generation and evolution of siliceous high magnesium basaltic magmas in the formation of the permian huangshandong intrusion xinjiang nw china abstract numerous permian orthopyroxene rich mafic ultramafic intrusions in the southern margin of the central asian orogenic belt caob have been emplaced in a post orogenic environment the huangshandong mafic ultramafic intrusion is the largest in the eastern tianshan orogenic belt of the caob and consists of a layered unit intruded by a massive unit the layered unit consists of lherzolite troctolite olivine gabbro hornblende gabbro gabbrodiorite and diorite whereas the massive unit is made of gabbronorite and olivine gabbronorite olivine from the layered unit has fo values of 75 to 84 and contains low nio and mno rocks from this unit have initial 87sr 86sr ratios 0 7031 0 7043 and end values 3 4 7 1 while plagioclase has initial 87sr 86sr ratios ranging from 0 70010 to 0 71569 olivine from the massive unit has relatively low fo values of 59 to 70 and high nio and mno rocks from this unit have relatively low and uniform initial 87sr 86sr ratios 0 7030 and high end values 6 0 7 5 and contain plagioclase with relatively low initial 87sr 86sr ratios 0 70042 to 0 70520 olivines from both units have low cao and cr2o3 both units formed from siliceous high magnesium basaltic magmas were derived from a hydrous depleted mantle source magma forming the layered unit experienced earlier removal of sulfides before emplacement with the earlier sulfide saturation caused by the addition of crustal material the magma that formed the massive unit has undergone fractional crystallization of olivine cr spinel siliceous high magnesium basaltic magmatism produced by melting of a hydrous depleted mantle source may be an important feature of orogenic belts

genesis of the huangshannan high ni tenor magmatic sulfide deposit in the eastern tianshan northwest china constraints from pge geochemistry and os s isotopes abstract the huangshannan magmatic ni cu sulfide deposit is one of a group of permian magmatic ni cu deposits located in the southern central asian orogenic belt in the eastern tianshan northwest china it is characterized by elevated ni tenor concentrations in recalculated 100 sulfide in sulfide within ultramafic rocks 9 19 wt with values much higher than other deposits in the region sulfides of the huangshannan deposit are composed of pentlandite chalcopyrite and pyrrhotite and the host rock is relatively fresh indicating that the high ni tenor is a primary magmatic feature rather than formed by alteration processes it is shown that sulfides with high ni tenor can be generated by sulfide olivine equilibrium at an oxygen fugacity of qfm 0 5 for magmas containing 450 ppm ni and 20 olivine ores with 10 wt sulfur have relatively low pge and ni tenors compared to other ores r factor mass ratio of silicate to sulfide liquid modeling of ni indicates that they formed at moderate r values 150 600 based on this constraint on r values ores with 4 5 wt as sulfide rich samples are also depleted in cu and as interstitial sulfides in those samples are physically interconnected at a scale of several cms the low pt and pd anomalies are attributed to solid pt and pd phases crystallization and retention with the monosulfide solid solution mss and cu rich sulfide liquid percolation during mss fractionation this finding indicates that pt anomalies in sulfide rich rocks from magmatic ni cu deposits in the eastern tianshan are the result of sulfide fractionation rather than a hydrothermal effect 187 os 188 os 278ma values of the lherzolite samples vary from 0 27 to 0 37 and os 278ma values vary from 110 to 189 indicating significant magma interaction with crustal sulfides rich in radiogenic os well constrained os values and 34 s values 0 4 to 0 8 indicate that crustal contamination occurred at depth before the arrival of the magma in the huangshannan chamber regionally deposits with high ni tenor have not been reported other than the huangshannan deposit however many intrusions with high ni contents in olivine are present in nw china such as the erhongwa poyi and poshi intrusions those intrusions are capable of forming high ni tenor sulfides due to olivine sulfide silicate equilibrium and relative high ni content in parent magma making them attractive exploration targets

origin of the adakite high nb basalt association and its implications for postsubduction magmatism in baja california mexico constraining the origin of the adakite high nb basalt hnb association in baja california mexico is critical to a better understanding of global arc magmatism currently the preferred explanation for the close spatial and temporal association of the two rock suites is through melting of the basaltic portion of the subducted farallon cocos plate thus providing support for the slab melting origin of adakites elsewhere moreover a tectono magmatic model involving the production of both adakite and hnb from slab melts offers a comprehensive explanation for the origin of the atypical arc related postsubduction magmatism in baja california this paper proposes alternative models for the origin of hnb and postsubduction magmatism in baja california wherein the unusual geologic setting of western mexico and westward movement of north america permitted the influx of pacific asthenosphere beneath the adjacent gulf of california after the cessation of subduction unlike the previous tectono magmatic model the new models propose that the asthenosphere provided a direct source for postsubduction tholeiitic and rare alkali magmas that were erupted in baja california as tholeiites and hnb respectively fractional crystallization of some of the hnb magmas plus assimilation of tholeiitic materials produced nb enriched basalts neb the influx of pacific asthenosphere after the cessation of subduction also provided thermal energy to melt the mafic lower baja california crust producing adakite rocks and the preexist ing metasomatized mantle wedge producing bajaites and calc alkaline magmas

40ar 39ar constraints on a temporal link between gold mineralization magmatism and continental margin transtension in the jiaodong gold province eastern china abstract the jiaodong gold province is the largest gold repository in china both mineralization and granitoid hosts are spatially related to the crustal scale tan lu strike slip fault system which developed along the mesozoic continental margin in eastern china a series of 40ar 39ar laser incremental heating analyses of hydrothermal sericite muscovite from three major gold deposits jiaojia xincheng and wangershan and igneous biotite from the granodiorite hosts were performed to establish a possible temporal link between gold mineralization magmatism and movement along the tan lu fault zone magmatic biotite crystals yield well defined and concordant plateau ages between documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewco

an evaluation of elemental mobility during metamorphism of a turbidite sequence greenland group new zealand the extent of elemental mobility during lower greenschist to amphibolite facies metamorphism of a uniform turbidite suite greenland group new zealand has been evaluated using data for major elements and 19 trace elements simple comparison of average compositions at 2 wt al 2 o 3 intervals in the data suites shows little contrast between lower greenschist protolith and upper greenschist and amphibolite facies equivalents except for enrichment of cao and sr and loss of ba and rb particularly in the sandier end members division into psammitic and pelitic suites using tio 2 al 2 o 3 and zr al 2 o 3 ratios allows delineation of individual residual enrichment models on ti reference element plots and both lithotypes can be used to assess potential elemental mobility these plots show that a large number of elements ti al fe mg ce cr ga la nb ni sc th v zn and zr constitute immobile reference species with abundances equal to their equivalent lithotype in the protolith and little mass loss or residual enrichment k and rb also largely conform to the residual model but in the amphibolite facies some exchange between pelite and psammite may occur a number of elements show enrichment mn cu pb u na p or depletion y in a small number of samples but the significance of these contrasts is questionable due to relatively large variation in the protolith in contrast ca and sr show progressive and marked enrichment with increasing grade and ba and as are clearly depleted in amphibolite facies psammites in the amphibolite facies some si may have been lost by psammites and gained by the pelites although there has been no mass change from the suite as a whole a large part of the ca and sr enrichment in the amphibolite facies can be accounted for by metasomatic homogenization of calcareous concretions which occur in the lower grade protolith the metamorphism of the greenland group is thus considered to be essentially isochemical

granulites in the tongbai area qinling belt china geochemistry petrology single zircon geochronology and implications for the tectonic evolution of eastern asia the tongbai area of the eastern qinling belt in china includes granulite grade metamorphic assemblages qinling complex which were previously regarded as archean to early proterozoic in age and belonging to the southern margin of the north china plate craton our petrological and geochemical data characterize these rocks as two pyroxene granulites and garnet granulites which formed at temperatures of 757 840 c and pressures of about 9 5 kbar and are now found as xenoliths in granodioritic gneisses the protoliths of these rocks were granodiorites and tholeiitic basalt or gabbro the 207pb 206pb ratios derived from evaporation of single zircons yield ages of 470 20 and 470 14 ma respectively for the basic granulites which we interpret to reflect the time of protolith emplacement these are intruded by a 435 14 ma granodioritic gneiss post dating granulite formation a metaquartzite sample contains detrital zircons as old as 2555 8 ma two samples of granitoid gneiss from the tongbai complex s of the qinling granulites have single zircon 207pb 206pb evaporation ages of 776 8 and 746 10 ma respectively and document late proterozoic igneous activity we suggest that the qinling granulites document an important and hitherto unknown phase of early silurian crustal thickening following subduction and continental collision and that both the qinling and tongbai complexes were part of the southern margin of the north china craton prior to this event and record late proterozoic igneous activity

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

zircon u pb and hf isotopic study of gneissic rocks from the chinese altai progressive accretionary history in the early to middle palaeozoic abstract gneissic rocks in the chinese altai mountains have been interpreted as either paleozoic metasedimentary rocks or precambrian basement this study reports geochemical and geochronological data for banded paragneisses and associated gneissic granitoids collected along a ne sw traverse in the northwestern chinese altai petrological and geochemical data suggest that the protoliths of the banded gneisses were possibly immature sediments with significant volcanic input and that the gneissic granitoids were derived from i type granites formed in a subduction environment three types of morphological features can be recognized in zircons from the banded gneisses and are interpreted to correlate with different sources zircons from five samples of banded paragneiss cluster predominantly between 466 and 528 ma some give neoproterozoic ages and a few yield discordant paleoproterozoic to archean ages zircon hf isotopic compositions indicate that both juvenile mantle and crust materials were involved in the generation of the source rocks from which these zircons were derived in contrast zircons occur ubiquitously as elongated euhedral prismatic crystals in the four samples of the gneissic granitoids and define single populations for each sample with mean ages between 380 and 453 ma the general absence of precambrian inheritance and positive zircon hf values for these granitoids suggest insignificant crustal contribution to the generation of the precursor magmas our data can be interpreted in terms of a progressive accretionary history in early to middle palaeozoic times and the chinese altai may possibly represent a magmatic arc built on a continental margin dominated by neoproterozoic rocks

a synthesis of zircon u pb ages and hf isotopic compositions of granitoids from southwest mongolia implications for crustal nature and tectonic evolution of the altai superterrane abstract granitoid magmatism is extensively developed in the altai superterrane but the timing of magmatic events is not so well determined in this study twelve granitoid intrusions from southwest mongolia were dated by the zircon u pb method and analyzed for zircon hf isotopic compositions the new age data indicated three phases of magmatic emplacement middle paleozoic 398 350 ma late paleozoic 317 289 ma and triassic ca 244 211 ma the first phase magmatism is mainly composed of hornblende bearing i type granitoids formed during a long lasting subduction process in the early to middle paleozoic the second phase magmatism was manifested by a few small intrusions primarily distributed along the erqis fault zone and probably formed in a transitional tectonic regime from subduction to post tectonic setting in the late paleozoic the third phase magmatism was represented by post tectonic granitoids emplaced in an intraplate extensional setting model ages that were calculated using the composition of new continental crust are predominantly neoproterozoic to early paleozoic indicating that the protoliths of granitoids were likely dominated by paleozoic juvenile crust with minor neoproterozoic materials the altai superterrane was a huge paleozoic tectonic collage produced by amalgamation of subduction accretion complexes in the margin of the siberian continent and the tuva mongol microcontinent

the tectonic evolution of the irtysh tectonic belt new zircon u pb ages of arc related and collisional granitoids in the kalaxiangar tectonic belt nw china abstract precise geochronological constraints of the irtysh tectonic belt situated between the saur island arc and the altay terrane are crucial to a better understanding of the tectonic evolution of the central asian orogenic belt caob recently we discovered repeatedly deformed arc related and collisional granitoids in the kalaxiangar tectonic belt ktb which is located in the eastern part of the irtysh tectonic belt in this study we report new whole rock geochemical zircon u pb and hf isotopic data of the arc related and collisional granitoids our data reveal that 1 arc related granodioritic porphyries formed at ca 382 374 ma recrystallized zircon grains from a ultra mylonitic granodiorite of the laoshankou zone in the southern ktb display a u pb age of ca 360 ma 2 syn collisional granodioritic porphyries which distribute along faults and parallel to the cleavage were emplaced at ca 367 356 ma with e hf t values varying from 7 8 to 14 2 and hf model ages from 873 to 459 ma 3 a post collisional a type granodioritic porphyry which crosscuts the nw nnw trending schistosity of the metasedimentary country rocks at a low angle has an age of ca 324 320 ma while the e hf t values range from 7 6 to 14 4 with hf model ages from 850 to 416 ma 4 post collisional strike slip a type granite dykes exposed along strike slip faults gave ages between 287 and 279 ma whereas the e hf t values range from 4 9 to 12 7 and the hf model ages from 995 to 500 ma and 5 a type biotite granite dykes which intruded along conjugate tension joints have ages of 274 271 ma and e hf t values from 1 5 to 13 2 with hf model ages from 1196 to 454 ma consequently we propose that the collision between the saur island arc and the altay terrane occurred in the early carboniferous ca 367 356 ma and the subsequent post collisional tectonic process continued to the late carboniferous ca 324 320 ma it is further suggested that the irtysh tectonic belt underwent large scale strike slip deformation during the mid permian between 287 and 279 ma the termination of the irtysh tectonic belt orogeny is thought to have also occurred during the mid permian between 274 and 271 ma

