petrology and geochronology of paleoproterozoic garnet bearing amphibolites from the dunhuang block eastern tarim craton abstract the dunhuang block located in the eastern margin of the tarim craton possesses a wealth of information about the geological evolution of the tarim craton in this study we conducted detailed petrological geochemical and geochronological investigations on garnet bearing amphibolites as well as granites in the dunhuang block which record important metamorphic and magmatic events that are poorly constrained based on field occurrence geochronological and geochemical characteristics the garnet bearing amphibolites can be divided into two groups group i garnet bearing amphibolite displays typical features of island arc basalts which may formed at 1 83 ga in a collisional setting during the assembly of the columbia supercontinent whereas group ii has geochemical characteristics of ocean island basalt which may formed at 1 61 ga in an ocean island setting the ca 1 83 ga amphibolite has positive hf t values 4 4 to 8 9 with tdm1 model ages of 2 02 1 83 ga indicating an important addition of juvenile mantle derived materials during the formation of its protolith the ca 1 61 ga amphibolite has hf t values ranging from 0 6 to 5 6 and corresponding tdm1 model ages varying from 2 03 ga to 1 84 ga which suggest that juvenile mantle derived materials were dominantly contributed to the formation of its protolith but were later contaminated by crustal materials the u pb ages obtained from the rims of the zircon grains in amphibolites and granites recorded a tectonothermal event at ca 400 ma which is coeval with the widespread magmatism in the beishan orogen we thus propose that these two groups of garnet bearing amphibolites were possibly not related to each other during paleoproterozoic time and a late paleozoic orogenic event brought them together during which both of them underwent metamorphism

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

late ordovician to early devonian adakites and nb enriched basalts in the liuyuan area beishan nw china implications for early paleozoic slab melting and crustal growth in the southern altaids abstract we report newly defined nb enriched basalts adakites and dacites from the beishan nw china of the southern altaids based on field geochemical isotopic and geochronology studies two phases of adakites adakite i and adakite ii have been defined which are calc alkaline and characterized by high na 2 o k 2 o ratios 1 49 1 71 and 2 32 3 64 and sr contents 494 1213 ppm and 325 494 ppm negligible to positive eu anomalies strong depletion of hree e g yb 0 48 0 93 ppm and 0 50 0 99 ppm and y 6 87 9 80 ppm and 6 02 10 30 ppm and enriched in rb sr ba k and depleted nb and ti they are characterized by relatively low e nd t values 0 8 to 0 9 and 0 6 to 3 8 and relatively constant high 87 sr 86 sr i ratios 0 70635 0 70636 and 0 70583 0 70651 the zircons of adakite i have relatively low e hf t 0 8 to 2 7 the nb enriched basalts are sodium rich n 2 o k 2 o 1 31 4 44 with higher tio 2 p 2 o 5 zr and nb contents and nb th pm nb la pm and nb u ratios than typical arc basalts they are relatively enriched in rb ba u pb and k depleted in nb and minor negative to positive ba zr sr and ti they have low positive e nd t 0 9 to 2 3 and relatively high 87 sr 86 sr i 0 70556 0 70691 ratios the dacites are typical arc magmas with moderately enriched lile distinctly negative eu nb sr and ti anomalies they have positive e nd t 2 2 and relatively high 87 sr 86 sr i 0 70786 we argue that the liuyuan adakites were most probably related to the melting of young hot subducted crust of the paleo asian ocean which included tectonically subducted radiogenic crustal material and or inheritance from highly radiogenic oceanic crust e g oib the nb enriched basalts likely resulted from mantle wedge peridotites metasomatized by adakites and or further changed by components other than adakites e g minor slab derived fluids based on own zircon sims u pb dating of these key rock types we further propose that from the late ordovician to early devonian large volumes of magma consisting of late ordovician nb enriched basalts 451 ma and dacites 442 ma late silurian adakite i 424 ma early devonian adakite ii 374 ma and i s a type granites 436 ma 380 ma developed in the southern altaids together with other geochronological data from the literature we conclude that subducted oceanic slab melting was frequent from 470 ma to 370 ma our results suggest that frequent hot and or young oceanic crustal subduction and slab melting were important mechanisms in the accretionary growth of the southern altaids

neogene vegetation of the meyer desert formation sirius group transantarctic mountains antarctica a tundra vegetation consisting of at least 18 plant species is described from the meyer desert formation which outcrops along the beardmore glacier in the transantarctic mountains about 500 km from the south pole the fossils include pollen seeds fruits flowers leaves wood and in situ plants of which wood and leaves of nothofagus and a pollen assemblage had been previously reported the plants include a cryptogamic flora of mosses and liverworts conifers and angiosperms in the families gramineae cyperaceae nothofagaceae ranunculaceae hippuridaceae caryophyllaceae and chenopodiaceae or myrtaceae the plants grew in a weakly developed soil formed within a complex periglacial environment that included moraines glacial outwash streams well drained gravel ridges and poorly drained depressions in which peat and marl were being deposited d 2004 elsevier b v all rights reserved

