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1

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

SciTech Connect

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.

Cousineau, P.A. (Universite du Quebec, Chicoutimi (Canada))

1991-01-01

2

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

Microsoft Academic Search

The Nellore–Khammam Schist Belt (NKSB) in South India is a Precambrian greenstone belt sited between the Eastern Ghats Mobile Belt (EGMB) to the east and the Cratonic region to the west. The belt contains amphibolites, granite gneisses and metasediments including banded iron formations. Amphibolites occurring as dykes, sills and lenses—in and around an Archaean layered complex—form the focus of the

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

2006-01-01

3

Origin, HP\\/LT metamorphism and cooling of ophiolitic melanges in southern Evia (NW Cyclades), Greece  

Microsoft Academic Search

Basic and ultrabasic blocks within ophiolitic melanges of the Cycladic Blueschist Unit in southern Evia provide a detailed insight into its ocean floor igneous and hydrothermal evolution, as well as the regional poly-metamorphism occurring during Alpine orogenesis. The upper structural levels (Mt. Ochi exposures) are dominated by metamorphosed wehrlites, gabbros and highly light rare earth element (LREE)-enriched pillow basalts, whereas

Y. K ATZIR; D. AV; A. M ATTHEWS; Z. G ARFUNKEL; B. W. E VANS

4

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

Microsoft Academic Search

Metre to decameter-size clasts of amphibolite are found embedded in ophiolitic melanges underlying the Yarlung Zangbo Suture Zone Ophiolites, South Tibet, China. These ophiolites and melanges occur at the limit between Indian and Tibetan-derived rocks and represent remnants of an Early Cretaceous intraoceanic supra-subduction zone domain, the Neo-Tethys. In the Saga-Dazuka segment (500 km along-strike), we discovered new occurrences of

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

2007-01-01

5

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

NASA Astrophysics Data System (ADS)

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

Gülmez, Fatma; Genç, Can; Prelevic, Dejan

2014-05-01

6

Geochronological and geochemical constraints on the Yazidaban ophiolitic melange at the southern Qimantage of the Kunlun orogen, Northern Tibet  

NASA Astrophysics Data System (ADS)

The Kunlun orogen is located at the northern Tibet plateau, and the western part of the Central China Orogenic Belt (CCOB) linking the Qinling-Dabie orogen, to the east, which represents the most important collisional belt between the North China and South China blocks. The Kunlun orogen is well documented that it was formed by convergence of the Tarim, North China, Yangtze, Qaidam and Qiangtang blocks associated to the closure of the Tethyan ocean. The tectonic framework and the convergence processes of the Kunlun orogen are keys for understanding the evolutionary history of the Tethys, as well as the formation of the Tibet plateau. The Kunlun orogen can be divided into the Qimantage, Central Kunlun, Central Kunlun and Songpan-ganze belts with distinctive litho-tectonic assemblages by the northern, central and southern Kunlun sutures which indicated by developing of ophiolite and subduction related volcanic rocks. These wide occurrences of ophiolitic and subduction-related volcanic mélanges provide important constraints on the tectonic evolution of the Kunlun oregen. The northern Kunlun suture, separating the Qimantage to the north and Central Kunlun belt to the south, is pronounced by the Yazidaban ophiolitic mélange which is predominantly exposure of serpentites, basalts, diabases and andesites. The serpentites outcropping at the eastern Yazidaban are characterized by low ?REE and depletion of mid-REE showing an ophiolitic ultramaifc affinity. The basalts overthrusting on the serpentites at the eastern Yazidaban show low ?REE and slightly enrichment of LREE, as well as depletion of LILE and insignificant fractionation of HFSE in the primitive mantle normalized trace elements distribution patterns. These geochemical features attributed to E-MORB. Both basalts from the south and central Yazidaban section exhibit similar geochemical compositions to that of the basalt from eastern Yazidaban attributed to E-MORB excepting lower SiO2, higher TiO2, P2O5, Zr and Nb contents, and higher ratios of Ti/V, Nb/Y and Ta/Yb compare to the basalts from eastern Yazidaban. The andesites mainly distributed in the northern Yazidaban section, which possess moderate ?REE and clear enrichment of LREE, as well as negative Nb-Ta, P and Ti, and positive Th-U anomalies. All these geochemical characteristics represent volcanic rocks associated to a typical island-arc/active continental margin. The diabases have high SiO2 content showing basalt-andesitic composition, and are characterized by high ?REE, strong enrichment of LREE, depletion of Nb-Ta, and significant fractionation of HFSE. These features indicate a subduction related tectonic setting. All above geochemical constraints on the ultramafic rocks, basalt, suggest an E-MORB tectonic setting, while geochemical features of the andesites and diabases pronounce a subduction related setting. Hence, we explain the Yazidaban mélange to represent remnants of subduction related volcanic rocks and associated oceanic crust of a back-arc basin. The zircon U-Pb age of a diabase from the northern Yazidaban yields an age of 420.6 × 3.6 Ma (MSWD = 0.36) representing the time of subduction and formation of back-arc basin. Acknowledgements: NSFC (41190074, 41225008), and MOST Special Fund from the SKLCD

DONG, Y.; Liu, X.; Zhou, X.; He, D.; Yue, Y.; Zhang, J.; Zhang, H.; Zhang, G.

2013-12-01

7

Melange genesis in the construction of the Anatolian subcontinent  

NASA Astrophysics Data System (ADS)

Melanges can be defined as chaotically deformed blocks with, or without, a matrix and may be of tectonic, or sedimentary origin, or both. Classic examples throughout Anatolia have mainly Carboniferous (Konya-Teke Dere-Karaburun), Late Triassic (Karakaya), Late Cretaceous (e.g. Ankara), Eocene (Pontides) and Oligo-Miocene (Misis-And?r?n) ages of formation. All of the major melanges reflect contractional processes and delineate convergent plate margins or suture zones. The Carboniferous melanges record southward (?) subduction of "older Palaeotethys" beneath the N margin of Gondwana. The U. Triassic melanges reflect late-stage northward subduction of "younger Palaeotethys" beneath Eurasia. The U. Cretaceous melanges record northward subduction of Mesozoic oceanic crust; i.e. Ankara-Izmir-Erzincan ocean in the N (e.g. Domuzda? Melange, central Pontides; Ankara Melange) and S Neotethys in the south (e.g. Berit Melange). The Eocene melanges in the Pontides reflect final closure of the Izmir-Ankara-Erzincan ocean. The Oligocene-Lower Miocene melanges in SE Turkey (e.g. Killan Melange) document later-stage subduction/closure of the S Neotethys. The melanges typically reflect the interaction of both tectonic and sedimentary processes. Tectonic processes include frontal accretion, forearc subcretion, subduction channel mixing, high pressure/low temperature metamorphism/deformation (e.g. Anatolide melanges), exhumation; also, re-imbrication to maintain a critical taper. Sedimentary processes include collapse of seamounts/continental margins into subduction trenches (e.g. Carboniferous mélanges), reworking as debris flows in trench/forearc basin settings (e.g. U. Cretaceous Anatolide melanges), also genesis of large-scale collision-related mega-debris flows, e.g. driven by seamount-trench collision (e.g. U. Triassic Karakaya melange), trench-passive margin collision (e.g. U. Cretaceous Tauride melanges), or continent-continent collision (e.g. Eocene Pontide melanges and Oligo-Miocene S Neotethyan melanges). Important controls of the melange genesis include: 1. Subduction setting, either continental margin or oceanic, with e.g. Carboniferous melanges representing near continental margin subduction with high terrigenous input, compared to e.g. some U. Cretaceous melanges that formed in sediment-starved more oceanic settings. 2. Age of subducting oceanic lithosphere: older ocean (e.g. Carboniferous melanges), was more easily subducted than young, buoyant, ocean (e.g. some U. Cretaceous melanges); 3. SSZ-type oceanic lithosphere: where present dismembered ophiolites including mantle rocks were accreted preferentially (e.g. U. Cretaceous Ankara Melange); 4. Igneous seamounts or continental fragments; where present (e.g. U Triassic Karakaya melange; U. Cretaceous Ankara melange) large volumes of ocean-island-type volcanics, cover sediments and flank facies were accreted. 5. Large-scale plate dynamics; slab roll-back favoured accretion but slab roll-forward subduction erosion. In the absence of situations favouring crustal accretion the normal condition was non-accretion such that melange genesis was skewed to exceptional geological settings (e.g. collision of plume-type seamounts or SSZ-type oceanic lithosphere with subduction trenches). Melanges generally lack incorporated oceanic lithosphere in cases where the subduction décollement was located at a high structural level in the subduction trench, well above oceanic basement (e.g. for the Carboniferous melanges). None of the major Anatolian mélanges formed by mainly sedimentary processes as "olistostromes" or in rift settings (either continental, or backarc), although relatively small-scale debris flow deposits characterise several extension-related settings (e.g. Triassic rifting of Antalya Complex; Eocene back-arc Maden Formation). Finally, the varied Anatolian melanges can be used as a model to infer modes of melange formation elsewhere (e.g. some Iapetus-related melanges).

Robertson, Alastair; Parlak, Osman; Ustaömer, Timur; Unlugenç, Ulvican; Nairn, Steven; Üçta? Özbey, Zeynep

2010-05-01

8

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

Microsoft Academic Search

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

G. M. Boone; E. L. Boudette

1985-01-01

9

The Obduction Direction of the Mersin Ophiolite: Structural Evidence from Subophiolitic Metamorphics in the Central Tauride Belt, Southern Turkey  

Microsoft Academic Search

The Mersin ophiolite is an ?6-km-thick oceanic crustal sequence in the central Taurides (southern Turkey), forming large outcrops on the southern flank of Bolkardag. It comprises a sequence of units and includes, in ascending structural order, the Mersin melange, subophiolitic metamorphics, tectonics, ultramafic and mafic cumulates, and pillow lavas interlayered with deep-marine sediments.Thin beds of ophiolite-related metamorphics occupy a constant

Osman Parlak; Erdin Bozkurt; M. Delaloye

1996-01-01

10

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

NASA Astrophysics Data System (ADS)

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

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

2009-03-01

11

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

SciTech Connect

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

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

1985-01-01

12

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

SciTech Connect

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

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

1993-03-01

13

DEFORMATION OF THE HURRICANE MOUNTAIN FORMATION MELANGE ALONG TOMHEGAN AND  

E-print Network

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

Beane, Rachel J.

14

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

USGS Publications Warehouse

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

Donato, M. M.

1987-01-01

15

Metamorphosed melange terrane in the eastern Piedmont of North Carolina.  

USGS Publications Warehouse

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

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

1986-01-01

16

Discovery of ophiolite in southeast Yunnan, China  

Microsoft Academic Search

The Babu ophiolite in Malipo County, SE Yunnan, consists of three units: ultramafic, mafic and basaltic rocks. Studies of\\u000a geological mapping, petrology and geochemistry reveal that the gabbro is similar to that in the Troodos ophiolite, and the\\u000a diabase and basalt belong to a normal MORB-type, analogous to the Shuanggou ophiolite in west Yunnan. The ophiolite studied\\u000a as a thrust

Dalai Zhong; Genyao Wu; Jianqing Ji; Qi Zhang; Lin Ding

1999-01-01

17

Metamorphosed melange in the central Piedmont of South Carolina  

SciTech Connect

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.

Mittwede, S.K.; Maybin, A.H. III (South Carolina Geological Survey, Columbia (USA))

1989-09-01

18

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

NASA Astrophysics Data System (ADS)

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

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

2013-03-01

19

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

NASA Astrophysics Data System (ADS)

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

Casey, John; Dewey, John

2013-04-01

20

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

USGS Publications Warehouse

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

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

1997-01-01

21

The Origin and Tectonic Significance of Ophiolites  

NASA Astrophysics Data System (ADS)

As defined by the 1972 Penrose Conference, ophiolites consist of a distinctive assemblage of mafic to ultramafic rocks, which, in complete bodies, includes mantle peridotites, gabbros, plagiogranites, sheeted dikes, mafic volcanic rocks and minor sedimentary rocks. Sheeted dyke complexes are taken as evidence of extension in a seafloor-spreading environment. Complete sequences are very rare and most ophiolites consist primarily of peridotite, gabbro and basalt, typically with tectonic contacts. Well-developed sheeted dyke complexes are rare suggesting either a lack of formation or tectonic removal during emplacement. We suggest that those ophiolites with poorly developed dyke complexes formed in an environment dominated by amagmatic extension. For many years ophiolites were thought to have formed at mid-ocean spreading ridges, a misconception fueled by the assumption of large-scale spreading and an abundance of MORB-like lavas. However, most ophiolite lavas have a suprasubduction zone signature, at least in part. The lavas range from MORB to arc tholeiite to boninite and many complexes contain two or more lava types. In some cases, the lavas are highly evolved and include andesites and rhyodacites, as well as basalts. We believe that the different lava types primarily reflect formation in different parts of a suprasubduction zone environment, however, pieces of old ocean lithosphere may also be incorporated in some ophiolites. We define ophiolites as fragments of suprasubduction zone lithosphere formed during subduction rollback that have been emplaced onto continental margins. Some ophiolites may also contain fragments of mid-ocean ridge lithosphere. Although ophiolites are distributed along continental margins, their emplacement does not necessarily reflect final closure of the ocean basin in which they formed.. Thus, several ophiolite belts may occur within a broad suture zone between continental blocks. Thus, ophiolites in the geologic record provide evidence primarily of subduction zone tectonics, not mid-ocean ridge spreading. Although they typically lie between continental blocks, their emplacement may predate final closure of the ocean basin in which they formed by many millions of years.

Robinson, P. T.; Malpas, J.; Flower, M. F.

2001-12-01

22

ETUDE DES EVOLUTIONS CHIMIQUES DANS UNE COUCHE DE MELANGE HYDROGENE -AIR Otto Leuchter  

E-print Network

ETUDE DES EVOLUTIONS CHIMIQUES DANS UNE COUCHE DE MELANGE HYDROGENE - AIR Otto Leuchter OFFICE chemical evolution i n an a i r - hydrogen mixing layer. The confluen- ce o f t w o p a r a l l e l flows://dx.doi.org/10.1051/jphyscol:1971559 #12;EVOLUTIONS CHIMIQUES DANS UNE COUCHE DE MELANGE HYDROGENE-AIR où l

Boyer, Edmond

23

Ophiolites as Analogs to Habitats on Mars  

NASA Technical Reports Server (NTRS)

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

Schulte, M.; Blake, D. F.

2003-01-01

24

Juxtaposed melanges: the result of polyphase tectonism in a forearc terrane  

SciTech Connect

The Mona Complex (Late Precambrian) of Anglesey, North Wales, can be modeled as a disrupted forearc complex. There are two distinct melanges in this complex. The Gwna melange contains large scale olistostromal masses, disrupted sequences that preserve a ghost stratigraphy, local syn-depositional slump and slide deposits, and sequences of pillowed basalts and associated sediments. Deformation was gravity driven. This unit developed in a forearc on a previously passive continental margin. The Aethwy Schists is belt of metabasite blocks in a pelitic matrix. The belt is metamorphosed; metabasites show a glaucophanic assemblage superimposed on an actinolitic precursor assemblage. Structures in the blocks developed when the rocks were highly ductile; block margins are highly sheared and recrystallized. These contrasting melanges are juxtaposed in two ways. The Aethwy Schists were underplated to the overlying plate. They were uplifted into the forearc high and stratigraphically overlain by characteristic Gwna melanges. Secondly, these units were faulted together along the Berw fault, a structure located along the edge of the ancient forearc high. These melanges and their juxtaposition demonstrate that when subduction is prolonged or episodic or when a complex terrane is rejuvenated, melanges formed by contrasting mechanisms can be juxtaposed and should not be treated as a single chaotic body.

Bieler, D.B.; Schuster, D.C.

1985-01-01

25

Structural evolution of the Thetford Mines Ophiolite Complex, Canada: Implications for the southern Québec ophiolitic belt  

Microsoft Academic Search

The Thetford Mines Ophiolite Complex (TMOC) preserves a complete ophiolitic sequence, and occupies the hanging wall of a major SE dipping normal fault, the Saint-Joseph fault. Preobduction, synobduction, and postobduction structures can be recognized in the TMOC. NS trending, preobduction, paleonormal faults are parallel to ultramafic minor intrusions, and to sheeted dykes, recording extension related to seafloor-spreading in a pericontinental

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

2005-01-01

26

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

NASA Astrophysics Data System (ADS)

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.

Shafaii Moghadam, Hadi; Stern, Robert J.

2014-09-01

27

Tethyan vs. Cordilleran ophiolites: a reappraisal of distinctive tectono-magmatic features of supra-subduction complexes in relation to the subduction mode  

NASA Astrophysics Data System (ADS)

Supra-subduction zone (SSZ) ophiolites deserve special attention because they represent fundamental markers of intraoceanic convergence and generation of new lithosphere above subduction zones. Moreover, owing to their structural characteristics and location in the overriding plate, these complexes are far better represented and preserved than Mid-Ocean-Ridge-Basalt (MORB) ophiolites in orogenic belts. In terms of their structure, tectonics, and magmatic features, SSZ ophiolites may be classified in two main types: (1) "Tethyan complexes" (such as those of the Albanide-Hellenide belt), which mostly consist of complete and extensive volcanic, dyke, plutonic, and mantle sections with prevalent Island Arc Tholeiitic (IAT) affinity and the significant presence of Boninites, obducted as relatively intact lithospheric slabs onto collisional continental margins; (2) "Cordilleran complexes" (including the circum-Caribbean ophiolites of Guatemala, Cuba, and Venezuela), mostly represented by dismembered sections of arc volcanic, plutonic, and subordinate mantle sequences with tholeiitic to Calc-Alkaline (IAC) magmatic affinity and acidic differentiates, commonly associated with metamorphic "subduction complexes" and tectonically emplaced onto or juxtaposed against the continental margin within polygenetic terranes. These two types appear to be related to significantly different subduction modes and intraoceanic plate dynamics whereby SSZ ophiolites were generated. The Tethyan complexes can best be compared with west Pacific-type subduction with accentuated steepening and retreat of the subducted slab, accompanied by progressive decoupling of the converging plates, intense mantle diapirism, and tensional events in the upper plate with generation of large IAT-sheeted dike complexes in "open" oceanic spreading systems. Continuous slab sinking and roll back allow increasing asthenospheric diapirism and interarc spreading from the arc axis to the forearc region with generation of boninites and Back Arc Basin Basalts (BABB) with intermediate MORB/IAT composition. By contrast, the genesis of the Cordilleran complexes requires a subduction mode characterized by a steady state regime, with moderate and constant dip of the subducted slab and limited extension in the backarc region. The magmatic evolution of these ophiolites from IAT to IAC and the significant presence of rhyodacite (and tonalite) differentiates coherently indicate a more mature stage of arc magmatism, as well as the occurrence of efficient differentiation processes developing under nearly "closed-system" conditions in independent magma chambers. Tethyan complexes are likely to be obducted as large and relatively intact slabs onto the continental margins through the interposition of metamorphic soles, which represent relics of the MORB lithosphere underplating the SSZ ophiolites. On the other hand, the common emplacement of Cordilleran complexes within polygenetic terranes appears to be controlled by prolonged accretionary mechanisms that trap, against the continental margin, parts of the arc structure, subduction complexes, melanges, and volcanoclastic products.

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

2004-11-01

28

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)

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.

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

2009-03-01

29

Ophiolites in European Variscides and Uralides: Geodynamic settings and metamorphism  

NASA Astrophysics Data System (ADS)

The most important tectonic and magmatic events in the history of ophiolites in the Uralides and the European Variscides occurred almost sinchronously. The rocks of paleooceanic complexes in the both regions have similar paleontological and isotopic ages. The formation of the Ural and Variscan ophiolites is brought into correlation with a wide range of geodynamic settings. The Ural ophiolites mainly correspond to the suprasubduction setting, whereas the Variscan ophiolites are commonly related to rifts and mid-ocean spreading centers of the oceanic crust. The main difference is related to the origin of metamorphic rocks associated with ophiolites. The high-pressure-low-temperature metamorphism of the Ural ophiolites mainly related to the island arc-continent collision, whereas the Barrow-type metamorphism of ophiolites in the European Variscides developed at a higher temperature under medium-low pressure.

Savelieva, G. N.

2011-11-01

30

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

SciTech Connect

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.

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

1985-01-01

31

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

USGS Publications Warehouse

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

Tabor, R.W.

1994-01-01

32

The Yarlung Zangbo Suture Zone Ophiolites, Tibet: A Synthesis  

Microsoft Academic Search

This paper presents the main results of a research project focused on the Yarlung Zangbo Suture Zone (YZSZ) ophiolites and related mélanges and flysch. Study area is representing a 300 km long and 30 km wide segment. The Cretaceous ophiolites are remnants of Neo-Tethys basin which was almost totally consumed in a northward oriented subduction zone and southerly transported onto

R. Hebert; C. Guilmette; C. Dupuis; V. Dubois-Cote; F. Huot; J. Dostal; C. Wang; Y. Li

2006-01-01

33

Magnetism of the oceanic crust: Evidence from ophiolite complexes  

Microsoft Academic Search

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

Subir K. Banerjee

1980-01-01

34

Ophiolite and Tectonic Development of the East Pacific Margin  

Microsoft Academic Search

Well-preserved ophiolites represent oceanic crust and mantle formed at a spreading center and emplaced by collision of a mantle-rooted thrust fault (subduction zone) with a continental margin or island arc. Ophiolite nappes thus represent remnants of lithospheric plates; their basal thrusts (fossil subduction zones) intrinscally cannot be balanced; their displacements are unknown but very large. Many environments of formation are

E. M. Moores

2001-01-01

35

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

E-print Network

Subsidence in magma chamber and the development of magmatic foliation in Oman ophiolite gabbros Keywords: Oman ophiolite fast spreading ridges magma chamber gabbro subsidence In the Oman ophiolite the ridge axis. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The Oman ophiolite is regarded

Demouchy, Sylvie

36

How Plate Kinematics Creates and Sweeps Away Supra-subduction ophiolites  

Microsoft Academic Search

A major characteristic of Circum Pacific belts is their integration of ophiolitic bodies of various size. A common point of these ophiolite is their origin as supra subduction ophiolites. They are therefore originated from convergent margins. In SE Asia, most of the ophiolitic bodies accreted during the Tertiary are remnant of arc-back arc systems that have developped along the edge

M. F. Pubellier; A. Deschamps; A. Loevenbruck; S. Lallemand; C. Monnier; K. Queano

2001-01-01

37

Magnetism of South Mongolian Middle Paleozoic ophiolites  

NASA Astrophysics Data System (ADS)

This paper presents new paleomagnetic data from South Mongolia, Middle Paleozoic rocks were studied from three different tectonic zones within the South Mongolian fold belt. From north to south these are the Gobi-Altai zone (in terms of paleotectonic position, the 'north' shelf of the paleobasin), the Trans-Altai zone (ophiolite complexes) and the South Gobia zone (the 'south' shelf). Several paleomagnetic field tests, including conglomerate, baked contact and fold tests, were carried out to determine magnetic stability. In addition, principal component analysis of the stepwise thermal demagnetization data was used. The ophiolites are characterized by three ancient components. One magnetic component is pre-folding and directed to the south with a very gentle inclination (declination ( D) = 180-210°, inclination ( I) = -6 ⋯ +2°, K = 120). Two post-folding magnetic components were observed also. The first post-folding magnetic component has D = 282.4°, I = -36.4°, k = 30.3, and the second has D = 318.6°, I = 33.0°, k = 60.5. The final magnetic component is very close to the Permian paleomagnetic direction expected from the North China platform. The first post-folding component of magnetization may have been acquired during the Variscan accretion in Early Carboniferous time when the basin was closed. The South Mongolian paleo-ocean was located in equatorial latitudes near the Tarim block, the South China platform and probably the North China platform in the Devonian.

Didenko, Alexei N.

1992-12-01

38

Geochemistry of the Bela Ophiolite, Pakistan  

NASA Astrophysics Data System (ADS)

The Bela ophiolite complex of Balochistan, Pakistan has been the subject of several geochemical and tectonic studies in the past. However until now there has never been a combined structural, geochemical and tectonic assimilation study which adequately explains the observed geochemistry and structural geology in a global tectonic framework. Here we present the geochemical findings of our work. The Bela ophiolite complex consists of two major units: the basal section or Lower Unit, and the Upper Unit, between the two is a mélange zone. The Lower Unit is relatively homogeneous and consists almost entirely of flow basalts and pillow basalts. The base of the Upper Unit is the metamorphic sole which is overlain by a sequence of massive basalts flows and intrusions of gabbro and granites. The entire Upper Unit is cut by doleritic dykes and sills. Geochemically the Lower Unit is comprised of basaltic lavas with E-MORB affinities. These lavas are tholeiitic, low-K series lavas with trace element signatures of E-type MORB. For example ratios such as V/Ti, Zr/Y, Nb/Th, Th/La and Nb/U all suggest these lavas are E-MORB. Previous workers have suggested these lavas are back-arc basin (BAB) however the samples lack the characteristic signatures of subduction modified MORB. This conclusion is supported by chondrite and N-MORB normalized spider diagrams where the Lower Unit lavas are enriched in the LILE with respect to the HFSE. The Upper Unit of the Bela Ophiolite sequence has a slightly more complex history. The older lavas sequences, the massive basalt flows, gabbros and granites, all formed in an oceanic arc environment. These lavas exhibit classic arc signatures such as a negative Nb and Ti anomalies, are enriched in LILE and LREE relative to HSFE, and plot in the volcanic arc and island arc fields in classic ternary plots such as 2Nb- Zr/4-Y and Y/15-La/10-Nb/8. The younger sequence of intrusions found in the Bela ophiolite appear to have BAB signatures. These lavas have relatively flat MORB normalized plots, are slightly depleted in the LILE relative the HFSE, and have a very small negative Nb anomaly. Source characteristics for both units have been determined using trace element data. This work suggests that the E-MORB lavas are derived from partial melting of enriched mantle. The lavas found in the Upper Unit have all been sourced from depleted or N-MORB mantle which has been modified by subducting fluids. It is possible that the younger BAB samples have a slightly more enriched source than the corresponding arc lavas which might indicate movement of the subduction zone allowing the influx of new mantle material below the wedge. In conclusion, our new geochemical work shows that the Bela ophiolite contains three distinct magmatic sequences: a lower E-MORB sequence over lain by a series of volcanic arc lavas which are cut by BAB-type sills and dykes.

Khan, M.; Nicholson, K. N.; Mahmood, K.

2008-12-01

39

Geology of the Zambales ophiolite, Luzon, Philippines  

USGS Publications Warehouse

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

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

1989-01-01

40

Supra-subduction zone ophiolites as favorable hosts for chromitite, platinum and massive sulfide deposits  

NASA Astrophysics Data System (ADS)

Supra-subduction zone ophiolites, as exemplified by the Zambales Ophiolite Complex, host extensive chromitite, volcanic-hosted massive sulfide deposits and, to a lesser degree, platinum-group minerals. In contrast, mid-ocean ridge basalt ophiolites are almost barren of economic mineral deposits. The high degree of partial melting, high water pressure, changes in oxygen fugacity, temperature, pressure and magma mixing could explain the chromitite deposits in zupra-subduction zone ophiolites. Platinum-group minerals are modeled to be derived from multi-stage melting events that also characterize supra-subduction zone ophiolites. The presence of capping rocks and the tectonics of emplacement are believed to be critical in the preservation of volcanic-hosted massive sulfides in ophiolites. Marginal basins (= SSZ ophiolites) are more easily emplaced than the large, open sea oceanic basin ophiolites which are usually subducted. Geochemical and tectonic controls point to supra-subduction zone ophiolites as more promising exploration targets.

Yumul, G. P.; Balce, G. R.

41

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

42

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

Microsoft Academic Search

The Zambales Ophiolite Complex, a supra-subduction zone ophiolite, is characterized by three volcanic-hypabyssal rock units: the Coto Block volcanic-hypabyssal rocks, the Coto dikes intruded into the Coto residual peridotites and the Acoje Block volcanic-hypabyssal rocks. The first two groups exhibit transitional mid-ocean ridge-island arc characteristics while the latter reveals island arc affinity. Furthermore, these three volcanic rock suites are characterized

G. P. Yumul

1996-01-01

43

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

USGS Publications Warehouse

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.

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

1998-01-01

44

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

USGS Publications Warehouse

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

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

1991-01-01

45

Ophiolite Generation and Emplacement in Subduction Rollback Systems  

NASA Astrophysics Data System (ADS)

Suprasubduction zone (SSZ) ophiolites in orogenic belts represent oceanic crust generation in subduction rollback cycles during the closing stages of basins prior to terminal continental collisions. Oceanic lithosphere created at mid-ocean ridges is generally recycled back into the mantle via subduction, and only rarely do fragments of this MOR lithosphere become accreted into continental margins (ridge-trench collision, accretion margin tectonics). Mantle flow and slab rollback may result in one or more episodes of arc splitting and basin opening, producing a collage of ‘proto-arc and forearc oceanic lithosphere' in suprasubduction zone settings. SSZ Tethyan ophiolites generally have Penrose-type oceanic crust and contain well-developed sheeted dike complexes indicative of magmatic extension beneath narrow rift zones during their seafloor spreading evolution. Igneous accretion of these SSZ Tethyan ophiolites involved upper plate extension and advanced melting of previously depleted asthenosphere in host basins, showing a progressive evolution from MORB-like to IAT (island arc tholeiite) to boninitic (extremely refractory) proto-arc assemblages. The IAT and boninitic magmas were derived from depleted peridotites that had already experienced previous MORB-type melt extraction during the early stages of ophiolite formation in the Tethyan subduction rollback systems. Rapid slab rollback and associated extension in the arc-forearc region caused increasing asthenospheric diapirism and corner flow toward the forearc mantle, resulting in shallow partial melting of the highly refractory harzburgites producing boninitic magmas. Differences in the geochemical evolution of Tethyan ophiolites resulted from variations in their subduction zone geodynamics. The production of increasingly more calc-alkaline rocks in the later stages of the generation of some Tethyan ophiolites suggests higher sediment input into the melting regimes via subduction and hence a longer period of subduction and arc maturity during their SSZ evolution. Emplacement of SSZ ophiolites onto rifted continental margins, particularly in the Tethyan realm, was a result of trench-continent collisions, followed by continental underplating, blueschist metamorphism, and slab breakoff in the downgoing plate.

Furnes, Harald; Dilek, Yildirim

2010-05-01

46

Dismembered Ophiolite in NYC Charles Merguerian, Geology Department, Hofstra University, Hempstead, NY 11549  

E-print Network

Dismembered Ophiolite in NYC Charles Merguerian, Geology Department, Hofstra University, Hempstead, Charles, 2006, Dismembered ophiolite in New York City: Geological Society of America Abstracts with Programs, v. 38, no. 2, p. 86. Filename: CM2006.doc #12;

Merguerian, Charles

47

Merguerian, Charles, 1979, Dismembered ophiolite along Cameron's Line, West Torrington, Connecticut.  

E-print Network

Merguerian, Charles, 1979, Dismembered ophiolite along Cameron's Line, West Torrington, Connecticut. To Cite This Abstract: Merguerian, Charles, 1979, Dismembered ophiolite along Cameron's Line, West Torrington, Connecticut (abs.): Geological Society of America Abstracts with Programs, v. 11, p. 45. Filename

Merguerian, Charles

48

Ophiolite and Tectonic Development of the East Pacific Margin  

NASA Astrophysics Data System (ADS)

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

Moores, E. M.

2001-12-01

49

Biomarkers in Tertiary me??lange, western Olympic Peninsula, Washington, U.S.A.  

USGS Publications Warehouse

Middle Eocene to middle Miocene me??lange and broken formations are exposed in the coastal outcrops along the west side of the Olympic Peninsula, Washington. A petroleum geochemical assessment of these geologic units has included the investigation of biomarker compounds. A comparison was made of biomarkers in an oil sample from a middle Miocene reservoir penetrated in the Medina No. 1 well with biomarkers in extracts from two samples of middle Eocene Ozette me??lange (one sample having a strong petroliferous odor, and the other sample lacking this characteristic odor). Distribution patterns of n-alkanes, tricyclic terpanes, pentacyclic triterpanes, steranes, and diasteranes are remarkably similar in the oil and rock extracts. Biomarker maturity parameters indicate higher maturity in the oil relative to the extracts. The presence of 17??(H)-23,28-bisnorlupane, 18??(H)- and 18??(H)-oleanane, and de-A-lupane and an odd-carbon-number dominance of the n-alkanes in the oil and extracts seems to tie the hydrocarbons to a common source that has a significant terrigenous component. ?? 1991.

Kvenvolden, K.A.; Hostettler, F.D.; Rapp, J.B.; Snavely, P.D., Jr.

