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

a late cretaceous tin metallogenic event in nanling w sn metallogenic province constraints from u pb ar ar geochronology at the jiepailing sn be f deposit hunan china abstract the jiepailing deposit located in southern hunan province china is a giant sn be f deposit in the nanling w sn province the sn be f mineralization is spatially associated with the jiepailing granite porphyry la mc icp ms zircon u pb dating of the jiepailing granite porphyry yielded a weighted mean 206 pb 238 u age of 90 5 0 9 ma mswd 0 32 which is interpreted as the emplacement age of the granite porphyry hydrothermal muscovite yields a plateau 40 ar 39 ar age of 92 1 0 7 ma mswd 0 9 which is well consistent with the zircon u pb age of the jiepailing granite porphyry responsible for the sn be f mineralization indicating a temporal link between the emplacement of the jiepailing granite porphyry and the sn be f mineralization our new high precise geochronological data suggest that the jiepailing giant sn be f deposit and related granite formed during the early late cretaceous 92 90 ma which provided convincing evidence for a previously unrecognized metallogenic event related to late cretaceous granitic magmatism in nanling region the occurrence of the late cretaceous sn metallogenic event identified in southern hunan province further highlights the importance of systematic metallogenic studies of the nanling w sn province integrated with high precise geochronological data obtained previously it is suggested that the nanling region mainly experienced three tectonomagmatic activities and lithospheric thinning events during mesozoic which are responsible for three sn w metallogenic events to form the world class nanling w sn metallogenic province

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

regional metallogeny of tungsten tin polymetallic deposits in nanling region south china abstract the nanling region gains its reputation for abundant mineral resources especially w sn sb bi pb zn li be ta nb and ion adsorption ree within weathering crust as well as uranium these dominant mineral resources are mainly formed in the yanshanian period with intimate genetic relationship to yanshanian granitoids spatially tungsten and tin deposits separately occurred in the eastern and western parts of nanling region respectively the complicated genetic relationships between tungsten tin polymetallic deposits and nanling granitoids demonstrate a regularity for mineralization we named this metallogenic specialization reflected in the aspects of specialized metallogenic compositions age spatiality alteration and its geological background the metallogenic specialization regional metallogeny is important to discover mineralization lay the foundation a new exploration model named five levels basement was proposed in this paper now it became a prospecting guidance in nanling region recently the ongoing prospecting of large tungsten deposits in nanling and its adjacent region using this model indicates a northward expanding for tungsten deposits in the south part of study area and westward expanding for that in the eastern part in practice the model also can be used in deep prospecting e g deep seated bedded or bed like orebodies as well as not limited to tungsten deposits other mineral deposits

geological and geochronological constraints on the formation of the jurassic maozaishan sn deposit dayishan orefield south china abstract the dayishan orefield in the central nanling belt is one of the most important sn districts in south china within this district there are more than seven medium to large sn sn cu and pb zn sn deposits the mineralization is associated with altered granites and greisen quartz veins both of which occur in the inner part of the dayishan granite batholith cassiterite is the principal ore mineral accompanied by subordinate molybdenite cassiterite separates from altered and mineralized granite and greisen quartz veins have concordia ages of 156 5 2 8 ma mswd 3 1 and 158 0 1 8 ma mswd 1 4 respectively molybdenite separates from molybdenite quartz veins that crosscut the greisen alteration have a re os isochron age of 157 9 7 7 ma mswd 26 whereas zircon separates from the porphyry monzogranite have a u pb age of 156 3 1 2 ma these ages are identical within the analytical error and together with the field relationships provide tight constraints on the sn mineralization at around 156 ma rhenium contents of the molybdenite suggests that the ore forming materials were derived from a crustal source whereas s and pb isotopic data point to mixed sources we suggest that the maozaishan deposit was formed by intrusion of the related pluton and occurred in the late jurassic during a period of lithospheric thinning and crustal extension of the south china block

volcanic markers of the post subduction evolution of baja california and sonora mexico slab tearing versus lithospheric rupture of the gulf of california the study of the geochemical compositions and k ar or ar ar ages of ca 350 neogene and quaternary lavas from baja california the gulf of california and sonora allows us to discuss the nature of their mantle or crustal sources the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement nine petrographic geochemical groups are distinguished regular calc alkaline lavas adakites magnesian andesites and related basalts and basaltic andesites niobium enriched basalts alkali basalts and trachybasalts oceanic morb type basalts tholeiitic transitional basalts and basaltic andesites peralkaline rhyolites comendites and icelandites we show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting three successive stages are distinguished between 23 and 13 ma calc alkaline lavas linked to the subduction of the pacific farallon plate formed the comondu and central coast of the sonora volcanic arc in the extensional domain of western sonora lithospheric mantle derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the basin and range province the end of the farallon subduction was marked by the emplacement of much more complex middle to late miocene volcanic associations between 13 and 7 ma calc alkaline activity became sporadic and was replaced by unusual post subduction magma types including adakites niobium enriched basalts magnesian andesites comendites and icelandites the spatial and temporal distribution of these lavas is consistent with the development of a slab tear evolving into a 200 km wide slab window sub parallel to the trench and extending from the pacific coast of baja california to coastal sonora tholeiitic transitional and alkali basalts of subslab origin ascended through this window and adakites derived from the partial melting of its upper lip relatively close to the trench calc alkaline lavas magnesian andesites and niobium enriched basalts formed from hydrous melting of the supraslab mantle triggered by the uprise of hot pacific asthenosphere through the window during the plio quaternary the no slab regime following the sinking of the old part of the farallon plate within the deep mantle allowed the emplacement of alkali and tholeiitic transitional basalts of deep asthenospheric origin in baja california and sonora the lithospheric rupture connected with the opening of the gulf of california generated a high thermal regime associated to asthenospheric uprise and emplaced quaternary depleted morb type tholeiites this thermal regime also induced partial melting of the thinned lithospheric mantle of the gulf area generating calc alkaline lavas as well as adakites derived from slivers of oceanic crust incorporated within this mantle

crustal structure of the peninsular ranges batholith from magnetic data implications for gulf of california rifting 1 a 70 km wide belt of magnetic highs extends 1200 km northwest from the southern tip of the baja california peninsula into southern california the anomalies are caused by the mafic western belt of the peninsular ranges batholith which is exposed extensively along the northern 800 km length of the magnetic belt modeling indicates that the source of the anomalies extends to mid to lower crustal depths the linearity and undisrupted nature of the magnetic belt support the hypothesis that the peninsula has behaved as a rigid block and has resisted significant 50 km strike slip deformation during the tertiary the absence of a corresponding magnetic belt along the coast of mainland mexico to the south suggests that the location of rifting in the gulf of california was influenced by the mafic western peninsular ranges batholithic crust perhaps because it is thermally and mechanically more resistant to deformation than the surrounding crust

