adakite like and normal arc magmas distinct fractionation paths in the east serbian segment of the balkan carpathian arc are used to assess petrogenetic and regional geodynamic processes associated with late cretaceous subvolcanic intrusions within the sparsely studied timok magmatic complex tmc and ridanj krepoljin zone rkz of eastern serbia the tmc and rkz form part of the apuseni banat timok srednogorie abts magmatic belt a cu au mineralized calc alkaline magmatic arc related to closure of thetethys ocean that extends through romania serbia and bulgaria in se europe zircon ages based on u pb laser ablation inductively coupled plasma mass spectrometry supplemented by existing isotope dilution thermal ionization mass spectrometry data respectively range from 89 to 79 ma and from 76 to 71 ma for the tmc and rkz this age pattern corresponds to cross arc younging away from the european continent adakite like trace element signatures y 18 ppm are linked with samples that extend across the arc these overlap in space and time with samples that conform to a normal arc differentiation trend we performed energy constrained assimilation fractional crystallization ec afc modeling of sr la nd yb concentrations and sr and nd isotopic data results suggest that the two distinct fractionation trends may be explained in terms of a common mantle derived parental magma but distinct fractionation and assimilation paths in the lower and upper crust petrogenesis of the adakite like magmas is consistent with extensive high pressure amphibole fractionation in the lower crust followed by ascent and plagioclase dominant fractionation and assimilation in the upper crust in contrast normal arc signatures appear to have evolved exclusively via an upper crustal differentiation process overall our interpretation supports mantle wedge melting related to weak extension during progressive rollback of a subducting slab

hydrothermal evolution and ore genesis of the hongshi copper deposit in the east tianshan orogenic belt xinjiang nw china constraints from ore geology fluid inclusion geochemistry and h o s he ar isotopes abstract the hongshi lode type copper deposit is located in the kalatag region in the east tianshan xinjiang nw china the lode ore bodies are hosted in the late ordovician to early silurian daliugou formation and the cu mineralization is closely associated with andesite minor amounts of intrusive rocks such as the quartz porphyry are also exposed the country rocks have undergone extensive silicification chlorite alteration and sericite alteration the metallic minerals are pyrite and chalcopyrite with minor magnetite sphalerite and galena the gangue minerals are quartz sericite epidote calcite and chlorite the mineralized veins in the hongshi deposit can be divided into an early quartz pyrite vein stage the main quartz chalcopyrite pyrite sphalerite vein stage and a late quartz calcite chlorite vein stage fluid inclusion petrography shows that two phase liquid rich type i two phase vapor rich type ii and daughter bearing multiphase type iii fluid inclusions are present in the hydrothermal quartz sulfide veins in the hongshi deposit the fluid inclusions in the early stage minerals are mainly type i and type ii with minor type iii their homogenization temperatures and salinities range from 198 c to 315 c with a peak value of 253 c and from 1 40 to 8 81 wt nacl equiv respectively the coexistence of type ii and type iii inclusions and their similar homogenization temperatures but contrasting salinities suggest that fluid boiling processes occurred in the early stage the main stage minerals feature dominantly type i and type ii inclusions with homogenization temperatures and salinities of 168 c 280 c with a peak value of 209 c and 1 40 6 16 wt nacl equiv respectively the late stage minerals feature dominantly type i and type ii inclusions with homogenization temperatures and salinities of 109 c 179 c with a peak value of 157 c and 0 53 3 23 wt nacl equiv respectively the initial ore forming fluids of the hongshi deposit were therefore associated with an h2o nacl system with a near neutral ph low intermediate temperatures and low salinities the oxygen and hydrogen stable isotopic analyses of fis in quartz from the different ore forming stages show that the ore forming fluids were of volcanic origin with gradually increasing proportions of meteoric water during after mineralization the 34s values of chalcopyrite and pyrite from the main ore forming stage vary between 1 9 and 11 9 suggesting a volcanic source the 3he 4he ratios of fluid inclusions in the main stage pyrite mainly range from 0 12 to 1 40 ra and the 40ar 36ar values vary from 310 to 337 all these isotopic characteristics suggest that the ore fluids were likely derived from a combination of crustal and mantle components combining the integrated analysis of ore deposit geology the fluid evolutionary process and h o s he ar isotopes we suggest that the hongshi deposit should be classified as a volcanic subvolcanic hydrothermal copper deposit processes including local fluid unmixing decreasing temperature and fluid mixing may have been responsible for the hydrothermal alteration and cu precipitation

