petrology and geochronology of paleoproterozoic garnet bearing amphibolites from the dunhuang block eastern tarim craton abstract the dunhuang block located in the eastern margin of the tarim craton possesses a wealth of information about the geological evolution of the tarim craton in this study we conducted detailed petrological geochemical and geochronological investigations on garnet bearing amphibolites as well as granites in the dunhuang block which record important metamorphic and magmatic events that are poorly constrained based on field occurrence geochronological and geochemical characteristics the garnet bearing amphibolites can be divided into two groups group i garnet bearing amphibolite displays typical features of island arc basalts which may formed at 1 83 ga in a collisional setting during the assembly of the columbia supercontinent whereas group ii has geochemical characteristics of ocean island basalt which may formed at 1 61 ga in an ocean island setting the ca 1 83 ga amphibolite has positive hf t values 4 4 to 8 9 with tdm1 model ages of 2 02 1 83 ga indicating an important addition of juvenile mantle derived materials during the formation of its protolith the ca 1 61 ga amphibolite has hf t values ranging from 0 6 to 5 6 and corresponding tdm1 model ages varying from 2 03 ga to 1 84 ga which suggest that juvenile mantle derived materials were dominantly contributed to the formation of its protolith but were later contaminated by crustal materials the u pb ages obtained from the rims of the zircon grains in amphibolites and granites recorded a tectonothermal event at ca 400 ma which is coeval with the widespread magmatism in the beishan orogen we thus propose that these two groups of garnet bearing amphibolites were possibly not related to each other during paleoproterozoic time and a late paleozoic orogenic event brought them together during which both of them underwent metamorphism

geochronology and geochemistry of late ordovician early devonian gneissic granites in the kumishi area northern margin of the south tianshan belt constraints on subduction process of the south tianshan ocean abstract the south tianshan belt has recorded the evolution of the south branches of the paleo asian ocean and is pivotal for understanding the formation of the southernmost part of the central asian orogenic belt in order to place constraints on the tectonic evolution of the south tianshan ocean three highly deformed granitic plutons monzonitic granites biotite k feldspar granites and two mica granites exposed in the kumishi area were collected for geochronological and geochemical studies zircon la icp ms u pb dating of the gneissic granites from the three plutons yielded crystallization ages at 446 2 2 2 ma 431 0 2 8 ma and 407 3 3 1 ma all of these gneissic granites have geochemical characteristics of calc alkaline igneous rocks the biotite monzonitic granites 446 ma are slightly peraluminous lree enriched and show relatively flat hree patterns mostly with negative eu anomalies the rocks have positive pb and negative nb ta ti anomalies in combination with their negative ehf t values 7 9 to 11 6 and the corresponding old tdmc ages 1 93 2 16 ga we suggest that the monzonitic granites were mainly derived from partial melting of the middle lower crust dominated by precambrian basement rocks the biotite k feldspar granites 431 ma are characterized by high k2o low fe2o3 and mgo contents they show ree patterns similar to the monzonitic granites as well as the nb ta ti depletions but have more enrichment in u pb zr and hf their depleted ehf t values 0 7 to 5 4 and young tdmc ages 1 07 1 37 ga imply significant input of mantle derived juvenile materials and thus suggest that the k feldspar granites were possibly derived from a k enriched crustal source mixed significantly with mantle derived juvenile materials the two mica granites 407 ma show a typical s type granite affinity with high al2o3 tio2 low cao na2o and k2o al2o3 ratios suggesting a pelitic source these characteristics together with their positive ehf t values 0 2 to 3 1 and relatively young tdmc ages 1 20 1 41 ga indicate that the two mica granites were most likely generated by partial melting of a pelitic source with less input of juvenile materials all of the samples from the three plutons show same geochemical characteristics of felsic magmatic rocks formed in an arc related tectonic setting together with the northward thrusting tectonics and the arc magmatic rocks exposed along the south nalati kawablak fault a northward subduction of the south tianshan ocean is suggested to interpret the formation of the three granitic plutons taking into account the distinct tectonic polarity and distribution of magmatic rocks in the western and eastern south tianshan belt we suggest that the south tianshan ocean probably underwent different subduction processes in its western and eastern parts a single northward subduction and a divergent bidirectional subduction are further proposed to interpret the formation of the western and eastern south tianshan belt respectively

the growth reworking and metamorphism of early precambrian crust in the jiaobei terrane the north china craton constraints from u th pb and lu hf isotopic systematics and ree concentrations of zircon from archean granitoid gneisses abstract archean granitoid tonalite trondhjemite granodiorite ttg gneisses are widely distributed in the early precambrian metamorphic basement of the jiaobei terrane the north china craton ncc and record the formation and evolution of the continental crust in the early precambrian in this study zircon cathodoluminescence cl images have assisted in obtaining u th pb and lu hf isotopic systematics and rare earth element ree concentrations from distinct zircon domains of complex zircons and the results have been integrated in order to unravel the growth reworking and metamorphism of the early precambrian crust in the jiaobei terrane zircons from seven representative archean granitoid samples collected from the jiaobei terrane record three groups of magmatic ages of 2 9 2 7 and 2 5 ga and two groups of metamorphic ages of 2 5 and 1 86 ga the 2 5 ga metamorphic event is thought to be linked to underplating of large amounts of mantle derived magma and the 1 86 ga metamorphic event to arc or continent continent collision magmatic zircons have positive hf t values 0 7 to 12 58 average 5 5 and two stage hf model ages cluster at 2 7 2 8 and 2 9 3 0 ga this indicates that the archean granitoid rocks were derived from partial melting of juvenile crust that was formed mainly during 2 7 2 8 and 2 9 3 0 ga which represent two periods of growth of juvenile crust the most significant period of crustal growth is 2 7 2 8 ga whereas 2 5 ga represents a period of reworking of the 2 7 2 8 ga juvenile crust in the jiaobei terrane as well as other areas in the ncc

