high temperature deformation during continental margin subduction exhumation the ultrahigh pressure western gneiss region of norway abstract a new dataset for the high pressure to ultrahigh pressure western gneiss region allows the definition of distinct structural and petrological domains much of the study area is an e dipping homocline with e plunging lineations that exposes progressively deeper more strongly deformed more eclogite rich structural levels westward although eclogites crop out across the wgr scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area the scandian reworking increases westward culminating in strong scandian fabrics with only isolated pockets of older structures the dominant scandian deformation was coaxial e w stretching the sinistrally sheared more trondelag fault complex and nordfjord mylonitic shear zone bound these rocks to the north and south there was moderate top e amphibolite facies deformation associated with translation of the allochthons over the basement along its eastern edge and the nordfjord sogn detachment zone underwent strong lower amphibolite facies to greenschist facies top w shearing a northwestward increase in exhumation related melting is indicated by leucosomes with hornblende plagioclase and scandian sphene in the western 2 3 of the study area exhumation related amphibolite facies symplectite formation in quartzofeldspathic gneiss postdated most scandian deformation further deformation was restricted to slip along biotite rich foliation planes and minor local folding that the western gneiss region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation implies that continental crust in general need not undergo pervasive deformation during subduction

observations and modeling of slow sinking particles in the twilight zone the biological carbon pump bcp transfers carbon from the surface ocean into the oceans interior mainly in the form of sinking particles with an organic component and thereby keeps atmospheric co2 at significantly lower levels than if the oceans were abiotic the depth at which these sinking particles are remineralized is a key control over atmospheric co2 particle sinking speed is likely to be a critical parameter over remineralization depth carbon export is usually controlled by large rapidly sinking particles 150 m d 1 however under some circumstances sinking velocity distributions are strongly bimodal with a significant fraction of total flux being carried by slowly 10 m d 1 sinking particles therefore there is an interest in determining sinking particle velocities and their variations with depth as well as in understanding the interplay between sinking velocity distributions and carbon export here we use profiles of total and particulate concentrations of the naturally occurring radionuclide pair 210po 210pb from the porcupine abyssal plain pap site 48 n 16 5 w to estimate depth variation in particle sinking speed using a one box model and inverse techniques average sinking speeds increase from 60 30 m d 1 at 50 m to 75 25 m d 1 and 90 20 m d 1 at 150 and 500 m furthermore a sensitivity analysis suggests that at the pap site the measured 210po profiles are inconsistent with the usually assumed sinking velocities of 200 m d 1 we hypothesize that a trend of increasing velocity with depth might be caused by a gradual loss of slow sinking material with depth a factor with significant implications for regional carbon budgets

upper ocean export of particulate organic carbon and biogenic silica in the southern ocean along 170 w abstract upper ocean fluxes of particulate organic carbon poc and biogenic silica bsi are calculated from four us jgofs cruises along 170 w using a thorium 234 based approach both poc and bsi fluxes exhibit large variability vs latitude during the seasonal progression of diatom dominated blooms poc fluxes at 100 m of up to 50 mmol c m 2 d 1 are found late in the bloom and farthest south near the ross sea gyre biogenic si fluxes also peak late in the bloom as high as 15 mmol si m 2 d 1 but this flux peak occurs at a different latitude just south of the antarctic polar front apf which is centered around 60 s along this cruise track the ratios of both poc and bsi export relative to their production rates are large suggesting an efficient biological pump at these latitudes the highest relative bsi poc flux ratios at 100 m are found just south of the apf coincident with a bsi poc flux peak seen in 1000 m traps during this same program by deep sea research ii honjo et al deep sea research ii 47 3521 3548 these data suggest that efficient export at these latitudes can support the high accumulation rates of bsi found in the sediments under and south of the apf despite the generally low biomass and productivity levels in this region

palaeomagnetism of cretaceous to tertiary sediments from southern tibet evidence for the extent of the northern margin of india prior to the collision with eurasia abstract palaeomagnetic studies have been carried out on mid cretaceous to latest palaeocene sediments from the tethyan himalaya on the northern margin of the indian plate near the localities of gamba 28 3 n 88 5 e and duela 28 0 n 89 2 e in southern tibet a total of 61 sites more than 700 cores have been sampled mainly from limestones zongshan formation jidula formation member ii zongpu formation sandstones jidula formation member iii and graywackes gamba group member ii with ages well defined from detailed lithological and biostratigraphic analyses carriers of characteristic remanences are magnetite in limestones and graywackes and hematite in sandstones reliable palaeodirections were obtained from 35 sites 356 specimens positive fold tests antiparallel normal and reverse polarities a northward trend of the palaeolatitudes with decreasing age and absence of any metamorphic overprint indicate a primary origin for the characteristic remanences the results have been compared with expected palaeodirections according to available data for the indian subcontinent based on its present day geometry the late maastrichtian sites fourteen sites 156 specimens and the middle late palaeocene sites fourteen sites 113 specimens yield palaeolatitudes of 5 7 s and 4 0 n for the sampling localities in southern tibet respectively these palaeolatitudes are much further north than expected and imply that crustal shortening between the tethyan himalaya and the indian shield since the time of india eurasia collision has occurred on the order of 1500 km at the longitude of gamba duela comparison of our observed palaeolatitudes with palaeolatitudes for the lhasa block during cretaceous and palaeocene times indicate that initial contact of the northern indian margin with the lhasa block was already established at 65 60 ma with suturing completed at about 55 50 ma

