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

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

a lower crust origin of some flood basalts of the emeishan large igneous province sw china abstract high seismic velocity layers within the lower crust i e 40 km of the yangtze block are interpreted as mafic underplated rocks derived from the late permian emeishan mantle plume however the region experienced a previous magmatic event during the neoproterozoic 800 ma that produced the kangdian basalts and associated mafic intrusions and therefore the lower crust seismic velocity layers may represent a mixture of two different magmatic episodes the identification of inherited neoproterozoic i e 750 to 850 ma zircons within emeishan magmatic rocks indicates either assimilation of older material during emplacement or that the rocks could be derived from a mafic neoproterozoic precursor equilibrium partial melt modeling of neoproterozoic kangdian basalts can produce compositions similar to emeishan basalt at a pressure of 1 2 gpa i e 40 km depth it is suggested that the injection of high temperature picritic magmas into the lower crust of the yangtze block could be sufficient to induce high degrees of melting of the lower crust and produce magmas that are similar in composition to some emeishan mafic rocks and or produce melts that could contaminate pristine emieshan magmas

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

iron rich fragments in the yamansu iron deposit xinjiang nw china constraints on metallogenesis abstract volcanic rock hosted iron deposits are among the important iron ores in china however the nature of primary magma and petrogenesis associated with these iron ores remains controversial here we report iron rich fragments irf from the yamansu iron deposit in eastern tianshan mountains nw china which occurs in association with volcanic breccia submarine volcanic breccia and ignimbrite the irf is composed of five types including oligoclase iron oxide type oio oligoclase albite iron oxide type oaio albite iron oxide type aio albite k feldspar iron oxide type akio and k feldspar iron oxide type kio these fragments display typical volcanic fabric features such as porphyritic texture hyalopilitic texture of the groundmass and vesicles filled by minerals to form amygdales the feldspar phenocrysts of irf are dominantly albite the groundmass of irf consists of magnetite and feldspar the magnetite is distributed in between the feldspar laths and together display hyalopilitic texture which could be observed only in volcanic rocks the vesicles are filled with magnetite feldspar chlorite and calcite from the margin to the interior the irf has high si al fe ca ti na and k contents and low mg content the average total fe is 26 wt the magnetite is mostly titanium vanadium magnetite with the tio2 content ranging up to 4 86 wt and v2o3 content up to 3 20 wt the irf probably came from iron rich melts and represent the products of the fenner magma evolution the basaltic magma evolved into the fe na rich residual melts by crystallization under low oxygen fugacity condition in a closed magma chamber after intruding into the shallow crust the fe na rich residual melts were emplaced in hypabyssal environments or erupted generating the orebodies or providing the material source for the generation of the high grade iron ores which were subsequently enriched by the late stage hydrothermal fluids

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

origin of the early permian wajilitag igneous complex and associated fe ti oxide mineralization in the tarim large igneous province nw china abstract the wajilitag igneous complex is part of the early permian tarim large igneous province in nw china and is composed of a layered mafic ultramafic intrusion and associated syenitic plutons in order to better constrain its origin and the conditions of associated fe ti oxide mineralization we carried out an integrated study of mineralogical geochemical and sr nd hf isotopic analyses on selected samples the wajilitag igneous rocks have an oib like compositional affinity similar to the coeval mafic dykes in the bachu region the layered intrusion consists of olivine clinopyroxenite coarse grained clinopyroxenite fine grained clinopyroxenite and gabbro from the base upwards fe ti oxide ores are mainly hosted in fine grained clinopyroxenite forsterite contents in olivines from the olivine clinopyroxenite range from 71 to 76 mol indicating crystallization from an evolved magma reconstructed composition of the parental magma of the layered intrusion is fe ti rich similar to that of the bachu mafic dykes syenite and quartz syenite plutons have end t values ranging from 1 4 to 2 9 identical to that for the layered intrusion they may have formed by differentiation of underplated magmas at depth and subsequent fractional crystallization magnetites enclosed in olivines and clinopyroxenes have cr2o3 contents higher than those interstitial to silicates in the layered intrusion this suggests that the cr rich magnetite is an early crystallized phase whereas interstitial magnetite may have accumulated from evolved fe ti rich melts that percolated through a crystal mush low v content in cr poor magnetite

the role of komatiitic and picritic magmatism and s saturation in the formation of ore deposits abstract the introduction of high temperature high magnesium komatiitic and picritic magmas into the earth s upper crust has given rise either directly or indirectly to many of the world s major ore deposits there is a direct association between komatiitic magmatism and kambalda style ni cu pge deposits the link between magma type and mineralization is not as clear cut in the case of the noril sk cu ni pge deposits but nevertheless there is an association between picritic magmatism and ore deposit development komatiitic and or picritic magmatism may also have contributed to the formation of many types of hydrothermal ore deposits recent nd isotope data generated by kent et al 1994 provide strong evidence for the hypothesis proposed by keays in 1983 that the au in archaean au deposits was derived either directly or indirectly from komatiites through the action of metamorphic fluids the importance of komatiitic and picritic magmas in ore forming processes arises from the fact that they are among the few mafic ultramafic magma types that are s undersaturated at the time of magma formation s saturation of magmas leads to marked depletion in the chalcophile metals for example czamanske and moore 1977 estimated that a third of the original cu and ni contents of morb were lost from the magmas due to extraction into magmatic sulphides the depletion in au and the pge is much greater due to their much larger partition coefficients keays and scott 1976 komatiitic and picritic magmas are s undersaturated because they are high temperature magmas produced by large degrees of partial melting of upper mantle sourc regions that were already depleted in s through earlier partial melting events most mafic magmas produced by less than 25 partial melting are s saturated and hence depleted in the ore forming chalcophile elements in contrast komatiitic and picritic magmas have their full complement of chalcophile metals when they do become s saturated they form sulphides that are strongly enriched in ni cu au pge and other strongly chalcophile metals these may accumulate to form ore deposits directly e g magmatic ni cu pge sulphides or be dispersed in komatiitic picritic rocks or their fractionation products making these source rocks for hydrothermal au or vms deposits

geochronology of layered mafic intrusions from the pan xi area in the emeishan large igneous province sw china the panzhihua xichang pan xi area hosts mafic ultramafic intrusions which are part of the permian emeishan large igneous province some of these intrusions host giant fe ti v deposits and minor ni cu pge mineralization in the present study zircon u pb ages of 259 3 1 3 and 260 7 0 8 ma have been obtained from the giant fe ti v ore bearing hongge and the unmineralized binggu intrusions respectively by isotope dilution thermal ionization mass spectrometry method in combination with the ages of other ore bearing intrusions this age shows that these mafic ultramafic intrusions were emplaced at ca 260 ma the hongge and binggu intrusions cut the lower part of the rapidly deposited emeishan flood basalt sequence but no further into the upper volcanic sequence in the pan xi area thus emplacement and mineralization of the mafic ultramafic intrusions were almost contemporaneous with the eruption of the emeishan flood basalts during a relatively short time span

