provenance and tectonic settings of permian turbidites from the beishan mountains nw china implications for the late paleozoic accretionary tectonics of the southern altaids the beishan orogenic belt which connects the tianshan suture on the west and solonker suture on the east contains key evidence for the termination time of the southern altaids critical for evaluating different controversial tectonic models are permian marine volcaniclastic arenites in the liuyuan and heishankou areas which are dominated by greywackes and lenticular pebbly litharenites that contain grading groove marks and erosional bases which provide evidence of turbidity action sandstones from the liuyuan section are dominated by angular basaltic andesitic and feldspar fragments but sandstones from the heishankou section mainly consist of andesitic and felsic volcanic fragments these relations suggest derivation from two different sources major element compositions suggest that the source rocks of the heishankou litharenites were more sio2 rich than those at liuyuan sandstones at heishankou are characterized by lower ni co cr v and slightly higher th and la contents than those at liuyuan this indicates that the litharenites in the liuyuan and heishankou areas were derived from intermediate mafic and intermediate felsic source rocks respectively tectonic setting discrimination plots suggest that the liuyuan sandstones were deposited as detritus from an oceanic island arc but the heishankou sediments from an andean continental margin our petrological and geochemical data from these two types of permian turbidites suggest that an arc continent collision took place in the early permian and this is consistent with the permian termination of the southern altaids

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

petrogenesis and tectonic implications of late devonian arc volcanic rocks in southern beishan orogen nw china geochemical and nd sr hf isotopic constraints abstract late devonian ca 370 ma volcanic rocks provide important information about the nature of magmatism during the tectonic transition between the early and late paleozoic in the beishan orogen southern central asian orogenic belt they are predominantly an andesitic dacitic rhyolitic assemblage characterized by alkali contents ranging from slightly calcic to slightly alkaline the rhyolitic rocks are generally ferroan whereas the andesitic rocks are magnesian these volcanic rocks exhibit similar trace element characteristics to those of continental arcs strongly negative e nd t values 2 8 to 3 6 and high sr isotopic compositions initial 87 sr 86 sr 0 7036 0 7108 suggest that they are mainly derived from an ancient crust however the positive zircon e hf t values 1 4 to 16 4 support the role of juvenile components in their genesis indicating the significant input of new mantle derived magmas these characteristics imply a hybrid derivation from an ancient crustal source with the addition of juvenile materials during magma genesis or perhaps heterogeneous contamination or hybridization during magma emplacement combined with the regional geology our results indicate that the late devonian magmatism resulted from a southward retreat of the subduction zone which records significant continental crustal growth in a transitional arc or an accretionary arc setting the distinct geochemical compositions especially the nd hf isotope decoupling of the dundunshan volcanic rocks imply a significant change in the geodynamic setting in the late paleozoic

dynamic processes from plate subduction to intracontinental deformation insights from the tectono sedimentary evolution of the zhaosu tekesi depression in the southwestern chinese tianshan abstract the zhaosu tekesi depression located to the northern margin of the south tianshan orogenic belt stb is an ideal region for investigating paleozoic orogenic processes and later modifications of the southwestern chinese tianshan in this study we employ 2d seismic and borehole data revealing the geological structure and tectonic evolution of the zhaosu tekesi basin and we report new zircon u pb geochronological and hf isotopic data and whole rock elemental and sr nd isotopic analytical results of the late silurian magmatic rocks as well as the whole rock trace element data of the early carboniferous carbonate rocks borehole data wells zc1 and zc2 and seismic imaging suggest that the zhaosu tekesi depression is composed of silurian lower carboniferous permian jurassic upper cretaceous and paleogene strata with six unconformity surfaces c 1 a s 3 p 1 c 1 a p 2 t p 1 j p 2 t k 2 d j and e k 2 d the silurian strata exhibit southward thrust structures and consist of basalt andesite dacite tuff minor muddy limestone and diorite intrusions zircon shrimp u pb dating for the well zc1 diorites yield a crystallization age of 422 ma with positive e hf t values 3 3 10 6 the diorites and andesites are calc alkaline and show enrichment in liles such as th and u and depletion in nb ta and ti relatively high initial 87 sr 86 sr ratios 0 703376 0 706737 and low e nd t values 0 5 to 1 4 and high th yb 0 29 0 35 and ta yb 1 31 1 58 ratios indicating their formation in an active continental margin setting the lower carboniferous strata mainly comprise oolitic limestone bioclastic micritic limestone and interbedded calcareous siltstone high sr ba 1 31 2 33 v sc 10 4 18 6 and low th u 0 01 0 23 ratios suggest that they were potentially deposited in the mixing near shore marine environment these results in combination with previous data indicate that the southwestern chinese tianshan is controlled by northward subduction of the south tianshan ocean during the late silurian to early carboniferous the presence of the lower permian terrigenous strata and rift basin structure constrains the timing of collision between the ctb kyb and the tarim craton as the late carboniferous thrusting reactivation and syn tectonic growth strata in the basin suggest that the southwestern chinese tianshan experienced late cretaceous compression and neogene modifications after the late permian post faulting thermal subsidence triassic peneplanation and jurassic regional subsidence intracontinental subduction due to distant effect is the main tectonic control on the miocene rapid uplift of the tianshan the zhaosu tekesi basin evolution sheds light on dynamic processes of the southwestern chinese tianshan from plate subduction to intracontinental deformation

re os molybdenite age of the cu mo skarn ore deposit at suoerkuduke in east junggar nw china and its geological significance the suoerkuduke cu mo ore is a medium sized skarn deposit in the east junggar orogen of the southern altaids xinjiang northwest china we measured re os isotopes of a molybdenite veinlet from the

electrical resistivity structure of the upper mantle beneath northeastern china implications for rheology and the mechanism of craton destruction abstract the north china craton ncc and central asian orogen belt caob in northeastern china experienced a range of tectonic events during the phanerozoic dominated by lithospheric thinning of the eastern ncc in the late mesozoic and cenozoic in order to better understand the tectonic evolution of the ncc and the caob new broadband and long period magnetotelluric data were collected along a north west to south east trending profile that extended from the caob across the yanshan belt the tanlu fault zone to the liaodong peninsula a two dimensional 2 d resistivity model was derived from inversion of the transverse electric mode transverse magnetic mode and vertical magnetic field data in the crust of the caob the resistivity model shows a northwest dipping low resistivity zone beneath the solonker suture that is identified as the suture zone formed by the collision between the siberian and north china cratons the upper mantle of the caob is characterized by moderate resistivity values 300 1000 m that are best explained by the presence of hydrogen dissolved in olivine the water concentration of the caob mantle is comparable to values reported for the asthenosphere and cratons that have been significantly hydrated the ncc upper mantle is generally lower in resistivity than the caob upper mantle and a zone of lower resistivity is observed in the upper mantle at the southeast end of the profile beneath the ncc superimposed on this southeast decrease in upper mantle resistivity three low resistivity zones were imaged 1 below the xilamulun fault 2 close to the north south gravity lineament and 3 between the northern yanshan belt and tanlu fault zone the low resistivities can be explained as regions of partial melts or fluids perhaps caused by asthenospheric upwelling together with seismic imaging results and geochemical data the resistivity model shows that the modification of the lithosphere associated with craton destruction has occurred in a spatially non uniform manner in the region of the ncc investigated in this study

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

chemically induced grain boundary migration in calcite temperature dependence phenomenology and possible applications to geologic systems chemically induced grain boundary migration cigm is a solid state reaction mechanism which involves grain boundary migration initially straight grain boundaries in calcite bicrystals exhibit cigm when exposed to a srco3 or baco3 rich melt at temperatures of 680 840 c in a 1 bar co2 atmosphere under these conditions cigm is apparently a much more efficient way to form solid solutions than lattice diffusion the maximum observed rate of migration 1 10 9 m s occurred at about 760 c below that temperature migration was thermally activated but above it the process was thermally inhibited some portions of the boundary swept through a given crystal volume more than once in the ca sr co3 system the solute concentrations incorporated during a single pass of the boundary were far less than values expected at chemical equilibrium and departed further from equilibrium at higher temperatures cigm may be geologically important when the distances characteristic of grain boundary migration and diffusion are much larger than those characteristic of lattice diffusion the great similarity between results in ionic calcite and metals systems suggests cigm is a very general process which may be expected to occur in a large number of geologic systems

influence of stress and strain on dolomite rim growth a comparative study triaxial compression and torsion experiments were performed to investigate the influence of non isostatic stress and strain on dolomite reaction rim growth using orientated natural calcite and magnesite single crystals at a temperature of 750 c 400 mpa confining pressure stresses between 7 and 38 mpa and test durations up to 171 h reaction products were composed of a polycrystalline magnesio calcite layer palisade shaped dolomite and granular dolomite grains in all experiments inelastic deformation was partitioned into calcite and reaction products while magnesite remained undeformed calcite deformed by twinning and dislocation creep where the activation of additional glide systems at high stress allowed high strain depending on grain size magnesio calcite deformed by diffusion creep and or grain boundary sliding twinning and dislocation creep dolomite deformed mainly by diffusion creep assisted by enhanced dislocation activity allowing ca enrichment in the granular rim a weak crystallographic preferred orientation of the reaction products was observed in triaxial compression dolomite rim growth was diffusion controlled and showed no influence of axial stresses up to 38 mpa on the reaction kinetics at high strain 0 1 the magnesio calcite layer is wider suggesting faster growth kinetics this may be related to additional diffusion pathways provided by enhanced dislocation activity at very high strain 0 3 0 6 twisted samples showed a gradual decrease in layer thickness of dolomite and magnesio calcite with increasing strain rate

