depleted mantle source for the ulungur river a type granites from north xinjiang china geochemistry and nd sr isotopic evidence and implications for phanerozoic crustal growth abstract the ulungur river granites from north xinjiang china display two petrological trends 1 a peralkaline trend composed of alkali amphibole and alkali pyroxene bearing granites and dykes and 2 an aluminous trend represented by alkali granites monzogranites and aluminous granite dykes within the peralkaline granites rb sr whole rock isochron dating yielded an age of 300 ma for the emplacement of the peralkaline granites and 270 ma for the aluminous granites the ulungur river granites share all the geochemical features common to a type granites of the world the ulungur river granites are post orogenic and emplaced in a post collisional extensional environment the peralkaline and aluminous rocks have indistinguishable nd isotopic compositions they have unusually positive and relatively uniform nd t values ranging from 5 1 to 6 7 for the peralkaline rocks at 300 ma and from 5 5 to 5 9 for the aluminous rocks at 270 ma the initial 87sr 86sr ratios cannot be determined precisely by the whole rock isochron technique mainly because most samples are highly radiogenic and have high rb sr ratios in addition the large variation in sr isotopic compositions can partly be explained by crustal contamination during magma differentiation the nd isotopic data clearly indicate that both the peralkaline and aluminous rocks were derived from a long lived depleted mantle reservoir the formation of the ulungur river a type granites is hypothesized as follows 1 a depleted mantle was first metasomatised then subjected to partial melting leading to the generation of basic magmas now represented by the kalatongke mafic ultramafic complex in the ulungur river area 2 the basic magmas have undergone high degrees of fractional crystallisation with separation of biotite hornblende plagioclase potassium feldspar and other accessory phases such as apatite and ilmenite with the residual magmas forming the peralkaline granites during the magmatic differentiation the liquids were probably contaminated by crustal materials as evidenced by nb depletion and nd sr isotopic systematics on the other hand the formation of aluminous granites involved a replenishment of new mantle derived magmas into the differentiating magma chamber and followed by fractional crystallisation with retention of hydrous phases hornblende and biotite the petrological evolution of the ulungur river granites is thus from peralkaline to calc alkaline and not from calc alkaline to peralkaline as more often observed elsewhere the ulungur river granites in common with the voluminous granitoids occurring in the central asian orogenic belt represent a significant juvenile addition of the continental crust in the phanerozoic

deficiencies in the diatom evidence for a pliocene reduction of the east antarctic ice sheet evidence for pliocene reduction of the east antarctic ice sheet eais has been inferred from the presence of pliocene planktic marine diatoms in tills of the sirius group now exposed along the slopes of the transantarctic mountains we highlight deficiencies in this diatom evidence for a pliocene reduction of the eais first we examine what diatom habitats are left behind by a retreating ice sheet in this case the fennoscandian ice sheet fis and find that diatoms should occur in planktic and benthic marine brackish water freshwater and terrestrial habitats we expect that during former reexpansions these diverse diatom assemblages were picked up by the advancing fis and deposited in tills this is shown to be the case and diatoms found in tills from sweden and finland reflect the many habitats over which the fis passed if there was a reduction of the eais during the pliocene followed by renewed glaciation diatoms from many different habitats should be found in the resulting tills of the sirius group this is not the case only marine planktic diatoms and a few freshwater forms are reported similarly vertical downsection distribution of diatoms in sirius group and fennoscandian tills differ while in the sirius group diatoms appear restricted to surface and near surface sediments in fennoscandia they are found throughout the tills such data do not support a pliocene reduction model for the eais

tectonics and hp uhp metamorphism of northern tibet preface abstract this special issue has been prepared for the 8th international eclogite conference iec to be held in the qilian qaidam regions western china during august 2009 most papers in this issue summarize results of many current petrological geochemical geochronological and tectonic studies of both qilian pacific type and qaidam alpine type lithotectonic assemblages important new occurrences include the qilian lawsonite eclogites which required an extremely cold subduction of the early paleozoic oceanic lithosphere and the mixing of oceanic and supracrustal protoliths in the qaidam continental subduction channel these two early paleozoic sutures in northern tibet provide additional constraints on the regional tectonic evolution of paleo and neo tethys in south central asia

diatoms in antarctic ice cores some implications for the glacial history of antarctica the presence of reworked pliocene and older marine microfossils in glacial tills of the sirius group suggests that these sediments were emplaced during the pliocene or later by glaciers riding up the eastern east antarctic slope of the transantarctic mountains this interpretation implies that there was considerable fluctuation in the size of the east antarctic ice sheet as recently as ca 3 ma here we test an alternate scenario eolian emplacement of the pliocene microfossils by comparing diatoms in the sirius group with diatoms in holocene and last glacial maximum ice from the dome c core from the central antarctic plateau since the diatoms in dome c ice are known to be eolian in origin we hypothesize that if the sirius diatoms are also eolian then the two antarctic sites dome c and the sirius group equally remote from the southern ocean will have the same exotic flora i e windblown diatoms from either a fresh water or marine source however whereas the dome c diatoms are predominantly fresh water those of the sirius are predominantly marine we conclude from these data that the sirius and dome c diatoms each reflect a different mode of emplacement and that the diatoms in the sirius till were not emplaced via wind transport this conclusion does not prove that the pliocene diatoms in the sirius till record a deglaciation and pliocene reglaciation of east antarctica but rather it removes an alternate mechanism eolian transport raised by those who oppose this scenario

geochronological evidence supporting antarctic deglaciation three million years ago the response of the antarctic ice sheets to increased global temperatures is an important unresolved issue in the assessment of future climate change in particular considerable controversy exists as to whether the east antarctic ice sheet suffered extensive deglaciation during the mid pliocene epoch 3 myr ago when temperatures were only slightly warmer than today although the ice sheet is widely assumed to have existed in something like its present form for the past 14 myr ref 1 marine diatoms eroded from the antarctic interior have been found in glacial till deposits high in the transantarctic mountains2 3 and have been bio stratigraphically dated at 3 myr before present this age has been disputed4 because it implies marine deposition in the antarctic interior and hence substantial deglaciation at a time when other evidence has been marshalled for the persistence of cold polar conditions4 here we report k ar and 40ar 39 ar ages for a volcanic ash bed in diatom bearing glaciomarine strata cored in ferrar fiord east antarctica by the ciros 2 drill hole5 which confirm the age of the diatoms at 3 myr and hence also confirm the mid pliocene deglaciation

paleozoic tectonic evolution of the northern xinjiang china geochemical and geochronological constraints from the ophiolites 1 the northern xinjiang of china is a composite orogenic belt in the south central part of the altaids formed by subduction accretion mainly during the paleozoic geochronologic and geochemical data from the paleozoic ophiolites and associated assemblages are reported here to place constrains on the tectonic evolution of this region and to reconstruct its history using a model of subduction accretion plus large scale map view strike slip faulting geologic and geochronologic review indicates that the paleozoic ophiolites and associated assemblages of the western and eastern junggar regions are tectonically related occurring as a pattern with the early paleozoic tangbale mayile hongguleleng aermantai complexes in the center the late paleozoic darbut karamaili and the northern tianshan complexes in the south and the late paleozoic kekesentao qiaoxiahala kuerti complexes in the north available geochemical data suggest that the paleozoic ophiolites of the northern xinjiang are mainly oceanic fragments of these three types mid ocean ridge basalt oceanic island basalts and island arc basalts the paleozoic subduction accretion complexes grew mainly along different segments of the kipchak arc and in associated with migration of magmatic arc the paleozoic ophiolite associated sedimentary sequences and subduction related volcanic rocks exhibit an evolution history from intraoceanic early paleozoic to marginal settings late paleozoic implying coeval strike slip stacking with subduction accretion processes and by which to transport the respective complexes together and further to amalgamate with neighboring units of the altaids subduction accretion processes terminated in the mid carboniferous in this region in the late permian early triassic a counterclockwise rotation of fault blocks in a giant sinistral strike slip and extension domain between altay and tianshan lastly generated present configuration of the paleozoic subduction accretion complexes in the northern xinjiang

