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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

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

NASA Technical Reports Server (NTRS)

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

Mustard, John F.; Pieters, Carle M.

1988-01-01

3

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

SciTech Connect

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

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

1985-01-01

4

Geochemistry and tectonic significance of the Koçali ophiolite and the related Koçali melange, Adiyaman region, SE Turkey  

NASA Astrophysics Data System (ADS)

The Koçali complex includes the Koçali Ophiolite and the volcanic-sedimentary Koçali melange. The ophiolitic rocks form E-W trending thrust sheets in which all of the components of a complete ophiolite sequence are present, although mainly separated by tectonic contacts; i.e. (from bottom to top), serpentinised harzburgite, layered cumulates, isotropic gabbros, sheeted dykes, pillow lavas, radiolarian and metalliferous sediments. Microgabbro-diabase dykes intrude various levels of the ophiolite pseudostratigraphy. The sheeted dykes and pillow lavas locally exhibit Cyprus-type hydrothermal mineralisation. The Koçali melange is a tectonic slice complex that structurally underlies the ophiolite on a regional scale, although ophiolite and melange are structurally intercalated in places related to emplacement onto the Arabian continental margin. The geochemistry of the crustal rocks of the Koçali ophiolite and the Koçali melange, taken together, shows that are all tholeiitic in composition except for the volcanic rocks which show both tholeiitic and alkaline features. New geochemical data from the ophiolitic isotropic gabbros, sheeted dikes, isolated dikes and volcanics indicate that there are three main types of parental basic magmas: (i) IAT series, comprising isotropic gabbros, sheeted dikes and the Group I isolated dikes; (ii) E-MORB series, characterized by the Group I volcanics and Group II isolated dikes; (iii) OIB series represented by the Group II volcanic rocks. The presence of highly magnesian olivines (Fo84-74), clinopyroxenes (Mg#92-65), orthopyroxenes (Mg#86-75), together with Ca-rich plagioclases (An95-86) in the cumulate rocks suggests that the plutonic suite was derived from an island arc tholeiitic (IAT) source rather than a mid-ocean ridge-type magmatic source. The Koçali Complex is interpreted to have formed during opening of the southern Neotethys, whereas the the Koçali ophiolite formed above a north-dipping intraoceanic subduction zone during ocean basin closure. Taking account of evidence from comparable ophiolites and melanges in Baer-Bassit (N-Syria) and the Mamonia Complex (SW Cyprus), the southern Neotethyan oceanic basin is interpreted to have rifted during the Late Triassic. The Koçali melange restores as the north-facing passive margin of the Arabian plate. The South Neotethyan ocean oceanic floor was covered by Jurassic-Lower Cretaceous continental margin-derived and pelagic/hemipelagic deep-sea sediment. Northward subduction began during the Late Cretaceous and the ophiolite formed above a northward-dipping subduction zone. The subduction trench collided with the Arabian margin during latest Cretaceous, driving southward emplacement of the Koçali, Hatay and Baer-Bassit ophiolites, finally during Maastrichtian time.

Yildirim, N.; Parlak, O.; Robertson, A.

2012-04-01

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

J. metamorphic Geol., 2000, 18, 699718 Origin, HP/LT metamorphism and cooling of ophiolitic melanges in  

E-print Network

J. metamorphic Geol., 2000, 18, 699­718 Origin, HP/LT metamorphism and cooling of ophiolitic me as the regional poly-metamorphism occurring during Alpine orogenesis. The upper structural levels (Mt. Ochi/LT) metamorphism (M1). Glaucophane, epidote, sodic clinopyroxene and high-Si phengite constitute the Eocene M1

Dov, Avigad

7

Oceanization of the northern Neotethys: Geochemical evidence from ophiolitic melange basalts within the ?zmir-Ankara suture belt, NW Turkey  

NASA Astrophysics Data System (ADS)

The remnants of the Neotethyan Izmir-Ankara Ocean, the main branch of Neotethys in the eastern Mediterranean are represented by the Dagküplü Melange Complex in Central Sakarya, NW Turkey. It comprises several blocks or tectonic slices of pillow lavas, some of which include mudstones and radiolarian cherts as intra-pillow-fillings or interlayers. In the Igdecik area, a huge basaltic block has been studied in detail. Geochemical data reveal three distinct basalt types separated by sheared contacts. The first of these groups is an enriched mid-oceanic ridge basalt (E-MORB) type which is enriched in the most incompatible trace elements relative to normal MORB (N-NORB) in addition to having heavy rare earth elements (HREE) depletion, suggesting the influence of residual garnet in their mantle source region. The second is back-arc basin basalt (BABB) type with relatively depleted trace element compositions with respect to N-MORB together with a negative Nb anomaly, suggesting generation above an intra-oceanic subduction zone where partial melts are derived from a depleted (MORB-like) mantle. The final group is island-arc tholeiite (IAT) type, displaying the most depleted trace element abundances among the studied groups in addition to marked Nb depletion, reflecting intra-oceanic supra-subduction zone (SSZ) signatures similar to the BABB-type but requiring a depleted mantle source which has experienced a previous melt extraction. Combined with a previously ascribed Late Triassic age of Tekin et al. (2002) (221 Ma, Late Carnian; based on the radiolarian fauna found in a chert layer alternating with mudstones), the associated basalts with E-MORB-type geochemical signatures, suggest formation of oceanic crust as early as Late Carnian. This age is the oldest thus far obtained from the basalts of the Izmir-Ankara Ocean. This new data provides constraints on tectonic models for the opening the Izmir-Ankara Ocean and its relationship to other branches of the Neotethyan ocean in the Eastern Mediterranean area.

Göncüoglu, M. Cemal; Sayit, Kaan; Tekin, U. Kagan

2010-04-01

8

Tectonic setting for ophiolite obduction in Oman.  

USGS Publications Warehouse

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

Coleman, R.G.

1981-01-01

9

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

10

Chunky Gal Melange and its tectonic significance  

SciTech Connect

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

Lacazette, A. Jr.; Rast, N.

1985-01-01

11

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

SciTech Connect

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

Patton, W.W. Jr. (Geological Survey, Menlo Park, CA (United States))

1993-04-01

12

Geochemistry of the ophiolite and island-Arc volcanic rocks in the Mianxian-Lueyang suture zone, southern Qinling and their tectonic significance  

Microsoft Academic Search

Ultrabasic rocks in the Mianxian-Lueyang ophiolitic melange zone include harzburgite and dunite which exhibit LREE depletion\\u000a with remarkable positive Eu anomaly. The diabase dike swarm shows LREE enrichment but slightly negative Eu anomaly. Metamorphosed\\u000a volcanic rocks can be divided into two groups in terms of their REE geochemistry and trace element ratios of Ti\\/V, Th\\/Ta,\\u000a Th\\/Yb and Ta\\/Yb. One is

Lai Shaocong; Zhang Guowei; Yang Yongcheng; Chen Jiayi

1999-01-01

13

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

SciTech Connect

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

Davidsen, R.K.; Cloos, M.

1985-01-01

14

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

SciTech Connect

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

Stetzer, L.M.; Dilek, Y. (Vassar Coll., Poughkeepsie, NY (United States). Dept. of Geology and Geography)

1993-03-01

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

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

SciTech Connect

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

Sedlock, R.L. (San Jose State Univ., CA (United States). Geology Dept.)

1993-04-01

17

Zambales ophiolite, Philippines  

NASA Astrophysics Data System (ADS)

The Acoje massif is part of a mafic-ultramafic complex, the Zambales ophiolite, and is a fragment of Mesozoic oceanic crust. This paper documents the occurrence and phase relations of sulfides and associated phases in the critical zone of the Acoje massif. The Acoje critical zone ( ACZ) forms the basal cumulate sequence of the massif and consists of a variably serpentinized lower ultramafic zone and a relatively less altered upper mafic zone. Two distinct sulfide associations have been identified: (1) a troilite (±pyrrhotite)-dominated group hosted by the mafic zone and (2) a pentlandite-dominated group hosted by the ultramafic zone. Troilite-dominated assemblages represent the original mineralogy of magmatically precipitated sulfides in the entire cumulate sequence. The pentlandite-dominated group appears to have evolved from the primary magmatic sulfides during low-temperature re-equilibration. The paragenetic evolution from the magmatic assemblage to the low-temperature assemblage appears to have proceeded as follows: (1) S-rich hexagonal pyrrhotite+pentlandite+chalcopyrite (or cubanite)+magnetite, (2) S-poor hexagonal pyrrhotite+pentlandite+intermediate solid solution ( iss) phase (and/or cubanite)+magnetite, (3) troilite (or mackinawite)+pentlandite+ iss+magnetite, (4) troilite (or mackinawite)+pentlandite+ iss+native Cu+magnetite, (5) pentlandite+native Cu+magnetite, and (6) pentlandite+native Cu+Fe-Ni alloy+magnetite. This evolutionary trend, in conjunction with the observed textural, chemical, and sulfur-isotopic relations, indicates that the native metal and alloy phases in the ACZ were produced by low-temperature reduction of the primary magmatic sulfides. Correlations between sulfide assemblages and coexisting silicate-hydrosilicate-oxide assemblages further indicate that this alteration occurred during retrograde serpentinization of the Acoje massif. Two end-member models that could explain the inferred low-temperature mineralogic evolution of the ACZ sulfides are described: (1) an isothermal reduction model and (2) a non-isothermal equilibration model. Both isothermal and non-isothermal effects apparently were involved in the development of variably reduced sulfide-oxide-metal assemblages from the initial magmatic sulfides.

Abrajano, Teofilo A.; Pasteris, Jill D.

1989-09-01

18

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

NASA Astrophysics Data System (ADS)

The Khoy ophiolite in northwestern Iran, along the Turkish border, represents a remnant of oceanic lithosphere formed in the Mesozoic Neotethys. This northwest-southeast trending ophiolite complex consists, from northeast to southwest of a well-defined basal metamorphic zone, peridotites (mainly harzburgites and dunites) and serpentinized peridotites, gabbros, dikes, and extensive amount of pillowand massive basalts and basaltic andesite. The associated sedimentary rocks include a variety of late Cretaceous deep- and shallow-marine rock. These include pelagic fossiliferous carbonates which are mixed with pillow basalts and the basaltic andesites as interlayers or exotic blocks. Also present are extensive units of radiolarian chert which are interbedded within the basalts and basaltic andesites. Analysis of the Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) imagery covering the Khoy ophiolite and published geological maps allowed us to distinguish between different rocks types and interpret regional structures. ASTER has three bands (1 to 3) in the Visible and Near Infrared (VNIR), six bands (4 to 9) in the Short Wave Infrared (SWIR) and 5 bands (10 to 14) in the Thermal Infrared (TIR) portions of the electromagnetic spectrum. We used 3-2-1, and 7-3-1 ASTER false-color images and 4/7-3/4-2/1 ASTER band-ratio image for this study. The complex is divided into three NW-trending, SW-verging nappes. Upper serpentinized ultramafic rocks and lower amphibolites forming an allochthon with will-developed klippes of ultramafic rocks dominate the northeastern nappe. This nappe is thrust across shelf sediments dominated by limestones of late Cretaceous age that occupy the region to the northeast of the Khoy ophiolite complex. Superimposition of ENE-trending folds on the nappe structures produced domical structures that eroded to form tectonic widows where the amphiolites are exposed within the ultramafic rocks. The central nappe is an allochthon made up of a tectonic melange that mixes different lithologies of the region. Two NW-trending basaltic belts that have distinctively different spectral signature dominate the southwestern nappe and perhaps structurally lower sub-complex. This nappe is interpreted as a para-authochthon below the central allochthon. Structural evolution of the Khoy ophiolite complex is attributed to an early tectonic event that caused the emplacement of nappes from northeast to southwest, and a late tectonic pulse that resulted in an overall N-S directed shortening manifested by the development of ENE-trending folds, shear zones, and mega kink bands .

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

2002-05-01

19

Early Proterozoic ophiolite, central Arizona  

SciTech Connect

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

Dann, J.C. (Washington Univ., St. Louis, MO (USA))

1991-06-01

20

Tectonic Setting for Ophiolite Obduction in Oman  

Microsoft Academic Search

The Samail ophiolite is part of an elongate belt in the Middle East that forms an integral part of the Alpine mountain chains that make up the northern boundary of the Arabian-African plate. The Samail ophiolite represents a portion of the Tethyan ocean crust formed at a spreading center of Middle Cretaceous age (Cenomanian). During the Cretaceous spreading of the

Robert G. Coleman

1981-01-01

21

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

22

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

23

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

E-print Network

STRUCTURE AND KINEMATICS OF THE SUZUME FAULT, OKITSU MELANGE, SHIMANTO ACCRETIONARY COMPLEX, JAPAN A Thesis by TAKAMASA KANAYA Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment... of the requirements for the degree of MASTER OF SCIENCE December 2006 Major Subject: Geology STRUCTURE AND KINEMATICS OF THE SUZUME FAULT, OKITSU MELANGE, SHIMANTO ACCRETIONARY COMPLEX, JAPAN A Thesis by TAKAMASA KANAYA Submitted...

Kanaya, Takamasa

2007-04-25

24

Thematic mapper study of Alaskan ophiolites  

NASA Technical Reports Server (NTRS)

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

Bird, John M.

1988-01-01

25

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

26

Newfoundland Ophiolites and the Geology of the Oceanic Layer  

Microsoft Academic Search

THE significance and origin of ophiolites is a matter of interest to geologists. Many investigators interpret ophiolites as slices of oceanic crust which have been technically emplaced in orogenic belts1-7. Moores and Vine5, Dewey and Bird6, and Coleman7 have proposed just such an origin for ophiolites as geographically diverse as Cyprus, Newfoundland and the Western United States. If ophiolites are

J. J. Peterson; P. J. Fox; E. SCHREIBER

1974-01-01

27

Contact metamorphism by an ophiolite peridotite from neyriz, iran.  

PubMed

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

Hall, R

1980-06-13

28

Mapping oceanic ridge segments in Oman ophiolite  

Microsoft Academic Search

This paper presents the results of detailed mapping of high-temperature flow structures in the mantle and crust of two massifs of the Oman ophiolite. In these massifs, the dominant structures, including large-scale folds, shear zones, and fractures, were generated at elevated temperatures and are ascribed to the ridge or ridge environment activity; this means that the structural maps presented can

A. Nicolas; F. Boudier

1995-01-01

29

Numerical Models of Ophiolite Genesis and Obduction  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

30

Ophiolite Tectonics, Rock Magnetism and Palaeomagnetism, Cyprus  

NASA Astrophysics Data System (ADS)

Magnetic properties of minerals may be sensitive indicators of provenance. Remanence-bearing minerals (RBM) such as iron-titanium oxides, and matrix-forming minerals such as paramagnetic phyllosilicate or diamagnetic calcite yield different clues to provenance, strain history and tectonics, and are essential supplements for the full interpretation of palaeomagnetic data. Moreover, mineral magnetic properties provide magnetic-petrofabric indicators of tectonic strain, determine the suitability of sites for palaeomagnetism, and permit the restoration of palaeomagnetic vectors in some strained rocks. In the Cretaceous Troodos ophiolite (~88 Ma) magnetic properties are dictated by the relative importance of mafic silicates and largely primary, ophiolite-derived RBM. In its cover of deformed pelagic sedimentary rock, magnetic properties are dictated by the balance of clastic RBM versus matrix calcite and in some cases clay. The two larger Cretaceous ophiolite outcrops (Troodos & Akamas) share a common orientation of their plutonic flow fabrics, determined by magnetic methods. The dike complex shows fabrics indicating plume-like feeders spaced along and perpendicular to the spreading axis, with longevities >0.5 Ma. South of the ophiolite, its Cretaceous-Miocene limestone cover possesses ubiquitous tectonic petrofabrics inferred from anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent susceptibility (AARM). Its foliation and maximum extension dip and plunge gently northward, sub-parallel to a common but previously unreported North-dipping stylolitic cleavage. In well-known localized areas, there are S-vergent thrusts and overturned folds. The S-vergent deformation fabrics are due to Late Miocene (pre-Messinian ~8 Ma) deformation. The structures are geometrically consistent with overthrusting of the Cretaceous Troodos-Akamas ophiolite, and its sedimentary cover, onto the underlying Triassic Mamonia terrane. The northern limit of pre-Messinian tectonic fabrics, the Troodos-Mamonia terrane boundary and the Arakapas-Transform fault form an approximate E-W composite boundary that we term the Troodos Tectonic Front. Miocene deformation remagnetized the ophiolite and its sedimentary cover in many places and also affects the Mamonia terrane to the SW, with which the Troodos terrane docked in the late Cretaceous. Magnetic mineralogy, particularly of the RBM traces the progressive un-roofing of the ophiolite during the deposition of its sedimentary cover. During the submarine exposure and erosion of the ophiolite, the contribution of RBM clasts to the overlying sedimentary cover changed qualitatively and quantitatively. Thus, magnetic mineralogy of the sedimentary rock cover records the progressive denudation of the ophiolite from lavas, down through dikes, to gabbros and deeper mantle rocks. Palaeomagnetic studies previously revealed the anticlockwise rotation of the Troodos terrane and its northwards migration. Characteristic remanent magnetism (ChRM) is most reliable for lavas and dikes although it is usually carried by recrystallized RBM. These correspond to the age of greenschist facies ocean-floor metamorphism, perhaps 7-15 Ma after igneous crystallization with an extent and depth dependent on depth and degree of hydrothermal circulation. The gabbros and mantle rocks commonly bear young (<12 Ma) remanences probably acquired (or re-acquired) during uplift of the Troodos terrane. In the cover of pre-Messinian deformed limestone (>8 Ma), the remagnetizing effects of penetrative strain have been under-estimated. Where strain has occurred, un-tilting procedures produce erroneous restorations for the remanence vectors, and thus for the associated paleopoles. We find that de-straining of limestone sites most appropriately restores ChRM vectors to their original orientation. The best-determined and restored ChRMs define an apparent polar wander path (APWP). Since the APWP terminates at the present N-pole, we inverted it to determine the true plate-motion of the Troodos-terrane. Thus, in prese

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

2010-06-01

31

Detrital zircon geochronology overlying the Naga Hills ophiolite  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

32

A vestige of Earth's oldest ophiolite.  

PubMed

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

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

2007-03-23

33

Thematic mapper study of Alaskan ophiolites  

NASA Technical Reports Server (NTRS)

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

Bird, John M.

1987-01-01

34

Actualistic Ophiolite Provenance: The Cyprus Case.  

PubMed

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

Garzanti; Andò; Scutellà

2000-03-01

35

Magnetic anisotropy and fabric of some progressively deformed ophiolitic gabbros  

Microsoft Academic Search

The ophiolites, considered remnants of oceanic seafloor, a fruitful source of information on the physical and chemical behavior of the deeper uncored lithosphere. A study of the gabbros in a rather well-preserved ophiolite from the western Alps (Montgenèvre, France) shows strong evidence for intraoceanic plate deformation. To characterize the different steps of deformation observed in the outcrops, a magnetic fabric

J.-J. Wagner; I. G. Hedley; D. Steen; C. Tinkler; m. Vuagnat

1981-01-01

36

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

37

Cretaceous radiolarians from the Luk-Ulo Melange Complex in the Karangsambung area, central Java, Indonesia  

NASA Astrophysics Data System (ADS)

The Luk-Ulo Melange Complex is a chaotic mixture of various kinds of sedimentary, igneous and metamorphic rocks, and is unconformably overlain by the Eocene Karangsambung Formation. Cretaceous radiolarians were extracted from shale and chert which are main constituents of the complex in the Karangsambung area, central Java. They are grouped into five assemblages (I-V). From the radiolarian data, the siliceous and argillaceous rocks of the Luk-Ulo Melange Complex are considered to have been deposited in Early to Late Cretaceous time, and accreted at a subduction trench during middle to latest Cretaceous or earliest Paleocene. As the complex is unconformably overlain by the Eocene, the fragmentation and mixing of these rocks with schist and quartz porphyry must have occurred during Paleocene time.

Wakita, Koji; Munasri; Bambang, Widoyoko

38

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

NASA Astrophysics Data System (ADS)

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

Sachau, Till; Koehn, Daniel

2012-12-01

39

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

40

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

Microsoft Academic Search

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.

Hisham A. Gahlan; Shoji Arai

2009-01-01

41

Photang thrust sheet: an accretionary complex structurally below the Spontang ophiolite constraining timing and tectonic environment of ophiolite obduction, Ladakh Himalaya, NW India  

Microsoft Academic Search

The pre-collisional tectonic evolution of the north Indian continental margin is best recorded in the few ophiolite complexes preserved, the largest of which occurs in the Spontang area of the Himalayas. Structural, sedimentological, palaeontological and geochemical work on the ophiolite and associated allochthonous thrust sheets has been carried out to constrain the timing and tectonic environment of ophiolite obduction. A

RICHARD I. CORFIELD; MIKE P. SEARLE; OWEN R. GREEN

1999-01-01

42

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

43

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

SciTech Connect

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

Smewing, J.D. (University Innovation Centre, Swansea (England)); Abbotts, I.L. (Clyde Petroleum plc, Herefordshire (England)); Dunne, L.A. (Amoco Production Co., Houston , TX (USA)); Rex, D.C. (Univ. of Leeds, Yorkshire (England))

1991-05-01

44

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

PubMed

(40)Ar/(39)Ar dates of emplacement-related metamorphic rocks beneath the Samail ophiolite in Oman show that cooling to <525 degrees C occurred within approximately 1 million years of igneous crystallization of the ophiolite. This unexpectedly short time span and rapid cooling means that old, cold continental or oceanic lithosphere must have been adjacent to the ophiolite during spreading and then been thrust beneath the ophiolite almost immediately afterward. PMID:17801534

Hacker, B R

1994-09-01

45

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

46

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

47

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

48

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

49

Slow spreading accretion and mantle denudation in the Mirdita ophiolite (Albania)  

Microsoft Academic Search

The Mirdita ophiolite in Albania occupies a N-S corridor which escaped most Alpine and Cenozoic deformation, possibly due to a thick ophiolitic basement. The sheeted dike complex strikes NS and dips steeply, indicating that the ridge was oriented parallel to the NS corridor and that the ophiolite has not been significantly tilted, although differential motion between individual massifs cannot be

Adolphe Nicolas; Françoise Boudier; Avni Meshi

1999-01-01

50

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

E-print Network

Birth, death, and resurrection: The life cycle of suprasubduction zone ophiolites John W. Shervais such ophiolites. This sequence includes the following: (1) birth, which entails the formation of the ophiolite which is continued melting of refractory asthenosphere (depleted during birth) occurs in response

Shervais, John W.

51

Role of tectonic-sedimentary melange and Permian-Triassic cover units, central southern Turkey in Tethyan continental margin evolution  

NASA Astrophysics Data System (ADS)

Melanges play a key role in the interpretation of orogenic belts, including those that have experienced deformation and metamorphism during continental collision. This is exemplified by a Palaeozoic tectonic-sedimentary melange (part of the Konya complex) that is exposed beneath a regionally metamorphosed carbonate platform near the city of Konya in central Anatolia. The Konya complex as a whole comprises three units: a dismembered, latest Silurian-Early Carboniferous carbonate platform, a Carboniferous melange made up of sedimentary and igneous blocks in a sedimentary matrix (also known as the Hal?c? Group or S?zma Group), and an overlying Volcanic-sedimentary Unit (earliest Permian?) . The Palaeozoic carbonates accumulated on a subsiding carbonate platform that bordered the northern margin of Gondwana, perhaps as an off-margin unit. The matrix of the melange was mainly deposited as turbidites, debris flows and background terrigenous muds. Petrographic evidence shows that the clastic sediments were mostly derived from granitic and psammitic/pelitic metamorphic rocks, typical of upper continental crust. Both extension- and contraction-related origins of the melange can be considered. However, we interpret the melange as a Carboniferous subduction complex that formed along the northern margin of Gondwana, related to partial closure of Palaeotethys. Blocks and slices of Upper Palaeozoic radiolarian chert, basic igneous rocks and shallow-water carbonates were accreted and locally reworked by gravity processes. Large (up to km-sized) blocks and slices of shallow-water limestone were emplaced in response to collision of the Palaeozoic Carbonate Platform with the subduction zone. The overlying Volcanic-sedimentary Unit (earliest Permian?) comprises alkaline lava flows, interbedded with volcaniclastic debris flows and turbidites, volcanogenic shales and tuff. The complex as a whole is overlain by shallow-water, mixed carbonate-siliciclastic sediments of mainly Late Permian age that accumulated on a regional-scale shelf adjacent to Gondwana. Successions pass transitionally into Lower Triassic rift-related shallow-water carbonates and terrigenous sandstones in the southwest of the area. In contrast, Triassic sediments in the southeast overlie the melange unconformably and pass upwards from non-marine clastic sediments into shallow-marine calcareous sediments of Mid-Triassic age, marking the base of a regional Mesozoic carbonate platform. During the latest Cretaceous-Early Cenozoic the entire assemblage subducted northwards and underwent high pressure/low temperature metamorphism and polyphase folding as a part of the regional Anatolide unit.

Robertson, Alastair H. F.; Ustaömer, Timur

2011-01-01

52

A mid-Archaean ophiolite complex, Barberton Mountain land  

NASA Astrophysics Data System (ADS)

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

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

53

Spectral mapping of Alaskan ophiolites using Landsat Thematic Mapper data  

NASA Technical Reports Server (NTRS)

Ophiolites are sections of oceanic lithosphere that have been exposed by thrusting in mountain belts. Thematic Mapper three-band color composites have been used to differentiate the main rock types of the ophiolites of the western Brooks Range, Alaska, and to map mineralogic variations produced by processes that affect oceanic lithosphere, such as mantle partial melting, hydration of ultramafic rock, formation of cumulate layers, and hydrothermal alteration and metamorphism of basalt. Scaled composites of TM Bands 7, 3, 1 and 5, 4, 2, and 3, 2, 1, displayed in red, green, and blue are most useful for visually differentiating rocks of the ophiolites. Ternary diagrams of digital number values are used to evaluate color variations observed in the three-band composites. Geologic maps, interpreted from TM images and reconnaissance field studies, are presented for the Asik Mountain, Maiyumerak Mountains, Avan Hills, and Misheguk Mountain ophiolites. The mapping results are being used to guide additional studies of the tectonics of the Brooks Range ophiolites.

Harding, David J.; Wirth, Karl R.; Bird, John M.

1989-01-01

54

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

E-print Network

chromitites both in the Proterozoic ophiolite, Eastern Desert, Egypt, and in the Phanerozoic Oman ophiolite contamination. The three chromitite types in the northern part of the Oman ophiolite are almost.1192 to 0.1459, in platinum-group minerals in Egyptian and Oman ophiolites can be attributed

55

Magnetism of the oceanic crust: Evidence from ophiolite complexes  

SciTech Connect

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

Banerjee, S.K.

1980-07-10

56

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

SciTech Connect

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

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

1987-05-01

57

The volcanic stratigraphy and petrogenesis of the Oman ophiolite complex  

Microsoft Academic Search

The volcanic stratigraphy and trace element geochemistry of the Oman ophiolite complex indicate a multistage magmatic origin comprising: (1) magmatism due to sea-floor spreading in a marginal basin; (2) magmatism associated with discrete submarine volcanic centres or seamounts; (3) magmatism associated with crustal uplift and rifting; and (4) magmatism associated with continent-arc collision.

T. Alabaster; J. A. Pearce; J. Malpas

1982-01-01

58

Mantle petrology and mineralogy of the Thetford Mines Ophiolite Complex  

E-print Network

ophiolite complex (TMOC) formed by boninite-fed seafloor-spreading, probably in a fore-arc environment­S, parallel to the orientation of seafloor-spreading related paleo-normal faults in the crust. These fabrics, and is interpreted to have resulted from upflow of the asthenosphere beneath the spreading ridge. This fabric

Long, Bernard

59

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

NASA Astrophysics Data System (ADS)

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

Kapsiotis, Argyrios N.

2014-12-01

60

Tectonic setting of the 1. 73 Ga Payson ophiolite  

SciTech Connect

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

Dann, J.C.

1993-04-01

61

Structural problems of the Brooks Range ophiolite, Alaska  

SciTech Connect

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

Harris, R.A.; Bickerstaff, D. (West Virginia Univ., Morgantown, WV (United States). Dept. of Geology); Stone, D.B. (Univ. of Alaska, Fairbanks, AK (United States). Geophysical Inst.)

