paleoclimatic and paleomagnetic constraints on the paleozoic reconstructions of south china north china and tarim abstract paleomagnetic and paleoclimatic data provide the most useful latitudinal constraints for plate reconstructions distributions through the paleozoic of five types of climatically sensitive sediments coals evaporites reefs dolomites and limestones for south china north china and tarim are shown on 15 maps that include 1578 reliable data points these paleoclimatic data agree reasonably well with available paleomagnetic directions although significant divergence between the two exists for the early paleozoic these data indicate the following 1 1 south china was in low latitudes during the entire paleozoic with a subtropical position in the cambrian 2 2 north china also remained near the equator in the early and late paleozoic except for the ordovian and the late permian when extensive evaporites suggest slightly higher latitudinal positions while its middle paleozoic position is uncertain due to the missing stratigraphie record 3 3 in south china local tectonics appears to have played a dominant role in determining paleogeography and therefore marine sedimentation especially after the late ordovician early silurian because the areal coverage of marine sediments through time is distinctly different from what would be expected from published global sea level curves 4 4 paleoclimatic and paleomagnetic data are compatible with biogeographic data which suggest that south china was part of eastern gondwana in the early paleozoic but was widely separated from gondwana in the late paleozoic and the split between the two probably happened in the devonian giving rise to a major break up unconformity in central south china

unexposed archean components and complex post archean accretion reworking processes beneath the southern yangtze block revealed by zircon xenocrysts from the paleozoic lamproites south china abstract integrated u pb dating and hf isotope analysis of zircon xenocrysts from the baifen lamproites of the zhenyuan area in the southeastern guizhou province offer a unique glimpse into the evolution of the deep continental crust beneath the southern part of the yangtze block south china a total of 148 zircons with 90 110 concordance yield u pb ages varying from 3458 9 ma to 252 2 ma they generally have high th u ratios 0 3 indicating a magmatic origin the two youngest magmatic zircons give a weighted mean 206pb 238u age of 253 0 2 8 ma which is interpreted as the emplacement age of the host baifen lamproites most zircons with pre eruption ages are xenocrysts derived from the deeper crust of the southern yangtze block they define two major paleoproterozoic age populations at 2 17 ga and 1 8 ga with similar hf model ages tcrust of 2 4 2 8 ga peak at 2 64 ga the oldest zircon has a 207pb 206pb age of 3458 9 ma with ehf t 1 47 and tcrust 3 7 ga this demonstrates the presence of archean crustal components or remnants as old as 3 7 ga beneath the southern yangtze block magmatic events involving significant addition of juvenile mantle derived materials mainly occurred at 2 2 2 1 ga and 2 0 1 8 ga but the latter event also involved reworking of the neoarchean crust and might represent the cratonization of the yangtze continent during the middle paleoproterozoic which could be correlated to the assembly of the columbia supercontinent this segment of the yangtze block became tectonically quiescent until 1 2 ga implying that the crustal stabilization lasted for ca 600 million years a few late mesoproterozoic zircon xenocrysts within the age range 1 3 1 0 ga suggest the occurrence of grenville aged events these neoproterozoic zircon xenocrysts 992 547 ma have a wide range of ehf t values 8 47 to 22 07 consistent with the zircon hf isotopic compositions of widespread coeval igneous rocks in the yangtze block the data show that the yangtze block has undergone important crustal growth and reworking during the neoproterozoic in response to the breakup of the rodinia supercontinent the early middle paleozoic zircon xenocrysts 513 357 ma have negative ehf t values 2 97 to 12 69 and tcrust hf model ages of 1 59 2 27 ga indicating reworking of the precambrian crustal basement without juvenile contributions during the caledonian orogeny the positive ehf t values 13 79 to 0 22 of twelve late paleozoic zircons 284 264 ma with a major age peak at ca 275 ma suggest that the precambrian basement beneath the southern yangtze block experienced an episode of strong rejuvenation during the permian we propose a possible linkage between the baifen lamproite magmatism 284 252 ma and the emeishan flood basalt volcanism

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

remagnetization in southeast china and the collision and suturing of the huanan and yangtze blocks abstract nearly all paleomagnetic studies in south china have encountered problems with remagnetization the region underwent widespread tectonic deformation during the mesozoic and the warm humid climate causes rapid weathering of surficial sediments paleomagnetic results from three carboniferous formations from three provinces in southeast china reveal two possible overprinting ages one is probably a post folding cretaceous remagnetization and the other appears to be a jurassic overprint which may be associated with the collision and suturing of the yangtze and the huanan or north china blocks the jurassic overprint is found in sediments with both magnetite and hematite as primary magnetic remanence carriers the remagnetization pre dates deformation in at least one of the formations further constraining the overprint to the jurassic or late triassic

intraplate extension prior to continental extrusion along the ailao shan red river shear zone left lateral movement of the ailao shan red river shear zone lends support to the hypothesis of continental extrusion resulting from the collision of india with asia our new observations from northwestern yunnan china and northwestern vietnam on different sides of the shear zone demonstrate that the sinistral offset was 600 km according to correlations of permian triassic flood basalt successions and late paleogene highly potassic mafic magmas we conclude that the shear was propagating on the south china continental margin and does not correspond to a suture between south china and indochina furthermore the highly potassic magmas were emplaced from ca 40 to 30 ma before the shear movement which was caused by the late oligocene to early miocene ca 27 22 ma extrusion activity this suggests that a late eocene to early oligocene intraplate extension possibly induced by delamination of thickened continental lithosphere took place in northwestern yunnan or eastern tibet as a response to the india asia collision this extension and sea floor spreading of the south china sea that began ca 30 ma could have accounted for the initiation of the ailao shan red river shear zone

intercalibration of 40ar39ar dating standards a number of standards used for k ar and 40ar39ar dating have been intercalibrated multiple splits 15 40 mg each of mmhb 1 sb 3 biotite lp 6 biotite 40 60 ga1550 biotite fish canyon tuff 3 biotite taylor creek rhyolite sanidine bern 4 muscovite bern 4 biotite and pennsylvania state university orthoclase 1a were irradiated in capsules fused and analyzed in replicate analyses most of these standards proved to be homogeneous in 40ar 39ark ratio age k ca and 36ar contents mmhb 1 is inhomogeneous in age at the 15 mg level indicating it is unsuitable for use as an interlaboratory standard using material taken straight from the bottle the orthoclase sample is also inhomogeneous at the 30 mg level quoted relative to an age of 162 9 ma for the sb 3 biotite standard total fusion ages are lp 6 biotite 40 60 128 1 ma ga1550 biotite 97 8 ma fish canyon tuff 3 biotite 27 95 ma taylor creek rhyolite sanidine 28 0 ma bern 4 muscovite 18 51 ma bern 4 biotite 17 21 ma these ages are in good agreement with the absolute k ar ages of these minerals different preparations of biotite and sanidine standards from a welded tuff such as the fish canyon tuff may not be of identical ages tests on submilligram splits reveals that sb 3 biotite ga1550 biotite fish canyon tuff 3 biotite and taylor creek rhyolite sanidine are suitable for use as monitors in laser work the sanidine specimen appears to be the most suitable for use as the primary monitor for 40ar39ar dating

severe remagnetization revealed from triassic platform carbonates near guiyang southwest china abstract an intensive paleomagnetic investigation has been conducted on the middle and lower triassic platform carbonates near guiyang southwest china in the hope of refining a middle triassic paleomagnetic pole previously reported from the region instead severe late cretaceous to recent remagnetizations have been revealed from all the three rock formations studied in all cases the in situ characteristic remanent magnetization chrm directions resemble or fall close to the local present day earth s magnetic field direction and fail the fold test despite the presence of dual polarity in two formations the major magnetic carriers of the remagnetizations are hematite and magnetite the cause s for the remagnetization events cannot be ascertained at present but weathering in recent time and migrating fluid in the past might be responsible it is suggested that caution should be exercised when investigating platform carbonate rocks which are prone to late remagnetization

