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1

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

Microsoft Academic Search

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

Pierre A. Cousineau

1991-01-01

2

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

3

Neo-Tethyan Ocean evolution of southwest Tibet inferred from a melange structurally beneath the Kiogar ophiolite  

Microsoft Academic Search

Pre-collisional tectonic evolution of Neo-Tethyan Ocean is best recorded in the Yarlung Tsangpo suture zone (YTSZ) ophiolite complexes and associated melanges, one of which occurs in the Kiogar area of SW Tibet. Structural, sedimentological, palaeontological and geochemical work on a subophiolitic melange has been carried out in order to better constrain the tectonic evolution of this part of the Neo-Tethyan

G. H. Chan; J. Aitchison; J. S. Chan; M. Searle

2007-01-01

4

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

5

Ophiolitic Melanges in the Yarlung-Tsangpo "Big Bend" Canyon, SE Tibet  

NASA Astrophysics Data System (ADS)

The geology of the Tsangpo's "Big Bend" canyon in southeasternmost Tibet is quite significant to collisional tectonics, but this rugged region has been little studied. New 1:250 000 scale mapping since 1999 has improved our knowledge of the area and revealed four major units: (a) Namche Barwa group. Plagioclase gneisses with amphibole-containing high-P granulite lenses, exposed in the central area of the canyon, and representing the lower Indian plate. (b) Gangdese group. Plagioclase-gneiss, amphibolite, marble, quartzite intruded by granites, exposed surrounding the Namche Barwa group, representing the lower Eurasian plate. (3) Ophiolitic melange of the Indus-Yarlung Tsangpo (IYT) suture. The former occurs roughly along the Tsangpo "Big Bend" canyon with good exposures at Pangxin, Jiaresa and between Pailong and Zhaqu; it separates the Namche Barwa and Gangdese groups. (4) Ophiolitic melange of the Jiali--Pailong Tsangpo (JPT) suture. The mafic rock assemblage of IYT includes radiolarian chert, basaltic pillow lava, diabase sills, cumulate complex and serpentinized peridotite. The ophiolitic zone has been metamorphosed and highly deformed to form a melange containing various blocks and matrix, including blocks of metamorphosed mafic and ultramafics, quartzite and mica quartz schist; marble, and both Namche Barwa and Gangdese gneiss. The petrochemistry and trace elements of IYT rocks show similar characteristics to other ophiolite suites in Tibet and western Sichuan. HREE and Cr, Co, Ni are strongly depleted in upper-mantle peridotite, which is slightly enriched in LREE and Rb, Sr, Ba, Nb, Ta. Meta-basalt and diabase are enriched in LREE with (La/Yb) ratios of 7.29 to 1.28, very different from MORB. Compared with MORB, IYT meta-basalts and diabases are enriched in lithophile elements and are depleted in transition elements, suggesting that the IYT ophiolite suite originated from a small oceanic basin. The initial magma formed in this environment could be LREE and lithophile element-enriched and might have been contaminated by more of these elements during the intrusion. Along the Jiali-Pailong fault zone near high mountain ridges, ultramafic and mafic blocks occur enclosed or intruded by Jurassic to Triassic granite batholiths. Blocks of silicate, marble, and hornstone also occur. Field and lab studies suggest that these blocks may be a remnant of an old ophiolite zone. Rb-Sr dating on mafic rocks suggests a rough age of 215+/-63Ma. Stratigraphic, petrologic, and tectonic studies suggest that the JPT zone originated from an inner-arc basin. Oceanic crust formed in late Triassic and subducted from south to north from late Triassic to Jurassic. Closing of this ocean and orogenesis started in middle Jurassic accompanied by granite intrusions. The ophiolitic melange developed during arc-to-arc collision. Our data support a new hypothesis that the Neo-Tethys ocean appeared between Paleozoic and early Cretaceous and closed along the Nujiang suture.

Geng, Q.; Zheng, L.; Pan, G.; Ou, C.; Sun, Z.; Dong, H.; Wang, X.; Liu, Y.; Li, S.

2002-12-01

6

Geology of the western Sierra de Santa Cruz, Guatemala, Central America: An ophiolite sequence  

Microsoft Academic Search

Mapping of the western end of the massif revealed that the base of the allochthon consists of a serpentinite matrix melange and a highly deformed argillite. The latter is metamorphosed in the prehnite pumpellyite facies and is interpreted to be a pelagic sedimentary unit overridden by the ophiolite during obduction. The ophiolite is composed of a basal tectonized and serpentinized

J. H. Rosenfeld

1981-01-01

7

Polygenetic ophiolite belt of the California Sierra Nevada: Geochronological and tectonostratigraphic development  

NASA Astrophysics Data System (ADS)

The assumption that ophiolite sequences are generated at essentially one point in geologic time by the process of sea-floor spreading is critical for modern concepts in the tectonics of ophiolites and for topics dealing with their structure and petrology. However, this assumption has only been verified in a few locations by an integrated geochronological and structural-stratigraphic approach. Many ophiolite sections are reconstructed from structurally disrupted sequences with the idealized ocean floor model in mind. Such reconstructions are prone to error without adequate age control on each of the reconstructed fragments. This is a significant problem in structurally complex regions where more than one generation of ophiolite may be present. In this paper new Pb/U zircon ages are presented for key locations along a 375 km segment of the western Sierra Nevada ophiolite belt. These age data are combined with structural-stratigraphic observations and published ages, and significant tectonic implications for the ophiolite belt emerge. Three different ophiolitic assemblages are recognized with igneous ages of about 300, 200 and 160 m.y. B.P. Rocks of the 300 m.y. assemblage are in a completely disrupted array of metamorphic tectonite slabs and serpentinite-matrix melange. Fragments of upper Paleozoic seamounts occur in association with the ophiolitic melange, and together these assemblages constitute the basement framework for the western Sierra. Pb/U and K/Ar isotopic systematics are complex within this framework and indicate a polymetamorphic history. Systematics in the 200 and 160 m.y. assemblages are less complex and give tighter igneous age constraints. Rocks of the 200 m.y. assemblage are in a semi-intact state with only local tectonite and melange zones. Rocks of the 160 m.y. assemblage are intact, but nevertheless deformed. Both the 200 and 160 m.y. assemblages have equivalent age basinal volcanic-sedimentary sequences that lie unconformably above the ophiolitic melange basement. In each case the basinal sequences locally extend conformably into the upper stratigraphic levels of the age-equivalent ophiolite sections. These relations along with vestiges of intrusive contacts between the edges of both younger ophiolites and the melange basement indicate that the younger ophiolites underwent igneous formation in proximity to the melange basement. The Sierran ophiolite belt is considered to have formed by a multistage process initiated by the early Mesozoic tectonic accretion of upper Paleozoic sea-floor in general proximity to the ancient continental margin. Regional metamorphism and ophiolitic melange resulted. This accretionary nucleus became the basement of Jurassic-age primitive volcanic arc terranes which underwent rifting episodes during the production of the 200 and 160 m.y. ophiolites. The rifting episodes resulted in the formation of sedimentary basins which were the depositional sites of volcanic-sedimentary sequences. Non-volcanic sources for the basinal sedimentary rocks include the melange basement and continental margin terranes. Contact zones between pre-existing basement and the juvenile ophiolitic sequences created during the rifting episodes consist of dynamothermal metamorphic aureoles, protoclastic deformation zones and cross-cutting dikes. Such edge-zone assemblages are in most localities obscurred or destroyed by superimposed deformations resulting from convergent and perhaps transform motions along basin edges. Both the 200 and 160 m.y. basins were destroyed by compressional orogenic episodes shortly after their formational episodes. Destruction of young ophiolite floored basins may be a common course of events when small oceanic-type plates are generated along continental margin environments. Such tectonic settings are ideal for the emplacement of young ophiolite sheets.

Saleeby, Jason B.

1982-03-01

8

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

SciTech Connect

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

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

1985-01-01

9

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

10

Geology of the western Sierra de Santa Cruz, Guatemala, Central America: An ophiolite sequence  

NASA Astrophysics Data System (ADS)

Mapping of the western end of the massif revealed that the base of the allochthon consists of a serpentinite matrix melange and a highly deformed argillite. The latter is metamorphosed in the prehnite pumpellyite facies and is interpreted to be a pelagic sedimentary unit overridden by the ophiolite during obduction. The ophiolite is composed of a basal tectonized and serpentinized harzburgite enclosing a gabbroic pluton at least two kilometers thick with associated ultramafic cumulates and quartz diorites. The pluton is capped by a zone of diabase which intrudes and feeds massive and pillowed basalt flows with intercalated pelagic chert and limestone. The intrabasalt sedimentary rocks were dated as Valaginian to Lower Cenomanian. The basalts are overlain by a sequence of tuffaceous cherts which coarsen upward to volcaniclastic flysch and epiclastic volcanic breccia containing andesitic and dacitic lava and welded tuff fragments.

Rosenfeld, J. H.

1981-08-01

11

In-situ pressure - temperature condition of tectonic melange : constraints from fluid inclusion analysis of syn-melange veins-  

NASA Astrophysics Data System (ADS)

One of the most reliable settings for the tectonic melange with composite planar fabric is along the decollement in the subduction zone (e.g. Cowan, 1985; Moore and Byrne, 1987; Kimura and Mukai, 1991). The decollement is developed from aseismic shallow part to seismogenic zone of several to tens of kilometer in depth. The decollement-related-melange, now exposed on land, might therefore record the deformation process along the aseismic to seismogenic decollement. Trapped pressure and temperature of fluid inclusion is estimated from "syn-melange" veins developed around the necks of boudin of sandstone blocks in the melange of the Shimanto Belt, SW Japan. The melange was decollement-related tectonic melange. Fluid inclusion analysis from "syn-melange" veins might be only tool to reveal the P-T condition during melange formation. The result of P-T estimation ranges from about 150 (}25)° Cto 220 (}31)° C for temperature and from 81 (+15) MPa to 235 (_}18) MPa for pressure. Geothermal gradient estimated from the result is between about 10.0 (+0.2/-1.5)° C/km (lithostatic) and 4.2 (+0.1/-0.9)° C/km (hydrostatic) if the fluid pressure ratio to lithostatic pressure is constant in each datum. The estimated lithostatic thermal gradient is much closer to that calculated from the oldness of the subducting oceanic plate. P-T range suggests that melange was formed enough within the seismogenic zone hypothesized by thermal model although the deformation mechanisms of melange, dominant diffusive mass transfer with minor brittle breakage, do not show the seismic deformation. One possible explanation for this discrepancy is that the melange was formed during the interseismic period. Alternative explanation is that melange formation was accomplished before subducting into the seismogenic zone but around the seismic front. Lithification front hypothesis for the seismic front suggests such frontal setting for the melange.

Hashimoto, Y.; Enjoji, M.; Sakaguchi, A.; Kimura, G.

2001-12-01

12

Melange a Six Ondes Degenere dans les Absorbants Saturables  

Microsoft Academic Search

Issus d'une generalisation du melange a quatre ondes degenere, les melanges a n ondes degeneres sont utiles pour la mesure des divers ordres de la susceptibilite nonlineaire. Nous avons procede a l'etude theorique et experimentale du melange a six ondes degenere dans des absorbants isotropes et anisotropes. Pour l'analyse theorique, nous avons developpe une methode de calcul basee sur une

Alain Blouin

1991-01-01

13

Melange a Six Ondes Degenere dans les Absorbants Saturables  

NASA Astrophysics Data System (ADS)

Issus d'une generalisation du melange a quatre ondes degenere, les melanges a n ondes degeneres sont utiles pour la mesure des divers ordres de la susceptibilite nonlineaire. Nous avons procede a l'etude theorique et experimentale du melange a six ondes degenere dans des absorbants isotropes et anisotropes. Pour l'analyse theorique, nous avons developpe une methode de calcul basee sur une approche holographique. Cette methode fut utilisee pour l'etude du melange a six ondes en regimes stationnaire et transitoire sous des conditions de faibles et de fortes saturations. Des experiences realisees a l'aide d'impulsions excitatrices de courte duree, soit 33 picosecondes, dans des verres dopes aux semi-conducteurs et dans la Rhodamine 6G en solution ont permis de verifier la validite de notre modele theorique. Nous avons aussi etudie les oscillateurs auto-pompes par melange a six ondes degenere.

Blouin, Alain

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

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

NASA Astrophysics Data System (ADS)

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

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

2013-03-01

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

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

18

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

NASA Astrophysics Data System (ADS)

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

Casey, John; Dewey, John

2013-04-01

19

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

20

Multistage melange formation in the Franciscan Complex, northernmost California  

NASA Astrophysics Data System (ADS)

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

Aalto, K. R.

1981-12-01

21

Example of ensialic ophiolites emplaced in a wrench zone: Innermost Hellenic ophiolite belt (Greek Macedonia)  

Microsoft Academic Search

The innermost Hellenic ophiolite belt of northern Greece comprises discontinuous ophiolites of the Guevgueli and Chalkidiki complexes. Field, structural, chemical, and geochronological data indicate that they are broadly autochthonous and were emplaced into continental crust along a northwest-southeast wrench zone during the Middle and Late Jurassic. Variations in structure and petrology along the belt can be related to changes in

Jean Bébien; Roland Dubois; Aline Gauthier

1986-01-01

22

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

23

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

NASA Astrophysics Data System (ADS)

The Cycladic blueschist belt (CBB) in the central Aegean Sea has experienced epidote blueschist to eclogite facies metamorphism in the Eocene (c. 50-40 Ma) during collisional processes between the Apulian microplate and Eurasia. The general geological framework is well established, but many details of the tectonometamorphic history still are not fully understood. Unresolved issues concern the importance of pre-Eocene HP metamorphism [1, 2] and the geochronological record that is preserved in melange sequences. These aspects are addressed in an ionprobe (SHRIMP) U-Pb zircon study focusing on block-matrix associations from the islands of Syros and Andros. Melanges comprise in variable mixtures eclogites, glaucophanites, meta-gabbros, ultrabasic rocks, meta-acidic gneisses and jadeitites in a serpentinitic and/or metasedimentary matrix. The origin of the block-matrix associations is controversial and in many cases it remains unclear whether these melanges record sedimentary or tectonic processes. A tectonic slab from Syros, consisting of an interlayered meta-acidic gneiss - glaucophanite sequence, yielded 206Pb/238U ages of c. 237-245 Ma, similar to ages determined for rocks collected from structurally coherent sequences on other Cycladic islands. On Syros, we have also studied zircons from a metasomatic alteration profile, which developed around a compound eclogite-jadeitite net-veined block enclosed in a serpentinite matrix. From the outside in, distinct blackwall alteration zones (c. 5-30 cm in thickness) can be distinguished, which predominantly consist either of actinolite- chlorite, glaucophane or omphacite. Zircon from the unaltered jadeitite and all reaction zones yielded 206Pb/238U ages of c. 80 Ma. Across this profile, systematic changes are observed in zircon morphology and CL patterns. U- and Th-concentrations in zircon decrease towards the peripheral rinds. These observations are difficult to reconcile with a magmatic origin of the zircons and instead we suggest a relationship of zircon formation to fluid infiltration and block-matrix interaction during subduction zone processes. Similar ages in meta-gabbros and associated felsic melts [2, this study] probably represent protolith ages. Thus, the Syros melange apparently records the coexistence of rocks documenting both magmatic and metamorphic processes of Cretaceous age. From Andros, we have dated a meta-gabbro and a meta-acidic gneiss that yielded Jurassic 206Pb/238U ages of 154.5 ± 0.7 Ma and 160.0 ± 0.5 Ma, respectively, which perfectly correspond to protolith ages reported for ophiolites from the Balkan peninsula and Crete [3]. Our study indicates that the Cycladic melanges comprise a heterogeneous age population of rocks derived from different sources. Up to now three magmatic (c. 240 Ma, c. 155-160 Ma, c. 75-80 Ma) and one metamorphic age group (c. 80 Ma) were recognized, but the number of dated blocks still is relatively small and the picture might be much more complex than implied by the available datasets. [1] Bröcker & Enders (1999): Geol. Mag. 136, 111-118. [2] Tomaschek et al. (2003): J. Petrol. 44, 1977-2002. [3] Koepke et al. (2002): Lithos 65, 183-203.

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

2005-12-01

24

Obduction-related ophiolitic polymict breccias covering the ophiolites of Antalya (southwestern Turkey)  

SciTech Connect

The ophiolites of the Antalya nappes, cropping out in the area if Kemer (southwestern Turkey), are capped by a spectacular detrital sequence composed of various ophiolitic breccias whose total thickness reaches 1000 m. Four main formations have been distinguished within the breccias. The basal formations are in depositional contact over the previously tectonized and eroded deepest part of the ophiolites. The upper breccias are dated as middle Maastrichtian, which is the age of the deposition of synchronous serpentinitic olistostromes onto the continental margin. These data suggest that the breccias formed as debris slides from bathymetric highs on the ocean crust as a result of renewed tectonic activity accompanying obduction onto the continental margin.

Lagabrielle, Y.; Whitechurch, H.; Marcoux, J.; Juteau, T.; Reuber, I.; Guillocheau, F.; Capan, U.

1986-09-01

25

Melange a Quatre Ondes Degenere dans les Absorbants Saturables EN Milieu Mince  

Microsoft Academic Search

Le melange a quatre ondes est utilise notamment pour determiner la duree des temps de reorientation moleculaire et mesurer la grandeur des susceptibilites nonlineaires du troisieme ordre. Nous avons mis en evidence les particularites du melange a quatre ondes degenere en milieu absorbant mince a l'aide du formalisme de l'optique nonlineaire et de l'approche holographique. Des experiences realisees avec des

Claire O'Neill

1995-01-01

26

Evolution of the Kid Group, southeast Sinai Peninsula: Thrusts, melanges, and implications for accretionary tectonics during the late Proterozoic of the Arabian-Nubian Shield  

NASA Astrophysics Data System (ADS)

The late Proterozoic age Kid Group in the southeast Sinai Peninsula is a volcano-sedimentary sequence of island-arc affinities. The group deformed in four main phases, with a thermal peak after D1, followed by a regional greenschist facies overprint during D2. Recumbent folding followed by thrusting began during D2, with the folds being propagated southward toward the arc foreland until D4, the last folding event. The thrusts are responsible for the juxtaposition of four megatectonic units above a substrate of unknown composition. These units, probably allochthonous, comprise a Basal Complex of infracrustal gneiss and three arc-related units, including inferred vestiges of a forearc accretionary prism and a marginal basin. Narrow slices of ophiolitic rocks were emplaced tectonically along the boundaries of some of these units. Melanges attributed to gravitational collapse of the forearc edifice and thrust tectonics formed, in particular, along the arc-forearc transition zone. The evolution of the Kid Group, coupled with data from the rest of the Arabian-Nubian Shield, can be well accommodated within the concept of oceanic-crust subduction, magmatic-arc development and ultimate collision between the arc complex and a continental margin.

Shimron, Aryeh E.

1984-04-01

27

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

NASA Astrophysics Data System (ADS)

Characteristics of the classic late Jurassic and late Cretaceous Tethyan-type ophiolites of the Eastern Mediterranean - Persian Gulf region are consistent with formation in a supra-subduction zone (SSZ); the most robust model for their production entails formation upon subduction initiation in the forearc or proto-forearc region of a nascent intra-oceanic arc (IOA). The ‘ophiolite rule’ however, demonstrates that a co-existence of MOR-like (in addition to SSZ) lavas is the norm and not the exception. Specifically, the ‘normal’ chemostratigraphy of Tethyan-type ophiolites usually exhibits a thick, lower section of tholeiitic MOR-like lavas in addition to an upper section of dominantly calc-alkaline, HFSE-depleted, VAB-like lavas and intrusives and lesser boninites; latest stage dikes of boninitic affinity commonly cut both suites. This association testifies to changes in the ophiolitic melt source. To date, explanations for the near ubiquitous relationship of these two distinct tectonomagmatic suites usually infer that conventional MOR or marginal basin formation was followed by tectonic reorganization and consequent IOA construction above the (much older) first-formed MORB seafloor. While such ad hoc tectonic interpretations might apply to a few ophiolites, these explanations surely can’t explain what appears to be a global ophiolite phenomenon as this chemostratigraphy is now being recognized in other major ophiolitic belts (e.g., of the SW Pacific) and IOA forearcs (e.g., the Izu-Bonin-Mariana arc). A lack of hiatus between sequences is shown by boundaries between the two main ophiolitic units that are stratigraphic and not fault-bounded; there exists no significant temporal hiatus between formation of the lower tholeiitic and upper calc-alkaline suites (no sediment horizons or obvious unconformities); furthermore, lavas compositionally intermediate to MORB- and VAB-like occur stratigraphically between the sequences. We present a new model consistent with these observations and detailed trace element chemistry characteristics. We interpret the normal ophiolite chemostratigraphy as described above to indicate that asthenospheric sources were systematically depleted and progressively metasomatized as the ophiolite formed. The model entails the decompression melting of a fertile, lherzolitic source and eruption of the lower MOR-like lavas in the earliest stages of subduction initiation; at this stage contribution of fluids from the sinking slab is small. Decompression melting of the asthenosphere leaves a depleted, OPX-rich source that is progressively metasomatized by slab-derived fluids; partial melting of increasingly depleted harzburgitic residue results in the production of VAB-like lavas and associated rocks of the upper unit and the latest stage dikes. Furthermore, the fact that many Neo-Teythan ophiolites display no affiliation with an evolved magmatic arc suggests formation in association with a narrow ocean basin, where subduction to ~100 km (needed to form a magmatic arc) did not occur prior to ophiolite emplacement.

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

2009-12-01

28

Arabian Shield ophiolites and Late Proterozoic microplate accretion  

NASA Astrophysics Data System (ADS)

Fragments of Late Proterozoic ocean crust and mantle (ophiolites) occur within six major fault zones that mark sutures between crustal blocks (microplates) that were accreted between about 630 and 715 Ma to form the Arabian Shield. We report new U-Pb zircon ages for ophiolitic gabbro, diorite, and plagiogranite that range from 840 to 700 Ma and establish these complexes among the oldest proven ophiolites. By dating the ophiolitic rocks we are able to monitor the magmatic ages of sea-floor spreading events during accretion of the Arabian Shield. Comparison of the ophiolitic dates with the ages of the adjacent crustal blocks provides a more complete basis for plate-tectonics reconstruction of the shield than has previously been possible. Our new zircon ages confirm earlier Sm-Nd mineral isochrons for ophiolites of the northwestern Arabian Shield, show that the ophiolites are among the oldest oceanic rocks in each terrane, support near- or within-arc tectonic settings for the ophiolites, suggest that the Bir Umq suture extends north along the Nabitah mobile belt into the northern shield, and suggest that older (>1250 Ma) continental crustal material is locally present in the dominantly “oceanic” western shield. We suggest that Indonesia and Alaska provide Phanerozoic analogues for stages in the accretion history. Note: Additional material for this article is Supplementary Data 8715, available on request from the GSA Documents Secretary (see footnote 1).

Pallister, John S.; Stacey, John S.; Fischer, Lynn B.; Premo, Wayne R.

1987-04-01

29

How Plate Kinematics Creates and Sweeps Away Supra-subduction ophiolites  

Microsoft Academic Search

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

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

2001-01-01

30

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

USGS Publications Warehouse

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

Tabor, R. W.

1994-01-01

31

Oscillatory zoning in eclogitic garnet and amphibole, Northern Serpentinite Melange, Cuba: a record of tectonic instability during subduction?  

Microsoft Academic Search

Exotic blocks of eclogite from distant localities along the Northern Serpentinite Melange of Cuba have comparable P-T histories that include high-pressure prograde sections (450-600 ? C, >15 kbar) associ- ated with subduction of oceanic lithosphere, and retrograde sections within the albite-epidote amphibolite facies (<500 ? C, <10 kbar) related to melange uplift. 40Ar ? 39Ar and Rb ? Sr cooling

A. G ARCIA-CASCO; R. L. TOR R ES-R O LDAN; G. M ILL; P. MO; J. S CHNEIDER

32

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

33

Origin of the amphibolite ``sole'' of the Josephine ophiolite: Emplacement of a cold ophiolite over a hot arc  

NASA Astrophysics Data System (ADS)

The amphibolite "sole" that occurs along the basal thrust of the Josephine ophiolite (Madstone Cabin thrust) is distinct from those described for ophiolites such as the Bay of Islands and Oman. Antigorite-bearing serpentinite mylonites occur directly above the amphibolite, and the presence of antigorite + brucite indicates formation between 300° and 400°C. The antigorite mylonites also contain relict lizardite/chrysotile, suggesting formation of antigorite by prograde metamorphism along the base of the ophiolite. Further evidence for prethrusting low-temperature serpentinization comes from a lizardite/chrysotile shear zone within fresh peridotite directly above the thrust that is cut by undeformed diabase dikes having geochemical affinities to the upper pillow lavas of the Josephine ophiolite. The prograde metamorphism of the serpentinite at the base of the ophiolite was apparently synchronous with greenschist facies retrogressive metamorphism of the underlying amphibolite. The amphibolite is chemically diverse and has been divided into two groups. Group 1 amphibolite appears to be derived from mafic plutonic rocks and has immobile trace element abundances indicating affinities to volcanic arc magmas. Group 2 amphibolite is metavolcanic (contains some quartzite) and has magmatic affinities to mid-ocean ridge basalt and within-plate basalt, an association that is distinctive of older basement rocks elsewhere in the Western Klamath terrane. We suggest the amphibolite was derived from the footwall of the Madstone Cabin thrust, including country rocks for the Chetco complex (Group 2) and possibly older intrusive phases of the Chetco complex itself (Group 1). Previous workers have shown that the base of the amphibolite is intruded by gneissic gabbro of the Chetco complex, that intrusion of the Chetco complex overlapped in time with deformation of the amphibolite, and that during and after emplacement the Josephine ophiolite and overlying flysch were intruded by abundant arc-related dikes and plutons. Emplacement of the Josephine ophiolite appears to have occurred by underthrusting of a cold ophiolite by an active magmatic arc.

Harper, Gregory D.; Grady, Kristen; Coulton, Angela J.

1996-04-01

34

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

Microsoft Academic Search

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

G. P. Yumul

1996-01-01

35

Melange a Quatre Ondes Degenere dans les Absorbants Saturables EN Milieu Mince  

NASA Astrophysics Data System (ADS)

Le melange a quatre ondes est utilise notamment pour determiner la duree des temps de reorientation moleculaire et mesurer la grandeur des susceptibilites nonlineaires du troisieme ordre. Nous avons mis en evidence les particularites du melange a quatre ondes degenere en milieu absorbant mince a l'aide du formalisme de l'optique nonlineaire et de l'approche holographique. Des experiences realisees avec des impulsions de 33 ps ont permis de caracteriser la cinetique de la nonlinearite en regime transitoire des molecules de rhodamine 6G incorporees dans des matrices d'alcool polyvinylique; un milieu aussi compose d'absorbants anisotropes, les molecules de rhodamine 6G en solution aqueuse et un autre forme d'absorbants isotropes, les films minces de cristaux de semiconducteur CdS_ {x}Se_{1-x} ont ete etudies. Nous avons observe pour ces trois milieux les signaux generes aux ordres superieurs par melange a quatre ondes degenere en milieu mince pour differentes conditions de polarisation.

O'Neill, Claire

1995-01-01

36

The Josephine Ophiolite—Remains of a Late Jurassic marginal basin in northwestern California  

NASA Astrophysics Data System (ADS)

The Josephine Ophiolite is a fully developed ophiolite exposed in the western Jurassic belt of the Klamath Mountains. The ophiolite is overlain by a thick flysch sequence derived from both an active volcanic arc and the continental margin. Many lines of evidence favor genesis of the ophiolite in an immature marginal basin by back-arc spreading rather than at a mid-ocean spreading center.

Harper, Gregory D.

1980-07-01

37

The carbon isotope composition of natural SiC (moissanite) from the Earth's mantle: New discoveries from ophiolites  

Microsoft Academic Search

Moissanite (natural SiC) has been recovered from podiform chromitites of several ophiolite complexes, including the Luobusa and Donqiao ophiolites in Tibet, the Semail ophiolite in Oman and the United Arab Emirates, and the Ray-Iz ophiolite of the Polar Urals, Russia. Taking these new occurrences with the numerous earlier reports of moissanite in diamondiferous kimberlites leads to the conclusion that natural

Robert B. Trumbull; Jing-Sui Yang; Paul T. Robinson; Simonpietro Di Pierro; Torsten Vennemann; Michael Wiedenbeck

2009-01-01

38

Episodic magma chambers and amagmatic extension in the Josephine ophiolite  

Microsoft Academic Search

Geochemical and structural aspects of the Josephine ophiolite indicate that it formed by alternating cycles of magmatic and structural (amagmatic) extension. Episodic magma chambers are indicated by the presence of very primitive lavas in the lower pillow lavas which would have mixed into a magma chamber. Fe-Ti basalts that occur in the uppermost extrusive rocks and plagiogranites that occur along

Gregory D. Harper

1988-01-01

39

Spreading structure of the Troodos ophiolite, Cyprus: Some paleomagnetic constraints  

Microsoft Academic Search

It has been suggested that grabens on the northern flank of the Troodos ophiolite formed by listric faulting in an oceanic environment and represent axial valleys fossilized by ridge-jumping. This paleomagnetic survey of the Solea graben was carried out to constrain the structural interpretation. An analysis was devised which restores both the dikes sampled to vertical and the primary remanent

Simon Allerton; F. J. Vine

1987-01-01

40

Ocean floor metamorphism in the Betts Cove ophiolite, Newfoundland  

Microsoft Academic Search

The mineralogical and geochemical features of the lower Ordovician Betts Cove ophiolite of northeastern Newfoundland indicate that hydrothermal circulation of seawater near a mid-ocean ridge has been involved in the metamorphism of the complex. The degree of greenschist facies metamorphism increases with stratigraphie depth in the ophioli te. Calcite, hematite and epidote distributions show that the metamorphosing fluid penetrated downward

R. A. Coish

1977-01-01

41

Ophiolite emplacement in NE Oman: constraints from magnetotelluric sounding  

NASA Astrophysics Data System (ADS)

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

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

2009-03-01

42

Emplacement of the Western Tethyan Ophiolites and Atlantic Spreading  

NASA Astrophysics Data System (ADS)

This contribution shows that the cessation of emplacement of the 'Tethyan' supra-suduction zone (SSZ) ophiolites of the circum-Mediterranean region, and Oman, is contemporaneous with a marked drop in the relative velocities between Africa and Eurasia. It is argued that this contemporaneity is to be expected from plate theory. The Hellenic-Dinaric SSZ ophiolites in Greece, Albania and former Yugoslavia, range in age from ~173-168 Ma (mid-Jurassic). They were emplaced, eroded and covered by younger sediments by ~140 Ma, or possibly earlier, on the plate margin between northwest Gondwana and southwest Laurasia (=Africa/Eurasia). During emplacement, a passive continental margin was partially subducted, but because large-scale subduction of a continent appears to be precluded by the low density of continental crust, the subduction zone eventually ceased to exist and moved elsewhere. The SSZ ophiolites of Turkey, Cyprus, Syria, Iran and Oman are of mid-Cretaceous age. The initial setting of the mid-Cretaceous ophiolites is clearest in Oman. The Oman ophiolite appears to have formed during roll-back of Triassic (or possibly Permian) ocean-floor from the Iranian margin at ~95 Ma and to have been emplaced by early Maastrichtian time, ~70 Ma, after which the Arabian passive margin could not undergo further subduction. Despite the complexities and uncertainties in the Atlantic spreading data, the relative motion of Africa to stable Europe in the region of the Tethyan ophiolites is quite simple. A major slowing of the relative motion from ~40-50 mm yr-1 to less than 5 mm yr--1 took place at ~140 Ma, broadly synchronous with the cessation of Hellenic-Dinaric ophiolite emplacement. Slow motion lasted from ~140-125 Ma and is attributed to the failure to subduct the passive continental margin; to the destruction of the Africa-Eurasian margin of that time; and to its eventual reestablishment elsewhere, ~15 m.y. later, after which the relative motion speeded up to ~20 mm yr-1. A second phase of very slow motion to ~4 mm yr-1, from 75 Ma to 55 Ma, is broadly synchronous with the cessation of emplacement of the mid-Cretaceous ophiolites. Like the first phase, the slowing down is attributed to the failure to subduct a continental margin (in the case of Oman, the NE Arabian margin) and the requirement to re-position the margin; processes that took some 20 m.y. to complete. Thus both slow phases of Africa to Europe motions are dictated by the tectonic setting of ophiolite emplacement. In this setting, subduction zone processes influence the spreading history of the parts of the Atlantic, rather than the other way round. There is no evidence that any large igneous provinces (LIPs) played a role in the emplacement of these western Tethyan ophiolites.

Smith, A. G.

2005-12-01

43

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

44

Paleo- and Neo-Tethyan ophiolites of Iran: a progress report  

NASA Astrophysics Data System (ADS)

The Bitlis-Zagros and Alborz stuture zones of Iran mark two collisional plate boundaries in the Alpine-Himalayan orogenic belt. The ophiolites of these zones together with the ophiolites of Makran accretionary prism and Central Iran form discontinuous linear belts of Tethyan oceanic fragments, which form a bridge between the Mediterranean and Himalayan ophiolites. Based on age alone these ophiolites have been divided into less abundant Paleozoic and much more abundant Mesozoic ophiolites. The Paleozoic ophiolites are located along the Alborz orogenic belt [i.e., Rasht and Mashhad ophiolites (297 Ma and 268 Ma)] and near Anarak in Central Iran. which are the remnants of the Paleo-Tethys ocean crust emplaced as result of closure of the Paleo-Tethys between the Turan and the Central Iranian Microplates (CIM). The Mesezoic ophiolites of Iran are more abundant and include the Zagros ophiolites (i.e., Neyriz and the Kermanshah ophiolites which appear to be coeval with the Oman ophiolite obducted onto the Arabian plate (˜96-92 Ma). The Khoy ophiolite in NW Iran which has formation age of Middle to Late Jurrasic (˜159-155 Ma), and emplacement age Albian ages (˜ 109-104 Ma) has a different tectonics than other Zagros ophiolites. Unfragmented ophiolites of the Makran accretionary prism which are located to the south of the Sanandaj-Sirjan microcontinental block, including complexes such as Band-e-Zeyarat/Dar Anar, Ganj and Remeshk/Mokhtarabad (˜140-98 Ma) are similar in age to the Masirah ophiolite (i.e., ˜150-120 Ma). The ophiolites of the Central Iran include those inside of the Sanandaj-Sirjan microcontinental block, such as Shahr-e-Babak (120 Ma), Naien (100 Ma), Baft, Sabzevar in north central Iran (98-70 Ma) and Tchehel Kureh on the eastern boundary of CIM.Geochemically, these ophiolites are quite diverse and show a significant variations in rock composition, representing a wide range of tectonic environment of formation. In terms of radiogenic isotopic data, basalt and gabbros from Neyriz (Zagros), Khoy (NW Zagros ?), and Band-e-Zeyarat (Makran) have Indian Ocean MORB signature.

Ghazi, M.; Hassanipak, A.; Babaie, H.

2003-04-01

45

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

46

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

Microsoft Academic Search

Sedimentology can shed light on the emplacement of oceanic lithosphere (i.e. ophiolites) onto continental crust and post-emplacement\\u000a settings. An example chosen here is the well-exposed Jurassic Mirdita ophiolite in southern Albania. Successions studied in\\u000a five different ophiolitic massifs (Voskopoja, Luniku, Shpati, Rehove and Morava) document variable depositional processes\\u000a and palaeoenvironments in the light of evidence from comparable settings elsewhere (e.g.

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

2010-01-01

47

Early-Middle Ordovician ridge-trench collision during the closure of Iapetus: Evidence from the Dunnage Melange tract, Newfoundland Appalachians  

Microsoft Academic Search

The Red Indian Line marks the boundary between the peri-Laurentian and peri-Gondwanan realms in the Northern Apppalachians and delimits the main Iapetus suture. The Middle Ordovician Dunnage Melange tract occurs immediately south of the Red Indian Line in north-central Newfoundland. The genesis of the melange belt has always been contentious, because it preserves structures both suggestive of an olistostromal (e.g.

