the giant dexing porphyry cu mo au deposit in east china product of melting of juvenile lower crust in an intracontinental setting the dexing porphyry cu mo au deposit in east china 1 168 mt at 0 45 cu is located in the interior of the south china craton scc made up of two lithospheric blocks the yangtze and cathaysia blocks the cu mo au mineralization is associated with mid jurassic granodioritic porphyries with three high level intrusive centers controlled by a series of lineaments at the southeastern edge of the yangtze block available age data define a short duration 172 170 ma of the felsic magmatism and the mineralization 171 1 ma the deposit shows broad similarities with deposits in volcanoplutonic arcs although it was formed in an intracontinental setting porphyries associated with mineralization are mainly granodiorites which contain abundant phenocrysts 40 60 and carry contemporaneous microgranular mafic enclaves mmes they are mainly high k calc alkaline and show geochemical affinities with adakite characterized by relatively high mgo cr ni th and th ce ratios the least altered porphyries yielded relatively uniform end t values from 0 9 to 0 6 and wide 87sr 86sr i range between 0 7046 and 0 7058 partially overlapping with the sr nd isotopic compositions of the mmes and mid jurassic mafic rocks in the scc zircons from the porphyries have positive ehf t values 3 4 to 6 9 and low 18o values 4 7 to 6 3 generally close to those of depleted mantle all data suggest an origin by partial melting of a thickened juvenile lower crust involving mantle components e g neoproterozoic mafic arc magmas triggered by invasion of contemporaneous mafic melts at dexing the mmes show textural mineralogical and chemical evidence for an origin as xenoliths formed by injection of mafic melts into the felsic magmas these mmes usually contain magmatic chalcopyrite and have original variable contents of cu up to 500 ppm their geochemical characteristics suggest that they were derived from an enriched mantle source metasomatized by proterozoic slab derived fluids and supplied a part of cu au and s for the dexing porphyry system during their injection into the felsic magmas the 171 1 ma magmatic hydrothermal event at dexing is contemporaneous with the mid jurassic extension in the scc followed by 160 90 ma arc like magmatism in southeastern china with respect to the tectono magmatic evolution of the scc the emplacement of cu bearing porphyries and the associated cu mineralization occurred in response to the transformation from a tensional regime related to mid jurassic extension to a transpressional regime related to the subduction of the paleo pacific oceanic lithosphere

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

late carboniferous to early permian magmatic pulses in the uliastai continental margin linked to slab rollback implications for evolution of the central asian orogenic belt abstract the paleo asian ocean underwent a protracted closure history during late paleozoic here we investigate the magmatic evolution during this process based on a detailed study in the baiyinwula region along the uliastai continental margin the major rock types in this area are late carboniferous early permian volcanic sequences and coeval intrusions we identified four stages of magmatic evolution based on the diverse assemblages and their precise isotopic ages the first stage is represented by andesites with a zircon 206pb 238u age of ca 326 12 ma these rocks are metaluminous to weakly peraluminous high k calc alkaline and possess high na2o k2o ratios in the range of 1 23 to 2 45 they also display enrichment of large ion lithophile elements lile and depletion of high field strength elements hfse with markedly positive zircon ehf t varying from 8 1 to 15 6 the geochemical features of these andesites are similar to those of typical arc volcanic rocks the second stage includes granodiorites emplaced at 318 6 1 8 ma the rocks are high k calc alkaline with a cnk values ranging from 0 95 to 1 06 and show enrichment in lile and depletion in hfse they show geochemical affinities to adakites with high sr and low y and yb contents indicating magma derivation from thickened lower crust zircon grains from these rocks display positive initial ehf t values ranging from 11 1 to 14 6 with corresponding two stage hf model ages tdm2 of 394 622 ma the third stage consists of syenogranite together with a volcanic suite ranging in composition from rhyolite todacite which formed during 303 4 1 2 to 285 1 2 2 ma they possess elevated silica and alkali contents high feot mgo and ga al ratios low al2o3 mgo and cao contents and high rb y nb ce zr y and ga contents strong negative ba sr and eu anomalies showing i to a type granitic affinities zircons in these rocks show elevated hf isotopic compositions ehf t 9 9 to 14 6 with tdm2 varying from 324 to 673 ma the fourth magmatic pulse is represented by k feldspar granite with zircon u pb ages from 283 2 1 9 ma to 280 0 1 4 ma and typical alkalic a type granite geochemistry these rocks possess positive ehf t values in the range of 9 7 to15 2 and a restricted range of hf model age from 327 to 684 ma the magmatic rocks from the four stages show comparable ehf t and t2dm suggesting that the magmas were derived from the same evolving mantle derived source we propose a tectonic model linking the evolution of the magmatism with the closure of the paleo asian ocean that involved the following stages the andesites were formed during the initial oceanic subduction stage with magma sourced from the metasomatized lithospheric mantle stage 2 adakite like rocks were derived from subduction induced thickened crust subsequent slab rollback resulted in asthenospheric upwelling and melting of residual juvenile crust to generate the i and a type syenogranite rhyolite and dacite suite finally followed by the a type k feldspar granite

multistage anatexis during tectonic evolution from oceanic subduction to continental collision a review of the north qaidam uhp belt nw china abstract the north qaidam ultra high pressure uhp metamorphic belt in the northern tibetan plateau is considered as a typical alpine type uhp metamorphic belt due to the early paleozoic subduction of the qaidam block beneath the qilian block the well preserved paleozoic metatexite migmatite diatexite migmatite felsic sheet dyke and anatectic granite in the north qaidam uhp metamorphic belt provide us with an excellent opportunity to study the generation transport and final fate of crustal melt and geodynamic processes associated with orogenesis based on structural relationships petrology geochronology whole rock geochemistry and sr nd isotope data the early paleozoic anatexis during oceanic subduction to continental collision in the north qaidam uhp metamorphic belt can be further divided into three stages of development 470 ma 446 428 ma and 432 420 ma the first stage granitic leucosomes are rich in k poor in na with low sr y and enrichment of large ion lithophile elements liles which were probably derived from partial melting of ancient felsic gneiss in continental arc circumstances during oceanic subduction the second stage na rich leucosomes and tonalite plutons are characterized by high na sr sr y and la yb and low heavy rare earth elements hrees with positive end t values of 0 1 4 3 and zircon ehf t values of 8 3 10 2 similar to typical tonalite trondhjemite granodiorites ttgs these ttg like magmas were produced through partial melting of newly emplaced gabbroic rocks with arc affinity under high pressure hp granulite facies conditions in a thickened lower crust during continental collision the volumetrically significant migrated plutons evolved from ttg like melt will become segments of continental crust and contribute to crustal growth with partial residual products of hp granulite and or garnet pyroxenite to the mantle by delamination the third stage of anatexis was preserved in both migmatitic uhp gneiss and eclogite in the xitieshan and luliangshan terranes the kfs rich felsic leucosomes inside uhp gneisses in the third stage are characterized by high alkali contents and low mafic component with feot mgo tio2 2 0 2 high la yb and sr y and low y and hrees 3 high al2o3 and low mg values and 4 enriched in liles e g rb ba k sr pb and poor in high field strength elements hfses partial melting of uhp eclogite and felsic gneiss during the initial retrogression stage with pl rich and kfs rich leucosome formation was triggered by dehydration melting involving predominant zoisite and muscovite the anatectic melts from partial melting of deeply subducted uhp gneiss have accumulated and migrated outside the deeply subducted crustal slice in the form of syn collisional plutons the melt evolution from the leucosomes produced at the early exhumation stage to syn collisional granitoids produced at the late exhumation might contribute greatly to the exhumation of the north qaidam uhp metamorphic belt from mantle depths to the lower crustal levels

