Origin of the troodos and other ophiolites: A reply to hynes

Geologic and tectonic evidence on the origin of the Troodos ophiolitic complex is ambiguous, being compatible with its creation in an island arc or a continent as well as in a mid-oceanic ridge. However, there is decisive petrologic and chemical evidence against its origin in a mid-oceanic ridge (and in a marginal sea). A large proportion (about half) of volcanic rocks in the sheeted complex and lower pillow lavas of Troodos have SiO₂ > 52.5% and FeO*/MgO > 2.0 in contrast to mid-oceanic ridge volcanics which are almost entirely basic (SiO₂ < 52.5%) with FeO*/MgO < 2.0. (FeO* means total iron as FeO.) A considerable proportion of volcanic rocks in Troodos belong to the calc-alkalic series. These facts indicate that the Troodos massif was formed probably in an island arc or a continental region.In order to rebut the above chemical evidence, Hynes resorts to an assumption of thorough compositional change of the Troodos volcanic rocks by metasomatism. However, this assumption is not plausible.     

An ophiolitic tectonic melange first discovered in Huashan area, south margin of Qinling Orogenic Belt, and its tectonic implications

On the basis of the synthetic studies of geology and geochemistry, an ophiolitic tectonic melange waa discovered in Sanligang-Sanyang area, the western part of Xiangfan-Guangji fault, the south margin of the Qinling Orogenic Belt. It is composed of different tectonic blocks with different lithological features and ages, mainly including the Huashan ophiolite blocks, Xiaofu Island-arc volcanic blocks, pelagic sediments, fore-arc volcanic-sedimentary system, and the massif of the basement and the covering strata of the Yangtze Block. These massifs were emplaced in the western part of Xiangfan-Guangji fault, the boundary between the Qinling Orogenic Belt and Yangtze Block, contacting each other by a shear zone or chaotic matrix. The characteristics of geochemistry indicate that the bash of the Huashan ophiolite are similar to mid-oceanic ridge basalts (MORB) formed in an initial oceanic baain, and the Xiaofu volcanic rocks are formed in a tectonic setting of island arc. The ophiolitic tectonic melange is the fragments of subduction wedge, which implies that there has been an oceanic basin between Qinling Block and Yangtze Block. Project supported by the National Natural Science Foundation of China (Grant Nos. 49773187, 49732080)     

Origin of the troodos and other ophiolites: A reply to moores

Advocates of the hypothesis of mid-oceanic ridge origin for ophiolites endeavored to show similarities between ophiolites and mid-oceanic ridges as evidence for their hypothesis. Some of the similarities claimed to exist appear to be true. However, this does not prove their hypothesis, because analogous similarities exist between ophiolites and some island arcs as well. Such advocates claim that rocks of the Troodos ophiolitic complex were subjected to such intense metasomatic changes that my hypothesis of island-arc origin for Troodos based on the bulk chemical analyses of rocks is not justified. However, a detailed examination of compositional variation in Troodos volcanic rocks has revealed that, though some components (e.g., K) were strongly mobile during secondary processes, other components were not, and the presently observed compositional variation resulted mainly from crystallization differentiation. Their assumption of large-scale silica metasomatism for Troodos cannot explain the compositional features of rocks and is contradictory to the hypothesis of mid-oceanic ridge origin, because such metasomatism does not appear to take place in mid-oceanic ridges.     

Geology of the southern Zambales Ophiolite Complex, Luzon, Philippines

Abstract Field, geochemical and geophysical evidence show that the southern Zambales Ophiolite Complex attained its present-day configuration through the juxtapositioning of an arc terrane (San Antonio massif) to a back-arc crust (Cabangan massif). The San Antonio massif manifests island arc-related characteristics (i.e. spinel XCr [Cr/(Cr + Al)] >0.60; mostly plagioclase An92–95; pyroxene crystallizing ahead of plagioclase; orthopyroxene as an early, major crystallizing phase) which cannot be directly parental to the Cabangan massif transitional mid-ocean ridge basalt to island arc tholeiitic volcanic carapace. The two massifs are believed to be separated by a left-lateral strike–slip fault, the Subic Bay Fault Zone. Apart from the presence of highly sheared, allochthonous outcrops, the Subic Bay Fault Zone is generally defined by northwest–southeast trending magnetic and bouguer anomalies. The San Antonio massif was translated southward from the northern part of the Zambales Ophiolite Complex through the Subic Bay Fault Zone. This resulted into its suturing with the Cabangan massif and could have led to the formation of the present-day Subic Bay.     

