copper mineralization prevented by arc root delamination during alpine himalayan collision in central iran contrasting geochemical signatures of copper ore hosting eocene and some barren undeformed miocene diorites to granites in central iran temporally overlap with the alpine himalayan collision and provide key implications on the existence and lack of cu mineralization during collisional magmatism high sr and low y and yb contents of eocene arc rocks in the natanz arc segment reflect thickened andean type orogenic arc crust 45 km whereas barren miocene natanz arc rocks 21 19 ma indicate thin arc crust similar to collisional volcanism in anatolia geochemical modeling indicates a change in the mineralogy of the melt residual from precollisional eocene basaltic garnet bearing 5 30 amphibolite to syn or postcollisional miocene metasomatized mantle peridotite which can be explained by collision induced delamination of the arc lithospheric root subsequent recharge of hot asthenosphere and melting of metasomatized mantle peridotite and lack of interaction with a garnet bearing arc crustal keel explain the low sr and high y and yb contents the relatively enriched initial sr isotope ratios of postcollisional miocene natanz rocks and the lack of copper mineralization in postcollisional miocene natanz arc rocks arc root delamination removes the copper and sulfur enriched metasomatized lithospheric arc root and hydrous cumulate reservoir required to form copper ore deposits lack of the dense melt residues also provides an alternative explanation for the elevated thin crustal iranian back arc plateau 38 km as a result of uplift by isostatic rebound rather than uplift by anomalous shortening miocene arc root delamination implies a minimum age of 21 ma for the alpine himalayan collision in central iran

examination of late palaeolithic archaeological sites in northern europe for the preservation of cryptotephra layers we report the first major study of cryptotephra non visible volcanic ash layers on late palaeolithic archaeological sites in northern europe examination of 34 sites dating from the last termination reveals seven with identifiable cryptotephra layers preservation is observed in minerogenic and organic deposits although tephra is more common in organic sediments cryptotephra layers normally occur stratigraphically above or below the archaeology nearby off site palaeoclimate archives peat bogs and lakes 0 3 km distant were better locations for detecting tephra however in most cases the archaeology can only be correlated indirectly with such cryptotephras patterns affecting the presence absence of cryptotephra include geographic position of sites relative to the emitting volcanic centre the influence of past atmospherics on the quantity direction and patterns of cryptotephra transport the nature and timing of local site sedimentation sampling considerations and subsequent taphonomic processes overall while tephrostratigraphy has the potential to improve significantly the chronology of such sites many limiting factors currently impacts the successful application

the giant dexing porphyry cu mo au deposit in east china product of melting of juvenile lower crust in an intracontinental setting the dexing porphyry cu mo au deposit in east china 1 168 mt at 0 45 cu is located in the interior of the south china craton scc made up of two lithospheric blocks the yangtze and cathaysia blocks the cu mo au mineralization is associated with mid jurassic granodioritic porphyries with three high level intrusive centers controlled by a series of lineaments at the southeastern edge of the yangtze block available age data define a short duration 172 170 ma of the felsic magmatism and the mineralization 171 1 ma the deposit shows broad similarities with deposits in volcanoplutonic arcs although it was formed in an intracontinental setting porphyries associated with mineralization are mainly granodiorites which contain abundant phenocrysts 40 60 and carry contemporaneous microgranular mafic enclaves mmes they are mainly high k calc alkaline and show geochemical affinities with adakite characterized by relatively high mgo cr ni th and th ce ratios the least altered porphyries yielded relatively uniform end t values from 0 9 to 0 6 and wide 87sr 86sr i range between 0 7046 and 0 7058 partially overlapping with the sr nd isotopic compositions of the mmes and mid jurassic mafic rocks in the scc zircons from the porphyries have positive ehf t values 3 4 to 6 9 and low 18o values 4 7 to 6 3 generally close to those of depleted mantle all data suggest an origin by partial melting of a thickened juvenile lower crust involving mantle components e g neoproterozoic mafic arc magmas triggered by invasion of contemporaneous mafic melts at dexing the mmes show textural mineralogical and chemical evidence for an origin as xenoliths formed by injection of mafic melts into the felsic magmas these mmes usually contain magmatic chalcopyrite and have original variable contents of cu up to 500 ppm their geochemical characteristics suggest that they were derived from an enriched mantle source metasomatized by proterozoic slab derived fluids and supplied a part of cu au and s for the dexing porphyry system during their injection into the felsic magmas the 171 1 ma magmatic hydrothermal event at dexing is contemporaneous with the mid jurassic extension in the scc followed by 160 90 ma arc like magmatism in southeastern china with respect to the tectono magmatic evolution of the scc the emplacement of cu bearing porphyries and the associated cu mineralization occurred in response to the transformation from a tensional regime related to mid jurassic extension to a transpressional regime related to the subduction of the paleo pacific oceanic lithosphere

