the paleoproterozoic magmatic metamorphic events and cover sediments of the tiekelik belt and their tectonic implications for the southern margin of the tarim craton northwestern china abstract the tiekelik belt in the southwestern part of the tarim craton nw china consists of archean paleoproterozoic orthogneisses and granitoids and younger sedimentary successions u pb dating of paleoproterozoic rocks in the tiekelik belt has provided new information on the southwestern margin of the tarim craton two orthogneisses from the heluositan complex yielded crystallization ages in the interval 2310 2260 ma and metamorphic ages of 2040 2007 ma and 1830 ma respectively two syenogranites from the buweituwei pluton yielded crystallization ages of 1900 and 1795 ma detrital zircon analyses from sedimentary rocks nonconformably overlying the buweituwei and akazi plutons indicate maximum depositional ages between 1950 ma and 1780 ma major detrital inputs are from the basement in the region and form prominent peaks at 1900 1800 ma and 2300 2200 ma our new data combined with previous results allow us to summarize the archean paleoproterozoic tectonic evolution of the tiekelik belt as follows 1 emplacement of archean aged ttg magmas during the 3 14 2 76 ga interval 2 a series of 2 41 2 26 ga magmatic events that represent an important period of crustal reworking within the tarim craton 3 2 03 1 80 ga magmatic and metamorphic events that include the emplacement of a 1 9 ga mafic dyke suite into the heluositan complex along with several episodes of intraplate magmatism the late magmatic events are coeval with metamorphism and may support an orogenic extensional model for the tiekelik belt during the 1 9 1 8 ga interval 4 post 1 8 ga subsidence and formation of a changcheng sequence in the tiekelik belt the paleoproterozoic ages of major tectonothermal events within the tiekelik north altyn dunhuang block and alashan block are almost identical and it is argued that they were connected at that time as north china and tarim were part of the larger columbia supercontinent

replenishment crystal accumulation and floor aggradation in the megacrystic kameruka suite australia detailed field evidence indicates that the kameruka suite plutons of the bega batholith eastern australia grew by crystal accumulation on the floor of a magma chamber depositional features in the plutons including mafic enclave channels asymmetric enclave pillows and exotic rafts load casts and flame structures and graded and trough cross beds indicate that the pluton built progressively upward the general eastward dip of depositional features in the main pluton implies a lower western and upper eastern contact consistent with a basal granite migmatite contact in the west and a sharp hornfelsic sidewall contact in the east mafic felsic and composite dykes most common near and below the basal western contact are interpreted as conduits for magma chamber replenishment and imply open system behaviour during pluton construction textural relations are also consistent with an open system cumulate origin typically centimetre scale grains of quartz plagioclase and megacrystic alkali feldspar form a touching framework with interstices filled with smaller biotite flakes and smaller intercumulus quartz and feldspar crystals alkali feldspar megacrysts vary from euhedral and unzoned to mantled and partially replaced by plagioclase to ovoid and completely pseudomorphed by quartz albite aggregates the common occurrence of mantled and pseudomorphed alkali feldspar in mafic enclaves and in hybrid tonalitic rocks forming the matrix to enclave swarms suggests that replacement or resorption of granitic primocrysts was associated with mafic replenishments the occurrence of all megacryst types at outcrop scale implies extended alkali feldspar crystallization in different parts of the chamber thorough stirring during or after periodic replenishment and final settling in a cumulate mush the bulk composition of the cumulate mush represented by granodiorite cannot represent the emplaced magma compositional variation can be modelled by variable degrees of crystal accumulation from a parental silica rich melt represented by the silicic dykes as dykes periodically fed the magma chamber crystals accumulated on the floor and more evolved melts probably erupted from its roof thus the average composition of the magma and the cumulus minerals may have remained relatively constant and the sublinear chemical trends that typify the kameruka suite simply reflect differing proportions of melt and cumulate material sublinear chemical trends can also be explained by a restite model however the distinctive ba light rare earth element and zr spikes at high silica can be explained only by a cumulate model which also explains why the low silica granites of the suite share the same chemical characteristics as the high silica granites

magnitude of post middle jurassic bajocian displacement on the central altyn tagh fault system northwest china the altyn tagh fault system a key structural feature in the tectonic collage of central asia is an active left lateral strike slip fault system however its age of initiation kinematic history and total magnitude of displacement are not well known middle jurassic nonmarine sedimentary rocks that crop out along the central segment of the altyn tagh fault were deposited in a contiguous tarim qaidam foreland style basin that was geodynamically linked to mesozoic contraction in the tian shan bei shan qilian shan and kunlun shan this study documents the best evidence of a piercing point to date consisting of an offset lacustrine shoreline demarcated by aalenian bajocian open lacustrine strata to the west and fluvial and alluvial strata of comparable age to the east detrital compositions palynology paleocurrents and lithostratigraphy support correlation of these sections restoration of this jurassic facies boundary indicates 400 60 km of post bajocian left lateral separation on the altyn tagh fault consistent with this result restoration of the jurassic shoreline aligns felsic plutons across the western segment of the altyn tagh fault system that may have been left laterally offset by 360 km these two pairs of offset features suggest that the current best estimate for net magnitude of post bajocian left lateral separation on the altyn tagh fault is about 360 km

correcting the cenozoic δ18o deep sea temperature record for antarctic ice volume abstract the oxygen isotope signal in benthic foraminifera from deep sea cores is mainly determined by deep ocean temperature and land ice volume separating the temperature and ice volume signals is a key step in understanding the evolution of cenozoic climate except for the last few million years fluctuations in land ice volume were determined mainly by the size of the antarctic ice sheet here a simplified antarctic ice sheet model and composite deep sea oxygen isotope records are used to derive a deep sea temperature history for the cenozoic the ice sheet model is forced with the oxygen isotope record which is instantaneously corrected for the calculated ice volume therefore the resulting deep sea temperature and antarctic ice volume curves are mutually consistent the temperature record of lear et al science 287 2000 269 derived from the mg ca ratio in benthic calcite is also used to force the ice sheet model the resulting ice volume does not agree well with the ice volume history inferred by lear et al from a comparison of mg ca and 18 o records the most important points emerging from this study are the cenozoic 18 o record from benthic forams is a truly mixed signal ice volume and deep sea temperature contribute equally to the variance for the period of 45 to 30 ma ago 18 o temperatures are significantly lower than mg ca temperatures rapid nonlinear build up of a continental scale ice sheet on the antarctic continent can easily occur for a linear cooling rate it is unlikely that the method of lear et al provides a reliable measure of ice volume the cooling rate at the beginning of the pliocene is much larger in the 18 o temperature curves than in the mg ca temperature curve

trace element modelling of pelite derived granites the presence or absence of a vapour phase during incongruent melt reactions of muscovite and biotite together with the composition of the protolith determines the trace element characteristics of the resulting melt provided that equilibrium melting occurs for those phases that host the tracc elements of interest for granitic melts rb sr and ba provide critical constraints on the conditions that prevailed during melting whereas ree are primarily controlled by accessory phase behaviour mass balance constraints for eutectic granites that are formed by the incongruent melting of muscovite in pelites indicate that melting in the presence of a vapour phase will result in a large melt fraction and deplete the restite in feldspar hence the melt will be characterized by low rb sr and high sr ba ratios in contrast vapour absent melting will result in a smaller melt fraction and an increase in the restitic feldspar consequently high rb sr and low sr ba ratios are predicted vapour absent melting will also enhance the negative eu anomaly in the melt granites that result from the incongruent melting of biotite in the source will be characterized by higher rb concentrations than those that result from the incongruent melting of muscovite the himalayan leucogranites provide an example of unfractionated crustally derived eutectic melts that are enriched in rb but depleted in sr and ba relative to their metasedimentary protoliths these compositions may be generated by the incongruent melting of muscovite as a low melt fraction f 0 1 from a pelitic source under vapour absent conditions

post collisional transition from calc alkaline to alkaline magmatism during transcurrent deformation in the southernmost dom feliciano belt braziliano pan african uruguay abstract in the southernmost dom feliciano belt of uruguay highly fractionated calc alkaline granites mildly alkaline granites shoshonitic volcanics and peralkaline intrusions and volcanics are spatially and temporal associated with the evolution of shear zones four representative magmatic unites of this diverse association were petrographic and geochemically investigated the solis de mataojo complex a medium to high k2o calc alkaline granite with signature typical of mature continental arcs and post collisional settings the maldonado granite highly fractionated calc alkaline to alkaline with characteristics that are transitional between both types of series the pan de azucar pluton with characteristics typical of post collisional alkaline granites and the las flores shoshonitic basalts geochemistry and geotectonic setting point out that slab breakoff was most likely the mechanism associated with the generation of high k calc alkaline magmas solis de mataojo and maldonado shortly after collision extension associated to the formation of molassic basins and emplacement of dolerites and basalt flows with shoshonitic affinity las flores 15and finally a shift to magmas with alkaline signatures pan de azucar simultaneous with a second transpressional phase were probably linked with lithospheric thinning through delamination this evolution took place between 615 and 575 ma according to available data contrary to previous proposals which considered this magmatism to represent the root of a continental magmatic arc a post collisional environment transitional from orogenic to anorogenic during transcurrent deformation is proposed

