prolonged magmatism juvenile nature and tectonic evolution of the chinese altai nw china evidence from zircon u pb and hf isotopic study of paleozoic granitoids abstract paleozoic granitoid magmatism played an important role in the tectonic evolution of the chinese altai and zircon u pb and hf isotopic compositions have been determined for samples from eleven granitic plutons batholiths the jiadengyu gneissic granitic pluton yielded a zircon u pb age of 479 ma and thus it does not represent precambrian basement as suggested previously our results and published data demonstrate that voluminous granitoids were continuously emplaced over more than 30 area of the chinese altai during the period from 447 ma to 368 ma with a climax at ca 400 ma ages for zircon overgrowth rims demonstrate additional thermal events at ca 360 and 280 ma respectively positive e hf t values 0 to 9 of normal magmatic zircons suggest that the granitoid magmas were derived from juvenile sources xenocrystic zircon cores are 543 421 ma old and also give positive e hf t values 2 5 to 12 suggesting their origin as early crystallized minerals in the magma chambers or as inherited cores from newly accreted meterials the strong magmatism at ca 400 ma significantly changed the hf isotopic composition of the magma source by substantial input of juvenile material in a relatively short period geophysical geological and geochemical data support that ridge subduction was a possible mechanism for the strong magmatism ca 400 ma and the above mentioned change of hf isotopic composition in the magma source

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

assessing the reliability of the modified three component spatial autocorrelation technique summary analysis of seismic ambient vibrations is becoming a widespread approach to estimate subsurface shear wave velocity profiles however the common restriction to vertical component wavefield data does not allow investigations of love wave dispersion and the partitioning between rayleigh and love waves in this study we extend the modified spatial autocorrelation technique mspac to three component analysis 3c mspac by determination of love wave dispersion curves this technique provides additional information for the determination of shear wave velocity depth profiles furthermore the relative fraction of rayleigh waves in the total portion of surface waves on the horizontal components is estimated tests of the 3c mspac method are presented using synthetic ambient vibration waveform data different types of surface waves are simulated as well as different modes in addition different spatial distributions of sources are used we obtain rayleigh and love wave dispersion curves for broad frequency bands in agreement with the models used for waveform simulation the same applies for the relative fraction of rayleigh waves dispersion curves are observed at lower frequencies for love waves than for rayleigh waves while 3c mspac has clear advantages for determination of love waves velocities 3c mspac and conventional vertical frequency wavenumber analysis complement each other in estimating the rayleigh wave dispersion characteristics inversions of the dispersion curves for the shear wave velocity depth profile show that the use of love wave velocities confirms the results derived from rayleigh wave velocities in the presence of higher mode surface waves love waves even can improve results application of 3c mspac to ambient vibration data recorded during field measurements pulheim germany show dominance of love waves in the wavefield existing shear wave profiles for this site are consistent with models obtained from inversion of rayleigh and love wave dispersion curves

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

seismic waves estimation and wavefield decomposition application to ambient vibrations summary passive seismic surveying methods represent a valuable tool in local seismic hazard assessment oil and gas prospection and in geotechnical investigations array processing techniques are used in order to estimate wavefield properties such as dispersion curves of surface waves and ellipticity of rayleigh waves however techniques presently in use often fail to properly merge information from three components sensors and do not account for the presence of multiple waves in this paper a technique for maximum likelihood estimation of wavefield parameters including direction of propagation velocity of love waves and rayleigh waves and ellipticity of rayleigh waves is described this technique models jointly all the measurements and all the wavefield parameters furthermore it is possible to model the simultaneous presence of multiple waves the performance of this technique is evaluated on a high fidelity synthetic data set and on real data it is shown that the joint modelling of all the sensor components decreases the variance of wavenumber estimates and allows the retrieval of the ellipticity value together with an estimate of the prograde retrograde motion

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

italian and alpine three dimensional crustal structure imaged by ambient noise surface wave dispersion we derive the 3 d crustal structure s wave velocity underneath italy and the alpine region expanding and exploiting the database of ambient noise rayleigh wave phase and group velocity of verbeke et al 2012 we first complement the database of verbeke et al 2012 with a dense set of new ambient noise based phase velocity observations we next conduct a suite of linear least squares inversion of both phase and group velocity data resulting in 2 d maps of rayleigh wave phase and group velocity at periods between 5 and 37 s at relatively short periods these maps clearly reflect the surface geology of the region e g low velocity zones at the po plain at longer periods deeper structures such as moho topography under alps and apennines and lower crust anomalies are revealed our phase and group velocity models are next inverted via the neighbourhood algorithm to determine a set of one dimensional shear velocity models one per phase group velocity pixel resulting in a new three dimensional model of shear velocity vs parameterized in the same way as the european reference crustal model epcrust we also show how well vs is constrained by phase and group dispersion curves the model shows the low velocity area beneath the po plain and the molasse basin the contrast between the low velocity crust of the adriatic domain and the high velocity crust of the tyrrhenian domain is clearly seen as well as an almost uniform crystalline crust beneath the alpine belt our results are discussed from the geological geodynamical standpoint and compared to those of other interdisciplinary studies

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

irarsite hollingworthite solid solution series and other associated ru os ir and rh bearing pgm s from the shetland ophiolite complex pure end and intermediate members of the irarsite hollingworthite solid solution series occur in the shetland ophiolite complex hollingworthite frequently rims irarsite their compositions are unusually pt poor compared with analyses of these minerals from elswhere suggesting the existence of a pt poor environment during their formation ir sb s and rh sb s have been identified as inclusions within irarsite ir sb s and rh sb s together with rh ni sb are thought to be new platinum group minerals pgm s in ophiolite complexes two types of laurite are present an os rich up to 22 os variety is entirely enclosed by chromite whereas an os free variety is located in the silicate matrix interstitial to the chromite laurites in the rims of chromite grains are os free but contain tiny inclusions of native osmium it is suggested that either the availability of os decreased during crystallisation of the laurites or that os has been removed from laurites not totally enclosed by chromite in a few cases laurite is surrounded by a ruthenian pentlandite containing up to 12 ru