Is the Vourinos Complex an island arc ophiolite?

A geochemical study has been undertaken on the Vourinos ophiolites, northern Greece, a complex long known for its unusual characteristics such as an environment of acidic rocks and a calc-alkaline chemical affinity. The Nd-Sr isotopic ratios and the Hf/Th and Ta/Th ratios are indicative of an island arc origin for Vourinos as opposed to the mid-oceanic ridge origin inferred for other ophiolites such as Inzecca, Corsica. Other data on trace elements confirm that the cumulative suite and the lavas originated from the same magma through a simple fractional crystallization process and show that this magma would have formed through partial melting of an already highly-depleted material. It is thus possible to distinguish ophiolites with MORB characteristics from island arc ophiolites such as the Vourinos Complex, the existence of the latter type imposing new constraints on the possible tectonic processes for emplacement.     

Ophiolites of the Eastern Peninsulas zone (Eastern Kamchatka): Age, composition, and geodynamic diversity

Abstract   The geological, geochemical and mineralogical data of dismembered ophiolites of various ages and genesis occurring in accretionary piles of the Eastern Peninsulas of Kamchatka enables us to discriminate three ophiolite complexes: (i) Aptian–Cenomanian complex: a fragment of ancient oceanic crust, composed of tholeiite basalts, pelagic sediments, and gabbroic rocks, presently occurring in a single tectonic slices (Afrika complex) and in olistoplaques in Pikezh complex of the Kamchatsky Mys Peninsula and probably in the mélange of the Kronotsky Peninsula; (ii) Upper Cretaceous complex, composed of highly depleted peridotite, gabbro and plagiogranite, associated with island arc tholeiite, boninite, and high-alumina tholeiitic basalt of supra-subduction origin; and (iii) Paleocene–Early Eocene complex of intra-island arc or back-arc origin, composed of gabbros, dolerites (sheeted dykes) and basalts produced from oceanic tholeiite melts, and back-arc basin-like dolerites. Formation of the various ophiolite complexes is related to the Kronotskaya intra-oceanic volcanic arc evolution. The first ophiolite complex is a fragment of ancient Aptian–Cenomanian oceanic crust on which the Kronotskaya arc originated. Ophiolites of the supra-subduction zone affinity were formed as a result of repeated partial melting of peridotites in the mantle wedge up to the subduction zone. This is accompanied by production of tholeiite basalts and boninites in the Kamchatsky Mys segment and plagioclase-bearing tholeiites in the Kronotsky segment of the Kronotskaya paleoarc. The ophiolite complex with intra-arc and mid-oceanic ridge basalt geochemical characteristics was formed in an extension regime during the last stage of Kronotskaya volcanic arc evolution.     

Geochemistry and tectonic implications of Proterozoic amphibolites in the northeastern part of the Yeongnam massif, South Korea

Abstract   Amphibolites in the Haenggongni area (Haenggongni amphibolite) and the Okbang area (Okbang amphibolite) in northeastern Yeongnam massif, South Korea occur as a sill-like body or inclusions within the metasedimentary sequences of the Proterozoic Wonnam Group. Major and trace element characteristics demonstrate that both amphibolites have tholeiitic chemical affinity. They are characterized by nearly flat rare earth element (REE) patterns, and low contents of immobile incompatible elements and have low values of Zr/Y, Ti/Y, La/Nb and Ta/Yb ratios, indicating enriched (E)-type mid-oceanic ridge basalt (MORB) affinities for their protoliths. This suggests that amphibolite protoliths formed in an extensional rift setting leading up to ocean opening. In combination with the previous studies in Yeongnam massif, three protolith types of amphibolites are assumed (E-type MORB, within-plate basalt and volcanic arc basalt). They could have been originated in different tectonic settings and/or different episodes. These characteristics are clearly different from the amphibolites in the Gyeonggi massif and Okcheon belt, in which most of the amphibolites show a within-plate basalt affinity that developed in continental rift zone.     

