gold in porphyry copper deposits its abundance and fate abstract porphyry copper deposits are among the largest reservoirs of gold in the upper crust and are important potential sources for gold in lower temperature epithermal deposits whether gold remains in porphyry copper deposits is important both to their economic attractiveness and to the distribution of gold in the upper crust cu au atomic ratios of porphyry copper ore deposits form a continuous range from about 5000 to 5 000 000 with a median near 40 000 which separates gold rich and gold poor deposits gold is found in porphyry copper deposits in solid solution in cu fe and cu sulfides and as small grains of native gold usually along boundaries of bornite sims ion probe analyses of ore minerals from the gold rich batu hijau kingking and skouries porphyry copper deposits show that bornite contains about 1 ppm au whereas chalcopyrite contains about an order of magnitude less chalcocite and covellite contain 10 20 ppm au but are not abundant enough to account for a significant part of the gold endowment in many porphyry copper deposits the amount of gold presently in solid solution in cu fe sulfides is not adequate to account for all the gold in porphyry copper deposits and the remainder is present as micron scale grains of native gold experiments in the cu fe s au system show that bornite and chalcopyrite can contain about 1000 ppm gold at typical porphyry copper formation temperatures of 600 700 c and indicate that bornite and chalcopyrite in porphyry copper deposits were saturated with respect to gold at temperatures of only 200 300 c in contrast cu au ratios of bulk ore in porphyry copper deposits would require bornite and chalcopyrite to be saturated with respect to gold at temperatures similar to those at which primary potassic ore and alteration are thought to form this indicates that the maximum gold endowment of porphyry copper deposits is probably fixed by the amount of gold that will go into solid solution in cu fe sulfides when the deposit forms at high temperature and that gold is not commonly added later from other sources although it can be redistributed during cooling or later events the experimental data also suggest that high temperature 600 700 c vapors can extract considerably more gold from porphyry copper systems than can low temperature 300 c alteration comparison to cu au ratios of volcanic emissions suggests further that high temperature processes remove copper relative to gold from porphyry copper systems whereas low temperature processes remove gold preferentially and that this can account for deposits with extremely low and high cu au ratios respectively deposits with cu au ratios between about 20 000 and 100 000 however probably reflect different degrees of removal of gold or copper by immiscible sulfides

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

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

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

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

a seismic structure study in the kaoping area southwestern taiwan the difference between s wave and s to p wave conversion s p phase arrival times is enhanced with rectilinear motion detector filtering to describe alluvial sediment thickness in the kaohsiung pingtung kaoping plains area a more complete understanding of the underground structures of the kaoping area is provided in this paper and explains why the surrounding regions in taiwan experience more earthquakes than the kaoping area data are based on seismic activity recorded by the portable array for numerical data acquisition panda for the period from 1995 to 1997 the difference between s wave and s p phase arrival times shows that the sedimentary layer is thicker along the west and southwest coasts p wave travel time residuals high frequency attenuation parameters kappa and quality factor q p q s and coda waves confirm this result we also determined the orientation of the chaochou fault using the first motion of p wave arrivals to the east of the chaochou fault stress trends southeast northwest while to the west it trends northeast southwest the change in stress trends east and west of chaochou fault suggests the presence of a highly fluid accretionary wedge in the kaoping area the chaochou fault forms a seismically active tectonic boundary with the uplift of the hanging wall leading to westward tilting of the basement of the kaoping plains we demonstrate that these features are the reason there are relatively few earthquakes in the kaoping area the presence of a highly fluid accretionary wedge is indicated by a thick alluvial layer in the west and southwest kaoping coasts the peikung high acts as the indenter that may allow seismic energy to escape and reduce the number of earthquakes in the region online material figures illustrating calculations of kappa q c p and s wave spectra q p and q s from ground motion data

