geochemical and sr nd isotopic characteristics of cretaceous to paleocene granitoids and volcanic rocks se tibet petrogenesis and tectonic implications abstract northward subduction of the neo tethyan oceanic lithosphere beneath south asia gave rise to an andean type convergent margin with arc magmatism starting since at least the early jurassic and lasting until the eocene this study reports geochemical and sr nd isotope data of the early cretaceous 133 110 ma to paleocene 60 ma magmatic belt that formed in the eastern lhasa terrane se tibet including three major transhimalayan batholiths namely azhagong demulha and chayu and an early cretaceous volcanic succession from ranwu area all these rocks show relative enrichments in lile and lree and depletions in hfse corresponding to the geochemical characteristics of arc magmas from modern subduction zones while the azhagong batholith consists typically of metaluminous i type granitoids that occasionally contain mafic enclaves highly fractionated peraluminous s type and a type granites are observed from the chayu and demulha batholiths respectively these granitoids as a whole show heterogeneous sr and nd isotope ratios with e nd t varying from 1 5 to 13 and i sr from 0 703 to 0 745 the ranwu volcanic rocks which range from basaltic to dacitic compositions are calc alkaline in nature they have e nd t from 3 to 6 and i sr from 0 705 to 0 707 overlapping the isotopic range of the azhagong mafic enclaves the overall geochemical and sr nd isotope data suggest three magma source components involved in the petrogenesis which are the mantle wedge and the lower and upper parts of the lhasa continental crust accordingly we propose a two stage petrogenetic model involving 1 deep level differentiation of the mantle wedge derived mafic magmas associated with assimilation by the lower continental crust of the lhasa terrane and 2 additional differentiation and upper crustal contamination of the evolved magmas as they rise and install in shallow level magma chambers while the first stage can explain how the ranwu volcanics and azhagong enclaves formed the second accounts for the emplacement of the azhagong and chayu batholiths the shallow intrusions moreover may have caused low pressure melting of existing calc alkaline rocks of paleoproterozoic nd isotope model age to produce the early cretaceous 125 ma a type granites in the demulha batholith a specific rock type that occurs rarely in subduction zones but echoes with the notion for an extension setting in the region this setting may have taken place after the late jurassic early cretaceous continental collision between the lhasa and qiangtang terranes and interacted with the back arc evolution in the neo tethyan subduction system

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

cretaceous tertiary shortening basin development and volcanism in central tibet the geologic map pattern of the qiangtang terrane in central tibet defines a 600 km long and up to 270 km wide east plunging structural culmination it is characterized by early mesozoic blueschist bearing melange and upper paleozoic strata in the core and mainly triassic jurassic strata along the limbs in the western qiangtang terrane 84 e the culmination is unconformably overlain by weakly deformed mid cretaceous volcanic flows and tuffs along the bangong suture to the south 32 n 84 e mid cretaceous nonmarine red beds and volcanic rocks lie unconformably on jurassic suture zone rocks these relationships demonstrate that west central tibet was above sea level during the mid cretaceous and experienced significant denudation prior to mid cretaceous time growth of the qiangtang culmination is inferred to have initiated during southward emplacement of a thrust sheet of early mesozoic melange and upper paleozoic strata during the early cretaceous lhasa qiangtang collision the north south width of the inferred thrust sheet provides a minimum slip estimate of 150 km at 84 e decreasing eastward to 70 km at 87 e paleogene deformation in the qiangtang terrane is characterized by widely distributed mainly north dipping thrust faults that cut eocene oligocene red beds and volcanic rocks in their footwall along the bangong suture the north dipping shiquanhe gaize amdo thrust system cuts 64 and 43 m y old volcanic tuffs in its footwall and accommodated 40 km of post mid cretaceous shortening the tertiary south dipping gaize siling co backthrust bounds the southern margin of the bangong suture and marks the northernmost limit of mid cretaceous marine strata in central tibet cretaceous deformation and denudation in central tibet is attributed to northward underthrusting of the lhasa terrane beneath the qiangtang terrane along the bangong suture this model implies that 1 cretaceous strata along the bangong suture and in the northern lhasa terrane were deposited in a flexural foreland basin system and derived at least in part from the qiangtang terrane and 2 the central tibetan crust was thickened substantially prior to the indo asian collision although its magnitude is poorly known tertiary shortening in the qiangtang terrane is more prevalent than in the lhasa terrane this difference may be attributed to the presence of underthrust melange in the deeper central tibetan crust which would have made it weaker than the lhasa terrane during the indo asian collision

radiolarian age constraints on mesotethyan ocean evolution and their implications for development of the bangong nujiang suture tibet radiolarian discoveries indicate that deep marine conditions prevailed in central tibet from the early middle jurassic until well into the early cretaceous late hauterivian early aptian 131 121 ma and help to constrain the temporal extent of oceanic conditions along the bangong nujiang suture these new fossils occur in chert or siliceous mudstone blocks associated with the lagkor tso ophiolitic melange basin inversion associated with closure of mesotethys between the qiangtang and lhasa terranes was accompanied by melange formation and predated regionally widespread deposition of overlying shallow marine late aptian albian orbitolinid limestones

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

genesis of high potassium calc alkaline peraluminous i type granite new insights from the gaoligong belt granites in southeastern tibet plateau abstract silicic magmas primarily felsic i and s type granite sensu lato play a key role in understanding the compositional evolution and differentiation of the continental crust however the genetic models of felsic i type granites remain controversial herein we investigate the gaoligong granite sensu lato belt in southern tibet and compare its features with an extensive dataset that we compiled on high potassium calc alkaline peraluminous i type granite from typical granitic belts around the word with a view to gain insights on the genesis of felsic i type granites the early cretaceous granites from gaoligong belt mostly belong to high k calc alkaline peraluminous i type series zircon lu hf ehf t 16 3 0 9 and whole rock sr nd isotopic data end t 14 6 3 0 on these rocks show highly variable and evolved nature suggesting that the granites were generated by partial melting of lower crustal materials with possible involvement of upper crustal materials followed by mixing leading to marked variation in source compositions compilation of the dataset and comparison with felsic i and s type granites show the following salient features 1 possibly distinct trends of al2o3 p2o5 k2o rb sr pb th u and ga vs mgo feot between the felsic i type granites and those s types and 2 there are notably higher and variable zircon hf ehf t 10e units and whole rock sr nd end t 5e units isotopic compositions of i type granites than those of the s types comparison with experimental liquids and other typical cases around the world indicate that the felsic i type granites from gaoligong are similar to those secondary i type granites where mixing processes and extensive partial melting of the heterogeneous lower crust by a mantle like magma control the variable components the range of compositional diversity is significantly higher for the studied felsic i type granites than those typical s type granites from north queensland our study provides further insights into the application of geochemical and isotopic proxies to differentiate felsic i and s type granites

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

a re evaluation of the stratigraphy and evolution of the kohistan arc sequence pakistan himalaya implications for magmatic and tectonic arc building processes new field mapping and structural data combined with published geochemical data from the kohistan arc in the nw himalaya enable a re evaluation of the arc stratigraphy key lithological units and their relationships are more clearly defined permitting the construction of a revised magmatic tectonic history for the arc the oldest units are transitional oceanic type basalts which form the basement to the subduction related sequence arc type gabbroic sheets and plutons intrude the oceanic basalts together these form the kamila amphibolite belt metasediments and basaltic lavas were deposited within an extensional basin onto the kamila amphibolite belt basement this sequence exposed across the arc forms a distinct stratigraphic unit which is formally defined here as the jaglot group sediment charged turbidity currents transported material into the basin whilst submarine eruptions contributed the basaltic component this period of extension culminated in the eruption of high mg boninites of the chalt volcanic group which overlie the rocks of the jaglot group the earliest granitoids of the kohistan batholith predate suturing and intrude the jaglot and chalt sequences at c 100 ma kohistan sutured to asia suturing being accompanied by thickening of the arc with the development of major intra arc shear zones and a penetrative regionally developed steep cleavage at c 85 ma intra arc rifting permitted the emplacement into the arc of the voluminous gabbronorites of the chilas complex which clearly intrudes the kamila amphibolite belt to the south and the jaglot group to the north the chilas complex has been regarded as part of the pre suturing juvenile arc sequence field evidence summarized here show this to be not so heat advection associated with emplacement of the complex caused amphibolite facies regional metamorphism melting of the lower arc and plutonism some of the resultant granitoid plutons were unroofed and eroded during a compressional phase at between 80 and 55 ma before emplacement of further plutons and extrusion of basaltic through to rhyolitic volcanic rocks at between 55 and 40 ma at least three phases of extension and rifting each separated by short lived phases of compression charac terized arc evolution much of the magmatism is controlled by extensional tectonics within the overriding plate of the kind commonly associated with a retreating subduction zone

age and genesis of the pusangguo skarn cu dominated polymetallic deposit gangdese metallogenic belt tibet abstract the pusangguo skarn cu polymetallic deposit is a newly discovered deposit in the western gangdese porphyry copper belt gpcb the orebodies are hosted along the contact between limestone of the early cretaceous takena formation and a biotite granodiorite intrusion the mineralization is associated with the biotite granodiorite intrusion and is divided into four stages 1 a prograde skarn stage 2 a pre ore retrograde skarn stage 3 a quartz sulfides stage and 4 a post ore carbonate stage these skarns have the characteristics of both calcareous and manganese skarns the mineral compositions of the garnets and pyroxenes indicate that the ore forming environment gradually evolved from a high temperature oxidative and alkaline environment into a low temperature and reductive environment the sulfur and lead isotopic compositions of the ore sulfides suggest a magmatic source for the sulfur and a crust dominated source for the ore forming metals the hydrogen and oxygen isotopic compositions of the quartz and garnets in the ores indicate that the ore forming fluids may have originated from a mixture of magmatic and meteoric fluids the geochronological data show that both the magmatism 14 8 0 4 ma and mineralization 13 2 0 7 ma occurred during the miocene in a post collisional tectonic setting the ore forming biotite granodiorite is a high k calc alkaline i type granite and has an adakitic geochemical affinity similar to the gangdese miocene adakites which were generated from thickened lower crust in a post collisional tectonic setting the low positive ehf t values 0 to 2 6 young two stage hf model zircon ages and whole rock geochemical and lead isotopic compositions indicate that the magma of the ore related biotite granodiorite may have originated from the thickened lower crust in the lhasa terrane

thermal evolution of the gangdese batholith southern tibet a history of episodic unroofing the gangdese batholith southern tibet was part of an andean type arc at the southern margin of asia prior to the collision of india and asia at approximately 50 to 40 ma fission track and 40ar 39ar analyses of 28 rocks from 10 gangdese granitoid plutons along an 250 km length of the batholith in the lhasa region provide a detailed understanding of the age and the postcrystallization erosional and tectonic history of these rocks these data suggest a range of ages for these plutons of 94 to 42 ma with the majority being of tertiary age the postcrystallization cooling histories of all of these plutons are characterized by marked discontinuities we conclude that most of these discontinuities and all of them after 40 ma reflect tectonic changes that produced brief pulses of rapid erosion which were distributed in both space and time in addition to the initial cooling of hot magma against cold country rock all of the rocks we studied showed evidence for at least one subsequent episode of rapid cooling dropping many tens of degrees in a few million years conversely these plutons all experienced intervals during which they cooled very slowly or not at all these slow cooling intervals lasted from 5 to 50 million years our data indicate that since the collision between india and asia began response to continued convergence has been quite variable in even this relatively small area the data reported here are consistent with a recently proposed model of oligo miocene crustal shortening along the gangdese thrust system in this area