measurement of size and settling velocity of suspended aggregates on the northern california continental shelf abstract a high resolution video camera attached to a miniature sediment trap was used to make time lapse images of suspended aggregates settling through the water column the video trap system was placed 2 m above the seabed in the strataform study area off eureka california and was programmed to take a 7 second time sequence of settling particles every 6 h over a 4 month period the camera field of view within the trap is 15 mm and the minimum particle resolution is approximately 130 m data analysis was carried out with a video frame grabber and digital imaging software to determine the particle size shape and settling velocity results of the first deployment of this system september 1995 show that particle sizes elliptical nominal diameter determined from the video images ranged from 130 to 740 m with a modal size of 300 m the average form factor ratio of short to long semi axis of the aggregates was 0 71 measured settling velocities varied from 0 09 to 8 13 mm s in terms of weight percent or mass the median size of the aggregate distribution is 600 m analysis of settled particles collected from the sediment trap shows disaggregated or component grains ranging from 1 0 to 63 m with a modal size of 26 m and a median size of 11 m additionally an analysis was carried out to separate the relative importance of particle shape and excess density on settling velocity and these results were compared to observations using the equations for settling velocity of spheres and prolate spheroids and substituting published relationships for size versus excess density into these equations the settling velocity for combinations of shape and excess density has been calculated results show that differences between sphere and prolate spheroid with major axis both parallel and perpendicular to flow approximations caused less than a 13 difference in computed settling velocity in contrast the various size excess density relationships used to compute settling velocity caused order of magnitude deviations from measured values in all cases however the approximations of settling velocity for aggregates provide a significantly better fit to data than approximations based on disaggregated sediment size distribution

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

late cretaceous high mg granitoids in southern tibet implications for the early crustal thickening and tectonic evolution of the tibetan plateau abstract this study presents new major and trace element plus sr nd and zircon u pb isotope data for the zhongcang granitic plutons which are located to the south of the yongzhu asuo ophiolite belt within the northwestern part of the central lhasa subterrane tibetan plateau these data provide new insights into the late cretaceous tectonic evolution of southern tibet the zhongcang plutons are dominated by granodiorites and granites that yield zircon u pb emplacement ages of 94 88 ma they can be further divided into metaluminous and peraluminous subtypes the metaluminous rocks have adakite like geochemical signatures including high sio 2 al 2 o 3 and sr concentrations and low yb and y concentrations and high sr y and la yb n ratios these rocks also have negative end t values 3 17 to 0 17 variable initial 87 sr 86 sr i ratios 0 705927 0 707668 and high k 2 o and th concentrations suggesting that they were not derived from the partial melting of subducted oceanic crust in an arc setting the zhongcang adakitic rocks have higher mgo and cr concentrations and mg values than do contemporaneous intrusive rocks derived from a region of thickened lower crust within the central lhasa subterrane these data suggest that the zhongcang adakitic rocks were generated by the partial melting of a delaminated thickened lower crust within a late cretaceous continental setting in comparison with the zhongcang adakitic rocks the peraluminous rocks have significant negative eu and sr anomalies and lower end t values 4 06 to 6 64 this combined with their high mg values and cr concentrations suggests that the peraluminous units formed from primitive magmas similar to those that formed the zhongcang adakitic rocks but modified by contamination with ancient crustal material and by fractional crystallization of plagioclase and apatite during uprising and or emplacement the zhongcang high mg granitoids provide robust evidence for late cretaceous crustal thickening prior to india asia collision

geographical seasonal and depth variation in sinking particle speeds in the north atlantic particle sinking velocity is considered to be a controlling factor for carbon transport to the deep sea and thus carbon sequestration in the oceans the velocities of the material exported to depth are considered to be high in high latitude productive systems and low in oligotrophic distributions we use a recently developed method based on the measurement of the radioactive pair 210po 210pb to calculate particle sinking velocities in the temperate and oligotrophic north atlantic during different bloom stages our estimates of average sinking velocities asvs show that slowly sinking particles 100 m d 1 contribute significantly to carbon flux at all the locations except in the temperate regions during the bloom asvs appear to vary strongly with season which we propose is caused by changes in the epipelagic community structure our results are the first field data to confirm the long standing theory that particle sinking velocities increase with depth with increases of up to 90 between 50 and 150 m depth

