petrology geochemistry and u pb geochronology of the betic ophiolites inferences for pangaea break up and birth of the westernmost tethys ocean abstract the betic ophiolitic unit part of the mulhacen complex in the betic cordillera se spain is composed of several tectonic slices of eclogitized mafic and ultramafic rocks together with oceanic metasediments a comprehensive review of the petrological characteristics of the betic ophiolites shows notable analogies with other ophiolites from the alps apennines system indicating that they represent relics of oceanic lithosphere created at the westernmost end of the western tethys u pb dating by shrimp sensitive high resolution ion microprobe on zircon grains carefully investigated by cathodoluminescence images has been carried out on an eclogitized metagabbro lugros outcrop and a metadolerite dyke intruded in ultramafic rocks almirez outcrop giving consistent magmatic ages of around 185 3 ma this age marks the beginning of the betic oceanic magmatism transitional to enriched morb that followed pangaea break up and the earliest oceanic accretion of the western tethys ocean subsequently ultraslow mid ocean ridge spreading developed from the betic oceanic basin northeastward to the ligurian and alpine tethys domains mainly around 165 to 140 ma

coupled oceanic oxygenation and metazoan diversification during the early middle cambrian the early middle cambrian fortunian to age 4 is characterized by a significant increase in metazoan diversification furthermore this interval is marked by a prominent environmental and ecological expansion of arthropod and echinoderm rich biotas recent redox work has suggested that this shift occurred during stable or decreasing marine oxygen levels suggesting that these paleobiological and paleoecological transformations were decoupled from a redox control we tested this idea by conducting new paleoredox analyses on age 2 age 4 cambrian outer shelf jiuqunao wangjiaping slope wuhe geyi and basinal zhalagou sections of the south china craton multiple sections indicate that mid depth waters transitioned from anoxic conditions during cambrian age 2 to stable oxic conditions during cambrian age 4 these findings suggest a stepwise expansion of oxic waters from shallow to deep settings during the early middle cambrian consistent with a redox control of metazoan diversification and ecological expansion more broadly despite the surge in redox work over the past decade this study highlights the need for continued coupled redox and paleontological studies to directly test models about the links between the evolution of animals ecosystems and marine redox conditions

new models for evolution of magma poor rifted margins based on a review of data and concepts from west iberia and the alps direct observation and extensive sampling in ancient margins exposed in the alps combined with drill hole and geophysical data from the present day iberia margin result in new concepts for the strain evolution and near surface response to lithospheric rupturing at magma poor rifted margins this paper reviews data and tectonic concepts derived from these two margins and proposes that extension leading to thinning and final rupturing of the continental lithosphere is accommodated by three fault systems each of them characterized by a specific temporal and spatial evolution during rifting of the margin by its fault geometry and its surface response the data presented in this paper suggest that margin architecture and distribution of rift structures within the future margin are controlled first by inherited heterogeneities within the lithosphere leading to a contrasting behaviour of the future distal and proximal margins during an initial stage of rifting the place of final break up appears to be determined early in the evolution of the margin and occurs where the crust has been thinned during a first stage to less than 10 kilometres during final break up the rheology of the extending lithosphere is controlled by the thermal structure related to the rise of the asthenosphere and by serpentinization and magmatic processes

depositional history and sequence stratigraphy of the precambrian cambrian boundary stratotype section chapel island formation southeast newfoundland abstract facies relationshps in the vendian to lower cambrian chapel island formation of southeast newfoundland are used to establish a sequence stratigraphic framework for this important boundary stratotype section the formation consists of five members of sandstone mudstone and minor limestone deposited in a variety of shoreline to shelf environments peritidal sandstone and shale units in member 1 are overlain by sandstone and silstone deltaic and shelf deposits of member 2 these make up a transgressive system tract in which small scale changes in relative sea level are superimposed on autocyclic deltaic processes the thickness 400 m of nearshore and inner shelf sediments resulted from high sediment accumulation rates which for member 2 nearly kept pace with subsidence upper member 3 and member 4 consist of inner shelf mudstone and peritidal limestone that were deposited in a low energy oxygen stratified basin these deposits are arranged in three large scale shoaling cycles representing three foresteping parasequences of one highstand systems tract an extensive limestone bed at the tope of member 4 contains a sequence boundary associated with or shortly postdating development of this surface is a significant epeirogenic event that resulted in basin reorganization increased subsidence and progradation of a thick clastic sequence represented by member 5 the deposits of this upper member consist of an upward coarsening storm dominated shelf to shoreface succession of sandstone and silstone deposits of nearly identical character are also found in new brunswick and suggest that these sediments and the underlying sequence boundary could be significant for interregional correlation global correlation of the paleobathymetric curve developed for the chapel island formation is considered unfeasible given the long history of epeirogenic movements during the vendian and early cambrian in this region

comparisons between tethyan anorthosite bearing ophiolites and archean anorthosite bearing layered intrusions implications for archean geodynamic processes

alpine ophiolites product of low degree mantle melting in a mesozoic transcurrent rift zone abstract the alpine ophiolites and their original sedimentary cover upper jurassic radiolarite limestone with ophiolite breccia occur as dismembered tectonic slices often inverted by alpine tectonics or olistoliths among the calc schists cretaceous original petrologic features of the ophiolites are largely masked by the alpine metamorphism but are discernible by the relic mineral chemistry and petrographic reconstruction the residual peridotite is generally plagioclase lherzolite resembling those from oceanic fracture zones but spinel lherzolite is present at davos switzerland and peas france the latter is characterized by higher na 2 o 1 6 of clinopyroxene and lower cr 2 o 3 8 of spinel than the former 30 respectively and resembles subcontinental lherzolites geologic evidence indicates that these two lherzolites were together exposed on the jurassic ocean floor the mafic ultramafic cumulates are of plagioclase type characterized by the scarcity of orthopyroxene and high tio 2 avg 0 8 of clinopyroxene the basaltic volcanics are alkalic rather than tholeiitic in respect to their relic clinopyroxene which is as rich in tio 2 and na 7 o as that in hawaiian alkali basalt the alpine ophiolite association of lherzolitic residual peridotite plagioclase type cumulates and alkalic basalt is reasonable as cogenetic products of low degree partial melting in the mantle geologic and petrologic features of the alpine ophiolites are consistent with a geotectonic model postulating the generation of the alpine ophiolites in an early mesozoic rift zone cut by many transform faults

new c isotope stratigraphy from southwest china implications for the placement of the precambrian cambrian boundary on the yangtze platform and global correlations a high resolution c isotope profile from a fossiliferous section spanning the precambrian cambrian pc c boundary on the yangtze platform southwest china is used to constrain c isotope chemostratigraphy and its global correlation the discovery of a strong negative c isotopic anomaly just below the china marker a indicates that this negative anomaly can be correlated globally and demonstrates that the pc c boundary of the chinese successions should be close to marker a on the basis of c isotope chemostratigraphic constraints and biostratigraphic data we find that c isotopic peak s2 also first reported here from the chinese successions can be correlated with c isotopic peak i in northern siberia and c isotope feature d in southwest mongolia by integrating the data from archaeocyaeothan fauna the c isotopic peak s3 in the middle dahai carbonates may be correlative with feature g in the mongolian section

new high resolution age data from the ediacaran cambrian boundary indicate rapid ecologically driven onset of the cambrian explosion the replacement of the late precambrian ediacaran biota by morphologically disparate animals at the beginning of the phanerozoic was a key event in the history of life on earth the mechanisms and the time scales of which are not entirely understood a composite section in namibia providing biostratigraphic and chemostratigraphic data bracketed by radiometric dating constrains the ediacaran cambrian boundary to 538 6 538 8 ma more than 2 ma younger than previously assumed the u pb ca id tims zircon ages demonstrate an ultrashort time frame for the lad of the ediacaran biota to the fad of a complex burrowing phanerozoic biota represented by trace fossils to a 410 ka time window of 538 99 0 21 ma to 538 58 0 19 ma the extremely short duration of the faunal transition from ediacaran to cambrian biota within less than 410 ka supports models of ecological cascades that followed the evolutionary breakthrough of increased mobility at the beginning of the phanerozoic

