new shrimp u pb zircon ages from the heilongjiang high pressure belt constraints on the mesozoic evolution of ne china the heilongjiang complex is a high pressure accretionary belt composed mainly of serpentinite glaucophane bearing metabasalt epidote glaucophane albite schist greenschist marble two mica schist muscovite albite schist quartz schist and quartzite chert located along the suture zone separating the jiamusi and songliao blocks in ne china shrimp u pb zircon analyses of two samples of granitic gneiss from yilan that are interleaved with the heilongjiang complex record diverse protolith ages that are consistent with derivation from regional permian granitoids 263 7 ma and the adjacent mashan complex 492 6 ma indicating tectonic incorporation of older granitic components into the accretionary complex the age of the basaltic protolith of two samples of glaucophane albite schist from yilan is 245 ma and is slightly younger than the protolith age of two samples of amphibole schist from mudanjiang located about 200 km south of yilan that define weighted mean 206pb 238u ages of 257 4 and 257 5 ma all four samples contain a few older inherited zircon grains and also show the effects of an isotopic disturbance at 200 to 190 ma most likely related to the subsequent high pressure metamorphism the timing of which has been determined in previous studies the basaltic rocks record rifting and ocean development from 260 to 210 ma based on previous studies in which both e morb and oib protolith signatures were identified detrital zircons in two samples of mica schist from mudanjiang show distinct populations at 700 ma 500 to 450 ma 350 to 213 ma peak at 255 ma 290 to 200 ma and 200 ma all except the youngest ages can be matched with rocks from the older components of the jiamusi block and with other areas in the central asia orogenic belt caob supporting the view that the jiamusi block was most likely the eastern segment of the caob prior to rifting the youngest concordant magmatic detrital grain with an age of 207 3 ma defines the maximum age of sedimentation this also limits the timing of high pressure metamorphism in the heilongjiang complex to post late triassic consistent with argon data obtained in previous studies final closure between the jiamusi and songliao blocks took place in the latest triassic early jurassic with the two blocks accreted as a result of pacific ocean subduction this suggests that the heilongjiang complex records the time when north south closure of the caob in china gave way to westward movement related to the pacific ocean subduction which has dominated the tectonics of ne china and far east russia since that time

early mesozoic southward subduction history of the mongol okhotsk oceanic plate evidence from geochronology and geochemistry of early mesozoic intrusive rocks in the erguna massif ne china abstract in this paper we present new zircon u pb ages hf isotope data and whole rock major and trace element data for early mesozoic intrusive rocks in the erguna massif of ne china and we use these data to constrain the history of southward subduction of the mongol okhotsk oceanic plate and its influence on ne china as a whole the zircon u pb dating indicates that early mesozoic magmatic activity in the erguna massif can be subdivided into four stages at 246 ma 225 ma 205 ma and 185 ma the 246 ma intrusive rocks comprise a suite of high k calc alkaline diorites quartz diorites granodiorites monzogranites and syenogranites with i type affinities the 225 ma intrusive rocks consist of gabbro diorites and granitoids and they constitute a bimodal igneous association the 205 ma intrusive rocks are dominated by calc alkaline i type granitoids that are accompanied by subordinate intermediate mafic rocks the 185 ma intrusive rocks are dominated by i type granitoids accompanied by minor amounts of a types these early mesozoic granitoids mainly originated by partial melting of a depleted and heterogeneous lower crust whereas the coeval mafic rocks were probably derived from partial melting of a depleted mantle modified by subduction related fluids the rock associations and their geochemical features indicate that the 246 ma 205 ma and 185 ma intrusive rocks formed in an active continental margin setting related to the southward subduction of the mongol okhotsk oceanic plate the 225 ma bimodal igneous rock association formed within an extensional environment in a pause during the subduction process of the mongol okhotsk oceanic plate every magmatic stage has its own corresponding set of porphyry deposits in the southeast of the mongol okhotsk suture belt taking all this into account we conclude the following 1 during the early mesozoic the mongol okhotsk oceanic plate was subducted towards the south beneath the erguna massif but with a pause in subduction at 225 ma and 2 the southward subduction of the mongol okhotsk oceanic plate not only caused the intense magmatic activity but was also favorable to the formation of porphyry deposits

methane derived authigenic carbonates formed by subduction induced pore water expulsion along the oregon washington margin authigenic magnesian calcite dolomite and aragonite are precipitated in the uppermost terrigenous sediments of the washington oregon accretionary prism by subduction induced dewatering these distinctive carbonates are methane derived and occur at sites of concentrated pore water expulsion unique biologic communities that subsist at least indirectly on methane suess et al 1985 are also found at some of these sites the methane which is dominantly biogenic is carried to the uppermost sediments of the prism by fluids and is oxidized by sulfate reducers before being incorporated into a carbonate cement carbonate precipitation occurs below the oxic layer probably no deeper than several centimetres to a few metres below the seabed cementation may be induced by three factors 1 increased carbonate alkalinity resulting from microbial sulfate reduction 2 decreased co 2 solubility resulting from a pressure decrease when the pore water escapes the prism and or 3 the addition of ca 2 and mg 2 ions from sea water near the sediment water interface the convergent margin setting engenders precipitation of authigenic carbonates in several ways compressive stresses induce anomalously rapid compaction and dewatering rates and they may cause overpressuring in migrating pore water thereby delaying precipitation of carbonates until pressure is released near the sediment water interface structural deformation of the accretionary prism creates pathways such as fault zones secondary fracture porosity and dipping permeable layers often exposed by mass movement for efficient advection and expulsion of methane enriched pore water these characteristic conditions which lead to the precipitation of methane derived carbonates may be found at other convergent margins

interpreting benthic oxygen levels in mudrocks a new approach quantified paleoecology and gamma ray spectrometry have been applied in the analysis of the kimmeridge clay a highly organic rich british jurassic mudrock decreasing benthic oxygen trends are reflected in decreasing species richness and dominance diversity values similarly the degree of fragmentation of the benthos reflects the benthic energy levels and covaries with benthic oxygen the calculation of authigenic uranium values from data gathered by gamma ray spectrometry shows enrichment in more oxygen deficient environments the good correlation between the independently derived paleoecological and authigenic u data indicates the importance of these techniques in environmental analysis of marine petroleum source rocks

compositional differences in enstatite chondrites based on carbon and nitrogen stable isotope measurements carbon and nitrogen abundance and isotopic compositions from four eh4 one eh5 five el6 chondrites and one aubrite were determined by using stepped pyrolysis n only and combustion n and c extractions in attempts to distinguish the components present carbon contents range from 0 15 to 0 70 wt with no systematic relationship between carbon content and meteorite group or petrologic type whole rock 13c values range from 28 5 to 4 1 most c occurs as graphite and when temperature steps above 700 c are considered there is a difference between eh4 5 13c 9 1 to 5 8 and el6 chondrites 13c 6 7 to 4 2 carbon in bustee aubrite is isotopically lighter 13c 24 than in any enstatite chondrite nitrogen occurs as osbornite sinoite and in isostructural substitution for oxygen in silicate lattice sites nitrogen abundances and isotopic compositions are more variable than c due to the heterogenous distribution of n bearing minerals three el6 s containing osbornite have higher n concentrations than other type 6 enstatite chondrites sinoite where present is depleted in 15n relative to osbornite nitrogen in the bustee aubrite has a similar abundance and 15n value to those of el6 s again dominated by the presence of osbornite in addition to the refractory c and n bearing minerals there is also organic material largely terrestrial contamination and evidence for at least two exotic components the first is a host for xe hl and is characterized by 13c 47 and 15n 73 whereas the second is less well defined but is marked by 15n 269

comparative evolutionary palaeoecology assessing the changing ecology of the past various palaeoecological trends have been identified in the phanerozoic each focusing on different aspects of the fossil record patterns that have been described include histories of tiering palaeocommunity species richness and guild occupation in evolutionary faunas as well as onshore offshore trends in origination expansion and retreat patterns of change through time have also been documented from biosedimentological features ichnofabrics microbial structures shell beds such trends can be compared and contrasted to yield unique insights into understanding the changing ecology of the past and in particular may be helpful in evaluating the relative degree of ecological degradation caused by a mass extinction this comparative approach can also shed light on a variety of fundamental palaeobiological problems for example why no new body plans phyla have evolved since the early phanerozoic causes of this phenomenon are thought to be either 1 ecospace was not sufficiently open after the early phanerozoic for survival of new body plans or 2 accumulating developmental constraints after the early phanerozoic have prevented the evolution of new body plans because the permian triassic mass extinction was the most devastating biotic crisis of the phanerozoic one might expect new body plans to appear if ecospace were the primary limiting factor and opened sufficiently by this mass extinction although previous studies have shown that ecospace availability in the cambrian and early triassic was indeed different this comparative approach indicates that ecological conditions in the early triassic were most like those of the late cambrian early ordovician thus if ecospace availability has constrained the survival of new body plans then ecospace has always been sufficiently filled after the cambrian explosion to inhibit their evolution

early paleozoic amalgamation of the songnen zhangguangcai range and jiamusi massifs in the eastern segment of the central asian orogenic belt geochronological and geochemical evidence from granitoids and rhyolites abstract to constrain the early paleozoic tectonic evolution of the songnen zhangguangcai range and jiamusi massifs we undertook zircon u pb dating and geochemical analyses major and trace elements and hf isotopic compositions of early paleozoic igneous rocks in the lesser xing an zhangguangcai range of eastern heilongjiang province ne china the zircons from one diorite two rhyolites and four granitoids are euhedral subhedral and display fine scale oscillatory growth zoning indicating a magmatic origin la icp ms zircon u pb age data indicate the occurrence of magmatic events in the late ordovician 443 451 ma and middle silurian 424 430 ma the late ordovician igneous rocks are composed mainly of diorite tonalite and rhyolite whereas the middle silurian igneous rocks consist mainly of monzogranites the early paleozoic igneous rocks have sio2 59 65 76 11 wt mgo 0 13 1 23 wt and na2o k2o 4 68 7 77 wt and are chemically a calc alkaline series they are characterized by enrichment in light rare earth elements and large ion lithophile elements and depletion in heavy rare earth elements and high field strength elements such as nb ta and ti consistent with the chemistry of igneous rocks from an active continental margin setting in situ hf analyses of zircons show that ehf t values of zircons that formed in the early ordovician captured and inherited zircons in late ordovician igneous rocks range from 0 1 to 7 1 those of zircons that formed in the late ordovician including captured and inherited zircons in middle silurian monzogranite vary from 2 6 to 5 6 and those of zircons that formed in the middle silurian vary from 3 0 to 3 2 the progressive decrease in zircon ehf t values from the early ordovician to the middle silurian suggests an increasing involvement of ancient crust in the magma sources corresponding to the transition from subduction to collision take together these data suggest that the formation of late ordovician diorite tonalite and rhyolite in the zhangguangcai range adjacent to the mudanjiang fault may have been related to the westward subduction of an oceanic plate between the songnen zhangguangcai range and jiamusi massifs in contrast coeval rhyolites from the lesser xing an range located far from the mudanjiang fault may have formed in the supra subduction zone the occurrence of a middle silurian monzogranite association combined with geochronological data for detrital zircons from early devonian sedimentary rocks in eastern heilongjiang province suggests that the amalgamation between the songnen zhangguangcai range and jiamusi massifs occurred in the middle silurian

