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

geochemical characteristics of the permian basins and their provenances across the solonker suture zone assessment of net crustal growth during the closure of the palaeo asian ocean abstract the solonker suture zone is commonly recognised as the location of the late permian to early triassic closure of the palaeo asian ocean in the southeastern segment of the central asian orogenic belt however the absence of typical suture related features as a consequence of uncommon collisional geometries gave it a cryptic nature thus the tectonic setting which led to suturing still remains enigmatic a geochemical characterisation of permian sedimentary and volcanic rocks across the suture was carried out supplemented with hf and nd isotopic analyses this approach enables not only a better definition of such regional suture but also estimates on the long controversial issue of net crustal growth in accretionary tectonic environments the results indicate short sedimentary transport distances between the arc basins and their provenances of which the studied volcanic rocks were a major contributor similar enrichment and depletion patterns with respect to n morb and average continental crust further corroborate a close source sediment relationship immobile element provenance analyses indicate that the active continental northern margin of the north china craton was a major source for arc basins to the south of the solonker suture zone to its north arc basins are interpreted to be sourced by a more complex mixture of provenances e g the baolidao volcanic arc suite and the heterogenous precambrian basement of southern mongolia an overall collisional tectonic setting across the suture is recognised the geochemical signature of sedimentary rocks to the south of the suture points at an active continental arc setting whereas the bimodal geochemical distribution of the samples to the north shows a contemporaneous active oceanic island arc as well as a passive margin environment these features favour a double sided subduction of the palaeo asian ocean beneath the north china craton and the mongolian arcs throughout the palaeozoic including back arc basin opening north of the suture in the permian analysis of hf and nd isotopic compositions revealed that magmas to the south were produced involving significant crustal contamination thus having less radiogenic compositions north of the suture however isotopic compositions tend to be more radiogenic implying a more juvenile contribution however the bulk isotopic compositions are close to present day chur suggesting that crustal reworking appears to be equally balanced by juvenile addition during the palaeozoic closure of the palaeo asian ocean across the solonker suture zone

highly evolved juvenile granites with tetrad ree patterns the woduhe and baerzhe granites from the great xing an mountains in ne china abstract in ne china voluminous granitoids were emplaced in late paleozoic and mesozoic times we report here sr nd o isotopic and elemental abundance data for two highly evolved granitic plutons woduhe and baerzhe from the great xing an mountains they show a rather juvenile sr nd isotopic signature and a spectacular tetrad effect in their ree distribution patterns as well as non charac charge and radius controlled trace element behavior the emplacement ages are constrained at 130 4 ma for the woduhe and 122 5 ma for the baerzhe granites by rb sr and sm nd isotope analyses both granites are also characterized by low but imprecise initial 87sr 86sr ratios of about 0 703 the nd sr isotope data argue for their generation by melting of dominantly juvenile mantle component with subordinate recycled ancient crust this is largely compatible with the general scenario for much of the phanerozoic granitoids emplaced in the central asian orogenic belt the parental magmas for both the woduhe and baerzhe granites have undergone extensive magmatic differentiation during which intense interaction of the residual melts with aqueous hydrothermal fluids probably rich in f and cl resulted in the non charac trace element behavior and the tetrad effect of ree distribution both the woduhe and baerzhe granites show the characteristic trace element patterns of rare metal granites but their absolute abundances differ by as much as two orders of magnitude the oxygen isotope compositions of the two granites have been severely disturbed significant 18o depletion in feldspar but not so much in quartz suggests that the hydrothermal alteration took place in a temperature condition of 300 500 c this subsolidus hydrothermal alteration is decoupled from the late stage magma fluid interaction at higher temperatures despite the two distinct and intense events of water rock interaction the rb sr and sm nd geochronological systems seem to have maintained closed hence suggesting that the two events occurred shortly after the plutonic emplacements

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

petrogenesis of the middle devonian gushan diorite pluton on the northern margin of the north china block and its tectonic implications the gushan diorite pluton located at the northern margin of the north china block was emplaced during middle devonian times shrimp u pb zircon age of 390 5 ma rocks from the pluton are characterized by low sio 2 and high alkali contents and they show monzodiorite compositions in a total alkali v silica tas plot they exhibit light ree enrichment no to slightly positive eu anomalies strong depletion in rb th u nb ta p zr hf and ti enrichment in ba k and sr low contents of y and yb and high sr y ratios they have a relatively narrow range of isotopic compositions with initial 87 sr 86 sr ratios of 0 7050 e nd t values of 9 5 to 7 5 and zircon e hf t values from 11 8 to 5 8 these features are remarkably similar to another middle devonian intrusion the shuiquangou syenitic complex at the northern margin of the north china block these similarities suggest that the two intrusions probably have a common origin they were considered to be derived from a type i enriched mantle ultimately with some involvement of ancient lower crustal components and were likely emplaced in a back arc extension environment related to southward subduction of the palaeo asian oceanic plate or during the cessation of the subduction aluminium in hornblende barometry studies of the middle devonian gushan pluton yielded emplacement depths of about 18 km combined with previous geobarometry results on the carboniferous plutons within the inner mongolia palaeo uplift on the northern margin of the north china block it is inferred that the uplift and exhumation of the plutons within the inner mongolia palaeo uplift during middle devonian to late carboniferous times were not as distinct as those during late carboniferous to early jurassic times and the strong uplift and exhumation of the inner mongolia palaeo uplift were achieved during late carboniferous to early jurassic times

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

the fluid factor as an avo indicator castagna and smith 1994 presented a very interesting comparison of some of the different amplitude variation with offset avo indicators used in the industry today they used a worldwide sample of data to compute their examples in their paper they alluded to the fluid factor of smith and gidlow 1987 but they did not compute this attribute for their data in the following we suggest how the fluid factor can be computed and compare the results with the rp rs and a b 2 techniques we have used castagna and smith s 1994 notation

zircon u pb and hf isotopic study of gneissic rocks from the chinese altai progressive accretionary history in the early to middle palaeozoic abstract gneissic rocks in the chinese altai mountains have been interpreted as either paleozoic metasedimentary rocks or precambrian basement this study reports geochemical and geochronological data for banded paragneisses and associated gneissic granitoids collected along a ne sw traverse in the northwestern chinese altai petrological and geochemical data suggest that the protoliths of the banded gneisses were possibly immature sediments with significant volcanic input and that the gneissic granitoids were derived from i type granites formed in a subduction environment three types of morphological features can be recognized in zircons from the banded gneisses and are interpreted to correlate with different sources zircons from five samples of banded paragneiss cluster predominantly between 466 and 528 ma some give neoproterozoic ages and a few yield discordant paleoproterozoic to archean ages zircon hf isotopic compositions indicate that both juvenile mantle and crust materials were involved in the generation of the source rocks from which these zircons were derived in contrast zircons occur ubiquitously as elongated euhedral prismatic crystals in the four samples of the gneissic granitoids and define single populations for each sample with mean ages between 380 and 453 ma the general absence of precambrian inheritance and positive zircon hf values for these granitoids suggest insignificant crustal contribution to the generation of the precursor magmas our data can be interpreted in terms of a progressive accretionary history in early to middle palaeozoic times and the chinese altai may possibly represent a magmatic arc built on a continental margin dominated by neoproterozoic rocks

