geochemical characteristics and magma fertility for the jurassic arc rocks in the gangdese belt tibet abstract jurassic arc magmatic rocks are widely distributed along the gangdese belt southern tibet however only one jurassic arc related porphyry cu deposit the xietongmen deposit has been found the jurassic arc magma fertility needs to be further constrained in order to understand the rarity of arc related porphyry cu deposits in this belt this paper seeks to combine new data on the jurassic granitoid rocks with all published data on the jurassic magmatic rocks in the gangdese belt to comprehensively explore their geochemical variation and magma fertility our new data show that the jurassic granitoid rocks have zircon u pb ages of 171 6 1 1 ma to 195 3 1 3 ma these jurassic granitoids are calc alkaline series with sio2 contents of 59 4 to 70 7 wt mg values of 35 to 53 and a cnk values of 0 72 1 25 mainly 20 v sc ratios 8 and high zircon eun eun 0 3 ce4 ce3 ratios 300 and low titanium in zircon temperatures

evidence for tibetan plateau uplift before 14 myr ago from a new minimum age for east west extension important changes in south asian climate occurred in the late miocene epoch 8 myr ago 1 2 and these have been attributed by some researchers to uplift of the tibetan plateau at about the same time3 5 unfortunately this link has been difficult to test because the timing of plateau uplift remains poorly constrained by independent evidence one way to determine the minimum age of uplift is to establish the initiation age of the north striking normal fault systems in southern tibet that are widely regarded6 10 as being related to gravitational collapse of the tibetan plateau here we report an40ar 39ar age of 14 myr for hydrothermal mica from an extensional fracture belonging to such a fault system in north central nepal this age implies that east west extension began before 14 myr ago in at least some parts of the tibetan plateau suggesting that the plateau attained its high mean elevation well before late miocene time

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

formation of al rich titanite catisio4o caalsio4oh reaction rims on ilmenite in metamorphic rocks as a function of fh2o and fo2 abstract reaction rims of titanite on ilmenite are described in samples from four terranes of amphibolite facies metapelites and amphibolites namely the tamil nadu area southern india the val strona area of the ivrea verbano zone northern italy the bamble sector southern norway and the northwestern austroalpine otztal complex the titanite rims and hence the stability of titanite catisio4o and al oh titanite i e vuaganatite hypothetical end member caalsio4oh are discussed in the light of fh2o and fo2 buffered equilibria involving clinopyroxene amphibole biotite ilmenite magnetite and quartz in the systems cao feo fe2o3 tio2 sio2 h2o o2 cftsh and cao feo fe2o3 al2o3 sio2 h2o o2 cfash present in each of the examples textural evidence suggests that titanite reaction rims on ilmenite in rocks from tamil nadu val strona and the bamble sector originated most likely due to hydration reactions such as clinopyroxene ilmenite quartz h2o amphibole titanite and oxidation reactions such as amphibole ilmenite o2 titanite magnetite quartz h2o during amphibolite facies metamorphism or as in the case of the otztal complex during a subsequent greenschist facies overprint overstepping of these reactions requires fh2o and fo2 to be high for titanite formation which is also in accordance with equilibria involving al oh titanite this study shows that in addition to p t bulk rock composition and composition of the coexisting fluid fo2 and fh2o also play an important role in the formation of al bearing titanite during amphibolite and greenschist facies metamorphism

u pb and re os geochronological evidence for the jurassic porphyry metallogenic event of the xiongcun district in the gangdese porphyry copper belt southern tibet prc abstract two large copper gold porphyry deposits no i and no ii were discovered in the last decade within the xiongcun district which is located along the south margin of the gangdese porphyry copper belt these two deposits are hosted by two porphyries that were emplaced into the early jurassic volcano sedimentary rock sequences of the xiongcun formation the porphyries are both quartz diorite bodies with one of these distinguished by its large quartz eyes our new molybdenite re os and zircon u pb dates for no i and no ii deposits and host intrusions reveal the presence of two middle jurassic ore forming episodes in the xiongcun district the no ii deposit is related to the 181 175 ma quartz diorite porphyry and formed ca 172 6 2 1 ma the no i deposit is related to the 167 161 ma quartz diorite porphyry with large quartz eyes and formed ca 161 5 2 7 ma geochronological and geochemical features of the jurassic ore bearing porphyries in the xiongcun district indicate that the porphyry copper gold mineralization formed in an island arc setting which is related to the northward subduction of the neo tethys oceanic plate these observations in combination with geochronological data reported at gangdese porphyry copper belt indicated four major metallogenic events existed in the gangdese porphyry copper belt at 172 161 ma 65 48 ma 40 23 ma and 20 12 ma we have identified two mineralization episodes comprise middle jurassic metallogenic event 172 161 ma in the xiongcun district which provides new constrains for the middle jurassic metallogenic event in the gangdese porphyry copper belt

