a new indicator of movement direction during orogenesis measurement technique and application to the alps the orientation of axes of sigmoidal staircase or spiral inclusion trails within porphyroblasts provides an indicator of the direction of movement during deformation that is synchronous with metamorphism a simple technique is presented in detail to find this axis in 3d by radially sectioning a horizontal slab cut from an oriented sample when viewed from one direction the switch in asymmetry of the porphyroblast inclusion trails in these sections defines the trend of the axis further radial sectioning of a vertical slab cut parallel to this trend determines the plunge of this axis this technique is independent of the model adopted for the formation of sigmoidal or spiral shaped inclusion trails and can be used to evaluate the mechanism by which they form it can also be used to evaluate theories of folding and orogenesis measurements of spiral and sigmoid axes in garnet porphyroblasts from the european alps show that they reflect the movement of the african plate relative to europe better than linear indicators preserved within the same rocks

geology of the sari gunay epithermal gold deposit northwest iran the sari gunay epithermal gold deposit is located within a mildly alkaline latitic to trachytic volcanic complex in central northwest iran intrusive and volcanic rocks that host the deposit have been dated at between 11 7 and 11 0 ma with one younger sample at 8 0 ma 40ar 39ar dating of igneous biotite and hornblende whereas sericitic alteration associated with an early stage of hydrothermal activity occurred between 10 8 and 10 3 ma the best age estimate is 10 7 ma obtained by 40ar 39ar dating of sericite the main hydrothermal system was structurally localized by a diatreme breccia complex centered on the sari gunay hill earliest hydrothermal activity only encountered at depth 300 m below surface produced weak potassic alteration and quartz sulfide magnetite veining with low grades of cu and au mineralization reminiscent of shallow level 1 5 km depth porphyry gold systems paragenetically later quartz tourmaline breccias and veins followed the south southwest north northeast trend of the diatreme breccias and produced strong sericitic alteration and silicification in igneous clasts and wall rocks fluid inclusions indicate temperatures of 246 to 360 c and salinities of 34 4 to 46 1 wt percent nacl equiv in hypersaline inclusions coexisting with low density co2 bearing vapor phase inclusions these fluids do not appear to have deposited gold but the breccias provided further structural permeability for flow of later mineralizing epithermal fluids invisible gold occurs in solid solution in fine grained arsenical pyrite and minor arsenopyrite deposited in the early stages of quartz adularia pyrite stibnite veins late stage minerals in these veins include realgar orpiment cinnabar and extremely rare wire gold liquid rich fluid inclusions in these veins have an average homogenization temperature trapping temperature of 199 24 c salinities of 3 6 1 1 wt percent nacl equiv and coexist with low density co2 bearing vapor phase inclusions suggesting low pressure conditions phase separation may have been a contributory cause of gold deposition but precipitation of sulfides in response to rapid cooling of the system may have been the main control on deposition of pyrite hosted gold the sari gunay gold deposit and its host volcanic system form part of the takab belt of structurally controlled middle miocene mildly alkaline volcano plutonic complexes the sediment hosted zarshuran gold deposit located 190 km to the north northwest along the belt is also related to middle miocene magmatism the takab belt is marginally younger than and displaced to the southwest of the extensive eocene miocene urumieh dokhtar calk alkaline volcanic arc which hosts several large porphyry cu deposits e g sar cheshmeh meiduk rocks of the takab belt are also distinctly more alkaline than the urumieh dokhtar rocks viewed in the context of plate tectonic models for neogene closure of a neo tethys ocean between the afro arabian and eurasian plates these differences may reflect a normal subduction zone origin for the urumieh dokhtar arc and contained porphyry cu deposits but a collisional origin for the slightly younger takab belt volcanic rocks and epithermal gold deposits the sari gunay deposit may thus be classified as a collision related alkalic type epithermal system although it is less alkaline than classic deposits of this group such as porgera emperor or cripple creek the latter apparently being related to postcollisional or back arc extensional rather than syncollisional processes

ore genesis of axi post collisional epithermal gold deposit western tianshan nw china constraints from u pb dating hf isotopes and pyrite in situ sulfur isotopes abstract the axi gold deposit has long been regarded as a typical paleozoic low sulfidation epithermal deposit located in the western tianshan in the central asian orogenic belt caob sims zircon u pb dating of the ore bearing andesite and dacite yielded concordia ages of 350 8 2 7 ma and 351 3 3 1 ma respectively in situ sims hydrothermal rutile u pb dating yielded two 207pb corrected ages as 306 1 16 9 ma and 303 8 14 6 ma respectively these ages are considerably younger than the zircon sims u pb ages of ore bearing volcanic rocks which precludes a genetic link between the epithermal gold mineralization and the subduction related late devonian early carboniferous magmatism in the western tianshan instead the gold mineralization is interpreted to be genetically related to a buried 300 ma pluton that formed in a post collisional environment combined with previously published zircon hf isotopic compositions of 300 ma felsic intrusive rocks from western tianshan the results show that 300 ma felsic rocks have higher zircon ehf t values than those from 351 ma felsic rocks in the axi deposit these elevated ehf t values suggest that mantle material inputs of 300 ma axi epithermal deposit in a post collisional environment pyrite the most common sulfide in the axi ores displays heterogeneous textures and has a large variation of as contents and sulfur isotopes suggesting complex ore forming processes in the axi deposit this study highlights the importance of in situ isotopes of hydrothermal minerals to decode the ore forming histories of epithermal systems

reply to comment by cole dai et al on climatic impact of the long lasting laki eruption inapplicability of mass independent sulfur isotope composition measurements here we respond to the comments by cole dai et al 2014 on our article schmidt et al 2012 specifically in response to section 2 of their reply we argued in schmidt et al 2012 that based on previously published estimates of the volatile release height during the 1783 1784 c e laki eruption the lack of a sulfur massindependent fractionation mif anomaly is expected in other words no previous study on laki ever argued that this eruption emitted so2 into altitudes 13 15 km in section 2 3 cole dai et al 2014 argue that the nonzero 33s value of their laki sample 1 may be explained by a short lived explosive phase at laki during which volatiles reached the stratosphere in schmidt et al 2012 in section 2 we argued in agreement with cole dai et al 2014 section 3 1 that for a mif anomaly to be preserved the laki volatiles would have had to be emitted in 20 km altitude our main point is that eruption column heights 20 km are unlikely based on the historical accounts and plume rise modeling for the laki eruption stothers et al 1986 woods 1993 thordarson and self 2003 in schmidt et al 2012 we argued that to deduce a short lived climatic impact of the laki eruption based on the lack of a mif anomaly and the length of the sulfate deposition in greenland ice cores may be misleading because the climatic impact will outlast the radiative forcing of the laki aerosol cloud cole dai et al 2014 acknowledge the latter in their reply in section 4 2 we agreewith cole dai et al 2014 in that themagnitude and length of the climatic impact during the winter of 1783 1784 depends on the altitude of the volatile release during the eruption sections 2 3 and 4 2 however even if we assumed that during laki all sulfur dioxide so2 would have been released in the troposphere then the aerosol cloud would still be present in the upper troposphere during march 1784 as is evident from independent model simulations of this tropospheric only scenario stevenson et al 2003 we acknowledge that there is uncertainty on the volatile release height for laki however it is worth considering that those climate model simulations that used an injection altitude between 9 km and 13 km for the laki so2 highwood and stevenson 2003 oman et al 2006a 2006b schmidt et al 2012 best match the observed temperature changes during summer of 1783 angell and korshover 1985 brzdil et al 2010 briffa et al 1998 d arrigo and jacoby 1999 jacoby et al 1999 kington 1988 manley 1974 parker et al 1992 thordarson and self 2003 based on these model simulations a climatic impact during the winter of 1783 1784 albeit weaker than during the climactic phases of laki is expected and our argument here does not exclude the role of natural variability in contributing to the cold winter of 1783 1784 as discussed in schmidt et al 2012 therefore we continue to argue that for high latitude eruptions such as laki the applicability of sulfur isotopic measurements to interpret the climatic relevance has yet to be demonstrated itmay transpire that the interpretation of mif signals for the climate relevance of an eruption is valid and unambiguous only for short lived explosive eruptions in the tropics in terms of the processes producing a mif anomaly section 3 3 in cole dai et al 2014 the works by hattori et al 2013 and ono et al 2013 suggest that there are remaining issues not discussed by cole dai et al 2014 for instance self shielding of so2 due to high column densities typical for eruptions of pinatubo scale and greater and the preservation of the mif signature in general

high latitude eruptions cast shadow over the african monsoon and the flow of the nile 1 nile river records indicate very low flow following the 1783 1784 laki volcanic eruption as well as after other high latitude volcanic eruptions as shown by climate model simulations of the laki eruption significant cooling 1 to 3 c of the northern hemisphere land masses during the boreal summer of 1783 resulted in a strong dynamical effect of weakening the african and indian monsoon circulations with precipitation anomalies of 1 to 3 mm day over the sahel of africa thus producing the low nile flow future high latitude eruptions would significantly impact the food and water supplies in these areas using observations of the flow of the nile river this new understanding is used to support a date of 939 for the beginning of the eruption of the eldgja volcano in iceland the largest high latitude eruption of the past 1500 years

a model of the plumes above basaltic fissure eruptions a simple model of the ascent of the volatiles above basaltic fissure eruptions shows that atmospheric moisture may play an important role in injecting volatiles high into the atmosphere as ambient water vapour is entrained and carried upwards by the plume it decompresses and some condensation may occur this causes the release of latent heat which heats up the air and thereby increases the buoyancy of the plume enabling it to ascend several kilometres higher than in a dry atmosphere the height of such plumes also increases with the mass fraction of fine ash in the fountain although very simple the model predictions are in accord with observations of plume heights during historical eruptions including the 1984 eruption of mauna loa

columbite group minerals and mica of peraluminous granite record the magmatic hydrothermal processes that formed the zhaojinggou tanb deposit in the north china craton abstract the newly discovered zhaojinggou ta nb deposit in the northern margin of the north china craton contains ore reserves of more than 38 mt at an average grade of 0 013 wt ta2o5 and 0 012 wt nb2o5 the ta nb mineralization is mainly hosted in a peraluminous albite granite body which is about 400 m long and 20 to 100 m wide with a surface exposure of 0 05 km2 granitic pegmatites and greisen veins that range from several centimeters to meters in width occur in the margin of the granite body and wolframite quartz veins intrude the wall rocks up to tens of meters away from the granite contact la icp ms u pb dating for columbite of the granite yields the lower intercept ages between 131 8 1 0 ma and 134 6 2 1 ma on the tera wasserburg concordia diagram indicating that the mineralization occurred in the early cretaceous coeval with the peak stage of lithospheric thinning and destruction of the north china craton the columbite grains in the granite are commonly euhedral and are surrounded or penetrated by irregular tantalite patches rims containing ta2o5 as high as 59 5 wt the ta concentrations within single columbite crystals commonly increase slightly from the center outwards consistent with a systematic decrease of nb ta caused by crystallization of columbite from the silicic magma mica in the granite shows disordered bright and dark domains in their bse images bright domains are f rich zinnwaldite crystallized from highly evolved melts that were relatively enriched in fe li rb nb and ta whereas the dark domains are f poor muscovite crystallized from later hydrothermal fluids that were relatively enriched in al mn ba sn and w it is thus likely that the tantalite patches rims around the columbite grains crystallized from a ta and f rich hydrosilicate melt in a magmatic hydrothermal transitional stage the wolframite quartz veins adjacent to the granite body likely formed from a w rich hydrothermal fluid

geology and geochemistry of the early permian axi low sulfidation epithermal gold deposit in north tianshan nw china abstract the axi gold deposit containing 12 mt of ore with an average au grade of 5 g t is the largest low sulfidation epithermal gold deposit in chinese tianshan it is hosted by late devonian to early carboniferous continental arc volcanic rocks in the tulasu volcanic sedimentary basin in north tianshan nw china the orebody is controlled by annular and radial faults associated with volcanic crater and later nw trending faults four stages of ore formation could be identified based on petrographic studies i chalcedony ii gold bearing pyrite quartz veins and breccias iii gold bearing marcasite quartz carbonate veins and iv quartz barite calcite veins electrum precipitated in stage ii 285 195 c log f s 2 6 7 to 13 and stage iii 190 95 c log f s 2 15 8 to 25 6 the chalcedony formed in stage i is strongly enriched in lree la yb 4 5 36 2 compared with later gold bearing pyrite quartz veins and quartz barite calcite veins la yb 1 2 2 0 o h isotopic data indicate that fluid mixing is a major mechanism causing electrum precipitation in stage ii hydrothermal breccias consisting of chalcedony altered wall rocks and gold bearing pyrite quartz veins stage ii are cemented by gold bearing marcasite quartz carbonate assemblages stage iii which implies that brecciation plays an important role in stage iii ore formation re os isotopic analyses of gold bearing pyrite samples yield an 187 re 188 os 187 os 188 os isochron age of 299 35 ma with an initial 187 os 188 os ratio of 0 57 0 37 k ar ages of four intensively sericitized andesitic wall rocks range between 286 and 293 ma these early permian ages are much younger than the late devonian early carboniferous host volcanic rocks but overlap the age range of postcollisional granitoid in the tulasu basin the re os isotopic compositions and the ages suggest that mantle derived components were involved in the axi ore forming system in a postcollisional setting

