molybdenite re os and muscovite 40 ar 39 ar dating of quartz vein type w sn polymetallic deposits in northern guangdong south china northern guangdong is an important part of nanling tungsten tin metallogenic belt south china the tungsten mineralization in this area consists of mainly quartz wolframite vein type mineralization with w sn polymetallic deposits mostly distributed at the outer contact zone between concealed late jurassic granitic stocks and cambrian ordovician low metamorphosed sandstones and shales molybdenite re os and muscovite 40ar 39ar isotopic dating of three typical tungsten vein type deposits yaoling meiziwo and jubankeng in northern guangdong show that two episodes of late jurassic w sn polymetallic mineralization occurred in this area an early episode during the late jurassic 158 159 ma represented by the yaoling hongling and meiziwo tungsten deposits and a younger event during the early cretaceous 138 ma represented by the jubankeng deposit analysis of available radiometric ages of several w sn deposits in the nanling region indicate that these deposits formed at several intervals during the mesozoic at 90 100 134 140 144 162 and 210 235 ma and that large scale w sn mineralization in this region occurred mainly between 150 and 160 ma

resurfacing of deeply buried oceanic crust in naga hills ophiolite north east india petrofabric microstructure and seismic properties abstract we have studied the microstructure texture and seismic anisotropy of a foliated blueschist eclogite rock in the naga hills ophiolite nho complex north east india the rock is composed essentially of omphacite sodic amphibole glaucophane sodic calcic amphibole barroisite phengite and garnet and represents oceanic crust that experienced neo tethyan subduction and metamorphism before it resurfaced and accreted in the ophiolite omphacite and glaucophane show weak shape preferred orientation spo while phengite displays a strong spo the crystallographic preferred orientation cpo of omphacite is characterized by the 001 axes gridle within the foliation and the 010 poles concentrated sub perpendicular to the foliation for glaucophane the 001 axis aligns parallel to lineation and the 100 axis and 110 pole plunge perpendicular to foliation these cpos correspond to sl type fabrics related to plane strain deformation geometry based on our analysis we propose that the observed texture had developed from plastic deformation through dislocation creep employing the textural data we have presented the seismic anisotropies of the individual minerals blueschist eclogite domains and discussed their contributions in the bulk rock anisotropy the calculated bulk rock seismic anisotropies avp and avs of the rock are 12 8 and 8 1 respectively which correlate with the observed seismic anisotropy of the subducting oceanic crust

stable isotope composition of magmatic and deuteric carbonate phases in hypabyssal kimberlite lac de gras field northwest territories canada abstract the oxygen and carbon isotope compositions of carbonates from thirteen late cretaceous to paleogene hypabyssal intrusions from the lac de gras kimberlite field indicate that the carbonates are well preserved and have a complex paragenetic formation history kimberlites at ten localities studied are calcite bearing based on the presence of phenocrysts microphenocrysts and or millimeter scale calcite only segregations of sr rich calcite the calcite bearing kimberlites also contain large cm scale sr poor calcite serpentine segregations that are texturally distinct from the smaller sr rich calcite only segregations kimberlites at three other localities studied are dolomite bearing based on the presence of calcite dolomite segregations some of which preserve complex oscillatory and banded textures of calcite dolomite and magnesian calcite 18 o values for whole rock calcite in calcite bearing kimberlite and for micro samples of calcite from individual segregations in calcite and dolomite bearing kimberlite generally vary from 6 to 9 consistent with their formation at magmatic temperatures e g 750 c from a kimberlite melt in contrast higher 18 o values of 9 to 14 characterize micro samples of calcite from calcite serpentine segregations in calcite bearing kimberlite as well as whole rock dolomite and micro samples of dolomite from segregations in dolomite bearing kimberlite these data are consistent with formation from deuteric fluids late stage magmatic at sub solidus temperatures e g 500 to 100 c these higher 18 o values for whole rock calcite also correlate with increased intensity of forsterite alteration increased abundance of calcite serpentine segregations textural overprint of primary groundmass minerals by anhedral calcite and serpentine and development of atoll spinels all interpreted as a result of deuteric fluids in contrast whole rock calcite in dolomite bearing kimberlite has notably lower 18 o values of 1 5 to 5 5 interpreted to have formed from or been overprinted by locally derived high co 2 h 2 o deuteric fluids at 500 to 100 c 13 c values of the various types of calcite and dolomite 8 to 3 have no systematic variation with 18 o values and therefore there is no isotopic record of volatile degassing from these hypabyssal bodies

the timing of marid metasomatism in the kaapvaal mantle an ion probe study of zircons from marid xenoliths abstract the precise timing of metasomatic events in the kaapvaal craton lithospheric mantle is poorly constrained and therefore it has proven difficult to relate mantle metasomatism in the source to igneous activity at the surface a suite of mica amphibole rutile ilmenite diopside bearing xenoliths marid represents the product of such metasomatic fluids or melts in this paper we report results of a shrimp and cathodoluminescence study on zircons from four marid samples from the kimberley cluster of pipes textures combined with experimental evidence suggest that zircons are late crystallizing phases in marids that form close to the solidus where the major precipitating phases are phlogopite and k richterite cathodoluminescence patterns reveal a complex growth history of the zircons involving strong deformation repeated events of crystallization resorption and or modification of the zircon composition possibly over several millions of years many of the u pb ages are significantly older than the intrusion age of the group i kimberlites of the kimberley cluster zircon ages in a marid xenolith from the wesselton kimberlite vary in the range 113 3 ma to 142 3 ma compared with a pipe age of 84 3 ma in two samples from the de beers bultfontein or dutoitspan pipes ages range from 85 5 to 119 2 ma in one and 80 4 to 90 2 ma in the other the ages obtained for the latter are indistinguishable from the pipe age of kimberlites belonging to the younger group i kimberlite event ca 80 95 ma zircons in this sample as well as the younger ages in the other may reflect real ages but most likely have lost radiogenic pb as a result of deformation and fluid or melt interaction the presence of pre group i ages in two of the samples demonstrates unambiguously that the metasomatism that yielded the older crystallization ages cannot be related to the group i kimberlite event instead minimum ages for the marid zircons coincide with the period of group ii kimberlite magmatism in the central to southeastern kaapvaal craton this coincidence in ages would be consistent with the contention based on experimental evidence that marid type rocks can be derived from group ii precursor magmas by olivine fractionation and exsolution of a carbonatitic component an alternative possibility not excluded by the zircon minimum ages is a genetic relation between marid metasomatism and karoo magmatism