ore genesis of the late cretaceous larong porphyry w mo deposit eastern tibet evidence from in situ trace elemental and s pb isotopic compositions abstract the late cretaceous ca 92 ma larong porphyry w mo deposit 39 44 mt wo3 0 1541 a newly discovered deposit in the easternmost bangong nujiang metallogenic belt bnmb is located in eastern tibet the w mo mineralization occurs as veins and stock works hosted in monzogranite porphyry granodiorite porphyry and surrounding quartz schist and can be divided into two main paragenetic stages the w mineralization stage stage i consists of various k feldspar quartz scheelite veins and the quartz sulfide stage stage ii is dominated by molybdenite and pyrite of which the latter can be further subdivided into three sub stages here we present la icpms in situ trace elements and s pb isotopes of sulfides from the larong deposit in order to constrain the sources of ore forming elements and the processes of mineralization our study shows that pyrite is rich in co and as molybdenite is rich in w pb and bi and chalcopyrite has high sn contents these results are consistent with the mineralization characteristics of the larong deposit where the main metallogenic elements are w and mo accompanied by bi sn and cu overall sulfides from the larong deposit have calculated 34svcdt values ranging from 0 25 to 6 37 suggesting a predominantly magmatic origin for sulfur from stage ii 1 to stage ii 2 to stage ii 3 the average 34svcdt values of pyrite change from 0 33 to 1 88 to 5 34 showing a progressive and gradual change in redox and temperature conditions molybdenite has high pb isotopic ratios 208pb 204pb 39 356 39 908 207pb 204pb 15 747 15 928 206pb 204pb 18 843 20 120 but low re contents and pyrite has low ni contents implying that the ore forming materials were mainly derived from the crust we propose that the w mo mineralization is genetically related to the late cretaceous monzogranite porphyry with the mineralization of w and mo occurring as a result of decreases in temperature and oxygen fugacity of the ore forming fluid exsolved from the magma in combination with previous studies our study led to the identification of three w dominated polymetallic metallogenic events and mineralization potential of igneous rocks in the bnmb

geochronology and geochemistry of mesoproterozoic granitoids in the lhasa terrane south tibet implications for the early evolution of lhasa terrane abstract the early history of the lhasa terrane remains poorly constrained due to the poor exposure of the proterozoic rocks we report here u pb zircon ages geochemical and hf isotopic data for granite gneisses and biotite gneisses from the bomi complex in the eastern part of the lhasa terrane south tibet petrological and geochemical data suggest that the protoliths of the granite gneisses and the biotite gneisses could be granites and tonalites respectively la icpms u pb zircon analyses yielded ages of 1343 27 ma mswd 0 3 and 1276 22 ma mswd 0 4 for two granite gneisses and a consistent age of ca 1250 ma for two biotite gneisses these ages are interpreted as the magma crystallization time of both the gneisses protoliths and thus the bomi complex represents the oldest rocks found in the lhasa terrane our data indicate that the mesoproterozoic detrital zircons from the paleozoic metasedimentary rocks in the lhasa terrane could be derived from the lhasa terrane itself or the tethyan himalaya rather than necessarily from the albany fraser belt in the australia geochemical characteristics show that the granite gneisses have an aluminous a type granite affinity the two granite gneisses dated in this study have zircon hf t values between 4 0 and 1 8 and between 2 6 and 0 2 respectively they have identical two stage hf model ages of 2 0 ga we suggest that the protoliths of the granite gneisses were produced by protracted high temperature partial melting of a felsic intracrustal source in an extensional setting in contrast the biotite gneisses have similar geochemical characteristics to those of calc alkaline granitoids that probably formed in a subduction related environment zircons from the two dated biotite gneisses have relatively higher hf t values of 8 1 to 3 6 and 10 5 to 5 7 respectively indicating a juvenile mantle contribution to their magma source earlier magmatism at 1343 1276 ma may formed in a continental rift setting related to the final breakup of supercontinent columbia while subsequent magmatism of 1250 ma resulted from subduction of ocean slab during the assemblage of rodinia we thus infer that the bomi complex was related to the contact zone between the eastern ghats belt and the archean cratons in southeastern india during the mesoproterozoic

combined paleomagnetic and geochronological study on cretaceous strata of the qiangtang terrane central tibet abstract a combined paleomagnetic and geochronological investigation has been performed on cretaceous rocks in southern qiangtang terrane 32 5 n 84 3 e near gerze central tibetan plateau a total of 14 sites of volcanic rocks and 22 sites of red beds have been sampled our new u pb geochronologic study of zircons dates the volcanic rocks at 103 8 0 46 ma early cretaceous while the red beds belong to the late cretaceous rock magnetic experiments suggest that magnetite and hematite are the main magnetic carriers after removing a low temperature component of viscous magnetic remanence stable characteristic remanent magnetization chrm was isolated successfully from all the sites by stepwise thermal demagnetization the tilt corrected mean direction from the 14 lava sites is d 348 0 i 47 3 k 51 0 95 5 6 corresponding to a paleopole at 79 3 n 339 8 e a 95 5 7 and yielding a paleolatitude of 29 3 5 7 n for the study area the chrm directions isolated from the volcanic rocks pass a fold test at 95 confidence suggesting a primary origin the volcanic data appear to have effectively averaged out secular variation as indicated by both geological evidence and results from analyzing the virtual geomagnetic pole vgp scatter the mean inclination from the late cretaceous red beds however is 13 1 shallower than that of the 100 ma volcanic rocks after performing an elongation inclination analysis on 174 samples of the red beds a mean inclination of 47 9 with 95 confidence limits between 41 9 and 54 3 is obtained which is consistent with the mean inclination of the volcanic rocks the site mean direction of the late cretaceous red beds after tilt correction and inclination shallowing correction is d 312 6 i 47 7 k 109 7 95 3 0 n 22 sites corresponding to a paleopole at 49 2 n 1 9 e a 95 3 2 yielding a paleolatitude of 28 7 3 2 n for the study area the chrm of the red beds also passes a fold test at 99 confidence indicating a primary origin comparing the paleolatitude of the qiangtang terrane with the stable asia there is no significant difference between our sampling location in the southern qiangtang terrane and the stable asia during 100 ma and late cretaceous our results together with the high quality data previously published suggest that an 550 km n s convergence between the qiangtang and lhasa terranes happened after 100 ma comparison of the mean directions with expected directions from the stable asia indicates that the gerze area had experienced a significant counterclockwise rotation after 100 ma which is most likely caused by the india asia collision

discovery of middle jurassic trench deposits in the bangong nujiang suture zone implications for the timing of lhasa qiangtang initial collision abstract the mesozoic trench deposits in the bangong nujiang suture zone in central tibet can provide critical information for reconstructing the paleotectonic evolution of the bangong nujiang ocean and constraining the timing of the lhasa qiangtang initial collision in this paper the gamulong formation which is well exposed in the dong co area western bangong nujiang suture central tibet was studied the sedimentological analysis shows that the gamulong formation was deposited in a submarine fan environment the youngest detrital zircon age and fossils from limestone pebbles within the gamulong formation combine to determine a middle jurassic age the pebbles in the conglomerate are dominated by limestones and sandstones which is consistent with the sandstone petrology showing limestone and sandstone fragments as the main framework components detrital zircon u pb ages from the sandstones yield age populations of 164 178 ma 200 500 ma 700 1000 ma 1700 2100 ma and 2500 ma the corresponding zircon ehf t values are distributed between 23 5 and 11 1 these zircon features are similar to those from the qiangtang terrane accordingly the provenance results imply that the gamulong formation was recycled from the accretionary complex mugagangri group melange in the bangong nujiang suture zone and the forearc basin deposits lower middle jurassic sewa shaqiaomu jiebuqu formations in the southern qiangtang subterrane the gamulong formation is characterized by sedimentary recycling in the trench likely in response to a tectonic event possibly the continental collision together with the previous studies this work on the occurrence of the gamulong formation indicates that the lhasa qiangtang initial collision could have occurred in middle jurassic time 166 ma

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

generation of leucogranites via fractional crystallization a case from the late triassic luoza batholith in the lhasa terrane southern tibet abstract leucogranites are important in understanding intracrustal differentiation and regional tectonic evolution but how these rocks form remains a matter of much debate the luoza batholith in the central lhasa subterrane provides an important opportunity to address these issues as this batholith consists of normal calc alkaline s type granodiorites group 1 and highly fractionated s type leucogranites group 2 laser ablation inductively coupled plasma mass spectrometry zircon u pb dating indicates that these rocks were emplaced at ca 221 205 ma the group 1 samples ca 205 ma have low sio2 content 64 67 wt and low differentiation index values di 75 76 and are slightly depleted in ba p sr and ti compared to the group 2 samples ca 221 ma and 213 ma which have high sio2 content 74 75 wt high differentiation index values 91 94 and are significantly depleted in ba p sr and ti samples from group 1 and group 2 exhibit similar zircon ehf t values 15 6 to 1 4 both of which are significantly higher than melts from the pure ancient basement in the central lhasa subterrane 20 5 to 10 6 the group 1 samples can be interpreted as having been derived from the partial melting of metagreywacke within the ancient basement in the central lhasa subterrane with contributions from mantle or juvenile crust derived components whereas the group 2 samples may have formed via varying degrees of fractional crystallization of different minerals e g plagioclase k feldspar biotite zircon allanite monazite titanite and apatite from the parental magmas represented by the granodioritic samples in group 1 in shallow crustal magma chambers in combination with sedimentary records the high temperature and low pressure conditions indicated by the group 1 samples can be geodynamically linked to a back arc extensional setting in response to the southward subduction of the bangong nujiang tethys ocean seafloor our work implies that fractional crystallization is likely a feasible mechanism for the development of leucogranites and for explaining the compositional diversity of the granite during intracrustal differentiation

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

slab breakoff a model for syncollisional magmatism and tectonics in the alps slab breakoff is the buoyancy driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision in a recent paper davies and von blanckenburg 1994 have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere melting of its enriched layers and thus to bimodal magmatism breakoff will also lead to thermal weakening of the subducted crustal lithosphere thereby allowing buoyant rise of released crustal slices from mantle depths in this paper we present a test of this model in the tertiary evolution of the european alps in the alps both basaltic and granitoid magmatism occur between 42 and 25 ma following the closure of oceanic basins by subduction and continental collision the granitoids are now well established to result from mixing of basalt with assimilated continental crust to identify the tectonically crucial origin of the partial mantle melts we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole alpine arc their trace element and isotopic composition suggests that they have been formed by low degree melting of the mechanically stable lithospheric mantle we see no evidence for melting of asthenospheric mantle it was thus not decompressed to depths shallower than 50 km once initiated rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust this explains why all alpine magmatic rocks intruded almost synchronously along a strike slip fault the periadriatic lineament a compilation of ages from penninic high pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55 40 ma followed by uplift at 40 35 ma from the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff the slab breakoff model fulfills its predictions in the case of the alps and therefore supports the assumptions made in the theoretical model on a geological basis we believe that the characteristic association of magmatic activity with the return of high pressure rocks to the surface allows the identification of this process in the earth s mountain belts