distinguishing and correlating multiple phases of metamorphism across a multiply deformed region using the axes of spiral staircase and sigmoidal inclusion trails in garnet schists from the appalachian orogen in south east vermont have undergone multiple phases of garnet growth these phases can be distinguished by the trend and relative timing of foliation inflexion or intersection axes fias of foliations preserved as inclusion trails in garnet porphyroblasts the relative timing of different generations of fias is determined from samples containing porphyroblasts with two or three differently trending fias developed outwards from core to rim multi fia porphyroblasts schists from south east vermont show a consistent pattern of relative clockwise rotation of fia trends from oldest to youngest four populations or sets of fias can be distinguished on the basis of their relative timings and trends from oldest to youngest the four sets have modal peaks trending sw ne w e nnw sse and ssw nne these peaks show that each of the four fia sets has a statistically consistent trend at all scales across a 35 125 km area containing numerous mesoscopic and macroscopic folds the fias of set 4 are defined by inclusion trails that are continuous with matrix foliations have trends subparallel to most folds and are inferred to have developed contemporaneously with these structures conversely sets 1 to 3 are oblique to and pre date most matrix foliations and folds all four fia sets occur in siluro devonian rocks and must have formed in the acadian orogeny the lack of statistically significant differences in the distribution of fia trends across the study area and their consistent relative timings in multi fia porphyroblasts despite a complex regional deformation history involving numerous phases of folding at all scales suggest the porphyroblasts have not rotated relative to one another the change in fia trend with time resulted from rotation of the kinematic reference frame of bulk flow possibly as a consequence of the reorganization of lithospheric plates responsible for acadian orogenesis recognition of distinct generations of fias provides a means of distinguishing different phases of porphyroblast growth four periods of garnet porphyroblast growth occurred in the schists of south east vermont this growth was heterogeneously distributed on the cm2 m2 scale no single porphyroblast records all stages of growth and adjacent samples from the same or dissimilar rock types commonly contain porphyroblasts that preserve different sequences of growth factors that may have been responsible for switching porphyroblast growth on and off at this scale include i subtle differences in bulk chemical composition ii oscillating levels of heat owing to the buffering effect of endothermic garnet forming reactions iii channelized infiltration of fluids with localized fluid buffering of bulk composition and iv cyclic controls on the rates of diffusion and material transport of reactants either by channelized fluid flow or by a changing pattern of microfracturing during foliation development consistency in fia trend and relative timing provide a new method for potentially distinguishing and correlating successive metamorphic events or even phases of metamorphism within a progressive tectonothermal event along and across orogens using a consistent pattern of core to rim changes in fia trend multiple phases of growth of a single porphyroblastic mineral can be quantitatively distinguished allowing correlation of different phases of growth around and across macroscopic folds the relative timing of growth of different porphyroblastic minerals can also be quantitatively determined using fia data and correlated around and across macroscopic folds conceptually the paragenetic history preserved in each generation of porphyroblast growth in the form of chemical zoning and the minerals in inclusion trails could be combined to produce a more detailed p t t deformation path than previously determined

hydrothermal processes at the axi epithermal au deposit western tianshan insights from geochemical effects of alteration mineralization and trace elements in pyrite abstract as the largest low sulfidation ls epithermal deposit in the tulasu basin western tianshan nw china the axi gold deposit has a poorly understood geologic history it is not clear whether hydrothermal mineralization occurred during a single event or multiple events in this paper new data consist of whole rock geochemistry and mass change calculations for hydrothermal alteration la icp ms trace element analysis of pyrite was conducted in order to develop a comprehensive metallogenic model the pyrite sericite quartz alteration at the axi deposit is characterized by replacement of plagioclase and mafic phenocrysts by sericite quartz pyrite py1 smectite and illite indicating slightly acidic and reducing conditions during the early ore stage medium subhedral pyrite crystals py1 with porous texture are typically sited adjacent to iron rich minerals e g hornblende and are characterized by co ni mn enrichment with high ratios of co ni avg 1 114 and cu as avg 0 655 implying preferential sulfidation reactions in magmatic hydrothermal systems the quartz calcite pyrite adularia assemblages in silicified rocks and the strong mobility of lree both reflect a slightly acidic chloride rich solution phase during the later ore stage coarse anhedral pyrites with cataclastic texture py2 and medium euhedral pyrites py3 have a mixed hydrothermal origin and were deposited with quartz during au pyrite quartz and au polysulfide quartz stages respectively py2 and py3 both have significantly lower cu as ratios avg 0 103 and avg 0 088 respectively than py1 indicating the input of meteoric fluid during the later ore stage the three generations of pyrite especially py3 have significant lattice bound gold contents py1 0 01 38 86 ppm median 1 51 ppm py2 0 03 234 37 ppm median 1 21 ppm py3 3 31 231 56 ppm median 42 72 ppm gold precipitation is associated with the destabilization of au bearing chlorine au cl 2 and bisulfide complexes au hs 2 via sulfidation and fluid boiling all of these characteristics indicate the axi epithermal deposit is the result of two stages of mineralization

origin of the mclaughlin mine sheeted vein complex metal zoning fluid inclusion and isotopic evidence the mclaughlin deposit is a large hot spring type gold deposit located in the northern coast ranges of california at the faulted lithologic contact between the coast range ophiolite and the great valley sequence the mclaughlin deposit is centered around the sheeted vein complex a large multistage vein swarm localized in a dilation zone formed by rheologic contrasts in the footwall polymictic melange the surface expression of the sheeted vein complex is a subaerial sinter terrace which merges into the sheeted vein complex metal zoning in the sheeted vein complex is highly telescoped the sinter is enriched in mercury whereas gold and silver are restricted to the upper 350 m with the proportion of gold to silver decreasing with depth gold is typically present as electrum and is associated with silver and base metal bearing sulfosalts below 350 m mineralization is dominated by small quartz veins with base metal sulfides fluid inclusion studies indicate that nacl is the main dissolved salt in concentrations averaging 2 4 wt percent nacl equiv the bulk of the salinity data can be explained by variable amounts of adiabatic boiling and mixing with steam heated ground water fluid inclusion homogenization data ranges from 121 degrees to 263 degrees c with the deepest portion of the system examined having the highest temperatures and the shallowest samples having the lowest temperatures fluid inclusion gas concentrations vary with depth below the sinter co 2 and to a lesser extent ch 4 shows a systematic increase with depth fluid pressures calculated from fluid inclusion homogenization temperatures and gas analysis are in excess of the hydrostatic pressure and approach or exceed lithostatic pressure self sealing of the system and tectonic dilation most likely resulted in catastrophic phase separation and hydrothermal eruption breccias geothermometry results based on fluid inclusion gas ratios show a reasonable correspondence with the highest fluid inclusion homogenization temperatures there is a strong trend for decreasing delta 18 o quartz values with depth due to the effect of temperature the calculated delta 18 o sub h 2 values are fairly constant at about 9 to 10 per mil oxygen and hydrogen isotope values as well as geologic relationships are consistent with those of hydrothermal fluids originating as meteoric water these evolved to increasingly high ones by interaction with great valley sequence sedimentary rocks lead isotope compositions of sulfides and potential source rocks indicate that lead and by inference other metals was derived from a crustal source the lead source may have been detritus from the sierra nevada batholith that comprises the great valley sequence or alternatively it may have been derived from a thorough mixture of radiogenic lead and nonradiogenic lead that occurred prior to mineralization

cenozoic kinematic history of the kohistan fault in the pakistan himalaya detailed mapping suggests a new scenario for the kinematic history of the late cretaceous kohistan arc complex and for the cause of cenozoic metamorphism and exhumation along the indian continental margin in pakistan the earliest deformational phases resulted in early cenozoic amphibolite facies metamorphism superposed tight to isoclinal f1 f2 folds and development of strong penetrative rock fabric within the indian plate in response to southwestward and then southeastward overthrusting of ophi olitic melange in the indus suture zone and west pakistan fold belt respectively this deformation included the development of the banna and malakand thrust faults prograde metamorphism ended following this deformational phase and the early structures were deformed along n trending open upright f3 folds and domes this was followed by ese to e directed thrusting of the kohistan arc complex onto the indian continent along the dominantly brittle kohistan fault thrusting ended in the late oligocene early miocene when the kohistan arc assumed its present position along the northern margin of india south verging structures began to form on the indian plate for the first time in the late oligocene early miocene deformation had expanded to the foreland by the middle to late miocene with development of the panjal khairabad thrust neogene structures in the metamorphic zone include n trending east side up high angle fault zones that deform or reactivate the kohistan fault and nw trending east side up reverse faults both of which are seismically active in the tectonic scenario outlined here shortening along the kohistan fault occurred subsequent to metamorphism and was directed toward the ese e throughout its history the southern termination of the fault is considered to be a right slip sidewall ramp such that the kohistan arc never advanced much father south on the indian plate than its present position broad flexure folding and erosion in front of and south of the actively advancing kohistan arc complex contributed to exhumation of the indian plate this differs from interpretations that state that kohistan was thrust in a southward direction that underthrusting beneath kohistan was the primary cause of early cenozoic metamorphism on the indian plate and that top side north extensional reactivation of the kohistan fault contributed to exhumation

two stage gold mineralization of the axi epithermal au deposit western tianshan nw china evidence from re os dating s isotope and trace elements of pyrite the axi gold deposit is one of the largest epithermal deposits in northwestern china and is characterized by multistage generations of pyrite associated with gold mineralization genetic models for superjacent epithermal au and porphyry cu systems involve either contemporaneous or episodic mineralization implying different metal sources and ore forming events nine pyrite separates from disseminated ores associated with pyrite sericite quartz alteration in the axi deposit yield a re os isochron age of 350 10 ma mswd 5 1 initial 187os 188os ios 0 20 0 11 this age likely reflects a mixed contribution of two subtypes of disseminated pyrite py1 based on zoned pyrite crystals the isochron age of 353 6 ma mswd 1 6 ios 0 107 0 021 n 6 excluding samples with pyrite cores overlaps previous ages for andesite from the dahalajunshan formation ca 356 ma indicating that the early mineralization event occurred shortly after the formation of host rocks ca 353 ma pyrite from disseminated ores shows little variation of sulfur isotopic compositions 2 9 to 4 0 and has high cu co ni and v contents which favors a magmatic hydrothermal origin pyrite grains from gray quartz veins yield a younger re os isochron age of 332 8 ma mswd 0 22 n 4 vein hosted pyrite shows characteristics of a mantle derived magma source ios 0 171 0 024 34s 0 10 3 10 which result from the intrusion driven convective hydrothermal circulation accompanying fluid exsolution and metal remobilization from basement and wallrocks our data demonstrate that the axi gold deposit was formed during two temporally separated hydrothermal events this model of ore formation may apply to other epithermal porphyry deposits within the axi tawuerbieke district disseminated au and porphyry style au cu mineralization occurred shortly after magmatic activity 356 353 ma whereas epithermal style au mineralization occurred at ca 332 ma

geochronology and geochemistry of late paleozoic magmatic rocks in the lamasu dabate area northwestern tianshan west china evidence for a tectonic transition from arc to post collisional setting abstract voluminous late paleozoic igneous rocks and associated cu au mo deposits occur in the northwestern tianshan district xinjiang west china however the tectonic setting and petrogenesis of these rocks remain controversial this paper reports zircon u pb and hf cuu isotopic data major and trace elements and sr nd pb isotopic data for the intrusive rocks and minor dacites in the lamasu dabate area of northwestern tianshan adjacent to the cu au mo deposits la icpms u pb zircon analyses suggest that the lamasu porphyries were formed at 366 3 ma and contain 907 738 ma inherited zircons the dabate dacites were formed at 316 4 ma and granite porphyries were formed at 289 3 ma with 319 ma inherited zircons the lamasu porphyries consist of plagioclase granite and granodiorite and are geochemically similar to adakites e g having high al2o3 14 54 19 75 wt and sr 308 641 ppm and low y 7 84 16 9 ppm contents with fractionated rare earth element ree patterns and slightly positive sr anomalies however they have variable initial ratios of 87sr 86sr 0 7072 0 7076 and 206pb 204pb 18 139 18 450 and variable end t 5 6 to 0 8 and positive ehf t 1 4 to 10 6 values they also have variable mg 100 mg2 mg2 fe2 41 73 and low th 3 13 8 09 and th ce 0 14 0 28 values we suggest that the lamasu adakitic magmas were generated through partial melting of southward subducted junggar oceanic crust with subsequent melt mantle interaction and assimilation of basement rocks the dabate dacites show typical arc like geochemical characteristics e g enrichment of large ion lithophile elements lile and strong negative anomalies of ta nb p and ti with variable end t 0 1 to 3 3 they were probably generated by melting of juvenile basaltic lower crust as a result of magma underplating the dabate granite porphyries are geochemically similar to a2 type granites e g high sio2 75 6 77 6 wt and alkalis na2o k2o 8 27 8 70 wt low cao 0 28 0 34 wt and mg 2 10 and obvious negative eu ba and sr anomalies they have variable values of end t 1 7 to 0 and 206pb 204pbi 18 567 18 721 and zircon ehf t 0 5 to 11 1 the dabate a type granite porphyries contain late carboniferous residual zircon cores suggesting that their source rock possibly contained carboniferous arc igneous rocks taking into account all available data from late paleozoic magmatic rocks and cu mo au mineralization in the northern tianshan district we suggest that the dabate lamasu area was a continental arc during the late devonian carboniferous but had entered a post collisional stage by the early permian 290 ma