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

geochronology and geochemistry of late paleozoic magmatic rocks in the lamasu dabate area northwestern tianshan west china evidence for a tectonic transition from arc to post collisional setting abstract voluminous late paleozoic igneous rocks and associated cu au mo deposits occur in the northwestern tianshan district xinjiang west china however the tectonic setting and petrogenesis of these rocks remain controversial this paper reports zircon u pb and hf cuu isotopic data major and trace elements and sr nd pb isotopic data for the intrusive rocks and minor dacites in the lamasu dabate area of northwestern tianshan adjacent to the cu au mo deposits la icpms u pb zircon analyses suggest that the lamasu porphyries were formed at 366 3 ma and contain 907 738 ma inherited zircons the dabate dacites were formed at 316 4 ma and granite porphyries were formed at 289 3 ma with 319 ma inherited zircons the lamasu porphyries consist of plagioclase granite and granodiorite and are geochemically similar to adakites e g having high al2o3 14 54 19 75 wt and sr 308 641 ppm and low y 7 84 16 9 ppm contents with fractionated rare earth element ree patterns and slightly positive sr anomalies however they have variable initial ratios of 87sr 86sr 0 7072 0 7076 and 206pb 204pb 18 139 18 450 and variable end t 5 6 to 0 8 and positive ehf t 1 4 to 10 6 values they also have variable mg 100 mg2 mg2 fe2 41 73 and low th 3 13 8 09 and th ce 0 14 0 28 values we suggest that the lamasu adakitic magmas were generated through partial melting of southward subducted junggar oceanic crust with subsequent melt mantle interaction and assimilation of basement rocks the dabate dacites show typical arc like geochemical characteristics e g enrichment of large ion lithophile elements lile and strong negative anomalies of ta nb p and ti with variable end t 0 1 to 3 3 they were probably generated by melting of juvenile basaltic lower crust as a result of magma underplating the dabate granite porphyries are geochemically similar to a2 type granites e g high sio2 75 6 77 6 wt and alkalis na2o k2o 8 27 8 70 wt low cao 0 28 0 34 wt and mg 2 10 and obvious negative eu ba and sr anomalies they have variable values of end t 1 7 to 0 and 206pb 204pbi 18 567 18 721 and zircon ehf t 0 5 to 11 1 the dabate a type granite porphyries contain late carboniferous residual zircon cores suggesting that their source rock possibly contained carboniferous arc igneous rocks taking into account all available data from late paleozoic magmatic rocks and cu mo au mineralization in the northern tianshan district we suggest that the dabate lamasu area was a continental arc during the late devonian carboniferous but had entered a post collisional stage by the early permian 290 ma

geochronology geochemistry and petrogenesis of the late palaeoproterozoic a type granites from the dunhuang block se tarim craton china implications for the break up of the columbia supercontinent the discovery of c 1 77 ga a type granite in the tarim craton tc provides the first evidence that supports an extensional event related to fragmentation of the columbia supercontinent in the late palaeoproterozoic we present laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb ages lu hf isotopic data and the whole rock geochemical and nd isotopic data of a type granites in the dunhuang area in the se tarim craton zircon u pb dating for three granite samples indicate that they were emplaced at c 1 77 ga zircons from these granites have e hf t values ranging from 5 9 to 8 7 corresponding to two stage model ages of 1 9 2 7 ga these granites exhibit the following petrological geochemical characteristics that are typical of a type granite a high content of sio 2 and alkalis i e high k 2 o na 2 o with k 2 o na 2 o 1 enrichment of high field strength elements hfse and rare earth elements ree except for eu and extreme depletion of ba sr p ti and eu b 10000 ga al ratios in the dunhuang granites of 3 5 4 4 with an average value of 3 79 which is similar to the global average of 3 75 for a type granites c the presence of characteristic minerals such as amphibole sphene and perthite and d zirconium saturation temperature results indicate that the dunhuang granites have high initial magmatic temperatures in the range 887 950 c similar to those of typical of a type granites whole rock e nd t values range from 2 5 to 6 2 and t dm model ages from 2 3 to 2 7 ga nd hf isotopic and whole rock geochemical data indicate that these granites were most likely derived from the late archean crustal source in a post collisional post orogenic extensional tectonic environment the late palaeoproterozoic a type granites in the tc could be correlated with those of the north china craton ncc india and the canadian shield thus demonstrating extensional tectonics and break up of the columbia supercontinent

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

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

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

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

chromitites platiniferes des complexes ophiolitiques de l estrie beauce appalaches du sud du quebec des chromitites contenant du platine du palladium et du rhodium viennent d etre mises a jour dans les appalaches du sud du quebec les complexes ophiolitiques hotes de ces mineralisations sont d ge ordovicien inferieur et d affinite boninitique de plus ils sont etroitement lies a un arc insulaire les chromitites a pt pd rh se situent dans les dunites de la sequence a cumulat a leur passage aux pyroxenites le platine le palladium et le rhodium sont exprimes soit sous forme d alliages et de sulfures complexes automorphes ou soit sous forme d alliages a pt pd et d arseniures rhodiferes xenomorphes cette derniere paragenese accompagne la serpentinisation des cumulats ultramafiques

ambient vibration analysis on large scale arrays when lateral variations occur in the subsurface a study case in switzerland the ambient vibration analysis is a non invasive and low cost technique used in site characterization studies to reconstruct the subsurface velocity structure depending on the goal of the research the investigated depth ranges from tens to hundreds of meters in this work we aimed at investigating the deeper contrasts within the crust and in particular down to the sedimentary rock basement transition located at thousands of meters of depth to achieve this goal three seismic arrays with minimum and maximum interstation distances of 7 9 m and 26 8 km were deployed around the village of schafisheim schafisheim is located in the swiss molasse basin a sedimentary basin stretching from lake constance to lake geneva with a thickness ranging from 800 to 900 m in the north to 5 km in the south to compute the multimodal dispersion curves for rayleigh and love waves and the rayleigh wave ellipticity angles the data were processed using two single station and three array processing techniques a preliminary analysis of the inversion results pointed out a good agreement with the fundamental modes of rayleigh and love waves used in the inversion and a quite strong disagreement with the higher modes the impossibility to explain at the same time most of the dispersion curves was interpreted as the co existence within the investigated area of portions of the subsurface with different geophysical properties the hypothesis was confirmed by the horizontal to vertical spectral analysis h v which indicated the presence of two distinguished areas the observation allowed a new interpretation and the identification of the rayleigh and love wave fundamental modes and of the s wave velocity profiles to be reconstructed for each investigated zone it results in two s wave velocity profiles with similar velocities down to 15 km deferring only in their shallow portions due to the occurrence of a low velocity zone at a depth of 50 150 m at the centre of the investigated area

architecture and mineral deposit settings of the altaid orogenic collage a revised model abstract the altaids are an orogenic collage of neoproterozoic paleozoic rocks located in the center of eurasia this collage consists of only three oroclinally bent neoproterozoic early paleozoic magmatic arcs kipchak tuva mongol and mugodzhar rudny altai separated by sutures of their former backarc basins which were stitched by new generations of overlapping magmatic arcs in addition the altaids host accreted fragments of the neoproterozoic to early paleozoic oceanic island chains and neoproterozoic to cenozoic plume related magmatic rocks superimposed on the accreted fragments all these assemblages host important many world class late proterozoic to early mesozoic gold copper molybdenum lead zinc nickel and other deposits of various types in the late proterozoic during breakup of the supercontinent rodinia the kipchak and tuva mongol magmatic arcs were rifted off eastern europe siberia and laurentia to produce oceanic backarc basins in the late ordovician the siberian craton began its clockwise rotation with respect to eastern europe and this coincides with the beginning of formation of the mugodzhar rudny altai arc behind the kipchak arc these earlier arcs produced mostly cu pb zn vms deposits although some important intrusion related orogenic au deposits formed during arc arc collision events in the middle cambrian and late ordovician the clockwise rotation of siberia continued through the paleozoic until the early permian producing several episodes of oroclinal bending strike slip duplication and reorganization of the magmatic arcs to produce the overlapping kazakh mongol and zharma saur valerianov beltau kurama arcs that welded the extinct kipchak and tuva mongol arcs this resulted in amalgamation of the western portion of the altaid orogenic collage in the late paleozoic its eastern portion amalgamated only in the early mesozoic and was overlapped by the transbaikal magmatic arc which developed in response to subduction of the oceanic crust of the paleo pacific ocean several world class cu mo porphyry cu pb zn vms and intrusion related au mineral camps which formed in the altaids at this stage coincided with the episodes of plate reorganization and oroclinal bending of magmatic arcs major pb zn and cu sedimentary rock hosted deposits of kazakhstan and central asia formed in backarc rifts which developed on the earlier amalgamated fragments major orogenic gold deposits are intrusion related deposits often occurring within black shale bearing sutured backarc basins with oceanic crust after amalgamation of the western altaids this part of the collage and adjacent cratons were affected by the siberian superplume which ascended at the permian triassic transition this plume related magmatism produced various deposits such as famous ni cu pge deposits of norilsk in the northwest of the siberian craton in the early mesozoic the eastern altaids were oroclinally bent together with the overlapping transbaikal magmatic arc in response to the northward migration and anti clockwise rotation of the north china craton the following collision of the eastern portion of the altaid collage with the siberian craton formed the mongol okhotsk suture zone which still links the accretionary wedges of central mongolia and circum pacific belts in the late mesozoic a system of continent scale conjugate northwest trending and northeast trending strike slip faults developed in response to the southward propagation of the siberian craton with subsequent post mineral offset of some metallogenic belts for as much as 70 400 km possibly in response to spreading in the canadian basin india asia collision rejuvenated some of these faults and generated a system of impact rifts