relative oxidation states of magmas inferred from ce iv ce iii in zircon application to porphyry copper deposits of northern chile major and trace element compositions of zircons and whole rocks from 14 barren and seven ore bearing calc alkaline intrusions from the chuquicamata el abra porphyry copper belt of northern chile have been measured in situ by excimer laser ablation ela icp ms these data permit the ce iv ce iii ratio within zircon to be calculated using a lattice strain model for mineral melt partitioning of ce iv and ce iii zircon ce iv ce iii and eun eun ratios and by inference magmatic oxidation states generally increase from older mafic to younger felsic units within this sequence porphyry copper mineralization is directly associated only with intrusions with zircon ce iv ce iii 300 and eun eun 0 4 such trends can be understood in terms of interdependent relations between oxygen fugacity sulfur speciation and solubility and chalcophile element partitioning in silicate magmas because zircon occurs in most calc alkaline intrusions and is resistant to subsolidus alteration zircon ce iv ce iii ratios provide a useful tool for evaluating the economic potential of such rocks for magmatic hydrothermal cu au mineralization the approach is general and may provide a means to infer relative oxidation state in a wide range of intermediate to felsic igneous rocks

the birth of the xigaze forearc basin in southern tibet abstract the stratigraphic succession of a forearc basin provides crucial information on the history of a convergent plate margin in particular it helps to establish the origin of the underlying ophiolites and to unravel the earliest evolutionary stage of arc trench systems which remain poorly understood the xigaze forearc basin in southern tibet is one of the best examples of a fossil forearc basin this study illustrates detailed stratigraphic and high precision sims u pb zircon geochronological and hf isotopic data from the chongdui formation representing the very base of the xigaze forearc basin succession and reconstructs when and how the basin was formed the chongdui formation includes tuffaceous chert and siliceous mudrocks deposited directly on top of pillow basalts of the xigaze ophiolite and conformably overlain by volcaniclastic turbidites tuff layers are interbedded throughout the unit and their u pb zircon ages range from 119 to 113 ma in the lower member and from 113 to 110 ma in the upper member broadly consistent with the established radiolarian biostratigraphy u pb ages and hf isotope signatures of zircons contained in both tuff layers and turbiditic sandstones indicate clear affinity with magmatic rocks of the lhasa terrane direct depositional and chronostratigraphic relationship with the underlying oceanic crust dated between 131 and 124 ma proves that the xigaze ophiolite is the basement of the xigaze forearc basin after an initial prolonged stage of starved siliceous sedimentation influx of terrigenous detritus began at 113 110 ma reflecting the onset of topographic growth and erosion of the lhasa terrane in response to intense magmatic activity formation of the ophiolitic basement during the early stage of subduction and the subsequent topographic growth of the arc source induced by subduction related magmatism are thus two critical factors for the birth of the xigaze forearc basin similar stratigraphies were identified in the great valley and luzon central valley forearc basins suggesting that the initial geodynamic evolution of the xigaze forearc basin may be common to many other forearc basins worldwide

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

a genetic linkage between subduction and collision related porphyry cu deposits in continental collision zones the genesis of continental collision related porphyry cu deposits pcds remains controversial the most common hypothesis links their genesis with magmas derived from subduction modified arc lithosphere however it is unclear whether a genetic linkage exists between collision and subduction related pcds here we studied jurassic subduction related cu au and miocene collision related cu mo porphyry deposits in south tibet the jurassic pcds occur only in the western segment of the jurassic arc which has depleted mantle like isotopic compositions e g 87 sr 86 sr i 0 7041 0 7048 e nd t as high as 7 5 and e hf t as high as 18 by contrast no jurassic pcds have been found in the eastern arc segment which is isotopically less juvenile e g 87 sr 86 sr i 0 7041 0 7063 e nd t 4 5 and e hf t 12 these results imply that incorporation of crustal components during underplating of jurassic magma induced copper accumulation as sulfides at the base of the eastern jurassic arc inhibiting pcd formation at this time miocene pcds are spatially confined to the jurassic arc and the giant miocene pcds cluster in its eastern segment where no jurassic pcds occur this suggests that the arc segment barren for subduction related pcds could be fertile for collision related pcds miocene ore forming porphyries have young hf model ages and sr nd hf isotopic compositions overlapping with those of the jurassic rocks in the eastern segment whereas contemporaneous barren porphyries outside the jurassic arc have abundant zircon inheritance and crustlike sr nd hf isotopic compositions these data suggest that remelting of the lower crustal sulfide bearing cu rich jurassic cumulates triggered by cenozoic crustal thickening and or subsequent slab break off led to the formation of giant miocene pcds the spatial overlap and complementary metal endowment between subduction and collision related magmas may be used to evaluate the mineral potential for such deposits in other orogenic belts

comparison of source rock geochemistry of selected rocks from the schei point group and ringnes formation sverdrup basin arctic canada abstract organic rich from the schei point group middle to late triassic in age and the ringnes formation late jurassic from the sverdrup basin of the canadian arctic archipelago have been geochemically evaluated for source rock characterization most samples from the schei point group are organic rich 2 toc and are considered as immature to mature oil prone source rocks kerogen types i i ii iia and ii iia these kerogen types contain abundant aom1 aom2 and alginite tasmanales nostocopsis leiosphaeridia acritarch and dinoflagellate with variable amounts of vitrinite inertinite and exinite samples from the ringnes formation contain dominant vitrinite and inertinite with partially oxidized aom2 alginite and exinite forming mostly immature to mature condensate and gas prone source rocks kerogen type ii iii iib iii and a few ii iia schei point samples contain higher bitumen extract saturate hydrocarbons and saturate aromatic ratio than the ringnes samples triterpane and sterane dominant c 30 distribution patterns and stable carbon isotope of bitumen and kerogen suggest that the analyzed samples from the schei point group are at the onset of oil generation and contain a mixture of sapropelic algal and minor terrestrial humic organic matter sterane carbon number distributions in the ringnes formation also suggest a mixed algal and terrestrial organic matter type there are some variations in hopane carbon number distributions but these are apparently a function of thermal maturity rather than significant genetic differences among samples pyrolysis gas chromatography mass spectrometry of the two samples with similar maturity shows that the schei point sample generates three times more pyrolyzate than the ringnes sample both samples have a dominant aliphatic character although the ringnes sample contains phenol and an aromaticity that is higher than that of the schei point sample

