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

evidence from earthquake data for a partially molten crustal layer in southern tibet earthquake data collected by the indepth ii passive source experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern tibet north of the zangbo suture beneath the southern lhasa block a midcrustal low velocity zone is revealed by inversion of receiver functions rayleigh wave phase velocities and modeling of the radial component of teleseismic p waveforms conversely to the south beneath the tethyan himalaya no low velocity zone was observed the presence of the midcrustal low velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern yadong gulu rift and possibly much of southern tibet

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

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

controls of alteration and mineralization in the sungun porphyry copper deposit iran evidence from fluid inclusions and stable isotopes the sungun porphyry copper deposit is located in northwestern iran azarbaijan province and is associated with diorite granodiorite to quartz monzonite of miocene age which intruded eocene volcano sedimentary and cretaceous carbonate rocks copper mineralization was accompanied by both potassic and phyllic alteration field observations and petrographic studies demonstrate that emplacement of the sungun stock took place in several intrusive pulses each with associated hydrothermal activity molybdenum was concentrated at a very early stage in the evolution of the hydrothermal system and copper somewhat later four main vein groups have been identified i quartz molybdenite anhydrite or k feldspar with minor pyrite chalcopyrite and bornite ii quartz chalcopyrite pyrite or molybdenite iii quartz pyrite calcite or chalcopyrite or anhydrite gypsum or molybdenite and iv quartz or calcite or gypsum or pyrite early hydrothermal alteration produced a potassic assemblage orthoclase biotite in the central part of the stock propylitic alteration occurred contemporaneously with potassic alteration but in the peripheral parts of the stock and phyllic alteration occurred later overprinting the earlier alteration the early hydrothermal fluids are represented by high temperature 340 degrees 500 degrees c high salinity up to 60 wt nacl equiv liquid rich fluid inclusions and high temperature 320 degrees 550 degrees c low salinity vapor rich inclusions these fluids are interpreted to represent an orthomagmatic fluid which boiled episodically the brines are interpreted to have caused potassic alteration and deposition of group i and ii quartz veins containing molybdenite and chalcopyrite propylitic alteration is attributed to a liquid rich lower temperature 240 degrees 330 degrees c ca rich evolved meteoric fluid influx of meteoric water into the central part of the system and mixing with magmatic fluid produced deep albitization transition alteration and shallow phyllic alteration this influx also caused dissolution of early formed copper sulfides and remobilization of cu into the sericitic zone where it was redeposited in response to a boiling induced decrease in temperature supergene alteration was minor and restricted to a thin blanket of cu sulfides below an argillic cap

tibetan tectonic evolution inferred from spatial and temporal variations in post collisional magmatism cenozoic magmatism on the tibetan plateau shows systematic variations in space and time that must be considered in models concerning tibetan tectonic evolution after the india asia collision which started in the early tertiary and terminated the gangdese arc magmatism in the lhasa terrane made of the southern tibetan plateau widespread potassium rich lavas and subordinate sodium rich basalts were generated from f50 to 30 ma in the qiangtang terrane of northern tibet subsequent post collisional magmatism migrated southwards producing ultrapotassic and adakitic lavas coevally between f26 and 10 ma in the lhasa terrane then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f13 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the indian continental lithospheric mantle started thrusting under asia by involving rollback and breakoff of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root we propose that only after the lithospheric removal which occurred at f26 ma could the indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the himalayan tibetan orogenesis consequently the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder owing to the push of the underthrust indian mantle lithosphere d 2004 elsevier b v all rights reserved