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

geochronological and geochemical constraints on the origin of the yunzhug ophiolite in the shiquanhe yunzhug namu tso ophiolite belt lhasa terrane tibetan plateau abstract the formation of the shiquanhe yunzhug namu tso ophiolite melange zone snmz within the lhasa terrane tibetan plateau is key to understanding the mesozoic tectonic evolution of this terrane which remains controversial we show that the yunzhug ophiolite in the central segment of the snmz formed at 150 ma based on u pb dating of zircons from a gabbroic sample in a well developed sheeted dike complex geochemically these mafic rocks are dominated by e morb type compositions along with minor amounts of rocks with p morb type compositions the samples also exhibit high end t values and lack negative nb and ta anomalies data for all the samples plot within the morb array on a th yb nb yb diagram therefore these mafic rocks most likely formed in either a slow spreading oceanic setting or an embryonic ocean and not in a back arc basin as has been previously assumed taking into account the regional geology we propose that the yunzhug ophiolite is part of a distinct ophiolitic belt and represents material formed in an embryonic ocean within the lhasa terrane which provides new insights into the jurassic tectonic evolution of the lhasa terrane

late cretaceous to middle tertiary basin evolution in the central tibetan plateau changing environments in response to tectonic partitioning aridification and regional elevation gain located in the bangong suture zone between the lhasa and qiangtang terranes of central tibet the nima basin records cretaceous late miocene sediment accumulation during a period of drastically changing paleogeography and paleoelevation the bangong suture zone originally formed during late jurassic early cretaceous time as the lhasa terrane collided with the qiang tang terrane during early to middle cretaceous time the region lay at northern near equatorial paleolatitudes near sea level by aptian time the nima basin was above sea level and was strongly influenced by nearby volcanic activity and crustal shortening in the reactivated bangong suture zone in the southern nima basin an 50 m y late cretaceous eocene depositional hiatus correlates with major crustal shortening and ensuing voluminous ignimbrite eruptions in the lhasa terrane in the northern nima basin deposition continued during latest cretaceous time recording arid paleoclimate in evaporitic lacustrine and eolian dunefield deposits by oligocene time the nima basin comprised two independent depocenters that accumulated coarse grained alluvial fluvial lacustrine evaporitic and fan delta deposits in close association with reactivated thrusts in the bangong suture zone carbon and oxygen isotope data from oligocene paleosol carbonate reported elsewhere indicate that regional paleoelevation during the late oligocene was 4 6 km as it is today overall the depositional record of the nima basin combined with ongoing structural and geochronologic studies demonstrates that the bangong suture zone was reactivated during middle cretaceous and middle tertiary time that the intervening 50 m y interval was a time of regional upper crustal shortening in the lhasa terrane followed by regional ignimbrite eruptions and that arid paleoclimate and high paleoelevation were established by the late cretaceous and late oligocene respectively within the context of other data sets from the lhasa terrane the record of deposition in the nima basin is consistent with low angle subduction of neotethyan oceanic lithosphere and reactivation of the bangong suture zone during the early cretaceous followed by shortening within the gangdese retroarc and northern lhasa terrane thrust belts during the late cretaceous lithospheric delamination dripping and regional magmatic flare up during latest cretaceous through early tertiary time and underthrusting of indian lower crust and lithosphere as far north as the bangong suture zone during late oligocene time

nyainqentanglha shan a window into the tectonic thermal and geochemical evolution of the lhasa block southern tibet 1 in the nyainqentanglha nqtl massif southern tibet a late cenozoic se dipping normal fault exhumed an oblique section of crust in its footwall u th pb dating of zircon and monazite from footwall exposures reveals a collage of felsic intrusions including cretaceous early tertiary and miocene granitoids ages of the latter span 10 m y suggesting semicontinuous or episodic miocene magmatism geochemical and isotopic analyses show a gangdese arc affinity indicating significant mantle heat and mass transfer in their formation and semicontinuous calc alkaline magmatism throughout the cenozoic indo asian collision the undeformed nature of the footwall cretaceous and miocene granitoids suggests that mesozoic cenozoic lhasa block deformation was thin skinned being concentrated in supracrustal assemblages this coupled with the lack of migmatites exposed in the nqtl implies the exposed crust was not a partial melt zone nor involved in large scale channel flow some 40ar 39ar thermochronologic studies of footwall k feldspars reveal that samples collected within several kilometers below the normal fault cooled prior to emplacement of young leucogranites indicating little perturbation of the background thermal structure since 15 ma this plus high melting temperatures and the lack of penetrative granitoid deformation requires that the melts formed at lower crustal levels and were emplaced rapidly to the midcrust seismic reflection results showing high bright spot anomalies in the midcrust along the nqtl rift may have imaged the youngest magmatic episode or its associated hydrothermal system

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

cretaceous tertiary structural evolution of the north central lhasa terrane tibet 1 in the north central lhasa terrane of tibet two distinct structural levels of an east west striking thrust system are exposed along the north trending late cenozoic xiagangjiang rift upper paleozoic strata deformed by the south directed langgadong la thrust and cretaceous strata involved in variably north and south directed thrusting characterize these lower and upper structural levels respectively these two structural levels are separated by the tagua ri passive roof thrust balanced cross section restoration suggests that the thrust system accommodated 103 km 53 shortening the 40ar 39ar results together with an interpretation of synthrust deposition of upper cretaceous strata suggest that the majority of shortening occurred during the late cretaceous paleocene cretaceous strata lie unconformable on permian rocks volcanic tuffs directly above the unconformity yield u pb zircon ages of 131 ma upper cretaceous strata record a change from shallow marine to nonmarine deposition indicating uplift above sea level during this time the overall south directed vergence of the thrust belt is consistent with substantial crustal thickening in central tibet by large scale northward underthrusting of lhasa terrane basement beneath the qiantang terrane prior to the indo asian collision the documented decoupling of contractional deformation at shallow crustal levels appears to be a regional characteristic of tibet from at least the bangong suture in the north to the tethyan himalaya to the south this style of deformation explains the absence of basement exposures and major denudation in this region despite substantial crustal shortening

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

magmatism and metallogenic mechanism of the ga erqiong and galale cu au deposits in the west central lhasa subterrane tibet constraints from geochronology geochemistry and sr nd pb hf isotopes abstract the ga erqiong galale ore district in the bangong nujiang metallogenic belt hosts two large skarn cu au deposits associated with voluminous late cretaceous volcanism and plutonism in this study we present new zircon u pb age and hf isotopic data whole rock geochemical and sr nd pb isotopic data to constrain the geodynamic mechanisms of late cretaceous magmatism and metallogenesis within the ga erqiong galale ore district the intrusive rocks in the district include diorite quartz diorite granodiorite granite and dioritic porphyrite zircon u pb analyses suggest those rocks mainly emplaced at early late cretaceous 84 95 ma which coincide with the ore forming age of ga erqiong and galale deposits 87 and 88 ma respectively geochemically the granodiorite and dioritic porphyrite are metaluminous with depletion of nb ta and ti and characterized by high mgo 2 68 4 16 wt mg 55 87 sr 414 616 ppm sr y 30 56 cr 22 5 105 0 ppm and ni 26 6 59 7 ppm low y 10 6 14 0 ppm and heavy rare earth elements hree showing geochemical signatures of high mg adakitic rocks four dioritic porphyrite and three granodiorite samples exhibit homologous sr nd pb isotope compositions and have positive whole rock end t values of 1 1 to 3 0 87sr 86sr i values of 0 7045 0 7048 206pb 204pb t ratios of 18 42 18 73 207pb 204pb t ratios of 15 58 15 63 and 208pb 204pb t ratios of 38 54 38 88 besides 116 analyses from the ga erqiong galale adakitic rocks give positive zircon ehf t values 4 3 to 12 1 the geochemical and isotopic data indicate that the early late cretaceous high mg adakitic rocks in the ga erqiong galale ore district originated from the delaminated juvenile lower mafic crust that has been modified by the previously subduction partial melting of previously subduction modified delaminated juvenile lower crust not only formed the hydrous adakitic melts in highly oxidation state but also provided the ore metals e g cu au mo for the subsequent mineralization such melts reacted with surrounding mantle peridotite to elevate the contents of metallic elements again then the melts experienced varying degrees of fractional crystallization process in the shallow magma chamber to form the dioritic and granodioritic magmas those magmas emplaced into the limestone of duo ai and jiega formation to form the ga erqiong and galale skarn cu au deposits respectively field observation together with geochronological and geochemical data indicate that the early late cretaceous mineralization events in the northern and central lhasa subterranes took place in a transition tectonic setting from collisional orogeny to extension and the input of subduction modified juvenile crust play an important role in the late cretaceous ore forming events

zircon u pb geochronology and geochemistry of early jurassic granodiorites in sumdo area tibet constraints on petrogenesis and the evolution of the neo tethyan ocean abstract the gangdese magmatic belt is oriented e w and located between the indus yarlung tsangpo and bangong nujiang suture zones in tibet it records the subduction of the neo tethyan oceanic crust indian eurasian collision and uplift of the tibetan plateau here we present new zircon u pb geochronology hf isotopic compositions and whole rock geochemistry of the granodiorites in the sumdo area of the gangdese belt our aim was to identify the petrogenesis of granitoid rcoks and to clarify the early stage evolution of the neo tethyan ocean during the late triassic to early jurassic the granodiorites yield zircon u pb ages of 193 201 ma indicating an early jurassic magmatic event they are metaluminous with medium to high k calc alkaline geochemistry and show characteristics of i type granitoids they show enrichment in light rare earth elements lrees and depletion in high field strength elements hfses in primitive mantle normalized spider diagrams indicating that their initial magma was generated in a subduction related active continental margin they have negative ehf t values 11 3 to 4 1 and crustal model ages tc dm of 1 50 1 95 ga hf isotopes and whole rock geochemistry indicate that the granodiorites were derived from partial melting of ancient lower crust material mixed with mantle material our data in combination with published geochronological and geochemical data show that the late triassic early jurassic arc magmatism in the gangdese magmatic belt was formed by the northward subduction of the neo tethyan oceanic crust beneath the lhasa terrane

mesozoic and cenozoic tectonic evolution of the shiquanhe area of western tibet 1 in the shiquanhe area of far western tibet mid cretaceous strata lie unconformable on ophiolitic melange and jurassic flysch associated with the bangong nujiang suture zone on the basis of our mapping and geochronologic studies we suggest that these cretaceous strata were shortened by 57 over a north south distance of 50 km during late cretaceous early tertiary time the late cretaceous narangjiapo thrust placed permian strata 20 km over ophiolitic melange and cretaceous strata north of the narangjiapo thrust 40 km of shortening was accommodated by the late cretaceous early tertiary south directed jaggang thrust system that involves jurassic flysch and cretaceous strata and roots into a decollement within ophiolitic melange the most recent shortening was accommodated to the south of the narangjiapo thrust along the north dipping shiquanhe thrust the shiquanhe thrust cuts flat lying 22 6 0 3 ma volcanic rocks and underlying folded tertiary nonmarine strata in its footwall and was likely active during slip along the oligocene gangdese thrust system of southern tibet ophiolitic melange and structurally overlying jurassic flysch near shiquanhe are interpreted to represent remnants of a subduction accretion complex and forearc basin respectively that were obducted southward onto the margin of the lhasa terrane during late jurassic early cretaceous closure of the bangong nujiang ocean subsequent imbrication of the obducted sheet could have produced the two east west trending belts of ophiolitic melanges separated by 100 km in western tibet late cretaceous early tertiary thin skinned shortening may have been accommodated in the deeper crust by northward underthrusting and duplexing of lhasa terrane rocks beneath the obducted ophiolitic melange and the qiangtang terrane to the north