continental growth of northwest china the northwest corner of china is the most interior part of central asia and includes parts of the siberian tarim and kazakhstan plates where the junggar and tarim basins are separated by the east west trending tianshan range a preliminary terrane map divides the area into thirteen separate areas which are classified into four types l continental 2 oceanic crust 3 island arc 4 composite the boundaries for these terranes are faults that have been previously defined either by geologic mapping or holocene activity passive consolidation of the southern margin of paleo asia is marked by accretion and subduction of the paleo tethys oceanic basins and by development of volcanic arcs paleomagnetic data indicate that the major plates and associated smaller terranes did not reach final consolidation until the permian or later the stratigraphic record reveals development of intracontinental basins in the late carboniferous followed by the pan asian thermal event that gave rise to widespread intrusion of a type granites of late paleozoic age the frontal collision of india along the southern border of asia in the paleocene reactivated nearly all of the older major strike slip and thrust faults formed during the late paleozoic consolidation of paleo asia the complex nature of the continental growth of central asia has produced and destroyed many varieties of sutures and reconstruction of the original plate and terrane configurations therefore is enigmatic

tidal heating in an internal ocean model of europa considerable evidence suggests that europe s internal structure might consist of a liquid water layer that decouples a thin 30 km overlying ice lithosphere from an underlying silicate core1 2 a lack of impact features extremely subdued topography and positive spectroscopic identification of h2o all imply recent resurfacing by water in addition curvilinear features resembling cracks are ubiquitous over the surface their orientations are broadly consistent with tidally controlled tectonic activity3 compositional models of europea indicate that the h2o layer could be over 100 km thick1 cassen et al 4 first showed how tidal heating if sufficiently intense might stabilize a thin 30 km ice lithosphere over the internal ocean and prevent the water from freezing other models of europa do not include an internal ocean and instead have an ice layer perhaps as thick as 100 km or as thin as a few kilometres resting directly on the silicate interior5 6 however based on studies of crater relaxation thomas and schubert7 have shown the version of this model with a thin ice layer to be unlikely the plausibility of the internal ocean model vis a vis the thick ice model depends critically on the level of tidal heating in europa here we present new calculations of tidal heating based on a more realistic three layer model of europa the tidal distortion of a decoupled ice lithosphere is only half that previously thought at the current value of orbital eccentricity tidal heating is only marginally able to prevent the internal ocean from freezing

the kokchetav massif kazakhstan type locality of diamond bearing uhp metamorphic rocks abstract after the discovery of metamorphic coesite in crustal rocks from the western alps italy and the western gneiss region norway in the mid 1980s of the last century metamorphic diamond was observed only a few years later in situ in the kokchetav massif kazakhstan findings of such coesite and diamond bearing ultrahigh pressure metamorphic uhp rocks with protoliths formed or embedded in crustal levels and subsequently experienced pt conditions within or even higher than the coesite stability field have dramatically changed our geodynamic view of orogenetic processes these occurrences provide evidence that crustal rocks were subducted into mantle depths and exhumed to the surface recent studies even suggest continental subduction to depths exceeding 300 km these rocks have been extensively studied and many new and important observations have been made thus far more than 350 papers have been published on various aspects of kokchetav uhp rocks the kokchetav massif of northern kazakhstan is part of one of the largest suture zones in central asia and contains slices of hp and uhp metamorphic rocks classical uhp rocks mainly occur in the kumdy kol barchi kol and kulet areas and include a large variety of lithologies such as calcsilicate rocks eclogite gneisses schists marbles of various compositions garnet pyroxene quartz rocks and garnet peridotite most of them contain microdiamonds some of which reach a grain size of 200 m most diamond grains show cuboid shapes but in rare cases diamonds within clinozoisite gneiss from barchi kol occur as octahhedral form microdiamonds contain highly potassic fluid inclusions as well as solid inclusions like carbonates silicates and metal sulfides which favour the idea of diamond formation from a c o h bearing fluid nitrogen isotope data and negative 13c values of kokchetav diamonds indicate a metasedimentary origin pt estimates of kokchetav uhp rocks yield peak metamorphic conditions of at least 43 kbar at temperatures of about 950 1000 c some zircon separates show inherited proterozoic cores and 537 530 ma uhp metamorphic mantle zones several ar ar ages on micas scatter around 529 528 and 521 517 ma and reflect different stages of the exhumation history migmatization occurred during exhumation at about 526 520 ma isotopic studies on calcsilicate rocks confirm a metasedimentary origin 18o values of garnet and clinopyroxene of a layered calcsilicate rock rule out the possibility having a primitive mantle protolith similar studies on eclogites indicate their basaltic protolith having experienced water rock interaction prior to uhp metamorphism a number of unique mineralogical findings have been made on kokchetav uhp rocks k feldspar exsolutions in clinopyroxene demonstrate that potassium can be incorporated into the cpx structure under upper mantle pressures other significant observations are coesite exsolutions in titanite quartz rods in cpx the discovery of k tourmaline as well as new minerals like kokchetavite a hexagonal polymorph of k feldspar and kumdykolite an orthorhombic polymorph of albite the kokchetav uhp rocks represent a unique and challenging stomping ground for geoscientists of various disciplines from crystallography petrology and geochemistry to geophysics and geodynamics geotectonics it concerns all who are interested in the diverse metamorphic processes under upper mantle conditions

tidal dissipation surface heat flow and figure of viscoelastic models of io abstract the distribution of hot spots on io s surface and io s figure constrain models of io s interior voyager observations indicate that most hot spots and volcanic plumes are close to the equator recently however it has been proposed that a large low albedo feature in the south polar region may be the source of 40 of the surface heat flow we calculate spatial distributions of the dissipation rate surface heat flow and the figures of three and four layer maxwell rheology models of io by using the correspondence principle the models are required to satisfy the observed surface heat flow constraint and are consistent with the composition of a dehydrated c2 chondrite they consist of an inviscid fe fes core and viscoelastic silicate mantle and lithosphere layers a comparison of the calculated equilibrium figure with the observed figure of io supports the chosen core radius of 980 km in addition the four layer model has a partially molten asthenosphere a few tens of kilometers thick the preferred three layer model has a mantle shear modulus of 10 10 pa and a viscosity of 2 10 16 pa s consistent with rock properties the tidal dissipation rate for this model is largest along the polar axis integration of the dissipation rate over radius gives a surface heat flow with maxima at the poles and minima near the sub jovian and anti jovian points in contrast the dissipation rate in a molten or partially molten incompressible asthenosphere has maxima near the equator and is close to zero at the poles the asthenosphere dissipation rate is mainly a function of asthenosphere viscosity and thickness the thinner the asthenosphere the smaller is the required viscosity a 50 km thick asthenosphere requires a viscosity of about 10 8 pa s a strong south polar hot spot if it can be confirmed by infrared observation favors dissipation in the mantle but does not exclude dissipation in a partially molten asthenosphere equatorial hot spots and the concentration of prometheus type plumes near the equator favor asthenosphere heating but do not rule out deep mantle dissipation both asthenosphere and deep mantle heating may contribute to io s surface heat flow the properties of magma transport through the lithosphere and the planform of mantle convection might exert a control on the surface expression of heating at greater depth

geochemistry and tectonic environment of basaltic rocks from the misis ophiolitic melange south turkey abstract the miocene misis tectonosedimentary melange was formed during the closing stages of the neotethys in what is now the adana basin of southern turkey a major part is characterized by large disorientated limestone blocks set in a predominantly volcanogenic matrix of bedded and chaotic units comprising basaltic pillow lavas associated hyaloclastites serpentinites and various acidic tuffs all the basaltic rocks are mildly metamorphosed zeolite facies variably vesicular and differentially altered to smectite chlorite fe oxides zeolites and carbonate petrographic and chemical data on the least altered pillow lavas indicate they were originally sparsely olivine plagioclase clinopyroxene phyric tholeiites and minor amphibole phyric basaltic andesites the basaltic clasts in the melange matrix are genetically related to the pillow lava sequences with the observed variation throughout the suite being governed by low pressure fractional crystallization as a group the basalts are characterized by low zr y 2 4 relatively high th yb 0 5 0 7 and la nb 1 5 2 5 ratios and moderate chondrite normalized light ree enrichment la yb n 3 patterns the basalts have a mildly enriched subduction related chemistry and were possibly formed initially in a back arc basin setting the supra subduction zone characteristics of the misis basaltic units are similar to other tethyan basalts in eastern mediterranean ophiolites

late cenozoic glacial history of the ross embayment antarctica abstract knowledge of the cenozoic terrestrial and marine glacial history of antarctica has been developed through studies of on land surface outcrops and subsurface drillcores and through examination of drillcores and piston cores obtained from a variety of offshore marine geological settings these field and laboratory endeavors have been a major target of the antarctic earth science community for more than thirty years the ross sea sedimentary basins including the byrd basin west antarctica wilkes and pensacola basins east antarctica and the transantarctic mountains provide significant data on the cenozoic glacial record a record that has been traced back to at least 38 ma close to the eocene oligocene boundary this paper reviews the late cenozoic history of the transantarctic mountains and flanking sedimentary basins with emphasis on the debate between the stabilist view of antarctic ice sheet history see clapperton and sugden 1990 and our favored multi glacial dynamic ice sheet model

