early devonian alkaline intrusive complex from the northern north china craton a petrological monitor of post collisional tectonics abstract laser ablation inductively coupled plasma mass spectrometry zircon u pb dating and geochemical data document the early devonian sandaogou alkaline complex 409 ma from the northern margin of the north china craton the rock suite includes pyroxene syenite quartz syenite and monzonite these rocks exhibit high contents of k 2 o 5 13 wt strong enrichments in large ion lithophile elements and light ree slightly negative eu anomalies and pronounced depletions in high field strength elements they are characterized by moderate 87 sr 86 sr i ratios of 0 7052 0 7071 low e nd t values of 12 7 to 17 9 and zircon e hf t values from 27 8 to 32 3 these geochemical features and quantitative isotopic modelling suggest that they might have been formed through simultaneous fractional crystallization and lower crustal assimilation of a metasomatized mantle derived alkali basaltic magma these trachytic rocks together with the middle devonian alkaline rocks and mafic ultramafic complex from neighbouring regions constitute a linear post collisional magmatic belt along the northern north china craton possibly formed under a tectonic regime of slab breakoff they serve not only as a magmatic milestone for marking the termination of early palaeozoic orogenic cycle around the northern north china craton but also as a spatial tracer for locating the position of the potential ancient slab breakoff at the surface supplementary material analytical methods mineral composition data field and zircon cathodoluminescence cl photographs are available at http www geolsoc org uk sup18403

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

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

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

location and sinistral displacement of the eastern liaoyuan accretionary belt along the tan lu fault zone ne china abstract the ne to nne striking continental scale tan lu fault zone tlfz in ne china is subdivided into the 900 km long yilan yitong fault yyf in the west and the 1000 km long dunhua mishan fault dmf in the east both faults record sinistral displacement during the earliest cretaceous leading to an obvious offset of the boundary between the liaoyuan accretionary belt lab and the north china craton ncc previous studies have demonstrated that the lab in the south of the eastern central asian orogenic belt caob extends eastwards into an area between the yyf and the dmf the boundaries between the lab and ncc indicate a 35 km sinistral displacement upon the yyf however it remains unclear whether the lab extends to the east of the dmf and the amount of sinistral displacement on the dmf is poorly constrained here we present lithological observations and u pb and hf isotopic data from zircon to constrain the age and position of the lab to the east of the dmf the data indicate an eastward extension of the lab across the dmf suggesting the continuous presence of the lab east of the tlfz in ne china the u pb ages ehf t values and two stage model ages of zircons from the study area constrain the boundary between the lab and ncc to an extension of the ne dipping chifeng kaiyuan fault to the east of the dmf the geological boundaries on both sides of the dmf indicate a 170 km sinistral displacement along the fault thus the tlfz has a total sinistral displacement of 205 km in ne china the eastern branch of the tlfz records significantly more movement due to its proximity to the pacific ocean where subduction of oceanic crust might have caused sinistral faulting along the tlfz

boundary lines within petrologic diagrams which use oxides of major and minor elements use of some petrologic diagrams applied to analyses of volcanic rocks is unnecessarily difficult due to lack of data for construction of discriminant lines between rock series coordinates are provided for sufficient points to enable accurate plotting of the boundary lines within seven diagrams viz 1 tas total alkalies na2o k2o vs sio2 2 k2o vs sio2 3 afm 4 jensen 5 ktp k2o tio2 p2o5 6 fms feo mgo vs sio2 and 7 taktip k2o na2o k2o vs tio2p2o5 different versions of these boundaries are collated to indicate their variable position and it is demonstrated that inter laboratory analytical precision suffices to account for almost all of their spread on the tas and k2o vs sio2 diagrams