Pacific-type orogeny revisited: Miyashiro-type orogeny proposed

Abstract The concept of Pacific-type orogeny is revised, based on an assessment of geologic data collected from the Japanese Islands during the past 25 years. The formation of a passive continental margin after the birth of the Pacific Ocean at 600 Ma was followed by the initiation of oceanic plate subduction at 450 Ma. Since then, four episodes of Pacific-type orogeny have occurred to create an orogenic belt 400 km wide that gradually grew both oceanward and downward. The orogenic belt consists mainly of an accretionary complex tectonically interlayered with thin (<2 km thick), subhorizontal, high-P/T regional metamorphic belts. Both the accretionary complex and the high-P/T rocks were intruded by granitoids ～100 million years after the formation of the accretionary complex. The intrusion of calc-alkaline (CA) plutons was synchronous with the exhumation of high-P/T schist belts. Ages from microfossils and K-Ar analysis suggest that the orogenic climax happened at a time of mid-oceanic ridge subduction. The orogenic climax was characterized by the formation of major subhorizontal orogenic structures, the exhumation of high-P/T schist belts by wedge extrusion and subsequent domed uplift, and the intrusion-extrusion of CA magma dominantly produced by slab melting. The orogenic climax ended soon after ridge subduction, and thereafter a new Pacific-type orogeny began. A single Pacific-type orogenic cycle may correspond to the interaction of the Asian continental margin with one major Pacific oceanic plate. Ophiolites in Japan occur as accreted material and are not of island-arc but of plume origin. They presumably formed after the birth of the southern Pacific superplume at 600 Ma, and did not modify the cordilleran-type orogeny in a major way. Microplates, fore-arc slivers, intra-oceanic arc collisions and the opening of back-arc basins clearly contributed to cordilleran orogenesis. However, they were of secondary importance and served only to modify pre-existing major orogenic components. The most important cause of cordilleran-type orogeny is the subduction of a mid-oceanic ridge, by which the volume of continental crust increases through the transfer of granitic melt from the subducting oceanic crust to an orogenic welt. Accretionary complexes are composed mainly of recycled granitic sediments with minor amounts of oceanic material, which indicate that the accretion of oceanic material, including huge oceanic plateaus, was not significant for orogenic growth. Instead, the formation and intrusion of granitoids are the keys to continental growth, which is the most important process in Pacific-type orogeny. Collision-type orogeny does not increase the volume of continental crust. The name ‘Miyashiro-type orogeny’ is proposed for this revised concept of Pacific-type or cordilleran-type orogeny, in order to commemorate Professor A. Miyashiro's many contributions to a better understanding of orogenesis.     

Geochemistry of basaltic and gabbroic rocks from the West Mariana basin and the Mariana trench

This paper describes the chemistry of 33 basaltic rocks dredged from the West Mariana basin and from the Mariana trench during the R/V “Dmitry Mendeleev” 1976 cruise in the western Pacific.The shipboard investigations were carried out by an international working group of 66 earth scientists under the IGCP Project “Ophiolites” and sponsored by the U.S.S.R. Academy of Sciences, Moscow. The purpose of the expedition was to investigate the structure and composition of the oceanic crust of marginal basins, remnant island arcs and deep-sea trenches. Tholeiitic basalts and gabbros as well as ultramafic rocks in various stages of alteration were dredged from the central part of the West Mariana basin demonstrating the presence of oceanic crust.The Pacific slope of the Mariana trench yielded altered basaltic rocks of tholeiitic and alkalic (? trachybasaltic to shoshonitic) composition. The lower part of the island arc slope contains typical tholeiitic basalts, dolerites and gabbros as well as ultramafics associated with flysch-type sediments. This is strong evidence for the formation of an “ophiolite-schuppenzone”, probably due to subduction of Pacific oceanic crust.Associated with these rocks are amygdaloidal, highly magnesian lavas (similar to boninites), which have not been recognized previously in oceanic ridge basalts.These rocks (together with the dolerites) are interpreted as parts of the Mariana island arc and are thought to be the first stage of island arc development (an immature island arc).     

