magmatism and metamorphism in the lhasa block tibet a geochronological study u pb systematics in small fractions of zircon monazite and sphene 5 to 20 grains each have been analyzed in order to decipher the age and origin of magmatism in the lhasa block and to date the tectono metamorphic events additionally we analyzed a few minerals for rb sr the results show that the granites and an ignimbrite were formed essentially through anatexis of continental crust the magma source regions of which consisted of palaeozoic to precambrian materials up to 2 b y in age the following episodes can be distinguished 1 an early tertiary plutonism volcanism in the southern lhasa block yangbajing region strongly related to intracontinental tectono metamorphism 2 a late jurassic to early cretaceous plutonism in the central and northern lhasa block and 3 a middle jurassic low grade metamorphism affecting the basement at the northern margin 171 6 m y ago the early tertiary magmatism and metamorphism in the south are probably a direct result of tethys subduction indian plate

miocene adakitic intrusions in the zhongba terrane implications for the origin and geochemical variations of post collisional adakitic rocks in southern tibet abstract post collisional adakitic rocks 26 9 ma have been recognized in southern tibet for almost 20 years however their origins and geodynamics remain highly controversial this paper reports geochronological and geochemical data for the lasa intrusion exposed in the zhongba terrane and proposes implications for the origins of the miocene adakitic rocks in southern tibet the lasa intrusion consists of granite and granodiorite porphyries with zircon u pb ages of ca 16 ma the porphyries have high sio2 65 14 67 61 wt al2o3 15 13 16 10 wt k2o 2 87 3 54 wt and sr 727 1046 ppm contents and low y 9 13 ppm and yb 0 82 1 19 ppm contents they are enriched in light rare earth elements lrees and depleted in heavy rare earth elements hrees the porphyries display relatively low mgo contents 1 39 1 98 wt high sr y 65 98 and la yb n 18 8 25 5 ratios initial 87sr 86sr i ratios of 0 70912 0 70805 end t values of 6 4 to 7 9 206pb 204pb t ratios of 18 7108 18 7665 207pb 204pb t ratios of 15 6837 15 7118 and 208pb 204pb t ratios of 39 1600 39 3034 these signatures indicate that the lasa porphyries are adakitic rocks derived from the partial melting of a thickened lower crust the miocene adakitic rocks in the zhongba terrane have similar sr nd isotopic compositions to those of western lhasa and the himalaya terrane which differ from coetaneous adakitic rocks of the eastern lhasa terrane in terms of sr nd isotopic compositions based on the spatial distributions and isotopic features of the post collisional adakitic rocks in southern tibet we suggest that all of the adakitic rocks were derived from the partial melting of thickened lower crust and that their geochemical variations are mainly caused by the heterogeneity of lower crust

early cretaceous subvolcanic calc alkaline granitoid magmatism in the nubra shyok valley of the shyok suture zone ladakh himalaya india evidence from geochemistry and u pb shrimp zircon geochronology abstract the lithounits constituting the ladakh himalaya are exposed along the indus and shyok sutures zones of northwest himalaya the shyok suture zone ssz in northern ladakh represents a highly tectonized zone of a back arc basin which is mainly composed of volcano sedimentary formations shyok and khardung formations intimately associated with intrusive granitoids in the nubra shyok valley of the ssz calc alkaline granitoids of batholithic dimension are exposed in the tirit region referred herein as the tirit granitoids which are intrusive evidently into the shyok volcanic rhyolite rocks belonging to the shyok formation in this valley the northern margins of granitoids of the ladakh batholith can also be found intrusive into the metasediments shale slate and metavolcanics of the shyok formation the compositions and crystallization pressures 66 to 91 mpa of amphiboles in the intrusive tirit granitoid corroborate a calc alkaline nature and solidification of tirit granitoid melt at subvolcanic level equivalent to a minimum of 2 5 km to a maximum of 3 5 km thick overburden of shyok volcanics u pb shrimp zircons from the tirit granitoids have yielded mean crystallization ages of 109 4 1 1 ma and 105 30 0 80 ma which strengthen the idea of early cretaceous subduction beneath the karakoram terrain inherited older zircon cores 278 393 ma 476 519 713 952 ma and 1933 ma suggest a contribution from heterogeneous palaeozoic and proterozoic sources in the generation of the tirit granitoids similar to those observed elsewhere in the karakoram kohistan region a mean crystallization age 105 30 0 80 of zircons in the tirit granitoid hosting xenoliths of porphyritic volcanics places a minimum eruption age of ca 105 ma for the shyok volcanics the ladakh granitoid tirit granitoids and porphyritic volcanic xenolith belong to a calc alkaline series a mean crystallization age 67 32 0 66 ma for zircon in the ladakh granitoid implies that the shyok metasediments shale slate were deposited and metamorphosed prior to ca 68 ma present and earlier determined ages on tirit granitoids revealed that the subduction related calc alkaline magmatism in the nubra shyok valley of the ssz prevailed between 110 and 68 ma and hence a minimum age of early cretaceous can be suggested for development of the ssz

structures uplift and magmatism of the western myanmar arc constraints to mid cretaceous paleogene tectonic evolution of the western myanmar continental margin abstract knowledge of trans himalayan tectono magmatic evolution is critical to understanding the complex pre collisional history of southern eurasia active continental margin it has been proposed that magmatic rocks of the trans himalayan batholith extending from southern tibet to southeast asia are now exposed as the western myanmar arc and central granite belt in myanmar yet origin emplacement and relationships of the two juxtaposed belts remain poorly constrained in this study 2d seismic and drilling data for the western myanmar arc zircon u pb age and hf isotope and whole rock geochemical data for magmatic rocks from the arc have been applied our seismic profiles borehole stratigraphic sequences and zircon u pb data show that a typical arc basin system was well developed along the western myanmar continental margin the magmatic arc has experienced at least three igneous events in the mid cretaceous 110 90 ma latest cretaceous early paleocene 69 64 5 ma and eocene 53 38 ma as well as three associated uplift processes in the late cretaceous eocene and late oligocene whole rock geochemical characteristics and zircons showing variable but predominately positive e hf t values suggest a significant juvenile mantle source involving a proportion of ancient subducted sediments and juvenile crustal materials for these typical arc related magmatic rocks the identification of mid cretaceous to paleogene magmatic rocks having positive e hf t values from the western myanmar arc 1 indicates that the magmatism can be correlated with the gangdese arc within the lhasa terrane of the southern tibetan plateau 2 provides evidence for the proximal derived model that paleogene sediments in the central myanmar basin were from the western myanmar arc but were not delivered by the paleo yarlung tsangpo irrawaddy river system from the gangdese arc and 3 enables a model of eastward subduction of the neo tethyan indian oceanic crust to reflect onset of the magmatism at the mid cretaceous and a long existed back arc extension in western myanmar

the yarlung zangbo paleo ophiolite southern tibet implications for the dynamic evolution of the yarlung zangbo suture zone the yarlung zangbo suture zone a major geological structure in tibet is well known as the locus of tectonic emplacement of the tethyan ophiolites current models propose that most of the east tethyan oceanic lithosphere was subducted within a single subduction zone active during the middle or late cretaceous which was completed during the paleogene collision between india and asia the early cretaceous sedimentary giabulin formation in southern tibet includes conglomeratic members that contain ultramafic and mafic plutonic pebbles as well as radiolarian chert clasts that record the erosion of oceanic lithosphere involved in a subduction event which occurred earlier than previously believed geochemical analyses mineral chemistry stratigraphic chronology and sedimentary analysis including source provenance suggest that the pebbly conglomerate was formed through erosion of an unknown ophiolitic source that was geochemically distinguishable from the xigaze ophiolites within the yarlung zangbo suture zone southern tibet we infer the existence of an older ophiolitic source termed the yarlung zangbo paleo ophiolite that was dismembered and eroded during an earlier subduction stage not taken into account in current models q 2000 elsevier science ltd all rights reserved

subduction zones an introduction to comparative subductology abstract the subduction model for trench arc back arc systems was deduced more or less as a logical consequence of sea floor spreading many features in these systems such as thrust type inter plate earthquakes and deep earthquakes along wadati benioff zones are readily explained by the subduction model but other features such as extensional spreading and high heat flow in back arc regions and arc volcanism are difficult to explain an approach to solve these problems may be to recognize the existence of the two basically different modes of subduction one chilean type causing a compressional stress regime in the arc and back arc regions the other mariana type causing a tensional stress regime implications of the two types of the mode of subduction for some tectonic problems such as vertical movement and sediment accretion at trenches are discussed with special reference to the results of recent dsdp active margin drilling in discussing the possible causes of the two modes it is suggested that the large scale thermal and stress regimes in the back arc areas may not be the direct products of simple subduction but of some additional factors involving much greater energy such as mantle flow associated with the motion of major plates finally the possible great importance of various types of collision accretion and erosion at subduction zones is emphasized

