subduction collision and exhumation in the ultrahigh pressure qinling dabie orogen c abstract high pressure metamorphism and ophiolite emplacement songshugou ophiolite attended suturing of the yangtze craton to rodinia during the c l o ga grenvillian oro geny the qinling microcontinent then rifted from the yangtze craton at c 750ma the erlangping intraoceanic arc formed in the early ordovician was emplaced onto the qinling microcontinent in the ordovician silurian and then both units were accreted to the sino

identifying time temperature and mineralogical effects on chemical compaction in shales by rock physics relations the seismic response of a shale is a direct result of the original mineral composition textural configuration at deposition and degree of compaction the aim of this article is to show how the compactional processes and mineralogical effects can be directly identified by rock physics techniques and to emphasize the importance of having a thorough geological understanding when doing rock physics analysis to succeed we must first understand the fundamental relationships between sedimentology the compactional processes and the expected rock physics response that are not restricted to a particular basin or data set to achieve this we used wireline logs from 100 wells on the mid norwegian shelf to establish robust rock physics background trends in shales that were compared with the local responses in and around a target area the target was the tulipan well in the southwestern part of the mid norwegian shelf where thick deposits from the glacial marine north sea fan are located

rhodoliths uniformitarianism and darwin pleistocene and recent carbonate deposits in the cape verde and canary archipelagos abstract visiting st jago santiago in the cape verde islands in 1832 and again in 1836 aboard hms beagle charles darwin was the first to trace and describe the tri part sequence of white limestone and sandstone beds stratigraphically located between two levels of basalt exposed almost uninterrupted for 10 km along coastal cliffs the pleistocene carbonate sediments dominated by rhodoliths and rhodolith debris accumulated on a basalt shelf and subsequently became buried by subaerial and submarine basalt on the southeast coastline of santiago the main goal of this contribution is to re examine darwin s stratigraphic sequence the secondary goal is to provide a general taphonomical model based on the observation of recent rhodolith deposits for evaluation of fossil rhodolith assemblages environmental uniformitarianism is employed to understand the depositional history of the southern santiago rhodolith bearing strata the mixed clastic carbonate sequence includes a basalt derived basal conglomerate with an intertidal to shallow subtidal fossil assemblage mainly denoted by limpets and oysters upper layers typically demonstrate swaley and hummocky cross stratification incorporating rhodolith debris further modified by bioturbation pillow basalts from 10 to 18 m in thickness succeeded by subaerial flows imply swift burial of the carbonate succession under equivalent water depths the calcareous nannofossil assemblage was investigated to more precisely date the deposits darwin s paleoshore is reinterpreted to represent two different transgressions occurring between approximately 1 1 and 0 7 ma taphonomic grades from whole rhodoliths to finely crushed rhodolith debris observed under present day conditions on maio cape verde islands and fuerteventura canary islands were used to model rhodolith preservation and to constrain the depositional settings to which rhodoliths may be transported from the offshore banks where they naturally thrive coastward transport of rhodoliths commonly ends with deposition in subtidal storm beds tidal pools and platform over wash deposits as well as beach berm hurricane tsunami and coastal dune deposits

tectonic evolution of a composite collision orogen an overview on the qinling tongbai hong an dabie sulu orogenic belt in central china abstract the formation of collisional orogens is a prominent feature in convergent plate margins it is generally a complex process involving multistage tectonism of compression and extension due to continental subduction and collision the paleozoic convergence between the south china block scb and the north china block ncb is associated with a series of tectonic processes such as oceanic subduction terrane accretion and continental collision resulting in the qinling tongbai hong an dabie sulu orogenic belt while the arc continent collision orogeny is significant during the paleozoic in the qinling tongbai hong an orogens of central china the continent continent collision orogeny is prominent during the early mesozoic in the dabie sulu orogens of east central china this article presents an overview of regional geology geochronology and geochemistry for the composite orogenic belt the qinling tongbai hong an orogens exhibit the early paleozoic hp uhp metamorphism the carboniferous hp metamorphism and the paleozoic arc type magmatism but the three tectonothermal events are absent in the dabie sulu orogens the triassic uhp metamorphism is prominent in the dabie sulu orogens but it is absent in the qinling tongbai orogens the hong an orogen records both the hp and uhp metamorphism of triassic age and collided continental margins contain both the juvenile and ancient crustal rocks so do in the qinling and tongbai orogens in contrast only ancient crustal rocks were involved in the uhp metamorphism in the dabie sulu orogenic belt without involvement of the juvenile arc crust on the other hand the deformed and low grade metamorphosed accretionary wedge was developed on the passive continental margin during subduction in the late permian to early triassic along the northern margin of the dabie sulu orogenic belt and it was developed on the passive oceanic margin during subduction in the early paleozoic along the northern margin of the qinling orogen three episodes of arc continent collision are suggested to occur during the paleozoic continental convergence between the scb and ncb the first episode of arc continent collision is caused by northward subduction of the north qinling unit beneath the erlangping unit resulting in uhp metamorphism at ca 480 490 ma and the accretion of the north qinling unit to the ncb the second episode of arc continent collision is caused by northward subduction of the prototethyan oceanic crust beneath an andes type continental arc leading to granulite facies metamorphism at ca 420 430 ma and the accretion of the shangdan arc terrane to the ncb and reworking of the north qinling erlangping and kuanping units the third episode of arc continent collision is caused by northward subduction of the paleotethyan oceanic crust resulting in the hp eclogite facies metamorphism at ca 310 ma in the hong an orogen and low p metamorphism in the qinling tongbai orogens as well as crustal accretion to the ncb the closure of backarc basins is also associated with the arc continent collision processes with the possible cause for granulite facies metamorphism the massive continental subduction of the scb beneath the ncb took place in the triassic with the final continent continent collision and uhp metamorphism at ca 225 240 ma therefore the qinling tongbai hong an dabie sulu orogenic belt records the development of plate tectonics from oceanic subduction and arc type magmatism to arc continent and continent continent collision

uhp metamorphism and exhumation of the dabie orogen china evidence from shrimp dating of zircon and monazite from a uhp granitic gneiss cobble from the hefei basin a shrimp u pb study of zircons separated from an ultrahigh pressure uhp granitic gneiss cobble sample ds12 from the upper jurassic fenghuangtai formation of the hefei basin north of the dabie orogen has identified three different domains 1 cores some of which show straight boundaries and strong oscillatory zoning with th u ratios of 0 12 0 70 and an imprecise upper intercept age of 777 220 ma mswd 2 4 2 mantles variable in shape and cathodoluminescence cl intensity and containing many uhp mineral inclusions including coesite and omphacite and with th u ratios of 0 02 0 26 and a weighted mean 206pb 238u age of 244 5 ma mswd 4 7 and 3 rims which are more homogeneous and luminescent in cl than the mantles and contain fewer uhp mineral inclusions with th u ratios of 0 01 0 05 and a weighted mean 206pb 238u age of 226 2 ma mswd 0 65 monazites from the same sample do not contain coesite and omphacite inclusions but they do show sector zones and are composed of two contrasting domains the light domains in backscattered electron bse images have a th u ratio of 7 3 18 9 and a weighted mean 206pb 238u age of 221 2 ma mswd 0 94 the dark domains in bse have a th u ratio of 2 8 7 2 and have a weighted mean 206pb 238u age of 218 2 ma mswd 0 96 based on these data and a review of the literature it can be concluded that 1 the protolith of gneiss sample ds12 is a granitoid of neoproterozoic age 2 the ages of 244 and 226 ma for the mantle and rim domains of the zircons are time records of two discrete stages in the uhp evolution of dabieshan 3 the ages of 221 and 218 ma for the light and dark gray domains respectively of the monazites record the times of two stages of retrogressive metamorphism during exhumation of the uhp granitic gneiss to a higher crustal level 4 the exhumation rate of the gneiss is estimated to be 6 km ma and 5 the uhp metamorphic blocks of the dabie orogen provided significant detritus to the hefei basin in the jurassic

pb isotopes of granitoids suggest devonian accretion of yangtze south china craton to north china craton feldspar separates of 30 granitoid intrusions ranging in age from neoproterozoic to mesozoic were analyzed for pb isotopic compositions together with associated precambrian basement whole rocks from the north and south qinling orogenic belts the two belts constitute the southern active margin of the north china craton and the northern passive margin of the yangtze craton respectively proterozoic basement rocks from the north and south qinling belts are clearly distinguished in terms of both present day pb isotopes and calculated initials at the time of granite magmatism with the former having considerably more radiogenic pb granitoids of the south qinling belt irrespective of age and specific granitoid rock type show a fairly homogeneous pb isotopic composition that is similar to that of the south qinling basement in contrast in the north qinling belt the pb isotopic composition of granitoids is correlated with their age neoproterozoic and early paleozoic granitoids have radiogenic pb isotopic compositions like those of the north qinling proterozoic basement rocks but late paleozoic and mesozoic granitoids show significantly less radiogenic pb isotopic compositions similar to those of the south qinling basement rocks the unusual change is interpreted to reflect underthrusting of the south qinling belt beneath the north qinling belt at ca 360 ma the process resulted in a change of the source for younger north qinling granitoids from the radiogenic north qinling to the less radiogenic south qinling basement rocks thus the yangtze craton accreted to the north china craton no later than 360 ma

