accelerated extension of tibet linked to the northward underthrusting of indian crust the tibetan plateau is extending numerical simulations suggest that regional scale extension is caused by gravitational collapse of the plateau whereas rapid extension in the south is caused by underthrusting of the indian slab

amphibole sponge in arc crust pressure temperature time paths followed by arc magmas ascending through the lithosphere dictate the phase assemblage that crystallizes and hence the compositions of liquid fractionates here we use la yb and dy yb versus sio 2 relationships from selected volcanoes to show that amphibole is an important mineral during differentiation of arc magma production of intermediate and silicic arc magmas occurs as magmas stall and cool in the mid lower crust where amphibole is stable because amphibole is rarely a phenocryst phase we term this cryptic amphibole fractionation if this process is as widespread as our investigation suggests then 1 amphibole cumulates may act as an effective filter for water dissolved in mantle derived magmas 2 amphibole cumulates may act as a fertile source of intracrustal melts and fluids and 3 recycling of amphibole cumulates has the potential to return incompatible trace elements and water to the mantle

provenance of xigaze fore arc basin clastic rocks cretaceous south tibet the xigaze group of south tibet was deposited in a fore arc basin produced by northward subduction of the neotethys under eurasia during middle cretaceous time clasts of andesite from the gangdese magmatic belt to the north and limestone clasts from a shelf rimming the basin to the north are the near exclusive detrital components in the xigaze group in the eastern part of the studied region and in older albian deposits in the west younger cenomanian coniacian deposits from western and central parts of the xigaze group show increasing amounts of tonalite pebbles in conglomerates and quartz sand these trends reflect differential arc uplift and erosional dissection with higher rates in the west some arenites show evidence for sediment contribution from an additional source probably from an orogen to the north of the lhasa block the presence of this orogen indicates that crustal thickening of tibet is not restricted to cenozoic times the middle cretaceous gangdese belt apparently lacked acid volcanic rocks this suggests a low degree of crustal contamination and or differentiation of primitive magma probably because the crust beneath the arc area was thin a thin crust is in accordance with a low elevation of the arc deduced from continuous carbonate production on and marine excursions beyond the arc magma eruption rates of the arc estimated from the xigaze basin sediment volume were probably low 7 5 km 3 km arc segment per million years but within the range of modern arcs the varying sediment compositions of the xigaze group may be typical of fore arc basin deposits in front of continental magmatic arcs the lack of a correlation of trends in sediment composition with fining upward sedimentary supercycles in the xigaze group is taken as evidence in favor of a eustatic cycle control

sedimentary facies distribution and genesis of a recent carbonate rich saline lake gallocanta lake iberian chain ne spain abstract the study focuses on the holocene sedimentary infill of the gallocanta lacustrine basin in the iberian chain ne spain the gallocanta lake is a saline wetland with a maximum length of 7 5 km and a maximum width of 2 85 km the water depth varies significantly from a maximum depth of 2 m to completely dry in the central areas central subenvironment sapropels and salts develop with halite gypsum anhydrite dolomite aragonite calcite magnesite and lesser amounts of quartz and clay minerals cyanobacteria filaments are related to the aragonite and dolomite crystals the marginal subenvironment either has a gradual or a sharp change from that of the central subenvironment an inner area with desiccated light grey lutites is present in this marginal subenvironment in sem and x ray diffraction analyses quartz clay minerals aragonite calcite and small quantities of dolomite gypsum anhydrite and halite can be identified this inner area is surrounded by an external fringe composed of light brown lutites and a high concentration of salicornia meadwod and microbial mats this zone is only occasionally submerged and contains sandy and conglomerate islets active palustrine areas are flood zones where grey lutites with a significant quantity of vegetation such as reeds are common in general this entire sector is being modified by human action five sedimentary facies have been defined and this has enabled the identification of three distinct stages in the general evolution of the basin the first stage is an alluvial period developing during an arid climate after this a more humid stage facilitated the installation of a shallow carbonate rich lake a reduction of the water level probably due to a more arid stage induces a salinity increase of the lacustrine brine and the change to the third stage which corresponds to the present conditions the water level experienced frequent oscillations and alternations between humid conditions with a high production of organic matter which favors carbonate formation and water level dropdowns even to total dryness with saline sedimentation

hydroclimatic and geothermal controls on the salinity of mbaka lakes sw tanzania limnological and paleolimnological implications summary the hydroclimatic and geothermal controls on the salinity of small tropical crater lakes masoko katubwi kyambangunguru ilamba and kingiri aligned with the mbaka fault line north of lake malawi are investigated by water stable isotopes which are used to trace evaporative processes as to establish lake water balances and by chloride concentrations allowing to identify the main salinity sources this region shows positive excess in the exchanges between atmosphere and lake surfaces p e 0 with the exception of lake ilamba the lakes are closed surface basins and their levels are relatively stable as catchment inflows cannot be neglected groundwater outflows have to compensate for this excess the isotopic budgets show that these lakes constitute a series of windows on the local shallow aquifer in addition the estimated losses by evaporation cannot solely account for the lake salinity observed as the most saline lake differs from not the most evaporated lake salinity is then investigated from chloride concentrations and seems to be controlled by inflows which results from the mixing between i fresh and ii hydrothermal ground waters the contrasts in lake salinity almost one order of magnitude are first related to the hydrothermal contribution which increases with the proximity of the mbaka fault second due to positive exchange with the atmosphere the lake salinity appears systematically diluted in regard to the respective inflows this dilution effect increases as the fraction of total inputs lost by evaporation or the lake water residence time increases this current hydrological study shows that local wetter conditions are not inconsistent with higher lake salinity and allows to conciliate the questioned and apparent contradictory lake masoko wetter and salty paleoenvironmental and paleoclimatic records for the last glacial maximum and younger dryas time intervals further in a general manner this work addresses the case of lake submitted to wet environment and highlights the potential contribution of hydrothermal water on lakes salinization occurring in active regions such as the east african rift system

miocene high sr y magmatism south tibet product of partial melting of subducted indian continental crust and its tectonic implication abstract post collisional 26 10 ma high sr y and hree depleted intrusive rocks occur in an e w trending belt paralleling the yarlung zangbo suture in south tibet their petrogenesis has been a subject of debate here we present chronological geochemical and sr nd hf isotopic data for two newly discovered large volume high sr y granitoids the pagu granodiorite and the nanmuqie granite and three high sr y porphyries intruding the granitoids in the lhasa terrane la icp ms zircon u pb dating for the pagu granodiorite and nanmuqie granite yielded identical magma crystallization ages of 14 0 14 4 ma which is indistinguishable from their associated high sr y porphyries 14 2 14 6 ma the granitoid was intruded at middle crust depth whereas the porphyry was intruded at upper crust depth indicating that the lhasa terrane has experienced a rapid crustal uplift during the high sr y magma emplacement zircon hf isotopic and whole rock sr nd isotopic compositions for these granitoids and porphyries suggest that their magmas were dominantly derived from partial melting of crustal materials the granitoids and porphyries have ehf t values overlapping with the mafic granulites in the himalayan terrane indian plate their sr nd isotopic compositions show two endmember mixing between the himalayan mafic lower crust and the ultrapotassic lavas the lhasa lower crust we suggest that the high sr y magmas in the lhasa terrane could be derived from partial melting of subducted indian mafic lower crust with incorporation of the ultrapotassic lava and or the lhasa lower crust components our study suggests a new model for the high sr y magma generation in the lhasa terrane and provides a line of geochemical evidence that the indian continental crust was subducted beneath the southern lhasa terrane in the early middle miocene

adakites from continental collision zones melting of thickened lower crust beneath southern tibet adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones here we report the first example of such magmas from southern tibet in an active continental collision environment the tibetan adakites were emplaced from ca 26 to 10 ma and their overall geochemical characteristics suggest an origin by melting of eclogites and or garnet amphibolites in the lower part 50 km of thickened tibetan crust this lower crustal melting required a significantly elevated geotherm which we attribute to removal of the tectonically thickened lithospheric mantle in late oligocene time the identification of collision type adakites from southern tibet lends new constraints to not only the himalayan tibetan orogenesis how and when the indian lithosphere started underthrusting asia can be depicted but also the growth of the early continental crust on earth that consists dominantly of the tonalite trondhjemite granodiorite suites marked by adakitic geochemical affinities

chemical sedimentology and paleoenvironmental history of lake olduvai a pliocene lake in northern tanzania lake olduvai was a saline highly alkaline closed basin lake in which lacustrine sediments of bed i and lower bed ii of olduvai gorge were deposited between ca 1 92 and 1 70 ma the lake consisted of a lacustrine area the central basin cb surrounded by lake margin terrain that was alternately flooded and dried the lacustrine deposits consist largely of authigenic clay and carbonate minerals zeolites k feldspar chert and pyrite claystones of the cb and most of the eastern lake margin elm terrain consist chiefly of mg rich authigenic illite smectite and interlayered illite smectite i s these clays were formed penecontemporaneously with detrital clay deposition by interaction of the detrital clays with the saline highly alkaline water the lake olduvai clays average 65 70 mg 2 sio 2 and alkali ions from lake water and the remaining 30 35 is detrital clay lacustrine clays of the elm indicate that this area was flooded by saline water for substantial periods of time hominids occupied the elm at times of lower lake levels when the clays were dried calcite is the most abundant carbonate mineral and occurs chiefly as sand size euhedral crystals dispersed in claystone and less commonly as calcite crystal limestones and nodules the sand size calcite crystals were formed diagenetically at shallow depths possibly from a gaylussite and or micritic calcium carbonate precursor dolomite occurs chiefly in the form of dolostones some of which are probably of replacement origin the oxygen isotope composition of lake water and pore fluid of the cb varied over 11 as determined from calculated delta 18 o h 2 o values of authigenic minerals clay minerals were formed chiefly in the more saline waters sand size calcite crystals were generated mostly from less saline pore fluids and limestone nodules were formed chiefly from dilute pore fluid dolostones were formed from fluids of varying salinity hydrogeology of the central basin was complex and postdepositional fluid movements are documented by differing delta 18 o h 2 o values for different authigenic minerals of the same samples pore fluids moved within the lake deposits for a considerable period of time after deposition as shown by 18 o values of quartz and k feldspar formed at different times downward flow of lacustrine fluids is indicated by zeolitic alteration of the underlying tuffaceous alluvial deposits lake olduvai became overall shallower during the deposition of bed i and the subaerial exposure increased leading to occasional playa conditions these changes very likely resulted from increasing aridity and tectonic activity the lake that deposited lower bed ii was larger than that responsible for upper bed i and episodes of wetter climate alternated with drier periods in which salinity and probably temperatures were higher than in the bed i lake