double layer structure of the crust beneath the zhongdian arc sw china u pb geochronology and hf isotope evidence abstract u pb ages and hf isotopes of zircons in late triassic and cretaceous intrusive rocks from the zhongdian arc sw china are used to decipher the tectonic magmatic and metallogenic processes that occurred during this period new u pb dating of zircons from late triassic porphyries yielded ages of ca 216 ma and ehf t values of 2 1 to 6 1 combined with previous results the data indicate that these late triassic rocks were most likely derived from a juvenile mafic lower crust with minor old crust material however the cretaceous granites 80 ma have lower ehf t values 7 6 to 2 4 than the late triassic rocks indicating that the former originated from old crust based on the new data and previous studies of mesozoic magmatic activity a plausible model for the tectono magmatism and metallogenesis of the zhongdian arc is proposed the westwards subduction of the ganzi litang oceanic crust began before 230 ma resulting in the formation of a juvenile lower crust beneath the zhongdian arc due to the underplating of mafic arc magmas during ca 230 216 ma at ca 216 ma break off or slab tearing of the west dipping ganzi litang oceanic slab led to partial melting of the juvenile lower crust which gave rise to cu bearing porphyries in the late cretaceous the zhongdian arc probably underwent post collision extension triggering the partial melting of the old middle upper crustal materials and producing various granites and related mo cu deposits according to this model the crust beneath the zhongdian arc probably has a double layer structure with older crust at shallow levels and juvenile crust at deeper levels

geochronology and geochemistry of late cretaceous igneous intrusions and mo cu w mineralization in the southern yidun arc sw china implications for metallogenesis and geodynamic setting abstract the sanjiang tethyan metallogenic domain stmd is an important part of the tethyan giant metallogenic belt the yidun arc is a part of the stmd in the eastern tibetan plateau recently four newly discovered mo cu w ore deposits related to granitic intrusions were found distributed along the north south strike in the southern yidun arc which are identified as the xiuwacu relin hongshan and tongchanggou deposits herein these four deposits formed along high angle north northwest or north west strike slip faults with vein type and porphyry type mo cu mineralization developed in the intrusions molybdenite re os and zircon u pb dating together with zircon hf isotopes and whole rock geochemistry of the intrusions were studied to discern the relationship between mineralization and magmatism metallogenesis and tectonic settings molybdenite from skarn type mineralization at the hongshan deposit has a re os isochron age of 81 2 2 6 ma mswd 1 3 n 5 consistent with previously published zircon u pb ages and re os ages of porphyry type mo mineralization these results indicate that the hongshan is a late cretaceous porphyry skarn cu mo deposit zircon u pb ages of the granitic intrusions in the xiuwacu relin and tongchanggou deposits varying from 87 4 ma to 82 7 ma combined with published molybdenite re os age spectrum 85 ma to 81 2 ma it is proposed that the mo cu w mineralization in the shangri la region is spatially temporally and probably genetically related to the late cretaceous granitic intrusions the relin hongshan and tongchanggou intrusions have high sio 2 65 2 70 0 wt sr 363 905 ppm sr y 22 72 and la yb 37 69 ratios and low y 11 6 17 0 ppm and yb 0 97 1 59 ppm which displayed adakitic affinities their low mgo 0 66 1 44 wt mg 25 46 variable negative zircon e hf t values 7 9 to 2 3 and proterozoic two stages hf model ages t dm2 1 13 1 62 ga suggest that they were probably dominantly derived from partial melting of thickened lower continental crust according to the tectonic evolution of the bangong meso tethys ocean during the late mesozoic the late cretaceous igneous event and mineralization in the yidun arc likely formed under a late or post collision extensional environment probably related to the collision between the lhasa and qiangtang terranes during the late cretaceous

