downward continuation of heat flow density data and thermal regime in eastern canada abstract heat flow density hfd data from eastern canada were analyzed by downward continuation the data cover parts of the superior and grenville provinces of the canadian shield and parts of the appalachian orogen the downward continuation eliminates the effect of shallow crustal radiogenic heat production and better reveals differences in the thermal regime of deeper origin the analysis shows that the deep heat flow beneath the superior and grenville provinces is low and the temperature in the lower crust is also low the thermal regime of the crust changes markedly across a boundary that follows the st lawrence river surface heat flow and heat production are markedly higher in the appalachian province with much higher temperature and heat flow the thermal regime in the appalachian lower crust is also distinctly different from that of the shield temperature and heat flow increase eastward toward the atlantic margin

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

variable eocene miocene sedimentation processes and bottom water redox conditions in the central arctic ocean iodp expedition 302 abstract in 2004 integrated ocean drilling program expedition 302 arctic coring expedition acex to the lomonosov ridge drilled the first central arctic ocean sediment record reaching the uppermost cretaceous 430 m composite depth while the neogene part of the record is characterized by grayish yellowish siliciclastic material the paleogene part is dominated by biosiliceous black shale type sediments the lithological transition between paleogene and neogene deposits was initially interpreted as a single sedimentological unconformity hiatus of 26 ma duration separating eocene from miocene strata more recently however continuous sedimentation on lomonosov ridge throughout the cenozoic was proclaimed questioning the existence of a hiatus in this context we studied the elemental and mineralogical sediment composition around the paleogene neogene transition at high resolution to reconstruct variations in the depositional regime e g wave current activity detrital provenance and bottom water redox conditions already below the hiatus mineralogical and geochemical proxies imply drastic changes in sediment provenance and or weathering intensity in the hinterland and point to the existence of another earlier gap in the sediment record the sediments directly overlying the hiatus the zebra interval are characterized by pronounced and abrupt compositional changes that suggest repeated erosion and re deposition of material regarding redox conditions euxinic bottom waters prevailed at the eocene lomonosov ridge and became even more severe directly before the hiatus with detrital sedimentation rates decreasing authigenic trace metals were highly enriched in the sediment this continuous authigenic trace metal enrichment under persistent euxinia implies that the arctic trace metal pool was renewed continuously by water mass exchange with the world ocean so the eocene arctic ocean was not fully restricted above the hiatus extreme positive ce anomalies are clear signs of a periodically well oxygenated water column but redox conditions were highly variable during deposition of the zebra interval significant mn enrichments only occur above the zebra interval documenting the miocene establishment of stable oxic conditions in the arctic ocean in summary extreme and abrupt changes in geochemistry and mineralogy across the studied sediment section do not suggest continuous sedimentation at the lomonosov ridge around the eocene miocene transition but imply repeated periods of very low sedimentation rates and or erosion

mineralogy and phase relations in the blueschist facies of the black butte and ball rock areas northern california coast ranges microprobe study of minerals in rocks of the blueschist facies from the black butte and ball rock areas of the south fork mountain schist northern california coast ranges is the basis in this report for analysis of reactions and phase relations among lawsonite pumpellyite epidote and associated minerals observed assemblages are intermediate in a progression from those observed elsewhere in high p low t schists of the franciscan e 9 panoche pass where lawsonite is present in nearly all lithologies to the relatively low phigh t assemblages of the shuksan blueschist washington where epidote is ubiquitous two successive reactions in this sequence in order of increasing temperature are l lawsonite na amphibole epidote quartz albite chlorite hzo 2 na amphibole pumpellyite quartz epidote ca amphibole albite chlorite h2o the pressure of metamorphism for the black butte and ball rock areas is estimated from the jadeite content of pyroxene and from the stability of aragonite to have been approximately 7 kbar temperature estimates are not tightly constrained by mineral equilibria but are suggested to be in the range of 250 to 300 c

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

evidence for late proterozoic subduction from 700 myr old blueschists in china in a comprehensive review of the world s blueschists ernst1 noted an apparent absence of these high pressure low temperature rocks in the precambrian and ascribed it to a cooling of the earth in the phanerozoic eon as blueschists are thought to be formed in subduction zones the absence of precambrian blueschists has been adduced as evidence that plate tectonics did not operate on the early earth2 although this conclusion is not inevitable in that ancient mobile belts are more likely to have been obliterated than younger ones3 the presence of paired metamorphic belts of kyanite sillimanite and andalusite sillimanite grade in proterozoic gondwanaland supports the existence of a subducted slab but with ancient geothermal gradients too high to form blueschists along the convergent plate boundary4 5 on the other hand data from inclusions in african diamonds support the opposing view that archaean continental geotherms were similar to those of today6 here we present rb sr and k ar ages of 698 718 myr for phengitic mica from high pressure pelitic schists from the aksu region of china which are interleaved with blueschists we interpret these dates as representing the time of the blueschist facies metatmorphism suggesting that subduction was occurring in the late proterozoic

geochemical discrimination of metabasalt rocks of the fan karategin transitional blueschist greenschist belt south tianshan tajikistan seamount volcanism and accretionary tectonics the fan karategin metamorphic belt south tianshan tajikistan is regarded to be an ancient subduction accretionary complex and is composed of three tectonostratigraphic units which display lithologies consistent with different tectonic settings the mafic schists which make up the major part of the older unit of the belt contain both alkali and tholeiitic metabasalts on the basis of rare earth and other immobile element characteristics the alkali metabasalts are akin to within plate ocean island basalts whereas the tholeiitic metabasalts resemble e type morb the association is interpreted to have been formed on seamount like structures under a within plate plume bedded cherts and marbles in the unit are regarded as ancient pelagic sediments and carbonate caps developed upon basaltic seamounts respectively dismemberment of the seamount related basalts and pelagic sediments and the high p low t prograde metamorphism of the unit rocks up to transitional blueschist greenschist facies was the result of paleoseamount submergence into a subduction zone this unit is tectonically overlain by arc derived metavolcanic unit and a disrupted mainly clastic unit of upper ordovician lower silurian age metavolcanic and metasedimentary rocks of the two upper units have geochemical characteristics compatible with subduction related origin the lithological assemblages of the individual units and their juxtaposition suggest an origin involving collision accretionary processes the fan karategin belt is a subduction accretionary complex which formed during subduction of oceanic crust under a volcanic arc and was subjected to tectonic juxtaposition and imbrication of seamount deep sea trench and volcanic arc sequences

