high precision isotopic characterization of usgs reference materials by tims and mc icp ms the pacific centre for isotopic and geochemical research pcigr at the university of british columbia has undertaken a systematic analysis of the isotopic sr nd and pb compositions and concentrations of a broad compositional range of u s geological survey usgs reference materials including basalt bcr 1 2 bhvo 1 2 andesite agv 1 2 rhyolite rgm 1 2 syenite stm 1 2 granodiorite gsp 2 and granite g 2 3 usgs rock reference materials are geochemically well characterized but there is neither a systematic methodology nor a database for radiogenic isotopic compositions even for the widely used bcr 1 this investigation represents the first comprehensive systematic analysis of the isotopic composition and concentration of usgs reference materials and provides an important database for the isotopic community in addition the range of equipment at the pcigr including a nu instruments plasma mc icp ms a thermo finnigan triton tims and a thermo finnigan element2 hr icp ms permits an assessment and comparison of the precision and accuracy of isotopic analyses determined by both the tims and mc icp ms methods e g nd isotopic compositions for each of the reference materials 5 to 10 complete replicate analyses provide coherent isotopic results all with external precision below 30 ppm 2 sd for sr and nd isotopic compositions 27 and 24 ppm for tims and mc icp ms respectively our results also show that the first and second generation usgs reference materials have homogeneous sr and nd isotopic compositions nd isotopic compositions by mc icp ms and tims agree to within 15 ppm for all reference materials interlaboratory mc icp ms comparisons show excellent agreement for pb isotopic compositions however the reproducibility is not as good as for sr and nd a careful sequential leaching experiment of three first and second generation reference materials bcr bhvo agv indicates that the heterogeneity in pb isotopic compositions and concentrations could be directly related to contamination by the steel mortar pestle used to process the materials contamination also accounts for the high concentrations of certain other trace elements e g li mo cd sn sb w in various usgs reference materials

eemian cooling in the norwegian sea and north atlantic ocean preceding continental ice sheet growth changing conditions in the north atlantic region may drive global climate changes1 2 according to previous reconstructions of the last interglacial the eemian north atlantic sea surface tempera tures ssts were similar to present day values3 in the norwegian sea even warmer conditions appeared as a single pulse of short duration4 5 whereas the greenland ice record suggests that the warm interglacial air temperatures were interrupted by several cold periods6 here we use faunal and stable isotope analyses of foraminifera in two sediment cores from the north atlantic ocean and norwegian sea to reconstruct high resolution records of sst and sea surface salinity sss during the eemian interglacial our results which differ significantly from the greenland record6 show a sharp decrease in sst and sss of the norwegian sea associated with a more moderate cooling and freshening of the north atlantic at the middle of isotope substage 5e several millennia before the beginning of continental ice sheet growth changes in the norweg ian sea surface conditions appear to have represented an important climate change affecting global atmospheric and thermohaline circulations

the closing of tethys and the tectonics of the himalaya recent geological and geophysical data from southern tibet allow refinement of models for the closing of southern neo tethys and formation of the himalaya shelf sediments of the indian passive continental margin which pass northward into deep sea tethyan sediments of the indus tsangpo suture zone were deposited in the late cretaceous an andean type margin with a 2 500 km long trans himalayan kohistan ladakh gangdese granitoid batholith formed parallel to the southern margin of the lhasa block together with extensive andesites rhyolites and ignimbrites lingzizong formation the southern part of the lhasa block was uplifted deformed and eroded between the cenomanian and the eocene in the western himalaya the kohistan island arc became accreted to the northern plate at this time the northern part of the lhasa block was affected by jurassic metamorphism and plutonism associated with the mid jurassic closure of the bangong nujiang suture zone to the north the timing of collision between the two continental plates ca 50 40 ma marking the closing of tethys is shown by 1 the change from marine flysch like to continental molasse like sedimentation in the indus tsangpo suture zone 2 the end of gangdese i type granitoid injection 3 eocene s type anatectic granites and migmatites in the lhasa block and 4 the start of compressional tectonics in the tibetan tethys and indus tsangpo suture zone south facing folds south directed thrusts after the eocene closure of tethys deformation spread southward across the tibetan tethys zone to the high himalaya deep crustal thrusting barrovian metamorphism migmatization and generation of oligocene miocene leucogranites were accompanied by south verging recumbent nappes inverting metamorphic isograds and by south directed intracontinental shear zones associated with the main central thrust continued convergence in the late tertiary resulted in large scale north directed backthrusting along the indus tsangpo suture zone more than 500 km shortening is recorded in the foreland thrust zones of the indian plate south of the suture and 150 km shortening is recorded across the indian shelf zanskar range and the indus suture in ladakh there was also large scale shortening of the karakoram and tibetan microplates north of the suture as much as 1 000 km shortening occurred in tibet the more recent deformation however involved the spreading of this thickened crust and the lateral motion of the tibetan block along major approximately east west trending strike slip fault zones

relicts of the early cretaceous seamounts in the central western yarlung zangbo suture zone southern tibet abstract despite more than 30 years studies in the yarlung zangbo suture zone yzsz southern tibet is still a region of discovery and the geological evolution of the neo tethys here remains controversial in this paper we present the new field observation petrography zircon u pb dating and geochemistry of the zhongba mafic rocks in the western segment of the yzsz zircon u pb analyses from a diabase using sims secondary ion mass spectrometry yielded a concordant age of 125 7 0 9 ma all the zhongba mafic rocks exhibit lree enrichment lan ybn 4 7 15 8 without eu anomalies eu 0 97 1 01 their primitive mantle normalized trace element patterns are similar to those of the average oib and hawaii alkaline basalts as well as other oib type rocks already reported in the central western yzsz these observations lead us to suggest the existence of a series of seamounts within the neo tethys during the early cretaceous these oib type rocks could be originated from 1 hotspot mantle 2 near ridge seamounts by melting induced mixing of two distinct mantle sources 3 late stage magmatism fed by melts that originated from an asthenospheric window due to slab delamination combining literature data from the entire yzsz which include studies of the morb and iab type rocks and mantle peridotites we prefer the third explanation that the oib type rocks represent a late stage magmatic activity above an intra oceanic supra subduction zone within the neo tethys

climate instability during the last interglacial period recorded in the grip ice core isotope and chemical analyses of the grip ice core from summit central greenland reveal that climate in greenland during the last interglacial period was characterized by a series of severe cold periods which began extremely rapidly and lasted from decades to centuries as the last interglacial seems to have been slightly warmer than the present one its unstable climate raises questions about the effects of future global warming

boninitic magmatism in a continental margin setting yukon tanana terrane southeastern yukon canada mid paleozoic mafic rocks in the finlayson lake region of the yukon tanana terrane southeastern yukon canada have the diagnostic geochemical signatures of boninites high mgo cr ni and co contents intermediate sio 2 contents high mg 9s mgo mgo feo al 2 o 3 tio 2 and zr hf middle rare earth element ree ratios low tio 2 ree and high field strength element contents and u shaped primitive mantle normalized trace element patterns however unlike most modern and ancient boninitic rocks that are typically associated with intraoceanic realms those from the finlayson lake region are part of a mid paleozoic continental margin arc backarc magmatic system we propose a model in which the boninitic rocks from the finlayson lake region formed as a result of spreading ridge propagation into an arc built on composite basement of oceanic and continental crust in the oceanic segment upwelling asthenosphere induced melting of a subducted slab metasomatized refractory mantle source to form boninitic magmatism in the continental sector upwelling asthenospheric mantle and or the melts derived thereof induced crustal melting which explains the large volume of temporally equivalent felsic volcanic and intrusive rocks

proterozoic melting in the northern peridotite massif zabargad island os isotopic evidence the geodynamic history of the three zabargad peridotite bodies is the key to their significance as samples of the upper mantle currently there are two main hypotheses that differ greatly in their implications for the origin and age of the peridotite complex as a whole in the first the zabargad peridotite bodies all represent young asthenospheric mantle that was juxtaposed with ancient crustal gneisses during the opening of the red sea in the second the complex may represent a single package of residual mantle and lower crust of pan african age that was uplifted during the miocene in order to distinguish between these two models we have analysed five samples of zabargad peridotite and two pyroxenites for os isotopes the 187os 188os results range from 0 1198 to 0 1320 and correlate with al2o3 allowing an age inference for the northern body based on the os isotopic evolution curve for the bulk earth of 1 4 0 3 gyr this is at least as old as the previously inferred pan african age and may be older and agrees with previous suggestions that the northern and central peridotite bodies probably represent ancient continental lithospheric mantle while the southern body represents recent asthenospheric mantle our results from both central and southern bodies are consistent with either crustal age hypothesis

origin of the northern atlantic s heinrich events as first noted by heinrich 1988 glacial age sediments in the eastern part of the northern atlantic contain layers with unusually high ratios of ice rafted lithic fragments to foraminifera shells he estimated that these layers are spaced at intervals of roughly 10 000 years in this paper we present detailed information documenting the existence of the upper five of these layers in odp core 609 from 50 n and 24 w their ages are respectively 15 000 radiocarbon years 20 000 radiocarbon years 27 000 radiocarbon years about 40 000 years and about 50 000 years we also note that the high lithic fragment to foram ratio is the result of a near absence of shells in these layers although we are not of one mind regarding the origin of these layers we lean toward an explanation that the heinrich layers are debris released during the melting of massive influxes of icebergs into the northern atlantic these sudden inputs may be the result of surges along the eastern margin of the laurentide ice sheet

zedong terrane revisited an intra oceanic arc within neo tethys or a part of the asian active continental margin abstract precise timing of the india asia collision is important to constrain the evolution history of both the himalayan orogen and the tibetan plateau it has been proposed that the indian plate first collided with an intra oceanic arc at 55 ma and then the composite terrane collided with the asian continent at 35 ma the zedong terrane has been suggested to represent the vestige of such an intra oceanic arc developed within the neo tethys ocean as some volcanic rocks with high k2o have been classified as shoshonites in this study we present detailed geochemical and geochronological data of various types of magmatic rocks including volcanic cumulate and granitic rocks widely exposed in the zedong terrane to constrain the formation age and tectonic setting of the zedong terrane we found that the zedong volcanic rocks belong to calc alkaline series rather than shoshonites and high k2o contents in some volcanic rocks resulted from alteration the basalts are highly enriched in lree and lile but strongly depleted in hfse indicating they were derived from a metasomatized mantle presence of hornblende phenocryst in both gabbros and hornblendites indicates that the cumulates were produced from hydrous basalts through crystallization the granitic rocks have adakite like compositional characteristics i e high sr y ratios but low y contents which were formed by melting of a thickened lower crust zircons from six samples including a volcanic rock an andesite three cumulates a hornblendites a hornblende bearing gabbro and a gabbro and two granitic a tonalite and a granodiorite rocks have been dated to yield identical ages of 155 160 ma this suggests that the volcanic eruption and plutonic emplacement were coevally developed in the zedong terrane zircons from both the andesite and the cumulates have similar positive ehf t values 11 6 to 16 7 indicating they were stemmed from similarly depleted mantle sources meanwhile zircons from the granitic rocks also have positive ehf t values of 12 6 to 15 2 implying their derivation from a juvenile lower crust therefore we proposed that the basalts in the zedong terrane were formed through partial melting of the mantle wedge metasomatized by slab released fluids melts a part of hydrous basalts were underplated in the thickened lower crust beneath the zedong terrane which gave rise to the cumulate and granitic rocks by comparison magmatic rocks in the zedong terrane show compositional similarities with the jurassic rocks exposed in the gangdese arc this suggests that the zedong terrane represents a slice of the active continental margin developed on the southern margin of the lhasa terrane as a result of the northward subduction of the neo tethys ocean during the late jurassic rather than the vestige of an intra oceanic arc

