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Sample records for ophiolitic melange nellore-khammam

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

    NASA Astrophysics Data System (ADS)

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

    2009-10-01

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

  2. 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.

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

    NASA Technical Reports Server (NTRS)

    Mustard, John F.; Pieters, Carle M.

    1988-01-01

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

  4. 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.

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

    NASA Astrophysics Data System (ADS)

    Robertson, Alastair; Karamata, Stevan; Šarić, Kristina

    2009-03-01

    The northern Balkan Peninsula, including Serbia, Montenegro, Bosnia, Croatia and the Former Yugoslavian Republic of Macedonia, represents an excellent region for the study of tectonic processes related to Mesozoic Tethyan ophiolite genesis and emplacement. We first summarise the main tectonic units of the northern Balkan Peninsula and then use this information to discuss tectonic processes, including rifting, sea-floor spreading, ophiolite genesis and emplacement, melange accretion, ocean-basin closure and collision. We then discuss alternative models of ophiolite genesis and emplacement for the region and suggest that multi-ocean-basin interpretations fit the data better than single-ocean-basin interpretations. Rifting of Adria (Gondwana) during the Triassic created a rift in the south (Budva zone) and opened a Triassic oceanic basin further north (Dinaride ocean). Occurrences of inferred sub-continental mantle lithosphere in the Dinaride ophiolite belt (e.g. Zlatibor) may record extensional exhumation within an ocean-continent transition zone bordering the Adria/Dinaride continent. This was followed by emplacement together with ophiolites and melange during Upper Jurassic-Early Cretaceous time. Upper Triassic radiolarites and mid-ocean ridge-type basalts formed at a spreading ridge after continental break-up. The oceanic lithosphere of the Dinaride ophiolite belt was partly generated above a subduction zone. The metamorphic soles of the Dinaride ophiolites formed during Mid-Late Jurassic mainly based on K/Ar dating. Widespread melange that is associated with the ophiolites represents a subduction complex, controlled by tectonic accretion and sedimentary reworking in trench and fore-arc basin settings. A possible cause of Jurassic Dinaride ophiolite emplacement was collision of a subduction trench with a continental margin. Further north, Mesozoic oceanic lithosphere subducted northeastwards (present coordinates) opening a Late Jurassic marginal basin in the Main

  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. 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.

  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

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

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

  16. 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.

  17. 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.

  18. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  20. Hydrocarbon gas potential of accretionary melange terranes: an example from the olympic peninsula, Washington

    SciTech Connect

    Kvenvolden, K.A.; Snavely, P.D. Jr.

    1985-01-01

    Convergence between the oceanic and North American plates during middle late Eocene and late middle Miocene times produced two principal accretionary terranes of melange and broken formation on the continental margin of Washington. Hydrocarbon analyses of these melange units were undertaken to evaluate their source rock potential for oil and gas and to assess the generative processes operating in these thick melange wedges. The results of pyrolysis, vitrinite reflectance, and visual kerogen analyses of samples of these melanges are consistent and in good agreement, showing mainly Type III organic matter that is marginally mature to mature with respect to gas generation. Coastal exposure of Ozette melange commonly have a petroliferous odor which contains methane through at least the pentanes as prominent constitutents. Hydrocarbon gases from seeps and from an abandoned well in the study area have been molecular compositions and methane carbon isotopic values indicating related sources. The authors evidence suggest that the Ozette assemblage melange is the principal source for thermogenic hydrocarbon gases. Potential exploration targets may exist in western Washington where melange and broken formation are thrust beneath the Eocene oceanic crust (Crescent Formation). Gas generated from the underplated rocks could have migrated through the upper plate into structures in the Tertiary strata that overlie these Eocene basalts.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  2. 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

  3. Origin of Zircon in Ophiolitic Mantle Rocks

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

  4. 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.

  5. 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.

  6. 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.

  7. The Tectonic Evolution of Caribbean Ophiolites

    NASA Astrophysics Data System (ADS)

    Draper, G.

    2001-12-01

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

  8. 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

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

    NASA Astrophysics Data System (ADS)

    Grubić, Aleksandar; Radoičić, Rajka; Knežević, Mirjana; Cvijić, Ranko

    2009-03-01

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

  10. Ophiolite Perspectives on Oceanic Mantle Heterogeneity

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  11. The ophiolite belts of northeastern China

    NASA Astrophysics Data System (ADS)

    Congzhou, Cao

    Four ophiolite belts, i.e. the Wendur Miao-Xar Moron River, the Solon Mt.-Hegen Mt., the Mudangjiang and the Raohe ophiolite belts, are recognized in northeastern China in various tectogenetic environments, with different ages and geological significance. They may be present in the Upper Proterozoic, lower Lower Paleozoic, lower Upper Paleozoic or Triassic, respectively. They are also formed in oceanic basin, mid-oceanic ridge, island-arc and back-arc basin, and either within plate or between two plates respectively.

  12. Mapping the Oman Ophiolite using TM data

    NASA Technical Reports Server (NTRS)

    Abrams, Michael

    1987-01-01

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

  13. Falls Lake melange, a polydeformed accretionary complex in the North Carolina piedmont

    SciTech Connect

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

    1985-01-01

    The Falls Lake melange in the eastern Piedmont of North Carolina is composed of mafic and ultramafic blocks and pods of diverse shapes and sizes, from about 1 cm to 7 km in length, dispersed without stratigraphic continuity in a matrix of pelitic schist and metagraywacke. The melange lies between the Carolina slate belt on the west and the Raleigh belt on the east. This terrane has been mapped in detail from near Wilton in Granville County, southward for about 40 km to west Raleigh in Wake County. Preliminary petrologic and geochemical data suggest an oceanic crustal origin for the mafic and ultramafic inclusions. Although the lenticular shapes of most fragments are attributable to ductile deformation, a few more equant fragments that are clearly overprinted by the earliest recognized matrix schistosity have round to angular shapes resembling sedimentary clasts. These observations suggest formation by a combination of sedimentary and tectonic processes, perhaps in the accretionary wedge of a convergent plate margin. The Falls Lake melange and the overlying late Proterozoic to Early Cambrian volcanic-arc terrane of the accreted Carolina slate belt were thrust upon a probably continental terrane of the Raleigh belt before overprinting by late Paleozoic folding and metamorphism. The melange and its folded basal decollement are truncated on the east by a later fault of the Nutbush Creek system and have not been found east of this fault.

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

    NASA Astrophysics Data System (ADS)

    Cloos, Mark; Ukar, Estibalitz

    2014-05-01

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

  15. Numerical Models of Ophiolite Genesis and Obduction

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  16. 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

  17. Thematic mapper study of Alaskan ophiolites

    NASA Technical Reports Server (NTRS)

    Bird, John M.

    1987-01-01

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

  18. 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.

  19. 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.

  20. 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

  1. The “ophiolite rule”, chemostratigraphy of Teythan-type ophiolites and subduction initiation

    NASA Astrophysics Data System (ADS)

    Whattam, S. A.; Stern, R. J.

    2009-12-01

    Characteristics of the classic late Jurassic and late Cretaceous Tethyan-type ophiolites of the Eastern Mediterranean - Persian Gulf region are consistent with formation in a supra-subduction zone (SSZ); the most robust model for their production entails formation upon subduction initiation in the forearc or proto-forearc region of a nascent intra-oceanic arc (IOA). The ‘ophiolite rule’ however, demonstrates that a co-existence of MOR-like (in addition to SSZ) lavas is the norm and not the exception. Specifically, the ‘normal’ chemostratigraphy of Tethyan-type ophiolites usually exhibits a thick, lower section of tholeiitic MOR-like lavas in addition to an upper section of dominantly calc-alkaline, HFSE-depleted, VAB-like lavas and intrusives and lesser boninites; latest stage dikes of boninitic affinity commonly cut both suites. This association testifies to changes in the ophiolitic melt source. To date, explanations for the near ubiquitous relationship of these two distinct tectonomagmatic suites usually infer that conventional MOR or marginal basin formation was followed by tectonic reorganization and consequent IOA construction above the (much older) first-formed MORB seafloor. While such ad hoc tectonic interpretations might apply to a few ophiolites, these explanations surely can’t explain what appears to be a global ophiolite phenomenon as this chemostratigraphy is now being recognized in other major ophiolitic belts (e.g., of the SW Pacific) and IOA forearcs (e.g., the Izu-Bonin-Mariana arc). A lack of hiatus between sequences is shown by boundaries between the two main ophiolitic units that are stratigraphic and not fault-bounded; there exists no significant temporal hiatus between formation of the lower tholeiitic and upper calc-alkaline suites (no sediment horizons or obvious unconformities); furthermore, lavas compositionally intermediate to MORB- and VAB-like occur stratigraphically between the sequences. We present a new model consistent

  2. 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

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

    NASA Astrophysics Data System (ADS)

    Sachau, Till; Koehn, Daniel

    2012-12-01

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

  4. Late Jurassic to Early Cretaceous Radiolaria from chert blocks in the Lubok Antu melange, Sarawak, Malaysia

    NASA Astrophysics Data System (ADS)

    Jasin, Basir

    The Lubok Antu melange is composed of blocks of mudstone, shale, sandstone, chert, limestone, hornfels, basalt, gabbro and serpentinite embedded in a strongly cleaved, pervasively sheared, chloritised mudstone matrix. Chert blocks are common and widespread in the melange. Fifty-three species of Radiolaria were identified from 14 samples collected from these chert blocks. The Radiolaria can be grouped into three assemblages. Assemblage I is composed of 17 species. The presence of Ristola altissima (Rüst) and Parvicingula excelsa Pessagno and Blome is indicative of late Tithonian (Late Jurassic). Assemblage II consists of 21 species. The occurrence of an index form Staurosphaera septemporata (Parona) indicates middle Valanginian to Barremian age. The presence of Squinabollum fossilis (Squinabol), Archaeodictyomitra vulgaris Pessagno, Obesacapsula somphedia (Foreman), Thanarla praeveneta Pessagno, Pseudodictyomitra pseudomacrocephala (Squinabol), Rhopalosyringium majuroensis Schaaf, Stichomitra communis Squinabol and Holocryptocanium tuberculatum Dumitrica in Assemblage III suggests that the age of this assemblage is late Albian to Cenomanian. All the results show that there are three different ages of the chert blocks present in the Lubok Antu melange.

  5. Modelling the impact of submarine frontal melting and ice melange on glacier dynamics

    NASA Astrophysics Data System (ADS)

    Krug, J.; Durand, G.; Gagliardini, O.; Weiss, J.

    2015-05-01

    Submarine melting of the calving face of tidewater glaciers and the mechanical back force applied by the ice melange layer are two mechanisms generally proposed to explain seasonal variations at the calving front of tidewater glaciers. However, the way these processes affect the calving rate and glacier dynamics remains uncertain. In this study, we used a finite element-based model that solves the full Stokes equations to simulate the impact of these forcings on two-dimensional theoretical flow line glacier configurations. The model, which includes calving processes, suggests that frontal melting affects the position of the terminus only slightly (less than a few hundred metres) and does not affect the multiannual glacier mass balance at all. However, the ice melange has a greater impact on the advance and retreat cycles of the glacier front (more than several kilometres) and its consequences for the mass balance are not completely negligible, stressing the need for better characterization of forcing properties. We also show that ice melange forcing against the calving face can mechanically prevent crevasse propagation at sea level and hence prevent calving. Results also reveal different behaviours in grounded and floating glaciers: in the case of a floating extension, the strongest forcings can disrupt the glacier equilibrium by modifying its buttressing and ice flux at the grounding line.

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

    NASA Astrophysics Data System (ADS)

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

    2016-09-01

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

  7. Ophiolitic basement to a forearc basin and implications for continental growth: The Coast Range/Great Valley ophiolite, California

    NASA Astrophysics Data System (ADS)

    Godfrey, Nicola J.; Klemperer, Simon L.