the genesis of early carboniferous adakitic rocks at the southern margin of the alxa block north china abstract adakite is a highly debated petrologic term that was originally proposed to describe igneous rocks formed by slab melting subsequent studies reported other ways to generate adakitic signatures such as the melting of mafic lower continental crust and fractional crystallization of basaltic magma we studied adakitic rocks from the taohuala mountain at the southern margin of the alxa block north china these rocks are characterized by high sr concentrations 300 450 ppm high sr y 20 75 ppm and la yb n 25 67 ppm ratios and low y

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

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

petrogenesis of the late archean 2 5 ga na and k rich granitoids in the zhongtiao wangwu region and its tectonic significance for the crustal evolution of the north china craton abstract petrogenesis of coexisting late archean 2 5 ga na and k rich granitoids in the zhongtiao wangwu region is significant for understanding the crustal evolution of the north china craton ncc in this paper we present new zircon u pb age and whole rock geochemical and isotope data on a 2 5 ga na and k rich granitoid suite from the zhongtiao wangwu region southern ncc the na rich granitoids ttg and k rich granitoids a type granites are zircon u pb dated to be 2 45 ga and 2 49 ga respectively integrated geochemical data indicate that the ttg were formed by partial melting of the mafic lower crust while the a type granites were formed by partial melting of a ttg like source in the shallower crust the petrogenesis likely occurred under lithospheric delamination and the subsequent asthenospheric upwelling and mafic magma underplating we proposed that the coeval occurrence of na and k rich granitoids in the southern ncc is probably ascribed to the partial melting of continental crust at different depths which could be an important way to induce the archean crustal reworking and compositional changes at 2 5 ga

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

experimental determination of trace element partitioning between pargasite and a synthetic hydrous andesitic melt in order to more fully establish a basis for quantifying the role of amphibole in trace element fractionation processes we have measured pargasite silicate melt partitioning of a variety of trace elements rb ba nb ta hf zr ce nd sm yb including the first published values for u th and pb experiments conducted at 1000 c and 1 5 gpa yielded large crystals free of compositional zoning partition coefficients were found to be constant at total concentrations ranging from 1 to 100 ppm indicating henry s law is oparative over this interval comparison of partition coefficients measured in this study with previous determinations yields good agreement for similar compositions at comparable pressure and temperature the compatibility of u th and pb in amphibole decreases in the order pb th u partial melting or fractional crystallization of amphibole bearing assemblages will therefore result in the generation of excesses in238u activity relative to230th similar in magnitude to that produced by clinopyroxene the compatibility of pb in amphibole relative to u or th indicates that melt generation in the presence of residual amphibole will result in the long term enrichment in pb relative to u or th in the residue this process is therefore incapable of producing the depletion in pb relative to u or th inferred from the pb isotopic composition of morb and oib comparison of partition coefficients measured in this study with previous values for clinopyroxene allows some distinction to be made between expected trace element fractionations produced during dry cpx present and wet cpx amphibole present melting rb ba nb and ta are dramatically less compatible in clinopyroxene than in amphibole whereas th u hf and zr have similar compatibilities in both phases interelement fractionations such as dnb dba are also different for clinopyroxene and amphibole changes in certain ratios such as ba nb ba th and nb th within comagmatic suites may therefore offer a means to discern the loss of amphibole from the melting assemblage elastic strain theory is applied to the partitioning data after the approaches of beattie and blundy and wood and is used to predict amphibole melt partition coefficients at conditions of p t and composition other than those employed in this study given values of dca dti and dk from previous partitioning studies this approach yields amphibole melt trace element partition coefficients that reproduce measured values from the literature to within 40 45 this degree of reproducibility is considered reasonable given that model parameters are derived from partitioning relations involving iron and potassium free amphibole

the cretaceous duimiangou adakite like intrusion from the chifeng region northern north china craton crustal contamination of basaltic magma in an intracontinental extensional environment abstract zircon u pb ages major and trace element and sr nd and pb isotope compositions of the duimiangou dmg quartz monzonite from the chifeng region on the northern north china craton ncc were studied to investigate its derivation evolution and geodynamic significance laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb dating yields an emplacement age of 128 1 ma for this intrusion with numerous mesozoic inherited zircons clustering at 219 12 ma and 161 3 ma along with some ancient zircons with ages of 2 5 ga 1 77 ga and 324 ma bulk rock analyses show that this intrusion is characterized by variable sio 2 63 4 69 4 wt al 2 o 3 14 5 16 3 wt na 2 o k 2 o 8 01 8 95 wt and mg 41 3 48 0 they are enriched in large ion lithophile elements and light rare earth elements without significant eu anomalies mostly between 0 89 1 10 and depleted in heavy rare earth elements and high field strength elements with high sr y 63 7 101 7 and la yb n 20 5 31 0 ratios the dmg intrusion formed in an intracontinental extensional setting contemporaneous with the formation of pull apart basins metamorphic core complexes and intense magmatism rather than in a convergent margin it has homogeneous sr 87 sr 86 sr i 0 7059 0 7066 nd e nd t 6 2 to 7 2 and pb 206 pb 204 pb i 17 289 17 375 207 pb 204 pb i 15 359 15 463 208 pb 204 pb i 37 130 37 472 isotope compositions sr nd isotope modeling results plus relatively young nd model ages 1522 1618 ma and the presence of relict zircons suggest that this intrusion could have originated from crustal contamination of newly formed basaltic melts derived from asthenospheric mantle accompanied by fractional crystallization of k feldspar biotite apatite fe ti oxides and minor hornblende and plagioclase thus the dmg adakite like intrusion may record the magmatic event associated with underplating of asthenospheric magma in an intracontinental extensional environment and both asthenospheric magma underplating and lithospheric delamination may have played important roles in the lithospheric thinning of the northern ncc