compression versus extension in the exhumation of the dora maira coesite bearing unit western alps italy abstract several recent papers suggest that uplift and exhumation of high pressure low temperature hp lt metamorphic rocks are due to crustal extension coeval with and or post dating the contraction responsible for the metamorphism a detailed survey of the coesite bearing unit of the southern dora maira massif offers an opportunity to discuss the relative importance of extension and compression in the exhumation tectonics of ultra high pressure uhp rocks this uhp unit consists of pre alpine basement rocks which equilibrated at a depth of 100 km probably 120 100 ma ago it is overlain by two units with a similar lithology and eoalpine metamorphic age and high p t gradients which equilibrated at 60 and 50 km respectively this eoalpine basement complex is intercalated between blueschist facies units the whole pile was affected by a mesoalpine greenschist facies mylonitic deformation during which the early contacts were reactivated as low angle shallow dipping shear zones extension is documented only during the mesoalpine collisional period and is referred to the gravity collapse of a thickened orogenic wedge with an exhumation rate of about 1 mm yr the most important part of the exhumation of the uhp rocks occurred earlier at a higher rate close to 4 mm yr a forced return flow or extrusion tectonics of imbricate slices in an on going subduction setting seems appropriate to account for this early stage the fact that the omission of intermediate pressure rocks between juxtaposed units is an unreliable criterion for demonstrating extension is discussed

precambrian blueschist terranes of the world blueschists are most common in post paleozoic orogenic belts higher geothermal gradients and a thinner lithosphere in the precambrian are widely thought to have disfavored blueschist formation however several precambrian blueschist belts have been reported from eurasia on the basis of established geologic and geochronologic constraints these belts are grouped as suspect possible and definite precambrian blueschists suspect ones include those in alaska in anglesey u k in the scandinavian caledonides and in yunnan china possible precambrian blueschist belts are those in delhi india in central china in heilongjiang china and in the urals and altai sayan tien shan u s s r definite precambrian blueschist terranes are those in western africa and in aksu china these blueschist belts are described with emphasis on the age constraints the recently investigated aksu blueschist terrane is presented as an example this high p t coherent blueschist greenschist terrane at aksu is unconformably overlain by a thick unmetamorphosed sedimentary sequence ranging in age from late sinian ca 600 800 ma through eocambrian to paleozoic the basal sinian conglomerates contain subrounded to rounded clasts of the underlying schist clearly demonstrating the pre sinian age of blueschist facies metamorphism characteristic features of the above mentioned blueschist belts include 1 1 the blueschist terranes are coherent and primary compositional lithological layers are well preserved melange belts are not common 2 2 protoliths include pelite sandstone bimodal volcanics limestone and small quantities of pillow basalt and pelagic sediment ultramafics are rare 3 3 the protoliths were subjected to multiple deformation and transitional blueschist greenschist facies metamorphism typical precambrian blueschist contains the assemblage crossitic amphibole epidote chlorite albite quartz actinolite rare lawsonite jadeitic pyroxene and aragonite have been described in few belts 4 4 the metamorphic facies series belongs to the high pressure intermediate type of miyashiro 1961 the precambrian blueschist and other pre mesozoic intracontinental blueschist of eurasia formed before rather than during final closure of intervening ocean basins between pairs of continents they do not usually mark the location of terminal sutures but are the result of earlier accretion and continental growth by subduction underplating and imbrication of accretionary wedges including both oceanic and continent derived elastics the rare preservation of precambrian blueschist is due to greenschist overprinting when the heat flow and geothermal gradient was high rather than reflecting lack of production their existence in china and africa demonstrates that requisite conditions for blueschist facies metamorphism existed at least 700 ma ago

palaeozoic collisional tectonics and magmatism of the chinese tien shan central asia abstract the chinese tien shan range is a palaeozoic orogenic belt which contains two collision zones the older southern collision accreted a north facing passive continental margin on the north side of the tarim block to an active continental margin on the south side of an elongate continental tract the central tien shan collision occurred along the qinbulak qawabulak fault southern tien shan suture the time of the collision is poorly constrained but was probably in in the late devonian early carboniferous we propose this age because of a major disconformity at this time along the north side of the tarim block and because the youshugou ophiolite is imbricated with middle devonian sediments a younger probably late carboniferous early permian collision along the north tien shan fault northern tien shan suture accreted the northern side of the central tien shan to an island arc which lay to its north the north tien shan arc this collision is bracketed by the middle carboniferous termination of arc magmatism and the appearance of late carboniferous or early permian elastics in a foreland basin developed over the extinct arc thrust sheets generated by the collision are proposed as the tectonic load responsible for the subsidence of this basin post collisional but palaeozoic dextral shear occurred along the northern suture zone this was accompanied by the intrusion of basic and acidic magmas in the central tien shan late palaeozoic basic igneous rocks from all three lithospheric blocks represented in the tien shan possess chemical characteristics associated with generation in supra subduction zone environments even though many post date one or both collisions rocks from each block also possess distinctive trace element chemistries which supports the three fold structural division of the orogenic belt it is unclear whether the chemical differences represent different source characteristics or are due to different episodes of magmatism being juxtaposed by later dextral strike slip fault motions because the southern collision zone in the tien shan is the older of the two the tarim block sensu stricto collided not with the eurasian landmass but with a continental block which was itself separated from eurasia by at least one ocean the destruction of this ocean in late carboniferous early permian times represented the final elimination of all oceanic basins from this part of central asia

