effect of aridification on carbon isotopic variation and ecologic evolution at 5 3 ma in the asian interior article history the cenozoic era is marked by dramatic climatic and ecological changes the timing of the emergence and the subsequent expansions of c4 grasses are prominent biological events on earth in china thick cenozoic deposits in the tarim and junggar basins which are located in the asian interior provide important geological archives for studying paleoenvironmental changes here we use carbon isotope compositions of organic matter to reconstruct the history of ecologic evolution during the late cenozoic in the tarim and junggar basins the results show that there is a shift to slightly higher 13 cv alues at 5 3 ma indicating a change in terrestrial ecosystems in the asian interior driven by an increased regional aridity rather than decreasing atmospheric pco2 levels the weakened water vapor transportation related to the retreat of paratethys ocean and the enhanced rain shadow effect of mountain uplift during the

u pb and hf isotopic compositions of detrital zircons from the paragneisses of the quanji massif nw china implications for its early tectonic evolutionary history abstract the quanji massif is a cratonic fragment located on the northeastern margin of the qinghai tibetan plateau in western qinghai province northwest china fig 1 this massif consists of pre neoproterozoic crystalline basement unconformably overlain by mid to late neoproterozoic and cambrian strata the basement is dominantly made up of the delingha complex and the supracrustal dakendaban group the delingha complex is made up of 2 24 2 39 ga granitic gneisses with enclaves of dominant amphibolite and minor felsic gneisses as well as granulites the supracrustal dakendaban group is in tectonic contact with the delingha complex and can be divided into the lower and upper dakendaban sub groups data from detrital zircons show that the protolith rocks of these two sub groups were deposited after 2 32 ga and 2 11 ga respectively the detrital zircon age and hf isotopic data and geological correlations suggest that the quanji massif was possibly fragmented from the tarim craton and the delingha complex was probably uplifted to become the major sedimentary source for the upper dakendaban sub group at 2 11 ga our zircon hf isotope compositions demonstrate important crustal growth at 2 6 2 7 ga and 2 5 ga together with the important magmatic activity at 2 2 2 4 ga and geological data our results seem to suggest that the tarim craton was part of the north china craton in its early evolutionary history but rifted away and joined the yangtze craton prior to the neoproterozoic

isotopic evidence of c4 grasses in southwestern europe during the early oligocene middle miocene c 4 plants are widely successful in the grass dominated ecosystems of tropical subtropical and warm temperate regions largely as a result of their ability to limit photorespiration and improve water use efficiency a widely held paradigm is that low 2 concentrations were an important factor selecting for the origin of c 4 plants although support in geological records is limited we determined the carbon isotopic composition of 686 individual grass pollen grains preserved in eight samples of lacustrine and shallow marine sediments from three basins spanning the early oligocene to middle miocene in southwestern europe grasses composed 4 grasses thus c 4 grasses occurred on the landscape as early as the earliest oligocene 14 m y earlier than previous isotopic evidence of first c 4 plants and before p co 2 fell during the oligocene

cenozoic moisture fluctuations on the northeastern tibetan plateau and association with global climatic conditions abstract the cenozoic history of aridification in the asian interior is thought to be intimately linked to uplift of the tibetan plateau and long term global cooling how fluctuating climatic conditions influenced regional moisture levels during the cenozoic remains largely unknown to infer moisture changes we present oxygen isotope values of carbonates 18ocarb and carbon isotope values of long chain n alkanes 13calk from a continuous geological section in the qaidam basin northwestern china the section spans most of the cenozoic era from 52 to 5 million years ago ma based on conventional chronology whereas recently a drastically revised chronology has placed its basal age at 26 ma the association of moisture changes with global climate largely depends on the chosen chronology with the revised chronology our records are inconsistent with existing moisture records from the asian interior and do not show clear association with global climatic conditions however with the conventional chronology our 18ocarb record shows a long term aridification trend over the cenozoic whereas the 13calk suggests reduced moisture levels during the oligocene both records consistently show enhanced aridity notably during the oligocene 20 17 ma and after 13 ma which largely corresponded to relatively cool global periods our records thus suggest that global climatic conditions during the cenozoic exerted more influence on the moisture levels in the asian interior than previously thought if the conventional chronology is correct our study also highlights the importance of constructing a reliable chronology in order to make an association of cenozoic terrestrial records with global climate

isotopic paleoecology of mammals and the middle miocene cooling event in the madrid basin spain abstract the middle miocene underwent profound climatic perturbations detected in worldwide marine and continental records the miocene climatic optimum 17 to 14 ma a warm and humid period was followed by the middle miocene cooling 14 13 8 ma characterized by a sharp drop in temperatures and an increase in aridity triggered by the reestablishment of the eastern antarctica ice sheet the madrid basin located in the central iberian peninsula has furnished a considerable number of miocene fossil sites with a very complete succession of localities from the lower middle miocene to the upper middle miocene middle and upper aragonian mn5 and mn6 local zones db to g mammalian fossil tooth enamel of different taxa including rhinoceros equids gomphothere suids and ruminants from 16 fossil sites spanning from 15 9 ma to 13 2 ma were analyzed for 13c 18oco3 and 18opo4 values with the aim of determining regional paleoecological paleoenvironmental and paleoclimatic changes across this important climate transition the 13c values vary between 11 9 and 6 4 vpdb indicating a c3 environment ranging from woodlands to more open conditions the rhinoceros alicornops simorrense and hoploaceratherium tetradactylum the gomphothere gomphotherium angustidens the cervid heteroprox moralesi and the paleomerycid triceromeryx pachecoi may be regarded as browsers inhabiting more closed environments whereas the equid anchitherium the rhinoceros hispanotherium matritense and the bovid tethytragus langai show a mixed feeding diet related to more open conditions furthermore the variation in 13c values across the fossil sites and local zones indicates drier conditions in local zones db and dc 15 9 14 8 ma a more humid period in local zone dd 14 8 14 1 ma before a return to a drier environment in local zones e f and g 14 1 13 2 ma the 18oco3 and 18opo4 values and calculated 18ow values all track a declining trend from local zones db to dd 15 9 14 1 ma with minimum values in local zone e 18oco3 26 5 2 1 18opo4 18 4 2 4 and 18ow 5 3 2 9 the minimum in 18o values in local zone e 14 1 13 7 ma appears to coincide with a drop in temperature at the time of the middle miocene cooling 14 13 8 ma after this minimum 18oco3 18opo4 and 18ow values recover in local zones f and g 13 7 13 2 ma the calculation of 13cleaf values allowed us to determine plausible biomes that might have existed in the madrid basin during the middle miocene i e a tropical rain forest a tropical deciduous forest a temperate evergreen forest and a xeric woodland scrubland although this method was not able to detect more savanna like environments finally 13cdiet meq values were used to infer mean annual precipitation with values ranging from 250 mm yr local zone dc to 670 mm yr local zone dd which are in accordance with the proposed biomes suggested by the 13cleaf values and fossil mammal assemblages

marked decline in atmospheric carbon dioxide concentrations during the paleogene the relation between the partial pressure of atmospheric carbon dioxide pco2 and paleogene climate is poorly resolved we used stable carbon isotopic values of di unsaturated alkenones extracted from deep sea cores to reconstruct pco2 from the middle eocene to the late oligocene approximately 45 to 25 million years ago our results demonstrate that pco2 ranged between 1000 to 1500 parts per million by volume in the middle to late eocene then decreased in several steps during the oligocene and reached modern levels by the latest oligocene the fall in pco2 likely allowed for a critical expansion of ice sheets on antarctica and promoted conditions that forced the onset of terrestrial c4 photosynthesis

