melting of newly formed mafic crust for the formation of neoproterozoic i type granite in the hannan region south china abstract neoproterozoic magmatism in the hannan region at the northwestern margin of the yangtze block is characterized by numerous felsic plutons associated with minor mafic ultramafic intrusions the felsic plutons are either adakitic or normal arc granitic in composition the adakitic plutons are 735 ma in age and are interpreted as having formed by partial melting of a thickened lower mafic crust among the normal arc related felsic plutons the tianpinghe pluton is the largest and has a shrimp zircon u pb age of 762 4 ma older than the adakitic plutons in the region rocks from the tianpinghe pluton have relatively high sio2 67 1 70 1 wt and k2o na2o 7 8 8 6 wt and relatively low mgo 0 7 1 3 wt and al2o3 contents 14 5 15 6 wt with al2o3 cao k2o na2o a cnk values ranging from 0 95 to 1 08 they have arc affinity trace element compositions that are characterized by enrichment of large ion lithophile elements and depletion of high field strength elements nb ta with strong

neoproterozoic adakitic plutons in the northern margin of the yangtze block china partial melting of a thickened lower crust and implications for secular crustal evolution abstract numerous neoproterozoic felsic and mafic ultramafic intrusions occur in the hannan region at the northern margin of the yangtze block among these the wudumen and erliba plutons consist of granodiorites and have shrimp zircon u pb ages of 735 ma the rocks have high k 2 o 0 8 3 6 wt and na 2 o 4 4 6 4 wt and low mgo 0 4 1 7 wt they also have high sr y 32 209 and la yb n ratios 4 4 38 6 their e nd values range from 0 41 to 0 92 and zircon initial 176 hf 177 hf ratios from 0 282353 to 0 282581 these geochemical features are similar to those of adakitic rocks produced by partial melting of a thickened lower crust our new analytical results combined with the occurrence of voluminous arc related mafic ultramafic intrusions emplaced before 740 ma lead us to propose that the crustal evolution in the northern margin of the yangtze block during neoproterozoic involved 1 rapid crustal growth and thickening by underplating of mafic magmas from the mantle which was modified by materials coming from the subducting oceanic slab from 1 0 to 0 74 ga and 2 partial melting of the thickened lower crust due to a thermal anomaly induced by upwelling of asthenosphere through an oceanic slab window producing the 735 ma adakitic wudumen and erliba plutons our model suggests that the crustal thickness was more than 50 km at the northern margin of the yangtze block at 735 ma and rule out the possibility of a mantle plume impact causing the 735 ma magmatism in the region

crustal deformation of the eastern tibetan plateau revealed by magnetotelluric imaging the ongoing collision of the indian and asian continents has created the himalaya and tibetan plateau through a range of deformation processes these include crustal thickening detachment of the lower lithosphere from the plate delamination and flow in a weakened lower crust 1 6 debate continues as to which of these processes are most significant 7 in eastern tibet large scale motion of the surface occurs but the nature of deformation at depth remains unresolved a large scale crustal flow channel has been proposed as an explanation for regional uplift in eastern tibet 6 8 9 but existing geophysical data 10 11 do not constrain the pattern of flow magnetotellurics uses naturally occurring electromagnetic waves to image the earth s subsurface here we present magnetotelluric data that image two major zones or channels of high electrical conductivity at a depth of 20 40 km the channels extend horizontally more than 800 km from the tibetan plateau into southwest china the electrical properties of the channels imply an elevated fluid content consistent with a weak crust 12 13 that permits flow on a geological timescale these findings support the hypothesis that crustal flow can occur in orogenic belts and contribute to uplift of plateaux our results reveal the previously unknown complexities of these patterns of crustal flow many previous studies of the indiaasia

structural style and metamorphic conditions of the jinshajiang metamorphic belt nature of the paleo jinshajiang orogenic belt in the eastern tibetan plateau abstract the jinshajiang metamorphic belt is a barrovian sequence distributed within the jinshajiang suture zone jsz which was formed in the thickened paleo jinshajiang orogenic belt after closure of the jinshajiang ocean we have identified three metamorphic phases m 1 m 2 and m 3 corresponding to deformation stages d 1 d 2 and d 3 in the barrovian sequences the metamorphic belt exhibits a metamorphic field gradient from chlorite to biotite garnet staurolite kyanite and sillimanite grades inclusions in garnet and staurolite chlorite mica quartz feldspar ilmenite and graphite indicate that m 1 reflects greenschist facies metamorphism pervasive m 2 metamorphism formed a dominant s 1 schistosity within the barrovian sequence peak metamorphic conditions for metapelites of the garnet staurolite and staurolite kyanite grade were 580 c and 0 65 gpa according to petrogenetic grids peak metamorphism was in conditions of 635 c and 0 50 gpa for the metapelites and 650 c and 0 61 gpa for amphibolites in sillimanite grade greenschist facies retrograde metamorphism m 3 followed d 2 deformation a top down to southeast shear in the jsz the d 3 deformation is characterized by well developed brittle faults with fault gouge and breccia zircon grains from an amphibolite sample have cores with igneous oscillatory zoning and metamorphic rims however the metamorphic rims are too narrow to analyze laser ablation inductively coupled plasma mass spectrometry la icpms analyses of the igneous cores of zircons yielded a crystallization age of 242 ma 40 ar 39 ar dating of white mica from a garnet schist gave a plateau age of 224 ma the peak metamorphism is thus limited to be between 242 and 224 ma we thus suggest a tectonic shift from collision to extension of for the permian to triassic paleo jinshajiang orogenic belt

proximal provenance of the western songpan ganzi turbidite complex late triassic eastern tibetan plateau implications for the tectonic amalgamation of china abstract the late triassic songpan ganzi turbidite complex on the eastern tibetan plateau which covers an area of 2 2 10 5 km 2 is one of the largest flysch turbiditic basins on earth it is juxtaposed with major chinese continental blocks across several outstanding tethyan sutures and is critical to elucidating the tectonic amalgamation of china however the provenance of the turbidites remains the subject of intense debate detrital modes and heavy mineral spectra of sandstone samples from three turbidite profiles in the western songpan ganzi complex were determined in an attempt to elucidate their provenance and the type of tectonic setting in which they were deposited upper triassic turbidites in the western songpan ganzi complex have an overwhelming derivation from an orogen source as shown by the average framework compositions of the sandstones q 57 f 21 l 22 qm 52 f 21 lt 27 qp 23 lvm 0 lsm 77 the compositional and textural immaturity of the sandstones suggests limited transport and nearby sources both light and heavy mineral and lithic fractions indicate dominant metamorphic source rocks and subordinate ophiolitic and sedimentary source rocks the occurrence of c type garnet omphacite rutile and si rutile and the high silica content of phengites reveal that the source rocks underwent ultrahigh pressure conditions therefore we advocate that the western songpan ganzi late triassic turbidites were derived from nearby central qiangtang triassic collisional orogen sources rather than distant dabie qinling sources eroded after north china south china collision as previously proposed the analogy of the songpan ganzi turbidites to the bengal indus fans is not favored the central qiangtang metamorphic belt is unlikely to have occurred as an early mesozoic melange underthrust from the jinsa suture and is more likely to have been an in situ triassic ultrahigh pressure orogen our provenance interpretation implies that the paleotethys represented by the present jinsa and kunlun sutures was not yet closed when central qiangtang was being exhumed to supply the sediments of the songpan ganzi basin this challenges the conventional model for sequential amalgamation of china during the phanerozoic

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

palaeomagnetic constraints on the collision and rotation of north and south china the eastern half of china is dominated by the north china block ncb and south china block scb separated by a complex fold system that trends roughly east west interpretation of the geology of this fold belt has led to a variety of estimates of the age of amalgamation of these two blocks1 7 palaeomagnetic results are few and have also been subject to conflicting interpretations8 11 here we present our own palaeomagnetic results for late permian rocks for both the scb and ncb and summarize other relevant palaeomagnetic data by reinterpreting the polarity given in some previous studies we are able to synthesize the results for the widespread emeishan basalts of the scb and to suggest a new model for the collision and relative rotation of the two blocks

indosinian deformation of the songpan garze fold belt northeast tibetan plateau abstract deformation of the songpan garze fold belt southwest china occurred progressively during the indosinian orogeny ca 230 200 ma two phases of south directed deformation are identified d 1 associated with pervasive thickening of the songpan garzi sedimentary pile and detachment of that portion of the fold belt onlapping the yangtze passive margin and d 2 a localized phase of deformation associated with transpressive shortening of the fold belt s southeast margin an arcuate form surface in the southeast fold belt reflects the distribution of d 1 and d 2 structural phases and the diachronous migration of these phases into the fold belt this migration accommodated lateral growth and cumulative sinistral transpression within the fold belt s eastern margin an enigmatic metamorphic complex the so called danba antiform is constrained to have formed during the early stages of the d 2 deformation this was associated with diapiric upwelling of buoyant migmatised basement and domal distortion of the overlying metasedimentary pile

metasomatic mantle source and crustal contamination for the formation of the neoproterozoic mafic dike swarm in the northern yangtze block south china abstract in the huangling region of the northern yangtze block south china the 820 ma huangling felsic pluton was intruded by 800 ma mafic dikes these dikes are composed of fine to medium grained diabase porphyritic diabase and olivine gabbro quartz xenocrysts occur locally in the olivine gabbro the rocks are alkaline to sub alkaline in composition and have sio2 ranging from 43 3 to 57 2 wt mgo from 3 7 to 9 2 wt and total alkali from 3 0 to 7 3 wt they exhibit lree enrichment la yb n 3 3 28 5 with slightly negative eu anomalies eu eu 0 6 1 0 their primitive mantle normalized trace element patterns are characterized by enrichment of large ion lithophile elements rb ba th and u depletion of high field strength elements nb ta zr and hf and positive pb and negative ti anomalies they have relatively high initial 87sr 86sr ratios ranging from 0 7052 to 0 7080 and variably negative end t values from 4 2 to 10 9 the rocks have a large range of whole rock ehf values 0 35 to 11 34 and a relatively narrow range of pb isotope ratios 206pb 204pb 16 96 to 17 97 207pb 204pb 15 41 to 15 58 and 208pb 204pb 38 08 to 39 66 initial 87sr 86sr ratios correlate negatively with 206pb 204pb ratios whereas 143nd 144nd and 208pb 204pb ratios and ehf values correlate positively with 206pb 204pb ratios these features are indicative of an em2 source region suggesting a lithospheric mantle source modified by subducted components during the neoproterozoic the quartz xenocrysts and linear isotopic variations indicate strong crustal contamination during magma emplacement the energy constrained assimilation and fractional crystallization ec afc modeling calculations reveal that the parental magmas were contaminated by continental crust including archaean amphibolites granitic gneisses metasedimentary rocks and granitic rocks the mafic dikes are 20 myrs younger than the huangling felsic pluton which was produced by partial melting of a thickened continental crust suggesting that the mafic dikes were a consequence of post orogenic extension the huangling region may have originally been located at the centre of the yangtze block the possible heat source for the generation of the huangling felsic pluton and mafic dikes was asthenospheric wedging resulting from oceanic slab subduction beneath the northern margin of the yangtze block this conclusion rules out the possibility that the yangtze block was located in the central part of the rodinia supercontinent the block was more likely situated along the margin of rodinia

geochemistry of the mian lue ophiolites in the qinling mountains central china constraints on the evolution of the qinling orogenic belt and collision of the north and south china cratons abstract the anzishan ophiolite a typical ophiolitic block of early carboniferous age in the mian lue suture zone of the qinling mountains central china consists of amphibolites metabasalts gabbros and gabbroic cumulates all of these rocks as well as those in the hunshuiguan zhuangke hz block have compositions similar to normal morb and back arc basin basalts babb with high end t values indicating that they were derived from a depleted mantle source the mian lue suture zone also contains blocks of other lithologies e g rift volcanic rocks in the heigouxia block and arc volcanic rocks in the sanchazi block although they are in fault contact with each other the presence of these different blocks in the mian lue suture zone may represent a complete wilson cycle from initial rifting to open ocean basin to final subduction and continent continent collision during the late paleozoic early triassic in this region the north and south china cratons were separated by paleo tethys at least until the early carboniferous and final amalgamation of both cratons along the qinling orogenic belt took place in the triassic

two phase growth of high topography in eastern tibet during the cenozoic high topography in eastern tibet is thought to have formed in response to weak lower crust flowing towards the plateau margin thermochronologic analyses of rocks exposed at the eastern plateau margin record periods of mountain growth early in the indo asian collision implying that crustal flow alone could not have created the high topography