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

mesozoic multiple magmatism and porphyry skarn cu polymetallic systems of the yidun terrane eastern tethys implications for subduction and transtension related metallogeny abstract most mineralized porphyries associated with large to giant oxidized porphyry cu deposits show an affinity with relatively high sr y features the mesozoic porphyry skarn cu polymetallic systems of the yidun terrane hosted by the late triassic and late cretaceous intrusions provide chances to investigate the spatial and genetic relationships between subduction and transtension related magmatism and mineralization and determine the petrogenesis and fertility of magmatic rocks zircon u pb ages indicate that the late triassic pre ore quartz diorite porphyries and syn ore quartz monzonite porphyries were emplaced at 225 and 215 ma respectively the late cretaceous syn ore monzogranite porphyries were emplaced at 83 78 ma our new data combined with previously published geochronological data show the spatially overlapping distribution of the multiple mesozoic porphyry systems in the yidun terrane although all the late triassic intrusive rocks share similar geochemical characteristics the pre ore quartz diorite porphyries have normal arc related chemical features with low sr y and la yb n ratios high y and yb abundances while the syn ore quartz monzonite porphyries exhibit high sr y and la yb n ratios low y and yb abundances all samples show similar sr nd hf isotopic compositions 87sr 86sr i 0 7060 0 7117 end t 6 7 to 0 0 zircon ehf t 4 0 to 3 0 suggesting that they were probably derived from partial melting of juvenile lower crust trace element patterns and partial melt modeling indicate that the quartz diorite porphyries were likely formed by partial melting of normal thick lower crust while the causative quartz monzonite porphyries were probably formed by partial melting of eclogitized thickened lower crust we propose that pre ore quartz diorite porphyries were probably generated earlier via the subduction of garze litang oceanic crust and syn ore quartz monzonite porphyries were formed later by partial melting of sulfide enriched thickened juvenile lower crust thus these high sr y quartz monzonite porphyries host several economically important porphyry cu deposits such as pualng xuejiping and songnuo however the late cretaceous syn ore monzogranite porphyries have lower ehf t values 8 4 to 2 9 and end t values 7 5 to 3 8 than the late triassic porphyries indicating that the former mainly originated from ancient crustal materials new dataset and previous studies suggest that the late cretaceous post collisional transtension triggered the asthenospheric upwelling and the underplating of mafic magmas induced the partial melting of garnet bearing amphibolite and thus caused the emplacement of monzogranite porphyries and associated porphyry skarn cu mo deposits

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

structural evidence for the permo triassic tectonic evolution of the yidun arc eastern tibetan plateau abstract deformation across the yidun arc occurred as a two phase process i early triassic deformation of paleozoic sequences resulting from closure of the jinsha jiang suture and ii late triassic deformation of middle to late triassic rocks as a result of final basin closure deformation in the western yidun arc is dominantly west vergent a north south striking west directed thrust the jinsha thrust forms the boundary between the western yidun arc and the eastern qiangtang block the jinsha thrust incorporates within it ophiolitic material of the jinsha jiang suture and the structural geometry across the jinsha thrust suggests that early triassic subduction was east dipping along the jinsha jiang suture deformation within middle to late triassic sequences of the eastern yidun arc eastern qiangtang block and western songpan garze fold belt is thin skinned being restricted largely to a single generation of upright folds and associated thrusts late triassic westwards subduction underneath the combined western yidun arc qiangtang block resulted in arc volcanism in the eastern yidun arc coeval arc volcanics in the northern margin of the qiangtang block suggest that the suture along the northern margin of the qiangtang block may be correlated with the garze litang suture rather than the jinsha jiang suture