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

porphyry cu mo au deposits related to melting of thickened mafic lower crust examples from the eastern tethyan metallogenic domain abstract most porphyry cu deposits in the world occur in magmatic arc settings and are formed in association with calc alkaline arc magmas related to subduction of oceanic lithosphere this contribution reviews a number of significant porphyry cu deposits in the eastern tethyan metallogenic domain they widely occur in a variety of non arc settings varying from post late collisional transpressional and extensional environments to intracontinental extensional environments related to orogenic and anorogenic processes their spatial temporal localization is controlled by strike slip faults orogen transverse normal faults lineaments and their intersections in these non arc settings these deposits are dominated by porphyry cu mo deposits with minor porphyry cu au and epithermal au deposits and exhibit a broad similarity with those in magmatic arcs the associated magmas are generally hydrous relatively high fo2 high k calc alkaline and shoshonitic and show geochemical affinity with adakites they are distinguished from arc magmas and or oceanic slab derived adakites by their occurrence as isolated complexes high k2o contents 1 2 8 5 and much wider range of end t values 10 to 3 and positive ehf t values 4 6 to 6 9 these potassic magmas are most likely formed by partial melting of thickened juvenile mafic lower crust or delaminated lower crust but also involving various amounts of asthenospheric mantle components key factors that generate hydrous fertile magmas are most likely crust mantle interaction processes at the base of thickened lower crust in non arc settings rather than oceanic slab dehydration as in arc settings breakdown of amphibole in thickened lower crust e g amphibole eclogite and garnet amphibolite during melting is considered to release fluids into the fertile magmas leading to an elevated oxidation state and higher h2o content necessary for development of porphyry cu mo au systems copper and au in hydrous magmas are likely derived from mantle derived components and or melts which either previously underplated and infiltrated at the base of the thickened lower crust or were input into the primitive magmas by melt mantle interaction in contrast mo and part of the s in the fertile magmas are probably supplied by old crust during melting and subsequent ascent

lower crustal granulite xenoliths from the pannonian basin hungary part 2 sr nd pb hf and o isotope evidence for formation of continental lower crust by tectonic emplacement of oceanic crust mafic granulite xenoliths from the lower crust of the pannonian basin are dominated by lree depleted bulk rock compositions many of these have morb like 143nd 144nd but 87sr 86sr is elevated relative to most morbs their 18o values cover a wide range from 3 8 to 9 5 a group of lree enriched mafic granulites have higher 87sr 86sr 0 704 0 708 and lower 143nd 144nd 0 5128 0 5124 with higher 18o values on average 7 8 to 10 6 than the lree depleted granulites the lree enriched granulites are however isotopically similar to newly discovered metasedimentary granulite xenoliths a sublinear correlation in ehf end isotope space has a shallower slope than the crust mantle array with the metasedimentary rocks forming the low ehf end member the radiogenic end is restricted to the lree depleted granulites and these overlap the field of morb pb isotopes for the lree depleted samples are less radiogenic on average than those of the lree enriched and metasedimentary xenoliths and metasedimentary granulites have consistently higher 208pb 204pb the wide range in 18o over a restricted range in nd and sr isotope values in combination with the predominance of lree depleted trace element compositions is consistent with an origin as a package of hydrothermally altered oceanic crust the existence of 18o values lower than average morb and or mantle peridotite requires that at least some of these rocks were hydrothermally altered at high temperature presumably in the oceanic lower crust the low 143nd 144nd of the lree enriched mafic granulites cannot be explained by simple mixing between a lree depleted melt and an enriched component represented by the recovered metasediments instead we interpret these rocks as the metamorphic equivalent of the shallowest levels of the ocean crust where pillow basalts are intimately intercalated with oceanic sediments a possible model is accretion of oceanic crustal slices during subduction and convergence followed by high grade metamorphism during the alpine orogeny

observations of regional phase propagation across the tibetan plateau we present observations of regional phase velocity and propagation characteristics using data recorded during a 1 year deployment of broadband digital seismic stations across the central tibetan plateau along the qinghai tibet highway from golmud to lhasa previous seismological studies within this region have had to rely on earthquakes recorded almost exclusively at stations outside of the plateau we have the opportunity to study numerous source receiver paths confined entirely within the tibetan plateau our analysis concentrates on travel time amplitude and frequency content measurements of the pg pn and sn phases pn can be clearly picked for all observed paths and propagates at an average velocity of 8 16 0 07 km s within the tibetan plateau sn however shows dramatic variations in propagation efficiency across the tibetan plateau that is strongly dependent on frequency we observe that sn rapidly decreases in frequency and amplitude as it passes through the northern portion of the plateau we show that in general sn propagation efficiency decreases with increasing frequency content we use 122 events from outside of the plateau and 61 from within to refine the boundaries of a region of inefficient high frequency sn propagation specifically we show that a larger portion of the northern tibetan plateau attenuates sn energy than was previously suggested in the southern plateau where high frequency sn is observed we computed an average velocity of 4 59 0 18 km s we also observed that the pn velocity within this region of inefficient high frequency sn propagation is nearly 4 slower than the pn velocity computed for paths restricted to the southern plateau and that the crust is about 10 km thinner than in the south the coincident locations of inefficient sn propagation and slow pn velocity is commonly observed in regions of active tectonics our results add constraints to the velocity structure of the lithosphere beneath the tibetan plateau and require first order lateral variations in the uppermost mantle structure despite the relatively uniform topography of the plateau

