constant elevation of southern tibet over the past 15 million years the uplift of the tibetan plateau an area that is 2 000 km wide to an altitude of about 5 000 m has been shown to modify global climate and to influence monsoon intensity mechanical and thermal models for homogeneous thickening of the lithosphere make specific predictions about uplift rates of the tibetan plateau but the precise history of the uplift of the plateau has yet to be confirmed by observations here we present well preserved fossil leaf assemblages from the namling basin southern tibet dated to approximately 15 myr ago which allow us to reconstruct the temperatures within the basin at that time using a numerical general circulation model to estimate moist static energy at the location of the fossil leaves we reconstruct the elevation of the namling basin 15 myr ago to be 4 689 895 m or 4 638 847 m depending on the reference data used this is comparable to the present day altitude of 4 600 m we conclude that the elevation of the southern tibetan plateau probably has remained unchanged for the past 15 myr

geochemistry and u pb detrital zircon dating of paleozoic graywackes in east junggar nw china insights into subduction accretion processes in the southern central asian orogenic belt abstract the southern central asian orogenic belt caob is characterized by multiple and linear accretionary orogenic collages including paleozoic arcs ophiolites and accretionay wedges a complex history of subduction accretion processes makes it difficult to distinguish the origin of these various terranes and reconstruct the tectonic evolution of the southern caob in order to provide constraints on the accretionary history we analyzed major and trace element compositions of paleozoic graywackes from the huangcaopo group hg and kubusu group kg in east junggar the hg graywackes have relatively low chemical index of alteration cia values 50 to 66 suggesting a source that underwent relatively weak chemical weathering the identical average index of compositional variability icv values 1 1 for both the kg and hg samples point to an immature source for the paleozoic graywackes in east junggar which is consistent with an andesitic felsic igneous source characterized by low la th ratios and relatively high hf contents these graywackes are geochemically similar to continental island arc sediments and therefore were probably deposited at an active continental margin u pb dating of detrital zircons from the lower subgroup of the hg yielded a young age peak at 440 ma indicating a post early silurian depositional age however the youngest populations of detrital zircons from the kg graywackes and the upper subgroup of the hg yielded 206 pb 238 u ages of 346 ma and 355 ma respectively which suggest a post early carboniferous depositional age because of similarities of rock assemblages these two units should be incorporated into the early carboniferous nanmingshui formation the detrital zircon age spectrum of the early paleozoic hg graywackes resembles that of the habahe sediments in the chinese altai which suggests that the ocean between east junggar and the chinese altai was closed before the deposition of the sediments and that the armantai ophiolite was emplaced prior to the early devonian the differences in age spectra for detrital zircons from the post early carboniferous graywackes in east junggar and the harlik arc indicate that the emplacement of the kalamaili ophiolite postdates the early carboniferous therefore a long lasting northward subduction accretion process is suggested for the formation of east junggar and the reconstruction of the early paleozoic evolution of the southern caob

geochemistry of miocene trachytes in bugasi lhasa block tibetan plateau mixing products between mantle and crust derived melts abstract it is generally believed that cenozoic potassic and ultrapotassic volcanic rocks of the tibetan plateau were generated by partial melting of an enriched mantle region or lower crustal materials the miocene bugasi volcanic rocks bvr in the western part of the lhasa block are composed mainly of trachyandesites and trachytes both of which belong to the shoshonite series the trachytes show somewhat transitional compositions between the mantle derived trachyandesites of the bvr and the crust derived potassic rocks of konglongxiang most evident in their sr ba and ni concentrations nb ta rb sr th nb zr nb and ba rb trace element ratios and sr and nd isotopic compositions these features coupled with the relatively high cr and ni concentrations and mg suggest that the trachytes are the product of mixing between mantle derived and lower crust derived melts

characteristics and formation mechanisms of large volcanic rock oil reservoirs a case study of the carboniferous rocks in the kebai fault zone of junggar basin china volcanic hydrocarbon reservoirs are rare and may be overlooked the carboniferous volcanic rocks of the kebai fault zone in the western junggar basin contain hydrocarbon hc reservoirs in volcanic rock with proven oil reserves of 9 76 108 bbl that have a complex filling history we have investigated the lithology and properties of these volcanic rock hc reservoirs as well as diagenesis and control of faults and fractures in oil reservoirs the lithology of these carboniferous volcanic rocks is primarily andesite and tuff also present were volcanic breccia and metamorphic rock in addition to rhyolite felsite diabase and granite in the volcanic lava on the basis of microscopic examination five types of pores and fractures were observed 1 fracture dissolved phenocrystal pore 2 fracture intergranular pore 3 fracture gas pore 4 fracture dissolved intragranular pore and 5 fracture dissolved matrix pore the fractures in these rocks are a significant factor in connecting the pores diagenetic processes that control reservoir quality include compaction filling of pores and fractures cementation metasomatism and grain dissolution the volcanic reservoirs show a variety of lithologies and oil has been discovered in all types of carboniferous rocks the controlling factors for oil distribution in these carboniferous volcanic rocks are faulting fracture development and degree of weathering when they were subaerially exposed in the permian the area in which these faults and fractures developed is the primary area of oil enrichment with high yields the objectives of this study were to 1 describe the characteristics of different types of volcanic rocks and reservoirs found in this basin and 2 characterize the diagenetic history of these rocks and document how diagenesis controls porosity and permeability

precise sm nd and u pb isotopic dating of the supergiant shizhuyuan polymetallic deposit and its host granite se china the supergiant shizhuyuan w sn bi mo deposit is hosted by the qianlishan granite a small highly fractionated granitic pluton 10 km 2 with multiple phases of intrusions within the early yanshanian granitoid province of se china strong alteration of skarn and greisen that formed in the contact zone between the first and second phases of granite intrusions and devonian limestone is responsible for the polymetallic mineralizations shrimp u pb zircon analysis indicates that the two early phases of the qianlishan granite formed contemporaneously at 152 2 ma metasomatic minerals garnet fluorite and wolframite separated from the skarn and greisen yield a sm nd isochron age of 149 2 ma that is interpreted as the formation age of the shizhuyuan deposit therefore the mineralization of the supergiant shizhuyuan polymetallic deposit formed contemporaneously with or very shortly after the intrusion of the small highly fractionated qianlishan granite

adakites from continental collision zones melting of thickened lower crust beneath southern tibet adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones here we report the first example of such magmas from southern tibet in an active continental collision environment the tibetan adakites were emplaced from ca 26 to 10 ma and their overall geochemical characteristics suggest an origin by melting of eclogites and or garnet amphibolites in the lower part 50 km of thickened tibetan crust this lower crustal melting required a significantly elevated geotherm which we attribute to removal of the tectonically thickened lithospheric mantle in late oligocene time the identification of collision type adakites from southern tibet lends new constraints to not only the himalayan tibetan orogenesis how and when the indian lithosphere started underthrusting asia can be depicted but also the growth of the early continental crust on earth that consists dominantly of the tonalite trondhjemite granodiorite suites marked by adakitic geochemical affinities

partitioning of sr ba rb y and lree between alkali feldspar and peraluminous silicic magma samples from well known rhyolitic systems in the western usa were analyzed to determine trace element partition coefficients between sanidine and coexisting melt both d sr and d eu correlate positively with an concentration in sanidine barium not surprisingly shows more correlation with or in sanidine all three show negative correlations with whole rock al2o3 partition coefficients for ree show a decreasing trend with strong positive d eu d lree correlate negatively with the whole rock al2o3 in peraluminous rocks al2o3 in magma seems to exert a major influence on partition coefficients of trace elements with increasing al2o3 concentration the melt composition becomes increasingly similar to that of feldspar incompatible trace elements rb y nb and zr in sanidine show positive trends whereas compatible trace elements sr eu and ba show negative correlations when plotted against whole rock al2o3 lrees which are expected to substitute for ca in the sanidine structure behave like sr and eu because trace element partitioning is influenced by both crystal and magma compositions multiple regression analysis has been used to predict trace element partition coefficients the equations will assist workers in the selection of partition coefficients used in petrogenetic models of silicic systems