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

geochemical sr nd and zircon u pb hf isotopic studies of late carboniferous magmatism in the west junggar xinjiang implications for ridge subduction abstract voluminous granitic intrusions are distributed in the west junggar nw china and they can be classified as the dioritic rocks charnockite and alkali feldspar granite groups the dioritic rocks sio 2 50 4 63 8 wt are calc alkaline and mg enriched average mgo 4 54 wt mg 0 39 0 64 with high sr y ratios average 21 2 weak negative eu average eu eu 0 80 and pronounced negative nb ta anomalies their sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7035 0 7042 e nd t 4 5 7 9 e hf t 14 1 14 5 show a depleted mantle like signature these features are compatible with adakites derived from partial melting of subducted oceanic crust that interacted with mantle materials the charnockites sio 2 60 0 65 3 wt show transitional geochemical characteristics from calc alkaline to alkaline with weak negative eu average eu eu 0 75 but pronounced negative nb ta anomalies sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7037 0 7039 e nd t 5 2 8 0 e hf t 13 9 14 7 also indicate a depleted source suggesting melts from a hot juvenile lower crust alkali feldspar granites sio 2 70 0 78 4 wt are alkali and fe enriched and have distinct negative eu and nb ta anomalies average eu eu 0 26 low sr y ratios average 2 11 and depleted sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7024 0 7045 e nd t 5 1 8 9 e hf t 13 7 14 2 these characteristics are also comparable with those of rocks derived from juvenile lower crust despite of the differences in petrology geochemistry and possibly different origins zircon ages indicate that these three groups of rocks were coevally emplaced at 305 ma a ridge subduction model can account for the geochemical characteristics of these granitoids and coeval mafic rocks as the slab window opened upwelling asthenosphere provided enhanced heat flux and triggered voluminous magmatisms partial melting of the subducting slab formed the dioritic rocks partial melting of the hot juvenile lower crust produced charnockite and alkali feldspar granite and partial melting in the mantle wedge generated mafic rocks in the region these results suggest that subduction was ongoing in the late carboniferous and thus support that the accretion and collision in the central asian orogenic belt took place in north xinjiang after 305 ma and possibly in the permian

shrimp zircon chronology of hp uhp rocks of the makbal metamorphic complex in the northern tien shan kyrgyzstan abstract in this paper we present the first results of sensitive high resolution ion microprobe shrimp u pb zircon dating obtained for the ultra high pressure uhp metamorphic rocks of the makbal metamorphic complex in the northern tien shan kyrgyzstan zircon grains from two hp eclogite samples and one sample of uhp garnet talk chloritoid schist have similar bright rims identified on cathodoluminescence cl images the rims are interpreted as zircon overgrowth during uhp metamorphism dating of zircon rims from the three samples yielded similar ages of 509 7 ma 502 10 ma and 498 7 ma the time span of 510 500 ma is considered as the minimum age of uhp metamorphism of the makbal complex this age is in accordance with middle cambrian faunal age of the kara archa ophiolite in the makbal area it is suggested that the makbal rocks were metamorphosed in the middle cambrian subduction zone dipping north under the northern tien shan microcontinent a deformed granodiorite associated with the makbal metamorphic rocks yielded an age of 514 5 ma indicating development of a magmatic arc simultaneously with the uhp metamorphism cores of zircon grains from the garnet talk chloritoid schist yielded a wide spectrum of ages 1850 800 ma characteristic for detrital zircons of continental origin a depositional age

when did the subduction first initiate in the southern paleo asian ocean new constraints from a cambrian intra oceanic arc system in west junggar nw china abstract we report here a cambrian southward subducting intra oceanic arc system in the southern west junggar nw china where an immature arc occurred initially on ssz type ophiolites and finally evolved into a more mature one the immature arc is dominantly represented by early middle cambrian 510 ma low k tholeiitic felsic rocks whereas the mature arc is characterized by late cambrian 495 ma medium and high k calc alkaline felsic and mafic rocks the ssz type ophiolites show remarkable depletion of nb and ta and contain high cr spinel cr 0 6 resembling these formed in the forearc altogether they record the initiation of subduction and transformation of crust during early subduction of the paleo asian ocean in the southern part of the central asian orogenic belt caob the subduction initiation might occur in the early cambrian 515 ma as constrained by both the ssz type ophiolites 516 ma and the oldest arc plutons 515 509 ma that crosscut the ophiolites the immature felsic plutons have high sio 2 72 wt contents and variable mgo 0 42 1 49 wt and mg values 22 62 crustal anatexis may be responsible for the genesis of those plutons and thus the transformation from oceanic to continental crust these results combined with regional data convincingly indicate that it is one of the oldest intra oceanic arc systems in the southern caob which may mark the initial subduction of the paleo asian ocean in its southern part much later than those reported in the north an archipelago type model is proposed for the evolution of the southern west junggar and has implications for the development of the southern part of the caob

intraplate volcanism off south greenland caused by glacial rebound plate tectonic mechanisms explain most of the dominant types of volcanism intraplate volcanism has been attributed to hot spots associated with mantle plumes though alternative explanations have been proposed for settings not fed by plumes we report young intraplate volcanism in an amagmatic margin south of greenland in a 40 ma old plate an elongated structure with a length of 110 km and a height of 2500 m above basement extends at an oblique angle to the mid atlantic ridge and the fracture zones we propose that this structure is related to reactivation of a rudimentary fracture zone by edge driven convection across minna fracture zone and partial melting caused by glacial rebound during the periodic melting of ice on greenland in the last 3 ma

ice age as a trigger of active quaternary volcanism and tectonism abstract the stress accumulation within the crust caused by the surface mass redistribution associated with the glaciation deglaciation cycle during the quaternary was numerically evaluated in order to examine the relationship between active quaternary volcanism and tectonism in island arc areas and ice age the vertical gradient of horizontal stress difference in the lithosphere for a meltwater of 130 m in equivalent sea level reaches a maximum value of 0 8 mpa km which is corresponding to the equivalent buoyancy of about 100 kg m3 for magma filled cracks for an earth model with a lithospheric layer of 20 30 km thickness and with a viscosity greater than 1023 pa s the changes in stress difference during the stages of deglaciation of 10 000 years amount to 13 mpa for both the top and bottom of the thin lithosphere thus the additional stress difference within the crust may be effective for island arc areas with thin lithospheric thickness we therefore speculate that the stress accumulation associated with ice age may be an important trigger and or accelerator on the active quaternary volcanism and tectonism for the areas along the circum pacific

middle paleozoic initial amalgamation and crustal growth in the west junggar nw china constraints from geochronology geochemistry and sr nd hf os isotopes of calc alkaline and alkaline intrusions in the xiemisitai saier mountains abstract as one of the important parts of the central asian orogenic belt the early middle paleozoic subduction accretion history of the west junggar is still needed to be further constrained although major progress has been made in the past years in particular the geodynamic setting for late silurian early devonian a type granites and associated mafic to felsic magmatism in the northern west junggar is still controversial in this study we report new geochronological and geochemical results for middle paleozoic plutons in the xiemisitai saier mountains northern west junggar including undeformed peralkaline granite alkali feldspar granite monzogranite quartz syenite syenite monzodiorite diorite and hornblende gabbro these plutons crop out in the eastern segment of the early middle paleozoic boshchekul chingiz arc and invade into early middle paleozoic marine volcanic and pyroclastic sequences new and published zircon u pb ages indicate that these intrusions were emplaced during the late silurian middle devonian 424 392 ma late silurian hornblende gabbro diorite and monzodiorite commonly contain abundant hornblendes and exhibit mantle like nb ta ratios 15 9 18 7 negligible eu anomalies with primitive sr nd hf i sr 0 70370 0 70375 end t 5 1 to 6 0 and ehf t 14 3 to 16 2 and radiogenetic os i os 0 2099 0 8795 isotopic signatures they may be originated from a fluid metasomatized h 2 o rich and isotopically depleted mantle source and suffered some crustal contamination early middle devonian calc alkaline monzogranite and alkaline peralkaline granitoids may have distinct origins the monzogranite is i type granite with high la yb ratios depleted end t values 5 1 and crust like nb ta ratios 8 5 8 9 which is suggested to be partial melt of juvenile lower crust containing massive hornblende garnet the peralkaline alkaline granitoids include a type and highly fractionated i type granites which contain mafic intermediate enclaves and chemically exhibit variable element ratios analogical end t values 5 1 to 6 2 and significantly high i os value 2 1247 they are interpreted to be produced by fractional crystallization of the hybrid magma contributed from mantle and crustal sources based on our new data along with other geological evidence it is speculated that these a type granites and associated mafic to felsic intrusions may occur in a post collisional extension setting since the end silurian after the initial amalgamation of the west junggar terrane during the middle late silurian on going lithosphere extension during the early middle devonian would result in the rifting of newly amalgamated west junggar and thus induce asthenosphere upwelling which gave rise to the mantle derived magmatism in the northern and southern west junggar and juvenile oceanic crust in central west junggar it is suggested that the middle paleozoic is an important episode of crustal growth in the west junggar