exhumation and preservation of porphyry cu au deposits at oyu tolgoi south gobi region mongolia field relationships and u pb data zircon are used to constrain the timing of tectonomagmatic events that occurred shortly after the formation of late devonian 372 ma porphyry cu au deposits at oyu tolgoi mongolia in the two northernmost of the several discrete porphyry centers hugo dummett deposits upper devonian rocks positioned stratigraphically above copper gold mineralization preserve a record of uplift exhumation and subsequent burial shortly after mineralization strongly altered and mineralized quartz monzodiorite and volcanic rocks at hugo dummett are overlain by an unmineralized dacite sequence 369 2 ma the basal breccia and conglomerate of the dacite sequence contain mineralized porphyry clasts derived from erosion of the porphyry deposits two zircon grains recovered from one of these mineralized porphyry clasts yielded chemical abrasion thermal ionization mass spectrometry ca tims 206 pb 238 u dates of 374 0 1 9 and 371 4 1 6 ma which are consistent with previously published ages for mineralized intrusions this implies that oyu tolgoi or other nearby porphyry deposits were exhumed shortly after the main mineralization event weakly mineralized granodiorite dikes intruded the postmineralization dacite sequence as well as devonian sedimentary and volcaniclastic rocks that structurally overlie the dacite a weighted mean 206 pb 238 u age of 370 6 0 5 ma for this rock is based on the three youngest grains analyzed by the ca tims technique mswd 1 14 burial of the cu au deposits at oyu tolgoi soon after their exhumation was critical to their preservation exhumation and subsequent burial occurred at 371 ma during a short time frame within the overlapping u pb zircon ages for the quartz monzodiorite mineralized porphyry clast unmineralized dacite sequence and granodiorite unit

dynamic recrystallization and chemical evolution of clinoamphibole from senja norway clinoamphibole from a mylonitic amphibolite exhibits microstructures characteristic of dynamic recrystallization including porphyroclasts in a finer grained matrix of needle shaped amphibole the matrix amphibole defines an ls fabric and porphyroclasts have core and mantle structures with a core containing undulose to patchy extinction and 100 deformation twinning surrounded by a mantle of recrystallized grains in addition intragranular grains also occur within the cores tem analyses of the porphyroclasts reveal that they contain a wide variety of lattice defects including high densities 5 108cm 2 of free dislocations and dislocation arrays dissociated dislocations stacking faults and 100 micro twins tem also shows that matrix grains and intragranular grains have relatively low defect densities and that the intragranular new grains occur at localities in the porphyroclasts characterized by high densities of dislocations these observations along with the chemical and orientation relationships between the recrystallized grains and porphyroclasts indicate that the new grains may have formed by heterogeneous nucleation and that further growth probably occurred by both strain assisted and chemically induced grain boundary migration or liquid film migration this recrystallization event is interpreted to be synkinematic based on the fact that no recrystallization textures are present in the matrix grains and that the matrix grains define an ls fabric however the low defect densities in the matrix grains and the lack of intracrystalline strain in other phases indicate that post kinematic recovery processes were active

late paleozoic arc magmatism in the southern yili block nw china insights to the geodynamic evolution of the balkhash yili continental margin central asian orogenic belt abstract the yili block represents the easternmost segment of the kazakhstan yili microcontinent late paleozoic magmatic rocks are extensively exposed in the yili block and are essential in understanding the tectonic development of the sw central asia orogenic belt caob we conducted field investigations zircon u pb dating whole rock geochemical and sr nd isotopic studies on the magmatic rocks from the wusun range southern yili block the late paleozoic volcanic and plutonic rocks of the wusun range show geochemical features consistent with that of calc alkaline series and they have positive end t values 0 9 to 5 6 neo to mesoproterozoic single stage nd model ages 0 80 1 33 ga and variable initial 87 sr 86 sr ratios 0 7041 0 7059 suggesting that these magmatic rocks are likely derived from partial melting of a moderately depleted mantle wedge with minor involvement of a precambrian continental crust these new data combined with previously published results put constraints on the late paleozoic magmatic and geodynamic evolution of the balkhash yili active continental margin the late paleozoic arc magmatism initiated from 386 ma along the northern yili block and migrated southward to have occurred since 340 ma along its southern margin subsequently the arc related magmatism moved back northward and terminated at 310 ma in the south and at 295 ma in the north the oib like geochemical features of some volcanic rocks indicate incipient marginal rifting occurred along the southern yili block during late carboniferous taking into account the regional structural and tectonic data the carboniferous yili magmatic arc may be laterally correlated with the balkhash yili active margin of the kazakhstan microcontinent which was likely resulted from the southward subduction of the junggar north tianshan ocean

aspects of the tectonic evolution of china the subject of this special publication is one of the most interesting in global geoscience the tectonic evolution of china the assemblage of terranes that underlie this part of the world provides outstanding opportunities to elucidate global processes and many of the factors that shape the earth s lithosphere are best exemplified by the geology of china and its immediately adjacent areas in addition there are geological features that are particular and unique to the region some have been the focus of recent attention and have attracted international interest because of their global importance this volume provides accounts of up to date research by chinese and international geological teams on key aspects of the tectonic evolution of china and its surrounding areas the papers describe the formation of the geological terranes that make up this part of east asia place constraints on plate tectonic models for their assembly and provide accounts of unique geological feature of the subcontinent

the effect of deformation on the titaniq geothermobarometer an experimental study we performed high strain up to 47 axial compression experiments on natural quartz single crystals with added rutile powder tio2 and 0 2 wt h2o to investigate the effects of deformation on the titanium in quartz titaniq geothermobarometer one of the objectives was to study the relationships between different deformation mechanisms and incorporation of ti into recrystallized quartz grains experiments were performed in a griggs type solid medium deformation apparatus at confining pressures of 1 0 1 5 gpa and temperatures of 800 1 000 c at constant strain rates of 1 10 6 or 1 10 7 s 1 mobility of ti in the fluid phase and saturation of rutile at grain boundaries during the deformation experiments are indicated by precipitation of secondary rutile in cracks and along the grain boundaries of newly recrystallized quartz grains microstructural analysis by light and scanning electron microscopy the latter including electron backscatter diffraction mapping of grain misorientations shows that the strongly deformed quartz single crystals contain a wide variety of deformation microstructures and shows evidence for subgrain rotation sgr and grain boundary migration recrystallization gbmr in addition substantial grain growth occurred in annealing experiments after deformation the gbmr and grain growth are evidence of moving grain boundaries a microstructure favored by high temperatures electron microprobe analysis shows no significant increase in ti content in recrystallized quartz grains formed by sgr or by gbmr nor in grains grown by annealing this result indicates that neither sgr nor moving grain boundaries during gbmr and grain growth are adequate processes to facilitate re equilibration of the ti content in experimentally deformed quartz crystals at the investigated conditions more generally our results suggest that exchange of ti in quartz at low h2o contents which may be realistic for natural deformation conditions is still not fully understood thus the application of the titaniq geothermobarometer to deformed metamorphic rocks at low fluid contents may not be as straightforward as previously thought and requires further research

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

continental growth of northwest china the northwest corner of china is the most interior part of central asia and includes parts of the siberian tarim and kazakhstan plates where the junggar and tarim basins are separated by the east west trending tianshan range a preliminary terrane map divides the area into thirteen separate areas which are classified into four types l continental 2 oceanic crust 3 island arc 4 composite the boundaries for these terranes are faults that have been previously defined either by geologic mapping or holocene activity passive consolidation of the southern margin of paleo asia is marked by accretion and subduction of the paleo tethys oceanic basins and by development of volcanic arcs paleomagnetic data indicate that the major plates and associated smaller terranes did not reach final consolidation until the permian or later the stratigraphic record reveals development of intracontinental basins in the late carboniferous followed by the pan asian thermal event that gave rise to widespread intrusion of a type granites of late paleozoic age the frontal collision of india along the southern border of asia in the paleocene reactivated nearly all of the older major strike slip and thrust faults formed during the late paleozoic consolidation of paleo asia the complex nature of the continental growth of central asia has produced and destroyed many varieties of sutures and reconstruction of the original plate and terrane configurations therefore is enigmatic

geochronology and geochemistry of basalts from the karamay ophiolitic melange in west junggar nw china implications for devonian carboniferous intra oceanic accretionary tectonics of the southern altaids new laser ablation inductively coupled plasma mass spectrometry la icp ms u pb zircon ages and geochemical data are reported for the karamay ophiolitic melange in the west junggar area at the southwestern margin of the altaids or central asian orogenic belt northwestern china the data were used to identify the petrogenesis of the melange and to evaluate its tectonic significance in addition this study aimed to assess the possible presence of devonian mantle plume in the region the karamay ophiolitic melange crops out at the boundary of the junggar basin but is partly hidden by mesozoic sediments with an exposure area of 40 km 2 the site contains ultramafic rocks cumulates gabbros pillow lavas abyssal radiolarian cherts turbidites and tuffs which show typical block in matrix structures zircon u pb analyses from the basalt and gabbro by la icp ms yielded weighted mean ages of 395 3 ma and 387 8 ma respectively these ages suggest a middle devonian emplacement all basalts bear the signature of ocean island basalt oib and are characterized by alkaline compositions with high concentrations of na 2 o k 2 o 3 7 8 5 wt and tio 2 1 5 3 1 wt large ion lithophile element and light rare earth element enrichment and heavy rare earth element depletion very weak or no eu anomalies eu eu 0 9 1 0 and no obvious nb ta or ti negative anomalies we propose that these basaltic rocks were derived from mantle plume related magmatism associated with the evolution of the paleoasian oceanic system the enriched mantle source could have contained 2 5 garnet and 3 spinel the rocks also display strong geochemical similarities with the xigaze seamount basalts which formed in intra oceanic settings compared to the basalts the gabbros display mid ocean ridge basalt type tholeiitic basalt features derived from a depleted mantle source with the addition of fluids from a subducted slab within a suprasubduction zone environment these observations are supported by previous work and they indicate devonian mantle plume related magmatism within the paleoasian ocean however as these rocks are mainly parts of accretionary complexes whether the karamay and darbut ophiolitic melange formed in a single belt remains equivocal and further work is required to resolve this issue thus there was a complex evolution by subduction accretion processes from the devonian to the carboniferous before final amalgamation and docking to the northern siberian block