Alexandre Zagorevski; Cees R. van Staal; Vicki McNicoll

2010-01-01

48

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

49

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

50

Ophiolites, the Sierra Nevada, “Cordilleria,” and Orogeny along the Pacific and Caribbean Margins of North and South America  

Microsoft Academic Search

Ophiolites are major expressions of orogeny; they are dominantly oceanic crust and mantle emplaced by collision of a mantle-rooted thrust (subduction zone) with a continental margin or island arc. Ophiolite nappes thus represent remnants of lithospheric plates; their basal thrusts (fossil subduction zones) intrinsically cannot be balanced; their displacements are unknown but very large.Two major belts of Mesozoic ophiolites and

E. M. Moores

1998-01-01

51

Rare earth element geochemistry of the Betts Cove ophiolite, Newfoundland: complexities in ophiolite formation  

NASA Astrophysics Data System (ADS)

The Betts Cove ophiolite includes the components of typical ocean crust: pillow lavas, sheeted dikes, gabbros and ultramafics. However, the trace element geochemistry of basaltic rocks is unusual. Three geochemical units are recognized within the lava and dike members. Within the pillow lavas, the geochemical units correspond to stratigraphic units. Upper lavas have `normal' ( i.e ., typical for ocean floor basalts) TiO 2 contents (0.75 to 2.0 wt%), heavy rare earth elements (HREE) values in the range 6-20× chondrites and chondrite-normalized REE patterns with relative LREE depletion. Intermediate lavas have TiO 2 contents between 0.30 and 0.50 wt%, HREE contents from 4-7× chondrites and extreme relative LREE depletion. Lower lavas have anomalously low TiO 2 contents (<0.30 wt%) and unusual convex-downwards REE patterns with REE abundances around 2-5 × chondrite. These geochemical differences can be explained if the three groups were derived from different mantle sources. Independent mantle sources for the three units are consistent with their different 143 Nd / 144 Nd ratios varying at 480 m.y.B.P. from 0.51222 in a lower lava to 0.51238 in an upper lava. The upper lavas may be partial melts of a source similar in composition to that of modern MORB, the intermediate lavas may be from a very depleted oceanic mantle (second stage melt), and the lower lavas may have formed by melting an extremely depleted mantle that had been invaded by a LREE-enriched fluid. A possible tectonic environment where these different sources could be juxtaposed is a back-arc or inter-arc basin.

Coish, R. A.; Hickey, R.; Frey, F. A.

1982-11-01

52

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

Microsoft Academic Search

Divergent paleogeographic origins have been proposed for the Coast Range ophiolite of western California which are testable using paleomagnetic methods. Paleomagnetic data for Middle Jurassic pillow lavas and diabase sills of the Coast Range ophiolite at Mount Diablo, northern California, indicate that they contain two components of remanent magnetization. The characteristic directions have normal and reversed polarities and apparently are

Jonathan T. Hagstrum; David L. Jones

1998-01-01

53

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

Microsoft Academic Search

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

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

2004-01-01

54

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".

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

2013-01-01

55

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

56

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

57

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

Microsoft Academic Search

Chromites separated from the upper mantle or lower crustal portions of 18 ophiolites ranging in age from 900 Ma to 50 Ma are examined for Re-Os isotopic systematics. The ophiolites include both MORB and back arc types, although most are from supra-subduction zone (SSZ) settings. The chromites are robust indicators of the initial Os isotopic compositions of the systems sampled.

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

2002-01-01

58

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

Microsoft Academic Search

Chromite separates from a set of historically important chromite deposits from the 90 Ma old Mayarí-Baracoa Ophiolitic Belt in eastern Cuba were inspected for Re-Os isotopic systematics in an attempt to quantify the extent of Os isotopic heterogeneities within a restricted upper mantle portion represented by a single ophiolite. Compositional variations of chromites indicate their crystallization from hydrous melts varying

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

2005-01-01

59

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

60

Serpentinization Of The Leka Ophiolite Complex, Norway: Geochemical And Physical Implications  

Microsoft Academic Search

The Leka Ophiolite Complex (LOC) contains all the principle components of an ophiolite and is a part of the Upper Allochthon of the Scandinavian Caledonides. The ultramafic lithologies (harzburgites, dunites, wehrlites and orthopyroxenite dykes) of the LOC undergo hydration dependent on the constituent primary mineral assemblages thus preserving the hydration history over a wide range of temperatures, where the orthopyroxenite

K. Iyer; H. Austrheim; T. John; B. Jamtveit

2008-01-01

61

Geology of the Smith River: A Virtual Trip Through The Josephine Ophiolite  

NSDL National Science Digital Library

This trip will take you through one of the world's best exposures of oceanic lithosphere (ophiolite). It begins in the mantle (peridotite) and works upward to pillow basalts and overlying sediments. A general overview of the formation and interpretation of an ophiolite is provided as background material to the specific examples found at the Smith River location in northern California.

Bazard, Dave

62

White K-mica evolution from phengite to muscovite in shales and shale matrix melange, Diablo Range, California  

Microsoft Academic Search

Eleven samples from high-pressure\\/low-temperature (HP\\/LT) shales and shale-matrix melange from four areas in the Diablo Range were studied using electron microprobe (EMP), transmission\\u000a electron microscopy (TEM), and analytical electron microscopy (AEM) to provide information about white K-mica evolution and\\u000a about the controls on illite “crystallinity” (IC) in these areas. The data indicate that: (1) compositional gradients from\\u000a phengitic to muscovitic

M. Dalla Torre; Kenneth J. T. Livi; David R. Veblen; M. Frey

1996-01-01

63

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

NASA Astrophysics Data System (ADS)

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

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

2013-03-01

64

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

65

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

66

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

67

Interesting Inclusions From Podiform Chromitites in Luobusa Ophiolite, Tibet  

NASA Astrophysics Data System (ADS)

For the past decade, diamonds and unusual mineral asemblages were reported in podiform chromitites of the Luobusa ophiolite, southern Tibet, China (Bai 1993, Bai 2000, Yan 2001). These minerals were found from heavy mineral separation of chromitites. These minerals include (1) native elements, (2) alloys, (3) carbide (SiC, CrC), (4) platinium group elements (PGE) and arsenides, (5) silicates (Ol, Opx, Cpx, Amp, Srp, Chl, Uv, Prp, Alm, Wo, Zrn, Ap, Bt, Spn, Rt, Pl, Kfs, Phl, Sil, Qz and octahedral serpentine (possible pseduomorph after ringwoodite?), (5) oxide (corundum and chromite), (6) carbonates. Despite many questions as to these minerals above still remain open, these mineral inclusions would provide us the important infomation on the formation of the podiform chromitites. In this study, octahedral serpentine was discovered both on a thin section and from the heavy mineral separation. These octahedral inclusions exist within chromites, forming a line. These minerals are approximately 5-15? m in diameter and have well octahedral morphology. EPMA, laser raman spectrometer and transmission electron microscopy (TEM) were used to determine the structure and chemical composition of this crystal. For the present, there are several interpretations of this octahedral silicate. One possibility is that if the octahedral structuer is euhedral so this octahedral serpentine may be pseudomorph after ringwoodite because of its chemical composition and octahedral crystal shape. Another is that ocahedral minerals are melt inclusions. Linear occurrence of octahedral minerals is similar to that of fuluid inclusions. If the octahedral structuer is negative crystal shape reflecting octahedral crystal of cromian spinel, then octahedral inclusions may be melt inclusions judging from linear occurrence. At the same time, zircons were obtained from the mineral separation from chromitites. U-Pb dating of these zircons by LA-ICP-MS yielded two different ages. One group has relatively younger age 107-534 Ma, which nearly plots on a concordia line. Another group has older age 1460-1822 Ma, which plots off the concordia line. Cathode luminescence images of these zircons indicate that some zircons have clear oscillatory zoning whereas other zircons show apparent homogeneous overgrowth. But any correlation between CL image and the U-Pb age was not identified in particular. Luobusa ophiolite has been recognized as fragment of Tethys oceanic crust formed in Cretaceous at 100-120 Ma (Allegre et al. 1984). The minimum age 107 Ma corresponds to the age of the formation of Luobusa ophiolite and all other age of zircons in chromitites is much older than that of ophiolite. In addition, the inclusions in the zircons were analyzed by EPMA and laser raman spectrometer. Several zircons contain some inclusions, which are quartz, feldsper, mica, apatite, titanite and others. These inclusions are the minerals composed of crustal material, which means that these zircons were crystalized in the low pressuer crustal condition. On the other hand, Yu et al. (2001) reported that zircons from chromitites in Luobusa ophiolite have shorter inter-atomic distances for Zr-O and Si-O bonds. They concluded that Tibetan-zircons were derived from the high-pressure mantle environment. Judging from the line of evidence mentioned avobe, it is highly possible that these zircons captured by chromitites were originated from recycled crustal materials convecting through upper mantle.

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

2003-12-01

68

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

69

Subsurface Investigation of the Coast Range Ophiolite, Northern California: Field Campaign Findings  

NASA Astrophysics Data System (ADS)

Our subsurface survey of the actively serpentinizing Coast Range Ophiolite, N. California, documents its mineralogy and aqueous geochemistry and provides qPCR results indicating that this subsurface habitat hosts microbial life.

Cardace, D.; Hoehler, T. M.

2010-04-01

70

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

NASA Astrophysics Data System (ADS)

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 a subduction-zone environment for their origin. The chemical similarity between the East Sulawesi ophiolite lavas and those from the Eocene Celebes Sea back-arc basin crust together with their identical age strongly suggest a back-arc tectonic environment for this ophiolite, which represents a fragment of the Eurasian plate obducted onto the East Sulawesi basement of Australian origin.

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

1995-09-01

71

Onset of subduction in eastern Cuba (Sierra del Convento melange): geochemical evidences for a partially melted young slab.  

NASA Astrophysics Data System (ADS)

Geochronological and regional geological arguments indicate the onset of subduction of the Protocaribbean (Atlantic) lithosphere during the Aptian (ca. 120 Ma). Exotic blocks of MORB-like amphibolite and associated tonalite-trondhjemite from Sierra del Convento melange in eastern Cuba document this early stage of subduction. Major and trace element geochemical signatures and Sr-Nd isotope data of these blocks indicate that oceanic crust underwent partial melting processes during subduction. Theoretical melting models suggest that the trondhjemites are not oceanic plagiogranite formed after differentiation or partial melting of MORB at oceanic ridges, but instead that they formed by 1-30 wt % water-saturated melting of epidote±garnet amphibolite. REE patterns of the trondhjemites are fractionated (1 < (La/Yb)n < 16.8), with LREE enrichment and flat or slightly depleted HREE patterns which compare well with adakitic melts formed by partial melting of mafic material at moderate to high pressure. Indeed, trondhjemites from Sierra del Convento have some of the geochemical features of Cenozoic adakites, including SiO2>56%, 3.5%melange (California) considered to be primary slab melts which did not react with the mantle wedge. In agreement with major and trace element geochemistry, the Sr-Nd isotope systematic of selected amphibolites and trondhjemites can be explained by a two stage model involving a) generation of MORB-like basalts and b) partial melting of subducted MORB. Partial melting of amphibolite in the Sierra del Convento melange is consistent with the onset of subduction of young oceanic lithosphere of the Protocaribbean.

Lazaro, C.; Garcia-Casco, A.

2007-12-01

72

Episodic magma chambers and amagmatic extension in the Josephine ophiolite  

NASA Astrophysics Data System (ADS)

Geochemical and structural aspects of the Josephine ophiolite indicate that it formed by alternating cycles of magmatic and structural (amagmatic) extension. Episodic magma chambers are indicated by the presence of very primitive lavas in the lower pillow lavas which would have mixed into a magma chamber. Fe-Ti basalts that occur in the uppermost extrusive rocks and plagiogranites that occur along the sheeted-dike/gabbro contact represent highly fractionated magmas produced during the last stages of crystallization of a magma chamber. During periods of amagmatic extension, breccias accumulated and the entire crustal sequence was tilted >50°, probably by faulting above an upper-mantle detachment. More than 1000 m of uplift and down-dropping between magmatic cycles is suggested by the presence of out-of-place screens in the sheeted-dike complex.

Harper, Gregory D.

1988-09-01

73

Scientific Drilling on the Diamond-Bearing Luobusa Ophiolite, Tibet  

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

74

Petrology of plagiogranite from Sjenica, Dinaridic Ophiolite Belt (southwestern Serbia)  

NASA Astrophysics Data System (ADS)

The Sjenica plagiogranite occurs in the southern part of the Dinaridic Ophiolite Belt, 5 km northwest of Sjenica. The main minerals are albite with strongly altered biotite (replaced with chlorite), with occasional amphibole (magnesio hornblende to tschermakite) and quartz. An enclave of fine-grained granitic rocks with garnet grains was noted too. Secondary minerals are calcite and chlorite (daphnite). Major, trace and REE geochemistry coupled with field observations support a model by which the Sjenica plagiogranite could be formed by fractional crystallization of mantle origin mafic magma in a supra-subduction zone setting. Occurrences of calcite and chlorite nests in the Sjenica plagiogranites revealed that these rocks underwent hydrothermal alteration due to intensive sea water circulation in a sub-sea-floor environment.

Milovanovi?, Dragan; Sre?kovi?-Bato?anin, Danica; Savi?, Marija; Popovic, Dana

2012-04-01

75

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

76

Sr isotope geochemistry and hydrothermal alteration of the Oman ophiolite  

NASA Astrophysics Data System (ADS)

The Sr isotopic behavior in hydrothermally altered rocks of 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. On the basis of rock-types, secondary mineral assemblage, alteration temperature, average 87Sr/86Sr of hydrothermal fluids, and degree of alteration, the section is divided into the following 5 sequences: (1) mainly basalts and clay minerals, low alteration temperature, average 87Sr/86Sr 0.70634, and 78% degree of alteration; (2) mainly basalts and chlorite, clay minerals and calcite, fairly low alteration temperature, average 87Sr/86Sr 0.70584, and 76% degree of alteration; (3) basalts and sheeted dike diabase and prehnite-actinolite facies, moderate alteration temperature, average 87Sr/86Sr 0.70519, and 85% degree of alteration; (4) dike diabase, plagiogranite, metagabbro and epidosite and prehnite-free greenschist facies, high alteration temperature, average 87Sr/86Sr 0.70514, and 65% degree of alteration; and (5) noncumulate and cumulate gabbro and amphibolite facies, very high alteration temperature, average 87Sr/86Sr 0.70413, and 85% degree of alteration. Alteration (metamorphic) grade deduced from secondary mineral assemblages generally increases versus stratigraphic depth. Only 15% of the rocks have escaped hydrothermal alteration. The estimated strontium isotopic compositions of hydrothermal fluids are much more water-dominated than those reported previously from modern and ophiolitic hydrothermal systems. On the basis of ideal model analysis we conclude that the whole sequence of oceanic crust in the Wadi Fizh section has experienced seawater recharge (downflow zone). The evidence suggests that the Wadi Fizh section was located close to a segment boundary of the oceanic crust along a spreading axis.

Kawahata, H.; Nohara, M.; Ishizuka, H.; Hasebe, S.; Chiba, H.

2001-06-01

77

Magnetization of upper mantle: Results from Oman Samail Ophiolite  

NASA Astrophysics Data System (ADS)

Based upon the new data on magnetization of upper lithosphere collected from sea floor and ophiolite, we present a new model for oceanic lithosphere magnetization. In our model, the thickness of each magnetic layer was assumed to be equal to that of a standard seismic model for oceanic crust: eg. 0.5km for layer 2A. However, the thickness of the upper mantle magnetic layer can not be obtained by the similar, simple assumption. To determine the thickness of magnetic layer of upper mantle, we studied magnetization of peridotites successively sampled from a series of outcrops exposed at the crust-mantle boundary section in Wadi al Hilti, Oman Ophiolite. The average NRM intensity calculated for 2km from crust-mantle boundary is 0.6 (+/- 0.4) A/m, while that of the lower portion being 0.1 (+/- 0.08) A/m, indicating that the effective thickness of the magnetic layer of the crust- mantle boundary/upper mantle section is about 2km: upper 2km portion from the crust-mantle boundary is much strongly magnetized than the lower portion. On the basis of this finding, together with the recent magnetization data from oceanic crust and upper mantle, we made a new magnetization model for upper oceanic lithosphere. The total oceanic lithosphere magnetization calculated from the new model is about 17,500 A, which can explain the MAGSAT magnetic anomalies over Pacific Cretaceous Quiet Zone. Our new model also explains the missing magnetization of upper oceanic lithosphere that has been a long standing problem in geophysics.

Kikawa, E.; Sakai, H.; Kudo, T.

2003-12-01

78

Subduction Initiation by Extrusion Tectonics? Evidence From the Palawan Ophiolite, Philippines  

NASA Astrophysics Data System (ADS)

There are few well-constrained geologic examples that can provide insight to the conditions under which subduction initiated. The Palawan ophiolite preserves evidence bearing on the initiation of subduction that can be linked with tectonic events in the surrounding areas. We report a ~34 Ma crystallization age for the Palawan ophiolite obtained by zircon U-Pb dating on plagiogranite. Previous hornblende and white mica Ar-40/Ar-39 dates from the high T and P metamorphic sole are indistinguishable from the crystallization age of the ophiolite. New major and trace element geochemical data from pillow basalts and mafic dikes from five separate areas of the ophiolite all indicate a predominantly N-MORB-like source for the ophiolite, although some trace element ratios are transitional to IAT (e.g., Hf/Ta and Th/Hf). Differentiation trends on plots of MgO vs. TiO2 and MgO vs. Al2O3 deviate from MORB trends and are more akin to trends for the Mariana and Lau backarc basins. The available evidence suggests that the ophiolite formed in latest Eocene-earliest Oligocene time in a "mature" backarc basin that opened within Early Cretaceous oceanic lithosphere (now preserved beneath the ophiolite). The concordance between the times of ophiolite and high T-P sole formation indicate that the ophiolite was detached at, or close to, the spreading axis. The transition from spreading to convergence requires that far-field compressional stresses were applied to the area of the ridge axis. Previous work has shown that this zone of convergence evolved into a subduction zone that spawned the Cagayan arc-Sulu Sea backarc system. This implies that the presence of the old, dense Early Cretaceous oceanic lithosphere was insufficient for subduction to begin and that external forces were required to initiate subduction. The new data, combined with recent thermochronologic data from the Red River Shear zone indicating shearing beginning at ~33 Ma and seafloor spreading of the South China Sea at 32 Ma suggest a scenario wherein extrusion of Indochina into the proto-South China Sea area caused compression in the Palawan back arc basin, ophiolite detachment and initiation of subduction.

Encarnacion, J.; Fernandez, D.; Mattinson, J.

2001-12-01

79

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

NASA Astrophysics Data System (ADS)

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

Haig, David W.; Bandini, Alexandre Nicolas

2013-10-01

80

The P-T-t history of blocks in serpentinite-matrix melange, west-central Baja California  

SciTech Connect

Thermochronologic, petrologic, and geochemical analyses of epidote-amphibolite-, amphilbolite-, eclogite-, and blueschist-facies blocks in serpentinite-matrix melanges from East San Benito Island and Cedros Island, west-central Baja California, provide constraints on the P-T-t history of this disrupted terrane. Results of {sup 40}Ar/{sup 39}Ar step heating experiments on minerals separated from these blocks vary according to metamorphic grade and indicate different P-T-t histories. Mid-Jurassic (160-170 Ma) epidote-amphibolite- and amphibolite-facies blocks were probably derived from oceanic crust and associated sediments that were metamorphosed during limitation of subduction. Coarse-grained blueschist blocks were likely metamorphosed during continued subduction in late Early Cretaceous time (that is, 115-100 Ma). Some epidote-amphibolite blocks are partially overprinted by blueschist-facies mineral assemblages and apparently record both metamorphic events. Fission-track analyses of apatites indicate that the blocks underwent significantly different post-metamorphic cooling histories. Epidote-amphilbolite and amphibolite blocks cooled below {approximately}100 {degrees}C form mid-Jurassic to Paleocene time; blueschist blocks cooled below {approximately}100 {degrees}C in Oligocene-Miocene time. In general, mafic blocks have trace-element concentrations and REE patterns characteristic of ocean-flood basalts. This study demonstrates that samples with very different P-T-t histories can evidently occur over relatively small length scales (<<1 km) in serpentinite-matrix melanges. 67 refs., 16 figs., 4 tabs.

Baldwin, S.L.; Harrison, T.M. [State Univ. of New York, Albany, NY (United States)

1992-01-01

81

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

82

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

Microsoft Academic Search

The Betic Ophiolitic Unit, part of the Mulhacén Complex in the Betic Cordillera (SE Spain), is composed of several tectonic slices of eclogitized mafic and ultramafic rocks, together with oceanic metasediments. A comprehensive review of the petrological characteristics of the Betic Ophiolites shows notable analogies with other ophiolites from the Alps–Apennines system, indicating that they represent relics of oceanic lithosphere

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

2011-01-01

83

The seismic velocity structure of a traverse through the Bay of Islands ophiolite complex, Newfoundland, an exposure of oceanic crust and upper mantle  

Microsoft Academic Search

Although the ophiolites are widely recognized as segments of oceanic crust emplaced on land, direct correlation between the ophiolites and the the oceanic crust has proven difficult owing to the near absence of common criteria on which to base a comparison; the ophiolites are defined petrologically, while the oceanic crust is defined largely in terms of seismic structure. To bridge

Matthew H. Salisbury; Nikolas I. Christensen

1978-01-01

84

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

85

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

86

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.

2012-06-01

87

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

NASA Astrophysics Data System (ADS)

Hornblendes from amphibolites in the sheet of metamorphic rocks beneath the peridotite member of the Samail ophiolite and phyllites farther from the peridotite contact have weighted mean 40Ar/39Ar total fusion ages of 90.0 ± 3.0 m.y. and 79.5 ± 3.0 m.y., respectively. The amphibolites 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 amphibolites 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 the 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, Marvin A.

1981-04-01

88

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

89

Spatial distribution of melt conduits in the mantle beneath oceanic spreading ridges: Observations from the Ingalls and Oman ophiolites  

Microsoft Academic Search

Ophiolites are on-land exposures of igneous crust and residual upper mantle formed beneath submarine spreading ridges. Upper mantle outcrops in ophiolites provide insight into focusing of melt transport from a ~100 km wide region of partial melting into an ~5 km wide zone of igneous crustal accretion beneath the ridges. Dunite veins, composed of the minerals olivine and spinel, mark

Peter B. Kelemen; Michael Braun; Greg Hirth

2000-01-01

90

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

Microsoft Academic Search

The middle Jurassic Coast Range Ophiolite (CRO) is one of the most important tectonic elements in western California, cropping out as tectonically dismembered elements that extend 700 km from south to north. The volcanic and plutonic sections are commonly interpreted to represent a supra-subduction zone (SSZ) ophiolite, but models specifying a mid-ocean ridge origin have also been proposed. These contrasting

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

2010-01-01

91

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

Microsoft Academic Search

The Meratus ophiolitic series (SE Borneo) present a specific assemblage that have recorded (1) a continental extensional episode mostly seen within the peridotites and 2) later subduction-related magmatic events marked by the emplacement of calc-alkaline magmas. These events relate the magmatic activity and geodynamic evolution of the SE Eurasia margin in Mesozoic times. The ophiolitic series comprise ultramafic rocks with

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

1999-01-01

92

Seawater alteration of accreting oceanic crust up to 1000°c -A physical model from the Oman ophiolite revisited  

Microsoft Academic Search

Following the discovery of a high temperature (800°C) and a very high temperature (1000°C) hydrothermal alteration in the crust of the Oman ophiolite, a systematic structural and petrological study has been conducted throughout the entire ophiolite (1). It was backed by isotopic geochemistry which showed the seawater origin of the fluids (2). The results show that the crustal gabbros are

F. Boudier; A. Nicolas; L. H. Cathles

2009-01-01

93

Burst of High Temperature Seawater Injection Throughout Accreting Oceanic Crust A Model Derived From the Oman Ophiolite  

Microsoft Academic Search

Following the discovery of a high temperature (800° C) and a very high temperature (1000° C) hydrothermal alteration in the crust of the Oman ophiolite, a systematic structural and petrological study has been conducted throughout the entire ophiolite, backed by isotopic geochemistry that is developed in a companion abstract. The results show that the crustal gabbros are extensively altered down

A. Nicolas; D. Mainprice

2003-01-01

94

Tectonic significance of 400 Ma zircon ages for ophiolitic rocks from the Lachlan fold belt, eastern Australia  

Microsoft Academic Search

New SHRIMP ion microprobe data provide constraints on the age of formation of ophiolitic rocks from the Tumut region of the Lachlan fold belt, southeastern Australia. Magmatic zircons from plagiogranites within both the Coolac serpentinite belt and Wambidgee serpentinite belt give 206Pb\\/238U ages of about 400 Ma. This Devonian age is considerably younger than those previously proposed for these ophiolitic

I. T. Graham; B. J. Franklin; B. Marshall; E. C. Leitch; M. Fanning

1996-01-01

95

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

96

Sr isotopic tracer study of the Samail Ophiolite, Oman  

NASA Astrophysics Data System (ADS)

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 cumulate gabbro from the Wadir Kadir section and nine samples from the Wadi Khafifah section have mean 87Sr/86Sr 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 87Sr/86Sr 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 87Sr/86Sr 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, Marvin A.; Coleman, Robert G.; Hopson, Clifford A.

1981-04-01

97

Spring and surface water quality of the Cyprus ophiolites  

NASA Astrophysics Data System (ADS)

A survey of surface, spring and borehole waters associated with the ophiolite rocks of Cyprus shows five broad water types (1) Mg-HCO3, (2) Na-SO4-Cl-HCO3, (3) Na-Ca-Cl-SO4-OH-CO3, (4) Na-Cl-SO4 and (5) Ca-SO4. The waters represent a progression in chemical reactivity from surface waters that evolve within a groundwater setting due to hydrolysis of the basic/ultrabasic rock as modified by CO2-weathering. An increase in salinity is also observed which is due to mixing with a saline end-member (modified sea-water) and dissolution of gypsum/anhydrite. In some cases, the waters have pH values greater than 11. Such high values are associated with low temperature serpentinisation reactions. The system is a net sink for CO2. This feature is related not only to the hydrolysis of the primary minerals in the rock, but also to CaCO3 or Ca-Mg-CO3 solubility controls. Under hyperalkaline conditions, virtually all the carbon dioxide is lost from the water due to the sufficiently high calcium levels and carbonate buffering is then insignificant. Calcium sulphate solubility controls may also be operative when calcium and sulphate concentrations are particularly high.

Neal, C.; Shand, P.

98

Time relations and structural-stratigraphic patterns in ophiolite accretion, west central Klamath Mountains, California  

NASA Astrophysics Data System (ADS)

New geochronological data and published structural and stratigraphic data show that two distinctly different ophiolitic assemblages formed in general proximity to one another at nearly the same time and were subsequently imbricated along a regional thrust zone. The Josephine ophiolite constitutes a complete oceanic crust and upper mantle sequence which lies within the western Jurassic belt of the Klamath province. Within the study area the Josephine ophiolite was formed by seafloor spreading at about 157 m.y. before present. It was immediately covered by a thin pelagic and hemipelagic sequence which grades into a thick flysch sequence, both of which comprise the Galice Formation. The Galice flysch was derived from volcanic arc and uplifted continental margin orogenic assemblages. A major nonvolcanic source for the Galice flysch appears to have been the western Paleozoic and Triassic belt of the Klamath province exposed to the east. Proximal volcanic arc activity migrated to the site of the Josephine-Galice section by 151 m.y. and is represented by numerous dikes and sills which intrude the ophiolite and Galice Formation. The Preston Peak ophiolite is a polygenetic assemblage consisting of (1) a pre-mid-Jurassic tectonitic peridotite-amphibolite substrate which represents disrupted and unroofed basement of the western Paleozoic and Triassic belt and (2) an upper mafic complex which was intruded through and constructed above the tectonite substrate at about 160 m.y. The mafic complex consists primarily of diabase hypabyssal rocks that are overlain by diabase-clast breccia and hemipelagic deposits. A major arc-plutonic complex was emplaced into the Preston Peak ophiolite in at least two pulses at 153 and 149 m.y. Major phases of this complex consist of wehrlite, gabbro, pyroxene diorite, and hornblende diorite. The Josephine ophiolite is interpreted as the remnants of interarc basin crust. The Preston Peak ophiolite is interpreted as either a primitive remnant arc complex or a rift edge facies for the Josephine interarc basin. The Galice Formation represents a submarine fan complex that was built on juvenile crust of the Josephine basin floor. During the time interval of 153 to 149 m.y. the locus or arc magmatism migrated to an area which included the interarc basin floor and the remnant arc or basin edge. The basin shortly thereafter closed by convergent tectonics during the Nevadan orogeny resulting in the imbrication of the Josephine and Preston Peak ophiolites and their superimposed arc assemblages. The transition from seafloor spreading generation of Josephine ophiolite to its tectonic accretion by convergence and basin closure occurred within 5 to 10 m.y. The process of rifting and ophiolite formation in series with convergence and ophiolite accretion is considered an important mechanism for generating and displacing allocthonous terranes in the Klamath Mountains-Sierra Nevada region, and perhaps throughout the western cordillera.

Saleeby, Jason B.; Harper, Gregory D.; Snoke, Arthur W.; Sharp, Warren D.

1982-05-01

99

The Hellenic ophiolites: eastward or westward obduction of the Maliac Ocean, a discussion  

NASA Astrophysics Data System (ADS)

Ophiolitic bodies in the Dinaro-Hellenic mountain belt are among the most important ones in the Peri-Mediterranean Alpine chains. The characteristic feature of this ophiolitic belt is its Middle to Late Jurassic age of obduction. The ophiolitic bodies form two major belts on each side of the Pelagonian zone: an east Pelagonian belt in the Vardarian domain and a Supra-Pelagonian ophiolitic belt (SPO) to the west. The different hypotheses relative to the origin of the SPO present geodynamic evolution models accounting for most of the available data: a mid-Triassic episode of rifting; a Ladinian-Jurassic episode of sea-floor spreading forming notably the Maliac Ocean; a Middle to Late Jurassic convergent period with subduction and obduction episodes, and finally, a late episode of Tertiary compressional deformation responsible for the westward thrusting of the Jurassic ophiolitic nappes over the external zones. Despite many studies dating from the early 1970s, the eastern or western Pelagonian origin of these ophiolites, especially the SPO, is still under dispute. Whatever the adopted hypothesis, we consider that the main SPO bodies (N-Pindos, Vourinos, Othris, Evia, Argolis) have the same origin because of their geographic continuity and of the similarities in their geological characteristics. We propose that this ocean corresponds everywhere to the Maliac Ocean, defined in Othris from the well-preserved sedimentary (oceanic margin) and ophiolitic nappes thrust during the Late Jurassic obduction onto the Pelagonian platform. There is strong evidence for the existence of two deep basins on both sides of the Pelagonian continental ridge during Triassic-Jurassic times. They correspond, respectively, to the Vardar area to the east and the Pindos domain to the west, one of these domains being at the origin of the SPO. The hypothesis of an eastward emplacement of the SPO from the Pindos domain is based mainly on sedimentological data from the margin series and on structural analyses of ophiolitic bodies. However, we conclude the westward obduction of the Maliac Ocean, originating from the Vardar area, to be the best fitting model. This westward model is supported by paleogeographic and structural constraints on regional scale. Notably, the absence of obducted ophiolites in the Jurassic series of the Koziakas units (units attributed to the western Pelagonian margin) and of the Parnassus domain (on the eastern side of the Pindos basin) is difficult to reconcile with an eastward obduction from the Pindos domain. Other observations, such as the distribution of ophiolitic detritus in the internal and external zones, also promote the westward Late Jurassic obduction of the Maliac Ocean. Our preferred model offers a consistent explanation for the mechanism and timing of the emplacement of the SPO, as well as providing insight on the origin and emplacement of the Vardarian ophiolites. Following this hypothesis, there is no need for a clear boundary between the SPO and the west Vardarian ophiolitic bodies as they were obducted from the same oceanic basin and during the same Jurassic tectonic event. In this paper, we develop evidence in favor of the eastern Pelagonian origin for the SPO (our adopted model) and provide discussion on the data supporting the main alternative hypothesis (western origin for the SPO).

Ferrière, Jacky; Chanier, Frank; Ditbanjong, Pitaksit

2012-09-01

100

Age and evolution of Western Brooks Range ophiolites, Alaska: Results from 40Ar/39Ar thermochronometry  

NASA Astrophysics Data System (ADS)

Basalt, gabbro, and peridotite in the western Brooks Range structurally overlie Mississippian through Late Jurassic platform sediments that were deformed during Late Jurassic - Cretaceous time. 40Ar/39Ar step-heating experiments on hornblende, biotite, plagioclase, and potassium feldspar from gabbro, diorite, plagiogranite, and granite yielded complex release spectra. On isotope correlation diagrams, the incremental-heating data indicate that gabbro, diorite, and plagiogranite cooled below the closure-temperature for argon in amphibole between 187 and 184 Ma. Because the closure temperature of argon retention in amphibole is relatively high, these ages are interpreted to be the time since crystallization. Biotite and hornblende from metamorphic rock exposed along faults separating peridotite and gabbro from underlying basalt yielded plateau and isochron dates of 169 - 163 Ma; these dates are interpreted to record postmetamorphic cooling following intraoceanic detachment and transport of "hot" peridotite and gabbro over "cold" basalt and sediment. Detachment and faulting of the gabbros and peridotites is interpreted to have occurred approximately 20 Ma after crystallization, and is longer than the 5-10 Ma interval commonly observed in studies of other ophiolites. Fossils in flysch underlying the ophiolites indicate that the ophiolites were emplaced onto the continental margin during the Tithonian, approximately 15-20 Ma after initial detachment. The ages of feldspars (149 - 137 Ma) in the ophiolites and associated volcanic rocks are similar to the fossil ages of the underlying flysch and might record uplift during emplacement of the ophiolites onto the continental margin. A fission-track age of apatite implies that the ophiolitic rocks were overlain by >4 km of rock until ˜59 Ma ago, the time of regional uplift in the Brooks Range. Supplement Appendix 1 is available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. document T92-005; $2.50. Payment must accompany order.

Wirth, Karl R.; Bird, John M.; Blythe, Ann E.; Harding, David J.; Heizler, Matthew T.

1993-01-01

101

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

NASA Astrophysics Data System (ADS)

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

Wright, James E.; Wyld, Sandra J.

1986-08-01

102

Forearcs and Supra-Subduction Zone Ophiolites: Some Similarities and Differences (Invited)  

NASA Astrophysics Data System (ADS)

Forearcs have long been recognised as important potential protoliths for ophiolites, given that they are the first lithologies to be obducted when a continent ‘collides’ with a trench in an intraoceanic setting. It is therefore perhaps surprising that oceanic forearcs and supra-subduction zone (SSZ) ophiolites are not more similar. One possible reason is that the best-understood present-day forearcs had time to evolve following the initiation of subduction of a large ocean (the Pacific) whereas most ophiolites represent extremely short-lived subduction events associated with the closure of small ocean basins. Thus only the first few million years of forearc evolution is critical to ophiolite formation and that is the least well understood part of forearc development in Western Pacific analogues. If there is anything systematic about supra-subduction zone ophiolites, it is that there is a general trend of increased depletion with time: this may be represented by a lava stratigraphy of true MORB, overlain by island arc tholeiites and then boninites (e.g. Pindos), or slightly anomalous MORB overlain by island arc tholeiities, then boninites (e.g. Oman) or island arc tholeiites overlain by boninites (Cyprus) or just boninites (e.g. Vourinos). The same progression can be seen in dykes, cumulates and (through spinel geochemistry) mantle tectonites. This may be explained by differences in subduction initation variables, best unravelled by detailed interpretation of each ophiolite in a forearc, subduction initiation context. We take as an example one ophiolite, the Troodos Massif, where we can use the large database of volcanic glass major element analyses to evaluate its setting in detail. We find that the lower (axis) lavas are high-Si8, medium-Fe tholeiities and that they are overlain by high-Ca boninites. This combination is characteristic of subduction initiation terranes and slab edges rather than ‘normal’ arc-basin systems. Low K8/H8 ratios support a near-trench setting where the mantle wedge can capture the water-rich, K-poor fluids normally lost through cold forearcs. Na8-Fe8 systematics demonstrate that the lower (axis) lavas of the ophiolite formed from mantle with a high potential temperature of about 1400 degrees-C, consistent with the rapid influx of hot mantle at a slab edge or during slab rollback after subduction initiation. Geological reconstructions indicate that slab tear (at a STEP) fault was likely needed to allow the influx of mantle into the wedge and hence slab roll-back. This was necessary in a small ocean bounded by continents where mantle flow was restricted, but may have been less important in the open ocean of the Western Pacific.

Pearce, J. A.; Robinson, P. T.