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

jurassic subsidence history in the hebrides n w scotland abstract geohistory analyses have been applied to the generally well dated triassic jurassic successions of three areas in the hebrides basin north west scotland and the importance of making corrections for decompaction palaeobathymetry and eustatic changes of sea level are assessed the thermo tectonic subsidence patterns in the skye areas north suggest a three phase history of crustal stretching though only the first phase is well developed in the mull area south episodes of relatively rapid subsidence are 1 1 late triassic c210 ma until earliest toarcian 186 ma 2 2 latest toarcian 180 ma until late bathonian c161 ma 3 3 early oxfordian 154 ma until at least early kimmeridgian incomplete these are separated by toarcian 186 180 ma and latest bathonian callovian c161 154 ma episodes of either little subsidence or of uplift but at slower rates than eustatic rise of sea level the intervals between phases at 30 ma and 26 ma are remarkably similar but the wider significance of this remains to be established

nature and timing of the solonker suture of the central asian orogenic belt insights from geochronology and geochemistry of basic intrusions in the xilin gol complex inner mongolia china the solonker suture zone of the central asian orogenic belt caob records the final closure of the paleo asian ocean the nature and timing of final collision along the solonker suture has long been controversial partly because of an incomplete record of isotopic ages and differing interpretations of the geological environments of key tectonic units the xilin gol complex consisting of strongly deformed gneisses schists and amphibolites is such a key tectonic unit within the caob lenticular or quasi lamellar amphibolites are dispersed throughout the complex intercalated with biotite plagioclase gneiss both rock types experienced amphibolite facies metamorphism the protolith of the amphibolite is a basic rock that intruded into the biotite plagioclase gneiss at 319 4 ma based on la icpms zircon u pb dating the basic intrusion was sourced from a modified magma that experienced crystal fractionation and was admixed with slab derived fluids the slab derived fluids which formed during early paleozoic oceanic subduction along the north dipping sonidzuoqi xilinhot subduction zone mixed with the magma source and produced subduction related geochemical signatures superimposed on volcanic arc chemistry after early paleozoic oceanic subduction and arc continent collision a transient stage of extension occurred between 313 and 280 ma in the sonidzuoqi xilinhot area deformation and recrystallization during the switch from compression to extension and reheating by the later magmatic intrusions reset the isotope systems of minerals in the xilin gol complex recorded by a 312 2 1 5 ma biotite 40ar 39ar age from biotite plagioclase gneiss a 309 12 ma zircon intercept age and a 307 5 3 5 ma hornblende 40ar 39ar age from amphibolites in the complex there was an arc forearc related marine basin at the southern margin of the xilin gol complex during the permian the closure of the oceanic basin led to late paleozoic middle triassic north dipping subduction beneath the xilin gol complex and induced the amphibolite facies metamorphism of the complex the final suturing of the solonker zone occurred from 269 to 231 ma this latest amphibolite facies metamorphism with pressures of 0 31 0 39 gpa and temperatures of 620 660 c was recorded at 263 4 1 4 ma to the xilin gol complex as indicated by the hornblende 40ar 39ar age from the amphibolites as well as several zircon ages of 260 3 231 3 ma the xilin gol complex documented the progressive accretion of a single long lived subduction system at the southern margin of the south mongolian microcontinent from the early paleozoic 452 ma to middle triassic 231 ma the caob shows protracted collision prior to final suturing

interactive interpretation of a submarine fan offshore ireland a case history during the eocene a series of submarine fans developed in the porcupine basin off the southwest coast of ireland interactive interpretation of one the clontarf fan shows the successive development of four sequences seismic facies analysis of these sequences suggests a progressive evolution from a more distal to proximal environment during deposition of the fan seismic facies mapping reveals an extensive series of sand prone distributary systems in the youngest fan sequence amplitude anomalies suggest that this and other fans in the basin are favorable stratigraphic and or structural traps for hydrocarbons

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

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

the 390 ma high t metamorphic event in the chinese altai a consequence of ridge subduction high grade rocks occur in the chinese altai but the timing of metamorphism is poorly constrained which hinders our understanding of the thermo tectonic history of the region representative high grade samples from the sillimanite zone extending from hanas to fuyun were selected for zircon u pb dating and temperature estimation la icp ms analyses of zircon overgrowth rims and recrystallized domains give consistent ages of 390 ma which is interpreted to record a regional metamorphic event temperature t estimations using the amphibole plagioclase quartz amp pl qtz and garnet biotite gb geothermometers give relatively high temperatures ranging from 650 to 700 c the zircon metamorphic rims yield temperature estimates of 720 c by using the ti in zircon thermometer these data suggest that a high temperature metamorphic event took place in the chinese altai in the middle devonian and may imply a tectonic environment involving an unusually elevated heat flux from a deep seated source our data support possible ridge subduction around 390 ma that caused upwelling of the hot asthenosphere and triggered the high t metamorphism this model can also account for coeval volcanic activity with a range of geochemical characteristics diverse mafic intrusions and extensive hydrothermal mineralization in the altai orogen

zircon u pb geochronology and hf isotopic composition of granitiods in russian altai mountain central asian orogenic belt the central asian orogenic belt caob consists of many tectonic terranes with distinct origin and complicated evolutionary history understanding of individual block is crucial for the reconstruction of the geodynamic history of the gigantic accetionary collage this study presents zircon u pb ages and hf isotopes for the granitoid rocks in the russian altai mountain range including gorny altai altai mongolian terrane and ctus suture zone between them in order to clarify the timing of granitic magmatism source nature continental crustal growth and tectonic evolution our dating results suggest that granitic magmatism of the russian altai mountain range occurred in three major episodes including 445 429 ma 410 360 ma and 241 ma zircons from these granitoids yield comparable positive ehf t values and neoproterozoic crustal model ages which favor the interpretation that the juvenile crustal materials produced in the early stage of the caob were probably dominant sources for the paleozoic magmatism in the region the inference is also supported by widespread occurrence of short lived juvenile materials including ophiolites seamount relics and arc assemblages in the northern caob consequently the paleozoic massive granitic rocks maybe do not represent continental crustal growth at the time when they were emplaced but rather record reworking of relatively juvenile proterozoic crustal rocks although mantle derived mafic magma was possibly involved to serve as heat engine during granitic magma generation the early triassic granitic intrusion may be a product in an intra plate environment as the case of same type rocks in the adjacent areas the positive ehf t values 1 81 7 47 and corresponding hf model ages 0 80 1 16 ga together with evidence of petrology are consistent with the interpretation that the magma source of the triassic granitic intrusion was derived from enriched mantle and melted under an usually high temperature condition likely due to basaltic magma that underplated the lower crust our data combined with evidence of the regional geology enable us to conclude that the gorny altai and altai mongolian terranes possibly have similar tectonic natures but represent two separate accretionary systems before devonian collision the accretion and amalgamation processes resulted in the paleozoic granitoid magmatism and caused the two terranes to merge as a composite tectonic domain at the siberian continental margin

microplate tectonics new insights from micro blocks in the global oceans continental margins and deep mantle abstract any plate has a growth process from small to large the micro blocks or micro plates are sometimes the precursors of large plates the origin growth aborting extinction and residual process of micro blocks are of great significance for the study of plate tectonics and pre plate tectonics the micro block can be divided into continental oceanic and mantle micro blocks according to their compositions in this paper the micro blocks in the global oceans have been summarized according to the following five environments mid ocean ridge system subduction system transform fault system deep sea intraplate system and extension rift system we first propose a genetic classification of micro blocks comprising detachment derived rifting derived transform derived propagation derived ridge jumping derived subduction derived accretion derived collision derived and delamination derived micro blocks the different types of micro block boundaries such as active or fossil detachment fault subduction zone mid ocean ridge transform fault fracture zone transfer fault accommodation zone lithosphere scale strike slip fault pseudofault intra oceanic convergent zone overlapping spreading centre non transform offset rheological crustal or mantle discontinuity are systematically discussed for different micro blocks thus the number of triple junctions will be more than the 16 in the traditional plate tectonic theory a stability analysis of these triple junctions is the key to understanding the causes of micro blocks however the micro block has no ultimate cause so it is unnecessary to pursue one ultimate cause or initiation of plate tectonics these micro blocks within oceanic basins along oceanic margins or within the deep mantle can not only be used to develop deep ocean fine structural analysis and plate tectonic reconstruction but also to explain the causes of micro blocks in some orogens this will enrich the research of more detailed pre orogenic or syn orogenic evolution of orogenic belts and even extend to the study of early precambrian pre plate tectonic mechanisms the micro block can be a transition among microplate plate and terrane under a plate tectonic regime it can also be formed in inter sphere tectonic processes it helps better our understanding of regimes of cratonic basin formation and intra continental deformation which are the difficulties faced by the plate tectonics theory we speculate that the micro block tectonics theory is a unified tectonic theory of cross layer cross phase cross space time scale and cross planet

