major types and time space distribution of mesozoic ore deposits in south china and their geodynamic settings the ore deposits of the mesozoic age in south china can be divided into three groups each with different metal associations and spatial distributions and each related to major magmatic events the first event occurred in the late triassic 230 210 ma the second in the mid late jurassic 170 150 ma and the third in the early mid cretaceous 120 80 ma the late triassic magmatic event and associated mineralization is characterized by peraluminous granite related w sn nb ta mineral deposits the triassic ore deposits are considerably disturbed or overprinted by the later jurassic and cretaceous tectono thermal episodes the mid late jurassic magmatic and mineralization events consist of 170 160 ma porphyry skarn cu and pb zn ag vein deposits associated with i type granites and 160 150 ma metaluminous granite related polymetallic w sn deposits the late jurassic metaluminous granite related w sn deposits occur in a ne trending cluster in the interior of south china such as in the nanling area in the early mid cretaceous from about 120 to 80 ma but peaking at 100 90 ma subvolcanic related fe deposits developed and i type calc alkaline granitic intrusions formed porphyry cu mo and porphyry epithermal cu au ag mineral systems whereas s type peraluminous and or metaluminous granitic intrusions formed polymetallic sn deposits these cretaceous mineral deposits cluster in distinct areas and are controlled by pull apart basins along the south china continental margin based on mineral assemblage age and space time distribution of these mineral systems integrated with regional geological data and field observations we suggest that the three magmatic mineralization episodes are the result of distinct geodynamic regimes the triassic peraluminous granites and associated w sn nb ta mineralization formed during post collisional processes involving the south china block the north china craton and the indo china block mostly along the dabie sulu and songma sutures jurassic events were initially related to the shallow oblique subduction of the izanagi plate beneath the eurasian continent at about 175 ma but i type granitoids with porphyry cu and vein type pb zn ag deposits only began to form as a result of the breakup of the subducted plate at 170 160 ma along the nne trending qinzhou hangzhou belt also referred to as qin hang or shi hang belt which is the neoproterozoic suture that amalgamates the yangtze craton and cathaysia block a large subduction slab window is assumed to have formed in the nanling and adjacent areas in the interior of south china triggering the uprise of asthenospheric mantle into the upper crust and leading to the emplacement of metaluminous granitic magma and associated polymetallic w sn mineralization a relatively tectonically quiet period followed between 150 and 135 ma in south china from 135 ma onward the angle of convergence of the izanagi plate changed from oblique to parallel to the coastline resulting in continental extensional tectonics and reactivation of regional scale ne trending faults such as the tan lu fault this widespread extension also promoted the development of ne trending pull apart basins and metamorphic core complexes accompanied by volcanism and the formation of epithermal cu au deposits granite related polymetallic sn w deposits and hydrothermal u deposits between 120 and 80 ma with a peak activity at 100 90 ma

the june 9 bolivia and march 9 fiji deep earthquakes of 1994 i source processes we examined the source characteristics of the 1994 deep earthquakes of bolivia june 9 and fiji march 9 over a period range of 1 1000 s using moment tensor and body wave analyses moment tensor inversions from 100 1000 s reveal that these two earthquakes were essentially point sources at very long period with insignificant isotropic and non double couple components with scalar moments mo 2 56 1021 nm for the bolivia and mo 0 26 1021 nm for the fiji earthquakes body wave analysis for the bolivia event reveals a 45 s duration a nearly linear e ne 1 km s rupture on the subhorizontal focal mechanism plane giving a stress drop 114 mpa for the fiji earthquake we find a change in focal mechanism and a linear 3 km s rupture plunging shallowly to the north on the steep dipping nodal plane with a duration of at least 20 s and 14 mpa both ruptures propagated parallel their null 2 axis and suggests that their width is controlled by the thickness of the seismogenic volume in deep slabs