2009-12-01

103

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

104

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

Microsoft Academic Search

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

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

2008-01-01

105

Geochemical evidence in clinopyroxenes from gabbroic sequence for two distinct magmatisms in the Oman ophiolite  

NASA Astrophysics Data System (ADS)

In the Oman ophiolite, one of the best preserved and most studied ophiolites in the world, two distinct petrogenetic suites of gabbroic rocks from the layered gabbro sequence of the Wadi Haymiliyah section is established using trace element chemistry of Ca-rich clinopyroxenes. The earlier GB1 suite is characterized by plagioclase with lower An (Ca/(Ca + Na)) content and clinopyroxene with low large-ion lithophile elements (LILE) concentrations. The later DWGB2 suite contains plagioclase with rather high An content and clinopyroxene with high LILE. This difference in clinopyroxene chemistry can be extended to the extrusive rocks in this section: lower, (earlier) HV1 suite with low LILE clinopyroxene and upper (later) HV2 suite with high LILE clinopyroxene. Difference in LILE concentration of clinopyroxenes is essentially due to geochemical difference in parental magmas. The GB1/HV1 suites formed at fast-spreading MOR setting and DWGB2/HV2 suites at SSZ setting, supporting a model of transition from mid-oceanic ridge to supra-subduction zone settings of the Oman ophiolite. Our results indicate that geochemical signature of clinopyroxene is a very strong tool for identification of tectonic setting of ophiolites.

Yamasaki, Toru; Maeda, Jinichiro; Mizuta, Toshio

2006-11-01

106

Geology and evolution of the magmatic rocks of the Vourinos Ophiolite, northern Greece  

Microsoft Academic Search

Magmatic stratigraphy and structure in an ophiolite complex is described. The integration of these features with chemical studies of the magmatic rocks resulted in a model detailing formation of the magmatic complex. This model requires that one or two parental magmas mix with fractionated liquids from older, partly solidified intrusions or with volatiles introduced by hydrothermal circulation during crystallization. One

A. E. Rassios

1981-01-01

107

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

108

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

Microsoft Academic Search

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

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

2010-01-01

109

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

110

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

111

Early Paleozoic Ophiolite Complexes of the Newfoundland Appalachians as Mantle-Oceanic Crust Sequences  

Microsoft Academic Search

The layered ultramarie-marie complexes of the Northwest platform and the Fleur de Lys orthotectonic zones of the Newfoundland Appalaehians are comparable to layered Tethyan ophiolite suites of the Piedmont Alps, the Dinarides, and the Oman. They can be interpreted as oceanic crust and mantle: the peridotires, with high-pressure mineral assemblages, kaersutire, and titaniferous phlogopite, represent the upper mantle; the overlying

W. R. Church; R. K. Stevens

1971-01-01

112

The application of remote sensing techniques to the study of ophiolites  

Microsoft Academic Search

Satellite remote sensing methods are a powerful tool for detailed geologic analysis, especially in inaccessible regions of the earth's surface. Short-wave infrared (SWIR) bands are shown to provide spectral information bearing on the lithologic, structural, and geochemical character of rock bodies such as ophiolites, allowing for a more comprehensive assessment of the lithologies present, their stratigraphic relationships, and geochemical character.

Shuhab D. Khan; Khalid Mahmood

2008-01-01

113

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

114

Chemical and chronologic complexity in the convecting upper mantle: Evidence from the Taitao ophiolite, southern Chile  

Microsoft Academic Search

Exposure of the ca. 6Ma Taitao ophiolite, Chile, located ?50km south of the Chile Triple Junction, allows detailed chemical and isotopic study of rocks that were recently extracted from the depleted mantle source of mid-ocean ridge basalts (DMM). Ultramafic and mafic rocks are examined for isotopic (Os, Sr, Nd, and O), and major and trace element compositions, including the highly

Ruth F. Schulte; Manuel Schilling; Ryo Anma; James Farquhar; Mary F. Horan; Tsuyoshi Komiya; Philip M. Piccoli; Lynnette Pitcher; Richard J. Walker

2009-01-01

115

A late Proterozoic ophiolite complex at Jabal Ess in northern Saudi Arabia  

Microsoft Academic Search

A folded and lightly-metamorphosed allochthonous thrust slice, representing a complete ophiolite succession (Penrose definition), has been mapped in the late Proterozoic shield of Saudi Arabia. This occurrence supports the suggestion that plate-tectonic motions operated during the late Proterozoic.

M. Shanti; M. J. Roobol

1979-01-01

116

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

117

U-Pb zircon geochronology of the Internal Liguride ophiolite (Northern Apennine, Italy)  

NASA Astrophysics Data System (ADS)

The ophiolitic bodies from the Alpine-Apennine belt are lithosphere remnants of the Middle to Late Jurassic Ligure-Piemontese basin. Most of these ophiolites preserve primary features recalling the magma-poor continental margin of Western Iberia. On the other hand, the Internal Liguride ophiolites from Northern Apennine display close structural and compositional similarities to modern slow and ultraslow spreading ridges. The Internal Liguride ophiolites preserve evidence for the presence of morphological highs made of gabbroic plutons within mantle peridotites (Cortesogno et al. 1987). The gabbroic plutons consist mostly of coarse-grained gabbros to olivine-gabbros interlayered with sill-like hectometre-scale lenses of olivine-rich troctolites (e.g. Tribuzio et al. 2000). The gabbro-peridotite association records a high-temperature deformation event in ductile shear zones and is locally crosscut by later dolerite dykes with chilled margins. Waning of the magma supply and onset of hydrothermalism was followed by the exposure of the gabbro-peridotite association at the seafloor by amagmatic tectonic extension. The morphological depressions from the Internal Liguride domain are recognised on the basis of massive and pillow basalt flows interlayered with breccias and cherts (Cortesogno et al. 1987). Whereas the geological and petrological evolution of the Internal Liguride ophiolites was extensively investigated, little is know about the timing of the magmatic events recorded by the Internal Liguride ophiolites. Previous geochronological studies yielded a discordant U-Pb zircon age of 153 +/- 1 Ma for a plagiogranite emplaced within the mantle peridotites (Borsi et al. 1996), and a Sm-Nd mineral isochron of 164 +/- 14 Ma for the gabbros (e.g. Rampone et al. 1998). On the basis of new single-grain U-Pb zircon datings by laser ablation ICP-MS, we therefore wish to constrain the timing of the evolution recorded by the Internal Liguride ophiolites. The preliminary U-Pb zircon data indicate that the formation of Internal Liguride igneous rocks cover a relatively long time span, thus gaining insights into absolute timing, duration and patterns of magma emplacement. Cortesogno L, Galbiati B, Principi G (1987) Ofioliti 12: 261-342 Borsi L, Schaerer U, Gaggero L, Crispini L (1996) Earth Planet Sci Lett 140:227-241 Rampone E, Hofmann AW, Raczek I (1998). Earth Planet Sci Lett 163: 175-89 Tribuzio R, Tiepolo M, Vannucci R (2000) Geol Soc Am Spec Paper 349: 129-138

Garzetti, F.; Tribuzio, R.; Tiepolo, M.

2009-04-01

118

Timing of ophiolite obduction and regional metamorphism in the Grampian orogen  

NASA Astrophysics Data System (ADS)

The Grampian terrane in the Caledonides of Scotland and NW Ireland is the type locality for Barrovian (regional) metamorphism, recognized in most mountain belts. It is thought to have resulted from the collision of the Laurentian margin with an oceanic arc and associated suprasubduction ophiolite during the Early Ordovician (van Staal et al., 1998). Here we address the timing and P-T conditions of Grampian ophiolite obduction and re-evaluate the link with regional metamorphism. Magmatic zircons from the Highland Border Ophiolite, Scotland define a 499 ± 8 Ma U-Pb Concordia age. Its metamorphism is dated by a 490 ± 4 Ma 40Ar-39Ar hornblende age, and a 488 ± 1 Ma 40Ar-39Ar muscovite age from a metasedimentary xenolith within it, from which P-T estimates of 5.3 kbar and 580 ° C relate to ophiolite obduction. Metamorphism of the Irish correlative of the Highland Border Ophiolite is constrained by a 514 ± 3 Ma 40Ar-39Ar hornblende age, while mica schist slivers within it yield detrital zircon U-Pb ages consistent with Laurentian provenance and Rb-Sr and 40Ar-39Ar muscovite ages of ca. 482 Ma. P-T values of 3.3 kbar and 580 ° C for these rocks constrain the conditions of ophiolite obduction (Chew et al., 2010). Peak Grampian metamorphism on the Laurentian margin (Dalradian Supergroup) is constrained to c. 475 - 465 Ma. This includes U-Pb zircon ages from Grampian syn-orogenic intrusives (a probable Andean-type arc which developed following subduction polarity reversal) and metamorphic mineral ages from Dalradian regions devoid of syn-orogenic intrusive rocks (Flowerdew et al., 2000; Chew et al., 2003). There is therefore a pronounced time gap between c. 470 Ma mineral ages in the Laurentian margin (Dalradian Supergroup) and c. 490 Ma mineral ages in the Grampian ophiolitic rocks. P-T conditions also differ markedly, with high T - low P metamorphism in the Grampian ophiolitic rocks and high P - low T (blueschist-facies) metamorphic conditions in the subducted Laurentian margin sediments of the Dalradian Supergroup. It is envisaged that subduction of the leading edge of the Laurentian plate initiated at c. 490 Ma, contemporaneous with the start of ophiolite obduction and resulted in high-pressure metamorphism of the Laurentian margin. The high-pressure rocks were transferred to the hanging-wall plate and thrust back onto the margin, and exhumed shortly afterwards by extensional collapse preserving mineral cooling ages as old as ca. 475 Ma close to the margin. Away from the Laurentian margin, collisional thickening created the thick Dalradian nappe stack and associated Barrovian metamorphism, with possibly minimal involvement of obducted oceanic lithosphere. If conductive heat transfer in over-thickened crust is the major heating mechanism, then collisional thickening may have initiated shortly after ophiolite obduction started at ca. 490 Ma in order to generate the ca. 470 Ma Grampian peak metamorphism in the Dalradian.

Chew, D. M.; Daly, J. S.; Page, L. M.; Whitehouse, M. J.

2012-04-01

119

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

NASA Astrophysics Data System (ADS)

Ophiolites are distinct assemblages of submarine volcanic rocks and plutonic rocks that include cumulate dunite, wehrlite, and gabbro, as well as isotropic gabbro and diorite, and peridotite tectonite, representing the underlying refractory mantle. They were originally thought to represent oceanic crust formed at mid-oceanic spreading centers, but their connection with island arcs has become increasingly apparent ever since it was proposed by Miyashiro (1973). Recognition that ophiolites are not normal arc assemblages, but form during unique, transient episodes of arc formation, has led to the concept of supra-subduction zone ophiolites (Pearce, 1984). SSZ ophiolites display a consistent development history from birth through death that implies a common origin and evolution in response to systematic, non-random processes (Shervais 2001). A review of modern volcanic rocks formed at mid-ocean ridges and back-arc basins shows that they have a limited range in major element compositions, and trace element systematics that range from depleted (“normal”) to enriched MORB, in which ratios of fluid-mobile LFS elements to fluid-immobile HFS elements are relatively constant. In contrast, volcanic rocks formed within regionally-extended fore-arcs (which may also form the basement of later arc complexes) have wider range in major element compositions and trace element systematics that are depleted in the HFS elements and enriched in fluid-mobile LFS elements (Metcalf and Shervais 2008). Most ophiolite volcanic suites are dominated by major and trace element systematics that are identical to those displayed by fore-arc volcanic suites, including the occurrence of boninites, which are only found within forearc settings. These systematics are consistent with fluid-enrichment of the mantle source region that had seen a prior extreme melt depletion event. Some ophiolites display more complex relations, with both SSZ and MORB or BAB-like compositions, but the SSZ components are generally dominant. These observations of volcanic rock chemistry are reinforced by recent studies of the mantle peridotite tectonites that underlie ophiolites. These studies show that ophiolite tectonites are strongly depleted in HFS and rare earth elements, requiring extensive melt extraction, and enriched in fluid-mobile elements, requiring a significant fluid flux that can only be sustained in a supra-subduction zone setting. This conclusion is enforced by recent isotopic studies, which document subduction-enriched isotopic compositions of Sr and Pb in SSZ ophiolites, and by ICP-MS studies of fluid mobile elements in relict pyroxene, which document enrichment in all fluid mobile elements. We conclude that ophiolites provide an analogue to modern fore-arc settings, and that their position in the upper plate of a subduction-zone leads to their preferential emplacement by obduction onto passive continental margins, or by accretionary uplift along continually active margins.

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

2009-12-01

120

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

121

Active compressive intraoceanic deformation: early stages of ophiolites emplacement?  

NASA Astrophysics Data System (ADS)

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

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

2010-05-01

122

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

Microsoft Academic Search

The middle Jurassic Coast Range Ophiolite (CRO) is one of the most important tectonic elements in western California, cropping\\u000a out as tectonically dismembered elements that extend 700 km from south to north. The volcanic and plutonic sections are commonly\\u000a interpreted to represent a supra-subduction zone (SSZ) ophiolite, but models specifying a mid-ocean ridge origin have also\\u000a been proposed. These contrasting interpretations

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

2010-01-01

123

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

Microsoft Academic Search

Remnants of ocean floor forming the Eastern Ophiolite Belt in Oman and the Western Ophiolite Belt in Pakistan have a common plate-tectonic history culminating in emplacement at the Cretaceous\\/Tertiary boundary. Fragments of ocean floor in these two belts have ages between 150 and 65 Ma and recorded tectonic events in the early Indian Ocean at 150 Ma, 130-120 Ma, 110-100

E. Gnos; A. Immenhauser; Tj. Peters

1997-01-01

124

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

Microsoft Academic Search

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

Stefan Claesson; John S. Pallister; Mitsunobu Tatsumoto

1984-01-01

125

Platinum-group element mineralogy and geochemistry of chromitite of the Kluchevskoy ophiolite complex, central Urals (Russia)  

Microsoft Academic Search

We report the results of investigation of chromitites occurring in the Kluchevskoy ophiolite complex of the Russian Urals. The chromite composition suggests crystallization from a boninitic magma in a supra-subduction zone geodynamic setting. The investigated chromitites are enriched in Os–Ir–Ru over Rh–Pt–Pd, as typical of the mantle hosted ophiolite chromitites. Consistent with the geochemical data, the Platinum Group Mineral (PGM)

F. Zaccarini; E. Pushkarev; G. Garuti

2008-01-01

126

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

127

Timing of accretion and collisional deformation in the Central Asian Orogenic Belt: implications of granite geochronology in the Bayankhongor Ophiolite Zone  

Microsoft Academic Search

Growing evidence suggests that the mechanism of Palaeozoic continental growth in Central Asia was by subduction–accretion with punctuated collisions that produced ophiolitic sutures between accreted blocks. The Bayankhongor ophiolite is the largest ophiolite in Mongolia and possibly all of Central Asia, and is interpreted to mark the collisional suture between the Baidrag and Hangai continental blocks. New 207Pb\\/206Pb zircon evaporation

Craig Buchan; Jörg Pfänder; Alfred Kröner; Timothy S Brewer; Onongin Tomurtogoo; Dondov Tomurhuu; Dickson Cunningham; Brian F Windley

2002-01-01

128

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

Microsoft Academic Search

U\\/Pb zircon ages are reported for four ophiolites and three crosscutting arc-related plutons from the Norwegian Caledonides. Plagiogranite differentiated from gabbro of the Karmøy ophiolite is dated at 493+7\\/-4 Ma whereas arc-related trondhjemite cutting this ophiolite crystallized at 485+\\/-2 Ma. A crosscutting clinopyroxene-phyric gabbro intrusion is dated at 470+9\\/-5 Ma by near concordant magmatic titanite (sphene) and discordant U-rich (2903–6677

G. R. Dunning; R. B. Pedersen

1988-01-01

129

Zircon U-Pb geochronology of the Zambales and Angat ophiolites, Luzon, Philippines: Evidence for an Eocene arc-back arc pair  

NASA Astrophysics Data System (ADS)

Two basement terranes, the Zambales ophiolite in the west, and the Angat ophiolite in the east, are exposed on the island of Luzon, separated by a circa 10 km thich and circa 100 km wide sedimentary basin. The structural and age relationships between the two ophiolitic blocks are center to understanding the geologic and tectonic development of the northern Philippines and evaluating models of terrane evolution proposed for this area of the western Pacific. We analyzed zircons from the Zambales and Angat terranes to better constrain their origin. Two zircon fractions from tonalite in the Acoje block of the Zambales ophiolite give concordant U-Pb ages at 44.2 (+/- 0.9) Ma. Two zircon fractions from plagiogranite and one fraction from diorite in the Coto block of the Zambales ophiolite give concordant U-Pb ages of 45.1 (+/- 0.6) Ma. These results provide a Middle Eocene age for the Zambales ophiolite, in agreement with the minimum Late Eocene age of the overlying Akistero Formation. No age difference is discerned between the arc-like Acoje block and MORB-like Coto block of the Zambales ophiolite. Four zircon fractions from two sample sites in the Angat ophiolite give concordant ages of 48.1 (+/- 0.5) Ma. This age is considerably younger than the Late Cretaceous age based on radiolarian fauna derived from a sheeted dike-pillow lava-sediment sequence south-southeast of the main ophiolite. The small age difference between the Zambales and Angat ophiolites suggests a common origin and obviates the need for a major structural discontinuity west of the Southern Sierra Mandre beneath the Central Valley of Luzon. The Cretaceous biostratigraphic ages of the ophiolitic rocks southeast of the Eocene Angat ophilolite implies that there are two ophiolitic basements exposed in the Southern Sierra Mandre. The relationship between the two ophiolites is constrainted by the overlying stratigraphic relations wich indicate that an Eocene volcanic arc and associated volcaniclastic apron was built on both the Eocene and Cretaceous ophiolitic basement. This suugests that the Zambales-Angat ophiolite represents a preserved Eocene back-arc basin that opened behind an Eocene arc that developed within Cretaceous oceanic basement. In this model, the Zambales-Angat ophiolites are therefore not allochthonous terranes but part of a single plate, generated in situ, forming part of the autochthonous basement of Luzon.

Encarnacion, John P.; Mukasa, Samuel B.; Obille, Eligio C., Jr.

1993-11-01

130

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

NASA Astrophysics Data System (ADS)

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

Pallister, John S.

1981-04-01

131

Suprasubduction Origin for the Nari Ophiolite, Southwestern Belt of Fariman, Northeast Iran  

NASA Astrophysics Data System (ADS)

In the west and southwest region of Fariman, northeast of Iran, two Late Cretaceous-Neotethys ophiolitic belts meet. The western belt of Fariman is the continuation of the eastern belt of the Sabzevar ophiolitic belt that runs parallel to the Miami fault and separates the Central Iran from the Alburz-Binalud structural zone. The continuation of the southwestern belt of Fariman, non-continuously and parallel to the Doruneh fault, joins to the Naein ophiolite. The study area, situated in the ophiolitic belt of southwest Fariman between 35o, 30'N and 35o 37'N latitudes, and 59o 37'E and 59o 45'E longitudes. The Nari Ophiolite exposes the tectonized mixture of ultramafic rocks, a few kilometers long mass of cumulate gabbro, sheeted dikes, pegmatite gabbro, individual diabase dikes and an extrusive sequence of pillow basalt and sheeted lava flow underlied by pelagic limestone and chert. The Eocene conglomerate is deposited on top of the sequence with a disconformity. Recent paleontology studies on the pelagic limestone give a Late Cretaceous, Coniasian-Santonian age for the limestone. Based on petrography studies, the ultramafic rocks are serpenticized and have harzburgite, clinopyroxene- bearing harzburgite, and lherzholite composition. Olivine porphyroclasts are affected by dense kink bands and the orthopyroxene porphyroclasts are stretched and suggest that the rocks are affected by mantle deformation. Massive gabbros have cumulated texture and display layering. The thickness varies from a few centimeters to one meter. Their composition is olivine gabbro and pyroxene gabbro. Pyroxenite is formed in thin non- continuous layers throughout the sequence however pegmatite gabbro only forms at the lower layers of the sequence. The microscopic texture of the majority of these masses is mesocumulate and heteradcumulate however pyroxenite shows an adcumulate texture. The presence of these textures suggests an open system for mineral crystallization. The layered gabbros in the Nari region are intersected with pegmatitic veins and dikes, individual diabase dikes, plagiogranite veins, and several epidote veins. The structural measurements for layers of gabbro show an E-W orientation while a N-S orientation is measured for dikes and veins, sheeted dikes, and individual diabase dikes. These findings suggest an opening of a basin while the ophiolitic belt of southwest Fariman was forming during the Late Cretaceous. Sheeted dikes are diabasic and the quartz contained in the majority of the dikes is the result of secondary processes. Pillow basalts and sheeted dikes have basaltic and basaltic andesitic composition. Even though in layered gabbros, the crystallization trend is olivine - plagioclase - clinopyroxene - orthopyroxene - amphibole, the study of the pattern of elements on primitive mantle-normalized spider diagrams show clear depletion of Nb in gabbros. The patterns of these elements in gabbros are less enriched than sheeted dikes and pillow basalts. However, sheeted dikes and pillow basalts also show clear negative anomalies. The tectonomagmatic diagrams also show that the Nari ophiolite separates from the MOR regions and they inclined more toward subduction tectonomagmatic regions. It seems that the N-S oriented Nari ophiolite of the southwest belt of Fariman is formed during the Late Cretaceous in a supra-subduction system. Our data is consistent with the model suggesting that the Arabian plate is continuously subducted beneath the margin of the Iranian plate and that the remnant of the Paleotethys ocean is subducting under northeastern Iran.

Khalatbari Jafari, M.; Hatami, M.

2009-05-01

132

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

Microsoft Academic Search

Both spinel and plagioclase harzburgites from the Yungbwa ophiolite, southwestern Tibet, have been studied for highly siderophile elements (HSEs) and Re–Os isotopes. The spinel harzburgites can be subdivided into low-Cr# and high-Cr# groups, according to the spinel Cr#. The low-Cr# harzburgites have been estimated to experience with ca. 13% degrees of partial melting, whereas the high-Cr# harzburgites have been subjected

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

133

Os isotopes and highly siderophile elements (HSE) in the Ligurian ophiolites, Italy  

NASA Astrophysics Data System (ADS)

The Os isotopic and highly siderophile element (HSE) concentration systematics of the upper mantle have been the focus of much recent interest. However, little systematic study has addressed the combined HSE and Os isotopes in mantle rocks from MOR ophiolites. The Ligurian ophiolites in northern Italy represent an important class of ophiolites representing, like Zabargad or the Galicia margin, crust with clear ocean ridge affinity floored by older mantle with arguably more continental affinities [Rampone et al., J. Petrol. 36, 18-105, 1995; Rampone et al., Contrib. Mineral. Petrol. 123, 61-67, 1996]. We have studied a suite of 15 geochemically and geologically well characterized mantle peridotites from this ophiolite. The samples have Os isotopic and HSE concentration characteristics in agreement with similar measurements made in abyssal peridotites [Snow and Schmidt, Nature 391, 166-169, 1998]. The observations are consistent with the notion that the depleted mantle shows a HSE signature consistent with the influence of core-derived metal. Robust age information could not be obtained for either body, however the data from the External unit are consistent with a Proterozoic melting age similar to the Ronda ultramafic massif. Os-Nd isotopic correlations observed are not, however, consistent with the evolution of variably depleted mantle. Such Os-Nd correlations could be modeled by addition of recycled sediment to depleted mantle, however this disagrees with current notions of the origin of the depleted mantle. Variable 187Os/ 188Os with essentially constant Al 2O 3 in mantle rocks can be explained by recent remelting of ancient mantle.

Snow, Jonathan E.; Schmidt, Gerhard; Rampone, Elisabetta

2000-01-01

134

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

Microsoft Academic Search

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

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

1986-01-01

135

Geochemistry and tectonic significance of the Tangihua Ophiolite Complex, New Zealand  

Microsoft Academic Search

The ophiolitic igneous complexes of the Tangihua Complex of Northland, New Zealand consist of massive and pillowed basaltic lava sequences with intercalated sediments and lesser gabbro, microgabbro and basaltic intrusions. The complex also includes minor felsic derivatives, younger alkalic intrusions (which exhibit within-plate characteristics) and rare ultramafic rocks.The lavas are relatively homogeneous and are dominantly tholeiitic basalts with minor calc-alkaline

K. N. Nicholson; P. M. Black; C. Picard

2000-01-01

136

Petrology and origin of primitive lavas from the Troodos ophiolite, Cyprus  

Microsoft Academic Search

Parental magmas to the Troodos ophiolite are characterised by low TiO2 and Al2O3 and high SiO2. Extremely fresh and chemically primitive (high MgO) rocks are found within the Upper Pillow Lavas and along the Arakapas Fault Belt of Cyprus and contain forsteritic olivine±enstatite and groundmass clinopyroxene set in glass or plagioclase, with accessory magnesiochromite and sometimes hornblende. They are quartz-normative

W. E. Cameron

1985-01-01

137

Uranium-lead isotopic ages of plagiogranites from the Troodos ophiolite, Cyprus, and their tectonic significance  

Microsoft Academic Search

Zircon populations from two plagiogranites in the plutonic section of the Troodos ophiolite yield U-Pb dates between 90.3 +\\/- 0.7 and 9.24 +\\/- 0.7 Ma. Zircons from one of the two samples yield U-Pb dates of 90.3 +\\/- 0.7 and 92.1 +\\/- 0.7 Ma. This difference is greater than their analytical error, and the zircons are therefore considered to be

Samuel B. Mukasa; John N. Ludden

1987-01-01

138

New constrains on the thickness of the Semail ophiolite in the Northern Emirates  

Microsoft Academic Search

Near-critical angle and refraction studies were performed at IFP as piggyback studies during a wider programme of crustal\\u000a imagery operated by WesternGeco on behalf of the Ministry of Energy of the United Arab Emirates. The main objective is to\\u000a illuminate the base of the Semail Ophiolite along part of a regional transect (D1) crossing the Northern Emirates from the\\u000a Gulf

Charles Naville; Martine Ancel; Paul Andriessen; Patrice Ricarte; François Roure

2010-01-01

139

A multi-technique study of platinum group element systematic in some Ligurian ophiolitic peridotites, Italy  

Microsoft Academic Search

Fe–Ni–Cu sulfide mineralogy has been investigated along with bulk-rock and in-situ PGE analyses by ICP MS and LA-ICP-MS in eight lherzolites from the Internal (IL) and External Liguride (EL) ophiolites (Italy). The two EL lherzolites are fertile (2–4% partial melting) and slightly serpentinized while the six IL cpx-poor lherzolites have experienced 5–10% of partial melting, impregnation by instantaneous melt fractions

Ambre Luguet; Jean-Pierre Lorand; Olivier Alard; Jean-Yves Cottin

2004-01-01

140

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

NASA Astrophysics Data System (ADS)

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

France, Lydéric; Ildefonse, Benoit; Koepke, Juergen

2013-08-01

141

Purtuniq ophiolite, Cape Smith belt, northern Quebec, Canada: A reconstructed section of Early Proterozoic oceanic crust  

NASA Astrophysics Data System (ADS)

The crustal part of a dismembered ophiolite is preserved in the structurally highest thrust sheets of the Early Proterozoic Cape Smith belt in northern Quebec, Canada. The rocks of the ophiolite represent two magmatic suites, each with a mantle source that is Nd isotopically distinct. The older suite comprises pillowed and massive volcanic rocks, sheeted dikes, dominantly gabbroic layered cumulate rocks, and rare plagiogranite sills and dikes in the volcanic sequence. This >5-km-thick tholeiitic suite is ca. 1998 Ma, and is compositionally and petrographically similar to rocks formed at modern oceanic spreading ridges. The younger suite (>4 km) comprises sheeted mafic dikes and layered ultramafic to mafic cumulate rocks that are similar to tholeiitic rocks found in modern plume- generated oceanic islands, such as Hawaii. The composite section is in excess of 9 km. Although the total thickness of the Purtuniq ophiolite may not be typical of that for oceanic crust formed at Early Proterozoic spreading ridges elsewhere, it does suggest that ancient ridge-formed crust was at least as thick as that in modern oceans.

Scott, D. J.; Helmstaedt, H.; Bickle, M. J.

1992-02-01

142

Geological features of a collision zone marker: The Antique Ophiolite Complex (Western Panay, Philippines)  

NASA Astrophysics Data System (ADS)

The Antique Ophiolite Complex exposed along the western side of Panay Island, central Philippines was derived from the Jurassic to Cretaceous proto-South China Sea oceanic leading edge of the Palawan microcontinental block. The subduction and ultimate closure of this ocean basin resulted in the emplacement and exposure of this lithospheric fragment along the collisional boundary of the microcontinental block and the oceanic- to island arc-affiliated Philippine mobile belt. The ophiolite complex has volcanic rocks having normal- to transitional mid-ocean ridge basalt (MORB) to island arc tholeiitic (IAT) geochemistry consistent with the transitional MORB-IAT characteristics of its peridotites. The chromitites manifest subduction signature suggestive of the involvement of water in its generation. All of these would be consistent with generation in a supra-subduction zone environment, specifically in a subduction-related marginal ocean basin. The collision of the Palawan microcontinental block with the Philippine mobile belt along western Panay resulted, aside from ophiolite emplacement, into arc curvature, island rotation, serpentinite diapirism and thrusting along the forearc side. The offshore bathymetric expression of the microcontinental block along the collision zone shows the leading edge of this oceanic bathymetric high to have spread laterally. This is indicative of its being buoyant resulting to non-subduction as supported by available earthquake hypocenter data.

Yumul, Graciano P.; Dimalanta, Carla B.; Tamayo, Rodolfo A.; Faustino-Eslava, Decibel V.

2013-03-01

143

Diamond and moissanite in ophiolitic mantle rocks and podiform chromitites: A deep carbon source?  

NASA Astrophysics Data System (ADS)

Diamonds are known from a variety of occurreces, mainly from mantle-derived kimberlites, meteorite impact craters, and continental deep subduction and collision zones. Recently, an unusual mineral group was discovered in the Luobusa ophiolitic chromitites from the Yarlung Zangbu suture, Tibet, which probably originated from a depth of over 300 km in the mantle. Minerals of deep origin include coesite apparently pseudomorphing stishovite, and diamond as individual grains or inclusions in OsIr alloy. To determine if such UHP and unusual minerals occur elsewhere, we collected about 1.5 t of chromitite from two orebodies in an ultramafic body in the Polar Urals. Thus far, more than 60 different mineral species have been separated from these ores. The most exciting discovery is the common occurrence of diamond, a typical UHP mineral in the Luobusa chromitites. These minerals are very similar in composition and structure to those reported from the Luobusa chromitites. So far diamond and/or moissanite have been discovered from many different ophiolitic ultramafic rocks, including in-situ grains in polished chromitite fragments. These discoveries demonstrate that the Luobusa ophiolite is not a unique diamond-bearing massif. Secondary ion mass spectrometric (SIMS) analysis shows that the ophiolite-hosted diamond has a distinctive 13C-depleted isotopic composition (?13C from -18 to -28‰, n=70), compatible to the ophiolite-hosted moissanite (?13C from -18 to -35‰, n=36), both are much lighter than the main carbon reservoir in the upper mantle (?13C near -5‰). The compiled data from moissanite from kimberlites and other mantle settings share the characteristic of strongly 13C-depleted isotopic composition. This suggests that diamond and moissanite originates from a separate carbon reservoir in the mantle or that its formation involved strong isotopic fractionation. Subduction of biogenic carbonaceous material could potentially satisfy both the unusual isotopic and redox constraints on diamond and moissanite formation, but this material would need to stay chemically isolated from the upper mantle until it reached the high-T stability field of diamond and moissanite. The origin of diamond and moissanite in the mantle is still unsolved, but all evidence from the upper mantle indicates that they cannot have formed there, except under special and local redox conditions. We suggest, alternatively, that diamond and moissanite may have formed in the lower mantle, where the existence of 13C-depleted carbon is strongly suspected.

Yang, J.; Xu, X.; Wiedenbeck, M.; Trumbull, R. B.; Robinson, P. T.

2010-12-01

144

Geology and geochemistry of the Neoproterozoic Tuludimtu Ophiolite suite, western Ethiopia  

NASA Astrophysics Data System (ADS)

The Kemashi Domain, a lithotectonic subdivision of the Neoproterozoic Tuludimtu Orogenic Belt of western Ethiopia, consists of a suite of mafic ultramafic volcanic and plutonic rocks, and interbedded deep marine sediments, mainly graphite-bearing pelitic schists and phyllites, and graphitic quartzites and cherts. Pillow structures indicate submarine extrusion of the volcanics, whilst partings within some of the basalts may represent sheeted dykes. An associated mélange unit, composed of blocks of the same rock types as above, set in a fine schistose matrix, also occurs. This assemblage is interpreted as a dismembered ophiolite—the Tuludimtu Ophiolite—formed in a deep oceanic environment. A turbiditic sequence is also present in the domain. The Tuludimtu Ophiolite underwent intense compression during the Neoproterozoic Pan African Orogeny, resulting in early recumbent folding and westwards-directed thrusting, followed by reactivation of steeper zones of the thrusts as N S orogen-parallel strike-slip shear zones, accompanied by refolding of early folds into upright horizontal folds. This was followed by development of deep crustal NNW SSE orogen-transecting shear zones, which were reactivated as brittle faults during orogenic collapse of the Tuludimtu Belt. Metamorphism to lower greenschist facies grade accompanied orogenesis. Major, trace and REE geochemistry of volcanic and some plutonic igneous rocks, has been employed to define the tectonic setting of the terrane. Tectonic discrimination diagrams, utilising REE and HFSE, indicate a wide distribution spectrum but with the majority of samples plotting in arc basalt and MORB fields, suggesting derivation from sources similar to N-MORB and depleted MORB (typical of many arc basalts). Most of the samples exhibit a slight depletion of immobile elements, relative to N-MORB values and also show depletion of Zr, Ti, Nb and Y, implying that their source had been depleted by an earlier melting episode. Overall, the geochemistry typifies spreading centre basalts with some compositional features transitional to those of arc basalts, a characteristic of back-arc basalts. Lithological association, structural style and geochemistry of the rock assemblage in the Kemashi Domain thus define an ophiolite interpreted to have formed within a deep marine environment. This is thought to have been due to rifting of continental crust within a back-arc basin regime in a continental margin type extensional setting. Comparison with other ophiolitic terranes within the Arabian Nubian Shield, suggests that many of these terranes may represent back-arc basin type tectonic settings, similar to the Kemashi Domain. This supports the multi-stage accretion model for closure of the Mozambique Ocean, for which the Pacific Ocean may be a present day analogue.

Tadesse, Gebremedhin; Allen, Alistair

2005-02-01

145

Petrology and geochemistry of the Cyclops ophiolites (Irian Jaya, East Indonesia): Consequences for the Cenozoic evolution of the north Australian margin  

Microsoft Academic Search

Summary The Cyclops massif (Irian Jaya - Western Indonesia) displays all components of an ophiolitic sequence including residual mantle peridotites (harzburgites and dunites), cumulate gabbros, dolerites, normal mid-oceanic ridge basalts (N-MORB) and minor amounts of boninitic lavas. This ophiolitic series tectonically overlies high temperature (HT)-high pressure (HP) mafic rocks metamorphosed during the Miocene.

C. Monnier; J. Girardeau; M. Pubellier; M. Polvé; H. Permana; H. Bellon

1999-01-01

146

Natural flows of H2-rich fluids in the ophiolites of Oman and the Philippines: Tectonic control of migration pathways and associated diagenetic processes  

Microsoft Academic Search

We compare the geological environments of sites of emission of natural hydrogen in the Oman ophiolite and the Zambales ophiolite (Luzon, Philippines). The genesis of natural H2 results from the interaction between ultrabasic rocks and aqueous solutions circulating in deep fracture networks, by oxidation of metals (Fe2+, Mn2+) and reduction of water, probably under high temperature conditions. This process generates

E. P. Deville; A. Prinzhofer; C. Vacquand; V. Chavagnac; C. Monnin; G. Ceuleneer; C. A. Arcilla

2009-01-01

147

530 Ma zircon age for ophiolite from the New England orogen: Oldest rocks known from eastern Australia  

NASA Astrophysics Data System (ADS)

New ion microprobe data provide constraints on the timing of formation of ophiolitic rocks in the New England tectonic collage in eastern Australia. Results for analyses of magmatic zircons from plagiogranite of the Weraerai terrane ophiolite at Upper Bingara give a 206Pb/238U age of 530 ±6 Ma (2?). This plagiogranite is the oldest rock from eastern Australia yet identified. Existing tectonic models suggest that progressively younger crust was accreted to the eastern margin of Gondwanan Australia throughout the Paleozoic. This cannot be reconciled easily with the Cambrian age for these ophiolitic rocks, which are juxtaposed between Devonian terranes and are at least 1000 km east of the nearest lithologically similar rocks of comparable age. We speculate that younger, thin-skinned terranes may have been thrust westward over the continental freeboard of eastern Australia during the late Paleozoic.