noble gas isotopic systematics of fe ti v oxide ore related mafic ultramafic layered intrusions in the panxi area china the role of recycled oceanic crust in their petrogenesis olivine and clinopyroxene grains have been separated from four large fe ti v oxide ore bearing intrusions panzhihua hongge baima and taihe in the panxi area emeishan large igneous province southwest china for he and ar isotope studies the samples examined revealed extremely low 3 he 4 he ratios 0 078 4 34 ra with the mean value 0 78 ra for gases extracted by stepwise heating this feature combined with low 40 ar 36 ar ratios can be interpreted as due to addition of subduction related fluids and melts that had been stored in the lithospheric mantle for long periods considering the regional geologic history such addition can be attributed to the paleo subduction that occurred along the western margin of the yangtze block during the neoproterozoic the subducted oceanic crust beneath the panxi area underwent eclogite facies metamorphism and subsequent exhumation the infiltration of subduction related melts and fluids into the lithospheric mantle led to enriched isotopic signatures from that of the slightly depleted asthenopheric mantle which has been suggested by the sr nd and pb isotopic data of the emeishan basalts and picrites in addition considerable amounts of eclogitic melts produced by partial melting of eclogite facies oceanic crust extensively contaminated the lithospheric mantle during the late permian partial melting of an upwelling mantle plume that contained an eclogite or pyroxenite component generated the parental fe rich magma that supplied the ore bearing intrusions the combination of these factors may have been the crucial reason that many world class fe ti v oxides deposits are clustered in the panxi area

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

formation of thick stratiform fe ti oxide layers in layered intrusion and frequent replenishment of fractionated mafic magma evidence from the panzhihua intrusion sw china 1 the panzhihua intrusion is one of the largest layered intrusions that hosts huge stratiform fe ti oxide layers in the central part of the emeishan large igneous province sw china up to 60 m thick stratiform massive fe ti oxide layers containing 85 modal of magnetite and ilmenite and overlying magnetite gabbro compose cyclic units of the lower zone of the intrusion the cyclic units of the middle zone consist of magnetite gabbro and overlying gabbro in these cyclic units contents of fe2o3 t tio2 and cr and fe3 ti4 ratio of the rocks decrease upward cr content of magnetite and forsterite percentage of olivine decrease as well the upper zone consists of apatite gabbro characterized by enrichment of incompatible elements e g 12 18 ppm la 20 28 ppm y and increasing of fe3 ti4 ratio from 1 3 to 2 3 upward these features indicate that the panzhihua intrusion was repeatedly recharged by more primitive magma and evolved magmas had been extracted calculations using melts indicate that extensive fractionation of olivine and clinopyroxene in deep level resulted in increasing fe and ti contents in the magma when these fe ti enriched magmas were emplaced along the base of the panzhihua intrusion fe ti oxides became an early crystallization phase leading to a residual magma of lower density we propose that the unusually thick stratiform fe ti oxide layers resulted from coupling of gravity settling and sorting of the crystallized fe ti oxides from fe ti enriched magmas and frequent magma replenishment along the floor of the magma chamber

a reappraisal of the high ti and low ti classification of basalts and petrogenetic linkage between basalts and mafic ultramafic intrusions in the emeishan large igneous province sw china abstract based on published data we reappraise the classification of high ti and low ti basalt from the emeishan large igneous province elip and the correlations between basalts and mafic ultramafic intrusions because of the lack of clear spatial and temporal variations of different types of basalts we suggest that the basalts in the elip cannot be classified into high ti and low ti groups by tio 2 contents and or ti y ratios the distinctive characteristics of these high ti and low ti lavas probably result largely from the different fractionating assemblages whether or not fractional crystallization of the fe ti oxides occurred probably is the key factor that controls the ti abundances and ti y ratios in the residual melts e g lavas although the nature of the mantle sources variable degrees of partial melting of mantle and crustal contamination also influence the geochemical signatures of the lavas therefore neither ti abundance nor ti y ratios in basalts can reflect the nature of their mantle source moreover the different types of mafic ultramafic intrusions in the elip cannot simply be attributed to be genetically related special types of basalts either high ti or low ti basalts it is likely that they are merely the cumulus phases i e chamber or conduit of the basaltic lavas hence caution should be exercised in the use of high ti or low ti basalts as prospecting vectors for ore deposits in the region potential implications are proposed that both the fe v ti oxide and cu ni pge sulfide mineralization in the elip intrusions is largely due to the variable differentiation and crustal contamination during magmatic processes

age of the emeishan flood magmatism and relations to permian triassic boundary events the permian triassic p t mass extinction the greatest biological mortality event in the earth s history was probably caused by dramatic and global forcing mechanisms such as the siberian flood volcanism here we present the first set of high precision 40 ar 39 ar dating results of volcanic and intrusive rocks from the emeishan traps south china which define a main stage of the flood magmatism at v251 253 ma and a subordinate precursory activity at v255 ma this time span is generally coeval with or slightly older than the age of the p t boundary estimated by the ash beds in the meishan stratotype section and the main eruption of the siberian traps our data reinforces the notion that the eruption of the emeishan traps rather than eruption of the siberian traps accounted for the formation of the p t boundary ash beds in south china the emeishan flood magmatism which occurred in the continental margin comprising thick marine limestone formations moreover may have triggered rapid release of large volumes of methane and carbon dioxide that could have been responsible for the global n 13 c excursion and associated environmental crisis leading to the mass extinction at the p t boundary fl 2002 elsevier science b v all rights reserved

a new plumbing system framework for mantle plume related continental large igneous provinces and their mafic ultramafic intrusions abstract the magmatic components of continental large igneous provinces lips include flood basalts and their plumbing system of giant mafic dyke swarms radiating linear and the recently discovered circumferential type mafic sill provinces a lower crustal magmatic underplate mafic ultramafic m um intrusions associated silicic magmatism and associated carbonatites and kimberlites this paper proposes a new plumbing system framework for mantle plume related continental lips that incorporates all of these components and provides a context for addressing key thematic aspects such as tracking magma batches upstream and downstream and their geochemical evolution assessing the setting of m um intrusions and their economic potential interpreting deep magmatic component identified by geophysical signatures and estimating magnitudes of extrusive and intrusive components with climate change implications this plumbing system model and its associated implications needs to be tested against the rapidly improving lip record

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

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

elemental and sr nd isotope geochemistry of microgranular enclaves from peralkaline a type granitic plutons of the emeishan large igneous province sw china abstract microgranular enclaves are common within intermediate to felsic granitic rocks that have i and s type affinity however they are rare within alkaline anorogenic granitoids of a type affinity the permian 260 ma emeishan large igneous province elip of southwest china contains two peralkaline silica saturated a type granitic plutons that host microgranular enclaves the enclaves from the baima pluton are intermediate in composition and have lower sio 2 and higher tio 2 cao and mg sio 2 57 2 to 63 0 wt tio 2 0 8 to 1 8 wt cao 1 7 to 3 3 wt mg 28 to 44 than their host sio 2 62 6 to 67 8 wt tio 2 0 5 to 1 4 wt cao 0 4 to 1 8 wt mg 15 to 31 the enclaves from the taihe pluton are more felsic sio 2 63 8 to 71 3 wt tio 2 0 3 to 0 6 wt cao 0 6 to 2 3 wt mg 8 to 22 but are still less evolved than their host sio 2 69 8 to 75 1 wt tio 2 0 2 to 0 6 wt cao 0 4 to 0 8 wt mg 3 to 12 in both cases the enclaves have very similar end t values baima end t 2 8 to 3 2 taihe end t 1 0 to 2 0 as their hosts baima end t 3 0 to 3 2 taihe end t 1 5 to 1 9 the major and trace element trends of the enclave host pairs suggest that fractional crystallization occurred and that element diffusion was likely minimal the enclaves are interpreted as entrained accumulations of early formed crystals of a silicic magma which was originally produced by fractional crystallization of a mafic magma