uplift of deep crust during orogenic extensional collapse a model based on field studies in the sogn sunnfjord region of western norway the western gneiss region wgr in norway experienced high pressure metamorphism during silurian devonian continent continent collision the eclogite bearing lower crust is separated from the middle and upper crust by major detachment zones formed during extensional collapse of the orogen formation of the devonian basins is related to the extension the footwall of the detachment zones comprises three structural and metamorphic zones the upper zone zone 1 is characterized by penetrative homogeneous down to the west simple shear developed under retrograde greenschist facies metamorphism zone 2 suffered inhomogeneous simple shear of the same polarity petrography and mineral chemistry data from the lower zone zone 3 show a record of initial eclogite facies metamorphism at 600 c and 16 kbar which was decompressed almost isothermally to amphibolite facies conditions at 550 c and 10 12 kbar both the eclogite and amphibolite facies metamorphism developed in a regime of pure shear with vertical shortening the rapid decompression records an approach of approximately 20 km to the surface related to uplift that was probably the result of the removal of a thickened thermal boundary layer in the mantle lithosphere the pure shear regime which developed initially in the lower crust was truncated by zones of simple shear as the lower crust was uplifted to middle and upper crustal levels extension by simple shear in the upper crust was rooted in the lower crust where extension occurred by pure shear the shear zones in zones 1 and 2 did not penetrate the pure shear regime of the lower crust a considerable amount of tectonic stripping of the orogenic welt predates deposition of the devonian sedimentary basins

modelled atmospheric temperatures and global sea levels over the past million years the climate has passed through a series of glacials and interglacials over the past million years but the nature of this cyclicity in terms of temperature ice volume and sea level and the underlying causes are not well known bintanja et al use a new method to deduce a one million year time series of these variables the reconstructed records are much longer than other methods have provided for any of these variables individually the most intense glacial stages were 17 c colder than today and most of the continental ice was present in north america strong cooling in the beginning of glacials was found to precede ice sheet build up these findings may shed light on the causes of ice age cycles marine records of sediment oxygen isotope compositions show that the earth s climate has gone through a succession of glacial and interglacial periods during the past million years but the interpretation of the oxygen isotope records is complicated because both isotope storage in ice sheets and deep water temperature affect the recorded isotopic composition1 2 3 4 5 separating these two effects would require long records of either sea level or deep ocean temperature which are currently not available here we use a coupled model of the northern hemisphere ice sheets6 and ocean temperatures forced to match an oxygen isotope record for the past million years compiled from 57 globally distributed sediment cores to quantify both contributions simultaneously we find that the ice sheet contribution to the variability in oxygen isotope composition varied from ten per cent in the beginning of glacial periods to sixty per cent at glacial maxima suggesting that strong ocean cooling preceded slow ice sheet build up the model yields mutually consistent time series of continental mean surface temperatures between 40 and 80 n ice volume and global sea level we find that during extreme glacial stages air temperatures were 17 1 8 c lower than present with a 120 10 m sea level equivalent of continental ice present

mafic magmatic enclaves and mafic rocks associated with some granitoids of the central sierra nevada batholith california nature origin and relations with the hosts abstract the calc alkaline granitoids of the central sierra nevada batholith are associated with abundant mafic rocks these include both country rock xenoliths and mafic magmatic enclaves mme that commonly have fine grained and less commonly cumulate textures scarce composite enclaves consist of either xenoliths enclosed in mme or of mme enclosed in other mme with different grain size and texture enclaves are often enclosed in mafic aggregates and form meter size polygenic swarms mostly in the margins of normally zoned plutons enclaves may locally divert schlieren layering mafic dikes which also occur in swarms are undisturbed composite or largely hybridized in central sierra nevada with the exception of xenoliths that completely differ from the other rocks host granitoids mafic aggregates mme and some composite dikes exhibit a bulk compositional diversity and at the same time important mineralogical and geochemical including isotopic similarities mme and host granitoids display distinct major and trace element compositions however strong correlations between mme host granitoid pairs indicate interactions and parallel evolution of mme and enclosing granitoid in each pluton identical mafic mineral compositions and isotopic features are the result of these interactions and parallel evolution mafic dikes have broadly the same major and trace element compositions as the mme although variations are large between the different dikes that are at distinctly different stages of hybridization and digestion by the host granitoids the composition of the granitoids and various mafic rocks reflects three distinct stages of hybridization that occurred respectively at depth during ascent and emplacement and after emplacement the occurrence and succession of hybridization processes were tightly controlled by the physical properties of the magmas the sequential thorough or partial mixing and mingling were commonly followed by differentiation and segregation processes unusual mme that contain abundant large crystals of hornblende resulted from disruption of early cumulates at depth whereas those richer in large crystals of biotite were formed by disruption of late mafic aggregates or schlieren layerings at the level of emplacement mme and host granitoids are considered cogenetic because both are hybrid rocks that were produced by the mixing of the same two components in different proportions the felsic component was produced by partial melting of preexisting crustal materials whereas the dominant mafic component was probably derived from the upper mantle however in the lack of a clear mantle signature the origin of the mafic component remains questionable

boundary lines within petrologic diagrams which use oxides of major and minor elements use of some petrologic diagrams applied to analyses of volcanic rocks is unnecessarily difficult due to lack of data for construction of discriminant lines between rock series coordinates are provided for sufficient points to enable accurate plotting of the boundary lines within seven diagrams viz 1 tas total alkalies na2o k2o vs sio2 2 k2o vs sio2 3 afm 4 jensen 5 ktp k2o tio2 p2o5 6 fms feo mgo vs sio2 and 7 taktip k2o na2o k2o vs tio2p2o5 different versions of these boundaries are collated to indicate their variable position and it is demonstrated that inter laboratory analytical precision suffices to account for almost all of their spread on the tas and k2o vs sio2 diagrams

formation and evolution of strike slip faults rifts and basins during the india asia collision an experimental approach the processes which have governed the formation and evolution of large tertiary strike slip faults during the penetration of india into eastern asia are investigated by means of plane strain indentation experiments on layered plasticine models the steady state deformation of plasticine obeys a power creep flow law e c t n the stress exponent n is between 6 and 9 at 25 c uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening a process observed for strain rates ranging from 3 5 10 5 to 3 6 10 3 s 1 fault or shear zones form after only 7 10 bulk strain subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine a series of nine plane strain indentation experiments shows the influence of boundary conditions as well as that of the internal structure of the plasticine model on the faulting sequence the ubiquity of strain softening in experimental deformation of a variety of rocks as well as the widespread occurrence of shear zones in nature suggest that long term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain the deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision induced strikeslip faulting in asia particularly to total offsets and rates of movements on the faults the experiments simulate the evolution of the western ends of the strike slip faults which have probably been analogous to trench fault fault triple junctions the experiments also illustrate mechanisms for the formation of extensional basins such as the south china sea north china basin and andaman sea near active continental margins the basins which appear to absorb terminal offsets along major strike slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts the existence of distinct phases of strike slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous non steady state deformation of the continental lithosphere

global ice volume variations through the last glacial cycle simulated by a 3 d ice dynamical model abstract a coupled ice sheet ice shelf bedrock model was run at 20 km resolution to simulate the evolution of global ice cover during the last glacial cycle the mass balance model uses monthly mean temperature and precipitation as input and incorporates the albedo mass balance feedback the model is forced by the grip and vostok derived temperature records and by sea level and solar radiation variations which are distributed spatially and seasonally model simulations show that at the last glacial maximum lgm a volume of ice representing about 65 m of sea level equivalent is stored on the eurasian continent whereas north america stores 42 m the other regions greenland tibet south america and antarctica contribute about 17 m to global sea level lowering at the lgm compared to present day sensitivity experiments show that the ice volumes of the eurasian and north american ice sheets are particularly sensitive to summer temperature changes finally the timing and mechanism of grounded ice formation and expansion for the last glacial were studied for the eurasian and north american continents ice sheet formation begins at 118 kyr bp in the barents and kara sea region over baffin island and in the high cordilleran range following a 4 c cooling alternating regions of ice sheet expansion by direct snowfall ice flow and grounding of ice shelves can be identified these relate to specific geomorphological traces and may as such help in model validation

ice sheet modelling characteristics in sea level based temperature reconstructions over the last glacial cycle a widely used method for investigating palaeotemperatures is to analyze local proxy records e g ice cores or deep sea sediment cores the interpretation of these records is often not straightforward and global or hemispheric means cannot be deduced from local estimates because of large spatial variability using a different approach temperature changes over the last glacial cycle can be estimated from sea level observations by applying an inverse method to an ice sheet model in order to understand the underlying physical mechanisms we used a 1 d ice sheet model and a 3 d coupled thermodynamic ice sheet ice shelf bedrock model to investigate the importance of several physical processes for the inverse temperature reconstructions results show that i temperature reconstructions are sensitive to the employed formulation of mass balance ii excluding thermodynamics in the ice sheet leads to a smaller temperature amplitude in the reconstruction and iii hysteresis in the non linear relation between sea level and temperature occurs as a consequence of ice redistribution in the process of merging and separation of ice sheets the ice redistribution does not occur if the geometry does not support the formation of a relatively flat dome which tends to be preserved in warming conditions

convective removal of thermal boundary layer of thickened continental lithosphere a brief summary of causes and consequences with special reference to the cenozoic tectonics of the tibetan plateau and surrounding regions england p 1993 convective removal of thermal boundary layer of thickened continental lithosphere a brief summary of causes and consequences with special reference to the cenozoic tectonics of the tibetan plateau and surrounding regions in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 67 73 the tectonics of tibet and its surroundings underwent marked changes at about 8 ma normal faulting of the plateau appears to have initiated at about this time widespread highly potassic volcanism interpreted as the result of rapid heating of the mechanical boundary layer began about 12 8 ma such rapid heating could be the consequence of convective removal of the lower lithosphere and its replacement by asthenosphere this process would have raised the surface height of the plateau and could have provided the driving force for extension the diffuse deformation in the indian ocean probably began at about 8 ma analysis of the buckling of a plastic lithosphere implies that the horizontal force per unit length required to drive this deformation is consistent with uplift of the tibetan plateau by 1 3 km the indian monsoon intensified at about 8 ma this intensification is consistent with the rapid passage of the surface height of the plateau across a threshold required to disrupt the pre existing atmospheric circulation of the region taken together these observations imply that the surface height of the tibetan plateau rose by between 1 and 3 km in a few million years around 8 ma such a rapid increase in surface height affecting an area the size of the tibetan plateau could not be achieved by crustal thickness changes but could have been caused by the convective removal of a substantial thickness of lower continental lithosphere of the region