transition from oceanic to continental lithosphere subduction in southern tibet evidence from the late cretaceous early oligocene 91 30 ma intrusive rocks in the chanang zedong area southern gangdese little is known about the detailed processes associated with the transition from oceanic to continental lithosphere subduction in the gangdese belt of southern tibet gbst here we report zircon u pb age major and trace element and sr nd hf isotopic data for late cretaceous early oligocene similar to 91 30 ma intermediate acid intrusive rocks in the chanang zedong area immediately north of the yarlung tsangpo suture zone these rocks represent five magmatic episodes at similar to 91 similar to 77 similar to 62 similar to 48 and similar to 30 ma respectively the 91 48 ma rocks have slightly lower initial sr 87 sr 86 0 7037 to 0 7047 and higher epsilon nd t 1 8 to 4 3 and epsilon hf t 3 5 to 14 7 values in comparison with those 0 7057 to 0 7062 3 3 to 2 5 and 2 2 to 6 6 of the similar to 30 ma intrusive rocks the similar to 91 similar to 62 and similar to 30 ma rocks are geochemically similar to slab derived adakites the similar to 91 ma somka adakitic granodiorites were likely derived by partial melting of the subducting neo tethyan oceanic crust with minor oceanic sediments and the similar to 91 ma somka dioritic rocks with a geochemical affinity of adakitic magnesian andesites likely resulted from interactions between adakitic magmas and overlying mantle wedge peridotite the similar to 77 ma luomu diorites were probably generated by partial melting of juvenile basaltic lower crust the similar to 62 ma naika and zedong adaldtic diorites and granodiorites were likely generated mainly by partial melting of thickened juvenile mafic lower crust but the source region of the zedong adakitic rocks also contained enriched components corresponding to indian continental crust the similar to 48 ma lamda granites were possibly generated by melting of a juvenile basaltic crust the younger similar to 30 ma chongmuda adakitic quartz monzonites and minor granodiorites were most probably derived by partial melting of early oligocene north ward subducted indian lower crust beneath the southern lhasa block taking into account the regional tectonic and magmatic data we suggest that the gangdese belt of southern tibet gbst underwent a tectonodynamic transition from oceanic subduction to continental subduction between 100 and 30 ma it evolved through four stages 100 65 ma roll back of subducted neo tethyan oceanic lithosphere 65 60 ma initial collision between indian and asian continents 60 40 ma breakoff of subducted neo tethyan oceanic lithosphere and similar to 30 ma northward subduction of the indian continent c 2014 elsevier b v all rights reserved

role of large particles in the transport of elements and organic compounds through the oceanic water column abstract during the past decade data from a variety of sources have been obtained which show conclusively that the relatively rare large particles sinking through the water column are responsible for the majority of the downward vertical mass flux in the sea this finding has important implications for understanding the transfer distribution and fate of elements and organic compounds in marine waters the large 100 m detrital particles responsible for vertical flux are primarily biogenic and range in size and composition from small discrete fecal pellets and plankton hard parts to large amorphous aggregates or snow which contain both organic and inorganic constituents depending on size shape and density these particles sink at rates ranging from 1000 m day 1 several methods have been developed for sampling these particles of which in situ sediment trapping has probably furnished the most comprehensive qualitative and quantitative information on the role large particles play in material transport flux studies have highlighted the importance of marine heterotrophs in packaging fine suspended particulate matter into large rapidly sinking particles which accelerate the movement of incorporated materials to depth large particle production via biological packaging is not restricted to the euphotic zone but can occur at all depths and information is now accruing on rates of production of large particles in the water column chemical analyses of sedimenting particles collected in sediment traps and those sampled by other means have allowed quantifying vertical fluxes and residence times of elements radionuclides and organic compounds natural and anthropogenic in various oceanic regimes pertinent studies dealing with the above aspects are reviewed and several areas for future research are suggested

in situ measurement of mesopelagic particle sinking rates and the control of carbon transfer to the ocean interior during the vertical flux in the global ocean vertigo voyages in the north pacific abstract among the parameters affecting carbon transfer to the ocean interior particle sinking rates vary three orders of magnitude and thus more than primary production f ratios or particle carbon contents e g boyd p w trull t w 2006 understanding the export of marine biogenic particles is there consensus progress in oceanography 4 276 312 doi 10 1016 j pocean 2006 10 007 very few data have been obtained from the mesopelagic zone where the majority of carbon remineralization occurs and the attenuation of the sinking flux is determined here we report sinking rates from 300 m depth for the subtropical station aloha june 2004 and subarctic station k2 july 2005 north pacific ocean obtained from short 6 5 day deployments of an indented rotating sphere irs sediment trap operating as an in situ settling column peterson m l wakeham s g lee c askea m a miquel j c 2005 novel techniques for collection of sinking particles in the ocean and determining their settling rates limnology and oceanography methods 3 520 532 to separate the flux into 11 sinking rate fractions ranging from 820 to 2 m d 1 that are collected by a carousel for further analysis functioning of the irs trap was tested using a novel programming sequence to check that all particles have cleared the settling column prior to the next delivery of particles by the 6 hourly rotation cycle of the irs there was some evidence from the flux distribution among the cups and photomicroscopy of the collected particles that very slow sinking particles may have been under collected because they were unable to penetrate the brine filled collection cups but good evidence for appropriate collection of fast settling fractions approximately 50 of the particulate organic carbon poc flux was sinking at greater than 100 m d 1 at both stations at aloha more than 15 of the poc flux sank at 820 m d 1 but low fluxes make this uncertain and precluded resolution of particles sinking slower than 137 m d 1 at k2 less than 1 of the poc flux sank at 820 m d 1 but a large fraction 15 45 of the flux was contributed by other fast sinking classes 410 and 205 m d 1 pic and bsi minerals were not present in higher proportions in the faster sinking fractions but the observations were too limited to rule out a ballasting contribution to the control of sinking rates photographic evidence for a wide range of particle types within individual sinking rate fractions suggests that biological processes that set the porosity and shape of particles are also important and may mask the role of minerals comparing the spectrum of sinking rates observed at k2 with the power law profile of flux attenuation with depth obtained from other vertigo sediment traps deployed at multiple depths buesseler k o lamborg c h boyd p w lam p j trull t w bidigare r r bishop j k b casciotti k l dehairs f elskens m honda m karl d m siegel d silver m steinberg d valdes j van mooy b wilson s e 2007b revisiting carbon flux through the ocean s twilight zone science 316 5824 567 570 doi 10 1126 science 1137959 emphasizes the importance of particle transformations within the mesopelagic zone in the control of carbon transport to the ocean interior

