zircon lu hf isotopic compositions of metaluminous and peralkaline a type granitic plutons of the emeishan large igneous province sw china constraints on the mantle source abstract permian a type granitoids of the emeishan large igneous province elip in the panxi region sw china are predominantly peralkaline and metaluminous in composition in situ zircon hf isotopic compositions along with whole rock trace element and nd isotopic data are used to examine the probable source characteristics of these a type granitic plutons zircons from the baima and taihe peralkaline plutons have weighted average ehf t values of 8 7 0 4 and 9 2 1 0 whereas the woshui and huangcao metaluminous plutons have differing ehf t values 8 6 0 2 and 5 8 0 3 respectively previous studies have suggested that the plutons were derived by differentiation e g fractionation or partial melting of mafic magmas from an enriched mantle source similar to oceanic island basalt with little or no contribution from the crust end t 1 3 to 3 2 zr nb 0 9 14 4 rb nb 0 1 8 4 and th ta t values of the woshui and baima plutons indicates that spatially and temporally associated peralkaline and metaluminous a type granitic rocks can originate from the same mantle source without crustal assimilation

40ar 39ar geochronological constraints on the age and evolution of the permo triassic emeishan volcanic province southwest china abstract the biostratigraphically constrained permo triassic emeishan volcanic province evp extending over wide areas in southwest china has been recently considered as a large igneous province contemporaneous with the siberian traps and the siliceous tuffs at the p t boundary in south china we report the first 40ar 39ar ages on this igneous province minimum ages have been obtained on phenocrystic plagioclase of the emeishan basalt which has undergone a pervasive metamorphism most likely during subsequent tectonization as a consequence of terrane amalgamation comparison between the ar release spectra obtained on clear vs cloudy plagioclase indicates a 40 30 ma sericite resetting time a minimum apparent age of 246 4 ma for plagioclase from a plagiogranite a late differentiate of the panzhihua layered complex and an age of 254 5 ma for phlogopite from a pyroxenite near lake erhai provide the first absolute age constraint on this igneous province additional ar ar age measurements on post emeishan alkaline and mafic magmatism yielded 104 2 and 100 2 ma for an alkaline complex near panzhihua and 42 1 ma for a gabbro sill emplaced near the ertan dam further study is still needed to determine the age of the emeishan volcanic emission accurately and to test the validity of the assumed short duration of the eruption

chalcophile element geochemistry and petrogenesis of high ti and low ti magmas in the permian emeishan large igneous province sw china sulfide poor mafic layered intrusions sills dykes and lava flows in the funing region sw china are part of the 260 ma emeishan large igneous province they belong to either a high ti group tio2 1 6 4 4 wt with elevated ti y ratios 351 1 018 or a low ti group tio2 1 low y pd pm ratios 0 2 7 1 and nearly constant cu pd pm ratios 1 5 3 8 the even and parallel chalcophile element patterns of the high pge low ti subgroup rocks are likely a result of olivine dominated fractionation under s undersaturated condition the low pge low ti group rocks have low mgo 4 5 8 9 wt and very poor pge pt pd 0 5 1 6 ppb ir 0 004 0 02 ppb with low cu zr ratios 0 1 0 5 high y pd pm 26 70 and variable cu pd pm ratios 2 8 14 the trough like chalcophile element patterns of the low pge low ti subgroup rocks indicate that the magmas were sulfide saturation and sulfide melts were extracted from the magmas the extracted sulfide melts might be potential ni cu sulfide ores at depth in the funing region

permian rifting related fayalite syenite in the panxi region sw china abstract within the panxi rift of the permian emeishan large igneous province elip sw china a 20 km 2 quartz bearing fayalite syenitic pluton intrudes the 260 ma baima igneous complex bic the fayalite syenite is dated using shrimp zircon u pb techniques at 252 2 5 ma the panxi fayalite syenite is mainly composed of alkali feldspar ferroan augite fayalite quartz and fe ti oxides and contains 61 8 63 7 wt sio 2 5 0 7 3 wt fe 2 o 3 t and 9 9 12 1 wt total alkalis belonging to an a type variety of ferroan alkalic metaluminous granitoids the fayalite syenite displays positive eu anomalies eu eu 0 9 8 9 and is enriched in lree la yb n 6 6 11 5 the nd values and 87 sr 86 sr initial ratios of the fayalite syenite range from 1 3 to 1 9 and from 0 7035 to 0 7068 respectively the fayalite syenite and the bic had the same mantle source however the 8 myr age gap between them suggests the fayalite syenite formed by remelting of underplated mafic magmas injected into the crust during elip magmatism the ages of the fayalite syenite and bic are coincident with two permian mass extinction events and suggest that elip may have influenced both

molybdenum behavior during early diagenesis insights from mo isotopes 1 we present molybdenum isotope and concentration data from 14 sites in the eastern pacific from the central california to the peru margin the environments studied have been chosen to represent a broad range in oxidation reduction redox potential which provide a framework for the behavior of this redox sensitive element manganese rich hemipelagic sediments from the eastern tropical pacific have a mean characteristic mo isotope signature 98 95mo 0 49 0 04 two times the standard deviation of the mean 2 sdom with n 14 that reflects fractionation between ocean water and authigenic mo associated with mn oxides authigenic fe mo s deposits from reducing continental margin settings also have a characteristic mo isotopic signature 98 95mo 1 64 0 04 2 sdom with n 136 both of these values are in contrast to highly sulfidic 11 m h2saq restricted basin environments which contain mo isotope values analytically indistinguishable from seawater in terms of the mo isotope composition the modern oceanic mo sink is dominated by continental margin type environments where 98 95mo 1 6 and mn rich sediments where 98 95mo approximately 0 5 with a minor contribution from euxinic settings where h2saq 11 m

two magma series and associated ore deposit types in the permian emeishan large igneous province sw china the late middle permian 260 ma emeishan large igneous province in sw china contains two magmatic series one comprising high ti basalts and fe rich gabbroic and syenitic intrusions the other low ti basalts and mafic ultramafic intrusions the fe rich gabbros are spatially and temporally associated with syenites each series is associated with a distinctive type of mineralization the first with giant fe ti v oxide ore deposits such as panzhihua and baima the second with ni cu pge sulfide deposits such as jinbaoshan limahe and zhubu new shrimp zircon u pb isotopic data yielded 263 3 ma for the limahe intrusion 261 2 ma for the zhubu intrusion and 262 2 ma for a syenitic intrusion these new age dates together with previously reported shrimp zircon u pb ages suggest that all these intrusions are contemporaneous with the emeishan flood basalts and formed during a major igneous event at ca 260 ma the oxide bearing intrusions have higher al2o3 feo as total iron and total alkalis na2o k2o but lower mgo than the sulfide bearing intrusions all intrusions are variably enriched in lree relative to hree the oxide bearing intrusions display positive nb and ti anomalies and in certain cases negative zr hf anomalies whereas the sulfide bearing intrusions have obvious negative nb and ti anomalies a feature of crustal contamination individual intrusions have relatively small ranges of nd t values all the intrusions however have nd t values ranging from 3 9 to 4 6 and initial 87sr 86sr ratios from 0 7039 to 0 7105 the syenites have very low mgo 2 wt but highly variable fe2o3 2 5 to 13 wt with initial 87sr 86sr ratios ranging from 0 7039 to 0 7089 magmas from both series could have derived by melting of a heterogeneous mantle plume the high ti series from a fe rich more fertile source and the low ti series from a fe poor more refractory source in addition the low ti series underwent significant crustal contamination the two magma series evolved along different paths that led to distinct mineralization styles

age and duration of the emeishan flood volcanism sw china geochemistry and shrimp zircon u pb dating of silicic ignimbrites post volcanic xuanwei formation and clay tuff at the chaotian section abstract the age and duration of the emeishan basalts sw china remain poorly constrained largely due to the severe thermo tectonic overprinting of the ar ar system and failure to obtain zircon separates from erupted basalts in an attempt to solve this problem geochemical analyses and shrimp zircon u pb dating have been carried out on rare felsic ignimbrite in the uppermost of the emeishan lava succession the xuanwei formation which immediately overlies the emeishan basalts and a clay tuff at the middle late permian boundary at the chaotian section clastic rocks of the lowermost xuanwei formation group 1 in eastern emeishan large igneous province lip have a geochemical affinity to the emeishan felsic volcanic rocks whereas the overlying sediments group 2 are compositionally more akin to mafic components of the emeishan traps this is the reverse of volcanic sequence of the central emeishan lip where the felsic extrusives sit above predominant mafic rocks it is likely that the clastic rocks are water transported sediments resulting from erosion of the volcanic rocks in the center of the emeishan lip this interpretation is further supported by the general agreement between the age of the lowermost xuanwei formation 257 4 ma 260 5 ma and that of the silicic ignimbrite 263 4 ma and the clay tuff at the middle late permian boundary at chaotian 260 5 ma these ages interpreted as the termination age of the emeishan volcanism are indistinguishable within error from the middle late permian boundary age 260 4 0 4 ma and the main stage 259 262 3 ma of the emeishan volcanism inferred from dating of mafic and alkaline intrusions in the emeishan lip all these suggest that the emplacement of the emeishan volcanism took place over a very short interval moreover the temporal link and geochemical analyses suggest that the chaotian clay at the middle late permian boundary was genetically related to the emeishan silicic volcanism this together with the fact at that both the emeishan basalts and the chaotian clay rest on the maokou formation leads us to infer that the emeishan basalt was emplaced at the middle late permian boundary in this sense the emeishan volcanism can be regarded as a boundary event and its age is presumed at 260 ma although more precise dating is required both stratigraphic correlation and chronologic data presented in this paper lend supports to the notion that the emeishan volcanism was one of likely causes of the end guadalupian mass extinction