post collisional orogen parallel large scale extension in the eastern alps abstract large scale extension affected the eastern alps during post collisional lateral tectonic extrusion in early and middle miocene times the tauern window was mainly tectonically exhumed by the 160 km pullapart of the rigidly behaving austroalpine basement blocks forming the tectonic lid of the penninic contents of the window an evaluation of the syn extrusion fault pattern reveals displacements of several tens of kilometers along the low angle extensional shear zones at the western and eastern margins of the window and along the important strike slip fault zones north and south of the window large scale shear is reflected by discontinuities in mineral cooling patterns across important shear zones the cooling histories of the tectonic blocks show that the window boundaries during exhumation coincide only partly with the classical boundaries of the window defined by thrust units reconstruction of the exhumation history shows an asymmetric evolution of the tauern window we argue that the eastern and western low angle extensional shear zones formed in sequence which is reflected by different evolutions of the fragmentation and uplift history of the austraolpine tectonic blocks to the east and west of the window in the central and eastern alps three structural domes the lepontin dome in the central alps and the tauern and rechnitz domes in the eastern alps were formed by large scale extension in miocene times total e w stretch amounts to more than 300 km and results in 70 extension we propose that the central and eastern alps are part of an extensional province that also includes the pannonian basin and has a counterpart separated by the moesian indenter in the rhodope mountains the aegean sea and the menderes massif in asia minor in these regions a number of mainly tectonically exhumed domes or deep sedimentary basins formed by miocene large scale extension

generation of metaluminous a type granites by low pressure melting of calc alkaline granitoids melting experiments on two calc alkaline rocks a tonalite and a granodiorite demonstrate that shallow dehydration melting of hornblende and biotite bearing granitoids generates metaluminous a type granitic melts profuse crystallization of calcic plagioclase orthopyroxene during low pressure p 4 kbar incongruent melting explains all of the major and trace element characteristics of a type granites e g their low al ca mg sr and eu contents and high ga al and k na ratios as well as their common association with anorthositic and mangeritic rocks the residual assemblage produced during melting of the same rocks in the deep crust p 8 kbar is dominated by clinopyroxene and as a consequence most of the chemical features diagnostic of a type granites are lost the high temperatures 900 c and low h 2 o contents 4 wt that are characteristic of a type magmas are adequately explained by the h 2 o poor nature of the granitoid source materials and require neither a granulitic nor a f rich source previous attempts to explain the origin of a type granites have focused on the nature of the source material these experiments show that pressure of melting is at least as important as source composition in engendering a type melts

mount dutton volcano alaska aleutian arc analog to unzen volcano japan abstract holocene eruptions from mount dutton a small late quaternary volcano near the tip of the alaska peninsula bear strong physical and petrologic similarities to the 1990 1995 unzen fugendake eruption in japan the volcano had a protracted phase of effusive calcalkaline andesitic 54 59 wt sio 2 cone building in the late pleistocene followed by an abrupt switch to more silicic 65 wt sio 2 lavas emplaced as a central summit cluster of steep sided domes beginning in the early holocene the flanks of the volcano are mantled by pyroclastic flows debris flows and talus formed as a result of gravitational dome collapse disequilibrium mineral assemblages including coexisting quartz and olivine in eruptive episodes ranging from the initial cone building basaltic andesite lavas to the latest holocene dacite domes suggest extensive magma mixing in addition up to meter sized pillow like cognate mafic enclaves of hornblende plagioclase glass are common in the latest of the summit dacite domes mineralogical evidence and bulk chemical data indicate the enclaves represent a high alumina basalt parent with variable and subordinate reservoir contaminant and the host lava is reservoir magma with variable and subordinate basaltic contaminant mount dutton s history and petrology can be interpreted as reflecting the monotonous repetitive intrusion of mantle derived mafic magma into a silicic crystal rich crustal reservoir during the holocene these injections resulted in the extrusion of partially crystallized viscous sticky central domes which typically failed by collapse resulting in small volume merapi type flowage deposits we speculate that slow introduction of mafic magma into the silicic chamber leads both to enclave formation and to the effusive eruption style mount dutton volcano experienced severe shallow earthquake swarms in 1984 1988 and to a lesser extent in 1991 although none of these swarms resulted in an eruption their epicenter distribution and volcanic tectonic character indicate they recorded the movement of magma at shallow depth beneath the volcano the lessons of unzen and montserrat suggest that despite limited tephra production these small volcanic centers with central dome complexes present a serious hazard to life and property

a new method to estimate ice age temperatures on glacial time scales the waxing and waning of the eurasian and north american ice sheets depend largely on variations in atmospheric temperature as global sea level is primarily determined by the volume of these ice sheets there is a direct yet complex relation between global sea level and the northern hemispheric nh temperature this relation is essentially represented by a model of the nh ice sheets we use a thermomechanical ice sheet ice shelf bedrock model in conjunction with an inverse method to deduce a time series of nh temperature from 120 kyr bp until present that is consistent with the observed global sea level record the advantage of this method is that it provides the annual mean surface air temperature averaged over the nh continents north of 40 n the results reveal that ice age temperatures were 4 10 c lower than today which agrees with other temperature reconstructions however reconstructed temperatures are comparitively low during the early stages of the glacial a feature that is consistent with the rapid growth of the ice sheets the sensitivity of the results for uncertainties in precipitation rate in observed sea level and in some other model parameters is examined to quantify the error in reconstructed temperature during the glacial period 120 15 kyr bp surface air temperatures in the nh north of 40 n were 7 2 1 5 c lower than today s interglacial temperatures

reconstruction of the altyn tagh fault based on u pb geochronology role of back thrusts mantle sutures and heterogeneous crustal strength in forming the tibetan plateau 1 reconstructing deformation along the northwestern margin of the tibetan plateau is critical for evaluating the relative importance of microplate versus continuum models of the indo asian collision questions regarding this margin s evolution are as follows 1 what is the total offset along the sinistral altyn tagh strike slip system 2 how has that offset been absorbed in the western kunlun shan 3 why does the n s width of the plateau vary along strike ion microprobe u pb zircon apparent ages of 17 plutons from nw tibet together with regional geologic observations define a discrete e w trending boundary between two tectonic belts that has been offset along the altyn tagh system by 475 70 km kinematic arguments indicate that this offset cannot be the result of north directed thrusting in the western kunlun shan therefore we propose that south directed faulting in the tianshuihai thrust belt both offset the tectonic boundary and produced the asymmetry in the plateau shortening appears to have been absorbed in the upper crust by thin skinned thrusting in the middle lower crust by east directed ductile flow and or subduction and in the mantle by north dipping subduction factors controlling the formation of the south directed thrust system appear to be the contrast between the rigid tarim and the weaker songpan ganzi flysch belt and a mantle suture inherited from late paleozoic subduction the evolution of western tibet leads to a view of continental deformation that integrates elements of the microplate model e g plate like mantle and crust mantle decoupling with aspects of the continuum model weak crustal flow beneath the plateau

petrology and geochronology of eclogites from the western segment of the altyn tagh northwestern china abstract the eclogites from the western segment of the altyn tagh occur as lens or boundins within quartzofeldspathic gneisses or pelitic gneisses characterized by amphibolite facies parageneses the petrographic features and reaction textures testify to three main metamorphic stages 1 the peak eclogite facies stage p 15 kbar t 665 880 c 2 the granulitic facies stage p 11 14 kbar t 670 800 c and 3 the amphibolite facies stage p 6 3 9 5 kbar t 619 738 c during decompression the trace element rare earth element and sm nd isotopic data suggest that most of the eclogites have protolith features resembling transitional type t type mid ocean ridge basalt morb the well preserved eclogite was selected for sm nd and u pb isotopic dating the sm nd isotopic data yield a whole rock garnet omphacite isochron of 500 10 ma age the u pb isotopic measurement of zircons from the same eclogite shows that the four grain populations are near concordant and are well plotted on concordia giving a weighted mean age of 503 9 5 3 ma the two different geochronological methods yield a similar age which is interpreted as the peak metamorphic age of eclogites and reflects the existence of a mountain root related to continental subduction during early paleozoic moreover based on the comparison with similar rocks in the northern margin of the qaidam basin a displacement of 350 400 km for the altyn tagh fault have been estimated

steady state characteristics of the greenland ice sheet under different climates the greenland ice sheet is modelled to simulate its extent and volume in warmer climates and also to find out whether the ice sheet would re form on theice free bedrock under present climatic conditions the ice sheet model is a three dimensional thermo mechanical model with a fine resolution grid thebedrock surface beneath the ice sheet was mapped using radio echo sounding measurements by the electromagnetic institute copenhagen the modelexperiments show that increased temperature will result in ice margin retreat but the ice sheet is relatively stable it takes a rather high temperature rise of atleast 6ic for the ice sheet to disappear completely which indicates that the ice sheet probably survived the last interglacial also it appears that the greenlandice sheet is not a mere relict ice mass from a previously colder climate but that the ice sheet will still re form on the bare bedrock under the present or evenslightly warmer climatic conditions