crustal structures revealed from a deep seismic reflection profile across the solonker suture zone of the central asian orogenic belt northern china an integrated interpretation abstract the solonker suture zone is one of the most important tectonic boundaries in the southeastern part of the central asian orogenic belt caob an 630 km long reflection seismic profile across this suture was recently completed by the chinese sinoprobe project the processed seismic data show clear crustal structures and provide new constraints on the tectonic and crustal evolution models the moho is delineated as a relatively flat boundary between a strongly reflective lower crust and a transparent mantle at a depth of 40 45 km 14 5 s two way travel time which is in agreement with the refraction data recorded along the same profile in a broad view the profile images an orogen that appears bivergent with and approximately centered on the solonker suture zone the southern portion of this profile is dominated by a crustal scale cratonward propagating fold and thrust system that formed during the late permian and triassic through collision and subsequent convergence in a post collisional stage the major thrust faults are truncated by mesozoic granitoid plutons in the upper crust and by the moho at the base of the crust this geometry suggests that the moho was formed after the thrusting event the northern portion of the profile although partially obliterated by post collisional magmatic bodies shows major south dipping folding and thrusting bands of layered reflectors immediately overlying the moho are interpreted as basaltic sills derived from the mantle episodic mafic underplating may have occurred in this region giving rise to post collisional magmatic events and renewal of the moho a few mantle reflectors are also visible the overall geometry of these mantle reflectors supports the tectonic models that the southern orogen manchurides experienced south directed subduction and the northern orogen altaids underwent north directed subduction prior to collision along the solonker suture zone

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

olivine dolerite intrusionsin the fastnet basin sills of olivine microgabbro and olivine dolerite 0 5 180 m thick intrude lower liassic hettangian sinemurian sediments in the fastnet basin a sw extension of the n celtic sea basin about lat 50 n long 10 w the evidence of 3 wells and 2300 km of seismic reflection profiles indicates 6 major sills and sill complexes and 3 possible igneous centres or plugs a high level sill may intrude lower cretaceous sediments and one plug apparently deforms cretaceous reflectors k ar ages on separated biotites from a thick sill intruding liassic sediments gave 170 4 ma bajocian this sill contains 30 olivine fo 70 73 20 clinopyroxene magnesian salite 45 plagioclase an 60 78 with accessory magnetite ilmenite edenitic hornblende ti phlogopite apatite zircon spinel in olivine and sulphides alteration to serpentine minerals chlorite sericitic mica and leucoxene is sporadic the magmatism in the fastnet province thus appears to have been active from the mid jurassic to the early tertiary related initially to the time of initial rifting between the european and american plates and subsequently to their final separation between greenland and rockall and associated thulean magmatism

petrogenesis of late paleozoic tholeiitic nb enriched calc alkaline and adakitic rocks in southwestern mongolia implications for intra oceanic arc evolution abstract intra oceanic subduction has been taken as one of the most important contributors to the crustal growth of the immense central asian orogenic belt caob however the detailed geological processes of the development and growth of numerous island arcs still remain unclear in order to explore the development history of representative ancient island arcs we have conducted geochronological and geochemical studies on an intrusive rock association in southwestern mongolia the studied rock association comprises a mafic intrusive complex consisting of norite and diabase porphyry diabase and diorite dykes new zircon geochronological data and whole rock geochemical compositions reveal sequential magmatic events of an island arc development the mafic intrusive complex has an emplacement age of 356 ma old geochemically its two components of norite and diabase porphyry have a common tholeiitic character but show striking differences in elemental contents the norites are characterized by relatively higher mgo 7 25 16 1 wt lower tio 2 0 54 0 93 wt and relatively lower nb contents 0 99 6 39 ppm than those of the nb enriched nb 9 ppm diabase porphyry these characters together with variable nb la ratios suggest that the parental magma of the norites was derived from a refractory mantle source metasomatized by slab derived fluids and or melts whereas the generation of the nb enriched diabase porphyry was associated with the mantle metasomatism by slab derived melts the 335 ma diabase dykes have negative nb ta ti anomalies and elevated lree and lile contents similar to calc alkaline subduction related magmas which together with high zircon e hf t values 10 6 to 15 8 indicate a derivation from a mantle wedge metasomatized by slab related fluids the 290 ma dioritic dykes show the highest sio 2 53 1 58 3 wt and total alkali k 2 o na 2 o 5 84 7 77 wt contents and their distinctively high sr y 56 1 97 9 ratios and low yb the timing and petrogenesis of the basic intermediate rock association may supply valuable information on tectonic evolution of an intra oceanic subduction accretion system of the caob the coexistence of the early carboniferous tholeiitic nb enriched mafic intrusive complex and boninite as well as picrite may represent an immature intra oceanic island arc the middle carboniferous calc alkaline diabase dykes were probably associated with the relatively mature island arc the permian adakitic rocks was likely produced by partial melting of a delaminated lower crust in an extensional regime when numerous subduction accretion complexes were likely amalgamated together and docked on the siberian active margin

geochemical study of the cambrian ordovician meta sedimentary rocks from the northern altai mongolian terrane northwestern central asian orogenic belt implications on the provenance and tectonic setting abstract the altai mongolian terrane am is a key component of the central asian orogenic belt caob but its tectonic nature has been poorly constrained this paper reports geochemical compositions of cambrian ordovician meta sedimentary rocks from the northern am to trace their source nature and depositional setting which in turn place constraints on the geodynamic evolution of the am the cambrian ordovician meta sedimentary rocks from the northern am show variable major element compositions with negative correlation between sio2 and tio2 al2o3 fe2o3t mgo and k2o their high icv values 1 18 2 53 and relatively low cia values 37 9 76 3 indicate that the sediments were immature and probably underwent mild to moderate chemical weathering the low sio2 samples are characterized by relatively restricted sio2 al2o3 mostly 2 60 6 07 and low rb sr ratios 0 02 1 89 implying their proximal deposition without obvious sedimentary sorting and recycling in contrast the high sio2 samples show much higher sio2 al2o3 ratios 15 4 19 9 possibly due to sedimentary sorting and or silicification all these samples yield relatively high al2o3 tio2 ratios 15 6 22 8 strong lrees hrees differentiation la yb n 4 86 10 7 and obvious negative eu anomalies eu 0 61 0 83 combined with their th sc zr sc la th and co th ratios comparable with intermediate acidic magmatic rocks we infer that these kinds of magmatic rocks served as a major source for the investigated meta sedimentary rocks the tio2 al2o3 and fe2o3t mgo concentrations are mostly higher than typical sediments from passive margin and the th u la sc th sc eu eu zr hf zr th and la th ratios are quite similar to sediments from continental arcs these data suggest that the cambrian ordovician meta sedimentary rocks from the northern am were most likely deposited in an environment related to a continental arc setting rather than a passive regime these rocks show strong similarities to their counterparts in the chinese altai ca southern am and tseel terrane southeastern extension of the ca in western mongolia in terms of geochemical compositions and depositional setting with combination of recent isotopic studies for detrital zircons our data suggest that the am probably represented a coherent accretionary prism along a continental arc in the early paleozoic