porphyroblast rotation versus nonrotation conflict resolution data on foliation intersection and or inflection axes preserved in porphyroblasts fias indicate that no porphyroblast rotation occurs during ductile deformation relative to spatial coordinates this contrasts with 99 of investigations of rigid objects in non coaxially deforming media where the objects rotate when anastomosing shear zone formation around relatively strong objects in a weaker matrix is modeled no porphyroblast rotation occurs formation of these anastomosing zones controls the development of this phenomenon called gyrostasis if such zones are absent porphyroblasts rotate in weak materials the gyrostatic situation arises because the superposition of simple shearing deformation normal to initial coaxial shortening results in only small rotations of principal axes of stress since shear zones are controlled by the orientations of principal axes of stress initial anastomosing zones retain their orientations and positions during subsequent non coaxial deformation the porphyroblast is isolated from the embedding non coaxially deforming matrix but this material close to the porphyroblast continues to deform coaxially no local rotation occurs this has major tectonic significance because allowing for the effects of rotation due to brittle deformation porphyroblasts can now be routinely used to access lengthy structural and or metamorphic histories destroyed in the matrix by reactivation such as movement directions shear senses and extended pressure temperature time paths

mystery of the volcanic mass independent sulfur isotope fractionation signature in the antarctic ice core 1 we simulate the record of sulfur mass independent fractionation mif in sulfates found in the antarctic ice cores from the 1991 eruption of mt pinatubo discovery of mif in sulfur isotopes in archean early proterozoic sediments and lack of mif in sulfur in younger rocks placed a strict upper limit on the amount of oxidants and oxygen in the ancient earth s atmosphere prior to 2 3 gyr ago however recent measurements in ice cores suggest that some non zero mif in sulfate can be delivered to antarctica following large volcanic eruptions current photochemical experiments do not identify the specific reaction responsible for sulfur mif production here we show that a time dependent non zero sulfur mif can be delivered to the ground in the present atmosphere if the mif producing process is so3 photolysis fractionation from such photolysis has not been considered before but is consistent with the available experimental data

geochemical and geochronological studies of granitoid rocks from the western tianshan orogen implications for continental growth in the southwestern central asian orogenic belt abstract the western tianshan orogen in northwestern china is part of the southwestern margin of the central asian orogenic belt caob which represents the largest tectonic assembly of accretionary complexes and the most significant area of phanerozoic continental growth in the world granitoid rocks have a widespread occurrence and occupy about 30 of the whole exposure of the orogen ages obtained for the granitoids vary from 896 ma to 247 ma while initial 87 sr 86 sr ratios vary from 0 70329 to 0 72070 and e nd values from 14 1 to 7 3 the occurrence of neoproterozoic granitic gneisses implies the presence of precambrian basement in this area during oceanic subduction continental growth occurred as a result of several combined processes i e by addition of oceanic crustal melts adakites the intrusion of basaltic magmas derived by partial melting of the metasomatized depleted mantle wedge and the upwelling of granitic magma derived from a mixed source of basaltic magmas and old continental basement however during the post collisional period vertical accretion of underplated juvenile mantle material may have been accomplished during slab breakoff delamination this study suggests a two stage model of continental growth of syn subduction lateral accretion of arc complexes and post collisional vertical accretion of underplated mantle material which may also be relevant for the western segment of the caob in the phanerozoic the reworking and involvement of old crust are more obvious in the western tianshan orogen compared to other parts of the caob comprising the western junggar eastern junggar alataw and altay terranes

rbsr dating of the kohistan arc batholith in the trans himalaya of north pakistan and tectonic implications the kohistan arc batholith in northern pakistan is situated between the indus suture and the northern suture it belongs to the trans himalayan belt which continues eastwards as the ladakh arc batholith in northwest india and the kangdese batholith in southern tibet in kohistan the island arc consists upward sequence of the chilas stratiform complex of norites noritic gabbros and chromite layered dunites which formed in the sub arc magma chamber plutons of tonalite and diorite one of which has a rbsr isochron age of 102 12 ma the chalt volcanics of basaltic tholeiites succeeded by andesites to rhyolites and the yasin group sediments which formed in overlying intra arc basins and which contain albian aptian faunas all these rocks were deformed by major fold structures which are correlated with the formation of the northern suture ne sw trending basic jutal nomal dykes north of gilgit cross cut all the above structures and have a39ar 40ar hornblende age of 75 ma thus the northern suture must have formed in the period 102 12 to 75 ma with the island arc attached to the eurasian plate further northward subduction of the tethyan plate gave rise to an andean type batholith two plutons of which have rbsr isochron ages of 54 4 ma and 40 6 ma the dir utror group of calc alkaline lavas and sediments with eocene fossils are remnants of the volcanic cover of the andean type batholith we suggest that late cretaceous blueschists formed during subduction under the active continental margin and that continental collision and formation of the indus suture was in the eocene the batholith was intruded by layered aplite pegmatite sheets at 34 14 ma and 29 8 ma rbsr ages in post collisional times the87sr 86sr initial ratios of all the dated rocks range between 0 7039 and 0 7052

modeling the distribution of the volcanic aerosol cloud from the 1783 1784 laki eruption 1 we conducted simulations of the atmospheric transformation and transport of the emissions of the 1783 1784 laki basaltic flood lava eruption 64 10 n 17 15 w using the nasa goddard institute for space studies modele climate model coupled to a sulfur cycle chemistry model the model simulations successfully reproduced the aerosol clouds of the 1912 katmai and 1991 mount pinatubo eruptions giving us confidence in the laki simulations simulations of the laki eruption produce peak zonal mean sulfate so4 concentrations of over 70 ppbv during august and into september 1783 in the upper troposphere and lower stratosphere at high latitudes while the majority of the sulfate aerosol was removed during the fall and early winter a significant aerosol perturbation remained into 1784 the peak so2 gas loading was just over 37 megatons mt in late june with the sulfate loading peaking in late august 1783 at 60 mt over the average of 3 runs this yielded a peak sulfate aerosol 75 h2so4 25 h2o loading of over 80 mt with the total aerosol produced during the entire eruption being about 165 mt the resulting sulfate deposition compares well with ice cores taken across greenland the top of atmosphere net radiative forcing peaks at 27 w m2 over the high latitudes during late summer 1783 and produces a global mean forcing of 4 w m2 the model results confirm that northern hemisphere high latitude volcanic eruptions produce aerosols that remain mostly confined north of 30 n latitude

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

from adria to africa driven orogenesis evidence from porphyroblasts in the betic cordillera spain abstract in order to unravel the kinematic evolution of the betic rif orogen we investigated the geometry of porphyroblast inclusion trails in the nevado filabride complex a high pressure unit situated at the base of the betic nappe stack a large number of quantitative orientation data for these microstructures and their mutual overprinting relationships reveal two age groups of inclusion trails in the study area with distinctive geographic trends coupled to a comprehensive compilation of field data in the study area these microstructures are shown to coherently witness the development of early ne sw trending structures and fabrics followed by younger nw se trending ones whereby the former experienced clockwise reorientations a major change in the direction of bulk crustal shortening from nw se to ne sw is inferred from these data and tentatively linked to the plate tectonic history of the mediterranean region paleomagnetic data indicate that prior to about 35 ma nw se convergence between adria eurasia and iberia produced orogenesis along a belt bordering greater iberia including sardinian and corsica and probably continuous in the alps brianconnais domain the roughly ne sw trend of this precursor appears to be preserved within porphyroblasts that grew during this period presumably due to the partitioning of deformation around them africa iberia relative motion had been mainly transcurrent until 35 ma azores transform but then rapidly changed to head on nne ssw convergence this gave rise to a second set of nw se trending structures which accompanied deformation of the precursor orogen into several oroclines western alps gibraltar and calabria arcs orogenic collapse and crustal extension initiated shortly after this transition in tectonic regime suggesting a causal relationship we propose that the waning of compressive stresses transmitted by adria and their substitution by a near perpendicular maximum stress direction resulting from africa eurasia convergence could have enhanced the sinking of a thick lithospheric root or roll back of a subducted slab which in turn would have provoked crustal extension and basin formation

age and source constraints for the giant muruntau gold deposit uzbekistan from coupled re os he isotopes in arsenopyrite the muruntau gold deposit uzbekistan is one of the largest gold deposits known worldwide but its origin remains controversial we used re os arsenopyrite geochronology to precisely determine the age of main stage gold mineralization at muruntau to be 287 5 1 7 ma which overlaps the emplacement of proximal post tectonic granitoid magmatism additionally we suggest that arsenopyrite growth may have occurred over an interval of at least 2 m y os initial ratios derived from arsenopyrite were coupled with he isotopic data from fluid inclusions within arsenopyrite to constrain the source of ore metals and fluids muruntau arsenopyrite yields relatively unradiogenic initial os 0 37 0 27 and elevated 3he 4he ratios 0 23 0 33 r a relative to purely crustal os he reservoirs these data suggest the presence of a mantle derived component in the ore system that was probably introduced during the generation of the granitoid magmas these new timing and source constraints provide important new insight into the generation of this giant gold deposit and they necessitate reexamination of genetic models for muruntau and potentially other giant orogenic gold deposits worldwide

stratigraphic evidence for an early collision between northwest india and asia the collision of india with asia1 had a profound influence on late cretaceous and cenozoic oceanography2 climate3 fauna extinctions4 and the motion of at least some of the earth s litho spheric plates5 as the collision ended a period of rapid indo asian convergence6 a precise knowledge of its timing when the crust of the neo tethys ocean was completely subducted7 at some point along the plate boundary is important for understanding its wider consequences but current estimates of the collision age range from 65 to 38 myr before present6 8 11 here we report the results of extensive biostratigraphic analyses from waziristan and kurram in northwest pakistan which show that accretionary prism and trench strata were first thrust onto the northwest indian passive margin after 66 myr but before 55 5 myr after this time volcanic arc fragments the accretionary prism trench material and imbricates of the north indian slope were raised to shallow water depths and overlapped by upper palaeocene shallow water carbonates and shales12 14 indicative of post collision thrusting in this region finally both the suture and the indian craton were overlapped by continuous unconformable upper lower eocene shallow marine strata demonstrating that suturing was complete by 49 myr

metallogenic potential of porphyries in tulasu basin northwest tianshan insight from magma nature and crustal thickness abstract tulasu basin in northwest tianshan hosts a lot of late paleozoic epithermal deposits such as axi jingxi yelmand and tawuerbieke deposits epithermal deposits are considered as telescoped systems of porphyry deposits however porphyry deposits are not discovered in tulasu basin though granitoid porphyries are widespread in this region tulasu pluton complex as one of the largest intrusions located in northeast part of tulasu basin is composed of the late devonian quartz diorite porphyry and pyroxene diorite porphyry and the late carboniferous granodiorite porphyry and granite porphyry the metallogenic potential of each rock type is evaluated in terms of magma nature and crustal thickness of source the magma nature including magmatic oxygen fugacity and water content is estimated based on trace elements compositions of zircons amphibole hygro barometer and ilmenite magnetite mineral pairs the mean ce4 ce3 ratio magmatic oxygen fugacity and water content of the late devonian quartz diorite porphyry are 291 fmq 1 0 1 8 fmq 0 2 1 5 and 0 9 2 2 wt respectively while those of pyroxene diorite porphyry are 96 fmq 2 6 3 3 and 0 2 1 3 wt the late carboniferous granodiorite porphyry has a mean ce4 ce3 ratio of 138 the crustal thicknesses in the tulasu area are estimated to be 30 35 km at the late devonian and 40 km at the late carboniferous based on whole rock geochemistry of the late devonian dioritic rocks and the late carboniferous granitic rocks considering magmas associated with typical porphyry deposits which have high oxygen fugacity of fmq 1 5 to 3 and water content 4 5 wt and are derived from crustal thickness of 30 km we think both dioritic and granitic rocks of tulasu porphyries have favorable conditions of oxidation state and crustal thickness for porphyry deposit formation however the magmatic water contents of these rocks are relatively low therefore the tulasu porphyries have low potential in forming porphyry cu deposits