modern halolites halite oolites in the tuz golu turkey abstract halite oolites halolites and pisoids halopisoids precipitate yearly in summer in the brine conduits of the saltpans in the tuz golu saline lake central anatolia turkey these halolites are well rounded and spherical ranging between 0 7 and 2 cm in size they are composed of coarse grained halite crystals as the nucleus and by concentric halite laminae with a radial fabric as the cortex the cortex is subdivided into inner middle and outer zones each zone showing different mineralogical and morphological features these features include the presence of organic matter particles native sulphur globules gypsum anhydrite calcite laminae quartz chlorite celestite thermonatrite laminae submicroscopic halite crystals and microborings cavities and corrosion like structures our observations in the tuz golu saltpan environment and in the halolite fabrics suggest that 1 an intermittent supply of heavy brines from the saline lake into the saltpan conduits which occur under agitated conditions during pumping operations is the main genetic reason for the halolite formation and that 2 physical chemical and biological factors exert a significant influence on the mineralogical textural complexity of the cortex

the xigaze forearc basin evolution and facies architecture cretaceous tibet abstract the mid cretaceous to eocene flysch deposits of the xigaze forearc basin in southern tibet were investigated in a 120 km segment along the indus yarlung suture zone the basin evolved south of the magmatic arc gangdise belt of the lhasa block on top of trapped oceanic or transitional crust remnants of shelf carbonates are preserved along the northern edge of the basin the mapped segment was shortened by about 65 metamorphism reached low grade conditions along the northern margin of the basin the flysch sequence reaches a thickness of at least 5 km and consists to a large degree of volcaniclastic andesitic to dacitic material shed from the magmatic arc gangdise belt particularly in the western part of the study area plutonic and sedimentary rocks from deep erosion levels and or more distal sources contributed to the basin fill rivers from the lhasa block acted as point sources and fed five major deep sea channel systems turbidity currents in the channels were directed towards the growing accretionary wedge of the subduction zone thus indicating that the basin was continuously filled up to outer ridge level and gradually shallowing the forearc flysch is subdivided into at least three megasequences which begin with wide up to several km incised 10 to 50 m coarse grained channel fills and their associated fan deposits the upper parts of the megasequences contain hemipelagic dark shales and marls deposited above the calcite compensation depth lateral channel migration channel lobe switching but also volcanic pulses generated a predominantly fining upward high frequency cyclicity after continental collision the marine sedimentation in the forearc basin was replaced by fluvial deposits of the eocene oligocene qiuwu formation which is time equivalent to the kailas and indus molasses farther west and rich in coarse gravel derived from the gangdise belt both forearc flysch and qiuwu formation were deformed simultaneously during the miocene we assume that the molasse type qiuwu formation represents the final continental facies of the forearc basin filling

a spring and wooded habitat at flk zinj and their relevance to origins of human behavior the 1959 discovery of the hominin fossil zinjanthropus boisei brought the world s attention to the rich records at olduvai gorge tanzania subsequent excavations of archaeological level 22 flk zinj bed i uncovered remains of homo habilis and a high density collection of fossils and oldowan stone tools the occurrence of this unusual collection of bones and tools at this specific location has been controversial for decades we present paleoecological data that provide new insights into the origin of flk zinj our recent excavations 200 m north of the site uncovered a 0 5 m thick tufa mound draped by tuff ic in the same stratigraphic horizon as level 22 stable isotope analyses indicate that the carbonates were deposited by a freshwater spring phytolith analysis of the waxy clay under tuff ic revealed abundant woody dicotyledon and palm phytoliths indicating that the site was wooded to densely wooded the time equivalency and close physical proximity of the two environments indicate the two are related this study has provided the first documented evidence of springs in bed i and these data have important implications for the interpretation of hominin behavior in meat acquisition and the ongoing debate on scavenging versus hunting

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

a pseudo adakite derived from partial melting of tonalitic to granodioritic crust kyushu southwest japan arc abstract cretaceous peraluminous granitic intrusions occur in kyushu in the southwest japan arc as batholiths or stocks accompanied by metaluminous tonalite to granodiorite the peraluminous intrusions are classed as adakitic based on sr y y ppm relationships but they contain abundant k feldspar a phase not common in typical adakites to consider their significance in the arc system we investigated the tsutsugatake stock as representative of the intrusions exposed in central kyushu the tsutsugatake rocks are mainly garnet bearing two mica leuco granitoids their sr y ratios and rare earth element concentrations are appropriate for adakite but k 2 o na 2 o ratios 0 53 1 43 and al 2 o 3 13 7 15 0 wt mgo 0 20 0 55 wt sr 207 462 ppm cr 12 ppm and ni 8 ppm contents differ from those of typical adakite geochemical modeling shows that the petrogenesis of the tsutsugatake stock can be explained by partial melting of arc type tonalite or adakitic granodiorite under middle to lower crustal conditions leaving amphibole and plagioclase as residual phases the differences between the tsutsugatake stock and classical adakites lie in their source rocks and melting conditions the tsutsugatake source was not basaltic and the melting pressure was insufficient to stabilize garnet the tsutsugatake rocks are here designated pseudo adakites such pseudo adakites may be found in both modern and ancient arcs and would play an important role in crustal evolution as a member of the recycled constituent

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

age and composition of dikes in southern tibet new constraints on the timing of east west extension and its relationship to postcollisional volcanism controversy exists over whether east west extension in southern tibet is related to plateau uplift or to the accommodation of plate boundary forces relationships between the onset of extension plateau uplift and the thermal state of the lithosphere are critical to this debate we present new data on postcollisional north south trending dikes in southern tibet their ages range from 18 3 2 7 ma to 13 3 0 8 ma and define the onset of regional east west extension in southern tibet dikes are compositionally indistinguishable from postcollisional lavas in southern tibet being either ultrapotassic having a source in the subcontinental lithospheric mantle or calc alkaline with a dominantly crustal origin the ultrapotassic dikes demonstrate that east west extension and subcontinental lithospheric mantle derived magmatism were temporally and spatially linked supporting models that relate the latest phase of plateau uplift to subcontinental lithospheric mantle thinning thus the onset of extension by 18 3 2 7 ma represents the time at which the potential energy of the plateau exceeded convergent boundary forces this places a new age limitation on the attainment of high elevation in southern tibet with implications for models that relate cenozoic monsoon intensification to plateau uplift

cenozoic volcanism and tectonic evolution of the tibetan plateau abstract cenozoic volcanism on the tibetan plateau which shows systematic variations in space and time is the volcanic response to the india asia continental collision the volcanism gradually changed from na rich k rich to potassic ultrapotassic adakitic compositions along with the india asia collision shifting from contact collision i e soft collision or syn collision to all sided collision i e hard collision the sodium rich and potasium rich lavas with ages of 65 40 ma distribute mainly in the lhasa terrane of southern tibet and subordinately in the qiangtang terrane of central tibet the widespread potassic ultrapotassic lavas and subordinate adakites were generated from 45 to 26 ma in the qiangtang terrane of central tibet subsequent post collisional volcanism migrated southwards producing ultrapotassic and adakitic lavas coevally between 26 and 8 ma in the lhasa terrane then potassic and minor adakitic volcanism was renewed to the north and has become extensive and semicontinuous since 20 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations provide important constraints on the geodynamic processes that evolved at depth to form the tibetan plateau these processes involve roll back and break off of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root and consequently northward underthrusting of the indian lithosphere the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder and detach owing to the persistently northward push of the underthrust indian lithosphere

geology and origin of the post collisional narigongma porphyry cu mo deposit southern qinghai tibet narigongma is a poorly studied mo rich similar to 0 06 wt post collisional porphyry cu deposit located in southern qinghai province tibet 400 km northwest of the yulong porphyry cu mo au belt the narigongma deposit has a similar age 43 40 ma to porphyry deposits in the yulong belt but different ore assemblages the narigongma deposit is associated with eocene granodiorite and granite intrusions that were emplaced into a permian volcanic sedimentary rock sequence an similar to 43 3 ma biotite granite stock p1 porphyry is the earliest eocene intrusion and this was itself intruded by a number of smaller similar to 43 6 ma fine grained granite porphyry stocks p2 porphyry and several post ore quartz diorite porphyry dikes similar to 41 7 ma the main cu mo mineralization at narigongma is associated with the p1 porphyry hydrothermal alteration surrounding the deposits is generally characterized by concentric zones that range from an inner potassic zone outward to phyllic and argillic alteration zones and an outer propylitic zone hypogene mineralization at narigongrna was characterized by early stage precipitation of molybdenite during potassic alteration and late stage deposition of chalcopyrite during phyllic alteration deposition of both the mo and cu mineralization stages was caused by decreasing temperature a high degree of crystallization of the p1 porphyry occurred prior to fluid saturation that produced mo enrichment in the residual melt due to the incompatible behavior of mo however compatible cu was sequestered by the crystallizing phases and resulted in the generation of a high mo cu magmatic hydrothermal fluid and the final mo cu mineralization assemblage zircon epsilon hf t values of 4 1 to 7 9 are indicative of magma derivation from a depleted source these isotopic data coupled with other geochemical characteristics of the narigongma porphyry such as high sio2 and k2o contents low mgo contents and compatible element abundances and highly fractionated rare earth element patterns indicate a mixing model for the origin of the porphyry bodies generation of the post collisional ore forming porphyries occurred in two stages 1 partial melting of metasomatized phlogopite bearing lithospheric mantle that generated potassic to ultra potassic mafic melts and 2 underplating of such melts beneath thickened juvenile lower crust which triggered partial melting of the lower crust to produce the ore forming high k adakitic magma c 2013 international association for gondwana research published by elsevier b v all rights reserved