tectonometamorphic evolution of the eastern tibet plateau evidence from the central songpan garze orogenic belt western china the songpan garze orogenic belt northeastern tibet plateau introduction experienced polyphase deformation and metamorphism that is best the songpan garze orogenic belt sgob occupies developed in the danba domal metamorphic terrane ddmt the northeastern portion of the tibet plateau fig 1a in which three tectonometamorphic events can be identified the first which comprises the tectonically distinct tethyan event d1 m1 is characterized by prograde sericiteto kyanite grade himalayan domain between india and eurasia and is barrovian metamorphism during late indosinian 205 190 ma characterized by a polyphase continent continent colcrustal thickening and shortening a subsequent early yanshanian lisional history dewey et al 1988 the tibet plateau 165 ma sillimaniteto migmatite grade event m2 developed has been the subject of a number of studies which have during predominantly east west compression d2 a final greenfocused on the himalayas themselves and on the major schist facies event m3 is best developed in shear zones of probable strike slip faults within or fault zones along the margin himalayan age p t conditions during m1 varied from 3 5 of the tibet plateau e g dirks et al 1994 roger et al kbar and 410 530 c biotite zone to 5 3 8 kbar and 1995 arne et al 1997 meng zhang 1999 in 570 600 c staurolite and kyanite zones and during m2 from comparison little work has focused on the interior of the 4 8 6 3 kbar and 640 680 c sillimanite zone to 5 8 6 2 sgob as the sgob resulted from the closure of the kbar and 660 725 c migmatite zone clockwise p t t segpalaeo tethys and subsequent continental collision and ments were inferred for the staurolite kyanite and sillimanite zones convergence associated with interaction between the timuscovite dehydration melting during m2 was largely responsible bet indochina south china and north china blocks for the generation of migmatites and locally voluminous pegmatites xu et al 1992 the multiphase deformational and metathe polyphase tectonometamorphic evolution of the eastern tibet morphic evolution of the sgob is critical for underplateau as documented in the danba area resulted from interactions standing the mesozoic tertiary tectonometamorphic between the indian tibet and the south and north china blocks evolution of the tibet plateau the eastern tibet plateau experienced limited uplift during the metamorphic studies of the sgob are hampered by its mesozoic followed by large scale uplift and rapid cooling during generally low grade lower greenschist facies however the tertiary himalayan orogeny poorly documented medium pressure barrovian type metamorphic complexes as high as upper amphibolite grade occur locally and are associated with basementcored structural domes mattauer et al 1992 the best examples are the danba domal metamorphic terrane

crustal response to continental collisions between the tibet indian south china and north china blocks geochronological constraints from the songpan garze orogenic belt western china we report shrimp u th pb monazite conventional u pb titanite sm nd garnet and rb sr muscovite and biotite ages for metamorphic rocks from the danba domal metamorphic terrane in the eastern songpan garze orogenic belt eastern tibet plateau these ages are used to determine the timing of polyphase metamorphic events and the subsequent cooling history the oldest u th pb monazite and sm nd garnet ages constrain an early barrovian metamorphism m1 in the interval c 204 190 ma coincident with extensive indosinian granitic magmatism throughout the songpan garze orogenic belt a second higher grade sillimanite grade metamorphic event m2 recorded only in the northern part of the danba terrane was dated at c 168 158 ma by a combination of u th pb monazite and titanite and sm nd garnet ages it is suggested that m1 was a thermal event that affected the entire orogenic belt while m2 may represent a local thermal perturbation rb sr muscovite ages range from c 138 100 ma whereas rb sr biotite ages cluster at c 34 24 ma these ages document regional cooling at rates of c 2 3 c myr 1 following the m1 peak for most of the terrane however those parts of the terrane affected by the higher temperature m2 event e g the migmatite zone experienced initially more rapid c 8 c myr 1 cooling after peak m2 before joining the regional slow cooling path defined by the rest of the terrane at c 138 ma regional slow cooling between c 138 and c 30 ma is thought to be the result of post tectonic isostatic uplift after extensive crustal thickening caused by collision of the south and north china blocks the clustering of biotite rb sr ages marks the onset of rapid uplift across the entire terrane commencing at c 30 20 ma this cooling history is shared with many other regions of the tibet plateau suggesting that uplift of the tibet plateau including the songpan garze orogenic belt occurred predominantly in the last c 30 myr as a response to the continuing northwards collision of india with eurasia

petrogenesis and tectonic significance of the baoxing granitic and mafic intrusions southwestern china evidence from zircon u pb dating and lu hf isotopes and whole rock geochemistry abstract voluminous neoproterozoic gabbros gabbro diorites and granites are exposed in the baoxing area along the western margin of the yangtze block we present here zircon u pb and lu hf isotope data as well as whole rock geochemical data for these rocks which help constrain the tectonic characteristics and evolution of the western margin of the yangtze block zircons from these rocks have high th and u contents and th u ratios 0 2 and show striped absorption patterns growth striations or zoning indicating a typical magmatic origin zircon u pb dating and lu hf isotope results show that the gabbros formed at ca 850 ma with e hf 5 44 to 11 94 and a single stage model age t hf of 1 04 ga suggesting a depleted mantle derived source the gabbro diorites formed at ca 800 ma with major e hf 3 99 to 14 62 and t hf 0 97 ga and minor e hf 0 68 to 5 66 and t hf 1 27 ga again suggesting a depleted mantle derived source the granites were emplaced at ca 770 ma with e hf 2 53 to 10 58 and t hf 1 07 ga indicating that they were derived from the partial melting of juvenile crust the geochemical data show that the gabbros and gabbro diorites are characterized by low sio 2 contents high mg values and cr co and ni contents significant enrichment in light rare earth elements and large ion lithophile elements depletion in high field strength elements e g nb ta zr and hf and relatively high nb u ta u and ce pb ratios these data suggest that the rocks were derived from the partial melting of depleted lithospheric mantle that had been metasomatized by subduction derived fluids or melts and which underwent very minimal crustal contamination during magma evolution and ascent the granites have high sio 2 and k 2 o and low mgo contents exhibit enrichment in rb ba th and pb and depletion in nb ta and ti and have low sr y and la yb n ratios and high y and yb contents which together with their hf isotopic compositions imply a juvenile crust source taking into account regional pre 750 ma arc related magmatism and the subsequent extensional setting it is proposed that a long lived active continental margin setting may have evolved along the western margin of the yangtze block during the neoproterozoic and that the coeval granitic and mafic rocks in the baoxing area were formed by the subduction of the western yangtze block

mesozoic cenozoic evolution of the danba dome songpan garze east tibet as inferred from la icpms u pb and fission track data abstract the eastern margin of the tibetan plateau is a key region to understand the formation and tectonic evolution of the plateau most studies concentrated on the longmen shan range that corresponds to the tectonically very active boundary between the tibetan plateau and the sichuan basin this work addresses the deformation and exhumation pattern in the poorly understood metamorphic danba dome situated at the conjunction between the southern longmen shan range and the major strike slip xian shui he fault the early mesozoic and cenozoic evolution of the dome are then compared to the regional evolution of eastern tibet the about 830 ma zircon u pb ages obtained on the gezong and gongcai gneiss as well as u pb ages on inherited zircons in the early mesozoic granites confirm the occurrence of neo proterozoic magmatic episodes in the danba basement at 800 900 ma and 650 750 ma following the closure of the palaeotethys ocean granitoids were emplaced in the danba metamorphic basement and triassic cover from about 230 ma to about 170 ma the western part of the lian he kau granitic massif emplaced between 210 and 200 ma within the triassic cover sequence of the dome was only slightly affected by the early mesozoic deformation however in the eastern deeper level compartment of the massif ductile deformation occurred between 200 and 190 ma contemporaneous with the onset of the barrovian type metamorphism dated at 200 180 ma that deformation is compatible with the early mesozoic deformation observed in the longmen shan range and with the development of the large scale decollement level in the songpan garze area apatite fission track data then indicate that following the oligocene middle eocene onset of eastward motion of the songpan garze terrane the danba dome developed as a crustal scale antiform during the late miocene early pliocene 10 4 ma the exhumation was controlled by the lian he kau fault a major transpressive structure parallel to the xian shui he fault and the associated manai thrust faults regionally while the oligocene middle miocene exhumation phase seems to have been of regional extent the late miocene to present day phase is localised along the longmen shan range thrusts as well as along the back thrusts formed by the lian he kau manai and associated faults

brittle modification of triassic architecture in eastern tibet implications for the construction of the cenozoic plateau abstract in the songpan ganzi fold belt east tibetan plateau post triassic deformation is localised within northwest trending sinistral and southwest trending dextral brittle fault zones between these zones large tracts of the fold belt preserving late triassic indosinian architecture have been transported effectively intact this transport was almost exclusively strike slip and did not accommodate significant differential exhumation or uplift right dihedra paleostress reconstructions from fault plane kinematic data indicate a rotation of the principal compressive stress from ne sw trending in the north to a dominant e w trend throughout the eastern songpan ganzi fold belt exceptions occur adjacent to competent proterozoic basement complexes and other rigid bodies these stress solutions are consistent with a compressive stress regime that radiates about the eastern himalayan syntaxis a secondary n s compressive paleostress identified in southern regions may record late southwards transportation of the songpan ganzi fold belt relative to the eastern himalayan syntaxis the lack of pervasive cenozoic recent upper crustal shortening in this region suggests that thickening of the east tibetan crust was predominantly accommodated by viscous deformation at depth we envisage decoupling of the lower and upper crust and partitioning of an e w compressive stress between 1 a laterally mobile thin skinned veneer and 2 a homogeneously thickened tectonic basement

detrital zircon provenance of the late triassic songpan ganzi complex sedimentary record of collision of the north and south china blocks the majority of the songpan ganzi triassic flysch sequence is believed to have derived from denudation of the dabie and sulu ultrahigh pressure uhp metamorphic belt in eastern china e g nie et al 1994 1 zhou and graham 1996 2 however intense debate still exists on the sources of

generation and emplacement of triassic granitoids within the songpan ganze accretionary orogenic wedge in a context of slab retreat accommodated by tear faulting eastern tibetan plateau china the eastern songpan ganze accretionary orogenic wedge northern tibetan plateau is characterized by extensive magmatism coeval with middle to upper triassic closure of the paleotethys ocean along a double subduction system traced by the kunlun anyemaqen suture to the north and the jinsha litang suture to the south new field petrographic and geochemical data on mesozoic plutons intrusive into triassic metasediments of the eastern songpan ganze wedge are presented and integrated in a review of available data for the region three types of granitoids are distinguished i high k calc alkaline granite to granodiorite yanggon maoergai markam sheng meng xue sheng taiyanghe menggu manai dusong xian tagong and jiulong plutons ii high k alkaline granite nyanbaoyeche and rilong plutons and the niuxingou shoshonitic syenite and iii peraluminous s type granite markam and manai leucogranites elemental and isotopic signatures of these granitoids are consistent with the implication of both crustal yangtze craton and songpan ganze metasediments and mantle asthenospheric mantle and metasomatised lithospheric mantle sources based on i the ages of the plutons that are concomitant with the end of the northern and southern subductions of the paleo tethys ii the position of the plutons intrusive in the crustal tectonically accreted units decoupled from the downgoing plate of this double subduction system iii the diversity of their petrological and geochemical signatures we propose that magma emplaced in the songpan ganze accretionary orogenic wedge were generated at the end of the paleotethys closure in a context of slab retreat accommodated by a tear fault along the passive margin of the south china block indeed we argue that this context is the most favourable to trigger synchronous partial melting of i the suprasubduction enriched mantle wedge owing to lowering of the mantle solidus by metasomatism ii the upwelled undepleted asthenospheric mantle into the tear fault owing to decompression iii the metasomatized lithospheric continental mantle of the yangtze craton owing to the temperature increase associated to the asthenospheric upwelling and iv the rocks of the songpan ganze accretionary wedge composed of metasediments and yangtze continental margin basement owing to the combined effects of increased radioactive heat production and mantle heat flux we propose that the concentration of plutons along the eastern margin of the songpan ganze results from focussed migration and emplacement of the granitic magmas guided by the development of a tear fault along the former passive eastern margin of the yangtze craton as a consequence of the paleotethys slab retreat

neoproterozoic high k granites produced by melting of newly formed mafic crust in the huangling region south china abstract the huangling igneous complex in the central yangtze block south china consists of the 850 ma tonalite and trondhjemite which were intruded by the 826 13 ma high k granite the granites are composed of plagioclase 30 60 alkali feldspar 10 30 quartz 20 30 biotite 2 63 1 73 3 wt k 2 o 2 58 4 79 wt and na 2 o 3 90 5 89 wt and low mgo 0 49 1 78 wt and are magnesian alkali calcic in composition their chondrite normalized ree patterns are characterized by enrichment of lree la yb n 14 5 32 8 and mostly negative eu anomalies eu eu 0 5 1 1 they have moderately high sr 202 698 ppm and low y 6 13 18 7 ppm with sr y ratios of 14 64 these large elemental variations probably resulted from fractional crystallization they also have negative whole rock nd 9 75 to 11 73 and zircon hf values 7 1 to 15 8 which are similar to those of the neoproterozoic mafic rocks in the same region suggesting that the granites may have been produced by melting of newly emplaced mafic rocks tonalites and trondhjemites of the huangling complex previously were considered as products by differentiation of water rich basaltic magmas and melting of the thickened archean continental crust in an arc setting respectively therefore the crustal growth and reworking of the yangtze block were probably wholly controlled by the subduction system which resulted in formation of the stable continent during the neoproterozoic