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

emeishan large igneous province sw china in recent years there have been major advances in our understanding of the emeishan large igneous province lip of sw china following publication of a number of lip focused investigations of the terrain and associated rocks this paper reviews the current state of knowledge the volcanic and upper intrusive portion of the province is relatively small 0 3 106 km3 even when offset fragments eroded sections and buried portions are included in the volume calculation the most reliable radiometric age dates zircon u pb shrimp from an associated layered intrusive body several kilometers in area indicate generation at 259 ma consistent with the end guadalupian end middle permian stratigraphic age in addition several ar ar dating studies have been carried out mainly on the volcanic rocks with a number of reported dates 253 251 ma late permian but a consensus is emerging that these ages are problematic because they are in conflict with the stratigraphic data possibly due to a monitor standard miscalibration the ar ar investigations have also yielded a large number of secondary ages which are clustered at 175 142 98 and 42 ma and these are inferred to record sub regional tectonic events that affected the western yangtze block as east asia was assembled and later deformed by india s collision indentation into asia magnetostratigraphic data and field observations suggest that the bulk of the volcanic sequence formed within 1 2 my the geochemistry of the volcanic rocks and bio lithostratigraphic studies of the underlying maokou formation suggests a mantle plume generated the province the basalts can be classified into low and high ti groups with different parental magmas the low ti basalts are confined to the western part of the province and are overlain by the high ti basalts the low ti magmas formed at shallow mantle depths in the spinel garnet field transition zone 60 80 km whereas the high ti magmas formed by low degrees of partial melting within the deeper garnet stability field this observation suggests a deepening of the melting column as the lithosphere thickened due to under plating and a transition from peak basalt generation to the waning stage outstanding issues which might focus future studies are also outlined

the origin of ni cu pge sulfide mineralization in the margin of the zhubu mafic ultramafic intrusion in the emeishan large igneous province southwestern china the zhubu magmatic ni cu pge sulfide deposit is hosted in a mafic ultramafic intrusion which is part of the emeishan large igneous province in southwest china the zhubu intrusion is composed of a layered sequence 750 x 400 x 600 m with subhorizontal modal layering and a subvertical marginal zone of 40 m across the marginal zone is composed of iherzolite and olivine websterite with minor gabbroic rocks the layered sequence is composed of iherzolite websterite gabbro and gabbrodiorite from the base to the top the zhubu intrusion can be explained by two stages of formation an early conduit stage for the marginal zone and a late in situ differentiation stage for the layered sequence most important ni cu pge mineralization in the intrusion occurs as disseminated sulfides within the marginal zone olivine crystals from the marginal zone contain 81 to 84 mol fo and 1 600 to 1 900 ppm ni the rims of zoned olivine phenocrysts in the emeishan picrites have similar fo contents but significantly higher ni contents 2 300 2 600 ppm the olivine data indicate that the parental magma of the zhubu ultramafic rocks is similar to the transporting magma of the emeishan picrites in mgo feo ratios but depleted in ni due to sulfide segregation before olivine crystallization the initial concentrations of pge in the zhubu magma estimated from bulk sulfide compositions are 7 ppb pd 9 3 ppb pt and 0 8 ppb ir similar to the values in the emeishan picrites like the emeishan picrites the zhubu intrusive rocks are characterized by light ree enrichments negative nb anomalies relative to th and ta which are rare in the emeishan picrites are present in the zhubu samples the sr 87 sr 86 i and epsilon nd values of the zhubu intrusive rocks vary from 0 709591 to 0 710692 and from 2 to 3 respectively the trace element and isotope compositions indicate that the zhubu magma was contaminated by crustal materials supporting the interpretation that sulfide saturation in the magma was triggered by crustal contamination the area where the lower part of the conduit may have been brought up by faulting should be the focus of future exploration at zhubu

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 paleomagnetic investigations of the emeishan basalts in ne yunnan southwestern china constraints on eruption history abstract paleomagnetic and magnetostratigraphic investigations have been conducted on two consecutive sections of the emeishan trap at shuimo sm and dadi dd in northeastern yunnan province southwestern china rock magnetic analysis indicates that the magnetic carrier of the basalts is dominantly low ti magnetite stepwise thermal demagnetization isolates a stable characteristic remanent magnetization chrm at high temperatures 400 585 c the chrms 301 323 at sm section are northeasterly upward of normal polarity with a mean direction of ds is 11 0 5 2 n 12 k 53 7 95 6 0 at dd section the chrms are southwesterly downward of reversed polarity with a mean direction of 205 9 10 0 n 5 k 78 4 95 8 7 the chrms from the basaltic gravels from the overlying conglomerate layer in late permian xuanwei formation at dd section pass the conglomerate test suggesting the primary nature of magnetization together with other magnetostratigraphic studies e g n r pattern of the emeishan basalts and the known radiometric ages ca 260 ma it suggests that the eruption period of the emeishan large igneous province is less than 3 myr

magnesium isotopic evidence for chemical disequilibrium among cumulus minerals in layered mafic intrusion abstract magnesium isotopic compositions of olivine clinopyroxene and ilmenite from the baima intrusion sw china for the first time are investigated to constrain the magnitude and mechanisms of mg isotope fractionation among cumulus minerals in layered mafic intrusions and to evaluate their geological implications olivine and clinopyroxene have limited mg isotope variations with 26 mg ranging from 0 33 to 0 05 and from 0 29 to 0 13 respectively similar to those of mantle xenolithic peridotites by contrast ilmenites display extremely large mg isotopic variation with 26 mg ranging from 0 50 to 1 90 the large inter mineral fractionations of mg isotopes between ilmenite and silicates may reflect both equilibrium and kinetic processes a few ilmenites have lighter mg isotopic compositions than coexisting silicates and contain high mgo contents without compositional zoning indicating equilibrium fractionation the implication is that the light mg isotopic compositions of lunar high ti basalts may result from an isotopically light source enriched in cumulate ilmenites on the other hand most ilmenites have heavy mg isotopic compositions coupled with high mgo concentration and chemical zoning which can be quantitatively modeled by kinetic mg isotope fractionations induced by subsolidus mg fe exchange between ilmenite and ferromagnesian silicates during the cooling of the baima intrusion the extensive occurrence of kinetic mg isotope fractionation in ilmenites implies the possibility of widespread compositional disequilibrium among igneous minerals in magma chambers consequently disequilibrium effects need to be considered in studies of basaltic magma evolution magma chamber processes and magmatic fe ti oxide ore genesis