zircon u pb geochronology and elemental and sr nd isotope geochemistry of permian mafic rocks in the funing area sw china mafic layered intrusions and sills and spatially associated andesitic basalts are well preserved in the funing area sw china the 258 3 ma layered intrusions are composed of fine grained gabbro gabbro and diorite the 260 3 ma sills consist of undifferentiated diabases both the layered intrusions and volcanic rocks belong to a low ti group whereas the diabases belong to a high ti group rocks of the high ti group have feo tio2 and p2o5 higher but mgo and th nb ratios lower than those of the low ti group they have initial 87sr 86sr ratios 0 706 0 707 lower and nd 1 5 to 0 6 higher than the low ti equivalents 0 710 0 715 and 9 6 to 4 0 respectively the high ti group was formed from relatively primitive high ti magmas generated by low degrees 7 3 9 5 of partial melting of an enriched oib type asthenospheric mantle source the low ti group may have formed from melts derived from an em2 like lithospheric mantle source the mafic rocks at funing are part of the emeishan large igneous province formed by a mantle plume at 260 ma

geochronology geochemistry and petrogenesis of late triassic dolerites associated with the nibao gold deposit youjiang basin southwestern china implications for post collisional magmatism and its relationships with carlin like gold mineralization abstract the relationship between gold mineralization and magmatism within the youjiang basin of sw china remains poorly constrained as a result of the lack of outcropping igneous units in this region this study presents new geological geochronological and geochemical data for recently discovered dolerite dykes associated with the nibao gold deposit which is located within the nw youjiang basin of sw china zircon laser ablation inductively coupled plasma mass spectrometry la icp ms u pb dating of these dykes yielded an age of 213 6 2 5 ma indicating that these dykes were emplaced during the late triassic and providing new evidence for late triassic magmatism within the study area these dykes also represent an opportunity to investigate the petrogenesis of mafic rocks and the tectonic setting of the late triassic magmatism in the youjiang basin these nibao dolerite dykes contain relatively low concentrations of sio 2 46 03 47 66 wt have moderate total alkali contents k 2 o na 2 o 2 71 3 15 wt and high total fe as fe 2 o 3 tfe 2 o 3 contents 13 49 14 96 wt their trace element compositions are indicative of an oib like geochemical affinity barring depletion in the high field strength elements hfse such as nb ta and th the nibao dolerites have homogeneous lu hf and sr nd isotopic compositions with 87 sr 86 sr i and 143 nd 144 nd i value of 0 70562 0 70677 and 0 51233 0 51239 respectively these dykes have associated e nd t and e hf t values that exhibit a narrow range from 0 70 to 0 51 and from 0 04 to 3 24 respectively these geochemical data indicate that the dolerites associated with the nibao gold deposit were derived from an enriched region of the asthenopheric mantle 90 km depth and the magmas that formed these dykes were generated by 5 10 partial melting of 75 garnet peridotite and 25 spinel peridotite material the ascending magmas underwent both crustal contamination and the fractional crystallization of olivine clinopyroxene and fe oxides e g magnetite or ilmenite prior to emplacement the nibao dolerites are geochemically distinct from mafic rocks related to the emeishan large igneous province suggesting that they are not linked to this mantle plume event instead they formed in an extensional setting generated by the subduction of the song ma oceanic plate beneath south china block and the subsequent collision between the indochina block and south china block the breakoff of the song ma oceanic slab was the process that caused the partial melting of the mantle source region for the nibao dolerite dykes these relationships suggest that the post collision extensional environment associated with this breakoff contributed to the deeply sourced mantle derived late triassic magmatism in this area and perhaps as well the gold mineralization in the youjiang basin most importantly the new geochronological data presented in this study the timing of extension in this region and the results of previous research together suggest that the carlin like gold deposits in the youjiang basin formed at around 213 ma i e the late triassic

late permian basalts in the yanghe area eastern sichuan province sw china implications for the geodynamics of the emeishan flood basalt province and permian global mass extinction abstract we report the finding of a 20 m thick sequence of massive pyroxene plagioclase phyric basalt lava flows in the yanghe area of the northeastern sichuan basin within the yangtze craton of sw china which were previously considered to be located outside the emeishan flood basalt province this basaltic sequence above the middle permian maokou formation fm is overlain by the late permian longtan fm thus the yanghe basalts should be stratigraphically correlated with the emeishan flood basalts the yanghe basalts show typical oceanic island basalt oib affinity and geochemically resemble emeishan basalts especially in the case of high ti ht basalts from the eastern domain of the emeishan flood basalt province the rocks have low age corrected 87sr 86sr t t 260 ma ratios 0 704158 0 704929 and pb isotopic ratios 206pb 204pb t 18 264 18 524 207pb 204pb t 15 543 15 58 and 208pb 204pb t 38 147 38 519 and positive end t values 3 15 to 3 61 suggesting that the lavas have not undergone any significant crustal contamination the crystallization temperature of clinopyroxene is estimated to be 1368 1420 c suggesting anomalously thermal inputs from a mantle source and a possible plume head origin the fractionation of middle rare earth elements mree to heavy ree hree suggests that these rocks were produced by small degrees of partial melting of mantle peridotite within the garnet spinel transition region the stratigraphic relationships and similar geochemical signatures with the emeishan flood basalts suggest that the yanghe basalts are part of the emeishan flood basalt province and can be considered as the northeastern limit of the emeishan flood basalt province our finding extends the diameter of the emeishan flood basalt province to 1200 1400 km covering an area of up to 7 105 km2 two times more than previously estimated the larger areal extent and giant eruption volume incorporating the sichuan basin lend support to the correlation between the end guadalupian mass extinction and the emeishan flood basalt province

a synthesis of magmatic ni cu pge sulfide deposits in the 260 ma emeishan large igneous province sw china and northern vietnam abstract magmatic ni cu pge sulfide deposits in the ca 260 ma emeishan large igneous province lip are all hosted in relatively small mafic ultramafic intrusions with surface areas usually less than 1 km 2 these deposits are mainly distributed in the danba panzhihua xichang panxi huili yuanmou midu funing and jinping regions in sw china and the ta khoa region in northern vietnam they include ni cu pge sulfide dominated ni cu sulfide dominated and pge dominated types sulfide ores of the ni cu pge and ni cu sulfide dominated deposits contain more than 10 vol sulfides and have low pge concentrations relative to the ores that contain 34 s values ranging from 6 7 to 3 4 whereas sulfides from sulfide ores of the baimazhai no 3 yingpanjie jinbaoshan and nantianwan intrusions in sw china have highly variable 34 s ranging from 0 2 to 21 4 indicating the addition of crustal sulfur into the mantle derived mafic magmas platinum group minerals pgm are abundant in the ni cu pge sulfide bearing intrusions and they span a wide range of composition more than 130 pgm grains have been identified in the pt pd rich jinbaoshan intrusion whereas only one small froodite pdbi 2 grain was observed in the ni cu sulfide dominated baimazhai no 3 intrusion overall the three types of ni cu pge sulfide deposits in the emeishan lip can be taken as a spectrum of ni cu pge sulfide mineralization the formation of which involved similar magmatic processes in open systems of magma conduits the magma conduits developed along the cross linking structures created by numerous strike slip faults and each intrusion appears to be part of a connecting trellis of conduits that formed complex pathways from the mantle to the surface the ni cu sulfide dominated deposits are attributed to a single sulfide segregation event in staging magma chambers whereas the pge dominated deposits were likely formed by a multistage dissolution upgrading process in the staging chambers the ni cu pge sulfide dominated deposits may have experienced interaction between successive pulses of s undersaturated mafic magma and early segregated sulfide melts in the staging chambers this study is intended to provide a better understanding of the magmatic processes related to the formation of conduit type ni cu pge sulfide deposits associated with continental flood basalt magmatism

geochemical constraints on the origin of the permian baimazhai mafic ultramafic intrusion sw china the 260 ma baimazhai mafic ultramafic intrusion is considered to be part of the emeishan large igneous province and consists of orthopyroxenite surrounded by websterite and gabbro the intrusion is variably mineralized with a massive sulfide ore body 20 vol in the core of the intrusion silicate rocks have ni cu ratios ranging from 0 3 to 46 with majority less than 7 and are rich in lree relative to hree and show nb and ta anomalies in primitive mantle normalized trace element patterns with low nb th 1 0 4 5 and nb la 0 3 1 0 ratios their nd t values range from 3 3 to 8 4 uniform pd pt 0 7 3 5 and cu pd 100 000 400 000 ratios throughout the intrusion indicate that all the sulfides in the rocks were formed in a single sulfide saturation event modeling suggests that the baimazhai rocks were formed when an mg rich magma became crustally contaminated in a deep seated staging chamber crustal contamination up to 35 drove the magma to s saturation and forced orthopyroxene opx onto the liquidus the crystal bearing magma forced out of the staging chamber was migrated by flow differentiation and consequently the denser sulfide melt and the opx crystals became centrally disposed in the flowing magma to form the baimazhai intrusion