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

different sources involved in generation of continental arc volcanism the carboniferous permian volcanic rocks in the northern margin of the north china block abstract new zircon u pb dating results on the carboniferous permian volcanic rocks in the northern margin of the north china block ncb indicate their eruption during the early carboniferous to late permian from 347 3 ma to 258 1 ma and a slight decrease of the upper limits of the volcanic sequences from west to east they have a main rock association of basalt basaltic andesite andesite dacite rhyolite tuff and tufaceous sandstone most of them have calc alkaline compositions and exhibit variable sio2 contents from 48 2 wt to 77 1 wt there is no significant gap between the mafic and felsic volcanic rocks in major and trace element classification diagrams indicating that they are not bimodal in composition the carboniferous permian volcanic rocks exhibit subduction related geochemical features such as negative nb and ta anomalies of mafic to intermediate rocks on primitive mantle normalized diagrams indicating they were formed in an andean type continental arc during southward subduction of the paleo asian oceanic plate beneath the northern ncb however their wide range of whole rock sr nd and zircon hf isotopic compositions indicate that their source areas are very complex and different sources were involved in generation of these volcanic rocks geochemical and sr nd hf isotopic results show that the basalt and some andesite were produced by fractional crystallization of mafic magma derived from partial melting of mantle wedge and subducted oceanic crust however most of the intermediate to felsic volcanic rocks were derived from partial melting of lower continental crust there is an increasing input of crustal materials from the carboniferous to permian as indicated by increasing volumes of felsic volcanic rocks in the volcanic sequences the results show that origin of the continental arc volcanism is very complex and both materials from the subducted oceanic crust and sediments mantle wedge and arc continental crust could be involved in their generation for the permian andesite dacite and rhyolitic volcanic rocks there is a striking correlation between their sr nd hf isotopic compositions and those of the underlying basement sr nd hf isotopic differences between the permian intermediate to felsic volcanic rocks from either side of the northern boundary fault of the north china craton suggest that mixture and homogenization of the magmas in the melting assimilation storage hybridization mash zone within this continental arc was incomplete and incomplete homogenization of magmas is likely a common feature of many mash zones within continental arcs our results on the volcanic rocks also show that the final closure of the paleo asian ocean along the solonker suture zone has not been finished prior to the middle permian and the decease of the paleo asian oceanic subduction beneath the northern ncb in its middle western parts is a little earlier than that in its eastern part

helium isotopes as a tool for detecting concealed active faults 1 a magnitude mj 7 3 crustal earthquake occurred in the western tottori area southwest japan on 6 october 2000 however there was no obvious prefaulting indication at surface of a fault corresponding to the western tottori earthquake in 2000 this study was undertaken to elucidate the geographic distribution of 3he 4he ratios around the seismic source region using helium isotope data obtained from groundwater samples from drinking water wells the maximum 3he 4he ratio observed was from the water well nearest to the epicenter of the main shock in addition there appears to be a clear trend of decreasing 3he 4he ratios with distance away from the main trace of the probable fault segments the observations provide significant evidence that the source fault of the earthquake in 2000 is associated with leakage of mantle volatiles through the crust to the earth s surface we suggest that helium isotopes can be regarded as a tool for investigating and or mapping concealed active faults with no surface expression

tarim and north china cratons linked to northern gondwana through switching accretionary tectonics and collisional orogenesis accretionary and collisional orogenesis in the tarim and north china cratons during paleozoic time can be correlated with events associated with the assembly and subsequent incipient dispersal of gondwana zircon u pb and hf isotopic data from the northern margins of the two cratons and neighbors have revealed comparable ehf t time patterns zircons with magmatic ages of 500 400 ma display a large spread and decreasing ehf t values with time whereas 400 310 ma zircons have dominantly positive ehf t values and an overall increasing trend the marked shift of the zircon hf array at ca 400 ma was most likely related to a major tectonic switch from advancing to retreating accretionary orogenesis corresponding to the development of regional extension the commencement of subduction at 500 ma and establishment of an advancing accretionary orogen along the preexisting passive margin was synchronous with early paleozoic continental collisional events along the southern margins of the two cratons the temporal agreement of these events and their accordance with collision and or accretion events during gondwana assembly suggest that the tarim and north china cratons likely collided with the northern australia margin of east gondwana at ca 500 ma they subsequently dispersed from gondwana in the early devonian coinciding with switching accretionary tectonics along the northern margins of the two cratons that were possibly induced by the slab rollback of the subducting paleo asian ocean plate