development of the porcupine seabight use of magnetic data to show the direct relationship between early oceanic and continental structures the porcupine seabight is a broad composite rectilinear embayment in the margin of the irish continental shelf which seems to be floored at least in the southern part by oceanic crust there is a geometrically straightforward relationship between areas of crustal thinning rifting and the position of probable 9oceanic crust9 it is proposed that an important crustal unit here termed the porcupine segment includes the porcupine bank and the western end of the slyne ridge interpretation of a new magnetic map indicates that opening in the porcupine seabight appears to have taken place along a set of transform faults which 9root9 in an older dextral shear fault set oriented at about 130 three major zones along the longitudinal axis of the seabight relate to the geometry of magnetic stripes in the subjacent basement and can be used in conjunction with opening related to a distant pole of rotation to explain the overall present physical configuration of the basin the porcupine seabight probably opened no earlier than about 170 ma following the establishment of a jurassic volcanic province in the vicinity of the southern seabight elbow

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

late cretaceous and cenozoic intra plate compressional deformations in the alpine foreland a geodynamic model abstract the mesozoic grabens and wrench induced troughs of western and central europe developed in response to triassic early cretaceous intra plate tensional stresses which affected the arctic north atlantic and the tethys borderlands during the rifting phase preceding the opening of the respective oceans this resulted in the fragmentation of the continental crust of the alpine foreland during the late cretaceous and tertiary alpine orogeny collision related compressive stresses exerted on the foreland induced the reactivation of these fracture systems causing the inversion of mesozoic grabens and the uplifting of major basement blocks at distances up to 1300 km to the north of the present alpine deformation front the structural style of these inverted basins and basement uplifts is indicative of compressional and transpressional deformations the total amount of crustal shortening associated with these deformations is unlikely to exceed a few tens of kilometres these displacements require a coupling between the foreland and the orogen at the alpine a subduction zones and within the foreland a decoupling at intra crustal levels between the crust and mantle and or at deeper lithospheric levels the example of the alpine foreland indicates that compressive stresses inducing major deformations can be transmitted from a collisional plate boundary through continental lithosphere over distances of up to 1300 km circumstantial evidence suggests that collision related compressive stresses not large enough to give rise to significant deformations can be transmitted through continental lithosphere over even greater distances

geochemistry of 2 5 ga granitoids at the northern margin of the yinshan block implications for the crustal evolution of the north china craton abstract in order to further constrain the crustal development and tectonic evolution of the north china craton ncc during late neoarchean to early paleoproterozoic we performed petrological and geochemical analyses on forty six granitoid samples from the northern margin of the craton at siziwangqi in central inner mongolia the granitoids commonly consist of a tonalite trondhjemite granodiorite ttg suite and alkali feldspar rich in potassium granites the geochemistry and isotopic characteristics suggest that they formed in a subduction related arc setting an andean type magmatic arc model is proposed to explain several unique geochemical features of the siziwangqi granitoids notably that the rocks are metaluminous to peraluminous and belong to the calc alkaline ttg and subalkaline to alkaline alkali feldspar granite series the ttg granitoids are characterized by light lree enrichment a weak positive eu anomaly and flat heavy hree profiles the alkali granite is also enriched in the lree but has a strong positive chondrite normalized eu anomaly and displays weak hree enrichment petrogenetic studies reveal that most samples are of i type granitoids the data further suggest that the subduction induced magmatism and slab rollback processes may have triggered reworking of both juvenile arc crust and minor older continental crust material along the north margin of yinshan block leading to the formation of granitoids from s to i type in the study area overall the ncc experienced a tectonic regime that changed from compressional to extensional during the early paleoproterozoic at 2 4 ga after the ca 2 5 ga cratonization event

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

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

evolution of the western approaches trough abstract the western approaches trough corresponds to a major mesozoic rift which became partly inverted during the late oligocene and miocene sediments contained in this basin range in age from permian to neogene and are some 7000 m thick the rifting stage of the western approaches trough spans permian to aptian times there is only minor evidence for volcanic activity important wrench deformations preceded the accumulation of wealden clastics in the rapidly subsiding southwest channel basin and in the brittany trough early aptian wrench deformations coincided with crustal separation in the bay of biscay during the albian to early oligocene the western approaches trough was tectonically inactive and subsided regionally late oligocene to miocene compressional deformations induced the upwarping of a major anticlinorium having a structural relief of some 3000 m

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

late jurassic early cretaceous reef trends on the continental margin sw of the british isles abstract summary multichannel seismic reflection data has been used to outline the distribution of reefs or carbonate banks on the continental margin sw of the british isles this distribution is discussed in terms of regional structure and stratigraphy all the reefal bodies are pre aptian in age with evidence for both late jurassic and early cretaceous phases of reef development reef growth was contemporaneous with rifting prior to the occurrence of sea floor spreading in the bay of biscay the distribution of reefs suggests that shallow marine conditions persisted in the goban spur area until middle cretaceous time while a deeper embayment may have occupied the bay of biscay as early as the late jurassic

western extension of the iberian european plate boundary during the early cenozoic pyrenean convergence a new model abstract a dense seismic net work allows of drawing the western prolongation of the northern boundary of the iberian plate this early cenozoic deformed boundary is disrupted by two prominent nw se transform faults a kinematic reconstruction with a nw late cretaceous lower eocene motion of the iberian plate is proposed the azores biscay rise is interpreted as a short lived subduction zone whereas king s trough could be a transform fault

a crustal model for a northeast southwest profile through ireland abstract the irish caledonian suture seismic project icssp consisting of a 250 km seismic refraction line was carried out in conjunction with and as an extension of the caledonian suture seismic project cssp of the university of durham 51 seismic recording sites at 5 km intervals were deployed on the northwest flank of the suture zone in ireland recordings were made from three shot points at the southwest of the line and from 31 shot points of the durham programme in the north irish sea a crustal model is presented which is based exclusively on a travel time interpretation the p g velocities recorded in eastern ireland rise quite rapidly with depth to 6 3 6 5 km s but in the western part of the profile lower velocities between 6 0 and 6 3 km s were recorded prominent energy in later arrivals can be correlated by two reflected phases from a mid crustal boundary and the moho a low velocity zone is present at the northeastern end of the line and a mid crustal layer with an average velocity of about 6 4 km s is separated from the lower crust mean velocity 6 8 km s by a well established mid crustal boundary zone of 1 2 km thickness at about 20 km depth the moho around 32 km at its deepest shallows towards the sea at both ends of the line it is a sharp discontinuity under western ireland but a transition zone of 3 4 km thickness in the east an interpretation is given which takes into account 1 a the geologic and tectonic history of ireland and 2 b data derived from the gravity and magnetic fields

zircon ages for a felsic volcanic rock and arc related early palaeozoic sediments on the margin of the baydrag microcontinent central asian orogenic belt mongolia abstract magmatic zircons from the basal part of an arc terrane in the accretionary complex ne of the baydrag microcontinental block in central mongolia were dated at 544 7 ma and reflect arc magmatism at the ediacaran cambrian boundary detrital zircon ages for clastic metasediments of the adjacent dzag zone suggest a depositional age

