boron geochemistry from some typical tibetan hydrothermal systems origin and isotopic fractionation the tibetan plateau is characterized by intense hydrothermal activity and abnormal enrichment of trace elements in geothermal waters hydrochemistry and b isotope samples from geothermal waters in tibet were systematically measured to describe the fractionation mechanisms and provide constraints on potential b reservoirs b concentrations range from 0 35 to 171 90 mg l and isotopic values vary between 16 57 and 0 52 geothermal fields along the indus yarlung zangbo suture zone and n s rifts are observed with high b concentrations and temperatures the similar hydrochemical compositions of high b geothermal waters with magmatic fluid and consistent modeling of b isotopic compositions with present 11b values imply that the b in high b geothermal waters is mainly contributed by magmatic sources probably through magma degassing in contrast geothermal fields in other regions of the lhasa block have relatively low b concentrations and temperatures after considering the small fractionation factor and representative indicators of na ca cl hco3 na k and si the conformity between modeling results and the isotopic compositions of host rocks suggests that the b in low temperature geothermal fields is mainly sourced from host rocks according to simulated results the b in some shallow geothermal waters not only originated from mixing of cold groundwater with deep thermal waters but it was also contributed by equilibration with marine sedimentary rocks with an estimated proportion of 10 it was anticipated that this study would provide useful insight into the sources and fractionation of b as well as further understanding of the relationships between b rich salt lakes and geothermal activities in the tibetan plateau

oligocene miocene burial and exhumation of the southernmost gangdese mountains from sedimentary and thermochronological evidence abstract the kailas conglomerates crop out ubiquitously along the southernmost boundary of the gangdese batholith they unconformably overlie the gangdese batholith and are displaced by the great counter thrust gct fault forming a fault contact with the xigaze forearc basin the associated subduction complex and the tethyan himalayan sequence these strata furnish a record of uplift and paleoenvironmental change in the indus yarlung suture zone during the oligocene miocene our new and previously published low temperature thermochronometric data from the gangdese batholith and the kailas conglomerates indicate a period of rapid exhumation beginning approximately 17 15 ma centered on the southern margin of the gangdese batholith whereas regional uplift commenced significantly earlier during the deposition of the kailas conglomerates based on the presence of an abrupt facies transition from deep water lacustrine deposits to red alluvial fan or fluvial deposits the period of rapid exhumation probably lagged behind the initiation of faster uplift while the related changes in the depositional environment were most likely recorded immediately in the basin stratigraphy subsequently the kailas conglomerates were buried in association with the development of the north directed great counter thrust while rapid exhumation was facilitated by efficient incision by the paleo yarlung river at approximately 17 15 ma

s wave residuals from earthquakes in the tibetan region and lateral variations in the upper mantle abstract mean s wave residuals from 46 earthquakes within and on the margins of the tibetan plateau exhibit systematic lateral variations that do not correlate well with elevation or with simple aspects of the geologic history the earliest s waves come from earthquakes in western tibet the karakorum and the western himalaya and the latest come from earthquakes in north central tibet although s waves from earthquakes in the himalaya tend to be early the east west variation in residuals across tibet is at least as large as the north south difference between the himalaya and northern tibet if the variations in residuals are a reflection of temperature variations in the upper mantle associated with convection then upwelling beneath north central tibet seems to be flanked by downwelling in western eastern and probably southern tibet this convective flow might reflect the detachment and removal of thickened mantle lithosphere beneath tibet

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

tectonic implications of felsic tuffs within the lower miocene gangrinboche conglomerates southern tibet felsic tuffs are interbedded with the gangrinboche conglomerates adjacent to the yarlung tsangpo suture zone in southern tibet laser ar ar dating of mineral separates indicates they are of early miocene age such tuffs are most likely an eruptive manifestation of geochemically indistinguishable coeval felsic adakitic intrusions that are widely reported across southern tibet the considerable lateral e w extent of the gangrinboche conglomerates and their depositional setting indicates sediment accumulation in an overall n s compressional regime and thereby places important constraints on the tectonic setting in which magmatism initiated the conglomerates were folded and truncated during back thrusting associated with development of the north directed great counter thrust as n s trending rifts associated with e w extension of the tibetan plateau cut both the conglomerates and this thrust system it can be inferred that post collisional volcanism is unlikely to have been genetically linked to later e w extension early miocene slab break off beneath tibet provides a model that appears to be consistent with petrogenesis of the associated magmatic suite which requires a lowermost crust or lithospheric mantle generation molasse accumulation and uplift and emplacement of north himalayan gneiss domes

miocene volcanism in the lhasa block tibet spatial trends and geodynamic implications miocene ultrapotassic up shoshonitic p and calc alkaline ca volcanism is recognized widely over the tibetan plateau this volcanism occurred after the end of india s oceanic lithosphere subduction beneath the plateau ambiguities about the distribution and significance of this volcanism arise in part from a paucity of geochronologic and geochemical data we have sampled two volcanic fields in the lhasa block 1 the eastern edge of the zabuye salt lake central part of the block 2 the southwestern edge of the yangbajin graben major and trace elements display high k2o 6 06 6 54 over a wide range of sio2 75 03 55 30 na2o 1 32 3 30 and lree hree enrichment with a significant rare earth element fractionation 266 la yb n 6 42 as well as negative nb ta and ti anomalies these features indicate that our samples are chemically identical to the post collision miocene up to p volcanic rocks already identified all over the tibetan plateau and the southern plateau specifically but distinct from the ca rocks found in the lhasa block the miocene volcanics found in the lhasa block present greater chemical variation than the rest of the tibetan plateau 40 ar 39 ar ages on both sanidine and biotite gave ages ranging from 16 16 g 0 12 ma to 16 01 g 0 12 ma and 10 60 g 0 14 ma to 10 88 g 0 17 ma 2 c excluding systematic errors relative to fish canyon sanidine at 28 02 ma for zabuye salt lake and yangying geothermal field volcanics respectively these results are consistent with the range of previously published ages from this part of the tibetan plateau the presence of n s trending dikes and the location of the volcanic fields suggest that local e w extension in the lhasa block was coeval we propose that the synchronous up p and ca volcanism in the lhasa block between 23 and 8 ma is the result of progressively deepening northward subduction of indian continental lithosphere producing asthenospheric upwelling convective thinning delamination of lithospheric mantle fueled by the hot asthenosphere induced partial melting of an enriched sub continental mantle lithosphere as well as eclogitic lower crust this mechanism could also explain the local e w extension observed in the same period 18 10 ma we also propose that lateral migration of the deepening indian continental lithosphere mantle slab could explain the observed younger volcanism in the eastern part of the lhasa terrane

two types of neoarchean basalts from qingyuan greenstone belt north china craton petrogenesis and tectonic implications abstract geochronological geochemical and isotopic studies were carried out on the basalts from the qingyuan greenstone belt to understand the petrogenesis of the basalts and the mantle source characteristics providing insights into neoarchean geodynamic evolution of the north china craton the qingyuan greenstone belt is analogous to the typical barberton type greenstone belt in lithological units u pb isotopic dating using the la icp ms method on zircons from the basalts and the associated andesite reveals that they erupted at 2510 2545 ma and were subjected to subsequent 2486 2491 ma metamorphism the basalts are high fe tholeiitic showing coherently positive end t and ehf t values the geochemical and isotopic studies strongly imply that they experienced significant fractional crystallization and minor no crustal contamination during magma evolution on the basis of chondrite normalized ree patterns the basalts can be further divided into two types viz the enriched and the depleted the enriched basalts are abundant in lree and rees contents depleted in hfses nb zr ti endowed with arc like geochemical peculiarities modeling reveals that the parent magma of the enriched basalts has been derived from 70 to the depleted basalts the basalts from the neoarchean qingyuan greenstone belt witness the episode of the vertical growth of continental crust at ca 2 5 ga combined with other geological considerations a large archean mantle plume model is favored to account for the generation of the late neoarchean basalts from qingyuan greenstone belt in the eastern block north china craton

origin of paired high pressure ultrahigh temperature orogens a ridge subduction and slab window model introductionmetamorphism at elevated pressure temperature conditions and distinct uid regimes generates ultrahigh pres sure uhp high pressure hp andultrahigh temperature uht rocks harley 2004 brown 2006 2007 2009 kelsey 2008 santosh andomori 2008a b santos et al 2009 santosh et al 2009a zhang et al 2009 although previous studies havecorrelated these di erent orogens todistinct tectonic settings recent inves tigations record high pressure anduht conditions from the same tec tonic environment within the sameorogenic belt and in some cases evenwithin the same samples mostoftheseexamples come from major collisionalzones along which continental blocksweresuturedatdi erenttimesinearthhistory including from the neoarch ean hp uht rocks of the limpopobeltinsouthafrica tsunogaeandvanreenen 2006 the palaeoproterozoichp guo et al 2004 obrien et al 2005 and uht santosh et al 2007a b 2008 2009b belts in thenorth china craton and the hp uht belts in the neoproterozoiceastern ghats belt as well as thecambrian gondwana suture in south ern india shimpo et al 2006 tsuno gaeet al 2008a santoshet al 2009c notably the thermal regimes for thesepaired hp and uht rocks are similar 1000 c and they also share acommon uid history characterizedby co

tibetan tectonic evolution inferred from spatial and temporal variations in post collisional magmatism cenozoic magmatism on the tibetan plateau shows systematic variations in space and time that must be considered in models concerning tibetan tectonic evolution after the india asia collision which started in the early tertiary and terminated the gangdese arc magmatism in the lhasa terrane made of the southern tibetan plateau widespread potassium rich lavas and subordinate sodium rich basalts were generated from f50 to 30 ma in the qiangtang terrane of northern tibet subsequent post collisional magmatism migrated southwards producing ultrapotassic and adakitic lavas coevally between f26 and 10 ma in the lhasa terrane then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f13 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the indian continental lithospheric mantle started thrusting under asia by involving rollback and breakoff of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root we propose that only after the lithospheric removal which occurred at f26 ma could the indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the himalayan tibetan orogenesis consequently 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 owing to the push of the underthrust indian mantle lithosphere d 2004 elsevier b v all rights reserved