electron microprobe monazite geochronological constraints on the late palaeozoic tectonothermal evolution in the chinese tianshan electron microprobe monazite data from the early palaeozoic granitoid gneisses in the central tianshan tectonic zone provide two main ages 1 an age of 355 12 ma obtained on the chemically complex grains which is good agreement with the high pressure metamorphic age obtained on eclogite and blueschist rocks from the south tianshan tectonic zone 2 an age of 323 7 ma obtained on the complex and the monogenetic grains which is also closely consistent with the age for a major episode of cooling recrystallization of these high pressure rocks monazite from the schist of the xingxingxia complex yields a younger th pb u pb centroid age of 282 9 ma this younger age is interpreted as being linked to a magmatic episode possibly related to underplating extension and or shear activities these data provide timing constraints on the episodes of metamorphism and high pressure metamorphism and evidence for transition from compression to extension during the carboniferous to early permian for the chinese tianshan region moreover this study shows that electron microprobe dating of monazite together with recent statistical treatment can provide a reliable method to reveal a sequence of events within an orogenic cycle

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

the mineralogy petrology metamorphic ptdt trajectory and exhumation mechanism of blueschists south tianshan northwestern china abstract the south tianshan blueschist belt extends for about 200 km along the south tianshan suture from changawuzhi to the upper kekesu river the high pressure low temperature high p low t metamorphic rocks contain sodic amphibole garnet phengite albite quartz epidote chlorite and stilpnomelane these blueschists occur as small blocks lenses laminae and bedded slabs or sheets in greenschist facies country rocks a few dismembered ophiolitic nappe slices and marble lenses are interlayered in the high p t metamorphic belt metabasite blueschists are mainly e type morb less commonly n type the protoliths of the meta sedimentary rocks are graywackes which contain volcaniclastic material of e type morbs and probably some continental basement materials derived from the tarim plate these protoliths may represent subduction complexes or meta melanges which formed within an accretionary wedge on the southern side of the yili central tianshan plate yctp at the end of the early paleozoic the mineral assemblages include sodic amphibole mainly crossite garnet phengite epidote chlorite albite estimated to have crystallized at 450 c and 9 10 kbar these conditions correspond to the epidote blueschist facies and are the peak metamorphic conditions the prograde metamorphism is represented by lawsonite blueschist facies and the retrograde is greenschist facies the metamorphic ptdt trajectory of the blueschists runs clockwise and reveals that the metamorphism revolved from laumontite facies lawsonite blueschist facies epidote blueschist facies 408 ma to greenschist facies 345 ma during the creation and exhumation of the blueschists in the south tianshan suture we have recognized six stages of deformation in the blueschists the earlier three stages were concomitant with metamorphism the later three postdated the metamorphism on the basis of the deformation characteristics of the blueschists and coherent rocks the regional geology and the metamorphic ptdt path we discuss the exhumation mechanism of the blueschists the post collisional ductile thrust within the suture probably exhumed the blueschists and coherent rocks allowing them to escape retrograde overprinting in the greenschist facies the influence of erosion on the exhumation of blueschists could not be considered directly and quantitatively because of the absence of devonian strata and the few exposures of early carboniferous clastic rocks all without detritus of blueschists

changes in dip of subducted slabs at depth petrological and geochronological evidence from hp uhp rocks tianshan nw china article high resolution seismic imaging has previously revealed sudden changes in the dip of subducted oceanic plates this kinking feature is crucial because in many subduction zones it is thought to coincide with the disappearance of the low velocity layers associated with subducted oceanic crust in this study we present petrological evidence for this phenomenon derived from oceanic blueschist and eclogite facies rocks from the chinese tianshan the investigated samples span a large range of peak metamorphic conditions with temperatures between 330 and 580 c at 1 5 to 2 3 gpa such variable peak metamorphic conditions together with the intimate interlayering of high and ultrahigh pressure rocks have also been reported from other tianshan localities these observations suggest that the rocks were derived from varying depths within the subduction zone and then juxtaposed during exhumation in the subduction channel multi point lu hf isochrons from four high pressure rocks yield consistent garnet growth ages of 313 12 315 8 2 9 313 9 4 8 and 315 2 1 6 ma these inconjunctionwiththe 311 maclusterof published 40

new 40ar 39ar age constraints on the late palaeozoic tectonic evolution of the western tianshan xinjiang northwestern china with emphasis on permian fluid ingress laser probe dating of mylonite whole rock samples from the north tianshan main tianshan fault zone that cross cuts the north tianshan domain s southern margin yielded 40ar 39ar spectra with 255 285 ma ages biotite from an undeformed early carboniferous granite which cuts the steep mylonitic foliation in the proterozoic basement of the yili arcs s southern margin gave a 263 4 0 6 ma plateau age 1 pre carboniferous metasediments overlying this basement yielded plateau ages 1 of 253 3 0 3 muscovite and 252 3 0 3 biotite ma the permian ages of mylonites date movement on these ductile dextral strike slip shear zones whereas the mica ages are interpreted by recrystallisation as a result of fluid flow around such transcurrent faults we propose that the tianshan s permian syn tectonic bimodal magmatism was created in a non plume related yellowstone like extensional transtensional tectonic regime gold mineralisation tracing aqueous flow in the crust peaked in permian time and continued locally into the triassic the picture is emerging that a convective fluid system partly driven by magmatic heat existed in a strongly fractured and weakened crust with an elevated heat flow leading to regional scale isotope resetting we suggest that surprisingly young isotopic ages in the literature for early orogenic ultra high pressure metamorphism are similarly due to fluid mediated recrystallisation