formation of the japan sea basin reassessment from ar ar ages and nd sr isotopic data of basement basalts of the japan sea and adjacent regions abstract many studies have examined the japan sea basalts recovered during ocean drilling program odp leg127 128 of these the 40ar 39ar dating undertaken is important in constraining the timing of the formation of the japan sea however the implications of their results do not appear to be fully appreciated by the geological community in this paper i reassess the 40ar 39ar age data of the basalts with reference to nd sr isotopic data the 40ar 39ar dating was performed on basalts somewhat enriched in large ion lithophile elements and recovered from odp sites 794 795 and the lower part of 797 yielding the plateau ages of 21 2 17 7 ma these basalts show the nd sr isotopic signature of a moderately depleted mantle source end 0 6 6 9 in contrast the basalts from the upper part of site 797 have yet to be dated due to their low k content although their nd isotopic compositions are similar to that of morb end 8 4 10 4 by analogy to the secular nd sr isotopic trends reported for sikhote alin and northeast japan the age of the upper basalts at site 797 may be inferred to be younger than the lower basalts probably around 16 ma the nd sr isotopic compositions of the japan sea basalts have been interpreted in terms of eastward asthenospheric flow as have the lavas of the sikhote alin and northeastern japan the timing of volcanic activity in the japan sea region i e from 21 2 to 14 86 ma is consistent with the timing of rotational crustal movements inferred from paleomagnetic studies of the japanese islands i e 14 8 4 2 ma for southwest japan and 16 5 14 4 ma for northeast japan

structure and geochemistry of tethyan ophiolites and their petrogenesis in subduction rollback systems suprasubduction zone ssz ophiolites in orogenic belts represent oceanic crust generation in subduction rollback cycles during the closing stages of basins prior to terminal continental collisions mantle flow and slab rollback result in one or more episodes of arc splitting and basin opening producing a collage of protoarc and forearc oceanic lithosphere in suprasubduction zone settings the jurassic cretaceous ssz tethyan ophiolites in the eastern mediterranean region i e mirdita pindos troodos kizildag oman generally have penrose type oceanic crust and contain well developed sheeted dike complexes indicative of magmatic extension beneath narrow rift zones during their seafloor spreading evolution igneous accretion of these ssz tethyan ophiolites involved upper plate extension and advanced melting of previously depleted asthenosphere in host basins showing a progressive evolution from morb like to iat island arc tholeiite to boninitic extremely refractory protoarc assemblages however there are some distinct differences in the geochemical evolution of these tethyan ophiolites that appear to have resulted from variations in their subduction zone geodynamics whereas a major part of the kizildag and troodos lavas shows island arc affinity similar to their counterparts in the pindos and mirdita ophiolites a significant component of the oman lavas indicates morb affinity and the majority of the kizildag and oman data plot within the mantle array between n morb and e morb on the nb yb th yb discriminant diagram furthermore the troodos and oman lavas do not show any particular th enrichment in their multi element patterns suggesting that fluid melt input from subducted sediments was not that significant in generation of their magmas although all ophiolites exhibit geochemical features indicating increased subduction influence during the melt evolution of their younger extrusive sequences and dike intrusions as evidenced by their negative nd values their overall characteristic trace element patterns seem to have been strongly affected by the maturity of the subduction systems in which they developed

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

initiation of fennoscandian ice sheet retreat during the last deglaciation although the retreat history of the southern margin of the laurentide ice sheet during the last deglaciation has been well known for several decades1 3 and recently supported by evidence from accelerator mass spectrometer ams radiocarbon dating of meltwater in the gulf of mexico4 and the north atlantic subtropical gyre5 the retreat history of the fennoscandian ice sheet before 13kyr ago is still poorly documented from ams 14c dating and isotopic analysis of a rapidly deposited series of supra till sediments in the norwegian channel north sea we find that the southern margin of the fennoscandian ice sheet was in retreat by 15 kyr bp before present approximately 2 000 yr earlier than previously supposed oxygen isotope analyses in the channel sediments confirm earlier evidence from deep norwegian sea sediment cores for low salinity due to ice sheet discharge beginning 15 14 5 kyr bp ref 6 recent recalibration of the 14c timescale indicates that the onset of deglaciation within the norwegian sea occurred during an interval of rapidly increasing summer insolation beginning 18 000 calendar yr bp ref 7 suggesting that ice sheet retreat may have been caused by increasing insolation acting on a particularly climate sensitive ice sheet configuration fresh water and icebergs discharged into the norwegian sea at this time reached the north atlantic and may have contributed to a brief interval of extreme oceanic cooling8 and reduced production of north atlantic deep water9 10 these factors may also have helped to briefly reverse the deglaciation trend erie interstade already underway in north america

multiple events in the neo tethyan oceanic upper mantle evidence from ru os ir alloys in the luobusa and dongqiao ophiolitic podiform chromitites tibet in situ analysis of major elements platinum group elements and os isotope composition data for 170 grains of ru os ir alloys from podiform chromitites in the luobusa and dongqiao ophiolites tibet provide new constraints on the sources of the alloy grains the os isotope composition of the neo tethyan oceanic upper mantle and the timing of melt rock reactions in the ultramafic peridotites of the host ophiolites most alloy grains are osmiridium and iridosmine based on elemental and isotopic compositions two populations can be identified group i found in both ophiolite bodies has fractionated pge patterns typical of alloys and homogeneous 187 os 188 os 0 12645 4 2s n 145 group ii alloys from the dongqiao ophiolites show smooth pge patterns and 187 os 188 os ranges from 0 12003 to 0 12194 yielding re depletion ages 1 1 ga both the pge patterns and the os isotopic compositions of group ii resemble those of sulfides residual after melting group i alloys represent an early cumulate phase during the crystallization of magmas and are interpreted as derived from an asthenospheric melt in contrast group ii alloys are inferred to derive from the breakdown of residual sulfides from the mantle section of the ophiolite which may represent relics of a sub continental lithospheric mantle the coexistence of the two groups of alloys in dongqiao samples reflects interaction between asthenosphere derived melts and old depleted lithospheric mantle and suggests that the chromitites originated through melt rock reactions the data on the group i alloys support the hypothesis that the re os evolution of the convective oceanic mantle resembles that of enstatite chondrites 2007 elsevier b v all rights reserved

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

seismic attenuation of atlantic margin basalts observations and modeling paleogene basalts are present over much of the northeastern atlantic european margin in regions containing significant thicknesses of layered basalt flows conducting seismic imaging within and beneath the volcanic section has proven difficult largely because the basalts severely attenuate and scatter seismic energy we use data from a vertical seismic profile vsp from well 164 07 1 that penetrated 1 2 km of basalt in the northern rockall trough west of britain to measure the seismic attenuation caused by the in situ basalts the effective quality factor qeff of the basalt layer is found from the vsp to be 15 35 which is considerably lower more attenuative than the intrinsic attenuation measured on basalt samples in the laboratory we then run synthetic seismogram models to investigate the likely cause of the attenuation full waveform 1d modeling of stacked sequences of lava flows based on rock properties from the same well indicates that much of the seismic attenuation observed from the vsp can b

forearc hyperextension dismembered the south tibetan ophiolites suprasubduction zone ophiolites are relics of oceanic upper plate forearcs and are typically preserved as discontinuous belts with discrete massifs along suture zones ophiolites usually contain an incomplete condensed section compared to average modern oceanic lithosphere the incompleteness and discontinuity of ophiolites are frequently attributed to dismemberment but tectonic causes remain poorly constrained here we show new paleomagnetic and field geological evidence for the preservation of extensional detachment faults that thinned and dismembered the south tibetan ophiolite belt during the early cretaceous similar to those documented in modern slow and ultraslow spreading ridges these detachments exhumed lithospheric mantle and subophiolitic melange to the seafloor which became unconformably covered by asia derived forearc strata we call this mechanism forearc hyperextension whereby widespread detachment faults accommodate upper plate extension above a subduction zone we propose that hyperextension is the key mechanism responsible for dismemberment of the south tibetan ophiolitic belt shortly after its magmatic accretion

precise radiolarian age constraints on the timing of ophiolite generation and sedimentation in the dazhuqu terrane yarlung tsangpo suture zone tibet well preserved abundant radiolarians provide high precision biostratigraphic age constraints on the timing of the eruption of ophiolitic basalts exposed along the yarlung tsangpo suture zone in southern tibet dazhuqu terrane ophiolites were generated in an intra oceanic supra subduction zone setting within a relatively short

crustal structure of the indus tsangpo suture zone and its ophiolites in southern tibet abstract the 2000 km long indus tsangpo suture zone itsz in southern tibet contains remnants of neotethyan oceanic lithosphere ophiolitic melanges flysch units and continental rocks and has been interpreted as the boundary between the india and eurasia plates we have used field based structural analyses and geological mapping in five key ophiolite massifs luobusa zedang xigaze jiding and dongbo and petrofabric studies of core samples recovered through the luobusa scientific drilling project to document the structural architecture and the deformation history of the itsz the structural and kinematic analyses of the suture zone units indicate the occurrence of superposed deformation fabrics which developed first during the ophiolite emplacement and then during the subsequent india asia continental collision the suprasubduction zone ophiolites were emplaced southward onto the northern margin of the proto indian plate in the late cretaceous the continental collision between the ophiolite laden proto indian margin and the lhasa terrane of the eurasian plate in the early paleogene resulted in backthrusting and inversion of the structural order of the itsz the ophiolite massifs ophiolitic melanges and flysch units were thrust northward as nappe sheets over the eocene and younger forearc basin strata the itsz shows therefore a complex structural anatomy characterized by both south and north directed thrust faults and ductile and brittle deformation fabrics

fore arc basalts and subduction initiation in the izu bonin mariana system recent diving with the jamstec shinkai 6500 manned submersible in the mariana fore arc southeast of guam has discovered that morb like tholeiitic basalts crop out over large areas these fore arc basalts fab underlie boninites and overlie diabasic and gabbroic rocks potential origins include eruption at a spreading center before subduction began or eruption during near trench spreading after subduction began fab trace element patterns are similar to those of morb and most izu bonin mariana ibm back arc lavas however ti v and yb v ratios are lower in fab reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid ocean ridges and back arc basins some fab also have higher concentrations of fluid soluble elements than do spreading center lavas thus the most likely origin of fab is that they were the first lavas to erupt when the pacific plate began sinking beneath the philippine plate at about 51 ma the magmas were generated by mantle decompression during near trench spreading with little or no mass transfer from the subducting plate boninites were generated later when the residual highly depleted mantle melted at shallow levels after fluxing by a water rich fluid derived from the sinking pacific plate this magmatic stratigraphy of fab overlain by transitional lavas and boninites is similar to that found in many ophiolites suggesting that ophiolitic assemblages might commonly originate from near trench volcanism caused by subduction initiation indeed the widely dispersed jurassic and cretaceous tethyan ophiolites could represent two such significant subduction initiation events

early tertiary deformation of the zhongba gyangze thrust in central southern tibet abstract as the boundary thrust between india and asia in southern tibet the zhongba gyangze thrust zgt emplaced the yarlung zangbo suture zone yzsz units in the hanging wall southward onto tethyan himalaya sequences ths of the northern indian continental margin in the footwall detailed field investigation electron backscatter diffraction ebsd analysis detrital zircon u pb geochronology and 40 ar 39 ar thermochronology were conducted to understand the evolution of the zgt in sangsang area central southern tibet the shear zone of zgt is located within the sedimentary matrix melange of yzsz that is mainly composed of matrix of blueschist with meta pelagic hemipelagic siliceous and siliciclastic rocks and blocks of basalt limestone and sandstone penetrative f1 foliation and kink band structure were recorded within the matrix both on outcrop and under microscope strong lattice preferred orientation lpo fabric initiated by the low temperature 350 450 c 010 001 slip system was detected by ebsd in the sodic amphiboles of the blueschist the 40 ar 39 ar ages of the phengites from blueschist and sericites from the phyllite in the shear zone indicate that the activity of zgt occurred between 71 and 60 ma in the ths a newly documented younger unit preserving detrital zircons from the southern asian margin lies above the triassic cretaceous sequences that carries only detrital zircons from the indian continent this unit is dated to be 61 ma by the detrital zircon ages similar to the sangdanlin formation and representing the yarlung zangbo foreland basin yzfb system the zgt had probably been active due to the initial india asia collision and acted as the frontal thrust controlling the development of yzfb