miocene volcanism in the lhasa block tibet spatial trends and geodynamic implications miocene ultrapotassic up shoshonitic p and calc alkaline ca volcanism is recognized widely over the tibetan plateau this volcanism occurred after the end of india s oceanic lithosphere subduction beneath the plateau ambiguities about the distribution and significance of this volcanism arise in part from a paucity of geochronologic and geochemical data we have sampled two volcanic fields in the lhasa block 1 the eastern edge of the zabuye salt lake central part of the block 2 the southwestern edge of the yangbajin graben major and trace elements display high k2o 6 06 6 54 over a wide range of sio2 75 03 55 30 na2o 1 32 3 30 and lree hree enrichment with a significant rare earth element fractionation 266 la yb n 6 42 as well as negative nb ta and ti anomalies these features indicate that our samples are chemically identical to the post collision miocene up to p volcanic rocks already identified all over the tibetan plateau and the southern plateau specifically but distinct from the ca rocks found in the lhasa block the miocene volcanics found in the lhasa block present greater chemical variation than the rest of the tibetan plateau 40 ar 39 ar ages on both sanidine and biotite gave ages ranging from 16 16 g 0 12 ma to 16 01 g 0 12 ma and 10 60 g 0 14 ma to 10 88 g 0 17 ma 2 c excluding systematic errors relative to fish canyon sanidine at 28 02 ma for zabuye salt lake and yangying geothermal field volcanics respectively these results are consistent with the range of previously published ages from this part of the tibetan plateau the presence of n s trending dikes and the location of the volcanic fields suggest that local e w extension in the lhasa block was coeval we propose that the synchronous up p and ca volcanism in the lhasa block between 23 and 8 ma is the result of progressively deepening northward subduction of indian continental lithosphere producing asthenospheric upwelling convective thinning delamination of lithospheric mantle fueled by the hot asthenosphere induced partial melting of an enriched sub continental mantle lithosphere as well as eclogitic lower crust this mechanism could also explain the local e w extension observed in the same period 18 10 ma we also propose that lateral migration of the deepening indian continental lithosphere mantle slab could explain the observed younger volcanism in the eastern part of the lhasa terrane

tibetan tectonic evolution inferred from spatial and temporal variations in post collisional magmatism cenozoic magmatism on the tibetan plateau shows systematic variations in space and time that must be considered in models concerning tibetan tectonic evolution after the india asia collision which started in the early tertiary and terminated the gangdese arc magmatism in the lhasa terrane made of the southern tibetan plateau widespread potassium rich lavas and subordinate sodium rich basalts were generated from f50 to 30 ma in the qiangtang terrane of northern tibet subsequent post collisional magmatism migrated southwards producing ultrapotassic and adakitic lavas coevally between f26 and 10 ma in the lhasa terrane then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f13 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the indian continental lithospheric mantle started thrusting under asia by involving rollback and breakoff of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root we propose that only after the lithospheric removal which occurred at f26 ma could the indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the himalayan tibetan orogenesis consequently the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder owing to the push of the underthrust indian mantle lithosphere d 2004 elsevier b v all rights reserved

timing of tibetan uplift constrained by analysis of volcanic rocks the tibetan plateau has had a central role in the development of recent models for the mechanics of mountain belts1 2 and cenozoic global climate change3 the present elevation and extensional deformation of the plateau probably result from uplift owing to convective thinning of the underlying lithospheric mantle1 2 an age for the uplift would provide a valuable constraint on these models but because recently proposed indicators of uplift are all climate dependent they are equivocal possibly reflecting global cooling rather than regional uplift4 here we present new geochemical data on post mid miocene lavas from the plateau which show that the lavas were derived from the lithospheric mantle simple thermal arguments indicate that the generation of such magmas also necessitates thinning of the lithospheric mantle thus volcanism is coincident with uplift providing a climate independent means of dating the onset of uplift dating by the laser 40ar 39ar technique places the beginning of this volcanism and therefore the time of uplift of the tibetan plateau at 13 myr ago

geochemistry of crustally derived leucocratic igneous rocks from the ulugh muztagh area northern tibet and their implications for the formation of the tibetan plateau this is the published version copyright 2012 american geophysical union all rights reserved

evidence for tibetan plateau uplift before 14 myr ago from a new minimum age for east west extension important changes in south asian climate occurred in the late miocene epoch 8 myr ago 1 2 and these have been attributed by some researchers to uplift of the tibetan plateau at about the same time3 5 unfortunately this link has been difficult to test because the timing of plateau uplift remains poorly constrained by independent evidence one way to determine the minimum age of uplift is to establish the initiation age of the north striking normal fault systems in southern tibet that are widely regarded6 10 as being related to gravitational collapse of the tibetan plateau here we report an40ar 39ar age of 14 myr for hydrothermal mica from an extensional fracture belonging to such a fault system in north central nepal this age implies that east west extension began before 14 myr ago in at least some parts of the tibetan plateau suggesting that the plateau attained its high mean elevation well before late miocene time

jurassic granitoids in the northwestern sanandaj sirjan zone evolving magmatism in response to the development of a neo tethyan slab window abstract voluminous jurassic granitoids within the sanandaj sirjan zone ssz provide insight into the magmatic arc formed in the active margin of eurasia here we present new in situ zircon u pb whole rock major and trace element and sr nd isotopic data for the gorveh plutonic complex gpc of the northwestern ssz in iran six samples from the plutons within the gpc yielded zircon u pb ages that range from 151 to 146 ma these plutons can be subdivided into two groups based on their geochemistry group 1 rocks the mobarak abad diorites and the gorveh gabbros and diorites contain relatively high concentrations of the high field strength elements hfse nb ta zr and ti and have low th nb 0 20 0 56 and moderate sm yb ratios 1 51 2 32 low 87sr 86sr i values 0 70354 0 70622 and high end t values 2 3 5 4 these features indicate that the group 1 rocks formed from magmas derived from a subduction modified region of the subcontinental lithospheric mantle the group 2 plutons the bolban abad granites and the gorveh quartz monzonites have a type granites affinities including high k2o na2o and zr nb ce y concentrations and high feotot mgo and 10 000 ga al ratios these a type rocks are enriched in rb th and k and depleted in ba u nb ta sr p and ti the group 2 plutons have different sr nd isotopic compositions to each other indicating they were derived from different sources and record different igneous processes the gorveh quartz monzonites have high 87sr 86sr i ratios 0 70552 0 70617 negative end t values 1 1 to 5 4 and extremely low concentrations of mgo 0 32 0 35 wt suggesting they were derived from an igneous quartzo feldspathic crustal source in comparison the bolban abad granites have positive end t values and contain high concentrations of sio2 and low concentrations of mgo suggesting that they formed from group 1 magmas that subsequently underwent assimilation and fractional crystallization processes combining these new data with the results of previous research we conclude that this jurassic magmatism was the result of the formation of a slab window within the subducting neo tethys slab a process that caused the partial melting of overlying continental lithospheric material

cenozoic k rich adakitic volcanic rocks in the hohxil area northern tibet lower crustal melting in an intracontinental setting it is generally accepted that the cenozoic potassic volcanic rocks of northern tibet were derived from a lithospheric mantle source here we report new chronological geochemical and isotopic data for the miocene ca 18 15 ma k rich adakitic volcanic rocks from the hohxil area of the songpan ganzi block in northern tibet we contend that these rocks were generated by partial melting of the mafic lower crust in an intracontinental setting unrelated to subduction of oceanic crust the hohxil rocks exhibit high sr y and la yb ratios high sr and la contents but low yb and y concentrations similar to adakites formed by slab melting associated with subduction however their relatively low e n d values 2 09 to 4 58 high 8 7 sr 8 6 sr 0 7072 0 7075 th u th ba and rb ba ratios and distinctive potassium enrichments k 2 o na 2 o are very different from the composition of typical adakites in addition those k rich adakitic rocks with the highest sio 2 contents 61 wt exhibit the lowest 8 7 sr 8 6 sr ratios and highest e n d values and are the oldest cenozoic volcanic rocks exposed in the songpan ganzi block suggesting that they were derived neither directly from a mantle source nor by differentiation of a shoshonitic magma taking into account the composition of lower crustal mafic xenoliths in tibet as well as the tectonic and geophysical evidence we conclude that the hohxil adakitic magmas were produced by partial melting of amphibole bearing eclogites with a k rich mafic bulk composition in the lower part 55 km of the thickened northern tibetan crust partial melting of the lower crust may have been triggered by dehydration release of fluids from sedimentary materials in the southward subducted continental lithosphere

