late paleozoic closure of the ob zaisan ocean along the irtysh shear zone nw china implications for arc amalgamation and oroclinal bending in the central asian orogenic belt the largest accretionary orogen in the world the central asian orogenic belt has evolved through the assembly of various oceanic and continental blocks understanding the processes associated with the development of this orogenic belt relies on precise recognition of the boundaries between various terranes one such major suture zone which records the collisional history of the siberian marginal arc system chinese altai with intra oceanic arc systems east west junggar is the irtysh shear zone in nw china the spatial continuity and the tectonic nature of this shear zone are still poorly understood but its development has supposedly made a significant impact on the architecture of the western central asian orogenic belt and the formation of the kazakhstan orocline here we provide new insight into the evolution of this shear zone based on detrital zircon ages hf isotope composition structural data and ar ar age constraints on the timing of deformation our results show a major discrepancy in detrital zircon populations and hf model ages across the southern chinese altai and the northern east west junggar thus allowing us to map the exact location of the tectonic boundary detrital zircon data constrain the initial closure of the ob zaisan ocean to the late carboniferous 323 ma and new structural and ar ar geochronological data shed light on the subsequent collisional processes we propose that the collisional zone between the chinese altai and the east west junggar was initially subjected to crustal thickening at ca 323 295 ma followed by orogen parallel extension ca 295 ma and transpressional deformation ca 286 253 ma the closure of the ob zaisan ocean in nw china postdates the initial phase of oroclinal bending in the western central asian orogenic belt thus indicating that oroclinal bending initiated during subduction based on our new constraints and other available geological data we suggest that the early stage of oroclinal bending was likely driven by slab rollback

depleted mantle source for the ulungur river a type granites from north xinjiang china geochemistry and nd sr isotopic evidence and implications for phanerozoic crustal growth abstract the ulungur river granites from north xinjiang china display two petrological trends 1 a peralkaline trend composed of alkali amphibole and alkali pyroxene bearing granites and dykes and 2 an aluminous trend represented by alkali granites monzogranites and aluminous granite dykes within the peralkaline granites rb sr whole rock isochron dating yielded an age of 300 ma for the emplacement of the peralkaline granites and 270 ma for the aluminous granites the ulungur river granites share all the geochemical features common to a type granites of the world the ulungur river granites are post orogenic and emplaced in a post collisional extensional environment the peralkaline and aluminous rocks have indistinguishable nd isotopic compositions they have unusually positive and relatively uniform nd t values ranging from 5 1 to 6 7 for the peralkaline rocks at 300 ma and from 5 5 to 5 9 for the aluminous rocks at 270 ma the initial 87sr 86sr ratios cannot be determined precisely by the whole rock isochron technique mainly because most samples are highly radiogenic and have high rb sr ratios in addition the large variation in sr isotopic compositions can partly be explained by crustal contamination during magma differentiation the nd isotopic data clearly indicate that both the peralkaline and aluminous rocks were derived from a long lived depleted mantle reservoir the formation of the ulungur river a type granites is hypothesized as follows 1 a depleted mantle was first metasomatised then subjected to partial melting leading to the generation of basic magmas now represented by the kalatongke mafic ultramafic complex in the ulungur river area 2 the basic magmas have undergone high degrees of fractional crystallisation with separation of biotite hornblende plagioclase potassium feldspar and other accessory phases such as apatite and ilmenite with the residual magmas forming the peralkaline granites during the magmatic differentiation the liquids were probably contaminated by crustal materials as evidenced by nb depletion and nd sr isotopic systematics on the other hand the formation of aluminous granites involved a replenishment of new mantle derived magmas into the differentiating magma chamber and followed by fractional crystallisation with retention of hydrous phases hornblende and biotite the petrological evolution of the ulungur river granites is thus from peralkaline to calc alkaline and not from calc alkaline to peralkaline as more often observed elsewhere the ulungur river granites in common with the voluminous granitoids occurring in the central asian orogenic belt represent a significant juvenile addition of the continental crust in the phanerozoic

new constraints on the pre permian continental crust growth of central asia west junggar china by u pb and hf isotopic data from detrital zircon in situ u pb geochronology of detrital zircons from various palaeozoic sedimentary rocks of west junggar accretionary complexes central asia suggests two distinct episodes of arc magmatism and an evolution in three steps i ordovician silurian subduction generating juvenile arc crust ii late silurian subduction jamming erosion and intraplate magmatism iii development of two new opposed devonian carboniferous subductions recycling the early palaeozoic crust zircon hf isotopes document three pre permian episodes of mantle derived magmatic input into the crust 1 neoproterozoic 850 550 ma 2 early palaeozoic 530 450 ma and 3 late palaeozoic 380 320 ma zircons also record the recycling of neoproterozoic and early palaeozoic juvenile crusts during the early and late palaeozoic these data support a model of episodic continental crust growth in central asia

final amalgamation of the tianshan and junggar orogenic collage in the southwestern central asian orogenic belt constraints on the closure of the paleo asian ocean abstract the tianshan and junggar orogenic collage occupies the southwestern part of the central asian orogenic belt and was assembled by collision accretion of several continental blocks and island arcs during late paleozoic early mesozoic time following the consumption of the junggar and south tianshan oceanic basins in the western segment of the paleo asian ocean considerable and continuing controversy has surrounded when where and how this orogenic collage was eventually amalgamated which plays a crucial role in understanding the formation of the central asian orogenic belt this review synthesizes the most recent data and geologic evidence that place critical constraints on the closure history of the oceanic domains a comprehensive data compilation indicates that the formation of ophiolites along major sutures in the tianshan and junggar region lasted to the early carboniferous for the kalamaili suture zone to the mid carboniferous for the west junggar region and the north tianshan suture zone and to the late carboniferous for the south tianshan suture zone ca 325 310 ma ultra high pressure metamorphism along the western part of the south tianshan suture occurred nearly synchronously with the collision between the tarim craton and the central tianshan yili block the oceanic closure and collision events were sequentially followed by widespread and intensive a type granitic and bimodal magmatism which commenced in the mid carboniferous along the kalamaili belt and in the latest carboniferous to earliest permian in other regions contemporaneously large scale strike slip shearing took place along major faults subparallel to main sutures with a dominant dextral sense and deformation time since the carboniferous permian transition available data and geological evidence support an eastward propagating scissor like closure of the western segment of the paleo asian ocean along the south tianshan and north tianshan suture zones over a period from the mid to end carboniferous with the south tianshan belt representing the final suturing site in the early permian the tarim mantle plume likely merely affected the southwestern part of the assembled tarim and tianshan region as indicated by regional discrepancy in type and geochemistry of early permian a type granitoids the amalgamation of the tianshan and junggar orogenic collage was associated with the tectonic bending of the kazakhstan orocline leaving a piece of trapped oceanic crust in its middle part beneath the present junggar basin

linking the southern west junggar terrane to the yili block insights from the oldest accretionary complexes in west junggar nw china abstract west junggar is known to tectonically correlate with east kazakhstan however the tectonic link of the southern west junggar terrane to adjacent regions still remains uncertain here we examined the oldest accretionary complexes thus constraining its tectonic evolution and link during the early middle paleozoic they have contrasting lithologic geochemical and geochronological features and thus provenances and tectonic settings the laba unit was derived from the late ordovician early devonian continental arc system peaking at 450 420 ma with precambrian substrate which formed as early as the early devonian and metamorphosed during the permian however the kekeshayi unit was accumulated in an intra oceanic arc setting and includes the pre late silurian and late silurian subunits with or without precambrian sources integrated with the regional data the southern west junggar terrane revealed a tectonic link to the northern yili block during the late silurian to early devonian as suggested by the comparable precambrian zircon age spectra between the southern west junggar terrane and the micro continents in the southern kazakhstan orocline the proximal accumulation of the laba unit in the continental arc atop the yili block and the sudden appearance of precambrian zircons in the kekeshayi unit during the late silurian this link rejects the proposals of the southern west junggar terrane as an extension of the northern kazakhstan orocline and the middle paleozoic amalgamation of west junggar a new linking model is thus proposed in which the southern west junggar terrane first evolved individually and then collided with the yili block to constitute the kazakhstan continent during the late silurian the independent and contrasting intra oceanic and continental arcs also support the paleozoic archipelago type evolution of the central asian orogenic belt