    1998-08-01

    We present a compilation of 18 published models from the length of the Great Valley forearc basin, California, based on seismic reflection, borehole, seismic refraction, gravity, and aeromagnetic data to address long-standing questions about the nature of the basement to the Great Valley, its origin, its tectonic history, and it's mechanism of incorporation into the North American continental margin. The geophysical models permit a 700-km-long, 70-km-wide, complete ophiolite sequence beneath the entire Great Valley. In the northern Great Valley, the ophiolite is overlain by ophiolitic breccia, the ophiolite crust is 7-8 km thick, and the mantle section is mostly unserpentinized. Beneath the southern Great Valley, there is no ophiolitic breccia, the crust may be up to 10 - 12 km thick, and the mantle section, if present at all, is serpentinized to such a degree that it cannot be distinguished from Sierran basement or mafic ophiolite members on the basis of velocity or density data. Geochemical, petrological, and paleomagnetic data support suprasubduction zone ophiolite formation at North American paleolatitudes, and geological data and geophysical models are consistent with ophiolite formation by back arc spreading behind an east facing arc. In the north, this was apparently followed by obduction of back arc crust onto older continental basement during the Late Jurassic Nevadan orogeny. In the south, the newly formed intraoceanic arc and back arc apparently collided with the continental margin during the Nevadan orogeny but were not obducted onto it. Instead, the arc and back arc "docked" with the continental margin leaving the arc itself to become the basement to the Great Valley basin. Cretaceous Sierran magmatism then intruded plutons beneath the docked ophiolite and mafic arc. Irrespective of the detailed accretionary history, our cross sections show a rapid pulse of continental growth by ophiolite accretion of more than 500 km³ km-1 in less than 10 Myr.

  8. 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.

  9. 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

  10. 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

  11. 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.

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

    NASA Astrophysics Data System (ADS)

    Robertson, Alastair

    2016-04-01

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

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

    SciTech Connect

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

    1991-05-01

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

  14. 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.

  15. Mapping oceanic ridge segments in Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Nicolas, A.; Boudier, F.

    1995-04-01

    This paper presents the results of detailed mapping of high-temperature flow structures in the mantle and crust of two massifs of the Oman ophiolite. In these massifs, the dominant structures, including large-scale folds, shear zones, and fractures, were generated at elevated temperatures and are ascribed to the ridge or ridge environment activity; this means that the structural maps presented can be viewed as those of partly dissected ridge segments. It has been possible in the two massifs to locate the paleoaxis of the oceanic ridge which created this crust. This location, which is constrained by several independent tests, is a prerequisite to reconstruct the structure and to investigate the dynamics of a fast spreading ridge. In the Nakhl-Rustaq massif, high temperature tectonic activity at the ridge rotated the Moho toward a vertical altitude and folded the layered gabbros on the scale of several hundred meters. This tectonism is attributed to a propagating ridge deforming a slightly older lithosphere. The propagating ridge segment extends in the field from a diapir area to a domain located along strike some 20 km away, where the sheeted dike complex roots directly in the mantle, without layered gabbros in between. The diapir area represents the mantle feeder for the ridge segment, and the rooted dikes represent the propagating tip. Other results include the detailed mapping of two mantle diapirs and of the diverging mantle flow issued from them. Magma chambers are centered over diapirs and are tent-shaped, in accord with our previous models.

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

    NASA Astrophysics Data System (ADS)

    Vearncombe, J. R.

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

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

    NASA Technical Reports Server (NTRS)

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

    1986-01-01

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

  18. 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.

  19. 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.

  20. Ophiolitic association of Cape Fiolent area, southwestern Crimea

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

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

  1. 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

  2. Dismembered Archean ophiolite in the SE. Wind River Mountains, Wyoming

    SciTech Connect

    Harper, G.D.

    1985-01-01

    Ophiolitic rocks occur as wall rocks of the 2.7 Ga Louis Lake batholith near Atlantic City, Wyoming. All of the Archean rocks are strongly deformed and metamorphosed to a greenschist and amphibolite facies, but relict structures and textures are commonly preserved. These include the following, from west to east: (1) metadiabase with rare coarse-grained metagabbro; (2) ultramafic rocks and metagabbro; (3) amphibolite, locally pillowed, overlain(.) by pelitic schist, banded iron formation, and quartzite; and (4) pillow lavas, massive sills or flows, and minor metasedimentary rocks. Slice 1 locally contains parallel dike margins and rare metagabbro screens; these features suggest that it may represent a sheeted dike complex. Slice 2 locally contains ultramafic rocks having relict cumulus textures and igneous layering, corresponding to the cumulus portion of an ophiolite. The pillow lavas of slice 4 and possibly slice 3 are interpreted as comprising the extrusive portion of the ophiolite. The immobile trace element chemistry (Ti, V, Zr, Y, Cr, Ni) of slice 1 and 4 is very similar and supports a cogenetic origin, whereas pillow lavas of slice 3 are somewhat distinct. The metadiabases and lavas of slices 1 and 4 are similar to modern mid-ocean ridge basalt, whereas lavas of slice 3 are more similar to island-arc tholeiites. Rare high-Ti basaltic komatiites occur in slices 1 and 4, but have very distinct trace element chemistry and probably represent later off-axis dikes. The ophiolitic rocks are interpreted to represent the remains of Archean oceanic crust.

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

    NASA Astrophysics Data System (ADS)

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

    2009-03-01

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

  4. 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.

  5. 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.

  6. 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.

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

    NASA Astrophysics Data System (ADS)

    Kapsiotis, Argyrios N.

    2014-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

    Obduction, whereby fragments of dense, oceanic lithosphere (ophiolites) are presumably 'thrust' on top of light continental ones, remains a poorly understood geodynamic process, in particular with respect to 1) obduction initiation and 2) effective ophiolite emplacement. Most of our knowledge on obduction initiation comes from the amphibolite to granulite facies high-temperature metamorphic soles welded to the base of non-metamorphic large-scale ophiolite thrusts (e.g., Oman, N.Caledonia, Balkans,...), which are interpreted as witnesses of the subduction inception stages preluding to obduction. We herein report for the first time the existence of deformed amphibolites near the base of the ophiolite (~ 50-100 m above), yet within the mantle peridotites proper, from the classic New Caledonia ophiolite (Plum beach, SE of Noumea). These amphibolites correspond to several cm thick thin bands of sheared mafic rocks showing highly deformed to mylonitic textures, which are embedded within well-preserved to strongly serpentinized peridotites. Large-scale shear bands (>100m long in places) show impressive, very consistent deformation patterns and shear senses on the outcrop scale in favor of reverse displacements. Amphibolites and peridotites are both strongly sheared with the common observation of strongly boudinaged peridotites in the core of the largest shear bands. Some gabbroic pods and plagioclase-rich veinlets are spatially associated to the amphibolites, but appear to have formed slightly later based on cross-cutting relationships. Preliminary mineralogical observations indicate complex, successive recrystallisations stages, with olivine-bearing clasts, at least three generations of amphiboles and late stage talc-chlorite associations. Although still preliminary, several interpretations can be proposed for the origin of such structures. They could correspond to (1) very-high temperature sole "amphibolites" deformed (tectonically mixed with the mantle; and possibly

  9. 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

  10. Lead isotopic studies of the Samail ophiolite, Oman

    NASA Technical Reports Server (NTRS)

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

    1981-01-01

    The isotopic composition of Pb and the concentrations of U, Th, and Pb are determined for samples from massive sulfides and lithologic units of the Samail ophiolite. It is shown that the magma that formed the ophiolite was probably derived from a depleted mantle, similar in Pb isotopic composition to the source area of mid-ocean ridge basalt. The gross geochemical characteristics of the mantle sources have remained almost unchanged for the last 100 million years. It is also shown that the U-Th-Pb systematics of the Samail formation indicate that local vertical heterogeneity in the oceanic crust can result from crystal fractionation and metasomatism at, and near, an oceanic spreading center.

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

    NASA Astrophysics Data System (ADS)

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

    2016-09-01

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

  12. Preliminary paleomagnetic study of the Thetford Mines Ordovician Ophiolite (Canada)

    NASA Astrophysics Data System (ADS)

    Di Chiara, Anita; Morris, Antony; Anderson, Mark

    2016-04-01

    Extension associated with oceanic ridges at divergent plate boundaries is characterized by normal faulting and episodic magma supply. Studies in modern oceanic settings suggest locally along ridges both lower crust and upper mantle peridotites may be exhumed to the seafloor in features known as oceanic core complexes (OCC). OCC are characterized by long-lived low-angle detachment faults that extend for 10s of km, and that are crosscut by high-angle normal faults oriented parallel to the rift axis. Here we present preliminary results from 12 paleomagnetic sites sampled on an example of fossilized Ordovician OCC preserved in the in the Canadian Appalachians, the Southern Quebec ophiolites. These were obducted and subjected to polyphase deformation during Palaeozoic orogeny along the Laurentian margin of Iapetus. Although locally obscured by tectonic fabrics and structures, the original relationships between the ophiolitic mantle, the overlying plutonic section, and onlapping Ordovician siliciclastic rocks can be reconstructed within the Thetford-Mines ophiolite. Preliminary results from AMS and Thermal demagnetization experiments record a remarkably consistent overprint from the youngest (Acadian) phase of Paleozoic orogeny. Although complicating further study of intra-oceanic deformation along the detachment fault in the original OCC, the results provide further insights into the progressive overprint of deformation events as recorded by AMS.

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2007-10-01

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

  15. 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

  16. 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

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  19. 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.

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

    NASA Astrophysics Data System (ADS)

    Acharyya, S. K.

    2015-07-01

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

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

    NASA Astrophysics Data System (ADS)

    Haig, David W.; Bandini, Alexandre Nicolas

    2013-10-01

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

  2. Mineralogy, geochemistry and geotectonic significance of mantle peridotites with high-Cr chromitites in the Neyriz ophiolite from the outer Zagros ophiolite belts, Iran

    NASA Astrophysics Data System (ADS)

    Rajabzadeh, Mohammad Ali; Nazari Dehkordi, Teimoor; Caran, Şemsettin

    2013-02-01

    The Neyriz ophiolite containing chromitite pods from the outer Zagros ophiolite belt was studied in the Abadeh Tashk area. It appears as four detached massifs in an area with 125 km2 in south of Iran and is comprised predominantly of peridotites. Harzburgites and dunites are the most ultramafics in the massifs with rare mafic and gabbroic rocks. Mineralogical composition of chromian spinel in chromitite and host peridotite of the Neyriz ophiolite in comparison with those of the Nain ophiolite, to tracing their geographical variations between outer and inner sectors of the Zagros ophiolite belt, shows that there are remarkable and striking compositional variations between these two ophiolites. Chromian spinels of the Neyriz mantle peridotite and chromitite pods are characterized by higher contents of Cr# (56-79; average, 71) in harzburgites and dunites, and higher Cr# (73-82), Mg# (62-71) and lower Al (9.1-13.9 wt% Al2O3) and Ti (up to 0.08 wt% TiO2) in chromitite pods with respect to available data on samples from the Nain ophiolite (Cr# 40-61 of spinels in mantle harzburgites; Cr#: 59-73, Mg#: 60-70, Al2O3: 13.6-22.37 wt%, TiO2: 0.13-0.40 wt% of spinels in chromitites). Based on geochemical affinities, we contend that the Neyriz mantle peridotites were largely affected by percolating hydrous boninitic melts, to produce high-Cr chromitites, whereas chromitites of the Nain mantle peridotites carry geochemical imprints of boninitic melts with MORB-like affinity, suggesting their distinct geotectonic setting. The presence of the high concentrations of wolfram (W: 275-1276 ppm) in the Neyriz mantle peridotites in comparison with those of the Nain mantle peridotites (W < 3 ppm), which is the one of the most striking geochemical features of these rocks, provides an additional evidence for the significant role of recycling of continental material such as subducted pelagic sediments into the sources of subduction zone magmas. Combining with the lithological variations of

  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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  5. Numerical model of ice melange expansion during abrupt ice-shelf collapse

    NASA Astrophysics Data System (ADS)

    Guttenberg, N.; Abbot, D. S.; Amundson, J. M.; Burton, J. C.; Cathles, L. M.; Macayeal, D. R.; Zhang, W.

