major types and time space distribution of mesozoic ore deposits in south china and their geodynamic settings the ore deposits of the mesozoic age in south china can be divided into three groups each with different metal associations and spatial distributions and each related to major magmatic events the first event occurred in the late triassic 230 210 ma the second in the mid late jurassic 170 150 ma and the third in the early mid cretaceous 120 80 ma the late triassic magmatic event and associated mineralization is characterized by peraluminous granite related w sn nb ta mineral deposits the triassic ore deposits are considerably disturbed or overprinted by the later jurassic and cretaceous tectono thermal episodes the mid late jurassic magmatic and mineralization events consist of 170 160 ma porphyry skarn cu and pb zn ag vein deposits associated with i type granites and 160 150 ma metaluminous granite related polymetallic w sn deposits the late jurassic metaluminous granite related w sn deposits occur in a ne trending cluster in the interior of south china such as in the nanling area in the early mid cretaceous from about 120 to 80 ma but peaking at 100 90 ma subvolcanic related fe deposits developed and i type calc alkaline granitic intrusions formed porphyry cu mo and porphyry epithermal cu au ag mineral systems whereas s type peraluminous and or metaluminous granitic intrusions formed polymetallic sn deposits these cretaceous mineral deposits cluster in distinct areas and are controlled by pull apart basins along the south china continental margin based on mineral assemblage age and space time distribution of these mineral systems integrated with regional geological data and field observations we suggest that the three magmatic mineralization episodes are the result of distinct geodynamic regimes the triassic peraluminous granites and associated w sn nb ta mineralization formed during post collisional processes involving the south china block the north china craton and the indo china block mostly along the dabie sulu and songma sutures jurassic events were initially related to the shallow oblique subduction of the izanagi plate beneath the eurasian continent at about 175 ma but i type granitoids with porphyry cu and vein type pb zn ag deposits only began to form as a result of the breakup of the subducted plate at 170 160 ma along the nne trending qinzhou hangzhou belt also referred to as qin hang or shi hang belt which is the neoproterozoic suture that amalgamates the yangtze craton and cathaysia block a large subduction slab window is assumed to have formed in the nanling and adjacent areas in the interior of south china triggering the uprise of asthenospheric mantle into the upper crust and leading to the emplacement of metaluminous granitic magma and associated polymetallic w sn mineralization a relatively tectonically quiet period followed between 150 and 135 ma in south china from 135 ma onward the angle of convergence of the izanagi plate changed from oblique to parallel to the coastline resulting in continental extensional tectonics and reactivation of regional scale ne trending faults such as the tan lu fault this widespread extension also promoted the development of ne trending pull apart basins and metamorphic core complexes accompanied by volcanism and the formation of epithermal cu au deposits granite related polymetallic sn w deposits and hydrothermal u deposits between 120 and 80 ma with a peak activity at 100 90 ma

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

tectonic evolution of a composite collision orogen an overview on the qinling tongbai hong an dabie sulu orogenic belt in central china abstract the formation of collisional orogens is a prominent feature in convergent plate margins it is generally a complex process involving multistage tectonism of compression and extension due to continental subduction and collision the paleozoic convergence between the south china block scb and the north china block ncb is associated with a series of tectonic processes such as oceanic subduction terrane accretion and continental collision resulting in the qinling tongbai hong an dabie sulu orogenic belt while the arc continent collision orogeny is significant during the paleozoic in the qinling tongbai hong an orogens of central china the continent continent collision orogeny is prominent during the early mesozoic in the dabie sulu orogens of east central china this article presents an overview of regional geology geochronology and geochemistry for the composite orogenic belt the qinling tongbai hong an orogens exhibit the early paleozoic hp uhp metamorphism the carboniferous hp metamorphism and the paleozoic arc type magmatism but the three tectonothermal events are absent in the dabie sulu orogens the triassic uhp metamorphism is prominent in the dabie sulu orogens but it is absent in the qinling tongbai orogens the hong an orogen records both the hp and uhp metamorphism of triassic age and collided continental margins contain both the juvenile and ancient crustal rocks so do in the qinling and tongbai orogens in contrast only ancient crustal rocks were involved in the uhp metamorphism in the dabie sulu orogenic belt without involvement of the juvenile arc crust on the other hand the deformed and low grade metamorphosed accretionary wedge was developed on the passive continental margin during subduction in the late permian to early triassic along the northern margin of the dabie sulu orogenic belt and it was developed on the passive oceanic margin during subduction in the early paleozoic along the northern margin of the qinling orogen three episodes of arc continent collision are suggested to occur during the paleozoic continental convergence between the scb and ncb the first episode of arc continent collision is caused by northward subduction of the north qinling unit beneath the erlangping unit resulting in uhp metamorphism at ca 480 490 ma and the accretion of the north qinling unit to the ncb the second episode of arc continent collision is caused by northward subduction of the prototethyan oceanic crust beneath an andes type continental arc leading to granulite facies metamorphism at ca 420 430 ma and the accretion of the shangdan arc terrane to the ncb and reworking of the north qinling erlangping and kuanping units the third episode of arc continent collision is caused by northward subduction of the paleotethyan oceanic crust resulting in the hp eclogite facies metamorphism at ca 310 ma in the hong an orogen and low p metamorphism in the qinling tongbai orogens as well as crustal accretion to the ncb the closure of backarc basins is also associated with the arc continent collision processes with the possible cause for granulite facies metamorphism the massive continental subduction of the scb beneath the ncb took place in the triassic with the final continent continent collision and uhp metamorphism at ca 225 240 ma therefore the qinling tongbai hong an dabie sulu orogenic belt records the development of plate tectonics from oceanic subduction and arc type magmatism to arc continent and continent continent collision

cretaceous ridge subduction along the lower yangtze river belt eastern china the lower yangtze river belt is one of the most important metallogenic belts in china the mechanisms responsible for ore genesis and the formation of related cretaceous igneous rocks such as adakite a type granitoid and nb enriched basalt remain controversial mesozoic granitoids in the lower yangtze river belt were mostly formed in the early cretaceous 140 125 ma and three granitoid belts the inner the south and the north have been defined according to petrological and geochemical characteristics previously based mainly on negative end and high initial sr isotope values the adakitic rocks were generally attributed to partial melting of thickened or delaminated lower crust both of which require crustal thickening mesozoic crustal thickening however is not supported by the development of extensional basins in the region from the late jurassic to cretaceous eastern china was closely associated with subduction of the pacific plate in the south and the izanagi plate in the north the midocean ridge mor between these two plates was drifting toward and likely subducting under the lower yangtze river belt a ridge subduction model can therefore explain the distribution of different magmatic rocks and ore deposits in the belt partial melting of subducting young hot oceanic slabs close to the ridge formed adakitic rocks the negative end values of adakitic rocks can be plausibly interpreted by mixing between adakitic magmas and enriched components in the lithospheric mantle and or crustal materials through afc process a slab window opened during ridge subduction as indicated by a type granitoids in the center of the inner belt nb enriched basalt found in the belt was likely formed by partial melting of a mantle wedge metasomatized by fluids released from the subducting slab at shallow depths