miocene adakitic intrusions in the zhongba terrane implications for the origin and geochemical variations of post collisional adakitic rocks in southern tibet abstract post collisional adakitic rocks 26 9 ma have been recognized in southern tibet for almost 20 years however their origins and geodynamics remain highly controversial this paper reports geochronological and geochemical data for the lasa intrusion exposed in the zhongba terrane and proposes implications for the origins of the miocene adakitic rocks in southern tibet the lasa intrusion consists of granite and granodiorite porphyries with zircon u pb ages of ca 16 ma the porphyries have high sio2 65 14 67 61 wt al2o3 15 13 16 10 wt k2o 2 87 3 54 wt and sr 727 1046 ppm contents and low y 9 13 ppm and yb 0 82 1 19 ppm contents they are enriched in light rare earth elements lrees and depleted in heavy rare earth elements hrees the porphyries display relatively low mgo contents 1 39 1 98 wt high sr y 65 98 and la yb n 18 8 25 5 ratios initial 87sr 86sr i ratios of 0 70912 0 70805 end t values of 6 4 to 7 9 206pb 204pb t ratios of 18 7108 18 7665 207pb 204pb t ratios of 15 6837 15 7118 and 208pb 204pb t ratios of 39 1600 39 3034 these signatures indicate that the lasa porphyries are adakitic rocks derived from the partial melting of a thickened lower crust the miocene adakitic rocks in the zhongba terrane have similar sr nd isotopic compositions to those of western lhasa and the himalaya terrane which differ from coetaneous adakitic rocks of the eastern lhasa terrane in terms of sr nd isotopic compositions based on the spatial distributions and isotopic features of the post collisional adakitic rocks in southern tibet we suggest that all of the adakitic rocks were derived from the partial melting of thickened lower crust and that their geochemical variations are mainly caused by the heterogeneity of lower crust

timing of tibetan uplift constrained by analysis of volcanic rocks the tibetan plateau has had a central role in the development of recent models for the mechanics of mountain belts1 2 and cenozoic global climate change3 the present elevation and extensional deformation of the plateau probably result from uplift owing to convective thinning of the underlying lithospheric mantle1 2 an age for the uplift would provide a valuable constraint on these models but because recently proposed indicators of uplift are all climate dependent they are equivocal possibly reflecting global cooling rather than regional uplift4 here we present new geochemical data on post mid miocene lavas from the plateau which show that the lavas were derived from the lithospheric mantle simple thermal arguments indicate that the generation of such magmas also necessitates thinning of the lithospheric mantle thus volcanism is coincident with uplift providing a climate independent means of dating the onset of uplift dating by the laser 40ar 39ar technique places the beginning of this volcanism and therefore the time of uplift of the tibetan plateau at 13 myr ago

models and mechanisms of drilling induced sinkhole in china the present paper distinguishes the four general types of collapsed sinkholes induced by drilling in regions where non indurated sediments clay silt and sand overlie unknown caves or open voids in underlying lithified karst rocks these sinkhole models are classified into balance arch hourglass vibration and subjacent drilling induced erosion forms a mechanical model was built for each type to assess the collapse probability drilling operations using boreholes facilitate rapid turbulent and erosive flow these activities resulted in the 26 sinkholes that have been attributed to constructions in china in the last several years awareness of potential collapse conditions should allow construction projects to avoid high risk settings that result in economic losses environmental concerns and life threatening accidents caused by such rapid collapse of the land surface awareness and close monitoring of high risk conditions during borehole monitoring should minimize the hazard

slab breakoff a critical appraisal of a geological theory as applied in space and time abstract the idea that prominent geological phenomena observed in the crust and at the surface may be caused by the detachment of subducting dense lithosphere has been anticipated by tectonic models in the sixties and seventies and imaged with improved geophysical techniques in the eighties in the mid nineties the model of slab breakoff was defined formally by davies and von blanckenburg initially proposed as a thought provoking working hypothesis the theory rapidly received wide a priori acceptance and was applied to virtually every mountain range around the world and even in orogens as old as paleoproterozoic and archean to explain a range of different phenomena these include magmatic flare ups rapid topographic uplift increasing sediment supply fast exhumation of metamorphic rocks ore mineralizations anomalous distribution of seismicity changes in stress regimes and inversion of subduction polarity even multiple breakoff events were assumed to have occurred at different times in the same orogenic belt or subduction zone in the last 20 years slab breakoff has been invoked in so many settings and time frames that it could have hardly taken place in each and every case in which it was called upon this article does not critically examine the theory but critically examines its use the extensive review of the vast literature presented here on the subject reveals how the model has been often applied to provide ad hoc explanations for a range of poorly understood observations based on incomplete evidence of deep seated processes our aim is to illustrate a paradigmatic example of how earth scientists in the face of evidence that challenges our capacity of understanding often recur to hypotheses based on other hypotheses such an approach may induce researchers to look for confirmation in the absence of compelling constraints or even in the face of conflicting evidence the faith in models should not lead us to confuse speculative theories with axiomatic truths and to build upon them theoretical edifices that are vulnerable and exposed to the risk of circular reasoning

contribution of mantle components within juvenile lower crust to collisional zone porphyry cu systems in tibet most porphyry cu mo au deposits are found in magmatic arcs worldwide and are associated with hydrous high fo2 calc alkaline magmas derived from a mantle wedge that was metasomatized by the fluids from a subducted oceanic slab recently such deposits have been documented as occurring widely in collisional settings where they are associated with potassic magmas generated during the collisional process but the genesis of the fertile magmas and the mechanism of metallic enrichment remain controversial here we present new geochemical and sr nd hf isotopic data from the post collisional fertile and barren porphyries of the miocene gangdese porphyry belt in the tibetan orogen an orogen formed by the collision of india and asia in the early cenozoic both types of porphyry are characterized by high k2o contents and have geochemical affinities with adakite but the fertile magmas were most likely derived from the melting of a thickened juvenile mafic lower crust formed by the underplating of earlier asthenospheric melts at the base of crust whereas the derivation of the barren magmas involved variable amounts of old lower crust in tibet the melting of sulfide bearing phases in the juvenile mantle components of the tibetan lower crust probably provided cu au and s to the fertile magmas the breakdown of amphibole during melting at the source released the fluids necessary for the formation of the porphyry cu deposits in tibet the thickened crust up to 70 80 km due to collision is thought to be responsible for a decrease in the fo2 of the fertile magmas during their ascent to the upper crust thus preventing the generation of more porphyry cu au and epithermal au deposits in this collisional zone

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

a three dimensional numerical investigation of a carolina coastal front and the gulf stream rainband abstract a three dimensional mesoscale planetary boundary layer pbl numerical model is used to investigate mesoscale circulations over the carolina coastal and gulf stream baroclinic zones idealized ambient onshore and offshore flows are investigated which represent the synoptic conditions during the intensive observation period 2 iop 2 of the 1986 genesis of atlantic lows experiment gale for the easterly onshore flow a confluence zone appears west of the gulf stream in response to the effect of the oceanic baroclinicity the confluence zone is nearly parallel to the coastline and the sst isotherms with northeasterly southwesterly flow to the west east a shallow coastal front forms below 2 km as the cyclonic shear of the ageostrophic flow becomes strong quasi stationary rainbands are produced by cumulus convection along the coastal front the northern part of the front and the rainbands later encroach inland as the cold air intensity over ground weakens due to onshore warm air advection

neoarchean granitoid gneisses in eastern hebei north china craton revisited abstract neoarchean granitoid gneisses are widely distributed throughout eastern hebei eastern north china craton which are dominated by deformed and metamorphosed tonalite trondhjemite granodiorite ttg diorite and granite this study presents the results of systematic zircon u pb geochronological whole rock geochemical and sm nd isotopic analyses of the neoarchean granitoid gneisses in eastern hebei these data provide insights into the archean paleoproterozoic multiple tectonothermal events as well as the petrogenesis of these gneisses zircon u pb isotopic dating indicate that the granitoid gneisses were contemporaneously emplaced between 2546 10 and 2510 10 ma reflecting a giant neoarchean igneous event throughout eastern hebei subsequently these rocks were subjected to regional amphibolite facies metamorphism at 2 48 2 45 ga the close spatial and temporal relationships in terms of magmatism and metamorphism at ca 2 5 ga suggest a uniform tectonothermal evolution of eastern hebei the granitic gneisses have mainly originated from the partial melting of juvenile metamorphosed greywackes with minor involvement of basalts the large geochemical and isotopic variations within the dioritic and ttg gneisses both provide evidence for the mixing of mafic and felsic magmas coupled with fractional crystallization however the chemical differences between the dioritic and ttg gneisses might result from individual mafic magma sources viz basaltic and high mg melts the mafic magma may also form the basalt or komatiite within the greenstone belt or evolve via fractional crystallization prior to the magma mixing large scale granitoid activities were possibly related to mafic magma underplating the combined geochronological geochemical and geological data support an archean proto mantle plume model for a geodynamic interpretation of the eastern north china craton during the neoarchean

metallogenesis and the minerogenetic series in the gangdese polymetallic copper belt abstract the gangdese is a newly explored porphyry cu ore belt in china in fact except for the porphyry cu deposits there are a lot of other deposits forming a polymetallic copper belt in the southern gangdese sg i e southern lhasa terrane and gangdese back arc fault uplift belt gbafub also called lungar nyainqentanglha arc the gangdese polymetallic copper belt contains porphyry cu mo au cu au and mo cu skarn cu mo au w pb zn ag fe cu and mo w breccia type and hydrothermal vein type pb zn ag deposits the porphyry cu au mineralization occurred at the xiongcun area of sg during the middle jurassic 175 160 ma and was associated with island arc magmatism related to the northward subduction of the neo tethyan oceanic slab the cretaceous ca 112 ma skarn fe cu mineralization appeared at the nixiong area of gbafub and was generated in a back arc extensional setting related to the neo tethyan oceanic subduction beneath the lhasa terrane the paleocene eocene 65 51 ma porphyry mo cu skarn mo w and skarn pb zn ag deposits are located in the eastern part of gbafub and related to granitoids originating from the partial melting of the old crust with contributions from the mantle in contrast the paleocene eocene 60 45 ma porphyry cu skarn breccia and hydrothermal vein type pb zn ag and skarn fe cu deposits at the namling area were associated with granitoids derived from the partial melting of the mantle with some input of the old crust the oligocene 30 23 ma porphyry skarn cu mo w au mineralization occurred at the zedong area adjacent to north of the indus yarlung zangbo suture zone in a post collisional setting the intensified miocene 21 13 ma porphyry skarn cu mo au and skarn hydrothermal vein type pb zn ag deposits formed in the sg whereas the porphyry mo cu and skarn hydrothermal vein type pb zn ag deposits formed in the gbafub the porphyry mo cu mineralization have more substantial contributions from the old crust in addition to the mantle than those with porphyry cu mo au mineralization the source region of the high sr y intrusions associated with post collisional porphyry cu mo au mineralization is probably the arc magmas that were generated at the subduction stage and have remained stalled at the boundary between the crust and mantle since the neo tethyan ocean closed the early subduction of the neo tethyan oceanic lithosphere played a crucial role in the generation of ore deposits formed in collisional and post collisional settings six minerogenic series or groups can be recognized in the gangdese polymetallic copper belt 1 jurassic porphyry cu au minerogenic series related to island arc magmatism 2 cretaceous skarn fe cu minerogenic series in back arc extensional setting 3 paleocene eocene granitoids related polymetallic minerogenic series in collisional setting 4 oligocene porphyry skarn cu w mo au minerogenic series in post collisional setting 5 miocene cu mo au pb zn ag minerogenic series related to granitoids with substantial contributions from the mantle in post collisional setting and 6 miocene mo cu pb zn minerogenic series related to granitoids with significant contributions from the old crust in post collisional setting during mineralization some deposits underwent superimposed mineralization such as the jiru porphyry cu deposit where the eocene and miocene porphyry type cu mineralization occurred the minerogenic series can be applied to ore prospecting for example when either pb zn ag veins or porphyry skarn mo w cu orebodies are known in an area one of them could provide a useful exploration vector for the other