metallogeny of the northeastern gangdese pb zn ag fe mo w polymetallic belt in the lhasa terrane southern tibet abstract the northeastern gangdese pb zn ag fe mo w polymetallic belt ngpb characterized by skarn and porphyry deposits is one of the most important metallogenic belts in the himalaya tibetan continental orogenic system this belt extends for nearly four hundred kilometers along the luobadui milashan fault in the central lhasa subterrane and contains more than 10 large ore deposits with high potential for development three major types of mineralization system have been identified skarn fe systems skarn breccia pb zn ag systems and porphyry skarn mo cu w systems in this study we conducted a whole rock geochemical u pb zircon geochronological and in situ zircon hf isotopic study of ore forming rocks in the ngpb specifically the jiangga jiaduopule and rema skarn fe deposits and the yaguila pb zn ag deposit although some of these deposits porphyry mo systems formed during the post collisional stage 21 14 ma the majority these three systems developed during the main soft collision stage of the india asia continental collision 65 50 ma the skarn fe deposits are commonly associated with granodiorites monzogranites and granites and formed between 65 and 50 ma the ore forming intrusions of the pb zn ag deposits are characterized by granite quartz porphyry and granite porphyry which developed in the interval of 65 55 ma the ore forming porphyries in the sharang mo deposit formed at 53 ma the rocks from fe deposits are metaluminous and have relatively lower sio2 and higher cao mgo feo contents than the intrusions associated with mo and pb zn ag mineralization while the pb zn ag deposits are peraluminous and have high sio2 and high total alkali concentrations they all exhibit moderately fractionated ree patterns characterized by lower contents of heavy ree relative to light ree and they are enriched in large ion lithophile elements and relatively depleted in high field strength elements ore forming granites from fe deposits display 87sr 86sr i 0 7054 0 7074 and end t 4 7 to 1 3 whereas rocks from the yaguila pb zn ag deposit have 87sr 86sr i 0 7266 0 7281 and end t 13 5 to 13 3 in situ lu hf isotopic analyses of zircons from fe deposits show that ehf t values range from 7 3 to 6 6 with tdm hf c model ages of 712 to 1589 ma and yaguila pb zn ag deposit has ehf t values from 13 9 to 1 3 with tdm hf c model ages of 1216 to 2016 ma combined with existing data from the sharang mo deposit we conclude that the ore forming intrusions associated with the skarn fe and porphyry mo deposits were derived from partial melting of metasomatized lithospheric mantle and rejuvenated lower crust beneath the central lhasa subterrane respectively melting of the ancient continental material was critical for the development of the pb zn ag system therefore it is likely that the source rocks play an important role in determining the metal endowment of intrusions formed during the initial stage of the india asia continental collision

differential crustal deformation across the cona oiga rift southern tibetan plateau abstract the cona oiga rift cor is the easternmost member of a series of n s trending cenozoic rifts in the southern tibetan plateau the yarlung river flows from west to east across the cor and exhibits diverse river morphology along strike we analyze tributary fluvial longitudinal profiles in the yarlung drainage and the published strain rate field on both sides of the rift in order to gain knowledge about its development and crustal deformation patterns within the plateau interior tributaries on the western side of the rift have relatively low normalized steepness index ksn concave curves in elevation plots and cross divide contrasts in mean relief and mean gradient indicate ongoing river reorganization in this region in contrast high topographic relief and steep stream channels developed on the eastern side in addition spatial variations in negative dilatational strain rates indicate diverse crustal shortening and regional uplift across the cor we suggest that the cona oiga rift plays an important role in accommodating deformation associated with the continuing indentation of the eastern himalayan syntaxis tectonic forcing might be the dominant factor controlling landscape evolution across the cor

the transhimalaya gangdese plutonism in the ladakh region a upb and rbsr study abstract the age and origin of the transhimalaya gangdese plutonic belt in the ladakh area has been studied by high resolution u pb analyses of accessory minerals zircon and monazite allanite and rb sr measurements on whole rock samples the ages determined of 101 2m y for a granodiorite and of 60 7 0 4m y for a granite substantiate that the transhimalaya plutonism was active at least in mid cretaceous albian and earliest tertiary palaeocene times a rb sr isochron of 73 4 2 4m y might be fortuitous because the rb sr systematics shows important heterogeneities on the whole rock scale the inherited radiogenic lead in zircon demonstrates that anatexis of continental crust was involved in magma genesis the thus recycled continental material was heterogeneous with respect to its primary ages and or metamorphic history the approximate minimum ages of the continental sources range from 350 to 590 m y the 101 2m y old transhimalaya granodiorite which intrudes a series of mainly basaltic island arc rocks dras series shows that this island arc was attached to the continental margin at that time this is consistent with the hypothesis that the ladakh tethys closed through two subduction regimes 1 an early cretaceous subduction which formed the dras island arc and 2 a late cretaceous to palaeocene subduction at the continental margin which caused the emplacement of the transhimalaya plutonic belt