1991-01-01

50

Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization  

E-print Network

Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D processes. We present maps of two areas of the Oman ophiolite where this transition zone is observed reheating stages, and assimilation processes observed in the Oman ophiolite are consistent

Demouchy, Sylvie

51

A Detailed Study of Mantle Flow away from Diapirs in the Oman Ophiolite  

E-print Network

lr t A Detailed Study of Mantle Flow away from Diapirs in the Oman Ophiolite B.ILDEFONSE, S diapirs. In the Oman ophiolite, however, the shear direc- tion inversion is not systematically developed in the Oman ophiolite' temperature flow presented a shear direction inversion at a few hundred meters to a few

Demouchy, Sylvie

52

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

E-print Network

Root zone of the sheeted dike complex in the Oman ophiolite Adolphe Nicolas and Franc¸oise Boudier´osciences Marines, IPG, CNRS, 4 Place Jussieu, F-75252 Paris Ce´dex 05, France [1] In the Oman ophiolite crustal. The Oman ophiolite is derived from a fast spreading ridge which had a melt lens located between the main

Demouchy, Sylvie

53

Geochemical variability of the Oman ophiolite lavas: Relationship with spatial distribution and paleomagnetic  

E-print Network

Geochemical variability of the Oman ophiolite lavas: Relationship with spatial distribution in the extrusive section of the Oman ophiolite was determined by inductively coupled plasma-source mass lavas types in Oman. Most of the Oman ophiolite extrusive sequence is composed of lavas of composition

Demouchy, Sylvie

54

Polychronous formation of mantle complexes in ophiolites  

NASA Astrophysics Data System (ADS)

The paper presents new determinations of the U-Pb zircon age of high-Al chromitite from dunite of the mantle section of the Voikar-Synya massif at the Kershor site in the boundary zone with rocks of the dunite-wehrlite-clinopyroxenite complex. The high-Cr chromitite from dunite in the central part of the same massif contains zircon dated at ca. 0.6 Ga [10]. It is suggested that Paleoproterozoic (2.0-1.9 Ga) zircons from chromitites of the mantle section near the petrological Moho boundary were formed in the course of partial melting of peridotites and/or their interaction with migrating MORB-type melts. The occurrence of Vendian and Paleoproterozoic zircons in chromitites from different parts of the mantle section, as well as previously published petrological, geochemical, and geological data [2, 11, 22] allow us to suggest a complex multistage evolution of the mantle section in ophiolites. The arguments stated below show that chromitites and host dunites could have been formed at different times and were probably related to different processes. Thus, not only various complexes of the pre-Paleozoic oceanic crust reworked in the suprasubduction setting differ in age, but also the mantle rock of similar petrography, vary in the time of their formation.

Savelieva, G. N.; Batanova, V. G.; Berezhnaya, N. A.; Presnyakov, S. L.; Sobolev, A. V.; Skublov, S. G.; Belousov, I. A.

2013-05-01

55

Thematic mapper study of Alaskan ophiolites  

NASA Technical Reports Server (NTRS)

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

Bird, J. M.

1986-01-01

56

The Habitability of Mars: Lessons From Ophiolites on Earth  

Microsoft Academic Search

Ophiolite sequences, sections of lower oceanic crust and upper mantle that have been thrust onto continental craton, are located in northern and central California. These rock suites provide easily accessible outcrops that likely parallel the rock types on Mars. We have begun investigating and characterizing these sites in order to understand better the processes that may be responsible for the

M. Schulte

2003-01-01

57

The Ophiolite Problem, Is It Really a Problem?  

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

58

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

USGS Publications Warehouse

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

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

1999-01-01

59

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

Microsoft Academic Search

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

James E. Wright; Sandra J. Wyld

1986-01-01

60

Project EARTH-13-MPS1: Constraining the origin and obduction history of the Oman Ophiolite: Structure, petrology and modeling of the ophiolite lower crust, Moho  

E-print Network

Project EARTH-13-MPS1: Constraining the origin and obduction history of the Oman Ophiolite: The Oman Ophiolite is a ~15 km thick unit of upper mantle peridotite (harzburgite, dunite, wehrlite in the foreland structures in UAE and Oman. Obduction processes and timing are directly relevant to the oil

Henderson, Gideon

61

Obduction initiation: evidence from the base of New Caledonia ophiolite  

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

62

Free SiO2 from Mantle of Ophiolite in Tibet, China  

Microsoft Academic Search

Free SiO2 and their host minerals were discovered from podiform chromitites of the Luobusa ophiolite, Tibet. The ophiolitic body is about 200 km east-southeast of Lhasa in the Indus-Yarlung Zangbo suture zone. It is a fault-bounded slab, 1-2 km thick, with about 70 km2 in area. Luobusa ophiolitic body consists of harzburgite, cumulative and m\\

W. Bai; J. Yang; Y. Ren; Q. Fang; B. Yan; H. Rong

2004-01-01

63

Lead isotopic studies of the Samail ophiolite, Oman  

NASA Technical Reports Server (NTRS)

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.

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

1981-01-01

64

Oceanic ridge metamorphism of the East Taiwan Ophiolite  

Microsoft Academic Search

Brecciated mafic+ultramafic plutonic rocks of the East Taiwan Ophiolite occur as detritus and slide blocks in the Pliocene\\u000a Lichi Mélange. These plutonic rocks have been subjected to two stages of post-magmatic recrystallization: (I) pre-brecciation\\u000a ridge-type metamorphism attended by high-grade greenschist and rare amphibolite facies physical conditions; and (II) later\\u000a off-axis metamorphism under zeolite to lowest greenschist facies conditions that postdated

J. G. Liou; W. G. Ernst

1979-01-01

65

The Eastern Carpathians “ophiolites” (Romania): Remnants of a Triassic ocean  

Microsoft Academic Search

Mesozoic ophiolitic and related rocks in the Eastern Carpathians occur in three areas, from north to south: Rar?u, H?ghima? and Per?ani Mts. They are found as blocks ranging from few metres to a few kilometers in size and as centimetre-sized in breccias, most likely embedded in the Late Barremian–Early Albian Wildflysch formation. Compositionally, they range from lherzolites and harzburgites to

Volker Hoeck; Corina Ionescu; Ioan Balintoni; Friedrich Koller

2009-01-01

66

The Yarlung Zangbo Suture Zone Ophiolites, Tibet: A Synthesis  

NASA Astrophysics Data System (ADS)

This paper presents the main results of a research project focused on the Yarlung Zangbo Suture Zone (YZSZ) ophiolites and related mélanges and flysch. Study area is representing a 300 km long and 30 km wide segment. The Cretaceous ophiolites are remnants of Neo-Tethys basin which was almost totally consumed in a northward oriented subduction zone and southerly transported onto the Indian plate colliding with Eurasia circa 50 Ma ago. The ophiolite massifs rest on an ophiolitic mélange, containing locally 124-127 Ma garnet- bearing and garnet-free foliated amphibolites. These amphibolites experienced high-temperature and relatively high pressure suggesting they were generated because of initiation of an intraoceanic subduction zone. Further south a Triasic flysch and a Cretaceous mélange contain igneous blocks derived from partial reworking of Indian passive margin and Tethyan ocean-floor. The amphibolites show arc tholeiitic affinities such La/Sm 0.8 and negative Ta and Nb anomalies and are geochemically similar to the ophiolitic mafic units. The mafic blocks found in the flysch and mélange zones are of intra-continental and intra-oceanic geochemical affinities respectively. The ophiolites are made of cpx-harzburgitic mantle and thin gabbro-lacking crust. Mantle harzburgites are subdivided into two groups. Samples from the western massifs are characterized by porphyroclastic textures, aluminous spinels (Cr/(Cr+Al) : 0.13-0.21), low average TiO2 content (0.04 wt.%), depleted REE patterns (average La/Sm ratios 0.4) and derive from 7-12 % melting of N-MORB-like source. Samples from the eastern massifs are granular peridotites with Cr-rich spinels (Cr/(Cr+Al) : 0.33-0.74), very low average TiO2 content (0.008 wt. %), enriched REE patterns (average La/Sm 5.6) and could originate from 20-30 % partial melting of a depleted source further enriched by an episode of metasomatism. Crustal samples are mainly basaltic flows, diabasic and rare gabbroic intrusions. They are subdivided into two groups. The western group is made of pl-cpx-phyric arc tholeiites with depleted REE patterns (average La/Sm 0.6) and slight Ta-Nb negative anomalies. The eastern group is made up of pl-cpx-(am)-phyric arc tholeiites having fractionated REE patterns (average La/Sm 4.6) and strong Ta-Nb negative anomalies. Boninitic component increases towards the east and is prominent in Luobusa area. These results are consistent with derivation of the ophiolites from dismemberment of back-arc (western portion) and intra-oceanic arc (eastern portion) sectors of the Neo-Tethys marginal basin.

Hebert, R.; Guilmette, C.; Dupuis, C.; Dubois-Cote, V.; Huot, F.; Dostal, J.; Wang, C.; Li, Y.

2006-12-01

67

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

Microsoft Academic Search

The origin and significance of ophiolites has been a major focus of research over the past three decades, with most attention focusing on whether they form at mid-ocean ridges or above subduction zones. The termination of igneous activity in ophiolites has received far less attention, since it is assumed that igneous activity ends when the ophiolite is obducted. The middle

C. A. Snow; J. W. Shervais

2002-01-01

68

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

Microsoft Academic Search

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

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

1995-01-01

69

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

E-print Network

Deep and High-temperature Hydrothermal Circulation in the Oman Ophiolite------- Petrological (Oman) shows that most of the gabbros have been affected by successive stages of alteration, starting the internal wall of the magma chamber. KEY WORDS: hydrothermal systems; Oman ophiolite; Sr and O isotopes

Demouchy, Sylvie

70

EPR microplates, a model for the Oman Ophiolite F. Boudier, A. Nicolas, B. lldefonseand D. Jousselin  

E-print Network

EPR microplates, a model for the Oman Ophiolite F. Boudier, A. Nicolas, B. lldefonseand D ci&x 05, France ABSTRACT Structural mapping in the Southern half of the Oman ophiolite has revealed. Such an activity supports the comparison of the Oman palaeoridge system with an oceanic microplate, and provides

Demouchy, Sylvie

71

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

E-print Network

Cooling rates in the lower crust of the Oman ophiolite: Ca in olivine, revisited Jill A. Van crust of the Khafifah section in the Wadi Tayin massif of the Oman ophiolite. Additionally, very high B.V. All rights reserved. Keywords: calcium; olivine; oman; hydrothermal; cooling; crust 1

VanTongeren, Jill A.

72

Rare-earth element geochemistry of the Samail ophiolite near Ibra, Oman  

Microsoft Academic Search

Rare-earth element (REE) analyses of 68 rocks and mineral separate samples from the Samail ophiolite clearly differentiate the various units of the ophiolite suite and indicate that the crustal suite is cogeneitc, produced by crystal fractionation of basaltic magma in a spreading ridge magma chamber. Mantle peridotities are residual in rare-earth character, but cannot be clearly related to the overlying

John S. Pallister; Roy J. Knight

1981-01-01

73

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

Microsoft Academic Search

A unique feature of the Circum Pacific orogenic belts is the occurrence of ophiolitic bodies of various sizes, most of which display petrological and geochemical characteristics typical of supra-subduction zone oceanic crust. In SE Asia, a majority of the ophiolites appear to have originated at convergent margins, and specifically in backarc or island arc settings, which evolved either along the

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

2004-01-01

74

The Habitability of Mars: Lessons From Ophiolites on Earth  

NASA Astrophysics Data System (ADS)

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

Schulte, M.

2003-12-01

75

Journal of the Geological Society, London, Vol. 147, 1990 pp. 59-68, 7 figs. Printed in Northern Ireland TheSunnfjordMelange,evidence of Silurianophioliteaccretion  

E-print Network

during the ophiolite accretion. A U-Pb zircon age of 443 f3 Ma from the ophiolite, the Silurian fossils (Caradoc)toupper (Ashgill) Ordovician oceaniccrustpresently recognized in the Scandinavian Caledonides will be discussed within the framework provided by the geochronology and fossil occurrences in the area

Andersen, Torgeir Bjørge

76

Geochemistry of the Boil Mountain ophiolitic complex, northwest Maine, and tectonic implications  

NASA Astrophysics Data System (ADS)

A coherent ophiolitic complex of pyroxenite, serpentinite, metagabbro, mafic volcanics, felsic volcanics and sediments crops out in NW Maine, adjacent to the Chain Lakes massif. The complex (here informally referred to as the Boil Mountain ophiolitic complex) is about 500 m.y. old. The volcanic sequence is not typical of ophiolites in that it contains a large proportion of felsic volcanics. The mafic volcanics are divided into two geochemical groups. A stratigraphically lower group is depleted in Ti, Zr, Y, Cr and REE contents similar to basalts from supra-subduction zone ophiolites. An upper mafic group has trace element contents similar to normal mid-ocean ridge basalts. The felsic volcanics are mostly rhyolitic and similar to low-K rhyolites found in the forearc of the Marianas trench and in an island arc sequence in the Klamath Mountains, California. The flat REE patterns of the felsic volcanic rocks are similar to those found in siliceous rocks in the Oman ophiolite. The presence of thick sequences of felsic volcanics, the abundance of pyroxenite, the low Ti, Zr and REE contents of some mafic rocks, the flat REE patterns of the felsic volcanics, and the composition of clinopyroxene all suggest the complex was formed in the vicinity of a subduction zone. The complex may be correlated with ophiolitic fragments in the eastern part of the Dunnage Zone in Newfoundland, rather than the main ophiolite belt of the western Appalachians.

Coish, R. A.; Rogers, N. W.

1987-09-01

77

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

NASA Astrophysics Data System (ADS)

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

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

2013-03-01

78

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

NASA Astrophysics Data System (ADS)

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

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

2013-05-01

79

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

NASA Astrophysics Data System (ADS)

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

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

2009-07-01

80

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

81

Overthrust Emplacement of New Caledonia Ophiolite: Geophysical Evidence  

NASA Astrophysics Data System (ADS)

Geophysical studies support inferences from outcrop geology that during the Late Eocene an ophiolite sheet exposed on New Caledonia was thrust southward over a rock complex consisting of sedimentary, volcanic, and metamorphic rocks that range in age from Pre-Permian to Eocene. The outcropping ultramafic complex consists of a layered sequence, approximately 3000 m thick, including harzburgite, dunite, wehrlite, serpentinite, and gabbro. The absence of pillow basalts and sheeted dikes on land suggests that during or after overthrusting, these units were removed either by thrust faulting or by erosion. Seismic refraction profiles, collected over the adjacent Loyalty Basin, show that the lower crust and upper mantle are characterized by velocities of 4.7, 5.8, 6.8, and 8.1-8.4 km/s, suggesting the presence of a complete oceanic section that includes sediment, basalt, gabbro, and peridotite. The lower crust is probably 6.5 km thick beneath the Loyalty Basin. A seismic reflection profile that extends across the Loyalty Basin is interpreted to show the oceanic crust rising toward New Caledonia. We suggest that the ophiolite on New Caledonia is continuous with this rising oceanic crust. Studies of gravity anomalies observed both on land and offshore indicate the presence of a short-wavelength, high-amplitude (+180 mGal), asymmetrical, free-air gravity anomaly along the northeastern coast of New Caledonia. A gravity anomaly profile, calculated for a geological model characterized by a 10-km-thick slab of oceanic crust and mantle material extending continuously from the ophiolite on New Caledonia to the oceanic crust of the Loyalty Basin, successfully matches the observed gravity data. We interpret the extension of the high-gravity anomaly over the whole length of the eastern lagoon as evidence for the lateral extension of the root zone of the ultramafic complex. A tentative geodynamic reconstruction suggests that the proto-Loyalty Basin crust formed a marginal basin along the eastern margin of Gondwanaland prior to Upper Cretaceous time; after the opening of the New Caledonia Basin during Upper Cretaceous to early Paleocene time a subduction zone developed along the western margin of New Caledonia. While this zone was active, the Loyalty Basin lithosphere overthrust New Caledonia in a southeasterly direction and locally rotated clockwise. This compressional tectonic event was terminated by upper Eocene time.

Collot, J. Y.; Malahoff, A.; Recy, J.; Latham, G.; Missegue, F.

1987-06-01

82

Feedbacks between deformation and melt distribution in the crustmantle transition zone of the Oman ophiolite  

E-print Network

Feedbacks between deformation and melt distribution in the crust�mantle transition zone of the Oman in the Oman ophiolite. This zone represents an `end member' case for analyzing feedbacks between deformation

Tommasi, Andrea

83

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

Microsoft Academic Search

We estimated metamorphic conditions for the ?6 Ma Taitao ophiolite, associated with the Chile triple junction. The metamorphic grade of the ophiolite, estimated from secondary matrix minerals, changes stratigraphically downwards from the zeolite facies, through the prehnite–actinolite facies, greenschist facies and the greenschist–amphibolite transition, to the amphibolite facies. The metamorphic facies series corresponds to the low-pressure type. The metamorphic zone boundaries

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

2007-01-01

84

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

Microsoft Academic Search

The Semail ophiolite in Oman was emplaced from NE to SW at least 200 km over the Arabian passive margin, probably over 450 km in total, during the late Cretaceous (95-70 Ma). The first phase of obduction involved NE-directed subduction of Triassic-Jurassic basalt at least 45-50 km beneath the ophiolite, whilst the crustal sequence was forming (U-Pb zircons from plagiogranites,

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

2003-01-01

85

Mineralogical and geochemical characterization of listwaenite from the Semail Ophiolite, Oman  

Microsoft Academic Search

The late Cretaceous-lower Tertiary hydrothermal alteration of serpentinized peridotite in the Semail ophiolite has formed two distinct types of listwaenite. Type I is characterized by the presence of calcite (Type IA) or dolomite (Type IB)+fuchsite±spinel. Type II is dominated by silicate minerals (quartz, chlorite, fuchsite)±calcite+dolomite±magnetite±apatite±plagioclase. Most listwaenites occur as veins along thrust fault zones within the ophiolite mélange. High Cr

Sobhi Nasir; Abdul Razak Al Sayigh; Abdulrahman Al Harthy; Salah Al-Khirbash; Omar Al-Jaaidi; Abdullah Musllam; Ali Al-Mishwat; Salim Al-Bu'saidi

2007-01-01

86

Geology, petrology and tectonic setting of the Late Jurassic ophiolite in Hokkaido, Japan  

Microsoft Academic Search

The Gokurakudaira Formation, which has a N–S zonal distribution within a latest Jurassic greenstone belt in Hokkaido Island, Japan, constitutes the uppermost ultramafic–mafic unit of the Horokanai Ophiolite. The following three hypotheses for the origin of the ophiolite have been proposed: (1) a mid-oceanic ridge; (2) an oceanic plateau; and (3) an island arc. The Gokurakudaira Formation can be subdivided

Reishi Takashima; Hiroshi Nishi; Takeyoshi Yoshida

2002-01-01

87

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

USGS Publications Warehouse

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

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

1975-01-01

88

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

NASA Astrophysics Data System (ADS)

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

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

2008-10-01

89

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

NASA Astrophysics Data System (ADS)

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

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

1988-10-01

90

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

NASA Astrophysics Data System (ADS)

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

Papanikolaou, Dimitrios

2009-03-01

91

Igneous Petrology, Geochemistry and Geochronology of the Dongcaohe Ophiolite From the Qilian Fold Belt, NW China  

NASA Astrophysics Data System (ADS)

The Dongcaohe ophiolite, a nappe-type ophiolite, occurs as a fault-bounded block within a volcanic belt in the Tuo-Lai Mountains, southern margin of the north Qilian fold belt. Field works show that the Dongcaohe ophiolite is composed of an intrusive sequence and an extrusive sequence. The intrusive sequence in turn consists of a layered cumulate dunite-troctolite-anorthosite-gabbro series and an isotropic gabbro-norite-hornblende gabbro series. The extrusive sequence consists of hornblende diabase and hornblende basalt. The lithological and structural features show that the Dongacohe ophiolite is analogous to the lherzolitic ophiolite type (LOT) and to the present-day slow-spreading oceanic ridge. Petrographic textures show that the sequence of mineral crystallization of the plutonic mafic-ultramafic sequence of the Dongcaohe ophiolite is olivine-Cr-spinel-plagioclase-clinopyroxene-orthopyroxene-Fe-Ti oxide minerals, similar to that of tholeiitic magma crystallizing in an anhydrous and low-pressure chamber. The Cr-spinels which occur commonly in the layered dunite-troctolite-anorthosite-gabbro cumulate series frequently carry solid inclusions of anhydrous silicate minerals such as clinopyroxene and orthopyroxene, hydrous high-temperature silicate minerals such as Na-phlogopite and pargasite, hydrous low-temperature silicate minerals such as chlorite, serpentine, hydrogarnet, and analcite, Fe-Ti oxide minerals, and base-metal sulfides. The compositions of the hydroxyl-sodic phases (pargasite and Na-phlogopite), similar to those of the Oman ophiolite, suggest a subduction-related origin. The REE patterns of basaltic lavas show slightly depleted LREEs, identical to that of typical normal-type mid-ocean ridge basalt (N-MORB). Also the tectonomagmatic discriminating diagrams of Ti/V, Zr/Y-Y, Zr-Ti-Y, Nb-Zr-Y, and Hf-Th-Nb suggest that they have both N-MORB and IAT (island arc tholeiite) geochemical characteristics simultaneously, similar to the BABB (backarc basin basalt). Separated zircons from the isotropic gabbro were studied with SHRIMP, and a 15-points concordant age of 497.3+/-5.5 Ma was obtained. The Dongcaohe ophiolite is associated with the island arc tholeiite, boninite, and ocean island basalts which crop out nearby, implying that the volcanic belt of the Tuo-Lai Mountain is part of an oceanic lithosphere in a supra-subduction zone. The geological significance of the Dongcaohe ophiolite is that the southward subduction of the paleo-Qilian oceanic lithosphere did occur in lower Ordovician period and gave rise to a backarc oceanic lithosphere.

Tseng, C.; Yang, H.; Liu, D.; Hsieh, C.; Wu, H.; Zuo, G.

2003-12-01

92

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

E-print Network

the emplacement of a tion occurred during emplacement. The Sulawesi large ophiolite belt, resulting from the Miocene ophiolite is only one of a number of ophiolites in collision between the Sulawesi island arc Sea and the NW Banda Introduction Sea, plus gravity and geology on the east arms of Sulawesi

McCaffrey, Robert

93

Modeling the seismic signature of structural data from the Oman Ophiolite: Can a mantle diapir be detected beneath  

E-print Network

Modeling the seismic signature of structural data from the Oman Ophiolite: Can a mantle diapir diapirs mapped in the Oman ophiolite and compare these results with those of active source seismic anisotropy that are consistent with Oman diapirs of different size. Forward calculations of the delay time

Toomey, Doug

94

The effect of dynamic recrystallization on olivine fabric and seismic anisotropy: Insight from a ductile shear zone, Oman ophiolite  

E-print Network

a ductile shear zone, Oman ophiolite Katsuyoshi Michibayashi a,, Toshiki Ina a , Kyuichi Kanagawa b a shear zone in a sinistral sense of shear within the northern Fizh mantle section of the Oman ophiolite in the region of the observation. © 2006 Elsevier B.V. All rights reserved. Keywords: ductile shear zone; Oman

Kanagawa, Kyuichi

95

Comment on ``dating the geologic history of Oman's Semail ophiolite: insights from UPb geochronology'' by C. J. Warren,  

E-print Network

COMMENT Comment on ``dating the geologic history of Oman's Semail ophiolite: insights from U of the Semail ophiolite of Oman. The new U­Pb data concern the eclogites preserved at the margin of the Arabian' geochemical signature has been questioned (e.g., Hofmann 2004), especially in the case of Oman (Godard et al

Demouchy, Sylvie

96

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

97

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

98

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)

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

Wright, James E.; Wyld, Sandra J.

1986-08-01

99

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

NASA Astrophysics Data System (ADS)

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

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

2009-05-01

100

Melt Migration in the Mantle Lithosphere: Evidence From Ophiolitic Peridotites  

NASA Astrophysics Data System (ADS)

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 from the Ligurian Tethys. Microstructural and compositional, both mineral modal and bulk rock mineral, features suggest that, in most cases, melt/peridotite interaction and addition of basaltic components are the main causes of formation of these Plg peridotites. The most evident microstructural feature is the interstitial crystallization of mm-size unstrained Plg crystals and microgabbroic aggregated between and within deformed and kinked mantle minerals. Frequently, these microtextures are coupled to replacements of new unstrained orthopyroxene(Opx) on kinked mantle Ol. This evidence suggests that the percolating/impregnating melt was Px(-silica)-saturated. Cpx trace element compositions indicate the MORB affinity of the percolating melt. In the studied peridotite massifs, m- to decametre-wide elongated bodies of spinel(Sp) dunites cut both reacted Sp peridotites and impregnated Plg peridotites, showing clear replacive textures and microstructures. This suggests that melt migration occurred along focused zones, most probably following previous compositional and structural discontinuities, and completely dissolved Px, transforming the pre-existing peridotite in high permeability, high porosity channels. Small interstitial and large megacrystic Cpx crystallized in these dunite channels: Cpx trace element composition clearly indicates that these channels were exploited by aggregate MORB melts to reach shallow crustal levels. Our data from the Alpine-Apennine ophiolitic peridotites illustrate a composite scenario of MORB-type melt migration during inception and opening of the Jurassic Ligurian Tethys ocean.

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

2006-12-01

101

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

NASA Astrophysics Data System (ADS)

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

Gahlan, Hisham A.; Arai, Shoji

2009-01-01

102

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

E-print Network

structure, ophiolite emplacement, continental growth, and the terrane hypothesis Joseph M. EnglishT, Stephen Triassic to Lower Jurassic time, arc magmatic rocks of the Stikine terrane, arc-marginal sediments sedimentary and volcanic rocks of the Cache Creek terrane represented a magmatic arc, forearc basin, forearc

Johnston, Stephen T.

103

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

Microsoft Academic Search

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

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

1983-01-01

104

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

Microsoft Academic Search

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

Chris Parkinson

1998-01-01

105

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

Microsoft Academic Search

New paleomagnetic results from the eastern Klamath Mountains of northern California show that Neoproterozoic rocks of the Trinity ophiolitic complex and overlying Middle Devonian volcanic rocks are latitudinally concordant with cratonal North America. Combining paleomagnetic data with regional geologic and faunal evidence suggests that the Trinity Complex and related terranes of the eastern Klamath plate were linked in some fashion

Edward A. Mankinen; Nancy Lindsley-Griffin; John R. Griffin

2002-01-01

106

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

Microsoft Academic Search

Detailed transects were sampled across dunite bodies in the Josephine and Trinity ophiolites. The major peridotite lithologies sampled in the Josephine transect are a sequence of dunite and harzburgite and in the Trinity transect a sequence of dunite, harzburgite, lherzolite, and plagioclase lherzolite (DHL-PL). Major, minor, and selected trace element abundances in olivine, orthopyroxene, clinopyroxene, and spinel were measured. The

Zachary Morgan; Yan Liang; Peter Kelemen

2008-01-01

107

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

Microsoft Academic Search

New data collected in central Crete on the ophiolite-bearing units, result in a better understanding of the role and tectonic significance of these units in the construction of this segment of the Hellenides. These nappes are composed of three main tectonic units that, characterized by different metamorphic facies conditions, are represented by an un-metamorphosed lower unit, a greenschist to HP

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

108

Mapping of an ophiolite complex by high resolution visible-infrared spectrometry  

E-print Network

Terrestre et Planétaire, Toulouse, France Abstract. The Sumail massif of the Oman peridotite has been and the development of appropriate processing methods for geological identifications. 1. Introduction The Oman massif of the Oman Ophiolite (Figure 1 a and b) was surveyed with the imaging spectrometer HyMap on 4

Combe, Jean-Philippe

109

TECTONICS, VOL. 15,NO. 6, PAGES 1230-1247,DECEMBER 1996 Rapid emplacementof the Oman ophiolite  

E-print Network

TECTONICS, VOL. 15,NO. 6, PAGES 1230-1247,DECEMBER 1996 Rapid emplacementof the Oman ophiolite,andtherate and durationof thatthrusting.Hornblende4øAr/39Arages demonstrate that the igneous oceanic crust in Oman]suggeststhattheOman ophioliteoriginatedasanintraoceanicarcor back

Hacker, Bradley R.

110

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

Microsoft Academic Search

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)

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

2010-01-01

111

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

E-print Network

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

Royal Holloway, University of London

112

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

NASA Astrophysics Data System (ADS)

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

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

2014-09-01

113

Mantle petrology and mineralogy of the Thetford Mines Ophiolite Complex  

NASA Astrophysics Data System (ADS)

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

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

2008-01-01

114

Microbiology of Ultrabasic Groundwaters of the Coast Range Ophiolite, California  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

115

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

USGS Publications Warehouse

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

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

1993-01-01

116

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

NASA Astrophysics Data System (ADS)

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.

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

2014-07-01

117

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

E-print Network

nappe in the Lesser Caucaus of Armenia helps us explain the obduction processes in the Caucasus region. The ophiolitic nappe preserves three distinct magmatic series, obducted in a single tectonic slice over the South

Demouchy, Sylvie

118

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

NASA Astrophysics Data System (ADS)

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

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

2010-05-01

119

Geochemical features of the two early paleozoic ophiolitic zones and volcanic rocks in the central-southern Tianshan Region, Xinjiang  

Microsoft Academic Search

This paper deals with the geochemical features of the two Early Paleozoic ophiolite zones in the central-southern Tianshan\\u000a region and the central Tianshan igneous rock belt between them. Study results suggest that the central Tianshan belt was an\\u000a Ordovician volcanic arc with an affinity of continental crust, and the Kumux-Hongliuhe ophiolitic zone that is located on\\u000a the southern margin of

Guo Jian; Shu Liangshu; Charvet Jaques; Laurent C. Sebastien; Sun Shuwen

2002-01-01

120

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

USGS Publications Warehouse

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

Lanphere, M. A.; Pamic, J.

1983-01-01

121

U\\/Pb, Sm\\/Nd and Rb\\/Sr geochronological and isotopic study of northern Sierra Nevada ophiolitic assemblages, California  

Microsoft Academic Search

Distinct ophiolitic assemblages occur as oceanic basement within three of the four regional tectonic belts of the northern Sierra Nevada. New U\\/Pb zircon, Sm\\/Nd and Rb\\/Sr data are presented for each assemblage, providing critical geochronological and isotopic constraints on the petrogenesis and tectonic evolution of the ophiolitic and associated ensimatic assemblages. Ophiolitic assemblages include from west to east the Smartville

J. B. Saleeby; H. F. Shaw; Sidney Niemeyer; E. M. Moores; S. H. Edelman

1989-01-01

122

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

PubMed

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

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

2004-11-11

123

Structure of the sheeted dike complex of the Samail Ophiolite near Ibra, Oman  

NASA Astrophysics Data System (ADS)

The sheeted dike complex is a regionally mappable geologic unit with a consistent position in the Samail ophiolite stratigraphy. Discontinuous exposures of the complex in the Ibra area of the southeast Oman Mountains provide structural data that allow reconstruction of a paleo-spreading ridge axis of 347° for the region and suggests a maximum spreading width of 275 km for the ophiolite. Contact relations within the well-exposed Al Ahmadi section indicate that the complex was formed by multiple parallel intrusion over a narrow zone at the paleo-ridge axis. Split dikes are abundant; facing directions indicate a spreading ridge southwest of the present outcrop area, in conflict with other spreading axis direction indicators.

Pallister, John S.

1981-04-01

124

Petrology and geochemistry of plagiogranite in the Canyon Mountain ophiolite, Oregon  

Microsoft Academic Search

Plagiogranites in the Canyon Mountain ophiolite, Oregon, include a wide range of rock types ranging from diorite to trondhjemite. The plagiogranites are mostly concentrated as an intrusive sill swarm at the top of a section of gabbroic cumulates. The plagiogranites are typically low in K2O and high in Na2O, and are enriched 10–20 times chondrites in REE, and overlap with

David C. Gerlach; William P. Leeman; Hans G. Avé Lallemant

1981-01-01

125

Ophiolites of the Kamchatsky Mys Peninsula, eastern Kamchatka: Structure, composition, and geodynamic setting  

Microsoft Academic Search

Ophiolites of the Afrika Mys Block of the Kamchatsky Mys Peninsula, eastern Kamchatka, are a fragment of an accretionary prism\\u000a that formed in the Late Cretaceous-Eocene on the southern side of the Kronotsky island arc as a result of its collision with\\u000a the Smagino volcanic uplift that arose at the post-Neocomian time on the subducting plate. On the basis of

M. Yu. Khotin; M. N. Shapiro

2006-01-01

126

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

Microsoft Academic Search

The Zambales ophiolite is the major source of chromite ore in the Philippines. The chromitites are concordant cumulates and are associated with distinct chromitite-bearing sequences within the mantle peridotites. Refractory and metallurgical chromite deposits are spatially separated and related to different lithologic associations, which crystallized from different parental magmas. — Refractory chromite ores (30–44 wt% Cr2O3; 20–30 wt% Al2O3) are

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

1986-01-01

127

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

NASA Astrophysics Data System (ADS)

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

France, Lydéric; Ildefonse, Benoit; Koepke, Juergen

2013-08-01

128

Deep origin of the Luobusa ophiolitic peridotites and chromitites in Tibet  

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

129

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

E-print Network

LE METAMORPHISME OCEANIQUE DANS LE COMPLEXE GABBROÏQUE DE L'OPHIOLITE D'OMAN. NICOLLET Christian l'ophiolite d'Oman, à la base du complexe filonien et dans les gabbros isotropes, les conditions et trajets PTt dans le complexe gabbroïque de l'ophiolite d'Oman ... le manteau est traversée de

Nicollet, Christian

130

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

131

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

NASA Astrophysics Data System (ADS)

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

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

2002-12-01

132

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

133

Earthquakes and ophiolite emplacement in the Molucca Sea Collision Zone, Indonesia  

NASA Astrophysics Data System (ADS)

The Molucca Sea in eastern Indonesia is the site of an active arc-arc collision, and among its interesting features is the exposure of ophiolitic rocks on islands of the Talaud-Mayu Ridge (TMR), a largely submarine ridge formed of highly deformed rocks that bisects the collision zone. To explore the relationship between earthquakes and uplift of the ophiolite, the centroid depths and fault plane solutions of 18 large earthquakes occurring in the past 20 years beneath the TMR are constrained by inversion of their teleseismic, long-period P and SH waveforms. Centroid depths range from 16 to 36 km, but uncertainties allow a range of 10 to 45 km. All events show thrust faulting; nodal planes strike parallel to the NNE trending TMR and dip 40±9° to the ESE and 53±9° to the WNW. Published seismic refraction and gravity data support the inference that the WNW dipping, steeper nodal planes are the fault planes. Dips do not change with depth, indicating either that the fault flattens below 40 km depth or that the steeply dipping fault penetrates the entire thickness of the lithosphere. I conclude that the Molucca Sea ophiolite is being lifted by high-angle thrust faults (45° to 60°) that extend at least 15 km into the upper mantle. Summing seismic moment tensors and assuming uniformly distributed deformation suggest that the closure across the Molucca Sea collision zone may account for 14% to 59% of the Pacific-Eurasia convergence vector or 15% to 63% of the Philippine-Eurasia convergence. Moment tensor sums provide estimates of crustal thickening rates beneath the TMR of 7 to 20 mm/yr and these imply uplift rates of 2 to 6 mm/yr if the increase in crustal thickness is isostatically compensated. Hence the ophiolite and an enormous volume of mélange are being lifted higher than the flanks of the island arcs and will be emplaced on them by gravity. I suggest that ophiolites occur in this tectonic setting because the concave down shape of the subducted oceanic lithosphere beneath the Molucca Sea increases its effective buoyancy, preventing it from sinking out of the way of the encroaching island arcs. This case suggests a family of convergent margin settings in which normal oceanic lithosphere is buoyed up so that ophiolites can be stripped from it and emplaced on a continental margin or island arc. These settings include those where (1) slab geometry restricts asthenospheric flow away from the convergent margin, (2) subduction of younger, less dense lithosphere decreases its negative buoyancy, (3) a passive continental margin resists subduction, or (4) the presence of a thick thermal boundary layer beneath a passive continental margin inhibits asthenospheric flow necessary to accommodate the sinking oceanic lithosphere.