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

petrogenesis and tectonic implications of early cretaceous s and a type granites in the northwest of the gan hang rift se china the gan hang rift trending at least 450 km in a ne sw direction is a part of a mesozoic basin and range province in southeastern china detailed shrimp zircon u pb chronology major and trace element and sr nd hf isotope data of three granitic plutons and a diabasic dike in the northwest of the gan hang rift are used to explore the origin of these granites and their relationship to the evolution of the gan hang rift shrimp zircon u pb dating shows that the granitic plutons and diabasic dike were emplaced in the early cretaceous 122 129 ma the tongshan and damaoshan plutons close to the gan hang rift consist mainly of weakly peraluminous granitic rocks which show a2 subtype affinity these granites have initial 87sr 86sr ratios of 0 7080 0 7103 end t values of 1 4 to 5 6 and ehf t in situ zircon values of 3 8 to 1 2 detailed elemental and isotopic data suggest that they were formed by partial melting of granulitized mesoproterozoic metamorphic basement including metasedimentary and metaigneous rocks in the shallow 30 km crust at a high temperature 823 c these pure crustal melts underwent crystal fractionation of biotite plagioclase and k feldspar forming the more felsic end member of granitic rocks the ehu pluton more distant from the gan hang rift is composed of two mica granites which are strongly peraluminous and are s type granites these granites have higher initial 87sr 86sr ratios 0 7162 0 7191 and lower end t 7 1 to 7 4 and ehf t 6 8 in situ zircon values than the tongshan and damaoshan granites detailed elemental and isotopic data suggest that the ehu granites were formed by partial melting of paleoproterozoic metasedimentary basement in the deep 40 50 km crust at a relatively low temperature 757 c with no fractional crystallization and mantle derived magma mixing in their evolution the coeval diabasic dike is calc alkaline and has initial 87sr 86sr ratio of 0 7085 end t value of 0 9 and ehf t in situ zircon value of 7 1 suggesting that it was derived from the asthenospheric mantle wedge that had been metasomatised via addition of subducted sediment derived melt with monazite in the residual phase the association of early cretaceous 122 129 ma s and a type granites in the northwest of the gan hang rift marks the onset of back arc extension or intra arc rift with ongoing extension the crust and lithospheric mantle became progressively thinned the upwelling of asthenosphere triggered partial melting of both metasedimentary and metaigneous rocks in the more thinned crust close to the gan hang rift forming the a type granitic magmas such as tongshan and damaoshan whereas partial melting of metasedimentary rocks in the less thinned crust farther from the gan hang rift formed the s type granitic magmas such as ehu the red sediments with the total thickness more than 10 000 m have been successively deposited in the gan hang rift valley since the late early cretaceous 105 ma suggesting that this region experienced the most back arc extension

petrogenesis and tectonic significance of late jurassic early cretaceous volcanic intrusive complex in the tianhuashan basin south china abstract the late mesozoic geological framework of south china is characterized by voluminous volcanic rocks and their intrusive counterparts however the ages petrogenesis and tectonic implications of the late mesozoic magmatism in south china especially those in the qinhang zone are poorly constrained the tianhuashan basin is one of the important volcanic basins along the southern margin of the qinhang zone and is mainly composed of the daguding and ehuling formations and their intrusive counterparts laser ablation inductively coupled plasma mass spectrometry la icpms u pb zircon data presented here in conjunction with our previously published data suggest that there are two distinct pulses of magmatism within the tianhuashan basin the granite porphyry was emplaced at 158 157 ma whereas large scale volcanic intrusive rocks were formed at 144 137 ma rather than in the late jurassic as previously regarded the tianhuashan volcanic intrusive rocks have similar whole rock element geochemical and sr nd and zircon hf isotopic compositions suggesting that they were probably derived from similar source regions these rocks exhibit clearly negative and variable bulk rock end t values in a range of 12 07 to 8 69 and low zircon ehf t values from 12 25 to 2 31 with paleo and mesoproterozoic two stage model ages for both nd and hf isotopes 1639 1915 ma and 1051 1970 ma respectively suggesting their derivation dominantly through partial melting of ancient crustal materials with involvement of a mantle derived component in variable proportions and followed by differential assimilation of upper crust during magma ascent an extensional setting associated with the roll back of the paleo pacific slab is favored to explain the petrogenesis of the tianhuashan volcanic intrusive complex the ehf t of zircon increases from late jurassic granite porphyry to early cretaceous magmatic rocks suggesting progressive crust mantle interaction in generating the episodic intermediate felsic magma eruption during late mesozoic our results suggest that the continental extension along the qinhang zone was weak during late jurassic and large scale lithospheric extension did not take place until early cretaceous with ongoing development of slab roll back

the mesozoic magmatic sources and tectonic setting of the zijinshan mineral field south china constraints from geochronology and geochemistry of igneous rocks in the southeastern ore segment abstract the zijinshan mineral field is a large porphyry epithermal cu au mo ag ore system located in the southwestern fujian province china the southeast cu mo ore segment soe is a newly discovered section located in the southeastern portion of the zijinshan mineral field zircon u pb and molybdenite re os ages as well as the geochemical compositions of intrusive and volcanic rocks and zircon hf isotope data are used to constrain the timing petrogenesis tectonic setting and the relationship between magmatism and metallogenesis in the soe and zijinshan mineral field major and trace element geochemistry indicates that igneous rocks from soe are mainly shoshonitic granitoids that are lile enriched rb ba th u with hfse depletion nb ta zr hf and y indicating a subduction related setting new u pb zircon analyses yield weighted mean ages of ca 160 ma for the caixi pluton ca 110 ma for the sifang granodiorite ca 107 105 ma for the luoboling granodiorite porphyry ca 105 ma for the quartz diorite porphyry ca 108 105 ma for the dacite porphyry ca 98 ma for the rhyodacite and 97 ma for the rhyolite in the soe these dates integrated with previous geochronological data indicate that there were two main magmatic events in this area 1 a middle to late jurassic event 165 150 ma and 2 a cretaceous event 110 95 ma which can be further divided into two groups i e the late early cretaceous event 110 105 ma and the early late cretaceous event 100 95 ma the late early cretaceous intrusions contain higher e hf t values 0 1 to 3 6 with t dm2 varying from 1 4 to 1 0 ga which may come from mixing between crustal and subduction related mantle derived melts in contrast to the middle to late jurassic intrusions that may come from the partial melting of the cathaysia block basement the early late cretaceous intrusions display a broader range of e hf t values 0 9 to 9 4 with t dm2 varying from 1 8 to 1 2 ga suggesting a similar source to the late early cretaceous rocks but with more crustal material the molybdenite re os ages for mineralization range from 110 ma to 105 ma consistent with ages of the late early cretaceous rocks zircon trace elements analyses imply that the early late cretaceous rocks ce 4 ce 3 113 1800 are characterized by higher oxygen fugacity with higher mineralization potential than the late early cretaceous ce 4 ce 3 9 545 and the middle to late jurassic rocks variability in ages geochemistry and oxygen fugacity indicate that the middle to late jurassic magmatic event represents pre mineralization intrusions whereas the late early cretaceous is syn mineralization and the early late cretaceous intrusions post mineralization we propose that both the middle to late jurassic and the cretaceous magmatic events may relate to the subduction of the pacific slab and the changing direction of the subducted slab may result in the different sources for the two magmatic events

foraminiferal stratigraphy and environment of late weichselian deposits on the continental shelf off troms northern norway abstract based on the analysis of 8 selected gravity cores from the upper sediment layers of andfjorden and malangsdjupet off northern norway six late weichselian foraminiferal assemblage zones are defined the two oldest assemblage zones 8 and 7 are found in basal tills and contain mixed faunas reworked and deposited by glaciers which reached the shelf edge during the early late weichselian during the earliest phase of the deglaciation of the continental shelf a sparse fauna characterized by buccella frigida zone 6 was deposited zone 5 is dominated by cassidulina laevigata and a fairly high content of planktic foraminifera indicating periods of ice free normal marine surface waters during a glacial advance between 14 000 and 13 000 b p the fauna zone 4 was dominated by elphidium excavatum this fauna reflects near glacial conditions characterized by unstable bottom water conditions the last c 500 years of this period suffered from oxygen depleted bottom waters zone 3 formed between 13 000 and 10 000 b p comprises several species e g cassidulina reniforme nonion barleeanum n labradoricum and cibicides lobatulus during the first 2000 years of this period the fauna reflects a climatic amelioration e g an increase in temperature salinity and current activity probably due to intrusion of atlantic water during the younger dryas zone 3 reflects a climatic deterioration