nitrogen isotopic compositions and concentrations in marid xenoliths abstract nitrogen isotopic compositions and concentrations in micas from kimberlite hosted marid mica amphibole rutile ilmenite diopside suite of xenoliths from the kimberley area of the kaapvaal craton south africa were measured using continuous flow isotope ratio mass spectrometry and fourier transform infrared spectroscopy fourier transform infrared spectroscopy data suggest that the nitrogen in marid phlogopite is from ammonium in the mica structure the replacement of potassium by ammonium is the major mechanism for the storage of nitrogen within these phlogopites the 15n values of micas from marid xenoliths range from 11 to 9 and nitrogen contents range between 113 and 272 ppm the 15n enrichment of most samples relative to the average upper mantle 15n value of 5 suggests that the nitrogen reflects a recycled reservoir from subducted material within their source region the presence of nitrogen from a recycled reservoir indicates that sedimentary organic nitrogen in the sub continental lithosphere is not effectively cycled to the surface resulting in a geochemically heterogeneous upper mantle

oxygen and hydrogen isotopic composition of phlogopites and amphiboles in diamond bearing kimberlite hosted marid xenoliths constraints on fluid rock interaction and recycled crustal material in the deep continental lithospheric mantle abstract marid mica amphibole rutile ilmenite diopside xenoliths are transported from the deep cratonic lithosphere to the earth s surface by cretaceous kimberlites emplaced in the kimberley area of the kaapvaal craton marid xenoliths have high modal abundances 70 80 vol of mica and amphibole indicating their origin from a hydrous source the 18o values 4 7 18o 6 9 of phlogopite micas from 14 marid samples indicate that these minerals are both 18o enriched and 18o depleted with respect to the average upper mantle 18o value of 5 8 0 6 the range of 2h values of phlogopites 83 2h 53 n 14 of marid xenoliths are slightly larger than the average mantle 2h value 70 10 the oxygen 18ophlogopites amphibole 0 4 and 0 4 and hydrogen 2hphlogopite amphibole 14 and 36 isotopic disequilibrium recorded from two marid xenoliths suggests the duration of the last isotopic exchange possibly just before the kimberlite emplacement between these minerals and metasomatic fluids was too short to reach isotopic equilibrium our model calculation indicates that the phlogopites of marid xenoliths underwent isotopic exchange with fluids of 18o 5 5 to 10 2h 62 to 90 the range of 18o value of the calculated metasomatic fluids resembles the oxygen isotopic composition of the primary mantle carbonate 6 9 suggesting interaction between carbonatic melt and marid xenoliths in the continental lithospheric mantle furthermore 18o values of phlogopites together with previously published nitrogen isotope data 11 15n 9 banerjee et al 2015 indicates incorporation of inhomogeneously distributed recycled crustal material from subducted crust within their source magma therefore o h n isotope data for marid xenoliths indicates their crystallization from geochemically heterogeneous magma in the upper continental mantle and subsequent metasomatism with mantle fluids

mean crustal velocity a critical parameter for interpreting crustal structure and crustal growth abstract mean crustal velocity is a critical parameter for genesis of continental crystalline crust because it is a function of mean crustal composition and therefore may be used to resolve continental crustal growth in space and time although the best values of mean crustal velocity are determined from wide angle reflection measurements most studied here necessarily come from vertical averages in crustal refraction determinations the mode of 158 values of mean crustal velocity is 6 3 km s a velocity which corresponds to a mean crustal composition of granodiorite to felsic quartz diorite archean crust may be slightly more mafic mean crustal velocities range from 5 8 to 7 0 km s the lowest values invariably are found in thermally disturbed rift zones and the highest values correspond to velocities in gabbro velocities in island arcs may be as low as 6 0 km s but are typically 6 5 6 9 km s which corresponds to andesitic composition estimates of island arc composition are andesitic if values of mean crustal velocity are not biased this observation suggests that continental crust did not grow simply by addition of island arc material possibilities are that crust formed from fusion of island arcs and was later changed to more felsic composition by addition of material from the mantle or that the late archean episode of major crustal growth did not involve processes similar to younger island arcs some crustal blocks might be changed in composition and thickness by such processes as underplating interthrusting necking and sub crustal erosion specially designed experiments are suggested to determine this parameter so critical for understanding genesis of continental crust

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

rapid magma emplacement in the karoo large igneous province abstract understanding the dynamics of continental large igneous provinces lips relies on precise dating of basaltic rocks lip research has traditionally focused on dating lavas often neglecting the volumetrically important sill intrusions in underlying sedimentary basins here we present u pb zircon and baddeleyite ages for fourteen new samples of karoo lip sills and dykes spaced by as much as 1100 km across the half million square kilometer karoo basin the samples yield remarkably coherent ages ranging from 183 0 0 5 to 182 3 0 6 myr probability modeling indicates that basin scale emplacement took place within an interval of about 0 47 myrs less than 0 90 myrs with 95 confidence and could even have represented a single magma emplacement event combining the new ages with the estimated volume of sills in the karoo basin gives an emplacement rate of 0 78 km3 yr which is higher than previous estimates upper crustal magma storage may account for these high rates the results challenge the view that melt emplacement in a sedimentary basin is a prolonged process support a scenario of pulsating catastrophic events within a narrow time frame and strengthens the hypothesis linking lips and sill emplacement to global environmental crises

molybdenite re os and muscovite 40ar 39ar dating of the xihuashan tungsten deposit central nanling district south china abstract the xihuashan tungsten deposit in the central nanling region south china is an important vein type ore deposit hosted in cambrian strata and mesozoic granitic intrusions wolframite and molybdenite are the principal ore minerals the gangue minerals are mainly quartz and muscovite wolframite and molybdenite are products of the first stage hydrothermal activity whereas muscovite formed dominantly at the second stage molybdenite re os and muscovite 40ar 39ar dating have been carried out to investigate the age of mineralization re os isotopic dating for molybdenite associated with wolframite yield a precise well constrained isochron age of 157 8 0 9 ma mswd 1 5 ar ar isotopic analyses of muscovite yield a plateau age of 152 8 1 6 ma in agreement with an inverse isochron age of 152 8 1 6 ma which is 5 mys younger than the re os age the molybdenite re os age is interpreted as the age of tungsten mineralization this age coincides well with the zircon u pb age of the host granitic intrusion reported previously the 5 mys difference between molybdenite re os and muscovite 40ar 39ar ages probably represents the duration of hydrothermal activity the results show that the xihuashan tungsten deposit is one of many important tungsten tin deposits formed during 150 to 160 ma associated with large scale lithospheric extension in south china