palaeozoic accretionary and convergent tectonics of the southern altaids implications for the growth of central asia the southern altaids present a unidirectional section from mongolia to china through an accretionary orogen that youngs progressively from neoproterozoic in the north to permian in the south the orogen formed by forearc accretion of island arcs accretionary wedges ophiolites and precambrian microcontinents this regularity was upset by early growth within the ocean of arcs that later collided at the accreting continental margin by imbrication of old ophiolites with young arcs and by himalayan style thrust nappe tectonics when an arc collided into a microcontinent lateral growth of the southern altaids represents a massive addition of juvenile material to the palaeozoic crust

geochemical and geochronological studies of granitoid rocks from the western tianshan orogen implications for continental growth in the southwestern central asian orogenic belt abstract the western tianshan orogen in northwestern china is part of the southwestern margin of the central asian orogenic belt caob which represents the largest tectonic assembly of accretionary complexes and the most significant area of phanerozoic continental growth in the world granitoid rocks have a widespread occurrence and occupy about 30 of the whole exposure of the orogen ages obtained for the granitoids vary from 896 ma to 247 ma while initial 87 sr 86 sr ratios vary from 0 70329 to 0 72070 and e nd values from 14 1 to 7 3 the occurrence of neoproterozoic granitic gneisses implies the presence of precambrian basement in this area during oceanic subduction continental growth occurred as a result of several combined processes i e by addition of oceanic crustal melts adakites the intrusion of basaltic magmas derived by partial melting of the metasomatized depleted mantle wedge and the upwelling of granitic magma derived from a mixed source of basaltic magmas and old continental basement however during the post collisional period vertical accretion of underplated juvenile mantle material may have been accomplished during slab breakoff delamination this study suggests a two stage model of continental growth of syn subduction lateral accretion of arc complexes and post collisional vertical accretion of underplated mantle material which may also be relevant for the western segment of the caob in the phanerozoic the reworking and involvement of old crust are more obvious in the western tianshan orogen compared to other parts of the caob comprising the western junggar eastern junggar alataw and altay terranes

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

geochronology and geochemistry of basaltic rocks from the sartuohai ophiolitic melange nw china implications for a devonian mantle plume within the junggar ocean abstract the west junggar domain in nw china is a distinct tectonic unit of the central asian orogenic belt caob it is composed of paleozoic ophiolitic melanges arcs and accretionary complexes the sartuohai ophiolitic melange in the eastern west junggar forms the northeastern part of the darbut ophiolitic melange which contains serpentinized harzburgite pyroxenite dunite cumulate pillow lava abyssal radiolarian chert and podiform chromite overlain by the early carboniferous volcano sedimentary rocks in this paper we report new geochronological and geochemical data from basaltic and gabbroic blocks embedded within the sartuohai ophiolitic melange to assess the possible presence of a devonian mantle plume in the west junggar and evaluate the petrogenesis and implications for understanding of the paleozoic continental accretion of caob zircon u pb analyses from the alkali basalt and gabbro by laser ablation inductively coupled plasma mass spectrometry yielded weighted mean ages of 375 2 ma and 368 11 ma geochemically the sartuohai ophiolitic melange includes at least two distinct magmatic units 1 a late devonian fragmented ophiolite which were produced by ca 2 10 spinel lherzolite partial melting in arc related setting and 2 contemporary alkali lavas which were derived from 5 to 10 garnet minor spinel lherzolite partial melting in an oceanic plateau or a seamount based on detailed zircon u pb dating and geochemical data for basalts and gabbros from the sartuohai ophiolitic melange in combination with previous work indicate a complex evolution by subduction accretion processes from the devonian to the carboniferous furthermore the alkali basalts from the sartuohai ophiolitic melange might be correlated to a devonian mantle plume related magmatism within the junggar ocean if the plume model as proposed here is correct it would suggest that mantle plume activity significantly contributed to the crustal growth in the caob

origin and accumulation mechanisms of petroleum in the carboniferous volcanic rocks of the kebai fault zone western junggar basin china abstract the kebai fault zone of the west junggar basin in northwestern china is a unique region to gain insights on the formation of large scale petroleum reservoirs in volcanic rocks of the western central asian orogenic belt carboniferous volcanic rocks are widespread in the kebai fault zone and consist of basalt basaltic andesite andesite tuff volcanic breccia sandy conglomerate and metamorphic rocks the volcanic oil reservoirs are characterized by multiple sources and multi stage charge and filling history characteristic of a complex petroleum system geochemical analysis of the reservoir oil hydrocarbon inclusions and source rocks associated with these volcanic rocks was conducted to better constrain the oil source the petroleum filling history and the dominant mechanisms controlling the petroleum accumulation reservoir oil geochemistry indicates that the oil contained in the carboniferous volcanic rocks of the kebai fault zone is a mixture the oil is primarily derived from the source rock of the permian fengcheng formation p 1 f and secondarily from the permian lower wuerhe formation p 2 w compared with the p 2 w source rock p 1 f exhibits lower values of c 19 tt c 23 tt c 19 20 tt tt ts ts tm and 20r sterane c 27 c 28 ratios but higher values of tt c 23 c 21 hhi gammacerane c 30 hopane hopane 20s c 34 c 33 c 29 and c 29 20s 20s 20r ratios three major stages of oil charge occurred in the carboniferous in the middle triassic late triassic to early jurassic and in the middle jurassic to late jurassic periods respectively most of the oil charged during the first stage was lost while moderately and highly mature oils were generated and accumulated during the second and third stages oil migration and accumulation in the large scale stratigraphic reservoir was primarily controlled by the top carboniferous unconformity with better porosity and high oil enrichment developed near the unconformity secondary dissolution pores and fractures are the two major reservoir storage space types in the reservoirs structural highs and reservoirs near the unconformity are two favorable oil accumulation places the recognition of the large scale carboniferous volcanic reservoirs in the kebai fault zone and understanding of the associated petroleum accumulation mechanisms provide new insights for exploring various types of volcanic reservoir plays in old volcanic provinces and will undoubtedly encourage future oil and gas exploration of deeper strata in the region and basins elsewhere with similar settings

late carboniferous high end t ehf t granitoids enclaves and dikes in western junggar nw china ridge subduction related magmatism and crustal growth abstract we report results of petrologic geochronological and geochemical investigation of the late carboniferous diorites granodiorites amphibole am bearing granites and associated dioritic and monzonitic enclaves and mafic and granitic dikes in the keramay area of the western junggar region of central asian orogenic belt caob zircon u pb dating suggests that they were generated in the late carboniferous 316 304 ma the diorite and granodiorite compositions extend over a wide range of sio2 53 70 wt sr 240 602 ppm and mg 41 58 values and are characterized by moderately fractionated rare earth element ree patterns nb ta depletion and relatively low y and yb contents the mafic dikes consist of dolerites diorite porphyries and minor granodiorite porphyries and have variable sio2 51 59 wt and high mg cr and ni values with the exception of two samples with relatively high heavy ree hree contents the mafic dikes exhibit trace element characteristics similar to diorites and granodiorites the am bearing granites and a granite porphyry dike sample have high levels of sio2 73 77 wt hrees e g yb 3 46 15 7 ppm and low mg cr and ni contents along with clearly negative eu ba and sr anomalies similar to typical a type granites all granitoids enclaves and dikes in this region have high positive end t 7 13 to 9 74 and zircon ehf t 10 to 16 values and moderate initial 87sr 87sr ratios 0 7004 0 7049 mineral composition data suggest that the parental magmas for mafic dikes are similar to cenozoic sanukitoids in the setouchi arc area japan and were possibly generated under water rich and high oxygen fugacity nno 1 5 to nno 2 7 conditions they most likely originated from partial melting of a mantle source variably modified by subducted oceanic crust derived melts and minor fluids and subsequently underwent fractional crystallization the diorites and granodiorites were possibly generated by magma mixing between enriched lithospheric mantle and juvenile lower crust derived magmas coupled with minor crystal fractionation the am bearing granites and granite porphyry dike were produced by partial melting of juvenile crustal materials at shallow crustal levels taking into account widespread contemporaneous magmatism including morb type basalts and slab derived adakites in western junggar we suggest that the keramay intrusive rocks were generated in a special arc setting related to ridge subduction and resultant slab window which played an important role in the crustal growth of the caob