1993-04-01

62

First palaeomagnetic results from the Mersin ophiolite (Turkey)  

NASA Astrophysics Data System (ADS)

The Mersin ophiolite of the central Taurides of Turkey formed by supra-subduction zone spreading within the northern Neotethyan ocean basin during the Late Cretaceous. No palaeomagnetic data currently exist for this ophiolite, and hence its relationship to other Neotethyan ophiolites that are known to have experienced tectonic rotation is currently unknown. Sampling of ultramafic and gabbroic cumulates took place along the Sorgun valley in the Mersin area. In addition, core samples were taken from the overlying Miocene carbonate sediments at a location to the south of the Sorgun valley. Palaeomagnetic and anisotropy of magnetic susceptibility (AMS) results from 19 sites in Late Cretaceous lower crustal ultramafic and gabbroic cumulates and Miocene carbonate sediments are presented. Most of the samples from the extensive cumulate section showed clustering of Kmax axes parallel to the strike of layering, suggesting that the magnetic fabric has been affected by tectonic deformation. The gabbros showed a mixture of oblate and prolate magnetic fabric shapes. In general ultramafic cumulates showed more scattering in AMS, probably as a result of variable serpentinization of these rocks. Stepwise thermal and alternating field demagnetization of cumulate samples showed high coercivity/high unblocking temperature components with a mean in situ direction of Dec = 053°, Inc = -25° and a tilt-corrected direction of Dec = 039°, Inc = 13°. In contrast, the samples from sub-horizontal Miocene carbonates showed low to intermediate coercivity/unblocking temperature components with a mean in situ direction of Dec = 341°, Inc = 58°. Together, these data suggest that the ophiolite was rotated clockwise by c. 60° prior to being covered by Miocene carbonate and then subsequently experienced c. 20° of anticlockwise rotation during regional neotectonic rotation of the Anatolian plate. The earlier clockwise rotation is potentially of composite origin (including components of intra-oceanic and emplacement-related rotations), and has a sense of rotation opposite to that seen in coeval ophiolites (Troodos, Hatay and Baer-Bassit) that formed in the southern Neotethyan basin.

Omer, Ahmed F.; Morris, Antony; Anderson, Mark W.

2013-04-01

63

Lead isotopic studies of the Samail ophiolite, Oman  

SciTech Connect

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

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

1981-04-10

64

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

65

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

E-print Network

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

Kidd, William S. F.

66

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

67

Albanian ophiolites as probes of a mantle heterogeneity study  

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

68

Mirdita Zone ophiolites and associated sediments in Albania reveal Neotethys Ocean origin  

Microsoft Academic Search

The Mirdita Ophiolite Zone in Albania is associated with widespread mélanges containing components of up to nappe-size. We\\u000a dated matrix and components of the mélange by radiolarians, conodonts, and other taxa. The components consist of radiolarites,\\u000a pelagic limestones and shallow-water limestones, all of Triassic age, as well as ophiolites. Triassic radiolarite as a primary\\u000a cover of ophiolite material proves Middle

Hans-Jürgen Gawlick; Wolfgang Frisch; Lirim Hoxha; Paulian Dumitrica; Leopold Krystyn; Richard Lein; Sigrid Missoni; Felix Schlagintweit

2008-01-01

69

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

NASA Astrophysics Data System (ADS)

Sedimentology can shed light on the emplacement of oceanic lithosphere (i.e. ophiolites) onto continental crust and post-emplacement settings. An example chosen here is the well-exposed Jurassic Mirdita ophiolite in southern Albania. Successions studied in five different ophiolitic massifs (Voskopoja, Luniku, Shpati, Rehove and Morava) document variable depositional processes and palaeoenvironments in the light of evidence from comparable settings elsewhere (e.g. N Albania; N Greece). Ophiolitic extrusive rocks (pillow basalts and lava breccias) locally retain an intact cover of oceanic radiolarian chert (in the Shpati massif). Elsewhere, ophiolite-derived clastics typically overlie basaltic extrusives or ultramafic rocks directly. The oldest dated sediments are calpionellid- and ammonite-bearing pelagic carbonates of latest (?) Jurassic-Berrasian age. Similar calpionellid limestones elsewhere (N Albania; N Greece) post-date the regional ophiolite emplacement. At one locality in S Albania (Voskopoja), calpionellid limestones are gradationally underlain by thick ophiolite-derived breccias (containing both ultramafic and mafic clasts) that were derived by mass wasting of subaqueous fault scarps during or soon after the latest stages of ophiolite emplacement. An intercalation of serpentinite-rich debris flows at this locality is indicative of mobilisation of hydrated oceanic ultramafic rocks. Some of the ophiolite-derived conglomerates (e.g. Shpati massif) include well-rounded serpentinite and basalt clasts suggestive of a high-energy, shallow-water origin. The Berriasian pelagic limestones (at Voskopoja) experienced reworking and slumping probably related to shallowing and a switch to neritic deposition. Mixed ophiolite-derived clastic and neritic carbonate sediments accumulated later, during the Early Cretaceous (mainly Barremian-Aptian) in variable deltaic, lagoonal and shallow-marine settings. These sediments were influenced by local tectonics or eustatic sea-level change. Terrigenous sediment gradually encroached from neighbouring landmasses as the ophiolite was faulted or eroded. An Aptian transgression was followed by regression, creating a local unconformity (e.g. at Boboshtica). A Turonian marine transgression initiated widespread Upper Cretaceous shelf carbonate deposition. In the regional context, the southern Albania ophiolites appear to have been rapidly emplaced onto a continental margin in a subaqueous setting during the Late Jurassic (Late Oxfordian-Late Tithonian). This was followed by gradual emergence, probably in response to thinning of the ophiolite by erosion and/or exhumation. The sedimentary cover of the south Albanian ophiolites is consistent with rapid, relatively short-distance emplacement of a regional-scale ophiolite over a local Pelagonian-Korabi microcontinent.

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

2012-09-01

70

Age, geochemistry and tectonic setting of Buqingshan ophiolites, North Qinghai-Tibet Plateau, China  

NASA Astrophysics Data System (ADS)

The Buqingshan ophiolite complex is a sector of the A'nyemaqen ophiolite belt in the East Kunlun southern marginal suture zone in the northern part of the Qinghai-Tibet Plateau. The WNW-trending ophiolite complex consists of metaperidotite, gabbro, diabase, pillow basalt, massive basalt and pelagic sedimentary rocks including radiolarian chert. We have determined that Early Paleozoic and Early Carboniferous-Early Permian ophiolites are present in this ophiolite complex that were previously thought of Permian-Middle Triassic age. A zircon U-Pb age of 467.2±0.9 Ma for gabbro, and a Rb-Sr isochron age of 481±130 Ma for diabase and gabbro have been obtained from the Early Paleozoic ophiolite slice, and Middle-Late Ordovician acritarchs have been found in the mélange matrix that envelopes the sliver. We interpret the first two ages to represent the formation age of the ophiolite. Granodiorite-tonalite plutons with a zircon U-Pb age of 402±24 Ma intrude the slice. Early Carboniferous-Early Permian radiolarians have been discovered in cherts and argillaceous cherts in the second ophiolite slice. Geochemistry indicates that metaperidotites in the first slice represent a depleted ocean lithosphere mantle. Most mafic rocks in the Early Paleozoic ophiolite are of N-MORB type with small amount of T-MORB, and they resulted from the magma from the depleted oceanic lithosphere mantle with little crustal contamination. The basalts in the Early Carboniferous-Early Permian ophiolite have similar characteristics to the mafic rocks and also show DUPAL anomaly and a mixed source of DMM and EM II. The original formation environments of the two-stage ophiolites are both mid-ocean ridges. These lines of evidence suggest that a mature Early Paleozoic Kunlun-Qilian-Qinling ocean basin and a mature Paleotethyan ocean basin once existed in the study area.

Bian, Qian-Tao; Li, Di-Hui; Pospelov, I.; Yin, Lei-Ming; Li, Hong-Sheng; Zhao, Da-Sheng; Chang, Cheng-Fa; Luo, Xiao-Quan; Gao, Shan-Lin; Astrakhantsev, O.; Chamov, N.

2004-08-01

71

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

USGS Publications Warehouse

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

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

2004-01-01

72

Geochemistry of the Jurassic Mirdita Ophiolite (Albania) and the MORB to SSZ evolution of a marginal basin oceanic crust  

Microsoft Academic Search

The Middle Jurassic Mirdita Ophiolite in northern Albania is part of an ophiolite belt occurring between the Apulian and Pelagonian subcontinents in the Balkan Peninsula. The upper mantle and crustal units of the Mirdita Ophiolite show major changes in thickness, rock types, and chemical compositions from west to east as a result of its complex evolution in a suprasubduction zone

Yildirim Dilek; Harald Furnes; Minella Shallo

2008-01-01

73

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

Microsoft Academic Search

We report new geochemical data from the Neoproterozoic ophiolite in the Wadi Ghadir area, Eastern Desert, Egypt. The Wadi Ghadir ophiolite (WGO) is composed of layered and isotropic gabbros and amygdaloidal to porphyritic pillow lavas. Both the gabbroic rocks and the pillow lavas are intruded by dike swarms with different chemical affinities and spatial orientations. The WGO occurs in an

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

2009-01-01

74

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.

75

Rare-Earth Element Geochemistry of the Samail Ophiolite near Ibra, Oman  

Microsoft Academic Search

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

John S. Pallister; Roy J. Knight

1981-01-01

76

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

77

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

78

Accretion of Oman and United Arab Emirates ophiolite – Discussion of a new structural map  

Microsoft Academic Search

This study is altogether an extended legend for the folded maps incorporated in this volume, a review of the current knowledge on the Oman-United Arab Emirates ophiolite belt, and a new synthesis at the scale of the entire belt. Following a brief description of the petrological and structural units composing the ophiolite, the content of the three structural maps (planar

A. Nicolas; F. Boudier; B. Ildefonse; E. Ball

2000-01-01

79

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

80

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

SciTech Connect

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

Wirth, K.R.; Bird, J.M. (Cornell Univ., Ithaca, NY (United States))

1992-01-01

81

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

Microsoft Academic Search

The origin of gabbro layering in ophiolites is widely debated because it is linked to the processes of melt circulation beneath spreading ridges. The Moho Transition Zone (MTZ) of the Oman ophiolite contains layered gabbro lenses that are tens of meters wide. At meter scale, these rocks are not distinguishable from crustal layered gabbros. We describe the first known occurrence

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

2010-01-01

82

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

83

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

84

Supersilicic clinopyroxene from Nidar Ophiolite ultramafics, Ladakh, India  

NASA Astrophysics Data System (ADS)

We discovered silica exsolution lamellae in clinopyroxenes from pyroxenites of Nidar ophiolite ultramafics, Indus Suture Zone in Eastern Ladakh, India. These signify former existence of supersilicic clinopyroxene, an indicator of deep mantle condition. The ophiolite ultramafic suit consists of varieties of peridotites (dunite, chromite, harzburgite etc.) and intrusive pyroxenites. The pyroxenites are heterogeneously altered and dominated in clinopyroxene (>70%) with minor carbonates. Recently numerous silica inclusions and lamellas are found in clinopyroxenes of pyroxenites. The lamellas have been classified on the basis of their various modes of occurrences, size and shape from different clinopyroxenes - (1) rectilinear, strongly oriented rods (2) straight needles of greater size and weaker orientation with respect to type 1 (3) curved lamellas with wider middle part and tapering ends restricted in outer margin of clinopyroxene. All the lamellas display diffused grain boundaries with the host clinopyroxenes. On the basis of higher silica content of the host clinopyroxene compared to the silica free matrix it is interpreted that silica lamellas are exsolution products from break down of primary supersilicic Ca - Eskola end member. The exsolved silica bearing clinopyroxenes are showing increase in concentration of Si along with K which is potential function of pressure in clinopyroxene (Shimizu 1971). Besides these, a quartz inclusion in clinopyroxene show palisade texture with radiating fractures in rim and a polycrystalline core, which may indicate phase transition of higher silica polymorphs during decompression. With the given line of evidences, the Himalayan Ophiolite may become a potential window for deep mantle study and could be equivocal for ultra-high pressure origin. Reference: Shimizu N, EPSL, 11, 374-380; 1971.

Das, S.; Mukherjee, B. K.

2011-12-01

85

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

NASA Astrophysics Data System (ADS)

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

García-Casco, A.

2012-04-01

86

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

87

Cretaceous radiolarians from Baliojong ophiolite sequence, Sabah, Malaysia  

NASA Astrophysics Data System (ADS)

The Baliojong ophiolite sequence exposed along the Baliojong River in Northern Sabah consists of volcanic rocks, mostly basalts, overlain by sedimentary layers consisting of well-bedded cherts, mudstones and sandstones. The ophiolite sequence occurs as steeply-dipping overturned thrust slices oriented approximately north-south. A total of 42 chert samples were collected from the sedimentary layers. However, most of the samples contain poorly preserved radiolarians. Only nine samples yielded moderately well-preserved radiolarians from three selected thrust slices. A total of 32 taxa were identified. Based on the stratigraphic distribution of selected taxa, the radiolarians can be divided into two assemblage zones. The first assemblage zone is Dictyomitra communis Zone characterized by the occurrence of Dictyomitra communis, Archaeodictyomitra (?) lacrimula, Sethocapsa (?) orca, Dictyomitra pseudoscalaris, and Pantanellium squinaboli. The assemblage indicates Barremian to Aptian in age. The second assemblage zone Pseudodictyomitra pseudomacrocephala Zone contains Pseudodictyomitra pseudomacrocephala, Dictyomitra gracilis, Dictyomitra montesserei, Xitus mclaughlini, and Dictyomitra obesa. This assemblage indicates an age of Albian and the presence of Pseudodictyomitra tiara suggests the age may extend up to Cenomanian. Each thrust slice yielded more or less similar radiolarian assemblages indicating that they all came from the same sedimentary layers.

Jasin, Basir; Tongkul, Felix

2013-10-01

88

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

NASA Astrophysics Data System (ADS)

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

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

2006-05-01

89

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

NASA Astrophysics Data System (ADS)

Satellite imagery of the February 2008 Wilkins Ice-Shelf Collapse event reveals that a large percentage of the involved ice shelf was converted to capsized icebergs and broken fragments of icebergs over a relatively short period of time, possibly less than 24 hours. The extreme violence and short time scale of the event, and the considerable reduction of gravitational potential energy between upright and capsized icebergs, suggests that iceberg capsize might be an important driving mechanism controlling both the rate and spatial extent of ice shelf collapse. To investigate this suggestion, we have constructed an idealized, 2-dimensional model of a disintegrating ice shelf composed of a large number (N~100 to >1000) of initially well-packed icebergs of rectangular cross section. The model geometry consists of a longitudinal cross section of the idealized ice shelf from grounding line (or the upstream extent of ice-shelf fragmentation) to seaward ice front, and includes the region beyond the initial ice front to cover the open, ice-free water into which the collapsing ice shelf expands. The seawater in which the icebergs float is treated as a hydrostatic fluid in the computation of iceberg orientation (e.g., the evaluation of buoyancy forces and torques), thereby eliminating the complexities of free-surface waves, but net horizontal drift of the icebergs is resisted by a linear drag law designed to energy dissipation by viscous forces and surface-gravity-wave radiation. Icebergs interact via both elastic and inelastic contacts (typically a corner of one iceberg will scrape along the face of its neighbor). Ice-shelf collapse in the model is embodied by the mass capsize of a large proportion of the initially packed icebergs and the consequent advancement of the ice front (leading edge). Model simulations are conducted to examine (a) the threshold of stability (e.g., what density of initially capsizable icebergs is needed to allow a small perturbation to the system evolve into full-blown collapse of the ice shelf? What proportion of uncapsizable icebergs prevent a collapse?), (b) the rates of mobilization and their dependence on iceberg geometry (e.g., what determines the speed at which the expanding ice melange moves into the open, ice-free water?), and (c) the factors that promote the arrest of the system (e.g., are there circumstances where only partial collapses can occur?). Results of simulations are compared with observational parameters derived from satellite imagery, seismic analysis and laboratory experiment to determine what aspects of the numerical model's physical formulation may have most relevance to the disappearance of ice shelves.

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

2010-12-01

90

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 crust­mantle transition zone of the Oman in the Oman ophiolite. This zone represents an `end member' case for analyzing feedbacks between deformation

Tommasi, Andrea

91

Ophiolites of the deep-sea trenches of the western Pacific  

SciTech Connect

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

Chudaev, O.

1990-06-01

92

TOWARDS A DEPOSIT MODEL FOR OPHIOLITE RIELATEC MESOTHERMAL GOLD IN BRITISH COLUMJ3IA  

E-print Network

The Listwanite Prqject was initiated in 1989 to develop a regional metallogenic deposit model for mesothermal gold veins associated wit11 ophiolitic ultramafic rocks in oceanic terranes throughout British Columbia. To date, investigations

C. H. Ash; R. W. J. Macdonald; R. L. Arksey

93

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

NASA Astrophysics Data System (ADS)

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

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

2009-08-01

94

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

95

Rift-Drift, Seafloor Spreading, and Subduction Tectonics of Albanian Ophiolites  

Microsoft Academic Search

Albanian ophiolites in the Dinaride-Albanide-Hellenide mountain system in the Balkan Peninsula represent oceanic remnants of the Mesozoic Pindos-Mirdita basin and structurally overlie the peripheral tectonic units, composed mainly of volcanosedimentary rift assemblages and conjugate passive margin sequences. These tectonic units and the ophiolites collectively record the rift-drift, seafloor spreading, and subduction evolution of the Pindos-Mirdita basin, which developed as a

Yildirim Dilek; Minella Shallo; Harald Furnes

2005-01-01

96

Geometry of accretion and oceanic thrusting of the Spongtang Ophiolite, Ladakh-Himalaya  

Microsoft Academic Search

Ductile mylonitic shear zones, which in ophiolitic peridotites over-print high-temperature ductile flow, have been commonly associated with intra-oceanic thrusting, if situated at the base of the peridotites1,2. In fewer cases, such shear zones have been related to the vicinity of transform faults cutting the spreading ridge3-9. In the Spongtang nappe, one of the few well preserved Himalayan ophiolite slabs, obducted

Ingrid Reuber

1986-01-01

97

Submarine hydrothermal metamorphism of the Del Puerto ophiolite, California.  

USGS Publications Warehouse

Metamorphic zonation overprinted on the volcanic member and overlying volcanogenic sediments of the ophiolite complex increases downward in grade and is characterized by the sequential appearance with depth of zeolites, ferric pumpellyite and pistacitic epidote. Metamorphic assemblages of the plutonic member of the complex are characterized by the presence of calcic amphibole. The overprinting represents the effects of hydrothermal metamorphism resulting from the massive interaction between hot igneous rocks and convecting sea-water in a submarine environment. A thermal gradient of 100oC/km is postulated to account for the zonal recrystallization effects in the volcanic member. The diversity and sporadic distribution of mineral assemblages in the amphibole zone are considered due to the limited availability of H2O in the deeper part of the complex. Details of the zonation and representative microprobe analyses are tabulated.-M.S.

Evarts, R.C.; Schiffman, P.

1983-01-01

98

Classification and origin of the Neoproterozoic ophiolitic mélanges in the Central Eastern Desert of Egypt  

NASA Astrophysics Data System (ADS)

The Eastern Desert of Egypt is part of the Neoproterozoic Arabian-Nubian Shield and displays different occurrences of Neoproterozoic ophiolitic mélanges. The mélanges contain exotic and native blocks and fragments of variable sizes and types set in a sheared and schistose volcaniclastic matrix. The main exotic blocks are ophiolitic and include metamorphosed ultramafic rocks, metagabbros, massive and pillowed metabasalts and pelagic sedimentary rocks. Based on the mode of occurrences of the ophiolitic components and the processes of mélange formation, the ophiolitic mélanges of the Central Eastern Desert are classified and mapped into tectonic mélange, olistostrome and olistostromal mélange. The whole rock associations of these mélanges were subjected to two different events of deformation (D1 and D2) and contemporaneous metamorphism (M1 and M2). The metagreywackes of the mélange matrix are similar to oceanic island arc sandstone and appear to have been deposited in a back-arc or inter-arc basin. The blocks of ophiolitic metabasalts within the mélange show tholeiitic affinity and have back-arc basin (BAB) tectonic origins. Metamorphosed ultramafic blocks in the mélange display both mid-ocean ridge (MOR) and suprasubduction zone (SSZ) affinities. The ophiolitic components represent fragments of oceanic lithosphere that formed in a back-arc/arc environment, and were incorporated into the mélange through tectonic and/or sedimentary processes. Both tectonic and sedimentary processes played a major role during mélange formation in a back-arc or inter-arc setting.

El Bahariya, Gaafar A.

2012-09-01

99

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

USGS Publications Warehouse

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

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

1988-01-01

100

Selenium content of sulfide ores related to ophiolites of Greece.  

PubMed

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

Economou-Eliopoulos, M; Eliopoulos, D G

1998-01-01

101

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

102

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

103

The South Ladakh ophiolites (NW Himalaya, India): an intra-oceanic tholeiitic arc origin with implication for the closure of the Neo-Tethys  

Microsoft Academic Search

In the Ladakh–Zanskar area, two groups of ophiolites document closure of the Neo-Tethys Ocean during India–Asia convergence. They are localized north of the Indus Suture Zone and in South Ladakh, respectively. The South Ladakh group includes the Spontang ophiolite, with both crustal and mantle units, the Nidar ophiolite, with mainly crustal units, and the small Karzog ophiolite, with abundant chromitites

Gweltaz Mahéo; Hervé Bertrand; Stéphane Guillot; Igor M. Villa; Francine Keller; Paul Capiez

2004-01-01

104

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

105

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

NASA Astrophysics Data System (ADS)

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

Anma, Ryo

2013-04-01

106

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

107

Neoproterozoic ophiolitic peridotites along the Allaqi-Heiani suture, South Eastern Desert, Egypt  

NASA Astrophysics Data System (ADS)

The Wadi Allaqi ophiolite along the Egyptian-Sudanese border defines the southernmost ophiolitic assemblage and suture zone in the Eastern Desert. Ophiolite assemblages comprise nappes composed mainly of mafic and ultramafic rocks that were tectonically emplaced and replaced by serpentine and carbonates along shear zones probably due to CO2-metasomatism. Serpentinites, altered slices of the upper mantle, represent a distinctive lithology of dismembered ophiolites of the western YOSHGAH suture. Microscopically, they are composed of more than 90 % serpentine minerals with minor opaque minerals, carbonate, brucite and talc. The mineral chemistry and whole-rock chemical data reported here indicate that the serpentinized peridotites formed as highly-depleted mantle residues. They show compositions consistent with formation in a suprasubduction zone environment. They are depleted in Al2O3 and CaO similar to those in fore-arc peridotites. Also, high Cr# (Cr/ (Cr+Al)) in the relict chrome spinels (average ~0.72) indicates that these are residual after extensive partial melting, similar to spinels in modern fore-arc peridotites. Therefore, the studied serpentinites represent fragments of an oceanic lithosphere that formed in a fore-arc environment, which belongs to an ophiolitic mantle sequence formed in a suprasubduction zone.

Azer, M. K.; Samuel, M. D.; Ali, K. A.; Gahlan, H. A.; Stern, R. J.; Ren, M.; Moussa, H. E.

2013-10-01

108

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

SciTech Connect

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

Moore, T.E. (Geological Survey, Menlo Park, CA (United States)); Aleinikoff, J.N.; Walter, M. (Geological Survey, Denver, CO (United States))

1993-04-01

109

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

SciTech Connect

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

Lanphere, M.A.

1981-04-10

110

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

USGS Publications Warehouse

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

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

1985-01-01

111

Diversity of platinum-group minerals in podiform chromitites of the late Proterozoic ophiolite, Eastern Desert, Egypt: Genetic implications  

Microsoft Academic Search

Podiform chromitites are frequently distributed as lensoidal pods in the central and southern parts of the Eastern Desert, Egypt. They are, in most cases, hosted by fully serpentinized peridotite which is a part of dismembered ophiolite complexes of the Pan–African belt of Late Precambrian age. Serpentinites are the predominant components in the ophiolitic mélange, either as matrix or as variably

Ahmed Hassan Ahmed

2007-01-01

112

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

113

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

114

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

115

Triassic and Jurassic radiolarians from sedimentary blocks of ophiolite mélange in the Avala Gora area (Belgrade surroundings, Serbia)  

NASA Astrophysics Data System (ADS)

Blocks of cherty rocks and Aptychus Limestone embedded into ophiolite mélange south of Avala Gora (Serbia) contain radiolarians of different ages. We distinguished here Late Jurassic (middle Oxfordianearly Tithonian), Middle-Late Jurassic (Bathonian-early Tithonian), and Middle Triassic (early Ladinian) radiolarian assemblages. The respective stratigraphic data suggest that the ophiolite mélange was formed after the early Tithonian.

Bragin, N. Yu.; Bragina, L. G.; Djeri?, N.; Tolji?, M.

2011-12-01

116

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

Microsoft Academic Search

The Leka Ophiolite Complex (LOC) is located on the island of Leka, Norway, and belongs to the Uppermost Allochthon of the Scandinavian Caledonides. The rocks of the adjacent mainland and most of the surrounding islands are basement gneisses and supracrustal rocks not related to the ophiolite complex. Paleostress analysis, gravity inversion, and regional geology support a fault-bounded rhombochasm geometry for

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

2002-01-01

117

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

NASA Astrophysics Data System (ADS)

The reduced status of mantle rocks is a possible controller and indicator of deep life habitat, due to interactions between water and ultramafic (Fe, Mg-rich) minerals, which, under reducing conditions, can yield copious free hydrogen, which is an energy source for rock-hosted chemosynthetic life. In this work, Mössbauer spectroscopy was used to parameterize the redox status of Fe in altering peridotites of the Coast Range Ophiolite (CRO) in California, USA and Zambales Ophiolite (ZO) in the Philippines. Fe-bearing minerals were identified and data were collected for the percentages of Fe(III)and Fe(II)and bulk Fe concentration. Thin section analysis shows that relict primary olivines and spinels generally constitute a small percentage of the ZO and CRO rock, and given satisfactory estimates of the volume of the ultramafic units of the ZO and CRO, a stoichiometric H2 production can be estimated. In addition, ZO serpentinites are ~63,000 ppm Fe in bulk samples; they contain ~41-58% Fe(III)and ~23-34% Fe(II) in serpentine and relict minerals along with ~8-30% of the total Fe as magnetite. CRO serpentinites are ~42,000 ppm Fe in bulk samples; they contain ~15-50% Fe(III), ~22-88% Fe(II) in serpentine and relict minerals, and ~0-52% of total Fe is in magnetite (Fe(II)Fe(III)2O4). Assuming stoichiometric production of H2, and given the following representation of serpentinization 2(FeO)rock + H2O ? (Fe2O3)rock +H2, we calculated the maximum quantity of hydrogen released and yet to be released through the oxidation of Fe(II). Given that relatively high Fe(III)/Fetotal values can imply higher water:rock ratios during rock alteration (Andreani et al., 2013), we can deduce that ZO ultramafics in this study have experienced a net higher water:rock ratio than CRO ultramafics. We compare possible H2 yields and contrast the tectonic and alteration histories of the selected ultramafic units. (M. Andreani, M. Muñoz, C. Marcaillou, A. Delacour, 2013, ?XANES study of iron redox state in serpentine during oceanic serpentinization, Lithos, Available online 20 April 2013)

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

2013-12-01

118

Sr isotopic tracer study of the Samail ophiolite, Oman.  

USGS Publications Warehouse

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

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

1981-01-01

119

Sr isotopic tracer study of the Samail ophiolite, Oman  

SciTech Connect

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

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

1981-04-10

120

Rooted Brooks Range ophiolite: Implications for Cordilleran terranes  

USGS Publications Warehouse

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

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

2001-01-01

121

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

122

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

NASA Astrophysics Data System (ADS)

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

Schiffman, Peter; Williams, Alan E.; Evarts, Russell C.

1984-10-01

123

Palaeogeographical evolution of the Arabian platform during the obduction of the Samail ophiolite  

NASA Astrophysics Data System (ADS)

Obduction of a dense oceanic lithosphere on top of a light continental lithosphere remains one of the oddest phenomenons in plate tectonics. In northern Oman, the emplacement of the Samail (or Oman) ophiolite took place during Upper Cretaceous. The Samail ophiolite is probably the best studied ophiolite in the world because of its large size and because it has been relatively well preserved from deformation after its emplacement. If many studies have been carried out on the ophiolite itself in order to better understand the structure of an oceanic crust, little is known on the processes leading to its emplacement on the top of the Arabian platform. The sedimentary series deposited on the Arabian platform during the emplacement of the ophiolite could help us to understand the processes involved during the obduction by providing tectono-stratigraphic constraints on the evolution of the platform affected by the obduction. Moreover the reconstitution of the palaeoenvironments and the identification of source areas would allow discussing the evolution of reliefs formed in such a unique context. However, these series, being located just below the allochthonous units, have been affected by deformation. They are grouped in one formation, the Muti Formation, consisting of various lithologies of which the age is uncertain. In consequence, sedimentological analyses require careful mapping of the sedimentary series and biostratigraphical determinations remain difficult. The first results of our sedimentological and stratigraphical investigations show (i) a diachronism of the onset of syn-tectonic sedimentation (chemostratigraphic and biostratigraphic data) and (ii) a high variation of the source areas within the basin. These data are used in order to present a new model of evolution of the basin formed during the emplacement of the Samail ophiolite on top of the Arabian Platform. Implications for the processes involved during the obduction are also discussed.

