early jurassic carbon isotope excursion in the qiangtang basin tibet the eastern tethys implications for the toarcian oceanic anoxic event abstract the early jurassic was characterized by a global disturbance of the carbon cycle known as the toarcian oceanic anoxic event t oae this event is recorded by a large negative carbon isotope excursion cie in marine carbonates and marine organic matter as well as fossil wood here we present new high resolution carbonate 13 c carb and organic 13 c org carbon isotope records and total organic content from the bilong co section in the qiangtang basin the eastern tethys this section is high in organic carbon and black shale facies are also identified the bilong co section shows heavy 13 c org values compared to other reported marine organic carbon isotope values for the t oae this is explained by variable mixing of terrestrial and marine organic matter compared to european sections the magnitude of the negative cie in the bilong co section is slightly different which could be explained by different local paleoenvironmental conditions and strata missing although these the carbon isotope profile from the bilong co area displays a similar trend to those in contemporaneous strata in the western tethyan and boreal realms of nw europe which is characterized by a distinct negative excursion in the early toarcian our new data from the bilong co section in n tibet provide the first high resolution record of the early toarcian oceanic anoxic event in the eastern tethys

the vast proto tibetan plateau new constraints from paleogene hoh xil basin abstract the surface uplift of the tibetan plateau is the key boundary condition in many cenozoic geological events ranging from global cooling to changes of asian environments during cenozoic however poorly constrained timing for the uplift of tibetan plateau makes these interpretations highly debatable here we report results from sedimentology detrital zircon u pb and lu hf isotopic compositions and paleomagnetic signatures from both the eastern and western hoh xil basins of north central tibetan plateau sedimentary lithofacies and facies associations analyzed in the western hoh xil basin indicate they were deposited in a braided fluvial system and alluvial fan similar with the fenghuoshan group eastern hoh xil basin provenance analyses from conglomerate clast compositions paleocurrent orientations and detrital zircon u pb and lu hf isotopic compositions document sediments in both western and eastern basins were derived from the qiangtang and lhasa blocks these observations in combination with comparative paleomagnetic results imply that hoh xil basin was a single wide basin during paleogene the period of hoh xil basin deposition was coeval with significant period of the early cenozoic uplift and erosion of the qiangtang and lhasa blocks these observations not only reinforce the suggestion that the qiangtang and lhasa blocks were uplifted during eocene to form a proto tibetan plateau but also imply that the proto tibet plateau is vast in areal extent the large dimension and high elevation of the proto tibetan plateau probably contributed to the global cooling during the early eocene

diachronous closure of the shuanghu paleo tethys ocean constraints from the late triassic tanggula arc related volcanism in the east qiangtang subterrane central tibet abstract the tectonic evolution of the shuanghu paleo tethys ocean is an important but poorly understood issue in the geology of the tibetan plateau and tethys here we present a new and detailed petrological geochronological major and trace elemental sr nd hf isotopic dataset for the late triassic tanggula volcanic sequence in the east qiangtang subterrane central tibet which together with previously published data would provide critical constraints on the evolution of the shuanghu paleo tethys ocean the volcanic sequence is constituted by two nearly concentric belts namely the outer belt of rhyolites and the inner belt of granite porphyries la icp ms zircon u pb dating indicates that the granite porphyries formed at ca 221 ma later than the rhyolites by ca 15 myr they are characterized by enrichments in large ion lithophile elements liles and light rare earth elements lrees with significant negative anomalies at nb ta and ti but positive anomalies at th u and pb isotopic compositions exhibit enrichment signatures with hf t values ranging from 14 1 to 5 7 and nd t values from 8 84 to 7 06 geochemical characteristics indicate that these volcanic rocks were formed by progressive melting of the subducted sediments accumulated on the shuanghu paleo tethys ocean floor which originated from the weathered paleozoic continental crust in the qiangtang terrane combined with the geological background we further propose that the eastern tanggula changdu segment of the shuanghu paleo tethys ocean was still northward subducting at ca 221 ma the shuanghu paleo tethys ocean has an diachronous closing process where the eastern tanggula changdu segment closed later than the western gangma co shuanghu segment by 20 myr

geochemistry of the upper triassic black mudstones in the qiangtang basin tibet implications for paleoenvironment provenance and tectonic setting abstract the qiangtang basin is the largest mesozoic marine basin in the tibetan plateau the upper triassic black mudstones are among the most significant hydrocarbon source rocks in this basin here we present geochemical data for the upper triassic black mudstones to determine their paleoenvironment conditions provenance and tectonic setting to achieve these 30 black mudstones formed in various sedimentary environments were collected from the zangxiahe zana and bagong formations the results show that the total ree concentrations of mudstones from these formations range from 169 to 214 ppm 204 to 220 ppm and 141 to 194 ppm respectively all samples have chondrite normalized ree patterns with enrichment of lree depletion of hree and negative eu and ce anomalies specifically mudstones from the bagong formation exhibit higher negative eu anomalies and lower ree contents than those from the zangxiahe and zana formations mudstones from the zangxiahe and zana formations with low sr ba and sr cu ratios indicate the humid climate whereas the high sr ba and sr cu ratios of rocks from the bagong formation suggest the arid climate the low u th cu mo zn v cr and ni co ratios of rocks from the zangxiahe zana and bagong formations are indicators of oxidized conditions the bivariate diagrams tio2 vs al2o3 tio2 vs zr la th vs hf and co th vs la sc reveal that mudstones from the zangxiahe and zana formations were potentially derived from intermediate igneous rocks whereas mudstones from the bagong formation were probably sourced from felsic igneous rocks their source rocks are mostly deposited in the collisional setting ree of mudstones from the zangxiahe zana and bagong formations were possibly originated from terrigenous detritus with minor non terrigenous contributions into the zana samples the ree contents of these mudstones are controlled mainly by terrigenous detrital minerals rather than by the paleoclimate paleoredox conditions or organic matter however calcite minerals could dilute ree therefore the ree contents of the bagong formation mudstones are significant lower than those of the zangxiahe and zana formations mudstones

sources and petrogenesis of late triassic zhiduo volcanics in the northeast tibet implications for tectonic evolution of the western jinsha paleo tethys ocean abstract the western jinsha suture representing a branch of the paleo tethys makes the junction between the songpan ganzi complex and the north qiangtang block nqb however the triassic tectonic evolution of the western jinsha paleo tethys ocean is poorly constrained this paper shows new la icp ms zircon u pb geochronology major and trace elements and sr nd hf isotopic compositions of the zhiduo dacitic volcanic rocks in the northern margin of the nqb la icp ms zircon u pb dating displays that they erupted between 227 2 and 228 1 ma these rocks exhibit high silica medium k calc alkalic signature and low mgo tio2 p2o5 cr and ni compositions they are enriched in light rare earth elements and large ion lithophile elements with moderate to strong negative eu anomalies eu eu 0 24 0 56 and depleted in heavy rare earth elements and high field strength elements they possess relatively low initial 87sr 86sr ratios 0 7055 0 7084 positive whole rock end t 0 2 to 1 4 and magmatic zircon ehf t values focus on 9 2 2 7 with young nd and hf model ages t2dm nd 885 983 ma t2dm hf 467 1035 ma these features are comparable to those of typical arc volcanic rocks and indicate that they were probably derived from partial melting of a juvenile crust source the eruption of these volcanic rocks could be attributed to southward subduction of the western jinsha paleo tethys ocean in combination with previous studies we propose that the subduction of the western jinsha paleo tethys ocean initiated at the beginning of the late triassic ca 228 ma and closed at the end of the late triassic ca 208 ma in addition there is a universal northeastward younging trend of arc magmas in the northern margin of the nqb which may indicate a model involving the initial shallow subduction and subsequent slab roll back