the geochemistry and petrology of an alkaline lamprophyre sheet intrusion complex on maio cape verde republic the atlantic island of maio in the cape verde archipelago has a basement complex of mesozoic and tertiary age which is invaded by an intense swarm of sheet intrusions ranging in age from c 16 to 8 ma these cut an alkaline ultramafic mafic and felsic plutonic complex central igneous complex and its host a segment of jurassic ocean floor with pillow lavas and abyssal sediments the basement complex is dissected and unconformably overlain by a sequence of lavas of basanitic and nephelinitic composition the sheet intrusions comprise sills and dykes of alkaline lamprophyre many of which are aphyric or sparsely porphyritic phenocrysts are predominantly clinopyroxene with smaller amounts of olivine and ti magnetite microphenocrysts are kaersutite apatite and biotite the groundmass is dominated by potassium feldspar and analcime calcic plagioclase is extremely rare chemically the intrusions are basanitic with major and minor element compositions directly comparable with the neogene and pleistocene basanitic lavas which cap the island calculations based on observed phenocryst phases suggest that the liquids involved in the crystallization of the sheet intrusions may have been derived by crystal fractionation from a range of basanitic parents these were similar to those from which the later lavas were generated but modified to some extent by variable water rock interactions enhanced by trapped volatiles

granulites in the tongbai area qinling belt china geochemistry petrology single zircon geochronology and implications for the tectonic evolution of eastern asia the tongbai area of the eastern qinling belt in china includes granulite grade metamorphic assemblages qinling complex which were previously regarded as archean to early proterozoic in age and belonging to the southern margin of the north china plate craton our petrological and geochemical data characterize these rocks as two pyroxene granulites and garnet granulites which formed at temperatures of 757 840 c and pressures of about 9 5 kbar and are now found as xenoliths in granodioritic gneisses the protoliths of these rocks were granodiorites and tholeiitic basalt or gabbro the 207pb 206pb ratios derived from evaporation of single zircons yield ages of 470 20 and 470 14 ma respectively for the basic granulites which we interpret to reflect the time of protolith emplacement these are intruded by a 435 14 ma granodioritic gneiss post dating granulite formation a metaquartzite sample contains detrital zircons as old as 2555 8 ma two samples of granitoid gneiss from the tongbai complex s of the qinling granulites have single zircon 207pb 206pb evaporation ages of 776 8 and 746 10 ma respectively and document late proterozoic igneous activity we suggest that the qinling granulites document an important and hitherto unknown phase of early silurian crustal thickening following subduction and continental collision and that both the qinling and tongbai complexes were part of the southern margin of the north china craton prior to this event and record late proterozoic igneous activity

settling and compaction of chromite cumulates employing a centrifuging piston cylinder and application to layered mafic intrusions the time scales and mechanics of gravitationally driven crystal settling and compaction is investigated through high temperature 1 280 1 500 c centrifuge assisted experiments on a chromite basalt melt system at 100 1 500g 0 5 gpa subsequently the feasibility of this process for the formation of dense chromite cumulate layers in large layered mafic intrusions lmis is assessed centrifugation leads to a single cumulate layer formed at the gravitational bottom of the capsule the experimentally observed mechanical settling velocity of a suspension of 24 vol chromite is calculated to be about half 0 53 of the stokes settling velocity with a sedimentation exponent n of 2 35 3 gravitational settling leads to an orthocumulate layer with a porosity of 0 52 all porosities as fraction formation times for such a layer from a magma with initial chromite contents of 0 1 1 vol are 140 3 5 days equal to a growth rate of 0 007 0 3 m day for grain sizes of 1 2 mm more compacted chromite layers form with increasing centrifugation time and acceleration through chemical compaction an increase of grain contact areas and grain sizes together with a decrease in porosity is best explained by pressure dissolution at grain contacts reprecipitation and grain growth into the intergranular space and a concomitant expulsion of intergranular melt the relation between the porosity in the cumulate pile and effective pressure integrated over time h a t is best fit with a logarithmic function in fact confirming that a pressure dissolution reprecipitation process is the dominant mechanism of compaction the experimentally derived equation allows calculating compaction times 70 80 chromite at the bottom of a 1 m thick chromite layer are reached after 9 250 years whereas equivalent compaction times are 0 2 0 9 years for olivine both for 2 mm grain size the experiments allow to determine the bulk viscosities of chromite and olivine cumulates to be of magnitude 109 pa s much lower than previously reported as long as melt escape from the compacting cumulate remains homogeneous fluidization does not play any role however channelized melt flow may lead to suspension and upward movement of cumulate crystals in lmis chromitite layers are typically part of a sequence with layers of mafic minerals compaction occurs under the additional weight of the overlying layers and can be achieved in a few years to decades

the archaean proterozoic palaeoboundary in the lulea area northern sweden field and isotope geochemical evidence for a sharp terrane boundary abstract extensive sm nd isotope work has been carried out in northern sweden with the intention of studying crustal reactivation and assimilation processes relative to juvenile arc accretion at an archaean cratonic margin previous regional work has shown a gradual change from negative nd t values in northernmost sweden to positive values towards the skellefte sulphide ore district farther south isotopic variation and geophysical data show a break in the character of the crust along a wnw trending zone it is suggested that this zone which passes close to the towns of lulea and jokkmokk represents the transition from the archaean craton in the northeast to proterozoic juvenile crust in the southwest a profile across the eastern part of this transition zone has been studied in detail in the lulea area where archaean rocks recently have been discovered the distribution and character of the archaean rocks have been studied these occur as fragments ranging in size from several km 2 to dm in scale and consist of ca 2 7 ga orthogneisses of granodioritic to dioritic composition and porphyritic granitoids of quartz monzodioritic to granodioritic composition plutonic rocks of ca 1 9 ga age and archaean rocks were analysed for major and trace elements and for sm nd isotopes the calc alkaline ca 1 9 ga granitoids are dominated by granodiorites tonalites quartz diorites and quartz monzodiorites and there is a distinct boundary rather than a transition zone between intrusions with postive nd t values in the southwest and intrusions with negative nd t values in the northeast the locations of the outcropping archaean fragments coincides with this distinct boundary which is interpreted as a terrane boundary separating a ca 1 9 ga juvenile arc terrane in the southwest from the archaean continent in the northeast a possible explanation for this is that the proterozoic juvenile terrane was obducted onto the archaean craton during the final stages of the svecofennian orogeny

la icp ms u pb zircon geochronology geochemistry and kinetics of the wenquan ore bearing granites from west qinling china abstract many hydrothermal cu mo au deposits related to granitoid intrusives were recently discovered in the west qinling orogenic belt wqob these deposits were mainly formed during the late indosinian epoch ca 214 ma and the regional geological setting of cu mo ore formation in wqob during this epoch is poorly understood until now this paper describes the geochronology and geochemistry of the wenquan ore bearing pluton a composite granite body to study the geologic background of magmatic emplacement and ore formation the mo mineralisation occurs at the contact between a fine grained biotite monzogranite and a medium to fine grained porphyritic monzogranite zircon 206pb 238u ages of 223 3 ma biotite monzogranite and 225 3 ma porphyritic monzogranite were obtained geochemical analyses show that the wenquan pluton is a high k calc alkaline to shoshonite series rock with relatively high lree and low hree and a moderate to weak negative eu anomaly relatively negative anomalies of ba ti p nb ta also exist these results imply that the wenquan pluton was emplaced during a transitional process from collision to extension between the yangtze craton and north china craton during the later indosinian epoch the east qinling orogenic belt eqob and wqob had similar tectonic settings and intensive magmatic activity and mo mineralisation occurred the eqob was then involved in the mesozoic subduction of the pacific plate and its subsequent tectonic evolution was different from that of the wqob

paleomagnetic constraints on the geodynamic history of the major blocks of china from the permian to the present paleomagnetic study of china and its environs has been the center of a major international effort for the last 10 years in this paper we critically review all available paleomagnetic poles of upper permian to tertiary age from the main blocks of china with the goal of placing constraints on models of the formation and the subsequent deformation of the region after selecting reliable poles by applying objective criteria we divide our analysis into first order motions of blocks and second order deformation within blocks for first order analysis apparent polar wander paths are constructed for the major blocks we discuss the compatibilities and contradictions between the geological and paleomagnetic records a sequence of paleogeographic configurations taking into account geological constraints but remaining within paleomagnetic uncertainties is presented in general the major blocks were probably in contact throughout the permian and triassic but the jurassic was the key age during which most of the movement toward china s present configuration took place our reconstructions include certain details which are suggested by the paleomagnetic record but whose geological signatures seem to have been hidden by subsequent events during the cretaceous chinese poles agree with poles from other continents transferred onto eurasia at the second order we observe that for almost each period with sufficient data the paleomagnetic poles are streaked along a small circle centered on the sampling region indicating that much of china has been affected by small 20 differential rotations this we interpret as deformation caused in part by the extrusion of the chinese blocks away from the indian collision the complete annotated list of poles is given as an appendix