contribution of mantle components within juvenile lower crust to collisional zone porphyry cu systems in tibet most porphyry cu mo au deposits are found in magmatic arcs worldwide and are associated with hydrous high fo2 calc alkaline magmas derived from a mantle wedge that was metasomatized by the fluids from a subducted oceanic slab recently such deposits have been documented as occurring widely in collisional settings where they are associated with potassic magmas generated during the collisional process but the genesis of the fertile magmas and the mechanism of metallic enrichment remain controversial here we present new geochemical and sr nd hf isotopic data from the post collisional fertile and barren porphyries of the miocene gangdese porphyry belt in the tibetan orogen an orogen formed by the collision of india and asia in the early cenozoic both types of porphyry are characterized by high k2o contents and have geochemical affinities with adakite but the fertile magmas were most likely derived from the melting of a thickened juvenile mafic lower crust formed by the underplating of earlier asthenospheric melts at the base of crust whereas the derivation of the barren magmas involved variable amounts of old lower crust in tibet the melting of sulfide bearing phases in the juvenile mantle components of the tibetan lower crust probably provided cu au and s to the fertile magmas the breakdown of amphibole during melting at the source released the fluids necessary for the formation of the porphyry cu deposits in tibet the thickened crust up to 70 80 km due to collision is thought to be responsible for a decrease in the fo2 of the fertile magmas during their ascent to the upper crust thus preventing the generation of more porphyry cu au and epithermal au deposits in this collisional zone

u pb zircon dating geochemical and sr nd hf isotopic compositions of motuo quartz monzonite implication for the genesis and diversity of the high ba sr granitoids in orogenic belt abstract early paleogene granitoids in southern lhasa subterrane have been widely investigated and many petrogenesis and geodynamic models have been proposed in the past few years however contemporaneous granitoids in the motuo tectono magmatic belt southeast lhasa terrane are still limitedly studied here we present the petrology zircon u pb geochronology whole rock geochemistry and sr nd hf isotope data of the damu and 52 k quartz monzonite in the motuo area la icp ms u pb zircon dating shows that they have magma crystallization ages of 49 and 69 ma respectively the damu quartz monzonite sio2 63 76 68 33 wt is high k calc alkaline k2o 2 54 4 02 wt with k2o na2o 0 59 1 09 and metaluminous to weakly peraluminous a cnk 0 99 1 07 the 52 k quartz monzonite sio2 61 12 66 12 wt shows slightly higher k2o contents 3 80 5 28 wt with k2o na2o 1 03 1 45 and metaluminous series a cnk 0 96 1 00 the analyzed samples are characterized by high ba 850 2573 ppm sr 534 986 ppm contents and fractionated ree patterns la yb n 22 72 and sm yb n 4 55 8 24 these geochemical features are comparable with those of high ba sr granite they display weakly evolved sr nd hf compositions whole rock 87sr 86sr 0 0 7068 to 0 7086 end t 4 20 to 3 41 and zircon ehf t 5 2 to 0 9 geochemical and sr nd hf isotopic data reflect that the damu and 52 k quartz monzonite represent residual magma from afc processes of lithospheric mantle derived mafic melts the over thickened lower crust in the eastern lhasa terrane had been delaminated during ca 83 70 ma which led to the replacement of ancient lithospheric mantle by the juvenile lithospheric mantle the juvenile mantle wedge in the study area was suspected to be metasomatized by melts that were derived from the foundering arc root rather than the subducted sediments thus the early paleogene high ba sr magmas from the se lhasa terrane may provide evidence for recycling of continental lithosphere by density destruction

paleomagnetic constraints on the geodynamic history of the major blocks of china from the permian to the present paleomagnetic study of china and its environs has been the center of a major international effort for the last 10 years in this paper we critically review all available paleomagnetic poles of upper permian to tertiary age from the main blocks of china with the goal of placing constraints on models of the formation and the subsequent deformation of the region after selecting reliable poles by applying objective criteria we divide our analysis into first order motions of blocks and second order deformation within blocks for first order analysis apparent polar wander paths are constructed for the major blocks we discuss the compatibilities and contradictions between the geological and paleomagnetic records a sequence of paleogeographic configurations taking into account geological constraints but remaining within paleomagnetic uncertainties is presented in general the major blocks were probably in contact throughout the permian and triassic but the jurassic was the key age during which most of the movement toward china s present configuration took place our reconstructions include certain details which are suggested by the paleomagnetic record but whose geological signatures seem to have been hidden by subsequent events during the cretaceous chinese poles agree with poles from other continents transferred onto eurasia at the second order we observe that for almost each period with sufficient data the paleomagnetic poles are streaked along a small circle centered on the sampling region indicating that much of china has been affected by small 20 differential rotations this we interpret as deformation caused in part by the extrusion of the chinese blocks away from the indian collision the complete annotated list of poles is given as an appendix

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

episodic tectonic plate reorganizations driven by mantle convection abstract periods of relatively uniform plate motion were interrupted several times throughout the cenozoic and mesozoic by rapid plate reorganization events r hey geol soc am bull 88 1977 1404 1420 p a rona e s richardson earth planet sci lett 40 1978 1 11 d c engebretson a cox r g gordon geol soc am spec pap 206 1985 r g gordon d m jurdy j geophys res 91 1986 12389 12406 d a clague g b dalrymple us geol surv prof pap 1350 1987 5 54 j m stock p molnar nature 325 1987 495 499 c lithgow bertelloni m a richards geophys res lett 22 1995 1317 1320 m a richards c lithgow bertelloni earth planet sci lett 137 1996 19 27 c lithgow bertelloni m a richards rev geophys 36 1998 27 78 it has been proposed that changes in plate boundary forces are responsible for these events m a richards c lithgow bertelloni earth planet sci lett 137 1996 19 27 c lithgow bertelloni m a richards rev geophys 36 1998 27 78 we present an alternative hypothesis convection driven plate motions are intrinsically unstable due to a buoyant instability that develops as a result of the influence of plates on an internally heated mantle this instability which has not been described before is responsible for episodic reorganizations of plate motion numerical mantle convection experiments demonstrate that high rayleigh number convection with internal heating and surface plates is sufficient to induce plate reorganization events changes in plate boundary forces or plate geometry are not required

dextral transpression and late eocene magmatism in the trans himalayan ladakh batholith north india implications for tectono magmatic evolution of the indo eurasian collisional arc the trans himalayan ladakh batholith is a result of arc magmatism caused by the northward subduction of the tethyan oceanic lithosphere below the edge of the eurasian plate the batholith dominantly consists of calc alkaline i type granitoids which are ferromagnetic in nature with the presence of magnetite as the principal carrier of magnetic susceptibility the mesoscopic and magnetic fabric are concordant and generally vary from wnw ese to ene wsw for different intrusions of ferromagnetic granites in different parts of the batholith strike of magnetic fabric is roughly parallel with the regional trend of the ladakh batholith in the present study area and is orthogonal to the direction of india eurasia collision in khardungla and changla section the magnetic fabric is distributed in a sigmoidal manner it is inferred that this sigmoidal pattern is caused by shearing due to transpression induced by oblique convergence between the two plates u pb zircon geochronology of a rhyolite from the southern parts of the batholith gives a crystallization age of 71 7 0 6 ma coeval with 68 ma magmatism in the northern parts of the batholith the central part of the batholith is characterized by s type two mica granites which gives much younger age of magmatism at 35 5 0 5 ma the magnetic fabric of these two mica granites is at a high angle to the regional trend of the batholith it is proposed that these two mica granites were emplaced well after the cessation of subduction and arc magmatism along fractures that developed perpendicular to the regional strike of the batholith due to shearing