re assessing the nitrogen signal in continental margin sediments new insights from the high northern latitudes organic and inorganic nitrogen and their isotopic signatures were studied in continental margin sediments off spitsbergen we present evidence that land derived inorganic nitrogen strongly dilutes the particulate organic signal in coastal and fjord settings and accounts for up to 70 of the total nitrogen content spatial heterogeneity in inorganic nitrogen along the coast is less likely to be influenced by clay mineral assemblages or various substrates than by the supply of terrestrial organic matter tom within eroded soil material into selected fjords and onto the shelf the 15 n signal of the inorganic nitrogen 15 ninorg in sediments off spitsbergen seems to be appropriate to trace tom supply from various climate and ecosystem zones and elucidates the dominant transport media of terrigenous sediments to the marine realm moreover we postulate that with the study of sedimentary 15 ninorg in the atlantic arctic gateway climatically induced changes in catchment s vegetations in high northern latitudes may be reconstructed the 15 norg signal is primarily controlled by the availability of nitrate in the dominating ocean current systems and the corresponding degree of utilization of the nitrate pool in the euphotic zone not only does this new approach allow for a detailed view into the nitrogen cycle for settings with purely primary produced organic matter supply it also provides new insights into both the deposition of marine and terrestrial nitrogen and its ecosystem response to paleo climate changes

mesozoic metallogeny in east china and corresponding geodynamic settings an introduction to the special issue abstract the giant east china mesozoic metallogenic province hosts some of the world s largest resources of tungsten tin molybdenum antimony and bismuth ores of gold silver mercury lead zinc copper uranium and iron are also of major importance the province and its constituent metallogenic belts or regions south china middle lower yangtze river valley east qinling dabie interior of north china craton yan liao and north east china are the products of several pulses of igneous activity and mineralisation between 240 and 80 ma each successive stage has produced a distinctive suite of deposits that can be readily related to the geodynamic evolution of the region during the mesozoic this geodynamic evolution is linked to a complex series of tectonic events involving far field subduction plate collisions crustal thickening post collision collapse and rifting

geology of gold bearing skarn deposits in the middle and lower yangtze river valley and adjacent regions abstract the middle and lower yangtze river valley and adjacent regions are the most important metallogenic belt of gold and copper bearing skarn deposits in china the total gold reserves in this belt have been estimated at more than 600 t the gold bearing skarns are mainly distributed in the southeastern hubei tongling and northern anhui regions favorable tectonic settings are depressions and fold zones of the platforms i e mobile belts these skarns are hosted by platformal limestone dolomitic limestone and dolomite of the triassic carboniferous permian and middle to lower cambrian formations the related intrusions are yenshanian 180 to 113 ma calc alkaline quartz monzodiorite granodiorite quartz monzonite monzogabbro and their hybabyssal facies the intrusions have high fe2o3 feo 0 5 and intermediate initial 87 sr 86 sr ratios 0 7046 to 0 7087 their ree distribution patterns are lree enriched and exhibit smooth right dipping curves these suggest that the source materials mainly came from upper mantle with contamination by sialic crustal components the auriferous skarns are both calcic and magnesian but calcic skarns are most common the constituent minerals of the calcic skarns are diopside garnet wollastonite vesuvianite and scapolite whereas magnesian skarns are dominated by forsterite spinel diopside phlogopite chondrodite and clinohumite with abundant superimposed serpentine clinochlore and brucite the compositions of coexisting pyroxenes and garnets are diopside and andradite indicating the high oxygen fugacity and low acidity conditions gold is closely associated with cu pb zn sulfides and exists mainly in the form of native gold and electrum arsenides tellurides bismuthides and selenides are present in many ore deposits therefore cu as bi te ag pb zn se and co are the major metals present in the deposits and are important geochemical ore searching indicators in some au fe cu magnesian skarns magnesiomagnetite magnesioferrite and ludwigite are locally abundant the metasomatic zoning in many gold skarn deposits is very distinct consisting of an outward sequence of fe cu cu mo cu au au cu au pb zn the geologic characteristics of au cu skarn deposits that formed in the mobile platformal setting of china have distinct differences compared to au skarns formed in orogenic belts at convergent plate margins in british columbia and the western usa

hydrogen isotope systematics of submarine basalts the d h ratios and water contents in fresh submarine basalts from the mid atlantic ridge the east pacific rise and hawaii indicate that the primary d h ratios of many submarine lavas have been altered by processes including 1 outgassing 2 addition of seawater at magmatic temperature and 3 low temperature hydration of glass decreases in d and h2o from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between d and h2o in basalts from the galapagos rise and the famous area are attributed to outgassing of ch4 and h2 a good correlation between d values and h2o is observed in a suite of submarine tholeiites dredged from the kilauea east rift zone where seawater added directly to the magma affected only the isotopic compositions of hydrogen and argon analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having d values as low as 100 d values vary with h2o contents of subaerial transitional basalts from molokai hawaii and subaerial alkali basalts from the society islands indicating that the primary d values were similar to those of submarine lavas extrapolations to possible unaltered d values and h2o contents indicate that the primary d values of most thoteiite and alkali basalts are near 80 5 the weight percentages of water are variable 0 15 0 35 for mor tholeiites about 0 25 for hawaiian tholeiites and up to 1 1 for alkali basalts the primary d values of 80 for most basalts are comparable to those measured for deep seated phlogopites these results indicate that hydrogen in marked contrast to other elements such as sr nd pb and o has a uniform isotopic composition in the mantle this uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the earth

seismic anisotropy from compositional banding in granulites from the deep magmatic arc of fiordland new zealand abstract we present calculated seismic velocities and anisotropies of mafic granulites and eclogites from the cretaceous deep lower crust 40 65 km of fiordland new zealand both rock types show a distinct foliation defined by cm scale compositional banding seismic properties are estimated using the asymptotic expansion homogenisation finite element aeh fe method that unlike the commonly used voigt reuss hill homogenisation incorporates the phase boundary network into calculations the predicted mean p and s wave velocities are consistent with previously published data for similar lithologies from other locations e g kohistan arc although we find higher than expected anisotropies a v p 5 0 8 0 a v s 3 0 6 5 and substantial s wave splitting along foliation planes in granulites this seismic signature of granulites results from a density and elasticity contrast between cm scale pyroxene garnet stringers and plagioclase matrix rather than from crystallographic orientations alone banded eclogites do not show elevated anisotropies as the contrast in density and elastic constants of garnet and pyroxene is too small the origin of compositional banding in fiordland granulites is primarily magmatic and structures described here are expected to be typical for the base of present day magmatic arcs hence we identify a new potential source of anisotropy within this geotectonic setting