documentation of benchmark photographs that show the effects of the 1983 borah peak earthquake with some considerations for studies of scarp degradation systematic benchmark photography of modern and prehistoric fault scarps provides an accurate and timely method of documenting details of ground breakage related to earthquakes and provides a near permanent record for future quantitative analysis of the degradation of these features during the geologic investigation of surface faulting associated with the 28 october 1983 borah peak earthquake of central idaho i established 14 benchmark stations for repeat photography of selected deformational features about 40 photographs most in stereopairs were made in november 1983 in order to show some of the details of the newly formed fault scarps grabens and an associated landslide one year later october 1984 h e malde and i repeated most of the 1983 photographs and photographed from 14 additional stations this paper provides an account of the location and subject of the photographs that are on file at the u s geological survey photo library in denver hopefully this study will provide the impetus for including systematic benchmark photography in future studies of ground deformation

cambrian to triassic geodynamic evolution of central qiangtang tibet abstract central qiangtang is a tectonic melange between the northern qiangtang terrane nqt and the southern qiangtang terrane sqt and it preserves a complete cambrian to triassic geological record related to tethyan evolution e g high pressure metamorphic rocks and ophiolites thus this region is the key to understanding the geodynamic processes that occurred during the dispersion of gondwana and the accretion of laurussia given previous studies on sedimentary paleomagnetic and seismic records this paper presents an integrated interpretation of ophiolites magmatism metamorphism and tectonism and reconstructs the cambrian triassic tectonic evolution of central qiangtang as described below 1 following the neoproterozoic early cambrian pan african orogeny and associated postcollisional extension the southward subduction of the proto tethys oceanic plate produced andean type magmatism along the northern margin of gondwana a back arc basin between the nqt and sqt further opened during the late cambrian early ordovician accompanying with trench retreat and overriding plate extension resulting from slab rollback 2 with the middle ordovician late silurian successive incorporation of micro continental terranes e g qaidam kunlun and tarim terranes into the northern margin of gondwana the back arc basin closed through subduction polarity reversal from southward to northward 3 during the late devonian early carboniferous the southward subduction of an oceanic plate to the north of the nqt separated the nqt and sqt through back arc extension consequently the nqt drifted northward rapidly and the back arc basin gradually evolved into a mature ocean basin i e longmu co shuanghu paleo tethys ocean lspto 4 subsequently a late carboniferous early permian continent continent collision between the nqt and the south china block scb resulted in the northward subduction of the lspto and the ongoing subduction probably induced the opening of a back arc basin between the nqt and the scb during the late permian 5 finally diachronous collision between the nqt and sqt closed the lspto during the early middle triassic and the termination of the collisional orogenic event was evidenced by strong late triassic magmatism related to postcollisional extension in the qiangtang block in this paper we propose that the opening and closure of back arc basins controlled the cambrian triassic tectonic evolution of central qiangtang which provides significant insights into the dispersion of gondwana and the accretion of laurussia

exhumation of the dabie shan ultra high pressure rocks and accumulation of the songpan ganzi flysch sequence central china the presence of coesite and diamond bearing ultra high pressure uhp metamorphic rocks in the dabie and sulu regions central china suggests that a 100 km thick crustal section 4 x 106 km6 in volume has been denuded this volume is comparable to that represented by the 10 15 km thick middle to upper triassic flysch rocks in the songpan ganzi region regional geology and radiometric dates are compatible with an interpretation that the majority of these sedimentary rocks were derived from denudation of the orogenic belt between north and south china blocks following their latest paleozoic to triassic collision this correlation implies a high denudation rate of 4 mm yr that may have been in part promoted by tropical precipitation and extreme topographic relief for as long as 25 m y

geochemistry of platinum group elements in marine oil shale from the changshe mountain area china implications for modes of occurrence and origins abstract the shengli river changshe mountain oil shale zone located in the north qiangtang depression northern tibet plateau represents a potentially largest marine oil shale resource in china eighteen samples including oil shale and micritic limestone were collected from the changshe mountain oil shale to determine the concentrations the modes of occurrence and origins of platinum group elements pges in marine oil shale the oil shale samples from the changshe mountain area exhibit low total pge contents ranging from 3 196 to 8 071 ng g with a weighted mean value of 5 383 ng g while the micritic limestone samples from the changshe mountain area exhibit a little lower ranging from 0 471 to 5 669 ng g pges in oil shale samples are characterized by high contents in pd 3 23 ng g pt 1 60 ng g and os 0 32 ng g as compared to rh 0 083 ng g ru 0 112 ng g and ir 0 043 ng g the individual pges of oil shale samples from the changshe mountain area exhibit various modes of occurrence ru and ir are mainly controlled by clay minerals and partly present as sulfide rh is mainly associated with sulfide and possibly present as clay minerals pt is associated with clay minerals and sulfide and pd is controlled by organic matter and pyrite while os is mainly present in organic matter three possible sources of pges were identified in oil shale seams in the changshe mountain area ru and ir are mainly terrigenous inputs rh and pt are derived from mixed sources seawater and terrigemnous supply while pd and os are mainly derived from seawater the terrestrial source of the oil shale seams in the changshe mountain oil shale was the suowa formation limestone and the nadi kangri felsic volcanic rocks

concentration and mode of occurrence of trace elements in marine oil shale from the bilong co area northern tibet china abstract the bilong co oil shale zone is located in the south qiangtang depression this zone together with the shengli river changshe mountain oil shale zone in the north qiangtang depression northern tibet plateau represents a potentially large marine oil shale resource in china with the aim of better understanding geochemistry of marine oil shale 18 samples from the bilong co area are studied and 56 elements in them are determined the contents of mo se cd cs as bi u rb pb th li sr and zn are enriched from 1 3 to 35 0 times as compared with the average concentration in the crust clarke values whereas the other elements are slightly higher lower than the respective clarke values compared to common australian oil shales the contents of ce co cs f ga hf nb nd pb pr rb sc sn th and zr in the bilong co oil shale samples exhibit relatively high levels whereas as br cd cr cu dy er eu ho lu mo ni se tb tm u v y yb and zn are relatively depleted however the distribution patterns of trace elements are quite similar indicating that trace element speciation in oil shale is governed by general processes rather than by individual geochemical mechanisms within one particular sample the elements in the bilong co oil shale may be classified into three groups of association according to their modes of occurrence i e groups a b and c group a si al k na ti zr sc th sn nb ga ta be hf cs li rb ash rees is strongly correlated with ash yield and mainly has an inorganic affinity group b cr w mg mn br f sr ba has weakly or negatively positive correlation coefficients with an exception of f with ash yield group c zn bi cd ca fe pb hg co cu ni p mo se u as v shows negative correlation coefficients with ash yield with an exception of bi and possibly has an organic affinity