characterization of shallow geology by high frequency seismic noise tomography summary to study the applicability of the passive seismic interferometry technique to near surface geological studies seismic noise recordings from a small scale 2 d array of seismic stations were performed in the test site of nauen germany rayleigh wave green s functions were estimated for different frequencies a tomographic inversion of the traveltimes estimated for each frequency from the green s functions is then performed allowing the laterally varying 3 d surface wave velocity structure below the array to be retrieved at engineering geotechnical scales furthermore a 2 d s wave velocity cross section is obtained by combining 1 d velocity structures derived from the inversion of the dispersion curves extracted at several points along a profile where other geophysical analyses were performed it is shown that the cross section from passive seismic interferometry provides a clear image of the local structural heterogeneities that are in excellent agreement with georadar and geoelectrical results such findings indicate that the interferometry analysis of seismic noise is potentially of great interest for deriving the shallow 3 d velocity structure in urban areas

platinum group elements co ni fe sulfarsenides and mineral paragenesis in cu ni platinum group element deposits copper cliff north area sudbury canada three deposits the offset sublayer 100 and 120 orebodies and contact sublayer lady violet deposit in the copper cliff north mine area sudbury were sampled with the aim of defining the mineral assemblages the ores are composed of pyrrhotite pentlandite chalcopyrite with minor titanomagnetite ilmenite accessory sulfarsenides bi te minerals and sperrylite sulfarsenides which are prominent accessory minerals contain the platinum group elements pge rh ru ir os and pt in decreasing order of abundance in solid solution some zoned sulfarsenide crystals have pge concentrated in the centers decreasing toward the edges nickeliferous cobaltite the common sulfarsenide contains as much as 16 at percent pge hollingworthite rh ru ass which occurs as cores or distinct crystals enclosed in cobaltite is a newly recognized occurrence in the 100 120 and lady violet deposits in the sudbury south range formation of primary pge bearing sulfarsenides is related to exsolution from monosulfide solid solution at about 550 to 600 c during late crystallization aqueous fluids interacted with the rocks resulting in precipitation of hydrous silicates epidote biotite chlorite and remobilization of pge bearing sulfarsenides

high pge contents and extremely abundant pge minerals hosted in chromitites from the veria ophiolite complex northern greece abstract more than 100 small isolated chromite occurrences form elongated lenses that follow the nw se trend of the host serpentinized dunite within a 60 km 2 area 15 km se of veria northern greece they occur only within mylonite shear zones characterized by a strong overprint of late brittle deformation detailed investigation of chromitites by scanning electron microscope with energy dispersive x ray analysis shows that chromite is dominantly of high cr composition with the atomic cr cr al ratio ranging from 0 55 to 0 84 and an average value of 0 7 interstitial primary silicates in chromitites have been replaced by secondary phases consisting of mostly cr andradite cr chlorite and serpentine flakes in lesser amounts the most salient feature is the high total platinum group element pge content up to 25 ppm coupled with the presence of extremely abundant solitary platinum group element minerals pgm within massive chromite samples located along a shear zone these pgm fragments are angular rarely corroded grains or fine grained aggregates and occur in a matrix of cr garnet they are mostly os ru ir fe alloys containing small amounts of cr co mn followed by irarsite laurite rus 2 with variable os and ir contents occurs as remnants surrounding by ru os ir oxides of varying composition available geological mineralogical and geochemical data indicate a prolonged multistage evolution at least four stages for the pge mineralization the relatively high ipge enrichment in chromitites seems to be related to post magmatic processes encompassing a long period of deformation episodes beginning with plastic deformation in the asthenospheric mantle and culminating with brittle deformation in the crust metasomatic fluids have during a process of rodingitization and serpentinization at a low state of sulfidation substantially modified the composition of primary laurite towards fe cr co and mn bearing fe os ir ru alloys and oxides the most prospective locations for chromitite pgm targets seem to exclusively be small chromite occurrences that are found along shear zones within ophiolite complexes these shear zones post date the initial magmatic deposition of the host chromitites

chromitites in ophiolites how where when why part i a review and new ideas on the origin and significance of platinum group minerals abstract platinum group minerals are the dominant control on the budget of platinum group elements in ophiolitic chromitites they vary widely in distribution morphology chemistry and os isotope compositions their distribution in different types of chromitites and different microstructural sites within them may reflect the role of chromite as a physical collector of the platinum group elements during crystallisation of both phases from melts the platinum group minerals show significant os isotope heterogeneity on the scale of single micrometric grains enclosed within individual chromite grains this micro scale isotopic heterogeneity and their random spatial distribution within the chromites argue strongly against an origin of the platinum group minerals via exsolution from the chromite lattice during post magmatic cooling a re interpretation of their microstructures and os isotopes analysed in situ had led us to review the traditional models and to propose several origins for the platinum group minerals in chromitites 1 the suites of platinum group minerals mainly record crystallisation during mixing of multiple different melts and physical entrapment in chromite 2 some platinum group minerals in chromitites may have been scavenged from wall rock peridotite during migration of the parental melts of the chromitite 3 other platinum group minerals may have precipitated from metasomatic fluid melts that infiltrated existing chromitites the recrystallization of pre existing platinum group minerals during polyphase metamorphism or recycling of the chromitite into deeper mantle levels can explain the presence of micrometric platinum group minerals with distinct os isotope compositions