late paleozoic magmatic record of east junggar nw china and its significance implication from zircon u pb dating and hf isotope abstract late paleozoic magmatic rocks including basic dykes basaltic andesite rhyolite keratophyre and syenite porphyry with minor tuff are widespread in the western margin of the east junggar terrane in situ zircon u pb dating and hf isotope analyses were carried out for these magmatic rocks from the baijiangou and zhangpenggou localities of east junggar integrating with geochemical data to investigate their tectonic evolution as well as crustal accretionary process of this region in the late paleozoic inherited zircons from basic dykes range in age from 435 ma to 300 ma the zhangpenggou rhyolite and keratophyre have typical arc like geochemical signatures and were formed in the early carboniferous 332 ma and 336 ma respectively suggesting they are products of subduction related magmatism the baijiangou rhyolites were formed in the late carboniferous 315 ma and 323 ma and their formation ages are similar to those of the syenite porphyries 307 ma and 312 ma the hf model ages and the formation ages of zircons from these magmatic rocks are alike with positive e hf t values vary from 0 7 to 16 6 implying that voluminous growth of juvenile crust happened in the east junggar terrane during the late paleozoic the absence of precambrian inherited zircons in basic dykes indicates the lack of precambrian basement beneath the east junggar terrane taking geochronological studies on regional ophiolites into account the east junggar terrane is considered as a devonian carboniferous oceanic island arc which has been continuously accreted to the southern active margin of the siberian craton since the early carboniferous

paleozoic crustal growth and metallogeny of central asia evidence from magmatic hydrothermal ore systems of central kazakhstan abstract the ore deposit spectrum of central kazakhstan reflects a range of lithospheric magmatism controlled by the polycyclic geodynamic evolution of an active continental margin which develops from back arc oceanic settings volcanic hosted massive sulphide cu au ore deposits to subduction controlled calcalkaline magmatism copper porphyries with subsequent stages of crust differentiation of low degree partial melting and extended intramagmatic fractionation molybdenum porphyries on to continental rifting peralkaline ree zr nb rich systems mesothermal gold deposits mark transcrustal shear zone controlled fluid conduits formed during arc slicing and thermal relaxation in a post orogenic setting the ore deposits characterize successive and spatially overlapping stages of crustal growth and consolidation the general subduction setting of this range of ore deposits is reflected by calc alkaline trends in the associated subalkaline high k felsic rocks with a wide range in silica contents the predominant metal spectrum of au cu and mo indicates relatively oxidized magmatic hydrothermal systems in accordance with the metaluminous i type nature of their igneous host rocks i e oxidized igneous precursor material despite of presumably highly variable ratios of mantle crust components in individual magmatic systems gabbros to leucogranites most rocks have very similar positive initial e nd values of 0 to 5 5 and depleted mantle model ages in the range of 500 800 ma the ubiquitous relatively young mantle extraction age is likely to characterize the lower crust of central kazakhstan this basement is interpreted as back arc oceanic crust that formed behind oceanwards drifting continental margin slivers during the initial stages of the active continental margin evolution of the late precambrian angara baltica supercontinent the early back arc oceanic crust was transformed into the present lower crust during the paleozoic orogenic evolution in which the back arc crust was buried by magmatic arc and sedimentary material and recurrently affected by high grade metamorphism basaltic melt injection underplating and granitic melt extraction anorogenic rift related permian peralkaline riebeckite granites with ree zr nb mineralization have very high positive e nd of 5 to 8 which is probably inherited from young subcrustal lithosphere at the mantle crust boundary that was affected by partial remelting due to asthenospheric upwelling during rifting

fast diffusion along mobile grain boundaries in calcite experimental measurements of grain boundary diffusion are usually conducted on static boundaries despite the fact that grain boundaries deep in the earth are frequently mobile in order to explore the possible effect of boundary mobility on grain boundary diffusion rates we have measured the uptake of 44ca from a layer of 44ca enriched calcite powder during the static recrystallization of a single crystal of calcite at 900 c a region about 500 m wide adjacent to the powder layer is heterogeneously enriched in 44ca and complex zoning patterns including sharp steps in composition and continuous increases and decreases in 44ca content are developed in metamorphic rocks these would normally be interpreted in terms of changes in pressure or temperature rayleigh fractionation or episodic fluid infiltration these explanations cannot apply to our experiments and instead the zoning patterns are interpreted as being due to variations in grain boundary migration rate we have applied an analytical model which allows the product of grain boundary diffusion coefficient and grain boundary width dgb to be calculated from the grain boundary migration rate and the compositional gradient away from the powder layer the value of dgb in the mobile grain boundaries is at least five orders of magnitude greater than the published value for static boundaries under the same conditions in order to allow the scale of chemical equilibrium and hence textural evolution to be predicted under both experimental and geological conditions we present quantitative diffusion regime maps for static and mobile boundaries in calcite using both published values and our new values for grain boundary diffusion in mobile boundaries enhanced diffusion in mobile boundaries has wide implications for the high temperature rheology of earth materials for geochronology and for interpretations of the length and time scales of chemical mass transport moreover zones of anomalously high electrical conductivity in the crust and mantle could be regions undergoing recrystallization such as active shear zones rather than regions of anomalous mineralogy water or melt content as is generally suggested

study of the lithospheric and upper mantle discontinuities beneath eastern asia by ss precursors summary we analyse broad bandss waveform data recorded by several networks in europe with sources mainly in the west pacific to study the underside reflections of teleseismic ss waves in the lithosphere and the upper mantle beneath eastern asia and the nw pacific ocean ss bounce points sample a corridor from the aleutian kamchatka and japan subduction zones through the north china craton and central asian orogenic belt to the tibetan plateau the corridor passes through different tectonic units such as subduction zones an old continental shield a fold belt and a high plateau we investigate the seismic structure of the lithosphere and the mantle transition zone beneath the different geotectonic units along the profile and infer the correlation of geodynamic processes at different depths we explore the short period frequency content in the ss waveform data and use moveout correction and common midpoint stack to acquire profiles with high lateral and depth resolution from the crust to the mantle transition zone clear ss precursors of the 410 and 660 km discontinuities show the effects of the interaction between the subducted oceanic lithosphere and the mantle transition zone beneath the nw pacific subduction zones a low velocity layer has also been detected beneath the 410 km discontinuity and can be traced along the entire profile due to the improved resolution acquired by the method presented here we have been able to study the shallower structures such as the moho and the lithosphere asthenosphere boundary by ss precursors the continental moho can be clearly seen along this corridor the depth variation agrees well with earlier receiver function results we also see negative reflectors along the profile at varying depths which can be interpreted as the lithosphere asthenosphere boundary

accretionary growth and crust formation in the central asian orogenic belt and comparison with the arabian nubian shield kroner a windley b f badarch g tomurtogoo o hegner e jahn b m gruschka s khain e v demoux a and wingate m t d 2007 accretionary growth and crust formation in the central asian orogenic belt and comparison with the arabian nubian shield in hatcher r d jr carlson m p mcbride j h and martinez catalan j r eds 4 d framework of continental crust geological society of america memoir 200 p 181 209 doi 10 1130 2007 1200 11 for permission to copy contact editing geosociety org 2007 the geological society of america all rights reserved kroner kroener mail uni mainz de windley brian windley btinternet com tomurtogoo igmr magicnet mn hegner hegner lmu de jahn jahn earth sinica edu tw khain khain ginras ru demoux demoux uni mainz de wingate mwingate tsrc uwa edu au

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

genesis of permian granites along the kangguer shear zone jueluotage area northwest china geological and geochemical evidence abstract permian mafic and felsic intrusions occur along the kangguer shear zone ksz in the jueluotage area eastern tianshan northwest china this study focuses on the permian felsic intrusions 288 276 2 ma systematic geological and geochemical investigations show that granite plutons in the western ksz dikan guandao and hongshi have lower al and higher alkali contents than those in the eastern ksz longdong huangshan and baishandong the western granite plutons experienced higher degrees of crystal fractionation than the eastern granite plutons both of the western and eastern permian granite intrusions along ksz were derived from juvenile crust the nd pb isotopic characteristics of which exhibit mixed compositions of dm and emii type mantles both of the western and eastern permian granites along ksz were formed within post collision background the dikan guandao and hongshi plutons in the west ksz sector were formed in an extensional tectonic setting whereas the longdong huangshan and baishandong plutons in the east ksz formed in a compressional tectonic setting

diffusion induced grain boundary migration in calcite grain boundaries in synthetic calcite bicrystals and slabs of yule marble exposed to srco3 at temperatures between 650 and 800 c undergo diffusion induced grain boundary migration digm as the boundary migrates through the crystal it produces a ca sr co3 solid solution digm is known to occur in metals at temperatures as low as one third the melting temperature where lattice diffusion rates are insignificant compared to grain boundary diffusion rates the similarity between microstructures in calcite and metals systems is striking in both systems grain boundaries migrate to increase their total area and reduce their radius of curvature migration occurs in the absence of significant storud internal strain energy and boundary migration is accompanied by changes in the chemical composition of the solid although a widely accepted theory for digm is lacking it is clear that the process involves the formation of a compositionally distinct phase which grows by material transport along an advancing grain or interphase boundary grain boundary migration during digm must be driven at least in part by the reduction of chemical free energy associated with the chemical composition change that takes place behind the advancing boundary although the reaction mechanisms by which multicomponent multiphase systems in the earth reach equilibrium are likely to be numerous and diverse it is possible that some mechanisms are analogous to the digm as observed in laboratory experiments on calcite bicrystals

the baikalide altaid transbaikal mongolian and north pacific orogenic collages similarity and diversity of structural patterns and metallogenic zoning