geochemical evidence for a magmatic co2 degassing event at mammoth mountain california september december 1997 recent time series soil co2 concentration data from monitoring stations in the vicinity of mammoth mountain california reveal strong evidence for a magmatic degassing event during the fall of 1997 lasting more than 2 months two sensors at horseshoe lake first recorded the episode on september 23 1997 followed 10 days later by a sensor on the north flank of mammoth mountain direct degassing from shallow intruding magma seems an implausible cause of the degassing event since the gas released at horseshoe lake continued to be cold and barren of other magmatic gases except for he we suggest that an increase in compressional strain on the area south of mammoth mountain driven by movement of major fault blocks in long valley caldera may have triggered an episode of increased degassing by squeezing additional accumulated co2 from a shallow gas reservoir to the surface along faults and other structures where it could be detected by the co2 monitoring network recharge of the gas reservoir by co2 emanating from the deep intrusions that probably triggered deep long period earthquakes may also have contributed to the degassing event the nature of co2 discharge at the soil air interface is influenced by the porous character of high sierra soils and by meteorological processes solar insolation is the primary source of energy for the earth atmosphere and plays a significant role in most diurnal processes at the earth surface data from this study suggest that external forcing due largely to local orographic winds influences the fine structure of the recorded co2 signals

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

estimates of shear wave q and κ0 for unconsolidated and semiconsolidated sediments in eastern north america measured and calculated values of the effective quality factor qef and the site attenuation parameter 0 for unconsolidated and semiconsolidated sediments in eastern north america ena indicate that the latter is strongly dependent on sediment thickness estimates of 0 for national earthquake hazard reduction program nehrp bc site profiles sediment plus hard rock in the mississippi embayment and the atlantic coastal plain were found to increase from about 9 to 31 msec for sedi ment thicknesses ranging from 116 to 600 m stochastic simulations using the 175 m thick hypothetical nehrp bc site profile used to estimate ena ground motions in the nationalseismichazardmapsbytheu s geologicalsurvey usgs indicatethat 0 20 msec provides a smaller estimate of amplification that agrees more closely with the low strain short period site coefficients in the nehrp recommended provisions for seismic regulations for new buildings and other structures nehrpprovisions than the10msecvalueusedbytheusgs alinearregressionofthe 0 estimatescompiledin this study indicates that 0 20 msec corresponds to a relatively thick bc sediment thickness of 460 116 m these same stochastic simulations indicate that the rela tivelyshallowusgssiteprofileprovidesestimatesofamplificationthataresmallerthan thelow strainlong periodsitecoefficientsinthenehrpprovisions thedependenceof bothsiteattenuationandsiteamplificationonsedimentthicknesssuggeststhattheuseof a single reference site condition for hazard mapping might not be appropriate instead theseresultsimplythateitheraregionalsetofreferencesiteprofilesshouldbedeveloped or that a more uniform site condition such as hard rock should be used to define a more stable reference site condition in ena

source spectral scaling and stress release estimates using strong motion records in greece a data set of 50 horizontal strong motion accelerograms from 18 main events from moderate and large earthquakes 5 2 mw 6 9 and distances 9 km r 116 km occurred at three different tectonic settings normal strike slip and thrust faulting were used to estimate stress parameters three different definitions of stress release were adopted the classical brune the dynamic rms root mean square and the apparent stress corner frequencies f0 based on the andrews 1986 methodology were computed and using these calculations seismic moments were estimated from independent sources teleseismic data the calculated corner fre quencies seem to be consistent based on seismic moments with previous estimates for the same area based on those source parameters a theoretical x square spectrum was derived from describing source radiation in terms of earthquake size frequency dependent site amplifications were included in the simulations separating the re cordings in three different categories b c and d corresponding to nehrp na tional earthquake hazards reduction program site classification the theoretical and observational spectra are in good agreement in a frequency range from 0 05 to 25 hz except for the cases where steep topography affects the recorded data a poor fit is also observed in cases where the distance of the recorded data is greater than 100 km and the attenuation model seems to be inadequate to describe the theoretical model the calculated parameters present a mean value of 55 16 bars for brune stress 72 26 bars for dynamic rms stress and 17 10 bars for apparent stress for all determined stresses the thrust faulting earthquakes exhibited higher values than the calculated normal and strike slip fault seismic events