zircon u pb and hf isotope constraints on the mesozoic tectonics and crustal evolution of southern tibet the first in situ hf and u pb isotope analyses of zircon separates from mesozoic granites in southern tibet identify a significant previously unknown stage of magmatism igneous zircons n 34 from a granite within the gangdese batholith show a weighted mean 206 pb 238 u age of 188 1 1 4 ma and e hf t the parts in 10 4 deviation of initial hf isotope ratios between the zircon sample and the chondritic reservoir values between 10 4 and 16 8 suggesting predominantly early jurassic intrusive activity with a juvenile mantle contribution of 40 inherited zircons from two cretaceous s type granites in the northern magmatic belt 23 delineate a slightly older 206 pb 238 u age cluster between 188 and 210 ma these zircons have e hf t values from 3 9 to 13 7 yielding crustal hf model ages from ca 1 4 to 2 1 ga suggesting a major episode of crustal growth in proterozoic time and remelting of this crust in early jurassic time combining these with literature data we interpret the jurassic gangdese magmatism as an early product of the neo tethyan subduction that played a long lasting role in the tectonic evolution of southern tibet prior to the india asia collision

jurassic u pb and re os ages for the newly discovered xietongmen cu au porphyry district tibet prc implications for metallogenic epochs in the southern gangdese belt the gangdese belt in southern tibet includes the cretaceous to early eocene gangdese continental magmatic arc a newly recognized middle jurassic magmatic arc and post gangdese arc magmatism of miocene age that is well known for its association with important copper molybdenum deposits although the tectonic history of the gangdese continental arc is relatively well constrained the tectonomagmatic setting of the jurassic and miocene magmatic events was poorly constrained previously geochronological analysis of samples from the xietongmen copper gold porphyry district has identified a previously unknown metallogenic event related to middle jurassic arc magmatism the occurrence of several metallogenic epochs within the southern gangdese belt highlights the importance of systematic metallogenic studies of this belt

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

petrogenesis of early to middle jurassic granitoid rocks from the gangdese belt southern tibet implications for early history of the neo tethys abstract the gangdese belt tibet records the opening and closure of the neo tethyan ocean and the resultant collision between the indian and eurasian plates mesozoic magmatic rocks generated through subduction of the tethyan oceanic slab constitute the main component of the gangdese belt and play a crucial role in understanding the formation and evolution of the neo tethyan tectonic realm u pb and lu hf isotopic data for tonalite and granodiorite from the xietongmen nymo segment of the gangdese belt indicate a significant pulse of jurassic magmatism from 184 ma to 168 ma the magmatic rocks belong to metaluminous medium k calc alkaline series characterized by regular variation in major element compositions with sio2 of 61 35 73 59 low to moderate mgo 0 31 2 59 with mg of 37 45 these magmatic rocks are also characterized by lree enrichment with concave upward trend in mree on the chondrite normalized ree patterns and also lile enrichment and depletion in nb ta and ti in the primitive mantle normalized spidergrams these rocks have high zircon ehf t values of 10 94 to 15 91 and young two stage depleted mantle model ages tdm2 of 192 ma to 670 ma the low mgo contents and relatively depleted hf isotope compositions suggest that the granitoid rocks were derived from the partial melting of the juvenile basaltic lower crust with minor mantle materials injected in combination with the published data it is suggested that northward subduction of the neo tethyan slab beneath the lhasa terrane began by the late triassic which formed a major belt of arc related magmatism

formation of the cretaceous skarn cu au deposits of the southern gangdese belt tibet case studies of the kelu and sangbujiala deposits abstract mesozoic subduction of neo tethyan oceanic crust caused magmatism that formed numerous jurassic and cretaceous intrusions within the southern gangdese belt of tibet these intrusions are associated with the formation of giant jurassic xiongcun porphyry cu deposits but the prospectivity of other cretaceous intrusions in this area remains unclear this study presents new molybdenite re os ages for the kelu and sangbujiala cu au skarn deposits within the southern gangdese belt as well as new geochemical zircon u pb geochronological and sr nd hf isotopic data for the intrusions associated with these deposits these new data indicate that 1 the kelu deposit formed at ca 92 ma coeval with emplacement of the diorite and biotite granodiorite intrusions ca 92 90 ma in this area and 2 the sangbujiala deposit formed at ca 95 ma contemporaneous with emplacement of the mineralized biotite granodiorite ca 95 92 ma in this area these data suggest that the skarn cu au mineralization in this area formed as a result of cretaceous magmatism in the southern gangdese belt the intrusions associated with the kelu and sangbujiala deposits have adakitic affinities with low contents of y and heavy rare earth elements and high sr y ratios 22 4 123 these intrusions also have more primitive sr nd isotopic compositions end t 3 99 4 81 87sr 86sr i 0 7040 0 7044 lower ehf t values 8 0 13 18 and higher zircon eun eu values mean of 0 50 than those of contemporaneous but barren adakitic intrusions in this region this suggests that the mineralization related intrusions were oxidized and formed from magmas that were generated predominantly by partial melting of subducted neo tethyan slab material without any significant crustal contamination or assimilation of sedimentary material however these intrusions contain elevated th ni and cr contents and have high mg ratios 43 4 56 9 suggesting that they formed from magmas that interacted with metasomatized mantle wedge peridotite material combining our new data with the results of previous research suggests that the southern gangdese belt records two stages of cu au mineralization related to neo tethyan subduction the first stage generated the large xiongcun porphyry cu au deposit and was associated with normal subduction during the jurassic whereas the second stage was associated with the formation of the kelu and sangbujiala skarn deposits during cretaceous slab rollback