geochemistry and nd isotopic composition of the early paleozoic flysch sequence in the chinese altai central asia evidence for a northward derived mafic source and insight into nd model ages in accretionary orogen abstract although nd isotopic composition of sedimentary rocks is considered as a useful tool to trace the provenance their nd model age tdm is usually misused to discuss the timing of crustal formation in accretionary orogen in the central asian orogenic belt an early paleozoic flysch sequence extensively occurs in the chinese altai and adjacent western mongolian terranes sedimentary rocks in the northwestern chinese altai were collected from the flysch sequence for geochemical and nd isotopic studies although these rocks show different geochemical characteristics e g arenaceous and argillaceous they all have relatively low cia chemical index of alteration values 49 75 indicating a relatively weak weathering source or the absence of compositionally mature alumina rich minerals produced by chemical weathering processes their relatively high icv index of compositional variability values 0 95 1 35 reveal an immature source and indicate that these rocks were probably deposited in an active tectonic setting the sedimentary rocks from the flysch sequence have high mgo sc co and low y nb which suggest significant addition of mafic rocks into the source modeling based on nb cr and zr mgo contents displays nearly 20 to 50 addition of mafic materials in combination with the age and distribution of mafic rocks in the chinese altai and adjacent terranes a northward derived mafic source is recommended for the sedimentary sequence although the sedimentary rocks have nd model ages tdm 1 44 1 57 ga older than devonian granites tdm 1 6 1 1 ga young detrital zircons from the sediments and inherited zircons from the granites 450 500 ma suggest a mixed source consisting of both old crust components and juvenile or less evolved crustal components this reveals that nd model ages tdm of metasedimentary rocks and igneous rocks can hardly provide close constrains on the timing of crustal formation in accretionary orogen with the progresses in adjacent western mongolian terranes a tectonic scenario dominated by subduction accretionary process is suggested to interpret the formation of the early paleozoic flysch sequence in the chinese altai and adjacent terranes

crust mantle mixing and crustal reworking of southern tibet during indian continental subduction evidence from miocene high silica potassic rocks in central lhasa block abstract collisional zones are commonly considered as important regions for crustal reworking but the reworking mechanism remains debated the well known himalayan southern tibetan orogen built by india asia collision and convergence has the thickest continental crust on earth and is therefore an ideal region for studying crustal reworking during collisional orogenesis here we revisit the miocene high silica potassic rocks trachytes in the konglong area of the central lhasa block southern tibet integrated studies of geochronology mineral compositions bulk rock major and trace element geochemistry and sr nd pb hf o isotopes unequivocally indicate that the konglong trachytes formed by mixing between enriched mantle derived ultrapotassic and thickened ancient crust derived magmas combined with post collisional magma mixing recently identified in the southern lhasa block we suggest that magma underplating and subsequent mantle crust interaction i e the matter and energy transfer from the mantle to the crust has been a common and important crustal reworking process in southern tibet during indian continental subduction this process may be related to indian plate flat subduction and subsequent foundering during the post collisional stage in combination with the nature of cenozoic magmatism in the himalaya block we suggest that in addition to partial melting of the subducted continental crust magma underplating and subsequent crust mantle mixing beneath the obducted continent has also played an important role in crustal reworking of the collisional zone

petrogenesis of the early paleozoic strongly peraluminous granites in the western south china block and its tectonic implications abstract strongly peraluminous sp granites have a cnk molecular al 2 o 3 cao na 2 o k 2 o ratios 1 1 indicating a predominant origin from the partial melting of metasedimentary rocks however an increasing number of studies have documented that mantle derived magmas can also be involved in the petrogenesis of some sp granites this is the case in the dulong batholith in the southeastern yunnan province southwestern south china block which is typically composed of sp granitic rocks with high a cnk values 1 1 zircon u pb dating of four samples from this batholith yielded consistent crystallization ages of ca 430 ma synchronous with the widespread late orogenic magmatism including s and i type granites and mafic igneous rocks in the wuyi yunkai orogen south china block scb all the granites have fractionated ree patterns la yb n 1 28 12 3 and conspicuous negative eu anomalies eu eu 0 07 0 43 with a similar depletion in hfse nb zr hf p ba and sr suggesting that these granitic magmas had a dominantly crustal source likely the neoproterozoic metasedimentary rocks i e the danzhou or banxi groups that are dominated by pelitic rocks with minor interlayered siltstones in the western yangtze block however their zircon hf isotopic results also reveal an important input of the mantle derived melts into their parental magma taken together their geochemical and isotopic compositions reflect a derivation by magma mixing between the crust and mantle derived 10 in volume magmas their chemical variations resulted from the fractionation process during the emplacement from the magma chamber after magmatic mixing in conjunction with regional data within the scb it is most likely that the magmatism younger than 435 ma was generated in a post collisional extensional regime related to the partial delamination of an overthickened lithospheric mantle root without lower crust

age and composition of dikes in southern tibet new constraints on the timing of east west extension and its relationship to postcollisional volcanism controversy exists over whether east west extension in southern tibet is related to plateau uplift or to the accommodation of plate boundary forces relationships between the onset of extension plateau uplift and the thermal state of the lithosphere are critical to this debate we present new data on postcollisional north south trending dikes in southern tibet their ages range from 18 3 2 7 ma to 13 3 0 8 ma and define the onset of regional east west extension in southern tibet dikes are compositionally indistinguishable from postcollisional lavas in southern tibet being either ultrapotassic having a source in the subcontinental lithospheric mantle or calc alkaline with a dominantly crustal origin the ultrapotassic dikes demonstrate that east west extension and subcontinental lithospheric mantle derived magmatism were temporally and spatially linked supporting models that relate the latest phase of plateau uplift to subcontinental lithospheric mantle thinning thus the onset of extension by 18 3 2 7 ma represents the time at which the potential energy of the plateau exceeded convergent boundary forces this places a new age limitation on the attainment of high elevation in southern tibet with implications for models that relate cenozoic monsoon intensification to plateau uplift

evolution of the bangong nujiang tethyan ocean insights from the geochronology and geochemistry of mafic rocks within ophiolites abstract the bangong nujiang suture zone bnsz is located in the central tibetan plateau and represents the relict of the bangong nujiang tethyan ocean bnto this paper presents zircon u pb ages and whole rock geochemical and sr nd isotope data for the ophiolitic rocks from the rutog dongco dongqiao amdo and dengqen areas from west to east in the bnsz zircon u pb ages obtained from five gabbros and one leucogabbro from the five areas are 169 2 167 2 187 2 184 2 177 3 and 164 2 ma respectively mafic rocks gabbros and basalts in the bnsz can be divided into morb like and oib type based on geochemical data the morb like rocks are tholeiitic and can be further divided into n and e morb subtypes marked by depleted and flat ree patterns respectively all morb like rocks show supra subduction zone ssz geochemical affinities such as mild enrichment in the large ion lithophile elements and depletion in the high field strength elements coupled with high and positive whole rock e nd t values 8 1 to 5 4 the oib type rocks which formed in the early cretaceous 132 108 ma are exposed in the dongco dongqiao and tarenben areas and they consist mostly of alkali basalts with within plate geochemical characteristics and positive e nd t values 5 9 to 4 5 on the basis of these new data combined with information from the literature we further support that the bnto was an intra oceanic arc backarc basin that existed between the north lhasa subterrane nlt and the south qiangtang subterrane sqt during the early middle jurassic this basin may have been active until the early cretaceous when the oib type basalts erupted

observational hints for a plume fed suboceanic asthenosphere and its role in mantle convection an asthenosphere layer which is entirely fed from below by plumes and which loses equal mass by accretion to the overlying oceanic lithosphere and at subduction zones may play a critical role in shaping the form of mantle convection in this study we discuss geochemical seismic and geoid depth evidence for lateral flow within this type of asthenosphere in particular we suggest that there are large scale layered horizontal flow structures that connect upward plume input beneath hotspots to near ridge regions of increased asthenosphere accretion into the growing oceanic lithosphere lateral asthenosphere flow is also shaped by oceanic subduction zones with a partial return flow from trenches and by deep continental roots that are migrating barriers to asthenosphere flow this alternative paradigm offers relatively simple explanations for several puzzles about mantle convection for example the low mantle heat flow beneath continents it also offers an explanation for why mid ocean ridges appear to be passive features that migrate with little geochemical or morphological change with respect to the lower mantle and seem to be uncoupled from large scale mantle flow while in contrast trenches appear to be strongly coupled to mantle thick regions of fast colder seismic velocity anomalies we also discuss several implications of this paradigm that should be testable in future studies such as the prediction of cogenetic off axis seamount volcanism that is created between an off axis hotspot and its neighboring ridge axis

mesozoic and cenozoic volcanic rocks from central and southern tibet 39ar 40ar dating petrological characteristics and geodynamical significance field relations radiometric ages and mineralogical and chemical characteristics of the orogenic volcanic rocks from the lhasa block tibet are studied volcanic activity started before the deposition of the albian aptian sedimentary units takena formation and xigaze group volcanism is volumetrically limited in the northern part of the lhasa block basalts andesites and dacites are present while ignimbrites are scarce radiometric ages of these rocks range between 110 and 80 ma conversely in the southern part of the lhasa block volcanism is largely developed gangdese belt there paroxysmal activity occurred later than in the north between 60 and 50 ma lingzizong formation this volcanism is mainly represented by andesites and ignimbrites a younger episode at about 15 10 ma took place in this southern region the mineralogical and geochemical characteristics indicate that as a whole the volcanic rocks from the lhasa block belong to a calc alkaline continental margin series enriched in k20 and other incompatible elements subordinate potassic alkaline lavas are present in the northern margin of the gangdese belt some basalts exhibit a tholeiitic affinity daquin area southern region the geodynamic significance of these cenozoic volcanic rocks lies in the relative motion of india and the lhasa block after accretion of the latter to the northern qantang block a geodynamic model based on changes of the type of subduction of the indian oceanic crust beneath the lhasa block for the period 120 40 ma is proposed to account for the abundance age and magmatic affinity of the volcanic products