geochronology of early mesozoic diabase units in southwestern china metallogenic and tectonic implications two phases of diabase sill forming magmatism are recorded within the badu anticline where magmas were emplaced into upper palaeozoic carbonates and clastic rocks of the youjiang fold and thrust belt in the sw south china block china zircons from these diabase units yield weighted mean u pb ages of 249 2 2 0 ma and 187 1 3 3 ma and magmatic oxygen fugacity values from 20 to 6 average of 12 equating to fmq 5 and 20 to 10 average of 15 equating to fmq 2 respectively these data indicate that the sills were emplaced during early triassic and early jurassic times the discovery of c 250 ma mafic magmatism in this area was probably related to post flood basalt extension associated with the emeishan mantle plume or rollback of the subducting palaeo tethys slab the c 190 ma diabase sills indicate that the southwestern south china block records early jurassic mafic magmatism and lithospheric extension that was likely associated with a transition from post collisional to within plate tectonic regimes the emplacement of diabase intrusions at depth may have driven hydrothermal systems enabling the mobilization of elements from sedimentary rocks and causing the formation of a giant epigenetic metallogenic domain the results indicate that high oxygen fugacity materials within basement rocks caused crustal contamination of the magmas contributing to the wide range of oxygen fugacity conditions recorded by the au bearing badu diabase in addition data from inherited xenocrystic zircons within the badu diabase and detrital zircons from basement rocks suggest that the neoproterozoic jiangshao suture extends to the south of the badu anticline

the significance of pge variations with sr nd isotopes and lithophile elements in the emeishan flood basalt province from sw china to northern vietnam abstract new analyses of siderophile lithophile elements and sr nd isotopes in the permian basalts and picrites from northern vietnam the southernmost occurrence of the emeishan flood basalt province together with previously published data are used to address the question of whether any meaningful correlation between these elements and isotopes exists at a province scale the available data show that negative correlations between e nd 87 sr 86 sr i and mantle normalized nb th n are present in the basalts but not in the associated picrites this indicates that crustal contamination is negligible in the picrites but significant in some of the basalts the picrites and basalts from the entire province show negative correlations between rh ru n pt ru n pd ru n and mg number this indicates that ru behaves compatibly whereas rh pt and pd behave incompatibly during magma differentiation the incompatible behavior of rh in natural basaltic systems is also supported by the fact that pt rh n remains constant with decreasing mg number in the lavas depletions of pd and pt and to a lesser degree cu in some basaltic samples characterized by relatively low e nd and nb th n support the notion that sulfide saturation in the magmas was triggered by a combination of siliceous crustal contamination and addition of external sulfur within the entire flood basalt province only the picrites from song da northern vietnam show clear depletion in ir relative to ru these picrites are also characterized higher al 2 o 3 tio 2 and lower mantle normalized la yb 0 2 2 4 than those from elsewhere in the province possibly due to the involvement of an ir depleted fertile mantle component in magma generation at this location

longevity of the permian emeishan mantle plume sw china 1 ma 8 ma or 18 ma after the formation of the 260 ma emeishan large igneous province there were two volumetrically minor magmatic pulses at 252 ma and 242 ma respectively alkaline mafic dykes intruding both 260 ma and 252 ma felsic plutons in the panxi region southwestern china have compositions similar to the emeishan flood basalts one dyke is dated using the shrimp zircon u pb technique at 242 2 ma 18 ma younger than the start of emeishan magmatism the dykes have enriched light rare earth element patterns la ybn 4 4 18 8 and trace element patterns similar to the emeishan flood basalts and average ocean island basalts some trace element ratios of the dykes zr nb 3 8 8 2 la nb 0 4 1 7 ba la 7 5 25 6 are somewhat similar to em1 source material however there are differences their end values end 2 6 and 2 7 and i sr i sr 0 704542 and 0 704554 ratios are indicative of a mantle source thus emeishan magmatism may have lasted for almost 20 ma after the initial eruption however geological evidence precludes the possibility that the post 260 ma magmatic events were directly related to emeishan magmatism which began at and ended shortly after 260 ma the 252 ma plutons and 242 ma dykes represent volumetrically minor melting of the fossil emeishan plume head beneath the yangtze crust the 252 ma magmatic event was likely caused by post flood basalt extension of the yangtze crust whereas the 242 ma event was caused by decompressional melting associated with the collision between the south china and north china blocks during the middle triassic

ni cu pge magmatic sulfide deposits in the yangliuping area permian emeishan igneous province sw china the ni cu pge sulfide deposits in the yangliuping area sw china are hosted in mafic ultramafic sills the four mineralized sills are located in the yangliuping tectonic dome and intrude devonian carbonaceous marble graphitic schist the sills are 200 300 m thick and 1 000 2 000 m in strike length and now consist chiefly of serpentinite talc schist tremolite schist and meta gabbro disseminated ni cu sulfide mineralisation occurs in the serpentinite in the lower parts of the sills massive sulfide mineralisation is located in the base of the sills and in the footwall along fractures beneath the mineralized serpentinite although the sulfide ores have been modified by hydrothermal activity there are relict cumulate textures in the disseminated sulfides indicating a magmatic origin for the ores the yangliuping intrusions and the dashibao formation have similar primitive mantle normalized trace element and platinum group element pge patterns indicating that they are derived from a common parental magma type the positive correlation between cu concentrations and cu zr ratios of the dashibao formation basalts indicates that the chalcophile elements were removed before eruption we propose that fractional crystallization of the yangliuping magma accompanied by the introduction of s and co2 from the wall rocks caused the magma to become s saturated leading to the segregation of magmatic sulfides that became enriched in ni cu pge the sills acted as conduits for the overlying dashibao formation basalts with the sulfide liquid along with early crystallizing olivine and pyroxene segregating from the magma as it passed through the conduits prior to eruption

geochronological and geochemical constraints on the coexistent n morb and ssz type ophiolites in babu area sw china and tectonic implications our detailed geological geochemical and geochronological investigations indicate the coexistence of late paleozoic 270 265 ma normal middle ocean ridge basalt n morb type and suprasubduction zone ssz type ophiolites in the babu area sw china metaperidotite samples from the babu ophiolites show high os concentrations and low re os 187 os 188 os and 187 re 188 os ratios and represent residual upper mantle metagabbro and metabasalt samples show light rare earth element lree depleted ree patterns and n morb like primitive mantle normalized spider diagrams without nb ta and zr hf anomalies and depleted sr nd isotopic compositions e nd t 6 9 8 3 these geochemical and re os sr nd isotopic data show that the babu ophiolites are derived from a depleted mantle source and represent ancient n morb like oceanic crust in addition peridotites basalts and boninites with ssz signatures e g nb ta negative anomalies were also reported in the babu ophiolites consistent with their generation in a forearc or arc setting in combination with the arc related high mg limburgite basalt and andesite these late paleozoic ultramafic mafic rocks are genetically related to paleotethyan subduction rather than being emeishan mantle plume related the coexisting n morb and ssz type ophiolites in the babu area show similar geochemical and geochronological characteristics to the jinshajiang ailaoshan song ma ophiolites supplementary material a description of the geology of the babu ultramafic mafic complex sample descriptions detailed analytical methods and results and a summary of previously published age data for late paleozoic magmatic rocks along the china vietnam border are available at https doi org 10 6084 m9 figshare c 4007569

precise age determination of mafic and felsic intrusive rocks from the permian emeishan large igneous province sw china abstract radiogenic isotopic dating using different mainly micro beam analyses techniques of rocks from the emeishan large igneous province elip has yielded a range of ages from the capitanian to the early triassic in contrast estimates based on paleontological and paleomagnetic data suggest the elip was emplaced rapidly 2 ma and that volcanism ended by 257 ma new zircon ca tims u pb ages that have superior precision compared to ages from micro beam analyses were obtained from strategic intrusive rocks of the panxi region inner zone of the elip the huangcao 258 9 0 7 ma daheishan 259 1 0 5 ma woshui 259 6 0 5 ma and cida 258 4 0 6 ma syenitic and granitic plutons and three mafic dykes 259 2 0 4 ma 259 5 0 8 ma 257 6 0 5 ma yielded a narrow range of ages between 257 ma and 260 ma the new results are consistent with the estimates from magneto biostratigraphic data and a rapid emplacement model the new wuchiapinqian ages of the huangcao and daheishan plutons suggests that magmatism may have lasted significantly less than 10 ma as previously suggested and the precise ages of the mafic dykes refute the hypothesis that emeishan high ti basalts represent the waning stages of the elip magmatism the short duration of magmatism also implies that degassing of the host sediments mostly carbonates and thus releasing of greenhouse gasses occurred within a substantially shorter interval than previously thought which significantly changes the loading of atmosphere per time unit the latter corollary potentially has severe environmental effects that may have contributed to causing the end guadalupian mass extinction

osl chronology of middle to late holocene aeolian activity in the st anthony dune field southeastern idaho usa abstract late quaternary aeolian dune systems in the great plains and western u s are valuable archives of past climate variability this study investigates the st anthony dune field located in the eastern portion of the snake river plain in southern idaho the current semi arid climate of the region allows for a central core of active dunes that is approximately 450 km 2 in area and includes compound crescentic blowout falling and climbing dunes that range in height from 15 to 121 m the surrounding region is characterized by 20 000 km 2 of relic parabolic linear and complex linear dunes that are currently fixed by vegetation this study examined nine samples collected from the fixed dunes that surround the active core of the st anthony dune field optically stimulated luminescence osl dating results indicate that these fixed dunes were mobile and actively accreting 6 6 ka middle holocene and 1 0 2 0 ka late holocene evidence for activation of a large region currently occupied by stable dunes suggests these periods were characterized by hydrologic deficit and or warmer climate conditions than currently exists this research provides new information regarding the depositional timing of the st anthony dunes and assists in providing evidence of palaeo climatic conditions for the eastern snake river plain during the middle to late holocene