re os isotopic dating of molybdenite and pyrite in the baishan mo re deposit eastern tianshan nw china and its geological significance the baishan mo re deposit is located in the eastern section of the eastern tianshan orogenic belt nw china the deposit has a grade of 0 06 mo and a high content of rhenium of 1 4 g t rhenium and osmium isotopes in sulfide minerals from the baishan deposit are used to determine the age of mineralization rhenium concentrations in molybdenite samples are between 74 and 250 g g analysis of eight molybdenite samples yields an isochron age of 224 8 4 5 ma 2 pyrite samples have rhenium and osmium concentrations varying in the range 33 4 330 6 ng g and 0 08 0 81 ng g respectively isotope data on seven pyrite samples yield an isochron age of 225 12 ma 2 on the 187re 188os versus 187os 188os plot and an age of 233 14 ma 2 on the 187os versus 187re correlation diagram the ages of molybdenite and pyrite are consistent within the analytical errors combined with field observations the data indicate that mo re mineralization in the baishan deposit is produced by a magmatic hydrothermal event in an intracontinental extensional setting after late paleozoic orogeny the initial 187os 188os ratio of pyrite is 0 3 0 07 the 34s values of molybdenite vary from 0 5 to 3 6 both data indicate that mineralization is derived mainly from a mantle source

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

melt components derived from a subducted slab in late orogenic ore bearing porphyries in the gangdese copper belt southern tibetan plateau abstract a porphyry type cu mo au polymetallic ore belt extends in an e w direction for 400 km along the gangdese magma arc in the southern tibetan plateau extensive field investigations and systematic geochemical study combined with s and pb isotopic tracing indicate that cu polymetallic mineralization of the copper belt is genetically related to late orogenic granitic porphyries formed in a late himalayan crustal extension stage 18 14 ma geochemistry of the porphyries shows the essential characteristics of adakite and suggests a dominant role of subduction related components in their genesis the genesis of these ore bearing porphyries is mainly related to melt components derived from subducted slabs and has little relationship with fluids released from them pb isotopes show a clear linear array in the plumbotectonics model and are manifested by a progressive variation from orogenic pb in the east segment of the copper belt to mantle pb in the west segment forming a mixing line of indian oceanic morb and indian oceanic sediments this suggests that the porphyry magma dominantly originated from partial melting of subducted oceanic crust and was mixed with a minor melts of sediments and mantle wedge components

the adakite connection of the tuwu yandong copper porphyry belt eastern tianshan nw china trace element and sr nd pb isotope geochemistry the tuwu yandong porphyry copper belt lies in the eastern tianshan mountains eastern section of the central asian orogenic belt the copper mineralization is mainly hosted in plagiogranite porphyries intruded into early carboniferous volcanic rocks of the paleozoic dananhu island arc between the tarim and siberian plates the plagiogranite porphyries have contents of 65 73 wt sio2 14 17 wt al2o3 0 9 2 2 wt mgo 3 16 ppm y 0 4 1 40 ppm yb 347 920 ppm sr and positive eu anomalies the rocks also exhibit positive nd t values 5 0 to 9 4 and low initial 87sr 86sr values 0 70316 0 70378 such features are similar to those of adakites derived from partial melting of a subduction related oceanic slab the mineralization age is early carboniferous 350 320 ma which is close to that of the porphyries the close relationship between the cu mineralization and the porphyry is also indicated by their similar sr nd pb isotopic compositions we suggest that the copper porphyry magma system in the dananhu island arc was formed by direct melting of an obliquely subducting early carboniferous oceanic slab