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

trace metal speciation in sea and pore water of the gotland deep baltic sea 1994 abstract studies on the speciation particulate colloidal anionic and cationic forms of trace metals cd co cu fe mn mo ni pb zn in the water column and in pore waters of the gotland deep following the 1993 94 salt water inflows showed dramatic changes in the total dissolved metal concentrations and in the ratios between different metal species in the freshly re oxygenated waters below 125 m changes in concentrations were greatest for those metals for which the solubility differs with the redox state fe mn co but were also noted for those metals which form insoluble sulphides cd pb cu zn and or stable complexes with natural ligands cu pore water data from segmented surface muds 0 200 mm indicated that significant redox and related metal speciation changes took place in the surface sediments only a few weeks after the inflow of the oxygenated sea water into the gotland deep

mayum an orogenic gold deposit in tibet china abstract located in western tibet china the mayum orogenic gold deposit was discovered in 2002 with estimated resources of 80 tonnes gold the gold mineralization is hosted by neoproterozoic cambrian schists and is controlled by nearly parallel e w trending bedding concordant fracture zones the au orebodies are composed of auriferous quartz veins and altered rocks with the au grades ranging from 2 23 g t to 69 56 g t and containing the 34s values of 18 separates of sulfides from auriferous quartz veins show a large variation from 0 2 to 16 8 the lead isotope compositions of sulfides from the gold ore are characterized by highly radiogenic values and very wide ranges of ratios 18 324 to 20 819 for 206pb 204pb 15 679 to 15 856 for 207pb 204pb and 38 401 to 41 204 for 208pb 204pb these data indicate that lead and sulfur in the quartz veins have multiple sources and were derived from the different wall rocks as the fluid flowed through them fluid inclusions indicate that the ore fluid was co2 rich with salinities mainly between 1 and 6 wt nacl equiv and homogenization temperatures predominantly ranging from 260 to 280 c the 18o values for quartz from auriferous veins range from 13 7 to 16 3 and the calculated 18oh2o values in equilibrium with quartz vary from 5 54 to 9 48 it suggests that the ore fluid may be derived from deep metamorphic deep crustal gold transporting fluids although a contribution from magmatic source cannot be ruled out 40ar 39ar age dating on the sericite from the alteration associated with the auriferous quartz veins in the mayum gold deposit gives a plateau like age of 59 34 0 62 ma later than the onset of the indo asian collision it is believed the gold mineralization was related to the indo asian collision and was formed during the early stage of orogenesis

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

petrology of the intrusive rocks within the sungun porphyry copper deposit azerbaijan iran abstract the sungun intrusive complex hosting the sungun porphyry copper stock is located about 75 km north west of ahar in azarbaijan province and intruded along the sungun anticline into cretaceous limestone and eocene tuff and agglomerate of andesitic to trachytic composition based on the petrography and geochemistry it is suggested that the sungun deposit as a composite stock comprising an early monzonite quartzmonzonite and a later diorite granodiorite phase the parental magma was a medium to high k andesite or diorite initially contained 3 wt h2o and was generated in a continental arc setting the high level emplacement of this magma led to its saturation with h2o and the exsolution of fluids at an early stage of crystallization and subsequently mo and then cu mineralization based on the major and trace element geochemistry especially the behaviour of the immobile trace elements y zr and nb fractional crystallization is suggested as the process that produced the present intrusive rocks the degree of fractionation was estimated using the rayleigh fractionation equation relating the concentration of zr nb and y in the least evolved sample to the concentrations of these elements in progressively more evolved samples the results of these calculations indicate that the compositions of the various rock types can be explained by up to 24 fractionation of ferromagnesian minerals e g pyroxene and amphibole of the diorite magma to produce the more evolved monzonite quartz monzonite suite