trapped nitrogen in individual chondrules nature of chondrule precursors and clues to formation mechanisms 1 nitrogen amount and isotopic composition have been measured in individual chondrules of six ordinary chondrites oc and two each of carbonaceous cc and enstatite chondrites ec the n ppm 15nt corrected for cosmogenic contribution for the chondrules have ranges of 0 8 to 61 97 to 164 19 to 140 21 to 31 and 2 to 77 27 to 116 among oc ec and cc respectively for oc while n contents of chondrules fall in the range of bulk oc 15n spans a wide range in contrast to the narrow range 25 to 14 for bulk for cc chondrules n contents are lower while 15n is different and heavier than bulk in contrast ec chondrules and bulk have comparable n and 15n values neither n nor 15nt of oc and cc chondrules show any size expected for nebular exchange or chemical mineralogical dependence ruling out the dominance of secondary effects of nebular or parent body origin the range of 15nt among oc and cc chondrules represents 15nt heterogeneity of chondrule precursors and their survival after a high temperature chondrule forming event on the basis of 15n of the chondrules we propose the presence of at least three n components carried by phase q presolar diamonds hl and insoluble organic matter iom the precursors of chondrules from oc and cc are different than the bulk oc and cc which represents the different environment for their formation compared to the bulk oc and cc however for ec chondrules precursors as well as the formation environment are the same as those of enstatite chondrites

late triassic bimodal igneous rocks in eastern heilongjiang province ne china implications for the initiation of subduction of the paleo pacific plate beneath eurasia abstract this paper reports new zircon la icp ms and sims u pb ages and hf isotope data and whole rock major and trace element data for late triassic igneous rocks of eastern heilongjiang province ne china these data provide new insights into the timing of the initiation of subduction of the paleo pacific plate beneath the eurasian continent the zircon u pb age data indicate that a suite of late triassic 228 202 ma igneous rocks is present within the songnen zhangguangcai range massif and within the western margin of the khanka massif the late triassic igneous rocks within the songnen zhangguangcai range massif consist of basalts basaltic andesites gabbro diabases and rhyolites whereas coeval igneous rocks in the western margin of the khanka massif consist of hornblende gabbros and syenogranitic porphyries these late triassic rocks constitute a geochemically bimodal igneous rock association that contains mafic rocks enriched in light rare earth elements lree and large ion lithophile elements lile and depleted in heavy rare earth elements hree and high field strength elements hfse such as nb ta zr hf and ti zircons from these mafic rocks have ehf t values and tdm1 ages of 2 8 to 9 8 and 477 733 ma respectively suggesting that they formed from a primary magma generated by the partial melting of depleted lithospheric mantle material that had been previously modified by subduction related fluids the coeval felsic rocks are characterized by enrichments in lrees and liles and depletions in hrees and hfses including nb ta and ti and their zircons have ehf t values and t dm2 ages of 0 6 to 7 9 and 766 to 1461 ma respectively implying that these rocks were generated during the partial melting of juvenile crustal material the late triassic bimodal igneous rocks in eastern heilongjiang province combined with the regional geologic information therefore record a post orogenic extensional environment related to the final late permian early triassic closure of the paleo asian ocean in addition the presence of late triassic bimodal igneous rocks within the eastern margin of the eurasian continent suggests that the subduction of the paleo pacific plate beneath the eurasian continent began after the late triassic

the marine permian of east and northeast asia an overview of biostratigraphy palaeobiogeography and palaeogeographical implications abstract the permian marine biostratigraphy faunal successions and mutual correlations where possible throughout east and northeast asia are synthesized region by region based on both published literature and the author s field observations in certain parts of the region the correlation of the permian marine successions of ne asia with the permian international timescale and in particular with gondwanan permian marine sequences remains a major challenge due to profound marine provincialism during the permian however by employing biogeographically mixed faunas from east asia se mongolia ne china south primorye of far east russia and the south kitakami terrane of japan as biostratigraphic gateways coupled with some bipolarly and bi temperately shared permian marine taxa and faunas it has been possible to correlate with reasonable confidence some of the high palaeolatitude permian marine rock units and faunas of ne asia with those of the tethyan region and gondwana palaeobiogeographically the permian marine faunas of east and ne asia are assigned to four major provinces verkolyman sino mongolian japanese cathaysian and panthalassan provinces on the basis of their palaeogeographical distribution patterns and characteristics of faunal assemblages of these the sino mongolian japanese province has considerable significance for regional palaeogeographical plate tectonic and palaeoceanographical reconstructions during the middle permian because of its conspicuously mixed cool and warm water marine biota the origin of this biogeographically mixed marine biota is interpreted to have resulted from a combination of some key factors including the increased tectonic convergence between the bureya jiamusi terrane and the sino korean platform during the permian and the intermingling of both warm and cold water ocean currents off the eastern coastal areas of the bureya jiamusi terrane and the sino korean platform during the middle permian

permian bimodal volcanism in the zhangguangcai range of eastern heilongjiang province ne china zircon u pb hf isotopes and geochemical evidence abstract in this paper we report on zircon u pb dating hf isotopes major and trace elements and sr nd isotope data with the aim of constraining the petrogenesis and regional tectonic evolution of late paleozoic volcanic rocks in the zhangguangcai range of eastern heilongjiang province ne china located in the eastern segment of the central asian orogenic belt caob between the siberian and north china cratons the late paleozoic volcanic rocks in the zhangguangcai range are composed mainly of basalt basaltic andesite rhyolite and minor dacite the results of la icp ms zircon u pb dating for two basaltic rocks three rhyolites and one dacite indicate that they formed in the early permian ca 292 ma the mafic rocks have sio2 50 13 53 80 wt k2o 0 98 2 28 wt mg 0 51 0 71 cr 144 541 ppm ni 74 260 ppm 87sr 86sr i 0 7044 and end t 4 28 whereas the felsic rocks have sio2 69 12 77 98 wt k2o 3 09 5 33 wt mg 0 17 0 36 87sr 86sr i 0 7032 and end t 4 32 these data are typical of bimodal volcanism the mafic volcanic rocks are characterized by a strong enrichment in large ion lithophile elements liles such as rb ba sr and pb depletion in high field strength elements hfses such as nb ta and ti depletion in heavy rare earth elements hrees and weak negative eu anomalies eu eu 0 88 0 94 on the other hand the felsic rocks show a strong depletion in nb ta sr p and ti enrichment in th u and k and relatively large negative eu anomalies eu eu 0 28 0 95 taken together these data suggest that the mafic magma was derived from the partial melting of a depleted lithospheric mantle modified by subducted slab derived fluids and that the felsic magma originated by partial melting of newly accreted crust the early permian bimodal volcanic rocks together with the coeval a type granites indicate an extensional environment similar to a back arc basin such a setting was possibly related to subduction of the paleo asian oceanic plate beneath the jiamusi and songnen zhangguangcai range massifs

stromatolite decline a reconsideration the prevailing concept of phanerozoic stromatolite decline is considered too simplistic changes in stromatolite character and proportion to other lithologies after the precambrian can be explained as responses to substrate competition and changing sedimentological conditions caused by the evolution of skeletal metazoans grazing pressure was likely insignificant as a limiting factor

structural characteristics of the yilan yitong and dunhua mishan faults as northern extensions of the tancheng lujiang fault zone new deep seismic reflection results abstract the tancheng lujiang fault zone tlfz can be subdivided into three segments that exhibit sharp contrasts in their deep structures a deep seismic reflection profile length 600 km across the north part of the tlfz which provides new constraints on the structural styles of the northern tlfz was recently completed by the chinese sinoprobe project here the tlfz branches into the yilan yitong fault yyf to the west and the dunhua mishan fault dmf to the east the yyf developed as an internal fault in the songnen zhangguangcai massif while the dmf serves as the tectonic boundary between the nadanhada terrane and the khanka massif both faults developed large scale flower structures with that of the yyf being negative and that of the dmf being positive with reverse faults the moho in the profile is at a depth of 25 39 km and is offset by the faults the north part of the tlfz extends into the upper mantle as thin shear zones with the reflectors truncated in the middle lower crust this feature differs from most crustal scale strike slip faults that distribute over a discrete shear zone in the lower crust such as the san andreas fault

radiolarian palaeoecology and radiolarites is the present the key to the past abstract radiolarian productivity pulses and related radiolarite deposition are phenomena difficult to understand from an exclusively actualistic viewpoint evolutionary selection pressure among silica secreting marine plankton both radiolarians and diatoms has led toward more economic usage of rapidly shrinking nutrient resources including dissolved silica of the photic zone in the late cenozoic oceans and in particular a substantial modification of oceanic cycle by the diatom explosive radiation even if there is a proved link between biomineralization and dissolved silica loading among the phytoplankton only the relative independence of modern siliceous planktic biotas from the available silica pool reflects mainly their progressive physiological specialisation during evolutionary history oceanic chemistry and productivity as well as patterns of circulation upwelling have changed radically during the phanerozoic radiolarites apparently represent an anachronistic facies as exemplified by their long lived and ocean wide distribution in palaeo pacific and hitherto highlighted actualistic models of localized intra oceanic wind driven upwelling loci are of largely questionable applicability in addition to plate drift hypersiliceous domains and intervals are explainable mostly by a large scale volcano hydrothermal activity during major plate boundary reconfigurations which in many ways favoured siliceous biotas acme and their skeletal remains accumulation and preservation factors tied to rapid voluminous submarine eruptions such as thermal buoyant megaplumes and basin overturns offer a viable alternative for traditional climatic circulation scenarios in case of hypersiliceous high productivity events irrelevant to greenhouse to icehouse climatic change the evolving carbon and silica cycles were coupled through the greenhouse effect and enhanced chemical weathering volcano hydrothermal and tectonic uplift events related mostly to extensive rifting and or accelerated oceanic spreading were the endogenous driving force that created this perturbation of the exogenous system the present biogeochemical cycle is representative only for the overall silica depleted post eocene oceanic ecosystems which broadly correlates with a major expansion of diatoms groups extremely efficient in silica removal and closely linking the silica budget with phosphorus and nitrogen cycles thus an orthodox uniformitarian approach to biosiliceous sedimentation based on a silica starved vigorous ocean is of limited significance when applied to the pre neogene settings especially in the peculiar planktic habitats of epeiric seas as well as during biotic crises marked by strong geotectonic overprint the major turnovers in marine siliceous biota composition in particular after the end permian radiolarite gap may have been coupled with discernible changes in an increasing biological control on the long term oceanic silica cycling punctuated equilibrium the evolutionary turnovers have induced a stepdown decrease of dissolved silica levels through the phanerozoic contemporaneously with the general secular trend of upward scaling of nutrient related ecological processes and increased effectiveness of resource utilization

nitrogen contents and isotopic ratios of clasts from the enstatite chondrite abee abstract nitrogen contents and isotopic ratios have been determined for three clasts from the enstatite chondrite abee by stepwise heating the clasts possess a wide range in nitrogen content ranging from 254 to 850 ppm whereas the nitrogen isotopic ratios are nearly identical at 15 n 29 2 0 6 a refractory inorganic nitrogen bearing phase contains about 90 of the nitrogen which is released at temperatures of 1000 c and above the stepwise heating experiments suggest the possible existence of two other distinct nitrogen components released at low 770 c and high 1500 c temperatures

high carbon concentrations in meteoritic chondrules a record of metal silicate differentiation abstract both reduced type i and oxidized type ii chondrules from the primitive chondrites allende semarkona and bishunpur were analysed with ion microprobe for the carbon and hydrogen concentrations of their silicates the high and variable h and c concentrations found i e from 100 to 1500 ppm h 2 o and from 100 to 3900 ppm c show that these volatile elements were present in chondrule precursors as solid grains mean c concentrations are 3 times higher in type i chondrules than in type ii chondrules therefore it seems likely that metal silicate oxydo reduction reactions were promoted by c rich grains whose heterogeneous distribution in the protosolar nebula caused the variation in the redox states of chondrules based on the present results an in situ reduction of the silicates during chondrule melting seems plausible although reduction in the gas phase before condensation cannot be ruled out calculations of the devolatilisation rate of co from hot chondrules account for the presently measured high c concentrations of type i chondrules if 2 6 wt c grains were originally present among chondrule precursors such a result is in agreement with the c concentration in c1 chondrites and with laboratory simulations which have demonstrated that up to 4 wt c grains were necessary to yield type i chondrules by in situ reduction connolly et al 1994 assuming a solar c o ratio in the chondrule forming regions of the protosolar nebula it is likely that the high density of c grains in the gas resulted from presolar chemistry this interpretation is supported by the corresponding c grain atom gas ratio 2 7 10 13 which is 10 times below the ratio measured in the prestellar clouds