formation of patterned ground and sublimation till over miocene glacier ice in beacon valley southern victoria land antarctica a thin glacial diamicton informally termed granite drift occupies the floor of central beacon valley in southern victoria land antarctica this drift is 40 ar 39 ar analyses of presumed in situ ash fall deposits that occur within granite drift at odds with the great age of this ice are high centered polygons that cut granite drift if polygon development has reworked and retransported ash fall deposits then they are untenable as chronostratigraphic markers and cannot be used to place a minimum age on the underlying glacier ice our results show that the surface of granite drift is stable at polygon centers and that enclosed ash fall deposits can be used to define the age of underlying glacier ice in our model for patterned ground development active regions lie only above polygon troughs where enhanced sublimation of underlying ice outlines high centered polygons the rate of sublimation is influenced by the development of porous gravel and cobble lag deposits that form above thermal contraction cracks in the underlying ice a negative feedback associated with the development of secondary ice lenses at the base of polygon troughs prevents runaway ice loss secondary ice lenses contrast markedly with glacial ice by lying on a d versus 18 o slope of 5 rather than a precipitation slope of 8 and by possessing a strongly negative deuterium excess the latter indicates that secondary ice lenses likely formed by melting downward percolation and subsequent refreezing of snow trapped preferentially in deep polygon troughs the internal stratigraphy of granite drift is related to the formation of surface polygons and surrounding troughs the drift is composed of two facies a nonweathered matrix supported diamicton that contains 25 striated clasts in the 16 mm fraction and a weathered clast supported diamicton with varnished and wind faceted gravels and cobbles the weathered facies is a coarse grained lag of granite drift that occurs at the base of polygon troughs and in lenses within the nonweathered facies the concentration of cosmogenic 3 he in dolerite cobbles from two profiles through the nonweathered drift facies exhibits steadily decreasing values and shows the drift to have formed by sublimation of underlying ice these profile patterns and the 3 he surface exposure ages of 1 18 0 08 ma and 0 18 0 01 ma atop these profiles indicate that churning of clasts by cryoturbation has not occurred at these sites in at least the past 10 5 and 10 6 yr although granite drift is stable at polygon centers low frequency slump events occur at the margin of active polygons slumping together with weathering of surface clasts creates the large range of cosmogenic nuclide surface exposure ages observed for granite drift maximum rates of sublimation near active thermal contraction cracks calculated by using the two 3 he depth profiles range from 5 m m y to 90 m m y sublimation rates are likely highest immediately following major slump events and decrease thereafter to values well below our maximum estimates nevertheless these rates are orders of magnitude lower than those computed on theoretical grounds during eruptions of the nearby mcmurdo group volcanic centers ash fall debris collects at the surface of granite drift either in open thermal contraction cracks or in deep troughs that lie above contraction cracks these deposits subsequently lower passively as the underlying glacier ice sublimes the fact that some regions of granite drift have escaped modification by patterned ground for at least 8 1 ma indicates long term geomorphic stability of individual polygons once established polygon toughs likely persist for as long as 10 5 10 6 yr our model of patterned ground formation which applies to the hyperarid cold desert polar climate of antarctica may also apply to similar sized polygons on mars that occur over buried ice in utopia planitia

pleistocene deglaciation chronology of the amery oasis and radok lake northern prince charles mountains antarctica abstract the east antarctic ice sheet is the largest ice mass on earth with a capacity to raise global sea level by up to 65 m as the lambert glacier amery ice shelf drainage system is the largest to reach the coast of antarctica quantifying its evolution over the quaternary is a vital component in developing an understanding of the antarctic response to future climate change here we present a deglaciation chronology based on 10 be and 26 al in situ cosmogenic exposure ages of the northern prince charles mountains which flank the lambert glacier amery system and that records the progressive emergence of mcleod massif and radok lake basin from beneath the mac robertson land lobe of the east antarctic ice sheet the exposure ages monotonically decrease with both decreasing altitude and increasing proximity to the amery ice shelf at the antarctic coast exposure ages from the crests of mcleod massif near the edge the amery ice shelf and from fisher massif 75 km further inland each at 1200 m above sea level are 2 2 0 3 and 1 9 0 2 ma respectively suggesting their continuous exposure above the ice sheet at least since close to the plio pleistocene boundary an extensive plateau at 800 m altitude on mcleod massif above battye glacier records the massif s increased emergence above the ice sheet surface at about between 880 and 930 ka ago indicating 400 m of ice volume reduction in the mid pleistocene correcting these apparent ages for a reasonable choice in erosion rate would extend this event to 1 15 ma a period identified from prydz bay odp core 1167 when sedimentation composition alters and rates decrease 10 fold exposure ages from boulder mantled erosional surfaces above and beyond the northern end of radok lake at 220 m range from 28 to 121 ka independent of choice of model interpretation to explain this age spread the most recent major reoccupation of radok lake by battye glacier ice occurred during the last glacial cycle moraine ridges at the lower altitude of 70 125 m were deposited during the final withdrawal of battye glacier ice from the lake basin between 11 and 20 ka ago this new chronology indicates that the highest amery oasis peaks have not been overridden by the mac robertson land lobe of the east antarctic ice sheet for at least the past 2 ma since this time we document 3 major periods of regional reduction in ice sheet volume at 1 1 ma during the last glacial cycle 120 to 30 ka and through the last glacial maximum 20 to 10 ka that resulted in an overall 1000 m of ice lowering in the battye glacier radok lake region

fragment of an ancient outlet glacier system near the top of the transantarctic mountains sirius group tillite near the summit of mount feather transantarctic mountains is critically located for reconstructing past behavior of the east antarctic ice sheet new ice direction indicators and tillite geometry show that the tillite accumulated beneath wet based ice on a transverse paleovalley floor a hanging remnant of a landscape largely removed by erosion we conclude that the mount feather tillite formed primarily by lodgment as part of an ancient wet based outlet glacier system when the transantarctic mountains were at least 1500 m lower than today till deposition took place before 20 ma on the basis of estimated past uplift rates

multiple cosmogenic nuclides document complex pleistocene exposure history of glacial drifts in terra nova bay northern victoria land antarctica geomorphological and glacial geological surveys and multiple cosmogenic nuclide analyses 10be 26al and 21ne allowed us to reconstruct the chronology of variations prior to the last glacial maximum of the east antarctic ice sheet eais and valley glaciers in the terra nova bay region glacially scoured coastal piedmonts with round topped mountains occur below the highest local erosional trimline they represent relict landscape features eroded by extensive ice overriding the whole coastal area before at least 6 ma pre dating the build up of the mt melbourne volcanic field since then summit surfaces were continuously exposed and well preserved under polar condition with negligible erosion rates on the order of 17 cm ma complex older drifts rest on deglaciated areas above the younger late pleistocene glacial drift and below the previously overridden summits the combination of stable and radionuclide isotopes documents complex exposure histories with substantial periods of burial combined with minimal erosion the areas below rounded summits were repeatedly exposed and buried by ice from local and outlet glaciers the exposure ages of the older drift s indicate multiple pleistocene glacial cycles which did not significantly modify the pre existing landscape

origin of high potassium talc alkaline i type granitoids many i type granitoid magmas are generated through partial melting of older metaigneous rocks and the compositions of such melts are broadly calc alkaline and metaluminous these melts are granitic to tonalitic and result from thermal extremes in their lower crustal source regions data on the experimental partial melting of common crustal rocks suggest that high k i type granitoid magmas can be derived only from the partial melting of hydrous calc alkaline to high k calc alkaline mafic to intermediate metamorphic rocks in the crust because of their low k2o contents metabasaltic rocks of all kinds are unsuitable as sources and models that propose mixing of mantle derived basaltic magmas and crustal melts are also inadequate there is no requirement that i type calc alkaline magmatism be related in any way to subduction processes