petrogenesis of early carboniferous adakitic dikes sawur region northern west junggar nw china implications for geodynamic evolution abstract adakitic dikes are widespread in the sawur region northern west junggar northwestern china zircon u pb analyses of the dikes have yielded consistent ages of ca 334 ma the dikes are characterized by relatively high sio 2 55 5 61 8 wt al 2 o 3 14 5 16 3 wt and sr 468 1005 ppm and low y 10 1 14 1 ppm and yb 0 93 1 39 ppm contents with high sr y 34 74 ratios and slight eu anomalies which are analogous to those of slab derived adakites in addition the dikes are relatively mgo rich 1 75 3 57 wt mg 44 56 with high th yb ratios and positive end 334 ma 6 2 to 6 5 and ehf 334 ma 11 3 to 15 8 values the data suggest that the dikes were generated by partial melting of subducted oceanic crust and overlying sediments however these early carboniferous adakitic rocks and associated i type granites in the northern part of west junggar are petrochemically distinct from the late carboniferous adakitic plutons from the southern part of west junggar the latter are associated with charnockites a type granites tholeiites and nb enriched alkaline basalts as well as magnesian dikes which were generated by ridge subduction during the late carboniferous the distinct lithological association of the early carboniferous adakites is consistent with flat slab subduction which was characterized by anomalous heating of the slab and consequential partial melting to generate adakitic magma after generation of the early carboniferous adakites the subducting slab cooled reverted to a more normal subduction angle in the late early carboniferous and was associated with more typical i type granite arc magmatism

fluid present melting of ocean crust in subduction zones phase equilibria were investigated in the pressure range 400 2000 mpa on the pinatubo dacite a representative example of natural cenozoic adakites slab melts to constrain the temperature versus wt h 2 o conditions of magma genesis the experimental results suggest that water contents of the melt were at least 10 wt and temperatures were 900 c during magma ascent from the slab natural aluminous amphiboles present in the dacite crystallized at pressures of 1000 mpa fixing an upper temperature limit of 950 c for the slab melts at the crust mantle boundary amphiboles experimentally crystallized at 1000 mpa are identical to those found in melt inclusions in metasomatized mantle xenoliths these natural inclusions represent samples of slab melts and contain glasses with a dacitic bulk composition which requires an h 2 o content in the melt of 10 wt at 1000 mpa and 900 c for conditions of magma generation of 800 900 c at 2000 2300 mpa melt h 2 o contents close to 15 wt are needed to generate melts with dacitic bulk composition overall these constraints are inconsistent with slab melts being produced by dehydration melting of amphibolite a mechanism unable to produce dacite melt compositions that are both hydrous and cool slab melting in modern subduction zones must occur under fluid present conditions at temperatures below 900 c in agreement with the thermal regime of subducted oceanic crust as deduced from numerical simulation of heat transfer using the present day mantle geotherm at such low temperatures ti bearing phases are stable during melting thus any metasomatizing slab melt will be strongly depleted in nb ta and ti a major characteristic of arc magmas produced in the mantle wedge

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

trace element evidence for growth of early continental crust by melting of rutile bearing hydrous eclogite the early continental crust is composed dominantly of archean tonalite trondhjemite granodiorite ttg gneisses and is generally explained as the product of melting of metabasalts in the subducted crust however whether the melting occurs in shallow level amphibolite facies or in relatively deep eclogite facies is debated here i present experimental partition coefficients ds for 27 trace elements between garnet amphibole and tonalitic melts they are used together with published mineral melt trace element ds to model the melting of metabasalt in order to delimit the conditions for ttg production the results clearly show that model melts with trace element characteristics that completely mimic the ttg are in equilibrium with rutile bearing anhydrous and hydrous amphibole bearing eclogitic residues but not rutile free amphibole dominated residues rutile appears to be a necessary residual phase to account for the characteristic negative nb ta anomaly in the ttg these results thus suggest that the early continental material was produced under eclogite facies conditions based on the modeling results obtained using appropriate partition coefficients and the archean geotherm the preferred process for the ttg production is the melting of rutile bearing hydrous eclogite triggered by the release of h2o from the progressive breakdown of amphibole the pressure temperature p t conditions for ttg production via this process are constrained to 1 5 2 5 gpa 50 80 km and 850 1050 c by the p t stability boundaries of amphibole and rutile in the basalt system

initial subduction of the paleo pacific oceanic plate in ne china constraints from whole rock geochemistry and zircon u pb and lu hf isotopes of the khanka lake granitoids abstract northeast china is located in the eastern part of the central asian orogenic belt caob and was influenced by paleo pacific subduction during the mesozoic abundant granitoids from the late paleozoic to early mesozoic in ne china record this process including the khanka lake granitoids which resulted in extensive growth of continental crust in the area however the question of how and when the paleo pacific tectonic system began to affect ne china is still highly controversial the khanka lake granitoids can be subdivided into two main components based on their geochemical characteristics namely granodiorite and syenogranite the granodiorite has a u pb age of 249 ma and is adakite like enriched in lree and liles with high mg sr la yb sr y and na 2 o k 2 o with zircon e hf t values of 0 65 to 1 61 produced by the magma mixing between melting of the lower continental crust and juvenile basaltic magma the syenogranite has zircon u pb ages of 209 to 199 ma and geochemical features of highly fractionated i type granites with high sio 2 total alkalis and low mg and mg fe cr and ni and positive zircon e hf t of 1 72 to 5 12 indicating an origin from remelting of juvenile crust the granitoids were intruded by felsic veins between 195 and 184 ma with positive zircon e hf t from 0 57 to 5 32 the e hf t values of the granitoids become more positive as the zircon u pb ages become younger suggesting continuous melting of juvenile crust during subduction it is concluded that the khanka lake granitoids record the early stage of subduction of the paleo pacific oceanic plate which commenced at least ca 250 ma ago

lithospheric structure and dynamic processes of the tianshan orogenic belt and the junggar basin abstract located at the center of the eurasian continent and accommodating as much as 44 of the present crustal shortening between india and siberia the tianshan orogenic belt tob is one of the youngest 7000 m orogenic belts in the world it provides a natural laboratory for examining the processes of intracontinental deformation in recent years wide angle seismic reflection refraction profiling and magnetotelluric sounding surveys have been carried out along a geoscience transect which extends northeastward from xayar at the northern margin of the tarim basin tb through the tianshan orogenic belt and the junggar basin jb to burjing at the southern piedmont of the altay mountain we have also obtained the 2d density structure of the crust and upper mantle of this area by using the bouguer anomaly data of northwestern xinjiang with these surveys we attempt to image the 2d velocity and the 2d electric structure of the crust and upper mantle beneath the tianshan orogenic belt and the junggar basin in order to obtain the small scale structure of the crust mantle transitional zone of the study area the wavelet transform method is applied to the seismic wide angle reflection refraction data combining our survey results with heat flow and other geological data we propose a model that interprets the deep processes beneath the tianshan orogenic belt and the junggar basin located between the tarim basin and the junggar basin the tianshan orogenic belt is a block with relatively low velocity low density and partially high resistivity it is tectonically a shortening zone under lateral compression a detachment exists in the upper crust at the northern margin of the tarim basin its lower part of the upper crust intruded into the lower part of the upper and the middle crust of the tianshan near the korla fault its middle crust intruded into the lower crust of the tianshan and its lower crust and lithospheric mantle subducted into the upper mantle of the tianshan in these processes the mass of the lower crust of the tarim basin was carried down to the upper mantle beneath the tianshan forming a 20 km thick complex crust mantle transitional zone composed of seven thin layers with a lower than average velocity the thrusting and folding of the sedimentary cover the intrusive layer in the upper and middle crust and the mass added by the subduction of the tarim basin into the upper mantle of the tianshan are probably responsible for the crustal thickening of the tianshan due to the important mass deficiency in the crust and the upper mantle of the tianshan buoyancy must occur and lead to rapid ascent of the tianshan the episodic tectonic uplift of the tianshan and tectonic subsidence of the junggar basin are closely related to the evolution of the paleozoic mesozoic and cenozoic tethys