formation of hp lt rocks and their tectonic implications in the western tianshan orogen nw china geochemical and age constraints the western tianshan high pressure low temperature metamorphic belt in northwestern china formed within a paleozoic accretionary wedge on the south side of the yili central tianshan plate it comprises various lithologies including mafic metavolcanic rocks metavolcaniclastic rocks metagreywacke marble and serpentinite the metavolcanics comprise eclogite omphacitite epidosite and blueschist the trace and rare earth element characteristics of these rocks indicate morb to oib affinities suggesting a seamount like setting for the protoliths of these rocks the end value of the eclogite 8 9 indicates that the protolith was derived from a long term depleted mantle i e an n morb source supporting the trace and rare earth element characteristics of these samples sm nd isochron dating of 343 44 ma omp gln grt wr and an age of 346 3 ma grt gln were determined using an eclogite boudin within blueschist a well defined 40ar 39ar plateau age of 344 1 ma was obtained for crossite from an omphacite phengite bearing blueschist and is concordant with the sm nd isochron age for the eclogite suggesting that neither argon loss nor excess argon affected the crossite a well defined 40ar 39ar plateau age of 331 2 ma was determined for phengite from the same sample the high pressure metamorphic rocks were formed by the b type subduction of a paleozoic south tianshan ocean about 344 ma ago they were however exhumed to higher crustal levels during the collision of the tarim and yili central tianshan plates about 331 ma ago similar to an alpine type tectonic regime

blueschists eclogites and subduction zone tectonics insights from a review of late miocene blueschists and eclogites and related young high pressure metamorphic rocks abstract in this paper late miocene blueschist and eclogite belts including the world s youngest blueschist belt in timor tanimbar island chain eastern indonesia and the world s youngest coesite bearing eclogite papua new guinea together with selected cenozoic high pressure and ultrahigh pressure metamorphic rocks are reviewed from a synthesis of the geology metamorphism and chronology of these rocks the formation and exhumation process are evaluated and the significance on tectonics at convergent plate boundaries is discussed

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

effects of mantle rheologies on viscous heating induced by glacial isostatic adjustment it has been argued that viscous dissipation from mantle flow in response to surface loading during glacial cycles can result in short term heating and thus trigger transient volcanism or changes in mantle properties which may in turn affect mantle dynamics furthermore heating near the earth s surface can also affect the stability of ice sheets we have studied the magnitude and spatial temporal distribution of viscous heating induced in the mantle by the realistic ice model ice 6g and gravitationally consistent ocean loads three types of mantle rheologies including linear non linear and composite rheologies are considered to see if non linear creep can induce larger viscous heating than linear rheology we used the coupled laplace finite element model of glacial isostatic adjustment gia to compute the strain stress and shear heating during a glacial cycle we also investigated the upper bound of temperature change and surface heat flux change due to viscous heating we found that maximum viscous heating occurs near the end of deglaciation near the edge of the ice sheet with amplitude as high as 120 times larger than that of the chondritic radioactive heating the maximum heat flux due to viscous heating can reach 30 mw m 2 but the area with large heat flux is small and the timescale of heating is short as a result the upper bound of temperature change due to viscous heating is small even if 30 glacial cycles are included the largest temperature change can be of the order of 0 3 c thus viscous heating induced by gia cannot induce volcanism and cannot significantly affect mantle material properties mantle dynamics nor ice sheet stability the author s 2017 published by oxford university press all rights reserved

isotopic studies bearing on the tectonics of the west junggar region xinjiang china isotopic investigations from the hercynian age fold belt between the kazakhstan and siberian cratons in the west junggar region determine the timing of tectonic evolution and the petrogenesis of the granitic rocks of the region sphene from the leucogabbro phase of the tangbale ophiolite melange the oldest member of the ophiolite sequences in the fold belt yields an isotopic pb pb age of 523 2 7 2 ma zircon from a postcollision alkali granite yields slightly discordant isotopic u pb ages that indicate magma crystallization at 321 4 6 7 ma dating it in the lower carboniferous period radiometric dating thus documents a time span of circa 200 ma for igneous activity in the area petrogenetic studies were made to test whether precambrian crustal rocks might underlie the junggar sedimentary basin initial lead isotope ratios determined from potassium feldspars from five alkali granites show clear affinity with ratios from mid ocean ridge basalts in pb isotope correlation diagrams sm nd data from sphene and apatite from one of the granites yield an initial end t 6 1 the granite sources are depleted mantle rocks of oceanic affinity that show no involvement of recycled aged granitic crustal rocks

space geodesy constrains ice age terminal deglaciation the global ice 6g_c vm5a model a new model of the last deglaciation event of the late quaternary ice age is here described and denoted as ice 6g c vm5a it differs from previously published models in this sequence in that it has been explicitly refined by applying all of the available global positioning system gps measurements of vertical motion of the crust that may be brought to bear to constrain the thickness of local ice cover as well as the timing of its removal additional space geodetic constraints have also been applied to specify the reference frame within which the gps data are described the focus of the paper is upon the three main regions of last glacial maximum ice cover namely north america northwestern europe eurasia and antarctica although greenland and the british isles will also be included if peripherally in the discussion in each of the three major regions the model predictions of the time rate of change of the gravitational field are also compared to that being measured by the gravity recovery and climate experiment satellites as an independent means of verifying the improvement of the model achieved by applying the gps constraints several aspects of the global characteristics of this new model are also discussed including the nature of relative sea level history predictions at far field locations in particular the caribbean island of barbados from which especially high quality records of postglacial sea level change are available but which records were not employed in the development of the model although ice 6g c vm5a is a significant improvement insofar as the most recently available gps observations are concerned comparison of model predictions with such far field relative sea level histories enables us to identify a series of additional improvements that should follow from a further stage of model iteration

using commercial finite element packages for the study of earth deformations sea levels and the state of stress summary modifications to commercial finite element fe packages must be applied before they can be used for geophysical studies involving long wavelength deformation or viscoelasticity this paper provides in detail how and why the commercial codes have to be modified when incompressibility is assumed both the non self gravitating flat earth and self gravitating spherical earth will be considered the latter involves an iterative procedure which converges within 5 iterations this is demonstrated both analytically and numerically in addition implementation of the gravitationally self consistent sea level equation on a self gravitating spherical earth is also described good agreement between numerical results obtained with this coupled finiteelement method and the conventional spectral method is also demonstrated in all cases the interpretation of the outputs of fe models are particularly important in modelling the state of stress

antarctic subglacial conditions inferred from a hybrid ice sheet ice stream model abstract subglacial conditions of large polar ice sheets remain poorly understood despite recent advances in satellite observation major uncertainties related to basal conditions such as the temperature field are due to an insufficient knowledge of geothermal heat flow here a hybrid method is presented that combines numerical modeling of the ice sheet thermodynamics with a priori information using a simple assimilation technique additional data are essentially vertical temperature profiles measured in the ice sheet at selected spots as well as the distribution of subglacial lakes in this way geothermal heat flow datasets are improved to yield calculated temperatures in accord with observations in areas where information is available results of the sensitivity experiments show that 55 of the grounded part of the antarctic ice sheet is at pressure melting point calculated basal melt rates are approximately 65 gt year 1 which is 3 of the total surface accumulation although these sensitivity experiments exhibit small variations in basal melt rates the impact on the ice age of basal layers is quite important in areas that are characterized by relatively low melt rates this may lead to standard deviations up to 50 in a model run over a period of 10 6 years subglacial water flow is concentrated underneath the large outlet glaciers of antarctica the larger subglacial lakes are perched at the head of these subglacial water systems an exception however is the lake system of the recovery ice stream catchment that receives a substantial amount of subglacial melt water from the upstream catchment