phase relations of epidote blueschists phase diagrams have been computed for minerals in the system ncmash using the optimised thermodynamic data base of berman 1988 augmented by data on glaucophane and pumpellyite to illustrate the relationships between rocks containing epidote and sodic amphibole and those containing assemblages characteristic of neighbouring metamorphic facies the stoichiometries of stable bounding equilibria yield a simple list of mineral parageneses that serves to define an epidote blueschist facies and surrounding metamorphic facies the calculations suggest that when iron free the pairs zoisite glaucophane and clinozoisite glaucophane do not possess a stability field the natural fe bearing paragenesis of epidote sodic amphibole occupies a closed field in the pt diagram whose size increases dramatically as the ratio fe3 al in the minerals is increased where garnet is involved in bounding reactions such as with respect to eclogite and garnet bearing albite epidote amphibolite the epidote blueschist field decreases in size as the ratio fe2 mg is increased the classic blueschist to greenschist transition reaction is best developed in rocks with large fe3 and fe2 contents and is possibly metastable in mgal rich rocks in the latter a wedge of albite epidote actinolite barroisite amphibolite separates the epidote blueschist from the greenschits paragenesis field a first occurrence of garnet divides the epidote blueschist field into a lower temperature garnet free zone and a higher temperature garnet bearing zone although the temperatures calculated for this garnet isograd seem high the locations of all the facies boundaries are influenced by h2o activity however even minor amounts of co2 in the fluid phase x co2 0 005 render the epidote sodic amphibole paragenesis metastable so that facies boundaries cannot be shifted by varying the co2 content of the fluid phase below 600 c no mutual boundaries exist between the amphibolite facies and the epidote blueschist and eclogite facies respectively the calculated low pressure limit for the eclogite paragenesis pyralspite omphacite jd 50 quartz typical metabasite compositions is 12 to 14 kbars with h2o activity 0 9 or 1 5 to 2 kbar lower for jd 30

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

exhumation of coesite bearing rocks in the dora maira massif western alps italy coesite bearing very high pressure rocks in the southern dora maira massif western alps italy emerged from a depth of 100 km during cretaceous time these rocks are now covered only by a thin fraction of the synmetamorphic overburden tectonic contacts above the coesite bearing rocks are retrograde ductile shear zones that operated with a normal sense of motion they cut out thick crustal sections from within the tectonometamorphic pile and caused the condensation of the eoalpine baric and thermal structure the result of this was the lateral dispersion of the eoalpine orogenic wedge and the facilitation of the unroofing of the very high p rocks extensional unroofing is consistent with the absence of deeply cutting cretaceous erosion and with pressure temperature paths that show that the very high p rocks cooled during decompression the very high p rocks reached relatively shallow crustal levels by ca 40 ma and the attenuated sequence overthrust a more external unit these indicate that the exhumation of the very high p rocks took place within an overall plate onvergence setting prior to the final suturing of europe and adria

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

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

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

controls on the first appearance of jadeitic pyroxene northern diablo range california a reaction producing jadeitic pyroxene in metagreywackes of the northern diablo range has been identified on the basis of mineral distribution isograd patterns and composition of coexisting minerals the appearance of jadeitic pyroxene jd80 is closely followed by the disappearance of pumpellyite which indicates that pumpellyite plays a major role in the pyroxene producing reaction a new projection from hematite lawsonite chlorite quartz and h2o on to the naalo2 feo mgo ternary confirms the role of pumpellyite in pyroxene production and suggests a reaction of the form 1 00 pumpellyite 0 31 chlorite 8 71 albite 0 70 hematite 2 00 h2o 8 54 jadeite 0 57 glaucophane 3 09 lawsonite 5 26 quartz metagreywackes of the northern diablo range were metamorphosed under conditions of ph2o ptotal at 200 300 c and 7 5 10 0 kbar despite the low temperatures attained during metamorphism the assumption of equilibrium yields results consistent with field observations and phase relations

tectonic evolution from subduction to arc continent collision of the junggar ocean constraints from u pb dating and hf isotopes of detrital zircons from the north tianshan belt nw china the junggar ocean once situated north present coordinate of the yili and central tianshan blocks during early paleozoic to late carboniferous time was a major southern branch of the paleo asian ocean the opening expansion and final closure of which led to the development of the central asian orogenic belt between eastern europe siberia and tarim north china however the detailed evolution of the junggar ocean has not been well constrained this paper reports u pb and lu hf isotopic data of detrital zircons from sandstones in the north tianshan belt which can provide new insights into understanding the paleozoic development of the junggar ocean most detrital zircons exhibit oscillatory zoning and high th u ratios typical of igneous origin the predominant paleozoic zircons yield major age populations at ca 294 313 327 338 375 440 455 and 474 502 ma and are interpreted to have been derived from the long lived volcanic and island arc systems formed by the southward subduction of the junggar ocean and subsequent collisional and postcollisional magmatism the minor precambrian zircons yield ages scattering at ca 550 680 765 890 970 990 1160 1250 1500 1690 1750 1840 1970 2440 2500 and 2615 2700 ma which are nearly but not fully congruent with those from the adjacent beishan and kuluketage precambrian terranes therefore our results indicate that the central tianshan block was once part of the tarim block during precambrian time most of the 541 440 ma zircons possess low negative ehf t values while the 440 300 ma zircons exhibit dominantly positive ehf t values which can be linked to the junggar oceanic slab rollback since ca 440 ma this event subsequently gave rise to the opening of the south tianshan back arc basin ocean between the central tianshan and tarim blocks exhumation of high pressure granulites and formation of a series of island arcs in the junggar ocean combined with previous studies we suggest that the junggar ocean was probably closed at ca 300 ma in association with arc continent collision followed by postcollisional magmatism it deserves mentioning that the ca 0 5 and 1 4 1 5 ga detrital zircons and contemporaneous magmatic rocks only occur in the central tianshan block not in the tarim block therefore more detailed investigations are needed to better elucidate the origin and precambrian evolution of the central tianshan block

tectonic juxtaposition of blueschists and greenschists in sifnos island aegean sea implications for the structure of the cycladic blueschist belt abstract in the cyclades kilometre thick high pressure rock sequences displaying blueschist and eclogite facies mineral assemblages overlie a rock sequence which was thoroughly overprinted in the greenschist and amphibolite facies during its exhumation both rock sequences subducted and metamorphosed at high pressures in eocene times have been considered by previous workers to have been exhumed as a coherent rock unit in contrast it is suggested here that the preserved high pressure rock sequences were exhumed more rapidly and prior to the underlying greenschists significant metamorphic structural and geochronological discontinuities exist across the blueschist greenschist contact and field evidence suggests that the high pressure metamorphic rocks in sifnos are tectonically juxtaposed above the greenschists these two rock sequences were juxtaposed by a low angle fault subsequent to the oligocene miocene greenschist facies overprint published geochronological data and petrological criteria are used to show that the high pressure sequence cooled below 350 c when the rocks now immediately underlying it suffered a greenschist facies overprint at temperatures of ca 450 c the section inferred to absorb this temperature difference is now missing and it is suggested that it has been cut out by the low angle fault