gravity anomalies flexure and deformation of the converging indian lithosphere in nepal and sikkim darjeeling himalayas researchers ubiquitously noted that the common processes of partitioning oblique convergence in response to drag from the trench hanging plate simultaneously produce radial slips along strike translation and extension parallel to the deformation front here we focus on the area between nepal and sikkim darjeeling himalayas and carry out gravity and finite element stress modeling of the strike orthogonal converging indian lithosphere we delineate the geometries of different layers and their interfaces through gravity modeling the optimum model parameters along with rheological parameters of different layers are used for finite element modeling finite element modeling is done with boundary conditions of keeping the upper surface free and rigidly fixing the section of the northern boundary below the main himalayan thrust we impart on its frontal section an amount of 6 1012 n m force equivalent to resistive force of the himalayan tibet system and analyze the maximum and minimum compressive stress fields evolved in the lithosphere we testify our observations with earthquake database and other geophysical and geological studies we note that an increasing flexing of the indian lithosphere beyond the main boundary thrust becomes maxima between the main central thrust and south tibetan detachment in both the areas however more steepening of the moho boundary is identified in the sikkim darjeeling himalaya this abrupt change in lithospheric geometry beneath the greater himalaya is likely correlated with the sharp elevation changes in the topography although the highest seismicity concentration is dominant in this zone the lesser and the tethys himalayas in sikkim darjeeling area also record relatively fair seismic activity more compressive stress field in different layers right within the sharp bending zone supports this observation we thus propose that the sharp bending zone beneath the greater himalaya is suffering maximum deformation and the deformation is continued in the mantle too we also identify both right lateral shear and radial vergence slip which are presumably associated with the general dynamics and kinematics of the himalaya

u pb zircon constraints on the tectonic evolution of southeastern tibet namche barwa area the eastern syntaxis of the himalayas is expressed in the crust as a pronounced southward bend in the orogen the change in strike of geologic features coincides with the high topography of the namche barwa region the exposure of granulite grade metamorphic rocks and a 180 degree bend in the yalu tsangpo we have conducted a geochronologic and geochemical investigation of several suites of granitoids collected from the namche barwa massif and subjacent terranes of southeastern tibet ranging from cm scale dikes and sills to larger outcrop scale intrusions u pb shrimp rg zircon ages establish at least five magmatic episodes 400 to 500 ma 120 ma 40 to 70 ma 18 to 25 ma and 3 to 10 ma these episodes broadly correlate to spatial patterns in sample localities as follows 400 to 500 ma ages occur in zircon cores collected from within the massif proper 120 ma granites related to early gangdese arc plutonism are primarily located northeast of namche barwa later 40 70 ma gangdese activity is expressed in granites west of namche barwa 18 to 25 ma granites occur both along the suture zone west of gyala peri and directly north of namche barwa along the area of the jiali fault zone and are attributed both to shearing within the jiali fault zone and to an early miocene gangdese thrust event exceptionally young 10 ma zircon ages are clustered near the core of the massif along the yalu tsangpo gorge trace element geochemical data indicates the presence of both fluid present and fluid absent melts with a fluid absent decompression melting regime dominating near the core of namche barwa

constraints to the timing of india eurasia collision a re evaluation of evidence from the indus basin sedimentary rocks of the indus tsangpo suture zone ladakh india deposited within the indus tsangpo suture zone the cenozoic indus basin sedimentary rocks have been interpreted to hold evidence that may constrain the timing of india eurasia collision a conclusion challenged by data presented here the eurasian derived 50 8 51 ma chogdo formation was previously considered to overlie indian plate marine sedimentary rocks in sedimentary contact thus constraining the timing of collision as having occurred by this time using isotopic analysis u pb dating on detrital zircons ar ar dating on detrital white mica sm nd analyses on detrital apatite sandstone and conglomerate petrography mudstone geochemistry facies analysis and geological mapping to characterize and correlate the formations of the indus basin sedimentary rocks we review the nature of these contacts and the identification and correlation of the formations our results reveal that previously interpreted stratigraphic contacts identifying chogdo formation unconformably overlying indian plate sedimentary rocks are incorrect rather we suggest that the inaccuracy of previous interpretations is most likely a result of formation misidentification and thus cannot be used to constrain the timing of india asia collision

the transhimalaya gangdese plutonism in the ladakh region a upb and rbsr study abstract the age and origin of the transhimalaya gangdese plutonic belt in the ladakh area has been studied by high resolution u pb analyses of accessory minerals zircon and monazite allanite and rb sr measurements on whole rock samples the ages determined of 101 2m y for a granodiorite and of 60 7 0 4m y for a granite substantiate that the transhimalaya plutonism was active at least in mid cretaceous albian and earliest tertiary palaeocene times a rb sr isochron of 73 4 2 4m y might be fortuitous because the rb sr systematics shows important heterogeneities on the whole rock scale the inherited radiogenic lead in zircon demonstrates that anatexis of continental crust was involved in magma genesis the thus recycled continental material was heterogeneous with respect to its primary ages and or metamorphic history the approximate minimum ages of the continental sources range from 350 to 590 m y the 101 2m y old transhimalaya granodiorite which intrudes a series of mainly basaltic island arc rocks dras series shows that this island arc was attached to the continental margin at that time this is consistent with the hypothesis that the ladakh tethys closed through two subduction regimes 1 an early cretaceous subduction which formed the dras island arc and 2 a late cretaceous to palaeocene subduction at the continental margin which caused the emplacement of the transhimalaya plutonic belt

geochemical variations in miocene adakitic rocks from the western and eastern lhasa terrane implications for lower crustal flow beneath the southern tibetan plateau abstract it is generally believed that miocene adakitic rocks in the lhasa terrane block of southern tibet were derived from a thickened lower crust and that their compositions will therefore reflect the geochemical characteristics of their lower crustal source s adakitic rocks in the western part of the lhasa terrane possess geochemical and sr nd isotopic characteristics that are clearly different from those in the eastern part of the lhasa terrane the former display a more enriched nd sr isotopic signature together with an older model age relative to the latter suggesting that the compositions of the lower crust under the western and eastern parts of the lhasa terrane also differ in addition the adakitic rocks in the himalayas on the southern side of the indus yalu suture iys have similar geochemical characteristics and sr nd isotopic compositions to the adakitic rocks of the western lhasa terrane but differ from rocks in the eastern lhasa terrane and from igneous rocks derived from crust beneath the himalayas we suggest that the lower and or middle crustal materials beneath the western lhasa terrane probably extend to the southern side of the iys due to channel flow if mid lower crustal flow has occurred below southern tibet the southeastward moving anatectic mid lower crustal material was probably derived from below the western part of the lhasa terrane rather than the eastern moreover the emplacement ages of adakitic rocks from mayum on the southern side of the iys imply that southeastward ductile flow of mid lower crust beneath western lhasa may have occurred as early as 17 ma

trans hudson orogen of north america and himalaya karakoram tibetan orogen of asia structural and thermal characteristics of the lower and upper plates 1 the trans hudson orogen tho of north america and the himalaya karakoram tibetan orogen hkto of asia preserve a paleoproterozoic and cenozoic record respectively of continent continent collision that is notably similar in scale duration and character in tho the tectonothermal evolution of the lower plate involves 1 early thin skinned thrusting and barrovian metamorphism 2 out of sequence thrusting and high t metamorphism and 3 fluid localized reequilibration anatexis and leucogranite formation the crustal evolution of the indian lower plate in hkto involves 1 early subduction of continental crust to ultrahigh pressure uhp eclogite depths 2 regional barrovian metamorphism and 3 widespread high t metamorphism anatexis and leucogranite formation the shallow depths of the high t metamorphism in hkto are consistent with early to mid miocene ductile flow of an indian lower plate midcrustal channel from beneath the southern tibetan plateau to the greater himalaya melt weakening of the lower plate in tho is not observed at a similar scale probably due to the paucity of pelitic lithologies tectonothermal events in the upper plate of both orogens include precollisional accretion of crustal blocks emplacement of andean type plutonic suites and high t metamorphism syncollisional to postcollisional events include emplacement of garnet biotite muscovite leucogranites anatectic granites and sporadic metamorphism up to 90 myr following the onset of collision in tho comparing the type and duration of tectonothermal events for tho and hkto supports the notion of tectonic uniformitarianism for at least the later half of dated earth history and highlights the complementary nature of the rock record in an older exhumed orogen compared to one undergoing present day orogenesis