Ducassou, Céline; Robin, Cécile; Wohlwend, Stephan; Piuz, André; Gorican, Spela

2014-05-01

124

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

125

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

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

126

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

NASA Astrophysics Data System (ADS)

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

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

2015-01-01

127

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

NASA Astrophysics Data System (ADS)

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

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

2003-04-01

128

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

NASA Astrophysics Data System (ADS)

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

Stern, R.

2003-04-01

129

Tectonic evolution of the Brooks Range ophiolite, Alaska  

SciTech Connect

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

Harris, R.A. (West Virginia Univ., Morgantown, WV (United States). Dept. of Geology)

1993-04-01

130

Bimodal stable isotope signatures of Zildat Ophiolitic Mélange, Indus Suture Zone, Himalaya: implications for emplacement of an ophiolitic mélange in a convergent setup  

NASA Astrophysics Data System (ADS)

Zildat Ophiolitic Mélange (ZOM) of the Indus Suture Zone, Himalaya, represents tectonic blocks of the fragmented oceanic metasediments and ophiolite remnants. The ZOM is sandwiched between the Zildat fault adjacent to a gneissic dome known as Tso Morari Crystalline (TMC) and thin sliver of an ophiolite called as the Nidar Ophiolitic Complex. The ZOM contain chaotic low-density lithologies of metamorphosed oceanic sediments and hydrated mantle rocks, in which carbonates are present as mega-clasts ranging from 100 meters to few centimeters in size. In this work, calcite microstructures, fluid inclusion petrography and stable isotope analyses of carbonates were carried out to envisage the emplacement history of the ZOM. Calcite microstructure varies with decreasing temperature and increasing intensity of deformation. Intense shearing is seen at the marginal part of the mélange near Zildat fault. These observations are consistent with the mélange as a tectonically dismembered block, formed at a plate boundary in convergent setup. The ?18O and ?13C isotope values of carbonates show bimodal nature from deeper (interior) to the shallower (marginal, near the Zildat fault) part of the mélange. Carbonate blocks from deeper part of the mélange reflect marine isotopic signature with limited fluid-rock interaction, which later on provide a mixing zone of oceanic metasediments and/or hydrated ultramafic rocks. Carbonates at shallower depths of the mélange show dominance of syn-deformation hydrous fluids, and this has later been modified by metamorphism of the adjacent TMC gneisses. Above observations reveal that the mélange was emplaced over the subducting Indian plate and later on synchronously deformed with the TMC gneissic dome.

Sen, Koushick; Das, Souvik; Mukherjee, Barun K.; Sen, Koushik

2013-06-01

131

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

132

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

E-print Network

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

Granot, Roi

133

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.

2015-02-01

134

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

135

Melt segregation and flow in mantle diapirs below spreading centers: evidence from the oman ophiolite  

Microsoft Academic Search

A mantle diapir of 8-km radius has been recognized by systematic structural mapping of the Oman ophiolite in the Maqsad district. This diapir chilled while still active under the ridge crest. Streamlines rotated in the diverging part of the diapir a few hundred meters under the Moho. This implies a decrease by several orders of magnitude in the effective viscosity

M. Rabinowicz; G. Ceuleneer; A. Nicolas

1987-01-01

136

Geology and seismic structure of the northern section of the Oman ophiolite  

Microsoft Academic Search

In the north Oman mountains, a continuous ophiolite succession is exposed, from tectonized harzuburgities and dunities at the base, through layered gabbros and peridotities, high-level gabbros and plagiogranite, to a dike swarm and pillowed volcanic overlain by pelagic shales. The upper part of this sequence possesses a static metamorphic overprint, which passes downward from greenschist facies in the lowermost volcanics

Nikolas I. Christensen; John D. Smewing

1981-01-01

137

Structure of the Sheeted Dike Complex of the Samail Ophiolite Near Ibra, Oman  

Microsoft Academic Search

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 ø for the region and suggests a maximum spreading width of 275 km for

John S. Pallister

1981-01-01

138

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.

139

Cross section through the peridotite in the Samail ophiolite, southeastern Oman Mountains  

Microsoft Academic Search

A continuous cross section through the peridotite of the Samail ophiolite south of Wadi Tayin offers a relatively undisturbed slice of oceanic mantle 9-12 km thick. The whole section consists of a regularly foliated harzburgite banded with orthopyroxenite except for a narrow zone at the base where the primary banding is dominantly dunitic. Websterite dikes, sometimes deformed and branching, are

F. Boudier; R. G. Coleman

1981-01-01

140

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

141

Magma chambers in the Oman ophiolite: fed from the top and the bottom  

Microsoft Academic Search

Recent models of magma chambers at fast-spreading ridges are based on the idea that the entire gabbro section of the oceanic crust crystallizes from a thin melt lens located just below the sheeted dike complex. The shape of the lens has been deduced from seismic reflection data at fast-spreading ridges. On the basis of structural studies in the Oman ophiolite,

Françoise Boudier; Adolphe Nicolas

1996-01-01

142

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

E-print Network

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

143

TiV plots and the petrogenesis of modern and ophiolitic lavas  

Microsoft Academic Search

Plots of Ti vs. V for many modern volcanic rock associations are diagnostic of tectonic setting and can be used to determine possible tectonic settings of ophiolites as well. The basis of this plot is the variation in the crystal\\/liquid partition coefficients for vanadium, which range with increasing oxygen fugacity from > 1 to << 1. Since the partition coefficients

John W. Shervais

1982-01-01

144

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

145

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

146

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

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

147

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

148

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

NASA Astrophysics Data System (ADS)

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

Tartarotti, Paola

2013-04-01

149

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

NASA Astrophysics Data System (ADS)

Suprasubduction zone (SSZ) ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites. This sequence includes the following: (1) birth, which entails the formation of the ophiolite above a nascent or reconfigured subduction zone; this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex; (2) youth, during which is continued melting of refractory asthenosphere (depleted during birth) occurs in response to fluid flux from the subducting slab, with extensional deformation of the older plutonic suite, eruption of refractory lavas, and the intrusion of wehrlite-pyroxenite; (3) maturity, with the onset of semistable arc volcanism, typically calc-alkaline, as the subduction zone matures and stabilizes, and the intrusion of quartz diorite and eruption of silicic lavas; and (4) death, which is the sudden demise of active spreading and ophiolite-related volcanism, which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis; expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites; (5) resurrection, with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction. The early stages (1-3) may be diachronous, and each stage may overlap in both time and space. The existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the SSZ tectonic setting.

Shervais, John W.

2001-01-01

150

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

151

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

152

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

USGS Publications Warehouse

Analyses of platinum-group elements (PGE) in rocks collected from the Voikar-Syninsky ophiolite in the Polar Urals suggest that the distribution and geochemistry of PGE in this Paleozoic ophiolite are similar to those in Mesozoic ophiolites from elsewhere. Chondrite-normalized PGE patterns for chromitite, the tectonite unit, and ultramafic and mafic cumulate unit have negative slopes. These results are similar to those found for chromitites from other ophiolites; stratiform chromities show positive slopes. If the magmas that form both types of chromitite originate from similar mantle source material with respect to PGE content, the processes involved must be quite different. However, the distinct chondrite-normalized PGE patterns may reflect differing source materials. ?? 1983 Springer-Verlag.

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

1983-01-01

153

Geologic Section Through the Samail Ophiolite and Associated Rocks Along a Muscat-Ibra Transect, Southeastern Oman Mountains  

Microsoft Academic Search

' Regional mapping at a 1:60,000 scale of a 30-kin strip from the Gulf of Oman (Muscat) across the Oman Mountains, 130 km to the south, provides the geologic setting for the (-95 m.y.) Ibra section of the Samail ophiolite. Where best preserved, the Ibra ophiolite section is an ---8 kin-thick section of oceanic consisting of --0.5 km of pillow

C. A. Hopson; R. G. Coleman; R. T. Gregory; J. S. Pallister; E. H. Bailey

1981-01-01

154

Copper deposits in the basal breccias and volcano-sedimentary sequences of the eastern ligurian ophiolites (Italy)  

Microsoft Academic Search

A large number of the copper deposits, associated with the eastern ligurian ophiolites, are linked to the volcano-sedimentary sequences and, in minor amounts, to the ophiolitic basal breccias partially covering the intrusive rocks (ultramafites and gabbros). Some of these Fe-Cu-Zn mineralizations were selected because of their well defined stratigraphic and structural features, which, together with the textural and mineralogical characters

A. Ferrario; G. Garuti

1980-01-01

155

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

156

Insight in Ridge Axial Melt Lens in the Oman Ophiolite  

NASA Astrophysics Data System (ADS)

As in fast spreading ridges, the Oman ophiolite had a melt lens perched on top of the magma chamber where the gabbro unit was crystallizing. This melt lens is now reduced to an horizon where its roof and floor are coinciding and this horizon is now identified in the field. It is generally marked by a sharp discordance between the isotropic gabbros from the root zone of sheeted dike complex (RZSDC) and steeply dipping foliated gabbros. These gabbros are issued from the mush settled on the floor of the melt lens, after subsidence inside the magma chamber. After stretching, compaction and rotation in the chamber, the mush has drifted through the wall of the chamber with, as a result, the observed steep foliated gabbros. Depending on its vertical distance beneath the lens horizon, a given gabbro derives from increasing distances inside the melt lens. Insights in the active melt lens are possible in three ways. 1) Looking at gabbros from the lens horizon, which virtually have not subsided. 2) Considering uncommon areas which display flat-lying foliated gabbros, below the contact with RZSDC and which grade down section into the steep foliated gabbros. Such situations are ascribed to a retreat of the melt lens, exposing gabbros which crystallized on its floor. Their good foliation points to a dynamic deposition on the floor, presumably by convection currents. 3) Considering the ubiquitous occurrence of anorthosites which are interlayered with the foliated gabbros. The anorthosites carry several important messages such as: - compaction of the mush at early stage of subsidence; - chemical nature of the rising melt which drops plagioclase first, followed by either olivine or clinopyroxene; - frequency and volume of melt intrusions, each one coming as short and massive melt surge; - spacing of areas of melt delivery on the lens floor. These results are essentially derived from anorthosites description and distribution in the field. It is concluded that melt lens activity is rhythmic, following different tempos. 1) The lower frequency of tens of thousands years, related to vertical migration of lenses, recognized at fast spreading ridges. 2) The frequency of a few hundreds years, related to horizontal displacements and to melt surges. This timing is compatible with the ~500 years deduced from the ~50 m spacing of abyssal hills, in the 10 cm/year western side of 18° S East Pacific Rise (Carbotte et al., 2003)1. Lens inflation, with possibly small swelling, is controlled by a melt surge, whereas lens deflation is illustrated by the "tide" effect, uncovering the flat foliations of the lens floor. 3) The highest frequency at the scale of ~10 years, related to basaltic extrusions on seafloor, as they are recorded by sheeted dikes. Deflation of the melt lens would result from these periodic basaltic extrusions. (1) Carbotte, S.M. et al., 2003., G3, vol.4/1, 1-21.

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

2008-12-01

157

Structure of Submarine Large Lobate Sheet from the Oman Ophiolite  

NASA Astrophysics Data System (ADS)

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

Umino, S.

2009-12-01

158

Exploring the deep biosphere through ophiolite-associated surface springs  

NASA Astrophysics Data System (ADS)

The process of serpentinization in the subsurface produces highly reduced, high pH fluids that provide habitats for microbial communities. At depth, these fluids likely contain appreciable hydrogen and methane gas, very little inorganic carbon, and limited electron acceptors. As fluids affected by serpentinization reach the oxygenated surface environment, microbial biomes may shift to include organisms capable of metabolizing oxygen. Our study of serpentinizing and hydrothermal fluids in the Zambales and Palawan regions of the Philippines includes a suite of geochemical analyses to define microbial habitats, targeted culturing efforts, and community analysis (the latter presented in a companion abstract). We measured temperature, pH, conductivity, ORP, dissolved oxygen, nitrate, ammonia, iron, sulfide, and silica in the fluids on site, and have analyzed dissolved gases, major ions, and trace elements. These analyses provide a description of the near subsurface and surface ecosystems of the study areas. Carbon and nitrogen isotopic ratios in solid biofilm and biominerals from the springs allow inference of metabolic cycling. Fluids expressed from the Zambales and Palawan ophiolites range from pH 9-11.5, 0.06-2 mg/L dissolved oxygen, 0-3.7 mg/L sulfide, and 30-800 mg/L silica in the source pools. Temperatures vary from 34-50C. Analysis of dissolved gases reveal concentrations of H2 and CH4 > 10uM, CO2 > 1 mM, and trace amounts of CO. This variability in fluid and gas composition indicates the potential for different mixing ratios with local meteoric water, local hydrothermal fluids, and deep-seated sediments. Results will be compared with other serpentinizing fluids to place these habitats in context of the ecology of serpentinizing springs globally. Anaerobic and aerobic growth media were designed to target heterotrophy, sulfate reduction, and iron reduction (the latter two with, and without organic carbon input). We have calculated that these processes have potential to yield energy for metabolism in these ecosystems. Media were diluted with filtered spring fluid to the desired concentration on location, and inoculated immediately. We found positive growth in cultures from all sample locations (seven in all), at temperatures ranging from 28-48C under both aerobic and anaerobic conditions. Growth on complex organic media was successful in five samples, and media including sugars or organic acids in two and four samples, respectively. Heterotrophic sulfate reduction was seen in six samples, and autotrophic sulfate reduction in only three samples. Four locations yielded heterotrophic iron reducers, and five locations host organisms capable of autotrophic iron reduction. This variety of positive growth indicates a metabolically flexible community, complementing data obtained from previously reported communities in serpentinizing systems. We begin to obtain a picture of community dynamics and functional diversity in these ecosystems that bridge the subsurface and surface biospheres.

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

2013-12-01

159

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

NASA Astrophysics Data System (ADS)

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

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

2013-01-01

160

Seismic anisotropy in an ophiolite peridotite: Application to oceanic upper mantle  

Microsoft Academic Search

Compressional velocity Vp to 8 kbar and 275°C in a homogeneous. 0.5% porosity, little-altered harzburgite from the Antalya ophiolite complex in Turkey was measured along the three principal kinematic axes as defined from the preferred orientations of olivine and enstatite. The observed Vp anisotropy was 0.7 km\\/s for pressures P from 2 to 8 kbar and temperatures T from 25°

L. Peselnick; A. Nicolas

1978-01-01

161

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

Microsoft Academic Search

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),

Antonios Koroneos; Giampiero Poli; Georgios Christofides

2010-01-01

162

Témoins d'un arc immature téthysien dans les ophiolites du Sud Ladakh (NW Himalaya, Inde)  

Microsoft Academic Search

The ophiolites of the South Ladakh are evidence of the Neo-Tethys obduction onto the Indian continental margin, during Late Cretaceous times. The mafic rocks are cogenetic and were extracted from a N-MORB like depleted source slightly metasomatized above a subduction zone. Thermobarometry on ultramafic rocks confirms this geodynamic setting. Considering their position in the Ladakh-Zanskar area and their geochemical signatures,

Gweltaz Mahéo; Hervé Bertrand; Stéphane Guillot; Georges Mascle; Arnaud Pêcher; Christian Picard; Julia De Sigoyer

2000-01-01

163

Geochemistry and Origin of Listwanites in the Sartohay and Luobusa Ophiolites, China  

Microsoft Academic Search

Gold-bearing listwanites of the Sartohay ophiolite, northwestern China, consist mainly of talc, magnesite, chlorite, quartz, and dolomite, locally accompanied by chromian mica (fuchsite). They formed along NE-trending faults by hydrothermal alteration of serpentinized peridotites. The original protoliths were clinopyroxene-bearing harzburgites and lherzolites containing relatively aluminous magnesiochromite. Compared to their protoliths, listwanites generally have lower MgO and significantly higher CaO, TiO2,

Paul T. Robinson; John Malpas; Mei-Fu Zhou; Chris Ash; Jing-Sui Yang; Wen-Ji Bai

2005-01-01

164

Modeling Enhanced In Situ CO2 Mineralization in the Samail Ophiolite Aquifer  

Microsoft Academic Search

The Samail Ophiolite aquifer in the Sultanate of Oman is a site of exceptionally well-developed naturally occurring in situ CO2 mineralization, and serves as a natural analog for an engineered CO2 sequestration process. Natural processes within the aquifer can be described by the following reactions [e.g.1,2]: near the surface, infiltrating rainwater dissolves peridotite, increasing dissolved Mg, Ca, and Si; interaction

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

2010-01-01

165

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

NASA Astrophysics Data System (ADS)

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

Cousineau, Pierre A.; St-Julien, Pierre

1992-08-01

166

Geochemistry and origin of plagiogranites from the Eldivan Ophiolite, Çank?r? (Central Anatolia, Turkey)  

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

167

1.0 GA Ophiolite on North Margin of the Yangtze Craton Clarifies South China's Amalgamagtion with Rodinia  

NASA Astrophysics Data System (ADS)

We report a Grenvillian ophiolite from the northern margin of the Yangtze craton. This is the first-recognized ophiolite from the northern Yangtze craton, and its recognition drastically changes models of how the Yangtze craton was incorporated into the Rodinia supercontinent. The ophiolite includes a complete sequence of circa 1.0 Ga N-MORB tholeiitic pillow lavas, boninites, sheeted dikes, gabbro, ultramafic cumulates, and harzburgite tectonite with podiform chromites. The amphibolitic basalts and diabase dikes exhibit TiO2 = 1.14%-1.48%, slightly depleted-flat type REE curves with no obvious Eu anomalies, (La / Yb) N = 0.87-1.12, average La / Nb, Ce / Zr, Zr / Nb, Zr / Y, Ti / Y are respectively 1.04,0.15,18.78,2.53,290.51, and the average Nb / Th is 9.88. These features show that these amphibolites are N-MORB type tholeiites typical of mid-ocean ridge tectonic settings. The metagabbro has typical cumulate textures, flat REE distributions and obvious positive Eu anomalies. The REE characteristics of serpentinized dunites show a U-shape of slight loss of middle REE, representing cumulates metasomatized by LREE slightly enriched mantle. The ophiolite is structurally dismembered and thrust over the Proterozoic shelf sequence covering the north margin of the Yangtze craton, and overrode a conglomerate-wildflysch unit shed from the ophiolite and a magmatic arc terrane. The youngest clasts in the conglomerate are circa 861-813 Ma (U-Pb zircon), giving a maximum age for ophiolite emplacement. The discovery of the Proterozoic Miaowan ophiolite supplies important evidence for the existence of a Neoproterozoic oceanic basin and accreted arc on the north margin of the Yangtze craton, and provides evidence for a Proterozoic suture between the north and south parts of the Yangtze craton. Further, it demonstrates that the Yangtze craton amalgamted with Rodinia through double-sided subduction on its northern and southern margins, explaining the complex tectonomagmatic evolution of the craton.

Kusky, T. M.; Peng, S.; Wang, L.; Jiang, X.; Wang, J.

2010-12-01

168

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

USGS Publications Warehouse

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

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

1999-01-01

169

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

NASA Astrophysics Data System (ADS)

Although the Oman ophiolite is classically regarded as being the direct analog of oceanic lithosphere created at fast spreading ridges, the geodynamic context of its formation is still highly debated. The other alternative end-member model suggests that this ophiolite entirely formed in a supra-subduction zone setting. Fluids involved in the hydration of the oceanic lithosphere and in the presence of a secondary boninitic and andesitic volcanism may provide a way to discriminate between these two interpretations: are they descending near-axis hydrothermal fluxes (first model) or ascending from a subducting slab (second model)? We herein focus on the base of the ophiolitic mantle in order to characterize the origin of fluids and decipher hydration processes. Samples were taken along hecto- to kilometre-long sections across the basal banded unit directly overlying the amphibolitic/granulitic metamorphic sole. We carried out a petrological, structural and geochemical study on these rocks and their constitutive minerals. Our results show that, unlike the generally refractory character of Oman harzburgites, all the basal mantle rocks display secondary crystallization of clinopyroxene and amphibole through metasomatic processes. The microstructures and the chronology of these secondary mineralizations (clinopyroxene, pargasitic amphibole, antigorite and then lizardite/chrysotile) suggest that these basal rocks have been affected by cooling from mantle temperatures (<1200°C) to low-T serpentinisation (<300°C). Furthermore, major elements required to crystallize these minerals and the observed fluid-mobile elements (FMEs) enrichments in the clinopyroxenes and in the amphiboles (B, Pb, Sr), as well as in the serpentines (B, Sr, Rb, Ba, As), are consistent with amphibolite-derived fluids (Ishikawa et al., 2005) and cannot be easily explained by other sources. Based on these observations, we propose a geodynamic model in which intense and continuous metasomatism of the cooling base of the ophiolitic mantle is due to the release of fluids coming from the progressive dehydration of underlying amphibolitic rocks. This process is compatible with the progressive subduction of the Arabian margin during the Upper Cretaceous (e.g., HP-LT units history, and tectonic structures observed on top of it). The basal part of the Oman ophiolite would thus represent a fossil incipient mantle wedge.

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

2014-05-01

170

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

171

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

172

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

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

173

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

USGS Publications Warehouse

Whole-rock and mineral samples from the Jabal al Wask and Jabal Ess ophiolites, northwestern Saudi Arabia, yield Sm-Nd isochron ages of 743+24 Ma and 782??38 Ma, respectively. These formation ages, which provide maximum limits for possible obduction ages, are in broad but not precise agreement with the previously known geologic history of the Arabian Shield. They indicate that the ophiolitic rocks are roughly coeval with nearby volcanic and plutonic rocks, supporting a back-arc origin for the two ophiolites. We suggest that the Jabal al Wask and Jabal Ess ophiolites were parts of the same northeast-southwest trending ophiolite belt, now offset along the Najd fault system. Initial e{open}Nd values range from +6.6 to+ 7.6, indicating derivation from a mantle source that has been LIL-depleted for at least 2 Ga. Reported e{open}Nd values from the Arabian Shield that are lower than this suggest the presence of older, reworked continental crust. ?? 1984 Springer-Verlag.

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

1984-01-01

174

Lower Permian formations of the Buqingshan Mountains in the A'nyemaqen ophiolitic zone (Eastern Kunlun, Qinghai Province, China): On the beginning of the Paleotethys closure  

Microsoft Academic Search

(1) Several Lower Permian formations are most completely represented within the Buqing- shan Mountains (eastern portion of the Eastern Kunlun orogenic belt), a part of the Paleotethyan ophiolite suture commonly considered as the A'nyemaqen ophiolite zone. The formations were formed in the following paleogeodynamic environments (in a southward direction): (1) shelf and slope of a passive continental margin of a

I. I. Pospelov; E. J. Leven; Qiantao Bian; V. A. Aristov; O. A. Korchagin

2005-01-01

175

Propagating rift tectonics of a Caledonian marginal basin: Multistage seafloor spreading history of the Solund-Stavfjord ophiolite in western Norway  

Microsoft Academic Search

The Late Ordovician Solund-Stavfjord ophiolite in western Norway represents a remnant of the Iapetus oceanic lithosphere that developed in a Caledonian marginal basin. The ophiolite contains three structural domains that display distinctively different crustal architecture that reflects the mode and nature of magmatic and tectonic processes operated during the multi-stage seafloor spreading evolution of this marginal basin. Domain I includes,

Yildirim Dilek; Harald Furnes; K. P. Skjerlie

1997-01-01

176

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

177

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

PubMed

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

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

2001-05-11

178

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

NASA Astrophysics Data System (ADS)

The Xigaze ophiolite crops out along the central Yarlung-Zangbo Suture Zone (YZSZ, Tibet), which also includes the Gangdese arc and the Xigaze forearc basin. Here we report new geochronological and geochemical data from this ophiolite. The Xigaze ophiolite is dominated by mantle peridotites with low CaO and Al2O3 contents, and U-shaped REE patterns. The petrological and geochemical characteristics of these peridotites indicate that they represent residues after moderate- to high-degrees of partial melting (15-24%) and that they were metasomatized by LREE-enriched boninitic melts in a mantle wedge beneath a forearc tectonic setting. The mafic rocks of the Xigaze ophiolite are particularly thin and can be divided into two groups based on their whole-rock compositions: (1) MORB-like, and (2) boninitic rocks. Both groups display negative Nb and Ta anomalies on MORB-normalized diagrams, consistent with the metasomatism of their mantle source by slab-derived fluids. LA-ICPMS zircon U-Pb data from five representative samples indicate formation ages of 124-127 Ma. The zircons are also characterized by positive ?Hf(t) values varying from +7.5 to +17.3. These observations, combined with the geological and geochronological characteristics of the central-western Yarlung-Zangbo ophiolites (YZO), the Gangdese arc and the Xigaze forearc basin, suggest that the central-western YZO might have formed in the forearc setting where rapid crustal accretion was caused by slab rollback during subduction initiation at 130-120 Ma. Subsequently, the rollback of the subducting slab slowed down and stabilized, and the zone of melting migrated to below the Gangdese arc, producing voluminous late Cretaceous granitoids with depleted mantle-type Hf isotopic characteristics. Our new model provides a good example for the generation of the forearc-type ophiolites.

Dai, Jingen; Wang, Chengshan; Polat, Ali; Warrier Santosh, Madhava; Ge, Yukui

2013-04-01

179

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

NASA Astrophysics Data System (ADS)

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

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

2008-12-01

180

Petrological and geochemical study of the Late Cretaceous ophiolite of Khoy (NW Iran), and related geological formations  

NASA Astrophysics Data System (ADS)

This paper, based on 113 new whole rock analyses and about 3500 electron microprobe analyses of the mineral phases, is dedicated to the petrography and geochemistry of the Khoy ophiolites and related formations, NW Iran. It is complementary to a previous paper published in this Journal, where we gave a detailed description of the geology of the Khoy area, including various geological field sections, two geological maps in colour, new micropaleontological data and 27 new 40K- 40Ar datings (Khalatbari-Jafari, M., Juteau, T., Bellon, H., Whitechurch, H., Cotten, J., Emami, H., 2004. New geological, geochronological and geochemical investigations on the Khoy ophiolites and related formations, NW Iran. J. Asian Earth Sci. 23, 507-535). Our conclusions are: (a) The petrographic study confirms the field data showing the existence of two ophiolite complexes in the region of Khoy. (b) The Late Cretaceous ophiolitic lavas of the Khoy region exhibit very homogeneous T-MORB-type multi-element plots, suggesting that they were formed at oceanic spreading centers, by partial melting of a depleted mantle source, probably contaminated by one or several regional mantle plumes, responsible for their moderate enrichment in LREE. They do not show any negative anomaly for Nb, Zr or Ti, which allows us to exclude a genesis in a 'supra-subduction' environment. (c) The Late Cretaceous ophiolite of Khoy was created at a slow-spreading oceanic ridge. (d) Cryptic variations along extrusive and layered gabbros sections suggest frequent replenishment and magma mixing events in the magma chambers. (e) The 'supra-ophiolitic turbiditic series' overlying the Late Cretaceous ophiolite was accumulated in a subduction trench running along the northwestern margin of the Iran Block. This trench was fed with detrital volcanic fragments from both sides: T-MORB basalt fragments from the ocean-side, and arc-type basalts from the continent-side. (f) The meta-ophiolites of Khoy probably also represent slow-spreading conditions, and the porphyroclastic to mylonitic tectonites preserved in these metamorphic slices attest to extreme conditions of ductile shearing, characteristic of oceanic fracture zones.

Khalatbari-Jafari, Morteza; Juteau, Thierry; Cotten, Joseph

2006-09-01

181

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

182

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

183

Gabbroic rocks in ophiolitic occurrences from East Othris, Greece: petrogenetic processes and geotectonic environment implications  

NASA Astrophysics Data System (ADS)

East Othris area consists of scattered ophiolitic units, as well as ophiolitic mélange occurrences, which encompass gabbroic rocks. These rocks have been affected by low-grade ocean floor metamorphism ( T < 350°C and P < 8 kbar). Based on their petrography, mineral chemistry and geochemistry gabbroic rocks have been distinguished into gabbros and diorites, with the latter being divided into two groups. Gabbros seem to have been formed from moderate to high partial melting degrees (~8-25%) of a highly depleted mantle source, while group (1) diorites have been differentiated after variable fractionation processes (up to 30%). Group (2) diorites seem to have been derived from low partial melting degrees (~3%) of a fertile or moderately depleted mantle source and with extensive fractionation processes (~50%). Geochemical results suggest that partial melting processes occurred at relatively shallow depths, in the plagioclase-spinel stability field, while amphibole chemistry data indicate shallow level crystallization. Chondrite and PM-normalized patterns, Th/Yb, and Nb/Th ratios as well as mineral chemistry analyses show that gabbros and group (1) diorites (with relatively low PM-normalized Nb and Ta values and negative Ti anomalies) suggest subduction processes, while group (2) diorites are MORB or BAB related. Some gabbros have been characterised as high-Mg, being compositionally similar to picrites or boninites. Variability in extent of partial melting of the mantle source and the different geotectonic environment affinities are consistent with a supra-subduction zone (SSZ) origin of the east Othris ophiolites. The fact that IAT related rocks are more abundant in east rather than in west Othris may possibly be explained by a slab rollback model retreating to the east within the Pindos oceanic basin.