how long lived is ultrahigh temperature uht metamorphism constraints from zircon and monazite geochronology in the eastern ghats orogenic belt india abstract along the coast of peninsular india the eastern ghats expose a deep crustal section through a composite proterozoic orogenic belt to quantify the late mesoproterozoic early neoproterozoic pressure p temperature t time t evolution of the eastern ghats province new shrimp u pb zircon and monazite age data from multiple localities are reported and integrated with the results of phase equilibria modelling samples of residual granulite migmatite and enderbite yield a spread of weighted mean 207pb 206pb zircon and monazite ages between ca 970 and ca 930 ma based on ranges of spot ages from several samples the late prograde to peak ultrahigh temperature uht metamorphism counter clockwise evolution ccw to t 950 c at p 8 kbar and initial cooling is interpreted to have occurred between ca 1130 and ca 970 ma regionally extensive enderbite and charnockite magmas were emplaced into the hot suprasolidus crust around the time of peak metamorphism for the residual granulites and migmatites the retrograde p t t path is characterized by close to isobaric cooling to the variable but elevated solidi for different samples weighted mean ages between ca 970 and ca 930 ma in several samples are interpreted to record the timing of crystallization of melt trapped by the percolation threshold in each of these samples two additional weighted mean ages of ca 980 ma from korhonen f j saw a k clark c brown m bhattacharya s 2011 new constraints on uht metamorphism in the eastern ghats province through the application of phase equilibria modelling and in situ geochronology gondwana research 20 764 781 extend this range back in time by 10 my the variability in the calculated weighted mean ages across the region is interpreted to be due mainly to differences in the temperature of the elevated solidus from sample to sample suggesting a slow cooling rate of 1 c my during the retrograde stage of this long lived uht metamorphism these results demonstrate that the eastern ghats province sustained uht conditions t 900 c for 50 my and perhaps for as long as 200 ma from ca 1130 to 930 ma during a single ccw tectono metamorphic event

early miocene repetitive vegetation and climatic changes in the lacustrine deposits of the rubielos de mora basin teruel ne spain pollen analysis of a lower miocene succession from the rubielos de mora basin ne spain has been carried out with the aim of reconstructing the flora vegetation and climatic changes previous paleobotanical studies on these sedimentary rocks and adjacent areas interpreted very diverse climates for the early miocene from humid temperate to dry subtropical in this study a rich thermophilous pollen spectrum and a diverse subarid flora including nitraria caesalpiniaceae ephedra and acacia indicative of a dry subtropical climate are identified on the other hand mesothermic taxa with high water requirements are also abundant therefore the pollen assemblages evidence the juxtaposition of very contrasted environments the presence of subdesertic taxa typical of plants growing in the lowlands and conditioned by a long warm dry season together with others with very high water requirements needing constant water this can be explained by the presence of the rubielos de mora lake providing local conditions for developing riparian forests the vegetation was clearly controlled by the water availability under a subtropical and dry seasonal climate pollen changes along the succession which coincide with sedimentological changes are related to climatic variations alternation in pollen taxa thermophilous dry vs mesothermic riparian reflects the influence of the cyclicity of temperature and precipitation on the lake level and vegetation

high resolution isotopic record of c4 photosynthesis in a miocene grassland abstract the origin and spread of grasslands is one of the key evolutionary events in the cenozoic which is characterized by long term cooling and drying climatic conditions one way to examine the relationship between vegetation and climate is to study the isotopic composition of organic carbon preserved in paleosols paleosols that preserve organic matter in the form of carbonaceous root traces provide direct evidence of the proportion of c3 to c4 biomass that grew in the soil in contrast with pedogenic carbonate 13c values which may also reflect complicating factors including mixing from atmospheric 13c in low productivity ecosystems a high temporal and spatial resolution reconstruction of past ecosystems was derived from thirty five paleosols in a thirty four meter section of the sixmile creek formation at timber hills montana usa that was deposited during the miocene 10 2 8 9 ma ago phytoliths were extracted from paleosol samples to compare vegetation assemblages to inferences based on isotopic compositions with both proxies giving similar results isotopic results from organic matter indicate both a small component of c4 photosynthetic plants locally prior to their regional expansion to dominance in the late miocene through the early pleistocene and large variation in the abundance of c4 plants 0 25 in this ecosystem both laterally and on a 100 kyr timescale in contrast pedogenic carbonate 13c values from this site indicate a high proportion of c4 photosynthesis that is at odds with both phytolith and 13corg results suggesting that the carbonate values are biased by diagenesis or diffusion of atmospheric co2 and that a similar issue may impact previous paleovegetation reconstructions based on pedogenic carbonates quantitative reconstructions of mean annual temperature and mean annual precipitation indicate little local variability through time and that the fluctuations in c4 proportion were not climatically driven instead the variable proportion of c4 photosynthesis is best explained by ecosystem scale variables such as succession fluvial avulsion and fire regime

the early origins of terrestrial c4 photosynthesis abstractthe c4 photosynthetic pathway is a series of structural and biochemical modifications around the more primitive c3 pathway that improve the photosynthetic efficiency under specific climatic conditions hence the origin and subsequent geographical expansion of the c4 plants likely reflects a record of climate change multiple paleoatmospheric pco2 proxies indicate a critical co2 threshold was breached 30 ma that potentially selected for co2 concentrating mechanisms to overcome photorespiratory stresses imposed on the basic c3 pathway details of the c4 pathway s earliest origins remain enigmatic given the paucity of the geologic record nonetheless 13c proxy records from paleosol carbonates ungulate teeth and plant derived compounds indicate c4 plants likely represented an important component of plant biomass by the early miocene low co2 levels appear to be a precondition for the development of the c4 photosynthetic pathway however comparisons of 13c proxy records indicate that the timin

tectonics or climate what drove the miocene global expansion of c4 grasslands investigations into the most plausible causes which triggered the late miocene global expansion of c4 grasslands have reached no consensus and are still being debated the global decline in co2 levels in the late miocene has been thought to be the most common driver although c4 grassland expansion was largely confined to the late miocene their first appearance varied significantly in different parts of the old and new world and therefore emphasizes role of local and or regional factors may or may not be in combination with one or more global factors the dynamic himalayan orogen constitutes a significant global tectonic event that is believed to have affected global climate hence a study of the himalayan foreland basin sediments could help in gleaning out possible causes behind this major paleoecologic event which could perhaps be directly or indirectly related stable carbon isotope analyses of a total of 141 pedogenic carbonates in the mio pleistocene siwalik paleosols of the ramnagar sub basin are coherent providing a better insight into paleovegetational changes across the sub basin on temporal and spatial scales furthermore paleovegetational history during oligocene is reconstructed from dagshai paleosols exposed in the subathu sub basin the isotopic results show dominance of c3 vegetation pre 7 ma and dominance of c4 vegetation post 5 ma percentage abundance of c4 vegetation was less than 20 pre 7 ma but increased to more than 40 post 5 ma reaching up to 100 in the youngest analyzed sediments there is exclusive dominance of c3 vegetation during oligocene these results conform to the pattern of change in vegetation documented in other parts of the himalayan belt the global expansion of c4 grasslands largely during late miocene have long been linked with then climate changes particularly brought by declining atmospheric co2 level large scale fires intensification of monsoon seasonality and aridity as demonstrated by various researchers these major hypotheses explaining expansion of c4 grasslands during late miocene are not very convincing due to significant shortcomings associated with them for example expansion of c4 grasslands in non monsoonal regions put serious constraints on the monsoonal intensification hypothesis likewise aridity as being the primary driver of c4 grassland expansion seems hard to support since there is no substantial evidence that could suggest global onset of arid conditions at about same geologic time and some most arid periods like the triassic show no evidence of c4 plants we suggest that the initial lowering of co2 below 450 ppmv created an environment for the beginning of c4 vegetation but the persistence of this threshold value for a considerable time during late miocene appears to be the probable cause of the late miocene global expansion of c4 grasslands irrespective of their time of first appearance it has been deduced through the correlation of himalayan tectonic events with atmospheric co2 levels and paleovegetational changes since upper miocene times that it was indeed the late miocene continuous intense tectonic instability of the himalayas that significantly decreased atmospheric co2 levels and which perhaps played a key role in changing the nature of photosynthetic pathways