sources of dissolved salts in the central murray basin australia abstract large areas of the australian continent contain scattered saline lakes underlain by shallow saline groundwaters of regional extent and debated origin the normative salt composition of subsurface pore fluids extracted by squeezing cores collected during deep drilling at piangil west 2 in the central murray basin in southeastern australia and of surface and shallow subsurface brines produced by subaerial evaporation in the nearby lake tyrrell systems helps constrain interpretation of the origin of dissolved solutes in the groundwaters of this part of the continent although regional sedimentation in the murray basin has been dominantly continental except for a marine transgression in oligocene pliocene time most of the solutes in saline surface and subsurface waters in the central murray basin have a distinctly marine character some of the tyrrell waters to the southwest of piangil west 2 show the increase in nacl and decrease in sulfate salts expected with evaporative concentration and gypsum precipitation in an ephemeral saline lake or playa environment the salt norms for most of the subsurface saline waters at piangil west 2 are compatible with the dilution of variably fractionated marine bitterns slightly depleted in sodium salts similar to the more evolved brines at lake tyrrell which have recharged downward after evaporation at the surface and then dissolved a variable amount of gypsum at depth apparently over the last 0 5 ma significant quantities of marine salt have been blown into the murray basin as aerosols which have subsequently been leached into shallow regional groundwater systems basin wide and have been transported laterally into areas of large evaporative loss in the central part of the basin this origin for the solutes helps explain why the isotopic compositions of most of the subsurface saline waters at piangil west 2 have a strong meteoric signature whereas the dissolved salts in these waters appear similar to a marine assemblage

adakitic rocks from slab melt modified mantle sources in the continental collision zone of southern tibet abstract major trace element and sr nd pb isotopic data are presented for newly discovered adakitic rocks in the western gangdese belt southern tibet the miocene 26 10 ma adakitic rocks from the southern tibetan continental collision zones exhibit distinct differentiation trends typical of arc magmas these rocks display geochemical affinities similar to those of cretaceous 136 80 ma adakitic rocks derived from the partial melting of subducted neotethyan slab in southern tibet the whole rock geochemical and isotope characteristics of the post collision adakitic rocks reveal that their magmas likely originated from an upper mantle region previously metasomatized by slab melts during the cretaceous subduction event an interesting observation is that the e w trending belt of adakitic rocks along the yarlung tsangpo suture zone occupies a fore arc position reflecting a geotectonic setting compatible with the genesis of adakitic magmas the widespread occurrences of cretaceous adakitic rocks in this region are interpreted to testify to the former location of the adakite metasomatized mantle our favoured interpretation is that the spatial isotopic variation in the post collision adakitic rocks is mostly linked to a westward increase in sediment input in the tibetan mantle region under the framework of our paleo subduction model a slab break off event that initiated at around 25 ma would have allowed an asthenospheric upwelling beneath southern tibet which was instrumental in generating the post collision adakitic magmatism in southern tibet

revision of the cretaceous paleogene stratigraphic framework facies architecture and provenance of the xigaze forearc basin along the yarlung zangbo suture zone abstract new field investigation structural analysis review of biostratigraphy stratigraphic framework facies architecture provenance and flow direction of the cretaceous paleogene sedimentary strata in southern tibet resulted in reinterpretation of the evolution of the xigaze forearc basin xfb the xfb is comprised by flysch dominant xigaze group consisting of ngamring chongdoi and sangzugang formations and of the shallow marine cuojiangding group padana qubeiya quxia and gyalaze formations with their ages herein revised as the albian coniacian and the santonian ypresian respectively the deep sea flysch represented by the ngamring formation constitutes the early sedimentary fill of the xfb it was subdivided into five megasequences ms1 ms5 in order the early submarine fan facies stage channel submarine fan stage the mature submarine fan stage the highstand submarine fan stage and outer fan pelagic stage the paleocurrent flows were eastwards and westwards during the early and late stages of basin development and northwards and southwards during its middle stage indicating typical forearc basin sedimentation with longitudinal supply and lateral filling provenance studies point to multiple sources with andesitic debris sourced from the jurassic lower cretaceous yeba formation and sangri group formed in the volcanic island arc chains in the southern gangdese the four periods of the xfb development are 1 the formation of the xfb basement by the xigaze ophiolite and completed before the aptian the coeval deposits on the shelf are the sangzugang formation carbonate 2 the earliest xfb deposits of the earliest aptian age ca 125 120 ma represented by the chongdoi formation related to the residual forearc 3 composite forearc developed represented by the late albian late cenomanian flysch megasequences ms1 ms2 and the turonian coniacian flysch megasequences ms3 ms5 respectively and 4 the final stage of the xfb development started the terminal forearc stage represented by the late late cretaceous early paleogene shallow marine cuojiangding group and terminated during middle ypresian when the gyalaze formation was deposited

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

tibetan plate overriding the asian plate in central and northern tibet at the southern boundary between india and tibet the indian tectonic plate subducts northwards beneath the tibetan plateau seismic imaging shows that at the northern boundary of tibet the asian plate is also subducting southwards beneath the plateau and the tibetan lithosphere is separate

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

cretaceous tertiary structural evolution of the north central lhasa terrane tibet 1 in the north central lhasa terrane of tibet two distinct structural levels of an east west striking thrust system are exposed along the north trending late cenozoic xiagangjiang rift upper paleozoic strata deformed by the south directed langgadong la thrust and cretaceous strata involved in variably north and south directed thrusting characterize these lower and upper structural levels respectively these two structural levels are separated by the tagua ri passive roof thrust balanced cross section restoration suggests that the thrust system accommodated 103 km 53 shortening the 40ar 39ar results together with an interpretation of synthrust deposition of upper cretaceous strata suggest that the majority of shortening occurred during the late cretaceous paleocene cretaceous strata lie unconformable on permian rocks volcanic tuffs directly above the unconformity yield u pb zircon ages of 131 ma upper cretaceous strata record a change from shallow marine to nonmarine deposition indicating uplift above sea level during this time the overall south directed vergence of the thrust belt is consistent with substantial crustal thickening in central tibet by large scale northward underthrusting of lhasa terrane basement beneath the qiantang terrane prior to the indo asian collision the documented decoupling of contractional deformation at shallow crustal levels appears to be a regional characteristic of tibet from at least the bangong suture in the north to the tethyan himalaya to the south this style of deformation explains the absence of basement exposures and major denudation in this region despite substantial crustal shortening

evidence in cerro pampa volcanic rocks for slab melting prior to ridge trench collision in southern south america late miocene approx 12 ma hornblende bearing andesitic to dacitic documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage wasysym pagestyle empty declaremathsizes 10 9 7 6 begin document mathrm 63 to 68 sio 2 1 2 to 1 9 k 2 o end document adakite flows at the small cerro pampa center in patagonia 47 55 s 71 25 have some of the clearest slab melt geochemical signatures yet seen in a phanerozoic center on continental crust these magmas formed in response to melting of the hot thin slab that was subducting beneath south america prior to the collision of the chile rise at approx 6 ma or at approx 10 ma their n morb like 87 sr 86 sr 0 70285 0 70309 and low 206 pb 204 pb 18 44 18 59 ratios show that they could have been generated by

paleoclimatic and paleoenvironmental framework of flk north archaeological site olduvai gorge tanzania abstract the multi component flk north archaeological site was discovered over 50 years ago and its interpretation has been highly controversial since explanations of the dense bone and stone tool accumulation range from a site on a featureless lake margin that is dominantly anthropogenic in origin to a site near a freshwater wetland that is dominated by carnivore activity e g felids and hyenas flk north occurs stratigraphically between the ng eju tuff 1 818 0 006 ma and tuff if 1 803 0 002 ma and is composed of 9 distinct levels analysis of newly recovered fossil bones and artifacts has shown that the bones of large animals are largely the product of felid hunting and feeding behavior followed by hyena gnawing and breakage of some bones the expanded sample of felid prey remains is significant for understanding the contrasts between the mortality profiles of fossil assemblages produced by carnivores and those produced by hominins geologic mapping in the environs of the site has revealed rich sedimentological and paleoecological records and a thin but persistent tuff here named kidogo tuff that is 1 5 m below tuff if electron microprobe analyses of the tuff mineralogy revealed a unique geochemical fingerprint that permits its use for correlation of widely separated outcrops and facilitates the high resolution reconstruction of the landscape at the time of site formation the 9 archaeological levels comprise a relatively continuous record through a milankovitch precession cycle dry wet dry as the lake receded into the central basin during the dry part of the cycle surface water supplies dwindled and groundwater fed springs and wetlands became the dominant freshwater supply the flk north archaeological record essentially ended when level 1 was covered with 0 4 m of tuff if in a violent volcanic eruption of nearby mt olmoti however the overlying bed ii sediments contain scattered archaeological material and a freshwater carbonate deposit that is similar to those found associated with other bed ii archaeological sites e g vek hwk and hwke the recognition of the ecological association of springs wetlands and archaeological remains is a powerful predictive tool for locating new archaeological sites in this region that is known for hominin remains

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

causes and consequences of protracted melting of the mid crust exposed in the north himalayan antiform gneiss domes exposed by the north himalayan antiform in southern tibet provide evidence for protracted melting of the mid crust during convergent tectonics anatexis was initiated during underthrusting and recurred during the southwards extrusion of a low viscosity crustal sheet sr nd isotope systematics establish that the cores of the domes dominated by cambrian gneiss form a window of the high himalayan crystalline series hhcs indian plate combined trace element and isotopic analyses from 4 two mica granites that intrude both core and the metasedimentary mantle of the domes indicate an origin from melting of the high himalayan crystalline series by the incongruent breakdown of muscovite and to a lesser extent biotite at temperatures from 680 to 760 c u pb dating of accessory phases extends the known period of granite magmatism from the late oligocene to mid miocene a ca rich garnet bearing leucogranite of oligocene age 27 5 ma provides a rare insight into the pre miocene history of the underthrust indian plate reflecting prograde melting at greater depths during a period of crustal thickening the origin of the miocene granites 15 10 ma can be modeled from the exhumation of an extruding channel at a rate of ca 1 mm year constrained by ar isotope data melting over a protracted time scale along the leading edge of the indian plate less than 50 km from the suture accelerated horizontal shortening within the collision zone and provided a positive feedback mechanism between magmagenesis and deformation