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

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

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

the spectral decay parameter kappa in northeastern sonora mexico we calculated the spectral decay parameter in the southern basin and range province using records from earthquakes located near the pitaycachi fault we found that for a given distance and recording site the values of vary considerably possibly as a result of the lateral heterogeneity of the crust individual values of do not show a clear trend with magnitude nor with the average values of the average values of tend to increase with distance up to about 70 80 km and then to decrease between 80 and 100 km suggesting higher s wave attenuation at shallow depths and lower attenuation for deeper paths we analyzed the possible dependence of on earthquake size by determining empirical curves that describe the behavior of with distance and magnitude our results indicate that is independent of earthquake size within the magnitude range m 3 5 of the events analyzed the nonparametric curves also provided site specific estimates of near the surface 0 for the stations and groups of stations analyzed we found that the average 0 value of 0 04 sec in northeastern sonora is similar to values of 0 reported in other regions we conclude that 0 depends not only on the rock type but also on the degree of fracturing and erosion of the rocks near the recording site

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

a study on kappa value in taiwan using borehole and surface seismic array since the inception of the borehole seismic array deployed by the central weather bureau cwb in taiwan in recent years a large quantity of strong motion records have been accumulated from frequently occurring earthquakes around taiwan that provide the opportunity to investigate the spectral decay parameter kappa and to understand further the site effects of seismic stations in this study we used 133 earthquakes recorded at 30 borehole seismic arrays each array includes two force balance accelerometers one at the surface and the other inside the borehole based on the regression analysis between value and hypocentral distance for each surface borehole station pairs the resulting derived from surface stations are higher than results from borehole stations the estimated values are found to range from 0 032 to 0 097 s at surface and 0 012 to 0 078 s in borehole in comparison with a study of site corrections for earthquake magnitude determination these higher values associated with negative station corrections can correspond to the effect of soil conditions to evaluate station corrections of borehole seismic array we collected records from the cosite stations of taiwan strong motion instrumentation program tsmip and central weather bureau station network cwbsn and then derived a linear relationship between the values and station corrections finally both resulting station corrections and values reveal high coherence in local site conditions

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

spectral scaling of the aftershocks of the tocopilla 2007 earthquake in northern chile summary we study the scaling of spectral properties of a set of 68 aftershocks of the 2007 november 14 tocopilla m 7 8 earthquake in northern chile these are all subduction events with similar reverse faulting focal mechanism that were recorded by a homogenous network of continuously recording strong motion instruments the seismic moment and the corner frequency are obtained assuming that the aftershocks satisfy an inverse omega square spectral decay radiated energy is computed integrating the square velocity spectrum corrected for attenuation at high frequencies and for the finite bandwidth effect using a graphical approach we test the scaling of seismic spectrum and the scale invariance of the apparent stress drop with the earthquake size to test whether the tocopilla aftershocks scale with a single parameter we introduce a non dimensional number that should be constant if earthquakes are self similar for the tocopilla aftershocks cr varies by a factor of 2 more interestingly cr for the aftershocks is close to 2 the value that is expected for events that are approximately modelled by a circular crack thus in spite of obvious differences in waveforms the aftershocks of the tocopilla earthquake are self similar the main shock is different because its records contain large near field waves finally we investigate the scaling of energy release rate gc with the slip we estimated gc from our previous estimates of the source parameters assuming a simple circular crack model we find that gc values scale with the slip and are in good agreement with those found by abercrombie and rice for the northridge aftershocks