late mesozoic intrusive complexes in the coastal area of fujian se china the significance of the gabbro diorite granite association abstract the pingtan and daiqianshan igneous complexes southeastern china are typical of calc alkaline series developed at active continental margins these two complexes are dominated by granites or gabbros with minor diorites and granodiorites and these rock types closely coexisted both temporally and spatially the major and trace element signatures of gabbros indicate they were crystallised from high aluminium basaltic magmas which were generated from metasomatised upper mantle the typical metaluminous calc alkaline composition trace element patterns and sr nd isotopes indicate the granites in the complexes are i type granites which may have been generated from the deeper crust by a thermal contribution from the mantle field relationships petrography and geochemistry favour a mechanical mingling together with a limited chemical mixing between high aluminium basaltic magma and granitic magma resulting in the formation of diorites or granodiorites the essentially bimodal character of these complexes reflects the general character of back arc extensional magmatism which was induced by the subduction of the pacific plate beneath the eurasian continent in late mesozoic time

high mantle temperature during cretaceous avalanche abstract a new sequence of axisymmetrical spherical mantle convection simulations has been computed to monitor the influence of mantle avalanches on the mantle thermal state during an avalanche that is characterized by a high rate of mixing between upper mantle and lower mantle a complex perturbation of the thermal field occurs due to the advection of cold upper mantle into the lower mantle and the hot return flows from the lower mantle the computation shows that the avalanches trigger global upper mantle warming of several tens of degrees and make it possible to explain both core mantle boundary and transition zone origins for mantle plumes recently the chemical compositions of dsdp odp drill holes on crust older than 80 myr have been interpreted to reflect a higher mean mantle temperature during the mesozoic humler et al earth planet sci lett 173 1999 7 23 here we have used and extended the data set for oceanic crust younger than 80 myr combining the mean chemistry for old oceanic crust and the mean depth of the ocean basin versus the square root of age we propose a continuous curve for the temperature evolution of the upper mantle for the last 130 myr it shows a mantle thermal high around 125 ma with amplitude 50 k and duration 30 myr comparable with those obtained from theoretical convection models it is proposed that such a high mantle temperature may be due to the thermal effects of an avalanche which may have started 180 myr ago

end permian to mid triassic termination of the accretionary processes of the southern altaids implications for the geodynamic evolution phanerozoic continental growth and metallogeny of central asia the altaids is one of the largest accretionary orogenic collages in the world with the highest rate of phanerozoic continental growth and significant metallogenic importance it is widely accepted that subduction related orogenesis of the altaids started in the late precambrian and gradually migrated southward present coordinates however it is uncertain when and how the building of the altaids was finally completed based on structural geology geochemical geochronological and paleomagnetic data this paper presents late paleozoic to early mesozoic accretionary tectonics of two key areas north xinjiang in the west and inner mongolia in the east together with neighboring mongolia the late paleozoic tectonics of north xinjiang and adjacent areas were characterized by continuous southward accretion along the wide southern active margin of siberia and its final amalgamation with the passive margin of tarim which may have lasted to the end permian to early mid triassic in contrast in inner mongolia and adjacent areas two wide accretionary wedges developed along the southern active margin of siberia and the northern active margin of the north china craton which may have lasted to the mid triassic the final products of the long lived accretionary processes in the southern altaids include late paleozoic to permian arcs late paleozoic to mid triassic accretionary wedges composed of radiolarian cherts pillow lavas and ophiolitic fragments and high pressure ultrahigh pressure metamorphic rocks permian alaskan type zoned mafic ultramafic complexes intruded along some major faults of the tien shan we define a new tarim suture zone immediately north of the tarim craton that is probably now buried below the tien shan as a result of northward subduction of the tarim block in the cenozoic the docking of the tarim and north china cratons against the southern active margin of siberia in the end permian to mid triassic resulted in the final closure of the paleoasian ocean and terminated the accretionary orogenesis of the southern altaids in this part of central asia this complex geodynamic evolution led to formation of giant metal deposits in central asia and to substantial continental growth

precise zircon u pb and molybdenite re os dating of the shuikoushan granodiorite related pb zn mineralization southern hunan south china abstract the shuikoushan ore district located in southern hunan province south china contains pb zn au mineralization hosted in the devonian to triassic strata and mesozoic granodiorite intrusions ore minerals are mainly pyrite sphalerite galena and minor molybdenite molybdenite usually intergrown with pyrite formed during pb zn hydrothermal mineralization in order to determine the precise age of pb zn mineralization and further understand the relationship between magma emplacement and hydrothermal mineralization in the ore district molybdenite re os dating and zircon sims u pb dating were undertaken the zircon u pb dating reveals that the granodiorite intrusion was emplaced at 158 8 1 8 ma mswd 0 40 re os isotopic age of seven molybdenite samples yields model ages ranging from 157 5 2 5 ma to 161 0 2 4 ma and gives a well defined 187re 187os isochron age of 157 8 1 4 ma 2 mswd 1 3 indicating the timing of pb zn mineralization in the shuikoushan ore district at about 158 ma this date coincides well with the zircon sims u pb age of the granodiorite revealing a genetic association between the pb zn mineralization and the granitic magmatism combined with geochronological data published for other pb zn cu deposits in southern hunan it can be concluded that the granodiorite related pb zn mineralization throughout southern hunan mainly occurred at 160 156 ma rather than 180 170 ma or 170 160 ma as considered previously the pb zn mineralization and major w sn mineralization in southern hunan are coeval and may be related to the same geological event

cenozoic k rich adakitic volcanic rocks in the hohxil area northern tibet lower crustal melting in an intracontinental setting it is generally accepted that the cenozoic potassic volcanic rocks of northern tibet were derived from a lithospheric mantle source here we report new chronological geochemical and isotopic data for the miocene ca 18 15 ma k rich adakitic volcanic rocks from the hohxil area of the songpan ganzi block in northern tibet we contend that these rocks were generated by partial melting of the mafic lower crust in an intracontinental setting unrelated to subduction of oceanic crust the hohxil rocks exhibit high sr y and la yb ratios high sr and la contents but low yb and y concentrations similar to adakites formed by slab melting associated with subduction however their relatively low e n d values 2 09 to 4 58 high 8 7 sr 8 6 sr 0 7072 0 7075 th u th ba and rb ba ratios and distinctive potassium enrichments k 2 o na 2 o are very different from the composition of typical adakites in addition those k rich adakitic rocks with the highest sio 2 contents 61 wt exhibit the lowest 8 7 sr 8 6 sr ratios and highest e n d values and are the oldest cenozoic volcanic rocks exposed in the songpan ganzi block suggesting that they were derived neither directly from a mantle source nor by differentiation of a shoshonitic magma taking into account the composition of lower crustal mafic xenoliths in tibet as well as the tectonic and geophysical evidence we conclude that the hohxil adakitic magmas were produced by partial melting of amphibole bearing eclogites with a k rich mafic bulk composition in the lower part 55 km of the thickened northern tibetan crust partial melting of the lower crust may have been triggered by dehydration release of fluids from sedimentary materials in the southward subducted continental lithosphere

the kalatongke magmatic ni cu deposits in the central asian orogenic belt nw china product of slab window magmatism the permian kalatongke ni cu deposits in the central asian orogenic belt are among the most important ni cu deposits in northern xinjiang western china the deposits are hosted by three small mafic intrusions comprising mainly norite and diorite its tectonic context petrogenesis and ore genesis have been highly contested in this paper we present a new model involving slab window magmatism for the kalatongke intrusions the origin of the associated sulfide ores is explained in the context of this new model minor amounts of olivine in the intrusions have fo contents varying between 71 and 81 5 mol which are similar to the predicted values for olivine crystallizing from coeval basalts in the region analytic modeling based on major element concentrations suggests that the parental magma of the kalatongke intrusions and the coeval basalts represent fractionated liquids produced by 15 of olivine crystallization from a primary magma itself produced by 7 8 partial melting of depleted mantle peridotite positive end values 4 to 10 and significant negative nb anomalies for both intrusive and extrusive rocks can be explained by the mixing of magma derived from depleted mantle with 6 18 of a partial melt derived from the lower part of a juvenile arc crust with a composition similar to coeval a type granites in the region plus up to 10 contamination with the upper continental crust our model suggests that a slab window was created due to slab break off during a transition from oceanic subduction to arc arc or arc continent collision in the region in the early permian decompression melting in the upwelling oceanic asthenosphere produced the primary magma when this magma ascended to pond in the lower parts of a juvenile arc crust it underwent olivine crystallization and at the same time triggered partial melting of the arc crust mixing between these two magmas followed by contamination with the upper crust after the magma ascended to higher crustal levels formed the parental magma of the kalatongke intrusions the parental magma of the kalatongke intrusions was saturated with sulfide upon arrival primarily due to olivine fractional crystallization and selective assimilation of crustal sulfur sulfide mineralization in the kalatongke intrusions can be explained by accumulation of immiscible sulfide droplets by flow differentiation gravitational settling and downward percolation which operated in different parts of the intrusions platinum group element pge depletion in the bulk sulfide ores of the kalatongke deposits was due to depletion in the parental magma which in turn was likely due to depletion in the primary magma pge depletion in the primary magma can be explained by a relatively low degree of partial melting of the mantle and retention of coexisting sulfide liquid in the mantle

40ar 39ar thermochronology from the northwestern dabie shan constraints on the evolution of qinling dabie orogenic belt east central china abstract seven structural units with distinct kinematics have been recognized in the northwestern dabie shan 114 e 115 e and 31 n 32 n east central china they are from north to south 1 the north verging mafan fold belt mfb involving early paleozoic metamorphic rocks 2 the liangting melange belt lmb which is separated from the mfb by a south dipping thrust and consists of gneissic and marble blocks surrounded by highly sheared greenschist 3 the north verging laoshan ductile fold belt lfb which was thrust northward over the liangting melange belt and involves proterozoic schists 4 the suhe thrust sheet sts which consists of phyllite and slate above a folded thrust fault dipping both to the north and south 5 the 5 8 km wide huwan shear belt hsb in the footwall of the folded thrust consisting of mylonitic gneisses and sheared ecologite 6 the kafang eclogite bearing gneiss dome kgd which is defined by broadly warped foliations and 7 the 10 km wide pendian ductile shear belt psb characterized by dextral strike slip deformation nine phengite and muscovite samples in eclogites mylonitic shear zones and metamorphic gneisses from four of the seven structural units have been dated by the 40ar 39ar step heating method phengites from deformed eclogites in the hsb yield complex age spectra that we interpret as most likely indicating closure with respect to ar loss between 350 and 430 ma the rock samples collected from the eclogite bearing gneiss display a significant static recrystallization of quartz which suggests temperatures in excess of ca 350 400 c the ages of 350 430 ma from samples 9669b and 9669c are therefore interpreted as retrograde metamorphic ages for the eclogites six additional phengites sampled from mylonitic rocks yield two groups of ages 263 249 and 236 214 ma the older group samples 96 5 9667 and 9692 were collected from mylonitic rocks with top to north shear sense in the lmb hsb and kgd respectively 9667 and 9692 exhibit a strong static recrystallization which means that ductile deformation took place at a temperature in excess of 350 400 c we therefore adopt the age of 263 249 ma as the cooling ages of white micas considering sample 96 5 shows microfabrics formed at lower temperature around or slight below the closure temperature of white mica there is a strong possibility that the age of 254 1 ma represents a crystallization age rather than a cooling age because these samples are taken from the top to north shear zones we also interpret the 263 249 ma event as a result of thrusting induced denudation the younger group samples 96106 96124 and 96113 collected from dextral shear belts in the hsb and psb exhibit a strong static recrystallization at a temperature in excess of 350 400 c their ages 236 214 ma are therefore interpreted as the cooling ages related to later dextral strike slip faulting sample 96101 collected from weakly foliated rock without penetrative deformation on the north limb of the kgd shows poorly recovered quartz microfabrics at the temperature around or slightly below the closure temperature of white mica its age of 187 1 ma is therefore interpreted as crystallization age during the downdip detachment event related to doming the 40ar 39ar data from the northwestern dabie shan suggest four distinctive cooling events on the basis of the local structural framework and regional tectonic relationships we suggest that the 410 350 ma cooling event was related to either subduction of an oceanic plate beneath the sino korean plate or collision between the sino korean plate and a microcontinental block in the middle paleozoic the three younger episodes of cooling between the late permian and the late triassic may all have been related to the collision between the sino korean and yangtze plates specifically the 263 249 ma event could have been a result of thrusting induced denudation whereas the 236 214 ma cooling event is likely related to strike slip faulting the younger ages of 195 187 ma obtained from the gneiss dome are interpreted to be a result of regional denudation related to doming during the late stage of collision between the sino korean and yangtze plates

post accretionary permian granitoids in the chinese altai orogen geochronology petrogenesis and tectonic implications the altai orogen is an important constituent of the central asian orogenic belt caob the main orogenic processes occurred mainly in the early to middle paleozoic and involved a series of northward subduction and terrane accretion however termination of the accretionary and post accretionary processes remains poorly defined the chinese altai is located in the southern part of this orogeny which is widely intruded by permian granitic plutons these plutons are approximately circular in shape free of deformation and generally cutting pre permian structures suggesting a post tectonic formation we report the results of geochronological and geochemical data from five specific granitic plutons buerjin xibodu daqiaonan aweitan and adenbluk which all yielded magmatic zircon u pb ages of about 270 ma these plutons are composed of high k calc alkaline rocks including k feldspar megaphyric granite biotite granite and monzogranite that have metaluminous to weakly lree enriched coupled with negative eu anomalies significant negative anomalies of ba sr p and ti are also observed in the primitive mantle normalized diagram they have positive whole rock end t 1 3 to 7 2 and zircon ehf t values 5 6 to 12 9 yielding sm nd model ages of 0 9 ga therefore these granitoids are proposed to have been generated by differentiation of mantle derived magmas with variable crustal contamination in view of the field occurrence structural analysis regional tectonics and geochemical characteristics these permian plutons are concluded to be post accretionary or post collisional asthenospheric upwelling after the collision and amalgamation of the altai and junggar blocks could have caused the mantle derived magmas that evolved to form the granitoids we note that permian intrusions are not only widespread but also voluminous in the caob they mostly are post collisional products and some of them might have been related to the large igneous province activity in the tarim block