Geochemical characteristics of Bikou volcanic group and Sr-Nd-Pb isotopic composition: Evidence for breakup event in the north margin of Yangtze plate,Jining era

The geodynamic setting of the Bikou volcanic group is a critical question to trace the Precambrain tectonic framework and evolution for the Yangtze plate. This study has suggested that the Bikou volcanic group is composed of several residual oceanic crust units: MORB (mid-ocean ridge basalt), Alk-OIB (alkaline ocean island basalt) and Th-OIB (tholeiitic ocean island basalt) as well as subduction-related volcanic rocks. According to field observation, those distinct rocks occurred collectively in form of tectonic contact, implying that the Bikou volcanic group was an ophiolitic mélange. Coupled with geochronological data, a perished oceanic basin at the northern margin of the Yangtze block during Neoproterozoic was tested by this ophiolitic mélange. Meanwhile, the isogeochemical data suggest that the ocean occurred in the Southern Hemisphere identical to Indian, South Atlantic and South Pacific oceans in terms of their Dupal anomalies, and the original source of the rocks could be probably mixing by EMI and EMII component caused by dehydration melting of subducting oceanic crust during subduction process. On the basis of geochemical characteristics of the studied rocks, the Bikou volcanic group could imply that a partial breakup event occurred in the northern margin of Yangtze plate during the Neoproterozoic era.

     

Forearc deformation and tectonic significance of the ultramafic Molucca central ridge, Talaud islands (Indonesia)

Abstract The Molucca Sea is a narrow basin located south of Mindanao (Philippines) and underlined by a north-south ophiolitic ridge. This ridge represents the outer ridge of the Sangihe subduction zone and emerges by uplift in the central part of the basin, in the Talaud Islands. Field studies indicate that forearc sediments rest uncomformably on (i) a dismembered ophiolitic series and (ii) thick melanges. Structural analysis indicates two deformation events, one of which is oriented east-west coaxial with the present state of strain. We interpret the earlier (N20°E) direction as a thrusting event that affected an ophiolitic basement associated with the edge of the Celebes Sea. Thrusting within the oceanic crust and sediments also generated olistostromes (melanges). The style of deformation is characterized by flattened rhombs of peridotites which exists in situ in the upper section of the crustal sequence and were also found inside the melange. Incipient Sangihe subduction around 15 Ma uplifted the deformed crust and buried the melanges beneath the forearc sediments. Recent east-west shortening during subduction of the Snellius Plateau reactivated the melanges within thrusts cutting the forearc series.     

Magmatism and metamorphism in the Ladakh Himalayas (the Indus-Tsangpo suture zone)

Ladakh (India) provides a complete geological section through the northwestern part of the Himalayas from Kashmir to Tibet. Within this section the magmatic, metamorphic and geotectonic evolution of the northern Himalayan orogeny has been studied using petrographic, geochemical and isotope analytical techniques.The beginning of the Himalayan cycle was marked by large basaltic extrusions (Panjal Trap) of Permian to Lower Triassic age at the “northern” margin of the Gondwana continent (Indian Shield). These continental type tholeiitic basalts were followed by a more alkaline volcanism within the Triassic to Jurassic Lamayuru unit of the Gondwana continental margin.Lower Jurassic to Cretaceous oceanic crust and sediments (ophiolitic mélange s.s.) accompany the Triassic to Cretaceous flysch deposits within the Indus-Tsangpo suture zone, the major structural divide between the Indian Shield (High Himalaya) and the Tibetan Platform. So far, no relic of Paleozoic oceanic crust has been found.Subduction of the Tethyan oceanic crust during Upper Jurassic and Cretaceous time produced an island arc represented by tholeiitic and calc-alkaline volcanic rock series (Dras volcanics) and related intrusives accompanied by volcaniclastic flysch deposits towards the Tibetan continental margin.Subsequent to the subduction of oceanic crust, large volumes of calc-alkaline plutons (Trans-Himalayan or Kangdese plutons) intruded the Tibetan continental margin over a distance of 2000 km and partly the Dras island arc in the Ladakh region.The collision of the Indian Shield and Tibetan Platform started during the middle to upper Eocene and caused large-scale, still active intracrustal thrusting as well as the piling up of the Himalayan nappes. The tectonically highest of these nappes is built up of oceanic crust and huge slices of peridotitic oceanic mantle (Spongtang klippe).In the High Himalayas the tectonic activity was accompanied and outlasted by a Barrovian-type metamorphism that affected Triassic sediments of the Kashmir-Nun-Kun synclinorium up to kyanite/staurolite grade and the deeper-seated units up to sillimanite grade. Cooling ages of micas are around 20 m.y. (muscovite) and 13 m.y. (biotite). Towards the Indus-Tsangpo suture zone metamorphism decreases with no obvious discontinuity through greenschist, prehnite-pumpellyite to zeolite grade. Remnants of possibly an Eo-Himalayan blueschist metamorphism have been found within thrust zones accompanying ophiolitic mélange in the suture zone.     