a high resolution middle to late permian paleotemperature curve reconstructed using oxygen isotopes of well preserved brachiopod shells abstract climatic warming has been widely cited as a driver for many mass extinction events in earth history including the middle late permian guadalupian lopingian boundary glb mass extinction and end permian mass extinction epme investigated in this study however the role of warming in driving the epme event is under debate because current paleotemperature studies indicate the rapid increase in sea surface temperature postdate the onset of the epme event and a reliable paleotemperature record across the glb event is lacking here we present a high resolution and successive 18 o record using low mg calcite shells of brachiopods blmcs from a single stratigraphic section in central china to reconstruct paleotemperature changes from the middle to the latest permian after a rigorous screening for diagenesis our data show a 1 0 decrease in 18 ocalcite and thus climatic warming during the middle capitanian followed by a gradual 2 0 increase climatic cooling during the late wuchiapingian from the latest wuchiapingian to the early changhsingian 18 o decreased gradually and reached the lowest values of 4 0 and thus rapid climatic warming immediately before the permian triassic boundary the two warming events are coeval with the onset of the emeishan large igneous province and the siberian traps large igneous province respectively suggesting the two volcanic events are the plausible drivers of these climatic warming events a close comparison of paleontological and geochemical records indicates that the warming events occurred predate the glb and epme events this is in contrast with conodont 18 oapatite data which document the rapid climatic warming lagged the onset of the extinction event the comparison between 18 o of blmcs and 18 o of conodonts from the same section shows that the long term trends are similar but at higher temporal resolution discrepancies are present entailing further investigation in the near future our study confirms that 18 o of the brachiopod shells underwent important changes during the end of paleozoic highlighting that sea surface temperature is a key factor to understand the biosphere history since it changes simultaneously with the biological crises and that blmcs are invaluable archive for tracking physical and chemical conditions of past oceans

disequilibrium iron isotopic fractionation during the high temperature magmatic differentiation of the baima fe ti oxide bearing mafic intrusion sw china abstract iron isotopic compositions of olivine clinopyroxene and titanomagnetite of the baima fe ti oxide bearing layered mafic intrusion sw china are used to investigate fe isotopic fractionation during the formation of the fe ti oxide ore bodies and to constrain the origin of silicate minerals and fe ti oxides the baima intrusion comprises the lower zone of interlayered troctolite clinopyroxenite and oxide ores and the upper zone of gabbros significant differences of fe isotope values between olivine fe 56 0 01 to 0 11 clinopyroxene fe 56 0 11 to 0 22 and titanomagnetite fe 56 0 20 to 0 31 were observed in rocks and ores of the lower zone iron isotopic fractionation between olivine and clinopyroxene olivine and titanomagnetite and clinopyroxene and titanomagnetite exhibits large variations of 0 06 to 0 22 0 12 to 0 27 and 0 00 to 0 20 respectively suggesting disequilibrium fractionation this disequilibrium cannot be explained by thermal or chemical gradient induced kinetic fractionation or by possible subsolidus exsolution of granular ilmenite from titanomagnetite instead it could be attributed to crystallization of silicates and titanomagnetite from two immiscible si rich and fe rich liquids continuous segregation of the fe rich liquid changed the fe isotopic composition of the si rich liquid and thus the fe isotopic compositions of olivine and clinopyroxene crystallized from it resulting in disequilibrium fe isotopic fractionation between them the effect of oxygen fugacity on the crystallization order of titanomagnetite and ilmenite from the fe rich melts on the other hand gave rise to the disequilibrium fe isotope fractionation between titanomagnetite and olivine clinopyroxene

genesis of the permian baimazhai magmatic ni cu pge sulfide deposit yunnan sw china the 260 ma old baimazhai ni cu pge sulfide deposit in the jinping region yunnan sw china is hosted in a small mafic ultramafic intrusion which intruded ordovician sandstone and slate the intrusion is concentric with lens shape about 530 m long 190 m wide and 24 to 64 m thick trends 296 and dips 22 ne the massive sulfide ore body forms the core of the intrusion and is surrounded by variably mineralized orthopyroxenite websterite and barren gabbro the proportion of gabbro websterite orthopyroxenite and massive ore is approximately 30 30 20 and 20 vol respectively magmatic pyrrhotite pentlandite and chalcopyrite make up more than 90 of the massive ores the massive ores contain high ni 1 6 to 4 2 wt and cu 0 4 to 6 5 wt and low pge contents 85 to 524 ppb they have pd ir ratios ranging from 6 7 to 530 pd pt ratios from 0 7 to 2 6 and cu pd 1 000 ratios from 31 to 400 which are comparable with those of the silicate rocks pd ir 4 to 183 pd pt 0 7 to 3 5 and cu pd 1 000 100 to 400 similar pd pt and cu pd ratios of the silicate rocks and massive ores throughout the intrusion indicate a single sulfide segregation event excess sulfide melt segregation resulted from intensive crustal contamination that formed si rich and mg rich basaltic magmas in a deep seated staging chamber before magma emplacement the immiscible sulfide melts and the silicate melts were eventually evacuated from the staging magma chamber by compressive forces flow differentiation under high velocity concentrated the sulfide melts toward the middle of the magma flow and consequently formed a massive sulfide ore body in the central part of the intrusion low concentrations of pges and general absence of platinum group minerals in the massive ores may have resulted from a relatively large mass fraction of the sulfide melts e g r factor 70 in baimazhai compared with other intrusions elsewhere such as noril sk talnakh with a r factor of 10 000

re os isotopic constraints on the genesis of the limahe ni cu deposit in the emeishan large igneous province sw china abstract there are three types of magmatic sulfide deposits sulfide poor pge deposit sulfide rich ni cu deposit and ni cu pge deposit associated with mafic ultramafic intrusions in the late permian emeishan large igneous province sw china the limahe deposit represents a sulfide rich ni cu deposit in the region re os concentrations and isotopic compositions of the sulfide ores and associated ultramafic rocks in the limahe intrusion are used to evaluate the relationships between magma evolution and oregenesis there are two types of sulfide barren olivine websterites in the limahe intrusion one is pge depleted and the other is pge undepleted the pge undepleted olivine websterites have os 260 ma between 5 and 8 whereas a pge depleted olivine websterite sample has a os 260 ma value of 30 a disseminated sulfide ore sample has a os 260 ma value close to 77 net textured sulfide ores and sulfide separates from net textured sulfide ores have the highest os 260 ma values varying between 102 and 124 the results can be explained by successive contamination of a mantle derived magma with the lower and upper crusts during magma ascent mixing calculations indicate that the total amount of crustal contamination is 15 the concentrations of os and re os isotopes in the sulfide ores of the limahe deposit are consistent with the sulfide liquids that segregated from a pge depleted magma due to previous sulfide segregation higher os 260 ma values in the sulfide ores than the sulfide barren ultramafic rocks in the limahe intrusion suggest that crustal contamination played a critical role in sulfide saturation significant variations in os concentrations and re os isotopes in the limahe intrusion suggest that multiple pulses of magma with variable crustal contamination were involved in the development of the intrusion the different pulses of magma are likely related to each other by variable crustal contamination and sulfide segregation our new results support the hypothesis that the limahe intrusion may represent a dynamic conduit through which magma successively ascended to higher crustal levels or to the surface

