geochemistry and geochronology of the amphibolite blocks in ophiolitic melanges along bangong nujiang suture central tibet abstract amphibolites occur as blocks within serpentinite matrix melange at dong tso and lagkor tso in the gertse area of central tibet where they are associated with the bangong nujiang suture bns geochemical data shows amphibolites have similar morb or arc like morb geochemical characteristics lagkor tso amphibolites show greater arc affinity whereas those from dong tso amphibolites are more morb like these characteristics are similar to basalts from continental back arc basin settings hornblendes from the amphibolites dated by the 40 ar 39 ar single grain laser fusion technique show similar metamorphic ages 170 177 ma which is interpreted to represent the timing of the main metamorphic event experienced in this region we suggest that the western and middle sectors of the bns contain the remnants of a short lived back arc basin the bangong nujiang ocean which partly opened and closed in the middle jurassic

palaeontology and u pb detrital zircon geochronology of upper triassic strata on the northern margin of the bangong co nujiang suture zone tibet constraints on the age of opening of the meso tethys abstract we present the results of field mapping paleontology and detrital zircon geochronology of newly found clastic rocks with intercalations of limestone in the gaerqin area of tibet gaerqin formation the gaerqin formation consists of sandstone siltstone and shale with intercalations of bioclastic limestone abundant late triassic norian fossils are found in the limestone beds the fossils include distichophyllia norica retiophyllia litangensis and mesozoic chaetetida in addition 273 detrital zircons were selected for u pb dating yielding the following age groups 210 300 500 600 800 1100 1600 1800 and 2400 2600 ma more than 80 of the detrital zircons are older than 550 ma and the most reliable youngest detrital zircon age for the gaerqin formation is 219 ma the results indicate that the gaerqin formation formed in a shallow marine to bathyal continental slope environment and represents the first reported upper triassic norian strata in the gaerqin area of tibet the findings also suggest that the bangong co nujiang tethys ocean was a bathyal basin by the late triassic

petrogenesis of ore bearing porphyries from the duolong porphyry cu au deposit central tibet evidence from u pb geochronology petrochemistry and sr nd hf o isotope characteristics abstract the duolong porphyry cu au deposit 5 4 mt at 0 72 cu 41 t at 0 23 g t au which is related to the granodiorite porphyry and the quartz diorite porphyry from the bangongco copper belt in central tibet formed in a continental arc setting here we present the zircon u pb ages geochemical whole rock sr nd whole rock and zircon in situ hf o isotopic data for the duolong porphyries secondary ion mass spectrometry sims zircon u pb analyses for six samples yielded consistent ages of 118 ma indicating a cretaceous formation age the duolong porphyries sio2 of 58 81 68 81 wt k2o of 2 90 5 17 wt belong to the high k calc alkaline series they show light rare earth element lree enriched distribution patterns with la yb n 6 1 11 7 enrichment in large ion lithophile elements e g cs rb and ba and depletion of high field strength elements e g nb with negative ti anomalies all zircons from the duolong porphyries share relatively similar hf o isotopic compositions 18o 5 88 7 27 ehf t 3 6 7 3 indicating that they crystallized from a series of cogenetic melts with various degrees of fractional crystallization this along with the general absence of older inherited zircons rules out significant crustal contamination during zircon growth the zircons are mostly enriched in 18o relative to mantle values indicating the involvement of an 18o enriched crustal source in the generation of the duolong porphyries together with the presence of syn mineralization basaltic andesite the mixing between silicic melts derived from the lower crust and evolved h2o rich mafic melts derived from the metasomatized mantle wedge followed by subsequent fractional crystallization fc and minor crustal contamination in the shallow crust could well explain the petrogenesis of the duolong porphyries significantly the hybrid melts possibly inherited the arc magma characteristics of abundant f cl cu and au elements and high oxidation state which contributed to the formation of the duolong porphyry cu au deposit

response of aeolian systems to holocene climatic and hydrologic changes on the northern margin of the sahara a high resolution record from the chott rharsa basin tunisia four millennial scale latest pleistocene to holocene climatic cycles are recorded by aeolian lacus trine strata along the southern margin of chott rharsa a playa situated within an active tectonic basin along the northern margin of the sahara desert in tunisia cycles are characterized by 1 a humid phase of dunefield stabilization and high lake levels and 2 an arid phase of playa desiccation and deflation with dunefield construction the cycles overall define a progressive drying trend and correlate well with a detailed chronology from morocco and with the sahara region in general for the latest pleistocene and early holocene climatic events appear more regional in nature for the later holocene an isolated outcrop of aeolian deposits dated at 86 ka bp argues for records of older cycles in the chott rharsa basin that were subsequently removed by long term deflation in the area