A rifted inside corner massif on the Mid-Atlantic Ridge at 5°S

The structure of the Mid-Atlantic Ridge at 5°S was investigated during a recent cruise with the FS Meteor. A major dextral transform fault (hereafter the 5°S FZ) offsets the ridge left-laterally by 80 km. Just south of the transform and to the west of the median valley, the inside corner (IC – the region bounded by the ridge and the active transform) is marked by a major massif, characterized by a corrugated upper surface. Fossil IC massifs can also be identified further to the west. Unusually, a massif almost as high as the IC massif also characterizes the outside corner (OC) south of the inactive fracture zone and to the east of the median valley. This OC massif has axis-parallel dimensions identical to the IC massif and both are bounded on their sides closest to the spreading axis by abrupt, steep slopes. An axial volcanic ridge is well developed in the median valley both south of the IC/OC massifs and in an abandoned rift valley to the east of the OC massif, but is absent along the new ridge-axis segment between the IC and OC massifs. Wide-angle seismic data show that between the massifs, the crust of the median valley thins markedly towards the FZ. These observations are consistent with the formation of the OC massif by the rifting of an IC core complex and the development of a new spreading centre between the IC and OC massifs. The split IC massif presents an opportunity to study the internal structure of the footwall of a detachment fault, from the corrugated fault surface to deeper beneath the fault, without recourse to drilling. Preliminary dredging recovered gabbros from the scarp slope of the rifted IC massif, and serpentinites and gabbros from the intersection of this scarp with the corrugated surface. This is compatible with a concentration of serpentinites along the detachment surface, even where the massif internally is largely plutonic in nature.     

Petrology of the greenstones of the Lower Sorachi Group in the Sorachi–Yezo Belt, central Hokkaido, Japan, with special reference to discrimination between oceanic plateau basalts and mid-oceanic ridge basalts

Abstract   The Lower Sorachi Group of the Sorachi–Yezo Belt in central Hokkaido, Japan is a peculiar accretionary complex characterized by numerous occurrences of greenstones (metabasalts and diabases), which are mostly composed of aphyric basalts. Clinopyroxene-rich phenocryst assemblage in phyric basalts is different from olivine–plagioclase assemblage in mid-oceanic ridge basalts (MORB). The greenstones are geochemically uniform, and show a lower-Ti trend than MORB in an FeO*/MgO-TiO₂ diagram, mostly plotting on the island arc tholeiite (IAT) field in a TiO₂−10MnO−10P₂O₅ diagram. In a MORB-normalized spider diagram, the greenstones show a flat pattern from P to Y, which are lower than those of normal mid-oceanic ridge basalt (N-MORB). These indicate that the greenstones were derived by a higher degree of partial melting from a depleted mantle similar to a N-MORB source, and experienced olivine–clinopyroxene fractional crystallization. However, a positive spike of Nb in the spider diagram cannot be explained, and may be attributed to mantle heterogeneity. These characteristics are analogous to those of oceanic plateau basalts (OPB) such as in Ontong Java Plateau, Manihiki Plateau and Nauru Basin, suggesting that the greenstones in the Lower Sorachi Group are of oceanic plateau origin. The present study proposes new field divisions to distinguish OPB from MORB in the conventional FeO*/MgO–TiO₂ and TiO₂−10MnO−10P₂O₅ diagrams.     

Thrusting of young lithosphere in subduction zones with special reference to structures in ophiolitic peridotites

Two contrasted types of structures have been recognized in peridotites from ophiolites and from the oceanic environment. The first one, typical of high-temperature/moderate-stress conditions, is observed in the upper part of ophiolitic peridotites and has been ascribed to plastic flow in an oceanic ridge environment. The second one, typical of moderate-temperature/high-stress conditions, is more specially dealt with here. It is printed in the peridotites above the basal metamorphic aureole found in many ophiolites. The strain increases downward over 1–2 km to produce peridotite mylonites at the contact with the metamorphic aureole. Similarities with rocks from trench and island arc environments suggest ascribing this deformation in ophiolites to a trench environment. We propose that shear fracturing in a young oceanic lithosphere is initiated by the compressive elastic stress in its lower part which is produced by bending of the subducted plate. An externally applied compressive stress is responsible for subsequent overthrusting of the fractured lithosphere. This interpretation is in good agreement with the available geophysical data on young subducted plates and with the physical data on ophiolitic peridotites.     