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

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

a κ model for mainland france an important parameter for the characterization of strong ground motion at high frequencies 1 hz is kappa which models a linear decay of the acceleration spectrum a f in log linear space i e a f a 0 exp f for f f e where f is frequency f e is a low frequency limit and a 0 controls the amplitude of the spectrum is a key input parameter in the stochastic method for the simulation of strong ground motion which is particularly useful for areas with insufficient strong motion data to enable the derivation of robust empirical ground motion prediction equations such as mainland france numerous studies using strong motion data from western north america wna an active tectonic region where surface rock is predominantly soft and eastern north america ena a stable continental region where surface rock is predominantly very hard have demonstrated that varies with region and surface geology with wna rock sites having a of about 0 04 s and ena rock sites having a of about 0 006 s lower s are one reason why high frequency strong ground motions in stable regions are generally higher than in active regions for the same magnitude and distance few if any estimates of s for french sites have been published therefore the purpose of this study is to estimate using data recorded by the french national strong motion network rap for various sites in different regions of mainland france for each record a value of is estimated by following the procedure developed by anderson and hough bull seismol soc am 74 1969 1993 1984 this method is based on the analysis of the s wave spectrum which has to be performed manually thus leading to some uncertainties for the three french regions where most records are available the pyrenees the alps and the cotes d azur a regional model is developed using weighted regression on the local geology soil or rock and source to site distance it is found that the studied regions have a mean between the values found for wna and ena for example for the alps region a value of 0 0254 s is found for rock sites an estimate reasonably consistent with previous studies

collision related genesis of the sharang porphyry molybdenum deposit tibet evidence from zircon u pb ages re os ages and lu hf isotopes article i nfo sharang is a low fluorine calc alkaline porphyry mo deposit hosted mainly in a granite porphyry of a multi stage plutonic complex in the northern gangdese metallogenic belt largely with stockwork and ribbon textured mineralization the observed age estimates suggest that the formation of the magmatic host complex 52 9 51 6 ma and the ore deposit itself 52 3 ma occurred during the main stage of the india asia collision the host rocks are characterized by lower zircon ehf t values than those of the pre ore and post ore rocks this suggests that the lhasa terrane basement might play an important role in the formation of sharang ore forming intrusions in view of the framework of magmatic metallogenic events we suggest that slab roll back may have induced melting of juvenile crust and ancient continental complexes during the india asia collision this proposal focuses exploration for additional molybdenum deposits on the collision zone

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

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

a comprehensive relocation of earthquakes in taiwan from 1991 to 2005 we have carried out a comprehensive relocation of a total of 267 210 earthquakes in taiwan that occurred during the past 15 yr we based our relocation process on the earthquake catalog of the taiwan central weather bureau seismic network cwbsn and made improvements in three aspects first we incorporated a large dataset of the s p times from 680 taiwan strong motion instrumentation pro gram tsmip stations distributed throughout the island of taiwan to improve the coverage of earthquakes on the island secondly we added 18 japan meteorological agency jma stations in the southern ryukyu island chain to enhance the station coverage for eastern offshore events especially around the subduction zone northeast of taiwan thirdly we adopted 3d vp and vp vs models in predicting the travel times of p and s waves the effectiveness of these improvements in earthquake re location can be seen in three aspects 1 the reduction in the residuals of p wave arrival times and s p times 2 a better understanding of the attenuation relationship between the peak ground acceleration and epicentral distance and 3 the geologically meaningful patterns of station corrections to p wave arrival times and s p times online material catalog of relocated earthquakes in taiwan from january 1991 to