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

tectonic evolution of the qinghai tibet plateau abstract the qinghai tibet plateau composed of several continental slivers within the eastern tethyan domain is one of the pivotal sites to examine to better understand the theory of plate tectonics and the orogenic evolution on earth this plateau is generally inferred to be a collage of several continental blocks that rifted from gondwanaland and subsequently accreted to the asian continent however recent recognition of over twenty ophiolite melange zones and their associated island arcs indicates that the traditional model of tectonic evolution requires revision based on 177 recently finished 1 250 000 scale geological maps and related studies we summarize the main tectonic context of the qinghai tibet plateau and propose a new integrated model to account for the new findings the complex orogen of the immense qinghai tibet plateau consisting of multiple island arc basin systems that developed at different stages while surrounded by the north china yangtze tarim and indian plates is emphasized the entire orogen surrounded by suture zones that mark the locations of oceanic closure is investigated by examining i the first order tectonic units and ophiolitic melanges including arc arc continent collision zones and ii their internally enclosed blocks as the second order tectonic units therefore the qinghai tibet plateau is divided into three major orogenic systems namely from northeast to southwest the early paleozoic qinling qilianshan kunlunshan qin qi kun the late paleozoic triassic qiangtang sanjiang and the late paleozoic to cenozoic gangdese himalaya orogenic systems which are separated by the kangxiwa muzitagh maqin mianxian and the bangong shuanghu changning menglian sutures respectively we propose that the formation and evolution of the qinghai tibet plateau to have been intrinsically related to those of the eastern tethys recorded by the longmu co shuanghu ophiolite melange zone the southern qiangtang paleozoic accretionary arc basin system the bangong nujiang suture zone and their associated composite island arc basin systems the present day bangong shuanghu changning menglian suture system marks the final closure of the tethyan ocean the yarlung zangbo ocean opened as a back arc basin in response to the southward subduction of the tethyan ocean lithosphere in the middle triassic and closed as a result of the india asia collision at the end of cretaceous followed by the northward indention of the indian plate that resulted in significant intra continental deformation and plateau uplift in the cenozoic

234th based carbon export during an ice edge bloom sea ice algae as a likely bias in data interpretation abstract total 234th and 238u vertical profiles in the water column were determined along a three station transect perpendicular to the eastern weddell sea ice shelf the transect was sampled during pre bloom 6 7 december 2003 and bloom 19 20 december 2003 conditions to follow changes occurring in the particulate organic carbon poc flux during an antarctic phytoplankton ice edge bloom during the pre bloom period 234th 238u ratios 1 up to 1 3 were measured just above the thermocline at all stations and throughout the entire mixed layer at the ice edge these water column data appear to be the first which support the hypothesis that sea ice algae could represent an input of 234th rich material in polar waters due to cellular uptake of the radionuclide and or its adsorption onto the extracellular polymeric substances eps produced by these organisms a simple conceptual model is proposed to link observed 234th 238u ratios to the input of 234th enriched sea ice algae caused by ice melting the extensive deep water 234th deficit observed during the second transect likely results from the combination of vertical particle export and water seafloor exchange processes poc steady state ss fluxes out of the photic layer were negative or nil 11 to 2 mmol m 2 d 1 under pre bloom conditions probably due to ice algae input at the ice edge and remineralization elsewhere whereas poc non steady state nss fluxes were generally higher 7 to 33 mmol m 2 d 1 when the bloom was developing we have estimated that these flux values could change by as much as 75 if a sea ice algae 234th input term were included in the simple one box model used this suggests that failure to consider the sea ice algae compartment when using 234th as a proxy to assess carbon export in seasonally ice covered regions could at times lead to significant biases in the interpretation of 234th based particle flux estimates

geological characteristics and genesis of the jurassic no i porphyry cu au deposit in the xiongcun district gangdese porphyry copper belt tibet abstract the xiongcun district located in the western segment of the gangdese porphyry copper belt gpcb hosts the only known jurassic mineralization in the gpcb tibet prc the no i deposit in the xiongcun district is related to the middle jurassic quartz diorite porphyry 167 161 ma and the mineralization was formed at ca 161 5 2 7 ma ore bearing middle jurassic quartz diorite porphyry emplaced into the early jurassic volcano sedimentary rock sequences of the xiongcun formation veinlets and disseminated mineralization developed within the middle jurassic quartz diorite porphyry and the surrounding metamorphosed tuff hosting a measured and indicated resource of 1 04 mt copper 143 31 t gold and 900 43 t silver with an average grade of 0 48 copper 0 66 g t gold and 4 19 g t silver the mineralization can be assigned to four stages including three main stages of hypogene mineralization and one epigenetic stage the main alteration associated with mineralization is potassic seven mineralization related hydrothermal veins have been recognized including quartz sulfide biotite sulfide magnetite sulfide quartz molybdenite sulfide chalcopyrite pyrite pyrrhotite pyrite and polymetallic veins the s and pb isotopic compositions of the ore sulfides and the re contents of the molybdenite suggest a mantle source for the ore forming materials with minor contamination from the subducted sediments hydrogen and oxygen isotope compositions of quartz in the ores suggest that both magmatic and meteoric waters were involved in the ore forming process the ore bearing porphyry 167 161 ma and ore forming 161 5 2 7 ma ages of the no i deposit correspond to the time of northward subduction of neo tethys oceanic slab the geochemical data of the ore bearing porphyry indicate that the no i deposit formed in an intra oceanic island arc setting and the ore bearing porphyry originated from the partial melting of mantle with limited contribution of subducted sediments the genesis of the ore bearing porphyry and no i deposit is interpreted as being related to northward intra oceanic subduction of neo tethys oceanic slab in the middle jurassic time 167 161 ma