manaslu leucogranite a collision signature of the himalaya a model for its genesis and emplacement the manaslu pluton is one of 10 leucogranites that formed in the overthrusted higher himalaya after the indo eurasian collision field and analytical data indicate that the underlying migmatites of the tibetan slab may be where the leucogranitic melts were generated the himalayan crustal thrusting of a hot slab over a rather cold volcano sedimentary pile le fort 1975a provides the necessary release of fluids these fluids cross the main central thrust mct induce the partial anatexis of the overheated tibetan slab and produce a leucogranitic magma the emplacement of the magma at first is located along the main disharmonic plane above the mct between the infrastructure and the superstructure there it generates a convective hydrothermal system extending very far laterally according to the stratification of permeabilities progressive emplacement of the granite proceeds as the convected fluids including the fluids released by the saturated magma dissolve the mainly calcareous host rocks of tibetan sedimentaries a frozen image of this caving out progression is given by the extensive network of granitic dikes outside the pluton the two quite independant fluid cycles of the generation and of the emplacement were triggered or guided by the tectonics due to collision this two fold model dominated by fluid activity may be of importance for other leucogranites and granites

siliceous islands in a carbonate sea modern and pleistocene spring fed wetlands in ngorongoro crater and oldupai gorge tanzania abstract a biogenic mechanism is proposed that produces a diagnostic geochemical signature of perennial spring fed wetlands in arid basins especially the saline and alkaline volcanic basins of east africa respiration by vegetation and decay of organic matter within marshes in ngorongoro crater tanzania locally increases aqueous equation the resulting lowered ph inhibits the precipitation of calcite and decreases the rate of silica dissolution steep geochemical gradients are produced these are represented mineralogically by siliceous biomineralization within the perennial marsh and carbonate precipitation in surrounding ephemeral wetlands mudflats and lacustrine environments as a test of this model of geochemical sedimentation the mineralogy of the early pleistocene siliceous earthy claystone of oldupai gorge is consistent with its interpretation as a paleo spring wetland deposit because such deposits are expressions of the paleo water table this approach has potential applications in many studies of basin paleohydrology paleoclimatology paleoecology and archaeology

origin of late oligocene adakitic intrusives in the southeastern lhasa terrane evidence from in situ zircon u pb dating hf o isotopes and whole rock geochemistry abstract we describe the whole rock geochemistry biotite ar ar and zircon u pb geochronology and in situ hf o isotopic compositions of the linzhi two mica granitoid in the southeastern lhasa terrane the granitoids are calc alkaline to high k calc alkaline in composition and they have geochemical affinities with adakites the whole rock geochemistry with high sio 2 and al 2 o 3 contents low mgo cr and ni contents low mg values negative e nd t 6 2 to 2 9 and low 87 sr 86 sr i 0 7059 0 7074 together with the in situ zircon hf o isotopic data e hf t 2 7 to 2 7 18 o 7 2 8 8 indicates that the granitoid was derived mainly from the partial melting of a rejuvenated lower crust underneath the lhasa terrane this lower crust was dominated by ancient crustal material with the minor involvement of juvenile mantle inputs 40 ar 39 ar ages for biotites and the presence of magmatic epidote suggest the granitoid was intruded at mid crustal depths indicating that the eastern lhasa terrane experienced a pulse of rapid uplift in the interval from 26 to 22 ma the geochemical features of the linzhi adakitic rocks are significantly different from those of eocene adakitic rocks from the north himalayan belt and this suggests that we can rule out the possibility of crustal thickening in the eastern part of lhasa terrane caused by impingement of the indian lower crust post collisional 26 10 ma adakitic rocks occur within an e w trending magmatic belt that runs parallel to the yarlung tsangpo suture in the lhasa terrane these rocks have distinctive variations in geochemistry along the strike of the lhasa terrane indicating that the lower crust is isotopically heterogeneous from west to east along the terrane these geochemical variations probably result from variable degrees of input along strike of juvenile mantle derived melts into an old lower crust the middle part of the lhasa terrane is marked by a juvenile lower crust that contains large amounts of mantle derived melt underplated during neo tethyan oceanic crust subduction in contrast the lower crust below the western and eastern parts of the terrane is relatively ancient therefore crustal thickening caused by underplating of mantle derived magmas could play an important role in the middle segment of the terrane whereas crustal compression and shortening as a result of the collision of the indian and asian plates could be the major cause for crustal thickening in the western and eastern segments

an aem tem study of weathering and diagenesis abert lake oregon i weathering reactions in the volcanics abstract abert lake in south central oregon provides a site suitable for the study of sequential weathering and diagenetic events in this first of two papers transmission electron microscopy was used to characterize the igneous mineralogy subsolidus alteration assemblage and the structural and chemical aspects of silicate weathering reactions that occur in the volcanic rocks basalts basaltic andesites and dacitic rhyolitic extrusive and pyroclastics that outcrop around the lake olivine and pyroxene replacement occurred topotactically whereas feldspar and glass alteration produced randomly oriented smectite in channels and cavities the tetrahedral octahedral and interlayer compositions of the weathering products largely dioctahedral smectites varied with primary mineral composition rock type and as the result of addition of elements released from adjacent reaction sites weathering of the highly evolved fe rich jug mountain complex at the north end of the lake produced a homogeneous smectite assemblage that contrasts with the heterogeneous smectite assemblage replacing the volcanics along the eastern margin of the lake the variability within and between the smectite assemblages highlights the microenvironmental diversity fluctuating redox conditions and variable solution chemistry associated with mineral weathering reactions in the surficial environment late stage exhalative and aqueous alteration of the volcanics redistributed many components and formed a variety of alkali and alkali earth carbonate chloride sulfate and fluoride minerals in vugs and cracks overall substantial mg si na ca and k are released by weathering reactions that include the almost complete destruction of the mg smectite that initially replaced olivine the leaching of these elements from the volcanics provides an important source of these constituents in the lake water the nature of subsequent diagenetic reactions resulting from the interaction between the materials transported to the lake and the solution will be described in part ii banfield et al 1991

chemical evolution of shallow groundwater along the northeast shore of mono lake california we attempted to quantitatively discriminate between hypothesized sources and geochemical processes responsible for the chemical evolution of shallow groundwater along the northeast shore of mono lake an alkaline saline lake located in a hydrologically closed basin in east central california shallow groundwater samples from 17 sites perpendicular and 11 sites parallel to the lakeshore were analyzed for major ions the shallow groundwater contains remnant solutes from higher lake stands which are transported toward the lake by lateral flow and surface runoff the flow system appears to be segregated into two different regions a concentrated highly saline groundwater underlying much of the northeast shore and pockets of more localized lower salinity groundwater the saturation state of the groundwater with respect to certain minerals was determined and simulations for both evaporative concentration of inflow and mixing of lake water with inflow coupled with mineral precipitation were performed solute trends in the shallow groundwater result primarily from the degree of mixing with historical lake water however evaporative concentration and redissolution cycles along with various chemical fractionation mechanisms including ca mg and na carbonate precipitation sulfate reduction ion exchange and potentially mg silicate formation are also important controls

origin of miocene cu bearing porphyries in the zhunuo region of the southern lhasa subterrane constraints from geochronology and geochemistry abstract the zhunuo cu bearing porphyries are located in the westernmost part of the miocene gangdese porphyry cu mo au deposit belt zircon u pb dating of the diorite porphyry k feldspar granite porphyry and monzonitic granite porphyry in zhunuo yielded crystallization ages of 12 5 0 4 ma 12 3 0 3 ma and 12 4 0 3 ma respectively the diorite porphyry is characterized by low sio 2 58 61 61 14 wt and th 0 30 0 76 ppm concentrations low th la 0 05 0 1 ratios and high mg 49 values coupled with low 87 sr 86 sr i 0 703777 0 703783 and high e nd t 4 07 to 4 90 values they also have adakite like affinities such as low y 10 5 12 0 ppm and high sr y ratios 61 65 they were probably derived from a thickened juvenile lower continental crust the k feldspar granite porphyry probably originated in the middle upper continental crust because of their high sio 2 73 59 74 98 wt and th 50 1 52 1 ppm concentrations high th la 1 67 2 10 and low sr y 20 2 20 7 ratios and mg 32 38 values combined with high 87 sr 86 sr i 0 710921 0 712008 low e nd t 8 47 to 9 26 isotopic compositions and old nd model ages 1 16 1 25 ga their magmas were most likely partial melts of the preserved ancient crust similar to the central lhasa subterrane the geochemical characteristics and sr nd isotopic compositions of the monzonitic granite porphyry display trends that lie between those of the diorite porphyry and k feldspar granite porphyry and they are therefore likely to be production of hybridization between the above two melts the ore bearing diorite porphyry and monzonitic granite porphyry have higher zircon ce 4 ce 3 ratios than the ore barren k feldspar granite porphyry indicating a higher oxygen fugacity in the ore bearing magmas we suggest that metals were released from the re melting of arc related cumulates which formed during lower crustal growth and thickening this mechanism provides a reasonable explanation for the significant flare up of mineralization during the miocene in the gangdese region the lower continental crust beneath southern lhasa subterrane probably was uniformly juvenile but the region to the west of zhunuo was not mineralized due to input of large ancient crustal materials in the source of these ore barren adakite like rocks

sedimentary geology and human origins a fresh look at olduvai gorge tanzania abstract recent field work at olduvai gorge tanzania using sedimentary geology in particular high resolution paleoenvironmental reconstruction and isotope geochemistry has revealed that freshwater was in proximity to a number of the rich fossil sites in beds i and ii 2 0 1 0 ma this paper presents the first geological evidence for springs associated with archaeological sites in this semiarid rift basin the springs appear to be limited to a small area within the basin and were likely connected to faults that acted as conduits for groundwater tufas associated with ten archaeological sites have stable isotope signatures occurring in a cluster bounded by 18o ratios from 6 to 1 and the 13c ratios from 5 to 2 the 18o values cluster around 4 that of precipitation in the region indicating a meteoric source the longevity of the spring record reflects a hydrologic system that apparently persisted for hundreds of thousands of years previous landscape reconstructions depicted the archaeological sites on the lake margin of paleo lake olduvai as an alkaline playa the discovery of springs at or near the archaeological sites provides fresh insights for interpreting hominin behavior during this key time in evolution with respect to procuring food water and materials for stone tools as well as hominin adaptation to climate change and paleoenvironmental change the idea that spring deposits may be in proximity to archaeological sites could lead to discovery of new sites at other hominin fossil localities in the east african rift system ears