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

some characteristics of the seismicity and the stress field in the panxi rift zone abstract some characteristics of the horizontal distribution of the earthquakes the focus depth distribution distribution of the earthquakes with time b values fault slip rates and stress field in the panxi rift zone have been investigated the results are as follows 1 1 earthquakes with m 4 7 occurred mainly along the major faults in the panxi rift zone earthquake activity is greater along the sides of the panxi rift zone than along its axis earthquakes with m 6 occurred along the bounding faults of the panxi rift zone the small earthquakes exhibit a random distribution 2 2 the focal depths along the axis of the panxi rift zone are shallow 5 20 km but the focal depths outside the rift zone are deeper 6 40 km 3 3 the b values in the panxi rift zone are 0 58 0 75 4 4 the correlation of the seismicity between the panxi rift zone and the eastern india eurasian collision zone is high 5 5 the fault planes in the panxi rift zone are nearly vertical and strike slip the fault slip rates in the panxi rift zone are 0 13 0 19 cm yr 6 6 the maximum principal compressive stress axis in the panxi rift zone is nearly horizontal nnw sse in direction according to the above results a dynamic model for the aborting process of the panxi rift zone is suggested

end permian extinction and subsequent recovery of the ophiuroidea echinodermata a worldwide stratigraphical survey of late palaeozoic and early mesozoic ophiuroids echinodermata reveals unique extinction and recovery patterns during and after the end permian mass extinction event the ophiuroidea suffered the loss of all palaeozoic genera however this clade profited greatly from the event by undergoing a significant increase in species number in the postextinction oceans although both sampling and taphonomic biases may have affected ophiuroid diversity changeover patterns across the permian triassic p tr transition to some extent both biases cannot explain adequately the abrupt increase in species numbers and geographically wide distributions of ophiuroids in the early triassic oceans the ophiuroidea recovery follows erwin s model of a logistic increase in diversity beginning immediately after the end of the mass extinction without the survival interval morphological innovation i e emergence of advanced canal systems diversification of life habitats and biogeographical dispersal over the palaeotethyan and gondwanan regions enabled the ophiuroidea to occupy efficiently vacant ecospace created by the end permian extinction enabling the clade to recover and radiate in the early triassic oceans previously reported ophiuroids from the upper permian to lower triassic of china are also taxonomically revised these ophiaulaxoides bijieensis feng huangzhishania gulinensis feng ophiolepis shanxiensis yang and ophioderma schistovertebrata yang were described from the lower triassic while aganaster qingchangensis feng and syntomospina kaiyangensis feng came from the upper permian

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

pressure dependence of melt viscosities on the join diopside albite abstract the pressure dependence of melt viscosities on the join diopside albite has been studied using falling sphere viscometry the five melt compositions investigated are diopside ab 25 di 75 ab 50 di 50 ab 75 di 25 and albite experiments were performed at 1500 and 1600 c and at pressures of 5 10 15 20 and 25 kbar the positive and negative pressure dependence of the viscosity of diopside and albite respectively were confirmed all intermediate compositions show an initial decrease in viscosity with increasing pressure however melt of ab 25 di 75 composition passes through a minimum viscosity at approximately 12 kbar and 1600 c this behavior is analogous to the variation in the viscosity of water with pressure at low temperature it is suggested that the three dimensional fully polymerized albite structure dominates flow at low pressures with increasing pressure disruption of this structure and decrease in the average size of the flow units leads to domination by the diopside structure the variation in viscosity with composition along the join at one atmosphere can be adequately modelled using the adam and gibbs 1965 configurational entropy model with an additional two lattice configurational entropy of mixing term the pressure dependence of viscosity in the diopside albite system however cannot be predicted by the model because there is an absence of information on the pressure dependence of the model parameters it is probable that relatively polymerized magmas e g rhyolites to sio 2 saturated basalts show a negative pressure dependence of viscosity to depths where they originate in the lower crust or upper mantle in contrast the most depolymerized naturally occurring melts such as strongly sio 2 undersaturated basalts and picrites may exhibit a viscosity minimum the viscosity of these melts may be sufficiently high at depths within the upper mantle to inhibit their segregation rise and eventual eruption at the surface

geodynamics and tectonic evolution of the panxi rift abstract this paper gives a report on the geophysical geological and geochemical studies of the panxi rift the results of wide scale gravity measurements deep seismic soundings aeromagnetic geomagnetic geothermal and seismicity investigations which have been carried out by the institute of geophysics of the academia sinica are presented and analysed thereby the results from geotraverse i are discussed in detail the following rift associated features have been found an intracrustal low velocity and resistivity layer lower than normal velocities of 7 6 7 9 km s and low resistivity of 250 m in the upper mantle elevated velocities and resistivities in the central region doming as analysed from the gravity and seismicity data n s rift oriented positive magnetic anomalies in the central region n s striking faults and n s oriented distribution of basic and ultrabasic proterozoic rocks and grabens it is shown that the region containing the panxi rift was subjected to extensional and compressional tectonics which started in the late proterozoic and found their climax in the late palaeozoic the recent compressional tectonics are considered to relate to the collision of the indian and eurasian plates rifting and igneous processes during the tectonic evolution of the panxi rift are compared with the major continental rifts and conceptual models for the various rifting episodes are presented

widespread late mesozoic to recent remagnetization of paleozoic and lower triassic sedimentary rocks from south china kent d v zeng x zhang w y and opdyke n d 1987 widespread late mesozoic to recent remagnetization of paleozoic and lower triassic sedimentary rocks from south china in d v kent and m krs editors laurasian paleomagnetism and tectonics tectonophysics 139 133 143 paleomagnetic results are described from eighteen formations sampled in the nanjing jiangsu province and yichang hubei province areas of south china the marine platform carbonates and elastics range in age from early cambrian to early triassic progressive alternating field and or thermal demagnetization analyses reveal predominantly single component magnetizations sometimes of both normal and reverse polarity these magnetizations isolated in 457 of the 549 samples studied typically have directions in geographic coordinates coincident with late mesozoic to recent fields a secondary origin for the magnetizations is supported by negative fold tests in several units a lower carboniferous and possibly a lower triassic limestone may have escaped the pervasive remagnetization if the remanences isolated are indeed prefolding a large counterclockwise tectonic rotation of the nanjing area is implied by comparison with available carboniferous and triassic paleopoles reported from elsewhere in south china this rotation may have been associated with mesozoic left lateral motion on the tancheng lujiang fault zone