palaeozoic accretionary and convergent tectonics of the southern altaids implications for the growth of central asia the southern altaids present a unidirectional section from mongolia to china through an accretionary orogen that youngs progressively from neoproterozoic in the north to permian in the south the orogen formed by forearc accretion of island arcs accretionary wedges ophiolites and precambrian microcontinents this regularity was upset by early growth within the ocean of arcs that later collided at the accreting continental margin by imbrication of old ophiolites with young arcs and by himalayan style thrust nappe tectonics when an arc collided into a microcontinent lateral growth of the southern altaids represents a massive addition of juvenile material to the palaeozoic crust

the geochemical evolution of anorthosite residual magmas in the laramie anorthosite complex wyoming olivine and pyroxene bearing fit enriched dioritic rocks in the 1434 ma laramie anorthosite complex are interpreted to represent variably fractionated and contaminated magmas residual after the crystallization of anorthosite geochemical characteristics of this suite include the following high contents ofti02 fe20s and p2o5 high incompatible trace element contents rare earth element patterns with a large range of eu anomalies and isotopic compositions that reflect the geographic location of individual samples with i r increasing and ejy decreasing from south to north after extraction from anorthosite fractionation offerrodioritic residual magmas resulted in secondary residual monzpdioritic melts and complementary oxide rich ferrodiorite cumulates geographic trends in isotopic composition reflect an increasing archean crustal component from south to north dioritic dikes and cumulates with isotopic compositions similar to associated anorthosites were derived locally large isotopic discrepancies between some diorites and their hosting anorthosites reflect preferential contamination of residual magma during ascent and emplacement of mantle derived plagioclase rich diapirs followed by subsequent extraction and isolation of fe enriched interstitial melt strong isotopic contrasts between anorthosite and associated fe enriched rocks in anorthosite complexes do not preclude a direct relationship between them and reflect the diversity and complexity of processes during their petrogenesis

neoproterozoic to paleozoic geology of the altai orogen nw china new zircon age data and tectonic evolution abstract we present a synthesis and a new account of the geological and tectonic history of the terranes of the chinese paleozoic altai orogen together with new single zircon ages for granitic and rhyodacitic rocks a central terrane consists of neoproterozoic to silurian amphibolite facies metasedimentary rocks and abundant devonian carboniferous granites the presence of precambrian basement is indicated by sinian fossils our xenocryst ages and published nd mean crustal residence ages of granites felsic arc type lavas on the southern margin of the terrane have a mean 207pb 206pb zircon age of 505 ma reflecting the time of arc volcanism and the presence of xenocysts with ages between 614 and 921 ma suggests derivation by intracrustal melting accordingly we suggest that a cambro ordovician continental magmatic arc was built on the southern margin of the central terrane by northward subduction a low grade ordovician andean type arc with a continental basement is situated above a normal fault on

the granites gold deposits northern territory australia evidence for an early syn tectonic ore genesis the ore deposits of the granites goldfield are shear hosted within palaeoproterozoic amphibolite facies metasedimentary rocks in the tanami region northern territory australia the ore bodies are located within a 5 to 35 m thick sequence of steeply dipping unit of metamorphosed iron rich metasedimentary rocks deformation at the granites was complex and is characterized by five successive deformation phases d1 5 shear veins central and oblique are the dominant type of vein geometry with minor development of extensional veins and reverse fault related veins four generations of syn tectonic veins corresponding to d1 d3 d4 and d5 have been recognized and are comprised of quartz quartz carbonate calc silicate and calcite in addition two generations of disseminated sulfide arsenide mineralization dominated by pyrrhotite arsenopyrite and loellingite with minor pyrite chalcopyrite and rare marcasite formed syn d1 and syn to post d3 textural and structural evidence indicates deposition of gold was contemporaneous with the syn d1 veins and sulfide arsenide mineralization four hydrothermal phases are proposed for the formation of the veins and disseminated sulfide arsenide assemblages the first phase d1 was responsible for transport and deposition of the majority of the gold minor remobilization and deposition of gold occurred during the d3 and d4 phases little is known about the nature of the d1 ore fluid although a relatively low sulfur content is indicated by the assemblage pyrrhotite arsenopyrite loellingite rare pyrite the growth of amphibolite facies metamorphic minerals andalusite and almandine garnet during d1 indicates a high temperature for the fluid the d3 hydrothermal phase coincided with peak metamorphism d4 fluids were hypersaline high temperature co2 poor and h2s poor

uniformity in sulfur isotope composition in the orogenic gold deposits from the dharwar craton southern india the sulfur isotope composition of sulfides mainly pyrite and arsenopyrite from gold deposits prospects of the dharwar craton such as hutti hira buddini uti kolar chigargunta ajjanahalli and jonnagiri has a narrow range 34s 1 1 to 7 1 such craton scale uniformity of the above gold camps is noteworthy in spite of the wide diversity in host rock compositions and their metamorphic conditions and suggests a magmatic or average crustal source of sulfur for all deposits studied in addition our study points towards gold precipitation from reduced ore fluids with near homogeneous sulfur isotope compositions

petrogenesis of late paleozoic tholeiitic nb enriched calc alkaline and adakitic rocks in southwestern mongolia implications for intra oceanic arc evolution abstract intra oceanic subduction has been taken as one of the most important contributors to the crustal growth of the immense central asian orogenic belt caob however the detailed geological processes of the development and growth of numerous island arcs still remain unclear in order to explore the development history of representative ancient island arcs we have conducted geochronological and geochemical studies on an intrusive rock association in southwestern mongolia the studied rock association comprises a mafic intrusive complex consisting of norite and diabase porphyry diabase and diorite dykes new zircon geochronological data and whole rock geochemical compositions reveal sequential magmatic events of an island arc development the mafic intrusive complex has an emplacement age of 356 ma old geochemically its two components of norite and diabase porphyry have a common tholeiitic character but show striking differences in elemental contents the norites are characterized by relatively higher mgo 7 25 16 1 wt lower tio 2 0 54 0 93 wt and relatively lower nb contents 0 99 6 39 ppm than those of the nb enriched nb 9 ppm diabase porphyry these characters together with variable nb la ratios suggest that the parental magma of the norites was derived from a refractory mantle source metasomatized by slab derived fluids and or melts whereas the generation of the nb enriched diabase porphyry was associated with the mantle metasomatism by slab derived melts the 335 ma diabase dykes have negative nb ta ti anomalies and elevated lree and lile contents similar to calc alkaline subduction related magmas which together with high zircon e hf t values 10 6 to 15 8 indicate a derivation from a mantle wedge metasomatized by slab related fluids the 290 ma dioritic dykes show the highest sio 2 53 1 58 3 wt and total alkali k 2 o na 2 o 5 84 7 77 wt contents and their distinctively high sr y 56 1 97 9 ratios and low yb the timing and petrogenesis of the basic intermediate rock association may supply valuable information on tectonic evolution of an intra oceanic subduction accretion system of the caob the coexistence of the early carboniferous tholeiitic nb enriched mafic intrusive complex and boninite as well as picrite may represent an immature intra oceanic island arc the middle carboniferous calc alkaline diabase dykes were probably associated with the relatively mature island arc the permian adakitic rocks was likely produced by partial melting of a delaminated lower crust in an extensional regime when numerous subduction accretion complexes were likely amalgamated together and docked on the siberian active margin

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

the early cretaceous yangzhaiyu lode gold deposit north china craton a link between craton reactivation and gold veining the yangzhaiyu gold deposit is one of numerous lode gold deposits in the xiaoqinling district southern margin of the north china craton gold mineralization is hosted in neoarchean to early paleoproterozoic amphibolite facies metamorphic rocks and consists of auriferous quartz veins and subordinate disseminated ores in the vein proximal alteration zone ore related hydrothermal alteration is dominated by sericite quartz sulfide assemblages close to gold veins and biotite quartz pyrite chlorite epidote alteration generally distal from mineralization pyrite is the predominant sulfide mineral locally coexisting with minor amounts of chalcopyrite sphalerite and galena gold occurs mostly as free gold enclosed in or filling microfractures of pyrite and quartz and is also present in equilibrium with au bearing tellurides mainly petzite and calaverite coexisting with hessite tellurobismuthite and altaite fluid inclusion studies suggest that gold veins were deposited at intermediate temperatures 175 313 c from aqueous or aqueous carbonic fluids with moderate salinity 5 14 wt nacl equiv 34s values of sulfide minerals range mainly from 2 0 to 4 4 whereas auriferous quartz vein samples have 18o values of 12 4 to 9 6 with calculated 18oh2o values of 6 0 to 3 2 gold related pyrite grains yield elevated 3he 4he ratios 1 51 0 32 ra relative to crustal reservoirs and mantle like 20ne 22ne and 21ne 22ne ratios 9 90 9 68 and 0 029 respectively the stable and noble gas isotopes thus suggest deep seated most likely magmatic and mantle derived sources for the ore fluids sulfur and by inference other components in the ore system 40ar 39ar dating of ore related sericite and biotite separates indicates two episodes of gold genesis at 134 5 to 132 3 and 124 3 to 123 7 ma the mineralization ages overlap zircon u pb ages of 141 0 1 6 to 125 8 1 4 ma 2 for the wenyu and niangniangshan monzogranite plutons and a number of mafic to intermediate dikes intruding these plutons all being proximal to the yangzhaiyu gold deposit the synchronism of gold genesis and magmatism provides additional weights of evidence for a magmatic derivation of ore fluids and sulfur the geochronologic data also suggest that gold veining took place billions of years after the stabilization of the north china craton and associated metamorphism in the late archean to early paleoproterozoic this contrasts sharply to lode gold deposits in other precambrian cratons that formed predominantly in late archean to paleoproterozoic temporarily and genetically related to regional high grade metamorphism and compressional or transpressional tectonism available data have demonstrated that the north china craton was reactivated in the late mesozoic as marked by voluminous igneous rocks faulted basin formation high crustal heat flow and widespread metamorphic core complexes in the eastern part of the craton it is thus suggested that the yangzhaiyu gold deposit together with other deposits of similar ages in the xiaoqinling district were products of this craton reactivation event lithospheric extension and extensive magmatism related to the craton reactivation may have provided sufficient heat energy fluid and sulfur required for the formation of the gold deposits

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

geochemical sr nd and zircon u pb hf isotopic studies of late carboniferous magmatism in the west junggar xinjiang implications for ridge subduction abstract voluminous granitic intrusions are distributed in the west junggar nw china and they can be classified as the dioritic rocks charnockite and alkali feldspar granite groups the dioritic rocks sio 2 50 4 63 8 wt are calc alkaline and mg enriched average mgo 4 54 wt mg 0 39 0 64 with high sr y ratios average 21 2 weak negative eu average eu eu 0 80 and pronounced negative nb ta anomalies their sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7035 0 7042 e nd t 4 5 7 9 e hf t 14 1 14 5 show a depleted mantle like signature these features are compatible with adakites derived from partial melting of subducted oceanic crust that interacted with mantle materials the charnockites sio 2 60 0 65 3 wt show transitional geochemical characteristics from calc alkaline to alkaline with weak negative eu average eu eu 0 75 but pronounced negative nb ta anomalies sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7037 0 7039 e nd t 5 2 8 0 e hf t 13 9 14 7 also indicate a depleted source suggesting melts from a hot juvenile lower crust alkali feldspar granites sio 2 70 0 78 4 wt are alkali and fe enriched and have distinct negative eu and nb ta anomalies average eu eu 0 26 low sr y ratios average 2 11 and depleted sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7024 0 7045 e nd t 5 1 8 9 e hf t 13 7 14 2 these characteristics are also comparable with those of rocks derived from juvenile lower crust despite of the differences in petrology geochemistry and possibly different origins zircon ages indicate that these three groups of rocks were coevally emplaced at 305 ma a ridge subduction model can account for the geochemical characteristics of these granitoids and coeval mafic rocks as the slab window opened upwelling asthenosphere provided enhanced heat flux and triggered voluminous magmatisms partial melting of the subducting slab formed the dioritic rocks partial melting of the hot juvenile lower crust produced charnockite and alkali feldspar granite and partial melting in the mantle wedge generated mafic rocks in the region these results suggest that subduction was ongoing in the late carboniferous and thus support that the accretion and collision in the central asian orogenic belt took place in north xinjiang after 305 ma and possibly in the permian