stable isotope s o h and c studies of the phyllic and potassic phyllic alteration zones of the porphyry copper deposit at sungun east azarbaidjan iran abstract the porphyry copper deposit pcd at sungun is located in east azarbaidjan nw of iran the magmatic suites in the sungun area are a part of the nw se trending cenozoic magmatic belt of iran the sungun porphyries occur as stocks and dikes the stocks are divided into two groups i and ii porphyry stock ii ranging in composition from quartz monzonite through granodiorite to granite hosts the sungun pcd four distinct types of hypogene alterations were recognized at sungun 1 potassic 2 potassic phyllic 3 phyllic and 4 propylitic stable isotope s o h and c studies were restricted to within the phyllic and potassic phyllic alteration zones where numerous cross cutting quartz sulfides carbonates and sulfate veinlets are present the objective of these studies was to determine the origin of the ore forming solutions and their important components e g sulfur and carbon twenty sulfide and four sulfate samples were taken from sulfide and gypsum veinlets within porphyry stock ii and the associated skarn zone for sulfur isotopic analyses the 34 s values of sulfides pyrite chalcopyrite molybdenite galena sphalerite and sulfate gypsum range from 4 6 to 0 2 mean of 1 5 and from 10 9 to 14 4 mean of 12 9 respectively these values are almost analogous to those from el salvador chile and ajo arizona and twin buttes arizona and strongly suggest a magmatic source for the sulfur at sungun twenty eight fluid inclusion rich quartz samples from quartz veinlets beneath the supergene zones of the porphyry stock ii were chosen for o and h isotopic analyses the 18 o of quartz and d of fluid inclusions in quartz values range from 8 3 to 10 2 mean of 9 2 and 58 to 75 mean of 66 relative to standard mean ocean water smow respectively the calculated 18 o values of the fluids range from 4 4 t 375 c to 7 6 t 570 c with a mean of 6 4 the 18 o and d values of the fluids lie within the typical range of magmatic waters based upon the lowest obtained d values the calculated maximum exchangeable meteoric water that might be involved in the phyllic and potassic phyllic alteration processes was 20 fourteen calcite samples from carbonate veinlets in the porphyry stock ii and two samples from associated marble aureole were collected for carbon and oxygen isotopic analyses the 13 c calcite from porphyry stock ii and associated marble aureole varies from 0 4 to 2 9 and 0 06 to 2 relative to the pdb standard of the university of chicago respectively the corresponding 18 o calcite values range from 6 to 13 2 relative to smow the 13 c values lie within the sedimentary range suggesting that there was little or almost no magmatic carbon in the hydrothermal system in contrast the 18 o calcite values are far lower than that of cretaceous sedimentary carbonates which suggest that the values were influenced by those of magmatic fluids through isotopic re equilibration these isotopic investigations demonstrated that 1 in contrast to some well known pcds about 80 of the fluids involved in the phyllic and potassic phyllic alterations at sungun had a magmatic o and h isotopic signature 2 the upper cretaceous limestone was the potential source of c and 3 the sulfur had a magmatic origin

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

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

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

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

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

the dynamics of thrust and normal faults over multiple earthquake cycles effects of dipping fault geometry we perform dynamic simulations of thrust and normal faults over multiple earthquake cycles our goal is to explore effects of asymmetric fault geometry on the long term seismicity and dynamics of dipping faults a dynamic finite element method is used to model the interseismic and coseismic processes with a dynamic relaxation technique for the former the faults are loaded by stable sliding along the downward continuation of the faults the asymmetric fault geometry of dipping faults with respect to the free surface cause changes in normal stress during the interseismic and coseismic periods these changes are of opposite sign in the two periods resulting in a stabilization of the normal and shear stresses over many earthquake cycles both faults develop relatively stable event patterns in which a large event that ruptures the entire fault is preceded by a number of small events with various rupture lengths a strong asymmetry in fault and ground motion exists between thrust and normal faults and between the hanging wall and the footwall of both faults on faults dipping less than 70 in both normal and thrust faults the horizontal component of ground motion dominates on the footwall while the vertical component dominates on the hanging wall the above results may have implications in seismic hazard analysis and building design in regions where dip slip faults predominate