the formation of chondrules petrologic tests of the shock wave model chondrules are millimeter sized rounded igneous rocks within chondritic meteorites their textures and fractionated mineral chemistries suggest that they formed by repeated localized brief minutes to hours melting of cold aggregates of mineral dust in the protoplanetary nebula astrophysical models of chondrule formation have been unable to explain the petrologically diverse nature of chondrites however a nebular shock wave model for chondrule formation agrees with many of the observed petrologic and geochemical properties of chondrules and shows how particles within the nebula are sorted by size and how rims around chondrules are formed it also explains the volatile rich nature of chondrule rims and the chondrite matrix

geochronology and geochemistry of the heilongjiang complex and the granitoids from the lesser xing an zhangguangcai range implications for the late paleozoic mesozoic tectonics of eastern ne china abstract an integrated study on two subduction related units the heilongjiang complex and magmatic rocks in the lesser xing an zhangguangcai range was carried out to explore the paleozoic to early mesozoic tectonic framework of the eastern northeast china with a focus on the jiamusi block the heilongjiang complex which is considered as the suture between the jiamusi and songliao blocks is composed of blueschist amphibolite and ultramafic rocks the blueschist from the yilan area shows geochemical features similar to oib and has a 206pb 238u age of 288 2 ma which is interpreted as the crystallization age of protolith formed in an ocean island setting in contrast amphibolite from the xiachengzi area shows geochemical features similar to n morb zircons in this amphibolite show magmatic character and yield a 206pb 238u age of 248 4 ma which is interpreted as the protolith age calcic amphiboles in the amphibolite give a 40ar 39ar plateau age of 195 3 ma which is interpreted as the metamorphic age of the heilongjiang complex magmatic rocks in the lesser xing an zhangguangcai range mostly belong to i type granites that were formed in an active continental margin setting zircon u pb ages of these granites range from 200 ma to 174 ma with a westward younging trend shown as two groups of emplacing ages of 200 ma in the east and 180 ma in the west our new data support an early occurrence likely since the early permian of a large ocean namely the mudanjiang ocean in the area whose subduction resulted in the metamorphic unit the heilongjiang complex and the magmatic unit the lesser xing an zhangguangcai range in the jurassic

evidence for an early nitrogen isotopic evolution in the solar nebula from volatile analyses of a cai from the cv3 chondrite nwa 8616 abstract nitrogen and noble gas ne ar abundances and isotope ratios determined by co 2 laser extraction static mass spectrometry analysis as well as al mg and o isotope data from secondary ion mass spectrometry sims analyses are reported for a type b calcium aluminum rich inclusion cai from the cv3 chondrite nwa 8616 the high 26 al 27 al i ratio of 5 06 0 50 10 5 dates the last melting event of the cai at 39 99 109 ka after time zero limiting the period during which high temperature exchanges between the cai and the nebular gas could have occurred to a very short time interval partial isotopic exchange with a 16 o poor reservoir resulted in 17 o 5 for melilite and anorthite whereas spinel and al ti pyroxene retain the inferred original 16 o rich signature of the solar nebula 17 o 20 the low 20 ne 22 ne 0 83 and 36 ar 38 ar 0 75 ratios of the cai rule out the presence of any trapped planetary or solar noble gases cosmogenic 21 ne and 38 ar abundances are consistent with a cosmic ray exposure cre age of 14 to 20 ma assuming cr fluxes similar to modern ones without any evidence for pre irradiation of the cai before incorporation into the meteorite parent body strikingly the cai contains 1 4 3 4 ppm n with a 15 n value of 8 to 30 even after correcting the measured 15 n values for cosmogenic 15 n produced in situ the cai is highly enriched in 15 n compared to the protosolar nebula 15 n psn 383 8 marty et al 2011 implying that the cai forming region was contaminated by 15 n rich material within the first 0 15 ma of solar system history or alternatively that the cai was ejected into the outer solar system where it interacted with a 15 n rich reservoir

triassic volcanism in eastern heilongjiang and jilin provinces ne china chronology geochemistry and tectonic implications abstract with the aim of constraining the early mesozoic tectonic evolution of the eastern section of the central asian orogenic belt caob we undertook zircon u pb dating and geochemical analyses major and trace elements sr nd isotopes of volcanic rocks of the luoquanzhan formation and daxinggou group in eastern heilongjiang and jilin provinces china the analyzed rocks consist mainly of dacite and rhyolite with sio2 contents of 68 52 76 65 wt three samples from the luoquanzhan formation and one from the daxinggou group were analyzed using laser ablation inductively coupled plasma mass spectrometry la icp ms u pb zircon techniques three zircons with well defined oscillatory zoning yielded weighted mean 206pb 238u ages of 217 1 214 2 and 208 1 ma and one zircon with oscillatory zoning yielded a weighted mean 206pb 238u age of 201 1 ma these ages are interpreted to represent the timing of eruption of the volcanic rocks the triassic volcanic rocks are characterized by high sio2 and low mgo concentrations enrichment in large ion lithophile elements liles and light rare earth elements lrees depletion in high field strength elements hfses and heavy rare earth elements hrees 87sr 86sr i 0 7040 0 7050 luoquanzhan formation and 0 7163 0 7381 daxinggou group and end t 1 89 3 94 luoquanzhan formation and 3 42 3 68 daxinggou group these geochemical features indicate an origin involving the partial melting of juvenile lower crust nd model ages tdm2 of 651 821 ma and that compositional variation among the volcanic rocks arose from mineral fractionation and minor assimilation these volcanic rocks formed within an extensional environment following collision of the ncc and jiamusi khanka massif during the late paleozoic early triassic

origin of distinct silicic magma types from the guachipelin caldera nw costa rica evidence for magma mixing and protracted subvolcanic residence abstract lower pleistocene pyroclastic deposits in nw costa rica represent sequential eruptions of distinct dacitic rhyolitic 69 79 sio2 magmas from the guachipelin caldera that were emplaced in a period of 0 75 myr this southern segment of the central american arc the chorotega block evolved on the modified caribbean large igneous province an overthickened oceanic plateau silicic magmatism is uncommon in settings such as this void of continental crust previously models have proposed the generation of silicic magmas in this type of setting by the partial melting or extreme crystal fractionation of intermediate source rocks e g tamura y tatsumi y 2002 remelting of an andesitic crust as a possible origin for rhyolite magma in oceanic arcs an example from the izu bonin arc journal of petrology 43 1029 1047 in fact recent melting experiments performed by sisson et al sisson t w ratajeski k hankins w b glazner a f 2005 voluminous granitic magmas from common basaltic sources contributions to mineralogy and petrology 148 5 542 565 suggest that melting of andesite or high k basalts common along the central american volcanic arc could produce silicic melts with the characteristic alkali content of these types of eruptives geochemical evaluation of the pumice clasts through the complete sequence of guachipelin eruptive products allows for subdivision into two major silica groups first the dacites are 66 72 wt sio2 pumice clast 75 2 76 5 wt sio2 glass orthopyroxene bearing with plagioclase an36 65 magnesiohornblende which are found intermittently from the base of the sequence through the uppermost ash flow deposit second the rhyolites are 72 79 wt sio2 pumice clast 74 5 78 6 wt sio2 glass biotite and quartz bearing with plagioclase an24 51 and magnesiohornblende chemical heterogeneities were discovered within each of these groups through evaluation of incompatible trace element ratios consequently seven units were defined using the nb ta ratio which has a significantly wide range from 3 8 to 29 4 and is interpreted to reflect contributions from more than one intermediate source most of these units also display extensive geochemical and petrological evidence of mixing oxygen isotopes phenocryst composition banded pumice clasts and different glass compositions in single pumice fragments the distinct geochemical characteristics of these deposits and the stratigraphic occurrence of each flow unit require both limited fractional crystallization within units coupled with melt extractions and extensive mixing hence individual melts would reside in relatively close proximity in subvolcanic zones for extended periods prior to the inception of a particular eruptive event considering the temporal

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

constraints on the timing of uplift of the yanshan fold and thrust belt north china this paper reports in situ u pb ages and hf isotopic data on detrital zircons from paleozoic to late mesozoic strata in the xishan area near beijing that provide important constraints on the evolution and paleogeography of the northern part of the north china craton ncc zircons from carboniferous to late mesozoic sandstones are characterized by three major groups of u pb ages 2 5 2 3 ga 1 9 1 7 ga and late phanerozoic ages the neoarchean and paleoproterozoic zircons and some phanerozoic zircons have negative ehf t values similar to igneous zircons from intrusive rocks known from the ncc indicating that the source of the sandstones was locally derived however late triassic to late jurassic 205 158 ma sandstones contain a group of phanerozoic zircons with positive ehf t values distinct from known igneous zircons in the ncc suggesting an exotic source the nearest potential source is the xing meng orogenic belt xmob to the north of the ncc where considerable amounts of juvenile crustal materials exist it is concluded that there are two sudden shifts in source provenances of the xishan sediments from the ncc before the late triassic to a mixed source from the ncc and xmob during the late triassic to late jurassic and then from the ncc again during the late jurassic 158 ma the first shift in source resulted from the exhumation and denudation of rocks due to uplift of the xmob which coincided with post collisional extension in the late triassic and intraplate extension of the ncc in the middle to late jurassic the second source shift resulted from the uplift of the yanshan fold and thrust belt thus constraining the timing of this to the late jurassic

a laminated hemipelagic facies from the wenlock and ludlow of the welsh basin a hemipelagic fades from the silurian succession in wales consists of finely laminated dominantly silt grade sediment it can be interbedded with sand or mud grade turbidites or can occur as an independent facies its occurrence and character was controlled mainly by low bottom water oxicity the pelagic fauna of the laminated hemipelagite supports this view many of the silt grade particles have an aggregate structure and probably represent the faecal pellets of oceanic zooplankton the fine lamination is thought to have formed by alternations of planktonic blooms and increased discharges of silt into the basin by comparison with modern examples this alternation was likely to have been annual or seasonal

giant subtidal stromatolites forming in normal salinity waters we report here the discovery of giant lithified subtidal columnar stromatolites 2 m high growing in 7 8 m of clear oceanic water in current swept channels between the exuma islands on the eastern bahama bank they grow by trapping ooid and pelletal carbonate sand and synsedimentary precipitation of carbonate cement within a field of giant megaripples the discovery is important to geologists and biologists because similar organo sedimentary structures built by a combination of cementation and the trapping of sediment by microbes were the dominant fossil types during the precambrian stromatolites are thought to have been responsible for the production of free oxygen and thus the evolution of animal life1 2 until the discovery of small lithified subtidal columnar stromatolites in the bahamas3 the only subtidal marine examples known to be living while undergoing lithification were in the hypersaline waters of hamelin pool at shark bay western australia4 7 shark bay stromatolites range from intertidal to the shallow subtidal with the larger columns reaching 1 m in height the shark bay stromatolites have strongly influenced geological interpretation by analogy many ancient stromatolites have been considered to have grown in intertidal and or hypersaline conditions8 although hypersalinity was not a necessity for growth during the precambrian because grazing metazoan life had not then evolved