magmatic evidence for middle late permian tectonic evolution on the northern margin of the north china craton abstract late paleozoic tectonic evolution of the northern margin of the north china craton ncc remains controversial especially for the mechanism and time during the late stage oceanic consumption of the paleo asian ocean pao we address this issue through geochronological geochemical and lu hf isotopic studies on felsic mafic volcanic rock suites and granite plutons from the damao area in the western segment of the ncc s northern margin zircon u pb dating reveals that the volcanic suites with eruptive unconformities in between erupted primarily at 275 260 ma while the granites emplaced later at 255 ma the volcanic rocks contain rhyolites and dacites having an a type affinity with high feot mgo k2o na2o cao and ga al ratios enriched th u zr hf and depleted ba sr and eu contents the basaltic rocks display low mgo and tio2 contents negative nb ta ti anomalies as well as negative hf t values we suggest that the 275 260 ma volcanic suites were generated in a post collisional setting with the dominant rhyolites and dacites derived from the mixing between mantle and crustal magmas and subordinate basalts from the partial melting of enriched sub continental lithospheric mantle the sudden and intensive occurrence of these 275 260 ma felsic dominant bimodal post collisional magmatism and their sudden increase of zircon hf t values imply a significant mantle contribution to their sources possibly due to the slab breakoff of the subducting pao plate the 255 ma granites also have an a type affinity and a large spread of negative hf t values 3 6 to 30 5 indicating a prominent involvement of continental crust materials that was likely related to crustal reworking after the slab breakoff overall these 275 255 ma magmatic suites document a significant post collisional crustal extension at the western segment of the northern ncc in this region the southward subduction of the pao likely ceased prior to 275 ma on the basis of a regional comparison

helium and carbon isotope variations in liaodong peninsula ne china chemical and c he isotopic compositions have been measured for n2 rich hydrothermal gases from the liaodong abbreviation of east liaoning province peninsula from which the oldest crustal rocks in china with 3 8 ga outcrop with the exception of one sample containing tritogenic 3he and atmospheric 4he in liaoyang the observed 3he 4he ratios from 0 1 ra to 0 7 ra indicate 1 8 helium from mantle 92 98 from crust and 0 1 0 8 from atmosphere despite the lack of quaternary volcanism such 3he 4he ratios suggest together with geophysical evidences the existence of intrusive magmas that contain mantle helium and heat within the liaodong middle lower crust the 3he 4he ratios are high along the ne trending jinzhou faults and gradually decrease with the increase of distance from the faults such a spatial distribution suggests that the mantle helium exsolves from magmatic reservoir in the middle lower crust becomes focused into the root zones of jinzhou faults and subsequently traverses the crust via permeable fault zones when transversely migrated by groundwater circulation in near surface mantle helium with high 3he 4he ratio may have been further diluted to the observed values by addition of radiogenic helium produced in the crust this pattern shows strong evidence that the major faults played an important role on mantle derived components transport from mantle upwards

solonker suture in east asia and its bearing on the final closure of the eastern segment of the palaeo asian ocean abstract the location and tectonic nature of the solonker suture in east asia and hence the late palaeozoic to early mesozoic closure of the palaeo asian ocean have notoriously been enigmatic in the past decades due to limited rock exposure and the absence of unambiguous collision related regional features several tectonic models have been proposed since but in many cases these models were derived only from single key exposures or methodologies often being over simplified or interpreted leading to questionable extrapolations on regional and global scales despite the complexity of the palaeozoic accretionary tectonic framework now the regionally consistent availability of geochronological geochemical stratigraphic and palaeo geographic data enables us to integrate these into a highly detailed and coherent palaeozoic tectonic review of the suture the region across the solonker suture can generally be subdivided into three major palaeozoic tectonic provinces e g i a sino cratonic province that resembles the active palaeozoic northern margin of the north china craton to the south of the suture ii a mongolian province that comprises the south eastern margin of the mongolian terrane to the north of the suture and iii an east asian pre pacific province in north east china that accreted with the late permian initiation of pacific plate subduction the sino cratonic province experienced episodic tectonic activity that includes the accretion of the initially ensialic bainaimiao arc onto the passive northern margin of the north china craton at 437 453 ma subsequent southward directed subduction activity beneath the amalgamated margin until at least 400 ma followed by a temporary cessation of magmatic activity accompanied by a switch from north to southward directed continental drift and finally concluded by renewed subduction activity until the final closure of the palaeo asian ocean the mongolian province is suggested to have been more closely associated with the siberian craton before the presumably late devonian opening of the mongol okhotsk ocean the east asian pre pacific province originally rifted away from the eastern margin of gondwana at 600 750 ma recorded early palaeozoic 500 ma orogenic processes along the then northern margin of the siberian craton the baolidao arc system successively developed along the present day south eastern margin of the mongolian province due to continued subduction activity throughout the palaeozoic after the middle carboniferous opening of the hegenshan back arc basin renewed subduction beneath a matured baolidao arc during carboniferous and permian times led to the obduction of the hegenshan ophiolite not earlier than 270 ma shortly after the palaeo asian ocean diachronously closed from west to east along the solonker suture in the late permian to early triassic the east asian pre pacific province re attached itself by the mid mesozoic followed by a change in the regional stress regime controlled by palaeo pacific plate subduction a divergent double sided subduction model for the closure of the palaeo asian ocean provides based on the reviewed data a tectonic model that is not only able to reasonably well explain the absence of typical collision related features in the region but also the abundance of mesozoic a type granitic magmatism in north east china