contribution a la datation de la croute oceanique de l atlantique central age valanginien inferieur des basaltes oceaniques et âge neocomien des calcaires maiolica de maio iles du cap vert abstract the discovery of calpionellids and radiolarians in thin sedimentary layers intercalated in the upper part of morb tholeiitic pillow basalts allows us to date the oceanic crust of maio island capo verde islands as early valanginian instead of as was hitherto believed late jurassic this new dating fits better with reconstructions of the geological history of the central atlantic based on magnetic anomalies the overlying light coloured pelagic limestones maiolica facies with radiolarians aptychi and ammonites are valanginian barremian in age these limestones are lithologically similar to the white limestones drilled at several dsdp sites in the central atlantic

avo classification of lithology and pore fluids constrained by rock physics depth trends elastic properties of rocks are strongly influenced by local geological trends and can change markedly even within a sedimentary basin critical geologic factors that control elastic properties are either related to depositional environment or burial history knowledge about the expected change in seismic response as a function of depositional or compactional trends will increase the ability to predict hydrocarbons especially in areas with little or no well log information in other words by understanding the geologic constraints in an area of exploration one can reduce the range of expected variability in rock properties and hence reduce the uncertainties in seismic reservoir prediction figure 1 depicts this problem where we assume there is well log control only in the shallow interval on the shelf edge before extending the exploration into more deeply buried zones or to more distal deepwater environments it is important to understand the rock physics trends in the area

the grenvillian songshugou ophiolite in the qinling mountains central china implications for the tectonic evolution of the qinling orogenic belt abstract the qinling mountains in central china mark a gigantic composite orogenic belt with a complex tectonic evolution involving multiple phases of rifting and convergence this belt separates the north china and south china blocks and consists of the south and north qinling terranes separated by the shangdan suture the suture is marked by the grenvillian songshugou ophiolite along the southern margin of the north qinling terrane which is key to understanding the proterozoic tectonic evolution of the belt the ophiolite consists of highly metamorphosed ultramafic and mafic rocks three groups of meta basalts are present group 1 rocks are lree depleted and have a morb compositional affinity their low ta yb ratios 0 12 and display slight enrichment of lree suggesting that the original magmas were derived from a depleted mantle source mixed with some enriched material samples from group 3 are enriched in lree and other incompatible elements ti zr ta nb suggesting derivation from an enriched mantle source possibly a plume all the basalts have high end t 4 2 to 6 9 variable esr t and high 207pb 204pb and 208pb 204pb ratios for given 206pb 204pb ratios these characteristics are compatible with formation at a mid ocean ridge system above an anomalous dupal mantle region the mafic rocks have a sm nd whole rock isochron age of 1030 46 ma the songshugou ophiolite was emplaced onto the southern margin of the north qinling terrane an active continental margin from the meso proterozoic to neo proterozoic

the position and nature of the gander avalon boundary southern new brunswick based on geochemical and isotopic data from granitoid rocks in southern new brunswick the gander avalon boundary is obscured by boundary parallel faults and various cover sequences siluro devonian granites which intrude unequivocal gander or avalon rocks display exclusively negative 1 9 1 0 and positive 1 9 0 7 end t signatures respectively such contrasting nd isotopic signatures combined with other geochemical differences between plutons are potentially valuable tools for terrane analysis nine small devonian plutons intruding the boundary zone fall into contrasting geochemical groups with la lu n 4 the former are topaz bearing granites while the latter are volcanic arc type granites except for one pluton with an end t signature of 2 0 end t values range from 0 4 to 0 7 spanning the gap between type avalon and type gander plutons these results suggest the plutons sampled either i stratigraphically overlapping or tectonically interleaved gander and avalon basement rocks or ii a distinct basement source beneath the b

mesozoic deep water and tertiary volcaniclastic deposition of maio cape verde islands implications for atlantic paleoenvironments and ocean island volcanism the oldest rocks exposed on maio in the cape verde islands eastern central atlantic are morb type ocean floor pillow basalts lava breccias and hyaloclastics generated at an inferred latest jurassic earliest cretaceous spreading axis possibly near a fossil fracture zone interlava sediments include distal terrigenous turbidites from the west african continental margin latest ocean floor volcanism was accompanied by precipitation of ferruginous oxide sediments from hydrothermal solutions comparable with modern spreading axes the overlying latest jurassic tithonian and lower cretaceous sediments are deep ocean nannoplankton radiolarian chalks coeval with the similar eastern atlantic white limestone and the tethyan maiolica facies by the mid cretaceous albian cenomanian maio had subsided below the calcite compensation depth with accumulation of the characteristic atlantic anoxic black shale facies thin interbeds of limestones and siltstones are pelagic calciturbidites and distal terrigenous turbidites lacking volcanic material after uplift the cretaceous pelagics are overlain apparently conformably without tilting by shallower marine to subaerial pyroclastics and epiclastics which include tuffs agglomerates conglomerates and limestones the existence not only of rounded water worn clasts of mesozoic ocean floor rocks but also of alkaline extrusive and intrusive igneous rocks points to derivation from an early plutonic complex of unknown age the earliest preserved extrusives which are for the most part fragmental basalts and trachytes are possible paleogene to miocene in age further dike and sill intrusion then occurred followed by westward directed folding and thrusting diapiric doming of the central igneous complex ensued followed by deep erosion and then eruption of now gently inclined ankaramites by late in the miocene fan deltas radiated from a rapidly eroding still volcanically active island the neogene saw more quiescent alkalic extrusion punctuated by peneplanation maio island is thus a clue to both mesozoic paleoenvironments in the eastern and central atlantic and to tertiary island building related to hot spot activity additional tabular matter for this article may be secured free of charge by requesting supplementary data 84 9 from the gsa documents secretary

an 40ar 39ar study of the cape verde hot spot temporal evolution in a semistationary plate environment the 40 ar 39 ar analyses of 28 groundmass separates from volcanic rocks from the islands of santiago sal and sao vicente cape verde archipelago are presented the new age data record the volcanic evolution for santiago from 4 6 to 0 7 ma for sal from around 15 to 1 1 ma and for sao vicente from 6 6 to 0 3 ma the major submarine constructional phase of santiago was erupted within a few hundred thousand years interval around 4 6 ma most of the subaerial santiago volcanic rocks were erupted in a second episode from 3 3 to 2 2 ma and late volcanism occurred at 1 1 0 7 ma volcanism on sal evolved in five stages 1 poorly constrained early miocene activity 2 16 14 ma 3 12 8 ma 4 around 5 4 ma and 5 1 1 0 6 ma sao vicente was constructed during three active periods 1 6 6 5 9 ma 2 4 7 4 5 ma and 3 0 3 ma sr isotope analyses of carbonates from maio confirm an early cretaceous age for limestones deposited on the seafloor and later uplifted the cape verde rise is indicated to have fully formed in the early miocene around 22 ma accompanied by the initial alkaline volcanism considerable volcanism on sal boa vista and maio took place in the miocene and pliocene and extended over much larger areas than the present islands whereas volcanism of the southwestern and northwestern island groups developed mainly during the pliocene and pleistocene and was mostly confined to the present island areas the periods of volcanic activity may be broadly correlated between the northwestern and southwestern groups of islands young volcanism 0 3 0 1 ma throughout the northwestern group extends along a 150 km long nw se trending lineament a relatively moderate average melting rate for the hot spot over the 22 ma period is estimated at 0 026 km 3 a corresponding to a total volume of 570 x 103 km 3 of magma emplaced in the crust and a mantle volume flux of 28 m 3 s much lower than iceland or hawaii the archipelago is situated to the south and sw of the center of the mantle plume anomaly and ahead of its relative movement the timing and location of volcanism suggest that mantle melting takes place in three channels an eastern one that has been active for 22 ma and in southwestern and northwestern channels since late miocene