deciphering caledonian events timing and geochemistry of the caledonian magmatic arc in the kyrgyz tien shan abstract in the kyrgyz tien shan also known as tian shan in literature the caledonian cambro silurian intrusions comprise an extensive magmatic arc stretching from east to west for more than 1000 km the characteristic feature of the arc is its relatively homogeneous composition of rock types over the whole structure the kichy naryn and djetim intrusions presented in this study are slightly elongated in an east west direction and occupy an area of ca 100 km2 the main rock types are diorite granodiorite and granite geological and geochemical features of the kichy naryn and djetim intrusions demonstrate characteristics of i type granite series rocks of the two intrusions define a continuous high k calc alkaline series diorite and granite of the kichy naryn intrusion yielded early silurian crystallisation ages of 436 2 ma u pb zircon diorite from pebbles in the conglomerate sampled close to the contact of the kichy naryn intrusion yielded a significantly older early ordovician crystallisation age of 466 10 ma the obtained ages of 466 and 436 ma match ages of two major regional magmatic pulses at ca 435 440 and 460 470 ma which took place during continuous subduction from the cambrian to the silurian the amount of granites in the northern tien shan their prolonged history of formation and pronounced i type geochemical characteristics suggest their formation in an andean type active margin environment

implications of palaeozoic ophiolites from western junggar nw china for the tectonics of central asia western junggar nw china contains fragments of several palaeozoic island arcs accretionary prisms and ophiolites which were accreted to each other and possibly to the southern margin of eurasia by the end of the palaeozoic the ophiolites of western junggar are important in reconstructions of these events four ophiolite belts are recognized from south to north these are tangbale mayila dalabute and barleik honggulueleng hebukesair they record the production and destruction of oceanic tracts in this region from the cambrian to the carboniferous the following characteristics are representative of them all 1 the ophiolitic successions are strongly deformed 2 sheeted dyke swarms are not preserved 3 basic igneous rocks show a range of chemical characteristics from depleted island arc tholeiite to ocean island basalt altered n type mid ocean basalts may be represented 4 large scale thrusts typically directed towards the southeast lie at the base of many individual ophiolitic bodies 5 the ophiolites rarely mark the position of possible sutures between arcs they are normally highly deformed and imbricated with other lithologies accretion of palaeozoic arcs and obduction of ophiolites was complete by the end of the carboniferous but the timing and sequence of events remain obscure no continental collisions occurred during this period of assembly crustal growth occurred by the amalgamation of non continental blocks

terrestrial gypsum from alaska and greenland in glacially influenced marine sediments abstract gypsum grains were identified in miocene pleistocene sediment cores from two deep water odp sites site 918 off the se greenland margin and site 887 in the gulf of alaska and in holocene sediment cores from shallow water localities in disenchantment bay and muir inlet in southern alaska although initial morphologic and textural observations suggested a complex system in which the gypsum may have had more than one origin quantitative sulfur isotope analyses of the gypsum provide evidence of its detrital nature 34s values in gypsum from southern alaska range between 0 0 and 7 1 gypsum has 34s values between 27 1 and 27 5 in the gulf of alaska and values between 28 5 and 0 2 off the se greenland margin all of these isotopic signatures are too highly depleted in 34s to have precipitated from seawater present or past in addition there is no significant change in 34s values for gypsum crystals with differing physical characteristics abraded vs unabraded from the same stratigraphic horizon suggesting all the gypsum is detrital regardless of the degree of abrasion the isotopic and physical evidence in combination with the onshore geology the environmental setting and site characteristics of the gypsum bearing marine localities lead us to propose that the ultimate source of the gypsum is precipitation from freeze induced terrestrial sediment or soil brines furthermore the combined evidence suggests that the subsequent occurrence of gypsum in glacimarine sediments results from ice rafting by icebergs or sea ice of the frozen regolith and or in the proximal glacimarine setting of southern alaska very rapid burial via turbidity currents

a neoproterozoic seamount in the paleoasian ocean evidence from zircon u pb geochronology and geochemistry of the mayile ophiolitic melange in west junggar nw china abstract the mayile ophiolitic melange mom is located in the southwestern part of the west junggar nw china and forms part of the southern altaids the mom comprises ultramafic rocks gabbro pillow and massive lavas abyssal radiolarian cherts and volcaniclastic rocks zircons with magmatic crystallization features including oscillatory zoning and high th u values from the isotropic gabbro within the mom yield la icp ms u pb age of 572 9 ma mswd 1 0 marking the timing of crystallization of these rocks as late neoproterozoic geochemically the basalts of the corresponding gabbros from mom display oib type alkali basalt and e morb type tholeiitic basalt features both of these groups are characterized by lile and lree enrichment and hree depletion very weak or no eu anomalies eu eu 0 9 1 and no obvious nb ta and ti negative anomalies suggesting a typical oib affinity we propose that these volcanic rocks were derived from a mantle plume related magmatism associated with the evolution of the paleoasian oceanic system with the mantle source containing 2 5 garnet 2 spinel and 2 amphibole the basalts show within plate affinity marked geochemical similarities with those from hawaii and xigaze seamount suggestive of their intra oceanic setting subduction of the oceanic lithosphere commenced during late cambrian to early ordovician with the eventual accretion of the seamounts in the fore arc together with oceanic fragments forming the mayile ophiolitic melange

age geochemistry and tectonic implications of a late paleozoic stitching pluton in the north tian shan suture zone western china the central asian orogenic belt is the largest tectonic assembly of continental and oceanic terranes on earth due to closure of the paleo asian ocean in the phanerozoic among major suture zones in the north xinjiang region of western china the north tian shan suture zone because of collision between the yili terrane in the south and the junggar terrane in the north contains the youngest ophiolitic rocks and may represent the terminal stage of development of the central asian orogenic belt in western china but the timing of the suture zone remains poorly constrained a sensitive high resolution ion microprobe shrimp zircon u pb age of 316 3 ma i e the beginning of the late carboniferous from the undeformed sikeshu pluton which crosscuts the suture zone places a crucial upperage bound for the time of collision between the yili and junggar terranes this event occurred later than or nearly concurrent with other accretion collision events in the north xinjiang region implying that fi nal terrane amalgamation was completed in the late carboniferous the sikeshu pluton shares geochemical characteristics of the widespread late carboniferous to permian postcollisional a type and i type granitoids with depleted mantle like sr nd isotopic signatures in the north xinjiang region they all occurred during a protracted ca 320 270 ma episode of postcollisional magmatism that may have been induced by basaltic under plating due to either slab breakoff or delamination of thickened mantle lithosphere beneath the central asian orogenic belt the same postcollisional magmatism also generated cu ni sulfi de bearing mafi c ultramafi c magmatic complexes adakites and porphyry type coppermolybdenum bearing magmatic rocks in the north xinjiang region

tectonic evolution of the west junggar region xinjiang china the west junggar region of northwestern china consists of a hercynian age fold belt occupying a late paleozoic zone of convergence between three major eurasian plates siberian tarim and kazakhstan the northern part of the west junggar area includes part of the irtysh zaysan fold belt that extends to the northwest into the ussr where it marks a broad boundary between the siberian and kazakhstan plates the central part of the west junggar area includes the southeastern extension of the chingiz tarbgatay early paleozoic island arc sequence the southern part of the west junggar area is truncated by the deep fundamental junggar alakol fault that has brought the yili microplate into juxtaposiition with the west junggar region in the vicinity of ebin nur lake to the east the junggar area is covered by sediments of the junggar basin paleogeographic reconstructions and geologic evidence indicates that these plates began converging in the middle carboniferous and by late carboniferous alkali granites began invading the imbricated leading edges of the coalescing plates paleomagnetic data reveals that the plates continued to move along strike slip faults bringing the tarim plate and yili microplate into their present positions sometime in the early mesozoic six separate ophiolite occurrences in the west junggar region are strongly tectonized and dismembered the tangbale ophiolite is considered to be late cambrian 508 20 ma and represents the earliest known oceanic crust in western china petrologic and chemical data indicate that the tangbale ophiolite represents a possible back arc or forearc tectonic setting situated close to a developing island arc the darbut ophiolite has been tectonized to a melange and is overlain by late devonian flysch ages of the radiolaria in the cherts associated with the pillow lavas indicate that it formed in the middle devonian other occurrences of ophiolite melange in the northern part of the west junggar region may be as young as late devonian these ophiolites record ocean crust formation in the early to middle paleozoic within the junggarian ocean and their present tectonic setting indicates continental accretion and imbrication shortly after their formation associated blueschist blocks in the ophiolite melange further supports the concept of imbrication brought about by b type subduction and associated obduction postcollisional alkali granites intrude the oceanic sequences of the west junggar region and are considered to represent melted lower crustal material lead isotopic systematics on feldspars from six separate granite plutons indicate ratios that could only be considered oceanic as far as the magmas are concerned the implications of this finding is that the junggar basin basement rocks in the north and west are probably oceanic evidence presented in this study shows a complex history for the development of the west junggar region major movements of consolidating plates in the late paleozoic followed by rejuvenation of these same zones of closure during the himalayan tectonic event of the late tertiary demonstrates that convergence zones and fold belts may respond to vertical and horizontal stress even after many million years of inactivity