crustal structure of the indus tsangpo suture zone and its ophiolites in southern tibet abstract the 2000 km long indus tsangpo suture zone itsz in southern tibet contains remnants of neotethyan oceanic lithosphere ophiolitic melanges flysch units and continental rocks and has been interpreted as the boundary between the india and eurasia plates we have used field based structural analyses and geological mapping in five key ophiolite massifs luobusa zedang xigaze jiding and dongbo and petrofabric studies of core samples recovered through the luobusa scientific drilling project to document the structural architecture and the deformation history of the itsz the structural and kinematic analyses of the suture zone units indicate the occurrence of superposed deformation fabrics which developed first during the ophiolite emplacement and then during the subsequent india asia continental collision the suprasubduction zone ophiolites were emplaced southward onto the northern margin of the proto indian plate in the late cretaceous the continental collision between the ophiolite laden proto indian margin and the lhasa terrane of the eurasian plate in the early paleogene resulted in backthrusting and inversion of the structural order of the itsz the ophiolite massifs ophiolitic melanges and flysch units were thrust northward as nappe sheets over the eocene and younger forearc basin strata the itsz shows therefore a complex structural anatomy characterized by both south and north directed thrust faults and ductile and brittle deformation fabrics

gas geochemistry of natural analogues for the studies of geological co2 sequestration geological sequestration of anthropogenic co2 appears to be a promising method for reducing the amount of greenhouse gases released to the atmosphere geochemical modelling of the storage capacity for co2 in saline aquifers sandstones and or carbonates should be based on natural analogues both in situ and in the laboratory the main focus of this paper has been to study natural gas emissions representing extremely attractive surrogates for the study and prediction of the possible consequences of leakage from geological sequestration sites of anthropogenic co2 i e the return to surface potentially causing localised environmental problems these include a comparison among three different italian case histories i the solfatara crater phlegraean fields caldera southern italy is an ancient roman spa the area is characterised by intense and diffuse hydrothermal activity testified by hot acidic mud pools thermal springs and a large fumarolic field soil gas flux measurements show that the entire area discharges between 1200 and 1500 tons of co2 per day ii the panarea island aeolian islands southern italy where a huge submarine volcanic hydrothermal gas burst occurred in november 2002 the submarine gas emissions chemically modified seawater causing a strong modification of the marine ecosystem all of the collected gases are co2 dominant maximum value 98 43 vol iii the tor caldara area central italy located in a peripheral sector of the quiescent alban hills volcano along the faults of the ardea basin transfer structure the area is characterised by huge co2 degassing both from water and soil although the above mentioned areas do not represent a storage scenario these sites do provide many opportunities to study near surface processes and to test monitoring methodologies

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

high magmatic flux during alpine himalayan collision constraints from the kal e kafi complex central iran voluminous eocene gabbros to granites of the kal e kafi backarc composite intrusion were emplaced prior to the alpine himalayan collision in the central iranian backarc but the reasons for precollisional high arc and backarc magmatic productivity 60 53 ma are unclear diagnostic geochemical signatures are high k2o sr contents and successively depleted middle rare earth element ree patterns reflecting a highly metasomatized source and an increasing role for amphibole and garnet 0 10 in the relatively younger granites release of concealed k sr rich fluids from oceanic fractures and faults during buckling and bulging of a precollisional choking oceanic slab and melting of phlogopite bearing lithosphere with subsequent interaction of the melt with lower crustal garnet amphibolite of a 40 km thick crust can explain the kal e kafi geochemical and isotopic signatures gravimetric data indicating a 39 km present day backarc crustal thickness are consistent with geochemical results but also imply little if any collisional crustal thickening of the central iranian plateau high eocene arc backarc melt flux prior to collision in this region reflects vigorous thermal convection which may in fact be diagnostic of collisional magmatism explaining the presence of postcollisional shoshonitic melts in this and other collisional orogenic settings