deformed continental arc sequences in the south tianshan new constraints on the early paleozoic accretionary tectonics of the central asian orogenic belt abstract the tectonic affinity and structural evolution of the south tianshan are key issues to understand the paleozoic accretionary or collisional orogenesis in the southwestern central asian orogenic belt but remain controversial geological and structural investigations on the early paleozoic low grade metamorphic volcanic sedimentary sequences around the bayinbuluk basin demonstrate that they were prevalently reworked by north vergent ductile deformation and overlain unconformably by devonian to carboniferous carbonates zircon u pb dating indicates that the volcanic rocks erupted during the middle ordovician to early devonian time 460 410 ma and contain neoarchean to neoproterozoic zircon xenocrysts 2 75 ga 2 46 ga 1 60 ga 997 963 ma and 827 ma their whole rock elemental sr nd isotopic end t 5 0 to 3 6 and zircon lu hf isotopic ehf t 13 5 to 11 6 features resemble those of andean type continental arc volcanic rocks the coexisting tuffaceous sedimentary rocks indicate a depositional environment amidst a continental volcanic arc at 421 to 404 ma the newly obtained and available geochronological geochemical and structural data suggest that 1 a wide active continental margin covering the south and central tianshan and northern tarim was formed due to early mid paleozoic southward subduction of the paleo tianshan ocean beneath the tarim craton and 2 a mid paleozoic transition from advancing to retreating accretionary orogeny was followed by opening of the south tianshan back arc basins and break up of the northern tarim continental margin the south tianshan was subsequently subjected to regional deformation during the successive diachronous closure of the major and back arc oceanic basins and locally reworked by the ductile strike slip faulting 307 255 ma

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

blueschists and eclogites a possible plate tectonic mechanism for their emplacement from the upper mantle the most significant problem of blueschists and eclogite origin is explaining the mechanism by which they were transported from great depths to the surface three aspects of this problem are discussed 1 1 prograde and retrograde trajectories in blueschist and eclogite bearing belts and the preservation conditions for such minerals as aragonite lawsonite and coesite 2 2 the mechanism by which low density crustal rocks with coesite and diamond descend to a depth of 100 150 km and the high rate of return from these depths to the surface 3 3 the origin of a tectonic mixture of rocks from different depths in melange zones using estimations of p t conditions and constructed p t t trajectories four types of blueschist belt are proposed these include two of the ernst types the alpine and franciscan and two additional types the dora maira and sanbagawa types key examples of geologically and mineralogically well studied complexes in the soviet union are discussed the kokchetav massif maksutov complex syumkeu belt borus belt the various models for the origin of different types of high pressure metamorphic belts are considered 1 1 slow uplift or uplifting of the thickened crust after cessation of subduction 2 2 more rapid tectonic pushing out 3 3 the same as in case 2 but with an additional champagne cork effect 4 4 the most complicated process with repeated deformation and metamorphism

zircon dating hf sr nd os isotopes and pge geochemistry of the tianyu sulfide bearing mafic ultramafic intrusion in the central asian orogenic belt nw china abstract the tianyu mafic ultramafic intrusion is one of many important sulfide bearing mafic ultramafic intrusions in the eastern tianshan terrane located in the southern margin of the central asian orogenic belt in north xinjiang nw china the origin of sulfide mineralization in these intrusions their relationship with permian basalts and a type granites and the geodynamic setting of the bimodal magmatism in the region are controversial in this paper we use zircon u pb age hf sr nd os isotope and lithophile chalcophile trace element constraints from the tianyu mafic ultramafic intrusion to address these important issues the u pb age of zircon from the tianyu intrusion determined by sims is 280 2 ma this confirms that the tianyu intrusion is coeval with most sulfide bearing mafic ultramafic intrusions and some basalts and a type granites in the tianshan region similar to coeval basalts in the nearby tuha basin the tianyu intrusive rocks are characterized by negative nb anomaly and elevated end values varying between 1 and 4 6 the negative nb anomaly in the tianyu intrusion is not as pronounced as in the coeval a type granites but the end values for these bimodal rocks are similar the ehf values of zircon from the tianyu intrusion are positive varying between 3 and 8 these values suggest that the parental magma of the tianyu intrusion was dominated by melt derived from a depleted source mantle the os values of pyrrhotite from the tianyu intrusion are high ranging from 725 to 995 which can be modeled by selective assimilation of sulfide from the lower and upper crusts negative pt anomaly is present in relatively sulfide rich samples such as semi massive and massive sulfide ores but not in sulfide poor samples which is more consistent with the effect of post magmatic hydrothermal alteration than the original signature of the parental magma the results of modeling using the compositions of selected disseminated sulfide samples suggest that the parental magma of the tianyu intrusion was depleted in all pge which is consistent with previous sulfide segregation at depth our data and modeling results support the model of decompression melting of upwelling asthenosphere due to slab break off during a transition from oceanic subduction to arc arc or arc continent collision in the permian decompression melting of the upwelling asthenosphere in the region produced mafic magma partial melting in the lower parts of a juvenile arc crust due to mafic magma underplating produced granitic melts mixing of the mafic magma with a small amount of granitic melts at depth followed by contamination with the upper crust at a higher level formed the parental magma of the tianyu intrusion pge depletion in the parental magma of the tianyu intrusion can be attributed to sulfide segregation associated to the first stage of crustal contamination at depth we suggest that the formation of sulfide ores in the intrusion was related to the second stage of crustal contamination in the upper crust