highly fractionated late eocene 35 ma leucogranite in the xiaru dome tethyan himalaya south tibet abstract the xiaru dome is located in the middle section of the north himalayan gneiss domes belt in southern tibet the leucogranite which crops out in the core of the xiaru dome is a typical medium grained garnet tourmaline muscovite leucogranite u th pb dating of zircon and monazite from the leucogranite yielded ages of approximately 35 ma this finding supports a growing body of evidence indicating that an extensive magmatic event occurred during the late eocene in the himalayas this leucogranite is strongly peraluminous with a cnk values of 1 08 1 52 and characterized by evolved geochemical composition with high contents of sio 2 and alkali elements low levels of cao mgo tio 2 and feot enriched large ion lithophile elements such as rb and depleted of high field strength elements such as nb zr and hf the non charac charge and radius controlled trace element behaviors which are typical of a highly fractionated granite system were recorded in the whole rock and the accessory minerals of the xiaru leucogranite furthermore the magmatic zircon overgrowths have extremely high content of hf consistent with those from the highly fractionated aqueous like fluid system in addition whole rock geochemical fractionation trends were observed which can be explained by crystal fractionation of biotite k feldspar zircon xenotime and monazite these geochemical features indicate that the xiaru leucogranite is a typical highly fractionated granite the geochronological and geochemical features of the inherited zircons from the xiaru leucogranite show a close affinity to those of the country rocks suggesting a certain degree of assimilation from the country rocks during melt ascent and emplacement although a restricted range of e hf t values from 12 8 to 6 6 with hf t dm2 model ages of 1 2 1 6 ga was obtained from the late eocene zircons it is invalid to constrain the source of the parental magma due to the strong fractionation and assimilation processes

spatial variability and hydrologic evolution of australian lake basins analogue for pleistocene hydrologic change and evaporite formation abstract processes evident in basins across hydrologic gradients from humid to arid regions today provide analogues for systems that may have operated in some individual basins during periods of past hydrologic change thus lakes in humid regions today with low salinity sedimentary facies and with morphology controlled largely by wave action may closely resemble past lacustral pluvial phases experienced in zones which today are relatively arid and saline conversely playas controlled today by groundwater and in some cases subject to defletion may resemble those arid and dune building stages experienced by basins in sub humid regions today as they underwent glacial age desiccation in any such sequence of spatial or temporal gradients from humid to more arid conditions the frequency of basin filling and drying modulates the rate of evaporitive loss of interstitial waters from the exposed basin surface this in turn controls the salinity evolution of the interstitial and local groundwaters the notion of closed systems for terminal lakes while satisfactory for analysis of surface water budgets is quite inapplicable to the evolution of brines where surfacewater groundwater exchange during the high salinity drying stages becomes a controlling factor affecting all salt budgets in these systems evaporites may only be preserved when the sub surface waters have evolved to salinites near saturation for the salts concerned if sub surface waters are frequently diluted by surfacewater groundwater exchange evaporites will not be preserved in the record even though the surface system appears hydrologically closed the process of interstitial water evolution which controls efflorescence deflation and salt weathering exerts a dominating influence on basin geomorphic expression as well as on hydrochemical fractionation

wetland diagenesis and traces of early hominids olduvai gorge tanzania lateral variations in whole rock and clay geochemistry of basal bed ii claystones in olduvai gorge tanzania reflect water quality differences across the eastern lacustrine plain 1 75 myr ago bulk ba sr and na2o k2o mgo al2o3 range from 1 4 to 4 2 and from 0 7 to 1 4 respectively and indicate leaching of lacustrine claystones beneath freshwater wetlands at times following lake retreat bulk mgo al2o3 0 3 1 0 and molar mg al 0 5 3 9 ratios of 0 2 m clays reflect alteration of mg rich lacustrine clays these indicators point to freshest conditions near locality 43 of hay 1976 hwk east leakey 1971 moderate conditions to the east locality 40 mck and high salinity and alkalinity to the west localities 85 vek 45 flk clay geochemistry and artifact abundances are well correlated r 0 67 p 0 005 suggesting a relationship between paleo water quality and hominid paleoecology this pattern is consistent with predictions of greatest artifact discard loss around freshwater sources where scavanging opportunities were greatest for hominids this quantifies a relationship between artifact density distribution and a paleoecological proxy over landscape scales for the first time in early stone age archaeology in contrast fossil bone abundance is uncorrelated r 0 14 p 0 6 reflecting more complex taphonomic processes quantitative tests of landscape scale land use models are important for understanding early hominid behavior and its evolution

tracking changes in crustal thickness during orogenic evolution with sr y an example from the north american cordillera global compilations indicate that the geochemistry of arc magmatism is sensitive to moho depth magmatic products are prevalent throughout the history of cordilleran orogenesis and can be employed to constrain the timing of changes in crustal thickness as well as the magnitude of those changes we investigate temporal variations in crustal thickness in the united states cordillera using sr y from intermediate continental arc magmas our results suggest that crustal thickening began during the late jurassic to early cretaceous and culminated with 55 65 km thick crust at 85 95 ma crustal thicknesses remained elevated until the mid eocene to oligocene after which time crustal thicknesses decreased to 30 40 km in the miocene the results are consistent with independent geologic constraints and suggest that sr y is a viable method for reconstructing crustal thickness through time in convergent orogenic systems

geochemical and nd pb isotopic characteristics of the tethyan asthenosphere implications for the origin of the indian ocean mantle domain abstract it is unclear why the pb nd and sr isotopic composition of the modern mid ocean ridge basalts morb from the indian ocean is different from that of the north atlantic and pacific oceans a possible explanation for this is that the indian morb type isotopic signature is a long lived regional feature of the mantle as evidently shown by the isotopic composition of the 350 ma morb like mian lue northern ophiolite which was formed in the same region presently occupied by the indian ocean however this hypothesis is in conflict with the lack of indian morb type isotopic signature in a number of 150 ma tethyan and indian ocean crusts to further constrain the origin of the indian morb type isotopic signature we analyze the geochemical and pb nd and sr isotopic composition of representative mafic rocks from four tethyan ophiolites ranging in age from 90 to 360 ma the sr isotopic composition of the samples is unreliable due to alteration but the age corrected nd and pb isotopic ratios and geochemical data indicate that these tethyan rocks were derived from a geochemically depleted asthenospheric source that had a clear indian morb type isotopic signature we therefore conclude that the bulk of the indian suboceanic mantle was most probably inherited from the tethyan asthenosphere a few regions in both the tethyan and indian oceans however are most probably underlain by pacific and north atlantic morb type mantle and vice versa because of the flow of the asthenosphere in response to tectonic plate reorganizations that lead to openings and closures of ocean basins the indian morb type isotopic signature of the western pacific marginal basin crusts could be due to either flow of the indian ocean mantle into the western pacific or to endogenous production of such an isotopic signature from delaminated east asian sublithospheric materials during closure of the tethys ocean

subduction of the indian lithosphere beneath the tibetan plateau and burma abstract subduction of the indian lithosphere under eurasia plays an important role in the tectonic evolution of the tibetan plateau and surrounding regions to improve our knowledge of pertinent mantle structures through tomographic imaging we combine p wave arrival time data from temporary arrays in tibet and stations of the chinese seismograph network with reprocessed data from the international seismological centre the new images reveal considerable spatial variations in mantle structure along the collision zone and the horizontal distance over which presumed continental indian lithosphere slides northward beneath the plateau decreases from west where it underlies the himalayas and the entire plateau to east where no indication is found for present day underthrusting beyond the himalayan block and indus tsangpo suture p wavespeed appears low in the shallow mantle beneath much of central and eastern tibet these observations suggest that indian lithosphere underlies only the southwestern part of the plateau and that the central and northeastern part is underlain by lithosphere of asian origin the continental parts of the indian plate that currently underthrust or subduct below the plateau appear generally detached from the oceanic slab fragments that subducted longer ago and that have been detected deeper in the mantle except perhaps in the central part of the collision zone 80 90 e this suggests that ongoing india eurasia collision must increasingly be driven by other forces such as subduction of the indian plate further west and east e g beneath indochina our images reveal structures associated with eastward subduction along the burmese arc southeast of the eastern himalayan syntaxis the tengchong volcanic complex in southwest china is marked by slow wave propagation to 150 km depth suggesting a causal link to subduction along the burmese arc the low velocities beneath the red river fault region extend to greater depth and may be related to upper mantle processes further southeast

first field evidence of southward ductile flow of asian crust beneath southern tibet there is lively debate on whether asian plate material was involved in southward flow of mid lower crust in a ductile channel beneath southern tibet one argument against such involvement is the apparent absence of material derived from asian lithosphere within the high himalayan series indian plate that could represent the putative channel a north south trending mid miocene dike swarm that intrudes the tethyan sedimentary cover of the sakya gneiss dome indian plate yields new sr nd isotopic data 87sr 86sr 0 7071 0 7079 nd 4 to 6 indicating that these melts share the same source as miocene dacitic dikes from north of the indus tsangpo suture moreover dikes on both sides of this suture represent crustal melts derived largely from mid lower crust of the asian plate exposed today as the nyainqentanglha gneisses that underlie the gangdese batholith we infer that melting of the asian lithosphere extended south of the surface trace of the suture requiring southward propagation of anatectic asian middle crustal material during the miocene the emplacement ages of the southern dike swarm 12 9 ma thus delimit the timing of active southward ductile flow of asian material

the salt ooids of berre salt works bouches du rhone france the role of bacteria in salt crystallisation abstract salt ooids that form in the solar salt works of berre france are studied principally through sem imagery they are compared to halite particles obtained in the laboratory by bacterial mediation under controlled conditions salt crystallisation appears to be induced and oriented by the growth of bacterial colonies the main bacterial processes involved are the multiplication of cells and the regulation of their internal salt concentration which tends to oversaturate the closely neighbouring environment such biogeological processes could have played a significant role in rocksalt deposition over geological times

paleoenvironmental and paleoecological reconstruction of a freshwater oasis in savannah grassland at flk north olduvai gorge tanzania the records of early hominins are commonly localized both temporally and spatially even in archaeologically rich basins like olduvai gorge tanzania the flk north site was discovered in 1960 but the reason for the exact location of this dense concentration of fossils and stone tools on a lake margin flat has not been explained we present new geological and geochemical information from six excavations in upper bed i which revealed up to 1 4 m thick carbonate deposits attesting to the presence of freshwater springs in the area surrounding flk north the 18o signatures of the tufa are typical for meteoric water that has evolved during evaporation tuff if that caps the sequence was deposited on uneven topography with the highest area a low relief ridge between two faults at the archaeological site and lowest areas being sites of groundwater discharge the model proposed here is that rainfall on adjacent highlands was transported to the basin where faults acted as conduits for water similar hydrogeological settings at modern lakes manyara and eyasi currently support lush groundwater forest and woodland despite arid climate flk north may have been an oasis offering freshwater and shelter for consuming meat by both carnivores and hominins

