite and granite dykes. They are thus hot sheared equivalents of the CED metavolcanics and basic arc plutonites, accompanied by sheared mafic and felsic intrusive rocks, and are not deep-seated crystalline basement rocks. The shear zones are interpreted as having formed by arc-rifting, not necessarily reaching the stage of marginal basin formation. Arc-accretion structures and those produced by later orogen squeezing are also described. The somewhat gneissic Barud Tonalitic is found to be entirely magmatic. Following intrusion of the Barud Tonalite, and before or during Hammamat and Dokhan deposition, the NED experienced a rapid uplift relative to the CED (˜620-600 Ma) that was not achieved by thrusting along the Qena-Safaga Line.

  17. From: Roma, Magistra Mundi. Itineraria culturae medievalis. Melanges offerts au Pere L. E Boyle al'occasion de son 71Je anniversaire. Federation Internationale des Instituts

    E-print Network

    Frischer, Bernard

    kilometers northeast of Rome. For students of antiquity, Saint'Odile's work at Licenza- never published /'intelligence de cet Auteur, & qui donne occasion de traiter d'une suite considerable de lieux antiques par M. l

  18. Influence des melanges complexes organiques sur le sort des dioxines et furanes: Implications dans le developpement de facteurs de caracterisation en analyse du cycle de vie

    NASA Astrophysics Data System (ADS)

    Taing, Eric

    The environmental fate of dioxins and furans, or polychlorodibenzo-p-dioxins and -furans (PCDD/Fs), leaching from wood poles treated with pentachlorophenol (PCP) oil is modified by the presence of oil. Interactions between co-contaminants, which also exist for other pollutants within the mixtures, were shown in the specific context of risk analysis, but have never been taken into account for the generic context of life cycle assessment (LCA). This decision-making tool relies on characterization factors (CF) to estimate the potential impacts of an emitted amount of a pollutant in different impact categories such as aquatic ecotoxicity and human toxicity. For these two impact categories, CFs are calculated from a cause-effect chain that models the environmental fate, exposure and effects of the pollutant (represented by a matrix of fate FF, exposure XF and effect EF, respectively), meaning that a modification of PCDD/Fs fate induces a change in PCDD/Fs CFs. The research question is therefore as follows: In life cycle impact assessment (LCIA), to what extent would the potential impacts of PCDD/Fs on aquatic ecotoxicity and human toxicity change when taking into account the influence of a complex organic mixture on PCDD/Fs fate?. Thus, the main objective is to develop CFs of PCDD/Fs when their fate is influenced by PCP oil and compare them with the CFs of PCDD/Fs without oil for the aquatic ecotoxicity and human toxicity impact categories. A mathematic approach is established to determine the new environmental distribution of PCDD/Fs in the presence of oil and a new FF' matrix is calculated from this new distribution to obtain new CFs' integrating oil influence. FF' and CF' are then compared to FF and CF of PCDD/Fs without the oil. Finally, potential (eco)toxic impacts of the PCDD/F Canadian emissions are calculated with the new CFs' of PCDD/Fs in presence of oil. By only focusing on the results for an emission into air, freshwater and natural soil on a continental scale, the overall elimination fractions of 2,3,7,8-TCDD changed significantly. For the three emissions, organic fractions increased the overall elimination fraction of 2,3,7,8-TCDD into the continental air compartment, induced by a higher volatility of organic fractions than 2,3,7,8-TCDD: for an emission into continental air, organic fractions increased the overall elimination fraction of 2,3,7,8-TCDD in the continental air from 29% to 49% at most. For an emission into continental freshwater, 2,3,7,8-TCDD fate was mainly influenced by two groups of organic fractions: the lightest ones that volatilize into continental air (overall elimination fraction of 2,3,7,8-TCDD increasing from 2% to 35%) and the heaviest ones that are removed by sedimentation (DTCDD,fwC,fwC from 87% up to 96%). Therefore, an approach has been proposed to represent the carrier behaviour of the oil for PCDD/Fs. PCDD/F potential impacts in aquatic ecotoxicity and human toxicity change in a range up to two orders of magnitude depending on the emitting compartment (except for the seawater and ocean compartments). As 2,3,7,8-TCDD is one of the most toxic pollutant, this changing is significant in LCA. To assess the validity of the model's result, it is recommended to carry out laboratory experiments on the PCDD/F volatilization with oil. In addition, it could be interesting to integrate the influence of PCP on PCDD/Fs fate and, more broadly, the influence of all cocontaminants on PCDD/Fs exposure and effects. Moreover having a unique CFeco and CF tox via a weighting of the 17 CF'eco and the 17 CF' tox, respectively, is necessary for a use in LCA. Unfortunately the variability of the composition makes the weighting difficult, so it is suggested to calculate the mean CF'eco and CF'tox. Finally this research could be carried out on other pollutants whose fate is known to be modified by a complex organic mixture in an effort to ensure that impact characterization better reflects reality. (Abstract shortened by UMI.).

  19. Apprentissage de l'expression orale en autonomie. Implications de l'approche fonctionelle (Learning Oral Expression in Independent Study. Implications of the Functional Approach). Melanges pedagogiques.

    ERIC Educational Resources Information Center

    Abe, D.; And Others

    Within the CRAPEL autonomous learning scheme, modular material is being developed for communicative oral expression. The purpose of this material is twofold: (1) to enable the learner to define his or her own needs in communicative terms, that is, to analyse a situation in terms of communicative acts needed in a given situation, the relationships…

  20. Bald Friar Metabasalt and Kennett Square Amphibolite: Two Iapetan Ocean Floor Basalts

    USGS Publications Warehouse

    Smith, R.C., II

    2006-01-01

    The Bald Friar Metabasalt (BFM) and Kennett Square Amphibolite (KSA) are basaltic units found in the Piedmont of southeastern Pennsylvania. The BFM is also recognized in northern Maryland. Both are believed to represent fragments of the floor of the Iapetus Ocean, but are not known occur in direct association with one another. The BFM typically occurs as small fragments having typical stratigraphic thicknesses of 2.5 m, and composed of greenish, fine-grained chlorite-epidote-actinolite-albite metabasalt in ophiolite me??lange. One bed of pillow basalt has been found at the type locality, Bald Friar, Cecil County, Maryland. Even though outcrops of BFM are highly discontinuous, they have a remarkable chemical uniformity over a strike length of 143 km and appear to be equivalent to the Caldwell Group 1b metabasalt of the Thetford, Quebec, area. The BFM is typically associated with ultramafic fragments and may be affiliated with the Baltimore Mafic Complex (BMC), from which a baddeleyite date of 442 +/- 7 Ma (Silurian) has been obtained. The BFM is probably a back arc basin basalt (BABB). Pod and schlieren chromite compositions suggest an island arc environment for the BMC itself. The poorly defined, informal "Conowingo Creek metabasalt" of Lancaster County, Pennsylvania, occurs on the north margin of the BMC and appears to be a fore arc boninite. The BFM and associated ultramafic fragments serve as a field-mappable marker for the structural equivalent of the Baie Verte-Brompton line in southeastern Pennsylvania and northern Maryland. Steatization of the associated ultramafic fragments has produced zones of extremely low competence that facilitated and localized thrusts of presumed Silurian age and later Alleghanian folding. The KSA typically occurs as much larger bodies having lengths of 3 km and composed of dark, medium-grained hornblende-plagioclase-clinopyroxene gneiss. No ultramafic rocks or me??lange have been recognized with the KSA. In Pennsylvania, the KSA appears to be restricted to a single belt on the south side of the Brandywine massifs. The KSA is transitional from N-OFB (Normal-Ocean Floor Basalt, which can be generated in a variety of oceanic spreading center environments) on the east to P=E-OFB (Plume=Enriched Ocean Floor Basalt, also generated in spreading centers) on the west, suggesting an evolving tectonomagmatic environment. It may be affiliated with the Wilmington Complex.

  1. ELSEVIER Earth and Planetary Science Letters 144(19961239-250 Magma chambers in the Oman ophiolite: fed from the top

    E-print Network

    Cattin, Rodolphe

    of magma chambers at fast-spreading ridges are based on the idea that the entire gabbro section in this way. We emphasize the contrast between: (1) upper level gabbros characterized by a magmatic foliation to the sheeted dikes; and (2) lower gabbros, flat-lying, magmatically deformed, and more or less strongly layered

  2. Serpentinization, iron oxidation, and aqueous conditions in an ophiolite: Implications for hydrogen production and habitability on Mars

    NASA Astrophysics Data System (ADS)

    Greenberger, Rebecca N.; Mustard, John F.; Cloutis, Edward A.; Pratt, Lisa M.; Sauer, Peter E.; Mann, Paul; Turner, Kathryn; Dyar, M. Darby; Bish, David L.

    2015-04-01

    Molecular hydrogen produced through iron oxidation during formation of serpentine and magnetite can sustain terrestrial subsurface ecosystems. The Fe3+ in serpentine partitions into octahedral and tetrahedral sites differently as serpentinization proceeds, and tetrahedral Fe3+ is present toward the end of serpentinization. We map Fe oxidation states in a serpentinite to determine the degree to which serpentinization progressed and where hydrogen production has been maximized to assess habitability at an abandoned chrysotile mine in Norbestos, Quebec, in association with the Canadian Space Agency's Mars Methane Analogue Mission. We also analyzed stable isotopes of carbon and oxygen in carbonates to constrain the conditions of water-rock interaction during serpentinization. Iron oxidation and coordination was determined through field imaging of rock walls with a visible hyperspectral imager (420-720 nm), and samples collected from imaged rocks and elsewhere in the mine were imaged in the laboratory (420-1100 nm). Sample chemistry, mineralogy, and oxidation state were determined with laboratory measurements of visible through mid-infrared reflectance spectra, major element chemistry, mineralogy, and Mössbauer spectroscopy. Mapping with hyperspectral imaging of outcrops and hand samples shows that tetrahedral Fe3+ is common in serpentinites at this site, and results are confirmed through other measurements. Major element chemistry and mineralogy are consistent with serpentine plus minor carbonate. Carbonate samples show an exceptional range in ?13C (-13.14 to + 16.12 ‰ VPDB) and ?18O (-15.48 to - 3.20 ‰ VPDB) that vary with location in the mine. Carbonates south of a shear zone (?13C more positive) likely formed during periods of serpentinization in a carbon-limited reservoir closed to carbon addition but open to methane escape. Carbonates in a shear zone (?13C more negative) probably formed later at low temperatures through CO2-metasomatism or atmospheric weathering, and isotopic trends are consistent with kinetic fractionation. The extensive presence of tetrahedral Fe3+ in serpentine shows the system liberally produced H2 while the isotope systematics have implications for preservation of indicators of the aqueous conditions that formed serpentinites on Mars and their habitability.