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

formation of the jurassic south china large granitic province insights from the genesis of the jiufeng pluton abstract the magma source and thermal regime for the formation of large granitic provinces lgps have been the subjects of much debate as mineralogical geochemical and isotopic variations of plutonic rocks preserve records of crustal thermal evolution the jiufeng pluton was comprehensively studied to gain insights into material and heat sources and tectonic process in the formation of the south china lgp sims zircon u pb analyses of a biotite granite 158 9 1 3 ma three two mica granites stage i 159 7 1 2 ma stage ii 158 6 1 2 ma and 156 8 1 2 ma and a muscovite granodiorite 159 0 4 4 ma of the jiufeng pluton suggest that they are coeval and were emplaced in a short period of no more than 5 3 m y limited variations in whole rock nd isotope end t 12 6 to 9 5 and zircon hf o isotopes ehf t 12 0 to 6 3 18 o 6 8 9 4 suggest that the jiufeng pluton was mainly derived from melting of a common metasedimentary source possibly with a minor basaltic contribution we consider the geochemical variations of the jiufeng pluton are primarily a result of incremental assembly of magma batches produced from rapid step like transition from fluid saturated to fluid absent melting of the source the muscovite granodiorite with high na 2 o 3 80 wt k 2 o na 2 o 1 is interpreted to have been produced by fluid saturated melting at low temperature 650 c high p 2 o 5 0 09 0 17 wt zircon saturation temperature t zr 769 816 c and la yb ratios 8 4 55 8 of the stage ii two mica granite support its formation from high temperature 800 c biotite dehydration melting whereas lower p 2 o 5 zr 685 742 c and la yb

a slab breakoff model for the neogene thermal evolution of south karakorum and south tibet on the south karakorum margin neogene high temperature medium pressure ht mp gneisses define an east west trending thermal anomaly these rocks have been heated from 600 to 750 c during a slight pressure drop from 0 7 to 0 5 gpa their retrogressive path cross cuts the relaxed geotherm of tectonically thickened crust such a p t evolution occurs only if an advective source of heat is involved involvement of an advective heat source is also implied by the occurrence of neogene granitoids and lamprophyres within the ht mp gneiss area these rocks are strongly enriched in large ion lithophile elements relative to primitive mantle and show negative high field strength element anomalies we interpret these geochemical characteristics to be the result of melting of metasomatized asian lithospheric mantle the nd and sr isotopic compositions of the south karakorum neogene magmatic rocks nd 12 to 7 and 87sr 86sr 0 705 0 725 further suggest they could have originated from mixing between asian variously metasomatized mantle and precambrian crust by contrast the origin of the youngest magmatic rocks 10 myr here exemplified by the hemasil syenite and associated lamprophyres requires involvement of a depleted mantle the combined hf nd signature of these rocks hf 10 4 11 5 and nd 3 4 4 3 suggests that the source of the hemasil syenite could have been depleted mantle contaminated by oceanic sediments likely during the earlier subduction of the tethyan ocean neogene magmatic rocks with the same geochemical characteristics and evolution as those of south karakorum have previously been described in south tibet based on their location and the geochemical evolution of their source region we here propose that the neogene magmatic and metamorphic evolution of the south asian margin was controlled by slab breakoff of the subducting indian continental margin starting at about 25 ma this model is supported by available geophysical data from south karakorum and south tibet

changes of provenance of permian and triassic sedimentary rocks from the ailaoshan suture zone sw china with implications for the closure of the eastern paleotethys abstract tectonic evolution of the ailaoshan ocean eastern paleotethys especially on the timing of opening and closure has long been debated published works on the paleotethyan evolution were mainly dedicated to structural tectonics and magmatic and metamorphic rocks whereas studies on sediment chemistry are rare in this study we present new chemical data on the permian and middle upper triassic clastic sedimentary rocks from the ailaoshan suture zone in southwestern yunnan sw china compared with the post archean australian shale paas and the average upper continental crust these ailaoshan clastic rocks are generally depleted in liles e g k and rb and hfses e g nb and zr and show varying lree hree enrichments la yb n 1 32 11 86 flat hree pattern and negative eu anomalies eu eu 0 3 0 9 in spite of these similarities the permian samples have relatively low sio2 avg 71 4 wt contents k2o na2o avg 0 65 and eu eu avg 0 65 ratios but high al2o3 avg 14 1 wt tio2 avg 0 60 wt fe2ot 3 mgo avg 6 96 wt contents al2o3 sio2 avg 0 20 and la yb n avg 6 9 ratios similar to greywackes from typical continental arcs in contrast the upper triassic samples have relatively high sio2 avg 81 9 wt contents k2o na2o avg 8 48 and lan ybn avg 9 74 ratios but low al2o3 avg 9 98 wt tio2 avg 0 54 wt fe2ot 3 mgo avg 5 26 wt contents al2o3 sio2 avg 0 12 and eu eu avg 0 64 ratios similar to greywackes from typical passive continental margin moreover the permian samples have lower cia values chemical index of alteration 47 74 avg 59 but higher icv values index of compositional variability 0 70 1 29 avg 1 03 than those of paas indicating relatively weak chemical weathering of the source rocks with low sediment maturity in contrast the upper triassic samples have relatively high cia 77 84 avg 80 but low icv values 0 52 1 12 avg 0 79 suggesting intense chemical weathering of the source rocks with high sediment maturity the wide range of cia 52 84 avg 69 and icv 0 50 1 14 avg 0 88 values for the middle triassic samples suggest multiple detrital sources with varying degree of chemical weathering and diverse sediment maturity as also supported by their wide range of detrital contents and chemical compositions based on the distinct chemical contrast between the permian and triassic sedimentary rocks we infer that a significant change in detrital provenance and depositional setting may have occurred across the middle and late triassic boundary which was likely caused by the closure of the ailaoshan ocean

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

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

non hotspot formation of volcanic chains control of tectonic and flexural stresses on magma transport the south pacific in the vicinity of both the superswell and the east pacific rise is repleat with volcanic chains that unlike the emperor hawaiian chain defy the hypothesis of formation via the relative motion of plates and hotspots we propose two nearly identical models for the origin of near axis and superswell chains assuming that both regions are underlain by significant quantities of more or less uniformly distributed partial melt given an initial volcanic load or a local anomaly in the melt source region volcanic chains form by magmatic hydrofracture at local tensile maxima of flexural and membrane stresses fracture wall erosion by magma flow provides a feedback which results in discrete edifices within the chains the model predicts island chains aligned with a deviatorically tensile tectonic stress near the ridge the elastic lithosphere is thin and observations and theoretical considerations indicate a strong deviatorically tensile stress field perpendicular to the ridge axis under such conditions the model results in parallel lines of volcanoes perpendicular to the spreading ridge later interstitial volcanism within the individual chains reduces the average spacing and results in nearly continuous ridges on the thicker lithosphere of the superswell membrane stresses are negligible and the model produces chains of much more widely spaced volcanoes a more isotropic stress field may result in broader chain like patterns of volcanoes in both cases the chains represent self propagating disturbances the resulting age progressions are thus independent of plate velocity but depend only on the dynamics of volcano formation and evolution