tectonics of the east pacific rise near 12 50 n a submersible study abstract using the diving saucer cyana a portion of the east pacific rise between 12 38 and 12 54 n has been studied and mapped four main zones are defined 1 an active volcanic zone located in the central graben the graben displays a variable relief along strike from less than 10 m up to 55 m and has an en echelon pattern in plan view 2 an active tectonic zone comprising the central graben and extending up to about 2 km from either side of the rise axis 3 an inactive tectonic zone starting 2 km away from the axis and formed by discontinuous horst and graben type of structures and 4 off axis volcanoes seamounts located less than 20 km on either side of the rise axis at 12 50 n the active tectonic zone is extremely narrow and the mode of deformation is through fissuring in strong contrast with that observed in slow or intermediate rate ridges the presence or absence of a magma reservoir at shallow depth explains the difference in the width of the active tectonic zone the state of isostatic equilibrium or lack of equilibrium explains the different modes of deformation

hidden eoarchean crust in the southwestern central asian orogenic belt abstract u pb and hf isotope analyses of zircon xenocrysts from drilling sampled carboniferous volcanic rocks suggest the presence of unexposed eoarchean crust beneath the junggar basin nw china southwestern central asian orogenic belt caob the oldest zircon population with u pb ages of 3 8 ga shows chondritic ehf t of 0 7 to 0 7 and hf model ages of 3900 3952 ma tdm and 3973 4062 ma tcrust three slightly younger populations at 3 7 3 62 and 3 45 ga have ehf t of 5 7 to 2 4 tdm of 3 6 3 9 ga and tcrust of 3 7 4 1 ga the youngest xenocrysts yield u pb ages of 2 55 ga with ehf t values of 3 7 to 8 0 crustal mineral inclusions and the trace element compositions of all these zircons show that they probably crystallized from granitic magmas the observation of deeply hidden archean crust significantly older than the exposed phanerozoic upper crust suggests that the juvenile origin in caob may be overestimated and the study of crustal growth rates through time for young orogens will require revision by comparing the hf isotopic signatures of ancient zircons 3 4 ga worldwide we infer that there may have been a great change in early crustal evolution at the hadean archean transition 3 9 ga possibly linked to the late heavy bombardment lhb

paleozoic tectonic evolution of the northern xinjiang china geochemical and geochronological constraints from the ophiolites 1 the northern xinjiang of china is a composite orogenic belt in the south central part of the altaids formed by subduction accretion mainly during the paleozoic geochronologic and geochemical data from the paleozoic ophiolites and associated assemblages are reported here to place constrains on the tectonic evolution of this region and to reconstruct its history using a model of subduction accretion plus large scale map view strike slip faulting geologic and geochronologic review indicates that the paleozoic ophiolites and associated assemblages of the western and eastern junggar regions are tectonically related occurring as a pattern with the early paleozoic tangbale mayile hongguleleng aermantai complexes in the center the late paleozoic darbut karamaili and the northern tianshan complexes in the south and the late paleozoic kekesentao qiaoxiahala kuerti complexes in the north available geochemical data suggest that the paleozoic ophiolites of the northern xinjiang are mainly oceanic fragments of these three types mid ocean ridge basalt oceanic island basalts and island arc basalts the paleozoic subduction accretion complexes grew mainly along different segments of the kipchak arc and in associated with migration of magmatic arc the paleozoic ophiolite associated sedimentary sequences and subduction related volcanic rocks exhibit an evolution history from intraoceanic early paleozoic to marginal settings late paleozoic implying coeval strike slip stacking with subduction accretion processes and by which to transport the respective complexes together and further to amalgamate with neighboring units of the altaids subduction accretion processes terminated in the mid carboniferous in this region in the late permian early triassic a counterclockwise rotation of fault blocks in a giant sinistral strike slip and extension domain between altay and tianshan lastly generated present configuration of the paleozoic subduction accretion complexes in the northern xinjiang

depositional models for moraine formation in east antarctic coastal oases this paper examines the origin of moraine ridges in east antarctic coastal oases and derives depositional models appropriate for the reconstruction of quaternary history on the basis of morphology structure and sedimentology four principal types of ridge may be identified 1 type a moraines form whenthe basal debris zone crops out near an ice margin 2 type b moraines form when large recumbent folds develop in the basal debris zone 3 type c moraines are ice contact screes and fans which form when debris accumulates at streep or cliffed ice margins and 4 type d moraines are thrust block moraines that form when unconsolidated sediment is entrained by freezing shearing and thrusting of sediment blocks at the base of the glacier simple calcumations of the rate of debris accumulation at ice margins suggest that type a b and c moraines take thousands of years to form and record stable ice margins type d moraines are structural features that may form relatively quickly when ice margins override unconsolidated sediment constructing models to explain the origin of the moraines is an important part of reconstructing the quaternary history of antarctic coastal oases because the models provide a basis for reconstructing the position and behaviour of the ice sheet during advance and retreat

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

large scale porphyry type mineralization in the central asian metallogenic domain a review abstract a series of porphyry type cu mo au deposits including some giant world class deposits such as kounrad aktogai kal makyr oyu tolgoi and chalukou occur in the central asian metallogenic domain camd the outline of the camd almost overlaps with that of the central asian orogenic belt caob which is located between the east europe siberia tarim and north china cratons the camd comprises sparsely exposed tectonic slices of precambrian high grade metamorphic basement rocks that are sandwiched between phanerozoic accretionary complexes and arc rocks according to geological metallogenic geochronological and tectonic setting characteristics the camd comprises three metallogenic provinces 1 the kazakhstan cu au mo 2 the mongolia cu au and 3 the northeast china mo cu metallogenic provinces early paleozoic 490 to 440 ma and late paleozoic 330 to 295 ma large scale porphyry type mineralization in the kazakhstan metallogenic province is intimately associated with the evolution of the central tianshan yili northern tianshan kokchetav chingiz paleozoic arc system which was terminated by the late permian closure of the western segment of the paleo asian ocean in contrast the 370 ma and 240 ma porphyry type mineralization of the mongolia metallogenic province is related to the tuva mongolia arc collage system that was formed during the evolution of the paleo asian ocean in the paleozoic and the mongol okhotsk ocean in the early mesozoic furthermore the northeast china metallogenic province was affected by three distinct geodynamic processes that were associated with the paleo asian mongol okhotsk and paleo pacific oceans during the paleozoic 250 ma the early mesozoic 200 ma and the late mesozoic 200 90 ma respectively except for the early paleozoic cu mineralization at duobaoshan 480 ma and bainaimiao 440 ma the porphyry type mineralization of this metallogenic province is predominantly characterized by widespread mesozoic mo mineralization in general porphyry type deposits of the camd have geological structural metallogenic alteration and fluid characteristics which are consistent either with those related to the oceanic subductional arc settings of the circum pacific metallogenic domain or with those of the continental collisional environment in the tethyan metallogenic domain the involvement of highly oxidized intermediate to felsic magmas is fundamental for the formation of porphyry deposits even though the prevalent mash and slab melting models can be employed to interpret the genesis of some porphyry cu deposits in the camd a here newly suggested model which is based on pre enriched mafic lower crust and the subduction of a relictic mid oceanic ridge is thought to explain the large scale copper mineralization in the balkhash west junggar metallogenic belt of the kazakhstan metallogenic province nevertheless most of the porphyritic suites associated with the mo deposits in the northeast china metallogenic province have relatively depleted mantle like neodymium and hafnium isotopic compositions suggesting that the ore forming granitic magma may have predominantly been derived from juvenile lower crust upwelling of asthenosphere may have triggered the melting of the lower crust during post collisional extension subsequent to the closure of the paleo asian ocean in the triassic the subduction of the paleo pacific ocean in the jurassic and the thinning of the lithosphere caused by rollback of the paleo pacific plate in the cretaceous ancient crustal components were only locally found to have been involved in the magma source of several medium large mo deposits