geochronology petrogenesis and tectonic significance of peraluminous granites from the chinese altai nw china abstract the paleozoic granites in the chinese altai are important for the study of tectonic evolution and crustal growth in the central asian orogenic belt caob four representative peraluminous granitic intrusions were selected for systematic studies of zircon u pb and hf isotopic compositions and whole rock geochemical and nd sr isotopic analyses these rocks have high asi alumina saturation index al 2 o 3 cao na 2 o k 2 o 1 01 1 46 molecular ratios with 0 6 5 6 wt of normative corundum and are characterized by moderately negative eu anomalies eu eu 0 38 0 98 and strong depletion in ba nb and sr elements our data suggest that these intrusions were emplaced from 419 to 393 ma consistent with a period of intensive magmatic activities and high temperature metamorphism in the chinese altai while in situ zircon hf isotopic analyses for these granites give predominantly positive e hf t values 0 8 to 12 8 a few inherited zircons yield negative e hf t values from 12 5 to 1 53 the u pb age and hf isotopic data of these inherited zircons are similar to that of the widespread metasediments in addition the peraluminous granitic rocks have near zero or negative e nd t values 3 3 to 0 5 and relatively high initial 87 sr 86 sr ratios 0 7079 0 7266 distinct from those of the i type granites in the study region but similar to the early paleozoic habahe sediments these isotopic compositions suggest that the newly accreted metasediments of habahe group may be the major source rock of the peraluminous granites the geochemical compositions indicate that their precursor magmas were derived from a relatively shallow crustal level p 5 kbar and zircon saturation temperatures suggest that these granitic intrusions were emplaced at 672 861 c the peraluminous granitic magmas were generated by dehydration melting of newly accreted materials which were possibly brought to at least middle crustal depth by subduction related processes in an active margin and were subsequently molten by strikingly high ambient temperature probably caused by upwelling of the hot asthenosphere associated with ridge subduction in the paleozoic

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

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

structural framework and tectono stratigraphic evolution of the porcupine seabight basin offshore western ireland abstract the porcupine seabight basin is an intra cratonic rift basin which evolved through recurrent episodes of lithospheric stretching structure contour and isopach maps presented here have been constructed following the interpretation of approx 10 000 km of seismic data together with all the available well data this integrated database has allowed the tectono stratigraphic history of the basin to be traced in detail from its earliest inception the basin was initially formed during the carboniferous when up to 3 7 km of autuno carboniferous sediments were deposited on a caledonian and hercynian structured basement late permian and early mesozoic sediments at least 1 km thick are preserved in contiguous grabens and their presence is also inferred within localized sub basins late liassic crustal doming heralded the onset of the final rifting episode and in the north promoted severe erosion and a rapid discharge of clastic sediments rapid subsidence block faulting and magmatism accompanied the deposition of up to 3 5 km of middle upper jurassic sediments in asymmetrical sub basins the transition from stretching to passive subsidence across the jurassic cretaceous boundary in combination with localized inversion abrupt eustatic and environmental changes generated a basin wide unconformity the rift drift transition in the adjacent proto atlantic rift system is recorded in the porcupine seabight basin by voluminous volcanism and a late lower cretaceous unconformity cretaceous sediments are strongly transgressive and up to 6 km thick these were superseded during the early palaeogene by several regressive transgressive cycles before the arrival of an upper slope fauna in the mid middle eocene cenozoic sediments are up to 4 km thick and record the interplay between 1 the location of the basin pn the pyrenean alpine foreland superimposed on the periphery of the icelandic plume and 2 its development as a marginal atlantic shelf region the former is thought to be responsible for eo oligocene faulting subsidence and igneous activity in the north east part of the basin and the latter for unconformities and a change in the style of sedimentation principally due to oceanic circulation changes

reconstructions of east asian blocks in pangea preface abstract pangea is the youngest supercontinent in earth s history forming about 300 250 ma ago as supported by large amounts of geological paleomagnetic and paleontonlogical data there is little debate on the positions of major continental blocks in pangea however controversy has long surrounded the relations of east asian blocks with pangea most pangea reconstructions assume that the east asian blocks were not the components of pangea but separated from pangea by the paleo tethys ocean these reconstructions were mainly based on geological and paleomagnetic data before the 1990 s but did not fully consider recent data especially for major orogenic belts that formed through the subduction and closure of the proto tethys paleo asian and paleo tethys oceans in east asia to determine where when and how these oceans were closed and whether or not the east asian blocks were added to pangea the national natural science foundation of china nsfc set up a nsfc major program entitled reconstructions of east asian continental blocks in pangea which started in 2012 and was completed in 2016 in the past five years the researchers of this project carried out extensive field based geological and paleomagnetic investigations on the east asian blocks and their intervening mountain belts and produced large amounts of data and competing interpretations which provide constraints on the paleogeographic evolution of the east asian blocks from the breakup of rodinia to the assembly of pangea this forms the justification for organizing this special issue in which we collected ten papers that timely summarize these new data and interpretations and present new models for the reconstructions of the east asian continental blocks in pangea

geodynamics asian continental growth the ancient central asian orogenic belt formed during a period of extensive continental crust formation comparison with modern continent building systems suggests that the processes that operate today were already active at that time

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 structure and subsidence of rockall trough from two ship seismic experiments coincident multichannel seismic reflection and refraction profiles acquired northwest of britain during a two ship seismic experiment in the southern rockall trough provide evidence for a thin 6 km syn rift crust below rockall trough the variation of seismic velocity with depth is consistent with either oceanic crust similar to that found in other parts of the north atlantic or with thinned continental crust heavily intruded by syn rift igneous rocks the syn rift crust beneath rockall trough is now buried by nearly 5 km of sediments that are intruded by tertiary volcanics varying in extent from isolated sills to a large sill complex extending 100 km laterally the basement imaged on seismic profiles exhibits a band of subhorizontal reflectors that extend laterally over distances of up to 40 km which they interpret as sediments intercalated with submarine lava flow generated during the opening of the trough estimates of stretching from crustal thinning and from subsidence indicate that rockall trough has undergone extensive rifting beta 6 upwelling asthenosphere beneath the thinned lithosphere generated at least 1 3 km thickness of melt as it decompressed the molten rock rose upward from the mantle until it was in part extruded as lava flows and in partmore intruded into the crust less

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

nd sr isotopic mapping of the chinese altai and implications for continental growth in the central asian orogenic belt article i nfo the chinese altai orogen is a typical domain of the central asian orogenic belt and in previous syntheses was divided into six tectonic units or terranes in this paper 41 new and 36 published whole rock nd and sr isotopic data for granitoid intrusions and mafic dykes within units 2 and 3 of the central altai and units 4 to 6 of the southern altai were used for isotopic mapping the age of the samples used is known from u pb zircon dating end t values range from 4 to 2 in the central altai units 2 and 3 and from 1 4 to 6 in the southern altai units 4 and 5 and up to 8 4 in unit 6 correspondingly nd model ages tdm decrease from 1 6 1 1 ga in the central altai to 1 0 0 5 ga in the southern altai except for a continental fragment in unit 4 isr is elevated in the central units decreasing to b0 705 in the southern units 5 and 6 these results demonstrate that the central altai contains widespread old crustal components in its basement whereas the southern altai comprises a higher proportion of juvenile crust when integrated with published u pb zircon ages our data show that new crust grew southwards in the chinese altai in addition our data suggest that units 2 and 3 are a single tectonic unit whereas the southern altai unit 4 which previously was regarded as a single young accretionary terrane should be subdivided into two units a younger arc and an older continental fragment the granitoids in the chinese altai can be divided into syn and post non orogenic groups the syn orogenic granitoids have ca 50 80 mantle derived component whereas the post non orogenic granitoids contain ca 70 90 the generation of syn and post non orogenic granitoids represents horizontal lateral and vertical crustal growth which contributed ca 18 28 and 7 8 respectively to the area of the chinese altai this work shows that isotopic mapping is an effective approach to elucidate terrane composition in the central asian orogenic belt and thereby better constrain palaeozoic crustal growth