birth of a volcanic passive margin in cambrian time rift paleogeography of the ossa morena zone sw spain abstract mapping at 1 10000 scale of both north n and south s flanks of the variscan monesterio antiform in the ossa morena zone southern spain in association with detailed petrographic and major element analyses of magmatic rocks has revealed the building of a passive but volcanically active oceanic margin in cambrian time this margin was located in northern gondwana and represents the beginning of an extensional tectonic regime that concluded with the opening of the rheic ocean at time of eruption middle cambrian the n flank was located on the northern coastline of a marine basin in the process of passive rifting the vents mainly located in its western part were subaerial continental and or shallow marine on the s flank eruptions and deposits were shallow marine occurring as lava flows and pillows interstratified within shallow marine sediments on both flanks intrusives were emplaced after the extrusives lateral thickness variations of the sedimentary deposits testify to the building during four distinct periods of a rifted passive margin with a shifting locus of volcanic activity over time the composition of magmas was bimodal the mafic magmas are silica saturated alkaline to sub alkaline and somewhat ti rich perhaps originating from partial melting of a subcontinental mantle that was less metasomatized with time the youngest magmatism with an affinity close to that of morb likely had an asthenospheric source the alkaline felsic products are diverse and may have originated from a very heterogeneous continental crust in each set partial melting relationships prevail suggesting that the melts rose along open fractures as soon as they formed in the context of lithospheric thinning it is possible that mantle and crustal melting was triggered by decoupling between rheologically different materials

extinction of cloudina and namacalathus at the precambrian cambrian boundary in oman biostratigraphic carbon isotope and u pb zircon geochronological data from the ara group of oman indicate an abrupt last appearance of cloudina and namacalathus coincident with a large magnitude but short lived negative excursion in the carbon isotope composition of seawater that is globally coincident with the precambrian cambrian boundary u pb zircon age data from an intercalated ash bed directly define this negative excursion to be at 542 0 0 3 ma consistent with previous age constraints from siberia and namibia combined with the global biostratigraphic record these new data strengthen hypotheses invoking mass extinction within terminal proterozoic ecosystems at or near the precambrian cambrian boundary

crystallization and age of zircon from corsican ophiolitic albitites consequences for oceanic expansion in jurassic times abstract in corsica albitites and albite granites are considered to be the latest products of an ophiolitic magma two albitites have been selected for a study of the morphology typology and age of their zircons applying for the first time the u pb method to western tethyan ophiolites both samples come from the rospigliani series possibly formed in a transform fault zone the first one shows a cumulate texture whereas the second has features indicative of more rapid cooling zircons have precipitated at the end of the fractional crystallization process after a strong enrichment in alkali and incompatible elements in a typological diagram they plot at high temperature and high agpaicity indexes these abundant large euhedral to subhedral zircons sometimes with fine zoning may also contain inclusions of granitic composition which testifies to k fractionation in ophiolitic magmas the u pb age of 161 3myr is in the range of the ages obtained for alpine and apennines ophiolites all these ophiolitic remnants belonging to the liguro piedmont basin may represent a portion of the central atlantic ocean created during middle and upper jurassic times

nitrogen cycle perturbations linked to metazoan diversification during the early cambrian abstract the rapid diversification of metazoans during the late ediacaran early cambrian is one of the most remarkable evolutionary events in earth history however the triggering mechanism remains highly contentious particularly concerning the relationship of this biological event to contemporaneous changes in ocean redox and nutrient cycling in this study we report new bulk nitrogen isotope 15n and organic carbon isotope 13corg data from the lower cambrian niutitang formation in two deep water sections in south china to provide a better understanding of the relationship of the nitrogen cycle ocean redox and biological evolution during this key period the 15n data from two study sections are characterized by consistently low values 1 to 2 throughout the niutitang formation despite large variations in 13corg we interpret these low 15n values to have resulted from assimilation of recycled ammonium nh4 and n2 fixation compilation of available 15n data reveals large temporal and spatial 15n variations from 1 to 7 in time equivalent strata in south china such 15n variations imply the co existence of aerobic and anaerobic n cycles and highly heterogeneous ocean redox conditions during the early cambrian in south china which is consistent with inferences from other geochemical proxies temporal fluctuations and spatial heterogeneity in ocean redox conditions may have controlled the distribution and evolution of metazoans during the early cambrian

age origin and geodynamic significance of plagiogranites in lherzolites and gabbros of the piedmont ligurian ocean basin abstract u pb zircon dating sr nd isotope tracing and major trace re element analyses were performed to constrain the age origin and geodynamic significance of plagiogranites that intrude lherzolites and gabbros in the ligurian alps and the northern apennines in addition a host fe diorite was investigated samples from the ligurian alps were collected from the voltri group and the sestri voltaggio zone whereas the plagiogranites from the northern apennines were taken in the bracco unit all these units have been affected by alpine metamorphism reaching eclogite facies in the voltri group blueschist degree in the sestri voltaggio samples and prehnite pumpellyite facies in the bracco unit which has additionally been affected by rodingitization u pb zircon ages of 150 1 153 1 and 156 ma were obtained respectively for two plagiogranites and the host fe diorite in the ligurian alps and an age of 153 1 ma was determined for the plagiogranite in northern apennines inherited components in zircon and initial pb in plagioclase indicate mixing of variously differentiated basaltic magmas with small amounts of roughly 1 7 2 1 ga old continental crust material ree patterns in both the plagiogranites and the host diorite are characterized by high ree abundance and moderate lree enrichment nd isotopic compositions lie in the range of n morb sources yielding initial epsilon nd values between 8 8 and 9 7 whereas sr is isotopically heterogeneous the geochemical pattern of the plagiogranites and the host fe diorite requires melting of a morb type mantle source that experienced lree enrichment shortly before melting the most likely explanation for such enrichment is the injection of melts derived by small degrees of melting from an adjacent mantle region the basaltic lree enriched parent magmas generated from this enriched domain have probably undergone up to about 72 of low pressure fractional crystallization prior to their emplacement into the gabbro peridotite complex the 156 150 ma magmatism occurred in close relation to normal faulting sedimentation of breccias and detachment of the mantle complex from its overlying continental crust followed by exposure on the ocean floor this tectono magmatic event in the ligurian alps and the northern apennines reflects rifting of the adriatic iberian continental plate segment preceding wider opening of the piedmont ligurian ocean basin and pillow basalt deposition

neoproterozoic to early cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the yangtze platform china abstract small shelly fossils ssfs occur widely on the yangtze platform and have great potential for biostratigraphic subdivision of pre trilobitic early cambrian strata based on the ssf record of the shallow water realm five biozones can be recognized for the meishucunian stage in ascending order the biozones are anabarites trisulcatus protohertzina anabarica assemblage zone paragloborilus subglobosus purella squamulosa assemblage zone watsonella crosbyi assemblage zone formerly heraultipegma yunnanensis zone poorly fossiliferous interzone sinosachites flabelliformis tannuolina zhangwentangi assemblage zone in addition one ssf biozone is recognized for the overlying qiongzhusian stage pelagiella subangulata taxon range zone the formerly used siphogonuchites triangularis paragloborilus subglobosus zone and heraultipegma yunnanensis zone are discussed and redefined approximately 80 species of ssfs were screened for their spatial and temporal distribution on the yangtze platform variations in lithofacies and biofacies can be recognized throughout the yangtze platform extending over an area of 2000 900 km in a deeper water shelf setting the first zone is represented by the protohertzina anabarica kaiyangites novilis assemblage zone while younger zones are not recognized in the southern region at the northern platform margin the qiongzhusian is represented by the ninella tarimensis cambroclavus fangxianensis assemblage zone and the rhombocorniculum cancellatum taxon range zone the southeastern shaanxi western hubei region followed a slightly different lithological and faunal development than the rest of yangtze platform indicating a stronger similarity with parts of east gondwana some taxa such as w crosbyi p subangulata r cancellatum microdictyon effusum a trisulcatus protohertzina unguliformis and p anabarica occur nearly worldwide and support an international correlation of early cambrian sequences between the yangtze platform and smaller west gondwanan blocks siberia newfoundland and australia the six investigated zones of the yangtze platform comprise an interval spanning the early nemakit daldynian to the late atdabanian early botoman stage on the siberian platform palaeobiogeographic analysis revealed a strong taxic similarity between the yangtze platform and the tarim platform a smaller number of species are shared with other west gondwanan platform fragments such as india and iran palaeobiogeographic results do not support the previously reported position of the south china block between australia and siberia during the early cambrian