sphalerite 40ar 39ar progressive crushing and stepwise heating techniques abstract it is difficult to determine the mineralization ages of sulfide deposits because no suitable minerals can be selected for the traditional isotopic dating methods the 40ar 39ar method rarely applies well to sulfide minerals because of their trace potassium content and strong radioactivity after irradiation this preliminary study investigates the possibility of dating the hydrothermal sphalerites by 40ar 39ar using crushing and heating techniques the 40ar 39ar crushing experiments of two sphalerite samples from the fankou lead zinc deposit yield well defined and concordant isochron ages of 265 8 1 0 1 ma and 269 7 6 2 ma corresponding to initial 40ar 36ar ratios of 290 7 2 4 and 294 6 8 1 respectively the crushed powders of the later sphalerite yields with stepwise heating a flat age spectrum with 40ar 39ar plateau age of 267 2 2 5 ma which define a good isochron with an age of 271 3 5 4 ma and initial 40ar 36ar ratio of 294 0 1 7 this is the first report to distinguish secondary and primary fluid inclusions within sphalerite to obtain their ages using the crushing technique and to obtain good concordant ages both of fluid inclusions by crushing and heating of k containing minerals we expect progressive crushing and stepwise heating techniques will become an effective approach to 40ar 39ar dating of the mineralization ages of hydrothermal sulfide deposits

origin of miocene cu bearing porphyries in the zhunuo region of the southern lhasa subterrane constraints from geochronology and geochemistry abstract the zhunuo cu bearing porphyries are located in the westernmost part of the miocene gangdese porphyry cu mo au deposit belt zircon u pb dating of the diorite porphyry k feldspar granite porphyry and monzonitic granite porphyry in zhunuo yielded crystallization ages of 12 5 0 4 ma 12 3 0 3 ma and 12 4 0 3 ma respectively the diorite porphyry is characterized by low sio 2 58 61 61 14 wt and th 0 30 0 76 ppm concentrations low th la 0 05 0 1 ratios and high mg 49 values coupled with low 87 sr 86 sr i 0 703777 0 703783 and high e nd t 4 07 to 4 90 values they also have adakite like affinities such as low y 10 5 12 0 ppm and high sr y ratios 61 65 they were probably derived from a thickened juvenile lower continental crust the k feldspar granite porphyry probably originated in the middle upper continental crust because of their high sio 2 73 59 74 98 wt and th 50 1 52 1 ppm concentrations high th la 1 67 2 10 and low sr y 20 2 20 7 ratios and mg 32 38 values combined with high 87 sr 86 sr i 0 710921 0 712008 low e nd t 8 47 to 9 26 isotopic compositions and old nd model ages 1 16 1 25 ga their magmas were most likely partial melts of the preserved ancient crust similar to the central lhasa subterrane the geochemical characteristics and sr nd isotopic compositions of the monzonitic granite porphyry display trends that lie between those of the diorite porphyry and k feldspar granite porphyry and they are therefore likely to be production of hybridization between the above two melts the ore bearing diorite porphyry and monzonitic granite porphyry have higher zircon ce 4 ce 3 ratios than the ore barren k feldspar granite porphyry indicating a higher oxygen fugacity in the ore bearing magmas we suggest that metals were released from the re melting of arc related cumulates which formed during lower crustal growth and thickening this mechanism provides a reasonable explanation for the significant flare up of mineralization during the miocene in the gangdese region the lower continental crust beneath southern lhasa subterrane probably was uniformly juvenile but the region to the west of zhunuo was not mineralized due to input of large ancient crustal materials in the source of these ore barren adakite like rocks

miocene adakites in south tibet partial melting of the thickened lhasa juvenile mafic lower crust with the involvement of ancient indian continental crust compositions

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

geochemical and nd pb isotopic characteristics of the tethyan asthenosphere implications for the origin of the indian ocean mantle domain abstract it is unclear why the pb nd and sr isotopic composition of the modern mid ocean ridge basalts morb from the indian ocean is different from that of the north atlantic and pacific oceans a possible explanation for this is that the indian morb type isotopic signature is a long lived regional feature of the mantle as evidently shown by the isotopic composition of the 350 ma morb like mian lue northern ophiolite which was formed in the same region presently occupied by the indian ocean however this hypothesis is in conflict with the lack of indian morb type isotopic signature in a number of 150 ma tethyan and indian ocean crusts to further constrain the origin of the indian morb type isotopic signature we analyze the geochemical and pb nd and sr isotopic composition of representative mafic rocks from four tethyan ophiolites ranging in age from 90 to 360 ma the sr isotopic composition of the samples is unreliable due to alteration but the age corrected nd and pb isotopic ratios and geochemical data indicate that these tethyan rocks were derived from a geochemically depleted asthenospheric source that had a clear indian morb type isotopic signature we therefore conclude that the bulk of the indian suboceanic mantle was most probably inherited from the tethyan asthenosphere a few regions in both the tethyan and indian oceans however are most probably underlain by pacific and north atlantic morb type mantle and vice versa because of the flow of the asthenosphere in response to tectonic plate reorganizations that lead to openings and closures of ocean basins the indian morb type isotopic signature of the western pacific marginal basin crusts could be due to either flow of the indian ocean mantle into the western pacific or to endogenous production of such an isotopic signature from delaminated east asian sublithospheric materials during closure of the tethys ocean

evolutionary model of the himalaya tibet system geopoem based on new modelling geological and geophysical data a two dimensional thermo mechanical laboratory modelling of continental subduction was performed the subducting continental lithosphere includes a strong brittle upper crust a weak ductile lower crust and a strong upper mantle the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is denser than the asthenosphere the lithospheric layers are composed of material whose strength is sensitive to and inversely proportional to temperature throughout the experiment the model surface was maintained under relatively low temperature and the model base at higher temperature the subduction rate satisfied the peclet criterion modelling confirms that the continental crust can be deeply subducted and shows that slab break off delamination and tectonic underplating are fundamental events with drastic consequences on the subsequent evolution of the convergent system combining these results with previous purely mechanical modelling we elaborate a new evolutionary model for the himalaya tibet convergent system the principal successive stages are 1 subduction of the indian continental lithosphere to 200 250 km depth following subduction of the tethys oceanic lithosphere 2 failure and rapid buoyancy driven uplift of the subducted continental crust from ca 100 km depth to some depth that varies along the mountain belt 20 30 km on average 3 break off of the indian subducted lithospheric mantle with the attached oceanic lithosphere 4 subduction underplating of the indian lithosphere under asia over a few to several hundred kilometers 5 delamination roll back and break off of the indian lithospheric mantle 6 failure of the indian crust in front of the mountain belt formation of the main central thrust and underthrusting of the next portion of indian lithosphere beneath tibet for a few hundred kilometers at the beginning of stage 6 the crustal slice corresponding to the crystalline himalayas undergoes erosion activated uplift and exhumation fl 2000 elsevier science b v all rights reserved

origin of potassic c type adakite magmas experimental and field constraints the postcollisional mesozoic c type or high k adakitic intrusions k adakites of china lack either temporal or spatial association with subduction and have k2o na2o around unity otherwise their geochemistry is very similar to ordinary adakites new experimental data geological observations and theoretical considerations lead us to suggest that these k adakites were high t 1050 and probably 1075 c rather hydrous magmas 6 wt h2o produced by fluid absent partial melting of metatonalites meta andesites or possibly potassic metabasalts at pressures exceeding 2 gpa their peculiar chemistry is a consequence of both the elevated k2o na2o in the protolith and the very high pressure of partial melting the most likely tectonic setting is one of extreme crustal thickening followed by delamination of the eclogitic keel and partial melting of this continental crustal material at mantle depths with high mantle heat flow during orogenic collapse

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

first field evidence of southward ductile flow of asian crust beneath southern tibet there is lively debate on whether asian plate material was involved in southward flow of mid lower crust in a ductile channel beneath southern tibet one argument against such involvement is the apparent absence of material derived from asian lithosphere within the high himalayan series indian plate that could represent the putative channel a north south trending mid miocene dike swarm that intrudes the tethyan sedimentary cover of the sakya gneiss dome indian plate yields new sr nd isotopic data 87sr 86sr 0 7071 0 7079 nd 4 to 6 indicating that these melts share the same source as miocene dacitic dikes from north of the indus tsangpo suture moreover dikes on both sides of this suture represent crustal melts derived largely from mid lower crust of the asian plate exposed today as the nyainqentanglha gneisses that underlie the gangdese batholith we infer that melting of the asian lithosphere extended south of the surface trace of the suture requiring southward propagation of anatectic asian middle crustal material during the miocene the emplacement ages of the southern dike swarm 12 9 ma thus delimit the timing of active southward ductile flow of asian material