platinum group element geochemistry of the hongge fe v ti deposit in the pan xi area southwestern china mafic and ultramafic intrusions in the pan xi area along the western margin of the yangtze block southwestern china are spatially associated with the late permian emeishan flood basalts of the emeishan large igneous province the hongge layered intrusion is one of the plutonic bodies of this province and hosts a giant fe v ti deposit this intrusion has three zones a lower olivine clinopyroxenite zone a middle clinopyroxenite zone and an upper gabbro zone each of these zones consists of one or two compositional cycles which have distinct mg values tio2 concentrations and total ree contents the middle clinopyroxenite zone and upper gabbro zone contain thick 14 84 m magnetite mineralized layers samples from the lower olivine clinopyroxenite zone and middle clinopyroxenite zone of the hongge intrusion are enriched in platinum and palladium relative to iridium and ruthenium rocks of the lower olivine clinopyroxenite zone have pd ir ratios 1 8 22 3 lower than those of the middle clinopyroxenite zone 6 2 to 83 in its lower part and 3 6 to 49 in its upper part the pd ir ratios increase progressively upwards in each cyclic unit chromite is the major phase controlling concentrations of iridium and ruthenium the cu pd ratios at the bottom of each cyclic unit are close to that of the mantle and increase upwards this variation suggests that each cyclic unit represents a new batch of magma and sulfide mineral segregation removes pge platinum group elements ni and cu each cyclic unit in the hongge layered intrusion might have resulted from crystal fractionation and mixing between a primary and evolved magma the high cu pd ti pd ni pd and cu ir ratios of the intrusion suggest that the sulfide mineral segregation may have played an important role in pge differentiation if this interpretation is correct then there is a potential to find economic pge rich horizon in the hongge type intrusions in the region

geochemistry of the siberian trap of the noril sk area ussr with implications for the relative contributions of crust and mantle to flood basalt magmatism the sequence investigated of the siberian trap at noril sk ussr consists of at least 45 flows that have been divided into six lava suites the lower three suites consist of alkalic to subalkalic basalts the ivakinsky suite overlain by nonporphyritic basalts the syverminsky suite and porphyritic and picritic basalts the gudchikhinsky suite the upper three suites are tholeiitic the uppermost 750 m of dominantly non porphyritic basalt belong to the mokulaevsky suite and are characterized by a nearly constant mg number 0 54 0 56 sio2 48 2 49 1 wt ce 12 18 ppm and ce yb 5 8 the underlying 1100 m of dominantly porphyritic basalt belong to the morongovsky and nadezhdinsky suites there is a continuous increase in sio2 48 1 55 2 wt ce 12 41 ppm and ce yb 5 18 from the top of the mokulaevsky to the base of the nadezhdinsky with little change in the mg number 0 53 0 59 mokulaevsky magmas have trace element signatures similar to slightly contaminated transitional type mid ocean ridge basalts the change in major and trace element geochemistry in the upper three suites is consistent with a decline in the degree of anatexis and assimilation of tonalitic upper crust by mokulaevsky magma the nadezhdinsky and underlaying lavas thicken within and thus appear to be related to an elongate basin centred on the noril sk talnakh mining camp the mokulaevsky and morongovsky lavas thicken to the east and appear to be related to a basin centred more than 100 km to the east of the noril sk region these magmas may have risen up out of a different conduit system

the behavior of molybdenum and its isotopes across the chemocline and in the sediments of sulfidic lake cadagno switzerland abstract molybdenum mo isotope studies in black shales can provide information about the redox evolution of the earth s oceans provided the isotopic consequences of mo burial into its major sinks are well understood previous applications of the mo isotope paleo ocean redox proxy assumed quantitative scavenging of mo when buried into sulfidic sediments this paper contains the first complete suite of mo isotope fractionation observations in a sulfidic water column and sediment system the meromictic lake cadagno switzerland a small alpine lake with a pronounced oxygen sulfide transition reaching up to h 2 s 200 m in the bottom waters or about 300 m total sulfide s 2 h 2 s hs s 2 we find that mo behaves conservatively in the oxic zone and non conservatively in the sulfidic zone where dissolved mo concentrations decrease from 14 nm to 2 8 nm across this transition dissolved mo in the upper oxic waters has a 98 mo oxic 0 9 0 1 which matches that of the riverine input 98 mo river 0 9 0 1 in the deeper sulfidic waters a subaquatic source delivers mo at 1 55 0 1 but the dissolved mo is even heavier at 98 mo sulfidic 1 8 sediment traps in the sulfidic zone of the lake collect particles increasingly enriched in mo with depth with 98 mo values significantly fractionated at 0 8 to 1 2 both near the chemocline and in the deepest trap suspended particulates in the sulfidic waters carry lighter mo than the ambient dissolved mo pool by 0 3 1 5 sedimentary mo concentrations correlate with total organic carbon and yield mo levels which are two orders of magnitude higher than typical crustal values found in rocks from the catchment area solid phase mo in the sediment shows a slightly positive 98 mo trend with depth from 98 mo 1 2 to 1 4 while the pore waters show dramatic enrichments of mo 2000 nm with a relatively light isotope signature of 98 mo 0 9 1 0 these data are explained if mo is converted to particle reactive oxythiomolybdates in the sulfidic waters and is fractionated during removal from solution onto particles isotope fractionation is expressed in the water column despite the high sulfide concentrations because the rate of mo removal is fast compared to the slow reaction kinetics of thiomolybdate formation however elemental and isotopic mass balances show that mo is indeed quantitatively removed to the lake sediments and thus the isotopic composition of the sediments reflects sources to the sulfidic water this efficient mo drawdown is expected to occur in settings where h 2 s is very much in excess over mo or in a restricted setting where the water renewal rate is slow compared to the mo burial rate we present a model for the mo isotope fractionation in sulfidic systems associated with the slow reaction kinetics and conclude that quantitative removal will occur in highly sulfidic and restricted marine systems

remobilisation of the continental lithosphere by a mantle plume major trace element and sr nd and pb isotope evidence from picritic and tholeiitic lavas of the noril sk district siberian trap russia the late permian to early triassic siberian traps have been sampled by drill core core sg 9 and from surface exposure section 1f in the noril sk region of the siberian platform russia combined major trace element and nd sr and pb isotope data on selected samples through the siberia trap offer new chemostratigraphic criteria for the identification and characterisation of two fundamentally different magma types and 9 of the 11 formations of lava developed near noril sk a lower sequence of sub alkalic basalts tholeiites and picritic basalts upwards these are the ivakinsky syverminsky and gudchichinsky formations are overlain by an upper sequence of picritic basalts and tholeiites interbedded with tuffs upwards these are the khakanchansky tuklonsky nadezhdinsky morongovsky mokulaevsky and kharayelakhsky formations the gudchichinsky and tuklonsky formations contain both picritic and tholeiitic lavas the tuklonsky formation tholeiites and picrites have moderate gd yb 1 6 1 8 low tio2 0 45 0 95 wt a significant negative ta and nb anomaly nb la 0 42 0 57 and unradiogenic nd nd chur to 4 6 in contrast both the gudchichinsky formation tholeiites and picrites have high gd yb 2 3 3 1 and tio2 1 2 2 3 wt no significant nb or ta anomaly nb la 0 8 1 1 and radiogenic nd nd chur to 7 3 the low ti and nb la high la sm and unradiogenic nd isotope signatures of the picritic tuklonsky formation lavas and the tholeiitic lavas of the upper sequence are characteristic of magmas strongly influenced by material from the continental lithosphere whereas the high ti and nb la low la sm and radiogenic nd isotope signatures of the lower sequence are more comparable to deeper asthenospheric mantle plume generated lavas similar to oceanic island basalts the lavas overlying the tuklonsky formation have mg numbers of 0 63 to 0 68 and are more evolved than the tuklonsky mg number 0 62 and have more radiogenic nd chur tuklonsky 0 03 to 4 66 mokulaevsky 0 60 to 1 61 but have many of the incompatible trace element features of the tuklonsky sky type magma these lavas show a progressive upwards decline in sio2 55 49 wt la sm 4 6 2 0 and ur sr 67 to 13 which has previously been attributed to a decrease in the proportion of crustal material contributed to the magma this paper explores and alternative model where a component of the crustal contribution might be derived from within an ancient region of the mantle lithosphere as recycled sediment rather than from the overlying continental crust

archean paleoproterozoic crustal evolution in the eastern north china craton zircon u th pb and lu hf evidence from the jiaobei terrane abstract the precambrian metamorphic basement of the north china craton ncc records a complex evolutionary history during the neoarchean and the paleoproterozoic the jiaobei terrane provides one of the best regions to evaluate the early precambrian crustal growth of the eastern ncc here we report the results of u pb zircon geochronology and hf isotopes of tonalite trondhjemite granodiorite ttg gneiss and associated mafic rocks from the jiaobei terrane sims dating of zircons from the ttg gneiss shows that the protolith was formed at 2912 6 ma and subjected to anatexis at 2488 9 ma zircons from the mafic rocks yield three age populations at 2484 7 ma 1855 5 ma and 1836 3 ma based on the differences in morphology internal texture and the u th compositions between magmatic and metamorphic zircons in the mafic suite we interpret the 2484 7 ma and 1836 3 ma ages obtained from two meta mafic intrusions as the crystallization age of their magmatic precursors and the 1855 5 ma from a garnet amphibolite as the metamorphic age magmatic zircons from ttg gneiss and meta gabbro yield hf model ages ranging from 3 2 to 2 8 ga suggesting an important period for crustal growth during mesoarchean the earlier mafic magmatic event 2 49 ga involved magma generation from a depleted mantle whereas the later phase 1 84 ga was derived from differentiation of a parent magma that originated from the partial melting of the mesoarchean lower curst the newly grown metamorphic zircons in the garnet amphibolite show initial 176hf 177hf and t dm c values from 0 281211 to 0 281343 and from 3 11 to 3 42 ga indicating that the mesoarchean and older crustal materials reworked at 1 86 ga the data reported in this study coupled with previous geochronological and lu hf isotopic data from other localities of the eastern ncc on the widespread emplacement of the mafic magmas and the coeval high grade metamorphism or melting events at 2 50 2 48 ga suggest that these regions were already part of a coherent tectonic unit with a common crustal evolution history during the end of neoarchean