elemental geochemistry of lower cambrian phosphate nodules in guizhou province south china an integrated study by la icp ms mapping and solution icp ms abstract the late ediacaran to early cambrian transition was a critical interval in earth history during which both the environment and life experienced dramatic change relationships between these events however are widely debated this time interval also witnessed the occurrence of massive phosphate deposits studies on the characteristics origins and depositional context of this geochemical archive may provide important insights into not only the ore forming process but also the coevolution of the earth surface system here phosphate nodules from two sections jinsha and songtao are investigated using in situ la icp ms mapping and solution icp ms methods elemental images and factor analyses of the la icp ms dataset show that ca p and sr are evenly distributed and co enriched within the nodule whereas iron related elements fe as and mo are roughly present as concentric layers the paas normalized ree patterns of the jinsha samples exhibit moderately negative ce anomalies and pronounced mree enrichments possibly related to uptake of ree released by iron oxyhydroxides during early diagenesis in contrast the songtao samples are characterized by more seawater like signals with significantly lower ce ce and higher eu eu values these positive eu anomalies are ascribed to active submarine hydrothermal vents along the yangtze platform margin based on a compilation of published ce ce data we suggest that the prominent negative ce anomalies during the ediacaran cambrian phosphogenesis give evidence of possible link between p dynamics and ocean oxygenation

fluid circulation related to deformation in the zabargad gneisses red sea rift abstract a fluid inclusion study in relation to textures has been performed on pan african granulite facies gneisses from zabargad island red sea rift egypt the gneiss samples were collected at a maximum distance of 60 m above the contact with mantle peridotites with which they are co structured by a high t plastic deformation during an isothermal decompression starting from conditions estimated at 35 km depth and 850 c base of the pan african crust a first metasomatism of the crustal section during decompression is represented by co2 fluid inclusions trapped at minimum conditions of 15 km depth and 800 c the co2 fluid inclusion planes are controlled in orientation by the high t plastic flow structures but have been subsequently deformed during medium t plastic deformation in quartz the second metasomatism is represented by solid rich aqueous fluid inclusions which were trapped at minimum p t conditions of 3 7 km depth and 450 c corresponding to a very high thermal gradient they trace the introduction either of seawater or of fluids resulting from leaching of the middle miocene evaporites within the gneisses during their residence at a shallow depth below the seafloor two models are proposed in the literature in order to reconstruct the geodynamic history of zabargad island one brueckner h k elhaddad m a hamelin b hemming s kroner a reisberg l seyler m 1995 a pan african origin and uplift for the gneisses and peridotites of zabargad island red sea a nd sr pb and os isotope study j geophys res 100 22283 22297 suggests that most of the decompression p t path is pan african in age and only the aqueous metasomatism is related to the rifting history of the red sea the second nicolas a boudier f montigny r 1987 structure of zabargad island and early rifting of the red sea j geophys res 92 461 474 boudier f nicolas a ji s kienast j r mevel c 1988 the gneiss of zabargad island deep crust of a rift tectonophysics 150 209 227 supposes the existence of an asthenospheric diapir intrusive through the deep pan african crust leading to a continuous deformation event during early rifting of the red sea although it is impossible to date trapping of fluid inclusions and to know with certainty the origin of the fluids our preference is for the second model but slightly modified the high t decompression path traced by co2 fluid inclusions relayed by solid rich aqueous inclusions traces a continuous process of crustal thinning such a continuous path would assign to a single event the thinning and uplift of the deep crust and uppermost lithosphere to shallow basin bottom during the red sea early rifting in such an interpretative model the northern and central peridotite bodies are representative of the uppermost continental lithosphere and only the southern body could belong to an ascending asthenospheric diapir centred on the southeast of zabargad island

highly dynamic marine redox state through the cambrian explosion highlighted by authigenic δ238u records abstract the history of oceanic oxygenation from the late neoproterozoic to the early cambrian is currently debated making it difficult to gauge whether and to what extent environmental triggers played a role in shaping the trajectory of metazoan diversification uranium isotope 238 u records from carbonates have recently been used to argue for significant swings in the global marine redox state from the late neoproterozoic to the early cambrian however geochemical signatures in carbonates the u isotope archive most commonly employed to argue for redox shifts are susceptible to diagenetic alteration and may have variable offsets from seawater values therefore there is an impetus to reconstruct seawater u isotopic evolution using another sedimentary archive in order to verify that these excursions can indeed be linked to global shifts in marine redox landscape here we report new u isotope data from two fine grained siliciclastic upper ediacaran to lower cambrian ca 551 515 ma successions in south china we find large 238 u swings between 0 63 and 0 39 for calculated values of authigenic u in the siliciclastic rocks consistent with correlative records from the carbonates the replication of these patterns in both carbonate and siliciclastic units provides confirmatory evidence that the early cambrian seawater was characterized by highly variable u isotope compositions these new 238 u data also provide higher resolution records of global oceanic redox conditions during cambrian age 3 coeval with a critical interval of the cambrian explosion these 238 u data bolster the case that the ediacaran cambrian transition experienced massive swings in marine redox state providing a dynamic environmental backdrop for and potentially even a key driver of the emergence and radiation of metazoans

st john s island red sea a new geophysical model and its implications for the emplacement of ultramafic rocks in fracture zones and at continental margins abstract new gravity and magnetic data from the northern red sea reveal the extent of the large gravity anomaly 164 mgal and the presence of significant magnetic anomalies over st john s island spectral transformation and three dimensional potential field modelling delineate the surface configuration and vertical extent of the causative body and the enormous density contrast required 1 2 g cm 3 suggests that it is composed of unserpentinised peridotite density 3 4 g cm 3 to a depth of at least 8 km st john s island is uniquely located not only at a passive continental margin but also within a fracture zone at the transition from plate separation by seafloor spreading to extension by lithospheric attenuation this precludes several suggested mechanisms for the emplacement of ultramafic bodies in fracture zones thermal contraction serpentinite diapirism and changes in the poles of rotation do not seem possible mechanisms in this tectonic environment and the emplacement is most probably related to the spreading readjustment necessary to create a continent to continent fracture zone a post mesozoic age of emplacement associated with the onset of continental rifting and the rejuvenation of a pre existing continental fracture seems most probable