u pb geochronology geochemistry and h o s pb isotopic compositions of the leqingla and xin gaguo skarn pb zn polymetallic deposits tibet china abstract the leqingla and xin gaguo deposits are two representative skarn pb zn polymetallic deposits of the gangdese pb zn polymetallic belt tibet china la icp ms zircon u pb dating of the mineralization related biotite granites from both the leqingla and xin gaguo deposits yielded weighted mean ages of 60 8 ma and 56 5 ma respectively which can be inferred as their mineralization ages the leqingla biotite granite is characterized by high al 2 o 3 total fe na 2 o and low k 2 o in comparison the xin gaguo biotite granite is characterized by relative higher k 2 o but lower al 2 o 3 total fe and na 2 o geochemical and mineralogical characteristics indicate that the leqingla and xin gaguo biotite granites are calc alkaline i type granite and high k calc alkaline i type granite respectively both the leqingla and xin gaguo biotite granites are enrichment in lree and liles and depletion in hfses and they were formed at the india asia collision stage 18 o and d values for the leqingla and xin gaguo deposits are 8 8 to 5 3 and 140 4 to 90 1 4 5 to 7 0 and 117 3 to 81 0 respectively indicating magma fluids mixed with meteoric water in ore forming fluids 34 s values 11 6 to 0 3 of ore sulfides from the leqingla deposit show characteristics of biogenetic sulfur isotope compositions suggesting sulfur for the leqingla deposit were sourced from wall rocks of the mengla and luobadui formation which are rich in organic materials 34 s values of ore sulfides from the xin gaguo deposits show bimodal distribution 5 0 to 1 6 and 1 6 2 1 indicating sulfur in the xin gaguo deposit were derived from both wall rocks and magma in the leqingla deposit most ore sulfides have the similar pb isotopic compositions with that of the mineralization related biotite granite suggesting the biotite granite supplied most of the ore forming metals pb isotopic compositions of ore sulfides and hf isotopic compositions of biotite granite show that the majority of ore forming metals are derived from mantle components of partial melting of the neo tethys ocean slab with some upper crust materials of the lhasa terrane pb isotopic compositions of ore sulfides from the xin gaguo deposit are similar to that of the leqingla deposit indicating they have the similar sources of ore forming metals

eocene neotethyan slab breakoff in southern tibet inferred from the linzizong volcanic record abstract formation of the linzizong volcanic successions in the lhasa terrane southern tibet has long been related to northward subduction of the neotethyan oceanic slab under asia here we report new 40ar 39ar age results for the volcanic rocks recovered from a large area 29 n to 32 n and 85 e to 93 e that together with literature data delineate two discrete stages of volcanism these are a widespread cretaceous stage and an intense but spatially confined paleogene stage the latter occurring only in the southern part of the lhasa terrane resulted in the linzizong volcanic successions our data furthermore suggest southward migration and intensification of the volcanism in the lhasa terrane with magmatic flare ups at ca 50 ma while the volcanic successions consist dominantly of calc alkaline rocks typical of arc lava geochemistry those formed during the flare up period show significant compositional variations from low k tholeiitic through calc alkaline to shoshonitic magma suites these observations enable us to interpret the volcanic southward migration and following flare ups as the consequences of rollback and breakoff of the subducted neotethyan slab that occurred ahead and in the early stage respectively of the india asia collision our interpretation that involves a major eocene tectonomagmatic activity and concomitant topographic uplift in southern tibet is consistent with himalayan metamorphic constraints regional sedimentary records and seismic tomography

zircon u pb geochronology and hf isotopic constraints on petrogenesis of the gangdese batholith southern tibet abstract during the mesozoic cenozoic northward neotethyan subduction and subsequent india asia collision gave rise to the extensive transhimalayan magmatism that stretches from burma and western yunnan through southern tibet to the ladakh and kohistan complexes to understand the age distribution and petrogenesis of the gangdese batholith the largest intrusive exposure along the transhimalayan magmatic belt fifty granitic samples were selected for in situ zircon u pb and hf isotopic analyses the u pb data suggest four discrete stages of magmatic activity i e 205 152 109 80 65 41 and 33 13 ma respectively with the 65 41 ma stage being the most prominent the hf isotopic data indicate that the gangdese batholith is overwhelmed by positive e hf t values which are comparable to those of the kohistan ladakh batholiths in the west but differ markedly from those of the chayu burma batholiths in the east most of the gangdese granites show similar and young hf model ages 1000 300 ma indicating their derivation from juvenile crust however those formed in the 65 41 ma stage exhibit more heterogeneous hf isotopic ratios with negative e hf t values being observed in some granites younger than 50 ma suggesting the involvement of old indian continental crust in their petrogenesis this age may thus mark the onset of the india asia collision the newly established zircon u pb age and hf isotope database of the gangdese batholith can be used as a powerful tracer or fingerprint when studying the source to sink relation of the sediments eroded from the southern tibetan plateau