zircon u pb geochronology and geochemistry of two episodes of granitoids from the northwestern zhejiang province se china implication for magmatic evolution and tectonic transition abstract granitoids 175 80 ma representing a prominent yanshanian jurassic to cretaceous magmatic event in south china widely intrude the precambrian crystalline basement and paleozoic strata here we report zircon u pb age data geochemical characteristics and sr nd isotopes of the late jurassic and early cretaceous granitoids from the northwestern zhejiang province zxb of southeastern china our results reveal two distinct episodes for the yanshanian magmatism the jiemeng and datongkeng granodiorites formed at 148 6 1 1 ma whereas the huangshitan jiuligang and ruhong aluminous a type granites were generated between 129 0 0 6 ma and 126 1 1 1 ma the two magmatic phases represent a tectonic transition from an active continental margin to post orogenic setting during the late jurassic ca 150 ma to early cretaceous ca 128 ma geochemically these intrusions are granodioritic to granitic in composition and show an affinity of s type and a type granitoids respectively the s type granodiorites of jiemeng and datongkeng are characterized by moderate sio 2 65 0 69 6 wt high k 2 o na 2 o 5 0 7 6 wt k 2 o na 2 o 1 2 1 5 zr 31 109 ppm sr 71 190 ppm and high field strength elements low to intermediate mg and moderate nb depletion the a type granites of huangshitan jiuligang and ruhong are characterized by high sio 2 72 7 77 2 wt k 2 o na 2 o 6 9 8 8 wt k 2 o na 2 o 1 3 2 1 fe t fe t mg ga 17 29 ppm 20 ppm commonly zr 96 197 ppm and sr 8 45 ppm with slight nb depletion the s type granodiorites have higher mg a nk sr sr ba sr y la yb n and lree hree and lower sio 2 k 2 o na 2 o ga and zr with weak negative eu anomalies compared to those of the a type granites with negative eu anomalies all these rocks show y nb ratios 1 2 high initial 87 sr 86 sr i sr ratios and low e nd t and are depleted in nb ti and sr indicating crustal origin with subduction zone signatures we suggested that the zxb s type granitic bodies might have been derived from the mesoproterozoic metamorphic basement rocks through partial melting induced by mantle derived magma followed by limited fractional crystallization the zxb aluminous a type granites were also derived from a similar magma source but underwent fractional crystallization at higher crustal levels the a type granites in the zxb correlate with a post orogenic tectonic setting a geological comparison between zxb and adjacent areas indicates that the geochemical features of the zxb a type granites are comparable with the baijuhuajian a type granites and also the adjacent areas a type granites but are distinct from the late cretaceous a type granites 105 90 ma distributed along the southeastern coastal area of south china we correlate the formation of the zxb s type granodiorites to inland compression associated with the subduction and collision of the paleo pacific plate in the late jurassic 170 145 ma in contrast the a type granites formed under a post orogenic setting during the early cretaceous period 145 120 ma resulting from lithospheric thinning and continent extension accompanied by slab roll back of the paleo pacific plate following the subduction collision event

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

introduction to tectonics of china abstract the continental crust of china is a mosaic of cratonic blocks and orogenic belts containing small cratons and terranes with various tectonic settings they have diverse origins and complex histories of amalgamation and often suffered repeated reworking after multiple episodes of amalgamation in the last three decades extensive geological geochemical and geophysical investigations have been carried out on these cratonic blocks and intervening orogenic belts producing an abundant amount of new data and competing interpretations this provides important insights into understanding the formation and evolution of the chinese continents the papers assembled in this volume present a timely and comprehensive overview on major advancements and controversial issues related to the formation and evolution of continental crust in china complex tectonic histories were experienced not only by the large scale cratonic blocks and orogenic belts but also by small scale terranes and orogens between and inside these blocks nevertheless our understanding of lithotectonic units and geological processes has been greatly advanced by recent studies of zirconology and geochemistry for various rock types from major petrotectonic units in china it has been further advanced from integrated interpretations of geochemical and petrological data for petrogenesis of magmatic rocks an overview of these observations and interpretations provides new insights into understanding the continental plate tectonics and the chemical geodynamics of subduction zones

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

building southeast china in the late mesozoic insights from alternating episodes of shortening and extension along the lianhuashan fault zone abstract understanding the mesozoic intraplate structural response of south china inboard of the paleo pacific south china convergent margin is a key issue in unravelling the continental tectonics of se asia the lianhuashan fault zone of south china represents a high strain structure pivotal for constraining the multi phased intraplate deformation strain localization and partitioning in response to the subduction of the paleo pacific plate new data coupled with published work describes the geometry kinematics and age of this fault zone establishing a multi stage tectono thermal history the first stage late jurassic shortening involved top to the w thrust sense shear along ne striking shear zones the associated regional e w shortening is interpreted to have been related to the advancing nw ward subduction of the paleo pacific plate the second stage early cretaceous deformation is poorly documented in the lianhuashan fault zone but is associated regionally with magmatism and early cretaceous basins formed under nw se extension this extension has been correlated with the rollback of the paleo pacific plate the third stage late early cretaceous nw se shortening produced top to the se thrust sense shear zones the shortening inverted the extensional basins and caused a lull in the magmatic activity it possibly originated from the re initiation of low angle paleo pacific subduction the fourth stage deformation generated the top to the w wuhua detachment normal sense ductile fabrics and distributed shear zones in its footwall and widespread hanging wall basins in the late cretaceous this regional e w extension has been associated with the re initiation of slab rollback our work predicts episodes of slab steepening and shallowing in retreating and advancing settings changes in the slab geodynamics may have caused the alternating contractional and extensional episodes as well as the flare up lull and resumption of magmatism during the late mesozoic evolution of south china

mesozoic large scale magmatism and mineralization in south china preface abstract south china is famous for its polymetallic deposits with more than 50 of the world s w and sb reserves and 20 of the sn reserves as well as abundant cu mo pb zn au ag ta nb hree and u this special issue reports recent progress on the tectonic evolution magmatism and ore deposits of south china which leads to a more comprehensive understanding of the mesozoic large scale magmatism and mineralization events 1 south china has been located in backarc settings during multiple subductions since the paleozoic which may have contributed to pre enrichments of the ore forming materials 2 the pacific plate subduction was a principle factor that controlled the late mesozoic large scale magmatism and mineralization in south china through flat subduction ridge subduction slab tearing and slab rollbacks

petrogenesis of the middle jurassic yinshan volcanic intrusive complex se china implications for tectonic evolution and cu au mineralization abstract volcanic subvolcanic rocks and associated cu au deposits in yinshan are located in the eastern part of the neoproterozoic jiangnan orogenic belt between the yangtze and cathaysia blocks constituting a large metallogenic district in southeastern china this paper presents new laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb dating element and sr nd hf isotopic data for yinshan igneous rocks in the dexing region our results show that the yinshan igneous rocks were produced during the middle jurassic ca 176 166 ma geochemically the yinshan igneous rocks are characterized by calc alkaline and show some adakitic affinities with high sr contents up to 903 ppm sr y up to 93 2 and la n yb n ratios 12 4 to 65 1 and low y 2 53 to 15 8 ppm and yb contents 0 22 to 1 39 ppm the absence of a negative eu anomaly extreme depletion in y and yb relatively low mgo contents 0 2 to 2 9 wt and relatively high 207 pb 204 pb ratio 15 555 15 575 indicate that yinshan igneous rocks were likely derived from the partial melting of thickened lower continental crust in addition the yinshan igneous rocks show similar element characteristics to arc derived igneous rocks with bulk earth like e nd t values 1 1 to 1 8 and initial 87 sr 86 sr 0 7036 0 7090 relatively depleted e hf t values 1 5 to 5 8 and hf model ages of 0 8 1 2 ga however there is no arc related evidence in the study area in the middle jurassic which indicates that these arc related elemental and isotopic compositions are possibly inherited from meso to neo proterozoic juvenile continental crust formed by the neoproterozoic oceanic crust subduction along the jiangnan orogen the geodynamic environment responsible for the development of the middle jurassic yinshan igneous rocks is probably a localized intra continental extensional environment along the ne jiangxi deep fault as a tectonic response to far field stress at the margins of south china plate during the early stage of the paleo pacific plate subduction regarding the cu au mineralization we infer that cu au were released from subducted oceanic slab and reserved in the juvenile crust during neoproterozoic subduction along the eastern jiangnan orogen partial melting of cu au rich neoproterozoic juvenile crust during the middle jurassic time in the yinshan area caused the formation of adakitic rocks and large scale cu au deposits our model provides a new insight into genesis of adakitic rocks and related cu au deposits in a non arc setting