sensitivity of a greenland ice sheet model to ice flow and ablation parameters consequences for the evolution through the last climatic cycle sensitivity experiments are conducted to test the influence of poorly known model parameters on the simulation of the greenland ice sheet by means of a three dimensional numerical model including the mechanical and thermal processes within the ice two types of experiments are performed steady state climatic conditions and simulations over the last climatic cycle with a climatic forcing derived from the grip record the experiments show that the maximum altitude of the ice sheet depends on the ice flow parameters deformation and sliding law coefficients geothermal flux and that it is low when the ice flow is fast on the other hand the maximum altitude is not sensitive to the ablation strength and consequently during the climatic cycle it is driven by changes in accumulation rate the ice sheet extension shows the opposite sensitivity it is barely affected by ice flow velocity and the ice covered area is smaller for large ablation coefficients for colder climates when there is no ablation the ice sheet extension depends on the sea level an interesting result is that the variations with time of the altitude at the ice divide summit do not depend on the parameters we tested the present modelled ice sheets resulting from the climatic cycle experiments are compared with the present measured ice sheet in order to find the set of parameters that gives the best fit between modelled and measured geometry it seems that compared to the parameter set most commonly used higher ablation rate coefficents must be used

tectonic history of the altyn tagh fault system in northern tibet inferred from cenozoic sedimentation the active left slip altyn tagh fault defines the northern edge of the tibetan plateau to determine its deformation history we conducted integrated research on cenozoic stratigraphic sections in the southern part of the tarim basin fission track ages of detrital apatites existing biostratigraphic data and magnetostratigraphic analysis were used to establish chronostratigraphy whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults implying a temporal link between sedimentation and deformation the results of our studies together with existing stratigraphic data from the qaidam basin and the hexi corridor suggest that crustal thickening in northern tibet began prior to 46 ma for the western kunlun shan thrust belt at ca 49 ma for the qimen tagh and north qaidam thrust systems bounding the north and south margins of the qaidam basin and prior to ca 33 ma for the nan shan thrust belt these ages suggest that deformation front reached northern tibet only 10 5 m y after the initial collision of india with asia at 65 55 ma because the aforementioned thrust systems are either termination structures or branching faults of the altyn tagh left slip system the altyn tagh fault must have been active since ca 49 ma the altyn tagh range between the tarim basin and the altyn tagh fault has been a long lived topographic high since at least the early oligocene or possibly late eocene this range has shed sediments into both the tarim and qaidam basins while being offset by the altyn tagh fault its continuous motion has made the range act as a sliding door which eventually closed the outlets of westward flowing drainages in the qaidam basin this process has caused large amounts of oligocene miocene sediments to be trapped in the qaidam basin the estimated total slip of 470 70 km and the initiation age of 49 ma yield an average slip rate along the altyn tagh fault of 9 2 mm yr remarkably similar to the rates determined by gps global positioning system surveys this result implies that geologic deformation rates are steady state over millions of years during continental collision

tectono magmatic rejuvenation of the qaidam craton northern tibet abstract ascertaining the enigmatic characteristics of the crystalline basement of the qaidam basin the deepest intracontinental basin yet located in the highest orogenic plateau with the thickest continental crust has broad implications for the cenozoic basin evolution in particular and intracontinental tectonics of asia in general we carried out a comprehensive investigation on the ages and geochemical isotopic characteristics of the qaidam and suganhu basement rocks obtained from drill cores petrological observations and u pb zircon dating la icp ms of granitoid samples from drill cores indicate that early neoproterozoic early paleozoic and late paleozoic mesozoic intrusions are largely represented suggesting that the qaidam and suganhu blocks were involved into three tectono magmatic episodes similar whole rock geochemistry and sr nd isotopic characteristics between the early neoproterozoic granitoids beneath the qaidam basin and the contemporaneous basement rocks in the surrounding orogenic belts confirm the existence of pervasive early neoproterozoic metamorphism and magmatism along the present day northern tibetan plateau by comparing the trace and rare earth elements patterns as well as the sr nd compositions of the ordovician arc related granitoids and silurian devonian post collisional granitoids beneath the qaidam and suganhu basins with the corresponding granitoids from the surrounding orogenic belts we assert that the proto tethys subduction events that were recorded within the eastern kunlun altyn tagh and the qilian shan orogenic belts were different they generated distinctive subduction related arc magmas whereas the closure of the oceanic basins resulted in the amalgamation of the orogenic belts inducing subsequent extensive post collisional magmatism within the qaidam block the widespread permian and triassic granitoids beneath the northwestern northern qaidam basin mainly involved a lower crust or a crust mantle source including mesoproterozoic crust they demonstrate that the influence of the paleo tethys evolution penetrated the qaidam block northwards reaching to the northern edges of the block we conclude that the qaidam block is not a craton as typical as the rigid tarim block but pertains to a tectono magmatic rejuvenated craton which is not mechanically stronger than its surrounding regions however trapped between the lithospheric left lateral strike slip altyn tagh fault system to the northwest and the qilian shan transpressional system to the northeast the qaidam block forms a relatively stable domain that escaped from significant cenozoic deformation

age geochemistry and tectonic setting of buqingshan ophiolites north qinghai tibet plateau china abstract the buqingshan ophiolite complex is a sector of the a nyemaqen ophiolite belt in the east kunlun southern marginal suture zone in the northern part of the qinghai tibet plateau the wnw trending ophiolite complex consists of metaperidotite gabbro diabase pillow basalt massive basalt and pelagic sedimentary rocks including radiolarian chert we have determined that early paleozoic and early carboniferous early permian ophiolites are present in this ophiolite complex that were previously thought of permian middle triassic age a zircon u pb age of 467 2 0 9 ma for gabbro and a rb sr isochron age of 481 130 ma for diabase and gabbro have been obtained from the early paleozoic ophiolite slice and middle late ordovician acritarchs have been found in the melange matrix that envelopes the sliver we interpret the first two ages to represent the formation age of the ophiolite granodiorite tonalite plutons with a zircon u pb age of 402 24 ma intrude the slice early carboniferous early permian radiolarians have been discovered in cherts and argillaceous cherts in the second ophiolite slice geochemistry indicates that metaperidotites in the first slice represent a depleted ocean lithosphere mantle most mafic rocks in the early paleozoic ophiolite are of n morb type with small amount of t morb and they resulted from the magma from the depleted oceanic lithosphere mantle with little crustal contamination the basalts in the early carboniferous early permian ophiolite have similar characteristics to the mafic rocks and also show dupal anomaly and a mixed source of dmm and em ii the original formation environments of the two stage ophiolites are both mid ocean ridges these lines of evidence suggest that a mature early paleozoic kunlun qilian qinling ocean basin and a mature paleotethyan ocean basin once existed in the study area

geochemical investigations of microgranitoid enclaves in the s type cowra granodiorite lachlan fold belt se australia the cowra granodiorite is a relatively mafic enclave rich s type pluton in the lachlan fold belt which has been cited as a type example of a restitic origin for all varieties of enclaves in lachlan fold belt s type granites microgranitoid enclaves from the pluton are subordinate to metasedimentary varieties and can be subdivided into two groups according to their mafic mineral assemblage pyroxene microtonalites and biotite microgranites no geochemical or isotopic distinction can be made between the two varieties petrographic evidence acicular apatites xenocrysts from the host granite suggests an origin as mingled more mafic magmas which have been variably contaminated by the more felsic host magma this is supported by the fact that the microgranitoid enclaves have isotopic compositions 87sr 86sr i 0 7095 to 0 7144 end i 9 2 to 6 9 that are generally more primitive than or similar to those of the host granite 87sr 86sr i 0 7142 end i 8 8 the spread in isotopic compositions like their trace element compositions is considered to be the consequence of variable degrees of diffusive exchange between the felsic and more mafic magmas during slow cooling several studied metasedimentary enclaves are not in isotopic equilibrium with their host granite and therefore cannot represent pristine samples of the bulk source region of the granite instead they represent portions of a lithologically and compositionally diverse source terrane or accidental xenoliths entrapped during emplacement

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

timing and nature of the partial melting processes during the exhumation of the garnet bearing biotite gneiss in the southern altyn tagh hp uhp belt western china abstract partial melting of the garnet grt bearing biotite gneiss from the jianggalesayi area in the southern altyn tagh high pressure hp ultra high pressure uhp metamorphic belt has been identified and reported here felsic veins were distributed along the foliation plane and deformed together with the grt bearing biotite gneiss which dominated the main outcrop and preserved diagnostic features of anataxis highly cuspate and irregular quartz qz melt and plagioclase pl melt surrounded or stretched into biotite and or quartz grains with obvious eroded boundaries further provided the microstructural evidences for partial melting of the gneiss cl images showed that zircons from the grt bearing biotite gneiss were characterized by multizone structures of core rim i rim ii rim iii the different zircon zones from the gneiss had integrated recordings of the protolith ages 836 1613 ma the peak metamorphic age of 503 5 ma the retrograded age of 456 4 ma and the anatectic age of 418 3 ma zircons from felsic veins also exhibited core rim structures on cl images the felsic veins yielded similar protolith ages 1099 1072 ma and anatectic age 417 2 ma as the host gneiss the zircons from the host gneiss and felsic veins had plotted identical crustal evolution curves in the ehf t vs u pb age diagram with identical tdm ages the u pb ages and lu hf isotope both suggested that the felsic veins were internally derived by the partial melting of the host gneiss moreover the high content of al2o3 and rb in felsic veins indicated that the decomposition of biotite accounted for the partial melting of the gneiss the temperatures calculated by ti in zircon thermometer for the anatectic zircons from the felsic veins 582 694 c were identical to the conditions of the amphibolite facies stage of the grt bearing biotite gneiss t 619 646 c which indicated that the crystallization of the melt had taken place during the amphibolite facies stage of the exhumation the anatectic ages 418 3 ma and 417 2 ma of the gneiss and felsic veins were determined to be within the range of the fourth episode of the early paleozoic magmatism 420 385 ma which had formed in a post collisional extensional setting therefore the combination of the magmatic heat events and stress release may have played an important role in the partial melting of the gneiss