Paleozoic submarine volcanoes in the high-P/T metamorphosed Chichibu system of Eastern Shikoku, Japan

The Sawadani greenstone in the Chichibu Paleozoic System is an ancient submarine volcanic complex consisting of pillow lavas and hyaloclastites. The volcanism is divided into two periods. Alkali basalt was erupted in the first period and two shield-shaped cones were formed. After a period of dormancy the volcanism of the second period took place and a cone was formed by eruptions of lavas ranging in composition from mildly alkaline to tholeiitic basalt. The top of the volcano nearly reached the sea surface and was finally about 3.7 km above the base. A limestone cap and volcanic conglomerate were deposited on the summit. The base rests conformably on upper Carboniferous sandstone and subordinate mudstone derived from a continent or mature island arc. Many feeding channels of lava cut the volcanic body and underlying sedimentary formation. Sedimentation proceeded concurrently on the surrounding sea floor, so that volcanic and sedimentary material is interlayered.The Sawadani greenstone, although it occurs in the high-P/T metamorphic belt, is not believed to be a fragment of oceanic crust (ophiolite complex) formed by oceanic ridge volcanism and later carried into a convergent zone. It is a seamount formed on and within a sedimentary sequence near a continent or island arc. The magma changed from alkaline to tholeittic as the volcano grew.It cannot be assumed that all metavolcanic rocks formed in high-pressure metamorphic terranes are fragments of oceanic crust.     

Amphibolite facies conditions in the oceanic crust: example of amphibolitized flaser-gabbro and amphibolites from the Chenaillet ophiolite massif (Hautes Alpes,France)

Petrological and mineralogical data on amphibolitized gabbros from an Alpine ophiolite massif (Chenaillet Massif, France) are presented. Comparison with metagabbros dredged from the ocean floor shows that synkinematic amphibolite facies conditions may be reached in gabbros after their initial crystallization in the vicinity of the ridge. It is suggested that sub-horizontal plastic flow took place in the gabbroic layer near the axis of a slowly spreading ocean ridge before the intrusion of diabase dykes. This thermo-tectonic regime which at the Chenaillet produced flaser-gabbros and layers of foliated amphibolites with brown hornblende and pargasite, probably also affected most of the other ophiolitic gabbros of the Piemont zone prior to the low-temperature/high-pressure Alpine metamorphism.     

Causes of spatial compositional variations in Mariana arc lavas: Trace element evidence

New inductively coupled plasma mass spectrometry (ICP-MS) trace element data are presented on a suite of arc lavas from the northern Mariana and southern Bonin island arcs. The samples were dredged from seamounts in the Central Island Province (CIP), the Northern Seamount Province (NSP) and the Volcano Arc (VA), and they range in composition from low-K tholeiites to shoshonites. Previous studies on these samples concluded that the primary compositional control was two-component mixing between a fluid-metasomatized mid-ocean ridge basalt (MORB) source and an enriched, ocean island basalt (OIB)-like, mantle component, with subducted sediment material playing a secondary role. However, the new trace element data suggest that the compositional variations along the Mariana arc can be better explained by the addition of spatially varying subduction components to a spatially varying mantle source. The data suggest that the subduction component in the CIP and VA is dominated by aqueous fluids derived from altered oceanic crust and a pelagic sediment component, while the subduction component in the NSP is dominated by more silicic fluids derived from volcanogenic sediments as well as from pelagic sediment and altered oceanic crust. The mantle wedge in the CIP and VA is depleted relative to a normal mid-ocean ridge basalt source by loss of a small melt fraction, while the mantle wedge in the NSP is enriched either by possible gain of a small melt fraction or addition of a sediment-derived melt. Because the subduction of seamounts controls the arc and back-arc geometries, so the concomitant variation between subducted material and mantle composition may be no coincidence. The high field strength element (HFSE) data indicate a high degree of melting (∼ 25–30%) throughout the arc, ∼ 10% of which may be attributed to decompression and ∼ 20% to fluid addition.     