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

late cretaceous neo tethyan slab roll back evidence from zircon u pb o and whole rock geochemical and sr nd fe isotopic data of adakitic plutons in the himalaya tibetan plateau

blocking and frontogenesis by two dimensional terrain in baroclinic flow part i numerical experiments the shallow atmospheric fronts that develop in the early winter along the east coast of north america have been attributed in various modeling and observational studies to the land sea contrasts in both surface heating and friction however typical synoptic conditions are such that these coastal fronts could also be a type of upstream influence by the appalachian mountain chain generalized models have suggested that relatively cold air can become trapped on the windward side of a mountain range during episodes of warm advection without a local contribution from differential surface fluxes such a process was proposed decades ago in a study of observations along the coast of norway could coastal frontogenesis be primarily a consequence of a mountain circulation acting on the large scale temperature gradient a two dimensional terrain following numerical model is used to find conditions under which orography may be sufficient to cause blocking and upstream frontogenesis in a baroclinic environment the idealized basic flow is taken to have constant vertical shear parallel to a topographic ridge and a constant perpendicular wind that advects warm or cold temperatures toward the ridge land sea contrasts are omitted in the observed cases the mountain is narrow in the sense that the rossby number is large this by itself increases the barrier effect but the experiments show that large scale warm advection is still crucial for blocking for realistic choices of ambient static stability and baroclinicity the flow can be blocked by a range like the northern appalachians if the undisturbed incident wind speed is around 10 m s21 cold advection weakens the barrier effect the long term behavior of the front in strongly blocked cases is described and compared to observations because of the background rotation and large scale temperature advection blocked solutions cannot become steady in the assumed environment however the interface between blocked and unblocked fluid can settle into a balanced configuration in some cases a simple argument suggests that in the absence of dissipation the frontal slope should be similar to that of the ambient absolute momentum surfaces

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

paleoproterozoic magmatic and metamorphic events in the hongcheon area southern margin of the northern gyeonggi massif in the korean peninsula and their links to the paleoproterozoic orogeny in the north china craton abstract the hongcheon area is located at the northern part of gyeonggi massif ngm in the korean peninsula the metamorphic rocks of the ngm at the hongcheon area are composed of the paleoproterozoic yongduri gneiss complex ygc euiam group eg and the euiam gneiss complex egc the quartz feldspathic gneisses in the northeastern part of the ygc record peak metamorphic temperature pressure condition of 790 840 c and 7 2 8 9 kbar whereas the granitic and garnet gneisses in the western part of the ygc record peak metamorphic temperature pressure conditions of 690 720 c and 6 1 6 9 kbar and 640 660 c and 5 0 5 4 kbar respectively the peak metamorphic conditions represent a regional low p t metamorphic event m2 in which metamorphic temperature and pressures increased towards the east shrimp zircon u pb age dating indicates that the m2 metamorphic stage occurred at ca 1886 1870 ma the presence of relict kyanite in the gneisses within the ygc suggests that the intermediate p t metamorphic stage m1 occurred in ca 1925 ma whole rock geochemistry of the metagranitoid in the egc indicates that they are originally post collision granitoid shrimp zircon u pb age dating indicates that the metagranitoid formed at ca 1885 1869 ma considering the similarity in age between the paleoproterozoic intrusion and the m2 low p t metamorphism the m2 metamorphism also occurred in a post collision tectonic setting and the m1 intermediate p t metamorphism with kyanite represents collision event further we suggest that the paleoproterozoic orogeny in the hongcheon area is closely linked to the paleoproterozoic orogeny in north korea and the jiao liao ji belt in the north china craton

mesozoic and cenozoic volcanic rocks from central and southern tibet 39ar 40ar dating petrological characteristics and geodynamical significance field relations radiometric ages and mineralogical and chemical characteristics of the orogenic volcanic rocks from the lhasa block tibet are studied volcanic activity started before the deposition of the albian aptian sedimentary units takena formation and xigaze group volcanism is volumetrically limited in the northern part of the lhasa block basalts andesites and dacites are present while ignimbrites are scarce radiometric ages of these rocks range between 110 and 80 ma conversely in the southern part of the lhasa block volcanism is largely developed gangdese belt there paroxysmal activity occurred later than in the north between 60 and 50 ma lingzizong formation this volcanism is mainly represented by andesites and ignimbrites a younger episode at about 15 10 ma took place in this southern region the mineralogical and geochemical characteristics indicate that as a whole the volcanic rocks from the lhasa block belong to a calc alkaline continental margin series enriched in k20 and other incompatible elements subordinate potassic alkaline lavas are present in the northern margin of the gangdese belt some basalts exhibit a tholeiitic affinity daquin area southern region the geodynamic significance of these cenozoic volcanic rocks lies in the relative motion of india and the lhasa block after accretion of the latter to the northern qantang block a geodynamic model based on changes of the type of subduction of the indian oceanic crust beneath the lhasa block for the period 120 40 ma is proposed to account for the abundance age and magmatic affinity of the volcanic products

the fault controlled skarn w mo polymetallic mineralization during the main india eurasia collision example from hahaigang deposit of gangdese metallogenic belt of tibet abstract the hahaigang w mo polymetallic skarn deposit is located in the central eastern part of gangdese tectono magmatic belt in lhasa terrane tibet the deposit was discovered in 2007 with currently proven 46 million tons of wo 3 ores 12 million tons of mo ores and 1 31 million tons of combined cu pb zn ores at an average grade of 0 20 wo 3 0 07 mo 0 026 cu 0 49 pb and 3 1 zn ore bodies occur in veins or disseminations and are confined within the ne striking dalong fault zone which is hosted by the lower permian pangna group of dominantly quartz sandstone and slate several granitic plutons are exposed in the area or known from drill holes ages of these granitic plutons are determined by using zircon u pb la icp ms method for example the biotite monzogranite yields a 206 pb 238 u 207 pb 238 u concordia age of 58 66 0 90 ma and a weighted mean 206 pb 238 u age of 57 02 0 42 ma the granite porphyry yields a 206 pb 238 u 207 pb 238 u concordia age of 109 1 8 9 ma and a weighted mean 206 pb 238 u age of 114 0 2 6 ma the biotite monzogranite yields a weighted mean 206 pb 238 u age of 56 1 1 1 ma re os isochron age of 63 2 3 2 ma from 5 molybdenite samples collected from the w mo skarn ores is also obtained in this study the zircon u pb and molybdenite re os geochronological data suggest that the w mo mineralization was not temporally associated with any of the dated igneous plutons however the molybdenite re os age of 63 2 3 2 ma indicates that the w mo mineralization might have occurred during the main india eurasia collision that was initiated around 65 ma microprobe analysis of ilvaite that occurs in two generations in the w mo skarn ores reveals a close relationship to ca fe f rich hydrothermal fluids which were probably derived from deeply seated magmas we suggest that ascent of the fluids was strictly controlled by the ore controlling dalong fault zone and that chemical interaction and metasomatism between the fluids and the lower permian pangna quartz feldspathic host rocks produced the ilvaite and the w mo polymetallic skarn deposit during the main india eurasia collision although the majority of the polymetallic deposits in the gangdese belt are reported to be either pre or post main collision it is evident from this study that the main collision also produced w mo polymetallic mineralization within the belt

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

hazards and treatment of karst tunneling in qinling daba mountainous area overview and lessons learnt from yichang wanzhou railway system from a global perspective karst geological disaster has great influence on the tunnel construction once it is not handled properly the karst problems in the tunnel area will cause great disaster such as water inrush mud gushing and collapse etc these karst disasters seriously affect the construction progress of the tunnel resulting in equipment damage casualties and so on the construction of yichang wanzhou yw railway tunnel has created a miracle in the field of chinese tunneling the geology of tunnel area is quite complex the karst disaster is remarkable and the engineering construction is extremely difficult in this paper the karst characteristics unfavorable geological disasters and treatment methods of yw railway tunnel were analyzed systematically the yw railway tunnel and zhengzhou wanzhou zw railway tunnel are both located in the qinling daba qb mountainous area and the regional geological conditions of them are very similar therefore the research in this paper can be extended to the qb mountainous area thus providing a reference for the construction of zw railway tunnel the investigation can also provide reference for the similar engineering