    2010-12-01

    evolve into full-blown collapse of the ice shelf? What proportion of uncapsizable icebergs prevent a collapse?), (b) the rates of mobilization and their dependence on iceberg geometry (e.g., what determines the speed at which the expanding ice melange moves into the open, ice-free water?), and (c) the factors that promote the arrest of the system (e.g., are there circumstances where only partial collapses can occur?). Results of simulations are compared with observational parameters derived from satellite imagery, seismic analysis and laboratory experiment to determine what aspects of the numerical model's physical formulation may have most relevance to the disappearance of ice shelves.

  6. Interesting Inclusions From Podiform Chromitites in Luobusa Ophiolite, Tibet

    NASA Astrophysics Data System (ADS)

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

    2003-12-01

    For the past decade, diamonds and unusual mineral asemblages were reported in podiform chromitites of the Luobusa ophiolite, southern Tibet, China (Bai 1993, Bai 2000, Yan 2001). These minerals were found from heavy mineral separation of chromitites. These minerals include (1) native elements, (2) alloys, (3) carbide (SiC, CrC), (4) platinium group elements (PGE) and arsenides, (5) silicates (Ol, Opx, Cpx, Amp, Srp, Chl, Uv, Prp, Alm, Wo, Zrn, Ap, Bt, Spn, Rt, Pl, Kfs, Phl, Sil, Qz and octahedral serpentine (possible pseduomorph after ringwoodite?), (5) oxide (corundum and chromite), (6) carbonates. Despite many questions as to these minerals above still remain open, these mineral inclusions would provide us the important infomation on the formation of the podiform chromitites. In this study, octahedral serpentine was discovered both on a thin section and from the heavy mineral separation. These octahedral inclusions exist within chromites, forming a line. These minerals are approximately 5-15μ m in diameter and have well octahedral morphology. EPMA, laser raman spectrometer and transmission electron microscopy (TEM) were used to determine the structure and chemical composition of this crystal. For the present, there are several interpretations of this octahedral silicate. One possibility is that if the octahedral structuer is euhedral so this octahedral serpentine may be pseudomorph after ringwoodite because of its chemical composition and octahedral crystal shape. Another is that ocahedral minerals are melt inclusions. Linear occurrence of octahedral minerals is similar to that of fuluid inclusions. If the octahedral structuer is negative crystal shape reflecting octahedral crystal of cromian spinel, then octahedral inclusions may be melt inclusions judging from linear occurrence. At the same time, zircons were obtained from the mineral separation from chromitites. U-Pb dating of these zircons by LA-ICP-MS yielded two different ages. One group has relatively

  7. 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.

  8. 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 ɛNd (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.

  9. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2009-07-01

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

  11. 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

  12. 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.

  13. 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

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  15. 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.

  16. Ophiolite sections of the Dzhalair-Nayman zone, South Kazakhstan: Their structure and age substantiation

    NASA Astrophysics Data System (ADS)

    Ryazantsev, A. V.; Degtyarev, K. E.; Kotov, A. B.; Sal'Nikova, E. B.; Anisimova, I. V.; Yakovleva, S. Z.

    2009-08-01

    The ophiolite complexes outcrop extending more than 700 km in the Dzhalair-Nayman zone located between the Chuya-Kendyk Tas (in the southwest) and Aktau-Dzhungar (in the northeast) Precambrian sialic massifs in South Kazakhstan. The most complete ophiolite sections are described in the central (Andassai massif) and southeastern (Dulankara massif) parts of the zone. Plagiogranites occur in a sheeted dyke complex in the upper part of ophiolite sections. A bimodal volcanic series occurs atop the section. The U/Pb zircons ages for the Andassai and Dulankara plagiogranites are 519 ± 4 and 521 ± 2 Ma, respectively. Ophiolites associated with bimodal volcanic series are also found in the north and northeast of Kazakhstan. They may form, together with the Lower Cambrian complexes of the Dzhalair-Nayman zone, a single paleostructure, whose fragments are well-traceable for over 2000 km

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

    NASA Astrophysics Data System (ADS)

    Falk, Elisabeth S.; Kelemen, Peter B.

    2015-07-01

    Extensive outcrops of listvenite-fully carbonated peridotite, with all Mg in carbonate minerals and all Si in quartz-occur along the basal thrust of the Samail ophiolite in Oman. These rocks can provide insight into processes including (a) carbon fluxes at the "leading edge of the mantle wedge" in subduction zones and (b) enhanced mineral carbonation of peridotite as a means of carbon storage. Here we examine mineralogical, chemical and isotopic evidence on the temperatures, timing, and fluid compositions involved in the formation of this listvenite. The listvenites are composed primarily of magnesite and/or dolomite + quartz + relict Cr-spinel. In some instances the conversion of peridotite to listvenite is nearly isochemical except for the addition of CO2, while other samples have also seen significant calcium addition and/or variable, minor addition of K and Mn. Along margins where listvenite bodies are in contact with serpentinized peridotite, talc and antigorite are present in addition to carbonate and quartz. The presence of antigorite + quartz + talc in these samples implies temperatures of 80-130 °C. This range of temperature is consistent with dolomite and magnesite clumped isotope thermometry in listvenite (average T = 90 ± 15 °C) and with conventional mineral-water oxygen isotope exchange thermometry (assuming fluid δ18O near zero). CO2-bearing fluids responsible for the formation of listvenite were likely derived from underlying calcite-bearing metasediment during emplacement of the ophiolite. An internal Rb-Sr isochron from one listvenite sample yields an age of 97 ± 29 Ma, consistent with the timing of emplacement of the ophiolite over allochthonous sediments of the Hawasina group, and autochthonous sediments of the Arabian continental margin. Most of the initial 87Sr/86Sr values in the listvenite, ranging from 0.7085 to 0.7135, are significantly higher than seawater values and consistent with values measured in the underlying metasediments

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

    NASA Astrophysics Data System (ADS)

    Khan, Shuhab D.; Mahmood, Khalid

    2008-08-01

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

  19. 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.

  20. 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

  1. Dismembered Archaean ophiolite in the southeastern Wind River Mountains, Wyoming: Remains of Archaean oceanic crust

    NASA Technical Reports Server (NTRS)

    Harper, G. D.

    1986-01-01

    Archean mafic and ultramafic rocks occur in the southeastern Wind River Mountains near Atlantic City, Wyoming and are interpreted to represent a dismembered ophiolite suite. The ophiolitic rocks occur in a thin belt intruded by the 2.6 Ga Louis Lake Batholith on the northwest. On the southeast they are in fault contact with the Miners Delight Formation comprised primarily of metagraywackes with minor calc-alkaline volcanics. The ophiolitic and associated metasedimentry rocks (Goldman Meadows Formation) have been multiply deformed and metamorphosed. The most prominant structures are a pronounced steeply plunging stretching lineation and steeply dipping foliation. These structural data indicate that the ophiolitic and associated metasedimentary rocks have been deformed by simple shear. The ophiolitic rocks are interpreted as the remains of Archean oceanic crust, probably formed at either a mid-ocean ridge or back-arc basin. All the units of a complete ophiolite are present except for upper mantle periodotities. The absence of upper mantle rocks may be the result of detactment within the crust, rather than within the upper mantle, during emplacement. This could have been the result of a steeper geothermal gradient in the Archean oceanic lithosphere, or may have resulted from a thicker oceanic crust in the Archean.

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

    NASA Astrophysics Data System (ADS)

    Karson, J. A.

    2014-12-01

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

  3. Obduction of Tethyan-type ophiolites-A case-study from the Thetford-Mines ophiolitic Complex, Quebec Appalachians, Canada

    NASA Astrophysics Data System (ADS)

    Tremblay, Alain; Ruffet, Gilles; Bédard, Jean H.

    2011-07-01

    Oceanic plagiogranites and peridotite-hosted granites (PHG) of the Thetford-Mines ophiolite (TMO) yield mean U-Pb zircon ages of 479.2 ± 1.6 Ma and 469.5 ± 2.8 Ma, and are interpreted to be the products of fractionation of basaltic magma and melting of the Laurentian margin, respectively. The existing 40Ar/39Ar age for the ophiolitic metamorphic sole of the TMO is a hornblende isochron age of 477 ± 5 Ma for amphibolite facies rocks. Sites previously sampled for U-Pb and 39Ar-40Ar analyses were resampled for more accurate 39Ar-40Ar dating, in order to better constrain the thermo-tectonic evolution of the ophiolite and underlying Laurentian margin rocks on which they were overthrust, and quantify the time required for obduction of Tethyan-type ophiolites in the Canadian Appalachians. Amphiboles from TMO gabbro and plagiogranite yield 39Ar-40Ar plateau ages of 477-475 Ma. In contrast, muscovites from PHG yield cooling ages of 466-465 Ma, and show evidence for thermal resetting at ca. 460 Ma for both a granitic mylonite and a crosscutting pegmatite facies. The amphibolitic metamorphic sole yields 39Ar-40Ar ages of ca. 471 Ma in amphibole, and high-temperature ages of ca. 466 Ma in muscovite, with evidence for recrystallization to ages as young as 457 Ma. Muscovites from the sub-ophiolitic Laurentian metasedimentary basement, and micaschist clasts in supraophiolitic debris flows of the TMO, yield similar pseudo-plateau and high-temperature ages of ca. 463-464 Ma. The U-Pb and 39Ar-40Ar data suggest that the obduction of peri-Laurentian oceanic crust lasted approximately 5 to 10 m.y. Since both facies formed approximately m.y. after the ophiolitic crust, this supports models whereby the still-hot ophiolitic mantle provided the heat needed to generate the PHG and the metamorphic sole between ca. 475 and 470 Ma. Muscovite ages of PHG record cooling below ca. 450-500 °C by ca. 465 Ma, but 39Ar-40Ar ages from both a mylonitized PHG facies and micaschist clasts suggest

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  5. Rotational deformation in the Jurassic Mesohellenic ophiolites, Greece, and its tectonic significance

    NASA Astrophysics Data System (ADS)

    Rassios, Anne E.; Dilek, Yildirim

    2009-03-01

    The Jurassic Pindos and Vourinos ophiolites in the Western Hellenides of Greece are part of the Mesohellenic mafic-ultramafic slab underlying the Eocene-Miocene sedimentary basin (Mesohellenic Trough). The tectonic incorporation of this oceanic slab into the western edge of the Pelagonian subcontinent occurred via trench - passive margin collision in the late Jurassic. Much of the tectonic architecture of these ophiolites, particularly Vourinos, was acquired during progressive inhomogeneous deformation associated with the initial displacement of the Jurassic oceanic crust from its original igneous environment of formation, and its subsequent tectonic emplacement eastward onto the Pelagonian margin. The heterogeneous deformation in the mantle and crustal sequences of the Pindos-Vourinos ophiolites occurred in ductile, ductile-brittle, and brittle fields synchronously, as the Jurassic oceanic lithosphere was translated eastward; it also resulted in differential horizontal rotations within the displaced oceanic slab. Areas retaining high-temperature (diapiric) mantle fabric appear to have been "passively" translated by lower temperature ductile shearing, commonly along mylonite zones. Individual dunite bodies in the harzburgite tectonites indicate movement distances of at least kilometer scale. Pervasively mylonitic domains within the upper mantle peridotites suggest elongation on the order of five to ten times in the direction of ophiolite emplacement. Seafloor-spreading related, high-temperature mantle structures within the Pindos and Vourinos ophiolites are sub-parallel. Imprinted ductile kinematic indicators and ductile shear zones pervasive to the mantle and lower crustal sections in both ophiolites are also sub-parallel, consistent with the direction of tectonic vergence, and appear traceable across the sedimentary overburden of the Mesohellenic Trough. These geometric relations imply that the Pindos-Vourinos ophiolites retain their relative orientations from

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

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

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

  7. 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 100-200μm in olivine bands vs 20-50μm 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

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

    USGS Publications Warehouse

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

    1991-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

  10. Original setting and emplacement history of the Zambales ophiolite, Luzon, Phillipines, from stratigraphic evidence

    NASA Astrophysics Data System (ADS)

    Schweller, W. J.; Karig, D. E.; Bachman, S. B.