Koutsovitis, Petros

2012-03-01

184

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

PubMed

Endemic PCs and high incidence of malignant mesothelioma from household use of asbestos have been reported in Metsovo in northwestern Greece ("Metsovo lung"). In the present study, we present similar findings in six more areas of Greece. Like Metsovo, all these areas are located within ophiolite belts. Like Metsovo, material similar to "Metsovo whitewash" has been used for various domestic uses. Asbestos fibers (chrysotile, antigorite and tremolite) were found in three of the six areas. Also, in two, MPM has been diagnosed. These findings suggest that "Metsovo lung" occurs in several areas of Greece and has similar etiology and epidemiology. PMID:2019171

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

1991-05-01

185

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

USGS Publications Warehouse

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

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

1995-01-01

186

Mapping in the Oman ophiolite using enhanced Landsat Thematic Mapper images  

NASA Technical Reports Server (NTRS)

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

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

1988-01-01

187

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

NASA Astrophysics Data System (ADS)

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

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

188

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

USGS Publications Warehouse

Fossil worm tubes of Cretaceous age preserved in the Bayda massive sulfide deposit of the Samail ophiolite, Oman, are apparently the first documented examples of fossils embedded in massive sulfide deposits from the geologic record. The geologic setting of the Bayda deposit and the distinctive mineralogic and textural features of the fossiliferous samples suggest that the Bayda sulfide deposit and fossil fauna are remnants of a Cretaceous sea-floor hydrothermal vent similar to modern hot springs on the East Pacific Rise and the Juan de Fuca Ridge.

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

1984-01-01

189

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

190

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

NASA Astrophysics Data System (ADS)

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

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

2010-05-01

191

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

E-print Network

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

Shervais, John W.

192

Platinum-Group Element Geochemistry in Podiform Chromitites and Associated Peridotites of the Precambrian Ophiolite, Eastern Desert, Egypt  

Microsoft Academic Search

The ultramafic portions of the Proterozoic ophiolite, Eastern Desert, Egypt, contain significant chromitite concentrations that are mainly located in the central and southern parts of the Eastern Desert as small and irregular bodies within a fully serpentinized dunite and harzburgite. Chromitite spinel exhibits a wide range of composition from high Cr to high Al varieties. The Cr# of chromian spinel

A. Hassan Ahmed; Shoji Arai

193

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

Microsoft Academic Search

Plagiogranites are a minor but widespread component of the Samail ophiolite plutonic member. They crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the Tethyan spreading ocean ridge, and their ages are thought to mark the time of ocean crust formation. Isotopic U--Pb ages of zircons from 13 plagiogranites collected

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

1981-01-01

194

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

195

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

E-print Network

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

Shervais, John W.

196

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

E-print Network

Chemical heterogeneity in the upper mantle recorded by peridotites and chromitites from associated harzburgites. As is common for ophiolitic peridotites, the harzburgites (gOs492Ma of �5.3 to þ2, of the bulk convecting upper mantle. The Shetland peridotite compositions reflect protracted melt depletion

Mcdonough, William F.

197

Evidence for forearc seafloor-spreading from the Betts Cove ophiolite, Newfoundland: oceanic crust of boninitic affinity  

Microsoft Academic Search

The Ordovician Betts Cove ophiolite of Newfoundland has a well-developed cumulate sequence, in which is rooted a sheeted dyke complex that grades up into pillow lavas. Dominant chromite + olivine + orthopyroxene cumulate peridotites and orthopyroxenites have phase assemblages and mineral chemistries consistent with crystallization from boninitic magmas. Dykes and lavas have phenocrysts of olivine + high-CrAl chromite ± orthopyroxene

J. H. Bédard; K. Lauzière; A. Tremblay; A. Sangster

1998-01-01

198

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

E-print Network

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

Cartigny, Pierre

199

From Aptian Onset to Danian Demise of Subduction along the Northern Margin of the Caribbean Plate (Sierra del Convento Melange, Eastern Cuba)  

NASA Astrophysics Data System (ADS)

The serpentinite-matrix melange of the Sierra del Convento, eastern Cuba, represents an oceanic subduction channel related to Mesozoic subduction in the Caribbean realm which provides evidence for a long-lasting history of subduction, accretion, melange formation, and uplift, and for Aptian onset of subduction in the region. Exotic blocks of MORB-derived plagioclase-free epidote±garnet amphibolite followed a hot subduction-related prograde P-T path, reaching ca. 750 °C, and 14-16 kbar at peak conditions. Fluid flux at this stage triggered melting of the amphibolites to yield peraluminous tonalitic-trondhjemitic melts, which appear intimately associated with the amphibolites. Trondhjemitic-granitic varieties richer in K2O suggest the local participation of a sedimentary source, likely diluted through the infiltrating fluid. Calculated conditions for the magmatic assemblages (plagioclase, quartz, epidote, ±paragonite, ±pargasite, ±muscovite) of the siliceous rocks yield pressures of ca. 15 kbar, indicating crystallization at depth in the subduction environment. SHRIMP U-Pb zircon dating of two granitoid samples gives crystallization ages of 113-114 Ma. Partial melting of subducted oceanic crust in eastern Cuba is unique in the Caribbean realm and is interpreted as the result of onset of subduction of young oceanic lithosphere during the Aptian (ca. 120 Ma), in agreement with regional geological data. Calculated P-T conditions for the retrograde blueschist-facies overprints present in all rocks indicate counterclockwise P-T paths during exhumation in a colder, syn-subduction scenario. Ar-Ar amphibole dating yielded two groups of cooling ages of 106-97 Ma (interpreted as cooling of metamorphic/magmatic pargasite) and 87-83 Ma (interpreted as growth/cooling of retrograde overprints). The above P-T-t data and additional ages of other rocks from the area suggest the following stages of evolution:(a) hot subduction during 120-115 Ma with heating and burial rates of 150 °C/Ma and 11 km/Ma, respectively, developed shortly after onset of subduction of young oceanic lithosphere; (b) relatively fast near-isobaric cooling (25 °C/Ma) during 115-107 Ma, developed after accretion of the blocks to the upper plate mantle; (c) slow syn-subduction cooling (4 °C/Ma) and exhumation (0.7 km/Ma) in the subduction channel in a colder (mature) subduction environment during 107-70 Ma, and d) fast cooling (70 °C/Ma) and exhumation (5 km/Ma) during 70-65 Ma, when arc-continent collision occurred and subduction terminated in the region.

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

2007-12-01

200

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

201

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

202

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

NASA Astrophysics Data System (ADS)

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

Belic, Maximilian; Hauzenberger, Christoph; Dong, Yunpeng

2013-04-01

203

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

204

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  

Microsoft Academic Search

Ultramafic cumulates, mainly crustal true wehrlites, were discovered and described in the mantle–crust transition zone (MTZ)\\u000a and the extremely lower layered gabbro sequence of the Ras Salatit ophiolite, Central Eastern Desert, Egypt. They form either\\u000a boudinaged lensoidal tabular bodies or interdigitated layers often concordant with the planolinear fabrics of the Ras Salatit\\u000a ophiolite rocks. The contact between wehrlites and the

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

205

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

206

The eroded Late Jurassic Kurbnesh carbonate platform in the Mirdita Ophiolite Zone of Albania and its bearing on the Jurassic orogeny of the Neotethys realm  

Microsoft Academic Search

Several Late Jurassic (Kimmeridgian?-Tithonian) to Early Cretaceous (Late Berriasian-Valanginian) shallow-water carbonate\\u000a clasts of different facies are contained in mass-flow deposits in a pelagic sequence in the Kurbnesh area of central Albania.\\u000a These clasts are used to reconstruct shallow-water carbonate platforms, which formed on top of the radiolaritic-ophiolitic\\u000a wildflysch (ophiolitic mélange) of the Mirdita Zone. Stratigraphic interpretation of the platform carbonates

Felix Schlagintweit; Hans-Jürgen Gawlick; Sigrid Missoni; Lirim Hoxha; Richard Lein; Wolfgang Frisch

2008-01-01

207

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

208

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

209

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

NASA Astrophysics Data System (ADS)

The basic premise of seafloor spreading is that magmatic activity at mid-ocean ridges is responsible for creation of new crust; however only several years after the formulation of plate tectonic theory we have recognised the importance of tectonic, rather than magmatic, processes in accommodating the divergence of the lithospheric plates in slow-spreading oceans. In these settings, magma supply is spatially and temporally discontinuous and the crust freezes solid between delivery of magma batches from the underlying mantle. Large-scale extensional detachment faults develop during these periods of reduced magma supply, and play a fundamental role in accommodating plate separation. Extensional faulting occurs at the corner between ridge and transform fault and is responsible for the generation of the Oceanic Core Complexes (OCCs). OCCs are the uplifted footwalls of oceanic detachment faults forming dome-shaped massifs consisting of mantle and lower crustal lithologies exposed at the seafloor. Numerical modelling and results from the Integrate Ocean Drilling Program (IODP) has shown that the footwalls beneath oceanic detachment faults rotate during their evolution, proving that they initiate at a steep angle at depth and then "roll-over" to their present day low angle orientations as a result of flexural isostasy during unroofing. Accordingly, information from paleomagnetic analyses provided strong constraints to the detachment kinematics, since a difference of 45-65° in paleomagnetic inclination across the fault has been documented by previous studies, confirming the rolling-hinge model for OCCs formation. Here we present the results of an extensive paleomagnetic study of an OCC preserved in the Mirditata ophiolite of the Albanian Dinarides, first recognized by Trembley et al. (2009). Ophiolites are slices of oceanic lithosphere which have been emplaced onto continental margins during the closure of ocean basins, and provide opportunities for the study of oceanic structures in three-dimensions from kilometric to millimetric spatial scales.The western part of Mirdita ophiolite is composed of a lherzolitic 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 ultramafics, lava flows, pillow lavas, and dykes, at the Puka and Krabbi massifs between the villages of Puka and Rreshen in northern Albania. Preliminary results showed a ca. 20° difference in paleomagnetic inclination between ultramafics and the volcanic sequence, quite minor than the 45-60° expected for detachment faults at modern OCCs. Now we discuss the results from all the samples sites and will provide a complete interpretation of the relationship between ultramafic and volcanics rocks from the northwestern part of the Mirdita ophiolite, in the framework of the OCC hypothesis.

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

2011-12-01

210

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

PubMed Central

The deepest rocks known from within Earth are fragments of normal mantle (?400 km) and metamorphosed sediments (?350 km), both found exhumed in continental collision terranes. Here, we report fragments of a highly reduced deep mantle environment from at least 300 km, perhaps very much more, extracted from chromite of a Tibetan ophiolite. The sample consists, in part, of diamond, coesite-after-stishovite, the high-pressure form of TiO2, native iron, high-pressure nitrides with a deep mantle isotopic signature, and associated SiC. This appears to be a natural example of the recently discovered disproportionation of Fe2+ at very high pressure and consequent low oxygen fugacity (fO2) in deep Earth. Encapsulation within chromitite enclosed within upwelling solid mantle rock appears to be the only vehicle capable of transporting these phases and preserving their low-fO2 environment at the very high temperatures of oceanic spreading centers. PMID:19880742

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

2009-01-01

211

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

212

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

213

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

214

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

215

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

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

216

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

Microsoft Academic Search

An airborne hyperspectral survey of the Oman ophiolite (Sumail Massif) has been conducted using the HyMap airborne imaging spectrometer with associated field measurements (GER 3700). An ASD FieldSpec3 spectrometer was also used in order to constrain the spectral signatures of the principal lithologies cropping out in the surveyed area. Our objective was to identify and map the various igneous lithologies

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

2009-01-01

217

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

Microsoft Academic Search

An airborne hyperspectral survey of the Oman ophiolite (Sumail Massif) has been conducted using the HyMap airborne imaging spectrometer with associated field measurements (GER 3700). An ASD FieldSpec3 spectrometer was also used in order to constrain the spectral signatures of the principal lithologies cropping out in the surveyed area. Our objective was to identify and map the various igneous lithologies

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

2009-01-01

218

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

NASA Astrophysics Data System (ADS)

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

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

2013-07-01

219

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

Microsoft Academic Search

Supra-subduction zone ophiolites in the Egyptian Central Eastern Desert (CED) occur as clusters in its northern (NCEDO) and southern (SCEDO) parts, displaying abundant island arc–boninitic and MORB\\/island-arc geochemical affinities, respectively. An amphibolite belt, including the investigated massive to slightly foliated Wadi Um Gheig (WUG) amphibolites, is exposed in the southeast most of the NCEDO thrusting over the El Sibai gneissic

E. S. Farahat

2011-01-01

220

Proterozoic ophiolites and mafic ultramafic complexes marginal to the ?stanbul Block: An exotic terrane of Avalonian affinity in NW Turkey  

NASA Astrophysics Data System (ADS)

Among the Proterozoic inliers in the ?stanbul Block, the lowest structural levels are exposed in the Sünnice Massif, north of Bolu. Amphibolite-facies mafic and subordinate ultramafic rocks of the Çele meta-ophiolite underlie the greenschist-facies Ediacaran calc-alkaline Yellice metavolcanics, which are intruded by the ˜ 565-576 Ma Dirgine granitoids. Hornblende gneisses of the Çele meta-ophiolite comprise island arc meta-tholeiites and transitional to calc-alkaline metabasalts which, together with minor serpentinite are disposed in a broadly antiformal structure. The meta-ophiolitic rocks are therefore the oldest ophiolites in NW Turkey, and are themselves thrust on to a putative pre-existing continental margin, now represented by the metasedimentary migmatites of the Demirci gneisses, which may thus be the oldest rocks of the complex. The ?stanbul Block is an exotic terrane. Unlike other western Turkish terranes, it lacks Variscan metamorphism: its different provenance, indicated by its geological record, faunal affinities, and inherited mid-Proterozoic isotopic dates, suggests a former link with Avalonian basement in England, NW Europe and the Maritime Provinces of Canada. Hence, together with other terranes now situated further east than the Avalonian terranes of NW Europe, the ?stanbul Block may represent the easternmost extremities of Avalonia, which were detached during its end-Ordovician collision with the Bruno-Silesian Promontory on the SW margin of Baltica. Subsequent migration of the ?stanbul Block to its present location occurred by eastward displacement by sinistral transpression along the southern margin of Baltica to a point east of the Dobrogea and south of the Scythian Platform, followed by collision with the Sakarya Block in the Mesozoic and Late Cretaceous southward displacement with the opening of the Black Sea basin.

Bozkurt, Erdin; Winchester, John A.; Yi?itba?, Erdinç; Ottley, Christopher J.

2008-12-01

221

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

222

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

PubMed

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

Abelson, M; Baer, G; Agnon, A

2001-01-01

223

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

224

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

225

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

NASA Astrophysics Data System (ADS)

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

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

2002-07-01

226

Metamorphic evolution of metadolerites from the Frido Unit ophiolites (Southern Apennine-Italy)  

NASA Astrophysics Data System (ADS)

The Southern Apennines chain is a fold-and-thrust belt resulting from the convergence of the African and European plates and simultaneous roll-back of SE-directed Ionian subduction (Upper Oligocene-Quaternary). Ophiolites in the Southern Apennines are related to northwest subduction of the oceanic lithosphere pertaining to the Ligurian sector of the Jurassic western Tethys. The ophiolitic sequences are enclosed within remnants of the Liguride accretionary wedge now incorporated in the Southern Apennine chain and they crop out in the north-eastern slope of the Pollino Ridge (Calabria-Lucania border zone). Mafic and ultramafic rocks, with garnet-bearing felses, amphibolites, gneiss and granitoides occur as tectonic slices within a matrix mainly composed of calcschists and phyllites. Metadolerites occur as dikes cutting through serpentinized peridotites. Metadolerites have different kinds of texture reflecting various degree of crystallinity and strain: porphyritic or aphyric, intersertal/intergranular, blastophitic, cataclastic to mylonitic. In all metadolerites primary plagioclase and clinopyroxene can be observed. The metamorphic mineral assemblage consists of brown amphibole, green amphibole, chlorite, blue amphibole, pumpellyite, prehnite, quartz, epidote, white mica, lawsonite and plagioclase (Pl2 and Pl3). Accessory phases are opaque minerals, Fe-hydroxides and zircon. Metadolerites are cross- cut by veins filled with pumpellyite, chlorite, prehnite, tremolite, plagioclase, white, mica, quartz, lawsonite, epidote and zircon. The veins are straight, a few millimetres in thickness and occur isolated or in closely spaced sets. The vein morphology ranges from planar to sinuous and irregular. On the basis of metamorphic mineral phases three different types of metadolerite can be distinguished: i) rocks with a high content of prehnite crystals in cataclastic-mylonitic bands, exhibiting an intersertal or a blastophitic texture or a mylonitic fabric and in some cases a seriate texture; ii) rocks with brown horneblende showing an intersertal or a blastophitic texture or a partially blastophitic and foliate texture in one specimen; iii) rocks with brown horneblende and blue amphibole with an intersertal or a blastophitic texture. Primary clinopyroxene is replaced by brown and green amphiboles interpreted as being of oceanic origin; brown amphiboles show Mg-hastingsite, edenite, pargasite, Fe-hastingsite, Mg-horneblende and tschermakite compositions, whereas green amphiboles show Mg-hastingsite, hastingsite, edenite, Mg-horneblende, tschermakite and Fe-tschermakite compositions. Other minerals developed in the amphibolite facies conditions are: oligoclase, titanite and apatite. The blue amphiboles have a winchite and barrowisite composition and are interpreted as being originated during the early stages of the orogenic metamorphism, since they rim the oceanic brown and green amphiboles. The mineral assemblage of orogenic metamorphism is typical of the LT-blueschist facies conditions and consists of glaucophane, Mg-riebekite, lawsonite, phengite, pumpellyite and aegerin-augite. Bulk-rock chemistry of metadolerites suggests that protoliths of the mafic rock have a N-MORB-type affinity. Ca-rich metadolerites are affected by ocean-floor rodingitic alteration, whereas Na-rich metadolerites show a spilitic alteration. The study of metadolerites from Frido Unit show evidence of the entire evolution from their origin in the ocean floor to their emplacement in the accretionary wedge. Textural and mineralogical observations suggest that the metadolerites of the Frido Unit have been affected by both ocean-floor metamorphism in the amphibolite to greenschist facies- and subsequent orogenic metamorphism under relatively HP/LT conditions. The HP/LT orogenic metamorphism reflecting underplating of the ophiolitic suite at the base of the Liguride accretionary wedge during subduction produced mineral assemblages typical of the lawsonite-glaucophane facies. Such polyphase metamorphic evolution has been entirely preserved in the metadolerites pro

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

2010-05-01

227

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

NASA Astrophysics Data System (ADS)

The ophiolitic peridotites in the Wadi Arais area, south Eastern Desert of Egypt, represent a part of Neoproterozoic ophiolites of the Arabian-Nubian Shield (ANS). We found relics of fresh dunites enveloped by serpentinites that show abundances of bastite after orthopyroxene, reflecting harzburgite protoliths. The bulk-rock chemistry confirmed the harzburgites as the main protoliths. The primary mantle minerals such as orthopyroxene, olivine and chromian spinel in Arais serpentinites are still preserved. The orthopyroxene has high Mg# [=Mg/(Mg + Fe2+)], ~0.923 on average. It shows intra-grain chemical homogeneity and contains, on average, 2.28 wt.% A12O3, 0.88 wt.% Cr2O3 and 0.53 wt.% CaO, similar to primary orthopyroxenes in modern forearc peridotites. The olivine in harzburgites has lower Fo (93-94.5) than that in dunites (Fo94.3-Fo95.9). The Arais olivine is similar in NiO (0.47 wt.% on average) and MnO (0.08 wt.% on average) contents to the mantle olivine in primary peridotites. This olivine is high in Fo content, similar to Mg-rich olivines in ANS ophiolitic harzburgites, because of its residual origin. The chromian spinel, found in harzburgites, shows wide ranges of Cr#s [=Cr/(Cr + Al)], 0.46-0.81 and Mg#s, 0.34-0.67. The chromian spinel in dunites shows an intra-grain chemical homogeneity with high Cr#s (0.82-0.86). The chromian spinels in Arais peridotites are low in TiO2, 0.05 wt.% and YFe [= Fe3+/(Cr + Al + Fe3+)], ~0.06 on average. They are similar in chemistry to spinels in forearc peridotites. Their compositions associated with olivine’s Fo suggest that the harzburgites are refractory residues after high-degree partial melting (mainly ~25-30 % partial melting) and dunites are more depleted, similar to highly refractory peridotites recovered from forearcs. This is in accordance with the partial melting (>20 % melt) obtained by the whole-rock Al2O3 composition. The Arais peridotites have been possibly formed in a sub-arc setting (mantle wedge), where high degrees of partial melting were available during subduction and closing of the Mozambique Ocean, and emplaced in a forearc basin. Their equilibrium temperature based on olivine-spinel thermometry ranges from 650 to 780 °C, and their oxygen fugacity is high (?log ƒO2 = 2.3 to 2.8), which is characteristic of mantle-wedge peridotites. The Arais peridotites are affected by secondary processes forming microinclusions inside the dunitic olivine, abundances of carbonates and talc flakes in serpentinites. These microinclusions have been formed by reaction between trapped fluids and host olivine in a closed system. Lizardite and chrysotile, based on Raman analyses, are the main serpentine minerals with lesser antigorite, indicating that serpentines were possibly formed under retrograde metamorphism during exhumation and near the surface at low T (<400 °C).

Khedr, Mohamed Zaki; Arai, Shoji

2013-10-01

228

Modeling Enhanced In Situ CO2 Mineralization in the Samail Ophiolite Aquifer  

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

229

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

230

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

231

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

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

232

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

NASA Astrophysics Data System (ADS)

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

Ghosh, Biswajit; Morishita, Tomoaki; Bhatta, Koyel

2013-04-01

233

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

234

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

235

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

236

Direct evidence for dynamic magma supply fossilized in the lower oceanic crust of the Troodos ophiolite  

NASA Astrophysics Data System (ADS)

Temporal and spatial variabilities of mantle upwelling and melt supply in mid-ocean ridges (MORs) have long been documented. Such variabilities span a range of scales and have a profound effect on the structure as well as the composition of the oceanic crust. Previous seismic and gravity studies have suggested that the lower oceanic crust plays a major role in accommodating these changes in melt supply. Here we report the first direct evidence for a sharp transition from coherent sub-horizontal to near vertical magma flows frozen in the lower oceanic crust of the Troodos ophiolite at the segment edge near a fossil ridge-transform intersection. We constrain the preferred petrofabric lineation directions at 13 gabbroic sites using anisotropy of magnetic susceptibility (AMS) verified by electron backscatter diffraction. Pre-emplacement accretion-related rotations were corrected using magnetic remanence directions. We identify two provinces of nearly uniform susceptibility directions (principal axes) and attribute them to two magmatic episodes. A more focused mantle upwelling and melting episode near the segment midpoint may have resulted in lower crustal lateral magma flows along the fossil segment-edge, whereas uniform mantle upwelling and melt supply along the entire axis may have resulted in vertical magma flows at the segment-edge. Overall, our data verify the vital role of the lower oceanic crust in accommodating changes in mantle upwelling and melt supply beneath MORs.

Granot, Roi; Abelson, Meir; Ron, Hagai; Lusk, Matthew W.; Agnon, Amotz

2011-08-01

237

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

SciTech Connect

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

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

1985-01-01

238

The chemical characterization and tectonic significance of ophiolite terrains in southeastern Papua New Guinea  

NASA Astrophysics Data System (ADS)

The evolution of the southeastern Gondwana margin in the Papua New Guinea segment is manifested by ophiolites signifying plate collision, volcanic arcs marking subduction, and metamorphic core complexes that have developed during extensional events and exhumation associated with sea floor spreading in the Woodlark Basin to the east. In detail the Papuan Ultramafic Belt marks a well-documented collision between continental crust and a subduction system. However, to the southeast, an extensive sequence of basaltic rocks known as the Milne Terrain is more problematic. Geochemical data indicate that these upper Cretaceous and Eocene rocks have MORB-type affinities, and their most likely tectonic association is with the opening of the Coral Sea Basin. Milne Terrain rocks represent the lower plate in the obduction system along which the Papuan Ultramafic Belt was emplaced, and thus they are the structural equivalent of the continental crust which was separated from the Australian continental block by the opening of the Coral Sea. Spectacular uplift (>4 km) of the oceanic basaltic crust of the Milne Terrain may be due to the underlying presence of underplated material associated with a Late Miocene-Pliocene episode of subduction immediately prior to the encroachment of the Woodlark spreading center into the Papuan area.

Smith, Ian E. M.

2013-03-01

239

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

240

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

USGS Publications Warehouse

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

Brouxel, M.

1991-01-01

241

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

USGS Publications Warehouse

30 samples of chromitite and chromite-rich rocks from two stratigraphic sections, 250 km apart, through the basal ultramafic member of the Samail ophiolite were spectrographically analysed for platinum-group elements (PGE) and for Co, Cu, Ni and V. These data are reported as are Cr/(Cr + Al), Mg/(Mg + Fe) and wt.% TiO2 for most samples. The chromitite occurs as pods or lenses in rocks of mantle origin or as discontinuous layers at the base of the overlying cumulus sequence. PGE abundances in both sections are similar, with average contents in chromite-rich rocks: Pd 8 ppb, Pt 14 ppb, Rh 6 ppb, Ir 48 ppb and Ru 135 ppb. The PGE data, combined with major-element and petrographic data on the chromitite, suggest: 1) relatively larger Ir and Ru contents and highest total PGE in the middle part of each section; 2) PGE concentrations and ratios do not correlate with coexisting silicate and chromite abundances or chromite compositions; 3) Pd/PGE, on average, increases upward in each section; 4) Samail PGE concentrations, particularly Rh, Pt and Pd, are lower than the average values for chromite-rich rocks in stratiform intrusions. 2) suggests that PGEs occur in discrete alloy or sulphide phases rather than in the major oxides or silicates, and 4) suggests that chromite-rich rocks from the oceanic upper mantle are depleted in PGE with respect to chondrites. L.C.C.

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

1982-01-01

242

Hydrothermal discharge zones beneath massive sulfide deposits mapped in the Oman ophiolite  

SciTech Connect

The area in the Oman ophiolite containing the volcanic-hosted Bayda and Aarja massive sulfide deposits exposes a cross section of ocean crust and reveals to an unprecedented extent the fossil zones of hydrothermal upwelling that fed these sea-floor deposits. The fossil discharge zones are elongate areas of alteration and mineralization characterized by numerous small (meters to tens of meters in length), linear, discontinuous gossans. The gossans result from oxidation of hydrothermal pyrite replacing primary igneous phases and filling voids and fractures in the altered host rocks. The two deposits have separate discharge zones that appear to be sub-sea-floor extensions of their stockworks. The Bayda zone extends through the volcanic section into the upper sheeted dike complex and is interpreted as having formed on the ridge crest above an axial magma chamber; the Aarja zone terminates against a plagiogranite pluton that intrudes the lower volcanic section and is thought to have formed after Bayda in an off-axis environment. Structural, stratigraphic, and compositional characteristics of the Bayda and Aarja massive sulfide bodies are consistent with this interpretation. The geometry of the discharge zones suggests that in both cases upflow occurred in broad zones (at least 400-600 m wide) that were elongated along strike (i.e., parallel to the spreading axis).

Haymon, R.M. (Univ. of California, Santa Barbara (USA)); Koski, R.A. (Geological Survey, Menlo Park, CA (USA)); Abrams, (California Institute of Technology, Pasadena (USA))

1989-06-01

243

Stratigraphy and radiolarians of upper cretaceous sedimentary cover of the Arakapas ophiolite massif (Cyprus)  

NASA Astrophysics Data System (ADS)

In the basal interval, sedimentary cover of the Arakapas ophiolite massif (southern Cyprus) is composed of metalliferous sediments of the Perapedhi Formation that is divided into three sequences based on diverse radiolarian assemblages. These are basal umbers of the Cenomanian age presumably (2-20 m), interlayering cherts and umbers of the Turonian-Coniacian (6-10 m), and opoka-like cherts of the Coniacian-Santonian. Higher in the succession, there are olistostrome deposits of the Moni Formation, which unconformably rest on the eroded underlying strata. In this formation also divisible into three sequences, the lower one 200 to 300 m thick is composed of variegated, presumably Campanian silty clays containing olistoliths of basic, presumably Upper Triassic volcanics, Lower Cretaceous sandstones, and opoka-like cherts and cherty limestones of the Albian-lower Cenomanian. Next sequence (100-200 m) is represented by alternation of variegated silty and green bentonitic clays of the Campaian, which enclose frequent olistoliths and horizons of fine-clastic olistostrome breccias. The upper sequence of upper Campanian-lower Maastrichtian bentonitic clays (50-100 m) contains interlayers of ash tuffs and clayey cherty sediments. Carbonate deposits of the upper Maastrichtian-Paleogene, conformably overlie the Moni Formation.