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

keketuohai mafic ultramafic complex in the chinese altai nw china petrogenesis and geodynamic significance abstract devonian magmatism was very intensive in the tectonic evolutionary history of the chinese altai a key part of the central asian orogenic belt caob the devonian keketuohai mafic ultramafic complex in the chinese altai is a zoned intrusion consisting of dunite olivine gabbro hornblende gabbro and pyroxene diorite the pyroxene diorite gives a zircon u pb age of 409 5 ma variations in mineral assemblage and chemical composition suggest that the petrogenesis of the keketuohai complex was chiefly governed by fractional crystallization from a common magma chamber low sio 2 k 2 o and na 2 o contents negative covariations between p 2 o 5 tio 2 and mg value suggest insignificant crustal assimilation contamination thus the positive e nd t values 0 to 2 7 and slight enrichments in light rare earth elements e g la yb n 0 98 3 64 suggest that their parental magma was possibly produced by partial melting of the lithospheric mantle model calculation suggests that their parental magma was high mg mg 66 tholeiitic basaltic melt the keketuohai intrusion was coeval with diverse magmatism high temperature metamorphism and hydrothermal mineralization which support a previously proposed model that ridge subduction most likely played an important role in the tectonic evolution of the chinese altai

major cretaceous volcanic province in southern rockall trough abstract a group of three large curvilinear ridges called the barra volcanic ridge system has been mapped in the acoustic basement of southern rockall trough typically each ridge is about 2 km high and 20 km wide at its base a crudely layered acoustic character moderate density but high strength of magnetisation point to a volcanic sedimentary ash composition for the ridges seismic continuity with the acoustic basement of the rest of rockall trough suggests that the trough basement is of similar composition an age for the ridges of lower cretaceous is indicated by well ties and consideration of regional geology volumetrically the ridges are on the scale of hot spot features such as the wyville thomson ridge

the hulan group its role in the evolution of the central asian orogenic belt of ne china the hulan group of central jilin province northeastern ne china consists of felsic volcanic and sedimentary rocks that underwent sub greenschist to amphibolite facies metamorphism and multiple phases of deformation during formation of the central asian orogenic belt young nd model ages of 1100 ma obtained in this study do not support earlier models that the hulan group was either rifted from the north china craton to the south or from the jiamusi block to the north since both these blocks have much older nd model ages detrital zircon u pb ages from a meta sedimentary rock of the hulan group define a maximum age of deposition of 287 6 ma not early palaeozoic as previously believed rb sr mineral isochron data indicate that metamorphism of the hulan group occurred at 250 ma this conclusion is further supported by an age of 248 4 ma for the dayushan granite which intruded the hulan group these new isotopic data together with evaluation of the regional geology indicate that final oceanic closure in the chinese segment of the central asian orogenic belt took place in the late permian the hulan area is located along the solonker xra moronchangchun suture that extends from inner mongolia across to central jilin province it is this zone that marks the final ocean closure resulting in the amalgamation of the north china and siberian cratons 2007 elsevier ltd all rights reserved

pre mesozoic geology of the western and north western irish continental shelf exploration wells drilled offshore west and north west of ireland penetrated a c 3 7 m cumulative thickness of permo carboniferous sediments overlying dalradian metasediments intruded by caledonian granitic plutons in the north late tournaisian to early visean clastics were deposited coeval with the rheic transgression upon an emergent craton in a continental to marginal marine environment they record an increasing marine influence upwards with the establishment of a carbonate shelf in the asbian clastic carbonate sedimentation which became cyclical in late visean to early namurian temporally correlates with the yoredale cyclothems younger namurian sediments are not preserved sedimentation in early westphalian progressed from prodelta to coal enriched delta top and abandonment facies and is conformably succeeded by brackish to freshwater sediments that contain evidence for local marine incursions of stephanian age an autuno silesian section that is preserved offshore may also have been deposited onshore prior to erosion seismic data enable partial delineation of n s and ne sw early sub basins marginal to the main porcupine basin these sub basins which contain silesian sediments may have styled successive extensional episodes in the basin and augment support for rift models of carboniferous basin development in the north atlantic nw european domain birps data provide supporting evidence for the offshore prolonga tion of pre mesozoic basement fault zones

structural constraints on the evolution of the central asian orogenic belt in sw mongolia we provide a detailed description of the structures along a 300 km long and 50 km wide transect across the central asian orogenic belt caob in southwestern mongolia covering the precambrian dzabkhan continental domain with overthrust neoproterozoic ophiolites in the north lake zone a silurian devonian passive margin association gobi altai zone and oceanic domain trans altai zone in the center and a continental area south gobi zone in the south structural analysis suggests late cambrian collapse of the thickened lake zone continental crust leading to stretching of the lithosphere and followed by silurian devonian formation of oceanic crust in the trans altai domain subsequent emplacement of devonian carboniferous and late carboniferous magmatic arcs occurred on the gobi altai and south gobi zone crusts respectively during e w shortening finally the entire system was affected by n s convergence from the permian to jurassic leading to heterogeneous shortening of the orogenic domain the model best fitting these observations is one of generalized westward drift of the tuva mongol dzabkhan baydrag ribbon continents during the silurian devonian associated with westward subduction of the mongol okhotsk ocean and sequential growth of syn convergent magmatic arcs back arc basins opened during this period in the area of the western paleoasian ocean the present day shape of the caob in southern mongolia was probably formed during permian to mesozoic anticlockwise rotation and folding of the tuva mongol dzabkhan baydrag continental ribbons combined with a strike slip transpressional reactivation of ancient transform boundaries in the paleoasian oceanic domain all continental and oceanic crustal domains were reactivated and intensely deformed during this convergence in a style controlled by crustal rheology and a heterogeneous permian magmatic thermal input the sequence of tectonic events is tested against published paleomagnetic data paleogeographic reconstructions and tectonic models leading to a revised model for the accretion of juvenile crust to a continental margin in the caob of southern mongolia

continental crust under the southern porcupine seabight west of ireland abstract two new seismic refraction wide angle reflection profiles demonstrate that the crust beneath the southern porcupine seabight out to water depths in excess of 4000 m is of continental type they also reveal the rifted margin of the porcupine basin on its eastern side crustal thickness under the seabight inclusive of sediments which are up to 6 km thick decreases from 23 km in the east to about 10 km at a sharp continent ocean transition in the west