arc rift transition volcanism in the puertecitos volcanic province northeastern baja california mexico the neogene puertecitos volcanic province of northeastern baja california records a transition from arc related volcanic activity to rift volcanism associated with opening of the gulf of california the eastern puertecitos volcanic province is divided into three volcanic sequences based on mapping petrology and 40 ar 39 ar geochronology the lowest sequence comprises early to middle miocene 20 16 ma arc related andesitic lava flows volcanic necks and proximal pyroclastic and epiclastic deposits up to 400 m in thickness with minor basaltic lava flows following the initiation of crustal extension in the region 11 6 ma synrift volcanism produced two rhyolitic sequences that discordantly overlie the arc related rocks the older synrift sequence 6 4 5 8 ma is composed of rhyolite domes and a series of pyroclastic flows up to 300 m thick the upper sequence 3 2 2 7 ma consists of ash flow tuffs and pumice lapilli pyroclastic flows collectively up to 200 m thick minor andesite eruptions followed each episode of silicic synrift volcanism synvolcanic faults produced topographic relief that controlled deposition of the pyroclastic flows and caused gentler dips upsection rhyolite domes are aligned parallel to the predominant north northwest to north northeast fault pattern all three volcanic sequences are calc alkaline however the synrift andesite is characterized by lower k 2 o lower incompatible element concentrations and less fractionation of light rare earth elements than the arc related basalt and andesite this suggests that the primary melts were more primitive for synrift andesite than for the arc related rocks

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

lithospheric extension near lake mead nevada a model for ductile flow in the lower crust small variations in gravity anomalies and topographic elevation observed in areas that have undergone highly variable amounts of upper crustal thinning can be satisfactorily explained by ductile flow of lower crustal material under the proper conditions in this study we examine the boundary between the unextended colorado plateau and a strongly extended domain in the basin and range province in the lake mead nevada region bouguer gravity and topography data suggest that both present and preextensional variations in crustal thickness between the unextended and extended regions are small analytic channel flow models show that viscosities required for ductile flow in a lower crustal channel to reduce discontinuities in crustal thickness associated with variable amounts of extension are highly dependent on the channel thickness and on the length scale of flow required finite element modeling of newtonian flow and power law creep shows that flow over the length scale of the eastern basin and range 500 km or more corresponding to upper crustal extension by a factor of 1 4 3 over 10 m y requires effective viscosities less than 10 18 10 20 pa s for ductile channels 10 25 km thick flow over shorter length scales 150 km may be accommodated with effective viscosities as high as 10 21 pa s modeling suggests that these effective viscosities may be sustained by lower crustal material deforming at laboratory derived power law creep rates the longer scale flow may require elevated crustal temperatures more than 700 c depending on the composition and material properties assumed under the boundary conditions assumed in this study the linear viscous flow models yield a satisfactory approximation to deformation by power law creep this work suggests that flow in the lower crust may be a viable mechanism for producing small variations in total crustal thickness between strongly extended and less extended regions and thereby explaining the relative uniformity in gravity and topography between such regions

age and genesis of the w bi cu f au nui phao deposit northeast vietnam constrains from u pb and ar ar geochronology fluid inclusions study s o isotope systematic and scheelite geochemistry abstract northeast vietnam comprises several tectonic units which host number of metallic deposits of different types from orogenic gold to intrusion hosted pge deposits the lo gam domain being part of the ne vietnam hosts the lang vai gold district a several pb zn deposits and the recently discovered super large w bi cu f au nui phao deposit the nui phao deposit is 87 9 mt at 0 19 wo3 7 95 caf2 0 18 cu 0 19 g t au and 0 09 bi while the measured resource is 25 1 mt at 0 25 wo3 8 4 caf2 0 23 cu 0 26 g t au and 0 108 bi the nui phao deposit is a very interesting object for research due to location of the orebodies between of two granite massifs of the cretaceous da lien and triassic nui phao age consequently the temporal and genetic relationships of mineralization with a certain magmatic events are unclear here we present mineralogical compositional and geochronological data of the nui phao deposit in conjunction with physical chemical researches to understand the ore genesis and its genetic link to a certain magmatic events according to our observations at least three different mineral associations are distinguished at the nui phao deposit which include greisens skarns and quartz sulfide ore skarn and sulfide ore mineral assemblages generally contain abundant scheelite associated with pyrrhotite chalcopyrite and native bismuth the greisens consist of fluorite muscovite and rare molybdenite the results of fluid inclusions study indicate that at the nui phao deposit the initial ore forming fluid was represented by the reduced co2 ch4 h2o nacl cacl2 system characterized by high temperatures and moderate salinities the fluid boiling is the dominant mechanism for ore deposition at the nui phao deposit relatively homogeneous isotopic compositions of oxygen 18oh2o 6 73 to 9 24 and sulfur 34s 1 6 to 3 5 that closely correspond to magmatic source suggest that the ore forming fluid was generated from cooling granite pluton without contribution meteoric water the systematic studies of scheelite composition show that ree contents decrease from early to late scheelite generations which suggest gradual ree depletion of the ore forming fluid without significant fractionation between lree mree and hree the rees of scheelite are mainly controlled by the substitution mechanisms ca2 w6 ree3 nb5 and 3ca2 2ree3 ca by combining ree features y ho ratios and other characteristics of the nui phao deposit we propose a single magmatic source for hydrothermal ore firming fluids without fluid mixing a significantly contribution of material during fluid rock interaction is also unlikely the combination of mineralogical and geochemical features as well as complex studies of the formation conditions allowed us to conclude that gold at the nui phao deposit occur as impurities in the native bismuth the ages of the da lien and nui phao granite massifs and the nui phao deposit have been estimated to be ca 84 ma ar ar 244 ma u pb and ca 85 ma ar ar respectively indicating a genetic relationship between the mineralization and the cretaceous magmatic event

silurian s type granite related w mo mineralization in the nanling range south china a case study of the pingtan w mo deposit abstract the pingtan deposit in southwestern hunan province is a newly discovered large sized w mo deposit this deposit is situated in the northwestern part of the miao ershan granite batholith western nanling range the tungsten mineralization is mainly hosted in biotite monzogranite and the alteration related to mineralization is controlled by a ne trending fault zone in this paper we present new molybdenite re os and zircon u pb ages zircon lu hf isotope and ree geochemical data aiming to constrain the w mo mineralization age and ore hosting granite petrogenesis in the pingtan w mo deposit molybdenite re os dating of eight ore samples yielded a weighted average model age of 427 0 5 4 ma mswd 0 24 the la icp ms zircon u pb analysis of two biotite monzogranite yielded 206pb 238u ages of 430 8 2 4 ma and 431 0 1 8 ma respectively suggesting that the w mo mineralization is spatial temporally associated with the biotite monzogranite the ore related granites have high contents of sio2 and k2o belong to weakly to strongly peraluminous a cnk 1 00 1 30 and geochemically classified as s type affinity molybdenite re contents 2 262 7 297 g g zircon ehf t 10 34 5 87 and 176hf 177hf 0 282241 0 282376 values of the biotite monzogranite suggest that the ore forming materials are predominantly derived from the crust this is the first identification of tungsten mineralization related to silurian s type granites caledonian granites in the miao ershan granite intrusion indicating caledonian is an important mineralization period of w in the western part of nanling range south china