Bragina, L. G.; Bragin, N. Yu.

2006-10-01

244

Mineral inclusions in corundum from the chromitites of the Luobusa ophiolite, Tibet  

NASA Astrophysics Data System (ADS)

Unusual mantle peridotites and chromitites with ultrahigh pressure (UHP) minerals have been identified in the Luobusa ophiolite of Tibet, suggesting a depth of formation >300 km. Heavy mineral separates from these rocks contain not only UHP minerals such as diamonds and moissanite but a wide range of mantle minerals including native Fe, Si, Ti; metal alloys such as Fe-Ni, Fe-Ni-Cr, and Mn-Ni; oxides such as FeO, Fe2O3, MgO, rutile, ilmenite and corundum; sulfides including pyrite, sphalerite, nickel sulfide, bismuth sulphide and tetrahedrite; and silicates, tungstates, phosphates and carbonates. Preliminary study of corundum grains from the chromitites shows that they contain abundant inclusions including native Ti, Ti-C, TiO2, Ti-Si, Si-Ti-Fe-Cr, Ti-B, Ti-N, Si-Ti-Fe, Ti-P-Si, silicates and silicates containing REEs. The inclusion mineral assemblage in the corundum is generally comparable to that derived by heavy mineral separation of the chromitites. Some of the unusual inclusions, such as Ti-N, Ti-C and Ti-B, range up to 50 ?m in size. TiN and BN were previously identified as inclusions coesite from the Luobusa chromitite (Dobrzhinetskaya, 2009). The new discovery of nitrides as inclusions corundum may offer a new window into the deep mantle and the distribution of nitrogen in Earth.

Xu, X.; Yang, J.; Guo, G.; Xiong, F.

2012-12-01

245

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

NASA Astrophysics Data System (ADS)

Metalliferous sediments of the Triassic siliceous formation of the Sikhote-Alin (manganese-silicate rocks and cherts with dispersed rhodochrosite, silicate-magnetite ores, and jasper) and skarns of the Dalnegorsk and Olginsk ore districts were initially the wash away products (Late Anisian-end of the Triassic) of the lateritic weathering crust on ophiolite in the islands. Manganese, iron, and other metals were deposited in the sediments of both lagoons (present-day, skarns) and island water areas (manganese-silicate and siliceousrhodochrosite rocks, silicate-magnetite ores, and jasper). Skarns contain boric and polymetallic ores thus indicating the occurrence of both shallow (periodically drying up) and quite deep (with hydrogen sulfide contamination zones) lagoons. Lead was deposited in protoliths of the skarn deposits in lagoons from the beginning of the Carboniferous to the beginning of the Late Anisian (initial island submergence). Tin, tin-leadzinc (with Ag), and silver-lead-zinc (with Sn and Au) vein deposits (Late Cretaceous-Paleogene) of the Taukha and Zhuravlevka Terrains contain lead deposited in the sediments flanking the islands of water areas with the hydrogen sulfide contamination zones, in the Carboniferous-Permian and Triassic metalliferous sediments.

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

2012-06-01

246

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

247

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

248

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

249

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

250

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

251

Listvenite from the Oman Ophiolite: complete carbonation of peridotite during hydrothermal alteration by CO2-rich fluids  

NASA Astrophysics Data System (ADS)

Hydrothermal alteration of peridotite in the Oman Ophiolite has resulted in the formation of listvenite, characterized by complete carbonatization, in which all of the Mg and Ca and much of the Fe has been incorporated into carbonate minerals. Here we examine chemical and isotopic evidence for the temperatures, timing, and fluid composition involved in the formation of this listvenite. Listvenites occur within 500m of the basal thrust of the ophiolite, frequently along the contact with underlying metasediments, but also as tabular bodies or bands, ~5-100m thick, within partially serpentinized peridotite. The listvenites are composed primarily of magnesite (and/or dolomite) + quartz + relict Cr-spinel. Clumped isotope data from magnesite and dolomite suggest that carbonate phases within the listvenite formed at peak temperatures ~150-170C. At these temperatures, thermodynamic stability of the magnesite + quartz assemblage (relative to talc + magnesite) requires that these minerals formed at relatively high PCO2. These CO2-rich fluids were likely derived from metamorphic dehydration of the underlying sediment during emplacement of the ophiolite. Carbon and oxygen stable isotopes are fairly uniform between listvenite samples and are consistent with values in some of the underlying metasediments. Initial 87Sr/86Sr values in the listvenite vary from 0.7085 to 0.7135, mostly significantly higher than seawater values, and are consistent with values within the underlying metasediments, suggesting mass-transfer. Trace and major element patterns are also suggestive of mass-transfer from underlying clastic sediments and limestones. Although the major mineral assemblages do not vary much between samples, bulk composition indicates that some listwanites have undergone significant silicification in addition to carbonation, while others have undergone very little mass transfer beyond the addition of CO2.

Streit, E.; Kelemen, P. B.; Eiler, J. M.; Goldstein, S. L.

2012-12-01

252

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

NASA Astrophysics Data System (ADS)

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

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

2011-10-01

253

Variation of magma source in the Oman ophiolite inferred from distribution of magmatic dykes in the mantle  

NASA Astrophysics Data System (ADS)

Magmatic relics are ubiquitous in the mantle of the Oman ophiolite. They include discordant and concordant dykes showing sharp contact with their host. Their lithological nature is extremely variable but can be classified into four lithological types: troctolites, olivine gabbros, pyroxenites and gabbronorites with euhedral Opx (i.e. where Opx appears early in the crystallisation sequence). The distribution map of the magmatic dykes show that troctolites and olivine gabbros crop out in two localized area, the main one is centred on the Maqsad mantle diapir. Pyroxenites are highly dominant in the mantle section of the Oman ophiolite but Opx-rich concordant and discordant dykes are abundant only in the northern massifs. On the other hand, these features are scarce or absent in the southern massifs where troctolite and olivine gabbro dykes are abundant. The dykes chemical composition show that they can be linked to two different magmatic series: one is similar to a typical MOR magma, and the other, richer in Si and poor in incompatible elements, is closer to an andesitic to boninitic magma. The dykes lithological nature is partly related to the nature of the parental magma and troctolites and Opx-free olivine gabbros crystallised exclusively from the MOR magma. However, pyroxenites are ubiquitous and may have crystallised from both types of magma. This is in particular reflected in Cr-spinel chemistry, which Cr# is higher and Ti contents lower, and the higher amount of Opx when the parental magma is andesitic. The dyke distribution all over the mantle section of the Oman ophiolite show a clear magmatic dichotomy between the southern and northern parts of the ophiolite. MOR-type magma was preponderant southward to the Nakhl massif while the northern areas are dominated by andesitic to boninitic magma. Mineral chemistry in the lower crust show characteristics intermediate between the two types of melt that circulated in the mantle, suggesting that magma mixing hardly occur in the mantle but may be an important process of ocean crust building. In the lower crust, the most MOR-like gabbros are to the area close to the Maqsad mantle diapir and the andesitic end-member becomes dominant in the northernmost massif, far from the Maqsad diapir. This North-South chemical gradation shows that mixing occurs along the ridge axis, implying that magma can circulate on significant distances along the ridge axis at crustal level.

Python, Marie; Abily, Bénédicte; Ceuleneer, Georges; Arai, Shoji

2014-05-01

254

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

255

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

NASA Astrophysics Data System (ADS)

The southern Apennine chain is a fold-and thrust belt formed between the upper Oligocene and Quaternary as a result of the convergence between the African and European plates and a simultaneous SE-directed rollback of the Ionian subducting lithospere. The ophiolitic sequences, which are part of the Southern Apennines, are remnants of the Ligurian oceanic lithosphere pertaining to the Jurassic western Tethys. The Liguride Units of the Southern Apennines include sequences characterized by an HP/LT metamorphic overprint in the Frido Unit and sequences lacking orogenic metamorphism North-Calabria Unit. The ophiolitic rocks occurring in the Frido Unit include serpentinite derived from a lherzolitic to harzburgitic mantle, as suggested by microstructural and petrographical features. The serpentinites are frequently associated to tectonic slices and dykes composed of diabase and medium to high-grade metamorphic rocks such as amphibolites, gneiss, granofels as well as gabbros and basalts with a pillow structure. The studied serpentinites of the Frido Unit show mesh, xenomorphic and mylonitic texture. Primary mantle minerals are represented by olivine, orthopyroxene, clinopyroxene and spinel. Pseudomorphic minerals are serpentine, magnetite and tremolite. Olivine is replaced by serpentine forming a mesh texture; orthopyroxene is mostly altered to bastite and in some cases shows exsolution lamellae of clinopyroxene and kink bands. Clinopyroxene is armoured by a tremolite rim. Spinel shows a holly-leaf habit and is often armoured by a corona of Cr-chlorite. The core of the analysed spinel has a Cr-Al spinel composition corresponding to chromite (Al2O3=29-31 wt %; Cr2O3= 28-37 wt%), whereas the rim has a Fe-Cr spinel composition corresponding to ferritchromite (Al2O3= 1-2% wt; Cr2O3=28-30 wt %). The Cr-Al spinel/ferritchromite ratio may be various in different spinel porphyroclasts. Serpentine has a fibrous stretched subidiomorphic habit, it is colourless or pale green. Tremolite is present as nematoblasts associated with orthopyroxene. Magnetite replaces spinel or occurs within the mesh textured serpentine. The metamorphic assemblages in the Frido Unit serpentinites allowed to infer the physical conditions operating during serpentinization. The mineralogical assemblages found are typical of the amphibolite facies, greenschist-amphibolite transition and greenschist facies conditions. Serpentinites are cut by veins filled with mineralogical assemblages typical of prehnite-pumpellyite facies likely related to the later orogenic Apennine evolution. The geochemical features of serpentinites show differences in compositions with respect to the Primitive Upper Mantle (PUM). These are likely related to serpentinization processes, since elements normalised to PUM show different trends, comparable to Residual MORB Mantle and to Primitive Upper Mantle respectively. HP/LT metamorphic conditions can be documented in mafic dykes enclosed in serpentinites, but similar conditions are not recorded in serpentinites. This suggests that P-T conditions during the orogenic event were not able to produce a HP/LT mineral assemblage in serpentinites.

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

2010-05-01

256

Melt Impregnation, Strain Localization, and Deformation Mechanisms in a Fossil Oceanic Fracture Zone (Ingalls Ophiolite)  

NASA Astrophysics Data System (ADS)

A steep mantle shear zone that deforms ultramafic rocks of the Jurassic Ingalls ophiolite is inferred to preserve a record of the interplay of melt impregnation, strain localization, and switching deformation mechanisms in a fossil oceanic fracture zone. This ~2-km-wide, E-W-striking shear zone separates harzburgite and dunite on the south from lherzolite and cpx-rich harzburgite on the north. Geochemical data from the lherzolite, which contains veinlets of plagioclase and cpx, suggest impregnation by infiltrating basaltic melt. The shear zone reworks the lherzolite unit, but also contains widespread plagioclase peridotite and hornblende peridotite, and shear-zone mylonites are less depleted than the adjacent units. Olivine is reduced in average grain size from 1.5-3.0 mm in the lherzolites to 50 µm in some mylonites. In the mylonites, opx and cpx porphyroclasts are set in a mosaic of olivine, cpx, opx +/- hornblende +/- plagioclase. Lattice preferred orientations (LPO) determined by EBSD indicate that olivine in the dunite-harzburgite and lherzolite units deformed by glide on [100] (010), a common mechanism for dislocation creep in the upper mantle. In contrast, olivine in the mylonites has much weaker, poorly defined LPOs. This weakening of the LPO and the microstructures are compatible with dynamic recrystallization and grain-size reduction resulting from dislocation creep leading to a change to a grain-size-sensitive deformation mechanism. We suggest that impregnation by infiltrating melts may have helped localize strain, and the formation of multiple phases, in part as a result of impregnation, may have stabilized the small olivine grain size.

Miller, R. B.; Gordon, S. M.

2010-12-01

257

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

NASA Astrophysics Data System (ADS)

Mars appears to have experienced little compositional differentiation of primitive lithosphere, and thus much of the surface of Mars is covered by mafic lavas. On Earth, mafic and ultramafic rocks present in ophiolites, oceanic crust and upper mantle that have been obducted onto land, are therefore good analogs for Mars. The characteristic mineralogy, aqueous geochemistry, and microbial communities of cold-water alkaline springs associated with these mafic and ultramafic rocks represent a particularly compelling analog for potential life-bearing systems. Serpentinization, the reaction of water with mafic minerals such as olivine and pyroxene, yields fluids with unusual chemistry (Mg-OH and Ca-OH waters with pH values up to ˜12), as well as heat and hydrogen gas that can sustain subsurface, chemosynthetic ecosystems. The recent observation of seeps from pole-facing crater and canyon walls in the higher Martian latitudes supports the hypothesis that even present conditions might allow for a rock-hosted chemosynthetic biosphere in near-surface regions of the Martian crust. The generation of methane within a zone of active serpentinization, through either abiogenic or biogenic processes, could account for the presence of methane detected in the Martian atmosphere. For all of these reasons, studies of terrestrial alkaline springs associated with mafic and ultramafic rocks are particularly timely. This study focuses on the alkaline Adobe Springs, emanating from mafic and ultramafic rocks of the California Coast Range, where a community of novel bacteria is associated with the precipitation of Mg-Ca carbonate cements. The carbonates may serve as a biosignature that could be used in the search for evidence of life on Mars.

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

2009-05-01

258

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

SciTech Connect

Mars appears to have experienced little compositional differentiation of primitive lithosphere, and thus much of the surface of Mars is covered by mafic lavas. On Earth, mafic and ultramafic rocks present in ophiolites, oceanic crust and upper mantle that have been obducted onto land, are therefore good analogs for Mars. The characteristic mineralogy, aqueous geochemistry, and microbial communities of cold-water alkaline springs associated with these mafic and ultramafic rocks represent a particularly compelling analog for potential life-bearing systems. Serpentinization, the reaction of water with mafic minerals such as olivine and pyroxene, yields fluids with unusual chemistry (Mg-OH and Ca-OH waters with pH values up to {approx}12), as well as heat and hydrogen gas that can sustain subsurface, chemosynthetic ecosystems. The recent observation of seeps from pole-facing crater and canyon walls in the higher Martian latitudes supports the hypothesis that even present conditions might allow for a rockhosted chemosynthetic biosphere in near-surface regions of the Martian crust. The generation of methane within a zone of active serpentinization, through either abiogenic or biogenic processes, could account for the presence of methane detected in the Martian atmosphere. For all of these reasons, studies of terrestrial alkaline springs associated with mafic and ultramafic rocks are particularly timely. This study focuses on the alkaline Adobe Springs, emanating from mafic and ultramafic rocks of the California Coast Range, where a community of novel bacteria is associated with the precipitation of Mg-Ca carbonate cements. The carbonates may serve as a biosignature that could be used in the search for evidence of life on Mars.

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

2008-10-01

259

Petrogenesis and tectono-magmatic significance of basalts and mantle peridotites from the Albanian-Greek ophiolites and sub-ophiolitic mélanges. New constraints for the Triassic-Jurassic evolution of the Neo-Tethys in the Dinaride sector  

NASA Astrophysics Data System (ADS)

The Albanide-Hellenide ophiolites and related ophiolitic mélanges include eight different types of volcanic and subvolcanic rocks: 1) Triassic, within-plate alkaline rocks (WPB); 2) Triassic high-Ti mid-ocean ridge basalts showing enriched compositions (E-MORB); 3) Triassic and Jurassic high-Ti mid-ocean ridge basalts showing normal compositions (N-MORB); 4) Jurassic basalts with geochemical features between MORB and island arc tholeiites; hereafter defined as medium-Ti basalts (MTB); 5) Jurassic low-Ti, island arc tholeiitic (IAT) rocks; 6) Jurassic very low-Ti (boninitic) rocks; 7) Jurassic backarc basin basalts and basaltic andesites (BABB); 8) Triassic and Jurassic calc-alkaline rocks (CAB). The geochemical and petrogenetic features of these rock-types, as well as the results from REE modelling of mantle sources, primary melt generation, and mantle residua indicate that they have formed in distinct tectonic settings within an oceanic environment. Both Triassic and Jurassic N-MORBs primary magmas derived from ~ 10 to 20% partial melting of a primitive asthenosphere, whereas Triassic alkaline WPB basalts originated from low degrees of partial melting of an OIB-type mantle source and were most likely erupted in seamounts. Triassic E-MORBs originated from ~ 12% partial melting of a primitive asthenosphere influenced by the OIB-type component. The residual MORB mantle is represented by depleted lherzolites, which are commonly found in the Albanide-Hellenide ophiolites. Mid Jurassic MTB and IAT primary magmas derived from ~ 10% and 10-20% partial melting of the MORB residual mantle, respectively with the variable addition of subduction components and were erupted in an intra-oceanic, supra-subduction zone setting. The residual mantle associated with these magmatic events is represented by harzburgites. Mid Jurassic boninitic primary magmas may have originated either from 10 to 20% partial melting of the MTB and IAT residual mantle or from ~ 30% partial melting of the MORB residual mantle. In both cases, the depleted mantle sources were enriched in light rare earth elements (LREE) by subduction-derived fluids. The extremely depleted harzburgites, which are widespread in the Albanide-Hellenide ophiolites, are interpreted as the residual mantle associated with boninite formation. Mid-Late Jurassic CABs originated from ~ 15 to 20% partial melting of a depleted peridotite mantle significantly enriched in Th and LREE by subduction-derived fluids, whereas BABBs originated from 10 to 20% partial melting of a primitive asthenosphere somewhat enriched in Th and LREE by a nearby subduction. Both these rock-types were erupted in a continental arc-backarc setting. The different rock-types of the Albanide-Hellenide ophiolites record the fundamental stages of the Triassic-Jurassic evolution of the Neo-Tethys in the Dinaride sector: from sea-floor spreading, after continental break-up, to intra-oceanic subduction initiation and supra-subduction zone (SSZ) lithospheric accretion.

Saccani, Emilio; Beccaluva, Luigi; Photiades, Adonis; Zeda, Ottavia

2011-06-01

260

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

261

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

USGS Publications Warehouse

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

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

1988-01-01

262

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

263

New 40Ar\\/39Ar Ages, Biostratigraphic and Geochemical Data from the Sabzevar Ophiolite, North Central Iran: Implications for Tectonic of Iranian Plate  

Microsoft Academic Search

The igneous rocks of the Sabzevar ophiolite in north central Iran composed of peridotites, serpentinite, minor pyroxenite, gabbros, and a volcanic sequence that exhibits a wide range of composition from basalts to basaltic andesites to rhyodacite-dacites, rhyolites and basanites. Sedimentary rocks include a variety of Upper Triassic to Lower Cretaceous deep- and shallow-marine rocks. These include pelagic fossiliferous carbonates, which

A. Hassanipak; M. Kariminia; K. Mobasher; M. Ghazi

2003-01-01

264

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

Microsoft Academic Search

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

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

2008-01-01

265

Variation of cooling rate with depth in lower crust formed at an oceanic spreading ridge: Plagioclase crystal size distributions in gabbros from the Oman ophiolite  

Microsoft Academic Search

(1) Abstract: Analysis of crystal size distributions (CSD) of plagioclase in gabbros from the Oman ophiolite indicates that cooling rates in the lower crust beneath a medium- to fast-spreading oceanic ridge did not vary smoothly with depth. Cooling rates in the upper half of the gabbro section were approximately 1.5 -2 times faster than in the lower half of the

Carlos J. Garrido; Peter B. Kelemen; Greg Hirth

2001-01-01

266

Origin of gabbro sills in the Moho transition zone of the Oman ophiolite: Implications for magma transport in the oceanic lower crust  

Microsoft Academic Search

The Moho transition zone (MTZ) of the Oman ophiolite commonly includes a number of gabbro sills surrounded by dunites. The petrology and geochemistry of these sills are investigated to provide constraints on how magma migrates from the subridge mantle to the oceanic crust. The gabbro sills have millimeter-scale to tens of centimeter-scale modal layering that closely resembles layering in lower

Jun Korenaga; Peter B. Kelemen

1997-01-01

267

Neoproterozoic arc-back-arc system in the Central Eastern Desert of Egypt: Evidence from supra-subduction zone ophiolites  

NASA Astrophysics Data System (ADS)

Ophiolites are widely distributed in the Central Eastern Desert (CED) of Egypt, occurring as clusters in the northern (NCEDO) and southern (SCEDO) segments. Mineralogical and geochemical data on the volcanic sections of Wizer (WZO) and Abu Meriewa (AMO) ophiolites as representatives of the NCEDO and SCEDO, respectively, are presented. The WZO volcanic sequence comprises massive metavolcanics of MORB-like compositions intruded by minor boninitic dykes and thrust over island-arc metavolcanic blocks in the mélange matrix. Such transitional MORB-IAT-boninitic magmatic affinities for the WZO metavolcanics suggest that they most likely formed in a protoarc-forearc setting. Chemical compositions of primary clinopyroxene and Cr-spinel relicts from the WZO volcanic section further confirm this interpretation. The compositional variability in the WZO volcanic sequence is comparable with the associated mantle rocks that vary from slightly depleted harzburgites to highly depleted harzburgites containing small dunite bodies, which are residues after MORB, IAT and boninite melt formation, respectively. Source characteristics of the different lava groups from the WZO indicate generation via partial melting of a MORB source which was progressively depleted by melt extraction and variably enriched by subduction zone fluids. MORB-like magma may have been derived from ~ 20% partial melting of an undepleted lherzolite source, leaving slightly depleted harzburgite as a residuum. The generation of island-arc magma can be accounted for by partial melting (~ 15%) of the latter harzburgitic mantle source, whereas boninites may have been derived from partial melting (~ 20%) of a more refractory mantle source previously depleted by melt extraction of MORB and IAT melts, leaving ultra-refractory dunite bodies as residuum. The AMO volcanic unit occurs as highly deformed pillowed metavolcanic rocks in a mélange matrix. They can be categorized geochemically into LREE-depleted (La/Yb CN = 0.41-0.50) and LREE-enriched (La/Yb CN = 4.7-4.9) lava types that show an island arc to MORB geochemical signature, respectively, signifying a back-arc basin setting. This is consistent, as well, with their mantle section. Source characteristics indicate depleted to slightly enriched mantle sources with overall slight subduction zone geochemical affinities as compared to the WZO. Generally, CED ophiolites show supra-subduction zone geochemical signature with prevalent island arc tholeiitic and minor boninitic affinities in the NCEDO and MORB/island-arc association in the SCEDO. Such differences in geochemical characteristics of the NCEDO and SCEDO, along with the abundance of mature island arc metavolcanics which are close in age (~ 750 Ma) to the ophiolitic rocks, general enrichment in HFSE of ophiolites from north to south, and lack of a crustal break and major shear zones, is best explained by a geotectonic model whereby the CED represents an arc-back-arc system above a southeast-dipping subduction zone.

Farahat, E. S.

2010-12-01

268

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

269

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

NASA Astrophysics Data System (ADS)

Serpentinization processes at slow- and ultraslow-spreading ridges control the exchange of various elements between seawater and the oceanic lithosphere and play a major role in marine geochemical cycles. We use opaque mineral assemblages and sulfur isotope geochemistry to reconstruct variations in fluid fluxes, redox conditions and microbial activity prevailing during serpentinization and carbonate precipitation of serpentinites and ophicalcites from an ophiolite sequence in the Northern Apennines (Italy). We then compare our results with calcite-veined serpentinites from the Iberian Margin formed during the opening of the North Atlantic, and with serpentinites sampled at the active peridotite-hosted Lost City hydrothermal field on the Atlantis Massif near the Mid-Atlantic Ridge. The sulfide and oxide mineral assemblage of the serpentinites from the Northern Apennines is dominated by pyrite, pentlandite, millerite, siegenite, magnetite, and hematite, reflecting relatively oxidizing conditions, while the sulfur geochemistry reveals a dominance of sulfide sulfur over sulfate sulfur. ?34S values of sulfide and sulfate indicate that microbial sulfate reduction, leaching, and oxidation are the main processes that affected the sulfur isotope signature of these serpentinites. The opaque mineralogy in the serpentinites from the Northern Apennines is similar to that of the Iberian Margin and the southern wall of the Atlantis Massif, and generally represents the late stages of serpentinization, where intense fluid circulation leads to fairly oxidizing conditions and alkaline fluids lead to carbonate precipitation. However, the mineral assemblages also indicate strong fluctuations in oxygen fugacity, likely caused by volume expansion during serpentinization and tectonic activity along a mid-ocean ridge. Additionally, local enrichment in 34S in sulfides suggests that fluids interacted at depth with gabbros and subsequently circulated along shear zones, similar to observations along detachment fault surfaces at the Atlantis Massif. Our results, therefore, give further evidence that the ophicalcites and serpentinites from the Northern Apennines formed in a tectonic setting similar to the Atlantis Massif. We estimate a global annual uptake of seawater sulfate of 5.6 to 12 × 1011 g S by serpentinization of ultramafic rocks exposed to seawater on the ocean floor. We suggest that the thermal structure of the exposed mantle rocks and, in turn, the efficiency of microbes to reduce seawater sulfate to sulfide is a major factor controlling the storage of sulfur in serpentinites. Thus, cycling of sulfur in peridotite-hosted hydrothermal systems during the Jurassic was probably very similar to the processes observed today in systems along the Mid-Atlantic Ridge.

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

2012-08-01

270

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

NASA Astrophysics Data System (ADS)

The gabbroic bodies from the Jurassic Ligurian ophiolites are structurally and compositionally similar to the gabbroic sequences from the oceanic core complexes of the Mid Atlantic Ridge. Initial cooling of the Ligurian gabbros is associated with local development of hornblende-bearing felsic dykes and hornblende vein networks. The hornblende veining is correlated with the widespread development of hornblende as coronas/pseudomorphs after the clinopyroxene in the host gabbros. In addition, the studied gabbroic body includes a mantle sliver locally containing hornblende gabbros and hornblendite veins. The hornblendes from the felsic dykes and the hornblende-rich rocks within the mantle sliver show a similar geochemical signature, characterized by low Mg#, CaO and Al2O3, negligible Cl, and high TiO2, K2O, REE, Y, Zr and Nb concentrations. The whole-rock Sm-Nd isotopic compositions of the felsic dykes and the hornblende-rich rocks define a Sm-Nd isochron corresponding to an age of 154 ± 20 Ma and an initial ?Nd of 9.2 ± 0.5. The ?18O of the hornblendes and coexisting zircons from these rocks (about +4.5‰ and +5.8‰, respectively) do not indicate the presence of a seawater component in these melts. The formation of the felsic dykes and of the hornblende-rich rocks within the mantle sliver involved SiO2-rich silicate melts with negligible seawater component, which presumably were derived from high degree fractional crystallization of MOR-type basalts. The vein and the coronitic/pseudomorphic hornblendes show high Mg# and CaO, significant Cl (0.02-0.17 wt%) and low TiO2 and K2O concentrations. The coronitic/pseudomorphic hornblendes have trace element compositions similar to those of the clinopyroxenes from the gabbros and ?18O values (+1.0‰ to 0.7‰) close to seawater, suggesting an origin by reaction between migrating seawater-derived fluids and the host gabbros. The vein hornblendes commonly show slight LREE enrichment, relatively high concentrations of Nb (up to 2.5 ppm) and ?18O ranging from +3.7‰ to +0.8‰. The crystallization of these hornblendes most likely involved both seawater and magmatic components.

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

2014-04-01

271

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

272

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

NASA Astrophysics Data System (ADS)

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

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

2011-06-01

273

Melanges Pedagogiques (Pedagogical Mixture), 1983.  

ERIC Educational Resources Information Center

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

Melanges Pedagogiques, 1983

1983-01-01

274

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

275

Melanges Pedagogiques (Pedagogical Mixture), 1984.  