end permian to mid triassic termination of the accretionary processes of the southern altaids implications for the geodynamic evolution phanerozoic continental growth and metallogeny of central asia the altaids is one of the largest accretionary orogenic collages in the world with the highest rate of phanerozoic continental growth and significant metallogenic importance it is widely accepted that subduction related orogenesis of the altaids started in the late precambrian and gradually migrated southward present coordinates however it is uncertain when and how the building of the altaids was finally completed based on structural geology geochemical geochronological and paleomagnetic data this paper presents late paleozoic to early mesozoic accretionary tectonics of two key areas north xinjiang in the west and inner mongolia in the east together with neighboring mongolia the late paleozoic tectonics of north xinjiang and adjacent areas were characterized by continuous southward accretion along the wide southern active margin of siberia and its final amalgamation with the passive margin of tarim which may have lasted to the end permian to early mid triassic in contrast in inner mongolia and adjacent areas two wide accretionary wedges developed along the southern active margin of siberia and the northern active margin of the north china craton which may have lasted to the mid triassic the final products of the long lived accretionary processes in the southern altaids include late paleozoic to permian arcs late paleozoic to mid triassic accretionary wedges composed of radiolarian cherts pillow lavas and ophiolitic fragments and high pressure ultrahigh pressure metamorphic rocks permian alaskan type zoned mafic ultramafic complexes intruded along some major faults of the tien shan we define a new tarim suture zone immediately north of the tarim craton that is probably now buried below the tien shan as a result of northward subduction of the tarim block in the cenozoic the docking of the tarim and north china cratons against the southern active margin of siberia in the end permian to mid triassic resulted in the final closure of the paleoasian ocean and terminated the accretionary orogenesis of the southern altaids in this part of central asia this complex geodynamic evolution led to formation of giant metal deposits in central asia and to substantial continental growth

history of ice sheet elevation in east antarctica paleoclimatic implications abstract the multi disciplinary study of past ice surface elevations in the grove mountains of interior east antarctica provides direct land based data on the behavior of the east antarctic ice sheet since the pliocene the glacial geology the ages of cold desert soils the depositional environment of younger moraine sedimentary boulders and their spore pollen assemblages combine to imply a possible significant shrinkage of the ice sheet before the middle pliocene epoch with the ice sheet margin retreating south of the grove mountains 450 km south from its present coastal position exposure age measurements of bedrock indicate that the elevation of the ice surface in the grove mountains region subsequently rose at about mid pliocene to at least 200 m higher than today s levels the ice surface then progressively lowered with some minor fluctuations middle to late pleistocene exposure ages found on the lowest samples at the ice bedrock contact line indicate a long period with ice surface elevations kept at the current level or complex fluctuation history during the quaternary epoch

antarctic climate during the middle pliocene model sensitivity to ice sheet variation significant controversy exists surrounding the nature of the antarctic ice sheet during the period of middle pliocene warmth this paper outlines the output from three sensitivity experiments for the middle pliocene using the ukmo gcm and the usgs prism2 data set different east antarctic ice sheet configurations were prescribed in each modelling experiment conforming to a dynamic sea level 35 m higher than at present intermediate pliocene control sea level 25 m higher than at present and stable sea level 12 15 in higher than at present scenario for the nature of the antarctic ice sheet during the middle pliocene all three scenarios would have provided middle pliocene sea levels across the spectrum of estimates provided by known geological data model outputs indicate that surface temperature increases are greatest in the pliocene dynamic experiment climatic conditions for the pliocene control and stable experiments remain harsh over antarctica with surface temperatures rising above freezing mean 4degreesc for a maximum of 2 months per year in contrast mean surface temperatures from the pliocene dynamic experiment rise above freezing mean 10degreesc for 4 months of the year climatic outputs from all of the pliocene experiments were used in a biome model biome 4 for the pliocene control and stable experiments biome 4 predicted the occurrence of exclusively tundra type vegetation on antarctica for the dynamic experiment climatic conditions ameliorated sufficiently to allow biome 4 to predict deciduous taiga montane forests on antarctica thus the results from the dynamic experiment are consistent with reported pliocene nothofagus occurrences on antarctica in the future more sophisticated palaeoclimate modelling studies need to employ atmospheric general circulation models gcms coupled to dynamic ocean and ice sheet models this will allow the gcm to predict rather than prescribe an antarctic ice sheet configuration in equilibrium with reconstructed middle pliocene sea surface temperatures such an approach may also shed light on how the antarctic ice sheet responded to climate variability during the middle pliocene c 2002 elsevier science b v all rights reserved

pliocene pleistocene diatoms in paleozoic and mesozoic sedimentary and igneous rocks from antarctica a sirius problem solved there are two competing scenarios on the behavior of the east antarctic ice sheet during the late tertiary in one scenario the ice sheet was very dynamic and underwent major drawdown and renewal as late as the pliocene in the other the ice sheet was relatively stable during the late neogene the presence of marine diatoms in sirius group sedimentary rocks in east antarctica is at the center of the disagreement one side regards the diatoms as the major piece of evidence to support the drawdown and renewal hypothesis and infers that they were introduced into the sirius during renewed glaciation of east antarctica others suggest that these diatoms were likely introduced into the sirius by atmospheric largely eolian processes we propose a simple test of the eolian hypothesis if diatoms were introduced into the sirius by eolian processes then they should also be present in older paleozoic and mesozoic sedimentary and igneous rocks samples from two units of the beacon supergroup devonian to jurassic from beacon valley east antarctica were analyzed the beacon heights orthoquartzite devonian and the feather conglomerate permian triassic also examined was sediment found in cracks of paleozoic and mesozoic devonian to cretaceous igneous rocks from marie byrd land west antarctica largely pliocene pleistocene planktonic marine diatoms were found in all sample sets because neither beacon supergroup sedimentary rocks nor igneous rocks from marie byrd land are pliocene pleistocene in age such findings strongly suggest that diatoms were introduced into them by eolian processes this same scenario can be applied to sirius group sedimentary rocks