ediacaran to paleozoic magmatism in west junggar orogenic belt nw china and implications for evolution of central asian orogenic belt abstract the formation of the central asian orogenic belt is thought to be related to the evolution of the paleo asian ocean surrounded by the siberian european and tarim north china cratons the west junggar orogenic belt wjob in the southern central asian orogenic belt is part of the kazakhstan junggar terrane between the altay orogenic belt to the north and the tianshan orogenic belt to the south it is characterized by the development of the latest ediacaran to paleozoic intra oceanic island arcs and accretionary complexes including the latest ediacaran to ordovician tangbale mayile arc accretionary system in the south and ordovician to early carboniferous boshchekul chingiz arc and zharma saur arc in the north and a remnant ocean between them our work reveals that the shinaizha kekesayi and barleik dioritic plutons within the tangbale mayile arc are typical of arc related magmatism and were formed between 572 and 505 ma which indicate the ediacaran to middle cambrian subduction of the southern paleo asian ocean however these arc plutons were strongly deformed and surrounded by younger rocks implying that the magmatic arc might be dismembered and involved into the ordovician to devonian accretionary complexes the boshchekul chingiz arc was recorded from late ordovician and its arc magmatism peaked in late silurian to early devonian the arc moved northward due to the retreat of the south facing irtysh zaysan subduction zone which resulted in the formation of contemporaneous a type or peralkaline magmatic rocks or bimodal volcanics in response to the within arc extension the northward migration of the subduction zone led to the formation of the devonian to early carboniferous zharma saur arc the earliest late carboniferous i type granitoids occurred sporadically in wjob followed by pervasive occurrences of late carboniferous to early permian a and i type granitoids in wjob and adjacent areas the late carboniferous granitoids combined with the late carboniferous stitching plutons within the irtysh zaysan suture zone to the north and the north tianshan suture zone to the south of wjob suggest the kazakhstan junggar terrane collided with altay and tianshan orogenic belts in the latest early carboniferous ca 320 ma

early silurian to early carboniferous ridge subduction in nw junggar evidence from geochronological geochemical and sr nd hf isotopic data on alkali granites and adakites abstract the central asian orogenic belt caob evolved through a long lived orogeny involving multiple episodes of subduction and accretion marking a major phase of continental growth during the paleozoic the northern part of the western junggar region nw junggar offers a window into these processes particularly to constrain the timing of closure of the paleo asian ocean here we report geochemical geochronological and isotopic data from k feldspar granites and adakitic rocks from the nw junggar region zircon u pb ages suggest that the granites were emplaced during early silurian to the early carboniferous 434 328 ma the granites show geochemical characteristics similar to those of a type granites with high sio2 71 13 76 72 wt na2o k2o 8 00 9 59 wt and al2o3 12 28 14 08 wt but depleted sr nb ta and eu they display moderate to high positive end t and ehf t values 4 26 8 21 and 7 69 14 60 respectively and young nd and hf model ages t2dm nd 489 740 ma and t2dm hf 471 845 ma suggesting magma derivation through partial melting of lower crust in the boshchekul chingiz and zharma saur arcs the adakites are characterized by high sr content 406 5 751 6 ppm and low y 13 8 16 4 ppm and yb 1 5 1 8 ppm content yielding relatively high sr y ratios 25 38 49 41 similar to those of modern adakites they have high positive end t and ehf t values 7 85 8 25 and 13 23 15 97 respectively and young nd and hf model ages t2dm nd 429 535 ma and t2dm hf 355 550 ma indicating that their source magmas were likely derived from partial melting of the oceanic crust beneath the boshchekul chingiz arc the petrogenesis and distribution of the a type granites and adakites as well as the tectonic architecture of the region suggest that a ridge subduction event might have occurred during the early silurian to early carboniferous in combination with previous studies in the chinese altai we suggest a two sided ridge subduction model for the junggar altai region

chromian spinels in highly altered ultramafic rocks from the sartohay ophiolitic melange xinjiang nw china abstract the sartohay ophiolitic melange is located in western junggar xinjiang province nw china which is a major component of the core part of the central asian orogenic belt caob chromian spinels in serpentinite talc schist carbonate talc schist and listwaenite in sartohay ophiolitic melange retain primary compositions with cr of 0 39 0 65 mg 0 48 0 67 and fe3

tectonic evolution of the paleozoic barluk continental arc west junggar nw china abstract the barluk arc situated in west junggar is an important component of the kazakhstan domain in the central asian orogenic belt caob however the nature of the barluk arc active continental margin or oceanic island arc is in debate the igneous rocks in the shiwu area barluk arc have diagnostic petrologic geochronologic and geochemical signatures that uncover the attributes of the barluk arc in this paper we report detailed petrology whole rock geochemistry and sr nd isotopic zircon u pb dating and hf isotopic data of igneous rocks in the shiwu area our results reveal that 1 except for 302 2 ma gabbro igneous rocks from barluk arc in the shiwu area are dioritic and granitic have ages ranging from 323 8 to 310 5 ma and they have geochemical signatures originated from typical continental arcs including calc alkaline series rocks enrichment of lile depletion of hfse and higher th yb and ta yb values 2 the granitic porphyry and rhyolite in barluk arc have old tdm2 hf ages up to 1091 ma suggesting the existence of precambrian crust or similar materials as their sources and 3 the 302 2 ma gabbro dike has high values of end t 7 54 and ehf t 12 8 to 15 5 and a young t2dm nd age 455 ma and therefore it was possibly related to upwelling of asthenosphere mantle in a post subduction extensional tectonic setting based on these parameters we suggest that the barluk arc is defined as a late paleozoic continental margin arc along the margin of the kazakhstan continent westward subduction of the junggar oceanic plate in the barluk area started in early devonian and continued to the late carboniferous 310 ma subsequent post collisional tectonic processes between the junggar oceanic plate and kazakhstan continent continued to the late carboniferous 302 ma