petrogenesis of the pt pd mineralized jinbaoshan ultramafic intrusion in the permian emeishan large igneous province sw china the jinbaoshan ultramafic intrusion is a sheet like body with a thick wehrlite unit in the center and thin pyroxenite units at the margins pge are enriched in several disseminated sulfide zones in the intrusion olivine from the intrusion has low fo and depleted ni contents compared to olivine from coeval emeishan picrites whole rock major and trace element concentrations suggest that the jinbaoshan wehrlites originally contained 30 trapped liquid the total amount of sulfide in the rocks exceeds that which could have been dissolved in the trapped liquid the jinbaoshan wehrlites are interpreted to represent residual assemblages formed by dissolution of plagioclase by passing magma no clear evidence of crustal contamination is indicated by s nd and os isotopes we envision that sulfide saturation occurred at depth due to olivine and chromite crystallization immiscible sulfide droplets were transported to the jinbaoshan conduit where they accumulated and reacted with magma successively passing through the conduit to achieve high pge concentrations

os pb and nd isotope geochemistry of the permian emeishan continental flood basalts insights into the source of a large igneous province abstract the nature of the source of continental flood basalts cfb is a highly debated topic proposed mantle sources for cfbs including both high and low ti basalts include subcontinental lithospheric mantle sclm asthenospheric mantle and deep plume related mantle re os isotope systematics can offer important constraints on the sources of both ocean island basalts oib and cfb and may be applied to distinguish different possible melt sources this paper reports the first re os isotope data for the late permian emeishan large igneous province lip in southwest china twenty one cfb samples including both low and high ti basalts from five representative sites within the emeishan lip have been analyzed for os nd and pb isotopic compositions the obtained os data demonstrate that crustal assimilation affected os isotopic compositions of some emeishan basalt samples with low os concentrations but not all of the samples and the emeishan basalts with high os contents likely experienced the least crustal contamination the low and high ti basalts yield distinct os signatures in terms of 187os 188os and os content the low ti basalt with the highest os concentration 400 ppt has a radiogenic os isotopic composition os t 6 5 similar to that of plume derived oib because the os isotopic composition of basalts with relatively high os concentrations typically 50 ppt likely represents that of their mantle source this result implies a plume derived origin for the low ti basalts on the other hand the high ti basalts with high os concentration over 50 ppt have unradiogenic os isotopic signatures os t values range from 0 8 to 1 4 suggesting that a subcontinental lithosphere mantle sclm component most likely contributed to the generation of these magmas combining pb and nd isotopic tracers with the os data we demonstrate that the low ti basaltic magmas in the emeishan cfb were mainly sourced from a mantle plume reservoir whereas the high ti basaltic magmas were most likely derived from a sclm reservoir or were contaminated by a significant amount of lithospheric mantle material during plume related magma ascent through the sclm

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

apatite hosted melt inclusions from the panzhihua gabbroic layered intrusion associated with a giant fe ti oxide deposit in sw china insights for magma unmixing within a crystal mush the 2 km thick panzhihua gabbroic layered intrusion in sw china is unusual because it hosts a giant fe ti oxide deposit in its lower zone the deposit consists of laterally extensive net textured and massive fe ti oxide ore layers the thickest of which is 60 m to examine the magmatic processes that resulted in the fe enrichment of parental high ti basaltic magma and the formation of thick fe ti oxide ore layers we carried out a detailed study of melt inclusions in apatite from a 500 m thick profile of apatite bearing leucogabbro in the middle zone of the intrusion the apatite hosted melt inclusions are light to dark brown in color and appear as polygonal rounded oval and negative crystal shapes which range from 5 to 50 m in width and from 5 to 100 m in length they have highly variable compositions and show a large and continuous range of sio2 and feot with contrasting end members one end member being fe rich and si poor 40 2 wt feot and 17 7 wt sio2 and the other being si rich and fe poor 74 0 wt sio2 and 1 20 wt feot this range in composition may be attributed to entrapment of the melt inclusions over a range of temperature and may reflect the presence of m scale and immiscible fe rich and si rich components in different proportions simulating results for the motion of si rich droplets within a crystal mush indicate that si rich droplets would be separated from fe rich melt and migrate upward due to density differences in the interstitial liquid when the magma unmixed migration of the si rich immiscible liquid component from the interstitial liquid caused the remaining fe rich melt in the lower part to react with plagioclase primocrysts an59 60 as evidenced by fine grained lamellar intergrowth of an rich plagioclase an79 84 clinopyroxene in the oxide gabbro of the lower zone therefore magma unmixing within a crystal mush combined with gravitationally driven loss of the si rich component resulted in the formation of fe rich melagabbro and major fe ti oxide ores in the lower part and si rich leucogabbro in the upper part of the intrusion

triassic high strain shear zones in hainan island south china and their implications on the amalgamation of the indochina and south china blocks kinematic and 40ar 39ar geochronological constraints abstract a kinematic and geochronological study has been carried out on the triassic high strain shear zones in hainan island the southern south china block there are wnw and ne trending high strain shear zones with greenschist to amphibolite facies metamorphism in this island kinematic indicators suggest a dextral top to the nne thrust shearing for the wnw trending high strain shear zones and a sinistral top to the se thrust shearing for the ne trending shear zones the quartz c axis orientations of mylonitic rocks exhibit the domination of basal slip and some activation of a rhombohedra gliding system the timing of shearing for these shear zones has been constrained by the 40ar 39ar dating analyses of synkinematic minerals middle triassic 242 250 ma and late triassic early jurassic 190 230 ma have been identified for the wnw and ne trending shear zones respectively a synthesis of these kinematic and thermogeochronological data points to a two stage tectonic model for hainan island that is top to the nne oblique thrusting at 240 250 ma followed by top to the se oblique thrusting at 190 230 ma in combination with the available data from the southern south china and indochina blocks it is inferred that south hainan and north hainan have affinity to the indochina and south china blocks respectively the tectonic boundary between south hainan and north hainan lies roughly along the wnw trending changjiang qionghai tectonic zone probably linking to the song ma and ailaoshan zones the middle triassic structural pattern of hainan island is spatially and temporally compatible with those of the south china and indochina blocks and thus might be a derivation from the amalgamation of the indochina with south china blocks in response to the closure of the paleotethys ocean and subsequent subduction collision

magmatic underplating and crustal growth in the emeishan large igneous province sw china revealed by a passive seismic experiment abstract in an attempt to characterize the subsurface structure that is related to fossil mantle plume activity a comprehensive geophysical investigation was conducted in the emeishan large igneous province elip the nature and geometry of the crust were examined within the scheme of the domal structure of elip which comprises the inner intermediate and outer zones which are defined on the basis of the biostratigraphy of pre volcanic sediments the bulk crustal properties within the inner zone are characterized by high density high p wave velocity high vp vs ratios and large crustal thickness a visible continuous seismic converter is present in the upper part of the crust in the whole intermediate zone and the eastern part of the inner zone but it is absent in the inner zone where another seismic converter is observed in the lower part of the crust the geometric configuration of these converters is attributable to the addition of mantle derived melts to the pre existing crust and subsequent interaction between them the crustal geometry which is delineated by the migrated image of receiver functions from the passive seismic experiment and the crustal properties collectively suggest that a mafic layer of 15 20 km thickness and 150 180 km width exists at the base of the crust in the inner zone such a mafic layer reflects a vertical crustal growth through magmatic underplating at the base of the crust and intraplating within the upper crust the salient spatial correlation between the deep crustal structure and the dome strongly supports a genetic link between crustal thickening and plume activity if the pre volcanic domal uplift is generated by the permian emeishan mantle plume this arrangement is further supported by the consistency of the extent of crustal uplift estimated by isostatic equilibrium modeling and sedimentary data this study therefore characterizes and provides evidence for a plume modified crust in a large igneous province