u pb zircon geochronology and tempo of the end permian mass extinction the mass extinction at the end of the permian was the most profound in the history of life fundamental to understanding its cause is determining the tempo and duration of the extinction uranium lead zircon data from late permian and early triassic rocks from south china place the permian triassic boundary at 251 4 0 3 million years ago biostratigraphic controls from strata intercalated with ash beds below the boundary indicate that the changhsingian pulse of the end permian extinction corresponding to the disappearance of about 85 percent of marine species lasted less than 1 million years at meishan a negative excursion in 13 c at the boundary had a duration of 165 000 years or less suggesting a catastrophic addition of light carbon

magnetostratigraphic investigations on an emeishan basalt section in western guizhou province china abstract a magnetostratigraphic investigation has been conducted on a late permian emeishan basalt section at duge in western guizhou province china the characteristic remanent magnetization chrm directions resolved at high temperatures from the basalts pass the fold test with dual polarity and define a normal magnetozone of five sixths of the total thickness in the lower part and a reversed magnetozone of one sixth of the thickness in the top part of the section the chrm directions from the basalt clasts of pyroclastic rocks fail the conglomerate test but the direction and polarity of the chrm of the clasts from the upper reversed zone are consistent with those of the basaltic lavas in the reversed zone and the normal directions also dominate the chrm of the clasts taken from the lower normal zone this suggests that either the original remanence of the clasts was reset by cementing basaltic lavas or the chrm of the basalts was acquired late but not beyond a field reversal the magnetostratigraphic pattern n r observed at duge is as simple as that observed from some major continental flood basalt provinces in the world where very few magnetic polarity zones were observed from volcanic sequences of great thickness the emplacement of the emeishan basalts in western guizhou is estimated to be less than two million years the normal polarity magnetozone observed indicates that the extrusion of the emeishan basalts in western guizhou postdates the permo carboniferous reversed superchron pcrs the apparent shortness of the reversed magnetozone explains why far fewer reverse directions than normal ones have been so far reported from the emeishan basalts

geochemistry and geochronology of native copper mineralization related to the emeishan flood basalts yunnan province china abstract native copper mineralization in northeastern yunnan is related to the emeishan flood basalts which have high cu background concentrations the deposits are located in the ludian maoling and yiche synclines between the flood basalts and the overlying xuanwei formation evidence from ree and hsfe concentrations oxygen isotope compositions bitumen reflectance and mineral paragenesis indicates that the high grade copper orebodies were deposited from moderate to low temperature hydrothermal solutions 420 to 100 c in a reducing environment 40 ar 39 ar spectra for the laumontites that coexist with native copper in the high grade ores yield consistent plateau and isochron ages 226 to 228 ma these are interpreted to represent the age of the first stage of hydrothermal mineralization which took place at temperatures of 200 to350 c concordant with that of zn pb ag ge mineralization also occurring in the area the heulandites yield a stable 40 ar 39 ar plateau age of 134 0 1 7 ma consistent with isochron ages indicated by u th pb isotopic systematics indicating a second stage of native copper mineralization at lower temperatures 100 to 200 c that took place during the early cretaceous

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

40ar 39ar geochronology of ferrar dolerite sills from the transantarctic mountains antarctica implications for the age and origin of the ferrar magmatic province the ferrar dolerite constitutes the hypabyssal phase of the tholeiitic ferrar group of antarctica sills with compositions representing most of the range of geochemical variation of the ferrar dolerite and separated by distances of as much as 1400 km have been analyzed by the 40 ar 39 ar method on feldspar and biotite separates the 40 ar 39 ar ages for five individual sills range from 176 2 to 177 2 ma and show no significant difference these ages reflect crystallization at 176 7 1 8 ma where the uncertainty includes provision for systematic uncertainty in the age of the neutron fluence monitor calibrated relative to mmhb 1 at 513 5 ma combining data from these sills with previous determinations on coeval lavas and underlying pyroclastic units indicates an age of 176 6 1 8 ma for the ferrar tholeiitic rocks as a whole the duration of magmatic activity was less than approximately 1 m y by extension other rocks in the ferrar magmatic province which occur from southeastern australia along the transantarctic mountains to the theron mountains are inferred to have this age the short duration of magmatic activity as well as the consistent pattern of geochemical variation and distinctiveness of the ferrar rocks suggest that magmas were transported laterally by an extensive dike swarm which is inferred to have originated in the weddell sea sector of the province

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

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

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

sedimentary evidence for a rapid kilometer scale crustal doming prior to the eruption of the emeishan flood basalts biostratigraphic and sedimentologic investigations in 67 sections have been carried out for the middle permian maokou formation that immediately underlies the emeishan flood basalts in southwest china the results suggest a domal crustal thinning before the emplacement of the emeishan large igneous province variably thinned carbonates in the maokou formation are capped by a subaerial unconformity which is generally manifested by karst paleotopography paleoweathering zone or locally by relict gravels and basal conglomerates provenance analysis indicates that these gravels and conglomerates were mainly derived from the uppermost maokou formation therefore the stratigraphic thinning likely resulted from differential erosion due to regional uplift iso thickness contours of the maokou formation delineate a subcircular uplifted area in accordance with the crustal doming caused by a starting mantle plume as predicted by experimental and numerical modeling the duration of this uplift is estimated to be less than 3 myr and the magnitude of uplift is greater than 1000 m the sedimentary records therefore provide independent supporting evidence for the mantle plume initiation model for the generation of the emeishan flood basalts

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

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

u pb dating on single detrital zircon grains from the triassic songpan ganze flysch central china provenance and tectonic correlations the songpan ganze flysch belt central china covers a huge triangular area of more than 200 000 km and is bounded by the continental blocks of south china north china and the tibetan plateau detrital zircons extracted from three flysch samples collected in the central part of the belt were analyzed grain by grain using the u pb method two samples of middle triassic sandstones collected at different locations in the belt provide identical results which suggests similar source regions the detrital zircons yield a wide range of ages and indicate their principal derivation from mid proterozoic 1 8 2 0 ga source rocks with minor contribution from late archean ca 2 5 2 6 ga material the discordance and pb loss patterns from low u zircons indicate disturbances during a subsequent event which may be of caledonian age as suggested by concordant zircon grains at ca 420 and 450 ma one sample collected within the palang shan pass zone provides concordant zircon grains at around 230 ma 231 1 ma and 233 1 ma these triassic ages are synchronous to flysch deposition and suggest intense geological activity calc alkaline volcanism at that time in the area close to the basin the data support an origin of the clastic material mainly from a northeastern landmass corresponding to the southern margin of the sino korean craton to a lesser degree inputs from the yangtze craton and possibly from the northern margin of the basin kunlun arc are also detected the age spectrum from the upper triassic sandstone is significantly different and shows predominance of sinian ca 760 ma grains probably derived from the yangtze craton this change in the source region is interpreted as reflecting the tectonic evolution of this area and in particular as being linked to the late triassic collision between south china and north china in the middle triassic while subduction of the songpan sea northward beneath the north china plate was still taking place continental subduction of south china in the dabie region was responsible for uplift of the overriding plate i e the sino korean craton which supplied large volumes of sediments during the late triassic clockwise rotation of the south china block uplifted the indo sinian part of the qinling belt and closed the basin as the accretionary wedge was thickening along the southern margin of north china detritus derived from this source region were unable to reach the flysch basin the age spectrum presented by detrital zircons indicates