stable carbon isotopic variation of long chain n alkanoic acids in the equatorial pacific sediments over the last 40 ma implications for expansion of c4 grassland in south america abstract the stable carbon isotope ratio 13 c values of sedimentary long chain c 20 n alkanoic acids from the equatorial pacific iodp sites u1331 1338 were measured for historical reconstruction of c 4 plant contribution from south america over the last 40 ma the values for c 30 and c 32 acids ranged between 36 9 and 25 4 suggesting that they likely originated from higher plant wax associated with c 3 dominated vegetation the molecular distribution of alkanoic acids in the sediments showed a marked change across the intertropical convergence zone itcz indicating a potential contribution from the northern hemisphere to the sediments located at 10 n in contrast the abundances of n alkanoic acids significantly decreased in the sediments at 13 c values suggest that the differences in the core locations possibly caused north and southward shifts of ca 1 0 n and ca 0 5 s from the equator respectively the corrected 13 c values of c 30 and c 32 acids did not cross the c 3 end member threshold until 1 7 ma despite the onset of c 4 plants in central south america at ca 7 ma this implies that vegetation in the low latitudes of south america was less affected by hydrological changes induced by the uplift of the andes during the late miocene

metamorphic evolution of the coesite bearing ultrahigh pressure terrane in the north qaidam northern tibet nw china widespread evidence for ultrahigh pressure uhp metamorphism is reported in the dulan eclogite bearing terrane the north qaidam altun hp uhp belt northern tibet this includes 1 coesite and associated uhp mineral inclusions in zircon separates from paragneiss and eclogite identified by laser raman spectroscopy 2 inclusions of quartz pseudomorphs after coesite and polycrystalline k feldspar quartz in eclogitic garnet and omphacite and 3 densely oriented sio2 lamellae in omphacitic clinopyroxene these lines of evidence demonstrate that the dulan region is a uhp metamorphic terrane in the north dulan belt ndb eclogites are characterized by the peak assemblage grt omp rt phn coe pseudomorph and retrograde symplectites of cpx ab and hbl pl the peak conditions of the ndb eclogites are p 2 9 3 2 gpa and t 631 687 c the eclogite shows a near isothermal decompression p t path suggesting a fast exhumation in the south dulan belt sdb three metamorphic stages are recognized in eclogites 1 a peak eclogite facies stage with the assemblage grt omp ky rt phn at p 2 9 3 3 gpa and t 729 746 c 2 a high pressure granulite facies stage with grt cpx jd 30 pl an24 29 scp at p 1 9 2 0 gpa t 873 948 c and 3 an amphibolite facies stage with the assemblage hbl pl ep czo at p 0 7 0 9 gpa and t 660 695 c the clockwise p t path of the sdb eclogites is different from the near isothermal decompression p t path from the ndb eclogites which suggests that the sdb was exhumed to a stable crustal depth at a slower rate in essence these two sub belts formed in different tectonic settings they both subducted to mantle depths of around 100 km but were exhumed to the earth s surface separately along different paths this uhp terrane plays an important role in understanding continental collision in north western china

island arc volcanic rocks in the north qaidam uhp belt northern tibet plateau evidence for ocean continent subduction preceding continent continent subduction abstract a suite of early paleozoic island arc volcanic rocks consisting mainly of basaltic lavas and some intermediate silicic rocks crops out in the north qaidam basin northeastern tibet plateau these rocks have been extensively metamorphosed under greenschist facies conditions and hence differ from the older proterozoic intermediate mafic volcanic rocks which have generally experienced amphibolite facies metamorphism the early paleozoic volcanic rocks are divided into three groups based on their geochemical characteristics 1 vtg i consists of island arc tholeiitic basalts basaltic andesites and andesites that crop out in jilusu and shuangkoushan 2 vtg ii which are high al alkaline and sub alkaline basalts in jilusu and 3 vtg iii an assemblage of highly depleted morb type basalts andesites and dacites that occurs mostly in shuangkoushan groups vtg i and ii are considered to be the products of mature island arcs produced in two stages in the first stage following initiation of ocean continent subduction the island arc tholeiites were generated by partial melting of the subsiding oceanic crust and mantle wedge beneath the island arc as subduction progressed and accelerated calc alkaline and high al basalts were formed and the crust beneath the island arc was thickened the highly depleted morb type lavas vtg iii have boninitic affinities and are believed to have been generated from depleted mantle material modified by subduction zone fluids possibly in the fore arc region the island arc tholeiites are dated at 514 2 8 5 ma whereas uhp eclogite in the north qaidam belt has been dated at 494 ma these ages suggest that ocean continent subduction took place in the north qaidam basin in the early paleozoic and was followed shortly by continent continent collision and subduction which produced the uhp rocks

mid miocene c4 expansion on the chinese loess plateau under an enhanced asian summer monsoon abstract atmospheric co2 starvation aridity fire and warm season precipitation have all been proposed as major contributors to c4 plant expansion during the late miocene however the driving factors responsible for the distribution of c4 plants in the early and mid miocene still remain enigmatic here we report pedogenic carbon and oxygen isotope data 13cpedo 18opedo along with magnetic susceptibility ms results from the zhuang lang drilling core on the chinese loess plateau clp elevated 13cpedo values 5 signal a prominent c4 expansion and substantially increased 18opedo and ms values indicate enhanced asian summer monsoon asm precipitation both of these conditions are observed during the mid miocene climatic optimum mmco 14 5 17 million years ago the marked increase in c4 plants associated with warm temperatures and increased precipitation strongly suggests the control of an enhanced asm on c4 expansion on the clp during the mmco this finding contrasts with the late miocene c4 expansion associated with cooling and drying conditions observed in low latitudes and argues for regionally specific control of c4 plant distribution expansion

ultrahigh temperature granulite metamorphism 1050 c 12 kbar and decompression in garnet mg70 orthopyroxene sillimanite gneisses from the rauer group east antarctica highly magnesian and aluminous migmatitic gneisses from mather peninsula in the rauer group eastern antarctica preserve ultrahigh temperature uht metamorphic assemblages that include orthopyroxene sillimanite quartz garnet sillimanite quartz and garnet orthopyroxene sillimanite garnet that ranges up to xmg of 71 5 coexists with aluminous orthopyroxene that shows zoning from cores with 7 5 8 5 wt al2o3 to rims with up to 10 6 wt al2o3 adjacent to garnet peak p t conditions of 1050 c and 12 kbar are retrieved from fe mg al thermobarometry involving garnet and orthopyroxene in very good agreement with independent constraints from petrogenetic grids in feo mgo al2o3 sio2 and related chemical systems sapphirine orthopyroxene and cordierite form extensive symplectites and coronas on the early phases the specific reaction textures and assemblages involving these secondary phases correlate with initial garnet xmg with apparent higher pressure reaction products occurring on the more magnesian garnet and are interpreted to result from an initial phase of ultrahigh temperature near isothermal decompression uht itd from 12 to 8 kbar at temperatures in excess of 950 c later textures that involved biotite formation and then partial breakdown along with garnet relics to symplectites of orthopyroxene cordierite or cordierite spinel may reflect hydration through back reaction with crystallizing melts on cooling below 900 850 c followed by itd from 7 to 8 kbar to c 5 kbar at temperatures of 750 850 c the tectonic significance of this p t history is ambiguous as the rauer group records the effects of archean tectonothermal events as well as high grade events at 1000 and 530 ma late stage biotite formation and subsequent itd can be correlated with the p t history preserved in the proterozoic components of the rauer group and hence with either 1000 or 530 ma collisional orogenesis however whether the preceding uht itd history reflects a temporally unrelated event e g archean or is simply an early stage of either the late proterozoic or pan african tectonism as recently deduced for similar uht rocks from other areas of the east antarctica remains uncertain