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

geochronological geochemical and sr nd hf isotopic constraints on the petrogenesis of late cretaceous a type granites from the sibumasu block southern myanmar se asia abstract the late cretaceous to paleogene granitoids occur widespread in the sibumasu block within myanmar se asia which show a close association with tin tungsten mineralization however the precise timing petrogenesis and tectonic significance of these granitoids are poorly constrained so far in this study we present a detailed study on geochronology elemental and sr nd hf isotopic geochemistry for the hermyingyi and taungphila granites in southern myanmar with the aim of determining their petrogenesis and tectonic implications la icp ms u pb dating of zircon grains from the two granites yield ages of 69 70 ma indicating a late cretaceous magmatic event these granitic rocks are weakly peraluminous and belong to the high k calc alkaline series they are both characterized by high sio2 k2o na2o feot feot mgo and ga al ratios and low al2o3 cao mgo p2o5 and tio2 contents enriched in rb th u and y but depleted in ba sr p and eu suggesting an a type granite affinity moreover they display prominent tetrad ree patterns and non charac trace element behavior which are common in late magmatic differentiates with strong hydrothermal interaction or deuteric alteration the granites belong to a2 type and probably formed at a high temperature and anhydrous condition they have zircon ehf t values from 12 4 to 10 0 and whole rock end t values from 11 3 to 10 6 with paleoproterozoic tdm2 ages 1741 1922 ma for both hf and nd isotopes geochemical and isotopic data suggest that these a type granites were derived from partial melting of the paleoproterozoic continental crust dominated by metaigneous rocks with tonalitic to granodioritic compositions without significant input of mantle derived magma and followed by subsequent fractional crystallization by integrating all available data for the regional tectonic evolution in se asia and adjacent regions we attribute the formation of the late cretaceous a type granites to a back arc extension in the hinterland behind the subduction zone which is induced by the rollback of the flat neo tethyan subducting slab around ca 70 ma

geology of the post collisional porphyry copper molybdenum deposit at qulong tibet porphyry deposits are usually thought to form from subduction related calc alkaline magmas in magmatic arc settings although some porphyry deposits also occur in post collisional extensional settings the post collisional deposits remain poorly understood here we describe the igneous geology alteration mineralogy and mineralization history of qulong a newly discovered porphyry cu mo deposit in southern tibet that belongs to the post collisional class the deposit is associated with miocene monzogranite granodiorite intrusions and is hosted partly by jurassic andesitic dacitic volcanic rocks the deposit contains 7 1 mt cu and 0 5 mt mo metal the 19 5 ma granodioritic monzogranitic rongmucuola pluton with diorite enclaves is the earliest miocene intrusive unit it was intruded by a regularly shaped stock p porphyry and then thin dikes x porphyry of monzogranite at about 17 7 ma the main cu mo mineralization is associated with the p porphyry a barren diorite porphyry intruded the p and x porphyries around 15 7 ma petrologic trends of the intrusions suggest that the miocene intrusions have similar origins and probably formed by fractionation of a deeper magma chamber emplacement of miocene porphyries controlled by the qulong anticline is in direct response to the rapid uplift erosion of the gangdese arc batholiths in southern tibet miarolitic cavities and unidirectional solidification textures key evidence for volatile separation have been recognized in the p and x porphyries respectively early potassic alteration characterized by quartz k feldspar anhydrite pervades the p porphyry and rongmucuola pluton laterally this alteration grades into quartz biotite anhydrite k feldspar which affects all miocene intrusions except the latest dioritic porphyry wall rocks of rongmucuola pluton and jurassic andesitic dacitic volcanics within 1 1 5 km of the porphyries are dominated by pervasive potassic alteration an outer halo of propylitic alteration epidote chlorite calcite extends up to 2 km away from the deposit feldspar destructive alteration sericite chlorite clay minerals has overprinted most of the potassic and part of the propylitic alteration the alteration is strongly pervasive in the interior of the porphyry bodies and occurs as vein halos away from the porphyry bodies the earliest quartz k feldspar alteration and veins are barren whereas approximately 60 of the cu reserves is associated with slightly later quartz biotite anhydrite alteration barren assemblages are related to irregular quartz k feldspar anhydrite veins which are truncated by the x porphyry cu sulfide bearing assemblages are associated with discontinuous chalcopyrite biotite and continuous quartz anhydrite chalcopyrite molybdenite veins deposition of cu mo with abundant anhydrite occurred during or between emplacement of closely related porphyries from high temperature magmatically derived fluids and was probably caused by the disproportionation of so

northern margin of the indian plate some litho tectonic constraints abstract the indus tsangpo suture zone represents crustal expression of a subduction zone in the himalaya all along its extension in kohistan ladakh and tibet the suture zone from south to north is constituted of litho tectonic assemblages referable to trench zone fore arc basin island arc and back arc basin these assemblages lie between the platform sediments deposited at the northern margin of indian and southern margin of tibet the trench assemblage consisting of ophiolitic assemblages exhibits high pressure glaucophanitic metamorphism the flysch sequence of the indus group represents fore arc sediments deposited in front of the ladakh magmatic arc comprising dras volcanics the ladakh plutonic complex and the khardung volcanics to the north of the magmatic arc the back arc basin assemblage represented by the shyok group consists of acid and basic volcanics and volcanoclastics intruded by granitic plutons the rocks of the arc and back arc region exhibit low pressure high temperature metamorphism the high pressure and low pressure metamorphic belts lying to the south and north respectively of the magmatic arc constitute paired metamorphic belts detailed analysis of tectonic and lithological characters of various litho tectonic units indicate that these developed at the northern margin of the indian plate during the mesozoic and early tertiary the miocene pliocene molassic rocks overlying various litho units unconformably were deposited after the collision of the indian and tibetan plates

tibetan tectonics from 40ar 39ar analysis of a single k feldspar sample ar 39ar data on an alkali feldspar sample from the quxu pluton gangdese batholith southern tibet allow a detailed assessment of unroofing and uplift history between 35 and 18 ma the 39ar arrhenius plot for this sample shows departures from a linear relationship between the effective diffusion parameter log d r2 and reciprocal temperature which we interpret to be the result of a distribution of distinct diffusion domain sizes we use an alternative way of plotting the arrhenius data that exhibits domain size versus cumulative 39ar released during step heating the 4 ar 39ar age spectrum of the sample has features such as local age plateaus that are most easily explained in terms of the distinctive closure age of particular domains the fact that the same distribution of diffusion domain sizes explains both the arrhenius data and the age spectrum is an indication that the diffusion properties operating in the laboratory are those of the sample while it was in its natural environment modelling of the age spectrum with a distribution of domain sizes results in the recovery of a continuous cooling history segment rather than a single time temperature datum we demonstrate the robustness of the cooling curve determination by showing the large misfits to the age spectrum that arise from relatively small changes in the cooling history the best fit cooling curve for the quxu sample shows a decreasing rate of cooling in the time interval 35 18 ma followed by a very rapid cooling beginning at about 18 ma we have used a thermal model for the conductive cooling of an unroofing pluton to estimate the rate of unroofing required to explain the quxu cooling curve and find that in the 35 20 ma time interval the primary control of the thermal evolution is the conductive loss of magmatic heat with little or no unroofing unroofing rates of approximately 0 05 mm yr followed by a brief period 5 ma of very rapid unroofing with rates of order 2 mm yr

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

tectonic and sedimentary basin evolution of the eastern bangong nujiang zone tibet a reading cycle the ophiolite bearing bangong nujiang zone bnz traversing central tibet from east to west separates the qiangtang block in the north from the lhasa block in the south their stratigraphic development indicates that both blocks once formed a continuous continental platform until the late triassic following late paleozoic triassic rifting ocean crust formed between both blocks during the late triassic creating the dongqiao naqu basin dnb among other basins yu et al 1991 the analysis of the rift flank sequences reveals that rifting was dominated by transtension the basin was shortened by post mid cretaceous transpression thus the overall basin evolution represents a reading cycle despite some active margin processes which gave this cycle a special imprint major basin parts were preserved despite transpressional shortening suggesting that the eastern bnz represents a remnant basin our understanding of the dnb solves the prior problem of viewing the bnz as a mid late jurassic collisional suture although typical collision related deformation thickening mountain building as well as related molasse formation are lacking our model also explains the scattered linear ophiolite distribution by local transpression of remnant oceanic basin floor without having to consider problematic long range ophiolite thrusting