dynamic link between the level of ductile crustal flow and style of normal faulting of brittle crust abstract in a rheologically layered crust compositional layers have an upper elasto plastic part and a lower viscous one when broken the upper elastic part undergoes flexure which is upward for the foot wall and downward for the hanging wall as a consequence of bending stresses will develop locally that can overcome the strength of the plate and therefore impose the migration of active fault in the lower viscous part of each compositional layer rocks can potentially flow numerical modelling of the behaviour of a crust made up of two compositional layers during and following extension shows that flow can take place not only in the lower crust but also and more importantly in the lower part of the upper crust the ability of crustal rocks to flow influences the style and kinematics of rifted regions when no flow occurs subsidence will affect the extending areas both hanging wall and foot wall will subside with respect to an absolute reference frame such as sea level and there will be a strict proportionality between extension and thinning in addition the downward movement of the fault blocks will decrease the local stresses created in the foot wall and increase those of the hanging wall thereby imposing a migration of fault towards the hanging wall this is the behaviour of extensional settings developed on stabilised crust and which evolved in a passive margin when flow does take place middle crustal rocks will move towards the rifting zone causing isostatically driven upward movements that will be superimposed on movements associated with crustal and lithospheric thinning consequently fault blocks will move upwards and the crust will show more extension than thinning the upward movements will decrease the stresses developed in the hanging walls and increase those of the foot wall faults will then migrate towards the foot wall such a mode of deformation is expected in regions with thickened crust and has its most apparent expression in core complexes

mineralogical evidence for magmatic and hydrothermal processes in the qitianling oxidized tin bearing granite hunan south china emp and mc la icpms investigations of three types of titanite abstract the qitianling granite which is located in hunan province southern china is a metaluminous sn granite that crystallized at relatively oxidized conditions between the ni nio nno and fe 2 o 3 fe 3 o 4 mh buffers titanite sphene catisio 5 is a common tin carrier mineral in the qitianling granite based on electron microprobe emp and multi collector mc laser ablation la icpms investigations we describe three types of titanite with various textures and sno 2 contents which probably formed during the magmatic and hydrothermal processes responsible for the granite crystallization and later evolution the form of type i titanite ranges from subhedral to euhedral coarse grained crystals and typically exhibits an envelope like shape and sector zoning it is characterized by a high fe 2 o 3 content on average 2 44 wt and low sno 2 0 43 wt which indicates crystallization during an early magmatic stage its formation temperature is estimated to have been 721 to 780 c from emp and la icpms using the zr in titanite geothermometer type ii titanite occurs as both discrete grains and symplectitic intergrowths and typically fills interstices between other minerals it contains up to 26 wt sno 2 which suggests a late magmatic origin chondrite normalized ree patterns for type ii titanite reveal a lanthanide tetrad effect that was linked to differentiation of the granite magma type iii titanite is restricted to cleavage cracks in chlorite and is rich in al 2 o 3 it is obviously the secondary product of biotite alteration at a temperature that is estimated to be lower than 400 c during the hydrothermal stage this hydrothermal activity also impacted the primary titanites type i titanite experienced alteration to different degrees via coupled dissolution reprecipitation which gave rise to the formation of secondary minerals including cassiterite micro grains some grains of the type ii titanite reacted with fluids along the grain boundary which led to the gradual leaching of sn based on information from these types of titanite and their sn behavior we discuss the progressive enrichment of sn during and after granite crystallization under a high oxidation state as well as the predominant role of fluids in the leaching concentration and transportation of sn during hydrothermal processes which favors vein type sn mineralization

convective upwelling in the mantle beneath the gulf of california in the past six million years baja california has rifted obliquely apart from north america opening up the gulf of california between transform faults seafloor spreading and rifting is well established in several basins other than hotspot dominated iceland the gulf of california is the only part of the world s seafloor spreading system that has been surrounded by enough seismometers to provide horizontal resolution of upper mantle structure at a scale of 100 kilometres over a distance great enough to include several spreading segments such resolution is needed to address the long standing debate about the relative importance of dynamic and passive upwelling in the shallow mantle beneath spreading centres here we use rayleigh wave tomography to image the shear velocity in the upper 200 kilometres or so of the mantle low shear velocities similar to those beneath the east pacific rise oceanic spreading centre underlie the entire length of the gulf but there are three concentrated locations of anomalously low velocities spaced about 250 kilometres apart these anomalies are 40 to 90 kilometres beneath the surface at which depths petrological studies indicate that extensive melting of passively upwelling mantle should begin we interpret these seismic velocity anomalies as indicating that partial melting triggers dynamic upwelling driven by either the buoyancy of retained melt or by the reduced density of depleted mantle

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

geological characteristics and re_os dating of molybdenites in hehuaping tin polymetallic deposit southern hunan province the hehuaping tin polymetallic deposit a large tin ore field newly discovered in southern hunan consists of skarn and cassiterite sulfide ores at present the controlled reserves amount to about 110 700 tons of tin 10 000 tons of bismuth 50 000 tons of lead and 10 000 tons of zinc with the advance of the exploration work four ore bodies have been explored which are located in the southeast portion of wangxianling indosinian gra nite pluton and hosted in granite granite porphyry and devonian limestone and sandstone respectively re os dating of molybdenites yields model ages between 223 3 2 9 and 225 2 2 7 ma and an isochron age of 224 0 1 9 ma for the skarn ore related to wangxianling granite indicating that w sn mineralization in southern hunan also took place in indosinian period the zircon shrimp u pb dating of the late granite porphyry is 142 3 ma the granite porphyry has been partly mineralized and altered and contains tin ore bodies all this implies that there existed late mineralization of yanshanian period in hehuaping area the early mineralization is supposed to correspond to underplating of the basic magma

geology and genesis of the hehuaping magnesian skarn type cassiterite sulfide deposit hunan province southern china abstract magnesian skarn type tin deposits are relatively rare in the world the hehuaping cassiterite sulfide deposit in southern china having a total reserve of approximately 130 000 t of tin 50 000 t of lead and 10 000 t of zinc is identified as such type the deposit is related to the late jurassic 157 ma hehuaping medium to coarse grained biotite granite that intruded the middle devonian qiziqiao dolomite formation and the tiaomajian sandstone formation four paragenetic stages of skarn and ore formation have been recognized i prograde stage ii retrograde stage iii cassiterite sulfide stage and iv carbonate stage alteration zoning between fresh granite and unaltered country rocks can be identified the skarn are typified by mg mineral assemblages of forsterite spinel diopside tremolite serpentine talc and phlogopite the geochemistry of various skarn minerals shows a gradually decrease of mg end member and correspondingly an increase of fe and especially mn end members along the process of skarn alteration tin mineralization developed during the late retrograde stage resulted in cassiterite magnetite diopside skarn however the deposition of cassiterite occurred predominantly as cassiterite sulfide veins along fractures and interlayer fracture zones during stage iii the petrogeochemistry of hehuaping granite as well as s and pb isotopic analyses suggest that the ore forming elements have a magmatic source originated from the upper crust the h o isotopic and fluid inclusion analyses indicate that high temperature ore forming fluids in early anhydrous skarn stage stage i are also magmatic origin in comparison the retrograde fluids are characterized by relatively low salinity 2 to 10 wt nacl equiv and low temperature 220 to 300 c suggesting a mixed origin of meteoric waters with magmatic fluids the major ore forming stage iii fluids are characterized by lower temperature 170 to 240 c and salinity 1 to 6 wt nacl equiv indicating fluid mixing could be an efficient tin mineralizing mechanism meteoric waters are dominant in stage iv resulting in a further lowering of temperature 130 to 200 c and salinity 0 4 to 1 wt nacl equiv