the age of the permian triassic boundary abstract the 5 cm boundary clay bed in the chinese stratotype section through the permian triassic boundary has been recognised as a bentonite shrimp ion microprobe dating of zircons in the bentonite indicates a magmatic age of 251 2 3 4 ma 2 this is the first direct constraint on the numerical age of the permian triassic boundary future refinements of ages at this important but poorly constrained level of the phanerozoic timescale may depend on re analysis of this uniquely placed volcanic horizon and other bentonites in the fossiliferous chinese upper permian and lower triassic the utility of defining the permian triassic boundary in the chinese stratotype section in the vicinity of known dateable horizons should be considered

nanpanjiang basin a window on the tectonic development of south china during triassic assembly of the southeastern and eastern asia abstract as the largest siliciclastic sink in the south china block the triassic nanpanjiang basin is generally thought of as a foreland basin related to indosinian collision between the south china and indochina blocks our basin analysis spanning an eight years period provides details of lopingian to middle triassic stratigraphic fill and tectonic subsidence of the basin field observation and sedimentological analyses reveal that the basin was not filled by a monotonous turbidite system and suggest instead the development of five primary depositional units constituted by deep water turbidite systems mass transport deposits condensed drapes shallow water isolated carbonate platforms and volcano volcaniclastic rocks the stratigraphic completeness of the basin the coexistence of widespread but localized carbonate platforms within axially filled deep water basin tectonically generated cyclothems of transverse mass transport deposits along intrabasinal faults and basin fill of extrusive rocks of bimodal composition together apparently call in question the claim that crustal shortening prevailed during the late permian to pre norian triassic patterns of basin fill differential tectonic subsidence and very high subsidence rate of the region adjacent to master fault are more closely match the pattern expected for extensional to transtensional basin in a back arc setting the crustal extension in the southwestern south china was not consistent with hypothesized indosinian collision between south china and indochina blocks but more likely developed in response to northwestward subduction of the paleo pacific plate our results combined with regional considerations lead to a new model for the triassic tectonic evolution of south china during triassic assembly of the southeastern and eastern asia

a temporal link between the emeishan large igneous province sw china and the end guadalupian mass extinction abstract previous studies have suggested that there were two mass extinction events in the late permian one that occurred at the permo triassic p t boundary 251 ma and a second smaller mass extinction that occurred 5 8 myr earlier at the end of the guadalupian many workers have argued that there is a causal relationship between large scale volcanic activity and mass extinctions the major mass extinction event at the p t boundary coincides with the outpouring of huge quantities of lava that formed the siberian flood basalt province in russia courtillot et al earth planet sci lett 166 1999 177 195 and wignall earth sci rev 53 2001 1 33 suggested that the earlier late permian mass extinction coincided with the eruption of the lavas that formed the emeishan flood basalt efb province in sw china however the age of eruption of the efb lavas is poorly constrained using the sensitive high resolution ion microprobe to analyze zircons we have established the age of the xinjie intrusion believed to be a feeder to the main phase of efb volcanism to be 259 3 ma hence the formation of the efb is coincident with a proposed extinction event at 256 259 ma this result supports a temporal link between the emeishan large igneous province and the end guadalupian mass extinction

detrital zircon u pb ages of paleogene deposits in the southwestern sichuan foreland basin china constraints on basin mountain evolution along the southeastern margin of the tibetan plateau

geochemistry petrogenesis and metallogenesis of the panzhihua gabbroic layered intrusion and associated fe ti v oxide deposits sichuan province sw china the panzhihua gabbroic layered intrusion is associated with the 260ma emeishan large igneous province in sw china this sill like body hosts a giant fe ti v oxide deposit with 1333 million ton ore reserves which makes china a major producer of these metals the intrusion has a marginal zone of fine grained hornblende bearing gabbro and olivine gabbro followed upward by lower middle and upper zones the lower and middle zones consist of layered melanogabbro and gabbro composed of cumulate clinopyroxene plagioclase and olivine these zones also contain magnetite layers the upper zone consists chiefly of leucogabbro composed of plagioclase and clinopyroxene with minor olivine most rocks in the body show variable scale rhythmic modal layering in which dark minerals primarily clinopyroxene dominate in the lower parts of each layer and lighter minerals primarily plagioclase dominate in the upper parts the oxide ores occur as layers and lenses within the gabbros and are concentrated in the lower parts of the intrusion ore textures and associated mineral assemblages indicate that the ore bodies formed by very late stage crystallization of v rich titanomagnetite from an immiscible oxide liquid in a fluidrich environment the rocks of the panzhihua intrusion become more evolved in chemistry upward and follow a tholeiitic differentiation trend with enrichment in fe ti and v they are enriched in light rare earth elements relative to heavy rare earth elements and exhibit positive nb ta and ti anomalies and negative zr and hf anomalies the silicate rocks and oxide ores of the panzhihua intrusion formed from highly evolved fe ti v rich ferrobasaltic or ferropicritic magmas the textures of the ores and the abundance of minor hydrous phases indicate that addition of fluids from upper crustal wall rocks induced the separation of the immiscible oxide melts from which the fe ti v oxide ore bodies in the lower part of the intrusion crystallized

contrasting molybdenum cycling and isotopic properties in euxinic versus non euxinic sediments and sedimentary rocks refining the paleoproxies abstract the redox sensitive transition element molybdenum is present in the earth s crust at trace concentrations but is abundant in seawater marine sediments and ancient sedimentary rocks these occurrences have led to the development of a suite of paleoredox proxies based on the bulk concentration of mo its isotopic composition and the covariation of mo with total organic carbon toc in ancient black shales however these proxies have almost exclusively targeted the identification and interpretation of euxinic environments where bottom waters are both anoxic and sulfidic here we present a discussion of the mo geochemistry of non euxinic sediments where hydrogen sulfide is sometimes present yet always restricted to pore waters we propose a new paleoredox application that uses mo concentrations to help distinguish between environments where sulfide was present in the bottom waters and environments where sulfide was restricted to pore waters under ideal conditions it is also possible to infer the presence of mn oxides recycling in non euxinic paleoenvironments based on the mo isotope composition of ancient black shales providing a critical constraint on bottom water oxygenation

giant radiating mafic dyke swarm of the emeishan large igneous province identifying the mantle plume centre in many continental large igneous provinces giant radiating dyke swarms are typically interpreted to result from the arrival of a mantle plume at the base of the lithosphere mafic dyke swarms in the emeishan large igneous province elip have not received much attention prior to this study we show that the geochemical characteristics and geochronological data of the mafic dykes are broadly similar to those of the spatially associated lavas suggesting they were derived from a common parental magma based on the regional geological data and our field observations we mapped the spatial distribution patterns of mafic dyke swarms in the elip and recognized six dyke sub swarms forming an overall radiating dyke swarm and converging in the yongren area yunnan province this location coincides with the maximum pre eruptive domal uplift and is close to the locations of high temperature picrites our study suggests that the yongren area may represent the mantle plume centre during the peak of emeishan magmatism

provenance of late permian volcanic ash beds in south china implications for the age of emeishan volcanism and its linkage to climate cooling abstract whole rock geochemistry and zircon trace element u pb and hf isotopic data for upper permian volcanic ash at shangsi show a significant change in source from the emeishan large igneous province lip to a convergent plate margin magmatic arc the lower stratigraphic samples display high al2o3 tio2 ratios strong negative eu anomalies and no depletion in high field strength elements zircons with ages of ca 260 ma from these ashes have a geochemical affinity to within plate type magmas and with felsic ignimbrites from the uppermost part of the emeishan volcanic succession at binchuan these features combined with relatively high ehf t values and low th nb ratios in the zircons imply a late stage felsic volcanic origin related to emeishan lip the upper stratigraphic samples at shangsi display significant depletion in nb and ti zircons separated from these volcanic ashes are chemically similar to those from arc related orogenic rocks indicative of a magmatic arc source combined with published high precision ca tims data for these volcanic ash layers the emeishan volcanism can be constrained to between ca 260 257 8 ma the age of emeishan volcanism is consistent with the wuchiapingian climate cooling event supporting a potential linkage between these two phenomena

mo total organic carbon covariation in modern anoxic marine environments implications for analysis of paleoredox and paleohydrographic conditions 1 sedimentary molybdenum mo s has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic rich marine facies deposited under oxygen depleted conditions a detailed analysis of mo s total organic carbon toc covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that 1 mo s while useful in distinguishing oxic from anoxic facies is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and 2 patterns of mo s toc covariation can provide information about paleohydrographic conditions especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal these inferences are based on data from four anoxic silled basins the black sea framvaren fjord cariaco basin and saanich inlet and one upwelling zone the namibian shelf representing a spectrum of aqueous chemical conditions related to water mass restriction in the silled basin environments increasing restriction is correlated with a systematic decrease in mo s toc ratios from 45 5 for saanich inlet to 4 5 1 for the black sea this variation reflects control of mo s by mo aq which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal the basin reservoir effect the temporal dynamics of this process are revealed by high resolution chemostratigraphic data from framvaren fjord and cariaco basin sediment cores which exhibit long term trends toward lower mo s toc ratios following development of water column stratification and deepwater anoxia mo burial fluxes peak in weakly sulfidic environments such as saanich inlet owing to a combination of greater mo aq availability and enhanced mo transport to the sediment water interface via fe mn redox cycling and are lower in strongly sulfidic environments such as the black sea and framvaren fjord these observations demonstrate that at timescales associated with deepwater renewal in anoxic silled basins decreased sedimentary mo concentrations and burial fluxes are associated with lower benthic redox potentials i e more sulfidic conditions these conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction e g silled basins and are not valid for low oxygen facies in open marine settings such as continent margin upwelling systems