gold and silver metallogeny of the south china fold belt a consequence of multiple mineralizing events abstract the south china fold belt is part of the south china block that is interpreted to be the result of multiple tectonic and magmatic events that formed a collage of accreted proterozoic and phanerozoic terranes the jurassic to early cretaceous yanshanian period 180 90 ma a time of major tectono thermal events that affected much of eastern and southeastern china is of great metallogenic importance in the fold belt this period is linked to subduction of the pacific plate beneath the eurasian continent and is manifested by voluminous volcano plutonic activity of predominantly calc alkaline affinity the distribution of gold and silver deposits in the south china fold belt indicates the presence of two distinct metallogenic provinces a region of basement uplifts which are controlled by shear zones and form neoproterozoic inliers of metamorphosed iron rich rock types defines the first province in this province orogenic lodes and volcanic related epithermal deposits represent the more significant precious metal mineralization the second province is essentially confined to a belt of yanshanian felsic intermediate volcanic and subvolcanic rocks that extends along most of the southeastern china coast in an area known as the coastal volcanic belt deposits in the coastal volcanic belt are silver and or copper rich volcanic hosted and epithermal in character the precious metal metallogeny of the south china fold belt is interpreted to have developed in at least three stages one as a result of collision events during the caledonian orogeny ca 400 ma the second during the indosinian orogeny ca 200 ma and the third during or soon after the formation of the yanshanian magmatic belt yanshanian orogeny 180 90 ma the latter was responsible for a hydrothermal event that affected large sections of the belt and its proterozoic substrate this may have resulted in the redistribution and enrichment of precious metals from preexisting orogenic gold lodes in neoproterozoic basement rocks which are now exposed as windows in zones of tectonic uplift the yanshanian hydrothermal activity was particularly widespread in the coastal volcanic belt and resulted in the formation of both low and high sulfidation epithermal gold and silver and locally copper and other base metal mineralization it is suggested that the coastal volcanic belt has greater potential for world class epithermal and porphyry deposits than previously realised

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

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

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

timing of the yuchiling giant porphyry mo system and implications for ore genesis the yuchiling mo deposit is a recently discovered giant porphyry system in the east qinling mo belt china its apparent causative intrusion i e the yuchiling granite porphyry is the youngest intrusion phase 4 of the heyu multiphase granite batholith which was emplaced between 143 and 135 ma new robust constraints on the formation of the yuchiling porphyry mo system are provided by combined zircon u pb biotite 40ar 39ar and molybdenite re os dating zircon grains from the mo mineralized granite porphyry yield weighted 206pb 238u age of 134 0 1 4 ma n 19 2 error mswd 0 30 magmatic biotite from the same sample yield a 40ar 39ar plateau age of 135 1 1 4 ma 2 error and an inverse isochron age of 135 6 2 0 ma n 7 2 error mswd 10 8 which are effectively coincident with the zircon u pb age within analytical error three pulses of mineralization can be deduced from the molybdenite re os ages namely 141 137 and 134 ma which agree well with the zircon u pb ages of granitic phases 1 2 and the yuchiling porphyry phase 4 respectively these well constrained temporal correlations indicate that mo mineralization was caused by pulses of granitic magmatism and that the ore forming magmatic hydrothermal activity responsible for the yuchiling porphyry mo system lasted about 8 ma the yuchiling mo deposit represents a unique style of porphyry mo system formed in a post collision setting and associated with f rich high k calc alkaline intrusions which differ from convergent margin associated porphyry mo deposits