  3. Metcalf, R.V., and Shervais, J.W., 2008, Suprasubduction-zone ophiolites: Is there really an ophiolite conundrum?, in Wright, J.E., and Shervais, J.W., eds., Ophiolites, Arcs, and Batholiths: A Tribute to Cliff Hopson: Geological Society of America Specia

    E-print Network

    Shervais, John W.

    that became central to the new theory of plate tectonics (Gass, 1968).A compelling aspect of this proposal detected (e.g., ridge-trench triple junctions). The historical contingency model is flawed for several modern ridge-trench triple junctions and subduction reversals are subtle and/or do not compare favorably

  4. The Grenville orogeny in the Llano Uplift, Texas: A record of collision and contraction along the southern margin of North America

    SciTech Connect

    Reese, J.F. . Dept. of Geological Sciences)

    1993-02-01

    Precambrian metamorphic rocks in the SE Llano Uplift record NE-directed ductile thrusting and regional-scale polyphase folding. This deformation is in response to Grenville-age shortening and crustal thickening associated with the collision of a south-lying tectonic block with the southern margin of North America. In the SE Llano Uplift, the most intense and pervasive deformational event, D2, is characterized in the Packsaddle Schist (PS) and Valley Spring Gneiss (VSG) by SSE-plunging, NE-verging isoclinal folds (F2) with an associated SW-dipping axial planar metamorphic layering (S2), and SW-dipping mylonite zones with kinematic indicators showing top-to-the-NE motion. In the Red Mountain and augen-bearing Big Branch gneisses, D2 structures are SW-dipping mylonite zones parallel to S2, and a SW-plunging stretching lineation. Taken together, this suite of structures indicates tectonic transport was to the NE, perpendicular to the NW trending regional structural and metamorphic grain. D2 structures were reoriented by at least two later phases of folding. Timing of all ductile deformation in the SE Llano Uplift is constrained from post-1,215 Ma (deformed PS) to pre-1,098 Ma (undeformed melarhyolite dike). From south to north, metatonalitic, arc-derived Big Branch Gneiss ([approximately] 1,303 Ma) and older mafic schist country rock, previously interpreted as possible ophiolitic melange, structurally overlie much younger, lithologically heterogeneous PS units (1,248-1,215 Ma), previously considered as arc flank deposits. In turn, the PS has been tectonically emplaced above the predominantly felsic VSG (1,270-1,232 Ma). The presence of older zircons in the VSG, of similar age ([approximately]1360 Ma) to Western Granite-Rhyolite Terrane rocks to the north, suggests that the VSG formed in a settling proximal to North America.

  5. Tectonic comparisons of Caucasus and California cordillera

    SciTech Connect

    Mulhern, M.E.

    1986-05-01

    Many parallels exist between the Caucasus region, shaped as the Eurasian and Arabian-African plates converged during the Cenozoic Alpine orogeny, and California, where the North American and Pacific plates interact. The Forecaucasian platform hosts many large oil and gas fields in molasse-filled foredeeps of high heat flow. Production is from Permian to Pliocene tight anticlines on reverse faults, diapiric structures, shoe-string sands of the prolific Maikop series, and Tertiary fanglomerates and sand lenses. Although not an exact parallel, this production suggests potential in Paleozoic and Mesozoic marine and Tertiary fanglomeratic, tuffaceous, and fluvial sediments in basins of the sparsely drilled Basin and Range. The Greater Caucasus, like the Sierra Nevada, show Paleozoic-Mesozoic geosynclinal sedimentation, subduction, and granitic magmatism. The former have undergone much folding, thrusting, and Quaternary volcanism. Jurassic bituminous limestone is exposed, a source for the petroliferous Transcaucasian Intermontane depression. The latter is filled with thick Mesozoic-Cenozoic flysch and molasse and, like California's Great Valley forearc basin, orogenic terrane and permeable soils favor enology. Sources also include Eocene and Miocene diatomaceous and bituminous clays. Reservoirs include prolific Pliocene sands, Eocene olistostromes, fractured tuffaceous rocks, and Cretaceous-Miocene sandstones in anticlines and stratigraphic traps. Migration to basin flanks was aided by compression. The Lesser Caucasus, a continuation of the main ophiolite foldbelt of Turkey, shares a subduction and accretionary history with the Franciscan melange. This microplate was obducted to Eurasia as the prong-shaped Arabian plate moved it and other microplates of the east Mediterranean in several directions (the Turkish microplate dextrally along the North Anatolian transform fault) and deformed wide areas.

  6. Aeromagnetic and aeromagnetic-based geologic maps of the Coastal Belt, Franciscan Complex, northern California

    USGS Publications Warehouse

    Langenheim, V.E.; Jachens, R.C.; McLaughlin, R.J.

    2011-01-01

    The Coastal belt of the Franciscan Complex represents a Late Cretaceous to Miocene accretionary prism and overlying slope deposits. Its equivalents may extend from the offshore outer borderland of southern California to north of the Mendocino Triple Junction under the Eel River Basin and in the offshore of Cascadia. The Coastal belt is exposed on land in northern California, yet its structure and stratigraphy are incompletely known because of discontinuous exposure, structural disruption, and lithologically non-distinctive clastic rocks. The intent of this report is to make available, in map form, aeromagnetic data covering the Coastal belt that provide a new dataset to aid in mapping, understanding, and interpreting the incompletely understood geology and structure in northern California. The newly merged aeromagnetic data over the Coastal belt of the Franciscan Complex reveal long, linear anomalies that indicate remarkably coherent structure within a terrane where mapping at the surface indicates complex deformation and that has been described as "broken formation" and, even locally as "melange". The anomalies in the Coastal belt are primarily sourced by volcanic-rich graywackes and exotic blocks of basalt. Some anomalies along the contact of the Coastal belt with the Central belt are likely caused by local interleaving of components of the Coast Ranges ophiolite. These data can be used to map additional exotic blocks within the Coastal belt and to distinguish lithologically indistinct graywackes within the Coastal terrane. Using anomaly asymmetry allows projection of these "layers" into the subsurface. This analysis indicates predominant northeast dips consistent with tectonic interleaving of blocks within a subduction zone.

  7. Dependence of frictional strength on compositional variations of Hayward fault rock gouges

    USGS Publications Warehouse

    Morrow, Carolyn A.; Moore, Diane E.; Lockner, David A.

    2010-01-01

    The northern termination of the locked portion of the Hayward Fault near Berkeley, California, is found to coincide with the transition from strong Franciscan metagraywacke to melange on the western side of the fault. Both of these units are juxtaposed with various serpentinite, gabbro and graywacke units to the east, suggesting that the gouges formed within the Hayward Fault zone may vary widely due to the mixing of adjacent rock units and that the mechanical behavior of the fault would be best modeled by determining the frictional properties of mixtures of the principal rock types. To this end, room temperature, water-saturated, triaxial shearing tests were conducted on binary and ternary mixtures of fine-grained gouges prepared from serpentinite and gabbro from the Coast Range Ophiolite, a Great Valley Sequence graywacke, and three different Franciscan Complex metasedimentary rocks. Friction coefficients ranged from 0.36 for the serpentinite to 0.84 for the gabbro, with four of the rock types having coefficients of friction ranging from 0.67-0.84. The friction coefficients of the mixtures can be predicted reliably by a simple weighted average of the end-member dry-weight percentages and strengths for all samples except those containing serpentinite. For the serpentinite mixtures, a linear trend between end-member values slightly overestimates the coefficients of friction in the midcomposition ranges. The range in strength for these rock admixtures suggests that both theoretical and numerical modeling of the fault should attempt to account for variations in rock and gouge properties.

  8. Neoproterozoic oceanic arc remnants in the Moroccan Anti-Atlas: reconstructing deep to shallow arc crustal sequence and tracking Pan-African subduction-accretion processes

    NASA Astrophysics Data System (ADS)

    Triantafyllou, Antoine; Berger, Julien; Baele, Jean-Marc; Bruguier, Olivier; Diot, Hervé; Ennih, Nasser; Plissart, Gaëlle; Monnier, Christophe; Spagna, Paul; Watlet, Arnaud; Vandycke, Sara

    2015-04-01

    The Pan-African belt of West and North Africa exposes many intra-oceanic arc complexes while they are rather uncommon in Phanerozoic orogenic belts. Intra-Oceanic Subduction Zone (IOSZ) in the Moroccan Anti-Atlas crop out in two tectonic windows moulded along the Anti-Atlas Major fault: the Sirwa (western-) and the Bou Azzer (eastern- part) inliers, associated with 760 Ma back-arc ophiolites. These arc sequences are located at the south of the ophiolites and are named the Iriri-Tachakoucht (Sirwa window) and the Asmlil arc complexes (Bou Azzer inlier). (i) The Iriri-Tachakoucht unit is composed of coarse grained hornblendite lenticular plugs, medium-grained hornblende gabbro dykes intruding andesitic to dacitic porphyroclastic gneiss. The contact between both lithologies is gradual and marked by an increasing migmatitization of the gneisses towards hornblendite intrusions. Phase diagram calculation were performed on garnet-bearing gneisses. Garnet cores have grown during a prograde P-T path up to upper amphibolite facies conditions (660°C at ~9 kbar) and recorded the burial of the Tachakoucht metavolcanics, while rims composition indicates that the rock recrystallized under higher temperature conditions (800°C at 4-5 kbar). These HT conditions match those for hornblendites igneous emplacement (850°C and 4 kbar) and this event leaded to more pronounced but still limited partial melting (< 10% melting) of the porphyroclastic gneisses. New geochronological data on the migmatitic gneiss (zircon U-Pb dating) constrain the protolith age at 733 ±7 Ma (zircons core) and the HT tectono-metamorphic event at 654 ±7 Ma (zircons rim). (ii) The Asmlil arc complex is made of hornblende gabbros and garnet-bearing gabbros intruded under HT conditions by dykes of medium-grained hornblendites, hornblende-gabbros and leucodiorites. These metagabbroic intrusions have been dated at 697 ± 8 Ma (U-Pb zircons). P-T pseudosections were calculated for garnet-bearing gabbros and established that they were recrystallized under garnet-granulites P-T conditions (up to ~1000°C at 12 kbar). Preliminary geochemical data of hornblende-gabbros and garnet-bearing granulites portray similar trace geochemical signatures ((La/Sm)N: 0.8-1.6 ; (Nb/La) < 0.46) as studied paleo-arc complexes. These P-T results and new geochemical data argue that Asmlil mafic complex could represent a deep arc root comparable to the deep section of exposed oceanic arcs (i.e. Kohistan, Talkeetna, Amalaoulaou). We propose that Iriri and Asmlil units depict the deep-to-shallow sequence of a single Cryogenian oceanic arc (760-740 Ma), as discrete exposures along the southern edge of Anti-Atlas ophiolitic assemblages. Nevertheless, this primary arc has been likely broke up and intruded by subsequent hydrous arc-related magmas under medium- to high-grade P-T conditions (700 to 650 Ma). We interpret this period as an oceanic pre-collision stage when subduction geometry is intensively perturbed (c.g. composite subductions, polarity inversion), doping production of typical hydrous arc magma that intrudes original arc. This complex arc melange has been lastly accreted and sealed on the West African Craton margin.