initiation and development of the laurentide and cordilleran ice sheets following the last interglaciation abstract fossil records from sites overridden by or adjacent to the laurentide ice sheet indicate that the climate of the last interglaciation oxygen isotope substage 5e ca 130 116 ka was warmer than today following the last interglaciation the laurentide ice sheet first developed during stage 5 over keewatin quebec and baffin island along its northern margin the ice sheet reached its maximum extent of the last glaciation during stage 5 the ice sheet advanced across baffin island onto the continental shelf early during stage 5 5d whereas the advance into the western canadian arctic occurred late during stage 5 5b the ice sheet also may have advanced into the st lawrence lowland during substage 5b although this event may be younger stage 4 the hudson bay lowland became ice free during substage 5a retreat of the ice sheet on baffin island occurred during late stage 5 probably substage 5a the exact timing of retreat from the western canadian arctic is unknown but it occurred before 48 ka the southern sector including the st lawrence lowland was ice free during late stage 5 the hudson bay lowland may have remained ice free through stage 4 and much of stage 3 because of conflicting chronologies however it is more likely that this area was glaciated throughout stage 3 and perhaps stage 4 nevertheless the data demonstrate that the lowland was ice free during part of the last glaciation an independent ice cap developed over the appalachian uplands and advanced across nova scotia during stage 4 perhaps as far as the edge of the continental shelf the ice cap remained active over nova scotia as a setellite to the laurentide ice sheet throughout the remainder of the last glaciation the ice sheet advanced into the st lawrence lowland during stage 4 and subsequently overwhelmed the local ice cap in the appalachian uplands advancing perhaps into northern new england but not farther south the lowland remained covered by the ice sheet until late stage 2 the ice sheet may also have advanced into the lake ontario basin during stage 4 the position of the northern margin of the laurentide ice sheet during stage 4 is not known but it remained an unknown distance behind its maximum position reached during stage 5 cores from baffin bay indicate a substantial decrease in high latitude glaciation during stage 4 following retreat the keewatin sector of the ice sheet may have remained over much of northwestern canada as a quasi stable ice mass until it readvanced during stage 2 similarly the baffin island sector of the ice sheet may have remained largely intact the southern margin of the ice sheet may have advanced into the lake ontario basin and upper mississippi valley during the middle of stage 3 ca 50 ka reaching its maximum extent of the last glaciation during stage 2 ca 18 21 ka advance of the northern margin was younger ca 8 13 ka than that of the southern margin this advance was less extensive than the penultimate advance stage 5 paleoenvironmental records indicate that the last interglaciation in areas covered by and near the cordilleran ice sheet was as warm as or warmer than present the cordilleran ice sheet appears to have developed during stage 5 or 4 at that time it advanced over southern british columbia and into the northern puget lowland there is no record of this event in northern areas that were later covered by the ice sheet the ice sheet disappeared before 59 ka at the beginning of a lengthy nonglacial interval paleoenvironmental records indicate that climate was similar to the present during part of this interval the ice sheet was absent and glaciers probably were confined to mountain areas throughout stage 3 climatic deterioration marking the end of this nonglacial interval may have begun as early as 29 ka by 14 15 ka the ice sheet had achieved its maximum extent of the last glaciation because there are few suitable dating methods capable of resolving events beyond the radiocarbon limit and because sites that preserve a record of events from the last glaciation are spatially restricted we consider this synthesis as tentative and subject to significant revision as dating methods improve nevertheless this perspective of the north american ice sheets through the last glaciation demonstrates their complex and dynamic behavior and attendant rapid fluctuations in ice volume

high field strength element ratios in archean basalts a window to evolving sources of mantle plumes abstract in terms of high field strength element ratios nb th zr nb nb y and zr y most basalts from non arc type archean greenstones are similar to oceanic plateau basalts suggestive of mantle plume sources a large number of these basalts have ratios similar to primitive mantle composition perhaps the archean mantle was less fractionated than at present and primitive mantle comprised much of the deep mantle and made a significant contribution to mantle plumes the near absence of archean greenstone basalts similar to nmorb in composition is also consistent with a relatively unfractionated mantle in which a shallow depleted source dm was volumetrically insignificant the element ratios in basalts also indicate the existence of recycled components himu em1 em2 in the mantle by the late archean this suggests that oceanic lithosphere was recycled into the deep mantle and became incorporated in some mantle plumes by the late archean high field strength element ratios also indicate an important contribution of continental crust or and subcontinental lithosphere to some non arc archean greenstone basalts this implies that at least thin continental lithosphere was relatively widespread in the archean

terminating the 100 kyr ice age cycle we report a simulation of the most recent 100 000 year glaciationdeglaciation cycle of the late pleistocene ice age a simulation that delivers an ice sheet chronology that is in close accord with that inferred from the geological record our analyses are performed with a reduced model of the climate system that incorporates significant improvements to the representation of both climate forcing and mass balance response in a previously described theory based upon a coupled one level energy balance model ebm and vertically integrated ice sheet model ism the theory fully incorporates the influences of orbital insolation forcing glacial isostatic adjustment and variations in the atmospheric concentrations of greenhouse gases it correctly predicts the main geographical regions of the northern hemisphere that were glaciated at last glacial maximum 21 000 years ago as well as the abrupt termination of the glacial epoch that occurred subsequently the latter feature of the ice age cycle is obtained without the need to incorporate unconstrained and therefore controversial physical processes into the model a limitation of all previous attempts to understand this global scale climate cycle our analyses suggest that the radiative impact on surface glaciation due to the changing atmospheric concentration of co2 is critical to the ability of the model to deliver a synthetic history of glaciation and deglaciation that is in accord with inferences based upon surface geological and geomorphological evidence with the incorporation of this influence model predicted ice thickness distributions at last glacial maximum lgm are very similar to those of the recently described ice 4g reconstruction that was based upon the inversion of postglacial relative sea level histories

geochemical and geochronologic constraints for paleozoic magmatism related to the orogenic collapse in the qimantagh south altyn region northwestern china abstract voluminous and discrete early paleozoic bimodal magmatic suites are thought to be the result of post collisional extension following the amalgamation of east kunlun and altyn tagh in this paper four representative magmatic units were studied for their geochemical fingerprint in conjunction with geochronological studies the 467 445 ma mangya mafic suite shows e morb type rare earth element ree patterns that are the result of asthenospheric interaction with a metasomatized subcontinental lithosphere high k calc alkaline granodiorites intruded at ca 450 ma are characterized by high mg the least fractionated ree pattern without an eu anomaly as well as high sr and low rb sr ratio we interpret these geochemical signals to result from lower crustal melting of garnet amphibolite at pressures between 16 and 22 kbar a 430 420 ma a type granite is interpreted to result from the melting of metaigneous rocks at middle to lower crustal depths lastly a late magmatic pulse occurs between 400 and 380 ma and is represented by the alk granite the alk granite is interpreted to be a product of metapelite melts and is associated with a smaller volume of mafic melts u pb zircon geochronology of paleozoic igneous rocks of the qimantagh south altyn reveals that most of the magmatic episodes are either coeval with or post extensional deformation this phase of extension is supported by the exhumation of hp uhp metamorphic rocks and crustal anatexis collectively the evolutionary stages documented in this study correspond to a succession of post collisional postorogenic and ultimately within plate magmatic episodes the overall features support orogenic collapse via removal of a thickened lithospheric root beneath the east kunlun altyn tagh collisional orogen during early paleozoic

zircon u pb dating and sulfide re os isotopes of the xiarihamu cu ni sulfide deposit in qinghai province northwestern china the xiarihamu cu ni sulfide deposit is the second largest cu ni deposit in china located in the eastern kunlun orogenic belt however despite previous study the formation and evolution of this d

early paleozoic orocline in the central china orogen abstract the altun qilian kunlun orogen as a western segment of the central china orogen has experienced a tectonic evolution of the proto tethys ocean and the early paleozoic orogenic movement to produce multiple stage early paleozoic structures and rocks the corresponding ophiolites metamorphic rocks and granites in the orogen show some similarities in petrology and geochemistry especially isotopic geochronology this indicates that they should be under the same subduction accretion system during the early paleozoic while there is diachronism of evolutionary time along strike combined with the paleomagnetic data and plate reconstruction the north qinling central qilian central altun oulongbuluke qaidam east kunlun and west kunlun micro continents from gondwana made up a chain like superterrane within the northern proto tethys ocean based on the detailed structural analysis and spatial and temporal distribution of the main rocks the proto tethys ocean subducted southward and formed a nearly linear subduction zone the tarim qaidam micro continent was likely to have drifted as a unit and mostly rotated relatively counterclockwise in the early paleozoic under the southward oblique subduction collision some large scale dextral strike slip faults developed and the relative displacement and rotation of continents micro continents and tectonic belts also took place in the region making the subduction zone bent to form a s shaped pattern in plan view in this paper we suggest that they are coupled oroclines and put forward a kinematic evolution model the shangdan ocean likely closed no earlier than the formation of the oroclines generating a curved suture zone bypassing east qaidam further linked southward to the central kulun suture although the coupled oroclines was displaced and destroyed by subsequent superimposed structures especially in the west of the altun orogen and east of the qaidam areas the early paleozoic tectonic unconformity and depositional systems developed in the tarim basin and the s shaped pattern of the paleostructures and hydrocarbon reservoirs provide evidence for its existence