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

geology of the post collisional porphyry copper molybdenum deposit at qulong tibet porphyry deposits are usually thought to form from subduction related calc alkaline magmas in magmatic arc settings although some porphyry deposits also occur in post collisional extensional settings the post collisional deposits remain poorly understood here we describe the igneous geology alteration mineralogy and mineralization history of qulong a newly discovered porphyry cu mo deposit in southern tibet that belongs to the post collisional class the deposit is associated with miocene monzogranite granodiorite intrusions and is hosted partly by jurassic andesitic dacitic volcanic rocks the deposit contains 7 1 mt cu and 0 5 mt mo metal the 19 5 ma granodioritic monzogranitic rongmucuola pluton with diorite enclaves is the earliest miocene intrusive unit it was intruded by a regularly shaped stock p porphyry and then thin dikes x porphyry of monzogranite at about 17 7 ma the main cu mo mineralization is associated with the p porphyry a barren diorite porphyry intruded the p and x porphyries around 15 7 ma petrologic trends of the intrusions suggest that the miocene intrusions have similar origins and probably formed by fractionation of a deeper magma chamber emplacement of miocene porphyries controlled by the qulong anticline is in direct response to the rapid uplift erosion of the gangdese arc batholiths in southern tibet miarolitic cavities and unidirectional solidification textures key evidence for volatile separation have been recognized in the p and x porphyries respectively early potassic alteration characterized by quartz k feldspar anhydrite pervades the p porphyry and rongmucuola pluton laterally this alteration grades into quartz biotite anhydrite k feldspar which affects all miocene intrusions except the latest dioritic porphyry wall rocks of rongmucuola pluton and jurassic andesitic dacitic volcanics within 1 1 5 km of the porphyries are dominated by pervasive potassic alteration an outer halo of propylitic alteration epidote chlorite calcite extends up to 2 km away from the deposit feldspar destructive alteration sericite chlorite clay minerals has overprinted most of the potassic and part of the propylitic alteration the alteration is strongly pervasive in the interior of the porphyry bodies and occurs as vein halos away from the porphyry bodies the earliest quartz k feldspar alteration and veins are barren whereas approximately 60 of the cu reserves is associated with slightly later quartz biotite anhydrite alteration barren assemblages are related to irregular quartz k feldspar anhydrite veins which are truncated by the x porphyry cu sulfide bearing assemblages are associated with discontinuous chalcopyrite biotite and continuous quartz anhydrite chalcopyrite molybdenite veins deposition of cu mo with abundant anhydrite occurred during or between emplacement of closely related porphyries from high temperature magmatically derived fluids and was probably caused by the disproportionation of so

partial melting of subducting oceanic crust the conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two dimensional thermal models of subduction zones for porosities of 1 containing h20 the amount of partial melt generated at the wet basaltic solidus is limited to 100 mpa can be maintained by rocks close to or above their melting temperatures in the absence of high shear stresses substantial melting of the oceanic crust will only occur during subduction of very young 5 ma oceanic lithosphere partial melting of hydrated basalt amphibolites derived from the mid ocean ridge has been proposed e g 1 3 as being responsible for the generation of certain recent high al andesitic to dacitic volcanic rocks adakites three of these volcanic suites mount st helens southern chile and panama occur in volcanic arcs where oceanic crust 25 ma is being subducted at rates of 1 3 cm yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments however oceanic lithosphere is not currently being subducted beneath baja and new guinea where recent adakites are also present suggesting that some adakite magmas may form by water undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately

hydrothermal evolution of the sar cheshmeh porphyry cu mo deposit iran evidence from fluid inclusions abstract the sar cheshmeh porphyry cu mo deposit is located in southwestern iran 65 km southwest of kerman city and is associated with a composite miocene stock ranging in composition from diorite through granodiorite to quartz monzonite field observations and petrographic studies demonstrate that the emplacement of the sar cheshmeh stock took place in several pulses each with associated hydrothermal activity molybdenum was concentrated at a very early stage in the evolution of the hydrothermal system and copper was concentrated later four main vein groups have been identified i quartz molybdenite anhydrite k feldspar with minor pyrite chalcopyrite and bornite ii quartz chalcopyrite pyrite molybdenite calcite iii quartz pyrite calcite chalcopyrite anhydrite gypsum molybdenite iv quartz calcite gypsum pyrite dolomite early hydrothermal alteration produced a potassic assemblage orthoclase biotite in the central part of the stock propylitic alteration occurred in the peripheral parts of the stock contemporaneously with potassic alteration and phyllic alteration occurred later overprinting earlier alteration the early hydrothermal fluids are represented by high temperature 350 520 c high salinity up to 61 wt nacl equivalent liquid rich fluid inclusions and high temperature 340 570 c low salinity vapor rich inclusions these fluids are interpreted to represent an orthomagmatic fluid which cooled episodically the brines are interpreted to have caused potassic alteration and deposition of group i and ii quartz veins containing molybdenite and chalcopyrite propylitic alteration is attributed to a liquid rich lower temperature 220 310 c ca rich evolved meteoric fluid influx of meteoric water into the central part of the system and mixing with magmatic fluid produced albitization at depth and shallow phyllic alteration this influx also caused the dissolution of early formed copper sulphides and the remobilization of cu into the sericitic zone the main zone of the copper deposition in sar cheshmeh where it was redeposited in response to a decrease in temperature