  9. Geologic Map of the Weaverville 15' Quadrangle, Trinity County, California

    USGS Publications Warehouse

    Irwin, William P.

    2009-01-01

    The Weaverville 15' quadrangle spans parts of five generally north-northwest-trending accreted terranes. From east to west, these are the Eastern Klamath, Central Metamorphic, North Fork, Eastern Hayfork, and Western Hayfork terranes. The Eastern Klamath terrane was thrust westward over the Central Metamorphic terrane during early Paleozoic (Devonian?) time and, in Early Cretaceous time (approx. 136 Ma), was intruded along its length by the massive Shasta Bally batholith. Remnants of overlap assemblages of the Early Cretaceous (Hauterivian) Great Valley sequence and the Tertiary Weaverville Formation cover nearly 10 percent of the quadrangle. The base of the Eastern Klamath terrane in the Weaverville quadrangle is a peridotite-gabbro complex that probably is correlative to the Trinity ophiolite (Ordovician), which is widely exposed farther north beyond the quadrangle. In the northeast part of the Weaverville quadrangle, the peridotite-gabbro complex is overlain by the Devonian Copley Greenstone and the Mississippian Bragdon Formation. Where these formations were intruded by the Shasta Bally batholith, they formed an aureole of gneissic and other metamorphic rocks around the batholith. Westward thrusting of the Eastern Klamath terrane over an adjacent body of mafic volcanic and overlying quartzose sedimentary rocks during Devonian time formed the Salmon Hornblende Schist and the Abrams Mica Schist of the Central Metamorphic terrane. Substantial beds of limestone in the quartzose sedimentary unit, generally found near the underlying volcanic rock, are too metamorphosed for fossils to have survived. Rb-Sr analysis of the Abrams Mica Schist indicates a metamorphic age of approx. 380 Ma. West of Weavervillle, the Oregon Mountain outlier of the Eastern Klamath terrane consists mainly of Bragdon Formation(?) and is largely separated from the underlying Central Metamorphic terrane by serpentinized peridotite that may be a remnant of the Trinity ophiolite. The North Fork terrane is faulted against the west edge of the Central Metamorphic terrane, and its northerly trend is disrupted by major left-lateral offsets along generally west-northwest-trending faults. The serpentinized peridotite-gabbro complex that forms the western base of the terrane is the Permian North Fork ophiolite, which to the east is overlain by broken formation of mafic-volcanic rocks, red chert, siliceous tuff, argillite, minor limestone, and clastic sedimentary rocks. The chert and siliceous tuff contain radiolarians of Permian and Mesozoic ages, and some are as young as Early Jurassic (Pliensbachian). Similar Pliensbachian radiolarians are found in Franciscan rocks of the Coast Ranges. The Eastern Hayfork terrane is broken formation and melange of mainly chert, sandstone, argillite, and various exotic blocks. The cherts yield radiolarians of Permian and Triassic ages but none of clearly Jurassic age. Limestone bodies of the Eastern Hayfork terrane contain Permian microfaunas of Tethyan affinity. The Western Hayfork terrane, exposed only in a small area in the southwestern part of the quadrangle, consists dominantly of mafic tuff and dark slaty argillite. Sparse paleontologic data indicate a Mesozoic age for the strata. The terrane includes small bodies of diorite that are related to the nearby Wildwood pluton of Middle Jurassic age and probably are related genetically to the stratified rocks. The terrane is interpreted to be the accreted remnants of a Middle Jurassic volcanic arc. Shortly after intrusion by Shasta Bally batholith (approx. 136 Ma), much of the southern half of the Weaverville quadrangle was overlapped by Lower Cretaceous, dominantly Hauterivian, marine strata of the Great Valley sequence, and to a lesser extent later during Oligocene and (or) Miocene time by fluvial and lacustrine deposits of the Weaverville Formation. This map of the Weaverville Quadrangle is a digital rendition of U.S. Geological Survey Miscellaneous Field

  10. Etude de l'effet du vieillissement sur les proprietes d'un tissu en melange KevlarRTM-PBI utilise dans le revetement exterieur des habits de protection contre le feu

    NASA Astrophysics Data System (ADS)

    Arrieta, Carlos

    The aim of this work is to study and model the effect of three aging factors, temperature, humidity and light radiation, on the properties of a fabric made of a blend of KevlarRTM and PBI fibers frequently used to manufacture fire-protective garments. Accelarated-aging treatments carried out at carefully chosen conditions for the three factors resulted in a sizeable loss of mechanical performance. The breaking force of both the fabric and the yarns extracted from it decreases to less than 50% after one month of continuous exposure. X-ray diffraction (XRD) tests performed on thermally-aged samples indicated an increase of the crystallinity of the fabric, whereas the disappearance of Raman spectral lines suggested instead a reduction of the crystallinity following thermal aging. To explain these seemingly contradictory results, a hypothesis was introduced, stating that two different processes occurred simultaneously during thermal aging. The first one, an increase of size of the crystallites in the direction of the fibers' axis, accounted for the increase in crystallinity observed in XRD tests. The second one, an increase in the gap separating lamellar crystallites that causes a non-measurable reduction of the crystallinity of the sample, was highlighted by the Raman analyses. The results of the dielectric spectroscopy analyses carried out on thermally-aged samples confirmed the XRD results showing a significant change in the Kevlar's morphology during thermal aging. Despite the important decrease of the breaking force that ensued thermal aging, no evidence of a chemical structure modification of KevlarRTM was found. On the other hand, differential thermal analyses conducted on thermally aged fabric samples indicated a reduction of the glass transition temperature of the other component of the blend, namely the PBI, a fact that suggests a decrease of molecular weight after thermal aging. Infrared spectroscopy analyses performed on samples exposed to high humidity levels showed the development of a new absorbing band in the spectrum of aged KevlarRTM yarns. This band was ascribed to the formation of carboxylic acid groups. Based on these results, the humidity aging mechanism was inferred. This mechanism corresponds to the hydrolysis of the amide bond of KevlarRTM catalyzed by an acid. The progress of the hydrolysis reaction was modeled mathematically using the evolution of the concentration of carboxylic acid groups. The mechanism of light radiation aging was also determined from infrared spectroscopy analyses as the absorbing band attributed to the carboxylic acid groups was once again observed. In the case of light radiation, the degradation mechanism corresponds to a photo-oxidation reaction initiated by the photolysis of the amide bond of KevlarRTM. The accumulation of Photo-Fries products on the surface of yarns is believed to slow down of the oxidation reaction, as indicated by the overlapping of breaking force vs. aging time curves for the light-radiation aged samples. Expressions based on the Arrhenius law were used to characterize both the thermal and hydrolytic aging, whereas an expression taking into account the irradiance as well as the temperature was used to model the light radiation aging. The global damage produced by the joint action of the three aging factors was modeled after Palmgren-Miner's linear cumulative damage theory.

  11. Lawrence Head Volcanics and Dunnage Melange, Newfoundland Appalachians: Ordovician ridge subduction or back arc rift? William S.F. Kidd1, Adam Schoonmaker2, Stephen E. DeLong1, John F. Bender3

    E-print Network

    Kidd, William S. F.

    Lawrence Head Volcanics (LHV), the underlying gabbro sills of the Exploits Group, and mafic blocks with the age of the LHV and gabbro sills. Blocks of volcanics and gabbro in the DM are lithologically similar to the LHV and related gabbro sills, but some blocks, mainly from the eastern half of the DM, do not resemble

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

    USGS Publications Warehouse

    Grenne, Tor; Slack, John F.

    2003-01-01

    The jaspers are interpreted to record colloidal fallout from one or more hydrothermal plumes, followed by maturation (ageing) of an Si-Fe-oxyhydroxide gel, on and beneath the Ordovician sea floor. Small hematitic filaments in the jaspers reflect bacteria-catalysed oxidation of Fe2+ within the plume. The larger tubular filaments resulted from either microbial activity or inorganic self-organized mineral growth of Fe-oxyhydroxide within the Si-Fe-oxyhydroxide gel after deposition on the sea floor, prior to more advanced maturation of the gel as represented by the spheroidal and botryoidal silica-hematite textures. Bleaching and hematite±epidote growth are interpreted to reflect heat and fluids generated during deposition of basaltic sheet flows on top of the gels.

  13. Age constraints on the formation and emplacement of Neoproterozoic ophiolites along the AllaqiHeiani Suture, South Eastern Desert of Egypt

    E-print Network

    Stern, Robert J.