east antarctic deglaciation and the link to global cooling during the quaternary evidence from glacial geomorphology and 10be surface exposure dating of the sor rondane mountains dronning maud land abstract reconstructing past variability of the antarctic ice sheets is essential to understand their stability and to anticipate their contribution to sea level change as a result of future climate change recent studies have reported a significant decrease in thickness of the east antarctic ice sheet eais during the last several million years however the geographical extent of this decrease and subsequent isostatic rebound remain uncertain in this study we reconstruct the magnitude and timing of ice sheet retreat at the sor rondane mountains in dronning maud land east antarctica based on detailed geomorphological survey cosmogenic exposure dating and glacial isostatic adjustment modeling three distinct deglaciation phases are identified for this sector during the quaternary based on rock weathering and 10 be surface exposure data we estimate that the ice sheet thinned by at least 500 m during the pleistocene this thinning is attributed to the reorganization of southern ocean circulation associated with global cooling into the pleistocene which reduced the transport of moisture from the southern ocean to the interior of eais the data also show that since the last glacial maximum the ice surface has lowered less than ca 50 m and that this lowering probably started after ca 14 ka this suggests that the eais in dronning maud land is unlikely to have been a major contributor to postglacial sea level rise and meltwater pulse 1a

a case for sirius group alpine glaciation at mount fleming south victoria land antarctica a case against pliocene east antarctic ice sheet reduction the sirius group comprises a suite of consolidated glacial deposits that are widespread in the transantarctic mountains some of the deposits contain pliocene marine diatoms because of the interpretation that the sirius group tills and the marine diatoms were deposited by the east antarctic ice sheet it has been inferred that the east antarctic ice sheet was much reduced in area and volume during the pliocene these interpretations were evaluated by studying sirius group lodgment tills on mount fleming in the dry valleys sector of the transantarctic mountains we infer that the sirius group lodgment tills at mount fleming do not support the hypothesis that the east antarctic ice sheet was much reduced during the pliocene we demonstrate that sirius group lodgment tills on mount fleming were deposited by alpine ice this conclusion is founded on ice flow directions inferred from rattails gravel fabric and the orientation of bullet boulders as well as on sediment and gravel characteristics of the tills only a very few unidentifiable diatom fragments were recovered better preserved identifiable pliocene marine diatoms occur in moderate abundances in a 5 10 cm thick unconsolidated and discontinuous drift that caps the lodgment tills on the basis of the decline in diatom preservation with depth into the till we dismiss transport of the better preserved diatoms by the ice that deposited the underlying sirius group lodgment till instead we attribute diatom presence to eolian deposition and recycling into lodgment tills at a later stage

sequence stratigraphy and onlap history of the donets basin ukraine insight into carboniferous icehouse dynamics abstract the degree to which permo carboniferous cyclothemic successions archive evidence for long term variations in ice volume during the late paleozoic ice age is insufficiently resolved here we develop the sequence stratigraphy and onlap offlap history for a 33 my interval of the carboniferous using the u pb calibrated succession of the donets basin ukraine in order to assess the relationship between sea level high latitude changes in glacial extent and climate integrated subsurface and outcrop data permit meter scale correlation of 242 biostratigraphically constrained limestones and coals and in turn individual cyclothems across 250 km of the donets basin rapid uniform subsidence and basinwide continuity of marker beds indicate pennsylvanian deposition under relatively stable tectonic conditions three scales of sequences avg durations of 140 ky 480 ky and 1 6 my are recognized on the basis of stratigraphic stacking patterns and basinwide architecture of marine to terrestrial facies assemblages the hierarchy of sequences and the geographic and stratigraphic positions of shifts in base level sensitive facies across the donets ramp permit the construction of an onlap offlap history at a sub 400 ky scale major sea level lowstands occur across the mid carboniferous boundary and during the early moscovian these lowstands coincide with glacial maxima inferred from high latitude glacigenic deposits the middle to late pennsylvanian is characterized by a stepwise onlap culminating in an earliest gzhelian highstand suggesting contraction of carboniferous ice sheets prior to the initiation of early permian glaciation the stratigraphic position of climate sensitive facies within individual donets cyclothems indicates a turnover from seasonal sub humid or semi arid climate to everwet conditions during the late lowstand and maximum ice sheet accumulation comparison of the stratigraphic and aerial distribution of coals and evaporites in the donets basin with the onlap offlap history further indicates everwet conditions during lowstands and inferred glacial maxima and drier climate during onlap and inferred ice sheet contraction at the intermediate 0 8 to 1 6 my and long 106 yr time scales taken together the relationship between inferred climate and glacioeustasy suggests a likely teleconnection between high latitude ice sheet behavior and low latitude atmospheric dynamics

middle pliocene sea surface temperatures a global reconstruction identification and analyses of pliocene marine microfossils from 64 globally distributed stratigraphic sequences have been used to produce a middle pliocene sea surface temperature reconstruction of the earth this reconstruction shows little or no change from current conditions in low latitude regions and significant warming of the ocean surface at mid and higher latitudes of both hemispheres this pattern of warming is consistent with terrestrial records and suggests a combination of enhanced meridional ocean heat transport and enhanced greenhouse effect were responsible for the middle pliocene warmth

paleozoic multiple accretionary and collisional processes of the beishan orogenic collage in the paper by xiao and others american journal of science v 310 p 1553 1594 1 please substitute figure 16 reproduced here for figure 16 that appears between pages 1573 and 1574 the original figure 16 2 was printed at an improper size figure 3 1 lookup volpage 310

the latest neoarchean paleoproterozoic evolution of the dunhuang block eastern tarim craton northwestern china evidence from zircon u pb dating and hf isotopic analyses abstract the dunhuang block in the easternmost segment of the tarim craton exposes precambrian mafic granulite and felsic gneiss to better understand the poorly known formation and evolution of the tarim craton we carried out detailed field based petrological and geochronological investigations on the major lithologies of the dunhuang block u pb dating and hf isotopic analyses on the mafic granulite and felsic gneiss ttg gneiss of the dunhuang block reveal two distinct age populations the latest neoarchaean 2 5 ga and the late palaeoproterozoic 1 82 1 85 ga the zircon magmatic cores and metamorphic rims of tonalitic gneisses yield similar ages of 2 5 ga supporting the existence of archean rocks in the dunhuang block the short time interval between latest neoarchean magmatism and high grade metamorphism suggests that they were related to the same latest neoarchean tectonothermal event the 2 5 ga zircons show hf depleted mantle model ages between 2 55 ga and 2 8 ga with a peak at 2 7 ga suggesting that 2 7 ga juvenile crust of the dunhuang block was reworked during a 200 300 m y period at the end of the neoarchean the age of 1 85 ga obtained from the mafic granulites and felsic gneisses is interpreted as the age of high pressure granulite facies metamorphism combined with the hf isotopic data these data suggest that the dunhuang block experienced 2 7 ga crustal growth a 2 5 ga magmatic metamorphic event and a 1 82 1 85 ga hp granulite facies metamorphic event this sequence of events is very similar to that of the north china craton implying that the evolutionary history of the dunhuang block was consistent with that of the north china craton from the neoarchean to paleoproterozoic