the foreslope hills large scale fine grained sediment waves in the strait of georgia british columbia abstract the foreslope hills are a set of 20 m high 5 km long ridges at 230 350 m water depth in the south central strait of georgia occupying a 60 km 2 area at the base of the fraser river delta foreslope they are composed of clay silt and fine sand previous authors have interpreted them as 1 a single mass slide deposit 2 mud diapirs 3 in situ rotational failures and 4 creep deformation features but collaborative evidence for these interpretations is lacking seafloor surface morphological renders derived from recent multibeam sonar bathymetry data show linear evenly spaced symmetrical ridges with ridge bifurcation resembling wave ripples but on a much larger scale detailed multichannel and single channel seismic reflection data show sinusoidal internal reflections suggesting upslope migration the ridges occur within the uppermost portion of the holocene section in a very high sedimentation rate environment consequently they are probably less than 3000 years old moored current meter records for the region show reversing currents largely oriented normal to the ridge crests averaging 10 cm s with a maximum of 50 cm s this evidence combined with increasing documentation of large scale sedimentary bedforms in slope and rise environments suggests that the foreslope hills should be re interpreted as sediment waves upslope migration is typical of many large scale fine grained sediment waves the preferred physical interpretation for the creation and maintenance of the foreslope hills as sediment waves is the lee wave model requiring a phase shift in density stratified flow as it passes over the wavy topography maximum measured bottom currents are easily strong enough to entrain silt and clay delivered to the site by turbidity currents or by sediment plume fallout from fraser river outflow either strong flood tides or severe biannual density intrusions due to their intense stratification may be susceptible to lee wave formation and the creation of sediment waves leading to the observed pattern of upslope migration

late jurassic early cretaceous continental convergence and intracontinental orogenesis in east asia a synthesis of the yanshan revolution abstract the basic tectonic framework of continental east asia was produced by a series of nearly contemporaneous orogenic events in the late middle jurassic to early cretaceous commonly the late mesozoic orogenic processes were characterized by continent continent collision large scale thrusting strike slip faulting and intense crustal shortening crustal thickening regional anatexis and metamorphism followed by large scale lithospheric extension rifting and magmatism to better understand the geological processes this paper reviews and synthesizes existing multi disciplinary geologic data related to sedimentation tectonics magmatism metamorphism and geochemistry and proposes a two stage tectono thermal evolutionary history of east asia during the late middle jurassic to early cretaceous ca 170 120 ma in the first stage three orogenic belts along the continental margins were formed coevally at ca 170 135 ma i e the north mongol okhotsk orogen the east paleo pacific coastal orogen and the west bangong nujiang orogen tectonism related to the coastal orogen caused extensive intracontinental folding and thrusting that resulted in a depositional hiatus in the late jurassic as well as crustal anatexis that generated syn kinematic granites adakites and migmatites the lithosphere of the east asian continent was thickened reaching a maximum during the latest jurassic or the earliest cretaceous in the second stage ca 135 120 ma delamination of the thickened lithosphere resulted in a remarkable 120 km lithospheric thinning and the development of mantle derived magmatism mineralization metamorphic core complexes and rift basins the middle jurassic early cretaceous subduction of oceanic plates paleo pacific meso tethys and mongol okhotsk and continent continent collision e g lhasa and qiangtang along the east asian continental margins produced broad coastal and intracontinental orogens these significant tectonic activities marked by widespread intracontinental orogeny and continental reconstruction are commonly termed the yanshan revolution movement in the chinese literature

petrogenesis of cretaceous adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china implications for geodynamics and cu au mineralization both adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china are associated with cretaceous cu au mineralization the shaxi quartz diorite porphyrites exhibit adakite like geochemical features such as light rare earth element lree enrichment heavy ree hree depletion high al2o3 mgo sr sr y and la yb values and low y and yb contents they have low end t values 3 46 to 6 28 and high 87sr 86sr i ratios 0 7051 0 7057 sensitive high resolution ion microprobe shrimp zircon analyses indicate a crystallization age of 136 3 ma for the adakitic rocks most volcanic rocks and the majority of monzonites and syenites in the luzong area are k rich or shoshonitic and were also produced during the cretaceous 140 125 ma they are enriched in lree and large ion lithophile elements and depleted in ti and nb and ba and exhibit relatively lower end t values ranging from 4 65 to 7 03 and relatively higher 87sr 86sr i ratios varying between 0 7057 and 0 7062 the shoshonitic and adakitic rocks in the luzong area have similar pb isotopic compositions 206pb 204pb 17 90 18 83 207pb 204pb 15 45 15 62 and 208pb 204pb 38 07 38 80 geological data from the luzong area suggest that the cretaceous igneous rocks are distributed along ne fault zones e g tanlu and yangtze river fault zones in eastern china and were likely formed in an extensional setting within the yangtze block the shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of 50 km i e 1 5 gpa possibly leaving rutile bearing eclogitic residue the shoshonitic magmas in contrast originated mainly from an enriched mantle metasomatized by subducted oceanic sediments they underwent early high pressure 1 5 gpa fractional crystallization at the boundary between thickened 50 km lower crust and lithospheric mantle and late low pressure 1 5 gpa fractional crystallization in the shallow 50 km crust the adakitic and shoshonitic rocks appear to be linked to an intra continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along ne fault zones e g tanlu and yangtze river fault zones in eastern china both the shoshonitic and adakitic magmas were fertile with respect to cu au mineralization

mesozoic felsic arc magmatism and continental olivine tholeiites in zhejiang province and their relationship with the tectonic activity in southeastern china abstract the mesozoic geology of zhejiang province in southeastern china is characterized by two distinct geodynamic settings separated by a major tectonic event during the late jurassic early cretaceous a continental magmatic arc developed with a westward subduction of the kula or izanagi plate under the south china block during the development of this magmatic arc microsyenite porphyries and quartz bearing porphyries were emplaced in western zhejiang about 124 ma ago these porphyritic rocks intruded felsic tuffs that rest unconformably on carboniferous and permian folded strata geochemically they are enriched in lil and light rare earth elements lree which are features of acid shoshonitic suites isotopically they are characterized by negative nd t 124 5 3 to 4 7 values and high initial 87 sr 86 sr ratios 0 7079 0 7084 later in eastern zhejiang quartz phyric ignimbrites and subordinate acid andesites were emplaced and intruded by nearly contemporaneous microsyenitic and rhyolitic domes during the cretaceous about 97 ma ago all these rocks belong to the high k calc alkaline suites they show geochemical features of volcanic arc granites emplaced in an active continental margin and are characterized by negative nd t 97 8 8 to 7 1 and high initial sr isotopic ratios 0 7081 0 7089 the negative nd t 97 values of all the cretaceous calc alkaline and shoshonitic rocks of intermediate to acid composition suggest that these rocks were probably derived from a mantle wedge source contaminated by variable amounts of old crustal components likely represented by the proterozoic basement rocks of the south china block the contemporaneous igneous rocks in vietnam as well as in the fujian and guangdong provinces china represent the lateral counterparts of the zhejiang calc alkaline plutonic and volcanic rocks in the late early cretaceous an important tectonic event occurred which provoked the ending of the inagazi or kula plate subduction and affected the continental magmatic arc this tectonic event is likely related to the collision of the china indochina margin with the west philippine block after this tectonic event in the beginning of the late cretaceous continental stretching and crustal attenuation affected the south china block leading to the development of ne sw faulted basins in which continental olivine tholeiites and minor alkaline rhyolites were erupted both characterized by low nd t 97 ratios ca 4 meanwhile continental conglomerates and sandstones were also deposited this rifting process began likely before the india asia indentation and lasted at least until the late tertiary