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

late paleozoic mafic ultramafic intrusions in southern central asian orogenic belt nw china insight into magmatic ni cu sulfide mineralization in orogenic setting abstract the mafic ultramafic intrusions hosting magmatic ni cu sulfide ore deposits usually present within the precambrian cratons and on their margin have been extensively studied whereas those occurring within the phanerozoic orogenic belts have received little attention the southern margin of the central asian orogenic belt is a region of late paleozoic mafic ultramafic intrusions emplaced during post orogenic stage in order to recognize the mineralization features this study compares barren or weakly mineralized mafic ultramafic intrusions in the beishan with those hosting ni cu sulfide ore deposit in the altay and eastern tianshan in petrology mineralogy and geochemistry aspects several mineralization features can be recognized with respects to petrology mineralogy and geochemistry from comprehensive comparisons among the mineralization indicators we highlight strong alteration accompanying sulfide emplacement post erosion exposure of magma conduit abundant primary hydrous minerals and potential transportation mechanism of ore forming elements other indicators are the presence of orthopyroxene and pyroxenite for potential sulfide formation through crystal fractionation restricted fo and ni ranges in olivines for sulfide segregation and some other special geochemical characteristics related to crustal contamination the variation in mineralization could be ascribed to different degrees of partial melting crustal assimilation and subduction related modification of a mantle source we suggest that the ni cu sulfide mineralization of mafic ultramafic intrusions within orogenic belts is remarkably different from that of cratonic and continental marginal settings in respects of petrogenesis and mineralizing mechanism magmatic conduit type ni cu sulfide ore deposit is a target for future prospecting and exploration for mafic ultramafic intrusions in the orogenic belts

integrated o sr nd isotope constraints on the evolution of four important fe ti oxide ore bearing mafic ultramafic intrusions in the emeishan large igneous province sw china abstract in the emeishan large igneous province sw china there are many layered mafic ultramafic intrusions such as the baima hongge panzhihua and taihe intrusions that host world class fe ti oxide ore deposits despite numerous studies the origin of these deposits still remains elusive this includes the role of crustal contamination especially addition of external co2 from carbonate country rocks during contact metamorphism in triggering fe ti oxide crystallization from high mg basaltic magma to address this important issue we have carried out an integrated o sr nd isotope study of these ore bearing intrusions and the country rocks our results show that in these intrusions clinopyroxene is much less susceptible to fluid mineral oxygen isotope exchange than coexisting plagioclase and fe ti oxides which is similar to other intrusions worldwide e g taylor 1967 gregory and taylor 1981 our calculations based on the least exchanged clinopyroxene oxygen isotope data show that the mean 18o values for the parental magmas of these intrusions are baima 5 7 panzhihua 6 1 taihe 5 9 the estimated mean 18o value for the parental magma of the upper and middle zones of the hongge intrusion is 6 2 which is similar to those for the parental magmas of the other intrusions baima panzhihua and taihe by contrast the estimated mean 18o value for the parental magma of the lower zone of the hongge intrusion is higher 6 9 this difference together with higher initial 87sr 86sr ratios 0 7057 to 0 7076 and lower endt values 2 82 to 0 07 for this zone can be attributed to higher degrees of contamination with siliceous crustal materials in this zone than elsewhere in this intrusion comparison of o sr nd isotope compositions between the intrusions and country rocks reveals that bulk assimilation of carbonate country rocks is negligible in all of these intrusions mixing calculations using the o sr nd isotope data are consistent with variable degrees of contamination with siliceous crustal materials in the intrusions panzhihua

rhenium osmium isotope and platinum group elements in the xinjie layered intrusion sw china implications for source mantle composition mantle evolution pge fractionation and mineralization abstract the xinjie mafic ultramafic layered intrusion in the emeishan large igneous province elip hosts cu ni platinum group element pge sulfide ore layers within the lower part and fe ti v oxide bearing horizons within the middle part the major magmatic cu ni pge sulfide ores and spatially associated cumulate rocks are examined for their pge contents and re os isotopic systematics the samples yielded a re os isochron with an age of 262 27 ma and an initial 187 os 188 os of 0 12460 0 00011 os t 0 5 0 1 the age is in good agreement with the previously reported u pb zircon age indicating that the re os system remained closed for most samples since the intrusion emplacement they have near chondritic os t values ranging from 0 7 to 0 2 similar to those of the lijiang picrites and song da komatiites exceptionally two samples from the roof zone and one from upper sequence exhibit radiogenic os t values 0 6 to 8 6 showing minor contamination by the overlying emeishan basalts the pge rich ores contain relatively high pge and small amounts of sulfides generally less than 2 and the abundance of cu and pge correlate well with s implying that the distribution of these elements is controlled by the segregation and accumulation of a sulfide liquid some ore samples are poor in s mostly t and cr tio 2 ratios immediately below units 2 4 together with high cu zr ratios decreasing from each pge ore layer within these cyclic units are consistent with multiple magma replenishment episodes the sulfides in the cumulate rocks show little evidence of pge depletion with height and thus appear to have segregated from successive inputs of fertile magma this suggests that the xinjie intrusion crystallized from in an open magma system e g a magma conduit the compositions of the disseminated sulfides in most samples can be modeled by applying an r factor silicate sulfide mass ratio of between 1000 and 8000 indicating the segregation of only small amounts of sulfide liquid in the parental ferropicritic magmas thus continuous mixing between primitive ferropicritic magma and differentiated resident magma could lead to crystallization of chromite cr bearing magnetite and subsequently abundant fe ti oxides thereby the segregation of pge rich cu sulfide when considered in the light of previous studies on plume derived komatiites and picrites worldwide the close to chondritic os isotopic composition for most xinjie samples lijiang picrites and song da komatiites suggest that the ferropicritic magma in the elip were generated from a plume this comprised recycled neoproterozic oceanic lithosphere including depleted peridotite mantle embedded with geochemically enriched domains the ascending magmas thereafter interacted with minor possibly