a slab breakoff model for the neogene thermal evolution of south karakorum and south tibet on the south karakorum margin neogene high temperature medium pressure ht mp gneisses define an east west trending thermal anomaly these rocks have been heated from 600 to 750 c during a slight pressure drop from 0 7 to 0 5 gpa their retrogressive path cross cuts the relaxed geotherm of tectonically thickened crust such a p t evolution occurs only if an advective source of heat is involved involvement of an advective heat source is also implied by the occurrence of neogene granitoids and lamprophyres within the ht mp gneiss area these rocks are strongly enriched in large ion lithophile elements relative to primitive mantle and show negative high field strength element anomalies we interpret these geochemical characteristics to be the result of melting of metasomatized asian lithospheric mantle the nd and sr isotopic compositions of the south karakorum neogene magmatic rocks nd 12 to 7 and 87sr 86sr 0 705 0 725 further suggest they could have originated from mixing between asian variously metasomatized mantle and precambrian crust by contrast the origin of the youngest magmatic rocks 10 myr here exemplified by the hemasil syenite and associated lamprophyres requires involvement of a depleted mantle the combined hf nd signature of these rocks hf 10 4 11 5 and nd 3 4 4 3 suggests that the source of the hemasil syenite could have been depleted mantle contaminated by oceanic sediments likely during the earlier subduction of the tethyan ocean neogene magmatic rocks with the same geochemical characteristics and evolution as those of south karakorum have previously been described in south tibet based on their location and the geochemical evolution of their source region we here propose that the neogene magmatic and metamorphic evolution of the south asian margin was controlled by slab breakoff of the subducting indian continental margin starting at about 25 ma this model is supported by available geophysical data from south karakorum and south tibet

late cretaceous gangdese intrusions of adakitic geochemical characteristics se tibet petrogenesis and tectonic implications article i nfo the gangdese batholith emplaced from the cretaceous to eocene in southern tibet has been widely regarded as the major constituent of an andean type convergent margin resulting from northward subduction of the neo tethyan oceanic lithosphere under asia while the gangdese batholith consists predominantly of calc alkaline rocks we identify from the eastern part of the batholith a suite of epidote bearing granodiorites that shows adakitic geochemical characteristics marked with apparently higher la yb and sr y and lower y and hree than other gangdese rocks and common arc magmas shrimp zircon u pb analyses of two of the samples yielded 206 pb 238 u dates of 80 4 1 1 and 82 7 1 6 2 ma which constrain the emplacement ages of the adakitic rocks trace element modeling suggests that these rocks originated from partial melting of a garnet amphibolite source that on the basis of the sr and nd isotope data isr 0 7044 0 7048 end t 3 2 to 0 9 we interpret to be a newly underplated mafic lower crust rather than the subducted neo tethyan oceanic crust this juvenile crust was produced by cretaceous basaltic underplating above the mantle wedge and then thickened by the tectonic contraction owing to flattening of the neo tethyan subduction a process that also led to the adakitic magmatism our interpretation involving a late cretaceous flat slab subduction and related orogenesis in southern tibet is consistent with petrographic data such as the occurrence of magmatic epidote and muscovite rimmed with resorption texture in the granodiorites which indicate deep seated emplacement followed by rapid tectonic exhumation

geochemical variations in miocene adakitic rocks from the western and eastern lhasa terrane implications for lower crustal flow beneath the southern tibetan plateau abstract it is generally believed that miocene adakitic rocks in the lhasa terrane block of southern tibet were derived from a thickened lower crust and that their compositions will therefore reflect the geochemical characteristics of their lower crustal source s adakitic rocks in the western part of the lhasa terrane possess geochemical and sr nd isotopic characteristics that are clearly different from those in the eastern part of the lhasa terrane the former display a more enriched nd sr isotopic signature together with an older model age relative to the latter suggesting that the compositions of the lower crust under the western and eastern parts of the lhasa terrane also differ in addition the adakitic rocks in the himalayas on the southern side of the indus yalu suture iys have similar geochemical characteristics and sr nd isotopic compositions to the adakitic rocks of the western lhasa terrane but differ from rocks in the eastern lhasa terrane and from igneous rocks derived from crust beneath the himalayas we suggest that the lower and or middle crustal materials beneath the western lhasa terrane probably extend to the southern side of the iys due to channel flow if mid lower crustal flow has occurred below southern tibet the southeastward moving anatectic mid lower crustal material was probably derived from below the western part of the lhasa terrane rather than the eastern moreover the emplacement ages of adakitic rocks from mayum on the southern side of the iys imply that southeastward ductile flow of mid lower crust beneath western lhasa may have occurred as early as 17 ma

the liaohe group an insight into the paleoproterozoic tectonic evolution of the jiao liao ji belt north china craton abstract the jiao liao ji belt represents one of the largest exposed regions of paleoproterozoic rocks along the eastern margin of the north china craton paleoproterozoic basement material in this area is dominated by deformed and metamorphosed volcano sedimentary successions with minor amounts of granitic and mafic intrusions this study presents the results of a systematic zircon u pb geochronological whole rock geochemical and sm nd isotopic analysis of the volcano sedimentary successions within the liaohe group of the jiao liao ji belt these data provide insights into the multiple paleoproterozoic tectonothermal events that have affected the belt as well as the petrogenesis of the paleoproterozoic meta volcanics and metasedimentary rocks on the liaodong peninsula accurate u pb age and trace element determinations of zircons from the meta volcanics in this region using laser ablation inductively coupled plasma mass spectrometry indicate that these units formed at ca 2 2 ga and were metamorphosed at ca 1 9 ga respectively these data indicate that the paleoproterozoic geochronological framework of this region is dominated by 2 2 2 0 ga magmatic 1 98 1 90 ga sedimentary ca 1 90 ga metamorphic and 1 87 1 82 ga post tectonic magmatic events the meta volcanics can be split into basalt basaltic andesite andesite dacite and rhyolite units that define a continuous calc alkaline dominated magmatic sequence that is similar to the sequences commonly found within phanerozoic magmatic arcs the meta basalts are enriched in the light rare earth elements cr co and ni and are depleted in the heavy rare earth elements and the high field strength elements e g nb ta and ti indicating that they formed from magmas generated by the partial melting of lithospheric mantle material that was previously metasomatized by subduction zone derived fluids melts the large geochemical and isotopic variations within the metamorphosed andesite dacite and rhyolite units all provide evidence of magma mixing between felsic melts generated by the partial melting of paleoarchean neoarchean basement material and the mafic melts that may also formed the meta basalts within the liaohe group or evolve via fractional crystallization prior to mixing the geochemistry of the metasedimentary rocks indicate that the pre metamorphism sedimentary units ranged from being compositionally mature to being immature and poorly sorted and were weakly to moderately weathered these sediments were derived primarily from metamorphosed felsic volcanics within the liaohe group as well as the paleoproterozoic liaoji granites they also contain archean continental crustal material that is derived mainly from ancient intermediate mafic basement units the combined geochronological geochemical and geological data indicate that a paleoproterozoic arc continent collision model can well explain the nature and formation of the jiao liao ji belt

middle triassic ultrapotassic rhyolites from the tanggula pass southern qiangtang china a previously unrecognized stage of silicic magmatism abstract there is a mid late mesozoic jurassic cretaceous arc magmatic belt in southern qiangtang subterrane associated with the north dipping subduction of the bangong nujiang tethyan ocean early mesozoic triassic magmatism tectonic evolution of the southern qiangtang subterrane and the mechanism of its crustal growth however remain unclear this paper reports zircon u pb age whole rock major trace elemental and sr nd hf isotopic components for the mesozoic tanggula pass rhyolites in southern qiangtang subterrane our new data reveal a significant previously unrecognized stage of magmatism in southern qiangtang subterrane the tanggula pass rhyolites are moderately strongly peraluminous a cnk 1 09 1 54 highly fractionated sio2 73 and ultrapotassic k2o na2o 44 zircon u pb dating by la icp ms yields a concordant age with a weighted mean 206pb 238u age of 235 9 0 86 ma n 18 mswd 0 25 the large variations of major and trace element concentrations are mainly attributed to fractional crystallization of plagioclase hornblende fe ti oxides and accessory minerals such as apatite allanite monazite and zircon in the shallow level k feldspar mainly occurs as cumulated mineral resulting in high k2o contents and k2o na2o ratios the rhyolites were derived by partial melting of mixed source rocks including older sedimentary rocks in the upper crust and juvenile materials triggered by north dipping subduction of the bangong nujiang tethyan oceanic lithosphere the eruption age of middle triassic suggests that the initiation of northward subduction of the bangong nujiang tethyan ocean took place earlier than is currently assumed whole rock nd ca 1 55 to 2 34 ga and zircon hf ca 1 77 to 2 33 ga data reflect the presence of the proterozoic basement in the southern qiangtang subterrane basaltic magma underplating and accretion of intra oceanic arc complexes and oceanic plateau contributed to vertical crustal growth and lateral crustal growth respectively

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

numerical simulation of a mesolow over a gulf stream filament a three dimensional mesoscale numerical model is used to investigate mesoscale circulation over a gulf stream filament two numerical experiments are performed with different initial uniform ambient wind speeds u 0 1 m s 1 3 5 m s 1 and 7 m s 1 for a typical winter day it is found that for both low and moderate winds a closed mesoscale circulation forms over the gulf stream filament when the gulf stream filament was removed the model did not predict a mesoscale circulation the modeled circulation over the filament is in agreement with the observations suggesting that the atmospheric circulations over the filaments may be an important mechanism in the u s east coast cyclogenesis