on ferroan a type granitoids their compositional variability and modes of origin we recognize eight types of ferroan granitoid that can be distinguished on the basis of major element chemistry these include alkalic granitoids that may be metaluminous or peralkaline alkali calcic granitoids that may be metaluminous peraluminous or peralkaline calc alkalic granitoids that may be metaluminous or peraluminous and rare calcic ferroan granitoids these granitoids may form by two distinct end member processes extreme differentiation of basaltic melts results in ferroan granitoids that are either peralkaline alkalic and alkali calcic or metaluminous alkalic alkali calcic and calc alkalic with alkalinity increasing with increasing pressure of differentiation partial melting of tonalitic to granodioritic crust produces alkali calcic to calc alkalic granitoids that are metaluminous at low pressures and peraluminous at high pressures it is likely that a combination of these two processes plays some role in the formation of most ferroan granitoids most granitoids that are referred to as ferroan however the term has become more confusing than edifying because it has been applied to a broad spectrum of granitoid compositions with varying petrogenesis for this reason we recommend that the term be discontinued and ferroan used in its stead

late quaternary geology and geochronology of the central continental shelf of western canada piston cores and high resolution seismic profiles from queen charlotte sound have yielded new information on the late quaternary history of this part of the western canada continental shelf cores in three major troughs goose island mitchell s and moresby up to 130 km apart sample a similar sedimentary sequence this sequence comprises five units the lowest unit a which is up to 40 m thick is a laminated to massive mud containing coarse ice rafted debris deposited between pre 13 6 ka and 12 9 ka during deglaciation of queen charlotte sound this unit is overlain by a sequence of stratified sediments units b1 b3 up to 20 m thick deposited during the transition from the pleistocene to the holocene unit b1 directly overlies and is similar to unit a but lacks ice rafted detritus unit b2 is sandy mud locally gravelly deposited during a period of lagging and winnowing from about 12 9 to 10 2 ka when sea level in queen charlotte sound was lower relative to the land than it is today unit b3 is a laminated to massive mud similar to unit b1 deposited when water depths in queen charlotte sound rapidly increased at the end of the pleistocene due to the combined effect of eustatic sea level rise and forebulge collapse the sedimentary sequence is capped by up to 10 m of mud unit c which is more organic rich and contains more clay than any of the other units it was accumulating under present day conditions as early as 9 7 k y ago

u pb and hf isotopic study of zircons from migmatised amphibolites in the cathaysia block implications for the early paleozoic peak tectonothermal event in southeastern china abstract amphibolites occur in a number of localities in the cathaysia block some of them have been migmatised and their protoliths represent basaltic magmas erupted in various tectonic settings four migmatised amphibolites were collected from jiangxi and fujian provinces cathodo luminescence images of zircons extracted from the representative amphibolites show unzoned or sector zoned structure la icp ms analysis indicates that most zircons have high th u ratios and yield u pb zircon ages of 446 5 435 2 434 4 and 423 2 ma respectively lu hf isotopic analysis on these zircons gives hf model ages ranging from 900 to 1200 ma based on lithological observations and previously published geochronological data we interpret that these u pb ages record an important tectonothermal event that led to the migmatization this early paleozoic caledonian tectonothermal event in the southeastern china has a great tectonic implication for the evolutionary history of the cathaysia block

petrogenesis of adakitic porphyries in an extensional tectonic setting dexing south china implications for the genesis of porphyry copper mineralization the dexing adakitic porphyries quartz diorite granodiorite porphyries associated with giant porphyry cu deposits are located in the interior of a continent south china they exhibit relatively high mgo cr ni and sr contents high la yb and sr y ratios but low yb and y contents similar to adakites produced by slab melting associated with subduction however they are characterized by bulk earth like nd sr isotope compositions end t 1 14 to 1 80 and 87 sr 86 sr i 0 7044 0 7047 and high th 12 6 27 2ppm contents and th ce 0 19 0 94 ratios which are different from those of cenozoic slab derived adakites sensitive high resolution ion microprobe shrimp geochronology studies of zircons reveal that the dexing adakitic porphyries have a crystallization age of 171 3ma this age is contemporaneous with middle jurassic extension within the shi han rift zone and within plate magmatism elsewhere in south china indicating that the dexing adakitic porphyries were probably formed in an extensional tectonic regime in the interior of the continent rather than in an arc setting their high th contents and th ce ratios and middle jurassic age argue against an origin from a neoproterozoic 1000ma stalled slab in the mantle taking into account available data for the regional metamorphic magmatic rocks and the present day crustal thickness 31km in the area we suggest that the dexing adakitic porphyries were most probably generated by partial melting of delaminated lower crust which was possibly triggered by upwelling of the asthenospheric mantle due to the activity of the shi hang rift zone moreover the dexing adakitic magmas must have interacted with the surrounding mantle peridotite during their ascent which elevated not only their mgo cr and ni contents but also the oxygen fugacity fo2 of the mantle the high fo2 could have induced oxidation of metallic sulfides in the mantle and mobilization of chalcophile elements which are required to produce associated cu mineralization therefore the cu metallogenesis associated with the dexing adakitic porphyries is probably related to partial melting of delaminated lower crust similar to the metallogenesis accompanying slab melting

an andean type retro arc foreland system beneath northwest south china revealed by sinoprobe profiling abstract in the mesozoic south china was situated along the convergent margin between the asian and pacific plates providing an excellent laboratory to understand the interactions between deformation sedimentation and magmatism in a retroarc environment the crustal architecture of northwest south china is displayed along the 600 km long sinoprobe deep seismic reflection profiles and reveals from east to west 1 highly folded and truncated reflectors in the upper crust of the yangtze fold zone which correspond to thin and thick skinned thrust systems and document large scale intraplate structural imbrication and shortening 2 a crustal scale flat ramp flat structure termed the main yangtze decollement which forms a weak viscous layer to accommodate strain decoupling and material transport in the thin and thick skinned systems and 3 nearly flat lying reflectors in the sichuan basin which support interpretation of the basin as a weakly deformed depocentre the yangtze fold zone and the sichuan basin represent a retro arc foreland basin system that is 800 km away from the continental margin magmatic arc we suggest that tectonic processes across the arc and retro arc systems including arc magma flare up basin sedimentation retroarc thrust propagation lithosphere underthrusting root foundering and extension related magmatism were interrelated and governed mass transfer age data and geological relations link the tectonic processes to evolving geodynamics of the subducting paleo pacific plate

orogeny and relative plate motions example of the japanese islands abstract the paleogeography of the eastern margin of asia near japan for the past 250 ma has been reconstructed on the basis of the relative motions between the oceanic plates and the eurasian plate engebretson et al 1982 and on land geology the relationships between the plate kinematics and orogeny are as follows calc alkaline magmatism is related to the presence of subduction when a plate boundary is a transform fault there is no volcanism very slow convergence rates do not give rise to magmatism migration of rft type triple junctions or ridge subduction produces forearc volcanism near the trench and also a wider volcanic zone uplift of low p t type regional metamorphic belts is related to collision episodes the uplift of the high p t blueschist belts on the contrary is not contemporaneous with them they appear to be related to the start of the subduction of a young plate after the migration of an rft type triple junction because the formation of a voluminous accretionary complex is contemporaneous with this and thrusting of this young accretionary complex beneath the uplifted metamorphic belt is often observed the underplating of the young accretionary complex beneath the older accretionary complex would be the cause of the uplift of the high p t type metamorphic belt the subduction of a young plate would thus be an important factor causing these orogenic events highly oblique convergence also results in formation of a serpentinite melange zone with fragments of blueschists presumably along a mechanical plate boundary in the forearc

paleoenvironments of the strait of georgia british columbia during the last deglaciation microfaunal and microfloral evidence the strait of georgia is situated between vancouver island and the western canadian mainland it was deglaciated quickly around 12 500 14c yr bp by in situ downwasting the subsequent marine invasion has left glacimarine sediments with foraminifers diatoms and dinocysts characterizing successive paleoenvironments recognizable basinwide the first paleoenvironment represents near ice conditions under the influence of the turbid meltwater plume marked by low algal productivity and few foraminifers mostly elphidium excavatum just off the plume the second environment shows a peak of nonionellina labradorica possibly resulting from an observed sudden increase in diatom numbers mostly ice dwelling forms implying a long annual sea ice cover further away cassidulina reniforme increases n labradorica decreases while other calcareous foraminifer species colonize the area there are fewer ice diatoms more marine planktonic diatoms and dinocysts become more abundant finally after 12 000 14c yr bp calcareous foraminifers are gradually replaced because of dissolution of calcium carbonate by a low diversity arenaceous assemblage at the same time abundant planktonic diatoms and dinocysts indicate warmer surface temperature the succession of fossils and environments compares well with modern horizontal off glacier successions observed in fjords of the european arctic