the distribution geochronology and geochemistry of early paleozoic granitoid plutons in the north altun orogenic belt nw china implications for the petrogenesis and tectonic evolution abstract abundant early paleozoic granitoid plutons are widely distributed in the north altun orogenic belt these rocks provide clues to the tectonic evolution of the north altun orogenic belt and adjacent areas in this paper we report an integrated study of petrological features u pb zircon dating in situ zircon hf isotope and whole rock geochemical compositions for the abei 4337 highland and kaladawan plutons from north to south in the north altun orogenic belt the dating yielded magma crystallization ages of 514 ma for the abei pluton 494 ma for the 4337 highland pluton and 480 460 ma for the kaladawan pluton suggesting that they are all products of oceanic slab subduction because of the age constraint the abei monzogranites derived from the recycle of paleoproterozoic continental crust under low pressure and high temperature conditions are products of subduction initiation the 4337 highland granodiorites have some adakitic geochemical signatures and are sourced from partial melting of thickened mafic lower continental crust the kaladawan quartz diorites are produced by partial melting of mantle wedge according to the positive e hf t values and the kaladawan monzogranite syenogranite are derived from partial melting of neoproterozoic continental crust mixing the juvenile underplated mafic material from the depleted mantle these results together with existing data provide significant information about the evolution history of oceanic crust subduction during the 520 460 ma the initiation of subduction occurred during 520 500 ma with formation of abei pluton subsequent transition from steep angle to flat slab subduction at ca 500 ma due to the arrival of buoyant oceanic plateaus which induces the formation of 4337 highland pluton with ongoing subduction the steep angle subduction system is reestablished to cause the formation of 480 460 ma kaladawan pluton meanwhile it is this model that account for the temporal spatial distribution of these early paleozoic magmatic rocks in the north altun orogenic belt

formation of earth s early archaean continental crust abstract subduction of oceanic crust at an unusually low angle has been proposed as a model for the growth of continental crust older than about 2 5 ga at modern zones of low angle or flat subduction magmatic additions to new crust come from partial melting of both the subducting oceanic crust slab and the thin wedge of mantle above the slab evidence for both a slab and wedge source is preserved in most late archaean 3 0 2 5 ga terrains but we find little evidence that a mantle wedge contributed to crustal growth prior to 3 1 ga this lack of evidence in part reflects a dearth of exposed crust aged between 3 0 and 3 3 ga but also suggests that subduction enriched mantle source regions did not develop before 3 3 ga and possibly not before 3 1 ga in contrast to most modern terrains and some late archaean terrains early archaean 3 3 ga continental crust evolved through direct melting of thick mafic crust we invoke a process of subduction that does not include the development of a mantle wedge and call this process archaean flat subduction to distinguish it from modern low angle subduction

geochronology geochemistry and geological implications of late carboniferous early permian mafic and felsic intrusive rocks from urad zhongqi western inner mongolia the mafic and felsic haertaolegai intrusions crop out in urad zhongqi western inner mongolia and are dominated by monzogranite porphyritic granite and gabbroic diorite intrusions we investigate the tectonic setting geochronology and anorogenic characteristics of the western inner mongolia through field investigation microscopic and geochemical analyses of samples from the haertaolegai bimodal intrusions and laser ablation multi collector inductively coupled plasma mass spectrometry la mc icp ms zircon u pb dating of gabbroic diorite and adakitic granites petrographic and geochemical studies of the bimodal intrusions indicate that the gabbroic diorites formed from a primary magma generated by the partial melting of lithospheric mantle material that had previously been modified by subduction related fluids the felsic rocks are high k calc alkaline and metaluminous have characteristics of adakitic rocks and were generated during the partial melting of juvenile crustal material zircon u pb dating indicates that the felsic portion of this pluton was emplaced during late carboniferous early permian time with the mafic portion of the pluton emplaced during early permian time the zircons of adamellites have hf t values and t dm2 ages of 1 0 to 2 7 and 1032 1128 ma respectively suggesting that they formed from magmas generated by partial melting of juvenile mesoproterozoic lower crust these data combined with the geology of the region indicate that the late carboniferous early permian bimodal intrusive rocks in western inner mongolia record a transitional period from collisional compression to post collisional extension these results indicate that the paleo asian ocean in western inner mongolia closed before 300 ma

zircon u pb ages and geochemistry of the huai an ttg gneisses terrane petrogenesis and implications for 2 5ga crustal growth in the north china craton abstract the huai an gneiss terrane is a typical archean terrane within the trans north china orogen of the north china craton ncc and is composed of tonalite trondhjemite granodiorite ttg suite and dioritic granulite facies gneisses precise shrimpii and cameca ims1280 zircon u pb ages demonstrate that the huai an gneisses formed around 2 55 2 50 ga and underwent granulite facies metamorphism at 1 85 1 80 ga the ttg gneisses are compositionally equivalent to high sio2 adakites with high sio2 and al2o3 concentrations strongly fractionated rare earth element ree patterns la yb n 10 54 with positive or negligible eu anomalies eu eu 0 9 3 4 low heavy ree hree and y concentrations high sr 435 938 ppm concentrations and high sr y ratios 48 261 high mg numbers 33 60 and high compatible element concentrations e g cr 11 183 ppm and ni 11 82 ppm indicate that the magmas that formed these gneisses were derived from interaction between adakitic melts and mantle material the dioritic gneisses have compositions similar to high mg diorites or sanukitoids with high al2o3 14 46 17 61 wt mgo 3 0 6 4 wt cr 100 304 ppm and ni 57 109 ppm concentrations and fractionated ree patterns with weak negative or no eu anomalies eu eu 0 8 1 0 all the ttg and dioritic gneiss samples analysed during this study have negative nb ta and ti anomalies compared to neighbouring elements in spider diagrams with the majority having high positive nd t and hf t values close to values of the depleted mantle these geochemical characteristics strongly suggest that protoliths of the huai an ttg and dioritic gneisses originated in a juvenile crust forming event at 2 55 2 50 ga through partial melting of subducted oceanic crustal material these magmas also assimilated mantle peridotite material during their ascent this indicates that 2 5 ga mantle derived magmatism led to significant crustal growth in the north china craton and was probably associated with subduction