juvenile versus recycled crust in the central asian orogenic belt implications from ocean plate stratigraphy blueschist belts and intra oceanic arcs abstract new or juvenile crust forms and grows mainly through mafic to andesitic magmatism at pacific type or accretionary type convergent margins as well as via tectonic accretion of oceanic and island arc terranes and translation of continental terranes during the last decades the juvenile or recycled nature of crust has been commonly evaluated using whole rock isotope and hf in zircon isotope methods however evidence for the accretionary or pacific type nature of an orogenic belt comes from geological data for example from the presence of accretionary complexes ac intra oceanic arcs ioa oceanic plate stratigraphy units ops and morb oib derived blueschist belts bsb the central asian orogenic belt caob represents the world s largest province of phanerozoic juvenile crustal growth during ca 800 m y between the east european siberian north china and tarim cratons from geological point of view the caob is a typical pacific type belt as it hosts numerous occurrences of accretionary complexes intra oceanic arcs ops units and morb oib derived blueschist belts in spite of its accretionary nature supported by positive whole rock nd isotope characteristics in caob granitoids the hf in zircon isotope data reveal a big portion of recycled crust such a controversy can be explained by presence of accreted microcontinents isotopically mixed igneous reservoirs and by the tectonic erosion of juvenile crust the most probable localities of tectonic erosion in the caob are the middle and southern tienshan and southern transbaikalia because these regions comprise a predominantly recycled crust based on isotope data but the geological data show the presence of intra oceanic arcs blueschist belts and accreted ops with oceanic island basalts oib and tectonically juxtaposed coeval arc granitoids and accretionary units this warrants combination of detailed geological studies with isotopic results as on their own they may not reflect such processes as tectonic erosion of juvenile crust and or arc subduction

geochronological and hf isotopic variability of detrital zircons in paleozoic strata across the accretionary collision zone between the north china craton and mongolian arcs and tectonic implications the central asian orogenic belt formed by accretion subsequent to the contraction of the paleo asian ocean and its southeastern segment terminated along the solonker suture zone amalgamating the mongolian arcs and the north china craton by the end of the early triassic since typical regional collisional features are absent its tectonic evolution remains speculative an investigation into the variability of detrital zircon u pb ages and hf ratios in permian strata along a southeast northwest transect from the mongolian arcs to the north china craton reveals distinct differences northern basins carry a broad mesoproterozoic to latest precambrian age signature and their sedimentary source terranes are of mixed juvenile to crustal magmatic origin in contrast southern basins contain detritus from the north china craton and their sources are of dominantly crustal contaminated magmatic origin provenance analysis suggests that in the early paleozoic ca 429 ma the paleo asian ocean was consumed along the uliastai arc and the north china craton initiating the formation of the northern and southern accretionary orogens respectively by the end of the middle carboniferous the mongolian arcs were consolidated after the accretion of the uliastai arc in the late carboniferous ca 314 ma the hegenshan back arc basin opened detaching the northern accretionary orogen while subduction continued there it may have temporarily ceased in the south after the accretion of a microcontinent ca 300 ma by the end of the middle permian ca 269 ma back arc basin closure led to the formation and obduction of the suprasubduction zone hegenshan ophiolite eventually the paleo asian ocean closed by double sided subduction such accretionary wedge wedge collision would not involve continental deep subduction unlike other continental collisions the early stages of the sequence likely resembled a pacific type scenario dominated by the subduction of oceanic lithosphere including a japanese sea type back arc basin opening the late stages involved the accretion of large scale continental blocks ultimately leading to final ocean closure which may be similar to the archipelago type setting of present day southeast asia