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

increasing magmatic oxidation state from paleocene to miocene in the eastern gangdese belt tibet implication for collision related porphyry cu mo au mineralization in the gangdese belt of southern tibet paleocene eocene magmas record the final stage of neo tethyan subduction and are associated with few economic porphyry deposits in contrast magmas formed during later stages of the india asia collision in the oligo miocene are associated with several large porphyry cu mo au deposits especially in the eastern part of the belt 89 e 93 e in a previous study we showed that these oligocene miocene magmas were more hydrous than the earlier paleocene eocene magmas in this study we show that the later magmas were also more oxidized paleocene eocene rocks from the eastern gangdese belt are characterized by low zircon ce4 ce3 ratios mostly 50 32 3 141 9 average 74 3 30 1 n 33 and higher f o2 values fmq 0 8 2 9 average 1 8 0 8 n 6 estimated from magnetite ilmenite pairs estimates of magmatic oxidation state from amphibole compositions also show an increase from fmq 1 2 to 2 1 average 1 6 0 2 n 40 in the paleocene eocene to 2 0 to 2 8 average 2 5 0 2 n 58 in the miocene sparse whole rock fe3 fe2 ratios show the same general trend the amphibole results are systematically shifted to higher fmq compared to data from magnetite ilmenite pairs but their trend is internally consistent the higher oxidation states and water contents of miocene igneous rocks from the eastern gangdese belt may explain their unique association with large porphyry deposits in tibet because it has been shown from other studies that the potential for hydrous calc alkaline magmas to transport au is maximized near fmq 1 0 and cu at higher f o2 fmq 1 0 in comparison the somewhat less oxidized and less hydrous paleocene eocene magmas would have been less fertile for the formation of such deposits we suggest that these differences reflect the tectonomagmatic evolution of the cenozoic gangdese collisional orogen from early collision related magmatism in the paleocene eocene to late collisional magmatism in the miocene asthenospheric upwelling following slab breakoff in the miocene caused interaction between mantle derived magmas and previously subduction modified tibetan lithosphere the resultant evolved magmas had high oxidation states and water contents which are favorable properties for the subsequent formation of magmatic hydrothermal porphyry cu mo au deposits

experimental constraints on the origin of the 1991 pinatubo dacite crystallisation and mantle interaction experiments have been performed on the 1991 pinatubo dacite to constrain its petrogenesis in the dacite h2o system at 1000 mpa magnetite and either clinopyroxene or amphibole are the liquidus phases no garnet is observed at this pressure dacite mantle interaction at 920 mpa produces massive orthopyroxene crystallisation in addition to amphibole phologopite amphibole crystallising in dacite at 1000 mpa faithfully reproduces aluminium rich hornblende preserved in the cores of amphibole phenocrysts of the 1991 dacite suggesting a high pressure stage of dacite crystallisation under high melt h2o contents 10 wt and low temperatures 950c altogether plagioclase amphibole and melt inclusion compositions suggest that the pinatubo dacite was water rich oxidized and not much hotter than 900c when emplaced in the shallow reservoir in which most phenocrysts precipitated until the onset of the 1991 eruptions the strongly fractionated ree pattern demands garnet somewhere during dacite genesis which in turn requires high pressure derivation partial melting of subducted oceanic crust gives melts unlike the pinatubo dacite interaction of these slab melts with sub arc peridotite is unable to produce such type of dacite nor is a direct mantle origin conceivable on the basis of mantle dacite interaction experimental results dehydration melting of underplated basalts requires temperatures too high to match with petrological constraints and does not allow production of dacite melts having the low feo mgo high h2o ni and cr contents typical of pinatubo like magmas on the face of chemical and phase equilibrium constraints the most plausible origin of the pinatubo dacite is via high pressure fractionation in the upper mantle of an hydrous oxidised primitive basalt that crystallises amphibole and garnet upon cooling as shown by recent phase equilibrium work dacite melts so produced are directly expelled from the uppermost mantle or lower crust to shallow level storage reservoirs from which they erupt occasionally following injection of h2o poor mantle derived basalts that can ascent without massive crystallisation unlike their hydrous counterparts parental to dacite magmas that rest at mantle depths dacites such as the pinatubo one may thus witness particular h2o rich conditions of the sub arc mantle rather than the melting of a young and hot subducting oceanic plate

miocene adakites in south tibet partial melting of the thickened lhasa juvenile mafic lower crust with the involvement of ancient indian continental crust compositions

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

southern tibetan oligocene miocene adakites a record of indian slab tearing abstract oligocene miocene granitoids exposed within the 1500 km long gangdese arc of southern tibet exhibit adakitic compositions five plutonic samples from the southeastern lhasa terrane near namche barwa area were analyzed to determine their geochemical characteristics and to better understand the geodynamic evolution of the eastern himalayan syntaxis orogen the samples yielded u pb zircon ages in the range of 30 26 ma are intermediate to silicic in composition and have elevated k2o 1 5 8 9 th la ratios 0 14 1 10 low mgo

petrogenesis of dongguashan skarn porphyry cu au deposit related intrusion in the tongling district eastern china geochronological mineralogical geochemical and hf isotopic evidence the dongguashan skarn porphyry cu au deposit located in the tongling district of the middle lower yangtze river valley metallogenic belt mlyb consists of skarn ore bodies in the upper part and porphyry ore bodies in the lower part both of which are hosted in quartz diorite and quartz monzodiorite zircon u pb age and geochemical studies show that the quartz diorite of the dongguashan intrusion formed at 140 3 2 0 ma mswd 0 19 and belongs to the high potassium calc alkaline series it is enriched in large ion lithophile elements lile and light rare earth elements lree depleted in high field strength elements hfse and heavy rare earth elements hree and has a slightly negative eu anomaly 176hf 177hf values of the rims of zircons show a variable range 0 282087 0 282391 corresponding with calculated ehf t values of 10 72 to 21 46 plagioclases in the quartz diorite have unbalanced structure characterized by bright andesine and labradorite an 37 0 65 5 cores with higher contents of fe and sr and are corroded by dark oligoclase an 13 8 27 6 rim major elements trace elements hf isotope and the composition of plagioclases indicate that the parental magma of the dongguashan intrusion was produced by the mixing of underplating mafic magma and felsic magma formed by remelting of paleoproterozoic and neoarchean crustal rocks neoproterozoic crust may also provide some material to the felsic magma mafic magma played a key role and made the parental magma rich in water sulfur metals cu au and gave it a high oxygen fugacity during its magmatic evolution the parental magma underwent fractional crystallization of hornblende apatite sphene and other mafic minerals some quartz diorite and quartz monzodiorite samples that show adakitic signatures may result from injection of mafic magma some inherited zircons of the quartz diorite in the dongguashan intrusion gave ages of 2 40 2 50 ga 1 95 2 05 ga and 0 74 0 81 ga coming from ultramafic mafic and andesitic igneous rocks and this indicates that there may have been three periods 2 4 2 0 and 0 8 ga of magmatic activity in the tongling district

geochronology and petrogenesis of miocene granitic intrusions related to the zhibula cu skarn deposit in the gangdese belt southern tibet abstract the zhibula cu skarn deposit 19 5 mt ore 1 64 near the qulong porphyry cu mo deposit is located in the gangdese porphyry copper belt in southern tibet the deposit is a typical metasomatic skarn that is related to the interaction of magmatic hydrothermal fluids and calcareous host rocks stratiform skarn orebodies are mainly distributed in the contact between tuff and marble in the lower part of the jurassic yeba formation endoskarn zonations for an outward trend are observed in the granodiorite which grade from a fresh granodiorite to a weakly chlorite altered granodiorite a green diopside bearing granodiorite and a dark red brown garnet bearing granodiorite the zhibula granodiorite and monzogranite have similar secondary ion mass spectrometry sims zircon u pb ages of 16 9 0 3 ma and 17 0 0 2 ma respectively they exhibit different fractional crystallization from granodiorite sio 2 64 8 69 3 wt to monzogranite sio 2 72 3 76 8 wt both the granodiorite and monzogranite are characterized by high al 2 o 3 12 6 16 7 wt and k 2 o 1 5 5 5 wt contents high sr y 35 151 and la yb 19 48 ratios and variable mgo 0 16 3 91 and mg 31 61 values they display features of enrichment in large ion lithophile elements liles e g rb ba sr and k depletion in high field strength elements hfses e g nb ta ti and p and moderate negative eu anomalies eu 0 58 0 98 they show restricted in situ zircon hf isotopic compositions 6 7 to 8 8 only one sample is 4 5 and consistent 18 o values 6 0 to 6 6 the geochemical data indicated that the miocene zhibula granitic intrusions formed by the magma that were characterized by high sr y ratios and were derived from the partial melting of the thickened juvenile lower crust which may have been metasomatized by the slab melts during subduction of the neo tethyan oceanic crust and were induced by the convective removal of the thickened lithosphere in addition the zhibula cu skarn deposit and the qulong cu mo deposit form a significant porphyry skarn ore system

a numerical study of a shallow sea front generated by buoyancy flux generation mechanism abstract we studied numerically the frontogenesis of shallow sea fronts such as are observed in the kii channel japan during winter under conditions of sea surface cooling and buoyancy influx from coast and open ocean numerical experiments were carried out in a vertically two dimensional basin with a new model nh model without using the hydrostatic approximation and the convective adjustment considering the vertical acceleration term in the momentum equation intermittent gravitational convections with a large aspect ratio were produced in the frontal region to intensify the horizontal convergence and to strengthen the horizontal density gradient consequently a front in the tracer distribution had a sharpness comparable to the observed front in the kii channel and 3 6 times the sharpness in an h model using the hydrostatic approximation and the convective adjustment in the present model situation this effect of gravitational convections on sharpening a front in the nh model is equivalent to t

gulf stream meanders in the south atlantic bight 1 scaling and energetics as the gulf stream flows along the outer edge of the southeastern united states continental shelf it meanders on and offshore creating frontal eddies which influence the exchange of water between the shelf and the adjacent atlantic ocean on weekly timescales the dynamics controlling the development of such features in this region the open quotes south atlantic bight close quotes or sab is the subject of this pair of papers in the present paper the authors provide the background on the subject summarize the length time and velocity scales associated with these features and discuss meander energetics as simulated by a numerical model the scaling indicates that significant ageostgrophic dynamics may be crucial to the development of these flows and that the semigeostrophic equations may provide the simplest description of the system these nonlinear dynamics may permit complex interactions among meander modes and with topography resulting in the observed behavior of the gulf stream in the sab the princeton ocean model was tuned for conditions which prevail in the southern south atlantic bight and idealized numerical simulations of gulf stream meanders and associated frontal eddies were performed the system was found to be sensitive to both the amplitude and period of a small perturbationmore applied at the southern boundary the energetics indicate that the development of these features is controlled by a mixture of both baroclinic and barotropic instabilities of the mean gulf stream flow tantalizing hints of a period doubling phenomenon are present in the energy conversion time series 47 refs 12 figs 4 tabs less

temporal evolution of magmatism in the northern volcanic zone of the andes the geology and petrology of cayambe volcanic complex ecuador in the northern volcanic zone of the andes the cayambe volcanic complex consists of 1 a basal mostly effusive volcano the viejo cayambe whose lavas andesites and subordinate dacites and rhyolites are typically calc alkaline and 2 a younger essentially dacitic composite edifice the nevado cayambe characterized by lavas with adakitic signatures and explosive eruptive styles the construction of viejo cayambe began 1 1 myr ago and ended at 1 0 ma the young and still active nevado cayambe grew after a period of quiescence of about 0 6 myr from 0 4 ma to holocene its complex history is divided into at least three large construction phases angureal cone main summit cone and secondary summit cone and comprises large pyroclastic events debris avalanches as well as periods of dome activity geochemical data indicate that fractional crystallization and crustal assimilation processes have a limited role in the genesis of each suite on the contrary field observations and mineralogical and geochemical data show the increasing importance of magma mixing during the evolution of the volcanic complex the adakitic signature of nevado cayambe magmas is related to partial melting of a basaltic source which could be the lower crust or the subducted slab however reliable geophysical and geochemical evidence indicates that the source of adakitic component is the subducted slab thus the viejo cayambe magmas are inferred to come from a mantle wedge source metasomatized by slab derived melts adakites whereas the nevado cayambe magmas indicate a greater involvement of adakitic melts in their petrogenesis this temporal evolution can be related to the presence of the subducted carnegie ridge modifying the geothermal gradient along the wadati benioff zone and favouring slab partial melting