Aitchison, J. C.; Ireland, T. R.; Blake, M. C., Jr.; Flood, P. G.

1992-02-01

148

Tectonic significance of 400 Ma zircon ages for ophiolitic rocks from the Lachlan fold belt, eastern Australia  

NASA Astrophysics Data System (ADS)

New SHRIMP ion microprobe data provide constraints on the age of formation of ophiolitic rocks from the Tumut region of the Lachlan fold belt, southeastern Australia. Magmatic zircons from plagiogranites within both the Coolac serpentinite belt and Wambidgee serpentinite belt give 206Pb/238U ages of about 400 Ma. This Devonian age is considerably younger than those previously proposed for these ophiolitic rocks (Cambrian-Ordovician to Early Silurian) and provides important tectonic data for this region of the Lachlan fold belt. We suggest that these rocks formed within a narrow marginal basin akin to those of Cretaceous age within the Chilean Andes of South America. The dates presented are within the 420 390 Ma range assigned to most of the granitic bodies of the Lachlan fold belt. We therefore suggest that the generation of both the ophiolitic rocks and granitic magmas of the Lachlan fold belt were part of the same major tectonothermal event.

Graham, I. T.; Franklin, B. J.; Marshall, B.; Leitch, E. C.; Fanning, M.

1996-12-01

149

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

150

Quantifying Strain Across a Paleotransform Fault in the Mantle Section of an Ophiolite, New Caledonia  

NASA Astrophysics Data System (ADS)

The Massif du Sud in New Caledonia exposes the mantle section of a Cretaceous-age ophiolite generated by spreading in a small basin northeast of the island. Most of the ophiolite has shallow SW-dipping foliations and subhorizontal NS lineations interpreted as reflecting fabric formed at a mid-ocean spreading ridge. Fabric intensity increases and foliation steepens to NS-striking and subvertical in the Bogota Peninsula shear zone while lineations remain shallowly S-plunging. The rotation of field foliation and pyroxenite dike orientations across a 50-km wide region is symmetric about a central high-strain zone and shear sense indicators suggest that relative displacement was dextral. The symmetry of fabrics is consistent with formation in the mantle section of a true paleotransform fault zone as opposed to a fracture zone, where fabric rotation would only be expected on one side of the shear zone. Thus, the New Caledonia ophiolite includes rocks that record both ridge- and transform-related deformation. To better understand the evolution of the transform fault, the fabrics within the Bogota Peninsula shear zone were subdivided into three regions (farfield, nearfield, and central high strain zone) of approximately homogeneous deformation. We used fabrics from these regions together with those from the Massif du Sud to model how kinematics change from ridge-dominated deformation outside the shear zone to transform- dominated inside the shear zone. Shear zone localization was modeled as superimposed increments of deformation using a grid search method that varied parameters including the orientation of the shear plane, shear sense, and transpressional versus transtensional kinematics. The best models were determined by comparing (1) the orientation of the strain ellipse with field foliation and lineation, and (2) the predicted and observed rotations of pyroxenite dikes across the shear zone. This approach allows quantitative estimation of the total deformation accumulated across this ridge-transform system.

Titus, S. J.; Davis, J. R.; Ferre, E. C.; Tikoff, B.

2008-12-01

151

Structure and dynamics of ridge axial melt lenses in the Oman ophiolite  

NASA Astrophysics Data System (ADS)

As oceanic fast spreading ridges, the Oman ophiolite had a melt lens perched on top of the magma chamber wherein the gabbro unit was crystallizing. In ophiolites, this melt lens is reduced to a horizon that is identified in the field. Insights in the active melt lens are possible, 1) looking at gabbros that subsided from the lens horizon and drifted out of the magma chamber with a steep magmatic foliation, as documented in another presentation in this session (Boudier and Nicolas, Subsidence in magma chamber and the development of magmatic foliation in Oman ophiolite gabbros). They record successive events within the lens, 2) considering uncommon areas, that are ascribed to a retreat of the melt lens, a “tide” effect uncovering flat-lying gabbros and exposing large areas of the lens floor. Their strong magmatic foliation and lineation point to a dynamic deposition on the floor, by convection rolls, traced by lineation trends in settled gabbros, 3) considering the ubiquitous occurrence of anorthosites that are interlayered with the foliated gabbros. They are issued from a rising melt having plagioclase on its liquidus and coming as sudden and massive melt surges. Their frequency of ~300 years and their large volume control the stability of melt lenses. A surge induces rapid inflation in the lens that migrates transversally with possibly some swelling. Melt lens deflation follows, that is due to continuous accretion processes by both subsidence of a mush from the lens floor, and basalt extrusions and sheeted dikes intrusions at the high frequency of ~10 years. Driven by a deficit in the melt surge activity feeding the lens, deflation conducting to death of the lens is documented.

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

2009-12-01

152

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

NASA Astrophysics Data System (ADS)

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

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

1981-02-01

153

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

NASA Astrophysics Data System (ADS)

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

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

1988-01-01

154

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

Microsoft Academic Search

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

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

2008-01-01

155

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

NASA Astrophysics Data System (ADS)

Fragments of subducted oceanic lithosphere returned along the plate interface convey crucial information regarding the thermal and rheological conditions of convergent plate boundaries. Geological evidence indicate that, unlike subduction, exhumation is non-steady (Agard et al., Earth Sci. Rev. 2009). We herein focus on deep processes along the plate interface (40-80 km depth), for which there is no counterflow (unlike in accretionary prisms) and no other known mechanisms to return eclogites than interplate friction or buoyancy. These eclogites are of two major types: large scale (>km) slices with coherent PT estimates (W. Alps) versus isolated fragments (frequently m-hm) in a serpentinite- or sedimentary-rich matrix showing contrasting equilibration depths (with hints of punctuated exhumation and even reburial; Franciscan, Cuba, Sistan; e.g., Garcia-Casco et al., Geol. Acta 2006). This latter type tends to show warmer equilibration paths, whereas the larger tectonic slices from the former type remain systematically cold. Serpentinites are crucial for both in permitting decoupling and acting as a buoy, and fluid budget is important too in enhancing floatability and allowing large slices to survive (Angiboust and Agard, Lithos 2010). Numerical models implementing free migration of fluids in the subduction zone also show that the plate interface is strongly localized in the absence of fluids: mechanical decoupling efficiently occurs along the sediment veneer and/or at the top of the highly hydrothermalized crust. Whenever fluids are released in greater amounts (depending on initial fluid content and/or thermal structure), deformation becomes much more distributed and affects both the mantle wedge and the top of the downgoing lithosphere (crust and hydrated mantle top), thereby increasing mechanical coupling between the two plates. Based on natural data and numerical modelling we herein propose that rheological contrast chiefly controls mechanical decoupling. On a steady-state basis the subduction interface is apparently efficiently decoupled. In this context, we hypothetize that the liberation of fluid through pulses (or a somewhat increased amount of fluids) is required to locally modify mechanical coupling and induce the slicing of large pieces of oceanic material along the subduction interface (type 1). By contrast, an extreme hydration of the subduction interface and mantle wedge will result in the formation of serpentinite melanges and extensive material mixing (e.g., cold plumes, mafic pods and localized melting; type 2). This latter situation may be promoted by young/fast/wet subduction, such as subduction initiation and/or subduction of young lithosphere or subduction of a particularly hydrated lithosphere section (e.g., at the ridge and/or prior to entering the trench), whereas cold, slow subduction (type 1) will result in irregular hydration and localized coupling able to detach large slices.

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

2011-12-01

156

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

NASA Astrophysics Data System (ADS)

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

Raznitsin, Yu. N.

2012-01-01

157

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

SciTech Connect

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

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

1986-01-01

158

The origin of oceanic plagiogranites from the karmoy ophiolite, western Norway  

NASA Astrophysics Data System (ADS)

Both field relationships and geochemical characteristics indicate two suites of plagiogranitic and related rocks coexisting in the higher parts of the Karmoy ophiolite of western Norway. The plutonic zone of this ophiolite can be subdivided into three complexes; the East-Karmoy Igneous Complex, the Visnes High Level Complex and the Veavagen Igneous Complex and plagiogranitic rocks are well developed in the first two of these. Within the East-Karmoy Igneous Complex, plagiogranites are associated with high temperature, pre-basic dyke, shear zones. Rare earth element modelling indicates that these plagiogranites were derived by anatexis of amphibolite (hydrated diabase) assuming a starting material consisting of 40% hornblende and 60% plagioclase and that batch melting occurred within the stability field of hornblende. In comparison, plagiogranite occurs in a number of bodies in the upper part of the Visnes High Level Complex and forms a sandwich horizon together with biotite diorites and epidosites between a roof assemblage of dykes, microgabbros and magnetite gabbros, and a floor assemblage of layered and non-layered gabbros. The R.E.E. modelling of the petrogenesis of this series of plagiogranites indicates that they were derived by filter pressing of a differentiated interstitial liquid to the vari-textured gabbros, although the distribution of highly hygromagmatophile elements such as K, Rb, Ba, etc. cannot be explained satisfactorily by this model alone. Depletion in these elements appears to be an autometasomatic effect.

Pedersen, Rolf B.; Malpas, John

1984-11-01

159

Anorthitic plagioclase and pargasitic amphibole in mantle peridotites from the Yungbwa ophiolite (southwestern Tibetan Plateau) formed by hydrous melt metasomatism  

Microsoft Academic Search

Plagioclase crystals with an anorthite content up to An99 have been discovered in mantle peridotites from the Yungbwa ophiolite, which crops out along the Indus–Yarlung Zangbo Suture, southwestern Tibetan Plateau. Amphiboles of pargasitic composition also occur in these plagioclase peridotites. Microtextures support the view that both plagioclase and amphibole were formed metasomatically by hydrous melts entering the Yungbwa peridotites, during

Chuan-Zhou Liu; Fu-Yuan Wu; Simon A. Wilde; Liang-Jun Yu; Ji-Liang Li

2010-01-01

160

Dike surface lineations as magma flow indicators within the sheeted dike complex of the Troodos Ophiolite, Cyprus  

Microsoft Academic Search

Mesoscopic flow lineations and anisotropy of magnetic susceptibility (AMS) have been measured for dikes within the Cretaceous-age Troodos ophiolite with the goal of comparing the direction of initial magma flow through dike conduits immediately following crack propagation with that of flow of subsequent magma emplaced during later stages of dike growth. Dike margin indicators of flow include cusp axes and

Robert J. Varga; Jeffrey S. Gee; Hubert Staudigel; Lisa Tauxe

1998-01-01

161

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

NASA Astrophysics Data System (ADS)

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

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

2013-07-01

162

Magma mixing and the origin of layered cumulates: evidence from the Oman ophiolite (Bahla and Wuqbah massifs)  

Microsoft Academic Search

Ultramafic cumulates, essentially wehrlitic, are an important component of the crustal section of the Oman ophiolite. They cannot be related to the most common gabbroic cumulates through simple fractional crystallization processes in anhydrous conditions. They occur frequently as intrusions post-dating the crystallization of the gabbros and, in such cases, are interpreted as \\

Bénédicte Abily; Georges Ceuleneer; Michel Gregoire; Mathieu Benoit

2010-01-01

163

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

Microsoft Academic Search

The Santa Elena Ophiolite Complex (SEOC) is located on the west coast of Northern Costa Rica, near the Nicaraguan border. It consists primarily of preserved oceanic crustal rocks and underlying upper mantle thrust onto an accretionary complex. The petrogenesis and tectonic origin of this complex have widely been interpreted to be either a preserved mantle portion of the Caribbean Large

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

2010-01-01

164

a Plate-Tectonic Model for Late Jurassic Ophiolite Genesis, Nevadan Orogeny and Forearc Initiation, Northern California  

NASA Astrophysics Data System (ADS)

Recently published age and structural data allow the reconciliation of previously conflicting models for Late Jurassic genesis of the Josephine, Smartville and Coast Range ophiolites, and the Nevadan orogeny in the Klamath Mountains and Sierra Nevada. The resulting model is consistent with the mode of initiation, location and geometry of the Great Valley forearc basin, and with the lack of a significant forearc basin west of the Klamath Mountains. The Coast Range ophiolite formed by backarc spreading west of an east-facing intraoceanic arc. Soon thereafter, a remnant arc was calved off the west side of this arc, and the Smartville ophiolite formed by backarc (interarc) spreading. During this time, the Sierran phase of the Nevadan orogeny began as the intraoceanic arc encountered the west-facing continental-margin arc of North America. An east-west-trending calcalkaline dike swarm in the Sierra Nevada foothills may mark the trajectory of the colliding arcs at the initiation of the collision. Simultaneously, a new subduction zone was initiated west of the collision (suture) zone, and this new trench propagated southward, thus trapping the Coast Range ophiolite in the new forearc area south of the Klamath area. Intense deformation in the Sierran region resulted from this collision, and both magmatic arcs became inactive as the last remnant of intervening oceanic crust was subducted. Continued westward relative movement of the North American arc was permitted north of the Sierra Nevada owing to the lack of a colliding intraoceanic arc. The result was the westward rifting of the continental-margin arc by intraarc spreading, which formed the Josephine ophiolite in the Klamath area. The Klamath phase of the Nevadan orogeny resulted from contraction of the west-facing intraoceanic arc and Josephine backarc basin beneath the continental margin. Basal sediments of the Great Valley forearc basin were derived primarily from the sutured arc/ophiolite terranes, and were deposited on top of the Coast Range ophiolite, the southern edge of the Klamaths, and the western side of the Sierra Nevada. A new (late Mesozoic) magmatic arc was superposed across the previously accreted terranes, and formed the primary sediment source for the Cretaceous forearc basin.

Ingersoll, Raymond V.; Schweickert, Richard A.

1986-10-01

165

Composition and U-Pb isotopic age determinations (SHRIMP II) of the ophiolitic assemblage from the Shaman paleospreading zone and the conditions of its formation (North Transbaikalia)  

NASA Astrophysics Data System (ADS)

The Upper Riphean Shaman ophiolitic assemblage was first distinguished and described in the territory of North Transbaikalia. Ophiolites found within a narrow suture (Shaman paleospreading zone) are represented by serpentinized ultrabasites with numerous plagiogranite veins having a U-Pb age of 971 ± 14 Ma, gabbros (939 ± 11 Ma), and basalts (892 ± 16 Ma). The ophiolite section also contains dikes of diabases and gabbro-diabases, siliceous-terrigenous stratum (black shale) of Upper Riphean age. The fragments of island-arc complexes (differentiated volcanites, gabbro-diorites, granites) of the Kelyan island-arc system are also found within the Shaman zone. The presence of Upper Riphean ophiolites in Baikalides of North Transbaikalia testifies to the formation of oceanic crust of the marginal spreading basins in the Precambrian Paleo-Pacific Ocean and the emerging Paleoasian Ocean.

Gordienko, I. V.; Bulgatov, A. N.; Lastochkin, N. I.; Sitnikova, V. S.

2009-12-01

166

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

NASA Astrophysics Data System (ADS)

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

Morgan, Zachary; Liang, Yan; Kelemen, Peter

2008-07-01

167

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

168

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

169

Oceanic versus emplacement age serpentinization in the Josephine ophiolite: Implications for the nature of the Moho at intermediate and slow spreading ridges  

NASA Astrophysics Data System (ADS)

We present field, petrographie, and geochemical evidence for oceanic serpentinization in the 162 Ma Josephine ophiolite of NW California and SW Oregon. Undeformed and unrodingitized dikes that intruded into serpentinized shear zones provide time markers for serpentinization and deformation. The dikes intruding serpentinites are of two types: (1) Fe-Ti enriched dikes with normal mid-ocean ridge basalt (NMORB) magmatic affinity which are geochemically linked to the uppermost lavas and a late Fe-Ti dike within the crustal sequence of the ophiolite; and (2) hornblende-bearing, calc-alkaline dikes intruded during ophiolite emplacement between 150 and 146 Ma. Based on crosscutting relationships between dikes and serpentinites, serpentinization of upper mantle peridotites took place at or near the ridge axis, during periods of amagmatic structural extension in the absence of a magma chamber. Lizardite-bearing seipentinites in the northern peridotite are constrained to be oceanic and indicate temperatures of <350°C within the oceanic upper mantle. Oceanic, lizardite-bearing, serpentinized shear zones, from the ultramafic cumulate section to the basal sole, indicate that the entire Josephine peridotite may have been transected by serpentinized shear zones prior to ophiolite emplacement The ultramafic cumulate sequence was completely serpentinized prior to ophiolite emplacement, and we suggest that the paleo-Moho in the Josephine ophiolite is a serpentinization boundary. The basal sole is interpreted to be a reactivated oceanic fault along which antigorite mylonites formed from a preexisting serpentinite during ophiolite emplacement. Oceanic serpentinization in the Josephine ophiolite took place beneath a 2-3 km crustal sequence and suggests that serpentinization may be an important feature at intermediate spreading rate mid-ocean ridges, as well as at slower spreading ridges.

Coulton, A. J.; Harper, G. D.; O'Hanley, D. S.

1995-11-01

170

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

Microsoft Academic Search

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

Peter B. Kelemen; Ken Koga; Nobu Shimizu

1997-01-01

171

Mineralogy and chemical composition of VMS deposits of northern Apennine ophiolites, Italy: evidence for the influence of country rock type on ore composition  

Microsoft Academic Search

In the ophiolites of the Italian northern Apennines, mantle rocks were exposed on the seafloor and eroded prior to the extrusion\\u000a of pillow basalt and the deposition of pelagic sediments. Various types of VMS deposits occur at different stratigraphic positions\\u000a in the ophiolite sequence. Stockwork-vein and seafloor-stratiform ore bodies are associated with serpentinized mantle peridotite\\u000a and serpentinite breccia. A second

F. Zaccarini; G. Garuti

2008-01-01

172

Evolution of arc crust and relations between contrasting sources: UPb (age), Nd and Sr isotope systematics of the ophiolitic terrain of SW Norway  

Microsoft Academic Search

The U\\/Pb dating of ophiolite and arc complexes in the Caledonides of SW Norway has demonstrated that these spatially associated\\u000a rocks are also closely related in time. A sequence of tholeiitic island arc volcanics, and an unconformably overlying sequence\\u000a of calc-alkaline volcanics have been dated as 494??2?Ma (2?) and 473??2?Ma respectively. Ophiolitic crust formed both prior\\u000a to, and during the

Rolf B. Pedersen; Greg R. Dunning

1997-01-01

173

Stable isotope geochemistry of Alpine ophiolites: a window to ocean-floor hydrothermal alteration and constraints on fluid–rock interaction during high-pressure metamorphism  

Microsoft Academic Search

The subduction of hydrated oceanic lithosphere potentially transports large volumes of water into the upper mantle; however,\\u000a despite its potential importance, fluid–rock interaction during high-pressure metamorphism is relatively poorly understood.\\u000a The stable isotope and major element geochemistry of Pennine ophiolite rocks from Italy and Switzerland that were metamorphosed\\u000a at high pressures are similar to that of unmetamorphosed ophiolites, suggesting that

I. Cartwright; A. C. Barnicoat

1999-01-01

174

Multi-Stage origin of the ophiolites along the Bangong-Nujiang suture zone: Implications for the evolution of central Tibet  

NASA Astrophysics Data System (ADS)

The history of the BNS ophiolite is important for models of the tectonic evolution of Central Tibet during the Mesozoic. Current petrologic, geochemical, and stratigraphic data all favor the interpretation of the ophiolite as having formed in a transitional MORB-IAT setting above a supra-subduction zone. Detailed studies of field relationships at several localities indicate that two distinct ophiolitic suites are present, especially in the western and middle sectors of the suture zone. One suite includes all elements of an entire ophiolite sequence albeit dismembered or disrupted through faulting. These rocks are mainly scattered along the northern part of the BNS, e.g. at Bangong Tso, Dong Tso and Dongqiao-Amdo. Radiometric ages for this ophiolitic suites are limited to the Early to Middle Jurassic. The other zone of ophiolitic rocks is located in the southern part of the BNS, such as Shiquanhe, Guchang, Lagkor Tso, Baila-Jueweng and Xainza, and is associated with a remnant basin allowed the deposition of Upper Jurassic to Lower Cretaceous cherts along the south part of the suture, and shallow marine deposition continued on the structural highs elsewhere along the suture.

Wang, W.; Aitchison, J. C.

2007-12-01

175

Tectonic evolution of the Cretaceous Ankara Ophiolitic Mélange during the Late Cretaceous to pre-Miocene interval in Central Anatolia, Turkey  

NASA Astrophysics Data System (ADS)

The chaotic tectonic belt, which is distinguished in northern Anatolia, is called the - Ankara Accretionary Complex - in the Ankara region, central Anatolia. The belt is differentiated into three imbricated tectonic subbelts, namely, pre-Triassic metamorphics, Mélange with calcareous blocks and Cretaceous mélange with ophiolitic blocks (Ankara Ophiolitic Mélange).The Ankara Ophiolitic Mélange (AOM) is a chaotic tectono-sedimentary mixture made up of detached blocks of Mesozoic ultramafic rocks, Cretaceous pillow basalts, Cretaceous radiolarites, Upper Jurassic-Cretaceous limestones and closely associated Upper Cretaceous basinal sequences. The detached and dismembered blocks lie within a highly sheared and brecciated ophiolitic detrital matrix or a block-on-block to sheared sedimentary matrix that varies along the mélange belt. Cenomanian-Turonian and Turonian-Santonian trench-linked basin deposits onlap the Cenomanian sedimentary and Cretaceous ophiolitic mélanges. The elements of the ophiolitic mélange were comixed as a result of tectonic recycling in the accretionary wedge. The belt is unconformably overlain by Campanian-Maastrichtian to Paleogene accretionary fore-arc basin deposits.The AOM developed in an accretionary wedge setting in which oceanic leading edge of the Anatolide-Tauride platform subducted toward north during the post-Barremian-pre-Campanian period. The AOM emplaced episodically and progressively as a result of in thrust tectonics with vergence ranging from SSW to SE during the post-Turonian to pre-Miocene in the Ankara terrain.

Rojay, Bora

2013-04-01

176

Detachment faulting and amagmatic extension at mid-ocean ridges: The Josephine ophiolite as an example  

NASA Astrophysics Data System (ADS)

Lithospheric extension at the Josephine paleo-spreading center occurred by a combination of magmatic and amagmatic processes. The amount of extension by amagmatic processes appears at least as large as that from magmatic processes. The structural processes responsible for amagmatic extension in the absence of a magma chamber appear to be normal faulting and block rotation in the brittle upper lithosphere and ductile flow in the lower lithosphere. An extensive detachment shear zone occurs beneath the fault blocks, approximately 1 km below the base of the crustal sequence. The amount of amagmatic extension can be approximated from a simple geometric model relating extension to rotations of sheeted dikes. Similarly, attenuated crustal thicknesses can be related to fault-block rotations. The results suggest that thin crustal sequences commonly observed in ophiolites and near fracture zones may result from attenuation during amagmatic extension. Furthermore, detachment faulting and block rotation may be characteristic of spreading centers where the magma budget is low.

Norrell, Gregory T.; Harper, Gregory D.

1988-09-01

177

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

NASA Astrophysics Data System (ADS)

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

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

2009-05-01

178

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

NASA Astrophysics Data System (ADS)

If confirmed, the recently-discovered methane (CH4) plume on Mars, in the Northern Summer of 2003, would reflect an emission of ~ 19 x103 tonnes y-1 and possibly even ~ 57 x104 tonnes y-1. Serpentinization in ophiolitic rocks is one of the main processes that are inferred for the origin of methane on Mars. Ophiolites or, hydrated mineral-bearing rocks in general on Earth could serve as analogs. So far, however, most of these “analog” studies focused on mineralogical and microbiological processes associated to ophiolitic environments. Analog studies specifically dealing with methane emissions to the Earth’s surface are missing. One of the observations of Mars methane is the transient release of large amounts of methane in a relatively short period, probably a few months. This would imply the existence of a mechanism of gas accumulations in the subsurface and episodic release to the surface. Such release mechanisms may be similar to certain weak and intermittent gas seeps or small mud volcanoes on Earth, rather than to steady, continuous degassing of methane from mineral reactions. Currently, it is not clear whether low-temperature serpentinization can be an abiogenic methane “kitchen” where methane might be generated fast enough to sustain vigorous and long-lasting seeps. In cases of fluxes of the order of several tonnes per year, a pressurized accumulation must exist. In case of lower fluxes, probably gas accumulations are not necessary and low temperature serpentinization can be fast enough to charge episodic seeps. These concepts are fundamental to our understanding of potential sources for the martian methane, and they need to be studied with the support of analog seepage data on Earth. Two examples are presented: (1) a case of terrestrial abiogenic CH4 seepage from ophiolitic rocks at the “eternal fires of Chimaera” in Turkey. Estimated flux data from the abiogenic gas seep of Chimaera in Turkey (>20 tonnes of CH4 per year) suggest a great potential of gas production and subsurface accumulation from low-temperature serpentinization. (2) Not related to serpentinization, we recently-discovered an extreme deuterium enrichment in methane from a gas seep in Romania, with delDCH4 value up to +124 ‰, that far exceed those reported for any terrestrial gas. This extraordinary value was interpreted as a result of abiogenic oxidation of biotic, thermogenic methane, generated from fossil organic matter. This finding shows that extremely positive delDCH4 values are still compatible with biotic sources and that the use of deuterium in CH4 as an indicator of abiogenic methanogenesis is invalidated. Although the D/H ratio is considered a fundamental biomarker, since biological systems prefer hydrogen (H) to deuterium (2H), our results indicate that this preference can be masked by abiogenic oxidation, occurring in the absence of oxygen. This is important in the context of identifying extraterrestrial biomarkers. Similar abiogenic alterations could, for example, occur on Mars, and this shall be taken into account when CH4 isotopic data will be hopefully available from the next Martian missions.

Schoell, M.; Etiope, G.

2010-12-01

179

Acadian remobilization of a Taconian ophiolite, Hare Bay allochthon, northwestern Newfoundland  

SciTech Connect

The Hare Bay fault is a major subhorizontal detachment at the base of the ophiolitic St. Anthony Complex in the Hare Bay allochthon, northwestern Newfounland. The fault is a postmetamorphic brittle detachment that truncates footwall structures related to both initial Ordovician (Taconian) assembly of the allochthon and subsequent Silurian-Devonian (Acadian) deformation. Although previously mapped as a thrust, the fault has an extensional rather than a contractional geometry; it cuts downsection to the west in the direction of transport, and it juxtaposes a hanging-wall sequence that contains little or no Acadian deformation against a footwall sequence that was pervasively deformed during the Acadian orogeny. The St. Anthony Complex lies on the western margin of the Acadian deformed zone. Its final emplacement, through movement on the Hare Bay fault, probably occurred through extensional faulting during gravitational collapse of the Acadian mountain front.

Cadwood, P.A. (Memorial Univ. of Newfoundland, St. Johns's (Canada))

1989-03-01

180

Diamond- and coesite-bearing chromitites from the Luobusa ophiolite, Tibet  

NASA Astrophysics Data System (ADS)

Diamonds and other ultrahigh pressure (UHP) minerals have been reported previously from the Luobusa ophiolite of Tibet, but these minerals have thus far been found only as individual grains. Here we report the occurrence of diamond as an inclusion in OsIr alloy and coesite as part of a silicate assemblage rimming a grain of FeTi alloy, both of which were recovered from chromitite. These occurrences confirm the presence of UHP minerals in the Luobusa chromitite requiring minimum pressures of ˜2.8-4 GPa. Individual coesite “crystals” have external form similar to that of stishovite and are polycrystalline, suggesting pseudo morphic replacement and implying a pressure >9 GPa. We propose that the UHP minerals were incorporated into the chromitites in the deep upper mantle or that they have an impact origin; the preponderance of evidence favors the former.

Yang, Jing-Sui; Dobrzhinetskaya, Larissa; Bai, Wen-Ji; Fang, Qing-Song; Robinson, Paul T.; Zhang, Junfeng; Green, Harry W., II

2007-10-01

181

The northern Samail ophiolite - An oxygen isotope, microprobe, and field study  

NASA Astrophysics Data System (ADS)

The paper presents geological, petrological, and oxygen isotopic data for 228 whole rock and mineral samples collected from a 100 x 20 km area of the northern Samail ophiolite in Oman. Most of these samples are from three detailed profiles through the pillow lavas, sheeted dikes, and layered gabbros of this laterally heterogeneous fragment of Cretaceous oceanic crust, down to and across the petrologic Moho. The least structurally complex transect (Wadi Hilti) also shows the simplest delta(O-18). The O-18 depletions in the Ibra area were produced by high-temperature hydrothermal activity at the spreading center during the cooling of middle- and upper-layered gabbros. Much more complex O-18/O-16 effects were observed in the Shafan-Kanut transect, which was sampled specifically to examine the effects of some major high-level shear zones associated with late-stage, off-axis, gabbro-diorite-plagiogranite intrusions emplaced into the sheeted dike complex.

Stakes, Debra S.; Taylor, Hugh P., Jr.

1992-05-01

182

Thin crust at the western Iberia Ocean-Continent transition and ophiolites  

NASA Astrophysics Data System (ADS)

Western Iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments. The age of breakup decreases from south to north. Seismic refraction and reflection profiles, and magnetic and gravity data from each segment, show a consistent pattern of geophysical observations across the ocean-continent transition (OCT) zone, which is a few tens of kilometers wide. We emphasize here the discovery of thin (2-4 km) oceanic crust underlain by 7.6 km s-1 material within the OCT. The available evidence favors the suggestion that the 7.6 km s-1 layer is serpentinized peridotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup. This thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities.

Whitmarsh, R. B.; Pinheiro, L. M.; Miles, P. R.; Recq, M.; Sibuet, J.-C.

1993-10-01

183

Melange: creating a \\  

Microsoft Academic Search

Most implementations of critical Internet protocols are written in type-unsafe languages such as C or C++ and are regularly vulnerable to serious security and reliability problems. Type-safe languages eliminate many errors but are not used to due to the perceived performance overheads. We combine two techniques to eliminate this performance penalty in a practical fashion: strong static typing and generative

Anil Madhavapeddy; Alex Ho; Tim Deegan; David Scott; Ripduman Sohan

2007-01-01

184

Melange: creating a \\  

Microsoft Academic Search

Most implementations of critical Internet protocols are written in type-unsafe languages such as C or C++ and are regularly vulner- able to serious security and reliability problems. Type-safe lan- guages eliminate many errors but are not used to due to the per- ceived performance overheads. We combine two techniques to eliminate this performance penalty in a practical fashion: strong static

Anil Madhavapeddy; Alex Ho; Tim Deegan; David Scott; Ripduman Sohan

2007-01-01

185

Melanges, No. 23.  

ERIC Educational Resources Information Center

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

Melanges, 1997

1997-01-01

186

Melanges, No. 23.  

ERIC Educational Resources Information Center

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

Melanges, 1997

1997-01-01

187

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

188

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

189

Dismembered ophiolites in Paleoproterozoic nappe complexes of Kandra and Gurramkonda, South India  

NASA Astrophysics Data System (ADS)

The constitution and structural architecture of ophiolites is important in the understanding of cratonic growth through subduction-collision-accretion history in the Precambrian terrains. The Nellore schist belt (NSB), sandwiched between the Eastern Ghats belt (EGB) and the Eastern Dharwar craton, South India is known to have deformed remnants of 'ocean-plate stratigraphy' (OPS) as at Kanigiri and Kandra suggesting multiple cycle of ocean opening and closure in the Paleoproterozoic-Mesoproterozic interval. The outcrop details and structural architecture of the Paleoproterozoic Kandra complex with geochemical character of subduction-related ophiolites is described and analyzed to better constrain the accretion-collision history along the southeastern margin of the Indian craton. Imbricate thrust slices between Kandra and Arimanupadu host slivers of hornblende metagabbro, thick massive to pillowed metabasalt sheets, sheeted meta-dolerite dykes and thin metapsammite and metapelite interstratified with metabasalt, comparable to Layer 2A of oceanic crust. Contractional deformation in interlayered metabasalt and quartzite around Kandra is accommodated by south dipping imbricate thrust systems and N(NE) vergent folds. The WNW trending thrust system with dominant tectonic transport toward northeast, is oblique to overall N-S trend of the NSB or the EGB. Further east, intercalated metabasalts, deep water turbidites and thin metacherts around Gurramkonda are deformed into SW vergent imbricate thrust system. Here, occurrence of metabasalt sheets along individual thrusts, suggest syntectonic emplacement of some basalts as the subducted and scraped upper oceanic crust was incorporated in the acrcetionary prism. The rock sequences around Kandra and Gurramkonda comprising incomplete OPS are tectonically stacked over granites of the Eastern Dharwar craton margin. Bivergent thrust systems around Kandra possibly root into a detachment separating Layers 1-2 from more rigid deeper layers (cf. Layer 3 of oceanic crust), as indicated by absence of significant volume of ultramafic rocks at the current outcrop level.

Saha, Dilip

2011-08-01

190

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

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

191

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

NASA Astrophysics Data System (ADS)

Plagiogranites in the Canyon Mountain ophiolite, Oregon, include a wide range of rock types ranging from diorite to trondhjemite. The plagiogranites are mostly concentrated as an intrusive sill swarm at the top of a section of gabbroic cumulates. The plagiogranites are typically low in K2O and high in Na2O, and are enriched 10 20 times chondrites in REE, and overlap with abundances in basic rocks from Canyon Mountain. All samples of plagiogranite are relatively depleted in LREE, with more silicic samples characterized by a slightly lesser degree of LREE depletion. Total REE content is not consistently correlated with contents of major and other trace elements. Fractional crystallization of basaltic magma may give rise to plagiogranites; however this model applied to Canyon Mountain plagiogranites is discounted because of the significant volume of plagiogranites relative to basic rocks, and the complete overlap of REE abundances of the basic rocks and the plagiogranites. The latter is also a major reason for rejecting the hypothesis of silicate liquid immiscibility in the generation of the plagiogranites. Field observations coupled with major-element and trace element chemistry lend support to a model by which the plagiogranites were produced by partial melting of basic rocks under hydrous conditions. REE data for the plagiogranites were used in calculations to delimit source REE contents. Relevant parameters in the calculations were estimated from experimentally determined phase relations of basalt under hydrous conditions. The resulting calculated source patterns are similar to those of basic rocks in ophiolites and oceanic settings, and suggest boundary conditions for the model. Partial melting as suggested for the Canyon Mountain plagiogranites probably occurred at relatively shallow depths (i.e., total pressures less than 5 kb).

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

1981-03-01

192

Emplacement of the ophiolitic units of the Grammos Massif (Dinaric-Hellenic chain, Greece): insights into processes of oceanic basin closure  

NASA Astrophysics Data System (ADS)

Ophiolite rocks from the Grammos Massif in northwestern Greece record a Mesozoic history of subduction, accretion and obduction deformation. Previous work focused on the Albanian (Mirdita) and Hellenic (Pindos- Vourinos, Koziakas, Orthys and Argolis) ophiolites located, respectively, to the north and to the south of Grammos Massif. Here the westward thrust of the ophiolites over the continental Pindos Unit has been unconformably covered by the Tertiary meso-hellenic molasses. The ophiolites show homogeneous distribution of MOR and SSZ ophiolites that record a series of coeval events: 1) Triassic oceanic-MOR basin opening, 2) Early-Middle Jurassic subduction and supra-subduction oceanic basins-SSZ; 3) Middle Jurassic intraoceanic obduction of young and still hot oceanic lithosphere (metamorphic sole), 4) Late Jurassic-Early Cretaceous collision and emplacement of the ophiolitic nappe on the continental margin and mélange formation underneath them, 5) Late-Early Cretaceous development of post-orogenic carbonate platform. Grammos Massif represents a key area to connect the tectonic evolution of the Dinaric-Hellenic ophiolites. Here we present new data on the evolution of both the metamorphic sole and the mélange unit underneath the ophiolites. In the Grammos Massif the exposure of the metamorphic sole and mélange consists, from top, underneath the ophiolitic units, to bottom, of: 1m of a thin, continuous band of mesoscopically not foliated metamorphic rocks; a discontinuous, usually thick assemblage of metasediments and amphibolites (metamorphic sole); a subophiolitic mélange with an upper portion of tectonic origin with very low grade metamorphic assemblage of ophiolites and a lower portion largely built by gravitational mechanisms with elements of the metamorphic sole embedded in the olistostromes. In particular we discuss the transition from metamorphic deformation to the block in matrix fabric in a regime of progressive shearing. We also report on the structural setting of the Early Tertiary terrigenous sediments of the continental margin (Pindos Flysch). In the Grammos area the flysch is cropping out as a folded unit with dismembered fold's limbs giving a block-in-matrix appearance to the formation. This structural setting disagree with the previous models of underthrusting lower plate.