early jurassic mafic magmatism in the lesser xing an zhangguangcai range ne china and its tectonic implications constraints from zircon u pb chronology and geochemistry abstract la icp ms zircon u pb dating and geochemical data have been obtained from five representative mafic ultramafic intrusions in the lesser xing an zhangguangcai range ne china with the aim of improving our understanding of the mesozoic tectonic evolution in the region and in particular determining the time of initiation of the circum pacific tectonic system the selected zircons exhibit striped absorption in cathodoluminescence cl images and have high th u ratios 0 20 3 16 indicating a magmatic origin the zircon u pb dates indicate that most of these magmatic zircons other than a few relics that were captured and entrained in the magma formed in the late early jurassic 186 182 ma and not as previously supposed in the middle caledonian the five mafic ultramafic plutons are composed of olivine gabbro hornblendite gabbro hornblende gabbro and gabbro diorite the olivine gabbro and hornblendite display cumulate textures implying that fractional crystallization of olivine and plagioclase took place during magma evolution these mafic ultramafic igneous rocks have sio2 37 3 55 7 mgo 3 05 13 3 al2o3 11 8 23 8 mg 42 69 mg 100mg mg fe2 total and eu 0 88 1 32 and they display three types of rare earth element ree distribution patterns right slipped flat patterns or dome like the trace element spider diagrams show that the rocks are enriched in large ion lithophile elements liles such as ba k and sr and depleted in high field strength elements hfses such as nb ta zr and hf the zircons have ehf 186 182 ma 2 7 to 12 0 and tdm1 366 732 ma the geochemical data indicate that the early jurassic mafic magma originated in an extensional environment from the partial melting of a depleted mantle wedge that had been metasomatized by fluids released from a fossil subducted slab these data combined with information on the spatial variation of coeval igneous rocks indicate that the formation of the early jurassic mafic ultramafic rocks in the lesser xing an zhangguangcai range was related to the subduction of the paleo pacific plate beneath the eurasian continent and this event would mark the beginning of the circum pacific tectonic system

abyssal molluscan colony of calyptogena in the pliocene strata of the miura peninsula central japan fossil calyptogena are found as colonial occurrence in pliocene abyssal sediments cropping out in the miura peninsula central japan the sediments containing calyptogena deposited on a talus during drastic rearrangement of the sedimentary basins caused by a collision process around a trench trench trench type triple junction the population density of calyptogena shells is 90 130 individuals m2 which is comparable to the living colony of calyptogena the oxygen and carbon isotopes of the fossil shells indicate that the shells lived at abyssal depth and under the influence of seepage of connate water with a high concentration of organic carbon

nitrogen abundance and isotope analysis of silicate glasses by secondary ionization mass spectrometry chondritic meteorites preserve extreme intra sample 15n 14n variations which exceed in some cases the range of nitrogen isotope ratios observed at the solar system scale these observations are based on in situ analyses of cn molecular ions by secondary ionization mass spectrometry sims in carbon rich phases the distribution of nitrogen and its isotopes in silicate minerals and glasses has not been investigated to this date due to the lack of an appropriate analytical protocol as well as of suitable n bearing standards in order to improve our knowledge of the nitrogen signature of both extraterrestrial and terrestrial silicate samples we have developed a protocol for determining precise and accurate nitrogen abundances and isotope ratios in basaltic glasses using high mass resolution sims twelve c n bearing synthetic basaltic glasses containing between 1 and 18 443 966 ppm n form the suite of reference materials for this study by targeting the cn no aln and sin secondary molecular ions nitrogen abundances can be detected down to the ppm level in both carbon bearing and carbon free glasses the analytical precision and reproducibility of isotope ratios in the form of 15n16o 14n16o is on the order of 11 and 10 to 17 2 respectively for reference glasses containing 100 ppm n thus nitrogen isotope ratios can be determined with an uncertainty that is small enough to resolve nitrogen isotope variations in extraterrestrial silicates the study of four chondrules of the ordinary chondrite semarkona ll3 0 reveals that the nitrogen distribution in the mesostasis is highly heterogeneous with concentrations ranging from 0 to 1099 168 ppm the 15n values in mesostasis olivine and pyroxene vary between 36 50 and 55 72 indicating that silicate phases in chondrules do not host particularly 15n poor nitrogen

palaeoenvironmental trends in the history of trace fossils tracks trails and burrows made by organisms are commonly preserved in phanerozoic sedimentary rocks these biogenic sedimentary structures or trace fossils have been most intensively studied from rocks deposited in marine environments and have proven to be extremely useful in palaeoenvironmental reconstruc tions our systematic analysis of occurrences in the stratigraphic record of two of the most common trace fossils zoophycos and ophiomorpha indicates that occurrences for each show an environmentally linked directional pattern through time zoophycos is commonly reported from palaeozoic strata deposited from nearshore to deep sea environments whereas in post palaeozoic stratait is generally reported only from rocks deposited in deeper water environments ophiomorpha first appeared in permian shallowwater environments and was common in deep sea fan environments by the cretaceous and has been distributed from nearshore to deep sea environments since then these onshore offshore trends which are similar to those previously reported for phanerozoic benthic macroinvertebrate assemblages and individual higher taxa require integration into future palaeoenvironmental reconstructions that use trace fossils

trace fossil model for reconstructing oxygenation histories of ancient marine bottom waters application to upper cretaceous niobrara formation colorado abstract trends of decreasing trace fossil diversity burrow size and penetration depth with diminishing oxygen availability in contemporary marine settings have been incorporated with the concept of trace fossil tiering to develop a model for reconstructing the oxygenation history of ancient basin bottom waters more sensitive than traditionally employed criteria the model permits detailed evaluation of relative degree of oxygenation through the recognition of oxygen related ichnocoenosis ori units or units of strata deposited under similar levels of bottom water oxygenation when employed in detailed vertical sequence analyses the model may also be used to construct interpreted oxygenation curves that reflect rates and magnitudes of temporal change in redox conditions the utility of the model is demonstrated by its application to rhythmically bedded calcareous strata of the niobrara formation upper cretaceous exposed in colorado non laminated strata of the niobrara are characterized by four recurring trace fossil suites referred to as the 1 chondrites 2 zoophycos teichichnus 3 planolites and 4 thalassinoides assemblages on the basis of trends in trace fossil diversity burrow diameter and penetration depth these assemblages are recognized as oxygen related ichnocoenoses interpreted oxygenation curves for the basal fort hays member reflect high magnitude redox cycles characterized by rapid deoxygenation events followed by gradual reoxygenation phases curves for the overlying smoky hill member reflect an overall lower degree of oxygenation with more irregular lower magnitude redox fluctuations significant correlation between interpreted levels of paleooxygenation and organic carbon content illustrate the model s potential for evaluating the distribution of organic rich fine grained strata co interpretation of these variables with carbonate content supports the premise that deposition was controlled by paleoceanographic perturbations modulated by milankovitch like climatic cycles comparison with traditionally emphasized macroinvertebrate body fossil criteria clearly demonstrates the greater utility of the trace fossil approach for evaluating paleo oxygenation

late triassic intrusive complex in the jidong region jiamusi khanka block ne china geochemistry zircon u pb ages lu hf isotopes and implications for magma mingling and mixing abstract whole rock major and trace element geochemistry together with zircon u pb ages and lu hf isotope compositions are reported for a late triassic intrusive complex in the jidong region jiamusi khanka block ne china zircon u pb dating yields ages between 211 and 208 ma for enclaves of microgranular diorite and quartz diorite and between 211 and 209 ma for the host granitoids these ages correlate with a previously established intensive late triassic magmatic event along the eastern asian margin field observations together with petrographic features geochemistry and zircon hf isotope data preclude simple crystal fractionation or restite unmixing as a genetic link for the various rock types within the intrusive complex the syenogranite suite has high sio 2 75 5 76 3 wt and low mgo 0 15 0 19 wt and yields enriched lile and lree patterns most of the zircons in the syenogranites have two stage model ages of 766 and 1461 ma together with positive e hf t values of 0 6 to 9 1 these results indicate that the granitoid magmas were generated by partial melting of meso to neoproterozoic lower crust the gabbro suite has a restricted range of sio 2 46 1 51 9 wt together with high mg values 49 70 and high concentrations of ni co and cr zircons from two diorite samples have single stage hf model ages of 557 787 ma and e hf t values of 1 9 to 8 3 that are consistent with the coeval gabbros previously studied in the jidong region these features together with the observation that all the gabbros are enriched in lree and lile suggest that the mafic magmas were derived from melting of depleted neoproterozoic lithospheric mantle that had been metasomatized by slab derived fluids it is concluded that the dominant igneous suites within the late triassic intrusive complex formed by mingling mixing of felsic and mafic magmas the geochemical data combined with regional geological investigations indicate that the late triassic intrusive complex formed during lithospheric extension caused by slab break off of the yuejinshan oceanic plate that was subducting westward beneath the jiamusi khanka block

manganese chromium isotope systematics of enstatite meteorites abstract we present results of a study of the 53 mn 53 cr isotope systematics in the enstatite chondrites and achondrites aubrites the goal of this study was to explore the capabilities of this isotope system to obtain chronological information on these important classes of meteorites and to investigate the original distribution in the inner solar system of the short lived radionuclide 53 mn our earlier work lugmair and shukolyukov 1998 shukolyukov and lugmair 2000a has shown that the asteroid belt bodies are characterized by essentially the same initial 53 mn abundance however we have found the presence of a gradient in the abundance of the radiogenic 53 cr between the earth moon system mars and the asteroid vesta if this gradient is considered as a function of the heliocentric distance a linear radial dependence is indicated this can be explained either by an early volatility controlled mn cr fractionation in the nebula or by an original radially heterogeneous distribution of 53 mn the enstatite chondrites are suggested to form in the inner zones of the solar nebula much closer to the sun than the ordinary chondrites therefore their investigation may be an important test on the hypothesis on a radial heterogeneity in the initial 53 mn we have studied the bulk samples of the eh4 chondrites indarch and abee and the el6 chondrite khairpur although these meteorites have essentially the same mn cr ratio as the ordinary chondrites the relative abundance of the radiogenic 53 cr is three times smaller than in the ordinary chondrites because these meteorites are primitive undifferentiated and no mn cr fractionation had occurred within their parent bodies this difference is a strong argument in favor of an initially heterogeneous distribution of 53 mn in the early inner solar system this finding is also consistent with formation of the enstatite chondrites in the inner zones of the solar nebula using the characteristic 53 cr excess of the enstatite chondrites and the observed gradient their place of origin falls at about 1 4 au or somewhat closer to the sun i e 1 0 1 4 au we also present chronological results for the enstatite chondrites and achondrites the absolute 53 mn 53 cr ages of the eh4 chondrites are old 4565 ma the el6 chondrite khairpur is 4 5 ma younger which is in good agreement with the 129 i 129 xe data from the literature the age of the aubrite pena blanca spring appears to be similar to those of the enstatite chondrites while that of the aubrite bishopville is at least 10 ma younger which is also in agreement with the 129 i 129 xe data the results from bulk samples of aubrites indicate that the last mn cr fractionation in their parent body occurred 4563 ma ago and imply an evolution of the mn cr isotope system in an environment with an higher than chondritic mn cr ratio for several millions of years

role of gas melt interaction during chondrule formation abstract it is generally believed that most chondrules formed in the protoplanetary disk by melting of fine grained solid precursor materials and that during these melting events chondrules behaved as closed systems both chemically and isotopically i e the observed range in chondrule bulk chemistry and modal mineralogy was entirely inherited from solid precursors that experienced melting to varying degrees followed by igneous crystallization accompanied by very limited if any evaporation recondensation our mineralogical and chemical study of magnesium rich porphyritic type i chondrules in primitive ordinary and carbonaceous chondrites revealed that chondrule glasses are not located on the subtraction lines of olivine or low ca pyroxene in appropriate phase diagrams indicating that type i chondrules do not obey closed system crystallization we suggest that gas melt interaction played a major role in evolution of mineralogy bulk chemical and isotopic compositions of type i chondrules chondrules are thus complex objects composed of an inherited solid component and a melt component that equilibrated with nebular gas suggesting that chondrules formed in regions with high ambient gas partial pressures as a result chondrule glasses melts are therefore thermochemical and isotopic sensors of the surrounding nebular gas in the chondrule forming regions of the protoplanetary disk and can be used for evaluating its physico chemical conditions