    A detailed study of sedimentary rocks associated with the Zambales ophiolite outlines its original setting and uplift history. The oldest sedimentary unit is upper Eocene pelagic limestone with thin ash layers that depositionally overlies the volcanic complex of the ophiolite. This limestone, the Aksitero Formation, was deposited at 1-4 km depth on newly formed ocean floor and has remained isolated from coarse volcanic and continental detritus for several million years. A comparison of this limestone to numerous DSDP sites from the western Pacific shows that the Aksitero Formation resembles pelagic sediments from marginal basins, but not those of island arcs. Sedimentation rates increase from 3-5 m/m.y. in the late Eocene to about 10 m/m.y. in the middle to late Oligocene, as volcaniclastic turbidites begin to dilute the pelagic limestone. Thick lower Miocene sandstones change from volcaniclastic to ophiolitic composition over a few million years, indicating rapid uplift and erosion of the ophiolite during this time. Seismic reflection profiles in the Central Valley, just east of the Zambales Mountains, show as much as 2 km of pre-Miocene strata onlapping the buried eastern flank of the ophiolite. This onlap apparently represents original bathymetric relief of the oceanic crust. The seismic reflection profiles also demonstrate that eastward tilting of the ophiolite began in the early Miocene and continued through the late Miocene. The uplift and eastward tilting is believed to be related to subduction of the Manila Trench, although the cause of the initial detachment of the ophiolite may be tied to transform faulting along the present western edge of the Zambales Mountains.

  11. 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.

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

    USGS Publications Warehouse

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

    1981-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  14. Kinematics and Ophiolite obduction in the Gerania and Helicon Mountains, central Greece

    NASA Astrophysics Data System (ADS)

    Kaplanis, A.; Koukouvelas, I.; Xypolias, P.; Kokkalas, S.

    2013-06-01

    New structural, petrofabric and palaeostress data from the Beotia area (central Greece) were used to investigate the tectonic evolution of the suture zone between the External (Parnassus microplate) and Internal Hellenides (Pelagonian microplate). Petrofabric studies of ultramafic rocks were done using conventional U-stage analysis and the electron backscatter diffraction (EBSD) technique. Detailed structural analysis enabled us to distinguish three main deformation phases that took place from the Triassic to the Eocene. Triassic-Jurassic deformation is related to continental rifting and the progressive formation of an ocean basin. Ophiolites formed above a westward-dipping supra-subduction zone (SSZ) in the Early-Late Jurassic. Trench-margin collision resulted in the southeastward emplacement of the ophiolite nappe over the Pelagonian margin. There is also evidence for a north-westward thrusting of ophiolitic rocks over the Gerania and Helicon units during Berriasian time. This latter tectonic process is closely related to the deposition of "Beotian flysch" into a foreland basin. An extensional phase of deformation accompanied by shallow-water carbonate sedimentation is documented in the Upper Cretaceous. Later, during Paleocene the area was subjected to a compressional deformation phase characterised by SW-directed thrusting and folding, as well as NE-verging backthrusts and backfolds. Our proposed geotectonic model suggests the consumption of the ocean between the Parnassus and Pelagonian microplates. This model includes Late Jurassic eastward ophiolite obduction followed by Early Cretaceous west directed ophiolite thrusting.

  15. 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.

  16. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2007-04-01

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

  18. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  20. 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

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

    NASA Astrophysics Data System (ADS)

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

    2014-08-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-01-01

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

  3. Melt Migration in the Mantle Lithosphere: Evidence From Ophiolitic Peridotites

    NASA Astrophysics Data System (ADS)

    Spagnolo, G.; Piccardo, G. B.; Poggi, E.

    2006-12-01

    Records of diffuse porous flow migration of asthenospheric melts through the lithospheric mantle are evident in mantle peridotites deriving from the oceanic lithosphere of the Jurassic Ligurian Tethys, exposed in the Alpine- Apennine orogenic system of Northern Italy. The migrating melts caused structural and chemical modifications, as a consequence of melt/peridotite interaction. Microstructures indicating pyroxene(Px)-dissolving/olivine(Ol)-forming reactions suggest that early percolating melts were Px(-silica)-undersaturated and their intergranular flow through the peridotite enhanced melt/peridotite interaction. Px dissolution modified: 1) the peridotite composition: in fact, the reacted peridotites changed their bulk rock characteristics to significantly SiO2-depleted, MgO-enriched compositions, and their mineral modal contents to significantly Ol-enriched compositions, with respect to any refractory residua after any kink of mantle partial melting; 2) the melt composition: in fact, the melt composition progressively attained Px(-silica)-saturation at the end of the reactive percolation, as evidenced by late Px interstitial crystallization. Depending on the degree of Px dissolution, the reacted peridotites from the same peridotite body have highly variable Px contents but their clinopyroxenes(Cpx) have closely similar trace element contents. This decoupling between mineral modal content and geochemical composition strongly suggests that these peridotites cannot have been originated by partial melting but it supports the evidence of melt/peridotite interaction. Thus, Cpx trace element composition depends on the geochemical equilibration with the percolating melt; it indicates, moreover, the MORB affinity of the percolating melt. Significant evidences of melt migration through lithospheric peridotites are represented by the plagioclase(Plg)- enriched peridotites, which are frequently present within the ophiolitic peridotites and particularly abundant in those

  4. 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

  5. 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.

  6. 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.

  7. 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.

  8. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    Ophiolites are interpreted as remnants of oceanic lithosphere. Many have a so-called supra-subduction zone (SSZ) geochemical signature, suggestive of formation at a spreading ridge overlying a subduction zone. Supra-subduction zone ophiolites frequently have a several-hundred-meter thick sequence of metamorphic rocks below their mantle section: the metamorphic sole. These dominantly mafic and generally heavily sheared metamorphic rocks have been shown to preserve an inverted metamorphic gradient with the highest pressures and temperatures at the top of the sole, decreasing downwards. Pressure estimates from rocks found at the top of metamorphic soles may be as much as 10-15 kbar with temperature estimates up to 875°C. The metamorphic grade varies from greenschist near the base, up to granulite facies at the top, with the bulk comprising of amphibolite facies rocks. At some locations a blueschist overprint of the amphibolite facies mineral assemblages has been described. The relative high pressures preserved in the metamorphic sole cannot simply result from overburden pressure of the currently overlying ophiolite, which is a long-standing problem. This raises the question of what process(es) can explain pressures up to 10-15 kbar in the top of metamorphic soles, in relation to the approximately synchronous formation of the SSZ oceanic lithosphere above the sole. One of the places to study the formation of SSZ ophiolites and their metamorphic soles is the Neotethyan Suture zone. Remnants of Neotethyan lithosphere are preserved as ophiolites that are discontinuously exposed from the Mediterranean region through the Himalaya to SE Asia. Supra-subduction zone ophiolites are particularly widespread in Turkey. The Pinarbaşi ophiolite is located in the SE of Central Anatolia, and overlies the Tauride fold-and-thrust belt that formed since the Late Cretaceous. It comprises mantle tectonites consisting of serpentinized harzburgite and dunite with remnants of gabbro to the

  10. 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

  11. Reappraisal of Peri-Arabic ophiolites and geodynamics: Why the Kermanshah ophiolite (SW Iran) is a Paleocene-Eocene magmatic arc at the foot of Eurasia

    NASA Astrophysics Data System (ADS)

    Whitechurch, Hubert; Omrani, Jafar; Agard, Philippe; Humbert, Fabien; Montigny, Raymond; Jolivet, Laurent

    2013-04-01

    The nature and significance of the Kermanshah ophiolite (Zagros, Iran), classically identified as one of the few remnants of the Peri-Arabic ophiolite system obducted onto Arabia during the Late Cretaceous, are reinvestigated in this study. Systematic major and trace element geochemistry was performed, mainly on magmatic rocks, from two distinct areas: the Kamaryan Paleocene to Eocene arc and the so-called Harsin-Sahneh "ophiolites". Both domains display low to medium-K calc-alkaline signatures with variably negative anomalies in Nb, Ta, and Ti and positive ones in Sr, Ba, Th, and U. The magmatic activity of the Palaeocene-Eocene magmatic group shows an evolution through time, with a geochemical signature close to tholeiitic Back-Arc Basin-Basalts (BAB) for Palaeocene rocks and a clear calc-alkaline arc signature for Eocene volcanics. The presumably ophiolitic gabbros of the Harsin-Sahneh complex intruding harzburgites, as well as the associated dykes, also show a BAB geochemical signature. Overall, field relationships and geochemical patterns suggest that these rocks were emplaced on a mantle substratum close to the ocean-continent transition. This Palaeocene-Eocene magmatic activity in Kermanshah, which extended further to the north-west into Turkey, coincided with a marked slowing of the Arabia/Eurasia convergence. It furthermore occurred after the stopping of the Sanandaj-Sirjan magmatism (Mesozoic arc) but before the development of the Urumieh-Dokhtar magmatic arc (Tertiary arc). We relate this transient magmatic activity to slab retreat and back-arc extension at the foot of the Eurasian margin and to lithospheric-scale reconstructions across the Neotethys between Turkey and Iran.

  12. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

    Samar island in the eastern part of Central Philippines is underlain by a complete ophiolite suite, the Samar Ophiolite. We present the first geochronological and paleomagnetic data for the Samar Ophiolite. Whole rock K-Ar dating of two basalt samples yielded an age of 100.2 ± 2.7 Ma and 97.9 ± 2.8 Ma. Thirteen sites in four localities yielded characteristic remanent magnetization with in situ direction of D = 340°, I = - 24°, k = 15, α95 = 11° and tilt-corrected direction of D = 342°, I = - 27°, k = 15, α95 = 11°. These values suggest that the ophiolitic basement rocks of Samar formed in the Late Cretaceous at a paleolatitude of 14°S ± 6°. The paleolatitude is several degrees south of the sub-equatorial positions calculated for the three other Mesozoic ophiolites of the Philippine Mobile Belt (PMB) whose paleomagnetism had been previously studied. The PMB ophiolites in eastern and central Philippines share a common age, geochemistry and paleolatitude with the Halmahera Ophiolite, suggesting that they originated from a Mesozoic supra-subduction zone that spanned a few degrees north of the equator to around 15°S.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

    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.

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

    NASA Astrophysics Data System (ADS)

    Morris, Antony; Maffione, Marco

    2016-04-01

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

  18. 3D Geometry of Fault Intersections in Central California: The Importance of Ultramafic Ophiolitic Rocks

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  19. 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 εHf(t) (+ 15.0 to + 15.9) and mantle-like δ18O (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.

  20. 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

  1. Petrogenesis of the Khabr-Marvast Tectonized Ophiolite in the middle part of Nain-Baft Ophiolite belt, NW Shahrebabak, Iran

    NASA Astrophysics Data System (ADS)

    Soltanmohammadi, A.; Rahgoshay, M.; Khalatbari-Jafari, M.

    2009-05-01

    The Late Cretaceous Khabr-Marvast tectonized Ophiolite, exposed in the middle part of Nain-Baft ophiolite belt, at the South edge of the Central Iran micro-continent. Intrusive sequence of this association comprise by: serpentinized harzburgite, isotropic gabbro, diabase sheeted dikes complex, which intruded by isolated diabase dikes, pegmatite gabbro and trondjhemite. Extrusive sequence are included: pillow lava, sheetflow and hyaloclastic breccias at passing upward. The relationship between the units of this association are tectonized.The primitive mantle-normalized multi-element plot (Sun and McDonough, 1989) indicate moderate enrichement in the LILE and show clear depletion for Nb in all samples, and depletion for Ta and Ti in most of samples. The chondrite-normalized multi-element plot indicate different enrichment of samples from LREE to HREE, and could be compare with subduction related magmatism.The variable rare earth elements patterns of the Khabr-Marvast tectonized ophiolite may reflect presence of heterogeneous source to generation of these rocks.The high Ba/Th(209-1460) ratios, low Th/Ta(0.5-1.7) and Nb/Ta(2.02-4.04) ratios in sheeted dikes and pegmatite gabbros, indicate more participant of fluids with partial melting of depleted mantle wedge in a subduction zone (Pearce et al., 2005; König et al., 2008 ). In contrast, sheetflows and isolated diabase dikes and trondjhemites have lower Ba/Th(20-170) ratios and higher Th/Ta(7.9-16.1), Nb/Ta(12.12-24) ratios which indicative of partial melting of depleted mantle and lower fluids to participating in generation of these rocks.The geological and geochemical studies suggest that the Khabr-Marvast tectonized Ophiolite had heterogeneous source and form in a supra-subduction zone system which is consistent with the model that assumes subduction of the Arabian plate under the Central Iran micro-continent.Keywords: Tectonized Ophiolite, Central Iran micro-continent, heterogeneous source, supra-subduction system.