paleomagnetism of eocene red beds in the eastern part of the qiangtang terrane and its implications for uplift and southward crustal extrusion in the southeastern edge of the tibetan plateau abstract a primary magnetic component with the acquisition time of 56 0 43 2 ma was isolated between 580 c and 685 c from the eocene gonjo and ranmugou formations in the eastern part of the qiangtang terrane tibetan plateau the tilt corrected site mean direction is d s 35 5 i s 29 3 k 45 9 95 3 2 the site mean inclination increased from 29 3 to 41 6 after multiple inclination shallowing corrections giving a paleopole of 57 9 n 192 1 e a 95 2 9 comparison of the paleogene paleomagnetic data for the qiangtang terrane and lhasa terrane reveals that both terranes experienced latitudinal crustal shortening before 54 43 ma indicating the uplift of southern and central tibet in the early eocene subsequently since 35 4 2 4 ma the north of the qiangtang terrane experienced 1300 410 km of crustal shortening indicating the uplift of the northern tibetan plateau the lhasa terrane and qiangtang terrane have not experienced further crustal shortening since the late eocene and the southeastern part of tibet cannot have provided abundant crustal material to accommodate the significant crustal southward extrusion in the southeastern edge of the tibetan plateau the crust of the tengchong terrane and shan thai block did not experience significant southward extrusion since the late oligocene early miocene the indochina block was situated in the north of the qiangtang terrane before the oligocene and since the early oligocene the indochina block began to experience southward extrusion from the north of the qiangtang terrane which absorbed part of the crustal shortening in the north of the qiangtang terrane

crustal thickening and uplift of the tibetan plateau inferred from receiver function analysis the india eurasia collision crustal thickening and northward convergence leading to uplift of the tibetan plateau have been topics of multidisciplinary studies here we employ the h k stacking technique and depth domain receiver function to constrain the crustal thickness and p and s wave velocity ratio as well as the upper mantle discontinuity our results identify the signature of slab break off and lower crustal delamination in southern tibet which might have induced the mantle upwelling and crustal melting the convergence of the indian plate inducing northward crustal flow might be a major factor that contributes to the uplift and crustal thickening of tibet we also evaluate the e w trending cumulative compression as another dominant factor that contributes to the increase in crustal thickness in the northern tibet 2015 elsevier ltd all rights reserved

zircon u pb ages geochemistry and sr nd pb hf isotopes of the nuri intrusive rocks in the gangdese area southern tibet constraints on timing petrogenesis and tectonic transformation abstract abundant magmatic rocks of various ages are exposed in gangdese southern tibet these rocks play an important role in understanding the tectonic transformation from the subduction of neo tethyan oceanic crust to the collision of the indian and asian continents based on zircon u pb ages geochemistry and sr nd pb hf isotopic data of the late cretaceous to early oligocene 96 30 ma intrusive rocks in the nuri cu w mo deposit we discuss the late cretaceous to early oligocene tectonic transformation of the region and the origin of oligocene cu w mo mineralization in southern gangdese the nuri intrusive rocks represent three magmatic episodes 96 91 56 52 and 33 30 ma the 96 91 and 56 52 ma rocks have relatively low 87sr 86sr i 0 7041 to 0 7060 and high end t 3 1 to 3 5 and ehf t values 3 7 to 15 the 33 30 ma rocks have relatively high 87sr 86sr i 0 7061 to 0 7063 and pb isotopes and low end t 3 8 to 1 8 and ehf t values 0 6 to 10 1 the three stages of intrusive rocks have geochemical characteristics that are similar to those of coeval rocks in gangdese the 96 91 and 33 30 ma rocks are adakitic whereas the 56 52 ma rocks have characteristics of arc calc alkaline magmatic rocks the 96 91 ma rocks were produced by the partial melting of neo tethyan basaltic oceanic crust and minor sediments whereas the 56 52 ma rocks were generated by the partial melting of juvenile crust and the 33 30 ma rocks were formed by the melting of indian plate lower crust contaminated with overlying mantle materials on the basis of the regional tectonic and magmatic characteristics we suggest that neo tethyan oceanic slab subduction and slab roll back occurred from 100 to 65 ma collision between the indian and asian continents occurred at 65 to 40 ma neo tethyan oceanic slab break off took place at 50 ma and the indian continent subducted northwards beneath the asian continent at 30 ma from the late cretaceous 96 91 ma to oligocene 30 ma the geodynamic setting of gangdese was changed from neo tethyan oceanic subduction to indian continental subduction in contrast to the miocene metallogenic rocks the magmatic source of oligocene metallogenic rocks contains old indian continental crust materials which result in the formation of intense unique cu w mo mineralization especially w mineralization in the southern gangdese

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

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

petrogenesis of the majiari ophiolite western tibet china implications for intra oceanic subduction in the bangong nujiang tethys abstract the majiari ophiolite lies in the western bangong nujiang suture zone which separates the qiangtang and lhasa blocks in central tibet the ophiolite consists of peridotite gabbro diabase and basalt zircon u pb dating yielded an age of 170 5 1 7 ma for the gabbro whereas 40 ar 39 ar dating of plagioclase from the same gabbro yielded ages of 108 4 2 6 ma plateau age and 112 2 ma isochron age indicating that the ophiolite was formed during the middle jurassic and was probably emplaced during the early cretaceous zircons from the gabbro have e hf t values ranging from 6 9 to 10 6 and f lu hf values ranging from 0 92 to 0 98 mafic lavas plot in the tholeiitic basalt field but are depleted in nb ta and ti and enriched in rb ba and th in the n morb normalized trace element spider diagram these lavas have whole rock e nd t values of 5 9 to 6 6 suggesting that they were derived from a depleted mantle source which was probably modified by subducted materials the majiari ophiolite probably formed in a typical back arc basin above a supra subduction zone ssz mantle wedge intra oceanic subduction occurred during the middle jurassic and collision of the lhasa and south qiangtang terranes likely occurred in the early cretaceous thus closure of the bangong nujiang tethys ocean likely occurred before the early cretaceous

tracing the transhimalayan magmatic belt and the lhasa block southward using zircon u pb lu hf isotopic and geochemical data cretaceous cenozoic granitoids in the tengchong block yunnan china abstract a number of cretaceous cenozoic granitoid bodies in the tengchong block were analyzed for their u pb zircon ages lu hf zircon isotopic compositions and whole rock geochemistry zircons from six different granite bodies in the central tengchong block have weighted mean 206 pb 238 u ages ranging from 76 to 65 ma and negative e hf t values ranging from 2 7 to 15 5 with t dm c ranging from 1312 to 1928 ma one dextrally sheared and gneissic granodiorite sample of the nabang shear zone in the western tengchong block yielded a weighted mean 206 pb 238 u age of 19 7 0 4 ma and positive e hf t values ranging from 0 2 to 10 7 with t dm c ranging from 418 to 1092 ma the late cretaceous granites have metaluminous and peraluminous affinities with strong enrichment of u and th and significant negative anomalies of eu ba sr nb p and ti the miocene granodiorite displays strong negative anomalies of nb ta p and ti and a weak negative eu anomaly the data suggest that the late cretaceous granites in the tengchong block were derived from partial melting of ancient most likely proterozoic continental crustal material whereas the miocene gneissic granodiorite was derived from partial melting of juvenile crustal material possibly within a supra subduction setting and intruded synkinematically into the active nabang shear zone lu hf isotopic data of late cretaceous paleocene granites are indistinguishable from those of granites of the central lhasa block thus we tentatively correlate the late cretaceous granites of the tengchong block with those of the central lhasa block the boundary between the south and central lhasa blocks in western tengchong may be exposed in the nne striking dextral nabang shear zone along the chinese myanmar border region which hosts early eocene granitoids with e hf t values 0 the lu hf data of the zircons from the gneissic miocene granodiorite indicate a juvenile crustal substrate in the western tengchong block as well the synkinematic miocene granodiorite provides an important age constraint for dextral shear at mid crustal levels which was active until at least c 19 20 ma in the nabang shear zone this miocene intrusion may be a first indication that active margin tectonics lasted 20 25 ma longer than in the gangdese belt and possibly was associated with deformation partitioning in the hinterland produced by increasing obliquity of subduction during clockwise rotation of sundaland during the india asia collision

nd isotopes of siliciclastic rocks from tibet western china constraints on provenance and pre cenozoic tectonic evolution nd isotope data from pre cenozoic siliciclastic rocks from tibet are used to determine variations of source regions with time and hence tectonic history on average the tdm and tchur ages of the tibetan samples are up to 1 83 0 36 ga and 1 25 0 45 ga respectively indicating that tibet is underlain by lower proterozoic basement and has close affinity with the himalayas an nd isotope crisis occurred at the early permian with older samples defining a trend of slight decrease in end values with time while the lower permian rocks sharply increasing 6 5 end units this is related to the coeval widespread basaltic magmatism in tibet which could be responsible for tibet rifting away from northern gondwanaland the central qiangtang meta sedimentary samples have distinct nd isotope characteristics end 0 19 87 1 51 tdm 2 07 0 13 ga tchur 1 78 0 21 ga from the songpan ganzi samples end 0 12 77 2 87 tdm 1 49 0 18 ga tchur 1 08 0 23 ga suggestive that these two groups were derived from differentsourcesandthatthe former werenot theequivalents ofthe latterthat wasunderthrust fromthe jinsasuture andmayrepresent subducted continental crust the mid jurassic flysch rocks in northern lhasa have typical upper crust nd isotopic signatures and could have formed under a passive margin environment their bangong counterparts averagely 6 35 units higher in end values unveil derivations from a likely magmatic arc along the northern bangong margin basaltic pebblewith low nd content and endvalue from the berriasian valanginian rocks in central tibet along with the samples with generally high end values shows that the bangong ophiolites could have been a main source of sediments during the earliest cretaceous time a significant transition of sediment sources occurred at the beginning of the hauterivian stage 120 ma sedimentary basaltic tuff end t 2 27 in the hauterivian lower cenomanian samples in central tibet and common nd signatures for input of mantle like materials in these rocks indicate that mantle derived magmatic rocks could have constituted a significant source the huge hauterivian lower cenomanian sediments in central tibet are thus inferred to have been accumulated under a back arc rifting setting 2007 elsevier b v all rights reserved

reconstructing in space and time the closure of the middle and western segments of the bangong nujiang tethyan ocean in the tibetan plateau when and how the bangong nujiang tethyan ocean closed is a highly controversial subject in this paper we present a detailed study and review of the cretaceous ophiolites ocean islands and flysch deposits in the middle and western segments of the bangong nujiang suture zone bnsz and the cretaceous volcanic rocks late mesozoic sediments and unconformities within the bnsz and surrounding areas our aim was to reconstruct the spatial temporal patterns of the closing of the middle and western segments of the bangong nujiang tethyan ocean our conclusion is that the closure of the ocean started during the late jurassic and was mainly complete by the end of the early cretaceous the closure of the ocean involved both longitudinal diachronous closure from north to south and transverse diachronous closure from east to west the spatial temporal patterns of the closure process can be summarized as follows the development of the bangong nujiang tethyan oceanic lithosphere and its subduction started before the late jurassic after the late jurassic the ocean began to close because of the compressional regime surrounding the bnsz along the northern margin of the bangong nujiang tethyan ocean collisions involving the arcs back arc basins and marginal basins of a multi arc basin system first took place during the late jurassic early early cretaceous resulting in regional uplift and the regional unconformity along the northern margin of the ocean and in the southern qiangtang terrane on the northern side of the ocean however the closure of the bangong nujiang tethyan ocean cannot be attributed to these arc arc and arc continent collisions because subduction and the development of the bangong nujiang tethyan oceanic lithosphere continued until the late early cretaceous the gradual closure of the middle and western segments of bangong nujiang tethyan ocean was diachronous from east to west starting in the east in the middle early cretaceous and being mainly complete by the end of the early cretaceous the bnsz and its surrounding areas underwent orogenic uplift during the late cretaceous