oxygen isotope studies of ordinary chondrites several stages in the evolution of ordinary chondritic meteorites are recorded in the oxygen isotopic composition of the meteorites and their separable components chondrules fragments clasts and matrix the whole rock isotopic compositions reflect the iron group of the meteorite h l or ll isotopic uniformity of h3 to h6 and l3 to l6 are consistent with closed system metamorphism within each parent body ll3 chondrites differ slightly from ll4 to ll6 implying a small degree of opensystem aqueous alteration and carbon reduction on the scale of individual chondrules the meteorites are isotopically heterogeneous allowing recognition of the solar nebular processes of chondrule formation chondrules for all classes of ordinary chondrites are derived from a common population which was separate from the population of chondrules in carbonaceous or enstatite chondrites chondrules define an isotopic mixing line dominated by exchange between 16o rich and 16o poor reservoirs the oxygen isotopic compositions of chondrites serve as fingerprints for identification of genetic association with other meteorite types achondrites and iron and for recognition of source materials in meteoritic breccias

pan african metamorphic and magmatic rocks of the khanka massif ne china further evidence regarding their affinity the khanka massif is a crustal block located along the eastern margin of the central asian orogenic belt caob and bordered to the east by late jurassic early cretaceous circum pacific accretionary complexes of the eastern asian continental margin it consists of graphite sillimanite and cordierite bearing gneisses carbonates and felsic paragneisses in association with various orthogneisses metamorphic zircons from a sillimanite gneiss from the hutou complex yield a weighted mean 206 pb 238 u age of 490 4 ma whereas detrital zircons from the same sample give ages from 934 610 ma magmatic zircon cores in two garnet bearing granite gneiss samples also collected from the hutou complex yield weighted mean 206 pb 238 u ages of 522 5 ma and 515 8 ma whereas their metamorphic rims record 206 pb 238 u ages of 510 500 ma these data indicate that the hutou complex in the khanka massif records early palaeozoic magmatic and metamorphic events identical in age to those in the mashan complex of the jiamusi massif to the west the older zircon populations in the sillimanite gneiss indicate derivation from neoproterozoic sources as do similar rocks in the jiamusi massif these data confirm that the khanka massif has a close affinity with other major components of the caob to the west of the dun mi fault based on these results and previously published data the khanka massif is therefore confirmed as having formed a single crustal entity with the jiamusi and possibly the bureya massif since neoproterozoic time

do carbonate reefs form due to fluid seepage buried carbonate reefs are favoured hydrocarbon prospecting targets mainly due to their high porosity and potential for containing large quantities of petroleum the question of the true relationship between reef structure and the internally trapped fluids hydrocarbons is here raised as one of cause and effect in other words which came first the hydrocarbons or the carbonate reef itself modern bioherms and seabed carbonate reefs in amongst other locations the north sea and the gulf of mexico are shown to form in close association with active hydrocarbon seepages mainly based on results from ecological studies at deep ocean vent communities a new model for carbonate reef formation is promoted that such reefs form at locations containing high concentrations of bacteria and other microorganisms suspended in the water column as a result of seeping fluids solutions and gases that provide some of the energy basis and carbon source for ecosystems independently of photosynthesis therefore on burial and effective sealing capping these carbonate reefs become hydrocarbon reservoirs trapping and accumulating the very minerals on which they in the first place were dependent

oxygen isotopic constraints on the origin of magnesian chondrules and on the gaseous reservoirs in the early solar system abstract we report in situ ion microprobe analyses of the oxygen isotopic composition of the major silicate phases olivine low ca pyroxene silica and mesostasis of 37 magnesian porphyritic type i chondrules from cv vigarano usnm 477 2 vigarano uh5 mokoia and efremovka and cr eet 92042 eet 92147 eet 87770 el djouf 001 mac 87320 and gra 95229 carbonaceous chondrites in spite of significant variations of the modal proportions of major mineral phases in cr and cv chondrules the same isotopic characteristics are observed i olivines are isotopically homogeneous at the level within a chondrule although they may vary significantly from one chondrule to another ii low ca pyroxenes are also isotopically homogeneous but systematically 16o depleted relative to olivines of the same chondrule and iii all chondrule minerals analyzed show 16o enrichments relative to the terrestrial mass fractionation line enrichments that decrease from olivine spinel to low ca pyroxene and to silica and mesostasis the observation that in most of the type i chondrules studied the coexisting olivine and pyroxene crystals and glassy mesostasis have different oxygen isotopic compositions implies that the olivine and pyroxene grains are not co magmatic and that the glassy mesostasis is not the parent liquid of the olivine the 18o and 17o values of pyroxene and olivine appear to be strongly correlated for all the studied cr and cv chondrules according to 18 o pyroxene 0 72 0 16 0 13 18 o olivine 1 21 0 54 0 79 17 o pyroxene 0 81 0 15 0 13 17 o olivine 0 60 1 00 0 95 these correlations are consistent with mass balance of oxygen isotopes calculated in the case of chondrule formation according to the following gas melt mineral reactions mg 2 sio 4 olivine sio 2 melt mg 2 si 2 o 6 pyroxene sio 2 melt sio gas 1 2 o 2 gas these equations imply that 2 3 of the oxygen incorporated by low ca pyroxene during its crystallization in the melt is derived from the dissolution of precursor olivine and 1 3 from the addition of chemical components such as sio from the nebular gas the composition of the nebular gaseous reservoir in which type i cr and cv chondrules formed is determined from the present data set to be 18o gas 3 6 1 and 17o gas 1 8 1 this isotopic composition is in excellent agreement with that of the gaseous reservoir previously inferred from the oxygen isotope systematics of bulk chondrules from various chondrite groups i e cv cr unequilibrated ordinary and eh thus the oxygen isotopic systematic strongly supports the idea that most of the olivines in type i chondrules are relict grains and that gas melt interaction played a major role in the formation of type i chondrules

compositional and genetic relationship between chondrules chondrule rims metal and matrix in the renazzo chondrite individual chondrules coarse and fine grained metal chondrule rims and matrix samples were separated from the renazzo chondrite cr2 and studied by instrumental neutron activation analysis both renazzo chondrule and matrix fractions have ci cr mg concentration ratios unlike chondrules from other classes of chondrites which have fractionated cr mg ratios this and other arguments suggest that renazzo chondrules and matrix formed in the same nebular region in renazzo fine grained metal the dominant metal component is mostly located inside chondrules or in chondrule rims matrix contains few coarse metal grains both types of metal fine and coarse exhibit similar chemical signatures comparatively high in cr and low in ni suggesting a genetic relationship although metal is mostly contained in chondrules chondrules and matrix of renazzo have similar common siderophile lithophile abundance ratios this may imply that renazzo chondrules and metal were formed by reduction of oxidized precursors compositionally similar to ci chondrites the ci like concentrations of sc mg se and zn in renazzo matrix are consistent with this inference the differences in siderophile element pattern between chondrules chondrule metal and matrix indicate evaporation and recondensation of volatile elements during chondrule formation recondensation of evaporated elements was incomplete and the degree of recondensation correlates with element volatility the higher than ci k mg na mg and ga fe ratios of renazzo matrix may reflect recondensation of k na and ga into the matrix the ci se sc and zn sc concentration ratios of the matrix however reflect very limited degrees of recondensation of se and zn into matrix the presence of abundant phyllosilicates and organic material in renazzo matrix requires a low background temperature 200 k during formation of the renazzo meteorite partial recondensation of volatile elements in a cold nebular environment implies instantaneous agglomeration of the renazzo chondrite following chondrule formation

boninite like volcanic rocks in the 3 7 3 8 ga isua greenstone belt west greenland geochemical evidence for intra oceanic subduction zone processes in the early earth abstract the 3 7 3 8 ga isua greenstone belt of southwest greenland is characterized by variably metamorphosed metasomatised and deformed lithotectonic successions of volcanic and sedimentary rocks the voluminous mafic volcanic rocks are composed primarily of pillow basalts intercalated with ultramafic units the sedimentary rocks consist mainly of banded iron formation cherts conglomerates and siliciclastic turbidites the least altered metavolcanic amphibolites the garbenschiefer unit from the central tectonic domain of the isua greenstone belt are characterized by high mg number 0 60 0 80 mgo 7 18 wt al2o3 14 20 wt ni 60 645 ppm and cr 60 1920 ppm contents but low tio2 0 20 0 40 wt zr 12 30 ppm y 6 14 ppm and rare earth element ree concentrations these compositional features collectively represent a coherent mafic to ultramafic suite chondrite normalized ree patterns are concave upward on the primitive mantle normalized extended trace element diagrams they are characterized by relative depletion of nb but with an enrichment of zr relative to neighboring rees alteration deformation and crustal contamination can be ruled out as the cause of the distinct and coherent composition the average initial end value of these metavolcanic rocks is 2 collectively these geochemical characteristics are comparable to those of phanerozoic boninites given the observation that in the tertiary boninites are exclusively associated with intra oceanic subduction environments e g izu bonin mariana subduction system this suggests that intra oceanic subduction zone like geodynamic processes were operating as early as 3 7 3 8 ga

oxygen isotopic compositions of enstatite chondrites and aubrites oxygen isotopic compositions have been measured for ten eh chondrites eight el chondrites and nine aubrites the mean 18o and 17o values are 5 31 and 2 74 5 57 and 2 96 5 24 and 2 70 respectively within the eh group more equilibrated meteorites are fractionated toward higher heavy isotope abundances than the less equilibrated meteorites small departures of individual meteorites from a common mass fractionation occur for some of the eh samples the aubrites may be genetically related to either the eh or el group or to both almost all of the measured isotopic compositions are indistinguishable from the terrestrial oxygen isotope fractionation line suggesting a genetic relationship among the earth the moon and the enstatite the meteorites

nitrogen isotopic compositions of iron meteorites abstract iron meteorites analyzed in this study have nitrogen concentrations 70 g g and 15n from 90 to 150 although the iron meteorites have a large range of 15n most have values more negative than 50 the nitrogen isotopic compositions were established by cosmochemical processes and were little modified by fractional crystallization or other chemical processes within the parent bodies the data do not suggest the existence of a well mixed solar nebular reservoir for nitrogen as was already inferred from data from stony meteorites the range of 1100 observed for 15n in bulk meteorites is probably too large to be accounted for by physical and chemical mass dependent fractionation processes in the solar nebula and thus reflects nebular inhomogeneities of nucleosynthetic origin nitrogen isotopes may serve as fingerprints for different reservoirs preserved in various parent bodies in a manner similar to that observed for oxygen isotopes in stony meteorites and oxygen bearing phases of stony irons

the anatomy of a deep water mud mound complex to the southwest of the dinantian platform in derbyshire uk an elongate waulsortian mud mound complex developed at dovedale on a ramp to the southwest of a developing carbonate platform in derbyshire during chadian early visean times the complex occupied an area of approximately 6 km2 and grew to a maximum relief of 80 m with longitudinal and transverse valleys developed near the southern margin five mound associated facies have been identified mound core mound flank fine mound flank coarse intermound fine and intermound coarse the mound core facies is a massive skeletal wackestone with comminuted sponge debris foraminifera ostracodes and crinoid debris set in a matrix of clotted micrite the mound flank sediments display moderately inclined bedding surfaces while the mound flank fine contains sponge debris the mound flank coarse is dominated by articulated crinoid columnals and includes algal encrusted micritized intraclasts and coarse peloids the well bedded intermound fine facies is bituminous and micritic while the intermound coarse facies is composed of skeletal peloidal intraclast grainstones which locally contain calcified algae although the fauna is diverse the density of colonization by metazoans was low and the supply of macrofossil debris modest the clotted micrite texture is interpreted as the product of micro organisms which precipitated and trapped fine grained sediment the mud mound complex is dominated by the bathymetric assemblages b and c proposed by lees hallet hibo which on their model of the belgian waulsortian indicate depths of between 220 and 280 m intercalation of assemblages b c and c d on the northern margin of the complex is interpreted as the result of local storm disturbance a deep water drift is postulated to explain the nw se alignment of the complex which probably fitted the export model of bosence rowlands quine beneath the sediment surface phreatic flow eroded unlithified sediments and developed interconnected cavities which were filled by cement and sediment relatively eariy mound instability triggered the opening of fissures which filled with crinoid debris peloids indurated lithoclasts and micrite