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

time limit for formation of main geological bodies in xiongcun copper gold deposit xietongmen county tibet evidence from zircon u pb ages and re os age of molybdenite the xiongcun copper gold deposit is a superlarge copper and gold deposit the main mineralization occurs in quartz augen diorite porphyry and exo contact zone of strongly altered fine grained tuff and is composed of veinlet disseminations in the porphyry and veinlet disseminations and veins in the tuff the genetic types of the ore deposit include porphyry ore type epithermal ore type sedex type fracture controlled altered rock type and structure altered rock type much controversy still exists concerning the rock forming and ore forming age based on zircon dating of ore bearing rocks intrusions and penetrating ore bodies as well as detailed geological mapping the authors carried out chronologic studies of ore bearing wall rocks of no and ore bodies the oldest hornblende quartz diorite porphyry j2 1 quartz augen diorite porphyry j2 biotite granodiorite e2 lamprophyre and molybdenite of no ore body according to the results of zircon u pb isotopic dating the weighted average age of 9 analytical data from ore bearing tuff zk5036 68 m is 175 5 ma with mswd being 0 63 the weighted average age of 16 analytical data from quartz augen diorite porphyry zk5036 303 m is 173 3 ma with mswd being 1 16 and the weighted average age of 14 analytical data from diorite granodiorite zk6187 335 m is 46 5 1 1 ma with mswd being 0 83 all the zircons have characteristics of magmatic origin zircon u pb isotopic dating data suggest that the ore bearing wall rocks were formed in early middle jurassic the mineralized intrusions were formed in middle jurassic and the batholiths were formed in eocene the dating results of four molybdenite re os isotope samples from xiongcun no ore body show that the content of 187re is 1 269 1 354 g g and the content of 187os is 3 587 3 993 ng g model ages of molybdenite are around 169 5 176 8 ma and model ages are consistent within the error range mswd 5 6 implying that mineralization took place in middle jurassic ar ar dating of the magmatic body and dyke show that 39ar 40ar plateau age of biotite from lamprophyre is 49 59 0 58 ma and 39ar 40ar plateau age of biotite from biotite granodiorite is 46 96 0 42 ma combined with the results obtained by other researchers it is thought that retrograde metamorphism is the cause for the ar ar dating of mica minerals around 46 50 ma the xiongcun copper gold deposit is an island arc porphyry copper gold deposit as evidenced by the study of petrologic and geochemical characteristics and the analysis of ore bearing features ore mineral assemblage element assemblages and chronology the discovery exploration and chronologic studies of the xiongcun island arc porphyry copper gold deposit have provided the information concerning the tethys subduction ore forming stage and also widened the new prospecting direction in the gangdese metallogenic belt of tibet

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

asynchronous formation of the adjacent epithermal au cu and porphyry cu mo deposits in the zijinshan orefield southeast china abstract adjacent high sulfidation epithermal hse and porphyry deposits often have similar cu au metal association suggesting they are cogenetic and formed contemporaneously the zijinshan orefield zof is the largest au producer in china and contains a series of hse au cu and porphyry cu mo deposits this study presents new 1 muscovite 40ar 39ar ages for the hse deposit in this area new 2 zircon laser ablation inductively coupled plasma mass spectrometry la icp ms u pb ages for felsic igneous rocks associated with the epithermal and porphyry mineralization and 3 new zircon lu hf isotopic data for these intrusions all of which provides insights into why these adjacent epithermal au cu and porphyry cu mo deposits have different metal associations the age data indicate that the alunite dickite alteration related to the hse au cu mineralization formed before ca 110 ma and the main hse au cu mineralizing event occurred at ca 113 ma in comparison the adjacent porphyry cu mo mineralization formed at ca 104 ma ca 9 myr after the hse event the volcanic rocks associated with the hse au cu mineralization yield zircon ehf t values from 4 1 to 0 3 whereas the cu mo mineralized porphyry has lower zircon ehf t values 4 5 to 1 2 suggesting that the magmas that formed the latter intrusion were derived from a source containing more crustal material than the magmas associated with the hse mineralization the differences in the timing of mineralization and the sources of the magmas indicate that the adjacent hse and porphyry deposits in the zof formed from different magmatic hydrothermal systems that controlled the different metal associations of the mineralizing systems this means that future mineral exploration within the zof should target porphyry au cu deposit at depths that is genetically related to the hse au cu deposit the results also show that not all adjacent porphyry and epithermal deposits especially those with different metal associations are cogenetic