sedimentation and lithofacies paleogeography in southwestern china before and after the emeishan flood volcanism new insights into surface response to mantle plume activity abstract investigations into permian sedimentation and reconstruction of paleogeography in sw china are aimed at characterizing sedimentary responses to the emeishan mantle plume in addition to erosional features on the sediments underlying the uplifted emeishan basalts unusual depositions of permian age are also present in the emeishan large igneous province lip specifically carbonate gravity flows and submarine incised canyon fillings were developed in the western margin of the postulated uplifted area and rifting trenches were developed along the eastern margin alluvial fan deposits occur at the boundary between the inner and intermediate zones these depositions all rest on the maokou formation and are in turn covered by the emeishan basalts implying synchronism between crustal uplift and depositional events these deposits and the associated extension and normal faulting along the margin of and within the lip represent sedimentary features resulting from dynamic behavior of mantle plume com

la icp ms u pb zircon geochronology of early neoproterozoic mafic intermediat intrusions from nw margin of the yangtze block south china implication for tectonic evolution abstract proterozoic magmatism along the margin of the yangtze block is extensive and provides important evidence in the plate tectonic reconstructions for the proterozoic the bikou terrane on the northwestern margin of the yangtze block consists of complex archean to paleozoic metamorphosed sedimentary and volcanic sequences intruded by several mafic intermediate bodies three major intrusions are dated using laser ablation icp ms u pb of zircons this dating yielded ages 884 5 5 ma 884 14 ma and 877 13 ma for the pingtoushan diorite guankouya diorite and liujiping gabbro respectively these are the oldest intrusions found along the margin of the yangtze block geochemical analyses of these mafic intermediate intrusions show they have negative nb ta anomalies that are best interpreted as having been formed in an arc setting the older magmatism and unique rock associations of the bikou terrane suggest that it was an isolated terrane that existed in a marginal sea to the yangtze block the eastward subduction present day orientation resulted in early arc magmatism 880 ma within the bikou terrane and lasted to ca 750 ma this study supports the model that the yangtze block was surrounded by ocean and developed arc magmatism along its western margins in neoproteroic time with continent directed subduction that commenced in the early neoproterozoic the bikou terrane was subsequently accreted onto the northwest margin of the yangtze block at about 750 ma

sedimentary evidence for a rapid kilometer scale crustal doming prior to the eruption of the emeishan flood basalts biostratigraphic and sedimentologic investigations in 67 sections have been carried out for the middle permian maokou formation that immediately underlies the emeishan flood basalts in southwest china the results suggest a domal crustal thinning before the emplacement of the emeishan large igneous province variably thinned carbonates in the maokou formation are capped by a subaerial unconformity which is generally manifested by karst paleotopography paleoweathering zone or locally by relict gravels and basal conglomerates provenance analysis indicates that these gravels and conglomerates were mainly derived from the uppermost maokou formation therefore the stratigraphic thinning likely resulted from differential erosion due to regional uplift iso thickness contours of the maokou formation delineate a subcircular uplifted area in accordance with the crustal doming caused by a starting mantle plume as predicted by experimental and numerical modeling the duration of this uplift is estimated to be less than 3 myr and the magnitude of uplift is greater than 1000 m the sedimentary records therefore provide independent supporting evidence for the mantle plume initiation model for the generation of the emeishan flood basalts

constraints on the depth geometry and kinematics of blind detachment faults provided by fault propagation folds an example from the mesozoic fold belt of south china abstract the mesozoic multi layered thin skinned fault related fold belt in south china is characterized by propagation chevron folding and thrust faulting we examine the geometry and depth of blind detachment faults in this belt using faults and folds that crop out along a well defined cross section a combination of parameters such as backlimb dip angle 2 interlimb angle height of anticline h thickness of the referenced layer h0 width of the anticline ab and thickness change ratio of layering within the forelimb tf t are used to develop new equations to estimate the geometry and depth of the detachment faults in the cross section the huangdushan chevron anticline has parameters revealing a blind fault with a ramp angle of 42 and a flattened segment at a depth of 4 9 km based on existing seismic data we interpret this blind fault as a backthrust developed over an in sequence imbricated duplex of cambrian to ordovician strata a chevron anticline at qiyueshan also reveals a backthrust fault however a broad anticline in lengzhuba and a small cylindrical sinusoidal anticline in yupize reveal a flat ramp flat detachment fault with flat segments at depths of 7 4 and 3 4 km respectively suggesting that the sinian strata were thickened probably by an out of sequence imbricated duplex our results indicate that the flat ramp flat detachment fault along the bottom of the sinian strata produced the out of sequence imbricated duplex and cylindrical sinusoidal folds the in sequence imbricated duplex produced backthrusts and related chevron anticlines we consider that this fault related fold belt was produced by collision between the yangtze block and north china blocks

where is the north china south china block boundary in eastern china the qinling dabieshan belt results from the collision of the north china and south china blocks the eastern extension of the belt the sulu area consists of several stacked units from top to bottom these are 1 a weakly metamorphosed slate sandstone unit of neoproterozoic paleozoic age 2 a gneiss quartzite unit metamorphosed under high pressure p conditions 3 a marble amphibolite unit 4 an ultrahigh p unit that was very deeply subducted and then exhumed and 5 a migmatitic dome resulting from the melting of ultrahigh p rocks all these units underwent the same deformation which was characterized by a top to the northwest extensional ductile shearing during their exhumation granulite facies restites in the migmatite are not significantly different from granulitized eclogites derived from the ultrahigh p rocks the migmatite exhibits the same structural and petrologic evolution as that determined for the other units the resemblance of petrologic and structural features and the lack of ocean basin rock shows that the boundary between the north china block and south china block must be placed north of the sulu area

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

possible correlation between a mantle plume and the evolution of paleo tethys jinshajiang ocean evidence from a volcanic rifted margin in the xiaru tuoding area yunnan sw china abstract several paleozoic sutures in southwestern china provide a record of the history of the paleo tethys ocean whose birth and final closure are associated with the breakup and assembly of gondwanaland recent studies indicate that there are widespread oib type mafic volcanic rocks within these suture zones and intervening terranes this paper examines the geology and geochemistry of volcanic rocks in the xiaruo tuoding area a remnant passive margin succession of the jinshajiang paleo tethyan ocean the sedimentary and volcanic stratigraphy of this area is interpreted as a seaward dipping margin with a few continentward dipping normal faults the available geochemistry of these volcanic rocks suggest that they are oib like basalts characterised by sio 2 42 78 50 46 wt high tio 2 contents tio 2 2 2 3 55 wt moderate mgo 4 15 6 49 wt mg 0 37 0 50 high ti y ratios mostly 450 large ion lithosphere elements enrichment high strength field elements and rare earth elements with la nb 1 04 1 39 ce yb 18 38 30 sm yb 2 16 3 52 87 sr 86 sr i 0 705350 0 707867 and nd t 1 43 1 90 these geochemical and isotopic signatures are generally similar to those of the emeishan flood basalts which together with stratigraphic constraints demonstrate that these volcanics were formed in a volcanic rifted margin probably associated with a mantle plume a new model is proposed to interpret the evolution of the jinshajiang paleo tethyan ocean and its possible relationship to the emeishan mantle plume in this model we argue that the opening of the jinshajiang paleo tethyan ocean in the carboniferous was caused by a mantle plume the mantle plume was active to the east along the western margin of the yangtze craton between 300 and 260 ma from which the voluminous emeishan flood basalts were erupted at 260 ma the closure of the jinshajiang ocean occurred since the middle permian continuous westward subduction generated the jiangda weixi magmatic arc to the west of the jinshajiang suture this subduction also partly destroyed and or tectonically sliced the volcanic rifted margin some seaward dipping volcanic sedimentary sequences on the east flank of the jinshajiang ocean were preserved but are strongly deformed

mesozoic sedimentary evolution of the northwest sichuan basin implication for continued clockwise rotation of the south china block we present new sedimentary data integrated into a regional mesozoic stratigraphic framework to provide a detailed picture of spatio temporal variations in deposition and depocenter migration of the northwest sichuan basin the mesozoic sedimentary evolution is utilized to interpret basin subsidence history and to unravel coeval basin margin tectonics the northwest sichuan basin together with the songpan ganzi terrane behaved as a passive margin south of the qinling paleo tethys from late paleozoic to early middle triassic times and then evolved into a peripheral foreland basin in response to collision of the north and south china blocks since the late middle triassic coeval with strong north south contraction of the songpan ganzi terrane in the late triassic sinistral transpressional deformation of the longmen shan belt led to flexural subsidence of the adjacent western sichuan basin renewed basin margin fold thrust activity triggered recurrence of flexural subsidence of the northwest sichuan basin since the middle jurassic with the depocenter eventually shifting to the northwestern corner of the basin in the early cretaceous sedimentary evolution of the northwest sichuan basin and the basin margin deformation imply that the south china block had been rotating clockwise relative to the north china block throughout the mesozoic with an interim period of early jurassic tectonic quiescence a model is advanced that invokes clockwise rotation of the south china block as a driver for tectonic evolution of both the basin and adjoining structural belts and provides an explanation for several salient features that are otherwise puzzling

constructing the longmen shan eastern tibetan plateau margin insights from low temperature thermochronology 1 contrasting models of upper crustal shortening versus lower crustal flow have been proposed to explain the formation of thickened crust in the longmen shan lms eastern tibetan plateau tp margin these models require different structural kinematics along the lms whose structural geometry is defined by three parallel nw dipping fault zones from foreland southeast to hinterland northwest they are the guanxian anxian fault yingxiu beichuan fault ybf and wenchuan maowen fault wmf newly derived and previously published low temperature thermochronology data from the lms were synthesized to constrain the spatial exhumation and test previous models the results show that 1 exhumation increases abruptly across the range bounding ybf suggesting the fault being the main thrust boundary between the lms and the sichuan basin to the east 2 younger late cenozoic cooling ages are found on the hinterland wmf where a dichotomy of ages on the hanging wall versus footwall suggests late cenozoic thrust activity and 3 toward the hinterland to the west exhumation rates decrease twofold over a distance of 30 40 km this exhumation pattern indicates a westward decrease of tectonic uplift providing the regional topography approached a steady state whereby exhumation is in balance with tectonic uplift the observed exhumation estimates support an upper crustal configuration where thrusts in the lms merge gradually into a gentle detachment seated at a depth of 20 30 km results of this study support a revised upper crustal thrusting model

provenance of the triassic songpan ganzi flysch west china abstract the songpan ganzi basin of western china is a triangular structure on the northeast corner of the qinghai tibet plateau filled with a great thickness of triassic flysch surrounded to the north by the eastern kunlun orogenic belt that extends farther eastwards as the qinling dabie belt to the southwest by the ganzi litang ophiolite and yidun volcanic arc and to the east by the longmenshan thrust belt we have carried out the most extensive petrographic and geochemical study so far undertaken on typical turbidites from the songpan ganzi basin including major element trace element rb sr and sm nd isotopic analyses in the baryan har region on the northern side of the basin petrographic study identifies the sandstones as typical graywackes whose matrix is fine grained detrital material recrystallized by greenschist facies metamorphism chemical weathering indices confirm that the fine grained turbidites were little altered during transport and deposition so compositions accurately represent their source areas major trace and isotope analyses displayed on various discriminant diagrams indicate that the flysch from the baryan har region is derived from the eastern kunlun belt to the north a conclusion supported by published paleocurrent directions comparison of the baryan har turbidites with published results from western sichuan on the eastern side of the songpan ganzi basin suggests that the latter is derived from a source area of varied sedimentary rocks and a few igneous rocks the most likely source is south qinling to the northeast presently available data do not support derivation of material in the songpan ganzi basin from the yidun volcanic arc or the dabie ultra high pressure metamorphic belt the present provenance studies on the triassic songpan ganzi flysch support a westward opening model for the paleotethyan ocean and reveal different evolutional histories for the qilian qinling dabie and eastern kunlun orogens

generation of metaluminous a type granites by low pressure melting of calc alkaline granitoids melting experiments on two calc alkaline rocks a tonalite and a granodiorite demonstrate that shallow dehydration melting of hornblende and biotite bearing granitoids generates metaluminous a type granitic melts profuse crystallization of calcic plagioclase orthopyroxene during low pressure p 4 kbar incongruent melting explains all of the major and trace element characteristics of a type granites e g their low al ca mg sr and eu contents and high ga al and k na ratios as well as their common association with anorthositic and mangeritic rocks the residual assemblage produced during melting of the same rocks in the deep crust p 8 kbar is dominated by clinopyroxene and as a consequence most of the chemical features diagnostic of a type granites are lost the high temperatures 900 c and low h 2 o contents 4 wt that are characteristic of a type magmas are adequately explained by the h 2 o poor nature of the granitoid source materials and require neither a granulitic nor a f rich source previous attempts to explain the origin of a type granites have focused on the nature of the source material these experiments show that pressure of melting is at least as important as source composition in engendering a type melts