molybdenite re os and albite 40ar 39ar dating of cu au mo and magnetite porphyry systems in the yangtze river valley and metallogenic implications abstract the area of the middle lower yangtze river valley eastern china extending from wuhan hubei province to western zhenjiang jiangsu province hosts an important belt of cu au mo and fe deposits there are two styles of mineralization i e skarn porphyry stratabound cu au mo fe deposits and magnetite porphyry deposits in several nne trending cretaceous fault bound volcanic basins the origin of both deposit systems is much debated we dated 11 molybdenite samples from five skarn porphyry cu au mo deposits and 5 molybdenite samples from the datuanshan stratabound cu au mo deposit by icp ms re os isotope analysis nine samples from the same set were additionally analyzed by ntims on re os results from the two methods are almost identical the re os model ages of 16 molybdenite samples range from 134 7 2 3 to 143 7 1 6 ma 2 the model ages of the five samples from the datuanshan stratabound deposit vary from 138 0 3 2 to 140 8 2 0 ma with a mean of 139 3 2 6 ma their isochron age is 139 1 2 7 ma with an initial os ratio of 0 7 8 1 mswd 0 29 these data indicate that the porphyry skarn systems and the stratabound deposits have the same age and suggest an origin within the same metallogenic system albite 40 ar 39 ar dating of the magnetite porphyry deposits indicates that they formed at 123 to 125 ma i e 10 20 ma later both mineralization styles characterize transitional geodynamic regimes i e the period around 140 ma when the main ns trending compressional regime changed to an ew trending lithospheric extensional regime and the period of 125 115 ma of dramatic ew trending lithospheric extension

origin and tectonic significance of a mesozoic multi layer over thrust system within the yangtze block south china abstract in the yangtze block south china a well developed mesozoic thrust system extends through the xuefeng and wuling mountains in the southeast to the sichuan basin in the northwest the system comprises both thin and thick skinned thrust units separated by a boundary detachment fault the dayin fault to the northwest the thin skinned belt is characterized by either chevron anticlines and box synclines to the northwest or chevron synclines to the southeast the former structural style displays narrow exposures for the cores of anticlines and wider exposures for the cores of synclines thrust detachments occur along silurian fs and lower cambrian fc strata and are dominantly associated with the anticlines to the southeast this style of deformation passes gradually into one characterized by chevron synclines with associated principal detachment faults along silurian fs cambrian fc and lower sinian fz strata there are however numerous secondary back thrusts therefore the thin skinned belt is like the valley and ridge province of the north american applachian mountains the thick skinned belt structurally overlies the thin skinned belt and is characterized by a number of klippen including the xuefeng and wuling nappes it is thus comparable to the blue ridge province of appalachia the structural pattern of this thrust system in south china can be explained by a model involving detachment faulting along various stratigraphic layers at different stages of its evolution the system was developed through a northwest stepwise progression of deformation with the earliest delamination along lower sinian strata fz analyses of balanced geological cross sections yield about 18 1 21 total 88 km shortening for the thin skinned unit and at least this amount of shortening for the thick skinned unit the compressional deformation from southeast to northwest during late jurassic to cretaceous time occurred after the westward progressive collision of the yangtze block with the north china block and suggests that the orogenic event was intracontinental in nature

an integrated model of kimberlite ascent and eruption diatremes are carrot shaped bodies forming the upper parts of very deep magmatic intrusions of kimberlite rock these unusual enigmatic and complex features are famous as the source of diamonds here we present a new model of kimberlite ascent and eruption emphasizing the extremely unsteady nature of this process to resolve many of the seemingly contradictory characteristics of kimberlites and diatremes dyke initiation in a deep co2 rich source region in the mantle leads to rapid propagation of the dyke tip below which co2 fluid collects with a zone of magmatic foam beneath when the tip breaks the surface of the ground gas release causes a depressurization wave to travel into the magma this wave implodes the dyke walls fragments the magma and creates a ringing fluidization wave together these processes form the diatreme catastrophic magma chilling seals the dyke no precursor to the eruption is felt at the surface and the processes are complete in about an hour

seismic velocities from the kohistan volcanic arc northern pakistan the rocks bounded by the main mantle thrust and the northern suture in northern pakistan constitute an exhumed section through a cretaceous volcanic arc samples have been collected from all the principal lithological groups of the arc and p and s wave seismic velocities have been measured in the laboratory with the prime objective of comparing the velocities with those determined by seismic refraction experiments on modern volcanic arcs velocities were measured at up to 0 7 gpa and consideration has been given to the effect of confining pressure pore pressure and temperature and to the deformation and metamorphism involved in the himalayan collision the reconstructed velocity section through the arc shows a distinct upper crust comprising granitic dioritic intrusions metasediments and volcanics with a p wave velocity of 6 2 6 4 km s 1 depending on the parameters used beneath the lower crust of amphibolitcs and pyroxene granulite has a velocity mainly about 6 4 6 7 kms 1 though the garnet granulitcs extend to 7 8 km s 1 the more mature arc with a higher proportion of granitic rocks would show a slightly lower upper crustal velocity than the younger arc velocity inversions might be expected with a thermal gradient of as little as 10 15 c km 1 depending on the pore pressure in general the proposed velocity structure is comparable with that of volcanic arcs and with many other sections of the continental crust

p and s wave velocities of the lowermost crustal rocks from the kohistan arc implications for seismic moho discontinuity attributed to abundant garnet abstract p vp and s wave vs velocities of garnet free two pyroxene granulite and garnet bearing garnet granulite and garnet pyroxenite lowermost crustal rocks collected from the kohistan arc northern pakistan were measured at 0 1 1 0 gpa and 25 400 c garnet granulite had higher vp 0 31 km s and vs 0 27 km s than two pyroxene granulite although vp and vs increased with increasing volume percent of garnet plagioclase free garnet pyroxenite showed significantly higher vp and vs than plagioclase rich garnet granulite mainly due to the low vp and vs of plagioclase in contrast we observed two quasi linear relationships between vp vs and sio2 content for the garnet bearing and garnet free rocks the garnet bearing rocks had relatively higher vp and vs and stronger sio2 dependences than the garnet free rocks the stronger sio2 dependences of vp and vs in the garnet bearing rocks suggest that the garnet formation in mafic to ultramafic rocks e g pyroxenite and hornblendite having relatively lower sio2 leads to more pronounced increases in vp and vs than that of relatively felsic rocks e g felsic to mafic granulite indeed the vp and vs of the garnet pyroxenite were significantly higher than those of garnet granulite but comparable to those of dunite the significantly high vp and vs of the garnet pyroxenite yielded high reflection coefficients between the garnet granulite and garnet pyroxenite of up to 0 13 for p waves and 0 14 for s waves comparable to values expected for moho reflection thus the lithological boundary between plagioclase rich garnet granulite and plagioclase free garnet pyroxenite in the lowermost crust of the kohistan arc corresponds to the seismic moho discontinuity