tectono magmatic evolution of late jurassic to early cretaceous granitoids in the west central lhasa subterrane tibet abstract there is ongoing debate as to the subduction direction of the bangong nujiang ocean during the mesozoic northward southward or bidirectional subduction arc related intermediate to felsic intrusions could mark the location of the subduction zone and more importantly elucidate the dominant geodynamic processes we report whole rock geochemical and zircon u pb and hf isotopic data for granitoids from the west central lhasa subterrane e80 to e86 all rocks show metaluminous to peraluminous calc alkaline signatures with strong depletion of nb ta and ti enrichment of large ion lithophile elements e g cs rb k a negative correlation between sio 2 and p 2 o 5 and a positive correlation between rb and th all these features are indicative of i type arc magmatism new zircon u pb results together with data from the literature indicate continuous magmatism from the late jurassic to the early cretaceous 160 to 130 ma zircon u pb ages for samples from the northern part of the west central lhasa subterrane e80 to e82 30 yielded formation ages of 165 to 150 ma whereas ages of 142 to 130 ma were obtained on samples from the south this suggests flat or low angle subduction of the bangong nujiang ocean consistent with a slight southward decrease in zircon ehf t values for late jurassic rocks considering the crustal shortening the distance from the bangong nujiang suture zone and a typical subduction zone melting depth of 100 km the subduction angle was less than 14 for late jurassic magmatism in the central lhasa interior consistent with flat or low angle subduction compared with late jurassic rocks main ehf t values of 16 to 7 early cretaceous rocks 145 to 130 ma show markedly higher ehf t values mainly 8 to 0 possibly indicating slab roll back likely caused by slab foundering or break off combined with previously published works on arc magmatism in the central lhasa and west part of the southern qiangtang subterranes our results support the bidirectional subduction of the bangong nujiang ocean along the bangong nujiang suture zone and indicates flat or low angle southward subduction 165 to 145 ma followed by slab roll back 145 to 130 ma

sequence and petrogenesis of the jurassic volcanic rocks yeba formation in the gangdese arc southern tibet implications for the neo tethyan subduction abstract the yeba formation volcanic rocks in the gangdese arc recorded important information regarding the early history of the neo tethyan subduction to explore their magmatic evolution and tectonic significance we performed a systematic petrological geochronological and geochemical study on these volcanic rocks our data indicated that the yeba formation documents a transition from andesite dominated volcanism which started before 182 ma and continued until 176 ma to bimodal volcanism 174 168 ma in the earliest middle jurassic the early stage andesite dominated volcanics are characterized by various features of major and trace elements and are interpreted as the products of interactions between mantle derived arc magmas and lower crustal melts their positive end t and ehf t values suggest a significant contribution of asthenosphere like mantle the late stage bimodal volcanism is dominated by felsic rocks with subordinate basalts geochemical signatures of the basalts indicate a composite magma source that included a subduction component an asthenosphere like upper mantle domain and an ancient subcontinental lithospheric mantle component the felsic rocks of the late stage were produced mainly by the melting of juvenile crust with some ancient crustal materials also involved we suggest that the occurrence and preservation of the yeba formation volcanic rocks were tied to a tectonic switch from contraction to extension in the gangdese arc which probably resulted from slab rollback of the subducting neo tethyan oceanic slab during the jurassic

zircon ce4 ce3 ratios and ages for yulong ore bearing porphyries in eastern tibet yulong ore bearing porphyries along the northwestern extension of the red river ailao shan fault system in eastern tibet consist of five porphyry deposits containing a total of more than 8 million tons of copper resources u th pb laser inductively coupled plasma mass spectrometry dating of zircon shows that the porphyries were emplaced in early tertiary 41 2 36 9 ma covering a period of 4 3 ma with formation ages decreasing systematically from northwest to southeast the start of porphyry magmatism coincided with the onset of transpressional movement along the red river ailao shan fault system implying a close link between these two events age sequence in intrusions can be plausibly explained by assuming that a region of melting in the lower northwestern plate moved southeasternward along the tuoba mangkang fault relative to the upper plate zircon grains from the yulong ore bearing porphyries have higher ce4 ce3 than those from barren porphyries in the region this suggests that the ore bearing porphyries crystallized from a relatively oxidized magma which has important implications for future ore exploration in the region and other cu deposits in convergent margin environments in general

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

the lhasa terrane record of a microcontinent and its histories of drift and growth article i nfo the lhasa terrane in southern tibet has long been accepted as the last geological block accreted to eurasia before its collision with the northward drifting indian continent in the cenozoic but its lithospheric architecture drift and growth histories and the nature of its northern suture with eurasia via the qiangtang terrane remain enigmatic using zircon in situ u pb and lu hf isotopic and bulk rock geochemical data of mesozoic early tertiary magmatic rocks sampled along four north south traverses across the lhasa terrane we show that the lhasa terrane has ancient basement rocks of proterozoic and archean ages up to 2870 ma in its centre with younger and juvenile crust phanerozoic accreted towards its both northern and southern edges this finding proves that the central lhasa subterrane was once a microcontinent this continent has survived from its long journey across the paleo tethyan ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the qiangtang terrane to the north and with the indian continent to the south zircon hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks e g 50 90 0 70 and 30 100 to the mesozoic magmatism in the southern central and northern lhasa subterranes respectively we suggest that much of the mesozoic magmatism in the lhasa terrane may be associated with the southward bangong nujiang tethyan seafloor subduction beneath the lhasa terrane which likely began in the middle permian or earlier and ceased in the late early cretaceous and that the significant changes of zircon ehf t at 113 and 52 ma record tectonomagmatic activities as a result of slab break off and related mantle melting events following the qiangtang lhasa amalgamation and india lhasa amalgamation respectively these results manifest the efficacy of zircons as a chronometer u pb dating and a geochemical tracer hf isotopes in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics

contrasting arc magma fertilities in the gangdese belt southern tibet evidence from geochemical variations of jurassic volcanic rocks abstract jurassic volcanic rocks are widely distributed along the gangdese belt of southern tibet forming a westerly extending southern belt bima formation with one large arc related porphyry cu au deposit xietongmen and an easterly extending northern belt yeba formation without any contemporary mineralization a compilation of new and published data on the bima formation is compared to published data on the yeba formation in order to understand their different relationships with mineralization the bima formation volcanic rocks comprise basalt andesite and dacite with zircon u pb ages of 195 0 ma 165 1 ma including new ages of 180 2 0 8 ma to 166 7 1 1 ma they are calc alkaline series lava flows and show enrichments in lile and lree depletions in hfse and hree which are features typical of arc rocks coherent geochemical variations and similar depleted sr nd hf isotopic characteristics between basaltic intermediate and felsic rocks new and published data 87sr 86sr i 0 703182 to 0 705489 end t 3 2 to 7 1 ehf t 9 3 to 16 0 suggest that they were formed from fractional crystallization of the basaltic magma in contrast the published yeba formation volcanic rocks although with a similar age range 192 7 ma to 168 0 ma have a wider compositional range from basalt to rhyolite and more evolved sr nd hf isotopic compositions 87sr 86sr i 0 700182 to 0 707723 end t 5 1 to 4 5 ehf t 18 5 to 17 6 new data on the bima formation andesite and dacite samples have higher zircon eu eu ce4 ce3 and ce nd y ratios and lower titanium in zircon temperatures than those from the yeba formation suggesting the bima formation magmas were more oxidized and hydrous and therefore more fertile for porphyry copper deposit formation the relatively reduced yeba formation magmas formed in a distal arc setting near the central lhasa old basement their evolved isotopic characteristics and reduced nature may have resulted from more extensive contamination by reduced crustal components rendering them infertile for porphyry copper mineralization exploration for arc related porphyry copper deposits in the gangdese belt should therefore focus on the fertile bima formation

metagabbros of the gangdese arc root south tibet implications for the growth of continental crust abstract the gangdese batholith of the southern tibet is regarded as a principal component of the pre himalayan collision andean type continental margin however the gangdese arc root remains poorly understood because most of previous studies concentrated on granitoids from the middle and upper crust of the arc in this paper we report new petrochemical and geochronological characteristics of metagabbros occurred as a part of the late cretaceous lilong batholith in the eastern gangdese arc the gabbros with protolith ages of 82 95 ma underwent slightly late 68 77 ma granulite facies metamorphism and nearly coeval anatexis typical of lower arc crusts the metagabbros and diorites of the lilong batholith show geochemical features of juvenile crust in continental magmatic arcs and an original magmatic differentiation trend with gabbros formed as the cumulates whereas diorites as the crystalline products of evolved magmas the lilong batholith forms an arc crustal section with a possible thickness of 20 30 km and may have an overall andesitic composition providing a support for the andesite model of continental crust growth

zircon compositional evidence for sulfur degassing from ore forming arc magmas porphyry cu mo au and epithermal au ag deposits are major sources of mined metals and are commonly formed by magmatic hydrothermal fluids derived from hydrous magmas in phanerozoic convergent margin settings the igneous rock assemblages associated with porphyry mineral deposits are common in modern convergent margin settings but while many have produced acidic magmatic fluids very few past or present have produced sufficient metal chlorine and sulfur enrichments necessary to engender an ore deposit the reasons for this remain uncertain we report shrimp rg ion microprobe analyses of hafnium titanium and rare earth element ree abundances in zircon a nearly ubiquitous and robust trace mineral in crustal magmas comparison of the compositions of zircons in ore forming and barren granitic plutons indicate that ore forming granites crystallized at relatively low temperature and have relatively small negative europium anomalies mostly eu n eu n 0 4 we interpret these small zircon europium anomalies to indicate oxidizing magmatic conditions and hypothesize that in many cases this reflects oxidation due to so 2 degassing from magmas with a relatively low fe s ratio oxidation of europium and iron in the melt is produced by reduction of magmatic sulfate s 6 to so 2 s 4 upon degassing this interpretation reinforces the important role of oxidized sulfur rich fluids in porphyry and epithermal mineral deposit formation zircon compositions thus may be used to identify ancient magmas that released significant amounts of so 2 rich gases and regional surveys of zircon composition are potentially a valuable tool for mineral exploration