the most ancient ophiolite of the central asian fold belt u pb and pb pb zircon ages for the dunzhugur complex eastern sayan siberia and geodynamic implications abstract ophiolitic rocks with a zircon age of 1020 ma occur in the dunzhugur complex of east sayan siberia and are part of a neoproterozoic to early palaeozoic segment of the central asian fold belt the most spectacular suite is exposed along the oka and bokson rivers where a complete ophiolite sequence with mantle tectonites a layered sequence composed of dunite wehrlite and pyroxenite a gabbro section a sheeted diabase dyke complex and basaltic pillow lavas are exposed petrologic and geochemical data suggest that all members of the ophiolite originally belonged to the same cogenetic mafic ultramafic crustal section and support a supra subduction zone setting in a fore arc rifting environment for its origin two multigrain zircon size fractions from a plagiogranite are both slightly discordant but yielded a combined mean 207pb 206pb age of 1020 10 ma evaporation of three additional fractions of three to four grains each from the same sample produced a mean 207pb 207pb age of 1019 9 0 7 ma that we consider to most closely reflect the time of igneous crystallization of the plagiogranite this is the oldest ophiolite so far dated from the central asian fold belt the southern margin of the siberian craton and the palaeo asian ocean were established at the end of the mesoproterozoic at least 1000 ma ago during the time interval 1000 570 ma one or several large ocean basins existed between baltica siberia kazakhstan tarim and northern china and these blocks are therefore unlikely to have been part of the supercontinent rodinia rifting initiation of subduction and marginal basin formation began prior to 1000 ma and continued through 570 ma the dunzhugur ophiolite of eastern sayan provides evidence for the early opening of the palaeo asian ocean not later than 1000 ma ago

the neogene east antarctic ice sheet a dynamic or stable feature abstract this paper is a review of the history and behaviour of the east antarctic ice sheet during the late neogene and is a response to the publication of sugden et al 1993a the case for a stable east antarctic ice sheet in geografiska annaler 75a 4 the antarctic ice sheet is a critical factor in the global climate and oceanographic system knowledge of its historical development is critical to understanding global palaeoceanographic and palaeoclimatic systems and their development the webb harwood hypothesis proposes that the ice sheet while being present in antarctica for more than 40 ma was until recently ca 2 5 ma a dynamic feature waxing and waning under a more temperate climatic and glaciologic regime the stabilistic hypothesis proposes that the current ice sheet has existed in its present cold and polar form for at least the last 14 ma the case for a stable east antarctic ice sheet is the clearest statement of the stabilistic argument to date the authors find support for their viewpoint by presenting evidence from several geologic data bases that does not support their own null meltdown hypothesisr which they deem to be equivalent to the webb harwood hypothesis thereby they conclude supporting the stabilistic hypothesis however their conclusions are weakened by failure to reject their null hypothesis conflict between the meltdown and webb harwood hypotheses infraction of geological laws and poor application of stratigraphic principles several issues are highlighted here as important for direction of future research and discussion concerning late cenozoic antarctic glacio climatic history

40ar 39ar u pb and sm nd constraints on the timing of metamorphic events in the maksyutov complex southern ural mountains new 40 ar 39 ar u pb and sm nd data from 13 metamorphic samples constrain the formation and exhumation of the high pressure maksyutov complex in the southern uralide orogen the maksyutov complex records the highest pressure metamorphism in the southern uralides and is part of the thrust stack that composes the footwall beneath the main ural fault the new isotopic data from eclogite in the lower unit of the maksyutov complex include 40 ar 39 ar data from phengite u pb data from rutile and apatite and sm nd data from garnet clinopyroxene rutile and apatite these data indicate that eclogite facies metamorphism occurred in the mid devonian epoch c 380 ma corresponding with eastward subduction of the east european craton beneath the magnitogorsk island arc after partial exhumation the lower unit was juxtaposed c 360 ma based on 40 ar 39 ar white mica data with the middle and upper units at mid crustal levels during the development of a d 3 shear zone within the subduction zone by the late carboniferous epoch the maksyutov complex was exhumed to upper crustal levels compatible with sedimentologic evidence that at this time the foreland thrust and fold belt began to form and material was shed off it into the foreland basin

in search of an ice history that is consistent with composite rheology in glacial isostatic adjustment modelling abstract global reconstructions of the earth s quaternary ice sheet history such as ice 6g assume that mantle rheology is linear and earth properties are laterally homogeneous however these assumptions are unrealistic because high temperature pressure creep experiments show that both linear and non linear creep laws operate simultaneously in the mantle also surface geology and seismic tomography show that mantle properties vary laterally this study uses the coupled laplace finite element method to find an ice model that is consistent with an earth model with composite rheology where both linear and non linear creep laws operate simultaneously in the mantle and the effective viscosity is laterally heterogeneous we use the ice 6g ice model as initial load on a composite rheology earth model and iteratively improve both the ice and earth model parameters until they fit observations of glacial isostatic adjustment or gia relative sea level and land uplift rate well simultaneously we thereby focus on north america and northern europe our best combination model called ice c m c also fits gravity rate of change data in both areas this finding removes the obstacle in using composite rheology in gia studies since previous work found it impossible for composite rheology to fit both relative sea level and land uplift rate data simultaneously it shows that composite rheology is clearly an alternative to linear rheology in future gia studies the ice c model is at the last glacial maximum lgm position from 26 ka bp thousand years before present to 19 ka bp which implies that composite rheology also supports the timing of lgm proposed by clark et al 2009 the model is found to be significantly thicker in the center of rebound in fennoscandia than ice 6g from the lgm to about 8 ka bp but significantly thinner around the fennoscandian coastline from lgm to about 12 ka bp these changes in the lower boundary conditions in northern europe may affect the reconstruction of past climate there regional differences in ice thickness between ice c and ice 6g are also found in laurentia during lgm there are negligible differences in total ice volume between ice c and ice 6g thus it remains questionable if a composite rheology may help in solving the missing ice problem ice c is also consistent with meltwater pulses 1a and 1b mantle viscosities and creep parameters of earth model m c are consistent with previous findings from gia modeling and microphysics experiments

tectonic evolution of palaeozoic terranes in west junggar xinjiang nw china nine separate cambrian to carboniferous terranes recognized in west junggar northwest china they were amalgamated as part of the central asian orogenic belt which records accretion of continental island arc and oceanic terranes to archaean proterozoic continental nuclei tangbale kekesayi ebinur and mayila terranes cambrian silurian evolved in intra oceanic settings and docked along a series of north dipping subduction zones on to the laba terrane to their south this southern continent was contiguous with lithosphere of the kulumudi ocean to the north devonian subduction on the northern edge of this ocean resulted in formation of a continental arc toli terrane and accretionary complex kulumudi terrane the karamay terrane formed as an accretionary complex during the carboniferous the ophiolitic sartuohai terrane was emplaced as melange between kulumudi and karamay terranes during the late carboniferous subduction migrated southward continuing beneath these terranes resulting in the intrusion of i type granites into the toli kulumudi sartuohai and karamay terranes these granites are closely associated with epithermal and porphyry style gold mineralization composite terranes either side of the kulumudi ocean collided in the late carboniferous marking the final consolidation of central asia collision was accompanied by anorogenic granite and diabase dyke intrusion followed by widespread latest carboniferous to permian extension and subsequently the formation of the junggar basin west junggar has been further disrupted by cenozoic strike slip faulting along junggar and dalabute faults