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

origin and tectonic setting of the giant duolong cu au deposit south qiangtang terrane tibet evidence from geochronology and geochemistry of early cretaceous intrusive rocks abstract we constrain the origin and tectonic setting of the giant duolong porphyry epithermal cu au deposit in the south qiangtang terrane of northern tibet based on new zircon u pb ages and hf isotopic data as well as whole rock major and trace element data from poorly studied ore associated intrusions in the duolong area the la icp ms zircon u pb dating indicates that the ore associated rocks formed between 121 and 126 ma these ore associated rocks are geochemically similar to low k tholeiitic m type granitoids and to mid to high k calc alkaline i type granitoids they have variable and predominantly positive zircon e hf t values 1 4 to 15 6 and variable crustal model ages t c dm hf 176 1122 ma taking into account previous data and the regional geology of the study area we propose that the ore associated rocks originated from fractional crystallization of mantle derived mafic melts and magma mixing of mantle derived mafic and hybrid lower crust derived felsic melts and the hybrid lower crust included a mix of juvenile and older continental material the duolong porphyry epithermal cu au deposit formed within an ensialic forearc of an active continental margin as a result of the northwards subduction of the bangong nujiang ocean crust beneath the south qiangtang terrane

predictive modelling of structure evolution in sandbox experiments in this paper a computational approach is presented that is able to forward model complex structural evolution with multiple intersecting faults that exhibit large relative movement the approach adopts the lagrangian method complemented by robust and efficient automated adaptive meshing techniques a constitutive model based on critical state concepts and global energy dissipation regularized by inclusion of fracture energy in the equations governing state variable evolution the efficacy of the approach is benchmarked by forward simulation of two extensional analogue experiments that exhibit the development of a roll over anticline with a series of superimposed crestal collapse graben systems these sandbox experiments are excellent benchmarks for computational models as both intersecting localisations with different rates of slip and large relative movement on localisations must be represented while other complex phenomena associated with structural evolution over geological timeframes are not present

magmatic hydrothermal evolution of the cretaceous duolong gold rich porphyry copper deposit in the bangongco metallogenic belt tibet evidence from u pb and 40ar 39ar geochronology abstract the duolong gold rich porphyry copper deposit was recently discovered and represents a giant prospect inferred resources of 4 5 mt fine cu with a grade of 0 72 cu 30 50 t fine gold with a grade of 0 23 g t au in the bangongco metallogenic belt tibet zircon shrimp and la icp ms u pb geochronology shows that the multiple porphyritic intrusions were emplaced during two episodes the first at about 121 ma bolong mineralized granodiorite porphyry bmgp and barren granodiorite porphyry bgp and the second about 116 ma duobuza mineralized granodiorite porphyry dmgp moreover the basaltic andesites also have two episodes at about 118 ma and 106 ma respectively one andesite yields an u pb zircon age of 111 9 1 9 ma indicating it formed after the multiple granodiorite porphyries by contrast the 40ar 39ar age of 115 2 1 1 ma hydrothermal k feldspar vein hosted in dmgp reveals the close temporal relationship of ore bearing potassic alteration to the emplacement of the dmgp the sericite from quartz sericite vein hosted in dmgp yields a 40ar 39ar age of 115 2 1 2 ma therefore the ore forming magmatic hydrothermal evolution probably persisted for 6 m y additionally the zircon u pb ages 106 121 ma of the volcanic rocks and the porphyries suggest that the neo tethys ocean was still subducting northward during the early cretaceous