sedimentary history of the tethyan basin in the tibetan himalayas after an epicontinental phase the sedimentary rocks in the tibetan himalayas document a complete wilson cycle of the neo tethyan tethys ill evolution between the gondwana supercontinent and its northward drifting margin lhasa block from the late permian to the eocene during the triassic rift stage the basin was filled with a huge clastic dominated sediment wedge with up to 5 000 m of flysch in the northern zone widespread deltaic clastics and shallow water carbonates of late norian to earliest jurassic age in the southern zone mark in conjunction with decreasing tectonic subsidence the transition to the drift stage some 4 500 m of jurassic and early cretaceous shallow water carbonates and siliciclastics accumulated on the tethyan indian passive margin deepening upward sequences with condensed beds at their tops alternate with repeated progradational packages of shelf sediments extensive abyssal sediments with basaltic volcanics in the northern deep water zone reflect continued ocean spreading and thermal subsidence paleomagnetic data gained separately for the northern indian plate and the lhasa block indicate that the neo tethys reached its maximum width about 110 ma ago with a spreading rate of 4 8 cm year before it commenced to close again during the remnant basin stage in the late cretaceous and paleogene a shallowing upward megasequence capped by a carbonate platform developed in the southern inner shelf realm in the northern slope basin plain zone turbidites and chaotic sediments derived from both the acretionary wedge and the steepening slope of the passive margin accumulated the depositional center of the remnant basin shifted southward as a result of flexural subsidence and southward overthrusting the sediments from the triassic to the paleogene are tentatively subdivided into five mega sequences which are controlled mainly by regional tectonics climatic influence e g carbonate deposition due to northward plate motion is partially subdued by terrigenous input and or increased water depth during the oligocene and miocene crustal shortening led to rapid uplift and the deposition of fluvial molasse in limited basins

western north atlantic evidence for millennial scale changes in ocean circulation and climate two late quaternary series of high resolution percent carbonate data from western north atlantic sediment drifts bermuda rise and bahama outer ridge show millennial scale oscillations superimposed on the familiar longer period oscillations of orbital origin the dominant high frequency oscillation in these records has a quasi period of about 4000 years these percent caco3 changes most likely result from the influence of climate change on the flux of terrigenous material from eastern canada the resuspension of continental margin sediment by deep eddy kinetic energy and carbonate dissolution sediment is transported to the bermuda rise by deep recirculating gyres and to the bahama outer ridge by the deep western boundary current system stable isotope results on foraminifera across several of these oscillations from interstadial climate conditions and from a glacial inception display variability similar to that of percent caco3 oxygen isotope ratios of planktonic foraminifera suggest large variations in near surface temperature and or salinity and carbon isotope ratios of benthic foraminifera indicate that there were significant oscillations in the flux of north atlantic deep water nadw these data support models which couple surface ocean conditions in the north atlantic production of nadw north atlantic heat flux and evidence for temperature oscillations in ice cores

lead isotopic study of the xigaze ophiolite tibet the problem of the relationship between magmatites gabbros dolerites lavas and tectonites harzburgites abstract the pb isotopic study of the xigaze ophiolite tibet shows that the tectonic harzburgites and magmatic gabbros dolerites lavas units bear different isotopic signatures the harzburgitic rocks show much higher207pb 204pb values than the samples from the magmatic unit analyses of primary magmatic clinopyroxenes separated from the harzburgites confirmed that this difference is real and not due to secondary alteration processes u pb analysis allowed determination of the crystallization age of the magmatites at 120 10 m y this age is compatible with paleontological age determinations and thus demonstrates that u pb behaved more or less as a closed system since formation of the ophiolite if an exchange with seawater has occurred it must necessarily have happened shortly after crystallization of the xigaze oceanic crust dolerite dykes crossing the harzburgites show similar pb isotopic composition as the magmatites indicating that they were emplaced when the ultramafic rocks were already brittle however the time difference of the emplacement was probably not larger than 280 m y this makes it evident that two sources with different isotopic signatures one similar to normal morb the other with much higher207pb 204pb ratios were active in the immediate neighborhood one possible environment which can account for this association is a propagating ridge

geochemistry zircon u pb geochronology and lu hf isotopes of metavolcanics from eastern hebei reveal neoarchean subduction tectonics in the north china craton abstract the neoarchean metavolcanic rocks of saheqiao occur within the central domain of the precambrian terrane in eastern hebei within the north china craton these rocks consist mainly of garnet two pyroxene granulites garnet clinopyroxene amphibolites amphibole plagioclase gneisses and biotite plagioclase gneisses these metavolcanic rocks are exposed as several kilometers scale of massive blocks or several tens of centimeters scale of enclaves within precambrian granitoid gneisses based on whole rock geochemistry we classify these intermediate mafic volcanic rocks into four groups group 1 exhibits low sio2 yb and nb ta ratios and high ni and cr and shows n morb like normal mid ocean ridge basalts geochemical features suggesting that the magmatic precursors of these rocks were derived through partial melting of depleted mantle compared to group 1 group 2 shows slightly right declined ree and nearly flat multi element patterns with variable negative nb anomalies matching with primitive arc basaltic rocks formed by partial melting of the mantle wedge group 3 exhibits chemical compositions of basaltic and andesitic rocks and is characterized by pronounced lree enrichment but hree depletion and shows distinctly negative nb and ta anomalies and weakly negative zr and hf anomalies these rocks may compare with island arc volcanic rocks and were probably generated by partial melting of the mantle wedge modified by dehydration of subducted oceanic slab the magmatic precursors of the andesites were derived from the crystallization differentiation of the basaltic magma group 4 is characterized by clearly right declined ree patterns and slightly positive or no nb ti anomalies which are analogous to nb enriched basalts neb and originated from melting of hfse enriched mantle sources la icp ms u pb dating of magmatic zircons from five samples from the three groups reveals that group 1 n morb like rocks formed at 2525 7 ma group 2 primitive arc basaltic rocks at 2518 15 ma and group 3 typical arc volcanic rocks at 2523 6 ma to 2614 18 ma zircons from all the dated samples also preserve signatures of 2450 2370 ma metamorphisms or thermal events the zircon lu hf data from the three dated samples in group 3 show ehf t values ranging from 1 4 to 9 0 and tdm values of 2627 ma to 2965 ma suggesting a significant episode of 2 6 2 9 ga crustal growth in this region the 2525 ma n morb like basaltic rocks are younger than the arc related rocks 2614 2518 ma suggesting that the morb like basaltic rocks were generated in a back arc environment on the basis of the rock association of back arc morb basalts group 1 arc related andesites basalts groups 2 and 3 and neb like rocks group 4 we suggest that the neoarchean volcanic rocks of the eastern hebei region formed in a subduction related setting within a convergent plate margin

geologic correlation of the himalayan orogen and indian craton part 2 structural geology geochronology and tectonic evolution of the eastern himalaya despite being the largest active collisional orogen on earth the growth mechanism of the himalaya remains uncertain current debate has focused on the role of dynamic interaction between tectonics and climate and mass exchanges between the himalayan and tibetan crust during cenozoic india asia collision a major uncertainty in the debate comes from the lack of geologic information on the eastern segment of the himalayas from 91 e to 97 e which makes up about one quarter of the mountain belt to address this issue we conducted detailed field mapping u pb zircon age dating and 40ar 39ar thermochronology along two geologic traverses at longitudes of 92 e and 94 e across the eastern himalaya our dating indicates the region experienced magmatic events at 1745 1760 ma 825 878 ma 480 520 ma and 28 20 ma the first three events also occurred in the northeastern indian craton while the last is unique to the himalaya correlation of magmatic events and age equivalent lithologic units suggests that the eastern segment of the himalaya was constructed in situ by basement involved thrusting which is inconsistent with the hypothesis of high grade himalaya rocks derived from tibet via channel flow the main central thrust in the eastern himalaya forms the roof of a major thrust duplex its northern part was initiated at ca 13 ma while the southern part was initiated at ca 10 ma as indicated by 40ar 39ar thermochronometry crustal thickening of the main central thrust hanging wall was expressed by discrete ductile thrusting between 12 ma and 7 ma overlapping in time with motion on the main central thrust below restoration of two possible geologic cross sections from one of our geologic traverses where one assumes the existence of pre cenozoic deformation below the himalaya and the other assumes flat lying strata prior to the india asia collision leads to estimated shortening of 775 km 76 strain and 515 km 70 strain respectively we favor the presence of significant basement topography below the eastern himalaya based on projections of early paleozoic structures from the shillong plateau i e the central shillong thrust located 50 km south of our study area since northeastern india and possibly the eastern himalaya both experienced early paleozoic contraction the estimated shortening from this study may have resulted from a combined effect of early paleozoic and cenozoic deformation

the youngest marine deposits preserved in southern tibet and disappearance of the tethyan ocean abstract fossil ages as young as priabonian 38 34 ma are reported for the last marine sedimentary rocks in southern tibet correlation is based on examination of foraminifers and nannofossil biostratigraphy of youngest preserved sediments in sections at gamba zongpu tingri qumiba as well as a previously unreported section at yadong our results demonstrate that a marine seaway remained in existence south of the yarlung tsangpo suture zone until at least priabonian time notably this remains a maximum age estimate in this area as all sections are truncated by erosion or faulting we compare our results with sections throughout the himalaya region to demonstrate that shallow marine conditions existed widely during the eocene period in fact it seems likely that the marine conditions in the tethyan himalaya did not entirely disappear by the end of priabonian especially in the eastern himalaya the data presented in this study place direct constraints on the elimination of the tethyan ocean and thus have important implications for timing of the india eurasia collision

the subduction initiation rule a key for linking ophiolites intra oceanic forearcs and subduction initiation we establish the subduction initiation rule sir which predicts that most ophiolites form during subduction initiation si and that the diagnostic magmatic chemostratigraphic progression for sir ophiolites is from less to more hfse depleted and lile enriched composi tions this chemostratigraphic evolution reflects formation of what ultimately becomes forearc lithosphere as a result of mantle melting that is progressively influenced by subduction zone enrichment during si the magmatic chemostratigraphic progression for the izu bonin mariana ibm forearc and most tethyan ophiolites is specifically from morb like to arc like volcanic arc basalts or vab boninites or bon because si progressed until establishment of a mature subduction zone morb like lavas result from decompression melting of upwelling asthenosphere and are the first magmatic expression of si the contribution of fluids from dehydrating oceanic crust and sediments on the sinking slab is negligible in early si but continued melting results in a depleted harzburgitic residue that is progressively metasomatized by fluids from the sinking slab subsequent partial melting of this residue yields typical ssz like lavas in the latter stages of si if si is arrested early e g as a result of collision morb only ophiolites might be expected consequently morb and ssz only ophiolites may represent end members of the si ophiolite spectrum the chemostratigraphic similarity of the mariana forearc with that of ophiolites that follow the sir intimates that a model linking such ophiolites oceanic forearcs and si is globally applicable

tectonic evolution of the qinghai tibet plateau abstract the qinghai tibet plateau composed of several continental slivers within the eastern tethyan domain is one of the pivotal sites to examine to better understand the theory of plate tectonics and the orogenic evolution on earth this plateau is generally inferred to be a collage of several continental blocks that rifted from gondwanaland and subsequently accreted to the asian continent however recent recognition of over twenty ophiolite melange zones and their associated island arcs indicates that the traditional model of tectonic evolution requires revision based on 177 recently finished 1 250 000 scale geological maps and related studies we summarize the main tectonic context of the qinghai tibet plateau and propose a new integrated model to account for the new findings the complex orogen of the immense qinghai tibet plateau consisting of multiple island arc basin systems that developed at different stages while surrounded by the north china yangtze tarim and indian plates is emphasized the entire orogen surrounded by suture zones that mark the locations of oceanic closure is investigated by examining i the first order tectonic units and ophiolitic melanges including arc arc continent collision zones and ii their internally enclosed blocks as the second order tectonic units therefore the qinghai tibet plateau is divided into three major orogenic systems namely from northeast to southwest the early paleozoic qinling qilianshan kunlunshan qin qi kun the late paleozoic triassic qiangtang sanjiang and the late paleozoic to cenozoic gangdese himalaya orogenic systems which are separated by the kangxiwa muzitagh maqin mianxian and the bangong shuanghu changning menglian sutures respectively we propose that the formation and evolution of the qinghai tibet plateau to have been intrinsically related to those of the eastern tethys recorded by the longmu co shuanghu ophiolite melange zone the southern qiangtang paleozoic accretionary arc basin system the bangong nujiang suture zone and their associated composite island arc basin systems the present day bangong shuanghu changning menglian suture system marks the final closure of the tethyan ocean the yarlung zangbo ocean opened as a back arc basin in response to the southward subduction of the tethyan ocean lithosphere in the middle triassic and closed as a result of the india asia collision at the end of cretaceous followed by the northward indention of the indian plate that resulted in significant intra continental deformation and plateau uplift in the cenozoic