middle triassic ultrapotassic rhyolites from the tanggula pass southern qiangtang china a previously unrecognized stage of silicic magmatism abstract there is a mid late mesozoic jurassic cretaceous arc magmatic belt in southern qiangtang subterrane associated with the north dipping subduction of the bangong nujiang tethyan ocean early mesozoic triassic magmatism tectonic evolution of the southern qiangtang subterrane and the mechanism of its crustal growth however remain unclear this paper reports zircon u pb age whole rock major trace elemental and sr nd hf isotopic components for the mesozoic tanggula pass rhyolites in southern qiangtang subterrane our new data reveal a significant previously unrecognized stage of magmatism in southern qiangtang subterrane the tanggula pass rhyolites are moderately strongly peraluminous a cnk 1 09 1 54 highly fractionated sio2 73 and ultrapotassic k2o na2o 44 zircon u pb dating by la icp ms yields a concordant age with a weighted mean 206pb 238u age of 235 9 0 86 ma n 18 mswd 0 25 the large variations of major and trace element concentrations are mainly attributed to fractional crystallization of plagioclase hornblende fe ti oxides and accessory minerals such as apatite allanite monazite and zircon in the shallow level k feldspar mainly occurs as cumulated mineral resulting in high k2o contents and k2o na2o ratios the rhyolites were derived by partial melting of mixed source rocks including older sedimentary rocks in the upper crust and juvenile materials triggered by north dipping subduction of the bangong nujiang tethyan oceanic lithosphere the eruption age of middle triassic suggests that the initiation of northward subduction of the bangong nujiang tethyan ocean took place earlier than is currently assumed whole rock nd ca 1 55 to 2 34 ga and zircon hf ca 1 77 to 2 33 ga data reflect the presence of the proterozoic basement in the southern qiangtang subterrane basaltic magma underplating and accretion of intra oceanic arc complexes and oceanic plateau contributed to vertical crustal growth and lateral crustal growth respectively

the exhumation history of collision related mineralizing systems in tibet insights from thermal studies of the sharang and yaguila deposits central lhasa abstract the large newly discovered sharang porphyry mo deposit and nearby yaguila skarn pb zn ag mo deposit reside in the central lhasa terrane northern gangdese metallogenic belt tibet multiple mineral chronometers zircon u pb sericite 40 ar 39 ar and zircon and apatite u th he reveal that ore forming porphyritic intrusions experienced rapid cooling 100 c ma during a monotonic magmatic hydrothermal evolution the magmatic hydrothermal ore forming event at sharang lasted 6 0 myr 1 8 myr for cooling from 900 to 350 c and 4 0 myr for cooling from 350 to 200 c whereas cooling was more prolonged during ore formation at yaguila 1 8 myr from 900 to 500 c and a maximum of 16 myr from 900 to 350 c all porphyritic intrusions in the ore district experienced exhumation at a rate of 0 07 0 09 mm yr apatite he ages between 37 and 30 ma combined with previous studies this work implies that uplift of the eastern section of the lhasa terrane expanded from central lhasa 37 30 ma to southern lhasa 15 12 ma at an increasing exhumation rate all available geochronologic data reveal that magmatic hydrothermal exhumation activities in the sharang yaguila ore district occurred within four periods of magmatism with related mineralization significant porphyry type mo mineralization was associated with late cretaceous eocene felsic porphyritic intrusions in the central lhasa terrane resulting from neotethyan oceanic subduction and india asia continental collision

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

geochronology and geochemistry of early cretaceous granitic rocks in the dongqiao area central tibet implications for magmatic origin and geological evolution abstractwe present here new geochronological and whole rock geochemical data for the dongqiao intrusive rocks six samples yield zircon u pb ages of 111 115 ma indicating that the dongqiao granitic rocks are contemporaneous with an early cretaceous 115 110 ma magmatic flare up event in central tibet on the basis of geochemical data granitic rocks from the dongqiao area can be divided into i and a type granites the a type granites contain higher na2o k2o zr hf and zr ce nb y contents than the i type granites the results indicate that i type granites were formed by partial melting of a juvenile mafic lower crustal source while a type granites were derived from a middle crustal source followed by fractional crystallization the formation of a type granites requires broad thermal anomalies indicating that early cretaceous magmatism occurred in a postcollision extensional setting likely related to the upwelling of hot asthenospheric mantle triggered by detachment and sinking of the bango

cretaceous plutonism in central tibet an example of post collision magmatism the magmatic province of the northern lhasa terrane includes an early cretaceous 120 130 ma plutonic event and a late cretaceous 80 110 ma volcanic event the plutonic association constitutes an older suite of granodiorites monzogranites and tonalites and a younger peraluminous leucogranite facies plutonism occurred about 20 ma after obduction of the banggong ophiolite following closure between the lhasa and qiantang terranes the earlier suite is of broadly calc alkaline in composition but differs from arc related magmas in that only more evolved compositions are represented sio2 58 and rb zr ratios are elevated relative to the gangdese batholith to the south trace element and isotopic constraints are consistent with derivation from a late proterozoic amphibole bearing crustal source requiring temperatures 950 c during anatexis the leucogranites require a pelitic source which is tentatively identified as the nyaingentanglha basement exposed south of the plutonic province unlike the high himalaya leucogranites trace elements and field relations require a high degree of melting at source 50 suggesting fluid absent melting at temperatures 850 c such high crustal temperatures indicate convective heat transfer from the mantle thermal constraints together with a tectonic setting of post emplacement uplift followed by a marine transgression in the northern lhasa terrane can not be reconciled with a model of tectonically thickened crust but are consistent with post collision attenuation of the lithosphere

tectono magmatic evolution of the gaoligong belt southeastern margin of the tibetan plateau constraints from granitic gneisses and granitoid intrusions abstract the gaoligong belt is located in the southeastern margin of the tibetan plateau and is bound by the tengchong and baoshan blocks this paper presents new data from zircon geochronology geochemistry and whole rock sr nd pb hf isotopes to evaluate the tectonic evolution of the gaoligong belt the major rock types analysed in the present study are granitic gneiss granodiorite and granite they are metaluminous to peraluminous and belong to high k calc alkaline series laser ablation inductively coupled plasma mass spectrometry la icp ms analyses of zircons from nine granitic rocks yielded crystallization ages of 495 487 ma 121 ma 89 ma and 70 63 ma the granitoids can be subdivided into the following four groups 1 early paleozoic granitic gneisses with high e nd t and e hf t values of 1 06 to 3 45 and 1 16 to 2 09 and model ages of 1 16 ga to 1 33 ga and 1 47 ga to 1 63 ga respectively their variable 87 sr 86 sr and pb values resemble the characteristics of the early paleozoic pinghe granite in the baoshan block our data suggest that the rocks were derived from the break off of the proto tethyan oceanic slab between the outboard continent and the baoshan block which induced the partial melting of mesoproterozoic pelitic sources mixed with depleted mantle materials 2 early cretaceous granodiorites with low e nd t and e hf t values of 8 92 and 4 91 with nd and hf model ages of 1 41 ga and 1 49 ga respectively these rocks have high initial 87 sr 86 sr 0 711992 and lower crustal pb values suggesting that they were derived from mesoproterozoic amphibolites with tholeiitic signature leaving behind granulite residue at the lower crust 3 early late cretaceous granites with low e nd t and e hf t values of 9 58 and 4 61 with nd and hf model ages of 1 43 ga and 1 57 ga respectively these rocks have high initial 87 sr 86 sr 0 713045 and lower crustal pb isotopic values these rocks were generated from the partial melting of mesoproterozoic metapelitic sources resulting from the delamination of thickened lithosphere following the closure of the bangong nujiang ocean and collision of the lhasa qiangtang blocks 4 late cretaceous to paleogene granitic gneisses with low e nd t and e hf t values of 4 41 to 10 and 5 95 to 8 71 nd model ages ranging from 1 08 ga to 1 43 ga and hf model ages from 1 53 ga to 1 67 ga respectively these rocks show high initial 87 sr 86 sr 0 713201 and 714662 and lower crustal pb values the data suggest that these rocks are likely related to the eastward subduction of the neo tethyan oceanic slab which induced partial melting of mesoproterozoic lower crustal metagreywacke the results presented in this study from the gaoligong belt offer important insights on the evolution of the proto tethyan bangong nujiang and neo tethyan oceans in the southeastern margin of the tibetan plateau

palaeoenvironmental reconstruction of the alluvial landscape of neolithic catalhoyuk central southern turkey the implications for early agriculture and responses to environmental change archaeological discussions of early agriculture have often used the neolithic village of catalhoyuk in central southern turkey as a key example of the restricting effect of environment on agricultural production and organization central to these discussions is the palaeoenvironmental reconstruction of the landscape surrounding the site this paper presents an important new dataset from an intensive coring programme undertaken between 2007 and 2013 in the immediate environs of the site designed to improve significantly the spatial resolution of palaeoenvironmental data using sediment analyses including organic content magnetic susceptibility particle size total carbon and nitrogen contents and carbon isotope analysis coupled with 3d modelling we are able to present a new reconstruction of the palaeotopography and sedimentary environments of the site our findings have major implications for our understanding of neolithic agricultural production and social practice we present four phases of environmental development phase 1 consists of the final phases of regression of palaeolake konya in the later parts of the pleistocene dominated by erosion due to wind and water that created an undulating surface of the marl deposited in the palaeolake phase 2 occurs in the latest pleistocene and early holocene and indicates increased wetness probably characteristic of a humid anabranching channel system in which there are localized pockets of wetter conditions in phase 3a this infilling continues producing a flatter surface and there are fewer pockets being occupied by wetter conditions the fluvial regime shifts from humid to dryland anabranching conditions the earliest period of occupation of the neolithic east mound coincides with this phase phase 3b coincides with the shift of occupation to the west mound in the chalcolithic when there is evidence for a very localized wetter area to the southeast of the west mound but otherwise a continuation of the dryland anabranching system finally phase 4 shows a shift to the pre modern style of fluvial environment modified by channelization this reanalysis demonstrates the importance of extensive spatial sampling as part of geoarchaeological investigations with this new evidence we demonstrate that the landscape was highly variable in time and space with increasingly dry conditions developing from the early holocene onwards in contrast to earlier landscape reconstructions that have presented marshy conditions during the early holocene that impacted agriculture we argue that localized areas of the floodplain would have afforded significant opportunities for agriculture closer to the site in this way the results have important implications for how we understand agricultural practices in the early neolithic