what drives orogeny in the andes the andes the world9s second highest orogenic belt were generated by the cenozoic tectonic shortening of the south american plate margin overriding the subducting nazca plate we use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening from the modeling we infer that the most important factor was accelerated westward drift of the south american plate changes in the subduction rate were less important other important factors are crustal structure of the overriding plate and shear coupling at the plates9 interface the model with a thick 40 45 km at 30 ma south american crust and relatively high friction coefficient 0 05 at the nazca south american interface generates 300 km of tectonic shortening during 30 35 m y and replicates the crustal structure and evolution of the high central andes the model with an initially thinner

episodic growth of the gondwana supercontinent from hafnium and oxygen isotopes in zircon it is thought that continental crust existed as early as 150 million years after planetary accretion but assessing the rates and processes of subsequent crustal growth requires linking the apparently contradictory information from the igneous and sedimentary rock records for example the striking global peaks in juvenile igneous activity 2 7 1 9 and 1 2 gyr ago imply rapid crustal generation in response to the emplacement of mantle super plumes rather than by the continuous process of subduction yet uncertainties persist over whether these age peaks are artefacts of selective preservation and over how to reconcile episodic crust formation with the smooth crustal evolution curves inferred from neodymium isotope variations of sedimentary rocks detrital zircons encapsulate a more representative record of igneous events than the exposed geology and their hafnium isotope ratios reflect the time since the source of the parental magmas separated from the mantle these model ages are only meaningful if the host magma lacked a mixed or sedimentary source component but the latter can be diagnosed by oxygen isotopes which are strongly fractionated by rock hydrosphere interactions here we report the first study that integrates hafnium and oxygen isotopes all measured in situ on the same precisely dated detrital zircon grains the data reveal that crust generation in part of gondwana was limited to major pulses at 1 9 and 3 3 gyr ago and that the zircons crystallized during repeated reworking of crust formed at these times the implication is that the mechanisms of crust formation differed from those of crustal differentiation in ancient orogenic belts

the sr and nd isotopic variations of the chinese loess plateau during the past 7 ma implications for the east asian winter monsoon and source areas of loess abstract 143 nd 144 nd and 87 sr 86 sr ratios of the acid insoluble residues of the red clay and overlying loess paleosols from the lingtai profile of the loess plateau china were investigated the results show that variations of the 87 sr 86 sr ratios of the lingtai profile can be divided into two stages from 7 ma b p to 2 5 ma b p the acid insoluble residues in the red clay rc formation are characterized by higher 87 sr 86 sr ratios with an average of 0 7230 from 2 5 ma b p to the present the acid insoluble residues in the wuchen loess wl4 ws1 lishi loess l15 s1 malan loess l1 and holocene loess s0 have relatively lower 87 sr 86 sr ratios and display an overall descending trend from 0 7223 at 2 5 ma b p to 0 7182 at the present among three possible interpretations for the variations of 87 sr 86 sr ratios of acid insoluble residues of the loess provenance change chemical weathering change or grain size distribution change only grain size distribution change provides a satisfactory interpretation this implies that the east asian winter monsoon strength was weak and relatively stable from 7 ma b p to 2 5 ma b p but became continuously enhanced from 2 5 ma b p to the present all the red clay and the overlying loess paleosols in the lingtai profile have generally identical e nd 0 implying that the source regions of these eolian deposits of the chinese loess plateau may be relatively stable and has not changed since 7 ma b p

anatomy and global context of the andes main geologic features and the andean orogenic cycle the andes make up the largest orogenic system developed by subduction of oceanic crust along a continental margin subduction began soon after the breakup of rodinia in late proterozoic times and since that time it has been intermittently active up to the present the evolution of the pacifi c margin of south america during the paleozoic occurred in the following stages 1 initial proterozoic rifting followed by subduction and fi nal re amalgamation of the margin in early cambrian times as depicted by the puncoviscana and tucavaca basins and related granitoids in southern bolivia and northern argentina 2 a later phase of rifting in the middle cambrian and subsequent collisions in middle ordovician times of parautochthonous terranes derived from gondwana such as paracas arequipa and antofalla and exotic terranes originating in laurentia such as cuyania chilenia and chibcha 3 fi nal permian collision between south america and north america to form pangea during the alleghanides orogeny leaving behind rifted pieces of laurentia as the tahami and tahuin terranes in the northern andes and other poorly known orthogneisses in the cordillera real of ecuador in the late permian early triassic and 4 amalgamation of the mejillonia and patagonia terranes in early permian times representing the last convergence episodes recorded in the margin during the gondwanides orogeny these rifting episodes and subsequent collisions along the continental margin were the result of changes of the absolute motion of gondwana related to global plate reorganizations during proterozoic to paleozoic times generalized rifting during pangea breakup in the triassic concentrated extension in the hanging wall of the sutures that amalgamated the paleozoic terranes the opening of the indian ocean in early jurassic times was associated with a new phase of subduction along the continental margin the northeastward absolute motion of western gondwana produced a negative trench roll back velocity that controlled subduction under an extensional regime until late early cretaceous times the northern andes of venezuela colombia and ecuador record a series of collisions of island arcs and oceanic plateaus from the early cretaceous to the middle miocene as a result of interaction with the caribbean plate the remaining central and southern andes record periods of orogenesis and mountain building alternating with periods of quiescence and absence of deformation as recorded in parts of the oligocene based on the generalized occurrence of fl at slab subduction episodes through time as recorded in most ramos v a 2009 anatomy and global context of the andes main geologic features and the andean orogenic cycle in kay s m ramos v a and dickinson w r eds backbone of the americas shallow subduction plateau uplift and ridge and terrane collision geological society of america memoir 204 p 31 65 doi 10 1130 2009 1204 02 for permission to copy contact editing geosociety org 2009 the geological society of america all rights reserved