tectonic evolution of the early mesozoic blueschist bearing qiangtang metamorphic belt central tibet this is the published version copyright 2003 american geophysical union all rights reserved

was late triassic tanggula granitoid central tibet western china a product of melting of underthrust songpan ganzi flysch sediments geochronological laser ablation inductively coupled plasma mass spectrometry la icp ms mineral chemistry major and trace element and sr nd hf isotopic data are presented for the tanggula granitoid the largest granitic pluton in the northern tibetan plateau the tanggula pluton yields a late triassic emplacement age 228 223 ma and is magnesian mg 29 10 49 81 high k calc alkaline k2o na2o 0 76 3 51 mostly 1 and strongly peraluminous molar al2o3 cao na2o k2o a cnk 1 06 1 42 mostly 1 1 high initial 87sr 86sr ratios 0 716849 to 0 730577 and low nd t 10 95 to 9 60 and hf t values 16 to 5 1 along with an abundance of aluminosilicate minerals e g biotite enrichments in light rare earth and large ion lithophile elements negative eu anomalies and depletion in high field strength elements suggest that the tanggula granitoid is an s type granite derived mainly from the anatexis of metasedimentary source rocks geochemical fingerprinting using sr nd hf isotopes and detrital zircon u pb age spectra shows that triassic flysch sediments from the songpan ganzi paleo tethyan ocean to the north of the pluton are the most likely source of the tanggula granitic magmas the tanggula granitoid could thus have been formed by the partial melting of the flysch sediments from the songpan ganzi complex as they were underthrust beneath the qiangtang terrane along the jinsha subduction zone during the late triassic

geophysical constraints on the link between cratonization and orogeny evidence from the tibetan plateau and the north china craton abstract understanding the geodynamic process of orogeny and cratonization and their transition is among the key topics of research in evaluating the link between plate tectonics and continental dynamics the tibetan plateau and the north china craton ncc two key areas in mainland china offer excellent laboratories to understand continental tectonics over a broad span of earth history particularly the deep structure of the lithosphere as imaged from geophysical data on the tibetan plateau and the ncc provide important clues in understanding orogeny and cratonization the tibetan plateau is the largest and highest plateau on earth in terms of mean altitude and is an important region for understanding the mechanisms of continent continent collision and cenozoic plateau uplift the ncc is an archean craton that underwent lithospheric disruption during the mesozoic here we reconstruct the main features of the structure of the crust and upper mantle from surface wave tomography and gravity modeling in tibet and its neighboring regions in order to understand the modality of the convergence and collision process between the indian and eurasian plates and the influence of this process on the uplift of the plateau in the ncc geological geochemical geophysical and tectonic investigations demonstrate that lithospheric destruction mainly occurred in the eastern block the crustal structure of the ncc is reconstructed from ambient noise surface wave tomography and the different possible disruption mechanisms are evaluated the vs shear wave velocity tomography results and the density structure of the crust and upper mantle to about 350 km depth demonstrate the lateral variation of the thickness of the metasomatic lid between the south and north of the bangong nujiang suture bns and the west and east of tibet which suggest that the leading edge of the subducting indian slab reaches the bns the subduction angle of indian plate indicates a transition from steep to shallow from the west to east tibet sections depicting the gravitational potential energy suggest that mantle flow contributes to the subduction of the indian plate as far as the bns and the transition from the asthenospheric layer to the metasomatic lid overlaps with the transition from north south shortening in south tibet to eastward tectonic escape in north tibet qiangtang and songpan ganzi blocks both vs and models suggest the following 1 north southward lower crust flow beneath the eastern ncc and interaction between the westward mantle flow and eastward escape flow beneath the central ncc in addition to the earlier proposed mechanisms of delamination and thermal erosion played important roles in the lithospheric disruption of the archean craton 2 mantle flow plays an important role in the continental tectonic transition between neighboring tectonic blocks and within the cycle between orogeny and cratonization

age and origin of high ba sr appinite granites at the northwestern margin of the tibet plateau implications for early paleozoic tectonic evolution of the western kunlun orogenic belt abstract the buya appinite granite is a typical high ba sr granite emplaced at the northern west kunlun orogenic belt along the northwestern margin of the tibetan plateau the granite is dated at ca 430 ma using the shrimp u pb zircon method it consists of alkaline feldspar granites with coeval appinite enclaves the granite possesses high sio2 69 77 72 69 k2o 4 44 5 10 and total alkalinity k2o na2o 8 80 9 92 sr 655 1100 ppm ba 1036 1433 ppm and lree and low hree and hfse contents and insignificant negative eu anomalies consequently the samples have very high sr y 74 141 and la yb n 37 96 ratios on the other hand they have low mgo or mg cr and ni contents and low radiogenic nd isotopes nd t 8 4 to 10 4 the high ba sr and other geochemical signatures of the granite also appear in the appinite enclaves except that the appinite enclaves have relatively higher abundances in these elements and higher nd t values 5 7 to 6 7 elemental and isotope compositions suggest that the appinites were derived from partial melting of an enriched lithospheric mantle source probably induced by upwelling of the asthenosphere due to the delamination of a subducted slab the granite was likely derived from partial melting of the mafic lower crust with residual garnet associated with involvement of minor lile enriched appinitic magma followed by crystal fractionation of hornblende biotite apatite and allanite in combination with previous investigations on the evolution of the western kunlun we suggest that the buya high ba sr plutons represent the end of an early paleozoic crust thickening event after a terrane accretion on southern tarim craton and the beginning of a post orogenic collapse phase in the paleozoic west kunlun orogenic belt

early mesozoic magmatism within the tibetan plateau implications for the paleo tethyan tectonic evolution and continental amalgamation

geochemistry of terrestrial oil shale from the lunpola area northern tibet china abstract the lunpola oil shale is the largest oil shale resource in tibet twenty four samples of oil shale mudstone and dolomite were collected from the lunpola oil shale for mineralogical and trace element analyses the vertical distribution and mode of occurrence of trace elements in this terrestrial oil shale were also discussed oil shale samples from the lunpola area have high toc contents 5 1 10 6 high ash yields 74 8 84 7 low total sulfur contents 0 19 1 03 and high shale oil contents 5 0 10 3 the major minerals in the oil shale samples are clays quartz calcite and dolomite with minor quantities of feldspar siderite hematite magnesite and pyrite other minerals such as halite zeolite barite anhydrite and mirabilite were detected in some oil shale samples the contents of as ba be bi cd ce cs hg la li mo nd pb pr rb se sm th u w and zr are enriched from 1 30 to 125 times as compared to their average crust abundance clarke values whereas the other elements are slightly higher or lower than their respective clarke values compared to common australian oil shale the lunpola oil shales contain higher as ba be ce co cs ga gd hf nb nd pb pr rb sc sm sn th and zr but lower cd cu lu mo ni se sr u v and zn elements associated with organic matter and or fe sulfides are highly enriched in all oil shale deposits suggesting that their speciation in oil shale is governed by general processes e g high organic matter content and anoxic environment whereas elements associated with clay minerals exhibit various enrichments indicating that their speciation in oil shale is governed by individual geochemical mechanisms e g source rocks the elements in the lunpola oil shale may be classified into three groups of associations according to their modes of occurrence group a elements tio 2 sc hf zr be li w v zn sio 2 al 2 o 3 rb ga sn cr ta cs k 2 o th na 2 o rees and ash yield have high positive correlations with ash yield and a mainly inorganic affinity group b elements as toc mno ni fe 2 o 3 cd co cu mo se mgo bi pb and hg show a negative correlation with ash yield and most elements possibly have a likely organic affinity and or fe sulfide affinity group c elements sr cao u and p 2 o 5 show a weak or negligible correlation with ash yield with the exception of cao elements of environmental concern as bi hg mo pb se and u are present in high contents and are clustered in the second association i e they are hosted mainly in fe sulfides and or organic matter