the origin of decoupled carbonate and organic carbon isotope signatures in the early cambrian ca 542 520 ma yangtze platform abstract the early cambrian ca 542 520 ma strata in south china record two prominent negative carbonate carbon isotope 13ccarb excursions of early nemakit daldynian n d and early tommotian ages across each of these excursions carbonate and organic carbon isotopes 13ccarb and 13corg are strongly decoupled regional correlation across a shelf to basin transect shows lateral heterogeneity of 13corg during the early middle n d but more homogenized 13corg values across the basin during the late n d and tommotian the temporal and lateral variations in 13corg suggest that decoupled 13ccarb and 13corg across the n d 13ccarb excursion were possibly caused by diagenetic alteration of organic matter and or amplification of detrital organic carbon isotope signature in low toc carbonates in contrast decoupled 13ccarb and 13corg of the upper n d and tommotian were likely resulted from chemoautotrophic methanotrophic biomass contribution to toc in organic rich black shale and carbonates the decoupled 13ccarb 13corg pattern from the lower n d strata ca 542 ma shows striking similarities with those from the basal ca 635 ma and upper ca 551 ma doushantuo formation in all three cases decoupled 13ccarb 13corg are seen in organic poor carbonates toc 0 1 and coupled 13ccarb 13corg occur in organic rich black shale and carbonates at the end of the negative 13ccarb excursion these similarities suggest that the shift from decoupled to coupled 13ccarb 13corg has no causal link with the terminal oxidation of a large oceanic doc reservoir given the pervasive anoxia euxinia in ediacaran early cambrian oceans local doc rich environments may have been common but a large oceanic doc reservoir capable of buffering the 13c of marine organic matter requires independent evidence

trace element geochemistry of iron titanium rich eclogitic rocks gruppo di voltri western liguria new analyses of rare earth and transition elements are summarized for 29 high pressure metamafic rocks which occur as lenses and layers in the beigua serpentinite of the ligurian alps included are data for 2 metarodingites 8 eclogites 5 glaucophanic eclogites 7 barroisitic amphibolites and 7 porphyroblastic greenschists trace element data similar to major element compositions previously presented show broadly intergradational chemistries among the several lithologies rare earth distributions are relatively flat at 10 30xchondrite with lree relative depletions transition elements exhibit a marked decrease in abundance with increasing atomic number but patterns are characterized by positive anomalies for ti mn fe and ni the bulk rock trace element concentrations are analogous to those of oceanic tholeiites similar ree patterns and overlapping ranges and sensibly identical transition element patterns demonstrate that a continuous spectrum of compositions is present in the analyzed rocks

dwindling vanadium in seawater during the early cambrian south china abstract elemental vanadium v an essentially redox sensitive metal in seawater has had a significant impact on the understanding of the evolution of the atmosphere ocean system throughout the history of the earth in fact the geochemical cycle of v in early cambrian seawater may have had an influence on the chengjiang biota in south china however it has not yet been well established given the authigenic vanadium accumulation is sensitive to the redox conditions of seawater here to constrain the geochemical cycle of v in seawater during the early cambrian the mo u and total organic carbon toc distributions with high resolution samples from both the outer shelf and slope facies e g the duoding and longbizui sections are applied to evaluate the redox conditions of ambient seawater the mo u relationships indicate that the redox conditions of the mid depth seawater evolved in a systematic way in south china transitioning from an fe mn reduction zone to anoxic intermittently euxinic states and then to oxic conditions during the early cambrian as a consequence the authigenic v enrichment constrained by the marine redox conditions was mainly controlled by the fe mn particulate shuttle and the reduction and adsorption of organic matter in anoxic euxinic conditions however the decoupling among v mo u and toc under anoxic euxinic conditions suggests a dwindling vanadium concentration in the early cambrian seawater of south china the scavenging efficiency of v from seawater is much higher than those of mo and u under anoxic euxinic conditions ultimately these trace elements e g mo u and especially v in seawater could effectively be regulated and adjusted to a reasonable level under the widespread anoxic euxinic conditions the drawdown of trace elements in seawater might provide an early stage preparation of the marine environment for the subsequent chengjiang biota

climate ocean control on the depositional watermass conditions and organic matter enrichment in lower cambrian black shale in the upper yangtze platform abstract the lower cambrian sediments including the qiongzhusi and maidiping formations in the sichuan basin are generally considered as the main source rocks of precambrian cambrian petroleum system a comprehensive study integrated with multiple geochemical methods was conducted on 119 lower cambrian drillcore samples collected from the upper yangtze platform the results show that the fluctuation of sea level and climate during the deposition of the qiongzhusi and maidiping formations act as the critical factors controlling the watermass conditions and thus om accumulation during the deposition of upper maidiping formation 1m the paleoclimate was arid and sea level was relatively low in the study area the depositional environment indexes uef moef gammacerane index carotane and vef suggested that the upper 1m was deposited in a strong restricted marine setting with high water salinity and anoxic water conditions in ziyang weiyuan area however during the early qiongzhusi formation period the paleoclimate became humid and sea level rose in the study area high paleoproductiviy caused by high concentrations of nutrients brought by seawater and freshwater plays an important role in om accumulation in the basal part of the qiongzhusi formation 1q furthermore the model of paleoproductivity promotion caused by the coordinated control of climate and extensive transgression in the lower part of 1q may also be applicable to interpret the origin of high organic carbon abundance in the strata deposited in the same period with the 1q such as shiyantou shuijingtuo and niutitang formations in other parts of the upper yangtze platform

organic geochemical characteristics of highly mature late neoproterozoic black shales from south china reappraisal of syngeneity and indigeneity of hydrocarbon biomarkers abstract during the late neoproterozoic to early cambrian period the transition from mainly microbial ecosystems to eukaryotic marine primary productivity was one of the most profound ecological revolutions in earth s history abundant biomarkers have been reported from high maturity late neoproterozoic sedimentary rocks in south china including the cryogenian datangpo and ediacaran doushantuo formations these biomarkers were suggested to be organic molecular evidence of the survival of photosynthetic eukaryotes in palaeo oceanic environments during the snowball earth era to advance the understanding of late neoproterozoic ecosystems and to re evaluate the provenance and validity of the biomarkers fresh drill core was collected from black shales of the datangpo and doushantuo formations in south china in order to ensure the indigeneity of the solvent extracted hydrocarbons their composition was determined by conducting experiments under strict laboratory procedures including using slice experiments on a precision saw that removed external surfaces the distribution of the recovered aliphatic and aromatic hydrocarbons indicates very a high thermal maturity for the analysed black shales which is consistent with the equivalent vitrinite reflectance 2 5 and the lack of specific biomarkers both prokaryotic bacterial and eukaryotic algal organisms are likely producers during the neoproterozoic however no long chain steranes c26 steranes were detected in the interior of any of the samples and only trace amounts of hopanes and pregnanes in just one sample from the doushantuo formation xs 199 dst on the contrary solvent extraction of millimetre thick slices cut from the external surfaces of the black shales reveals strikingly high level of contamination from organic residues on the sample surfaces these contaminants include n alkanes c10 c33 isoprenoids monomethylalkanes aromatic hydrocarbons naphthalene phenanthrene and their alkyl isomers and abundant biomarkers such as tricyclic terpanes hopanes steranes and diasteranes the concentrations of these biomarkers in the exterior sub samples exceed blank concentrations by more than three orders of magnitude due to surficial contamination these results indicate that previously studied late neoproterozoic samples from south china likely contain mixtures of abundant biomarker contaminants and some indigenous over mature hydrocarbons therefore special care need to be taken with regard the existing biomarker evidence from south china that has been used to illustrate the contribution of eukaryotic photosynthesis to marine primary productivity during the snowball earth period no late neoproterozoic rocks from south china within the appropriate thermal maturity window for survival of indigenous biomarkers are currently known but these may be found during future exploration