two stages of granulite facies metamorphism in the eastern himalayan syntaxis south tibet petrology zircon geochronology and implications for the subduction of neo tethys and the indian continent beneath asia the eastern himalayan syntaxis in southeastern tibet consists of the lhasa terrane high himalayan rocks and indus tsangpo suture zone the lhasa terrane constitutes the hangingwall of a subduction zone whereas the high himalayan rocks represent the subducted indian continent our petrological and geochronological data reveal that the lhasa terrane has undergone two stages of medium p metamorphism an early granulite facies event at c 90 ma and a late amphibolite facies event at 36 33 ma however the high himalayan rocks experienced only a single high p granulite facies metamorphic event at 37 32 ma it is inferred that the late cretaceous c 90 ma medium p metamorphism of the southern lhasa terrane resulted from a northward subduction of the neo tethyan ocean and that the oligocene 37 32 ma high p 1 8 1 4 gpa rocks of the high himalayan and coeval medium p 0 8 1 1 gpa rocks of the lhasa terrane represent paired metamorphic belts that resulted from the northward subduction of the indian continent beneath asia our results provide robust constraints on the mesozoic and cenozoic tectonic evolution of south tibet

stable isotope stratigraphy and paleoclimatology of the paleogene bighorn basin wyoming usa abstract climatic warming from the late paleocene into the early eocene had profound effects on atmospheric and marine circulation marine thermal gradients and benthic biota in addition marine carbon isotope values decreased substantially in both surface and deep waters because carbon is rapidly exchanged between reservoirs at the earth s surface such as marine surface water the atmosphere land plants and materials that obtain carbon from plants e g soil minerals and herbivores carbon isotope fluctuations provide time lines linking the marine and continental records we analyzed the carbon isotope composition of paleosol carbonates and mammalian tooth enamel from stratigraphic sections in northwestern wyoming carbon isotope correlation demonstrates that a short interval of extreme high latitude warming coincided precisely with the first appearance of several important modern mammalian orders in addition oxygen isotope analyses of fossils and paleosol carbonates provide information about climatic conditions on land in the paleogene we reconstructed the oxygen isotope composition of local meteoric water using biogenic minerals paleocene eocene meteoric water was significantly 18 o depleted indicating substantial loss of water vapor during its transport to the region soil temperature was calculated as a proxy for mean annual temperature assuming oxygen isotope equilibrium between soil carbonate and meteoric water calculated temperatures were plausible 10 25 c but highly variable prohibiting high resolution analysis of local temperature variations in response to global climatic warming

transient hot channels perpetrating and regurgitating ultrahigh pressure high temperature crust mantle associations in collision belts abstract two dimensional numerical modeling of early continental collision associated with subduction of the lithospheric mantle shows that the formation and exhumation of coesite and diamond bearing rocks metamorphosed at 700 to 900 c in the presence of dense supercritical silicate fluids and melts may be explained by a transient hot channel effect anomalously high temperature is caused by intense viscous and radiogenic heating in the channel composed of deeply subducted radiogenic upper crustal rocks especially sediments of passive margin origin and mantle rocks heating is associated with partial melting of crustal rocks caused by pervasive flow of aqueous fluids relieved by rapid dehydration deserpentinization of the overriding mantle lithosphere that has been hydrated during earlier subduction stages the channel penetrates the plate interface to the bottom of the lithosphere of the overriding plate 150 200 km and is characterized by metamorphic temperatures reaching 700 to 900 c low effective viscosity of rocks subsequent to increased temperature partial melting and fluid infiltration permits profound mixing of hydrated mantle and crustal rocks the hot channel exists only during early collision but rapidly produces large amounts of ultrahigh pressure high temperature rocks further collision closes the channel by squeezing rheologically weak partially molten buoyant rocks between the strong lithospheric mantles of the two colliding plates assemblages of complicated p t paths with repetitive loops characterize exhumation of ultrahigh pressure rocks in the convoluted flow pattern of the hot channel

neogene biomarker record of vegetation change in eastern africa the evolution of c4 grassland ecosystems in eastern africa has been intensely studied because of the potential influence of vegetation on mammalian evolution including that of our own lineage hominins although a handful of sparse vegetation records exists from middle and early miocene terrestrial fossil sites there is no comprehensive record of vegetation through the neogene here we present a vegetation record spanning the neogene and quaternary periods that documents the appearance and subsequent expansion of c4 grasslands in eastern africa carbon isotope ratios from terrestrial plant wax biomarkers deposited in marine sediments indicate constant c3 vegetation from 24 ma to 10 ma when c4 grasses first appeared from this time forward c4 vegetation increases monotonically to present with a coherent signal between marine core sites located in the somali basin and the red sea the response of mammalian herbivores to the appearance of c4 grasses at 10 ma is immediate as evidenced from existing records of mammalian diets from isotopic analyses of tooth enamel the expansion of c4 vegetation in eastern africa is broadly mirrored by increasing proportions of c4 based foods in hominin diets beginning at 3 8 ma in australopithecus and slightly later kenyanthropus this continues into the late pleistocene in paranthropus whereas homo maintains a flexible diet the biomarker vegetation record suggests the increase in open c4 grassland ecosystems over the last 10 ma may have operated as a selection pressure for traits and behaviors in homo such as bipedalism flexible diets and complex social structure

expansion of open landscapes in northern china during the oligocene induced by dramatic climate changes paleoecological evidence abstract the mammalian faunas of the mongolian plateau were affected by an important turnover at the eocene oligocene transition caused by a global cooling and a local aridification which could have strongly affected the environment however paleoenvironmental indices remain scarce in this area during the oligocene the knowledge of the main paleoecological trends of mammals can thus constitute essential clues in order to better evaluate the impact of these climate changes on east asian environment variations as fossils of the primary consumer macro mammals are unavailable to reconstruct past environments we proposed to infer the feeding habits of well described rodent communities by studying their dental microwear patterns we assumed that their diet range reflected their past micro habitats 80 dental specimens of cricetidae and ctenodactylidae excavated from ulantatal sites inner mongolia china were investigated through the oligocene period our results reveal that the main wear features observed in their dental patterns are fine scratches which correspond to the ingestion of abrasive matter such as grasses or dust present in herbaceous to shrubby layers in addition to these plants they could also have consumed insects feeding behaviors which included a high component of abrasive intakes were probably linked to important environmental changes that occurred between the eocene and the oligocene that is stressed by the appearance and the relative expansion of open habitats involving mainly shrubs and probably to a lesser extent grasses these environmental changes are supported by some dental characteristics of the oligocene mammalian paleocommunities of the mongolian plateau e g high crown similarly oligocene paleoenvironmental records of the north american great plains indicate the presence of woodlands and savannas at the same latitudes including grasses although preliminary these results concur with the assumption of the diversification of grasses in open landscapes well before their expansion during the late miocene