triassic continental subduction in central tibet and mediterranean style closure of the paleo tethys ocean the qiangtang metamorphic belt qmb in central tibet is one of the largest and most recently documented high pressure hp to near ultrahigh pressure near uhp belts on earth lu hf ages of eclogite and blueschist facies rocks within the qmb are 244 223 ma indistinguishable from the age of uhp metamorphism in the qinling dabie orogen results of a u pb detrital zircon study suggest that protoliths of the qmb include upper paleozoic qiangtang continental margin strata and sandstones that were derived from a paleozoic arc terrane that developed within the paleo tethys ocean to the north we attribute qmb hp metamorphism to continental collision between the qiangtang terrane and a paleo tethys arc terrane this collision and the coeval south china north china collision may have slowed convergence between laurasia and gondwana derived terranes and initiated mediterranean style rollback and backarc basin development within much of the remnant paleo tethys ocean realm

triassic nb enriched basalts magnesian andesites and adakites of the qiangtang terrane central tibet evidence for metasomatism by slab derived melts in the mantle wedge new chronological geochemical and isotopic data are reported for triassic 219 236 ma adakite magnesian andesite nb enriched basaltic rock associations from the tuotuohe area central qiangtang terrane the adakites and magnesian andesites are characterized by high sr y 25 45 la yb 14 42 and na2o k2o 12 49 ratios high al2o3 15 34 18 28 wt and moderate to high sr concentrations 220 498 ppm and end t 0 86 to 1 21 values low enrichments of th rb relative to nb and subequal normalized nb and la contents and enrichments of light rare earth elements combine to distinguish a group of nb enriched basaltic rocks nebs they have positive end t 2 57 to 5 16 values positive correlations between th la and nb and an absence of negative nb anomalies on mantle normalized plots indicate the nebs are products of a mantle source metasomatized by a slab melt rather than by hydrous fluids a continuous compositional variation between adakites and magnesian andesites confirms slab melt interaction with mantle peridotite the spatial association of the nebs with adakites and magnesian andesites define an adakitic metasomatic volcanic series recognized in many demonstrably subduction related environments e g mindanao arc philippines kamchatka arc russia and southern baja california arc mexico the age of the touhuohe suite and its correlation with triassic neb to the north indicates that volcanism derived from subduction modified mantle was abundant prior to 220 ma in the central qiangtang terrane

the late triassic rift related volcanic rocks from eastern qiangtang northern tibet china age and tectonic implications abstract the late triassic nadi kangri volcanic rocks with nearly ew trending outcrops within the qiangtang basin northern xizang tibet china are composed mainly of acid tuff dacite rhyolite and minor basic volcanic rocks there exists a significant depositional hiatus between the nadi kangri volcanic rocks and underlying strata therefore the nadi kangri volcanic rocks represent a new evolution history of the mesozoic qiangtang basin the magma emplacement age of the nadi kangri volcanic rocks in the geladaindong area is 220 4 2 3 ma representing the onset of the mesozoic qiangtang basin the nadi kangri basalts have high nb zr 0 049 0 058 ta hf 0 12 0 15 and zr y 4 95 6 01 ratios in the tectonic discrimination diagrams such as zr vs zr y and th hf vs ta hf the nadi kangri basaltic rocks mostly plot in the within plate setting field the geological background and the geochemical characteristics suggest that the nadi kangri volcanic rocks were formed in a continental rift setting

geochemistry and petrogenesis of the permian mafic dykes in the panxi region sw china abstract numerous intrusive bodies of ultramafic mafic to felsic compositions are exposed in association with volcanic rocks in the emeishan large igneous province lip southwestern china in this paper we present new elemental and isotopic data for the permian mafic dykes from the panxi region which is located in the western part of the emeishan lip the characteristics of major and trace elements and sr nd isotopes in combination with isotopic ages suggest that these mafic dykes were originated from a similar mantle source related to an upwelling mantle plume and their formation was involved in a process of variable degrees of fractional crystallisation coupled with assimilation of enriched crustal materials the mafic dyke samples from the panxi region show some systematic variations in geochemical compositions the gabbros in yanyuan contain the lowest ree e g la 5 12 ppm the highest mgo 8 38 21 33 and compatible elements e g cr 104 2610 ppm ni 135 496 ppm and display depleted sr nd isotopic ratios initial 87sr 86sr 0 703752 0 703844 end t 4 87 to 5 13 these gabbros likely represent an undifferentiated composition similar to primary melts derived from a depleted mantle source in contrast the gabbro samples from panzhihua and huili have more enriched lree abundances and incompatible elements and display depleted to slightly enriched sr and nd isotopic signatures initial 87sr 86sr 0 704354 0 705436 end t 0 29 to 4 58 though their trace element spidergram patterns are similar to those of the yanyuan samples these gabbros were formed probably through the removal of a few percent of olivine clinopyroxene plagioclase and apatite from the melts that formed the yanyuan gabbros all the mafic intrusives in the region exhibit elemental and sr nd isotopic features generally comparable with those of the nearby volcanic rocks in addition their emplacement immediately preceded or was synchronous with the main pulse of volcanism that formed lavas over a large area along the western margin of the yangtze craton these lines of evidence suggest that there is a genetic link between the mafic dykes and flood basalts in the emeishan lip and the both share a common mantle source related to the emeishan plume

upper triassic reef coral fauna in the renacuo area northern tibet and its implications for palaeobiogeography abstract upper triassic reef corals from the riganpeicuo formation in northern tibet represent important scleractinian coral fauna that help explain the palaeobiogeography of the eastern tethys region during the late triassic period the corals were discovered in bedded limestone in patch reefs or biostromes of the renacuo area in this paper 15 genera and 25 species are identified and categorized the systematic composition of these corals and their relationships with other triassic coral faunas are also discussed the results show that these corals are composed of the typical elements of the western tethys with the following genera and species that are endemic to china radiophyllia cf astylatus margarosmilia zogangensis and conophyllopsis qamdoensis and the genera retiophyllia margarosmilia hydrasmilia procyclolites pamiroseris araiophyllum stylophyllopsis stylophyllum and guembelastraea provide important links to the tethys province the coral fauna also highlights the connection between the qiangtang terrane and the songpan ganzi fold belt but shows that the areas are distinct from the himalayan terrane it has been interpreted that the qiangtang terrane and the songpan ganzi fold belt were in the vicinity of the gradually closed paleo tethys ocean which resulted in the free transmigration of the benthonic organisms of these areas on the other hand the himalayan terrane was separated from the qiangtang terrane by a wide ocean meso tethys during the late triassic period which made it impossible for the benthonic organisms on both flanks to freely migrate toward the opposite continental margins