suborbital timescale variability of north atlantic deep water during the past 200 000 years we generated 200 kyr long proxy records of surface and deepwater variability from a subpolar north atlantic core v29 202 enabling us to assess the linkage between surface and deepwater changes on suborbital timescales in particular we used a benthic 13c record to evaluate the deep water response to dansgaard oeschger temperature oscillations and to heinrich events times of massive iceberg delivery to the north atlantic we found that the reduction of north atlantic deep water nadw production was generally associated with cold or dropping sea surface temperatures ssts as indicated by planktonic foraminiferal assemblages the nadw contribution to the site did not drop appreciably during heinrich events h4 through h2 probably because these events followed intervals of prolonged surface cooling already characterized by low rates of nadw production by contrast nadw reduction appears to have been synchronous with h5 sst rise associated with both dansgaard oeschger oscillations and heinrich events was usually accompanied by increasing nadw strength in a few cases the nadw recovery appeared to lag the sst rise however the apparent delay is most likely an artefact of the sedimentary record low concentrations of benthic foraminifera as a result of low benthic foraminiferal abundances during stage 6 the stage 6 benthic foraminiferal 13c record is of lower resolution than the younger part of the record the stage 6 proxy records for surface hydrography nevertheless reveal millennial scale oscillations similar to those seen in stage 3 the available 13c data suggest that nadw weakened in association with the cold portions of stage 6 sst oscillations we also sought to confirm a recent study which concluded that there was little nadw variability during the peak of the last interglaciation marine oxygen isotope substage 5e eemian isotope stage 5 was marked by a trend of increasing benthic 13c in v29 202 rising 13c through isotope stage 5 is also seen in tropical surface water records and at some deep atlantic sites and may reflect the common derivation of these water masses variations of 0 5 superimposed on this rising 13c trend within substage 5e in v29 202 are so far not evident in tropical feed waters and may therefore indicate that nadw production was weaker during the late than mid eemian an electronic supplement of this material may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type quit to leave the system paper 95pa02089 suborbital timescale variability of north atlantic deep water during the past 200 000 years by d w oppo and s j lehman diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order

geochemical constraints on the origin of bimodal magmatism at the okinawa trough an incipient back arc basin abstract the magmatism at the axial zone of the middle okinawa trough a young continental back arc basin comprises a bimodal basaltic rhyolitic suite accompanied by minor intermediate rocks we report major and trace element and sr nd isotopie data for the intermediate to silicic suites to provide constraints on their petrogenesis the rhyolites recovered as lava and pumice fall into three geochemical groups type 1 2 and 3 rhyolites type 1 rhyolites have 87sr 86sr 0 7040 0 7042 and 143nd 144nd 0 5128 0 5129 identical to those of associated basalts and are characterized by highly fractionated ree patterns petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt in contrast type 2 rhyolites and andesite have slightly higher 87sr 86sr 0 7044 0 7047 but similar 143nd 144nd 0 5128 compared to those of the basalts the compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization afc processes of the basalt magma quantitative analysis suggests assimilation fractional crystallization ma mc ratios of 0 05 hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present we emphasize that mechanical extension in this part of the okinawa trough involves gabbro ic lower crust that resulted from fractionation of mantle derived basaltic magmas type 3 rhyolite occurs only as pumice which makes its derivation questionable this rhyolite has major and trace element compositions and sr nd isotopie ratios which suggests that it may be derived from volcanic activity on the southern ryukyu volcanic front and arrived in the okinawa trough by drifting on the kuroshio current

zircon u pb and hf isotope constraints on the mesozoic tectonics and crustal evolution of southern tibet the first in situ hf and u pb isotope analyses of zircon separates from mesozoic granites in southern tibet identify a significant previously unknown stage of magmatism igneous zircons n 34 from a granite within the gangdese batholith show a weighted mean 206 pb 238 u age of 188 1 1 4 ma and e hf t the parts in 10 4 deviation of initial hf isotope ratios between the zircon sample and the chondritic reservoir values between 10 4 and 16 8 suggesting predominantly early jurassic intrusive activity with a juvenile mantle contribution of 40 inherited zircons from two cretaceous s type granites in the northern magmatic belt 23 delineate a slightly older 206 pb 238 u age cluster between 188 and 210 ma these zircons have e hf t values from 3 9 to 13 7 yielding crustal hf model ages from ca 1 4 to 2 1 ga suggesting a major episode of crustal growth in proterozoic time and remelting of this crust in early jurassic time combining these with literature data we interpret the jurassic gangdese magmatism as an early product of the neo tethyan subduction that played a long lasting role in the tectonic evolution of southern tibet prior to the india asia collision

subduction zone decoupling retreat modeling explains south tibet xigaze and other supra subduction zone ophiolites and their uhp mineral phases abstract the plate tectonic setting in which proto ophiolite oceanic lithosphere is created remains controversial with a number of environments suggested recent opinions tend to coalesce around supra subduction zone ssz forearc extension with a popular conceptual model in which the proto ophiolite forms during foundering of oceanic lithosphere at the time of spontaneous or induced onset of subduction this mechanism is favored in intra oceanic settings where the subducting lithosphere is old and the upper plate is young and thin we investigate an alternative mechanism namely decoupling of the subducting oceanic lithosphere in the forearc of an active continental margin followed by subduction zone trench retreat and creation of a forearc oceanic rift basin containing proto ophiolite lithosphere between the continental margin and the retreating subduction zone a template of 2d numerical model experiments examines the trade off between strength of viscous coupling in the lithospheric subduction channel and net slab pull of the subducting lithosphere three tectonic styles are observed 1 c continuous subduction without forearc decoupling 2 r forearc decoupling followed by rapid subduction zone retreat 3 b breakoff of subducting lithosphere followed by re initiation of subduction and in some cases forearc decoupling b r in one case ba b r where ba denotes backarc subduction zone retreat follows backarc rifting subduction zone decoupling is analyzed using frictional plastic yield theory and the stefan solution for the separation of plates containing a viscous fluid the numerical model results are used to explain the formation of xigaze group ophiolites southern tibet which formed in the lhasa terrane forearc likely following earlier subduction and not necessarily during subduction initiation either there was normal coupled subduction before subduction zone decoupling or precursor slab breakoff subduction re initiation and then decoupling rapid deep upper mantle circulation in the models during subduction zone retreat can exhume and emplace material in the forearc proto ophiolite from as deep as the mantle transition zone thereby explaining diamonds and other 10 15 gpa uhp phases in tibetan ophiolites

the indus yarlung zangbo ophiolites from nanga parbat to namche barwa syntaxes southern tibet first synthesis of petrology geochemistry and geochronology with incidences on geodynamic reconstructions of neo tethys the purpose of this first synthesis is to summarize findings on the yarlung zangbo suture zone yzsz ophiolites and discuss still remaining problems the yzsz studied for almost 30 years and is the youngest of the sutures recognized on tibet plateau it is now acknowledged that the yzsz is a complex assemblage of sedimentary metamorphic and igneous rocks created during and shortly after the collision between india and eurasia the ages of the various lithologies span a time interval from the jurassic to the middle miocene with some permian and devonian exotic blocks from melange zone the yzsz is characterized by ophiolitic complexes and ophiolitic melange the ophiolites are of two types non dismembered and dismembered sections the non disturbed sections although tectonically reworked are observed along the segment from dazhuqu to jiding in xigaze area and spontang ophiolite the dismembered sequences are found in various locations such as nidar kiogar jungbwa saga sangsang xigugabu and luobusa the incomplete stratigraphic log could be connected to intraoceanic or orogenetic origins the ophiolites are distributed into two groups of ages the luobusa zedang and kiogar sequences being jurassic lower cretaceous whereas all other sequences are of lower cretaceous age compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70 my from their initial genesis to obduction which occurred around 70 90 my ago ophiolites differ in terms of petrological and geochemical aspects however they were all generated in suprasubduction zone and more specifically in arc few fore arc and back arc settings synthesis of more than 700 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 while the lower cretaceous ones show little to strong mixing in addition most ophiolites were created in short lived 30 my basins and generated close to the eurasiatic continental margin we propose that ladakh tibet ophiolites were generated in a suprasubduction context similar to mariana arc interarc and back arc or tonga lau system the variable arc signature of these ophiolites is directly related to their initial position within the suprasubduction system

origin of mesozoic ophiolitic melanges in the western yarlung zangbo suture zone sw tibet abstract the petrogenesis and tectonic evolution of the mesozoic ophiolitic melanges in the western section of the yarlung zangbo suture zone yzsz remain controversial in this paper we present the results of whole rock geochemical and sr nd isotope analyses zircon u pb ages and in situ lu hf isotopic data obtained from mafic rocks of the northern and southern sub belts of the western yzsz mesozoic ophiolitic melanges to help us understand these controversial issues diabases and dolerites from the northern sub belt and gabbros from the southern sub belt exhibit variable fore arc basalt fab like geochemical compositions and have zircon u pb ages of 126 4 120 3 ma in addition gabbro diabases from the northern sub belt have boninite series affinities and yield a zircon u pb age of 125 7 ma these results along with previous studies on the yzsz mesozoic ophiolitic melanges and the gangdese arc reveal that the western yzsz mesozoic ophiolites were likely generated over multiple stages in the epicontinental gangdese fore arc basin as the yarlung zangbo neo tethyan ocean subducted northward in front of the lhasa terrane the early cretaceous fab like and boninite series mafic rocks were formed by the reinitiation of subduction which was followed by a retreat of the subduction zone and the creation of the fore arc basin and strong hyperextension accompanied by asthenosphere upwelling at 130 120 ma during this process the upwelling asthenosphere underwent decompressional melting with limited penetration of slab derived fluids and gave rise to the n morb normal mid ocean ridge basalt like basaltic magmas that intruded the overlying previously generated depleted mantle as fab like gabbro diabase and dolerite sills or dykes then boninitic magmas represented by boninitic gabbro diabases were generated by remelting the extremely depleted residual mantle source which was metasomatized by a small amount of slab derived fluids following previous extractions of fab like magma

the δ13c of equatorial atlantic surface waters implications for ice age pco2 levels carbon isotopic records of nutrient depleted surface water place constraints on the past fertility of the oceans and on past atmospheric pco2 levels the best records of nutrient depleted 13c are obtained from planktonic foraminifera living in the thick mixed layers of the western equatorial and tropical atlantic ocean we have produced a composite stacked globigerinoides sacculifer 13c record from the equatorial atlantic which exhibits significant spectral power at the 100 000 and 41 000 year milankovitch periods but no power at the 23 000 year period similar to the record presented by shackleton and pisias 1985 surface deep ocean 13c produced with the g sacculifer record leads the 18o ice volume record however the glacial interglacial amplitudes of 13c differ between our record and shackleton and pisias 1985 record although large changes in 13c occur in the equatorial atlantic during early stages of the last three glacial cycles surface deep 13c at glacial maxima 18o stage 2 late stage 6 and late stage 8 was only about 0 2 greater than during the subsequent interglacial our results imply that nutrient driven pco2 changes account for about one third of the pco2 decrease observed in ice cores and consequently 13c should not be used as a proxy pco2 index enough variance in the ice core pco2 records remains to be explained that conclusions about pco2 and ice volume phase relationships should also be reexamined as much as 40 ppm pco2 change still has not been accounted for by models of past physics and chemistry of the ocean