age of the emeishan flood magmatism and relations to permian triassic boundary events the permian triassic p t mass extinction the greatest biological mortality event in the earth s history was probably caused by dramatic and global forcing mechanisms such as the siberian flood volcanism here we present the first set of high precision 40 ar 39 ar dating results of volcanic and intrusive rocks from the emeishan traps south china which define a main stage of the flood magmatism at v251 253 ma and a subordinate precursory activity at v255 ma this time span is generally coeval with or slightly older than the age of the p t boundary estimated by the ash beds in the meishan stratotype section and the main eruption of the siberian traps our data reinforces the notion that the eruption of the emeishan traps rather than eruption of the siberian traps accounted for the formation of the p t boundary ash beds in south china the emeishan flood magmatism which occurred in the continental margin comprising thick marine limestone formations moreover may have triggered rapid release of large volumes of methane and carbon dioxide that could have been responsible for the global n 13 c excursion and associated environmental crisis leading to the mass extinction at the p t boundary fl 2002 elsevier science b v all rights reserved

neoproterozoic tonalite and trondhjemite in the huangling complex south china crustal growth and reworking in a continental arc environment the huangling igneous complex in the central yangtze block of south china is composed of 850 ma tonalite and trondhjemite intruded by 820 ma granite tonalites mainly consist of plagioclase and quartz with abundant hornblende and biotite and contain fine to coarse grained hornblende rich igneous enclaves they have variably low sio2 56 4 67 5 wt and sr 230 517 ppm and high mgo 1 47 4 02 wt and high y 8 87 15 7 ppm with low sr y 17 43 unlike archean tonalites their whole rock end values range from 8 3 to 10 3 and zircon ehf values from 3 9 to 12 6 similar to those of spatially associated neoproterozoic mafic rocks the tonalites are thus proposed to have been produced by three stages involving partial melting of subduction modified lithospheric mantle 5 to 20 percent crustal contamination of hydrous basaltic magmas in the middle to lower crust and fractional crystallization of amphibole plagioclase biotite magnetite trondhjemites are composed of plagioclase quartz and alkali feldspar with minor biotite and hornblende they have high sio2 67 3 75 6 wt and sr 340 840 ppm and low mgo 0 09 0 78 wt and y 4 8 11 2 ppm with high sr y 32 162 similar to archean trondhjemites and high pressure partial melts of mafic rocks these rocks have extremely negative end 18 7 to 19 9 and zircon ehf values 18 9 to 49 2 with two stage hf isotope model ages of 2 46 to 3 90 ga indicating their formation in the lower crust by melting of archean amphibolites and granulites which were probably widely distributed beneath the yangtze block their highly variable chemical compositions also resulted from fractional crystallization of plagioclase hornblende biotite thus the association of the tonalite and trondhjemite is different from the archean ttg series this difference may reflect different geothermal gradients however modeling using tonalitic and trondhjemitic samples of the huangling complex suggests that mixing of the two magmas can generate a typical ttg affinity series suggesting that the origin of some ttg series may have involved both crustally derived and mantle derived melts the generation of the huangling igneous complex was linked to slab subduction beneath the northern margin of the yangtze block during the neoproterozoic similar to the cenozoic arc magmatism in the andes therefore melting of metasomatized hydrous mantle in arc settings can generate mafic magmas that not only under plated and contributed to continental crustal growth but also caused melting of thickened ancient crust and newly formed lower mafic crust

hydrodynamic and suspended sediment transport controls on river mouth morphology 1 river mouths building into standing bodies of water have strikingly varied growth habits this presents a compelling pattern formation problem that is also of great practical relevance for subsurface prediction and managing coastal wetlands here we present a generalized 2 5 dimensional potential vorticity pv theory that explains sedimentation patterns of a sediment laden stationary jet by coupling an understanding of vorticity with suspended sediment concentration fields we explore the physical meaning of this new sediment pv definition and its impact on outflow depositional patterns by analyzing data from a shallow wall bounded plane jet experiment and by discussing new theoretical insights a key result is that lateral advection and diffusion of suspended sediment are directly proportional to jet vorticity a feature that reveals the mechanistic process that forms elongated channels by focused levee deposition the new pv theory constitutes a more generalized mathematical framework that expands the rouse theory for the equilibrium of suspended sediment

constraints of detrital zircon u pb ages and hf isotopes on the provenance of the triassic yidun group and tectonic evolution of the yidun terrane eastern tibet abstract eastern tibet to the west of the yangtze block consists of the yidun and songpan ganzi terranes separated by the ganzi litang suture zone the yidun terrane includes the zhongza massif to the west but the eastern part of the yidun terrane is covered by the yidun group extending from the south shangri la region to the north changtai region the yidun group from the base upward includes the lieyi qugasi tumugou and lanashan formations which are mainly composed of volcanic flysch successions based on the ages of volcanic interlayers and plutonic intrusions depositional ages of the qugasi and tumugou formations are considered to be slightly older than 230 ma and ca 220 230 ma respectively which are prominently older than the previous estimates the yidun group in the changtai region has two prominent detrital zircon age peaks at 400 480 and 880 980 ma and a minor peak at 2 45 2 50 ga this pattern suggests a detritus source from the zhongza massif which was a micro continent separated from the western yangtze block in contrast the yidun group in the shangri la region has various zircon age spectra among different formations the qugasi formation in this region has detrital zircon age patterns similar to the yidun group in the changtai region however the overlying tumugou formation shows distinct age peaks at triassic 220 240 ma neoproterozoic 720 880 ma and paleoproterozoic 1 75 1 90 ga this age pattern is similar to that of the xikang group of the songpan ganzi terrane to the east the detrital zircon age difference between the qugasi and tumugou formations in this region indicates a transition of sedimentary sources from the zhongza massif to locally distributed triassic magmatic rocks at 230 ma it is thus suggested that the songpan ganzi terrane may have been connected to or collided with the southern part of the yidun terrane during the late triassic whereas the songpan ganzi terrane and the northern part of the yidun terrane were still separated by the ganzi litang ocean

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

thermochronology of the yidun arc central eastern tibetan plateau constraints from 40ar 39ar k feldspar and apatite fission track data abstract four k feldspar samples from the yidun arc eastern tibetan plateau were analysed by the 40ar 39ar method with the aim of recovering information on their thermal history using multiple diffusion domain mdd theory arrhenius plots for each of the samples reveal low retentivity early in the heating experiments a property that is attributed to their recrystallised nature this low argon retentivity appears to violate the mdd assumption that volume diffusion is the only mechanism for argon transport within the crystals thus the thermal histories derived from these analyses are considered suspect nevertheless the age spectra themselves suggest that the majority of samples had cooled below 200 c prior to the eocene collision of india with asia thermal history modelling from apatite fission track analyses from the same and nearby samples shows slow cooling through the apatite fission track partial annealing zone during the cenozoic in samples from the high elevation low relief areas of the yidun arc while samples from the major jinsha river valley show rapid cooling through the partial annealing zone beginning in the miocene these results suggest that significant cenozoic denudation has been localised and that most parts of the yidun arc have experienced very little denudation during the cenozoic

neoproterozoic arc related mafic intrusions along the northern margin of south china implications for the accretion of rodinia abstract south china has been considered as part of the rodinian supercontinent during neoproterozoic time although its paleogeographic position within this supercontinent is still a matter of debate the wangjiangshan and bijigou complexes along the northern margin of south china are among the largest mafic intrusions in china new shrimp zircon u pb results indicate that these two intrusions have crystallization ages of 820 ma and 780 ma respectively enrichment of large ion lithophile and light rare earth elements and depletion of high field strength elements in these intrusions suggest derivation from an active arc along a continental margin this interpretation of these intrusions as part of a continental arc assemblage is in contrast with the previous view that they were products of a neoproterozoic mantle plume that initiated the breakup of rodinia the presence of a neoproterozoic magmatic arc suggests that rodinian oceanic lithosphere was subducted beneath the present northern margin of south

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

origin and tectonic significance of a mesozoic multi layer over thrust system within the yangtze block south china abstract in the yangtze block south china a well developed mesozoic thrust system extends through the xuefeng and wuling mountains in the southeast to the sichuan basin in the northwest the system comprises both thin and thick skinned thrust units separated by a boundary detachment fault the dayin fault to the northwest the thin skinned belt is characterized by either chevron anticlines and box synclines to the northwest or chevron synclines to the southeast the former structural style displays narrow exposures for the cores of anticlines and wider exposures for the cores of synclines thrust detachments occur along silurian fs and lower cambrian fc strata and are dominantly associated with the anticlines to the southeast this style of deformation passes gradually into one characterized by chevron synclines with associated principal detachment faults along silurian fs cambrian fc and lower sinian fz strata there are however numerous secondary back thrusts therefore the thin skinned belt is like the valley and ridge province of the north american applachian mountains the thick skinned belt structurally overlies the thin skinned belt and is characterized by a number of klippen including the xuefeng and wuling nappes it is thus comparable to the blue ridge province of appalachia the structural pattern of this thrust system in south china can be explained by a model involving detachment faulting along various stratigraphic layers at different stages of its evolution the system was developed through a northwest stepwise progression of deformation with the earliest delamination along lower sinian strata fz analyses of balanced geological cross sections yield about 18 1 21 total 88 km shortening for the thin skinned unit and at least this amount of shortening for the thick skinned unit the compressional deformation from southeast to northwest during late jurassic to cretaceous time occurred after the westward progressive collision of the yangtze block with the north china block and suggests that the orogenic event was intracontinental in nature

subduction related origin of the 750 ma xuelongbao adakitic complex sichuan province china implications for the tectonic setting of the giant neoproterozoic magmatic event in south china abstract the xuelongbao plutonic complex in the eastern margin of the tibetan plateau is dated at 748 7 ma using the shrimp zircon u pb method and represents part of the neoproterozoic igneous assemblage of south china rocks in the complex include tonalite and granodiorite and have sio2 ranging from 62 0 to 74 8 wt and al2o3 from 14 3 to 20 9 wt their na2o contents range from 4 2 to 6 7 wt and k2o from 0 47 to 1 96 wt indicating that they belong to the na series these rocks show chondrite normalized ree patterns depleted in hree and variably enriched in lree they have positive sr and negative nb and ti anomalies in the primitive mantle normalized trace elemental spider diagram their sr contents range from 320 to 780 ppm and y contents are lower than 10 ppm resulting in high sr y ratios 52 320 characteristic of typical adakites their end t and initial sr isotopic compositions range from 0 36 to 2 88 and from 0 7033 to 0 7054 respectively the geochemical features of the xuelongbao plutonic complex are consistent with an origin from adakitic magmas that were likely derived from partial melting of a subducted oceanic slab together with arc signatures of other granites and mafic intrusions in the region the 750 ma xuelongbao adakitic complex provides evidence for a major subduction related neoproterozoic magmatic event of south china

deformation partitioning and foliation reactivation during transpressional orogenesis an example from the central longmen shan china progressive localization of strain within the songpan garze fold belt at the eastern margin of the tibetan plateau has resulted in the formation of the wenchuan maowen shear zone and preservation of excellent overprinting relationships between three major phases of ductile and semi ductile deformation these relationships show that deformation in the eastern songpan garze fold belt evolved from crustal thickening during sw directed compression d1 to localized sinistral transcurrent shear d2 and finally to increasingly localized high temperature d3p and retrograde d3r reverse shear in response to se directed compression both d2 and d3 deformations were co planar and reactivation of the s2 foliation led to the formation of a composite s2 3 fabric however inclusion trails in garnet porphyroblasts preserve evidence of a transition between these strike slip and reverse shear end members indicating that d2 and d3 were part of an episode of progressive transpressional shear metamorphic grade increases from chlorite to kyanite zone within the wenchuan maowen shear zone reaching syn d3p peak conditions of 600 c and 10 kbar the d3p and d3r reverse shears were responsible for tectonic exhumation of the shear zone and resulted in the formation of retrograde d3r high strain zones along the margins of the xuelongbao granite and adjacent to the wenchuan maowen fault