palaeozoic and mesozoic tectonic evolution and palaeogeography of east asian crustal fragments the korean peninsula in context east and southeast asia comprises a complex assembly of allochthonous continental lithospheric crustal fragments terranes together with volcanic arcs and other terranes of oceanic and accretionary complex origins located at the zone of convergence between the eurasian indoaustralian and pacific plates the former wide separation of asian terranes is indicated by contrasting faunas and floras developed on adjacent terranes due to their prior geographic separation different palaeoclimates and biogeographic isolation the boundaries between asian terranes are marked by major geological discontinuities suture zones that represent former ocean basins that once separated them in some cases the ocean basins have been completely destroyed and terrane boundaries are marked by major fault zones in other cases remnants of the ocean basins and of subduction accretion complexes remain and provide valuable information on the tectonic history of the terranes the oceans that once separated them and timings of amalgamation and accretion the various allochthonous crustal fragments of east asia have been brought into close juxtaposition by geological convergent plate tectonic processes the gondwana derived east asia crustal fragments successively rifted and separated from the margin of eastern gondwana as three elongate continental slivers in the devonian early permian and late triassic late jurassic as these three continental slivers separated from gondwana three successive ocean basins the palaeo tethys meso tethys and cenotethys opened between these and gondwana asian terranes progressively sutured to one another during the palaeozoic to cenozoic south china and indochina probably amalgamated in the early carboniferous but alternative scenarios with collision in the permo triassic have been suggested the tarim terrane accreted to eurasia in the early permian the sibumasu and qiangtang terranes collided and sutured with simao indochina east malaya in the early middle triassic and the west sumatra terrane was transported westwards to a position outboard of sibumasu during this collisional process the permo triassic also saw the progressive collision between south and north china with possible extension of this collision being recognised in the korean peninsula culminating in the late triassic north china did not finally weld to asia until the late jurassic the lhasa and west burma terranes accreted to eurasia in the late jurassic early cretaceous and proto east and southeast asia had formed palaeogeographic reconstructions illustrating the evolution and assembly of asian crustal fragments during the phanerozoic are presented d 2005 international association for gondwana research published by elsevier b v all rights reserved

episodic sedimentation how normal is average how rare is rare does it matter where repeated sharp changes of sedimentation are inescapable periodic cycles are commonly invoked to preserve uniform orderly variations from some supposed norm the sedimentary record rarely reflects such uniformity however as sedimentologists have gradually realized magnitude versus frequency of processes has received little attention in sedimentology recurrence interval recovery time and preservation potential are critical factors for evaluating their significance for the sedimentary record large magnitude processes which represent positive deviations from the norm and are rare on the human time scale must be significant over geologic time everyday processes may have obliterated much of the evidence the sedimentological importance of rare events is difficult to assess because the record of such events may be very subtle this is especially true if a deposit has been thoroughly bioturbated or if the record of an event is simply an erosional surface it has now become possible to evaluate quantitatively ancient episodic sedimentation using modern process rates as well as refined biostratigraphic and isotopic dating i predict that such evaluation will necessitate revision of our favorite depositional models which have become so important for exploration as well as for research modified journal abstract

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

subduction of a spreading ridge within the bangong co nujiang tethys ocean evidence from early cretaceous mafic dykes in the duolong porphyry cu au deposit western tibet abstract the intrusion of mafic dykes into a near trench accretionary prism and continental margin magmatism with characteristics that differ from those of adjacent arc magmatism are direct manifestations of the subduction of a spreading ocean ridge and the formation of a slab window in this paper we investigated mafic dykes intruded into the accretionary prism that hosts the duolong porphyry cu au deposit dcad of western tibet la icp ms analysis of u pb in zircon indicates that the dykes formed during the early cretaceous 126 127 ma the dykes are characterized by ehf t values from 2 44 to 11 8 twenty nine mafic dyke samples were divided into three groups based on their locations and geochemical compositions group i has nb 8 31 10 2 ppm nb la 0 71 1 20 and nb u 21 4 37 9 group ii has nb 40 5 52 6 ppm nb la 0 84 1 58 and nb u 18 8 47 8 and group iii has nb 65 7 105 ppm nb la 1 35 2 08 and nb u 36 5 73 8 group i is classified as nb enriched basalts enbs whereas groups ii and iii are classified as high nb basalts hnbs both the enbs and hnbs were derived from an adakite metasomatized mantle wedge that subsequently underwent crystallization of olivine and clinopyroxene the compositional variations of the studied dykes resulted mainly from mantle source heterogeneity the volume of the slab melts gradually increases from group i enbs to group ii hnbs and group iii hnbs leading to gradually increasing incompatible element concentrations considering their geochemical characteristics and field relationships as well as the unique characteristics of continental margin magmatism in the dcad we propose that the dykes emplaced in an extensional accretionary prism were derived from the northward subduction of a spreading ridge in the bangong co nujiang tethys ocean during the early cretaceous the ridge subduction event was also responsible for the generation of coeval adakites intermediate felsic intrusions the maierze bimodal volcanic rocks as well as the metallogenesis of the dcad