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

provenance analysis of cretaceous peripheral foreland basin in central tibet implications to precise timing on the initial lhasa qiangtang collision abstract mesozoic strata along northern margin of the lhasa terrane near dingqing tibet provide a semi continuous record of the bangong nujiang ocean closure and the subsequent lhasa qiangtang collision in this study we present results of sandstone petrographic detrital zircon u pb and cr spinel geochemical data to determine the provenance of the mesozoic strata from the triassic quehala group and the mid jurassic to lower cretaceous xihu lagongtang and duoni formations which are young from 220 ma to 100 ma in this region thereby allowing for interpretation of their tectonic setting the similar 1200 900 age cluster from the lower xihu formation to that of the triassic quehala group and the distinct age peaks at 200 ma and 146 ma from the upper xihu formation suggest a lhasa terrane provenance to the south distinctive age clusters of 300 210 ma and 1800 ma and cr spinel composition analysis of the lagongtang formation indicate a provenance shift from the lhasa terrane to the qiangtang terrane and the bangong nujiang suture zone to the north the lagongtang formation was deposited in a bathyal abyssal to shallow slope environment from north to south that we interpret as the foredeep depozone and distal foredeep of a peripheral foreland basin system that developed due to flexural subsidence related to the lhasa qiangtang collision and terrane accretion the age of the lhasa qiangtang collision is constrained by a 140 ma tuffite u pb age at the base of the foreland basin strata an angular unconformity between the xihu and lagongtang formation which we interpret as the result of the lhasa qiangtang collision our results indicate that the lhasa qiangtang collision initiated around 140 ma in the dingqing region simultaneously with previous determinations 1200 km to the west near gaize area therefore we prefer quasi simultaneous onset of collision along strike to zippering collision models wherein the east collision age is older than the west

eocene neotethyan slab breakoff in southern tibet inferred from the linzizong volcanic record abstract formation of the linzizong volcanic successions in the lhasa terrane southern tibet has long been related to northward subduction of the neotethyan oceanic slab under asia here we report new 40ar 39ar age results for the volcanic rocks recovered from a large area 29 n to 32 n and 85 e to 93 e that together with literature data delineate two discrete stages of volcanism these are a widespread cretaceous stage and an intense but spatially confined paleogene stage the latter occurring only in the southern part of the lhasa terrane resulted in the linzizong volcanic successions our data furthermore suggest southward migration and intensification of the volcanism in the lhasa terrane with magmatic flare ups at ca 50 ma while the volcanic successions consist dominantly of calc alkaline rocks typical of arc lava geochemistry those formed during the flare up period show significant compositional variations from low k tholeiitic through calc alkaline to shoshonitic magma suites these observations enable us to interpret the volcanic southward migration and following flare ups as the consequences of rollback and breakoff of the subducted neotethyan slab that occurred ahead and in the early stage respectively of the india asia collision our interpretation that involves a major eocene tectonomagmatic activity and concomitant topographic uplift in southern tibet is consistent with himalayan metamorphic constraints regional sedimentary records and seismic tomography

reworked precambrian metamorphic basement of the lhasa terrane southern tibet zircon titanite u pb geochronology hf isotope and geochemistry abstract due to the paucity of exposure the formation and evolution of the precambrian basement of the lhasa terrane remain poorly known here we report zircon and titanite in situ u pb ages bulk rock geochemical and zircon hf isotopic data on the orthogneisses from the dongjiu area of the southern lhasa subterrane slt southern tibet geochemical data suggest that the protoliths of the biotite amphibole gneiss and biotite gneiss are granodiorite and granite respectively inherited magmatic zircon cores from these orthogneisses give protolith crystalline ages of 1520 1506 ma whereas the overgrown zircon rims give metamorphic ages of 605 590 ma the mesoproterozoic granitic rocks have bulk rock end t values of 3 6 to 0 1 and zircon core ehf t values of 4 5 to 2 6 which give similar tdm2 ages of 2 35 2 05 ga and 2 54 2 10 ga respectively suggesting their derivation from partial melting of paleoproterozoic crustal material the granitic rocks are also local provenance for the mesoproterozoic detrital zircons in the paleozoic strata in the lhasa terrane titanite in situ u pb ages further indicate that the dongjiu orthogneiss experienced more recent metamorphism at 26 ma the mineral assemblage and thermobarometry calculations indicate that the oligocene metamorphism occurred under medium pressure mp amphibolite facies conditions 5 4 7 2 kbar 691 765 c we propose that the dongjiu gneisses represent the precambrian metamorphic basement of the lhasa terrane but have been intensively reworked by metamorphism in the slt in response to the continued india asia convergence since the collision

late cretaceous granitic intrusions and associated deposits in the yidun arc of the eastern tibetan plateau abstract the yidun arc is an important tectonic unit and polymetallic metallogenic belt in the sanjiang tectono magmatic belt in which many cretaceous 138 75 ma granites and associated polymetallic deposits are distributed along a nw trending belt the queershan cuomolong rongyicuo and genie granitic intrusions and the associated sn pb zn ag polymetallic deposits are distributed in the changtai and xiangcheng arcs whereas the xiuwacu relin hongshan and tongchanggou granitic intrusions and the associated cu mo w deposits are mainly distributed in the geza arc this study presents new zircon u pb hf o isotopic data for the queershan cuomolong rongyicuo genie xiuwacu and tongchanggou granitic intrusions combined with the structural analysis of the xiuwacu granitic complex to discuss the differences in the petrogenesis and metallogenesis of these granitic intrusions as well as the geodynamic processes that controlled the formation of the late cretaceous intrusions and related deposits hf o isotopic characteristics indicate that the highly fractionated granites hfg in the changtai and xiangcheng arcs were mainly derived from the partial melting of metasedimentary crustal materials with a minor input from mantle components while the i type granites ig in the geza arc were predominantly derived from the partial melting of ancient lower crustal material the spatial distributions of the different types of deposits in the yidun arc were probably controlled by the magma sources reduced magmas produced by the partial melting of the sedimentary crust in the changtai and xiangcheng arcs are favorable for w sn pb and zn mineralization whereas oxidized magmas produced by the partial melting of the lower crust in the geza arc are conducive to the enrichment of cu au and mo the geochronological data indicate that the emplacement ages of the late cretaceous granites in the yidun arc gradually decrease from north to south ranging from 106 to 73 ma reflecting the southward lateral tectonic migration the structural analysis of the xiuwacu w mo deposit in combination with previous studies demonstrated that the late cretaceous granites in the yidun arc were likely formed in a regional extensional setting which was probably related to the slab break off of the bangong nujiang oceanic slab or the subduction of the yarlung zangbo oceanic slab

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

origin and deformation of high porosity bands in the takanoobane rhyolite lava of aso volcano japan abstract in rhyolite lavas the high porosity bands are often developed they potentially act as pathways for gas movement to the lava surface since explosive activities of lavas are generally considered to be controlled by degassing system understanding the origin and deformation process of the high porosity bands is important to assessing volcanic hazards the takanoobane rhyolite lava in the middle of kyushu island in sw japan is effused at 51 5 ka the volume flow length and thickness are 0 14 km 3 2 km and about 90 m respectively the central crystalline part of the lava is characterized by the light colored bands defined by the high porosity zone hpz on the basis of geological and petrographical studies we revealed that the hpz was primary formed by ductile brittle tearing of the lava known as cavitation according to the ams results the hpzs were subsequently stretched and flattened laterally during the concentric spreading of the lava this deformation process could stretch the hpz not only radially but also laterally this effective stretching developed the hpz into pervasive thin bands since the hpzs act as degassing pathways to the lava surface the pervasive hpz bands may play a role in providing volcanic gasses to void spaces created in surface fold hinges of rhyolite lavas thus this degassing system may promote explosive activity of the lava surface

discovery of an eclogite belt in the lhasa block tibet a new border for paleo tethys abstract a newly discovered eclogite belt in the eastern part of the lhasa block tibet is about 500 1000 m wide and at least 60 km long in an e w direction the eclogites occur as tectonic slices in garnet bearing mica quartz schist they are generally fresh and form thick massive layers that consist chiefly of garnet grt omphacite omp phengite phe rutile rut quartz qtz p t calculations based on the grt omp phe mineral assemblage yielded peak metamorphic conditions of 2 7 gpa and 730 c close to the phase boundary between coesite and quartz and thus the eclogites can be regarded as part of a very high pressure metamorphic belt petrochemical data suggest that the eclogite protoliths were typical morb basalts derived from depleted mantle shrimp u pb dating of zircons from the eclogite yielded metamorphic ages ranging from 242 15 to 292 13 ma with an average value of 262 5 ma the morb eclogites are interpreted to be remnants of paleo tethyan oceanic lithosphere the eclogites along with permian island arc volcanic rocks to the north are believed to mark a carboniferous permian suture zone dividing the lhasa block into a northern and southern segment this newly identified suture zone suggests that the border of the paleo tethyan ocean jumped southward from north of the bangong nujiang suture to within what is now the lhasa block

assembly of the pamirs age and origin of magmatic belts from the southern tien shan to the southern pamirs and their relation to tibet 1 magmatic rocks and depositional setting of associated volcaniclastic strata along a north south traverse spanning the southern tien shan and eastern pamirs of kyrgyzstan and tajikistan constrain the tectonics of the pamirs and tibet the northern pamirs and northwestern tibet contain the north facing kunlun suture the south facing jinsha suture and the intervening carboniferous to triassic karakul mazar subduction accretion system the latter is correlated with the songpan garze hoh xi system of tibet the kunlun arc is a composite early paleozoic to late paleozoic triassic arc arc formation in the pamirs is characterized by 370 320 ma volcanism that probably continued until the triassic the cryptic tanymas suture of the southern northern pamirs is part of the jinsha suture a massive 227 ma batholith stitches the karakul mazar complex in the pamirs there are striking similarities between the qiangtang block in the pamirs and tibet like tibet the regional structure of the pamirs is an anticlinorium that includes the muskol and sares domes like tibet the metamorphic rocks in these domes are equivalents to the karakul mazar songpan garze system granitoids intruding the qiangtang block yield 200 230 ma ages in the pamirs and in central tibet the stratigraphy of the eastern pshart area in the pamirs is similar to the bangong nujiang suture zone in the amdo region of eastern central tibet but a triassic ocean basin sequence is preserved in the pamirs arc type granitoids that intruded into the eastern pshart oceanic basin arc sequence 190 160 ma and granitoids that cut the southern qiangtang block 170 160 ma constitute the rushan pshart arc cretaceous plutons that intruded the central and southern pamirs record a long lasting magmatic history their zircons and those from late miocene xenoliths show that the most distinct magmatic events were cambro ordovician 410 575 ma triassic 210 250 ma likely due to subduction along the jinsha suture middle jurassic 147 195 ma subduction along rushan pshart suture and mainly cretaceous middle and late cretaceous magmatism may reflect arc activity in asia prior to the accretion of the karakoram block and flat slab subduction along the shyok suture north of the kohistan ladakh arc respectively before india and asia collided the pamir region from the indus yarlung to the jinsha suture was an andean style plate margin our analysis suggests a relatively simple crustal structure for the pamirs and tibet from the kunlun arc in the north to the southern qiangtang block in the south the pamirs and tibet likely have a dominantly sedimentary crust characterized by karakul mazar songpan garze accretionary wedge rocks the crust south of the southern qiangtang block is likely of granodioritic composition reflecting long lived subduction arc formation and cretaceous cenozoic underthrusting

the tethyan himalayan detrital record shows that india asia terminal collision occurred by 54 ma in the western himalaya the himalayan orogen is a type example of continent continent collision knowledge of the timing of india asia collision is critical to the calculation of the amount of convergence that must have been accommodated and thus to models of crustal deformation sedimentary rocks on the indian plate near the suture zone can be used to constrain the time of collision by determining first evidence of asian derived material deposited on the indian plate however in the himalaya for this approach to be applied successfully it is necessary to be able to distinguish between asian detritus and detritus from oceanic island arcs that may have collided with india prior to india asia collision zircons from the indian plate asian plate and kohistan ladakh island arc can be distinguished based on their u pb ages combined with hf signatures we undertook a provenance study of the youngest detrital sedimentary rocks of the tethyan himalaya of the indian plate in the western himalaya we show that zircons of asian affinity were deposited on the indian plate at 54 ma we thus constrain terminal india asia collision when both sutures north and south of the kohistan ladakh island arc were closed to have occurred in the western himalaya by 54 ma