palaeozoic collisional tectonics and magmatism of the chinese tien shan central asia abstract the chinese tien shan range is a palaeozoic orogenic belt which contains two collision zones the older southern collision accreted a north facing passive continental margin on the north side of the tarim block to an active continental margin on the south side of an elongate continental tract the central tien shan collision occurred along the qinbulak qawabulak fault southern tien shan suture the time of the collision is poorly constrained but was probably in in the late devonian early carboniferous we propose this age because of a major disconformity at this time along the north side of the tarim block and because the youshugou ophiolite is imbricated with middle devonian sediments a younger probably late carboniferous early permian collision along the north tien shan fault northern tien shan suture accreted the northern side of the central tien shan to an island arc which lay to its north the north tien shan arc this collision is bracketed by the middle carboniferous termination of arc magmatism and the appearance of late carboniferous or early permian elastics in a foreland basin developed over the extinct arc thrust sheets generated by the collision are proposed as the tectonic load responsible for the subsidence of this basin post collisional but palaeozoic dextral shear occurred along the northern suture zone this was accompanied by the intrusion of basic and acidic magmas in the central tien shan late palaeozoic basic igneous rocks from all three lithospheric blocks represented in the tien shan possess chemical characteristics associated with generation in supra subduction zone environments even though many post date one or both collisions rocks from each block also possess distinctive trace element chemistries which supports the three fold structural division of the orogenic belt it is unclear whether the chemical differences represent different source characteristics or are due to different episodes of magmatism being juxtaposed by later dextral strike slip fault motions because the southern collision zone in the tien shan is the older of the two the tarim block sensu stricto collided not with the eurasian landmass but with a continental block which was itself separated from eurasia by at least one ocean the destruction of this ocean in late carboniferous early permian times represented the final elimination of all oceanic basins from this part of central asia

types geological features and geodynamic significances of gold copper deposits in the kanggurtag metallogenic belt eastern tianshan nw china occupying the middle of the central asia paleozoic accretionary and collisional orogenic belt the eastern tianshan area has a great economic potential due to au cu mineralization during syn and post orogenic events in the kanggurtag au cu metallogenic belt three major types of gold deposits have been recognized ductile shear zone hosted gold deposits magmatic hydrothermal gold deposits and epithermal gold deposits four kinds of copper deposits have also been identified recently the porphyry type the skarn type the magmatic type and volcanic sedimentary type tectonically the development of these late paleozoic gold and copper deposits was closely associated with the subduction and collision of the ancient tianshan ocean that intervened between the tarim craton and the siberian block in the early to mid carboniferous n dipping subduction beneath the dananhu arc generated magmatic intrusions leading to formation of the porphyry cu deposits the magmatic front migrated southward to form the yamansu arc upon the kanggurtag accretionary wedge in the latest carboniferous to early permian during the closure of the ancient tianshan ocean large mafic ultramafic complexes were emplaced resulting in several magmatic copper nickel deposits gold deposits of the shear zone type are controlled by the kanggurtag ductile shear zone which is related to collisional orogenesis the epithermal gold deposits are associated with extensional tectonics and post tectonic volcanic activity the tectonic settings geological features and temporal and spatial distributions of these different types of gold and copper deposits reflect to a great extent the accretionary and collisional tectonics that occurred between the northern margin of the tarim block and the southern margin of the siberian block