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

stable isotope geochemistry of the archean val d or canada orogenic gold vein field a systematic study of the auriferous quartz veins of the val d or vein field abitibi quebec canada demonstrates that the c o s isotope composition of silicate carbonate borate oxide tungstate and sulphide minerals have a range in composition comparable to that previously determined for the whole superior province the oxygen isotope composition of quartz from early quartz carbonate auriferous veins ranges from 9 4 to 14 4 whereas later quartz tourmaline carbonate veins have 18oquartz values ranging from 9 2 to 13 8 quartz carbonate veins have carbonate 18o 6 9 12 5 13c 6 2 1 9 and pyrite 34s 1 2 and 1 9 isotope compositions comparable to those of quartz tourmaline carbonate veins 18o 7 9 11 7 13c 8 0 2 4 34s 0 6 6 0 18oquartz values in quartz tourmaline carbonate veins have a variance comparable to analytical uncertainty at the scale of one locality irrespective of the type of structure the texture of the quartz or its position along strike across strike or down dip a vein in contrast the oxygen isotope composition of quartz in quartz tourmaline carbonate veins displays a regional distribution with higher 18o values in the south central part of the vein field near the cadillac tectonic zone and which 18o values decrease regularly towards the north another zone of high 18o values in the northeast corner of the region and along the trace of the senneville fault is separated by a valley of lower 18o values from the higher values near the cadillac tectonic zone oxygen isotope isopleths cut across lithological contacts and tectonic structures this regional pattern in quartz tourmaline carbonate veins is interpreted to be a product of reaction with country rocks and mixing between 1 a deep seated hydrothermal fluid of metamorphic origin with minimum 18o 8 5 13c 0 6 and 34s 0 4 and 2 a supracrustal fluid most likely archean seawater with a long history of water rock exchange and with maximum 18o 3 9 13 c 5 6 and 34s 5 0

high pressure metamorphic rocks from tongbaishan central china u pb and 40ar 39ar age constraints on the provenance of protoliths and timing of metamorphism abstract the tongbaishan area is situated at the junction between the paleozoic north qinling orogen and the mesozoic south qinling dabie sulu orogen the area contains an important geological section for understanding the assembly of the sino korean and yangtze cratons two eclogite zones were recently recognized in this area the peak metamorphic conditions are estimated to be 530 610 c and 17 20 kbar for the northern eclogite zone and 460 560 c and 13 19 kbar for the southern eclogite zone shrimp u pb analyses on zircons of eclogite metagabbro and orthogneiss indicate that their protoliths were formed during the neoproterozoic and experienced high pressure hp recrystallization during the permian triassic period therefore the tongbaishan area belongs to the western extension of the dabieshan hp uhp terrane in addition u pb ages of detrital zircons from quartzite paragneiss and schist show three age populations of 2 49 ga 1 93 ga and 1 85 1 82 ga suggesting that extensive paleoproterozoic orogeny or tectonothermal events took place in the northern margin of the yangtze craton the shrimp dating and complementary 40ar 39ar dating of muscovite from quartzofeldspathic gneiss and quartzite reveal that the crustal rocks were subducted to great depths and underwent hp eclogite facies metamorphism at ca 255 ma slightly earlier than the timing of deep subduction of the uhp slice the hp rocks were later exhumed to mid to upper crustal levels and suffered from retrograde recrystallization at ca 238 ma the dating also discloses a still later phase of deformation and recrystallization at ca 215 ma the available age data throughout the tongbai dabie sulu orogen reflect diachronous exhumation from ca 238 ma for the hp slice to ca 213 ma for the uhp slice such a phenomenon could be explained by successive subduction and exhumation of the broken slices of the subducting yangtze slab

contribution of mafic melt to porphyry copper mineralization evidence from mount pinatubo philippines and bingham canyon utah usa mount pinatubo in the philippines known for its cataclysmic eruption in 1991 hosts several porphyry copper deposits and active geothermal systems an underlying mafic melt supplied much of the sulphur for the dacitic magma and its injection into the dacitic magma chamber triggered the eruption the eruption caused purging of the sulphur rich fluid from the dacite to the atmosphere and extensive fracturing similar events took place at bingham canyon utah site of the largest copper and gold deposit in north america at 38 ma the bingham canyon mineralization took place beneath an active stratovolcano and pyroclastic flows contemporaneous with the mineralization show evidence for magma mingling ascent of mafic melt supplied sulphur and chalcophile elements to the felsic magma which consolidated to form the bingham stock and its underlying magma chamber injections of the mafic melt caused periodic eruptions of felsic magma to form the stratovolcano and deposition of sulphide minerals in highly fractured rocks in and around the stock

constraints of sr isotopic compositions of apatite and carbonates on the origin of fe and cu mineralizing fluids in the lala fe cu mo lree deposit sw china abstract numerous fe cu deposits with mineralization styles similar to iron oxide copper gold iocg deposits form the kangdian fe cu metallogenic province southwestern sw china as one of the largest deposits in the region the 1 0 ga lala fe cu deposit is hosted in a paleoproterozoic volcanic sedimentary succession named the hekou group which is alternately intruded by 1 0 ga doleritic plutons this deposit has a paragenetic sequence evolving from stage i of na alteration to stage ii of fe mineralization and finally to stage iii of cu mo ree mineralization coeval with mafic felsic intra plate magmatism in the region this study conducted in situ sr isotopic analyses on apatite and carbonate aiming to resolve the long controversial issue regarding the origin of the fe and cu mineralizing fluids in the deposit apatite of stage ii has 87sr 86sr ratios varying from 0 71380 to 0 72733 much higher than those of synchronous igneous rocks in the region 0 7074 to 0 7091 but similar to the paleoproterozoic host rocks 0 71368 to 0 71837 at 1 0 ga this similarity indicates that radiogenic sr of the fe mineralizing fluid was dominantly sourced from the host rocks apatite and calcites of stage iii have 87sr 86sr ratios 0 75758 0 79293 much higher than apatite of stage ii and the host rocks but similar to the archean basement rocks as high as 0 80 at 1 0 ga beneath the cover of the yangtze block suggesting that the highly radiogenic sr isotopic composition of the cu mineralizing fluid was mainly inherited from the old basement rocks in combination with previous c o s isotopic data indicating a magma hydrothermal origin it was suggested that the fe mineralizing fluid was exsolved from a mafic magma that generated the 1 0 ga doleritic plutons and inherited radiogenic sr from the host rocks during fluid rock interaction by contrast the cu mineralizing fluid might have been sourced from another pulse of magmatic cu mo ree and co2 rich fluid which have once interacted with archean basement rocks prior to mineralization the source of such a cu mo ree rich fluid was not well constrained in current study but was inferred to be exsolved from a hidden felsic magma we propose that intrusions of the bimodal magmas in kangdian are responsible for regional hydrothermal circulation which led to fe cu mo ree mineralization in the kangdian province

recognition of yanshanian magmatic hydrothermal gold and polymetallic gold mineralization in the laowan gold metallogenic belt tongbai mountains new evidence from structural controls geochronology and geochemistry abstract the laowan metallogenic belt in china is an important metallogenic belt within the tongbai orogenic belt and contains the medium sized laowan and shangshanghe gold deposits the small huangzhuyuan lead zinc silver gold deposit and some gold and cu pb occurrences these deposits are hosted in mesoproterozoic plagioclase amphibolite or schist and mica quartz schist the gold ores are mainly quartz veins and veinlets and disseminated altered ores subordinate ore types include massive sulfides and breccias the laowan gold deposit is characterized by three right stepping en echelon fracture controlled alteration zones that dip gently to the south and includes disseminated sheeted and stockwork ores these lodes were formed by the interaction of ore forming fluid with foliated to laminated cataclasite within the transpressional faults the shangshanghe gold deposit is characterized by parallel ore lodes that dip steeply to the north and includes quartz veins and breccias in addition to ores in altered wallrocks these lodes were formed by focusing of fluids into transtensional faults these ore controlling faults displaced early barren quartz veins 10 m horizontally with a dextral sense of motion the ore hosting structures at the laowan and shangshanghe deposits correspond to the p and r type shears of a brittle dextral strike slip fault system respectively which make angles of about 15 and 15 to the laowan and songpa boundary faults the ore controlling fault system post dated formation of a ductile shear zone and peak regional metamorphism this precludes a genetic relationship between hydrothermal mineralization and regional metamorphism and ductile shear deformation these gold deposits are not typical orogenic gold deposits the metallogenic belt displays district scale zoning of mo cu pb zn ag au relative to songpa granite porphyry dike zone suggesting the mineralization may be closely related to the granite porphyry measured 34 s of sulfides and 18 o and d of fluid inclusion waters in auriferous quartz also are consistent with a magmatic source for sulfur and ore fluids the similarity of pb isotope ratios between the ores and yanshanian granitoids suggests a similar source as the age 139 3 ma of granite porphyry obtained by zircon u pb isotope overlaps the mineralization age 138 1 ma zhang et al 2008a the gold and polymetallic metallogenesis of the laowan gold belt has close spatial temporal and possibly genetic relationships with yanshanian high level magmatism

tectonics of the dabieshan eastern china and possible exhumation mechanism of ultra high pressure rocks the dabieshan is divided into three tectonic domains the southern dabieshan is a stack of allochtons from top to bottom i unmetamorphosed cambrian early triassic series unconformably covered by jurassic sandstone ii weakly metamorphosed proterozoic slate iii hp rocks mostly retrogressed into greenschist facies iv uhp allochton v uhp free gneisses these units exhibit a nw se lineation and top to the nw shearing reworked by s verging folds the central dabieshan is a migmatitic dome superimposed on uhp metamorphism and bounded by a detachement fault responsible for the exhumation of the uhp unit during its retrogression into amphibolite facies in the northern dabieshan early foliation and n s trending compressional lineation are deformed by n verging folds coeval to the syn exhumation ductile structures of the central dabieshan a geodynamic model involving coeval thrusting and normal faulting is discussed

isotopic responses to basaltic injections into silicic magma chambers a whole rock and microsampling study of macrorhythmic units in the pleasant bay layered gabbro diorite complex maine usa the pleasant bay layered gabbro diorite complex 420 ma formed via repeated injections of mafic magma into a felsic magma chamber it is dominated by repeating sequences macrorhythmic units with chilled gabbroic bases which may grade upward into medium grained gabbro diorite and granite each unit represents an injection of mafic magma into the chamber followed by differentiation increases in sri and decreases in endi with stratigraphic height indicate open system isotopic behaviour and exchange between the mafic and felsic magmas isotopic variations of whole rock samples in individual macrorhythmic units do not conform to bulk mixing or afc models between potential parental magmas sr isotopic studies of single feldspar crystals from one macrorhythmic unit indicate that exchange of crystals between the resident felsic magma and mafic influxes was important that some of the rocks contain feldspar xenocrysts and that the rocks are isotopically heterogeneous on an intercrystal scale xenocryst abundance increases with stratigraphic height suggesting that crystal exchange occurred in situ the lack of disequilibrium textures in the xenocrystic feldspar indicates the evolved macrorhythmic magma and resident silicic magma were of a similar composition and likely in thermal equilibrium at the time of crystal transfer mafic chilled margins are enriched in alkalis and isotopically evolved compared with mafic dikes representing the parental melts and suggest rapid in situ diffusional exchange following emplacement of individual mafic replenishments

highly depleted hadean mantle reservoirs in the sources of early archean arc like rocks isua supracrustal belt southern west greenland abstract growing evidence from the accessible geological record reveals that crust mantle differentiation on earth started as early as 4 4 ga in order to assess the extent of early archean mantle depletion we obtained 176lu 176hf 147sm 143nd and high field strength element hfse concentration data for the least altered well characterized boninite like metabasalts and associated metasedimentary rocks from the isua supracrustal belt southern west greenland the metasediments exhibit initial ehf 3720 values from 0 7 to 1 5 and initial end 3720 values from 1 6 to 2 1 initial ehf 3720 values of the least altered boninite like metabasalts span a range from 3 5 to 12 9 and initial end 3720 values from 0 3 to 3 2 these initial hf isotope ratios display coherent trends with sio2 al2o3 tio2 and other relatively immobile elements indicating contamination via assimilation of enriched components most likely sediments derived from the earliest crust in the region this model is also consistent with previously reported initial os 3720 values for some of the samples in addition to the positive ehf 3720 values the least disturbed samples exhibit positive end 3720 values and a co variation of ehf 3720 and e d 3720 values based on these observations it is argued that the most depleted samples with initial ehf 3720 values of up to 12 9 and high 176lu 177hf of 0 05 to 0 09 tap a highly depleted mantle source with a long term depletion history in the garnet stability field high precision high field strength element hfse data obtained for the isua samples confirm the contamination trend even the most primitive samples display negative nb ta anomalies and elevated nb ta indicating a subduction zone setting and overprint of the depleted mantle sources by felsic melts generated by partial melting of eclogite collectively the data for boninite like metabasalts support the presence of strongly depleted mantle reservoirs as previously inferred from hf isotope data for hadean zircons and combined 142nd 143nd isotope data for early archean rocks

magma mixing origin for the post collisional adakitic monzogranite of the triassic yangba pluton northwestern margin of the south china block geochemistry sr nd isotopic zircon u pb dating and hf isotopic evidences petrogenesis of high mg adakitic rocks in intracontinental settings is still a matter of debate this paper reports major and trace element whole rock sr nd isotope zircon u pb and hf isotope data for a suite of adakitic monzogranite and its mafic microgranular enclaves mmes at yangba in the northwestern margin of the south china block these geochemical data suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived from subcontinental lithospheric mantle sclm may account for the origin of high mg adakitic rocks in the intracontinental setting the host monzogranite and mmes from the yangba pluton have zircon u pb ages of 207 2 and 208 2 ma respectively the mmes show igneous textures and contain abundant acicular apatite that suggests quenching process their trace element and evolved sr nd isotopic compositions 87sr 86sr i 0 707069 0 707138 and end t 6 5 indicate an origin from sclm some zircon grains from the mmes have positive ehf t values of 2 3 8 2 with single stage hf model ages of 531 764 ma thus the mmes would be derived from partial melts of the neoproterozoic sclm that formed during rift magmatism in response to breakup of supercontinent rodinia and experience subsequent fractional crystallization and magma mixing process the host monzogranite exhibits typical geochemical characteristics of adakite i e high la yb and sr y ratios low contents of y 9 5 14 5 ppm and yb no significant eu anomalies eu eu 0 81 0 90 suggesting that garnet was stable in their source during partial melting its evolved sr nd isotopic compositions 87sr 86sr i 0 7041 0 7061 and end t 3 1 to 4 3 and high contents of k2o 3 22 3 84 and th 13 7 19 0 ppm clearly indicate an origin from the continental crust in addition its high mg 51 55 cr and ni contents may result from mixing with the sclm derived mafic magma most of the zircon grains from the adakitic monzogranite show negative ehf t values of 9 4 to 0 1 with two stage hf model ages of 1 043 1 517 ma some zircon grains display positive ehf t of 0 1 3 9 with single stage hf ages of 704 856 ma these indicate that the source region of adakitic monzogranite contains the neoproterozoic juvenile crust that has the positive ehf t values in the triassic thus the high mg adakitic granites in the intracontinental setting would form by mixing between the crustal derived adakitic magma and the sclm derived mafic magma the mafic and adakitic magmas were generated coevally at late triassic temporally consistent with the exhumation of deeply subducted continental crust in the northern margin of the south china block this bimodal magmatism postdates slab breakoff at mantle depths and therefore is suggested as a geodynamic response to lithospheric extension subsequent to the continental collision between the south china and north china blocks