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

geochronology of the hongniu hongshan porphyry and skarn cu deposit northwestern yunnan province china implications for mineralization of the zhongdian arc abstract the hongniu hongshan porphyry and skarn copper deposit is located in the triassic zhongdian island arc northwestern yunnan province china single zircon laser ablation inductively coupled plasma mass spectrometry u pb dating suggests that the diorite porphyry and the quartz monzonite porphyry in the deposit area formed at 200 ma and 77 ma respectively a re os isotopic date of molybdenite from the ore is 78 9 ma which indicates that in addition to the known triassic cu au porphyry systems a late cretaceous porphyry cu mo mineralization event also exists in the zhongdian arc the quartz monzonite porphyry shows characteristics of a magnetite series intrusion with a high concentration of al k rb ba and pb low amount of ta ti y and yb and a high ratio of sr y average 26 42 the cretaceous porphyry also shows a strong fractionation between light and heavy rare earth elements average la yb n 37 9 which is similar to those of the triassic subduction related diorite porphyry in the hongniu hongshan deposit and the porphyry hosting the pulang copper deposit however in contrast to the older intrusions the quartz monzonite porphyry contains higher concentrations of large ion lithophile elements and co and lesser sr and zr therefore whereas the triassic porphyry cu au mineralization is related to slab subduction slab in an arc setting the quartz monzonite porphyry in the hongniu hongshan deposit formed by the remelting of the residual oceanic slab combined with contributions from subduction modified arc lithosphere and continental crust which provided the metals for the late cretaceous mineralization

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

geochronology and magmatic oxygen fugacity of the tongcun molybdenum deposit northwest zhejiang se china the tongcun mo porphyry deposit in northwest zhejiang is hosted in three porphyry units huangbaikeng songjiazhuang and tongcun from southwest to northeast u pb zircon ages of 162 3 0 ma for the huangbaikeng porphyry 159 9 3 0 ma for the songjiazhuang porphyry and 167 6 155 6 ma for the tongcun porphyry indicate that these intrusions formed during the jurassic and are most likely associated with the northwestward subduction of the izanagi plate trace element compositions of zircons from the tongcun deposit constrain the oxygen fugacity fo2 of the magma using zircon ce anomalies and ti in zircon temperatures the average magmatic fo2 for the porphyries in the tongcun deposit is fayalite magnetite quartz fmq 2 7 which is similar to the shapinggou fmq 3 2 and dabaoshan fmq 3 5 mo porphyry deposits but much higher than that of the reduced cretaceous ore barren shangjieshou porphyry fmq 1 1 around 8 km away from the tongcun deposit the distinct difference in magmatic oxygen fugacity between the jurassic and cretaceous porphyries may help to explain the absence of mo porphyry mineralization in northwest zhejiang during the cretaceous