McCaffrey, Robert

1991-04-01

134

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

NASA Astrophysics Data System (ADS)

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

Koutsovitis, P.; Magganas, A.

2012-04-01

135

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

NASA Astrophysics Data System (ADS)

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.

Xiong, Yingqian

136

Mineral and whole rock geochemistry of the spinel peridotites from the East Taiwan Ophiolite, Southeast Taiwan  

NASA Astrophysics Data System (ADS)

The East Taiwan Ophiolite (ETO) is hosted within Miocene and Pliocene turbidites and agglomerates of southeast Taiwan. The ophiolite suite of rocks is dismembered and appears as coherent ';blocks' within the Lichi mélange distributed around the southern Coastal Range. The original stratigraphy of the ETO, as exposed in the larger blocks, consists of a sequence of incompletely metamorphosed gabbro, diabase and peridotite capped by pelagic red shale and an overlying sequence of originally glassy basaltic pillow lavas and volcanic breccias with intercalated red shale. The spinel peridotites consist mostly of serpentinized harzburgite with minor amounts of lherzolite. The peridotites have bulk rock Al2O3 content between 5.5 wt% and 8.9 wt% and Mg-numbers between 90.1 and 91.3 whereas the Cr-numbers of the spinel range between 40 and 55. The initial results indicate the peridotites were formed at an ocean-ridge setting as they are similar to other peridotites dredged at ocean-ridge settings and collected from ophiolites interpreted to be ocean-ridge settings. A previous interpretation suggests that the ETO represents the western terminus of the Philippine Sea plate and was analogous to mature, marginal oceanic crust however recent work based on geochemistry of the basaltic rocks suggests that the ETO formed in a mid-ocean ridge environment related to the opening of the South China Sea during the middle Miocene. The new geochemical data from this study are consistent with a mid-ocean ridge setting and that the ETO likely represents new oceanic crust formed during the opening of the South China Sea.

Hsieh, R. B.; Shellnutt, J. G.

2013-12-01

137

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

NASA Astrophysics Data System (ADS)

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

Coveney, R. M.

2008-12-01

138

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

NASA Astrophysics Data System (ADS)

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

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

2012-09-01

139

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

NASA Astrophysics Data System (ADS)

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

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

2010-03-01

140

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

NASA Astrophysics Data System (ADS)

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

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

1981-02-01

141

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

NASA Astrophysics Data System (ADS)

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

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

2006-05-01

142

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

143

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

Microsoft Academic Search

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

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

2008-01-01

144

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

145

Melanges, No. 23.  

ERIC Educational Resources Information Center

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

Melanges, 1997

1997-01-01

146

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

147

Spring and surface water quality of the Cyprus ophiolitesHydrology and Earth System Sciences, 6(5), 797817 (2002) EGS Spring and surface water quality of the Cyprus ophiolites  

E-print Network

originate in the Troodos Massif. Spring discharge is generally highest in the plutonic rocks of the Troodos waters associated with the ophiolite rocks of Cyprus shows five broad water types (1) Mg-HCO3 , (2) Na to hydrolysis of the basic/ultrabasic rock as modified by CO2 -weathering. An increase in salinity is also

Paris-Sud XI, Université de

148

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

SciTech Connect

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

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

1986-01-01

149

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

NASA Astrophysics Data System (ADS)

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.

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

2014-02-01

150

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

NASA Technical Reports Server (NTRS)

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.

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

1980-01-01

151

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

NASA Astrophysics Data System (ADS)

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

Raznitsin, Yu. N.

2012-01-01

152

Modelling Chemical Diffusion with Extraction - Application to Observed Geochemical Gradients in the Trinity Ophiolite of Northern California  

Microsoft Academic Search

The Trinity Ophiolite contains numerous outcrops of dunite progressively surrounded by harzburgite, spinel lherzolite, and plagioclase lherzolite. The dunites are interpreted as pathways of focussed melt ascent with incompatible trace element contents decreasing closer to the dunite resulting from melt extraction (Kelemen et al. 1992); Lundstrom (Nature, 2000) has proposed that alkali elements diffuse from dunite into the surrounding peridotite

A. T. Hsui; C. C. Lundstrom

2002-01-01

153

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

Microsoft Academic Search

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

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

154

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

E-print Network

EPSL ELSEVIER Earth and Planetary Science Letters 144(19961239-250 Magma chambers in the Oman studies in the Oman ophiolite, we suggest that the accretion of the lower crust may not proceed entirely unit at all scales. If this interpretation is correct, the gabbro section of the oceanic crust in Oman

Demouchy, Sylvie

155

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

NASA Astrophysics Data System (ADS)

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

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

2013-07-01

156

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

E-print Network

Osmium isotope systematics of the Proterozoic and Phanerozoic ophiolitic chromitites: In situ ion © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.03.021 #12;1. Introduction The rhenium­osmium

157

Significance of age relations above and below upper Jurassic ophiolite in the Geysers--Clear Lake region, California  

Microsoft Academic Search

In The Geysers--Clear Lake area of northern California, a fragmented Upper Jurassic ophiolite overlain depositionally by the Great Valley sequence is juxtaposed over deformed and metomorphosed rocks of the Franciscan assemblage along the Coast Range thrust. The basal strata of the Great Valley sequence consist of thick breccias of mafic clasts, identical in composition to the upper part of the

R. J. McLaughlin; E. A. Jr. Pessagno

1978-01-01

158

New ophiolite occurrences in Sudan and constraint on the western boundary of the Nubian Shield: Petrographical and geochemical evidence  

NASA Astrophysics Data System (ADS)

Important mafic-ultramafic masses have been located for the first time in the intersection area between the Keraf Shear Zone and the Nakasib Suture Zone of the Nubian Shield. The masses, comprising most of the members of the ophiolite suite, are Sotrebab and Qurun complexes east of the Nile, and Fadllab complex west of the Nile. The new mafic-ultramafic masses are located on the same trend of the ophiolitic masses decorating the Nakasib Suture. A typical complete ophiolite sequence has not been observed in these complexes, nevertheless, the mafic-ultramafic rocks comprise basal unit of serpentinite and talc chlorite schists overlain by a thick cumulate facies of peridotites, pyroxenites and layered gabbros overlain by basaltic pillow lavas with dolerite dykes and screens of massive gabbros. Associated with pillow lavas are thin layers of carbonates and chert. The best section of cumulate mafic-ultramafic units has been observed in Jebel Qurun and El Fadlab complexes, comprising peridotites, pyroxenites and layered gabbros. Dolerite dykes and screens of massive gabbros have been observed with basaltic pillow lava sections in Wadi Dar Tawaiy. The basal ultramafic units of the complexes have been fully or partly retrograded to chlorite magnetite schist and talc to talc-carbonate rocks (listowenites), especially in the Jebel Qurun and Sotrebab complexes. Petrographically, the gabbros (layered and massive) and the basaltic pillow lavas show mineral assemblages of epidote amphibolite facies. The mafic members from the three complexes show a clear tholeiitic trend and oceanic floor affinity. The pillow lavas plot in the field of oceanic floor basalt, namely in the back arc field. Primitive mantle normalized spider diagram of the pillow lavas reveals a closer correspondence to Enrich-Mid-Oceanic Ridge Basalt (E-MORB) type, which is confirmed by the flat chondrite normalized Rare Earth Elements (REE) pattern. Field, petrographical and geochemical evidence supports ophiolitic origin of the three complexes. The newly discovered ophiolitic complexes mark the western continuation of the Nakasib Suture Zone.

Ali, E. A.; Abdel Rahman, E. M.

2011-01-01

159

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

160

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

161

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

162

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

E-print Network

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

Rioux, Matthew

163

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

E-print Network

New high-precision single grain U-Pb zircon geochronology and whole rock Nd isotopic data provide insight into the magmatic and tectonic development of the Samail ophiolite. The analyzed rocks can be broadly divided into ...

Rioux, Matthew

164

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

NASA Astrophysics Data System (ADS)

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

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

2009-05-01

165

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

166

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

NASA Technical Reports Server (NTRS)

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.

Smith, Susan E.; Elthon, Don

1988-01-01

167

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

NASA Astrophysics Data System (ADS)

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

Koutsovitis, Petros; Magganas, Andreas

2013-04-01

168

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

USGS Publications Warehouse

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

Page, Norman J.; Talkington, Raymond W.

1984-01-01

169

Extreme Nd isotopic variation in the Trinity Ophiolite Complex and the role of melt\\/rock reactions in the oceanic lithosphere  

Microsoft Academic Search

.  ?Peridotites, dykes and gabbros from the 470–420 Ma Trinity Ophiolite Complex of northern California exhibit large geochemical\\u000a rare earth element (REE) and Nd isotopic variations on the small scales which are indicative of a complex history. The Trinity\\u000a Ophiolite, which covers an area of ?1600 km2, consists of three distinct units: (1) a ?2–4 km-thick sheet of plastically deformed peridotites,

G. Gruau; J. Bernard-Griffiths; C. Lécuyer; O. Henin; J. Macé; M. Cannat

1995-01-01

170

Geochemistry of gabbro sills in the crust-mantle transition zone of the Oman ophiolite: implications for the origin of the oceanic lower crust  

Microsoft Academic Search

Gabbroic sills intruding dunite in the crust-mantle transition zone (MTZ) of the Oman ophiolite have textures and compositions very similar to those in modally layered gabbros that form the lower part of the gabbro section in the ophiolite, and different from those in non-layered gabbros near the dike-gabbro transition. The presence of gabbroic sills in the MTZ indicates that modally

Peter B. Kelemen; Ken Koga; Nobu Shimizu

1997-01-01

171

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

Microsoft Academic Search

Unusual silicate lamellae were discovered within chromites of podiform chromitites in the Luobusa ophiolite, southern Tibet, especially from massive- and nodular-type chromites. Using analytical transmission electron microscopy (ATEM), we found coesite, clinopyroxene and MgSiO3 phase as exsolution lamellae from the host chromites. There is no evidence that the Luobusa ophiolite itself formed at great depth, and the presence of coesite

S. Yamamoto; T. Komiya; S. Maruyama

2007-01-01

172

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

173

Tracking flux melting and melt percolation in supra-subduction peridotites (Josephine ophiolite, USA)  

NASA Astrophysics Data System (ADS)

Here, we investigate the scale and nature of melting and melt percolation processes recorded by 17 supra-subduction peridotites collected in a ~70 km2 area in the northern portion of the Josephine ophiolite (Western USA). We present major and trace element variations in whole rocks; major elements in olivine, orthopyroxene, clinopyroxene and spinel; and trace elements [including rare earth element (REE)] in clinopyroxene and orthopyroxene. In the Josephine peridotites, compositional variability occurs at different scales. On the one hand, large systematic changes from depleted to fertile peridotites occur on large kilometer scales. Field, petrological and geochemical data can be consistently explained if the Josephine mantle experienced variable degrees of hydrous flux melting (10 to >20-23 %), and we argue that small fractions of subduction-derived fluids (0.015-0.1 wt%) were pervasive in the ~70 km2 studied area, and continuously supplied during wedge melting. Fluid localization probably led to increased extent of flux melting in the harzburgitic areas. On the other hand, in single outcrops, sharp transitions from dunite to harzburgite to lherzolite and olivine websterite can be found on meter to centimeter scales. Thus, some fertile samples may reflect limited degrees of refertilization at the outcrop scale. In addition, clinopyroxene and orthopyroxene in ultra-depleted harzburgites (Spinel Cr# > 58) show variable degrees of LREE enrichment, which reflect percolation of and partial re-equilibration with, small fractions of boninite melt. Because the enriched samples also show the highest spinel Cr#, we argue that these enrichments are local features connected to the presence of dunite channels nearby. Lastly, trace element concentrations of pyroxenes in Josephine harzburgites show that they are one of the most depleted harzburgites among worldwide ophiolitic peridotites, indicating particularly high degrees of melting, potentially past the exhaustion of clinopyroxene.

Le Roux, V.; Dick, H. J. B.; Shimizu, N.

2014-10-01

174

Geochemistry of High-Ca Boninite Dike Swarms and the Related Plutonic Rocks in the Oman Ophiolite  

NASA Astrophysics Data System (ADS)

It has been debated whether the tectonic setting of the Oman ophiolite is mid-ocean ridge (MOR) or supra- subduction zone (SSZ). The study of the timing and geochemical modeling of boninite magmatism in the Oman ophiolite suggested that the tectonic setting changed from MOR to SSZ (island arc) setting due to an intra oceanic thrusting (Ishikawa et al., 2004). If this model is acceptable, it is expected that the crustal section of the Oman ophiolite contains the early stage products of island arc magmatism. In the Fizh block of the northern part of the Oman ophiolite, ultramafic cumulate, gabbronorite, plagiogranite, and boninitic dike swarms are recognized as late intrusive rocks, which are largely intruding into MOR type gabbroic crust. The boninitic dikes and the olivine-clinopyroxene ultramafic cumulates have the geochemical similarity of their mineral and parental melt compositions. Their Cr-spinels have an island arc character with high Cr# (> 65) and low TiO2 (< 0.5 wt%). Chondrite normalized multi-elements patterns of the parental magma calculated from clinopyroxene composition of the ultramafic cumulates have a closely resemblance to boninitic dikes with depletion of HFS elements and enrichment of LIL elements than MORB volcanic rocks of the Oman ophiolite. Boninite melt generation requires hydration melting of refractory mantle peridotite under an extremely high temperature and low pressure condition. This condition is generally explained by the addition of slab-derived fluids into a hot young oceanic crust, which previously experienced MORB melt extraction. In this area, boninitic dikes form 500 m to 2 km width dike swarms, which are rooted in the ultramafic cumulates, and strike WNW direction oblique to N-S direction of MORB sheeted dike complex. This indicates that the principal stress direction of Oman ophiolite changed from E-W extension to E-W compression (Yanai et al., 1989). These evidence support a model in which the tectonic setting changes from MOR to SSZ (island arc). And the ultramafic cumulates with the boninitic affinity are interpreted as the early stage products of island arc magmatism. Refference: Ishikawa et al. (2005), EPSL, 240, 355-377; Yanai et al. (1989), jornal of Geography (Japanese), 98, 278-289.

Yamazaki, S.; Miyashita, S.

2008-12-01

175

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

176

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

NASA Technical Reports Server (NTRS)

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

Dixon, S.; Rutherford, M. J.

1979-01-01

177

The Bangong Lake ophiolite (NW Tibet) and its bearing on the tectonic evolution of the Bangong–Nujiang suture zone  

Microsoft Academic Search

The Jurassic Bangong Lake ophiolite, NW Tibet, is a key element within the western part of the Bangong–Nujiang suture zone, which marks the boundary between the Lhasa and Qiangtang blocks. It is a tectonic mélange consisting of numerous blocks of peridotite, mafic lavas and dikes. The mantle peridotites include both clinopyroxene-bearing and clinopyroxene-free harzburgites. The Cpx-bearing harzburgite contains Al-rich spinel

Rendeng Shi; Jingsui Yang; Zhiqin Xu; Xuexiang Qi

2008-01-01

178

The effect of dynamic recrystallization on olivine fabric and seismic anisotropy: Insight from a ductile shear zone, Oman ophiolite  

Microsoft Academic Search

Subhorizontal mantle structures subparallel to the Moho are rotated into NW–SE subvertical orientations across a shear zone in a sinistral sense of shear within the northern Fizh mantle section of the Oman ophiolite. Dynamic recrystallization resulted in grain size reduction of olivine and the development of porphyroclastic texture. Mean olivine grain size stabilized at ?0.7 mm within the shear zone center;

Katsuyoshi Michibayashi; Toshiki Ina; Kyuichi Kanagawa

2006-01-01

179

Nd-Sr-Pb systematics and age of the Kings River ophiolite, California: implications for depleted mantle evolution  

Microsoft Academic Search

Sm-Nd whole-rock and mineral data for the Kings River ophiolite define two isochrons of 485±21 Ma and 285±45 Ma age with ? Nd (483)= +10.7±0.5 and ?Nd (285)= +9.9±1.1, respectively. The 483 Ma isochron is defined by samples of the main igneous construct. Samples from crosscutting diabase dikes and flaser gabbro sheets within the peridotite unit yield the 285 Ma

H. F. Shaw; J. H. Chen; J. B. Saleeby; G. J. Wasserburg

1987-01-01

180

Nd-Sr-Pb systematics and age of the Kings River ophiolite, California: implications for depleted mantle evolution  

Microsoft Academic Search

Sm-Nd whole-rock and mineral data for the Kings River ophiolite define two isochrons of 485±21 Ma and 285±45 Ma age with eNd (483)= +10.7±0.5 and eNd (285)= +9.9±1.1, respectively. The 483 Ma isochron is defined by samples of the main igneous construct. Samples from crosscutting diabase dikes and flaser gabbro sheets within the peridotite unit yield the 285 Ma isochron.

H. F. Shaw; J. H. Chen; J. B. Saleeby; G. J. Wasserburg

1987-01-01

181

A conceptual hydrogeological model of ophiolite hard-rock aquifers in Oman based on a multiscale and a multidisciplinary approach  

Microsoft Academic Search

Ophiolites are found all over the world: from the Alps to the Himalayas, in Cuba, Papua-New Guinea, New Caledonia, Newfoundland, etc. They are composed of hard rocks—basalt, dolerite, gabbro and peridotite, which are formed at the mid-oceanic ridges, with specific ridge-related tectonic fracturing and intense hydrothermal alteration. Their geological and thus their hydrogeological properties differ from those of both granite

Benoît Dewandel; Patrick Lachassagne; Françoise Boudier; Saïd Al-Hattali; Bernard Ladouche; Jean-Louis Pinault; Zaher Al-Suleimani

2005-01-01

182

Ophiolite emplacement by strike-slip tectonics between the Pontide Zone and the Sakarya Zone in northwestern Anatolia, Turkey  

NASA Astrophysics Data System (ADS)

Northwestern Anatolia contains three main tectonic units: (a) the Pontide Zone in the north which consists mainly of the ?stanbul-Zonguldak Unit in the west and the Ball?da?-Küre Unit in the east; (b) the Sakarya Zone (or Continent) in the south, which is juxtaposed against the Pontide Zone due to the closure of Paleo-Tethys prior to Late Jurassic time; and (c) the Armutlu-Ovac?k Zone which appears to represent a tectonic mixture of both zones. These three major tectonic zones are presently bounded by the two branches of the North Anatolian Transform Fault. The two tectonic contacts follow older tectonic lineaments (the Western Pontide Fault) which formed during the development of the Armutlu-Ovac?k Zone. Since the earliest Cretaceous, an overall extensional regime dominated the region. A transpressional tectonic regime of Coniacian/Santonian to Campanian age caused the welding of the ?stanbul-Zonguldak Unit to the Sakarya Zone by an oblique collision. In the Late Campanian, a transtensional tectonic regime developed, forming a new basin within the amalgamated tectonic mosaic. The different tectonic regimes in the region were caused by activity of the Western Pontide Fault. Most of the ophiolites within the Armutlu-Ovac?k Zone belong to the Paleo-Tethyan and/or pre-Ordovician ophiolitic core of the ?stanbul-Zonguldak Unit. The Late Cretaceous ophiolites in the eastern parts of the Armutlu-Ovac?k Zone were transported from Neo-Tethyan ophiolites farther east by left-lateral strike-slip faults along the Western Pontide Fault. There is insufficient evidence to indicate the presence of an ocean (Intra-Pontide Ocean) between the ?stanbul-Zonguldak Unit and the Sakarya Zone during Late Cretaceous time.

Elmas, Ali; Yi?itba?, Erdinç

2001-06-01

183

The Upper Cretaceous ophiolite of North Kozara - remnants of an anomalous mid-ocean ridge segment of the Neotethys?  

NASA Astrophysics Data System (ADS)

This study sheds new light on the origin and evolution of the north Kozara ophiolite, a part of the Sava-Vardar Zone. The Sava-Vardar Zone is regarded as a relict of the youngest Tethyan realm in the present-day Balkan Peninsula. The north Kozara ophiolite consists of a bimodal igneous association comprising isotropic to layered gabbros, diabase dykes and basaltic pillow lavas (basic suite), as well as relicts of predominantly rhyodacite lava flows and analogous shallow intrusions (acid suite). The rocks of the basic suite show relatively flat to moderately light-REE enriched patterns with no or weak negative Eu-anomaly, whereas those of the acid suite exhibit steeper patterns and have distinctively more pronounced Eu- and Sr- negative anomalies. Compared to the known intra-ophiolitic granitoids from the Eastern Vardar Zone, the acid suite rocks are most similar to those considered to be oceanic plagiogranites. The new geochemical data suggest that the basic suite rocks are similar to enriched mid-ocean ridge basalts. The geochemical characteristics of the acid suite rocks indicate that their primary magmas most probably originated via partial melting of gabbros from the lower oceanic crust. Our study confirms the oceanic nature of the north Kozara Mts rock assemblage, and suggests that it may have formed within an anomalous ridge setting similar to present-day Iceland.

Cvetkovi?, Vladica; Šari?, Kristina; Grubi?, Aleksandar; Cviji?, Ranko; Miloševi?, Aleksej

2014-04-01

184

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

185

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

186

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

NASA Astrophysics Data System (ADS)

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

Dilek, Y.

2001-12-01

187

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

NASA Astrophysics Data System (ADS)

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

Sonzogni, Y.; Treiman, A. H.

2012-12-01

188

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

189

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

NASA Astrophysics Data System (ADS)

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 ?Nd = 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) ?Nd values of 7.6-8.9, again consistent with a MORB source. This Sm-Nd isotopic homogeneity may indicate a close approach to solid-melt equilibration on a 1-kilometer scale in the mantle, or it may represent an inherited isotopic homogeneity of the mantle source. In either case, melting and melt-rock interaction were the most likely processes to produce nearly complete equilibration between peridotite and melt. In contrast with recently accumulated evidence of apparent melt-source isotopic disequilibrium, generally on scales of several kilometers, here we show a setting where isotopic homogeneity between source rock and melt was attained. Whether this result can be applied elsewhere will depend on additional detailed studies in other regions.

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

2009-11-01

190

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

191

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

192

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

NASA Astrophysics Data System (ADS)

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 later and in lower temperatures than the E- and D-type rocks. The Mikabu and Sorachi Plateaus were formed in a low-latitude region of the Late Jurassic Pacific Ocean possibly near a subduction zone, partially experienced high P/ T metamorphism during subduction, and then uplifted in association with (or without, in case of Mikabu) the supra-subduction zone ophiolite. The Mikabu and Sorachi Plateaus may be the Late Jurassic oceanic LIPs that could have been formed in brotherhood with the Shatsky Rise.

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

2014-07-01

193

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

NASA Astrophysics Data System (ADS)

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

Casey, J.; Dewey, J. F.

2013-12-01

194

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

NASA Astrophysics Data System (ADS)

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

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

195

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

196

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

USGS Publications Warehouse

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

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

1997-01-01

197

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

NASA Astrophysics Data System (ADS)

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

Kryza, Ryszard; Beqiraj, Arjan

2014-04-01

198

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

NASA Astrophysics Data System (ADS)

A clinopyroxene-rich dike of the Trinity ophiolite sheeted-dike complex shows three different magmatic pulses, probably injected in a short period of time (no well developed chilled margin) and important variations of the clinopyroxene and plagioclase percentages between its core (highly porphyritic) and margins (aphyric). This variation, interpreted as related to a flow differentiation phenomenon (mechanical phenocryst redistribution), has important geochemical consequences. It produces increases in the FeO, MgO, CaO, Cr and Ni contents from the margin to the core, together with increases in the clinopyroxene percentage, and decreases in the SiO 2, Zr, Y, Nb and REE contents together with a decrease in the percentage of the fine-grained groundmass toward the core of the dike. This mineralogical redistribution, which also affects the incompatible trace element ratios because of the difference in plagioclase and clinopyroxene mineral/liquid partition coefficients, illustrate the importance of fractionation processes outside of a magma chamber.

Brouxel, Marc

1991-01-01

199

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

NASA Astrophysics Data System (ADS)

The Fizh massif, Oman Ophiolite, contains a ductile shear zone at the Moho Transition Zone. The dunites in the shear zone are classified based on microstructures into coarse granular texture, medium-grained texture, mylonite, and ultramylonites toward a gabbro contact. The average grain size of olivine decreases toward the shear zone, which contains a zone of high strain (˜15 m wide). The proportion of hydrous minerals (amphibole and chlorite) in the shear zone show an increase toward the gabbro contact, suggesting that water infiltrated the shear zone from the gabbro contact. Equilibrium temperatures indicate a higher deformation temperature (˜900?°C) outside of the high strain zone compared with inside this zone (˜750?°C). Under these geochemical and temperature conditions, the temporal evolution of olivine crystal-preferred orientations (CPO) indicates the following continuous deformation scenario. First, deformation by dislocation creep under higher temperatures resulted in slip by D-type {0kl}[100] and then weak E-type (001)[100] slip. Next, deformation by dislocation creep under lower temperatures and higher stress conditions produced a C-type (100)[001] CPO. Finally, superplastic deformation by grain boundary sliding resulted in a random CPO.

Michibayashi, Katsuyoshi; Oohara, Tatsuya

2013-09-01

200

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

SciTech Connect

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

Bickerstaff, D.; Harris, R.A.; Miller, M.A. (West Virginia Univ., Morgantown, WV (United States). Dept. of Geology and Geography)

1993-04-01

201

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

USGS Publications Warehouse

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

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

2002-01-01

202

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

NASA Astrophysics Data System (ADS)

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

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

2013-11-01

203

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

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

204

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

205

The Fuchuan ophiolite in Jiangnan Orogen: Geochemistry, zircon U-Pb geochronology, Hf isotope and implications for the Neoproterozoic assembly of South China  

NASA Astrophysics Data System (ADS)

The Fuchuan ophiolite occurs along the easternmost domain of the Jiangnan Orogen, considered as the suture between Yangtze and Cathaysia Blocks in South China. Here we report results from our new field observations, zircon U-Pb ages and Hf isotope of the cumulate units, chemistry and tectonic discrimination of Cr-spinel from the harzburgite, and whole rock geochemistry of the pillow lava from the ophiolite in an attempt to evaluate the Neoproterozoic tectonic evolution of South China. The zircon data indicate that the ophiolite suite formed at 840-820 Ma, and the high ?Hf(t) (8-13) and ?Nd(t) (3.3-5.7) values suggest that the cumulates were derived from significantly depleted mantle sources. The pillow lavas and the keratophyres, show typical calc-alkalic continental arc-signatures including the enrichment in LREE and LILE, and depletion in HFSE (Nb/La = 0.3 to 0.5) and a wide range of ?Nd(t) values (- 1.2 to 2.5). The chromium-spinels from the serpentinized harzburgite show Cr# ranging from 40 to 67 and Mg# from 33 to 60 comparable with the chemistry of spinels from suprasubduction zone (SSZ)-type ophiolites. The geochemical features as well as the association of coeval I-S type granites and several thousand meters of flysch-type sedimentary sequences correlate the Fuchuan ophiolite to SSZ-type. We suggest that the emplacement of the Fuchuan ophiolite suite marks the time of final welding between the Yangtze and Cathaysia blocks in South China at ca.820 Ma

Zhang, Chuan-Lin; Santosh, M.; Zou, Hai-Bo; Li, Huai-Kun; Huang, Wen-Cheng

2013-10-01

206

Age and composition of meta-ophiolite from the Rhodope Middle Allochthon (Satovcha, Bulgaria): A test for the maximum-allochthony hypothesis of the Hellenides  

NASA Astrophysics Data System (ADS)

metamorphosed thrust stack of the Rhodopes comprises a level with ophiolites (Middle Allochthon) underlain and overlain by continent-derived allochthons. The Upper Allochthon represents the European margin, but the origin of the Lower Allochthon remains controversial, with suggestions that it may be derived from an inferred microcontinent (Drama) or from the margin of Adria. Trace element compositions and Sr and Nd isotope ratios of metagabbroic amphibolites and enclosed meta-plagiogranites from the Satovcha Ophiolite, Middle Allochthon, show that they are cogenetic and represent suprasubduction zone ophiolites. U-Pb dating using laser ablation sector field inductively coupled plasma mass spectrometry of zircons from two meta-plagiogranites and a metagabbro yielded identical Jurassic ages (160 ± 1 Ma, 160.6 ± 1.8 Ma, and 160 ± 1 Ma, respectively), similar to ophiolites in the eastern Vardar Zone bordering the Rhodopes to the SW. The trace element patterns also closely resemble those of the Vardar ophiolites. The association with Late Jurassic arc-type granitoids is another feature that applies both to eastern Vardar and Satovcha. This strongly suggests that the Middle Allochthon comprises the metamorphosed northeastward continuation of the Vardar Zone. The Jurassic age of the Satovcha Ophiolite contradicts the hypothesis of Early Jurassic suturing between Europe (Upper Allochthon) and the assumed Drama microcontinent (Lower Allochthon) but is in line with the "maximum allochthony hypothesis," i.e., the assumption that the Lower Allochthon represents Adria and that the "root" of the Vardar-derived thrust sheets is at the NE boundary of the Rhodopes.

Froitzheim, Nikolaus; Jahn-Awe, Silke; Frei, Dirk; Wainwright, Ashlea N.; Maas, Roland; Georgiev, Neven; Nagel, Thorsten J.; Pleuger, Jan

2014-08-01

207

Carbonation rates of peridotite in the Samail Ophiolite, Sultanate of Oman, constrained through 14C dating and stable isotopes  

NASA Astrophysics Data System (ADS)

Detailed 14C dating as well as stable C and O isotope analyses were conducted on carbonates formed during alteration of the peridotite layer of the Samail Ophiolite, Sultanate of Oman. 14C results obtained in this and previous studies indicate that surface travertines range in age from modern to >45,000 yr BP, indicating long-term deposition and preservation. Travertine deposition rates in two localities were ˜0.1 to 0.3 mm/yr between ˜30,000 and 45,000 yr BP. Using an estimate of total travertine area, this would result in a maximum of ˜1000 to 3000 m3/yr of travertine being deposited throughout the ophiolite during this time period. This travertine deposition would have sequestered a maximum of ˜1 to 3 × 106 kg CO2/yr. Ca-rich carbonate veins that are associated with the surface travertine deposits have ages ranging from ˜4000 to 36,000 yr BP (average: 15,000 yr BP). Mg-rich carbonate veins exposed in outcrops have ages ranging from ˜8000 to 45,000 yr BP (average: 35,000 yr BP). Detailed sampling from numerous locations (3 locations in this study and 10 locations in the previous studies) indicates that no carbonate veins from the natural peridotite weathering surface are older than the ˜50,000 yr BP dating limit of 14C. However, 14C dating of Mg-rich carbonate veins from three roadcut exposures (Qafeefah, Fanja, and Al-Wuqbah) indicates that a significant number of roadcut veins are 14C dead (>50,000 yr BP). A location weighted average indicates that ˜40% of veins sampled at the three roadcuts are 14C dead. An average including veins sampled at both roadcuts and outcrops indicates that overall ˜8% of Mg-rich carbonate veins are 14C dead. Mg-rich carbonate veins are estimated to sequester on the order of 107 kg CO2/yr throughout the ophiolite.

Mervine, Evelyn M.; Humphris, Susan E.; Sims, Kenneth W. W.; Kelemen, Peter B.; Jenkins, William J.

2014-02-01

208

Nd-Sr-Pb systematics and age of the Kings River ophiolite, California: implications for depleted mantle evolution  

NASA Astrophysics Data System (ADS)

Sm-Nd whole-rock and mineral data for the Kings River ophiolite define two isochrons of 485±21 Ma and 285±45 Ma age with ? Nd (483)= +10.7±0.5 and ?Nd (285)= +9.9±1.1, respectively. The 483 Ma isochron is defined by samples of the main igneous construct. Samples from crosscutting diabase dikes and flaser gabbro sheets within the peridotite unit yield the 285 Ma isochron. The 483 Ma data provide the first evidence of lower Paleozoic oceanic crust in the Sierran ophiolite belt. New U-Pb analyses of zircons from a plagiogranite lying on the 483 Ma Sm-Nd isochron yield upper and lower intercepts with the concordia of 430{-60/+200}and 183±15 Ma. Published zircon ages have underestimated the primary age of the ophiolite by 200 300 m.y. due to the effects of polymetamorphism. The 483 Ma samples have initial 87Sr/86Sr=0.7023 0.7030, 206Pb/204Pb=17.14 17.82, 207Pb/204Pb=15.37 15.52, 208Pb/204Pb=36.80 37.38. The 285 Ma samples have similar initial 87Sr/86Sr, but more radiogenic Pb. The range in Sr and Pb compositions is probably due to introduction of radiogenic Sr and Pb during multiple post-emplacement metamorphic events. The high ? Nd, low 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb of the least disturbed samples are clearly diagnostic of a midocean ridge origin for the 483 Ma portion of the ophiolite. Igneous activity at 285 Ma is thought to have occurred in an arc or back-arc setting, or perhaps along a leaky transform. The initial ? Nd (483)=+10.7 is indistinguishable from that of the similar age Trinity Peridotite (Jacobsen et al. 1984). This value is the highest yet reported for the Mesozoic or Paleozoic depleted mantle and requires either a mantle source that was depleted ˜ 850 m.y. earlier than average or a source more highly depleted than average. Alternatively, if such values were more typical of the early Paleozoic mantle than is currently thought, then there has been little evolution of the depleted mantle over the last ˜ 500 m.y. This requires that the modern mantle has been refluxed by material with low ?Nd, such as continental crust.