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

cretaceous fore arc basalts from the tonga arc geochemistry and implications for the tectonic history of the sw pacific the tonga fore arc preserves a complex history of subduction initiation back arc basin formation and arc volcanism which has extended from the cretaceous to the present in this paper we discuss the geochemistry of a cretaceous basalt dolerite gabbro suite recovered in two dredges from the tonga fore arc at 19 s the geochemistry of the tonga fore arc suite is unlike that of the uniformly depleted morb basalts of the subducting pacific plate and therefore is unlikely to be accreted from pacific cretaceous crust the 102 ma age obtained for one tongan fore arc dolerite is contemporaneous with a major phase of cretaceous subduction related volcanism recorded both in detrital zircon age populations and associated volcanics from new caledonia and new zealand we believe that the tonga fore arc basalts are a remnant of a hypothesized once extensive cretaceous back arc basin called the east new caledonia basin which we propose to have existed from 102 to 50 ma the allochthonous poya terrane of new caledonia is geochemically very similar to the tonga fore arc basalts and represents a younger 84 55 ma remnant of the same basin subduction related cretaceous volcanics from the sw pacific representing both arc and back arc settings all appear to have similar zr nb values suggesting a common mantle component in their petrogenesis the tonga fore arc basalts are also similar to fore arc basalts recovered from the izu bonin mariana fore arc but unlike these basalts they are not associated with subduction initiation

paleomagnetism of the xigaze ophiolite and flysch yarlung zangbo suture zone southern tibet latitude and direction of spreading abstract the xigaze ophiolite 29 2 n 89 5 e which outcrops in the yarlung zangbo suture zone represents the remnants of an oceanic lithosphere formed in middle cretaceous times between the lhasa block to the north and the indian plate to the south in an attempt to define the paleo orientation and latitude of the spreading center at which it has been created a paleomagnetic study has been done on three sites in volcanics and overlying or interbedded radiolarites forming the upper part of the ophiolite sequence and also on seven sites in the xigaze group flysch which stratigraphically overlies the volcanics to the north in each site hand blocks carefully oriented both with sun and magnetic compass have been sampled the paleomagnetism data combined with structural data on the ophiolite dolerite intrusives allow a partial reconstruction of south eurasia at the time of formation of the xigaze ophiolite the paleolatitude of accretion and deposition of the xigaze ophiolite and overlying sediments is found to be 10 20 n both ophiolite and basin have encountered a 85 20 anti clockwise rotation the corresponding ridge was close to the southern margin of the lhasa block and was oriented n175 25

deglacial meltwater discharge north atlantic deep circulation and abrupt climate change high resolution paleogeochemical data from the north atlantic ocean indicate that in the interval 15 000 to 10 000 14c years before present b p north atlantic deep water nadw production was decreased or eliminated four times at about 14 500 and probably older 13 500 12 000 and 10 500 years b p each of these changes occurred at the same time as abrupt events of meltwater discharge to the surface ocean inferred from oxygen isotope studies of planktonic foraminifera and from glacial geological studies on land in addition each of these times may be associated with brief episodes of cooler climate in the north atlantic region the best example of which is the younger dryas cooling of 10 500 years ago these results support models linking meltwater discharge decreased nadw production and decreased north atlantic heat flux

basalts erupted along the tongan fore arc during subduction initiation evidence from geochronology of dredged rocks from the tonga fore arc and trench a wide variety of different rock types were dredged from the tonga fore arc and trench between 8000 and 3000 m water depths by the 1996 boomerang voyage 40ar 39ar whole rock and u pb zircon dating suggest that these fore arc rocks were erupted episodically from the cretaceous to the pliocene 102 to 2 ma the geochemistry suggests that mor type basalts and dolerites were erupted in the cretaceous that island arc tholeiites were erupted in the eocene and that back arc basin and island arc tholeiite and boninite were erupted episodically after this time the ages generally become younger northward suggesting that fore arc crust was created in the south at around 48 52 ma and was extended northward between 35 and 28 ma between 9 and 15 ma and continuing to the present day the episodic formation of the fore arc crust suggested by this data is very different to existing models for fore arc formation based on the bonin marianas arc the bonin marianas based models postulate that the basaltic fore arc rocks were created between 52 and 49 ma at the beginning of subduction above a rapidly foundering west dipping slab instead a model where the 52 ma basalts that are presently in a fore arc position were created in the arc back arc transition behind the 57 35 ma loyalty three kings arc and placed into a fore arc setting after arc reversal following the start of collision with new caledonia is proposed for the oldest rocks in tonga this is followed by growth of the fore arc northward with continued eruption of back arc and boninitic magmas after that time

cambrian stratigraphy and depositional history of the northern indian himalaya spiti valley north central india recent work on himalayan tectonics indicates that prior to the cenozoic collision of india and asia an enigmatic cambrian ordovician event may have strongly infl uenced the regional geology of the himalaya stratigraphic and sedimentological analyses of well preserved cambrian deposits are critical for understanding the nature of this early tectonic event and its infl uence on the later tectonic evolution of the himalaya the parahio formation defi ned herein of the parahio valley spiti region in the tethyan himalaya of india is the best biostratigraphically resolved section of cambrian strata in the entire himalaya this formation consists of 1350 m of dominantly siliciclastic deltaic deposits the formation ranges from uppermost lower cambrian lungwangmiaoan stage to middle middle cambrian hsuchuangian stage representing a time span of 5 10 m y it contains numerous mediumscale shoaling cycles that range from storminfl uenced offshore deposits to thick trough cross bedded fl uvial facies many thin carbonate beds with abundant trilobite fossils directly overlie the fl uvial facies and represent transgressive systems tract deposits the cycles are interpreted to have resulted from delta lobe switching based on a lack of systematic stratigraphic changes in cycle or facies thicknesses this paleoenvironmental reconstruction contradicts previous interpretations of this unit that range from deep sea fl ysch to shallow marine tidalites in addition our paleoenvironmental analysis and paleocurrent data suggest that the uppermost lower to middle cambrian deposits of the lesser and tethyan himalaya are parts of the same ancient northward prograding fl uvial deltaic depositional system of the paleo tethys margin of india an angular unconformity with overlying ordovician conglomeratic rocks has considerable local relief with meter scale scours and a valley fi ll 100 m thick the scours have northeast southwest orientations which parallel both the paleocurrents in the underlying parahio formation as well as published paleocurrent readings from the coarse red beds of the overlying ordovician strata the cambrian ordovician unconformity is of regional extent and our recent biostratigraphic database indicates that the minimum hiatus associated with the unconformity in spiti is 15 m y our sedimentological analysis and associated paleocurrent data from the parahio formation along with additional data from units both above and below the unconformity indicate that published models portraying foreland basin development at this time with southward directed thrusting are problematic an alternate possibility that uplift took place south of the tethyan himalaya is also problematic because no published stratigraphic or structural evidence exists for such an uplift to the south for either the greater or the lesser himalaya lithotectonic zones

greenland ice core studies abstract greenland deep ice cores represent continuous sequences of deposits spanning the last up to several hundred thousand years isotopes and impurities in the ice reveal a broad spectrum of information on past environmental conditions in the north atlantic region for example climatic changes in terms of surface temperature and annual precipitation turbidity and chemical composition of the atmosphere including its co 2 concentration volcanic activity and its climatic impact and cosmic radiation flux changes in the past one of the fundamental parameters in ice core studies is the isotopic composition of the ice for example the 18 o concentration on the relative scale which varies in phase with the temperature of snow formation the seasonal variations allow identification of annual layers in the core and hence absolute dating back to at least 8000 yrs b p with some reservations smoothed records may be interpreted in terms of climatic temperature changes cp fig 1 which shows profiles along two deep ice cores from dye 3 southeast greenland and camp century northwest greenland the latter spans all of the last glaciation and the preceding interglacial eem the time scale in units of 10 3 yrs b p has been derived by comparison of the most predominant features with their analogues in sea sediment cores dansgaard et al 1982 the dramatic shifts in times of full glacial severity seem to recur with a quasi periodicity of 2550 yrs perhaps caused by abrupt changes of the north atlantic sea ice cover spectral data analysis by the maximum entropy method shows similar but greatly damped oscillations in the well dated post glacial part of the camp century record in antiphase with independent estimates on world wide degree of glaciation dansgaard et al 1984 figure 2 left shows the deepest 340 m of the dye 3 profile plotted on a linear depth scale the very deepest 25 m of ice is silty and the s are higher than in holocene ice indicating a time of deposition with unusually high temperature of formation probably the eemian interglacial however the record is hardly continuous that far back in time fig 2 middle shows some of the oscillations in detail and fig 2 right gives further details of three representative shifts along with a dust concentration profile increasing towards left the dust is probably brought to greenland mainly from distant areas covered by loss the dust content changes less abrupt than and not at exactly the same depth which indicates that the measured shifts are not due to temporal discontinuities in the ice core the general antiphase between dust content and suggests higher storminess in times of lower s the circles are co 2 concentrations in air inclusions in the ice but it is still doubtful if they can be interpreted in terms of past atmospheric co 2 concentrations hammer et al 1980 great volcanic eruptions inject acid gases into the stratosphere particularly sulphuric gases which are spread all over the hemisphere under oxydation the strong acids re enter the troposphere and are subsequently washed out by precipitation causing acid layers in the ice sheets hence acidity profiles along the holocene part of the ice cores reveal violent volcanic activity in the northern hemisphere and comparison with the profiles suggest a substantial cooling impact on the climate hammer et al 1980

multicyclic history of the northern india continental margin northwestern himalaya 1 the geologic evolution of northern india is best recorded in the stratigraphic succession of the zanskar range northwestern himalaya which represents the most complete transect through this ancient continental margin sedimentary history began in the late proterozoic and recorded a late pan african orogenic event around the cambrian ordovician boundary when the gondwana supercontinent was eventually assembled the following long period of epicontinental deposition in shallow seas linked to palaeo tethys lasted until the early permian when a neo tethyan rift began to open between paleo india and the cimmerian microcontinents neo tethyan history can be subdivided into two sedimentary megasequences both recording a major tectonic and magmatic event in the lower part the first one began with breakup in the late permian and lasted until the end of the jurassic the second one started in the early cretaceous with the final detachment of india from gondwana and the opening of the indian ocean and ended with the india eurasia collision in the early eocene the two megasequences can be in turn subdivided into six transgressive regressive supersequences bounded by tectonically enhanced unconformities basal sandstone units of early permian late permian norian callovian early cretaceous and paleocene age are invariably associated with oolitic ironstones or reworked glauco phosphorites and mark the transgre sive part of each supersequence next condensed nodular carbonates or shales with pelagic fauna are typically overlain by thick shallowing upward marly units capped by regressive platformal carbonates the six tectono eustatic supercycles reflect successive rifting episodes which punctuated the progressive separation of india from the rest of gondwana and document the combination of plate microplate reorganizations and eustatic climatic and oceanographic changes in the tethyan domain after the onset of collision between india and asia close to the paleocene eocene boundary obduction of the remnants of the neo tethys ocean floor onto the indian margin began and the latter underwent multiphase deformation with fold thrust shortening followed by heating and extension after the main metamorphic event ophiolitic nappes were re thrusted and finally emplaced with their sedimentary sole on top of the passive margin succession

geochemistry and geochronology of the metamorphic sole underlying the xigaze ophiolite yarlung zangbo suture zone south tibet abstract strongly foliated amphibolite clasts are found embedded within the ophiolitic melange underlying the xigaze ophiolite near bainang and angren yarlung zangbo suture zone southern tibet these high grade amphibolites are interpreted as remnants of a dismembered subophiolitic metamorphic sole that would have formed during the inception of a subduction they include garnet clinopyroxene amphibolites clinopyroxene amphibolites and common amphibolites petrographic descriptions mineral chemistry and thermobarometry of these rocks can be found in a companion paper guilmette c hebert r dupuis c wang c s li z j 2008 metamorphic history and geodynamic significance of high grade metabasites from the ophiolitic melange beneath the yarlung zangbo ophiolites xigaze area tibet journal of asian earth sciences 32 423 437 the geochemistry of the amphibolites confirms that their protoliths were igneous mafic rocks of basaltic to pyroxenitic composition that were likely part of an upper oceanic crust rare earth elements contents are suggestive of an n morb origin however enriched liles and depleted nb ta ti when compared to n morbs rather suggest a suprasubduction zone influence a large proportion of the overlying ophiolitic mafic rocks share the same geochemical characteristics suggesting the protoliths of the amphibolites might have crystallized in the same environment as the xigaze ophiolitic crust likely in a back arc basin 40 ar 39 ar step heating dating of hornblende concentrates from three samples yielded ages of 123 6 2 9 ma 127 7 2 2 ma and 127 4 2 3 ma these cooling ages are slightly younger or overlapping magmatic and sedimentary ages obtained from the overlying ophiolite all these new data support a model in which the ophiolitic crust and the protolith of the amphibolites were formed along the same spreading center above a subduction zone the demise of the early subduction circa 130 ma forced the inception of a new subduction zone at the ssz spreading axis burying ssz mafic rocks underneath a ssz ophiolitic mantle