geochemical and geochronological study of early carboniferous volcanic rocks from the west junggar petrogenesis and tectonic implications abstract a series of early carboniferous volcanic rocks i e basalt andesite and dacite occur in the west junggar zircon u pb isotopic data indicate that these rocks coevally erupted at 331 344 ma these volcanic rocks have comparable trace element patterns characterized by the enrichment of large ion lithophile elements lile and the depletion of high field strength elements hfse isotopically they display consistently depleted sr nd isotopic compositions initial 87sr 86sr ratios 0 7034 0 7054 end t 4 2 7 7 these rocks are interpreted to be derived from a slightly metasomatized depleted mantle in a normal subduction environment in the context of a previously proposed ridge subduction model this study constrains that ridge subduction if it indeed occurred commenced between 331 and 310 ma in the central west junggar

giant mesozoic gold provinces related to the destruction of the north china craton abstract lode gold deposits in precambrian cratons represent the world s major gold source and were mostly generated during formation and stabilization of the cratons however there is an extraordinary exception in the north china craton ncc where lode gold deposits formed after prolonged stabilization of the craton molybdenite re os and hydrothermal sericite and biotite 40 ar 39 ar dating of major gold deposits from the xiaoqinling district southern ncc bracket their emplacement in the range of 154 1 1 1 to 118 9 1 2 ma n 23 postdating formation of the craton by more than 1 7 billion years fluid inclusions extracted from gold bearing pyrite have elevated 3 he 4 he ratios 1 52 0 22 ra and mantle like ne isotopes 20 ne 22 ne 10 02 9 22 and 21 ne 22 ne 0 033 0 027 indicating presence of mantle derived fluids in the ore system measured 34 s of pyrite and d and 18 o of hydrothermal micas and fluid inclusion waters in auriferous quartz further confirm a magmatic mantle source for sulfur and ore fluids gold deposits of similar ages also widely occur in the eastern and northern margins of the ncc which together with those in the xiaoqinling district have a total reserve of 2500 t gold forming the only known giant late mesozoic gold province in the world s precambrian cratons these deposits formed coevally with extensive felsic to mafic magmatism development of intracontinental rift basins and exhumation of metamorphic core complexes across the eastern ncc events interpreted as indicating thinning and destruction of the lithosphere beneath the craton rising of asthenosphere coupled with destruction of the lithosphere has generated voluminous mafic and felsic magmas that provided sufficient fluids sulfur and by inference other ore components to form the giant gold provinces

tectonics of an ultrahigh pressure metamorphic terrane the dabie shan tongbai shan orogen china ultrahigh pressure metamorphic rocks with coesite and diamond form a tectonic slice over 20 km thick called the eclogite zone within the dabie shan complex in the qinling orogen in central china the orogen separates the sino korean block in the north from the yangtze block in the south the dabie shan complex is a composite terrane made up of eclogite facies and amphibolite facies gneiss slices and represents fragments of the lower continental crust of the yangtze block the dabie shan complex is bounded in the south by a triassic foreland fold thrust belt and in the north by a greenschist facies metaclastic unit the foziling group which probably represents the passive continental apron deposits of the yangtze block farther north is a granulite facies gneiss complex the qinling group which has ultramafic slivers and includes the remnants of an island arc with two bounding suture zones north of the qinling group are early paleozoic active margin deposits of the sino korean block the eclogite zone in the dabie shan complex is sandwiched between amphibolite facies gneiss slices dating by sm nd rb sr and ar ar of two eclogite samples from the eclogite zone gives early to middle triassic ages 236 246 ma the initial end values indicate reworking of a 2 11 and 1 55 ga continental crust a himalayan type tectonic evolution is envisaged for the qinling orogen with the creation of a 100 km thick crustal thrust wedge through continuous underplating during the subduction of the yangtze continental lithosphere exhumation of the ultrahigh pressure metamorphic rocks was chiefly achieved by the southward propagation of the thrust planes thereby isostatically uplifting and eroding the earlier deeply subducted parts of the orogen a total of 680 km of southward thrusting in front of dabie shan is inferred based on the abrupt termination of the tanlu fault normal faulting possibly caused by gravitational collapse probably also had a role in the exhumation process

dynamics of the development of the isle au haut gabbro diorite layered complex quantitative implications for mafic silicic magma interactions the isle au haut igneous complex provides a unique opportunity to examine in detail the in situ physical and chemical interactions between contemporaneously emplaced mafic and silicic magmas the complex contains a 600 m thick sequence of 11 alternating layers of gabbro and diorite typically 15 40 m thick purely on the basis of density contrasts 2 65 g cm 3 gabbro vs 2 55 g cm 3 diorite the entire system should have undergone wholesale instability and mixing it is instead arrested in a grossly unstable state of interaction while molten chilled margins along the lower contacts of the gabbros and structural integrity of the diorite layers indicate that near liquidus gabbroic magma invaded partly crystalline cooler diorite mineral assemblages chemical analyses and phase equilibria calculations indicate initial temperatures during emplacement of 11808c gabbro and 10008c diorite conductive thermal models yield solidification timescales of 15 60 years for single gabbro layers and about a thousand years for the entire complex there is ample evidence for two phases of small scale interfacial rayleigh taylor type instabilities of dioritic melt into the gabbros phase i occurred immediately upon gabbro emplacement whereas evenly spaced slender more silicic pipes represent a much later stage phase ii pipe geometry and spacing estimated viscosities of the gabbroic magma and silicic melt and the sudden increase in silica near the upper contact of the diorite all indicate a thin 18 53 cm buoyant layer at the upper contact of the diorite as the source of the pipes compaction of the diorite produced this layer over a period of about 10 years simultaneous solidification along the lower contact of the overlying gabbro thickening inwards increased viscosity enough to arrest pipe ascent after a few meters crystal size distribution analyses of the gabbro layers yield crystal growth rates go1 4 2 4 10 10 cm s and nucleation rates jo1 410 10 6 cm 3 s indicative of conductive cooling coupled with some sluggish convective stirring owing to collapse of the roof ward gabbro solidification fronts were the complex larger with a much longer solidification time all this evidence would have been lost thus suggesting that in larger systems similar processes may commonly take place leaving little direct evidence of their operation apart from the ultimate final petrological product

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

petrogenesis of adakitic porphyries in an extensional tectonic setting dexing south china implications for the genesis of porphyry copper mineralization the dexing adakitic porphyries quartz diorite granodiorite porphyries associated with giant porphyry cu deposits are located in the interior of a continent south china they exhibit relatively high mgo cr ni and sr contents high la yb and sr y ratios but low yb and y contents similar to adakites produced by slab melting associated with subduction however they are characterized by bulk earth like nd sr isotope compositions end t 1 14 to 1 80 and 87 sr 86 sr i 0 7044 0 7047 and high th 12 6 27 2ppm contents and th ce 0 19 0 94 ratios which are different from those of cenozoic slab derived adakites sensitive high resolution ion microprobe shrimp geochronology studies of zircons reveal that the dexing adakitic porphyries have a crystallization age of 171 3ma this age is contemporaneous with middle jurassic extension within the shi han rift zone and within plate magmatism elsewhere in south china indicating that the dexing adakitic porphyries were probably formed in an extensional tectonic regime in the interior of the continent rather than in an arc setting their high th contents and th ce ratios and middle jurassic age argue against an origin from a neoproterozoic 1000ma stalled slab in the mantle taking into account available data for the regional metamorphic magmatic rocks and the present day crustal thickness 31km in the area we suggest that the dexing adakitic porphyries were most probably generated by partial melting of delaminated lower crust which was possibly triggered by upwelling of the asthenospheric mantle due to the activity of the shi hang rift zone moreover the dexing adakitic magmas must have interacted with the surrounding mantle peridotite during their ascent which elevated not only their mgo cr and ni contents but also the oxygen fugacity fo2 of the mantle the high fo2 could have induced oxidation of metallic sulfides in the mantle and mobilization of chalcophile elements which are required to produce associated cu mineralization therefore the cu metallogenesis associated with the dexing adakitic porphyries is probably related to partial melting of delaminated lower crust similar to the metallogenesis accompanying slab melting

mineral chemistry trace elements and sr nd hf isotope geochemistry and petrogenesis of cailing and furong granites and mafic enclaves from the qitianling batholith in the shi hang zone south china abstract the shi hang zone is an important ne trending mesozoic magmatic belt composed of granites with relative high end t values and young tdm model ages in south china however the petrogenesis and the tectonic environment for the shi hang zone magmatic rocks remain controversial we report here mineral chemistry geochemical and sr nd hf isotopic data for the cailing and furong granites and mafic microgranular enclaves mmes from the qitianling granite batholith in southern hunan province south china the qitianling granite batholith is a multi staged composite pluton with three phases cailing furong and huangtangling according to their ages and petrography the cailing 163 160 ma and furong 157 153 ma phases are mainly composed of porphyritic amphibole biotite monzogranite and they share similar geochemical and isotopic characteristics both of them show similar sio2 contents from 66 50 to 70 28 and metaluminous a cnk values of 0 80 to 0 98 the granites are characterized by high contents of large ion lithosphile elements lile such as rb th u pb high field strength elements hfse such as nb ta zr hf and zr nb ce y contents 350 ppm and high 10 000 ga al ratios 2 6 chondrite normalized ree patterns show relative enrichment of light rare earth elements lrees and significant negative eu anomalies mineralogical and geochemical features suggest that the cailing and furong granites are a type which can be further classified as a2 subtype they have relatively lower 87sr 86sr i ratios 0 7091 0 7132 higher end t values 5 5 to 7 6 and younger nd isotopic model ages 1 48 1 56 ga than those common s type granites in south china zircon ehf t values vary from 8 1 to 3 7 the mmes in the cailing phase show similar trace element and sr nd isotopic characteristics with the host granites but zircons from the mmes show different ehf t values 6 4 2 6 with those from the host granites 8 1 to 3 7 this indicates that the mmes and host granites were crystallized from different sources of magmas providing direct evidence for mafic felsic magma mixing processes the isotope data indicate that the cailing and furong granites from the qitianling batholith were derived from a hybrid magma consisting of about 80 felsic magma derived from old crust and about 20 mantle derived mafic magma the strong magma mixing at about 160 155 ma caused by intra arc rifting or back arc extension related to subduction of the paleo pacific plate is favored to explain the petrogenesis of the cailing and furong granites as well as the shi hang zone

magma mixing recorded by sr isotopes of plagioclase from dacites of the quaternary tengchong volcanic field se tibetan plateau abstract the tengchong volcanic field east of the burma arc in sw china comprises numerous quaternary volcanoes the volcanic rocks can be grouped into four units numbered 1 4 from oldest to youngest units 1 3 and 4 are composed of trachybasalt basaltic trachyandesite and trachyandesite respectively whereas unit 2 consists of hornblende dacite primary minerals in the dacite include plagioclase clinopyroxene amphibole and magnetite all set in a glassy groundmass some of the dacites are moderately to highly weathered and show effects of low temperature alteration with loss on ignition loi up to 9 6 wt and positive correlations of loi with al2o3 but negative correlations with sio2 cao and na2o fresh dacites loi plagioclase in the fresh dacite has relatively high sr contents and 87sr 86sr ratios ranging from 0 7050 to 07125 except for one value of 0 7516 whereas plagioclase in the altered dacite has low sr contents and highly variable sr isotopic compositions 0 7039 0 7138 indistinguishable from those of the fresh rocks both an values and sr isotopic compositions of the plagioclase indicate mixing of mantle and crustally derived magmas we therefore propose that mantle derived basaltic magma caused partial melting of the lower middle crust and that mixing of the mafic and felsic magmas produced the dacite in staging magma chambers the dacites contain minerals crystallized from both the mafic and felsic magmas as well as a few xenocrysts plucked from the country rocks

recognition of early carboniferous alkaline granite in the southern altai orogen post orogenic processes constrained by u pb zircon ages nd isotopes and geochemical data the altai orogen forms the southern part of the central asian orogenic belt caob the world s largest accretionary orogen however its tectonic evolution par ticularly during the late paleozoic is still not well under stood u pb zircon analyses for the bulgen alkaline granite yield crystallization ages of 358 4 ma shrimp and 354 4 ma la icp ms these ages are significantly younger than published emplacement ages for subduction collision related syn orogenic granitoids 460 375 ma in this region the bulgen granite has high sio2 total alkalis rare earth elements hfse th zr hf nb and ce and low ba sr with pronounced negative anomalies in eu ba sr p and ti showing a clear a type geochemical signa ture the granite records high end t values of 6 3 to 6 4 and young model ages tdm of ca 600 ma the bulgen alkaline granite is largely undeformed as opposed to the early middle paleozoic counterparts which form elongated deformed bodies parallel to the prevailing tec tonic fabric nw direction available data suggest that magmatism in the southern altai region evolved from early middle paleozoic i type tholeiitic and calc alkaline granitoids to late paleozoic a type alkaline granitoids the high end t values of the bulgen alkaline granite indicate a homogeneous juvenile mantle source whereas the early middle paleozoic granitoids are characterized by lower and more variable end t values 2 6 to 4 2 these differ ences provide an important insight into the late paleozoic orogenic processes of the chinese altai and indicate a significant change of the tectonic regime from a syn oro genic regional compression setting to a post orogenic extensional one major tectonic movements in this region ceased after the early carboniferous