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

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    The process of serpentinization in the subsurface produces highly reduced, high pH fluids that provide habitats for microbial communities. At depth, these fluids likely contain appreciable hydrogen and methane gas, very little inorganic carbon, and limited electron acceptors. As fluids affected by serpentinization reach the oxygenated surface environment, microbial biomes may shift to include organisms capable of metabolizing oxygen. Our study of serpentinizing and hydrothermal fluids in the Zambales and Palawan regions of the Philippines includes a suite of geochemical analyses to define microbial habitats, targeted culturing efforts, and community analysis (the latter presented in a companion abstract). We measured temperature, pH, conductivity, ORP, dissolved oxygen, nitrate, ammonia, iron, sulfide, and silica in the fluids on site, and have analyzed dissolved gases, major ions, and trace elements. These analyses provide a description of the near subsurface and surface ecosystems of the study areas. Carbon and nitrogen isotopic ratios in solid biofilm and biominerals from the springs allow inference of metabolic cycling. Fluids expressed from the Zambales and Palawan ophiolites range from pH 9-11.5, 0.06-2 mg/L dissolved oxygen, 0-3.7 mg/L sulfide, and 30-800 mg/L silica in the source pools. Temperatures vary from 34-50C. Analysis of dissolved gases reveal concentrations of H2 and CH4 > 10uM, CO2 > 1 mM, and trace amounts of CO. This variability in fluid and gas composition indicates the potential for different mixing ratios with local meteoric water, local hydrothermal fluids, and deep-seated sediments. Results will be compared with other serpentinizing fluids to place these habitats in context of the ecology of serpentinizing springs globally. Anaerobic and aerobic growth media were designed to target heterotrophy, sulfate reduction, and iron reduction (the latter two with, and without organic carbon input). We have calculated that these processes have

  6. 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

  7. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  10. 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

  11. 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

  12. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

  14. Constraints on the magmatic evolution of the oceanic crust from plagiogranite intrusions in the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Haase, Karsten M.; Freund, Sarah; Beier, Christoph; Koepke, Jürgen; Erdmann, Martin; Hauff, Folkmar

    2016-05-01

    We present major and trace element as well as Sr, Nd, and Hf isotope data on a suite of 87 plutonic rock samples from 27 felsic crustal intrusions in seven blocks of the Oman ophiolite. The rock compositions of the sample suite including associated more mafic rocks range from 48 to 79 wt% SiO2, i.e. from gabbros to tonalites. The samples are grouped into a Ti-rich and relatively light rare earth element (LREE)-enriched P1 group [(Ce/Yb) N > 0.7] resembling the early V1 lavas, and a Ti-poor and LREE-depleted P2 group [(Ce/Yb) N < 0.7] resembling the late-stage V2 lavas. Based on the geochemical differences and in agreement with previous structural and petrographic models, we define phase 1 (P1) and phase 2 (P2) plutonic rocks. Felsic magmas in both groups formed by extensive fractional crystallization of olivine, clinopyroxene, plagioclase, apatite, and Ti-magnetite from mafic melts. The incompatible element compositions of P1 rocks overlap with those from mid-ocean ridges but have higher Ba/Nb and Th/Nb trending towards the P2 rock compositions and indicating an influence of a subducting slab. The P2 rocks formed from a more depleted mantle source but show a more pronounced slab signature. These rocks also occur in the southern blocks (with the exception of the Tayin block) of the Oman ophiolite implying that the entire ophiolite formed above a subducting slab. Initial Nd and Hf isotope compositions suggest an Indian-MORB-type mantle source for the Oman ophiolite magmas. Isotope compositions and high Th/Nb in some P2 rocks indicate mixing of a melt from subducted sediment into this mantle.

  15. 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.

  16. 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

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

    NASA Astrophysics Data System (ADS)

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

    2008-01-01

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

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

    USGS Publications Warehouse

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

    1998-01-01

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

  19. 40Ar/ 39Ar dating of the emplacement of the Muslim Bagh ophiolite, Pakistan

    NASA Astrophysics Data System (ADS)

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

    1995-11-01

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

  20. 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.

  1. 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.

  2. 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…

  3. Petrogenetic and Geotectonic Study of Early Formed (Triassic?) Volcanics in Ophiolitic Complexes in Central Greece: Examples from Koziakas, Othris, Iti and Kallidromo Regions

    NASA Astrophysics Data System (ADS)

    Koutsovitis, P.; Magganas, A.; Pomonis, P.; Karipi, S.; Tsikouras, B.

    2009-04-01

    The early formed volcanics occurring in ophiolitic complexes in Central Greece are suggested to be mostly of Triassic age. They occur within Triassic sedimentary sequences as well as in mélange formations, being interpreted to be parts of the Pindos Ocean, which formed after breakup of a Gondwana continental margin. In the studied regions of Koziakas, Othris, Iti and Kallidromo the volcanic rocks consist mainly of pillow lavas with extreme variable geochemical affinities (alkaline, tholeiitic and calc-alkaline) and in rare cases ultramafic lavas. The igneous sequence is complemented by another wide compositional range rocks consisting of wehrlites, mafic subvolcanic rocks (picrites, transitional boninites, dolerites) and pyroclastic tuffs compositionally intermediate to felsic. The alkaline lavas are present in Othris in close association with tholeiitic pillow lavas and within mélange formations in the regions of Koziakas, Iti and Kallidromo. In the Triassic formations of Othris most of the igneous rocks are represented by pillow lavas which are mainly tholeiitic, ranging in composition from E-MORB (most lavas) to IAT types. Similar E-MORB volcanic rocks are not present in Koziakas, Iti and Kallidromo regions. The calc-alkaline lavas have a broad composition which varies from basaltic to trachyandesitic and felsic. They occur sandwiched between middle Triassic sedimentary formations in Koziakas, in melange formations in central Othris, while they are missing in Iti and Kallidromo. The ultramafic lavas, which are accompanied by picrites, transitional boninites and dolerites, are found in Othris only, intruding the E-MORB pillow lavas. Geochemical and mineralogical data evaluation as well as petrogenetic modelling show the formation of the studied compositionally wide, unusual rock association can be explained through complex and multistage events, which have taken place in a rift- and subduction-related geotectonic environment of the Pindos Ocean in Middle

  4. 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

  5. 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

  6. 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.

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

    SciTech Connect

    Hagstrum, J.T. )

    1992-06-01

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

  8. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  10. 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

  11. 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

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

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

    The Northern Cordillera of the Dominican Republic records the (oblique) subduction and collisional processes occurred between the Caribbean and North-American plates during Upper Cretaceous to Lower Paleogene times. The boundary between these two plates can be traced within this range disrupted by an Upper Paleogene to present intense left-lateral strike-slip tectonism, onset after collision. In the western part of the range this boundary might be defined by the Camu fault. In the coastal area of Puerto Plata, located on the northern block of the Camu fault, basement rocks belonging to the subducting plate (the Puerto Plata Basement Complex) and several related units probably formed in an accretionary prism, preserve in a large (300 km2) outcrop of chaotic formations presumably formed (and exhumed) during collision. They include from broken formations and tectonic melanges to olistostromes and other coeval sedimentary deposits. The Puerto Plata Basement Complex (PPBC) consists of highly faulted and dismembered blocks formed by discontinuous but sometimes coherent outcrops of serpentinized or massive peridotite, pods of ultramafic cumulates, massive or banded gabbros and Los Caños Fm, a thick sequence of gross bedded volcaniclastic material with interbedded basaltic (sometimes pillowed) or andesitic flows. All these rocks bear low grade metamorphism and lack a general deformation fabric apart from occasional transformation to mylonites due to localized shearing. The PPBC has been interpreted as a fragment of oceanic crust, belonging to the subducting (North-American) slab that has been exhumed as a tectonic melange or a broken complex. The Imbert Fm, of Palaeocene-Eocene age, is formed by a well bedded succession of white very fine grained porcelaneous tuffs, with eventual intercalations of cherts, limestones and marls that, towards the lower part, is interbedded with volcanic-derived graywackes and limolites, and more occasionally, thick beds of conglomerates and

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

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

    NASA Astrophysics Data System (ADS)

    Xiong, Yingqian

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

  16. 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.

  17. 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

  18. 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.

  19. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-09-01

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

  1. 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.

  2. 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

  3. 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

  4. U/Pb ages of ophiolites and arc-related plutons of the Norwegian Caledonides: implications for the development of Iapetus

    NASA Astrophysics Data System (ADS)

    Dunning, G. R.; Pedersen, R. B.

    1988-01-01

    U/Pb zircon ages are reported for four ophiolites and three crosscutting arc-related plutons from the Norwegian Caledonides. Plagiogranite differentiated from gabbro of the Karmøy ophiolite is dated at 493+7/-4 Ma whereas arc-related trondhjemite cutting this ophiolite crystallized at 485+/-2 Ma. A crosscutting clinopyroxene-phyric gabbro intrusion is dated at 470+9/-5 Ma by near concordant magmatic titanite (sphene) and discordant U-rich (2903 6677 ppm) zircon. Lower intercepts of 247+/-68 and 191+/-70 Ma defined by the plagiogranite and clinopyroxene-phyric gabbro best-fit lines may reflect a real low-T alteration/rift-related event. A plagiogranite differentiate of the Gullfjellet ophiolite complex is dated at 489+/-3 Ma and a crosscutting arc-related tonalite is 482+6/-4 Ma. Both of these ages overlap with those of the correlative rocks at Karmøy suggesting that they are parts of one ophiolitic terrane with a common history. Trondhjemite associated with the Leka ophiolite is dated at 497+/-2 Ma, indicating that supra-subduction zone magmatism there may be coeval with spreading which formed the Karmøy axis sequence. The U/Pb zircon ages of Norwegian ophiolites reported here, combined with ages of other Appalachian-Caledonian ophiolite complexes in Britain and Canada, indicate a narrow age range for the generation of at least two marginal basins in the Tremadoc-Arenig. Two spreading episodes documented at Karmøy are separated in time by intrusion of arc-related trondhjemite magmas at 485+/-2 Ma and may correlate with two separate spreading events documented in other ophiolites. The Solund/Stavfjorden ophiolite, at 443+/-3 Ma, is the only late Ordovician ophiolite yet documented in the entire Appalachian-Caledonian Orogen and it probably represents a small, short-lived marginal basin late in the history of the Iapetus Ocean. It is correlative with Caradocian ensialic marginal basin magmatism in Wales and the Trondheim region, and with tholeiitic gabbro

  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

  6. Petrology of the Hegenshan ophiolite and its implication for the tectonic evolution of northern China

    NASA Astrophysics Data System (ADS)

    Nozaka, Toshio; Liu, Yan

    2002-08-01

    Petrographic observations, and mineralogical and geochemical analyses, have revealed that the Hegenshan ophiolite is of mid-ocean ridge origin and has been subjected to dynamothermal metamorphism at medium P/T conditions. The metamorphism is characterized by a prograde change in paragenesis from the greenschist to epidote-amphibolite facies, with peak temperature conditions of 570-640°C at pressures of 4-10 kbar. The amphiboles formed by this metamorphism show K-Ar ages of 110-130 Ma. The metamorphic conditions and K-Ar ages suggest that the Hegenshan ophiolite is located at the suture between the Siberian and North China continental blocks, where the continental collision in this area took place in middle Mesozoic time. Given the temporal and spatial distribution of the igneous activity around the Da Hinggan Ling Mountains, it is suggested that the extensive Yanshanian magmatism in this region resulted predominantly from a southward subduction of an oceanic plate prior to collision. Alternatively, it may possibly have resulted from the collision itself, at the final stage.