middle late triassic magmatism in the hutouya fe cu pb zn deposit east kunlun orogenic belt nw china implications for geodynamic setting and polymetallic mineralization abstract the hutouya pb zn mining district in the qiman tagh region in nw china is a zoned skarn district with inner fe sn cu co ores and outer pb zn skarn mineralization temporal coincidence of 40ar 39ar plateau age of ore stage phlogopite 226 9 1 5 ma with weighted mean la icp ms u pb ages of zircons 230 3 3 7 and 221 6 1 3 ma and 40ar 39ar plateau ages of biotite 229 6 2 3 and 224 7 2 6 ma in the spatially associated monzogranite strongly suggest a genetic relationship between skarn formation in the hutouya metallogenic district and intrusion of the monzogranite suite major and ree analyses indicate the hutouya monzogranites are a type granites that belong to the high k calc alkaline series hafnium isotope data are consistent with a crust mantle mixing source geochemical characteristics of the middle late triassic indosinian 235 204 ma granitoids associated with mineralization in the qiman tagh region are consistent with an origin as post collisional mantle derived magmas that underplated the lower crust in the east kunlun orogen in an extensional setting causing lower crustal melting and subsequent mixing and assimilation that produced magmas fertile for mineralization

geomechanical simulation of low order fracture of tight sandstone abstract the core factors in the study of fractures within tight sandstone reservoirs are tectonics geomechanics and fluid mobility which are not independent but are coupled to each other in this paper taking the upper paleozoic gas reservoir within the low amplitude tectonic zone in the sulige gas field of the ordos basin as an example the developmental characteristics and geomechanical model of fractures and their relationships with the productivity of tight gas sandstone reservoirs were systematically studied the results show that this low amplitude structural zone develops mainly low order ruptures the paleotectonic stress field of the target layer in the yanshanian period was restored using a 3d finite element method the horizontal principal stresses exhibited stress diffusion and were affected by local low amplitude structures there was a positive correlation between h and h and h h was distributed primarily between 1 35 and 1 95 with an average of 1 62 the strain energy density u was calculated to quantitatively characterize the microfracture distribution of the target layer based on the principle of energy conservation there are certain positive correlations between u and the horizontal principal stresses the geostresses of high u regions underwent a certain degree of stress torsion which manifested as a stress deflection reaching up to 30 during stress propagation finally we proposed the concept of u value geomechanical sweet spots in a tight gas sandstone based on this criterion commercial gas wells can be effectively identified

ice sheet inception subgrid hypsometric parameterization of mass balance in an ice sheet model studies of ice sheet inception and mountain glaciation are limited by model resolution in both atmospheric and glaciologic models the accumulation and ablation of snow and ice are controlled by local climate which is sensitive to topographic features that are subgrid in continent scale investigations climate patterns are complex in areas of high relief but the first order influence of elevation on temperature and atmospheric humidity is quantifiable and is central to glacier mass balance we make substantial improvements to the descriptions of accumulation and ablation in a large scale ice sheet model by incorporating subgrid topographic information the distribution of relief in the large scale model is estimated through a hypsometric parameterization based on the minimum maximum and median topographic elevation this permits accounting of high elevation accumulation and melting in deep valleys we combine this with a parameterization of downslope subgrid ice flux simulating but not explicitly describing the role of alpine glaciers the detailed budgeting of mass balance greatly improves our skill in modelling present day ice distribution in north america we anticipate similar improvements in our ability to model nucleation centres of the quaternary ice sheets this approach may also prove useful in projections of glacier and ice field response to climate change

paleozoic granitic magmatism and tectonic evolution of the south altun block nw china constraints from zircon u pb dating and lu hf isotope geochemistry abstract the south altun orogenic belt saob consisting of the south altun block sab and the south altun continental block sacb is one of several important orogenic belts in nw china numerous granite bodies are widely distributed in the sab however the petrogenesis and tectonic environment of these granites and their relationships to those of the sacb central altun continental block cacb north altun block nab qilian block qlb and north qaidam block nqb remain unclear in this investigation we selected the sab granites for detailed study and explore the petrogenesis employing bulk rock geochemistry zircon u pb dating and hf isotopic analysis zircon u pb la icp ms dating of the granites from the sab identified seven spatially distinct paleozoic plutonic suites a yigan 352 343 ma b baiganhu 448 444 ma c west mangya 444 435 ma d aketishan 265 ma e chaishuigou 406 ma f changchungou 469 ma and g north mangya 462 ma based on previous research combined with our new u pb dating and bulk rock geochemical investigations in the sab we identdified six distinct episodes of granitoid emplacement episodes 1 and 2 quartz diorite granodiorite monzogranite with i type affinities and ages of 503 497 ma and 483 458 ma respectively episode 3 granodiorite monzogranite syenogranite with i and s type affinities and an age of 456 432 ma episode 4 monzogranite syenogranite alkali feldspar granite with a type affinities and an age of 424 385 ma episode 5 granodiorite monzogranite with s type affinities and an age of 352 338 ma and episode 6 quartz diorite monzogranite syenogranite with i type affinities and an age of 265 264 ma zircon lu hf isotope analyses from various episodes show that ehf t values are positive in most cases but negative for a small number of inherited xenocrystic zircons source rocks were mainly juvenile crust evidently mixed with minor ancient continental material based on our new results integrated with regional geological data we conclude that episodes 1 and 2 granitic magma generations were related to the subduction of oceanic crust episode 3 to post collisional partial melting episode 4 to the isostatic adjustment of orogenic blocks after plate suturing episode 5 to petrotectonic thickening of the altun crust and episode 6 to altun fault transcurrent slip attending kunlun paleo tethys subduction

the dynamic response of the greenland and antarctic ice sheets to multiple century climatic warming new calculations were performed to investigate the combined response of the greenland and antarctic ice sheets to a range of climatic warming scenarios over the next millennium use was made of fully dynamic 3d thermomechanic ice sheet models which were coupled to a two dimensional climate model the experiments were initialized with simulations over the last two glacial cycles to estimate the present evolution and were subsequently forced with temperature scenarios resulting from greenhouse emission scenarios which assume equivalent co2 increases of two four and eight times the present 1990 a d value by the year 2130 a d and a stabilization after that the calculations brought to light that during the next century short term effect the background evolution trend would dominate the response of the antarctic ice sheet but would be negligible for the greenland ice sheet on that timescale the greenland and antarctic ice sheets would roughly balance one another for the middle scenario similar to the ipcc96 is92a scenario with respective contributions to the worldwide sea level stand on the order of about 610 cm on the longer term however both ice sheets would contribute positively to the worldwide sea level stand and the most important effect would come from melting on the greenland ice sheet sensitivity experiments highlighted the role of ice dynamics and the height massbalance feedback on the results it was found that ice dynamics cannot be neglected for the greenland ice sheet not even on a century timescale but becomes only important for antarctica on the longer term the latter is related to an increased outflow of ice into the ice shelves and to the grounding line retreat of the west antarctic ice sheet which are both found to be sensitive to basal melting below ice shelves and the effective viscosity of the ice shelves stretching parameters to their limits yielded a combined maximum rate of sea level rise of 85 cm century21 of which 60 cm would originate from the greenland ice sheet alone

some geodynamic and compositional constraints on postorogenic magmatism postorogenic magmatic suites are common to many orogens in many cases apparently just postdating the cessation of deformation they differ from preceding orogenic suites in that they have higher temperatures more primitive isotopic signatures and bimodal natures and thus are compositionally similar to suites found in extensional regimes we propose that thinning of the lithospheric mantle which may be an automatic response to orogenic lithospheric thickening is responsible for these magmatic suites mantle lithospheric thinning moves the asthenospheric lithospheric thermal boundary higher in the lithospheric column thereby raising the overall thermal budget of the orogen and the likelihood of basaltic magmatism this removal of much of the dense lithospheric mantle root of the orogen also invokes uplift capable of producing horizontal orogenic buoyancy forces that will oppose and potentially terminate deformation extreme fractionation of the magmas is promoted by high temperatures and a noncompressional or tensional lithospheric stress regime to produce accompanying felsic a type magmas this model explains why the cessation of deformation is coincident with high temperature bimodal magmatism and inferred uplift as well as the long noted similarity and confusion between postorogenic and extensional magmatic suites this arises because both reflect thinning of the lithospheric mantle not because both reflect extensional tectonics we suggest that these magmatic episodes may be important in transferring lithospheric mantle material and its compositional signatures into the crust

methodology for use of isotopic climate forcings in ice sheet models in ice sheet models an internally consistent calculation of climatic temperature forcing from ice core isotopic records requires that corrections be made for an ice core site s elevation changes such corrections can be important for two reasons first elevation change at an ice divide can be large compared to elevation change at the migrating ice margin resulting in erroneous estimates of marginal melt rate second the of precipitation at an ice divide responds less to regional climatic temperature changes than to local temperature changes of the same magnitude that are caused by elevation changes significant elevation change corrections may also be necessary for inferring the relationships used to calculate climatic temperature from a simple methodology incorporating both of these elevational effects is presented here further i recommend that the to temperature conversion relations used in ice sheet models should derive from independently reconstructed temperature and time series rather than from spatial correlations and justify this recommendation based on current understanding of the atmospheric isotopic distillation system