experimentally determined mineral melt partition coefficients for sc y and ree for olivine orthopyroxene pigeonite magnetite and ilmenite our current lack of understanding of the partitioning behavior of sc y and the ree rare earth elements can be attributed directly to the lack of a sufficiently large or chemically diverse experimental data set to address this problem we conducted a series of experiments using several different natural composition lavas doped with the elements of interest as starting compositions microprobe analyses of orthopyroxene pigeonite olivine magnetite ilmenite and co existing glasses in the experimental charges were used to calculate expressions that describe ree partitioning as a function of a variety of system parameters using expressions that represent mineral melt reactions versus element ratio distribution coefficients it is possible to calculate terms that express low ca pyroxene melt partitioning behavior and are independent of both pyroxene and melt composition compositional variations suggest that sc substitution in olivine involves either a paired substitution with al or more commonly with vacancies the partitioning of sc is dependent both on melt composition and temperature our experimentally determined olivine melt ree ds partition coefficients are similar to but slightly higher than those reported by mckay 1986 and support their conclusions that olivines are strongly lree depleted y and ree mineral melt partition coefficients for magnetite range from 0 003 for la to 0 02 for lu ilmenite partition coefficients range from 0 007 for la to 0 029 for lu these experimental values are two orders of magnitude lower than many of the published values determined by phenocryst matrix separation techniques

nd sr isotopic mapping of the chinese altai and implications for continental growth in the central asian orogenic belt article i nfo the chinese altai orogen is a typical domain of the central asian orogenic belt and in previous syntheses was divided into six tectonic units or terranes in this paper 41 new and 36 published whole rock nd and sr isotopic data for granitoid intrusions and mafic dykes within units 2 and 3 of the central altai and units 4 to 6 of the southern altai were used for isotopic mapping the age of the samples used is known from u pb zircon dating end t values range from 4 to 2 in the central altai units 2 and 3 and from 1 4 to 6 in the southern altai units 4 and 5 and up to 8 4 in unit 6 correspondingly nd model ages tdm decrease from 1 6 1 1 ga in the central altai to 1 0 0 5 ga in the southern altai except for a continental fragment in unit 4 isr is elevated in the central units decreasing to b0 705 in the southern units 5 and 6 these results demonstrate that the central altai contains widespread old crustal components in its basement whereas the southern altai comprises a higher proportion of juvenile crust when integrated with published u pb zircon ages our data show that new crust grew southwards in the chinese altai in addition our data suggest that units 2 and 3 are a single tectonic unit whereas the southern altai unit 4 which previously was regarded as a single young accretionary terrane should be subdivided into two units a younger arc and an older continental fragment the granitoids in the chinese altai can be divided into syn and post non orogenic groups the syn orogenic granitoids have ca 50 80 mantle derived component whereas the post non orogenic granitoids contain ca 70 90 the generation of syn and post non orogenic granitoids represents horizontal lateral and vertical crustal growth which contributed ca 18 28 and 7 8 respectively to the area of the chinese altai this work shows that isotopic mapping is an effective approach to elucidate terrane composition in the central asian orogenic belt and thereby better constrain palaeozoic crustal growth

guidelines for reporting zircon hf isotopic data by la mc icpms and potential pitfalls in the interpretation of these data invited technical paper high quality analytical data are essential for the development of sound scientific interpretations to ensure the quality of the data published in chemical geology a new type of contribution has been introduced the invited technical paper when a particular technical issue needing discussion or clarification is identified specialists in the technique will be invited to share their expertise as is true for all articles published in the journal these contributions will be subject to peer review before publication the first topic to be treated concerns in situ hf isotopic measurements in zircon by laser ablation inductively coupled plasma mass spectrometry this is an extremely powerful and promising technique that has been adopted by many laboratories throughout the world however if insufficient care is taken during analysis and data processing inaccurate results can be obtained notably because of the existence of very large isobaric interferences on the isotope of interest to address this issue we have asked christopher fisher jeffery vervoort and john hanchar to provide a set of guidelines that can be adopted to assure that reliable hf isotopic data are obtained by this technique abstract over the past decade the hf isotope composition of zircon as determined in situ by laser ablation multicollector inductively coupled plasma mass spectrometry la mc icpms has been applied increasingly to a wide range of geological problems and has proven to be a valuable analytical tool there has been no uniformity however in the reporting of in situ hf isotopic data and quite often reviewers and readers of papers are not provided with sufficient information to assess data quality the goal of this invited contribution is to provide the non specialist with a brief outline and explanation of what is required for proper presentation and documentation of in situ hf isotopic data from zircon including details of how the challenging large isobaric interference corrections have been made in addition we discuss a number of potential pitfalls vis a vis the assignment of the incorrect age to the measured hf isotope composition non specialists should be aware of these important issues when doing their own analyses and evaluating the analyses done by others

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

adakites without slab melting high pressure differentiation of island arc magma mindanao the philippines abstract new geochemical data for pleistocene magmatic rocks from the surigao peninsula eastern mindanao the philippines demonstrate typical adakitic traits including elevation of sr y and depletion of the heavy rare earth elements 87sr 86sr and 143nd 144nd ratios of the adakites do not support melting of the subducted philippine sea plate but resemble pliocene arc lavas generated in the same subduction zone excepting the heavy rare earth elements the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge the wide range of sio2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature 1 adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet bearing assemblage 2 solidified basaltic rock containing garnet melted to yield adakitic magma in either case the basaltic precursor was generated from fluid modified mantle then differentiated within the garnet stability field in surigao this requires differentiation within mantle the surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth this has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks

u pb zircon ages constrain the architecture of the ultrahigh pressure qinling dabie orogen china new shrimp and tims zircon ages 40ar 39ar ages and eclogite locations contribute significantly to our understanding of the ultrahigh pressure dabie shan 1 the geographic extent of the yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the dabie shan 2 the northern half of the dabie shan is a magmatic complex intruded over a 10 myr interval between 137 and 126 ma that accommodated 100 n s stretching of the pre existing collisional architecture 3 granitic orthogneisses and enclosing ultrahigh pressure paragneisses have indistinguishable zircon populations the population of triassic zircon ages ranges from 219 to 245 ma leading us to question the prevailing assumption that 219 ma zircons formed at ultrahigh pressure and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation

oscillatory sr isotopic signature in plagioclase megacrysts from the damiao anorthosite complex north china implication for petrogenesis of massif type anorthosite abstract formation of proterozoic massif type anorthosites is known to be related to polybaric process involving early high pressure crystallization of plagioclase and high al orthopyroxene megacrysts at mantle crust boundary followed by emplacement of plagioclase dominated mushes to shallow level crust therefore the plagioclase megacrysts record important information about the magmatic sources and crystallization process of anorthosites the 1 74 ga damiao complex north china comprises 90 vol anorthosites and leuconorites containing abundant plagioclase megacrysts associated with minor high al orthopyroxene megacrysts haoms the plagioclase megacrysts are generally euhedral to subhedral mostly 1 to 30 cm in diameter and some of them contain very fine lamellae of orthoclase and undefined fe ti rich minerals 1 5 m the haoms occur as subhedral grains or as angular grains with an intercumulus relationship to the plagioclase megacrysts they contain abundant thin regular lamellae of exsolved plagioclase 15 20 vol indicative of originally high al features originally 6 0 7 3 wt al2o3 based on the al in orthopyroxene geobarometry the haoms and associated plagioclase megacrysts are constrained to be crystallized together at pressures of 9 4 11 2 kbar 33 36 km indicative of their crystallization at lower crust depths or crust mantle boundary in contrast orthopyroxene grains from late differentiation phases such as oxide apatite gabbronorites do not contain plagioclase lamellae and have much lower al2o3 contents 1 5 to 1 7 wt indicating final crystallization at

granites in the sawuer region of the west junggar xinjiang province china geochronological and geochemical characteristics and their geodynamic significance abstract the sawuer region is located in the west junggar jimunai county of altay district and hefeng county of tacheng district xinjiang autonomous region in northwest china in the study area granitic intrusions are widespread and can be subdivided into i type granites tasite pluton sentasi pluton wokensala pluton and kaerjia pluton and a type a 2 granites kuoyitasi pluton and qiaqihai pluton the i type granites consist of granite adamelite granodiorite and the a type granites are mainly represented by alkali granite shrimp u pb dating results indicate that the granites in sawuer region were formed between 337 ma and 290 7 ma late carboniferous and early permian with the i type granites being formed between 337 ma and 302 6 ma and the a type between 297 9 and 290 7 ma from the older to the youngest granites their composition changes from calc alkalic to high k calc alkalic to alkali series and from i type to a type based on the characteristics of whole rock trace element and rare earth elements geochemistry this compositional range is the result of a change in the geodynamic regime from compressional to tensional during the late paleozoic strontium nd pb o isotope systematics suggest that both the i type and a type granites in sawuer were sourced from the mantle dm but with some crustal contamination for the kuoyitasi pluton the i type granites were formed by equilibrium partial melting while the a type granites were formed mainly by fractional crystallization based on the rb y nb and nb y ce discrimination criteria by eby eby g n 1992 chemical subdivision of the a type granitoids petrogenetic and tectonic implications geology 20 641 644 and pearce pearce j a 1996 sources and settings of granitic rocks episodes 19 120 125 and comparisons with granites in other regions of northern xinjiang the i type granites were emplaced during a transition from compressional to extensional settings in a post collision regime whereas the a type granites were emplaced in an extensional setting we concluded that the sawuer as well as the whole junggar region in northern xinjiang were involved in tectonism that gradually changed from compressional to extensional between the late carboniferous and early permian this late paleozoic granitic magmatism contributed to vertical crustal growth in the region

accretionary orogenesis of the chinese altai insights from paleozoic granitoids zircon u pb dating and whole rock major oxide trace element and nd sr isotope compositions have been determined for four representative granitic intrusions in the sw chinese altai in order to understand the orogenesis and history of crustal growth in the central asian orogenic belt caob the ashile and halatas bodiesare relatively small intrusions formed in the late carboniferous 318 6ma andpermian 267 4ma respectively thelargertarlang tb andkeketuohaibatholiths kb havemagmaticagesof 359 to 412 ma the granitoids consist chiefly of tonalite granodiorite and granite and are metaluminous to weakly peraluminous in composition asi 0 8 1 1 the ashile pluton is characterized by relatively high mgo 2 73 3 54 wt and sr 395 456 ppm contents with low k2o na2o ratios 0 31 0 42 and mantle like nd sr isotopic compositions endt 2 85 to 3 26 initial 87 sr 86 sr 0 7047 0 7051 it probably formed by fractional crystallization of a mantle derived magma with limited assimilation of oldcrustalmaterial theotherintrusionshavenear zeroornegative endtvalues 0 67to 4 41 andhighersrisotopiccompositions initial 87 sr 86 sr 0 7067 0 7092 indicating a mixture of juvenile and old components in their genesis these intrusions are characterized by relatively low k rb cs contents and low rb sr ratios mostly b0 6 and generally exhibit moderate to high heavy rare earth element hree concentrations ybn 8 26 and weak hree fractionation dy ybnb1 5 with or without negative europium anomalies eu eu 0 5 1 0 these characteristics are consistent with an origin by dehydration melting of a hornblendebearing mid crustal source above the garnet stability field dominated by mafic to intermediate rocks this suggests that the deep crust of the chinese altai may contain a considerable proportion of juvenile material mantle derived magmatism probably in an active extensional continental margin setting played an important role in the formation of the chinese altai acting not only as a heat sourceforcrustalmeltingbutasasourceofjuvenilecomponents therelativelysmallvolumesanddistinctgeochemicalcompositions of the ashile and halatas plutons may imply a substantial change in the geodynamic setting in the late paleozoic 2007 elsevier b v all rights reserved

structural and lithological controls on coastline profiles in fife eastern britain the coastline of southern fife on the east coast of the british isles is presented here as an illustration of geological influences on spatial variability at different lengthscales the coastline profile of britain is widely referred to as an example of fractal geometry as presented in mandelbrot s 1967 original paper on statistical self similarity i show here that at different lengthscales different geological processes dominate the character of the coastline profile and that estimates of the fractal dimension are strongly dependant on the scale and choice of sample significant periodic components in the tortuosity of the coastline are evident and can be related to the causative scale specific geological processes

origin of mesozoic adakitic intrusive rocks in the ningzhen area of east china partial melting of delaminated lower continental crust to the best of our knowledge modern adakites have not been documented in a nonarc environment we report geochemical and isotopic data for early cretaceous anjishan adakitic intrusive rocks that are in a continental setting unrelated to subduction the anjishan adakitic intrusive rocks which are exposed in the ningzhen area of east china have high sr y and la yb ratios coupled with low yb and y as well as relatively high mgo contents and mg numbers mg 0 4 0 6 similar to products from slab melting however low n d t values 6 8 to 9 7 and high 8 7 sr 8 6 sr i 0 7053 0 7066 are inconsistent with an origin by slab melting the tectonics and geochemistry lead us to conclude that adakitic magmas were most likely derived from partial melting of mafic material at the base of the continental crust high sr y and la yb ratios of the adakitic intrusive rocks suggest that garnet was stable as a residual phase during partial melting implying that the crustal thickness exceeded 40 km in the early cretaceous the present thickness of the crust in the ningzhen area is only 30 km and therefore the crust appears to have been thinned by at least 10 km since the early cretaceous the relatively high mgo contents and mg of the anjishan intrusive rocks suggest that adakitic magmas interacted with mantle rocks possibly coinciding with lower crustal delamination which would also account for the observed thinning