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

syn and post collisional granitoids in the central tianshan orogen geochemistry geochronology and implications for tectonic evolution abstract the eastern part of the central tianshan belt in china is characterized by abundant granitoids which are not only indicators of the interaction between crust and mantle but also can be used for tracking the tectonic evolutionary history of the orogen four granitic plutons muscovite granites mg biotite monzonitic granites bmg biotite granites bg and alkali granites ag have been recognized to have intruded in the mishigou area central tianshan belt the mg has pronounced a s type affinity which is strongly peraluminous with a high aluminum saturation index and displays positive rb th u and lree anomalies with a strong negative ba nb sr p ti and eu anomalies therefore the magma of the mg is considered to have been derived from melting of thickened continental crust which is composed mainly of the sediments eroded from a pre silurian continental island arc the u pb zircon age of 424 5 2 6 ma constrains the age of mg and crust thickening the bg and bmg granitoids display i type geochemical features and have affinities to subduction arcs both of them are characterized by calc alkaline peraluminous granites with a significant lree enrichment and a negative eu anomaly as well as a depletion in ba nb ta sr p and ti and an enrichment in rb th u nd zr and sm these characteristics indicate that they were derived from a mixed magma source between lower continental crust and the input of components derived from the mantle wedge above the subduction zone the la icpms zircon u pb ages of 411 4 7 ma and 402 3 4 ma represent the formation ages of the bg and bmg granitoids respectively which also constrain that the subduction of the south tianshan oceanic crust also occurred during early devonian time the ag shows a typical a type granite affinity with depletion of nb ta sr p ti and y it is interpreted that ag was formed in a within plate tectonic setting and related to continental up doming and rifting zones as a consequence of the extensional collapse after the collision of the tianshan orogen the u pb zircon age of 290 5 1 ma represents the formation age of the ag and the time of the extensional collapse and rifting

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

tectonic history of subduction zones inferred from retrograde blueschist p t paths many phanerozoic convergent plate junctions are marked by discontinuous blueschist belts reflecting relatively high pressure p prograde trajectories common blueschist paragneisses such as those of the western alps exhibit widespread overprinting by greenschist and or epidote amphibolite facies assemblages for this type of high p belt retrograde metamorphism involved fairly rapid nearly isothermal decompression some terranes underwent continued heating during early stages of pressure release uplift probably occurred as a consequence of the entrance of an island arc oceanic plateau or segment of continental crust into the subduction zone collision resulting in marked deceleration or cessation of underflow and buoyant approximately adiabatic rise of the stranded recrystallized subduction complex other high p belts such as the franciscan of western california preserve metamorphic aragonite and lack a low p overprint retrogression approximately retraced the prograde p t temperature path or for early formed high grade blocks occurred at even higher p t ratios parts of this type of metamorphic belt evidently migrated slowly back up the subduction zone in response to isostatic forces during continued plate descent and refrigeration upward motion took place as tectonically imbricated slices as laminar return flow in melange zones and perhaps partly a lateral spreading extension of the underplated accretionary prism retrograde p t trajectories of high pmore belts therefore provide important constraints on the tectonic evolution of convergent plate junctions less

uplift and exhumation of high pressure metamorphic terrains the example of the cycladic blueschist belt aegean sea the cycladic blueschists emerged from a depth of more than 50 km during the tertiary these rocks are now covered only by a thin fraction of the synmetamorphic overburden upper plate remnants of the upper plate include low p crystalline rocks and sediments which have been juxtaposed onto the lower plate blueschists in several stages since the early miocene the low angle contacts operated as normal faults which removed the overburden from above the cycladic blueschists detailed investigations on tinos island indicate that the fractured blocks of the upper plate lying above a low angle normal fault did not rotate this shows that the faults initiated as rather gently dipping fractures similar structures were also reported from other blueschist terrains kinematic analysis of such flat lying faults implies that the synmetamorphic overburden should be mostly located several hundred kilometers away from the present exposure of the blueschists considering that the cycladic blueschist belt exposes successively shallower units in the west and that rocks of the upper plate have an anatolian nature we suggest that these blueschists were dragged from beneath turkey we recognize that the blueschist unit has undergone further thrusting in the final stage of the exhumation as have its counterparts elsewhere in the hellenides the emergence of the blueschists in the tertiary was concomitant with continental collision and thrusting in the hellenides the operation of low angle normal faults with large extensional displacements occurred within this context of collision tectonics this extension occurred parallel to the strike of the convergent orogen

blueschists in major suture zones of china blueschist facies rocks and c type eclogites occur as allochthonous terranes or as small tectonic blocks in almost all the accretionary fold belts and major suture zones within the interior of china and along the circum pacific and tethys himalayan belts at least forty blueschist localities have been described blueschists around the pacific margins and the himalaya alpine belts are mostly mesozoic in age only those in taiwan and new caledonia are cenozoic intracratonal blueschists of china are older than late permian when the collision amalgamation presumably occurred several alleged precambrian blueschist localities ye 1987 are shown in the recently published metamorphic map of china a precambrian blueschist belt in anhui hubei central china extends for about 2300 km however except for the pre sinian blueschist terrane in aksu the age determination for precambrian blueschist metamorphism elsewhere has not been confirmed characteristic features of chinese blueschists include the following 1 blueschist facies metamorphism predates the continental collision amalgamation 2 most blueschists have undergone multistage metamorphism commonly with increasing temperature and or decreasing pressure for later events 3 assemblages include sodic amphibole epidote albite quartz phengite chlorite sphene lawsonite jadeitic pyroxene and aragonite are not common and 4 protoliths are mainly mafic pelagic and clastic rocks blueschist facies metamorphism in china may be divided into two types on the basis of the imposed geothermal gradient hence the progressive change in mineral assemblage the rare lawsonite bearing blueschists produced at lower temperatures and higher pressures are confined to the inner mongolia and yarlung zangbo suture zones where aragonite and possibly jadeitic pyroxene quartz occur and the blueschists are associated with subgreenschist facies rocks most other blueschists of china formed at higher temperatures along the transitional blueschist greenschist facies boundary where epidote sodic and calcic amphiboles are ubiquitous and the blueschists are interlayered with greenschist facies rocks the common occurrence of intracratonal blueschists in major suture zones of china and elsewhere in eurasia indicates pre mesozoic subduction accretion of oceanic crust and flysch sediments these zones of accretion represent growth around the periphery of major precambrian cratons this fact together with the distribution of ophiolite belts and available paleomagnetic data provides evidence for an extremely mobile history of plate movement in eurasia however the paleozoic intracontinental blueschists and ophiolites of china have formed before rather than during final closure between cratons they do not usually mark the location of terminal sutures but are the result of earlier accretion and continental growth by subduction underplating and imbrication of oceanic materials similar to the accretion history of westernmost north america