development and evolution of a euxinic wedge on the ferruginous outer shelf of the early cambrian yangtze sea abstract the early cambrian was a critical interval in geological history and featured profound oceanic and biotic changes to unravel the oceanic redox conditions high resolution analyses of iron speciation and redox sensitive trace elements mo and u within the framework of sequence stratigraphy were carried out on the niutitang formation 528 521 ma the two examined sections the daotuo and bahuang sections were respectively located in a mid upper slope setting and a lower slope to basin settings behind a seaward submerged sill on the middle yangtze block south china at daotuo the fepy fehr values display moderate positive covariances with the total organic carbon contents toc and the mo toc ratios notably in the basal part of the niutitang formation therefore at this locality the euxinic water mass wedge was developed in association with a high primary organic productivity burial rate likely within a high productivity zone in contrast at bahuang the fepy fehr values show weak to negative covariances with the mo toc ratios and toc contents in the basal part indicating that the biogeochemical cycles of fe mo with c were decoupled in a lower productivity ferruginous deeper basinal setting these spatial changes in marine redox structures and biogeochemical cycles can be reasonably explained by the existence of oceanic upwelling in the presence of strong offshore currents and a seaward submarine sill reconciling the oxygen minimum zone omz in modern oceanic margins to some extent this spatial redox pattern also accounts well for previous data based on which the euxinic state intermittently invaded upward onto the shelf margin and evolved into a ferruginous dominant anoxia in the inner shelf subbasins in addition the euxinic wedge dynamically fluctuated upslope and downslope along the transect from the ferruginous outer shelf slope to the basin in response to rise and fall of sea level consequently the temporal evolution of the redox conditions was driven in part by eustatic variations moreover the paleogeographic position of the yangtze block in the mid lower latitudes suggests that the block was associated with relatively strong offshore currents induced by trade winds which notably enhanced the oceanic upwelling near the transgression maximum on the other hand the dominance of deep ferruginous waters with the local occurrence of a euxinic wedge on the outer shelf slope indicates relatively low concentrations of seawater sulfate and atmospheric oxygen during the deposition of the niutitang black shales this redox model thus highlights the important roles of paleogeographic paleoclimatic and eustatic sea level changes in controlling the spatiotemporal fluctuations in marine redox conditions and biogeochemical cycling in the early cambrian ocean

high primary productivity during the ediacaran period revealed by the covariation of paired c isotopic records from south china abstract remarkable carbonate carbon isotope 13ccarb excursions accompanied by invariant organic carbon isotope 13corg values have been observed in many late neoproterozoic successions this decoupled relationship between 13ccarb and 13corg in the precambrian differs from most phanerozoic records which have been substantially imprinted by aqueous photosynthesis and it has been tentatively interpreted as resulting from exotic organic matter inputs or diagenesis in this study we present high resolution 13corg records in combination with previously published 13ccarb data from a continuous drill core from the yangtze platform of south china the studied interval spans the cryogenian nantuo formation the ediacaran doushantuo and dengying formations and the base of the cambrian yanjiahe formation the 13ccarb profile of this drill core can be reliably correlated with contemporaneous sequences and it reveals multiple negative 13ccarb excursions associated with phosphorite deposits intriguingly the 13corg profile exhibits a co varying trend with the 13ccarb profile and the coupled relationship between 13ccarb and 13corg is rare in geological records of the precambrian the 13corg values of the drill core vary from 22 13 to 34 12 and the overall isotopic difference 13ccarb org between 13ccarb and 13corg ranges between 19 61 and 34 58 most of the 13corg and 13ccarb org data can be interpreted in terms of oxygenic photosynthesis probably mainly by eukaryotic algae however it is possible that exotic organic matter most likely doc from the deep ocean may also have contributed to the sedimentary organic matter composition of specific intervals studied herein moreover high primary productivity in shallow water may have been sustained for a long period in phosphate rich conditions and the organic matter generated by photosynthesis may have been quantitatively sufficient to overwhelm the doc buffering effect in the total 13corg signal throughout the ediacaran period the enhanced burial of organic matter in sediments would gradually result in the accumulation of oxygen both in the ocean and atmosphere which would likely have paved the way for macroscopic animal evolution

precambrian cambrian boundary global stratotype ratified and a new perspective of cambrian time the precambrian cambrian boundary global stratotype is the base of the phycodes pedum zone at fortune head southeastern newfoundland the initial two stage radiation of the phanerozoic fauna the appearance of phanerozoic aspect trace producers and subsequent origin of diverse skeletalized metazoans was earliest cambrian fossil assemblages with deep complex burrows or originally mineralized skeletal fossils with exception of cloudina are early cambrian or younger correlation into avalon indicates that strata once referred to the proterozoic e g manykaian or nemakit daldynian and rovno stages are lowest cambrian and that the siberian tommotian stage is middle lower cambrian the thick sequence and complex epeirogenic history of the placentian series as compared with the trilobite bearing branchian series suggest that most early cambrian time is represented by the subtrilobitic lower cambrian

tectono depositional pattern and evolution of the middle yangtze platform south china during the late ediacaran abstract the depositional environments and evolution of the yangtze block during the ediacaran cambrian transition remain uncertain preventing a thorough understanding of the coeval ocean stratification redox heterogeneity and organism distribution in this study detailed facies and sequence stratigraphic analyses were carried out on the dengying formation and its correlatives to better constrain the tectono depositional pattern and evolution of the middle yangtze area in the late ediacaran twenty five lithofacies were identified in nineteen sections and grouped into six depositional facies intrashelf basin intrashelf shoal restricted platform interior platform margin upper middle slope and lower siliceous slope basin the spatiotemporal distributions of these depositional facies reveal an isolated rimmed middle yangtze carbonate platform embayed with a westward opening intrashelf basin this intrashelf basin gradually became wider and deeper westwards linking to a nne trending basinal seaway that separated the middle yangtze platform from its upper yangtze counterpart the enclosure of the marginal shoal reef complex with an intrashelf shoal chain formed a large scale restricted lagoon and associated tidal flat in the platform interior based on key sequence boundaries and cycle stacking patterns two and a half transgressive regressive sequences were distinguished in the dengying formation and its correlatives in the twelve representative sections furthermore these sequences can be correlated well with those identified in other upper ediacaran successions elsewhere in upper yangtze namibia india and oman indicating a eustatic origin for these sequences however the complex platform basin configuration with significant facies and thickness variations reflects a transtensional tectonic regime in the middle yangtze area during the late ediacaran the abrupt changes in lithofacies and thickness across the southern principal platform margin and those from sandy shoals banks to storm influenced shelf along the northern portion of the intrashelf basin in the three gorges western hubei suggest synthetic extensional faulting across the margins during deposition the two synthetic extensional fault zones might have been induced by nne trending transtensional faulting along the eastern margin of the upper yangtze platform this structural pattern fundamentally controlled the depositional differentiation of the middle yangtze platform during the late ediacaran and also laid the foundation for the subsequent facies distribution and depositional evolution during the early cambrian terreneuvian epoch

structure age and tectonic development of the huoshishan niujuanzi ophiolitic melange beishan southernmost altaids abstract the huoshishan niujuanzi ophiolitic melange hno is located near the central part of the beishan orogenic belt in the southernmost altaids the hno consists of ultramafic rocks cumulate gabbros gabbros plagiogranites diorites diabases basalts andesites rhyolitic volcaniclastic rocks and siliceous sedimentary rocks many of which are in a schist matrix gongpoquan group geochemical data of the mafic rocks indicate a calc alkaline or a mixture of calc alkaline and tholeiitic rocks with negative nb ta and positive pb ba and la anomalies suggesting formation in an island arc or supra subduction zone setting a gabbro from a block in the melange in the niujuanzi area has a zircon age of 435 0 1 9 ma and a plagiogranite with an age of 444 3 1 9 ma and another gabbro from the huoshishan area has an age of 410 5 3 7 ma the schist matrix has a zircon age of 512 5 3 ma and contains silurian devonian and carboniferous fossils thus the melange formed in the late carboniferous or later our structural analysis of fault planes in the hno the crenulation cleavages s 2 of the schist and fold axial planes of early permian sandy limestone quartz veins and late permian sandstones indicates that the melange underwent a north to south compression and the orientation of stretching lineations slickensides and fold hinge lines implies that the hno experienced top to the north or northwest movement the entire planar and linear structural data set suggests that the subduction polarity was probably to the south in the late paleozoic the emplacement age of the hno was probably near the end permian based on the age of the youngest rocks in the ophiolitic melange and by the presence of a late permian unconformity from our work integrated with published regional data we outline a comprehensive geodynamic model for the central boc