evolution of a permian intraoceanic arc trench system in the solonker suture zone central asian orogenic belt china and mongolia abstract the identification of a fossil arc trench system from the ophiolite decorated solonker suture zone in the southernmost central asian orogenic belt caob enables us to constrain the timing of pre subduction extension ca 299 290 ma subduction initiation ca 294 280 ma ridge trench collision ca 281 273 ma and slab break off ca 255 248 ma in the permian a fraction of proto arc crust ca 45 km long up to 8 km wide is preserved as a volcanic plutonic sequence and is juxtaposed against a wide ca 30 80 km forearc melange this proto arc crust comprises two distinct magma series island arc tholeiite iat and mid ocean ridge basalt morb both of which have strong supra subduction zone ssz geochemical signatures zircons from a gabbro and a plagiogranite yielded weighted mean 206pb 238u ages of 284 0 4 0 and 288 0 6 0 ma the forearc melange consists of numerous ophiolite fragments and continental margin derived olistoliths blocks that predate the ophiolite the olistoliths are best represented by a gabbroic block 291 8 2 3 ma that contains granite xenoliths 312 6 1 8 and 313 6 3 1 ma other dated blocks include a trondhjemite 323 9 2 7 ma a gabbro 296 6 1 7 ma and a tonalite 294 9 2 4 ma small bodies of diabase andesite and diorite in the forearc melange exhibit a wide variety of geochemical signatures we dated zircons from an n morb like diabase 274 4 2 5 ma an e morb like diabase 252 5 2 3 ma a transitional sanukitoid adakite andesite 250 2 2 4 ma a sanukitoid high mg diorite 251 8 1 1 ma and an anorthosite 252 2 1 7 ma the n morb like diabase contains ca 301 394 ma zircon xenocrysts suggesting assimilation of trench sediments when a spreading ridge intersected a trench the other dated rocks simultaneously formed near the permian triassic boundary and captured abundant zircon xenocrysts ca 269 295 ma ca 301 495 ma and ca 923 2501 ma our new formation ages constrain a magmatic episode in response to slab break off beneath a fossil forearc in a young post collisional setting and the youngest xenocryst ages ca 269 273 ma may define the maximum depositional age of trench sediments

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

geochemical and geochronological study of early carboniferous volcanic rocks from the west junggar petrogenesis and tectonic implications abstract a series of early carboniferous volcanic rocks i e basalt andesite and dacite occur in the west junggar zircon u pb isotopic data indicate that these rocks coevally erupted at 331 344 ma these volcanic rocks have comparable trace element patterns characterized by the enrichment of large ion lithophile elements lile and the depletion of high field strength elements hfse isotopically they display consistently depleted sr nd isotopic compositions initial 87sr 86sr ratios 0 7034 0 7054 end t 4 2 7 7 these rocks are interpreted to be derived from a slightly metasomatized depleted mantle in a normal subduction environment in the context of a previously proposed ridge subduction model this study constrains that ridge subduction if it indeed occurred commenced between 331 and 310 ma in the central west junggar

geochemical changes in baslts and andesites across the archean proterozoic boundary identification and significance abstract to identify accurately changes in rock composition across the archean proterozoic boundary it is necessary to compare rocks from similar lithologic associations to constrain the effect of tectonic setting most basalts and andesites from the greenstone association volcanic dominated submarine supracrustal rocks possess a geochemical subduction zone component similar to their counterparts from modern arc systems basalts with island arc geochemical affinities dominate in archean greenstones while those with calc alkaline affinities are most abundant in proterozoic greenstones basalts with morb or oceanic within plate geochemical characteristics are rare in precambrian greenstones of all ages preserved early archean greenstone basalts 3500 ma reflect mantle sources less depleted than late archean greenstone basalts 2500 3500 ma proterozoic greenstone basalts are derived from relatively enriched mantle sources compared to all archean sources a feature which may be due to recycling of continental sediments into the mantle following rapid late archean continental growth precambrian andesites are geochemically similar to andesites from modern arcs but archean andesites are unique in that they are depleted in heavy ree and y results are consistent with archean andesite production by partial melting of descending mafic crust with amphibole garnet remaining in the residue while proterozoic and younger andesites are produced by fractional crystallization of basalts