diversity of platinum group minerals in podiform chromitites of the late proterozoic ophiolite eastern desert egypt genetic implications abstract podiform chromitites are frequently distributed as lensoidal pods in the central and southern parts of the eastern desert egypt they are in most cases hosted by fully serpentinized peridotite which is a part of dismembered ophiolite complexes of the pan african belt of late precambrian age serpentinites are the predominant components in the ophiolitic melange either as a matrix or as variably sized blocks and are derived from harzburgite and subordinate dunite the central eastern desert ced chromitites have a wide compositional range from high cr to high al varieties whilst those of the southern eastern desert sed have a very restricted compositional range the cr ratio of spinel ranges from 0 5 up to 0 8 in the former while it is around 0 8 in the latter platinum group element pge mineralization has been recently reported in podiform chromitites from the late proterozoic pan african ophiolite of the eastern desert of egypt the populations of platinum group minerals pgm in the studied ced and sed chromitites are quite distinguishable they are mainly sulfides os rich laurite in the former and os ir alloy in the latter sulfarsenides and arsenides are also found in subordinate amounts from both chromitites the most abundant base metal sulfides bms in the eastern desert chromitites of egypt are millerite heazlewoodite pentlandite chalcopyrite and pyrite the sulfur fugacity and temperature conditions are the main controlling factors of pge mineralogy in the host chromitite at the initial stage within the upper mantle os rich laurite is stable at high sulfur fugacity and low temperature conditions whereas os ir alloy is stable at lower sulfur fugacity and higher temperature conditions the diversity of pge mineralogy combined with the differences in petrological characteristics of chromian spinels from ced chromitites to sed ones suggests different degrees of partial melting of the mantle rocks of this ophiolite which in turn may be attributed to different tectonic settings during the post magmatic processes i e serpentinization the primary pgm e g os rich laurite and os ir alloy can be modified at low temperatures to secondary pgm

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

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

on the diversity of the pge content in chromitites hosted in ophiolites and in porphyry cu systems controlling factors abstract the present study on the diversity of the platinum group elements pge content in chromitites and porphyry cu deposits from greece and everywhere literature data aims to define factors controlling precious metal enrichment in ophiolite complexes and the unknown pd and pt potential in porphyry cu systems through an integrated set of mineralogical and geochemical approaches the pge content in chromitites associated with ophiolite complexes is commonly low a few 100 s of ppb however a significant pge enrichment up to 10 s of ppm is a feature of relatively small chromite occurrences regardless their major element composition plots of the pd ir versus normalized pt pt ratios defined separate fields corresponding to various chromitite types large chromite deposits exhibit low pd ir values reflecting low degree of fractionation primitive magmas negative pt pt partial melting of already depleted mantle and increasing partial melting trend from high al towards to high cr deposits the association with hydrous silicates cr bearing hydrogrossular chlorite and serpentine and texture features of os ir ru rich compatible or ipge minerals coupled with very low pd ir and negative pt pt values were attributed to an origin by partial melting of already depleted mantle and increased partial melting degree during a subsequent crustal mantle recycling stage via fluid driven processes assuming that the increasing pd ir reflects a fractionation trend of parent magma a significant enrichment in pt and pd incompatible or ppge occurring in small occurrences either high cr or high al type may be a sensitive fingerprint for the orientation in the mantle sequence of the ophiolites and exploration for chromite commonly the pge content in porphyry cu mo au deposits is less than 10 ppb however elevated pd pt contents over 6 ppm in sulfide rich samples are a characteristic feature of certain porphyry cu au alkaline intrusions evidence of a favorable tectonic setting post collisional for the origin of porphyry cu au pd pt elevated porphyry deposits include elevated up to hundreds of ppm contents in cr co ni and re and elevated 187 os 188 os ratios reflecting mixing and contribution to the magma composition the oxidized nature of parent magmas abundant magnetite and the evolved mineralized fluid system in porphyry deposits are considered to be critical requirements for the transportation and deposition of sufficient au and pge elevated pd pt and au contents in porphyry deposits combined with the association of the major pd mineral merenskyite with bornite and chalcopyrite major cu minerals are encouraging economic factors for the contribution to the global pd pt production

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

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

spiral arm seismic arrays abstract seismic arrays have many uses for signal enhancement from surface wave characterization of the near surface to teleseismic detection in the context of monitoring nuclear tests many variants of the geometrical configuration of stations have been used with the objective of maximizing potential resolution of the incoming wavefronts direction of arrival a versatile class of array configurations with good resolution properties can be constructed with multiple spiral arms the array response is comparable with the same number of full circles but with far fewer stations and is robust to minor position changes in emplacement the desirable properties of the spiral arm arrays are illustrated for a permanent array in the precambrian pilbara craton in northwestern australia and for a temporary array on ancient sediments in southern queensland australia in each case the practical array response is very good and matches the theoretical expectations the spiral arm configuration allows the deployment of relatively large aperture arrays with a limited number of stations which is advantageous in a broad range of seismic applications including near surface characterization online material figures illustrating the relation between spiral arm and multiring circular arrays

early paleozoic ca 465 ma eclogites from beishan nw china and their bearing on the tectonic evolution of the southern central asian orogenic belt abstract the beishan orogen is situated in the southern margin of the central asian orogenic belt caob it is connected with the tianshan orogen to the west and the mongolia xing anling orogen to the east this orogen exhibits a well preserved neoproterozoic to late paleozoic sequence containing ophiolites accretionary complexes and eclogites in this paper we provide new geochronological data for eclogites and their country paragneiss to constrain the tectonic evolution of the beishan orogen u pb zircon dating of eclogites yielded two concordant ages 886 4 ma for the cores and 465 10 ma for the rims the zircon cores display bright luminescence with homogeneous or planar sector zoning intermediate th u ratios 0 17 0 47 and hree enriched patterns with negative eu anomalies eu eu 0 16 0 58 these features coupled with their formation temperatures of 680 710 c suggest that the core age represents the time of an earlier upper amphibolite facies metamorphic episode by contrast the zircon rims have dark luminescence with homogeneous or fir tree zoning low th u ratios 12 kbar for the eclogite facies metamorphism on the other hand zircon dating of a paragneiss near the eclogite produced two major groups of ages ca 1450 ma for a magmatic event and ca 900 ma for a metamorphic event it seems that eclogites and their country rocks have suffered the same earlier metamorphic episode based on the available age data obtained for eclogites ophiolites and granitoids from the beishan area a tectonic scenario can be constructed as follows the ca 1000 ma oceanic crust representing by protoliths of the eclogites was involved in early neoproterozoic orogenesis to have formed a microcontinental block during the early paleozoic island arcs and microcontinental blocks were progressively accreted to the northern margin of the tarim craton dunhuang block this was followed by the closure of paleoasian ocean and final amalgamation between the tarim craton and kazakhstan block in the late paleozoic