intrinsic and scattering seismic wave attenuation in the central apennines italy abstract recently an earthquake sequence formed by a series of subsequences characterized by large earthquakes three events m 6 0 and five events m 5 0 occurred in the central apennines in 6 7 months during 2016 2017 several studies interpreted this cascading type of earthquake occurrence as associated with fluid movements through an interconnected network of cracks around the active faults that triggered the subsequences in different time and location in the present study we aim to verify if the presence of fluids is confirmed by the pattern of seismic wave attenuation to do so we separately estimate the space averaged parameters intrinsic qi 1 and scattering qs 1 proportional to the correspondent attenuation parameters by applying a modified multiple lapse time window analysis mltwa in order to establish reference background values for the study region mltwa measures the decay of s wave energy with distance and lapse time and fit it to the energy transport equation solution for scattering yielding the seismic albedo b0 qs 1 qt 1 and the extinction length le qt 1 parameters as a function of frequency the knowledge of the separated attenuation parameters allows determining which process of seismic energy dissipation dominates in the region in the present study b0 results to be around0 6 at 1 5 hz and decreases with increasing frequency down to 0 2 indicating that the attenuation of the seismic energy carried by s wave is controlled by intrinsic dissipation rather than by scattering moreover the intrinsic attenuation is frequency dependent f 0 9 for the uniform model and f 0 4 for the crust moho model this behavior well matches with the presence of fluids in the faults and fracture zones in the central apennines in agreement with results from other independent studies in the region finally a comparison among the attenuation parameters estimated for this area and those measured in several areas of italy and other parts of the world is presented together with a discussion on the comparison between the present mltwa attenuation estimates and homologous results obtained for the same area with different approaches

destruction of ancient lower crust through magma underplating beneath jiaodong peninsula north china craton u pb and hf isotopic evidence from granulite xenoliths abstract zircons from granulite xenoliths entrained in a late cretaceous mafic dike in the jiaodong peninsula north china craton ncc show three distinct u pb age populations part of the old zircon grains yield discordant data that project to ages of about 2 4 to 2 5 ga a few grains indicate growth at about 2 0 ga and a third group yield cretaceous ages with peaks at 120 and 90 ma the oldest zircons give hf t dm model ages of 2 6 2 8 ga these results demonstrate the existence of original archean lower crust in the jiaodong region zircons of 2 0 ga have similar hf t dm model ages as the neoarchean paleoproterozoic grains suggesting that these zircons were products of metamorphic recrystallization due to thermal event without juvenile input early cretaceous zircons yield ehf t values of 21 to 12 and late cretaceous zircons large variable ehf t from 4 to 50 these data suggest that magmatic underplating occurred in the neoarchean to earliest proterozoic lower crust of the ncc both in the early and late cretaceous it is suggested that the mesozoic magma underplating which also provided the heat source for the voluminous mesozoic magmatism in the ncc significantly modified the composition of the archean to paleoproterozoic lower crust of the ncc

a tale of amalgamation of three permo triassic collage systems in central asia oroclines sutures and terminal accretion the central asian orogenic belt records the accretion and convergence of three collage systems that were finally rotated into two major oroclines the mongolia collage system was a long n s oriented composite ribbon that was rotated to its current orientation when the mongol okhotsk orocline was formed the components of the kazakhstan collage system were welded together into a long single composite arc that was bent to form the kazakhstan orocline the cratons of tarim and north china were united and sutured by the beishan orogen which terminated with formation of the solonker suture in northern china all components of the three collage systems were generated by the neoproterozoic and were amalgamated in the permo triassic the central asian orogenic belt evolved by multiple convergence and accretion of many orogenic components during multiple phases of amalgamation followed by two phases of orocline rotation

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

reassessment of continental growth during the accretionary history of the central asian orogenic belt we argue that the production of mantle derived or juvenile continental crust during the accretionary history of the central asian orogenic belt caob has been grossly overestimated this is because previous assessments only considered the palaeozoic evolution of the belt whereas its accretionary history already began in the latest mesoproterozoic furthermore much of the juvenile growth in central asia occurred in late permian and mesozoic times after completion of caob evolution and perhaps related to major plume activity we demonstrate from zircon ages and nd hf isotopic systematics from selected terranes within the caob that many neoproterozoic to palaeozoic granitoids in the accreted terranes of the belt are derived from melting of heterogeneous precambrian crust or through mixing of old continental crust with juvenile or short lived material most likely in continental arc settings at the same time juvenile growth in the caob occurred during the latest neoproterozoic to palaeozoic in oceanic island arc settings and during accretion of oceanic island arc and precambrian terranes however taking together our data do not support unusually high crust production rates during evolution of the caob significant variations in zircon ehf values at a given magmatic age suggest that granitoid magmas were assembled from small batches of melt that seem to mirror the isotopic characteristics of compositionally and chronologically heterogeneous crustal sources we reiterate that the chemical characteristics of crustally derived granitoids are inherited from their source s and cannot be used to reconstruct tectonic settings and thus many tectonic models solely based on chemical data may need re evaluation crustal evolution in the caob involved both juvenile material and abundant reworking of older crust with varying proportions throughout its accretionary history and we see many similarities with the evolution of the sw pacific and the tasmanides of eastern australia

end permian to mid triassic termination of the accretionary processes of the southern altaids implications for the geodynamic evolution phanerozoic continental growth and metallogeny of central asia the altaids is one of the largest accretionary orogenic collages in the world with the highest rate of phanerozoic continental growth and significant metallogenic importance it is widely accepted that subduction related orogenesis of the altaids started in the late precambrian and gradually migrated southward present coordinates however it is uncertain when and how the building of the altaids was finally completed based on structural geology geochemical geochronological and paleomagnetic data this paper presents late paleozoic to early mesozoic accretionary tectonics of two key areas north xinjiang in the west and inner mongolia in the east together with neighboring mongolia the late paleozoic tectonics of north xinjiang and adjacent areas were characterized by continuous southward accretion along the wide southern active margin of siberia and its final amalgamation with the passive margin of tarim which may have lasted to the end permian to early mid triassic in contrast in inner mongolia and adjacent areas two wide accretionary wedges developed along the southern active margin of siberia and the northern active margin of the north china craton which may have lasted to the mid triassic the final products of the long lived accretionary processes in the southern altaids include late paleozoic to permian arcs late paleozoic to mid triassic accretionary wedges composed of radiolarian cherts pillow lavas and ophiolitic fragments and high pressure ultrahigh pressure metamorphic rocks permian alaskan type zoned mafic ultramafic complexes intruded along some major faults of the tien shan we define a new tarim suture zone immediately north of the tarim craton that is probably now buried below the tien shan as a result of northward subduction of the tarim block in the cenozoic the docking of the tarim and north china cratons against the southern active margin of siberia in the end permian to mid triassic resulted in the final closure of the paleoasian ocean and terminated the accretionary orogenesis of the southern altaids in this part of central asia this complex geodynamic evolution led to formation of giant metal deposits in central asia and to substantial continental growth