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

water in the lower crustal granulite xenoliths from nushan eastern china 1 nominally anhydrous clinopyroxene cpx orthopyroxene opx and plagioclase pl from 10 lower crustal granulite two pyroxene granulite and hypersthene granulite xenoliths in cenozoic basalts from the nushan volcano eastern china have been analyzed for their hydrogen content by microscopic fourier transform infrared spectroscopy micro ftir the results demonstrate that hydrogen was incorporated in all these minerals in the manner of oh and that the content h2o weight is up to 2360 ppm for cpx 1170 ppm for opx and 880 ppm for pl on the basis of the water content of constitutive minerals and their proportions whole rock water contents of the nushan granulites were estimated to be 150 950 ppm estimated equilibrium temperatures of the nushan granulites are in the range of 810 892 c corresponding to the lowermost crust at nushan about 25 30 km therefore this study provides direct evidence that the lower continental crust even the lowermost part devoid of hydrous minerals can contain a certain amount of water in nominally anhydrous minerals

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

early eocene crustal thickening in southern tibet new age and geochemical constraints from the gangdese batholith abstract the lhasa terrane southern tibet is where the thickest part of the earth s crust occurs as a result of the india asia collision this paper reports zircon u pb age and geochemical data of samples collected from the gangdese batholith southern lhasa terrane a subset of which has already been dated at 65 41 ma using zircon u pb geochronology ji w q wu f y chung s l li j x liu c z 2009a zircon u pb chronology and hf isotopic constraints on the petrogenesis of gangdese batholiths southern tibet chemical geology 262 229 245 to constrain the timing and mechanism s responsible for crustal thickening beneath southern tibet our new data indicate that the gangdese magmatism lasted continuously from ca 65 to 34 ma during which geochemical evidence for crustal thickening is observed crustal thickening is best illustrated by the progressive increase in la yb and sr y of the early to latest eocene magmatic rocks associated with the decrease in hf isotopic ratios of paleocene to eocene magmatic zircons we also identify in this study the oldest ca 51 ma post collisional adakitic granitoids in the gangdese batholith interpreted as products from partial melting of underplated basaltic lower crust therefore a thickened crust likely existed by that time at least locally in the southern lhasa terrane tibet

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

spectral analysis of k net and kik net data in japan part ii on attenuation characteristics source spectra and site response of borehole and surface stations abstract in this study we apply a nonparametric spectral inversion scheme to a data set of accelerograms recorded by the k net and kik net networks in japan in order to derive attenuation characteristics source spectra and site response for this purpose we use a total of more than 67 000 s wave records from 2178 earthquakes m jma 2 7 8 obtained at 1555 stations at the earth s surface and more than 29 000 records from 1826 events recorded at 637 borehole stations at depths of 100 to 3000 m attenuation characteristics are investigated in five separate regions showing that crustal q depicts lower values in central compared to southern japan and a significant frequency dependence is observed in every region the source spectra follow the 2 model with higher stress drops for subcrustal earthquakes as compared with crustal ones while strong amplification effects dominate the site contributions for the surface sensors those for the borehole sensors are characterized by smaller variability nevertheless consistent with observations from deconvolution of borehole surface recording pairs downgoing wave effects are visible in the site contributions for many borehole stations finally the site amplification functions obtained at the surface are compared with surface to borehole s b and horizontal to vertical h v spectral ratios showing that the s b ratios generally provide better estimates of the horizontal amplification than the h v ratios due to amplification of the vertical component of ground motion

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

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

the significance of cenozoic magmatism from the western margin of the eastern syntaxis southeast tibet the major and trace element geochemistry sr nd bulk rock isotopes u pb zircon chronology and lu hf isotopic compositions are described for three granitic bodies which intrude the nyingchi gneisses lhasa terrane along the western margin of the eastern himalayan syntaxis the bayi two mica granite and lunan granite granodiorite were intruded at 22 1 and 25 4 0 3 ma respectively whereas the confluence biotite granite was emplaced at 49 1 0 4 ma all share strong depletions in y and hree requiring a garnet bearing source both during and following the eocene collision of the indian plate with the lhasa terrane the isotope geochemistry of these intrusives e nd t 3 to 5 87sr 86sr t 0 706 0 707 indicates a crustal source within the lhasa terrane sr nd systematics of the garnet bearing nyingchi gneisses together with the u pb and lu hf isotopic ratios of detrital zircons recovered from this unit identifies it as a potential melt source the combined element and isotope geochemistry of the plutons indicate a mixed source the gneisses provide the older component whereas the gangdese batholith provides a younger siliceous component the involvement of garnet bearing crustal material in melt sources from the cretaceous 80 ma to the miocene 20 ma is consistent with the presence of a thicker continental crust in the eastern lhasa terrane as is the presence of magmatic epidote in several plutons which indicates a regional deepening level of exposure eastwards post collision crustal melting is synchronous with proposed slab break off during the early miocene suggesting advective heating by rising asthenospheric melts