india asia convergence insights from burial and exhumation of the xigaze fore arc basin south tibet the composite fore arc syncollisional xigaze basin in south tibet preserves a key record of india asia collision new apatite fission track and zircon u th he data from an n s transect across the preserved fore arc basin sequence near xigaze show a consistent northward late cretaceous to middle miocene younging trend while coexisting apatite u th sm he ages are all miocene corresponding detrital zircon u pb data are also reported for constraining the cretaceous depositional ages of the xigaze basin sequence in the region thermal history modeling indicates that the basin experienced northward propagating episodic exhumation along with a northward migration of the depocenter and a pre existing cenozoic syncollisional basin sequence which had been removed in the southern part fore arc exhumation commenced in the late cretaceous 89 2 ma following transition to a syncollisional basin in the paleocene sedimentation in the central and northern xigaze basin continued until the latest eocene 34 4 ma ongoing folding and thrusting e g great counter thrusts caused by progressive plate convergence during late oligocene early miocene time resulted in regional uplift and considerable basin denudation which fed two fluvial basins along its northern and southern flanks and exposed the basement ophiolite subsequent incision of the yarlung river resulted in miocene cooling in the region different episodes in the exhumation history of the xigaze basin caused by thrusting of an accretionary wedge and ophiolitic basement can be linked to changes in india asia convergence rates and the changing subduction pattern of the indian and neo tethyan slabs

the nature and timing of crustal thickening in southern tibet geochemical and zircon hf isotopic constraints from postcollisional adakites abstract rising as the roof of the world the tibetan plateau is now underlain with the thickest continental crust on earth how and when was this crust formed which would have exerted pivotal controls to the formation of the plateau has long been an issue of hot debates this paper reports zircon u pb ages and hf isotope data for postcollisional 30 9 ma adakites in the southern lhasa terrane southern tibet a comparative analysis of whole rock rare earth element geochemistry and zircon hf isotopes between the adakites and associated gangdese igneous rocks suggests that the tibetan crust underwent a major phase of tectonic thickening between ca 45 and 30 ma in the region the lower part of the thickened crust consisted prevailingly of mafic lithologies which we argue to have resulted from intense basaltic underplating and subsequent remelting that took place during the late cretaceous and eocene time related to the neotethyan subduction processes including breakoff of the subducted slab at ca 50 ma in the early stage of the india asia collision these processes were responsible for not only the juvenile crust formation but also for the creation of a thermally softened lithosphere in the southern lhasa terrane the indentation of india consequently caused distributed lithospheric thickening with formation of an orogenic root beneath southern tibet root foundering during the oligocene gave rise to the adakitic magmatism regional topographic uplift and onset of northward underthrusting of the indian plate that has since played a key role in forming the entire tibetan plateau

extent of underthrusting of the indian plate beneath tibet controlled the distribution of miocene porphyry cu mo au deposits miocene igneous rocks in the 1 600 km long e w gangdese belt of southern tibet form two groups separated at longitude 89 e the eastern group is characterized by mainly intermediate felsic calc alkaline plutons with relatively high sr y ratios 23 to 342 low 87sr 86sr i ratios 0 705 to 0 708 and high endi values 5 5 to 6 1 in contrast the western group is characterized by mainly potassic to ultrapotassic volcanic rocks with relatively high th and k2o contents low sr y ratios 11 to 163 high 87sr 86sr i ratios 0 707 to 0 740 and low endi values 4 1 to 17 5 the eastern plutonic group is associated with several large porphyry cu mo au deposits whereas the western group is largely barren we propose that the sharp longitudinal distinction between magmatism and metallogenic potential in the miocene gangdese belt reflects the breakoff of the greater india slab and the extent of underthrusting by the indian continental lithosphere at that time magmas to the east of 89 e were derived by partial melting of subduction modified tibetan lithosphere mostly lower crust triggered by heating of hot asthenospheric melt following slab breakoff these magmas remobilized metals and volatile residual in the crustal roots from prior arc magmatism and generated porphyry cu mo au deposits upon emplacement in the upper crust in contrast magmas to the west of 89 e were formed by smaller volume partial melting of tibetan lithospheric mantle metasomatized by fluids and melts released from the underthrust indian plate they are less hydrous and oxidized and did not have the capacity to transport significant amounts of metals into the upper crust

hominin use of springs and wetlands paleoclimate and archaeological records from olduvai gorge 1 79 1 74 ma abstract an interdisciplinary study of the geologic and archaeological records of a 50 ky year period at the plio pleistocene boundary in olduvai basin tanzania revealed that a small groundwater fed wetland was a consistent focus of hominin activity during periods of climate change wet dry cycles paleoenvironmental analysis of sedimentary records from 33 excavations in lowermost bed ii lmb ii olduvai gorge 3 s revealed evidence of significant climate fluctuations that produced two and one half lake cycles between 1 79 and 1 74 ma the region experienced wet dry cycles in response to combined astronomical forcing effects of eccentricity 100 ky cycle and precession of the equinox 19 and 23 ky cycles rhythmic variability of the monsoon system produced wet periods followed by a transition into dry periods followed by a transition back into wet climate a high resolution study of the stratigraphy documented a groundwater fed wetland complex 1 km2 that flourished during low lake levels dry periods and was drowned during high lake levels wet periods stratigraphic analysis temporal changes of 916 artifacts from 13 excavations selected from a landscape transect across alluvial fan spring wetland and lake margin environments showed no notable differences in artifact type frequency and raw material type when comparing wet periods as a group and dry periods as a group however a lateral shift of hominin activity on the landscape did occur between wet and dry periods the oldowan tool kit did not vary at spring sites between wet and dry periods whereas the tool kit did vary at other lake margin sites between wet and dry periods both analyses suggest that climate affected the location of an activity not type of activity these data suggest that oldowan hominins adapted their land use patterns to exploit spring wetland resources available during dry periods a switch from wet to dry conditions likely altered surface and groundwater systems affecting food and potable water availability forcing hominins to adjust their subsistence patterns

copper enrichment in arc magmas controlled by overriding plate thickness the characteristics of magmas typically associated with porphyry copper deposits are thought to be imparted in the mantle statistical assessment of over 40 000 geochemical analyses of magmatic rocks formed in subduction zones worldwide however shows that the characteristics of these magmatic rocks are systematically controlled by the thickness of the arc crust

eocene neo tethyan slab breakoff constrained by 45 ma oceanic island basalt type magmatism in southern tibet slab breakoff is one of the primary processes in the evolution of many collisional orogens in the tibet himalaya orogen the timing of breakoff of the neo tethyan slab remains controversial because of a scarcity of solid evidence this study reports the discovery of eocene gabbros dated at 45 0 1 4 ma in situ u pb age of titanite using secondary ion mass spectrometry from the eastern segment of tethyan himalaya in southern tibet these rocks show geochemical characteristics similar to those of himu high type oceanic island basalt and have depleted sr nd isotopes 87sr 86sr t 0 70312 0 70317 end t 4 9 to 5 0 it is suggested that the gabbros stand as the first direct evidence for partial melting of the asthenosphere followed by rapid magma ascent with negligible crustal contamination this event combined with results from relevant studies along the indus yarlung suture zone is best explained by a sudden and full scale detachment of subducted neo tethyan slab at great depth the breakoff model may account for coeval tectonomagmatic activities development of small scale short lived magmatism and subsequent termination of the gangdese arc magmatism in southern tibet and for the abrupt slowdown ca 45 ma of indo asia convergence

bacterially induced mineralization of calcium carbonate in terrestrial environments the role of exopolysaccharides and amino acids this study stresses the role of specific bacterial outer structures such as glycocalix and parietal polymers on calcium carbonate crystallization in terrestrial environments the aim is to compare calcium carbonate crystals obtained in bacterial cultures with those obtained during abiotically mediated synthesis to show implications of exopolysaccharides and amino acids in the mineralogy and morphology of calcium carbonate crystals produced by living bacteria this is done using various amounts of purified exopolysaccharide xanthan eps and l amino acids with a range of acidities amino acids and increasing xanthan content enhance sphere formation in calcite and vaterite regarding calcite the morphology of crystals evolves from rhombohedral to needle shape this evolution is characterized by stretching along the c axis as the amino acid changes from glutamine to aspartic acid and as the medium is progressively enriched in eps regarding vaterite the spherulitic habit is preserved throughout the morphological sequence and starts with spheres formed by the agglomeration of short needles which are produced in a xanthan free medium with glutamine monocrystals forming spheres increase in size as xanthan is added and the acidity of amino acids glutamic and aspartic acids is increased at high xanthan concentrations amino acids and mainly aspartic and glutamic acids induce vaterite precipitation the role of the carboxyl group is also probably critical because bacterial outer structures associated with peptidoglycan commonly contain carboxyl groups this role combined with the results presented here clearly demonstrate the influence of bacterial outer structure composition on the morphology and mineralogy of bacterially induced calcium carbonate this point should not be neglected in the interpretation of calcite cements and carbonate accumulations in terrestrial environments