late paleozoic tectonics of the solonker zone in the wuliji area inner mongolia china insights from stratigraphic sequence chronology and sandstone geochemistry abstract the geology in the wuliji area including the enger us and quagan qulu areas is important for understanding the late paleozoic tectonics of the solonker zone ultramafic mafic rocks in the enger us area previously interpreted as an ophiolitic suture are actually lava flows and sills in a permian turbiditic sequence and a small body of fault breccia containing serpentinite subduction zone features such as accretionary complexes magmatic arc volcanics or lp hp metamorphism are absent early permian n morb mafic rocks and late permian radiolarian cherts accompanied by turbidites and tuffeous rocks indicate a deep water setting in the quagan qulu area outcrops of the late carboniferous to permian amushan formation are composed of volcano sedimenary rocks and guyot like reef limestone along with a late permian volcano sedimentary unit a dacite lava in the late permian volcano sedimentary unit yields a zircon u pb age of 254 ma the gabbros in the quagan qulu area are intruded into the amushan formation and caused contact metamorphism of country rocks sandstones in the upper member of the amushan formation contain detrital clasts of volcanic fragments and mineral clasts of crystalline basement rocks i e biotite muscovite and garnet geochemical analysis of volcaniclastic sandstones shows a magmatic affinity to both continental island arc cia and active continental margin acm tectonic settings a late permian incipient rift setting is suggested by analyzing the lithostratigraphic sequence and related magmatism in the wuliji area the volcano sedimentary rocks in the wuliji area experienced a nearly n s shortening that was probably related to the early mesozoic nearly n s compression well developed in other areas close to the wuliji area

fluid evolution and mineralization mechanism of the east kounrad porphyry mo w deposit in the balkhash metallogenic belt central kazakhstan abstract the east kounrad mo w deposit is located in the eastern part of the balkhash metallogenic belt central kazakhstan the intrusion in the east kounrad deposit is a permain complex that includes medium and fine grained granites both the primary magma and hydrothermal fluids of the east kounrad deposit are characterized by high fluorine and high oxygen fugacity which would be important for the formation of high grade large mo deposit four mineralization stages are identified in this study stage i quartz wolframite molybdenite fluorite veins which were associated with intensively greisen alteration were formed from fluids with high temperature 340 460 c high pressure 250 400 bars high low salinity 54 5 wt and 3 4 10 2 wt respectively and high oxygen fugacity with a fluid system of h2o nacl co2 stage ii quartz molybdenite pyrite veins which were accompanied by k feldspars muscovite fluorite alteration were formed from fluids with medium high temperature 260 400 c medium pressure 100 250 bars high low salinity 39 1 58 4 wt and 2 4 11 7 wt respectively and relatively low oxygen fugacity after the main stage of w and mo mineralization stage iii quartz pyrite molybdenite veins associated with quartz sericite alteration were formed from fluids with medium low temperature 160 340 c low pressure less than 100 bars various salinity 4 5 46 2 wt and high oxygen fugacity stage iv quartz polymetallic sulfide veins related to argillic alteration were formed under temperature of 120 220 c and salinity of 1 7 8 1 wt remarkably fluid boiling is the most important precipitation mechanism of molybdenite and wolframite in addition the decreasing oxygen fugacity and temperature also play a vital role in triggering deposition of molybdenite the pb isotopic composition of sulfides shows that the ore forming materials at east kounrad are dominantly derived from the lower crust

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

architecture and evolution of accretionary orogens in the altaids collage the early paleozoic west junggar nw china understanding the development and evolution of accretionary orogens is crucial for characterizing continental crust growth in time and space in the altaids tectonic collage of central asia conflicting geodynamic models have been proposed about the paleozoic evolution during which juvenile continental crust has been extensively formed this study focuses on west junggar nw china a key region that has not been extensively studied yet a multidisciplinary approach including detrital zircon provenance study geochemistry and field structural analysis provides new constraints on the evolution of three early paleozoic units of west junggar oceanic subduction predominates in early paleozoic time as indicated by ophiolitic melanges fore arc volcaniclastic turbidites and magmatic arc suites however the development of an olistostrome supplied by a sub contemporaneous carbonate platform the occurrence of continental molasse and the chronology and geochemistry of magmatic events reveal interruptions of subduction discrete collisions of volcanic arcs and micro continent are inferred from the tectonic structure of the belt which displays allochthonous units rooted in the suture zone early paleozoic magmatic and sedimentary rocks of west junggar bear some resemblance to contemporaneous analogues in eastern kazakhstan and suggest a lateral connection on the basis of these new results we propose a geodynamic evolution in four steps 1 early to middle ordovician intra oceanic subduction forming island arcs 2 late ordovician tectonic accretion of island arcs against the kazakhstan continental margin 3 early to middle silurian resumption of subduction and active continental margin magmatism and 4 late silurian early devonian micro continent collision this scenario consistent with the formation of the kazakhstan block supports a model of multiple episodic accretions for the altaids tectonic collage rather than a continuous continental margin evolution these results also suggest that in central asia continental crust growth results from alternating vertical and horizontal growth episodes throughout the paleozoic

extension and application of discontinuous deformation analysis with a damped contact spring model abstract discontinuous deformation analysis dda is widely used in geological and geotechnical engineering to provide an alternative approach to considering damping effect in dynamic analysis a damped contact spring model is implemented the model assumes that the damping component linear dashpot takes place in joints of dda models the normal and shear damping forces are determined according to the relative velocities of the two contacted points a semi analytical solution is also proposed to calculate damping coefficient from a given damping ratio the viscous damping effect produced by the damped contact spring model is independent to step size the proposed model is incorporated into dda program and then several classic damping dynamic examples are constructed to validate the contact model showing good agreement with analytical solutions after that a seismic spectrum of a recorded ground motion is analyzed to show the contact model is suitable for response spectrum generation finally the donghekou landslide case is studied under different parameters suggesting that the proper consideration of damping is essential to natural hazard analysis

geochronological and geochemical study of the darbut ophiolitic complex in the west junggar nw china implications for petrogenesis and tectonic evolution abstract the darbut ophiolitic complex consists of ultramafic rocks cumulates pillow lavas and abyssal radiolarian cherts and occurs in the sartuohai suluqiaoke area which is situated in the eastern part of the west junggar nw china southern altaids in the present study we report new geochronological and geochemical data obtained for gabbro and basalt blocks embedded in the darbut ophiolitic complex zircon u pb analyses from one representative gabbro sample by la icp ms yielded a weighted mean age of 391 6 ma showing that it was emplaced in the middle devonian the samples bear n morb e morb and arc like geochemical signatures they are tholeiitic in composition with low concentrations of na 2 o k 2 o varying from 1 36 to 4 09 wt mg and ti depleted large ion lithophile elements liles enriched with insignificant negative eu they also show negative nb ta anomalies and high positive e nd t values with relative high la nb ba nb ba la ba th and zr nb ratios we interpret that these volcanic rocks were derived from a slightly metasomatized depleted mantle source in a back arc basin setting which corresponds to 10 30 partial melting of a depleted spinel lherzolite mantle the data presented here in combination with previously published data suggest a northwestward subduction model for the west junggar during the period of the early devonian to early carboniferous the west junggar was dominated by normal northwestward subduction and the ridge subduction commenced in the late carboniferous

geochemistry and geochronology of the delinggou intrusion implications for the subduction of the paleo asian ocean beneath the north china craton abstract the formation and evolution of the central asian orogenic belt caob are intimately related to the opening and closure of the paleo asian ocean the final closure led to amalgamation or collision between the caob and north china craton however the issue about the subduction polarity and final closure of the paleo asian ocean has long been debated in order to resolve this issue we carried out a detailed geochemical and age study on the delinggou intrusion from the siziwangqi area of central inner mongolia at the northern margin of the north china craton the delinggou intrusion consists predominantly of diorite and quartz diorite with minor gabbroic diorite u pb zircon dating indicates that the intrusion was emplaced during early carboniferous 330 to 343 ma geochemically the rocks are metaluminous and belong to the calc alkaline series they are enriched in large ion lithophile elements lile e g rb sr and ba and depleted in high field strength elements hfse e g zr nb hf ta and ti the delinggou rocks were probably generated by partial melting of an enriched subcontinental lithospheric mantle further contaminated by crustal material during the magmatic differentiation in a continental arc setting we conclude that the paleo asian ocean underwent southward subduction beneath the north china craton and the subduction initiated in the carboniferous 343 330 ma but the final ocean closure time is not yet constrained

3820 ma zircons from a tonalitic armisoq gneiss in the godthab district of southern west greenland abstract zircons from a sample of the tonalitic amitsoq gneisses near godthhb west greenland exhibit several stages of crystal growth measurements of the u th pb isotopic composition of 30 m areas within individual grains using the ion microprobe shrimp reveal that components of magmatic zircon from the original igneous protolith are at least 3822 5 ma l old slightly but significantly older than the age of zircons from a fclsic volcanic unit in the isua supracrustal belt as measured at high precision by the same technique and older than all ages previously determined by mineral and whole rock isotopic techniques for amitsoq gneisses at godthab and at isukasia overgrowths of younger zircon formed at ca 3630 ma coinciding with an episode of major pb loss from the older grains these results indicate that previous whole rock isochron ages refer to later isotopic redistribution and or that other phases of the amitsoq gneisses intruded during this later period and dominate the isotopic systematics comparison of the ion probe results with previous conventional u th pb data for the same zircon sample demonstrates the strong influence of discordant high uranium parts of zircon on the mean isotopic composition of multi grain samples

grain growth control of isotope exchange between rocks and fluids isotope exchange between fluid and rocks has been traditionally considered to be rate limited by two elementary processes lattice diffusion in the matrix minerals and dissolution into the fluid followed by precipitation from it in this study we show the results of high pressure experiments on 18 o water infiltration into quartzite that point to a third highly efficient process grain growth accompanied by migration of the grain boundaries gbs that are isotopically enriched by gb diffusion and surface exchange we predict on the basis of a mass transfer mechanism discrimination diagram that this gb sweeping is the primary control on isotopic equilibration under hydrothermal conditions in various fine grained rocks with low fluid fraction grain growth should be considered when interpreting and simulating isotope and chemical composition of rocks and fluids