nitrogen isotopes in bulk ordinary chondrites nitrogen isotopic compositions of twenty one ordinary chondrites were measured using a stepwise combustion technique we observed a good correlation between the abundances of cosmogenic15n and cosmogenic 38ar we obtained a relationship between the abundances of these two cosmogenic nuclides as follows 15nc atoms g 16 0 3 7 38arc atoms g using this relationship the cosmogenic nitrogen production rate in l chondrites is calculated to be 5 0 1 2 10 12 g 15n g myr assuming the cosmogenic38ar production rate given by eugster 1988 this value is consistent with the production rate on the lunar surface estimated by becker et al 1976 isotopic ratios of trapped nitrogen in ordinary chondrites seem to be heterogeneous among both unequilibrated and equilibrated meteorites heterogeneity among the latter are well observed in this study the isotopic ratios of the trapped components seem to differ by more than 110 ranging from 18 observed in guaren a h6 to 95 observed in jilin h5 a lack of correlation between nitrogen abundances and isotopic ratios indicates that differences in nitrogen isotopic ratios are not produced by mass dependent isotopic fractionation an inverse correlation between nitrogen abundances and the degree of thermal metamorphism is observed among most of the unequilibrated chondrites nitrogen abundances decrease from 8 25 p p m to 1 p p m with increasing petrographic subtype from 3 4 3 9 the nitrogen abundance in equilibrated chondrites is generally higher than the amount expected by extending the trend observed among unequilibrated ones this is probably because contaminating nitrogen masked the trend at small amounts of trapped nitrogen

evidence for the presence of planetesimal material among the precursors of magnesian chondrules of nebular origin abstract chondrules are the major high temperature components of chondritic meteorites which are conventionally viewed as the samples from the very first generation of undifferentiated planetesimals growing evidences from long and short lived radionuclide chronologies indicate however that chondrite parent asteroids accreted after or contemporaneously with igneous activities on differentiated asteroids questioning the pristine nature of chondrites here we report a discovery of metal bearing olivine aggregates with granoblastic textures inside magnesian porphyritic type i chondrules from the cv carbonaceous chondrite vigarano formation of the granoblastic textures requires sintering and prolonged high temperature 1000 c annealing conditions which are not expected in the solar nebula during chondrule formation but could have been achieved on parent bodies of olivine rich differentiated or thermally metamorphosed meteorites the mineralogy and petrography of the metal olivine aggregates thus indicate that they are relict dunite like lithic fragments which resulted from fragmentation of such bodies the very old pb pb absolute ages and al mg relative model ages of bulk cv chondrules suggest that such planetesimals may have formed as early as the currently accepted age of the solar system 4567 2 0 6 ma

the exaerobic zone a new oxygen deficient marine biofacies classical biofacies models1 2 for reconstructing palaeoenvironments of strata deposited in oxygen deficient marine settings define three principal facies aerobic 1 0 ml 1 1 o2 dysaerobic 1 0 to 0 1 ml 1 1 o2 and anaerobic 0 1 ml 1 1 o2 zones these models have postulated a decrease in organism size and degree of calcification as well as a drastic reduction in the relative percentage of fauna possessing calcified skeletons as the dysaerobic anaerobic boundary is approached through use of evidence independent of that provided by body fossils we demonstrate here that in portions of the monterey formation miocene california the bivalve anadara montereyana occurs in situ almost exclusively in strata deposited at the dysaerobic anaerobic boundary the occurrence of fossils of large well calcified benthic macroinvertebrates at this redox boundary contradicts classical biofacies models1 2 and there fore provides the basis for definition of a new oxygen related biofacies the exaerobic zone the exaerobic zone concept when adjusted for differences in basin configuration and palaeoceano graphic conditions provides a potentially useful model for explain ing occurrences of shelly benthic fossils within laminated organic rich strata of other phanerozoic marine sequences

stromatolitic biotic systems in the mid triassic of israel a product of stress on an epicontinental margin abstract phanerozoic microbialites are carbonate precipitating biotic forms that are often described in modern systems from inter or supratidal hypersaline or fresh water environments and from beds associated with a variety of ecologically stressed environments over geologic time middle to late triassic microbialitic successions from southern israel were commonly subtidal and thrived under cryptic ecological stress conditions related to the regional configuration of the levant margin at that time the ladinian and carnian sequence at the ramon outcrop contains 18 successions with 28 microbialitic horizons these microbialites form isolated patches small bioherms and biostromes interleaved with other calcareous sediments including poorly diverse molluscan accumulations three distinct microbialitic facies and two associated molluscan facies were identified most were distinctly subtidal the two associated molluscan biofacies include a subtidal bivalve boundstone forming mounds and a bedded packstone of minute gastropods the ladinian carnian microbialites were not climatically forced environmental stress was caused by partial restriction of marine circulation due to tectonic movements and biotic accumulation on the outer edge of the levant margin comparison of coeval successions from central and northern israel and the north calcareous alps and southern alps in europe indicates a tethyan or global carbonate system of microbial producers separated from the open marine environment by shelf edge barriers therefore we conclude that the ramon microbialites represent a response to ecological stress of the carbonate system within the subtidal environment rather than solely to normal stresses of the intertidal environment

early triassic stromatolites as post mass extinction disaster forms aftermaths of mass extinctions have been thought to be characterized by relaxation of ecological constraints accompanied by increased prominence of opportunistic generalists such taxa termed disaster forms have been shown to increase dramatically in range and abundance after several mass extinction events the cambrian ordovician stromatolite decline in normal marine level bottom environments has been explained as a direct or indirect consequence of increases in ecological constraints such as greater levels of predation and or bioturbation of microbial communities caused by early paleozoic benthic invertebrate evolution and diversification thus one would predict that in post ordovician strata stromatolites might appear in normal marine level bottom environments as disaster forms in the aftermaths of mass extinction particularly devastating to the benthic biota such as during early triassic time mounded stromatolites are present in two beds up to 1 5 m thick of the lower triassic spathian virgin limestone member moenkopi formation in the southwestern spring mountains of nevada stromatolites from level bottom normal marine subtidal environments have also been described from other lower triassic strata in north america europe and asia these stromatolites unusual in level bottom normal marine settings may have developed locally during the long aftermath 4 5 m y of the permian triassic mass extinction because of partial relaxation of the ecological constraintsmore that typically restricted them from unstressed subtidal normal marine level bottom environments less

early carboniferous low temperature hydrothermal vent communities from newfoundland living vent communities dependent on microbial chemosynthesis rather than photosynthesis were discovered only in the 1970s1 since then six fossil vent communities have been identified they range in age from early carboniferous to early tertiary2 7 here we describe fossil tubes an abundant low diversity fauna and sulphide mineralization in 340 myr old carboniferous carbonate mounds in newfoundland fig 1a these features together with evidence for microbial activity point to the existence of a seventh chemosynthetic community clustered around low temperature hydrothermal vents the remarkable preservation of this newfoundland community allows a direct comparison to be made with the composition and structure of modern vent communities

early cretaceous lower crustal reworking in ne china insights from geochronology and geochemistry of felsic igneous rocks from the great xing an range this paper presents new zircon la icp ms u pb ages and whole rock geochemical data for two granitic plutons and rhyolites of the baiyingaolao formation in the western xing an range ne china the two syenogranite granitic plutons yield identical zircon u pb age of 142 1 ma and the baiyingaolao rhyolites yield zircon u pb age of 138 2 ma the granites contain some hornblendes and show low zr and zr nb ce y contents and low a cnk 0 98 1 11 mg 6 55 and feot mgo values rhyolite samples show similar geochemical characteristics with a cnk of 0 99 1 10 and mg of 14 21 in combination with the high k2o contents 4 43 5 61 wt and negative correlations between p2o5 and sio2 both the granites and rhyolites were classified as high k calc alkaline i type granitoids all samples give high zirconium saturation temperature of 794 964 c with few initially inherited zircons and belong to high temperature i type granitoids they were generated by dehydration melting of biotite muscovite from sub alkaline meta basalts in lower crust depth leaving garnet amphibole and plagioclase as the major residual minerals the syenogranites and rhyolites are likely formed in mongol okhotsk oceanic subduction setting incorporating other lower crust originated felsic rocks in erguna and xing an massifs and songliao basin it is argued that lower crustal reworking is pronounced in ne china during early cretaceous

microbial carbonates as indicators of environmental change and biotic crises in carbonate systems examples from the late devonian alberta basin canada microbial precipitation of calcium carbonate has played a vital role in the development of carbonate platforms since their initiation in the proterozoic we report here the varied roles that microbial carbonates played in late devonian carbonate platforms in the alberta basin canada we recognize microbial carbonates as important contributors within the carbonate system during times of major environmental change including transgressive events in platform environments and the recovery interval following the frasnian famennian mass extinction detailed sequence stratigraphic analysis of two isolated platforms in the canadian rockies was used to document their evolution from a regional ramp to isolated platforms with phases of progradation aggradation and backstepping and renewed progradation related to rates of second order and third order sea level change and basin infill the carbonate system was reorganized following annihilation of many carbonate producing biota during the frasnian famennian mass extinction microbial carbonates figure prominently in both frasnian platform development and the famennian recovery of the carbonate system following the frasnian famennian mass extinction during transgressive platform phases deeper water facies with a crinoid rugose coral and microbe dominated biota abruptly overlie prograding stromatoporoid framestones microbial carbonates consist of abundant girvinellid oncoids that nucleated around other fossil clasts the temporary replacement of stromatoporoid communities during transgression implies a shift from oligotrophic to mesotrophic conditions and the microbes likely capitalized on the change in nutrient supply microbes played a significant role in highstand reef margin facies where renalcis was a binder and cementer in stromatoporoid framestones and downslope rugose coral stromatoporoid mounds microbially laminated carbonates also form stromatolitic mats in peritidal shallowing upward cycles and repetitive stromatolitic intervals commonly indicate sea level lowstands associated with the development of sequence boundaries stromatoporoid reefs and their associated reef interior facies indicate an oligotrophic ecosystem where microbial carbonates were relegated mainly to cryptic or stressed marine habitats an end frasnian sea level lowstand exposed the isolated carbonate platforms transgression and mass extinction characterize the frasnian famennian boundary event and microbial carbonates occur at this horizon in western canada stromatolites oncoids and large scale microbial thrombolites appear to be

environmental oxygen fluctuations and the adaptive response of marine benthic organisms oxygen depletion is widespread in modern and ancient benthic environments most modern examples exhibit a significant cyclic fluctuation in o 2 availability ranging from tidal to seasonal cycles and longer term periods e g el nino in contrast only long term changes have been generally recognized in ancient environments benthic organisms adapt along two pathways 1 to low o 2 values and free h 2 s including chemosymbiosis 2 synchronization of individual life cycles to the o 2 fluctuation cycle by living as planktonic larvae in oxygenated surface waters using oxygenated periods in bottom waters during the adult stage examples of both adaptive pathways can be recognized in the benthic faunas of fossil oxygen controlled environments thus recording short and longer term fluctuations