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

synchronous n s and e w extension at the tibet to himalaya transition in nw bhutan despite 50 myr of continuous continent continent collision contractional structures in the himalayan tibetan orogen are today limited to the northern and southern margins of the system while extension dominates much of the interior on the tibetan plateau cenozoic e w extension has been accommodated by strike slip faults and extensional grabens while n s extension at the tibet to himalaya transition has been accommodated by the south tibetan fault system stfs the genetic relationship between n s and e w extension is disputed although age constraints indicate temporal overlap of at least 7 myr in nw bhutan the two intersect where the stfs basal detachment is cut by the yadong cross structure ycs an extensional half graben that provides a rare opportunity to constrain relative timings we report u pb zircon dates from four stfs footwall leucogranites consistent with episodic magmatism during the middle late miocene and in situ u th pb monazite and xenotime dates from three metasedimentary rocks ranging from late oligocene to middle miocene we suggest that amphibolite facies footwall metamorphism was ongoing at the time the basal stfs detachment initiated as a ductile structure in the middle late miocene late stage granitic intrusions may reflect footwall melting during extensional exhumation along the stfs but post metamorphic and post intrusion fabrics suggest that most displacement occurred after emplacement of the youngest granites some of the oldest ycs related fabrics are found in a deformed 14 ma leucogranite implying middle miocene ductile deformation this observation along with evidence for subsequent brittle ycs deformation suggests that n s and e w extensional structures in the area had protracted and overlapping deformation histories

petrologic case for eocene slab breakoff during the indo asian collision the greater himalayan sequence is the metamorphic core of the himalaya and has been a focus of considerable study yet its petrologic evolution remains controversial pre oligocene metamorphism was nearly obliterated by miocene metamorphism and melting which many workers ascribe to shear heating unusually high concentrations of radioactive elements accompanying burial and or several kilometers of miocene exhumation due to extensional faulting sparse oligocene to eocene ages are often assigned to a tectonically unspecified event an alternative slab breakoff model subduction of greater himalayan rocks to 100 km depth in the eocene followed by buoyant extrusion due to decoupling of the oceanic lithosphere has also been proposed based on rare eocene eclogites but without explanation of miocene melting or metamorphic petrogenesis we argue that slab breakoff readily explains the eocene eclogites miocene partial melts and late eocene k rich magmas in southeastern tibet and that metamorphic and plutonic ages help define the timing and rates of breakoff and extrusion this model implies that 1 much of the greater himalayan sequence was subducted to depths greater than commonly considered 2 fluid absent decompression melting at 30 35 km depth was the consequence of as much as 100 km of extrusion rather than radioactive heating or a smaller crustal level extensional or erosional event and 3 eruption of eocene k rich tibetan plateau lavas has no implication for topography of the tibetan plateau

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

cenozoic volcanism in tibet evidence for a transition from oceanic to continental subduction geochronological k ar or ar ar major and trace element sr nd pb isotopic and mineral chemical data are presented for newly discovered cenozoic volcanic rocks in the western qiangtang and central lhasa terranes of tibet alkali basalts of 65 45ma occur in the western qiangtang terrane and represent primitive mantle melts as indicated by high mgnumbers 100 mg mg fe 54 65 cr 204 839 ppm and ni 94 218 ppm contents and relatively low ratios of sr sr 0 7046 0 7061 pb pb 18 21 18 89 pb pb 15 49 15 61 and pb pb 38 42 38 89 and high ratios of nd nd 0 5124 0 5127 in contrast younger volcanic rocks in the western qiangtang terrane 30ma and the central lhasa terrane 23 13 and 8ma are potassic to ultrapotassic and interpreted to have been derived from an enriched mantle source they are characterized by very high contents of incompatible trace elements negative ta nb and ti anomalies and radiogenic pb isotopic compositions pb pb 18 43 19 10 pb pb 15 64 15 83 pb pb 39 14 39 67 sr sr 0 7088 0 7092 and nd nd 0 5122 ratios of the western qiangtang terrane potassic lavas are similar to those of 45 29ma potassic volcanic rocks in the north central qiangtang terrane whereas sr sr 0 7167 0 7243 and nd nd 0 5119 ratios of central lhasa terrane lavas are similar to those of 25 16ma ultrapotassic volcanic rocks in the western lhasa terrane the 65 45ma alkali basalts in the western qiangtang terrane along with widespread calc alkaline volcanic rocks of this age in the lhasa terrane may be related to roll back of a previously shallow north dipping slab of tethyan oceanic lithosphere beneath tibet subduction as opposed to convective thinning of continental lithosphere is favored to explain potassic volcanism in tibet because of its occurrence in distinct east west trending belts 45 29ma in the qiangtang terrane 25 17ma in the northern lhasa terrane 16 8ma in the southern lhasa terrane and temporal and spatial relationships with major thrust systems

saline lake diagenesis as revealed by coupled mineralogy and geochemistry of multiple ultrafine clay phases pliocene olduvai gorge tanzania mineralogical and geochemical analyses of ultrafine 0 1 m extracts of pliocene clays from olduvai gorge reveal the complexities of clay diagenesis in saline alkaline paleo waters multiple authigenic phases are routinely present these may be distinguished and quantified by decomposition of xrd 060 peaks coupled with geochemical microprobe hrtem aem and crystallographic ir investigation by assigning geochemistry based on xrd results we identify three principal phases by octahedral composition dioctahedral al rich dioctahedral relatively fe rich and mg rich with a trioctahedral component average octahedral formulae are al 1 30 fe 0 57 mg 0 13 al rich al 0 56 fe 0 80 mg 0 62 fe rich and al 0 09 fe 0 23 mg 2 09 mg rich per half formula unit ir analyses support these octahedral structures the mg rich phase either has coexisting dioctahedral and trioctahedral domains within individual sheets or a homogeneous octahedral sheet of intermediate composition this is to our knowledge the first report of an intermediate octahedral occupancy in a 2 1 phyllosilicate hrtem observations indicate both solid state octahedral and illitization reactions and dissolution precipitation layer charge and interlayer cations suggest that during illitization layer charge increase was due to interaction with na rich brines but later k uptake may have been with either saline or fresher fluids with higher k na ratios all three phases occur throughout the basin relative proportions and octahedral compositions vary total mg content of bulk 0 1 m clay fractions a paleochemical indicator in this and other basins is the product of both the relative abundance and the mg content of the mg rich phase in addition to shedding light on the crystal chemistry of authigenic 2 1 phyllosilicates these results demonstrate the paleolimnologic importance of discriminating the presence of multiple authigenic clay phases in lacustrine deposits

postsubduction porphyry cu au and epithermal au deposits products of remelting of subduction modified lithosphere porphyry cu mo au and some epithermal au deposits are formed from hydrothermal fluids exsolved from cooling water rich calc alkaline magmas emplaced in volcanoplutonic arcs above subduction zones these magmas originate by partial melting of the metasomatized asthenospheric mantle wedge however there is increasing evidence for the existence of a suite of porphyry cu au and epithermal au deposits related to magmas generated after subduction beneath the arc has ceased associated magmas tend to be mildly alkaline relatively sulfur poor and emplaced as isolated complexes rather than in voluminous volcanoplutonic arcs they are likely formed by remelting of previously subduction modified arc lithosphere triggered by postsubduction lithospheric thickening lithospheric extension or mantle lithosphere delamination metasomatized mantle lithosphere or hydrous lower crustal cumulates residual from first stage arc magmatism contain small amounts of chalcophile and siderophile element rich sulfides and constitute a fertile source for hydrous au rich but relatively sulfur poor magmas during later remelting the recognition that porphyry cu au and related epithermal au systems can also form in postsubduction and collisional tectonic settings expands the range of geological environments and geographical terranes that are prospective for such deposits

characteristics and genesis of gangdese porphyry copper deposits in the southern tibetan plateau preliminary geochemical and geochronological results porphyry type cu mo au deposits have been discovered along the gangdese magmatic arc in the southern tibetan plateau extensive field investigations and systematic studies of geochemistry s pb isotopic tracing together with re os and 40ar 39ar isotopic dating indicate that the mineralisation of the copper belt is genetically related to emplacement of late orogenic granitic porphyries during the post collisional crustal relaxation period of the late himalayan epoch these porphyries are petrochemically k enriched and belong to shoshonitic to high k calc alkaline series they display enrichment of large ion lithophile elements lile rb k u th sr pb and depletion of high field strength elements hfse nb ta ti and the heavy rare earth elements hree and y without eu anomalies these characteristics demonstrate that subduction played a dominant role in their petrogenesis and residual garnet was left in the magma sources pb isotope data show a linear correlation in the plumbotectonic framework diagram ranging from orogenic pb in the eastern segment of the copper belt to mantle pb in the western segment these constitute a mixing line of the indian oceanic morb with indian oceanic sediments and suggest that the porphyry magmas were dominantly derived from partial melting of subducted oceanic crusts mixed with a minor quantity of sediments and mantle wedge components the gangdese porphyry copper polymetallic belt has alteration characteristics and zonation typical of porphyry type copper deposits which include potassic alteration k feldspathisation and biotitisation silicification sericitisation and propylitisation mineralisation mainly occurs in strongly altered granitic cataclasite at the exo contact with veinlet disseminated textures the porphyries themselves are weakly mineralised with disseminated pyrite and chalcopyrite the copper deposits contain simple ore mineral associations consisting of chalcopyrite pyrite bornite molybdenite sphalerite and oxidised minerals of malachite covellite and molybdite during supergene oxidation primary ores underwent secondary enrichment to form economic orebodies with cu grade ranging from 1 to 5 ore sulphides of the copper belt display s and pb isotopic compositions identical to the ore bearing porphyries their 34s values vary between 3 8 and 2 4 and are typical of mantle sulphur the 206pb 204pb 207pb 204pb and 208pb 204pb ratios vary in the ranges 18 106 to 18 752 15 501 to 15 638 and 37 394 to 39 058 respectively and yielded radiogenetic lead enriched signatures twelve molybdenite samples from the copper belt yielded isochron ages of 14 76 0 22 ma and 13 99 0 16 ma for the nanmu and chongjiang deposits and model ages of 13 5 to 13 6 ma for the lakang e deposit meanwhile 40ar 39ar isotopic dating of two biotite phenocrysts from the chongjiang and lakang e deposits give plateau ages of 13 5 1 0 ma and 13 42 0 10 ma respectively during the geodynamic evolution of the gangdese collision orogenic belt intrusion of the ore bearing porphyries took place just before the rapid uplift and e w extension of the southern plateau and the ore forming process may have occurred simultaneously with the uplift and extension 14 0 1 ma

dating the indian continental subduction and collisional thickening in the northwest himalaya multichronology of the tso morari eclogites multichronometric studies of the low temperature eclogitic tso morari unit ladakh india place timing constraints on the early evolution of the northwest himalayan belt several isotopic systems have been used to date the eclogitization and the exhumation of the tso morari unit lu hf sm nd rb sr and ar ar a ca 55 ma age for the eclogitization has been obtained by lu hf on garnet omphacite and whole rock from mafic eclogite and by sm nd on garnet glaucophane and whole rock from high pressure metapelites these results agree with a previously reported u pb age on allanite and together these ages constrain the subduction of the indian continental margin at the paleocene eocene boundary during exhumation the tso morari rocks underwent thermal relaxation at about 9 3 kbar characterized by partial recrystallization under amphibolite facies conditions ca 47 ma as dated by sm nd on garnet calcic amphibole and whole rock from metabasalt rb sr on phengite apatite and whole rock and ar ar on medium si phengite from metapelites ar ar analyses of biotite and low si muscovite from metapelites which recrystallized at 5 kbar toward the end of the exhumation show that the tso morari unit was at upper crustal levels ca 30 ma these results indicate variable exhumation rates for the tso morari unit beginning with rapid exhumation while the indian margin subduction was still active and later proceeding at a slower pace during the crustal thickening associated with the himalayan collision