a review of the western part of the altaids a key to understanding the architecture of accretionary orogens abstract the altaids is one of the largest accretionary collages in the world and the tectonic styles of the accretionary processes have been interpreted in several ways including as an amalgamation of multiple terranes as a result of oroclinal bending of a long single arc or as a caledonian continental collision based on recent tectonostratigraphic analyses together with paleomagnetic data the tectonic styles of the neoproterozoic to paleozoic accretionary processes of the altaids are discussed the western altaids is the main focus of the study which was mainly composed of several independent linear components such as arcs and microcontinents with proterozoic basement and cover rocks various kinds of arcs existed in the paleo asian ocean including a complicated type of arc alaskan type which is a combination of the japan and mariana type intra oceanic arcs and the cordillera type continental arcs these linear components rotated and collided with each other with multiple subduction polarities which could have been an important result of multiple linear element amalgamation and which has contributed greatly to the architecture of the eurasian continent the basic tectonic styles of the altaids can be summarized as arc arc collision oroclinal bending and large scale rotation and multiple subductions with a complicated archipelago paleogeography these basic features of accretionary orogens in general can be attributed to the amalgamation of complicated multiple linear elements some mesozoic to cenozoic accretionary orogens in the world are also characterized by processes of multiple linear element amalgamation more attention should be paid to the multiple linear element amalgamation of ancient accretionary orogens which will shed light on lateral and vertical continental growth

cenozoic k rich adakitic volcanic rocks in the hohxil area northern tibet lower crustal melting in an intracontinental setting it is generally accepted that the cenozoic potassic volcanic rocks of northern tibet were derived from a lithospheric mantle source here we report new chronological geochemical and isotopic data for the miocene ca 18 15 ma k rich adakitic volcanic rocks from the hohxil area of the songpan ganzi block in northern tibet we contend that these rocks were generated by partial melting of the mafic lower crust in an intracontinental setting unrelated to subduction of oceanic crust the hohxil rocks exhibit high sr y and la yb ratios high sr and la contents but low yb and y concentrations similar to adakites formed by slab melting associated with subduction however their relatively low e n d values 2 09 to 4 58 high 8 7 sr 8 6 sr 0 7072 0 7075 th u th ba and rb ba ratios and distinctive potassium enrichments k 2 o na 2 o are very different from the composition of typical adakites in addition those k rich adakitic rocks with the highest sio 2 contents 61 wt exhibit the lowest 8 7 sr 8 6 sr ratios and highest e n d values and are the oldest cenozoic volcanic rocks exposed in the songpan ganzi block suggesting that they were derived neither directly from a mantle source nor by differentiation of a shoshonitic magma taking into account the composition of lower crustal mafic xenoliths in tibet as well as the tectonic and geophysical evidence we conclude that the hohxil adakitic magmas were produced by partial melting of amphibole bearing eclogites with a k rich mafic bulk composition in the lower part 55 km of the thickened northern tibetan crust partial melting of the lower crust may have been triggered by dehydration release of fluids from sedimentary materials in the southward subducted continental lithosphere

isotope provinces mechanisms of generation and sources of the continental crust in the central asian mobile belt geological and isotopic evidence abstract the available geological geochronological and isotopic data on the felsic magmatic and related rocks from south siberia transbaikalia and mongolia are summarized to improve our understanding of the mechanisms and processes of the phanerozoic crustal growth in the central asian mobile belt camb the following isotope provinces have been recognised precambrian t dm 3 3 2 9 and 2 5 0 9 ga at the microcontinental blocks caledonian t dm 1 1 0 55 ga hercynian t dm 0 8 0 5 ma and indosinian t dm 0 3 ga that coincide with coeval tectonic zones and formed at 570 475 420 320 and 310 220 ma continental crust of the microcontinents is underlain by or intermixed with juvenile crust as evidenced by its isotopic heterogeneity the continental crust of the caledonian hercynian and indosinian provinces is isotopically homogeneous and was produced from respective juvenile sources with addition of old crustal material in the island arcs or active continental margin environments the crustal growth in the camb had episodic character and important crust forming events took place in the phanerozoic formation of the camb was connected with break up of the rodinia supercontinent in consequence of creation of the south pacific hot superplume intraplate magmatism preceding and accompanying permanently other magmatic activity in the camb was caused by influence of the long term south pacific plume or the asian plume damping since the devonian

timing of destruction of the north china craton abstract the north china craton ncc is the typical example of destruction of an ancient craton however the destruction mechanism and geodynamic controlling factors still remain enigmatic due to controversy on the timing of destruction which is the key to understanding the destruction processes based on temporal and spatial distributions of the igneous rocks and their sources and tectonic settings it is recognized that six stages of tectono magmatism occurred in the ncc during the phanerozoic i e carboniferous to early permian late permian to middle triassic late triassic jurassic early cretaceous and cenozoic among them cenozoic magmatism mainly consists of alkali basalts and is found only occasionally in the eastern ncc the first four stages of magmatism and tectonism related to the southward subduction of the paleo asian plate and the assembly of sino korean and yangtze cratons are locally distributed in limited parts of the ncc reflecting a multiple stage modification of the ncc from the late carboniferous to jurassic however the intensive development of early cretaceous magmatism extensional deformation and associated gold mineralization with significant continental crustal growth indicate that the eastern ncc was destroyed during this time period this destruction was the result of paleo pacific subduction beneath the eastern asian continent with lithospheric removal and or replacement of an ancient cratonic lithosphere by a juvenile oceanic lithosphere

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

paleozoic multiple accretionary and collisional processes of the beishan orogenic collage in the paper by xiao and others american journal of science v 310 p 1553 1594 1 please substitute figure 16 reproduced here for figure 16 that appears between pages 1573 and 1574 the original figure 16 2 was printed at an improper size figure 3 1 lookup volpage 310