paleogeographic and tectonic evolution of southern tibet since middle cretaceous time new paleomagnetic data and synthesis during the second french chinese paleomagnetic campaign in tibet in the summer of 1981 we sampled the middle cretaceous red beds of the takena formation and the cenozoic volcanics of the lingzizong formation a detailed analysis of about 400 specimens of red beds reveals a stable component of normal magnetization n in the north of the lhasa block north of the nyainqentanglha range and two stable components in the south lt ht where the red beds are more strongly folded and are unconformably overlain by the lingzizong volcanics all three components are carried by hematite blocking temperatures range from 400 to 600 c for lt and up to the neel temperature of hematite for both n and ht lt is shown to postdate part of the folding of the takena whereas n and ht both indicate a positive fold test at the 99 probability level in addition a small less than 10 of the natural remanent magnetization antiparallel component is superimposed in most samples and may be consistent with a model of long term acquisition of a chemical remanent magnetization for the red beds analysis of the volcanics reveals a single stable component with mixed polarities carried either by hematite or magnetite the paleomagnetic directions indicate that 1 the lhasa block stood at 12 5 3 n by upper cretaceous and at 13 5 6 5 n in the paleocene eocene thus confirming the limited displacement of the margin of eurasia between albian aptian deposition of the red beds and eocene onset of the india asia collision these values imply crustal shortening by as much as 1900 850 km north of the block since the upper cretaceous and by 1400 1200 km south of the block since the onset of the collision 2 southern tibet formed a single tectonic unit by middle cretaceous time and 3 differential rotations are observed within the block and are related to the collision these rotations are discussed in the frame of a recent propagating extrusion tectonics model this study confirms the close paleogeographic relationship between indochina khorat plateau and southern tibet prior to the india asia collision

lower cretaceous xigaze ophiolites formed in the gangdese forearc evidence from paleomagnetism sediment provenance and stratigraphy the india asia suture zone of southern tibet exposes lower cretaceous xigaze ophiolites and radiolarian cherts and time equivalent asian derived clastic forearc sedimentary rocks xigaze group these ophiolites have been interpreted to have formed in the forearc of the north dipping subduction zone below tibet that produced the gangdese magmatic arc around 15 20 n or in the forearc of a sub equatorial intra oceanic subduction zone to better constrain the latitude of the ophiolites we carried out an integrated paleomagnetic geochronologic and stratigraphical study on epi ophiolitic radiolarites chongdui and bainang sections and xigaze group turbiditic sandstones unconformably overlying the ophiolite s mantle units sangsang section detrital zircon u pb geochronology of tuffaceous layers from the chongdui section and sandstones of the xigaze group at the sangsang sectionprovides maximum depositional ages of 116 5 3 1maand128 8 3 4ma respectively for the chongdui sectionand an asian provenance signature for the xigaze group paleomagnetic analyses integrated with rock magnetic experiments indicate significant compaction related inclination shallowing of the remanence within the studied rocks two independent methods are applied for the inclination shallowing correction of the paleomagnetic directions from the sangsang section yielding consistent mean paleolatitudes of 16 2 n 13 n 20 9 n and 16 8 n 11 1 n 23 3 n respectively these results are indistinguishable from recent paleolatitude estimates for the gangdese arc in southern tibet radiolarites from the chongdui and bainang sections yield low paleomagnetic inclinations that would suggest a sub equatorial paleolatitude but the distribution of the paleomagnetic directions in these rocksstrongly suggests a low inclination bias by compaction our data indicate that spreading of the xigaze ophiolite occurred in the gangdese forearc and formed the basement of the forearc strata

xigaze forearc basin revisited south tibet provenance changes and origin of the xigaze ophiolite our new stratigraphic sedimentological and micropaleontological analysis integrated with basalt geochemistry sandstone petrography and detrital zircon u pb and hf isotope data suggests the revision of current models for the geological evolution of the asian active margin during the cretaceous the xigaze forearc basin began to form in the late early cretaceous south of the gangdese arc during the initial subduction of the neotethyan oceanic lithosphere under the lhasa terrane well preserved stratigraphic successions document the classical upwardshallowing pattern of the forearc basin strata and elucidate the origin of the associated oceanic magmatic rocks the normal midocean ridge basalt n morb geochemical signature and stratigraphic contact with the overlying abyssal cherts chongdui formation indicate that the xigaze ophiolite formed by forearc spreading and represents the basement of the forearc sedimentary sequence volcaniclastic sedimentation began with thick turbiditic sandstones and interbedded shales in the late albian santonian ngamring formation followed by shelfal deltaic and fl uvial strata padana formation with fi nal fi lling of the basin by the campanian age forearc sandstones do not show the classical trend from feldspatholithic volcaniclastic to quartzo feldspathic plutoniclastic compositions indicating limited unroofi ng of the gangdese arc prior to collision u pb age spectra of detrital zircons are unimodal with a 107 ma peak in the lower ngamring formation 104 99 ma bimodal with a subordinate additional peak at 157 ma in the middle ngamring formation 99 88 ma and multimodal with more abundant pre mesozoic ages in the upper ngamring and padana formations 88 76 ma these three petrofacies with distinct provenances document the progressive erosional evolution of the gangdese arc with uplift of the central lhasa terrane and expanding river catchments to include the central lhasa terrane during the late cretaceous

zircon u pb geochronology and hf isotopic constraints on petrogenesis of the gangdese batholith southern tibet abstract during the mesozoic cenozoic northward neotethyan subduction and subsequent india asia collision gave rise to the extensive transhimalayan magmatism that stretches from burma and western yunnan through southern tibet to the ladakh and kohistan complexes to understand the age distribution and petrogenesis of the gangdese batholith the largest intrusive exposure along the transhimalayan magmatic belt fifty granitic samples were selected for in situ zircon u pb and hf isotopic analyses the u pb data suggest four discrete stages of magmatic activity i e 205 152 109 80 65 41 and 33 13 ma respectively with the 65 41 ma stage being the most prominent the hf isotopic data indicate that the gangdese batholith is overwhelmed by positive e hf t values which are comparable to those of the kohistan ladakh batholiths in the west but differ markedly from those of the chayu burma batholiths in the east most of the gangdese granites show similar and young hf model ages 1000 300 ma indicating their derivation from juvenile crust however those formed in the 65 41 ma stage exhibit more heterogeneous hf isotopic ratios with negative e hf t values being observed in some granites younger than 50 ma suggesting the involvement of old indian continental crust in their petrogenesis this age may thus mark the onset of the india asia collision the newly established zircon u pb age and hf isotope database of the gangdese batholith can be used as a powerful tracer or fingerprint when studying the source to sink relation of the sediments eroded from the southern tibetan plateau

dissolution reprecipitation of igneous zircon in mid ocean ridge gabbro atlantis bank southwest indian ridge abstract zircons recovered from oceanic gabbro exposed on atlantis bank southwest indian ridge typically display oscillatory and sector zoning consistent with igneous crystallization from mafic magmas in one rock of twenty investigated weak oscillatory zonation patterns are overprinted by secondary textural features characterized by mottled convoluted and wavy internal zonation patterns that are frequently associated with secondary micron to submicron scale micro porosity these zircons are hosted in a felsic vein that intruded an oxide gabbro both of which are cross cut by monomineralic amphibole and quartz rich veinlets zircons with weak oscillatory zonation patterns record a weighted average 206pb 238u age of 12 76 0 20 ma mswd 1 5 and have high trace element concentrations e g rees 0 4 2 2 wt y 0 6 2 8 wt p 0 4 0 9 wt and th u 0 1 0 5 these zircons are anomalously old 1 myr relative to the magnetic age for this portion of oceanic crust 11 75 ma in contrast zircons with non igneous secondary textures have a younger weighted average 206pb 238u age of 12 00 0 16 ma mswd 1 7 and have lower trace element concentrations e g rees 0 2 0 8 wt y 0 3 1 0 wt p 0 1 0 3 wt and slightly lower th u 0 1 0 3 the weighted average age of these zircons is similar to the magnetic anomaly age and other 206pb 238u ages of nearby rocks we do not observe a correlation between crystallographic misorientation internal texture or trace element chemistry we suggest that the decrease in trace element concentrations associated with the development of non igneous alteration textures is attributed to the purging of non essential structural constituent cations from the zircon crystal lattice at amphibolite facies conditions the mechanism of alteration re equilibration was likely an interface coupled dissolution reprecipitation processes that affected pre existing anomalously old zircons during shallow level magmatic construction of atlantis bank at 12 0 ma

a pan african origin and uplift for the gneisses and peridotites of zabargad island red sea a nd sr pb and os isotope study a sr nd pb and os isotopic study of peridotites and granulite facies gneisses from zabargad island in the red sea suggests that the tectonothermal petrogenetic and geochemical evolution of these rocks occurred largely during the pan african orogeny rather than the recent opening of the red sea sm nd model ages and whole rock errorchrons indicate that spinel iherzolites and gneisses differentiated from a common depleted mantle source about 700 ma the iherzolites were mylonitized metasomatized and amphibolitized during a structural event that juxtaposed the peridotites with the gneiss complex and uplifted the gneiss peridotite complex to relatively shallow crustal levels most radiometric dating schemes suggest a pan african age for this event the gneisses generally have lower 143nd 144nd 87sr 86sr 208pb 204pb 207pb 204pb and 206pb 204pb ratios than the peridotites they extend linear trends defined by the spinel and amphibole peridotites on sr nd sm nd and pb pb diagrams suggesting the gneisses were either the source or buffering medium for the pan african metasomatism only one post pan african event had a significant effect on the geochemistry of the gneiss peridotite complex shallow level metasomatism by ultrahot 750 900 c hypersaline solutions with high 87sr 86sr 0 710 ratio led to the development of gem quality olivine crystals as well as low pressure mineral assemblages in the peridotites gneisses and younger rocks plagioclase rich assemblages with apparent igneous textures troctolites that are most common in the southern peridotite body may have formed by interaction of these fluids with peridotite i e are pseudo troctolites metasomatism changed the 87sr 86sr sm nd and re os ratios of the plagioclase peridotites making them unsuitable representatives of the pan african mantle

assembly of the lhasa and qiangtang terranes in central tibet by divergent double subduction abstract integration of lithostratigraphic magmatic and metamorphic data from the lhasa qiangtang collision zone in central tibet including the bangong suture zone and adjacent regions of the lhasa and qiangtang terranes indicates assembly through divergent double sided subduction this collision zone is characterized by the absence of early cretaceous high grade metamorphic rocks and the presence of extensive magmatism with enhanced mantle contributions at ca 120 110 ma two jurassic cretaceous magmatic arcs are identified from the caima duobuza rongma kangqiong amdo magmatic belt in the western qiangtang terrane and from the along tso yanhu daguo baingoin daru tso magmatic belt in the northern lhasa terrane these two magmatic arcs reflect northward and southward subduction of the bangong ocean lithosphere respectively available multidisciplinary data reconcile that the bangong ocean may have closed during the late jurassic early cretaceous most likely ca 140 130 ma through arc arc soft collision rather than continent continent hard collision subduction zone retreat associated with convergence beneath the lhasa terrane may have driven its rifting and separation from the northern margin of gondwana leading to its accretion within asia

the lhasa terrane record of a microcontinent and its histories of drift and growth article i nfo the lhasa terrane in southern tibet has long been accepted as the last geological block accreted to eurasia before its collision with the northward drifting indian continent in the cenozoic but its lithospheric architecture drift and growth histories and the nature of its northern suture with eurasia via the qiangtang terrane remain enigmatic using zircon in situ u pb and lu hf isotopic and bulk rock geochemical data of mesozoic early tertiary magmatic rocks sampled along four north south traverses across the lhasa terrane we show that the lhasa terrane has ancient basement rocks of proterozoic and archean ages up to 2870 ma in its centre with younger and juvenile crust phanerozoic accreted towards its both northern and southern edges this finding proves that the central lhasa subterrane was once a microcontinent this continent has survived from its long journey across the paleo tethyan ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the qiangtang terrane to the north and with the indian continent to the south zircon hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks e g 50 90 0 70 and 30 100 to the mesozoic magmatism in the southern central and northern lhasa subterranes respectively we suggest that much of the mesozoic magmatism in the lhasa terrane may be associated with the southward bangong nujiang tethyan seafloor subduction beneath the lhasa terrane which likely began in the middle permian or earlier and ceased in the late early cretaceous and that the significant changes of zircon ehf t at 113 and 52 ma record tectonomagmatic activities as a result of slab break off and related mantle melting events following the qiangtang lhasa amalgamation and india lhasa amalgamation respectively these results manifest the efficacy of zircons as a chronometer u pb dating and a geochemical tracer hf isotopes in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics

stratigraphy and sedimentary history of the nepal tethys himalaya passive margin abstract the sedimentary history of the nepal tethys himalaya began with deposition of thick carbonates in the cambro ordovician followed by a mixed siliciclastic carbonate epicontinental succession recording two major deepening events in the early silurian and late devonian fossiliferous carbonate ramp deposits in the tournaisian were disconformably followed by white quartzose sandstones and black mudrocks with locally intercalated diamictites derived from sedimentary rocks and deposited in asymmetric tectonic basins rift stage break up in the mid early permian locally associated with effusion of tholeiitic lava flows was followed by a transgressive sandy to shaly locally coal bearing or bioclastic unit capped by condensed pelagic carbonates in the middle to late permian juvenile ocean stage subsidence of the cooling stretched crust led close to bathyal water depths in the olenekian but then slowed down in the middle triassic to increase again sharply in the late triassic owing to renewed extensional tectonic activity and sediment loading during up and out building of the indian continental terrace deposition of tropical platform carbonates finally became widespread in the middle liassic mature passive margin stage the initial fragmentation of gondwana in the middle jurassic led to rejuvenation of the indian craton and deposition of quartzo feldspathic hybrid arenites capped by condensed oolitic ironstones deposited at warm subtropical latitudes in the late bathonian middle callovian next a discontinuous pelagic grey marly limestone unit was followed by the ammonoid rich offshore spiti shale in the late jurassic the final disintegration of gondwana began in the berriasian when quartzose siliciclastics derived again from the rejuvenated indian craton and partly from recycling of older clastic successions were followed by thick deltaic to shelf volcaniclastics documenting eruption of alkali basalts in the valanginian followed in the hauterivian to albian by more felsic differentiates such as the trachyandesites exposed in the lesser himalaya 120 km to the south a widespread drowning episode fostered by waning volcaniclastic supply during a global eustatic rise is documented by a major glauconitic horizon deposited at middle southern latitudes in the late albian overlain by scaglia like pelagic limestones in the latest albian the final part of sedimentary history during the rapid northward flight of india and its collision with eurasia is not documented anywhere in nepal due to later erosion of upper cretaceous to lower tertiary strata

crustal structure of the mid atlantic ridge at 23 n from seismic refraction studies an explosive seismic refraction experiment using four ocean bottom hydrophone receivers was carried out along a 120 km long section of the mid atlantic ridge mar median valley at latitude 23 n immediately south of its intersection with the kane fracture zone the data are interpreted in terms of laterally homogeneous horizontally layered structures using travel time inversion and synthetic seismogram modeling techniques these solutions are combined using two dimensional ray tracing to produce a single model defining the major along axis structural changes occurring beneath this section of the median valley with the exception of slightly low layer 3 velocities the crust beneath several tens of kilometers of the mar median valley has all the characteristics of simple mature ocean crust including a well defined moho transition zone 8 km s upper mantle velocities and a total crustal thickness of 6 7 km no evidence is found for the presence of a steady state axial magma chamber in the crust or upper mantle and the excellent propagation characteristics and orderly amplitude versus range relationships imply the existence of a considerable degree of lateral homogeneity beneath the median valley over distances of several tens of kilometers along axis we infer from these observations that the basic seismic characteristics of the ocean crust are frozen in in a time period less than that separating the major volcanic injection events 10 000 50 000 years by hydrothermal circulation penetrating to the base of the crust along the 100 km long ridge segment studied here two major along axis structural changes occur the first is a 10 to 15 km wide along axis zone of lowered velocities in the lower crust centered beneath a major along axis topographic high this is interpreted to be the remnants of the most recent phase of injection that has temporarily left behind it a region of elevated temperatures and pervasive cracking and thus reduced velocities the second occurs abruptly at latitude 23 15 n and is accompanied by both a major change in rift valley and crestal mountain morphology and the apparent noncoincidence of the median valley and central magnetic anomaly north of this boundary the 30 km long ridge segment that abuts the kane fracture zone has a crustal thickness of 4 5 km and no distinctive layering to produce the characteristic amplitude patterns that typify mature oceanic crust interpretation of the cause of this structural change is uncertain but may be related to local tectonic events perhaps a recent 10 to 20 km ridge jump to the east

geochemistry and geochronology of mafic rocks from the luobusa ophiolite south tibet abstract this study presents geochemical compositions of mafic rocks outcropped in the luobusa ophiolite that locates at the eastern part of the yarlung zangbo suture zone yzsz the studied mafic rocks include gabbroic diabase dykes cutting the peridotites and foliated amphibolites embedded within the subophiolitic melange the diabases have low k 2 o contents and display n morb like geochemical characteristics i e with flat ree patterns and weak enrichment in lile e g rb ba th and u the gabbros show lree depleted patterns and variable enrichment in rb and ba foliated amphibolites mainly consist of hornblendes and plagioclases with minor titanites they commonly show lree depleted patterns with strong enrichment in lile three diabases have depleted sr nd hf isotope compositions with initial 87 sr 86 sr ratios of 0 703009 0 703502 e nd t of 5 0 to 8 5 and e hf t of ca 14 two gabbros have similar nd hf isotopes with the diabases but slightly higher initial 87 sr 86 sr ratios i e 0 704820 and 0 704550 compared to both diabases and gabbros the amphibolites have higher initial 87 sr 86 sr ratios i e 0 705131 0 705825 but more depleted nd hf isotope compositions with e nd t of 9 1 to 11 6 and e hf t of 18 2 to 21 9 geochemical compositions of the diabase dykes indicate that they were formed in a mid ocean ridge setting zircon sims u pb dating of the gabbroic dyke cutting the serpentinites yields an age of 128 4 0 9 ma which is identical within uncertainty to the zircon u pb age of the amphibolite i e 131 0 1 2 ma low u and th contents of zircons in the amphibolite support their metamorphic origin titanites in the amphibolites have been dated by la icpms and give u pb ages of 131 134 ma which are similar to the zircon u pb ages of the dated gabbro and amphibolite therefore we suggest that the luobusa ophiolite was generated at the early cretaceous and underwent the intra oceanic emplacement immediately after its formation this implies that ophiolites in the eastern segment of the yzsz were formed roughly synchronously with those in both central and western segments of the yzsz

boninitic volcanism in the oman ophiolite implications for thermal condition during transition from spreading ridge to arc the discovery of boninite a typical high mgo andesite in the oman ophiolite is reported the boninites in the oman ophiolite occur as lavas and dikes of the alley volcanic sequence that overlie or crosscut the spreading ridge derived lavas geotimes volcanic sequence and sheeted dikes the phenocryst mineral assemblage and the major and trace element compositions observed for these boninites resemble those of the izu mariana forearc boninites indicating that the alley boninites represent primitive melt generated by partial melting of hydrous peridotite the occurrence of boninite provides strong thermal and chemical constraints on the formation of the oman ophiolite that require hot hydrous shallow mantle 1250 c at 30 km depth to have underlain the proto oman ophiolite at the time of boninite generation the initiation of subduction of the young hot oceanic lithosphere and obduction of the future oman ophiolite near the spreading ridge and the resultant melting of the highly depleted shallow mantle wedge metasomatized by slab derived fluid represent the most favorable mechanism for the genesis of the alley boninites

early cretaceous subduction related adakite like rocks of the gangdese belt southern tibet products of slab melting and subsequent melt peridotite interaction abstract the limited geochronology and geochemistry data available for the early cretaceous igneous rocks of the southern gangdese belt southern tibet has resulted in the proposal of conflicting geodynamic models for the generation of the widespread cretaceous igneous rocks in the middle and northern parts of the belt to explore this issue we present shrimp u pb zircon data and geochemical and sr nd pb hf isotopic data for the mamen andesites from the southern margin of the gangdese belt the mamen andesites emplaced at 136 5 ma are sodic na2o k2o 1 2 2 3 and have geochemical characteristics typical of adakites i e high al2o3 high la yb ratios and sr contents low y and hree contents and positive eu anomalies except for high cr ni and mgo contents the andesites have initial 87sr 86sr t ratios of 0 70413 0 70513 positive end t values of 3 7 5 8 and 206pb 204pb t ratios of 18 37 18 51 207pb 204pb t ratios of 15 59 15 65 and 208pb 204pb t ratios of 38 43 38 72 in situ hf isotopic analyses of zircons that had previously been dated by shrimp yielded positive initial ehf t values ranging from 11 0 to 15 5 a model calculation using trace element and sr nd pb isotopic data indicates that several percent of subducted sediment is required to generate the mamen andesites which were derived via the partial melting of subducted neo tethyan slab morb sediment fluid and subsequently hybridized by peridotite in the mantle wedge our data indicate that the neo tethyan oceanic crust was subducted northward beneath the gangdese belt during the early cretaceous at a high angle our results are inconsistent with a tectonic model that advocates the low angle or flat slab subduction of neo tethyan oceanic crust in generating the widespread cretaceous magmatism recorded in the gangdese belt

rapid forearc spreading between 130 and 120 ma evidence from geochronology and geochemistry of the xigaze ophiolite southern tibet article i nfo the cretaceous xigaze ophiolite is best exposed at the central yarlung zangbo suture zone yzsz tibet which also includes the gangdese arc and the xigaze forearc basin this study reports new geochronological and geochemical data for this ophiolite to revisit its geodynamic and petrogenetic evolution the xigaze peridotites have low cao and al2o3 contents and u shaped rare earth element ree patterns suggesting that they are residues after moderate to high degrees of partial melting and were modified by infiltration of light rare earth element lree enriched boninitic melts the xigaze crustal rocks belong to two groups mid ocean ridge basalt morb like rocks and boninitic rocks showing a uniform lree depletion and flat to lree enrichment on chondrite normalized patterns respectively geochemically both groups show the influence of subducting oceanic slab derived fluids la icpms zircon u pb and lu hf analyses from dolerite and quartz diorite dikes which intruded into the mantle peridotite and dolerite sheeted sills show that they were generated between 127 and 124 ma the zircons possess positive ehf t values ranging from 7 5 to 17 3 taking into account the geological and geochronological characteristics of the central western yzsz we propose that ophiolites in this region formed in a forearc spreading setting through rapid slab roll back during subduction initiation between 130 and 120 ma following this stage of spreading the forearc was stabilized and the zone of melting migrated beneath the gangdese arc producing the voluminous late cretaceous granitoids displaying depleted mantle type hf isotopic compositions our model provides a new explanation for the generation and evolution of forearc type ophiolites

hot paleocene eocene gangdese arc growth of continental crust in southern tibet abstract the 1600 km long gangdese magmatic belt features extensive paleocene eocene i type intrusive rocks and coeval volcanic successions which can be divided into group i 69 53 ma group ii 53 49 ma and group iii 49 43 ma corresponding to neo tethyan slab rollback neo tethyan slab breakoff and ongoing indian asian collision respectively the magmas from these three groups show significant variations in geochemical and isotopic compositions which provide the information of the growth of continental crust in southern tibet the most voluminous magmatism in the gangdese belt occurred during 53 49 ma high zircon saturation temperature up to 800 c and ti in zircon temperature up to 980 c estimations suggest there is a period of thermal anomaly during 53 49 ma starting from 53 ma magmas have increased k2o contents and their zircons have decreased th u ratios and y and yb contents zircons from group ii have the most heterogeneous hf isotopic compositions ehf t 5 3 to 15 1 these are evident of ingress of asthenosphere mantle in the arc extensive crustal melting and magma mixing magma underplating during this time is the main mechanism for the growth of continental crust with the indian asian collision going on the magmas in group iii show high th y and la yb ratios and k2o contents but significantly low tzr and t ti zr values mostly below 750 c these features suggest the water fluxed melting of early arc residues occurred in the late stage of growth of continental crust the crust has been thickened and nearly mature at this stage this study has great implication on understanding of growth of continental crust in orogenic belts