three fe ti oxide ore bearing gabbro granitoid complexes in the panxi region of the permian emeishan large igneous province sw china the permian 260 ma emeishan large igneous province of sw china contains three nearly identical gabbro granitoid complexes that host giant fe ti oxide deposits the fe ti oxide deposits are within the lower portions of evolved layered gabbroic intrusions and are spatially and temporally associated with a type granitic plutons the 264 3 ma taihe layered gabbroic intrusion hosts a large magmatic fe ti oxide deposit and is coeval with the taihe peralkaline a type granitic pluton which is dated at 261 2 ma within the a type granitic pluton are microgranular enclaves which have compositions intermediate between the gabbro and host granite primitive mantle normalized incompatible element plots show corresponding reciprocal patterns between the mafic and felsic rocks the chondrite normalized ree patterns show eu anomalies changing from positive eu eu 1 5 to 5 9 in the gabbroic intrusion to negative in the enclaves eu eu 0 4 to 0 6 and granites eu eu 0 2 to 0 5 whole rock end t values of the gabbroic intrusion end t 2 5 to 3 3 are similar to those of the enclaves end t 1 0 to 2 0 and granite end t 1 5 to 1 9 whereas the zircon ehf t values of the gabbro ehf t 8 1 0 8 are indistinguishable from those of the granites ehf t 9 2 1 0 suggesting that the parent magmas for all rock types originated from the same mantle source geochemical modeling indicates that the gabbros and granites can be generated by fractional crystallization of a common parental magma similar to high ti emeishan flood basalt the compositional jump from the gabbro to the enclaves is attributed to the crystallization of fe ti oxide minerals the results of this study and other studies suggest that the magmatic conditions for example pressure composition fo2 which led to the formation of at least three fe ti oxide bearing gabbro granitoid complexes were relatively common during the development of the emeishan large igneous province

petrogenesis and metallogenesis of the xinjie layered mafic ultramafic intrusion china modeling of recharge assimilation and fractional crystallization abstract the xinjie layered mafic ultramafic intrusion in the central emeishan large igneous province elip sw china hosts fe ti v oxide ore in the upper part and ni cu platinum group element pge sulfide deposits in the lower part in this study we use published sr nd isotopic data to simulate and evaluate the energy constrained recharge assimilation and fractional crystallization ec rafc model with a view to track the petrogenesis and mineralization in contrast to the energy constrained assimilation fractional crystallization ec afc model the ec rafc modeling shows that the xinjie intrusion may represent a shallow crustal magma chamber system where it experienced the rafc processes in the early stage of the magmatic process tm 1460 c a pulse of magma was injected into an actively evolving magma chamber minor melting ma 0 257 and assimilation of the wallrock underlying emeishan basalts occurred when the temperature of magma tm 1245 c decreased close to the equilibration temperature teq 1165 c the mass temperature plots indicate that fractional crystallization was significantly affected by the recharge and assimilation processes whereas the contaminated magma recharge favored the early crystallization of fe ti oxides as well as the formation of the pge bearing immiscible melt in the lower part of the intrusion in contrast the formation of the fe ti oxides ores in the upper part occurred probably through fractional crystallization and or immiscibility of the evolved fe ti rich magma resulting in the paragenesis of fe ti v oxides and ni cu pge sulfides in the xinjie layered intrusion

permian peralkaline peraluminous and metaluminous a type granites in the panxi district sw china their relationship to the emeishan mantle plume abstract numerous permian granitic intrusions plutons in the panxi region sw china are spatially and temporally associated with the emeishan large igneous province elip granites from the 260 ma panzhihua and taihe plutons are peralkaline asi 0 76 1 12 na k al 0 78 1 2 and have high fe values 0 92 0 99 with negative eu anomalies eu eu 0 22 0 98 contrasted with the peralkaline plutons syenites from the 260 ma woshui pluton are metaluminous asi 0 84 1 0 na k al 0 91 1 01 and have moderately high fe 0 79 0 81 and positive eu anomalies eu eu 0 82 1 91 both the peralkaline and metaluminous rocks have mantle isotopic signatures end t 1 3 3 2 and were derived from the elip mantle plume source the peralkaline rocks were likely derived by fractional crystallization of the temporally spatially and chemically associated layered mafic intrusions which host giant magmatic fe ti v oxide deposits the metaluminous granitic rocks although spatially related to the peralkaline granites and layered gabbros were likely generated by partial melting of underplated mafic rocks granites from the 251 ma ailanghe pluton are peraluminous asi 0 97 1 29 and have high fe 0 82 0 91 values and negative eu anomalies eu eu 0 10 0 37 their crustal isotopic signature end t 5 7 to 6 7 indicates possible derivation from melting of yangtze block basement rocks the ailanghe pluton is contemporaneous with other a type granites in the region supporting the view that elip magmatism was sporadic for at least 9 million years the coexistence of three chemically exclusive a type granites in the elip suggests that a variety of a type granites can be generated in the same tectonic setting

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

temporal evolution of the permian large igneous province in tarim basin in northwestern china abstract the temporal relations of different types of rocks in the tarim large igneous province tlip are studied using stratigraphic correlation lithologically spatial distribution and isotopic ages the tlip covers an area 250 000 km2 in the western and central part of the tarim basin the lithological units of the tlip include basalt diabase layered intrusive rock breccia pipe mica olivine pyroxenite olivine pyroxenite gabbro ultramafic dyke quartz syenite quartz syenite porphyry and bimodal dyke this is the first report for the spatial distribution of the tlip from the spatial section lines which shows the stratigraphic correlation among basaltic lava tuff interlayered mudstone siltstone and sandstone and the thicknesses of the basaltic lavas from different field sections and drill holes and the basalts from the kupukuziman and kaipaizileike formations were subdivided in the drill hole sections this indicates that the basaltic lavas were widely distributed in the tarim basin combined with previous study of geochemical and sr nd pb isotopic characterization from the basalts and their genetic link with mantle plume activity and large diabase swarms developments the permian igneous units constitute a tarim large igneous province zircon shrimp u pb dating firstly yields 284 3 2 8 ma for the quartz syenite porphyry recent shrimp and la icp ms zircon u pb and 40ar 39ar ages are used to discuss temporal relations of different rock units in the tlip formed between 290 and 274 ma and these data are more reliable than the k ar ages of 220 310 ma reported previously the sequence of magmatism of the tlip in the central and western parts of the tarim basin are basaltic lava in the kupukuziman and kaipaizileike formations 285 290 ma layered mafic ultramafic rock mica olivine pyroxenite breccia pipe diabase and ultramafic dyke quartz syenite quartz syenite porphyry and bimodal dyke 274 284 ma

paleointensity results from the jurassic new constraints from submarine basaltic glasses of odp site 801c 1 tholeiite of the oldest oceanic crust was drilled during odp legs 129 and 185 at hole 801c in the western pacific fresh appearing submarine basaltic glass sbg was recovered from the tholeiites 167 ma which has been shown to be nearly ideal for determining absolute paleointensity paleointensities of the younger off axis alkalic basalts 160 ma overlying the tholeiites had been studied earlier here we report results from the older tholeiitic on axis sequence we subjected a total of 73 specimens from 17 cooling units to absolute paleointensity experiments of these 30 specimens and six cooling unit averages met our strictest reliability criteria yielding an average of 11 9 3 9 t the bulk of evidence suggests a paleolatitude of the site of 14 s with an uncertainty of 10 this translates the intensity to a value for the virtual axial dipole moment of 28 zam2 slightly lower than values determined from the plagioclase crystals in the three cooling units of the younger alkalic basalts overlying the tholeiites tarduno cottrell 2005 our value is low when compared to the long term median value of the field of 42 zam2 our results and those of the published literature therefore support the contention of a low magnetic field strength in the jurassic average of 28 14 zam2 n 138 individual estimates as initially suggested by prevot et al 1990 our interpretation of the body of available data argue for low field strengths for the entire jurassic extending into the early cretaceous