the himalayan arc large scale continental subduction oroclinal bending and back arc spreading abstract palaeomagnetic results from the himalayan arc and southern tibet compared with simulated apparent polar wander paths for the indian plate show a consistent pattern of rotations of the himalayan arc relative to the indian shield varying gradually from 45 clockwise in the northwestern himalaya to slightly counterclockwise in the lhasa region this pattern is consistent with continental underthrusting of greater india beneath the tibetan plateau since the early miocene over at least 650 km at the longitude of western nepal and oroclinal bending since the latest miocene available palaeomagnetic observations support the steady state model for the formation of the himalayan arc with refinements as follows 1 collision between greater india s northern boundary and southern asia occurred at equatorial latitudes with progressive suturing from palaeocene in the northwestern himalaya until early eocene in the eastern himalaya 2 continuing convergence and indentation of greater india into southern asia over about 2000 km up to the early miocene resulted in southeastward extrusion of indochina and 3 neogene counterclockwise rotational underthrusting of greater india along the main central thrust with pliocene quaternary oroclinal bending of the himalayan arc

geological characteristics and genesis of the jurassic no i porphyry cu au deposit in the xiongcun district gangdese porphyry copper belt tibet abstract the xiongcun district located in the western segment of the gangdese porphyry copper belt gpcb hosts the only known jurassic mineralization in the gpcb tibet prc the no i deposit in the xiongcun district is related to the middle jurassic quartz diorite porphyry 167 161 ma and the mineralization was formed at ca 161 5 2 7 ma ore bearing middle jurassic quartz diorite porphyry emplaced into the early jurassic volcano sedimentary rock sequences of the xiongcun formation veinlets and disseminated mineralization developed within the middle jurassic quartz diorite porphyry and the surrounding metamorphosed tuff hosting a measured and indicated resource of 1 04 mt copper 143 31 t gold and 900 43 t silver with an average grade of 0 48 copper 0 66 g t gold and 4 19 g t silver the mineralization can be assigned to four stages including three main stages of hypogene mineralization and one epigenetic stage the main alteration associated with mineralization is potassic seven mineralization related hydrothermal veins have been recognized including quartz sulfide biotite sulfide magnetite sulfide quartz molybdenite sulfide chalcopyrite pyrite pyrrhotite pyrite and polymetallic veins the s and pb isotopic compositions of the ore sulfides and the re contents of the molybdenite suggest a mantle source for the ore forming materials with minor contamination from the subducted sediments hydrogen and oxygen isotope compositions of quartz in the ores suggest that both magmatic and meteoric waters were involved in the ore forming process the ore bearing porphyry 167 161 ma and ore forming 161 5 2 7 ma ages of the no i deposit correspond to the time of northward subduction of neo tethys oceanic slab the geochemical data of the ore bearing porphyry indicate that the no i deposit formed in an intra oceanic island arc setting and the ore bearing porphyry originated from the partial melting of mantle with limited contribution of subducted sediments the genesis of the ore bearing porphyry and no i deposit is interpreted as being related to northward intra oceanic subduction of neo tethys oceanic slab in the middle jurassic time 167 161 ma

magmatism and metamorphism in the ladakh himalayas the indus tsangpo suture zone abstract ladakh india provides a complete geological section through the northwestern part of the himalayas from kashmir to tibet within this section the magmatic metamorphic and geotectonic evolution of the northern himalayan orogeny has been studied using petrographic geochemical and isotope analytical techniques the beginning of the himalayan cycle was marked by large basaltic extrusions panjal trap of permian to lower triassic age at the northern margin of the gondwana continent indian shield these continental type tholeiitic basalts were followed by a more alkaline volcanism within the triassic to jurassic lamayuru unit of the gondwana continental margin lower jurassic to cretaceous oceanic crust and sediments ophiolitic melange s s accompany the triassic to cretaceous flysch deposits within the indus tsangpo suture zone the major structural divide between the indian shield high himalaya and the tibetan platform so far no relic of paleozoic oceanic crust has been found subduction of the tethyan oceanic crust during upper jurassic and cretaceous time produced an island arc represented by tholeiitic and calc alkaline volcanic rock series dras volcanics and related intrusives accompanied by volcaniclastic flysch deposits towards the tibetan continental margin subsequent to the subduction of oceanic crust large volumes of calc alkaline plutons trans himalayan or kangdese plutons intruded the tibetan continental margin over a distance of 2000 km and partly the dras island arc in the ladakh region the collision of the indian shield and tibetan platform started during the middle to upper eocene and caused large scale still active intracrustal thrusting as well as the piling up of the himalayan nappes the tectonically highest of these nappes is built up of oceanic crust and huge slices of peridotitic oceanic mantle spongtang klippe in the high himalayas the tectonic activity was accompanied and outlasted by a barrovian type metamorphism that affected triassic sediments of the kashmir nun kun synclinorium up to kyanite staurolite grade and the deeper seated units up to sillimanite grade cooling ages of micas are around 20 m y muscovite and 13 m y biotite towards the indus tsangpo suture zone metamorphism decreases with no obvious discontinuity through greenschist prehnite pumpellyite to zeolite grade remnants of possibly an eo himalayan blueschist metamorphism have been found within thrust zones accompanying ophiolitic melange in the suture zone

the palung granite himalaya high resolution upb systematics in zircon and monazite abstract u pb analyses of fractions of zircon and monazite 3 8 grains each and of single zircon grains resolve a lower ordovician age of 470 4m y for the palung granite which occurs in the high himalayan nappes south of kathmandu its thrusting during the alpine orogeny under lower greenschist facies conditions did not affect the u pb systems in zircon and monazite the granite crystallized from a magma which was mainly generated by anatexis of precambrian continental crust the magma was heterogeneous with respect to primary ages and or metamorphic histories of the magma source rocks this indicates either a derivation from meta sediments or an intense mixing of different crustally derived magmas the genesis of the palung granite is possibly related to an orogeny which affected the indian shield in lower palaeozoic times the detected inherited radiogenic lead in the palung zircons occurs in perfectly homogeneous transparent crystals i e this radiogenic excess lead is not related to the presence of old microscopically visible overgrown zircon cores the minimum ages of the inherited lead components range from about 800 to 1700 m y

argon 39 argon 40 dating of metamorphic and plutonic events in the north and high himalaya belts southern tibet china new 39ar 40ar radiometric data are presented they concern the metasedimentary series and the orthogneiss pluton of kangmar ages as young as 13 ma are found which may be related either to the metamorphism or to the resetting of the minerals during the emplacement of the numerous intrusive bodies into the north himalaya belt inherited excess argon is found in micas from the orthogneiss previously dated at 562 ma these bodies are constituted by leucogranitic magmas the biotites and the muscovites of which the granites intrude lhagoi kangri massif same range of ages is found in the high himalaya belt for micas from leucogranites ages as young as 4 ma are found on micas from the southern part of the cross section between nyalam and zham the recent uplifts and or the movement of recent normal faults may be related to these young ages

crustal structure of the himalayan orogen at 90 east longitude from project indepth deep reflection profiles international deep profiling of tibet and the himalaya indepth deep seismic reflection profiles show that the indian lithosphere is underthrusting the central himalaya along a gently north dipping decollement that is traceable northward beneath the tethyan belt to 28 6 n and to a depth of about 45 km the decollement carries in its hanging wall a near crustal thickness slice of internally deformed indian continental basement and cover represented in outcrop by the greater himalayan belt and structurally overlying tethyan belt geometric relationships suggest that the decollement probably ramps downward beneath the northern tethyan belt to near the base of the crust and that the hanging wall crustal slice was detached from indian mantle lithosphere that presently underlies southern tibet to the north the north himalayan anticlinorium kangmar dome appears to be a large duplex ramp anticline in the hanging wall of the decollement the south tibetan detachment appears to have been imaged in two areas in the wagye la area it appears to follow the tethyan belt greater himalayan belt contact northward in the subsurface and to have been folded over the kangmar dome in the zherger la area it appears to have been active relatively recently as a 30 north dipping normal fault that cuts deeply into the greater himalayan belt allochthon palinspastic reconstruction indicates a minimum of about 326 km of shortening of the indian basement across the himalaya since the initiation of the main central thrust hence indian continental lithosphere largely stripped of its overlying crust can extend northward beneath the tibetan plateau to at least 32 n where earthquake seismological observations show that the properties of the upper mantle change markedly e g mcnamara et al 1995 taken together the seismic reflection and earthquake seismological observations lend support for the view that the indian mantle lid mechanically underplates roughly the southern half of the tibetan plateau the indepth reflection data do not show that indian crust attached to this lithosphere extends north of southernmost tibet kangmar dome nor do they negate this possibility similarly the reflection data do not yield direct evidence for or against lower crustal subduction beneath the himalaya