north atlantic thermohaline circulation during the past 20 000 years linked to high latitude surface temperature during a surface cooling event 10 000 to 12 000 years ago higher cd ca and lower 13c 12c ratios are observed in benthic foraminifera shells from rapidly accumulating western north atlantic sediments data from sediment cores show that marked nutrient depletion of intermediate waters occurs in association with reduced glacial north atlantic deep water flux it is proposed that cold high latitude sea surface temperatures enhance intermediate water formation at the expense of deep water formation

the role of the deep ocean in north atlantic climate change between 70 and 130 kyr ago the suggestion1 that changes in north atlantic deep water nadw production are linked through surface heat flux to the atmospheric temperature over greenland is supported by earlier indications2 3 that nadw production decreased during glacial times and by the subsequent finding4 6 that it declined during the younger dryas cool period at the end of the last glaciation changes in north atlantic surface temperatures have been found7 to mirror high frequency temperature changes recorded in greenland ice cores over the past 80 kyr but the connection to abyssal circulation has yet to be established except for one or two isolated oscillations8 9 here we present carbon and oxygen isotope analyses of benthic foraminifera in a high resolution north atlantic deep sea sediment core for the period 70 130 kyr ago these data allow us to reconstruct the history of nadw production which shows a close correlation with greenland climate variability for much of this time interval suggesting that the climate influence of nadw variability was widespread we see no evidence however for changes in nadw production during substage 5e the eemian interglacial period in contrast with recent ice core data10 which suggest severe climate instability in greenland during this time period our results may support suggestions based on data from a second ice core that this apparent instability is an artefact caused by ice flow11 alternatively the eemian climate instability may have had a different origin from the subsequent climate events

sedimentary evidence of a cambro ordovician orogenic event in the northwestern himalaya abstract the late proterozoic early paleozoic sequence of the tethys himalaya provides evidence of an orogenic episode close to the cambrian ordovician boundary late pan african event pre middle cambrian sediments were deposited on a terrigenous tidal flat gradually passing upward to a peritidal carbonate platform the succession shows an overall transgressive trend which is reproduced at smaller scale by second and third order coarsening upward cycles terminated by the landward shift of carbonate facies during highstands of sea level sandstone petrography suggests a passive continental margin setting for this part of the sequence in middle cambrian times the carbonate platform underwent gradual drowning and outer shelf slope shales with intercalated tuffaceous material erupted from a nearby immature volcanic arc were deposited the shales are overlain by basinal turbidites attesting to the growth of a sandstone lobe enclosed in lobe fringe siltstones in the middle ordovician unconformable alluvial fan conglomerates and braid plain sandstones derived from uplifted sedimentary sequences indicate erosion of a newly formed orogenic belt comparison with early paleozoic and recent counterparts allows to interpret the cambro ordovician sedimentary evolution in terms of foreland basin development the thermal subsidence of a passive margin temporarily reduced by the passage of a peripheral bulge was enhanced strongly by flexural downwarping towards a subduction zone collision of a buoyant continental margin at the trench hampered subduction rapid uplift ensued with formation of a foreland fold thrust belt shedding sedimentary detritus into the adjacent molasse basins

palaeomagnetic results from the tibetan plateau palaeomagnetic measurements were carried out on 1325 oriented samples collected from 246 sites on a traverse of the tibetan plateau from lhasa to golmud in 1985 crossing the lhasa terrane qiangtang terrane and kunlun terrane high blocking temperature high coercivity statistically grouped magnetizations were isolated from the following units lhasa terrane cretaceous takena formation mid cretaceous nagqu volcanics mid cretaceous qelico volcanics qiangtang terrane norian batang group volcanics kimmeridgian yanshiping group paleocene to eocene fenghuoshan group kunlun terrane visean to namurian dagangou formation dykes of the triassic igneous province the triassic data from the kunlun terrane triassic and lower tertiary data from the qiangtang terrane and the cretaceous data from the lhasa terrane indicate palaeolatitudes ca 20 s of their present position within the eurasian frame of reference a possible interpretation is that the terranes successively accreted to eurasia and remained in the southern position until the convergence of india drove them northward via a process of tectonic shortening and or displacement of continental crust the carboniferous data from the kunlun terrane are consistent with moderate southern hemisphere latitude well separated from eurasia which was in the northern hemisphere at this time implying the existence of ocean crust between these blocks during the carboniferous

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

secular geochemical variations of the lower cretaceous siliciclastic rocks from central tibet china indicate a tectonic transition from continental collision to back arc rifting abstract fifty five lower cretaceous siliciclastic rocks from central tibet were analyzed for major and trace elements in an attempt to evaluate their provenance and the type of tectonic setting in which they were deposited the spatial temporal variation of both sediment maturity indices sio 2 al 2 o 3 la v and zr tio 2 the ratios of immobile trace elements cr th th sc th co la sc and la co and sio 2 k 2 o na 2 o eu eu gd yb n and ferromagnesian contents displays that the berriasian valanginian bv sediment could have been derived from northern orogenic source areas whereas the hauterivian lower barremian hb sediment could have been closely related to the syndepositional volcanism in the northern lhasa block itself the na 2 o al 2 o 3 k 2 o al 2 o 3 relations also place approximately half of hb samples in the field of volcanic rocks of back arc rifting origin whereas little volcanic clasts exist in the bv rocks there are distinct signatures that the mafic ultramafic materials in the bv rocks were derived from oceanic island basalts ophiolites in the bangong nujiang suture therefore the collisional orogen due to the qiangtang lhasa collision should have played a key role in supply of the sediments during bv time bimodal sedimentary basaltic and rhyolitic tuffs with respective distinguished geochemical features exist in the hb sequence therefore a distinct tectonic transition from continental collision to back arc rifting in central tibet is believed to have occurred during early cretaceous time extreme depletion of na of most lower cretaceous samples compared to the upper crust and their high average chemical index alteration cia values and th u ratios point out the source rocks formed in tropical climatic regions with little tectonic uplift although the weathering degree of the source rocks for hb samples could likely have been affected by the syndepositional volcanism during middle cretaceous time the entire southern eurasian margin could have been characterized by a series of back arc rift basins including those in kohistan and ladakh

geochemical and geochronological constraints on the origin and emplacement of the yarlung zangbo ophiolites southern tibet

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved

island arc tholeiite to boninitic melt evolution of the cretaceous kizildag turkey ophiolite model for multi stage early arc forearc magmatism in tethyan subduction factories article i nfo the kizildag ophiolite in turkey is a remnant of the late cretaceous suprasubduction zone ssz oceanic lithosphere that was developed in southern tethys and is part of a peri arabian ophiolite belt including the troodos cyprus baer bassit syria and semail oman ophiolites in the eastern mediterranean region the internal structure petrology and geochemistry of the kizildag intrusive and extrusive rocks show a magmatic progression from island arc tholeiite iat to boninitic compositions similar structural architecture and geochemical trends in the coeval troodos and baer bassit ophiolites suggest that this multi stage ssz magmatism was common in most tethyan ophiolites the sheeted dikes and the majority of extrusive rocks in kizildag are composed of basalt basaltic andesite and andesite with relatively low la smn 0 5 0 7 low mgo 7 9 wt and high tio2 0 7 1 0 wt and exhibit high dy ybn ratios these iat suites also include rare younger dacitic dikes showing strong enrichment in the lree the late stage basaltic dikes and lavas sakalavites have high la smn 0 8 1 0 high mgo 9 11 wt and low tio2 0 3 0 4 wt contents and display strong depletion in both lree and hree this suite of rocks represents the boninitic component of the kizildag ophiolite that developed from shallow and greater degrees of melting of hydrous and highly refractory peridotites beneath the extended forearc environment the progressive evolution of kizildag magmas from iat to boninitic compositions in time and space was a result of variable mixing aggregation and differentiation of geochemically different melt batches which formed in different levels polybaric within the melting column rapid slab rollback induced corner flow mantle return flow and asthenospheric diapirism providing more fertile melt directly affected the evolution of this melting column and may have triggered advanced degrees of shallow melting of the hydrous and highly depleted forearc mantle producing refractory liquids of the boninitic stage these geochemical patterns of the kizildag ophiolite reflect a multi stage evolution of ssz magmatism in tethyan subduction factories that is analogous to the earlier arc volcanism in the eocene oligocene izu bonin mariana system

zabargad and the isotopic evolution of the sub red sea mantle and crust abstract zabargad island in the red sea exposes ultramafic bodies in contact with partially retrograded granulite facies lower crustal rocks the contact could represent the sub red sea moho prior to the opening of the red sea the ultramafic rocks are largely protogranular spinel lherzolites that grade locally to porphyroclastic to mylonitic amphibole peridotites some of the peridotites are infiltrated by high mg gabbros troctolites the peridotites and the lower crustal rocks are cut by diabase dikes the zabargad peridotites are isotopically heterogeneous on a scale of meters or less sr and nd isotopic ratios of clinopyroxenes cpx from spinel lherzolites lie in the depleted portion of the 143 nd 144 nd 87 sr 86 sr mantle array whereas cpx and amphiboles from amphibole peridotites fall off the array towards the low 143 nd 144 nd and sol 87 sr 86 sr field that may characterize portions of the sub continental mantle cpx from troctolites plot in the middle of the 143 nd 144 nd 87 sr 86 sr mantle array and have higher 87 sr 86 sr ratios and more depleted lree patterns than the intruded spinel lherzolites and amphibole peridotites all whole rock lherzolites and peridotites and most diabase dikes from zabargad as well as a variety of rocks from other parts of the red sea area morb glasses a diabase dike from the brothers island basanites and clinopyroxenes from associated nodules from the ataq diatreme in yemen define a linear array on a sm nd isochron diagram with a best fit age of ca 675 ma and an initial nd of 6 7 this array is probably a mixing line between depleted morb type mantle and a low 143 nd 144 nd 87 sr 86 sr metasomatic component the age defined by the linear array may date either an episode of pan african mantle homogenization or in our preferred model the pan african segregation of a previously homogeneous mantle into two or more reservoirs of identical 143 nd 144 nd but different sm nd ratios the subsequent partial remixing of these isotopic reservoirs during metasomatism associated with the opening of the red sea did not perturbed the slope of the array the lree enriched reservoir may have been the pan african lower crust since there is field and textural evidence that the metasomatic fluids were derived from or equilibrated with the adjacent crustal complex alternatively the lree enriched component may have been a mantle reservoir similar to that which generated basalts and nodules from the nearby ataq diatreme the troctolites are isotopically distinct from the host peridotites and retain igneous textures suggesting they intruded the peridotites during the opening of the red sea their isotopic patterns are similar to those of recent volcanics from afar portions of the ataq diatreme and some zabargad dikes indicating derivation from a similar mantle source their very lree depleted patterns suggest they are either re melted fractions of previously very depleted spinel lherzolites or early formed mineral aggregates that became enmeshed in the peridotites as fractionating melt ascended to higher levels our results indicate that at least two major processes metasomatism and intrusion have modified the isotopic pat terns of the spinel lherzolites of zabargad had the resultant chemically and isotopically heterogeneous bodies stayed in the mantle instead of being uplifted to their present positions they would have proven to be enigmatic sources for future basalts and xenoliths

u pb zircon ages for yarlung tsangpo suture zone ophiolites southwestern tibet and their tectonic implications ophiolite complexes preserved along the yarlung tsangpo suture zone ytsz and obducted onto the northern continental margin of india in southern tibet represent the remnants of the once extensive permian mesozoic neo tethyan ocean that separated india from asia complete ophiolite successions are preserved near xigaze whereas the rest of the belt is essentially represented by mantle rocks with subordinate disrupted lower crustal rocks u pb zircon la mc icp ms geochronology on two gabbro samples from the luobusa ophiolite yielded concordant data with mean 206pb 238u ages of 149 9 1 4 2 ma and 150 0 5 0 ma these ages are in contrast to a younger age of 131 8 1 0 ma obtained from a pegmatitic gabbro in xigaze five u pb zircon tims ages from gabbroic samples in the western portion of the ophiolite belt reveal that the dangxiong ophiolite formed between 126 7 0 4 ma and 123 4 0 8 ma zircons from the jungbwa ophiolite have similar ages of 123 4 0 8 ma and 123 9 0 9 ma a single zircon analysed from a gabbro in kiogar has an age of 159 7 0 5 ma geochronological data reported here show ytsz ophiolites formed in association with intra oceanic subduction zone systems and are related a significant tectonic episode within the tethyan ocean during late jurassic to early to mid cretaceous times