  7. Palaeomagnetic studies within the Ballantrae Ophiolite; southwest Scotland: magnetotectonic and regional tectonic implications

    NASA Astrophysics Data System (ADS)

    Trench, A.; Bluck, B. J.; Watts, D. R.

    1988-11-01

    The Early Arenig Slockenray Formation within the Ballantrae Ophiolite, southwest Scotland, displays a multivectorial magnetisation structure. Two components (S and M) are identified delineated by differing blocking temperature/ coercivity spectra. Component S is removed around 200°C/10 mT, and is regarded to be of recent viscous origin. Component M forms the characteristic formation magnetisation and resides in both magnetite and haematite. Extensive sampling of all exposed lithologies reveals an (in situ) non-Fisherian distribution of the characteristic magnetisation defining an envelope from SE moderate positive to SW shallow negative directions. A negative infra-formation conglomerate test identifies this component as a pervasive overprint. A second conglomerate test performed in the overlying Benan Conglomerate of Llandeilo age, reveals dispersely directed magnetisation with a stability range equivalent to that of component M. This field test therefore defines a maximum remagnetisation window of 30 million years for the characteristic remanence. "Hard" viscous magnetisations are identified both in the Benan Conglomerate and at some sites within the Slockenray Formation. Structurally corrected site mean results from the Slockenray Formation define a non-Fisherian distribution and form a small circle partial arc centred on a vertical axis (NW moderate positive to SW moderate positive directions). A combined palaeomagnetic fold and fault test suggests that acquisition of component M pre-dates both folding and faulting. The resulting palaeolatitude of remanence acquisition (28.8°S) implies a tectonic position close to the southern Laurentian margin for the Ballantrae ophiolite in Arenig times.

  8. Age and petrogenesis of the Sarmiento ophiolite complex of southern Chile

    NASA Astrophysics Data System (ADS)

    Stern, C. R.; Mukasa, S. B.; Fuenzalida P., R.

    1992-08-01

    Zircon fractions separated from fine-grained plagiogranites, interpreted to be cogenetic with the mafic rocks of the Sarmiento ophiolite complex in southern Chile, yield slightly to grossly discordant age patterns for which the lower concordia intercept U-Pb ages of 140.7 ± 0.7 Ma (Lolos Fjord) and 137.1 ± 0.6 Ma (Encuentro Fjord) are well constrained. These dates are interpreted as formation ages for the northern portion of the igneous floor of the Rocas Verdes basin, and they are younger than the age of 150 Ma determined for a more southern portion of the floor of this basin on South Georgia Island. Coarse-grained trondjemites within the gabbro units of the Sarmiento complex yield a lower concordia intercept U-Pb age of 147 ± 10 Ma and a poorly defined upper intercept reflecting an inherited zircon component, possibly of Proterozoic age. These rocks are interpreted as remobilized fragments of country rocks entrapped within the essentially mantle-derived rocks of the ophiolite complex.

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

    NASA Astrophysics Data System (ADS)

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

    2014-02-01

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

  10. Genesis of the Smartville arc-ophiolite, Sierra Nevada foothills, California

    NASA Technical Reports Server (NTRS)

    Menzies, M.; Blanchard, D.; Xenophontos, C.

    1980-01-01

    Rare earth element analyses of metavolcanic rocks from the Smartville, Calif. ophiolite divide the uniform suite of pillowed and massive lavas into the lower part consisting of massive, brecciated tholeiites, an intrusive dike-sill complex, and part of a plutonic suite. The tholeiites are light REE depleted with a (Ce)N range of 6.5 to 26.0 and (Yb)N of 6.0 to 30.0; it is suggested that the tholeiites were produced by partial melting of a LREE depleted source similar to MORB. The upper part of the volcanic pile is comprised of basaltic-andesitic flows and interbedded coarse to fine volcanic sediments; these 'calc-alkaline' rocks are light REE enriched with the (Ce)N range of 17.0-28.0 and (Yb)N of 8.0 to 12.0. Finally, petrographic investigations of volcanic sediments and sulfide ore deposits in the lava show that the ophiolite formed near a group of active submarine and subareal volcanoes.

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

    NASA Astrophysics Data System (ADS)

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

    2001-06-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2005-12-01

    In Japan, an older ophiolite thrust onto younger ophiolite with tectonic intercalation of accreted oceanic sediments (chert, limestone, shale and sandstone forming _gocean plate stratigraphy_h deposited on the basaltic basement) or their high-P/T metamorphosed varieties. For example, the Yakuno ophiolite (SW Japan) of early Permian igneous age and supra-subduction zone (SSZ) origin (Ichiyama & Ishiwatari, Island Arc, 13, 157-) is tectonically underlain by the Ultra-Tamba nappe (chert, shale, sandstone) accreted in Late Permian, which is further underlain by the Tamba nappes (greenstone, chert, limestone, shale and sandstone) accreted in Jurassic. Major occurrence of the greenstones (mainly Permian) in the Upper Tamba nappe (consisting of 3 sub-nappes) is more than 1 km thick intact sheet of >200 km extension forming the structurally basal part of each sub-nappe, originated in an oceanic edifice composed of pillow lava, massive lava, hyaloclastite and dikes (_gBasal TypEh). Another minor occurrence is greenstone fragments of a few cm to 100 m size in the muddy matrix (_gMixed TypEh), constituting structurally upper part of each sub-nappe. The Basal Type greenstones show uniform E-MORB affinity, but the Mixed Type ones show diverse features such as N-MORB, OIT and OIA. This clear correlation between the occurrence of greenstones and their chemistries suggests the accretion of thick crust of oceanic plateau (E-MORB) to make Basal Type greenstones and the accretion of thin normal oceanic crust (N-MORB) with disseminated small seamounts (OIT and OIA) to make Mixed Type greenstones (Koizumi & Ishiwatari, Island Arc, in submission.). We discovered HFSE-rich picrite (meimechite) sills and hyaloclastites as well as ferropicrite and picritic ferrobasalt dikes emplaced in the Basal Type greenstones and its chert-dolomite cover of Late Permian age. Zr/Y and Ti/Al signatures of these ultramafic volcanic rocks are intermediate between Polynesian picrites and Siberian meimechites

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

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2001-08-01

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

  15. 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

  16. Tectono-metamorphic processes beneath an obducted ophiolite: evidence from metamorphic soles and accreted units from western Turkey

    NASA Astrophysics Data System (ADS)

    Plunder, A.; Agard, P.; Chopin, C.; Okay, A. I.; Whitechurch, H.

    2015-12-01

    The convergence between the Anatolide Tauride block and Eurasia during Cretaceous times lead to the closure of a branch of the Neotethyan ocean and to ophiolite obduction. After reconstruction, obducted ophiolite and their sub-ophiolitic units can be found along a 200 kilometre-long north to south transect in western Anatolia. If related to a single event, the dimension of this ophiolite makes it an exceptional object of interest to study obduction and early subduction dynamics. This contribution aim to: (1) (re)-appraise the metamorphic pressure-temperature (PT) conditions and evolution of the sub-ophiolitic units of western Anatolia; (2) reconstruct the Anatolian ophiolite and (3) 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 of is this sole is made of garnet ± clinopyroxene amphibolites. In the northern part the sole is characterised by an important blueschist-facies overprint destabilizing the amphibolite paragenesis whereas it is lacking in the south. PT conditions were refined at 10.5 kbar and 780°C for the south and at 11 kbar and 725°C using pseudosection modelling. Field and petrological observations recognize three principal units in the accretionary complex (from top to bottom, OC1, 2 and 3) 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 pristine oceanic rocks to very low grade metamorphics rocks (lawsonite - pumpelltyite facies). Blueschist facies rocks including Fe-Mg carpholite-bearing layers were found in OC2 and attest high-pressure and low-temperature conditions (~10 kbar - 350°C). OC3 exhibit a clear blueschist facies metamorphism, but higher PT conditions (17 kbar - 450°C). Both OC2 and 3 were only found in the northern area close to the suture zone. Combining these data and recent advances

  17. 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

  18. 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.

  19. 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

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  1. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2002-01-01

    Chromites separated from the upper mantle or lower crustal portions of 18 ophiolites ranging in age from 900 Ma to 50 Ma are examined for Re-Os isotopic systematics. The ophiolites include both MORB and back arc types, although most are from supra-subduction zone (SSZ) settings. The chromites are robust indicators of the initial Os isotopic compositions of the systems sampled. There is very limited range in calculated initial γ Os values, with the entire group averaging +1.31. Least squares linear regression of the age of chromite formation (in Ga) versus initial 187Os/ 188Os of a filtered suite yields a slope of -0.0058±0.0019 (2σ) and a present day intercept of 0.12809±0.00085 (2σ), equivalent to a γ Os value of +0.9±0.6. Of the suite of 51 samples analyzed, 68% lie within ±1% of this evolution trajectory. Although most of the samples formed in SSZ environments, there is little evidence to suggest modification of the mantle Os isotopic composition via radiogenic melts or fluids derived from subducting slabs. The ophiolite data are interpreted as representative of the convecting upper mantle and suggest that the present isotopic composition of the convecting upper mantle averages approximately 1.2% less radiogenic than the estimated minimum composition of the primitive upper mantle of 0.1296±8 (Meisel et al., 2001). The most likely explanation for the difference is the formation, subduction and isolation of some portion of the mafic oceanic crust. Using models based on the assumption that the convecting upper mantle comprises 50% of the total mass of the mantle, and that the average isolation period for subducted oceanic crust is 1.5 to 2.0 Ga, it is estimated that approximately 2 to 3% of the total mass of the mantle is composed of subducted mafic oceanic crust that remains isolated from the convecting upper mantle. Because the isotopic compositions of the DMM and PUM overlap within uncertainties, however, the results do not require any isolated slab

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

    NASA Astrophysics Data System (ADS)

    Ishiwatari, Akira; Ichiyama, Yuji; Ganbat, Erdenesaikhan

    2013-04-01

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

  4. 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

  5. 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

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

    NASA Astrophysics Data System (ADS)

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

    2015-06-01

    Petrographic and geochemical analyses of partly and completely serpentinized and carbonated peridotites within the ultramafic section of the Leka Ophiolite Complex have been used to elucidate the evolution of alterations and identify possible fluid sources. The alterations show no evidence for any major deformation and are located along low-angle structures that were formed in a late stage of the structural evolution of the ophiolite complex. Modeling of mineral equilibria in the SiO2-MgO-FeO-Fe2O3-CaO-H2O ± CO2 system has been utilized to constrain the conditions during serpentinization and carbonation. The partly altered peridotites consist of the mineral assemblage olivine-clinopyroxene-serpentine-magnetite-brucite and formed at temperatures < 410 °C by infiltration of aqueous fluids. Completely serpentinized rocks with the mineral assemblage serpentine-magnesite-magnetite-dolomite formed at temperatures < 500 °C and low XCO2 (≤ 0.05) by the breakdown of the minerals in the partly altered peridotites. Talc-carbonate rocks formed by the breakdown of the serpentine in the previously formed serpentinite rock at temperatures < 550 °C and elevated XCO2, which resulted in the stabilization of the assemblage talc-magnesite-magnetite-dolomite. Carbon isotope values determined for dolomite from crosscutting carbonate lenses within the talc-carbonate rock yield δ13C values of ~- 5 indicative of a mantle source for the carbon required for the carbonation. Oxygen isotope values δSMOW18O of ~ 10.8-11.3‰ together with initial 87Sr/86Sri = 400Ma values of 0.7029 and 0.7063, suggest dehydration of rocks with mantle affinity as a source for the fluids. Based on analytical results and field observations we propose that the formation of the talc- and carbonate-bearing alteration zones is caused by the focused infiltration of fluids that originated at the bottom of already partly serpentinized ophiolite complex during extension-driven burial at the late stage of the

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  8. 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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  10. The geologic history of the Nicoya Ophiolite Complex, Costa Rica, and its geotectonic significance

    NASA Astrophysics Data System (ADS)

    Kuijpers, Eric P.