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

dehydration melting of a basaltic composition amphibolite at 1 5 and 2 0 gpa implications for the origin of adakites this study presents the results of dehydration melting experiments on a basaltic composition amphibolite under conditions appropriate to a hot slab geotherm 1 5 and 2 0 gpa and temperatures of 850 to 1150 c dehydration melting produces an omphacitic augite and garnet bearing residue coexisting with rhyolitic to andesitic composition melts at 1 5 gpa the amphibolite melts in two stages between 800 and 1025 c the 2 0 gpa data also define two melting stages at 2 0 gpa the first stage involves nearly modal melting of the original amphibolite minerals qtz pl amp to produce melt cpx grt during the second stage the eclogite restite melts non modally 0 86 cpx 0 14 grt 1 melt the experimental results were combined with data from the literature to generate a composite p t phase diagram for basaltic composition amphibolites over the 800 to 1100 c temperature range for pressures up to 2 0 gpa comparison of the major element compositions of the experimentally produced melts with compositions of presumed slab melts adakites shows that partial melting of amphibolite at conditions appropriate to a hot slab geotherm produces melts similar to andesitic and dacitic adakites except for significant mgo and cao depletions trace element modelling of amphibolite dehydration melting using the 2 0 gpa melting reactions produces ree abundances similar to those of adakites at 10 15 wt batch melting but the models do not reproduce the high sr y ratios characteristic of adakites taken together the major and trace element results are not consistent with the derivation of adakites by dehydration melting of the subducted slab with little or no interaction with the mantle wedge or crust if adakites are partial melts of the subducted slab they must undergo significant interaction with the mantle and or crust during which they acquire a number of their distinctive characteristics

partial melting of thickened or delaminated lower crust in the middle of eastern china implications for cu au mineralization abstract field relations isotope systematics and plate tectonic reconstructions require that felsic adakites in the yangtze block and the dabie orogen eastern china were not derived from a subducting slab despite the signature of a mantle component in the contemporaneous mafic adakite hosts of cu au deposits in the same areas the apparently contradictory requirements are accounted for by a a deep crustal melting origin for barren adakites and b a crustal delamination origin followed by ascent through lithospheric mantle for adakites associated with mineralization the crustal delamination process associated with the prospective porphyries duplicates the metallogenically essential aspects of the subduction environment the importance of adakitic magmas in the genesis of porphyry style cu au deposits is affirmed by these findings but the range of prospective tectonic environments is extended to include an important intraplate postsubduction setting the porphyries of eastern china demonstrate