paleozoic to mesozoic micro block tectonics in the eastern central asian orogenic belt insights from magnetic and gravity anomalies abstract the ne china region involves a complex accretionary belt formed by multiple stages of assembly of micro blocks from the paleozoic to mesozoic the tectonics of which provides important clues on the evolution of the east asian continental margin with a view to evaluating the micro block evolution in ne china we used gravity and magnetic methods to analyze the geophysical characteristics of the micro blocks and their boundaries combining with regional geology paleomagnetism and paleontology we reconstructed the micro block assembly history in the eastern central asian orogenic belt five paleozoic to mesozoic micro blocks are recognized in ne china the erguna xing an songnen jiamusi and nadanhada and their boundaries show significant differences in gravity and magnetic fields the boundary between the erguna block eb and the xing an block xb shows as a continuous tectonic line in the gravity field whereas the one between the xb and the songnen block sb exhibits a rather discrete but strong magnetic anomaly belt the boundary between the sb and the jiamusi block jb is characterized by both distinct gravity and magnetic anomalies the nadanhada micro block nmb appears as an independent micro block in the fourth order detail field of the bouguer gravity anomaly where it shows as a short axis or circinate structure in this anomaly field the paleozoic to mesozoic tectonic evolution of these micro blocks in ne china is closely related to five sutures in the eastern central asian orogenic belt the xinlin xiguitu suture was first formed at 500 ma leading to the assembly of the eb and the xb the heihe hegenshan suture was formed at 320 290 ma marking the amalgamation of the xb and the sb the mudanjiang yilan suture was formed at 200 ma marking the collision of the jb and the sb the paleo asian ocean closed during 250 230 ma in a scissor style the subduction of the paleo pacific ocean was initiated at 200 ma based on the deep structural architecture imaged from gravity and magnetic anomalies we propose an evolutionary model of the micro blocks in the ne china which provides important insights into the tectonic evolution of the ne china region during the paleozoic to mesozoic

geochemical sr nd and zircon u pb hf isotopic studies of late carboniferous magmatism in the west junggar xinjiang implications for ridge subduction abstract voluminous granitic intrusions are distributed in the west junggar nw china and they can be classified as the dioritic rocks charnockite and alkali feldspar granite groups the dioritic rocks sio 2 50 4 63 8 wt are calc alkaline and mg enriched average mgo 4 54 wt mg 0 39 0 64 with high sr y ratios average 21 2 weak negative eu average eu eu 0 80 and pronounced negative nb ta anomalies their sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7035 0 7042 e nd t 4 5 7 9 e hf t 14 1 14 5 show a depleted mantle like signature these features are compatible with adakites derived from partial melting of subducted oceanic crust that interacted with mantle materials the charnockites sio 2 60 0 65 3 wt show transitional geochemical characteristics from calc alkaline to alkaline with weak negative eu average eu eu 0 75 but pronounced negative nb ta anomalies sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7037 0 7039 e nd t 5 2 8 0 e hf t 13 9 14 7 also indicate a depleted source suggesting melts from a hot juvenile lower crust alkali feldspar granites sio 2 70 0 78 4 wt are alkali and fe enriched and have distinct negative eu and nb ta anomalies average eu eu 0 26 low sr y ratios average 2 11 and depleted sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7024 0 7045 e nd t 5 1 8 9 e hf t 13 7 14 2 these characteristics are also comparable with those of rocks derived from juvenile lower crust despite of the differences in petrology geochemistry and possibly different origins zircon ages indicate that these three groups of rocks were coevally emplaced at 305 ma a ridge subduction model can account for the geochemical characteristics of these granitoids and coeval mafic rocks as the slab window opened upwelling asthenosphere provided enhanced heat flux and triggered voluminous magmatisms partial melting of the subducting slab formed the dioritic rocks partial melting of the hot juvenile lower crust produced charnockite and alkali feldspar granite and partial melting in the mantle wedge generated mafic rocks in the region these results suggest that subduction was ongoing in the late carboniferous and thus support that the accretion and collision in the central asian orogenic belt took place in north xinjiang after 305 ma and possibly in the permian

siliceous volcanic associations of the northern balkhash ophiolite zone central kazakhstan biostratigraphy sedimentation and tectonic evolution in the middle late ordovician abstract the northern balkhash zone is the largest and the most complex ophiolite zone of the junggar balkhash region in the west of the central asian orogenic belt the zone encompasses serpentinite melange containing fragments of dunite harzburgite dunite verlite pyroxenite complexes layered and isotropic gabbro sheeted dikes and plagiogranites along with stacks of tectonic slivers composed of siliceous siliceous basaltic and siliceous volcanic tuffaceous units conodont finds suggest the middle late darriwilian age of the siliceous basaltic itmurundy fm while the siliceous basalt tiuretai siliceous kazyk and the siliceous tuffaceous uzuntobek formations are close in age extend from the top of darriwilian to the base of katian the siliceous clastic tuffaceous sediments of the obaly fm are the sandbian and katian in age and the volcanic sedimentary dzhamanshuruk fm is broadly upper ordovician dating of these units has also permitted recognition of the units with intricate thrust and fold structure species of the genera periodon pygodus and scabbardella dominate in the conodont assemblages in cherts an uneven distribution of the conodonts in sections is caused by variable siliceous sedimentation rate that depends on productivity of radiolarians the petrochemistry of the volcanics in the itmurundy fm suggests that they correspond to oib and e morb being formed within seamounts or oceanic plateaus while the tiuretai basalts are close to n morb erupted within a spreading center the felsic volcanics in the dzhamanshuruk fm have demonstrated an island arc affinity the proposed evolution model suggests that during the middle late ordovician tectonic settings changed successively from the spreading center to island arc within the northern balkhash zone of the junggar balkhash region

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

when did the subduction first initiate in the southern paleo asian ocean new constraints from a cambrian intra oceanic arc system in west junggar nw china abstract we report here a cambrian southward subducting intra oceanic arc system in the southern west junggar nw china where an immature arc occurred initially on ssz type ophiolites and finally evolved into a more mature one the immature arc is dominantly represented by early middle cambrian 510 ma low k tholeiitic felsic rocks whereas the mature arc is characterized by late cambrian 495 ma medium and high k calc alkaline felsic and mafic rocks the ssz type ophiolites show remarkable depletion of nb and ta and contain high cr spinel cr 0 6 resembling these formed in the forearc altogether they record the initiation of subduction and transformation of crust during early subduction of the paleo asian ocean in the southern part of the central asian orogenic belt caob the subduction initiation might occur in the early cambrian 515 ma as constrained by both the ssz type ophiolites 516 ma and the oldest arc plutons 515 509 ma that crosscut the ophiolites the immature felsic plutons have high sio 2 72 wt contents and variable mgo 0 42 1 49 wt and mg values 22 62 crustal anatexis may be responsible for the genesis of those plutons and thus the transformation from oceanic to continental crust these results combined with regional data convincingly indicate that it is one of the oldest intra oceanic arc systems in the southern caob which may mark the initial subduction of the paleo asian ocean in its southern part much later than those reported in the north an archipelago type model is proposed for the evolution of the southern west junggar and has implications for the development of the southern part of the caob

aeolian sediment fingerprinting using a bayesian mixing model identifying sand provenance in depositional aeolian environments e g dunefields can elucidate sediment pathways and fluxes and inform potential land management strategies where windblown sand and dust is a hazard to health and infrastructure however the complexity of these pathways typically makes this a challenging proposition and uncertainties on the composition of mixed source sediments are often not reported this study demonstrates that a quantitative fingerprinting method within the bayesian markov chain monte carlo mcmc framework offers great potential for exploring the provenance and uncertainties associated with aeolian sands eight samples were taken from dunes of the small 58 km2 ashkzar erg central iran and forty nine from three distinct potential sediment sources from the surrounding area these were analyzed for 61 tracers including 53 geochemical elements trace major and rare earth elements ree and 8 ree ratios kruskal wallis h tests and stepwise discriminant function analysis dfa allowed the identification of an optimum composite fingerprint based on six tracers rb sr 87sr la yb n ga and ce and a bayesian mixing model was applied to derive the source apportionment estimates within an uncertainty framework there is substantial variation in the uncertainties in the fingerprinting results with some samples yielding clear discrimination of components and some with less clear fingerprints quaternary terraces and fans contribute the largest component to the dunes but they are also the most extensive surrounding unit clay flats and marls however contribute out of proportion to their small outcrop extent the successful application of these methods to aeolian sediment deposits demonstrates their potential for providing quantitative estimates of aeolian sediment provenances in other mixed source arid settings and may prove especially beneficial where sediment is derived from multiple sources or where other methods of provenance e g detrital zircon u pb dating are not possible due to mineralogical constraints