tethys tectonic evolution and its bearing on the distribution of important mineral deposits in the sanjiang region sw china abstract the sanjiang region in se tibet plateau and nw yunnan is known to have formed by amalgamation of gondwana derived continental blocks and arc terranes as a result of oceanic subduction followed by continental collision from paleozoic to mesozoic in this paper we provide a synthesis of tectonic evolution magmatism and metallogeny in the region based on data from literatures early paleozoic ophiolites 473 439 ma in the changning menglian belt indicate the existence of a proto tethys ocean in this region two episodes of subduction related magmatism in the early paleozoic one occurred in the baoshan and tengchong blocks at 502 455 ma and the other occurred in the simao block at 421 401 ma are regarded as evidence for two different events of subduction of the proto tethys ocean at different locations the proto tethys was succeeded in early devonian by the paleo tethys which comprised the main ocean and three branches ailaoshan jinshajiang and garze litang the changning menglian main ocean existed from middle devonian to middle triassic the remnants of the oceanic crust are preserved in a few places in the longmu tso shuanghu suture as well as in the changning menglian ophiolite belt the eastward subduction of the main oceanic plate from early permian to early triassic formed a prominent arc terrane stretching 1500 km from yunnan to eastern tibet from the waning stage of subduction to post subduction numerous s type granite plutons with ages varying between 230 and 219 ma such as the lincang batholith in yunnan were emplaced at or close to the suture this event produced several hydrothermal w sn deposits in the region the tectonic evolution and associated magmatism of the jinshajiang and ailaoshan branch oceans are generally comparable to those of the main ocean however the branch oceans were subducted westward instead the garze litang branch ocean also underwent westward subduction from middle devonian to late triassic arc related high sr y porphyry intrusions and associated porphyry skarn cu mo au deposits are common in the jinshajiang ailaoshan region especially in the yidun arc which formed prior to jurassic the vms deposits in the sanjiang region formed in diverse tectonic settings including middle silurian back arc basins carboniferous oceanic islands paleozoic subduction zones and triassic post subduction rifting environments the mesozoic and early cenozoic evolution of the baoshan and tengchong blocks was largely influenced by eastward oceanic subduction of the meso and neo tethys from late permian to middle cretaceous and from late cretaceous to 50 ma respectively abundant early cretaceous granitoids and associated skarn type pb zn and sn fe deposits in the baoshan and tengchong blocks were produced in the background of the shan boundary oceanic slab subduction to the west and the break off of the nujiang bitu oceanic slab to the north the subduction of the neo tethys oceanic plate beneath the tengchong block from late cretaceous to paleogene formed abundant s type granitoids and many skarn type and greisen type sn w deposits granitoids formed at 105 to 81 ma and contemporaneous hydrothermal w mo ag and au deposits which temporally coincided with the subduction of the neo tethys are common in the yidun arc terrane

geochemical differences between subduction and collision related copper bearing porphyries and implications for metallogenesis abstract porphyry cu mo au deposits occur not only in continental margin arc settings subduction related porphyry cu deposits such as those along the eastern pacific rim eprim but also in continent continent collisional orogenic belts collision related porphyry cu deposits such as those in southern tibet these cu mineralized porphyries which develop in contrasting tectonic settings are characterized by some different trace element e g th and y concentrations and their ratios e g sr y and la yb suggesting that their source magmas probably developed by different processes subduction related porphyry cu mineralization on the eprim is associated with intermediate to felsic calc alkaline magmas derived from primitive basaltic magmas that pooled beneath the lower crust and underwent melting assimilation storage and homogenization mash whereas k enriched collision related porphyry cu mineralization was associated with underplating of subduction modified basaltic materials beneath the lower crust with subsequent transformation into amphibolites and eclogite amphibolites and resulted from partial melting of the newly formed thickened lower crust these different processes led to the collision related porphyry cu deposits associated with adakitic magmas enriched by the addition of melts and the subduction related porphyry cu deposits associated with magmas comprising all compositions between normal arc rocks and adakitic rocks all of which were associated with fluid dominated enrichment process in subduction related cu porphyry magmas the oxidation state fo2 the concentrations of chalcophile metals and other volatiles e g s and cl and the abundance of water were directly controlled by the composition of the primary arc basaltic magma in contrast the high cu concentrations and fo2 values of collision related cu porphyry magmas were indirectly derived from subduction modified magmas and the large amount of water and other volatiles in these magmas were controlled in part by partial melting of amphibolite derived from arc basalts that were underplated beneath the lower crust and in part by the contribution from the rising potassic and ultrapotassic magmas both subduction and collision related porphyries are enriched in potassium and were associated with crustal thickening their high k2o contents were primarily as a result of the inheritance of enriched mantle components and or mixing with contemporaneous ultrapotassic magmas