late triassic batang group arc volcanic rocks in the northeastern margin of qiangtang terrane northern tibet partial melting of juvenile crust and implications for paleo tethys ocean subduction the batang group btg volcanic rocks in the zhiduo area with nw trending outcrops along the northeastern margin of the qiangtang terrane northern tibet are mainly composed of volcaniclastic rocks dacite and rhyolite major and trace element sr and nd isotope zircon u pb and hf isotope data are presented for the btg dacites laser ablation inductively coupled plasma mass spectrometry zircon u pb dating constrains the timing of volcanic eruption as late triassic 221 1 ma major and trace element geochemistry shows that the btg volcanic rocks are classified as calc alkaline series all samples are enriched in large ion lithophile elements and light rare earth elements with negative slightly positive eu anomalies eu eu 0 47 1 15 and depleted in high field strength elements and heavy rare earth elements in addition these rocks possess less radiogenic sr 87sr 86sr i 0 7047 0 7078 much radiogenic nd end t 4 2 to 1 3 and hf ehf t 4 0 6 6 isotopes suggesting that they probably originated from partial melting of a crustal source containing a mantle derived juvenile component the inferred magma was assimilated by crustal materials during ascending and experienced significant fractional crystallization by combining previously published and the new data we propose that the btg volcanic rocks were genetically related to southwestward subduction of the ganzi litang ocean a branch of paleo tethys in the northeastern margin of the qiangtang terrane given the coeval arc affinity magmatic rocks in the region we envisage that the ganzi litang ocean may extend from the zhongdian arc through the yidun terrane to the zhiduo area probably even further northwest to the tuotuohe area

blueschist bearing metamorphic core complexes in the qiangtang block reveal deep crustal structure of northern tibet a 500 km long belt of metamorphic exposures in the qiangtang block provides an opportunity to study the internal structure of northern tibetan crust metamorphic rocks exposed at two widely separated areas along this belt consist of blueschist bearing melange and are bounded by late triassic early jurassic domal low angle normal faults we propose that this melange was underplated to the qiangtang block and was subsequently exhumed by detachment faulting both the underplating and the exhumation occurred during early mesozoic southward subduction of oceanic lithosphere along the jinsha suture this model predicts that the deeper crust of much of northern tibet consists of accretionary melange in contrast to the continental crystalline crust of southern tibet and may account for north south variations of cenozoic tectonism in tibet

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

new insights about petroleum geology and exploration of qiangtang basin northern tibet china a model for low degree exploration abstract the qiangtang basin is the biggest petroleum bearing basin in the qinghai tibet plateau this basin experienced a foreland basin evolution during the early middle triassic and a rift basin evolution during the late triassic early cretaceous triassic and jurassic hydrocarbon source rocks were widely distributed throughout the basin the triassic tumen gela formation coal bearing mudstones represent the best source rocks because of high total organic carbon toc content 1 25 3 45 and hi values 2 8 123 mg g toc and the xiali formation mudstones are moderately good source rocks with an average toc content ranging from 0 55 to 7 30 and hi values ranging from 7 0 to 165 mg g toc the jurassic buqu formation and suowa formation carbonates however exhibit poor to fair quality as hydrocarbon source rocks excellent dolomite and paleokarst reservoirs and mudstones and bearing evaporite marl cap rocks together with well developed structural traps are recognized in the basin additionally a large paleo oil reservoir zone has also been discovered based on an integrated petroleum systems analysis nine favorable hydrocarbon exploration areas are proposed of which the tuonamu area and badaohu area are selected as the potential targets for the exploration for oil and gas resources in the basin good reservoir quality dolomites in the buqu formation are considered to have a significant exploration potential

tethys tectonic evolution and its bearing on the distribution of important mineral deposits in the sanjiang region sw china abstract the sanjiang region in se tibet plateau and nw yunnan is known to have formed by amalgamation of gondwana derived continental blocks and arc terranes as a result of oceanic subduction followed by continental collision from paleozoic to mesozoic in this paper we provide a synthesis of tectonic evolution magmatism and metallogeny in the region based on data from literatures early paleozoic ophiolites 473 439 ma in the changning menglian belt indicate the existence of a proto tethys ocean in this region two episodes of subduction related magmatism in the early paleozoic one occurred in the baoshan and tengchong blocks at 502 455 ma and the other occurred in the simao block at 421 401 ma are regarded as evidence for two different events of subduction of the proto tethys ocean at different locations the proto tethys was succeeded in early devonian by the paleo tethys which comprised the main ocean and three branches ailaoshan jinshajiang and garze litang the changning menglian main ocean existed from middle devonian to middle triassic the remnants of the oceanic crust are preserved in a few places in the longmu tso shuanghu suture as well as in the changning menglian ophiolite belt the eastward subduction of the main oceanic plate from early permian to early triassic formed a prominent arc terrane stretching 1500 km from yunnan to eastern tibet from the waning stage of subduction to post subduction numerous s type granite plutons with ages varying between 230 and 219 ma such as the lincang batholith in yunnan were emplaced at or close to the suture this event produced several hydrothermal w sn deposits in the region the tectonic evolution and associated magmatism of the jinshajiang and ailaoshan branch oceans are generally comparable to those of the main ocean however the branch oceans were subducted westward instead the garze litang branch ocean also underwent westward subduction from middle devonian to late triassic arc related high sr y porphyry intrusions and associated porphyry skarn cu mo au deposits are common in the jinshajiang ailaoshan region especially in the yidun arc which formed prior to jurassic the vms deposits in the sanjiang region formed in diverse tectonic settings including middle silurian back arc basins carboniferous oceanic islands paleozoic subduction zones and triassic post subduction rifting environments the mesozoic and early cenozoic evolution of the baoshan and tengchong blocks was largely influenced by eastward oceanic subduction of the meso and neo tethys from late permian to middle cretaceous and from late cretaceous to 50 ma respectively abundant early cretaceous granitoids and associated skarn type pb zn and sn fe deposits in the baoshan and tengchong blocks were produced in the background of the shan boundary oceanic slab subduction to the west and the break off of the nujiang bitu oceanic slab to the north the subduction of the neo tethys oceanic plate beneath the tengchong block from late cretaceous to paleogene formed abundant s type granitoids and many skarn type and greisen type sn w deposits granitoids formed at 105 to 81 ma and contemporaneous hydrothermal w mo ag and au deposits which temporally coincided with the subduction of the neo tethys are common in the yidun arc terrane