deep sea rock record exhumed on oceanic volcanic islands the cretaceous sediments of maio cape verde abstract revising the deep water stratigraphy exposed on maio offers a key section recording the sedimentary evolution in the distal domain of the central atlantic and the related north west african atlantic margin nwaam the oceanic volcanic island is one of nine islands in the cape verde archipelago and is unique to the central atlantic due to the exposures of ophiolites and the overlying mesozoic deep water sediments uplifted during the cenozoic this provides the opportunity to assess at outcrop the exhumed sediments stratigraphy and palaeo environments of the early central atlantic and contribute towards the knowledge of passive margin evolution along the nwaam part of the largest continental remnant of gondwana combined with the previous recording of calpionellids the first collection of lower valanginian ammonites from the lowest sedimentary succession provides conclusive evidence that jurassic sediments are not present on maio bed by bed sampling and ensuing micro palaeontological analysis of these 71 samples together with a re interpretation of stahlecker s remaining historical palaeontological collection providing a comprehensive dataset to build a high resolution stratigraphic framework for the mesozoic sediments this reveals pelagic deep water limestones of the morro fm were deposited until the upper barremian a major lithological change from carbonate to siliciclastic dominated facies corresponding to the morro carquiejo fm boundary is recognised as a regional hiatus spanning part of the aptian identified across the central atlantic the overlying albian and younger carquiejo fm is seen as an equivalent to the albian cenomanian black shales of dsdp leg 41 yet organic content is absent due to degradation future studies can build on this multi disciplinary investigation revising the stratigraphy of maio

an overview of the earth crust under china abstract we assess the results of a number of deep seismic soundings performed in china over the last few decades and study the variations in crustal structure in 18 tectonic units comprised of three platforms and 15 fold systems thickness data on 344 mesozoic cenozoic sedimentary basins as well as data on moho depth are collected in order to discuss the relationship between the thickness of the basins and the average thickness of the consolidated crust in each tectonic unit the degree of mirror image symmetry between mesozoic cenozoic sedimentary basins and the uplifting topmost parts of the mantle is herein analyzed using deep geophysical data on sedimentary basins by applying standard methods of least squares analysis to both datasets we have obtained both the average thickness of the consolidated crust and the mirror image symmetry factor for every platform and tectonic fold system thereby allowing us to explore the correlation between the depths of the bottom of the sedimentary basins and the top of the uplifting mantle the thickness of the consolidated crust in china is found to be between 20 and 63 km following a pattern of gradual thickening from east to west expressed in terms of spatial seismicity and the concentration of seismic energy and according to the sharing out of earthquake hypocenters in the top 80 km of the earth the rheology of the area does not appear to suit the widely accepted jelly sandwich model for the continents which does not seem to be entirely valid in china the findings on the mirror image symmetry factor show that this parameter varies mainly in the range 0 5 to 1 8 compared with each other tectonic unit most of the tectonic systems in china appear to be isostatically compensated tibet is an exception in that the symmetry factor is positive for the gangdise nyainqentanglha 1 0 and himalayan 0 1 fold systems implying that these tectonic zones are far from being in the isostatic equilibrium of the other regions we have also analyzed the zoning characteristics through the logarithmic relationship ln r ln h a using the average thickness of the consolidated crust and the absolute value of the symmetry factor the key finding is that regardless of geographical location all the values of r h a for those sedimentary basins in which oil gas reservoirs have to date been found fall into the narrow range of values between 19 38 and 37 40 there is some appeal in a possible relationship involving the ratio of crustal thickness to symmetry factor more so when the results obtained appear to suggest a prognostic tool for exploratory practice in relation to oil gas reservoirs

timing of collision of the north and south china blocks controversy and reconciliation the qinling orogen was formed by the joining of the north and south china blocks but the timing of their integration has been debated for more than a decade the controversies obviously stem from different approaches to reconstruction of the integration history two contrasting lines of evidence yield two different ages for collision of the north and south china blocks middle paleozoic and late triassic the shangdan suture within the qinling was regarded in previous studies as the trace along which the north and south china blocks collided our studies however demonstrate that there are two sutures within the qinling the well documented shangdan suture and the newly discovered mianlue suture we show in this paper that the late proterozoic to early mesozoic evolution of the qinling involved interactions between the north china block the north and south qinling orogens and the south china block the middle paleozoic collision along the shangdan suture as constrained by some evidence accreted only the south qinling orogen to the southern part i e the north qinling of the north china block contemporaneous rifting of the south china block and subsequent drifting separated the south qinling from the south china block during the middle to late paleozoic the separation of the south from the north china blocks is supported by other evidence in particular geomagnetic data evidently it was the late triassic collision of the south china block with the south qinling orogen along the mianlue suture that led to final integration of the north and south china blocks

evidence for ocean continent crust boundary beneath the abyssal plain of the east central atlantic abstract a survey of geophysical results and basalt characteristics of the east central atlantic suggests that such data are at present unable to define the seaward limit of the thinned continental crust the combined evidence from margin sedimentation deep sea diapirism salinity concentration in dsdp ipod cores and the distribution of deep sea barite and palygorskite sepiolite assemblages indicate that the central atlantic developed from a wide rift basin within a normal continental setting the notion of an extensive pre drift basin gains additional support from the occurrences of lower cretaceous black shales which are interpreted as resulting from a tectonomagmatic forerunner phase to the actual continental separation process seafloor spreading which appears to have commenced at around 90 ma b p cenomanian turonian following major phases of subsidence and crustal attenuation in jurassic and lower cretaceous is identified by an apparently sharp change over from reducing to oxygenated deep sea environment as well as by the onset of a major sedimentary hiatus the new development model of the east central atlantic is regarded as a representative example of a global pattern commencement of seafloor spreading in the upper cretaceous probably explains the world wide cenomanian transgression as well as the formation of extensive upper cretaceous sedimentary basins in the interior of the major continental blocks a consequence of this model is that vertical crustal dynamics seem to be as important as seafloor spreading in the development of the oceanic lithosphere thus in the central atlantic spreading is probably confined only to the region of the elevated mid atlantic ridge

triassic diorites and granitoids in the foping area constraints on the conversion from subduction to collision in the qinling orogen china abstract the western qinling is characterized by triassic granitic plutons which are keys to constrain the timing of subduction and collisional processes between the north china and south china blocks this paper presents a set of new la icpms u pb zircon geochronology major and trace element geochemistry for several representative diorite and granitoid plutons in the foping area the diorites are characterized by high mgo al2o3 y and yb contents but low sio2 sr y and la yb ratios as well as depletion of nb ta p and ti suggesting derived from a subduction related setting la icpms zircon u pb age of 216 9 1 5 ma mswd 0 69 from the diorite constrains the time of the subduction event the medium and fine grained monzogranites as well as the biotite granites exhibit relative high sio2 mgo and rb low y and yb contents and low sr y and la yb ratios as well as various depletions of nb ta p and ti these features indicate a derivation from lower continental crust but had been influenced by slab released fluids in a syn collisional setting the medium grained monzogranite and biotite granite yield la icpms zircon u pb ages of 207 3 2 1 ma mswd 1 8 and 201 6 1 2 ma mswd 1 04 respectively proposing the syn collision during ca 207 201 ma geochemical and geological evidences suggest that the granodiorites intruding in the syn collisional monzogranites were related to the post orogenic collapse after the collision between the north china and south china blocks the la icpms u pb zircon age of 189 3 2 9 ma mswd 5 6 for the granodiorite constrains the time of the post orogenic collapse our geochronological and geochemical data provide important constraints on the triassic evolution from subduction to collision then up to post collisional collapse along the qinling orogen

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

sediment sorting in the deposits of turbidity currents created by experimental modeling of explosive subaqueous eruptions abstract deposits of turbidity currents induced by subaqueous volcanic eruptions are increasingly recognized in settings ranging from lakes to the deep sea these eruption fed density currents have high current temperatures and low density particles which affect current dynamics and deposit features field data indicate that deposit thickness and grain size decrease downflow at a changing rate this trend is similar for deposits of noneruptive turbidity currents but with volcanic activity the deposits often exhibit a break in slope of the profile of deposit thickness and grain size distribution here we present laboratory results on density currents that address the effects of variations in grain density and water temperature on flow properties and depositional processes parallel runs were performed under identical flow conditions using quartz sand and pyroclastic sediment each grain type was used in one series of runs in which a hot density current enters a cold environment and one series in which

constraints on the early mesozoic denudation of the qinling orogen from upper triassic lower jurassic successions in the zigui basin central china abstract the qinling mountains are situated in central china and mark the collisional assembly between the north and south china blocks to understand the early mesozoic evolution of the qinling orogenic belt we present new paleocurrent data sandstone petrology and detrital zircon u pb ages for upper triassic lower jurassic sedimentary successions in the zigui basin in the northern margin of the south china block the deltaic sandstones in the upper triassic jiuligang formation have high concentrations of quartz grains and the basal alluvial conglomerates of the lower jurassic tongzhuyuan formation contain abundant chert gravels with sandy matrixs rich in quartz grains both the upper triassic sandstones and lower jurassic conglomerate have two major detrital zircon u pb age groups of 2100 1700 ma and 950 700 ma with very few phanerozoic ages the overlying fluvial sandstones of the tongzhuyuan formation display relative enrichment in volcanic and metamorphic rock fragments they are characterized by primary zircon u pb age groups of 250 200 ma and 950 700 ma consistent with the northwestward paleocurrents zircon grains of 2100 1700 ma and 900 700 ma were likely derived directly or recycled from the paleoproterozoic and neoproterozoic basement of the south china block with the latter also possibly sourced from the qinling orogen the 250 200 ma age group is understood to be associated with volcanic rock fragments and considering the southward paleocurrents indicates a volcanic source within the qinling orogen along with the increased low grade metamorphic rock fragments in sandstones this significant provenance change in the early jurassic indicates the initial massive erosion and rapid southward propagation of the south qinling orogen