timing and formation of porphyry cu mo mineralization in the chuquicamata district northern chile new constraints from the toki cluster the recently discovered toki cluster which includes the toki quetena genoveva miranda and opache porphyry cu mo prospects is located 15 km south southwest of the chuquicamata radomiro tomic mines in northern chile these prospects occur in an area of 5 6 km and are completely covered with neogene alluvial deposits inferred resources for the cluster are estimated at about 20 mt of fine copper with toki and quetena contributing 88 of these resources mineralization in these deposits is associated with tonalite porphyries that intruded andesites and dacites of the collahuasi group and intrusions of the fortuna los picos granodioritic complex hypogene mineralization in the toki cluster consists mainly of chalcopyrite bornite with minor molybdenite with mineralization grading outward to a chalcopyrite pyrite zone and ultimately to a pyrite halo alteration is dominantly of the potassic type with k feldspar and hydrothermal biotite sericitic alteration is relatively restricted to late quartz pyrite veins d type veins previous k ar geochronology for the cluster yielded ages within a range of 34 to 40 ma four new re os ages for toki indicate that molybdenite mineralization occurred in a single pulse at 38 ma re os ages for three different molybdenite samples from quetena are within error of the toki mineralization ages these ages are concordant with a new zircon u pb age of 38 6 0 7 ma from the tonalite porphyry in quetena two re os ages for genoveva 38 1 0 2 and 38 0 0 2 ma are also within error of the toki and quetena molybdenite ages four re os molybdenite ages for opache range between 36 4 and 37 6 ma the miranda prospect is the youngest with an age of 36 ma four new re os ages for the chuquicamata deposit range between 33 and 32 ma whereas nine new 40ar 39ar ages of biotite muscovite and k feldspar range between 32 and 31 ma analyzed molybdenites have re and os concentrations that vary between 21 3 099 ppm and 8 1 231 ppb respectively the highest re and os concentrations are found in the toki prospect three new 40ar 39ar ages for the toki cluster are younger than the re os mineralization ages the age spectra for these three samples show evidence of excess argon and have similar inverse isochron ages of 35 ma that probably reflect a late hydrothermal phyllic event the new geochronological data presented here for the toki cluster indicate that molybdenite mineralization occurred within a very short period probably within 2 ma and synchronously at 38 ma in three mineralization centers toki quetena and genoveva furthermore mineralization at the toki cluster preceded the emplacement of the chuquicamata deposit 35 31 ma and indicates that porphyry cu mo mineralization occurred episodically over a period of several million years in the chuquicamata district

no evidence for a large mesozoic overthrust in the lantian area of anhui province south china abstract this paper presents new geological data from the lantian area of anhui province se china that contradict the hypothesis of the lantian tectonic window one of the key argument in hsu s mesozoic collision tectonic model for south china the 1 50 000 scale geological mapping of the lantian area located along the eastern part of the neoproterozoic jiangnan belt confirms that the sinian cambrian strata unconformably overlies the pre sinian formations and that they form a syncline preserved from early paleozoic deformation pre sinian formations including the banxi group distributed in the jiangnan belt do not form a mesozoic tectonic melange but a normal pre sinian stratigraphic sequence the subsequent tectonic events only slightly transformed the structural framework of this area which does not exhibit a tectonic window thus these geological data call into question the model of huanan alps that invokes large mesozoic overthrust tectonics in south china we believe that early paleozoic tectonic activity has clearly affected the sinian early paleozoic formations in the lantian area and given rise to the jiangnan belt

late and mid cretaceous mineralization in the northern canadian cordillera constraints from re os molybdenite dates new re os molybdenite dates are presented from three deposits casino cash and mt nansen in the dawson range yukon northern canadian cordillera the re os molybdenite dates provide constraints on the timing of sulfide mineralzation and together with 40 ar 39 ar and k ar mineral dates yield estimates for the longevity of the porphyry hydrothermal system at the casino deposit re os molybdenite dates associated with the potassic 75 05 0 28 74 69 0 28 ma alteration are nominally older to overlapping with re os molybdenite dates 74 4 0 28 ma n 2 with phyllic alteration and suggest that it is likely that potassic and phyllic mineralization alteration was contemporaneous the re os molybdenite dates are significantly older than hydrothermal k feldspar 40 ar 39 ar and k ar biotite dates 72 ma which most likely reflect the cessation of the porphyry hydrothermal system the molybdenite age data combined with silicate hydrothermal mineral dates suggest a porphyry hydrothermal system longevity of 4 16 to 1 94 casino 8 66 to 6 44 cash and 2 47 to 1 48 ma mt nansen these longevity estimates for porphyry hydrothermal systems are greater than previously proposed 50 c km may control the disparate re os molybdenite and 40 ar 39 ar k feldspar dates of the porphyry systems of this study the contrasting re os and 40 ar 39 ar dates clearly show the re os isotope system in molybdenite is resistant to prolonged 1 ma interaction with a high temperature 400 500 c hydrothermal fluid and that the re os isotope systematics in molybdenite unambiguously record directly the timing of sulfide mineralization this ability to directly date sulfide mineralization has identified mid pattison idaho creek 102 104 ma and late cretaceous zappa koffee 74 5 ma mineralization which is hosted by the mid cretaceous 104 102 ma casino plutonic suite the late cretaceous date is very similar to that for molybdenite mineralization at casino and we suggest that the zappa koffee mineralization is an area of late cretaceous porphyry mineralization for which the host pluton has not yet been discovered