pervasive melt migration from migmatites to leucogranite in the shuswap metamorphic core complex canada control of regional deformation abstract the shuswap metamorphic core complex exhumed in the hinterland of the canadian cordillera displays a 15 km structural section from migmatites to leucogranite it comprises a lower unit dominated by anatectic migmatites exposed in the core of domes at higher structural level the transition from a middle amphibolite facies unit to an upper crustal unit is marked by a low angle detachment associated with the emplacement of laccoliths of leucogranite the amphibolite facies middle unit is permeated by a network of granitic veins feeding the laccoliths and structurally connected to the migmatites of the lower unit this paper describes the geometric textural and structural characteristics and the distribution of the granitic fraction at the outcrop scale in the amphibolite facies unit separating the migmatites from the leucogranite this analysis constrains the mechanisms of melt migration within a plurikilometer scale section across high grade rocks of the middle crust in fertile lithologies metapelites felsic amphibolites the granitic fraction in part generated in situ forms a diffuse network of concordant veins feeding discordant veins and structural sites such as the foliation shear zones and boudin necks discordant granitic veins are oriented perpendicular and to a lesser extent parallel to the mineral and stretching lineation in refractory lithologies the granitic fraction is dominantly intrusive and displays sharp contacts with the host rock however distribution of the granitic fraction shows the same characteristics as for fertile lithologies the melt fraction migrated through a network of fractures taking advantage of mechanical weaknesses such as the foliation plane or forming dikes oriented dominantly perpendicular to the regional stretching and mineral lineations these geometric and textural characteristics suggest that granite migration in these rocks is achieved dominantly by viscous flow of the melt solid through the solid matrix driven by its buoyancy and controlled by mechanical anisotropy of the rock and local dilation created by heterogeneous deformation the relationship between the regional fabric and the distribution of the granitic fraction indicates that regional deformation incremental and finite played a major role in providing pathways for melt migration the formation of laccoliths of leucogranite at higher structural levels suggests that upward melt migration led to accumulation of granitic magmas these observations are consistent with a model of pervasive melt migration through a network of interconnected concordant and discordant granitic veins feeding laccolithic plutons extracted from the dominantly solid matrix during vertical thinning and compaction at a regional scale the presence of both transposed and intact granitic networks indicate an overlap between deformation and granite migration as a continuum or in pulses the methodology and terminology proposed in this paper provide some insights on the behavior of the granitic fraction in the middle crust this approach should be applicable to other high grade terrains exhumed in the cores of orogenic belts

emeishan basalt ar ar overprint ages define several tectonic events that affected the western yangtze platform in the mesozoic and cenozoic abstract ar ar whole rock dating was carried out as part of a detailed stratigraphical investigation of the emeishan basalt large igneous province lip in the stratotype area sichuan china thirteen from twenty one specimens from three sections yielded reliable reversed isochron and plateau ages maximum 1 errors of 2 1 and 1 6 m y respectively and form two clusters centered on the late jurassic early cretaceous and early late cretaceous with a tail spanning 82 40 ma however all are appreciably younger than the magnetobiostratigraphically constrained late middle permian 255 260 ma age of the basalts reflecting varying degrees of thermal resetting during the mesozoic and cenozoic recently middle jurassic early cretaceous ages reported from elsewhere in the lip have been used to infer a single 40 m y long tectonic episode that affected the western yangtze platform developing this idea the new information have been combined with data we hold for other parts of the terrain and results from three emeishan lip ar ar dating studies published during 2002 to give 32 reliable age dates three 10 12 m y events appear to be recorded in different parts of the province middle jurassic late jurassic early cretaceous and early late cretaceous with a fourth shorter middle eocene episode the 40 m y mesozoic episode appears to be a sampling artifact following a review of the regional deformation tectonic features it is argued that activity related to various phases of deformation in the longmen shan thrust belt is the most likely cause of resetting the final suturing of the north and south china blocks may have also been responsible for the middle jurassic event however the oligocene present indentation of india into asia appears not to have had an impact possibly due to the large scale related strike slip faults that have effectively shielded the lip sichuan basin

detrital zircon u pb geochronological and lu hf isotopic constraints on the precambrian magmatic and crustal evolution of the western yangtze block sw china abstract in situ u pb geochronological and lu hf isotopic studies of detrital zircon from a variety of precambrian strata along the western margin of the yangtze block of south china show that the rocks have maximum depositional ages of ca 1000 740 ma all of the strata probably formed in a subduction related tectonic setting the kunyang group heishantou formation and huili group tongan formation have maximum depositional ages of 960 1000 ma and contain zircons whose ages are dominantly paleoproterozoic 2500 2300 1900 1700 and 1650 1500 ma the yanbian group has a zircon population ranging mainly from 860 to 1000 ma different from that of the bikou group with ages from 745 to 890 ma ages of zircons from the lieguliu and guanyinya formations range from 740 to 900 ma the paleoproterozoic detrital zircons commonly have negative or slightly positive hf values from 8 0 to 2 3 whereas most of the 1000 740 ma zircons have moderately to significantly high positive hf values from 0 3 to 13 0 the new detrital zircon u pb and lu hf isotopic dataset suggests four major episodes of magmatism in the region 2500 2300 1900 1700 1650 1500 and 1000 740 ma and the presence of widespread archean basement of the western yangtze block the data also suggest an important period of juvenile magmatism at 1000 740 ma our results favor a model in which continuous subduction 1000 740 ma along the western margin of the yangtze block lasted for 260 million years

stratigraphic and u pb shrimp detrital zircon evidence for a neoproterozoic continental arc central china rodinia implications abstract the bikou terrane of central china is located within the southwest qinling mountains along the northwest margin of the yangtze plate the terrane consists of a volcanic assemblage the bikou group and a correlative volcaniclastic turbidite sequence the hengdan group u pb shrimp detrital zircon dating of 181 grains collected from the hengdan group indicates ages from ca 850 to 700 ma with a peak near 770 ma given the predominance of volcanic single cycle zircons within the hengdan group and the thick succession of coarsening upward volcaniclastic marine sediments the sequence is best interpreted as a fore arc basin this interpretation is supported by previous u pb shrimp and conventional u pb zircon dating performed within the bikou group that indicates subduction related magmatism persisted from at least ca 850 to as recently as 700 ma together these data indicate that the bikou terrane is a fragment of a neoproterozoic continental arc system and that the yangtze plate was bordered by

longmen shan fold thrust belt and its relation to the western sichuan basin in central china new insights from hydrocarbon exploration the longmen shan fold thrust belt is one of the key regions of demonstrable mesozoic cenozoic tectonic evolution in china and the sichuan basin was the first natural gas producing area in china in this article the structural features of the longmen shan belt are presented using both seismic profiles and field data the complex structures of the northeast trending longmen shan fold thrust belt and its foreland in the western sichuan basin are formed by southeast directed thrusting several eastward verging rootless thrust sheets and imbricates of cambrian triassic rocks have been recognized in the northern longmen shan belt evidence suggests that the northern longmen shan belt experienced at least two major periods of deformation in the late triassic and cenozoic however the southern longmen shan belt is represented by the basement involved thrust structures and klippen and its major periods of deformation were in the latest cretaceous early cenozoic sedimentary features in the western sichuan basin reflect a two phase flexural loading history and illustrate that the late triassic foreland basin extends along the foredeep of the entire length of the longmen shan belt but the uppermost cretaceous paleogene rejuvenated foreland basin is restricted in the southern part of the western sichuan basin structural geometries suggest that prospective traps are mainly developed in the frontal zone of the longmen shan fold thrust belt and in the southern part of the western sichuan basin one of the major contributions of this article is finding preexisting paleozoic rift basins under the cenozoic thin skinned thrust belts which represent a new potential hydrocarbon play

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

collision leading to multiple stage large scale extrusion in the qinling orogen insights from the mianlue suture abstract the geologic framework of the phanerozoic qinling dabie orogen was built up through two major suturing events of three blocks from north to south these include the north china craton including the north qinling block the qinling dabie microblock and the south china craton including the bikou block separated by the shangdan and mianlue sutures the mianlue suture zone contains evidence for mesozoic extrusion tectonics in the form of major strike slip border faults surrounding basement blocks a late paleozoic ophiolite and a ca 240 200 ma thrust belt that reformed by 200 150 ma thrusts during a type intracontinental subduction the regional map pattern shows that the blocks are surrounded by complexly deformed devonian to early triassic metasandstones and metapelites forming a regional scale block in matrix melange fabric five distinct tectonic units have been recognized in the belt 1 basement blocks including two types of precambrian basement crystalline and transitional 2 continental margin slices including early paleozoic strata and late paleozoic fluviodeltaic sedimentary rocks proximal and distal fan clastics reflecting the development of a north facing rift margin on the edge of the south china plate 3 out of sequence oceanic crustal slices including strongly deformed postrift deep water sedimentary rocks sheeted dikes basalts and mafic ultramafic cumulates of a late paleozoic ophiolite suite developing independent of the rift margin in a separate basin 4 out of sequence island arc slices 5 accretionary wedge slices all the tectonic units were deformed during three geometrically distinct deformation episodes d 1 d 2 and d 3 during 240 200 ma units 2 4 involved southward thrusting and vertical then southward extrusion of about 20 km of horizontal displacement above the autochthonous basement during the d 1 episode thrust slices 20 km south of the mianlue suture are related to this vertical extrusion due to the same rock assemblages ages and kinematics the d 2 and d 3 episodes folded all the units in a thick skinned style about east west d 2 and west northwest d 3 axes in the mianlue suture zone an early foreland propagating sequence of accretion of late paleozoic rocks deposited above the yangtze craton is not involved in d 1 deformation but is temporally equivalent to the d 2 and d 3 deformation in the mianlue suture two stages of strike slip faulting mainly occurred at the end of d 2 and d 3 respectively during d 2 deformation the bikou block was obliquely indented to the ese into the mianlue suture rather than being thrust over the mianlue suture from the north as a part of the qinling dabie microblock during d 3 deformation however the bikou block was bounded by the south boundary fault of the mianlue suture and the yangpingguan fault on the south these faults are coeval strike slip faults but of opposite senses and accommodated minor southwestward extrusion of the bikou block into songpan ganze orogen the other basement blocks north of the mianlue suture were extruded eastward by about 20 km of lateral displacement based on the offset of the wudang dome during the d 3 episode due to the northeastward indentation of the hannan complex of the south china craton post d 3 emplacement of granite cutting across the strike slip faults such as the mianlue suture provides a minimum age of 200 ma for d 3 deformation therefore based on insights from the evolution of the mianlue suture the d 2 and d 3 episodes in the mianlue suture and its neighbors are not responsible for and associated with the two stage extrusion of the dabie uhp hp terranes from the foping dome to the present erosional surface more than 350 km

late paleoproterozoic to early mesoproterozoic tangdan sedimentary rock hosted strata bound copper deposit yunnan province southwest china sedimentary rock hosted strata bound copper deposits are widespread in the kangdian region of the central yunnan and southern sichuan provinces southwest china the deposits occur within weakly metamorphosed rocks of the late paleoproterozoic to early mesoproterozoic dongchuan group and are spatially associated with discordant breccia bodies that are interpreted as having formed from salt diapirs the tangdan deposit the largest in the region consists of stratiform and discordant orebodies predominantly hosted in dolostones immediately above hematitic sandstones and siltstones and in black shales above the dolostones host rocks for the deposit display a complex paragenetic sequence of alteration and mineralization early sodic alteration resulted in the growth of both albite and ferroan dolomite later potassic alteration resulted in the precipitation of potassium feldspar and locally biotite in argillaceous layers sulfide mineralization was temporally and spatially associated with silicification that postdated both sodic and potassic alteration textures suggest that silicification may have preferentially affected evaporite minerals in the dolostones copper sulfides form bedding parallel disseminations veinlets and to a lesser extent stockworks copper sulfides are dominated by chalcopyrite with lesser bornite and chalcocite hypogene chalcopyrite and bornite from the tangdan deposit have 34s values that range from 12 7 to 9 3 and cluster between 3 to 5 the values suggest derivation from mesoproterozoic marine sulfates the dolostone host rocks have relatively homogeneous c and o isotope values ranging from 0 2 to 1 3 13cv pdb and from 19 1 to 22 4 18ov snow carbonate minerals in quartz sulfide veins display both a trend toward lighter oxygen isotope values and a trend to significantly lighter carbon isotope values the light carbon isotope values suggest involvement of organic carbon in the mineralizing process alteration and mineralization at tangdan probably occurred via interaction of oxidized saline brines derived from the underlying red bed sequence with partially to wholly lithified dolostones sulfide precipitation was likely due to both redox reactions and mineralizing fluid ph changes resulting from dolomite and sulfate dissolution the styles of alteration and mineralization at the tangdan deposit are similar to those observed in the central african copperbelt particularly the dolostone hosted orebodies in the mines series of the democratic republic of congo