geochemical evidence for origin of magma mixing for the triassic monzonitic granite and its enclaves at mishuling in the qinling orogen central china abstract whole rock major trace elements and sr nd isotopes mineral chemistry and zircon u pb and lu hf isotope compositions are reported for i type monzogranite and its mafic microgranular enclaves from the triassic mishuling granite in the qinling orogen central china zircon u pb dating yields an age of 212 5 ma for the mafic enclaves and 213 3 ma for the host monzogranite whole rock major trace elements sr nd isotopes and zircon hf isotopes preclude simple crystal fractionation or restite unmixing for a genetic link between the enclaves and the monzogranite mixing between mafic and felsic magmas is compatible with the geochemical data the monzogranite displays an enriched lile and lree pattern high 87sr 86sr i ratios of 0 7068 to 0 7071 and low end t values of 9 2 to 5 7 suggesting a crustal origin its high mg 47 6 to 50 7 cr and ni contents clearly indicate its derivation from mafic refractory lower crust most of the zircons in the monzogranite have negative ehf t values of 12 5 to 0 1 and two stage hf model ages of 1049 to 1683 ma and some zircons in the host monzogranite have positive ehf t values of 0 3 to 4 8 and single stage hf model ages of 665 to 845 ma this indicates that the monzogranite is most likely to be formed by partial melting of a proterozoic lower continental crust the mafic enclaves have fine grained igneous textures and contain abundant acicular apatite that indicates rapid cooling they also have a high 87sr 86sr i ratio of 0 7071 and a low end t value of 6 5 suggesting a derivation from ancient subcontinental lithosphere mantle sclm zircons in the mafic microgranular enclaves have ehf t values of 1 6 to 5 1 and single stage hf model ages of 700 to 910 ma suggesting a possible origin from partial melting of the neoproterozoic sclm that may form in association with contemporaneously supercontinental rifting in the south china block therefore the mishuling i type granite may form by anatexis of meso neoproterozoic lower crust with mixing between the mafic and felsic magmas in particular we interpret the positive ehf t values as being derived from reworking of neoproterozoic juvenile lithosphere in response to tectonic extension in the late stage of the triassic continental collision between the south china block and the north china block

sedimentologic and geochemical evidence for middle ordovician near trench volcanism in the central appalachian orogen the jonestown volcanic rocks are a minor but important component of the greenwich slice of the hamburg klippe of eastern pennsylvania sedimentary rocks of the greenwich slice are arranged in faultbounded coarsening upward sequences of pelagic deposits early to middle ordovician overlain by hemipelagic and turbidite deposits middle ordovician petrographic and geochemical characteristics of the sedimentary rocks indicate derivation from continental rather than volcanic sources the coarsening upward sequences of the greenwich slice are inferred to record advance of a site on oceanic lithosphere toward and ultimately into a trench by middle ordovician time rocks of the jonestown volcanic suite were extruded onto and intruded into the inferred middle ordovician near trench and trench axis mudstone and sandstone immobile element concentrations of the jonestown suite are typical of mid oceanic ridge basalts and along with the lack of associated shallow water and or volcaniclastic sedimentary rocks a

a middle silurian early devonian magmatic arc in the qinling mountains of central china an early to middle paleozoic magmatic arc was discovered in the qinling orogenic belt of central china that marks the boundary between the north china craton and the south china craton the arc was built on the deformed southern passive continental margin of the north china craton which has been modified by the accretion of an oceanic island arc in early paleozoic time u pb analyses of zircons from granites indicate short lived arc activity from middle silurian to early devonian time trace element analyses as well as ree analyses of i type granites are consistent with the interpretation that the magmatic arc developed above a north dipping subduction zone termination of magmatic activity was probably caused by the initiation of collision between the north china craton and the south china craton this interpretation is consistent with faunal data and places new constraints on the paleozoic tectonic evolution of the qinling belt

mechanical compaction in heterogeneous clastic formations from plastic poroelastic deformation principles theory and applications mechanical compaction or loss of porosity due to increase in effective stress is a fundamental geological process that governs many of the rock elastic and transport parameters all of great importance in exploring and developing subsurface reservoirs the ability to model the compaction process enables us to improve our understanding of the seismic signature of the basin and better relate the geology of deposition to current porosity velocity pore pressure and other mechanical parameters that depend on the state of compaction of the sediment in this paper a set of mathematical equations that can be used to model the plastic deformation associated with primary and secondary loading curves is presented compaction laws are posed in terms of natural strain increment formulation often used in plasticity theory to model large deformation laboratory and field estimates of constitutive plastic deformation relations for sand shale mixtures are used in a numerical model that generates estimates of porosity under various pore pressures shale content and loading scenarios these estimates can be used in a variety of settings to predict various basin and reservoir properties associated with different loading conditions and or sedimentation processes

tectonic affinity of the west qinling terrane central china north china or yangtze 1 our ignorance about the tectonic affinity of the western qinling songpan ganzi tectonic region which is strategically located between the northeastern corner of the tibetan plateau the northwestern corner of the yangtze block and the southwestern corner of the north china block limits our understanding of the tectonic evolution of east asia basaltic volcanic rocks in the duofutun area within the west qinling terrane in qinghai province china the northernmost part of the songpan ganzi region contain coeval magmatic zircons that constrain the eruption age of the host basalts to 14 ma more significantly the basalts have entrained zircon xenocrysts from the deep crust that record the presence of unexposed neoarchean 2 7 2 5 ga basement u pb and hf isotope data from the xenocrysts reveal that this basement has undergone a complex evolution that includes the addition of new mantle derived material at 2 7 2 4 and 1 1 0 8 ga and crustal reworking events at 1 8 and 1 4 ga phanerozoic thermal events at 320 300 230 and 160 ma have also modified reworked the basement using these data we interpret at least the western part of the west qinling orogenic terrane as a microcontinental block that originally separated from the north china block closed with the northern yangtze block during the meso neoproterozoic and then redocked with the southern part of the north china block in the phanerozoic i e early paleozoic the west qinling terrane was then affected by the northward subduction and collision of the yangtze block in the paleozoic and early mesozoic and underwent lithospheric extension in jurassic time

tectonic evolution of the east qinling mountains china in the palaeozoic a review and new tectonic model east asia is a composite assemblage built up from various plates and the qinling mountains are prominent among the orogenic belts dividing these plates because of its complex and prolonged history interpretation of the geology of the qinling mountains has led to a variety of tectonic models we review and summarize recently published data and previous work and develop a new tectonic model for the eastern part of this mountain range four tectonic zones are recognized from north to south the lesser qinling the north qinling the central qinling and the south qinling zones the lesser and north qinling zones are interpreted as a late proterozoic to early palaeozoic passive continental margin bordering the north china block the central qinling separated from the lesser north qinling zone by a suture zone mainly consists of volcanic arcs and a high grade metamorphic complex the south qinling zone forms part of the northern margin of the yangtze block and has a long latest proterozoic to middle triassic depositional history as revealed by its sedimentary cover available data do not support the hypothesized existence of a major suture divide between the central and the south qinling zones these two zones seem to have collectively formed an early palaeozoic destructive continental margin on the northern edge of the yangtze block with the central qinling as an island arc system and the south qinling as a wide highly attentuated marginal basin late ordovician to early silurian collision between the north china and the yangtze block built up the palaeozoic orogenic belt in the northern part of the qinling and gave rise to the embryonic east asian continent yet the south qinling was transformed to a remnant basin and remained undeformed until the late triassic when the palaeo tethys collision to the south of the yangtze block drove the yangtze block to move northeastwards deformation in a transpressive regime during this late event largely reconstructed the qinling belt and hence further complicated its tectonics

fracture zones across the cape verde rise ne atlantic more accurate locations have been made for 13 fracture zones across the cape verde rise and four previously unknown fracture zones mapped evidence for their existence is from analysis of the m series of magnetic anomalies together with evidence from seismic reflection profiles where these data exist the cape verde islands themselves lie among a series of five fracture zones the continuity of magnetic lineations and fracture zones across the region of the cape verde islands indicates not only that the islands lie on oceanic crust but also that the original mesozoic seafloor features and magnetization persist and were not obliterated by the neogene reheating uplift and volcanism