flow of mantle fluids through the ductile lower crust helium isotope trends heat and mass are injected into the shallow crust when mantle fluids are able to flow through the ductile lower crust minimum 3he 4he ratios in surface fluids from the northern basin and range province western north america increase systematically from low crustal values in the east to high mantle values in the west a regional trend that correlates with the rates of active crustal deformation the highest ratios occur where the extension and shear strain rates are greatest the correspondence of helium isotope ratios and active transtensional deformation indicates a deformation enhanced permeability and that mantle fluids can penetrate the ductile lithosphere even in regions where there is no substantial magmatism superimposed on the regional trend are local high 3he 4he anomalies indicating hidden magmatic activity and or deep fluid production with locally enhanced permeability identifying zones with high resource potential particularly for geothermal energy development

geochemistry and zircon u pb hf isotopes of paleozoic intrusive rocks in the damao area in inner mongolia northern china implications for the tectonic evolution of the bainaimiao arc abstract the tectonic evolution of the central asian orogenic belt caob is related to the subduction and closure of the paleo asian ocean pao considerable controversy still surrounds whether when and how the pao subducted underneath the northern margin of the north china craton ncc this study carries out whole rock geochemical and zircon u pb hf isotopic investigations on paleozoic intrusive rocks in both the bainaimiao arc and the northern margin of the ncc zircon u pb dating of gabbroic diorite and diorite in the bainaimiao arc and the granite and diorite at the northern margin of the ncc yielded crystallization ages of 458 2 ma 428 2 ma 326 2 ma and 258 2 ma respectively in the bainaimiao arc the middle silurian diorite as the main body of the arc related intrusive rocks there show geochemical characteristics of continental margin arc magma with negartive hf t values and old hf model ages on the other hand the middle ordovician gabbroic diorite mainly shows typical calc alkaline continental arc geochemical characteristics with positive hf t values and relative young hf model ages indicative of the southward propagating reworking and arc accretion processes thus we interpret these rocks as a consequence of southward subduction of the pao beneath the bainaimiao arc here termed as bainaimiao arc terrane bat the early carboniferous granite and late permian diorite at the northern margin of the ncc show geochemical characteristics similar to post collisional adakites and typical continental arc rocks respectively combined with previous work the early carboniferous post collisional adakite shows limited distribution and corresponds to the subsequent effect of the amalgamation between the bat and the ncc whereas most of the carboniferous permian magmatic rocks ca 342 258 ma at the northern margin of the ncc are arc related and resulted from the continued effect of the southward subduction of the pao moreover the southward subduction in the early paleozoic was transformed from relative high angle subduction beneath the bat to shallow subduction beneath the unity of the ncc and the bat in the late paleozoic which lasted until the late permian ca 258 ma

u pb baddeleyite ages distribution and geochemistry of 925 ma mafic dykes and 900 ma sills in the north china craton evidence for a neoproterozoic mantle plume numerous neoproterozoic mafic dykes referred to as the dashigou swarm are identified in the central and southeastern parts of the north china craton ncc they are 305 010 degrees trending dykes with widths of similar to 10 100 m and exposed lengths of several to 10 km precise u pb isotope dilution thermal ionization mass spectrometry id tims measurements on baddeleyite grains separated from three dykes yield 207 pb 206 pb average ages of 924 0 3 7 ma dashigou dyke 921 8 2 6 ma yangjiaogou dyke and 925 8 1 7 ma taohuagou dyke baddeleyite grains from a late stage pegmatite vein in the dashigou dyke were analyzed by secondary ion mass spectrometry sims methods these yield a 207 pb 206 pb average age of 920 4 5 7 ma the dashigou dyke swarm exhibits an overall radiating geometry the overall fan angle is about 60 degrees with a focal point located along the southern margin of the eastern ncc where a large ca 900 ma sill swarm was previously recognized the rift system hosting these slightly younger sills named the xu huai rift system could represent two breakup parallel arms of a rift rift rift triple junction related to the initiation of the magma center that produced the dashigou dykes the sills have similar characteristics and could be cogenetic with the dashigou dykes the dashigou dykes are coarse grained composed mainly of clinopyroxene and plagioclase feldspar with or without olivine one of the most primitive dykes has 47 79 wt sio 2 6 41 wt mgo 1 38 wt tio 2 17 77 wt al 2 o 3 10 47 wt cao and 0 62 wt k 2 o it shows slight enrichment in light rare earth elements and a slightly positive eu eu anomaly 1 1 and is slightly depleted in high field strength elements compared to neighboring elements on a primitive mantle normalized spidergram the dashigou dykes show some similarities with enriched mid ocean ridge basalts e morb or ocean island basalts oib and have epsilon nd t values of 1 8 to 3 1 and 87 sr 86 sr t values of 0 7019 0 7047 t 920 ma all these characteristics indicate that they are not likely derived from the ancient lithospheric mantle under the ncc but rather from a mantle source below in the asthenosphere this 925900 ma magmatism represents a second sub lithospheric mantle upwelling event following the 1780 1730 ma event that occurred shortly similar to 70 ma after the formation of the subcontinental lithospheric mantle sclm therefore the sclm of the ncc was multiply metasomatized during asthenospheric upwellings at least at 1780 1730 and 925 900 ma before most of its eastern part was finally removed during the mesozoic collectively these similar to 925 ma dykes and similar to 900 ma sills constitute a large igneous province lip with an areal extent of about 0 5 mkm 2 and a diameter of about 1000 km this lip probably resulted from a neoproterozoic mantle plume centered along the present southern margin of the eastern ncc and probably resulted in the break off and rifting away of a separate crustal block we speculate that this conjugate block could have been the combined sao francisco congo craton on the basis of precisely matched ages for the bahia dykes sao francisco craton and gangila mayumbian volcanic associations western part of the congo craton c 2011 elsevier b v all rights reserved less