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

the first occurrence of platinum group minerals pgm in a chromite deposit in the eastern desert egypt the platinum group mineralogy pgm of the chromitite from gebel lawi in the southeastern desert has been investigated the most abundant base metal sulfides bms associated with the lawi chromite are pentlandite millerite and heazlewoodite the major platinum group minerals identified were as follows laurite irosru s2 osmian iridium osir hollingworthite rhass tellurian arsenopalladinite pdtesbas polarite pdhg besides cuprian palladian gold cupdau a pd sb hg and hgte phases laurite and osmian iridium occur preferentially in chromite os ir commonly forms composite pgm with laurite hollingworthite and tellurian arsenopalladinite are included within serpentine and close to the base metal sulfides the cuprian palladian gold shares boundaries with chromite polarite together with the pd sb hg and hgte phases are embedded in serpentine palladium is the most abundant pge in the gebel lawi chromite

c 1 85 ga hp granulite facies metamorphism in the dunhuang block of the tarim craton nw china evidence from u pb zircon dating of mafic granulites mafic granulites of the dunhuang block eastern tarim craton record high pressure granulite facies metamorphism followed by a medium to low pressure granulite facies metamorphic overprint suggesting a clockwise p t path and implying an environment of collisional orogenesis zircon u pb ages of two samples are 1834 12 ma and 1842 5 ma mineral inclusions in zircon indicate that these ages record the high pressure granulite facies event suggesting that the tarim craton may be related to the c 1 85 ga assembly of the columbia supercontinent similar c 1 85 ga metamorphism in both the dunhuang block and the alxa block of the north china craton supports their correlation across the altyn tagh fault supplementary material analytical procedures mineral abbreviations mineral chemistry and u pb dating data are available at www geolsoc org uk sup18541

analysis of three component ambient vibration array measurements summary both synthetic and observed ambient vibration array data are analysed using high resolution beam forming in addition to a classical analysis of the vertical component this paper presents results derived from processing horizontal components we analyse phase velocities of fundamental and higher mode rayleigh and love waves and particle motions ellipticity retrieved from h v spectral ratios a combined inversion with a genetic algorithm and a strategy for selecting possible model parameters allow us to define structural models explaining the data the results from synthetic data for simple models with one or two layers of sediments suggest that in most cases the number of layers has to be reduced to a few sediment strata to find the original structure generally reducing the number of soft sediment layers in the inversion process with genetic algorithms leads to a class of models that are less smooth they have a stronger impedance contrast between sediments and bedrock combining love and rayleigh wave dispersion curves with the ellipticity of the fundamental mode rayleigh waves has some advantages scatter is reduced when compared to using structural models obtained only from rayleigh wave phase velocity curves by adding information from love waves some structures can be excluded another possibility for constraining inversion results is to include supplementary geological or borehole information analysing radial components also can provide segments of rayleigh wave dispersion curves for modes not seen on the vertical component finally using ellipticity information allows us to confine the total depth of the soft sediments for real sites considerable variability in the measured phase velocity curves is observed this comes from lateral changes in the structure or seismic sources within the array constraining the inversion by combining love and rayleigh wave information can help reduce such problems frequency bands in which the rayleigh wave dispersion curves show considerable scatter are often better resolved by love waves information from the horizontal component can be used to correctly assign the mode number to the different phase velocity curve segments especially when two modes seem to merge at osculation points such merging of modes is usually observed for rayleigh waves and thus can be partly solved if additional information from the love waves and the horizontal component of rayleigh waves is considered whenever a site presents a velocity inversion below the top layer love wave data clearly helps to better constrain the solution

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

progressive accretionary tectonics of the beishan orogenic collage southern altaids insights from zircon u pb and hf isotopic data of high grade complexes abstract high grade gneissic rocks of the southern altaids in central asia bear one of the key elements needed to solve a basic architectural controversy in accretionary orogenesis high grade gneissic rocks widely crop out across the beishan orogenic collage southern altaids mapped as the beishan complex or dunhuang group which was previously regarded as neoarchean to paleoproterozoic crystalline basement but without systematic field geochronological and geochemical data this study provides field geochemical and zircon u pb and hf isotopic data for gneissic rocks from the beishan complex geochemical data show that the gneissic rocks were probably formed in a subduction environment most zircons from the beishan complex are magmatic origins because their th u ratios are mostly in the range of 1 0 0 1 they show age peaks at 494 ma 464 ma and 375 ma with a few precambrian ages zircon hf isotopic compositions indicate that both old and juvenile materials were involved in the formation of the beishan complex which is different from the basement rocks of the tarim craton both in radiometric age and hf isotopic composition all these data thus do not support a uniform precambrian basement in the beishan area as previously proposed but instead the beishan complex probably formed in a subduction accretion environment combined with regional geology and geochemical data we conclude that the beishan orogenic collage was characterized by progressive long lived accretionary tectonics during late precambrian to paleozoic time therefore shedding light on the controversy regarding the architecture of accretionary orogens

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

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

the hongliuhe fold and thrust belt evidence of terminal collision and suture reactivation after the early permian in the beishan orogenic collage northwest china abstract the early permian strata of the hongliuhe formation are located in the beishan orogenic collage a tectonically significant region between the tianshan orogen and the southern mongolian accretionary system of the central asian orogenic belt also called the altaids stratigraphic reconstruction of the formation shows a general fining upwards sequence of rhythmically interbedded sandstones and siltstones with thick basal successions of conglomerates all unconformably overlying an earlier mid paleozoic volcanic assemblage the clast lithotypes of conglomerates successively change from being polymictic metamorphic tectonites at the base to a nearly monomictic granitoids mid section illustrating the unroofing sequence of an arc related provenance the intra oceanic gongpoquan composite arc that comprises most of the central beishan fits the clast lithotype profile of the conglomerates and is the most probable provenance the hongliuhe formation stratigraphy experienced a thin skinned fold and thrust belt style of deformation that has a consistent southward vergence deformation in the lower stratigraphic levels created thrust imbrication of formation scale blocks each separated by steep north over south brittle ductile shear zones the blocks are internally deformed with large scale folding with one significant block containing a sheath fold 15 km in breadth the upper stratigraphic levels accommodated strain through low angle thrust ramping shear folding imbricate fanning and duplexing this indicates that the hongliuhe formation was syn orogenically deposited in a progressively deepening terrestrial to nearshore environment within the gongpoquan composite arc our observations and data indicate that the basin overlies an earlier paleozoic suture zone this implies that the northern half of the gongpoquan composite arc overrode the southern half in a fault system reactivation centered about this zone of weakness and created a stratigraphic and structural response consistent with a subduction or collision driven fold and thrust belt its southward structural vergence is supporting evidence of a north dipping subduction related fault system under the northern gongpoquan the timing of both the genesis of the hongliuhe formation and this deformation event coincides with the subduction of the liuyuan ocean or docking of the dunhuang block further south at the southern margin of the gongpoquan composite arc

effects of mother lode type gold mineralization on 187os 188os and platinum group element concentrations in peridotite alleghany district california osmium isotope compositions and concentrations of re platinum group elements pge and au were determined for host peridotites serpentinites and harzburgites and hydrothermally altered ultramafic wall rocks associated with mother lode type hydrothermal gold quartz vein mineralization in the alleghany district california the host peridotites have os isotope compositions and re pge and au abundances typical of the upper mantle at their presumed formation age during the late proterozoic or early paleozoic the hydrothermally altered rocks have highly variable initial os isotope compositions with os values deviation of 187os 188os from the chondritic average calculated for the approx 120 ma time of mineralization ranging from 1 4 to 8 3 the lowest os isotope compositions are consistent with re depletion of a chondritic source e g the upper mantle at ca 1 6 ga most of the altered samples are enriched in au and have depleted and fractionated abundances of re and pge relative to their precursor peridotites geochemical characteristics of the altered samples suggest that re and some pge were variably removed from the ultramafic rocks during the mineralization event in addition to re the pt and pd abundances of the most intensely altered rocks appear to have been most affected by mineralization the 187os depleted isotopic compositions of some altered rocks are interpreted to be a result of preferential 187os loss via destruction of re rich phases during the event for these rocks os evidently is not a useful tracer of the mineralizing fluids the results do however provide evidence for differential mobility of these elements and mobility of 187os relative to the initial bulk os isotope composition during hydrothermal metasomatic alteration of ultramafic rocks