basaltic fissure eruptions plume heights and atmospheric aerosols convective plumes that rise above hawaiian style fire fountains consist of volcanic gases aerosols fine ash and entrained heated air plume theory has been applied to observational estimates of the rate of thermal energy release from large fire fountains the theoretically predicted heights of maintained plumes agree very well with the heights found from actual observations predicted plume heights for both central vent point source and fissure line source eruptions indicate a stratospheric penetration by plumes that form over vents with very high magma production rates flood basalt fissure eruptions that produce individual lava flows with volumes 100 km at very high mass eruption rates are capable of injecting large quantities of sulfate aerosols into the lower stratosphere with potentially drastic short term atmospheric consequences like acid precipitation darkening of the sky and climatic cooling

hydrothermal breccias and veins at the kelian gold mine kalimantan indonesia genesis of a large epithermal gold deposit mineralized hydrothermal breccias and veins formed during and after the waning stages of maar diatreme related volcanic activity at kelian kalimantan indonesia subsurface phreatic explosions occurred around the margins of the diatreme breccia complex focusing high temperature fluid flow and generating several large mineralized hydrothermal breccia bodies tectonic phreatomagmatic and hydraulic processes also contributed to hydrothermal brecciation explosive phreatic brecciation was followed by in situ hydraulic brecciation and then by minor veining as the system returned to steady state geothermal conditions brecciation and mineralization mostly occurred 400 to 700 m below the paleowater table the kelian deposit contained more than 240 t au prior to mining precious metals occur with sulfide minerals as disseminations in sheeted and conjugate veins and as breccia cement there is a progression from pyrite dominated stage 1 to base metal sulfide dominated stages 2 and 3 to sulfosalt dominated hydrothermal breccias and veins stage 4 in terms of gangue minerals the system evolved from illite quartz to adularia and or quartz and or illite dominated and then carbonate dominated boiling produced abundant bladed carbonate during stages 3 and 4 overall carbonate and base metal sulfide minerals are abundant whereas quartz is comparatively minor free gold is most abundant in stages 3 and 4 but also occurs in stages 1 and 2 native gold occurs principally as inclusions in pyrite sphalerite galena arsenopyrite quartz bladed carbonate and sulfosalts hydrothermal alteration assemblages are zoned about contacts faults breccias and veins and consist of secondary quartz illite pyrite chlorite and various carbonate minerals kelian preserves evidence of a magmatic component to the mineralizing fluids fluid inclusion analyses revealed the presence of saline fluid inclusions in particular from stage 3 isothermal mixing of low salinity 0 to 2 wt nacl equiv with moderate salinity fluids 10 to 25 wt nacl equiv rather than boiling resulted in the range of salinity and homogenization temperatures of fluid inclusions in sphalerite carbonate and quartz deposited during stage 3 salinities of 4 to 6 wt percent nacl equiv for inclusions in adularia and quartz from stages 2 and 4 and in rhodochrosite and proustite pyrargyrite from stage 4 may also reflect a component of mixing of low salinity water with a moderately saline fluid sulfur isotope values from kelian sulfides 34s 1 4 to 5 5 per mil are consistent with a magmatic sulfur source both boiling and fluid mixing contributed to high grade au mineralization at kelian rapid formation hundreds of years or less is inferred for the individual hydrothermal breccia bodies 2008 society of economic geologists inc

middle cambrian to permian subduction related accretionary orogenesis of northern xinjiang nw china implications for the tectonic evolution of central asia abstract middle cambrian to permian subduction related records are widely distributed in northern xinjiang which can be grouped into the chinese altay east junggar eastern tien shan west junggar yili and tarim domains by integrating paleogeographic and geological data we suppose that the chinese altay east junggar eastern tien shan domain was more closely located to siberia while the west junggar and yili domains occupied an intermediate position near the kazakhstan block in the early paleozoic paleoasian ocean distribution of andean type magmatic arcs island arcs accretionary wedges ophiolitic slices and or microcontinents shows an archipelago paleogeography forming a huge accretionary active margin sequences the tarim domain was on the opposite side of the early paleozoic paleoasian ocean remaining passive margin these tectonic units drifted northwards and approached the southern active margin of the siberian craton in the late paleozoic leading to termination of the paleoasian ocean and formation of a complicated orogenic collage between siberian craton and the tarim block between the end permian and triassic these multiple accretion processes significantly contributed to the lateral growth of central asia

microstructural evidence for n s shortening in the mount isa inlier nw queensland australia the preservation of early w e trending foliations in porphyroblasts revealed by independent 3d measurement techniques 3d microstructural analyses of porphyroblast inclusion trails using both the asymmetry switch method for determining foliation intersection axes preserved in porphyroblasts fias and the recently developed fitpitch method reveal w e and n s trending fia sets in the white blow formation of the mount isa inlier each method reveals two subsets of fias centered on each of these major trends these were distinguished based on the relative timing trend and orientation of inclusion trail patterns thirty six samples were analyzed using both techniques and produced very similar results pitches of the inclusion trails preserved within the porphyroblasts in vertically oriented thin sections and trends in horizontal sections yield distinct near orthogonal modal orientations from all the analyzed samples this indicates that the porphyroblasts host successive fabrics as crenulation foliations and did not rotate with respect to geographical axes w e and n s trending fias have been obtained from both garnet and staurolite porphyroblasts hosting differentiated crenulation cleavages garnet and staurolite growth during bulk north south shortening recorded the development of multiple foliations and an associated succession of metamorphic events at middle amphibolite facies conditions that predates the metamorphic history generally recognized in this terrain this period of bulk shortening and associated metamorphism formed during a period of orogenesis called o1 w e shortening formed n s striking foliations that preserve a period of orogenesis o2 and another succession of metamorphism involving more phases of porphyroblast growth preserving n s trending fias overprinting of successive fia trends wsw ene wnw ese nnw sse and ssw nne suggests a relative clockwise rotation of the bulk shortening direction through time as it switches from n s to w e overall with a major tectonic break or decompression between o1 and o2 the porphyroblast inclusion trail patterns preserving w e trending fias provide a window into the lengthy period of earlier deformation and metamorphism that is no longer preserved within the matrix foliations

a long lived metamorphic history in the contact aureole of the mooselookmeguntic pluton revealed by in situ dating of monazite grains preserved as inclusions in staurolite porphyroblasts emplacement of the mooselookmeguntic pluton located in the western maine region of the northern appalachians was thought to have occurred towards the end of the acadian deformation at around 370 ma crystallization ages from different parts of the pluton suggest a more sequential emplacement history over a period of c 20 myr foliation inflection intersection axes fias within porphyroblasts from its aureole reveal at least five periods of garnet and staurolite growth the orientation of fias in both garnet and staurolite porphyroblasts trend successively from ese wnw nnw sse e w ene wsw to ne sw electron probe microanalysis dating of monazite grains included in staurolite porphyroblasts containing one of these five periods of fia development reveals a succession of apparent ages from 410 ma to 345 ma a similar spread of crystallization ages can be observed for plutons from maine and adjacent regions this succession indicates that deformation and metamorphism began well before and continued long after what is classically regarded as the acadian orogeny the thermal structure of the orogen progressively evolved to enable pluton emplacement and it continued to develop afterwards with magmatic fluids still forming at depth

the northern suture pakistan margin of a cretaceous island arc the northern suture is a fault separating the cretaceous kohistan island arc terrain northwest himalayas from palaeozoic sediments of the asian plate to the north the kohistan arc includes volcanic and sedimentary rocks andesitic lavas tuffs volcaniclastics slates and limestones metamorphosed to greenschist facies and intruded by the two phase kohistan batholith asian continental margin sediments are mainly of shelf type are variably metamorphosed and intruded by the karakoram batholith the northern suture is a zone of melange from 150 m to 4 km wide and contains blocks of volcanic greenstone limestone red shale conglomerate quartzite and serpentinite in a slate matrix it has a strong planar fabric but in many places bedding is preserved in blocks and matrix and depositional rather than tectonic contacts are seen between the two the melange is inferred to be an olistostrome largely derived from the kohistan arc formed in a small back arc basin between kohistan and asia limestone blocks in the melange are dated as aptian albian post tectonic intrusions yield radiometric ages from 111 to 62 ma the northern suture therefore probably formed in the early late cretaceous during closure of the back arc basin the tethys ocean lay south of kohistan where the main mantle thrust represents the westward continuation of the indus tsangpo suture

age geochemistry and tectonic implications of a late paleozoic stitching pluton in the north tian shan suture zone western china the central asian orogenic belt is the largest tectonic assembly of continental and oceanic terranes on earth due to closure of the paleo asian ocean in the phanerozoic among major suture zones in the north xinjiang region of western china the north tian shan suture zone because of collision between the yili terrane in the south and the junggar terrane in the north contains the youngest ophiolitic rocks and may represent the terminal stage of development of the central asian orogenic belt in western china but the timing of the suture zone remains poorly constrained a sensitive high resolution ion microprobe shrimp zircon u pb age of 316 3 ma i e the beginning of the late carboniferous from the undeformed sikeshu pluton which crosscuts the suture zone places a crucial upperage bound for the time of collision between the yili and junggar terranes this event occurred later than or nearly concurrent with other accretion collision events in the north xinjiang region implying that fi nal terrane amalgamation was completed in the late carboniferous the sikeshu pluton shares geochemical characteristics of the widespread late carboniferous to permian postcollisional a type and i type granitoids with depleted mantle like sr nd isotopic signatures in the north xinjiang region they all occurred during a protracted ca 320 270 ma episode of postcollisional magmatism that may have been induced by basaltic under plating due to either slab breakoff or delamination of thickened mantle lithosphere beneath the central asian orogenic belt the same postcollisional magmatism also generated cu ni sulfi de bearing mafi c ultramafi c magmatic complexes adakites and porphyry type coppermolybdenum bearing magmatic rocks in the north xinjiang region

conjugate shear folding a model for the relationships between foliations folds and shear zones abstract microstructural mapping of whole thin sections cut from two samples of micaschist containing cm scale folds plus garnet porphyroblasts has provided new insight in the relationships between folding shearing and foliation development the garnets exhibit coherent inclusion trail patterns that place important constraints on the kinematic development of both samples which are shown to be representative of coaxial versus non coaxial deformation in rocks containing a pre existing schistosity a comparison of crenulations cleavages geometries in both samples and a review of the geometry of natural and experimental multilayer folds leads to the conclusion that folding involves conjugate shearing at different scales at microscopic scales crenulation cleavages nucleate as conjugate kink or shear instabilities and develop further as a function of the macroscopic partitioning of deformation in fold hinge domains bulk coaxial deformation results in equal development of conjugate crenulations that progressively coalescence into symmetrical crenulation patterns so that macroscopically parallelism is achieved between foliation fold axial planes and long axes of strain ellipses fold limb domains represent a system of conjugate shear zones where single sets of crenulation instabilities with synthetic shearing component preferentially develop producing oblique relationships between the aforementioned elements cleavage fanning is inferred as a direct consequence of this conjugate shear origin of folds the model implies that crenulation cleavages and s c fabrics in shear zones form by analogous processes in both cases involving a component of shearing along foliation planes the development of conjugate sets of foliation planes surrounding porphyroblasts during early relatively coaxial stages of deformation explains continued gyrostatic behaviour during more advanced non coaxial stages as indicated by consistently oriented inclusion trails in the studied samples

an early india asia contact paleomagnetic constraints from ninetyeast ridge odp leg 121 new paleomagnetic results from sedimentary rock and basement of the ninetyeast ridge ocean drilling program leg 121 sites 756 758 detail the northward movement of the indian plate for the past 80 m y analysis of the combined paleolatitude age profile indicates a distinct reduction in india9s northward movement rate at 55 ma interpreted as completion of suturing of greater india and asia india9s northward motion slowed from 18 19 5 cm yr to 4 5 cm yr for the location of site 758 comparison of this profile with paleomagnetic data from the wider himalayan region indicates that initial contact between northwestern greater india and southern asia was already established by cretaceous tertiary time this supports a possible causal link between the india asia convergence and the deccan traps extrusion

geology and geochemistry of axi gold deposit xinjiang the axi gold deposit is a large epithermal gold deposit in west tianshan mountain of xinjiang its ore forming process can be divided into four stages namely chalcedony quartz stage pyrite quartz stage marcasite carbonate quartz stage and barite carbonate stage electrum occurs as the most important gold bearing mineral stage and are the major periods for gold deposition from stage 195 285 logf s2 6 7 13 to stage 95 190 logf s2 15 8 25 6 the temperature and sulfur fugacity of the ore forming fluid decrease obviously the reactions between the ore forming fluid and the wall rocks caused the increase of lile contents in the ore forming fluid the variations of eu behaviors in veins formed at different stage suggest that stage i vein crystallized in a rather oxidized fluid relative to stage vein stage i vein is strongly enriched in lree la yb 4 5 36 2 in comparison with veins formed at stages and la yb 1 2 2 0 stage vein ree 67 5 10 6 is different from stage vein ree 1 0 10 6 4 2 10 6 co2 3 and so2 4 are important in transporting and enriching ree in the ore forming fluid geochemistry of volcanic rocks and veins formed at different stage suggests that the magmatic fluid was the primary ore forming fluid and meteoric water participated in the mineralization from stage to stage which caused obvious decrease of temperature and sulfur fugacity and hence induced the electrum precipitation at stages and