surface wave studies of the sierra madre occidental of northern mexico abstract seismograms originating from earthquakes in the gulf of california and recorded in west texas txar were used to construct rayleigh wave group velocity dispersion curves for propagation paths in northern mexico these paths were approximately orthogonal to boundaries of several different tectonic provinces including the gulf of california the sonoran basin and range the central plateau of mexico and the sierra madre occidental smo thus the effects of horizontal refraction on the rayleigh waves were neglected prior surface wave studies have determined velocity structures for the crust and upper mantle for all of these provinces except the smo and the results of these studies were used to generate rayleigh wave dispersion curves for each segment of the path the group delays associated with each province were calculated and subtracted from the observed dispersion curve for the entire path thus isolating the effects of the smo on group velocity dispersion between periods of 12 and 40 sec linearized inversions showed that the most realistic geophysical model that fits the smo dispersion data consisted of a 55 km thick crust the crustal velocity structure for the smo consists of three layers a thin 5 km low velocity 2 8 km sec 1 l upper layer a middle layer approximately 20 km with a mean shear velocity near 3 6 km sec 1 l and a third lower crustal layer 30 km with shear velocity of about 4 0 km sec 1 l the mantle has shear velocities between 4 55 and 4 7 km sec 1 l but the thickness of the mantle lid is poorly resolved

shear wave velocity structure in the northern basin and range province from the combined analysis of receiver functions and surface waves a new method based on the joint inversion of receiver functions and surface wave phase velocities results in well determined shear velocity structures that are consistent with the compressional wave structure gravity heat flow and elevation data in the northern basin and range this new inversion method takes advantage of average velocity information present in the surface wave method and differential velocity information contained in the receiver function method thus min imizing the nonuniqueness problem that results from the velocity depth trade off an unusually thick 38 kin and relatively faster crust and upper mantle are found in central and eastern nevada compared to the thin 28 to 34 km and slower crust and upper mantle of the western basin and range we interpret the regions of thicker and faster crust and upper mantle as zones that have undergone less cenozoic ex tension relative to the surrounding regions to the west and north the thick crust and consequently greater depth to the dense mantle material is consistent with the gravity low pattern in central and eastern nevada simple gravity modeling shows both local and regional isostatic compensation occur within 40 km of the surface indicating a near classical airy type of compensation in the province we analyze in detail the shear wave s wave velocity model derived from the receiver functions at station bmn and compressional wave p wave velocity models derived from the 1986 passcal experiment in northwestern nevada the most interesting feature of these models is the presence of negative velocity gradients in the s wave model with no corresponding velocity decrease in the p wave models between depths of 10 and 24 km this combined velocity model may be explained by high pore fluid pressures at these depths this model favors a layered fluid porosity model proposed in the literature to explain extensive middle to lower crust conti nental seismic reflections and high electrical conductivity an upper mantle grada tional low velocity zone is present between 32 and 38 km in the s wave model this upper mantle shear wave low velocity zone is consistent with partial melt which may be the source material for magmatic underplating in this region

crustal thickness of the peninsular ranges and gulf extensional province in the californias we estimate crustal thickness along an east west transect of the baja california peninsula and gulf of california mexico and investigate its relationship to surface elevation and crustal extension we derive moho depth estimates from p to s converted phases identified on teleseismic recordings at 11 temporary broadband seismic stations deployed at 318n latitude depth to the moho is 33 63 km near the pacific coast of baja california and increases gradually toward the east reaching a maximum depth of 40 64 km beneath the western part of the peninsular ranges batholith the crust then thins rapidly under the topographically high eastern peninsular ranges and across the main gulf escarpment crustal thickness is 15 18 62 km within and on the margins of the gulf of california the moho shallowing beneath the eastern peninsular ranges represents an average apparent westward dip of 258 this range of moho depths within the peninsula ranges as well as the sharp east west gradient in depth in the eastern part of the range is in agreement with earlier observations from north of the international border the moho depth variations do not correlate with topography of the eastern batholith these findings suggest that a steeply dipping moho is a regional feature beneath the eastern peninsular ranges and that a local airy crustal root does not support the highest elevations we suggest that moho shallowing under the eastern peninsular ranges reflects extensional deformation of the lower crust in response to adjacent rifting of the gulf extensional province that commenced in the late cenozoic support of the eastern peninsular ranges topography may be achieved through a combination of flexural support and lateral density variations in the crust and or upper mantle

seismic structure beneath the gulf of california a contribution from group velocity measurements rayleigh wave group velocity dispersion measurements from local and regional earthquakes are used to interpret the lithospheric structure in the gulf of california region we compute group velocity maps for rayleigh waves from 10 to 150 s using earthquakes recorded by broad band stations of the network of autonomously recording seismographs in baja california and mexico mainland unm in mexico bor dpp and gor in southern california and tuc in arizona the study area is gridded in 120 longitude cells by 180 latitude cells with an equal spacing of 10 10 km assuming that each gridpoint is laterally homogeneous for each period the tomographic maps are inverted to produce a 3 d lithospheric shear wave velocity model for the region near the gulf of california rift axis we found three prominent low shear wave velocity regions which are associated with mantle upwelling near the cerro prieto volcanic field the ballenas transform fault and the east pacific rise upwelling of the mantle at lithospheric and asthenospheric depths characterizes most of the gulf this more detailed finding is new when compared to previous surface wave studies in the region a low velocity zone in northcentral baja at 28on which extends east south eastwards is interpreted as an asthenospheric window in addition we also identify a well defined high velocity zone in the upper mantle beneath central western baja california which correlates with the previously interpreted location of the stalled guadalupe and magdalena microplates we interpret locations of the fossil slab and slab window in light of the distribution of unique post subduction volcanic rocks in the gulf of california and baja california we also observe a high velocity anomaly at 50 km depth extending down to 130 km near the southwestern baja coastline and beneath baja which may represent another remnant of the farallon slab