ERIC Educational Resources Information Center

The 1984 issue of the journal on second language teaching and learning contains five articles in French and two in English. These include the following: "Enseignement individuel vs. enseignement de masse (Individual Instruction vs. Large Group Instruction)" (Jacqueline Billant, Pascal Fade); "Systemes 'autonomisants' d'apprentissage des langues…

Melanges Pedagogiques, 1984

1984-01-01

276

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

NASA Astrophysics Data System (ADS)

The Bracco-Levanto ophiolitic complex (Eastern Liguria) represents one of the largest and better-exposed ophiolitic successions in the Northern Apennines. It is considered to be a fragment of heterogeneous Jurassic lithosphere that records tectono-magmatic and alteration histories similar to those documented along the Mid-Atlantic Ridge, such as at the 15°20'N area and the Atlantis Massif at 30°N. Structural and petrological studies on these rocks provide constraints on metamorphic/deformation processes during formation and hydrothermal alteration of the Jurassic oceanic lithosphere. We present a petrological and geochemical study of deformation processes and fluid-rock interaction in the Bracco-Levanto ophiolitic complex and compare these to modern oceanic hydrothermal systems, such as the Lost City Hydrothermal Field hosted in ultramafic rocks on the Atlantis Massif. A focus is on investigating mass transfer and fluid flow paths during high and low temperature hydrothermal activity, and on processes leading to hydrothermal carbonate precipitation and the formation of ophicalcites, which are characteristic of the Bracco-Levanto sequences. Major element and mineral compositional data allow us to distinguish a multiphase history of alteration characterized by: (1) widespread SiO2 metasomatism during progressive serpentinization, and (2) multiple phases of veining and carbonate precipitation associated with circulation of seawater and high fluid-rock ratios in the shallow ultramafic-dominated portions of the Jurassic seafloor. We observe regional variations in MgO, SiO2 and Al2O3, suggesting Si-flux towards stratigraphically higher units. In general, the ophicalcites have higher Si, Al and Fe concentrations and lower Mg than the serpentinite basement rocks or serpentinites with minimal carbonate veins. Bulk rock trace element data and Sr isotope ratios indicate seawater reacting with rocks of more mafic composition, then channeled towards stratigraphically higher units, leading to Si metasomatism in the serpentinites and ophicalcites. Channelling of Si-rich fluids is also indicated by amphibole and talc growth in shear zones and wall rock around the ophicalcites. ?18O-values of the carbonate veins indicate temperatures up to 150°C and document a decrease in temperature with ongoing serpentinization. Comparison with serpentinites from the Atlantis Massif and 15°20'N indicates a similar degree of Si enrichment in the modern seafloor and suggests that Si-metasomatism may be a fundamental process associated with serpentinization at slow-spreading ridge environments.

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

2014-05-01

277

Fault-related oceanic serpentinization in the Troodos ophiolite, Cyprus: Implications for a fossil oceanic core complex  

NASA Astrophysics Data System (ADS)

Ultramafic rocks are exposed at the core of a domal structure near a fossil ridge-transform intersection (RTI) in the Troodos ophiolite. A sequence of heavily serpentinized rocks occurs adjacent to a major axis-parallel fault, the Amiandos Fault (AF), which juxtaposes them against gabbro. Previously, serpentinization and faulting were not considered to be related to the Cretaceous ocean spreading history of the Troodos ophiolite, but instead were interpreted as associated with late, emplacement-related tectonics and diapirism. Unusually high ?18O values previously measured in Troodos serpentine (Srp) supported this view. Here, petrographic and isotope tracers ( ?D, ?18O) of water-rock interaction are examined in three profiles across the AF to determine the spatial distribution, temperature, and the type of water involved in serpentinization in the Troodos RTI. Complete serpentinization, widespread chrysotile veining and very high ?18O (Srp), 10.6 to 12.6‰, characterize the rocks along a 2.5 km long profile at the central part of the serpentinite exposure. Much lower ?18O(Srp) values, 4.6 to 6.6‰, were measured at the AF fault zone in the highly serpentinized 300 m-long northern profile. Hydrogen isotope ratios also spatially vary: from - 70 to - 86‰ and - 57 to - 75‰ in the central and northern profiles, respectively. The bimodal distribution of isotope ratios calls for two distinct serpentinization events: localized 'oceanic-type' hydrothermal (100-200 °C) alteration initiated by deep infiltration of seawater during seafloor spreading followed by pervasive 'ophiolite-type' low-temperature hydration and veining along the AF. Superposition of the two serpentinization events is evident at a third, 2 km long southernmost profile, where ?18O (Srp) values decrease gradually from 10.7‰ 2 km away from the fault to 5.0‰ at the AF fault zone. Post-magmatic decrease of ?18O(plagioclase) in gabbros in the footwall of the AF and the apparent lack of amphibolitization suggest high-temperature, off-axis gabbro-water interaction and focused fluid flow to the lower crust through the AF zone. These observations suggest that the Amiandos fault was active during seafloor spreading, possibly operating as a detachment fault in a core complex structure, exhuming progressively deeper levels of the oceanic lithosphere. This scenario is supported by additional observations such as the proximity to RTI and association with highly-rotated blocks in the sheeted dikes.

Nuriel, Perach; Katzir, Yaron; Abelson, Meir; Valley, John W.; Matthews, Alan; Spicuzza, Michael J.; Ayalon, Avner

2009-05-01

278

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

NASA Astrophysics Data System (ADS)

Oceanic core complexes (OCCs) are dome-shaped massifs commonly associated with the inside corners of the intersection of transform faults and slow (and ultra-slow) spreading centres. They represent the uplifted footwalls of large-slip oceanic detachment faults (e.g. Cann et al., 1997; Blackman et al., 1998) and are composed of mantle and lower crustal rocks exhumed during fault displacement (Smith et al., 2006, 2008). Recent paleomagnetic studies of core samples from OCCs in the Atlantic Ocean (Morris et al., 2009; MacLeod et al., in prep) have confirmed that footwall sections undergo substantial rotation around (sub-) horizontal axes. These studies, therefore, support “rolling hinge” models for the evolution of OCCs, whereby oceanic detachment faults initiate at a steep angle at depth and then “roll-over” to their present day low angle orientations during unroofing (Buck, 1988; Wernicke & Axen, 1988; Lavier et al., 1999). However, a fully integrated paleomagnetic and structural analysis of this process is hampered by the one-dimensional sampling provided by ocean drilling of OCC footwalls. Therefore, ancient analogues for OCCs in ophiolites are of great interest, as these potentially provide 3-D exposures of these important structures and hence a more complete understanding of footwall strain and kinematics (providing that emplacement-related phases of deformation can be accounted for). Recently, the relationship between outcropping crustal and upper mantle rocks led Tremblay et al. (2009) to propose that an OCC is preserved within the Mirdita ophiolite of the Albanian Dinarides (northern Albania). This is a slice of Jurassic oceanic lithosphere exposed along a N-S corridor which escaped the main late Cenozoic Alpine deformation (Robertson, 2002, 2004; Dilek et al., 2007). Though in the eastern portion of the Mirdita ophiolite a Penrose-type sequence is present, in the western portion mantle rocks are in tectonic contact with upper crustal lithologies. This main fault has been interpreted by Tremblay et al. (2009) as originally an oceanic detachment fault that exhumed mantle rocks and put them in contact with upper crustal basalts according to the “rolling-hinge” model. In order to test this model and document the kinematics of the proposed detachment fault, we carried out a preliminary paleomagnetic and structural sampling campaign in July 2010. The principal aims were: (i) to determine whether paleomagnetic remanences provide evidence for early relative rotation of footwall and hanging wall sequences either side of the proposed detachment that may be consistent with rolling-hinge models for OCCs; & (ii) to provide insights into the broader tectonic evolution of the Mirdita units. We collected c. 200 oriented samples at 32 localities distributed within a 30 km x 15 km area located between the Puka and Krabbi massifs, near the villages of Puka and Reps. Here we present the preliminary results of this study and discuss their geological implications for the history of the Mirdita ophiolite, including the interpretation of the Puka and Krabbi massifs as a fossil OCC and the primary orientation of the Mirdita spreading axis.

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

2010-12-01

279

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

280

Mineralogy and distribution of Platinum-Group Minerals (PGM) and other solid inclusions in the Faryab ophiolitic chromitites, Southern Iran  

NASA Astrophysics Data System (ADS)

High-Cr podiform chromitites hosted by upper mantle depleted harzburgite were investigated for PGM and other solid inclusions from Faryab ophiolitic complex, southern Iran. Chemical composition of the chromian spinels, Cr#[100*Cr/(Cr+Al) = 77-85], Mg# [100*Mg/(Mg+Fe2+) = 56-73], TiO2?0.25wt%, and the presence of abundant primary hydrosilicates included in the chromian spinels indicate that the deposits were formed from aqueous melt generated by high degree of partial melting in a suprasubduction zone setting. Solid phases hosted by chromian spinel grains from the Faryab ophiolitic chromitites can be divided into three categories: PGM, base-metal minerals and silicates. Most of the studied PGM occurred as very small (generally less than 20 ?m in size) primary single or composite inclusions of IPGE-bearing phases with or without silicates and base metal minerals. The PGM were divided into the three subgroups: sulfides, alloys and sulfarsenides. Spinel-olivine geothermometry gives the temperatures 1,131-1,177 °C for the formation of the studied chromitites. At those temperatures, fS2 values ranged from 10-3 to 10-1 and provided a suitable condition for Ru-rich laurite formation in equilibrium with Os-Ir alloys. Progressive crystallization of chromian spinel was accompanied by increase of fS2 in the melt. The formation of Os-rich laurite, erlichmanite and then sulfarsenides occurred by increase of fS2 and slight decrease in temperature of the milieu. The compositional and mineralogical determinations of PGM inclusions respect to their spatial distribution in chromian spinels show that the minerals regularly distributed within the chromitites, reflecting cryptic variation consistent with magmatic evolution during host chromian spinel crystallization.

Rajabzadeh, Mohammad Ali; Moosavinasab, Zohreh

2013-12-01

281

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

NASA Astrophysics Data System (ADS)

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

Cunningham, W. D.

1994-04-01

282

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

283

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

284

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

285

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

286

Evolution of the Ligurian Tethys in the Western Alps: Sm\\/Nd and U\\/Pb geochronology and rare-earth element geochemistry of the Montgenèvre ophiolite (France)  

Microsoft Academic Search

We provide geochemical and geochronological data for gabbro, diorite and albitite samples from the Montgenèvre ophiolite in the Western Alps. This well-preserved remnant of the Piemont–Ligurian oceanic basin shows evidence of intra-oceanic deformation and metamorphism, but has suffered minor ductile deformation and metamorphism during the Alpine orogeny. The gabbros have geochemical features and initial Nd isotopic signatures similar to that

Sylvie Costa; Renaud Caby

2001-01-01

287

Infiltration of refractory melts into the lowermost oceanic crust: evidence from dunite and gabbro-hosted clinopyroxenes in the Bay of Islands Ophiolite  

Microsoft Academic Search

Up to 3?km of dunitic rocks occur below crustal gabbro in the Blow Me Down massif (Bay of Islands Ophiolite, Newfoundland).\\u000a Analyses of dunite- and gabbro-hosted clinopyroxene grains (cpx) for rare earth elements (REE), Zr, and Ti reveal three types\\u000a of chondrite-normalized patterns: N-group patterns are similar to cpx grains as they would form by fractionation from a range\\u000a of

G. Suhr; H. A. Seck; N. Shimizu; D. Günther; G. Jenner

1998-01-01

288

Trace element distribution within olivine-bearing gabbros from the Northern Apennine ophiolites (Italy): evidence for post-cumulus crystallization in MOR-type gabbroic rocks  

Microsoft Academic Search

The trace element distribution in three selected olivine-bearing gabbros from the Northern Apennine ophiolites has been determined.\\u000a These rocks consist of euhedral plagioclase and olivine, and subhedral to poikilitic clinopyroxene. Fe-Ti-oxides, titanian\\u000a pargasite, orthopyroxene and apatite occur as interstitial accessory minerals. Plagioclase, clinopyroxene and accessory minerals\\u000a were analysed for rare earth (REE) and selected trace elements by secondary ion mass

Riccardo Tribuzio; Massimo Tiepolo; Riccardo Vannucci; Piero Bottazzi

1999-01-01

289

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

Microsoft Academic Search

Summary  Chromitites sampled from four different pseudostratigraphic levels of the Mesozoic Shebenik Ophiolite Complex, Albania, have\\u000a low PGE totals Burgath et al., 2003) as well as differing mineral chemistry, PGM mineralogy and Os isotopic signatures. To circumvent analytical\\u000a problems with low PGE abundances, representative samples were analyzed using HPA-digestion followed by isotope dilution ICP-MS.\\u000a Osmium isotopes were determined by ICP-QMS and

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

2007-01-01

290

Geochemistry and petrogenesis of the Yishak Volcanic Sequence, Kudi ophiolite, West Kunlun (NW China): implications for the magmatic evolution in a subduction zone environment  

Microsoft Academic Search

The Yishak Volcanic Series (YVS) is part of the Late Cambrian Kudi ophiolite in the western Kunlun Mountains, NW China, which\\u000a marks the oldest suture zone in the Tibetan Plateau. The YVS is a well-preserved sequence comprising five conformable units\\u000a (A to E) of basalts to andesites, with mafic dykes in Units A to D. These rocks are sequentially characterized

Chao Yuan; Min Sun; Mei-Fu Zhou; Wenjiao Xiao; Hui Zhou

2005-01-01

291

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

292

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

293

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

NASA Astrophysics Data System (ADS)

The boninites in the Oman ophiolite occur as lavas and dikes of the Alley volcanic sequence (Ishikawa et al., 2002). Moreover, Yamazaki and Miyashita (2008) reported about boninitic dike swarms in the Fizh crustal section. The boninitic melt generation requires hydrous melting of refractory mantle peridotite under an extremely high temperature and low pressure condition. This condition is generally explained by the addition of slab-derived fluids into a hot young oceanic lithosphere, which previously experienced MORB melt extraction. In this study, we report an ultramafic complex mainly composed of dunite which is in equilibrium with chemical composition of boninites in the southwestern part of the Salahi mantle section in the northern Oman ophiolite. Based on the study by Nomoto and Takazawa (2013) the complex consists mainly of massive dunite associated with minor amounts of harzburgite, pyroxenites and wehrlite. We use spinel Cr# (=Cr/[Cr+Al] atomic ratio) as an indicator of extent of melt extraction in harzburgites. For dunites spinel Cr# varies as a function of extent of reaction and of melt composition (Dick and Bullen, 1984; Arai, 1994; Ozawa, 2008). The spinels in the dunites from the complex have Cr# greater than 0.7 indicating highly refractory signature. The range of spinel Cr# is similar to those of spinels in boninites reported worldwide (Umino, 1986; van der Laan et al., 1992; Sobolev and Danyushevsky, 1994; Ishikawa et al., 2002). The complex might be a section of dunite channel that formed by flux melting of harzburgites as a result of infiltration of a voluminous fluid from the basal thrust. We determined the abundances of rare earth elements (REE) in the peridotite clinopyroxenes (cpxs) by LA-ICP-MS to estimate the compositions of the melts in equilibrium with these clinopyroxenes. The chondrite-normalized patterns for clinopyroxenes in the dunites are characterized by enrichments in light REE (LREE) relative to those of the harzburgite clinopyroxenes. The chondrite-normalized REE patterns for the calculated melts in equilibrium with clinopyroxenes in the dunites do not resemble to the pattern of N-MORB (Sun and McDonought, 1989) but fit very well to the patterns of the boninites (Cameron et al., 1983; Cameron, 1985; Taylor et al., 1994; Ishikawa et al., 2005). With increasing the spinel Cr# from harzburgite to dunite, the Yb content of clinopyroxenes decreases whereas the Ce content increases. Chondrite-normalized REE patterns of clinopyroxenes in dunites indicate that the dunites are not a residue of closed system melting but a product of open system melting with addition of a LREE-enriched fluid. Our results supports a hypothesis that the dunites formed as residue after flux melting of harzburgite accompanied with LREE-enriched fluid infiltrated from the base of the ophiolite. The pyroxenites frequently occur in the periphery of the voluminous dunite mass. These pyroxenites were considered as crystal cumulates from the boninitic melt.

Nomoto, Y.; Takazawa, E.

2013-12-01

294

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

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

295

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

NASA Astrophysics Data System (ADS)

Supra-subduction zone ophiolites in the Egyptian Central Eastern Desert (CED) occur as clusters in its northern (NCEDO) and southern (SCEDO) parts, displaying abundant island arc-boninitic and MORB/island-arc geochemical affinities, respectively. An amphibolite belt, including the investigated massive to slightly foliated Wadi Um Gheig (WUG) amphibolites, is exposed in the southeast most of the NCEDO thrusting over the El Sibai gneissic association and intruded by late- to post-orogenic granitoids and gabbros. The WUG rocks are metamorphosed under epidote amphibolite to common amphibolite facies. The amphiboles are calcic and represented by actinolitic hornblende to magnesio-hornblende in the epidote amphibolites and magnesio- to ferro-hornblende in the amphibolites. Plagioclase composition varies from pure albite (An3-8) in the epidote amphibolites to andesine and labradorite (An36-65) in the amphibolites. The estimated P-T conditions are in favor of their metamorphism under epidote amphibolite (c. 550-600 °C and 2-3 ± 1.5 kbar) and amphibolite (c. 618-720 °C and 3-6 ± 1.5 kbar) facies. The peak metamorphic conditions point to a burial depth of c.15-20 km. Geochemically, the WUG amphibolites show basaltic to andesitic compositions of tholeiitic affinity. They display LILE-enriched MORB-normalized patterns with negative Nb anomalies characteristic of the subduction-related rocks. However, their chondrite-normalized rare-earth element (REE) patterns vary from LREE-depleted (LaN/YbN = 0.29 to 0.49) to LREE-enriched (LaN/YbN = 2.97 to 3.74). Few samples show major and trace element contents typical of boninitic rocks, including U-shaped REE pattern. On the standard tectonic discrimination diagrams the WUG amphibolites plot mostly in the island-arc fields with some samples of MORB and boninitic affinities. Greenschist facies metamorphosed NCEDO obviously share these geochemical characteristics, implying formation in the same tectonic environment, i.e. forearc basin. This argues that the WUG amphibolites likely represent remnants of a dismembered metamorphic sole beneath the NCEDO. Their formation possibly involves intra-forearc basin thrusting followed by emplacement of ophiolite as imbricated stack of dismembered thrust slices in an accretionary wedge setting. This revives interests in geotectonic model in which the CED represents a forearc-arc-back-arc system above a southeast-dipping subduction zone.

Farahat, E. S.

2011-07-01

296

The exhumation of an HP ophiolitic massif (Voltri Massif, Western Alps): insight from 3D numerical models  

NASA Astrophysics Data System (ADS)

The high-pressure ophiolitic Voltri Massif outcrops at the eastern sector of the Ligurian Western Alps. Highly deformed serpentinite and metasediments wrap eclogite to blueschist facies metagabbro and metabasalt bodies; these lenses have foliated rims and preserve undeformed textures in their cores revealing a strong strain-partitioning. The mechanism that drove the exhumation of these high-pressure rocks has been already analyzed in detail using 2D numerical simulations (Malatesta et al., 2011). In particular they reproduce the subduction process that in the Mesozoic affected the Alpine branch of the Western Tethys. In the Ligurian area subduction was intraoceanic and involved a non-layered oceanic lithosphere with gabbro as discrete bodies inside serpentinized peridotites that were overlain by a limited basaltic cover. The comparison of field and petrologic evidence with model results showed that the exhumation of the high-pressure Voltri rocks was related to the formation of a serpentinite channel above the downgoing slab. This low-viscosity area formed after the hydration of the mantle-wedge rocks by the uprising fluids that migrate from the slab. Buoyancy of the high-pressure serpentinitic mélange that included slices of the slab finally triggered their exhumation. Alpine subduction was however oblique (Marroni and Treves, 1998; Malusà et al., 2011) thus including a trench-parallel left-lateral motion and not only a trench-normal motion as in 2D simulations. We have studied this particular setting through 3D numerical simulations starting from the setup of 2D models. We therefore designed an oceanic basin (500 km-wide) surrounded by continental margins and floored by a non-layered oceanic lithosphere. Subduction starts at a prescribed weak zone in the mantle; the weak zone defines the plate margins geometry. We test different "lateral" geometries of the weak zone (e.g. continuous, segmented). We designed "continuous" weak zones either parallel or increasingly moving away from the continental margins. Moreover, we tested the effect on subduction/exhumation dynamics of varying values of the trench-parallel component of convergence-rate vector. The comparison among field and petrologic data of the Voltri Massif rocks with 3D numerical models results will finally provide a more detailed description of the subduction dynamics acting in the Ligurian-Piedmontese basin and in particular will shed more light on the mechanism that drove the exhumation of the high-pressure ophiolitic Voltri Massif.

Malatesta, C.; Gerya, T.; Crispini, L.; Federico, L.; Scambelluri, M.; Capponi, G.

2012-04-01

297

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

NASA Astrophysics Data System (ADS)

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

Angiboust, Samuel; Oncken, Onno; Agard, Philippe

2014-05-01

298

Os isotope heterogeneity of the upper mantle: Evidence from the Mayarí Baracoa ophiolite belt in eastern Cuba  

NASA Astrophysics Data System (ADS)

In an attempt to quantify the extent of geochemical heterogeneity within a restricted and well dated portion of the upper mantle, 27 chromite separates from the 90 My old chromite deposits in the Mayarí-Baracoa ophiolite belt in eastern Cuba have been investigated for platinum group element (PGE) concentrations and Re-Os isotopic systematics. The samples are characterized by systematically subchondritic initial 187Os/ 188Os ratios and substantial heterogeneity. The initial 187Os/ 188Os ratios vary with chromite chemistry and with geographical distribution, reflecting differences in the Os isotopic evolution for the different upper mantle sections represented by the ophiolite. Accordingly, the Os isotope data might be divided into three groups. In the Moa-Baracoa district, where the chromite bodies are located in the mantle-crust transition zone, the calculated initial ?Os values average - 0.97 ± 0.69 ( n = 13). In the Sagua de Tanamo district, where chromite chemistry is highly variable and their location in relation the mantle sequence is less clear, the initial ?Os values are intermediate, with an average of - 1.77 ± 0.80 ( n = 7). In the Mayarí district, where the chromite bodies are located in the lower part of the mantle sequence, initial ?Os values average - 2.66 ± 0.29 ( n = 7). These subchondritic (i.e. negative) initial ?Os values are most simply explained by Re depletion during ancient partial melting and/or melt percolation events. The Os isotope heterogeneity documented here indicates a high degree of geochemical complexity on small to intermediate length scales in the upper mantle. Our results, in combination with data on chromites from the literature, show that an "average present-day Os isotopic composition" for the hypothetical depleted MORB mantle (DMM) reservoir cannot be precisely established beyond the statement that it is "broadly chondritic". Indeed, the upper mantle cannot be considered a sufficiently homogeneous geochemical "reservoir" to serve meaningfully as a baseline against which geochemical "anomalies" are evaluated. On the other hand, our findings are consistent with the "Statistical Upper Mantle Assemblage" or "SUMA"-concept, according to which a high level of geochemical heterogeneity is maintained in the upper mantle at all relevant length scales, as a result of the plate-tectonic cycle and intra-mantle processes such as melt-migration and metasomatism.

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

2006-01-01

299

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

PubMed

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

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

1999-09-01

300

Formation of anorthosite-Gabbro rhythmic phase layering: an example at North Arm Mountain, Bay of Isands ophiolite  

USGS Publications Warehouse

Rhythmically layered anorthosite and gabbro are exposed in a 4-10-m thick interval at the base of the layered gabbro unit on North Arm Mountain, one of four massifs that compose the Bay of Islands ophiolite, Newfoundland. The rhythmically layered interval is sandwiched between thick layers of adcumulate to orthocumulate uniform gabbro. Calculated fractional crystallization paths and correlated cryptic variation patterns suggest that uniform and rhythmically layered gabbros represent 20-30% in situ crystallization of two distinct magma batches, one more evolved and the other more primitive. When the more primitive magma entered the crystallization site of the NA300-301 gabbros, it is estimated to have been ~40??C hotter than the resident evolved magma, and may have been chilled by contact with a magma chamber margin composed of uniform gabbro. In this model, chilling caused the liquid to become supercooled with respect to plagioclase nucleation temperatures, resulting in crystallization of gabbro deficient in plagioclase relative to equilibrium cotectic proportions. Subtraction of a plagioclase-poor melagabbro enriched the liquid in normative plagioclase, which in turn led to crystallization of an anorthosite layer. -from Authors

Komor, S.C.; Elthon, D.

1990-01-01

301

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

302

Rheic Ocean ophiolitic remnants in southern Iberia questioned by SHRIMP U-Pb zircon ages on the Beja-Acebuches amphibolites  

NASA Astrophysics Data System (ADS)

The Rheic Ocean was a major oceanic domain between Avalonia and Gondwana in Ordovician-Silurian times. Most of the Paleozoic plate reconstructions assume that the Rheic Ocean suture lies within southern Iberia, coinciding with the contact between the South Portuguese Zone and the Ossa-Morena Zone. This paper reports four Sensitive High Resolution Ion Micro-Probe (SHRIMP) U-Pb zircon ages from mid-ocean ridge basalt (MORB)-featured rocks of the Beja-Acebuches Amphibolite unit, which crops out along the boundary between the Ossa-Morena and the South Portuguese Zone, and is considered its most conspicuous suture unit. The obtained ages range from 332 ± 3 to 340 ± 4 Ma, corresponding to the crystallization of the mafic protoliths. These Early Carboniferous ages for the Beja-Acebuches amphibolites imply that this unit can no longer be viewed as an ophiolite belonging to the Rheic Ocean suture, since this oceanic domain was presumably closed in Devonian times. Tectonic reconstructions joining in a single suture line the Beja-Acebuches Amphibolite unit in southern Iberia to either the Devonian Lizard ophiolite in southern England or the root zone of the Devonian/Ordovician ophiolitic units in northwest Iberia must be therefore reconsidered because of the age difference. We interpret the Beja-Acebuches Amphibolite unit to represent a narrow and very ephemeral realm of oceanic-like crust that opened in Early Carboniferous times, after total consumption of the Rheic Ocean. We suggest that a mantle plume underneath southern Iberia in Early Carboniferous times is the most plausible large-scale geodynamic scenario for the formation of these MORB-featured rocks.

Azor, A.; Rubatto, D.; Simancas, J. F.; GonzáLez Lodeiro, F.; MartíNez Poyatos, D.; MartíN Parra, L. M.; Matas, J.

2008-10-01

303

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

304

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

305

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

NASA Astrophysics Data System (ADS)

is a planetary process that has important consequences on geochemical cycles, supporting microbial activity through the formation of H2 and CH4 and having the potential to sequester atmospheric CO2. We present geochemical evidence of active serpentinization in the Santa Elena Ophiolite, Costa Rica which is sustained by peridotites with a degree of serpentinization less than 50% with no evidence of an internal heat source. Average spring water temperatures are 29.1°C. Two hyperalkaline spring systems were discovered, with a spring fluid pH up to 11.18. The fluids are characterized by low Mg (1.0-5.9 mg/L) and K (1.0-5.5 mg/L) and relative high Ca (29-167 mg/L), Na (16-27 mg/L), Cl (26-29 mg/L), hydroxide (41-63 mg/L), and carbonate (31-49 mg/L). Active CH4 (24.3% v/v) vents coupled with carbonate deposits (?13CCO2 =-27 to -14‰; ?18OCO2 =-17 to - 6‰) also provide evidence for active serpentinization and carbonation. Isotope ratios of the alkaline fluids (?18O = -7.9‰, ?2H = -51.4‰) and groundwater (?18O = -7.6‰; ?2H = -48.0‰) suggests that, during base flow recession, springs are fed by groundwater circulation. Methanogenic Archaea, which comprises a relatively high percentage of the 16S rRNA gene tag sequences, suggests that biological methanogenesis may play a significant role in the system. Santa Elena's extreme varying weather results in a scenario that could be of significant importance for (a) improving the knowledge of conditions on a humid early Earth or Mars that had periodic changes in water supply, (b) revealing new insights on serpentinizing solute transport, and (c) modeling hydrogeochemical responses as a function of recharge.

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

2014-05-01

306

Thermodynamic modelling of Sol Hamed serpentinite, South Eastern Desert of Egypt: implication for two serpentinization stages in the Arabian-Nubian Shield ophiolites  

NASA Astrophysics Data System (ADS)

The Arabian-Nubian Shield is the largest tract of juvenile continental crust of Neoproterozoic age on Earth. This crust was generated due to arc-arc collision associated with the closing of the Mozambique Ocean. Distribution of ophiolitic rocks marks fossils suture zones in the shield. Petrological, mineral chemistry, whole-rock chemistry and thermodynamic studies are carried out to examine the serpentinite component of Sol Hamed ophiolite in south Eastern Desert of Egypt. The protolith mantle was harzburgite and formed in subduction zone of forearc setting. Serpentinization occurred in two stages. The first by intrusion of high concentrated CO2 fluid released from carbonate-bearing sediments and altered basalt at the subduction zone. The serpentinization achieved during isobaric cooling path at pressure of 1 kbar and before the emplacement. The minimum temperature limit of the serpentinization is above the breakdown of lizardite to antigorite and brucite (170 °C). The fluid composition during the isobaric cooling path was buffered by the metamorphic reactions. The second stage of serpentinization took place through prograde path which led to formation of chrysotile after lizardite. The increasing in the pressure during this stage occurred as a result of extensive duplex array and thrusting of oceanic crust. The crust in the forearc basin was overloaded by 28 km of obducted and thrusted oceanic crust from both mid-oceanic and forearc basins, respectively.