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

a petrological and fluid inclusion study of magnetite scheelite skarn mineralization at kara northwestern tasmania implications for ore genesis abstract the kara magnetite scheelite deposit is located about 40 km south of burnie in northwestern tasmania the deposit consists of a number of orebodies and the total mineable reserves are 1 7 mt of magnetite at 30 fe and 0 3 mt of scheelite at 0 52 wo3 the major orebodies at kara are hosted by the ordovician gordon limestone at the southern end of the devonian housetop granite adjacent to the granite or separated from it by the ordovician moina sandstone at least four paragenetic stages of skarn formation and ore deposition have been recognized stage i clinopyroxene garnet vesuvianite wollastonite quartz scheelite stage ii garnet vesuvianite magnetite scheelite apatite quartz stage iii magnetite amphibole epidote fluorite quartz chlorite garnet vesuvianite scheelite carbonate pyrite clinopyroxene and stage iv hematite fluorite calcite quartz fluid inclusions in clinopyroxene from stage i homogenize at 460 620 c mode 520 c stage ii gave homogenization temperatures of 349 578 c mode 500 c from scheelite and 360 570 c mode 500 c from vesuvianite the stage i clinopyroxene gave a salinity range of 10 0 12 0 equiv wt nacl with only one low value of 2 0 equiv wt nacl the stage ii mineral assemblages also showed moderate salinities of 12 0 and 17 8 equiv wt nacl from scheelite and vesuvianite fluid inclusions in quartz scheelite calcite and fluorite from the main scheelite mineralization phase stage iii homogenize at 230 360 c with a mode at 300 c and salinity varies from 0 2 to 19 8 equiv wt nacl the mineral paragenetic and fluid inclusion studies indicate that magnetite scheelite mineralization at kara was formed as a proximal skarn assemblage in carbonate host the skarn formation and ore deposition occurred in stages starting with prograde anhydrous clinopyroxene garnet metasomatism at relatively high temperatures 500 c this early assemblage was replaced by the subsequent retrograde deposition of hydrous mineral phases e g amphibole epidote and chlorite as temperatures decreased to 300 c or less deposition of scheelite predominantly occurred in association with hydrous minerals e g amphibole in stage iii the high temperature and moderate to high salinity ore fluids in early anhydrous skarn formation stages i and ii are consistent with fluids of magmatic origin in comparison the lower temperature and variable salinity from high 19 8 equiv wt nacl to as low as 0 2 equiv wt nacl in stage iii suggests a possible involvement of low salinity meteoric waters mixing with high salinity fluids originating from the nearby granite in the later hydrous skarn and ore formation

how long can a hydrothermal system be sustained by a single intrusive event published numerical calculations show that the convective systems which produce near surface geothermal systems or ore deposits are likely to cool their intrusion heat source in at most a few tens of thousands of years even if the intrusion is large generally hydrothermal circulation near surface geothermal activity and magmatic intrusion should be closely tied in time and space long lived approximately 1 m y hydrothermal activity normally suggests multiple pulses of intrusion with associated pulses of hydrothermal circulation however if the permeability of the intruded environment is just above that which allows convection and the volume of the intrusion is very large calculations presented here show that a single episode of intrusion can sustain hydrothermal circulation and near surface geothermal activity for approximately 800 000 yr for a deep sill heat source these unusually long lived hydrothermal systems can vent through a few very widely separated discharge sites and have the potential to produce isolated massive sulfide deposits of unusually large size