tectonics of the qinling central china tectonostratigraphy geochronology and deformation history abstract the qinling orogen preserves a record of late mid proterozoic to cenozoic tectonism in central china high pressure metamorphism and ophiolite emplacement songshugou ophiolite assembled the yangtze craton including the lower qinling unit into rodinia during the 1 0 ga grenvillian orogeny the lower qinling unit then rifted from the yangtze craton at 0 7 ga subsequent intra oceanic arc formation at 470 490 ma was followed by accretion of the lower qinling unit first to the intra oceanic arc and then to the sino korea craton subduction then imprinted a 400 ma andean type magmatic arc onto all units north of the northern liuling unit oblique subduction created silurian devonian wnw trending sinistral transpressive wrench zones e g lo nan shang dan and late permian early triassic subduction reactivated them in dextral transpression lo nan shang xiang shang dan and subducted the northern edge of the yangtze craton exhumation of the cratonal edge formed the wudang metamorphic core complex during dominantly pure shear crustal extension at 230 235 ma post collisional south directed shortening continued through the early jurassic cretaceous reactivation of the qinling orogen started with nw se sinistral transtension coeval with large scale early cretaceous crustal extension and sinistral transtension in the northern dabie shan it presumably resulted from the combined effects of the siberia mongolia sino korean and lhasa west burma qiangtang indochina collisions and pacific subduction regional dextral wrenching was active within a ne sw extensional regime between 60 and 100 ma an early cretaceous andean type continental magmatic arc with widespread early cretaceous magmatism and back arc extension was overprinted by shortening related to the collision of yangtze indochina block with the west philippines block strike slip and normal faults associated with eocene half graben basins record paleogene nne ssw contraction and wnw ese extension the neogene is characterized by normal faults and nne trending sub horizontal extension pleistocene quaternary nw se extension and ne sw contraction comprises sinistral strike slip faults and is part of the nw se extension imposed across eastern asia by the india asia collision

zircon u pb ages and geochemistry of the wenquan mo bearing granitioids in west qinling china constraints on the geodynamic setting for the newly discovered wenquan mo deposit abstract east qinling is the largest porphyry molybdenum province in the world these mo deposits have been well documented in west qinling however few mo deposits have been discovered although granitic rocks are widespread recently the wenquan porphyry mo deposit has been discovered in gansu province which provides an insight into mo mineralization in west qinling in this paper we report pb isotope compositions for k feldspar and sulfides s isotope ratios for sulfides the results obtained from petrochemical study and from in situ la icp ms zircon u pb dating and hf isotopes the granitoids are enriched in lile and lree with ree and trace element patterns similar to continental crust suggesting a crustal origin the mg 40 05 to 56 34 and cr and ni contents are high indicating a source of refractory mafic lower crust the e hf t values of zircon grains from porphyritic monzogranite range from 2 9 to 0 6 and from granitic porphyry vary from 3 3 to 1 9 the zircons have t dm2 of 1014 to 1196 ma for the porphyritic monzogranite and 954 to 1224 ma for the granitic porphyry implying that these granitoids were likely derived from partial melting of a late mesoproterozoic juvenile lower crust the pb isotope compositions of the granitoids are similar to granites in south china showing that the magma was sourced from the middle lower crust in the southern qinling tectonic unit the pb isotopic contrast between the mo bearing granitoids and ores shows that the pb in the ore forming solution was derived from fractionation of a triassic magmatic system 34 s values of sulfides are between 5 02 and 5 66 similar to those associated with magmatic hydrothermal systems la icp ms zircon u pb dating yields crystallization ages of 216 2 1 7 and 217 2 2 0 ma for the granitoids consistent with a previously reported molybdenite re os isochron age of 214 4 7 1 ma this suggests that the mo mineralization is related to the late triassic magmatism in the west qinling orogenic belt in view of these geochemical results and known regional geology we propose that both granitoid emplacement and mo mineralization in the wenquan deposit resulted from the triassic collision between the south qinling and the south china block along the mianlue suture since triassic granitoid plutons commonly occur along the qinling orogenic belt the triassic wenquan mo bearing granitoids highlight the importance of the triassic tectono magmatic belt for mo exploration in order to apply this metallogenic model to the whole qinling orogen further study is needed to compare the wenquan deposit with other deposits

the lhasa terrane record of a microcontinent and its histories of drift and growth article i nfo the lhasa terrane in southern tibet has long been accepted as the last geological block accreted to eurasia before its collision with the northward drifting indian continent in the cenozoic but its lithospheric architecture drift and growth histories and the nature of its northern suture with eurasia via the qiangtang terrane remain enigmatic using zircon in situ u pb and lu hf isotopic and bulk rock geochemical data of mesozoic early tertiary magmatic rocks sampled along four north south traverses across the lhasa terrane we show that the lhasa terrane has ancient basement rocks of proterozoic and archean ages up to 2870 ma in its centre with younger and juvenile crust phanerozoic accreted towards its both northern and southern edges this finding proves that the central lhasa subterrane was once a microcontinent this continent has survived from its long journey across the paleo tethyan ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the qiangtang terrane to the north and with the indian continent to the south zircon hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks e g 50 90 0 70 and 30 100 to the mesozoic magmatism in the southern central and northern lhasa subterranes respectively we suggest that much of the mesozoic magmatism in the lhasa terrane may be associated with the southward bangong nujiang tethyan seafloor subduction beneath the lhasa terrane which likely began in the middle permian or earlier and ceased in the late early cretaceous and that the significant changes of zircon ehf t at 113 and 52 ma record tectonomagmatic activities as a result of slab break off and related mantle melting events following the qiangtang lhasa amalgamation and india lhasa amalgamation respectively these results manifest the efficacy of zircons as a chronometer u pb dating and a geochemical tracer hf isotopes in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics

different melting conditions and petrogenesis of peraluminous granites in western qinling china and tectonic implications abstract the qinling orogen records the prolonged amalgamation history between the north china block ncb and the south china block scb accompanied by major granitoid intrusions during the triassic however the petrogenesis of these granitoids especially that of the peraluminous granites remains a matter of debate detailed study of these peraluminous granites which are the products of crustal melting will give insights into the melting process and petrogenesis of peraluminous magmas and will further constrain the regional geological evolution in this study xiahe xh tourmaline bearing two mica granite and baijiazhuang bjz two mica granite were selected for systematic analysis of their mineralogy petrology zircon u pb geochronology and geochemistry the zircon u pb dating showed that the xh and bjz granites were emplaced at approximately 242 ma and approximately 214 ma respectively the presence of muscovite and tourmaline high peraluminosity at 1 1 low oxygen fugacity o2 conditions below the fayalite magnetite quartz fmq buffer and low zircon saturation temperatures indicate sedimentary protolith s type granitic affinity the low cao na2o ratios

geologic framework and tectonic evolution of the qinling orogen central china abstract the geologic framework of the qinling orogen was built up through interplay of three blocks the north china block including the north qinling the south qinling and the south china block separated by the shangdan and mianlue sutures the shangdan suture resulted from middle paleozoic collision of the north china block and the south qinling the mianlue suture resulted from late triassic collision of the south qinling and the south china block present upper crust of the qinling is structured dominantly by thrust fold systems the north qinling displays thick skinned deformation with crystalline basement involved whilst the south qinling is characterized by thin skinned thrusts and folds detached above the lower sinian two types of precambrian basement crystalline and transitional are defined according to lithology and metamorphic grade and different in age stratigraphic and sedimentary architecture is characterized by distinct zonation the qinling orogen experienced a prolonged continental divergence and convergence between blocks during the period from late neoproterozoic to early paleozoic times the south qinling was the northern margin of the south china block and the north qinling was the southern margin of the north china block separated by a proto tethyan qinling ocean the north qinling evolved into an active margin when the proto tethyan qinling ocean subducted northward during ordovician time collision of the south and north qinling took place in middle paleozoic along the shangdan suture synchronous with the collision rifting occurred at the southern rim of the south qinling and was followed by the opening of the paleo tethyan qinling ocean during the late paleozoic resulting in the splitting of the south china block from the south qinling collision of the south qinling and the south china block came about in the late triassic along the mianlue suture the late triassic collisional orogeny caused extensive fold and thrust deformation and granitoid intrusions throughout the qinling and led to final amalgamation of the north and south china blocks

crustal structure in the junction of qinling orogen yangtze craton and tibetan plateau implications for the formation of the dabashan orocline and the growth of tibetan plateau

timing of synorogenic granitoids in the south qinling central china constraints on the evolution of the qinling dabie orogenic belt abstract the ca 400 km long granitoid belt in the south qinling is believed to be a synorogenic product of the collision between the north and south china blocks along the qinling dabie orogenic belt in central china single and multigrain zircon u pb dating of six of these granitoid bodies indicate that the granitoids were formed between documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 220 pm 1 end document and documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage m

heterogeneous sources of the triassic granitoid plutons in the southern qinling orogen an e w tectonic division in central china 1 the qinling orogen is an important orogenic belt formed by the collision between the north and south china blocks along the mianlue suture during the triassic the orogen is customarily divided into the western and eastern qinling terranes however the boundary has long been a matter of debate there are many triassic granitoid plutons in the orogen especially in south qinling along the southern part of the shangdan suture systematic analysis of u pb ages and hf isotopes in zircons frxom six granitoid plutons including guangtoushan gaoqiaopu xiba laocheng dongjiangkou and zhashui from west to east allows us to trace their formation ages and source regions all plutons yield ages that vary only from 218 to 211 ma however zircon hf isotope data for these triassic plutons cluster in two distinct groups granitoids from the western segment i e guangtoushan gaoqiaopu and xiba of the south qinling belt have negative ehf t values 20 9 to 5 2 and relatively old two stage hf model ages 1 58 to 2 57 ga in contrast those from the eastern segment i e laocheng dongjiangkou and zhashui show higher ehf t values 5 4 to 6 8 and younger two stage hf model ages 0 82 to 1 60 ga integrating these analyses with published sr nd pb hf isotopic data we suggest that the division between the western and eastern south qinling segments is located near the taibai chenggu line between the baoji chengdu railway and 108 e longitude the integrated data suggest that the western south qinling segment separated from the north china block during the paleoproterozoic and early mesoproterozoic and then switched into continental convergence with the eastern south qinling segment northern margin of the yangtze block during the late mesoproterozoic to early neoproterozoic finally the two segments amalgamated during the late neoproterozoic