a model of velocity structure beneath calabria southern italy based on laboratory data abstract laboratory velocity measurements of representative rock samples from across a nearly complete crustal section from the serre mountains of southern calabria have been carried out as an aid to interpreting in situ seismic velocity data the section is composed of interlayered acidic mafic and pelitic rocks metamorphosed to granulite and amphibolite facies conditions in addition a suite of post tectonic granitoids intrudes the section the experimental data include the measurement of compressional wave vp and shear wave vs velocities at confining pressure in conjunction with structural and stratigraphic field data the experimentally determined data are used to construct a seismic velocity model for the crust beneath calabria in the upper crustal levels vp and vs vary in different manners below the uppermost amphibolite facies gneisses a weak velocity inversion of vp is associated with the post tectonic granites at the granite granodiorite and the tonalite dioritic gneiss boundary vp increases in steps of about 200 and 300 m s respectively in contrast vs remains constant or even decreases slightly through this same sequence of rocks this contrasting behaviour is primarily related to the quartz feldspar ratio in the rocks the most pronounced velocity jump occurs at the top of the granulite facies metapelite unit here both vp and vs increase markedly due to mineralogic changes that accompany increasing metamorphic grade and the greater abundance of mafic and garnet bearing restites of partially melted pelite the mafic rocks of the lowermost crustal layer have p and s wave velocities that are significantly lower than those in the overlying metapelites the variation of poisson s ratio with depth suggests that this parameter which is a combination of vp and vs is more sensitive to lithology than the p and s wave velocities alone in rock units of upper and mid crustal levels with average p wave velocities

mantle heterogeneities and their significance results from lithoprobe seismic reflection and refraction wide angle reflection studiesthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent lithoprobe contribution 1486 within lithoprobe s 10 transects data from more than 20 000 km of multichannel seismic mcs reflection profiling and 12 refraction wide angle reflection r war surveys were acquired while the

infrared microthermometric and stable isotopic study of fluid inclusions in wolframite at the xihuashan tungsten deposit jiangxi province china the xihuashan tungsten deposit jiangxi province china is a world class vein type ore deposit hosted in cambrian strata and mesozoic granitic intrusions there are two major sets of subparallel ore bearing quartz veins the ore mineral assemblage includes wolframite and molybdenite with minor amounts of arsenopyrite chalcopyrite and pyrite there are only two phase aqueous rich inclusions in wolframite but at least three major types of inclusions in quartz two or three phase co2 rich inclusions two phase pure co2 inclusions and two phase aqueous inclusions indicating boiling fluid inclusions in wolframite have relatively higher homogenization temperatures and salinities 239 380 c 3 8 13 7 wt nacl equiv compared with those in quartz 177 329 c 0 9 8 1 wt nacl equiv these distinct differences suggest that those conventional microthermometric data from quartz are not adequate to explain the ore formation process enthalpy salinity plot shows a linear relationship implying mixing of different sources of fluids although boiling occurred during vein type mineralization it seems negligible for wolframite deposition mixing is the dominant mechanism of wolframite precipitation in xihuashan 34s values of the sulfides range from 1 6 to 0 1 indicative of a magmatic source of sulfur 18o values of wolframite are relatively homogeneous ranging from 4 8 to 6 3 oxygen isotope modeling of boiling and mixing processes also indicates that mixing of two different fluids was an important mechanism in the precipitation of wolframite

the lower changjiang yangzi yangtze river metallogenic belt east central china intrusion and wall rock hosted cu fe au mo zn pb ag deposits abstract the lower valley of changjiang from wuhan of the hubei province in the west to zhenjiang of the jiangsu province in the east contains more than 200 polymetallic cu fe au mo zn pb ag deposits and is one of the most important metallogenic belts in china this metallogenic belt situated at the northern margin of the yangzi craton and bordered by the dabieshan ultrahigh pressure metamorphic belt to the north consists mainly of cambrian triassic marine clastic sedimentary rocks and carbonate and evaporite rocks which overlay a precambrian basement and are intruded by yanshanian 205 to 64 ma granitoid intrusions and subvolcanic complexes repeated tectonism from late proterozoic to triassic resulted in extensively developed networks of faults and folds involving the cambrian triassic sedimentary strata and the precambrian basement the yanshanian granitoid intrusions and subvolcanic complexes in the lower changjiang metallogenic belt are characterized by whole rock 18 o of 8 to 10 initial 87 sr 86 sr of 0 704 to 0 708 and ndt from 10 to 17 and have been interpreted to have originated from mixing between juvenile mantle and old crustal materials also the yanshanian granitoids exhibit eastward younging and increase in alkalinity i e from older calc alkaline in the west to younger subalkaline alkaline in the east which are related to oblique collision between the yangzi and sino korean cratons and tectonic evolution from early compressional to late extensional or rifting regimes most polymetallic deposits in the lower changjiang metallogenic belt are clustered in seven districts where the yanshanian magmatism is particularly extensive from west to east edong jiurui anqing guichi luzhong tongling ningwu and ningzhen mineralization is characterized by the occurrence of three distinct types of orebodies in individual deposits orebodies in yanshanian granitoid intrusions skarn orebodies at the contact zones between the yanshanian intrusions and late paleozoic early mesozoic sedimentary rocks and stratabound massive sulfide orebodies in the late paleozoic early mesozoic sedimentary strata the most important host sedimentary strata are the middle carboniferous huanglong formation lower permian and lower middle triassic carbonate and evaporite rocks the intrusion hosted and skarn orebodies exhibit well developed zonation in alteration assemblages metal contents and isotopic compositions within individual deposits and apparently formed from hydrothermal activities related to the yanshanian magmatism the stratabound massive sulfide orebodies in the late paleozoic early mesozoic sedimentary strata have long been suggested to have formed from sedimentary or volcano sedimentary exhalative processes in shallow marine environments however extensive research over the last 40 years failed to produce unequivocal evidence for syngenetic mineralization on the basis of geological relationships and isotope geochemical characteristics we propose a carbonate hosted replacement deposit model for the genesis of these stratabound massive sulfide orebodies and associated skarn orebodies this model suggests that epigenetic mineralization resulted from interactions between magmatic fluids evolved from the yanshanian intrusions with carbonate and evaporite wall rocks mineralization was an integral but distal part of the larger hydrothermal systems that formed the proximal skarn orebodies at the contact zones and the intrusion hosted orebodies the stratabound massive sulfide deposits of the lower changjiang metallogenic belt share many features with the well studied high temperature carbonate hosted replacement deposits of northern mexico and western united states particularly with respect to association with small shallow granitoid complexes structural and stratigraphic controls on mineralization alteration assemblages geometry of orebodies metal association metal zonation and isotopic systematics