    ­Heiani Suture, South Eastern Desert of Egypt K.A. Ali a,d, , M.K. Azer b , H.A. Gahlan c , S.A. Wilde d , M the Gabgaba­Gebeit­Hijaz terranes to the south and the SE Desert­Midyan terranes to the north occurred

  14. doi:10.1016/j.gca.2004.08.004 Observations of Li isotopic variations in the Trinity Ophiolite: Evidence for isotopic

    E-print Network

    ; accepted in revised form August 11, 2004) Abstract--The Trinity peridotite (northern CA) contains numerous the Trinity peridotite. Specifically, measurements of average 7 Li for single thin sections along the traverse conduit to the surrounding peridotite, or from the surrounding peridotite into the dunite conduit

  15. Nitrogen isotopes in ophiolitic metagabbros: A re-evaluation of modern nitrogen fluxes in subduction zones and implication for the early Earth atmosphere

    NASA Astrophysics Data System (ADS)

    Busigny, Vincent; Cartigny, Pierre; Philippot, Pascal

    2011-12-01

    Nitrogen contents and isotope compositions together with major and trace element concentrations were determined in a sequence of metagabbros from the western Alps (Europe) in order to constrain the evolution and behavior of N during hydrothermal alteration on the seafloor and progressive dehydration during subduction in a cold slab environment (8 °C/km). The rocks investigated include: (i) low-strain metagabbros that equilibrated under greenschist to amphibolite facies (Chenaillet Massif), blueschist facies (Queyras region) and eclogite facies (Monviso massif) conditions and (ii) highly-strained mylonites and associated eclogitic veins from the Monviso Massif. In all samples, nitrogen (2.6-55 ppm) occurs as bound ammonium ( NH4+) substituting for K or Na-Ca in minerals. Cu concentrations show a large variation, from 73.2 to 6.4 ppm, and are used as an index of hydrothermal alteration on the seafloor because of Cu fluid-mobility at relatively high temperature (>300 °C). In low-strain metagabbros, ?15N values of +0.8‰ to +8.1‰ are negatively correlated with Cu concentrations. Eclogitic mylonites and veins display Cu concentrations lower than 11 ppm and show a ?15N-Cu relationship that does not match the ?15N-Cu correlation found in low-strain rocks. This ?15N-Cu correlation preserved in low-strain rocks is best interpreted by leaching of Cu-N compounds, possibly of the form Cu(NH 3) 22+, during hydrothermal alteration. Recognition that the different types of low-strain metagabbros show the same ?15N-Cu correlation indicates that fluid release during subduction zone metamorphism did not modify the original N and Cu contents of the parent hydrothermally-altered metagabbros. In contrast, the low Cu content present in eclogitic veins and mylonites implies that ductile deformation and veining were accompanied either by a loss of copper or that externally-derived nitrogen was added to the system. We estimate the global annual flux of N subducted by metagabbros as 4.2 (±2.0) × 10 11 g/yr. This value is about half that of sedimentary rocks, which suggests that gabbros carry a significant portion of the subducted nitrogen. The net budget between subducted N and that outgassed at volcanic arcs indicates that ˜80% of the subducted N is not recycled to the surface. On a global scale, the total amount of N buried to the mantle via subduction zones is estimated to be three times higher than that released from the mantle via mid-ocean ridges, arc and intraplate volcanoes and back-arc basins. This implies that N contained in Earth surface reservoirs, mainly in the atmosphere, is progressively transferred and sequestered into the mantle, with a net flux of ˜9.6 × 10 11 g/yr. Assuming a constant flux of subducted N over the Earth's history indicates that an amount equivalent to the present atmospheric N may have been sequestered into the silicate Earth over a period of 4 billion years.

  16. Dynamic jamming of iceberg-choked fjords

    E-print Network

    Ivo R. Peters; Jason M. Amundson; Ryan Cassotto; Mark Fahnestock; Kristopher N. Darnell; Martin Truffer; Wendy W. Zhang

    2014-12-03

    We investigate the dynamics of ice m\\'elange by analyzing rapid motion recorded by a time-lapse camera and terrestrial radar during several calving events that occurred at Jakobshavn Isbr\\ae, Greenland. During calving events (1) the kinetic energy of the ice m\\'elange is two orders of magnitude smaller than the total energy released during the events, (2) a jamming front propagates through the ice m\\'elange at a rate that is an order of magnitude faster than the motion of individual icebergs, (3) the ice m\\'elange undergoes initial compaction followed by slow relaxation and extension, and (4) motion of the ice m\\'elange gradually decays before coming to an abrupt halt. These observations indicate that the ice m\\'elange experiences widespread jamming during calving events and is always close to being in a jammed state during periods of terminus quiescence. We therefore suspect that local jamming influences longer timescale ice m\\'elange dynamics and stress transmission.

  17. Evaluation of ultramafic deposits in the Eastern United States and Puerto Rico as sources of magnesium for carbon dioxide sequestration

    SciTech Connect

    Fraser Goff; George Guthrie; Bruce Lipin; Melissa Fite; Steve Chipera; Dale Counce; Emily Kluk; Hans Ziock

    2000-04-01

    In this report, the authors evaluate the resource potential of extractable magnesium from ultramafic bodies located in Vermont, the Pennsylvania-Maryland-District-of-Columbia (PA-MD-DC) region, western North Carolina, and southwestern Puerto Rico. The first three regions occur in the Appalachian Mountains and contain the most attractive deposits in the eastern United States. They were formed during prograde metamorphism of serpentinized peridotite fragments originating from an ophiolite protolith. The ultramafic rocks consist of variably serpentinized dunite, harzburgite, and minor iherzolite generally containing antigorite and/or lizardite as the major serpentine minor phases. Chrysotile contents vary from minor to major, depending on occurrence. Most bodies contain an outer sheath of chlorite-talc-tremolite rock. Larger deposits in Vermont and most deposits in North Carolina contain a core of dunite. Magnesite and other carbonates are common accessories. In these deposits, MgO ranges from 36 to 48 wt % with relatively pure dunite having the highest MgO and lowest H{sub 2}O contents. Ultramafic deposits in southwestern Puerto Rico consist of serpentinized dunite and harzburgite thought to be emplaced as large diapirs or as fragments in tectonic melanges. They consist of nearly pure, low-grade serpentinite in which lizardite and chrysotile are the primary serpentine minerals. Chlorite is ubiquitous in trace amounts. Magnesite is a common accessory. Contents of MgO and H{sub 2}O are rather uniform at roughly 36 and 13 wt %. Dissolution experiments show that all serpentinites and dunite-rich rocks are soluble in 1:1 mixtures of 35% HCl and water by volume. The experiments suggest that low-grade serpentinites from Puerto Rico are slightly more reactive than the higher grade, antigorite-bearing serpentinites of the Appalachian Mountains. The experiments also show that the low-grade serpentinites and relatively pure dunites contain the least amounts of undesirable insoluble silicates. Individual ultramafic bodies in the Appalachian Mountains are as great as 7 km{sup 3} although typically they are {le}1 km{sup 3}. In contrast, ultramafic deposits in southwestern Puerto Rico have an estimated volume of roughly 150 km{sup 3}. Based on the few detailed geophysical studies in North Carolina and Puerto Rico, it is evident that volume estimates of any ultramafic deposit would benefit greatly from gravity and magnetic investigations, and from corehole drilling. Nevertheless, the data show that the ultramafic deposits of the eastern United States and southwestern Puerto Rico could potentially sequester many years of annual CO{sub 2} emissions if favorable geotechnical, engineering, and environmental conditions prevail.

  18. Hydrogeochemical and Isotopic Composition of Pasinler Geothermal Water (Erzurum, Turkey)

    NASA Astrophysics Data System (ADS)

    Hatipoglu, E.; Sunnetci, M. O.; Gultekin, F.

    2013-12-01

    In this investigation Pasinler (Erzurum) hot water spring has been studied from the point of geology and environmental isotopes. The Pasinler geothermal field is located 37 km east of Erzurum Province. The basement of Pasinler Geothermal field consists of Upper Cretaceous ophiolitic melange, shale, claystone, marl, and limestones, Eocene gabbro, andesite, basalt, trachyandesite, Oligocene andesite and basalt, Lower Miocene reef limestones, Upper Miocene pyroclastics, Plio- Quaternary (sandstone, marl, conglomerate) and Quaternary alluvium. The rhyolite is the reservoir for the geothermal fluid. The tuffs and marls are cap rocks of the system. The fault and related fractures around the Pasinler geothermal field provide pathways for the upward flow of geothermal fluid to the surface. The Alluvium around the Hasankale River is the most important unit as cold groundwater deposits in the study area. The thermal waters in the Pasinler geothermal fields have outlet temperatures of 23 to 35°C in springs. But discharge temperatures in the wells vary between 38-52°C. Geothermal well waters belong to the Na-Ca-Cl-HCO3 type. The Pasinler geothermal water has discharge pH values of 6 to 6.6, electrical conductivity (EC) of 970 to 6233 ?S/cm, and TDS values between 635 and 4304 mg/l. ?18O, ?2H and ?3H isotope analyses were carried out to determine the origin of waters, recharge altitude, precipitation types, and groundwater circulation. In the 18O-? 2H diagram all of the waters in the study area situated near the Globol Meteoric Water Line (GMWL) and indicate meteoric origin with little to no evaporation. According to the ?18O - temperature relation all water samples recharged at the same elevation in the plain. Low tritium coupled with high electrical conductivity and high Cl-value in the Pasinler thermal spring indicate that this spring has deep circulation. In order to determinate the origin of sulphure (SO4) and carbon in the waters, all waters were analysed for their 13CVPDB (Bee Pee Belemnite) and 34SCDT (Canyon Diablo Triolite). In the ? 13C-HCO3 diagram, HCO3 content of the thermal spring shows an enriched value of ? 13CVPDB with respect to the cold spring and ?34S CDT values show that the sulphure has come different sources in cold and hot springs.

  19. New age constraints on the evolution of the Karakorum Fault, West Tibet

    NASA Astrophysics Data System (ADS)

    Valli, F.; Arnaud, N.; Lacassin, R.; Paquette, J.; Leloup, P. H.; Li, H.; Tapponnier, P.; Guillot, S.; Deloule, E.; Maheo, G.; Xu, Z.

    2003-12-01

    Results of detailed mapping and dating of sheared rocks along southern fault-half helps assess the total offset, lifespan, slip-rate and geodynamic importance of the Karakorum Fault (KFZ). South of 33° N, along SW edge of Tashikang-Gar basin, active right-lateral normal faults, belonging to KFZ, exhume metamorphic and magmatic footwall rocks forming the SE termination of Ladakh-Karakorum range. Close to active fault, gneisses and mylonites are affected by strong ductile dextral shear and inter-layered with leucocratic melt veins sheared to various degrees. Several generations of veins are often observed on a same outcrop, with late, weakly deformed veins cutting highly sheared ones. This implies that anatexy and intrusion were coeval with dextral shearing. Direct dating of leucogranites reveals Oligo-Miocene formation ages. U/Pb thermo-ionization dating on zircons yields concordant ages at 22.7+-0.1Ma. Discordant zircons ages are consistent with a poorly defined lower intercept at 32+-3 Ma, and a Proterozoic (1300+-100 Ma) upper intercept. In situ ion microprobe dating of 17 zircons (24 spots) confirms these results. Four mylonite zircons yield comparable, concordant ages ranging between 20 and 25 Ma in their cores and rims, in good agreement with conventional U/Pb dating. Inherited zircon ages range from Paleozoic to Precambrian (ca.1200 Ma). Some of them recrystallized partially during Tertiary deformation, yielding discordant ages, the youngest being 34+-0.8Ma. Cooling was delayed until 12 to 8Ma, at which time very rapid cooling is recorded by 40Ar/39Ar and fission track data. Purely strike-slip ductile shear was thus already in progress along the fault at 23Ma and possibly earlier (ca.34Ma). A marked kinematic change from purely dextral to dextral-normal motion occurred around or just after 12 Ma. Best offset estimate along this main (northern) branch of KFZ is given by correlation of the ophiolite-bearing melange of Shiquanhe with the Shyok suture zone in Pakistan (minimum offset of 250km). Assuming that this offset accrued in time span of about 23Ma suggests average long-term rates of at least 1 cm/yr. Evidence for strike-slip faulting is also clear within the Ladakh-Karakorum range and along its SW border where mapping shows large-scale boudinage of ophiolite units implying a maximum offset of 400km along a southern branch of KFZ. South of Baer, several active strike-slip branches bound the Kailas-Ponri range to the south and continue eastwards at least up to 82.5E (lake Kunggyu). The southern part of the range is made of steep, fault-bounded slivers with clear field for brittle dextral shear. Our structural study of the area does not support inference that the Karakorum Fault terminates west of Manasarovar lake and merges with the Gurla Mandatha detachment. Instead, both deformation and large-scale geometry concur to show that the Karakorum Fault Zone continues east of 81E as a dextral, transpressive flower structure reactivating the Indus-Tsangpo suture.