late paleozoic subduction and collision processes during the amalgamation of the central asian orogenic belt along the south tianshan suture zone abstract the provenance of late paleozoic siliciclastic sedimentary strata on the northern margin of the tarim craton and the south tianshan orogenic belt provides important insights into subduction and collision processes during the formation of the southern central asian orogenic belt detrital zircons from carboniferous and permian sedimentary rocks in the south tianshan belt show two predominant age populations of 500 400 and 305 270 ma and three subordinate clusters around 2 5 ga 2 0 1 7 ga and 1 2 0 6 ma such age patterns are similar to major magmatic episodes in the tarim craton but are distinct from those in the central tianshan yili block implying that the carboniferous permian strata in the south tianshan belt were deposited on the northern margin of the tarim craton these data in combination with carboniferous passive margin deposition along the south tianshan and northern tarim regions and intense arc magmatism in the central tianshan area support the northward subduction of the south tianshan oceanic crust the abrupt decrease of zircon e hf t values at 310 ma indicates the tectonic transformation from oceanic subduction to continental collision however syn and post collisional sediments in the south tianshan and northern tarim regions did not contain detritus from the central tianshan yili block as evidenced by the absence paucity of 380 310 ma detrital zircons in the late paleozoic strata and by the stratigraphic record in the regions insignificant surface erosion and uplifting during the collision and ultra high pressure rock exhumation require divergence in the frontal wedge we propose that the switch from a convergent to a divergent regime was triggered by the arrival of the tarim mantle plume in the latest carboniferous which possibly had profound effects on regional sedimentation and exhumation of ultra high pressure rocks in the orogenic belt

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

paleozoic accretionary orogenesis in the paleo asian ocean insights from detrital zircons from silurian to carboniferous strata at the northwestern margin of the tarim craton a detrital zircon u pb and lu hf isotopic study was carried out in the middle silurian to late carboniferous sedimentary strata of the northwestern tarim craton in order to understand accretionary processes in the southern part of the central asian orogenic belt detrital zircons from these strata yielded u pb ages clustering around 2 8 2 3 ga 2 0 1 7 ga 1 3 0 9 ga 880 600 ma and 500 400 ma with age populations and hf isotopic signatures matching those of magmatic rocks in the tarim craton and the central tianshan block abundant 500 400 ma detrital zircons most likely reflect deposition in a retroarc foreland basin inboard of an andean type magmatic arc to the north supporting the northern tarim central tianshan connection during early paleozoic time the absence of 380 310 ma zircon population in the carboniferous siliciclastic rocks suggests that the central tianshan block may have been separated from the tarim craton in the early devonian caused by the interarc back arc opening of the south tianshan ocean we propose an accretionary orogenic model switching from advancing to retreating mode during paleozoic time in the southwestern part of the paleo asian ocean this transition most likely occurred coevally with the rifting of southeast asian blocks from the northeastern margin of gondwana