provenance of ediacaran sinian sediments in the helanshan area north china craton constraints from u pb geochronology and hf isotopes of detrital zircons abstract the tectonic relationship between the alxa block and the north china craton has long been controversial the helanshan area lies at the western margin of the ordos block and to the east of the alxa block and it contains eidacaran zhengmuguan formation and tuerkeng formation the zhengmuguan formation is made up of tillites limestone and dolomine and the tuerkeng formation consists of silty slate a parallel unconformity marks the boundary between the tuerkeng formation and the early cambrian suyukou formation which is composed mainly of phosphatic pebbly sandstone and sandstone u pb dating ages of detrital zircons in the zhengmuguan and suyukou formations reveal two neoproterozoic age peaks of 818 4 ma n 88 and 905 8 ma n 20 neoproterozoic magmatic events have yet been reported in the basement of the ordos block while some neoproterozoic igneous rocks have been found in the basement of the alxa block moreover the two neoproterozoic age peaks correspond well with the ages of neoproterozoic igneous rocks in alxa block the hf isotopic characteristics of a part of neoproterozoic zircons from the ediacaran zhengmuguan formation in the helanshan area ehf t 7 812 to 3 274 tdmc 2211 1578 ma n 10 are similar to those neoproterozoic igneous zircons from the langshan area ehf t 1 105 to 5 928 tdmc 1 75 1 38 ga n 23 and bayinnuoergong area in the alxa block the parental magmas had a long residency time in the crust it seems therefore that alxa block provenance provided neoproterozoic clastics for helanshan area in ediacaran and alxa block had already been a part of the north china craton by the late neoproterozoic even earlier

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

provenance and tectonic setting of late carboniferous clastic rocks in west junggar xinjiang china a case from the hala alat mountains abstract the late carboniferous palaeo tectonic setting of the west junggar region is comprised of arcs alternating with basins geochemical analysis of the sedimentary rocks associated with these arc related basins was conducted to better constrain the provenance and tectonic setting major and trace element geochemistry data of late carboniferous mudstones and sandstones in the hala alat mountains suggest that these sedimentary rocks and their source areas are characterized by the following four features 1 sediments experienced a simple recycling process 2 a low degree weathering conditions in the source areas 3 compositional immature of the sedimentary rocks 4 dominated by intermediate to felsic provenance with a few intermediate to mafic sediments integrated with the palaeo flow data and previous authors works a fore arc basin model is proposed for the tectonic setting of the sedimentary rocks the sawur arc is the primary provenance and supplies the major intermediate to felsic detrital fragments the bozchekul chingiz arc and kexia oceanic island arc are the other two secondary sources

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

zircon response to diamond pressure metamorphism in the kokchetav massif ussr ultrahigh pressure metamorphism of the diamond bearing gneisses in the kokchetav massif ussr has been dated at 530 7 ma 2 sgr by shrimp ion microprobe analysis of metamorphic zircons zircon xenocrysts as old as 2000 ma have retained original isotopic compositions through the extreme metamorphic conditions and indicate that the protolith that was buried to diamond forming pressures included early proterozoic components highly variable reset ting of u pb isotopic systems in zircon xenocrysts at the time of metamorphism suggests that diffusive lead loss had commenced but was incomplete a disequilibrium observation consistent with relatively transient exposure of the rocks to extreme crustal depths

petrogenesis of the kekesai composite intrusion western tianshan nw china implications for tectonic evolution during late paleozoic time abstract the late carboniferous to permian is a critical period for final amalgamation of the central asian orogenic belt caob which is characterized by voluminous igneous rocks particularly granitoids the kekesai composite granitoid porphyry intrusion situated in the chinese western tianshan southwest part of caob includes two intrusive phases a monzogranite phase intruded by a granodiorite phase la icpms u pb zircon analyses suggest that the monzogranitic rocks were formed at 305 5 1 1 ma with a wide age range of inherited zircons 358 488 ma and 1208 1391 ma whereas the granodioritic rocks formed at 288 7 1 5 ma the monzogranitic and granodioritic phases have similar geochemical features and sr nd hf isotopic compositions they exhibit high and variable sio 2 66 71 wt and mgo 0 41 2 14 wt contents with some arc like geochemical characteristics e g enrichment of large ion lithophile elements and negative anomalies of nb ta and ti and relatively high initial 87 sr 86 sr ratios i sr 0 7055 0 7059 low positive e nd t 0 84 to 1 03 as well as a large variation in hf isotopic compositions with e hf t between 3 43 and 14 8 implying both of them were derived from similar source materials these geochemical characteristics suggest that they might be mainly derived from the partial melting of arc derived mesoproterozoic mafic lower crust with involvement of a mantle derived component in variable proportions by mantle derived magma underplating the presence of late ordovician to earliest early carboniferous inherited zircons and the hf isotopic compositions in the monzogranitic sample similar to that of the widespread juvenile arc rocks indicates some crust contamination during magma emplacement our new data combined with previous studies imply that extensive post collisional magmatism due to underplating of mantle derived magma could plausibly be explained by slab break off regime

paleozoic accretionary and collisional tectonics of the eastern tianshan china implications for the continental growth of central asia this paper deals with the various tectonic units in the chinese eastern tianshan orogenic collage in the central asian orogenic belt and discusses the paleozoic geological history of the several periods of accretion and collision of archipelago systems lying between the tarim and southern angaran continental margins the late ordovician silurian to early devonian eastern tianshan archipelago was characterized by a the harlik dananhu subduction system with a s dipping polarity in the north b a southerly n dipping subduction system beneath the central tianshan arc in the middle and c the south tianshan ocean against tarim in the south during the devonian to early carboniferous n dipping subduction led to the harlik dananhu arc and the kanggurtag forearc basin accretionary complex in the early to mid carboniferous the magmatic front associated with the n dipping subduction beneath the dananhu harlik arc migrated southwards forming the yamansu arc constructed upon the kanggurtag accretionary forearc by the late carboniferous the dananhu harlik arc was attached northwards to the angaran margin resulting in lateral enlargement of the angaran continent in the latest carboniferous to early permian a multiple soft collision left wide suture zones in the south that include the ophiolite strewn aqikkuduk shaquanzi and kumishi accretion collision complexes which were stitched by early permian post collisional plutons by re defining and re interpreting the various tectonic terranes this paper presents a new improved model for the paleozoic evolution of this part of central asia

contrasting ore styles and their role in understanding the evolution of the altaids abstract the altaids is a paleozoic orogenic collage in central asia it occupies 10 of the eurasian continent and it is one of the richest concentrations of mineral deposits in the world comparable to those of the circum pacific and tethyan orogenic belts in particular it contains many types of hydrothermal ore deposits that are sensitive to the nature and origin of their host terranes including porphyry copper gold molybdenum volcanogenic massive sulfide copper lead zinc and orogenic gold deposits which are all diagnostic of convergent continental margins extensional arc related submarine basins and transpressional terrane boundaries within the altaids porphyry copper deposits are mostly concentrated within the kazakhstan and tuva mongolia micro continents where they formed almost continuously from the early ordovician to permian only a few are cambrian and triassic and most are late carboniferous most volcanogenic massive sulfide ore deposits are distributed along the south urals and rudny altai in the western altaids where they formed in a short time window from the early to middle devonian important orogenic gold deposits formed along regional strike slip faults between craton island arc or micro continent boundaries from the neoproterozoic to permian mostly in the early permian in limited key areas volcanogenic massive sulfide deposits formed upon much older 100 ma basement rocks than those of coeval porphyry deposits indicating that the altaids could not have developed from a single arc furthermore the porphyry deposits outline the distribution of several continental fragments the combined area of which is far less than that of the altaids which suggests that extensive new juvenile material has been added into island arc systems which may have been accreted to surrounding continental margins these metallogenic distributions in localized areas were a result of the mechanisms of crustal growth within the overall tectonic evolution of the altaids and they are consistent with the interpretation that the altaids is a multiple subduction accretion collage

provenance and tectonic setting of the carboniferous sedimentary rocks of the east junggar basin china evidence from geochemistry and u pb zircon geochronology abstract the modern junggar basin was located in the central asian orogenic belt caob during the devonian and carboniferous few data concerning the weathering provenance and tectonic setting of the carboniferous sedimentary rocks in the junggar basin have been published the recent discovery of an oil gas field in the east junggar basin provides the impetus for this study twenty eight sediment samples were recovered from eight boreholes and all were analysed for major and trace element geochemistry two samples of sedimentary rocks were selected for zircon u pb dating low chemical index of alteration cia values 44 71 and relatively high index of compositional variability icv values 0 72 1 83 suggested weak chemical weathering and an immature source for these samples the relatively low th u 2 16 10 55 and rb sr 0 03 5 44 ratios of the drill core samples reflect a simple recycling history two samples indicated upper carboniferous ages 314 6 ma and a lower middle devonian age 397 9 ma geochemistry and detrital zircon geochronology from the east junggar basin reveal a distinct difference in the composition and source rock ages of clastic rocks from the lower and the upper carboniferous major and trace element compositions of the lower carboniferous sedimentary rocks lcrs were derived from typical mafic rocks and an intermediate ocean island arc of early to middle devonian age whereas the upper carboniferous sedimentary rocks ucrs were derived from a continental island arc or active continental margin setting with a dominantly felsic to intermediate late carboniferous age the current data provide little support for tectonic models that show a passive continental margin of a precambrian crystalline basement our study suggests that a volcanic arc accretion or archipelago early carboniferous to early middle devonian model for the east junggar basin is more viable