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

hf isotope systematics in granitoids from the central and southern alps first initial hf isotopic compositions for samples from the alpine domain are presented and discussed the results are mainly based on zircons and a few whole rocks with ages between 30 and 450 ma of those so far analyzed the present day hf isotopic compositions of zircons from non metamorphic and metamorphic granitoid rocks vary between 0 2824 and 0 2829 zircon populations with concordant u pb ages have much higher initial 176hf 177hf than inversely discordant populations which have been contaminated with older zircons containing less radiogenic hf correlated nd hf crustal residence ages have been found involving model parameters of ehf end f lu hf f sm nd 1 6 for the depleted mantle and f lu hf f sm nd 1 2 for elemental fractionations in the crust the model implies 176lu 177hf of 0 017 for the bulk crust it is suggested that the granitoid rocks are the result of mixing of subcontinental mantle derived magmas with 1 7 ga old recycled and partially molten crustal material the continental mantle component mass ratio values for the granitoids range between 0 3 and 2

mesozoic cooling across the yidun arc central eastern tibetan plateau a reconnaissance 40ar 39ar study abstract 40 ar 39 ar dating of hydrous minerals from igneous and metamorphic rocks was undertaken with the aim of characterizing the thermal history of the yidun arc eastern tibetan plateau regional metamorphism of palaeozoic rocks of the western yidun arc occurred during the early triassic collision with the qiangtang block along the jinsha jiang suture hornblende muscovite and biotite 40 ar 39 ar ages from syn tectonic igneous rocks within this collision zone suggest that the region had cooled to below 300 c by the late triassic muscovite samples from mica schists within this collision zone yield variably discordant age spectra with generally triassic 40 ar 39 ar ages consistent with cooling of the region to below muscovite closure temperatures by the close of the triassic hornblende and biotite 40 ar 39 ar ages from granite and granodiorite plutons in the eastern yidun arc suggest post emplacement cooling of these plutons occurred rapidly consistent with these granites being high level intrusions in sum these results suggest that by the beginning of the jurassic the yidun arc as a whole had cooled to below 300 c two cretaceous granites in the central yidun arc also experienced rapid cooling suggesting they too were emplaced at shallow crustal depths post triassic deformation of the region must have occurred at or below the brittle ductile transition within the crustal levels now exposed

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

petrogenesis of late cretaceous i type granites in the southern yidun terrane new constraints on the late mesozoic tectonic evolution of the eastern tibetan plateau abstract the collision between the lhasa and qiangtang terranes prior to the indo asian collision is a critical aspect in terms of development of the tibetan plateau it has been demonstrated that the occurrence of the late cretaceous granites 110 80 ma in the yidun terrane eastern tibetan plateau etp associates with the lhasa qiangtang collision the xiuwacu late cretaceous pluton in the southern yidun terrane consists three phases including biotite granitic porphyry phase 1 monzogranite phase 2 and alkali feldspar leucogranite phase 3 which have zircon u pb ages ranging from 85 5 ma to 84 4 ma all these three phases are metaluminous or slightly peraluminous granites a cnk 0 96 1 07 with high sio2 70 0 76 0 wt k2o na2o 7 5 10 7 wt and ga al 2 5 4 7 and relatively low cao 0 39 1 67 wt mgo 0 01 0 57 wt and p2o5 0 01 0 17 wt the granites are enriched in light rare earth elements lrees rb th u and ta but depleted in heavy rees hrees ba sr p and ti with significantly negative eu anomalies eu eu 0 24 0 59 comparing to classic a type granites these samples present higher sr 10 1 256 ppm mostly 100 ppm and lower feo mgo ratios 1 2 9 9 and zr nb ce y 248 483 ppm mostly

giant versus small porphyry copper deposits of cenozoic age in northern chile adakitic versus normal calc alkaline magmatism cenozoic magmatic activity in northern chile led to the formation of two contrasting porphyry copper belts 1 a paleocene early eocene belt comprising small porphyry copper deposits e g lomas bayas of normal calc alkaline affinity and 2 a late eocene early oligocene belt hosting huge porphyry copper deposits e g chuquicamata of adakitic affinity although the first belt comprises both volcanic and plutonic rocks andesitic basaltic and rhyolitic lavas and tuffs and associated sub volcanic porphyries and felsic stocks the latter only includes intrusions mostly granodioritic types including porphyry copper deposits we suggest that the late eocene early oligocene belt formed when fast and oblique convergence between the south america and farallon plates led to flat subduction and direct melting of the subducting plate hence giving rise to plutonic rocks of adakitic affinity the absence of volcanism under prevailing compressional conditions prevented the escape of so2 from the adakitic sulfur rich highly oxidized magmas closed porphyry system which allowed formation of huge mineral deposits on the contrary coeval volcanic activity during formation of the paleocene early eocene calc alkaline porphyries allowed development of open systems hence to outgassing and therefore to small mineral deposits