thermal evolution of a sheared continental margin insights from the ballenas transform in baja california mexico abstract the ballenas transform margin in central baja california offers an unparalleled opportunity to study the thermal behaviour of a sheared continental margin during various stages of its evolution apatite fission track and u th he results from two transects perpendicular to the coast reveal a pronounced latest pliocene to pleistocene 1 8 ma heating event related to the neogene opening of the gulf of california proximity to a regional pre rift unconformity indicates that samples remained at near surface levels since paleogene unroofing despite having experienced reheating to maximum paleotemperatures within or above the fission track partial annealing zone in general maximum paleotemperatures during overprinting decrease from 100 120 c near the coast to below 60 c ca 5 8 km further inland suggesting lateral heat flow from a source within the gulf of california heat sources related to the structural development of the ballenas transform fault located approximately 1 5 4 5 km offshore from the two sample transects most likely controlled the observed reheating overprinting patterns do not support conductive reheating due to reburial magmatism or frictional shear instead a pronounced thermal spike in between much less overprinted neighbouring samples strongly favours convective heating by hydrothermal fluids as the dominant overprinting process hydrothermal activity may be caused by either deep fluid circulation along newly formed shear zones of the transform fault or more likely magmatic leaking along the transform fault latest pliocene to pleistocene 1 8 ma activity on the ballenas transform fault is closely linked to extension in the lower and upper delfin basins and provides a minimum age for the structural reorganisation and the relocation of extension in the northern gulf of california this study shows that hydrothermal activity can cause significant thermal events in a transform margin before the passage of the spreading centre paleotemperatures from the ballenas transform are similar to those of other transform margins after passage of a spreading centre which suggests that hydrothermal fluids may have an important thermal buffering effect moderating the maximum temperatures of the ridge segments near their intersection with a continental transform margin

in situ sims u pb dating of hydrothermal rutile reliable age for the zhesang carlin type gold deposit in the golden triangle region sw china the contiguous region between guangxi guizhou and yunnan commonly referred to as the golden triangle region in sw china hosts many carlin type gold deposits previously the ages of the gold mineralization in this region have not been well constrained due to the lack of suitable minerals for radiometric dating this paper reports the first sims u pb age of hydrothermal rutile crystals for the zhesang carlin type gold deposit in the region the hydrothermal u bearing rutile associated with gold bearing sulfides in the deposit yields an u pb age of 213 6 5 4 ma which is within the range of the previously reported arsenopyrite re os isochron ages 204 19 to 235 33 ma for three other carlin type gold deposits in the region our new and more precise rutile u pb age confirms that the gold mineralization was contemporaneous with the triassic w sn mineralization and associated granitic magmatism in the surrounding regions based on the temporal correlation we postulate that coeval granitic plutons may be present at greater depths in the golden triangle region and that the formation of the carlin type gold deposits is probably linked to the coeval granitic magmatism in the region this study clearly demonstrates that in situ rutile u pb dating is a robust tool for the geochronogical study of hydrothermal deposits that contain hydrothermal rutile

temporal consistency between granite evolution and tungsten mineralization in huamei ao southern jiangxi province china evidence from precise zircon u pb molybdenite re os and muscovite 40ar 39ar isotope geochronology abstract the large huamei ao tungsten deposit with total wo 3 reserves of 67 400 tons at an average grade of 1 334 wo 3 is located in the convergent zone of the eastern nanling e w trending tectono magmatic belt and the western wuyishan nne ssw trending tectono magmatic belt in southern jiangxi province china the tungsten mineralization in this deposit is mainly found in quartz wolframite veins with most orebodies distributed at the outer contact zone between concealed late jurassic granitic stocks and sinian weakly metamorphosed sandstones and phyllites zircons collected from medium to fine grained biotite granite in a diamond drill hole at a sea level of ca 10 m yield a crystallization age of 159 9 1 2 ma through laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms u pb dating molybdenite and muscovite that were both separated from quartz wolframite veins yield a re os isochron age of 158 5 3 3 ma and an 40 ar 39 ar weighted plateau age of 157 9 1 1 ma respectively these dates obtained via three independent geochronological techniques constrain the ore forming age of the huamei ao deposit and link the genesis of the ore and the underlying granite analyses of available high precision zircon u pb molybdenite re os and muscovite 40 ar 39 ar radiometric ages of major w sn deposits in southern jiangxi province indicate that there is no significant time interval between w sn mineralization and its intimately associated parent granite emplacement interval of 0 6 ma these deposits formed over three intervals during the mesozoic 240 210 170 150 and 130 90 ma with large scale w sn mineralization occurring mainly between 160 and 150 ma the majority of w sn deposits in this region are located in southern jiangxi and southern hunan provinces

rayleigh wave dispersion measurements reveal low velocity zones beneath the new crust in the gulf of california rayleigh wave tomography provides images of the shallow mantle shear wave velocity structure beneath the gulf of california low velocity zones lvzs are found on axis between 26 and 50 km depth beneath the guaymas basin but mostly off axis under the other rift basins with the largest feature underlying the ballenas transform fault we interpret the broadly distributed lvzs as regions of partial melting in a solid mantle matrix the pathway for melt migration and focusing is more complex than an axis centered source aligned above a deeper region of mantle melt and likely reflects the magmatic evolution of rift segments we also consider the existence of solid lower continental crust in the gulf north of the guaymas basin where the association of the lvzs with asthenospheric upwelling suggests lateral flow assisted by a heat source these results provide key constraints for numerical models of mantle upwelling and melt focusing in this young oblique rift

carbon release by off axis magmatism in a young sedimented spreading centre continental rifting creates narrow ocean basins where coastal ocean upwelling and enhanced silicate weathering remove carbon dioxide from the atmosphere evidence from seismic data sonar backscatter and seafloor images and geochemical water analyses suggest that in young sedimented rifts active magmatism occurs in a broader region than appreciated and releases carbon from the sediments

thick deltaic sedimentation and detachment faulting delay the onset of continental rupture in the northern gulf of california analysis of seismic reflection profiles 1 the transition from distributed continental extension to the rupture of continental lithosphere is imaged in the northern gulf of california across the obliquely conjugate tiburon upper delfin basin segment structural mapping on a 5 20 km grid of seismic reflection lines of petroleos mexicanos demonstrates that 1000 extension is accommodated on a series of nne striking listric normal faults that merge at depth into a detachment fault the detachment juxtaposes a late neogene marine sequence over thinned continental crust and contains an intrabasinal divide due to footwall uplift two northwest striking dextral oblique faults bound both ends of the detachment and shear the continental crust parallel to the tectonic transport a regional unconformity in the upper 0 5 s two way travel time and crest erosion of rollover anticlines above the detachment indicates inversion and footwall uplift during the lithospheric rupture in the upper delfin and lower delfin basins the maximum length of new crust in both delfin basins is less than 40 km based on the lack of an acoustic basement and the absence of a lower sedimentary sequence beneath a wedge shaped upper sequence that reaches 5 km in thickness a fundamental difference exists between the tiburon delfin segment and the guaymas segment to the south in terms of presence of low angle normal faults and amount of new oceanic lithosphere which we attribute to thermal insulation diffuse upper plate extension and slip on low angle normal faults engendered by a thick sedimentary lid