a possible middle paleozoic suture in the altyn tagh nw china preliminary 40ar 39ar thermochronologic results from two transects across the altyn tagh at range allow new interpretations for the genesis of the range and provide constraints for its subsequent displacement recent reports of middle to upper ordovician pillow lavas in the ne portion of the range as well as the first reports of high pressure metamorphic rocks within the range are interpreted as the remnants of a lower paleozoic ocean subducted within the at muscovite and biotite ages from schist gneiss and granodiorite document a significant metamorphic and intrusive event throughout the range at 435 20 ma this fabric is crosscut by undeformed postorogenic microgranite plutons dated at 383 7 ma by muscovite these data suggest that an ocean basin closed after the early silurian and prior to the middle devonian forming the herein named lapeiquan suture the lack of similary aged plutons to the north and the suggestion of a synchronous arc to the south implies that subduction was south dipping present geography if the altyn tagh contains a middle paleozoic suture lateral continuations should be identifiable possibilities to the west include the kudi suture in the western kunlun shan comparisons of lithologies and radiometric data reveal significant similarities between the kudi suture and the altyn tagh suggesting that the at originated as part of the same middle paleozoic orogenic belt reconstruction of the paleozoic structure of northern tibet is complicated by cenozoic deformation particularly along the altyn tagh fault although this could be used as a measure of the finite post devonian deformation a more thorough understanding of paleozoic geodynamics is a prerequisite to unraveling subsequent displacements

a three dimensional climate ice sheet model applied to the last glacial maximum a quasi three dimensional climate model sellers 1983 was used to simulate the climate of the last glacial maximum in order to provide climatic input forthe modelling of the northern hemisphere ice sheets the climate model is basically a coarse gridded gcm with simplified dynamics and was subject toappropriate boundary conditions for ice sheet elevation atmospheric co2 concentration and orbital parameters when compared with the present daysimulation the simulated climate at the last glacial maximum is characterized by a global annual cooling of 3 5deg c and a reduction in global annualprecipitation of 7 5 which agrees well with results from other more complex gcm s also the patterns of temperature change compare fairly with mostother gcm results except for a smaller cooling over the north atlantic and the larger cooling predicted for the summer rather than for the winter overeurasia the climate model is able to simulate changes in northern hemisphere tropospheric circulation yielding enhanced westerlies in the vicinity of thelaurentide and eurasian ice sheets however the simulated precipitation patterns are less convincing and show a distinct mean precipitation increase over thelaurentide ice sheet nevertheless when using the mean monthly fields of lgm minus present day anomalies of temperature and precipitation rate to drive athree dimensional thermomechanical ice sheet model it was demonstrated that within realistic bounds of the ice flow and mass balance parameters veryreasonable reconstructions of the last glacial maximum ice sheets could be obtained

proto tethys magmatic evolution along northern gondwana insights from late silurian middle devonian a type magmatism east kunlun orogen northern tibetan plateau china abstract the east kunlun orogen records the geological evolutions of the neoproterozoic early paleozoic proto tethyan ocean and late paleozoic mesozoic paleo tethys ocean along northern gondwana however the late stage evolution of the proto tethyan ocean and the configuration of peri gondwana microcontinents during the silurian devonian is under debate here we report new geochronological and geochemical data of a type granites from the western wulonggou and the eastern gouli areas in the east kunlun orogen to deepen our understanding of these problems zircon la icp ms u pb data reveal that the danshuigou monzogranite and shenshuitan syenogranite from the western wulonggou area were emplaced simultaneously at 418 3 ma while the niantang syenogranite from the eastern gouli area was emplaced at 403 2 ma all these rocks display high k calcic alkalic to shoshonitic and metaluminous to slight peraluminous signatures with relatively low cao al2o3 mgo and sr and high feot mgo ga al zr and nb indicating their a type affinity their moderate whole rock end t 5 3 to 0 6 and zircon ehf t 6 3 6 4 are different from those of depleted mantle and old basement rocks but similar to those of the ordovician silurian granitoids in the east kunlun orogen these chemical signatures together with the anhydrous low pressure and high temperature characteristics of the magmas indicate that partial melting of the ordovician silurian granitoids generated these a type granites regionally these a type granites and previously reported a type granites in the east kunlun orogen compose a late silurian middle devonian a type granite belt this belt together with the regionally coeval molasse formation and mafic ultramafic rocks indicate a post collisional extensional regime for the east kunlun orogen during the late silurian middle devonian given that extensive contemporaneous post collision related magmatic rocks have also been revealed in the neighboring west kunlun altyn qilian and qinling blocks terranes we contend that the neoproterozoic early paleozoic proto tethyan ocean that separated these blocks terranes from gondwana had closed by the late silurian middle devonian which resulted in the re welding of the above blocks terranes to northern gondwana or gondwana derived microcontinents

a 3 d model for the antarctic ice sheet a sensitivity study on the glacial interglacial contrast on the longer climatic time scales changes in the elevation and extent of the antarctic ice sheet have an important role in modulating global atmospheric and oceanographic processes and contribute significantly to world wide sea levels in this paper a 3 d time dependent thermomechanical model for the entire ice sheet is presented that is subsequently used to examine the effects of glacial interglacial shifts in environmental boundary conditions on its geometry the model takes into account a coupled ice shelf grounding line dynamics basal sliding and isostatic bed adjustment and considers the fully coupled velocity and temperature fields ice flow is calculated on a fine mesh 40 km horizontal grid size and 10 layers in the vertical for grounded and floating ice and a stress transition zone in between at the grounding line where all stress components contribute in the effective stress in the flow law there is free interaction between ice sheet and ice shelf so that the entire geometry is internally generated a simulation of the present ice sheet reveals that the model is able to yield realistic results a series of sensitivity experiments are then performed in which lower temperatures reduced accumulation rates and lower global sea level stands are imposed either singly or in combination by comparing results of pairs of experiments the effects of each of these environmental changes can be determined in agreement with glacial geological evidence we found that the most pronounced changes show up in the west antarctic ice sheet configuration they appear to be essentially controlled by variations in eustatic sea level whereas typical glacial interglacial changes in temperature and ice deposition rates tend to balance one another these findings support the hypothesis that the antarctic ice sheet basically follows glacial episodes in the northern hemisphere by means of sea level teleconnections grounding occurs more readily in the weddell sea than in the ross sea and long time scales appear to be involved it may take up to 30 40000 years for these continental shelf areas to become completely grounded after an initial stepwise perturbation in boundary conditions according to these reconstructions a steady state antarctic ice sheet may contribute some 16 m to global sea level lowering at maximum glaciation

mantle unrooting in collisional settings abstract we present a two dimensional numerical model to study the thermo mechanical evolution of the lithosphere under a convergence regime in order to define the conditions that lead to lithospheric mantle break up and consequent unrooting a newtonian rheology with a temperature dependent viscosity is considered the system is not closed and horizontal flow through lateral boundaries is permitted a horizontal velocity is imposed at the top of the model to simulate compression whereas velocity vanishes at the bottom of the model the initial conditions correspond to a homogeneous lithosphere with a constant heat production in the crust the analysis of variations of maximum shear stress strain rate and total kinetic energy allowed us to define four major stages during the mantle unrooting process orogenic growth initiation of gravitational instability until lithospheric failure sinking of the detached lithosphere and relaxation of the system numerical results also show that the conditions for lithospheric unrooting strongly depend on the convergence velocity the wideness of the deformation zone and the imposed rheology

tectonic evolution of the south altyn nw china constraints by geochemical zircon u pb and lu hf isotopic analysis of the palaeozoic granitic plutons in the mangya area the south altyn orogenic belt saob is one of the most important orogenic belts in nw china consisting of the south altyn continental block and the apa mangya ophiolitic melange belt however its palaeozoic tectonic evolution is still controversial here we present petrological geochemical zircon u pb and lu hf isotopic data for the mangya plutons with the aim of establishing the palaeozoic tectonic evolution we divide the early palaeozoic magmatism in the apa mangya ophiolitic melange belt into four episodes and propose a plate tectonic model for the formation of these rocks during 511 494 ma the south altyn ocean sao was in a spreading stage and some shoshonite series i type granitic rocks were generated from 484 to 458 ma the oceanic crust of the sao subducted northward accompanied by large scale magmatic events resulting in the generation of vast high k calc alkaline series i type granitic rocks during 450 433 ma the sao closed and break off of the subducted oceanic slab occurred with the generation of some high k calc alkaline series i s transitional type granites the saob was in post orogenic extensional environment from 419 to 404 ma and many a type granites were generated

a comparison of different ways of dealing with isostasy examples from modelling the antarctic ice sheet during the last glacial cycle the bedrock isostatic response exerts a strong control on ice sheet dynamics and is therefore always taken into account in ice sheet models this paperreviews the various methods normally used in the ice sheet modeling community to deal with the bedrock response and compares these with a moresophisticated full earth model each of these bedrock treatments five in total are coupled with a three dimensional thermomechanical ice sheet model underthe same forcing conditions to simulate the antarctic ice sheet during the last glacial cycle the outputs of the simulations are compared on the basis of thetime dependent behavior for the total ice volume and the mean bedrock elevation during the cycle and of the present rate of uplift over antarctica thiscomparison confirms the necessity of accounting for the elastic bending of the lithosphere in order to yield realistic bedrock patterns it furthermoredemonstrates the deficiencies inherent to the diffusion equation in modeling the complex deformation within the mantle nevertheless when characteristicparameters are varied within their range of uncertainty differences within one single method are often of the same order as those between the variousmethods this overview finally tries to point out the main advantages and drawbacks of each of these methods and to determine which one is mostappropriate depending on the specific modeling requirements