mesozoic episodic magmatism in south korea and its tectonic implication 1 ne trending granitoid batholiths in south korea are part of the huge phanerozoic granitoid belt in east asia the crystallization ages of granitoid plutons show three episodes of mesozoic magmatism in south korea triassic 248 210 ma jurassic 197 158 ma and late cretaceous early tertiary 110 50 ma with a late jurassic early cretaceous 50 m y hiatus the granitoid ages and kinematic analyses of the mesozoic structures in south korea suggest that the middle jurassic 180 170 ma dextral strike slip ductile shear zones formed by the northwest orthogonal subduction of the izanagi plate whereas the cretaceous sinistral faults formed by the northward oblique subduction of the same plate which generally agree with proposed reconstruction of plate motions albeit uncertain around east asia among the models proposed to explain magmatic lull in a continental arc shallow angle subduction associated with high convergence rate and subduction of oceanic plateaus might be the probable cause for the magmatic hiatus in south korea and japan the huge width 500 km of the east asian granitoid belt may be explained by oceanward retreat of subduction zone in the cretaceous in association with change in the movement direction of the subducting plate

middle to late jurassic felsic and mafic magmatism in southern hunan province southeast china implications for a continental arc to rifting abstract extensive middle to late jurassic felsic and mafic magmatism occurred in the southern hunan province southeast china shrimp zircon u pb dating mineral chemical element geochemical and sr nd hf isotopic data have been determined for these rocks the results indicate that the middle jurassic 178 170 ma changchengling and ningyuan basaltic rocks belong to tholeiitic series and alkaline series respectively and were formed by fractional crystallization of the parent magmas that were derived from partial melting of asthenospheric mantle triggered by the slab released fluids of the subducted paleo pacific plate the late jurassic 152 146 ma daoxian basalts and guiyang lamprophyre dikes are low ti high mg potassic rocks they were derived directly by partial melting of shallower 60 100 km lithospheric mantle composed of amphibole and phlogopite bearing lherzolite in response to thermal perturbations associated with the rifting of continental arc the middle jurassic 164 ma tongshanling granitic rocks belong to calc alkaline series and are strongly peraluminous they were formed by partial melting of palaeoproterozoic metasedimentary basement in the lower crust plus additional input from coeval basaltic magma the late jurassic 156 ma jinjiling and xishan granitic rocks show a2 subtype affinity they were generated by partial melting of granulitized palaeoproterozoic metamorphic basement in the lower crust in response to injection of coeval low ti high mg potassic magmas a moderate degree 56 58 of fractionation of these pure crustal melts could account for more felsic end member of granitic rocks detailed petrologic and geochemical data for the middle to late jurassic felsic and mafic rocks from the southern hunan imply that during the middle jurassic time se china on the southeast of the shi hang zone was a continental arc coupled with the subduction of the palaeo pacific plate and since the beginning of late jurassic time an intra arc rift has been formed along the shi hang zone as a consequence of slab roll back

the kuroshio large meander formation in 2004 analyzed by an eddy resolving ocean forecast system 1 the recent formation of the kuroshio large meander southeast of the kii peninsula during the period from may to july 2004 has been simulated successfully in an eddy resolving ocean forecast system japanese coastal ocean predictability experiment the formation process was examined from the viewpoint of eddy kuroshio interactions using sensitivity experiments that had various initial conditions for eddies in march and may 2004 we confirmed that the large meander does not occur when the trigger meander southeast of the kyushu island has small amplitude and that the cyclonic eddy on the offshore side of the kuroshio contributes to the large meander formation our results show that the large amplitude of the trigger meander southeast of kyushu island in april 2004 was caused by downstream advection of anticyclonic eddies along the kuroshio in the east china sea from east of taiwan the results suggest that the successive westward propagation of strong anticyclonic eddies in the subtropical frontal region from 2003 to 2004 was the origin of the formation of the kuroshio large meander south of japan in summer 2004

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

a preliminary description of the gan hang failed rift southeastern china abstract the gan hang failed rift as defined by present day topography extends at least 450 km in length and 50 km in width it is a northeast southwest trending series of features spanning from hangzhou bay in zhejiang province into jiangxi province through fuzhou city southwest of fuzhou the rift splits into two portions one continuing along the southwestern trend and the other diverging westward the total extent of the rift cannot be defined at this time the rift is superimposed upon a major suture zone of caledonian or early mesozoic age the suture represents the fusing of the south china huanan and yangtze cratons perhaps in late triassic but for sure by late middle jurassic time the rifting was initiated and followed this older suture in part this time corresponds roughly to the middle stage of the yanshanian orogeny and to the subduction of the postulated pacific kula ridge southeast of the continental margin the total thickness of the sediments and volcanics filling the rift valley reaches more than 10 000 m peak intensity of extension was between late middle jurassic and middle to late cretaceous sedimentation within the rift was not continuous and is marked with periodic unconformities sediments within the rift include red beds sandstones siltstones mudstones conglomerates breccias tuffs and ignimbrites vertebrate fossils and dinosaur eggs are also found contemporaneous volcanics within and flanking the rift include basalts rhyolites granites gabbros dacites and andesites silicic volcanics are mostly attributed to caldera systems early basalts are tholeiitic and later change to alkaline olivine basalt bimodal volcanism is recognized peak intensity of volcanism ranges between 135 and 75 ma in early cenozoic time the area was a topographic low paleocene eocene sediments and evaporites are the last rocks to be deposited in the rift today the rift is delineated by major high angle faults the pingxiang guangfeng deep fault belt red beds and volcanically derived u deposits scismic study of p wave velocities has produced an earth model which shows substantial crustal thinning under the gan hang rift the region has been included in the southeastern china mesozoic fault depression system and has not generally been recognized as a distinct failed rift

isotopic and paleomagnetic constraints on the mesozoic tectonic evolution of south china in order to better constrain the paleogeographic evolution of south china we measured sm nd and rb sr isotopic compositions for 23 mesozoic granites that crop out throughout the area tightly grouped neodymium depleted mantle model ages 1 4 0 3 ga suggest the region is underlain by relatively homogeneous proterozoic crust and fail to define crustal provinces neither the isotopic nor geologic data suggest that a mesozoic suture exists however granites possessing anomalously high sm 8 ppm and nd 45 ppm concentrations relatively high initial epsilon neodymium 4 to 8 and high but variable initial 87sr 86sr 0 759 to 0 713 form a northeast trending zone that coincides with two prominent mesozoic basins southeast of the zone lie the majority of mesozoic intrusives and upper triassic to lower cretaceous extensional basins found in south china mesozoic paleomagnetic poles are well clustered northwest of the zone pre cretaceous poles southeast of it are discordant with respect to those from the northwest the only recognized tectonostratigraphic terrane in south china lies southeast of the zone the terrane is bordered by a northeast trending sinistral fault that was active in the mesozoic other faults in south china have similar attitudes ages and sense of shear together the observations suggest that the mesozoic tectonic regime in south china consisted of strike slip activity plus concomitant rifting as terranes or fragments of similar crust were transported north along sinistral faults the zone defined by the granites enriched in nd and sm demarcates displaced terranes to the southeast from relatively stable land to the northwest