structural overprints of early paleozoic arc related intrusive rocks in the chinese central tianshan implications for paleozoic accretionary tectonics in sw central asian orogenic belts abstract the chinese central tianshan constitutes the centric and vital component of the tianshan orogen it contains important information for understanding the accretionary orogeny of the central asian orogenic belt caob to better constrain the paleozoic evolution of the chinese central tianshan we present new field observations fabric analyses zircon u pb ages zircon lu hf isotopic and whole rock geochemical data of mafic intermediate and felsic intrusive rocks in the baluntai area chinese central tianshan field scale and microscopic structural patterns of the intrusive rocks reveal that the early paleozoic gabbro diorite intrusions underwent multistage structural reworking and generally exhibit gently south or north dipping foliations and microscopic kinematic fabrics indicating top to the north shearing these ductilely deformed intrusive rocks were intruded by granitic sills or dikes that are generally sub parallel or with low angle to the main foliations of the host rocks gentle foliations and top to the north kinematics can be rarely noticed within the granitic intrusions at the sites close to the baluntai fault locally intense mylonitization occurred in the felsic intrusions and deformed mafic intermediate ones where earlier shallow foliations were completely or partially transformed to steep ones bearing sub horizontal stretching lineations and asymmetric kinematic indicators suggest dextral shearing zircon la icp ms u pb datings constrain the emplacement ages of the gabbroic and dioritic intrusive rocks at 452 420 ma and the crystallization age of one mylonitic k granite at 417 ma two samples of granitic sill contain plenty of zircons inherited from the host gabbroic and dioritic rocks and a few such zircons have dark rims probably formed during granitic injection dating on these rims roughly constrains the crystallization ages of the felsic sills at ca 370 360 ma the ehf t values of a gabbro 420 ma two diorite samples 425 and 445 ma and a mylonitic granite 417 ma vary from 15 45 to 2 06 corresponding to two stage crustal model ages tdm c ranging from 2 4 ga to 1 5 ga suggesting variable degrees of involvement of the proterozoic crustal basement and the juvenile mantle components during the early paleozoic magmatic generation the geochemical data of the mafic intermediate plutons exhibit features of magmatic rocks formed in continental arcs the mylonitic granite and one undeformed granite resemble a type granite in terms of geochemical features suggesting that these granites were produced in a post collisional extensional environment considering the previously published data the early paleozoic arc type magmatic rocks within the chinese central tianshan likely resulted from the southward subduction of an oceanic basin located to the north and ceased progressively during 420 400 ma the amalgmation between chinese central tianshan and tarim most likely completed at ca 360 ma following the closure of the south tianshan back arc basin this event reworked the chinese central tianshan arc and formed the top to the north kinematics thereafter the chinese central tianshan was affected by the early permian dextral strike slip faulting due to the regional large scale transcurrent tectonics and overprinted by the permian mafic magmatism

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

tectonic evolution of the chinese tianshan orogen from subduction to arc continent collision insight from polyphase deformation along the gangou section central asia the central asian orogenic belt as the largest accretionary orogen on earth is an ideal candidate to study the geodynamics of convergent plate boundaries through a prolonged period the evolution of this orogen has been explained by different tectonic models which incorporated one or a combination of the following mechanisms lateral stacking of arc systems along major shear zones arc amalgamation oroclinal bending and trench migration here we elucidate major mechanisms responsible for the tectonic evolution of the central asian orogenic belt focusing on the chinese tianshan orogen in the southern central asian orogenic belt structural observations from the 50 km long gangou section show evidence of polyphase deformation the earliest episode of orogen parallel sinistral shearing constrained to the early devonian ca 399 ma by syn deformational intrusions was possibly controlled by oblique subduction this was followed by an episode of ne sw contractional deformation dated at ca 356 ma 40ar 39ar age of syn deformational hornblende and likely associated with an episode of trench advance the third stages of deformation during the latest carboniferous and permian involved ne sw contraction orogen parallel extension and dextral transpression our new geochronological data constrain the timing of orogen parallel extension to ca 303 293 ma and confirm that dextral activation along shear zones occurred during the permian the results highlight the role of trench migration oblique tectonics and syn collisional orogen parallel extension in the build up of the central asian orogenic belt and contribute to the pre collisional reconstruction of this orogenic system

late paleozoic strike slip shear zones in eastern central asia nw china new structural and geochronological data 1 new structural studies and 40ar 39ar dating in northwest china provide information about late paleozoic strike slip motions subsequent to accretional events which built eastern central asia during the paleozoic two principal areas were affected by these large transcurrent motions first in the tianshan range main east west ductile shear zones are dextral and coeval with an eastward decreasing greenschist retrograde metamorphism associated biotites give ages ranging from 290 ma to 245 ma the earlier n110 shearing occurred in western tianshan while the last one dextral in whole tianshan occurred 250 245 myr ago second in the chinese altay region several nw se shear zones structured the area the main motion is sinistral and occurred along the erqishi zone at 280 290 ma it is followed by a complex succession of dextral and sinistral shearing episodes leading to the northwestward structuring dated at 245 ma of a metamorphic zone that was folded during a compressive event