metamorphic evolution of diamond bearing and associated rocks from the kokchetav massif northern kazakhstan representative diamond bearing gneisses and dolomitic marble eclogite and ti clinohumite bearing garnet peridotite from unit i at kumdy kol and whiteschist from unit ii at kulet eastern kokchetav massif northern kazakhstan were studied diamond bearing gneisses contain variable assemblages including grt bt qtz pl kfs zo chl tur cal and minor ap rt and zrn abundant inclusions of diamond graphite chlorite or calcite phengite clinopyroxene k feldspar biotite rutile titanite calcite and zircon occur in garnet diamond bearing dolomitic marbles consist of dol di grt phl inclusions of diamond dolomite graphite biotite and clinopyroxene were identified in garnet whiteschists carry the assemblage ky tlc grt rt garnet shows compositional zoning and contains abundant inclusions of talc kyanite and rutile with minor phlogopite chlorite margarite and zoisite inclusions and zoning patterns of garnet delineate the prograde p t path inclusions of quartz pseudomorphs after coesite were identified in garnet from both eclogite and gneiss other ultrahigh pressure uhp indicators include na bearing garnet up to 0 14 wt na2o with omphacitic cpx in eclogite occurrence of high k diopside up to 1 56 wt k2o and phlogopite in diamond bearing dolomitic marble and cr bearing kyanite in whiteschist these uhp rocks exhibit at least three stages of metamorphic recrystallization the fe mg partitioning between clinopyroxene and garnet yields a peak temperature of 800 1000 c at p 40 kbar for diamond bearing rocks and about 740 780 c at 28 35 kbar for eclogite whiteschist and ti bearing garnet peridotite the formation of symplectitic plagioclase amphibole after clinopyroxene and replacement of garnet by biotite amphibole or plagioclase mark retrograde amphibolite facies recrystallization at 650 680 c and pressure less than about 10 kbar the exsolution of calcite from dolomite and development of matrix chlorite and actinolite imply an even lower grade greenschist facies overprint at c 420 c and 2 3 kbar a clockwise p t path suggests that supracrustal sediments together with basaltic and ultramafic lenses apparently were subjected to uhp subduction zone metamorphism within the diamond stability field tectonic mixing may have occurred prior to uhp metamorphism at mantle depths during subsequent exhumation and juxtaposition of many other tectonic units intense deformation chaotically mixed and mylonitized these lithotectonic assemblages

how was the carboniferous balkhash west junggar remnant ocean filled and closed insights from the well tacan 1 strata in the tacheng basin nw china abstract the balkhash west junggar remnant ocean is a conspicuous tectonic feature that had an important effect on the paleozoic tectonic evolution of the central asian orogenic belt how the carboniferous balkhash west junggar remnant ocean was filled and closed is poorly understood in this study we focus on the integrated tectonic sedimentary evolution of the tacheng basin which is located in the eastern part of the balkhash west junggar remnant ocean using high resolution seismic reflection well line geochronological and geochemical data to establish the filling of the basin by sediments and its evolution the carboniferous balkhash west junggar remnant ocean basin was filled with early carboniferous deep marine mudstone and interbedded sandstone with minor andesite basalt and tuff and late carboniferous basalt andesite breccia and tuff and shallow marine mudstone and sandstone the carboniferous igneous assemblage mainly consists of basalt with high nb content and nb la pm values and concomitant magnesian andesites with adakite geochemical characteristics of high sr and low y and yb suggesting a subduction related tectonic setting zircons in the tuff that underlies the basalts and magnesian andesites were dated to ca 315 ma by la icp ms u pb the identified unconformity between the carboniferous and lower permian from seismic reflection profile in well tacan 1 subdivided the carboniferous early permian strata into two tectonostratigraphic units the lower permian sequences overlap the carboniferous strata and the facies transition from the submarine carboniferous to the terrigenous lower permian indicates that the balkhash west junggar remnant ocean closed during the early permian these new data suggest that the sedimentary filling of the basin was in response to subduction and accretionary processes and that oceanic subduction was an important mechanism that led to the shrinking filling and closure of the balkhash west junggar remnant ocean because of compression therefore lateral crust growth through subduction accretionary processes still played an important role in the carboniferous construction of western junggar

formation of the kokchetav subduction collision zone northern kazakhstan insights from zircon u pb and lu hf isotope systematics abstract the kokchetav subduction collision zone is located in the western part of the ancestral central asian orogenic belt this zone is built up by the precambrian kokchetav microcontinent which includes a hp uhp metamorphic belt and the north kokchetav tectonic zone nktz which represents an accretionary complex between the kokchetav microcontinent and the adjacent stepnyak island arc the entire region is widely intruded by palaeozoic granitoids which were emplaced after the collision of the stepnyak island arc with the kokchetav microcontinent we present zircon u pb ages and lu hf systematics in zircon to better characterize the tectonic evolution of the kokchetav subduction collision zone the lu hf results indicate that the kokchetav basement rocks are derived from late neoarchaean early palaeoproterozoic 2 5 ga crust for the granite gneiss basement of the kokchetav microcontinent early mesoproterozoic grenville age 1 17 1 14 ga zircon u pb crystallization ages were obtained for the nktz two main age components were recognized 1 an oldest mesoproterozoic age component 1 20 1 05 ga similar as for the kokchetav microcontinental zircons and 2 a younger early cambrian 540 520 ma or late cambrian early ordovician 490 480 ma age component th u ratios 0 1 and are typical of a magmatic protolith the distinction between both zircon types is supported by characteristic cathodoluminescence images these results confirm previous observations indicating early palaeozoic high grade metamorphism 540 520 ma and collisional deformation 490 480 ma of the mesoproterozoic kokchetav zone as a result of subduction accretion and collision of the stepnyak island arc to the kokchetav microcontinent for two additional samples of the balkashin granitic complex early devonian 415 395 ma zircon crystallization ages magmatic th u ratios were obtained the occurrence of a mesozoic xenocryst within these leucogranites may indicate that they were emplaced in a continental arc setting

microfabrics in carbonate mylonites along a large scale shear zone helvetic alps abstract carbonate mylonites with varying proportions of second phase minerals were collected at positions of increasing metamorphic grade along the basal thrust of the morcles nappe helvetic nappes switzerland variations of temperature stress and strain rate changes in chemistry of solid and fluid phases and differing degrees of strain localization and annealing were tracked by measuring the shapes mean sizes and size distributions of both matrix and second phase grains as well as crystal preferred orientation cpo of the matrix field structures suggest that strain rate was constant along the fault the mean and distribution of the calcite grain sizes were affected most profoundly by temperature increased temperature presumably accompanied by decreased stress correlated with larger mean sizes and wider size distributions at a given location the matrix grains in mylonites with more second phase particles are on average smaller have narrower size distributions and have more elongate shapes for example mylonites with 50 vol of second phases have matrix grain sizes half that of pure mylonites changes in calcite chemistry and the presence of synkinematic fluids seemed to influence microfabric only weakly temporal variations in conditions such as exhumation induced cooling apparently provoke changes in temperature stress and strain rate along the nappe these changes result in further strain localization during retrograde conditions and cause the grain size to be reduced by an additional 50 the matrix cpo strengthens with increasing temperature or strain but weakens and rotates with increasing second phase content these fabric changes suggest differing rates of grain growth grain size reduction and development of cpo owing to variations in the deformation conditions and perhaps mechanisms to interpret natural mylonite structures or to extrapolate mechanical data to natural situations requires careful characterization of the microfabric and in particular second phase minerals

effect of grain growth on cation exchange between dunite and fluid implications for chemical homogenization in the upper mantle the effect of grain growth on the cation exchange between synthesized forsterite aggregates i e dunite and nickel rich aqueous fluid was evaluated experimentally at 1 2 gpa and 1 200 c the grain boundary gb migration caused nickel enrichment in the area swept by the gbs in a fashion similar to that reported for stable isotope exchange in the quartz aggregates the progress of the grain growth resulted in an increase in the average nickel concentration in the dunites of up to 80 times that was calculated for a system having stationary gbs the overall diffusivity of the nickel along the wet gbs and interconnected fluid networks was found to be 6 5 10 19 6 7 10 18 m3 s which is 4 5 orders of magnitude higher than the grain boundary diffusivity in the dry dunite these results show that the grain growth rate is a fundamental factor in the evaluation of the time scale of chemical homogenization in the upper mantle

sources of phanerozoic granitoids in the transect bayanhongor ulaan baatar mongolia geochemical and nd isotopic evidence and implications for phanerozoic crustal growth abstract the central asian orogenic belt caob is renowned for massive generation of juvenile crust in the phanerozoic mongolia is the heartland of the caob and it has been subject to numerous investigations particularly in metallogenesis and tectonic evolution we present new petrographic geochemical and sr nd isotopic analyses on phanerozoic granitoids emplaced in west central mongolia the data are used to delineate their source characteristics and to discuss implications for the phanerozoic crustal growth in central asia our samples come from a transect from bayanhongor to ulaan baatar including three tectonic units the baydrag cratonic block late archean to middle proterozoic the eo cambrian bayanhongor ophiolite complex and the hangay hentey basin of controversial origin the intrusive granitoids have ages ranging from ca 540 to 120 ma the majority of the samples are slightly peraluminous and can be classified as granite s s including monzogranite syenogranite and alkali feldspar granite most of the rocks have initial 87sr 86sr ratios between 0 705 and 0 707 late paleozoic to mesozoic granitoids 250 ma are characterized by near zero end t values 0 to 2 whereas older granitoids show lower end t values 1 5 to 7 the data confirm the earlier observation of kovalenko et al geochemistry international 34 1996 628 who showed that granitoids emplaced outside of the pre riphean basement rocks are characterized by juvenile positive end t values whereas those within the pre riphean domain and the baydrag cratonic block as for the present case show a significant effect of contamination by precambrian basement rocks nevertheless mass balance calculation suggests that the granitoids were derived from sources composed of at least 80 juvenile mantle derived component despite our small set of new data the present study reinforces the general scenario of massive juvenile crust production in the caob with limited influence of old microcontinents in the genesis of phanerozoic granitoids