late triassic syn exhumation magmatism in central qiangtang tibet evidence from the sangehu adakitic rocks abstract the geodynamic setting of late triassic magmatic activity along the longmu co shuanghu suture zone lssz in central qiangtang tibet is a matter of debate this paper presents zircon la icp ms u pb ages zircon hf isotopic compositions and whole rock geochemical data for the sangehu sgh granitic intrusion in central qiangtang and addresses the petrogenesis of late triassic magmatism and the history of collision between the northern and southern qiangtang terranes the sgh pluton consists mainly of biotite adamellite with mafic microgranular enclaves mmes and small amounts of k feldspar granite the biotite adamellite mmes and k feldspar granite give ages of 207 8 3 0 ma 212 4 31 ma and 211 6 3 8 ma respectively the mmes show magmatic textures and acicular apatite and are coeval with the host biotite adamellite suggesting they were produced by magma mixing all samples from the sgh pluton show high sr and low y contents and positive eu anomalies similar to adakitic rocks the high k2o contents and low mg cr and ni contents and enriched hf isotopic characteristics of the zircons indicate that these magmas were derived from the partial melting of thickened crust however the whole rock geochemical data and zircon hf isotopic compositions also reveal heterogeneity at the source the combined magmatic and metamorphic records suggest that triassic magmatic activity in central qiangtang was closely related to the collision of the northern and southern qiangtang terranes the large scale late triassic 225 200 ma magmatic event in central qiangtang may have resulted from the breakoff of the longmu co shuanghu tethys ocean lithospheric slab in the early late triassic 236 230 ma the late triassic magmatic rocks including adakitic rocks are coeval with retrograde high pressure hp to ultrahigh pressure uhp metamorphic rocks in central qiangtang and show characteristics of syn exhumation magmatism the early adakitic rocks 220 ma were generated by melting of the edge of the slab which was heated by upwelling asthenospheric mantle after subduction slab breakoff and the late adakitic rocks

lithium isotopic composition and concentration of himalayan leucogranites and the indian lower continental crust abstract the lithium isotopic compositions of adakitic rocks and k rich volcanic rocks in southern tibet range from 1 3 to 7 5 and 4 9 to 3 5 respectively the subduction of the indian plate beneath the lhasa terrane means that traditional lithium isotopic compositions of various reservoirs are unable to explain the aforementioned 7 li data therefore it is necessary to determine the li isotopic compositions of the different geological endmembers underneath southern tibet here we report the lithium isotopic composition and concentration of indian upper and lower continental crust on the basis of whole rock major and trace element data and sr nd pb isotope data leucogranites from luozha and longzi are considered as representative of indian upper crust whereas two mica granites from quedang and dala and granulites and gneisses from nyalam are considered as representative of indian lower crust the li concentration of indian upper crust varies from 23 to 45 ppm with a mean of 34 ppm consistent with a weighted mean li concentration for the upper crust of 35 11 ppm in contrast the li abundance of indian lower crust is estimated to be 33 84 ppm with a mean of 58 ppm much higher than the average li concentration for the lower crust of 8 ppm because of the high modal abundance of biotite and muscovite the li isotopic compositions 0 9 to 5 6 of indian upper continental crust are relatively heavy compared with the average li isotopic composition of upper continental crust on the other hand the li isotopic compositions of indian lower continental crust 4 4 to 0 1 are lighter than those of indian upper continental crust the isotopically heavy signature of indian upper crust is produced by high 7 li fluids released from the indian lower crust slab whereas the lighter signature of indian lower crust is generated by the partial melting of residual indian lower crust slab during metamorphic dehydration of indian lower crust

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

evidence for late pleistocene thinning of siple dome west antarctica 1 we use a two dimensional full stress thermomechanical flow band model to investigate late pleistocene and holocene histories of ice thickness and ice flow at siple dome in west antarctica measurements obtained from an ice core and borehole near the ice divide and measurements of radar detected internal stratigraphy are used as targets for modeling models that account for the climate history accumulation rate and temperature alone yield a poor match to observations histories of ice dynamics ice thickness ice divide location and ice flow enhancement also need to be included our favored model includes 350 m of thinning at siple dome between 15 000 and 14 000 years ago onset of divide flow 3000 years ago ongoing divide migration starting 2500 years ago and evolution of ice fabric over the past 30 000 years the timing of thinning is earlier than that inferred from geologic evidence from the margins but is coincident with a 20 m rise in global sea level about 14 500 years ago meltwater pulse 1a this opens the possibility that thinning was a result of increased activity of the ice streams in the late pleistocene forced by meltwater pulse 1a the contribution to meltwater pulse 1a as a result of this thinning was likely small

did the indo asian collision alone create the tibetan plateau it is widely believed that the tibetan plateau is a late cenozoic feature produced by the indo asian collision however because tibet was the locus of continental accretion and subduction throughout the mesozoic crustal thickening during that time may also have contributed to growth of the plateau this portion of the geologic history was investigated in a traverse through the central lhasa block southern tibet together with earlier studies our mapping and geochronological results show that the lhasa block underwent little north south shortening during the cenozoic rather our mapping shows that 60 crustal shortening perhaps due to the collision between the lhasa and qiangtang blocks occurred during the early cretaceous this observation implies that a significant portion of southern tibet was raised to perhaps 3 4 km elevation prior to the indo asian collision

late triassic granitic magmatism in the eastern qiangtang eastern tibetan plateau geochronology petrogenesis and implications for the tectonic evolution of the paleo tethys abstract triassic granites are widely exposed in the central qiangtang northern tibetan plateau they therefore made an important contribution to the growth of the tibetan plateau in this paper we present zircon u pb dating and hf isotopic results and whole rock elemental and sr nd isotopic analyses of the dongdashan dds batholith in the central qiangtang in order to understand their petrogenesis and tectonic setting la mc icp ms zircon u pb results show that the dds batholith was emplaced at 220 3 0 7 ma all the granites in the dds possess high a cnk values 1 0 and display peraluminous characteristics similar to s type granite their strongly fractionated ree patterns la yb n 6 98 13 9 with conspicuous negative eu anomalies eu eu 0 51 0 67 together with negative e nd t and e nd t values and depleted nd and hf model ages suggest that the dds granitic magma had a dominantly crustal source likely the paleoproterozoic basement i e the ningduo and caoqu group metasediments in the area in combination with regional studies our new geochemical data and geochronological results demonstrate that the late triassic magmatism was generated in a post collisional tectonic setting the spatial distribution pattern of the mesozoic igneous rocks coupled with the exhumation of high pressure metamorphic rocks in the central qiangtang favors a slab breakoff model which resulted in post collisional extension and asthenospheric upwelling that induced large scale partial melting of the middle lower crust to produce voluminous amounts of felsic magma therefore the occurrence of the late triassic post collisional magmatism and particularly the exhumation of high pressure eclogites and blueschists as well as their presence in the lowermost portion of the late triassic volcanoclastics in the qiangtang basin clearly indicate that the final closure of the of paleo tethys ocean and associated continent continent collision between the gondwana derived western qiangtang and eastern qiangtang terranes had not been completed until the early mid triassic

crustal origin of early proterozoic syenites in the congo craton ntem complex south cameroon two petrographically different syenite suites can be distinguished in the mostly archean ntem complex of cameroon a suite of two pyroxene syenites and a suite of clinopyroxene syenites the syenites occur as discrete intrusions surrounded by archean gneisses based on structural relationships the syenites are of late to post orogenic origin the pb pb zircon evaporation ages indicate that the syenites intruded at ca 2 3 ga and thus postdate significantly the archean episode of crust formation and differentiation that generated the surrounding gneiss terrane the isotope signatures and the trace elements provide unambiguous evidence that the two syenite suites are of crustal origin the syenite suites do not show simple chemical trends have highly variable trace element contents and display only a small variation in their major element contents particularly in their sio2 contents the isotope characteristics including 143nd 144nd 87sr 86sr and common pb point to a heterogeneous source or multiple sources for the syenites the sr isotope compositions calculated for the time of intrusion vary over a wide range from an unrealistically low 87sr 86sr ratio of 0 690 to high values of 0 713 in contrast the initial 143nd 144nd ratios are more homogenous with end 2320 from 5 8 to 7 5 the pb isotopes from leached k feldspars indicate that the sources of the syenites had a history of u depletion and th u enrichment the syenites show the same isotope characteristics as the surrounding gneiss terrane thus the deep continental crust of the ntem complex itself is the most likely source for the syenite melts this conclusion is also supported by the heterogeneity of the trace elements in the syenites that exclude a homogenous source in addition inherited zircons from the syenites yield pb pb evaporation ages that are consistent with ages obtained for zircons extracted from the surrounding gneisses in contrast to many other syenites worldwide the syenites in the ntem complex do not contain juvenile mantle material but are the sole products of crustal reworking thus the syenites did not contribute to the crustal growth in the ntem complex however this conclusion may not preclude the possibility that the syenites were generated in the deep crust during a time of addition of mantle material to the continental crust the cause for the generation of these large syenite bodies in the ntem complex is most likely an episode of deep seated thermal activity that was so far not known from this part of the congo craton