re os ages for archean molybdenite and pyrite kuittila kivisuo finland and proterozoic molybdenite kabeliai lithuania testing the chronometer in a metamorphic and metasomatic setting seven 187re 187os ages were determined for molybdenite and pyrite samples from two well dated precambrian intrusions in fennoscandia to examine the sustainability of the re os chronometer in a metamorphic and metasomatic setting using a new 187re decay constant 1 666 10 11y 1 with a much improved uncertainty 0 31 we determined replicate re os ages for molybdenite and pyrite from the kuittila and kivisuo prospects in easternmost finland and for molybdenite from the kabeliai prospect in southernmost lithuania these two localities contain some of the oldest and youngest plutonic activity in fennoscandia and are associated with newly discovered economic au mineralization ilomantsi finland and a cu mo prospect kabeliai lithuania two re os ages for vein hosted kabeliai molybdenite average 1486 5 ma in excellent agreement with a 1505 11 ma u pb zircon age for the hosting kabeliai granite pluton the slightly younger age suggests the introduction of cu mo mineralization by a later phase of the kabeliai magmatic system mean re os ages of 2778 8 ma and 2781 8 ma for kuittila and kivisuo molybdenites respectively are in reasonable agreement with a 2753 5 ma weighted mean u pb zircon age for hosting kuittila tonalite these re os ages agree well with less precise ages of 2789 290 ma for a rb sr whole rock isochron and 2771 75 ma for the average of six sm nd tdm model ages for kuittila tonalite three re os analyses of a single pyrite mineral separate from the same sample of kuittila pluton that yielded a molybdenite separate provide individual model ages of 2710 27 2777 28 and 2830 28 ma re 17 4 12 1 and 8 4 ppb respectively with a mean value of 2770 120 ma in agreement with the kuittila molybdenite age the re and 187os abundances in these three pyrite splits are highly correlated r 0 9994 and provide a 187re 187os isochron age of 2607 47 ma with an intercept of 21 ppt 187os mswd 1 1 it appears that the re os isotopic system in pyrite has been reset on the millimeter scale and that the 21 ppt 187os intercept reflects the in situ decay of 187re during the 160 to 170 m y interval from 2778 ma time of molybdenite pyrite deposition to 2607 ma time of pyrite resetting when the re os data for molybdenites from the nearby kivisuo prospect are plotted together with the kuittila molybdenite and pyrite data a well constrained five point isochron with an age of 2780 8 ma and a 187os intercept 2 4 3 8 ppt of essentially zero results mswd 1 5 we suggest that the pyrite isochron age records a regional metamorphic and or hydrothermal event possibly the time of au mineralization a proposed re os age of 2607 ma for au mineralization is in good agreement with radiometric ages by other methods that address the timing of archean au mineralization in deposits worldwide so called late au model molybdenite in contrast provides a robust re os chronometer retaining its original formation age of 2780 ma despite subsequent metamorphic disturbances in archean and proterozoic time

reliable re os age for molybdenite abstract re os ages for four molybdenites were determined by our newly developed method with isotope dilution mass spectrometry special caution was given to achieve osmium isotopic equilibration between sample and spike during chemical treatment re os data for molybdenites from mt mulgine australia matasvaara finland preissac canada and climax usa define ages of 2790 ma 2600 ma 2680 ma and 26 6 ma respectively the reproducibilities of our re os age determination are shown to be within 2 by the replicate analyses for mt mulgine matasvaara and preissac samples the ages obtained are in good agreement with reference ages estimated for wall rocks of molybdenite deposits concerned thus with careful experimental procedure it appears that the re os geochronometer can be used to reliably determine the age of molybdenite