zircon u pb and hf isotope constraints on crustal melting associated with the emeishan mantle plume abstract shrimp zircon u pb dates combined with in situ hf isotopic data provide new constraints on the petrogenesis and protolith of peralkaline metaluminous and peraluminous intrusions and rhyolitic tuffs in the emeishan large igneous province with significant bearing on crustal melting associated with mantle plumes syenite and a type granitic intrusions from huili miyi and taihe in the center of this large igneous province yield u pb dates at 260 ma consistent with the ages obtained for mafic layered intrusions in the same province zircon from these rocks exhibits a wide range of initial hf isotope ratios ehf t 1 4 to 13 4 with corresponding tdm1 of 400 900 ma the highest ehf t value is only marginally lower than that of depleted mantle reservoir at 260 ma suggesting that their source is primarily juvenile crust added during emeishan volcanism with incorporation of variable amounts of neoproterozoic crust the trigger of crustal melting is most likely related to advective heating associated with magmatic underplating in contrast the 255 251 ma peraluminous granites from ailanghe and 238 ma rhyolitic tuff from binchuan have negative initial ehf values of 1 3 to 4 4 and of 7 7 to 14 respectively hf isotopic model ages and presence of inherited zircons indicate their derivation from mesoproterozoic and paleoproterozoic crust respectively given the time lag relative to the plume impact 260 ma and insignificant mantle contribution to 255 238 ma magmatism conductive heating is suggested as the trigger of crustal melting that resulted in formation of delayed felsic magmas the involvement of older crust in younger felsic magmas is consistent with upward heat transfer to the lithosphere during plume impregnation if the age of crust is inversely stratified i e changes from paleoproterozoic to mesoproterozoic to neoproterozoic to permian with increasing depth such crust may have resulted from episodic downward crustal growth during the evolution of the western yangtze craton

the structures of albite and jadeite composition glasses quenched from high pressure abstract the structures of jadeite and albite composition glasses quenched from 10 and 15 kbar respectively have been studied using radial distribution analysis it is found that aluminum does not undergo a coordination increase at these pressures in agreement with recent raman and nmr studies of these glass compositions radial distribution functions rdf s indicate that the structural response of albite and jadeite composition melts to pressure up to 10 15 kbar has to do primarily with a distortion of the tetrahedral sites a slight collapse of the tetrahedral rings forming the framework and a small loss of short range order resulting in a more densely packed framework structure the rdf s do not provide any clues concerning t o t angular changes at these pressures it is suggested that the observed structural changes should be closely related to those that take place in melts of rhyolitic composition such changes in rhyolitic melts would result in a density increase and viscosity decrease with increasing pressure similar to that observed for melts of albite and jadeite composition

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

pyrobitumen in south china organic petrology chemical composition and geological significance abstract pyrobitumen has been discovered in the dolostone reservoir of the central sichuan basin southwest china accumulates coevally with hydrothermal minerals in pores the pyrobitumen has strong anisotropy with an evolutionary sequence from mosaic to a fibrous texture a similar sequence of anisotropy has also been observed in mesophase pitch during the process of coking coal reflectance of the pyrobitumen shows a positive correlation with anisotropic evolution and increasing depth hopane and sterane biomarkers are abundant in extracts of highly anisotropic pyrobitumen with smooth rough hyperplastic surface and honeycomb micropores methylpyrene and methylchrysene maturity indicators and the reflectance values of pyrobitumen show a positive linear correlation in cambrian samples but show random correlation in sinian samples fluid inclusion microthermometry of hydrothermal minerals together with the burial and heat flow history of the reservoir indicate an abrupt thermal event that may have impacted the generation and evolution of pyrobitumen this appears to be confirmed by the reflectance values of pyrobitumen in the source rock and reservoir the pyrobitumen in source rock was less heated by the hydrothermal fluid due to its poor porosity and have lower reflectance values however the pyrobitumen in weathering crust reservoir was more heated by the hydrothermal fluid and have higher reflectance values because the reservoir rocks have higher porosity the occurrence and characteristics of pyrobitumen indicate an in situ pyrolysis of oil which was triggered by an abrupt thermal event in the reservoir the reconstruction of burial history and heat flow as well as the hydrothermal fluid minerals of the dolomite reservoir all indicate the thermal event of the sichuan basin might correlate with the emei mantle plume in late permian

seismic tomography in the southern margin of the sichuan basin insight into the plateau craton interaction and seismotectonics in the se tibetan plateau abstract the expansion of the tibetan plateau in its southeastern margin causes high seismic hazards in southwest china even if some earthquakes are located far away from the expansion frontier in the southern sichuan basin in this study we determined the high resolution p wave velocity structure of the crust beneath the southern margin of the sichuan basin with an updated data set we find low velocity anomalies in the upper and lower crust whereas high velocity anomalies between them large earthquakes are spatially related to the low velocity anomalies in the lower crust the results indicate that the southern margin of the sichuan basin has been weakened laterally by the deep crustal extrusion related to the expansion of the se tibetan plateau we infer that the shallow crust might have been broken in association with the strike slip faults during the plateau expansion leading to large earthquakes

petrogenesis and tectonic implications of neoproterozoic highly fractionated a type granites from mianning south china abstract the origin of neoproterozoic intrusions ca 860 750 ma along the western part of the yangtze craton has been the subject of debate in recent years with two competing models proposed the plume model argues for an extensional setting and emphasizes the role of a superplume in the rodinia breakup whereas the arc model argues for the presence of a subduction zone in the yangtze craton as a contribution to this animated dispute geochronologic and geochemical analyses have been carried out on the mianning granite which is the largest pluton 700 km2 in the northern kangdian rift of the western yangtze craton it is shown that the mianning granites were emplaced at ca 780 ma and display highly fractionated feature i e sio2 75 wt eu eu 0 03 0 50 enrichment of k rb th u zr hf y and rees depletion of nb ta ba sr p eu and ti they are metaluminous to strongly peraluminous a cnk 0 93 1 55 and contain abundant perthite and minor alkali riebeckite and sphene sharing the petrological and geochemical characters of a2 type granites positive nd t 2 97 5 24 and zircon hf t 9 2 12 1 values are consistent with a derivation by partial melting of a relatively young crust formed about 1000 900 ma given the general absence of a type granites in arc settings the mianning a type granites are suggestive of an anorogenic crustal extensional environment for the western yangtze craton during the neoproterozoic the data presented in this study are therefore consistent with an intracontinental rift model but are not sufficient to identify plume involvement in the neoproterozoic magmatism