the multi stage tectonic evolution of the xitieshan terrane north qaidam orogen western china from grenville age orogeny to early paleozoic ultrahigh pressure metamorphism abstract the geodynamic evolution of the early paleozoic ultrahigh pressure metamorphic belt in north qaidam western china is controversial due to ambiguous interpretations concerning the nature and ages of the eclogitic protoliths within this framework we present new la icp ms u pb zircon ages from eclogites and their country rock gneisses from the xitieshan terrane located in the central part of the north qaidam uhp metamorphic belt xitieshan terrane contains clearly different protolith characteristics of eclogites and as such provides a natural laboratory to investigate the geodynamic evolution of the north qaidam uhp metamorphic terrane la icp ms u pb zircon dating of three phengite bearing eclogites and two country rock gneiss samples from the xitieshan terrane yielded 424 427 ma and 917 920 ma ages respectively the age of 424 427 ma from eclogite probably reflects continental lithosphere subduction post dating oceanic lithosphere subduction at 440 460 ma the 0 91 0 92 ga metamorphic ages from gneiss and associated metamorphic mineral assemblages are interpreted as evidence for the occurrence of a grenville age orogeny in the north qaidam uhpm belt using internal microstructure geochemistry and u pb ages of zircon in this study combined with the petrological and geochemical investigations on the eclogites of previous literature s data three types of eclogitic protoliths are identified in the xitieshan terrane i e 1 subducted early paleozoic oceanic crust 440 460 ma 2 neoproterozoic oceanic crust material emplaced onto micro continental fragments ahead of the main early paleozoic collision event 440 420 ma and 3 neoproterozoic mafic dikes intruded in continental fragments rifted away from the former supercontinent rodinia these results demonstrate that the basement rocks of the north qaidam terrane formed part of the former supercontinent rodinia attached to the yangtze craton and or the qinling microcontinent and recorded a complex tectono metamorphic evolution that involved neoproterozoic and early paleozoic orogenies

evolution of grasses and grassland ecosystems the evolution and subsequent ecological expansion of grasses poaceae since the late cretaceous have resulted in the establishment of one of earth s dominant biomes the temperate and tropical grasslands at the expense of forests in the past decades several new approaches have been applied to the fossil record of grasses to elucidate the patterns and processes of this ecosystem transformation the data indicate that the development of grassland ecosystems on most continents was a multistage process involving the paleogene appearance of c3 and c4 open habitat grasses the mid late cenozoic spread of c3 grass dominated habitats and finally the late neogene expansion of c4 grasses at tropical subtropical latitudes the evolution of herbivores adapted to grasslands did not necessarily coincide with the spread of open habitat grasses in addition the timing of these evolutionary and ecological events varied between regions consequently region by region investigations using both direct plant fossil

regional scale variability in the spread of grasslands in the late miocene abstract a new paleovegetation reconstruction based on both phytolith and paleosol carbon isotopic data is presented for the middle late miocene 11 2 9 5 ma of southwestern montana both sources of data indicate an open habitat c3 grassland ecosystem with a small c4 component

gold distribution in as deficient pyrite and telluride mineralogy of the yangzhaiyu gold deposit xiaoqinling district southern north china craton the mesozoic yangzhaiyu lode gold deposit is situated in the southern edge of the north china craton gold mineralization is hosted in archean amphibolite facies metamorphic rocks and consists mainly of auriferous quartz veins pyrite is the predominant sulfide mineral with minor amounts of chalcopyrite sphalerite and galena based on morphology and paragenesis there are three generations of pyrite termed as first generation g1 second generation g2 and third generation g3 they have distinct contents occurrences and distribution patterns of gold the coarse grained euhedral g1 pyrite contains negligible to low levels of gold whereas both invisible and visible gold are present in the fine to medium grained g2 pyrite that is characterized by abundance of microfractures and porosities forming a foam like texture laser ablation inductively coupled plasma mass spectrometry la icp ms depth profiles indicate that invisible gold occurs either as solid solution or as nanoparticles of gold bearing tellurides in the g2 pyrite visible gold is widespread and present as irregular grains and stringers of native gold mostly along grain boundaries or filling microfractures of pyrite likely resulting from remobilization of invisible gold once locked in the g2 pyrite the g3 pyrite invariably intergrown with chalcopyrite sphalerite and galena contains the highest levels of invisible gold there is a positive correlation between au ag and te indicating that gold occurs as submicroscopic au bearing telluride inclusions in the host minerals whenever gold either invisible or visible is present as is always below or only marginally higher than the detection limit of la icp ms this indicates that as played an insignificant role in gold mineralization tellurides are widespread in the auriferous quartz veins consisting mainly of petzite calaverite hessite altaite and tellurobismuthite native gold commonly occurs as intergrowths with tellurides textural evidence indicates a precipitation sequence in a temporal order of calcaverite petzite altaite tellurobismuthite and hessite little amount of sulfide phases has been found in association with the tellurides indicating that tellurides were deposited under low s fugacity fs2 and or high te fugacity fte2 conditions the textural relationships when combined with fluid inclusion microthermometric data of auriferous quartz veins and tellurides thermodynamic data permit estimation for logfte2 during telluride formation which are 6 8 to 10 8 at 300 c and 9 6 to 17 6 at 250 c available geochronological and geochemical data suggest that te was most likely derived from the late mesozoic magmatic rocks widespread in the xiaoqinling district and other parts of the southern north china craton which were emplaced broadly contemporaneous with gold mineralization at yangzhaiyu this study highlights the role of te and tellurides as important gold scavengers in as deficient ore fluids

constraining the prograde and retrograde p t t path of eocene hp rocks by shrimp dating of different zircon domains inferred rates of heating burial cooling and exhumation for central rhodope northern greece ion microprobe shrimp dating was carried out on different zircon domains from metamorphic rocks of the hp ht terrane of central rhodope northern greece to constrain the timing of prograde and retrograde stages within a single tectono metamorphic cycle a well determined p t t relative path for the metamorphic rocks of this terrane was used as a petrological basis for the geochronological investigations ion microprobe work was assisted by cathodoluminescence cl images of the zircon crystals the geochronological results revealed that hercynian continental crust was subducted during the eocene several stages of the eocene tectono metamorphic cycle including both the prograde and retrograde parts of the p t path above ca 300 c 0 3 gpa were dated using zircons from the following rock types 1 a deformed quartz vein probably formed at ca 300 c 0 3 gpa zircons in this vein precipitated from a hydrothermal fluid they yielded an age of 45 3 0 9 ma which corresponds to the time of a low t prograde stage of metamorphism 2 in kyanite eclogites zircons were entirely reset during eclogite facies metamorphism resetting was very probably enhanced by the presence of fluids derived by h2o liberating reactions close to the p peak they yielded an age of 42 2 0 9 ma 3 orthogneisses surrounding the kyanite eclogites contained zircons with magmatic oscillatory zoned cores which yielded hercynian ages of 294 8 ma age of granitic protolith formation whereas cl bright metamorphic rims yielded like the eclogite zircons ages of 42 0 1 1 ma therefore both the eclogites and orthogneisses are interpreted to have approached maximum depth at around 42 ma 4 in a leucosome of a migmatized orthogneiss oscillatory zoned zircons yielded an age of 40 0 1 0 ma at this time the rocks reached maximum temperatures during early decompression 5 a late pegmatite crosscutting the schistosity of amphibolites contained oscillatory zoned zircons that yielded a crystallization age of 36 1 1 2 ma thus the whole tectono metamorphic cycle above ca 300 c 0 3 gpa lasted from 45 3 0 9 ma to 36 1 1 2 ma that is 9 2 ma with an extreme error value of 2 1 ma based on combined shrimp and petrological data the average rates of heating and burial during subduction above ca 300 c 0 3 gpa are 94 c ma and 15 mm a respectively rates of cooling and exhumation also above 300 c 0 3 gpa are calculated to be 128 c ma and 7 7 mm a the eocene age of metamorphism in central rhodope implies that the terrane of at least central rhodope and the cyclades very probably was part of the same continental crust