the zedong window a record of superposed tertiary convergence in southeastern tibet determining the timing magnitude and location of deformation due to the indo asian collision is widely acknowledged as an important step in understanding how the lithosphere responds during continental collision a puzzling result of geological investigations of the lhasa block over the past 2 decades has been the apparent lack of significant tertiary deformation there perhaps the most important structural feature of the lhasa block is the south directed gangdese thrust system which developed along its southern edge the thrust system which separates the andean type batholith of southern asia from rocks of indian affinity is obscured at most locations across southeastern tibet by back thrusts of the younger north directed renbu zedong thrust system the best documented site where both thrusts are exposed occurs near zedong zedong window systematic geochronologic analyses were conducted in this area u pb zircon dating of three samples of a synkinematically to postkinematically deformed hanging wall granitoid the yaja granodiorite cut by the gangdese thrust indicates a crystallization age of 30 4 0 4 ma 2 thus placing an upper bound on the initiation of the thrust u pb zircon dating of granitoid samples structurally higher in the gangdese hanging wall yields emplacement ages of 42 5 1 0 ma and 48 9 0 8 ma 2 similar to other magmatic complexes within the gangdese arc geochemical results are consistent with these plutons forming in the same environment as precollisional intrusions within the gangdese batholith suggesting a significant post collisional input of juvenile heat the 40ar 39ar thermochronology of samples from a vertical section through the hanging wall within the yaja granodiorite coupled with results of a numerical thermal model indicate an average slip rate along the gangdese thrust of 7 mm yr between 30 and 23 ma and a minimum displacement of 50 km farther east in the zedong window thermal effects produced by later north directed thrust sheets of the renbu zedong system appear to have obscured thermal history signatures in the gangdese hanging wall related to earlier south directed thrusting thermochronological results from this region indicate that thermal overprinting related to the north directed thrusting occurred between 25 and 10 ma consistent with previous estimates

the tibetan side of the india eurasia collision results are reported from the 1980 joint french chinese field expedition in tibet the area covered was from the high himalaya in the south to the region of nagqu 250 km north of yangbajain ophiolites in the zangbo valley represent remnants of the crust of an ocean basin which lay adjacent to the gangdise granodiorite belt in the late mesozoic the ophiolites were thrust to the south onto the cretaceous melange and triassic flysch but the age of this event is unclear in the tertiary sediments of the indian margin and the xigaze basin were folded with steeply dipping cleavage which indicates some north south shortening of the crust however the tertiary volcanic sequence on the lhasa block and further north shows comparatively little folding and thrusting in the quaternary deformation of tibet was characterized by east west extension

india asia convergence insights from burial and exhumation of the xigaze fore arc basin south tibet the composite fore arc syncollisional xigaze basin in south tibet preserves a key record of india asia collision new apatite fission track and zircon u th he data from an n s transect across the preserved fore arc basin sequence near xigaze show a consistent northward late cretaceous to middle miocene younging trend while coexisting apatite u th sm he ages are all miocene corresponding detrital zircon u pb data are also reported for constraining the cretaceous depositional ages of the xigaze basin sequence in the region thermal history modeling indicates that the basin experienced northward propagating episodic exhumation along with a northward migration of the depocenter and a pre existing cenozoic syncollisional basin sequence which had been removed in the southern part fore arc exhumation commenced in the late cretaceous 89 2 ma following transition to a syncollisional basin in the paleocene sedimentation in the central and northern xigaze basin continued until the latest eocene 34 4 ma ongoing folding and thrusting e g great counter thrusts caused by progressive plate convergence during late oligocene early miocene time resulted in regional uplift and considerable basin denudation which fed two fluvial basins along its northern and southern flanks and exposed the basement ophiolite subsequent incision of the yarlung river resulted in miocene cooling in the region different episodes in the exhumation history of the xigaze basin caused by thrusting of an accretionary wedge and ophiolitic basement can be linked to changes in india asia convergence rates and the changing subduction pattern of the indian and neo tethyan slabs

the cretaceous tertiary deformation of the lhasa block and its implications for crustal thickening in tibet the strong tertiary deformation of the northern margin of the indian plate and of the tsangpo suture zone is not matched by comparable deformation within the lhasa block yet the southern margin of the block was the site of large scale intrusive activity the gangdese or transhimalayan batholith throughout the late cretaceous and was therefore presumably weaker than its surroundings this poses a problem for any approach that interprets the deformation of tibet in terms of diffuse crustal thickening however there is evidence that the lhasa block was in elevation as well as in magma type an andean margin by the end of the cretaceous and we suggest that the buoyancy force associated with the elevated crust was sufficient to inhibit compressional deformation within the southern lhasa block this suggestion is tested quantitatively by treating the continental lithosphere as a thin viscous sheet containing an isostatically compensated elevation contrast when subjected to boundary conditions representing the collision between india and asia the sheet accommodates convergence by diffuse deformation over a region comparable in size with the tibetan plateau in the absence of an elevation contrast this deformation consists of approximately 100 thickening strain over the region a precollision elevation contrast of 3000 m in the lhasa block would have resulted in its experiencing roughly 40 rather than 100 postcollisional thickening smaller elevation contrasts would have resulted in greater thickening and vice versa these calculations have implications for the mode of crustal thickening in northern tibet as the available evidence suggests that the topographic expression of the earlier mesozoic tectonic activity in southern asia was erased before the india asia collision we expect that there has been widespread and relatively homogeneous crustal shortening migrating northward from the lhasa block during the tertiary

hirnantian sea level changes in the baltoscandian basin a review abstract correlation of hirnantian sections of the baltoscandian basin reveals four major transgressive regressive sea level cycles related to waxing and waning of ice sheets at high southern hemisphere paleolatitudes the correlation between stratigraphic members in estonia and latvia shows transgressions regressions in the first half of the hirnantian when sea level fluctuated between shallow and deep shelves during the second half of the hirnantian the sea regressed into the former deep shelf leaving north estonia as a mainland after that the sea retreated to the central baltoscandian basin and the whole baltic shelf stayed subaerial this final regression brought the coastline to the vicinity of scania in the west and of lublin basin in the east in the latest hirnantian during ice cap melting sea level began to ascend the baltic shelf submerged again in the early silurian this transgression of latest hirnantian earliest silurian indicates fast melting of ice in gondwanan ice sheets we are proposing that the destabilisation of great amount of gas hydrates due to diminishing of water column pressure during the last deepest regression acted as a trigger for rapid warming of climate the ascertainment of sea level changes presumes correlations between different parts of the baltoscandian basin comparison of published data on stratigraphy lithology faunal finds chemical elements and stable carbon isotopes was the ground for tracking hirnantian changes

collision tectonics in the himalaya as evidenced by the indus and shyok rock assemblages abstract the himalayan orogeny is closely associated with the tectonic developments which occurred along the indus suture zone during the last decade many new data on sediment stratigraphy and volcanic plutonic suites have been obtained these data have been interpreted in terms of plate tectonics the meso tethys was generated following the permo triassic rifting and spreading during the jurassic the tethys ocean is postulated in accordance with geophysical data to have been consumed during northward subduction along the indus suture resulting in the himalayan orogeny through collision between india and eurasia however the nature of tectonic processes and their chronology appear to be different in the western and eastern himalayan segments in the western himalaya subduction in an island arc setting and eocene collision are evident mainly by the occurrence of the island arc dras volcanics the sangeluma group srikantia and rajdhan 1980 trench deposits ophiolitic blocks older 60 m y calc alkaline plutons and blueschist facies rocks the indus group srikantia and rajdhan 1980 continental deposits and the younger

upb geochronology of gangdese transhimalaya plutonism in the lhasa xigaze region tibet abstract a series of different plutons from the lhasa xigaze segment of the gangdese transhimalaya belt has been studied by high resolution u pb analyses of zircon using zircon fractions of 5 100 grains each selected upon specific grain characteristics for two diorites located east of xigaze dazhuka the zircons yield concordant ages of 93 4 1 0 and 94 2 1 0 m y respectively also concordant ages of 41 1 0 4 and 41 7 0 4 m y have been obtained for two granodiorites collected southwest of lhasa qushui the precision on the ages of two granites from the xigaze and lhasa area is limited by two factors the presence of inherited radiogenic lead and the occurrence of subsequent lead loss however some concordant zircons detected in both granites define approximate ages of about 67 and 53 m y respectively the inherited lead components show that melting of precambrian material was involved in magma genesis the u pb ages substantiate a magmatic activity lasting from mid cretceous cenomanian to eocene lutetian time such a period of plutonism at the southern margin of eurasia as well as the occurrence of magma generation from continental crust suggest that the gangdese range results from the subduction of tethys oceanic lithosphere indian plate underneath eurasia eurasian plate if this model is true the collision of india with eurasia along the lhasa xigaze sector postdates the emplacement of the 41 m y old gangdese granodiorites i e the collision occurred after lutetian time