Principi, G.; Vannucchi, P.; Menna, F.; Nirta, G.; Garfagnoli, F.; Photiades, A.

2006-12-01

193

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

194

Helium and neon isotopic compositions in the ophiolites from the Yarlung Zangbo River, Southwestern China: The information from deep mantle  

Microsoft Academic Search

The ophiolites from the Yarlung Zangbo River (Tibet), Southwestern China, were analysed for the contents of helium and neon\\u000a and their isotopic compositions by stepwise heating. The serpentinites from Bainang showed a high 3He\\/4He value of 32.66R\\u000a a (R\\u000a a is referred to the 3He\\/4He ratio in the present air) in 700°C fraction. At lower temperature, all of the dolerites

XianRen Ye; MingXin Tao; ChuanAo Yu; MingJie Zhang

2007-01-01

195

Geochemistry and geodynamic significance of the dike series of the Aluchin ophiolite complex, Verkhoyansk-Chukotka fold zone, Northeast Russia  

Microsoft Academic Search

The Aluchin ophiolites represent a tectonomagmatic complex, the upper crustal part of which is made up of two dike series.\\u000a One series includes diabases and gabbrodiabases, which are exposed in the Late Triassic Atamanov Massif (226 Ma) and subdivided\\u000a into low-potassium, low and moderate-titanium varieties. In terms of rare-earth element (REE) distribution pattern, these\\u000a rocks correspond to the mid-ocean ridge

A. V. Ganelin

2011-01-01

196

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

NASA Astrophysics Data System (ADS)

Suprasubduction-zone ophiolites are a characteristic feature of the Palaeozoic Caledonian-Appalachian orogenic belt, and mark the onset of convergence and closure of one or more oceans separating the Baltican, Laurentian and Avalonian continents, ending with continent-continent collision in the Mid Palaeozoic. The Bymarka-Løkken ophiolite in the Central Norwegian Caledonides is a variably tectonised ophiolite fragment, locally with an incomplete, but nevertheless well-preserved ophiolite pseudostratigraphy. Previous work has concluded that the ophiolite formed in an Early Ordovician suprasubduction-zone environment, most likely in an oceanic arc/back-arc basin system, but little is know about the evolution of the ophiolitic crust. There has also been some debate as to the whether obduction of the ophiolite upon the subjacent Gula Complex was onto Laurentia, Baltica, or a microcontinent of Baltican affinity. Here we present new, high-precision TIMS and SHRIMP zircon analyses from felsic rocks in the ophiolite. Combined with geochemical and Sm-Nd whole-rock and Lu-Hf zircon analyses from the same rocks, these data allow us to elucidate the timing of various stages in the evolution of the ophiolite. Plagiogranite bodies range in age from 493 to 480 Ma and have relatively juvenile isotopic compositions. Geochemical data suggest subduction-zone influence and we interpret this stage to represent formation of the ophiolite in an oceanic back-arc setting. At 480 Ma, a large granitoid body with an unradiogenic isotopic composition and strong subduction-zone geochemical signature intruded the ophiolite. We interpret this stage to reflect convergence in the back-arc basin and formation or migration of an oceanic arc. The unradiogenic isotopic composition probably reflects subduction of back-arc basin crust with sediments derived from the nearby continent or microcontinent. At 480-470 Ma, a greenstone-dominated conglomerate and an overlying volcaniclastic sequence was deposited unconformably on the eroded, metamorphosed ophiolite, indicating obduction between 480 and 470 Ma, either onto a microcontinent close to Baltica at intermediate to high latitudes or directly onto Laurentia. By Late Arenig (Dapingian) time, the sediments were hosting Laurentian faunas, and detrital zircon data from these rocks reveal a significant Archaean component, unlike tectonostratigraphically lower allochthonous, parautochthonous and autochthonous sedimentary rocks that generally lack Archaean input. This suggests that the volcaniclastic succession and its ophiolite base had reached the equatorial latitudes of Laurentia by this stage. At 469-467 Ma, the ophiolite and its sedimentary cover was intruded by trondhjemite dykes and calc-alkaline volcanic rocks with intermediate isotopic compositions. We interpret this magmatism to reflect the establishment of a magmatic arc close to the continental margin of Laurentia at this time. Data from other parts of the Central Norwegian Caledonides suggest this arc might have been active until Late Silurian collision between Laurentia and Baltica.

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

2012-04-01

197

Spatial distribution of melt conduits in the mantle beneath oceanic spreading ridges: Observations from the Ingalls and Oman ophiolites  

NASA Astrophysics Data System (ADS)

Ophiolites are on-land exposures of igneous crust and residual upper mantle formed beneath submarine spreading ridges. Upper mantle outcrops in ophiolites provide insight into focusing of melt transport from a ~100 km wide region of partial melting into an ~5 km wide zone of igneous crustal accretion beneath the ridges. Dunite veins, composed of the minerals olivine and spinel, mark conduits for melt transport through at least the uppermost 30 km of the mantle. New data in this paper, on dunite veins in the Ingalls ophiolite, central Washington Cascades, show a power law relationship between frequency and width, in which frequency/m ~0.02 width-3 over a size interval from ~0.1 to 2 m. There may be several ways to generate this relationship, but we favor the hypothesis that the dunites represent a coalescing melt transport network. This conclusion is broadly consistent with the related hypothesis that mantle melt extraction occurs in a fractal, branching network, and with recent results on formation of a coalescing network of dissolution channels via flow of a solvent through a partially soluble, compacting porous medium.

Kelemen, Peter B.; Braun, Michael; Hirth, Greg

2000-07-01

198

Origin and serpentinization of ultramafic rocks of Manipur Ophiolite Complex in the Indo-Myanmar subduction zone, Northeast India  

NASA Astrophysics Data System (ADS)

The Manipur Ophiolite Complex (MOC) is part of the Manipur-Nagaland ophiolite belt (MNOB). The belt is exposed in the eastern margin of the Indo-Myanmar Ranges (IMRs), which formed by the collision between the India and Myanmar continental plates. Several contrasting views were put forward concerning the origin of the MNOB. The complex represents a dismembered ophiolite sequence with serpentinite as the largest litho-unit formed. Petrography and Raman spectroscopy of the serpentinite suggest that they are serpentinized ultramafic cumulate and peridotite. The serpentinization may have occurred at a condition of low pressure and low temperature metamorphism. Geochemical signatures of the rocks and spinel grains revealed that the protolith be an abyssal peridotite, derived from a less depleted fertile mantle melt at a MORB setting after low degree (10-15%) partial melting. The study concluded that the serpentinite may have been created at a slow-spreading ridge, rather than a supra-subduction-zone setting. These rocks were later obducted and incorporated into the IMR of Indo-Myanmar suture zone.

Ningthoujam, P. S.; Dubey, C. S.; Guillot, S.; Fagion, A.-S.; Shukla, D. P.

2012-05-01

199

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

NASA Astrophysics Data System (ADS)

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

Dilek, Y.

2001-12-01

200

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

201

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

2012-06-01

202

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

203

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

204

Papuan Ultramafic Belt (PUB) Ophiolite: Field Mapping, Petrology, Mineral chemistry, Geochemistry, Geochronology, And Experimental Studies Of The Metamorphic Sole  

NASA Astrophysics Data System (ADS)

The Papuan Ultramafic Belt (PUB) ophiolite in Papua New Guinea (PNG) is a large and well known section of former oceanic crust and upper mantle exposed in the western Pacific region. The PUB ophiolite was emplaced onto the southeast PNG continental crust possibly in the early Cenozoic . Detailed west-east transects during field mapping in the Musa-Kumusi divide area has shown that the Emo Metamorphics grade into amphibolites which grade up into the granulites which grade up into or are in contact with the ultramafic base of the PUB ophiolite. The ultramafics at the base of the PUB consist of harzburgites, and banded peridotites consisting of lherzolite, pyronenite and harzburgite layers. The harzburgites in the ultramafics have Fo92 olivine but there are small and correlated differences in Cr/Al ratio of both spinel and orthopyroxene and in CaO content. The pyroxenes in the lherzolite are very Ca-rich diopside and co-existing orthopyroxene with low CaO content, but higher than that of orthopyroxene in harzburgite. Temperature of equilibration by two pyroxene thermometry is 814-865oC, at 3 kbar. Hornblende is the dominant mineral phase in the granulites and amphibolites coexisting with olivine, orthopyroxene, clinopyroxene, plagioclase, ilmenite, magnetite. Lower SiO2, CaO, Al2O3 and higher MgO, TiO2, (Na2O+K2O), P2O5 contents of the granulites and the high MgO content and normative olivine (10% ) suggest that the sole granulites are essentially picritic in composition and are similar or transitional to the basic rocks of the Emo Metamorphics, and differ from the gabbroic rocks of the PUB ophiolite in lower TiO2, lower Na2O, higher Al2O3, lower FeO and lower P2O5 of the PUB, at similar MgO contents. A conventional A K-Ar, 40Ar-39Ar total fusion and incremental step-heating 40Ar-39Ar geochronological study on the metamorphic sole using amphiboles from emplacement-related granulites and amphibolites have been concluded. Sandwich melting experiments have been conducted to understand the mechanism for generation of boninite melts within the mantle wedge above subduction zone. Eruption of the Cape Vogel boninites and the emplacement of the PUB ophiolite and the formation of the metamorphic sole occurred in the Paleocene between 60 Ma to 58 Ma.

Lus, W. Y.; Green, D.; McDougall, I.; Eggins, S.; Davies, H.

2001-12-01

205

Relationship between the early Paleozoic SSZ ophiolite complex and North Qaidam UHP metamorphic belt in NW China  

NASA Astrophysics Data System (ADS)

The presence of an supra-subduction zone ophiolite complex in close proximity to ultrahigh-pressure rocks in the North Qaidam of northern Tibet suggests a complex early Paleozoic collisional geometry. This newly described ophiolite complex is found in the hanging wall of a Paleozoic detachment considered to be the structure that emplaced the North Qaidam UHP rocks into the lower crust. In recent years, significant progress has been made in deciphering the Paleozoic tectonic history of northern Tibet. Accretion of the Qaidam and Qilian terranes to the Tarim and Sino-Korean cratons is known to have occurred during Silurian-Devonian time, as terrestrial Devonian and younger strata unconformably overlie all terranes and the adjacent craton. This event resulted in the emplacement of the early Paleozoic North Qilian Franciscan-type oceanic-subduction complex, ~400 km to the northeast. The presence of ophiolitic material emplaced in the North Qaidam, in contact with UHP rocks in the lower crust suggests an additional basin closure. The North Qaidam ophiolite complex was deformed and metamorphosed to epidote-amphibolite facies by a late Ordovician ductile folding event, after juxtaposition with the UHP rocks. Despite the strong fabric development, four mappable units can be readily identified: (1) a serpentinized ultramafic layer containing broad quartz-phlogopite veins, (2) vari-textured metagabbro with rare plagiogranite, (3) a sheeted dike complex which is now mafic schist, and (4) a thin and discontinuous sequence of vesicular metabasalt interlayed with metasediments and marble. Zircons were separated from the plagiogranite and analysed by SIMS to have a U-Pb age of 545.5 + 6.0 Ma (MSWD 1.1), which is interpreted to be the crystallization age. The North Qaidam ophiolite complex was determined to have supra-subduction zone affinity using major and trace element whole rock geochemistry and is envisioned to have formed in a back-arc basin setting prior to the closure of the North Qilian Ocean. This interpretation is supported by thermochronological data which places the UHP rocks in the lower crust by 477 ± 8 Ma (Ar/Ar hornblende). The exhumation timing predates final closure in the North Qilian and requires a second subduction zone in the early Paleozoic, north Tibet.

Menold, C. A.; Walsh, E. O.; Manning, C. E.; Yin, A.

2009-12-01

206

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

NASA Astrophysics Data System (ADS)

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 along a 270-km segment of the Samail ophiolite subparallel to the regional trend of the sheeted dike complex (the former spreading ridge axis direction) define a narrow time interval of 93.5-97.9 m.y., with a pronounced clustering about 95 m.y. The zircon ages of the plagiogranites agree remarkably well with the early Cenomanian to early Turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them (Tippit et al., 1981). The agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite U-Pb ages closely date the time span of ocean crust formation. No step changes in age patterns are observed along the ridge axis (sheeted dike) direction, suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse. One possible exception occurs at the southeastern end of the sampled interval (Ibra area), where a 3 m.y. discontinuity might be caused by an unmapped fault. Assuming that the regional trend of the sheeted dikes (N10°-25°W) marks the direction of the former spreading ridge axis, the present array of sample localities spans a distance of 130 to 195 km normal to that axis (i.e., in the spreading direction). The data as a whole do not define a clear-cut age trend normal to the spreading axis, but by eliminating samples that may be aberrant due to faulting, the data array suggests a pattern of increasing ages from east to west. This would indicate that the Samail ophiolite is derived from the western (southwestern) side of the Tethyan spreading system—a conclusion reached independently from geological evidence. Also, the gentle age gradient in the spreading direction (i.e., between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m.y.) suggests a very fast spreading rate. This conclusion also is supported by independent geologic evidence.

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

1981-04-01

207

Dormation of the Xigaze ophiolite (Tibet): implications for the geodynamic evolution of the Neo-Tethys Ocean  

NASA Astrophysics Data System (ADS)

Recent tomography studies and paleo-geographic reconstructions argue for the presence of an active intra-oceanic subduction zone in the Neo-Tethys ocean. The inferred subduction zone extended from in front of the Arabian margin of Africa to southern Tibet via Ladakh and Kohistan. This intra-oceanic subduction zone implies the presence of an island arc in which the ophiolite massifs present along the Tethyan suture zoned may have evolved. Here we present the results of an extensive major, trace element and isotopic study of mineral separates from the ultramafic sequence of the Xigaze Ophiolite. This geochemical data set is used to identify and quantify the mantle processes that formed the Xigaze massif. These results constrain the geodynamic environment in which the massif has evolved and hence contribute to the paleo-reconstruction of the Neo-Tethys Ocean. The Xigaze ophiolite is situated along the Yarlung-Zangbo suture zone, 250 km west of Lhasa (Tibet). The massif forms an east-west 250 km long sub-linear belt with a N-S width ranging from 1-30 km. The harzburgitic mantle sequence is 5 km and was sampled along three sub-massifs: Luqu, Dazhuqu and Dazhuka sections. Mineral data define systematic east-west trends as do clinopyroxenes REE abundances. The Luqu clinopyroxenes are the most depleted in MREE and HREE with (Dy)N and (Yb)N as low as 0.5 and 2.1 respectively. They also record the lowest degree of LREE abundances but have variable LREE/MREE enrichment. The Dazhuku clinopyroxenes have spoon-shaped REE patterns characterised by sub-parallel HREE and constant MREE depletion but more variable LREE. In contrast, the Dazhuka clinopyroxenes are characterised by LREE depletion. The clinopyroxenes REE patterns are best modelled by 2-8% fractional melting in the garnet stability field followed by variable degree of hydrous melting also involving trace element refertilisation. The east-west increase in the degree of LREE enrichment, high degree of partial melting in hydrous conditions all attest to changes in melting condition along the ophiolite. These variation are interpreted as a record of changing tectonic environment. Evolution from MOR to back-arc setting with decreasing influence of the subduction zone is proposed. This study provides the first geochemical evidence that an intra-oceanic subduction was present in the Neo-Tethys Ocean at the time of formation of the Xigaze Ophiolite (110Ma) and confirm recent tomography and paleo-geographic studies.

Griselin, M.; Davies, G. R.; Pearson, D. G.

2003-04-01

208

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

209

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

NASA Astrophysics Data System (ADS)

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 peridotites are residual in rare-earth character, but cannot be clearly related to the overlying mafic rocks. Chromian spinel is probably insignificant in its effect on REE distribution during partial melting and crystal fractionation, as indicated by the low REE concentrations in chromitite from the Samail. Layered gabbro REE patterns are dominated by cumulus clinopyroxene and plagioclase. Large positive Eu anomalies demonstrate plagioclase accumulation. Modal mixing (mass balance) calculations reveal that most of these adcumulus gabbros have REE patterns that are the products of the REE concentrations of their constituent cumulus phases in the observed modal proportions; hence no appreciable REE-rich mesostasis is present. Such calculations also allow the prediction of mineral REE concentrations not actually determined by mineral separate analyses. Several high-level (noncumulus?) gabbros yield patterns with positive Eu anomalies suggesting relative plagioclase accumulation, probably due to liquid fractionation (filter pressing). Dike complex REE patterns show light rare-earth element depletions and are similar to, but not necessarily diagnostic of, midocean ridge basalt. They cluster at similar abundance levels; however, the absolute variation in abundance is large, indicative of modification by crystal fractionation. Calculations using partition coefficient data indicate that many dikes represent liquids that could have existed in equilibrium with cumulus minerals of the plutonic suite. Mineral separate REE data from layered cumulus gabbro of the Khafifah stratigraphic section reveal cryptic variation trends that are correlated with major element variation, in support of a long-lived (periodically replenished) magma chamber model (Pallister and Hopson, 1978, 1979, 1981). (Cryptic variation as used herein is defined as the change in mineral composition with respect to stratigraphic position.) The REE cryptic variation shows both direct and inverse correlation to major element variation, indicating that magmatic replenishment was complicated by changes in parent melt REE abundance. A crystal fractionation origin for the small plagiogranite bodies of the Ibra area is favored by REE modeling, although larger bodies (Dasir) may also be related to magmatic inclusion of roofrock with diabase level REE distribution (Gregory and Taylor, 1979).

Pallister, John S.; Knight, Roy J.

1981-04-01

210

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

NASA Astrophysics Data System (ADS)

In the north Oman mountains, a continuous ophiolite succession is exposed, from tectonized harzburgites and dunites at the base, through layered gabbros and peridotites, high-level gabbros and plagiogranite, to a dike swarm and pillowed volcanics 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 and most of the dike swarm to amphibolite facies in the lowermost dike swarm and in the high-level intrusives. These pervasively altered rocks are underlain by layered gabbros and peridotites, where hydration is restricted to fractures. Compressional and shear wave velocities have been measured to confining pressures of 6 kbar for 139 water-saturated cores from this sequence. The samples were field-oriented and collected from known stratigraphic levels, which allows the construction of velocity-depth profiles and an examination of anisotropy within the ophiolite. Compressional wave velocities measured for the basalt section are quite variable, ranging from 4.5 to 6.0 km/s at in situ pressures. Within the sheeted dike section, Compressional and shear wave velocities vary from 5.4 to 6.3 km/s and from 2.9 to 3.6 km/s, respectively. From the basalt-sediment contact through the dike section, velocities increase rapidly with depth. This gradient is related to increasing metamorphic grade and a decrease in grain boundary porosity. The top of seismic layer 3 is located near the lower boundary of the sheeted dike swarm and marks the transition from greenschist facies to amphibolite facies metamorphics. The dikes decrease in abundance downward into the high-level intrusives: coarse-grained gabbroic and granitoid rocks, which often contain abundant hornblende, with isotropic textures and lacking compositional layering. Mean compressional and shear wave velocities increase uniformly from 6.7 and 3.6 km/s, respectively, in the highlevel intrusives, to 7.5 and 3.9 km/s, respectively, at the base of the layered sequence. Measured velocities of the ultramafic tectonites are highly variable because of the anisotropy and serpentinization. Petrofabrics of relic olivine in the Wadi Ragmi region show strong preferred fabrics for the tectonites, and calculated compressional wave velocities for the serpentine-free rocks range from 7.8 to 8.5 km/s, with the fast direction parallel to the direction of spreading inferred from dike orientations, in excellent agreement with observed upper oceanic mantle seismic anisotropy. The crust-mantle boundary, which is defined by the abrupt increase in seismic velocities, coincides with the petrological contact between layered gabbros and peridotites and tectonized harzburgite and dunites.

Christensen, Nikolas I.; Smewing, John D.

1981-04-01

211

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

212

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

NASA Astrophysics Data System (ADS)

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

2002-10-01

213

Fracture and permeability analysis in magma-hydrothermal transition zones in the Samail ophiolite (Oman)  

NASA Astrophysics Data System (ADS)

Many numerical models have stressed the key importance of processes operative within magma-hydrothermal transition zones of spreading centers, but few studies have attempted a geologic characterization of the key physical parameters of this zone. With this in mind, fracture-related data have been collected in the Samail ophiolite (Oman) within gabbro/dike transition zones and major plagiogranite plutons, and at each site the nature, abundance, orientation, filling, and attitude of the fractures were determined. Two main hydrothermal vein groups can be distinguished: an amphibole vein system and a quartz-epidote-sulfide vein system. Amphibole veins are restricted to the gabbro, whereas quartz-epidote-sulfide veins mainly appear in the sheeted-dike complex and plagiogranite. The amphibole vein system is strongly anisotropic (perpendicular to the layering and subparallel to the sheeted dikes) and heterogeneous. The quartz-epidote-sulfide veins,too, are subparallel to the sheeted dikes. they are characterized by a sharp density decrease of macrosopic veins: from an average 0.5/m (sheeted dikes) to 0.2/m (transition zone) to less than 0.01/m (gabbro). In plagiogranite, where no concentric or radiating vein systems were seen, they are 0.1 to 0.3/m. The ubiquitous presence of ridge parallel hydrothermal veins implies a major contribution of regional tectonic stresses associated with spreading in addition to other mechanisms involved in fracture generation: thermal cracking, volatile-rich magma expansion, differential expansion of pore fluids.

Nehlig, P.

1994-01-01

214

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

NASA Astrophysics Data System (ADS)

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 (metres to tens of metres 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 upfiow occurred in broad zones (at least 400-600 m wide) that were elongated along strike (i.e., parallel to the spreading axis).

Haymon, Rachel M.; Koski, Randolph A.; Abrams, Michael J.

1989-06-01

215

Two-phase separation of fossil hydrothermal fluids in the Mid-Indian Ridge ophiolites  

NASA Astrophysics Data System (ADS)

Rocks of the ophiolite suite were dredged from the axis of the Mid-Indian Ridge at 1 °N. Three stages of their hydrothermal alteration were identified: (1) actinolite-epidote-albite (420-450°C), (2) epidote-chlorite-quartz, and (3) chlorite-smectite (190°C). Epidote-chlorite-quartz veins cutting basaltic breccias include an opaque, amorphous, Na-bearing silica rock that grades into cryptocrystalline quartz. Sometimes it contains rare transparent microcrystalline inclusions, which may be inferred from their high Na, Cl, and Si contents to be quartz intergrown with halite. The specific bulk composition, structure, and texture of the matter allow the interpretation that it is a lithified fossil colloid, precipitated from a highly saline hydrothermal solution as a result of phase separation in the Na?Si?Cl?H 2O system at elevated PT-conditions. The phase separation of hydrothermal fluids is proposed as a mechanism generating supersaturated salt-silicate solutions with simultaneous precipitation of ore components. The latter is confirmed by the negligible Fe and Mn concentrations in both the colloidal matrix and the microcrystalline inclusions in the presence of disseminated ore mineralization in the examined basalt breccias.

Plyusnina, L. P.; Vysotsky, S. V.

1994-05-01

216

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

NASA Astrophysics Data System (ADS)

In situ 187Os/ 188Os ratios are determined on Os-rich platinum-group minerals in podiform chromitites both in the Proterozoic ophiolite, Eastern Desert, Egypt, and in the Phanerozoic Oman ophiolite. Because they have very low Re/Os, these primary minerals reflect the initial 187Os/ 188Os ratios of their parental magmas. The platinum-group minerals (PGM) in the central Eastern Desert chromitites exhibit sub-chondritic to chondritic 187Os/ 188Os ratios, 0.1226 on average, which is lower than the primitive upper mantle evolution trend of a comparable age. Those of the southern Eastern Desert chromitites have more radiogenic Os, with supra-chondritic 187Os/ 188Os ratio of about 0.1293 on average, which could be due to crustal contamination. The three chromitite types in the northern part of the Oman ophiolite are almost indistinguishable in terms of their 187Os/ 188Os ratios; they have overlapping values ranging from sub-chondritic to supra-chondritic ratios. The PGE-rich, mantle chromitite samples have a wide range of 187Os/ 188Os ratio from 0.1230 up to 0.1376, with an average of 0.1299. The values of the PGE-poor mantle chromitites overlap in their 187Os/ 188Os ratios with PGE-rich chromites, but are less variable and have a significantly higher average ratio. The Moho transition zone (MTZ) chromitites are highly variable in the 187Os/ 188Os ratio, ranging from 0.1208 up to 0.1459. The wide range of 187Os/ 188Os ratios, from 0.1192 to 0.1459, in platinum-group minerals in Egyptian and Oman ophiolites can be attributed to the diversity of origin of their podiform chromitites. The Os-isotope data combined with spinel chemistry indicate that the way involved in podiform chromitite formation was not substantially different between the Proterozoic ophiolite of Egypt and the Phanerozoic ophiolite in northern Oman. The Os-isotope compositions of the mantle chromitites in the Proterozoic ophiolite of Egypt clearly suggest crustal contamination. The heterogeneity of 187Os/ 188Os ratios combined with the spinel chemistry and high PGE contents of the PGE-rich chromitite in the Oman ophiolite may give reliable evidence for high degree partial melting at a supra-subduction zone setting. Crustal contamination from the subducted slab, and assimilation of previously altered, lower crustal gabbro, may have contributed to the high Cr# spinel and radiogenic Os characteristics in chromitite formed in the mantle section and along the Moho transition zone, respectively.

Ahmed, Ahmed H.; Hanghøj, Karen; Kelemen, Peter B.; Hart, Stanley R.; Arai, Shoji

2006-05-01

217

Link between SSZ ophiolite formation, emplacement and arc inception, Northland, New Zealand: U Pb SHRIMP constraints; Cenozoic SW Pacific tectonic implications  

NASA Astrophysics Data System (ADS)

New U Pb age-data from zircons separated from a Northland ophiolite gabbro yield a mean 206Pb/238U age of 31.6 ± 0.2 Ma, providing support for a recently determined 28.3 ± 0.2 Ma SHRIMP age of an associated plagiogranite and ˜ 29 26 Ma 40Ar/39Ar ages (n = 9) of basalts of the ophiolite. Elsewhere, Miocene arc-related calc-alkaline andesite dikes which intrude the ophiolitic rocks contain zircons which yield mean 206Pb/238U ages of 20.1 ± 0.2 and 19.8 ± 0.2 Ma. The ophiolite gabbro and the andesites both contain rare inherited zircons ranging from 122 104 Ma. The Early Cretaceous zircons in the arc andesites are interpreted as xenocrysts from the Mt. Camel basement terrane through which magmas of the Northland Miocene arc lavas erupted. The inherited zircons in the ophiolite gabbros suggest that a small fraction of this basement was introduced into the suboceanic mantle by subduction and mixed with mantle melts during ophiolite formation. We postulate that the tholeiitic suite of the ophiolite represents the crustal segment of SSZ lithosphere (SSZL) generated in the southern South Fiji Basin (SFB) at a northeast-dipping subduction zone that was initiated at about 35 Ma. The subduction zone nucleated along a pre-existing transform boundary separating circa 45 20 Ma oceanic lithosphere to the north and west of the Northland Peninsula from nascent back arc basin lithosphere of the SFB. Construction of the SSZL propagated southward along the transform boundary as the SFB continued to unzip to the southeast. After subduction of a large portion of oceanic lithosphere by about 26 Ma and collision of the SSZL with New Zealand, compression between the Australian Plate and the Pacific Plate was taken up along a new southwest-dipping subduction zone behind the SSZL. Renewed volcanism began in the oceanic forearc at 25 Ma producing boninitic-like, SSZ and within-plate alkalic and calc-alkaline rocks. Rocks of these types temporally overlap ophiolite emplacement and subsequent Miocene continental arc construction.

Whattam, Scott A.; Malpas, John; Smith, Ian E. M.; Ali, Jason R.

2006-10-01

218

New isotopic constraints on age and magma genesis of an embryonic oceanic crust: The Chenaillet Ophiolite in the Western Alps  

NASA Astrophysics Data System (ADS)

The Chenaillet Ophiolite is one of the best-preserved remnants of the Piemont-Liguria oceanic basin, a branch of the Central Atlantic that opened during the separation of Adria/Africa from Europe. Despite numerous studies of structure, petrology, geochemistry and isotope geochronology, the timing and genesis of various magmatic rocks within the Chenaillet Ophiolite are still controversial. We provide in this study integrated in situ analyses of zircon U–Pb age and O–Hf isotopes for the troctolite and albitite within the Chenaillet Ophiolite. Our new results indicate that the troctolite and albitite crystallized synchronously at ~ 165 Ma. Zircons from the troctolite have homogeneous Hf and O isotopic compositions, with ?Hf(T) = + 13.5 ± 1.0 (2SD) and ?18O = 5.4 ± 0.4‰ (2SD), indicating crystallization from magmas that were derived from a depleted, MORB-like mantle. The albitite zircons give consistent ?Hf(T) values (+ 13.0 to + 13.5) within errors with those of troctolite zircons, but variable ?18O values. The altered zircon domains have relatively low ?18O values of 4.7 ± 0.6‰ (2SD) due to subsolidus hydrothermal alteration, whilst the least-altered zircon domains give ?18Ozir values of 5.1 ± 0.4‰ (2SD), indistinguishable within errors with the troctolite zircons and the igneous zircons from the Mid-Atlantic and Southwest Indian Ridges gabbros, norites, and plagiogranites of modern oceanic crust. In situ zircon O–Hf isotopic data suggest that the troctolites and albitites are most likely cogenetic, with the albitites being formed by extreme fractional crystallization from the basaltic magma.

Li, Xian-Hua; Faure, Michel; Lin, Wei; Manatschal, Gianreto

2013-02-01

219

Hf Isotope Constraints on the Longevity of the Indian Ocean Mantle Signature: New Results from Tethyan Ophiolites  

NASA Astrophysics Data System (ADS)

Studies over the last decade of drilled seafloor basalts have shown that the distinct Sr-Nd-Pb isotope compositions exhibited by Indian Ocean MORB extend well into the geologic past. Investigations of Tethyan seafloor exposed in ophiolites extending from eastern Europe to China indicate that the 'Indian Ocean' isotopic signature existed well before the opening of the Indian Ocean basin. The Lu-Hf system is particularly useful for testing the longevity and magnitude of the Indian Ocean isotopic signature because of its resistance to the effects of alteration and because the Indian Ocean signature is strongly expressed as having radiogenic Hf for a given Nd isotopic composition relative to oceanic basalts worldwide. Hf isotope data will be presented for MORB-like basalts from 20 DSDP sites throughout the Indian Ocean and for Neotethyan ophiolites (up to 200 Ma) from Iran and the Indus-Yarlung Suture Zone in Tibet. These data show that Indian Ocean type Nd-Hf isotopic compositions (outside the range of Pacific & Atlantic MORB) do indeed extend back to at least 200 Ma, but the maximum decoupling observed between Hf and Nd isotopes in Tethyan seafloor basalts (e.g., from ophiolites and Wharton Basin) is less than that measured in Indian Ocean seafloor basalts. Thus, although similar processes (e.g., assimilation of subduction modified mantle wedge and/or lower continental crust) may have been responsible for the distinct Sr-Nd-Pb-Hf isotopic composition of both Indian Ocean and Tethyan seafloor basalts, the closing of the Tethyan ocean and opening of the Indian Ocean basin appears to have significantly increased the heterogeneity of the upper mantle now located beneath the Indian Ocean.

Janney, P. E.

2008-12-01

220

Distribution of platinum-group elements and Os isotopes in chromite ores from Mayarí-Baracoa Ophiolitic Belt (eastern Cuba)  

Microsoft Academic Search

The Mayarí-Baracoa ophiolitic belt in eastern Cuba hosts abundant chromite deposits of historical economic importance. Among\\u000a these deposits, the chemistry of chromite ore is very variable, ranging from high Al (Cr#=0.43–0.55) to high Cr (Cr#=0.60–0.83)\\u000a compositions. Platinum-group element (PGE) contents are also variable (from 33 ppb to 1.88 ppm) and correlate positively with\\u000a the Cr# of the ore. Bulk PGE abundances correlate

F. Gervilla; J. A. Proenza; R. Frei; J. M. González-Jiménez; C. J. Garrido; J. C. Melgarejo; A. Meibom; R. Díaz-Martínez; W. Lavaut

2005-01-01

221

Fragment of the Coast Range ophiolite and the Great Valley sequence in the San Juan Islands, Washington  

SciTech Connect

The Decatur terrane is a small, allochthonous fragment of Upper Jurassic oceanic crust with an overlying blanket of Upper Jurassic-Lower Cretaceous sedimentary rocks in the San Juan Islands. The stratigraphy and petrotectonic elements are dissimilar to those of coeval sequences in the Pacific Northwest (Oregon, Washington, and southern British Columbia), but are virtually identical to those of the Coast Range ophiolite and overlying Great Valley sequence south of San Francisco, California. This stratigraphic correlation, together with regional considerations, suggests that the Decatur terrane was detached and transported during Cretaceous time. The resultant displacement is in excess of 1000 km.

Garver, J.I. (Univ. of Washington, Seattle (USA))

1988-10-01

222

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

NASA Astrophysics Data System (ADS)

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

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

1987-07-01

223

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

NASA Astrophysics Data System (ADS)

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

Bernoulli, Daniel; Jenkyns, Hugh C.

2009-05-01

224

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

225

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

NASA Astrophysics Data System (ADS)

Dikes of the eastern Troodos ophiolite of Cyprus intruded at slow ocean-spreading axes with dips ranging up to 15° from vertical and with bimodal strikes (now NE SW and N S due to post-88 Ma sinistral microplate rotation). Varied dike orientations may represent local stress fields during dike-crack propagation but do not influence the spatial-distributions or orientation-distributions of dikes' magnetic fabrics, nor of their palaeomagnetic signals. Anisotropy of magnetic susceptibility (AMS) integrates mineral orientation-distributions from each of 1289 specimens sampled from dikes at 356 sites over ˜400 km2 in the eastern Troodos ophiolite of Cyprus. In 90% of dikes, AMS fabrics define a foliation (kMAX kINT) parallel to dike walls and a lineation (kMAX) that varies regionally and systematically. Magma-flow alignment of accessory magnetite controls the AMS with a subordinate contribution from the mafic silicate matrix that is reduced in anisotropy by sea-floor metamorphism. Titanomagnetite has less influence on anisotropy. Occasionally, intermediate and minimum susceptibility axes are switched so as to be incompatible with the kinematically reasonable flow plane but maximum susceptibility (kMAX) still defines the magmatic flow axis. Such blended subfabrics of kinematically compatible mafic-silicate and misaligned multidomain magnetite subfabrics; are rare. Areas of steep magma flow (kMAX plunge ? 70°) and of shallow magma-flow alternate in a systematic and gradual spatial pattern. Foci of steep flow were spaced ˜4 km parallel to the spreading axes and ˜6 km perpendicular to the spreading axes. Ridge-parallel separation of steep flow suggest the spacing of magma-feeders to the dikes whereas ridge-perpendicular spacing of 6 km at a spreading rate of 50 mm/a implies the magma sources may have been active for ˜240 Ka. The magma feeders feeding dikes may have been ? 2 km in diameter. Stable paleomagnetic vectors, in some cases verified by reversal tests, are retained by magnetite and titanomagnetite. In all specimens, the stable components were isolated by three cycles of low-temperature demagnetization (LTD) followed by ? 10 steps of incremental thermal demagnetization (TD). 47% of primary A-components [338.2 /+ 57.2 n = 207, ?95 = 3.9; mean TUB = 397 ± 8 °C] are overprinted by a B-component [341.4 /+ 63.5, n = 96, ?95 = 8.7; mean TUB = 182 ± 11 °C]. A- and B-components are ubiquitous and shared equally by the N S and NE SW striking dikes. A-component unblocking temperatures (TUB) are zoned subparallel to the fossil spreading axis. Their spatial pattern is consistent with chemical remagnetization at some certain off-axis distance determined by sea-floor spreading. A-components indicate less microplate rotation and more northerly palaeolatitudes that are consistent with metamorphic remagnetization after some spreading from the ridge-axis. Thus, their magnetizations are younger than those of the overlying volcanic sequence for which ChRMs are commonly reported as ˜274 /+ 33 (88 Ma).

Borradaile, G. J.; Gauthier, D.