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

the latest neoarchean paleoproterozoic evolution of the dunhuang block eastern tarim craton northwestern china evidence from zircon u pb dating and hf isotopic analyses abstract the dunhuang block in the easternmost segment of the tarim craton exposes precambrian mafic granulite and felsic gneiss to better understand the poorly known formation and evolution of the tarim craton we carried out detailed field based petrological and geochronological investigations on the major lithologies of the dunhuang block u pb dating and hf isotopic analyses on the mafic granulite and felsic gneiss ttg gneiss of the dunhuang block reveal two distinct age populations the latest neoarchaean 2 5 ga and the late palaeoproterozoic 1 82 1 85 ga the zircon magmatic cores and metamorphic rims of tonalitic gneisses yield similar ages of 2 5 ga supporting the existence of archean rocks in the dunhuang block the short time interval between latest neoarchean magmatism and high grade metamorphism suggests that they were related to the same latest neoarchean tectonothermal event the 2 5 ga zircons show hf depleted mantle model ages between 2 55 ga and 2 8 ga with a peak at 2 7 ga suggesting that 2 7 ga juvenile crust of the dunhuang block was reworked during a 200 300 m y period at the end of the neoarchean the age of 1 85 ga obtained from the mafic granulites and felsic gneisses is interpreted as the age of high pressure granulite facies metamorphism combined with the hf isotopic data these data suggest that the dunhuang block experienced 2 7 ga crustal growth a 2 5 ga magmatic metamorphic event and a 1 82 1 85 ga hp granulite facies metamorphic event this sequence of events is very similar to that of the north china craton implying that the evolutionary history of the dunhuang block was consistent with that of the north china craton from the neoarchean to paleoproterozoic

tectonic evolution of the south tianshan orogen and adjacent regions nw china geochemical and age constraints of granitoid rocks geochemical and geochronological evidence was obtained from granitoids of the south tianshan orogen and adjacent regions which consist of three individual tectonic domains the kazakhstan yili plate the central tianshan terrane and the tarim plate from north to south the central tianshan terrane is structurally bounded by the early paleozoic nikolaev line north nalati fault and late paleozoic atbashy inyl chek south nalati qawabulak fault zones against the kazakhstan yili and tarim plates respectively the meta aluminous to weakly peraluminous granitic rocks which are exposed along the kekesu river and the bikai river across the central tianshan terrane have a tholeiitic calc alkaline or high potassium calc alkaline composition i type geochemical trace element characteristics and the y versus rb nb or y versus nb discrimination diagrams favor a continental arc setting for these granitoid rocks shrimp u pb and la icp ms u pb zircon age data indicate that the magmatism started at about 480 ma continued from 460 to 330 ma and ended at about 275 ma the earlier magmatism 470 ma is considered to be the result of a simultaneous southward and northward subduction of the terskey ocean beneath the northern margin of the tarim plate and the kazakhstan yili plate respectively the later magmatism 460 330 ma is related to the northward subduction of the south tianshan ocean beneath the southern margin of the kazakhstan yili central tianshan plate the dataset presented here in conjunction with previously published data support a late paleozoic tectonic evolution of the south tianshan orogen not a triassic one as recently suggested by shrimp u pb zircon dating for eclogites

paleozoic intrusive rocks from the dunhuang tectonic belt nw china constraints on the tectonic evolution of the southernmost central asian orogenic belt abstract the dunhuang tectonic belt dtb is of great importance for understanding the tectonic evolution of the southernmost central asian orogenic belt caob in this study the temporal spatial distribution petrogenesis and tectonic setting of the paleozoic representative intrusive rocks from the dtb were systematically investigated to discuss crustal evolution history and tectonic regime of the dtb during paleozoic our results reveal that the paleozoic magmatism within the dtb can be broadly divided into two distinct episodes of early paleozoic and late paleozoic the early paleozoic intrusive rocks represented by a suite metaluminous slight peraluminous and medium to high k calc alkaline i type granitoids crystallized at silurian ca 430 410 ma are predominantly distributed along the northern part of the dtb they were probably produced with mineral assemblage of eclogite or garnet amphibole rutile in the residue and were derived from magma mixing source of depleted mantle materials with various proportions of archean mesoproterozoic continental crust the late paleozoic intrusive rocks can be further subdivided into two stages of late devonian stage ca 370 360 ma and middle carboniferous stage ca 335 315 ma the former stage is predominated by metaluminous to slight peraluminous and low k tholeiite to high k calc alkaline i type granitic rocks distributed in the central part of the dtb they were also generated with mineral assemblage of amphibolite to eclogite facies in the residue and originated from magma source of depleted mantle materials mixed with different degrees of old continental crust the later stage is represented by adakite and alkali rich granite exposed in the southern part of the dtb the alkali rich granites studied in this paper were possibly produced with mineral assemblage of granulite facies in the residue and were generated by partial melting of thickened lower continental crust zircon hf isotopes and field distribution of those paleozoic intrusive rocks reveal that both the silurian and the late devonian magmatic activities predominantly represent crustal growth processes in the dtb accompanied by different degrees of reworking of pre existing continental crust however the middle carboniferous ca 335 315 ma magmatic activity reflects a crustal reworking process the silurian and late devonian intrusive rocks were most likely formed in the arc related subduction zones whereas the middle carboniferous intrusive rocks were possibly formed in a transitional tectonic setting from compression to extension representing the final stage of paleozoic orogeny in the dtb these paleozoic magmatic rocks further suggest that the dtb has reactivated from a stable block to an orogen and undergone two episodes the early paleozoic and the late paleozoic of orogeny during paleozoic it represents a paleozoic accretionary orogen of the southernmost margin of the caob between the tarim craton and north china craton and tectonically extends northward to the beishan orogen and westward to the eastern south tianshan belt

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

formation of hp lt rocks and their tectonic implications in the western tianshan orogen nw china geochemical and age constraints the western tianshan high pressure low temperature metamorphic belt in northwestern china formed within a paleozoic accretionary wedge on the south side of the yili central tianshan plate it comprises various lithologies including mafic metavolcanic rocks metavolcaniclastic rocks metagreywacke marble and serpentinite the metavolcanics comprise eclogite omphacitite epidosite and blueschist the trace and rare earth element characteristics of these rocks indicate morb to oib affinities suggesting a seamount like setting for the protoliths of these rocks the end value of the eclogite 8 9 indicates that the protolith was derived from a long term depleted mantle i e an n morb source supporting the trace and rare earth element characteristics of these samples sm nd isochron dating of 343 44 ma omp gln grt wr and an age of 346 3 ma grt gln were determined using an eclogite boudin within blueschist a well defined 40ar 39ar plateau age of 344 1 ma was obtained for crossite from an omphacite phengite bearing blueschist and is concordant with the sm nd isochron age for the eclogite suggesting that neither argon loss nor excess argon affected the crossite a well defined 40ar 39ar plateau age of 331 2 ma was determined for phengite from the same sample the high pressure metamorphic rocks were formed by the b type subduction of a paleozoic south tianshan ocean about 344 ma ago they were however exhumed to higher crustal levels during the collision of the tarim and yili central tianshan plates about 331 ma ago similar to an alpine type tectonic regime