giant porphyry deposits characteristics distribution and tectonic controls more than half of the 25 largest known porphyry copper deposits defined in terms of contained copper metal formed during three time periods the paleocene to eocene eocene to oligocene and middle miocene to pliocene these giant deposits are clustered within three provinces central chile northern chile and southwest arizona northern mexico other giant deposits occur in montana utah panama peru argentina irian jaya mongolia and iran compressive tectonic environments thickened continental crust and active uplift and erosion were associated with the formation of many of these deposits calc alkalic magmas are most favorable for the formation of giant porphyry copper deposits although several of the largest systems are associated with high k calc alkalic intrusions the 25 largest gold rich porphyry deposits are concentrated in the southwest pacific and south america with other occurrences in eurasia british columbia alaska and new south wales many of the deposits formed in the last 13 m y the largest of the deposits are associated with high k calc alkalic intrusions many calc alkalic porphyritic intrusions have also produced giant gold rich porphyries in the last 20 m y the formation of giant porphyry copper molybdenum and copper gold deposits in the circum pacific region has been closely associated with subduction of aseismic ridges seamount chains and oceanic plateaus beneath oceanic island and continental arcs in several examples these tectonic perturbations have promoted flat slab subduction crustal thickening uplift and erosion and adakitic magmatism coeval with the formation of well endowed porphyry and or epithermal mineral provinces similar tectonic features are inferred to be associated with the giant porphyry copper molybdenum provinces of northern chile eocene oligocene and southwest united states cretaceous paleocene topographic and thermal anomalies on the downgoing slab appear to act as tectonic triggers for porphyry ore formation other factors such as sutures in the overriding plate permeability architecture of the upper crust efficient processes of ore transport and deposition and in some cases formation and preservation of supergene enrichment blankets are also vital for the development of high grade giant ore deposits a low grade geochemical anomaly may be the final product of mineralization if ore forming processes do not operate efficiently even in the most favorable geodynamic settings

the different sources and petrogenesis of jurassic intrusive rocks in the southern lhasa subterrane tibet evidence from the trace element compositions of zircon apatite and titanite abstract the jurassic magmatism distributed in the southern lhasa subterrane slt has been recognized as a record of the early stage evolution of the neo tethys ocean however the magma compositions and detailed crystallization histories remain an enigma zircon apatite and titanite in igneous rocks can serve as effective petrogenetic indicators in this paper we present the whole rock geochemical zircon u pb data zircon hf isotopic analyses and major and trace elemental compositions of zircon apatite and titanite from baiga granite porphyry egp shanba granodiorite egd pengcuolin granodiorite porphyry mgp and dazhuqu monzonite lm rocks in the slt to constrain their source and petrogenesis the results from this study show that a combination of zircon ti th u hf and rare earth element ree contents apatite halogen ree and sr contents and titanite rees can be used to track the detailed magmatic crystallization histories higher sr contents are found in apatite srap from egp ca 192 ma and egd ca 176 ma than those in their host rocks srwr indicating that their magma genesis may have involved some mafic magma the magma genesis of lm ca 159 ma may have involved more reduced mafic or least evolved magma as evidenced by the higher th u but lower yb gd ratios higher zircon saturation and ti in zircon temperatures in zircon highest srap srwr ratios and large positive eu anomalies in titanite from lm the lower cl contents and higher la sm n values in apatite from mgp ca 172 ma are inherited from the low cl melts the mgp rocks have higher zircon ce4 ce3 and eun eun ratios a greater apatite so3 content and a higher titanite total ree content which indicates that mgp has a higher oxidation state than do egp egd and lm the mgp also exhibits a higher h2o content 11 wt dissolved in the melt greater ba la and nb ta ratios in whole rock and higher nb ta ratios in zircon however mgp demonstrates lower zircon saturation and ti in zircon temperatures which suggest that mgp was generated by fluid fluxed melting below 750 c the low ti in zircon temperatures but high la yb and sr y ratios of mgp indicate that a local thickened crust likely existed in the middle jurassic period in the slt due to mafic magma underplating during this time

is the present the key to the past a global characterization of modern sedimentary basins the stratigraphic record is heavily biased because it is uniquely composed of sediments that were laid down in basins whereas the majority of the present and historic land surface of the planet is composed of areas that are in net long term erosion with no preservation potential global mapping and quantification of the distribution of currently active sedimentary basins suggest that only 16 of earth s terrestrial land surface is within sedimentary basins the remainder of the land is in upland areas that will not be represented in the future rock record furthermore 60 of the modern basin area has an arid climate as opposed to 27 of the land surface tectonic classification of modern basins indicates that intracratonic and foreland basins cover the greatest area whereas forearc extensional and strike slip basins are the least represented by area while this modern snapshot does not account for differences in subsidence rate or basin longevity the mapping and quantification of modern basins highlight the incompleteness of the stratigraphic record and the importance of caution when assuming the present is the key to the past

porphyry copper gold mineralization at yulong china promoted by decreasing redox potential during magnetite alteration the yulong porphyry cu au deposit occurs on the eastern margin of the tibetan plateau and is one of the largest porphyry cu au deposits in china containing more than 6 million tons of cu metal similar to other porphyry cu deposits in the world the yulong porphyry is highly oxidized sulfate is the dominant sulfur species in fluid inclusions hosted by magmatic quartz phenocrysts and no sulfide is observed indicating an oxygen fugacity above the sso buffer during magmatic processes the sulfur species changed from sulfate dominant during magmatic processes to sulfide dominant during the main mineralization processes with sulfate and sulfide mineral assemblages observed in fluid inclusions in mineralized quartz veinlets and only sulfides in the orebodies corresponding to a decrease in redox potential from the porphyry to the hydrothermal fluid magnetite crystallization was coincident with the onset of major sulfate reduction indicating that magnetite isolated trivalent iron reducing sulfate and consequently leading to the formation of porphyry cu au mineralization most porphyry cu au ore deposits are spatially associated with and genetically related to oxidized felsic magmas our results indicate that sulfate reduction promoted by magnetite crystallization is essential for the final precipitation of cu au bearing sulfides i e decreasing redox potential of the fluid is the key to and direct cause of ore formation in yulong initial high redox potential is a prerequisite for and may be an indirect indicator of most mineralization of this kind because it enables efficient transportation of cu au redox potential fluctuations can be well preserved in zircon a resistant accessory mineral commonly found in porphyries which might be developed as a handy and reliable exploration tool for porphyry cu deposits associated with magnetite