miocene post collisional shoshonites and their crustal xenoliths yarlung zangbo suture zone southern tibet geodynamic implications the convergence between the indian plate and the southern margin of the eurasian continent created an active continental margin from late jurassic until about 40 ma ago which then evolved to form the himalaya and the tibetan plateau during the continental collision stage post collisional magmatism in southern tibet north of the yarlung zangbo suture zone yzsz has been active since 45 ma and is related to normal faulting and extensional tectonism to date no such magmatism was reported within the yzsz itself this paper reports on the discovery of miocene shoshonites within the yzsz they are significant because the magma traveled at least in part through oceanic crust thus limiting interaction with the continental crust to the mid crustal level and which affected the post collisional magmatic rocks occurring in the northern part of the subduction system in addition xenoliths and xenocrysts of crustal origin in these rocks constrain the nature of metamorphic rocks underlying the yzsz at mid crustal level the geochemical signatures of the shoshonitic rocks including nd and sr isotope systematics indicate derivation from a garnet bearing middle continental crustal source crustal imprint complicates modeling of the petrogenetic processes which occurred prior to mid crustal ponding of the magma which took place between 11 and 17 ma at depths of 40 to 50 km the significant role of crustal contamination raises serious concerns about models proposed for similar magmatic activity elsewhere in the himalaya and the tibetan plateau

discovery and significance of cretaceous fossils from the xigaze forearc basin tibet abstract the xigaze cretaceous forearc basin located between the lhasa block and indus yarlung suture was filled with flysch deposits during the mid cretaceous as the tethys oceanic crust was being subducted underneath eurasia the flysch comprises the xigaze group dominated by the angren formation well preserved newly discovered foraminifera within the angren formation constrain its age to late albian to late coniacian radiometric dating of the ophiolite belt impinging on the southern edge of the forearc basin yielded an age of 120 10 ma which implies that the deposition in the forearc basin occurred shortly after initiation of the subduction of the neotethys

early late cretaceous ca 93 ma norites and hornblendites in the milin area eastern gangdese lithosphere asthenosphere interaction during slab roll back and an insight into early late cretaceous ca 100 80 ma magmatic flare up in southern lhasa tibet abstract at more than 500 km in length the mainly jurassic early eocene gangdese batholith is one of the most important constituents of the southern lhasa sub block and provides an ideal site for study of tibetan orogenesis recent studies on gangdese intermediate felsic intrusive rocks mainly granites demonstrate that remarkable crustal growth as well as an early late cretaceous ca 100 80 ma magmatic flare up event occurred in southern tibet however the mechanism that drove this magmatic event and its relationship to crustal growth event are not yet clear here we report detailed petrological geochronological geochemical and sr nd hf isotopic data for recently identified norites and hypersthene bearing hornblendites in the milin area southern lhasa sub block these mafic rocks are dated at early late cretaceous ca 93 ma and are characterized by relatively uniform sr nd hf isotopic compositions 87sr 86sr i 0 7042 to 0 7047 end t 2 9 to 3 6 and ehf t zircon 10 9 to 17 0 suggesting that they evolved from similar parental magmas the milin norites and hornblendites are likely to be the products of mineral fractionation and accumulation from a common parental magma during the early and late stages of the magma evolution respectively thermometric calculations indicate that pyroxenes from the milin norites have high crystallization temperatures 1240 1349 c the parental magmatic compositions calculated from pyroxene trace element compositions in the milin norites show slightly flat to enriched light rare earth element lree patterns la yb n 2 9 3 4 la gd n 2 1 2 9 with variable negative nb anomalies nb la n 0 18 0 81 indicating their dual or hybrid geochemical characteristics we suggest that their parental magmas may have been generated by the interaction of upwelling asthenospheric and metasomatized lithospheric mantle taking into account the spatial and temporal distribution of the mesozoic magmatic rocks and regional paleomagnetic data we further suggest that the early late cretaceous magmatic flare up in the southern lhasa sub block was also triggered by the asthenospheric upwelling which resulted from roll back of subducted tethyan oceanic slab

uranium lead system in fragments of a single zircon grain since the demonstration that u pb analyses of zircon fractions1 from a few grains up to several thousand grains or fragments of grains2 or single zircon grains3 4 yield reliable radiometric age data this method has been widely applied to igneous metamorphic and sedimentary rocks the present investigation of u pb systems for a suite of distinct fragments of a single zircon grain from a 1 093 2118 myr old syenite of the pikes peak batholith colorado usa demonstrates that uranium is heterogeneously distributed in this zircon crystal most parts of the grain suffered various degrees of lead loss the others show a closed system behaviour we show that degrees of lead loss are not correlated with uranium concentrations of the fragments

cretaceous tertiary geology of the gangdese arc in the linzhou area southern tibet abstract knowledge of the cretaceous tertiary history of upper crustal shortening and magmatism in tibet is fundamental to placing constraints on when and how the tibetan plateau formed in the lhasa terrane of southern tibet the widely exposed angular unconformity beneath uppermost cretaceous lower tertiary volcanic bearing strata of the linzizong formation provides an excellent geologic and time marker to distinguish between deformation that occurred before vs during the indo asian collision in the linzhou area located 30 km north of the city of lhasa a 3 km thick section of the linzizong formation lies unconformably on cretaceous and older rocks that were shortened by both northward and southward verging structures during the late cretaceous the linzizong formation dips northward in the footwall of a north dipping thrust system that involves triassic jurassic strata and a granite intrusion in the hanging wall u pb zircon geochronologic studies show that the linzizong formation ranges in age from 69 ma to at least 47 ma and that the hanging wall granite intrusion crystallized at 52 ma coeval with dike emplacement into footwall cretaceous strata 40ar 39ar thermochronologic studies suggest slow cooling of the granite between 49 and 42 ma followed by an episode of accelerated cooling to upper crustal levels beginning at 42 ma the onset of rapid cooling was coeval with the cessation of voluminous arc magmatism in southern tibet and is interpreted be a consequence of either 1 tertiary thrusting in this region or 2 regional rock uplift and erosion following removal of overthickened gangdese arc lower crust and upper mantle or break off of the neo tethyan oceanic slab

origin of the xigaze ophiolite yarlung zangbo suture zone southern tibet abstract the xigaze ophiolite tibet displays unusual lithological petrological textural and structural characteristics there are no large masses of cumulate gabbros but dolerite intrusives throughout the whole ophiolite sequence some of which were intruded into already serpentinized peridotites only minor residual harzburgites and dunites in dominantly iherzolitic peridotites equilibrated at low temperatures and pressures and relatively low temperature deformation structures in the uppermost peridotites these features suggest a very low heat flow at the spreading center where the xigaze ophiolite was formed in good agreement with a discontinuous and slowly accreting spreading center origin however this ophiolite does not represent a typical mid oceanic ridge ophiolite rather it was formed in a small basin located at the southern margin of eurasia hence within a preexisting oceanic lithosphere the opening of the nearly n s xigaze paleo ridge resulted from the w e drift of africa relative to eurasia from 180 to 110 ma the n s emplacement onto the continent of the xigaze ophiolite formed 120 to 110 ma ago can be correlated to changes in direction of motion of the african and indian plates a primary intra oceanic thrusting event probably occurring at 110 or 85 ma and the final obduction near 50 ma during the india eurasia collision