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

upb geochronology of gangdese transhimalaya plutonism in the lhasa xigaze region tibet abstract a series of different plutons from the lhasa xigaze segment of the gangdese transhimalaya belt has been studied by high resolution u pb analyses of zircon using zircon fractions of 5 100 grains each selected upon specific grain characteristics for two diorites located east of xigaze dazhuka the zircons yield concordant ages of 93 4 1 0 and 94 2 1 0 m y respectively also concordant ages of 41 1 0 4 and 41 7 0 4 m y have been obtained for two granodiorites collected southwest of lhasa qushui the precision on the ages of two granites from the xigaze and lhasa area is limited by two factors the presence of inherited radiogenic lead and the occurrence of subsequent lead loss however some concordant zircons detected in both granites define approximate ages of about 67 and 53 m y respectively the inherited lead components show that melting of precambrian material was involved in magma genesis the u pb ages substantiate a magmatic activity lasting from mid cretceous cenomanian to eocene lutetian time such a period of plutonism at the southern margin of eurasia as well as the occurrence of magma generation from continental crust suggest that the gangdese range results from the subduction of tethys oceanic lithosphere indian plate underneath eurasia eurasian plate if this model is true the collision of india with eurasia along the lhasa xigaze sector postdates the emplacement of the 41 m y old gangdese granodiorites i e the collision occurred after lutetian time

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

the miocene gangdese porphyry copper belt generated during post collisional extension in the tibetan orogen abstract the mid miocene gangdese orogenic belt in southern tibet contains porphyry cu deposits that developed after the indian asian continental collision in the early cenozoic field work confirms that these porphyry cu deposits are temporally and spatially associated with mid miocene monzogranitic and quartz monzonitic stocks which intruded mesozoic cenozoic gangdese granitoid batholiths and surrounding triassic tertiary volcano sedimentary sequences in the lhasa terrane and formed a 50 km wide 400 km long e w belt magmatism occurred during the interval 11 2 to 19 7 ma peaking at 16 ma for the porphyry stocks this is consistent with the age 10 to 18 ma of the east west extension and subsequent ns striking normal fault systems 13 5 ma in the tibetan orogen the stocks typically occur as multiple small diameter intrusions 50 km breakdown of hornblende in the juvenile lower crust source during melting is regarded as fundamental for the formation of the fertile adakite and porphyry cu systems in south tibet the gangdese belt contains four large and three intermediate and small sized porphyry cu mo deposits and shows a continuous spectrum from typical porphyry type cu mo to skarn type zn pb mineralization twenty three samples collected from four deposits yielded a limited range of molybdenite re os isochron ages from 15 2 to 16 0 ma similar to previously reported re os ages 13 8 to 14 9 ma for other porphyry cu mo deposits in the belt suggesting that cu mo mineralization in the belt occurred in a post collisional extension environment the characteristics of porphyry cu mo deposits in the collisional zone are similar in many aspects to those in arc settings i e mineralization style alteration zoning and metal association preliminary fluid inclusion studies indicate that the early hydrothermal fluids are preserved as high temperature 450 c high salinity 40 to 47 wt nacl equiv and high pressure 1100 to 1500 bar fluid inclusions these fluids are interpreted to represent an orthomagmatic fluid that boiled episodically due to fluid overpressuring and hydrofracturing resulting in potassic alteration and precipitation of chalcopyrite molybdenite assemblages on the basis of comparison with porphyry cu deposits in arc settings we proposed that melting of thickened newly formed basaltic crust resulted in generation of cu bearing magmatic systems in the tibetan collision zone and both orogen transverse normal faults and orogen parallel thrust zones during the miocene post collisional extension constrain the temporal and spatial localization of the porphyry cu systems in the collision zone

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

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

tertiary structural evolution of the gangdese thrust system southeastern tibet structural and thermochronological investigations of southern tibet xizang suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost tibetan plateau since late oligocene two thrust systems are documented in this study the north dipping gangdese system gts and the younger south dipping renbu zedong system rzt west of lhasa the gangdese thrust juxtaposes the late cretaceous forearc basin deposits of the lhasa block the xigaze group over the tethyan sedimentary rocks of the indian plate whereas east of lhasa the fault juxtaposes the late cretaceous eocene andean type arc the gangdese batholith over tethyan sedimentary rocks near zedong 150 km southeast of lhasa the gangdese thrust is marked by a 200 m thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks a major south dipping backthrust in the hanging wall of the gangdese thrust puts the xigaze group over tertiary conglomerates and the gangdese plutonics north of xigaze and west of lhasa a lower age bound for the gangdese thrust of 18 3 0 5 ma is given by crosscutting relationships the timing of slip on the gangdese thrust is estimated to be 27 23 ma from 40 ar 39 ar thermochronology and a displacement of at least 46 9 km is indicated near zedong the age of the gangdese thrust gt is consistent with an upper age limit of 24 ma for the initiation of movement on the main central thrust in places the younger renbu zedong fault is thrust over the trace of the gt obscuring its exposure the rzt appears to have been active at circa 18 ma but had ceased movement by 8 1 ma the suture between india and asia has been complexly modified by development of the gts rzt and locally strike slip and normal fault systems