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

geochemistry of miocene trachytes in bugasi lhasa block tibetan plateau mixing products between mantle and crust derived melts abstract it is generally believed that cenozoic potassic and ultrapotassic volcanic rocks of the tibetan plateau were generated by partial melting of an enriched mantle region or lower crustal materials the miocene bugasi volcanic rocks bvr in the western part of the lhasa block are composed mainly of trachyandesites and trachytes both of which belong to the shoshonite series the trachytes show somewhat transitional compositions between the mantle derived trachyandesites of the bvr and the crust derived potassic rocks of konglongxiang most evident in their sr ba and ni concentrations nb ta rb sr th nb zr nb and ba rb trace element ratios and sr and nd isotopic compositions these features coupled with the relatively high cr and ni concentrations and mg suggest that the trachytes are the product of mixing between mantle derived and lower crust derived melts

late triassic granitoids of the eastern margin of the tibetan plateau geochronology petrogenesis and implications for tectonic evolution abstract late triassic granitoids in the songpan garze fold belt sgfb on the eastern margin of the tibetan plateau formed at 230 to 220 ma and can be divided into two groups group 1 are high k calc alkaline rocks with adakitic affinities k adakites with sr 400 ppm y n 32 105 and high k 2 o na 2 o 1 group 2 are ordinary high k calc alkaline i types with lower sr 18 ppm and weakly fractionated ree patterns la yb n 0 50 to 0 94 and initial 87 sr 86 sr 0 70528 to 0 71086 but group 1 rocks have higher e nd t 1 01 to 4 84 than group 2 3 11 to 6 71 calculated initial pb isotope ratios for both groups are 206 pb 204 pb 18 343 to 18 627 207 pb 204 pb 15 610 to 15 705 and 208 pb 204 pb 38 269 to 3759 group 1 magmas were derived through partial melting of thickened and then delaminated ttg type eclogitic lower crust with some contribution from juvenile enriched mantle melts group 2 magmas were generated by partial melting of shallower lower crustal rocks the inferred magma sources of both groups suggest that the basement of the sgfb was similar to the exposed kangding complex and that the sgfb was formed in a similar manner to the south china basement here passive margin crust was greatly thickened and then delaminated all within a very short time interval 20 myr such post collisional crustal thickening could be the tectonic setting for the generation of many adakitic magmas especially where there is no spatial and temporal association with subduction

mesozoic plutons of the yidun arc sw china u pb geochronology and hf isotopic signature abstract the yidun arc is a triassic volcanic arc located between the songpan garze fold belt and the qiangtang block southwest china to constrain the age of a number of the major granitic plutons from the yidun arc laser ablation icp ms u pb analysis of zircon was conducted hafnium isotope data was also acquired through laser ablation multicollector icpms analysis of zircon with the aim of gaining insight into the age and nature of the source region of the plutons three age groups have been identified from seven granite samples early middle triassic 245 to 229 ma late triassic 219 to 216 ma and cretaceous 105 to 95 ma hafnium analysis shows the triassic granites to have negative and variable e hf values and mesoproterozoic 1 6 ga depleted mantle model ages which is interpreted to reflect derivation from an isotopically heterogeneous largely crustal source the cretaceous granite shows higher and less variable e hf values and slightly younger model ages 1 3 ga and is interpreted to be derived from melting of a more homogeneous crustal source a depleted mantle model age of 1 5 ga is calculated from the pooled triassic and cretaceous samples the source region for these magmas may be tentatively correlated with mesoproterozoic material of the yangtze craton which has been suggested to underlie the yidun arc however further work is necessary to demonstrate this suggestion