Shaw, H. F.; Chen, J. H.; Saleeby, J. B.; Wasserburg, G. J.

1987-07-01

209

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

210

Coexisting serpentine and quartz from carbonate-bearing serpentinized peridotite in the Samail Ophiolite, Oman  

NASA Astrophysics Data System (ADS)

Tectonically exposed mantle peridotite in the Oman Ophiolite is variably serpentinized and carbonated. Networks of young carbonate veins are prevalent in highly serpentinized peridotite, particularly near low-temperature alkaline springs emanating from the peridotite. An unusual feature in some samples is the coexistence of serpentine and quartz, which is not commonly observed in serpentinites. This assemblage is unstable with respect to serpentine + talc or talc + quartz under most conditions. Serpentine in the carbonated serpentinites in this study is more iron rich than in most serpentinites reported in previous studies, and samples with co-existing quartz contain the most iron-rich serpentines. Calculations of thermodynamic equilibria in the MgO-SiO2-H2O-CO2 system suggest that serpentine + quartz may be a stable assemblage at low temperatures (e.g., <~15-50 °C) and is stabilized to higher temperatures by preferential cation substitutions in serpentine over talc. Based on these calculations, serpentine + quartz assemblages could result from serpentinization at near-surface temperatures. Clumped isotope thermometry of carbonate veins yields temperatures within error of the observed temperatures in Oman groundwater for all samples analyzed, while the ?18O of water calculated to be in equilibrium with carbonate precipitated at those temperatures is within error of the observed isotopic composition of Oman groundwater for the majority of samples analyzed. As groundwater geochemistry suggests that carbonate precipitation and serpentinization occur concomitantly, this indicates that both hydration and carbonation of peridotite are able to produce extensive alteration at the relatively low temperatures of the near-surface weathering environment.

Streit, Elisabeth; Kelemen, Peter; Eiler, John

2012-11-01

211

Petrogenesis of boninites in the Ordovician Ballantrae Complex ophiolite, southwestern Scotland  

NASA Astrophysics Data System (ADS)

Primitive lava and hyaloclastite with unusual, highly refractory compositions, form part of the Early Ordovician Balcreuchan Group within the ophiolitic Ballantrae Complex, southwestern Scotland. They are identified as likely high-Ca boninites on the basis of new XRF and INAA results and are the first unambiguous boninites to be discovered in the British Isles. The boninites are interbedded with low-Ti tholeiitic lavas with which they share some distinctive geochemical characteristics suggestive of a close petrogenetic relationship. The low-Ti tholeiite lavas have been interpreted as island-arc tholeiites but they also resemble back-arc basin basalts. The newly discovered boninites confirm an intra-oceanic environment of eruption; their distinctive features include relatively high SiO 2, MgO, Cr and Ni but low Al 2O 3 and HFSE abundances, U-shaped REE patterns, low {Ti}/{Zr} and high {Zr}/{Hf} ratios. Bulk geochemical trends are indicative of low-temperature, seawater-dominated alteration of the lavas but these alteration conditions apparently had little effect on the distribution of critical diagnostic elements such as Zr, Ti, Sc, Ta and the mid-heavy rare earths. We suggest that the Ballantrae boninites and low-Ti tholeiites represent different batch melts derived from a common, depleted mantle source region variably modified compositionally (i.e., made "streaky") by fluids and/or melts during slab interaction (subduction metasomatism). A contribution from slab-derived pelagic sediments and/or a carbonatite melt is necessary to account for the fractionated, non-chondritic {Zr}/{Hf} ratios in the boninites. In view of the close compositional similarity of the Ballantrae lavas to Cenozoic boninite suites, we believe that these interpretations may have wider application to the petrogenesis of boninites in general.

Smellie, J. L.; Stone, P.; Evans, J.

1995-12-01

212

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

NASA Astrophysics Data System (ADS)

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

Koroneos, Antonios; Poli, Giampiero; Christofides, Georgios

2010-05-01

213

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

NASA Astrophysics Data System (ADS)

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

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

2014-01-01

214

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

NASA Astrophysics Data System (ADS)

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

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

2013-07-01

215

Sub-seafloor hydrothermal alteration of oceanic crust of the Oman ophiolite - Interaction with global environmental change -  

NASA Astrophysics Data System (ADS)

The chemical change of rocks during hydrothermal alteration was investigated in a complete section through the Wadi Fizh oceanic crust in the Oman ophiolite was investigated in order to evaluate seawater-rock interaction through the oceanic crust. 20 elements analyzed in this study are classified into 6 groups: Group 1 (Ca, P, Al), Group 2 (Mg, Ni, Co, Cr), Group 3 (Fe, Ti, Na, Mn, Zn, Y), Group 4 (K, Rb, Ba), Group 5 (Li, Cd) and Group 6 (other elements such as Sr, Cu). The degree of alteration is assessed by 87Sr/86Sr ratios of rocks and the abundance of secondary minerals. Heavy metals (Cu, Zn, Ni, Cr, Co and Cd) and phosphorus were leached from the lower oceanic crust during amphibolite facies (> 450uC) alteration. Even if the scavenging and/or removal by hydrous Fe and Mn oxides is taken into consideration, the alteration of ophiolite complex, as a whole, works as a source of phosphorus to the ocean during the mid-Cretaceous. This period is characterized by deposition of black shales and oil generation caused by superplume activity. Although such carbon should have been supplied directly by mantle outgassing, nutrients such as phosphorus and silica required for the production of marine organic matter would have been supplied from enhanced hydrothermal activity.

Kawahata, H.; Nohara, M.

2003-12-01

216

Enriched and depleted characters of the Amnay Ophiolite upper crustal section and the regionally heterogeneous nature of the South China Sea mantle  

NASA Astrophysics Data System (ADS)

The volcanic section of the Middle Oligocene Amnay Ophiolite in Mindoro, Philippines has previously been shown to be of normalmid-oceanic ridge basalt (NMORB) composition. Here we report for the first time an enriched mantle component that is additionally recorded in this crustal section. New whole rock major and trace element data are presented for nine mafic volcanic rocks from a section of the ophiolite that has not been previously examined. These moderately evolved tholeiitic basalts were found to have resulted from the bulk mixing of ˜10% ocean island basalt components with depleted mantle. Drawing together various geochemical characteristics reported for different rock suites taken as representatives of the South China Sea crust, including the enriched MORB (EMORB) and NMORB of the East Taiwan Ophiolite, the NMORB from previous studies of the Amnay Ophiolite and the younger ocean floor eruptives of the Scarborough Seamount-Reed Bank region, a veined mantle model is proposed for the South China Sea mantle. The NMORB magmatic products are suggested to have been derived from the more depleted portions of the mantle whereas the ocean island basalt (OIB) and EMORB-type materials from the mixing of depleted and veined/enriched mantle regions.

Perez, Americus d. C.; Faustino-Eslava, Decibel V.; Yumul, Graciano P.; Dimalanta, Carla B.; Tamayo, Rodolfo A.; Yang, Tsanyao Frank; Zhou, Mei-Fu

2013-03-01

217

Geochemical investigation of serpentinized oceanic lithospheric mantle in the Feather River Ophiolite, California: Implications for the recycling rate of water by subduction  

Microsoft Academic Search

The petrology and geochemistry of serpentinized harzburgites within the Feather River Ophiolite in northern California were investigated to constrain the origin of serpentinization. Trace-element systematics indicate that serpentinization was associated almost solely with relatively low temperature hydrothermal addition of seawater and not with the addition of metamorphic fluids associated with subduction or tectonic obduction. Major element systematics show almost negligible

Zheng-Xue Anser Li; Cin-Ty Aeolus Lee

2006-01-01

218

The high temperature reaction zone of the Oman ophiolite: new field data, microthermometry of fluid inclusions, PIXE analyses and oxygen isotopic ratios  

Microsoft Academic Search

The present study is focused on the so-called High Temprature Reaction Zone of the Oman ophiolite, a thin zone located between the roots of the sheeted dyke complex and the high-level gabbros marking the roof of the fossil magma chambers. The distribution of diabases, chloritised dykes, spilitized dykes and epidosites (in the order of increasing hydrothermal alteration) was studied along

Thierry Juteau; Gilles Manac'h; Olivier Moreau; Christophe Lécuyer; Claire Ramboz

2000-01-01

219

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

NASA Astrophysics Data System (ADS)

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

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

2008-06-01

220

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

221

Melanges Pedagogiques (Pedagogical Mixture), 1985.  

ERIC Educational Resources Information Center

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

Melanges Pedagogiques, 1985

1985-01-01

222

Melanges Pedagogiques (Pedagogical Mixture), 1988.  

ERIC Educational Resources Information Center

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

Melanges Pedagogiques, 1988

1988-01-01

223

STRUCTURAL FEATURES OF THE ALADA? MOUNTAINS IN THE N??DE - ÇAMARDI REGION (EASTERN TAURIDES)  

Microsoft Academic Search

The Alada? mountains exhibit a napped structure in the west end of the Eastern Taurides that occurred during the Late Cretaceous - Paleocene period. The tectono-stratigraphic units of the Alada? mountains are called from bottom to top as Yahyali, Siyah Alada?, Çobanda?i, Minaretepeler, Çataloturan, Beyaz Alada?, ophiolitic melange and Alada? ophiolite nappes. The study area consists mainly of lithological units

Alper GÜRBÜZ

224

Petrogenetic significance of spinel-group minerals from the ultramafic rocks of the Iti and Kallidromon ophiolites (Central Greece)  

NASA Astrophysics Data System (ADS)

The peridotites occurring at Iti and Kallidromon ophiolites comprise lherzolite with lensoid dunitic bodies (only at Kallidromon) and harzburgite. The lherzolite contains spinel of aluminous composition forming subhedral to anhedral grains with lobate boundaries. The dunite and harzburgite contain chromite and magnesiochromite, respectively, forming subhedral to euhedral grains. The dunitic chromite displays commonly richer-in-Cr core compositions rimmed by poorer-in-Cr ones. Infrequently, the magnesiochromitic rims show irregular distribution of small areas of poorer-in-Cr composition. On the basis of their Cr#, the lherzolitic spinels and the harburgitic magnesiochromites are analogous to those from abyssal peridotites and oceanic (including back-arc basins) ophiolites, whereas the chromites in the dunite resemble those from arc-related ophiolitic sequences. The lherzolitic spinels show a linear covariation of Cr# with Mg#, compatibly with their being products of restricted partial melting. The richer-in-Cr chromites cluster at high Cr#, as a result of their crystallization from a boninitic melt while the poorer-in-Cr chromites and the magnesiochromites show a linear covariation, with the latter being the Cr-poor end-member of that trend, suggesting crystallization during the evolution of the boninitic melt that interacted with the mantle peridotites. The dunite formation is assigned to the interaction of that melt with the lherzolite after consuming pyroxenes and crystallizing olivine and chromite. This melt subsequently migrated upwards in the harzburgite and evolved, thus magnesiochromite, lower in Cr#, was crystallized. Textural evidence and mineral chemistry data from the harzburgitic magnesiochromite are in agreement with a melt-peridotite interaction after a partial melting event. Later hydrothermal alteration imprinted in the samples by forming ferrian chromite and magnetite at the expense of the spinel-group minerals and developing the silicate assemblage antigorite + talc + tremolite, occurred at conditions analogous to the boundary of greenschist-low amphibolite up to low amphibolite facies. The crystallization of spinel-group minerals induced from a boninitic melt that interacted with mantle peridotites, along with chemical affinities of the magnesiochromite and chromite and plots of pairs of either chromite or magnesiochromite and olivine from the dunitic and harzburgitic samples, are consistent with the origin of the Iti and Kallidromon peridotites in a supra-subduction zone (SSZ) environment.

Karipi, Sofia; Tsikouras, Basilios; Hatzipanagiotou, Konstantin; Grammatikopoulos, Tassos A.

2007-11-01

225

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

226

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

USGS Publications Warehouse

Mineralization composed dominantly of primary troilite, maucherite, pentlandite, and chalcopyrite, and secondary valleriite occurs in serpentinized transition zone rocks of the Limasol Forest segment of the Troodos ophiolite complex, Cyprus. Whole-rock and electron microprobe analyses of this mineralization gives ranges of Cu/(Cu+Ni)=0.16 to 0.47, Pt/(Pt+Pd)=0.66 to 0.51, Ni/Co=6.33 to 13.4, and chondrite normalized plots with low concentrations of Rh, Pt, and Pd, but relatively high Au. Estimated distribution coefficients of nickel and iron between olivine and ore range from 0.5 to 7.4. Most of these data are unlike values from magmatic sulfide deposits and indicate either a complete alteration of a preexisting magmatic sulfide concentration or, more likely, a nonmagmatic origin for this mineralization. ?? 1985 Springer-Verlag.

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

1985-01-01

227

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

NASA Astrophysics Data System (ADS)

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

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

2010-05-01

228

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

NASA Astrophysics Data System (ADS)

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

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

2010-05-01

229

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

NASA Astrophysics Data System (ADS)

The hypocentres of intermediate-depth earthquakes have been shown to overlap with the regions in subducting slabs that contain high abundances of hydrous minerals. This relationship was initially revealed using geophysical and numerical modelling and until recently has lacked corroboration from direct field-based research. We evaluated the relationship of the coincidence of intermediate-depth earthquakes with hydrous minerals in the slab by undertaking detailed geochemical analyses of blueschist to lawsonite to eclogite facies pseudotachylytes and their hostrocks located within an exhumed ophiolite, the Eocene Schistes Lustres Complex in Corsica. These units comprise incompletely metamorphosed metagabbro and peridotite. The wallrocks of the pseudotachylytes contain variable amounts of hydrous minerals: tremolite, Mg-hornblende, glaucophane in the metagabbro, and serpentine, tremolite and chlorite in the peridotite. Back-scatter-electron images show that the hydrous minerals entrained in the melt undergo fusion rather than dehydration. Vesicular and H2O-rich melt veins are observed cross-cutting partially molten pseudotachylyte fault veins and show evidence of H2O exsolution during melt solidification. The crystallisation products of these melts indicate formation under high temperature, high pressure conditions (1400-1700 °C; 1.5 GPa). The peridotite-hosted pseudotachylytes crystallised olivine, orthopyroxene and diopside, which are surrounded by interstitial Al- and H2O-rich glass. The metagabbro pseudotachylyte is dominated by Al-rich omphacite, ilmenite and high-Fe anorthite. XRF bulk analyses of the wallrock of the pseudotachylyte and electron microprobe analyses of the pseudotachylyte matrix, entrained survivor clasts and the crystallisation products show that near-total disequilibrium melting took place. The peridotite-hosted pseudotachylyte composition is skewed strongly towards chlorite; however, the preservation of delicate dendritic diopside and olivine hopper crystals suggests that the pseudotachylyte is unaltered, indicating that preferential fusion of chlorite took place. The metagabbro-hosted pseudotachylyte matrix composition is very similar to the bulk wallrock composition but slightly skewed by the preferential melting of Mg-hornblende and tremolite. Not all the pseudotachylytes are hydrous as the H2O content of the melts is highly variable; the metagabbro-pseudotachylyte ranges from 0 to 4 wt.% and the peridotite-pseudotachylyte ranges from 0 to 14 wt.%. The range in H2O content of the pseudotachylytes has lead us to conclude that the localised dehydration of hydrous minerals may be a second order factor in initiating intermediate-depth seismicity. However, we have observed that the pseudotachylytes with the most chaotic vein networks, thickest fault veins and most comminuted material have the highest abundances of hydrous wallrock minerals, possibly owing to repeated fluid ingress in between pseudotachylyte-generating events. This implies that free fluids enhance pseudotachylyte generation and possibly seismicity, but are not a first order requirement. Microtextural and geochemical results from this study suggest that the presence of abundant H2O-rich minerals in the slab exerts a strong rheological control during high strain-rate deformation, facilitating thermally-triggered localising shear instabilities. These field-based observations allow us to explore the assumption of the causal link between slab hydration and earthquake nucleation, and offer fresh insight into the debate of how intermediate-depth earthquakes take place.

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

2014-10-01

230

Modelling Chemical Diffusion with Extraction - Application to Observed Geochemical Gradients in the Trinity Ophiolite of Northern California  

NASA Astrophysics Data System (ADS)

The Trinity Ophiolite contains numerous outcrops of dunite progressively surrounded by harzburgite, spinel lherzolite, and plagioclase lherzolite. The dunites are interpreted as pathways of focussed melt ascent with incompatible trace element contents decreasing closer to the dunite resulting from melt extraction (Kelemen et al. 1992); Lundstrom (Nature, 2000) has proposed that alkali elements diffuse from dunite into the surrounding peridotite increasing the degree of melting. Three dunite to plagioclase lherzolite transects in this ophiolite all show a trough in Li isotopic ratio as a function of distance away from dunite. Although a simple diffusion model accounting for the difference in the diffusion rates of 6Li and 7Li can explain the existence of the trough, it cannot explain the decreasing concentrations of many incompatible elements towards the dunite-harzburgite boundary. Explaining both the incompatible element profile and the Li isotope changes requires a combined model of diffusion and melt extraction. We developed a mathematical model to investigate the combined effect of diffusion and extraction. Here, a phenomenological model with an extraction strength decreasing away from the ascending magma is used to simulate the extraction effects. Consequently, extraction becomes a compositional sink with a parameterised and spatially varying strength. Based on fluid dynamic considerations, an exponentially decaying sink appears appropriate, and the effects of different extraction models can be examined by simply varying the exponent of the sink function. To account for the isotopic fractionation and extraction simultaneously, diffusion of the Li isotopes must be treated as a coupled system to be solved numerically. Our results show that a coupled diffusion and extraction model is able to account for the observed trough in Li isotopic ratio and the decreasing incompatible element concentrations closer to the dunite. The model also produces a slightly higher 7Li to 6Li ratio on the far side of the trough. This occurs due to a reversal in diffusion direction in response to the Li extraction. Unfortunately, existing data cannot resolve this feature at the present time.

Hsui, A. T.; Lundstrom, C. C.

2002-12-01

231

The Bangong Lake ophiolite (NW Tibet) and its bearing on the tectonic evolution of the Bangong Nujiang suture zone  

NASA Astrophysics Data System (ADS)

The Jurassic Bangong Lake ophiolite, NW Tibet, is a key element within the western part of the Bangong-Nujiang suture zone, which marks the boundary between the Lhasa and Qiangtang blocks. It is a tectonic mélange consisting of numerous blocks of peridotite, mafic lavas and dikes. The mantle peridotites include both clinopyroxene-bearing and clinopyroxene-free harzburgites. The Cpx-bearing harzburgite contains Al-rich spinel with low Cr#s (20-25), resembling peridotites formed in mid-ocean ridge settings. On the other hand, the Cpx-free harzburgite is highly depleted with Cr-rich spinel (Cr# = 69-73), typical of peridotites formed in subduction zone environments. Mafic rocks include lavas of N-MORB and E-MORB affinity and boninites. The N-MORB rocks consist of pillow lavas and mafic dikes, whereas the E-MORB rocks are brecciated basalts. The boninites have high SiO 2 (53.2-57.9 wt%), MgO (6.5-12.5 wt%), Cr (166-752 ppm) and Ni (63-213 ppm) and low TiO 2 (0.22-0.37 wt%) and Y (5.34-8.10 ppm), and are characterized by having U-shaped, chondrite-normalized REE patterns. The N-MORB and E-MORB lavas probably formed by different degrees of partial melting of primitive mantle, whereas the boninites reflect partial melting of depleted peridotite in a suprasubduction zone environment. The geochemistry of the ophiolite suggests that it is a fragment of oceanic lithosphere formed originally at a mid-ocean ridge (MOR) and then trapped above an intraoceanic subduction zone (SSZ), where the mantle peridotites were modified by boninitic melts. The Bangong-Nujiang suture zone is believed to mark the boundary between two blocks within Gondwanaland rather than to separate Gondwanaland from Eurasia.

Shi, Rendeng; Yang, Jingsui; Xu, Zhiqin; Qi, Xuexiang

2008-04-01

232

Formation of gabbronorites in the Purang ophiolite (SW Tibet) through melting of hydrothermally altered mantle along a detachment fault  

NASA Astrophysics Data System (ADS)

The Purang ophiolite tectonically was thrust over an Upper Cretaceous mélange south of the Yarlung-Zangbo Suture Zone (YZSZ) in SW Tibet. It comprises a large ultramafic massif of ~ 800 km2, but plutonic and volcanic sections are notably absent. The Purang peridotite massif is dominated by harzburgites with minor clinopyroxene-poor lherzolites, which are occasionally intruded by gabbronoritic and basaltic dykes. They are characterized by low Al2O3 and CaO, which can be modeled by ~ 15-25% degrees of isobaric batch melting at a pressure of 20 kbar. Spinel Cr# values give an estimated ~ 12-19% degrees of fractional melting for the Purang peridotites. Clinopyroxenes in Purang peridotites show variable LREE patterns and their HREE contents could be reproduced by ~ 10-16% degrees of fractional melting from a depleted mantle source. The Purang gabbronorites are olivine-free, with early crystallization of orthopyroxene and clinopyroxene relative to plagioclase. Pyroxenes in gabbronorites have more variable Mg# than those in peridotites. Plagioclases are characterized by very high anorthite contents of 94-99. Mineral compositions suggest that the Purang gabbronorites crystallized from high-silica hydrous melts, which are strongly depleted in incompatible elements. Such hydrous melts could be derived from serpentinized peridotite via melting along a detachment fault at a slow-spreading ridge that was triggered by the upwelling asthenosphere. The detachment fault exhumed the Purang peridotite massif as an oceanic core complex at the seafloor, which was emplaced and preserved in the suture zone. Therefore, we suggest that the Purang ophiolite was formed in the mid-ocean ridge rather than subduction-related settings.

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

2014-09-01

233

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

NASA Astrophysics Data System (ADS)

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

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

2014-01-01

234

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

235

The origin of U-shaped rare earth patterns in ophiolite peridotites: assessing the role of secondary alteration and melt\\/rock reaction  

Microsoft Academic Search

Ten samples of serpentinized harzburgite and lherzolite from the Trinity ophiolite complex (California) can be classified into two groups using their REE patterns and initial neodymium and strontium isotopic ratios. Samples with the most refractory major element compositions (Al2O3 = 0.6–1.8%; CaO < 1.2%) have U-shaped REE patterns and show low to very low ?Nd(T) values (+2 to ?7), but

Gérard Gruau; Jean Bernard-Griffiths; Christophe Lécuyer

1998-01-01

236

Peridote-water interaction generating migration pathways of H2-rich fluids in subduction context: Common processes in the ophiolites of Oman, New-Caledonia, Philippines and Turkey  

Microsoft Academic Search

The occurrence of H2 flows which were punctually known notably in the ophiolites of Oman, Zambales (Philippines) and Antalya (Turkey) appears to be a widespread phenomenon in these major peridotite massifs associated with ancient or active subduction processes. Similar H2-rich gas flows have been discovered also in the peridotite of New-Caledonia. H2 concentrations are locally high (commonly 60 to90% in

E. P. Deville; A. Prinzhofer; D. Pillot; C. Vacquand; O. Sissmann

2010-01-01

237

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

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

238

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

NASA Astrophysics Data System (ADS)

The Khawr Fakkan block of the Semail ophiolite (United Arab Emirates) exhibits a suite of 10-100m scale metaluminous to peraluminous granitic intrusions, ranging from cordierite-andalusite-biotite monzogranites to garnet-tourmaline leucogranites, which intrude mantle sequence harzburgites and lower crustal sequence cumulate gabbros. Structural constraints suggest that the subduction of continental sedimentary material beneath the hot proto-ophiolite in an intra-oceanic arc environment led to granulite facies metamorphism at the subduction front and the generation of granitic melts which were emplaced up to the level of the ophiolite Moho. Compositions indicate the analysed granitoids were largely minimum melts that crystallised at variable aH2O and pressures of 3 to 5kbar. The LILE (Sr, Rb and Ba) covariation modelling suggests that the granitoids formed largely by the dehydration melting of muscovite rich metasediments. Initial 87Sr/86Sr ratios of analysed dykes vary between 0.710 and 0.706at initial ?Nd values of between -6.3 and -0.5. Cogenetic units of a composite sill from Ra's Dadnah yield a Sm-Nd isochron age of 98.8+/-9.5Ma (MSWD=1.18). Geochemical and isotopic characteristics of the analysed granitic intrusions indicate that the subducted continental material was derived from oceanic trench fill (Haybi complex) sediments, preserved as greenschist (Asimah area) to granulite facies (Bani Hamid area) ophiolitic metamorphic sole terranes. The Sr-Nd isotope systematics suggest that hybrid granitic melts were derived from pre-magmatic mixing of two contrasting subduction zone sources.

Cox, Jon; Searle, Mike; Pedersen, Rolf

239

Jurassic formation and Eocene subduction of the Zermatt–Saas-Fee ophiolites: implications for the geodynamic evolution of the Central and Western Alps  

Microsoft Academic Search

The Zermatt–Saas-Fee ophiolites (ZSFO) are one of the best preserved slices of eclogitic oceanic crust in the Alpine chain.\\u000a They formed during the opening of the Mesozoic Tethys and underwent subduction to HP\\/UHP conditions during Alpine compression. A cathodoluminescence-based ion microprobe (SHRIMP) dating of different zircon domains\\u000a from metagabbros and oceanic metasediments was carried out to constrain the timing of

Daniela Rubatto; Dieter Gebauer; Mark Fanning

1998-01-01

240

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

NASA Astrophysics Data System (ADS)

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

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

2011-06-01

241

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

242

U/Pb, Sm/Nd and Rb/Sr geochronological and isotopic study of northern Sierra Nevada ophiolitic assemblages, California  

NASA Astrophysics Data System (ADS)

Distinct ophiolitic assemblages occur as oceanic basement within three of the four regional tectonic belts of the northern Sierra Nevada. New U/Pb zircon, Sm/Nd and Rb/Sr data are presented for each assemblage, providing critical geochronological and isotopic constraints on the petrogenesis and tectonic evolution of the ophiolitic and associated ensimatic assemblages. Ophiolitic assemblages include from west to east the Smartville complex, Central belt and Feather River belt. The Smartville complex represents an island arc volcanic-plutonic sequence with a major late-stage sheeted dike swarm. The Sm/Nd systems from a wide compositional spectrum of rocks record a 178±21 Ma petrogenetic age and an ? Nd(T)=+9.2±0.6. Zircon U/Pb systems on an uppermost dacite yield a 164±2 Ma age, and on a number of plagiogranite screens and dikes from the sheeted complex 162±1 Ma ages. The Central and Feather River belts are structurally complex polygenetic assemblages. The U/Pb zircon and Sm/Nd systems record major ˜205 Ma and ˜315 Ma petrogenetic events respectively both involving depleted mantle derived magmas. Such magmatism probably occurred in marginal basin/transform systems developed within an older oceanic depleted mantle basement regime. Both Sm/Nd and U/Pb zircon systems show local components of Proterozoic sialic material. The sialic contaminants were probably introduced into the system as craton derived detritus. It is doubtful that any of the ophiolitic assemblages studied represent genetically related crust-upper mantle sequences generated during the development of new oceanic lithosphere. Integration of the geochronological data with geological relations reveals a pattern of petrogenesis and tectonics whereby progressively younger ensimatic terranes were added to the continental margin through time by plate convergence, and were ultimately welded into North American sial by a crosscutting batholithic belt. This accretionary pattern is reflected in both the protolith ages and deformation-metamorphic ages of each of the regional belts which progressively young westward. Crustal components of the accreted ensimatic terranes grew by mainly basaltic igneous activity within island arc, marginal basin and leaky transform systems adjacent to the continent edge prior to final tectonic accretion. Such complexities are suggested to be typical of Cordilleran-type ophiolites and representative of the circum-Pacific erogenic style.

Saleeby, J. B.; Shaw, H. F.; Niemeyer, Sidney; Moores, E. M.; Edelman, S. H.

1989-06-01

243

Melt extraction and melt refertilization in mantle peridotite of the Coast Range ophiolite: an LA-ICP-MS study  

NASA Astrophysics Data System (ADS)

The middle Jurassic Coast Range Ophiolite (CRO) is one of the most important tectonic elements in western California, cropping out as tectonically dismembered elements that extend 700 km from south to north. The volcanic and plutonic sections are commonly interpreted to represent a supra-subduction zone (SSZ) ophiolite, but models specifying a mid-ocean ridge origin have also been proposed. These contrasting interpretations have distinctly different implications for the tectonic evolution of the western Cordillera in the Jurassic. If an SSZ origin is confirmed, we can use the underlying mantle peridotites to elucidate melt processes in the mantle wedge above the subduction zone. This study uses laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to study pyroxenes in peridotites from four mantle sections in the CRO. Trace element signatures of these pyroxenes record magmatic processes characteristic of both mid-ocean ridge and supra-subduction zone settings. Group A clinopyroxene display enriched REE concentrations [e.g., Gd (0.938-1.663 ppm), Dy (1.79-3.24 ppm), Yb (1.216-2.047 ppm), and Lu (0.168-0.290 ppm)], compared to Group B and C clinopyroxenes [e.g., Gd (0.048-0.055 ppm), Dy (0.114-0.225 ppm), Yb (0.128-0.340 ppm), and Lu (0.022-0.05 ppm)]. These patterns are also evident in orthopyroxene. The differences between these geochemical signatures could be a result of a heterogeneous upper mantle or different degrees of partial melting of the upper mantle. It will be shown that CRO peridotites were generated through fractional melting. The shapes of REE patterns are consistent with variable degrees of melting initiated within the garnet stability field. Models call for 3% dry partial melting of MORB-source asthenosphere in the garnet lherzolite field for abyssal peridotites and 15-20% further partial melting in the spinel lherzolite field, possibly by hydrous melting for SSZ peridotites. These geochemical variations and occurrence of both styles of melting regimes within close spatial and temporal association suggest that certain segments of the CRO may represent oceanic lithosphere, attached to a large-offset transform fault and that east-dipping, proto-Franciscan subduction may have been initiated along this transform.

Jean, Marlon M.; Shervais, John W.; Choi, Sung-Hi; Mukasa, Samuel B.

2010-01-01

244

Origin and geodynamic significance of metabasic rocks from the Antrona ophiolites (Western Alps): new insights from SHRIMP-dating  

NASA Astrophysics Data System (ADS)

Subduction/collision and exhumation of continental and oceanic fragments between the European and Adriatic plates resulted in highly complex nappe structures in the Alpine chain. Geotectonic segments bearing metabasic rocks are key domains for unraveling the pre-metamorphic paleogeography. Metabasic rocks in the Alps commonly represent remnants of the Piemont-Ligurian (PL) and the Valais oceanic domains. We dated by SHRIMP (GSC, Ottawa and ANU, Canberra) zircon domains from two metabasites of the Western Alps, areas of Quarata (S of Domodossola) and at Guggilihorn (SE of Simplon). The samples belong to the Antrona ophiolite unit, which was attributed to the Valais domain, based on structural/geological arguments. The Quarata amphibolite (metagabbro) contains exclusively magmatic zircons with weak oscillatory zoning in cathodoluminescence. They show no metamorphic domains, probably due to the relatively low-T and/or limited amounts of fluids during metamorphism. 7 analysed spots yield a weighted mean (crystallisation) age at 155.6 ± 2.1 Ma. The Simplon metagabbro contains magmatic, oscillatory-zoned zircons that also yield a crystallisation age at 155.0 ± 2.0 Ma (10 spots). No age of metamorphism could be obtained from the relatively thin metamorphic domains. The ca. 155 Ma age obtained for both metabasites attributed to the Valais domain is within the time span accepted for the formation of Piemont-Ligurian oceanic crust and older than assumed for Valaisan oceanic crust. This apparent contradiction can be resolved if we invoke Cretaceous opening of the Valais ocean oblique to the NW margin of the Piemont-Ligurian ocean (as suggested earlier by Stampfli 1993). Re-rifting of PL oceanic crust, together with the eastward movement of Iberia caused older PL oceanic crust to be locally positioned to the N of the newly formed Brianconnais peninsula within the younger Valais domain. This scenario can explain the presence of oceanic crust of PL age in a position occupied by the Valais domain. This view is also supported by a 38.5 ± 0.7 Ma age of an eclogite at Passo del Mottone (Antrona ophiolites), attributed to subduction metamorphism in the Valais domain. New data on the protolith age of this eclogite are in agreement with the ca. 155 Ma protolith ages obtained for the Quarata and Simplon metabasites. The age of subduction for the Valais domain can thus be clearly differentiated from that of the PL domain (ca. 44--45 Ma) and that of the European margin (ca. 35 Ma). Stampfli, G., Eclogae Geol. Helv. 86: 1--45 (1993).

Liati, A.; Gebauer, D.; Froitzheim, N.; Fanning, M.