  20. New geological data of New Siberian Archipelago

    NASA Astrophysics Data System (ADS)

    Sobolev, Nikolay; Petrov, Evgeniy

    2014-05-01

    The area of New Siberian Archipelago (NSA) encompasses different tectonic blocks is a clue for reconstruction of geological structure and geodynamic evolution of East Arctic. According to palaeomagnetic study two parts of the archipelago - Bennett and Anjou Islands formed a single continental block at least from the Early Palaeozoic. Isotope dating of De Long Islands igneous and sedimentary rocks suggests Neoproterozoic (Baikalian) age of its basement. The De Long platform sedimentary cover may be subdivided into two complexes: (1) intermediate of PZ-J variously deformed and metamorphosed rocks and (2) K-KZ of weakly lithified sediments. The former complex comprises the Cambrian riftogenic volcanic-clastic member which overlain by Cambrian-Ordovician turbiditic sequence, deposited on a continental margin. This Lower Palaeozoic complex is unconformably overlain by Early Cretaceous (K-Ar age of c.120 Ma) basalts with HALIP petrochemical affinities. In Anjou Islands the intermediate sedimentary complex encompasses the lower Ordovician -Lower Carboniferous sequence of shallow-marine limestone and subordinate dolomite, mudstone and sandstone that bear fossils characteristic of the Siberian biogeographic province. The upper Mid Carboniferous - Jurassic part is dominated by shallow-marine clastic sediments, mainly clays. The K-KZ complex rests upon the lower one with angular unconformity and consists mainly of coal-bearing clastic sediments with rhyolite lavas and tuffs in the bottom (117-110 Ma by K-Ar) while the complexe's upper part contains intraplate alkalic basalt and Neogene-Quaternary limburgite. The De-Long-Anjou block's features of geology and evolution resemble those of Wrangel Island located some 1000 km eastward. The Laptev Sea shelf outcrops in intrashelf rises (Belkovsky and Stolbovoy Islands) where its geology and structure may be observed directly. On Belkovsky Island non-dislocated Oligocene-Miocene sedimentary cover of littoral-marine coal-bearing unconformably overlies folded basement. The latter encompasses two sedimentary units: the Middle Devonian shallow-marine carbonate and Late-Devonian-Permian olistostrome - flysch deposited in transitional environment from carbonate platform to passive margin. Dating of detrital zircons suggests the Siberian Platform and Taimyr-Severnaya Zemlya areas as the most possible provenance. The magmatic activity on Belkovsky Island resulted in formation of Early Triassic gabbro-dolerite similar to the Siberian Platform traps. Proximity of Belkovsky Island to the north of Verkhoyansk foldbelt allows continuation of the latter into the Laptev Sea shelf. The geology of Bolshoy Lyakhovsky Island is discrepant from the rest of the NSA. In the south of Bolshoy Lyakhovsky Island the ophiolite crops complex out: it is composed of tectonic melange of serpentinized peridotite, bandedf gabbro, pillow-basalt, and pelagic sediments (black shales and cherts). All the rocks underwent epidot - amphibolite, glaucophane and greenschist facies metamorphism. The ophiolite is intruded by various in composition igneous massifs - from gabbro-diorite to leuco-granite, which occurred at 110-120 Ma. The Bolshoy Lyakhovsky Island structure is thought to be a westerly continuation of the South Anui suture of Chukchi.

  1. LE METAMORPHISME OCEANIQUE DANS LE COMPLEXE GABBROQUE DE L'OPHIOLITE D'OMAN. NICOLLET Christian, DEBRET Baptiste Laboratoire Magmas et Volcans, Universit Blaise Pascal -UMR 6524 -Clermont-Ferrand

    E-print Network

    Nicollet, Christian

    gabbros isotropes, au sommet du complexe gabbroïque. Cette hydratation devient faible en dessous dans les gabbros foliés et lités, mais persiste jusqu'au manteau où l'on trouve encore des minéraux hydratés. Les-Cpx permettent de quantifier les trajets rétrogrades de refroidissement de ces (méta)gabbros. Les Gabbros

  2. COMPRENDRE LE FONCTIONNEMENT DE LA CROUTE OCEANIQUE GRACE A L'ETUDE DES OPHIOLITES. NICOLLET Christian, Laboratoire Magmas et Volcans, Universit Blaise Pascal -UMR 6524. 5, rue Kessler, 63 038 Clermont-Ferrand FRANCE

    E-print Network

    Nicollet, Christian

    le Vert Punta Rascia 2200 2200 2545 Sommet de la Loubatière péridotites serpentinisées gabbros du Chenaillet, à l'est de Briançon, en est un exemple très bien préservé. Gabbros et laves (basalte) en forme de coussin sont associés aux roches du manteau serpentinisé (figure 2) Gabbros Laves en

  3. Alpine-type sensu strictu(ophiolitic) peridotites: Refractory residues from partial melting or igneous sediments? A contribution to the discussion of the paper: "The origin of ultramafic and ultrabasic rocks" by P.J. Wyllie

    USGS Publications Warehouse

    Thayer, T.P.

    1969-01-01

    Although Alpine peridotites and basaltic lavas are widely associated in eugeosynclines and oceanic areas, their genetic ties are obscure. Three major characteristics of olivine-rich Alpine peridotite and dunite-relict cumulus textures, aggregated masses of chromitite, and intimate association with magnesium-rich gabbro - cannot be explained by partial melting of garnet peridotite to form tholeiite. Association of magnesium-rich gabbro with the chromite-bearing and so-called high-temperature Alpine peridotites is believed to present problems that have not been considered by advocates of the partial-melting hypothesis. The chromite-bearing Alpine peridotites and related feldspathic rocks are believed to have formed near the top of the mantle by gravitational differentiation processes which are largely independent of the melting processes that produce basaltic magma at depths of 50 km or more. ?? 1969.

  4. L'Ordinateur, Un Nouveau Pedagogue? Quelques Exemples d'Application de l'Ordinateur a l'Enseignement des Langues Vivantes (The Computer: A New Teacher? Some Examples of the Use of the Computer in Second Language Teaching). Melanges Pedagogiques, 1975.

    ERIC Educational Resources Information Center

    Zerling, J. P.

    The aim of this paper is to present linguists--both members of research teams and language teachers--with some possible applications of computing science to language teaching. First, it is possible to use a computer's output system merely in order to print any type of information, e.g., a list of students' names and their marks. The computer also…

  5. Review of past and present geotectonic concepts of eastern indonesia

    NASA Astrophysics Data System (ADS)

    Katili, John A.

    By the turn of the last century Dutch geoscientists already were comparing the Indonesian island arcs to the complicated structures of the European Alps and recognized that the Indonesian Archipelago possessed a dual character, both as the intersection of two of the largest and youngest mountain systems, and as an intercontinental zone between the Asiatic and Australian continents. About half a century ago they discovered in Indonesia the largest negative gravity anomalies at sea, and established that the depth of earthquake epicenters increases landward from the trenches. Despite the limited marine technology then, they discerned that the Indonesian island arcs represent a mountain belt in statu nascendi, exhibiting a systematic relationship of active tectonic and magmatic features to the deep submarine trenches. The geological and geophysical findings at sea by the first Snellius Expedition were integrated with the theories born out of the results of geological mapping on land. This is why the tectonic theories proposed by Dutch and other European geologists before the second World War were superior to those proposed by others. Though most of these theories can no longer be accepted without modification or refinement, they constitute part of the basis of the new global tectonics. Since the advent of the plate-tectonic concept, active subduction zones, transform faults and spreading centers have been recognized in Indonesia with reasonable confidence, by their physiographic, geologic and geophysical characteristics. In contrast to this, in much of the interior of the Eurasian continent the structural complexity of similar rock assemblages which have been folded, thrust and crumpled together by nearby subduction and collision is far more difficult to unravel. Consequently, the sort of geologic events deduced from the Indonesian Archipelago are of a type that should be recorded in older tectonic belts around the globe. The modern tectonic setting of the whole equatorial Indo-Pacific region, for example, has recently been compared to the terrane map of the North American Cordillera. The position of eastern Indonesia within the plate-tectonic framework is the key to resolving contradictory views on the tectonics of the Banda Sea. For example, did the Indonesian orogeny take place at the Gondwana margin or the Asian margin, are Timor and Seram a tectonic melange and thus part of the Tertiary Indonesian island arcs, or are these two islands a part of the passive Australian margin? Oceanic magnetic stripes from the Sulu, Celebes and Banda Seas all trend NE-SW. These new data suggest that the Sulu, Celebes and probably the Banda Sea represent areas of trapped Indian Ocean crust. Deep sea drilling in the Banda Sea can resolve much controversy. The Banda Sea occupies a critical position in the complex convergent zone between Australia, Southeast Asia and the Philippine Sea Plate. The determination of the stratigraphy and basement ages of the Banda Sea will constrain evolutionary models which have been proposed. Another unsolved question of key importance in our understanding of the evolution of Sulawesi and the Moluccas is the function and timing of events of the Birdhead 'bacon slicer', or the tectonic shaving in Irian Jaya. Once this mechanism is understood, the development and timing of the various structural features of Sulawesi, Halmahera and the Banda Arc will be classified. Opinions still differ regarding the 'birthplace' of the micro-continents in the Banda Sea. Some regard them as a result of Jurassic rifting of Gondwana in northwestern Australia while others consider them displaced westward from northern Irian Jaya along the Sorong transform fault. Several authors suggested that the eastern parts of Sulawesi, Buru and Seram represent micro-continents which originated from Irian Jaya, while others considered East Sulawesi and north Sulawesi remnants of ophiolite belts or fragments of island arcs that originate from the Pacific Ocean. Contrary to those who positioned Sulawesi close to Kalimantan in Miocene time or who sep

  6. Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England

    NASA Astrophysics Data System (ADS)

    Moench, Robert H.; Aleinikoff, John N.