pre eruptive uplift in the emeishan in their correspondence he and colleagues question our conclusion of little or no uplift preceding emeishan volcanism that we reported in our letter1 debate concerns the nature of the contact between the maokou limestone and emeishan volcanics the depositional environment and volumetric significance of mafic hydromagmatic deposits mhds and evidence for symmetrical domal thinning mhds in the daqiao section are separated from the maokou limestone by 100 m of subaerial basaltic lavas but elsewhere mhds previously interpreted as basal conglomerates2 3 directly overlie the maokou2 3 mhds thus feature strongly in basal sections of the emeishan lava succession as also recently shown4 elsewhere in the emeishan an irregular surface at the top of the maokou limestone has been interpreted as an erosional unconformity2 3 but clastic deposits presented as evidence of this erosion2 3 are mhds produced by explosive magma water interaction1 a clear demonstration that this irregular top surface is an erosional truncation of limestone reef facies slope rim flat lagoonal is currently lacking but is critical because reefs and carbonate platforms show considerable natural relief of tens of metres the persistent hot wet climate since the oligocene has produced well developed weathering profiles on exposed palaeozoic marine sedimentary sequences5 but weathering and karst relief of the uppermost maokou limestone underlying the flood basalts have not been properly documented nor shown to be of middle permian age and immediately preceding emplacement of the large igneous province

silicic magmas from the emeishan large igneous province southwest china petrogenesis and their link with the end guadalupian biological crisis abstract the emeishan large igneous province in sw china comprises a bimodal mafic silicic suite with the silicic rocks occurring at the uppermost part of the thick lava sequence the silicic rocks have an age of 257 263 ma and are thus roughly coeval with the guadalupian loping g l boundary event most silicic rocks trachyte and rhyolite from the emeishan province have rather uniform e nd values 1 to 2 9 that are comparable with the uncontaminated high ti basalts this fact and the remarkably narrow ranges of incompatible element ratios e g zr nb of the basalt silicic suite indicate a genetic relationship between basalt and silicic members the significant difference between the emeishan rocks and the experimental melts of hydrated basaltic crust suggests the fractional crystallization of basaltic magma rather than crustal melting as the major petrogenetic process for the formation of silicic rocks indeed their major and trace element trends can be modeled by fractionation of the observed mineral phases feldspar clinopyroxene fe ti oxide and apatite in contrast to the virtually closed system differentiation processes associated with trachytes the rhyolites may have experienced interaction with upper crustal material during ascent the geochemical characteristics and recent assessment of timing of the emeishan volcanism suggest the emeishan rhyolites as the potential source of the widespread clay bed at the g l boundary in south china this enhanced the causal link between the emeishan eruption and the end guadalupian biological crisis

age and origin of magmatism along the cenozoic red river shear belt china to decipher the geodynamic significance of cenozoic magmatism along the red river shear belt geochemical analyses u pb and rb sr dating and pb sr nd isotope tracing were undertaken zircon monazite titanite and a ti u oxide from foliated granitoid intrusions in the shear belt gneisses yield u pb emplacement ages of 33 1 0 2 2 31 9 0 3 25 8 0 2 and 24 7 0 2 ma and an age of 35 0 0 3 ma was obtained for the roughly 100 km long adjacent jinping phan si pang alkali granite together with our previous data the new ages suggest that magmatism and left lateral strike slip movements occurred coevally during latest eocene oligocene times from 33 to 22 ma the rb sr dating of muscovite and biotite from the northernmost gneisses indicates that cooling to 500 c occurred at 52 6 1 1 ma pre dating the onset of magmatism whereas further cooling to 300 c took place at 28 9 0 6 this shows that unroofing in the north took place almost 9 million years earlier than in the central gneiss segments of the shear zone geochemical data substantiate two types of magmas 1 amphibole bearing intrusions of alkaline trend which are derived from sources with isr 0 7065 0 7089 and i nd 3 7 to 6 6 2 leucogranitic layers and bodies having isr 0 7084 0 7354 and i nd 3 3 to 13 4 the former type of intrusion is found in both the gneisses and the adjacent unmetamorphosed cover rocks whereas leucogranites are restricted to the shear belt gneisses source signatures of the alkaline intrusions lie adjacent to the those of oib plotting at the lower end of the mantle array contamination of these melts by continental material seems to be very limited on the other hand the leucogranitic layers are essentially crustal derived but none of the them has country rock isotope signatures requiring melting of crust different from the actually exposed gneisses magma sources similar to those of ocean island basalt indicate magmatism to involve melting of light rare earth element and large ion lithophile element enriched mantle domains most likely present in the lithosphere underneath the region since lithospheric thickening or subduction can be ruled out to produce both types of magmas the presence of an important thermal anomaly is required which is coevally active with left lateral strike slip shear adiabatic decompression and melting within the rising anomaly is the most plausible mechanism to produce the mantle magmas which successively migrate through the crust to induce anatectic melting at 20 15 km crustal depth alkaline magmas largely dominate the volume of magmatism along the belt being continuously present in the shear zone for millions of years such lubrication potentially explains how very large amounts of displacement can be absorbed in surprisingly narrow shear zones such as the red river belt possibly also playing a role for where and when zones of plate scale lateral extrusion develop

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

a positive conglomerate test for the characteristic remanent magnetization chrm of the emeishan basalt from southwest china a positive conglomerate test has been achieved on ten basalt clasts from a pyroclastic breccia in the emeishan basaltic sequence from southwest china nine basalt samples collected from a nearby lava flow yield chrm directions similar to the northeasterly and shallow up directions reported from elsewhere in the basalt province this effectively refutes the suggestion that this group of directions represents a remagnetization of as late as jurassic or triassic age the difference between different sets of results from the emeishan basalt and the non antipodality between the northeasterly directions and the reversed southwesterly down directions are likely to be due to a combination of minor structural rotations about vertical axes between the sampled localities inaccuracy in determination of bedding attitude in massive basaltic rocks and insufficient paleomagnetic sampling

high precision u pb zircon age constraints on the guadalupian in west texas usa abstract the guadalupian epoch was characterized by major changes in paleogeography paleoclimate and biodiversity yet the paucity of precise and accurate radioisotopic dates from the guadalupian stages in their type area guadalupe mountains national park in west texas has rendered their calibration inadequate in this study we report high precision u pb zircon geochronology by the ca id tims method from three ash beds 2 internal errors only in the rader member of the bell canyon formation at the back ridge section 262 127 0 097 ma mswd 0 89 n 3 the lower pinery member of the bell canyon formation at the frijole section 264 23 0 13 ma mswd 0 89 n 8 and the basal south wells member of the cherry canyon formation at the monolith canyon section 266 525 0 078 ma mswd 0 62 n 5 the bayesian interpolation statistics method is used to establish an age stratigraphy model that estimates the base of the capitanian to be 264 28 0 16 ma serving as the best age estimate for the capitanian stage at present in addition we review the existing geochronology from the guadalupian series in west texas and seek to propose more precise temporal estimates of guadalupian geological and biological events these data constrain the high frequency sequences of the cherry canyon and bell canyon formations in the guadalupe mountains national park area accordingly the base of the wordian is estimated at 266 9 0 4 ma and the illawarra geomagnetic polarity reversal in west texas at 267 4 0 4 ma to 266 5 0 3 ma the global end guadalupian extinction began in the conodont zone of jinogondolella altudaensis above the base of the reef trail member of the bell canyon formation and might continue to the clarkina postbitteri postbitteri zone in the earliest wuchiapingian the conodonts display a rapid evolutionary rate during this interval this constrains the biotic crisis from ca 260 ma to 259 ma based on our conodont age estimation the emplacement of the emeishan large igneous province elip in south china has been constrained to ca 260 ma to 257 4 ma based on zircon u pb geochronology by the ca id tims method overlapping with the end guadalupian extinction which provides support for the temporal relationship between them additionally the elip persisted into the early wuchiapingian and may have hampered ecosystem restoration during the post extinction interval