zircon ages of metamorphic and magmatic rocks within peridotite bearing melanges crucial time constraints on early carboniferous extensional tectonics in the chinese tianshan abstract we dated and geochemically characterized peridotite bearing melanges in the chinese south tianshan and within the main tianshan shear zone the yushugou tonghuashan melange in the chinese south tianshan exposes a tectonic juxtaposition of a diapirically emplaced metaperidotite predominantly lherzolite massif with a high grade metamorphic terrane ca 10 km long protolith age ca 445 466 ma metamorphic zircons of a mafic granulite end t 5 0 yielded a weighted mean 206pb 238u age of 341 8 ma that we interpret as the time of granulite facies metamorphism the youngest zircon rims of an intermediate granulite end t 4 3 have a mean age of 332 13 ma that records a retrogressive metamorphic event these ages determine the timing ca 341 332 ma of mantle diapirism through continental crust a dolerite dike end t 2 3 emplaced into metaperidote has a crystallization age of 335 5 ma that on the basis of geochemistry we interpret as representing e morb oib magmatism that accompanied mantle diapirism an undeformed pink granite end t 3 6 intrudes the melange matrix and has an emplacement age of 324 5 ma thus providing an upper time limit for a tectonic movement that led to melange formation the older time limit of deformation ca 362 352 ma is constrained by the youngest ages of thermo tectonically modified zircons in a mylonitized metagabbro and a foliated meta andesite magmatic zircons in the meta andesite end t 3 3 and a dacite end t 5 6 have eruption ages of 433 4 ma and 435 3 ma that date formation of the melange matrix a microgabbro dike end t 10 0 cutting metasediments i e melange matrix has an emplacement age of 279 3 ma and contains abundant zircon xenocrysts ranging in age from archean to late paleozoic similarly a dolerite dike a meta andesite an intermediate granulite and a metagabbro all contain inherited precambrian to paleozoic zircons we accordingly conclude that the melange was formed by mantle diapirism in the early carboniferous and was subsequently affected by an early permian volumetrically minor mafic episode two foliated garnet granites end t 7 0 and 6 9 in direct contact with metaperidotite within the main tianshan shear zone have emplacement ages of 341 2 ma and 329 4 ma that reflect an early carboniferous episode of crustal anatexis they also contain neoproterozoic zircon xenocrysts also within the main tianshan shear zone a gabbro end t 8 7 has an emplacement age of 297 3 ma and contains abundant neoproterozoic to late paleozoic zircon xenocrysts based on the regional geological relationships we associate early carboniferous mantle diapirism and crustal anatexis with lower crustal delamination induced asthenospheric upwelling the numerous precambrian zircon xenocrysts support a chinese tianshan tarim connection the early permian mafic episode coincided in time with the development of an early permian unconformity in the chinese north tianshan

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

s wave velocity profiling by joint inversion of microtremor dispersion curve and horizontal to vertical h v spectrum a joint inverse analysis using both microtremor dispersion curve and horizontal to vertical h v spectrum is proposed for estimating the s wave velocity vs profiles of subsurface soils in the inversion both microtremor dispersion and h v data are assumed to be the rayleigh wave dispersion curve and the surface both rayleigh and love wave h v spectrum that have been theoretically derived by taking into account the effects of their fundamental and higher modes the proposed joint inversion as well as the conventional one using dispersion data alone is per formed at four sites where shallow vs profiles down to engineering bedrock are available the vs profiles estimated by the proposed joint inversion are more con sistent with available down hole velocity logs than those by the conventional method in particular the proposed inversion shows significant improvement in estimating bedrock vs structures compared to the conventional inversion sensitivity analyses indicate that the surface wave h v ratio is sensitive to the bedrock vs structure more than the rayleigh wave phase velocity confirming that the proposed joint inversion including h v spectrum is promising

paragenesis of multiple platinum group mineral populations in shetland ophiolite chromitite 3d x ray tomography and in situ os isotopes abstract chromitite from the harold s grave locality in the mantle section of the shetland ophiolite complex is extremely enriched in ru os and ir at g g concentrations high resolution x ray computed tomography on micro cores from these chromitites was used to determine the location size distribution and morphology of the platinum group minerals pgm there are five generations of pgm in these chromitites small average 5 m in equivalent sphere diameter esd euhedral laurites often with os ir alloys are totally enclosed in the chromite and are likely to have formed first by direct crystallisation from the magma as the chromite crystallised also within the chromitite there are clusters of larger 50 m esd aligned elongate crystals of pt rh ir os and ru bearing pgm that have different orientations in different chromite crystals these may have formed either by exsolution or by preferential nucleation of pgms in boundary layers around particular growing chromite grains thirdly there is a generation of large 100 m esd composite os ir ru rich pgm that are all interstitial to the chromite grains and sometimes form in clusters it is proposed that os ir and ru in this generation were concentrated in base metal sulfide droplets that were then re dissolved into a later sulfide undersaturated magma leaving pgm interstitial to the chromite grains fourthly there is a group of almost spherical large 80 m esd laurites hosting minor os ir ru rich pgm that form on the edge or enclosed in chromite grains occurring in a sheet crosscutting a chromitite layer these may be hosted in an annealed late syn or post magmatic fracture finally a few of the pgm have been deformed in localised shear zones through the chromitites the vast majority of the pgm including small pgm enclosed within chromite larger interstitial pgm and elongate aligned pgm have os isotope compositions that give re depletion model ages approximately equal to the age of the ophiolite at 492 ma a number of other pgm not confined to a single textural group fall to more or less radiogenic values with four pgm giving anomalously unradiogenic os corresponding to an older age of 1050 ma the 187 os 188 os isotopic ratios for pgm from cliff and quoys from the same ophiolite section are somewhat more radiogenic than those at harold s grave this may be due to a distinct mantle source history or possibly the assimilation of radiogenic crustal os

joint inversion of h v ratios and dispersion curves from seismic noise estimating the s wave velocity of bedrock 1 a joint inversion of phase velocity and h v ratio curves both obtained from seismic noise recordings permits the retrieval of the shear wave velocity structure of local sedimentary cover our inversion scheme uses a genetic algorithm and considers the influence of higher modes on the data sets encouraged by the results published previously on joint inversion parolai et al 2005 we went one step further we found using a synthetic data set that the impedance contrast at the sediment bedrock interface has a strong influence on the shape of the h v ratio curve which therefore allows the bedrock s wave velocity to be well constrained in the joint inversion procedure our observations were further confirmed using a real data set