nature and evolution of hydrothermal fluids in the formation of the tuwu porphyry copper deposit in the eastern tianshan mountains nw china abstract the tuwu porphyry copper deposit is located in the eastern tianshan mountains xinjiang china lentiform like ore bodies are hosted in the early carboniferous qi eshan group and cu mineralization shows a close association with tonalite porphyry porphyry and country rocks have undergone extensive potassic chlorite sericite phyllic and propylitic alteration the mineralized veins in the tuwu deposit can be divided into earlier quartz biotite magnetite chalcopyrite veins stage i quartz chalcopyrite bornite pyrite veins stage ii and quartz molybdenite chalcopyrite pyrite calcite veins stage iii and later calcite quartz veins stage iv the stage ii and iii are the main cu mineralization stages at the tuwu deposit three types of fluid inclusions have been distinguished liquid rich l type vapor rich v type and halite bearing s type respectively laser raman spectra identified magnetite hematite chalcopyrite and pyrite at s type inclusions the quartz veins in stage i and ii contain all inclusion types l type and v type inclusions are recognized in stage iii quartz minerals but only l type inclusions are present in stage iv quartz minerals fluid inclusions in quartz of stage i ii iii and iv are characterized by homogenization temperature vary of 275 492 c 232 477 c 199 287 c and 133 227 c with corresponding nacl equiv salinities of 2 2 58 7 wt nacl equiv 1 4 58 3 wt nacl equiv 1 1 6 0 wt nacl equiv and 1 1 4 2 wt nacl equiv respectively initial ore forming fluids of the tuwu deposit belong to relatively oxidized h2o nacl system with moderate high temperature and salinity sulfur isotope data from chalcopyrite and pyrite suggest a magmatic source two types of chlorite type 1 and type 2 are recognized based on the relationship with metal sulfides electron microprobe analysis shows that chlorites are plotted in ripidolite and pycnochlorite decreasing fe fe mg ratios of chlorites indicate that the meteoric water was involved in the magmatic water combined with the positive correlation with aliv implying that the chlorite formed in a relatively reduced and acidic environment the hydrothermal fluids in type 1 chlorite bring large quantities of reduced metallic minerals compared to the barren fluids in type 2 chlorite the formation temperatures of chlorite are roughly coincident with the results that obtained from fluid inclusion studies which show a decrease trend changes in temperature redox ph and fluid rock interaction likely play an important role in the transportation and precipitation of metal sulfides of the tuwu deposit

is the gulf stream responsible for europe s mild winters is the transport of heat northward by the gulf stream and north atlantic drift and its subsequent release into the midlatitude westerlies the reason why europe s winters are so much milder than those of eastern north america and other places at the same latitude here it is shown that the principal cause of this temperature difference is advection by the mean winds south westerlies bring warm maritime air into europe and north westerlies bring frigid continental air into north eastern north america further analysis of the ocean surface heat budget shows that the majority of the heat released during winter from the ocean to the atmosphere is accounted for by the seasonal release of heat previously absorbed and not by ocean heat flux convergence therefore the existence of the winter temperature contrast between western europe and eastern north america does not require a dynamical ocean two experiments with an atmospheric general circulation model coupled to an ocean mixed layer confirm this conclusion the difference in winter temperatures across the north atlantic and the difference between western europe and western north america is essentially the same in these models whether or not the movement of heat by the ocean is accounted for in an additional experiment with no mountains the flow across the ocean is more zonal western europe is cooled the trough east of the rockies is weakened and the cold of north eastern north america is ameliorated in all experiments the west coast of europe is warmer than the west coast of north america at the same latitude whether or not ocean heat transport is accounted for in summary the deviations from zonal symmetry of winter temperatures in the northern hemisphere are fundamentally caused by the atmospheric circulation interacting with the oceanic mixed layer copyright 2002 royal meteorological society

pulses of enhanced continental weathering associated with multiple late devonian climate perturbations evidence from osmium isotope compositions anomalously high rates of continental weathering have frequently been proposed as a key stimulus for the development of widespread marine anoxia during a number of late devonian environmental and biospheric crises which included a major mass extinction during the frasnian famennian transition marked by the upper and lower kellwasser horizons here this model is investigated by presenting the first stratigraphic record of osmium isotope trends 187os 188os in upper devonian strata from the kowala quarry holy cross mountains poland changes in reconstructed 187os 188os seawater values to more radiogenic compositions are documented at the base of both the lower 0 42 to 0 83 and upper 0 31 to 0 81 kellwasser horizons characteristic of the frasnian famennian transition and additionally within upper famennian shales that record a more minor environmental perturbation known as the annulata event 0 20 to 0 53 these shifts indicate the occurrence of extremely enhanced continental weathering rates at the onsets of the kellwasser crises and during the later annulata event the similarity of 187os 188os values in this study from frasnian famennian boundary and lower famennian strata between 0 4 and 0 5 to those from north american stratigraphic equivalents suggests that the 187os 188os values record global trends these findings support a causal relationship between increased continental weathering and thus nutrient supply to the marine shelf and the environmental perturbations that occurred during numerous late devonian events including both of the biospherically catastrophic kellwasser crises as well as other less severe oceanic anoxic events