40ar 39ar geochronology of santo antao cape verde islands santo antao the northernmost island of the cape verde archipelago consists entirely of silica undersaturated volcanic products and minor intrusions 40ar 39ar incremental heating experiments have been carried out on 24 samples that cover the entire exposed chronological sequence the oldest lavas 7 57 0 56 ma representing an older volcanic basement are exposed about 620 m above mean sea level after an interval of quiescence of up to 4 3 ma the volcanic activity resumed and continued at low eruption rates the older basement is unconformably overlain by a ca 810 m thick lava sequence that spans an age range from 2 93 0 03 to 1 18 0 01 ma this sequence is cut by many dykes and sills simultaneous volcanic activity occurred in the northeastern central and eastern part of the island a phonolitic pumice deposit that forms a noteworthy feature over most of the island has an estimated age of 0 20 ma this predates volcanic activity that formed the highest point of the island tope de coroa which has an age of 0 17 0 02 ma the most recent eruption on the island formed nephelinitic lavas in the porto novo region at 0 09 0 03 ma the oldest volcanism exposed on santo antao which took place about 7 6 ma ago was simultaneous with waning activity on maio at the eastern end of the cape verde archipelago

tectonics of the qinling belt build up and evolution of eastern asia the structure and history of the qinling range which separates the north and south china cratons are a key to the tectonic evolution of eastern asia the mountain range is composed of a northern palaeozoic belt and a southern mesozoic belt the palaeozoic orogeny probably involved the closure of an oceanic basin along a north dipping subduction zone followed by south vergent obduction of ophiolites and intracontinental thrusting during the devonian the mesozoic belt appears to be the product of purely intracontinental shallow crustal deformation over a large decollement with at least 100 km of horizontal transport since the devonian both belts have been sliced by huge east west sinistral strike slip faults

rock physics diagnostics of depositional texture diagenetic alterations and reservoir heterogeneity in high porosity siliciclastic sediments and rocks a review of selected models and suggested work flows rock physics has evolved to become a key tool of reservoir geophysics and an integral part of quantitative seismic interpretation rock physics models adapted to site specific deposition and compaction help extrapolate rock properties away from existing wells and by so doing facilitate early exploration and appraisal many rock physics models are available each having benefits and limitations during early exploration or in frontier areas direct use of empirical site specific models may not help because such models have been created for areas with possibly different geologic settings at the same time more advanced physics based models can be too uncertain because of poor constraints on the input parameters without well or laboratory data to adjust these parameters a hybrid modeling approach has been applied to siliciclastic unconsolidated to moderately consolidated sediments specifically in sandstones a physical contact theory such as the hertz mindlin model combined with theoretical elastic bounds such as the hashin shtrikman bounds mimics the elastic signatures of porosity reduction associated with depositional sorting and diagenesis including mechanical and chemical compaction for soft shales the seismic properties are quantified as a function of pore shape and occurrence of cracklike porosity with low aspect ratios a work flow for upscaling interbedded sands and shales using backus averaging follows the hybrid modeling of individual homogenous sand and shale layers different models can be included in site specific rock physics templates and used for quantitative interpretation of lithology porosity and pore fluids from well log and seismic data

from pore pressure prediction to reservoir characterization a combined geomechanics seismic inversion workflow using trend kriging techniques in a deepwater basin to optimize drilling decisions and well planning in overpressured areas it is essential to carry out pore pressure predictions before drilling knowledge of pore pressure implies knowledge of the effective stress which is a key input for several geomechanics applications such as fault slip and fault seal analysis and reservoir compaction studies it is also a required input for 3d and 4d seismic reservoir characterization because the seismic response of shales and sand depends on their compaction history the effective stress will govern the sedimentary seismic response this is in contrast to normally pressured regimes where the depth below mudline or overburden stress is typically used to characterize the compaction effect

an introduction to velocity based pore pressure estimation abnormal pore pressures are encountered worldwide often resulting in drilling problems such as kicks blow outs borehole instability stuck pipe and lost circulation because of this a quantitative predrill prediction of pore pressure is required for the safe and economic drilling of wells in overpressured formations in this paper the basic concepts used in pore pressure prediction are defined and the way in which pore pressure can be estimated using velocity measurements is explained a predrill estimate of pore pressure can be obtained from seismic velocities using a velocity to pore pressure transform calibrated with offset well data however velocities obtained from processing seismic reflection data often lack the spatial resolution needed for accurate pore pressure prediction due to assumptions such as layered media and hyperbolic moveout in the following paper in this section chopra and huffman review the available methods of velocity model building and discuss the advantages and disadva

crustal structures across the western weihe graben north china implications for extrusion tectonics at the northeast margin of tibetan plateau the stable ordos plateau extensional weihe graben and qinling orogenic belt are located at the northeast margin of the tibetan plateau they have been thought to play different roles in the eastward expanding of the tibetan plateau peking university deployed a linear seismic array across the western end of the weihe graben to investigate the crustal structures of the tectonic provinces of this structure receiver function analyses revealed low to moderate poisson s ratios and anticorrelations between poisson s ratios and topography beneath the qinling orogen these features may indicate a tectonic thickening of the felsic upper crust by folding and thrusting within the qinling orogen we observed a strong horizontal negative signal at the midcrust beneath the ordos plateau which may indicate a low velocity zone this observation would suggest the stable cratonic ordos plateau had been modified due to the compression between the tibetan plateau and the ordos plateau we also observed an abrupt 4 km moho offset across the weihe fault changing from 44 km beneath the ordos plateau to 40 km beneath the qinling orogen we conclude that the weihe fault is a lithosphere scale fault shear zone which extends into the upper mantle beneath the weihe graben it acts as the major boundary separating the stable ordos plateau and the active qinling orogen

triassic granitoids of the qinling orogen central china genetic relationship of enclaves and rapakivi textured rocks abstract granitoids varying in composition from biotite amphibole granodiorite to monzogranite with rapakivi textures have recently been recognized in the qinling orogen central china the granitoids are magnesian calc alkalic and metaluminous a cnk 0 71 0 95 they contain many mafic to intermediate microgranular enclaves the enclaves are quenched mafic melts in granitic host magma as evidenced by fine grained sinuous margins against or transitional contacts with the host granitoids as well as disequilibrium textures and mineral assemblages u pb zircon dating of the granitoids yields 209 2 ma and 212 2 ma and the enclaves 210 2 ma and 211 2 ma respectively the host granitoids and enclaves show obvious silica gap but similar ree and trace element patterns the whole rock 87sr 86sr t end t and zircon ehf t values for the granitoids vary from 0 70516 to 0 70634 2 3 to 4 4 and 0 7 to 7 6 and for the enclaves from 0 70467 to 0 70600 1 1 to 4 0 and 1 2 to 8 8 respectively showing overlapped end t and zircon ehf t values all these suggest that the two magmatic systems represented by the host rock and enclave are probably derived from distinct sources with sufficient interaction or common origin but underwent different degrees of crustal contamination mineralogical and geochemical data point to enclave dissolution in silicic magma as a major process accounting for the origin of the rapakivi texture the possible bimodal magmatism suggested by the coeval granitoids and lamprophyre dykes combined with the structural pattern geochemical features and regional tectonics suggest a post collision setting for the qinling rapakivi textured granitoid plutons