    1980-10-01

    Costa Rica forms part of an intra-oceanic arc between the Pacific and Caribbean oceans; the Nicoya Ophiolite Complex is located along its Pacific border. In this study, evidence is given that the Nicoya Complex is composed of ridge-formed oceanic crust that suffered a strong compressional stress during Late Santonian times. As a result of this, isoclinal folding and large-scale nappe emplacement occurred at a shallow crustal depth. The principal component of this compressional stress was E-W-directed. It is also demonstrated that, from this time, the complex was situated between a subducting plate and a volcanic arc. From that Campanian until the Middle Eocene the zone was undulated, and generally at a great depth below sea level. During the Eocene—Oligocene epoch a new tectonic stress affected the area. It produced open folding with upthrusting in the ophiolite complex and overthrust folding of the overlying rock series. As a result of crustal thickening during this tectonic phase, the area was uplifted. From Miocene times, the zone was shaped into a dome and a synform. These undulations are attributed to compression of the subducting Coco Plate, west of the area. The Upper Santonian tectonic phase demonstrates how compressional stress produced the break-up of the Caribo-Pacific plate west of the study area, as a result of which, a Caribbean plate without an associated oceanic ridge and a Pacific plate originated. The compressional stress in question was presumably generated by the opposed spreading directions of the new Mid-Atlantic Ridge and an older ridge to the west of the study area. Furthermore, it is argued that the Cretaceous obduction of the ophiolite belt along the Pacific coast of the American continents, was produced by the directional change of these continents during the birth of the Mid-Atlantic Ridge. This created intra-plate compressional stress and converted originally passive continental margins into active zones, where thrusting of oceanic

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

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Schoell, M.; Etiope, G.

    2010-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    The basic premise of seafloor spreading is that magmatic activity at mid-ocean ridges is responsible for creation of new crust; however only several years after the formulation of plate tectonic theory we have recognised the importance of tectonic, rather than magmatic, processes in accommodating the divergence of the lithospheric plates in slow-spreading oceans. In these settings, magma supply is spatially and temporally discontinuous and the crust freezes solid between delivery of magma batches from the underlying mantle. Large-scale extensional detachment faults develop during these periods of reduced magma supply, and play a fundamental role in accommodating plate separation. Extensional faulting occurs at the corner between ridge and transform fault and is responsible for the generation of the Oceanic Core Complexes (OCCs). OCCs are the uplifted footwalls of oceanic detachment faults forming dome-shaped massifs consisting of mantle and lower crustal lithologies exposed at the seafloor. Numerical modelling and results from the Integrate Ocean Drilling Program (IODP) has shown that the footwalls beneath oceanic detachment faults rotate during their evolution, proving that they initiate at a steep angle at depth and then "roll-over" to their present day low angle orientations as a result of flexural isostasy during unroofing. Accordingly, information from paleomagnetic analyses provided strong constraints to the detachment kinematics, since a difference of 45-65° in paleomagnetic inclination across the fault has been documented by previous studies, confirming the rolling-hinge model for OCCs formation. Here we present the results of an extensive paleomagnetic study of an OCC preserved in the Mirditata ophiolite of the Albanian Dinarides, first recognized by Trembley et al. (2009). Ophiolites are slices of oceanic lithosphere which have been emplaced onto continental margins during the closure of ocean basins, and provide opportunities for the study of oceanic

  15. 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.

  16. 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.

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

    NASA Astrophysics Data System (ADS)

    Koutsovitis, Petros; Magganas, Andreas

    2013-04-01

    Three small occurrences of extensively carbonate-altered serpentinite (listwanite) have been identified in East Othris at the ophiolitic formation of Vrinena, and in South Othris at the ophiolitic mélange formations of Agios Georgios and Paleokerasia. Their mineral assemblage includes calcite + quartz + chlorite + spinel ± clinopyroxene, as well as accessory Fe-oxides, titanite and apatite. Based upon their mineralogical composition they belong to the Type IB listwanite, characterized by the predominance of calcite and the presence of Mg-rich chlorite, mainly clinochlore and diabantite. In the ophiolitic mélange formation of Agios Georgios listwanite was found in proximity with serpentinite (former harzburgite), which could be considered as the precursor protolith. Changes of major and minor elements between the listwanite and the protolith have been calculated based on the method for mass-balance analyses[1]. Results show that this listwanite resulted after metasomatic processes dominated by Ca enrichment. Sr, Y and Pb contents were also significantly increased, whereas rather moderate enrichments of Al, Mn, Cr and Cu also took place. Small reductions were observed for Mg and Ni. Si, Ti and Fe remained relatively immobile. The chondrite normalized REE patterns reveal significant enrichment of all analysed REE, and especially of the LREE [(La/Yb)CN=20.4], and also with a negative Eu anomaly (EuCN/Eu*=0.79). Spinel grains from the Agios Georgios listwanite and the adjacent serpentinite are in most elements compositionally similar. The listwanites from Agios Georgios, Paleokerasia and Vrinena all contain spinel grains. Their mineral chemistry is respectively: TiO2=0.18-0.25; 0.04-0.10; 0.22-0.54 wt%, Al2O3=23.13-25.03; 27.69-29.70; 5.69-7.35 wt%, FeO=18.24-22.98; 16.44-19.49; 21.47-24.61 wt%, CaO= 0.01-0.07; 0.03-0.15; 0.01-0.28 wt%, Cr#=52.28-54.93; 45.57-48.85; 83.58-87.59, Mg#=51.07-65.39; 56.68-65.62; 46.77-55.35. Their rims exhibit slightly higher FeO and Ca

  18. Mantle flow patterns and magma chambers at ocean ridges: Evidence from the Oman ophiolite

    NASA Astrophysics Data System (ADS)

    Nicolas, A.; Boudier, F.; Ceuleneer, G.

    1988-12-01

    As a result of an extensive program of structural mapping in the ultramafic section of the Oman ophiolite, maps of mantle flow below the spreading center of origin have been drawn. They reveal a mantle diapiric system in which the uppermost mantle flow diverges from diapirs 10 15 km across, which could have been spaced by an average distance of 50 km. Some diapirs could have been located off-axis. The rotation of flow lines in the diapirs occurs within the few hundred meters of the transition zone separating the mantle and crustal formations. The importance of this zone is stressed. The structure of the layered gabbros of the crustal unit in most places reflects a large magmatic flow induced by the solid state flow in the underlying peridotites. The magmatic foliation of the gabbros steepens upsection and becomes parallel to the sheeted dike attitude. A new model of a tent-shaped magma chamber is derived from these structural data.

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

    PubMed

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

    2013-07-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2005-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Saccani, Emilio; Principi, G.

    2016-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

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

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

    NASA Astrophysics Data System (ADS)

    Yuxiu, Z.; Kaijun, Z.

    2009-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  5. 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

  6. 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

  7. Differentiation of Neotethyan ophiolitic mélange and an approach revealing its surficial chromite deposits using ASTER image and spectral measurements (Sivas, Turkey)

    NASA Astrophysics Data System (ADS)

    Kavak, Kaan Şevki; Töre, Yavuz; Temiz, Haluk; Parlak, Osman; Çığla, Hande; Yakan, Mustafa

    2010-10-01

    This work is aimed at differentiation of ophiolitic mélange rocks which were outcropped 60 km far from Sivas city center using image processing and spectral measurement methods. These rocks are known as oceanic crust remnants which were made up of different rocks. Turkey hosts several paleo-oceans and their realms in Alpine-Himalayan orogenic belt. The Neotethyan ophiolites in Turkey are characterized by supra subduction zone (SSZ-type) ophiolites. Ophiolitic rocks are generally coloured with greenish tones and human eye could not separate these tone differences. But satellite images such as ASTER can realize these separation utilizing spectral enhancement methods such as classification and decorrelation stretching. Chromite is a valuable mineral and is formed in only ophiolitic rocks. Dunites and harzburgites named as also ultramafic tectonits of ophiolitic serie mainly contain these deposits in study area. In this study, an approach was also realized to find target regions of chromite deposits with the aid of spectral methods. Spectral measurements were realized to determine boundaries between different mélange rocks using spectroradiometer. Reflectance curves collected from field and laboratory analysis were evaluated together and compared with ASTER image of the study area respectively. A detailed differentiation generally was accompanied with petrographic and geochemical analyses.

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

    NASA Astrophysics Data System (ADS)

    Blank, J. G.

    2015-12-01

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

  9. 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).

  10. Petrology and geochemistry of the Saga and Sangsang ophiolitic massifs, Yarlung Zangbo Suture Zone, Southern Tibet: Evidence for an arc-back-arc origin

    NASA Astrophysics Data System (ADS)

    Bédard, É.; Hébert, R.; Guilmette, C.; Lesage, G.; Wang, C. S.; Dostal, J.

    2009-12-01

    The Saga and Sangsang ophiolites are located about 600 and 450 km west of Lhasa and represent a western extention of the central portion of the Yarlung Zangbo Suture Zone (YZSZ) ophiolite belt. The Saga massif comprises fresh mantle lherzolite and cpx-harzburgite, an ophiolite mélange (± amphibolite), metamorphosed mafic crustal rocks (meta-gabbro, meta-basalts and amphibolites) and a sequence of uppermost crustal rocks (chert, basaltic lavas, diabase sills and dikes). The Sangsang ophiolite consists of an ophiolite mélange (harzburgite) and upper mantle harzburgite with minor lavas and gabbro. Peridotites from both massifs show variable degrees of serpentinization. Their Mg# varies between 0.89 and 0.91. All peridotites show distinct flat REE (rare earth elements) patterns with La/Yb N ratios close to 1, probably indicative of a refertilized mantle. The Olivine-Spinel equilibrium and the spinel chemistry for the Saga (Cr# ~ 0.10-0.22) and Sangsang (Cr# ~ 0.30-0.55) peridotites suggest that the Saga peridotites have a deeper mantle provenance (> 20 kbar) and have undergone lower degrees of partial melting (5-12%) than the Sangsang peridotites (< 15 kbar; 17-30%). The composition of the Saga peridotites is similar to the composition of pre-oceanic peridotites while peridotites from the Sangsang massif resemble abyssal and subduction-related peridotites. Mafic rocks from both ophiolites have basalt and basaltic andesite compositions. They are slightly depleted in light REE with respect to the heavy REE, with La/Yb N between 0.5 and 0.8 and with a small negative Ta-Nb anomaly suggesting the presence of a subduction component. The abundances of incompatible elements in these mafic rocks are similar to N-MORB (mid-ocean ridge basalt) or back-arc-basin basalts (BABB). Our data suggest that the Saga and Sangsang ophiolites belong to an intraoceanic suprasubduction zone segment as postulated for other ophiolitic massifs in the eastern portion of the YZSZ. However, the

  11. 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

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

    NASA Astrophysics Data System (ADS)

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

    2015-06-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2014-06-01

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

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

    NASA Astrophysics Data System (ADS)

    Sonzogni, Y.; Treiman, A. H.