geology fluid inclusion and isotope constraints on ore genesis of the post collisional dabu porphyry cu mo deposit southern tibet abstract the dabu cu mo porphyry deposit is situated in the southern part of the lhasa terrane within the post collisional gangdese porphyry copper belt gpcb it is one of several deposits that include the qulong and zhunuo porphyry deposits the processes responsible for ore formation in the dabu deposit can be divided into three stages of veining stage i quartz k feldspar biotite chalcopyrite pyrite stage ii quartz molybdenite pyrite chalcopyrite and stage iii quartz pyrite molybdenite three types of fluid inclusions fis are present liquid rich two phase l type vapor rich two phase v type and solid bearing multi phase s type inclusions the homogenization temperatures for the fis from stages i to iii are in the ranges of 272 475 c 244 486 c and 299 399 c and their salinities vary from 2 1 to 49 1 1 1 to 55 8 and 2 9 to 18 0 wt nacl equiv respectively the coexistence of s type v type and l type fis in quartz of stage i and ii with similar homogenization temperatures but contrasting salinities indicate that fluid boiling is the major factor controlling metal precipitation in the dabu deposit the ore forming fluids of this deposit are characterized by high temperature and high salinity and they belong to a h2o nacl magmatic hydrothermal system the h o s pb isotopic compositions indicate that the ore metals and fluids came primarily from a magmatic source linked to miocene intrusions characterized by high sr y ratios similar to other porphyry deposits in the gpcb the fluids forming the dabu deposit were rich in na and cl derived from metamorphic dehydration of subducted oceanic slab through which nacl brine or seawater had percolated the inheritance of ancient subduction associated arc chemistry without shallow level crustal assimilation and or input of the meteoric water was responsible for the generation of fertile magma as well as co2 poor and halite bearing fis associated with post collisional porphyry deposits the estimated mineralization depths of qulong dabu and zhunuo deposits are 1 6 4 3 km 0 5 3 4 km and 0 2 3 0 km respectively displaying a gradual decrease from eastern to western gangdese deep ore forming processes accounted for the generation of giant sized qulong deposit because the exsolution of aqueous fluids with large fraction of water and chlorine in deep or high pressure systems can extract more copper from melts than those formed in shallow systems however the formation of small sized dabu deposit can be explained by a single magmatic event without additional replenishment of s metal or thermal energy in addition the ore forming conditions of porphyry cu mo deposits in gpcb are comparable to those of porphyry cu au mo deposits formed in oceanic subduction related continental or island arcs but differ from those of porphyry mo deposit formed in the dabie qinling collisional orogens the depth of formation of the mineralization and features of primary magma source are two major controls on the metal types and ore fluid compositions of these porphyry deposits

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

response of surface water masses and circulation to late quaternary climate change east of new zealand a series of cores from east of new zealand have been examined to determine the paleoceanographic history of the late quaternary in the sw pacific using planktonic foraminiferal data distinct shifts of species can be seen between glacial and interglacial times especially south of chatham rise east of south island foraminiferal fragmentation ratios and benthic planktonic foraminiferal ratios both show increased dissolution during glacials especially isotope stage 2 to the south of chatham rise the present day subtropical convergence appears to be tied to the chatham rise at 44 s but during glacial times this rise separated cold water to the south from much warmer water to the north with an associated strong thermal gradient across the rise we estimate that this gradient could have presented as much as an 8 c temperature change across 4 of latitude during the maximum of the last ice age there is only weak evidence of the younger dryas cool event but there is a clear climatic optimum between 8 and 6 4 ka with temperatures 1 2 c higher than the present day the marine changes compare well with vegetational changes on both south and north island

a numerical investigation of the gulf stream and its meanders in response to cold air outbreaks the three dimensional princeton ocean model is used to examine the modification of the gulf stream and its meanders by cold air outbreaks two types of gulf stream meanders are found in the model meanders on the shoreward side of the gulf stream are baroclinically unstable they are affected little by the atmospheric forcing because their energy source is stored at the permanent thermocline well below the influence of the surface forcing meanders on the seaward side of the stream are both barotropically and baroclinically unstable the energy feeding these meanders is stored at the surface front separating the gulf stream and the sargasso sea which is greatly reduced in case of cold air outbreaks thus meanders there reduce strength and also seem to slow their downstream propagation due to the southward ekman flow heat budget calculations suggest two almost separable processes the oceanic heat released to the atmosphere during these severe cooling episodes comes almost exclusively from the upper water column transport of heat by meanders from the gulf stream to the shelf though it is large does not disrupt the principal balance it is balanced nicely with the net heat transport in the downstream direction

two phases of post onset collision adakitic magmatism in the southern lhasa subterrane tibet and their tectonic implications

petrogenesis of the early eocene adakitic rocks in the napuri area southern lhasa partial melting of thickened lower crust during slab break off and implications for crustal thickening in southern tibet abstract cenozoic adakitic rocks in the lhasa block southern tibet have been widely used to trace the lateral extent of crustal thickening however their petrogenesis remains controversial here we report geochronological and geochemical data for the napuri intrusive rocks in the core area of the quxu batholith southern lhasa zircon u pb dating suggests that they were generated at approximately 48 ma the studied samples show significant geochemical variations manifested by the coexistence of three types of igneous rocks groups i and ii rocks exhibit variable and high sio 2 66 4 73 9 wt high al 2 o 3 14 0 17 4 wt k 2 o 3 9 5 3 wt sr 273 718 ppm and sr y 18 3 to 81 3 values and low y 3 6 to 16 ppm heavy rare earth element ree e g yb 0 48 to 1 8 ppm mgo 0 4 1 0 wt cr 2 9 7 4 ppm and ni 1 6 4 5 ppm contents which are similar to those of thickened lower crust derived adakitic rocks the group i rocks show higher sr y 77 5 81 3 ratios and lower total ree 55 5 63 2 ppm contents with clearly positive eu and sr anomalies whereas the group ii rocks have relatively lower sr y 18 3 65 7 ratios and higher total ree 115 375 ppm contents with negligible or slightly negative eu and sr anomalies group iii rocks have the highest sio 2 74 5 76 0 wt y 17 0 23 7 ppm and yb 2 91 3 30 ppm contents and the lowest al 2 o 3 12 5 13 2 wt sr 81 3 141 ppm and sr y 4 8 5 9 values with distinctly negative eu and sr anomalies compared with the jurassic cretaceous granitoids in southern lhasa the relative enrichment in sr nd hf isotopic compositions 87 sr 86 sr i 0 7049 0 7055 e nd t 0 3 0 7 and e hf t zircon 3 6 11 4 for the napuri intrusive rocks indicates that they likely contained indian continental components the group i and group ii rocks most probably originated from thickened mafic lower crust amphibolite eclogites or garnet amphibolites with garnet rutile plagioclase as residual minerals in the source at 1 5 gpa corresponding to depths of 50 km and group iii rocks were probably generated by fractional crystallization of plagioclase from the adakitic magmas taking into account the narrow linear nature of the eocene magmatic belt and reported synchronous asthenosphere derived basaltic rocks in southern lhasa we suggest that upwelling asthenosphere triggered by the break off of subducted neo tethyan slab probably provided the required thermal conditions for lower crustal melting the identification of indian continental components in the napuri intrusive rocks probably indicates that the asia india collision had taken place prior to their emplacement the dramatic changes in the la yb n ratios and e nd t and e hf t values of magmatic rocks in the gangdese area at ca 51 46 ma indicate that the cenozoic crustal thickening associated with the indentation of the indian continent began in the early eocene ca 51 46 ma at the latest

tectonic and deformation history of the gyeonggi massif in and around the hongcheon area and its implications in the tectonic evolution of the north china craton abstract a careful literature review reveals that the gyeonggi massif in korea extensively undergoes the collision related metamorphism during the triassic and thus represents a subduction complex with the south china affinity the triassic subduction complex is bounded by the imjingang belt to the north and the hongseong odesan belt to the south and can be well correlated with the sulu dabie belts of china in the p t t paths therefore the gyeonggi massif is a part of the south china block and the boundary between the north china craton and the south china block lies on the imjingang belt the paleoproterozoic tectonic events in and around the hongcheon area of the gyeonggi massif can be attributed to the accretion of the south china block onto the columbia supercontinent during the paleoproterozoic such paleoproterozoic tectonothermal events are well documented in the south china block in particular along its northern margin

early cretaceous subduction related adakite like rocks of the gangdese belt southern tibet products of slab melting and subsequent melt peridotite interaction abstract the limited geochronology and geochemistry data available for the early cretaceous igneous rocks of the southern gangdese belt southern tibet has resulted in the proposal of conflicting geodynamic models for the generation of the widespread cretaceous igneous rocks in the middle and northern parts of the belt to explore this issue we present shrimp u pb zircon data and geochemical and sr nd pb hf isotopic data for the mamen andesites from the southern margin of the gangdese belt the mamen andesites emplaced at 136 5 ma are sodic na2o k2o 1 2 2 3 and have geochemical characteristics typical of adakites i e high al2o3 high la yb ratios and sr contents low y and hree contents and positive eu anomalies except for high cr ni and mgo contents the andesites have initial 87sr 86sr t ratios of 0 70413 0 70513 positive end t values of 3 7 5 8 and 206pb 204pb t ratios of 18 37 18 51 207pb 204pb t ratios of 15 59 15 65 and 208pb 204pb t ratios of 38 43 38 72 in situ hf isotopic analyses of zircons that had previously been dated by shrimp yielded positive initial ehf t values ranging from 11 0 to 15 5 a model calculation using trace element and sr nd pb isotopic data indicates that several percent of subducted sediment is required to generate the mamen andesites which were derived via the partial melting of subducted neo tethyan slab morb sediment fluid and subsequently hybridized by peridotite in the mantle wedge our data indicate that the neo tethyan oceanic crust was subducted northward beneath the gangdese belt during the early cretaceous at a high angle our results are inconsistent with a tectonic model that advocates the low angle or flat slab subduction of neo tethyan oceanic crust in generating the widespread cretaceous magmatism recorded in the gangdese belt

close interactions between two pairs of heton like vortices under sea ice close interactions between two pairs of vertically stacked counterrotating eddies under sea ice were investigated in numerical experiments the numerical model contains a stratified ocean capped by an ice layer under the ice layer a shallow brine source produces a top cyclone and a submerged anticyclone while a shallow freshening source generates a top anticyclone and a submerged cyclone ice exerted friction dissipates the top eddies in time leaving submerged ones in lone existence in this work winning vortices are sought from a brine brine pair a freshening brine pair and a freshening freshening pair brine and freshening sources are made equal in strength to level the playing field when closely paired fission an increase in the number of subsurface vortices occurs from a brine brine pair and a freshening brine pair but not from a freshening freshening pair consequences of fission invariably increase the number of submerged anticyclones a heuristic argument is given to explain this polarity the strong interaction among closely packed eddies operates in timescales of tens of days helping to explain the predominance of submerged anticyclones under the arctic sea ice