2006-05-01

226

Modeling the Evolution of a Transform Fault in the Mantle Section of the New Caledonia Ophiolite  

NASA Astrophysics Data System (ADS)

The Bogota Peninsula shear zone has been interpreted as a paleotransform fault in the mantle section of the New Caledonia ophiolite. The shear zone was originally identified based on stronger fabric development on the Bogota Peninsula and the rotation of foliation from shallow NW-striking outside the shear zone to subvertical NNE-striking inside the shear zone. This rotation is generally symmetric and occurs across a 50 km-wide region centered on a 3 km-wide high strain core. To better understand the evolution of this dextral transform fault, we develop a kinematic model constrained by several types of field and laboratory data. Field fabrics were used to empirically define three distinct domains within the shear zone: a farfield, nearfield, and central high-strain core. Our model assumes that the shear zone localized during its development so that farfield features preserve the first increment of deformation and the center records the last increment. We specify the orientation of the shear zone boundary based on the lattice preferred orientation of olivine within the center. For each increment of deformation, the orientation of the finite strain ellipsoid is constrained by foliation and lineation while the magnitude is constrained the shape preferred orientation of macroscopic orthopyroxene. Our modeling also incorporates the changing orientation of orthopyroxene dikes that, like field fabrics, rotate systematically across the shear zone. Because many dikes also show increasing boudinage towards the center of the shear zone, we estimate stretches recorded by these dikes using the length, width, and separation between boudins along a variety of outcrop surfaces. This data-rich approach to modeling is challenging, but may ultimately provide more insight into the development of a transform fault at a deeper lithospheric level than is commonly observed.

Titus, S. J.; Davis, J. R.

2010-12-01

227

Mineralogical and geochemical investigation of layered chromitites from the Bracco-Gabbro complex, Ligurian ophiolite, Italy  

NASA Astrophysics Data System (ADS)

The Bracco-Gabbro Complex (Internal Liguride ophiolite), that intruded subcontinental mantle peridotite, contains layers of chromitite that are associated with ultramafic differentiates. The chromitites and disseminated chromites in the ultramafics have Al contents similar to the Al-rich podiform chromitites [0.40 < Cr# = Cr/(Cr + Al) < 0.55]. TiO2 contents of the chromitites are unusually high and range up to 0.82 wt%. The calculated Al2O3 and TiO2 content of the parental melt suggest that the melt was a MORB type. Geothermobarometrical calculations on few preserved silicate inclusions revealed formation temperatures between 970 and 820 °C under a relatively high oxygen fugacity (?log fO2 at +2.0-2.4). Chromitites were altered during the post-magmatic tectono-metamorphic uplift and the final exposure at the seafloor, as evidenced by the formation of ferrian chromite. The PGE contents of the chromitites and associated ultramafics are unusually low (PGEmax 83 ppb). The chondrite-normalized PGE spidergrams show positive PGE patterns and to some extent similarities with the typical trend of stratiform chromitites. No specific PGM have been found but low concentrations of PPGE (Rh, Pt, and Pd) have been detected in the sulphides that occur interstitially to or enclosed in chromite. Recently, it has been shown that the Internal Liguride gabbroic intrusions have formed by relatively low degrees of partial melting of the asthenospheric mantle. We conclude that the low degree of partial melting might be the main factor to control the unusual low PGE contents and the rather unique PGE distribution in the Bracco chromitites.

Baumgartner, R. J.; Zaccarini, F.; Garuti, G.; Thalhammer, O. A. R.

2013-03-01

228

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

NASA Astrophysics Data System (ADS)

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

Streit, Elisabeth; Kelemen, Peter; Eiler, John

2012-11-01

229

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

230

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

NASA Astrophysics Data System (ADS)

The platinum group minerals (PGM) in chromite ores of the Kempirsai ophiolite massif, located south of the Ural Mountains, are extremely varied in composition and represented predominantly by alloys, sulfides, arsenides, and sulfosalts of the iridium-group PGE (IPGE). The earlier Ir-Os-Ru alloys prevail over the later Cu-Os-Ru, Cu-Ir, Ni-Ir, Ni-Os-Ir-Ru, and Ni-Ru-Os-Fe alloys rich in base metals (BM). The earlier Ru-Os disulfides crystallize coevally with Ir-Os-Ru alloys, whereas the later sulfides are represented by compounds with a variable stoichiometry and a wide miscibility of Ni, Cu, Ir, Rh, Os, and Fe. Phase relations of PGE alloys with PGE-BM alloys, sulfides and sulfoarsenides confirm that deposition of these minerals was defined by a general evolution of PGE fractionation in the mineral-forming system but not by a super-imposed process. The leading mechanism of PGM crystallization is thought to be their dendritic growth during gas-transport reactions from low-density gaseous fluid enriched in PGE. The representative technological sampling of 0.5 million tons of an ore showed that the average PGE content in chromite ore is 0.71 ppm which leads to an evaluation of the PGE resources to be no less than 250 tons. Hence, the Kempirsai deposit is not only a giant chromium deposit, but also a giant deposit of IPGE: Ir, Ru, and Os. The size parameters of PGM and their aggregates suggests that the PGE may be recoverable in separate concentrates.

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

2008-01-01

231

Emplacement mechanism of off-axis large submarine lava field from the Oman Ophiolite  

NASA Astrophysics Data System (ADS)

Large submarine lava flows with thicknesses >100 m and volumes exceeding a few km3are not uncommon volcanic constructs of mid-ocean ridges and around Hawaii Islands, yet details of the physical processes of eruption of these large lava flows are poorly understood. The V3 unit of the Oman Ophiolite extruded onto pelagic sediment far off the paleospreading axis as thick lava flows with an areal extent of >11 km by 1.5 km and the maximum thickness >270 m, yielding an estimated volume >1.2 km3. The feeder dike for the V3 flow field has an unusual thickness up to 60 m, striking at a high angle to the paleospreading axis. The skewed variation in thickness suggests an intrusion at the apparent level of neutral buoyancy in off-axis crust. The thick sheet flows consist of massive core and columnar jointed crust. Finer-grained, finely jointed layers and lenses embedded in coarser-grained, roughly jointed lava show a complex cooling and growth history of the lava crust. In the periphery of the flow field, sheet flows lack developed core textures and change laterally into compound flows of pillow and subaqueous pahoehoe lobes, which are occasionally intermingled with pelagic shale. On the other hand, core lacks fine columnar joints and shows a typical doleritic texture. We conclude that the V3 flow field formed by extrusion of lava at low to moderate rates onto a subhorizontal seafloor covered with thick pelagic sediment, burying topographic relief >100 m of abyssal hills and fault grabens.

Umino, Susumu

2012-11-01

232

The Eastern Himalayan syntaxis: major tectonic domains, ophiolitic mélanges and geologic evolution  

NASA Astrophysics Data System (ADS)

Geologic mapping in the eastern Himalayan syntaxis confirmed the three regional tectonic elements outlined by previous geologic workers. Our studies, however, show that the Indus-Yarlung Tsangpo suture (IYS) is a continuous mélange zone that forms an inverted U in map view around the Namche Barwa antiform. The Namche Barwa and Nyainqentanglha crystalline complexes lie below and above the IYS suture, respectively, and both were parts of the northern Indian plate basement rocks with petrologically and geochronologically correlative protoliths. Both units were deformed, metamorphosed and intruded at the end of the Proterozoic. The Zhibai Formation, the lower part of the Namche Barwa Group, extends along the northwest slope of the Himalaya, and is mainly composed of highly deformed aluminous felsic gneiss containing sporadic boudins of high-pressure granulite in the Namche Barwa antiform. The upper part of the Namche Barwa Group includes a calcareous rock assemblage characteristized by marble and diopside-bearing calcsilicate rocks interlayed with felsic gneiss. Petrochemical studies show that the IYS contains lenses of oceanic crustal rocks originated from fore-arc trench, island arc, and back-arc basin environments, which implies they were derived from a SSZ-type (supra-subduction zone) ophiolite. Our field mapping identifies the Jiali-Parlung Tsangpo remnant suture (JPS) that lies north of the Namche Barwa antiform as a possible branch of the Neo-Tethyan oceanic realm. Subduction of the Mesozoic Neo-Tethyan oceanic plate resulted in both Mesozoic and Cenozoic granite intrusions in the northwest-trending Gangdise magmatic belt along the southern edge of Asia. Uplift and exhumation have been the most recent dominant tectonic processes in the late Cenozoic for the High Himalayan crystalline rocks (Namche Barwa Group) in the core of the Namche Barwa antiform.

Quanru, Geng; Guitang, Pan; Zheng, Lailin; Chen, Zhiliang; Fisher, Richard D.; Sun, Zhiming; Ou, Chunsheng; Dong, Han; Wang, Xiaowei; Li, Sheng; Lou, Xiongying; Fu, Heng

2006-08-01

233

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

NASA Astrophysics Data System (ADS)

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

Koroneos, Antonios; Poli, Giampiero; Christofides, Georgios

2010-05-01

234

Low silica activity for hydrogen generation during serpentinization: An example of natural serpentinites in the Mineoka ophiolite complex, central Japan  

NASA Astrophysics Data System (ADS)

The textural evolution in the serpentinite of the Mineoka ophiolite complex has been investigated to constrain the natural environment for hydrogen production in the serpentinite-hosted hydrothermal vent systems. Textural relations of the serpentinites from the Mineoka ophiolite indicate at least two stages in the process of serpentinization, with the replacement of olivine by a mesh texture of serpentine and brucite, followed by the development of magnetite-bearing or -free serpentine veins. The generation of hydrogen during serpentinization, which accompanies the formation of magnetite, involves a silica-depletion reaction, as evidenced by the low abundance of serpentine in the magnetite-bearing veins and the absence of magnetite in pseudomorphs of orthopyroxene. Direct evidence for the production of hydrogen and strongly reducing conditions is provided by CH 4 and H 2-bearing inclusions in relic olivine crystals; the production of methane and hydrogen may have provided a suitable environment for microbial activity in hydrothermal vent systems along the seafloor. Our results indicate that low silica activity plays a key role in the generation of hydrogen during serpentinization, and that low silica activity environments are possible in olivine-rich rocks such as dunite, or during local disequilibrium in other silica-poor rocks in the mantle lithosphere.

Katayama, Ikuo; Kurosaki, Iori; Hirauchi, Ken-ichi

2010-09-01

235

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

NASA Astrophysics Data System (ADS)

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

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

2013-02-01

236

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

Microsoft Academic Search

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

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

2000-01-01

237

L’ophiolite de la chaîne centrale d’Irian Jaya (Indonésie) : évidences pétrologiques et géochimiques pour une origine dans un bassin arrière-arc  

Microsoft Academic Search

Since the Mesozoic, the north–south oblique convergence between the Australian and Pacific plates produced the obduction of pieces of oceanic lithospheres now incorporated into the New Guinea Island. The largest one corresponds to Central Ophiolite Belt (COB), located in the central part of Irian Jaya, that displays well-preserved peridotites, gabbros, dolerites and basalts. The ultramafic rocks, characterized by the abundance

Christophe Monnier; Jacques Girardeau; Manuel Pubellier; Haryadi Permana

2000-01-01

238

Geochemistry of Cretaceous to Eocene Ophiolitic Rocks of the Central Cordillera: Implications for Mesozoic-Early Cenozoic Evolution of the Northern Philippines  

Microsoft Academic Search

The Central Cordillera in northern Philippines is underlain mainly by ophiolitic basement of Cretaceous to Eocene age. In the central portion of the range, these complexes consist mainly of pillow basalt and basaltic feeder dikes that are unconformably overlain by epiclastic rocks, including turbidites. Field and geochemical evidence reveals that these rocks have a supra-subduction signature, and were generated in

Karlo L. Queaño; Jason R. Ali; Jonathan C. Aitchison; Graciano P. Yumul; Manuel Pubellier; Carla B. Dimalanta

2008-01-01

239

Reconstructing the Mesozoic-early Cenozoic evolution of northern Philippines: clues from palaeomagnetic studies on the ophiolitic basement of the Central Cordillera  

Microsoft Academic Search

The first reliable palaeomagnetic data from the Cretaceous to Eocene ophiolitic basement rocks in the Philippines are presented. A total of 12 drill core sites from five localities in the Central Cordillera in northern Luzon, Philippines were sampled. Eight drill core sites were from pillow basalts, and four were from diabase feeder dykes. Combining the characteristic remanent magnetization direction from

Karlo L. Queaño; Jason R. Ali; Manuel Pubellier; Graciano P. Yumul Jr.; Carla B. Dimalanta

2009-01-01

240

Igneous and Metamorphic Processes Associated With the Formation of Chilean Ophiolites and Their Implication for Ocean Floor Metamorphism, Seismic Layering, and Magnetism  

Microsoft Academic Search

A metamorphic overprint on the pseudostratigraphy of ophiolite complexes in southern Chile shows an extremely steep vertical metamorphic gradient passing downward from zeolite to amphibolite facies in 2 km, followed by a transition to fresh gabbros. Burial metamorphism does not explain either the steep metamorphic gradient or the abrupt termination of this metamorphic effect. A combination of hydrothermal and contact

Charles Stern; Maarten J. de Wit; James R. Lawrence

1976-01-01

241

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

Microsoft Academic Search

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

Zheng-Xue Anser Li; Cin-Ty Aeolus Lee

2006-01-01

242

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

NASA Astrophysics Data System (ADS)

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

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

2013-03-01

243

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

244

Chemical diversity of MORB primary melts: ultra-enriched endmember from the Macquarie Island ophiolite  

NASA Astrophysics Data System (ADS)

The ophiolitic assemblage of Miocene Macquarie Island (54o30`S, 158o56`E) is the only above sea-level exposure of the Macquarie Ridge that was created at the Australia-Pacific plate boundary within a slow-spreading, waning system. A unique feature of the Macquarie Island MORB glasses is the occurrence of very primitive, weekly fractionated compositions (Mg# 63-69 mol%, liquidus olivine Fo 87-90) that show extreme diversity in abundances and ratios of incompatible major, trace and volatile elements (e.g., K2O 0.14-1.76 wt%; H2O 0.25-1.5 wt%; Rb 3.6-60 ppm; La/Sm 2.1-7.9, Kamenetsky et al., 2000). The predominance of highly enriched varieties and absence of fractionated counterparts for the most enriched varieties suggests that early magma batches erupted with little or no mixing and fractionation. Primary melt compositions are calculated using primitive glass compositions and equilibria with the most forsteritic (90.5) olivine of the Macquarie Island picritic lavas. These melts are inferred to represent a new ultra-enriched endmember in the MORB compositional spectrum, far more enriched than any P- or E-MORB known to date. Macquarie Island primary melts are characterised by well-correlated major and trace elements and Sr, Nd and Pb isotope compositions (Kamenetsky &Maas, 2002). These relationships are inconsistent with simple binary mixing of different melt batches, but may reflect concomitant compositional evolution of the spinel lherzolite mantle source at low degrees of melting (< a few %) and mantle-derived partial melts. Radiogenic isotope ratios of MORB primary melts are thus a complex function of small, grain-size mantle compositions, the extent of mantle melting and the degree of re-equilibration of mantle constituents between each other and with low degree melt fractions. Kamenetsky et al., 2000. Enriched end-member of primitive MORB melts: petrology and geochemistry of glasses from Macquarie Island (SW Pacific): J. Petrol., 41, 411-430. Kamenetsky &Maas, 2002. Mantle-melt evolution (dynamic source) in the origin of a single MORB suite: a perspective from magnesian glasses of Macquarie Island: J. Petrol, 43, 1909-1922.

Kamenetsky, V.; Maas, R.; Eggins, S.

2003-04-01

245

A recent trend in sciences on mantle-derived materials, with special emphases on refertilization, rheology, and ophiolite problems: a report of the Fifth International Conference on Orogenic Lherzolite  

Microsoft Academic Search

A recent trend in sciences on mantle-derived materials, with special emphases on refertilization, rheology, and ophiolite problems: a report of the Fifth International Conference on Orogenic Lherzolite

Tomoaki Morishita; Kazuhito Ozawa; Masaki Obata

2010-01-01

246

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

247

Spontaneous Nucleation of Subduction Zones in the Western Pacific During Middle Eocene Time: Evidence From the IBM Forearc Ophiolite  

NASA Astrophysics Data System (ADS)

Subduction zones nucleate in two fundamentally different ways. Induced nucleation is a response to continuing plate convergence following a collision event and requires the lithosphere to fail under compression; no change in plate motion is expected. Spontaneous nucleation of a subduction zone (SNSZ) manifests failure of old lithosphere due to gravitation instability. SNSZ doesn't require plate convergence to occur but major changes in plate motion are expected. SNSZ is possible where old oceanic lithosphere is unusually dense (old continental margins) or weak (along fracture zones). The western edge of the Pacific plate spontaneously reorganized as a convergent margin during Middle Eocene time ( ~50-42 Ma) and is the best known example of SNSZ. The unusual nature of this episode is preserved in the Izu-Bonin-Mariana (IBM) forearc, where pillow basalts, dyke complexes, gabbro, and harzburgitic mantle define an in situ ophiolite. The IBM forearc ophiolite requires that SNSZ was accompanied by a strongly magmatic episode of seafloor spreading. Spreading so close to the present trench requires asthenospheric upwelling where strong mantle downwelling now occurs. Abundant boninite, formed by melting harzburgite, in IBM forearc sections further demonstrates the unique nature of the IBM subduction initiation event. IBM SNSZ spans the period from beginning of magmatic construction of the IBM `forearc ophiolite' about 50 Ma to the change in Pacific Plate motion at 43 Ma marking the start of true subduction. Events during this stage are very poorly understood but can only be explained by subsidence of part of the lithosphere to a depth such that asthenosphere flowed over it. Stern and Bloomer (1992 BGSA 104, 1621-1636) argue that this occurred along a zone of weakness associated with a N-S fracture zone but this has been criticized on the basis of paleomagnetic models requiring ~90 o CW rotation of the Philippine Sea Plate (PSP) since 43Ma. The youngest parts of the paleomagnetic reconstructions fail because geologic evidence refutes arguments for significant rotations since ~15 Ma. Regardless of paleogeographic controvesies, the essential geologic evidence for IBM SNSZ are robust. Furthermore, the prediction that SNSZ will simultaneously develop along the entire margin of the affected plate is satisfied. Development of physical models are now required in order to make further advances in understanding SNSZ.

Stern, R. J.

2001-12-01

248

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

249

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

NASA Astrophysics Data System (ADS)

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

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

2010-05-01

250

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

NASA Astrophysics Data System (ADS)

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, similar to sulfide-depositing hot springs on modern spreading centers. 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 hyalo-clastite altered the rock to quartz + Fe-chlorite and quartz + sericite + Fe-chlorite. Intense alteration resulted in replacement of the protolith by quartz + sulfide. The fluid responsible for the hydrothermal alteration was evolved seawater with low pH and Mg and high Fe. Sulfide ?534S values average 4.7‰ and indicate contribution of sulfur from both basalt and seawater sulfate. The average value of sulfide ?34S and the difference between sulfide and contemporaneous seawater sulfate ?34S values are similar to ophiolite-hosted sulfide deposits in Cyprus. Chlorite-rich hydrothermally altered basalt has ?18O values of 3.0-6.8‰ due to high-temperature (250°-350°C) reaction with a seawater-derived ore fluid. Quartz in silicified basalt has higher ?18O values of 12.3-15.5‰, probably reflecting lower-temperature recrystallization of amorphous silica formed during initial alteration of basalt. 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‰.

Zierenberg, Robert A.; Shanks, Wayne C., III; Seyfried, William E., Jr.; Koski, Randolph A.; Strickler, Michael D.

1988-05-01

251

Geochemistry of mafic dykes from the Southeast Anatolian ophiolites, Turkey: Implications for an intra-oceanic arc-basin system  

NASA Astrophysics Data System (ADS)

The Late Cretaceous-Tertiary accretionary prism in Eastern Turkey includes several ophiolitic megablocks and/or tectonic slivers (Mehmetalan, Mollatopuz and Alabayir) within a mélange complex, mainly comprising harzburgite, dunite and cumulate-textured gabbro. The diabases, which are the main focus of this study, cut across the ophiolites as parallel and variably thick dyke-swarms. Geochemistry of the diabases reveals three distinct groups, including a) supra-subduction zone (SSZ) type, which is characterized by marked Nb-anomaly and normal mid-ocean ridge basalt (N-MORB) like HFSE distribution, b) enriched MORB (E-MORB) type, showing some degree of enrichment relative to N-MORB, c) oceanic-island basalt (OIB) type with characteristic hump-backed trace element patterns, coupled with fractionated REE distribution. Among these groups, SSZ- and E-MORB-type signatures are acquired from the Mehmetalan and Mollatopuz suites, whereas OIB-type characteristics are found in the Alabayir suite. The melting models indicate involvement of both depleted and enriched sources for the genesis of the studied dykes. The close spatial relationship, similar ages (based on Ar-Ar dating) and the presence of variable subduction component displayed by Mehmetalan and Mollatopuz suites may indicate melt generation in an intra-oceanic SSZ within the southern branch of Neotethys. In spite of the solely OIB-like character of the Alabayir suite, the similar age obtained from these dykes may suggest their formation in a similar SSZ setting. Alternatively, the Alabayir suite may have represented an oceanic island or seamount formed in an intra-plate setting with or without plume influence. We suggest that decompression melting triggered by slab roll-back mechanism during the closure of the southern branch of the Neotethys during the Late Cretaceous may have been the main process that led to generation of magmas of both depleted and enriched characteristics.

Colakoglu, Ali Riza; Sayit, Kaan; Günay, Kurtulus; Göncüoglu, M. Cemal

2012-02-01

252

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

NASA Astrophysics Data System (ADS)

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

Schnur, S.; Gilbert, L. A.

2009-12-01

253

Tectonic mélanges and the exhumation of HP ophiolites: a case-study from the Ligurian Alps  

NASA Astrophysics Data System (ADS)

Mélanges form in a variety of geodynamic settings and can be related to either sedimentary, tectonic or diapiric processes, or a combination of them. We studied in detail a 100 m-scale tectonic mélange formed in the context of the alpine subduction/collision and we tested if the local-scale pattern could be applicable at larger scale in the Ligurian Western Alps. The studied mélange crops out inside metamorphic serpentinites belonging to the high-pressure (HP), meta-ophiolitic Voltri Massif (southern end of the Western Alps). It is made up of a foliated chlorite-actinolite greenschist matrix enclosing 10m-scale lenses of metabasites and metasediments. These blocks appear to be exotic because similar rocks do not outcrop in the surrounding HP-units. The matrix records three sets of superposed folds from blueschist to greenschist-facies conditions. The metabasite lenses preserve internal HP schistosities forming high angles with the greenschist matrix foliation. The lenses equilibrated at different peak metamorphic conditions (ranging from eclogite- to blueschist-facies). The matrix is widely retrogressed in greenschist facies, but it contains rare relics of Na-amphibole. Individual lenses display different segments of typical subduction PT paths which apparently converge in the blueschist facies. Moreover, geochronological data for the different HP blocks show that two undistinguishable blueschist samples display distinct peak ages of 43 and 40 Ma. One blueschist age is contemporaneous with the eclogitic equilibration of another block (43.2 ± 0.5 Ma) (Federico et al., 2007). The described structural, metamorphic and geochronological features suggest that this mélange formed at depth in a subduction channel and was active at least from blueschist- to greenschist-facies conditions, but possibly also at higher pressures. The subduction channel formed between the overriding and the subducting plates, as a consequence of progressive hydration of the mantle wedge by fluids released from the slab (e.g. Gerya et al., 2002). Here a forced flow inside serpentinites sampled different HP lenses from different depths and forced them to flow towards surface. The pattern of the studied mélange could be applicable at larger scale in the Ligurian Alps, where various HP units may be larger-scale equivalents of blocks, and may have been exhumed in a subduction channel inside serpentinites. Different units of the Voltri massif display metamorphic peaks at conditions ranging from eclogite- to blueschist-facies and available geochronological constraints reveal the heterogeneous timing of eclogite- facies metamorphic re-equilibrations (Federico et al., 2005; Rubatto & Scambelluri, 2003). However, pervasive retrograde - stage tectonics and greenschist imprint (Capponi & Crispini, 2002) mask structural relationships among units acquired at high pressure conditions. As a consequence, we discuss formation of the studied mélange and the feasibility of the subduction channel mode of exhumation at larger scale, in the Voltri massif, as already invoked for the Monviso Massif of the Western Alps (Guillot et al., 2004). Capponi, G., Crispini, L. - 2002 - Eclogae Geologicae Helvetiae, v. 95, 31-42. Federico, L., Capponi, G., Crispini, L., Scambelluri, M., & Villa, I.M. - 2005 - EPSL, 240, 668-680 Federico L., Crispini L., Scambelluri M. & Capponi G. - Geology, 35 (6), 499 - 502. Gerya, T.V., Stockhert, B. & Perchuk, A.L. - 2002 - Tectonics, 21(6), 1056. Guillot, S., Schwartz, S., Hattori, K., Auzende, A.L. & Lardeaux, J.M. - 2004 - in Beltrando, M., Lister, G., Ganne, J., and Boullier, A., eds., Evolution of the western Alps: Insights from metamorphism, structural geology, tectonics and geochronology, Journal of the Virtual Explorer, Electronic Edition, ISSN 1441-8142, v. 16, Paper 3. Rubatto D., Scambelluri M. - 2003 - Contrib. Mineral. Petrol. 146 (3), 341- 355.

Federico, Laura; Crispini, Laura; Scambelluri, Marco; Capponi, Giovanni; Malatesta, Cristina

2010-05-01

254

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

NASA Astrophysics Data System (ADS)

The Yishak Volcanic Series (YVS) is part of the Late Cambrian Kudi ophiolite in the western Kunlun Mountains, NW China, which marks the oldest suture zone in the Tibetan Plateau. The YVS is a well-preserved sequence comprising five conformable units (A to E) of basalts to andesites, with mafic dykes in Units A to D. These rocks are sequentially characterized upward by geochemical features of N-MORB (Unit A), E-MORB (Unit B), island-arc basalt (Unit C), BABB (Unit D and mafic dykes) and boninite series rocks (Unit E), reflecting complex interactions among various source components. The evolution of magma compositions from MORB-dominated to boninite series rocks clearly indicates temporally increasing subduction signatures, which, in combination with the extensional tectonic background, suggests that the Kudi ophiolite most likely formed in a spontaneous nucleation of subduction zone.

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

2005-09-01

255

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

256

The supra-subduction zone Pozanti–Karsanti ophiolite, southern Turkey: evidence for high-pressure crystal fractionation of ultramafic cumulates  

Microsoft Academic Search

The Pozanti–Karsanti ophiolite (PKO), forming one of the late Cretaceous discontinuous oceanic lithosphere remnants in the eastern Tauride belt in southern Turkey, is characterized by mantle tectonites, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes and volcanics. Well-preserved crustal cumulate rocks are mainly composed of dunite±chromite, wehrlite, olivine clinopyroxenite, clinopyroxenite, olivine websterite and low-Ti gabbro. The crystallization order in the

Osman Parlak; Volker Höck; Michel Delaloye

2002-01-01

257

Variation of geochemical risk associated with the use of ophiolitic washing mud as refilling material in a basalt quarry of the Northern Apennine (Italy)  

Microsoft Academic Search

Ophiolitic sequences in Northern Apennines are usually exploited as source of raw material for civil engineering works. Grinding\\u000a procedures of basalts imply the production of dusts with relatively high concentration of PHES. This paper studied the increase\\u000a of geochemical risk when washing mud produced at Sasso di Castro quarry site (Tuscany) is reused as rock keeper in a near\\u000a dismissed

M. Voltaggio; M. Spadoni

2007-01-01

258

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

259

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

Microsoft Academic Search

Sedimentary beds of jasper (red hematitic chert) in the Ordovician Løkken 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 Løkken (25–30 Mt) and small Høydal (0.1 Mt) VMS deposits, and are exposed discontinuously for several kilometres along strike. Massive or laminated types predominate; jasper-sulphide debris-flow

Tor Grenne; John F. Slack

2002-01-01

260

Isotopic and trace element composition of volcanic glasses from the Akaki Canyon, Cyprus: implications for the origin of the Troodos ophiolite  

Microsoft Academic Search

We report major elements, K, Rb, Cs, Sr, Ba, Sc, Cr, Ni, Hf, Ta, Th, and REE concentrations and isotopic compositions of Sr, Nd, Pb and O of carefully handpicked volcanic glasses from the Akaki River section of the Troodos ophiolite complex. On the basis of Sr and O isotopic composition and Fe2O3\\/FeO ratios, nine of the ten glasses analyzed

M. Rautenschlein; G. A. Jenner; J. Hertogen; A. W. Hofmann; R. Kerrich; H.-U. Schmincke; W. M. White

1985-01-01

261

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

262

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

NASA Astrophysics Data System (ADS)

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

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

2004-12-01

263

Significance of Nain-Baft ophiolitic belt (Iran): Short-lived, transtensional Cretaceous back-arc oceanic basins over the Tethyan subduction zone  

Microsoft Academic Search

Four dismembered massifs belonging to the Nain-Baft ophiolitic belt (Central Iran) stretch in a NW-SE direction parallel to the fossil active margin of the Iranian Continental Block (Sanandaj-Sirjan Zone). They are separated by huge transcurrent faults. The Nain, Dehshir, Shahr-e-Babak and Baft massifs are composed of associated slices of harzburgites, small bodies of gabbros and dike swarm complexes, accompanied by

Hadi Shafaii Moghadam; Hubert Whitechurch; Mohamad Rahgoshay; Iman Monsef

2009-01-01

264

On Arcs and Ophiolites - Petrologic Comprison of Zambales Range and the Mariana Forearc - Evidence of Early Stages of Island Arc Development  

Microsoft Academic Search

The Zambales Range ophiolite (ZRO), a relatively intact, coherent, series of ocean crust and upper mantle rocks, comprises depleted harzburgite-dunite tectonites, overlain by Ol-Opx-Cpx layered cumulates, and Ol-Px-Pl layered and massive rocks. All display evidence for near-solidus, dry, penetrative ductile deformation. Higher crustal levels of ZRO have massive gabbro\\/norite, diabase sills and dikes, pillow basalts. Many of these have primary

J. W. Hawkins

2005-01-01

265

Melanges Pedagogiques (Pedagogical Mixture), 1985.  

ERIC Educational Resources Information Center

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

Melanges Pedagogiques, 1985

1985-01-01

266

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

267

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

268

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

269

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

270

Melanges Pedagogiques (Pedagogical Mixture), 1985.  

ERIC Educational Resources Information Center

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

Melanges Pedagogiques, 1985

1985-01-01

271

Magma mixing and the origin of layered cumulates: evidence from the Oman ophiolite (Bahla and Wuqbah massifs)  

NASA Astrophysics Data System (ADS)

Ultramafic cumulates, essentially wehrlitic, are an important component of the crustal section of the Oman ophiolite. They cannot be related to the most common gabbroic cumulates through simple fractional crystallization processes in anhydrous conditions. They occur frequently as intrusions post-dating the crystallization of the gabbros and, in such cases, are interpreted as "late intrusive" attributed to subduction or obduction related processes. Many authors have generalized this conclusion to all kinds of ultramafic cumulates, whatever their mode of occurrence, including those that are interlayered with the gabbros and that are supposed to be injected as sills between pre-existing gabbroic layers. In order to better constrain the origin of the ultramafic cumulates interlayered with gabbros, we have conducted a field, petrographic, and geochemical study of the lower crustal section of Bahla and Wuqbah massifs (westernmost part of the Oman ophiolite) where gabbroic and ultramafic cumulates occur in roughly equal proportions. We have performed detailed geochemical profiles with the SEM and with the LA-ICP-MS across the boundaries between gabbroic and ultramafic layers. In terms of modal composition, individual layers are quite homogeneous and the boundaries between layers are clear-cut down to the microscopic scale. In spite of the sharp nature of the lithological boundaries, the chemical composition in both major and trace elements defines progressive evolutions as they are approached. The thickness of these "cryptic transition zones" varies considerably according to the element and to the mineral considered. It ranges typically from a few mm to a few cm. By the same way, the shape of the geochemical profiles is curved (close to hyperbolic) for some elements (mostly major elements), but is almost linear for many other elements, especially some trace elements. These characteristics are inconsistent with solid-state intra-crystalline diffusion but call for diffusion in a liquid phase and/or for magma mixing. Mixing between either the parent melts of each cumulate suite or between intercumulus liquids percolating in the crystal mush may be envisioned. Whatever the details of the processes, our observations show that the crystallization of interlayered ultramafic and gabbroic cumulates are contemporary events. Both gabbroic and ultramafic rocks from these massifs are characterized by early crystallization of orthopyroxene, implying that their parent melts were significantly richer in SiO2 than MORB. Moreover, for a same degree of differentiation, their plagioclases are richer in An% and their Cpx are poorer in Al2O3 than MOR gabbros. These characteristics are reminiscent of boninitic-andesitic parent melts. However, Bahla and Wuqbah ultramafic and gabbroic cumulates do not present the extreme depletion in HFSE typical of supra subduction zone boninites. Their trace element signature implies that a MORB source was actually an important ingredient of their parent melts. The simplest way to account for these observations is to invoke mixing between N-MORB extracted from an asthenospheric upwelling and silica enriched melts produced by hydrated re-melting of the lithosphere in response to the deep penetration of hydrothermal fluids. Magma mixing may account for the coexistence of clear-cut mineralogical boundaries and of progressive geochemical transitions, the change in the nature of the cotectic assemblage occurring suddenly after a certain amount of mixing. We conclude that interlayered ultramafic and gabbroic cumulates characteristic of some cumulate sections in the Oman ophiolite bear witness of complex hybridization processes related to interaction between anhydrous and hydrous magmas.

Abily, Bénédicte; Ceuleneer, Georges; Gregoire, Michel; Benoit, Mathieu

2010-05-01

272

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

NASA Astrophysics Data System (ADS)

The Santa Elena Ophiolite Complex (SEOC) is located on the west coast of Northern Costa Rica, near the Nicaraguan border. It consists primarily of preserved oceanic crustal rocks and underlying upper mantle thrust onto an accretionary complex. The petrogenesis and tectonic origin of this complex have widely been interpreted to be either a preserved mantle portion of the Caribbean Large Igneous Province (CLIP) as it drifted between North and South America from the Galapagos hotpot into the present day Caribbean Ocean around 80 Ma or as the mantle section to the nearby Nicoya complex. Previous structural work suggests that SEOC is a supra-subduction complex, not related to the CLIP or Nicoya. Our preliminary results agree. Mantle peridotites collected from the Santa Elena Ophiolite Complex consist primarily of spinel lherzolite (61 %) with minor amounts of harzburgite and dunite (22 % and 16 % respectively). Spinel Cr# [molar Cr / (Cr+Al)*100] is widely accepted to constrain mantle partial melting and lithospheric melt stagnation. Cr# of spinels within Santa Elena lherzolites fall between 12 and 35, suggesting an extent of 3 % to 13 % partial melting. Cr# of harzburgites range from 35 to 39, suggesting 13 % to 14 % partial melting. This range of partial melting suggests only modest depletion of this exposed portion of the ancient uppermost mantle. TiO2 concentrations of the lherzolite and harzburgite range from 0.004% to 0.128%, with the exception of one sample, SE10 - 17 (0.258%), and fall within the normal melting trend for mantle peridotites. The presence of dunite indicates that melt flow and associated melt - rock reaction with the surrounding peridotite took place within this portion of the mantle. A Cr# of 84.5 from one of these dunite samples indicate that significant melt rock reaction with refractory melts took place. Such results are rarely found in mid-ocean ridge abyssal peridotite settings, and are currently found primarily in forearc tectonic settings. However, due to the overall "normal" TiO2 concentrations in all but one spinel peridotite requires that if melt flow did occur, that the melt be nearly depleted in titanium. The relatively low Cr#'s and TiO2 concentrations of spinel in these peridotites that suggest low degrees of partial melting along with the paleo presence of melt flow and melt-rock reaction by low titanium melts, such as boninites, point toward a young fore-arc model for the tectonic origin of this ophiolite body rather than a preserved mantle portion of the CLIP. Additionally, two lines of evidence suggest SEOC was emplaced prior to the collision of the CLIP with North and South America. The SEOC is 1) capped by a Campanian (83.5 - 70.6 Ma) rudist limestone and 2) lies uncomformably atop Cenomanian (93.6 - 99.6 Ma) radiolarite beds. This suggests that the mantle portion of the SEOC was emplaced and exposed at the Caribbean ocean floor prior to the Late Cretaceous (Campanian), but no earlier than the Cenomanian. This combined tectonic and geochemical evidence suggests SEOC may be a portion of the proto-arc that existed between the Americas in the Cretaceous prior to assault by the CLIP.