39 ar 40 ar dating of the trans himalayan calc alkaline magmatism of southern tibet the age of the trans himalayan calk alkaline magmatism has not been clearly established on the basis of geological control it is possible to infer a pre late cretaceous age for the plutonites at the southern edge of the batholith belt and a late cretaceous or younger age for the lava series on the top and or to the north of the plutons k ar radiometric data previously obtained by chinese groups agree with these geological ages the 39ar 40ar method has been used to analyse more accurately some of the rocks which have been sampled during the joint french chinese field work in southern tibet in the summer of 1980 the incremental degassing of argon gives generally more information about the thermal history of the sample using this method we now show that the earlier basic plutons of the southern edge of the trans himalayan belt have ages of 90 110 myr albian to cenomanian whereas the lavas of the upper unconformable series are clearly younger 60 myr palaeocene the ladakh and kohistan magmatic belts further west are probable equivalents of the trans himalaya north of the indus suture their precise ages are also poorly known orbitolinas of middle cretaceous aptian age have been found in the drosh yasin series which lie unconformably on dioritic plutons in the northern edge of the belt the few radiometric data that are available give ages in fairly good agreement with those we have obtained in the trans himalaya so we conclude that a subduction zone has been active during the cretaceous between india and asia the only outstanding question is whether this active margin was at that time close to india or to asia or somewhere in between

investigation of the archaean crust by single grain dating of detrital zircon a greywacke of the slave province canada u pb dating of single zircon grains from a graywacke of the point lake area of the archaean slave structural province in canada indicates that this sediment is derived from two sources 1 the ancient shield parts of which are at least 3100 ma old which is the basement of the slave province supracrustal rocks and 2 younger rocks about 2700 ma old this implies the creation of large amounts of acidic to intermediate rocks during the 2700 ma event it also indicates that magma generation and intracontinental sedimentary basin formation and deepening are strongly related

identification of early carboniferous granitoids from southern tibet and implications for terrane assembly related to the paleo tethyan evolution abstractthis article presents zircon u pb and hf isotope data together with the whole rock major and trace element composition of early carboniferous granitoids newly identified from the jiacha and langxian areas in the southern lhasa terrane southern tibet the jiacha rocks are monzogranites that yield zircon u pb ages of 347 345 ma and ehf t values from 5 4 to 4 9 the langxian rocks are granodiorites with slightly older zircon u pb ages of 355 352 ma and lower ehf t values from 6 8 to 6 5 our data suggest that these granitoids were generated largely by reworking of paleoproterozoic ga basement materials in conjunction with literature data it is further argued that the southern and central parts of the lhasa terrane separated by the sumdo eclogite belt should have been an integrated block before the late paleozoic our study supports the notion that the lhasa terrane was derived from the northern margin of gondwanaland in association with formation of at least two stages of tethyan oc

geochemical investigations of the origin of the manaslu leucogranite himalaya nepal the himalayan orogeny is characterized by an intraplate subduction zone the thrusting of the tibetan slab over the foreland is probably responsible for the origin of the manaslu type leucogranites this geochemical study tries to examine the crustal origin of the granite to verify whether the paragneisses of the tibetan slab are the most probable parent material and to determine the subsequent petrogenetic processes the rb sr systematics suggest that 1 1 the scatter of data points in the 87sr86srvs 87rb86sr diagram may be due to the heterogeneity of the initial 87sr86sr ratios 2 2 the initial isotopic ratios are very high 0 730 0 760 and are in agreement with those of the gneisses of the tibetan slab at the time of the melting pb isotopic ratios are homogeneous they are also consistent with a crustal prehistory the ree and th contents are unexpectedly low in comparison with the much higher contents of the parent material monazite left in the residue of melting or more probably extracted in an early stage of the magmatic history may explain the observed ree and th abundances alternatively field observations show that the emplacement of the granite has been accompanied by a very strong hydrothermal activity in consequence the low ree and th contents of the granite could also be due to the loss by solution during prolonged fluid circulation throughout the granitic melt and the sr and pb isotope distribution to the effect of late to post magmatic fluids the isotopic character of which was determined by the underlying tibetan slab

characteristics and genesis of gangdese porphyry copper deposits in the southern tibetan plateau preliminary geochemical and geochronological results porphyry type cu mo au deposits have been discovered along the gangdese magmatic arc in the southern tibetan plateau extensive field investigations and systematic studies of geochemistry s pb isotopic tracing together with re os and 40ar 39ar isotopic dating indicate that the mineralisation of the copper belt is genetically related to emplacement of late orogenic granitic porphyries during the post collisional crustal relaxation period of the late himalayan epoch these porphyries are petrochemically k enriched and belong to shoshonitic to high k calc alkaline series they display enrichment of large ion lithophile elements lile rb k u th sr pb and depletion of high field strength elements hfse nb ta ti and the heavy rare earth elements hree and y without eu anomalies these characteristics demonstrate that subduction played a dominant role in their petrogenesis and residual garnet was left in the magma sources pb isotope data show a linear correlation in the plumbotectonic framework diagram ranging from orogenic pb in the eastern segment of the copper belt to mantle pb in the western segment these constitute a mixing line of the indian oceanic morb with indian oceanic sediments and suggest that the porphyry magmas were dominantly derived from partial melting of subducted oceanic crusts mixed with a minor quantity of sediments and mantle wedge components the gangdese porphyry copper polymetallic belt has alteration characteristics and zonation typical of porphyry type copper deposits which include potassic alteration k feldspathisation and biotitisation silicification sericitisation and propylitisation mineralisation mainly occurs in strongly altered granitic cataclasite at the exo contact with veinlet disseminated textures the porphyries themselves are weakly mineralised with disseminated pyrite and chalcopyrite the copper deposits contain simple ore mineral associations consisting of chalcopyrite pyrite bornite molybdenite sphalerite and oxidised minerals of malachite covellite and molybdite during supergene oxidation primary ores underwent secondary enrichment to form economic orebodies with cu grade ranging from 1 to 5 ore sulphides of the copper belt display s and pb isotopic compositions identical to the ore bearing porphyries their 34s values vary between 3 8 and 2 4 and are typical of mantle sulphur the 206pb 204pb 207pb 204pb and 208pb 204pb ratios vary in the ranges 18 106 to 18 752 15 501 to 15 638 and 37 394 to 39 058 respectively and yielded radiogenetic lead enriched signatures twelve molybdenite samples from the copper belt yielded isochron ages of 14 76 0 22 ma and 13 99 0 16 ma for the nanmu and chongjiang deposits and model ages of 13 5 to 13 6 ma for the lakang e deposit meanwhile 40ar 39ar isotopic dating of two biotite phenocrysts from the chongjiang and lakang e deposits give plateau ages of 13 5 1 0 ma and 13 42 0 10 ma respectively during the geodynamic evolution of the gangdese collision orogenic belt intrusion of the ore bearing porphyries took place just before the rapid uplift and e w extension of the southern plateau and the ore forming process may have occurred simultaneously with the uplift and extension 14 0 1 ma

reconstructing carbonate alteration histories in orogenic sedimentary basins xigaze forearc southern tibet abstract carbonate clumped isotope thermometry t 47 and the oxygen stable isotope record 18 o provide critical constraints on the temperature of carbonate mineral formation and isotopic composition of ancient waters used in reconstructions of past climate tectonics and ecological and environmental change the robust use of these proxies requires that carbonate minerals in the rock record retain primary isotopic compositions through often complex post depositional thermal histories new carbonate clumped isotope thermometry and sedimentology data are paired with existing thermochronology and isotope exchange reaction modeling to provide unique constraints on the alteration history of the carbon and oxygen isotopes in tethyan marine carbonates jialazi fm tso jianding group of the xigaze forearc southern tibet within the framework of the textural and isotopic data the effects of early and late burial alteration on 47 and 18 o values of marine rocks that were buried to 4 km above sea level are modeled the alteration products are considered within two well studied mechanistic frameworks water rock recrystallization and solid state reordering of 13 c 18 o bonds at high temperature the proposed alteration model is as follows 1 shallow early diagenesis with water of similar composition and temperature to formation waters infills primary porespace with microspar 2 high temperature rock buffered water rock exchange and partial solid state reordering at 3 4 km depth and 3 near surface low temperature water rock exchange on the exhumation pathway the 47 heterogeneity within individual samples 0 456 to 0 721 requires significant differential alteration on the retrograde path high to low temperature low water rock ratio in which primary compositions and compositions inherited during early burial diagenesis are overprinted with modern meteoric values a low t 47 value is often assumed to record the primary temperature of the depositional environment however samples yielding low t 47 values in the jialazi fm have undergone extensive high temperature water rock alteration and therefore require a mechanism for increasing 13 c 18 o bond ordering on the retrograde pathway while preserving many primary biogenic structures and micritic textures this study highlights the potential significance of late stage low temperature alteration of carbonates derived from orogenic sedimentary basins for studies of terrestrial paleoenvironments and tectonics this study also provides an example framework for combining observational and analytical data to reconstruct a carbonate alteration history of a buried sedimentary package