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

tectonic magmatic and metallogenic evolution of the tethyan orogen from subduction to collision abstract this paper reviews the tectonic magmatic and metallogenic history of the tethyan orogen from the carpathians to indochina focus is placed on the formation of porphyry cu mo au deposits as being the most characteristic mineral deposit type formed during both subduction and collisional processes in this region relatively little is known about the history of the paleotethys ocean which opened and closed between gondwana and eurasia in the paleozoic and few ore deposits are preserved from this period the neotethyan ocean opened in the permian early triassic as the cimmerian continental fragments the cores of turkey iran tibet and indochina rifted from the northern gondwana margin and drifted northwards these microcontinents docked with the eurasian margin at various points in the mesozoic and cenozoic and formed a complex archipelago involving several small back arc basins and remnants of the paleotethyan ocean the main neotethyan ocean and these smaller basins were largely eliminated by collision with india and africa arabia in the early eocene and early mid miocene respectively although neotethyan subduction continues beneath the hellenic arc and the makran the majority of porphyry type deposits are found in association with neotethyan subduction mainly in the mesozoic and paleogene and syn to post collisional events in the mid paleogene to neogene they are found throughout the orogen but some sections are particularly well endowed including the carpathians balkans rhodopes eastern turkey lesser caucasus nw iran se iran sw pakistan southern tibet and se tibet indochina other sections that appear barren may reflect deeper levels of erosion young sedimentary cover or lack of exploration although there may also be real reasons for low prospectivity in some areas such as minimal subduction e g the western mediterranean region or lithospheric underthrusting as proposed in western tibet over the last decade improved geochronological constraints on the timing of ore formation and key tectonic events have revealed that many porphyry deposits that were previously assumed to be subduction related are in fact broadly collision related some forming in back arc settings in advance of collision some during collision and others during post collisional processes such as orogenic collapse and or delamination of subcontinental mantle lithosphere while the formation of subduction related porphyries is quite well understood collisional metallogeny is more complex and may involve a number of different processes or sources these include melting of orogenically thickened crust previously subduction modified lithosphere including metasomatized mantle underplated mafic rocks or lower crustal arc plutons and cumulates or upwelling asthenosphere e g in response to delamination slab breakoff back arc extension or orogenic collapse the most fertile sources for syn and post collisional porphyry deposits appear to be subduction modified lithosphere because these hydrated lithologies melt at relatively low temperatures during later tectonomagmatic events and retain the oxidized and relatively metalliferous character of the original arc magmatism unusually metallically enriched lithospheric sources do not seem to be required but the amount of residual sulfide phases in these rocks may control metal ratios e g cu au in subsequent magmatic hydrothermal ore deposits relatively au rich deposits potentially form in these settings as observed in the carpathians e g rosia montan turkey kisladag copler and iran sari gunay dalli although the majority of syn and post collisional porphyries are cu mo rich and resemble normal subduction related deposits e g in the gangdese belt of southern tibet this similarity extends to the associated igneous rocks which being derived from subduction modified sources largely retain the geochemical and isotopic character of those original arc magmas while still retaining a broadly calc alkaline character these rocks may extend to mildly alkaline shoshonitic compositions and may display adakite like trace element signatures high sr y and la yb ratios reflecting melting of deep crustal garnet amphibolitic sources but they are otherwise hard to distinguish from normal subduction related magmas small post collisional mafic alkaline volcanic centers are common throughout the orogen but for the most part appear to be barren however similar rocks in other post subduction settings around the world are associated with important alkalic type porphyry and epithermal au cu deposits and the potential for discovery of such deposits in the tethyan orogen should not be overlooked

mantle contributions to crustal thickening during continental collision evidence from cenozoic igneous rocks in southern tibet the tibetan crust is twice as thick as average continental crust crustal compression and shortening as a result of indian asian collision isoftenconsideredtobetheprimarycauseforthecrustalthickening inthispaper weshowthatmagmaticcontributionisalsoimportant we come to this conclusion by documenting the paleogene linzizong volcanic succession lvs its coeval granitoid batholiths and the miocene adakitic rocks along the gangdese magmatic belt in southern tibet it has been widely accepted that the indian asian collision proceeded from a soft phase at 65 70 ma to a hard phase at 45 40 ma followed by continued post collisional convergence to the present in response to the collision and post collision convergence are a series of tectono magmatic events recorded in the gangdese magmaticbelt theseinclude 1 thesyn collisionallvsvolcanism 65 40ma andtheemplacementofsoutherngangdesebatholiths a peakageof 50ma 2 aperiod 40mato25ma thatismagmaticallyquiescent yettectonicallydominatedbyactivecompressionand crustal shortening and 3 the emplacements of post collisional adakitic rocks 25 12 ma potassic ultrapotassic volcanics 25 10 ma and peraluminous muscovite bearing granites between 24 and 18 ma these three major events contribute in different ways to the crustal thickness phase i formation of the lower juvenile crust from 65 ma to 50 ma with crustal thickening largely concentrated at 50 40maviainputofmantle derivedmagmas phaseii crustalthickeningbytectonicshorteningat 40 25ma andphaseiii retaining crustal thickness but thinning of the lithospheric mantle since 25 ma in response to crustal extension and upwelling and lateral flow of asthenosphericmantle weemphasizethatcollision inducedcrustalthickeningtookplacemainlyintheperiodof 50 40maand 25ma i e the period between the late stage of the lvs volcanism and the beginning of the adakitic rock emplacement most of the lvs rocks and the collision related granitoids in southern gangdese have end 0 attesting to the significance of mantle input most likely through remelting of mantle derived basaltic rocks including the subducted neo tethyan ocean crust the petrologic and geochemical characteristics of the miocene potassic adakitic rocks support the idea that the lower portion of the thickened tibetan crust is mafic and is genetically associated with the earlier lvs magmatism we estimate that the mantle material input contributed about 30 of the total thickness of the present day tibetan crust by assuming a pre collision crustal thickness of 35 km then the tectonic contribution would be about 20 km

shoreward intrusion of upper layer warm water by prescribed shelf break temperature perturbation and surface wind stress a new mechanism for mid shelf frontogenesis in shelf waters off the carolina capes is proposed based on results from three numerical experiments using a three dimensional coastal ocean model we find that quasi geostrophic currents induced by a density temperature anomaly at the shelf break may advect upper layer waters from the shelf break to the mid shelf this process can cause mid shelf frontogenesis even in the absence of favorable wind forcing however persistent northerly winds can substantially enhance mid shelf frontogenesis and increase the along shore scale of the mid shelf fronts off the carolina capes

increased magmatic water content the key to oligo miocene porphyry cu mo au formation in the eastern gangdese belt tibet the eastern gangdese magmatic belt between 88 e and 94 e in southern tibet preserves a series of cenozoic collision related igneous rocks that have formed since the start of the india asia collision at 55 to 50 ma the paleocene eocene intrusive rocks have intermediate la yb n ratios average 12 0 8 6 n 150 range 1 15 64 0 intermediate to low sr y ratios mostly 40 and weak or absent eu anomalies oligocene average eun eu 0 99 0 27 n 35 range 0 67 1 58 miocene average eun eu 0 95 0 13 n 93 range 0 44 1 46 and their mineralogy consists mainly of plagioclase quartz and amphibole as phenocrysts these geochemical and mineralogical characteristics suggest that the paleocene eocene magmas were relatively dry and evolved primarily by fractionation of pyroxene and plagioclase whereas the oligo miocene magmas were more hydrous and fractionated significant amounts of hornblende and lesser plagioclase prior to upper crustal emplacement oligocene and miocene magmatic rocks crop out as small volume intrusions and are associated with several large to giant sized porphyry cu mo au deposits as well as numerous smaller porphyry and skarn deposits in contrast earlier more voluminous paleocene eocene magmatism is only known to be associated with three small porphyry deposits erosional loss of subvolcanic porphyry systems from the older paleocene eocene sequence is not thought to be a controlling factor in this temporal difference of porphyry deposit distribution because exhumation rates after 55 ma were relatively low and coeval linzizong volcanic rocks are extensively preserved in the gangdese belt instead we suggest that differences in magmatic history and petrogenesis led to this restricted temporal distribution of porphyry deposits the paleocene eocene magmas were generated during the onset of collision between india and asia and were triggered by rollback of the neo tethyan oceanic slab this magmatism records the final stage of subduction of oceanic lithosphere beneath southern tibet and the relatively low water contents of these magmas may reflect final dehydration of the remnant neo tethyan slab after oceanic slab breakoff at 40 to 38 ma the greater india slab began to subduct beneath the gangdese belt until hard collision with the indian continent at 35 ma and breakoff of the greater india slab at 25 to 10 ma this last event is suggested to have caused asthenospheric mantle upwelling and partial melting of subduction modified hydrated tibetan lithosphere the resultant partial melts may have remobilized metals residual in the deep lithosphere from previous arc magmatism giving rise to a suite of postsubduction porphyry type ore deposits in the oligo miocene