molybdenite re os and shrimp zircon u pb dating of duobuza porphyry copper deposit in tibet and its geological implications the duobuza porphyry copper deposit the first large size porphyry copper deposit discovered along the bangong nujiang suture zone is situated in the duobuza tectonic magmatic zone on the southern margin of the qiangtang sanjiang massif the main mineralized porphyry in the ore deposit is a granodiorite porphyry whose sio2 content varies from 61 37 to 67 73 65 16 on the average its ree content is 41 4 94 10 6 and the lree content is higher than the hree content suggesting the enrichment of lree as for trace elements the porphyry shows enrichment of rb k th ba la ce sr and depletion of nb ta p ti the major rare earth and trace elements of the mineralized porphyry have characteristics of volcanic rocks formed in the island arc the zircon shrimp dating 14 samples of duobuza granodiorite porphyry gave 206pb 238u concordant ages from 116 5 4 6 ma to 125 3 4 7 ma with a weighted mean age of 120 9 2 4 ma which indicates the age of the copper mineralized porphyry rock meanwhile the 187re and 187os content of molybdenite from the ore deposit was determined by inductively coupled plasma mass sperctrometry icp ms and the results show that the re os model ages of the deposit are from 117 6 1 3 to 118 5 1 4 ma and its isochron age 118 0 1 5 ma which have accurately determined the timing of mineralization the age of the ore deposit is consistent with the evolution of the bange nujiang archipelagic arc basin system in early cretaceous implying that the duobuza copper deposit was formed in an archipelagic are basin system similar to the system of southeast asia

geochemistry and pb sr nd isotopic features of ore bearing porphyries in bangong lake porphyry copper belt western tibet detailed geochemical and pb sr nd isotopic researches on the duobuza copper deposit and the garqiong copper deposit were conducted in the bangong lake copper belt the result shows that compared with the porphyries of the gandese copper belt the ore bearing porphyries of the bangong lake copper belt are weaker in the enrichment of large ion lithophile elements lile and are also weaker in the depletion of high field strength elements and high rare elements thus having weak positive eu anomaly viewed from pb sr nd isotopic features the ore bearing porphyries are characterized by the enrichment of radiogenic pb the obviously low ratios of 143nd 144nd and the source nearer to the indian ocean sedimentary area indicating that the magma source of the ore bearing porphyries is much shallower with the mixture of more crustal materials the forming tectonic environment of the ore bearing porphyries in the bangong lake copper belt belonged to the active continental ridge whereas that in the gangdese copper belt was far away from the active continental ridge the bangong lake porphyry copper belt was formed in a post collisional crustal up lift stage whereas the gangdise porphyry copper belt in a stage of post collisional crustal extensional collapse when the crust rose to the maximum height

permo carboniferous sequences of gondwana affinity in southwest china and their paleogeographic implications abstract in southwest china permo carboniferous depositional sequences of gondwana affinity were first discovered in the himalayan region by the expedition team of acadimia sinica in 1975 by 1983 gondwana affinity permo carboniferous sequences had been identified also in the lhasa block the south qiangtang block in tibet and the tengchong and baoshan blocks in yunnan these sequences are characterised by the presence of successions which are interpreted as glacio marine deposits and by the presence of gondwana affinity biota the distribution of gondwana affinity permo carboniferous indicates that basins whose remnants are now represented by the yarlung zangbo suture and the bangong nujiang suture did not exist at that time and the vast area south of the lungmu shuanghu jinshajiang zone in tibet and west of the changning menglian belt in yunnan formed a part of the northern margin of gondwana