modelling late oligocene c4 grasses and climate evidence suggests that c4 grasses adapted to conditions of low co2 high temperatures and water stressed environments expanded relatively quickly and in several geographically distinct regions during the late miocene approximately 9 7 ma using a variety of modelling tools we attempt to explain this expansion in terms of changes in global climate we use a fully coupled atmosphere ocean general circulation model aogcm hadcm3l to simulate global climate before and after the expansion the late oligocene 27 ma and pre industrial we then use the simulated climates to drive a process based global vegetation model lpj contrary to indications from palaeo data the vegetation model predicts a greater c4 distribution during the late oligocene than the pre industrial the principal reason for this is that the relatively high tropical temperatures which we predict for the late oligocene confer a competitive advantage on c4 grasses over c3 grasses which outweighs the disadvantage imposed by the higher co2 concentrations we find that this result is robust to prescription of different co2 levels in the late oligocene aogcm simulation and to the vegetation model used of the several possible explanations proposed for the model data discrepancy three seem most likely first that the vegetation models do not adequately represent certain processes under extreme climate conditions such as fire disturbance or key parameters related to the competition between c3 and c4 grasses second that there existed an evolutionary niche for c4 grasses prior to their expansion in the late miocene which for some reason was not filled third that we have not adequately modelled the late oligocene climate with the aogcm future research should concentrate on the collection and analysis of additional data for the assessment of palaeo tropical land temperatures and on further testing of the sensitivity of the aogcm results

cenozoic continental climatic evolution of central europe continental climate evolution of central europe has been reconstructed quantitatively for the last 45 million years providing inferred data on mean annual temperature and precipitation and winter and summer temperatures although some regional effects occur the european cenozoic continental climate record correlates well with the global oxygen isotope record from marine environments during the last 45 million years continental cooling is especially pronounced for inferred winter temperatures but hardly observable from summer temperatures correspondingly cenozoic cooling in central europe is directly associated with an increase of seasonality in contrast inferred cenozoic mean annual precipitation remained relatively stable indicating the importance of latent heat transport throughout the cenozoic moreover our data support the concept that changes in atmospheric co2 concentrations although linked to climate changes were not the major driving force of cenozoic cooling

hualong complex south qilian terrane u pb and lu hf constraints on neoproterozoic micro continental fragments accreted to the northern proto tethyan margin abstract the neoproterozoic hualong complex dominates the south qilian terrane it contains mafic to felsic magmatic rocks cherts and turbiditic sediments some of which have been metamorphosed to amphibolite grade and is traditionally regarded as a pre cambrian micro continental block rifted from the south china plate new u pb geochronology of morphologically complex zircons sheds light on the history of the complex garnet bearing amphibolite and quartzite have detrital zircon u pb main peak ages of 1 47 1 78 ga with youngest u pb ages of 967 ma and 964 ma they are intruded by neoproterozoic orthogneisses that crystallized between ca 850 and 940 ma psammitic paragneisses are dominated by 940 780 ma detrital zircon populations with a youngest u pb weighted mean age of 721 3 ma and a main peak at 906 ma metamorphic overgrowths on older zircons record amphibolite facies metamorphism of the hualong complex and correspond to the emplacement of the younger plutonic rocks during the early paleozoic northward subduction of proto tethyan oceanic lithosphere 426 ma and 455 ma detrital zircon populations that originated from synkinematic granitoids dominate quartz mica schist negative to positive hf t values for both 940 850 ma granitic orthogneisses and intruding 460 410 ma granites suggest a crustal mantle mixture source hf t dm1 model ages and inherited zircon u pb ages are the same as those for garnet bearing amphibolites and psammitic gneiss indicating that late magmatic rocks were formed by melting of the accreted neoproterozoic rocks

thermal modelling in low pressure high temperature metamorphic belts abstract low pressure high temperature metamorphic belts occur throughout the world they display a characteristic metamorphic style which has remained unchanged with time they are found in disparate tectonic settings including magmatic arcs regions of extension regions of thickened crust in continent continent collision zones and accretionary wedges they are commonly characterized by an abundance of granitoid plutons pressures of metamorphism are primarily less than or equal to that of the aluminosilicate triple point approximately 4 kbar while temperatures are in the range 500 750 c with rare exceptions inferred ptt paths are nearly isobaric and recrystallization was rapid and prograde metamorphic thermal gradients are commonly in the range 60 150 c km these conditions necessitate the presence of a large component of advective heat transfer and or anomalously high basal heat flow during metamorphism the large magmatic fluxes associated with the formation of magmatic arcs provide an adequate heat source for low pressure metamorphism lpm lithospheric extension is likely to produce lpm only if the degree of thinning is large 3 and there is additional heat from magmas the generation of lpm by crustal thickening requires either a large component of advective heating by magmas anomalously high basal heat flow or pre thickening lithospheric extension lpm in this environment must necessarily be accompanied by plutonic activity advection of heat by aqueous fluids may provide a significant contribution to the thermal budget but acting along is unlikely to produce lpm regional lpm induced by advective sources rarely occurs along a geotherm which increases monotonically with depth or synchronously across the region at a given depth it occurs over time as a series of overlapping temporally separate and spatially localized thermal events

magmatic history of the northeastern tibetan plateau 1 the northeastern margin of the tibetan plateau is underlain by the qaidam and qilian terranes which consist primarily of mid proterozoic through lower paleozoic oceanic and arc type assemblages that have been accreted to the southern margin of the tarim sino korean craton most previous models suggest that these assemblages formed along a northeast dipping subduction system constructed along the margin of the tarim sino korean craton during early paleozoic time the main components are interpreted to have formed either as an archipelago of volcanic arcs and back arc basins or as a broad expanse of accretionary complexes our geochronologic data support a model suggested by sobel and arnaud 1999 in which the qaidam and qilian terranes are separated from the tarim sino korean craton by a mid paleozoic suture that closed along a southwest dipping subduction zone the basement to these terranes consists of oceanic assemblages that were amalgamated into a coherent crustal fragment prior to emplacement of 920 930 ma granitoids early paleozoic arc type magmatism occurred between 480 and 425 ma apparently sweeping southwestward across much of the qilian and qaidam terranes accretion related magmatism along the inboard margin of the qilian terrane occurred between 423 ma and 406 ma following silurian devonian accretion the region has experienced late paleozoic and mesozoic uplift and erosion and has been severely overprinted by tertiary thrusting uplift and strike slip motion along the altyn tagh fault correlation of geologic features and magmatic histories between the altun shan and the nan shan suggests that the eastern altyn tagh fault has a total left lateral offset of 375 km

a description of metamorphic ptt paths with implications for low p high t metamorphism the terms clockwise and anticlockwise are well entrenched in the metamorphic geology literature where they are used to describe respectively decompression or compression of rocks at and after the metamorphic temperature peak accordingly we define the clockwiseness of a pt path c as the rate of pressure change at the temperature maximum c dp dt i rmax and suggest that c may be used to characterise metamorphic pt paths we illustrate this approach using a simple one dimensional kinematic model with a prescribed strain rate evolution and a thermal model in which the thermal evolution is governed by a cooling intrusion and we consider both the lithostatic and the deviatoric stress contributions to the pressure field in a parallel paper we have shown that because of the consideration of deviatoric stresses in this model pt paths may vary from clockwise to anticlockwise depending on distance form the intrusion and or subtle changes of the assumptions of the strain rate evolution this result is important as it challenges many current interpretations of pt paths in particular those from the low p high t metamorphic environment here we use the simple parameter c to investigate some of the governing controls on the shape variations of such paths