particle settling rates increase with depth in the ocean abstract time and depth scales of particulate organic carbon degradation and caco3 and biogenic opal dissolution are critical to understanding the depth distribution of co2 and dissolved nutrients in the ocean the speed at which particles sink and the factors that control sinking speed are of primary importance to the distribution of oceanic nutrient concentrations and to the preservation of biogenic material in the sediment record sequencing sediment trap collectors deployed at us jgofs sites in the equatorial pacific and arabian sea provided a time series of particle fluxes from which particle settling rates were estimated a comparison of settling velocities obtained from 100 500 m pilskaln et al deep sea research 45 1998 1803 to settling velocities obtained for depths between 1000 and 3500 m indicate an increase of a factor of 2 10 between 100 and 2000 m and an increase of 15 60 between 2000 and 3500 m the increase in settling velocity in the deep ocean is generally correlated with the loss of corg with depth lithogenic content does not appear to impact particle settling rate variability in particle settling rate is systematically related to physical forcing of the surface ocean in the equatorial pacific but not in the arabian sea the increase in particle settling rate with depth likely influences the delivery of caco3 to the sea floor

sampling the vertical particle flux in the upper water column using a large diameter free drifting nettrap adapted to an indented rotating sphere sediment trap abstract further development of the large surface tethered sediment trap nettrap employed as part of the medflux program is described whereby the large collection capacity of the nettrap is combined with an indented rotating sphere sample carousel irsc sediment trap irsc nt this trap is capable of collecting particle flux either in a time series or settling velocity mode settling velocity mode allows the collection of particles that fall within discrete settling velocity intervals during short field deployments in the mediterranean sea the irsc nt configured in the settling velocity mode successfully collected unpoisoned samples for chemical and microbiological experiments in addition to the development of the irsc nt particle settling behavior above and below the swimmer excluding irs valve was tested during on deck experiments using a specially constructed water tight trap chemical analyses of settling materials published elsewhere suggested that separation of particles by settling velocity was achieved however due to the motion of the ship it was not possible to directly measure particle settling velocities within the trap particle release from the irs did not bias the apparent settling velocity spectrum rotation of the irs did not engender turbulence at the surface of the sphere or within the skewed funnel below tests of different ball designs over the course of the medflux program showed that a ridge and saddle pattern was optimal for efficiently transferring particles under the irs seal while still reducing swimmer entrance to the collection funnel the large size of the irsc nt did not prevent it from drifting effectively with the current several modifications of the present design are proposed that should improve the accuracy of the settling velocity measurements

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved

syn collisional magmatism at the longgen pb zn deposit western nyainqentanglha belt tibet petrogenesis and implications for regional polymetallic metallogeny abstract syn collisional orogeny generally lacks adequate fluids and has a low potential to form magmatic hydrothermal ore deposits however some syn collisional pb zn deposits have been recognized in the nyainqentanglha polymetallic belt npb central lhasa subterrane and ore related magma petrogenesis and the metallogenic geodynamic setting is limited in this study detailed petrology geochemical and geochronological data of ore related syn collisional granite porphyries are evaluated to constrain the timing of mineralization and discuss the genesis and evolution of ore related magma using the longgen skarn pb zn deposit as a case study zircon u pb dating of ore related porphyries provides a weighted mean age of 61 2 0 8 ma which is slightly earlier than the sphalerite rb sr age of 59 1 3 0 ma indicating that a paleocene pb zn mineralization event occurred during syn collisional orogeny the porphyries have high sio2 k2o na2o and are enriched in rb th u pb lrees and a cnk 0 94 1 15 but have low p2o5 and are depleted in nb ta and ti this suggests that the longgen granite porphyries are metaluminous to weakly peraluminous high k calc alkaline arc like granites these porphyries have highly radiogenic isotopic compositions with 87sr 86sr i 0 709513 0 713587 207pb 204pb i 15 657 15 713 and 208pb 204pb i 38 964 39 163 but low end t values from 6 7 to 6 0 as well as negative zircon ehf t values from 5 1 to 3 2 corresponding to tdm2 nd ages of 1 37 to 1 42 ga and zircon hf tdmc ages of 1 30 to 1 42 ga this demonstrates that the longgen granite porphyries may have been generated by the partial melting of mesoproterozoic basement continental crust materials with significant amounts 30 50 of mantle derived melt input paleocene eocene 65 45 ma magmatic rocks associated with pb zn and fe cu deposits are compiled in the npb the pb zn ore related granitoids have relatively lower whole rock end t and zircon ehf t values higher 87sr 86sr i values and older tdmc ages than the coeval granitoids associated with fe cu mineralization this indicates that mantle material is more critical for the development of fe cu ore related granitoids than granitoids associated with pb zn mineralization the final results show that different sources of paleocene eocene syn collisional magma in the npb are beneficial for different mineralization events during the syn collisional orogeny of the indian and asian plates