geochemical and zircon u pb and hf isotopic study of the baijuhuajian metaluminous a type granite extension at 125 100 ma and its tectonic significance for south china abstract the early cretaceous baijuhuajian pluton is an a type granitic intrusion emplaced along the jiangshan shaoxing jssx fault zone in western zhejiang province se south china it intruded into a late jurassic volcanic basin bounded by proterozoic sandstone and siltstone the granite has a coarse grained porphyritic texture and is composed of alkali feldspar phenocrysts in a matrix of k feldspar quartz plagioclase and biotite the pluton has a clear a type geochemical signature e g it is metaluminous a cnk 1 0 and has high sio2 75 to 78 wt total alkalis na2o k2o 4 8 to 8 5 wt rare earth elements total ree 228 to 438 ppm hfse and fe feot feot mgo 0 87 to 0 97 on the other hand it is characterized by low cao 0 51 to 1 01 wt tio2 0 06 to 0 13 wt p2o5 below detection and sr 7 0 to 9 5 ppm the hypersolvus texture and fine grained groundmass indicate emplacement at a shallow depth shrimp u pb zircon dating yielded a weighted mean 206pb 238u age of 126 3 ma and zircons from this pluton give ehf t values between 0 52 and 4 24 suggesting the importance of juvenile material in the magma source most of the plutonic rocks in southeast china are calc alkaline subduction related intrusions of jurassic and cretaceous age previous studies suggested that the sparse a type granites in south china were emplaced in two episodes 190 155 ma in the inland area and 100 ma along the coastal area in se south china our study of the baijuhuajian granite together with the coeval suzhou a type granite 123 ma in jiangsu province defines an important extensional event between 125 and 100 ma considering the available data for the mesozoic igneous rocks in south china we suggest that subduction of the pacific plate in the mesozoic produced voluminous calc alkaline granitoids and volcanic rocks with a younging trend from west to east the a type granites were generated during periods of local extension at 190 155 ma and 125 100 ma perhaps corresponding to localized rollback of the pacific plate

a geochemical and geochronological study of the early cretaceous extension related honggong ferroan a type granite in southwestern zhejiang province southeast china the honggong pluton is the largest ferroan alkalic a type granite intrusion emplaced along the jiangshan shaoxing fault zone in southwestern zhejiang province and has important implications for understanding the late mesozoic tectonic evolution of se china u pb ages of 138 7 0 8 134 2 1 1 128 5 1 5 and 126 1 0 9 ma were obtained from zircon by laser ablation inductively coupled plasma mass spectrometry indicating that the honggong pluton formed in the early cretaceous the honggong pluton has a clear ferroan alkalic a type granite geochemical signature with for example high total alkali contents and feo t feo t mgo values the sr nd hf isotopic compositions suggest that there was juvenile material in the magma source geochemical evidence indicates that the pluton was derived through extensive fractionation of melts that contained both asthenospheric mantle and mesoproterozoic crustal components these rare granites in southern china were emplaced during five episodes at 235 225 190 165 155 100 90 and 140 120 ma the age of the honggong pluton suggests that localized extension in southwestern zhejiang province began as early as 138 ma and continued to 126 ma this early cretaceous extensional event was triggered by localized rollback of the subducting pacific plate

zircon u pb geochronology and hf isotopic compositions of the mesozoic granites in southern anhui province china abstract mesozoic magmatism is extensively developed in eastern china along the eastern asian continental margin but the geochronological framework is not well understood for each area in this study seventy eight samples were collected from the mesozoic igneous rocks in south anhui province east central china in order to precisely constrain their formation ages and contributions to crustal growth zircon u pb age determination using ion probe and laser ablation techniques indicate that magmatism in the area can be divided into two separate stages at 150 136 and 136 120 ma respectively the first stage of granitic rocks not documented in other areas of eastern china is mostly composed of hornblende bearing i type granodiorite and monzogranite and is considered to be formed by slab tearing during pacific subduction the second stage of magmatism coeval to that in other areas of eastern china is characterized by the occurrence of a type granite and syenite and their volcanic counterparts although i type granitic rocks are also developed locally these rocks are mostly distributed around and within the fault controlled volcanic basins and some are closely related to detachment shearing indicating their formation in an extensional setting zircon hf isotopic analyses indicate that archean material might be present in the area but that the main episode of crustal growth took place during the mesoproterozoic with very little juvenile material in the mesozoic

origin of mesozoic adakitic intrusive rocks in the ningzhen area of east china partial melting of delaminated lower continental crust to the best of our knowledge modern adakites have not been documented in a nonarc environment we report geochemical and isotopic data for early cretaceous anjishan adakitic intrusive rocks that are in a continental setting unrelated to subduction the anjishan adakitic intrusive rocks which are exposed in the ningzhen area of east china have high sr y and la yb ratios coupled with low yb and y as well as relatively high mgo contents and mg numbers mg 0 4 0 6 similar to products from slab melting however low n d t values 6 8 to 9 7 and high 8 7 sr 8 6 sr i 0 7053 0 7066 are inconsistent with an origin by slab melting the tectonics and geochemistry lead us to conclude that adakitic magmas were most likely derived from partial melting of mafic material at the base of the continental crust high sr y and la yb ratios of the adakitic intrusive rocks suggest that garnet was stable as a residual phase during partial melting implying that the crustal thickness exceeded 40 km in the early cretaceous the present thickness of the crust in the ningzhen area is only 30 km and therefore the crust appears to have been thinned by at least 10 km since the early cretaceous the relatively high mgo contents and mg of the anjishan intrusive rocks suggest that adakitic magmas interacted with mantle rocks possibly coinciding with lower crustal delamination which would also account for the observed thinning

high precision u pb zircon ages for mesozoic igneous rocks from hong kong abstract sixteen new high precision u pb zircon ages are reported from jurassic and early cretaceous silicic volcanic and plutonic rocks of hong kong when combined with the existing age dataset the new ages constrain more tightly the timing of major periods of volcanism and plutonism at 162 6 4 5 ma 146 7 1 1 ma 143 0 1 0 ma and 140 8 0 6 ma however two ages of 151 9 0 2 ma and 148 1 0 2 ma from eastern new territories and southern hong kong indicate additional and therefore more continuous albeit pulsed magmatic activity than previously thought

chemical composition and sr isotopes of apatite in the xiangshan a type volcanic intrusive complex southeast china new insight into petrogenesis abstract a series of late mesozoic a type granitoids including the xiangshan volcanic intrusive complex was recently identified along the gan hang volcanic belt which was close to the boundary of the yangtze and cathaysia blocks in south china however significant issues on the petrogenesis of the xiangshan complex especially how mantle derived magma contributes if any still remain controversial in this study we focused on the apatite geochemistry of the xiangshan complex to understand the characteristics and evolution of its primary magma in the xiangshan complex mainly including rhyolite porphyritic lava granite porphyry and quartz monzonite porphyry apatites occur mostly as inclusions in plagioclase and biotite and exhibit high srap sr content in apatite srhr sr content in host rock from 0 97 to 3 22 1 85 on average indicating these apatites crystalized before plagioclase in the melt therefore the negative eu anomalies from 0 06 to 0 83 in apatites of the xiangshan complex indicate that plagioclase is a remaining stable mineral phase in the source rocks during the partial melting in addition most of apatites in the xiangshan complex with varied whole rock asi aluminum saturation index values from 0 88 to 1 19 are characterized by high and relatively homogeneous f content from 3 14 to 3 55 therefore f rich mineral should be a primary phase in the source compared with apatites in three earlier rocks of the complex apatites from quartz monzonite porphyry have higher mn and cl contents stronger degree of negative eu anomalies and lower initial 87sr 86sr 0 70642 0 70714 suggesting that before apatites crystalized the magma to form quartz monzonite porphyry had been mixed with a cl rich magma derived from a mantle source that received addition of cl from subducting oceanic sediments of the paleo pacific plate the sr isotope ratios and halogens contents of apatite from quenched dark enclaves found in granitic porphyry imply that the enclaves were formed from this cl rich mantle derived magma through injecting part of this magma into the granitic magma and remaining as quenched enclaves without mechanic mixing and no significant chemical influence on the host granitic magma thorough mixing between this mantle derived magma and the felsic crustal magma did not occur until the latest stage of the xiangshan volcanism when quartz monzonite porphyry was forming