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

2010-12-01

273

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

NASA Astrophysics Data System (ADS)

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

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

1989-06-01

274

Oceanic faulting and fault-controlled subseafloor hydrothermal alteration in the sheeted dike complex of the Josephine Ophiolite  

NASA Astrophysics Data System (ADS)

Based upon detailed mapping (1:10 and 1:100) of a large water-polished outcrop of the sheeted dike/gabbro transition zone in the Josephine ophiolite of NW California and SW Oregon, the following history of alternating episodes of magmatic, structural, and hydrothermal events has been documented using crosscutting relationships, petrography, geochemistry, and strontium and oxygen isotopic data: (1) crystallization of gabbro and later subvertical mafic dike injection, (2) amphibolite facies metamorphism, (3) extensional faulting and tilting of dikes, (4) continued faulting, tilting, and dike injection associated with retrograde metamorphism at greenschist facies conditions, (5) continued extensional faulting and tilting synchronous with the development of a variety of hydrothermal veins at decreasing temperature and increasing fluid/rock ratios, and (6) subvertical injection of a highly fractionated dike which truncates all previous features (features 1-5). Trace element geochemistry indicates the highly fractionated dike (feature 6) is genetically related to the other dikes and thus was intruded at or near the paleospreading axis. Hence, all previous events (features 1-5) can be constrained to have occurred at the rift axis, including large-scale tilting (˜ 50°) of the sheeted dikes and extensional faulting. The [87Sr/86Sr]initial ratios from recrystallized whole rocks and from hornblende, epidote, and prehnite separates from veins display a systematic increase with relative age, from 0.7033 for altered gabbro screens and mafic dikes to 0.7049 for prehnite in the youngest oceanic fault rock. Calculated oxygen isotope fluid compositions for the same suite of samples range from ?18Ofluid = +5 to -1 with time, indicating a change to a seawater-dominated hydrothermal system with time, consistent with observed increased permeability due to seafloor extensional faulting. The crosscutting relationships, alteration mineral assemblages, and isotopic data suggest (1) an early stage of high-temperature (?450°C) hydrothermal alteration with low permeability (i.e., grain-scale flow), followed by (2) a decrease in temperature (˜350to ?200°C), and an increase in permeability due to faulting and accompanied by tectonic tilting at the rift axis. The consistency of these crosscutting relationships at similar pseudo-stratigraphic levels at different localities in the Josephine ophiolite suggests that alternating magmatic and structural extension with synchronous retrograde alteration is common in crust formed at similar rates of spreading (slow- to intermediate-spreading centers), such as the modern Mid-Atlantic Ridge.

Alexander, Robert J.; Harper, Gregory D.; Bowman, John R.

1993-06-01

275

Os Isotopes in the ultramafic Sequence of the Zermatt-Saas Ophiolite (ZSO): Petrogenesis and Age Constraints  

NASA Astrophysics Data System (ADS)

The Re-Os isotopic system has been shown to provide a unique view of mantle processes that contrasts markedly with that provided by lithophile element isotope systems. In this study, Re and Os abundances and the Os isotopic composition of peridotites from the Zermatt-Saas ophiolite (ZSO) have been determined in order to asses the behaviour of these elements during mantle melting processes, and the geodynamic evolution of the ZSO. The ZSO is a well preserved slice of eclogitic oceanic crust in the Alpine chain. The ophiolitic sequence is thought to be formed during the opening of the Mesozoic Tethys (˜165 Ma) that underwent subduction to high-pressure/ultra-high-pressure conditions during the Alpine compression. Major and trace element data suggest that the peridotites represent residues after the extraction of ˜2-25% of melts from a primitive mantle source. The Os abundances vary between 3.39 and 5.18 ppb, and are negatively correlated with the Al_2O_3 content. Rhenium abundances vary from 0.171 to 0.593 ppb, and display a positive correlation with the Al_2O_3 content. The peridotites from the ZSO show a wide range in the 187Os/188Os ratios between 0.116 and 0.127. With the exception of three harzburgites with very low 187Os/188Os ratio, the peridotites exhibit a strong positive correlation with Al_2O_3, and intercepts the 187Os/188Os axis at an initial Os isotopic ratio of 0.1205 ± 0.0012. This calculated initial 187Os/188Os ratio leads to a model age of 965 ± 177 Ma, assuming a chondritic evolution for the source. The lower osmium isotopic ratio (0.1165-0.1178) of three samples indicate an even older Re depletion event (TRD: 1.36 ˜ 1.55 Ga). Based on an isochron calculation, the initial Os isotopic ratio is computed to be 0.1201 ± 0.0017 with a slope corresponding to an age of 788 ± 240 Ma. Both results, the calculated model age (˜965 Ma) and the errorchron age (˜744 Ma) corroborate that the age of the last melting event recorded by the ultramafic rocks is significantly older than the age of the associated metagabbros (˜164 Ma). Additionally, the older Re depletion ages (TRD =1.36-1.55 Ga) for three harzburgites is inconsistent with a common melting event leading to the formation of the ZSO peridotites. These results raise major questions on the geodynamic processes leading to the formation of the ZSO, and on the nature of the protolith of the ZSO peridotites.

Philipp, H.; Brügmann, G.; Baumgartner, L. P.

2003-04-01

276

Tectonic setting for the genesis of oceanic plagiogranites: evidence from a paleo-spreading structure in the Oman ophiolite  

NASA Astrophysics Data System (ADS)

In the Oman ophiolite, detailed structural and lithological mapping of the 60 × 45 km Maqsad area has revealed several plagiogranite intrusions, ranging from metre scale dykes to hectometre scale bodies. These plagiogranites are spatially related to kilometre scale mafic plutons, displaying generally pegmatitic textures and locally deformed in amphibolite facies conditions along shear zones up to a few hundred metres thick. Isotope data demonstrate the mantle origin of these pegmatites and plagiogranites: ?Nd ranges from 6.3 to 9.9 and has an average value (26 samples) of 8.1, within the field of present-day MORBs, and similar to the average ?Nd (7.8) of the Oman ophiolite primary igneous sequence. These intrusions are not restricted to a given structural level: their paleovertical extent exceeds 5-6 km, from 2 km below the Moho up to the base of the sheeted dyke complex. Their modal composition evolves up section, from predominantly pyroxenites and gabbro norites in the mantle harzburgites to gabbro norites, diorites and quartz diorites at crustal level. Petrography, whole rock and mineral chemistry show that this vertical succession of lithologies can be simply explained by low pressure fractional crystallization of a hydrated basaltic melt. Plagiogranites, although more abundant at upper crustal level, are not restricted to this horizon. Plagioclase composition in plagiogranites presents a remarkable evolution with paleodepth, from highly calcic (70-95% an) in the mantle section, intermediate and highly scattered in the lower crust (10-85% an), to quite sodic at upper crustal level (5-20% an). Highly calcic plagioclases in quartz-bearing rocks is a puzzling character, inconsistent with fractional crystallization. Even if high degrees of fractional crystallization of a hydrated basaltic melt remain, the simplest way to account for the chemistry of the upper crustal plagiogranites, processes such as fluid-induced remelting or assimilation of country rocks must be invoked to account for the chemistry and field characteristics of the deeper plagiogranites. Mantle and crustal structures in the Maqsad area indicate that the pegmatites and plagiogranites emplaced in an ocean ridge setting, when a mantle diapir, soaked with basaltic melt, intruded a cool, hydrothermally altered, lithosphere. They were eventually transposed in an off-axis setting as the ascension of the diapir progressed up to Moho level. It is concluded that the formation of large volumes of buoyant leucocratic rocks is possible at ocean ridges, provided periods of amagmatic spreading are long enough to induce the growth and alteration of an axial lithospheric lid. This is likely more frequent at slow spreading centres where mantle upwelling is discontinuous in space and time.

Amri, Isma; Benoit, Mathieu; Ceuleneer, Georges

1996-03-01

277

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

278

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

NASA Astrophysics Data System (ADS)

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

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

2012-10-01

279

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

280

Shallow intrusive directions of sheeted dikes in the Troodos ophiolite: Anisotropy of magnetic susceptibility and structural data  

SciTech Connect

Sheeted dikes play a central role in the formation of oceanic crust. It is commonly assumed that sheeted dikes intrude vertically upward, from elongated mid-ocean ridge (MOR) magma chambers, but there are no direct observational data bearing on this hypothesis. This assumption contrasts with the intrusive behavior of subaerial volcanoes where magmas rise into shallow central magma chambers that laterally feed vertically oriented dikes. The authors have studied intrusive directions of sheeted dikes in a structural analogue to oceanic crust, the Troodos ophiolite. Structural and magnetic fabric data of 65 dikes provide consistent results and suggest a broad distribution of shallow (< 20[degree]) to nearly vertical, upward magma-transport directions. These data suggest that horizontal emplacement has to be considered for sheeted dikes at MORs, implying more centralized MOR plumbing systems than previously thought. Such plumbing systems provide ample opportunity for complex mixing, fractionation, and contamination of MOR lavas in magma chambers and tabular magma-storage volumes. Whether the MOR magma supply is linear or centralized also has a fundamental effect on crustal accretion processes and the geometry of hydrothermal convection systems.

Staudigel, H.; Gee, J.; Tauxe, L. (Scripps Inst. of Oceanography, La Jolla, CA (United States)); Varga, R.J. (College of Wooster, OH (United States))

1992-09-01

281

The Gothara plagiogranite: evidence for oceanic magmatism in a non-ophiolitic association, North Khetri Copper Belt, Rajasthan, India?  

NASA Astrophysics Data System (ADS)

The Gothara granitoid, a small pluton in the vicinity of Khetri town, is one of several granitoid bodies that intrude the Palaeo-Mesoproterozoic Delhi Supergroup of the Khetri Copper Belt (KCB) in north Rajasthan, India. Detailed petrological and geochemical investigations characterise the Gothara granitoid as a plagiogranite. It exhibits profuse granophyric intergrowth between quartz and chessboard albite, besides having the low Al2O3, extremely low K2O, Rb, Ba, and high Na2O that are characteristic of plagiogranites, and matches some typical oceanic plagiogranites of the world in totality. However, unlike the other known plagiogranites, it is mineralogically and chemically exceptionally homogeneous, almost free from alteration and occurs in a non-ophiolitic setting. The field relationships of the Gothara plagiogranite, particularly its dual and overlapping relationship with the mafic magmatic rocks, its accessory mineral assemblage, I-type characteristics and the major, trace and REE geochemistry, all point towards its fractionation-related petrogenesis from a mafic magma of mantle origin in an oceanic ridge tectonic regime. This discovery is of importance not only for the petrotectonic evolution of the Khetri Copper Belt but also for the petrogenesis of plagiogranites elsewhere in the world.

Kaur, Gurmeet; Mehta, P. K.

2005-08-01

282

Podiform chromitite classification revisited: A comparison of discordant and concordant chromitite pods from Wadi Hilti, northern Oman ophiolite  

NASA Astrophysics Data System (ADS)

Two types of podiform chromitite, concordant and discordant, were examined in the mantle section of northern Oman ophiolite along Wadi Hilti, to revisit the structural classification of podiform chromitite. They are contrasted in mineral chemical characteristics, in addition to the difference in attitude; the Cr/(Cr + Al) atomic ratio of spinel is around 0.6 for the concordant chromitite and surrounding peridotites, but is around 0.7 for the discordant one and surrounding peridotites. Chromian spinel grains contain pargasite-rich inclusions of primary origin from the both types, but they are far less abundant and smaller in size in the concordant chromitite than in the discordant one. Thin lamellae of pyroxenes in chromian spinel, similar to those in ultrahigh-pressure (UHP) chromitites from Tibet, are available only from the concordant chromitite. The dunite enveloping the concordant chromitite is extraordinarily high in NiO (up to >0.5 wt.%), suggesting subsolidus Ni diffusion from the chromitite. The involved melt was quite different between the two types of chromitite; the melt to precipitate the discordant one was more hydrous than that for the concordant one because of far more abundance of hydrous minerals in the former. The difference in duration of subsolidus cooling, and probably decompression, is prominent between the two types of chromitite. The concordant chromitite cannot be formed from the discordant one simply by metamorphic conversion: the former is of deep magmatic origin whereas the latter, of shallow magmatic origin.

Miura, Makoto; Arai, Shoji; Ahmed, Ahmed H.; Mizukami, Tomoyuki; Okuno, Masayuki; Yamamoto, Shinji

2012-10-01

283

New palaeomagnetic results from Tertiary carbonates of NW Syria and their implications for the timing of extreme tectonic rotations in the Baer-Bassit ophiolite  

NASA Astrophysics Data System (ADS)

The eastern Mediterranean/Middle East segment of the Tethyan orogenic belt is marked by several chains of ophiolites, the most prominent of which consists of the Troodos (Cyprus), Baer-Bassit (Syria), Hatay (Turkey), Kermanshah (Iran) and Semail (Oman) units. These are interpreted as fragments of Neotethyan oceanic lithosphere formed during the Late Cretaceous. The Baer-Bassit and Hatay ophiolites are closely related spatially and represent parts of a single unit emplaced onto the Arabian continental margin during the Maastrichtian. The Baer-Bassit units are highly deformed by thrust faulting (reflecting their inferred position at the leading edge of the ophiolite) and by post-emplacement strike-slip faulting. Our recent palaeomagnetic results have demonstrated extreme (>90°) tectonic rotations within Baer-Bassit (Morris et al., 2002, EPSL 202, 247-261). Here we present new palaeomagnetic data from the post-emplacement Tertiary sedimentary cover of the ophiolite which place important constraints on the timing of rotational deformation. Sites are located in two sub-areas: (i) Palaeogene (Palaeocene-Lower Eocene) carbonates in the west; and (ii) Miocene carbonates in the east. Stable components of magnetization are successfully isolated despite very low intensities of remanence (c. < 0.5 mA/m). Demagnetization characteristics are consistent with remanences being carried by magnetite. Tilt correction of data from Miocene sites produces no significant change in dispersion and so a tilt test cannot be performed. The in situ mean direction of magnetization at these sites, however, is indistinguishable from both the present day GAD field direction in northern Syria and the Miocene direction derived from the 10 Ma African pole of Besse and Courtillot (1991). This suggests that the sampled Miocene sequences record either Recent (?viscous) components of magnetization or a post-tilting Miocene remagnetization. In the latter case, remagnetization may be related to Late Miocene initiation of the Latakia-Killis lineament, the extension of which traverses the sampling area. Application of an inclination-only tilt test to data from Palaeogene sites indicates a clear syn-folding age of magnetization, with minimum dispersion occurring at ~50% of untilting. Gentle folding in the Baer-Bassit area occurred in the late Eocene - Oligocene in response to initial suturing between Arabia and Anatolia to the north. We tentatively relate the syn-tilting remagnetization event to diagenetic growth of low concentrations of magnetite during this deformation. The Palaeogene sites can be divided into two groups which subsequent to remagnetization have been rotated by c. 30° CCW and c. 50° CW. Rotations are likely to be linked to deformation within a strike-slip fault zone, which initiated in response to transpression associated with final suturing in the latest Miocene and may still be active today. These data suggest that only a fraction of the rotations in the underlying ophiolite may be attributed to post-emplacement deformation. This supports a model in which the most significant phase of rotation of the Baer-Bassit/Hatay ophiolite occurred in an intra-oceanic setting, possibly as part of a wider "Troodos microplate".

Morris, A.; Inwood, J.; Anderson, M. W.; Robertson, A. H. F.

2003-04-01

284

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

NASA Astrophysics Data System (ADS)

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

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

2010-05-01

285

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

286

Timescales of Crustal Accretion at a Medium to Fast Spreading Ridge: High Precision U- Pb Zircon Dating of the Intrusive Crust of the Cretaceous Oman Ophiolite  

NASA Astrophysics Data System (ADS)

The Oman ophiolite is one of the largest subaerial exposures of oceanic lithosphere on Earth. We present new high-precision single grain U-Pb zircon dates from both gabbro and plagiogranite plutons in the ophiolite, which provide new insight into the temporal evolution of oceanic crust. In the Ibra area, at the southern end of the ophiolite, a large and laterally coherent block of oceanic lithosphere exposes a cross section through mantle harzburgites, layered gabbros, massive gabbros, sheeted dikes and rare basaltic flows and pillows. We have dated five upper level massive gabbros and a single plagiogranite from a ~25 km transect perpendicular to the inferred ridge axis. The 206Pb/238U dates range from ~96.2-95.7 Ma and generally confirm that the crust formed at a medium to fast spreading ridge. However, the spatial distribution and spread of dates do not follow the pattern predicted by current models of simple spreading, and suggest that either i) there was significant off-axis magmatism, ii) older plutons crystallized in the mantle and were incorporated into the crust at the ridge axis, or iii) the crust was imbricated by post magmatic strike- slip faulting. Two new dates from the Samail Massif provide insight into the origin of oceanic plagiogranites. A plagioclase- hornblende vein that crosscuts layered gabbros yielded a 206Pb/238U date of ~95.8 Ma, providing a minimum age for the layered gabbros. In contrast, an adjacent plagiogranite pluton generated a range of 206Pb/238U dates of ~95.4-95.2 Ma, indicating that the plagiogranite intrusion post- dates gabbro crystallization by a minimum of ~400,000-600,000 years and likely reflects protracted off- axis modification of the crust.

Rioux, M.; Bowring, S. A.; Kelemen, P. B.

2008-12-01

287

A Cretaceous forearc ophiolite in the Shyok suture zone, Ladakh, NW India: Implications for the tectonic evolution of the Northwest Himalaya  

NASA Astrophysics Data System (ADS)

The northwestern Himalaya in India contains critical evidence of the convergent margin and collision tectonics between the Ladakh arc and the Karakoram block. Here we present new petrochemical and geochronological data from a forearc ophiolite at the Shyok-Nubra river confluence. Whole-rock geochemical data show relatively high large-ion lithophile elements (LILE), light rare earth elements (LREE), and Ba, U and Pb anomalies, and depletions in La, Ce and Zr, particularly, the high-field strength elements (e.g. Nb, Ta); these geochemical characteristics are similar to those in modern ophiolites that formed in arc-related environments. Meta-volcanic greenschists that contain spinel layers have high MgO, Ni, Co, and Cr contents. In contrast, they have low contents of TiO2, very low Nb and Zr that are diagnostic of high-Ca boninitic magmas in modern forearc settings, as in Izu-Bonin-Mariana (IBM) and Tonga. The spinels have high Cr# [Cr / (Cr + Al)] and Mg#[Mg / (Mg + Fe2 +)], which are characteristic of spinels in forearc boninite-type of melts. The spinel-free meta-volcanic samples have incompatible trace element abundance patterns similar to those of MORB. However, their enrichments in Cs, Rb, Pb, U and depletion in HFSE may reflect an input of subduction fluids that are different from MORB; these MORB-like basalts are suggestive of a forearc complex that erupted prior to the formation of boninitic rocks.Our data from the Shyok ophiolite indicate the existence of supra-subduction rocks on the southern Karakoram margin. Albite porphyroblasts in greenschists yield a K-Ar age of 104.4 ± 5.6 Ma that represents the time of early exhumation of the greenschists. The boninite-type melts formed prior to 104 Ma, thereby indicating that the time of initiation of subduction on the southern margin of the Karakoram block was before 104 Ma. These geochemical signatures together with the spatial and temporal distribution of the arc rocks on the southern margin of Karakoram block suggest northward subduction of NeoTethys in the Early Cretaceous. The subsequent collision between the Ladakh arc and the Karakoram block thrusted/obducted the forearc ophiolite onto the southern edge of the Karakoram block probably between 74 Ma and 97 Ma.

Thanh, Ngo Xuan; Rajesh, V. J.; Itaya, Tetsumaru; Windley, Brian; Kwon, Sanghoon; Park, Chan-Soo

2012-12-01

288

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

NASA Astrophysics Data System (ADS)

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

Bourgois, J.; Witt, C.

2008-12-01

289

Mineralogy, composition and PGM of chromitites from Pefki, Pindos ophiolite complex (NW Greece): evidence for progressively elevated fAs conditions in the upper mantle sequence  

Microsoft Academic Search

The Pindos ophiolite complex, located in the northwestern part of continental Greece, hosts various chromite deposits of both\\u000a metallurgical (high-Cr) and refractory (high-Al) type. The Pefki chromitites are banded and sub-concordant to the surrounding\\u000a serpentinized dunites. The Cr# [Cr\\/(Cr?+?Al)] of magnesiochromite varies between 0.75 and 0.79. The total PGE grade ranges\\u000a from 105.9 up to 300.0 ppb. IPGE are higher than

Argirios Kapsiotis; Tassos A. Grammatikopoulos; Basilios Tsikouras; Konstantin Hatzipanagiotou; Federica Zaccarini; Giorgio Garuti

2011-01-01

290

Melt-rock reaction an melt impregnation in oceanic peridotites: insights from the Ligurian-Piemontese ophiolites  

NASA Astrophysics Data System (ADS)

Mantle peridotites from Alpine-Apennine ophiolites, deriving from the Jurassic Ligurian Tethys, record signatures of the complex petrogenetic evolution, other than partial melting, that the lithospheric mantle suffered during pre-oceanic extension and melt percolation, melt-rock interaction and refertilization of early melts from decompression melting of the almost adiabatically upwelling asthenosphere (Piccardo et al., 2008). Lithosphere extension by far field tectonic forces lead to thinning of the lithospheric mantle and its progressive exhumation. Field and petrographic-structural data indicate that lithosphere extension was driven by extensional shear zones during the whole evolution of the mantle lithosphere, from garnet- to plagioclase-facies conditions. The pristine sub-continental lithospheric mantle is still preserved in ophiolites deriving from the passive margins (ocean-continent transition zones) of the basin, whereas melt-reacted and refertilized peridotites are dominant in more internal oceanic domains. OCT peridotites maintain structural-paragenetic features indicating their provenance from the deep lithosphere (P > 2.5 GPa) (Piccardo et al., 2009). Km-scale extensional shear zones in spinel peridotites (e.g., Vissers et al., 1991; Hoogerduijn Strating et al., 1993) have been dated to 220 Ma (Lu-Hf age) (Montanini et al., 2006) and 225 Ma (40Ar/39Ar amphibole age) (Müntener & Hermann, 2001) indicating that significant lithosphere extension and mantle exhumation was already active during Triassic times. Passive upwelling asthenosphere underwent fractional melting under spinel-facies conditions forming MORB-type depleted single melt increments that were injected into the lithospheric spinel-facies shear zones. Porous flow percolation of the silica-undersaturated melt fractions and melt-peridotite interaction (pyroxene dissolution and olivine precipitation) formed reactive spinel harzburgites and dunites. Melt-peridotite interaction led to silica saturation of the rising melts and further extension led to formation of plagioclase-facies shear zones and porous flow percolation under plagioclase-facies conditions. Melt-peridotite interaction (olivine corrosion and ortopyroxene + plagioclase precipitation) and melt interstitial crystallization (impregnation) enriched in gabbro-noritic components the lithospheric peridotites that were widely impregnated when the conductive heat loss prevailed on percolation heating and the depleted melts stagnated in the upper lithospheric mantle. Sometime pockets of melts formed strongly depleted gabbro-norite intrusions (Ross & Elthon, 1993; Piccardo & Guarnieri, 2011). The early depleted MORB-type melts were completely entrapped into the shallow mantle lithosphere and refertilized it. Later plagioclase-facies shear zones were frequently infiltrated by aggregated MORB-type melts and transformed by reactive percolation to dunite channels, sometimes 10-100-meters wide. These high porosity channels are considered the fastest way to deliver oceanic MORB to shallow levels. These melts form the gabbroic intrusions and the basaltic extrusions of the oceanic crustal rocks of Jurassic Ligurian Tethys.

Piccardo, G. B.

2011-12-01

291

Boron and oxygen isotope systematics for a complete section of oceanic crustal rocks in the Oman ophiolite  

NASA Astrophysics Data System (ADS)

Boron content and the isotopic composition of boron and oxygen were determined for a complete 5.3-km-thick oceanic crust in the Wadi Fizh area of the Oman ophiolite, a fragment of Cretaceous oceanic lithosphere obducted onto the Arabian shield. The depth profiles of the ?18O values and the secondary mineral assemblages are consistent with successively higher temperature hydrothermal alterations occurring with increasing depth. The upper pillow basalts underwent low temperature alteration (<60 °C) or seafloor weathering (?18O > +10‰); the lower pillow basalts and upper sheeted dikes (<2000 m of stratigraphic depth) were altered at 250-350 °C at the spreading axis and subsequently experienced retrograde alteration (<200 °C) in the flank provinces (?18O = +6‰ to +10‰); the lowermost sheeted-dike complex and upper gabbros underwent high-temperature alteration at 300-450 °C (?18O < +6‰); the lower gabbros were altered at very high temperatures of >450 °C (?18O < +6‰). Plagiogranites from the uppermost gabbro section show exceptionally high ?18O values compared with the adjacent rocks, suggesting the production of 18O-enriched melt. The boron content of the rocks in the oceanic crust decreases in both abundance and range with increasing stratigraphic depth; 1.4-29.1 ?g/g in pillow lava (7.9 ?g/g on average), 1.5-11.6 ?g/g in sheeted dike complex (5.3 ?g/g on average), 1.6-5.0 ?g/g in dolerite dike in gabbro (2.9 ?g/g on average), 0.25-3.8 ?g/g in gabbro (1.3 ?g/g on average). Considering an original boron content of 0.72 ± 0.47 ?g/g for basalt and 0.06 ± 0.09 ?g/g for gabbro, boron from seawater was incorporated into the rocks through hydrothermal alteration, even at temperatures higher than 300 °C. The ?11B values systematically increase with stratigraphic depth; -1.1‰ to +11.9‰ in pillow lava (+5.5‰ on average), +1.1‰ to +17.5‰ in sheeted dike complex (+6.3‰ on average), +8.3‰ to +18.6‰ in dolerite dike in gabbro (+13.9‰ on average), +7.3‰ to +17.7‰ in gabbro (+12.0‰ on average). The whole-rock ?11B values negatively correlate with the ?18O values, suggesting that the ?11B values of altered rocks are essentially controlled by isotopic equilibrium with hydrothermal fluids, and the increase in the ?11B values is caused by a decrease in the rock-fluid boron isotopic fractionation factor with increasing alteration temperatures. The ?11B values estimated for hydrothermal fluids from rocks completely altered at 300-450 °C range from +28‰ to +33‰, values indistinguishable from those of vent fluids observed at modern mid-ocean ridges. The boron content of the bulk oceanic crust of the Oman ophiolite is estimated to be 3.6 ?g/g, and the ?11B value is estimated at +7.9‰. In contrast to previous views, the hydrothermally altered gabbro section is a large boron sink, accounting for ˜30% of the total boron in the oceanic crust with a high ?11B value of +13‰. This boron-enriched, high-?11B lower oceanic crust may impact estimates of the ?11B value of fluid liberated from the subducted oceanic slab, which is believed to largely control the ?11B value of arc magma generated in the mantle wedge.

Yamaoka, Kyoko; Ishikawa, Tsuyoshi; Matsubaya, Osamu; Ishiyama, Daizo; Nagaishi, Kazuya; Hiroyasu, Yuko; Chiba, Hitoshi; Kawahata, Hodaka

2012-05-01

292

Building of the deepest crust at a fossil slow-spreading centre (Pineto gabbroic sequence, Alpine Jurassic ophiolites)  

NASA Astrophysics Data System (ADS)

This work presents new field and petrological data on a poorly known lower crustal section from the Alpine Jurassic ophiolites, the Pineto gabbroic sequence from Corsica (France). The Pineto gabbroic sequence is estimated to be ~1.5 km thick and mainly consists of clinopyroxene-rich gabbros to gabbronorites near its stratigraphic top and of troctolites and minor olivine gabbros in its deeper sector. The sequence also encloses olivine-rich troctolite and mantle peridotite bodies at different stratigraphic heights. The composition and the lithological variability of the Pineto gabbroic sequence recall those of the lower crustal sections at slow- and ultra-slow-spreading ridges. The gabbroic sequence considered in this study is distinct in the high proportion of troctolites and olivine gabbros, which approximately constitute 2/3 of the section. In particular, the lower sector of the Pineto gabbroic sequence shows the existence of large-scale fragments of the deepest oceanic crust displaying a highly primitive bulk composition. The mineral chemical variations document that the origin and the evolution of the Pineto gabbroic rocks were mostly constrained by a process of fractional crystallisation. The clinopyroxenes from the olivine gabbros and the olivine-rich troctolites also record the infiltration of olivine-dissolving, Cr2O3-rich melts that presumably formed within the mantle, into replacive dunite bodies. Cooling rates of the troctolites and the olivine gabbros were evaluated using the Ca in olivine geospeedometer. We obtained high and nearly constant values of -2.2 to -1.7 °C/year log units, which were correlated with the building of the Pineto gabbroic sequence through multiple gabbroic intrusions intruded into a cold lithospheric mantle.

Sanfilippo, Alessio; Tribuzio, Riccardo

2013-04-01

293

Geochemical Characterization of Subsurface Microbial Habitat Produced by Serpentinization: Preliminary Results from Coast Range Ophiolite Formation Fluids  

NASA Astrophysics Data System (ADS)

We examined the mineralogy and aqueous geochemistry associated with five wells sunk (up to 45 m deep) in actively serpentinizing bodies of the Coast Range Ophiolite to assess their potential as subsurface habitats for life. Through production of hydrogen, serpentinization has the potential to support a variety of subsurface microbial metabolisms (e.g., methanogenesis), and is thus of interest both in charting the deep biosphere on Earth and in assessing the habitability of other planetary bodies with ultramafic components. This site, at which repeated monitoring and ready access to the subsurface are possible, offers great potential for characterizing the long-term life-hosting potential of serpentinizing systems. Inspection of archived core cuttings from the same locality, sampling serpentinite up to 110 meters below the surface, reveals nearly completely serpentinized parent rock, with polished and massive serpentine, and prominent bastite (serpentinized pyroxene) grains. Historical records of fluid geochemistry from the wells, dating to their installation in the 1980's, show consistently elevated pH (9.12-10.15) and Ca2+ concentrations (12-84 ppm), consistent with formation under serpentinizing conditions. In general, these formation fluids are depleted in major ions and enriched in Fe and Mn with respect to seawater. New data, including fluid concentrations of methane and hydrogen, are similarly consistent with active serpentinization. Geochemical modeling of these data constrains the free energy yields that could be accessed by H2 - consuming microbial metabolisms, including methanogenesis. These models constitute the first in a series of quantitative assessments of the habitability of a subsurface serpentinizing system.

Cardace, D.; Hoehler, T. M.

2008-12-01

294

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

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

295

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

Microsoft Academic Search

In the preceding report we showed experimentally that the dunite-harzburgite-lherzolite (DHL) sequence found in the mantle sections of ophiolite could be formed by reactive dissolution of lherzolite in a basaltic liquid. The most striking results of our lherzolite dissolution experiments are the sharp mineralogical boundaries between adjacent lithologies and simple monotonic composition variations in minerals across the DHL sequence. Here

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

2004-01-01

296

The most ancient ophiolite of the Central Asian fold belt: U-Pb and Pb-Pb zircon ages for the Dunzhugur Complex, Eastern Sayan, Siberia, and geodynamic implications  

NASA Astrophysics Data System (ADS)

Ophiolitic rocks with a zircon age of ~1020 Ma occur in the Dunzhugur complex of East Sayan, Siberia, and are part of a Neoproterozoic to early Palaeozoic segment of the Central Asian fold belt. The most spectacular suite is exposed along the Oka and Bokson rivers, where a complete ophiolite sequence with mantle tectonites, a layered sequence composed of dunite, wehrlite, and pyroxenite, a gabbro section, a sheeted diabase dyke complex and basaltic pillow lavas are exposed. Petrologic and geochemical data suggest that all members of the ophiolite originally belonged to the same cogenetic mafic-ultramafic crustal section and support a supra-subduction zone setting in a fore-arc rifting environment for its origin. Two multigrain zircon size fractions from a plagiogranite are both slightly discordant but yielded a combined mean 207Pb/206Pb age of 1020+/-10 Ma. Evaporation of three additional fractions of three to four grains each from the same sample produced a mean 207Pb/207Pb age of 1019.9+/-0.7 Ma that we consider to most closely reflect the time of igneous crystallization of the plagiogranite. This is the oldest ophiolite so far dated from the Central Asian fold belt. The southern margin of the Siberian craton and the palaeo-Asian ocean were established at the end of the Mesoproterozoic, at least 1000 Ma ago. During the time interval 1000-570 Ma, one or several large ocean basins existed between Baltica, Siberia, Kazakhstan, Tarim and northern China, and these blocks are therefore unlikely to have been part of the supercontinent Rodinia. Rifting, initiation of subduction, and marginal basin formation began prior to 1000 Ma and continued through 570 Ma. The Dunzhugur ophiolite of Eastern Sayan provides evidence for the early opening of the palaeo-Asian ocean not later than 1000 Ma ago.

Khain, E. V.; Bibikova, E. V.; Kröner, A.; Zhuravlev, D. Z.; Sklyarov, E. V.; Fedotova, A. A.; Kravchenko-Berezhnoy, I. R.

2002-06-01

297

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

298

Processes of brine generation and circulation in the oceanic crust: Fluid inclusion evidence from the Troodos Ophiolite, Cyprus  

NASA Astrophysics Data System (ADS)

Detailed temporal, thermal, and compositional data on aqueous fluid inclusions from a suite of plutonic and diabase samples from the Troodos ophiolite, Cyprus provide the first documentation that generation of high-temperature brines may be common at depth in the oceanic crust. Anastomosing arrays of fluid inclusions in rocks of the upper intrusive sequence record episodic fracturing events. The earliest fracturing event, at temperatures >450-600°C resulted in entrapment of brine-rich aqueous fluids with salinities of 36-61 wt % NaCl equivalent. Homogenization of the brine inclusions by haute dissolution, the virtual absence of vaporrich fluid inclusions throughout the upper level plutonic sequence, and the restriction of brine inclusions to the most evolved plutonic rocks suggests that exsolution of brines off of the late stage gabbro and plagiogranite melts played a significant role in generating the quartz-hosted, high-salinity inclusions. Cooling of the fluids during pulses of fluid migration associated with episodic fracturing events, resulted in entrapment of the brines in the deep-seated, high-temperature portion of the hydrothermal system. In localized areas, the high-temperature brines (NaCl±KCl±CaCl2) caused extreme alteration of the plagiogranite bodies and in the formation of podiform epidosites. Arrays of low-temperature, low-salinity fluid inclusions, which in some samples crosscut fractures dominated by brine inclusions, indicate downward propagation of a cracking front subsequent to collapse of the high-temperature magmatic system, resulting in penetration of seawaterlike fluids into the plutonic sequence at temperatures >200-400°C. Hydration reactions under greenschist facies conditions, or limited mixing with brine-rich fluids, may have resulted in salinity variations from 70% below to 200% above seawater concentrations. Temperatures and compositions of the low-salinity inclusions are similar to those found in stockwork systems beneath Troodos ore deposits and to those of fluids exiting active submarine hydrothermal vents at mid-ocean ridge spreading centers. The low-temperature fracture networks may represent an extensive deep-seated feeder system which coalesced to form zones of concentrated hydrothermal upflow.

Kelley, Deborah S.; Robinson, Paul T.; Malpas, John G.

1992-06-01

299

Trace element characteristics of the fluid liberated from amphibolite-facies slab: Inference from the metamorphic sole beneath the Oman ophiolite and implication for boninite genesis [rapid communication  

NASA Astrophysics Data System (ADS)

Major and trace element compositions of amphibolites and quartzose rocks in the 230-m-thick metamorphic sole underlying the mantle section of the Oman ophiolite in Wadi Tayin area were determined to investigate the chemical characteristics of the hydrous fluid released from subducted amphiboltie-facies slab. The fluid-immobile element compositions indicate that protoliths of these rocks are mid-ocean ridge basalt-like tholeiite and deep-sea chert, which is consistent with the idea that these rocks represent Tethyan oceanic crust overridden during the early, intraoceanic thrusting stage of the Oman ophiolite emplacement. The rare-earth element (REE) and high field-strength element concentrations of the amphibolites show limited variations, within a factor of two except for a few evolved samples, throughout transect of the sole. On the other hand, concentrations of fluid-mobile elements, especially B, Rb, K and Ba, in amphibolites are highly elevated in upper 30 m of the sole (> 600 °C in peak metamorphic temperature), suggesting the equilibration with evolved, B-Rb-K-Ba-rich fluids during prograde metamorphism. The comparison with amphibolites in the lower 150 m (500 to 550 °C) demonstrates that the trace element spectra of the fluids equilibrated with the high-level amphibolites may vary as a function of metamorphic temperature. The fluids are characterized by striking enrichments of B, Rb, K and Ba and moderate to minor enrichments of Sr, Li, Be and Pb. At higher temperature (up to 700 °C), the fluids become considerably enriched in light REE and Nb in addition to the above elements. The estimated trace element spectra of the fluids do not coincide with the compositions of basalts from matured intra-oceanic arcs, but satisfactorily explain the characteristics of the low-Pb andesites and boninites found in the Oman ophiolite. Compositional similarity between the boninites of Oman and other localities suggests that the fluids estimated here well represent the amphibolite-derived fluids involved in the magmatism of immatured, hot, shallow subduction zones.