geochemical variations of the late mesozoic granitoids in the southern margin of north china craton a possible link to the tectonic transformation from compression to extension abstract the composition of the continental crust of the north china craton ncc is more felsic than that of the average bulk crust which is regarded to be the result of the delamination of the thickened lower crust during mesozoic however whether the thickened continental crust existed and when the delamination event happened along the southern margin of the ncc are still debated here we report geochronological geochemical and sr nd hf pb isotopic evidence that granitoids from the late jurassic wuzhangshan pluton and the early cretaceous huashani complex were derived by partial melting of the lower crust with different thickness our new data shows that the two lithofacies of the wuzhangshan pluton were mainly formed between ca 157 and 156 ma whereas the five lithofacies of the huashani complex were mainly emplaced between ca 132 and 125 ma the wuzhangshan pluton and the earlier four lithofacies granitoids of the huashani complex ca 160 125 ma both display adakitic geochemical features which are characterized by as high sio 2 63 26 72 71 wt al 2 o 3 13 97 16 89 wt and sr 413 1218 ppm contents and low y 6 30 14 98 ppm and yb n 1 55 4 43 and high sr y 33 112 and la yb n 11 53 29 72 ratios they also have high 87 sr 86 sr i 0 7066 0 7086 and low end t 9 9 to 18 8 and ehf t 11 to 26 values and two stage nd and hf model ages ranging from 2 4 to 1 7 ga and 2 7 to 1 7 ga respectively in contrast the late early cretaceous ca 125 110 ma granitoids have higher sio 2 71 30 76 78 wt and lower sr 64 333 ppm contents and lower sr y 17 29 and la yb n 13 25 18 36 ratios and similar end t 10 to 16 and relatively higher ehf t 10 to 14 values these geochemical variations suggest that the ca 160 125 ma granitoids were most probably produced by partial melting of thickened crust 45 km with eclogite garnet amphibolite or amphibolite residues whereas that the ca 125 110 ma granitoids were formed by partial melting of the thinner crust

neoarchean to paleoproterozoic continental growth in the southeastern margin of the north china craton geochemical zircon u pb and hf isotope evidence from the huoqiu complex abstract the huoqiu complex in the southeastern margin of the north china craton ncc is dominated by neoarchean gray gneisses amphibolites and voluminous metasediments here we report the occurrence of neoarchean to paleoproterozoic rocks from drill core samples the gneisses are similar to ttg tonalite trondhjemite granodiorite in composition and show close spatial association with amphibolites geochemical characteristics such as high sr y and la yb n with steep ree patterns and trace element modeling suggest that these rocks were generated by partial melting of hydrous meta basalts amphibolites at the base of a thickened mafic continental crust leaving a rutile bearing eclogite residue la mc icp ms u pb age data from magmatic zircon grains show protolith emplacement ages of 2 76 2 71 ga subsequently widespread migmatization took place at 1 91 1 82 ga generating voluminous migmatites and high k granites hf isotopic compositions of zircon grains from the amphibolite and gneiss show ehf t values of 2 4 15 5 and 3 0 1 5 respectively the tdm2 hf model ages of the gneisses range from 2 87 to 3 14 ga and are identical to the tdm1 hf ages of amphibolites 2 84 3 16 ga within analytical uncertainty suggesting that the gneisses formed by partial melting of amphibolite and attest to large scale reworking of the ancient continental crust during neoarchean the zircon grains from the granites define two groups with regard to their hf isotopic composition the older group 1916 42 ma has ehf t values and tdm2 hf ages of 10 5 2 4 and 2 40 3 20 ga respectively whereas the younger one 1823 41 ma shows a large variation in ehf t values ranging from 18 1 to 12 5 with tdm2 hf model ages of 1 70 3 59 ga a couple of zircon grains from the younger group display consistent u pb ages and tdm2 indicating accretion of juvenile crust from depleted mantle sources during 1 82 to 1 91 ga however the dominant hf isotope features are consistent with the reworking of preexisting continental crust we therefore infer that only limited accretion of juvenile crust occurred during this time and that the paleoproterozoic 1 82 to 1 91 ga tectonics in the southeastern margin of the ncc witnessed extensive reworking of older continental crust

internal differentiation of the archean continental crust fluid controlled partial melting of granulites and ttg amphibolite associations in central finland fault bound blocks of granulite and enderbite occur within upper amphibolite facies migmatitic tonalitic trondhjemitic granodioritic ttg gneisses of the iisalmi block of central finland these units record reworking and partial melting of different levels of the archean crust during a major tectonothermal event at 2 6 2 7 ga anhydrous mineral assemblages and tonalitic melts in the granulites formed as a result of hydrous phase breakdown melting reactions involving amphibole at peak metamorphic conditions of 8 11 kbar and 750 900 c a nominally fluid absent melting regime in the granulites is supported by the presence of carbonic fluid inclusions the geochemical signature of light rare earth element lree depleted mafic granulites can be modelled by 10 30 wt partial melting of an amphibolite source rock leaving a garnet bearing residue the degree of melting in intermediate granulites is inferred to be less than 10 wt and was restricted by the availability of quartz pressure temperature estimates for the ttg gneisses are significantly lower than for the granulites at 660 770 c and 5 6 kbar based on the p t conditions melting of the ttg gneisses is inferred to have occurred at the wet solidus in the presence of an h 2 o rich fluid a hydrous mineralogy abundant aqueous fluid inclusions and the absence of carbonic inclusions in the gneisses are in accordance with a water fluxed melting regime low ree contents and strong positive eu anomalies in most leucosomes irrespective of the host rock composition suggest that the leucosomes are not melt compositions but represent plagioclase quartz assemblages that crystallized early from felsic melts furthermore similar plagioclase compositions in leucosomes and adjacent mesosomes are not a migmatite paradox as both record equilibration with the same melt phase percolating along grain boundaries

trace element partitioning in the granulite facies analyses of trace elements in the mineral phases of granulites provide important information about the trace element distribution in the lower crust since granulites are often considered residues of partial melting processes trace element characteristics of their mineral phases may record mineral melt equilibria thus giving an opportunity to understand the nature and composition of melts in the lower continental crust this study provides an extensive set of mineral trace element data obtained by la icp ms analyses of mafic and intermediate granulites from central finland mass balance calculations using the analytical data indicate a pronounced contribution of the accessory minerals apatite for the ree and ilmenite for the hfse coherent mineral mineral ratios between samples point to a close approach to equilibrium except for minerals intergrown with garnet porphyroblasts mineral trace element data were used for the formulation of a set of d mineral melt partition coefficients that is applicable for trace element modelling under lower crustal conditions d mineral melt were derived by the application of predictive models and using observed constant mineral mineral ratios the comparison of the calculated d mineral melt with experimental data as well as the relationship between mineral trace element contents and a leucosome with a composition close to an equilibrium melt provides additional constraints on mineral melt partitioning the d values derived in this study are broadly similar to magmatic partition coefficients for intermediate melt compositions they provide a first coherent set of d values for sc v cr and ni between clinopyroxene amphibole garnet orthopyroxene ilmenite and melt in addition they emphasize the strong impact that ilmenite exerts on the distribution of nb and ta