tibetan basement rocks near amdo reveal missing mesozoic tectonism along the bangong suture central tibet the u pb and 4 0 ar 3 9 ar studies of a unique exposure of crystalline basement along the jurassic early cretaceous bangong suture of central tibet reveal previously unrecognized records of mesozoic metamorphism magmatism and exhumation the basement includes cambrian and older orthogneisses that underwent amphibolite facies metamorphism coeval with extensive granitoid emplacement at 185 170 ma the basement cooled to 300 c by 165 ma and was exhumed to upper crustal levels in the hanging wall of a south directed thrust system during early cretaceous time we attribute jurassic metamorphism and magmatism to the development of a continental arc during bangong ocean subduction and early cretaceous exhumation to northward continental underthrusting of the lhasa terrane beneath the qiangtang terrane we speculate that a jurassic arc extended regionally along the length of the bangong suture but in all other places in tibet has been buried either depositionally or structurally beneath supracrustal assemblages

fluid inclusion studies in quartz veinlets in the porphyry copper deposit at sungun east azarbaidjan iran abstract the porphyry copper deposit pcd at sungun is located in east azarbaidjan in the nw of iran the sungun porphyries occur as stocks and dikes ranging in composition from quartz monzodiorite through quartz monzonite and granodiorite to granite the stocks are divided into two groups 1 porphyry stocks i and 2 porphyry stock ii porphyry stock ii hosting the copper ore experienced intense hydro fracturing leading to the formation of stockwork type and anastomozing veinlets and micro veinlets of quartz sulfides carbonates and sulfates three distinct types of hydrothermal alteration and sulfide mineralization are recognized at sungun 1 hypogene 2 contact metasomatic skarn and 3 supergene four types of hypogene alteration are developed at sungun potassic propylitic potassic phyllic and phyllic four types of inclusion are common at sungun based upon their phase content 1 mono phase vapor 2 vapor rich 2 phase 3 liquid rich 2 phase and 4 multi phase solid halite is the principal solid phase the distribution pattern shape and phase contents of fluid inclusions in quartz veinlets at sungun are analogous to those from bingham and globe miami in western usa the fluid inclusion data at sungun showed that the liquid vapor homogenization temperature t h l v values for liquid rich 2 phase vapor rich 2 phase and halite bearing inclusions vary from 160 to 580 c from 200 to 600 c and from 160 to 580 c respectively the ascending unboiled fluid at the onset of the phyllic alteration episode had temperatures 580 c and was moderately saline 15 wt with the gradual decrease in temperature the salinity of this fluid gradually decreased so that its salinity at temperatures of 370 and 350 c the residual fluid remained undersaturated with respect to nacl however at temperatures t s nacl t h l v showed that they were developed under the maximum hydrostatic pressure head of 1500 m during the boiling events

mid eocene high sr y granites in the northern himalayan gneiss domes melting thickened lower continental crust abstract within the himalayan collisional belt granotoids occur along two sub parallel belts the northern himalayan gneiss domes nhgd and the high himalayan crystalline series hhcs in the yardoi area of nhgd two mica granite a new type granite occurs in the core of the yardoi gneiss dome ygd dala and quedang from north to south and extends at least 50 km long these granites have similar mineral composition elemental and radiogenic isotope geochemistry and age of formation shrimp zircon u pb dating indicates that the yardoi and the quedang two mica granites formed at 42 6 1 1 ma and 42 8 0 6 ma respectively similar to the dala pluton these two mica granites have 1 high sio2 68 wt al2o3 15 wt and a cnk 1 0 2 relatively high sr and lree but low y 40 and up to 250 and la yb 30 4 very weak or no eu anomalies and 5 as compared with those in the himalayan leucogranites low initial sr 87sr 86sr i