2003-04-01

245

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

NASA Astrophysics Data System (ADS)

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

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

2012-10-01

246

The Armenian and NW Anatolian ophiolites: new insights for the closure of the Tethys domain and obduction onto the South Armenian Block and Anatolian-Tauride Platform before collision through dynamic modeling  

NASA Astrophysics Data System (ADS)

In the Lesser Caucasus three main domains are distinguished from SW to NE: (1) the South Armenian Block (SAB), a Gondwanian-derived continental terrane; (2) scattered outcrops of ophiolites coming up against the Sevan-Akera suture zone; and (3) the Eurasian plate. The Armenian ophiolites represent remnants of an oceanic domain which disappeared during Eurasia-Arabia convergence. Previous works using geochemical whole-rock analyses, 40Ar/39Ar and paleontological dating have shown that the ophiolite outcrops throughout this area were emplaced during the Late Cretaceous as one non-metamorphic preserved ophiolitic nappe of back-arc origin that formed during Middle to Late Jurassic. From these works, tectonic reconstructions include two clearly identified subductions, one related to the Neotethys subduction beneath the Eurasian margin and another to intra-oceanic subduction responsible for the opening of the back-arc basin corresponding to the ophiolites of the Lesser Caucasus. The analysis of the two stages of metamorphism of the garnet amphibolites of the ophiolite obduction sole at Amasia (M1: HT-LP peak of P = 6-7 kbar and T > 630°C; M2; MP-MT peak at P = 8-10 kbar and T = 600°C) has allowed us to deduce the onset of subduction of the SAB at 90 Ma for this locality, which age coincides with other paleontological ages at the obduction front. A preliminary paleomagnetic survey has also brought quantification to the amount of oceanic domain which disappeared by subduction between the SAB and Eurasia before collision. We propose a dynamic finite element model using ADELI to test the incidence of parameters such as the density of the different domains (or the interval between the densities), closing speed (or speeds if sporadic), the importance and interactions of mantle discontinuities with the subducting lithosphere and set a lithospheric model. Our field observations and analyses are used to validate combinations of factors. The aim is to better qualify the predominant factors and quantify the conditions leading to the onset of obduction, the paradox of dense oceanic lithosphere emplaced on top of a continental domain, after subduction and prior to collision. The results of this modeling are also compared to new observations of the assumed eastward extension of this ophiolitic nappe in NW Anatolia. Analyses of the Refahiye ophiolites show similar geochemical signatures as the Armenian ophiolites, due to a similar setting of formation (back-arc). The impact of the obduction of such a vast oceanic domain is not to be taken for granted when considering the following collision stage.

Hässig, Marc; Rolland, Yann; Sosson, Marc; Hassani, Riad; Topuz, Gultekin; Faruk Çelik, Ömer; Gerbault, Muriel; Galoyan, Ghazar; Müller, Carla; Sahakyan, Lilit; Avagyan, Ara

2013-04-01

247

The petrology of the Ortakoy district and its ophiolite at the western edge of the Middle anatolian Massif, Turkey  

NASA Astrophysics Data System (ADS)

The geological history of a regionally metamorphosed sequence of Silurian on early Devonian sediments, now calcite marbles, quartzites, semipelitic, psammitic and migmatitic gneisses containing K-feldspar, sillimanite, and garnet with later andalusite and cordierite, is outlined with chemical analyses of the rocks and minerals, the latter suggesting peak metamorphic conditions about 600 to 700° C at 4kb. Deposition was in part at an active continental margin and in part probably at a passive margin and was followed by basic intrusions, now amphibolites. Later gabbroic rocks are shown for the first time by their geochemistry, especially REE, to be probably part of a major ophiolite sheet, tectonically emplaced and containing late magmatic or metamorphic hornblende. Later intrusive hornblende diorite, with appinitic (diorite dykes with voluminous euhedral hornblende) and calc-alkaline lamprophyric affinites, immediately preceded the intrusion of voluminous granitoids (granite, tonalite, quartz monzonite) of probably late Cretaceous to Palaeocene age, whose geochemistry indicates a continental volcanic arc setting. Neogene formations completed the rock sequence.

Kocak, K.; Leake, B. E.

1994-02-01

248

Olivine fabric evolution in a hydrated ductile shear zone at the Moho Transition Zone, Oman Ophiolite (Invited)  

NASA Astrophysics Data System (ADS)

The Fizh massif, Oman ophiolite, contains a ductile shear zone at the Moho Transition Zone. The dunites in the shear zone are classified based on microstructures into coarse granular texture, medium-grained texture, mylonite, and ultramylonites toward a gabbro contact. The average grain size of olivine decreases toward the shear zone, which contains a zone of high strain ( 15 m wide). The proportion of hydrous minerals (amphibole and chlorite) in the shear zone show an increase toward the gabbro contact, suggesting that water infiltrated the shear zone from the gabbro contact. Equilibrium temperatures indicate a higher deformation temperature ( 900 degree C) outside of the high strain zone compared with inside this zone ( 750 degree C). Under these geochemical and temperature conditions, the temporal evolution of olivine crystal-preferred orientations (CPO) indicates the following continuous deformation scenario. First, deformation by dislocation creep under higher temperatures resulted in slip by D-type {0kl}[100] and then weak E-type (001)[100] slip. Next, deformation by dislocation creep under lower temperatures and higher stress conditions produced a C-type (100)[001] CPO. Finally, superplastic deformation by grain boundary sliding resulted in a random CPO.

Michibayashi, K.

2013-12-01

249

Early Cretaceous amphibolite dehydratation melting preserved within the Tertiary Sabzevar ophiolitic suture (NE Iran): significance for the closure of the Alpine Tethyan oceans in central Iran  

NASA Astrophysics Data System (ADS)

The tectono-metamorphic signature of the oceanic-derived units marking orogenic suture zones provides key elements to decipher modes and regimes of oceanic subduction and continental accretion, and to constrain tectonic reconstructions at paleo-convergent margins. The remnants of the Tethyan oceanic realm form the most remarkable of these suture zones, running from the Mediterranean through East Europe, Middle East to Asia. These ophiolitic rocks record a polyphase and prolonged history of oceanic construction (the Paleozoic-Early Mesozoic Paleo-Tethys and the Mesozoic-Tertiary Neo-Tethys oceanic realms) and consumption during a sequence of Late Paleozoic to Cenozoic subduction/obduction/collision stages localized along the Eurasian active plate margin (e.g. Stampfli and Borel, 2002). The Iranian ophiolites are an integrant part of this evolving scenario, with the Neotethyan remnants distributed to mark diachronous closures of various oceanic branches during the Alpine-Himalayan convergence history. Despite these peculiar characteristics, few modern studies have addressed the characterization of the tectono-metamorphic evolution of the Neotethyan Iranian ophiolites. Furthermore, most of these studies focused on the Zagros orogen (e.g. Agard et al., 2006), and the ophiolitic mélanges surrounding the Central East Iranian Microcontinent are still lacking of a full petrological and geochronological characterisation. The ophiolitic mélange exposed in the Sabzevar Range of NE Iran is a remnant of one of the Neo-Tethyan oceanic branches of Central Iran, closed during the Paleocene-Eocene Arabia-Eurasia convergence. In this study, we document occurrence of km-scale, variably retrogressed mafic high-pressure granulitic (Am + Grt + Cpx + Pl + Qtz) slices embedded within this suture zone. Granulites record an episode of amphibole-dehydratation melting and felsic (tonalite/throndhjemite) melt segregation at 1.1 ± 0.1 GPa and 810 ± 80 °C. In situ U(-Th)-Pb geochronology of zircon and titanite grains hosted in melt segregations points to an Early Cretaceous (Albian) age for the metamorphic climax. This provides evidence for an unknown episode of high-grade subduction zone metamorphism in the region and argues for juxtaposition of an older ophiolitic suture along the Paleocene-Eocene Sabzevar orogen. When combined with the existing reconstructions, these new data (i) impose reconsideration of the current paleotectonic models of the Eurasia convergent margin during the Early Cretaceous, and (ii) argue that punctuated events of subduction of short-lived back-arc oceanic basins accompanied the long-lasting history of the Neotethyan subduction in the region. Agard P., P. Monié, W. Gerber, J. Omrani, M. Molinaro, L. Labrousse, B. Vrielynck, B. Meyer, L. Jolivet and P. Yamato (2006): J. Geophys. Res., 111, doi: 10.1029/2005JB004103. Stampfli, G. M. and G. D. Borel (2002): Earth Planet. Sci. Lett., 196, 17-33.

Rossetti, Federico; Nasrabady, Mohsen; Vignaroli, Gianluca; Theye, Thomas; Gerdes, Axel; Razavi, Mohammad Hossein; Moin Vaziri, Hosein

2010-05-01

250

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

251

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

NASA Astrophysics Data System (ADS)

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

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

2013-05-01

252

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

NASA Astrophysics Data System (ADS)

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

Bourgois, J.; Witt, C.

2008-12-01

253

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

254

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

NASA Astrophysics Data System (ADS)

The presence of aqueously altered, olivine-rich rocks along with carbonate on Mars suggest that serpentinization may have occurred in the past and may be occurring presently in the subsurface, and possibly contributing methane (CH4) to the martian atmosphere. Serpentinization, the hydration of olivine in ultramafic rocks, yields ultra-basic fluids (pH ? 10) with unique chemistry (i.e. Ca2+-OH- waters) and hydrogen gas, which can support abiogenic production of hydrocarbons (i.e. Fischer-Tropsch Type synthesis) and subsurface chemosynthetic metabolisms. Mars analogue sites of present-day serpentinization can be used to determine what geochemical measurements are required for determining the source methane at sites of serpentinization on Earth and possibly on Mars. The Tablelands Ophiolite is a continental site of present-day serpentinization and a Mars analogue due to the presence of altered olivine-rich ultramafic rocks with both carbonate and serpentine signatures. This study describes the geochemical indicators of present-day serpentinization as evidenced by meteoric ultra-basic reducing groundwater discharging from ultramafic rocks, and travertine and calcium carbonate sediment, which form at the discharge points of the springs. Dissolved hydrogen concentrations (0.06-1.20 mg/L) and methane (0.04-0.30 mg/L) with ?13CCH4 values (-28.5‰ to -15.6‰) were measured in the spring fluids. Molecular and isotopic analyses of CH4, ethane, propane, butane, pentane and hexane suggest a non-microbial source of methane, and attribute the origin of methane and higher hydrocarbon gases to either thermogenic or abiogenic pathways.

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

2013-06-01

255

Feedbacks between deformation and melt distribution in the crust-mantle transition zone of the Oman ophiolite  

NASA Astrophysics Data System (ADS)

This study presents microstructural evidence for deformation-controlled melt organization and for changes in olivine deformation associated with the presence of melt in an 80 m vertical section of the crust-mantle transition zone in the Oman ophiolite. This zone represents an ‘end member’ case for analyzing feedbacks between deformation and melt distribution in the upper mantle, since it experienced strong shear strains in presence of large melt fractions. It is characterized by a subhorizontal compositional layering at the mm to meter scale, from weakly impregnated dunites to olivine-rich gabbros, which parallels a pervasive foliation containing a strong stretching lineation. The parallelism between the compositional layering and the foliation, the diffuse limits of the layers, the alignment of elongated plagioclase-rich aggregates devoid of internal deformation structures with the elongation of olivine crystals in the dunitic layers, and the sharp compositional changes across some, but not all layer limits suggest deformation plays an essential role on the development of the layering. The variation on a mm-scale of the olivine crystal preferred orientation (CPO) symmetry as a function of the modal content: from axial-[100] symmetry in layers with <70% modal olivine to axial-[010] in more gabbroic levels (<40% olivine), which is repeated over the entire section, implies deformation in presence of variable melt fractions. Axial-[100] olivine CPO in olivine-rich layers is consistent with deformation by dislocation creep under high temperature, low pressure, dry conditions. Axial-[010] olivine CPO patterns imply additional sliding along preferentially wetted (010) grain boundaries, increase in the activity of [001] glide, or transpression localized in the melt-rich layers. Since the change in CPO symmetry is not accompanied by dispersion, instantaneous melt fractions must have remained <30-40% in all layers. The continuous variation in olivine CPO symmetry with decreasing olivine content implies therefore that the former depends on the cumulated strain in presence of melt rather than on the instantaneous melt fraction.

Higgie, Katherine; Tommasi, Andréa

2012-12-01

256

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

NASA Astrophysics Data System (ADS)

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

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

2012-05-01

257

U/Pb and Sm/Nd dating on ophiolitic rocks of the Song Ma suture zone (northern Vietnam): Evidence for upper paleozoic paleotethyan lithospheric remnants  

NASA Astrophysics Data System (ADS)

The Sm/Nd isochron mineral dating technique, applied on lenses of ophiolitic rocks of the Song Ma suture zone, reveals crystallization ages of 387-313 Ma for titanites extracted from the mafic components (metagabbros, metabasalts) of the ophiolite suite. These ages correspond to a large time interval within the Carboniferous period. Such results mean that these blocks are lithospheric relics of an eastern branch of the Paleotethys. They however do not exclude that an older early Paleozoic ocean has previously existed in the area. The overprinted metamorphism that affect these rocks and the metasedimentary host rocks, including locally HP granulite and eclogite facies conditions (Nakano et al., 2006, 2008, 2010), took place during the Triassic Indosinian orogeny after closure of the ocean, continental subduction and collision, leading to the suturing of the Indochina and South China blocks. U/Pb and Ar/Ar data reveal that peak metamorphic conditions were attained 266-265 Ma ago, being then followed by cooling at 250-245 Ma.

V. V??ng, Nguy?n; Hansen, Bent T.; Wemmer, Klaus; Lepvrier, Claude; V. Tích, V?; Tr?ng Th?ng, T?

2013-09-01

258

A Neoproterozoic seamount in the Paleoasian Ocean: Evidence from zircon U-Pb geochronology and geochemistry of the Mayile ophiolitic mélange in West Junggar, NW China  

NASA Astrophysics Data System (ADS)

The Mayile ophiolitic mélange (MOM) is located in the southwestern part of the West Junggar (NW China) and forms part of the Southern Altaids. The MOM comprises ultramafic rocks, gabbro, pillow and massive lavas, abyssal radiolarian cherts and volcaniclastic rocks. Zircons with magmatic crystallization features including oscillatory zoning and high Th/U values from the isotropic gabbro within the MOM yield LA-ICP-MS U-Pb age of 572 ± 9 Ma (MSWD = 1.0) marking the timing of crystallization of these rocks as late Neoproterozoic. Geochemically, the basalts of the corresponding gabbros from MOM display OIB-type alkali basalt and E-MORB-type tholeiitic basalt features. Both of these groups are characterized by LILE and LREE enrichment and HREE depletion, very weak or no Eu anomalies (Eu/Eu* = 0.9-1), and no obvious Nb, Ta and Ti negative anomalies, suggesting a typical OIB affinity. We propose that these volcanic rocks were derived from a mantle plume-related magmatism associated with the evolution of the Paleoasian Oceanic system, with the mantle source containing 2%-5% garnet, ˜ 2% spinel and ˜ 2% amphibole. The basalts show within-plate affinity marked geochemical similarities with those from Hawaii and Xigaze seamount, suggestive of their intra-oceanic setting. Subduction of the oceanic lithosphere commenced during late Cambrian to early Ordovician, with the eventual accretion of the seamounts in the fore-arc together with oceanic fragments forming the Mayile ophiolitic mélange.

Yang, Gaoxue; Li, Yongjun; Santosh, M.; Gu, Pingyang; Yang, Baokai; Zhang, Bing; Wang, Haibo; Zhong, Xing; Tong, Lili

2012-05-01

259

Evidence for transition from mid-oceanic ridge setting to supra-subduction zone setting in the lower crustal gabbroic sequence, Haylayn block, Oman ophiolite  

NASA Astrophysics Data System (ADS)

Origin of ophiolites has been a major focus of research over the past several decades, with most attention focusing on whether they form at mid-ocean ridges or above subduction zones. Oman ophiolite is well known as one of the greatest and the most complete ophiolites in the world, and occurrences of arc tholeiites, calc-alkaline rocks and boninites as well as MORB-type lavas have been reported. In the case of the lower crustal gabbroic sequence, however, clear identification of both arc- and mid-ocean ridge (MOR)- type gabbros has not been documented until now. In the Haylayn block of central part of the Oman ophiolite, we recognized two distinct magmatic suites from the lower gabbroic sequence. The earlier suite (GB1) consists of troctolites and olivine gabbros in the Layered Gabbro Sequence. The GB1 is characterized by MOR gabbro-type trend (Fo87: An93-Fo81: An83) in the plot of olivine (Fo) and plagioclase (An) compositions. On the other hand, the later suite (GB2) consists of olivine gabbros, olivine gabbronorites and gabbronorites from the Layered Gabbro Sequence and also from the upper Laminated Gabbronorite Sequence. The GB2 is characterized by more calcic plagioclase (Fo83: An91-Fo*72: An88, Fo* is calculated from Mg# of orthopyroxene in olivine-free gabbronorites) and clinopyroxene poorer in Ti and Na compared to those in the GB1. One of the most significant results of our new observations is that the GB2 completely continues in mineral composition to the ultramafic cumulate intrusives (dunites and wehrlites, DW; Fo92-83) within the lower portion of the Layered Gabbro Sequence. Calcic nature of the GB2 plagioclase and the crystallization sequence from the DW and the GB2 suggest that both the DW and the GB2 crystallized from a common hydrous magma. The isotopic compositions are consistent with those observations. The highest value of Fo from the DW is the same as the values from the harzburgites (Fo93-91) in the Mantle Sequence. These lines of evidence imply that the existence of the mantle-derived hydrous primary magma of the DW-GB2 suite rather than the injection of seawater from the roof of the magma chamber by hydrothermal circulation. On the contrary, the GB1 can be explained by crystallization from an essentially anhydrous magma. Since the GB2 constitutes the Layered Gabbro Sequence together with the GB1, very long time interval between the intrusions of the GB1 and the GB2 cannot be suggested.This transition probably resulted from the change from mid-ocean ridge setting to supra-subduction zone setting of the Oman ophiolite.

Yamasaki, T.; Maeda, J.

2003-12-01

260

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

NASA Astrophysics Data System (ADS)

Oman ophiolite is one of the well-known ophiolites for excellent exposures not only of the mantle section but also of the crustal section including effusive rocks and the underlying metamorphic rocks. In the Oman ophiolite, three types of effusive rocks (V1, V2 and V3 from the lower sequences) are recognized: i.e., V1, MORB-like magma, V2, island-arc type lava, and V3, intra-plate lava (Godard et al., 2003 and references there in). V1 and V2 lavas are dominant (> 95 %) as effusive rocks and have been observed in almost all the blocks of northern part of the Oman ophiolite (Godard et al., 2003), but V3 lava has been reported only from Salahi area (Alabaster et al., 1982). It is clear that there was a time gap of lava eruption between V1-2 and V3 based on the presence of pelagic sediments in between (Godard et al., 2003). In addition, V3 lavas are fed by a series of doleritic dikes crosscutting V2 lava (Alley unit) (Alabaster et al., 1982). We found clinopyroxenite (CPXITE) dikes crosscutting deformation structure of basal peridotites just above the metamorphic sole in Wadi Ash Shiyah. The sole metamorphic rock is garnet amphibolite, which overlies the banded and deformed harzburgite and dunite. The CPXITE is composed of coarse clinopyroxene (CPX) with minor amount of chlorite, garnet (hydrous/anhydrous grossular-andradite) with inclusions of titanite, and serpentine formed at a later low-temperature stage. The width of the CPXITE dikes is 2-5 cm (10 cm at maximum) and the dikes contain small blocks of wall harzburgite. Almost all the silicates are serpentinized in the harzburgite blocks except for some CPX. The Mg# (= Mg/(Mg + Fe) atomic ratio) of the CPX is almost constant (= 0.94-0.95) in the serpentinite blocks but varies within the dikes, highest at the contact with the block (0.94) and decreasing with the distance from the contact to 0.81 (0.85 on average). The contents of Al2O3, Cr2O3, and TiO2 in the CPX of the dikes are 0.5-2.0, 0.2-0.6, and 0.2-0.7, respectively. Garnets, both andradite and glossular, contain high amounts of TiO2 (up to 18 wt%). The TiO2 content should be higher in the primary CPX if we consider the formation of secondary Ti-rich garnet and titanite. The La/Yb ratio, normalized to C1 chondrite (subscript CN) (McDonough and Sun, 1995), of CPX in the dikes is high (1.1-2.0) and (La/Yb)CN of calculated melt in equilibrium with the CPX is 6.0-9.6. The REE patterns differ completely from those of diopsidite dikes, (La/Yb)CN < 1, related with hydrothermal fluid (Python et al., 2007), but are similar to those of V3 lava, (La/Yb)CN ? 5, (Godard et al., 2003). We can judge the CPXITE dikes are cumulates from alkaline basalt based on the mineral assemblages and mineral chemistry of the dikes. Based on the similarity of the REE pattern between this CPXITE dike and V3 lava, the CPXITE dike thought to be cumulates from the primary or closely related V3 lava, filling its conduit. These dikes are the clear evidence for that the V3 magma came from outside of the Oman ophiolite after its obduction.

Ishimaru, Satoko; Arai, Shoji; Tamura, Akihiro

2013-04-01

261

OmanDB: The Role of Water in Axial Lavas And Dykes From The Oman Ophiolite And Geochemical Segmentation Of The Palaeo-Spreading Ridge  

NASA Astrophysics Data System (ADS)

The ~500km-long Semail nappe of the Sultanate of Oman and United Arab Emirates is the largest and best-preserved ophiolite complex known. It is generally accepted that it formed at a fast-spreading ridge in NeoTethys in the Late Cretaceous and was obducted immediately afterwards. It has a 'MORB-like' sheeted dyke complex and associated 'Geotimes' extrusive sequence, overlain by two further suites of lavas derived by the hydrous melting of variably depleted mantle sources: the 'Lasail' and 'Alley' units. However, there is a lack of consensus as to the geodynamic setting of the Oman spreading ridge: opinions vary as to whether it formed: [1] entirely in a marginal basin above a long-lived, pre-existing subduction zone; [2] entirely by a short-lived 'infant arc' spreading episode immediately following the initiation of intraoceanic subduction; [3] at an open-ocean spreading centre that was subsequently over-thrusted; or [4] entirely in an open-ocean mid-ocean ridge setting. The nature of the Geotimes lavas and sheeted dykes is key to distinguishing the above models. We have compiled a geochemical database - 'OmanDB' - of more than 1100 dyke and lava analyses from the length and breadth of the ophiolite, including more than 200 new analyses of our own. We interrogate this database to address the controversy over the geodynamic setting of the ophiolite. We show that Geotimes lavas and sheeted dykes differ subtly but significantly from N-MORB across the entire ophiolite. Relative to MORB they show more rapid depletion of Cr and increase of SiO2 during fractional crystallisation, and saturate in oxides at much lower TiO2 contents. These differences indicate an enhanced role of cpx and suppression of plag during fractionation of the Geotimes, consistent with a primary water content significantly higher than that of an open-ocean MOR source. This is supported by consistently elevated Th/Nb ratios. These characteristics can be reconciled only with models in which both the initial seafloor spreading phase of the Oman ophiolite as well as subsequent magmatism took place above a pre-existing or nascent subduction system, i.e. [1] and [2] above. We use the OmanDB data further to investigate along-strike variability in the Geotimes sheeted dykes and lavas. Compositional variations correlate well with the ridge axial segmentation pattern established by field studies. Major discontinuities spaced 50-100km apart along-axis are characterised by cross-cutting sheeted dykes and tectonic complexities that have been interpreted as propagating rifts. We find many of the older sheeted dykes and lavas in these regions to be abnormally fractionated, consistent with enhanced cooling at the retreating rift tip; in contrast, individual cross-cutting dykes from the younger propagating rift tips are typically very primitive. Several further smaller-scale peaks in the proportion of highly-fractionated lavas and dykes are also present along axis on a 10-20km scale, and probably correspond to lower-order segmentation of the Oman palaeo-spreading ridge. This shows that ridge axial segmentation of fast-spreading supra-subduction systems occurs in a similar manner to modern open-ocean MOR.

Bibby, L. E.; MacLeod, C. J.; Lissenberg, J. C.

2011-12-01

262

Chemical transfer between mantle xenoliths and basic magmas: Evidence from oceanic magma chambers. The trinity ophiolite (northern California)  

NASA Astrophysics Data System (ADS)

The Trinity ophiolite consists of small magma chambers inside a large mantle body. Xenoliths of mantle peridotite occur both in gabbroic cumulates along the walls and in the matrices of ultrabasic breccias on the floors of the magma chambers. Field relationships and petrographic data suggest that these fragments of original mantle peridotite were modified by contact with basic magmas by modal metasomatism. Quantitative elemental mass transfers determined from the composition, volume and density variations of reacting minerals demonstrate both closed and open system conditions for the major (Si, Al, Ti, Na, Ca, Fe and Mg) and trace elements (Cr, Ni). In the open system, material gains and losses provide information on the composition of the fluid taking part in the metasomatic reaction. During a first stage of metasomatism the mantle xenoliths were affected by high-temperature reactions at 600 to 925°C. They resulted from the interaction between solid mantle lherzolites and basic melts. The reactions are: (1)those forming orthopyroxene-magnetite simplectite (2)those forming plagioclase-magnetite corona (3)clinopyroxene+spinel I?pargasitic hornblende+spinel II. Chemical interactions between the upper mantle and oceanic magma chambers occurred as soon as the basic magmas had ascended through the upper mantle. The chemically modified magmas, within oceanic magma chambers, were depleted in Ti, Fe and Na. This could partly explain regional variations of the chemical compositions of primary magmas produced beneath slow-spreading ridges. The breakdown of olivine to orthopyroxene and magnetite participates in the control of the partition of magnetic Fe?Ti oxides between oceanic crust and mantle. During the second stage, the serpentinization of olivine and the production of talc were superimposed on the products of the first stage. These reactions require large amounts of H 2O. The hydrothermal fluid was probably seawater. It circulated in the brecciated area along the walls and floors of the magma chambers located at shallow depths. Such structural discontinuities thus played the role of penetration channels favoring seawater circulation in the oceanic crust. All the chemical reactions examined suggest a significant open-system element transfer by infiltrating melts or circulating fluids. The results of this study suggest that caution is required in the interpretation of mineralogical and chemical information provided by mantle xenoliths carried to the surface by ascending magmas.

Lécuyer, Christophe

1990-11-01

263

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

NASA Astrophysics Data System (ADS)

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

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

2003-12-01

264

The 190Pt-186Os Decay System Applied to Dating Platinum-Group Element Mineralization in Layered Intrusions, Ophiolites and Detrital Deposits  

NASA Astrophysics Data System (ADS)

Discrete platinum-group minerals (PGM) occur as accessory phases in mafic-ultamafic intrusions and ophiolitic chromitites, as well as numerous detrital deposits globally. The 190Pt-186Os decay system, measured by laser ablation MC-ICPMS (LA-MC-ICPMS) provides a useful geochronometric tool for direct dating of PGM. Here we present two examples that verify the accuracy of the technique in geologically well constrained situations and demonstrate the potential for using the 190Pt-186Os PGM method to accurately date layered mafic intrusions, ophiolitic chromitites and detrital PGM deposits. Fifty PGM grains from three different horizons within the Bushveld complex yield a Pt-Os isochron age of 2012 ± 47 Ma (2?, MSWD = 1.19, 186Os/188Osi = 0.119818 ± 0.000006). This is consistent with the published U-Pb zircon age of 2054 Ma (Scoates and Friedman, 2008). The younger PGM isochron age is not likely to be a function of difference in blocking temperatures in the different systems. Pt-Os model ages are possible in high pt grains because initial 186Os/188Os can be well constrained. Using this approach we obtained Pt-Os model ages of 2113 ± 106 Ma and 2042 ± 102 Ma for a Bushveld Pt-Fe alloy and sperrylite respectively. Detrital PGM derived from the Meratus ophiolite, southeast Borneo yield a 190Pt-186Os isochron age of 202.5 Ma ± 8.3 Ma (2?, n = 260, MSWD = 0.90, 186Os/188Osi = 0.119830 ± 0.000003), consistent with radiometric and biostratigraphic age constraints (Wakita et al., 1998). We interpret this as the age of formation of the PGM grains in during chromitite genesis in the lower oceanic lithosphere. Our combined data demonstrate the utility of the LA-MC-ICPMS method as a tool for accurate Pt-Os dating of detrital PGM as well as their igneous parent bodies. We can constrain Pt/Os fractionation at the ablation site as being < 2.5%, while within-grain heterogeneity is ultimately one of the strongest controls on isochron and single-grain ages given the partial sampling represented by laser ablation. Scoates, J.S. and Friedman, R.M. 2008. Precise age of the platiniferous Merensky reef, Bushveld Complex, South Africa, by the U-Pb zircon chemical abrasion ID-TIMS technique; Economic Geology 103, p. 465-471. Wakita, K., Miyazaki, K., Zulkarnain, I., Sopaheluwakan, J. and Sanyoto, P. 1998. Tectonic implications of new age data for the Meratus complex of south Kalimantan, Indonesia; Island Arc 7, p. 202-222.

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

2010-12-01

265

Fluid-Mobile Element Enrichment in the Mantle Wedge of Subduction Zones: A View From the Coast Range Ophiolite, California  

NASA Astrophysics Data System (ADS)

It is now commonly accepted that island arc magmas in response to hydrous melting of a metasomatized mantle wedge that overlies the subducting oceanic plate. Fluids released from this plate lowers the mantle solidus, leading to enhanced melting compared to the anhydrous melting that characterizes mid-ocean ridge spreading centers. These fluids also enrich the refractory mantle in lithophile elements derived from the subducting oceanic crust and its carapace of pelagic sediment, and lead to the eruption of volcanic rocks that are compositionally distinct from mid-ocean ridge basalts. We have shown that supra-subduction zone ophiolites, which form in the mantle wedge of nascent subduction zones, preserve mantle lithologies that formed in response to hydrous melting and represent the refractory residuum of that process [Choi et al CMP 2008, Geology 2008; Jean et al CMP 2010]. In this study we document the concentrations of fluid-mobile elements in unaltered residual pyroxenes and show how these concentrations may be used to calculate the composition and flux of slab-derived fluids in the mantle wedge during melting. We use high-precision laser ablation ICP-MS analyses (Element 2 ICP-MS with 213 nm laser) of relic pyroxenes in supra-subduction zone peridotites for B, Be, Rb, Th, Ba, Li, and Pb - made in conjunction with a suite of non-fluid mobile elements including the REE and high-field strength elements, which are used to assess melt extraction and melt percolation. Results show that pyroxenes in all samples display enrichments in fluid-mobile elements relative to depleted MORB mantle, with B ~20-80x DMM and Pb ~4-20x DMM. In contrast, melting models based on the RE and HFS elements show that most fluid-mobile elements should have concentrations that are well below detection limits and concentrations that are effectively zero. Fluid-Cpx partition coefficients suggest fluid phase with fluid-mobile element concentrations of 30-500x DMM. Melt models require large mass fractions of fluid if the peridotite and fluid are “mixed” prior to melting. However, if we assume that the calculated fluid represents an “instantaneous” composition in equilibrium with the observed refractory pyroxenes, more reasonable melt models may be constructed. Our results show that high concentrations of fluid-mobile elements in supra-subduction peridotites can be attributed to a flux of aqueous fluid or fluid-rich melt phase derived from the subducting slab. The “hot zone” of the mantle wedge where melts form, lies outside the stability field of hydrous phases such as hornblende or phlogopite, therefore, the aqueous fluid forms a separate phase within the wedge until is dissolves into newly generated silicate melt.

Shervais, J. W.; Jean, M. M.

2010-12-01

266

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

NASA Astrophysics Data System (ADS)

Biologically habitable systems on the surface of Mars are deemed impossible for two reasons: the pressure on the surface is too low to permit liquid water to persist, and the levels of cosmic radiation that reach the surface would sterilize any living organisms. Thus any habitable systems are limited to subsurface environment. Serpentinization-fueled life is an example of life thriving in the subsurface, where appropriate parent rock and liquid water co-exist. Serpentinization is the reaction to alter olivine end members fayalite and forsterite to serpentine. The iron hydroxide product can be oxidized to form magnetite as water reduces to form diatomic hydrogen. 5Mg2SiO4 (forsterite) + 9H2O + Fe2SiO4 (fayalite) ? 3Mg3Si2O5(OH)4 (serpentine) Mg(OH)2 (brucite) + 2Fe(OH)2 (iron hydroxide) + 3Fe(OH)2 (iron hydroxide) ? Fe3O4 (magnetite) + 2H2O + H2 H2 is a necessary component for some chemosynthetic life, and is a product of serpentinization in which Fe-rich olivines alter to magnetite-rich serpentinite. Since serpentinites are known host rocks for chemosynthetic life (e.g., the submarine Lost City Hydrothermal Field hosts a methane-cycling microbial consortium, Kelley et al., 2005), we can intimate that Mars could very possibly contain an environment supportive of life. Our focus is on the Martian subsurface environment, which is protected from radiation and likely host to groundwater flow. Evidence for such flow is seen in larger Martian valleys, which display geomorphic features associated with groundwater seepage and surface runoff (Malin and Edgett 2000); and from fluvial sediments composed of sulfate-rich grains (Andrews-Hanna et al 2007). Given support for subsurface groundwater flow, was serpentinization common on Mars in the past? Can modern serpentinization on Mars occur? If so, what would the host rocks look like? We approach these questions by studying an analog for serpentinizing settings on Mars at the McLaughlin Reserve (MCL) in the Coast Range Ophiolite in California, where altered peridotites crop out in serpentine barrens. The same rocks have been detected in certain regions of Mars (Ehlmann et al., 2009), and we seek field, mineral, and geochemical evidence that MCL is an apt point of comparison. Specifically, we will be looking at two cores from MCL, each cored to ~50 m depth and 5 cm in diameter. Assays of host rock and altered horizons using XRD, XRF, and thin-section petrography will be employed to constrain the hydrogen production possible with this particular serpentinite assemblage. We will make the case for MCL as a strong terrestrial analog for serpentinite bodies on Mars, and consider the implications of a similar mineral and geochemical "habitat" for the Martian subsurface. Schulte, M et al (2006) Astrobiology, Vol 6, No 2 Ehlmann et al. (2009) Journal of Geophysical Research, Vol 114 E00D08 Kelley et al, (2005) Science 307 (5714): 1428-1434 Malin and Edgett (2000) Science, 288, 2330-2335 Andrews-Hannah and Phillips (2007) Journal of Geophysical Research Vol 12 E08001

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

2011-12-01

267

The origin of U-shaped rare earth patterns in ophiolite peridotites: assessing the role of secondary alteration and melt/rock reaction  

NASA Astrophysics Data System (ADS)

Ten samples of serpentinized harzburgite and lherzolite from the Trinity ophiolite complex (California) can be classified into two groups using their REE patterns and initial neodymium and strontium isotopic ratios. Samples with the most refractory major element compositions (Al 2O 3 = 0.6-1.8%; CaO < 1.2%) have U-shaped REE patterns and show low to very low ? Nd(T) values (+2 to -7), but high to very high 87Sr/ 86Sr(T) ratios (up to 0.715), whereas rocks with more fertile major element compositions (Al 2O 3 = 2-3.2%; CaO = 2-2.6%) give REE patterns showing increasing depletion from Lu to La and have high ? Nd(T) values (+6 to +8), but low 87Sr/ 86Sr(T) ratios (0.703-0.704). The low neodymium and high strontium isotopic ratios of those peridotites having U-shaped REE patterns demonstrate that the LREE enrichments observed in these rocks are the result of contamination by continental crust and not the consequence of peculiar fractionation during melting or chromatographic effects during mantle ascent. A clinopyroxene concentrate prepared from a bulk sample of lherzolite with ? Nd(T) = -7 and 87Sr/ 86Sr(T) = 0.708 gives MORB-like isotopic signatures: ? Nd(T) = +14 and 87Sr/ 86Sr(T) = 0.7035. Furthermore, two highly serpentinized samples with U-shaped REE patterns show strongly negative ?D values (-95 and -110‰) much typical of peridotites altered by continental metamorphic fluids or/and meteoritic waters. From these results, we conclude that the crustal contamination corresponds to a low-temperature alteration event which took place during or after obduction of the Trinity ophiolite complex onto the continent and is not the consequence of any high-temperature invasion of the ultramafic rocks by melts/fluids derived from subducted continental materials.