    2012-12-01

    Serpentinization of ocean crustal peridotite, both beneath the seafloor and as ophiolites on land, has been identified as a source of hydrogen that can support microbial activity. The similarity of Mars' crust to terrestrial ocean lithosphere thus suggests that ophiolites may be good analogs to some martian environments where life might have existed and may persist today. However, peridotite-water reactions are poorly understood in detail, and serpentinization is commonly idealized as isovolumetric or isochemical hydration of olivine to form serpentine, brucite, magnetite, and H2 gas. Here, a net-veined serpentinite from the Josephine ophiolite, California, was studied in order to characterize in detail the physical-chemical nature of its serpentinization. The extent of serpentinization in the studied sample is ~60%. Remnants of the original harzburgite include ~30% olivine, ~10% orthopyroxene, and accessory augite and chromite. Two generations of serpentinite veins are present, the distinction between them being in their textures (in SEM imagery); type 1 veins appear striated, while type 2 veins are massive. Both types of veins consist almost entirely of serpentine. Both types contain <5% magnetite, which occurs as one or more distinct bands near the veins' centers, and as rare isolated grains elsewhere. No brucite has been identified by optical petrography, BSE/SEI/EDS imagery, or by electron microprobe point analyses. The serpentine mineral in both types of vein was identified as lizardite based on its foliate texture (as shown in SEM images), suggesting that serpentinization occurred at T<300°C. The lizardite in type 1 veins is more magnesian (Mg# 96) than the lizardite in type 2 veins (Mg# 93). Based on the mineral proportions in the serpentinite and original harzburgite and their mineral compositions, this reaction approximates the formation of type 1 serpentine veins: 22.5 Mg1.80Fe0.20SiO4 + 7.5 Mg0.91Fe0.09SiO3 + 31.15 H2O → 15 Mg2.88Fe0.12Si2O5(OH)4 + 1

  15. 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

  16. 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

  17. The Mirdita Eastern Ophiolite, Albania: petrological evidence for MORB Mantle in the Tropoje peridotite massif?

    NASA Astrophysics Data System (ADS)

    Ntaflos, Theodoros; Onuzi, Kujtim; Hauzenberger, Christoph; Rice, A. Hugh N.; Proßegger, Peter; Mekshiqi, Nazir; Kaza, Gjon

    2016-04-01

    The Mirdita ophiolites in northern Albania are divided into the Western Mirdita Ophiolite (WMO), with MORB geochemical affinities, and the Eastern Mirdita Ophiolite (EMO), for which suprasubduction geochemical affinities have been reported. Ultramafic massifs in the WMO are often plagioclase bearing lherzolites/harzburgites whereas those in the EMO are strongly depleted spinel harzburgites. The Tropoje ultramafic-mafic complex, which forms the northernmost part of the EMO, consists of spinel harzburgites, wherlites, orthopyroxenites and gabbros. The spinel harzburgites, which experienced variable degrees of serpentinization, are coarse- to medium-grained. One of the most striking features of the Tropoje ultramafic rocks is an outcrop of exceptionally fresh coarse- to medium-grained spinel harzburgites. While occasionally the peridotites exhibit a weak foliation, the prevalent texture is protogranular. Olivine is mainly coarse-grained (up to 6 mm size), typically showing kink-bands that frequently contain submicron-sized spinel exsolution lamellae. Both orthopyroxene and clinopyroxene, with grain size up to 3 mm and 1.5 mm, respectively, carry very thin exsolution lamellae of the other pyroxene. Spinel, up to 1 mm in size, is interstitial between boundaries of the associated silicates. The mineral compositions of the spinel harzburgites indicate that they are strongly depleted in basaltic components. The rock-forming minerals (olivine, orthopyroxene and clinopyroxene) are all highly magnesian and chemically homogenous. The magnesium numbers (Mg#=100xMg/[Mg+Fe]) for olivine and orthopyroxene are fairly homogeneous and vary within the narrow range of 90.9-91.6 and 91.5-91.7, respectively. The clinopyroxene is exceptionally highly magnesian, with Mg# ranging between 93.6 and 95.4. In both orthopyroxene and clinopyroxene, the Al2O3 contents range from 0.95 to 1.75 and from 0.62 to 2.27 wt%, respectively. Spinel shows a considerable variation in Al2O3 and Cr2O3. The

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

    NASA Astrophysics Data System (ADS)

    Khedr, Mohamed Zaki; Arai, Shoji

    2013-10-01

    The ophiolitic peridotites in the Wadi Arais area, south Eastern Desert of Egypt, represent a part of Neoproterozoic ophiolites of the Arabian-Nubian Shield (ANS). We found relics of fresh dunites enveloped by serpentinites that show abundances of bastite after orthopyroxene, reflecting harzburgite protoliths. The bulk-rock chemistry confirmed the harzburgites as the main protoliths. The primary mantle minerals such as orthopyroxene, olivine and chromian spinel in Arais serpentinites are still preserved. The orthopyroxene has high Mg# [=Mg/(Mg + Fe2+)], ~0.923 on average. It shows intra-grain chemical homogeneity and contains, on average, 2.28 wt.% A12O3, 0.88 wt.% Cr2O3 and 0.53 wt.% CaO, similar to primary orthopyroxenes in modern forearc peridotites. The olivine in harzburgites has lower Fo (93-94.5) than that in dunites (Fo94.3-Fo95.9). The Arais olivine is similar in NiO (0.47 wt.% on average) and MnO (0.08 wt.% on average) contents to the mantle olivine in primary peridotites. This olivine is high in Fo content, similar to Mg-rich olivines in ANS ophiolitic harzburgites, because of its residual origin. The chromian spinel, found in harzburgites, shows wide ranges of Cr#s [=Cr/(Cr + Al)], 0.46-0.81 and Mg#s, 0.34-0.67. The chromian spinel in dunites shows an intra-grain chemical homogeneity with high Cr#s (0.82-0.86). The chromian spinels in Arais peridotites are low in TiO2, 0.05 wt.% and YFe [= Fe3+/(Cr + Al + Fe3+)], ~0.06 on average. They are similar in chemistry to spinels in forearc peridotites. Their compositions associated with olivine’s Fo suggest that the harzburgites are refractory residues after high-degree partial melting (mainly ~25-30 % partial melting) and dunites are more depleted, similar to highly refractory peridotites recovered from forearcs. This is in accordance with the partial melting (>20 % melt) obtained by the whole-rock Al2O3 composition. The Arais peridotites have been possibly formed in a sub-arc setting (mantle wedge), where

  19. 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

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

    NASA Astrophysics Data System (ADS)

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

    2002-07-01

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

  1. 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

  2. Petrological constraints on the mantle peridotites from the Cretaceous ophiolites in southern Turkey and northern Cyprus

    NASA Astrophysics Data System (ADS)

    Yıldız Yüksekol, Özlem; Aldanmaz, Ercan; Güçtekin, Aykut; van Hinsbergen, Douwe J. J.; Mason, Paul R. D.

    2016-04-01

    In this study we present geochemical variations of peridotites from the ophiolite suites exposed within the Tauride Belt of southern Turkey and in Northern Cyprus with an aim to document the nature of mantle melting and possible effects of melt movement on element behavior in supra-subduction zone (SSZ) mantle. The ultramafic rocks representing the mantle sections of these ophiolites are variably serpentinized spinel-bearing harzburgites and dunites with major element compositions indicating variable depletions in basaltic components. Major and trace element systematics of primary mantle minerals indicate that the peridotites are likely the residual products left behind after relatively high-degree of mantle melting (16 - 23%). These mantle relicts, however, display also compositional and textural evidence indicative of extensive melt-rock interaction. Olivine-orthopyroxene-spinel equilibria indicate that the peridotites are characterized by high oxygen fugacity (QFM+2), which may be indicative of extensive interaction of the peridotites with highly oxidized melts. Precise determination of trace elements from in situ measurements of primary mantle minerals by laser-ablation ICP-MS reveals important features about the petrogenetic evolution of these mantle representatives. Trace element signatures in clinopyroxenes indicate that the peridotites are strongly depleted in Ti and HREE relative to Zr and MLREE, respectively. Uneven distribution of REE among coexisting opx - cpx pairs in the peridotites reflects chemical disequilibrium, which can be interpreted to have resulted from either diffusive exchange during melt movement or interaction with metasomatizing agents. Based on Ga concentrations and Ga-Ti-Fe+3# variations in chrome-spinels the peridotites have been inferred to have experienced significant compositional modification through melt-solid interaction following partial melting. Overall, mineral chemical variations in the peridotites indicate that the

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Punturo, Rosalda; Bloise, Andrea; Cirrincione, Rosolino

    2016-04-01

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

  5. Jurassic plume-origin ophiolites in Japan: accreted fragments of oceanic plateaus

    NASA Astrophysics Data System (ADS)

    Ichiyama, Yuji; Ishiwatari, Akira; Kimura, Jun-Ichi; Senda, Ryoko; Miyamoto, Tsuyoshi

    2014-07-01

    The Mikabu and Sorachi-Yezo belts comprise Jurassic ophiolitic complexes in Japan, where abundant basaltic to picritic rocks occur as lavas and hyaloclastite blocks. In the studied northern Hamamatsu and Dodaira areas of the Mikabu belt, these rocks are divided into two geochemical types, namely depleted (D-) and enriched (E-) types. In addition, highly enriched (HE-) type has been reported from other areas in literature. The D-type picrites contain highly magnesian relic olivine phenocrysts up to Fo93.5, and their Fo-NiO trend indicates fractional crystallization from a high-MgO primary magma. The MgO content is calculated as high as 25 wt%, indicating mantle melting at unusually high potential temperature ( T p) up to 1,650 °C. The E-type rocks represent the enrichment in Fe and LREE and the depletion in Mg, Al and HREE relative to the D-type rocks. These chemical characteristics are in good accordance with those of melts from garnet pyroxenite melting. Volcanics in the Sorachi-Yezo belts can be divided into the same types as the Mikabu belt, and the D-type picrites with magnesian olivines also show lines of evidence for production from high T p mantle. Evidence for the high T p mantle and geochemical similarities with high-Mg picrites and komatiites from oceanic and continental large igneous provinces (LIPs) indicate that the Mikabu and Sorachi-Yezo belts are accreted oceanic LIPs that were formed from hot large mantle plumes in the Late Jurassic Pacific Ocean. The E- and D-type rocks were formed as magmas generated by garnet pyroxenite melting at an early stage of LIP magmatism and by depleted peridotite melting at the later stage, respectively. The Mikabu belt characteristically bears abundant ultramafic cumulates, which could have been formed by crystal accumulation from a primary magma generated from Fe-rich peridotite mantle source, and the HE-type magma were produced by low degrees partial melting of garnet pyroxenite source. They should have been formed

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

    NASA Astrophysics Data System (ADS)

    Olds, E. P.

    2015-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2009-11-01

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

  8. Significance of chromian spinels from the mantle sequence of the Andaman Ophiolite, India: Paleogeodynamic implications

    NASA Astrophysics Data System (ADS)

    Ghosh, Biswajit; Morishita, Tomoaki; Bhatta, Koyel

    2013-04-01

    The mantle section of the Andaman Ophiolite between Rutland Island in the south and north Andaman in the north has been studied. The restitic peridotite of Rutland Island is comprised of depleted harzburgite to clinopyroxene-bearing harzburgite whereas that in middle and north Andaman is mostly less-depleted, lherzolite-dominated mantle. Chromian spinels from the mantle section of the Andaman Ophiolite belong to four major groups: Group-1 — spinels from Rutland chromitite pods, characterized by high Cr# [= Cr/(Cr + A1) atomic ratio] (~ 0.71); Group-2 — spinels from north Andaman chromitite pods with two subgroups, 2A — characterized by high Cr# (0.70-0.79) and 2B — characterized by medium Cr# (0.48-0.51) and high Mg# [= Mg/(Mg + Fe2+) atomic ratio] (0.74-0.77); Group-3 — residual accessory grains from Rutland peridotite, characterized by medium Cr# (0.37-0.55) and low Mg# (0.41-0.54); and Group-4 — residual accessory grains from middle and north Andaman peridotites, characterized by low Cr# (0.09-0.23) and high Mg# (0.71-0.81). Group-1 chromian spinels of Rutland Island and Group-2A of north Andaman are likely to have crystallized from boninitic melts, formed by relatively high degrees of partial melting, whereas Group-2B varieties from north Andaman are from tholeiitic melts, formed by lower degrees of melting. The chemistry of the residual accessory chromian spinels (Groups 3 and 4) suggests that the mantle peridotites of Rutland Island towards the south are similar to fore-arc peridotites of suprasubduction zone environments whereas those of north Andaman are less depleted. This spatial and/or temporal directional change in spinel compositions may reflect variations linked to the melting history where the same sliver of oceanic mantle underwent different styles of melting in different tectonic settings at different points in time. The coexistence of both chromitite types, with high- and low-Cr spinels, in the same area from north Andaman possibly

  9. 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

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

    NASA Astrophysics Data System (ADS)

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

    2005-12-01

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