A Jurassic-Cretaceous island arc in the Ladakh-Himalayas

The Ladakh Mesozoic ophiolite belt (western Himalaya) contains a pile of volcanic thrust sheets (Dras unit) which differ significantly in structure and composition from the ophiolitic mélange zones. The Dras unit is composed of pillow lavas, doleritic sills, very irregular basaltic (?basaltic andesites) and dacitic flows intercalated with pyroclastics, volcanoclastic sediments and radiolarian cherts. According to fossil evidence, this volcanism must have been active between Upper Jurassic and Upper Cretaceous.The presence of relict primary minerals, such as magnesiochromite, clinopyroxene, hastingsitic hornblende and Ti-magnetite as well as distinctive bulk chemistries, suggests that the volcanics belong to island arc tholeiite and to calc-alkaline rock series, typical of present island arcs in the Caribbean and Pacific.Model calculations incorporating probed phenocryst phases indicate that in addition to olivine, clinopyroxene and plagioclase, amphibole and titanomagnetite are crucial fractionating phases in the development of the dacites from a primitive tholeiitic melt. The latter process must have taken place at about 1000°C and at moderate depth of 5–15 km within or underneath the island arc. Today, hornblende-bearing mafic cumulates appear in the vicinity of Kargil within and close to the Dras volcanics.In a Sr-evolution diagram, the Dras volcanics have yielded a “pseudo-isochron” with a low initial ratio of 0.7035 ± 0.0003, which is in the same range as the mean of modern island arc volcanics. However, a geologically unrealistic age of 263 m.y., is obtained from the slope of this isochron.The upper mantle is regarded as the source material for the island arc tholeiitic magmas. Enrichment in K, Ba, Sr and LREE supports the involvement of components derived from dehydration or incipient melting of subducted Tethyan oceanic crust in the mantle.     

Dredge petrology of the boninite‐ and adakite‐bearing Hahajima Seamount of the Ogasawara (Bonin) forearc: An ophiolite or a serpentinite seamount?§

Abstract During the Hakuho‐Maru KH03‐3 cruise and the Tansei‐Maru KT04‐28 cruise, more than 1000 rock samples were dredged from several localities over the Hahajima Seamount, a northwest–southeast elongated, rectangular massif, 60 km × 30 km in size, with a flat top approximately 1100 m deep. The rocks included almost every lithology commonly observed among the on‐land ophiolite outcrops. Volcanic rocks included mid‐oceanic ridge basalt (MORB)‐like tholeiitic basalt and dolerite, calc‐alkaline basalt and andesite, boninite, high‐Mg adakitic andesite, dacite, and minor rhyolite. Gabbroic rocks included troctolite, olivine gabbro, olivine gabbronorite (with inverted pigeonite), gabbro, gabbronorite, norite, and hornblende gabbro, and showed both MORB‐type and island arc‐type mineralogies. Ultramafic rocks were mainly depleted mantle harzburgite (spinel Cr? 50–80) and its serpentinized varieties, with some cumulate dunite, wehrlite and pyroxenites. This rock assemblage suggests a supra‐subduction zone origin for the Hahajima Seamount. Compilation of the available dredge data indicated that the ultramafic rocks occur in the two northeast–southwest‐oriented belts on the seamount, where serpentinite breccia and gabbro breccia have also developed, but the other areas are free from ultramafic rocks. Although many conical serpentinite seamounts 10 km in size are aligned along the Izu–Ogasawara (Bonin)–Mariana forearc, the Hahajima Seamount may be better interpreted as a fault‐bounded, uplifted massif composed of ophiolitic thrust sheets, resembling the Izki block of the Oman ophiolite in its shape and size. The ubiquitous roundness of the dredged rocks and their thin Mn coating (<2 mm) suggest that the Hahajima Seamount was uplifted above sealevel and wave‐eroded, like the present Macquarie Is., a rare example of ophiolite exposure in an oceanic setting. The Ogasawara Plateau on the Pacific Plate is adjacent to the east of the Hahajima Seamount, and collision and subduction of the plateau may have caused uplift of the forearc ophiolite body.