paleomagnetic results from the upper permian of the eastern qiangtang terrane of tibet and their tectonic implications abstract paleomagnetic samples were collected from red intercalations within the upper permian tuoba formation from the eastern qiangtang terrane of tibet progressive thermal demagnetization has revealed a prefolding possibly primary characteristic remanent magnetization chrm which yields a tilt corrected mean direction ofd i 25 2 6 7 95 8 6 the equatorial paleolatitude for the qiangtang terrane indicated by this result and the similarity of late permian fauna and flora shared by the qiangtang terrane and south china block scb suggest that the two terranes were in close proximity during the late permian a late permian paleogeographic reconstruction involving the major terranes of the present day east asia is constructed in which the songpan garze sea is surrounded by laurasia the north china block ncb the scb and the qiangtang terrane the permian development of the songpan garze sea appears to be related to the rifting of the qiangtang terrane off the scb its triassic evolution may resemble that of the solomon sea today in the southwestern pacific

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

paleomagnetism of middle triassic redbeds from hubei and northwestern hunan provinces south china abstract paleomagnetic investigations have been conducted on redbeds from the middle triassic badong formation at badong hubei and sangzhi hunan as well as from the coeval puxi formation from puxi southeastern hubei province south china the characteristic remanent magnetization chrm determined from badong and sangzhi passes the fold test with dual polarity the tilt corrected chrm direction from puxi is congruent with those from badong and sangzhi but is based on a smaller number of samples collected from a monocline these chrm directions are believed to be primary the data indicates that badong has been rotated clockwise by 13 6 8 5 relative to sangzhi which in turn has possibly been rotated in the same sense with respect to other parts of the yangtze block yb except eastern sichuan based on the bend of the sampled fold axis and the lower triassic paleomagnetic data published for the yb the lower and middle triassic paleomagnetic data together appear to indicate that eastern sichuan and the border area between sichuan guizhou hubei and hunan provinces have been affected by differential rotations probably due to oroclinal bending the paleomagnetic data obtained from this study further constrain the timing of the final suturing of the yb with the north china block ncb to be post middle triassic

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

permo triassic palaeomagnetism in the emei mountain region southwest china summary a palaeomagnetic study has been made of samples collected from permo triassic type sections in the emei mountain region of southwest china no results were obtained from the lower permian qixia formation limestones or the lower upper permian maokou formation limestones the upper permian longtanian emeishan basalt formation shows excellent stability to both alternating field and thermal demagnetization results from 24 flows 125 samples are all of normal polarity have a positive fold test cretaceous or tertiary folding and give a pole position at 54 5 n 254 3 e with dp 1 8 dm 3 5 the basalts were thus presumably extruded after the end of the kiaman reversed superchron the overlying upper permian xuanwei formation of red mudstones and sandstones changxingian in age also shows a positive fold test from n 12 samples after thermal treatment and has a positive reversals test at level c the pole position is close to that of the emeishan basalt and lies at 57 0 n 260 9 e with dp 4 3 dm 8 5 the lower triassic griesbachian to mid nammalian feixianguan formation is primarily a red sandstone sequence at this locality thermal treatment of the collection of 114 samples reveals two main components the a component is always observed below 600 c and probably represents a drm either from magnetite or specular haematite the 5 component resides in haematite and is often of the opposite polarity superimposed on the a component it is revealed only above 600 c and is a crm acquired during diagenesis although normal components dominate both the a and b components also have reverse polarities the normal a and normal b components are statistically indistinguishable and the normal b component shows a positive fold test the reverse b component is however not quite antiparallel to the normal b component and may therefore be a second crm acquired at a slightly different time to the normal b component all these components are thought to have arisen during deposition and diagenesis over a few million years as has been observed in other studies of redbeds the dominance of normal components is consistent with the observation that the earlier part of the griesbachian stage of the lower triassic is mainly a normal polarity chron the overall mean of the n 101 both normal and reverse a and 5 components is considered to be the best estimate of the palaeofield in the early triassic giving a pole position at 56 2 n 227 6 e with dp 1 t dm 3 4 this pole lies 15 2 away from the emeishan basalt pole suggesting a polar shift occurred near the permo triassic boundary a review of late permian and early triassic poles from the south china block shows a similar polar shift

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

variation of viscosity with temperature and composition in the plagioclase system the viscosities of supercooled melts of 13 different plagioclase compositions have been measured in the range 1013 108 dpas with a micro penetration viscometer application of the most common empirical and theoretical viscosity temperature models to the present data and plagioclase viscosity data available from the literature showed that the configurational entropy theory represents the best approach using this theory together with an entropy of mixing term evaluated from a modified two lattice mixing model the viscosities of intermediate plagioclase compositions have been calculated as a function of temperature and anorthite content solely from the appropriate data of the end members albite and anorthite the agreement between experimental and calculated viscosity data is excellent

some aspects of the permian triassic boundary ptb and of the possible causes for the biotic crisis around this boundary abstract the first appearance datum fad of hindeodus parvus is an excellent datum very close to the base of the otoceras woodwardi zone priority base of the triassic for the first time it allows an exact correlation of the ptb in all marine facies and faunal realms the following features of the extinction and recovery patterns near the ptb are most important for the search for causes of the ptb biotic crisis 1 most strongly affected were the siliceous plankton radiolarians and the warm water benthos and nekton 2 most of the cold water faunas were not significantly affected 3 the recovery of the warm water benthos and siliceous plankton occurred only after an unusually long time during the late olenekian and middle triassic 4 the productivity of the terrestrial plants dropped strongly and the recovery was mainly in the upper olenekian and in the middle triassic 5 several terrestrial extinction events occurred at different levels within the upper permian considerable before the marine ptb at or close to the fad of lystrosaurus no extinction event in the terrestrial faunas can be observed 6 terrestrial faunal elements that survived the dzhulfian dorashamian extinctions were able to survive some months of extreme climatic conditions by hibernation like life stages vertebrates or by dry and freezing resistant eggs conchostracans 7 about 50 of the genera that disappeared at the ptb re appeared in the olenekian middle triassic interval lazarus taxa or in this time interval genera appeared that had undoubtedly evolved from genera that had disappeared at the ptb and 8 the ptb is preceded by mass occurrences of marine and continental fungi and the fungal spike ends abruptly a little before the ptb a scenario for the ptb biotic crisis is elaborated that takes into consideration the real not interpolated diversity patterns of different marine and continental fossil groups the exact correlation of bioevents in different faunal realms and facies the strong biotic crisis in the tethyan warm water faunas in contrast to the much weaker biotic event in cold water faunas the severe dzhulfian dorashamian climate in many parts of the world caused by the continent ocean configuration and by siberian trap volcanism the cooling of the boreal area by the northward drift of pangaea that interrupted the exchange of warm water benthos between the tethys and western north america during the guadalupian the extinction event at the guadalupian lopingian boundary that restricted most of the affected warm water faunas to the tethys the importance of lower scythian anoxia that reached an unusually shallow level and above all the effects of the contemporaneous huge volcanic events about 250 m y ago on the siberian platform and in the eastern tethys that caused a 3 6 months lasting volcanic winter with strong cooling in low latitudes followed by a strong global warming