Gruau, Gérard; Bernard-Griffiths, Jean; Lécuyer, Christophe

1998-11-01

268

Origin of wehrlite cumulates in the Moho transition zone of the Neoproterozoic Ras Salatit ophiolite, Central Eastern Desert, Egypt: crustal wehrlites with typical mantle characteristics  

NASA Astrophysics Data System (ADS)

Ultramafic cumulates, mainly crustal true wehrlites, were discovered and described in the mantle-crust transition zone (MTZ) and the extremely lower layered gabbro sequence of the Ras Salatit ophiolite, Central Eastern Desert, Egypt. They form either boudinaged lensoidal tabular bodies or interdigitated layers often concordant with the planolinear fabrics of the Ras Salatit ophiolite rocks. The contact between wehrlites and the host MTZ dunite or layered gabbro is razor sharp, lobate and/or sinuous, without chilled margins or any visible deformations. The Ras Salatit wehrlites are orthopyroxene-free and composed mainly of olivine and clinopyroxene. They are texturally equilibrated and show a characteristic poikilitic texture. Crystallization order of the Ras Salatit wehrlites is olivine/spinel followed by clinopyroxene with the absence of plagioclase. Olivine and clinopyroxene of the Ras Salatit wehrlites are compositionally uniform and conspicuously high in Mg#, mostly around 0.93 and 0.92, respectively. Moreover, the clinopyroxene shows low Ti and Al contents coupled with marked depletion in LILE. The calculated melt in equilibrium with clinopyroxene from the Ras Salatit wehrlites is largely similar to lavas from the Izu-Bonin forearc. Given the above characteristics, the Ras Salatit wehrlites were produced by crystal accumulation from a hydrous depleted basaltic/tholeiitic melt corresponding to temperatures between 1,000 and 1,100°C at the oceanic crustal pressure (~2 kbar). The involved hydrous tholeiitic melt has been probably formed by fluid-assisted partial melting of a refractory mantle source (similar to the underlying harzburgites) in a somewhat shallow sub-arc environment.

Gahlan, Hisham A.; Arai, Shoji; Abu El-Ela, Fawzy F.; Tamura, Akihiro

2012-02-01

269

An Oxygen Isotope Profile in a Section of Cretaceous Oceanic Crust, Samail Ophiolite, Oman: Evidence for ? 18 O Buffering of the Oceans by Deep (>5 km) Seawater-Hydrothermal Circulation at Mid-Ocean Ridges  

Microsoft Academic Search

Isotopic analyses of 75 samples from the Samail ophiolite indicate that pervasive subsolidus hydrothermal exchange with seawater occurred throughout the upper 75% of this 8-km-thick oceanic crustal section; locally, the H20 even penetrated down into the tectonized peridotite. Pillow lavas (8280 -- 10.7 to 12.7) and sheeted dikes (4.9 to 11.3) are typically enriched in 280, and the gabbros (3.7

Robert T. Gregory; Hugh P. Taylor

1981-01-01

270

Evidence for Paleocene-Eocene evolution of the foot of the Eurasian margin (Kermanshah ophiolite, SW Iran) from back-arc to arc: Implications for regional geodynamics and obduction  

NASA Astrophysics Data System (ADS)

The nature and significance of the Kermanshah ophiolite (Zagros Mountains, Iran), traditionally identified as one of the remnants of the Peri-Arabic ophiolite system obducted onto Arabia in the Late Cretaceous, is reinvestigated in this study. We assess the geochemistry of magmatic rocks from two distinct areas: the Kamyaran Paleocene-Eocene arc and the so-called Harsin-Sahneh ophiolite complex. Volcanic rocks associated with Triassic to Liassic sediments display a clear alkali signature, whereas the Paleocene volcanic rocks show a geochemical signature similar to that of tholeiitic back-arc basin basalts. The presumed ophiolitic gabbros of the Harsin-Sahneh complex and some of the associated dykes that intrude harzburgites or gabbros also have a back-arc basin signature. Eocene volcanics, gabbros and dykes intruding the harzburgites display clear low to medium-K calc-alkaline signatures with variable negative Nb, Ta, and Ti and positive Sr, Ba, Th, and U anomalies. Field relationships and geochemical evidence indicate that the Eocene magmatic rocks were intruded into a mantle substratum close to the ocean-continent transition. The geochemistry of magmatic rocks from Paleocene to Eocene suggests that an Eocene arc was constructed in a Paleocene back-arc basin along the Eurasian continental margin. In the Kermanshah region this magmatic activity, which extended further to the northwest into Turkey, coincided with a marked slowing down of the convergence of Arabia with Eurasia. Furthermore, it occurred after the Mesozoic Sanandaj-Sirjan magmatism had ceased but before the development of the Tertiary Urumieh-Dokhtar magmatic arc. We tentatively relate this transient magmatic activity to a slab retreat and a back-arc extension at the Eurasian continental margin.

Whitechurch, H.; Omrani, J.; Agard, P.; Humbert, F.; Montigny, R.; Jolivet, L.

2013-12-01

271

Melanges pedagogiques 1989 (Pedagogical Mixtures 1989).  

ERIC Educational Resources Information Center

Seven articles, presented in English or French, address aspects of second language instruction. (Articles written in English have an abstract in French and articles in French have an accompanying English abstract.) They include the following: "Bilan d'une experience de sensibilisation interculturelle pour enseignants" (Report of an Experiment in…

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

272

Melanges Pedagogiques (Pedagogical Mixture), 1986/87.  

ERIC Educational Resources Information Center

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

Melanges Pedagogiques, 1987

1987-01-01

273

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

Microsoft Academic Search

New U-Pb zircon ages for nine samples of tonalite and pegmatitic trondhjemite from the Trinity ophiolite and associated melange reveal a complex history of magmatic activity extending back into the earliest Cambrian, much older than previously believed. Earlier investigations, based on limited data, recognized lower Paleozoic crustal elements in the eastern Klamath terrane (EKT) ranging in age from Middle Ordovician

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

1988-01-01

274

Nature and distribution of ultramafic layers in the mantle section of the Oman ophiolite: implications for early magma genesis below a spreading centre  

NASA Astrophysics Data System (ADS)

Pyroxenitic dykes transposed into parallelism with the high-temperature plastic flow structures of their host harzburgites (layerings) are common features in the mantle section of the Oman ophiolite. In order to better constrain their origin, we performed a systematic survey of these layerings (524 sampling stations distributed all along the Oman range). They generally crop out as series of several parallel layers, a few mm (one crystal) to several cm thick, with a characteristic spacing of a few cm. Their host is usually harzburgite showing, in one third of the cases, a gradual increase in the Opx content toward the layer and, elsewhere, no variation in the Opx mode. Locally, concordant dunites (a few mm to a few cm thick) are in contact or associated with the pyroxenites as isolated layers. The modal composition of the layers themselves is quite variable, covering a wide part of the ultramafic domain: most of them are orthopyroxenites and websterites. Clinopyroxenites, wehrlites, clinopyroxene-bearing harzburgite and lherzolite are less common. Mineral composition of the layers is globally within the field of the mantle harzburgites, i.e. primitive in terms of Mg# and highly depleted in incompatible elements (HFSE and LILE). The Al content of the pyroxenes from the layers is, on average, slightly higher than the one of mantle harzburgites, and much higher than the Al content of pyroxene in discordant pyroxenitic mantle dykes. At the local scale, most layers are in chemical equilibrium with their host harzburgite and composition variation within the layer itself or its host are not observed. Cpx trace elements content shows compositions richer in REE than the Cpx from Oman harzburgite with chondrite normalised profiles slightly dipping in the HREE field suggesting magmatic evolution in the presence of garnet. Two-pyroxenes geothermometer show equilibrium temperatures between 950 and 1100°C, indicating high temperature of final equilibration and transposition. The distribution map shows that layerings appear at any level in the mantle section, close to the basal thrust plane as well as a few tens of meters below the Moho. They are, however, unevenly distributed at the scale of the ophiolite: abundant only in the northernmost massifs from the Wuqbah to the Fizh blocks with exceptionally low abundance in the Hilti block. They are rare to non-existent in the south-eastern massifs (Sumail, including the Maqsad diapirs, Wadi Tayin, etc.). The conditions for their genesis or preservation were thus not encountered everywhere. Clearly, this distribution map mimics the one of discordant (later) mantle dykes: pyroxenitic layers are almost absent in the south-easternmost massifs and in Hilti where discordant dykes belong to the MORB kindred. Layers are abundant in massifs where discordant mantle dykes crystallized from depleted andesites. Accordingly, it is tempting to attribute the origin of the pyroxenite layers to igneous processes similar to those leading to the genesis of the depleted magma suites in the Oman ophiolite, although pyroxene crystallization and equilibration occurred at greater depth in the case of the layers, consistently with their chemical composition and transposed nature.

Python, M.; Ceuleneer, G.; Tamura, A.; Arai, S.

2012-04-01

275

Extreme Nd isotopic variation in the Trinity Ophiolite Complex and the role of melt/rock reactions in the oceanic lithosphere  

NASA Astrophysics Data System (ADS)

Peridotites, dykes and gabbros from the 470 420 Ma Trinity Ophiolite Complex of northern California exhibit large geochemical rare earth element (REE) and Nd isotopic variations on the small scales which are indicative of a complex history. The Trinity Ophiolite, which covers an area of ?1600 km2, consists of three distinct units: (1) a ˜2 4 km-thick sheet of plastically deformed peridotites, including various ultrabasic lithologies (plagioclase and spinel lherzolite, harzburgite, wherlite and dunite); the peridotite unit is a fragment of mantle lithosphere of oceanic affinity; (2) a series of small (˜1 km diameter) undeformed gabbroic massifs; (3) several generations of basic dykes. The peridotites display the largest geochemical and isotopic variations, with ?Nd( T) values ranging from +10 down to 0. In the gabbroic massifs and intrusive dykes, the variation in model ?Nd( T) values is reduced to 7 ?Nd units: 0 to +7. As a general rule, peridotites, gabbros and dykes with ?Nd( T) values around 0 or +3 give less depleted L(light)REE patterns than do those with ?Nd( T) values in the range +7 to +10. In the peridotites, the Nd isotopic variations take place over very short distances, with jumps as large as 7 ?Nd units occurring on scales of less than 20 m. Comparison with available age data indicates that the peridotites with ?Nd( T)?+10 could be slightly older than the intrusive gabbro massifs and basic dykes (470 Ma vs. 420 Ma). Strontium isotopic data used in connection with Sm-Nd results demonstrate that the 10 ?Nd units variation displayed by the Trinity Peridotite is a primary feature and not an artefact due to REE mobility during seawater interaction. The variable Nd isotopic signatures and variable LREE patterns in the Trinity Peridotite cannot represent mantle source characteristics as there is evidence that this unit was partially melted when it rose as part of the upwelling convecting mantle. Field, petrographic, geochemical and isotopic data rather suggest that the observed heterogeneity is due to local reactions between a 470 Ma proto-peridotite with ?Nd( T)=+10 and younger (420 Ma) basaltic melts with lower ?Nd( T) values (i.e. the gabbroic massifs and the dykes). The gabbros and basic dykes of the Trinity Complex have geochemical and isotopic compositions similar to the arc basalts from the adjacent Copley Formation, so it is proposed that the younger melts are related to arc magmatism.

Gruau, G.; Bernard-Griffiths, J.; Lécuyer, C.; Henin, O.; Macé, J.; Cannat, M.

1995-10-01

276

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

USGS Publications Warehouse

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

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

2011-01-01

277

Geochronology and geochemistry of the Hegenshan ophiolitic complex: Implications for late-stage tectonic evolution of the Inner Mongolia-Daxinganling Orogenic Belt, China  

NASA Astrophysics Data System (ADS)

The Hegenshan ophiolite in the Inner Mongolian-Daxinganling Orogenic Belt (IMDOB), northern China, consists of several discontinuous blocks composed dominantly of serpentinized ultramafic rocks with subordinate cumulate gabbros, mafic lavas and dikes, intruded by younger granodiorite dikes. The ultramafic rocks are highly depleted, serpentinized harzburgites with minor dunite, characterized by relative enrichment in large ion lithophile elements (LILE, e.g., Ba and Rb) and light rare earth elements (LREE). They are interpreted to be oceanic mantle that has undergone extensive melt extraction and variable degrees of metasomatism. The cumulate rocks consist mainly of gabbro and troctolite with LREE-depleted chondrite-normalized REE patterns showing significant positive Eu anomalies. They are enriched in LILE, depleted in Nb, and have high positive ?Nd( t) (+8 to +11), suggesting derivation from a subduction-modified N-MORB-like source. The gabbros and mafic dikes have essentially the same age (295 ± 15 and 298 ± 9 Ma, respectively). The mafic dikes have flat to LREE-depleted, chondrite-normalized REE patterns, are depleted in Nb, enriched in LILE and have N-MORB-type Nd isotopic signatures ( ?Nd( t) = +8.1 to +10). The mafic lavas, erupted at 293 ± 1 Ma, can be divided into two groups; one composed of strongly deformed metabasalts similar in chemical and Nd-Sr isotopic compositions to the mafic dikes, and the other composed of undeformed and unmetamorphosed basalts with oceanic island basalt (OIB)-like trace element signatures and Nd isotopic compositions. The granodiorite dikes, which were intruded at 244 ± 4 Ma, have LREE-enriched, chondrite-normalized REE patterns with no Eu anomalies. Their abnormally high ?Nd( t) values (+6.3 to +6.8) and low ISr (0.70412 to 0.70450) suggest formation from melts derived from thickened oceanic crust during or shortly after closure of the Paleo-Asian Ocean. The structure, lithology and geochemistry of the Hegenshan ophiolite suggest that it is a Cordilleran-type body formed in a supra-subduction zone (SSZ) environment and amalgamated by collision of several fragments of Paleo-Asian lithosphere. Final emplacement and amalgamation occurred in the latest Permian or earliest Triassic.

Miao, Laicheng; Fan, Weiming; Liu, Dunyi; Zhang, Fuqin; Shi, Yuruo; Guo, Feng

2008-04-01

278

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

NASA Astrophysics Data System (ADS)

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

Guoqing, Zhou

279

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

NASA Astrophysics Data System (ADS)

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

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

2014-09-01

280

The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge  

NASA Astrophysics Data System (ADS)

Amphibolite lenses that locally crop out below the serpentinite sole at the base of the ophiolite of New Caledonia (termed Peridotite Nappe) recrystallized in the high-temperature amphibolite facies and thus sharply contrast with blueschists and eclogites of the Eocene metamorphic complex. Amphibolites mostly display the geochemical features of MORB with a slight Nb depletion and thus are similar to the youngest (Late Paleocene-Eocene) BABB components of the allochthonous Poya Terrane. Thermochronological data from hornblende (40Ar/39Ar), zircon, and sphene (U-Pb) suggest that these mafic rocks recrystallized at ˜56 Ma. Using various geothermobarometers provides a rough estimate of peak recrystallization conditions of ˜0.5 GPa at ˜800-950°C. The thermal gradient inferred from the metamorphic assemblage (˜60°C km-1), geometrical relationships, and geochemical similarity suggest that these mafic rocks belong to the oceanic crust of the lower plate of the subduction/obduction system and recrystallized when they subducted below young and hot oceanic lithosphere. They were detached from the down-going plate and finally thrust onto unmetamorphosed Poya Terrane basalts. This and the occurrence of slab melts at ˜53 Ma suggest that subduction inception occurred at or near to the spreading ridge of the South Loyalty Basin at ˜56 Ma.

Cluzel, Dominique; Jourdan, Fred; Meffre, SéBastien; Maurizot, Pierre; Lesimple, StéPhane

2012-06-01

281

Armorican provenance for the mélange deposits below the Lizard ophiolite (Cornwall, UK): evidence for Devonian obduction of Cadomian and Lower Palaeozoic crust onto the southern margin of Avalonia  

NASA Astrophysics Data System (ADS)

Devonian sedimentary rocks of the Meneage Formation within the footwall of the Lizard ophiolite complex in SW England are thought to have been derived from erosion of the over-riding Armorican microplate during collision with Avalonia and the closure of the Rheic Ocean. We further test this hypothesis by comparison of their detrital zircon suites with those of autochthonous Armorican strata. Five samples analysed from SW England (Avalonia) and NW France (Armorica) have a bimodal U-Pb zircon age distribution dominated by late Neoproterozoic to middle Cambrian (c. 710-518 Ma) and Palaeoproterozoic (c. 1,800-2,200 Ma) groupings. Both can be linked with lithologies exposed within the Cadomian belt as well as the West African craton, which is characterized by major tectonothermal events at 2.0-2.4 Ga. The detrital zircon signature of Avalonia is distinct from that of Armorica in that there is a much larger proportion of Mesoproterozoic detritus. The common provenance of the samples is therefore consistent with: (a) derivation of the Meneage Formation mélange deposits from the Armorican plate during Rheic Ocean closure and obduction of the Lizard Complex and (b) previous correlation of quartzite blocks within the Meneage Formation with the Ordovician Grès Armoricain Formation of NW France.

Strachan, Rob A.; Linnemann, Ulf; Jeffries, Teresa; Drost, Kerstin; Ulrich, Jens

2014-07-01

282

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

283

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

NASA Astrophysics Data System (ADS)

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

Karkalis, Christos; Magganas, Andreas; Koutsovitis, Petros

2014-05-01

284

Compositional variations and tectonic settings of podiform chromitites and associated ultramafic rocks of the Neoproterozoic ophiolite at Wadi Al Hwanet, northwestern Saudi Arabia  

NASA Astrophysics Data System (ADS)

Wadi Al Hwanet area in NW of Saudi Arabia is part of the Jebel Ess ophiolite constituting the northeastern part of the ˜700 Ma Yanbu-Sol Hamed-Onib-Allaqi-Heiani suture of the northern Arabian-Nubian Shield. The mantle section of Wadi Al Hwanet ophiolite consists mainly of voluminous harzburgites overlain by thick, massive transition-zone dunites, and small-scale chromitite pods. The harzburgites and massive dunites are exceptionally fresh; primary magmatic textures and silicate minerals are still preserved. Two modes of podiform chromitites exist; small lensoidal pods (group I), and relatively large dike-like pods (group II). Geochemically, the former chromitite type contains chromian spinels with high Cr# (0.79-0.81) and displays a PGE-poor character, with steep negatively-sloped PGE distribution patterns, whereas the latter chromitite type contains chromian spinels with relatively lower Cr# (0.61-0.71) and is PGE-rich (up to 1000 ppb), with positively-sloped PGE distribution patterns. The group II chromitites have much higher sulfide content than the group I suite. Parental melt compositions, in equilibrium with podiform chromitites, vary in Al2O3, FeO*/MgO and TiO2 contents from group I to group II chromitites, although both of them are in the range of the boninitic melts. The differences in the chromitites chemistry are most probably due to variable degrees of partial melting of the involved melts. Two stages of a magmatic activity were inferred for the chromitites genesis. The group I chromitites, of high Cr# of chromian spinels and PGE-poor negatively-sloped patterns, were precipitated in the first stage from a boninitic melt produced by a high degree of partial melting at a supra-subduction zone setting. The second chromitite-forming stage involves a relatively low degree of partial melting under high activities of sulfur and oxygen to produce the group II chromitites with enrichment in sulfides and PGE contents, possibly in a supra-subduction zone setting. In contrast to the chromitites, the harzburgites have low PGE contents, with characteristic unfractionated patterns, and low Cr# (0.46-0.57) of the chromian spinels suggesting mantle residues after low degrees of mantle melting beneath a mid-ocean ridge setting. Together with the entire plotting within the olivine-spinel mantle array, the similarity of olivine and spinel chemistry of dunites with those of harzburgites suggests a replacement origin for the dunites by the consumption of pyroxenes. It is likely that Wadi Al Hwanet mantle section was initially derived from a mid-ocean ridge environment and modified later, under a supra-subduction zone regime, to form podiform chromitites.

Ahmed, Ahmed Hassan; Harbi, Hesham M.; Habtoor, Abdelmonem M.

2012-08-01

285

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

286

Chromitites in ophiolites: How, where, when, why? Part I. A review and new ideas on the origin and significance of platinum-group minerals  

NASA Astrophysics Data System (ADS)

Platinum-group minerals are the dominant control on the budget of platinum-group elements in ophiolitic chromitites. They vary widely in distribution, morphology, chemistry and Os-isotope compositions. Their distribution in different types of chromitites, and different microstructural sites within them may reflect the role of chromite as a physical "collector" of the platinum-group elements during crystallisation of both phases from melts. The platinum-group minerals show significant Os-isotope heterogeneity on the scale of single micrometric grains enclosed within individual chromite grains. This micro-scale isotopic heterogeneity, and their random spatial distribution within the chromites, argue strongly against an origin of the platinum-group minerals via exsolution from the chromite lattice during post-magmatic cooling. A re-interpretation of their microstructures and Os isotopes analysed in situ had led us to review the traditional models, and to propose several origins for the platinum-group minerals in chromitites. (1) The suites of platinum-group minerals mainly record crystallisation during mixing of multiple different melts and physical entrapment in chromite. (2) Some platinum-group minerals in chromitites may have been scavenged from wall-rock peridotite during migration of the parental melts of the chromitite. (3) Other platinum-group minerals may have precipitated from metasomatic fluid/melts that infiltrated existing chromitites. The recrystallization of pre-existing platinum-group minerals during polyphase metamorphism or "recycling" of the chromitite into deeper mantle levels can explain the presence of micrometric platinum-group minerals with distinct Os-isotope compositions.

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

2014-02-01

287

Fluid inclusion evidence for boiling at approx. 370/sup 0/C in the stockwork of the Lasail ophiolitic hydrothermal massive sulfide deposit, Oman  

SciTech Connect

Four samples of stockwork material from DDHnumber9 through the Lasail ophiolitic massive sulfide deposit in Oman were found to contain satisfactory densities of primary fluid inclusion. Primary fluid inclusions in the latter three samples show evidence typical of boiling: (i) variable phase rations, and (ii) inclusions which homogenize into the liquid or vapor phases over the same temperature interval (360/sup 0/C-400/sup 0/C). The pooled data show two salinity populations: one with a modal composition near that of seawater, (3.5 wt.%TDS), and the other characterized by the bulk of the data concentrated between 4.9 and 6.4 equiv. wt.% NaCl (x 1.4-1.8 seawater) with values as high as 8.6 (x 2.5 seawater). The latter high salinities are interpreted to have been produced by the boiling process. For boiling conditions, fluid inclusion homogenization temperatures = trapping temperatures, with some perturbations. Hence, the mode (372/sup 0/C) and range (360/sup 0/C-400/sup 0/C) of the homogenization temperatures for the high salinity population are estimates of the fluid temperatures during ore deposition. The values are at the upper end of the measured range for black smokers. The pressure given by these boiling temperatures is approx. 230 bars, giving an estimate for original seawater depth of approx. 2-2 1/2 km; figures typical of active spreading ridges. The sample from immediately below massive ore (OM2064) shows a lower hom. T range of 330/sup 0/C-350/sup 0/C, no high salinity population, and no evidence for boiling. These observations are interpreted to reflect high level mixing with cold seawater immediately below the original sea floor.

Spooner, E.T.C.; Bray, C.J.

1985-01-01

288

Mantle Peridotites from the Rehafiye-Erzincan Ophiolite, NE-Turkey: REE and mineral chemical modeling of partial melting and refertilization processes  

NASA Astrophysics Data System (ADS)

We have identified two types of upper mantle peridotites; cpx-rich harzburgites and depleted harzburgites from the Refahiye-Erzincan ophiolite, NE Turkey. Less depleted, cpx-rich mantle harzburgites have higher concentrations of Al and Ca, and contain spinel phases with Cr# ranging between 33 and 38. These samples display Lanthanide Group Elements (LGE) patterns that are highly similar to those of abyssal peridotites, the low degree partial melting residues of primitive mantle. Some peridotite samples, on the other hand, are highly depleted in cpx and display extremely low contents of Al and Ca. Spinel phases in these samples have Cr# ranging between 57 and 73, indicating that they represent the residues of high degrees of partial melting. The less depleted cpx-harzburgites can simply be modeled by 12-10% melt extraction from a primitive mantle (PM). However, Cr# of spinel in these samples, indicate higher degrees of depletion, close to ~18%, and cannot be explained with this model. This observation implies that the depletion ratios indicated by spinel Cr# and LGE abundances show an apparent contradiction. We, therefore, suggest a later, low-degree re-melting and refertilization of the old, MOR-type residue in a back arc environment that better explain the higher Cr# of spinel against their higher concentrations of LGE. Consistent with their low content of LGE, the depleted harzburgites contain spinel with high Cr#, and resemble highly depleted SSZ mantle residues. They can be reproduced by 17 to 22% melt extraction from the previously depleted (~12%) old, MOR-type mantle residue in a fore-arc tectonic setting. Some spinels in these samples contain Ti contents higher than those experimentally determined, and indicate interaction with Ti-rich bonititic melt.

Uysal, I.; Yalcin Ersoy, E.; Dilek, Y.; Sarifakioglou, E.; Aydin, F.; Ottley, C. J.

2012-04-01

289

The low-grade Canal de las Montañas Shear Zone and its role in the tectonic emplacement of the Sarmiento Ophiolitic Complex and Late Cretaceous Patagonian Andes orogeny, Chile  

NASA Astrophysics Data System (ADS)

The Canal de las Montañas Shear Zone (CMSZ), southern Patagonian Andes (51-52°S), is a low-grade mylonite belt generated from felsic ignimbritic, pelitic and basaltic protoliths of the Late Jurassic-Early Cretaceous Rocas Verdes basin. The different types of rock fabrics across the CMSZ are thought to be associated with relatively intermediate and high strain conditions, characterized by the development of a narrow western belt of S-?-type mylonites and phyllonites interpreted as the metamorphic sole thrust of the Sarmiento Ophiolitic Complex. Highly strained rocks of the CMSZ display a reverse, continent-ward tectonic transport, with a minor dextral component of shearing. Transitional pumpellyite-actinolite and upper greenschist facies metamorphic conditions at ca. 5-6 kbar and 230-260 °C indicate that the primary shearing event occurred in a subduction zone setting. In-situ 40Ar/39Ar laserprobe chronology yielded ages of ca. 85 Ma on syntectonic phengite which are interpreted as representing cooling synchronous with mica crystallization during the main compressive deformational event. The 78-81 Ma U-Pb zircon crystallization ages of cross-cutting plutonic and hypabyssal rocks and 40Ar/39Ar amphibole age of ca.79 Ma from lamprophyric dikes within the fold-thrust belt constrain an upper age limit of the ophiolite tectonic emplacement deformation.

Calderón, M.; Fosdick, J. C.; Warren, C.; Massonne, H.-J.; Fanning, C. M.; Cury, L. Fadel; Schwanethal, J.; Fonseca, P. E.; Galaz, G.; Gaytán, D.; Hervé, F.

2012-02-01

290

The Jelly Sandwich Bites Back: A Case Study of the Viscosity Contrast Between the Lower Crust and Upper Mantle From the Oman Ophiolite  

NASA Astrophysics Data System (ADS)

The viscosity contrast between the lower crust and upper mantle controls key tectonic processes such as lithospheric coupling between the crust and mantle and the origin and transmission of plate driving forces. Recent reviews have emphasized data that suggest a strong, mafic lower crust overlies a relatively weak, peridotite upper mantle in continental plates, in contrast to the "Jelly Sandwich" concept in which weak lower crust is underlain by a relatively strong shallow mantle. While experimental data provide a basis to predict viscosity contrast, this involves substantial extrapolation from laboratory to natural conditions. Pertinent geologic observations of synchronously deformed gabbro and peridotite are not well documented. We analyzed microstructures in deformed gabbronorite dikes (~10-50 cm thick) and host harzburgite from the Oman ophiolite. Outcrop scale observations demonstrate that deformation is highly localized in 1-50 mm wide mylonitic shear bands formed within portions of the gabbronorite that are plagioclase rich. We constrained the conditions of deformation using pyroxene thermometry, petrology, grain size piezometry and EBSD analyses of lattice preferred orientation (LPO). Syn-deformation temperature ranged from ~600- 800°C, based on Fe-Mg-Ca exchange between recrystallized orthopyroxene-clinopyroxene pairs using QUILF (Andersen et al., 1993) and tremolite/actinolite thermal stability. Pressure was ~300 MPa based on structural thickness of the section. We observe a range of plagioclase grain sizes indicating that strain localized into progressively smaller volumes within the gabbronorite as deformation continued within the finest, most localized plagioclase grain size indicating a stress of ~80 MPa. In contrast, olivine grain size in peridotite corresponds to a stress of ~10 MPa, indicative of earlier, high T, distributed deformation. Plagioclase exhibits a LPO throughout the analyzed samples, though variations in orientation and strength of the LPO are observed. This observation suggests that deformation of the gabbronorite involved a significant component of dislocation creep. Olivine fabrics in the harzburgite, indicative of deformation by dislocation creep, also display variations in orientation, with some samples showing alignment of [100] parallel to the foliation while others have [100] inclined ~45° to the foliation. The presence of deformed amphibole in the gabbronorite suggests that deformation occurred at a relatively high water fugacity. The dikes are small, so H2O fugacity during deformation must have been high and approximately constant in host peridotites as well. Calculations of the strain rate ratio between plagioclase and olivine dominated lithologies predict that plagioclase rich gabbronorite is significantly weaker than harzburgite at the observed conditions of deformation. They suggest that localized deformation and grain size reduction in plagioclase began at high temperature, possibly "dry" conditions, and progressed down temperature during the hydrothermal alteration that formed syn-kinematic amphibole. Additionally they predict that - at a given grain size - plagioclase is weaker than olivine under high temperature conditions, wet or dry, and under low temperature, hydrous conditions as well. Our observations and calculations demonstrate that plagioclase dominated gabbroic rocks are weaker than mantle peridotite during low T, hydrous deformation.

Homburg, J. M.; Hirth, G.; Kelemen, P. B.

2008-12-01

291

Sutures in the Altaids: Implications for Continental Suturing and Growth (Invited)  

NASA Astrophysics Data System (ADS)

Ophiolites and melanges are remnants of vanished oceans in orogenic belts; therefore they are very important for defining sutures. However, as illustrated in the Altaids ophiolitic fragments can be formed at different tectonic settings. Some ophiolites in the Altaids may be substrate of oceanic island arcs or accreted fragments, such as those in Kazakhstan and East Junggar. Others are mainly melanges and mostly occur as blocks or slices in accretionary prisms, such as those in the southern Chinese Altay and southern Tianshan. Ages of ophiolites and melanges should be used to constrain the existence time of ancient oceans, which predate the accretionary process after their emplacement. These ophiolitic fragments or melanges do not necessarily represent real sutures. Ages of some ophiolitic fragments and melanges in Central Asia such as those in the Tianshan predate accretion/collision event and the incorporation of these ophiolitic fragments into accretionary complexes may reflect merely different phases of accretion instead of closure of a major ocean. Major oceans or real sutures can only be recognized by paleogeographical separation of typical faunas and/or floras as many orogenic systems contain archipelagos with second-hand ocean basins, a tectonic scenario similar to the present-day SE Asia. Paleomagnetic data and tectonic analysis enable us to conduct palinspastic reconstructions and can help define real sutures, such as the one in the Southern Tianshan as a main cryptic plane separating the Tarim craton to the south and the accretionary collages to the north from the late Paleozoic to the early Triassic. During most of the Paleozoic time, the Siberia Craton was distributed in the northwest and the Tarim Craton was in the southeast, while the Kazakhstan arc chain was in the west. The active margin of the Siberia Craton had wide accretionary complexes and accreted intra-oceanic arcs and terranes, the Kazakhstan arc chain was characterized by multiple subductions, while the northern margin of the Tarim Craton remained mostly a passive margin. The multiple convergence and accretion among these orogenic systems generated huge orogenic collages in the late Paleozoic and even early Triassic, in which two types of amalgamation happened: (1) orthogonal interactions and oroclinal stacking of the Kazakhstan composite arc chain, and (2) parallel collision of the Altai, East Junggar, and Tianshan arcs. Complicated orogensis may shed light on a better understanding of orogenic suturing and continental growth. Acknowledgements: NSFC (41230207, 41190070, 41190075)

Xiao, W.