cenozoic volcanism in tibet evidence for a transition from oceanic to continental subduction geochronological k ar or ar ar major and trace element sr nd pb isotopic and mineral chemical data are presented for newly discovered cenozoic volcanic rocks in the western qiangtang and central lhasa terranes of tibet alkali basalts of 65 45ma occur in the western qiangtang terrane and represent primitive mantle melts as indicated by high mgnumbers 100 mg mg fe 54 65 cr 204 839 ppm and ni 94 218 ppm contents and relatively low ratios of sr sr 0 7046 0 7061 pb pb 18 21 18 89 pb pb 15 49 15 61 and pb pb 38 42 38 89 and high ratios of nd nd 0 5124 0 5127 in contrast younger volcanic rocks in the western qiangtang terrane 30ma and the central lhasa terrane 23 13 and 8ma are potassic to ultrapotassic and interpreted to have been derived from an enriched mantle source they are characterized by very high contents of incompatible trace elements negative ta nb and ti anomalies and radiogenic pb isotopic compositions pb pb 18 43 19 10 pb pb 15 64 15 83 pb pb 39 14 39 67 sr sr 0 7088 0 7092 and nd nd 0 5122 ratios of the western qiangtang terrane potassic lavas are similar to those of 45 29ma potassic volcanic rocks in the north central qiangtang terrane whereas sr sr 0 7167 0 7243 and nd nd 0 5119 ratios of central lhasa terrane lavas are similar to those of 25 16ma ultrapotassic volcanic rocks in the western lhasa terrane the 65 45ma alkali basalts in the western qiangtang terrane along with widespread calc alkaline volcanic rocks of this age in the lhasa terrane may be related to roll back of a previously shallow north dipping slab of tethyan oceanic lithosphere beneath tibet subduction as opposed to convective thinning of continental lithosphere is favored to explain potassic volcanism in tibet because of its occurrence in distinct east west trending belts 45 29ma in the qiangtang terrane 25 17ma in the northern lhasa terrane 16 8ma in the southern lhasa terrane and temporal and spatial relationships with major thrust systems

tectonic magmatic and metallogenic evolution of the tethyan orogen from subduction to collision abstract this paper reviews the tectonic magmatic and metallogenic history of the tethyan orogen from the carpathians to indochina focus is placed on the formation of porphyry cu mo au deposits as being the most characteristic mineral deposit type formed during both subduction and collisional processes in this region relatively little is known about the history of the paleotethys ocean which opened and closed between gondwana and eurasia in the paleozoic and few ore deposits are preserved from this period the neotethyan ocean opened in the permian early triassic as the cimmerian continental fragments the cores of turkey iran tibet and indochina rifted from the northern gondwana margin and drifted northwards these microcontinents docked with the eurasian margin at various points in the mesozoic and cenozoic and formed a complex archipelago involving several small back arc basins and remnants of the paleotethyan ocean the main neotethyan ocean and these smaller basins were largely eliminated by collision with india and africa arabia in the early eocene and early mid miocene respectively although neotethyan subduction continues beneath the hellenic arc and the makran the majority of porphyry type deposits are found in association with neotethyan subduction mainly in the mesozoic and paleogene and syn to post collisional events in the mid paleogene to neogene they are found throughout the orogen but some sections are particularly well endowed including the carpathians balkans rhodopes eastern turkey lesser caucasus nw iran se iran sw pakistan southern tibet and se tibet indochina other sections that appear barren may reflect deeper levels of erosion young sedimentary cover or lack of exploration although there may also be real reasons for low prospectivity in some areas such as minimal subduction e g the western mediterranean region or lithospheric underthrusting as proposed in western tibet over the last decade improved geochronological constraints on the timing of ore formation and key tectonic events have revealed that many porphyry deposits that were previously assumed to be subduction related are in fact broadly collision related some forming in back arc settings in advance of collision some during collision and others during post collisional processes such as orogenic collapse and or delamination of subcontinental mantle lithosphere while the formation of subduction related porphyries is quite well understood collisional metallogeny is more complex and may involve a number of different processes or sources these include melting of orogenically thickened crust previously subduction modified lithosphere including metasomatized mantle underplated mafic rocks or lower crustal arc plutons and cumulates or upwelling asthenosphere e g in response to delamination slab breakoff back arc extension or orogenic collapse the most fertile sources for syn and post collisional porphyry deposits appear to be subduction modified lithosphere because these hydrated lithologies melt at relatively low temperatures during later tectonomagmatic events and retain the oxidized and relatively metalliferous character of the original arc magmatism unusually metallically enriched lithospheric sources do not seem to be required but the amount of residual sulfide phases in these rocks may control metal ratios e g cu au in subsequent magmatic hydrothermal ore deposits relatively au rich deposits potentially form in these settings as observed in the carpathians e g rosia montan turkey kisladag copler and iran sari gunay dalli although the majority of syn and post collisional porphyries are cu mo rich and resemble normal subduction related deposits e g in the gangdese belt of southern tibet this similarity extends to the associated igneous rocks which being derived from subduction modified sources largely retain the geochemical and isotopic character of those original arc magmas while still retaining a broadly calc alkaline character these rocks may extend to mildly alkaline shoshonitic compositions and may display adakite like trace element signatures high sr y and la yb ratios reflecting melting of deep crustal garnet amphibolitic sources but they are otherwise hard to distinguish from normal subduction related magmas small post collisional mafic alkaline volcanic centers are common throughout the orogen but for the most part appear to be barren however similar rocks in other post subduction settings around the world are associated with important alkalic type porphyry and epithermal au cu deposits and the potential for discovery of such deposits in the tethyan orogen should not be overlooked

potassic volcanic rocks and adakitic intrusions in southern tibet insights into mantle crust interaction and mass transfer from indian plate abstract elucidating geodynamic processes at depth relies on a correct interpretation of petrological and geochemical features in magmatic records in southern tibet both potassic volcanic rocks and adakitic intrusions exhibit high sr y and la yb and low y and yb concentrations but these two rock types have contrasting temporal spatial distributions and isotopic variations here we present a systematic study on the postcollisional potassic and adakitic rocks in order to investigate their petrogenetic links with the coeval mantle derived ultrapotassic rocks and shed light on the potential input from underthrusted indian continental crust we found that adakitic intrusions with higher k2o na2o tend to display lower y and higher sio2 suggesting that the mantle derived ultrapotassic melts showing relatively high y and yb concentrations only played a minor role in adakitic magmatism therefore the unradiogenic 143nd 144nd and the dramatic decrease of zircon ehf t values since 35 ma shown by postcollisional adakites should be interpreted as reflecting the crustal input from indian plate unlike adakitic intrusions in southern lhasa subterrane potassic volcanic rocks share similar spatial distributions with ultrapotassic rocks and their isotopic discrepancy is diminishing with volcanic activity becomes younger and migrates eastward evidence from whole rock pb and zircon hf isotopes further indicates that potassic volcanic rocks are more likely to originate from partial melting of the overthickened and isotopically heterogeneous lhasa terrane crust rather than the underthrusted indian continental crust the elevated rb sr and varying sr cao in potassic volcanic rocks provide an argument for sanidine plagioclase clinopyroxene as the major fractionating phases during magmatic differentiation these findings not only highlight the significance of potassic and adakitic rocks in providing constraints on the geodynamic processes beneath southern tibet but also imply that special caution is needed if we attempt to probe into the nature of mantle lithosphere using isotopic tracers of the tibetan ultrapotassic rocks

palaeo altimetry of the late eocene to miocene lunpola basin central tibet the elevation history of the tibetan plateau provides direct insight into the tectonic processes associated with continent continent collisions here we present oxygen isotope based estimates of the palaeo altimetry of late eocene and younger deposits of the lunpola basin in the centre of the plateau which indicate that the surface of tibet has been at an elevation of more than 4 kilometres for at least the past 35 million years we conclude that crustal but not mantle thickening models combined with plate kinematic solutions of india asia convergence are compatible with palaeo elevation estimates across the tibetan plateau