the longmenshan tectonic complex and adjacent tectonic units in the eastern margin of the tibetan plateau a review abstract the longmenshan tectonic complex lstc along the eastern margin of the tibetan plateau the site of devastating earthquakes such as the magnitude 8 0 wenchuan earthquake on 12 may 2008 preserves an exceptionally complete history of the tectonic evolution of the yangtze block and its relations to adjacent tectonic units due to sequential tectonic superposition and tectonic reactivation the tectonic nature of the lstc and in particularly the older history has been profoundly debated and many different tectonic models have been proposed herein we summarize the current understandings of the major tectonic events that have shaped this important tectonic complex highlighting problems left to be solved by future work including 1 the nature and constraints for at least 6 regional tectonic events i e building of the metamorphic basement art the columbia nuna supercontinent pt1t the rodinia supercontinent pt3t the paleozoic passive continental margin pzt the paleotethys orogeny mzt and the neotethys orogeny kzt 2 metamorphic basement exposures and their tectonic implications including rock types and geochronological constraints for the archean paleoproterozoic and neoproterozoic basements 3 nature of the present lstc and its affinity with adjacent tectonic units 4 consideration of the ne striking longmenshan thrust belt and arcuate shape yanyuan muli thrust belt as parts of a single tectonic feature 5 mountain basin coupled systems recording past tectonic eposides we draw the following conclusions and tectonic models based on published research combined with our own recent studies 1 the well preserved archean yudongzi gneiss group in the lstc has a genetic affinity with the kongling group and thus belongs to the yangtze block 2 the paleoproterozoic hejiayan group juxtaposed adjacent to the archean yudongzi group may represent a 2000 1800 ma orogenic belt which corresponds to the supercontinent nuna columbia amalgamation event 3 a neoproterozoic trench arc basin system which is reconstructed based on identification of a neoproterozoic ophiolite complex arc type magmatic rock assemblages and volcaniclastic basinal deposits along the western margin of the yangtze block and the lstc may represent the record of eastward subduction of the neoproterozoic mozambique oceanic lithosphere beneath the yangtze block during the assembly of the rodinia supercontinent 4 a complete bidirectional wilson cycle was reconstructed by the formation of the late permian to the middle late triassic back arc ganze litang rift and ocean following the early paleozoic mianlue continental rift and ocean and subsequent closure of the ocean basin by simultaneous bidirectional northward and southwestward subduction and later collision this relatively uncommon bidirectional wilson cycle might be attributed to the formation of the three armed rift system in the eastern paleotethys associated with the late permian eemeishan large igneous province in the lstc 5 a three stage tectonic sequence of in sequence imbricate thrust in the lstc during india eurasian collisional orogeny at 55 15 ma extrusion from 15 to 5 ma and the plateau uplift since 5 ma resulting from lower crustal channel flow is proposed for the formation of the present lstc

the yanbian terrane southern sichuan province sw china a neoproterozoic arc assemblage in the western margin of the yangtze block abstract the magmatic and tectonic history of the yangtze block and its possible affinity with other neoproterozoic arc terranes are important elements in the reconstruction of neoproterozoic plate tectonics the yanbian terrane in the western margin of the yangtze block is a typical arc assemblage composed of a flysch type sedimentary sequence intruded by gabbroic and granodioritic plutons the sedimentary sequence consists chiefly of tuffaceous material with interlayered chert sandstone and pillow basalts laser ablation icp ms u pb dating of detrital zircons from the sandstones yield ages as young as 840 ma the gaojiacun and lengshuiqing mafic intrusions are dated at 812 3 ma and 806 4 ma respectively using the shrimp zircon u pb technique geochemical data show that both the gaojiacun and lengshuiqing intrusions have arc signatures with nd t values of 1 5 to 6 0 initial 87 sr 86 sr ratios of 0 705 0 706 and pronounced negative nb ta and zr hf anomalies their geochemical variations are best explained by fractional crystallization without major crustal contamination the yanbian terrane represents a typical arc assemblage formed on the western edge of the yangtze block during neoproterozoic time the sedimentary sequence was deposited in an oceanic setting probably in a back arc basin environment the depleted subduction modified lithospheric mantle wedge above the subduction zone was the source of melts from which the mafic plutons were crystallized this scenario suggests subduction of oceanic lithosphere eastward present day orientation underneath the yangtze block

formation of rotliegend ca cl brines in the north german basin compared to analogues in the geological record abstract the formation of terrestrial ca cl brines is controlled by two major processes chemical reactions between halite brines and igneous and metamorphic rocks and or their debris in sedimentary basins and the abstraction of h 2 o by evaporation freezing or formation of hydrous minerals albitization dominates in environments of low h 2 o activity i e halite saturated systems chloritization serpentinization and formation of smectites seems to be related to less saline systems i e enhanced water activity the rotliegend brines of the north german basin evolved by interaction of infiltrating halite brines with volcanic debris of the permian acid volcanic rocks by albitization and chloritization of plagioclase at present their temperatures are between 130 and 150 c at 3000 4000 m depth although they developed in a different environment they chemically and isotopically resemble the basinal brines from the mississippi salt dome basin or even the deep brines from the red sea all these brines have in common that halite brines interacted with plagioclase an sio 2 2na 2alb ca 2 or possibly with clay minerals ca clay 2na na 2 clay ca 2 at enhanced temperatures the trends of stable isotopes show slopes in the d 18 o diagram which are typical for hydrothermal reactions of evaporation brines with rock or sediments the genesis of rotliegend brines differ from ca cl brines in the canadian shield siberian platform the dry valleys in antarctica and even the local brines from the chilean salars and the californian bristol dry lake the antarctic dry valley brines seem to be dominated by melting of ice cements of their permafrost sections the ca cl brines of springs in the chilean salars and the bristol dry lake ascend due to thermal convection along tectonic faults although the canadian shield brines chemically resemble those of the siberian platform their stable isotope fractionation suggest that they developed by different processes of water abstraction exotic stable isotope values in brines are gained by freezing or ultrafiltration processes in the precursor sediments of the present crystalline rocks of the canadian shield and by small volume ratios of liquids and metamorphic rocks such as of ktb vb

petrogenesis and tectonic implications of the triassic volcanic rocks in the northern yidun terrane eastern tibet abstract triassic paleo tethyan subduction related volcanic rocks are extensively distributed in the xiangcheng and changtai regions northern yidun terrane eastern tibet the 228 ma volcanic rocks in the xiangcheng region have intermediate felsic composition 60 1 to 64 7 wt sio 2 with relatively low y 14 18 ppm and high sr 630 1830 ppm yielding high sr y ratios 43 94 they have lree enriched and hree depleted ree patterns la yb 19 26 typical of adakites sr nd isotopic compositions of these volcanic rocks are similar to those of the contemporaneous high silica adakitic rocks in the southern yidun terrane the 231 230 ma volcanic rocks in the changtai region are composed of basalts andesites dacites and rhyolites with sio 2 ranging from 47 7 to 81 0 wt basalts in the changtai region have initial 87 sr 86 sr ratios from 0 7052 to 0 7058 and e nd t from 1 1 to 1 5 they display oib like geochemical signatures and were likely derived from low degrees of partial melting of an oib like mantle source with subordinate input of subduction components indicated by increasing th content andesites and dacites show elemental and isotopic compositions that evolved from the basalts indicative of derivation via crustal contamination and fractional crystallization afc of basaltic magmas rhyolites have lower initial 87 sr 86 sr ratios and higher e nd t values than andesites dacites inconsistent with an afc process and are suggested to have been derived from anatexis of crustal materials emplacement of volcanic rocks in the changtai and xiangcheng regions could be attributed to the subduction of the ganzi litang ocean a branch of the paleo tethys ocean volcanic rocks in the changtai and xiangchang regions have zircon u pb ages about 4 6 myr older than that of arc granites in the eastern yidun terrane the spatio temporal distribution of the volcanic rocks indicates that the subduction was initiated under the southern yidun terrane in the middle triassic and extended to the northern yidun terrane at the beginning of the late triassic

zircon u pb ages and geochemistry of the wenquan mo bearing granitioids in west qinling china constraints on the geodynamic setting for the newly discovered wenquan mo deposit abstract east qinling is the largest porphyry molybdenum province in the world these mo deposits have been well documented in west qinling however few mo deposits have been discovered although granitic rocks are widespread recently the wenquan porphyry mo deposit has been discovered in gansu province which provides an insight into mo mineralization in west qinling in this paper we report pb isotope compositions for k feldspar and sulfides s isotope ratios for sulfides the results obtained from petrochemical study and from in situ la icp ms zircon u pb dating and hf isotopes the granitoids are enriched in lile and lree with ree and trace element patterns similar to continental crust suggesting a crustal origin the mg 40 05 to 56 34 and cr and ni contents are high indicating a source of refractory mafic lower crust the e hf t values of zircon grains from porphyritic monzogranite range from 2 9 to 0 6 and from granitic porphyry vary from 3 3 to 1 9 the zircons have t dm2 of 1014 to 1196 ma for the porphyritic monzogranite and 954 to 1224 ma for the granitic porphyry implying that these granitoids were likely derived from partial melting of a late mesoproterozoic juvenile lower crust the pb isotope compositions of the granitoids are similar to granites in south china showing that the magma was sourced from the middle lower crust in the southern qinling tectonic unit the pb isotopic contrast between the mo bearing granitoids and ores shows that the pb in the ore forming solution was derived from fractionation of a triassic magmatic system 34 s values of sulfides are between 5 02 and 5 66 similar to those associated with magmatic hydrothermal systems la icp ms zircon u pb dating yields crystallization ages of 216 2 1 7 and 217 2 2 0 ma for the granitoids consistent with a previously reported molybdenite re os isochron age of 214 4 7 1 ma this suggests that the mo mineralization is related to the late triassic magmatism in the west qinling orogenic belt in view of these geochemical results and known regional geology we propose that both granitoid emplacement and mo mineralization in the wenquan deposit resulted from the triassic collision between the south qinling and the south china block along the mianlue suture since triassic granitoid plutons commonly occur along the qinling orogenic belt the triassic wenquan mo bearing granitoids highlight the importance of the triassic tectono magmatic belt for mo exploration in order to apply this metallogenic model to the whole qinling orogen further study is needed to compare the wenquan deposit with other deposits

tectonics of the qinling central china tectonostratigraphy geochronology and deformation history abstract the qinling orogen preserves a record of late mid proterozoic to cenozoic tectonism in central china high pressure metamorphism and ophiolite emplacement songshugou ophiolite assembled the yangtze craton including the lower qinling unit into rodinia during the 1 0 ga grenvillian orogeny the lower qinling unit then rifted from the yangtze craton at 0 7 ga subsequent intra oceanic arc formation at 470 490 ma was followed by accretion of the lower qinling unit first to the intra oceanic arc and then to the sino korea craton subduction then imprinted a 400 ma andean type magmatic arc onto all units north of the northern liuling unit oblique subduction created silurian devonian wnw trending sinistral transpressive wrench zones e g lo nan shang dan and late permian early triassic subduction reactivated them in dextral transpression lo nan shang xiang shang dan and subducted the northern edge of the yangtze craton exhumation of the cratonal edge formed the wudang metamorphic core complex during dominantly pure shear crustal extension at 230 235 ma post collisional south directed shortening continued through the early jurassic cretaceous reactivation of the qinling orogen started with nw se sinistral transtension coeval with large scale early cretaceous crustal extension and sinistral transtension in the northern dabie shan it presumably resulted from the combined effects of the siberia mongolia sino korean and lhasa west burma qiangtang indochina collisions and pacific subduction regional dextral wrenching was active within a ne sw extensional regime between 60 and 100 ma an early cretaceous andean type continental magmatic arc with widespread early cretaceous magmatism and back arc extension was overprinted by shortening related to the collision of yangtze indochina block with the west philippines block strike slip and normal faults associated with eocene half graben basins record paleogene nne ssw contraction and wnw ese extension the neogene is characterized by normal faults and nne trending sub horizontal extension pleistocene quaternary nw se extension and ne sw contraction comprises sinistral strike slip faults and is part of the nw se extension imposed across eastern asia by the india asia collision

geologic framework and tectonic evolution of the qinling orogen central china abstract the geologic framework of the qinling orogen was built up through interplay of three blocks the north china block including the north qinling the south qinling and the south china block separated by the shangdan and mianlue sutures the shangdan suture resulted from middle paleozoic collision of the north china block and the south qinling the mianlue suture resulted from late triassic collision of the south qinling and the south china block present upper crust of the qinling is structured dominantly by thrust fold systems the north qinling displays thick skinned deformation with crystalline basement involved whilst the south qinling is characterized by thin skinned thrusts and folds detached above the lower sinian two types of precambrian basement crystalline and transitional are defined according to lithology and metamorphic grade and different in age stratigraphic and sedimentary architecture is characterized by distinct zonation the qinling orogen experienced a prolonged continental divergence and convergence between blocks during the period from late neoproterozoic to early paleozoic times the south qinling was the northern margin of the south china block and the north qinling was the southern margin of the north china block separated by a proto tethyan qinling ocean the north qinling evolved into an active margin when the proto tethyan qinling ocean subducted northward during ordovician time collision of the south and north qinling took place in middle paleozoic along the shangdan suture synchronous with the collision rifting occurred at the southern rim of the south qinling and was followed by the opening of the paleo tethyan qinling ocean during the late paleozoic resulting in the splitting of the south china block from the south qinling collision of the south qinling and the south china block came about in the late triassic along the mianlue suture the late triassic collisional orogeny caused extensive fold and thrust deformation and granitoid intrusions throughout the qinling and led to final amalgamation of the north and south china blocks