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

distinct deformational history of two contrasting tectonic domains in the chinese altai their significance in understanding accretionary orogenic process abstract the chinese altai a key component of the western central asian orogenic belt is considered to be formed through multiple accretions of different terranes however the deformational histories of each terrane tectonic domain i e structural records before and after the accretion are rarely studied which has hindered our understanding of the accretionary processes to fill the gap a systematic macro and microscopic structural analysis was carried out on two contrasting litho tectonic units i e the early paleozoic low grade alegedayi ophiolitic complex aoc juxtaposed to the high grade tarlang granitic massif tgm selected rock samples were analyzed using zircon u pb isotopic dating to constrain the timing of polyphase deformation our structural and geochronological data suggest that the two litho tectonic units were initially detached and located in different crustal levels and experienced distinct phases of deformation under contrasting p t conditions they were mutually accreted with each other in the early devonian and jointly underwent a wnw ese directed shortening deformational event d 1 at 390 ma the change of tectonic regime was further enhanced by a subsequent nne ssw directed shortening deformation d 2 after 380 ma the shortening process ended before the crustal scale sinistral strike slip shearing deformation along the erqis fault zone at 290 240 ma results of this study provide solid field based evidence for a model that the chinese altai initially underwent a nearly e w oriented subduction accretional event in the middle paleozoic before it was reoriented to a nearly n s oriented convergence

a geochemical traverse across the north chilean andes evidence for crust generation from the mantle wedge major and trace element and sr and nd isotope analyses are presented on 186 0 ma magmatic rocks along an east west traverse across north chile at 22 s esr ranges from 25 to 100 and end from 6 to 9 but the low end and high esr values are in rocks generated in the last 15 ma it is argued that previous discussions of petrogenesis in north chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one but two processes one results in a progressive shift of esr from 25 to 20 and end from 6 to 6 in jurassic to recent rocks which is accompanied by increasing ta sm and sr and decreasing th ta the second is largely confined to the younger rocks and it is characterised by esr increasing up to 100 with increasing sio2 and decreasing sr and it results in relatively shallow trends on an end esr diagram the preferred interpretation is that trend 1 is due to the mobilisation of old late proterozoic mantle lithosphere as magmatism migrated eastwards and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust it follows that the mantle wedge is the principal site of crust generation and it is argued that 20 of the sr in the recent northern chile rocks is derived from the subducted ocean crust

graben hydrocarbon occurrences and structural style major hydrocarbon occurrences types of traps and structural styles have been synthesized from the sirte basin the suez and viking grabens and from other normal faulted regions hydrocarbons occur in a stacked succession of one or more basins pregraben graben fill and interior sag preservation of pregraben accumulations depends on late initiation of crustal arching and limitation of uplift to the graben shoulders furthermore a stable pregraben tectonic environment is required to ensure internal continuity of subsequent fault block closures hydrocarbon traps in the pregraben and graben fill sediments are primarily dependent on the multidirectional orientation of normal faults tilting of fault blocks and in many cases flexing or erosion parallel with block edges the fault pattern is dominated by longitudinal faults subparallel with the graben axis and secondarily by oblique faults the direction and degree of block rotation are influenced by fault profile planar or listric degree of extension fault pattern doglegs junctions terminations downwarping of the subsequent sag basin and isostatic adjustments between large blocks fold closures result from the upward termination of faults into forced folds which may be accentuated by drag during renewed faulting passive drape and differential compaction extend he folds to shallower depths because they form along block edges the flexures have trends that mimic the multidirectional fault patterns truncation traps are usually developed from the erosional retreat of fault scarps and they can also mimic the fault patterns where the sedimentary succession is complete an interior sag basin is superimposed across the graben its lower part is structured mostly by drape and differential compaction across the underlying graben fault blocks and fault block topography the upper part is usually an unsegmented syncline and hydrocarbon traps at this level include depositional buildups reefs turbidite mounds and salt diapirs or other structures caused by superimposed deformation unrelated to graben tectonics

permian geodynamic setting of northeast china and adjacent regions closure of the paleo asian ocean and subduction of the paleo pacific plate abstract northeast china and adjacent regions are located in the central east asian continent and consist tectonically of both the paleo asian and paleo pacific orogens between the siberian platform and sino korean north china block this paper discusses some hotly debated issues concerning the permian geodynamic setting of these regions based on a comprehensive analysis of available geological geochemical paleobiogeographical and paleomagnetic data spatial and temporal distribution of ophiolites and associated continental marginal sequences permian sedimentary sequences spatial distribution and geochemistry of permian magmatic rocks and the evolution of paleobiogeographical realms imply 1 that the permian marine basins in northeast china and adjacent regions include remnants of the paleo asian ocean in southeastern inner mongolia and central jilin province and active continental margins of the paleo pacific ocean 2 that the suture between the siberian and sino korean paleoplates was finally emplaced in the permian and is located in areas from suolunshan solonker eastwards through regions north to the xar moron river in southeastern inner mongolia and then central jilin province to the yanji area and 3 that the permian crustal evolution of northeast china and adjacent regions as well as parts of the siberian paleoplate was influenced by subduction of the paleo pacific oceanic plate finally the permian tectonic framework and paleogeography of northeast china and adjacent regions in central east asia are discussed briefly and early and late permian palinspastic reconstruction maps are provided

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

early permian plutons from the northern north china block constraints on continental arc evolution and convergent margin magmatism related to the central asian orogenic belt recent zircon dating identified several late carboniferous to early permian hornblende gabbro diorite quartz diorite granodiorite tonalite granite plutons in lithological assemblages at the northern margin of the north china block ncb that were previously regarded as archaean to palaeoproterozoic our geochronological results indicate that emplacement of these plutons was a continuous process during the late carboniferous to early permian from 324 6 to 274 6 ma and lasted for at least 50 ma in this paper the early permian components with compositions from gabbro to granite within the intrusive complex were studied the early permian plutons exhibit calc alkaline or high k calc alkaline metaluminous geochemical features and highly variable sio2 contents they have no significant eu anomaly in their ree patterns and in primitive mantle normalized spidergrams they display depletion in th u nb ta p and ti and enrichment in ba k pb and sr the granitoid bodies within these plutons display i type and adakitic geochemical signatures the early permian rocks exhibit low whole rock initial 87sr 86sr ratios from 0 70520 to 0 70615 and have negative whole rock e nd t values ranging from 17 4 to 9 3 and zircon e hf t values of 23 2 to 10 5 the gabbros exhibit higher e nd t values from 11 1 to 9 3 and e hf t values from 16 5 to 10 5 and one granodiorite exhibits an even lower e nd t value of 17 4 and zircon e hf t values of 23 2 to 15 1 geochemical sr nd and in situ zircon hf isotopic compositions suggest that the hornblende gabbros were derived from a metasomatized lithospheric mantle and the diorite and quartz diorite were generated from a gabbroic magma by fractional crystallization coupled with differential assimilation of ancient lower crustal material the granodiorite was likely derived from partial melting of ancient lower crust with involvement of some mantle components involvement of both lithospheric mantle and ancient lower crust in the generation of the early permian plutons indicates strong crust mantle interaction in the northern ncb petrological associations as well as geochemical and sr nd hf isotopic results show that the early permian plutons were emplaced along an andean type active continental margin during southward subduction of the palaeo asian oceanic plate beneath the ncb integration of our results with previously published data for late carboniferous and late permian to middle triassic intrusions suggests that the continental arc on the northern margin of the ncb existed for at least 50 ma during the late palaeozoic and final amalgamation of the mongolian arc terranes with the northern ncb likely occurred during a period from 270 to 250 ma i e in the late permian to earliest triassic

a precise u pb zircon age for the inishtrahull syenitic gneiss county donegal ireland smd zircon fractions from the deformed alkaline igneous complex on inishtrahd county donegal ireland yield near concordant u pb ages of 1779 3 ma interpreted as dating the igneous crystallization of the gneiss9s precursor sm nd depleted mantle model ages range from 1912 ma to 1978 ms the similarity of the model ages and the crystallization age indicates that the protoliths were derived from a depleted mantle source essentially uncontaminated by older crust the isotopic data confirm previous tentative correlations of the inishtrahull rocks with those of islay in southwestern scotland archaean crust is apparently absent from this region consistent with recent models placing a southern limit on the archaean along the line of the great glen fault zone pre caledonian orthogneisses in scotland and ireland south of the great glen fault zone have generally been correlated with the late archaean lewisian complex of northwestern scotland e g wilkinson 1907 mccallien 1930 bowes hopgood 1975 dixon et al 1990 although aftalion max 1987 concluded that the lewisian was absent from the annagh division gneisses of north mayo ireland there are now several lines of evidence to suggest that archaean crust may be absent southeast of the great glen fault zone e g fitches et al 1990 daly 1990 halliday et al 1985 a large area of pre caledonian basement here termed the rhinns complex is thought to be present within the fault bounded colonsay west islay block fig 1 bentley et al 1988 fitches et al 1990 marcantonio et al 1988 which extends from colonsay in