the jurassic danba hypozonal orogenic gold deposit western china indirect derivation from fertile mantle lithosphere metasomatized during neoproterozoic subduction the genesis of hypozonal orogenic gold deposits is highly controversial with both crustal metamorphic fluids and variously derived sub crustal fluids suggested as fluid and metal sources the lower jurassic danba gold deposit a rare phanerozoic hypozonal orogenic gold deposit on the western margin of the yangtze craton china provides an opportunity to resolve these controversies danba is characterized by biotite amphibole alteration and pyrrhotite dominant ore assemblages formed at 500 600 c the 34s values of ore related pyrrhotite range from 3 1 to 9 9 in sharp contrast to those of syn sedimentary pyrrhotite from devonian host rocks which are between 6 8 and 9 5 the pb isotope compositions of ore related pyrrhotite with 206pb 204pb 17 85 18 25 and 207pb 204pb 15 48 15 67 are less radiogenic and more variable than those of unaltered host rocks these differences combined with the hypozonal feature of the deposit indicate that ore metals and sulfur were derived from a sub crustal rather than a local crustal source the pge patterns of ore related pyrrhotites are similar to those of sulfides in baltic mantle lithosphere that was metasomatized by aqueous fluids in terms of marked enrichment of pd and ru but significantly differ from those of magmatic intrusions and related magmatic hydrothermal deposits this suggests that the metals that formed the danba deposit were transported via aqueous fluid from metasomatized mantle lithosphere rather than from a magmatic source the 18o of mantle fluid calculated from hydrothermal quartz and biotite ranges from 10 to 12 the anomalously high s and o isotope ratios and variable pb isotope ratios together with pge data support a model in which the inferred mantle lithosphere source was significantly hydrated metasomatized and fertilized by fluid derived from subducted oceanic sediments with elevated 34s and 18o values geological evidence suggests that this subduction event was neoproterozoic in age this study defines a model in which hydrated and metasomatized mantle lithosphere was formed during early subduction but it was over 500 million years later during later lower jurassic asthenosphere upwelling that metal and sulfur rich fluid was released into the crust to form the danba gold deposit

oxygen isotope geochemistry of oceanic arc lavas variations of oxygen isotope ratios in arc related lavas can constrain the contributions of subducted crustal igneous rocks sediments and fluids to the sub arc mantle we have measured oxygen isotope ratios in 72 arc and back arc lavas from five ocean ocean subduction zone systems using laser fluorination analyses of olivine and other phenocrysts and glass eighty percent of our samples have delta 18o values for any given phase olivine plagioclase glass or biotite within 0 2 per thousand of the average value for that phase in upper mantle peridotites and mid ocean ridge basalt morb the range for each phase is 1 0 per thousand this result contrasts with previous studies of whole rock samples which are significantly more variable even after exclusion of samples believed to be altered or fractionated by magmatic differentiation and demonstrates that most arc related lavas contain 1 2 of 18o enriched crustal oxygen from any source i e assimilation or subducted contributions elevations in delta 18o that do occur in these basic arc derived magmas relative to values most common for mantle derived lavas are associated both with enriched radiogenic isotope signatures and even more strongly with chemical indices consistent with high integrated extents of melting of their peridotite sources we interpret these relationships as evidence that melting in the sources of the high delta 18o lavas we have studied was fluxed by addition of high delta 18o aqueous fluid or perhaps a hydrous melt from the subducted slab such that sources that contain relatively large components of slab derived fluid or melt are both relatively 18o enriched and also experienced relatively large amounts of melting we have developed a quantitative model linking the amount of melting to the extents of 18o radiogenic isotope and trace element enrichment in a mantle source being fluxed by addition of aqueous fluid comparison of this model with observed variations in the geochemistry of lavas from the vanuatu fiji new caledonia region the suite of related samples showing the greatest range in delta 18o observed in this study constrains the amounts and chemical and isotopic compositions of slab derived phases in the sources of these arc related lavas assuming a delta 18o value of 20 per thousand for the slab derived fluid 0 5 1 0 wt is added to the sources of most mantle derived arc magmas the maximum amount of slab derived flux in the sources of arc magmas according to our results is 2 5 wt

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

metallogeny of copper and iron deposits in the eastern yangtse craton east central china abstract the eastern yangtse craton of central to eastern china is an important fe cu metallogenic province based on studies of the regional geology and tectonic evolution this metallogenic belt is controlled by faults and aulacogens in the continental plate in the early yanshan epoch jurassic the dominant west northwest and east west lithospheric faults controlled the distribution of cu mo and au mineralization whereas in the late yanshan epoch cretaceous the north northeast and northeast lithospheric faults controlled cu fe mineralization in a mineral district the distribution of ore fields is under the dual control of basement fault s and near surface fault s based on the relation of ring and lineament faults to intrusions a three level model of vertical zonation is proposed systematic petrochemical studies of mesozoic igneous rocks and mineralization identify three subseries 1 a calc alkaline intermediate to silicic subseries rich in potassium 2 a calc alkaline intermediate to felsic subseries rich in sodium and 3 a calc alkaline intermediate to mafic subseries rich in sodium genetic types of ore deposits are diverse including fe and cu skarns cu porphyries fe ore magma injections ningwu luzong fe deposits from late orthomagmatic to pegmatitic pneumato hydrothermal and to mesothermal epithermal associated with subvolcanic rocks and remobilized sedimentary fe and or cu deposits the genetic types of ore deposits form two metallogenic series 1 a sedimentary metallogenic series active mainly in the paleozoic and 2 a magmatic hydrothermal metallogenic series consisting of three subseries active predominantly in the jurassic and cretaceous with the latter being more important in this region the superposition and compounding of these two metallogenic series contribute to many styles of mineralization in this mineral province

exposed cross sections through the continental crust implications for crustal structure petrology and evolution abstract the continental crust is exposed in cross section at numerous sites on the earth s surface these exposures which appear to have formed by obduction along great faults during continental collision may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle from an examination of five complexes which meet these criteria it is concluded that the most prominent layering in the crust is not compositional but metamorphic the lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks respectively post metamorphic granitic intrusions are common at intermediate to shallow levels position of discontinuities in refraction velocity where present commonly correspond to changes in composition or metamorphic grade with depth the continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by cocorp field observations coupled with geochemical data indicate a complex evolution of the lower crust which can include anatexis multiple deformation polymetamorphism and reworking of older crustal material the complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments

precise molybdenite re os and mica ar ar dating of the mesozoic yaogangxian tungsten deposit central nanling district south china the yaogangxian deposit in the central nanling region south china consists of vein type ore bodies hosted in cambrian to jurassic strata and mesozoic granitic intrusions wolframite and molybdenite are the dominant ore minerals intergrown with gangue minerals of quartz feldspar phlogopite and muscovite we have carried out molybdenite re os and phlogopite and muscovite 40ar 39ar dating to better understand the timing and genesis of mineralization re os dating of eight molybdenite samples yielded model ages ranging from 152 0 3 5 to 161 1 4 5 ma with an average of 156 0 ma the re os analyses give a well defined 187re 187os isochron with an age of 154 9 2 6 ma mswd 2 4 hydrothermal phlogopite and muscovite display extremely flat 40ar 39ar age spectra phlogopite yields a 40ar 39ar plateau age of 153 0 1 1 ma whereas muscovite yields a plateau age of 155 1 1 1 ma both 40ar 39ar ages are in good agreement with the re os ages placing the timing of tungsten mineralization at about 154 ma this age is consistent with the field relationships our new data when combined with published geochronological results from other major deposits in this region suggest that large scale w sn mineralization occurred throughout the central nanling region in the late jurassic

pore pressure and oceanic crustal seismic structure summary marine refraction studies during the past decade have found considerable lateral variability in the seismic properties of the upper basaltic regions of the oceanic crust in many localities compressional and shear wave velocities are quite low at the top of the basalt section and velocities increase rapidly with depth it is concluded that pore pressure may be at least in part responsible for the low upper crustal velocities and contribute significantly to the lateral variability this is supported by compressional and shear wave velocity measurements as functions of confining pressure and pore pressure for basalt from the juan de fuca ridge and dolerite from the samail ophiolite oman within the oceanic crust regions of overpressure and underpressure will possess anomalous velocities the magnitude of which will depend upon the porosity and the deviation of the pore pressure from hydrostatic the influence of pore pressure on velocities is expected to diminish with depth and is unlikely to be significant at lower crustal depths where porosity is extremely low of significance poisson s ratio is shown to be dependent on pore pressure as well as confining pressure at constant confining pressure poisson s ratio increases with increasing pore pressure thus overpressured regions within the upper oceanic crust are likely to have relatively high poisson s ratios as well as low compressional and shear wave velocities

geo tectonic framework of the himalaya of n pakistan in the karakorum range there is a structurally complicated cretaceous are comprising the kohistan sequence on its northern side the northern suture consists of a mega melange and is bounded to the s by tightly folded pillow bearing volcanics and sediments to the s the kohistan plutonic belt consists of southwards a early foliated and late post tectonic tonalites and diorites b aplites and pegmatites up to 30 of rock volume c basic dykes up to 10 m thick d the chilas complex a stratiform cumulate body over 300 km long and 8 km thick chromite layered dunites gabbros and norites with a low pressure granulite facies mineral fabric of tectonic origin e an amphibolite belt with a complex mixture of other rocks and f the jijal complex a 200 km 2 tectonic wedge of high pressure granulites and chromite layered dunites cumulate graded units in the chilas complex show that it is folded by an isoclinal anticline f 1 the mid upper crust of the are is folded by a 50 km half wavelength f 2 syncline the whole kohistan sequence with its two phases of isoclinal folds was tilted during himalayan collision so that the structures are now subvertical the southern suture main mantle thrust has a wedge of glaucophane schists the indian plate contains a basement of psammites and schists intruded by cambrian granites and overlain by isoclinally folded and metamorphosed carbonates and shales

a review of the geochronological constraints on the evolution of the kaapvaal craton south africa abstract all the published u pb geochronological data from zircon titanite sphene and monazite and pb pb evaporation data applicable to the palaeoarchaean to neoarchaean geology of the kaapvaal craton are employed to reconstruct the history and crustal architecture of the ca 3600 2500 ma cratonic basement in south africa only data interpreted as representative of the age of crystallisation or the 207 pb 206 pb ages from zircon xenocrysts have been considered in constructing graphs of cumulative probability versus age of rock formation in this manner it has been possible to identify the main magmatic events that have contributed to the assembly of the craton for convenience of data handling the kaapvaal craton has been subdivided into eastern central northern and western geographic domains these domains are not intended as discrete geological terranes although each one has its own geological and chronological character although consanguinity of events is also evident the oldest rocks so far recognised are located in the swaziland barberton areas where ages 3600 ma have been recorded the early stages of shield development are best exposed in the barberton mountain land where it is now apparent that continent formation took place by magmatic accretion and tectonic amalgamation of small protocontinental blocks at barberton several diachronous blocks formed between 3600 and 3200 ma have been identified each of which represents a cycle of arc related magmatism and sedimentation this phase of crustal development was followed by a period of mesoarchaean cratonic magmatism particularly prevalent between approximately 3100 3000 ma and marked by the development of a major crescent shaped juvenile arc that was accreted onto the northern and western margins of the evolving kaapvaal shield cratonisation was accomplished by the emplacement of major granitoid batholiths which thickened and stabilised the continental crust during the early stages of this cycle subsequent evolution of the craton between 3000 and 2700 ma was associated with continent arc collision during which time the witwatersrand basin and its correlatives were deposited as foreland sequences followed by episodic extension and rifting when the gaborone kanye and ventersdorp sequences were developed

structural signatures and 40ar 39ar geochronology of the indosinian xuefengshan tectonic belt south china block abstract to advance our understanding of the kinematics of the xuefengshan tectonic belt xfstb and the mesozoic tectonics of the south china block scb this paper presents new data and interpretations from our structural studies and thermochronological dating the xfstb is characterized by greenschist facies metamorphism and development of s and s l tectonic fabrics an asymmetric positive flower structural pattern is composed of nw wnw and se ese dipping cleavages faults and shear zones kinematic indicators indicate a dominant top to wnw thrusting with a sinistral strike slip component on the ese dipping shear zones and top to ese back thrusting on the wnw dipping shear zones the quartz c axis orientations of mylonitic rocks exhibit monoclinic point maximum asymmetry indicative of a sinistral shear sense under the a basal gliding condition the timing of the major deformation event has been constrained to the middle triassic to early jurassic 244 195 ma on the basis of 40ar 39ar geochronology and other geological observations the structural characteristics of the xfstb are probably related to an indosinian oblique convergent zone in combination with tectonic wedging and associated back thrusting all above a low angle se dipping basal detachment oblique northwestward and upward movement along the basal detachment was partitioned into the nw wnw directed thrusting se ese directed back thrusting and subsidiary strike slip movement along the ne and nne trending faults and shear zones the xfstb may represent part of a huge structural fan between the yangtze and cathaysian blocks and can be interpreted as a product of the indosinian intracontinental collision involving a weak zone