carbonate carbon isotope chemostratigraphy and u pb zircon geochronology of the liuchapo formation in south china constraints on the ediacaran cambrian boundary in deep water sequences abstract the ediacaran cambrian transition was one of the most significant intervals in geological history witnessing the origin of the metazoans and major environmental changes establishing a precise chronostratigraphic framework for the ediacaran cambrian transition has great significance for understanding the geological and evolutionary processes during this transition an integrated study on carbonate carbon isotopes and u pb zircon dating was conducted within the liuchapo formation in the sifangjing section in northeastern guizhou province south china to recognize the ediacaran cambrian boundary in slope facies of the yangtze block a substantial negative carbonate carbon isotope excursion was recognized within the lower part of the liuchapo formation moreover a weighted mean u pb zircon age of 540 7 3 8 6 6 ma was obtained from a tuffaceous interlayer approximately 2 m above the interval of the negative carbon isotope excursion the carbon isotope and age dating results indicate that the ediacaran cambrian boundary within the sifangjing section should be placed in the lower part of the liuchapo formation additionally in the sifangjing section a distribution of horodyskia minor extends up to the basal cambrian while the last appearance datum lad of palaeopascichnus jiumenensis correlates to the large negative carbon isotope excursion within the liuchapo formation this relationship indicates that the lad of the palaeopascichnus jiumenensis is a potential tool for the stratigraphic correlation of the ediacaran cambrian boundary successions in slope and deep water basin settings of the yangtze block

a horodyskia nenoxites dominated fossil assemblage from the ediacaran cambrian transition liuchapo formation hunan province its paleontological implications and stratigraphic potential abstract because of the scarcity of fossil data life evolution in deep water settings through the ediacaran cambrian transitional interval is still poorly understood this study describes a horodyskia nenoxites dominated fossil assemblage from the chert of the liuchapo formation in liujiata village western hunan province which was deposited in a slope to basinal setting at the terminal ediacaran and earliest cambrian in this assemblage horodyskia covers the longest range possibly from the upper part of the lower ediacaran to the base of the cambrian other members including three types of nenoxites helanoichnus shaanxilithes and two affinity undetermined forms appear in the upper part of the liuchapo formation the whole assemblage exhibits an upward increasing complexity and fossil abundance which reaches its acme at a few meters below the cambrian xiaoyanxi formation regardless of the lithology of the host rock these fossils are exclusively preserved with clay minerals and devoid of any detectable concentration of carbon such a preservation mode is dominant among the known records of these fossils worldwide and possibly related to an unknown biological commonality of these organisms microscope observations support a body fossil origin of nenoxites and contradict the trace fossil interpretation meanwhile some large horodyskia show features implying a close relation to testate amoebae however despite some resemblances the differences between these fossils and any suggested relatives are considerable taking all the information together we tentatively interpret this fossil assemblage as a diverse and complex deep water community of protist grade organisms the worldwide occurrence of some members in this assemblage endows them with a biostratigraphic potential nevertheless to establish such a tool more quality data of fossil occurrences and referential radiometric ages and a better understanding of the interplay between these organisms and sedimentary environments are required

oceanic redox condition during the late ediacaran 551 541 ma south china abstract the late ediacaran 551 541 ma oceanic environment could represent a significant preparation for the enrichment of redox sensitive elements during the early cambrian and even for the evolution of life however the marine environment during the late ediacaran has not been thoroughly characterized to date to better understand late ediacaran oceanic redox conditions we investigated the fe and s isotopic compositions of cherts from the slope yinjiang and basin silikou sections of the nanhua basin in south china the slope yinjiang section is characterized by a wide range of 56fet values 0 12 to 1 06 and by continuously decreasing 34spy values from 40 4 to 4 1 the variable 56fet values in the yinjiang section primarily reflect the various oxidation degrees of dissolved fe2 in ferruginous seawater the homogeneous 56fet signatures 0 02 0 40 along the silikou section and the decreasing trend of 56fehr values from 0 86 to near zero primarily depend on the mixing of fe3 hydroxide oxide precipitates in a ferruginous water column possibly followed by partial reduction the authigenic framboidal pyrite trapped within an intermittent euxinic condition and detrital silicate fe minerals a 20 30 discrepancy in 34spy values between the yinjiang and silikou sections may be ascribed to the formation of diagenetic pyrites and framboidal pyrites respectively therefore the continuously decreasing 34spy values in the yinjiang and silikou sections may reflect a significant increase of sulfate diffusion into sediments at the slope location and the enlarged marine sulfate reservoir at the basin location these new results indicate that the deep water in the nanhua basin is primarily characterized by a ferruginous condition with intermittent euxinic conditions in the basin location during the late ediacaran

an early miocene age for a high temperature event in gneisses from zabargad island red sea egypt mantle diapirism in the last decade much attention has been focused on the tectonothermal and geochemical evolution of rocks outcropping on zabargad island red sea egypt this island though of limited size 4 km has an almost unique geodynamic setting 50 kmwest of the red sea axis and reveals a close association of mafic ultramafic rocks with a metamorphic gneiss complex understanding the geodynamic evolution of the island was expected to elucidate the processes operating in a young rift setting environment and crust mantle interface tectonics since the first work of bonatti et al 1981 however genetic models have been limited by the the scant data on the timing of juxtaposition between the peridotites and the gneisses as well as the nature of the mafic ultramafic rocks one interpretation considers the peridotites as an asthenospheric mantle diapir intruding a pan african continental crust during the early stages of the red sea opening e g nicolas et al 1987 the fewattempts todate the time of intrusion of the peridotites are consistent with this view nicolas et al 1985 for example inferred a k ar age of 23 7 myr for amphiboles extracted fromanamphibolite this value concurs with the more precise ar ar age 18 7 1 3 myr given by hornblende extracted from a pegmatoidal pocket of a coarse grained hornblendite in harzburgite trieloff et al 1997 both ages are identical within errors of a 18 4 1 0myr zircon age obtained by oberli et al 1987 although there is a good agreement between the three methods oberli s zircon value has been presented only in an abstract thus making it difficult to properly evaluate k ar and ar ar ages have also been questioned for decoupling of parent and daughter nuclides villa 1990 or for their meaning in terms of a geological event uplift hydrothermalism or emplacement age due to their known susceptibility to low grade events an alternative interpretation considers the peridotites and the gneisses to be panafrican brueckner et al 1988 1995 and that juxtaposition of both rock types occurred shortly after differentiation from a common depleted mantle source 700 ma this is apparently supported by the so called slap errorchron resulting from alignment of whole rock peridotite samples and cpx separates in the sm nd isochron diagram brueckner et al 1988 this model however has been challenged in two recent papers on the grounds of fluid inclusion boullier et al 1997 and noble gases and argon chronological studies trieloff et al 1997 in both models gneisses are seen to represent remnants of the pan african continental deep crust left behind during opening of the red sea rift pan african ages for the metamorphic gneiss complex are well established by sm nd and rb srmineral isochrons lancelot and bosch 1991 brueckner et al 1995 as well as by a zircon pb pb evaporation date brueckner et al 1995 from a felsic gneiss located a few hundred metres from the contact zone between the central peridotite body and the gneisses these ages however provide no constraints on the timing of juxtaposition of gneisses and peridotites in order to solve this fundamental problem and because of the known resistance to alteration of the mineral zircon we focused on the u pb dating of single zircon grains extracted from a felsic gneiss located at the contact zone with the central peridotite body