rapid cenozoic glaciation of antarctica induced by declining atmospheric co2 the sudden widespread glaciation of antarctica and the associated shift towards colder temperatures at the eocene oligocene boundary 34 million years ago refs 1 4 is one of the most fundamental reorganizations of global climate known in the geologic record the glaciation of antarctica has hitherto been thought to result from the tectonic opening of southern ocean gateways which enabled the formation of the antarctic circumpolar current and the subsequent thermal isolation of the antarctic continent5 here we simulate the glacial inception and early growth of the east antarctic ice sheet using a general circulation model with coupled components for atmosphere ocean ice sheet and sediment and which incorporates palaeogeography greenhouse gas changing orbital parameters and varying ocean heat transport in our model declining cenozoic co2 first leads to the formation of small highly dynamic ice caps on high antarctic plateaux at a later time a co2 threshold is crossed initiating ice sheet height mass balance feedbacks that cause the ice caps to expand rapidly with large orbital variations eventually coalescing into a continental scale east antarctic ice sheet according to our simulation the opening of southern ocean gateways plays a secondary role in this transition relative to co2 concentration

rapid discharge connects antarctic subglacial lakes a hidden subglacial world consisting of more than 140 lakes lies beneath the antarctic ice sheet it is widely assumed that these lakes are a static ecosystem cut off from each other and the rest of the world far from it according to a 16 month study of surface elevation changes in the region changes indicating a rapid discharge of water from one lake to another were detected and it is possible that the entire subglacial drainage basin is flushed through periodically by massive water transfers observation of surface elevation changes over 16 months in an area of the east antarctic ice sheet overlying subglacial lakes suggests a rapid discharge of one lake to at least two other lakes involving a water volume of 1 8 km3 in conflict with previous expectations that subglacial lakes have long residence times and slow circulations the existence of many subglacial lakes1 provides clear evidence for the widespread presence of water beneath the east antarctic ice sheet but the hydrology beneath this ice mass is poorly understood2 such knowledge is critical to understanding ice flow basal water transfer to the ice margin glacial landform development and subglacial lake habitats here we present ice sheet surface elevation changes in central east antarctica that we interpret to represent rapid discharge from a subglacial lake our observations indicate that during a period of 16 months 1 8 km3 of water was transferred over 290 km to at least two other subglacial lakes while viscous deformation of the ice roof above may moderate discharge the intrinsic instability of such a system3 suggests that discharge events are a common mode of basal drainage4 if large lakes such as lake vostok or lake concordia1 are pressurizing it is possible that substantial discharges could reach the coast5 6 our observations conflict with expectations that subglacial lakes have long residence times and slow circulations2 7 8 and we suggest that entire subglacial drainage basins may be flushed periodically the rapid transfer of water between lakes would result in large scale solute and microbe relocation and drainage system contamination from in situ exploration is therefore a distinct risk

mid late pleistocene glacial evolution in the grove mountains east antarctica constraints from cosmogenic 10be surface exposure dating of glacial erratic cobbles abstract glacial histories from the east antarctic ice sheet eais provide keys to understanding correlations between the eais and global climate they are especially helpful in the assessment of global sea level change and as a means of quantifying the magnitude of past glacial activity and the rate at which ice responded to climate change given the significance of eais glacial histories it is imperative that more glacial chronologic data for this region be obtained especially for the mid to late pleistocene we report cosmogenic 10 be surface exposure dating results from glacially transported cobbles embedded in blue ice moraine material at mount harding the grove mountains eais forty exotic cobbles sampled along two profiles a and b on this blue ice moraine present apparent exposure ages ranging from 7 2 to 542 2 ka we explore this scattered dataset by using principal component analysis pca to identify statistically significant trends in the data we identify a correlation between exposure age and distance of the cobbles from mount harding in profile a cobbles further from mount harding yield older exposure ages than those that are relatively close in profile b cobbles closer to mount harding are found to have relatively older exposure ages in term of glacial history we suggest that the direction of ice flow changed during the period from 60 to 200 ka and that multiple glacial fluctuations occurred in the mid late pleistocene

cenozoic glacigene sedimentation and erosion at the menzies range southern prince charles mountains antarctica the menzies range in the southern prince charles mountains antarctica records at least four intervals of cenozoic terrestrial glacigene sedimentation and two periods of glacial erosion the oldest cenozoic strata here named the pardoe formation are 240 m thick and consist of variable diamicts with subordinate sandstones and minor laminated lacustrine siltstones the pardoe formation overlies a rugged erosion surface cut into precambrian basement two subsequent cenozoic sequences are here named informally the trail diamicts and the younger amphitheatre diamicts the latter infilled the lower regions of an extremely rugged erosion surface many components of which still dominate the present topography the palaeodrainage of this erosion surface is markedly discordant with that of the older erosion surface underlying the pardoe formation these three depositional events and the two associated erosion surfaces record warmer climates and increased snow accumulation under conditions of temperate wet based glaciation during the excavation of the sub amphitheatre diamict erosion surface the east antarctic ice sheet was either absent further inland or the height of its surface relative to the menzies range was considerably lower than at present the fourth and youngest depositional episode recorded by a veneer of boulder gravel distributed along the northern flank of the menzies range is from dry based glacier ice and assumed to be 2 6 myr

geochronology petrogenesis and tectonic implications of triassic granitoids from beishan nw china abstract five triassic granitic plutons have been identified from the beishan orogen by zircon u pb dating that defines two age groups 240 238 ma and 225 217 ma they belong to the high k calc alkaline and shoshonitic series and have alkalic and alkali calcic characteristics and are weakly peraluminous metaluminous these granitoids are mainly highly fractionated i type granite and a type granite they have high i sr values 0 7049 0 7096 and negative e nd t values of 0 9 to 2 8 with nd model ages of 0 89 1 13 ga suggesting a crustal contribution they also exhibit relatively high zircon e hf t values 1 3 to 6 1 and two stage hf model ages of 1 34 0 87 ga suggesting a mixture of mantle and various amounts of crustal components the mantle component probably originated from underplated magmas produced by partial melting of lithospheric mantle metasomatized by fluids or melts possibly derived from pre existing subducted materials the mid triassic 240 238 ma pluton has relatively high sr y 15 9 41 8 ratios and mgo 1 22 3 77 contents with negligibly to weakly negative eu anomalies eu 0 57 0 80 indicating adakite like features this was likely the result of partial melting of crustal components in response to voluminous underplated mantle derived magmas probably linked to lithospheric delamination and asthenospheric upwelling the late triassic 217 225 ma granitic plutons show strong negative anomalies of eu ba nb ta sr p and ti these were likely the result of fractional crystallization and crustal contamination for the granitic magma emplaced in a post orogenic setting