chalcophile element geochemistry of the baima layered intrusion emeishan large igneous province sw china implications for sulfur saturation history and genetic relationship with high ti basalts the permian baima mafic layered intrusion believed to be related to the s undersaturated emeishan high ti basalts hosts a giant fe ti v oxide deposit in the lower part of the intrusion uniformly high cu pd 1 9 106 6 1 104 and low pd zr 0 1 indicate that the baima parental magma experienced prior sulfide segregation mantle liked 34s values and low s se values indicate negligible external sulfur addition primitive mantle normalized pge patterns and melts calculations indicate that extensive fractional crystallization 59 of chromite olivine and pyroxene at depth drove the primitive picritic magma to s saturation strong positive correlation between ipge and ppge and between pge and v cr and s suggest that magmatic sulfide is the dominant mineral controlling the distribution of pge in the baima intrusion a positive correlation between s and cr feot tio2 and v content together with melts calculations indicate that the parental magma of the baima intrusion reached a second stage of s saturation in the shallower baima magma chamber which was likely triggered by decreasing fe2 accompanying magnetite precipitation primitive mantle normalized pge patterns for baima intrusion rocks display similar trends to high ti basalts inside the panxi area suggesting that they are comagmatic and following a similar differentiation trend however the lavas erupted before they reached sulfide saturation the more evolved nature of high ti basalts outside the panxi area indicate that they experienced more extensive pre eruption fractional crystallization further fractional crystallization process led these lavas show more pge fractionated feature

geothermal constraints on emeishan mantle plume magmatism paleotemperature reconstruction of the sichuan basin sw china the middle late permian emeishan large igneous province elip in southwestern china represents a classic example of a mantle plume origin to constrain the thermal regime of the elip and contemporaneous magmatic activity in the northeastern sichuan basin maximum paleotemperature profiles of deep boreholes were reconstructed using vitrinite reflectance ro and apatite fission track data two heating patterns were identified 1 heating of the overlying lithosphere by magma storage regions and or magmatic activity related to the mantle plume which resulted in a relatively strong geothermal field and 2 direct heating of country rock by stock or basalt borehole ro data and reconstructed maximum paleotemperature profiles near the elip exhibit abrupt tectonothermal unconformities between the middle and late permian the profiles in the lower subsections i e pre middle permian exhibited significantly higher gradients than those in the upper subsections distal to the basalt province high paleo geotemperatures hereafter paleotemperatures were inferred despite deformation of the paleogeothermal curve due to deep faults and igneous rocks within the boreholes in contrast ro profiles from boreholes without igneous rocks i e late permian contained no break at the unconformity paleotemperature gradients of the upper and the lower subsections and erosion at the middle late permian unconformity revealed variations in the thermal regime the inferred spatial distribution of the paleothermal regime and the erosion magnitudes record the magmatic and tectonic thermal response to the emeishan mantle plume

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

understanding the physics of kappa κ insights from a downhole array at high frequencies the acceleration spectral amplitude decreases rapidly this has been modelled with the spectral decay factor its site component 0 is used widely today in ground motion prediction and simulation and numerous approaches have been proposed to compute it in this study we estimate for the euroseistest valley a geologically complex and seismically active region with a permanent strong motion array consisting of 14 surface and 6 downhole stations site conditions range from soft sediments to hard rock first we use the classical approach to separate local and regional attenuation and measure 0 second we take advantage of the existing knowledge of the geological profile and material properties to examine the correlation of 0 with different site characterization parameters 0 correlates well with vs30 as expected indicating a strong effect from the geological structure in the upper 30 m but it correlates equally well with the resonant frequency and depth to bedrock of the stations which indicates strong effects from the entire sedimentary column down to 400 m third we use our results to improve our physical understanding of 0 we propose a conceptual model of 0 with vs comprising two new notions on the one hand and contrary to existing correlations we observe that 0 stabilizes for high vs values this may indicate the existence of regional values for hard rock 0 if so we propose that borehole measurements almost never used up to now for 0 may be useful in determining these values on the other hand we find that material damping as expressed through travel times may not suffice to account for the total 0 measured at the surface we propose that apart from material damping additional site attenuation may be caused by scattering from small scale variability in the profile if this is so then geotechnical damping measurements may not suffice to infer the overall crustal attenuation under a site but starting with a regional value possibly from a borehole and adding damping we might define a lower bound for site specific 0 more precise estimates would necessitate seismological site instrumentation

contrasting parental magma compositions for the hongge and panzhihua magmatic fe ti v oxide deposits emeishan large igneous province sw china many 260 ma mafic ultramafic layered intrusions including hongge and panzhihua in the emeishan large igneous province southwestern china host world class fe ti v oxide ore deposits these two most important ore bearing intrusions show differences in lithology and mineral chemistry the most important orebodies in the hongge intrusion occur as concordant layers in the middle part of the intrusion closely associated with clinopyroxenites titanomagnetite and mg rich ilmenite are the major ore minerals of the hongge deposit coexisting clinopyroxene contains 1 7 wt tio2 these data indicate high ti parental magma for the hongge ore bearing clinopyroxenites in the panzhihua intrusion the most important orebodies also occur as concordant layers but in its lower part instead of middle part in contrast with the hongge deposit the most important host rocks of the panzhihua deposit are gabbros not clinopyroxenites in addition ilmenite is rare and titanomagnetite is predominant in the panzhihua deposit coexisting clinopyroxene in the panzhihua deposit contains 1 6 wt tio2 the contrasting lithologic and mineral compositions indicate that the parental magma for the hongge deposit has higher tio2 than that for the panzhihua deposit the compositions of clinopyroxene from the panzhihua and hongge deposits indicate that their parental magmas are also different in mgo feo and trace element ratios as well calculations using average experimental mg fe exchange coefficient and trace element partition coefficients between clinopyroxene and magma show that the hongge parental magma has higher mgo feo ratios and more fractionated mantle normalized trace element patterns than the panzhihua parental magma the estimated compositions of the parental magmas for the hongge and panzhihua ore bearing lithologies resemble the average compositions of the longzhoushan type high ti basalts and ertan type intermediate ti basalts in the emeishan large igneous province respectively the new trace element data from this study together with available sr nd isotope data for the ore bearing intrusions from literature support a new petrogenetic model involving selective assimilation of newly subducted stagnant oceanic gabbroic slab above the deep seated emeishan mantle plume this process and subsequent contamination with the upper crust played an important role in the variation of parental magma compositions between the hongge and panzhihua magmatic oxide ore deposits abundant fe ti oxide ore deposits associated with less evolved basaltic magma in the emeishan large igneous province than elsewhere in the world are attributed to selective assimilation of newly subducted stagnant oceanic lithospheric slab by the ascending mantle plume derived picritic magma that was originally undersaturated with fe ti oxides