the jinshajiang ailaoshan suture zone china tectonostratigraphy age and evolution abstract the jinshajiang suture zone is important for enhancing our understanding of the evolution of the paleo tethys and its age tectonic setting and relationship to the ailaoshan suture zone have long been controversial based on integrated tectonic biostratigraphic chemostratigraphic and isotope geochronological studies four tectono stratigraphic units can be recognized in the jinshajiang suture zone the eaqing complex the jinshajiang ophiolitic melange the gajinxueshan group and the zhongxinrong group isotope geochronology indicates that the redefined eaqing complex composed of high grade metamorphic rocks might represent the metamorphic basement of the jinshajiang area or a remnant micro continental fragment eaqing complex protolith rocks are pre devonian and probably of early middle proterozoic age and are correlated with those of the ailaoshan complex two zircon u pb ages of 340 3 and 294 3 ma separately dated from the shusong and xuitui plagiogranites within the ophiolitic assemblage indicate that the jinshajiang oceanic lithosphere formed in latest devonian to earliest carboniferous times the oceanic lithosphere was formed in association with the opening and spreading of the jinshajiang oceanic basin and was contiguous and equivalent to the ailaoshan oceanic lithosphere preserved in the shuanggou ophiolitic melange in the ailaoshan suture zone the latter yielded a u pb age of 362 41 ma from plagiogranite the re defined gajinxueshan and zhongxinrong groups are dated as carboniferous to permian and latest permian to middle triassic respectively on the basis of fossils and u pb dating of basic volcanic interbeds the gajinxueshan group formed in bathyal slope to neritic shelf environments and the zhongxinrong group as bathyal to abyssal turbidites in the jinshajiang ailaoshan back arc basin latest permian earliest middle triassic synorogenic granitoids with ages of 238 18 and 227 5 255 8 ma respectively and an upper triassic overlap molasse sequence indicate a middle triassic age for the jinshajiang ailaoshan suture formed by collision of the changdu simao block with south china

remnants of late permian middle triassic ocean islands in northern tibet implications for the late stage evolution of the paleo tethys ocean abstract in this paper we present new data for the tianquan tq and dabure db ocean islands in the western segment of the longmuco shuanghu lancangjiang suture zone northern tibet including the results of major and trace element analyses zircon u pb dating and hf isotope analyses our aim was to assess the genesis of these ocean islands and to consider the implications for the tectonic evolution of the region as a whole both tq and db retain an ocean island type double layered structure comprising a volcanic basement basalt and andesite and an oceanic sedimentary cover sequence conglomerate limestone and chert the basalts and andesites in the tq and db are enriched in light rare earth elements and high field strength elements nb ta zr hf and ti yielding chondrite normalized ree patterns and primitive mantle normalized trace element patterns that are similar to those of ocean island basalts given the small and generally positive ehf t values of the tq andesites 4 25 to 6 22 and db andesites 0 59 to 1 97 mostly 0 we conclude that the basalts were derived from the partial melting of garnet peridotite in the mantle and that the andesites were formed by fractional crystallization of the mafic parent magma derived from the garnet peridotite mantle the ascending magmas underwent varying degrees of fractional crystallization but were not contaminated by crustal material these features indicate that both tq and db are typical ocean islands that formed in an ocean basin geochemical analyses of cherts from tq and db show that they contain terrigenous material indicating the proximity of a continental margin the andesites of tq contain zircons that yield two u pb ages of 251 ma given that ages of 246 247 and 254 ma had been reported previously we conclude that tq formed during the late permian early triassic the andesites of db contain zircons that yield u pb ages of 242 and 246 ma taking into account the youngest age of 244 ma from the db basalt we conclude that db formed during the middle triassic these data combined with the geological history of the region indicate that the development of the longmuco shuanghu lancangjiang paleo tethys ocean continued after the early permian and that the closure of this ocean was diachronous from east to west the eastern segment of the ocean closed during the early triassic however the western segment remained at least partially open until the middle triassic although the ocean was relatively small at this time the ocean finally closed in the late triassic

redox conditions and climate control on organic matter accumulation and depletion during the toarcian in the qiangtang basin eastern tethys the toarcian early jurassic sediments are regarded as the most significant hydrocarbon source rocks in the qiangtang basin qinghai tibet plateau in this study geochemical and mineralogical data are reported from the middle to late toarcian organic lean sediments in the qiangtang basin eastern tethys combined with previous studies the factors that control organic matter om accumulation during the toarcian are investigated the early toarcian organic rich sediments were deposited in stratified water column with oxygen deficient bottom water which was conducive to om preservation a warm and humid paleoclimatic condition favored the growth of organisms which in turn could enhance the surficial biotic productivity during the middle to late toarcian oxic waterbody was widely distributed in the qiangtang basin this condition was unbeneficial to the preservation of om during this interval a cold arid climate had an adverse effect on organism growth which in turn would reduce the initial productivity of surface water the consistently low ti al and si al ratios revealed restricted but rather homogeneous clastic influx conditions during the toarcian this may suggest that the dilution of terrigenous material input had a little influence on om accumulation in addition the differential enrichment of om between shales and marls should attribute to carbonate dilution these studies emphasize that redox conditions and paleoclimatic conditions were likely responsible for om accumulation during the toarcian

trace elements and their behaviour during the combustion of marine oil shale from changliang mountain northern tibet china the shengli river changshe mountain oil shale zone represents a potentially large marine oil shale resource in china with the aim of better understanding the geochemistry of trace elements in marine oil shale and its combustion residues 40 raw samples 27 oil shale combustion residues and 29 selected minerals from changliang mountain oil shale are studied for geochemical analyses the contents of se cd mo as cs pb sr and u in the oil shale samples are enriched from 1 47 to 33 91 times as compared with the clarke values whereas the concentrations of other elements are slightly higher lower than the respective worldwide means the most enriched elements in oil shale combustion residues are se cd mo as and u with enrichment values from 4 78 to 50 92 trace elements with high volatile behaviour such as as co ni sc sn and v occur predominantly in organic matter and or sulphides other non volatile or slightly volatile trace elements e g be bi cs cu ga hf li nb rb ta th w zr and rees may occur mainly in original and relatively refractory minerals in raw oil shale the potentially hazardous trace elements in changliang mountain oil shale include as cd mo and se arsenic and se are controlled mainly by fe bearing minerals probably pyrite in changliang mountain oil shale cadmium is present mainly in dolomite while mo occurs mainly in organic matter

mayer kangri metamorphic complexes in central qiangtang tibet western china implications for the triassic early jurassic tectonics associated with the paleo tethys ocean the mesozoic orogeny in central qiangtang metamorphic belt northern tibet provides important insights into the geological evolution of the paleo tethys ocean however the triassic early jurassic tectonics particularly those associated with the continental collisionstage remains poorly constrained here we present results from geological mapping structural analysis p t data and ar ar geochronology of the mayer kangri metamorphic complex our data reveal an e w trending 2 km wide dome like structure associated with four successive tectonic events during the middle triassic and early jurassic field observations indicate that amphibolite and phengite schist complexes in this complex are separated from the overlying lower greenschist melange by normal faulting with an evident dextral shearing component open antiform like s2 foliation of the footwall phengite schist truncates the approximately north dipping structures of the overlying melange microtextures and mineral chemistry of amphibole reveal three stages of growth geothermobarometric estimates yield temperatures and pressures of 524 c and 0 88 gpa for pargasite cores 386 c and 0 34 gpa for actinolite mantles and 404 c and 0 76 gpa for winchite rims peak blueschist metamorphism in the phengite schist occurred at 0 7 1 1 gpa and 400 c our ar ar dating of amphibole reveals rim ward decreasing in age bands including 242 4 241 2 ma 202 6 196 8 and 192 9 189 8 ma the results provide evidence for four distinct phases of mesozoic tectonic evolution in central qiangtang 1 northward oceanic subduction beneath north qiangtang 244 220 ma 2 syn collisional slab break off 223 202 ma 3 early collisional extension driven by buoyant extrusion flow from depth 202 6 197 ma and 4 post collision contraction and reburial 195 6 188 7 ma