adjacent releases of mantle helium and soil co2 from active faults observations from the marmara region of the north anatolian fault zone turkey volatile releases in terms of co2 efflux and noble gas of mantle origin are investigated in the marmara region of the seismically active north anatolian fault naf zone a total of 180 co2 efflux measurements 1 to 309 g m 2d 1 were carried out along four transects across the naf spatial variability of surface co2 effluxes was higher at faulted sites relative to comparable background areas the 13c values of soil co2 are 24 0 to 15 6 without correlation with co2 efflux values which suggest that co2 efflux anomaly is caused by fault related biogenic gas flow furthermore 36 gas and water samples from spring sites were investigated for 3he 4he ratios which resulted in a range from 0 26 r ra to 4 22 r ra large mantle helium contributions were observed from several sources along the naf the high co2 efflux sector on the gazikoy saros fault along the naf coincides with the area of high mantle helium flux this shows that fault zones play an important role both in transferring mantle helium to the surface and in causing fault related biogenic gas flow

neoproterozoic accretionary tectonics along the northwestern margin of the yangtze block china constraints from zircon u pb geochronology and geochemistry abstract the neoproterozoic mafic intrusions and granitoids of the hannan and micangshan massifs located at the northwestern margin of the yangtze block in south china are composed mainly of subduction related plutons here we present la icpms zircon u pb data on the granitoids from wudumen xishenba xixiang and hanshan within the hannan massif which show crystallization ages of 718 9 ma 706 9 ma 707 20 ma and 777 8 ma respectively the arc related mujiaba gabbros display crystallization age of 746 4 ma in the micangshan massif the crystallization ages of the granodiorite at daheba and the granites at xihe and guangwushan are 871 77 ma 832 5 ma and 838 17 ma respectively whereas the shatan diorite xihe and zhengyuan gabbroic intrusions yield crystallization ages of 840 6 ma 824 4 ma and 857 46 ma respectively the tonalites in the wudumen pluton display adakitic affinity characterized by strong depletion of nb ta p and ti and enrichment of large ion lithophile elements lile i e cs rb ba th u and k as well as positive pb and sr anomalies suggesting an origin by the melting of a thickened mafic lower crust at ca 718 ma the syenogranite at xixiang is characterized by depletion of nb ta sr p and ti and marked enrichment of lile k and pb indicating a post collisional orogenic collapse at ca 707 ma the results from our study suggests that the arc related magmatism migrated from micangshan area during ca 870 820 ma through huijunba xixiang in ca 840 820 ma and to hannan during 825 706 ma based on which we propose an accretionary tectonic model for the northwestern margin of the yangtze block

ten kilometer vertical moho offset and shallow velocity contrast along the denali fault zone from double difference tomography receiver functions and fault zone head waves we examine the structure of the denali fault system in the crust and upper mantle using double difference tomography p wave receiver functions and analysis spatial distribution and moveout of fault zone head waves the three methods have complementary sensitivity tomography is sensitive to 3d seismic velocity structure but smooths sharp boundaries receiver functions are sensitive to quasi horizontal interfaces and fault zone head waves are sensitive to quasi vertical interfaces the results indicate that the mohorovicic discontinuity is vertically offset by 10 to 15 km along the central 600 km of the denali fault in the imaged region with the northern side having shallower moho depths around 30 km an automated phase picker algorithm is used to identify 1400 events that generate fault zone head waves only at near fault stations at shorter hypocentral distances head waves are observed at stations on the northern side of the fault while longer propagation distances and deeper events produce head waves on the southern side these results suggest a reversal of the velocity contrast polarity with depth which we confirm by computing average 1d velocity models separately north and south of the fault using teleseismic events with m 5 1 we obtain 31 400 p receiver functions and apply common conversion point stacking the results are migrated to depth using the derived 3d tomography model the imaged interfaces agree with the tomography model showing a moho offset along the central denali fault and also the sub parallel hines creek fault a suture zone boundary 30 km to the north to the east this offset follows the totschunda fault which ruptured during the m7 9 2002 earthquake rather than the denali fault itself the combined results suggest that the denali fault zone separates two distinct crustal blocks and that the totschunda and hines creeks segments are important components of the fault and cretaceous aged suture zone structure