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

new constraints on the onset age of the emeishan lip volcanism and implications for the guadalupian mass extinction abstract the onset of the middle permian emeishan large igneous province elip is commonly implicated in the guadalupian mass extinction one of the largest phanerozoic extinction events yet the contribution of the elip to this biotic event remains unresolved because of a lack of appropriate age constraints in this study we examined claystone beds overlying the cherty limestone of the maokou formation and underlying the flood basalts of the elip that were emplaced around the guadalupian lopingian boundary integrated petrographic mineralogical elemental and isotopic features indicate that the studied claystones contain significant components from mafic volcanic ash that was likely associated with the emplacement of the elip zircons from the claystones yield u pb ages of 262 5 ma which predates the subaerial eruptive phase of the elip 259 1 260 4 ma but is comparable to the j altudaensis conodont biostratigraphic zone 263 ma middle capitanian stage which is contemporaneous with the emplacement of early stage emeishan subaqueous pillow basalts and mafic volcaniclastics our data provide a crucial new constraint on the lower age limit for the onset of the elip which was likely associated with violent submarine eruptions and thus could have contributed to the loss of marine fauna during the guadalupian mass extinction

geochemistry and petrogenesis of the permian mafic dykes in the panxi region sw china abstract numerous intrusive bodies of ultramafic mafic to felsic compositions are exposed in association with volcanic rocks in the emeishan large igneous province lip southwestern china in this paper we present new elemental and isotopic data for the permian mafic dykes from the panxi region which is located in the western part of the emeishan lip the characteristics of major and trace elements and sr nd isotopes in combination with isotopic ages suggest that these mafic dykes were originated from a similar mantle source related to an upwelling mantle plume and their formation was involved in a process of variable degrees of fractional crystallisation coupled with assimilation of enriched crustal materials the mafic dyke samples from the panxi region show some systematic variations in geochemical compositions the gabbros in yanyuan contain the lowest ree e g la 5 12 ppm the highest mgo 8 38 21 33 and compatible elements e g cr 104 2610 ppm ni 135 496 ppm and display depleted sr nd isotopic ratios initial 87sr 86sr 0 703752 0 703844 end t 4 87 to 5 13 these gabbros likely represent an undifferentiated composition similar to primary melts derived from a depleted mantle source in contrast the gabbro samples from panzhihua and huili have more enriched lree abundances and incompatible elements and display depleted to slightly enriched sr and nd isotopic signatures initial 87sr 86sr 0 704354 0 705436 end t 0 29 to 4 58 though their trace element spidergram patterns are similar to those of the yanyuan samples these gabbros were formed probably through the removal of a few percent of olivine clinopyroxene plagioclase and apatite from the melts that formed the yanyuan gabbros all the mafic intrusives in the region exhibit elemental and sr nd isotopic features generally comparable with those of the nearby volcanic rocks in addition their emplacement immediately preceded or was synchronous with the main pulse of volcanism that formed lavas over a large area along the western margin of the yangtze craton these lines of evidence suggest that there is a genetic link between the mafic dykes and flood basalts in the emeishan lip and the both share a common mantle source related to the emeishan plume

the role of fe ti oxide crystallization in the formation of a type granitoids with implications for the daly gap an example from the permian baima igneous complex sw china abstract the baima igneous complex bic is a member of the permian emeishan large igneous province sw china and is composed of a layered gabbroic unit and an isotropic syenitic unit the gabbroic unit consists of four distinct zones 1 lower cumulate zone 2 massive fe ti v oxide ore zone 3 olivine gabbro zone and 4 upper gabbro zone shrimp zircon u pb dating results yield an age of 261 2 ma for the gabbroic unit contemporaneous with the spatially associated syenitic unit the gabbroic rocks show slightly lree enriched patterns with la yb n values between 1 6 and 12 1 and positive eu anomalies eu eu 1 1 3 6 in contrast the syenites display stronger enrichments of lree la ybn 6 8 51 3 and negative eu anomalies eu eu 0 3 1 1 rocks of the syenitic unit are chemically similar to the ferroan alkalic a type granitoids k2o na2o 0 58 0 80 feot feot mgo 0 80 0 91 and have a within plate geochemical signature both units have similar trace element ratios zr nb th u ni co cu ni hf ta and zn ga resembling an ocean island basalt source the end t values for the gabbroic unit end t 1 6 4 2 and syenitic unit end t 2 5 3 2 are within the range of the high ti emeishan flood basalts the geochemical and geochronological data indicate that the gabbroic and syenitic units are comagmatic and cogenetic geochemical modeling suggests that the two units originated by fractional crystallization of a common parental magma that resembles the high ti emeishan flood basalts the formation of the two units by fractional crystallization was likely responsible for the enrichment of fe ti v oxide minerals in the gabbroic unit and thus the development of the daly gap the results of this study indicate that silica saturated peralkaline a type granitoids can be formed by fractionation of mantle derived mafic magmas

emeishan basalt ar ar overprint ages define several tectonic events that affected the western yangtze platform in the mesozoic and cenozoic abstract ar ar whole rock dating was carried out as part of a detailed stratigraphical investigation of the emeishan basalt large igneous province lip in the stratotype area sichuan china thirteen from twenty one specimens from three sections yielded reliable reversed isochron and plateau ages maximum 1 errors of 2 1 and 1 6 m y respectively and form two clusters centered on the late jurassic early cretaceous and early late cretaceous with a tail spanning 82 40 ma however all are appreciably younger than the magnetobiostratigraphically constrained late middle permian 255 260 ma age of the basalts reflecting varying degrees of thermal resetting during the mesozoic and cenozoic recently middle jurassic early cretaceous ages reported from elsewhere in the lip have been used to infer a single 40 m y long tectonic episode that affected the western yangtze platform developing this idea the new information have been combined with data we hold for other parts of the terrain and results from three emeishan lip ar ar dating studies published during 2002 to give 32 reliable age dates three 10 12 m y events appear to be recorded in different parts of the province middle jurassic late jurassic early cretaceous and early late cretaceous with a fourth shorter middle eocene episode the 40 m y mesozoic episode appears to be a sampling artifact following a review of the regional deformation tectonic features it is argued that activity related to various phases of deformation in the longmen shan thrust belt is the most likely cause of resetting the final suturing of the north and south china blocks may have also been responsible for the middle jurassic event however the oligocene present indentation of india into asia appears not to have had an impact possibly due to the large scale related strike slip faults that have effectively shielded the lip sichuan basin

silicic ash beds bracket emeishan large igneous province to 1 m y at 260 ma this research was supported by the open fund plc201404 of state key laboratory of oil and gas reservoir geology and exploitation chengdu university of technology the national natural science foundation of china 41502109 project funded by china postdoctoral science foundation 2015 m582528 and the fundamental research funds for the central universities chengdu university of technology

key factors controlling the accumulation of the fe ti oxides in the hongge layered intrusion in the emeishan large igneous province sw china abstract the hongge layered intrusion hosts the largest fe ti v oxide ore deposit in the central part of the emeishan large igneous province sw china it is divided into lower zone lz middle zone mz and upper zone uz from the bottom to the top for the lz relatively higher cr 250 3000 ppm and ni 50 200 ppm contents of clinopyroxene lower end259ma and higher 87sr 86sr 259ma values 2 82 to 0 07 and 0 7057 0 7076 respectively and plenty of hornblende demonstrate a more primitive parental hydrous magma in contrast relatively low cr