aeolian n alkane isotopic evidence from north pacific for a late miocene decline of c4 plant in the arid asian interior abstract aeolian deposition in the central north pacific has been well recognized to originate from arid asian interior while there is no doubt about the transport of organic matters along with the mineral dust from the source region little is known about the nature and changes of the terrestrial organic compounds preserved in the deep sea sediments in this study higher plant leaf wax n alkanes from odp site 1208 and site 886 in the north pacific since the middle miocene were analyzed to explore long term changes in vegetation and climate in the source region accumulation rates of leaf wax n alkanes show an increasing trend consistent with the documented climatic drying of the asian interior since the late miocene the records of carbon isotopic enrichment factors of c29 n alkane relative to atmospheric co2 ec29 co2 show a prominent decrease from 12 to 8 ma the average ec29 co2 value prior to 8 ma is 0 8 heavier than after 8 ma although almost all values of ec29 co2 25 3 to 21 3 are well within the range of c3 plants adjustment of isotope discrimination by c3 plants is not considered as the main cause of the observed variations instead changes in relative abundance of c3 vs c4 plants are invoked to interpret the ec29 co2 records higher c4 contribution 17 7 5 3 to the local vegetation is inferred for the period prior to 8 ma implying a slightly warmer climate in the source region a marked decline in c4 plants from 12 to 8 ma interpreted as a result of regional temperature drop coincides with the prominent growth of northern tibetan plateau around 8 ma along with the global cooling climate our results therefore point to apparently close links among plateau uplift development of drying and cooling climates and vegetation changes in the asian interior

trench proximal volcanism following ridge crest trench collision along the antarctic peninsula on the basis of multichannel seismic reflection magnetics and bathymetry data a pleistocene holocene 0 1ma volcanic seamount has been identified at a location very close to the shelf edge off the west coast of the antarctic peninsula the seamount overlies the subducted trace of a transform fracture zone which divides segments of the margin along which collision took place 3 1 and 6 0 ma ago new trace element and isotopic data from dredged samples demonstrate that the seamount appears to be predominantly formed of relatively primitive undersaturated alkali basalt the basalts have geochemical signatures that are indistinguishable from ocean island basalts and some continental alkali basalts e g th ta 1 0 1 5 rb nb 0 5 87sr 86sr 0 70265 0 7028 143nd 144nd approximately 0 5129 and are totally lacking in any evidence for interaction with subduction enriched mantle geochemical similarities with other more trench distal postsubduction alkalic basalts along the antarctic peninsula are striking strongly implying that all the postsubduction basalts were derived from a chemically similar asthenospheric source region the basalts were most likely to have been generated as a result of the formation of a slab window beneath the antarctic peninsula following ridge crest trench collision subduction component free subslab asthenosphere upwelled into the incipient void left by the continued sinking of the leading plate following collision and decompressional melting resulted this type of trench proximal volcanism following ridge crest trench collision differs from that in other locations where calc alkaline volcanism persisted or ophiolite obduction occurred

variscan extension in the pyrenees evidence for variscan extension in the pyrenees is summarized and used with other geological and geochronological data to reconstruct the variscan extensional geometry and kinematics at a crustal scale a regionally consistent pattern of ese wnw trending synmetamorphic stretching directions indicates variscan extension roughly parallel to the present alpine trend of the belt the spatial distribution of variscan low pressure high temperature metamorphic domes is interpreted as being possibly due to the activity of few moderately dipping crustal scale shear zones allowing heterogeneous ductile extension of the variscan middle and lower crust extension occurred synchronous with uplift and emergence of large parts of the crust and deposition during the upper carboniferous and permian of up to 2500 m of continental sediments and intercalated basaltic andesitic volcanics in fault bounded extensional halfgrabens the extensional structures and related metamorphism postdate a namurian to early westphalian stage of crustal thickening characterized by thrusting and steep folding it is suggested that variscan extension in the pyrenees was driven by the gravitational potential energy of an initially thickened lithosphere enhanced by detachment of a cold and gravitationally unstable lithospheric root which caused progressive uplift tectonic denudation and a highly anomalous thermal structure of the variscan crust leading to extensive lower crustal melting and granodioritic intrusion during the early permian

pressure temperature time paths and a tectonic model for the evolution of granulites petrologic studies and application of well calibrated mineralogic thermometers and barometers reveal several important features common to many granulite terrains 1 peak metamorphic pressure and temperature conditions cluster around values of 7 5 pm 1 kbar and 800 pm 50 c implying average geothermal gradients of 30 35 c km 2 in comparison with upper amphibolite facies rocks especially in paired amphibolite granulite terrains it is evident that granulites are distinguished from amphibolites by the former s higher metamorphic temperatures but not higher pressure 3 initial cooling of granulite terrains from peak conditions is nearly isobaric for garnet closure temperatures of 600 pm 50 c the dp dt retrograde paths inferred from compositionally zoned garnet rims are 2 8 b c in most granulites 4 primary coarse sillimanite is found in most granulites and retrograde kyanite is known in a few these data suggest that granulites form as a result of anomalous thermal gradients

early paleozoic to devonian multiple accretionary model for the qilian shan nw china abstract the qilian shan located in the northern tibetan plateau of nw china has an excellent record of early to middle paleozoic subduction accretion that resulted from convergence between the alax and qaidam blocks but there is no consensus about its detailed tectonic history this paper summarizes its tectonic divisions and discusses its tectonic evolution from the cambrian to devonian the belt has the following tectonic divisions in the far north the southern passive margin of the alax block is juxtaposed against an early cambrian to ordovician marianan type intra oceanic arc north qilian the north qilian arc is separated from a japanese type arc central qilian to the south by a high pressure metamorphic belt composed mostly of ophiolitic slices and oceanic crusts the central qilian is bounded to the south by a wide melange zone south qilian consisting of ophiolitic slices and continental margin sequences farther to the south lies the oulongbuluk microcontinent that is separated from the qaidam block and farther south by an ultrahigh pressure metamorphic belt tectonostratigraphic analysis together with geochemical geochronological and geophysical data indicates a complex evolution by subduction accretion processes from the cambrian to the devonian before final amalgamation and docking to the northern alax block this model solves the long lasting discussion on the polarity of subduction in paleozoic time this multiple subduction accretion history sheds light on the continuity of paleozoic sutures along the qilian shan and the nature of the altyn tagh fault and thus contributes to an improved understanding of the tectonic architecture of central asia

productivity and paleoclimatic controls on source rock character in the aman trough north central sumatra indonesia abstract the central sumatra basin csb is a prolific oil producing basin in indonesia where a single source rock has been identified the brown shale formation of the paleogene pematang group despite the presence of a single source rock the oils in the central sumatra basin and particularly in the aman trough exhibit variable molecular and isotopic compositions previously described as reflective of lateral facies variations within the source rock analyses by gas chromatography gc gas chromatography mass spectrometry gc ms and gas chromatography isotope ratio mass spectrometry gc irms of 15 oil samples from the aman trough allowed identification of vertical and lateral facies changes within the brown shale formation as well as paleoproductivity and paleoclimatic changes that took place during deposition of this prolific source rock the molecular and isotopic data indicate the source rock for these oils was deposited in a fresh to brackish water stratified lake in which co 2 limiting conditions prevailed compound specific isotopic data also indicate that changes in paleoclimatic conditions possibly associated with the eocene oligocene paleoclimatic transition had an effect on the geochemical composition of the source rock in the aman trough