Abu-Alam, T.; Hamdy, M.

2012-04-01

307

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

308

A fossil melt lens in the Oman ophiolite: Implications for magma chamber processes at fast spreading ridges  

NASA Astrophysics Data System (ADS)

From seismic evidence we know that a small body of magma is present at mid-crustal levels beneath most fast spreading ridges, and that this small sill or melt lens overlies a much broader area of hot but largely solid material (a 'crystal mush') that constitutes the lower ocean crust. Beyond this, however, we have little direct knowledge of the physical and chemical processes that operate in and below the melt lens, nor the role the melt lens plays in the storage and aggregation of mid-ocean ridge basalts (MORB). We here offer constraints on the processes of melt transport, modification and residence beneath fast spreading ridges from a coupled structural and geochemical study through the crustal section of the Oman ophiolite, concentrating on the uppermost plutonic rocks and transition into sheeted dykes. We find that almost the entire Oman plutonic suite - the layered gabbro, and most of the 'high-level gabbro' - is dominated by cumulate rocks containing very low proportions of trapped interstitial melt. The exception to this is the very uppermost part of the section, within ˜150 metres of the base of the sheeted dyke complex, at which level the gabbros instead display a marked heterogeneity of structure, texture and composition, and plutonic rocks of generally basaltic composition occur for the first time. Associated pegmatitic ferrogabbros formed by in situ fractionation of these liquids. This horizon passes up into a more homogeneous microgabbroic facies, also of basaltic composition, within which isolated asymmetric doleritic chilled margins are found, and thence into doleritic sheeted dykes. We argue that the thin heterogeneous gabbro horizon is a fossilised melt lens, containing pooled liquids of net basaltic composition (Mg# of ˜65) expelled from the crystal mush beneath. Existing models have suggested that steep fabrics observed in homogeneous gabbros beneath this horizon formed by subsidence of originally horizontal cumulus layers formed at the base of the melt lens, but this is not supported by the field relationships. Instead we propose that the fabrics record the buoyant ascent of magma through the mush pile (probably in part by porous flow) and reflect crystallisation essentially in situ, without significant vertical transport of the mush. Low residual melt porosities in the sub-melt lens region imply efficient extraction of interstitial liquid and at least episodically high permeabilities in the upper part of the lower crust.

MacLeod, C. J.; Yaouancq, G.

2000-03-01

309

Channelized Fluid Flow and Metasomatism in Subducted Oceanic Lithosphere recorded in an Eclogite-facies Shear Zone (Monviso Ophiolite, Italy)  

NASA Astrophysics Data System (ADS)

The Monviso ophiolite Lago Superiore Unit (LSU) constitutes a well-preserved, almost continuous fragment of upper oceanic lithosphere subducted down to ca. 80 km (between 50 and 40 Ma) and later exhumed along the subduction interface. The LSU is made of (i) a variably thick (50-500 m) section of eclogitized mafic crust (associated with minor calcschist lenses) overlying a 100-400 m thick metagabbroic body, and of (ii) a serpentinite sole (ca. 1000 m thick). This section is cut by two 10 to 100m thick eclogite-facies shear zones, found at the boundary between basalts and gabbros (Intermediate Shear Zone: ISZ), and between gabbros and serpentinites (Lower Shear Zone: LSZ). Fragments of mylonitic basaltic eclogites and marbles were dragged and dismembered within serpentinite schists along the LSZ during eclogite-facies deformation [Angiboust et al., Lithos, 2011]. Metasomatic rinds formed on these fragments at the contact with the surrounding antigorite schists during lawsonite-eclogite facies metamorphism, testifying to prominent fluid-rock interaction along with deformation. We present new petrological and geochemical data on four types of metasomatically altered eclogites (talc-, chlorite-, lawsonite- and phengite-bearing eclogites) and on a (serpentinite-derived) talc schist from the block rind. Bulk-rock compositions, in situ LA-ICP-MS analysis and X-ray Cr/Mg maps of garnet demonstrate that (i) these samples underwent significant B, Cr, Mg, Ni and Co enrichment and Fe, V and As depletion during eclogite-facies metamorphism (while Li and Pb behaved inconsistently) and (ii) garnet composition and chemistry of inclusions show extreme variation from core to rim. These compositional patterns point to a massive, pulse-like, fluid-mediated element transfer along with deformation, originating from the surrounding serpentinite (locally, with contributions from metasediments-equilibrated fluids). Antigorite breakdown, occurring ca. 10 km deeper than the maximum depth reached by these eclogites, could have provided significant amounts of fluid promoting extensive fluid/rock interaction. We therefore propose that the LSZ witnesses the existence of a highly anisotropic, large-scale, prominent, subduction-parallel fluid migration pathway within the subducted oceanic lithosphere, active at eclogite facies conditions (in line with recent permeability experiments; [Kawano et al., Geology, 2011]). The strongly focused deformation also enhanced fluid/rock interaction that significantly contributed to the mechanical weakening attested by the extreme degree of block fragmentation and disaggregation observed along the LSZ in the field.

Angiboust, S.; Agard, P.; Pettke, T.

2012-04-01

310

Depth profiles of trace elements and stable isotopic compositions (O, H, B, Sr) of the hydrothermally altered oceanic crust in the Oman ophiolite  

NASA Astrophysics Data System (ADS)

Trace elements and stable isotopic compositions of hydrothermally altered oceanic crust could be useful tracers during geochemical processes, such as seawater-rock interaction, arc magmatism at convergent zone, and heterogeneity of mantle. Although previous studies have been reported the chemical compositions of oceanic crustal rocks from dredged and/or drilled modern seafloor and ophiolite, available depth-successive data is still limited. The ancient oceanic crust in the Oman ophiolite is a good material to investigate the geochemistry associated with hydrothermal alteration at fast spreading ridge system because of its continuous exposure and less metamorphism. In this study, we present the depth profiles of trace elements with stable isotopic compositions including oxygen and hydrogen [1], boron [2], and strontium [3] for the complete sequence of the oceanic crust from the Wadi Fizh area, in the northern part of the Oman ophiolite. Based on the types of alteration and oxygen isotopic compositions of bulk rocks, the oceanic crust from the Wadi Fizh area was divided into four zones as follows: (1) Zone I (100-350 m; basalt altered at <60oC, partly seafloor weathering; clay mineral, calcite; ?18O >10‰), (2) Zone II (350-2000 m; basalt and dolerite altered at 60-350oC; actinolite, prehnite, albite, chlorite, epidote; ?18O = 6-10‰), (3) Zone III (2000-3560 m; dolerite dike, plagiogranite, metagabbro and epidosite altered at 350-450oC; actinolite, albite, chlorite, epidote, quartz; ?18O <6‰, except for plagiogranite), (4) Zone IV (3560-5340 m; noncumulate and cumulate gabbro altered at >450oC; hornblende, albite, chlorite; ?18O <6‰). The 87Sr/86Sr ratios and ?11B values of the lower gabbros were highly elevated relative to MORB, indicating that seawater-originated hydrothermal fluid penetrated into deep section of the oceanic crust at or near a spreading center. Furthermore, the obviously D-depleted chlorite and hornblende from the lower oceanic crust suggested interaction with saline brine generated by supercritical phase separation of seawater. Taking into these findings, we discuss the behavior of trace element during hydrothermal alteration of the oceanic crust along mid-ocean ridges. [1] Yamaoka et al., submitted. [2] Yamaoka et al., submitted. [3] Kawahata et al. (2001) JGR 106, 11,083-11,099.

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

2010-12-01

311

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  

NASA Astrophysics Data System (ADS)

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 Oman). H2 is frequently degassing in hyperalkaline springs but the highest flows were found directly expelled from fractures in the peridotites. Obviously, within the fracture systems, gas and associated hyperalkaline water separate at shallow depth close to the top of the upper aquifer level. Locally high flows of gas migrate vertically in the fractures, whereas water with degassing H2 tends to migrate laterally in the fracture network toward the creeks where most of the hyperalkaline springs are found. The genesis of natural H2 is interpreted as the result of the interaction, at depth, between ultrabasic mantle rocks in the upper plate and water expelled by the subducted sediments by oxidation of metals (Fe2+, Mn2+) and reduction of water during serpentinisation. CH4 is commonly associated to the H2-rich fluids and it is interpreted as the result of the reduction of available CO2 at depth. N2 is also commonly associated to the H2-rich fluids in the ophiolites, whereas N2 flows (within H2) were found in the subducted sediments (below the sole décollement of the peridotite) where it can be observed (Oman and New-Caledonia). Within the peridotites, the hyperalkaline water is rich in ions OH- and Ca2+ and characterized by high pH (between 11 and 12). Most alkaline springs are found in the vicinity of major faults and/or lithological discontinuities like the basal décollement of the ophiolites and the peridotite-gabbro contact (Moho). This hyperalkaline water migration induces a chain of diagenetic reactions starting at depth within the fracture systems by the precipitation of Mg-carbonates (dolomite, magnesite) and continuing up to the surface where it leads mostly to the precipitation of Ca-carbonates (calcite, aragonite), and brucite. This chain of diagenetic reactions is associated with the capture and sequestration of the atmospheric CO2 during the precipitation of the carbonates.

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

2010-12-01

312

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

313

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

NASA Astrophysics Data System (ADS)

In Northern Apennines, the Internal Liguride units are characterized by an ophiolite sequence that represents the stratigraphic base of a Late Jurassic-Early Paleocene sedimentary cover. The Bocco Shale represents the youngest deposit recognized in the sedimentary cover of the ophiolite sequence, sedimented just before the inception of subduction-related deformation history. The Bocco Shale has been interpreted as a fossil example of deposits related to the frontal tectonic erosion of the alpine accretionary wedge slope. The frontal tectonic erosion resulted in a large removal of material from the accretionary wedge front reworked as debris flows and slide deposits sedimented on the lower plate above the trench deposits. These trench-slope deposits may have been successively deformed and metamorphosed during the following accretion processes. The frontal tectonic erosion can be envisaged as a common process during the convergence-related evolution of the Ligure-Piemontese oceanic basin in the Late Cretaceous-Early Tertiary time span. In the uppermost Internal Liguride tectonic unit (Portello Unit of Pandolfi and Marroni. 1997), that crops-out in Trebbia Valley, several isolated blocks of authigenic carbonates, unidentificated corals and intrabasinal carbonatic arenites have been recognized inside the fine-grained sediments that dominate the Early Paleocene Lavagnola Fm. (cfr. Bocco Shale Auctt.). The preliminary data on stable isotopes from blocks of authigenic carbonates (up to 1 m thick and 3 m across) and associated corals archive a methane signatures in their depleted carbon isotope pattern (up to ?13C -30‰ PDB) and suggest the presence of chemosynthetic paleocommunities. The seep-carbonates recognized at the top of Internal Liguride succession (cfr. Bocco Shale Auctt.) occur predominantly as blocks in very thick mudstone-dominated deposits and probably developed in an environment dominated by the expulsion of large volume of cold methane-bearing fluids focused in the frontal part of the Early Paleocene alpine accretionary wedge.

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

2014-05-01

314

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

315

Geochemistry and jasper beds from the Ordovician L??kken ophiolite, Norway: Origin of proximal and distal siliceous exhalites  

USGS Publications Warehouse

Stratiform beds of jasper (hematitic chert), composed essentially of SiO2 (69-95 wt %) and Fe2O3 (3-25 wt %), can be traced several kilometers along strike in the Ordovician L??kken ophiolite, Norway. These siliceous beds are closely associated with volcanogenic massive sulfide (VMS) deposits and are interpreted as sea-floor gels that were deposited by fallout from hydrothermal plumes in silica-rich seawater, in which plume-derived Fe oxyhydroxide particles promoted flocculation and rapid settling of large (???200 ??m) colloidal particles of silica-iron oxyhydroxide. Concentrations of chalcophile elements in the jasper beds are at the parts per million level implying that sulfide particle fallout was insignificant and that the Si-Fe gel-forming plumes were mainly derived from intermediate- (100??-250??C) to high-temperature (>250??) white smoker-type vents with high Fe/S ratios. The interpreted setting is similar to that of the Lau basin, where high-temperature (280??-334??C) white smoker venting alternates or overlaps with sulfide mound-forming black smoker venting. Ratios of Al, Sc, Th, Hf, and REE to iron are very low and show that the detrital input was <0.1 percent of the bulk jasper. Most jasper beds are enriched in U, V, P, and Mo relative to the North American Shale Composite, reflecting a predominantly seawater source, whereas REE distribution patterns (positive Eu and negative Ce anomalies) reflect variable mixing of hydrothermal solutions with oxic seawater at dilution ratios of ???102 to 104. Trace element variations in the gel precursor to the jasper are thought to have been controlled by coprecipitation and/or adsorption by Fe oxyhydroxide particles that formed by the oxidation of hydrothermal Fe2+ within the variably seawater-diluted hydrothermal plume(s). Thick jasper layers near the H??ydal VMS orebody show distinct positive As/Fe and Sb/Fe anomalies that are attributed to near-vent rapid settling of Si-Fe particles derived from As- and Sb-rich hydrothermal fluids prior to extensive mixing with seawater in the buoyant plume. Particles that formed later in the highly diluted nonbuoyant plume formed relatively As and Sb poor distal jasper. The large particle sizes and accordingly high settling rates of the particles, together with mass-balance calculations based on modern vent field data, suggest that individual meter-thick jasper beds formed within a plume lifetime of 200 years or less. The lack of thick jasper beds near the L??kken VMS orebody, which is larger than the H??ydal orebody by more than two orders of magnitude, probably reflects a shift to anoxic conditions during L??kken mineralization. This environment limited oxidation of iron in the hydrothermal plume and formation of the ferric oxyhydroxides necessary for the flocculation of silica and sea-floor deposition of the gel precursor of the jasper beds. Distal pyritic and iron-poor cherts are more common than jasper in ancient VMS-hosting sequences. The origin of these other types of siliceous exhalite is enigmatic but at least in some cases involved sulfidation, reduction to magnetite, or dissolution of the original ferric iron in precursor Si-rich gels, either by hydrothermal or diagenetic processes. ?? 2005 Society of Economic Geologists, Inc.

Grenne, T.; Slack, J.F.

2005-01-01

316

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

317

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).

318

Melanges pedagogiques 1990 (Pedagogical Miscellany 1990).  

ERIC Educational Resources Information Center

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

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

319

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

320

Application of airborne LiDAR to the detailed geological mapping of mineralised terrain: the Troodos ophiolite, Cyprus  

NASA Astrophysics Data System (ADS)

The identification of mineral prospects is highly dependent upon the acquisition and synthesis of a wide variety of geological information, e.g., lithological, structural, geophysical and geochemical data. Conventionally, the majority of this information is acquired through field-based surveys. However, the quality of data collected in this manner is often affected by subjectivity and lack of detail due to coarse sampling over vast areas or inaccessible terrain. Both multi- and hyperspectral satellite remote sensing and the interpretation of aerial photography are typically used to help try and overcome some of the limitations associated with field-based surveys. However, the use of these approaches for the extraction of exploration data can be hindered by spatial and spectral limitations and by dense forest cover. A relatively new active remote sensing technology—known as airborne Light Detection And Ranging (LiDAR)—offers the possibility of acquiring accurate and high-resolution (ca. 1-4 m) topographic data through dense forest cover. The ability of LiDAR systems to detect multiple returns from the emission of a single laser pulse can be utilised to generate a high-resolution digital elevation model (DEM) of the ground beneath the forest canopy. Airborne LiDAR is an important tool for geoscience research, with a wide spectrum of applications including the mapping of landslides and faults to help inform hazard assessment studies. A LiDAR system can also provide an insight into the spectral and textural properties of surface materials using intensity data—a ratio of the reflected laser energy to the emitted laser energy. Where rocks outcrop, these properties are linked to the surface mineralogy and weathering at the LiDAR footprint scale. The ability to acquire two high-resolution datasets simultaneously from a single survey makes airborne LiDAR an attractive tool for the extraction of detailed geological information in terrain with either sparse or dense forest cover. To examine the efficacy of LiDAR in mineral exploration, an airborne survey was flown over approximately 375 km2 of the Troodos ophiolite, Cyprus—a region noted for its volcanogenic massive sulphide (VMS)-style mineralisation. Although most commonly found at the Lower Pillow Lava-Upper Pillow Lava interface, sulphide mineralisation occurs throughout the pillow lava sequence. Therefore, accurate identification of geological contacts is a key parameter for VMS exploration in the Troodos complex. However, the existing geological maps, produced using a combination of conventional field mapping and aerial photograph interpretation, have significant differences and do not adequately represent the geological complexity in high detail. In this study, we present a semi-automated algorithm for the detailed lithological mapping of a 16 km2 study area using high-resolution (4 m) airborne LiDAR topographic data in which non-ground features such as trees and buildings have been removed (i.e., bare-earth). Differences in the geomorphological characteristics of each major lithological unit result in each unit having a distinctive topographic signature in the bare-earth LiDAR DEM. Thematic maps (slope, curvature and surface roughness) are derived from the LiDAR DEM in order to quantify the topographic signatures associated with each lithological unit. With the thematic maps as the input layers, Kohonen's Self-Organising Map is used as a supervised artificial neural network to assign each pixel to a lithology to produce a geological map. The algorithm successfully identifies the major lithological units—Basal Group (> 50 % dykes and < 50 % pillow lavas), pillow lavas, alluvium and Lefkara Formation (chalks and marls)—in excellent detail and highlights geological features to a 20 m resolution. Although the ability to distinguish between lithologies in some areas is affected by anthropogenic activity (e.g., farming), the resultant lithological map easily surpasses the quality and detail of the existing geological maps for the area. As well as providing a q

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

2009-04-01

321

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

322

A new finding of Cu-Au alloy in association with rodingite minerals in the Kaa-Khem ophiolitic belt, Tuva  

NASA Astrophysics Data System (ADS)

A new Cu-Au alloy occurrence is located at the southeastern flank of the Malye Kopty massif of ultramafic rocks in the Vendian-Early Cambrian Kaa-Khem ophiolitic belt. Lithic clasts with Cu-Au alloy segregations (up to 15 mm in size) intergrown with other minerals were found in alluvium of the Kara-Oss Creek valley, which extends along the fault zone crosscutting ultramafic rocks. Cu-Au alloy occupies the main volume of clasts and fills the network of veinlets in grained aggregates consisting of andradite (2-18% grossular component) and diopside ( X Fe = 0.01-0.05). Cu-Au alloy contains small ingrowths of andradite (up to 43% grossular component), diopside ( X Fe = 0.14-0.19), chlorite (penninite), chalcocite that contains up to 1.5 wt % Au, Cu-bearing greenockite (6.07-13.67 wt % Cu, 0.48-1.56 wt % Zn, and 0.76-1.06 wt % Au), and magnetite. The chemical composition of Cu-Au alloy is nonuniform. The central parts of large Cu-Au alloy segregations consist of Ag-bearing tetraauricupride (AuCu) blocks (3.2-6.4 wt % Ag). They contain veinlet-shaped AuCu zones with 13.3-14.5 wt % Ag. The AuCu blocks are cemented by late Cu-Au alloy, whose composition is close to auricupride (AuCu3). Taking into account the limits of component miscibility in the Au-Ag-Cu system, the temperature of the Cu-Au alloy formation was estimated at 350-600°C. This temperature corresponds to the formation conditions of garnet-pyroxene rodingite mineral assemblage (Plyusnina et al., 1993). The studied Cu-Au alloy samples from the Malye Kopty massif are very similar to Cu-Au alloy minerals hosted in the Alpine-type ultramafic rocks of the Karabash massif in the southern Urals. This similarity is confirmed by identical chemical compositions of pyroxene, garnet, and chlorite, and similar PT conditions of their formation. The data show that primary ore mineralization of gold-rodingite type occurs in the Kaa-Khem ophiolitic belt.

Murzin, V. V.; Oydup, Ch. K.; Varlamov, D. A.

2009-12-01

323

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

324

Behavior of MORB magmas at uppermost mantle beneath a fast-spreading axis: an example from Wadi Fizh of the northern Oman ophiolite  

NASA Astrophysics Data System (ADS)

Relationships of lithologies in uppermost mantle section of Oman ophiolite are highly complicated, harzburgites especially being closely associated with dunites, wehrlites, and gabbros. The petrology and geochemistry of the uppermost mantle section provide constrains on how MORB (mid-ocean ridge basalt) magmas migrate from the mantle to crust. We conducted detailed sampling at the uppermost mantle section of the northern Oman ophiolite (along Wadi Fizh), and it provides us with centimeter-scale lithological and mineral chemical heterogeneity. In particular, we found peculiar plagioclase-free harzburgites that have not been recorded from the current ocean floor, which contain high-Mg# [Mg/(Mg + Fe2+) atomic ratio] clinopyroxenes that are almost in equilibrium (saturated) with MORB in terms of REE concentrations. They are from the uppermost mantle section underlying the wehrlite-dunite layer (=Moho transition zone; MTZ) just beneath the layered gabbro. MORBs cannot be in equilibrium with harzburgites; however, we call the peculiar harzburgites as "MORB-saturated harzburgite" for simplicity in this paper. The MORB-saturated harzburgites exhibit slightly enriched mineralogy (e.g., spinels with higher Ti and ferric iron, and clinopyroxenes with higher Ti and Na) and contain slightly but clearly more abundant modal clinopyroxene (up to 3.5 vol.%) than ordinary Oman depleted harzburgites (less than 1 vol.% clinopyroxene), which are similar to abyssal harzburgites. Gabbro-clinopyroxenite bands, which were melt lenses beneath the ridge axis, are dominant around the MTZ. Detailed sampling around the gabbro-clinopyroxenite bands revealed that the MORB-saturated harzburgites appear around the bands. The interaction between a melt that was MORB-like and an ordinary harzburgite induced incongruent melting of orthopyroxenes in harzburgites, and the melt chromatographically intruded into the wall harzburgite and was modified to coexist with olivine and two pyroxenes at low melt/harzburgite ratios. The modified melt left clinopyroxene (not clinopyroxene + plagioclase as in plagioclase-impregnated abyssal harzburgite) to form the MORB-saturated harzburgites in the vicinity (harzburgite) of the fracture, which are left as gabbro-clinopyroxenite bands. This local modification mimics the whole lithological and chemical variation of the MTZ and makes chemical variation of MORB suite at fast-spreading ridge.

Akizawa, Norikatsu; Arai, Shoji; Tamura, Akihiro

2012-10-01

325

Ophiolite-contaminated andesites, trachybasalts, and cognate inclusions of Mount Lamington, Papua New Guinea: anhydrite-amphibole-bearing lavas and the 1951 cumulodome  

NASA Astrophysics Data System (ADS)

The catastrophic explosive eruptions in January 1951 from Mount Lamington were followed by a year-long episode of activity which was thoroughly described by Taylor (1958). However, only preliminary major-element analyses and petrographic observations have previously been reported from this volcano. The present study was prompted by the description of anhydrite and numerous ultramafic inclusions in the lavas of Mount Lamington, together with reports of similar inclusions and anomalously high Ni and Cr contents in other volcanics of southeastern Papua. Mount Lamington, together with three other major andesitic centers — Mount Victory, Mount Trafalgar and Hydrographers Range — rest on a northeast-dipping ophiolite sheet, the Papuan Ultramafic Belt (PUB); there is no evidence for a Benioff-Wadati zone beneath this part of Papua. The lavas of Mount Lamington are porphyritic trachybasalt and high-Si, medium-to-high-K andesite. Phenocrysts are plagioclase, amphibole, and biotite, and less common minerals are magnetite, pyroxene, apatite, cristobalite, zircon and sulphide. Olivine and orthopyroxene grains and clusters are found as foreign fragments, and anhydrite is a minor constituent, particularly of pristine samples. Numerous inclusions of xenolithic harzburgite and amphibole-biotite-plagioclase-dominated, coarse-grained igneous inclusions are also present. The lava suite is characterized by enrichments in several incompatible elements relative to intermediate lavas from Bougainville Island and New Britain. Products of the 1951 eruption have Ni and Cr concentrations in the ranges 23-45 ppm and 54-111 ppm, respectively. There are dramatic compositional zonations in the constituent phases of both lavas and inclusions. For example, amphibole zones range from compositions compatible with crystallization from pristine intermediate magmas, to Mg and Si-rich and alkali-poor components. Spinels range from refractory, Al-poor chromite to magnetite in composition. Contamination of rising andesitic magmas by the PUB is demonstrated conclusively for Mount Lamington lavas, and overall interaction with crustal rocks in other andesitic provinces may be more generally prevalent than has previously been recognized. The high Ni and Cr contents of the lavas have resulted from partial digestion of PUB material, and are not the result of direct partial melting of an upper-mantle source; neither do the high values exclude derivation of the intermediate magmas by fractional crystallization of basalt below the ophiolite. Isotopic analysis indicates the source of the sulphur in anhydrite was of primary magmatic rather than sedimentary origin. Two possible sources for the sulphur are either remobilized sulphides in the PUB, or, more speculatively, low-pressure breakdown of sulphate meionite (scapolite), possibly present in the lower crust of southeastern Papua.

Arculus, R. J.; Johnson, R. W.; Chappell, B. W.; McKee, C. O.; Sakai, H.

1983-10-01

326

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

NASA Astrophysics Data System (ADS)

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

Magganas, Andreas; Koutsovitis, Petros

2015-01-01

327

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

NASA Astrophysics Data System (ADS)

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

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

2015-02-01

328

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

NASA Astrophysics Data System (ADS)

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

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

2015-01-01

329

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

330

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

331

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

332

Westward prograding metamorphism in mantle peridotites from the Eastern Desert of Egypt: clues to the subduction polarity of the Arabian Nubian Shield intra-oceanic arc ophiolite  

NASA Astrophysics Data System (ADS)

Neoproterozoic arc mantle beneath the Arabian Nubian Shield (ANS) in the Eastern Desert (ED) of Egypt exhumed due to intra-oceanic upthrusting are represented mainly by exposed ophiolitic peridotites serpentinized to different degree. Metamorphism is related to the Pan-African collision and the subduction of oceanic lithosphere. However, polarity of the Pan-African intra-oceanic subduction is still questionable. We here trace the variation of the degree of serpentinization and regional metamorphism of six serpentinite masses, widely distributed in the ED (from the east to the west: W (Wadi). Alam, W. Igla, W. Mubarak, G. El-Maiyit, W. Um El Saneyat and W. Atalla). This is based on their mineralogy, textures and mineral chemistry. The studied rocks have harzburgite composition and they all formed in oceanic mantle wedge in the fore-arc setting, except those from W. Atalla that formed in MOR-arc transition setting. Much difference in the degree of serpentinization is obvious among these rocks. They are mainly partly serpentinized containing primary olivine and orthopyroxene at W. Alam and W. Igla, while they are completely serpentinized in the other localities. With the increased degree of metamorphism, textures were transformed from the pseudomorphic to the non-pseudomorphic. The most common retrograde assemblage is composed of lizardite ± chrysotile± brucite± magnetite. The serpentine prograde textures can be viewed as a continuum from retrograde lizardite pseudomorphic textures, to very fine-grained transitional texture of lizardite and chrysotile, to chrysotile-antigorite interlocking texture and finally to antigorite interpenetrating texture. These textures appear to represent successive stages in a recrystallization event. In late subduction-related metamorphism and early collisional emplacement stage, mylonitic-antigorite serpentinites formed and antigorite became the major phase in G. El-Maiyit, Um El-Saneyat and W. Atalla. The polygonal units of the hourglass texture and the penetrative fabric of the serrate veins in all serpentinized peridotites indicate that fracturing of these rocks was developed in a dynamic regime. The late emplacement of veins of brucite, carbonates and oxides were most probably formed during the final stage of exhumation and under a stress regime in the brittle-ductile transition. As the grade of metamorphism increases Fe released from olivine and orthopyroxene and Cr released from chromite are accommodating in antigorite-rich serpentinites. Serpentine in veins also tends to have less substitutions, which is consistent with the fact that Al, Cr and Ni are relatively immobile during alteration and therefore remain in their original microstructural site. Compositional zoning in spinel grains in all serpentinites reflect variation in the degree of alteration. The biggest variation of spinel compositions are among serpentinites from Um El-Saneyat and W. Atalla. With increasing the degree of alteration, size of the aluminian chromite core decreases while width of the intermediate Fe3+-rich aluminian chromite to ferrian-chromite zone and the outer Cr-magnetite to magnetite zones increase. The alteration zones were formed in a temperature < 400 ° C to 550 ° C corresponding to the low green-schist to the lower amphibolite facies. We propose that this is concordant with a westward polarity of the subducting oceanic lithosphere, associating the intra-oceanic arc ophiolite during the closure of the Mozambique ocean.