middle cambrian to permian subduction related accretionary orogenesis of northern xinjiang nw china implications for the tectonic evolution of central asia abstract middle cambrian to permian subduction related records are widely distributed in northern xinjiang which can be grouped into the chinese altay east junggar eastern tien shan west junggar yili and tarim domains by integrating paleogeographic and geological data we suppose that the chinese altay east junggar eastern tien shan domain was more closely located to siberia while the west junggar and yili domains occupied an intermediate position near the kazakhstan block in the early paleozoic paleoasian ocean distribution of andean type magmatic arcs island arcs accretionary wedges ophiolitic slices and or microcontinents shows an archipelago paleogeography forming a huge accretionary active margin sequences the tarim domain was on the opposite side of the early paleozoic paleoasian ocean remaining passive margin these tectonic units drifted northwards and approached the southern active margin of the siberian craton in the late paleozoic leading to termination of the paleoasian ocean and formation of a complicated orogenic collage between siberian craton and the tarim block between the end permian and triassic these multiple accretion processes significantly contributed to the lateral growth of central asia

middle cambrian granites in the dunhuang block nw china mark the early subduction of the southernmost paleo asian ocean abstract the formation and evolution of the central asian orogenic belt caob were closely associated with the opening and closure of the so called paleo asian ocean pao the subduction of the southern pao is expected to have resulted in widespread paleozoic arc magmatism in the southernmost caob however the precise location and timing of such subduction remain uncertain the dunhuang block dhb in the southernmost caob is thought to have recorded such subduction in this contribution we report the results of the zircon u pb ages hf isotopes and whole rock geochemical data for the lianghu dapokouzi and xiaowan plutons in the northeastern dhb nw china zircon u pb dating reveals that these plutons formed at ca 510 ma the middle cambrian and may represent the oldest arc related magmatism in the dhb geochemically the dapokouzi pluton is characterized by high sio2 70 7 71 2 wt na2o 5 11 5 26 wt al2o3 15 5 15 6 wt and sr 730 733 ppm contents high sr y 380 398 ratios moderate mg 49 50 values low mgo 0 61 wt k2o 2 00 2 01 wt and y 1 84 1 93 ppm contents and weakly positive eu 1 39 1 44 anomalies indicating geochemical affinities of adakitic rocks they have positive ehf t values 11 2 to 13 9 close to the depleted mantle line and young two stage model ages tdm2 571 742 ma all the geochemical characteristics show that the dapokouzi pluton was derived from the remelting of juvenile crust with addition of minor metasomatized mantle derived materials and a residue phase dominated by garnet bearing amphibolite similarly the lianghu and xiaowan plutons have high sio2 70 2 74 4 wt and al2o3 12 8 14 3 wt low mgo 0 61 1 01 wt with mg values of 35 43 and k2o 0 43 1 55 wt contents compared to the dapokouzi pluton they exhibit low sr 121 155 ppm and y 15 3 23 1 ppm contents low sr y 5 25 9 54 ratios and cr ni co concentrations and pronouncedly positive ehf t values 11 0 to 14 7 indicating that the magmas originated from the remelting of juvenile lower crust followed by significant fractional crystallization at relatively shallow crustal depths data from this and previous studies convincingly suggest the dhb was located in a thickened continental arc setting during the early paleozoic probably related to the early subduction of the pao lithosphere crustal thickening as indicated by the geochemical signature of the dapokouzi pluton was most likely associated with the underplating of mantle derived mafic magmas during the slab subduction this study therefore highlights that the subduction of the southern pao might have initiated prior to 510 ma possibly dating back to the early cambrian

northwestern junggar basin xiemisitai mountains china a geochemical and geochronological approach abstract we report sims u pb zircon elemental and nd sr pb isotopic data for the volcanic rocks and their subvolcanic units from the xiemisitai mountains in the west junggar region xinjiang china aiming to determine their ages source regions and tectonic setting zircon u pb dating results of the volcanic rocks 422 5 1 9 ma 411 2 2 9 ma indicate that the volcanic strata occurred in the xiemisitai mountains at late silurian early devonian time rather than middle devonian as suggested previously they are probably the eastern extension of the early paleozoic cambrian silurian boshchekul chingiz volcanic arc of east kazakhstan in china the xiemisitai volcanic rocks include andesite rhyolite and their pyroclastic equivalents the subvolcanic units are felsite and granite porphyry andesite is moderately lree enriched with a marked negative nb anomaly and th yb enrichment rhyolite felsite and granite porphyry are enriched in lree and th and depleted nb the initial sr isotope values of the xiemisitai volcanic rocks and their subvolcanic units range from 0 702294 to 0 704456 e nd 420 ma from 0 19 to 1 88 with nd model ages ranging from 600 to 803 ma initial pb isotope ratios 206 pb 204 pb are 17 77 17 96 207 pb 204 pb are 15 49 15 54 and 208 pb 204 pb are 37 56 37 80 these features suggest that the xiemisitai volcanic rocks and their subvolcanic units derived from a mantle wedge significant mixed by subducted material emi in a subduction zone they also indicate significant involvement of neoproterozoic primitive crust in the formation of the xiemisitai magmatic arc these rocks are associated with a late silurian early devonian southward subduction of the oceanic lithosphere in the northern west junggar region this gives rise to an ew trending boshchekul chingiz volcanic arc and associated metallogenic belt in the northern west junggar region and adjacent kazakhstan

implications of palaeozoic ophiolites from western junggar nw china for the tectonics of central asia western junggar nw china contains fragments of several palaeozoic island arcs accretionary prisms and ophiolites which were accreted to each other and possibly to the southern margin of eurasia by the end of the palaeozoic the ophiolites of western junggar are important in reconstructions of these events four ophiolite belts are recognized from south to north these are tangbale mayila dalabute and barleik honggulueleng hebukesair they record the production and destruction of oceanic tracts in this region from the cambrian to the carboniferous the following characteristics are representative of them all 1 the ophiolitic successions are strongly deformed 2 sheeted dyke swarms are not preserved 3 basic igneous rocks show a range of chemical characteristics from depleted island arc tholeiite to ocean island basalt altered n type mid ocean basalts may be represented 4 large scale thrusts typically directed towards the southeast lie at the base of many individual ophiolitic bodies 5 the ophiolites rarely mark the position of possible sutures between arcs they are normally highly deformed and imbricated with other lithologies accretion of palaeozoic arcs and obduction of ophiolites was complete by the end of the carboniferous but the timing and sequence of events remain obscure no continental collisions occurred during this period of assembly crustal growth occurred by the amalgamation of non continental blocks

age geochemistry and tectonic implications of a late paleozoic stitching pluton in the north tian shan suture zone western china the central asian orogenic belt is the largest tectonic assembly of continental and oceanic terranes on earth due to closure of the paleo asian ocean in the phanerozoic among major suture zones in the north xinjiang region of western china the north tian shan suture zone because of collision between the yili terrane in the south and the junggar terrane in the north contains the youngest ophiolitic rocks and may represent the terminal stage of development of the central asian orogenic belt in western china but the timing of the suture zone remains poorly constrained a sensitive high resolution ion microprobe shrimp zircon u pb age of 316 3 ma i e the beginning of the late carboniferous from the undeformed sikeshu pluton which crosscuts the suture zone places a crucial upperage bound for the time of collision between the yili and junggar terranes this event occurred later than or nearly concurrent with other accretion collision events in the north xinjiang region implying that fi nal terrane amalgamation was completed in the late carboniferous the sikeshu pluton shares geochemical characteristics of the widespread late carboniferous to permian postcollisional a type and i type granitoids with depleted mantle like sr nd isotopic signatures in the north xinjiang region they all occurred during a protracted ca 320 270 ma episode of postcollisional magmatism that may have been induced by basaltic under plating due to either slab breakoff or delamination of thickened mantle lithosphere beneath the central asian orogenic belt the same postcollisional magmatism also generated cu ni sulfi de bearing mafi c ultramafi c magmatic complexes adakites and porphyry type coppermolybdenum bearing magmatic rocks in the north xinjiang region