u pb and re os geochronology and geodynamic setting of the dabaoshan polymetallic deposit northern guangdong province south china abstract the dabaoshan polymetallic deposit in northern guangdong province contains iron copper lead zinc molybdenum tungsten and sulfur mineral resources porphyry type mo w and skarn type mo w mineralization occurs along the internal and external contact zones of the granodioritic porphyry respectively la icp ms u pb dating of zircons from two granodioritic porphyry samples yielded a weighted mean 206 pb 238 u age of 175 8 1 5 ma mswd 0 037 and 175 0 1 7 ma mswd 0 41 they can be pooled together to yield a combined weighted age of 175 4 1 6 ma mswd 0 26 which is interpreted as the emplacement age of the granodioritic porphyry re os dating of three molybdenite samples from porphyry and skarn ores yielded consistent model ages of 163 2 2 3 ma to 165 2 2 4 ma with a weighted mean of 163 9 1 3 ma mswd 0 81 which is the age of mo w mineralization these ages are consistent with the molybdenite re os model age 164 7 3 ma measured by mao et al 2004a for the stratiform cu pb zn orebody and they can yield a weighted mean of 164 0 2 5 ma mswd 0 16 this implies that mo w and cu pb zn mineralization in the dabaoshan polymetallic deposit are the products of one mineralization event the mineralization in the deposit coincides closely with that of mo polymetallic mineralization 164 149 ma elsewhere in the nanling region comprising an important polymetallic metallogenic belt of south china and corresponds to the second episode of mesozoic metallogenesis in south china combined with previous studies we suggest that the dabaoshan polymetallic deposit is related to post collisional lithosphere extension in the nanling region of south china geological data and pb isotopic evolution diagrams together with stable isotopic data of fluid inclusions 18 o 3 75 7 0 d 50 7 to 56 1 and ore sulfides 34 s 2 3 suggest a genetic relationship between the dabaoshan polymetallic deposit the granodioritic porphyry and the dacitic porphyry these data combined with the re content 64 7 to 102 4 ppm of molybdenite indicate that the ore forming components were derived from mixed crustal and mantle sources

the tectonic history of kohistan and its implications for himalayan structure the tectonic history of kohistan northern pakistan involves two collisional events cleavage and folding developed at 90 100 ma along the northern suture between the kohistan island arc and the asian plate at the same time there was major folding and shearing of the lower part of the kohistan arc approximately 100 km south of the suture this deformation was followed by ocean subduction south of the kohistan arc generating the kohistan calc alkaline batholith with subsequent ocean closure during the eocene and obduction of the kohistan arc together with the adjacent part of the asian plate over the indian continental crust the construction of balanced cross sections through the imbricated upper part of the indian continental crust in the footwall to this southern suture indicates a minimum displacement of 470 km requiring the western himalayan hinterland to be underlain by a large wedge of indian middle to lower crust there is some shortening of the overriding kohistan and asian plates by thrusts and shear zones but it is insufficient to satisfy the palaeomagnetic data there must be major crustal shortening involving thrusts in the hindu kush and pamirs north of kohistan the post eocene thrust direction which for most of pakistan is towards 160 is almost perpendicular to that immediately to the east in the himalayan belt generating complex refolded thrust patterns in the hazara syntaxis and large scale folding and rapid uplift with associated brittle faulting and seismic activity adjacent to the nanga parbat syntaxis these different thrust trends indicate that major thrust movement as well as the folds and deformation fabrics cannot always be related to plate movement vectors but are modified by or develop from complex rotations during place collision or from the gravitational spreading of a thickened crust a regional approach is required to recognize and correctly attribute the various components in thrust displacements

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

determination of poisson s ratio from pre critical wide angle seismic reflection data from the babel project summary the pre critical wide angle seismic reflection data collected during the babel survey using multireceiver spreads contain high amplitude p wave and modeconverted s wave reflected arrivals the two phases can be partially separated and their signal to noise ratio emphasized by exploiting the multifold nature and the increased sensitivity of velocity analysis afforded by these data to produce zero offset corrected stacked sections using these sections a method has been developed to derive estimates of poisson s ratio by optimizing the cross correlation of p wave and s wave arrivals and hence to constrain petrological models of the crust sampled by these data although the apparently high degree of temporal and spatial resolution of reflected arrivals that are visible within the stacked sections must be reduced to achieve a satisfactory degree of correlation robustness the results shed some light on crustal petrology and concur with existing estimates of poisson s ratio for the baltic shield

multiple mesozoic mineralization events in south china an introduction to the thematic issue mesozoic mineral deposits in south china include world class deposits of w sn and sb and those that provide the major sources of ta cu hg as tl pb zn au and ag for the entire country these deposits can be classified into polymetallic hydrothermal systems closely related to felsic intrusive rocks sn w mo granites cu porphyries polymetallic and fe skarns and polymetallic vein deposits and low temperature hydrothermal systems with no direct connection to igneous activities mvt deposits epithermal au and sb deposits recent studies have shown that they formed in the triassic indosinian jurassic cretaceous early yanshanian and cretaceous late yanshanian stages indosinian deposits include major mvt pb zn ag deposits and granite related w sn deposits early yanshanian deposits are low temperature sb au and high temperature w sn and cu porphyry types many late yanshanian deposits are low temperature au as sb hg and u deposits and also include high temperature w sn polymetallic deposits the formation of these deposits is linked with a specific tectonothermal evolution and igneous activities this special issue brings together some of the latest information in eight papers that deal with the origins and tectonic environments of mineral deposits formed in these stages we anticipate that this issue will stimulate more interests in these ore deposits in south china

emplacement ages of kimberlite occurrences in the prieska region southwest border of the kaapvaal craton south africa abstract rb sr mica and u pb perovskite emplacement ages for kimberlites from the southwest part of the kaapvaal craton near prieska south africa vary from as old as 170 ma to as young as 74 ma similar to ages elsewhere in southern africa isotopically defined group i and ii kimberlites are found on the kaapvaal craton northeast of the doornberg and brakbos lineaments domains i and ii as defined by skinner et al 1992 with ages of 74 108 and 118 125 ma kimberlites in the craton margin area bounded by the two lineaments domain iii are exclusively group ii with ages of 116 119 ma kimberlites in domain iv to the south of the craton boundary are of group i affinity emplaced in two episodes at 74 and 100 103 ma southernmost domain v is dominated by kimberlites petrographically chemically and isotopically transitional between group i and ii types but with the oldest ages at 140 and possibly 170 ma phlogopite from a peridotite xenolith from the sanddrift kimberlite has retained an age of 2250 ma presumably the enrichment age of the nodule