chemical evolution of ore forming brines basement leaching metal provenance and the redox link between barren and ore bearing hydrothermal veins a case study from the schwarzwald mining district in sw germany abstract six mineralogically exemplary barren and mineralized hydrothermal veins with pb and zn ores of jurassic cretaceous and cenozoic age in the schwarzwald mining district sw germany were chosen to shed light on the origin of their mineralogical diversity the selection of the veins was guided by the fact that they represent the largest number of veins in the region are very well known mineralogically and geochemically and they provide nice examples of barren and mineralized veins of similar age fluid inclusion data from the individual veins overlap implying their diverse mineralogy is not caused by different fluid compositions participating during fluid mixing la icpms data of single fluid inclusions indicate no systematic variations in major elements but significant changes in fluid mixing ratios which caused variable trace element concentrations of different fluid inclusion assemblages in one sample we suggest that different ore gangue assemblages can be produced by mixing of identical fluid endmembers but variable mixing ratios la icpms analyses of single fluid inclusions in barren and mineralized veins record similar base metal and sulfur concentrations hence sulfide mineralization in the veins appears not to be controlled by metal solubility thermodynamic modeling based on the fluid data indicates that the presence of a reducing phase during fluid mixing is required for sulfide mineralization to precipitate la icpms trace element analyses of feldspars biotites chlorites and clay minerals in granites and paragneisses which are the source of the metal content in the ore forming fluids demonstrate that the dominant provenance for pb zn as sb ba tl mo fe and mn are probably paragneisses while co cu and ni are probably sourced from s type granite a rough quantification indicates that

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

sedimentary and tectonic evolution of the southern qiangtang basin implications for the lhasa qiangtang collision timing the mesozoic stratigraphic record of the southern qiangtang basin in central tibet records the evolution and closure of the bangong nujiang ocean to the south the jurassic succession includes toarcian aalenian shallow marine limestones quse formation aalenian bajocian feldspatho litho quartzose to feldspatho quartzo lithic sandstones shallow marine sewa formation and deep sea gaaco formation and bathonian outer platform to shoal limestones buqu formation this succession is truncated by an angular unconformity overlain by upper bathonian to lower callovian fan delta conglomerates and litho quartzose to quartzo lithic sandstones biluoco formation and callovian shoal to outer platform limestones suowa formation sandstone petrography coupled with detrital zircon u pb and hf isotope analysis indicate that the sewa and gaaco formations contain intermediate to felsic volcanic detritus and youngest detrital zircons 183 170 ma with ehf t ranging widely from 13 to 25 pointing to continental arc provenance from igneous rocks with mixed mantle and continental crust contributions an arc trench system thus developed towards the end of the early jurassic with the southern qiangtang basin representing the forearc basin above the angular unconformity the biluoco formation documents a change to dominant sedimentary detritus including old detrital zircons mainly 500 ma ages in the lower part of the unit with age spectra similar to those from paleozoic strata in the central qiangtang area a major tectonic event with intense folding and thrusting thus took place in late bathonian time 166 1 ma when the qiangtang block collided with another microcontinental block possibly the lhasa block

magmatic expression of tectonic transition from oceanic subduction to continental collision insights from the middle triassic rhyolites of the north qiangtang block abstract the tectonic transition from subduction to collision is a fundamental process during orogenesis yet the magmatic expression of this transition and related deep geodynamic processes remain unclear this study focuses on a newly identified volcanic belt within the moyun zaduo sulu area of the north qiangtang block and presents new zircon u pb data that indicate that this belt formed during the middle triassic 247 241 ma a time characterized by a regional transition from subduction to collisional tectonism the volcanic belt is located to the south of a permian to early triassic arc and is dominated by high k calc alkaline and peraluminous rhyolites these rhyolites have low mg nb ta and eu values contain low contents of sr have high rb sr and whole rock end t values and show positive zircon ehf t values all of which suggest that they formed from magmas generated by the dehydration melting of juvenile crustal material the migration of middle triassic volcanism in this region was most likely caused by rollback of the subducting longmucuo shuanghu tethyan oceanic slab combining our new data with previously published results of numerical modeling of subduction collisional processes and regional data from north central tibet yields insights into the magmatic expressions and related deep geodynamics of the transition from oceanic subduction to continental collision this combination of data also suggests that variations in oxygen fugacity can be used as a proxy for the discrimination of magmatism related to subduction the transition from subduction to collision and collisional tectonism

reply to the comment by zhou et al on revisiting the yanbian terrane implications for neoproterozoic tectonic evolution of the western yangtze block south china precambrian res 151 2006 14 30 precambrian res 154 2007 153 157

minerals and potentially hazardous trace elements in the late triassic coals from the qiangtang basin china abstract the tumen coal mine has the largest coal resources in tibet which have been developed and utilized as an important energy source in the last decades it has raised some health problems during the coal exploration and utilization in tibet fifty late triassic coal or coaly samples were picked up from the qiangtang basin tumen mine woruoshan and hongshuihe areas to determine the minerals potentially hazardous elements and their mode of occurrence and possible sources coal samples from the qiangtang basin have medium and high ash yields 15 20 47 88 with low or medium high total sulfur content 0 04 4 86 minerals in qiangtang basin coal include clay minerals dolomite quartz pyrite siderite and hematite and trace amounts of halite feldspar anhydrite barite chromite and galena potentially hazardous trace elements in coal samples include as hg pb and se arsenic is controlled mainly by aluminosilicate minerals mercury occurs mainly as an organic bound form the organically bound pb is dominant selenium is controlled mainly by fe bearing probably pyrite a proportion of the elevated concentrations of as hg pb and se in qiangtang basin coal may be related to underlying shale bed these elements might have been eroded or leached from the shale bed which were subsequently transported and accumulated syngenetically in the coal forming peat swamps the enrichment of as hg pb and se in qiangtang basin coal however is also partly to be related to magmatic hydrothermal fluids