nd isotopic mapping of the archaean proterozoic boundary in southwestern tanzania implication for the size of the archaean tanzania craton abstract nd isotopic data are presented for the granitoids that straddle the archaean proterozoic a p boundary in southwestern tanzania along the itigi makongolosi road traverse on the basis of their nd depleted mantle tdm ages two groups of granitoids can be identified those which show tdm ages of 2541 2894 ma and are therefore belonging to the archaean and those which have proterozoic tdm ages of 2031 2430 ma archaean granitoids are deformed grayish in colour and consist of plagioclase biotite and quartz and include a few microcline rich rare biotite pink alkali granites whereas the proterozoic ones include mainly undeformed microcline rich pink alkali granites although there are several compositional overlaps between the two proterozoic granitoids differ from those of the archaean in showing enrichment in incompatible elements and overall higher abundances of the rare earth elements both archaean and proterozoic granitoids show negative to slightly positive eu anomalies these geochemical characteristics are attributed to the involvement of plagioclase in their magmagenesis indicating their generation at low pressures and shallow depths with the proterozoic granitoids being derived from a more felsic protolith nd isotopic data coupled with petrography and lithological field relationships as well as major and trace elements geochemistry of the granitoids places the a p boundary approximately 150 km inside from the southern traditionally accepted boundary near lake rukwa in southwestern tanzania this implies that the size of the archaean tanzania craton is smaller than hitherto understood small vestiges domains showing archaean ages within the proterozoic regions could be explained as being slivers of tectonically interleaved archaean material found within the proterozoic terrane in southwestern tanzania

geochemistry and palaeo oceanography of metalliferous and pelagic sediments from the late cretaceous oman ophiolite abstract metalliferous and pelagic sediments are exposed within and above the extrusive successions of the upper cretaceous oman ophiolite which on the basis of mostly geochemical evidence is believed to have formed in an incipient marginal basin setting located above a ne dipping subduction zone the ophiolitic extrusives document various volcano tectonic settings which include the axial zones of a spreading ridge fault controlled seamounts and off axis volcanic edifices most of the fe mn and trace metal enriched sediments studied are interpreted as precipitates formed by oxidation of solutions derived from high temperature sulphide precipitating vents the trace element content e g ree and sr was largely scavenged from seawater the sediments are similar to the dispersed metalliferous sediments on the flanks of modern spreading ridges and the basal sediments of dsdp wells and of other ophiolite complexes e g troodos cyprus distinctive mound structures located low in the lavas are attributed to percolation of sulphide rich solutions into already deposited metalliferous oxide sediments the resulting iron silica rock was probably originally precipitated as ferruginous silicates major massive sulphides formed off axis at the base of intermediate basic edifices of volcanic arc affinities fe mn and trace metal enrichment in the sediment cover of a flat topped seamount of axial lavas is interpreted as a dispersion halo around the largest massive sulphide orebody which is situated 5 km away lasail small massive sulphide bodies are common in the axial lavas particularly along major seafloor fault zones the metalliferous sediments locally precipitated near these vents are ferromanganiferous but trace metal depleted the metalliferous and pelagic sediment cover of the extrusive successions generally documents waning hydrothermal input after volcanism ended in the area a model is discussed in which the ophiolite was created at a spreading axis above a subduction zone dipping away from the arabian continental margin with progressive subduction this crust approached the margin initially calcareous sediment accumulated above the calcite compensation depth ccd but then non calcareous radiolarites were deposited as the ophiolitic crust approached the continental margin where the ccd was higher and marginal upwelling possibly enhanced productivity as the edge of the arabian continental margin entered the trench the over riding ophiolite was regionally uplifted allowing short lived chalk accumulation above the ccd this was followed by volcaniclastic deposition related to the tectonic emplacement

linking the tengchong terrane in sw yunnan with the lhasa terrane in southern tibet through magmatic correlation abstract new zircon u pb data along with the data reported in the literature reveal five phases of magmatic activity in the tengchong terrane since the early paleozoic with spatial and temporal variations summarized as cambrian ordovician 500 460 ma to the east minor triassic 245 206 ma in the east and west abundant early cretaceous 131 114 ma in the east extensive late cretaceous 77 65 ma in the central region and paleocene eocene 65 49 ma in the central and western tengchong terrane in which the cretaceous eocene magmatism migrated from east to west the increased zircon ehf t of the early cretaceous granitoids from 12 3 to 1 4 at ca 131 122 ma to 4 6 to 7 1 at ca 122 114 ma identified for the first time in this study and the magmatic flare up at ca 53 ma in the central and western tengchong terrane indicate increased contributions from mantle or juvenile crust derived components the spatial and temporal variations and changing magmatic compositions over time in the tengchong terrane closely resemble those of the lhasa terrane in southern tibet such similarities together with the data of stratigraphy and paleobiogeography enable us to propose that the tengchong terrane in sw yunnan is most likely linked with the lhasa terrane in southern tibet both of which experienced similar tectonomagmatic histories since the early paleozoic

elastic and resistivity anisotropy of shale during compaction and diagenesis joint effective medium modeling and field observations abstractshales are anisotropic materials which have been observed to have both electrical and elastic anisotropy shales also comprise most of the sedimentary column in clastic basins and thus the ability to model shale response to electrical and seismic fields may improve our ability to better resolve anisotropy by using different measurements compacting shale effective media modeling which captures the geological process of porosity reduction and geochemical compaction was formulated in this research in microstructural parameters associated with particle alignment and pore shape deformation the state of compaction captured by the porosity of the soft sediment governs the pore aspect ratio and the amount of particle alignment diagenetic processes are captured using temperature dependent mineral elasticity and conductivity the model uses the same microstructural parameters and equivalent effective medium approximation scheme to predict electrical and elastic anisotropy in situ measurements of e

a detrital record of continent continent collision in the early middle jurassic foreland sequence in the northern yangtze foreland basin south china abstract the mesozoic northern yangtze foreland basin system was formed by continental collision between the north china and south china plates along the mianlue suture synorogenic stratigraphic sequences of late triassic to early middle jurassic age were developed in the northern yangtze foreland basin the upper middle jurassic shaximiao formation consists mainly of thick bedded terrestrial successions that serve as the main body of the basin filling sequences suggesting intense tectonism in the peripheral orogeny of the foreland basin laser icp ms u pb analysis of 254 detrital zircon grains from sandstone samples and several published lower middle jurassic samples detrital compositions petrofacies and paleocurrent reconstructions in the northern yangtze foreland basin indicate that discrete source areas included the qinling dabieshan ranges and the mianlue suture zone to the north and the south china plate to the south the stratigraphic succession and sediment provenance of the foreland basin imply that the early mianlue oceanic basin magmatic arc and nonmarine molasse foreland basin during the period of deposition were modified or buried by the subsequent continent continent collision between the north china qinling dabieshan plate and the yangtze plate during the jurassic which followed the oblique amalgamation between these plates during the middle late triassic

silurian devonian provenance changes of south qinling basins implications for accretion of the yangtze south china to the north china cratons abstract the north china and yangtze south china cratons are the most important tectonic units of china the two cratons finally coalesced through collision with the formation of the e w trending qinling mountains that are considered to be the eastern extreme of the paleotethys the north qinling and south qinling belts constitute the southern active margin of the north china craton and the northern passive margin of the yangtze craton respectively we have studied chemical compositions of 573 fine grained clastic sedimentary rocks of all the early proterozoic to neogene ages from the north china craton the yangtze craton and the north and south qinling orogenic belts as well as their ten tectonostratigraphic subunits the purpose of this study is to investigate possible provenance changes that are expected to have been recorded in the phanerozoic clastic sediments of the south qinling basins if north and south china had approached and accreted because the north china craton north qinling belt and yangtze craton have distinct upper crustal and provenance compositions both the north china and yangtze cratons and the north qinling belt are characterized by a relatively uniform non evolutionary composition of clastic sediments in la th sc co systematics during post archean times they can be however distinguished from each other in provenance in terms of the la th sc th th co as well as k2o na2o ratios in contrast there is a significant provenance change in the silurian devonian clastic sediments from the baiyun zhashui liuba yunxian and western xichuan subunits of the south qinling belt the late sinian 700 600 ma cambrian and ordovician pelites show characteristics that are consistent with the yangtze craton being high in la th 3 5 10 and sc th 1 4 2 4 and low in th co 0 2 1 0 and lacking la th ratios 3 5 these are accompanied by the increasing immaturity of the silurian devonian clastic sediments illustrated by the decreasing k2o na2o ratio which is attributed to the near source of the north qinling belt dominated by low k2o island arc igneous suites and immature metagraywackes we propose that the south qinling basins except the eastern xichuan subunit had been in a close proximity and accreted to the north qinling belt in silurian devonian times the result agrees well with recent studies of benthic faunas granite distribution age of granulite and clastic provenance directions of the south qinling devonian sediments but it is apparently inconsistent with the age of coesite and diamond bearing ecologite in the dabie mountains to the east and with paleomagnetic data both suggesting an important triassic jurassic collision event between north and south china it is likely that the silurian devonian and the mesozoic represent two different episodes phases of accretion and collision between north and south china new models regarding the phanerozoic tectonic evolution of east china are needed to reconcile the contrasting evidence