late cretaceous evolution of the coqen basin lhasa terrane and implications for early topographic growth on the tibetan plateau the tectonic evolution of the lhasa terrane southern tibetan plateau played a fundamental role in the formation of the tibetan plateau however many uncertainties remain with regard to the tectonic and paleogeographic evolution of the lhasa terrane prior to the india asia collision to determine the early tectonic processes that controlled the topographic evolution of the lhasa terrane we analyze the cretaceous strata exposed in the coqen basin northern lhasa subterrane which comprises the langshan and daxiong formations the langshan formation unconformably overlies the volcanic rocks of the lower cretaceous zelong group and consists of similar to 80 m of orbitolina bearing limestones which were deposited in a low energy shallow marine environment micropaleontological analysis indicates that the langshan formation in the coqen basin was deposited from late aptian to early cenomanian times ca 113 96 ma the overlying daxiong formation similar to 1700 m thick consists of conglomerate coarse sandstone and siltstone with interbedded mudstone and represents deposits of alluvial fans and braided rivers the daxiong formation was deposited after the early cenomanian ca 96 ma and accumulated until at least ca 91 ma indicating accumulation rates of greater than 0 3 km m y 1 by combining paleocurrent data sandstone petrology detrital zircon u pb ages and hf isotope analysis we demonstrate that the daxiong formation was derived from lower cretaceous volcanic rocks and pre cretaceous strata in the northern lhasa subterrane during late cretaceous time two thrust systems with opposite vergence were responsible for transforming the northern lhasa subterrane into an elevated mountain range this process resulted in the evolution from a shallow marine environment langshan formation into a terrestrial depositional environment daxiong formation on the southern margin of the northern lhasa subterrane given the regional paleogeographic context we conclude that the daxiong formation in the coqen basin records local crustal shortening and flexure resulting in foreland basin development on the southern margin of the northern lhasa subterrane which implies early topographic growth of the northern lhasa subterrane in southern tibet prior to the india asia collision

tracing magma mixing in granite genesis in situ u pb dating and hf isotope analysis of zircons in situ zircon u pb and hf isotopic data have been determined for mafic microgranular enclaves and host granitoids from the early cretaceous gudaoling batholith in the liaodong peninsula ne china in order to constrain the sources and petrogenesis of granites the zircon u pb age of the enclaves 120 1 ma is identical to that of the host monzogranite 120 1 ma establishing that the mafic and felsic magmas were coeval the hf isotopic composition of the enclaves ehf t 4 5 to 6 2 is distinct from the host monzogranite ehf t 15 1 to 25 4 indicating that both depleted mantle and crustal sources contributed to their origin the depleted mantle component was not previously revealed by geochemical and nd and sr isotopic studies showing that zircon hf isotopic data can be a powerful geochemical tracer with the potential to provide unique petrogenetic information some wall rock contamination is indicated by inherited zircons with considerably older u pb ages and low initial hf isotopic compositions hafnium isotopic variations in early cretaceous zircons rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead mixing of mantle derived mafic magmas with crustal derived felsic magmas coupled with assimilation of wall rocks is compatible with the data

zircon u pb and hf isotopic constraints from eastern transhimalayan batholiths on the precollisional magmatic and tectonic evolution in southern tibet abstract before the indian collision with asia northward subduction of the neo tethyan oceanic lithosphere produced an andean type convergent margin in south asia characterized by arc magmatism starting from the early jurassic and lasting until the eocene the magmatic arc is now represented by widespread intrusive bodies or the so called transhimalayan batholiths in the lhasa terrane of southern tibet that have been divided into two main magmatic suites i e the northern plutonic belt and the southern gangdese batholith their temporal distribution concerning how exactly the magmatic suites correlate eastwards and then southeastwards around the eastern himalayan syntaxis however remains poorly constrained here we report the first combined zircon u pb and hf isotopic study of the transhimalayan batholiths from the eastern part of the lhasa terrane 95 97 5 e and 28 5 30 n se tibet zircon u pb dating results of 24 granitoids indicate that the rocks were emplaced principally in the early cretaceous 133 110 ma and subordinately in the paleocene 66 57 ma long after an older granite intrusion in the earliest jurassic 198 ma the zircon ehf t values range from 5 to 20 yielding hf crustal model ages tdmc between 0 8 and 2 4 ga that peak at 1 7 ga suggesting a major episode of crustal growth in the proterozoic and a predominantly or even exclusively crustal source for the granitoid petrogenesis these zircon u pb and hf isotopic constraints together with whole rock geochemical characteristics allow us to correlate them to s type granitoids in the northern plutonic belt rather than to the i type gangdese batholith from the central part of the southern lhasa terrane in contrast to the gangdese magmatism that formed with a significant juvenile mantle input related to the neo tethyan subduction these s type granitoids show geochemical similarities to the north american cordilleran interior batholiths that did not form directly from a subduction setting thus we attribute the petrogenesis of the widespread early cretaceous granitoids in the northern belt and eastern himalayan batholiths to a postcollisional regime due to the late jurassic early cretaceous continental collision between the lhasa and qiangtang terranes under this framework the s type magmatism was generated in response to collision induced crustal thickening which may have capabilities of causing crustal anatexis by itself in the region however continued interplay with the neo tethyan subduction zone processes such as back arc extension could also have played a crucial and long lasting role in the magma generation

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

early jurassic and late cretaceous granites in the tongka micro block central tibet implications for the evolution of the bangong nujiang ocean abstract the tongka micro block located in the eastern section of the bangong nujiang suture and characterized by mesozoic granitoids and high grade metamorphic rocks holds the key to the evolution of the bangong nujiang ocean bno here we present a systematic petrologic geochemical zircon u pb hf and whole rock sr nd pb isotopic investigation on six new granitoids in tongka the results suggest these granitoids yielded two stages of crystallization 171 177 ma and 74 ma and two stages of metamorphism 201 215 ma and 170 175 ma combined with the existing work within bns the magmatism along the bns can be divided into five stages stage 1 240 250 ma stage 2 200 230 ma stage 3 150 190 ma stage 4 100 135 ma and stage 5 73 100 ma both the early jurassic and late cretaceous tongka granitoids in this study are high k calc alkaline series with enrichments in large ion lithophile elements e g cs rb and k and depletions of large ion lithophile elements e g nb ta and ti the variable ehf t values 17 3 to 7 6 end t values 12 08 to 11 15 and ancient tdm2 1 48 2 00 ga for the early jurassic granitoids and weakly negative ehf t values 6 3 to 1 5 end t values 8 75 to 8 60 and old tdm2 ages 1 07 1 33 ga for the late cretaceous granitoids suggest that they are both mainly derived from the melting of ancient crust with minor mantle material input however the formation of the early jurassic and late cretaceous granitoids in tongka is related to different tectonic settings roll back after the tongka micro block subducted beneath the southern qiangtang subterrane and extension collapse after the tongka micro block collided with the northern lhasa subterrane the initial subduction of the bno may start from 190 ma and the collision between the lhasa and qiangtang terranes may continute to the latest cretaceous

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

zircon u pb dating and in situ hf isotopic analysis of permian peraluminous granite in the lhasa terrane southern tibet implications for permian collisional orogeny and paleogeography article i nfo the lhasa terrane has long been interpreted as a simple tectonic block rifted from gondwana during the late paleozoic and then drifted northward before finally amalgamating with the qiangtang terrane during the early cretaceous in this paper we document permian peraluminous granites near pikang in the southern margin of the central lhasa terrane close to the recently documented songdo eclogite of permian age zircon shrimp and la icpms u pb dating for a pikang granite sample gives an identical crystallization age of about 263 ma and a wide age range of inherited zircons 283 2141 ma in situ hf isotopic analyses for 20 zircons of 263 ma yielded ehf t values of 4 5 to 1 9 the pikang granites have high a cnk values 1 08 and high normative corundum 1 3 2 0 indicative of peraluminous s type granite they are characterized by moderately negative eu anomalies eu eu 0 48 0 61 and strongly negative ba nb sr p and ti anomalies the granites have high end t values 6 4 to 6 0 and low initial 87 sr 86 sr ratios 0 7082 0 7096 relative to melts derived from mature continental crust these rocks are interpreted to have been generated by mixing between mantle melts and their induced melting of mature crustal materials we interpret the pikang peraluminous granite magmatism the regional angular unconformity between the middle and the upper permian and the eclogite of the same metamorphic age 262 ma from the same geotectonic location to represent different products of a common event in time and space we tentatively term this common event as syncollisional orogeny i e the permian gangdese orogeny we further hypothesize that the orogeny may be genetically associated with the collision between the lhasa terrane and the northern margin of australia following the closure of the paleo tethyan ocean south of the lhasa terrane

organic matter accumulation of the lacustrine lunpola oil shale central tibetan plateau controlled by the paleoclimate provenance and drainage system abstract the cenozoic lacustrine lunpola oil shale located in the central tibetan plateau has received little research interest in its depositional history although it is estimated to be a huge resource reserve this study investigates the palaeoclimate provenance and regional drainage system of the ancient lakes using a combined analysis of element geochemistry and stratigraphy to elucidate the depositional history of the lunpola oil shale most of the samples in the lower part of the section have chemical index of alteration cia values between 65 and 75 reflecting a moderate weathered source while samples in the upper part the section have cia values between 60 and 65 which indicate relatively weak chemical weathering in the source area this stratigraphic grouping suggests a paleoclimate transition from a relatively warm or humid period to a cold or dry period the zr sc and th sc ratios of the samples reveal that sorting and recycling were minor during deposition of the oil shale the geochemical results al2o3 cao na2o k2o a cn k diagrams high contents of th plots of co th la sc and cr th sc th demonstrate that the sedimentary sources were felsic rocks plots of la th sc th sc zr 10 th co zr 10 and ti zr versus la sc illustrate that the source rocks were from continental arc and active continental margin the baingoin batholiths to the south of the lunpola basin consisting mainly of cretaceous granodiorite and adamellite are likely responsible for supplying materials to the lunpola oil shale similar special ree patterns further suggest the affiliation of the oil shale with the batholiths provenance analyses indicate a main river from the south moreover stratigraphic study confirms an upstream ancient baingoin lake between the source area and the ancient lunpola lake in this drainage system paleoclimate clearly restricted the development of rivers around the lakes although terrestrial organic matter higher plants dominated the drainage area the main river originating from the south could only bring a certain amount of them to the lakes moreover considering the filtration of the upstream ancient baingoin lake minor terrestrial organic matter could be carried and deposited in the ancient lunpola lake therefore lacustrine algae could proliferate as the main organic source and preserve in the stratified hypersaline lake to form the largest oil shale resource in tibet

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

tibetan basement rocks near amdo reveal missing mesozoic tectonism along the bangong suture central tibet the u pb and 4 0 ar 3 9 ar studies of a unique exposure of crystalline basement along the jurassic early cretaceous bangong suture of central tibet reveal previously unrecognized records of mesozoic metamorphism magmatism and exhumation the basement includes cambrian and older orthogneisses that underwent amphibolite facies metamorphism coeval with extensive granitoid emplacement at 185 170 ma the basement cooled to 300 c by 165 ma and was exhumed to upper crustal levels in the hanging wall of a south directed thrust system during early cretaceous time we attribute jurassic metamorphism and magmatism to the development of a continental arc during bangong ocean subduction and early cretaceous exhumation to northward continental underthrusting of the lhasa terrane beneath the qiangtang terrane we speculate that a jurassic arc extended regionally along the length of the bangong suture but in all other places in tibet has been buried either depositionally or structurally beneath supracrustal assemblages