warm water pathways transports and transformations in the northwestern north atlantic and their modification by cold air outbreaks 1 this paper presents a numerical study of the warm water pathways transports and water mass transformation in the newfoundland basin region and an investigation of the rectification effects of a series of cold air outbreaks caos on the above processes an initial mean state simulation was compared with observations and showed good agreement then repeated cao events were explicitly included in the surface fluxes and illustrated the following rectification effects the thermal regime of the entire baroclinic layer was impacted as the thermocline deepened and the temperature anomaly was noticeable down to 500 m different mechanisms were responsible for the propagation of the temperature anomaly at different stages during the onset of the cao vertical diffusion propagated the temperature anomaly downward near the surface then vertical advection further propagated the anomaly downward between cao events to about 500 m depth with caos the north atlantic current carried more warm water and its pathway in the northwest corner shifted towards the southeast the volume averaged mean and eddy kinetic energy increased by 30 and 20 respectively and the water mass transformation rate in the newfoundland basin was doubled the gulf stream carried more heat but heat transport to the eastern basin decreased owing to the increase in heat release to the atmosphere

when and where did india and asia collide timing of the collision between india and asia is the key boundary condition in all models for the evolution of the himalaya tibetan orogenic system thus it profoundly affects the interpretation of the rates of a multitude of associated geological processes ranging from tibetan plateau uplift through continental extrusion across eastern asia as well as our understanding of global climate change during the cenozoic although an abrupt slowdown in the rate of convergence between india and asia around 55 ma is widely regarded as indicating the beginning of the collision most of the effects attributed to this major tectonic episode do not occur until more than 20 ma later refined estimates of the relative positions of india and asia indicate that they were not close enough to one another to have collided at 55 ma on the basis of new field evidence from tibet and a reassessment of published data we suggest that continent continent collision began around the eocene oligocene boundary 34 ma and propose an alternative explanation for events at 55 ma

crustal thickening prior to 38 ma in southern tibet evidence from lower crust derived adakitic magmatism in the gangdese batholith article thepetrogenesisandgeodynamicimplicationsofthecenozoicadakitesinsoutherntibetremaintopicsofdebate here we report geochronological and geochemical data for host granites and mafi ce nclaves from wolong in the easterngangdesebatholith southerntibet zirconla icp ms datingindicatesthatthe wolonghostgranitesand enclaves were synchronously emplaced at ca 38 ma the host granites are medium to high k calc alkaline metaluminous a cnk 0 93 0 96 with high al2o3 15 47 17 68 low mgo 0 67 1 18 very low abundances of compatible elements e g cr 3 87 8 36 ppm ni 3 04 5 71 ppm and high sr y ratios 127 217 similar to those typical of adakite the mafi ce nclaves sio2 51 08 56 29 have 3 83 5 02 mgo

the largest karst cave discovered in a tunnel during motorway construction in slovenia s classical karst kras karst researchers of the karst research institute zrc sazu are regularly consulted during the planning of slovenia s motorways and invited to observe and monitor construction in the karst areas more than 350 caves have been discovered in the course of building 60 km of new motorways over the past decade access to the most important caves is preserved by concrete tubes closed with metal covers at the roadside the largest cave system in a tunnel named lc s647 is almost entirely preserved the protection of this cave will serve as a good example for the preservation of natural heritage in the future construction work for the motorway uncovered a high degree of karstification in particular the discovery of this cave in the kastelec tunnel lc s647 showed the existence of a major cave system in the geological and speleological past a number of unconnected passages have also been found caves discovered during highway construction have brought new knowledge about the cavernosity and the geological history of this part of the karst

oligocene magmatism in the eastern margin of the east himalayan syntaxis and its implication for the india asia post collisional process abstract post collisional high sr y type magmatism in the southern lhasa terrane extends 1500 km along the strike of the orogen the origin and magma generation mechanism for these rocks remain controversial we report a detailed study of such high sr y type magmatism in the east himalayan syntaxis se tibet and explain its significance for understanding post collisional geodynamical processes the two mica granites from the beibeng damu bolonggong and bomi plutons emplaced along the eastern margin of the east himalayan syntaxis represent a high k calc alkaline suite sio 2 67 75 wt al 2 o 3 13 3 15 8 wt k 2 o na 2 o 0 7 1 9 an outstanding geochemical feature is that they have high sr y ratios 19 88 mostly 30 coupled with low y and hree indicating that they were derived from partial melting of thickened lower crustal materials sr nd isotopic data show that the potential magma sources are the intermediate intrusive rocks related to the jurassic eocene gangdese magmatism and basement metasedimentary rocks the contribution of the latter source increases from south to north the two mica granites from the beibeng damu bolonggong and bomi intrusions have magma crystallization ages of 29 9 27 5 ma 27 6 ma 27 1 26 5 ma and 23 7 ma respectively our data show decreasing magma crystallization ages both from south to north along the eastern margin of the east himalayan syntaxis and from east to west along the gangdese belt we suggest that such variations resulted from the diachronous breakoff of the subducted indian continental slab beneath the east himalayan syntaxis the breakoff of the subducted indian continental slab was initiated in the south and propagated towards north probably due to the thermal impact of a widening window opened by slab breakoff along the gangdese belt the breakoff of the subducted indian continental slab commenced in east and then propagated westwards

magma mixing in the genesis of the kalatongke dioritic intrusion implications for the tectonic switch from subduction to post collision chinese altay nw china abstract the kalatongke complex located in south chinese altay of the central asian orogenic belt caob consists of the 308 ma dioritic suite intruded by the 287 ma noritic suite the noritic suite is composed of olivine norite and hornblende norite and hosts sulfide mineralization the dioritic suite is composed of ferrodiorite diorite and quartz diorite ferrodiorites have 43 6 to 51 8 wt sio 2 8 2 to 12 1 wt fe 2 o 3 t 2 87 to 4 86 wt mgo with mg ranging from 42 to 53 they have highly variable al 2 o 3 17 4 20 7 wt na 2 o 2 78 4 50 k 2 o 0 58 3 70 sr 280 1300 ppm and ba 219 3100 ppm diorites and quartz diorites have 15 2 19 6 wt al 2 o 3 239 890 ppm sr and170 1100 ppm ba 11 2 25 5 ppm y and1 2 2 2 ppm yb all rocks have lree enriched ree patterns and do not display eu anomalies they have primitive mantle normalized trace element patterns enriched in large ion lithosphile elements lile e g ba and sr and depleted in ti nb and ta both ferrodiorites and diorites have relatively constant initial 87 sr 86 sr ratios of 0 7037 to 0 7040 but highly variable e nd t values of 4 5 to 11 6 whereas quartz diorites have higher initial 87 sr 86 sr ratios 0 7056 and lower e nd t values 0 5 plagioclase from ferrodiorites diorites and quartz diorites has similar initial 87 sr 86 sr ratios of 0 7015 to 0 7055 the dioritic suite formed from a mixed magma of an evolved mantle derived basaltic magma by partial melting of subduction modified mantle source in response to slab breakoff and a crustal derived granitic magma at 308 ma before mixing with granitic magmas basaltic magmas underwent fractional crystallization of olivine to form ferrodiorite the mixed magma also formed diorite and quartz diorite combining fractionation clinopyroxene amphibole and plagioclase the noritic suite formed from a mantle derived high mg magma in a post collisional environment both the dioritic and noritic suites in kalatongke suggest the switch from subduction to extension and the final collision between the junggar block and chinese altay at 308 ma

petrogenesis and geological implications of the oligocene chongmuda mingze adakite like intrusions and their mafic enclaves southern tibet abstractthe oligocene to the miocene was a critical period for the growth of the tibetan plateau this growth is commonly considered to have been controlled by deep geodynamic processes the ultrapotassic and adakite like igneous rocks that developed during this period offer constraints on these deep seated processes in southern tibet whole rock geochemistry u pb zircon geochronology and in situ zircon hf isotopes have been determined for the mafic enclaves and host adakite like granitoids in the oligocene chongmuda mingze intrusive complex of southern tibet the host rocks including granodiorite and monzogranite are mainly high k and calc alkaline in composition their whole rock geochemistry low mgo ni and cr contents negative end t values 2 5 to 3 4 and low 87sr 86sr i values 0 7061 0 7066 and in situ zircon ehf t values 0 6 6 1 indicate that they were derived by partial melting of a juvenile lower crust implying that the southern tibetan crust was already thickened to up to 50 k

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

constraints on the origin of adakites and porphyry cu mo mineralization in chongjiang southern gangdese the tibetan plateau chongjiang is a low grade porphyry cu deposit located in the gangdese belt south tibet the petrogenesis and geodynamic settings of the miocene intrusions associated with the deposit remain controversial this study presents new results on in situ zircon hf o isotopic compositions and u pb ages whole rock major and trace elements and sr nd isotopes for the adakitic intrusions from chongjiang deposit the ore bearing biotite monzogranite porphyry has adakitic characteristics with enriched large ion lithophile elements lile and light rare earth elements lree and depleted in high field strength elements hfse p and ti la icp ms zircon u pb dating indicates that the ore bearing and barren adakites were emplaced at 14 9 0 3 ma and 12 9 0 3 ma respectively the porphyry is characterized by relatively high initial sr 87 sr 88 ratios 0 7059 to 0 7066 and negative whole rock epsilon nd t values 3 8 to 2 6 zircon delta 18 0 is slightly higher than mantle values 5 0 to 7 2 parts per thousand with varied epsilon hf t 1 0 to 7 6 most of the in situ zircon hf o isotopic data plot in a binary mixing trend between morb and lower continental crust derived melts these results indicate contributions from mixing of a mantle like source e g slab melts with continental crust interestingly most of the samples plot in the field defined by dabie adakites representing partial melting of the lower continental crust with several samples near in the circum pacific adakite field representing partial melting of subducted oceanic slabs which seemingly indicates that chongjiang adaldtes mostly formed through partial melting of lower continental crust with a small amount derived from oceanic slab melts these may be plausibly explained by plagioclase retention in the thickened tibetan continental crust which lowers sr contents in the magmas during crustal assimilation such a model is supported by other adakite discrimination diagrams which all point towards slab melting crustal contamination can compellingly explain the low grade of the chongjiang deposit considering the temporal spatial distribution of porphyry cu deposits geochemical characteristics and high oxygen fugacity we propose that the subducting ninetyeast ridge probably played a critical role in controlling the formation of miocene adakites and porphyry copper deposits in the eastern gangdese belt c 2017 elsevier b v all rights reserved