high pressure granulites formation recovery of peak conditions and implications for tectonics high pressure granulites are characterised by the key associations garnet clinopyroxene plagioclase quartz in basic rocks and kyanite k feldspar metapelites and felsic rocks and are typically orthopyroxene free in both basic and felsic bulk compositions in regional metamorphic areas two essential varieties exist a high to ultrahigh temperature group and a group representing overprinted eclogites the high to ultrahigh temperature type formerly contained high temperature ternary feldspar now mesoperthite coexisting with kyanite is associated with garnet peridotites and formed at conditions above 900 c and 1 5 gpa clinopyroxene in subordinate basic rocks is al rich and textural evidence points to a high pressure high temperature melting history the second variety contains symplectite like or poikilitic clinopyroxene plagioclase intergrowths indicating former plagioclase free i e eclogite facies assemblages this type of rock formed at conditions straddling the high pressure amphibolite high pressure granulite field at around 700 850 c 1 0 1 4 gpa importantly in the majority of high pressure granulites orthopyroxene is secondary and is a product of reactions at pressures lower than the peak recorded pressure in contrast to low and medium pressure granulites which form at conditions attainable in the mid to lower levels of normal continental crust high pressure granulites of nonxenolith origin mostly represent rocks formed as a result of short lived tectonic events that led to crustal thickening or subduction of the crust into the mantle short times at high temperature conditions are reflected in the preservation of prograde zoning in garnet and pyroxene high pressure granulites of both regional types although rare are known from both old and young metamorphic terranes e g c 45 ma namche barwa e himalaya 400 340 ma european variscides 1 8 ga hengshan china 1 9 ga snowbird saskatchewan and 2 5 ga jianping china this spread of ages supports proposals suggesting that thermal and tectonic processes in the lithosphere have not changed significantly since at least the end of the archean

pge rich podiform chromitites in the al ays ophiolite complex saudi arabia an example of critical mantle melting to extract and concentrate pge the al ays ophiolite complex in saudi arabia is an example of an ophiolite that contains anomalous concentrations of all six platinum group elements pge in podiform chromitite with maximum values of 2 570 ppb pt 6 870 ppb pd 840 ppb rh 5 800 ppb ru 6 200 ppb ir and 3 300 ppb os smooth chondrite normalized pge profiles indicate igneous pge ratios these suggest that in situ alteration of the pgm caused only minor mobility of pge during secondary modification of the mineralogy thus the geochemistry of the igneous concentration processes can be examined despite the mineralogical changes caused by subsequent alteration three main types of pge mineralization are observed that are defined by their relative abundances of individual pge type 1 has ru both pt and pd with negative slope chondrite normalized profiles type 2 has ru either pt or pd pt pd ir ratios of 1 to 5 and convex upward chondrite normalized profiles type 3 has ru either pt or pd pt pd ir ratios of 5 to 60 positive slope chondrite normalized profiles and is associated with elevated cu and ni concentrations the unaltered centers of chromite grains in the chromitite within this complex have an unusually large range of composition for example cr2o3 varies from 39 to 69 wt percent pge mineralization types 1 2 and 3 are related to the composition of the chromite type 1 occurs across the range of chromitite compositions from 39 to 69 wt percent cr2o3 type 2 occurs in chromitite having a range of 53 to 61 wt percent cr2o3 and type 3 occurs in chromitite having a range of 39 to 51 wt percent cr2o3 the pge form a great variety of platinum group minerals pgm and they differ among the three types of pge mineralization type 1 is characterized by euhedral os ir and ru ipge alloys and laurite both commonly enclosed in chromite as well as members of the irarsite hollingworthite solid solution series and pt ipge bearing pgm both commonly interstitial to the chromite grains type 2 pge enrichment is characterized by ipge pt and rh bearing pgm type 3 pge enrichment hosts predominantly pd and pt bearing pgm associated with ni and cu bearing minerals where exposed to the serpentinization process the pgm are altered to alloys arsenides antimonides and oxides that form irregular shapes or may form pseudomorphs of former pgm they are commonly associated with ni and cu bearing minerals including ruthanian pentlandite millerite arsenides and pge bearing awaruite mantle melting and subsequent crystallization were at an optimum to concentrate pge in the al ays ophiolite complex crystallization of ipge commonly prior to chromite crystallization and latterly with some pt and rh occurred across the range of chromitite composition crystallization of pd with some remaining pt occurred during sulfur saturation in chromitite formed from a more evolved magma we propose that this crystallization was from a magma that was enriched in pge because the degree of mantle melting was just sufficient to extract the pge but not dilute them in a melt that includes further mantle melting this feature is likely to be common to other pge rich ophiolite complexes such as in shetland in the united kingdom leka in norway thetford in canada pindos in greece tropoja in albania and in new caledonia if equilibrium partial melting had continued in al ays then the magma would have been diluted by subsequent pge poor melt this would have prevented sulfur saturation until much higher in the sequence producing pt and pd bearing base metal sulfides in the crustal wehrlite and gabbro as has occurred in the cyprus and oman ophiolites

first report of coupled early permian paleomagnetic and geochronologic data from the dunhuang block nw china and implications for the tectonic evolution of the paleo asian ocean abstract in order to better understand the tectonic relationship between the dunhuang block dhb and its adjacent blocks and to constrain the timing of the closure of the paleo asian ocean a combined geochronologic and paleomagnetic investigation has been undertaken on early permian tuff basalt flows and sandstones from the shuangbaotang formation in the northwestern part of the dhb u pb zircon dating indicates that the age of the strata is between 280 6 2 9 ma and 291 4 2 6 ma thermal demagnetization of a three component isothermal remanent magnetization irm and curie point experiment suggest that magnetite dominates in the rock samples analyzed in addition there is a minor amount of hematite in some sandstones stepwise thermal demagnetization successfully isolated stable characteristic remanent magnetization chrm from 11 tuff layers two lava flows and nine sandstone beds two components were isolated from all samples a lower temperature component ltc and a higher temperature component htc the ltc is near the direction of the present day geomagnetic field and produced a negative fold test indicating it is a viscous remanent magnetization in the present day geomagnetic field most of the htc are reverse polarity normal 4 reverse 158 which is in accordance with the kiaman reversed superchron that spans the late carboniferous permian interval in addition the htc of all studied sites passed the fold and reverse tests suggesting that they likely represent primary remanent magnetization the tilt corrected mean direction from all sites tuff basalts and sandstones is ds 1 7 is 43 1 ks 403 95 1 5 n 22 the mean paleopole of the site mean direction corresponded vgps lies at 74 5 n 268 5 e with a95 1 6 considering the consistent inclination values recorded among the studied tuff basalts and sandstones and the low degrees of anisotropy within all samples we suggest that there is no significant inclination shallowing caused by depositional compaction in the sedimentary layers of the studied section taking into account the results from this study as well as previous late paleozoic paleomagentic studies from adjacent tectonic blocks we conclude that the dhb formed part of the amalgamated dunhuang north china alxa qaidam mega block during the early permian but was separated from the tarim block by a small ocean here named the qiemo xingxingxia paleo ocean at this time a comparison of the early permian paleolatitudes of these and other adjacent blocks suggest that the paleo asian ocean sensu lato was still open at this time combined with other geological evidence a paleogeographic reconstruction of the paleo asian ocean has been reconstructed for the early permian