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

a tectono genetic model for porphyry skarn stratabound cu au mo fe and magnetite apatite deposits along the middle lower yangtze river valley eastern china abstract the middle lower yangtze river valley metallogenic belt yrb situated along the northern margin of the yangtze craton is characterized by porphyry skarn stratabound cu au mo fe deposits in the areas of uplift and magnetite apatite deposits in cretaceous fault basins following detailed field investigations and a review of published data we recognize two episodes of magmatism and mineralization in the yrb 1 156 137 ma high k calc alkaline granitoids associated with 148 135 ma porphyry skarn stratabound cu au mo fe deposits and 2 135 123 ma shoshonitic series associated with 134 9 122 9 ma magnetite apatite deposits a type granitoids and associated alkaline volcanic have a small age range from 126 5 to 124 8 ma and are temporally spatially and genetically associated with the second episode the geodynamic history of the yrb did not experience the paleozoic to mesozoic lithospheric thickening that took place in the north china craton this process is inferred to be linked to partial melting of the delaminated lower crust at high pressures resulting in the development of c type adakitic rocks the petrochemical and sr nd isotopic data show that both the shoshonitic series and a type granitoids are quite different from adakites with only some of the k calc alkaline granitoids having adakitic signatures previous ore genesis models were established based on an assumed relationship with adakites and a continuous tectono thermal evolution from 150 to 100 ma all data obtained for the middle lower yangtze river region consistently show that the tan lu regional strike slip fault zone initiated at 233 6 to 225 6 ma from the collision between the north china and yangtze cratons and was reactivated at ca 160 ma the tan lu fault was caused by the oblique subduction of the izanagi plate which along the yrb the low angle subducted slab and the overlying crust was disrupted or broken due to the disharmonious movement of the two blocks the high k calc alkaline granitoids magmas were derived from melting of the subducted slab with some input of crustal material these magmas were emplaced at the intersections between ne and ew trending faults and formed porphyry skarn stratabound cu au mo fe deposits between 156 and 137 ma after 135 ma the subducted plate changed its direction of motion to northeast now running parallel to the eurasian continental margin and leading to large scale continental extension the shoshonitic series and subsequent a type granitoids magmatism and the development of magnetite apatite ores in the yrb took place in both fault basins and ne trending rifts between 135 and 124 ma

petrology and geochemistry of metamorphosed komatiites and basalts from the sula mountains greenstone belt sierra leone the sula mountains greenstone belt is the largest of the late archaean greenstone belts in the west african craton it comprises a thick 5 km lower volcanic formation and a thinner 2 km upper metasedimentary formation komatiites and basalts dominate the volcanic formation and komatiites form almost half of the succession all the volcanic rocks are metamorphosed to amphibolite grade and have been significantly chemically altered two stages of alteration are recognised and are tentatively ascribed to hydrothermal alteration and later regional amphibolite facies metamorphism ratios of immobile trace elements and ree patterns preserve for the most part original igneous signatures and these are used to identify five magma types these are low ti komatiites depleted in light ree low ti komatiites with flat ree patterns high ti komatiitic basalts with flat ree low ti basalts depleted in light ree high ti basalts with flat ree patterns much of the variation between the magma types can be explained in terms of different melt fractions of the mantle source although there were two separate mantle sources one light ree depleted the other not the interleaving of the basalts and komatiites produced by this melting indicates that the two mantle sources were melted simultaneously the simplest model with which to explain these complex melting processes is during melting within a rising mantle plume in which there were two different mantle compositions the very high proportion of komatiites in the sula mountains relative to other greenstone belts suggests either extensive deep melting and or the absence of a thick pre existing crust which would have acted as a filter to komatiite eruption

dehydration melting of a basaltic composition amphibolite at 1 5 and 2 0 gpa implications for the origin of adakites this study presents the results of dehydration melting experiments on a basaltic composition amphibolite under conditions appropriate to a hot slab geotherm 1 5 and 2 0 gpa and temperatures of 850 to 1150 c dehydration melting produces an omphacitic augite and garnet bearing residue coexisting with rhyolitic to andesitic composition melts at 1 5 gpa the amphibolite melts in two stages between 800 and 1025 c the 2 0 gpa data also define two melting stages at 2 0 gpa the first stage involves nearly modal melting of the original amphibolite minerals qtz pl amp to produce melt cpx grt during the second stage the eclogite restite melts non modally 0 86 cpx 0 14 grt 1 melt the experimental results were combined with data from the literature to generate a composite p t phase diagram for basaltic composition amphibolites over the 800 to 1100 c temperature range for pressures up to 2 0 gpa comparison of the major element compositions of the experimentally produced melts with compositions of presumed slab melts adakites shows that partial melting of amphibolite at conditions appropriate to a hot slab geotherm produces melts similar to andesitic and dacitic adakites except for significant mgo and cao depletions trace element modelling of amphibolite dehydration melting using the 2 0 gpa melting reactions produces ree abundances similar to those of adakites at 10 15 wt batch melting but the models do not reproduce the high sr y ratios characteristic of adakites taken together the major and trace element results are not consistent with the derivation of adakites by dehydration melting of the subducted slab with little or no interaction with the mantle wedge or crust if adakites are partial melts of the subducted slab they must undergo significant interaction with the mantle and or crust during which they acquire a number of their distinctive characteristics