4 ma coarsening of sediments from baikal chinese loess plateau and south china sea and implications for the onset of nh glaciation abstract intense tectonic movement occurred along the northeastern margin of the tibetan plateau 3 4 ma during the same time period global climate changed from a period of stability to a period of frequent and abrupt changes which prevented fluvial and glacial systems from establishing a state of equilibrium thus it is difficult to directly attribute the 3 4 ma coarse grained sediment accumulation in east asia to climate change or tectonic activity this study compares late cenozoic multi proxy records from the chinese loess plateau clp to the low latitude grain size sea level and benthic 18 o records from the south china sea scs and the high latitude grain size record from lake baikal all records suggest a steady and persistent cooling since 4 ma which correlates well with those cooling records from other regions around the world this coincided in timing with the modeling results that the closure of the central american seaway cas initiated strengthening of atlantic meridional overturning circulation between 4 8 and 4 0 ma which led to both warming of the northern hemisphere nh and cooling of the southern hemisphere sh cooling of the sh would induce a marked development of the antarctic ice sheets at 4 ma pushing the intertropical convergence zone itcz northward this was superimposed on warming of the nh and brought more precipitation to the middle latitudes of the nh resulting in increases in coarse grained sediments in the sikouzi section from the western clp since 4 2 ma notably the absence of coarse grained sedimentation in the sikouzi section during 3 0 2 1 ma and the gradual decrease in coarse grained peaks since 2 1 ma reflected by the sikouzi grain size record probably resulted from stepwise increase in the nh ice volume pushing the itcz southward on the other hand development of the antarctic ice sheets would induce global cooling and enhancement of physical weathering initiating increases in sedimentation rates as well as increases in grain size from lake baikal to the clp to the scs therefore the closure of the cas during 4 8 4 0 ma and its influence on ocean heat transport was possibly the major forcing factor for global cooling since 4 ma a persistent and steady cooling during 4 3 ma probably made a significant contribution to the establishment of the nh ice sheets at 2 75 ma ago

middle cambrian to permian subduction related accretionary orogenesis of northern xinjiang nw china implications for the tectonic evolution of central asia abstract middle cambrian to permian subduction related records are widely distributed in northern xinjiang which can be grouped into the chinese altay east junggar eastern tien shan west junggar yili and tarim domains by integrating paleogeographic and geological data we suppose that the chinese altay east junggar eastern tien shan domain was more closely located to siberia while the west junggar and yili domains occupied an intermediate position near the kazakhstan block in the early paleozoic paleoasian ocean distribution of andean type magmatic arcs island arcs accretionary wedges ophiolitic slices and or microcontinents shows an archipelago paleogeography forming a huge accretionary active margin sequences the tarim domain was on the opposite side of the early paleozoic paleoasian ocean remaining passive margin these tectonic units drifted northwards and approached the southern active margin of the siberian craton in the late paleozoic leading to termination of the paleoasian ocean and formation of a complicated orogenic collage between siberian craton and the tarim block between the end permian and triassic these multiple accretion processes significantly contributed to the lateral growth of central asia

geochemical character and petrogenesis of pan african amspoort suite of the boundary igneous complex in the kaoko belt nw namibia abstract in the kaoko belt of northwestern namibia syn tectonic granitoids of the boundary igneous complex intruded a nnw sse trending suture between the neoproterozoic coastal terrane and archaean neoproterozoic congo craton derived foreland typical representatives of this syn collision igneous association are ca 550 ma old k feldspar phyric bt cam granites granodiorites of the amspoort suite with minor cpx gabbro and rare two pyroxene dolerite bodies the scarce opx cpx bt dolerites could come from a chur like mantle source 87sr 86sr550 0 7045 e550nd 0 the rest of the amspoort suite is interpreted as a product of ht anatexis of a heterogeneous lower crust built mainly by immature metapsammites rich in arc derived detritus with minor metabasite and intermediate metaigneous bodies the most likely source appears to be the anatectic coastal terrane gneisses yet partial melting of so far little constrained congo craton cover if formed by immature and youthful detritus unrelated to the basement cannot be discounted in any case the rather primitive sr nd isotopic compositions of the amspoort suite apart from dolerites e550nd 3 4 to 5 3 87sr 86sr550 0 7063 0 711 rule out any major role for the congo craton basement derived material which is in sharp contrast to other more southerly pan african granitoid complexes of the kaoko belt

a deglacial model for antarctica geological constraints and glaciological modelling as a basis for a new model of antarctic glacial isostatic adjustment abstract we present a new reconstruction of the antarctic ice sheets between 20 ka bp and the present day our reconstruction is derived using a numerical model to generate a physically consistent ice surface across the whole of the continent we define the extent of the ice sheet at five time slices 20 15 10 5 and 0 ka bp assuming an equilibrium state for the 20 ka bp time slice and a transient state for the deglacial time slices the evolution of the ice sheet within the numerical model is driven by variations in temperature accumulation rate and relative sea level in order to reconstruct the concave profile of the ice sheet in marine grounded regions such as the ross and weddell seas we force our model to develop channels of faster flow by defining greater basal sliding along the trajectory of former ice streams we find a strong dependence upon the basal sliding parameters and also the position of the grounding line we use an extensive data base of geological and glaciological data to constrain our ice sheet reconstruction grounding line extent is prescribed from marine geological data and we test ice sheet thickness against onshore geological data at 62 sites of the five time slices considered our 20 ka bp reconstruction is the best constrained by data and has an rms misfit of 147 6 m when compared to observations of ice thickness change between 20 ka bp and the present day across all time slices there are large regions of the ice sheet which are poorly constrained especially after 20 ka bp we estimate the spatial distribution of uncertainty in our ice sheet reconstruction and note that the solutions are least reliable in regions of complex topography we predict that the antarctic ice sheets contributed 9 1 5 m of eustatic sea level to the global ocean between 20 ka bp and the present and our reconstruction with minimum misfit contributes 8 m eustatic sea level during this period these values which we argue are an upper bound are lower than many previous estimates the reconstructed pattern of ice unloading can serve as a new input for glacial isostatic models