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

late cretaceous crustal growth in the gangdese area southern tibet petrological and sr nd hf o isotopic evidence from zhengga diorite gabbro abstract recent studies of gangdese granitic magmatism demonstrate a mantle contribution to crustal growth in southern tibet during the jurassic early eocene however the specific mechanism for adding such juvenile crust has been disputed owing to a lack of reliable evidence for contemporaneous deep mantle geodynamic processes here we report on the zhengga diorite gabbro suite from the gangdese area they consist of plagioclase labradorite and anorthite amphibole clinopyroxene biotite and minor magnetite epidote zircon and apatite la icp ms zircon u pb dating for two samples gives a ca 94 ma age for the zhengga intrusive rocks i e the late cretaceous apart from one high sio 2 52 2 wt diorite sample with slightly high k 2 o 1 72 wt and initial 87 sr 86 sr 0 7068 and low e nd t 5 6 values the gabbro samples are geochemically characterized by low sio 2 39 8 50 1 wt and k 2 o 0 3 1 1 wt strongly negative nb ta and positive sr anomalies and uniform initial 87 sr 86 sr 0 7043 0 7048 the gabbros can be divided into two groups group i gabbros with relatively low total rare earth element ree rb and sio 2 contents and positive eu anomalies and group ii gabbros with slightly higher total ree rb and sio 2 contents and negative eu anomalies the group i gabbros have e nd t 1 7 to 4 1 and zircon e hf t 6 5 to 11 1 and 18 o 5 89 to 7 24 values which are slightly different to those of the group ii gabbros 0 2 to 2 0 2 9 to 6 5 and 6 24 to 7 05 trace element compositions of amphibole and clinopyroxene grains suggest that the zhengga mafic magmas contained a significant fluid transported component probably released from subducted oceanic lithosphere we suggest that the parental magmas of the zhengga gabbros were generated by the hydrous partial melting of lithospheric mantle metasomatized by sediment melts fluids the group i gabbros were likely generated by the fractional crystallization of olivine or clinopyroxene from such parental magmas with insignificant crustal contamination whereas the group ii gabbros were probably produced by assimilation and fractional crystallization afc processes from mafic magmas that were geochemically similar to the group i gabbros pre collisional underplating of mantle derived mafic magmas likely played an important role in crustal growth and supplied the source materials for some late late cretaceous cenozoic granitoids of the gangdese batholiths this study also demonstrates that the hydrous partial melting of mantle wedge triggered by the dehydration in a subduction setting has a capacity to create significant volumes of juvenile continental crust

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

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

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

earthquake scaling characteristics and the scale in dependence of seismic energy to moment ratio insights from kik net data in japan 1 we investigate earthquake source characteristics and scaling properties using the results of a spectral inversion of more than 29 000 accelerometric borehole recordings from 1 826 earthquakes mjma 2 7 8 throughout japan we find that the calculated source spectra can be well characterized by the omega square model and show on average self similar scaling over the entire magnitude range with median stress drops of 1 1 and 9 2 mpa for crustal and subcrustal events respectively the seismic energy to moment ratio as theoretically expected if the omega square model is valid shows a strong dependency on stress drop only which in conjunction with data selection practice in some studies to cope with limited recording bandwidth can explain the often observed apparent scale dependence our observations suggest that there is no significant deviation from similarity of the energy radiation in the investigated magnitude range and that the observed scatter is mainly related to the scatter in stress drop

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

seismic tomography of taiwan improved constraints from a dense network of strong motion stations in this study a large collection of 41 141 s p times from the untapped records of the taiwan strong motion instrumentation program tsmip network is combined with the p and s wave arrival times from the taiwan central weather bureau seismic network cwbsn to image the v p and v p v s structures beneath taiwan the records from the 680 tsmip stations throughout taiwan in the past 15 years enhance the path coverage and the resolution in the tomography inversions tremendously our result for the v p structure largely confirms previous studies but brings better constraint on the v p v s structure the colliding luzon volcanic arc is characterized by a belt of high v p and high v p v s with high seismicity that includes the offshore islands of lutao and lanyu and the coastal ranges in eastern taiwan at the depth between about 13 and 25 km this high v p v s belt can be traced to the subduction zone in the region between hualian and ilan in the deeper portion the shallow portions of the southwestern coastal plain and the pingtung region are also characterized by a belt of high v p v s with lower seismicity most of the events occurred at the base of the high v p v s zones we suggest that material strength in those regions may be too low to accumulate stress which may indicate water saturated young sediments finally the central range region is characterized by a low v p v s belt