2013-12-01

292

The geodynamic evolution of the Alpine orogen in the Cyclades (Aegean Sea, Greece): insights from diverse origins and modes of emplacement of ultramafic rocks  

Microsoft Academic Search

The Alpine orogen in the Cyclades, wherein both high-pressure metamorphic rocks and ultramafic rocks co-occur, is a key area in studying the emplacement of mantle rocks into the crust. Within the Cyclades three distinct ultramafic associations occur: (1) HP-LT ophiolitic melanges of the Cycladic Blueschist Unit (CBU) on Evia and Syros; (2) meta-peridotites associ- ated with migmatized leucogneisses on Naxos,

Y. Katzir; Z. Garfunkel; D. Avigad; A. Matthews

2007-01-01

293

Geology of Sedimentary Rock-Hosted Disseminated Gold Deposits in Northwestern Sichuan, China  

Microsoft Academic Search

Sixteen sedimentary rock-hosted disseminated gold deposits (e.g., Dongbeizhai, Qiuluo, Manaoke, Laerma, Gala and Lianhecun) and about 50 prospects have been discovered in northwestern Sichuan Province, China since the late 1970s. They are hosted in complex rock series such as fine-grained clastic rocks, subvolcanic rocks, and ophiolitic melange, mainly of Triassic age. Mineral associates include pyrite, arsenopyrite, marcasite, realgar, orpiment, stibnite,

Xiao-Chun Wang; Zhe-Ru Zhang

2001-01-01

294

Mesozoic and Cenozoic tectonic evolution of the Shiquanhe area of western Tibet  

Microsoft Academic Search

In the Shiquanhe area of far-western Tibet, mid-Cretaceous strata lie unconformable on ophiolitic melange and Jurassic flysch associated with the Bangong-Nujiang suture zone. On the basis of our mapping and geochronologic studies, we suggest that these Cretaceous strata were shortened by >57% over a north south distance of 50 km during Late Cretaceous-early Tertiary time. The Late Cretaceous Narangjiapo thrust

Paul Kapp; Michael A. Murphy; An Yin; T. Mark Harrison; Lin Ding; Jinghu Guo

2003-01-01

295

Stratigraphy and geochemistry of pillow basalts within the ophiolitic mélange of the Izmir-Ankara-Erzincan suture zone: implications for the geotectonic character of the northern branch of Neotethys  

NASA Astrophysics Data System (ADS)

The Izmir-Ankara-Erzincan suture zone of Turkey is a broad zone of ophiolitic mélange containing numerous fragmented blocks ranging in age from Triassic to Cretaceous. Stratigraphic sequences for various mélange units are compared, together with the geochemistry of associated basaltic pillow lavas of Cretaceous age. A review of geochemical data for the pillow lavas demonstrate: (a) a dominant group of alkalic basalts with enriched incompatible elements, variable Zr/Y and Zr/Nb ratios, and (b) a range of tholeiitic basalts with slightly depleted to mildly enriched (normalized) rare earth patterns, (La/Yb)N 0.4-3.0, and generally low Zr/Y ratios. The alkalic basalts can be chemically matched to Pacific Ocean seamounts, although the close association of red radiolarites and cherts suggests that many basalts represent the margins of such structures, rather than the main seamount edifice. Nd-Sr isotope data are typical for ocean island basalts and represent an admixture of a dominant EM-1 source and a depleted MORB-like source. Enhanced ?18O compositions are a consequence of submarine alteration and not crustal contamination. Tholeiitic compositions have affinities with both N- and E-type MORB, although most are probably representative of tholeiitic ocean islands. Overall the basalts are mainly representative of structures built on the ocean floor, rather than the oceanic crust itself, being scraped off the subducting crust and preserved in the mélange of the accretionary wedge.

Gökten, Ergun; Floyd, P. A.

2007-08-01

296

Le Discours Oral (Oral Discourse). Melanges Pedagogiques, 1972.  

ERIC Educational Resources Information Center

The contrast between written language and oral language did not really begin to attract attention until second language teaching defined as its goal the acquisition of a communicative tool rather than a literary tool. This focus on communication made necessary the distinction between language used for oral communication and language used for…

Boulanger, C.; And Others

297

Pourquoi les mod`eles de melange pour la classification ?  

E-print Network

'estimation que de choix de mod`eles des informations suppl´ementaires en rap- port avec l'objectif initial d´eveloppements m´ethodologiques. L'objectif exploratoire est typiquement repr´esent´e par la

Biernacki, Christophe

298

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

ERIC Educational Resources Information Center

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

Duda, R.; And Others

299

Pedagogie et Communication (Pedagogy and Communication). Melanges Pedagogiques, 1971.  

ERIC Educational Resources Information Center

This paper reviews the traditional relationship established between teaching and learning, in which teaching and teachers' concerns take focus. The maintenance of this hierarchical relationshp has resulted in a communication breakdown between educators and students, and a situation into which students have little or no input in terms of changing…

Kuhn, M.

300

Determination of Fe3+/?Fe ratios in chrome spinels using a combined Mössbauer and single-crystal X-ray approach: application to chromitites from the mantle section of the Oman ophiolite  

NASA Astrophysics Data System (ADS)

We present the results of a comparative study in which we have measured Fe3+/?Fe ratios in chromites from mantle chromitites in the Oman ophiolite using Mössbauer spectroscopy and single-crystal X-ray diffraction. We have compared these results with ratios calculated from mineral stoichiometry and find that mineral stoichiometry calculations do not accurately reflect the measured Fe3+/?Fe ratios. We have identified three groups of samples. The majority preserve Fe3+/?Fe ratios which are thought to be magmatic, whereas a few samples are highly oxidized and have high Fe3+/?Fe ratios. There is also a group of partially oxidized samples. The oxidized chromites show anomalously low cell edge ( a 0) values and their oxygen positional parameters among the lowest ever found for chromites. Site occupancy calculations show that some chromites are non-stoichiometric and contain vacancies in their structure randomly distributed between both the T and M sites. The field relationships suggest that the oxidation of the magmatic chromitites took place in association with a ductile shear zone in mantle harzburgites. Primary magmatic Fe3+/?Fe ratios measured for the Oman mantle chromitites are between 0.193-0.285 (X-ray data) and 0.164-0.270 (Mössbauer data) and preserve a range of Fe3+/?Fe ratios which we propose is real and reflects differences in the composition of the magmas parental to the chromitites. The range of values extends from those MORB melts (0.16 ± 0.1) to those for arc basalts (0.22-0.28).

Lenaz, Davide; Adetunji, Jacob; Rollinson, Hugh

2014-01-01

301

Magmatic relationships between depleted mantle harzburgites, boninitic cumulate gabbros and subduction-related tholeiitic basalts in the Puerto Plata ophiolitic complex, Dominican Republic: Implications for the birth of the Caribbean island-arc  

NASA Astrophysics Data System (ADS)

The Lower Cretaceous Puerto Plata ophiolitic complex (PPC) occurs west of the main collisional suture between the Caribbean and North American plates in the northern Dominican Republic, and imposes important constraints on the geochemical and tectonic processes associated with the birth of the Caribbean island-arc. The PPC exposes a tectonically dismembered 3.0-km-thick section of upper mantle harzburgites, lower crustal cumulate gabbroic rocks and upper crustal basaltic pillow lavas, volcanic breccias and pelagic sediments. The harzburgites exhibit a highly depleted signature in terms of their modal compositions, mineral chemistry and whole rock major and trace element contents, suggesting that they are residues after high-degrees of partial melting. Melt modeling suggests that they were similar in trace element characteristics to a boninite. In the crustal sequence, three magmatic episodes have been recognized based on field, mineral and geochemical data. The first phase is composed of the lower layered gabbronorites, which are variably deformed and recrystallized at high-temperature conditions. Trace element modeling suggests that the gabbronorites crystallized from LREE-depleted island-arc tholeiitic (IAT) melts. The second phase is composed of the intermediate layered troctolites (126 Ma), which are undeformed and preserve igneous cumulate textures. Modeling indicates that the troctolites crystallized from boninitic melts. The gabronorite-troctolite substrate was intruded by a third, supra-subduction zone tholeiitic magmatic phase at < 126 Ma, which formed the upper olivine gabbros and gabbronorites. These gabbroic rocks formed from melts similar in composition to the IAT basalts and basaltic andesites of the overlying Los Caños Fm. Contemporaneous Aptian to lower Albian mafic volcanic rocks of the Los Ranchos Fm and Cacheal complex have comparable IAT geochemical and isotopic signatures, suggesting that all of them may have erupted over a single piece of the Caribbean oceanic lithosphere.

Escuder-Viruete, Javier; Castillo-Carrión, Mercedes; Pérez-Estaún, Andrés

2014-05-01

302

In-situ LA-ICP-MS Analysis of Pyroxene in the Peridotite Section of the Coast Range Ophiolite: Diverse Trace Element Compositions and Cryptic Garnet Field Melting in the Cordilleran Mantle Wedge  

NASA Astrophysics Data System (ADS)

The Coast Range Ophiolite (CRO) is tectonically dismembered and widely distributed in western California. We measured trace element, rare-earth element (REE), and fluid mobile element (FME) contents of pyroxenes in peridotites from four mantle sections from the CRO. This study utilized Laser Ablation- Inductively Coupled Plasma-Mass Spectrometry. The pyroxenes record magmatic processes characteristic of both mid-ocean ridges and supra-subduction zone (SSZ) settings. On average, the abyssal group display enriched clinopyroxene REE concentrations [Gd (0.938-1.663 ppm), Dy (1.79-3.24 ppm), Yb (1.216-2.047 ppm), Lu (0.168-0.290 ppm)], compared to SSZ clinopyroxene [Gd (0.048-0.055 ppm), Dy (0.114-0.225 ppm), Yb (0.128-0.340 ppm), Lu (0.022-0.05 ppm)]. The differences between these geochemical signatures could be a result of differing upper mantle composition, or different degrees of partial melting of the upper mantle (Shojaat et al., 2003). Fractional melting was important in the generation and evolution of CRO peridotites. Models call for 2% dry partial melting of MORB-source asthenosphere in the garnet lherzolite field for abyssal peridotites. SSZ peridotites, represents further partial melting (18%-23%) in the spinel lherzolite field, possibly formed by hydrous melting; evidenced by elevated FME concentrations (Li, B, Rb, Pb). These geochemical variations and occurrence of both styles of melting regimes within close spatial and temporal association (Barth et al., 2007) suggest that certain segments of the CRO may represent oceanic lithosphere, attached to a large-offset transform fault and that east-dipping, proto-Franciscan subduction may have been initiated along this transform (Choi et al., 2008).

Jean, M. M.; Shervais, J. W.; Mukasa, S. B.; Choi, S.

2008-12-01

303

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

SciTech Connect

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

Barnes, C.G.; Tomlinson, S.L. (Texas Tech Univ., Lubbock, TX (United States). Dept. of Geosciences); Donato, M.M. (Geological Survey, Menlo Park, CA (United States))

1993-04-01

304

Remnants of the Rheic SSZ Oceanic Lithosphere (320 Ma) Within the Izmir-Ankara-Erzincan Suture Zone in NE Turkey: New Geochemical and Re-Os Isotope Data From the Rehafiye-Erzincan Ophiolite  

NASA Astrophysics Data System (ADS)

We report on new major-trace-REE and Re-Os isotope compositions and mineral chemistry data from upper mantle peridotites and ultramafic-mafic cumulate rocks in the Rehafiye-Erzincan ophiolite (REO) in NE Turkey, and discuss their siginificance for the tectonic evolution of various oceanic tracts in the eastern Mediterranean region. The REO is part of the Izmir-Ankara-Erzincan Suture Zone (IAESZ) between the Gondwana-derived Tauride-South Armenian ribbon continent to the south and the Rhodope-Pontide micro-continent to the north. It shows bidivergent thrusting along its southern and northern boundaries, resting tectonically on the margins of these continental masses. The IAESZ includes fragments of oceanic lithosphere with WPB, MORB, IAT-Boninite, OIB and LIP affinities that range in age from the Permo-Triassic to the latest Crecateous, although it is commonly interpreted as Neotethyan in origin. The REO consists of upper mantle peridotites including harzburgite with dunite bands/lenses and crosscutting dolerite dikes, ultramafic-mafic cumulate rocks making up a transitional Moho, isotropic gabbro, plagiogranites, and sheeted dikes. Extrusive rocks are missing in the ophiolite sequence but occur as blocks of pillow basalts in an ophiolitic mélange structurally beneath the REO. We have identified two types of upper mantle peridotites, abyssal and SSZ, in the REO. Less depleted, clinopyroxene-rich mantle harzburgites have higher concentrations of Al (1.75-2.12 wt.% Al2O3) and Ca (0.43-1.53 wt.% CaO) and contain spinel phases with Cr# ranging between 33.2 and 37.8. These abyssal peridotites represent a mantle residue of low degrees of partial melting of primitive upper mantle during MOR-type oceanic crust formation. Some peridotite samples, on the other hand, are highly depleted in clinopyroxene and display extremely low contents of Al (0.16-0.89 wt.% Al2O3) and Ca (0.07-0.77 wt.% CaO), characteristic of SSZ peridotites. Spinel phases in these samples have Cr# ranging between 57.0 and 73.5, indicating high degrees of partial melting (in a mantle wedge) of previously depleted peridotites. Both types of peridotites are characterized by low REE contents. Abyssal-type peridotites display a flat pattern from Lu to Tb and negative-flat pattern from Tb to La, although some samples show slight LREE enrichment. SSZ-type, depleted peridotite samples are characterized by nearly similar LREE concentrations, whereas their HREE and especially MREE concentrations are significantly depleted with respect to those of the abyssal peridotites. These rocks show U-shaped REE patterns, reminiscent of forearc peridotites. The PGE content of mantle tectonites in the REO, although low, are significantly higher than those presented by the cumulate gabbros. The 187Os/188Os isotope ratio of these peridotites (0.1195-0.1240) is typical of the depleted mantle and much lower than those of the cumulate gabbros (0.2074-0.5842). Whole-rock samples from the mantle tectonites and cumulate gabbros display a well-defined linear trend in a 187Re/188Os vs. 187Os/188Os isochron diagram. These samples collectively define an isochron with a best-fit age of about 320±16 Ma, suggesting that the melt evolution and crystallization of the SSZ ultramafic-mafic units of the REO may be as old as the late Carboniferous. This new finding of a late Carboniferous SSZ mantle lithosphere between the Gondwana and Eurasia continental masses in NE Turkey indicates: (1) the existence of Rheic Ocean relics farther away in the Tethyan realm of the eastern Mediterranean region than previously considered; (2) a highly heterogeneous character of the IAESZ, containing some lithospheric material that evolved in rift-drift, plume and subduction-accretion tectonic settings of the Rheic, Paleotethyan and Neotethyan Ocean basins. We discuss the geodynamic implications of these inferences.

Uysal, Ibrahim; Dilek, Yildirim; Sarifakioglu, Ender; Meisel, Thomas

2010-05-01

305

The Jurassic Rhodope subduction-accretion history: temporal relations between the Circum-Rhodope Belt Evros ophiolite, and the UHP-HP events and the granitoid magmatism in the underlying high-grade metamorphic basement, Thrace region, NE Greece  

NASA Astrophysics Data System (ADS)

The Circum-Rhodope Belt in Thrace region of NE Greece includes arc-related Evros ophiolite (EO) and arc to continental margin greenschist to unmetamorphosed sedimentary rocks, all thrust emplaced to the north in Late Jurassic time (Bonev et al. 2010) onto the Rhodope high-grade basement that contains ultrahigh and high-pressure (UHP-HP) relics. Ages of the EO are only available for the Petrota gabbro crystallization at 169±2 Ma, whose low-temperature evolution spans 160-140 Ma. UHP relics are considered to have formed before 170-160 Ma, while the HP event together with granitoid magmatism have occurred around 150 Ma (Liati et al., 2011). Although scarce the time constraints for EO and UHP relics both witnesses subduction setting with intricate temporal relationships of island arc system related to the Rhodope continental margin basement. Here, we provide new U-Pb LA-ICP-MS zircon geochronology in the intrusive section of the EO and the underlying high-grade basement complemented by EO geochemistry. Zircons from the plagiogranite intruding the basalts of the EO at Didymotycho, together with cross-cutting gabbro yielded crystallization ages of 172.1±1.5 Ma and 159.8±2.2 Ma, respectively. The gabbro chemistry, plus the presence of granodiorite, confirms arc signature of the EO intrusive suite that resulted via extreme fractional crystallization of basaltic magma (Bonev and Stampfli, 2009). Zircons from migmatitic gneiss intruded into the mafic eclogite in the high-grade basement of a UHP locality near the village of Sidiro yielded granitoid protolith age of 160.2±1.2 Ma. Muscovite gneiss near the village of Micro Derion, with the same relations to the host mafic rock from the basement, supplied ages in the range 155±1.97Ma-159.1±2.06 Ma. The field and radiometric results indicate that EO formation and the UHP-HP relics are temporally indistinguishable and spatially related, and therefore, both are coupled at mantle depth to the same subduction setting. The granitoid magmatism recorded by gneisses postdate UHP-HP events within the subduction zone wedge, but temporarily overlaps late stage of EO formation. Thus, the EO formation, UHP-HP metamorphism and granitoid magmatism, all temporarily relate to a single common south-dipping subduction system near the Rhodope margin. The results reveal for the first time a clearly separate Jurassic subduction system related to N-directed arc-margin collision and accretion, from a Late Cretaceous subduction system responsible for the Rhodope S-directed nappe stacking that persisted into the early Paleogene. Acknowledgments: The study was supported by NSF (Bulgaria) grant no. DDVU 02/94.

Bonev, N.; Marchev, P.; Moritz, R.

2012-04-01

306

Chromite alteration processes within Vourinos ophiolite  

NASA Astrophysics Data System (ADS)

The renewed interest in chromite ore deposits is directly related to the increase in Cr price ruled by international market trends. Chromite, an accessory mineral in peridotites, is considered to be a petrogenetic indicator because its composition reflects the degree of partial melting that the mantle experienced while producing the chromium spinel-bearing rock (Burkhard in Geochim Cosmochim Acta 57:1297-1306, 1993). However, the understanding of chromite alteration and metamorphic modification is still controversial (e.g. Evans and Frost in Geochim Cosmochim Acta 39:959-972, 1975; Burkhard in Geochim Cosmochim Acta 57:1297-1306, 1993; Oze et al. in Am J Sci 304:67-101, 2004). Metamorphic alteration leads to major changes in chromite chemistry and to the growth of secondary phases such as ferritchromite and chlorite. In this study, we investigate the Vourinos complex chromitites (from the mines of Rizo, Aetoraches, Xerolivado and Potamia) with respect to textural and chemical analyses in order to highlight the most important trend of alteration related to chromite transformation. The present study has been partially funded by the Aliakmon project in collaboration between the Public Power Corporation of Greece and Institute of Geology and Mineral Exploration of Kozani.

Grieco, Giovanni; Merlini, Anna

2012-09-01

307

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

NASA Astrophysics Data System (ADS)

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

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

1988-02-01

308

Geology of the Ulugh Muztagh area, northern Tibet  

USGS Publications Warehouse

Within the Ulugh Muztagh area, north central Tibet, an east-west-trending ophiolitic melange marks a suture that apparently was formed during a late Triassic or slightly younger collision between a continental fragment to the south and the rest of Asia. The southern continental fragment carries a thick sequence of upper Triassic sandstone, but the contact between the sandstone and the ophiolitic melange is covered by a younger redbed sequence of unknown age. A suite of 2-mica, tourmaline-bearing leucogranite plutons and dikes intruded the Triassic sandstone at shallow crustal levels 10.5 to 8.4 Ma. These rocks range from granite to tonalite in composition, are geochemically very similar to slightly older High Himalayan leucogranite and are interpreted to have been derived by the partial melting of crustal material. We interpret this to mean that crustal thickening began in this part of the Tibetan plateau at least by 10.5 Ma. Welded rhyolitic tuff rests on a conglomerate that consists of abundant debris from the Ulugh Muztagh intrusive rocks and has yielded Ar Ar ages of about 4 Ma. The tuffs are geochemically identical to the intrusive rocks suggesting that crustal thickening may have continued to 4 Ma. Crustal thickening probably occurred by distributed crustal shortening similar to shortening now occurring north of Ulugh Muztagh along the northern margin of the Tibetan Plateau. ?? 1989.

Burchfiel, B. C.; Molnar, P.; Zhao, Ziyun; Liang, K'uangyi; Wang, Shuji; Huang, Minmin; Sutter, J.

1989-01-01

309

Paleozoic tectonic evolution of the Tianshan Orogen, northwestern China  

NASA Astrophysics Data System (ADS)

The Chinese Tianshan orogenic belt which extends west-east for about 1500 km is one of the most important Paleozoic orogenic belts in central Asia. It includes parts of the Tarim, Yili-Central Tianshan and Junggar plates, which are microcontinental blocks with Precambrian basements and Meso-Cenozoic depositional covers. The Tarim plate is separated from the Yili-Central Tianshan plate by the Southern Central Tianshan suture (Early Carboniferous). Ophiolitic melanges and blueschists are widespread in this suture and are believed to be produced by the collision between a passive continental margin on the north side of the Tarim plate and an active continental margin on the south side of the Yili-Central Tianshan plate. The collision occurred at the end of Early Carboniferous. The Yili-Central Tianshan and the Junggar plates are separated by the Northern Central Tianshan suture, also considered as an Early Carboniferous collisional zone, along which the Middle Ordovician ophiolitic melanges, blueschists and Late Devonian-Early Carboniferous ophiolites are exposed. In early Paleozoic times, the subduction of the paleo-Junggar ocean took place in an active margin on the north side of the Yili-Central Tianshan plate along the Northern Central Tianshan suture. At the end of Early Carboniferous, the north side of the Yili-Central Tianshan plate collided with the North Tianshan Arc along this suture. The paleo-oceanic crust in the Tianshan and adjacent region was entirely consumed at the end of Early Carboniferous. Meanwhile, the Tarim, Yili-Central Tianshan and Junggar plates were accreted to Eurasia. Epicontinental sediments were then deposited throughout the Tianshan region during the Middle-Late Carboniferous times. In the Early Permian, Tianshan began to uplift with accompanying volcanic eruption and granitic intrusion. The uplift was accelerated in the Late Permian and continental molasse was extensively deposited.

Jun, Gao; Maosong, Li; Xuchang, Xiao; Yaoqing, Tang; Guoqi, He

1998-03-01

310

Melange: Supporting Heterogeneous QoS Requirements in Delay Tolerant Sensor Networks  

E-print Network

], and wildlife tracking networks [6]. Epidemic protocols have recently been proposed for routing information probability p to every other node that they encounter, similar to the way that humans infect each other that is used affects both the speed and reliability with which each packet is delivered. A high value of p

Whitehouse, Kamin

311

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

NASA Astrophysics Data System (ADS)

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

Broustail, Guillaume

312

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

E-print Network

of sandstone (ss) in interbedded shale (sh) and sandstone. (b) The thickness of sand boudins. Thickness is assumed to represent original sandstone layer thickness (see text). (c) The size distribution of sandstone boudins represented by equivalent radius... of sandstone (ss) in interbedded shale (sh) and sandstone. (b) The thickness of sand boudins. Thickness is assumed to represent original sandstone layer thickness (see text). (c) The size distribution of sandstone boudins represented by equivalent radius...

Kanaya, Takamasa

2007-04-25

313

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

NASA Astrophysics Data System (ADS)

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

Delannoy, Thomas; Burov, Evgueni; Wolf, Sylvie

2014-05-01

314

Local melange in a global world: From transferred Southern European lifestyles to transnational Sydney streets  

Microsoft Academic Search

Summary The dynamics of urban diversity is analysed in three distinctive commercial streets in two neighbouring inner-Sydney local government areas. The social landscape of these streets is defined by a dynamic migration process over the last sixty years and later cultural changes. Different images tell about past developments, settlement difficulties, reactions, aspirations and achievements. These visible symbols of diversity are

Walter F. Lalich

315

29 Figure 5: Type 2 melange containing a block of quartzite.  

E-print Network

of quartzite. #12;#12;#12;#12;#12;#12;#12;36 Figure 7: Bedding in wacke of the Hatch Hill Formation. #12 of silty quartzite are elongated in the plane of the S1-cleavage and define a stretching lineation. #12. The quartzite pebble is oriented in the plane of the S1-cleavage. A weak crenulation-cleavage (S2) trends from

Kidd, William S. F.

316

Modelisation des elements de melange en extrusion par la methode des elements frontieres  

NASA Astrophysics Data System (ADS)

In order to obtain new plastic materials, polymer mixing is a very commonly used process. Polymer processing using single-screw extrusion makes it possible by modifying the screw of extrusion. For this reason, the Maddock mixing head is used in order to study its effect on quality of the mixing. The mixing of polymers is characterized by two mechanisms: the dispersive mixing and the distributive mixing. In this work, the dispersive mixing is especially studied. The experimental part of this project studies the dispersion of the secondary phase in the polymer matrix. First, the dispersion characterization of various types of agglomerates in PVC is carried out. Second test has consisted in characterizing the dispersion of the CaCO3 loads in the polypropylene (PP) matrix. Besides the experimental study, the numerical methods can be used to provide more detailed information on the mixing flows. In this study the Boundary Element Method (BEM) is used. In the case of the Newtonian simulations, BEM is characterized by easy meshing and calculation at the internal points and for these reasons is very advantageous compared to other numerical methods. Whereas, the polymer flows exhibit non-Newtonian behaviors and consequently non-linear constitutive equation, the BEM simulations have to take that into account. The classical BEM enables to solve this type of problems. Specific techniques have to be developed in order to adapt the BEM resolution. For that, the technique of multi-domain decomposition together with the Dual Reciprocity Method (DRM) has been developed. Software called "POLY_BEM" has been developed based upon BEM. Newtonian and non-Newtonian computations have been performed in 3D and 2D respectively. The dispersion performances of mixing devices are calculated using the flow number definition. In order to validate these results, preliminary comparisons have been carried out showing the beneficial effect of the mixing head. The polymer flow in specific zones of the mixing head (the entry and the exit of the narrow gap between channels) exhibits elongational flows improving dispersion more than in other zones.

Busuladzic, Ibrahim

317

Late Palaeozoic-Cenozoic assembly of the Tethyan orogen in the light of evidence from Greece and Albania  

NASA Astrophysics Data System (ADS)

The objective here is to use the geology and tectonics of a critical part of the Tethyan orogen, represented by Greece and Albania, to shed light on the tectonic development of Tethys on a regional, to global scale, particularly the history of convergence during Late Palaeozoic to Cenozoic time. For Carboniferous time much evidence suggests that the Korabi-Pelagonian crustal unit as exposed in Albania and Greece formed above a northward-dipping subduction zone along the Eurasia continental margin, with Palaeotethys to the south. However, there is also some evidence of southward subduction beneath Gondwana especially from southern Greece and central southern Turkey. Palaeotethys is inferred to have closed in Europe as far to the east as the longitude of Libya, while remaining open beyond this. There is still uncertainty about the Pangea A-type reconstruction that would restore all of the present units in the area to within the E Mediterranean region, versus the Pangea B-type reconstruction that would require right-lateral displacement of exotic terranes, by up to 3,500 km eastwards. In either reconstruction, fragments of the Variscan collisional orogen are likely to have been displaced eastwards (variable distances) in the Balkan region prior to Late Permian-Early Triassic time. From ~Late Permian, the Greece-Albania crustal units were located in their present relative position within Tethys as a whole. From the mid-Permian, onwards the northern margin of Gondwana was affected by crustal extension. A Mesozoic ocean (Pindos-Mirdita ocean) then rifted during Early-Middle Triassic time, culminating in final continental break-up and seafloor spreading during the Late Triassic (Carnian-Norian). Subduction-influenced volcanics of mainly Early-Middle Triassic age probably reflect the extraction of magma from sub-continental lithosphere that was enriched in subduction-related fluids and volatiles during an earlier, ?Variscan subduction event. The existence of Upper Triassic mid-ocean ridge-type igneous rocks, known locally in Albania and Greece, points to rifting of a Red Sea-type oceanic basin rather than a back-arc basin related to contemporaneous subduction. After initial, inferred slow spreading at an Upper Triassic, rifted ocean ridge and spreading during the Early Jurassic, the ocean basin underwent regional convergence. Subduction was initiated at, or near, a spreading axis perhaps adjacent to an oceanic fracture zone. The Jurassic supra-subduction zone-type ophiolites of both Greece and Albania largely relate to melting of rising asthenosphere in the presence of volatiles (water) that originated from subducting oceanic lithosphere. High-magnesian boninite-type magmas that are present in both the Albanian and Greece ophiolites and some underlying melanges reflect remelting of previously depleted oceanic upper mantle. Localised MOR-type ophiolites of Late Middle Jurassic age, mainly exposed in NE Albania, were created at a rifted spreading axis. The amphibolite-facies metamorphic sole of the ophiolites was mainly derived from oceanic crust (including within-plate type seamounts), whereas the underlying lower-grade, greenschist facies sole was mainly sourced from the rifted continental margin. The melange, dismembered thrust sheets and polymict debris flows ("olistostromes") beneath the ophiolites formed by accretion and gravity reworking of continental margin units. The in situ radiolarian chert cover of the ophiolites in northern Albania is overlain by polymict debris flows ("olistostromes"). Pelagic carbonate deposition followed during Tithonian-Berriasian time and then restoration of a regional carbonate platform during the Cretaceous. Exhumation of deeply buried parts of the over-ridden continental margin probably took place during the Early Cretaceous. Structural evidence, mainly from northern Greece (Vourinos, Pindos and Othris areas), indicates that the ophiolites, the metamorphic sole, the accretionary melange, and the underlying continental margin units were all deformed by top-to-the-northeast

Robertson, A. H. F.

2012-04-01

318

The South China block-Indochina collision: Where, when, and how?  

NASA Astrophysics Data System (ADS)

This study uses new field observations and existing studies to shed new light on the origin and significance of two NW-SE striking orogens in NW and NE Vietnam. We conclude that the architecture of each belt is a stack of NE-directed nappes formed either under deep ductile synmetamorphic conditions, or under shallow depth in the SW and NE parts, respectively. The Song Ma zone and Song Chay ophiolitic melange represent two ophiolitic sutures. However, the Late Permian Song Da and Babu mafic rocks are not ophiolites but intraplate basalts related to the Emeishan plume. A Late Triassic unconformity, the 225-205 Ma postorogenic plutonism, and the 250-230 Ma syntectonic metamorphism support an Early to Middle Triassic age for these tectonic events. Both NW and NE belts are due to SW-directed subduction with arc magmatism, ocean closure, and continental collision. Though two contemporary S-dipping subduction zones might explain the structural evolution of the two belts, a single convergent system, offset by the Tertiary Red River fault, is preferred as this S-directed subduction better accounts for the Late Permian intraplate magmatism. This scenario is discussed in the general geodynamic framework of SE Asia.

Faure, Michel; Lepvrier, Claude; Nguyen, Vuong Van; Vu, Tich Van; Lin, Wei; Chen, Zechao

2014-01-01

319

Mantle petrology and mineralogy of the Thetford Mines Ophiolite Complex  

E-print Network

August 2006; accepted 8 June 2007 Available online 16 August 2007 Abstract The Ordovician Thetford Mines­S, parallel to the orientation of seafloor-spreading related paleo-normal faults in the crust. These fabrics

Long, Bernard

320

Alpine Jurassic ophiolites resemble the modern central Atlantic basement  

Microsoft Academic Search

Recent developments in knowledge of the structure of the central Atlantic clearly indicate that the magmatic crust is very thin and locally discontinuous in large areas of this ocean. Specifically, extensive outcrops of serpentinized mantle peridotites have been discovered along the present-day spreading axis away from fracture-zone scarps. These peculiar aspects of the present-day structure of a slow-spreading major ocean

Yves Lagabrielle; Mathilde Cannat

1990-01-01

321

Mesozoic petrotectonic development of the Sawyers Bar suprasubduction-zone arc, central Klamath Mountains, northern California  

SciTech Connect

The Sawyers Bar area of the central Klamath Mountains, California, consists of three tectonically juxtaposed supracrustal units: (1) on the east, the Stuart Fork metabasalt-metachert-metagraywacke terrane above the low-angle, east-dipping Soap Creek Ridge thrust; (2) the medial North Fork ophiolitic terrane, composed of intercalated St. Clair Creek laminated cherts and fine-grained quartzofeldspathic argillites, interstratified with and overlain by two mafic igneous, largely extrusive suites--North Fork (sensu stricto) mildly alkaline basalts, and Salmon River basaltic-diabasic-gabbroic arc tholeiites; and (3) the cherty, Eastern Hayfork metagraywacke melange terrane west of the minor, high-angle Twin Sisters fault. Mineral and bulk-rich elemental and isotopic data, integrated with geologic mapping, document deformation and fluid-rock interaction in the upper few kilometers of a suprasubduction-zone basaltic arc during tectonic accretion to the western margin of North America or a nearby offshore arc. The geologic history is advanced.

Ernst, W.G.

1999-08-01

322

The oroclinal bend in the South Island, New Zealand  

NASA Astrophysics Data System (ADS)

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

Mortimer, N.

2014-07-01

323

Oceanic Remnants In The Caribbean Plate: Origin And Loss Of Related LIPs.  