neogene fjordal sedimentation on the western margin of the lambert graben east antarctica neogene glacial sedimentation east antarctica

tundra environments in the neogene sirius group antarctica evidence from the geological record and coupled atmosphere vegetation models the neogene meyer desert formation sirius group at oliver bluffs in the transantarctic mountains contains a sequence of glacial deposits formed under a wet based glacial regime within this sequence fluvial deposits have yielded fossil plants that along with evidence from fossil insects invertebrates and palaeosols indicate the existence of tundra conditions at 85 s during the neogene mean annual temperatures of c 12 c are estimated with short summer seasons with temperatures up to 5 c the current published date for this formation is pliocene although this is hotly debated reconstructions produced by the triffid and biome 4 vegetation models utilizing a pliocene climatology derived from the hadam3 general circulation model running with prescribed boundary conditions from the us geological survey prism2 dataset also predict tundra type vegetation in antarctica the consistency of the model outputs with geological evidence demonstrates that a pliocene age for the meyer desert formation is consistent with proxy environmental reconstructions and numerical model reconstructions for the mid pliocene if so the east antarctic ice sheet has behaved in a dynamic manner in the recent geological past

mid pliocene deep sea bottom water temperatures based on ostracode mg ca ratios abstract we studied magnesium calcium mg ca ratios in shells of the deep sea ostracode genus krithe from a short interval in the middle pliocene between 3 29 and 2 97 ma using deep sea drilling sites in the north and south atlantic in order to estimate bottom water temperatures bwt during a period of climatic warmth results from dsdp and odp sites 552a 610a 607 658a 659a 661a and 704 for the period ma reveal both depth and latitudinal gradients of mean mg ca values shallower sites 552a 610a and 607 have higher mean mg ca ratios 10 3 9 7 10 1 mmol mol than deeper sites 661a 6 3 mmol mol and high latitude north atlantic sites 552a 610a 607 have higher mg ca ratios than low latitude 658a 9 8 mmol mol 659a 7 7 mmol mol 661a 6 3 mmol mol and southern ocean 704 8 0 mmol mol sites converting mg ca ratios into estimated temperatures using the calibration of dwyer et al 1995 dwyer g s cronin t m baker p a raymo m e buzas j s correge t 1995 north atlantic deepwater temperature change during late pliocene and late quaternary climatic cycles science 270 1347 1351 suggests that mean middle pliocene bottom water temperatures at the study sites in the deep atlantic were about the same as modern temperatures however brief pulses of elevated bwt occurred several times between 3 29 and 2 97 ma in both the north and south atlantic ocean suggesting short term changes in deep ocean circulation

clay mineral assemblages in the northern south china sea implications for east asian monsoon evolution over the past 2 million years abstract clay mineral assemblages at odp site 1146 in the northern south china sea are used to investigate sediment source and transport processes and to evaluate the evolution of the east asian monsoon over the past 2 myr clay minerals consist mainly of illite 22 43 and smectite 12 48 with associated chlorite 10 30 kaolinite 2 18 and random mixed layer clays 5 22 hydrodynamic and mineralogical studies indicate that illite and chlorite sources include taiwan and the yangtze river that smectite and mixed layer clays originate predominantly from luzon and indonesia and that kaolinite is primarily derived from the pearl river mineral assemblages indicate strong glacial interglacial cyclicity with high illite chlorite and kaolinite content during glacials and high smectite and mixed layer clay content during interglacials during interglacials summer enhanced monsoon southwesterly currents transport more smectite and mixed layer clays to site 1146 whereas during glacials enhanced winter monsoon northerly currents transport more illite and chlorite from taiwan and the yangtze river the ratio smectite mixed layers illite chlorite was adopted as a proxy for east asian monsoon variability higher ratios indicate strengthened summer monsoon winds and weakened winter monsoon winds during interglacials in contrast lower ratios indicate a strongly intensified winter monsoon and weakened summer monsoon during glacials spectral analysis indicates the mineral ratio was dominantly forced by monsoon variability prior to the development of large scale glaciation at 1 2 myr and by both monsoon variability and the effects of changing sea level in the interval 1 2 myr to present

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

major neogene fluctuations of the east antarctic ice sheet stratigraphic evidence from the lambert glacier region stratigraphic and sedimentological data are presented from a region likely to directly record the evolution of the east antarctic ice sheet along the western margin of the lambert graben which now drains 1 106 km2 or one eighth of the total area of the east antarctic ice sheet extensive remnants of cenozoic glaciomarine fjordal sediment known as the pagodroma group are preserved as far as several hundred kilometers inland from the open coast the sediments resemble those produced by tidewater glaciers in the arctic rather than those of modern antarctica four separate formations of the pagodroma group spanning the interval from early miocene or older to pliocene or early pleistocene are preserved the oldest and highest crop out at nearly 1500 m above sea level each formation was deposited in close proximity to an ice margin grounded on the fjord bottom during major recessional phases of the lambert glacier underlying each formation is a glacially cut erosion surface each surface records a separate stage of fjord excavation when ice expanded onto the continental shelf in prydz bay evidence from offshore drilling in prydz bay and these data indicate fluctuations of the lambert glacier terminus in excess of 500 km in neogene time thereby reflecting large changes in the volume of the ice sheet depositional episodes were succeeded by phases of uplift totalling more than 1500 m

paleoproterozoic crustal evolution of the tarim craton constrained by zircon u pb and hf isotopes of meta igneous rocks from korla and dunhuang abstract the tarim craton is one of three large cratons in china presently there is only scant information concerning its crustal evolutionary history because most of the existing geochronological studies have lacked a combined isotopic analysis especially an in situ lu hf isotope analysis of zircon in this study precambrian basement rocks from the kuluketage and dunhuang blocks in the northeastern portion of the tarim craton have been analyzed for combined in situ laser ablation icp mc ms zircon u pb and lu hf isotopic analyses as well as whole rock elements to constrain their protoliths forming ages and magma sources two magmatic events from the kuluketage block at 2 4 ga and 1 85 ga are revealed and three stages of magmatic events are detected in the dunhuang block i e 2 0 ga 1 85 ga and 1 75 ga the 1 85 ga magmatic rocks from both areas were derived from an isotopically similar crustal source under the same tectonic settings suggesting that the kuluketage and dunhuang blocks are part of the uniform precambrian basement of the tarim craton zircon hf model ages of the 2 4 ga magmatism indicate that the crust of the tarim craton may have been formed as early as the paleoarchean period the 2 0 ga mafic rock from the dunhuang block was formed in an active continental margin setting representing an important crustal growth event of the tarim craton in the mid paleoproterozoic that coincides with the global episode of crust formation during the assembly of the columbia supercontinent the 1 85 ga event in the kuluketage and dunhuang blocks primarily involved the reworking of the old crust and most likely related to the collisional event associated with the assembly of the columbia supercontinent while the 1 75 ga magmatism in the dunhuang block resulted from a mixture of the reworked archean crust with juvenile magmas and was most likely related to a post collisional episode