volcanic rocks of the 1985 tibet geotraverse lhasa to golmud volcanic rocks encountered during the tibet geotraverse have been studied in the field in thin section and by major and trace element geochemistry in order to determine their most probable original eruptive environment rocks from a total of eleven distinct volcanic provinces were studied in this way they provide evidence for an active continental margin or post collision province of probable devonian early carboniferous age in the northern kunlun mountains an active continental margin of late carboniferous age in the southern lhasa terrane permian continental rifts in the central qiangtang and central kunlun terranes triassic volcanic arcs in the southern lhasa and northern qiangtang terranes a triassic active continental margin dyke swarm in the northern kunlun mountains a jurassic post collision or back arc rifting province in the southern qiangtang terrane a jurassic island arc in the northern lhasa terrane a cretaceous post collision province in the northern lhasa terrane possibly extending into the southern qiangtang terrane and a palaeogene active continental margin in the southern lhasa terrane an oligocene trachyte plug in the northern qiangtang terrane was the only evidence encountered during the geotraverse of volcanism post dating the palaeogene india eurasia collision however the composition of this plug coupled with new and published analyses from miocene volcanics in the southern lhasa terrane and from the pliocene recent volcanic province of northwest tibet places important constraints on models for post collision underplating of tibet by continental lithosphere any underplating is likely to have been a much later than the start of collision b directed beneath tibet from the north as well as the south and c limited in extent

late paleozoic calc alkaline to shoshonitic magmatism and its geodynamic implications yuximolegai area western tianshan xinjiang article i nfo continuous late paleozoic magmatic activity in the western tianshan xinjiang china temporally overlapped the late carboniferous collision between the tarim and the kazakhstan yili plates isotopic dating whole rock geochemistry and isotopic characteristics of a suite of calc alkaline to shoshonitic rocks from yuximo legai area in the eastern end of the awulale mountains help define the tectonic evolution of this part of the tianshan zircon u pb ages of a quartz diorite and a quartz syenite porphyry are 310 8 2 1 ma and 284 4 3 6 ma respectively the geochemical and isotopic characteristics of late carboniferous igneous rocks including the quartz diorite as well as basaltic andesite and k rich trachyandesite indicate an evolu tion from calc alkaline towards shoshonitic series this was generated by the partial melting of the hydrated mantle wedge induced by changing subduction angles the younger quartz syenite porphyry however was more likely derived from the partial melting of a thickened crust in a post collisional setting or the partial melting of hot juvenile basalt in the lower crust during early permian delamination of the subcontinental lithospheric mantle therefore the generation of the calc alkaline to shoshonitic igneous rocks in yuximole gai area is genetically linked to the geodynamic evolution of the western tianshan from convergent subduc tion to collision during the late paleozoic

discrete alternating hotspot islands formed by interaction of magma transport and lithospheric flexure the large scale geometry and age progression of many hotspot island chains such as the hawaiian emperor chain are well explained by the steady movement of tectonic plates over stationary hotspots but on a smaller scale hotspot tracks are composed of discrete volcanic islands whose spacing correlates with lithospheric thickness1 moreover the volcanic shields themselves are often not positioned along single lines but in more complicated patterns such as the dual line known as the kea and loa trends of the hawaiian islands2 3 here we make use of the hypothesis that island spacing is controlled by lithospheric flexure1 to develop a simple nonlinear model coupling magma flow which feeds volcanic growth to the flexure caused by volcanic loads on the underlying plate for a steady source of melt underneath a moving lithospheric plate magma is found to reach the surface and build a chain of separate volcanic edifices with realistic spacing if a volcano is introduced away from the axis of the chain as might occur following a change in the direction of plate motion the model perpetuates the asymmetry for long distances and times thereby producing an alternating series of edifices similar to that observed in the kea and loa trends of the hawaiian island chain

origin of cenozoic alkaline potassic volcanic rocks at konglongxiang lhasa terrane tibetan plateau products of partial melting of a mafic lower crustal source abstract it is generally believed that cenozoic potassic and ultrapotassic volcanic rocks of the tibetan plateau were generated by partial melting of enriched mantle this paper reports new data regarding the elemental and isotopic compositions of the konglongxiang volcanic rocks kvr k2o na2o 2 and low mgo contents in the middle western lhasa terrane tibetan plateau ar ar isotope dating of a sample from the kvr yields an age of 21 3 ma indicating that the kvr lavas erupted during the early miocene the kvr consists mainly of trachyte and rhyolite a comparison of the kvr with cenozoic potassic volcanic rocks of the tibetan plateau and the results of melting experiments involving k rich volcanic rocks as well as other k rich volcanic rocks from throughout the world reveals that the kvr trachytes originated from a mafic lower crust containing k rich minerals rather than from a mantle source the kvr rhyolites were probably derived from the middle upper crust based on age data compositional data and the regional tectonic setting we propose that the formation of the kvr and associated n s grabens was triggered by early miocene e w regional extension in the area of the southern tibetan plateau

the high k2o volcanism of northwestern tibet geochemistry and tectonic implications cenozoic volcanic rocks sampled during the 1989 sino french kunlun geotraverse have been analyzed to constrain the origin of widespread tertiary to neogene tibetan basic magmatism samples ranging from phonotephrites to trachyandesites are high k lavas k na always greater than 1 which exhibit typical calc alkaline characteristics although they are highly enriched in mobile trace elements and ree 600 times chondrites for la isotopic compositions exhibit radiogenic sr and pb 87sr 86sr 0 708 208pb 204pb 39 39 4 207pb 204pb 15 6 15 7 206pb 204pb 18 7 and unradiogenic nd 143nd 144nd 0 5123 0 5124 constancy of sr and nd isotopic values contrasts with variations in pb isotopes and highlights the problem of crustal contamination afc and simple mixing models are compatible with isotopic variations but were probably buffered by elemental concentration in sr and nd while pb isotopic values for the most primitive magmas are probably uncontaminated isotopic features are therefore proposed to be representative of a mantle source in which heterogeneity could be highlighted in the pb isotopic values such a source highly enriched in trace elements and radiogenic sr and pb is proposed to lay in the subcontinental mantle isolated from the overall convection movements the calc alkaline characteristics can date from the succession of enrichment phases linked to the numerous subduction events that have affected this part of asia while the isotopic characteristics are proposed to be the result of the aging of the concomitant lile enrichments the broad geographical geochemical and timing aspects of recent basic cenozoic magmatism are discussed in the light of our new data and we suggest that two mechanisms are acting together 1 a limited delamination of the base of the lithosphere or a thermal reequilibration of isotherms subsequent to thickening and 2 intracontinental subduction under the kunlun such mechanisms appear the most probable as they respect at the same time recent geophysical data and other geochemical studies

normal faulting in central tibet since at least 13 5 myr ago tectonic models for the evolution of the tibetan plateau interpret observed east west thinning of the upper crust to be the result of either increased potential energy of elevated crust1 or geodynamic processes that may be unrelated to plateau formation2 3 4 5 6 a key piece of information needed to evaluate these models is the timing of deformation within the plateau the onset of normal faulting has been estimated to have commenced in southern tibet between about 14 myr ago7 and about 8 myr ago8 and in central tibet about 4 myr ago9 here however we report a minimum age of approximately 13 5 myr for the onset of graben formation in central tibet based on mineralization ages determined with rb sr and 40ar 39ar data that post date a major graben bounding normal fault these data along with evidence for prolonged activity of normal faulting in this and other tibetan grabens support models that relate normal faulting to processes occurring beneath the plateau thinning of the upper crust is most plausibly the result of potential energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath tibet this is supported by recent geophysical and geological data10 11 12 13 14 15 16 17 which indicate that spatial heterogeneity exists in both the tibetan crust and lithospheric mantle

geochemistry of eocene high mg adakitic rocks in the northern qiangtang terrane central tibet implications for early uplift of the plateau it is generally believed that the tibetan plateau is the result of crustal thickening in response to the collision of the asian and indian plates however the specific timing and uplift mechanism remain controversial the widespread occurrence of cenozoic lavas in the northern qiangtang terrane provides a unique opportunity to constrain the dynamic processes that resulted in uplift of the northern tibetan plateau eocene lavas from the northern qiangtang terrane display adakitic geochemical characteristics such as high sio 2 and al 2 o 3 contents low y and yb contents positive sr anomalies and high sr y and la yb ratios in combination with high mg 43 69 and negative anomalies for nb and ta which suggest a garnet rutile in and plagioclase free source residue the same samples also have high k 2 o and th contents high th ce ratios and low nb u ce pb ti eu and nd sm ratios as well as high 87 sr 86 sr i 0 7062 0 7075 and low e nd t 6 3 to 2 9 which show a clear continental crust affinity these high mg adakitic rocks combined with other characteristics of tibetan cenozoic lavas indicate that they were derived from partial melting of delaminated lower continental crust which subsequently reacted with surrounding mantle peridotites during ascent to crustal depths the eocene high mg adakitic rocks 46 38 ma north south trending shoshonitic dikes 47 38 ma and contemporaneous mantle derived mg rich potassic and shoshonitic lavas indicate that the thickness of the crust was at least 50 km before ca 46 ma at which time rapid uplift and extension occurred most likely caused by small scale delamination of the lithospheric mantle at 46 38 ma eocene in central tibet