deposition and early alteration of evaporites summary the depositional settings for primary and early diagenetic evaporite deposits generally fall into three categories marginal mixed shallow subaqueous and subaerial shallow and deep subaqueous these three environmental groupings hold for both marine and nonmarine settings although the details of continental evaporites may be far more complex than in most marine fed water bodies the primary evaporite morphologies from many continental playa hypersaline marine and marine marginal depositional settings are reasonably well understood because of the numerous detailed studies of recent holocene and cenozoic deposits that serve as models for sedimentary interpretation the sedimentological features that develop in deeper water settings are inferred based on examination of unaltered cenozoic deposits each environmental setting develops characteristic depositional features and patterns and one facies grades into the next because there may be significant physiochemical changes in water composition during deposition as well as sudden change s in both water depth and basinal circulation description and interpretation of evaporative rocks should not be based on mineralogy alone but on the distinct sedimentary characteristics of each part of a deposit in cases where sediments still reflect their primary mineralogy and morphology most of the environments can now be recognized however geochemical studies are commonly required to determine the source s of the original water the determinative geochemical techniques that presently serve as a support to sedimentologic study include studies of fluid inclusions bromine content of chloride salts and the stable isotopes of strontium sulphur carbon and oxygen only after these and perhaps other chemical analyses are considered can the full depositional history of a region be reasonably unravelled

an aem tem study of weathering and diagenesis abert lake oregon ii diagenetic modification of the sedimentary assemblage abstract this paper compares the mineralogy and chemistry of clay minerals in sediments from various depths and positions in abert lake and surrounding playa with those of the weathered materials entering the lake in order to reveal the nature and extent of post depositional mineralogical modification analytical electron microscope aem data from individual clay particles reveal that each sample is comprised of a highly inhomogeneous smectite assemblage the thin clay flakes commonly less than 10 nm wide display a complete range in octahedral sheet compositions from nearly dioctahedral to nearly trioctahedral the very abundant mg rich lake smectites with an estimated composition k 0 29 al 0 23 mg 2 16 fe 0 30 si 3 80 al 0 20 o 10 oh 2 are not formed by weathering this confirms the importance of diagenetic mg uptake lattice fringe imaging failed to reveal distinct brucite like or vermiculite like layers suggesting that interstratifications of this type are rare or absent siliceous coatings on clay particles identified by silica excess in smectite analyses seem to favor topotactic overgrowth of stevensite rather than addition of brucite like layers to the dioctahedral nuclei the growth of k stevensite dilutes the al content of the crystal and thus the increasing diagenetic modification reduces rather than supplements its illite component smectite compositions within individual samples were highly variable yet source related characteristics such as the abundance of fe rich smectite were apparent little evidence for systematic k or mg enrichment with depth was identified in samples from depths of down to 16 feet below the sediment water interface the most magnesian assemblages are associated both with weathering sources of mg rich smectite and playa environments subjected to repeated wetting and drying cycles thus the observations suggest that clay compositions primarily reflect changes in lake levels brine composition and source characteristics rather than time and depth compaction effects other diagenetic reactions in the sediment include recrystallization of na rich silica gel and diatom fragments abundant submicron sized untwinned euhedral crystals of k feldspar are interpreted to be authigenic in origin

adakites without slab melting high pressure differentiation of island arc magma mindanao the philippines abstract new geochemical data for pleistocene magmatic rocks from the surigao peninsula eastern mindanao the philippines demonstrate typical adakitic traits including elevation of sr y and depletion of the heavy rare earth elements 87sr 86sr and 143nd 144nd ratios of the adakites do not support melting of the subducted philippine sea plate but resemble pliocene arc lavas generated in the same subduction zone excepting the heavy rare earth elements the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge the wide range of sio2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature 1 adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet bearing assemblage 2 solidified basaltic rock containing garnet melted to yield adakitic magma in either case the basaltic precursor was generated from fluid modified mantle then differentiated within the garnet stability field in surigao this requires differentiation within mantle the surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth this has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks

tomographic imaging of the underthrusting indian slab and mantle upwelling beneath central tibet abstract to better understand the pattern of convergence between the indian and eurasian plates we determine a high resolution p wave tomography of the crust and upper mantle under southern to central tibet using a large number of high quality data of local earthquakes and teleseismic events collected by the antilope ii and hi climb projects a significant low velocity zone is detected above the northward underthrusting indian slab beneath the indus tsangpo suture which may reflect fault zones or incomplete fragmentation as well as melts and or fluids associated with the dehydration of the underthrusting indian slab the variations in the diving depth and extending distance of the indian slab under the antilope ii and hi climb seismic profiles may be caused by the differences in the viscosity contrast between the indian slab and the surrounding mantle as well as strong structural heterogeneities in the upper mantle under the study region both high and low velocity anomalies are revealed in the mid to lower crust under southern tibet which may reflect the complex pattern of crustal structure the rifts in southern tibet exhibit different velocity images suggesting that different structures exist beneath those rifts

in situ u pb zircon dating using laser ablation multi ion counting icp ms high resolution in situ u pb zircon geochronology on zoned grains can obtain isotope signatures from multiple growth or thermal events we present a method using laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms to overcome complications associated with intricately zoned zircon crystals through in situ sampling of zircon volumes as small as 12 m 25 m and 40 m in diameter by about 10 m in depth high precision u pb age of a series of zircon standard covering a wide age range of 30 to 1 065 ma was acquired using la mc icp ms the precision of measured pb u ratios in homogeneous zircon is about 2 2 resulting in routinely achieved precision of u pb ages obtained by external calibration of 1 2 or better all masses of interest can be simultaneously recorded with a multi ion counting system mic operating in static mode and the short ablation required to achieve such precision results in spatial resolution that is superior to comparable u pb zircon analyses by single collector icp ms the resulting present u pb age for five zircon reference samples and two geological samples show an excellent agreement with the previously reported id tims or shrimp data

geochemical characteristics and petrogenesis of miocene high sr y rocks in xigatze fore arc basin southern tibet abstract the xigatze fore arc basin one of the well preserved fore arc basins worldwide consists of interesting nearly e w trending dike swarms of basaltic to granitic compositions mid miocene more felsic dikes to the responses of fore arc lithosphere provide important records to the post collisional processes miocene 15 11 ma zircon u pb ages high sr y magmatic rocks from the xigatze area in this study are characterized by 1 rich in na as compared with k contents na2o k2o 1 75 5 62 and high al2o3 content 16 wt 2 high sr but low y contents thus high sr y 81 0 ratios resembling those of adakites 3 enrichment in lree and depletion in hree with weakly negative or no eu anomalies eu eu 0 80 1 02 4 small range in 87sr 86sr t 0 705080 0 706031 and end t 3 1 0 4 values indicating that the xigatze miocene magmatic rocks are derived from partial melting of juvenile lower crust with minor contributions from the ancient crust in the lhasa terrane 5 thickened crust with a crustal thickness of 52 69 km estimated from la yb n values as compared with those in the gangdese high sr y suite the xigatze ones have lower sio2 and k2o contents but higher na2o k2o sr y and k rb ratios the higher sr y and k rb ratios implying a weaker effect by amphibole either as a residual in the source or as a fractionation phase during the magma ascent than that in the gangdese suite

petrology of the chilliwack batholith north cascades washington generation of calc alkaline granitoids by melting of mafic lower crust with variable water fugacity calc alkaline granitoid rocks of the oligocene pliocene chilliwack batholith north cascades range from quartz diorites to granites 57 78 sio2 and are coeval with small gabbroic stocks modeling of major element trace element and isotopic data for granitoid and mafic rocks suggests that 1 the granitoids were derived from amphibolitic lower crust having ree rare earth element and sr nd isotopic characteristics of the exposed gabbros 2 lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting the main effect of fh 2 o variation is to change the relative proportions of plagioclase and amphibole in the residuum the ree data for intermediate granitoids quartz diorite granodiorite eu eu 0 84 0 50 are modeled by melting with fh 2 o 1 kbar leaving a plagioclase pyroxene residuum in contrast data for leucocratic granitoids leuco granodiorites and granites eu eu 1 0 0 54 require residual amphibole in the source and are modeled by melting with fh 2 o 2 3 kbar consistent with this model isotopic data for the granitoids show no systematic variation with rock type 87sr 86sri 0 7033 0 7043 end 0 3 3 to 5 5 and overlap significantly with data for the gabbroic rocks 87sr 86sri 0 7034 0 7040 end 0 3 3 to 6 9 the fh 2 o variations during melting may reflect additions of h2o to the lower crust from crystallizing basaltic magmas having a range of h2o contents chillwack gabbros document the existence of such basalts one dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large scale melting of amphibolitic lower crust provided that ambient wallrock temperatures exceed 800 c based on lithologic and geochemical similarities this model may be applicable to other cordilleran batholiths

hydrogeochemistry in the ngorongoro crater tanzania and implications for land use in a world heritage site the chemistry of surface and ground waters in the ngorongoro crater tanzania home to thousands of large mammals and a world heritage site is controlled by the volcanic host rock lithology evaporative concentration mineral precipitation and redissoluton and biological factors three groups of waters are informally differentiated based on their ranges of concentration 1 dilute inflow meteoric runoff and springs from short flowpaths ph 9 cl 100 mg l evaporative concentration and biological activity in swamps commonly produce strong geochemical gradients between dilute sources and peripheral concentrated ephemeral wetlands dilute inflow is found in the lerai munge and oljoro nyuki streams and several large springs near the crater wall such as ngoitokitok and seneto concentrated inflow is found in downstream reaches of the munge stream discharging from springs away from the crater wall such as engitati and mti moja and in dryseason pools brackish waters are found discharging from springs on the southern margin of lake makat in mudflats surrounding marshes in ephemeral pools and in the lake itself although few hydrologic data are available the persistence of relatively fresh water in vegetated wetlands is consistent with lower sedge dominated wetland evapotranspiration rates compared with open water this suggests that wetlands may play an important role in ensuring fresh water availability in the basin and it demonstrates the need for future hydrologic study most of the ngorongoro waters originate as rainfall outside the crater and travel into the basin as surface or ground water flow emphasizing the need for a watershed scale approach to land management