last exposure process of the larsemann hills and adjacent area east antarctica based on bedrock exposure ages abstract the east antarctic ice sheet eais is the largest glacial system in the world and the evolution history of the eais strongly affects global climate change as one of the most extensive ice free areas in coastal east antarctica the larsemann hills represent an important site for understanding the glacial evolution of east antarctica generally the last exposure process in the larsemann hills was considered to occur in the last glacial period however the initial time of the last exposure process in the larsemann hills remains unclear this study presents the 10be exposure ages of eight bedrock samples with glacial striae which means that the original uneroded surfaces are preserved from the larsemann hills and neighboring areas the 10be exposure ages ranging from 70 7 4 7 to 13 5 1 2 ka indicate that the larsemann hills have become ice free since mis4 the 10be exposure ages with the characteristics of zonal distribution in the e w direction and ages generally change from north to south which reveals the combination of glacial retreat and topography differences the activity of the ice sheet in the larsemann hills did not respond to the last glacial maximum lgm in contrast the deglaciation in the larsemann hills occurred as a diachronous process during the global last glacial periods which began long before the lgm and lasted after the lgm according to the exposure ages in the larsemann hills and the distance from the edge of the ice sheet the average glacial retreat rate since mis4 has been 71 57 m ka

exposure ages from mountain dipsticks in mac robertson land east antarctica indicate little change in ice sheet thickness since the last glacial maximum past changes in east antarctic ice sheet eais volume are poorly known and diffi cult to measure yet are critical for predicting the response of the ice sheet to modern climate change in particular it is important to identify the sources of sea level rise since the last glacial maximum lgm and ascertain the present day stability of the world s largest ice sheet we present altitudinal transects of 10 be and 26 al exposure ages across the framnes mountains in mac robertson land that allow the magnitude and timing of eais retreat to be quantifi ed our data show that the coastal eais thinned by at most 350 m in this region during the past 13 k y this reduction in ice sheet volume occurred over a 5 k y period and the present icesheet profi le was attained ca 7 ka in contrast to the west antarctic ice sheet which continues to retreat today combined with regional offshore and terrestrial geologic evidence our data suggest that the reduction in eais volume since the lgm was smaller than that indicated by contemporary ice sheet models and added little meltwater to the global oceans stability of the ice margin since the middle holocene provides support for the hypothesis that eais volume changes are controlled by growth and decay of northern hemisphere ice sheets and associated global sea level changes

paleozoic sanweishan arc in the northern dunhuang region nw china the dunhuang block is a phanerozoic orogen not a precambrian block abstract the dunhuang region is located to the south of the central asian orogenic belt and connecting the tarim craton westerly and north china craton easterly respectively it is an important position for understanding the geodynamics and tectonic framework of central asia however the issue about whether it is a phanerozoic orogen or a precambrian block is still controversial magmatic arcs are the most prominent and laterally persistent structural elements of orogenic belts and used to reconstruct now disrupted orogenic belts this paper concerns the petrogenesis and tectonic setting of andesitic dacitic rhyolitic volcaniclastic rocks and dacitic porphyries in the sanweishan area northern dunhuang region volcaniclastic rocks overlie the metasedimentary rocks by fault and dacitic porphyries intrude into the metasedimentary rocks the calc alkaline signature and relative enrichment of large ion lithophile elements of volcaniclastic rocks and dacitic porphyries seemingly favor a subduction setting zircon u pb dating results reveal the deposition at 424 414 ma for volcaniclastic rocks and emplacement at ca 364 ma for dacitic porphyries the coeval granitoid plutons in the sanweishan area show arc geochemical signature and are originated from partial melting of the subducted sediments based on the available data paleozoic volcanic subvolcanic rocks and plutons in this area probably represent a magmatic arc developed on the accretionary complex resembling the arc magmatic rocks formed in the nankai complex sw japanese island arc our results provide constraints on the existence of the paleozoic magmatic arc in the northern dunhuang region and suggest the so called dunhuang block is virtually a phanerozoic orogen rather than a precambrian block

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

origin of the granodiorite in the forearc region of southwest japan melting of the shimanto accretionary prism abstract in the forearc region of southwest japan felsic to intermediate volcano plutonic complexes of middle miocene age intruded into the shimanto accretionary complex possibly in association with megatectonic events such as the opening of the japan sea and resultant underthrusting of the newly borne shikoku basin philippine sea plate beneath the southwest japan arc activities of mantle derived magmas are also distinct in the forearc of middle miocene southwest japan tholeftic basalts are closely related to igneous activity of the shikoku basin and high mg andesites probably formed under anomalously high temperature conditions caused by the opening of japan sea two plutons uwajima and miuchi were chosen for understanding the origin of felsic magmas in the forearc region of southwest japan all granitic rocks of the plutons show enriched isotopic characteristics with higher 87 sr 86 sr and lower 143 nd 144 nd tha bulk earth which suggests that the contribution of crustal materials has a magma source however they have lower 87 sr 86 sr and higher 143 nd 144 nd than shimanto sediments and hence an additional component with lower 87 sr 86 sr and higher 143 nd 144 nd than granitic rocks must be required possible candidates for such components are coeval mantle derived magmas mass balance modelling between mantle derived magmas and shimanto sediments on the basis of their sr nd isotopic compositions suggested that mixing of anatectic melt of the shimanto sediments and porphyritic andesite magma evolved from high mg andesite as a possible mechanism of the origin of the granodiorites of the uwajima pluton injection of high mg andesite into the shimanto accretionary prism may triggered the melting of sediments calculated composition of anatectic melt of the shimanto sediment and p t condition of the gneissose enclave in the granodiorite suggest that the melting of sediments may have occurred at higher temperature conditions than those of the melting reactions involving biotite breakdown