neoarchean to paleoproterozoic continental evolution and tectonic history of the north china craton a review rocks of archean age are widely distributed in the north china craton ncc and can be divided into high grade regions and granite greenstone belts even though most underwent amphibolite to granulite facies metamorphism quartzo feldspathic gneisses occupy 70 to 85 of the total exposure of archean rocks in the craton supracrustal rocks commonly consist of intercalated volcano sedimentary rocks including metabasites meta intermediate to acid rocks banded iron formations metapelites and minor marbles the oldest rocks outcrop over a small area of 4 km2 and are a quartzite amphibolite sequence that yield 3 8 3 5 ga isotopic ages chemical and geochronological data constrain the earliest main crust forming episode at ca 2 9 2 7 ga when most rocks were metamorphosed to upper amphibolite or granulite facies at moderate pressures mafic granulites can be classified into two textural types referred to as near isobaric cooling ibc and near isothermal decompression itd types high pressure granulites and retrograded eclogites occur mainly near the junction of the provinces of hebei shanxi and inner mongolia in the central ncc and in eastern shandong province of the eastern ncc as well as locally in henan province in the southern ncc these high pressure rocks show clockwise p t paths indicating decompression from 14 kb to 5 6 kb an exposed section interpreted as representing precambrian continental lower crust has also been identified in the northern part of the craton although some mafic ultramafic metamorphic rocks have been interpreted as remnants of ancient oceanic crust and mantle by a number of geologists geochemical and petrological evidence demonstrates that many supracrustal orthogneiss associations have island arc affinities therefore several tectonic models have been proposed including continent continent collision and island arc terrain accretion achieved by arc arc or arc microcontinent collision the ncc underwent multiple high grade metamorphic events of which the two most important took place during the intervals 2600 2450 ma and 1950 1750 ma which are considered to be related to early precambrian supercontinental cycles

the western central asian orogenic belt a window to accretionary orogenesis and continental growth abstract the architecture of accretionary orogens is a key to understand continental growth here we present an overview of the orogenic components and their amalgamation in the western central asian orogenic belt caob the caob records the convergence and interactions among various types of orogenic components including the japan type mariana type and alaska aleutian type arc systems as well as the active marginal sequences of the siberia craton which incorporated wide accretionary complexes and accreted arcs and terranes during construction of the caob the kazakhstan arc chain was characterized by multiple subduction whereas the northern fringe of the tarim craton remained mostly as a passive margin the multiple convergence and accretions among these various orogenic components generated huge orogenic collages in the late paleozoic and even in the early triassic involving parallel amalgamation circum microcontinent amalgamation and oroclinal bending the preservation of trapped basins played a significant role in orogenesis with some parts of the oceanic plate being subducted and others behaving as rigid units the orogenesis in the caob was long lived lasting for more than 800 m y involving multiple subduction and long continuous accretion and featuring the complexity of accretionary orogenesis and continent growth

crust mantle interaction beneath the luxi block eastern north china craton evidence from coexisting mantle and crust derived enclaves in a quartz monzonite pluton abstract the laiwu quartz monzonite in the luxi block of eastern north china craton ncc is characterized by the presence of abundant plagioclase amphibolite and gabbro diorite enclaves here we present la icpms zircon u pb ages which show that the host quartz monzonite was emplaced at 129 8 1 0 ma whereas the protolith of the plagioclase amphibolite enclaves formed during early paleoproterozoic the gabbro diorite enclaves were produced simultaneously with or slightly earlier than the formation of the host quartz monzonite combined with the archean and paleoproterozoic zircons as well as the low e nd 0 values 18 4 to 18 0 in the plagioclase amphibolite enclaves the equilibrium temperature and pressure conditions 645 670 c and 4 8 6 5 kb suggest that the plagioclase amphibolite enclaves are fragments of the middle crust the gabbro diorite enclaves mainly originated from an enriched lithospheric mantle metasomatized by melts fluids derived from the continental crust as indicated by their low sio 2 54 4 54 7 wt and high mgo 10 9 11 1 wt contents as well as the negative e nd t values 13 5 to 10 7 and enrichment of liles e g ba and sr and depletion of hfses e g nb ta p and ti compared with the ancient crustal rocks and the mafic plutons considered to have been derived from lithospheric mantle in the luxi block the moderate e nd t 15 7 to 15 1 and e hf t 20 7 to 13 0 values of the quartz monzonite in our study suggest that both mantle and crust derived melts were involved in the magma generation thus we propose a model involving magma mixing between mantle and crust derived melts for the formation of the quartz monzonite since significant crust mantle interaction is recorded not only in the quartz monzonite and its enclaves in the luxi block but also in the other granitoids widespread in the ncc it is considered that large scale crust mantle interaction and magmatic underplating were associated with the mesozoic lithospheric mantle thinning and crustal reactivation in the eastern ncc

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

the collision between the yili and tarim blocks of the southwestern altaids geochemical and age constraints of a leucogranite dike crosscutting the hp lt metamorphic belt in the chinese tianshan orogen article i nfo a ca 600 m long 0 5 20 m wide nw se trending granite dike crosscuts the high pressure low temperature hp lt tianshan metamorphic belt the foliation of which is parallel to the main ene regional trend in the chinese south tianshan orogen it is mainly composed of plagioclase k feldspar quartz muscovite biotite and secondary chlorite while fluorite zircon and xenotime occur as accessories the immediate country rock is a quartz biotite plagioclase schist which grades several tens of meters away from the granite dike into a chlorite mica albite schist the latter schist is intimately intercalated with blueschist layers and boudins the a cnk value of the granite dike samples varies from 1 15 to 1 27 indicating a strongly peraluminous composition cao na2o ranges from 0 06 to 0 17 and al2o3 tio2 from 240 to 525 similar to the ratios of strongly peraluminous sp granites exposed in high pressure collision zones such as the himalayas a zircon u pb age of 285 ma was obtained for the granite dike thus constraining the upper limit for the age of hp lt metamorphism the petrological and geochemical data suggest that the sp leucogranite dike intruded during the exhumation of overthickened crust in the post collisional setting between the yili central tianshan and tarim blocks the dataset presented here in conjunction with previously published data corroborate that the hp lt metamorphism must have occurred earlier than the permian in the tianshan orogen therefore the collision between the yili central tianshan and tarim blocks and the final amalgamation of the southwestern altaids must have been terminated in late paleozoic and not in triassic times as previously suggested