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

mantle contributions to crustal thickening during continental collision evidence from cenozoic igneous rocks in southern tibet the tibetan crust is twice as thick as average continental crust crustal compression and shortening as a result of indian asian collision isoftenconsideredtobetheprimarycauseforthecrustalthickening inthispaper weshowthatmagmaticcontributionisalsoimportant we come to this conclusion by documenting the paleogene linzizong volcanic succession lvs its coeval granitoid batholiths and the miocene adakitic rocks along the gangdese magmatic belt in southern tibet it has been widely accepted that the indian asian collision proceeded from a soft phase at 65 70 ma to a hard phase at 45 40 ma followed by continued post collisional convergence to the present in response to the collision and post collision convergence are a series of tectono magmatic events recorded in the gangdese magmaticbelt theseinclude 1 thesyn collisionallvsvolcanism 65 40ma andtheemplacementofsoutherngangdesebatholiths a peakageof 50ma 2 aperiod 40mato25ma thatismagmaticallyquiescent yettectonicallydominatedbyactivecompressionand crustal shortening and 3 the emplacements of post collisional adakitic rocks 25 12 ma potassic ultrapotassic volcanics 25 10 ma and peraluminous muscovite bearing granites between 24 and 18 ma these three major events contribute in different ways to the crustal thickness phase i formation of the lower juvenile crust from 65 ma to 50 ma with crustal thickening largely concentrated at 50 40maviainputofmantle derivedmagmas phaseii crustalthickeningbytectonicshorteningat 40 25ma andphaseiii retaining crustal thickness but thinning of the lithospheric mantle since 25 ma in response to crustal extension and upwelling and lateral flow of asthenosphericmantle weemphasizethatcollision inducedcrustalthickeningtookplacemainlyintheperiodof 50 40maand 25ma i e the period between the late stage of the lvs volcanism and the beginning of the adakitic rock emplacement most of the lvs rocks and the collision related granitoids in southern gangdese have end 0 attesting to the significance of mantle input most likely through remelting of mantle derived basaltic rocks including the subducted neo tethyan ocean crust the petrologic and geochemical characteristics of the miocene potassic adakitic rocks support the idea that the lower portion of the thickened tibetan crust is mafic and is genetically associated with the earlier lvs magmatism we estimate that the mantle material input contributed about 30 of the total thickness of the present day tibetan crust by assuming a pre collision crustal thickness of 35 km then the tectonic contribution would be about 20 km

thickened juvenile lower crust derived 90 ma adakitic rocks in the central lhasa terrane tibet abstract the questions of why the late cretaceous magmatism generated and how the nature of the lower crust evolves in central tibet remain poorly constrained in this paper we report the presence of early late cretaceous adakitic rocks from the azhang area northern edge of the central lhasa subterrane central tibet these rocks are rhyodacites dacites in composition and have geochemical characteristics of adakitic rocks e g high sr 554 836 ppm sr y 66 100 and la yb n 20 21 low y 7 96 8 96 ppm and heavy rare earth elements hree in situ zircon u pb dating for two samples yields an early late cretaceous age 90 1 ma and 87 1 ma the low mgo 1 4 1 9 wt contents and compatible element abundances e g cr 22 30 ppm ni 19 25 ppm indicate that these rocks were most likely derived from the partial melting of a garnet bearing amphibolite under a thickened lower crust condition the positive whole rock e nd t 2 5 to 5 6 and zircon e hf t 8 9 to 16 0 values suggest that this thickened lower crust was juvenile the crust beneath the central lhasa subterrane may have been significantly thickened due to tectonic shortening in response to the lhasa qiangtang collision and magma underplating before the emplacement of azhang adakitic rocks 90 ma we argue that regional lithospheric delamination at 90 ma triggered the partial melting of the lowermost garnet bearing crust that is still attached to the middle crust to generate the azhang adakitic rocks the presence of the 90 ma azhang adakitic rocks provides valuable constraints on the origin of the early late cretaceous magmatism in the central lhasa subterrane and on the crustal evolution beneath the lhasa qiangtang collisional zone prior to the cenozoic india asia collision

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

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

early cretaceous na rich granitoids and their enclaves in the tengchong block sw china magmatism in relation to subduction of the bangong nujiang tethys ocean abstract the na rich intermediate to felsic granitic rocks provide insights into the generation of magmas in subduction zones this paper presents zircon la icp ms u pb ages as well as whole rock geochemical mineral chemical and in situ zircon hf isotopic data on na rich granitic rocks from the tengchong block sw china the granodiorites and associated mafic magmatic enclaves mmes from the menglian batholith yield zircon u pb ages of 116 1 0 8 to 117 8 0 6 ma and 117 7 0 7 ma respectively both host granodiorites and enclaves show calc alkaline and sodium rich nature enrichment in large ion lithophile elements liles and variable depletion in zircon hf isotopic compositions euhedral amphiboles in both granodiorites and associated enclaves are magnesian hornblende with high mg and ca and contain euhedral plagioclase inclusions of labradorite to andesine an36 57 composition the granodiorite was most likely derived through the mixing of partial melts derived from juvenile basaltic lower crust and a minor evolved component of ancient crustal sources the quartz monzodiorite granodiorites and associated mmes from the xiaotang mangdong batholith yield zircon u pb ages of 120 3 1 3 to 122 6 0 8 ma and 120 7 1 5 ma these rocks are also sodium rich and show calc alkaline trend with negative zircon hf isotopic compositions 5 55 to 0 58 the mmes in the host intrusions are monzogabbro with variable and depleted zircon hf isotopic compositions the amphiboles in the both host intrusions and the enclaves show al rich ferro tschermakite composition we infer that the quartz monzodiorite granodiorites were derived from magmas generated by the melting of ancient basaltic rocks in the lower arc crust induced by the underplating of mantle derived mafic magmas the formation of the different types of na rich granitic rocks is correlated to the subduction of bangong nujiang tethyan ocean a comparison with magmatism in the northern magmatic belt suggests that mantle derived magmas played an important role in the genesis of early cretaceous intrusions from tengchong to lhasa blocks although crustal melting is the dominant contributor

a type granite and adakitic magmatism association in songpan garze fold belt eastern tibetan plateau implication for lithospheric delamination abstract the songpan garze fold belt covers a huge triangular area 200 000 km2 confined by the south china yangtze north china and tibetan plateau continental blocks in the songpan garze fold belt triassic adakitic granitoids have been identified however whether there are triassic a type granites is unclear here we report our first finding of an a type granite nianbaoyeche which occurs in the central part of the songpan garze fold belt the nianbaoyeche granite 820 km2 is characterized by arfvedsonite in its mineral assemblage using both la icpms and tims u pb zircon dating methods we obtain a magma crystallization age of 211 1 ma which is slightly younger than triassic adakitic granitoids 216 221 ma in the songpan garze fold belt the nianbaoyeche granite is enriched in si k na rb ree hfse nb ta zr hf with elevated feotot feotot mgo and ga al ratios but is depleted in al mg ca ba and sr the ree compositions show moderately fractionated patterns with la yb n 2 67 7 54 and eu eu 0 09 0 34 these geochemical characteristics indicate that the nianbaoyeche granite has an a type affinity geochemical data and u pb zircon age combined with regional studies show that the nianbaoyeche granite formed in a post collisional tectonic setting sr nd isotopic data for the granite exhibit isr 0 7090 0 7123 and end t 2 72 to 4 26 with tdm 1 15 1 51 ga suggesting that the magma has a dominantly crustal source though a minor contribution from the mantle cannot be ruled out melting to produce an a type granite may have resulted from triassic lithospheric delamination after triassic crustal thickening of the songpan garze fold belt due to convergence between the yangtze north china and north tibet continental blocks the lithospheric delamination model also helps to explain the triassic adakitic magma generation in the songpan garze belt we conclude that association of a type granite and adakitic granitoids in post collisional environment could be a useful indicator of lithospheric delamination

sedimentology palaeoecology and palaeoenvironmental evolution of the 1985 lhasa to golmud geotraverse vertical and horizontal measurements of almost 30 km of sections were made along the geotraverse route at 113 localities ranging in age from ordovician to tertiary over 280 palaeocurrent measurements were taken and 200 thin sections were studied ordovician strata occur only in the kunlun terrane w here thick metamorphosed sequences of elastics and carbonates occur these are tentatively interpreted as platform margin and slope deposits during carboniferous times in the kunlun terrane transgressive late dinantian marine limestones with tropical to subtropical eurasian reefoidal faunas overlie fluviatile redbeds derived from an unroofed orogenic belt the lhasa terrane contains shelf basin elastics with low diversity faunas succeeded by thick late carboniferous early permian glacio marine mixtites deposited by iceberg meltout permian carbonate ramp and shelf facies with reefoidal developments occur over both the lhasa and qiangtang terranes with coal bearing elastics and fluviatile redbeds also occurring in the latter

geochemical investigation of early cretaceous igneous rocks along an east west traverse throughout the central lhasa terrane tibet article i nfo bulk rock major and trace element sr nd isotope zircon u pb and hf isotope data are reported for early cretaceous igneous rocks sampled along an east west traverse through the central lhasa subterrane e80 00 e89 in southern tibet these results offer new insights into the pre cenozoic geology and tectonomagmatic evolution of the lhasa terrane and provide boundary conditions for the origin and evolution of the tibetan plateau our new data together with data from the literature indicate that the early cretaceous magmatism in the central lhasa subterrane lasted for a long period 143 102 ma with a magmatic flare up at 110 ma the latter is accompanied by mafic magmatism in this terrane the silicic rocks are metaluminous to peraluminous and enrichedinrb th and u and depleted inba nb ta sr p and ti withvarying initial 87

petrogenesis and tectonic significance of a mesozoic granite syenite gabbro association from inland south china article i nfo a granite syenite gabbro association from inland south china has been studied for zircon u pb ages and hf isotopic compositions as well as whole rock elemental and sr nd isotopic compositions to constrain their sources and tectonic settings these rocks have distinctive crystallization ages the longyuanba biotite granites have a mean age of 240 ma the longyuanba two mica granites pitou alkali feldspar granites and amphibole bearing alkali feldspar granites tabei and huangbu syenites and chebu and chenglong gabbros have a similar age of 178 ma whereas the dafengnao syenite gives a mean age of 165 ma the longyuanba biotite granites and two mica granites exhibit s type characteristics probably derived from a neoproterozoic crustal source the chenglong gabbro has oib like trace element and highly depleted isotope compositions ehf t 10 0 1 3 end t 5 2 suggesting its parental melt to be of asthenospheric origin with insignificant crustal assimilation however crustal contamination is required to explain the isotopic compositions of the chebu gabbro the syenites are shoshonitic in composition and have depleted sr nd hf isotopic signatures which we interpret to have resulted from a mixed source of asthenospheric mantle and metasomatized lithospheric mantle as expected the signals of crustal assimilation are conspicuous in the petrogenesis of the more evolved syenites the pitou alkali feldspar granite and amphibole bearing alkali feldspar granite exhibit i type and a type characteristics respectively they are isotopically more depleted than the s type longyuanba granites we suggest that they may have formed through magma mixing of predominantly mantle derived melts with the neoproterozoic crust the jurassic granite syenite gabbro association was the product of asthenosphere lithosphere crust interactions which records the primary role of asthenospheric mantle upwelling in magma generation both in the mantle and in the crust in the early yanshanian time in south china they probably occurred in an intraplate rift like environment as a tectonic response to far field stress at plate margins during the early stage of the paleo pacific plate subduction the tectonic transition from the tethys orogenic regime to the paleo pacific regime was accomplished in the early jurassic and the early yanshanian magmatism should be genetically associated with the paleo pacific tectonic regime