oxygen isotope geochemistry of oceanic arc lavas variations of oxygen isotope ratios in arc related lavas can constrain the contributions of subducted crustal igneous rocks sediments and fluids to the sub arc mantle we have measured oxygen isotope ratios in 72 arc and back arc lavas from five ocean ocean subduction zone systems using laser fluorination analyses of olivine and other phenocrysts and glass eighty percent of our samples have delta 18o values for any given phase olivine plagioclase glass or biotite within 0 2 per thousand of the average value for that phase in upper mantle peridotites and mid ocean ridge basalt morb the range for each phase is 1 0 per thousand this result contrasts with previous studies of whole rock samples which are significantly more variable even after exclusion of samples believed to be altered or fractionated by magmatic differentiation and demonstrates that most arc related lavas contain 1 2 of 18o enriched crustal oxygen from any source i e assimilation or subducted contributions elevations in delta 18o that do occur in these basic arc derived magmas relative to values most common for mantle derived lavas are associated both with enriched radiogenic isotope signatures and even more strongly with chemical indices consistent with high integrated extents of melting of their peridotite sources we interpret these relationships as evidence that melting in the sources of the high delta 18o lavas we have studied was fluxed by addition of high delta 18o aqueous fluid or perhaps a hydrous melt from the subducted slab such that sources that contain relatively large components of slab derived fluid or melt are both relatively 18o enriched and also experienced relatively large amounts of melting we have developed a quantitative model linking the amount of melting to the extents of 18o radiogenic isotope and trace element enrichment in a mantle source being fluxed by addition of aqueous fluid comparison of this model with observed variations in the geochemistry of lavas from the vanuatu fiji new caledonia region the suite of related samples showing the greatest range in delta 18o observed in this study constrains the amounts and chemical and isotopic compositions of slab derived phases in the sources of these arc related lavas assuming a delta 18o value of 20 per thousand for the slab derived fluid 0 5 1 0 wt is added to the sources of most mantle derived arc magmas the maximum amount of slab derived flux in the sources of arc magmas according to our results is 2 5 wt

properties of chondrules in el3 chondrites comparison with eh3 chondrites and the implications for the formation of enstatite chondrites the study of chondrules provides information about processes occurring in the early solar system in order to ascertain to what extent these processes played a role in determining the properties of the enstatite chondrites the physical and chemical properties of chondrules from three el3 chondrites and three eh3 chondrites have been examined by optical cathodoluminescence cl and electron microprobe techniques properties examined include size texture cl and composition of both individual phases and bulk chondrules the textures distribution of textures and composition of silicates of the el3 chondrules resemble those of eh3 chondrules however the chondrules from the two classes differ in that 1 the size distribution of the el chondrules is skewed to larger values than eh chondrules 2 the enstatite in el chondrules displays varying shades of red cl due to the presence of fine grained sulfides and metal in the silicates and 3 the mesostasis of eh chondrules is enriched in na relative to that of el chondrules the similarities between the chondrules of the two classes suggest similar precursor materials while the differences suggest that there was not a single reservoir of meteoritic chondrules but that their origin was fairly local the differences in the size distribution of chondrules in eh and el chondrites may be explained by aerodynamic and gravitational sorting during accumulation of the meteoric material while differences in cl and mesostasis properties may reflect differences in formation conditions and cooling rate following chondrule formation we argue that our observations are consistent with the formation of enstatite chondrites in a thick dynamic regolith on their parent body

chemostratigraphy and magnetic susceptibility of the late devonian frasnian famennian transition in western canada and southern china implications for carbon and nutrient cycling and mass extinction we investigate the late devonian frasnian famennian extinction interval in western alberta and south china to shed light on the palaeoecological and palaeoceanographic conditions that characterize this biotic crisis both the lower and upper kellwasser events are documented in western canada only the upper kellwasser event has been evaluated in south china our multi proxy geochemical approach reveals that these events are characterized by positive d 13 c and d 15 n excursions and increasing magnetic susceptibility canada china and increases in detrital al si ti zr productivity cu ni zn and redox mo u v elemental proxies canada we interpret these trends as part of a systemic palaeoecological shift associated with the development of wide spread terrestrial forests and their alteration of chemical mechanical weathering patterns increase in detrital proxies is thus interpreted as resulting from pedogenically driven weathering on the con tinents that nutrified epeiric and continental margin seas high biological productivity led to eutro phication and development of suboxic to anoxic conditions during both events and probably euxinic conditions during the upper kellwasser event in western canada positived 13 c excursions are the telltale signature of excessive carbon burial while redox proxies and d 15 n records indicate

collision belt between the khanka block and the north china block in the yanbian region northeast china the pre jurassic geotectonics in northeast china and its adjacent regions are characterized by collision of microcontinental blocks the yanbian region lies between the north china block ncb and the khanka block kb along the dashanzui antu kaishantun collision belt the yanbian pre jurassic fold belt can be divided into two parts one part represents the western margin of the kb which consists of late proterozoic metamorphic basement and a permian active continental margin the other one is located at the eastern margin of the ncb and is composed of proterozoic passive margin deposits where the seluohe and qinglongcun groups are developed and rocks of the permian passive margin collision of these two parts took place along the dashanzui antu kaishantun collision zone during the late permian to early triassic as evidenced by the emplacement of syncollision granites and a type granites in northeast china and adjacent regions a series of microcontinental blocks collided with one other from the end of early paleozoic to the early triassic the collision event that took place in the yanbian region may represent the latest collision event in the paleo asian oceanic domain c 2003 elsevier ltd all rights reserved

geochronology and geochemistry of igneous rocks in the southeastern lesser xing an range northeastern china petrogenesis and implications for the early mesozoic tectonic evolution we present systematic u pb age data collected by laser ablation multi collector inductively coupled plasma mass spectrometry precise geochemical data and nd isotope data for igneous rocks from th

trace fossil model for reconstruction of paleo oxygenation in bottom waters recognition of fluctuations in the degree of paleo oxygenation of bottom waters recorded in fine grained pelagic strata is important for interpretation of paleoceanographic and paleoclimatologic conditions general sedimentary fabric composition of trace fossil assemblages and burrow size and crosscutting relationships have been incorporated into a trace fossil tiering model that permits detailed reconstruction of changes in paleo oxygenation of bottom waters applications of this model to the miocene monterey formation california and the cretaceous niobrara formation colorado indicate that the ichnologic approach is more sensitive to both magnitude and rates of change in oxygenation levels compared to macrobenthic body fossil information

sources of phanerozoic granitoids in the transect bayanhongor ulaan baatar mongolia geochemical and nd isotopic evidence and implications for phanerozoic crustal growth abstract the central asian orogenic belt caob is renowned for massive generation of juvenile crust in the phanerozoic mongolia is the heartland of the caob and it has been subject to numerous investigations particularly in metallogenesis and tectonic evolution we present new petrographic geochemical and sr nd isotopic analyses on phanerozoic granitoids emplaced in west central mongolia the data are used to delineate their source characteristics and to discuss implications for the phanerozoic crustal growth in central asia our samples come from a transect from bayanhongor to ulaan baatar including three tectonic units the baydrag cratonic block late archean to middle proterozoic the eo cambrian bayanhongor ophiolite complex and the hangay hentey basin of controversial origin the intrusive granitoids have ages ranging from ca 540 to 120 ma the majority of the samples are slightly peraluminous and can be classified as granite s s including monzogranite syenogranite and alkali feldspar granite most of the rocks have initial 87sr 86sr ratios between 0 705 and 0 707 late paleozoic to mesozoic granitoids 250 ma are characterized by near zero end t values 0 to 2 whereas older granitoids show lower end t values 1 5 to 7 the data confirm the earlier observation of kovalenko et al geochemistry international 34 1996 628 who showed that granitoids emplaced outside of the pre riphean basement rocks are characterized by juvenile positive end t values whereas those within the pre riphean domain and the baydrag cratonic block as for the present case show a significant effect of contamination by precambrian basement rocks nevertheless mass balance calculation suggests that the granitoids were derived from sources composed of at least 80 juvenile mantle derived component despite our small set of new data the present study reinforces the general scenario of massive juvenile crust production in the caob with limited influence of old microcontinents in the genesis of phanerozoic granitoids

eocene deep sea communities in localized limestones formed by subduction related methane seeps southwestern washington densely populated communities of soft bottom dwelling taxa similar to those found today along subduction zones off the coasts of japan and oregon have been discovered in very localized deep water limestones of late middle to late eocene age along the southwestern margin of washington subduction was prevalent in this area during this time and compressive forces squeezed subsurface methanerich waters onto the ocean floor where opportunistic bivalves especially modiolus calyptogena and thyasira vestimentiferan tube worms serpufid tube worms siliceous sponges very small limpets trochid and turbinid archaeogastropods and other macrobenthos colonized these assemblages are the earliest recorded biologic communities formed in response to methane seeps in subduction zones

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

petrologic geochemical and experimental constraints on models of chondrule formation abstract the petrologic and geochemical properties of chondrules as well as results of experimental studies provide strong constraints on chondrule formation models nebular formation is indicated by the non mass fractionated oxygen isotopic compositions of bulk chondrules chondrule formation from a melt is required by the prototypical spheroidal shapes and the presence of euhedral phenocrysts and glassy mesostases incomplete melting is indicated by the abundance of porphyritic chondrules which experiments demonstrate require relict nuclei and coarse relict grains the length of time that chondrules were hot is constrained by their retention of relict grains and moderately volatile elements rapid cooling of chondrules after formation is supported by the presence of zoned phenocrysts isotopic anomalies and dynamic crystallization experiments it is clear from the presence of relict grains enveloping compound chondrules and igneous rims that many chondrules were heated again after cooling the heating mechanism responsible for chondrule formation seems to have operated at varying intensities over large regions of the inner solar nebula for at least the time it took ambient nebular temperatures to cool from above 900 to below 600 k the chondrule formation mechanism provided a repeatable source of energy capable of highly localized melting characteristic of flash heating the occurrence of ferroan microchondrules with low melting temperatures within some chondrule rims indicates that chondrule formation did not occur exclusively in high temperature regions near the sun as required in bipolar outflow models mechanisms for forming chondrules that are consistent with the constraints include various flash heating models nebular lightning magnetic reconnection flares gas dynamic shock waves and radiative heating

spatial temporal relationships of mesozoic volcanic rocks in ne china constraints on tectonic overprinting and transformations between multiple tectonic regimes abstract la icp ms zircon u pb ages and geochemical data are presented for the mesozoic volcanic rocks in northeast china with the aim of determining the tectonic settings of the volcanism and constraining the timing of the overprinting and transformations between the paleo asian ocean mongol okhotsk and circum pacific tectonic regimes the new ages together with other available age data from the literature indicate that mesozoic volcanism in ne china can be subdivided into six episodes late triassic 228 201 ma early middle jurassic 190 173 ma middle late jurassic 166 155 ma early early cretaceous 145 138 ma late early cretaceous 133 106 ma and late cretaceous 97 88 ma the late triassic volcanic rocks occur in the lesser xing an zhangguangcai ranges where the volcanic rocks are bimodal and in the eastern heilongjiang jilin provinces where the volcanics are a type rhyolites implying that they formed in an extensional environment after the final closure of the paleo asian ocean the early middle jurassic 190 173 ma volcanic rocks both in the erguna massif and the eastern heilongjiang jilin provinces belong chemically to the calc alkaline series implying an active continental margin setting the volcanics in the erguna massif are related to the subduction of the mongol okhotsk oceanic plate beneath the massif and those in the eastern jilin heilongjiang provinces are related to the subduction of the paleo pacific plate beneath the eurasian continent the coeval bimodal volcanic rocks in the lesser xing an zhangguangcai ranges were probably formed under an extensional environment similar to a backarc setting of double direction subduction volcanic rocks of middle late jurassic 155 166 ma and early early cretaceous 145 138 ma age only occur in the great xing an range and the northern hebei and western liaoning provinces limited to the west of the songliao basin and they belong chemically to high k calc alkaline series and a type rhyolites respectively combined with the regional unconformity and thrust structures in the northern hebei and western liaoning provinces we conclude that these volcanics formed during a collapse or delamination of a thickened continental crust related to the evolution of the mongol okhotsk suture belt the late early cretaceous volcanic rocks widely distributed in ne china belong chemically to a low to medium k calc alkaline series in the eastern heilongjiang jilin provinces i e the eurasian continental margin and to a bimodal volcanic rock association within both the songliao basin and the great xing an range the volcanics in the eastern heilongjiang jilin provinces formed in an active continental margin setting related to the subduction of the paleo pacific plate beneath the eurasian continent and the bimodal volcanics formed under an extensional environment related either to a backarc setting or to delamination of a thickened crust or both late cretaceous volcanics limited to the eastern heilongjiang jilin provinces and the eastern north china craton ncc consist of calc alkaline rocks in the eastern heilongjiang jilin provinces and alkaline basalts in the eastern ncc suggesting that the former originated during subduction of the paleo pacific plate beneath the eurasian continent whereas the latter formed in an extensional environment similar to a backarc setting taking all this into account we conclude that 1 the transformation from the paleo asian ocean regime to the circum pacific tectonic regime happened during the late triassic to early jurassic 2 the effect of the mongol okhotsk suture belt on ne china was mainly in the early jurassic middle late jurassic and early early cretaceous and 3 the late early cretaceous and late cretaceous volcanics can be attributed to the subduction of the paleo pacific plate beneath the eurasian continent