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

native gold and native copper grains enclosed by olivine phenocrysts in a picrite lava of the emeishan large igneous province sw china a native gold bleb found in an olivine phenocryst in a picrite lava from the emeishan large igneous province elip may be the first documented case of the transport of gold as a distinct precious metal phase in a mantle derived magma four picrite layers have been recognized in the lower part of the volcanic succession in the lijiang area in the western part of the elip the native gold bleb was found enclosed in an olivine phenocryst in the second picritic layer of a basalt picrite succession in the elip the gold bleb is spheroidal about 30 m in diameter consists of pure gold and does not contain any other elements in addition native copper grains were also discovered in the serpentinized olivine phenocrysts and native zinc and moissanite sic were separated from an 20 kg sample of picrite the paragenesis of these minerals suggests that the primary magmas were s unsaturated the native gold and native copper grains are considered to be xenocrysts from the mantle transported to shallow depths by a rising plume and then captured by the picritic melts the discovery of native gold and native copper grains provides direct evidence that the gold in the hydrothermal gold deposits and the native copper deposits in the elip lavas ultimately may be derived from a mantle plume

synchrony and causal relations between permian triassic boundary crises and siberian flood volcanism the permian triassic boundary records the most severe mass extinctions in earth9s history siberian flood volcanism the most profuse known such subaerial event produced 2 million to 3 million cubic kilometers of volcanic ejecta in approximately 1 million years or less analysis of 40 ar 39 ar data from two tuffs in southern china yielded a date of 250 0 0 2 million years ago for the permian triassic boundary which is comparable to the inception of main stage siberian flood volcanism at 250 0 0 3 million years ago volcanogenic sulfate aerosols and the dynamic effects of the siberian plume likely contributed to environmental extrema that led to the mass extinctions

volcanic rocks of the 1985 tibet geotraverse lhasa to golmud volcanic rocks encountered during the tibet geotraverse have been studied in the field in thin section and by major and trace element geochemistry in order to determine their most probable original eruptive environment rocks from a total of eleven distinct volcanic provinces were studied in this way they provide evidence for an active continental margin or post collision province of probable devonian early carboniferous age in the northern kunlun mountains an active continental margin of late carboniferous age in the southern lhasa terrane permian continental rifts in the central qiangtang and central kunlun terranes triassic volcanic arcs in the southern lhasa and northern qiangtang terranes a triassic active continental margin dyke swarm in the northern kunlun mountains a jurassic post collision or back arc rifting province in the southern qiangtang terrane a jurassic island arc in the northern lhasa terrane a cretaceous post collision province in the northern lhasa terrane possibly extending into the southern qiangtang terrane and a palaeogene active continental margin in the southern lhasa terrane an oligocene trachyte plug in the northern qiangtang terrane was the only evidence encountered during the geotraverse of volcanism post dating the palaeogene india eurasia collision however the composition of this plug coupled with new and published analyses from miocene volcanics in the southern lhasa terrane and from the pliocene recent volcanic province of northwest tibet places important constraints on models for post collision underplating of tibet by continental lithosphere any underplating is likely to have been a much later than the start of collision b directed beneath tibet from the north as well as the south and c limited in extent

fragmentation of asia in the permian asia is a composite continent formed by the accretion of crustal blocks1 3 a likely sequence of events is that siberia collided with a kazakhstan block which together then collided with europe along the urals in permo triassic times2 this was followed by the sucessive accretion of various blocks of south and east asia to siberia culminating in the collision of india with asia during the cenozoic palaeomagnetic data have already demonstrated that the kolyma and sikhote alin blocks of eastern asia collided with the main continent along the late mesozoic fold belts of verkhoyansk and sikhote alin4 furthermore results from the malay peninsula5 and japan6 suggest that south east asia and japan were situated near the equator in permian times and were therefore separated from the asian continent we report here some palaeomagnetic results from the permian of the sino korean and yangtze blocks of china that show they were also situated near the equator were not in their present relationship to one another and were widely separated from siberia

paleomagnetism and electron microscopy of the emeishan basalts yunnan china abstract many normal and a few reversed characteristic directions of magnetization have been obtained by predominantly thermal demagnetization from ten sites of the late permian emeishan basalts collected near kunming yunnan province the normal magnetization directions pass a fold test at the 99 confidence level and yield declination inclination 26 12 k 46 95 6 and a paleopole at 50 n 241 e however the reversed polarity directions with declination inclination at 244 3 are not antipodal to the normal ones which is also noted in other studies of the emeishan basalts of the yangtze paraplatform of the south china block speculations about the cause of this lack of antipodality center on 1 1 local relative rotations 2 2 incomplete demagnetization 3 3 unusually large non dipole fields secular variation or asymmetric reversals in the late permian 4 4 errors in sample orientation 5 5 later remagnetization represented by one or the other polarity group the first two causes are ruled out by our observations and the third cannot be tested with data from china alone but is unlikely because it has not been observed elsewhere errors in sample orientation may be present in those studies that used a magnetic compass because sample intensities of magnetization are on the high side however for the normal polarity directions a solar compass has been used in some of the studies and all normal polarity directions observed in the area are in agreement thus sample orientation errors can be invoked only for the reversed polarity directions in order to test cause 5 we have examined the magnetic carriers in eight samples with scanning electron microscopy titanium poor to titanium rich magnetite commonly cruciform in crystal habit indicates primary igneous crystallization in two samples that have ne and wsw declinations in contrast six normal polarity samples with nne declinations show pervasive replacement of original titanomagnetite by titanium free magnetite and sphene indicating a high degree of alteration we ascribe this alteration to late hydrothermal circulation and argue that it has caused remagnetization in post permian times this study suggests therefore that the ne sw directions are more likely to be representative of the late permian paleomagnetic field than the nne directions