petrogenesis of the kekesai composite intrusion western tianshan nw china implications for tectonic evolution during late paleozoic time abstract the late carboniferous to permian is a critical period for final amalgamation of the central asian orogenic belt caob which is characterized by voluminous igneous rocks particularly granitoids the kekesai composite granitoid porphyry intrusion situated in the chinese western tianshan southwest part of caob includes two intrusive phases a monzogranite phase intruded by a granodiorite phase la icpms u pb zircon analyses suggest that the monzogranitic rocks were formed at 305 5 1 1 ma with a wide age range of inherited zircons 358 488 ma and 1208 1391 ma whereas the granodioritic rocks formed at 288 7 1 5 ma the monzogranitic and granodioritic phases have similar geochemical features and sr nd hf isotopic compositions they exhibit high and variable sio 2 66 71 wt and mgo 0 41 2 14 wt contents with some arc like geochemical characteristics e g enrichment of large ion lithophile elements and negative anomalies of nb ta and ti and relatively high initial 87 sr 86 sr ratios i sr 0 7055 0 7059 low positive e nd t 0 84 to 1 03 as well as a large variation in hf isotopic compositions with e hf t between 3 43 and 14 8 implying both of them were derived from similar source materials these geochemical characteristics suggest that they might be mainly derived from the partial melting of arc derived mesoproterozoic mafic lower crust with involvement of a mantle derived component in variable proportions by mantle derived magma underplating the presence of late ordovician to earliest early carboniferous inherited zircons and the hf isotopic compositions in the monzogranitic sample similar to that of the widespread juvenile arc rocks indicates some crust contamination during magma emplacement our new data combined with previous studies imply that extensive post collisional magmatism due to underplating of mantle derived magma could plausibly be explained by slab break off regime

himalayan structure and metamorphism south of the main mantle thrust lower swat pakistan abstract during the eocene oligocene the indian plate collided with the kohistan arc along the main mantle thrust mmt zone the structure of the lower swat rock sequence on the indian plate directly south of the mmt is a dome with a basement of granitic gneiss and quartz rich schist unconformably overlain by amphibolitic and calcareous schist the earliest superposed small scale folds f1 f2 represent a progressive f1 f2 deformation that is associated with a single set of wsw vergent large scale folds termed f2 these folds are inferred to have developed during oblique wsw directed overthrusting of the mmt suture complex onto the lower swat rock sequence metamorphism began during f1 f2 as indicated by an s1 foliation that developed during biotite grade metamorphism s1 is preserved as a relict texture in porphyroblasts that grew during a subsequent interkinematic phase during garnet and higher grade metamorphism the dominant regional foliation s2 developed following the interkinematic phase s2 is associated with transposition of s1 and rotation or dismemberment of porphyroblasts annealing recrystallization followed s2 and continued during f3 thereby destroying or masking possible pre existing stretching fabrics superposed f3 folds are upright and open with n s axial trends they may correlate with early doming of the lower swat rock sequence and with strike slip displacement in the northern part of the mmt zone north of the lower swat area f3 was followed by retrograde metamorphism and development of e w trending s vergent f4 folds f4 may be associated with a final phase of southward directed thrusting and inactivity in the mmt zone correlation of published 40ar 39ar ages with the metamorphic fabrics suggests that f1 f2 and f3 occurred in the eocene and that f4 developed in the oligocene f4 is the earliest indication of southward verging structures on this part of the indian plate

combining compositional zoning and foliation intersection axes fias in garnet to quantitatively determine early p t t paths in multiply deformed and metamorphosed schists north central massachusetts usa quantitative compositional and microstructural analysis of garnet porphyroblasts in kyanite staurolite garnet grade rocks from the northeastern flank of the pelham dome north central massachusetts distinguishes the effects of acadian deformation and metamorphism from extensive overprinting alleghanian shearing the p t conditions and the metamorphic path during the acadian were determined using samples preserving well defined stages in a lengthy tectonic history revealed by a succession of five foliation intersection axis trends preserved within porphyroblasts fias this acadian succession extends at least 120 km to the north into rocks where no evidence has been found of an alleghanian overprint for each sample where garnet first nucleated during one of these stages in the tectonic history the pt of core growth was determined by plotting the intersection of the mn fe and ca isopleths calculated for the core composition on a p t pseudosection for that sample using thermocalc combining the pt data from all these samples indicates that the temperature and pressure increased throughout acadian orogenesis causing episodic garnet growth during the alleghanian locally intense shearing especially against the margin of the pelham dome formed the dominant schistosity which truncated all foliations defined by inclusion trails in porphyroblasts and obliterated all remains of acadian deformation and metamorphism in the rock matrix shearing was accompanied by near complete homogenization of the compositional zoning in garnet porphyroblasts and an associated apparent increase in the temperature of the matrix to 700 c in those rocks lying directly adjacent to the pelham dome and resulted from the rocks of the northfield syncline being thrust a large distance southwards over the gneisses in the dome

time constraints on deformation and metamorphism from epma dating of monazite in the proterozoic robertson river metamorphics ne australia epma dating of monazite confirms the relative timing of a succession of foliation intersection axis trends in porphyroblasts fias and two periods of progressive metamorphism intervened by retrogression in the robertson river metamorphics ne australia a complete pressure temperature deformation time p t d t path has been deciphered involving an early clockwise p t loop followed by high temperature low pressure metamorphism successive generations of monazite were identified using microstructure and fia trends and then dated the succession of four fias trending ene wsw fia1 e w fia2 n s fia3 ne sw fia4 plus four matrix foliations s1 s4 were accompanied by three periods of garnet and two periods of staurolite growth monazite grew in response to most of these events isopleth thermobarometery using p t pseudosections in the mnnckfmash system for garnet porphyroblasts selected based on fia trends in their core reveal that pressure increased progressively from 3 4 kb at 530 550 c to 6 7 kb at 600 620 c during medium pressure temperature metamorphism this was accompanied by changing bulk shortening directions from nnw sse to n s to e w perpendicular to fias 1 2 and 3 the development of fia1 and fia2 occurred between 1655 2 2 ma and 1625 15 ma fia3 developed over 30 50 ma periods from 1625 15 ma to 1586 6 ma retrogressive metamorphism decompression and exhumation occurred over a not vert similar45 ma period further metamorphism accompanied regional granite intrusion and generated lower pressure higher temperature metamorphism during the development of fia4 nw se bulk shortening at 1542 8 ma this period possibly extended for another 30 50 ma based on the monazite ages obtained from the youngest foliations in the matrix correlation of these data with other ne australian proterozoic inliers suggests that they were all once part of a single orogen that developed from ca 1650 to 1500 ma

further evaluation of the re os geochronometer in organic rich sedimentary rocks a test of hydrocarbon maturation effects in the exshaw formation western canada sedimentary basin abstract re os isotopic analyses of a single organic rich sedimentary rock unit ors of known depositional age and at three levels of regional hydrocarbon maturity show that hydrocarbon maturation does not affect the ability the 187re 187os chronometer to yield a depositional age for such rocks we present re os isotope analyses from the late devonian exshaw formation in the subsurface of the western canada sedimentary basin alberta and obtain a re os isochron age of 358 10 ma 2 model 3 1 666 10 11 a 1 for samples ranging from hydrocarbon immature to overmature this age is within uncertainty of the established absolute age for the exshaw formation hydrocarbon immature and mature plus overmature samples show no significant age differences if regressed individually indicating that hydrocarbon maturation did not greatly disturb the re os isotope system in the exshaw formation as such we propose that the re os geochronometer may be used as a reliable tool for measuring the depositional ages of ors regardless of their level of hydrocarbon maturity we find that minimizing natural variation in the initial 187os 188os ratio is more important than avoiding hydrocarbon maturation in obtaining precise re os ages in particular the exshaw formation appears to contain a nonhydrogenous component of unradiogenic os in addition to the hydrogenous os load a subset of exshaw formation samples with 5 total organic carbon toc which should best reflect the hydrogenous os load alone yields a very well fitted isochron having a depositional age of 358 9 ma 2 1 666 10 11 a 1 with an initial 187os 188os ratio of 0 59 0 05 model 3 mean square of weighted deviates mswd 1 8 the initial 187os 188os ratio of this regression may provide an estimate of the os isotopic composition of local seawater at the time of deposition

metamorphic pressure temperature conditions of indian plate rocks south of the main mantle thrust lower swat pakistan the lower swat rock sequence in northern pakistan is composed of greenschist and amphibolite facies indian plate rocks that crop out in a dome directly south of the main mantle thrust zone and kohistan arc terrane the metamorphism is eocene to oligocene in age and records the collision of the indian plate with the kohistan arc this paper presents mineral assemblage mineral composition and garnet zoning data on the lower swat rock sequence in order to estimate the pressure temperature conditions of metamorphism and to infer a possible pressure temperature time path the stratigraphic sequence consists of precambrian to cambrian manglaur formation unconformably overlain by late paleozoic to early mesozoic alpurai group the alpurai group is further subdivided into the marghazar kashala saidu and nikanai ghar formations the manglaur formation in the core of the dome is at kyanite grade metamorphic grade decreases in the overlying alpurai group where a garnet isograd is mapped in calcareous schist of the kashala formation mineral composition geothermometers and geobarometers indicate final metamorphic equilibrium conditions of about 600 700 c and 9 11 kbar in the manglaur and marghazar formations these conditions are associated with a phase of static recrystallization which postdates the main phases of deformation correlation of the structural tectonic history with the pressure temperature estimates and with the garnet zoning analysis suggests that in the eocene the lower swat rock sequence was subducted to a depth of 35 to 45 km beneath the mmt suture melange subduction was followed immediately by exhumation the initiation of exhumation is believed to have been triggered by a change from subduction to strike slip motion in the mmt thereby allowing the relatively buoyant lower swat crust to rise the entire metamorphic deformational cycle lasted from 7 to 16 m y ending in the late eocene about 38 ma when the rocks cooled through the argon blocking temperature in hornblende 550 c

geology geochemistry and genesis of axi a paleozoic low sulfidation type epithermal gold deposit in xinjiang china abstract axi is a low sulfidation type epithermal gold deposit hosted in paleozoic subaerial volcanic rocks in the western tianshan orogenic belt xinjiang china the resource is more than 50 t gold at an average grade of 4 4 ppm the deposit occurs in the tulasu volcanic fault basin in the paleozoic active continental margin on the northern side of the yili central tianshan plate the host rocks are andesitic volcaniclastic rocks of the paleozoic dahalajunshan formation and the orebodies occur as veins in annular faults of a paleocaldera mineralization at axi can be subdivided into five stages quartz and or chalcedony vein quartz vein quartz carbonate vein sulfide vein and carbonate vein there are two types of ore host quartz vein and altered rocks ore minerals are native gold electrum pyrite marcasite arsenopyrite hematite limonite and trace amounts of pyrargyrite polybasite naumannite cerargyrite sphalerite chalcopyrite tetrahedrite galena pyrrhotite and clausthalite gangue minerals are mainly quartz chalcedony illite calcite siderite dolomite adularia and laumontite the main wall rock alteration is silicification and phyllic alteration carbonatization and propylitization the deposit is characterized by an enrichment relative to crustal abundance of au ag as sb bi hg se te and mo depletion in base metals cu pb and zn and a low ag au ratio 0 5 3 7 three types of fluid inclusions were recognized in quartz from the major mineralization stages liquid aqueous inclusions liquid rich two phase inclusions and small amounts of vapor rich two phase inclusions microthermometric measurements indicate that the final ice melting temperatures are 0 3 to 4 4 c corresponding to salinities of 0 5 6 9 wt nacl equiv 2 2 wt nacl equiv in average the peak temperatures of ice melting varies from 0 4 to 1 9 c corresponding to salinities of 0 7 3 1 wt nacl equiv homogenization temperatures range mainly between 120 and 240 c with an average of 190 c and a maximum of 335 c the fluid density is 0 73 to 0 95 g cm 3 and thus the estimated maximum mineralization depth is about 700 m hydrogen and oxygen isotopic compositions of the ore fluids lie within a narrow range d h2o is 98 to 116 and 18 o h2o 1 8 to 0 4 3 he 4 he ranges from 0 0218 to 0 138 ra with an average of 0 044 ra indicating that he derived predominantly from crust with negligible mantle he in the ore fluids by contrast the 40 ar 36 ar ranges from 317 7 to 866 0 suggesting that crust derived radioactive 40 ar accounts for 7 0 to 66 and atmospheric 40 ar about 43 to 93 in the ore fluids hydrogen oxygen carbon sulfur and noble gas isotopes indicate that the ore forming fluids of the axi gold deposit consisted predominantly of circulating meteoric water ore forming metals may have derived mainly from the host volcaniclastic rocks of the dahalajunshan formation and basement rocks the occurrence of adularia platy calcite and quartz or sulfide aggregates as pseudomorphs after bladed calcite in ore veins and occurrence of aqueous liquid and liquid rich and vapor rich two phase inclusions indicates that boiling of the ore forming fluid have occurred leading to supersaturation of the hydrothermal solution and deposition of ore metals this is the main mineralization mechanism for quartz vein type ores in axi the ore forming fluid was buffered to a near neutral ph in a reduced environment during mineralization the preservation of this paleozoic axi deposit and its discovery required a rapid accumulation of sediments in the basin after formation of the deposit and minimal amount of erosion after late cenozoic uplift