tectonic evolution of the west junggar region xinjiang china the west junggar region of northwestern china consists of a hercynian age fold belt occupying a late paleozoic zone of convergence between three major eurasian plates siberian tarim and kazakhstan the northern part of the west junggar area includes part of the irtysh zaysan fold belt that extends to the northwest into the ussr where it marks a broad boundary between the siberian and kazakhstan plates the central part of the west junggar area includes the southeastern extension of the chingiz tarbgatay early paleozoic island arc sequence the southern part of the west junggar area is truncated by the deep fundamental junggar alakol fault that has brought the yili microplate into juxtaposiition with the west junggar region in the vicinity of ebin nur lake to the east the junggar area is covered by sediments of the junggar basin paleogeographic reconstructions and geologic evidence indicates that these plates began converging in the middle carboniferous and by late carboniferous alkali granites began invading the imbricated leading edges of the coalescing plates paleomagnetic data reveals that the plates continued to move along strike slip faults bringing the tarim plate and yili microplate into their present positions sometime in the early mesozoic six separate ophiolite occurrences in the west junggar region are strongly tectonized and dismembered the tangbale ophiolite is considered to be late cambrian 508 20 ma and represents the earliest known oceanic crust in western china petrologic and chemical data indicate that the tangbale ophiolite represents a possible back arc or forearc tectonic setting situated close to a developing island arc the darbut ophiolite has been tectonized to a melange and is overlain by late devonian flysch ages of the radiolaria in the cherts associated with the pillow lavas indicate that it formed in the middle devonian other occurrences of ophiolite melange in the northern part of the west junggar region may be as young as late devonian these ophiolites record ocean crust formation in the early to middle paleozoic within the junggarian ocean and their present tectonic setting indicates continental accretion and imbrication shortly after their formation associated blueschist blocks in the ophiolite melange further supports the concept of imbrication brought about by b type subduction and associated obduction postcollisional alkali granites intrude the oceanic sequences of the west junggar region and are considered to represent melted lower crustal material lead isotopic systematics on feldspars from six separate granite plutons indicate ratios that could only be considered oceanic as far as the magmas are concerned the implications of this finding is that the junggar basin basement rocks in the north and west are probably oceanic evidence presented in this study shows a complex history for the development of the west junggar region major movements of consolidating plates in the late paleozoic followed by rejuvenation of these same zones of closure during the himalayan tectonic event of the late tertiary demonstrates that convergence zones and fold belts may respond to vertical and horizontal stress even after many million years of inactivity

ophiolitic melanges in crustal scale fault zones implications for the late palaeozoic tectonic evolution in west junggar china the baijiantan and darbut ophiolites in west junggar are exposed in steep fault zones 70 containing serpentinite melange in contact on either side with regionally distributed upper devonian lower carboniferous ocean floor peperitic basalts and overlying sedimentary successions the ophiolitic melanges show classic structural features created by strike slip faulting and consistent shear sense indicators of left slip kinematics sandstone blocks within the melanges resemble the surrounding sediments in lithology and age indicating that the ophiolitic melanges consist of locally derived rocks the ophiolitic melanges therefore originated from left slip fault zones within a remnant basin and are not plate boundaries nor subduction suture zones sandstone is the youngest lithology involved in the melange and provides a maximum age for the melange of 322 ma whereas stitching plutons are younger than 302 ma multiple clusters in zircon ages from single gabbro blocks in the melange at 375 360 354 and 340 ma are inconsistent with accretionary incorporation of subducting ocean crust but rather suggest that episodic movement of the faults provided pathways for magma from the mantle into magma chambers late paleozoic tectonic evolution of west junggar involved late devonian to carboniferous relative motion between the junggar block and west junggar ocean basin which triggered the left slip fault zones within a remnant ocean basin along which the oceanic crust was disrupted to form linear ophiolitic melanges final filling of this remnant ocean basin and its dismemberment by strike slip faulting occurred in the late carboniferous followed by crustal thickening by juvenile granites at the carboniferous permian boundary

evolution of cambrian and early ordovician arcs in the kyrgyz north tianshan insights from u pb zircon ages and geochemical data abstract geochronological geochemical and structural studies of magmatic and metamorphic complexes within the kyrgyz north tianshan nts revealed an extensive area of early palaeozoic magmatism with an age range of 540 475 ma during the first episode at 540 510 ma magmatism likely occurred in an intraplate setting within the nts microcontinent and in an oceanic arc setting within the kyrgyz terskey zone in the south during the second episode at 500 475 ma the entire nts represented an arc system these two phases of magmatism were separated by an episode of accretionary tectonics of uncertain nature which led to obduction of ophiolites from the kyrgyz terskey zone onto the microcontinent the occurrence of zircon xenocrysts and predominantly negative whole rock nd t values and hf t values of magmatic zircons suggest a continental setting and melting of precambrian continental sources with minor contributions of palaeozoic juvenile melts in the generation of the magmatic rocks the late cambrian to early ordovician 500 475 ma arc evolved mainly on mesoproterozoic continental crust in the north and partly on oceanic crust in the south arc magmatism was accompanied by spreading in a back arc basin in the south where supra subduction ophiolitic gabbros yielded ages of 496 to 479 ma the relative position of the arc and active back arc basin implies that the subduction zone was located north of the arc dipping to the south variably intense metamorphism and deformation in the nts reflect an early ordovician orogenic event at 480 475 ma resulting from closure of the djalair naiman ophiolite trough and collision of the djel tau microcontinent with the northern margin of nts comparison of geological patterns and episodes of arc magmatism in the nts and chinese central tianshan indicate that these crustal units constituted a single early palaeozoic arc and were separated from the tarim craton by an oceanic basin since the neoproterozoic

tectonic evolution of palaeozoic terranes in west junggar xinjiang nw china nine separate cambrian to carboniferous terranes recognized in west junggar northwest china they were amalgamated as part of the central asian orogenic belt which records accretion of continental island arc and oceanic terranes to archaean proterozoic continental nuclei tangbale kekesayi ebinur and mayila terranes cambrian silurian evolved in intra oceanic settings and docked along a series of north dipping subduction zones on to the laba terrane to their south this southern continent was contiguous with lithosphere of the kulumudi ocean to the north devonian subduction on the northern edge of this ocean resulted in formation of a continental arc toli terrane and accretionary complex kulumudi terrane the karamay terrane formed as an accretionary complex during the carboniferous the ophiolitic sartuohai terrane was emplaced as melange between kulumudi and karamay terranes during the late carboniferous subduction migrated southward continuing beneath these terranes resulting in the intrusion of i type granites into the toli kulumudi sartuohai and karamay terranes these granites are closely associated with epithermal and porphyry style gold mineralization composite terranes either side of the kulumudi ocean collided in the late carboniferous marking the final consolidation of central asia collision was accompanied by anorogenic granite and diabase dyke intrusion followed by widespread latest carboniferous to permian extension and subsequently the formation of the junggar basin west junggar has been further disrupted by cenozoic strike slip faulting along junggar and dalabute faults