paleozoic accretionary and collisional tectonics of the eastern tianshan china implications for the continental growth of central asia this paper deals with the various tectonic units in the chinese eastern tianshan orogenic collage in the central asian orogenic belt and discusses the paleozoic geological history of the several periods of accretion and collision of archipelago systems lying between the tarim and southern angaran continental margins the late ordovician silurian to early devonian eastern tianshan archipelago was characterized by a the harlik dananhu subduction system with a s dipping polarity in the north b a southerly n dipping subduction system beneath the central tianshan arc in the middle and c the south tianshan ocean against tarim in the south during the devonian to early carboniferous n dipping subduction led to the harlik dananhu arc and the kanggurtag forearc basin accretionary complex in the early to mid carboniferous the magmatic front associated with the n dipping subduction beneath the dananhu harlik arc migrated southwards forming the yamansu arc constructed upon the kanggurtag accretionary forearc by the late carboniferous the dananhu harlik arc was attached northwards to the angaran margin resulting in lateral enlargement of the angaran continent in the latest carboniferous to early permian a multiple soft collision left wide suture zones in the south that include the ophiolite strewn aqikkuduk shaquanzi and kumishi accretion collision complexes which were stitched by early permian post collisional plutons by re defining and re interpreting the various tectonic terranes this paper presents a new improved model for the paleozoic evolution of this part of central asia

characteristics and genesis of kanggur gold deposit in the eastern tianshan mountains nw china evidence from geology isotope distribution and chronology abstract the kanggur gold deposit lies in east tianshan mountains eastern section of central asia orogenic belt the gold mineralization occurs on the northern margin of the aqishan yamansu island arc in the tarim plate it is mainly hosted in lower carboniferous volcanic rocks and controlled by syn tectonic intrusions and the ductile shear zone the host rock is a calc alkaline volcanic rock series consisting of andesite dacite tuff and subvolcanic rocks the gold ore bodies were distributed in transition zone from strong ductile deformation to weak deformation ore controlling structure is ew trending normal brittle ductile shear zone the gold ore bodies are composed of au bearing altered rocks and a few au bearing sulfide quartz veins the mineralization shows vertical zonation in which au was enriched in the upper orebodies au pb zn were enriched in the middle and au cu were enriched in the lower orebodies according to mineral assemblages and alteration intensity hydrothermal alteration could be divided into three zones silicification chloritization pyritization and magnetization zone major ore body pyrite phyllic rock and or sericite alteration zone and sericite chlorite alteration zone five mineralization stages are identified on the basis of mineral assemblage and crosscutting relationship of veins the 34 s values of sulfide in the ores range from 0 9 to 3 3 similar to those of the volcanic and subvolcanic rocks 3 21 1 3 indicating that the sulfur isotope composition is characterized by unfractionated mantle sulfur the lead isotope compositions of ores are between those of the volcanic rocks and subvolcanic rocks the initial 87 sr 86 sr ratios of the ores vary between 0 7077 and 0 7106 similar to those of the host rocks 0 7079 0 7125 according to the sr s and pb isotopic data it is indicated that the ore forming materials were derived from the surrounding volcanic rocks and feature deep source the 18 o values of the ore fluids vary from 3 83 to 1 77 and the d values from 46 to 66 the 18 o vs d and 18 o vs 87 sr 86 sr diagrams indicate that ore fluids were derived from dynamic metamorphic water magmatic water and meteoric water the ore forming ages of kanggur gold deposit are determined by rb sr and sm nd isochron methods the results show that the main ore forming stages of kanggur gold deposit are 290 282 ma secondary mineralization stages are 275 254 ma studies on regional geology ore geology isotope geochemistry and ore forming ages have proved that the kanggur gold deposit was formed in the compression extension transition stage during the latest paleozoic continental collision and is characterized by orogenic gold deposits