    The Ammonoosuc Volcanics, Partridge Formation, and the Oliverian and Highlandcroft Plutonic Suites of the Bronson Hill anticlinorium (BHA) in axial New England are widely accepted as a single Middle to Late Ordovician magmatic arc that was active during closure of Iapetus. Mapping and U-Pb dating indicate, however, that the BHA contains two volcano-sedimentary-intrusive sequences of probable opposite subduction polarity, here termed the Ammonoosuc and Quimby sequences. The Ammonoosuc sequence is defined by the Middle Ordovician Ammonoosuc Volcanics near Littleton, NH, the type area, northeast to Milan, NH, and Oquossoc, ME; it also includes black slate of the Partridge Formation ( C. bicornis--zone graptolites, ?457 Ma). Related metamorphosed intrusives are the tonalitic Joslin Turn pluton (469 ± 2 Ma), the Cambridge Black granitic pluton (468 ± 3 Ma), and gabbro, tonalite (467 ± 4 Ma), and sheeted diabase of the Chickwolnepy intrusions. These intrusives cut lowermost Ammonoosuc (therefore >469 Ma). Probable uppermost Ammonoosuc is dated at 465 ± 6 and 461 ± 8 Ma. Successively below the Ammonoosuc are the Dead River and Hurricane Mountain Formations (flysch and melange), and the Jim Pond Formation (484 ± 5 Ma) and Boil Mountain Complex (both ophiolite), which are structurally underlain by the Neoproterozoic(?) Chain Lakes massif. The Quimby sequence is defined by the Lower Silurian(?) to Upper Ordovician Quimby Formation, composed of bimodal volcanics (443 ± 4 Ma) and sulfidic shale and graywacke that lie conformably to unconformably above the Ammmonoosuc Volcanics and Partridge Formation. Also in the Quimby sequence are several granitic to sparsely gabbroic plutons of the Highlandcroft (441-452 Ma) and Oliverian (435-456 Ma) Plutonic Suites, which intrude the Dead River, Ammonoouc and Partridge, but not the Quimby Formation. Based on faunal, paleolatitude, and isotopic data, the Ammonoosuc sequence and its correlatives and underlying sequences formed off the southern Laurentian margin, but northwest of the principal Iapetan suture, or Red Indian line. The Boil Mountain-Jim Pond-Hurricane Mountain sequence was ramped northwestward over the Chain Lakes massif at ?475 Ma, on the basal Boil Mountain surface. This obduction probably occurred slightly before obduction on the Baie Vert-Brompton surface (BBL), farther NW, over the Laurentian margin, and was followed by Dead River flysch sedimentation, which ended with the abrupt onset of Ammonoosuc-sequence arc magmatism at ?470 Ma. Ammonoosuc eruptions probably ended at ?460 Ma, when Iapetus closed along the Red Indian line. During a following magmatic hiatus of ?3-5 m.y., now represented by portions of the Partridge Formation that overlie the Ammonoosuc Volcanics, subduction polarity reversed, and subduction resumed below the northwest-dipping Brunswick subduction complex (BSC) of New Brunswick, Canada. Quimby-sequence magmatism (?456-435 Ma) on the newly accreted Laurentian margin occurred above the BSC, whose footwall is now buried to the southeast by mainly Silurian clastic sediments of the Merrimack-Fredericton trough, deposited in the “Fredericton Sea”. In Silurian to Early Devonian time, the NW-dipping BSC footwall was paired with a SE-dipping subduction zone that produced arc magmas of the Coastal Volcanic belt, built on the composite Avalon and adjacent peri-Avalonian terranes. Orogen-normal extension produced by rapid rollback of both subduction zones narrowed the Fredericton Sea, produced the Central Maine and Connecticut Valley-Gaspé basins, and culminated in the Acadian orogeny when the sea completely closed in Early Devonian time.

  7. Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England

    USGS Publications Warehouse

    Moench, R.H.; Aleinikoff, J.N.

    2002-01-01

    The Ammonoosuc Volcanics, Partridge Formation, and the Oliverian and Highlandcroft Plutonic Suites of the Bronson Hill anticlinorium (BHA) in axial New England are widely accepted as a single Middle to Late Ordovician magmatic arc that was active during closure of Iapetus. Mapping and U-Pb dating indicate, however, that the BHA contains two volcano-sedimentary-intrusive sequences of probable opposite subduction polarity, here termed the Ammonoosuc and Quimby sequences. The Ammonoosuc sequence is defined by the Middle Ordovician Ammonoosuc Volcanics near Littleton, N.H., the type area, northeast to Milan, N.H., and Oquossoc, Me.; it also includes black slate of the Partidge Formation (C. bicornis zone graptolites, ???457 Ma). Related metamorphosed intrusive are the tonalitic Joslin Turn pluton (469 ?? 2 Ma), the Cambridge Black granitic pluton (468 ?? 3 Ma), and gabbro, tonalite (467 ?? 4 Ma), and sheeted diabase of the Chickwolnepy instructions. These intrusives cut lowermost Ammonoosuc (therefore>469 Ma). Probable uppermost Ammonoosuc is dated at 465 ?? 6 and 461 ?? 8 Ma. Successively below the Ammonoosuc are the Dead River and Hurricane Mountain Formations (flysch and melange), and the Jim Pond Formation (484 ?? 5 Ma) and Boil Mountain Complex (both ophiolite), which are structurally underlain by the Neoproterozoic(?) Chain Lakes massif. The Quimby sequence is defined by the Lower Silurian(?) to Upper Ordovician Quimby Formation, composed of bimodal volcanics (443 ?? 4 Ma) and sulfidic shale and graywacke that lie conformably to unconformably above the Ammonoosuc Volcanics and Partridge Formation. Also in the Quimby sequence are several granitic to sparsely gabbroic plutons of the Highlandcroft (441-452 Ma) and Oliverian (435-456 Ma) Plutonic Suites, which intrude the Dead River, Ammonoosuc and Partridge, but not the Quimby Formation. Based on faunal, paleolatitude, and isotropic data, the Ammonoosuc sequence and its correlative and underlying sequences formed off the southern Laurentian margin, but northwest of the principal Iapetan suture, or Red Indian line (RIL). The Boil Mountain-Jim Pond-Hurricane Mountain sequence was ramped northwestward over the Chain Lakes massif at ???475 Ma, on the basal Boil Mountain surface. This obduction probably occurred slightly before obduction on the Baie Verte-Brompton surface (BBL), farther NW, over the Laurentian margin, and was followed by Dead River flysch sedimentation, which ended with the abrupt onset of Ammonoosuc-sequence arc magmatism at ???470 Ma. Ammonoosuc eruptions probably ended at ???460 Ma, when Iapetus closed along the Red Indian line. During a following magmatic hiatus of ???3-5 m.y., now represented by portions of the Partridge Formation that overlie the Ammonoosuc Volcanics, subduction polarity reversed, and subduction resumed below the northwest-dipping Brunswick subduction complex (BSC) of New Brunswick, Canada. Quimby-sequence magmatism (???456-435 Ma) on the the newly accreted Laurentian margin occurred above the BSC, whose footwall is now buried to the southeast by mainly Silurian clastic sediments of the Merrimack-Fredericton trough, deposited in the "Fredericton Sea". In Silurian to Early Devonian time, the NW-dipping BSC footwall was paired with a SE-dipping subduction zone that produced arc magmas of the Coastal Volcanic belt, built on the composite Avalon and adjacent peri-Avalonian terranes. Orogen-normal extension produced by rapid rollback of both subduction zones narrowed the Fredericton Sea, produced the Central Maine and Connecticut Valley-Gaspe?? basins, and culminated in the Acadian orogeny when the sea completely closed in Early Devonian time. Published by Elsevier Science Ltd.

  8. Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England

    USGS Publications Warehouse

    Moench, R.H.; Aleinikoff, J.N.

    2003-01-01

    The Ammonoosuc Volcanics, Partridge Formation, and the Oliverian and Highlandcroft Plutonic Suites of the Bronson Hill anticlinorium (BHA) in axial New England are widely accepted as a single Middle to Late Ordovician magmatic arc that was active during closure of Iapetus. Mapping and U-Pb dating indicate, however, that the BHA contains two volcano-sedimentary-intrusive sequences of probable opposite subduction polarity, here termed the Ammonoosuc and Quimby sequences. The Ammonoosuc sequence is defined by the Middle Ordovician Ammonoosuc Volcanics near Littleton, NH, the type area, northeast to Milan, NH, and Oquossoc, ME; it also includes black slate of the Partridge Formation ( C. bicornis--zone graptolites, ???457 Ma). Related metamorphosed intrusives are the tonalitic Joslin Turn pluton (469 ?? 2 Ma), the Cambridge Black granitic pluton (468 ?? 3 Ma), and gabbro, tonalite (467 ?? 4 Ma), and sheeted diabase of the Chickwolnepy intrusions. These intrusives cut lowermost Ammonoosuc (therefore >469 Ma). Probable uppermost Ammonoosuc is dated at 465 ?? 6 and 461 ?? 8 Ma. Successively below the Ammonoosuc are the Dead River and Hurricane Mountain Formations (flysch and melange), and the Jim Pond Formation (484 ?? 5 Ma) and Boil Mountain Complex (both ophiolite), which are structurally underlain by the Neoproterozoic(?) Chain Lakes massif. The Quimby sequence is defined by the Lower Silurian(?) to Upper Ordovician Quimby Formation, composed of bimodal volcanics (443 ?? 4 Ma) and sulfidic shale and graywacke that lie conformably to unconformably above the Ammmonoosuc Volcanics and Partridge Formation. Also in the Quimby sequence are several granitic to sparsely gabbroic plutons of the Highlandcroft (441-452 Ma) and Oliverian (435-456 Ma) Plutonic Suites, which intrude the Dead River, Ammonoouc and Partridge, but not the Quimby Formation. Based on faunal, paleolatitude, and isotopic data, the Ammonoosuc sequence and its correlatives and underlying sequences formed off the southern Laurentian margin, but northwest of the principal Iapetan suture, or Red Indian line. The Boil Mountain-Jim Pond-Hurricane Mountain sequence was ramped northwestward over the Chain Lakes massif at ???475 Ma, on the basal Boil Mountain surface. This obduction probably occurred slightly before obduction on the Baie Vert-Brompton surface (BBL), farther NW, over the Laurentian margin, and was followed by Dead River flysch sedimentation, which ended with the abrupt onset of Ammonoosuc-sequence arc magmatism at ???470 Ma. Ammonoosuc eruptions probably ended at ???460 Ma, when Iapetus closed along the Red Indian line. During a following magmatic hiatus of ???3-5 m.y., now represented by portions of the Partridge Formation that overlie the Ammonoosuc Volcanics, subduction polarity reversed, and subduction resumed below the northwest-dipping Brunswick subduction complex (BSC) of New Brunswick, Canada. Quimby-sequence magmatism (???456-435 Ma) on the newly accreted Laurentian margin occurred above the BSC, whose footwall is now buried to the southeast by mainly Silurian clastic sediments of the Merrimack-Fredericton trough, deposited in the "Fredericton Sea". In Silurian to Early Devonian time, the NW-dipping BSC footwall was paired with a SE-dipping subduction zone that produced arc magmas of the Coastal Volcanic belt, built on the composite Avalon and adjacent peri-Avalonian terranes. Orogen-normal extension produced by rapid rollback of both subduction zones narrowed the Fredericton Sea, produced the Central Maine and Connecticut Valley-Gaspe?? basins, and culminated in the Acadian orogeny when the sea completely closed in Early Devonian time. Published by Elsevier Science Ltd.