patagonian origin of glacial dust deposited in east antarctica vostok and dome c during glacial stages 2 4 and 6 abstract the source area of continental dust deposited at both vostok and dome c sites east antarctica during the glacial stages stage 2 18 ka last glacial maximum stage 4 60 ka and stage 6 160 ka of the last two climatic cycles has remained constant the isotopic composition 87 sr 86 sr and 143 nd 144 nd of the ice core dust has been compared with the isotopic composition of the potential source areas antarctica new zealand southern africa australia and south america this comparison reveals the southern south american provenance of the dust for all three glacial periods as has already been shown for the lgm 1 we show that the patagonian loess and the marine shelf sediments from the argentine continental shelf which was variably emerged during glacial periods display different isotopic compositions and that the composition of the patagonian loess best matches the signature of the ice core dust the identification of the patagonian region as the source of the windblown dust deposited over east antarctica during all the latest glacial periods permits a better understanding of paleo atmospheric circulations of the last climatic cycles and a better constraint on the parametrization of dust sources for paleoatmospheric general circulation models

the cambrian initiation of intra oceanic subduction in the southern paleo asian ocean further evidence from the barleik subduction related metamorphic complex in the west junggar region nw china abstract in this study we present new evidence from the barleik subduction related metamorphic complex in the southern west junggar region northwestern china for the cambrian initiation of intra oceanic subduction in the southern paleo asian ocean the barleik metamorphic complex is mainly composed of blueschist and amphibolite blocks within an ophiolitic melange and their protoliths are calc alkaline andesite and alkali and tholeiitic basalts the calc alkaline andesite has a zircon u pb age of 502 2 ma obtained from magmatic cores of zircon grains and shares geochemical features similar to the 515 485 ma intra oceanic arc magmatic rocks in the west junggar region by contrast the alkali and tholeiitic basalts have trace element features similar to ocean island and enriched mid ocean ridge basalts respectively rutile and sodic calcic amphibole from the amphibolite have a u pb age of 502 25 ma and a 40 ar 39 ar age of 504 ma respectively which are in good agreement within errors with a 40 ar 39 ar age of 492 4 ma for phengite from the blueschist these metamorphic ages of 500 ma are interpreted to represent the timing of pacific type subduction related metamorphism and are also compatible with ages of the oldest supra subduction zone ophiolites 531 512 ma and intra oceanic arc plutons 515 485 ma in the southern west junggar region being one of the oldest subduction related metamorphic complexes 509 490 ma in the southern central asian orogenic belt the barleik metamorphic complex together with the oldest arc plutons definitely indicate the initial intra oceanic subduction in the southern paleo asian ocean at least in the early cambrian

formation of the kokchetav subduction collision zone northern kazakhstan insights from zircon u pb and lu hf isotope systematics abstract the kokchetav subduction collision zone is located in the western part of the ancestral central asian orogenic belt this zone is built up by the precambrian kokchetav microcontinent which includes a hp uhp metamorphic belt and the north kokchetav tectonic zone nktz which represents an accretionary complex between the kokchetav microcontinent and the adjacent stepnyak island arc the entire region is widely intruded by palaeozoic granitoids which were emplaced after the collision of the stepnyak island arc with the kokchetav microcontinent we present zircon u pb ages and lu hf systematics in zircon to better characterize the tectonic evolution of the kokchetav subduction collision zone the lu hf results indicate that the kokchetav basement rocks are derived from late neoarchaean early palaeoproterozoic 2 5 ga crust for the granite gneiss basement of the kokchetav microcontinent early mesoproterozoic grenville age 1 17 1 14 ga zircon u pb crystallization ages were obtained for the nktz two main age components were recognized 1 an oldest mesoproterozoic age component 1 20 1 05 ga similar as for the kokchetav microcontinental zircons and 2 a younger early cambrian 540 520 ma or late cambrian early ordovician 490 480 ma age component th u ratios 0 1 and are typical of a magmatic protolith the distinction between both zircon types is supported by characteristic cathodoluminescence images these results confirm previous observations indicating early palaeozoic high grade metamorphism 540 520 ma and collisional deformation 490 480 ma of the mesoproterozoic kokchetav zone as a result of subduction accretion and collision of the stepnyak island arc to the kokchetav microcontinent for two additional samples of the balkashin granitic complex early devonian 415 395 ma zircon crystallization ages magmatic th u ratios were obtained the occurrence of a mesozoic xenocryst within these leucogranites may indicate that they were emplaced in a continental arc setting

a vendian cambrian island arc system of the siberian continent in gorny altai russia central asia an extended vendian cambrian island arc system similar to the izu bonin mariana type is described in the gorny altai terrane at the margin of the siberian continent three different tectonic stages in the terrane are recognized 1 a set of ensimatic active margins including subducted oceanic crust of the paleo asian ocean the uimen lebed primitive island arc oceanic islands and seamounts the set of rocks is assumed to be formed in the vendian 2 a more evolved island arc comprising calc alkaline volcanics and granites a fore arc trough in middle late cambrian time was filled with disrupted products of pre middle cambrian accretionary wedges and island arcs 3 collision of the more evolved island arc with the siberian continent folding metamorphism and intrusion of granites occurred in late cambrian early ordovician time in the late paleozoic the above mentioned caledonian accretion collision structure of the siberian continent was broken by large scale strike slip faults into several segments this resulted in the formation of a typical mosaic block structure

modelling the effect of arbitrary p t t histories on argon diffusion in minerals using the macargon program for the apple macintosh argon diffusion in mineral grains has been numerically modelled using p t t histories that may be relevant to multiply metamorphosed orogenic terranes and for rocks that have resided at high ambient temperatures in the earth s crust for long durations the macargon program1 generates argon concentration profiles in minerals assuming argon loss occurs via volume diffusion it can be run on an apple macintosh computer with arbitrary p t t histories used as input finite difference equations are used in the calculation of 40ar concentration profiles across individual diffusion domains the associated macspectrometer generates model spectra after a p t t history has been specified the form of model 40ar39ar apparent age spectra suggests that considerable caution needs to be exercised in the use of the closure temperature concept and in the interpretation of the significance of plateaux observed in many 40ar39ar apparent age spectra particularly in cases involving metamorphic rocks where complex p t t histories might apply although modelled spectra cannot be directly compared to experimentally determined 40ar39ar age spectra especially when hydrous phases are involved or in cases where loss of argon has not occurred via volume diffusion they do provide insight into theoretically expected age spectra for samples that have experienced complex p t t histories

cenozoic microfossils in northern finland local reworking or distant wind transport abstract allochthonous cenozoic microfossils have been reported from late pleistocene lake and mire host sediments across an area of 30 000 km2 in northern finland two main groups of microfossils are recognised palaeogene marine diatoms silicoflagellates and ebridians that include taxa from around the time of the palaeocene eocene thermal maximum and pliocene to early pleistocene freshwater diatoms the presence of these microfossils has been regarded as evidence that eocene marine and late neogene freshwater sediments formerly existed on the shield surface both groups have been referred to frequently in reconstructions of the sea level tectonic and erosion history of the northern fennoscandian shield the questions raised by the presence of allochthonous cenozoic microfossils in northern finland are however strongly resonant of the debate over the biota origin and age of the pliocene sirius group in antarctica where competing hypotheses have been put forward of local deposition and reworking versus distant wind transport of marine diatoms from the continental shelf this review explores alternative origins for the allochthonous cenozoic microfossils in northern finland local reworking of palaeogene marine sediments during pleistocene glaciation is unlikely as no source rocks of palaeogene age are known from the shield surface or from surrounding sedimentary basins in the baltic and white sea moreover at all sites except akanvaara the marine diatom taxa cover wide age ranges and occur only as minor components in diatom assemblages that are dominated by quaternary freshwater taxa local reworking of pliocene pleistocene freshwater diatoms is however compatible with the widespread survival of pre pleistocene deep weathering although no in situ or unmixed ice rafted pliocene pleistocene lacustrine sediment has yet been found an alternative origin for the marine palaeogene microfossils by distant wind transport is proposed in this hypothesis palaeogene diatomites on the barents sea shelf were exposed to deep glacial and fluvioglacial erosion during the pliocene and early pleistocene and in low sea level stages of the middle and late pleistocene intense wind action acting on comminuted mudstones on outwash plains carried dust including microfossils into northern fennoscandia to be deposited by rain out in lakes and wetlands this material may have been later further recycled by glacial and meltwater transport and more localised wind action processes that also may help to account for the distribution of eemian marine diatoms well beyond eemian shorelines the distant wind transport hypothesis implies that the presence of marine palaeogene diatoms on the shield surface in northern finland cannot be regarded as vestiges of former marine sediments and so do not constrain the tectonic and geomorphic history of the northern fennoscandian shield in the cenozoic