the study of molybdenites through the187re 187os chronometer we determined the re and os concentrations in 11 molybdenites of various ages 30 2700 m y and from various areas no common osmium has been detected and os was found to be purely radiogenic therefore by comparing the concentrations we were able to determine 187 re 187 os ages for most samples these ages are in good agreement with those measured in surrounding rocks and probably correspond to the age of mineralization however two or three re os ages seem unreliable being probably related to some rhenium leaching through later metamorphic and or hydrothermal events this shows that the petrographic characteristics of ores and the type of crystallization of mos 2 are important in choosing samples suitable for dating re and os determinations combined with careful petrographic investigations may give important clues on the genesis and evolution of molybdenum ores

a preliminary description of the gan hang failed rift southeastern china abstract the gan hang failed rift as defined by present day topography extends at least 450 km in length and 50 km in width it is a northeast southwest trending series of features spanning from hangzhou bay in zhejiang province into jiangxi province through fuzhou city southwest of fuzhou the rift splits into two portions one continuing along the southwestern trend and the other diverging westward the total extent of the rift cannot be defined at this time the rift is superimposed upon a major suture zone of caledonian or early mesozoic age the suture represents the fusing of the south china huanan and yangtze cratons perhaps in late triassic but for sure by late middle jurassic time the rifting was initiated and followed this older suture in part this time corresponds roughly to the middle stage of the yanshanian orogeny and to the subduction of the postulated pacific kula ridge southeast of the continental margin the total thickness of the sediments and volcanics filling the rift valley reaches more than 10 000 m peak intensity of extension was between late middle jurassic and middle to late cretaceous sedimentation within the rift was not continuous and is marked with periodic unconformities sediments within the rift include red beds sandstones siltstones mudstones conglomerates breccias tuffs and ignimbrites vertebrate fossils and dinosaur eggs are also found contemporaneous volcanics within and flanking the rift include basalts rhyolites granites gabbros dacites and andesites silicic volcanics are mostly attributed to caldera systems early basalts are tholeiitic and later change to alkaline olivine basalt bimodal volcanism is recognized peak intensity of volcanism ranges between 135 and 75 ma in early cenozoic time the area was a topographic low paleocene eocene sediments and evaporites are the last rocks to be deposited in the rift today the rift is delineated by major high angle faults the pingxiang guangfeng deep fault belt red beds and volcanically derived u deposits scismic study of p wave velocities has produced an earth model which shows substantial crustal thinning under the gan hang rift the region has been included in the southeastern china mesozoic fault depression system and has not generally been recognized as a distinct failed rift

trace elements u pb ages and hf isotopes of zircons from mesozoic granites in the western nanling range south china implications for petrogenesis and w sn mineralization abstract the e w trending nanling range located in the southern part of the south china block scb is characterized by widespread intra continental mesozoic granitoids and accompanying w sn polymetallic ore deposits detailed studies are presented on trace elements u pb ages and lu hf isotopes in zircons from the mesozoic a2 type like granitoids including the qianlishan qitianling xianghualing huashan guposhan niumiao tong an jinziling and jinjiling plutons from the western nanling range la icp ms u pb analyses of zircon indicate that these granites intruded mainly from ca 160 150 ma two types of zircon magmatic and late magmatic hydrothermal with similar ages can be distinguished based on their zircon trace element compositions the late magmatic hydrothermal zircons have high contents of metallogenic elements e g w and sn and may be closely related with the ore deposits the ehf t values of zircons from the mesozoic granitoids are mildly negative to positive indicating the reworking of both old and young crustal components the significant involvement of mesozoic juvenile mantle derived materials in the source can be excluded because of the rare occurrences of mafic rocks with radiogenic isotopic signatures in the area the average crustal residence age patterns of the mesozoic granites in the western nanling range are similar to those of the neoproterozoic magmatic rocks in the eastern part of the jiangnan orogen which may represent the neoproterozoic collisional belt between the yangtze and cathaysia blocks considering the specific isotopic and metallogenic features of the mesozoic granitoids in the study area we propose that the western nanling range may represent a western extension of neoproterozoic arc continent collisional belt in south china the mesozoic granitoids of this area may have been generated mainly by reworking of deeply buried neoproterozoic arc related volcanic sedimentary materials due to thermal perturbations triggered by the subduction of the paleo pacific plate in mesozoic time