a deep seismic reflection transect across the irish caledonides new reflection profiles recorded to 18s c 60km along a 600 km north south transect west of ireland wire lines provide a cross section of the irish caledonides from lewisian basement in the north to the variscan fold and thrust belt in the south and constrain the offshore extrapolations of major caledonide structures we interpret the profiles as showing the great glen fault and associated ne trending strike slip faults as near vertical structures which cut the whole crust and the uppermost mantle beneath the surface trace of the fair head clew bay line we image a north dipping zone which soles into horizontal layers just above the reflection moho the north dipping zone separates south dipping reflections above and to the north from north dipping reflections below and to the south from the fair head clew bay line as far south as the iapetus suture the lower crust contains north dipping packages of reflectors possibly related to acadian accretionary underplating the iapetus suture zone can be identified as a zone more than 50 km wide dipping north at 30 from 5 km depth offshore dingle peninsula to sole into the moho at c 30 km depth west of the shannon estuary in the upper crust the shallow part of the iapetus suture zone appears to be transported to the north by late caledonian retrocharriage or by variscan thrusting the iapetus suture does not penetrate through the present day reflection moho but there are scattered reflectors in the mantle not continuous with crustal reflectors perhaps relics of a subducted slab we recognize lower crustal reflectivity and the reflection moho along almost all the wire profiles regionally the travel time to the reflection moho varies from 9 s c 28 km offshore donegal northwest of ireland to 12 s c 35 km offshore kerry and cork southwest of ireland apparent moho offsets rapid lateral changes in the travel time to the reflection moho north of donegal and north of mayo are probably due to the great glen fault and associated faults the lower crustal reflectivity varies considerably in thickness and prevalent dip direction along the wire profiles in donegal bay this reflectivity is observed at depths less than 5 km c 1 5s whereas west of kerry prominent reflectivity is only observed beneath 6s c 17 km though the age of crustal reflectors cannot in general be determined beneath central ireland from donegal to dingle the lower crustal reflectors mimic in dip direction the structural vergence of the exposed dalradian and early palaeozoic rocks so that the lower crustal reflectivity patterns and probably much of the crust must be caledonian in age

interpretation of avo anomalies we investigate the effects of changes in rock and fluid properties on amplitude variation with offset avo responses in the slope intercept domain reflections from wet sands and shales fall on or near a trend that we call the fluid line reflectionsfromthetopofsandscontaininggasorlight hydrocarbonsfallonatrendapproximatelyparalleltothefluidline reflectionsfromthebaseofgassandsfallonaparallel trendontheopposingsideofthefluidline thepolaritystandard of the seismic data dictates whether these reflections from the top of hydrocarbon bearing sands are below or above the fluid line typically rock properties of sands and shales differ and therefore reflections from sand shale interfaces are also displaced from the fluid line the distance of these trends from the fluid line depends upon the contrast of the ratio of p wave velocity vp and s wave velocity vs this ratio is a function of pore fluid compressibility and implies that distance from the fluid line increases with increasing compressibility reflections from wet sands are closer to the fluid line than hydrocarbon related reflections porosity changes affect acoustic impedance but do not significantly impactthevp vscontrast asaresult porositychangesmove theavo response along trends approximately parallel to the fluidline theseobservationsareusefulforinterpretingavo anomaliesintermsoffluids lithology andporosity

carboniferous mantle derived felsic intrusion in the chinese altai nw china implications for geodynamic change of the accretionary orogenic belt abstract this paper reports whole rock sr nd and zircon u pb and hf isotopic compositions as well as major and trace element data for the granodiorites and their mafic microgranular enclaves mmes in the tuokesalei area northwestern chinese altai the granodiorites are characterized by peraluminous compositions a cnk 1 01 1 45 with relatively high sio 2 64 4 to 70 9 wt and low mgo 0 75 to 1 97 wt contents these rocks possess low total rare earth element ree contents 67 187 ppm with weakly negative eu anomalies and mantle like whole rock nd sr isotopic compositions e nd t 2 5 to 4 5 87 sr 86 sr i 0 7038 to 0 7048 and zircon e hf t values 5 93 to 12 9 the mmes contain abundant magmatic amphiboles and needle like apatites have low sio 2 51 7 to 54 3 wt and high mgo 2 86 to 3 75 wt and ree contents 162 298 ppm and are characterized by moderately negative eu anomalies eu eu 0 40 0 60 and slightly depleted to enriched lree la nd n 0 74 1 41 and flat hree gd yb n 1 16 1 39 patterns the mmes have whole rock nd sr isotopic compositions e nd t 2 6 to 2 7 87 sr 86 sr i 0 7034 0 7050 similar to those of the host granodiorites their similarities in crystallization age mineral assemblage and isotopic composition document that the mmes may represent an early crystallized phase which was broken into pieces and brought to the shallow level during the host magma ascent and emplacement their precursor magmas were probably derived from an oceanic lithospheric mantle which was characterized by pronounced nd ta depletions mineral compositions of mmes suggest that amphibole was a major early fractional mineral which effectively differentiated the mantle derived basaltic liquids to silica rich melts zircon u pb dating yielded an age of 313 5 ma for the host granodiorites and 313 13 ma for their mmes the geochemical data of this study records an important episode of magmatism in an environment transitional from a subduction related regime to a post tectonic extension results of this study suggest that the accretionary orogenic process did not completely terminate until the late carboniferous to permian in this region

a comparison between methods that discriminate fluid content in unconsolidated sandstone reservoirs three seismic attributes commonly used to predict pore fluid and lithology are the fluid factor f poisson impedance pi and lambda rho we evaluated the pore fluid sensitivity of these attributes with both well log and seismic data in tertiary unconsolidated sediments from the gulf of mexico where sand and shale are the only expected lithologies while the sensitivity of one attribute versus another to discriminate pore fluid is often debated in the literature the sensitivities of the three attributes are not independent but can be traced back to the fluid factor which is a function of the p and s wave normal incident reflection coefficients interestingly the fluid factor which is a reflectivity attribute at the top of a hydrocarbon saturated reservoir is basically independent of the shale properties above the reservoir it is a function of the brine and hydrocarbon impedances of the reservoir the next attribute poisson impedance is thenequal to the fluid factor times the sum o

late permian to early mesozoic rifting and sedimentation offshore nw ireland abstract a late permian triassic and early lias rift succession up to 1 km thick is revealed from offshore nw ireland sediments are preserved in deep narrow grabens from the donegal basin in the north southwestwards through the erris and slyne troughs to a series of newly defined basins collectively forming the north porcupine basin with a further structure the galway graben on the porcupine bank this line of basins superimposed on a caledonoid fractured basement underwent a complex structural evolution but since the late permian they developed primarily as a wrench modified rift system during the latest lias crustal doming west of britain may have produced a regional angular unconformity and created a source for thick middle jurassic alluvial clastics deposited in the main porcupine basin to the south early tertiary transpression of the north porcupine area caused late stage fragmentation of the basin the rift succession consists of continental red beds bounded by two dateable transgressive intervals these intervals provide valuable stratigraphic control for constraining the timing and scale of seaways propagating across the fracturing pangean supercontinent from the late permian to the early jurassic evidence is also presented for an episode of syn rift volcanism temporally unique to western britain and ireland