mode of extension and rifting history of upper tiburon and upper delfin basins northern gulf of california 1 the crustal structure of the northern gulf of california transtensional margin has been investigated by a 280 km long nw se profile including deep multichannel seismic reflection and densely sampled refraction wide angle reflection seismic information combined with gravity modeling the seismic and gravity modeling constrains two thinned crustal areas corresponding to the upper delfin and the upper tiburon basins on both sides of the profile toward the baja california peninsula and the mexico mainland a progressive thickening of the continental crust is observed our results indicate that the crustal thickness is 19 km below the coastline and it decreases to 14 and 17 km below the upper delfin and upper tiburon basins respectively in the area between both basins the crust thickens to 19 5 km there are significant lateral thickness variations for the different levels of the crust the interpreted structure is consistent with the existence of an aborted rift below the upper tiburon basin prominent dipping reflections in the multichannel data under upper tiburon basin and the ridge between upper tiburon and upper delfin basins can be explained as a mylonite like zone related to a detachment fault this interpretation suggests that the structural evolution of upper tiburon basin could be controlled by a major fault that cuts through the upper crust and merges into a zone of subhorizontal reflections in the lower crust the mode and locus of extension have evolved from a core complex in upper tiburon to a narrow rift mode in upper delfin basin

rapid localization of pacific north america plate motion in the gulf of california correlation of late miocene volcaniclastic strata across the northern gulf of california shows that the pacific north america plate boundary localized east of the baja california peninsula ca 6 ma dextral offset of the 12 6 ma tuff of san felipe and a pair of overlying ca 6 3 ma pyroclastic flows indicate at least 255 10 km of displacement along an azimuth of 310 isopach and facies trends of the tuff of san felipe support no more than a few tens of kilometers of additional dextral displacement between 12 6 and 6 3 ma these constraints indicate that nearly all of the dextral displacement between the pacific and north american plates prior to 6 3 ma was accommodated outside of the gulf region and by 4 7 ma the plate boundary motion was localized in the gulf of california although continental extension has accounted for a component of plate boundary motion in northwestern mexico since cessation of subduction offshore of southern baja california at 12 5 ma transfer of baja california to the pacific plate was delayed by at least 6 7 m y

constraints of multiple dating of the qingshan tungsten deposit on the triassic w sn mineralization in the nanling region south china abstract the qingshan tungsten deposit is a quartz vein type ore deposit in the nanling region south china and consists of orebodies along the contact between ordovician sandstone and the mesozoic keshuling granitic pluton there are more than 54 ore bearing quartz veins controlled by ne and nne trending faults the principal ore minerals are wolframite and cassiterite with subordinate molybdenite which formed dominantly during the early oxide and oxide sulfide ore forming stage together with extensive greisenization adjacent to the w bearing quartz veins the gangue minerals are mainly quartz and muscovite zircon grains from the keshuling pluton have a u pb age of 231 9 1 0 ma mswd 4 0 molybdenite separates from the ore bearing quartz veins have a well defined 187re 187os isochron age of 228 7 2 5 ma mswd 0 36 with model ages ranging from 227 6 3 3 to 229 7 3 2 ma hydrothermal muscovite from the greisen between the granite and country rocks has a flat 40ar 39ar plateau age spectra yielding a 40ar 39ar plateau age of 229 4 1 4 ma mswd 0 34 the u pb re os and 40ar 39ar ages are nearly identical within analytical uncertainties and these ages together with the field relationships provide tight constraints on the w sn mineralization at around 230 ma this multiple dating study reflects that the duration of the hydrothermal activity could be less than several million years our new results suggest that the triassic large scale w sn mineralization occurred throughout the nanling region

ore forming mechanism of quartz vein type w sn deposits of the xitian district in se china implications from the trace element analysis of wolframite and investigation of fluid inclusions abstract the xitian w sn district is located in eastern hunan province of se china and comprises the heshuxia goudalan and longshang w sn deposits our study on the occurrence mineral assemblage and geochemistry of the orebodies shows that quartz vein orebodies are dominant mode of mineralization and can be divided into three stages stage i quartz molybdenite wolframite i stage ii quartz wolframite ii polymetallic sulfides and stage iii quartz pyrite fluorite the elements nb ta sc sn zn cr v mo cu and zr are abundant in both wolframite i and wolframite ii wolframite i contains higher concentrations of nb 1408 9245 ppm ta 241 2407 ppm and sc 123 521 ppm than wolframite ii nb 144 1080 ppm ta 3 76 315 ppm and sc 1 86 11 44 ppm these results indicate that wolframite i was formed closer to the host rock and at a greater ore forming depth than wolframite ii the total ree concentrations of wolframite i and ii range from 34 96 to 133 36 ppm and 21 30 to 43 73 ppm respectively both wolframite i and wolframite ii have low lree concentrations 0 00 0 05 and 0 08 8 18 ppm respectively and high hree concentrations 34 96 133 31 and 21 23 41 87 ppm respectively the enrichment of hrees in wolframite may be due to the fact that the sizes of the hrees3 0 94 1 02 a in combination with nb5 or ta5 0 72 a are closer to that of w6 0 68 a in combination with ca2 1 08 a or mg2 0 80 a than that of the lrees3 1 03 1 13 a making the coupled substitution easier a study of fluid inclusions in the coexisting gangue minerals shows that the homogenization temperatures of the fluid inclusions decrease from stage i to stage iii stage i 187 382 c average 275 c stage ii 122 278 c average 175 c stage iii 92 172 c average 139 c whereas the salinities of the fluid inclusions increase from stages i to ii and decrease from stages ii to iii stage i 1 8 18 2 wt nacleqv average 10 3 wt nacleqv stage ii 7 3 24 4 wt nacleqv average 14 9 wt nacleqv stage iii 0 2 5 3 wt nacleqv average 1 5 wt nacleqv raman spectroscopy reveals that the fluid inclusions mainly contain h2o co2 is minor and h2s ch4 and n2 are rare we conclude that the formation of wolframite was driven by post magmatic thermodynamic processes the ore forming fluid flowed through a multi fractured low pressure zone in the fault system in the district and a temperature pressure decrease led to fluid immiscibility characterized by co2 escaping in a low ph and high eh environment in stage i crust mantle mixed fluid and meteoric water mixing in stage ii and natural cooling of the fluid system in stage iii