geochemistry of the kalatongke ni cu pge sulfide deposit nw china implications for the formation of magmatic sulfide mineralization in a postcollisional environment the kalatongke also spelt as karatungk ni cu platinum group element pge sulfide deposit containing 33 mt sulfide ore with a grade of 0 8 wt ni and 1 3 wt cu is located in the eastern junggar terrane northern xinjiang nw china the largest sulfide ore body which occupies more than 50 vol of the intrusion y1 is dominantly comprised of disseminated sulfide with a massive sulfide inner zone economic disseminated sulfides also occur at the base of the intrusions y2 and y3 the main host rock types are norite in the lower part and diorite in the upper part of each intrusion enrichment in large ion lithophile elements and depletion in heavy rare earth elements relative to mid ocean ridge basalt indicate that the mafic intrusions were produced from magmas derived from a metasomatized garnet lherzolite mantle the average grades of the disseminated ores are 0 6 wt ni and 1 1 wt cu whereas those of the massive ores are 2 wt ni and 8 wt cu the pge contents of the disseminated ores 14 69 ppb pt and 78 162 ppb pd are lower than those of the massive ores 120 505 ppb pt and 30 827 ppb pd however on the basis of 100 sulfide pge contents of the massive sulfides are lower than those of the disseminated sulfides very high cu pd ratios 4 5 104 indicate that the kalatongke sulfides segregated from pge depleted magma produced by prior sulfide saturation and separation a negative correlation between the cu pd ratio and the pd content in 100 sulfide indicates that the pge content of the sulfide is controlled by both the pge concentrations in the parental silicate magma and the ratio of the amount of silicate to sulfide magma the negative correlations between ir and pd indicate that the massive sulfides experienced fractionation

petrogenesis of the mesozoic shuikoushan peraluminous i type granodioritic intrusion in hunan province south china middle lower crustal reworking in an extensional tectonic setting abstract the mesozoic shuikoushan granitoid intrusion in the jiangnan orogen between the yangtze and the cathaysia blocks in southern hunan province south china provides an opportunity to probe deep crustal composition and structure and to investigate the geodynamic evolution of south china the intrusion is composed of granodiorite with i type geochemical affinities and contains zircon with an u pb concordant age of 158 3 1 2 ma rocks from the intrusion have variable sio2 from 58 4 wt to 65 2 wt al2o3 from 15 0 wt to 17 2 wt tio2 from 0 57 wt to 0 76 wt and p2o5 from 0 27 wt to 0 52 wt they have a cnk from 0 97 to 1 23 1 04 on average and are dominantly peraluminous they are characterized by enrichment in lree la sm n 3 47 5 19 and lil e g k rb and depletion in hfse e g u th in primitive mantle normalized trace element diagrams these geochemical features indicate that the rocks are i type granodiorites and have undergone extensive fractional crystallization of hornblende biotite titanite and apatite all samples show negative ehf t 10 6 to 8 1 and end t 5 92 to 6 13 values with relatively high initial 87sr 86sr isr 0 7101 0 7102 and 18o ratios 8 4 9 7 these values significantly differ from mantle values suggesting that they are generated by the amphibole dehydration melting of a mafic source in the middle to lower crust beneath the jiangnan orogen the emplacement of the shuikoushan granitoid intrusion is contemporaneous with the late jurassic to early cretaceous extension within the shi hang rift zone indicating that it may have been formed under an extensional regime in response to both the pacific superplume activity and westward subduction of the pacific plate the extensional processes along with asthenosphere upwelling and basaltic underplating may have triggered the widespread late jurassic granitoid intrusion in southern hunan province and possibly in south china

late kinematic timing of orogenic gold deposits and significance for computer based exploration techniques with emphasis on the yilgarn block western australia abstract orogenic gold deposits are a widespread coherent group of epigenetic ore deposits that are sited in accretionary or collisional orogens they formed over a large crustal depth range from deep seated low salinity h2o co2 ch4 n2 ore fluids and with au transported as thio complexes regional structures provide the main control on deposit distribution in many terranes first order faults or shear zones appear to have controlled regional fluid flow with greatest ore fluid fluxes in and adjacent to lower order faults shear zones and or large folds highly competent and or chemically reactive rocks are the most common hosts to the larger deposits focusing of supralithostatic ore fluids into dilatant zones appears to occur late during the evolutionary history of the host terranes normally within d3 or d4 in a d1 d4 deformation sequence reactivation of suitably oriented pre existing structures during a change in far field stress orientation is a factor common to many deposits and repeated reactivation may account for multiple mineralization episodes in some larger deposits absolute robust ages of mineralization support their late kinematic timing and in general suggest that deposits formed diachronously towards the end of the 100 to 200 m y long evolutionary history of hosting orogens for example in the yilgarn block a region specifically emphasised in this study orogenic gold deposits formed in the time interval between 40 and 90 m y with most about 60 to 70 m y after the youngest widespread basic ultrabasic volcanism and towards the end of felsic magmatism the late timing of orogenic gold deposits is pivotal to geologically based exploration methodologies this is because the present structural geometries of i the deposits ii the hosting goldfields and iii the enclosing terranes are all essentially similar to those during gold mineralization at least in their relative position to each other thus interpretation of geological maps and cross sections and three dimensional models can be used to accurately simulate the physical conditions that existed at the time of ore deposition it is particularly significant that the deposits are commonly related to repetitive and predictable geometries such as structural heterogeneities within or adjacent to first order structures around rigid granitoid bodies or in specific locked up fold thrust structures importantly the two giant greenstone hosted goldfields kalgoorlie and timmins show a remarkably similar geometry at the regional scale computer based stress mapping and gis based prospectivity mapping are two computer based quantitative methodologies that can utilize and take advantage of the late timing aspect of this deposit type to provide important geological aids in exploration both in broad regions and more localized goldfields both require an accurate and consistent solid geology map stress mapping requires knowledge of the far field stresses during mineralization and the empirical prospectivity mapping requires data from a significant number of known deposits in the terrane the kalgoorlie terrane in the yilgarn block meets these criteria and illustrates the potential of these methodologies in the exploration for orogenic gold deposits low minimum stress anomalies interpreted to represent dilational zones during gold related deformation coincide well with the positions of known goldfields rather than individual gold deposits in the terrane and there are additional as yet unexplained anomalies the prospectivity analysis confirms that predictable and repetitive factors controlling the siting of deposits are i proximity to and orientation and curvature of granitoid greenstone contacts ii proximity to segments of crustal faults which strike in a preferred direction iii proximity to specific lithological contacts which have similar preferred strike iv proximity to anticlinal structures and v the presence of preferred reactive host rocks e g dolerite the prospectivity map defines a series of anomalous areas which broadly conform to those of the stress map 78 correspondence the most prospective category on this map covers less than 0 3 of the greenstone belts and yet hosts 16 of the known deposits which have produced 80 of known gold thus it discriminates in favour of the larger economically more attractive deposits in the terrane the successful application of stress mapping and prospectivity mapping to geology based exploration for orogenic gold deposits indicates that more quantitative analysis of geological map data is a profitable line of research the computer based nature of these methodologies is ideal for the production of an ultimate integrated deposit target map which can be compared to other more conventional targeting parameters such as geophysical and geochemical anomalies such an integrated strategy appears the way forward in the increasingly difficult task of cost effective global exploration for orogenic gold deposits in poorly exposed terranes

characteristics of mo mineralization and zircon la icp ms u pb geochronology of mushan mo deposit from luoshan county henan province mushan mo deposit a typical porphyry molybdenum deposit occurs in the northern margin of dabie mountain is characterized by veinlet disseminated mineralization and typical porphyry hydrothermal system alteration envelope centering the unmineralized quartz core formed by silicification early at the top of granitic porphyry the coexistence of vapour rich and halite bearing three phase inclusions indicates that the primitive magmatic fluid has experienced phase separation under low pressure the results of homogenization temperatures of fluid inclusions show that the ore forming element including mo cu deposited by cooling of ore forming hydrothermal system mushan granitic porpphry related to mineralization exhibits low sr 400 10 6 low yb 1 8 10 6 strong fractionation of lree and hree la yb n 32 13 68 69 moderate to slight eu negative anomaly eu 0 66 0 92 and high 87sr 86sr i 0 70 962 0 71 076 extremely low end t 14 77 13 36 the result of zircon la icp ms u pb geochronology that mushan granitic porphyry emplaced at 142 0 1 8 ma it concluded that rock forming and mo mineralization of mushan mo deposit are resulted from the thickened continental crust by partial melting during post collision of dabie mountain and it happened nearly 10 ma earlier than granitic magmatism peak 130 ma in early cretaceous in dabie region

reappraisal of the ages of neoproterozoic strata in south china no connection with the grenvillian orogeny the jiangnan fold belt separates the yangtze and cathaysia blocks in south china and has long been considered grenvillian in age in order to place south china in central rodinia it consists of deformed early neoproterozoic strata that are unconformably overlain by undeformed late neoproterozoic strata and intruded by undeformed and unmetamorphosed granitic plutons zircons from the early neoproterozoic strata yield u pb ages as young as 830 ma and one granitic pluton has a zircon u pb age of ca 827 ma the 830 ma mafic rocks along the southeastern margin of the yangtze block have arc affinity geochemical characters whereas mafic rocks younger than 830 ma have typical ocean island basalt oib like compositions thus we suggest that the early neoproterozoic strata were deposited on an active continental margin prior to amalgamation of the yangtze and cathaysia blocks at ca 830 ma the overlying late neoproterozoic strata were deposited in the intracontinental rifted nanhua basin at 820 730 ma and probably reflect backarc spreading above the long lived 950 735 ma oceanic subduction zone along the northern and western margin of the yangtze block this model is consistent with the secular tectonic evolution of south china during the neoproterozoic the jiangnan fold belt is therefore not a grenvillian feature as previously suggested and there is no evidence to place south china in central rodinia instead we believe that south china was located in a marginal position relative to this supercontinent

the pleasant bay layered gabbro diorite coastal maine ponding and crystallization of basaltic injections into a silicic magma chamber

permian geodynamic setting of northeast china and adjacent regions closure of the paleo asian ocean and subduction of the paleo pacific plate abstract northeast china and adjacent regions are located in the central east asian continent and consist tectonically of both the paleo asian and paleo pacific orogens between the siberian platform and sino korean north china block this paper discusses some hotly debated issues concerning the permian geodynamic setting of these regions based on a comprehensive analysis of available geological geochemical paleobiogeographical and paleomagnetic data spatial and temporal distribution of ophiolites and associated continental marginal sequences permian sedimentary sequences spatial distribution and geochemistry of permian magmatic rocks and the evolution of paleobiogeographical realms imply 1 that the permian marine basins in northeast china and adjacent regions include remnants of the paleo asian ocean in southeastern inner mongolia and central jilin province and active continental margins of the paleo pacific ocean 2 that the suture between the siberian and sino korean paleoplates was finally emplaced in the permian and is located in areas from suolunshan solonker eastwards through regions north to the xar moron river in southeastern inner mongolia and then central jilin province to the yanji area and 3 that the permian crustal evolution of northeast china and adjacent regions as well as parts of the siberian paleoplate was influenced by subduction of the paleo pacific oceanic plate finally the permian tectonic framework and paleogeography of northeast china and adjacent regions in central east asia are discussed briefly and early and late permian palinspastic reconstruction maps are provided

the structure of the crust and uppermost mantle beneath south china from ambient noise and earthquake tomography summary two years of continuous recordings of ambient seismic noise observed at 354 stations in south china from 2009 to 2010 are used to estimate rayleigh wave group and phase velocity maps from 6 to 40 s period these results are merged with rayleigh wave phase velocity maps from 25 to 70 s period derived from earthquakes in the same time frame eikonal tomography generates the dispersion maps which by monte carlo inversion are used to estimate a 3 d vsv model of the crust and upper mantle down to a depth of 150 km across all of south china with attendant uncertainties a clear image emerges of the west yangtze block a region of the western yangtze craton characterized by relatively thick crust 40 km overlying a seismic mantle lithosphere that extends to at least 150 km that may have been the nucleus for the formation of the yangtze craton in the archean and may present a present day obstacle to the eastward expansion of tibet the west yangtze block contrasts with the thinner crust 30 km and mantle lithosphere 70 80 km beneath the eastern yangtze craton and south china foldbelt these observed differences are consistent with processes associated with flat slab subduction in the mesozoic that may have eroded the lithosphere of the eastern yangtze craton and the south china foldbelt

evolving metasomatic agent in the northern andean subduction zone deduced from magma composition of the long lived pichincha volcanic complex ecuador geochemical studies of long lived volcanic complexes are crucial for the understanding of the nature and composition of the subduction component of arc magmatism the pichincha volcanic complex northern andean volcanic zone consists of 1 an old highly eroded edifice the rucu pichincha whose lavas are mostly andesites erupted from 1 100 to 150 ka and 2 a younger essentially dacitic guagua pichincha composite edifice with three main construction phases basal guagua pichincha toaza and cristal which developed over the last 60 ka this structural evolution was accompanied by a progressive increase of most incompatible trace element abundances and ratios as well as by a sharp decrease of fluid mobile to fluid immobile element ratios geochemical data indicate that fractional crystallization of an amphibole rich cumulate may account for the evolution from the guagua pichincha andesites to dacites however in order to explain the transition between the rucu pichincha andesites and guagua pichincha dacites the mineralogical and geochemical data indicate the predominance of magma mixing processes between a mafic trace element depleted mantle derived end member and a siliceous trace element enriched adakitic end member the systematic variation of trace element abundances and ratios in primitive samples leads us to propose that the rucu pichincha magmas came from a hydrous fluid metasomatized mantle wedge whereas guagua pichincha magmas are related to partial melting of a siliceous melt metasomatized mantle this temporal evolution implies a change from dehydration to partial melting of the slab which may be associated with an increase in the geothermal gradient along the slab due to the presence of the subducted carnegie ridge at the subduction system this work emphasizes the importance of studying arc magma systems over long periods of time of at least 1 million of years in order to evaluate the potential variations of the slab contribution into the mantle source of the arc magmatism