evaluating episodic hydrothermal activity in south china during the early cambrian implications for biotic evolution abstract the early cambrian 541 514 ma was a crucial interval for the evolution of life on earth popularly known as the cambrian explosion here we report the timing of changes in hydrothermal and depositional inputs as well as paleo redox state which may have influenced biogeochemical changes according to high resolution petrology fossil distributions isotopic records and inorganic geochemistry the lower cambrian of the south china can be subdivided into four intervals the lowermost cambrian zhujiaqing formation cam i the cambrian stage 2 shiyantou formation cam ii the lower part of the cambrian stage 3 yu anshan formation cam iii and the mid upper cambrian stage 3 and the middle upper part of the yu anshan formation continuing into the canglangpu formation cam iv hydrothermal events are detected during the early cam i cam ii and cam iii intervals during the early cam i and cam ii intervals these events coincided with extensive bottom water euxinia which in turn may have restricted the spread or proliferation of ediacaran fauna and small shelly fauna through the whole cam iii interval further hydrothermal events occurred concurrently with euxinic and ferruginous conditions probably within a single spatially stratified water column again plausibly restricting the spread of aerobic organisms in conjunction with the cessation of hydrothermal events and the gradual lowering of sea level during the late cam iii oxic water environments gradually spread into relatively deep water regions concurrent with the emergence of the chengjiang and qingjiang faunas these data suggest that periodic hydrothermal events may have had a significant impact on the spread radiation and extinction of macroscopic fauna during the early cambrian in south china

marine redox stratification on the earliest cambrian ca 542 529 ma yangtze platform abstract in order to explore the marine redox structure on the earliest cambrian ca 542 529 ma yangtze platform south china new fe mo c s geochemical data from the slope xugongping section for the liuchapo and the base of niutitang formation are reported here integrated fe speciation and mo abundance data from the xugongping section in combination with previously published data from slope basinal longbizui section and others suggest oxic surface waters and ferruginous deep waters coexisting with occasional mid depth euxinic waters during the fortunian stage and with widespread euxinic waters during the early stage 2 marine transgression a compilation of new and published sulfur isotope data of pyrite for the liuchapo and the base of niutitang formations presents high isotopic values mostly 10 and distinctly different spatiotemporal gradient characteristics during the fortunian stage and the beginning of the stage 2 we propose that redox stratification developed in the earliest cambrian protected basin and eustatic change and the connectivity of the protected basin with the open ocean controlled the materials supply from riverine input and open ocean reservoir and thus spatiotemporal variability of ocean chemistry

the magmatic hydrothermal transition in the lower oceanic crust clues from the ligurian ophiolites italy abstract the gabbroic bodies from the jurassic ligurian ophiolites are structurally and compositionally similar to the gabbroic sequences from the oceanic core complexes of the mid atlantic ridge initial cooling of the ligurian gabbros is associated with local development of hornblende bearing felsic dykes and hornblende vein networks the hornblende veining is correlated with the widespread development of hornblende as coronas pseudomorphs after the clinopyroxene in the host gabbros in addition the studied gabbroic body includes a mantle sliver locally containing hornblende gabbros and hornblendite veins the hornblendes from the felsic dykes and the hornblende rich rocks within the mantle sliver show a similar geochemical signature characterized by low mg cao and al2o3 negligible cl and high tio2 k2o ree y zr and nb concentrations the whole rock sm nd isotopic compositions of the felsic dykes and the hornblende rich rocks define a sm nd isochron corresponding to an age of 154 20 ma and an initial end of 9 2 0 5 the 18o of the hornblendes and coexisting zircons from these rocks about 4 5 and 5 8 respectively do not indicate the presence of a seawater component in these melts the formation of the felsic dykes and of the hornblende rich rocks within the mantle sliver involved sio2 rich silicate melts with negligible seawater component which presumably were derived from high degree fractional crystallization of mor type basalts the vein and the coronitic pseudomorphic hornblendes show high mg and cao significant cl 0 02 0 17 wt and low tio2 and k2o concentrations the coronitic pseudomorphic hornblendes have trace element compositions similar to those of the clinopyroxenes from the gabbros and 18o values 1 0 to 0 7 close to seawater suggesting an origin by reaction between migrating seawater derived fluids and the host gabbros the vein hornblendes commonly show slight lree enrichment relatively high concentrations of nb up to 2 5 ppm and 18o ranging from 3 7 to 0 8 the crystallization of these hornblendes most likely involved both seawater and magmatic components

cr isotopic composition of the laobao cherts during the ediacaran cambrian transition in south china abstract in this study we present the cr isotopic compositions from the cherts in the laobao formation 551 522 ma that were deposited in deep water in south china the cr concentrations and isotopic compositions in the laobao cherts were dominated by the contemporaneous seawater and detrital phases the lower part of the laobao cherts show that the 53cr of contemporaneous deep seawater in the huanan basin was approximately 0 1 and the 53cr of detrital materials was approximately 0 2 these findings imply that surface 53cr enriched cr vi species did not affect the deep ocean until the late ediacaran because of the strong redox stratification in the ocean although the oxidative cr cycle in terrestrial settings had emerged before that time a slight rise in 53cr in the deep ocean to approximately 0 08 was estimated for the upper part of the laobao cherts if the 53cr rise and its amplitude can be further determined this will imply that shallow 53cr enriched cr vi species had an impact on the deep ocean and will suggest an oxidation of the ocean in the huanan basin around the ediacaran cambrian boundary 542 ma

the rift to drift transition in the southern north atlantic a stuttering start of the morb machine comment and reply reply beard s 2008 1 welcome comment offers us the opportunity to discuss and clarify certain aspects of our paper jagoutz et al 2007 2 beard raises points about our reinterpretation of his originally postulated genetic link between albitite clasts recovered from a breccia and a coarse

paleo sedimentary environment in relation to enrichment of organic matter of early cambrian black rocks of niutitang formation from xiangxi area china abstract early cambrian organic rich black rocks are extensively distributed in the southern china in this study we have discussed the relation of paleo sedimentary environments with enrichment of organic matter based on toc data we divided the niutitang formation into three parts upper middle and lower the amount of total organic carbon in the lower 1 55 wt and the upper part 1 35 wt of niutitang formation is significantly smaller than the middle part 8 0 wt which suggest initial transgression and regression stages of the sea level respectively the middle part from both sections longbizui and sancha contains higher toc content which is a clue of intense transgression stage of sea level with high bio productivity kerogen type index kti using different macerals specify the organic matter in these black rock series is mainly type i with a minor amount of type ii1 moreover these black rocks were deposited in weak to moderately restricted hydrographic conditions the heterogenic mineral composition was also observed in these black rocks including quartz clay gypsum pyrite barite dolomite and plagioclase quartz content in the middle part is relatively higher than the upper and the lower part displays a strong positive relation with toc which suggests the main source of quartz during deposition of the middle part was biogenic quartz during early cambrian period due to extension between the yangtze and the cathaysian plate the hydrothermal fluids rich in uranium vanadium molybdenum zinc barium etc from the deeper part of the earth crust and entered the ocean basin through extensional remnant fissures and cracks and due to upwelling phenomena entered to the shelf area these nutrient rich deep crustal fluids at the surface of the sea enhanced the evolution and breeding of planktons and other marine life meanwhile it upsurges the bio productivity additionally it creates bottom water hypoxia which is favourable for the preservation of organic matter in the sedimentary rocks the main factors responsible for the enrichment of organic matter in these early cambrian black rock series of niutitang formation were a hydrographic restriction hydrothermal activities and paleo redox conditions respectively