from magma replenishment to epithermal mineralization combined evidence from zircon u pb u th he and trace element data abstract the dahalajunshan formation volcano volcanoclastic rocks in chinese western tianshan host specific paleozoic epithermal gold deposits as well as volcanogenic fe cu mineralization although numerous studies have been taken to constraint its geological geochemical geochronological features as well as petrogenesis the link between volcanism and mineralization is not well constrained in this contribution accessory zircon was collected from the intermediate to felsic volcanic rocks outcropped at the jingxi yelmand gold deposit detailed cathodoluminescence imaging illustrates ubiquitous resorption surfaces and embayments and in situ la icpms trace element analysis further indicates periodic recharge of magma chamber by mafic magma input from depths u pb geochronology constrains two main pulses of magmatism at 370 ma and 365 362 ma respectively the duration of magmatic activity in the magma chamber is estimated to 52 6 myrs using the u pb age of the oldest zircon antecryst 406 9 5 4 ma and the youngest zircon autocryst 354 6 3 0 ma as cutting edge and taking the analytical uncertainty into consideration u th he dating of zircon from the first pulse yields an age of 354 15 ma which is identical to u pb age within analytical uncertainty however zircons from the second pulse of volcanic rocks yield much younger u th he centered at 292 6 7 6 ma due to significant post eruption reheating events we propose that the intermediate to acid volcanic rocks are constructed by incremental addition of small magma batches the periodic injection of more mafic magma and subsequent mixing may act as important trigger of not only volcanic eruption but also epithermal mineralization gold mineralization at the deposit should be no later than 292 6 ma

k ar and ar ar geochronology of the himalayan collision in nw pakistan constraints on the timing of suturing deformation metamorphism and uplift mica and hornblende k ar and ar ar data are presented from each of the three crustal components of the himalayan collision zone in north pakistan the asian plate the kohistan island arc and the indian plate together with u pb and rb sr data published elsewhere these new data 1 date the age of suturing along the northern suture which separates kohistan from asia at 102 85 ma 2 establish that the basic magmatism in kohistan which postdates collision along the northern suture predates 60 ma and that the later granite magmatism spanned a range of 60 25 ma 3 show that uplift amounts within kohistan are greater toward the nanga parbat syntaxis than away from it and that rate of uplift near the syntaxis increased over the last 20 ma to a current figure of about 5 5 mm a year 4 show that much of southern kohistan had cooled to below 500 c by 80 ma and that the major deformation which imbricated kohistan probably predated 80 ma and certainly predated 60 ma and was related to the kohistan asia collision rather than the kohistan india one 5 imply that uplift along the hunza shear in the asian plate together with imbrication of the metamorphics in its hanging wall took place at about 10 ma and was associated with breakback thrusting in the hanging wall of the main mantle thrust 6 suggest that the indian plate has a lengthy pre himalayan history with an early metamorphism at about 1900 ma major magmatism at 500 550 ma and early jurassic lithospheric extension or inversion and 7 show that the indian plate rocks were metamorphosed shortly after the collision within kohistan which occurred at circa 50 ma and subsequently cooled back through 500 c at circa 38 ma and 300 c at 30 35 ma with ages of cooling through 200 and 100 c as determined by fission track data locally controlled by nanga parbat related uplift tectonics

an early permian epithermal gold system in the tulasu basin in north xinjiang nw china constraints from in situ oxygen sulfur isotopes and geochronology abstract the axi and jingxi yelmand gold deposits being the largest gold deposits in the chinese north tianshan nw china are located ca l0 km apart in the tulasu basin and are hosted by the late devonian early carboniferous dahalajunshan formation in situ la icp ms titanium analyses on quartz from the axi and jingxi yelmand deposits are broadly identical accordingly the calculated ore forming temperatures by ti in quartz thermometer give average temperatures of 279 c and 294 c respectively results of in situ sims analyses of oxygen and sulfur isotopes on quartz and pyrite from these two deposits are similar temperature corrected fluids of the axi deposit have 18 o values of 2 6 8 1 and 34 s values of 0 8 2 4 whereas the fluids of the jingxi yelmand deposit have 18 o of 6 4 8 9 and 34 s of 0 4 to 4 0 the oxygen and sulfur isotopes from the two deposits indicate a magmatic origin la icp ms zircon u pb ages of aqialehe formation sandstone provided a lower limit for the mineralization timing of the axi deposit 288 ma in situ sims u pb analyses on entrapped zircon 297 ma and newly recognized 284 5 ma columnar rhyolite implies that the jingxi yelmand deposit formed in the early permian based on the magmatic affinity of the ore fluids similar age and ore formation temperatures we propose that the axi and jingxi yelmand deposits comprise an epithermal gold system which was driven by the same permian magma in the tulasu basin the ore geological features together with our new results indicate that the axi and jingxi yelmand deposits are intermediate and high sulfidation type epithermal deposits respectively

paleozoic tectonic evolution of the tianshan orogen northwestern china abstract the chinese tianshan orogenic belt which extends west east for about 1500 km is one of the most important paleozoic orogenic belts in central asia it includes parts of the tarim yili central tianshan and junggar plates which are microcontinental blocks with precambrian basements and meso cenozoic depositional covers the tarim plate is separated from the yili central tianshan plate by the southern central tianshan suture early carboniferous ophiolitic melanges and blueschists are widespread in this suture and are believed to be produced by the collision between a passive continental margin on the north side of the tarim plate and an active continental margin on the south side of the yili central tianshan plate the collision occurred at the end of early carboniferous the yili central tianshan and the junggar plates are separated by the northern central tianshan suture also considered as an early carboniferous collisional zone along which the middle ordovician ophiolitic melanges blueschists and late devonian early carboniferous ophiolites are exposed in early paleozoic times the subduction of the paleo junggar ocean took place in an active margin on the north side of the yili central tianshan plate along the northern central tianshan suture at the end of early carboniferous the north side of the yili central tianshan plate collided with the north tianshan arc along this suture the paleo oceanic crust in the tianshan and adjacent region was entirely consumed at the end of early carboniferous meanwhile the tarim yili central tianshan and junggar plates were accreted to eurasia epicontinental sediments were then deposited throughout the tianshan region during the middle late carboniferous times in the early permian tianshan began to uplift with accompanying volcanic eruption and granitic intrusion the uplift was accelerated in the late permian and continental molasse was extensively deposited

comment on climatic impact of the long lasting laki eruption inapplicability of mass independent sulfur isotope composition measurements by schmidt et al laki eruption inapplicability of mass independent sulfur isotope composition measurements by schmidt et al jihong cole dai joel savarino mark h thiemens and alyson lanciki department of chemistry and biochemistry south dakota state university brookings south dakota usa cnrs lgge umr5183 grenoble france university grenoble alpes lgge umr5183 grenoble france department of chemistry and biochemistry university of california san diego la jolla california usa

major optical depth perturbations to the stratosphere from volcanic eruptions pyrheliometric period 1881 1960 a detailed chronology of major stratospheric dust veils from 1881 to 1960 has been constructed by searching the primary literature for relevant observational data of various kinds especially pyrheliometry data from 23 observing sites in both hemispheres have been reduced in a rigorous fashion to yield a table of pyrheliometric optical depth perturbations as a function of year month and latitude band to convert to visual reference wavelengths the tabular entries should be multiplied by a time dependent factor that is somewhat greater than unity ten measurable volcanic dust veils have been established in this manner and have been analyzed as to their generation transport decay and total mass these clouds arose from eruptions of krakatau 1883 an unidentified volcano 1890 soufriere and pelee 1902 santa maria 1902 ksudach 1907 katmai 1912 puyehue 1921 paluweh 1928 komagatake 1929 and quizapu 1932 other turbidity indicators have also been used in the analysis including starlight extinction purple twilight glows color of sun and moon polarization of blue sky light bishop s ring around the sun and dark lunar eclipses global stratospheric aerosol loadings have been computed from the peak turbidities agreement with the aerosol masses derived from polar ice cores is good in the important cases of krakatau santa maria and after a major correction of the ice core value katmai during the long period 1881 1992 about 80 of all stratospheric aerosols generated by the largest sulfur producing eruptions were emplaced during the two short time intervals 1883 1902 and 1982 1991 the long term 1881 1992 average annual production rate of stratospheric so2 from the largest eruptions was 0 8 tg yr 1 about half the average rate since 1981 implications of the principal results are discussed

stratigraphy and structure of the indus suture in the lower swat pakistan nw himalaya abstract we present a geological map detailed structural description and new subdivisions of the indus suture zone lithologies in northeast pakistan lower swat region the indus suture zone is a dominantly fore arc related assemblage obducted onto the indian plate initial southward thrusting was followed by the reactivation of the indus suture as a ductile brittle normal fault significant strike slip faulting may have taken place at this stage later brittle reverse faults demonstrate that shortening outlasted normal faulting the comparison of the structural evolution of the indus suture zone and the indian plate shows that both terrains were sharing the deformational history possibly since the earliest stage of collision the lower swat area records a full orogenic cycle from early rifting through continent arc collision to orogenic collapse we present a geodynamic model that integrates all these elements

re os isotope systematics of the taklimakan desert sands moraines and river sediments around the taklimakan desert and of tibetan soils abstract reported here are the first 187os 188os ratios and abundances of os and re for taklimakan desert sands and glacial moraines from the kunlun mountains osmium isotopic data are also reported for river sediments around the taklimakan desert river sediments from the kunlun and tianshan mountains tibetan soils and loesses from the loess plateau as well as sr and nd isotopic data for these samples the taklimakan desert sands from various regions show surprisingly homogeneous os isotopic ratios 187os 188os 1 29 0 08 and abundances os 11 ppt with some variations in re abundances re 130 to 260 ppt and 187re 188os ratios 60 to 140 the 187os 188os ratios for the taklimakan desert sands are close to the average for kunlun moraines river sediments around the taklimakan desert sands and the tibetan soils supporting the idea that the taklimakan desert sands are derived from moraines and river sediments around the desert or from tibetan soils and are homogenized by aeolian activity in the desert furthermore the os isotopic data for the sediments studied here are compared with those 187os 188os 1 04 os 32 ppt re 206 ppt 187re 188os 35 of loesses from the loess plateau reported elsewhere and it is concluded that the re os data for the loess can be used as proxy for the upper continental crust

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

climatic impact of the long lasting 1783 laki eruption inapplicability of mass independent sulfur isotopic composition measurements 1 the long lasting 1783 1784 ce laki flood lava eruption in iceland released around 120 tg of sulfur dioxide into the upper troposphere lower stratosphere northern hemisphere temperature proxy records of the 1780s indicate below average temperatures for up to three years following the eruption the very warm summer of 1783 in europe which was followed by a very cold winter may have been caused by the eruption but the mechanisms are not yet well understood some studies attributed the cold winter 1783 1784 to natural variability of climate however our climate model simulations show that the laki radiative effects lasted long enough to contribute to the winter cooling we suggest that sulfur isotopic composition measurements obtained using samples from greenland ice cores do not provide evidence of either a short lived volcanic aerosol cloud or a short lived climatic impact of the laki eruption in fact the applicability of mass independent sulfur isotopic composition measurements for interpreting the climatic impact of any high latitude eruption remains yet to be demonstrated

foliation intersection axes preserved in garnet porphyroblasts from the swat area nw himalaya a record of successive crustal shortening directions between the indian plate and kohistan ladakh island arc abstract we have investigated the geometry and tectonic significance of porphyroblast inclusion trails in 61 oriented samples from the swat region of north pakistan nw himalaya the studied samples represent the indian plate cover sequence deformed and metamorphosed to medium and high grade during the himalayan orogeny two complementary techniques were applied to measure the orientation of foliation intersection inflection axes fia in porphyroblasts in the studied samples and reveal the existence of three principal fia sets in the study area measuring approximately 60 30 km each fia set is characterized by a distinctive regionally consistent trend and relative timing from old to young ese wnw fia set1 e w fia set 2 and broadly nne ssw fia set 3 this sequence is inferred to record an anticlockwise rotation of the direction of tectonic transport or bulk crustal shortening perpendicular to fia through time either progressively or in a stepwise fashion consistent with previous kinematic and geochronological data regarding collisions between the indian plate kohistan ladakh island arc klia and eurasia between ca 55 and 40 ma fia set 1 and 2 correlate well with early formed ese wnw and e w trending thrust related structures in the study area respectively whereas the trend of fia 3 matches that of relatively young regional scale folds and shear zones with a conspicuous n s trend hence the fia succession has successfully resolved the relative timing of formation between the e w versus n s trending structures in the region which was a longstanding question in the tectonics of the nw himalaya in addition a comparison of orientation data for symmetrical millipede inclusion trails versus asymmetrical curved trails indicates that porphyroblasts experienced very limited differential rotations in the region despite the complex polyphase deformation history recorded perhaps the most fascinating aspect of our findings is the direct relationship that is suggested between fia trends and successive vectors of relative indian plate motion since collision with the klia