an attenuation study of body waves in the south central region of the gulf of california mexico abstract we studied the seismic attenuation of body waves in the south central region of the gulf of california goc with records from the network of autonomously recording seismographs of baja california nars baja from the centro de investigacion cientifica y de educacion superior de ensenada s broadband seismological network of the goc resban and from the ocean bottom seismographs obs deployed as part of the sea of cortez ocean bottom array experiment scooba we examine 27 well located earthquakes reported in sumy et al 2013 that occurred from october 2005 to october 2006 with magnitudes m w between 3 5 and 4 8 we estimated s wave site effects by calculating horizontal to vertical spectral ratios and determined attenuation functions with a nonparametric model by inverting the observed spectral amplitudes of 21 frequencies between 0 13 and 12 59 hz for the scooba obs stations and 19 frequencies between 0 16 and 7 94 hz for nars baja and resban stations we calculated the geometrical spreading and the attenuation 1 q factors for two distance intervals 10 120 and 120 220 km respectively for each frequency considered the estimates of q obtained with the scooba obs records for the interval 10 120 km indicate that the p waves attenuate more than s waves q p 34 1 2 f 0 82 0 10 q s 59 1 1 f 0 90 0 03 for frequencies between 0 6 and 12 6 hz whereas for the 120 220 km interval where ray paths travel deeper s waves attenuate more than p waves q p 117 1 3 f 0 44 0 19 q s 51 1 2 f 1 12 0 11 the estimates of q obtained using nars baja and resban records within 10 120 km indicate that p waves attenuate more than s waves q p 69 1 2 f 0 87 0 16 q s 176 1 4 f 0 61 0 26 at frequencies between 0 3 and 6 3 hz whereas at the 120 220 km distance interval s waves attenuate slightly more than p waves q p 39 1 1 f 0 64 0 06 q s 48 1 1 f 0 37 0 07 at high frequencies f 3 hz these results based on a unique obs dataset provide an indirect mean to constrain future models of the thermal structure beneath the goc

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

an early cretaceous w sn deposit and its implications in southeast coastal metallogenic belt constraints from u pb re os ar ar geochronology at the feie shan w sn deposit se china the feie shan w sn deposit is in eastern guangdong province which is a newly explored deposit in the southeastern coastal metallogenic belt in order to constrain the age of mineralization in the feie shan w sn deposit we collected molybdenites and hydrothermal biotite of the feie shan w sn deposit for dating the tungsten mineralization is spatially associated with the biotite monzonitic granite porphyry la mc icp ms zircon u pb dating of the biotite monzonitic granite porphyry yielded a weighted mean pb u age of 139 2 1 7 ma which was interpreted as the emplacement age of the biotite monzonitic granite porphyry the re os model ages ranged from 137 3 2 0 ma to 146 1 2 1 ma with a weighted mean age of 140 6 1 9 ma and ar ar isotopic analyses of hydrothermal biotite yield a plateau ar ar age of 135 1 0 8 ma these dates obtained via three independent geochronological techniques constrained the ore forming age of the feie shan deposit and linked the genesis of the ore to the underlying biotite monzonitic granite porphyry combined with previous studies it is suggested that there is a period at 144 132 ma for w sn mineralization in the southeastern coastal metallogenic belt the feie shan w sn deposit belongs to the 144 132 ma metallogenic events in the scmb in a geodynamic setting of lithospheric extension

zircon u pb molybdenite re os and muscovite ar ar isotopic dating of the xitian w sn polymetallic deposit eastern hunan province south china and its geological significance the xitian tungsten tin w sn polymetallic deposit located in eastern hunan province south china is a recently explored region containing one of the largest w sn deposits in the nanling w sn metallogenic province the mineral zones in this deposit comprise skarn greisen structurally altered rock and quartz vein types the deposit is mainly hosted by devonian dolomitic limestone at the contact with the xitian granite complex the xitian granite complex consists of indosinian late triassic 230 215 ma and yanshanian late jurassic early cretaceous 165 141 ma granites zircons from two samples of the xitian granite dated using laser ablation inductively coupled mass spectrometer la icpms u pb analysis yielded two ages of 225 6 1 3 ma and 151 8 1 4 ma representing the emplacement ages of two episodic intrusions of the xitian granite complex molybdenites separated from ore bearing quartz veins yielded a re os isochron age of 149 7 0 9 ma in excellent agreement with a weighted mean age of 150 3 0 5 ma two samples of muscovites from ore bearing greisens yielded 40ar 39ar plateau ages of 149 5 1 5 ma and 149 4 1 5 ma respectively these isotopic ages obtained from hydrothermal minerals are slightly younger than the zircon u pb age of 151 8 1 4 ma of the yanshanian granite in the xitian area indicating that the w sn mineralization is genetically related to the late jurassic magmatism the xitian deposit is a good example of the early yanshanian regional w sn ore forming event 160 150 ma in the nanling region the relatively high re contents 8 7 to 44 0 ppm average of 30 5 ppm in molybdenites suggest a mixture of mantle and crustal sources in the genesis of the ore forming fluids and melts based upon previous geochemical studies of early yanshanian granite and regional geology we argue that the xitian w sn polymetallic deposit can be attributed to back arc lithosphere extension in the region which was probably triggered by the break off of the flat slab of the palae pacific plate beneath the lithosphere

constraints of in situ zircon and cassiterite u pb molybdenite re os and muscovite 40ar 39ar ages on multiple generations of granitic magmatism and related w sn mineralization in the wangxianling area nanling range south china abstract the nanling range in south china hosts numerous w sn rare metal deposits associated with several cycles of polygenetic granitic intrusions in the wangxianling area of the middle nanling range igneous intrusions include the wangxianling and hehuaping granite plutons and more than 20 granite porphyry dykes the wangxianling pluton has a central phase of medium grained tourmaline two mica granite and a marginal phase of medium grained tourmaline muscovite granite the pluton hosts several tungsten deposits including shuiyuanshan and yejiwo mineralization type is dominantly scheelite greisen with subordinate scheelite skarn and wolframite quartz veins the hehuaping pluton composed of coarse grained biotite granite and granite porphyry dykes to the east of the wangxianling pluton have an associated hehuaping tin deposit with tin skarn and subordinate porphyry and greisen mineralization in situ zircon and cassiterite u pb dating molybdenite re os and muscovite 40ar 39ar dating techniques are used to refine the age spectra of granitic rocks and w sn mineralization in the wangxianling area granites of the wangxianling pluton have zircon u pb ages of 224 ma tungsten bearing quartz veins from both the shuiyuanshan and yejiwo tungsten deposits have an average molybdenite re os isochron age of 224 ma and muscovite of scheelite greisen from the shuiyuanshan tungsten deposit yields a 40ar 39ar plateau age of 214 ma the two mineralization ages are consistent with the zircon u pb age of the wangxianling pluton indicating a close association between pluton emplacement and w mineralization in the late triassic both the hehuaping pluton and granite porphyry dykes have zircon u pb ages of 156 ma in situ la icp ms u pb dating of cassiterite in both the tin skarn and tin greisen yields an average age of 156 ma showing that the sn mineralization occurred at a similar time to the late jurassic granitic magmatism thus two mineralization events are identified in the wangxianling area and include an earlier w dominant mineralization event and a later sn dominant mineralization event the earlier event is related both spatially and temporally to intrusion of the late triassic s type wangxianling granite pluton whereas the later mineralization is associated both spatially and temporally with intrusion of the late jurassic hehuaping a2 type granite pluton and a2 type granite porphyry dykes