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

dating the giant zhuxi w cu deposit taqian fuchun ore belt in south china using molybdenite re os and muscovite ar ar system abstract the recently discovered zhuxi w cu ore deposit is located within the taqian fuchun ore belt in the southeastern edge of the yangtze block south china its inferred tungsten resources based on new exploration data are more than 280 mt by 2016 at least three paragenetic stages of skarn formation and ore deposition have been recognized prograde skarn stage retrograde stage and hydrothermal sulfide stage secondly greisenization marmorization and hornfels formation are also observed scheelite and chalcopyrite are the dominant metal minerals in the zhuxi deposit and their formation was associated with the emplacement of granite stocks and porphyry dykes intruded into the surrounding carboniferous carbonate sediments huanglong and chuanshan formations and the neoproterozoic slate and phyllites the scheelite was mostly precipitated during the retrograde stage whereas the chalcopyrite was widely precipitated during the hydrothermal sulfide stage a muscovite 40ar 39ar plateau age of about 150 ma is interpreted as the time of tungsten mineralization and molybdenite re os model ages ranging from 145 9 2 0 ma to 148 7 2 2 ma for the subsequent hydrothermal sulfide stage of activity as the time of the copper mineralization our new molybdenite re os and muscovite 40ar 39ar dating results along with previous zircon u pb age data indicate that the hydrothermal activity from the retrograde stage to the last hydrothermal sulfide stage lasted up to 5 myr from 150 6 1 5 to 145 9 1 ma and is approximately coeval or slightly later than the emplacement of the associated granite porphyry and biotite granite the new ages reported here confirm that the zhuxi tungsten deposit represents one of the mesozoic magmatic hydrothermal mineralization events that took place in south china in a setting of lithospheric extension during the late jurassic 160 150 ma it is suggested that mantle material played a role in producing the zhuxi w cu mineralization and associated magmatism

tunnel boring machine collision with an ancient boulder beach during the excavation of the barcelona city subway l10 line a case of adverse geology and resulting engineering solutions the existence of a layer of hard boulders up to 1 m in size within the soft sediments of the holocene llobregat delta in the sw of barcelona city caused the damage and stoppage of the epb type tunnel boring machine that excavated the subway l10 line this layer constitutes a detrital deposit of exceptionally large grain size developed in the base of the delta it originated as an alluvial fan in the northern margin of the delta during the last fall of the mediterranean sea level at the end of the pleistocene period the subsequent sea level rise holocene transgression caused the flooding of this alluvial fan and the reworking of its materials by sea waves thereby a retrogradant boulder beach developed in which the largest rock fragments of the former alluvial fan remained densely packed the tunnel boring machine precisely collided with the main boulder concentration area of this ancient beach the cutter tools and the structure of the cutterhead were seriously affected the repair tasks required the excavation of a mine in the tunnel face under atmospheric pressure and a sealing and stable environment to get these conditions a jet grouting block from the surface was performed to consolidate the deltaic sediments around the tunnel face additional tam injections were carried out to improve some deficiencies in the sealing of the block subsequently the shield of the tunnel boring machine was embedded into the block and the water table was depressed by pumping wells after 13 months of complicated works the tunnel boring machine resumed the excavation encountering further problems although the layer of boulders was early identified by the geological and geotechnical studies of the l10 constructive project its precise geological characteristics and potential adverse influence on tunnelling was then not valued in depth a better knowledge of the geology of this layer would have been helpful in order to foresee the incident and propose alternatives