geology and geochemistry of intrusions and flood basalts of the noril sk region ussr with implications for the origin of the ni cu ores geochemical studies indicate that five principal magma types characterize the 3 8 km vertical thickness of flood basalt of the noril9sk region from base to top these are 1 a fractionated type with alkalic affiliations ivakinsky and syverminsky 2 an ni rich suite including picritic basalt gudchichinsky 3 a primitive but ni depleted suite with fiat rare earth element ree profiles and also including picritic basalts tuklonsky 4 a light ree enriched crustally contaminated suite lower nadezhdinsky nd 1 above 4 there is a transitional suite upper nadezhdinsky nd 2 and morongovsky mr 1 and mr 2 which is overlain by the fifth type a second primitive suite without picrites mokulaevsky similar but not identical in its chemistry to the tuklonsky type this is followed by tuffs and basalts of the kharayelakhsky kumginsky and samoyedsky suites which are largely composed of similar uncontaminated tholeiitic magma the latter three suites have not been studied by us intrusions of the region fall into five main groups 1 alkalic intrusions 2 ti rich dolerite dikes 3 dolerite sills and dikes 4 differentiated bodies not related to ore junctions and 5 differentiated bodies related to ore junctions the ni cu ores are associated with ni and cr rich type 5 olivine rich intrusions with flat ree profiles that are geochemicaly similar to the mokulaevsky magma light ree enriched ni and cr poor olivine rich type 5 intrusions with uneconomic mineralization occur closely associated with the strongly mineralized variants and resemble geochemically the basalts transitional between the lower nadezhdinsky and upper morongovsky they are cut by the strongly mineralized bodies indicating that the latter are of late morongovsky to mokulaevsky age the mineralized intrusions are associated with about 1 5 to 1 10 of their mass in sulfides this is a factor of 25 to 50 more than could have been carried in solution and deposited by in situ liquation from the magma of these intrusions the platinum group element pge concentrations in the ores require that the sulfides giving rise to them equilibrated with 15 to 200 times more magma than is represented by the intrusions much of the ore is very massive and crosscuts the host intrusions the sequence of igneous activity and mineralization in the immediate vicinity of noril9sk and talnakh from the base of the nadezhdinsky upward is thus interpreted to be 1 extrusion of highly crustally contaminated ni and cu poor basalt 2 extrusion of a series of basalt flows each progressively less contaminated and less ni and cu depleted than the preceding one accompanied by intrusion of contaminated ni depleted magma with weak mineralization 3 extrusion of uncontaminated ni and cu rich basalt accompanied by intrusion of uncontaminated ni rich magma with strong mineralization followed very closely by introduction of associated massive ore and 4 continued extrusion of ni and cu rich basalt it is proposed that crustal contamination of the original ni and cu rich magma occurred at the top of a vertically extensive fault controlled magma chamber and caused segregation of immiscible sulfides with a low r factor these depleted this magma in chalcophile elements and settled deeper in the chamber as they settled they reacted with less contaminated magma scavenging additional chalcophile elements they finally came to rest near the base of the chamber to form a zone of sulfide and thus chalcophile element enriched magma eruption and in some instances intrusion of progressively stratigraphically lower less contaminated and thus less depleted layers in the chamber gave rise to the observed sequence of basalts and intrusions the answer to the strong enrichment in ni cu and pge associated with the ore bearing intrusions is that these metals came from the magma that is now represented by the depleted basalts

remobilization of the continental lithosphere by a mantle plume major element trace element and sr isotope nd isotope and pb isotope evidence from picritic and tholeiitic lavas of the norilsk district siberian trap russia

the giant south china mesozoic low temperature metallogenic domain reviews and a new geodynamic model abstract the south china craton was formed by amalgamation of the yangtze and cathaysia blocks during the neoproterozoic during the mesozoic voluminous granitic plutons and associated w sn polymetallic deposits were formed in the cathaysia block the giant south china low temperature metallogenic domain ltmd includes an area of 500 000 km 2 in the yangtze block and is composed of the chuan dian qian pb zn youjiang au as sb hg and xiangzhong sb au metallogenic provinces the chuan dian qian pb zn province contains numerous mvt pb zn deposits whereas the other two provinces are characterized by carlin type au deposits and vein type sb hg and as deposits these epigenetic deposits which formed under low temperature conditions 100 250 c are typically hosted in sedimentary rocks and are locally controlled by faults and fractures the deposits formed dominantly at 200 230 ma and 130 160 ma corresponding to indosinian triassic and yanshanian jurassic to cretaceous orogenies respectively indosinian mineralization is recognized in all three provinces but yanshanian mineralization occurred only in the youjiang and xiangzhong provinces the indosinian orogeny which involved collision of the indochina block with the south china craton resulted in circulation of basinal brines that leached ore forming elements from adjacent sedimentary strata to form the chuan dian qian pb zn province deep seated granitic magmas generated during this orogeny caused extensive circulation of meteoric water that mobilized ore forming elements from the sedimentary strata to form the carlin type au deposits in the youjiang province and the sb au deposits in the xiangzhong province the indosinian orogeny was the key factor in establishing the metallogenic framework of the ltmd it produced widespread mineralization in the three metallogenic provinces each of which has unique features reflecting differences in the nature and composition of the basement rocks the yanshanian metallogeny was less important and overprinted the older ore deposits in the youjiang and xiangzhong provinces

multiple magma evolution and ore forming processes of the hongge layered intrusion sw china insights from sr nd isotopes trace elements and platinum group elements abstract the hongge layered intrusion 259 ma which is located in the inner zone of the emeishan large igneous province elip is one of the most typical fe ti v ore deposits in the pan xi area mafic ultramafic layered intrusions of the elip have attracted a lot of attention lately because these intrusions host world class fe ti v oxide deposits plus interesting cu ni pge mineralization which may have economic potential this paper reports new whole rock major and trace element compositions pge abundances and sr nd isotopic data for selected cumulate rocks and basalts we use these data to investigate the nature of parental magmas and the controls on its evolution from the source mantle en route to the surface involving the hongge ore bearing intrusion two abrupt changes in mt ilm and trace element ratios such as ba th with depths in the hongge layered intrusion indicate that this intrusion formed by at least two pluses of relatively primitive magma the whole rock sr nd isotopic data of basaltic and intrusive rocks plot in the region of emeishan low ti basalts and the compositions of residual liquid at 1260 c and 1155 c calculated by melts are similar to our actual high ti bfq 2 and low ti bc 1 basltic samples indicate they are co magmatic rather than derivation from a distinct source total pge abundances in the hongge samples are extremely low ranging from 0 5 to 10 ppb sulfide bearing rocks in the hongge intrusion and the nearby coeval banfangqing and baicao basalts have similar mantle like pd pt ratios 2 6 and extremely high cu pd ratios 3 104 to 4 105 indicating that sulfide segregation took place at depth prior to emplacement at hongge and eruption in this region sulfide saturation in the hongge magma may have resulted from such crustal contamination event crystallization of silicate minerals under the anhydrous magma magma hydration plus fe ti enrichments in the parental magma are three critical factors for the formation of fe ti oxide ore layers in the hongge intrusion

platinum group elements of the meishan permian triassic boundary section evidence for flood basaltic volcanism abstract permian triassic boundary sections record the most severe mass extinction event in geological history however there is a long standing controversy of whether bolide impact and or basaltic flood volcanism triggered the mass extinction platinum group elements pges are enriched in most extraterrestrial materials but highly depleted in the terrestrial crust materials we analyzed ir ru rh pt and pd in a set of samples from the global stratotype section and point gssp of the permian triassic boundary at meishan china using isotope dilution inductively coupled plasma mass spectrometry id icp ms and nickel sulfide fire assay nis fa combined with te coprecipitation the samples have no ir anomaly 5 53 pg g and their pge patterns normalized to chondrites are highly fractionated with ir pd ratios of 0 02 0 03 ci distinct from most extraterrestrial materials in contrast these patterns are closely parallel to those of the siberian and probably emeishan flood basalts suggestive of possible sources of pges from the basalts the abundances of pges increase in order of the pyrite lamina on the top of bed 24 bed 25 and bed 26 and then decrease to bed 28 probably indicative of a maximum eruption of the flood basalts during deposition of bed 26 the new data favor massive volcanism rather than extraterrestrial impact as a major cause of the permian triassic boundary mass extinction

paleotethyan evolution of the indochina block as deduced from granites in northern laos abstract the paleotethyan evolution of the southeast asia has become better understood in recent years questions remain however over the role of the dien bien phu suture zone in the evolution of the indochina block and whether the song ma suture represents the boundary between the indochina block and the south china block granitoid geochronological and geochemical data obtained in northern laos provide new information vis a vis these arguments zircon u pb ages together with whole rock trace and rare earth element data from 27 granitic rocks from five complexes allow us to conclude that these granites are typical of i type indosinian volcanic arc granites however the 234 256 ma i type granites mismatch the initiation age obtained from the ductile shear zone of the dien bien phu fault thus repudiating the existence of the dien bien phu suture zone this then implies that the qamdo simao and indochina blocks were united the geochemical and geochronological data further suggest that the main crust in the indochina block formed in the late paleoproterozoic to early mesoproterozoic much later than the archean crustal formation age identified east of the song ma suture moreover the 440 404 ma and 234 256 ma i type granites suggest that the boundary between indochina and south china should be the jinsha river suture song ma suture kontum massif instead of the jinsha river suture song chay suture finally the emeishan basalt and granite complexes both form part of the south china tectonic units subducting westward under the qamdo simao and indochina blocks

the formation of p rich fe ti oxide ore layers in the taihe layered intrusion sw china implications for magma plumbing system process abstract the taihe intrusion is one of the layered intrusions that host giant fe ti oxide deposit in the central part of the emeishan large igneous province sw china different from the other layered intrusions the taihe intrusion has a massive fe ti oxide ore layer at the top of the lower zone lz particularly the rocks of both middle and upper zones contain as much as 5 12 apatite and the most important fe ti oxide ore layers apatite magnetite clinopyroxenite occur at the bases of cyclic units in the middle zone mz rather than in the lower zone lz high cr and ni concentrations of clinopyroxene 65 263 ppm and 66 83 ppm respectively and high an contents of plagioclase 79 80 of the lz rocks indicate their more primitive parental magmas the clinopyroxene and plagioclase of the mz rocks are characterized by low cr and ni contents 1 21 ppm and 0 4 10 ppm respectively and low an 51 67 indicating more evolved parental magma the fe ti oxide ore layers in the mz are not only high in fe and ti but also enriched in p2o5 and rare earth elements indicating an unusual fe ti p rich parental magma it is speculated that the mz represents a later intrusion of fe ti p rich magmas that were produced by the fe ti enriched magma from a deep seated magma chamber mixing with the extensively evolved p rich magma in a middle level magma chamber early crystallization and cumulation of fe ti oxide and apatite together with clinopyroxene from the fe ti p rich magmas from the middle level magma chamber resulted in the formation of fe ti oxide ore layers in the mz fe ti p rich magma and apatite magnetite clinopyroxenites occur in the base of cyclic units whereas apatite gabbros are presented at the upper parts of each cyclic unit the upper zone uz is the product of the crystallization of highly differentiated magma consists chiefly of apatite gabbro and minor intercalated apatite clinopyroxenite