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

middle triassic volcanic rocks in the northern qiangtang central tibet geochronology petrogenesis and tectonic implications abstract although voluminous magmatism occurred during the triassic in the qiangtang terrane the petrogenesis of the volcanic rocks and the associated tectonic scenarios remain mysterious this study focuses on new identified primitive volcanic rocks from the yanshiping area northern qiangtang subterrane the whole rock major trace elemental sr nd isotopic data and zircon u pb age of volcanic rocks are reported in this paper in order to understand their petrogenesis and tectonic setting the studied volcanic rocks can be grouped into two types i e nb enriched basalts and basaltic andesites nebs in the low sequence and arc basalts in the upper sequence zircon u pb dating using la icp ms techniques yields the concordant age with a weighted mean 206 pb 238 u age of ca 242 241 ma for the nebs and ca 240 ma for the arc basalts the distinct geochemical and isotopic characteristics of whole rock and varying th u ree components of zircon grains from the volcanic rocks suggest that the nebs were derived from partial melting of relatively enriched mantle wedge that have been metasomatised by slab related melts and that the arc basalts originated from partial melting of mixing of mantle wedge and depleted asthenospheric mantle in response to the slab breakoff our new geochemical and geochronological results in combination with regional studies imply that the middle triassic magmatism was generated in the tectonic setting of northward subduction of the paleo tethys or bangong nujiang ocean subduction related processes that include melting of the slab and the slab breakoff during the permian middle triassic made an important contribution to crustal growth in the qiangtang terrane whereas vertical crustal growth associated with collision related setting during the late triassic is nonsignificant

petrogenesis and tectonic setting of triassic granitoids in the qiangtang terrane central tibet evidence from u pb ages petrochemistry and sr nd hf isotopes abstract triassic granitoids including the 220 ma shuanghu and 210 ma rongma granitoids studied here are widely distributed around the longmu shuanghu suture and in the qiangtang terrane central tibet the majority of these granitoids can be classified as high k calc alkaline in nature and yield negative ba and sr anomalies on primitive mantle normalized diagrams in addition they are enriched in light rare earth elements lree la yb n 1 61 21 79 strongly enriched in large ion lithophile elements lile e g cs rb and k and depleted in high field strength elements hfse e g nb and ti magma mixing played a role in the genesis of the shuanghu granodiorites as indicated by the occurrence of dioritic enclaves and the wide range in zircon hf compositions ehf t 15 0 to 2 5 the i type shuanghu granodiorites and s type shuanghu and rongma granites might have been derived from melting of southern qiangtang crust given the high initial sr 0 7131 0 7272 low end t 8 9 to 11 1 and zircon ehf t values 15 to 7 2 the granitoids may have formed during melting of southern qiangtang crust heated by upwelling asthenosphere mantle a result of break off and delamination of the paleo tethys slab in a collisional setting

mineralogical composition of and trace element accumulation in lower toarcian anoxic sediments a case study from the bilong co oil shale eastern tethys the sediments of organic rich oil shales in the bilong co area can be correlated with those of the early toarcian anoxic black shale events in europe the bilong co sediments are rich in trace elements se mo cd as and ni and to a lesser extent li f v co cu cs hg and bi in comparison to the upper continental crust thirty two oil shale samples were collected from the bilong co oil shale to evaluate the controlling factors of trace element enrichment in the lower toarcian anoxic sediments minerals identified in the bilong co oil shale include calcite quartz illite feldspar and dolomite and trace amounts of siderite magnesite halite haematite zeolite amphibole gypsum anhydrite apatite pyrite sphalerite barite and mixed layer illite smectite mineralogical and geochemical data show that seawater and hydrothermal activities are the dominant influences on the mineralogical composition and elevated trace element concentrations in the oil shale the clay minerals quartz and feldspar in the bilong co oil shale were derived from the nadi kangri volcanic rocks input of sediment from this source may have led to enrichment of trace elements li cr and cs in the oil shale carbonate minerals and nodular and framboidal pyrite are authigenic phases formed from seawater the enrichment of v co ni cu mo as se bi and u in the oil shale was owing to marine influence barite sphalerite and fracture filling pyrites were derived from hydrothermal solutions high concentrations of f zn and cd were probably derived from hydrothermal fluids

minerals and potentially hazardous trace elements in marine oil shale new insights from the shengli river north surface mine northern tibet china the shengli river changshe mountain oil shale zone including the changliang mountain shengli river oil shale the shengli river north oil shale and the changshe mountain oil shale represents potentially the largest marine oil shale resource in china new geochemical data from the shengli river north oil shale are presented to evaluate potentially hazardous trace elements in oil shale and to discuss possible factors controlling the geochemical anomalies oil shale samples from the shengli river north area are characterized by high ash yield 59 25 66 27 and toc content 4 77 9 35 with low total sulfur content 0 1 1 56 and intermediate shale oil content 3 60 16 30 minerals in the shengli river north oil shale include calcite clay minerals quartz dolomite and fe bearing minerals e g pyrite limonite and hematite and trace amounts of zeolite apatite and barite potentially hazardous trace elements in oil shale samples include as se mo f ni cd and hg arsenic is present probably in both organic and inorganic matter selenium is controlled mainly by fe bearing probably pyrite while f and cd occur mainly as apatite the organically bound mo and ni are dominant and hg probably occurs as a substitution in the structure of pyrite apatite and calcite the elevated concentrations of f and cd in the shengli river north oil shale may be related to sediment source water chemistry may have played an important role for ni and mo enrichments while the enrichment of se in the oil shale samples is possibly related to hydrothermal fluids other elements including hg and as are probably related to several factors

organic accumulation in lacustrine rift basin constraints from mineralogical and multiple geochemical proxies the lunpola basin in the central tibet is the highest petroliferous basin in the world the basin is a cenozoic lacustrine rift basin with widespread oil shale depositions which provide a valuable example for understanding the mechanisms of organic accumulation thirty five samples of oil shale and mudstone were collected from one of the organic rich oil shale intervals in the lunpola basin to discuss the controlling factors of organic enrichment in the investigated oil shale interval many redox proxies including v v ni ratios mo u covariations and pyrite framboid size distribution suggest anoxic conditions however the almost consistent variation patterns of toc p and mo contents indicate that organic matter accumulation was controlled mainly by primary productivity the relatively low sr ba average 0 74 and sr cu ratios average 4 75 suggest a warm humid climate during the oil shale deposition these conditions are suitable for living of organisms which would raise the initial productivity of lake and in turn could have led to the oxygen depleted conditions the detrital input together with fast sedimentary rate during the oil shale deposition would result in dilution of organic matter

geochemical characteristics redox conditions and organic matter accumulation of marine oil shale from the changliang mountain area northern tibet china abstract the organic rich changliang mountain oil shale located in the north qiangtang depression northern tibet is considered to be excellent mineral resource in china the changliang mountain oil shale reported here was deposited in a tidal flat lagoon environment and is characterized by black thin layered oil shales intercalated with dark gray marls here we present geochemical data from the changliang mountain oil shale profile in order to investigate the mechanism of organic matter om accumulation and to establish the formation model for the marine oil shale deposition total organic carbon toc values range from 2 96 to 23 47 in the oil shale samples while the marl samples contains low toc contents ranging from 0 06 to 0 21 in the organic rich oil shale sediments many redox indicators including mo al ratios v cr ratios th u ratios ni co ratios and the relationship of mo to toc suggest a deposition under dysoxic to anoxic environments subsequently the bottom water evolved into an oxic water body when the organic poor marls were deposited however the negative and or weak relationship between toc and productivity indices p ti and ba al indicates that om accumulation was not controlled mainly by primary productivity but dysoxic anoxic bottom water environment a stratified water column may be initiated by the supply of fresh water from the continent nearby combining with warm and humid climate which is beneficial to the reproduction of marine organisms the death and bury of these organisms could lead to the formation of dysoxic anoxic bottom waters and enhance the preservation of om in this study a preservation model of the changliang mountain oil shale was established the model implies that excellent preservation is the major controlling factor for om enrichment in the oil shale layer in addition factors such as mixed deposition with clay minerals and detrital matter input cannot be ignored for their influence on om enrichment