magma mixing in the kalaqin core complex northern north china craton linking deep lithospheric destruction and shallow extension abstract the widespread mesozoic magmatism in the north china craton ncc has received considerable attention as a trigger for large scale lithospheric destruction here we investigate the early cretaceous jiguanzi adamellite from the northern part of the ncc which is contemporaneous with shallow extensional deformation and deep lithospheric destruction this intrusion emplaced at ca 133 ma is located in the foot wall of the kalaqin metamorphic core complex mcc and occurs as a synextensional ring complex with numerous magmatic equigranular group 1 and porphyritic group 2 enclaves hornblende and plagioclase from the host adamellite and xenocrysts of group 2 enclaves show distinct inverse zoning with mg and ca rich mantle the group 2 enclaves are characterized by plagioclase xenocrysts hosting hornblende biotite and apatite inclusions quartz ocelli with fine grained rim enriched in biotite and hornblende and poikilitic biotite surrounded by hornblende geochemically the host intrusion is calc alkaline to alkaline and metaluminous with variable contents of sio 2 60 70 72 20 wt al 2 o 3 14 19 17 22 wt na 2 o k 2 o 6 16 9 42 wt and mg values 28 0 47 7 whereas the group 2 enclaves exhibit low sio 2 54 05 55 55 wt high fe 2 o 3 8 18 8 64 wt and tio 2 2 08 2 28 wt and moderate mg 44 0 44 1 both the host intrusion and group 2 enclaves are enriched in large ion lithophile and light rare earth elements and depleted in high field strength elements and heavy rare earth elements except that the latter has lower ba and high nb ta and ti contents the major and trace element contents of the group 1 enclaves are broadly similar to those of the host intrusion analyses of sr nd hf isotopes in the host intrusion and in group 1 and group 2 enclaves show 87 sr 86 sr i host 0 70600 0 70618 e nd t host 8 2 to 9 6 t 2dm nd host 1592 1706 ma e hf t host 9 2 to 12 0 87 sr 86 sr i group 1 0 70590 0 70635 e nd t group 1 9 6 to 10 4 t 2dm nd group 1 1705 1775 ma 87 sr 86 sr i group 2 0 70586 0 70587 e nd t group 2 0 6 to 0 7 t 2dm nd group 2 971 978 ma and e hf t group 2 4 5 to 10 0 the mineralogical and geochemical features suggest that the host jiguanzi adamellite and group 2 enclaves were generated through synchronous mixing and fractional crystallization accompanied by mechanical and chemical exchanges the group 1 enclaves showing cogenetic affinity with their felsic host represent the mechanical concentrations of mafic minerals and accessory phases from evolved hybrid host magma magma sources of the host jiguanzi adamellite and its group 1 enclaves are dominated by lower crust lcc components whereas those for the group 2 enclaves are dominated by metasomatized lithospheric mantle the magma genesis involved complex multi stage crust mantle interaction processes including 1 lithospheric mantle modification induced by triassic subduction of the paleo asian ocean slab and or triassic cretaceous asthenospheric melt underplating and 2 melting of the ancient lcc and lithospheric mantle and hybridization of mantle and crust derived melts in conjunction with regional geological and geochemical data we argue that the late jurassic early cretaceous shallow extension events represented by the kalaqin mcc triggered deep seated multi stage magmatism and lithospheric destruction and the continuous generation of magma further strengthened the extension and result in more extensive lithospheric thinning

geochronological and geochemical constraints on the formation of chizhou cu mo polymetallic deposits middle and lower yangtze metallogenic belt eastern china abstract the cu mo polymetallic deposits are widespread in the chizhou region the middle and lower yangtze river metallogenic belt eastern china but the formation of cu mo polymetallic deposits remains unclear geochronological geochemical and sr nd hf isotopic compositions of the cu mo polymetallic deposits in the chizhou region are systemically investigated to reveal the formation of cu mo polymetallic deposits and petrogenesis of the granodiorite porphyry the geochronological dating results show the formation ages of cu mo polymetallic deposits molybdenite re os age 151 148 ma and the granodiorites porphyries zircon u pb age 150 146 ma in the chizhou region are simultaneous within error the chizhou granodiorites porphyries sio2 62 9 65 7 wt are high k calc alkaline peraluminous i type granodiorites they have high mg mainly 40 values and low na2o 0 2 wt and high log fo2 values 28 3 to 6 22 the distinctly chemical and isotopic compositions indicate that the chizhou granodiorites formed from metasomatized mantle derived magmas mixing with neoproterozoic crustal components the results indicate that the formation of the cu mo polymetallic deposits in the chizhou region may depend on the slab derived cl enriched fluids an oxidizing environment and a mantle crust mixed source

petrogenesis of early permian sanukitoids from west junggar northwest china implications for late paleozoic crustal growth in central asia abstract sanukitoids and their equivalents are rare subduction related rock types that have been found in modern arc settings and in late archean sequences the investigation of sanukitoids is of particular importance to a better understanding of crust forming processes and continental growth in this paper we report zircon u pb and ar ar ages and major element trace element and sr nd hf isotope data for the bieluagaxi dioritic pluton and dikes from the southern part of the west junggar nw china these rocks formed in the early permian and show a remarkable geochemical affinity with cenozoic sanukitoids of the setouchi volcanic belt of sw japan with high mg 48 73 values cr 54 539 ppm ni 21 197 ppm contents and th la 0 15 0 37 ratios and low sr y ratios 16 27 and sr 263 442 ppm contents they may be generated by the partial melting of subducting sediments and subsequent melt mantle interaction additionally the sanukitoids are also widespread in the karamay baogutu area of west junggar accompanied by high sr average 713 ppm contents and sr y 50 130 ratios and low y 6 9 12 6 ppm contents the difference in petrochemical characteristics between the baogutu karamay and bieluagaxi sanukitoids can be explained by the difference in depth of initial melting origin composition and fractional crystallization the baogutu karamay sanukitoids were probably formed under eclogitic conditions while the bieluagaxi sanukitoids were at a shallower depth moreover the compositional similarity between continental crust forming rocks and the bieluagaxi sanukitoids suggests that the sanukitoids genesis could be closely related to the process of continental crust formation the late carboniferous early permian sanukitoids in the west junggar may be an indicator of anomalous thermal activity ridge subduction may play a crucial role in the evolution and growth of the continental crust in central asia

zircon u pb ages geochemistry and nd hf isotopes of the ttg gneisses from the jiaobei terrane implications for neoarchean crustal evolution in the north china craton abstract the precambrian basement in the jiaobei terrane is largely composed of tonalite trondhjemite granodiorite ttg suite of rocks and offers important insights into the crustal evolution history of the north china craton ncc the la icp ms zircon u pb age data presented in this study show that the magmatic protoliths of the ttg gneisses formed during 2508 2547 ma and recorded the paleoproterozoic metamorphism 1905 ma the rocks are enriched in lile rb ba and sr and depleted in hfse nb ta zr and hf they are characterized by high sr contents 406 2906 ppm sr y ratios 31 3 355 and subchondritic nb ta ratios 18 5 68 9 the ttgs show relatively high ree contents 72 0 266 ppm with strongly enriched lree la yb n 11 5 121 and positive or negligible negative eu anomalies eu eu 0 84 1 89 these geochemical features suggest that the magma source might have been rutile bearing amphibole eclogite their high mg numbers 42 56 and high cr 153 285 ppm and ni contents 22 2 74 5 ppm indicate interaction with the mantle wedge during magma ascent the whole rock end t values 2 6 to 3 8 and most of the magmatic zircon ehf t values 1 3 to 7 6 suggest juvenile to evolved isotopic signatures all these lines of evidence suggest that the ttg rocks in this study formed through partial melting of subducted oceanic slab in a continental arc environment the drill holes in the jiaobei terrane are dominated by 2 5 ga ttg gneisses suggesting that the ttg magma at 2 5 ga is more widely distributed deep underground than that of 2 7 2 9 ga at least within the approachable depth range of our research some zircon grains from jiaobei ttgs give high ehf t values plotting above the curve of 0 75 ehf of dm and their tcdm ages are very close to the time of the zircon crystallization however the majority of the ehf t values fall below the curve of 0 75 ehf of dm and their tcdm ages are concentrated between 2 7 2 9 ga therefore combining with other related studies we propose that the 2 5 2 6 ga magmatism in the ncc probably represents a coherent event of crustal growth and major reworking remelting