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

the crustal accretion history and tectonic evolution of the ne china segment of the central asian orogenic belt abstract the basement rocks in parts of ne china constitute a khondalitic sequence of sillimanite and garnet bearing gneisses hornblende plagioclase gneiss and various felsic paragneisses zircon u pb dating of garnet sillimanite gneiss samples from the erguna xing an jiamusi and khanka blocks indicates that high grade metamorphism occurred at 500 ma evidence from detrital zircons in paleozoic sediments from the songliao block also indicates the former presence of a 500 ma component this uniformity of u pb ages across all crustal blocks in ne china establishes a 1300 km long late pan african khondalite belt which we have named the ne china khondalite belt this indicates the blocks of ne china were amalgamated prior to 500 ma contrary to current belief one scenario is that this amalgamated terrane had a tectonic affinity to the siberia craton once forming part of the late pan african 500 ma sayang baikal orogenic belt extensively developed around the southern margin of the siberia craton this belt was the result of collision between currently unidentified terranes with the southeastern angara anabar province at about 500 ma where the rocks were deformed and metamorphosed to granulite facies it appears likely that at sometime after 450 ma the combined ne china blocks rifted away from siberia and moved southward to form what is now ne china the combined block collided with the north china craton along the solonker xar moron changchun suture zone at 230 ma rather than in the end permian as previously thought local rifting at the eastern extremity of the developing central asian orogenic belt caob resulted in the splitting away of the jiamusi khanka bureya blocks however this was only transient and sometime between 210 and 180 ma these were re united with the caob by the onset of pacific plate subduction which has dominated the tectonic evolution of the region since that time

paleozoic multiple accretionary and collisional tectonics of the chinese tianshan orogenic collage abstract subduction related accretion in the junggar balkash and south tianshan oceans paleo asian ocean mainly in the paleozoic gave rise to the present 2400 km long tianshan orogenic collage that extends from the aral sea eastwards through uzbekistan tajikistan kyrgyzstan to xinjiang in china this paper provides an up to date along strike synthesis of this orogenic collage and a new tectonic model to explain its accretionary evolution the northern part of the orogenic collage developed by consumption of the junggar balkash ocean together with paleozoic island arcs northern ili issyk kul and chatkal located in the west which may have amalgamated into a composite arc in the paleozoic in the west and by addition of another two roughly parallel arcs dananhu and central tianshan in the east the western composite arc and the eastern dananhu and central tianshan arcs formed a late paleozoic archipelago with multiple subduction zones the southern part of the orogenic collage developed by the consumption of the south tianshan ocean which gave rise to a continuous accretionary complex kokshaal kumishi which separated the central tianshan in the east and other paleozoic arcs in the west from cratons tarim and karakum to the south cross border correlations of this accretionary complex indicate a general southward and oceanward accretion by northward subduction in the early paleozoic to permian as recorded by successive southward juxtaposition of ophiolites slices of ophiolitic melanges cherts island arcs olistostromes blueschists and turbidites which are mainly paleozoic in age with the youngest main phase being late carboniferous permian the initial docking of the southerly tarim and karakum cratons to this complicated late paleozoic archipelago and accretionary complexes occurred in the late carboniferous early permian in the eastern part of the tianshan and in the late permian in the western part which might have terminated collisional deformation on this suture zone the final stages of closure of the junggar balkash ocean resembled the small ocean basin scenario of the mediterranean sea in the cenozoic in summary the history of the altaids is characterized by complicated multiple accretionary and collisional tectonics

cratonic reactivation and orogeny an example from the northern margin of the north china craton abstract reactivation of cratonic basement involves a number of processes including extension compression and or lithospheric delamination the northern margin of the north china craton ncc adjacent to the inner mongolian orogenic belt was reactivated in the late paleozoic to early mesozoic during this period the northern margin of the ncc underwent magmatism n s compression regional exhumation and uplift including the formation of e w trending thick skinned and thin skinned south verging folds and south verging ductile shear zones zircon u pb shrimp ages for mylonite protoliths in shear zones which show ages of 310 290 ma mid carboniferous early permian constraining the earliest possible age of deformation muscovite within carbonate and quartz feldspar muscovite mylonites from the kangbao weichang and fengning longhua shear zones defines a stretching lineation and gives 40ar 39ar ages of 270 250 ma 250 230 ma 230 210 ma and 210 190 ma deformation developed progressively from north to south between the late paleozoic and triassic exhumation of lower crustal gneisses high pressure granulites and granites occurred at the cratonic margin during post ductile shearing 220 210 ma an undeformed early jurassic 190 180 ma conglomerate overlies the deformed rocks and provides an upper age limit for reactivation and orogenesis deformation was induced by convergence between the southern mongolia and north china cratonic blocks and the location of this convergent belt controlled later deformation in the yanshan tectonic province this province formed as older e w trending archean proterozoic sequences were reactivated along the northern margin of the ncc this reactivation has features typical of cratonic basement reactivation compression crustal thickening remelting of the mid to lower crust and subsequent orogenesis adjacent to the orogenic belt

oxygen isotope geochemistry of oceanic arc lavas variations of oxygen isotope ratios in arc related lavas can constrain the contributions of subducted crustal igneous rocks sediments and fluids to the sub arc mantle we have measured oxygen isotope ratios in 72 arc and back arc lavas from five ocean ocean subduction zone systems using laser fluorination analyses of olivine and other phenocrysts and glass eighty percent of our samples have delta 18o values for any given phase olivine plagioclase glass or biotite within 0 2 per thousand of the average value for that phase in upper mantle peridotites and mid ocean ridge basalt morb the range for each phase is 1 0 per thousand this result contrasts with previous studies of whole rock samples which are significantly more variable even after exclusion of samples believed to be altered or fractionated by magmatic differentiation and demonstrates that most arc related lavas contain 1 2 of 18o enriched crustal oxygen from any source i e assimilation or subducted contributions elevations in delta 18o that do occur in these basic arc derived magmas relative to values most common for mantle derived lavas are associated both with enriched radiogenic isotope signatures and even more strongly with chemical indices consistent with high integrated extents of melting of their peridotite sources we interpret these relationships as evidence that melting in the sources of the high delta 18o lavas we have studied was fluxed by addition of high delta 18o aqueous fluid or perhaps a hydrous melt from the subducted slab such that sources that contain relatively large components of slab derived fluid or melt are both relatively 18o enriched and also experienced relatively large amounts of melting we have developed a quantitative model linking the amount of melting to the extents of 18o radiogenic isotope and trace element enrichment in a mantle source being fluxed by addition of aqueous fluid comparison of this model with observed variations in the geochemistry of lavas from the vanuatu fiji new caledonia region the suite of related samples showing the greatest range in delta 18o observed in this study constrains the amounts and chemical and isotopic compositions of slab derived phases in the sources of these arc related lavas assuming a delta 18o value of 20 per thousand for the slab derived fluid 0 5 1 0 wt is added to the sources of most mantle derived arc magmas the maximum amount of slab derived flux in the sources of arc magmas according to our results is 2 5 wt