NASA Astrophysics Data System (ADS)

The modern Caribbean Plate is an independent lithospheric entity, occupying more than 4 Mkm2 and consisting of the remnants of little deformed Cretaceous oceanic plateau of the Colombia and Venezuela Basins (almost 1 Mkm2) and the Palaeozoic-Mesozoic Chortis continental block (about 700,000 km2), both bounded by deformed marginal belts. The northern (Guatemala and Greater Antilles) and the southern (northern Venezuela) plate margins are marked by collisional zones, whereas the western (Central America Isthmus) and the eastern (Lesser Antilles) margins are represented by convergent boundaries and their magmatic arcs, all involving ophiolitic terranes. The evolutionary history of the Caribbean Plate since the Jurassic-Early Cretaceous encompasses plume, accretionary, and collisional tectonics, the evidence of which has been recorded in the oceanic remnants of lost LIPs, as revealed in: i) the MORB to OIB thickened crust of the oceanic plateau, including its un-deformed or little deformed main portion, and scattered deformed tectonic units; ii) ophiolitic tectonic units of MORB affinity and the rock blocks in ophiolitic melanges; iii) intra-oceanic, supra subduction magmatic sequences with IAT and CA affinities. The Mesozoic oceanic LIPs, from which the remnants of the Caribbean Plate have been derived, have been poorly preserved during various episodes of the intra-oceanic convergence, either those related to the original proto-Caribbean oceanic realm or those connected with two eo-Caribbean stages of subduction. The trapped oceanic plateau of the Colombia and Venezuela Basins is likely to be an unknown portion of a bigger crustal element of a LIP, similar to the Ontong-Java plateau. The Jurassic-Early Cretaceous proto-Caribbean oceanic domain consists of oceanic crust generated at multiple spreading centres; during the Cretaceous, part of this crust was thickened to form an oceanic plateau with MORB and OIB affinities. At the same time, both South and North American continental margins, inferred to be close to the oceanic realm, were affected by rifting and within-plate tholeiitic magmatism (WPT); this interpretation supports a near mid-America original location of the "proto-Caribbean" LIP. The MORB magmatic sections and rock blocks in the ophiolitic melanges are interpreted as exhumed tectonic sheets of the normal proto-Caribbean oceanic lithosphere, or part of a back-arc crust, both deformed in the eo-Caribbean stages. The SSZ complexes, considered as Cordilleran-type deformed ophiolites, were derived from a LIP that experienced two superimposed eo-Caribbean stages of intra-oceanic subduction. The older (Mid-Cretaceous) stage involved the eastward subduction of the un-thickened proto-Caribbean lithosphere, resulting in IAT and CA magmatism accompanied by HP-LT metamorphism and melange formation. The second, Late Cretaceous stage involved a westward dipping intra-oceanic subduction, which generated tonalitic arc magmatism. The eastward wedging of the Caribbean Plateau between the North and South American plates progressively trapped remnants of the Colombia and Venezuela Basins between the Atlantic and Pacific subduction zones and their new volcanic arcs (Aves-Lesser Antilles and Central American Isthmus). Unlike the proto-Caribbean, it appears that this LIP did not involve the main continental margins, even though the northern and southern Caribbean borders experienced different evolutionary paths. It was largely lost by superimposed accretionary and collisional events producing the marginal belts of the Caribbean Plate; its evolution has been dominated by a strongly oblique tectonic regime, constraining seafloor spreading, subduction, crustal exhumation, emplacement, and dismembering processes.

Giunta, G.

2005-12-01

324

Olistostromes are the Source of Melange in Diapirs in the Cascadia-Olympics Accretionary Wedge , NW USA  

NASA Astrophysics Data System (ADS)

Diapirs consisting of block-in-matrix mélange are common in the ocean-ward part of the active Cascadia-Olympics wedge. Some of these bodies and similar Neogene mélanges ["Hoh mélange"] have been interpreted as having originated in shear zones related to accretion as oceanic crust of the Juan de Fuca plate was thrust beneath the wedge. However, this interpretation is untenable. The Hoh mélange contains fragments and blocks, ranging from centimeters to kilometers in size, of basalt. The chemistry of the basalt, and the microfossils in associated mudstone, prove that the basalt is Eocene: these basalts were derived from the Crescent Formation, not the much younger oceanic crust of the Juan de Fuca plate. The Crescent basalts originally formed the lid beneath which the Cascadia-Olympics wedge of sediments was underplated. Much of the lid has been eroded, but in Miocene time it extended to the coast and contributed fragments and blocks to muddy debris flows, which were deposited as olistostromes on the subducting Juan de Fuca plate. Younger sediments buried the olistostromes, which became overpressured and mobilized as mobile masses that have intruded as diapirs and anticlinal ridges. Analogous diapiric bodies, in the broad sense, are present in other active accretionary wedges, such as the in the Lesser Antilles.

Cowan, D. S.; Brandon, M. T.

2011-12-01

325

High-Pressure\\/Low-Temperature Melanges in the Cycladic Blueschist Belt, Greece: Results of an Ionprobe (SHRIMP) UPb Study  

Microsoft Academic Search

The Cycladic blueschist belt (CBB) in the central Aegean Sea has experienced epidote blueschist to eclogite facies metamorphism in the Eocene (c. 50-40 Ma) during collisional processes between the Apulian microplate and Eurasia. The general geological framework is well established, but many details of the tectonometamorphic history still are not fully understood. Unresolved issues concern the importance of pre-Eocene HP

M. Bröcker; A. Keasling; R. T. Pidgeon

2005-01-01

326

Une experience d'auto-enseignement de groupe (An Experiment in Group Self-Instruction). Melanges pedagogiques.  

ERIC Educational Resources Information Center

The principles and organization of a group learning scheme involving 100 members of the Universite du Troisieme Age are described. The particular group of interest here was involved in a conversational course in English. The average age of the group was 59 and generally the educational level was the baccalaureate. In this form of learning, the…

Riley, P.; Sicre, M.

327

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

ERIC Educational Resources Information Center

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

Abe, D.; And Others

328

Apprentissage des Valeurs Communicatives de l'Intonation Anglaise (Learning the Communicative Functions of English Intonation). Melanges Pedagogiques, 1977.  

ERIC Educational Resources Information Center

This paper discusses an ongoing experiment in teaching the communicative values of intonation to second-year students in an English department at a French university. The communication needs of the learners are described in terms of academic activities (e.g., formal discussion) and social contact. A basic three-term structure -- opening,…

Heddesheimer, C.; Roussel, F.

329

Disrupted peridotites and basalts from the Neoproterozoic Araguaia belt (northern Brazil): Remnants of a poorly evolved oceanic crust?  

NASA Astrophysics Data System (ADS)

Fault-bound slices of serpentinized peridotites and pillow basalts interbedded with the metasedimentary rocks of the Tocantins Group in the central western Araguaia belt are probably remnants of poorly developed oceanic crust dismembered into a melange. The hydrothermally altered basalts display a fairly well-defined T-MORB, whereas the serpentinite lenses appear to be derived from moderately depleted upper mantle rocks, such as harzburgites and dunites. Neither sheeted dykes nor gabbros, which characterize a normal ophiolitic sequence, have been identified. The restricted volume of mafic volcanics, their moderate REE fractionation, the LREE enrichment of the serpentinized upper mantle peridotites, and the lack of other ophiolitic lithologies suggest that no appropriate geotectonic conditions were reached for normal oceanic crust development. Instead, it appears that the rifting of a large cratonic block resulted in local mantle exhumation and the formation of a partially mantle-floored basin, with sporadic basaltic magma supply. Thus, only a thin and discontinuous oceanic crust formed, somewhat similar to the Alpine-Apennine poorly evolved oceanic basins. Basin inversion occurred during the Neoproterozoic Brasiliano tectonic event before it reached an advanced ocean stage. The sedimentary sequence underwent intense deformation and greenschist facies metamorphism and was transformed into a complex system of westward to northwestward, overthrusted, imbricated slabs with slivers of basalt and serpentinized peridotites.

Kotschoubey, Basile; Hieronymus, Bernard; de Albuquerque, Carlos Alberto R.

2005-12-01

330

Fragments of the Vendian-Paleozoic oceanic crust of the Paleo-Asian Ocean in foldbelts (Altai-Sayan, Central Asia)  

NASA Astrophysics Data System (ADS)

Four main accretion-collision stages of the evolution of the Paleo-Asian ocean have been recognized in Altai-Sayan: 1) Early-Middle Cambrian, 2) Late Cambrian-Early Ordovician, 3) Devonian-Early Carboniferous and 4) Late Paleozoic. The 1st and 2nd stages characterize evolution of the Kuznetsk-Altai island-arc system, which was accreted to the Siberian continent. The third stage refers to two collisional events during the closing of the Paleo-Asian Ocean: the Gondwana-derived Altai-Mongolian terrane collided with the Siberian continent (D3), and the latter collided with the Kazakhstan continent (C2). Each stage was recorded in folded rock units. Paleooceanic crust fragments occur within accretionary wedges and suture zone. Their geological identification was supported by geochemical data. The better studied areas are Kurai and Katun accretionary wedges, Charysh-Terekta shear zone, and Chara ophiolitic suture zone. Identification of the Vendian-Early Carboniferous oceanic crust extended our knowledge about the Paleo-Asian Ocean evolution. The Kurai and Katun accretionary wedges recorded the Vendian-Cambrian stage of the Kuznetsk-Altai island arc evolution. The Charysh-Terekta zone resulted from the Late Devonian collision of the Altai-Mongolian terrane and the Siberian continent. The Chara ophiolitic suture was formed after the Late Carboniferous-Permian collision of the Siberian and Kazakhstan continents. The Kurai accretionary wedge is composed of the tectonic sheets of the Baratal paleoisland and Chagan-Uzun ophiolites. The Katun accretionary wedge involves paleo-oceanic island rock units: limestones, dolomites, siliceous shales, and basaltic flows. The Charysh-Terekta zone is composed of several tectonic lenses (e.g. Zasurin Formation) comprising sandstones, cherts, pillow-basalts, volcanoclastics, and gabbro-diabase sills and dikes. The Chara ophiolitic belt consists of several melange zones with high-pressure metamorphic rocks (metabasaltic rocks) metamorphosed deep-water siliceous sediments, eclogites, amphibolites, glaucophane schists and volcanic-siliceous rocks with radiolarians and conodonts. The geochemical study of metabasaltic rocks from those four entities indicates that they were formed in two major settings: at mid-oceanic ridges and in oceanic islands. The fragments of the oceanic crust of the Paleo-Asian Ocean were involved in accretionary wedges and shear zones as a result of accretion-collision processes and subduction.

Safonova, I.; Buslov, M.

2003-04-01

331

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

E-print Network

rocks (300 km long, 50 km wide and 2 km thick) is thrust upon a smaller nappe (Poya terrane) composed to the MORB of the Poya terrane. This is used to infer that the ultramafic nappe and the mafic Poya terrane

Boyer, Edmond

332

Comparing Sulfur and Carbon Speciation in Ophiolites and Active Peridotite-hosted Hydrothermal Systems  

NASA Astrophysics Data System (ADS)

At slow and ultraslow mid-oceanic ridges, tectonic extension and crustal thinning lead to the exposure of ultramafic rocks on the ocean floor resulting in serpentinization and precipitation of carbonates in open fractures. Serpentinization processes play a major role in the global marine bio-geochemical cycle and account for an important part of the exchange of sulfur and carbon between seawater and the oceanic crust. The opaque mineral assemblages and the sulfur and carbon isotopic compositions of the serpentinites strongly depend on fluid flux, redox conditions and microbial activity prevailing during serpentinization. Here we present a carbon and sulfur geochemical study that compares Lost City, the only as yet known active peridotite-hosted hydrothermal system, with ancient systems preserved in drill cores of the Iberian margin (ODP Leg 149) and ophicalcites from Liguria (northern Apennines, Italy). Carbon speciation and isotope analyses conducted on serpentinized peridotites from Leg 149 show distinct changes with depth, indicating a decrease in seawater penetration and marine carbonate precipitation downhole, but constant precipitation temperatures of <20°C. Samples from Liguria show similar marine carbon isotope values, but higher carbonate precipitation temperatures (<144°C), which are in the range of serpentinization temperatures measured at the Iberian Margin (~150°C). Values of ?13CTOC suggest the presence of organic matter and lie within a narrow range of -28 to -24 permil at both localities. The occurrence of magnetite, pyrite, pentlandite and millerite in serpentinites from Liguria are similar to the opaque mineral assemblages observed by Alt and Shanks [1] in samples from the Iberian Margin, where serpentinization took place at low temperatures (<200°C) and high water-rock ratios. In addition, they are very similar to assemblages in serpentinite samples from the southern wall at the Atlantis Massif, where microbial activity below active hydrothermal chimneys is indicated by negative ?34S-values. The range in carbon compositions and a dominance of depleted carbon isotope compositions is similar to trends in serpentinites from Lost City and indicates that organic carbon is an important component of the carbon budget in marine serpentinites. Furthermore, carbon and sulfur isotope compositions indicate that microbial activity has a significant influence on the development of active and ancient peridotite-hosted hydrothermal systems. [1] Alt, J. C. and Shanks, W. C. 1998: Sulfur in serpentinized oceanic peridotites: Serpentinization processes and microbial sulfate reduction. Journal of geophysical research 103, B5, 9917- 9929.

Schwarzenbach, E. M.; Frueh-Green, G. L.; Bernasconi, S. M.; Delacour, A.; Plas, A.

2008-12-01

333

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

Microsoft Academic Search

Hydrogen gas is emitted by springs associated with serpentinites and extensive carbonate deposits in Oman, The Philippines, the USA and other continental locations. The hydrogen springs contain unusually alkaline fluids with pH values between 11 and 12.5. Other workers have described off-ridge submarine springs with comparably alkaline fluid compositions, serpentinite, abundant free hydrogen gas, and associated carbonate edifices such as

R. M. Coveney

2008-01-01

334

Comparing Sulfur and Carbon Speciation in Ophiolites and Active Peridotite-hosted Hydrothermal Systems  

Microsoft Academic Search

At slow and ultraslow mid-oceanic ridges, tectonic extension and crustal thinning lead to the exposure of ultramafic rocks on the ocean floor resulting in serpentinization and precipitation of carbonates in open fractures. Serpentinization processes play a major role in the global marine bio-geochemical cycle and account for an important part of the exchange of sulfur and carbon between seawater and

E. M. Schwarzenbach; G. L. Frueh-Green; S. M. Bernasconi; A. Delacour; A. Plas

2008-01-01

335

Harzburgite Peak: A large mantle tectonite massif in ophiolite from southwest Yukon  

Microsoft Academic Search

Detailed mapping of bedrock in the northern Wellesley basin adjacent to the Donjek River revealed a coherent sequence of cumulus-textured gabbros, sheeted dykes, and a large massif of spinel harzburgite. The coarse-textured harzburgite tectonite covers an area of ~75 km2, and is generally well preserved, making it one of the largest and most exceptional mantle tectonite bodies yet recognized in

D. Canil; S. T. Johnston

336

Paleocene–Eocene record of ophiolite obduction and initial India-Asia collision, south central Tibet  

Microsoft Academic Search

Uppermost Cretaceous to Eocene marine sedimentary sequences occur both to the south and north of the Yarlung Zangbo suture in south central Tibet. They consist of Indian-margin strata of the northern Tethyan Himalaya and Asian-margin strata of the Gangdese forearc. Both assemblages are characterized by major changes in depositional environment and sedimentary provenance at ?65 Ma and an appearance of

Lin Ding; Paul Kapp; Xiaoqiao Wan

2005-01-01

337

Paleocene-Eocene record of ophiolite obduction and initial India-Asia collision, south central Tibet  

Microsoft Academic Search

Uppermost Cretaceous to Eocene marine sedimentary sequences occur both to the south and north of the Yarlung Zangbo suture in south central Tibet. They consist of Indian-margin strata of the northern Tethyan Himalaya and Asian-margin strata of the Gangdese forearc. Both assemblages are characterized by major changes in depositional environment and sedimentary provenance at ~65 Ma and an appearance of

Lin Ding; Paul Kapp; Xiaoqiao Wan

2005-01-01

338

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

E-print Network

. We complement studies at Troodos that have reconstructed the brittle deformation of the upper crust. The spatial distribution of rotation axes allow recognition of three regions to which deformation sufficiently hot so the upper crust can decouple from the substrate. Clockwise rotation in the gabbro increases

Granot, Roi

339

Geochemical variations in a depleted fore-arc mantle: The Ordovician Thetford Mines Ophiolite  

E-print Network

by harzburgite (5­6% clinopyroxene), cut by dunitic to orthopyroxenitic dykes. The plutonic crust is dominated isotopic data on closely related rocks. However, bulk sediment contamination yields model residues at depth reacted with mantle rocks on their way to the surface, enriching the mantle and depleting

Long, Bernard

340

Spectroscopic characterization of chromite from the Moa-Baracoa Ophiolitic Massif, Cuba  

NASA Astrophysics Data System (ADS)

The Cuban chromites with a spinel structure, FeCr 2O 4 have been studied using optical absorption and EPR spectroscopy. The spectral features in the electronic spectra are used to map the octahedral and tetrahedral co-ordinated cations. Bands due Cr 3+ and Fe 3+ ions could be distinguished from UV-vis spectrum. Chromite spectrum shows two spin allowed bands at 17390 and 23810 cm -1 due to Cr 3+ in octahedral field and they are assigned to 4A 2g(F) ? 4T 2g(F) and 4A 2g(F) ? 4T 1g(F) transitions. This is in conformity with the broad resonance of Cr 3+ observed from EPR spectrum at g = 1.903 and a weak signal at g = 3.861 confirms Fe 3+ impurity in the mineral. Bands of Fe 3+ ion in the optical spectrum at 13700, 18870 and 28570 cm -1 are attributed to 6A 1g(S) ? 4T 1g(G), 6A 1g(S) ? 4T 2g(G) and 6A 1g(S) ? 4T 2g(P) transitions, respectively. Near-IR reflectance spectroscopy has been used effectively to show intense absorption bands caused by electronic spin allowed d -d transitions of Fe 2+ in tetrahedral symmetry, in the region 5000-4000 cm -1. The high frequency region (7500-6500 cm -1) is attributed to the overtones of hydroxyl stretching modes. Correlation between Raman spectral features and mineral chemistry are used to interpret the Raman data. The Raman spectrum of chromite shows three bands in the CrO stretching region at 730, 560 and 445 cm -1. The most intense peak at 730 cm -1 is identified as symmetric stretching vibrational mode, A 1g(? 1) and the other two minor peaks at 560 and 445 cm -1 are assigned to F 2g(? 4) and E g(? 2) modes, respectively. Cation substitution in chromite results various changes both in Raman and IR spectra. In the low-wavenumber region of Raman spectrum a significant band at 250 cm -1 with a component at 218 cm -1 is attributed F 2g(? 3) mode. The minor peaks at 195, 175, 160 cm -1 might be due to E g and F 2g symmetries. Broadening of the peak of A 1g mode and shifting of the peak to higher wavenumber observed as a result of increasing the proportion of Al 3+O 6. The presence of water in the mineral shows bands in the IR spectrum at 3550, 3425, 3295, 1630 and 1455 cm -1. The vibrational spectrum of chromite gives raise to four frequencies at 985, 770, 710 and 650 cm -1. The first two frequencies ? 1 and ? 2 are related to the lattice vibrations of octahedral groups. Due to the influence of tetrahedral bivalent cation, vibrational interactions occur between ? 3 and ? 4 and hence the low frequency bands, ? 3 and ? 4 correspond to complex vibrations involving both octahedral and tetrahedral cations simultaneously. Cr 3+ in Cuban natural chromites has highest CFSE (20,868 cm -1) when compared to other oxide minerals.

Reddy, B. Jagannadha; Frost, Ray L.

2005-06-01

341

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

Microsoft Academic Search

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

Y. Dilek

2001-01-01

342

PaleoceneEocene record of ophiolite obduction and initial India-Asia collision, south central Tibet  

E-print Network

Tibet Lin Ding Institute of Tibetan Plateau Research and Institute of Geology and Geophysics, Chinese suture in south central Tibet. They consist of Indian-margin strata of the northern Tethyan Himalaya in depositional environment and sedimentary provenance at $65 Ma and an appearance of detrital chromium

343

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

E-print Network

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

Mervine, Evelyn Martinique

2012-01-01

344

Etude Electrochimique de Proprietes Chimiques Dans L 'ethanolamine et Ses Melanges Avec L 'eau (Electrochemical Study of Chemical Properties in Ethanolamine and Its Mixtures with Water).  

National Technical Information Service (NTIS)

The work was concerned with the study of acid-base reactions and of complex formation in ethanolamine and its mixtures with water. The ionic product of the solvent was determined by an electrochemical study of the H(+)/H2 system. The reduction curves for ...

M. Grall

1966-01-01

345

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

E-print Network

on the historical record in his adulthood as a functionary in the Habsburg foreign service. For students of Horace /'intelligence de cet Auteur, & qui donne occasion de traiter d'une suite considerable de lieux antiques par M. l

Frischer, Bernard

346

La Recherche en Pedagogie des Langues aux Adultes: Compte Rendu d'un Seminaire (Research in Language Teaching to Adults: Conference Report). Melanges Pedagogiques, 1977.  

ERIC Educational Resources Information Center

This article describes the seminar on research in adult language education held in Nancy, France, in June, 1977. It includes reports on the four roundtables and gives details concerning organization and membership, as well as a list of the topics that were discussed. The four roundtables addressed the issues of student needs and motivation,…

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

347

mentales. Les couches sont generalement consti-tuees d'un melange de Cu,O et de CuO avec une  

E-print Network

] WAGNER et GRUNEWALD, Z. Phys. Chemie (B), 1938, 40, 457. �TUDE DE LA CONSTITUTION DES FILMS D'OXYDE oxydation, anodique a été effectuée par spectrométrie d'absorption infrarouge. Les spectres ont montré que les anions constitutifs de l'électrolyte se trouvent inclus dans la couche d oxyde et peuvent être

Paris-Sud XI, Université de

348

Contributions au modele de melange Gaussien pour l'estimation de densites d'histogrammes et applications a l'analyse d'images radar  

NASA Astrophysics Data System (ADS)

This thesis is a publication thesis. It presents several of our contributions to the problem of image histogram density estimation. This thesis is divided into three parts. In the first part, we propose two algorithms for estimating the pdf of an image histogram with a mixture model. Both algorithms are designed to estimate the number of modes in a multi-modal histogram and the parameters of each component of the mixture. The algorithms are categorized under two different approaches. The second part of this thesis concerns the generation of test data, and particular emphasis on the generation of artificial histograms. Valid test histograms are essential in the development of mixture pdf estimation algorithms. For this purpose, we first give a formal definition for the concept of overlap between two adjacent components mixtures, and then propose two different algorithms for controlling this overlap. Finally, in the third part of this thesis we present two real applications dealing with target detection and segmentation in SAR (Synthetic Aperture Radar) images. In the first application, we propose an algorithm for the segmentation of small vehicle targets, and in the second we propose an algorithm for ship target detection in SAR images. Six research papers are included in this thesis. Three of them have been published in refereed journals (the Journal of Neural, Parallel and Scientific Computation, the International Journal of Pattern Recognition and Image Analysis and the Canadian Journal of Remote Sensing), two have been submitted to refereed journals (the International Journal of Pattern Recognition and Image Analysis and the International Journal of Remote Sensing), and one is an article published in an international conference (the International Conference on Signal Processing, Application and Technology (ICSPAT'2000)).

Aitnouri, Elmehdi

349

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

NASA Astrophysics Data System (ADS)

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

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

2006-10-01

350

Accretionary nature of the crust of Central and East Java (Indonesia) revealed by local earthquake travel-time tomography  

NASA Astrophysics Data System (ADS)

Reassessment of travel time data from an exceptionally dense, amphibious, temporary seismic network on- and offshore Central and Eastern Java (MERAMEX) confirms the accretionary nature of the crust in this segment of the Sunda subduction zone (109.5-111.5E). Traveltime data of P- and S-waves of 244 local earthquakes were tomographically inverted, following a staggered inversion approach. The resolution of the inversion was inspected by utilizing synthetic recovery tests and analyzing the model resolution matrix. The resulting images show a highly asymmetrical crustal structure. The images can be interpreted to show a continental fragment of presumably Gondwana origin in the coastal area (east of 110E), which has been accreted to the Sundaland margin. An interlaced anomaly of high seismic velocities indicating mafic material can be interpreted to be the mantle part of the continental fragment, or part of obducted oceanic lithosphere. Lower than average crustal velocities of the Java crust are likely to reflect ophiolitic and metamorphic rocks of a subduction melange.

Haberland, Christian; Bohm, Mirjam; Asch, Günter

2014-12-01

351

Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications  

NASA Astrophysics Data System (ADS)

This paper deals with the tectonic events that result in the accretion of mafic terranes in the fore-arc region and a close juxtaposition of ultramafic rocks, low grade and high-grade mafic terranes in many collisional orogens. The example is taken from New Caledonia where tectonic accretion, subduction, underplating and obduction of mafic terranes took place during the late Eocene in an intra-oceanic forearc setting. The late Eocene tectonic complex comprised three major terranes: an overlying ultramafic, mainly harzburgitic allochthon named the Ophiolitic Nappe, an intermediate mafic, mainly basaltic off-scraped melange, composed of kilometre-scale slices of oceanic upper crust, called the Poya Terrane, parts of which have been metamorphosed into an eclogite/blueschist facies complex, the Pouebo Terrane; and a lower, continental basement formed by the Norkolk Ridge terranes. Based upon exhaustive sampling of the mafic terranes and field surveys, our tectonic, micropaleontologic and geochemical data reveal that Poya and Pouebo terranes rocks originally formed within one single Campanian to late Paleocene oceanic basin, floored by tholeiitic basalt associated with some minor seamount-related intraplate alkali basalt. The tholeiitic basalt displays a continuous range of compositions spanning between "undepleted" and "depleted" end-members; the former being volumetrically predominant. The overall geochemical and isotopic features indicate an origin from a prominently heterogeneous mantle source during the opening of a marginal basin, the South Loyalty Basin, which almost completely disappeared during Eocene convergence. The opening of this basin originally located to the east of the Norfolk Ridge was synchronous with that of Tasman Sea basin as a consequence of oceanward migration of the west-dipping Pacific subduction zone. Establishing the origin of the ultramafic Ophiolitic Nappe is beyond the scope of this paper; however, it appears to be genetically unrelated to the mafic Poya and Pouebo terranes. Although it was located in the Late Eocene fore-arc, the Ophiolitic Nappe and the corresponding oceanic lithosphere originated before the Late Cretaceous, to the east of the South Loyalty Basin in a back-arc setting; or alternatively in a much older, trapped basin. For reasons that remain unclear, a new east-dipping subduction started in the Eocene and consumed most of the South Loyalty Basin, forming the intra-oceanic Loyalty Arc. Due to a changing subduction regime (underplating of the Diahot Terrane?), the mafic slices that now form the Poya Terrane were tectonically accreted in the Loyalty fore-arc region and remained under low pressure-low temperature conditions (possibly at the subsurface) until the Norfolk Ridge reached the subduction zone diachronously. This resulted in the final obduction of the fore-arc area. The two-step obduction involved first the mafic complex forming the Poya Terrane and thereafter the lithospheric mantle that now forms the Ophiolitic Nappe. In contrast, pieces of the accretionnary complex were dragged down into the subduction zone, underplated at depth ca. 70 km and metamorphosed into high-temperature eclogite to form the Pouebo Terrane metamorphics that display the same geochemical features as the Poya Terrane basalt. A mid-to-late Eocene syntectonic piggy-back sedimentary basin (the Nepoui flysch basin) mainly filled with mafic clastic material and shallow water carbonates that record the progressive uplift of the fore-arc region due to the accretion and underplating of mafic ocean-related and other material. In contrast, a slightly younger foreland basin located upon the Norfolk Ridge (the Priabonian Bourail Flysch basin) received a massive input of detrital material derived from the Norfolk Ridge itself and a time-increasing amount of mafic, Poya-derived material that recorded the first step of obduction. Thereafter, the Bourail Flysch was overthrust by the Poya Terrane and finally by the Ophiolitic Nappe. At the same time, buoyancy-driven uplift and exhumation of the

Cluzel, Dominique; Aitchison, Jonathan C.; Picard, Christian

2001-10-01

352

International Geology Review, Vol. 46, 2004, p. 11031118. Copyright 2004 by V. H. Winston & Son, Inc. All rights reserved.  

E-print Network

with major ophiolite units such as the Coast Range ophiolite (CRO), Josephine ophiolite, Trinity ophiolite and Coast Range ophiolite in California. The Franciscan Complex, an assemblage of variably deformed rocks Bodies, San Francisco Bay Area, California: Derivation from the Subducting Plate vs. Mantle Hanging Wall

Wang, Zhi "Luke"

353

Ophiolite emplacement by strike-slip tectonics between the Pontide Zone and the Sakarya Zone in northwestern Anatolia, Turkey  

Microsoft Academic Search

Northwestern Anatolia contains three main tectonic units: (a) the Pontide Zone in the north which consists mainly of the Gstanbul-Zonguldak Unit in the west and the BallLda<-Küre Unit in the east; (b) the Sakarya Zone (or Continent) in the south, which is juxtaposed against the Pontide Zone due to the closure of Paleo-Tethys prior to Late Jurassic time; and (c)

Ali Elmas; Erdinç Yigitbas

2001-01-01

354

Precambrian Ophiolites and Related Rocks Edited by Martin J. van Kranendonk, R. Hugh Smithies and Vickie C. Bennett  

E-print Network

the crust, hydrosphere, atmosphere and the emergent biosphere, on the geochemical cycles of the biologically and hydrosphere is dominated by exchange of sulphur between seawater and rocks by fluids, weathering, dissolution

Mojzsis, Stephen J.

355

Structure and composition of mantle peridotites at the boundary with crustal complexes of ophiolites in the Syumkeu massif, Polar Urals  

NASA Astrophysics Data System (ADS)

Intense viscous-ductile deformations with multiorder flow folds and thin banding have been established in lherzolite and harzburgite of the Syumkeu massif 1.0-1.5 km below the boundary with crustal complexes. Intense shear deformation of mantle restites is traced along the entire boundary zone. The mineral composition of lherzolite and harzburgite in this zone occupies a transitional position between peridotite restites and olivine websterite from the lower part of the banded dunite-wehrlite-pyroxenite-gabbro complex. This implies that the mantle rocks from the crust-mantle transition zone were substantially transformed under transpressional intense shear stress settings along with a high-temperature ductile flow of mantle restites interacting with the supplied melt at a depth of more than 10 km. This type of transition zones differs from those known elsewhere in the Urals and supplements our knowledge on modes of mantle restite juxtaposition with crustal plutonic rocks.

Savelyeva, G. N.; Suslov, P. V.

2014-09-01

356

The geology, geochemistry and emplacement of the Cretaceous—Tertiary ophiolitic Nicoya Complex of the Osa Peninsula, southern Costa Rica  

NASA Astrophysics Data System (ADS)

The Nicoya Complex of the Osa Peninsula is essentially an obducted segment of oceanic crust comprising basaltic lavas and associated intrusive dolerite and gabbro, interstratified with lesser amounts of pelagic limestones, cherts and argillites. The sediments contain a minor clastic component and were deposited on an ocean floor of considerable relief and distant from a major landmass. The extrusive and intrusive basaltic rocks have geochemical affinities to large ion lithophile (LIL) element-enriched oceanic crust, and are interpreted to have formed in a back-arc basin analagous to the Mariana Trough, Lau Basin or Gulf of California. One sample has distinctly different geochemical characteristics and may represent a younger within-plate seamount. In the Late Cretaceous, an E-W-trending intra-oceanic trench/volcanic/back-arc system developed in association with an active southward-dipping subduction zone located south of the present-day southern Central American isthmus. Pelagic sediments and basaltic lavas accumulated in the back-arc over a period of at least 34 Ma spanning the Late Cretaceous and Early Tertiary. During this period there were three major volcanic events dated respectively as Santonian-Campanian (78.0 ± 2 Ma), Palaeocene (60.2 ± 7.6 Ma) and Middle Eocene (44.0 ± 4.4 Ma). Continuing northward movement of the southern plate caused overthrusting of the volcanic arc onto the northern plate and production of a thickened embryonic continental crust. Inferred reorganization of crustal stress in the Late Eocene caused fragmentation of the single ancestral plate into the Caribbean and "East Pacific" plates, with a flipping of the subduction zone accompanying development of the NE-dipping Middle America subduction zone and andesitic volcanism. During the Oligocene, the ancestral East Pacific plate split into the NE-moving Cocos plate and the eastward-moving Nazca plate, separated by the E-W-trending Colón spreading ridge and a series of N-S-trending transforms. The Cocos plate was subsequently split into two blocks separated by the Costa Rica Fracture Zone that extends northeastwards from the western end of the Colón Ridge to Costa Rica, which it divides into two distinctive volcanotectonic domains. To the north of the Costa Rica Fracture Zone, the Cocos plate is moving northeastwards and being consumed by the Middle America subduction zone, whereas the southern block is under the influence of the Colón spreading ridge with a N-S-oriented main stress axis. The N-S-trending Panama Fracture Zone can be extrapolated northwards via three submarine canyons on the continental slope to merge into a braided system of curved NW-trending coast-parallel wrench faults on which predominantly dextral strike-slip movement has produced pull-apart and tipped wedge basins with adjacent uplifted mini-horsts dating back to the Middle Pliocene. Therefore, although the Nicoya Complex of the Osa Peninsula was originally emplaced by accretion and thrusting related to pre-Oligocene plate movement, it owes its present-day exposure to post-Late Miocene wrench fault tectonics, with superimposed isostatic uplift.

Berrangé, J. P.; Thorpe, R. S.

1988-04-01

357

Newly Discovered Ophiolite Scrap in the Hartland Formation of Midtown Charles Merguerian, Geology Department, 114 Hofstra University, Hempstead, NY 11549  

E-print Network

, highly deformed ellipsoidal serpentinite body that has been uncovered during recent excavation of a deep February 2005 we had four opportunities to examine the bedrock geology of a large construction excavation, west of Sixth Avenue and was ultimately excavated to a depth of ~60' below street level (Figure 1

Merguerian, Charles