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

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

paleozoic magmatism and metamorphism in the central tianshan block revealed by u pb and lu hf isotope studies of detrital zircons from the south tianshan belt nw china as a major precambrian microcontinent in the southernmost central asian orogenic belt caob the central tianshan block cts in the chinese tianshan is essential for understanding the final assembly of the southern caob it experienced multistage paleozoic magmatism and metamorphism but the detailed processes are still controversial and far from being completely understood this paper reports coupled u pb and lu hf isotopic data of detrital zircons from late paleozoic meta sedimentary strata in the south tianshan belt which can provide new insight into deciphering the paleozoic evolution of the eastern segment of the cts block characterized by typical oscillatory zoning and high th u ratios 0 2 detrital zircons in the permian sedimentary samples yield dominant age populations at ca 505 490 ma 475 440 ma 430 400 ma and 340 250 ma pinpointing the development of four episodes of magmatism in the eastern cts block particularly ordovician silurian 475 440 ma zircons possessing low negative ehf t values predominate in sedimentary strata in and surrounding the cts block indicating that the 475 440 ma magmatic rocks probably constitute the main body of the cts block the origin of this early paleozoic episode of magmatism was most likely related to the southward subduction of the junggar ocean beneath the cts block carboniferous triassic 340 250 ma zircons have dominantly positive ehf t values probably derived from the post collisional juvenile rocks in the cts block combined with previous studies our data suggest that the single source terrane for the sampled strata was the cts block which had been a topographic high providing substantial detritus to the surrounding areas at least since the early permian in the metasedimentary sample detrital zircons mostly show partially fully recrystallized internal textures and low th u ratios 0 2 probably sourced from the amphibolite to granulite facies metamorphosed rocks in the eastern cts block the youngest zircons suggest that the sampled metasedimentary strata were carboniferous 341 305 ma in age rather than neoproterozoic or early sinian in chinese literatures as previously interpreted based on u pb hf isotopes and ti in zircon thermometer of metamorphic zircons in this study as well as previous investigations we suggest that the eastern cts block has experienced a a 400 370 ma collisional event related to the closure of the eastern segment of the south tianshan ocean and b a 360 340 ma extensional event as a result of post collisional mantle upwelling therefore the final assembly of the eastern tianshan took place during late carboniferous time

early paleozoic tectonic evolution of the chinese south tianshan orogen constraints from shrimp zircon u pb geochronology and geochemistry of basaltic and dioritic rocks from xiate nw china traditionally the chinese south tianshan has been regarded as a late paleozoic orogenic belt however little is known about the early paleozoic tectonic architecture of the region this paper presents the first evidence of cambrian ordovician morb type basalts and adakitic diorites on the southern margin of the yili plate in china basalts from xiate in southwestern tianshan show a typical transitional t morb and ferrobasalt composition which indicate a formation at a propagating spreading ridge the basalts give a weighted mean 206pb 238u crystallization age of 516 3 7 4 ma by shrimp u pb zircon dating and have experienced contact metamorphism due to the intrusion of a dioritic pluton the dioritic pluton has a weighted mean 206pb 238u crystallization age of 470 12 ma and geochemical characteristics resembling that of adakitic rocks the pluton is considered to have been formed by partial melting of garnet amphibolites from thickened lower crust in arc or continental collision settings the basalts and diorites are considered to outline the eastern extension of the early paleozoic suture zone the nikolaev line which stretches east west for hundreds of kilometers between the northern tianshan and central tianshan terranes of kyrgyzstan our findings substantiate that the yili and central tianshan plates were separated by the early paleozoic terskey ocean the terskey ocean probably closed during the early stage of the late ordovician lomize et al in geotectonics 31 6 463 482 1997 resulting in the final amalgamation of the yili and central tianshan plates consequently an early paleozoic suture zone is documented in the chinese tianshan region which is most likely represented by the north nalati fault

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

petrography and geochemistry of lower carboniferous greywacke and mudstones in northeast junggar china implications for provenance source weathering and tectonic setting abstract northeast junggar occupies an important position that links east junggar and chinese altai numerous magmatic and sedimentary rocks of the paleozoic in this area recorded the final amalgamation processes of east junggar and chinese altai this study analyzes the petrological and geochemical characteristics of sandstones and mudstones from the early carboniferous nanmingshui formation in northeast junggar the provenance and tectonic setting of these clastic rocks are discussed petrography indicates that the composition and texture maturity of the sandstones are low the components of the sandstones are mainly volcanic fragments 61 87 feldspars 9 30 and monocrystalline quartz 2 18 with a few polycrystalline quartz and other minerals slice observation indicates that the majority of the volcanic fragments of sandstones are basic intermediate volcanic rocks with a few dacite and felsic plutonic fragments the detrital modes of the sandstones reflect that these sandstones are derived from undissected arcs a low to moderate chemical index of alteration and the al2o3 cao na2o k2o diagram reflect a low to moderate weathering degree in the source area trace and rare earth element e g la th hf sc cr co and eu contents and their ratios suggest that the source rocks of the clastic rocks are intermediate basic rocks with some felsic rocks compared with sandstones the source rocks for mudstones are more felsic the petrography and geochemistry characteristics of the clastic rocks suggest that the proximal dulate arc should be the primary source area mixing calculations based on ree data suggest that approximately two thirds of the sandstone fragments are intermediate basic volcanic rocks the contents of the major and trace elements as well as the stratum features of the clastic rocks manifest that these clastic rocks resemble sedimentary rocks in a back arc basin the formation of this back arc basin is caused by the southward subduction of the zaysan erqis ocean

exhumation of high pressure metamorphic rocks the exhumation of high pressure metamorphic minerals necessitates the removal of overburden which was present at the time of metamorphism assuming metamorphism during an orogenic phase and exhumation during an uplift phase the model presented here is based on postulating continuous deformation at the active margin of an ocean continent interaction accepting the conclusion that coesite bearing eclogites and other high pressure minerals were formed when a sliver of continental crust was subducted down to 100 km i propose that their uplift was caused by a shift of the beniofff zone the high pressure slab now accreted to the continental lithosphere was lifted up the ramp of a benioff zone when ocean lithosphere was thrust under the continent the geology of the western alps illustrates this model