discovery of late triassic mineralization in the gangdese metallogenic belt tibet the banduo pb zn deposit somdo area abstract the newly discovered banduo pb zn deposit in the somdo area of tibet is hosted by a late triassic granodiorite a combination of zircon u pb dating hf isotopic compositions sulfide rb sr dating and whole rock geochemistry was used to determine the age of the mineralization and characterize the metallogenesis of the deposit the weighted mean u pb ages of zircon from the granodiorite 203 8 1 7 ma mswd 0 53 206 1 0 4 ma mswd 0 098 and the weighted mean rb sr isochron age of sulfides from the pb zn mineralization 202 7 2 3 ma mswd 1 3 suggest that the timing of mineralization was close to that of granodiorite crystallization during the late triassic the average 87sr 86sr i value of 0 71026 implies that the ore forming metals were sourced from a mixture of mantle and crustal material the weakly altered granodiorite of the deposit is characterized by sio2 66 78 67 85 wt k2o 2 57 3 37 wt al2o3 13 53 15 15 wt and mg 45 5 51 3 it is enriched in lrees and liles and depleted in hrees and hfses and it exhibits a negative eu anomaly the granodiorite has positive and negative ehf t values 2 5 to 1 7 and crustal model ages tc dm of 1 14 1 40 ga hf isotopic analyses and whole rock geochemistry indicate that the late triassic granodiorite was likely sourced from of a mixture of mantle and ancient lower crust material that had undergone variable degrees of fractional crystallization based on regional data the banduo deposit is the oldest pb zn deposit in the gangdese belt and likely formed as a result of the northward subduction of the neo tethys ocean the metallogenic epoch indicates that deposits related to the northward subduction of the neo tethys oceanic crust exist in this area the late triassic age of granitoid hosted mineralization provides additional prospects for exploration in the gangdese metallogenic belt

normal faulting in central tibet since at least 13 5 myr ago tectonic models for the evolution of the tibetan plateau interpret observed east west thinning of the upper crust to be the result of either increased potential energy of elevated crust1 or geodynamic processes that may be unrelated to plateau formation2 3 4 5 6 a key piece of information needed to evaluate these models is the timing of deformation within the plateau the onset of normal faulting has been estimated to have commenced in southern tibet between about 14 myr ago7 and about 8 myr ago8 and in central tibet about 4 myr ago9 here however we report a minimum age of approximately 13 5 myr for the onset of graben formation in central tibet based on mineralization ages determined with rb sr and 40ar 39ar data that post date a major graben bounding normal fault these data along with evidence for prolonged activity of normal faulting in this and other tibetan grabens support models that relate normal faulting to processes occurring beneath the plateau thinning of the upper crust is most plausibly the result of potential energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath tibet this is supported by recent geophysical and geological data10 11 12 13 14 15 16 17 which indicate that spatial heterogeneity exists in both the tibetan crust and lithospheric mantle

estimates of internal tide energy fluxes from topex poseidon altimetry central north pacific energy fluxes for first mode m2 internal tides are deduced throughout the central north pacific ocean from topex poseidon satellite altimeter data temporally coherent internal tide signals in the altimetry combined with climatological hydrographic data determine the tidal displacements pressures and currents at depth which yield power transmission rates for a variety of reasons the deduced rates should be considered lower bounds internal tides are found to emanate from several large bathymetric structures especially the hawaiian ridge where the integrated flux amounts to about 6 gigawatts internal tides are generated at the aleutian trench near 172 w and propagate southwards nearly 2000 km

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

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

geochemical characteristics of intrusive and volcanic rocks in no ii ore body of xiongcun copper gold deposit tibet constraints on rock genesis and tectonic setting the xiongcun porphyry copper gold deposit is located in southern gangdise belt the ore deposit is related to jurassic magmatic event petrographic and geochemical studies of andesitic volcanic rock hornblende diorite porphyrite corroded quartz phenocrysts bearing quartz diorite porphyrite and late coarse grained corroded quartz phenocrysts bearing diorite porphyrite show that the tuff and intrusive rocks of no ore body are of calc alkaline series with w sio2 ranging from 61 97 to 66 57 w al2o3 ranging from 16 40 to 17 57 w mgo ranging from 1 07 to 2 89 and w tfeo ranging from 1 75 5 72 mg values vary from 19 44 37 46 28 79 on average w tio2 values are lower than 1 total ree concentrations range from 35 51 10 6 to 103 85 10 6 lan ybn ratios vary from 1 76 to 6 57 with the enrichment of lree mineralized tuff and intrusion are enriched with lile elements such as rb sr th and u and depleted in hfs elements such as nb ta ti and y with no or weak negative eu anomalies and no ce anomalous features petrographic and geochemical studies indicate that volcanic and intrusive rocks are products of tethys subduction to north lhasa tectonic body and magma originated from partial melting of the mantle