structural style and tectonic significance of the jianglang dome in the eastern margin of the tibetan plateau china abstract the jianglang tectonic dome jtd lies on the eastern margin of the tibetan plateau and consists of three tectono stratigraphic units separated by two extensional detachment faults a schist complex a middle ductile slab and an overlying sedimentary unit the schist complex is composed of quartz mica schists and amphibolites of possible mesoproterozoic age and exhibits two early foliations s 1 and s 2 overprinted by two later foliations s 3 and s 4 the ductilely deformed middle slab consists of paleozoic quartzite phyllite marble amphibolite and meta basalt it is characterized by pervasive intrafolial and intraformational recumbent folds with a penetrative axial foliation s 1 an equivalent of s 3 in the basement rocks the cover unit the triassic xikang group of flysch sedimentary rocks experienced shallow level buckling and flattening deformation with an axial cleavage defining s 1 an equivalent of s 4 in the middle slab and basement rocks the jtd appears to have experienced three stages of extension immediately after orogeny the earliest d 1 recorded by an axial surface s 1 of s 0 intrafolial folds and sub parallel to s 0 in the schist complex probably occurred prior to the collision between the south china and north china blocks this extensional event was followed by subduction of the yangtze block to the west towards the eastern margin of the tibetan plateau the two detachment faults are characterized by structures formed by ductile brittle to brittle shear deformation which are sub parallel to the layer of s 2 in the lower detachment and to s 0 in the upper detachment these faults mark the second stage of extension and may have caused thinning and selective removal of the sinian and paleozoic strata as well as exhumation of the schist complex and the middle slab in the middle jurassic 177 ma the final stage of extension d 5 shown by brittle deformation and thermo metamorphism from the lower detachment to the xikang group was associated with the indian eurasia collision

paleomagnetic evidence for post cretaceous internal deformation of the chuan dian fragment in the yangtze block a consequence of indentation of india into asia abstract paleomagnetic samples of paleocene eocene red sandstones were collected at 36 sites from the jiangdihe 4 and zhaojiadian formations around the yongren 26 1 n 101 7 e and dayao areas 25 7 n 101 3 e these areas are located in the chuxiong basin of the chuan dian fragment southwestern part of the yangtze block after stepwise thermal demagnetization a high temperature component with unblocking temperature of about 680 c is isolated from 26 sites the primary nature of this magnetization is ascertained through positive fold and reversal tests at 95 confidence level the tilt corrected mean paleomagnetic directions for the yongren and dayao areas are d 17 2 i 26 6 with 95 5 8 and d 16 5 i 31 1 with 95 4 8 respectively easterly deflected declinations from this study are consistent with those reported from other areas of the chuxiong basin indicating its wide presence in the cretaceous eocene formations of the said basin comparison with declination values expected from the cretaceous eocene apwp of eurasia indicates that the magnitude of clockwise rotation systematically increases toward the southeast within the chuxiong basin as well as in the chuan dian fragment this trend of the differential tectonic rotation in the chuan dian fragment is consistent with curvature of the xianshuihe xiojiang fault system deformation of the chuxiong basin can fairly be associated with the formation of eastward bulge in the southern part of the chuan dian fragment during southward displacement the chuan dian fragment was probably subjected to tectonic stresses as a result interaction with the yangtze and indochina blocks which resulted into east west extension and north south shortening

mesozoic intracontinental orogeny in the qinling mountains central china abstract the qinling orogenic belt has been well documented that it was formed by multiple steps of convergence and subsequent collision between the north china and south china blocks during paleozoic and late triassic times following the collision in late triassic times the whole range evolved into an intracontinental tectonic process the geological geophysical and geochronological data suggest that the intracontinental tectonic evolutionary history of the qinling orogenic belt allow deduce three stages including strike slip faulting during early jurrassic n s compressional deformation during late jurassic to early cretaceous and orogenic collapse during late cretaceous to paleogene the strike slip faulting and the infills in early jurassic along some major boundary faults show flower structures and pull apart basins related to the continued compression after late triassic collision between the south qinling belt and the south china block along the mianlue suture late jurassic to early cretaceous large scale of n s compression and overthrusting progressed outwards from inner of qinling orogen to the north china block and south china block due to the renewed southward intracontinental subduction of the north china block beneath the qinling orogenic belt and continuously northward subduction of the south china block respectively after the late jurassic early cretaceous compression and denudation the qinling orogenic belt evolved into late cretaceous to paleogene orogen collapse and depression and formed many large fault basins along the major faults

miocene emplacement and deformation of the konga shan granite xianshui he fault zone west sichuan china geodynamic implications the presently active sinistral xianshui he strike slip fault xsh is a lithospheric scale strike slip fault in the eastern himalaya in the study area this fault affects the eastern edge of the konga shan granitic massif where it has caused both brittle and ductile deformation a rbsr isochron and nd and pb isotope study of three samples and a upb zircon study of a single sample were completed on the granite upb data indicate a granite emplacement age of 12 8 1 4 ma the rbsr isochrons show that the granite emplacement and the deformational event were synchronous at around 12 10 ma minimum age for deformation the nd and pb isotope compositions of whole rocks and k feldspars indicate the involvement of proterozoic continental crust which is confirmed by upb systematics indicating inherited zircons sinistral faulting along the xsh began at the latest at 12 ma and marks the extrusion toward the east of the west sichuan and south china blocks following their extrusion along the red river fault zone between 50 and 21 ma

emplacement of the longmen shan thrust nappe belt along the eastern margin of the tibetan plateau abstract the topographic descent from the eastern margin of the tibetan plateau to the sichuan basin passes through the longmen shan thrust nappe belt which is sub divided by six nw dipping major listric thrusts with accompanying duplexes and imbricate fans into five large scale nappes each nappe has its own stratigraphic and deformational features in the inner longmen shan longmen mountains allochthonous nappe units have incorporated both mesoproterozoic basement and sinian neoproterozoic to triassic cover sequences as thick skinned horses whereas in the frontal longmen shan sinian cretaceous cover sediments have been stripped from the basement as thin skinned fold and thrust sheets including the extensively distributed klippen structures pre thrusting extension during devonian to middle late triassic times resulted in syn depositional normal faults structural inversion of these faults initiated the peng xian guan xian basement complex jiuding shan and tangwangzhai nappes during the early episode of the indosinian orogeny norian to rhaetian this was followed by episodic thrusting during latest triassic to early cretaceous times to develop the guan xian an xian and southeastern marginal nappes that have incorporated sediments from the neighbouring foreland basin into the frontal part of the thrust nappe belt during a late miocene reactivation of the pre existing thrusts differential thrusting occurred across the thrust nappe belt

permian peralkaline peraluminous and metaluminous a type granites in the panxi district sw china their relationship to the emeishan mantle plume abstract numerous permian granitic intrusions plutons in the panxi region sw china are spatially and temporally associated with the emeishan large igneous province elip granites from the 260 ma panzhihua and taihe plutons are peralkaline asi 0 76 1 12 na k al 0 78 1 2 and have high fe values 0 92 0 99 with negative eu anomalies eu eu 0 22 0 98 contrasted with the peralkaline plutons syenites from the 260 ma woshui pluton are metaluminous asi 0 84 1 0 na k al 0 91 1 01 and have moderately high fe 0 79 0 81 and positive eu anomalies eu eu 0 82 1 91 both the peralkaline and metaluminous rocks have mantle isotopic signatures end t 1 3 3 2 and were derived from the elip mantle plume source the peralkaline rocks were likely derived by fractional crystallization of the temporally spatially and chemically associated layered mafic intrusions which host giant magmatic fe ti v oxide deposits the metaluminous granitic rocks although spatially related to the peralkaline granites and layered gabbros were likely generated by partial melting of underplated mafic rocks granites from the 251 ma ailanghe pluton are peraluminous asi 0 97 1 29 and have high fe 0 82 0 91 values and negative eu anomalies eu eu 0 10 0 37 their crustal isotopic signature end t 5 7 to 6 7 indicates possible derivation from melting of yangtze block basement rocks the ailanghe pluton is contemporaneous with other a type granites in the region supporting the view that elip magmatism was sporadic for at least 9 million years the coexistence of three chemically exclusive a type granites in the elip suggests that a variety of a type granites can be generated in the same tectonic setting

reappraisal of the ages of neoproterozoic strata in south china no connection with the grenvillian orogeny the jiangnan fold belt separates the yangtze and cathaysia blocks in south china and has long been considered grenvillian in age in order to place south china in central rodinia it consists of deformed early neoproterozoic strata that are unconformably overlain by undeformed late neoproterozoic strata and intruded by undeformed and unmetamorphosed granitic plutons zircons from the early neoproterozoic strata yield u pb ages as young as 830 ma and one granitic pluton has a zircon u pb age of ca 827 ma the 830 ma mafic rocks along the southeastern margin of the yangtze block have arc affinity geochemical characters whereas mafic rocks younger than 830 ma have typical ocean island basalt oib like compositions thus we suggest that the early neoproterozoic strata were deposited on an active continental margin prior to amalgamation of the yangtze and cathaysia blocks at ca 830 ma the overlying late neoproterozoic strata were deposited in the intracontinental rifted nanhua basin at 820 730 ma and probably reflect backarc spreading above the long lived 950 735 ma oceanic subduction zone along the northern and western margin of the yangtze block this model is consistent with the secular tectonic evolution of south china during the neoproterozoic the jiangnan fold belt is therefore not a grenvillian feature as previously suggested and there is no evidence to place south china in central rodinia instead we believe that south china was located in a marginal position relative to this supercontinent

neoproterozoic adakitic plutons and arc magmatism along the western margin of the yangtze block south china abstract neoproterozoic adakitic plutons that crop out along the western margin of the yangtze block south china from kangding on the north to panxi on the south provide constraints on the origin of the giant jinningian magmatic event of south china representative plutons include the xuelongbao 750 ma datian 760 ma and dajianshan intrusions the latter two bodies consist mainly of granodiorite with relatively high sio2 51 0 73 4 wt and mg s 0 36 0 55 they have fractionated rare earth element patterns with la yb n ratios ranging from 2 6 to 101 8 and are characterized by high sr 344 1018 ppm and low y 4 3 17 9 ppm yielding sr y ratios ranging from 27 to 111 on primitive mantle normalized trace element diagrams these rocks show enrichment of large ion lithophile elements and depletion of high field strength elements nb ta with positive zr hf and negative ti anomalies consistent with an arc related setting they have relatively constant initial whole rock 87sr 86sr ratios 0 70

paleokarst on the top of the maokou formation further evidence for domal crustal uplift prior to the emeishan flood volcanism abstract the 260 ma emeishan large igneous province elip in southwest china has previously been demonstrated to provide compelling evidence for pre volcanic crustal doming in support of the mantle plume hypothesis however this has been questioned by ukstins peate and bryan 2008 by showing hydrothermal magmatic activity at the daqiao section to solve this argument a detailed characterization of the contact between the emeishan basalts and the maokou formation was carried out the contact is shown to be an unconformity which is characterized by paleokarst on top of the maokou formation including paleokarst relief sinkholes caves tower karst and its corresponding rocks such as kaolinite bauxite and ferruginous duricrust and collapsed breccias etc this paleokarst unconformity was in turn covered or infilled by the emeishan basalts and tuffs suggesting that uplift and erosion occurred prior to the eruption of the elip the extent of erosion of the maokou formation indicates the elip can be divided into three roughly concentric zones the inner intermediate and outer zones the paleokarst features on the top of maokou formation vary across the elip in the inner zone a likely sinkhole and an incision valley with 450 m relief in height are found in the intermediate zone various paleokarst landforms such as karst relief sinkholes and tower karsts are well developed some sinkholes that developed in the qixia formation below the maokou formation imply that the paleorelief is more than 350 m in height in the outer zone the paleokarstic surface is a paleo weathering layer with minor karstification and development of caves at 10 50 m this spatial variation of the paleokarst reflects variation of uplift height across the elip the extent of minimal uplift is estimated to be at least 450 m in the inner zone 350 m in the intermediate zone whereas uplift is minor tens 50 m in the outer zone the magnitude and shape of the uplift is roughly consistent with that predicted by mantle plume models the paleokarst was formed after the deposition of the maokou formation and the eruption of the emeishan basalts at the end guadalupian and indicates a short duration of uplift thus this study lends further support to domal uplift prior to the emeishan flood volcanism but also to the mantle plume initiation model for the generation of the elip

timing of granite emplacement and cooling in the songpan garze fold belt eastern tibetan plateau with tectonic implications abstract new u pb and rb sr geochronology on syn and post orogenic granites provide constraints on the timing of major tectonic events in the songpan garze fold belt west sichuan china the ma nai granite was probably syn kinematic with the main deformation and yields an age of 197 6 ma that is interpreted as an upper age limit of the indosinian event zircons and apatites from the post kinematic rilonguan granite also yield jurassic ages 195 6 and 181 4 ma the post orogenic markam massif gives two ages of 188 1 and 153 3 ma both granites are undeformed and cut structures in the triassic sedimentary rocks these results demonstrate that the major deformation and decollement tectonics in the songpan garze fold belt occurred prior to the early jurassic the wide range of ages obtained for post kinematic granites from early jurassic to late jurassic suggests that locally magmatic activity persisted for a long time at least 50 ma after the indosinian compressional tectonism no tertiary ages have been obtained suggesting that these granites were not affected strongly by the india asia collision