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

mesoproterozoic grenville age terranes in the kyrgyz north tianshan zircon ages and nd hf isotopic constraints on the origin and evolution of basement blocks in the southern central asian orogen abstract the north tianshan orogenic belt in kyrgyzstan consists predominantly of neoproterozoic to early paleozoic assemblages and tectonically interlayered older precambrian crystalline complexes and formed during early paleozoic accretionary and collisional events one of the oldest continental fragments of late mesoproterozoic grenvillian age occurs within the southern part of the kyrgyz north tianshan using shrimp zircon ages we document two magmatic events at 1 1 and 1 3 ga the younger event is characterized by voluminous granitoid magmatism between 1150 and 1050 ma and is associated with deformation and metamorphism the older event is documented by 1 3 ga felsic volcanism of uncertain tectonic significance and may reflect a rifting episode geochemical signatures as well as nd and hf isotopes of the mesoproterozoic granitoids indicate melting of still older continental crust with model ages of ca 1 2 to 2 4 ga the mesoproterozoic assemblages are intruded by paleozoic diorites and granitoids and nd and hf isotopic systematics suggest that the diorites are derived from melts that are mixtures of the above mesoproterozoic basement and mantle derived material their source is thus distinct from that of the mesoproterozoic rocks emplacement of these plutons into the precambrian rocks occurred between 461 and 441 ma this is much younger than previously assumed and indicates that small plutons and large batholiths in north tianshan were emplaced virtually synchronously in the late ordovician to early silurian the mesoproterozoic rocks in the north tianshan may be remnants of a once larger continental domain whose fragments are preserved in adjacent blocks of the central asian orogenic belt comparison with broadly coeval terranes in the kokchetav area of northern kazakhstan the chinese central tianshan and the tarim craton point to some similarities and suggests that these may represent fragments of a single mesoproterozoic continent characterized by a major orogenic event at 1 1 ga known as the tarimian orogeny

zircon u pb geochronology and hf isotopic composition of the hongqiyingzi complex northern hebei province new evidence for paleoproterozoic and late paleozoic evolution of the northern margin of the north china craton abstract the hongqiyingzi complex located at the northern margin of the north china craton was previously defined as a paleoproterozoic unit it consists of volcanic and sedimentary rocks which underwent greenschist to amphibolite facies metamorphism this study presents zircon u pb ages and hf isotopic composition and whole rock nd isotopic composition of metamorphosed igneous and sedimentary rocks from the hongqiyingzi complex the age dating results show that different gneisses of this complex contain mainly late paleoproterozoic 1800 ma and late paleozoic 275 to 300 ma zircons and minor late archean zircons late paleozoic zircons either occur as magmatic grains or as metamorphic overgrowth and both types of zircon can be distinguished by their th u ratios and internal structures most zircon grains of a paleozoic magmatic rock sample hq14 have high e hf t values of 1 5 to 10 8 suggesting significant contribution of mantle or juvenile crustal material to the magma source s the results imply that the hongqiyingzi complex contains precambrian supracrustal sequences and paleozoic rocks of magmatic origin that underwent strong metamorphism and deformation during the late paleozoic related to the closure of the paleo asian ocean between siberia and the north china craton

structural and geochronological study of high pressure metamorphic rocks in the kekesu section northwestern china implications for the late paleozoic tectonics of the southern tianshan abstract blueschist and eclogite facies high to ultrahigh pressure hp uhp metamorphic rocks occur in the southern tianshan belt their deformation and metamorphic history is important for understanding the paleozoic tectonics of the central asian orogenic belt our study focuses on the structural analysis and geochronology of the hp metamorphic rocks and the surrounding rocks in the kekesu section in the southern chinese tianshan geometric and kinematic analyses indicate three ductile deformation events a top to the north thrusting a top to the south shearing and a dextral wrenching new 40ar 39ar laser probe plateau ages were obtained on white mica from retrograde blueschist documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr

the dunhuang block is a paleozoic orogenic belt and part of the central asian orogenic belt caob nw china abstract the dunhuang region is located in the east of the tarim craton and has long been considered as part of a precambrian terrane of the tarim or north china craton aiming to further understand the tectonic evolution of the dunhuang region and its relations with the adjacent tectonic units this study is to investigate the metamafic rocks in the sanweishan area dunhuang region and in turn to summarize the tectono thermal events and re examine the tectonic characteristics of the region the metamafic rocks in the sanweishan area include three types of amphibolites they are garnet clinopyroxene amphibolites garnet free amphibolites and garnet mica amphibolites the whole rock major and trace element compositions suggest that their protoliths probably are e morb type tholeiitic basalts with some crustal addition the la icp ms zircon u pb data of the garnet clinopyroxene amphibolite and the garnet free amphibolite yielded lower intercept ages of 410 11 ma and 403 8 ma respectively while the garnet mica amphibolite gave three metamorphic ages of 441 3 ma 408 12 ma and 370 2 ma three suites of metamorphic conditions have been estimated at 1 14 1 55 gpa and 458 548 c 0 67 0 82 gpa and 590 600 c and 0 47 0 66 gpa and 557 586 c for garnet clinopyroxene amphibolite using different mineral thermobarometers which illustrate a collisional orogeny related clockwise p t path integrated with the study on the high pressure granulite in the mogutai area it can be inferred that the metamafic rocks in the sanweishan area obviously experienced amphibolite facies metamorphism at ca 410 400 ma and they could also underwent high grade metamorphism at ca 440 430 ma therefore we suggest that the e w trending collisional orogeny of ca 440 400 ma could sweep cross the dunhuang region the dunhuang region records multiple tectono thermal events occurring at ca 3 1 2 5 ga ca 2 3 1 6 ga ca 440 400 ma and ca 370 310 ma however compared with the limited exposure of the precambrian complex the early silurian late carboniferous ca 440 310 ma orogeny related metamorphic and magmatic rocks are widely distributed in the dunhuang region making up nearly 70 of the total exposure thus it is proposed that the geological bodies in the dunhuang region were mainly produced by a paleozoic orogeny and represent a paleozoic orogen while the limited precambrian complex probably is a microcontinental fragment or part of the margin of the tarim or north china cratons that was involved in this paleozoic orogeny during the early silurian late carboniferous we infer that the dunhuang orogenic belt is part of the central asian orogenic belt caob and thus the caob probably extends beyond the beishan orogenic collage and southward into the dunhuang region