late neogene sirius group strata in reedy valley antarctica a multiple resolution record of climate ice sheet and sea level events late neogene sirius group strata from tillite spur and quartz hills in the reedy glacier area antarctica demonstrate the variability in sirius group facies and contrasts sirius group strata deposited at high and low paleo elevation respectively the tillite spur and quartz hills formations pliocene are formally defined here the tillite spur formation type section crops out on the edge of the wisconsin plateau overlooking tillite spur it comprises 32 m of alternating coarse gray conglomerate and muddy olive brown diamictites the quartz hills formation type section crops out above the western margin of reedy glacier in a pre existing cirque towards the southern end of the quartz hills it comprises c 100 m of alternating massive diamictites and rhythmically interbedded sandstone and laminated mudstones which were deposited close to sea level and subsequently rapidly uplifted 500 m myr 1 to their present elevation at c 1500 m three orders of paleoclimatic variability are recorded in the sirius group strata from reedy valley 1 recycled marine microfloras in glacial diamictites indicate intervals of marine incursion into the antarctic cratonic interior co occurring with reductions in the east antarctic ice sheet 2 an advancing and retreating paleo reedy glacier deposited a glacial interglacial sequence alternating on a 10 100 kyr scale 3 centimeter and millimeter stratification in strata of the quartz hills formation record annual kyr scale variability

formation of the maturin foreland basin eastern venezuela thrust sheet loading or subduction dynamic topography 1 the maturin basin in eastern venezuela is considered a good example of a peripheral foreland basin earthquake and tomographic data indicate that eastern venezuela is affected by the oblique subduction of the south american plate underneath the caribbean plate new forward flexural isostatic modeling of eastern venezuela has been carried out in order to determine whether the maturin basin was generated purely by thrust sheet loading from the serrania and monagas foreland thrust belts a sequence of forward models from middle miocene to present was generated for 3 profiles across the serrania del interior thrust belt the monagas foreland thrust belt and the maturin foreland basin the predictions of these models are constrained using seismic reflection and well data the flexural isostatic modeling shows that thrust sheet loading associated with the serrania del interior and monagas foreland thrust belts is insufficient to generate the observed subsidence within the maturin basin dynamic fluid flow modeling of subduction related dynamic topography of eastern venezuela has been used to investigate the influence of south american plate subduction on the generation of the accommodation space observed in the maturin basin fluid flow modeling of subduction related dynamic topography suggests that the subduction of the south american lithospheric mantle caused downward deflection of the south american crust affecting the maturin basin and the serrania thrust belt this modeling suggests that the maturin basin subsidence has two components 55 related to thrust sheet loading and 45 driven by continental subduction

sims u pb rutile age of low temperature eclogites from southwestern chinese tianshan nw china article i nfo the geological evolution in the southwestern chinese tianshan orogen in nw china is poorly understood because of controversial geochronological results of the low temperature eclogites it is likely due to sm nd and rb sr isotopic disequilibrium in different mineral fractions excess 40 ar in the high pressure k bearing minerals and multi stage growths of zircon in u pb system rutile as a typical eclogite facies mineral with high u pb ratio is a feasible candidate to constrain the eclogite facies metamorphic event in this study we measured u pb rutile age data using sims cameca ims 1280 for low temperature eclogites from southwestern chinese tianshan orogen nw china we demonstrate that sims is a powerful technique for in situ u pb dating of rutile with u content as low as 1 ppm sims u pb rutile dating yielded an age of 318 7 ma which may record the time when the rocks cooled down to the closure temperature of pb in rutile ca 500 c but should be very close to the time of eclogite facies metamorphism in terms of common fast exhumation of coesite bearing eclogite this new proof strongly suggests a late carboniferous collision between kazakhstan yili block and central tianshan microcontinent

elemental and nd sr isotopic geochemistry of granitoids from the west junggar foldbelt nw china with implications for phanerozoic continental growth we report elemental and nd sr isotopic data for two coeval postcollisional magmatic suites 300 ma the miaogou and karamay suites from west junggar foldbelt nw china aiming to determine their source regions and implications for continental growth the miaogou rocks monzonitic to granitic in composition show low mg 0 30 and depletion of hfse and are highly depleted in isotopic compositions with isr ranging from 0 7035 to 0 7045 end 300 ma from 8 4 to 6 6 and young nd model ages 0 37 0 70 ga that coincide with the ages of the ophiolites in the area these features suggest that the miaogou rocks were derived essentially by partial melting of a juvenile basaltic lower crust formed in the early to middle palaeozoic the karamay suite contains a variety of rock types from gabbroic diorite to granite these rocks show enrichment of lree and lile and depletion of hfse and have chemical composition e g mg 0 63 0 41 rather different from the miaogou suite though isotopically they are indistinguishable this suggests that the parental magma of the karamay suite originated from melting of a young lithospheric mantle that had previously been metasomatized during the palaeozoic subduction the west junggar magmatic rocks represent production of juvenile continental crust in the phanerozoic triggered by upwelling of asthenophere in an extensional regime the basement beneath the area is dominated by trapped palaeozoic arc series and oceanic crust

causes of dust size variability in central east antarctica dome b atmospheric transport from expanded south american sources during marine isotope stage 2 abstract we here investigate the spatial and temporal variability of eolian dust particle sorting recorded in the dome b 77 05 s 94 55 e ice core central east antarctica during marine isotope stage mis 2 we address the question whether such changes reflect variable transport pathways from a unique source area or rather a variable apportionment from diverse southern hemisphere sources transported at different elevation in the troposphere the sr nd radiogenic isotope composition of glacial dust samples as well as single particle raman mineralogy support the hypothesis of a single dust provenance both for coarse and fine mode dust events at dome b the southern south american provenance of glacial dust in antarctica deduced from these results indicate a dust composition coherent with a mixture of volcanic material and minerals derived from metamorphic and plutonic rocks additionally dome b glacial samples contain aragonite particles along with diatom valves of marine benthic epiphytic species and freshwater species living today in the northern antarctic peninsula and southern south america these data suggest contribution from the exposed patagonian continental shelf and glacial outwash plains of southern patagonia at the time when sea level reached its minimum our results confirm that dust sorting is controlled by the relative intensity of the two main patterns of tropospheric dust transport onto the inner plateau i e fast low level advection and long range high altitude transport including air subsidence over antarctica

back arc basin basalt systematics the mariana east scotia lau and manus back arc basins babs have spreading rates that vary from slow 100 mm yr and extension axes located from 10 to 400 km behind their island arcs axial lava compositions from these babs indicate melting of mid ocean ridge basalt morb like sources in proportion to the amount added of previously depleted water rich arc like components the arc like end members are characterized by low na ti and fe and by high h2o and ba la the morb like end members have the opposite traits comparisons between basins show that the least hydrous compositions follow global morb systematics and an inverse correlation between na8 and fe8 this is interpreted as a positive correlation between the average degree and pressure of mantle melting that reflects regional variations in mantle potential temperatures lau manus hotter than mariana scotia this interpretation accords with numerical model predictions that faster subduction induced advection will maintain a hotter mantle wedge the primary compositional trends within each bab a positive correlation between fe8 na8 and ti8 and their inverse correlation with h2o 8 and ba la are controlled by variations in water content melt extraction and enrichments imposed by slab and mantle wedge processes systematic axial depth as a proxy for crustal production variations with distance from the island arc indicate that compositional controls on melting dominate over spreading rate hydrous fluxing enhances decompression melting allowing depleted mantle sources just behind the island arc to melt extensively producing shallow spreading axes flow of enriched mantle components around the ends of slabs may augment this process in transform bounded back arcs such as the east scotia basin the re circulation by mantle wedge corner flow to the spreading axes of mantle previously depleted by both arc and spreading melt extraction can explain the greater depths and thinner crust of the east lau spreading center manus southern rifts and mariana trough and the very depleted lavas of east scotia segments e8 e9 the crust becomes mid ocean ridge mor like where the spreading axes further away from the island arc and subducted slab entrain dominantly fertile mantle