high pressure melting of metapelite and the formation of ca rich granitic melts in the namche barwa massif southern tibet abstract a detailed geochemical and geochronological study of anatectic migmatites from the namche barwa massif nbm southern tibet has been carried out to place important constraints on the thermal and tectonic evolution of the eastern himalayan syntaxis shrimp zircon u pb dating indicates that the granulite facies metapelite underwent metamorphism at 21 8 0 7 ma and 24 5 0 7 ma respectively the latter is similar to the timing of partial melting and the formation of ca rich leucosomes at 24 25 ma these leucosomes are characterized by 1 high cao na 2 o and na k ratios 2 radiogenic sr 87 sr 86 sr t 0 7407 0 7904 but unradiogenic nd e nd t 7 0 to 21 2 isotope compositions 3 depleted hfse and 3 variable but depleted hree relative to their host pelites some of the leucosomes show large degrees of nd isotopic disequilibrium up to 10 epsilon units with respect to their hosts these high cao and na 2 o leucosomes were derived from fluxing melting of metapelite at high pressures a similar process could have operated during the formation of the himalayan leucogranites and contributes to the heterogeneities in such granites

cretaceous tectonic evolution of south china a preliminary synthesis abstract the cretaceous tectonic evolution of south china is characterized by widespread extensional basin and dome generation voluminous magma intrusion eruption and associated polymetallic mineralization all of which are of world wide interests that have stimulated the attention of geologists for more than half a century due to the lack of a comprehensive understanding of regional tectonic evolution and geodynamics of south china many controversies regarding the nature and origin of these features remain this paper attempts to make a review by synthesizing existing structural petrological geochronological and geochemical data of the cretaceous structures and magmas which guides us to propose a three stage tectono thermal evolutionary history of south china during the cretaceous period the earliest cretaceous 145 137 ma tectonic stage was characterized by syn orogenic shortening deformation and metamorphism under a nw se compressional setting which generated voluminous porphyry cu au ore bearing adakitic rocks in the lower yangtze river belt and gneissic granites in the coastal area its geodynamic origin was interpreted as combined effects of the pacific izanagi ridge subduction beneath the lower yangtze river belt and the pacific subduction beneath the cathaysia block this syn orogenic shortening stage was followed by a post orogenic stage 136 86 ma comprising two episodes of alternate extensional and shortening events the extension 136 118 ma in the earlier episode was dominated by a nw se extensional regime it led to significant taphrogenesis manifested by large scale extensional basins and voluminous magma intrusion eruption this extensional event was associated with a combination of the slab window opening during the ridge subduction and the rollback of the subducted pacific slab the subsequent nw se transpressional event led to cessation of active marginal magmatism and resulted in tectonic inversion of previous rift basins this transpression lasted from 117 ma to 108 ma and its driving mechanism was attributed to the collision between the eastern asian margin and the west philippine block the extension 107 86 ma in the later episode was dominated by a wnw ese extensional regime which led to a second phase of basin subsidence and produced numerous a and i type granites and bimodal volcanoes the tectonic regime then changed at ca 85 ma to wnw ese oriented transpression causing inversion of the late cretaceous rift basins and cessation of extension related magmatism the change of stress field from wnw ese extension to wnw ese transpression was related to the variations of the subducted slab dynamics i e the transition from either the ese ward retreat to wnw ward subduction of the pacific slab or the alternate stress during slab break off during the latest cretaceous a drastic change of regional tectonic stress orientation from wnw ese to n s occurred in south china the third stage predominated by n s extension was developed which caused a new phase of regional crustal subsidence along the youngest e w trending extensional structures

remelting of neoproterozoic relict volcanic arcs in the middle jurassic implication for the formation of the dexing porphyry copper deposit southeastern china article i nfo the dexing copper deposit in southeastern china is a typical non arc porphyry deposit the origin of which has been a topic of debate for several decades here we present new results from u pb geochronology whole rock chemistry and sr nd hf o isotopic investigations on the ore forming granodioritic porphyry la icpms zircon u pb data suggest that the granodioritic porphyry was formed in the middle jurassic ca 172 5 ma probably associated with lithospheric thinning driven by either sub continental lithospheric mantle delami nation or asthenospheric upwelling the porphyry displays both arc like and adakitic trace element signa tures the adakitic features suggest that hree heavy rare earth elements rich minerals such as garnet and hornblende in the absence of plagioclase resided in the source region the arc like signatures are broadly comparable with those of the proximal neoproterozoic island arc rocks including the keratophyre from shuangxiwu group and associated granitoids indicating a potential genetic relationship the porphyry has chondritic end t of 0 28 to 0 25 and radiogenic ehf t of 2 to 7 and correspondingly uniform two stage depleted mantle nd model ages of 940 980 ma and hf model ages of 800 1100 ma mean 920 ma on nd and hf isotopic evolution diagrams these values are markedly similar to those of the adjacent neoproterozoic arc rocks when calculated forward to the mid jurassic zircons of the porphyry show mantle like oxygen isotope characters with 18 o values clustering in the range of 4 7 5 9 similar to the values for the neoproterozoic arc rocks mentioned above the geochemical and isotopic features recorded in our study suggest mantle derived magmas with no significant supracrustal input for the source of the por phyry with regard to the source of the cu ore we consider a model involving the remelting of sulfide bearing arc related lower crustal source furthermore the occurrence of a neoproterozoic vms volcanic massive sulfide type copper deposit the pingshui cu deposit in the shuangxiwu group might suggest that the lower crustal rocks related to a neoproterozoic relict island arc provided the source for copper during a second stage melting event we propose a new geodynamic model for the dexing porphyry cu deposit which envis ages that the sulfide bearing arc lower crustal rocks were generated during oceanic slab subduction in the early neoproterozoic the remnants of which were preserved at the crust mantle boundary subsequently in the middle jurassic these rocks were heated by asthenospheric upwelling and remelted to produce fertile magmas the magmas ascended along the northeast jiangxi fault and intruded into the jiuling terrane where cu precipitation occurred upon subsequent magma cooling and fluid exsolution

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

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry

metallogeny of the zoujiashan uranium deposit in the mesozoic xiangshan volcanic intrusive complex southeast china insights from chemical compositions of hydrothermal apatite and metal elements of individual fluid inclusions abstract the zoujiashan u deposit in the mesozoic xiangshan volcanic intrusive complex is one of the largest volcanic related type u deposits in china chemical composition of hydrothermal apatite and metal contents in individual fluid inclusions of quartz and fluorite are investigated in order to understand the precipitation mechanism of u and to further constrain the origin of u and ore forming fluids of the zoujiashan deposit illitization hematitization and u mineralization successively occurred within rhyolite and porphyritic lava three types of hydrothermal apatite are identified apatite type 1 ap1 and apatite type 2 ap2 are both euhedral subhedral crystals found in hematitization zone while only ap2 shows evident irregular chemical zoning along its rims apatite type 3 ap3 occurs as anhedral aggregates closely associated with u minerals in u ore veins implying synchronous precipitation of ap3 and u ores comparable chemical compositions of ap1 and cores of ap2 including similar high cl content indicate both of them were precipitated from a cl rich fluid that induced hematitization the elevated f contents of ap3 and rims of ap2 implies that a f rich fluid was involved and altered ap2 during the main stage of u deposition ap1 and cores of ap2 have lower mn contents than ap3 and rims of ap2 indicating an oxidized character for the cl rich fluid and a reducing character for the f rich fluid metal contents of fluid inclusions in fluorite and quartz occurring in u ores suggest that two end members of fluids including a u rich and a u poor were involved in the main stage of u mineralization mixing between these two fluids led to the deposition of u and th minerals through oxidation reduction reactions and destabilization of th f complexes respectively the u rich fluid is depleted in th and sr compared with the u poor fluid based on comparison of chemical compositions of apatite and fluid inclusions we suggest that the cl rich fluid precipitating ap1 and ap2 corresponds to the u rich end member fluid while the f rich fluid precipitating ap3 represents a mixture between the two end members the high u 12 7 58 5 ppm in the u rich end member fluid is in agreement with a relatively high oxygen fugacity and a surface derived origin the u poor end member fluid has lower rb cs than the u rich end member and the unaltered host rocks indicating the u poor fluid was derived from a mantle wedge that experienced addition of cs by fluids generated from the devolatilization of the subducted paleo pacific slab