late cenozoic to holocene deformation in southwestern sichuan and adjacent yunnan china and its role in formation of the southeastern part of the tibetan plateau from at least 2 4 ma to present crust in the southeastern part of the tibetan plateau west of the convex east xianshuihe xiaojiang fault system has deformed internally and rotated clockwise around the eastern himalayan syntaxis the northwest striking ganzi fault zone bounds the rotating crust on the north and has a total left slip of 78 100 km of which 60 km is transferred to the xianshuihe fault zone across a diffuse transfer zone and 22 40 km is absorbed by bending of older structures and crustal shortening crustal shortening is expressed along and east of the eastern end of the ganzi fault zone by mountains capped by permanent glaciers locally rising nearly 1000 m above the average elevation of the tibetan plateau a similar transfer of left slip into shortening occurs farther south across the xianshuihe fault in the high mountains around and east of gongga shan 7556 m the northwest striking convex east left lateral litang fault zone lies southwest of the ganzi xianshuihe xiaojiang fault zone and appears to be less well developed but otherwise similar to the ganzi fault zone the batang chenzhi and other northeast striking right lateral faults of small displacement occur within the rotating crustal fragment together with the left slip faults they accommodate east west shortening northeast of the eastern himalayan syntaxis south of this region of shortening the crust is extending to form grabens within the dali and southern xiaojiang fault systems and in the tengchong volcanic province the progressive change from shortening southward into extension is related to variations in strain that characterize the region from northeast to southeast of the eastern himalayan syntaxis the assemblage of structures in southwestern sichuan geometrically resembles structures of eocene to miocene age in southern yunnan that were positioned northeast of the eastern himalayan syntaxis similar to present day southwestern sichuan at the time of their development the similarity in the structural development in the two areas indicates that crust northeast of the syntaxis underwent a common evolution as the syntaxis migrated northward during the past 50 m y structures in sichuan are less fully developed than older structures in southwestern yunnan and can serve as a guide to reconstruct the progressive tectonic development in the region of the syntaxis deformation in these areas indicates that plateau formation has been complex inhomogeneous and diachronous at scales from 1000 km to less than 100 km

structural and geochronological constraints on the tectono thermal evolution of the danba domal terrane eastern margin of the tibetan plateau the songpan ganze terrane of the tibetan plateau is underlain by neoproterozoic crystalline basement rocks of the yangtze block these basement rocks are exposed as a series of extensional tectonic domes that form a nearly north south trending extensional belt more than 1000 km long in the eastern margin of the tibetan plateau in the danba area detachment faults separate the basement core complexes e g the gezong and gongcai complexes from the paleozoic strata which have been thinned or removed completely the cover sequences have undergone upper greenschist to lower amphibolite facies metamorphism to form the danba schist and are overlain by the triassic xikang group a thick flysch sequence both the basement rocks and the paleozoic rocks have undergone multiple stages of deformation and thus provide an excellent opportunity to study the tectono thermal evolution of the eastern margin of the tibetan plateau two stages of deformation corresponding to three generations of foliation s1 view the mathml source and view the mathml source have been recognized on the basis of structural and microscopic observations we selected amphibole and biotite separates associated with distinct generations of foliation for 40ar 39ar dating using laser microprobe incremental heating technique to place numerical constraints on the major tectono thermal events within the danba area the geochronogical results reveal an earliest metamorphic event at 258 6 0 5 ma s1 biotite and 263 6 0 8 ma s1 amphibole coinciding temporally with the mantle plume that produced the voluminous emeishan flood basalts the second event was a progressive extensional deformation first occurred at 159 166 ma view the mathml source amphibole responsible for the earlier tectonic doming of the crystalline basement and then the final tectono thermal overprint recorded by view the mathml source foliation and view the mathml source metamorphism locally in the core complexes at 47 58 ma for the gezong complex and 64 81 ma for the gongcai complex this major post orogenic extensional event is believed to be a consequence of collision between the north china and south china blocks the apparent discrepancy of the 40ar 39ar ages observed between localities suggests a slow cooling process associated with progressive uplift

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

effects of water and fluorine on the viscosity of albite melt at high pressure a preliminary investigation the viscosities of fluorine and water bearing melts based on albite composition have been determined at 7 5 15 and 22 5 kbar by the falling sphere method all melt viscosities decrease isothermally with increasing pressure at 1200 c the viscosity of the fluorine bearing melt albite 5 8 wt fluorine substituted for oxygen denoted abf2o 1 decreases from5000 750p at7 5kbar to1600 240p at22 5kbar at 1400 c the viscosity of this melt decreases from1300 200p at7 5kbar to430 65p at22 5kbar at 1400 c the viscosity of albite 2 79 wt water denoted abh2o decreases from650 100p at7 5kbar to400 60p at22 5kbar fluorine as f2o 1 and water strongly decrease the viscosity of albite melt over the entire range of investigated pressures the ratio of the effects of 5 8 wt fluorine f f o molar 0 10 and 2 79 wt water oh oh o molar 0 10 on the log of melt viscosity log abf2o 1 log abh2o equals0 90 0 05 0 84 0 05and0 97 0 05at7 5 15and22 5kbar respectively comparison with available data on the high pressure viscosity of albite melt indicates that both f2o 1 and h2o maintain their viscosity reducing roles to lower crustal pressures the difference between the viscosities of melts of albite abf2o 1 and abh2o may be explained in terms of the relatively depolymerized structures of abf2o 1 and abh2o melts the depolymerization of albite melt by the addition of water results from the formation of sioh bonds the depolymerization of albite melt by f2o 1 substitution results from the formation of non bridging oxygens associated with network modifying aluminum cations that are formed upon fluorine solution the strong viscosity reducing effects of water and fluorine in albite melt at pressures corresponding to the mid to lower continental crust indicate that these two components will strongly influence the dynamic behavior of anatectic melts during initial magma coalescence and restite melt segregation

40ar 39ar study of plagioclases from the rogaland anorthosite complex sw norway an attempt to understand argon ages in plutonic plagioclase abstract the combined use of stepwise heating and uv laser ablation 40 ar 39 ar techniques with microprobe analyses on plutonic plagioclase allows the investigation of complex argon release and retention the complexity arises from the multiphase nature of the samples comprising plagioclase k feldspar in antiperthite secondary sericite and various inclusions in which some of the components also exhibit variable ages fragments of plagioclase megacrysts and plagioclase separates from whole rock samples were selected from a well constrained series of anorthosite leuconorite intrusions of the rogaland igneous complex sw norway intruded at ca 930 ma all samples exhibit saddle like staircase shaped argon release spectra ages rise from around 400 600 to 750 900 ma though three samples exhibit excess argon released at high temperatures the most remarkable aspect of this data is however the apparently linear correlation of 40 ar 39 ar with ca k the stepwise heating data reveal late stage cooling ages considerably younger than the intrusive age and low temperature reheating of the rogaland igneous complex during caledonian overthrusting arrhenius plots for the stepwise heated samples show a linear trend of diffusion data at low temperatures followed by a systematic break around 900 1060 c single spot ages obtained using a uv laser show extreme age variations with heterogeneous excess argon contained in inclusions but no correlation of 40 ar 39 ar with ca k this lack of correlation indicates that the two component mixing model with two end members of distinct ages suggested by the stepwise heating data is an artefact resulting from a more complex mechanism of argon release the most apparent interpretation of this data is that both plagioclase and another potassium rich minor component probably k feldspar antiperthite yield variable but correlated ages because the excess argon is found exclusively in samples from marginal leuconorite bodies and never in the core of the megacrysts it is probably derived from the outgassing of the k rich country rock