  9. Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England

    NASA Astrophysics Data System (ADS)

    Moench, Robert H.; Aleinikoff, John N.

    2002-01-01

    The Ammonoosuc Volcanics, Partridge Formation, and the Oliverian and Highlandcroft Plutonic Suites of the Bronson Hill anticlinorium (BHA) in axial New England are widely accepted as a single Middle to Late Ordovician magmatic arc that was active during closure of Iapetus. Mapping and U-Pb dating indicate, however, that the BHA contains two volcano-sedimentary-intrusive sequences of probable opposite subduction polarity, here termed the Ammonoosuc and Quimby sequences. The Ammonoosuc sequence is defined by the Middle Ordovician Ammonoosuc Volcanics near Littleton, N.H., the type area, northeast to Milan, N.H., and Oquossoc, Me.; it also includes black slate of the Partidge Formation ( C. bicornis zone graptolites, ?457 Ma). Related metamorphosed intrusive are the tonalitic Joslin Turn pluton (469±2 Ma), the Cambridge Black granitic pluton (468±3 Ma), and gabbro, tonalite (467±4 Ma), and sheeted diabase of the Chickwolnepy instructions. These intrusives cut lowermost Ammonoosuc (therefore >469 Ma). Probable uppermost Ammonoosuc is dated at 465±6 and 461±8 Ma. Successively below the Ammonoosuc are the Dead River and Hurricane Mountain Formations (flysch and melange), and the Jim Pond Formation (484±5 Ma) and Boil Mountain Complex (both ophiolite), which are structurally underlain by the Neoproterozoic(?) Chain Lakes massif. The Quimby sequence is defined by the Lower Silurian(?) to Upper Ordovician Quimby Formation, composed of bimodal volcanics (443±4 Ma) and sulfidic shale and graywacke that lie conformably to unconformably above the Ammonoosuc Volcanics and Partridge Formation. Also in the Quimby sequence are several granitic to sparsely gabbroic plutons of the Highlandcroft (441-452 Ma) and Oliverian (435-456 Ma) Plutonic Suites, which intrude the Dead River, Ammonoosuc and Partridge, but not the Quimby Formation. Based on faunal, paleolatitude, and isotropic data, the Ammonoosuc sequence and its correlative and underlying sequences formed off the southern Laurentian margin, but northwest of the principal Iapetan suture, or Red Indian line (RIL). The Boil Mountain-Jim Pond-Hurricane Mountain sequence was ramped northwestward over the Chain Lakes massif at ?475 Ma, on the basal Boil Mountain surface. This obduction probably occurred slightly before obduction on the Baie Verte-Brompton surface (BBL), farther NW, over the Laurentian margin, and was followed by Dead River flysch sedimentation, which ended with the abrupt onset of Ammonoosuc-sequence arc magmatism at ?470 Ma. Ammonoosuc eruptions probably ended at ?460 Ma, when Iapetus closed along the Red Indian line. During a following magmatic hiatus of ?3-5 m.y., now represented by portions of the Partridge Formation that overlie the Ammonoosuc Volcanics, subduction polarity reversed, and subduction resumed below the northwest-dipping Brunswick subduction complex (BSC) of New Brunswick, Canada. Quimby-sequence magmatism (?456-435 Ma) on the the newly accreted Laurentian margin occurred above the BSC, whose footwall is now buried to the southeast by mainly Silurian clastic sediments of the Merrimack-Fredericton trough, deposited in the “Fredericton Sea”. In Silurian to Early Devonian time, the NW-dipping BSC footwall was paired with a SE-dipping subduction zone that produced arc magmas of the Coastal Volcanic belt, built on the composite Avalon and adjacent peri-Avalonian terranes. Orogen-normal extension produced by rapid rollback of both subduction zones narrowed the Fredericton Sea, produced the Central Maine and Connecticut Valley-Gaspé basins, and culminated in the Acadian orogeny when the sea completely closed in Early Devonian time.

  10. . JOURNAL OF GEOPHYSICAL RESEARCH, YOLo 95, NO. 84, PAGES 4895-4907, APRIL 10,1990 Simulationof the MetamorphicandDeformationalHistory of the

    E-print Network

    Hacker, Bradley R.

    into the development of the metamorphic rocks at the base of the ophiolite. The thermal evolution was calculated of rock types in the metamorphic sole of Tethyan ophiolites might be used to infer the emplacement fragmentswIthin accretionarywedges;the t COnsISt of hIgh-temperature metamorphIc rocks. These soles I f h. d

  11. Variation of cooling rate with depth in lower crust formed at an oceanic spreading ridge: Plagioclase

    E-print Network

    : Plagioclase crystal size distributions in gabbros from the Oman ophiolite Carlos J. Garrido Woods Hole: Analysis of crystal size distributions (CSD) of plagioclase in gabbros from the Oman ophiolite indicates with depth. Cooling rates in the upper half of the gabbro section were approximately 1.5­2 times faster than

  12. ORIGINAL PAPER Supra-subduction and abyssal mantle peridotites

    E-print Network

    Shervais, John W.

    ORIGINAL PAPER Supra-subduction and abyssal mantle peridotites of the Coast Range ophiolite compositions in mantle peridotites that underlie crustal sections of the ophiolite, and show. Abyssal peridotite (characterized by high- Al spinels and relatively high Ti, Na, Nd, Sm, Lu, and Hf

  13. doi:10.1144/GSL.SP.2003.218.01.22 2003; v. 218; p. 427-447Geological Society, London, Special Publications

    E-print Network

    Dundas, Robert G.

    in the formulation of plate tectonic theory, because ophiolites provide a critical link between the sea- floor been recognized as on-land frag- ments of oceanic crust since the advent of plate tectonics (e.g. Gass of their incorporation into land con- stitute a first-order tectonic problem in plate tectonics. Ophiolite emplacement

  14. Workshop Reports Scientific Drilling and Related Research in the Samail

    E-print Network

    Manning, Craig

    Workshop Reports Scientific Drilling and Related Research in the Samail Ophiolite, Sultanate.2204/iodp.sd.15.10.2013 64 Scientific Drilling, No. 15, March 2013 Workshop Reports Summary This workshop report describes plans for scientific drill- ing in the Samail ophiolite in Oman in the context of past

  15. Is the West Karmøy complex igneous or metasedimentary?

    NASA Astrophysics Data System (ADS)

    Rodgers, John

    1994-03-01

    The island of Karmøy in southwestern Norway is famous among geologists for the Ophiolite, one of the first ophiolites to be recognized and described in the Scandinavian Caledonides. Much of the island is underlain by the West Karmøy Complex, presently interpreted as an igneous complex that intrudes the Karmøy Ophiolite. There is a striking resemblance of some rocks of the Complex with the Sykesville "granite" of Maryland, which was shown by Cloos and by Hopson (1964) to be a metadiamictite. After local examination of the rocks and after comparison with the 1980 report of Ledru, the conclusion was drawn that — except for the Risdal granodiorite, pegmatite and aplite dikes — the Complex is a metasedimentary, compositionally variable succession of meta-arkose (the "quartz-augen gneiss"—Ledru's Diorite quartzique et Granodiorite du nord) and metadiamictite (his various inclusion-rich "granite" units). Both units were derived by erosion from advancing thrust sheets, including the Karmøy Ophiolite, which then overrode the sedimentary succession. The high modal quartz and normative corundum contents of the "granitic" rocks resemble those of the Sykesville and favor a metasedimentary origin; the chemistry is also similar and rather far from a granite minimum melt, though certain mixtures of quartz-sandy matrix and mafic blocks may have melted locally while being overridden by the Ophiolite. Reports that quartz diorite of the West Karmøy Complex intrudes metagabbro of the Karmøy Ophiolite could not be confirmed, although trondhjemite dikes belonging to the Ophiolite do intrude the metagabbro; the contact of the Ophiolite with the Complex was faulted wherever observed. As recognized by almost all observers, both Ophiolite and Complex are overlain unconformably by Upper Ordovician sediments of the Skudeneset Group, which was later deformed and metamorphosed in the greenschist facies; at least a great part of the fault separating the Karmøy Ophiolite and the West Karmøy Complex dates from that later episode.

  16. Local Geology Lithostructural Units Folded and Faulted Flysch 25 Figure 6 Three yellow-brown unlithified bentonite beds (top, center, bottom) in gently

    E-print Network

    Kidd, William S. F.

    cleavage. Many individual blocks can be recognized as well as bands of carbonate breccia blocks. Large carbonate breccia slab at eastern (right) limit is visible at center of cliff between vegetation]. Figure 45 Spherical carbonate breccia clast in band of carbonate conglomerate in exotic melange [267

  17. Store-consciousness (Alaya-Vijnana) - A Grand Concept of the Yogacara Buddhists

    E-print Network

    Sastri, N. A.

    1972-02-15

    result of good and bad acts of previous life. (see Trimsika, ver. 1). Vasubandhu in his Karmasiddhi (Et. lamotte's French Translation in Melanges Chinois Et. Buddhique. Vol. IV. further remarks: It is called Adana-vijnana as it assumes the body...