on the formation of granulites the tectonic settings for the formation and evolution of regional granulite terranes and the lowermost continental crust can be deduced from pressure temperature time p t time paths and constrained by petrological and geophysical considerations p t conditions deduced for regional granulites require transient average geothermal gradients of greater than 35 c km 1 implying minimum heat flow in excess of 100 mwm 2 such high heat flow is probably caused by magmatic heating tectonic settings wherein such conditions are found include convergent plate margins continental rifts hot spots and at the margins of large deep seated batholiths however particular p t time paths do not allow specific tectonic settings to be distinguished at this time under different conditions both clockwise cw pmax attained before tmax and anticlockwise acw pmax attained slightly after tmax paths are possible in the same tectonic setting both cw and acw end member paths can yield nearly isobaric cooling ibc paths such cooling paths are clearly not an artefact of thermobarometry but can be constrained by solid solid and devolatilization equilibria and geophysical modelling in terms of understanding the evolution of the deep crust a potentially significant group of regional granulite terranes are those that show evidence for acw ibc paths such paths are the likely result of i episodic igneous activity resulting in intrusions within all levels of the crust ii thickening of the crust by magmatic underplating iii slow uplift as a result of the formation of a deep garnet rich crustal root and iv excavation resulting from a later tectonic event unrelated to that resulting in the formation of the granulites the later event might be triggered by the delamination of the garnet rich lowermost crust

a cold early palaeozoic subduction zone in the north qilian mountains nw china petrological and u pb geochronological constraints the north qilian high pressure hp low temperature lt metamorphic belt is composed mainly of blueschists eclogites and greenschist facies rocks it formed within an early palaeozoic accretionary wedge associated with the subduction of the oceanic crust and is considered to be one of the best preserved hp lt metamorphic belts in china here we report new lawsonite bearing eclogites and eclogitic rocks enclosed within epidote blueschists in the north qilian mountains five samples contain unaltered lawsonite coexisting with omphacite and phengite as inclusions in garnet indicating eclogite facies garnet growth and lawsonite pseudomorphs were observed in garnet from an additional 11 eclogites and eclogitic rocks peak pressure conditions estimated from lawsonite omphacite phengite garnet assemblages were 2 1 2 4 gpa at temperatures of 420 510 c in or near the stability field of lawsonite eclogite and implying formation under an apparent geothermal gradient of 6 8 c km 1 consistent with metamorphism in a cold subduction zone shrimp u pb dating of zircon from two lawsonite bearing eclogitic metabasites yields ages of 489 7 ma and 477 16 ma respectively cl images and mineral inclusions in zircon grains indicate that these ages reflect an eclogite facies metamorphism an age of 502 16 ma is recorded in igneous cores of zircon grains from one lawsonite pseudomorph bearing eclogite which is in agreement with the formation age of early ordovician for some ophiolite sequences in the north qilian mountains and may be associated with a period of oceanic crust formation the petrological and chronological data demonstrate the existence of a cold early palaeozoic subduction zone in the north qilian mountains

u pb zircon geochronology of coesite bearing eclogites from the southern dulan area of the north qaidam uhp terrane northwestern china spatially and temporally extensive uhp metamorphism during continental subduction coesite bearing eclogites from 100 km 2 in the southern dulan area north qaidam mountains nqm of western china contain zircon that records protolith crystallization and ultra high pressure uhp metamorphism sensitive high resolution ion microprobe mass spectrometer and laser ablation inductively coupled plasma mass spectrometry u pb analyses from cathodoluminescence cl dark zircon cores in a coesite bearing eclogite yield an upper intercept age of 838 50 ma and oscillatory zoned cores in a kyanite bearing eclogite gave a weighted mean 206 pb 238 u age of 832 20 ma these zircon cores yield steep heavy rare earth element hree slopes and negative eu anomalies that suggest a magmatic origin thus c 835 ma is interpreted as the eclogite protolith age unzoned cl grey or bright zircon and zircon rims from four samples yield weighted mean ages of 430 4 438 2 446 10 and 446 3 ma flat hree patterns without eu anomalies and contain inclusions of garnet omphacite rutile phengite and rare coesite these ages are interpreted to record 16 5 myr of uhp metamorphism these new uhp ages overlap the age range of both eclogite and paragneiss from the northern dulan area suggesting that all uhp rock types in the dulan area belong to the same tectonic unit our results are consistent with slow continental subduction but do not match oceanic subduction and diapiric exhumation uhp model predictions these new data suggest that similar to eclogites in other hp uhp units of the nqm and south altyn tagh protoliths of the eclogites in the dulan area formed in a continental setting during the neoproterozoic and then subducted to mantle depth together with continental materials during the early palaeozoic

high cr and high al chromitites from the sagua de tanamo district mayari cristal ophiolitic massif eastern cuba constraints on their origin from mineralogy and geochemistry of chromian spinel and platinum group elements article i nfo several chromitite bodies of variable sizes are hosted in dunite harzburgite of the small b3 km mining district of sagua de tanamo in the mayari cristal ophiolitic massif eastern of cuba the chromian spinel in these bodies displays a large range in cr cr cr al atomic ratio from 0 74 to 0 45 in high cr chromitites cr and ti contents increase from harzburgite to dunite to chromitite a roughly opposite trend of variation in cr is observed in high al chromitites although in this case ti distributes randomly the differences in the type of chromian spinel i e high cr or high al in the chromitite coincide with a different behavior of the platinum group elements pge whereas high cr chromitites are rich in pge and contain abundant grains of platinum group minerals pgm high al chromitites are systematically poor in pge and in pgm the calculated melts in equilibrium with chromian spinel of high cr chromitite are island arc thoeliites iat with boninitic affinity whereas those in equilibrium with chromian spinel of high al chromitites are back arc basin basalts babb the formation of high cr chromitites is interpreted as a result of the extensive reaction of harzburgite with migrating island arc tholeiite melts of boninitic affinity melt rock reaction produces boninitic melts with variable composition and porous dunitic channels in which the mixing mingling of melts promotes crystallization of mononomineralic high cr chromian spinel in contrast high al chromitites formed by the mixing mingling of babb melts within conduits not in equilibrium with dunite percolation of primitive babb melts through pre existing dunite dissolved olivine producing melt conduits in which babb melts mixed and formed high al chromian spinel the higher pge and pgm in high cr chromitites is not only a function of the degree of partial melting but is linked to interaction between the migrating melt and mantle harzburgite during the formation of the chromitite the coexistence of both types of chromitites in one small mining district reflects the temporal and or spatial variations of separate melt intrusions emplaced in different subarc mantle domains during the opening of a back arc basin in a supra subduction zone environment

tomography of the alpine region from observations of seismic ambient noise we use correlations of the ambient seismic noise to study the crust in western europe cross correlation of 1 year of noise recorded at 150 three components broadband stations yields more than 3 000 rayleigh wave group velocity measurements these measurements are used to construct rayleigh group velocity maps of the alpine region and surrounding area in the 5 80 s period band in the 5 10 s period band the seismic noise recorded in europe is dominated by surface waves originating from the northern atlantic ocean this anisotropy of the noise and the uneven station distribution affect the azimuthal distribution of the paths where we obtain reliable group velocity measurements as a consequence our group velocity models have better resolution in the northeast direction than in the southwest direction finally we invert the resulting rayleigh wave group velocity maps to determine the moho depth our results are in good agreement with the result of the numerous active experiments in the alps and provide a continuous image of the alpine structure