geochronology geochemistry and sr nd hf isotopic compositions of mafic ultramafic intrusions in the niubiziliang ni cu sulfide deposit north qaidam orogenic belt nw china implications for magmatic source geodynamic setting and petrogenesis abstract whole rock geochemical and sr nd isotopic data and zircon u pb hf isotopic data are reported for mafic ultramafic intrusions in the niubiziliang ni cu sulfide deposit located in the westernmost segment of the north qaidam orogenic belt nqob nw china ni cu sulfide ores of economic interest are found within these intrusions with the niubiziliang iii and iv intrusions hosting most of the ni cu mineralization the niubiziiang mafic ultramafic intrusions intrude biotite plagioclase gneiss of the paleoproterozoic dakendaban group zircons separated from the gabbro plagioclase bearing clinopyroxenite and olivine websterite yield u pb ages of 389 2 373 2 and 380 2 ma respectively indicating a genetic linkage with middle late devonian regional magmatism 390 370 ma gabbro zircons have ehf t values of 4 2 10 9 with older tdm1 ages 0 86 0 59 ga the niubiziliang intrusions contain many xenoliths of paleoproterozoic biotite plagioclase gneiss and the intrusions have similar trace element patterns with strong enrichment in liles e g k rb and th and moderate depletion in hfses e g nb ta p and ti the major and trace element characteristics and sr nd hf isotopic compositions indicate the parental magmas originated from a metasomatized asthenospheric mantle source that had previously been modified by subduction related fluids the magmas experienced significant crustal contamination both in the magma chamber and during ascent triggering sulfur over saturation that resulted in the deposition and enrichment of sulfides considering the tectonic evolution of this region we infer that the niubiziliang mafic ultramafic intrusions formed in a post collisional extensional setting related to delamination of the subcontinental lithosphere

magma mixing in the genesis of the kalatongke dioritic intrusion implications for the tectonic switch from subduction to post collision chinese altay nw china abstract the kalatongke complex located in south chinese altay of the central asian orogenic belt caob consists of the 308 ma dioritic suite intruded by the 287 ma noritic suite the noritic suite is composed of olivine norite and hornblende norite and hosts sulfide mineralization the dioritic suite is composed of ferrodiorite diorite and quartz diorite ferrodiorites have 43 6 to 51 8 wt sio 2 8 2 to 12 1 wt fe 2 o 3 t 2 87 to 4 86 wt mgo with mg ranging from 42 to 53 they have highly variable al 2 o 3 17 4 20 7 wt na 2 o 2 78 4 50 k 2 o 0 58 3 70 sr 280 1300 ppm and ba 219 3100 ppm diorites and quartz diorites have 15 2 19 6 wt al 2 o 3 239 890 ppm sr and170 1100 ppm ba 11 2 25 5 ppm y and1 2 2 2 ppm yb all rocks have lree enriched ree patterns and do not display eu anomalies they have primitive mantle normalized trace element patterns enriched in large ion lithosphile elements lile e g ba and sr and depleted in ti nb and ta both ferrodiorites and diorites have relatively constant initial 87 sr 86 sr ratios of 0 7037 to 0 7040 but highly variable e nd t values of 4 5 to 11 6 whereas quartz diorites have higher initial 87 sr 86 sr ratios 0 7056 and lower e nd t values 0 5 plagioclase from ferrodiorites diorites and quartz diorites has similar initial 87 sr 86 sr ratios of 0 7015 to 0 7055 the dioritic suite formed from a mixed magma of an evolved mantle derived basaltic magma by partial melting of subduction modified mantle source in response to slab breakoff and a crustal derived granitic magma at 308 ma before mixing with granitic magmas basaltic magmas underwent fractional crystallization of olivine to form ferrodiorite the mixed magma also formed diorite and quartz diorite combining fractionation clinopyroxene amphibole and plagioclase the noritic suite formed from a mantle derived high mg magma in a post collisional environment both the dioritic and noritic suites in kalatongke suggest the switch from subduction to extension and the final collision between the junggar block and chinese altay at 308 ma

carboniferous mantle derived felsic intrusion in the chinese altai nw china implications for geodynamic change of the accretionary orogenic belt abstract this paper reports whole rock sr nd and zircon u pb and hf isotopic compositions as well as major and trace element data for the granodiorites and their mafic microgranular enclaves mmes in the tuokesalei area northwestern chinese altai the granodiorites are characterized by peraluminous compositions a cnk 1 01 1 45 with relatively high sio 2 64 4 to 70 9 wt and low mgo 0 75 to 1 97 wt contents these rocks possess low total rare earth element ree contents 67 187 ppm with weakly negative eu anomalies and mantle like whole rock nd sr isotopic compositions e nd t 2 5 to 4 5 87 sr 86 sr i 0 7038 to 0 7048 and zircon e hf t values 5 93 to 12 9 the mmes contain abundant magmatic amphiboles and needle like apatites have low sio 2 51 7 to 54 3 wt and high mgo 2 86 to 3 75 wt and ree contents 162 298 ppm and are characterized by moderately negative eu anomalies eu eu 0 40 0 60 and slightly depleted to enriched lree la nd n 0 74 1 41 and flat hree gd yb n 1 16 1 39 patterns the mmes have whole rock nd sr isotopic compositions e nd t 2 6 to 2 7 87 sr 86 sr i 0 7034 0 7050 similar to those of the host granodiorites their similarities in crystallization age mineral assemblage and isotopic composition document that the mmes may represent an early crystallized phase which was broken into pieces and brought to the shallow level during the host magma ascent and emplacement their precursor magmas were probably derived from an oceanic lithospheric mantle which was characterized by pronounced nd ta depletions mineral compositions of mmes suggest that amphibole was a major early fractional mineral which effectively differentiated the mantle derived basaltic liquids to silica rich melts zircon u pb dating yielded an age of 313 5 ma for the host granodiorites and 313 13 ma for their mmes the geochemical data of this study records an important episode of magmatism in an environment transitional from a subduction related regime to a post tectonic extension results of this study suggest that the accretionary orogenic process did not completely terminate until the late carboniferous to permian in this region

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

permian alkaline granites in the erenhot hegenshan belt northern inner mongolia china model of generation time of emplacement and regional tectonic significance abstract the central asian orogenic belt caob is known to be the earth s largest phanerozoic accretionary orogenic belt its southern margin particularly in the southern mongolia and inner mongolia smim shows an extensive distribution of alkaline granites study of these granites could shed light on long debated hypotheses on the late paleozoic tectonic evolution of this region in this work we performed a detailed zircon age determination and whole rock geochemical analysis on alkaline granites from four granitic plutons in northern inner mongolia hongol saiyinwusu baolag and baiyinwula u pb zircon dating yielded early permian ages ca 280 ma for the four plutons whole rock geochemical analyses show chemical characteristics typical of alkaline granites coeval alkaline granites from southern mongolia to northern inner mongolia constitute a gigantic 900 km permian 292 275 ma alkaline granite belt in the southern caob furthermore these alkaline granites have whole rock e nd t varying between 3 6 and 6 4 and zircon e hf t from 4 9 to 20 3 the geochemical and nd hf isotopic signatures suggest derivation by partial melting of a protolith assemblage dominated by metasomatised mantle and followed by fractional crystallization in a post collisional extensional environment

ore forming fluids associated with granite hosted gold mineralization at the sanshandao deposit jiaodong gold province china the sanshandao gold deposit with total resources of more than 60 t of gold is located in the jiaodong gold province the most important gold province of china the deposit is a typical highly fractured and altered disseminated gold system with high grade quartz sulphide vein veinlet stockworks that cut mesozoic granodiorite there are four stages of veins that developed in the following sequence 1 quartz k feldspar sericite 2 quartz pyrite arsenopyrite 3 quartz base metal sulfide and 4 quartz carbonate fluid inclusions in quartz and calcite in vein veinlet stockworks contain c o h fluids of three main types the first type consists of dilute co2 h2o fluids coeval with the early vein stage molar volumes of these co2 h2o fluid inclusions ranging from 50 60 cm3 mol yield estimated minimum trapping pressures of 3 kbar homogenization temperatures obtained mainly from co2 h2o inclusions with lower co2 concentration range from 267 375 c the second inclusion type with a co2 h2o ch4 composition was trapped during the main mineralizing stages these fluids may reflect the co2 h2o fluids that were modified by fluid rock reactions with altered wallrocks isochores for co2 h2o ch4 inclusions with homogenization temperatures ranging from 204 325 c and molar volumes from 55 to 70 cm3 mol provide an estimated minimum trapping pressure of 1 2 kbar the third inclusion type aqueous inclusions trapped in cross cutting microfractures in quartz and randomly in calcite are post mineralization and have homogenization temperatures between 143 228 c and salinities from 0 71 7 86 wt nacl equiv stable isotope data show that the metamorphic fluid contribution is minimal and that ore fluids are of magmatic origin most likely sourced from 120 126 ma mafic to intermediate dikes this is consistent with the carbonic nature of the fluid and the cross cutting nature of those deposits relative to the host mesozoic granitoid

paleozoic metamorphism in the qinling orogen tongbai mountains central china the collision of the sino korean and yangtze blocks to form a significant part of china is recorded in the qinling tongbai and dabie mountains radiometric ages of the ultrahigh pressure metamorphic rocks in the south qinling orogenic belt suggest that subduction and collision took place during the triassic period our new 40 ar 39 ar geochronology of units in the north qinling orogenic belt confirms that high grade metamorphism and deformation took place also during the silurian devonian and carboniferous periods these results imply that the amalgamation of eastern china was a multistage process extending over at least 200 m y

revisiting the age and emplacement process of the huangshandong ni cu deposit in the central asian orogenic belt northwestern china implications for multiple magma extractions from a short lived staging magma chamber abstract magmatic ni cu deposits that associated with mafic ultramafic intrusions in convergent settings could form from multiple magma pulses with age difference varying from 1 ma to 10 ma years the huangshandong deposit in nw china of the southern central asian orogenic belt was formed in such a setting the intrusion is composed of four units formed by four magma pluses a gabbroic sequence phase i crystallized from an evolved magma a sheet like ultramafic body phase ii crystallized from a relatively primitive magma a dike like gabbronorite body phase iii crystallized from an evolved magma and an irregular ultramafic unit phase iv crystallized from a relatively primitive magma our zircon sims u pb dating shows that the phase i and phase iv magmas of the huangshandong deposit were emplaced at 280 3 1 9 ma and 281 4 2 2 ma respectively indicating that the four magma pulses were intruded over a very short period and cannot be distinguished within the uncertainty of 2 ma moreover the zircon crystals in the four units record identical hf t values average of 12 1 13 5 and 18o values average of 6 3 6 5 however the intrusive relationships of the four units illustrate that the huangshandong complex cannot form by in situ fractionation therefore the indistinguishable hf o isotopic values together with their contemporaneous emplacements indicate that the four magma pulses could originate from a same staging magma chamber the overall enrichment of hf t value relative to that of the depleted mantle 16 and elevation of 18o values relative to the mantle value 5 3 suggest 15 20 crustal material incorporation additionally the magmas may have been contaminated prior to their arrival at the staging magma chamber where the magma has subsequently experienced variable olivine fractionations and relatively primitive magma replenishment for most mafic ultramafic intrusions which are characterized by long duration magmatism 10 ma in convergent settings e g intrusions in the kalatongke area tulaergen intrusions xiarihamu intrusions the most important ni cu mineralization are closely related to the last stage magmatism this finding suggests that the intrusions emplaced at the late stage of subduction and post collisional period of the orogenic process may have more potential for ni cu mineralization exploration

geochemistry of the permian kalatongke mafic intrusions northern xinjiang northwest china implications for the genesis of magmatic ni cu sulfide deposits the kalatongke cu ni sulfide deposit in northern xinjiang northwest china is located on the southern side of the regional irtysh fault zone which is the boundary between the early paleozoic altai orogenic belt to the north and the late paleozoic junggar terrane to the south in the kalatongke region there are 11 mafic intrusions which were emplaced in lower carboniferous strata economic ni cu sulfide ores are found within three of these intrusions which are well differentiated and compositionally zoned four rock types are recognized which from top to base of the three mineralized intrusions include biotite diorite biotite hornblende norite biotite hornblende olivine norite and biotite hornblende dolerite the biotite hornblende olivine norites and biotite hornblende norites are the most favorable host rocks for ni cu mineralization the initial 87sr 86sr t t 280 ma ratios of the intrusions vary from 0 70375 to 0 70504 and end t from 6 3 to 8 2 implying that the magmas originated from depleted asthenospheric mantle however the strong enrichment of lithophile elements such as k rb th u and lree the negative nb and ta anomalies and the high 18o values of whole rocks 5 4 10 6 suggest significant crustal contamination these patterns are present in almost all samples from the intrusions indicating that the contamination took place before the magma was emplaced at its present level in the crust crustal contamination is interpreted to have driven the magma to s saturation and brought orthopyroxene onto the liquidus convection or perhaps flow differentiation of the crystal bearing magma during ascent caused the dense sulfide melt with entrained olivine and orthopyroxene crystals to become concentrated in the center of the intrusions whereas the fractionated magmas formed other less mafic intrusions which intrude the nearby strata