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

adakites without slab melting high pressure differentiation of island arc magma mindanao the philippines abstract new geochemical data for pleistocene magmatic rocks from the surigao peninsula eastern mindanao the philippines demonstrate typical adakitic traits including elevation of sr y and depletion of the heavy rare earth elements 87sr 86sr and 143nd 144nd ratios of the adakites do not support melting of the subducted philippine sea plate but resemble pliocene arc lavas generated in the same subduction zone excepting the heavy rare earth elements the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge the wide range of sio2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature 1 adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet bearing assemblage 2 solidified basaltic rock containing garnet melted to yield adakitic magma in either case the basaltic precursor was generated from fluid modified mantle then differentiated within the garnet stability field in surigao this requires differentiation within mantle the surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth this has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks

petrogenesis of archean ttg series rocks from the jiaodong complex eastern china implications for crustal evolution in the north china craton abstract rocks of the tonalite trondhjemite granodiorite ttg series with associated supracrustal rocks are dominant components of many archean terranes but their petrogenesis remain controversial here we present whole rock major and trace elements zircon u pb ages sm nd and zircon hf isotopes of the three phases of ttg gneisses 2 9 2 7 and 2 5 ga and amphibolites in the jiaobei terrane china to investigate their petrogenesis and implications for crustal evolution the 2 5 ga ttgs are characterized by high sr sr y and la yb n values enrichment in lile and depletion in hfse positive or negligible eu anomalies and similarity to medium to high pressure mp hp ttgs in combination with their geochemical modeling results and high contents of mgo mg cr and ni it is considered that the 2 5 ga ttgs were subduction related and mainly from melting of rutile bearing eclogite and garnet amphibolite in contrast the 2 7 and 2 9 ga ttgs show similarity to low to medium pressure lp mp ttgs and have lower sr contents and negligible eu anomalies combining their y yb and ho yb n ratios the geochemical modeling results and their mgo mg cr and ni contents it can be inferred that they were mostly produced by melting of garnet amphibolite and amphibolite at the base of thickened lower crust the available geochronological and zircon hf isotopic data suggest that the major crustal growth period was around 2 7 ga with the subordinate growth peroid at 2 9 3 0 ga synthesizing all lines of evidence we propose that the jiaodong complex possibly witnessed a transition of tectonic regime during the meso neoarchean

petrogenesis and geological implications of the oligocene chongmuda mingze adakite like intrusions and their mafic enclaves southern tibet abstractthe oligocene to the miocene was a critical period for the growth of the tibetan plateau this growth is commonly considered to have been controlled by deep geodynamic processes the ultrapotassic and adakite like igneous rocks that developed during this period offer constraints on these deep seated processes in southern tibet whole rock geochemistry u pb zircon geochronology and in situ zircon hf isotopes have been determined for the mafic enclaves and host adakite like granitoids in the oligocene chongmuda mingze intrusive complex of southern tibet the host rocks including granodiorite and monzogranite are mainly high k and calc alkaline in composition their whole rock geochemistry low mgo ni and cr contents negative end t values 2 5 to 3 4 and low 87sr 86sr i values 0 7061 0 7066 and in situ zircon ehf t values 0 6 6 1 indicate that they were derived by partial melting of a juvenile lower crust implying that the southern tibetan crust was already thickened to up to 50 k

multiple sources for archean granitoids in the yalgoo area yilgarn craton western australia geochemical and isotopic evidence abstract in the yalgoo area of the north central yilgarn craton three small granitoid bodies named the lady lydia llg yalgoo east yeg and yalgoo north granitoids yng intruded 2 8 ga mafic supracrustal rocks they form two suites i granodiorites from both the llg and yeg with strongly fractionated ree patterns high lree contents low hree contents weakly positive eu anomalies and of archean ttg affinity and ii biotite monzogranites from the yng with very high lree contents and prominent negative eu anomalies all the granitoids are calc alkaline with the granodiorites recording ages of 2763 2752 ma and the biotite monzogranites recording ages of 2636 2634 ma the ree and hfse characteristics of the granodiorites favor an archean subduction environment abundant xenocrystic zircons 3 00 2 94 ga in the granodiorites and their lu hf isotopic characteristics imply a long history of crustal recycling in the north central yilgarn the lu hf isotopes indicate that the 2 76 ga granodiorites and xenocrysts were formed from a composite source ranging in age from 3 5 to 3 0 ga increased crustal thickening due to convergence after cessation of subduction caused melting in the upper mid crustal levels with generation of parental melt at 2 64 ga to source the biotite monzogranites the latter are characterized by strongly ve ehf t 10 to 12 and largely eoarchean tdm2 hf model ages 3 85 3 6 ga significantly they provide evidence of eoarchean crustal material in this part of the yilgarn craton

partial melting of metabasic rocks and the generation of tonalitic trondhjemitic granodioritic ttg crust in the archaean constraints from phase equilibrium modelling abstract rocks of tonalitic trondhjemitic granodioritic ttg composition preserved in archaean terranes represent fragments of the earth s earliest formed continental crust and are thought to have formed via partial melting of hydrated metabasalt the geodynamic environments in which such high grade metamorphism and anatexis may have occurred in the early earth is strongly debated constraining the pressure p and temperature t conditions at which melts of appropriate composition can be derived from protoliths containing plausible mineral assemblages is central to addressing this question phase equilibrium modelling has been undertaken for an enriched archaean tholeiite bulk composition a suggested protolith for early earth ttg magmas using newly parameterised thermodynamic models that were specifically developed to evaluate the anatectic behaviour of metabasalt assuming minimal h 2 o saturation at the wet solidus the potential fertility of the studied metabasalt is greatest if the solidus is crossed at a pressure of 11 kbar where the solidus temperature reaches a minimum of 610 c major element compositions and proportions of calculated partial melts show the best correlation with those of natural archaean ttgs when in the p t range 800 950 c and 10 18 kbar which we suggest are optimal conditions for their petrogenesis normative geochemistry suggests that these melts would crystallise to broadly trondhjemitic or tonalitic lithologies calculated modal proportions of garnet plagioclase amphibole and rutile in the residuum which control diagnostic trace element signatures in the melt also show the best agreement with natural and experimental data at these conditions importantly although partial melts calculated outside of this p t range still produce ttg like major element compositions and normative mineralogies they would poorly match the diagnostic trace element signatures of natural archaean examples owing to the absences of key minerals in the residuum this optimal p t range of 800 950 c and 10 18 kbar defined herein is most likely to characterise metamorphism of hydrated basalt at the base of a 40 km thick archaean oceanic plateau overthickened crust or else tectonic underplating via shallow subduction which we therefore suggest are the most likely tectonic settings for the formation of the first voluminous continental crust on the early earth