review of nd isotopic data and xenocrystic and detrital zircon ages from the pre variscan basement in the eastern bohemian massif speculations on palinspastic reconstructions

modelling of porphyroclasts in simple shear and the role of stress variations at grain boundaries abstract grain scale numerical experiments involving simple shear of a two phase non linear viscous material are described and compared with mineral fish or lozenge shaped porphyroclasts such as muscovite two types of 2d models are considered either a single elongate grain or two parallel elongate grains in both cases supported by a weaker polygonal grain matrix the relative viscosities of the contrasting grain structures were systematically varied allowing us to observe the effects of non linear viscous rheology on the resulting microstructure and flow patterns the results show that the finite rotation of the hard elongate grain was similar within any one experiment but was largely influenced by viscosity contrast the geometry of the model and the imposed shear strain models involving single elongate hard grains show increasing instability at their ends and less strain compatibility with the deforming matrix grains as the viscosity contrast is increased in the paired grain models the greatest variation in the matrix grain microstructure is seen in the region where the two hard grains are oriented at a high angle to the direction of shear finally we consider the changes in intragranular stress by comparing microstructural observations using different viscosities with the distribution of stress in space and during progressive shear in the plane approximately parallel to the maximum principal stress direction 1 a localised change of stress occurs across and along the interface between the hard and soft grains variations in the mean stress at these boundaries are directly attributable to changes in the minimum principal stress we propose that with shear strains greater than 2 it is the minimum principal stress that can control diffusion processes at the grain boundary rather than mean stress in conclusion we suggest that our models have the potential for providing useful insights into why metamorphic reactions can occur at the interface between a porphyroclast and matrix at high shear strains and how stress distribution can control the initiations of such reactions

late paleozoic post collisional magmatism in the eastern tianshan belt northwest china new insights from geochemistry geochronology and petrology of bimodal volcanic rocks abstract the late carboniferous to permian marks an important phase of terrane amalgamation through accretion collision in the central asian orogenic belt caob the tianshan belt forms part of this sequence where the post collisional event generated an e w trending bimodal volcanic zone that extends for ca 500 km from the southern bogda mt to the easternmost tianshan belt here we report the petrology geochemistry and geochronology of the bimodal volcanic rocks to characterize the post collisional tectonic evolution in late paleozoic the major element chemistry of the basalt and rhyolite samples indicates a bimodal high k calc alkaline affinity the basalts are characterized by high abundance in tio2 th u and pb and depletion in nb ta and p together with slight enrichment in lree contents and low hfse lree ratios the rhyolites show a close affinity to aluminous a type granites with enrichment in k na zr ce y and depletion in p nb ta and ti they also exhibit fractionated ree patterns with prominent negative eu anomalies the petrographic and geochemical data suggest that this bimodal rock series was generated in a post collisional setting the mafic rocks are likely derived from a metasomatized lithospheric mantle assimilation of different crustal components played a minor role in the genesis of these basaltic rocks the la icp ms analysis on zircons from four rhyolites and two basalts yielded similar ages ranging from 295 8 2 8 to 293 3 1 7 ma suggesting an early permian event zircons from the basalts show a wide ehf t range from 0 49 to 13 whereas those from the rhyolites show two groups of ehf t values from 5 to 3 and from 6 to 12 suggesting a mixing of crust mantle material the magmatic zircons from the bimodal volcanic rocks yielded two peaks of tdm2 between 600 820 ma and 1160 1360 ma probably representing two significant episodes of continental crust growth the bimodal volcanic rocks are correlated with the early stage of post collision our results suggest that vertical crustal growth through mantle derived magmas in post collisional setting also played a major role in the construction of the caob

origin of high sr y type granitic magmatism in the southwestern of the alxa block northwest china abstract the petrogenesis of high sr y type magmas is still open to debate usually such magmas could result from melting under high pressure settings 12 kbar in this paper we gave an example that some high sr y type magmas could originate from melting of crustal materials at pressure of 10 12 kbar we carried out a study of petrogenesis for devonian high sr y granites from the beidashan batholith 397 411 ma southwestern alxa block northwest china the beidashan granites have sio 2 69 21 74 60 wt and al 2 o 3 14 01 16 20 wt with a cnk ratios of 0 99 1 08 according to their trace element compositions and whole rock zirconium saturation temperatures t zr the beidashan granites can be divided into two groups group i dy yb n 1 2 3 0 eu eu 0 77 1 3 t zr 761 856 c resulted from fluid absent partial melting of mafic to intermediate crustal materials leaving garnet residuum at pressure of 12 kbar and group ii dy yb n 0 76 2 16 eu eu 1 7 5 3 t zr 651 785 c formed by fluid present melting of mafic to intermediate crustal materials with residual amphibole in the source at pressure of 10 kbar both group i and group ii show high sr y and la yb n features they show i sr 0 7134 0 7180 e nd t 6 61 to 9 71 t 2dm 1 7 1 9 ga e hf t 5 6 to 9 9 and t dm c 1 7 2 0 ga indicating that the beidashan high sr y granites were derived from melting of crustal basement materials our results suggest that some high sr y type granites formed under relatively lower pressure conditions 10 12 kbar and they could not be an indicative of partial melting of thickened crust

diffusion in polycrystalline materials grain boundaries mathematical models and experimental data geological relevance of grain boundary diffusion as a rule of thumb diffusion along grain boundaries is several orders of magnitude faster than within minerals and hence it could enable mass transport on much larger spatial scales within geological time scales therefore earth scientists are interested in the transport properties of grain and interphase boundaries and other short circuit diffusion paths such as dislocations or a fluid network since they may provide very efficient transport paths controlling a variety of processes operating on very different spatial scales for example on a micron scale corona formation around minerals is a common phenomenon that is typically controlled by grain boundary diffusion similarly element and isotopic exchange between a physically separated mineral pair used for example as a geothermometer requires transport along grain boundaries metasomatic reaction zones on the centimeter to meter scale are not always simply related to fluid flow but are also induced by fluid assisted diffusive fluxes between two different rock types imposed on each other due to tectonic processes e g miller et al 2009 it has been also argued that the observed strong gradual fractionation of li isotopes on the meter scale was a result of diffusion where a strong contribution comes from grain boundary diffusion within these rocks teng et al 2006 further new data on grain boundary diffusion of siderophile elements and carbon in polycrystalline mgo suggest that grain boundary diffusion provides an efficient mechanism to exchange these elements between the mantle and the core even on the kilometer scale within reasonable time scales hayden and watson 2007 2008 in addition to the significant role of grain boundary diffusion for element transfer in rocks grain boundary diffusion can also dominate the bulk physical properties of rocks e g their viscosity coble creep or electrical conductivity which is for example relevant for the interpretation of

palaeozoic tectonics of the south western chinese tianshan new insights from a structural study of the high pressure low temperature metamorphic belt the south western chinese tianshan orogenic belt is famous for its omphacite bearing blueschists and associated eclogite facies metavolcanic rocks although numerous petrologic geochemical and geochronological studies are available structural data and interpretations are still rare this paper provides new structural data including bulk geometry of structures and kinematic analyses based on field and laboratory studies along the akyazhi keburt and muzaert rivers the study area is divided into three tectonic units namely 1 a southern unit composed of weakly metamorphosed sedimentary rocks of silurian age 2 a central hp lt unit composed of blueschist eclogite facies metamorphic rocks derived from basalts pelites and volcaniclastic rocks 3 a northern unit which consists of a carboniferous magmatic arc developed upon an amphibolite facies metamorphic continental basement our structural analysis documents a polyphase deformation the main event d1 is reflected by devonian to carboniferous top to the northwest ductile shearing coeval with hp lt metamorphism this is followed by north directed thrusting d2 of the southern unit over the central hp lt unit coeval with retrogression of the high pressure rocks a top to the s se deformation d3 overprints the earliest events and is observed in the northern and central units lastly permian dextral ductile brittle wrenching d4 overprints the older flat lying fabrics d4 is conspicuous along the nalati fault that separates the northern unit from the central hp lt unit the absolute timing of these deformation events is discussed in the light of available radiometric dating the structural metamorphic and geochronological data are integrated into a geodynamic model of the south western chinese tianshan that emphasizes south directed subduction of microcontinents located between tarim and junggar

sims u pb rutile age of low temperature eclogites from southwestern chinese tianshan nw china article i nfo the geological evolution in the southwestern chinese tianshan orogen in nw china is poorly understood because of controversial geochronological results of the low temperature eclogites it is likely due to sm nd and rb sr isotopic disequilibrium in different mineral fractions excess 40 ar in the high pressure k bearing minerals and multi stage growths of zircon in u pb system rutile as a typical eclogite facies mineral with high u pb ratio is a feasible candidate to constrain the eclogite facies metamorphic event in this study we measured u pb rutile age data using sims cameca ims 1280 for low temperature eclogites from southwestern chinese tianshan orogen nw china we demonstrate that sims is a powerful technique for in situ u pb dating of rutile with u content as low as 1 ppm sims u pb rutile dating yielded an age of 318 7 ma which may record the time when the rocks cooled down to the closure temperature of pb in rutile ca 500 c but should be very close to the time of eclogite facies metamorphism in terms of common fast exhumation of coesite bearing eclogite this new proof strongly suggests a late carboniferous collision between kazakhstan yili block and central tianshan microcontinent

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