  18. rsted Lecture at DTU, 1 November 2013 at 14:00 Mechanics on our planet: Ice sheets,

    E-print Network

    . Williams, Jr., and C. Tracey) Average sea level rises due to melting: Greenland Ice Sheet: ~ 6.5 m West front, glacier terminusMelange of calved icebergs Jakobshavn Isbrae Glacier Greenland Ice Sheet 10 km #12;What causes glacial EQs? · Fast sliding at bed of ice sheet? -- analogous to normal EQs. · Simple

  19. Structure of Franciscan complex in the Stanley Mountain window, Southern Coast ranges, California

    SciTech Connect

    Korsch, R.J.

    1982-11-01

    Three sets of deformational events are recognized in the Franciscan Complex of the Stanley Mt. area, S. Coast ranges, California. First, in pre-melange time, shortening of the relatively cohesive sequence of interbedded graywacke and mudstone formed isoclinal folds and an axial-plane slaty cleavage. Second, fragmentation of the once cohesive sequence, probably over a considerable period of time, produced the configuration now considered a melange. Third, after the melange developed, the Franciscan Complex was deformed along with the surrounding upper Mesozoic Great Valley sequence into the Stanley Mt. antiform. In the cohesive Upper Cretaceous Carrie Creek Formation, macroscopic and mesoscopic folds have 2 predominant orientations. The less cohesive Franciscan Complex attempted to fold, as shown by the distribution of shear foliations on stereographic projections, but lack of lithologic continuity and slip along previously formed shear fractures prevents the recognition of macroscopic folds. Hence, in the Franciscan Complex of the Stanley Mt. window, several lines of evidence show that the melange structure is tectonic in origin, not just a tectonic imprint superimposed upon already chaotic rocks of sedimentary origin (olistostromes). 43 references.

  20. Structure and metamorphism of the Franciscan Complex, Mt. Hamilton area, Northern California

    USGS Publications Warehouse

    Blake, M.C., Jr.; Wentworth, C.M.

    1999-01-01

    Truncation of metamorphic isograds and fold axes within coherent terranes of Franciscan metagraywacke by intervening zones of melange indicate that the melange is tectonic and formed after the subduction-related metamorphism and folding. These relations are expressed in two terranes of blueschist-facies rocks of the Franciscan Complex in the Mt. Hamilton area, northern California-the Jurassic Yolla Bolly terrane and the structurally underlying Cretaceous Burnt Hills terrane. Local preservation in both terranes of basal radiolarian chert and oceanic basalt beneath continent-derived metagraywacke and argillite demonstrates thrust repetition within the coherent terranes, although these relations are scarce near Mt. Hamilton. The metagraywackes range from albite-pumpellyite blueschists to those containing well-crystallized jadeitic pyroxene, and a jadeite-in isograd can be defined in parts of the area. Primary bedding defines locally coherent structural orientations and folds within the metagraywacke units. These units are crosscut by thin zones of tectonic melange containing blocks of high-grade blueschist, serpentinite, and other exotic rocks, and a broader, but otherwise identical melange zone marks the discordant boundary between the two terranes.

  1. Diversity Trumps Freedom on Campus

    ERIC Educational Resources Information Center

    Talkington, Scott W.

    2006-01-01

    The tally of occurrences of a PC buzzword on academic and other websites seems to corroborate the suspicion that we're quickly abandoning "e pluribus unum" as the governing principle for treating individuals justly. In its place is the new image of America as a melange of competing interest groups. In this NAS research on comparative usage,…

  2. Chromitite Prospecting Using Landsat TM and Aster Remote Sensing Data

    NASA Astrophysics Data System (ADS)

    Beiranvand Pour, A.; Hashim, M.; Pournamdari, M.

    2015-10-01

    Studying the ophiolite complexes using multispectral remote sensing satellite data are interesting because of high diversity of minerals and the source of podiform chromitites. This research developed an approach to discriminate lithological units and detecting host rock of chromitite bodies within ophiolitic complexes using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Landsat Thematic Mapper (TM) satellite data. Three main ophiolite complexes located in south of Iran have been selected for the study. Spectral transform techniques, including minimum noise fraction (MNF) and specialized band ratio were employed to detect different rock units and the identification of high-potential areas of chromite ore deposits within ophiolitic complexes. A specialized band ratio (4/1, 4/5, 4/7) of ASTER, MNF components and Spectral Angle Mapper (SAM) on ASTER and Landsat TM data were used to distinguish ophiolitic rock units. Results show that the specialized band ratio was able to identify different rock units and serpentinized dunite as host rock of chromitites within ophiolitic complexes, appropriately. MNF components of ASTER and Landsat TM data were suitable to distinguish ophiolitic rock complexes at a regional scale. The integration of SAM and Feature Level Fusion (FLF) used in this investigation discriminated the ophiolitic rock units and prepared detailed geological map for the study area. Accordingly, high potential areas (serpentinite dunite) were identified in the study area for chromite exploration targets.The approach used in this research offers the image processing techniques as a robust, reliable, fast and cost-effective method for detecting serpentinized dunite as host rock of chromitite bodies within vast ophiolite complexes using ASTER and Landsat TM satellite data.

  3. Geologic map of the Sauk River 30- by 60-minute quadrangle, Washington

    USGS Publications Warehouse

    Tabor, R.W.; Booth, D.B.; Vance, J.A.; Ford, A.B.

    2002-01-01

    Summary -- The north-south-trending regionally significant Straight Creek Fault roughly bisects the Sauk River quadrangle and defines the fundamental geologic framework of it. Within the quadrangle, the Fault mostly separates low-grade metamorphic rocks on the west from medium- to high-grade metamorphic rocks of the Cascade metamorphic core. On the west, the Helena-Haystack melange and roughly coincident Darrington-Devils Mountain Fault Zone separate the western and eastern melange belts to the southwest from the Easton Metamorphic Suite, the Bell Pass melange, and rocks of the Chilliwack Group, to the northeast. The tectonic melanges have mostly Mesozoic marine components whereas the Chilliwack is mostly composed of Late Paleozoic arc rocks. Unconformably overlying the melanges and associated rocks are Eocene volcanic and sedimentary rocks, mostly infaulted along the Darrington-Devils Mountain Fault Zone. These younger rocks and a few small Eocene granitic plutons represent an extensional tectonic episode. East of the Straight Creek Fault, medium to high-grade regional metamorphic rocks of the Nason, Chelan Mountains, and Swakane terranes have been intruded by deep seated, Late Cretaceous granodioritic to tonalitic plutons, mostly now orthogneisses. Unmetamorphosed mostly tonalitic intrusions on both sides of the Straight Creek fault range from 35 to 4 million years old and represent the roots of volcanoes of the Cascade Magmatic Arc. Arc volcanic rocks are sparsely preserved east of the Straight Creek fault, but dormant Glacier Peak volcano on the eastern margin of the quadrangle is the youngest member of the Arc. Deposits of the Canadian Ice Sheet are well represented on the west side of the quadrangle, whereas alpine glacial deposits are common to the east. Roughly 5000 years ago lahars from Glacier Peak flowed westward filling major valleys across the quadrangle.

  4. Processes of Late Cretaceous to Late Miocene episodic thrust-sheet translation in the Lycian Taurides, SW Turkey 

    E-print Network

    Collins, Allan S; Robertson, Alastair H F

    1998-01-01

    of structural analysis, sedimentology, palaeontology and geochemistry allows subdivision of this orogenic event into: (1) latest Cretaceous trench–passive margin collision causing ophiolite obduction and detachment of the Köycegiz Thrust Sheet; (2) Mid...

  5. Physics ofthe Earth and Planetary Interiors, 78 (1993) 257-280 Elsevier Science Publishers B.V., Amsterdam

    E-print Network

    Cattin, Rodolphe

    1993-01-01

    ) The seismic properties of five South African kimberlite nodules have been calculated from petrofabric the orthopyroxene component is taken into account. The ophiolite samples are twice as anisotropic as the kimberlite

  6. GeoArabia, Vol. 13, No. 2, 2008 Gulf PetroLink, Bahrain

    E-print Network

    Ali, Mohammed

    could be due to the collision of the Arabian and Eurasian Plates and the opening of the Red Sea which Ophiolite, Haybi, Hawasina and Sumeini sheets onto the continental margin of the Arabian Plate. Near Al Ain

  7. Petrogenetic and tectonic significance of xenocrystic Precambrian zircon in Lower Cambrian tonalite, eastern Klamath Mountains, California

    SciTech Connect

    Wallin, E.T. )

    1990-11-01

    U-Pb analyses of zircon from a Lower Cambrian tonalite in the Yreka terrane, Klamath Mountains, California, reveal an inherited Precambrian zircon component within rocks that have generally been believed to be part of the ophiolitic Trinity complex. The isotopic data indicate that Lower Cambrian plutonic rocks in the eastern Klamath Mountains are probably not ophiolitic in origin. These Lower Cambrian rocks may be either tectonic blocks within a lower Paleozoic accretionary complex or part of a disrupted Lower Cambrian volcanoplutonic terrane.

  8. Podiform chromitites do form beneath mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Arai, Shoji; Miura, Makoto

    2015-09-01

    Podiform chromitites are commonly found within the Moho transition zone to mantle section of ophiolites, as well as in non-ophiolitic peridotite massifs. However, they have rarely been observed in the present-day ocean floor, even though some ophiolites are considered slices of oceanic lithosphere. One of the factors controlling podiform chromitite formation is the chemistry of the host mantle peridotite. A moderately refractory harzburgite that contains chromite with an intermediate Cr# (Cr/(Cr + Al)) of 0.4-0.6 is the optimum host for chromitites. Such a harzburgite represents the most typical lithology of oceanic lithosphere where peridotite-melt reactions, another requirement for chromitite formation, are possibly common. Thus, the oceanic upper mantle is potentially a suitable host for podiform chromitites. In particular, off-ridge magmatism may lead to the formation of podiform chromitite. The apparent rarity of chromitites in the present-day ocean floor is simply a reflection of the under-sampling of mantle material from fast-spreading ridges and the center segment of slow-spreading ridges. However, in addition to ophiolitic chromitites forming at ordinary mid-ocean ridges from genuine MORB, they also form in the typical ophiolite tectonic setting, at supra-subduction zone spreading centers, from wet MORB. Future mantle drilling as an active way of sampling, on the ocean floor will possibly reveal the occurrence of podiform chromitite in present-day oceanic lithosphere, analogous to ophiolitic chromitite.