u pb zircon age geochemical and lu hf isotopic constraints of the southern gangma co basalts in the central qiangtang northern tibet abstract a recent study suggests that the central qiangtang is a key locality to investigate the evolution of the opening and closure of the paleo tethys ocean basalts are commonly associated with supercontinent fragmentation and they have the potential to indicate the tectonic environment into which they were erupted in this study laser ablation inductively coupled plasma mass spectrometry la icp ms u pb zircon dating whole rock geochemistry and zircon hf isotope analysis were used to study newly discovered basalts in the southern gangma co area of central qiangtang dating of magmatic zircons from three basalt samples indicates that the basalts erupted in the late devonian to early carboniferous early tournaisian at 360 350 ma the basalts are geochemically similar to within plate basalts and formed from a depleted mantle source we suggest that the basalts may have been a product of the breakup of the northern margin of gondwana in the late devonian to early carboniferous 360 350 ma which may be linked to the initial rifting and opening of the paleo tethys ocean in the qiangtang area

a paleoclimatic review of southern south america during the late paleozoic a record from icehouse to extreme greenhouse conditions abstract this paper provides a review of the late mississippian to permian paleoclimatic history for southern south america based on lithologic indicators biostratigraphic information and chronostratigraphic data the region is divided into three major types of basins 1 eastern intraplate basins e g parana basin 2 western retroarc basins e g paganzo basin and 3 western arc related basins e g rio blanco basin four major types of paleoclimatic stages are recognized in these basins 1 glacial late visean early bashkirian 2 terminal glacial bashkirian earliest cisuralian 3 postglacial cisuralian early guadalupian and 4 semiarid arid late guadalupian lopingian the glacial stage began in the late visean and continued until the latest serpukhovian or early bashkirian in almost all of the basins in southern south america during the bashkirian earliest cisuralian terminal glacial stage glacial deposits disappeared almost completely in the western retroarc basins e g paganzo basin but glaciation persisted in the eastern basins e g parana and sauce grande basins a gradual climatic amelioration postglacial stage began to occur during the earliest permian when glacial deposits completely disappeared across all of south america during this interval glacial diamictites were replaced by thick coal beds in the parana basin while north south climatic belts began to be delineated in the western basins which were likely controlled by the distribution of mountain belts along the panthalassan margin of south america towards the late permian climatic belts became less evident and semiarid or arid conditions dominated in the southern south america basins eolian dunes playa lake deposits and mixed eolian fluvial sequences occur in the parana basin and in the western retroarc basins volcanism and volcaniclastic sedimentation dominated along the western margin of south america at that time the stratigraphic record obtained in southern south america supports a long duration transition from icehouse to extreme greenhouse conditions

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

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved

geochronology geochemistry and zircon hf isotopic compositions of mesozoic intermediate felsic intrusions in central tibet petrogenetic and tectonic implications abstract mesozoic intermediate felsic intrusions are widely distributed in the southern qiangtang terrane central tibet zircon u pb dating shows that these intrusions formed in two periods in the jurassic 169 150 ma and cretaceous 127 113 ma they mostly belong to the high k calc alkaline series and show strong enrichments in large ion lithophile elements e g cs rb and k depletions in nb ta and ti and negative ba anomalies on primitive mantle normalized diagrams p 2 o 5 contents decrease with increasing sio 2 content and th contents increase with increasing rb content consistent with the evolution trend of i type magmas these intrusions show a wide range of zircon ehf t values from 19 4 to 11 2 including in rocks with similar sio 2 contents suggesting variable contributions from mantle and qiangtang crustal sources fine grained mafic to intermediate igneous enclaves in jurassic intrusions have similar zircon u pb ages and similar or slightly higher zircon ehf t values to the host rocks suggesting that the enclave magmas were derived from mixed magmas at depth and injected into more evolved magmas in upper crustal magma chambers magma mixing is also supported by the wide range of zircon hf isotopic compositions ehf t 19 4 to 2 5 from within individual jurassic and cretaceous intrusions and jurassic enclaves the jurassic cretaceous magmas likely formed in a continental arc setting during subduction of the bangong nujiang ocean between 170 and 110 ma and evolved in the upper plate crust by mash processes