provenance and tectonic setting of the neoproterozoic yanbian group western yangtze block sw china abstract tectonic evolution of the yangtze block of south china during neoproterozoic time has been a major focus of debate and is important in the reconstruction of the rodinian supercontinent the yanbian group in the western margin of the yangtze block is a well preserved neoproterozoic volcanic sedimentary sequence and provides a rare opportunity to examine the provenance and tectonic setting it consists of a lower part composed of 1500 m thick basaltic lavas and an upper part of 3500 m thick flysch deposits the flysch sequence contains typical deep marine turbidites consisting mainly of fine grained volcaniclastic sandstones and mudstones indicating a submarine fan depositional system detrital zircon dating yields ages ranging from ca 865 ma to 1000 ma with two peaks at ca 900 ma and 920 ma respectively the yanbian group is intruded by the 860 ma guandaoshan dioritic pluton constraining its depositional age at ca 870 ma the sandstones contain abundant feldspar and lithic fragments with minor amounts of quartz and have an average composition of q16f35l49 suggesting a proximal source and an undissected to transitional arc setting the lithic fragments in the sandstones point to andesitic and felsic volcanic rock sources the sandstones and mudstones have intermediate sio2 al2o3 typically 3 6 high fe2o3 mgo contents 5 12 wt and moderate to high k2o na2o ratios generally 0 1 1 and 1 10 for sandstones and mudstones respectively in comparison with average upper continental crust they show strong negative nb ta anomalies slight depletion of zr hf la and th but moderate enrichment of v cr ni and sc these rocks show lree enrichment la ybn 5 3 7 4 with flat hree and pronounced negative eu anomalies eu eu 0 6 0 74 in chondrite normalized ree patterns similar to post archean shales geochemical data for these rocks suggest an arc setting and intermediate felsic volcanic source consistent with a back arc basin scenario for the basaltic lavas in the lower yanbian group together with regional geological evidence the turbidites of the yanbian group strongly support a model in which the western margin of the yangtze block was a major magmatic arc active probably from 920 ma to 740 ma a period of more than 180 million years

late cenozoic to holocene deformation in southwestern sichuan and adjacent yunnan china and its role in formation of the southeastern part of the tibetan plateau from at least 2 4 ma to present crust in the southeastern part of the tibetan plateau west of the convex east xianshuihe xiaojiang fault system has deformed internally and rotated clockwise around the eastern himalayan syntaxis the northwest striking ganzi fault zone bounds the rotating crust on the north and has a total left slip of 78 100 km of which 60 km is transferred to the xianshuihe fault zone across a diffuse transfer zone and 22 40 km is absorbed by bending of older structures and crustal shortening crustal shortening is expressed along and east of the eastern end of the ganzi fault zone by mountains capped by permanent glaciers locally rising nearly 1000 m above the average elevation of the tibetan plateau a similar transfer of left slip into shortening occurs farther south across the xianshuihe fault in the high mountains around and east of gongga shan 7556 m the northwest striking convex east left lateral litang fault zone lies southwest of the ganzi xianshuihe xiaojiang fault zone and appears to be less well developed but otherwise similar to the ganzi fault zone the batang chenzhi and other northeast striking right lateral faults of small displacement occur within the rotating crustal fragment together with the left slip faults they accommodate east west shortening northeast of the eastern himalayan syntaxis south of this region of shortening the crust is extending to form grabens within the dali and southern xiaojiang fault systems and in the tengchong volcanic province the progressive change from shortening southward into extension is related to variations in strain that characterize the region from northeast to southeast of the eastern himalayan syntaxis the assemblage of structures in southwestern sichuan geometrically resembles structures of eocene to miocene age in southern yunnan that were positioned northeast of the eastern himalayan syntaxis similar to present day southwestern sichuan at the time of their development the similarity in the structural development in the two areas indicates that crust northeast of the syntaxis underwent a common evolution as the syntaxis migrated northward during the past 50 m y structures in sichuan are less fully developed than older structures in southwestern yunnan and can serve as a guide to reconstruct the progressive tectonic development in the region of the syntaxis deformation in these areas indicates that plateau formation has been complex inhomogeneous and diachronous at scales from 1000 km to less than 100 km

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

geology of the ulugh muztagh area northern tibet abstract within the ulugh muztagh area north central tibet an east west trending ophiolitic melange marks a suture that apparently was formed during a late triassic or slightly younger collision between a continental fragment to the south and the rest of asia the southern continental fragment carries a thick sequence of upper triassic sandstone but the contact between the sandstone and the ophiolitic melange is covered by a younger redbed sequence of unknown age a suite of 2 mica tourmaline bearing leucogranite plutons and dikes intruded the triassic sandstone at shallow crustal levels 10 5 to 8 4 ma these rocks range from granite to tonalite in composition are geochemically very similar to slightly older high himalayan leucogranite and are interpreted to have been derived by the partial melting of crustal material we interpret this to mean that crustal thickening began in this part of the tibetan plateau at least by 10 5 ma welded rhyolitic tuff rests on a conglomerate that consists of abundant debris from the ulugh muztagh intrusive rocks and has yielded ar ar ages of about 4 ma the tuffs are geochemically identical to the intrusive rocks suggesting that crustal thickening may have continued to 4 ma crustal thickening probably occurred by distributed crustal shortening similar to shortening now occurring north of ulugh muztagh along the northern margin of the tibetan plateau

structural and geochronological constraints on the tectono thermal evolution of the danba domal terrane eastern margin of the tibetan plateau the songpan ganze terrane of the tibetan plateau is underlain by neoproterozoic crystalline basement rocks of the yangtze block these basement rocks are exposed as a series of extensional tectonic domes that form a nearly north south trending extensional belt more than 1000 km long in the eastern margin of the tibetan plateau in the danba area detachment faults separate the basement core complexes e g the gezong and gongcai complexes from the paleozoic strata which have been thinned or removed completely the cover sequences have undergone upper greenschist to lower amphibolite facies metamorphism to form the danba schist and are overlain by the triassic xikang group a thick flysch sequence both the basement rocks and the paleozoic rocks have undergone multiple stages of deformation and thus provide an excellent opportunity to study the tectono thermal evolution of the eastern margin of the tibetan plateau two stages of deformation corresponding to three generations of foliation s1 view the mathml source and view the mathml source have been recognized on the basis of structural and microscopic observations we selected amphibole and biotite separates associated with distinct generations of foliation for 40ar 39ar dating using laser microprobe incremental heating technique to place numerical constraints on the major tectono thermal events within the danba area the geochronogical results reveal an earliest metamorphic event at 258 6 0 5 ma s1 biotite and 263 6 0 8 ma s1 amphibole coinciding temporally with the mantle plume that produced the voluminous emeishan flood basalts the second event was a progressive extensional deformation first occurred at 159 166 ma view the mathml source amphibole responsible for the earlier tectonic doming of the crystalline basement and then the final tectono thermal overprint recorded by view the mathml source foliation and view the mathml source metamorphism locally in the core complexes at 47 58 ma for the gezong complex and 64 81 ma for the gongcai complex this major post orogenic extensional event is believed to be a consequence of collision between the north china and south china blocks the apparent discrepancy of the 40ar 39ar ages observed between localities suggests a slow cooling process associated with progressive uplift

differential exhumation in response to episodic thrusting along the eastern margin of the tibetan plateau abstract thermochronological data from the songpan ganze fold belt and longmen mountains thrust nappe belt on the eastern margin of the tibetan plateau in central china reveal several phases of differential cooling across major listric thrust faults since early cretaceous times differential cooling indicated by distinct breaks in age data across discrete compressional structures was superimposed upon a regional cooling pattern following the late triassic indosinian orogeny 40ar 39ar data from muscovite from the central and southern longmen mountains thrust nappe belt suggest a phase of differential cooling across the wenchuan maouwen shear zone during the early cretaceous the zircon fission track data also indicate differential cooling across a zone of brittle re activation on the eastern margin of the wenchuan maouwen shear zone during the mid tertiary between 38 and 10 ma apatite fission track data from the central and southern longmen mountains thrust nappe belt reveal differential cooling across the yingxiu beichuan and erwangmiao faults during the miocene forward modelling of apatite fission track data from the northern longmen mountains thrust nappe belt suggests relatively slow regional cooling through the mesozoic and early tertiary followed by accelerated cooling during the miocene beginning at ca 20 ma to present day regional cooling is attributed to erosion during exhumation of the evolving longmen mountains thrust nappe belt lmtnb following the indosinian orogeny differential cooling across the wenchuan maouwen shear zone and the yingxiu beichuan and erwangmiao faults is attributed to exhumation of the hanging walls of active listric thrust faults thermochronological data from the longmen mountains thrust nappe belt reveal a greater amount of differential exhumation across thrust faults from north to south this observation is in accord with the prevalence of proterozoic and sinian basement in the hanging walls of thrust faults in the central and southern longmen mountains the two most recent phases of reactivation occurred following the initial collision of india with eurasia suggesting that lateral extrusion of crustal material in response to this collision was focused along discrete structures in the lmtnb

viscous collision in channel explains double domes in metamorphic core complexes in hot orogens gneiss domes are a response to upper crustal stretching and lower crustal flow two dimensional thermal mechanical modeling shows that localization of extension in the upper crust triggers in the deep crust oppositely verging horizontal flows that converge beneath the extended region upon viscous collision both flowing regions rotate upward to form two upright domes of foliation double domes separated by a steep median high strain zone in such systems horizontal shortening in the infrastructure develops in an overall extensional setting dome material follows a complex depth dependent strain history from shearing in the deep crustal channel to contraction upon viscous collision in the median high strain zone to extension upon advection into the shallow crust this depth dependent strain history is likely a general feature of dome evolution and is arguably well preserved in double domes such as the montagne noire france and naxos greece gneiss domes

neoproterozoic crustal growth and reworking of the northwestern yangtze block constraints from the xixiang dioritic intrusion south china abstract neoproterozoic magmatism along the northwestern margin of the yangtze block south china produced voluminous adakitic and normal arc felsic intrusions associated with many mafic ultramafic bodies dioritic plutons are rare but are important for understanding the crustal growth and secular evolution of the region the xixiang dioritic intrusion at hannan on the northwestern margin of the yangtze block provides a good opportunity to investigate the petrogenesis of intermediate plutons and to document the neoproterozoic continental crustal evolution of the region the pluton has a zircon u pb age of 764 9 ma rocks from the pluton have relatively low sio 2 59 3 63 0 wt and k 2 o na 2 o 5 8 6 2 wt and high mgo 2 3 3 1 wt and al 2 o 3 15 8 17 1 wt they have chondrite normalized ree patterns enriched in lree la ybn 7 2 14 7 with negative eu anomalies their primitive mantle normalized trace element patterns are characterized by enrichment of lile and depletion of hfse nb and ta with positive pb and negative ti anomalies they have a narrow range of initial 87 sr 86 sr ratios 0 7033 to 0 7035 with positive end values 1 94 to 3 16 their zircon ehf values range from 5 5 to 9 8 with an average of 7 32 these isotopic data are similar to those of the neoproterozoic mafic ultramafic intrusions in the hannan region suggesting that the diorites were generated by partial melting of newly emplaced basaltic rocks the diorites have mg values 50 3 54 4 higher than those of experimental melts of mafic igneous rocks in addition they contain fe rich clinopyroxene suggesting that partial melts of the newly formed mafic lower crust interacted with the mafic residue based on its arc related geochemical affinity and emplacement preceding voluminous adakitic magmatism the xixiang intrusion is proposed to have been generated in a neoproterozoic arc setting during crustal growth and reworking that lasted from 950 ma to

mesozoic plutons of the yidun arc sw china u pb geochronology and hf isotopic signature abstract the yidun arc is a triassic volcanic arc located between the songpan garze fold belt and the qiangtang block southwest china to constrain the age of a number of the major granitic plutons from the yidun arc laser ablation icp ms u pb analysis of zircon was conducted hafnium isotope data was also acquired through laser ablation multicollector icpms analysis of zircon with the aim of gaining insight into the age and nature of the source region of the plutons three age groups have been identified from seven granite samples early middle triassic 245 to 229 ma late triassic 219 to 216 ma and cretaceous 105 to 95 ma hafnium analysis shows the triassic granites to have negative and variable e hf values and mesoproterozoic 1 6 ga depleted mantle model ages which is interpreted to reflect derivation from an isotopically heterogeneous largely crustal source the cretaceous granite shows higher and less variable e hf values and slightly younger model ages 1 3 ga and is interpreted to be derived from melting of a more homogeneous crustal source a depleted mantle model age of 1 5 ga is calculated from the pooled triassic and cretaceous samples the source region for these magmas may be tentatively correlated with mesoproterozoic material of the yangtze craton which has been suggested to underlie the yidun arc however further work is necessary to demonstrate this suggestion