antarctic ice volume and deep sea temperature during the last 50 myr a model study a simple quasi analytical model is used to study the sensitivity of the antarctic ice sheet to climate change the model is axisymmetrical and has a profile that only depends on the ice sheet radius the climatic conditions are represented by three parameters the altitude of the runoff line the accumulation rate above the runoff line and the balance gradient below the runoff line the ice sheet may extend into the sea at the grounding line the ice velocity is assumed to be proportional to the water depth for this set up an explicit algebraic expression for the total mass budget of the ice sheet can be derived after calibration of the model with respect to the present day ice sheet equilibrium states are studied for a wide range of temperatures the model predicts a maximum ice volume 3 4 for a temperature that is 2 5 k above the present value for a temperature increase of 7 k mass loss by runoff and calving are about the same in this case the ice volume is about 82 of the current value the ice sheet model is used to correct the cenozoic deep sea temperature record 18o record from benthic foraminifera in ocean sediments for antarctic ice volume the model is forced with the oxygen isotope record which is then corrected for the calculated ice volume therefore the resulting deep sea temperature and antarctic ice volume curves are mutually consistent it is concluded that for the last 35 106 years the 18o record truly is a mixed temperature ice volume record in which the contributions from these parameters have the same order of magnitude

the importance of thermodynamics for modeling the volume of the greenland ice sheet two different kinds of ice flow models one two dimensional 2 d and the other three dimensional 3 d have been used to test the importance of the thermodynamic response of the greenland ice sheet the basic difference between the two models is that the thermodynamics are neglected in the two dimensional model whereas these are incorporated in the three dimensional model the 2 d and 3 d models are compared in terms of volume response time and total volume of the ice sheet results indicate that the sawtooth character of the ice volume on glacial interglacial timescales is among other possible reasons a result of the thermodynamic coupling the changes in elevation over the last 130 000 years calculated with the three dimensional version are 230 m for the summit drill site and 190 m for the n grip site the standard deviation of the changes in elevation is 55 m for the summit site and 43 m for the n grip site the present day imbalance is merely a result of the rather constant climate over the last 10 kyr and is not determined by the thermodynamics consequently pleistocene temperatures do still exist in the ice sheet but are not important for volume calculations of the present day ice sheet for short term perturbation experiments in the future the pronounced sensitivity of the mass balance will determine the response of the greenland ice sheet whereas thermodynamics will play only a minor role

line source simulation for shallow seismic data part 1 theoretical background equivalent line source seismograms can be obtained from shallow seismic field recordings by 1 convolving the waveforms with sqrt 1 t 2 applying a sqrt 1 t time domain taper where t is traveltime and 3 scaling the waveform with r offset sqrt 2 where roffset is source to receiver offset we require such a procedure when applying algorithms of 2 d adjoint full waveform inversion fwi to shallow seismic data although derived from solutions for acoustic waves in homogeneous full space this simple procedure performs surprisingly well when applied to vertical and radial components of shallow seismic recordings from hammer blows or explosions this is the case even in the near field of the force although the procedure is derived from a far field approximation similar approximative procedures recommended in literature are optimized for reflected waves and do not convert the amplitudes of all shallow seismic wavefield constituents equally well we demonstrate the suitability of the proposed method for the viscoelastic case by numerical examples as well as analytical considerations in contrast to the proposed single trace procedure integral transform approaches are exact for all viscoelastic wavefield constituents of the near and far field in unknown 1 d heterogeneous structure unfortunately integral transform approaches suffer from artefacts in applications to data sampled on 2 d structures here we use the fourier bessel integral transformation as a reference in 1 d heterogeneous structures we unroll the wave theoretical background of both approaches in order to demonstrate why the simplistic single trace simulation approach derived from the asymptotic acoustic case can perform so well when applied to the shallow elastic wavefield further we give recommendations for practical implementation and application to field data of the proposed simulation method and compare to the results of alternative conversion rules the performance of the conversion procedure to data recorded on 2 d heterogeneous structures is presented in a companion study by fwi reconstruction tests

age petrogenesis and tectonic implications of early devonian bimodal volcanic rocks in the south altyn nw china abstract in this paper we report zircon u pb dating hf isotopes geochemical and sr nd isotopic data with the aim the petrogenesis and regional tectonic evolution of early devonian bimodal volcanic rock in the south altyn nw china new la icpms zircon u pb isotopic data constrained them at ca 406 ma the mafic samples are characterized by high fe cr and ni contents low ti and mg contents slightly enriched lree patterns and low la yb n la nb and la ta ratios and positive e nd t values 3 3 to 3 4 indicating that they were likely derived from strong batch melting of the asthenosphere in the spinel facies field the felsic rocks show an a type affinity with high alkalis fe ga zr nb ce and y contents low mg sr content high rb sr and ga al ratios enrichment in lile e g rb k th u and lree and depletion in ba sr nb ta p and ti and fractionated ree patterns with very strong negative eu anomalies these features along with distinct e nd t values 0 5 to 2 3 and mostly positive e hf t 0 29 to 5 18 indicate that the felsic rocks were mainly generated by partial melting of the crust in low pressure and high temperature conditions and simultaneously underwent slight magma mixing of such melts with mantle magma according to the petrogenetic schemes and geological background of the early devonian bimodal volcanic rocks tholeiite and a type dacite rhyolite they should have formed in a post collisional extensional setting moreover on the basis of spatial and temporal distribution and formation mechanism the tectonic magmatic evolution of the early paleozoic south altyn tagh could be divided into three stages 505 472 ma continental collision the magmatite formed under high pressure conditions due to the deep subduction and initial tearing of continental slab 467 450 ma continental slab break off the magmatite formed at high temperature and low pressure in virtue of felsic upper crust uplifting and mantle magma underplating and 432 385 ma post collisional extension the magmatite consists of a type granites and bimodal volcanic rocks which are the products of the interaction between mantle and crust and asthenosphere upwelling

geological geochemical and geochronological characteristics of caledonian w sn mineralization in the baiganhu orefield southeastern xinjiang china abstract the newly discovered large scale baiganhu w sn orefield consisting of the kekekaerde baiganhu bashierxi and awaer deposits is located in ruoqiang county southeastern xinjiang china these deposits comprise mainly three types of w sn mineralization early stage skarn type middle stage greisen type and late stage quartz vein type in this study we classified seven major vertical zones on the basis of petrographic characteristics roughly from the bottom of the parental granitic intrusions upward as a fresh syenogranite b argillic alteration c muscovite dominated greisenization d tourmaline dominated greisenization e marginal facies including k feldspar pegmatite and fine grained granite f aplitic apophysis and g1 skarn or g2 infilled silification zones according to the alteration mineralization assemblages and cross cutting relationships five stages of mineralization are recognized in the orefield i skarn stage ii greisen stage iii quartz vein stage iv argillic alteration stage and v supergene stage and reverse alteration zonation in the altered intrusion is also observed the w sn deposits are spatially associated with syenogranite which is part of the caledonian bashierxi magmatic series laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb dating of the syenogranite yields a weighted mean 206 pb 238 u age of 413 6 2 4 ma mswd 0 36 n 30 hydrothermal muscovite from three samples associated with w sn mineralization yields plateau 40 ar 39 ar ages of 411 7 2 6 ma mswd 0 21 412 8 2 4 ma mswd 0 22 and 413 8 2 6 ma mswd 0 22 which is consistent with the zircon u pb age thus indicating a temporal link between the emplacement of the syenogranite and the w sn mineralization our age data and previously published ages along with geochemical data for the granitoids in the bashierxi magmatic series show a nearly contemporaneous evolution of a and s type granites which were emplaced in a post orogenic setting at ca 432 413 ma as compared with the a type granites the syenogranites with s type affinities probably resulted from a lower degree of partial melting of metagreywackes which was more likely to be enriched in the ore forming elements w and sn as well as volatiles such as b and h 2 o in addition the syenogranites exhibit low oxidation states and underwent high degrees of factional crystallization both of which favor post magmatic w sn mineralization we suggest that more attention should be given to buried syenogranites of s type affinities during mineral exploration in this area and that the proposed model of a vertical alteration zoning can act as a guide to the targeting of similar ore systems

geochemistry and zircon u pb geochronology of granitoids in the east kunlun orogenic belt northern tibetan plateau origin and tectonic implications abstract the east kunlun orogenic belt ekob in the northern margin of the tibet plateau is characterized by widespread granitic plutons which are keys to understanding the tectonic evolution of the ekob the zhiyu pluton newly recognized in the central part of the ekob mainly consists of monzogranites biotite granites and quartz diorites their la icpms zircon u pb results show formation ages of 447 1 6 ma 448 2 5 ma and 408 1 8 ma the monzogranites and biotite granites are characterized by relatively high sr 208 631 ppm low y 4 28 15 82 ppm and yb 0 44 1 59 ppm contents thus resulting in elevated sr y 30 105 and la yb n 4 79 ratios indicating geochemical features of adakitic rocks these adakitic granites are medium to high k calcic or calc alkaline in composition and display a weak peraluminous character they have low mgo 0 57 1 84 wt average 1 01 wt mg 40 53 average 45 as well as low cr 3 67 17 98 ppm average 7 19 ppm and ni 2 59 9 30 ppm average 4 71 ppm contents these rocks are enriched in lree and show negligible or variable positive eu anomalies eu eu 0 61 3 80 average 1 45 and significant negative nb and ta anomalies majority of the zircon grains from these adakitic granitic rocks have positive ehf t values of 0 09 5 21 with two stage model ages ranging from 1 1 ga to 1 6 ga these features are compatible with those of adakitic rocks derived from a thickened lower crust in the garnet stability field their formation is mainly controlled by the process of crust thickening following the closure of the qimantag ocean the younger quartz diorites belong to medium to high k calc alkalic or alkali calcic and metaluminous series and exhibit a relatively high mgo 2 23 5 18 wt and mg 40 56 average 50 11 with significant lree enrichment and negative eu anomalies as well as depletion of nb ta in addition the quartz diorites have an enriched ehf t values ranging from 5 25 to 3 19 combining the regional tectonic data we infer that the younger quartz diorites were derived from enriched lithospheric mantle sources and contaminated by the crustal materials in a post collision extensional setting