petrography geochemistry and geochronology of the host porphyries and associated alteration at the tuwu cu deposit nw china a case for increased depositional efficiency by reaction with mafic hostrock tuwu is the largest porphyry copper deposit discovered in the eastern tianshan mountains xinjiang china a newly recognized volcanic complex in the early carboniferous qi eshan group at tuwu consists of basalt andesite and diorite porphyry the plagiogranite porphyry was emplaced into this complex at 332 8 2 5 ma u pb zircon sims determination whole rock element geochemistry shows that the volcanic complex and plagiogranite porphyry formed in the same island arc although the complex was derived by partial melting of the mantle wedge and the plagiogranite porphyry by partial melting of a subducting slab the diorite and the plagiogranite porphyries have both been subjected to intense hydrothermal alteration and associated mineralization but the productive porphyry is the plagiogranite porphyry three alteration and mineralization stages including pre syn and post ore stages have been recognized the pre ore stage formed a barren propylitic alteration which is widespread in the volcanic complex the syn ore stage is divided into three sub stages stage 1 is characterized by potassic alteration with chalcopyrite bornite chalcocite stage 2 is marked by chlorite sericite albite alteration with chalcopyrite pyrite bornite stage 3 is represented by phyllic alteration with chalcopyrite pyrite molybdenite the post ore stage produced a barren argillic alteration limited to the diorite porphyry a specific feature of the tuwu deposit is that the productive porphyry was emplaced into a very mafic package and reaction of the resulting fluids with the ferrous iron rich hostrocks was a likely reason that tuwu is the largest porphyry in the district

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

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

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab

paleozoic accretion and cenozoic redeformation of the chinese tien shan range central asia the tien shan range in central asia contains two late paleozoic sutures the older southern suture marks the collision of a passive margin at the north of the tarim block and an active continental margin subduction under the latter was to the north the younger northern suture separates a northern carboniferous island arc from an active continental margin developed over a south dipping subduction zone the subduction direction under the island arc is unknown mesozoic elastics were deposited over the doubly sutured orogen rate and energy of sedimentation waned until deposition of oligocene conglomerates above a regional unconformity interpreted as marking the onset of deformation induced by the india asia collision molasse deposition accelerated in pliocene and quaternary time and deposition continues today as active thrusts generate relief paleozoic structures control the gross divergence of cenozoic thrusts across the orogen

underplating related adakites in xinjiang tianshan china abstract intermediate to acidic porphyries and dacites in the awulale and sanchakou area of the xinjiang tianshan region china conform to the definition of high sio2 adakite hsa these volcanic or subvolcanic rocks are characterized by high na2o na2o k2o high sr y 51 327 and strong depletions in hree and hfse nb ta and ti despite these characteristics they are distinct from a suite of adakites in the same area that are inferred to be derived from partial melting of subducted slab differences between the two suites are evident mainly in their rock association isotopic ages and compositions of sr and nd isotopes the subducted slab related adakites are associated with common island arc rocks niobium enriched basalt neb and high mg andesite hma formed in the late carboniferous 320 334 ma and are characterized by relative high nd t 3 4 11 6 and low 87sr 86sr i ratios 0 7032 0 7063 in contrast the awulale and sanchakou adakites were generated during the middle to late permian 260 278 ma possess a wide range of low nd t 0 75 5 69 a greater range of 87sr 86sr i 0 7039 0 7054 and have young nd model ages t2dm 472 699 ma in addition their mg 35 56 and compatible element contents mgo 0 93 2 22 wt cr 4 95 16 41 ppm ni 2 9 25 8 ppm are relative low compared to the subducted slab related adakites mg 55 71 mgo 1 22 6 78 wt cr 24 132 ppm ni 2 28 45 61 ppm based on these characteristics the regional association of igneous rocks and evidence from a global geoscience transect ggt the awulale and sanchakou adakites represent a genetically distinct suite in the xinjiang region we proposed that the source of these adakites was basaltic rock underplated to the base of the lower crust that partial melted at the rutile bearing amphibole eclogite facies 650 c and 1 5 gpa minimum pressure 50 km