Ishikawa, Tsuyoshi; Fujisawa, Shiori; Nagaishi, Kazuya; Masuda, Toshiaki

2005-12-01

300

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

301

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

NASA Astrophysics Data System (ADS)

Unusual silicate lamellae were discovered within chromites of podiform chromitites in the Luobusa ophiolite, southern Tibet, especially from massive- and nodular-type chromites. Using analytical transmission electron microscopy (ATEM), we found coesite, clinopyroxene and MgSiO3 phase as exsolution lamellae from the host chromites. There is no evidence that the Luobusa ophiolite itself formed at great depth, and the presence of coesite lamellae in a chromite directly indicate that podiform chromitites originate from deep mantle environment. On the other hand, magmatic structures are still observed in the podiform chromitite ore-body, such as banding chromites and the sharp contact between host peridotite and dunite envelopes. The characteristic features of the disseminated- and banded-type chromites, such as their interstitial distribution, euhedral to subhedral morphology and absence of exsolution lamellae, suggest they formed under low-pressure magmatic conditions. According to petrographic investigations, nodular-type chromites with numerous lamellae seem to be changing into disseminated-type chromite with no lamellae. Therefore, we conclude that the podiform chromitites at Luobusa retain evidence of their multi-stage development from ultrahigh-pressure environment to low-pressure magmatic processes under a ridge. On the basis of our results, we propose that a significant component of the podiform chromitite at Luobusa originate from the ultra-deep environment, and that chromitites were transported with mantle upwelling from a deep mantle to a shallow level under a mid-ocean ridge. Subsequently, the mantle peridotite with its podiform chromitite underwent partial melting, and the chromites without exsolution lamellae were largely recrystallized or newly precipitated by a shallow-level magmatic process under the mid-ocean ridge. As chromite is a highly refractory mineral, the petrological ultrahigh-pressure evidence can be preserved in spite of its long evolved history. Moreover, our new approach of nano-scale measurement with ATEM for refractory chromites may become a new tool to trace ultra-high pressure signature not only for ophiolitic massifs but also for ultra-high pressure massifs or mantle xenoliths.

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

2007-12-01

302

Stable isotope geochemistry of Alpine ophiolites: a window to ocean-floor hydrothermal alteration and constraints on fluid-rock interaction during high-pressure metamorphism  

NASA Astrophysics Data System (ADS)

The subduction of hydrated oceanic lithosphere potentially transports large volumes of water into the upper mantle; however, despite its potential importance, fluid-rock interaction during high-pressure metamorphism is relatively poorly understood. The stable isotope and major element geochemistry of Pennine ophiolite rocks from Italy and Switzerland that were metamorphosed at high pressures are similar to that of unmetamorphosed ophiolites, suggesting that they interacted with little pervasive fluid during high-pressure metamorphism. Cover sediments also have oxygen isotope ratios within the expected range of their protoliths. In the rocks that escaped late greenschist-facies retrogression, different styles of sub-ocean-floor alteration may be identified using oxygen isotopes, petrology, and major or trace element geochemistry. Within the basalts, zones that have undergone high- and low-temperature sub-ocean-floor alteration as well as relatively unaltered rocks can be distinguished. Serpentinites have ?18O and ?2H values that suggest that they were formed by hydration on or below the ocean floor. The development of high-pressure metamorphic mineralogies in metagabbros occurred preferentially in zones that underwent sub-ocean-floor alteration and which contained hydrated, fine-grained, reactive assemblages. Given that the transformation of blueschist-facies metabasic rocks to eclogite-facies assemblages involves the breakdown of hydrous minerals (e.g. lawsonite, zoisite, and glaucophane), and will thus liberate considerable volumes of fluids, metamorphic fluid flow must have been strongly channelled. High-pressure (quartz+calcite+/-omphacite+/-glaucophane+/-titanoclinohumite) veins that cut the ophiolite rocks represent one possible channel; however, stable isotope and major element data suggest that they were not formed from large volumes of exotic fluids. Fluids were more likely channelled along faults and shear zones that were active during high-pressure metamorphism. Such strong fluid channelling may cause fluids to migrate toward the accretionary wedge, especially along the slab-mantle interface, which is probably a major shear zone. This may preclude all but a small fraction of the fluids entering the mantle wedge to flux melting. Additionally, because fluids probably interact with relatively small volumes of rock in the channels, they cannot "scavenge" elements from the subducting slab efficiently.

Cartwright, I.; Barnicoat, A. C.

303

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

304

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

NASA Astrophysics Data System (ADS)

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. The new data come from a well-preserved and exposed crustal section in the center of the Wadi Tayin massif. Single grain and grain fragment 206Pb/238U dates from upper-level gabbros, tonalites/trondhjemites and gabbroic pegmatites, corrected for initial Th exclusion, range from 112.55 ± 0.21 to 95.50 ± 0.17 Ma, with most data clustered between 96.40 ± 0.17 to 95.50 ± 0.17 Ma. Zircon dates from upper-level gabbros are most consistent with the ophiolite forming at a fast spreading ridge with half-rates of 50-100 km/Ma. Dates from tonalites/trondhjemites and from a gabbroic pegmatite associated with a wehrlite intrusion overlap with dates from adjacent upper-level gabbros, suggesting that any age differences between these three magmatic series are smaller than the analytical uncertainties or intrasample variability in the dates. Three of the dated upper-level gabbros and a single gabbroic pegmatite from the base of the crust have >1 Ma intrasample variability in single grain dates, suggesting assimilation of older crust during the formation or crystallization of the magmas. Whole rock ?Nd(t) of seven samples, including the upper-level gabbros with variable zircon dates, have tightly clustered initial values ranging from ?Nd(96 Ma) = 7.59 ± 0.23 to 8.28 ± 0.31. The ?Nd values are similar to those from other gabbros within the ophiolite, suggesting that any assimilated material had a similar isotopic composition to primitive basaltic magmas. The new dates suggest that the studied section formed at a fast spreading mid-ocean ridge between ˜96.4-95.5 Ma. The large intrasample variability in zircon dates in some samples is unexpected in this setting, and may be related to propagation of a younger ridge into older oceanic lithosphere.

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

2012-07-01

305

Tubular textures in pillow lavas from a Caledonian west Norwegian ophiolite: A combined TEM, LA-ICP-MS, and STXM study  

NASA Astrophysics Data System (ADS)

Tubular alteration textures, mineralized by titanite, in glassy rims of pillow lavas from a Norwegian ophiolite (Solund-Stavfjord ophiolite complex (SSOC)) are described and characterized by a multimethod approach. Tubular alteration textures, mineralized by titanite, have been previously proposed to result from bioalteration. The microstructure of the titanite and the tubes is investigated using focused ion beam milling in combination with transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM). These indicate an assemblage of submicrometer-sized (about 500 nm) titanite single crystals with no organic film or residue in between the grains. In situ U-Pb radiometric dating of the titanite, using laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS), yielded a metamorphic age of 442 ± 13 Ma. An isotope dilution-thermal ionization mass spectrometric age obtained previously for zircons from the SSOC plutonic rocks yielded a magmatic age of 443 ± 3 Ma. The overlap in ages indicates that subseafloor metamorphism, responsible for titanite formation, occurred during seafloor or subseafloor formation of the tubular alteration textures. The rare earth element contents of the titanite were determined using LA-ICP-MS and chondrite-normalized patterns are similar to those of the SSOC volcanics; hence these do not reflect hydrothermal or seawater influence. The Y/Ho ratio of ˜20-30 in the titanite is also consistent with an upper mantle-derived origin. The sum of all of the spatial resolved data reported here neither supports nor refutes a biogenic origin for the tubular textures.

Fliegel, Daniel; Wirth, Richard; Simonetti, Antonio; Schreiber, Anja; Furnes, Harald; Muehlenbachs, Karlis

2011-02-01

306

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

307

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

308

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

Microsoft Academic Search

The Coast Range Ophiolite (CRO) is tectonically dismembered and widely distributed in western California. We measured trace element, rare-earth element (REE), and fluid mobile element (FME) contents of pyroxenes in peridotites from four mantle sections from the CRO. This study utilized Laser Ablation- Inductively Coupled Plasma-Mass Spectrometry. The pyroxenes record magmatic processes characteristic of both mid-ocean ridges and supra-subduction zone

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

2008-01-01

309

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  

Microsoft Academic Search

The Balkan-Carpathian ophiolite (BCO), which outcrops in Bulgaria, Serbia and Romania, is a Late Precambrian (563Ma) 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;

Ivan Savov; Jeffrey G Ryan; Ivan Haydoutov; Johan Schijf

2001-01-01

310

Coupled evolution of back-arc and island arc-like mafic crust in the late-Neoproterozoic Agardagh Tes-Chem ophiolite, Central Asia: evidence from trace element and Sr-Nd-Pb isotope data  

Microsoft Academic Search

. We report major-element, trace-element and isotopic data of volcanic rocks from the late-Neoproterozoic (570 Ma) Agardagh Tes-Chem ophiolite in Central Asia, south-west of Lake Baikal (50.5°N, 95°E). The majority of samples are high-alumina basalts and basaltic andesites having island-arc affinities. They were derived from an evolved parental magma (Mg#̾.60, Cr~180 ppm, Ni~95 ppm) by predominantly clinopyroxene fractionation. The parental

Jörg A. Pfänder; Klaus Peter Jochum; Ivan Kozakov; Alfred Kröner; Wolfgang Todt

2002-01-01

311

Petrology and metamorphic evolution of ultramafic rocks and dolerite dykes of the Betic Ophiolitic Association (Mulhacén Complex, SE Spain): evidence of eo-Alpine subduction following an ocean-floor metasomatic process  

Microsoft Academic Search

The Betic Ophiolitic Association, cropping out within the Mulhacén Complex (Betic Cordilleras), is made up of numerous metre- to kilometre-sized lenses of mafic and\\/or ultramafic and meta-sedimentary rocks. Pre-Alpine oceanic metasomatism and metamorphism caused the first stage of serpentinization in the ultramafic sequence of this association, which is characterized by local clinopyroxene (Cpx) breakdown and Ca-depletion, and complementary rodingitization of

E. Puga; J. M Nieto; A D??az de Federico; J. L Bodinier; L Morten

1999-01-01

312

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

313

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

314

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

315

Mixing characteristics and compositional differences in mantle-derived melts beneath spreading axes: Evidence from cyclically layered rocks in the ophiolite of North Oman  

NASA Astrophysics Data System (ADS)

Within the ophiolite of north Oman, interlayered gabbros and peridotites form a unit up to 3.5 km thick between pervasively foliated harzburgites and dunites of mantle origin and isotropic gabbros and plagiogranites. Spatial and chemical relationships within this sequence require the existence of a large reservoir (20 km wide, 3.5 km high) of tholeiitic magma that was periodically replenished by mantle-derived picritic to tholeiitic melts during the spreading process. Cyclicity in the appearance of minerals during cooling of the magma is marked by the repetition of stratigraphies in the layered rocks. Within these cyclic units, basal peridotites with magnesian olivines and pyroxenes are abruptly overlain by gabbros with less magnesian mafic phases. This abrupt discontinuity is explained by density stratification set up at the base of the chamber whenever a picritic melt influx occurs. By virtue of its greater density than tholeiitic magma (Sparks et al., 1979) any picritic magma emplaced into the base of the tholeiitic magma reservoir would spread out along the floor of the chamber with only limited mixing taking place across the liquid-liquid interface. Cooling of this basal layer by contact with the cooler, overlying magma would result in the precipitation of olivine + chrome spinel ± pyroxene from the picritic liquid. This process would continue until the now more evolved input liquids were incorporated into the tholeiitic magma resevoir. The precipitation of a gabbroic layer would then take place until a new magma input occurred to repeat the cycle. Variable thicknesses of the cyclic units, different liquid lines of descent, and various entry points within the liquid line of descent represented by the lowermost layered rocks reveal that the input liquids varied in volume, primary composition, and the extent and nature of prefractionation in the mantle. Liquids expelled from the chamber as lavas and dykes are shown to be more fractionated than liquids precipitating the layered rocks and are considered to have undergone additional fractionation within a zone of underplated gabbro and plagiogranite beneath the dyke swarm. The existence of a large magma reservoir supports earlier interpretations based on age (Tilton et al., 1981) and geochemical data (Pearce, 1980) that the Oman ophiolite is an analogue for fast spreading ridges.

Smewing, John D.

1981-04-01

316

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

317

Relationships between geochemistry and structure beneath a palaeo-spreading centre: a study of the mantle section in the Oman ophiolite  

NASA Astrophysics Data System (ADS)

The Oman ophiolite exposes a large and well-preserved mantle section beneath a palaeo-spreading centre. The mantle section is mainly composed of extremely refractory harzburgites with relatively homogeneous modal and major element compositions. Nevertheless, our trace element data exhibit variations connected with the main mantle structures, which allow us to define three geochemical and structural domains. The main harzburgitic mantle section, mainly constituted of strongly refractory harzburgites characterised by chondrite-normalised REE patterns that are steadily depleted from HREE to LREE. These rocks are interpreted as mantle residues after >15% melt extraction. Their REE signature can be explained by melt transport associated with partial melting. The diapir areas (mainly the Maqsad diapir), defined by plunging lineations. They are constituted of harzburgites with roughly the same modal composition as the main mantle section but distinct, concave-upward REE patterns. The regions of most active upwelling (characterised by sub-vertical lineations) are further distinguished by higher Al 2O 3/CaO ratios and TiO 2 contents. This character is ascribed to focused partial melt upwelling. The diapirs are interpreted as local instabilities in upwelling mantle, possibly triggered by feedback mechanisms between deformation and melt percolation. The Maqsad diapir is topped by a thick, dunitic, mantle-crust transition zone (MTZ) that displays the same trace-element signature as the diapir. However, the dunites are distinguished by low Mg# values and Ni contents. Together with structural evidence, this allows us to interpret the MTZ dunites as diapir harzburgites that were strongly modified by olivine-forming melt-rock reactions at high melt/rock ratios. The MTZ is thought to act as a major collecting zone for mantle melts. The cpx-harzburgites from the base of the mantle section. These rocks are distinguished by high clinopyroxene contents (>5%), low AL 2O 3/CaO and 'spoon-shaped' REE patterns. They were individualised from the rest of the harzburgite mantle section by a cpx-forming melt-rock reaction at decreasing malt mass. This reaction probably occurred at near-solidus conditions along the lithosphere-asthenosphere boundary. The formation of these three domains may be integrated in a geodynamic scenario involving the reactivation of an oceanic lithosphere, a process that would be related to the ridge propagator identified in the Oman ophiolite.

Godard, Marguerite; Jousselin, David; Bodinier, Jean-Louis

2000-07-01

318

Radioactivity and distribution of U and Th in some granitic masses, wadi El-Saqia area, central eastern desert, Egypt  

Microsoft Academic Search

Radioactivity measurements and U and Th content determinations were carried out on 3 small granitic plutons, Gabal Abu Aqarib, Gabal El-Himeiyer and Gabal Um Zarabit, in the Central Eastern Desert. The Abu Aqarib and Um Zarabit alkali feldspar granites are elenogated bodies intruded into the Dokhan Volcanics, whereas El-Himeiyir is intruded into an ophiolitic melange section. Compared to the average

A. A. Abdel-Monem; H. A. Hussein; Z. M. Abdel-Kader; H. T. Abu Zied; S. E. Ammar

1996-01-01

319

Radioactivity and distribution of U and Th in some granitic masses, Wadi El-Saqia Area, Central Eastern Desert, Egypt  

Microsoft Academic Search

Radioactivity measurements and U and Th content determinations were carried out on 3 small granitic plutons, Gabal Abu Aqarib, Gabal El-Himeiyer and Gabal Um Zarabit, in the Central Eastern Desert. The Abu Aqarib and Um Zarabit alkali feldspar granites are elenogated bodies intruded into the Dokhan Volcanics, whereas El-Himeiyir is intruded into an ophiolitic melange section. Compared to the average

A. A. Abdel-Monem; H. A. Hussein; Z. M. Abdel-Kader; H. T. Abu Zied; S. E. Ammar

1996-01-01

320

NEW DATA ON THE UPPER AGE OF THE INTRA-PONTIDE OCEAN FROM NORTH OF ?ARKÖY (THRACE)  

Microsoft Academic Search

Upper Eocene elastics in the southern part of the Thrace Basin north of ?arköy contain large number of olisto- liths. The olistoliths are largely derived from the ophiolitic melange that forms the basement of the Eocene sedimentary se- quence in this region. They comprise serpantinite, metadiabase, radiolarian chert and pelagic limestone. Some of the pelagic limestone blocks contain abundant Late

Aral I. OKAY

321

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

2013-09-01

322

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

NASA Astrophysics Data System (ADS)

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

Lü, Zeng; Bucher, Kurt; Zhang, Lifei

2013-10-01

323

Variation of geochemical risk associated with the use of ophiolitic washing mud as refilling material in a basalt quarry of the Northern Apennine (Italy)  

NASA Astrophysics Data System (ADS)

Ophiolitic sequences in Northern Apennines are usually exploited as source of raw material for civil engineering works. Grinding procedures of basalts imply the production of dusts with relatively high concentration of PHES. This paper studied the increase of geochemical risk when washing mud produced at Sasso di Castro quarry site (Tuscany) is reused as rock keeper in a near dismissed quarry and highlighted geochemical fractionation produced on the base of different mineral hardness. Co, Cr, Ni and V concentration measured in washing mud were higher than the limits fixed by the Italian law but compatible with background values. The mobility of these four elements during future weathering processes were estimated by considering the element transfer coefficients and assuming weathered rocks and soils as two different natural analogues of the future state of washing mud. The future concentration was estimated by considering the average lifetime of mineral grains calculated through their dissolution rate, molar volume and grain diameter. The variations of geochemical concentrations were used to estimate the percentage increase of the geochemical risk at the displacement place. After 50 years the associated geochemical risk is still considerably lower than the probability to be damaged by a single landslide event.

Voltaggio, M.; Spadoni, M.

2007-10-01

324

Mantle podiform chromitites do not form beneath mid-ocean ridges: A case study from the Moho transition zone of the Oman ophiolite  

NASA Astrophysics Data System (ADS)

Podiform chromitites from the mantle transition zone at Maqsad in the Oman ophiolite have MORB-like cr# (0.48–0.59). However they show other features which are not in keeping with a MORB-origin: some parental melts have TiO2 values lower than typical MORB, they have a higher Fe3 +/?Fe ratio and a wider range of Fe3 +/?Fe ratios than might be expected in MORB and their parental melts have a higher fO2 than expected for MORB. The chromitites also contain silicate melt inclusions which are hydrous and atypical of MORB and inclusions of higher cr# chromites (cr# = 0.55–0.71). These inclusions are derived from a more depleted harzburgite source than that of the harzburgites which now host the Maqsad chromitites. In addition associated MORB-like lavas display geochemical traits which are atypical of MORB. It is proposed therefore that the Maqsad chromitites were derived by mixing from parental melts which were produced by the interaction of a MORB-like melt and depleted mantle. The MORB-like melt was produced by the hydrous melting of asthenospheric mantle above a subducting slab. The trigger for chromite to appear on the liquidus of the melt was the reaction between the primary melt and depleted harzburgite, illustrating the more general relationship in which chromite appears on the liquidus in mafic and ultramafic melts as a consequence of some form of mixing process.

Rollinson, Hugh; Adetunji, Jacob

2013-09-01

325

Development of a brine-dominated hydrothermal system at temperatures of 400-500°C in the upper level plutonic sequence, Troodos ophiolite, Cyprus  

NASA Astrophysics Data System (ADS)

Microthermometric analyses of quartz-hosted fluid inclusions in fresh and epidotized plagiogranites from the Troodos ophiolite, Cyprus, indicate that brine-rich fluids (46-56 wt% NaCl equivalent) at temperatures >400-500°C were pervasive throughout many of the plagiogranite bodies. High-salinity, Fe-bearing inclusions along healed microfractures homogenize by halite dissolution at temperatures of 400-500°C. Liquid-dominated, low-salinity (2-7 wt% NaCl) secondary fluid inclusions are ubiquitous in all plagiogranite and gabbro samples studied. Homogenization of the low-salinity inclusions occurs at uncorrected temperatures of 200-400°C. The quartz-hosted, high-salinity fluids represent either phase separation of hydrothermal seawater or an exsolved magmatic aqueous phase at uncorrected temperatures of 400-500°C. Migration and segregation of brine and vapor phases into fractures near the margins of the crystallizing plagiogranite bodies resulted in preferential entrapment of the Fe-rich brines in the deep-seated, high-temperature portions of the hydrothermal system. Subsequent fracturing of the upper plutonic rocks at temperatures of 200-400°C allowed penetration by seawater, which pervasively altered the plagiogranites. Hydration reactions involving formation of secondary minerals and/or mixing with phase-separated fluids resulted in fluid salinities two times that of seawater.

Kelley, Deborah S.; Robinson, Paul T.

1990-03-01

326

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).

327

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

328

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).

329

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).

330

Trace- and major-element zoning in garnets from eclogites of the Zermatt-Saas ophiolite, western Alps: LA-ICP-MS data and implications for Lu-Hf geochronology  

NASA Astrophysics Data System (ADS)

Trace element contents, inclusion assemblages, and element zoning patterns found in garnets from several eclogite-facies metabasalts sampled across the Zermatt-Saas ophiolite complex, western Alps, record large segments of the prograde P-T-t path to HP/UHP conditions. Ca, Fe, Mn, and Mg contents in garnet from core to rim are indicative of prograde growth zoning. Laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) on garnets cut to expose the core using 3-D x-ray tomography indicates strong HREE (Er, Yb, Lu) zoning, where the highest concentrations occur in the garnet core. The core-to-rim zoning of HREE is very similar to that of Mn determined by both electron microprobe and LA-ICP-MS along the same transect. The HREE and Mn patterns tend to be bell-shaped and have a moderate spike in concentrations toward the rim that is followed by an increase in Gd concentrations. These complex zoning patterns suggest that a reaction or multiple reactions took place that liberated HREE and possibly MREE as well during growth of garnet. Hf concentrations, estimated from Zr concentrations measured by LA-ICP-MS, are typically constant across garnet traverses. The Lu and Hf garnet zoning patterns have important implications for Lu-Hf geochronology because they indicate that the Lu-Hf ratios are highest in the core of the garnet. The high Lu-Hf ratio in garnet cores results in a measured Lu-Hf age that is strongly skewed toward the early prograde growth, if bulk garnet separates are used for geochronology for units such as the Zermatt-Saas ophiolite that were not metamorphosed to temperatures above the Lu-Hf blocking temperature. Sm-Nd and Lu-Hf geochronology using bulk garnet separates suggests that the duration of prograde metamorphism of the Zermatt-Saas ophiolite in the Lago di Cignana area, Italy, is between 50-38 Ma (Amato et al., 1999; Lapen et al., 2003). An eclogite sample collected from the Pfulwe area, Switzerland, yielded a bulk garnet-cpx-whole rock Lu-Hf age of 54 ± 3 Ma (2-sigma). Based on our Lu and Hf zoning data, we interpret the age from the Pfulwe area as representing a stage in its P-T-t history that precedes attainment of peak metamorphic conditions. The range in ages between Cignana and Pfulwe may record the different partitioning behavior of Lu, Hf, Sm, and Nd in specific samples or the diachronous subduction of the ophiolite sheet.

Mahlen, N. J.; Skora, S.; Johnson, C. M.; Baumgartner, L. P.; Lapen, T. J.; Pilet, S.; Beard, B. L.

2004-12-01

331

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

332

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

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

Origin of titanian pargasite in gabbroic rocks from the Northern Apennine ophiolites (Italy): insights into the late-magmatic evolution of a MOR-type intrusive sequence  

NASA Astrophysics Data System (ADS)

Gabbroic rocks from Northern Apennine ophiolites contain accessory titanian pargasite, in interstices between plagioclase and clinopyroxene, and as rims around interstitial Fe-Ti-oxide phases. The origin of titanian pargasite has been evaluated by combining major, volatile and trace element microanalyses. Titanian pargasites show variable amounts of F and low Cl (0.03-0.23 and ?0.03 wt%, respectively), and mg# value ranging from 0.78 to 0.70 and from 0.61 to 0.53 in Mg- and Fe-rich rocks, respectively. Geothermometric calculations based on amphibole-plagioclase equilibrium yield temperatures of 900±50°C and 840±50°C for Mg- and Fe-rich rocks, respectively. Titanian pargasites are characterized by LREE depletion, nearly flat HREE and variable negative Eu anomalies. Total REE contents in titanian pargasites are higher and lower than in associated clinopyroxenes and apatites, respectively. In the chondrite-normalized patterns of titanian pargasites, Ba, K and Sr are markedly depleted relative to LREE, whereas Nb, Zr and Ti are slightly enriched to slightly depleted relative to neighboring REE. A separate of titanian pargasite from a Fe-rich gabbroic rock was analyzed for Sr isotopic composition; its initial 87Sr/ 86Sr falls within the range of modern N-MORB and is consistent with those of fresh Mg-rich gabbroic rocks. Titanian pargasite most likely formed by an igneous liquid with relatively high H 2O content (4.4-6.1 wt%) and a slight LREE enrichment. The origin of this liquid has been ascribed to the percolation in the gabbroic crystal mush of a H 2O-rich agent of igneous origin, WHICH could be a trondhjemite liquid or an exsolved fluid. Probably, such interaction triggered a post-cumulus crystallization process that finally yielded the precipitation of titanian pargasite.

Tribuzio, Riccardo; Tiepolo, Massimo; Thirlwall, Matthew F.

2000-03-01

335

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

336

A neodymium, strontium, and oxygen isotopic study of the Cretaceous Samail ophiolite and implications for the petrogenesis and seawater-hydrothermal alteration of oceanic crust  

NASA Astrophysics Data System (ADS)

In the Samail ophiolite, 147Sm-143Nd, 87Rb-87Sr, and 18O/16O isotopic systems have been used to distinguish between sea-floor hydrothermal alteration and primary magmatic isotopic variations. The Rb-Sr and 18O/16O isotopic systems clearly exhibit sensitivity to hydrothermal interactions with seawater while the Sm-Nd system appears essentially undisturbed. Internal isochrons have been determined by the 147Sm-143Nd method using coexisting plagioclase and pyroxene and give crystallization ages of 130 +/- 12 m.y. from Ibra and 100 +/- 20 m.y. from Wadi Fizh. These ages are interpreted as the time of formation of the Samail oceanic crust and are older than the inferred emplacement age of 65-85 m.y. The initial 143Nd/144Nd ratios for a tectonized harzburgite, cumulate gabbros, plagiogranite, sheeted dikes and a basalt have a limited range in ?Nd of from 7.5 to 8.6 for all lithologies, demonstrating a clear oceanic affinity and supporting earlier interpretations based on geologic observations and geochemistry. The 87Sr/86Sr initial ratios on the same rocks have an extremely large range of from 0.70296 to 0.70650 (?Sr = -19.7 to +30.5) and the ? 18O values vary from 2.6 to 12.7. These large variations are clearly consistent with hydrothermal interaction of seawater with the oceanic crust. A model is presented for the closed system exchange of Sr and O, that in principle illustrates how the Sr isotopic data may be utilized to estimate the water/rock ratio and subsequently used to evaluate the temperature of equilibration between the water and silicates from the 18O/16O water-rock fractionation.

McCulloch, Malcolm T.; Gregory, Robert T.; Wasserburg, G. J.; Taylor, Hugh P.

1980-01-01

337

Contrasting rift and subduction-related plagiogranites in the Jinshajiang ophiolitic mélange, southwest China, and implications for the Paleo-Tethys  

NASA Astrophysics Data System (ADS)

The Jinshajiang ophiolitic mélange zone in southwest China represents a remnant of the eastern Paleo-Tethys Ocean. Field, geochronological and geochemical studies have identified two distinct suites of plagiogranites within the mélange, the Dongzhulin trondhjemite and Jiyidu tonalite, which represent rift and subduction settings, respectively, related to opening and closing of the ocean. SHRIMP U-Pb analysis on zircons extracted from the Dongzhulin trondhjemite yields a mean 206Pb/238U age of 347 ± 7 Ma. REE and isotopic characteristics suggest an origin from low pressure partial melting of an amphibolitic protolith. Highly variable Hf isotopic compositions for zircons from this body may indicate a heterogenous source involving both depleted mantle and enriched continental components. This, together with geologic relations, suggests formation near an embryonic spreading center in a continent-ocean transition setting. The Jiyidu tonalite has a U-Pb zircon age of 283 ± 3 Ma, and geochemical data indicates high Sr/Y, (La/Yb)N, Nb/Ta and low Y, and marked heavy REE depletion. These signatures suggest derivation from low degree partial melting of subducted slab at pressure high enough to stabilize garnet and rutile. A slab-melt origin is also supported by in situ Hf and O data for zircon that show isotopic compositions comparable with typical altered oceanic crust. Thus, the crystallization age of the Jiyidu high Sr/Y tonalite provides a constraint for the subduction of the Jinshajiang ocean floor. The rift-related Dongzhulin trondhjemite and subduction-related Jiyidu high-Sr/Y tonalite constrain the timing and setting of opening and closing of this segment of the Paleo-Tethys Ocean.

Zi, Jian-Wei; Cawood, Peter A.; Fan, Wei-Ming; Wang, Yue-Jun; Tohver, Eric

2012-04-01

338

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

339

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

NASA Astrophysics Data System (ADS)

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

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

2012-02-01

340

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

NASA Astrophysics Data System (ADS)

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

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

2004-12-01

341

Late Triassic subduction-related ultramafic-mafic magmatism in the Amasya region (eastern Pontides, N. Turkey): Implications for the ophiolite conundrum in Eastern Mediterranean  

NASA Astrophysics Data System (ADS)

The eastern Pontides orogenic belt of northeastern Turkey offers critical clues on convergent margin tectonics associated with the late Mesozoic-early Cenozoic geodynamic evolution of the eastern Mediterranean region. Here we report the geology, petrology, geochemistry, Nd-Sr-Pb isotopes and U-Pb zircon ages from a ultramafic-mafic complex in Aksalur which forms part of a series of Alaskan-type ultramafic-mafic bodies in the Amasya region located at the south-western corner of the eastern Pontides orogenic belt. These ultramafic-mafic intrusions occur as minor (30-300 m in diameter), scattered, roughly circular bodies within the low-grade metamorphic rocks of the Tokat metamorphic massif. Petrographic studies characterise the rock types as gabbro and wehrlite, with the latter showing typical cumulate texture and mainly comprising serpentinized olivine, clinopyroxene, hornblende, mica and opaque minerals. The gabbroic rocks are characterized by relatively high initial ratios of 143Nd/144Nd (0.512618-0.512633) and low initial ratios of 87Sr/86Sr (0.70420-0.70435). Their initial Pb isotope ratios range from 17.306 to 17.771. The gabbros and cumulate wehrlites display geochemical distribution patterns analogous to the Alaskan-type intrusions that were derived from a subduction-related, high-alumina, hydrous basaltic parental magma in a convergent plate setting. LA-ICPMS analyses of zircons separated from the gabbros show high Th/U ratios (1.35-5.26) typical of magmatic origin. The U-Pb isotopic data define a weighted mean 206Pb/238U age of 203.3 ± 4.9 Ma. Detailed evaluation of the magma tectonics suggests that the Alaskan-type Aksalur ultramafic-mafic intrusions were accumulated in magma chamber(s) located at relatively shallow crustal levels. Two distinct magmatic pulses are identified: the first phase produced the gabbroic intrusions and the second phase was characterized by numerous small-scale werhlitic cumulate intrusions. In contrast to the previous models which assigned a mid-oceanic ridge-derived ophiolite affinity for ultramafic-mafic complexes in the eastern Pontides orogenic belt, our study reveals that these intrusions were derived from subduction-related high-alumina hydrous basaltic magmas transferred into the overlying relatively thin continental crust along deep-seated fractures.

Eyuboglu, Yener; Santosh, M.; Bektas, Osman; Chung, Sun-Lin

2011-08-01

342

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

USGS Publications Warehouse

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

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

1997-01-01

343

Samail Ophiolite plutonic suite: Field relations, phase variation, cryptic variation and layering, and a model of a spreading ridge magma chamber  

NASA Astrophysics Data System (ADS)

Geologic mapping of an intact plutonic sequence within the Samail ophiolite in the Ibra area, southeastern Oman Mountains, reveals stratigraphic, structural, and petrologic details of oceanic layer 3. Four measured stratigraphic sections, each spaced about 5 km apart across the southern flank of Jabal Dimh, define a time-transgressive progression within the ophiolite and reveal geometric and petrologic features of a spreading-ridge magma chamber. The sections show the following vertical sequence: (1) dunite (chr-ol cumulates ± harzburgite xenoliths) 0-200 m thick, grading up from a transition zone with harzburgite tectonite, (2) interlayered wehrlite-melagabbro-gabbro (cpx-ol and ol-cpx-pl cumulates) 0-100 m thick, (3) layered gabbro (chiefly ol-cpx-pl cumulates but including recurrent intervals of cumulus wehrlite and melagabbro) 2.6-5.5 km thick, (4) planar laminated nonlayered gabbro (chiefly ol-cpx-pl cumulates) 100-400 m thick, (5) hypidiomorphic (ol)-hb-cpx gabbro (high-level gabbro) 200-800 m thick, (6) small, discontinous diorite to plagiogranite bodies at or near the top of the gabbro. Cumulus textures (adcumulus > mesocumulus), planar lamination, and cumulus layering (phase, ratio, and grain size layers at mm to 10-m scale, commonly graded) within this sequence show that crystals accumulated from the base of the magma chamber upward to within a few hundred meters of the top; downward solidification from the roof was minor. Cyclicity within the cumulus sequence is represented by the recurrence of olivine-rich intervals (melagabbo and wehrlite) up to high stratigraphic levels and by hundreds of phase-graded layers (ol-rich at the bases to pl-rich at the tops), individually up to 5 m thick. Limited cryptic variation relative to closed-system layered intrusions and the limited range in solid-solution components of olivine (Fo69-90), plagioclase (An62-95) and clinopyroxene (En40-54, Fs4-16, Wo37-49) from the cumulus suite require replenishment of the magma during its crystallization history. Zig-zag normal and reverse cryptic variation differentiation trends (both in major solid-solution components and in minor element concentration) indicate that the replenishment took place in pulses followed by periods of magma mixing and crystal fractionation. Recurrent olivine-rich intervals commonly (but not always) coincide with reverse cryptic variation trends. They are considered to be the products of fresh draughts of primitive magma, with olivine and chromite as the only liquidus phases prior to extensive mixing with the more fractionated resident magma within the chamber. The sequence of crystallization is explained using the simplified tholeiitic basalt tetrahedron of Presnall et al. (1979). The plutonic sequence is roofed by sheeted dikes that are overlain by submarine basalt, indicating that the magma crystallized beneath a spreading ocean ridge. Major and trace element geochemistry of the dike complex is similar in many respects to that of mid-ocean ridge basalt (MORB) and yield Mg/Mg + Fe+2 ratios that overlap with ratios predicted for parent liquids of the cumulus suite based on crystal/liquid equilibria. Field evidence, including continuity of layering over large areas, distinctive layer sets that are mapped for several km along strike, and lack of chamber edge contacts indicates that crystallization of Jabal Dimh plutonics took place in a single, large, long-lived magma chamber rather than in small transitory chambers. Similarity of mineral compositions and tentative correlation of cryptic variation trends between stratigraphic sections favors a large, single-chamber model. The chamber shape, controlled by floor growth upward greatly exceeding roof growth downward as the chamber halves diverged from the paleo-spreading axis, must have been funnel-shaped in cross section; i.e., the floor sloped inward to the center from a sandwich horizon that developed just beneath the roof at both sides. This conclusion is based on the occurrence of cumulates up to a very high level in the stratigraphic sections and on geol

Pallister, J. S.; Hopson, C. A.

1981-04-01

344

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

NASA Astrophysics Data System (ADS)

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

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

2008-12-01

345

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 garn