a review of the geological characteristics and geodynamic mechanisms of late paleozoic epithermal gold deposits in north xinjiang china article i nfo in this review we describe the geological characteristics of late paleozoic epithermal gold deposits in north xinjiang and place these into their metallogenic tectonic context these gold deposits can be divided into high sulfidation and low sulfidation types with the latter being predominant the spatial distribution of epithermal gold deposits in north xinjiang is controlled by calderas craters diatremes and related faults the volcanic and sub volcanic host lithologies of the gold deposits are mainly of calc alkaline affinity and dominated by basalt andesite dacite rhyolite suites and associated pyroclastic rocks alteration assemblages are dominated by adularia chalcedony sericite for low sulfidation gold deposits and alunite kaolinite quartz chalcedony forhigh sulfidationgolddeposits homogenization temperaturesof fluid inclusions inthe high sulfidation type epithermal deposits range from 200 to 300 c and ore forming fluids were mainly of magmatic hydrothermal origin e g jingxi yelmand with some contributions from meteoric water homogenization temperatures of fluid inclusions in the low sulfidation type epithermal deposits range mainly from 120 to 210 c and ore forming fluids were largely derived from meteoric water the high sulfidation type and low sulfidation type epithermal deposits have similar salinities ranging from 0 2 to 19 2 wt nacl equiv with a cluster between 0 3 and 13 wt nacl equiv based on isotopic age data the epithermal gold deposits essentially appear to have formed during three broad intervals 346 300 ma with a peak at 340 330 ma 290 270 ma and 240 226 ma each of these episodes corresponds to a period of syncollision postcollision and intraplate tectonics during latepaleozoic accretionaryand collisional evolution of the siberian junggar kazakhstan and tarim plates

the tianger bingdaban shear zone hosted gold deposit west tianshan nw china petrographic and geochemical characteristics abstract the wangfeng tianger saridala ore district consists of over 20 orebodies with total proven gold reserves exceeding 15 tonnes the district is located on the ew striking tianger bingdaban shear zone in west tianshan mountains northwest china the lensoid orebodies are distributed along the shear zone in a region about 25 km in length and 1 km in width enhanced fluid flow and fracturing controlled location and orientation of mineralized zones during deformation along narrow shear bands oriented oblique to the overall shear zone margins fabrics in fractures and strain shadow overgrowths generally plunge steeply parallel to the lineation in the surrounding cataclasite matrix consisting of new grains of quartz and minor amounts of micas native gold occurs in mylonitized granites or mylonitized sulfide mica quartz veins gold grade is closely related with the occurrence of sulfide quartz veins pyrite and pyrrhotite in gold bearing sulfide quartz veins contain inclusions of native gold chondrite normalized rare earth element ree distribution patterns for pyrite from the orebodies show light ree enrichments and negative eu anomalies relationships between gold contents and th u zr hf and la yb n values for pyrites imply that gold rich pyrites are different from gold poor pyrites thus indicate that the pyrites in the orebodies have a different origin compared with those in wall rocks a rb sr isochron for the mylonitized au bearing quartz veins indicates an age of 224 14 ma with an initial 87 sr 86 sr ratio of 0 7294 0 0089 mswd 1 1 this age is conformed by 40 ar 39 ar dating on muscovites 220 9 to 222 5 ma which is interpreted as the age of ore formation in the tianger deposit high initial 87 sr 86 sr ratios 0 7294 high 34 s values 11 2 to 16 5 low 18 o h2o values 1 67 to 3 07 and low d values 84 to 104 indicate that the ore forming fluid was unrelated to any magmatic process

so2 photoexcitation mechanism links mass independent sulfur isotopic fractionation in cryospheric sulfate to climate impacting volcanism natural climate variation such as that caused by volcanoes is the basis for identifying anthropogenic climate change however knowledge of the history of volcanic activity is inadequate particularly concerning the explosivity of specific events some material is deposited in ice cores but the concentration of glacial sulfate does not distinguish between tropospheric and stratospheric eruptions stable sulfur isotope abundances contain additional information and recent studies show a correlation between volcanic plumes that reach the stratosphere and mass independent anomalies in sulfur isotopes in glacial sulfate we describe a mechanism photoexcitation of so2 that links the two yielding a useful metric of the explosivity of historic volcanic events a plume model of s iv to s vi conversion was constructed including photochemistry entrainment of background air and sulfate deposition isotopologue specific photoexcitation rates were calculated based on the uv absorption cross sections of 32so2 33so2 34so2 and 36so2 from 250 to 320 nm the model shows that uv photoexcitation is enhanced with altitude whereas mass dependent oxidation such as so2 oh is suppressed by in situ plume chemistry allowing the production and preservation of a mass independent sulfur isotope anomaly in the sulfate product the model accounts for the amplitude phases and time development of 33s 34s and 36s 33s found in glacial samples we are able to identify the process controlling mass independent sulfur isotope anomalies in the modern atmosphere this mechanism is the basis of identifying the magnitude of historic volcanic events

late paleozoic metallogenesis and evolution of the chinese western tianshan collage nw china central asia orogenic belt abstract the chinese west tianshan has a prominent position in china with respect to mineral resources and mineral production including copper nickel molybdenum iron gold zinc and many other mineral resources these resources are represented by a vast array of mineral systems and deposit styles in their respective terranes spanning the precambrian and phanerozoic geological history these mineral systems include vms iron zinc orogenic gold magmatic copper nickel porphyry copper and copper molybdenum and epithermal gold deposits some of the deposits are large and superlarge including the well known axi gold and dunde iron zinc deposits the zhibo chagangnuoer beizhan iron deposits and the katebahasu gold deposit apart from the above mentioned giant deposits several others are poorly known and or unknown to western geoscience the study of these mineral systems can significantly contribute to our further understanding of the metallogeny of cratons and orogenic belts orogenic collages and anorogenic settings this provides additions to and further development of existing classifications and genetic models of mineral systems allowing researchers to elucidate unknown or poorly studied mineral systems and formation styles found in the chinese west tianshan and to discover some other important styles that appear to be missing although they are present in other regions with similar geological and tectonic settings based on the study of the of the west tianshan domain five types of endogenous metal deposits are identified 1 magmatic cu ni deposits 2 porphyry cu mo deposits 3 vms iron deposits 4 epithermal au deposits and 5 orogenic au cu deposits tectonically the development of these metal deposits was closely associated with accretionary and convergent processes that occurred in the chinese western tianshan collage the formation of the deposits involved three main stages briefly described as follows the jingbulake intrusion associated with the cu ni deposits suggests subduction of the south tianshan oceanic lithosphere beneath the yctb during the early to late carboniferous the north dipping subduction led to the development of porphyry cu au deposits epithermal gold deposits and vms iron deposits in the earliest late carboniferous the south tianshan oceanic crust was probably consumed leading to the collision between the yctb and the tarim craton the large suites of granites were widely distributed in the ntas and yctb accompanied by the formation of orogenic au cu deposits

structural evolution of the footwall of the indus suture in malakand n pakistan during the himalayan collision abstract we report new data on the geometry and kinematics of structures in the rocks of the indian plate below the indus suture in the area of malakand n pakistan the study area forms part of the exposed northwestern promontory of the indian plate indenting eurasia to form the himalayan orogenic belt earlier widespread structures i e regional foliation and lineation are consistent with an approximate n s convergence direction in coincidence with the movement direction of the indian plate younger structures such as nne trending open folds that laterally become syntaxes i e besham or nanga parbat and relatively minor e w dextral strike slip shear zones are related to a regional tectonic shortening direction subparallel to the trend of the suture these folds are radial to the northwestern corner of the indian plate and contemporaneous with the general bulk n s convergence it is only very recently in the structural record that the irregular outline of the indian plate played a role in determining the orientation of new structures during its indentation the structural pattern presents similarities to arcuate megastructures in ancient collisional orogens such as the ibero armorican arc of the variscan belt in europe where indentation tectonics as well as post orogenic oroclinal folding are invoked to explain the present curvature

re os isotopic systematics of the voisey s bay ni cu co magmatic ore system labrador canada abstract re and os concentrations and os isotopic compositions have been obtained for massive matrix and disseminated sulphide ores from three environments within the voisey s bay intrusion the ovoid eastern deeps and discovery hill zone in order to assess the role of crustal contamination in the genesis of this large cu ni co deposit these samples have high re concentrations 148 to 288 ppb in 100 sulphide for their common os concentrations 4 8 to 24 ppb in 100 sulphide yielding high re os ratios 12 to 33 these data confirm that the magma parental to the voisey s bay ore system was broadly basaltic in major element chemistry rather than picritic consistent with the low ni cu ratio of the ores 1 5 re os isotopic data exhibit a limited spread in 187 re 188 os 57 to 157 and define an imprecise 1323 135 ma model 3 isochron likely the result of small r factor variations within the ore system the re os isochron age is within error of 1334 ma u pb ages obtained for baddeleyite from the ore bearing troctolites demonstrating that whole rock re os isotopic systematics have remained closed since crystallisation the initial os isotopic composition of the isochron os 1040 200 implies significant magma interactions with radiogenic os that most likely resides in the nain churchill province crust these data are therefore consistent with the parental magma achieving sulphide saturation as a result of contamination by radiogenic crustal components with further addition of base and precious metals as a function of r factor analyses of sulphide separates from the proterozoic tasiuyak churchill and archaean nain gneisses confirm that both units contained significant os 8 6 ppb and 0 38 ppb respectively that was very radiogenic at 1334 ma os 1908 and 5202 respectively yielding crustal residence tchur model ages of 2200 to 2400 ma however these model ages may have been affected by re and or os mobility during the 1 85 ga torngat and 1 1 ga grenville orogeny re os isotopic modelling suggests that a reasonable fit to the ore data may be obtained via bulk crustal contamination of magmas parental to the nain plutonic suite nps with either tasiuyak 2 contamination or nain 16 contamination gneiss followed by an r factor process 200 to 1000 which improved the tenor of the sulphide liquid during transport in active magma conduits or after deposition in an active replenished voisey s bay intrusion magma chamber however the dominance of plagioclase rich high al2o3 magmas in the mafic members of the nps troctolites and anorthosites may indicate that other deeper forms of lithospheric interaction potentially involving mafic lower crust may be crucial to understanding this style of magmatic sulphide ore system

re os dating of sulfides associated with gold mineralization in central victoria australia we report re os isotope ages that are the first for sulfides closely associated with gold mineralization at the bendigo and maldon goldfields in central victoria two arsenopyrite samples from bendigo contain 11 ppb re and 0 08 ppb highly radiogenic os llhr samples stein et al 2000 and have model ages of 444 7 and 447 7 ma with a mean model age of 446 5 ma a five point isochron based on these arsenopyrite analyses plus three pyrite analyses including one replicate gives an age of 438 6 ma molybdenite associated with au te cu and sb mineralization at maldon gives an re os age of 376 2 ma similar to the age of the nearby har court granite these results support previous 40 ar 39 ar data indicating that initial emplacement of gold mineralization in central victoria occurred in the late ordovician during the earliest stages of accretionary development of the southern lachlan fold belt the molybdenite data are also consistent with the mobilization and or introduction of gold during late devonian magmatism

climatic response to high latitude volcanic eruptions 1 strong volcanic eruptions can inject large amounts of so2 into the lower stratosphere which over time are converted into sulfate aerosols and have the potential to impact climate aerosols from tropical volcanic eruptions like the 1991 mount pinatubo eruption spread over the entire globe whereas high latitude eruptions typically have aerosols which remain in the hemisphere in which they where injected this causes their largest radiative forcing to be extratropical and the climate response should be different from that of tropical eruptions we conducted a 20 member ensemble simulation of the climate response to the katmai eruption 58 n of 6 june 1912 using the nasa goddard institute for space studies modele climate model we also produced an additional 20 member ensemble for a 3 times katmai 3x katmai eruption to see the impact the strength of the eruption has on the radiative as well as the dynamical responses the results of these simulations do not show a positive arctic oscillation response like past simulations of tropical volcanic eruptions but we did find significant cooling over southern asia during the boreal winter the first winter following katmai and the second winter following 3x katmai showed strong similarities in lower stratospheric geopotential height anomalies and sea level pressure anomalies which occurred when the two cases had similar optical depth perturbations these simulations show that the radiative impact of a high latitude volcanic eruption was much larger than the dynamical impact at high latitudes in the boreal summer however strong cooling over the northern hemisphere landmasses caused a decrease in the asian monsoon circulation with significant decreases of up to 10 in cloud cover and warming over northern india thus the main dynamical impact of high latitude eruptions is in the summer over asia