emplacement of the ophiolitic melanges in the west karamay area implications for the late paleozoic tectonic evolution of west junggar northwestern china abstract the darbut and karamay ophiolitic melanges in the west karamay area at the southeastern margin of west junggar southern altaids represent the relict fragments of the paleozoic junggar ocean basin the melanges contain structurally complex tectonic blocks of ultramafic rocks gabbro basalt and chert that are dispersed within an extensively sheared serpentinite matrix this study presents new field observations structural analyses geochronological and geophysical data for the melanges and the overlying carboniferous strata three stages of deformation d1 d3 have been identified d1 is only locally preserved in the melanges and is characterized by thrust imbricated faults recording the subduction and accretion of the paleo oceanic crust in the early carboniferous d2 mainly consists of map scale upright folds f1 in the carboniferous strata and ne sw striking sub vertical right lateral shear zones in the melanges d3 is characterized mainly by ne sw striking transcurrent left lateral strike slip faults and s shaped drag folds f2 the extensive tectonic reworking during d2 and d3 deformations eradicated most of the d1 structures our structural observations combined with geophysical data suggest that the basement of the west karamay area consists of trapped oceanic crust or dismembered ophiolitic fragments and that the darbut and karamay ophiolitic melanges were extruded through right lateral shear zones our results and the published data further suggest that the emplacement of the melanges and the formation of f1 folds represent regional scale amalgamation processes between different tectonic blocks and or plates in the late carboniferous these processes resulted in the closure of the west junggar remnant ocean basin and other sub oceanic basins in the paleo asian ocean leading to the formation of the kazakhstan orocline furthermore the d3 deformation was associated with anticlockwise rotation of the west junggar region in a post collisional setting in the permian

mineral chemistry of ultramafic cumulates from the north arm mountain massif of the bay of islands ophiolite evidence for high pressure crystal fractionation of oceanic basalts the mineral chemistry of ultramafic cumulates forming the basal portions of the plutonic section of the north arm mountain massif bay of islands ophiolite complex is not consistent with crystal liquid fractionation of a primitive mid ocean ridge basalt at low pressures the appearance of clinopyroxene and orthopyroxene having very high mg numbers 100 mg mg fe 85 to 92 and the absence of plagioclase as early fractionating phases coprecipitating with forsteritic olivine fo86 92 suggest that moderate to high pressure phase relationships are applicable we propose on the basis of mineral chemistry crystallization sequences and structural relationships that the basal ultramafic cumulates on the north arm massif represent the products of high pressure 10 kbar polybaric crystal fractionation of primary basaltic melts generated within the mantle beneath a mid ocean spreading center these basal ultramafic cumulates were probably precipitated due to in situ crystallization processes along the margin of a vertical conduit extending downward from a crustal level magma chamber to mantle depths that exceed 30 km after their formation the cumulates were transported to the near surface along the axis of upwelling and divergence as the lithosphere was separating beneath a mid ocean ridge

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

island arc tholeiite to boninitic melt evolution of the cretaceous kizildag turkey ophiolite model for multi stage early arc forearc magmatism in tethyan subduction factories article i nfo the kizildag ophiolite in turkey is a remnant of the late cretaceous suprasubduction zone ssz oceanic lithosphere that was developed in southern tethys and is part of a peri arabian ophiolite belt including the troodos cyprus baer bassit syria and semail oman ophiolites in the eastern mediterranean region the internal structure petrology and geochemistry of the kizildag intrusive and extrusive rocks show a magmatic progression from island arc tholeiite iat to boninitic compositions similar structural architecture and geochemical trends in the coeval troodos and baer bassit ophiolites suggest that this multi stage ssz magmatism was common in most tethyan ophiolites the sheeted dikes and the majority of extrusive rocks in kizildag are composed of basalt basaltic andesite and andesite with relatively low la smn 0 5 0 7 low mgo 7 9 wt and high tio2 0 7 1 0 wt and exhibit high dy ybn ratios these iat suites also include rare younger dacitic dikes showing strong enrichment in the lree the late stage basaltic dikes and lavas sakalavites have high la smn 0 8 1 0 high mgo 9 11 wt and low tio2 0 3 0 4 wt contents and display strong depletion in both lree and hree this suite of rocks represents the boninitic component of the kizildag ophiolite that developed from shallow and greater degrees of melting of hydrous and highly refractory peridotites beneath the extended forearc environment the progressive evolution of kizildag magmas from iat to boninitic compositions in time and space was a result of variable mixing aggregation and differentiation of geochemically different melt batches which formed in different levels polybaric within the melting column rapid slab rollback induced corner flow mantle return flow and asthenospheric diapirism providing more fertile melt directly affected the evolution of this melting column and may have triggered advanced degrees of shallow melting of the hydrous and highly depleted forearc mantle producing refractory liquids of the boninitic stage these geochemical patterns of the kizildag ophiolite reflect a multi stage evolution of ssz magmatism in tethyan subduction factories that is analogous to the earlier arc volcanism in the eocene oligocene izu bonin mariana system

an example of phanerozoic continental crustal growth the west junggar orogenic belt northwest china abstract it is generally accepted that continental crustal growth occurred principally during precambrian whereas the phanerozoic growth seems insignificant however the west junggar orogenic belt wjob a key part of central asian orogenic belt in nw china underwent significant crustal growth during phanerozoic a compilation of 683 whole rock sr nd isotopic data shows that wjob has different crustal structures and or compositions from adjacent areas all igneous rocks in wjob are characterized by depleted sr nd isotopes with initial 87sr 86sr ratios from 0 7022 to 0 7060 end t values from 3 0 to 9 2 and nd model ages from 0 9 to 0 3 ga these features are obviously different from the large variations in sr nd isotopes in adjacent tianshan orogenic belt tianshan to the south and altay orogenic belt altay to the north the continental crustal growth in wjob resulted from the transformation of pre existing juvenile crust and mantle wedge during the successively southward subduction of oceanic lithosphere beneath the ediacaran to ordovician tangbale mayile arc in southern wjob the silurian to devonian boshchekul chingiz arc and early carboniferous zharma saur arc in northern wjob followed by the addition of mantle derived materials in a post collisional setting throughout wjob during late carboniferous to permian with little or no involvement of precambrian crust such a pattern of continental crustal growth in wjob is significantly different from those of the adjacent tianshan and altay as well as the hercynian orogenic belt in french massif central where precambrian crustal materials are largely involved in thus wjob would probably be a good example of phanerozoic continental crustal growth around the world

granites in the sawuer region of the west junggar xinjiang province china geochronological and geochemical characteristics and their geodynamic significance abstract the sawuer region is located in the west junggar jimunai county of altay district and hefeng county of tacheng district xinjiang autonomous region in northwest china in the study area granitic intrusions are widespread and can be subdivided into i type granites tasite pluton sentasi pluton wokensala pluton and kaerjia pluton and a type a 2 granites kuoyitasi pluton and qiaqihai pluton the i type granites consist of granite adamelite granodiorite and the a type granites are mainly represented by alkali granite shrimp u pb dating results indicate that the granites in sawuer region were formed between 337 ma and 290 7 ma late carboniferous and early permian with the i type granites being formed between 337 ma and 302 6 ma and the a type between 297 9 and 290 7 ma from the older to the youngest granites their composition changes from calc alkalic to high k calc alkalic to alkali series and from i type to a type based on the characteristics of whole rock trace element and rare earth elements geochemistry this compositional range is the result of a change in the geodynamic regime from compressional to tensional during the late paleozoic strontium nd pb o isotope systematics suggest that both the i type and a type granites in sawuer were sourced from the mantle dm but with some crustal contamination for the kuoyitasi pluton the i type granites were formed by equilibrium partial melting while the a type granites were formed mainly by fractional crystallization based on the rb y nb and nb y ce discrimination criteria by eby eby g n 1992 chemical subdivision of the a type granitoids petrogenetic and tectonic implications geology 20 641 644 and pearce pearce j a 1996 sources and settings of granitic rocks episodes 19 120 125 and comparisons with granites in other regions of northern xinjiang the i type granites were emplaced during a transition from compressional to extensional settings in a post collision regime whereas the a type granites were emplaced in an extensional setting we concluded that the sawuer as well as the whole junggar region in northern xinjiang were involved in tectonism that gradually changed from compressional to extensional between the late carboniferous and early permian this late paleozoic granitic magmatism contributed to vertical crustal growth in the region