oyu tolgoi mongolia siluro devonian porphyry cu au mo and high sulfidation cu mineralization with a cretaceous chalcocite blanket the oyu tolgoi porphyry cu au mo deposit is located in the gobi desert of southern mongolia the deposit consists of three main mineralized zones north central and south oyu interpreted to constitute at least two separate porphyry copper centers oyu tolgoi is associated with a series of small structurally controlled stocks and dikes of monzonitic and dioritic composition which intrude a sequence of siluro devonian volcanic and sedimentary rocks country rocks consist of pillow basalt and basaltic andesite interbedded with fine grained clastic sedimentary horizons the sequence is interpreted to have formed in a subaqueous island arc volcanic environment which formed part of the extensive paleozoic tuva mongol arc these island arc rocks are covered by carboniferous terrigenous sequences and are intruded by carboniferous syenite and granite and permian alkaline granite much of the area is covered by poorly consolidated cretaceous sedimentary sequences central oyu consists of a multiple phase hydrothermal breccia crosscutting an altered fine grained feldspar porphyry containing porphyry type alteration and mineralization the breccia underwent texture destructive advanced argillic alteration characterized by several associations of quartz alunite dickite pyrophyllite sericite and zunyite plus lesser svanbergite and fluorite these associations overprint earlier formed k silicate and quartz sericite illite assemblages copper mineralization at central oyu is present in a supergene chalcocite blanket that formed at the expense of a pyrite rich hypogene chalcocite covellite tennantite arsenosul vanite sulvanite chalcopyrite bornite sulfide suite that accompanied the advanced argillic alteration event at south oyu a feldspar hornblende porphyry of monzonitic composition intrudes a sequence of fine grained andesite and basaltic andesite hypogene copper gold mineralization occurs as early magnetite rich pyrite poor quartz chalcopyrite bornite stockworks and sheeted veinlets hydrothermal alteration is composed of biotite and k feldspar with apatite and minor albite magnetite averages 7 to 10 vol percent and copper and gold grades vary sympathetically these early assemblages are overprinted by a sericite chlorite event with ore grade copper and erratic gold and molybdenum values and also by structurally controlled pyrite rich advanced argillic alteration k ar ages show that the hypogene copper gold mineralization at south oyu is latest silurian earliest devonian 411 3 ma and that the entire district was intruded by postmineralization syenite during the carboniferous 307 4 ma the supergene chalcocite blanket at central oyu formed in the early cretaceous earliest late cretaceous based on ages for supergene alunite of 117 1 and 93 1 ma which makes central oyu one of the oldest preserved chalcocite blankets in the world chalcocite blanket preservation was favored by climatic desiccation into the late cretaceous and the tertiary it is inferred that significant regional uplift took place at the silurian devonian boundary this synmineral ization uplift and erosion may have been in part responsible for the telescoping of the hydrothermal systems at oyu tolgoi including the superposition of epithermal style associations over higher temperature deeper seated assemblages typical of the porphyry environment

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

partial melting of slab window margins genesis of adakitic and non adakitic magmas abstract when a mid ocean spreading ridge subducts it typically splits apart at depth to form two tapered slab edges separated by asthenospheric mantle within a slab window we examine the fate of the slab edges using simplified slab window geometries specific thermal parameters and assumptions regarding shear stress and slab hydration six fundamental zones of slab anatexis are identified and classified according to expected melt and restite compositions non adakitic melts of granodioritic to tonalitic composition are generated along the plate edges at depths of 5 65 km whereas adakitic melts form proximal to the edges at depths of 25 90 km as anatexis proceeds the subducted crust is converted to a migmatite of slab melt and eclogitic restite much of the migmatite may shear away from the slab and become incorporated into the mantle the melts will rise and leave behind fragments of restite within mantle peridotite if the peridotite is part of the overriding plate then the restite fragments may become long term residents of the continental lithospheric mantle however if the restite becomes entrained in the asthenosphere it may undergo upwelling and melting or flow away as ductile streaks to become long term mantle heterogeneities slab windows are thereby identified as important sites for slab melting and geochemical replenishment of the mantle