using spiral single pollen isotope ratio analysis to estimate c3 and c4 grass abundance in the paleorecord abstract c 3 and c 4 grasses differ greatly in their responses to environmental controls and influences on biogeochemical processes e g water carbon and nutrient cycling difficulties in distinguishing between these two functional groups of grasses have hindered paleoecological studies of grass dominated ecosystems stable carbon isotopic analysis of individual grains of grass pollen using a spooling wire microcombustion device interfaced with an isotope ratio mass spectrometer holds promise for improving c 3 and c 4 grass reconstructions this technique spiral single pollen isotope ratio analysis has only been evaluated using pollen of known c 3 and c 4 grasses to test the ability of spiral to reproduce the abundance of c 3 and c 4 grasses on the landscape we measured 13 c values of 1500 individual grains of grass pollen isolated from the surface sediments of ten lakes in areas that span a large gradient of c 3 and c 4 grass abundance as determined from vegetation surveys results indicate a strong positive correlation between the 13 c based estimates of c 4 grass pollen and the abundance of c 4 grasses on the landscape the c 4 grass pollen slightly underestimates the actual abundance of c 4 grasses at sites with high proportions of c 4 grasses which can be corrected using regression analysis comparison of the c 4 grass pollen with c n and 13 c measurements of bulk organic matter illustrates the distinct advantages of grass pollen 13 c as a proxy for distinguishing c 3 and c 4 shifts within the grass family thus spiral promises to advance our understanding of grassland ecology and evolution

hot orogens tectonic switching and creation of continental crust comment and reply comment in a stimulating paper collins 2002 1 p 535 asserted that many granulite terrains were too hot to have formed during continental collision in spite of evidence to the contrary e g harley 1989 2 o brien and rotzler 2003 3 based on this assertion collins 2002 1 p

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

paired metamorphic belts revisited abstract the modern plate tectonics regime is characterized by a duality of thermal environments one representing the subduction zone and the other representing the arc backarc or orogenic hinterland this duality is the hallmark of one sided asymmetric subduction and the characteristic imprint of one sided subduction in the geological record is predicted to be the broadly contemporaneous occurrence of two contrasting types of metamorphic belt one of high dt dp type and the other low dt dp type the broadly contemporaneous occurrence of granulite and ultrahigh temperature metamorphism with eclogite high pressure granulite metamorphism in the geological record since the neoarchean era is evidence of dual thermal environments and indicates that subduction has operated on earth since that time classic paired metamorphic belts in which an inboard high dt dp metamorphic belt is juxtaposed against an outboard low dt dp metamorphic belt along a tectonic contact such as the ryoke and sanbagawa belts in japan are found in phanerozoic accretionary orogens of the circum pacific generally they appear to result from juxtaposition of terranes with different metamorphic facies series that may or may not be exactly contemporaneous and that may or may not be far traveled this is a consequence of the difference between globally continuous subduction generating a low to intermediate dt dp environment in the subduction zone and a high dt dp environment in the arc backarc system and metamorphic imprints in the geological record that represent discrete events due to changes in plate kinematics or subduction boundary dynamics or as a result of collision of ridges arcs or continents with the upper plate at the trench the concept of paired metamorphic belts may be generalized and extended more widely than in the original proposition to subduction to collision orogenic systems in addition to accretionary orogenic systems in this wider application the term paired metamorphic belts may be used for penecontemporaneous belts of contrasting type of metamorphism that record different apparent thermal gradients one warmer and the other colder juxtaposed by plate tectonics processes brown 2009 this extends the original concept of miyashiro 1961 beyond the simple pairing of high dt dp and low dt dp metamorphic belts in circum pacific accretionary orogens and makes it more useful in the context of our better understanding of the relationship between thermal regimes and tectonic settings this is particularly useful in subduction to collision orogenic systems where the suture and lower plate materials will register the imprint of low to intermediate dt dp and the upper plate will register penecontemporaneous high dt dp metamorphism commonly manifested at shallow crustal levels by the occurrence of granites in the rock record

high temperature clockwise p t paths and melting in the development of regional migmatites an example from southern brittany france the southern brittany migmatite belt sbmb which evolved through the metamorphic peak between c 400 ma and c 370 ma ago consists of a heterogeneous suite of high grade gneisses and anatectic migmatites both metatexites and diatexites rare garnet cordierite gneiss layers record evidence of an early prograde p t path in these rocks growth zoned garnet cores and a sequence of included mineral assemblages in garnet from core to rim of qtz ilm ky pl ky st rt bt and pl sil st rt bt constrain a prograde evolution during which the reactions ilm ky qtz aim rt ms chl st bt qtz v and st qtz grt sil v were crossed parts of this prograde evolution are preserved as inclusion assemblages in garnet in all other rock types in all rock types garnet has reverse zoned rims and garnet replacement by cordierite and or biotite and plagioclase suggests the following reactions have occurred grt sil qtz crd hc ilm bt sil qtz crd hc ilm kfs v and na ca k ti grt bt pl qtz microstructural analysis of reaction textures in conjunction with a petrogenetic grid has enabled the construction of a tightly constrained clockwise p t path for the sbmb the high temperature part of the path has a steep dt dp slope characteristic of near isothermal decompression it is proposed that the p t path followed by the sbmb is the result of the inversion by overthrusting of a back arc basin and that such a tectonic setting may be applicable to other high temperature migmatite terranes the near isothermal decompression is at least partly driven by the upward diapiric movement of the diatexite anatectic granite core of the sbmb

zircon u pb ages and hf isotopic analyses of migmatite from the paired metamorphic belt in chinese sw tianshan constraints on partial melting associated with orogeny abstract a paired metamorphic belt with ubiquitous migmatites and rare granulites in its high dt dp part formed due to oceanic subduction in chinese southwestern sw tianshan although several geochronological studies have addressed it in recent years the exact timing of the high grade metamorphism and partial melting is still controversial for this study we selected nine samples obtained from three valleys transecting the sw tianshan migmatite complex for u pb geochronology and lu hf isotope analysis to provide constraints on partial melting and the mid crustal evolution during oceanic subduction and subsequent continental collision on the basis of internal morphology th u ratios and ree patterns three types of zircons are discriminated dating them reveals five age groups 1 inherited zircons either are detrital with ages scattering from 432 ma to 3261 ma or yield ages concentrated at 406 410 ma reflecting early devonian magmatism 2 recrystallized zircon has been transformed from pre existing zircon under subsolidus conditions at 400 ma and 360 ma 3 metamorphic rims have grown on zircons at 400 ma 290 ma and 270 ma corresponding to three phases of anatexis the partial melting phase at 400 ma is probably related to the emplacement of voluminous mafic to felsic magmas in a continental arc region above the subduction zone consuming the south tianshan paleo ocean that triggered the most extensive regional scale thermal event however the two subsequent partial melting phases at 290 ma and 270 ma representing post collisional stages were not penetrative p t conditions of the studied migmatites estimated by hb pl thermobarometry and ti in zircon thermometry are 682 763 c 4 7 8 0 kbar together with composite kfs q pl kfs q and kfs q ab inclusions representing entrapped melt and kfs inclusions with ab exsolution lamellae in zircon rims the p t estimates support the occurrence of anatexis zircon lu hf isotope analysis indicates that zircon rims formed under high grade metamorphic conditions and during partial melting by dissolution precipitation of pre existing zircons in a close system zircon two stage hf model ages constrain the development of the protoliths of the migmatites at four major crustal formation periods 1871 ma 1368 1426 ma 1099 1276 ma and 743 ma combined with previous researches in this area we conclude that the main crustal formation of the yili and central tianshan plates has mostly occurred in proterozoic times but not or only subordinately during the archean

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

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

spatiotemporal variation in the origin of c4 grasses δ13c analysis of grass pollen from the southeastern united states abstract the c4 plant functional type is a major evolutionary and ecological success however the relative importance of environmental factors such as climatic conditions and atmospheric co2 concentrations in driving the origin of c4 grasses in different parts of the world remains poorly understood we determined the carbon isotope composition of 612 individual grains of grass pollen in seven sediment samples from the southeastern united states from the middle late eocene by applying a bayesian model to these data we determined the probability of rejecting at p