crustal thickness variations in the margins of the gulf of california from receiver functions receiver functions rfs from teleseismic events recorded by the nars baja array were used to map crustal thickness in the continental margins of the gulf of california a newly forming ocean basin although the upper crust is known to have split apart simultaneously along the entire length of the gulf little is known about the behaviour of the lower crust in this region the rfs show clear p to s wave conversions from the moho beneath the stations the delay times between the direct p and p to s waves indicate thinner crust closer to the gulf along the entire baja california peninsula the thinner crust is associated with the eastern peninsular ranges batholith prb crustal thickness is uncorrelated with topography in the prb and the moho is not flat suggesting mantle compensation by a weaker than normal mantle based on seismological evidence the approximately w e shallowing in moho depths is significant with extremes in crustal thickness of 21 and 37 km similar results have been obtained at the northern end of the gulf by lewis et al who proposed a mechanism of lower crustal flow associated with rifting in the gulf extensional province for thinning of the crust based on the amount of pre pliocene extension possible in the continental margins if the lower crust did thin in concert with the upper crust it is possible that the crust was thinned during the early stages of rifting before the opening of the ocean basin in this case we suggest that when breakup occurred the lower crust in the margins of the gulf was still behaving ductilely alternatively the lower crust may have thinned after the gulf opened the implications of these mechanisms are discussed

hydrothermal zircon geochronology age constraint on nanling range tungsten mineralization southeast china abstract the nanling range southeast china is well known for its wolframite bearing quartz vein wqv tungsten deposit this study focuses on the geochemistry and geochronology of zircons from the wqv and challenges the current view of the tungsten mineralization in the nanling range the features of the wqv zircons include 1 pale brown murky brown or orange red color and translucence under microscope 2 110 101 type crystal form 3 weak cathodoluminescence 4 enrichment of hf ranging from 1 97 to 7 83 wt hfo2 u ranging from 0 02 to 3 97 wt uo2 th ranging from 0 to 0 65 wt tho2 and p ranging from 0 to 1 82 wt p2o5 and 5 presence of solid hydrothermal and ore minerals and fluid inclusions these features indicate that the wqv zircons crystallized from hydrothermal fluids during tungsten mineralization the in situ la icpms u pb results of the wqv zircons from five different tungsten deposits in the nanling range yield similar ages ranging from 134 4 1 9 ma to 132 9 1 5 ma approximately 20 million years younger than proposed tungsten ore ages 155 5 ma several mineralization characteristics and field observations also cast doubt on the current model nanling range tungsten ore is the result of orthomagmatic processes the zircon characterization method provided in this study could be applied to tungsten metallogenic research in other parts of the world

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

shear wave splitting observations at the regions of northern baja california and southern basin and range in mexico 1 recent installation of 3 component broadband digital stations around the gulf of california allowed us to make shear wave splitting observation from records of 73 sks and skks phases from 30 events recorded at 12 stations since january of 2001 stations in the southern basin and range province yield upper mantle fast shear wave direction from ne sw to e w consistent with both local direction of miocene extension and north american absolute plate motion the shear at the pacific north american plate limit seems to control tightly the anisotropy pattern of the closest station ne70 the anisotropic pattern along the peninsular range east of the former trench between the north american plate and the now subducted farallon plate is almost uniform and is interpreted as asthenospheric flow induced by the sinking fragment of the farallon plate although frozen anisotropy associated with in situ captured fragment of this plate cannot be ruled out

strengthened east asian summer monsoons during a period of high latitude warmth isotopic evidence from mio pliocene fossil mammals and soil carbonates from northern china the east asian monsoons have fluctuated in concert with high latitude warmth during the past several hundred thousand years with humid summer monsoon dominant climates characterizing warm intervals including interglacials and interstadials and arid winter monsoon dominant climates characterizing cool intervals including glacials and stadials of the states comprising the mid pleistocene to recent climatic regime interglacials are most similar in terms of high latitude ice volumes and temperatures to those extant during the late miocene and early pliocene thus an important question is whether mio pliocene climates in northern china were analogous to a hypothetical prolonged interglacial state with increased summer monsoon precipitation and expansion of forest and steppe environments at the expense of desert environments we utilize new and previously published carbon isotopic data from fossil teeth and soil carbonates to place constraints on paleovegetation distributions and to help infer the behavior of the monsoon system between 7 and 4 ma we find that plants using the c 4 photosynthetic pathway which today are largely grasses found in regions with warm season precipitation were present in northern china by late miocene time demonstrating that the c 4 expansion in china was not significantly delayed compared to the global c4 event during the late miocene early pliocene interval soil carbonate and tooth enamel 13 c data indicate 1 that nearly pure c 3 plant ecosystems existed in the southern chinese loess plateau clp and therefore ecosystems there were dominated by woody dicot herbaceous dicot or cool season grass vegetation or a combination of these and 2 that the clp was characterized by a pattern of northward increasing c 4 vegetation and aridity utilizing a broadened conceptual model for interpreting 13 c data and citing independent faunal floral and lithostratgraphic data we suggest that these patterns reflect northward expansion of forest and steppe ecosystems and relatively humid monsoon climates during the late miocene and early pliocene an important implication of this interpretation is that the forcing mechanism illuminated by the temporal correlation during the pleistocene between warm high latitudes and strong east asian summer monsoons is a robust feature of the eurasian tectonic climatic system that predates the plio pleistocene climatic reorganization

frequency dependent shear wave splitting and heterogeneous anisotropic structure beneath the gulf of california region abstract the gulf of california region has undergone a major evolution over the past 15 ma since subduction of the now extinct farallon plate ceased and transform motion began leading to the initiation of rifting and the opening of the gulf of california 5 ma ago the character of dynamic processes in the upper mantle beneath the region remains poorly understood but constraints on seismic anisotropy in the upper mantle can shed light on contemporary mantle flow processes in order to characterize more fully the anisotropic structure of the upper mantle beneath the region i present measurements of sks splitting at 14 broadband stations of the nars baja array at periods between 8 and 50 s the measured splitting parameters t at nars baja stations exhibit a high degree of complexity with dramatic lateral variations significant backazimuthal variations at individual stations and a large number of well constrained null measurements over a range of backazimuths the dependence of splitting parameters on frequency is evaluated by applying a series of bandpass filters to high signal to noise sks arrivals i find that while fast directions are generally insensitive to the sks frequency content delay times are frequency dependent at several stations the measurements presented in this study indicate that the anisotropic structure of the upper mantle beneath the gulf of california region is complex and exhibits a high degree of both lateral and vertical heterogeneity the observed splitting is consistent with a scenario in which a large scale reorganization of the mantle flow field has been inhibited by the presence of a stalled fragment of oceanic lithosphere associated with farallon subduction the pattern of mantle flow appears to be controlled by more localized processes likely including small scale buoyant upwelling associated with partial melting beneath the central part of the gulf of california rift