Salam Abu El-Ela, Abdel; Hamdy, Mohamed; Abu-Alam, Tamer; Hassan, Adel; Gamal El Dien, Hamed

2013-04-01

333

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

334

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

335

Melt Transfer Mechanisms in the Lower Ophiolitic Crust: Examples from the Bay of Islands, Thetford-Mines, Betts Cove and Annieopsquotch  

NASA Astrophysics Data System (ADS)

Field evidence from the Bay of Islands (BOI) and Annieopsquotch (AN) ophiolites (dominantly tholeiitic, boninitic subordinate) implies that `open' sill-like melt bodies were up to 30m thick, and were emplaced at all crustal levels. At BOI, melt segregation appears to have proceeded in tandem with near-pervasive high-temperature deformation, leading to dynamic recrystallization of cumulates. Comparing results of inverse trace element models to compositions of potential lavas suggests that cumulates retained low residual porosities (5-7%). Expelled Fe-H2O-rich residual magma migrated laterally and up along shear zones. At AN, 10-30m thick tabular bodies are composed either of massive granular micro-diabasic gabbro, or are zoned from coarse olivine-gabbro bases to gabbroic tops. Downward-growing feldspar dendrites in the coarser bodies, and coomb-layered apophyses issuing from their upper contacts, imply that they are sills. The granular to micro-diabasic bodies have textures similar to some AN dykes and lavas; and dykes issuing from their upper contacts also imply an intrusive origin. There is no evidence for deformation in any AN rocks, but residual melt porosities of the order of 10-15% yield inverse trace element model solutions similar to those of dykes and lavas. In contrast, at Betts Cove (BC), which is dominated by boninitic magmas, macrocyclic sequences 300+m thick that show progressive cumulus modal evolution suggest the existence of substantial open chambers. Primary size-graded beds 0.5-2m thick of harzburgite suggest deposition from episodic crystal-charged influxes of replenishing magma into an ambient melt-filled chamber. In one harzburgitic cumulate bed, cm-scale faceted orthopyroxene phenocrysts filled with olivine inclusions are aligned parallel to bedding, but abundant (20-30%) intercumulus clinopyroxene and plagioclase oikocrysts show no sign of flattening, suggesting post-compaction crystallization of a porous cumulate. Inverse trace element models from these rocks yield good matches with the dominant low-Ti boninite lavas for residual porosities of c.25%. At BC, many orthopyroxenite layers embedded within the dominant harzburgitic-lherzolitic cumulates jog up-sequence, or link up with discordant pyroxenitic dykes when traced along-strike, and may represent fossil melt-escape channels. At BC, the junction between the layered cumulates and the sheeted-dyke complex is interpreted as an extensional decollement zone which has been injected by km-scale bodies of gabbro, which can be related to slightly younger intermediate-Ti boninitic lavas. This gabbro has major, 200-m scale apophyses that penetrate the overlying sheeted dykes and underlying cumulates. Cross-cutting relationships between sheeted dykes and intrusive gabbros show that these gabbros also belong to the spreading phase. Gabbros injected into the cumulates reacted with them to form pyroxenitic reaction zones. At Thetford-Mines (TM), another boninite-related ophiolite, cumulates at the base of the crust are affected by high-temperature plastic deformation and are layered parallel to the moho. Low assumed residual melt porosities (c.10%) yield inverse trace element models that match lava compositions. The upper half of the crust is dissected into tilted (40-90 degrees) Km-scale blocks by syn-volcanic, dyke-parallel paleo-normal faults. The faults are surrounded by igneous breccias, and are injected by undeformed peridotitic and pyroxenitic intrusions, demonstrating their syn-oceanic timing and role in the transfer of melt towards the surface

Bedard, J. H.; Page, P.; Lissenberg, J.

2003-12-01

336

Diopside and Coesite Lamellae in Cr-spinel of Podiform Chromitite in Luobusa Ophiolite, Tibet: Evidence for Ultra-High Pressure Origin of the Chromite  

NASA Astrophysics Data System (ADS)

The Luobusa ophiolite lies in the Indus-Tarlung Zangbo suture zone, which separates Indian continent from Eurasia. In this study, we discovered needle-shaped clinopyroxene and coesite within Cr-spinel of podiform chromitite in the Luobusa ophiolite, Southern Tibet. The Cr-spinels contain abundant cpx-needles, which are almost 1? m wide and a few ten ? m long under the microscopic observation. Crystal orientation of the cpx-neeldes seems random at a glance, but some cpx needles are parallel each other. These textures of the cpx needles within Cr-spinel are very similar to that of pyroxene lamellae within garnet in ultramafic mantle xenoliths (Haggerty and Sautter 1990). Hence, the similarity suggests that these cpx needles were formed as exsolution lamellae from the host Cr-spinel. We investigated the chemical composition, crystal structure of the needle in Cr-spinel and the topotaxy between the needles and the host Cr-spine using SEM-EDS, Laser-Raman and Analtical Transmission Electron Microscopy (ATEM) at Tokyo Institute of Technology. EDS spectra of ATEM showed that most needles have diopside composition. A rod-shaped SiO 2 phase was also observed and identified as coesite by electron diffraction pattern. Examination by selected area electron diffraction and nano-beam electron diffracrion showed that the topotaxy between diopside and Cr-spinel and between coesite and Cr-spinel are [100]sp // [103]di and [011]sp // [010]di, [100]sp // [012]coe and [110]sp // [100]coe respecivly. These observations indicate that diopside and coesite were formed as exsolution lamellaes from Cr-spinel. Although it is well-known that ilmenite occurs as exsolution lamellae in Cr-spinel from layered intrusion (e.g. Rollinson et al., 2002), diopside and coesite lamellae in Cr-spinel have not been known yet. Previous experimental and mineralogical works showed that CaO and SiO2 were not incorporated in Cr-spinel. How were the diopside and coesite lamellae generated? Recently, high-pressure polymorphs of Cr-spinel, CaFe2O4 (CF) and CaTi2O4 (CT) structures, were discovered in the shock veins of the Suizhou meteorite (Chen et al. 2003a). And then, they performed laser-heated diamond anvil cell experiments to confirm that Cr-spinel transforms to CF at 12.5 GPa, and to CT structures above 20GPa. They mentioned the possibility that the Cr-spinel polymorphs, CF and CT phases, incorporate Ca, Si, Ti, and Fe as CaFe2O4 and CaTi2O4 solid solutions (Chen et al. 2003b). It suggests that diopside and SiO2 component can be solved in high-pressure polymorphs of Cr-spinel. Actually coesite lamallae is the direct evidence of the high-pressure condition for the formation of these lamellaes. On the basis of these arguments, we conclude that the Cr-spinel with the diopside and coesite lamellae were formed as the CaO- and SiO2-bearing polymorphs under the high-pressure condition, over 12.5 GPa.

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

2005-12-01

337

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

338

The origin and age of the metamorphic sole from the Rogozna Mts., Western Vardar Belt: New evidence for the one-ocean model for the Balkan ophiolites  

NASA Astrophysics Data System (ADS)

This study brings new geochronological and petrochemical data from the metamorphic sole beneath the Rogozna Mts., Western Vardar ophiolite belt. The Rogozna metamorphic sole is located at the base of a serpentinite nappe and consists of amphibolites and talc-chlorite schists. The Rogozna amphibolites are medium- to fine-grained rocks with nematoblastic texture and pronounced foliation. They consist of green amphibole (~ 70 vol.%) with variable silica contents (6.4 to 7.8 Si apfu), as well as Mg# (molMg/[Mg + Fetot]; 0.53 to 0.77) and variably albitized plagioclase (~ 30 vol.%; Ab24-Ab98). Amphibolites are overprinted by a retrograde assemblage containing actinolite, epidote, clinoclore, sericite, chlorite, and magnetite. The amphibolites formed due to metamorphism of two basaltic suites: subalkaline/tholeiitic and alkaline. Subalkaline/tholeiitic amphibolites possess low Zr, Nb, Y, Th, Hf, TiO2, and P2O5 values and a LREE-depleted pattern, typical for the N-MORB (normal mid ocean ridge basalt) to BAB (back-arc basalt) origin. Alkaline amphibolites show elevated concentrations of Zr, Nb, Y, Th, Hf, TiO2, and P2O5 with a LREE-enriched pattern typically displayed by OIB (ocean island basalt). Amphibolites were crystallized during intra-oceanic thrusting at temperatures between 685 °C and 765 °C and at a depth of 12-17 km. 40Ar/39Ar cooling ages of amphibole, ranging from 165 to 170 Ma, slightly postdate the sole formation. Talc-chlorite schists are related to retrograde greenschist-facies metamorphism. They consist of Fe-rich talc and Cr-rich chlorite (peninite-diabantite) pseudomorphs after amphibole and MORB-type Cr-Al spinel, surrounded by Al- and Mg-poor ferrit chromite. The occurrence of ferrit chromite is related to earlier, amphibolite facies metamorphism. Chlorite pseudomorphs after amphibole were formed at 300-410 °C.

Borojevi? Šoštari?, S.; Palinkaš, A. L.; Neubauer, F.; Cvetkovi?, V.; Bernroider, M.; Genser, J.

2014-04-01

339

Characterization of the thermally metamorphosed mantle-crust transition zone of the Neoproterozoic ophiolite at Gebel Mudarjaj, south Eastern Desert, Egypt  

NASA Astrophysics Data System (ADS)

A suite of mantle-crust transition zone (Moho transition zone = MTZ) rocks are exceptionally well exposed in Gebel Mudarjaj area, southeastern desert of Egypt. The MTZ rocks were thermally metamorphosed by younger granitic intrusion, forming mafic-ultramafic hornfels with characteristic metamorphic mineral assemblages. The MTZ rocks are remarkably thin (30-50 m thick) and are composed mainly of dunites, troctolites, gabbroic rocks and pyroxenite masses overlying a basal serpentinized mantle harzburgite section. The Cr# of spinels of the basal serpentinized harzburgites and the MTZ dunites are on average 0.76 and 0.74, respectively, which is consistent with the range for arc peridotite spinels. The melt in equilibrium with these MTZ rocks is compositionally similar to boninitic magmas produced by high degrees of partial melting. The basal harzburgites and MTZ dunites have been produced by 19-23% mantle melting, and are compositionally similar to supra-subduction zone (SSZ) peridotites. The mantle melt in equilibrium with pyroxenites was formed after 16-17% partial melting, which subsequently reacted with the lower crustal gabbroic rocks to produce pyroxenites. The occurrence of pyroxenite masses at the crust-mantle boundary suggests a medium- to high-pressure accumulation of pyroxenes in mid- to lower crustal magma chambers. The original MTZ rocks were partially or fully hydrated, prior to the granitic intrusion, during the regional metamorphism, tectonic disruption and emplacement as various fragments of a dismembered ophiolite, to form antigorite-bearing serpentinized mafic-ultramafic rocks. Progressive metamorphic assemblages then overprinted the primary features due to the contact metamorphism of the MTZ rocks. The resultant metamorphic mineral assemblages are: (1) olivine + anthophyllite + tremolite ± chlorite ± talc (in the basal serpentinites), (2) olivine + enstatite ± chlorite (in the MTZ dunites), and (3) olivine + aluminous spinel (pleonaste) + chlorite + magnetite ± enstatite (in the troctolites). The peak of thermal metamorphism was about 650°-700 °C and < 7 kb, equivalant to the upper amphibolite facies.

Ahmed, Ahmed Hassan; Gharib, Moustafa E.; Arai, Shoji

2012-06-01

340

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

NASA Astrophysics Data System (ADS)

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

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

2008-11-01

341

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

SciTech Connect

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

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

1981-04-10

342

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

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

343

Bedded jaspers of the Ordovician L??kken ophiolite, Norway: Seafloor deposition and diagenetic maturation of hydrothermal plume-derived silica-iron gels  

USGS Publications Warehouse

Sedimentary beds of jasper (red hematitic chert) in the Ordovician L??kken ophiolite of Norway are closely associated with volcanogenic massive sulphide (VMS) deposits. The jaspers occur in the immediate hangingwall and laterally peripheral to the large L??kken (25-30 Mt) and small H??ydal (0.1 Mt) VMS deposits, and are exposed discontinuously for several kilometres along strike. Massive or laminated types predominate; jasper-sulphide debris-flow deposits are also abundant near VMS deposits. The jaspers contain hematite-rich laminae showing soft-sediment deformation structures and microtextural evidence that record the presence of a colloidal precursor and an origin as gels. Early textures include: (1) straight or curved chains of hematitic filaments 3-10 ??m in diameter and 20-100 ?? long; (2) branching networks of 15-25 ??m-thick, tubular structures surrounded by cryptocrystalline hematite and filled with quartz and euhedral hematite; (3) small (up to 10 ??m) spherules composed of cryptocrystalline hematite and silica; and (4) up to 50 ??m silica spherules with hematitic cores. The small filaments seem to have been deposited in varying proportions in the primary laminae, possibly together with hematitic and siliceous microspheroids. Diagenetic changes are represented by polygonal syneresis cracks, and the presence of cryptocrystalline (originally opaline) silica, chalcedony, quartz, carbonate and cryptocrystalline hematite and/or goethite forming botryoidal masses and spheroids < 10 ??m to 5 mm in diameter. Coarser euhedral grains of quartz, carbonate, and hematite are integral parts of these textures. Bleached, silica-rich jaspers preserve only small relics of fine-grained hematite-rich domains, and locally contain sparse pockets composed of coarse euhedral hematite ?? epidote. The jaspers are interpreted to record colloidal fallout from one or more hydrothermal plumes, followed by maturation (ageing) of an Si-Fe-oxyhydroxide gel, on and beneath the Ordovician sea floor. Small hematitic filaments in the jaspers reflect bacteria-catalysed oxidation of Fe2+ within the plume. The larger tubular filaments resulted from either microbial activity or inorganic self-organized mineral growth of Fe-oxyhydroxide within the Si-Fe-oxyhydroxide gel after deposition on the sea floor, prior to more advanced maturation of the gel as represented by the spheroidal and botryoidal silica-hematite textures. Bleaching and hematite ?? epidote growth are interpreted to reflect heat and fluids generated during deposition of basaltic sheet flows on top of the gels.

Grenne, T.; Slack, J.F.

2003-01-01

344

Metasomatism and Channelized Fluid Flow in Subducted Oceanic Lithosphere: the Record from an Eclogite-facies Shear Zone (Monviso Ophiolite, Italy)  

NASA Astrophysics Data System (ADS)

The Monviso ophiolite Lago Superiore Unit (LSU) constitutes a well-preserved, almost continuous fragment of upper oceanic lithosphere subducted down to ca. 80 km (between 50 and 40 Ma) and later exhumed along the subduction interface. The LSU is made of (i) a variably thick (50-500 m) section of eclogitized mafic crust (associated with minor calcschist lenses) overlying a 100-400 m thick metagabbroic body, and of (ii) a serpentinite sole (ca. 1000 m thick). This section is cut by two 10 to 100m thick eclogite-facies shear zones, found at the boundary between basalts and gabbros (Intermediate Shear Zone), and between gabbros and serpentinites (Lower Shear Zone: LSZ). Fragments of mylonitic basaltic eclogites and calcschists were dragged and dismembered within serpentinite schists along the LSZ during eclogite-facies deformation [Angiboust et al., Lithos, 2011]. Metasomatic rinds formed on these fragments at the contact with the surrounding antigorite schists during lawsonite-eclogite facies metamorphism, testifying to prominent fluid-rock interaction along with deformation. We present new petrological and geochemical data on four types of metasomatically altered eclogites (talc-, chlorite-, lawsonite- and phengite-bearing eclogites) and on a (serpentinite-derived) magnesite-bearing talc schist from the block rind. Bulk-rock compositions, in situ LA-ICP-MS analysis and X-ray Cr/Mg maps of garnet demonstrate that (i) these samples underwent significant Cr, Mg, Ni and Co enrichment and Fe, V (and to a lesser extent As) depletion during eclogitic metasomatism and (ii) garnet composition and chemistry of inclusions show extreme variation from core to rim. These compositional patterns point to a massive, pulse-like, fluid-mediated element transfer along with deformation, originating from the surrounding serpentinite (locally, with possible contributions from metasediments-equilibrated fluids). Antigorite breakdown, occurring ca. 15 km deeper than the maximum depth reached by these eclogites, could have provided significant amounts of fluid promoting extensive fluid/rock interaction. The strongly focused deformation also enhanced fluid/rock interaction that significantly contributed to the mechanical weakening attested by the extreme degree of block fragmentation and disaggregation observed along the LSZ in the field. We finally propose that transient slip behaviour along the LSZ under eclogite-facies conditions could have enhanced this highly anisotropic, pulse-like, subduction-parallel fluid migration pathway within the downgoing oceanic lithosphere.

Angiboust, S.; Pettke, T.; Agard, P.; Oncken, O.

2012-12-01

345

Late Precambrian Balkan-Carpathian ophiolite — a slice of the Pan-African ocean crust?: geochemical and tectonic insights from the Tcherni Vrah and Deli Jovan massifs, Bulgaria and Serbia  

NASA Astrophysics Data System (ADS)

The Balkan-Carpathian ophiolite (BCO), which outcrops in Bulgaria, Serbia and Romania, is a Late Precambrian (563 Ma) mafic/ultramafic complex unique in that it has not been strongly deformed or metamorphosed, as have most other basement sequences in Alpine Europe. Samples collected for study from the Tcherni Vrah and Deli Jovan segments of BCO include cumulate dunites, troctolites, wehrlites and plagioclase wehrlites; olivine and amphibole-bearing gabbros; anorthosites; diabases and microgabbros; and basalts representing massive flows, dikes, and pillow lavas, as well as hyaloclastites and umbers (preserved sedimentary cover). Relict Ol, Cpx and Hbl in cumulate peridotites indicate original orthocumulate textures. Plagioclase in troctolites and anorthosites range from An 60 to An 70. Cumulate gabbro textures range from ophitic to poikilitic, with an inferred crystallization order of Ol-(Plag+Cpx)-Hbl. The extrusive rocks exhibit poikilitic, ophitic and intersertal textures, with Cpx and/or Plag (Oligoclase-Andesine) phenocrysts. The major opaques are Ti-Magnetite and Ilmenite. The metamorphic paragenesis in the mafic samples is Chl-Trem-Ep, whereas the ultramafic rocks show variable degrees of serpentinization, with lizardite and antigorite as dominant phases. Our samples are compositionally and geochemically similar to modern oceanic crust. Major element, trace element and rare earth element (REE) signatures in BCO basalts are comparable to those of MORB. In terms of basalt and dike composition, the BCO is a 'high-Ti' or 'oceanic' ophiolite, based on the classification scheme of Serri [Earth Planet. Sci. Lett. 52 (1981) 203]. Our petrologic and geochemical results, combined with the tectonic position of the BCO massifs (overlain by and in contact with Late Cambrian island arc and back-arc sequences), suggest that the BCO may have formed in a mid-ocean ridge setting. If the BCO records the existence of a Precambrian ocean basin, then there may be a relationship between the BCO and the Pan-African ophiolites from the Arabian-Nubian Shield. We suggest that the BCO is the missing link between the Pan-African and the Avalonian-Cadomian peripheral orogens of Murphy and Nance [Geology 19 (1995) 469].

Savov, Ivan; Ryan, Jeff; Haydoutov, Ivan; Schijf, Johan

2001-10-01

346

Applying Rb/Sr isochron geochronology on phengite included in garnet: a case study on the meta-sediments of the Zermatt-Saas Fee ophiolite (Western Alps)  

NASA Astrophysics Data System (ADS)

We report Rb/Sr isochron ages obtained from phengites included in garnets of meta-sediments of the Zermatt-Saas Fee ophiolite (ZSFO) in the Western Alps. The ZSFO represents the remnants of the Mesozoic Piemonte-Ligurien Ocean. It consists of several sheets of ultramafic, mafic and meta-sedimentary rocks. The ZSFO underwent (U)HP peak metamorphic conditions (P ~15-25 kbar; T~500-600°C) in the Tertiary, followed by a greenschist-facies overprint during exhumation. Phengites included in garnets of two meta-pelites and one calcschist sample from two different localities were separated for Rb/Sr geochronology. The phengite inclusions-whole rock pairs give Rb/Sr isochron ages of 44.25 ±0.48 Ma and 43.19 ±0.32 Ma for one locality and 40.58 ±0.24 Ma for the other locality. Rb/Sr isochron ages using different size phengites from the matrix and a combination of bulk rock, tourmaline, zoisite or calcite yield ages of 40.12 ±0.51 Ma, 39.37 ±0.66 Ma and 39.8 ±1.5 Ma for the samples above. The phengite inclusion ages are hence about 4 and 1 Ma older than the phengite matrix isochrones. It is important to note that isochron initial ratios from phengite inclusion and phengite matrix isochrones are consistent with closed system behavior. The differences in initial 87Sr/86Sr ratios are proportional to the differences in time between them and the 87Rb/86Sr of the bulk rock. We interpret the phengite matrix isochrones to date early stages of exhumation (closure temperature of ca. 500°C). Older phengite inclusion ages are the result of garnet preventing Rb-Sr isotope exchange between the phengite inclusion and the rest of the rock matrix. For an even distribution of inclusions, there will be more inclusions recovered from the outer parts of the garnet (larger volume). Thus, we conclude that the phengite inclusion ages date the later part of the garnet growth. Hence ages slightly older than the peak of HP metamorphism. Previous studies of the prograde geochronology at Zermatt-Saas Fee using eclogitic garnets have obtained ages of ~40 Ma and ~50 Ma for Lu/Hf and ~40 Ma for Sm/Nd (e.g. [1]). Integrating garnet zoning pattern with the prograde garnet growth history suggests that a) Sm/Nd isochron ages reflect the end of the garnet growth, and thus peak metamorphic conditions, and b) Lu/Hf ages are either dating near peak metamorphic conditions or are skewed towards the onset of garnet growth, depending on the total time of growth [2]. Our results are in agreement with the above mentioned former data and show that using inclusions in garnet for Rb/Sr geochronology combined with petrological modeling is a powerful tool for reconstruction of (U)HP metamorphic terranes. [1] Lapen et al. 2003 EPSL [2] Skora et al. 2009 EPSL

de Meyer, C. M.; Baumgartner, L.; Johnson, C.; Beard, B. L.; Skora, S. E.

2011-12-01

347

Magmatism and metamorphism at the sheeted dyke-gabbro transition zone: new insight from beerbachite from ODP/IODP Hole 1256D and Oman ophiolite  

NASA Astrophysics Data System (ADS)

During IODP Expedition 335, two-pyroxenes bearing granulites (beerbachites) were extensively recovered as drilling cuttings at the gabbro-sheeted dyke transition zone of ODP Hole 1256D (East Pacific Rise, 6°44.163'N, 91°56.061'W). This lithology results from high-temperature metamorphism of previously hydrothermally altered diabases, basalts and/or gabbros; the heat source likely stems from the melt lens located at the top of the magmatic chambers imaged along present-day fast-spreading ridges. This lithology, associated with gabbroic bodies, characterises the transition zone between the sheeted dyke complex and the uppermost gabbroic section and represents the interface between magmatic and hydrothermal convecting systems in an oceanic crust formed at fast-spreading ridges. Samples acquired during IODP Exp. 335 show a particularly high degree of recrystallisation and are characterised by the absence of hydrous phases like amphibole, suggesting very high-T metamorphism. The Beerbachites mineral chemical characteristics are rather homogeneous compared to gabbros or dolerite from the sheeted dyke but pyroxenes Mg#, Ti, Al and Cr contents as well as the anorthite content of plagioclase are closer to gabbro than dolerite. This similarity may be explained by two hypothesis: either beerbachites in Hole 1256D are metamorphosed gabbros, or they underwent a melt-rock reaction process with the gabbros parental magma and were re-equilibrated at high temperature until their mineral composition become similar to that of gabbros. The gabbro-sheeted dyke transition zone in the Oman ophiolite is also outlined by the presence of high grade metamorphic rocks. Fine grained granulites and amphibolites that may be derived from the transformation of altered sheeted dyke diabases are in direct contact with fresh gabbroic and troctolitic bodies which are themselves cross-cut by dolerite dykes. The observation of textures show that high-T recrystallisation occurred in the fine grained beerbachite and amphibolites as well as in gabbros and troctolites. In completely recrystallised zones, the texture show a mosaic of small (<100 µm) Cpx and Pl grains with strait boundaries, and subrounded Opx. Textures and mineralogical assemblage are alike, independently of the nature of the protolith, suggesting that fine grained strongly recrystallised beerbachites may have formed from gabbro as well as from diabase. Moreover, the occurrence of unusual lithologies like Opx or amphibole-bearing troctolites and strongly metamorphosed amphibolites show that metamorphic processes at the sheeted dyke-gabbro boundary in the oceanic crust is not a simple reheating of hydrothermally altered diabases but involve complex interactions between newly injected magma and altered and non altered host. Partial melting of hydrothermally hydrated diabases and/or gabbros during reheating and the resulting probable magma mixing, together with melt-rock reaction are likely to play a critical role at this interface and, thus, in the ocean crust building.

Python, Marie; Abily, Bénédicte; France, Lydéric

2014-05-01

348

Gravitational collapse origin of shear zones, foliations and linear structures in the Neoproterozoic cover nappes, Eastern Desert, Egypt  

Microsoft Academic Search

The Um Esh–Um Seleimat area lies to the west of the Meatiq Core Complex (MCC), in the Central Eastern Desert (CED), Egypt, which forms part of the Neoproterozoic Arabian–Nubian Shield in NE Africa and Western Arabia. The study area is a NW-trending zone of intensely foliated ophiolitic melange and molasse sedimentary rocks. There is a single regional foliation, S1, defined

Abdel-Rahman Fowler; Baher El Kalioubi

2004-01-01

349

Diagenetic and metamorphic history of the Umm Nar BIF, Eastern Desert, Egypt  

Microsoft Academic Search

The Umm Nar BIF was formed in a sedimentary environment. It is confined to an upper stratigraphic zone of pre-Pan-African metamorphosed shelf deposits. During the Pan-African deformational history, the BIF and the host metasediments were tectonically' overlain by ophiolitic melange succession. The metasediments and the mélange were subjected to a major folding phase and then thrust over the ``Shaitian'' sheared

M. M. El Aref; A. El Dougdoug; M. Abdel Wahed; A. W. El Manawi

1993-01-01

350

Diagenetic and metamorphic history of the Umm Nar BIF, Eastern Desert, Egypt  

Microsoft Academic Search

The Umm Nar BIF was formed in a sedimentary environment. It is confined to an upper stratigraphic zone of pre-Pan-African\\u000a metamorphosed shelf deposits. During the Pan-African deformational history, the BIF and the host metasediments were tectonically'\\u000a overlain by ophiolitic melange succession. The metasediments and the mélange were subjected to a major folding phase and then\\u000a thrust over the “Shaitian” sheared

M. M. El Aref; A. El Dougdoug; M. Abdel Wahed; A. W. El Manawi

1993-01-01

351

Structural and tectonic evolution of the Umm Gheig\\/El-Shush region, central Eastern Desert of Egypt  

Microsoft Academic Search

The rocks of the Wadi Umm Gheig\\/El-Shush area in the central Eastern Desert of Egypt form part of the Nubian Shield, a component of the Neoproterozoic Pan-African Orogeny. The rocks have been divided into three units: (i) low-grade metamorphosed rocks, which consist of metavolcanic rocks interleaved with slices of ophiolitic melange; (ii) high-grade metamorphic rocks, which consist of syn-tectonic granitoids;

S. Ibrahim; J. Cosgrove

2001-01-01

352

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

NASA Astrophysics Data System (ADS)

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

Saccani, Emilio; Allahyari, Khalil; Rahimzadeh, Bahman

2014-05-01

353

Dynamics of sediment subduction, melange formation, and prism accretion  

Microsoft Academic Search

The descending plate and overriding block in a subduction zone are analogous to the guide surface and slide block in a slipper bearing, and subducted sediment is analogous to the lubricant. Subduction is more complex and varied, however, because the overriding block is not rigid, the sediment is buoyant, underplating can occur, and sediment supply can vary widely. A model

Ronald L. Shreve; Mark Cloos

1986-01-01

354

The Modulation of Discursive Functions. Melanges Pedagogiques, 1974.  

ERIC Educational Resources Information Center

Discursive functions are seldom expressed in an absolutely neutral way. In most cases, various colorings - expressive, affective or social - are superimposed on the utterance by which a function is conveyed. In so far as these colorings are not random shades, but can be regarded as graded nuances within given ranges, selected in order to fit the…

Roussel, F.

355

Impasses and Reversals in Foreign Language Instruction. Melanges Pedagogiques, 1971.  

ERIC Educational Resources Information Center

All proposed programs in foreign language instruction should be scrutinized and questioned in liberal-minded research centers, because this field seems to have arrived at an impasse. Two types of impasse are evident: "intrinsic" and "emergent.""Intrinsic" refers to built-in difficulties, those that are not artificially imposed on the…

Baskan, O.

356

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

357

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

358

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 s