thermal controls on early tertiary short lived rapid regional metamorphism in the nw himalaya pakistan abstract during tertiary collision in the nw himalaya the leading edge of the indian plate was subducted beneath the kohistan island arc along the main mantle thrust mmt metamorphism within indian plate cover sediments was synchronous with ductile shearing and took place along a path of increasing pressure during subduction beneath the island arc initial collision cannot have pre dated 65 ma and probably shortly pre dated 50 ma radiometric data constrain the metamorphic peak as shortly post dating 50 ma as firstly initially subducted units are now probably located beneath tibet secondly the subduction thrust separating the kohistan arc terrane from the indian plate was probably cooled by continued underthrusting and thirdly the heat producing indian plate cover sediments were delaminated from the basement during collision metamorphism was more rapid than can be predicted by purely conductive models of thermal relaxation although dissipative shear heating along the mmt doubtless contributed to early stages of heating of the footwall rocks the temperatures attained in the footwall are too high to support the shear stresses required to generate them solely through shear heating a model is derived to account for both the rapid regional metamorphism and the equally rapid post metamorphic cooling dissipative shear heating along the mmt generated an early inverted thermal profile in the upper units of the indian plate as the hanging wall mafic rocks have a low thermal conductivity they would have acted as a thermal reflector and the heat would have been conducted away only slowly as footwall temperatures increased through the brittle ductile transition the role of dissipative shear heating decreased and continued heating became a function of internal heat generation within the footwall rocks together with hanging wall thermal reflectivity the metamorphic inversion was reinforced by imbrication of the metamorphic stack as it accreted onto the mmt footwall during early stages of uplift and exhumation dissipative shear heating within thrust systems provides a potentially important mechanism by which areas of large regional extent can undergo regional metamorphism over short timescales in the absence of magmatic heating

re os isotope systematics in carbonates from serpent mound ohio implications for re os dating of crustal rocks and the osmium isotopic composition of ordovician seawater 1 eleven core samples of ordovician carbonates and shale from within and nearby the serpent mound ohio cryptoexplosion structure were analyzed for re os isotopes whole rock samples span a large range in measured 187os 188os 0 714 to 6 083 and exhibit a linear correlation of 187os 188os with 187re 188os the linear relationship is inconsistent with a meteorite crust mixing trend but could represent an errorchron with an age of 485 35 ma and an initial 187os 188os of 0 54 0 15 in order to assess whether the errorchron slope and initial 187os 188os ratio may provide constraints on depositional age and the 187os 188os ratio of ordovician seawater acetic acid and hcl leaching studies were done on selected samples the 87sr 86sr ratios of the leachates vary from 0 7081 to 0 7091 within the range found in middle upper ordovician brachiopods shields et al 2003 and consistent with the leachable fraction of the carbonates representing ordovician seawater compositions 187os 188os signatures for the leached samples are generally lower than those of respective whole rocks however five of six leachate samples produce a strong linear correlation on a re os isochron diagram the slope of which reflects an age of 443 34 ma consistent with biostratigraphic constraints on the depositional ages of the sampled rock units 445 472 ma the calculated initial 187os 188os of 0 53 0 04 is distinctly lower than present day seawater and if representative of ordovician seawater may reflect relatively low continental weathering rates due to waning pan african orogenies as proposed to explain the slow decrease in seawater 87sr 86sr starting in early ordovician qing et al 1998

auriferous pyrite re os geochronology and he ar isotopic compositions of the jinchangyu au deposit in the northern margin of the north china craton abstract the jinchangyu au deposit is the largest quartz vein gold deposit in eastern hebei province china in the northern margin of the north china craton nmncc three mineralization stages are identified based on crosscutting relationships and mineral assemblages with au occurring mainly in pyrites in quartz sulphide veinlets and quartz albite veins five low level highly radiogenic pyrite samples in the quartz sulphide veinlets yielded a re os isochron age of 223 5 6 0 ma auriferous pyrite he ar isotopic analyses indicate that the ore forming fluid was a mixture of mantle derived and crustal fluids 3he 4he ratios are in the range 1 72 3 92 ra corresponding to 22 50 mantle he contribution these high 3he 4he ratios are related to lithospheric thinning in the nmncc during the middle late triassic with initial thinning during that period being supported by the wide distribution of indosinian au deposits and the regional tectonic and magmatic evolution the mineralization of the jinchangyu deposit and of other similar triassic deposits was a response to post orogenic to intraplate lithospheric extension after collision between the ncc and the southern margin of the siberian plate

garnet porphyroblast timing and behaviour during fold evolution implications from a 3 d geometric analysis of a hand sample scale fold in a schist detailed 3 d analysis of inclusion trails in garnet porphyroblasts and matrix foliations preserved around a hand sample scale tight upright fold has revealed a complex deformation history the fold dominated by interlayered quartz mica schist and quartz rich veins preserves a crenulation cleavage that has a synthetic bulk shear sense to that of the macroscopic fold and transects the axis in mica rich layers garnet porphyroblasts with asymmetric inclusion trails occur on both limbs of the fold and display two stages of growth shown by textural discontinuities garnet porphyroblast cores and rims pre date the macroscopic fold and preserve successive foliation inflection intersection axes fias which have the same trend but opposing plunges on each limb of the fold and trend nne ssw and ne sw respectively the fias are oblique to the main fold which plunges gently to the wsw inclusion trail surfaces in the cores of idioblastic porphyroblasts within mica rich layers define an apparent fold with an axis oblique to the macroscopic fold axis by 32 whereas equivalent surfaces in tabular garnet adjacent to quartz rich layers define a tighter apparent fold with an axis oblique to the main fold axis by 17 this potentially could be explained by garnet porphyroblasts that grew over a pre existing gentle fold and did not rotate during fold formation but is more easily explained by rotation of the porphyroblasts during folding tabular porphyroblasts adjacent to quartz rich layers rotated more relative to the fold axis than those within mica rich layers due to less effective deformation partitioning around the porphyroblasts and through quartz rich layers this work highlights the importance of 3 d geometry and relative timing relationships in studies of inclusion trails in porphyroblasts and microstructures in the matrix

an x ray computed tomography study of inclusion trail orientations in multiple porphyroblasts from a single sample inclusion trails in 58 garnet porphyroblasts in a single sample from the cram hill formation in southeast vermont were imaged using high resolution x ray computed tomography texturally the porphyroblasts have a large core with a sub vertical foliation that has a well defined geometry with a mean orientation of 113 72e for a 95 confidence cone semi angle of 5 3 this steep foliation curves into a sub horizontal foliation the foliation intersection inflection axes fias defined by hinge lines for this curvature are tightly clustered with a mean plunge of 11 towards 201 with a 95 confidence cone semi angle of 4 1 eigenvalue analysis indicates that the clustered distribution of the foliation and fia data are unlikely to be the result of a random event there is evidence in the specimen for multiple foliation forming events subsequent to garnet nucleation and the preservation of these clustered distributions in their wake strongly suggests that the porphyroblasts have not rotated with respect to a geographic reference frame fias represent a measurable structural element that can provide information on tectonic events at the time the porphyroblasts grew a comparison of fia data collected using high resolution x ray computed tomography with data collected using the asymmetry method demonstrates that the asymmetry method is a valid technique for defining the mean fia orientations in a sample

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

increased magmatic water content the key to oligo miocene porphyry cu mo au formation in the eastern gangdese belt tibet the eastern gangdese magmatic belt between 88 e and 94 e in southern tibet preserves a series of cenozoic collision related igneous rocks that have formed since the start of the india asia collision at 55 to 50 ma the paleocene eocene intrusive rocks have intermediate la yb n ratios average 12 0 8 6 n 150 range 1 15 64 0 intermediate to low sr y ratios mostly 40 and weak or absent eu anomalies oligocene average eun eu 0 99 0 27 n 35 range 0 67 1 58 miocene average eun eu 0 95 0 13 n 93 range 0 44 1 46 and their mineralogy consists mainly of plagioclase quartz and amphibole as phenocrysts these geochemical and mineralogical characteristics suggest that the paleocene eocene magmas were relatively dry and evolved primarily by fractionation of pyroxene and plagioclase whereas the oligo miocene magmas were more hydrous and fractionated significant amounts of hornblende and lesser plagioclase prior to upper crustal emplacement oligocene and miocene magmatic rocks crop out as small volume intrusions and are associated with several large to giant sized porphyry cu mo au deposits as well as numerous smaller porphyry and skarn deposits in contrast earlier more voluminous paleocene eocene magmatism is only known to be associated with three small porphyry deposits erosional loss of subvolcanic porphyry systems from the older paleocene eocene sequence is not thought to be a controlling factor in this temporal difference of porphyry deposit distribution because exhumation rates after 55 ma were relatively low and coeval linzizong volcanic rocks are extensively preserved in the gangdese belt instead we suggest that differences in magmatic history and petrogenesis led to this restricted temporal distribution of porphyry deposits the paleocene eocene magmas were generated during the onset of collision between india and asia and were triggered by rollback of the neo tethyan oceanic slab this magmatism records the final stage of subduction of oceanic lithosphere beneath southern tibet and the relatively low water contents of these magmas may reflect final dehydration of the remnant neo tethyan slab after oceanic slab breakoff at 40 to 38 ma the greater india slab began to subduct beneath the gangdese belt until hard collision with the indian continent at 35 ma and breakoff of the greater india slab at 25 to 10 ma this last event is suggested to have caused asthenospheric mantle upwelling and partial melting of subduction modified hydrated tibetan lithosphere the resultant partial melts may have remobilized metals residual in the deep lithosphere from previous arc magmatism giving rise to a suite of postsubduction porphyry type ore deposits in the oligo miocene

rhenium osmium geochronology of arsenopyrite in meguma group gold deposits meguma terrane nova scotia canada evidence for multiple gold mineralizing events rhenium osmium geochronology using arsenopyrite was undertaken for three gold deposits in the meguma terrane nova scotia canada in order to better constrain their age of formation and to assess the utility of arsenopyrite for dating similar deposits globally analyses of arsenopyrite from bedding concordant veins from the ovens locality southwest meguma terrane yield a precise re os isochron age of 409 5 ma arsenopyrite analyses from a bedding discordant vein at this locality indicate an identical re os age of 407 4 ma saddle reef veins from the dufferin deposit northeast meguma terrane contain arsenopyrite with a precise re os isochron age of 380 3 ma the lack of common os in some arsenopyrite samples from both the ovens and dufferin permit calculation of single mineral model ages for each deposit which are identical to those determined using the isochron method initial os compositions for the two vein types at the ovens suggest a predominately crustal source of os in the mineralizing fluids whereas a less radiogenic initial os composition for arsenopyrite from dufferin does not as clearly define a crustal metal source at a third locality the touquoy deposit the re os systematics of arsenopyrite associated with disseminated gold mineralization do not define a precise formation age possibly as a result of mixing of re and os derived from the mineralizing fluid and the shale host rock the re os ages of arsenopyrite indicate that there were at least two distinct periods of gold deposition in the meguma terrane coinciding with widespread tectonothermal events regional deformation and metamorphism associated with acadian orogenesis and widespread generation of meta and peraluminous granites and high grade metamorphism within the basement rocks under the meguma terrane the ca 407 ma age for the ovens arsenopyrite provides the best estimate for the timing of regional acadian deformation in the meguma terrane and is slightly older than previous estimates based solely on 40ar 39ar dating

timing of himalayan ultrahigh pressure metamorphism sinking rate and subduction angle of the indian continental crust beneath asia coesite relics were discovered as inclusions in clinopyroxene in eclogite and as inclusions in zircon in felsic and pelitic gneisses from higher himalayan crystalline rocks in the upper kaghan valley north west himalaya the metamorphic peak conditions of the coesite bearing eclogites are estimated to be 27 32 kbar and 700 770 c using garnet pyroxene phengite geobarometry and garnet pyroxene geothermometry respectively cathodoluminescence cl and backscattered electron bse imaging distinguished three different domains in zircon inner detrital core widely spaced euhedral oscillatory zones and thin broadly zoned outermost rims each zircon domain contains a characteristic suite of micrometre sized mineral inclusions which were identified by in situ laser raman microspectroscopy core and mantle domains contain quartz apatite plagioclase muscovite and rutile in contrast the rim domains contain coesite and minor muscovite quartz inclusions were identified in all coesite bearing zircon grains but not coexisting with coesite in the same growth domain rim domain 206 pb 238 u zircon ages reveal that the quartz bearing mantle domains and the coesite bearing rim were formed at c 50 ma and 46 2 0 7 ma respectively these facts demonstrate that the continental materials were buried to 100 km within 7 9 myr after initiation of the india asia collision palaeomagnetic data from the indian oceanic floor supports an initial india asia contact at 55 53 ma combination of the sinking rate of 1 1 1 4 cm year 1 with indian plate velocity of 4 5 cm year 1 suggests that the indian continent subducted to about 100 km depth at an average subduction angle of 14 19