an early cambrian plume induced subduction initiation event within the junggar ocean insights from ophiolitic melanges arc magmatism and metamorphic rocks abstract the central asian orogenic belt caob is one of the largest accretionary orogens in the world the mechanism of continental growth and tectonic evolution of the caob remain debated here we present an overview of early paleozoic ophiolitic melanges calc alkaline intrusions and metamorphic rocks in west junggar with an aim to provide constraints on the time and mechanism of subduction initiation in the junggar ocean a branch of the southern paleo asian ocean pao the early paleozoic ophiolitic melanges are composed of ultramafic mafic rocks cherts pelagic limestones basaltic breccias and tuffs the mafic rocks from these ophiolitic melanges are divided into morb like and oib like types the morb like rocks were formed in a fore arc setting but the oib like mafic rocks were formed by the intra plate magmatism related to mantle plume activities the early paleozoic intrusions are occurred as small stocks with a dominant composition of diorite trondhjemite and granodiorite these granitoids display high k calc alkaline affinities and have high and positive end t and ehf t values formed in an arc related setting the metamorphic complex is mainly composed of blueschist and amphibolite blocks with metamorphism ages ranging from 500 ma to 460 ma their protoliths are calc alkaline andesite and tholeiitic and alkaline basalts formed in an arc related and seamount setting respectively it is clearly show that the west junggar was under an intra oceanic subduction regime during the early paleozoic and the initial subduction of the southern pao might have occurred in the early cambrian based on our observations and in combination with previous work we propose the plume induced subduction initiation model for the early middle cambrian tectonic evolutionary of the junggar ocean our new model not only shed light on subduction initiation dynamics of the southern pao but also contribute to tectonic evolution of the caob

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

genesis of post collisional granitoids and basement nature of the junggar terrane nw china nd sr isotope and trace element evidence the post collisional magmatism of the junggar terrane is characterized by intrusion of large amounts of granitoids and minor basic ultrabasic rocks the granitoids comprise two magmatic suites calc alkaline and alkaline which were emplaced contemporaneously at about 294 ma the calc alkaline rocks are typically sodium rich na2o k2o 1 1 3 5 and metaluminous a nk 1 0 a cnk 0 78 1 04 they show mildly fractionated ree patterns la yb n 15 and spidergrams with strong depletion of nb ti and to a lesser extent sr the alkaline granites have high contents of sio2 75 78 alkalis nb hree y sn f and high feo mgo ratios and huge sr and eu depletion in the spidergrams moreover they display characteristic tetrad ree patterns and non charac trace element behaviour the two rock suites have similar initial nd and sr isotopic compositions with end t in the range 5 2 to 7 1 and isr mostly in the range 0 7031 0 7041 this points to a predominance of juvenile components in their sources the calc alkaline rocks are most probably derived by dehydration melting of a basic lower crust leaving behind a granulite residue the process was probably triggered by underplating of mantle derived basic magmas in an extensional regime the alkaline granites are considered to have formed by differentiation of the calc alkaline granitoids our study argues for a juvenile continental crust for the basement of the junggar terrane which is likely dominated by early paleozoic oceanic crust and arc complex that were deeply buried during the late paleozoic subduction and accretion

as sb bi au mineralization in the baogutu gold deposit xinjiang nw china here we report the occurrence of some uncommon mineral assemblages including paakonenite aurostibite native arsenic native antimony and native bismuth found in the baogutu gold deposit in the western junggar xinjiang nw china the mineralization could be generally subdivided into two types the gold bearing quartz vein type mineralization and disseminated mineralization in the wall rocks the sulfide minerals in gold lodes commonly include pyrite arsenopyrite marcasite and stibnite however the l7 lode in no 4 orebody and the l1 lode in no 11 orebody of the baogutu gold deposit are quite different in terms of their mineral assemblages the l7 lode contains native arsenic quartz veins in shallow levels and stibnite quartz veins at depth gold bearing minerals electrum native gold and rarely aurostibite mainly coexist with paakonenite stibnite native arsenic and native antimony the crystallization of as and sb bearing minerals was likely to have consumed h2s from the hydrothermal fluid which probably triggered the precipitation of native gold the l1 lode consists of several discontinuous sulfide dominated lensoid orebodies the massive sulfide ores that produced most of the gold resource are characterized by an intimate association between native bismuth and native gold mineralization

remelting of an andesitic crust as a possible origin for rhyolitic magma in oceanic arcs an example from the izu bonin arc

elemental and nd sr isotopic geochemistry of granitoids from the west junggar foldbelt nw china with implications for phanerozoic continental growth we report elemental and nd sr isotopic data for two coeval postcollisional magmatic suites 300 ma the miaogou and karamay suites from west junggar foldbelt nw china aiming to determine their source regions and implications for continental growth the miaogou rocks monzonitic to granitic in composition show low mg 0 30 and depletion of hfse and are highly depleted in isotopic compositions with isr ranging from 0 7035 to 0 7045 end 300 ma from 8 4 to 6 6 and young nd model ages 0 37 0 70 ga that coincide with the ages of the ophiolites in the area these features suggest that the miaogou rocks were derived essentially by partial melting of a juvenile basaltic lower crust formed in the early to middle palaeozoic the karamay suite contains a variety of rock types from gabbroic diorite to granite these rocks show enrichment of lree and lile and depletion of hfse and have chemical composition e g mg 0 63 0 41 rather different from the miaogou suite though isotopically they are indistinguishable this suggests that the parental magma of the karamay suite originated from melting of a young lithospheric mantle that had previously been metasomatized during the palaeozoic subduction the west junggar magmatic rocks represent production of juvenile continental crust in the phanerozoic triggered by upwelling of asthenophere in an extensional regime the basement beneath the area is dominated by trapped palaeozoic arc series and oceanic crust

early paleozoic tectonic evolution of the northern west junggar nw china constraints from early cambrian middle silurian felsic plutons of the chagantaolegai ophiolitic melange abstract the early paleozoic tectonic evolution of the junggar ocean a major branch of the southern paleo asian ocean pao remains a topic of debate this study mapped the chagantaolegai ophiolitic melange com in the northern west junggar nwj on a large scale and identified several tectonically juxtaposed lithotectonic units including the ophiolite suite and felsic plutons the ophiolite suite mainly comprises serpentinized ultramafic rock pyroxenolite gabbro dolerite plagiogranite basalt volcanic rock and radiolarian chert zircon u pb ages for two plagiogranite samples yielded ages of 515 4 ma and 513 6 ma constraining the existence of the junggar ocean to the early cambrian at least the felsic plutons can be subdivided into two groups group i 503 481 ma which intruded into the com shows low k calc alkaline features with remarkable depletion of nb ta and ti resembling rocks formed in an intra oceanic arc group ii 435 428 ma was found on both sides of the com and displays high k calc alkaline series characteristics similar to the silurian to early devonian a type granites in the nwj implying a post collisional environment based on a combination of existing observations and our new observations and data with regional geological evidence our new data indicate that the nwj underwent a transition from the tectonic processes of seafloor spreading and subduction to arc arc collision and post collisional extension during the early cambrian early devonian