a revised late eocene age for porphyry cu magmatism in the escondida area northern chile new u pb zircon and 40 ar 39 ar single crystal incremental heating biotite dates are reported for porphyritic rocks from three mineralized centers in the escondida region of northern chile all dates are reported with 2sigma errors at the chimborazo porphyry cu prospect a weighted mean 40 ar 39 ar plateau age of 38 09 or 0 30 ma has been obtained for igneous biotite from a syn or postmineralization feldspar biotite quartz porphyry intrusion at the zaldivar porphyry cu mine zircon and biotite have been analyzed from a similar synmineralization feldspar biotite quartz porphyry the llamo porphyry the zircon u pb age is 38 7 or 1 3 ma whereas the weighted mean 40 ar 39 ar plateau age of igneous biotite is slightly younger at 37 40 or 0 18 ma u pb dating of two other rhyolitic quartz feldspar porphyry bodies that host much of the ore at zaldivar shows that these rocks are paleozoic in age antigua porphyry 290 or 4 ma zaldivar porphyry broadly coeval with antigua but age poorly defined at the escondida deposit the reportedly synmineralization escondida porphyry is dated by u pb at 37 9 or 1 1 ma whereas the reportedly postmineralization rhyolitic porphyry returned a u pb age of 34 7 or 1 7 ma these results indicate a single pulse of synmineralization magmatism in the escondida district at approximately 38 ma followed by minor largely nonmineralizing intrusive events the timing of main stage porphyry mineralization at escondida is apparently bracketed by the ages of the escondida 37 9 or 1 1 ma and rhyolitic porphyries 34 7 or 1 7 ma previously published k ar dates for intrusive rocks and alteration minerals at escondida which range from 39 to 31 ma are interpreted to reflect variable degrees of disturbance of the k ar system by later intrusive events and or by supergene weathering processes an age of approximately 38 ma for ore forming porphyry magmatism in the escondida region is significantly older than previously accepted best estimates based on k ar and u pb dating 32 34 ma but still falls within the age range of other large porphyry systems within the north south trending west fissure zone of northern chile 31 42 ma a common feature of the porphyry intrusions at zaldivar and escondida and also at chuquicamata and el salvador is the presence of zircons with inherited upper paleozoic cores these cores are interpreted to reflect interaction between arc magmas and crustal rocks during melting assimilation storage and homogenization mash and assimilation and fractional crystallization afc processes at various crustal levels

u pb zircon geochemical and sr nd hf isotopic constraints on age and origin of jurassic i and a type granites from central guangdong se china a major igneous event in response to foundering of a subducted flat slab abstract the mesozoic geology of se china is characterized by widespread jurassic to cretaceous igneous rocks consisting predominantly of granites and rhyolites and subordinate mafic lithologies however the tectonic regime responsible for the inland jurassic granites remains controversial we report here u pb zircon ages geochemical and sr nd hf isotopic data for the nankunshan alkaline granite and the fogang granitic batholith in central guangdong mineralogical and geochemical features suggest that the fogang and nankunshan rocks are i and aluminous a type granites respectively shrimp u pb zircon analyses yield consistent ages ranging from 159 3 ma to 165 2 ma for four samples from the fogang batholith and an age of 158 5 ma for the nankunshan granite the fogang granites having i sr 0 7098 0 7136 e nd t 4 3 to 12 2 and e hf t 11 5 to 3 1 for the magmatic zircons were derived from paleoproterozoic mafic intermediate igneous protolith with minor addition of mantle derived magmas the nankunshan rocks have relatively lower i sr 0 706 0 708 higher e nd t 0 3 to 2 4 and e hf t 5 7 to 1 1 for the magmatic zircons and some oib like trace element ratios they were likely generated through extensive fractional crystallization of mantle derived alkaline parental magma associated with crustal assimilation these 160 ma i and a type granites in central guangdong were emplaced coeval with the widespread 165 155 ma i and a type granites and syenites following the initiation of intraplate basaltic and or bimodal igneous magmatism at 180 170 ma in the adjacent regions we interpret these jurassic igneous rocks as anorogenic magmatism formed during a major igneous event in response to foundering of an early mesozoic subducted flat slab beneath se china continent