chemosymbiont dominated seafloor communities in modern and cretaceous upwelling systems support a new high productivity variant of standard low oxygen models faunal analysis of modern benguela upwelling system namibia and ancient upper cretaceous israel sedimentary records rich in organic matter biogenic silica carbonate and phosphate indicates that contrary to stereotypes of upwelling systems as dead zones macrobenthic communities are present but highly variable biofacies vary with distance from the upwelling core and as a function of both the supply of food to the seafloor and the oxygen demand it creates there under a high organic supply large bodied chemosymbiotic bivalves dominate exaerobic and lower dysaerobic seafloors and deposit feeders are abundant in upper dysaerobic and aerobic zones macrobenthic biofacies under upwelling thus contrast strongly with the overwhelmingly filter feeder dominated biofacies encountered in the dark siliciclastic shales on which standard oxygen restricted biofacies models have been based and argue that mechanistically the low oxygen conditions characterizing those shales reflect ordinary levels of water column productivity and arise largely from water column stratification our biofacies model requires testing in other upwelling records however we expect it to be robust and useful given the long evolutionary histories of the chemosymbiotic and deposit feeding guilds providing a new means for discriminating the relative roles of high organic flux and low ventilation in creating low oxygen conditions at the seafloor

cretaceous cold seep communities and methane derived carbonates in the canadian arctic lower cretaceous cold seep fossil assemblages have been found in the canadian arctic archipelago serpulid worm tubes and bivalves are most abundant in these communities in contrast fossils are scarce in the surrounding strata the fossils are contained in an isotopically light delta 13 c 25 to 50 per mil carbonate rock groundmass that is interpreted to have formed from bacterial oxidation of methane the rocks were deposited at intermediate depth 400 meters in a cold marine environment nearby normal faults may have provided a conduit for seeping methane and hydrogen sulfide needed to fuel chemosynthetic bacteria and in turn the higher life forms

iodine xenon dating of chondrules from the qingzhen and kota kota enstatite chondrites abstract initial 129 i 127 i values i xe ages have been obtained for individual mineralogically characterized chondrules and interchondrule matrix from the enstatite chondrites qingzhen eh3 and kota kota eh3 in view of the absence of aqueous alteration and the low peak metamorphic temperatures experienced by these meteorites we suggest that the i xe ages for the chondrules record the event in which they were formed these ages are within the range recorded for chondrules from ordinary chondrites demonstrating that chondrules formed during the same time interval in the source regions of both ordinary chondrites and enstatite chondrites the timing of this chondrule forming episode or episodes brackets the i xe closure age of planetesimal bodies such as the shallowater aubrite parent body although chondrule formation need not have occurred close to planetesimals the existence of planetesimals at the same time as chondrule formation provides constraints on models of this process whichever mechanisms are proposed to form and transport chondrules they must be compatible with models of the protosolar nebula which predict the formation of differentiated bodies on the same timescale at the same heliocentric distance

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

a type granites in northeastern china age and geochemical constraints on their petrogenesis abstract a type granites are widely distributed in northeastern china ne china they were emplaced during three major episodes the permian late triassic to early jurassic and early cretaceous and evolved in different tectonic regimes according to their mineralogical and geochemical characteristics two subgroups of a type granites aluminous and peralkaline can be recognized the peralkaline subgroup contains alkali mafic minerals such as riebeckite arfvedsonite and sodic pyroxene while the aluminous subgroup contains annite and fe rich calcic or sodic calcic amphibole with respect to the aluminous subgroup the peralkaline granites contain higher rb ga and total rare earth elements ree but lower mgo cao al 2 o 3 ba and sr based on the discrimination criteria of eby geology 20 1992 641 the permian and late triassic to early jurassic a type granites belong to the a 2 post orogenic type whereas the early cretaceous granites are of the a 1 anorogenic type nd isotopic compositions of these a type granites indicate their derivation from a dominantly juvenile crustal source their origin is thought to have involved partial melting of an underplated lower crustal source because the generation of a type granites requires high melting temperature we propose models involving slab break off lithospheric delamination and extension the permian a type granites have the same age range as those in eastern junggar southern mongolia and central inner mongolia they occur along a major suture and form a narrow belt between the north china and siberian cratons we suggest that their formation was associated with post collisional slab break off the late triassic to early jurassic a type granites are likely to be the product of lithospheric delamination after the final collision of the major crustal blocks in the late paleozoic to early triassic the early cretaceous a type granites have an anorogenic affinity and were possibly associated with rifting in eastern china at this time associated with the onset of paleo pacific subduction consequently we conclude that the a type granites in ne china were generated at three different times involving multiple processes operative in different tectonic environments

zircon u pb hf isotopes bulk rock geochemistry and petrogenesis of middle to late triassic i type granitoids in the xing an block northeast china implications for early mesozoic tectonic evolution of the central great xing an range abstract we report zircon u pb age data hf isotopes and bulk rock geochemical data for the middle to late triassic granitoids in the taerqi chabaqi kutihe region within the xing an block of the central great xing an range northeast china euhedral to subhedral zircon grains were extracted from the nine representative granitoids these zircons exhibit oscillatory zoning typical of a magmatic origin the zircon u pb determinations on the monzogranite syenogranite and quartz diorite samples yielded ages between 244 ma and 206 ma which essentially suggests that the magmatism in the xing an block occurred during the middle to late triassic period geochemically these granitoids have sio2 62 97 76 31 wt a cnk 1 03 1 51 and k2o na2o 0 40 2 39 we infer that they belong to high k calc alkaline series and are peraluminous i type granites in nature the major and trace element systematics indicate that these granitoids have different origins among them the 244 ma syenogranite of taerqi and the 230 206 ma monzogranite and syenogranite of kutihe were probably generated from partial melting of pre existing juvenile arc type rocks in a relatively shallow crustal level the 210 ma quartz diorite of chabaqi likely originated from the hybridization between a depleted mantle component and the juvenile subducted oceanic crustal materials that were buried to depths of the middle to lower continental crust the 212 ma monzogranite and syenogranite of kutihe were probably generated from the partial melting of miscellaneous lower crustal materials at high pressure conditions by combining these new data with regional geological data we conclude that the early mesozoic evolution of the central great xing an range was governed by two superimposed tectonic regimes i e 1 post orogenic extension due to slab break off after the closure of the paleo asian oceanic basin along the hegenshan heihe suture belt and 2 back arc extension associated with the southward subduction of mongol okhotsk oceanic plate

zircon u pb geochronology and petrogenesis of the late paleozoic early mesozoic intrusive rocks in the eastern segment of the northern margin of the north china block abstract zircon u pb dating and whole rock geochemical analysis have been undertaken on late paleozoic early mesozoic intrusive rocks of central jilin province ne china with the aim of constraining the tectonic evolution of the eastern segment of the northern margin of the north china block ncb during the late paleozoic early mesozoic zircon u pb dating indicates that late paleozoic early mesozoic magmatic events can be subdivided into four stages middle permian 270 ma late permian 259 255 ma early triassic 249 ma and late triassic 222 ma the middle permian magmatic event formed peraluminous garnet bearing monzogranites indicative of formation under a compressional tectonic regime related to collision between the ncb and the songnen zhangguangcai range massif the late permian magmatic event formed gabbros and syenogranites a bimodal association that is typical of magmatism within post collisional extensional setting the early triassic magmatic event formed adakitic monzogranites suggesting that they formed from magmas derived from partial melting of a thickened mafic lower crust the late triassic magmatic event formed a series of mafic ultramafic intrusive rocks together with coeval granitoids in adjacent regions and make up a typical bimodal association suggesting that they formed under an extensional environment related to the final amalgamation of the ncb and the songnen zhangguangcai range massif taken together we conclude that the northern margin of the ncb underwent multiple orogenic events between the middle permian and the late triassic with final amalgamation of the ncb and the songnen zhangguangcai range massif occurring in the early triassic an event that led to the final suturing of the solonker xra moron changchun zone

detrital zircon u pb geochronology and stratigraphy of the cretaceous sanjiang basin in ne china provenance record of an abrupt tectonic switch in the mode and nature of the ne asian continental margin evolution abstract the age spectra obtained from 505 spots of detrital zircon u pb ages of five representative sandstone samples from the sanjiang basin in ne china point to a significant change in its provenance during the coniacian santonian the predominant detrital source for the sanjiang basin during the early cretaceous was the zhangguangcai range magmatic belt and jiamusi block along its western and southern periphery whereas it changed in the late cretaceous to its eastern periphery the timing of these inferred changes in the detrital source regions and drainage patterns nearly coincide with the age of a regional unconformity in and across the basin the time interval of non deposition and unconformity development was coeval with a transitional period between an extensional tectonic regime in the early cretaceous and a contractional deformation episode in the late cretaceous the sanjiang basin evolved during this time window from a backarc to a foreland basin the migration of the coastal orogenic belt and the fold and thrust belt development farther inland during the late cretaceous marked the onset of regional scale shortening and surface uplift in the upper plate of a flat or very shallow dipping subduction zone the stratigraphic record the detrital source and geochronology of the basinal strata and the internal structure of the sanjiang basin present therefore an important record of a tectonic switch in the nature of continental margin evolution of northeast asia during the late mesozoic

the origin of chondritic macromolecular organic matter a carbon and nitrogen isotope study the n and c abundances and isotopic compositions of acid insoluble carbonaceous material in thirteen primitive chondrites five unequilibrated ordinary chondrites three cm chondrites three enstatite chondrites a ci chondrite and a cr chondrite have been measured by stepped combustion while the range of c isotopic compositions observed is only delta 13c 30 the n isotopes range from delta 15n approximately 40 to 260 after correction for metamorphism presolar nanodiamonds appear to have made up a fairly constant 3 4 wt of the insoluble c in all the chondrites studied the apparently similar initial presolar nanodiamond to organic c ratios and the correlations of elemental and isotopic compositions with metamorphic indicators in the ordinary and enstatite chondrites suggest that the chondrites all accreted similar organic material this original material probably most closely resembles that now found in renazzo and semarkona these two meteorites have almost m shaped n isotope release profiles that can be explained most simply by the super position of two components one with a composition between delta 15n 20 and 40 and a narrow combustion interval the other having a broader release profile and a composition of delta 15n approximately 260 although isotopically more subdued the ci and the three cm chondrites all appear to show vestiges of this m shaped profile how and where the components in the acid insoluble organics formed remains poorly constrained the small variation in nanodiamond to organic c ratio between the chondrite groups limits the local synthesis of organic matter in the various chondrite formation regions to at most 30 the most 15n rich material probably formed in the interstellar medium and the fraction of organic n in renazzo in this material ranges from 40 to 70 the isotopically light component may have formed in the solar system but the limited range in nanodiamond to total organic c ratios in the chondrite groups is consistent with most of the organic material being presolar

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