chalcophile element constraints on magma differentiation of quaternary volcanoes in tengchong sw china abstract the tengchong volcanic field comprises numerous quaternary volcanoes in sw china the volcanic rocks are grouped into units 1 4 from the oldest to youngest units 1 3 and 4 are composed of trachybasalt basaltic trachyandesite and trachyandesite respectively and unit 2 consists of hornblende bearing dacite this rock assemblage resembles those of arc volcanic sequences related to oceanic slab subduction rocks of units 1 and 3 contain olivine phenocrysts with fo contents ranging from 65 to 85 mole indicating early fractionation of olivine and chromite prior to the eruption of magma all the rocks from units 1 3 and 4 have very low pge concentrations with

open magma chamber processes in the formation of the permian baima mafic ultramafic layered intrusion sw china abstract the baima mafic ultramafic layered intrusion of the 260 ma emeishan large igneous province elip hosts the second largest fe ti v oxide deposit in the panxi region sw china it is a 1600 m thick layered body intruded by slightly younger syenitic and granitic plutons the intrusion includes the lower and upper zones troctolite and olivine pyroxenite of the lower zone contains conformable oxide ore layers whereas the upper zone consists of olivine gabbro and gabbro with abundant apatite in the higher level the crystallization order of the silicates in the baima intrusion is olivine plagioclase clinopyroxene fe ti oxides titanomagnetite and ilmenite crystallized after olivine and possibly plagioclase the oxide ores in the lower zone show slightly lree enriched patterns with la yb n values between 2 0 and 6 4 and positive eu anomalies eu eu of 1 0 to 2 7 in contrast olivine gabbros in the lower zone display stronger lree enrichments la ybn 7 7 14 0 and positive eu anomalies eu eu 2 8 3 3 gabbros in the upper zone have ree profiles characterized by intermediate lree enrichments with la yb n values of 3 2 to 11 2 and positive eu anomalies of 2 1 to 3 0 primitive mantle normalized trace element patterns are characterized by negative la ce th sm and positive nb ta ba and ti anomalies in oxide ores and negative th u zr hf and positive ba sr and ti anomalies in olivine gabbro and gabbro fo of olivine and an of plagioclase remain roughly constant from 0 to 90 m in the lower zone indicating that the magma chamber was continuously filled by compositionally similar magmas during the initial stage three magma replenishments occurred afterwards in the upper part of the lower zone and the upper zone based on compositional reversals of plagioclase olivine and sr isotope mass balance calculations show that the baima parental magma can produce all oxide ores under closed system conditions a wide range of an values of plagioclase within thin sections and disequilibrium sr isotopic compositions of plagioclase both along the stratigraphic profile 87sr 86sri 0 70312 0 70510 and within thin sections indicate convection and co accumulation of cumulus plagioclase that had crystallized from different magmas we propose that the 1600m thick baima intrusion formed in an open and isotopically heterogeneous magma chamber that was periodically recharged by compositionally similar or more primitive magmas in each replenishment cycle the magma underwent progressive crustal contamination and fractional crystallization convection and sorting of crystals based on density differences result in igneous layering characterized by intervals of oxide ores troctolite and olivine gabbro

geochemistry of the emeishan flood basalts at yangliuping sichuan sw china implications for sulfide segregation a well developed 1 000 m thick basaltic sequence in the yangliuping region northern part of the emeishan basalt province includes the lower and middle units of tholeiitic basalts and an upper unit of both tholeiites and subalkalic basalts the basalts contain 42 55 wt sio2 and 4 1 8 3 wt mgo most of these lavas have gd yb 2 0 zr nb 12 and nd 260 ma values from 2 5 to 4 7 the platinum group elements pge are very mildly depleted in most of the basalts which contain 8 19 ppb pt and 7 27 ppb pd however a significant proportion of the middle unit basalts are strongly depleted in pge with some samples having concentrations lower than detection limits they have extremely high zr nb ratios up to 14 5 and low nd 260 ma values 3 21 to 0 65 features of extensive lower crustal contamination some samples in this unit have high ni pd 3 965 61 198 and low pd cr 410 000 3 930 000 ratios indicating sulfide segregation and pge depletion prior to eruption the primary magmas were s undersaturated and derived from partial melting at variable depths in the upper mantle the early and late stage magmas as represented by the lower and upper units underwent afc processes which induced mild s saturation and pge depletion in some of the basalts whereas the magmas represented by the middle unit experienced more extensive crustal contamination resulting in stronger s saturation and in most cases significant pge depletion

geochemistry and origin of the intrusive hosts of the noril sk talnakhcu ni pge sulfide deposits the world class deposits of cu ni and platinum group elements pge of the noril sk talnakh district in russia are hosted by relatively small complex mafic ultramafic bodies that intrude permian sedimentary rocks and the lowermost suites of the siberian continental flood volcanic sequence the noril sk type ore bearing intrusions immediately overlie and are probably penecontemporaneous with weakly mineralized intrusions of the lower talnakh type which have similar petrological features but contrasting and much more variable nd os and sr isotopic compositions several types of spatially associated intrusions were emplaced after the ore bearing intrusions and the flood volcanic sequence to clarify relations between the various types of igneous bodies flood volcanic rocks noril sk type intrusions lower talnakh type intrusions and later intrusions and to help understand the formation of the ore deposits we undertook a detailed geochemical and nd os and sr isotopic study of eight of the more important intrusions the data indicate that these intrusions formed by the input of a series of magma pulses each of which produced rocks with distinctive petrology and chemical compositions these compositions are interpreted to have formed from mixtures of mantle derived magmas and different types of crustal rock which were encountered and assimilated during passage to the surface some of these variations may be superimposed on heterogeneities inherited from the mantle source the magmas that formed the lower talnakh type intrusions became contaminated with large amounts of crustal rock in a deep staging chamber the chemical and isotopic characteristics of the resultant hybrid magmas are broadly similar to those of basalts of the lower and middle nadezhdinsky lava subsuites nd 1 2 which are located about one third of the way through the overlying lava sequence these magmas became saturated in sulfur during contamination and segregation of sulfides resulted in the depletion of chalcophile elements recorded in the nd 1 2 lavas magmas feeding the ore bearing intrusions had trace element and isotopic compositions similar to those of the basalts in the upper two thirds of the volcanic sequence these magmas underwent lower degrees of contamination in a deep staging chamber and sulfur saturation was not reached these magmas therefore contained a full compliment of chalcophile elements when they encountered anhydrite rich sediments during passage toward the surface assimilation of these sediments led to the segregation of sulfides to form the ores two aspects of our data impose important constraints on the ore forming process the first is the os isotopic composition of the ore sulfides avg os 7 which is similar to that of mantle rocks and very different from those of sulfides that segregate from highly contaminated magma the second is the absence of ni depletion in olivine in rocks from the ore bearing intrusions most notably in the ore rich picritic gabbrodolerites the simplest explanation is that the sulfides segregated from magmas that were not highly contaminated and were transported into the intrusions by magmas different from those that yielded the sulfide if this explanation is correct it imposes some modifications to earlier models of sulfide ore deposition in the conduits of flood basalts

late triassic granitoids of the eastern margin of the tibetan plateau geochronology petrogenesis and implications for tectonic evolution abstract late triassic granitoids in the songpan garze fold belt sgfb on the eastern margin of the tibetan plateau formed at 230 to 220 ma and can be divided into two groups group 1 are high k calc alkaline rocks with adakitic affinities k adakites with sr 400 ppm y n 32 105 and high k 2 o na 2 o 1 group 2 are ordinary high k calc alkaline i types with lower sr 18 ppm and weakly fractionated ree patterns la yb n 0 50 to 0 94 and initial 87 sr 86 sr 0 70528 to 0 71086 but group 1 rocks have higher e nd t 1 01 to 4 84 than group 2 3 11 to 6 71 calculated initial pb isotope ratios for both groups are 206 pb 204 pb 18 343 to 18 627 207 pb 204 pb 15 610 to 15 705 and 208 pb 204 pb 38 269 to 3759 group 1 magmas were derived through partial melting of thickened and then delaminated ttg type eclogitic lower crust with some contribution from juvenile enriched mantle melts group 2 magmas were generated by partial melting of shallower lower crustal rocks the inferred magma sources of both groups suggest that the basement of the sgfb was similar to the exposed kangding complex and that the sgfb was formed in a similar manner to the south china basement here passive margin crust was greatly thickened and then delaminated all within a very short time interval 20 myr such post collisional crustal thickening could be the tectonic setting for the generation of many adakitic magmas especially where there is no spatial and temporal association with subduction

molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the early jurassic relatively brief periods of severe paleoenvironmental change during the jurassic and cretaceous were associated with the widespread accumulation of organic rich marine deposits termed oceanic anoxic events oaes these intervals involved abrupt global warming of 5 10 c higher rates of continental weathering elevated extinction rates and large scale perturbations to the global carbon cycle the major oaes also overlapped temporally the emplacement of large igneous provinces however despite being known as oaes the extent of seawater anoxia at those times is undefined and the causative processes remain unclear here we show how changes in seawater molybdenum isotope ratios a proxy for seawater anoxia during the toarcian early jurassic oae define the onset and expansion of oxygen deficient conditions our data also place constraints on the areal extent of marine anoxia during the event and demonstrate that anoxia expanded and contracted periodically broadly in line with precession driven changes in 13 corg despite their intermittent occurrence over geological history oaes have an important contemporary relevance because the magnitude and high rates of environmental change then were broadly similar to those occurring at the present day