paleoproterozoic crustal evolution of the tarim craton constrained by zircon u pb and hf isotopes of meta igneous rocks from korla and dunhuang abstract the tarim craton is one of three large cratons in china presently there is only scant information concerning its crustal evolutionary history because most of the existing geochronological studies have lacked a combined isotopic analysis especially an in situ lu hf isotope analysis of zircon in this study precambrian basement rocks from the kuluketage and dunhuang blocks in the northeastern portion of the tarim craton have been analyzed for combined in situ laser ablation icp mc ms zircon u pb and lu hf isotopic analyses as well as whole rock elements to constrain their protoliths forming ages and magma sources two magmatic events from the kuluketage block at 2 4 ga and 1 85 ga are revealed and three stages of magmatic events are detected in the dunhuang block i e 2 0 ga 1 85 ga and 1 75 ga the 1 85 ga magmatic rocks from both areas were derived from an isotopically similar crustal source under the same tectonic settings suggesting that the kuluketage and dunhuang blocks are part of the uniform precambrian basement of the tarim craton zircon hf model ages of the 2 4 ga magmatism indicate that the crust of the tarim craton may have been formed as early as the paleoarchean period the 2 0 ga mafic rock from the dunhuang block was formed in an active continental margin setting representing an important crustal growth event of the tarim craton in the mid paleoproterozoic that coincides with the global episode of crust formation during the assembly of the columbia supercontinent the 1 85 ga event in the kuluketage and dunhuang blocks primarily involved the reworking of the old crust and most likely related to the collisional event associated with the assembly of the columbia supercontinent while the 1 75 ga magmatism in the dunhuang block resulted from a mixture of the reworked archean crust with juvenile magmas and was most likely related to a post collisional episode

permian geodynamic setting of northeast china and adjacent regions closure of the paleo asian ocean and subduction of the paleo pacific plate abstract northeast china and adjacent regions are located in the central east asian continent and consist tectonically of both the paleo asian and paleo pacific orogens between the siberian platform and sino korean north china block this paper discusses some hotly debated issues concerning the permian geodynamic setting of these regions based on a comprehensive analysis of available geological geochemical paleobiogeographical and paleomagnetic data spatial and temporal distribution of ophiolites and associated continental marginal sequences permian sedimentary sequences spatial distribution and geochemistry of permian magmatic rocks and the evolution of paleobiogeographical realms imply 1 that the permian marine basins in northeast china and adjacent regions include remnants of the paleo asian ocean in southeastern inner mongolia and central jilin province and active continental margins of the paleo pacific ocean 2 that the suture between the siberian and sino korean paleoplates was finally emplaced in the permian and is located in areas from suolunshan solonker eastwards through regions north to the xar moron river in southeastern inner mongolia and then central jilin province to the yanji area and 3 that the permian crustal evolution of northeast china and adjacent regions as well as parts of the siberian paleoplate was influenced by subduction of the paleo pacific oceanic plate finally the permian tectonic framework and paleogeography of northeast china and adjacent regions in central east asia are discussed briefly and early and late permian palinspastic reconstruction maps are provided

paleozoic accretion and cenozoic redeformation of the chinese tien shan range central asia the tien shan range in central asia contains two late paleozoic sutures the older southern suture marks the collision of a passive margin at the north of the tarim block and an active continental margin subduction under the latter was to the north the younger northern suture separates a northern carboniferous island arc from an active continental margin developed over a south dipping subduction zone the subduction direction under the island arc is unknown mesozoic elastics were deposited over the doubly sutured orogen rate and energy of sedimentation waned until deposition of oligocene conglomerates above a regional unconformity interpreted as marking the onset of deformation induced by the india asia collision molasse deposition accelerated in pliocene and quaternary time and deposition continues today as active thrusts generate relief paleozoic structures control the gross divergence of cenozoic thrusts across the orogen

palaeozoic arc magmatism in the central asian orogenic belt of kazakhstan shrimp zircon ages and whole rock nd isotopic systematics abstract early palaeozoic tonalite to granodiorite intrusions in northern kazakhstan are associated with lode gold mineralization and have shrimp zircon ages of 457 3 6 6 ma aksu 452 9 5 6 ma and 447 4 5 4 ma both zholymbet the stepnyak intrusion contains large xenoliths with an age of 480 6 5 0 ma one early palaeozoic zircon from a porphyritic diorite at stepnyak has a core with a near concordant 207pb 206pb age of 3888 1 5 ma whereas other xenocrystic grains are between 983 and 2698 ma old the early archaean age is probably inherited from unexposed basement of the kokchetav massif and represents the oldest crustal material so far known from the asian continent it appears that the aksu zholimbet and stepnyak granitoids were emplaced in the late ordovician in an andean or japan type continental arc environment on the margin of the kokchetav massif late palaeozoic granitoids in central kazakhstan have devonian zircon ages of 407 1 3 9 381 1 3 1 and 369 2 4 9 ma whereas the youngest sample from the topar massif has a late carboniferous emplacement age of 314 1 5 1 ma initial end values range from 5 to 1 corresponding to nd model ages of 1 1 0 6 ga the isotopic data are similar to those of other phanerozoic granitoids of the central asian orogenic belt and corroborate melting of predominantly juvenile crustal protoliths our data also confirm that devonian and carboniferous arc magmatism in central kazakhstan assimilated older and already accreted island arc systems yet is derived from primitive sources probably in a subduction setting the range of end values with corresponding nd mean crustal residence ages in the range of 0 6 1 1 ga for most samples indicates variable recycling of late proterozoic largely juvenile continental crust in the formation of the palaeozoic magmatic arcs of central kazakhstan the orogenic evolution terminated with permian anorogenic rift related alkaline granite magmatism end 2 to 8 which reflects melting of juvenile mantle derived lower crust