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

recycling of orogenic arc crust triggers porphyry cu mineralization in kerman cenozoic arc rocks southeastern iran pre collisional eocene oligocene arc diorites quartzdiorites granodiorites and volcanic equivalents in the kerman arc segment in central iran lack porphyry cu mineralization and ore deposits whereas collisional middle late miocene adakite like porphyritic granodiorites without volcanic equivalents host some of the world s largest cu ore deposits petrological and structural constraints suggest a direct link between orogenic arc crust evolution and the presence of a fertile metallogenic environment ore hosting kuh panj porphyry intrusions exhibit high sr 400 ppm low y 20 no negative eu anomalies eu eu 1 and relatively non radiogenic sr isotope signatures 87sr 86sr 0 7042 0 7047 relative to eocene oligocene granitoids mainly sr 12 la yb 15 eu eu 1 87sr 86sr 0 7053 0 7068 trace element modeling indicates peridotite melting for the barren eocene oligocene intrusions and a hydrous garnet bearing amphibolite source for middle late miocene ore hosting intrusions the presence of garnet implies collisional arc crustal thickening by shortening and basaltic underplating from about 30 35 to 40 45 km or 12 kbar the changes in residual mineralogy in the source of eocene to miocene rocks in the kerman arc segment reflect probing of a thickening arc crust by recycling melting of the arc crustal keel underplating of cu and sulfur rich melts from fertile peridotite generated a fertile metallogenic reservoir at or near the crust mantle boundary and dehydration melting under oxidizing conditions produced syn and post collisional ore hosting intrusions while the lack of post collisional volcanism prevented the venting of volatiles to the atmosphere from sulfur rich and oxidized adakitic magmas

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

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

large scale hydration of the lithosphere above subducting slabs during subduction oceanic lithosphere capped by variably hydrated uppermost mantle oceanic crust and sediments undergoes progressive metamorphism and devolatilization the petrologic evolution of subducting oceanic lithosphere and the overriding continental oceanic lithosphere can be predicted by integrating thermal models with petrologic phase equilibria in mature subduction zones that have achieved thermal steady state the subducting slab lies at subsolidus conditions for depths 100 150 km most subducting oceanic crust passes through the blueschist to eclogite facies transition where large amounts of h2o are released by the breakdown of na amphibole lawsonite or clinozoisite and chlorite the large amounts of h2o released by continuous blueschist eclogite reactions can cause large scale hydration of the overriding lithosphere and can trigger partial melting of the mantle wedge lowermost lithosphere and asthenosphere upward migration of slab derived fluids add h2o co2 sio2 and incompatible elements to the overlying lithospheric plate that result in the formation of new phases e g serpentine amphibole and mica and chemical modification of existing phases e g trace element enrichment of clinopyroxene evidence for large scale lithospheric hydration above subducting slabs includes serpentinite diapirs observed in the marianas forearc low p wave velocities observed in the hanging wall of the alaskan and japanese subduction zones and studies of high pressure metamorphic terrains

adakite like signature of late miocene intrusions at the los pelambres giant porphyry copper deposit in the andes of central chile metallogenic implications adakite like features are recognized in the late miocene 10 ma porphyritic intrusions of the los pelambres giant porphyry copper deposit central chile 32 s located within the southern portion of the flat slab segment 28 33 s of the chilean andes the al and na rich porphyries of los pelambres display distinctly higher sr y 100 300 and lan ybn 25 60 ratios than contemporaneous and barren magmatic units e g la gloria pluton cerro aconcagua volcanic rocks of the same andean magmatic belt strong fractionation of heavy rare earth elements hree absence of eu anomalies high sr y and zr sm and low nb ta ratios suggest melt extraction from a garnet amphibolite source the late miocene adakite like porphyritic intrusions at los pelambres formed closely related in time and space to the subduction of the juan fernandez ridge jfr hotspot chain along the chilean margin current tectonic reconstructions reveal that at the time of formation of the los pelambres rocks a w e segment of the jfr started to subduct beneath them producing a slow down of a previously rapid southward migration of a ne ridge trench collision these particular tectonic conditions are favorable for the origin of the los pelambres porphyry suite by melting of subducting young hotspot rocks under flat slab conditions the incorporation of crustal components into the oceanic lithopheric magma source by subduction erosion is evidenced by the sr nd isotope composition of the los pelambres rocks different from the morb signatures of true adakites a close relationship apparently exists between the origin of this adakite like magmatism and the source of the mineralization in the los pelambres porphyry copper deposit