evidence for marine redox control on spatial colonization of early animals during cambrian age 3 c 521 514 ma in south china the early cambrian period was a key interval in earth history with regard to changes in both ocean chemistry and animal evolution although increasing ocean ventilation has been widely assumed to have played a key role in the rapid appearance diversification and spatial colonization of early animals this relationship is in fact not firmly established here we report a high resolution fe c s al ti geochemical study of the lower cambrian wangjiaping section from an outer shelf setting of the yangtze sea of south china iron speciation data document a redox transition from dominantly euxinic to ferruginous conditions during cambrian age 3 c 521 514 ma interpretation of coexisting pyrite sulphur isotope 34 s py records from wangjiaping reveals relatively high marine sulphate availability at wangjiaping furthermore wangjiaping section shows lower 34 s py 2 1 5 3 and lower toc 2 4 1 1 values but higher positive correlation r 2 0 66 p 0 01 between toc and fe py fe hr relative to deeper sections reported previously suggesting that euxinia developed at wangjiaping in response to increasing marine productivity and organic matter sinking fluxes our reconstructed redox conditions and fossils at wangjiaping in comparison with previously well studied strata in the inner shelf xiaotan and shatan sections suggest that planktonic and benthic planktonic trilobites with bioturbation appeared in the oxic water columns whereas only planktonic trilobites without bioturbation occurred within the anoxic even euxinic water columns during cambrian age 3 this finding indicates that spatial heterogeneity of redox conditions in the shelves had an important effect on early animal distribution in the yangtze block

cavity filling dolomite speleothems and submarine cements in the ediacaran dengying microbialites south china responses to high frequency sea level fluctuations in an aragonite dolomite sea speleothems mostly composed of calcium carbonate are widely present in modern karst originated caves but have rarely been reported in palaeokarst systems this study presents a novel type of dolomite speleothem and subsequent submarine dolomite cement which are widely present in the upper ediacaran dengying formation in the upper yangtze area these precipitated materials occur in the cavity system that cuts across several peritidal cycles the interconnected cavity networks with irregular shapes embayed walls internal breccias on cavity floor and their preferential development in the shallower cycle tops for example tepee deformed beds suggest that they were initially generated by subaerial dissolution as the earliest infills the hemispherical protrusions icicle like pendants and ground up columns show similar morphological features and occurrence patterns to the cave popcorn stalactites and stalagmites respectively thus these earliest infills are speleothems resulting from associated meteoric precipitation during subaerial exposure the isopachous growth pattern of subsequent more extensive fibrous dolomite cements points to a submarine diagenetic environment in which they were precipitated microscopically the micritic to micro crystalline dolomite acicular dolomite in speleothems and the subsequent fibrous dolomite share similar crystal fabrics to metastable precursors for example mg calcite meanwhile the carbon oxygen isotope compositions of the speleothem and fibrous dolomite although partly altered by burial diagenesis share a large overlap with host rock and coeval marine carbonates all over the yangtze platform for these reasons these speleothems and fibrous cements are considered to have been initially precipitated as metastable carbonate precursors in meteoric and submarine environments respectively and stabilized during submarine mimetic dolomitization the cyclic occurrence of cavity systems filled with speleothems and submarine cements reflects periodic subaerial exposure and marine flooding of broad tidal flat in the upper yangtze area driven by high frequency sea level fluctuations furthermore the neoproterozoic seawater chemistry that favoured early dolomitization of carbonate precursor mineralogies was an advantage for the preservation of fabrics from metastable precursor minerals

tracking the evolution of seawater mo isotopes through the ediacaran cambrian transition abstract the rapid increase in animal diversity and abundance over the ediacaran cambrian boundary has been attributed to among other parameters pronounced earth surface oxygenation at that time however whether atmospheric oxygen elevated to modern levels or whether the extent was smaller and oxygen remained relatively muted until the late paleozoic is currently debated here we present new mo isotope datasets of drill core material for two key black shale units the ediacaran doushantuo formation and the lower cambrian jiumenchong formation located on the yangtze block these data capture a 0 7 isotope fractionation between sediments experiencing quantitative and non quantitative mo removal by identifying this offset we provide integrated estimates for global seawater mo isotope compositions during the deposition of these two formations 1 2 1 4 and 1 8 2 respectively when paired with a mass balance model our results indicate that a large proportion of the early cambrian ocean was bathed under oxic waters we also discuss how recent discoveries have provided insights into the geochemical processes underpinning the oxygenation of the oceans

rare earth elements in oligo miocenic pelitic sediments from lagonegro basin southern apennines italy implications for provenance and source area weathering the lagonegro units are a part of the southern apennines orogenic wedge the age of the lagonegro successions ranges from lower middle triassic to oligo miocene during late cretaceous and oligocene the deposition of calcareous clastic sediments occurred interbedded with shales flysch rosso fm during oligocene and early miocene in the mediterranean area an important variation of the tectonic regime occurred and siliciclastic sediments of the numidian basin unconformably lay on the meso cenozoic units of the lagonegro basin in the lucanian apennine the aquitanian langhian numidian flysch fm overlies the flysch rosso fm the shales of the flysch rosso fm have a peculiar geochemical fingerprint relative to typical shales of post archean age the abundance of ni and cr is significantly higher and the hree chondrite normalized patterns are steep with a gd yb ch 2 a supply of material from the african archean terranes could be the cause the palaeo weathering indices record changes at the source reflecting variations in the tectonic regime the oldest samples are derived from an environment in which steady state weathering conditions prevailed whereas the youngest samples are related to non steady state weathering conditions this difference could record deformational events that affected the mediterranean area during the oligocene and early miocene the sample at the top of the studied log has very high silica content and an abundant coarse grain sized fraction this suggests that this sample belongs to the numidian flysch fm the geochemical proxies of this sample are different from those associated with samples from the flysch rosso fm indicating that the source area of the numidian flysch fm did not include the archean terranes

further shrimp geochronology on the early cambrian of south china zircon 206pb 238u ages are reported for bentonites from two cambrian sections in south china the meishucun and the fandian the analyzed zircons vary widely in age because of the presence of detrital and altered grains spot ages were corrected for calibration drift during analysis and the age of reference zircon sl13 was calibrated as 580 ma against other standards using sims use of mixture modeling was essential to resolve the different age components and to identify the volcanic ages two age groups occur within the meishucun bed 5 bentonite of which the larger at 518 7 2 5 ma is interpreted as a post depositional zircon growth event and the other at 539 4 2 9 ma as the volcanic age bentonites from the meishucun bed 9 and its fandian equivalent have two age groups 526 5 1 1 ma and 540 2 1 0 ma we interpret the younger of these as the tuff magmatic age and the older as detrital small shelly fossils indicate meischucunian bed 5 is an equivalent of the late nemakit daldynian and its 539 4 2 9 ma age both closely times the base of the cambrian and is a maximum for the basal tommotian the 526 5 1 1 ma for bed 9 and its equivalents approximates the close of the tommotian

paleo environmental variation and its control on organic matter enrichment of black shales from shallow shelf to slope regions on the upper yangtze platform during cambrian stage 3 abstract to understand the evolution of oceanic redox conditions and organic matter om accumulation in the shelf and slope regions of the upper yangtze platform during cambrian stage 3 high resolution geochemical data were obtained from two wells respectively representing shelf and slope environments based on redox proxies widespread anoxic euxinic waters with intermittent sulfidic water columns were formed from shallow shelf to deep water slope and basinal regions during stage 2 and early stage 3 then widespread anoxic waters were reduced and oxic waters expanded during the middle stage 3 after which fluctuating suboxic and oxic occasionally anoxic environments became predominant during late stage 3 the presence of hyalophane and positive eu anomalies suggest strong ba rich hydrothermal activities in the shelf and deep basin regions during early and middle stage 3 the ba rich hydrothermal fluids and upwelling events enhanced the primary productivity in surface waters and subsequently promoted organic matter enrichment during stage 2 and early stage 3 the high primary productivity driven by coastal upwelling and hydrothermal activity were the main factors controlling om enrichment during middle stage 3 the local anoxic water condition became responsible for om enrichment when oxic waters began to gradually expand during late stage 3 both primary productivity and redox conditions became the main control factors for om enrichment according to the analysis of primary productivity redox conditions hydrothermal activity and upwelling events an evolution model is proposed to account for the paleo environmental variation and the om enrichment from shallow shelf to slope regions during cambrian stage 3

petrological and geochemical characteristics of the botryoidal dolomite of dengying formation in the yangtze craton south china constraints on terminal ediacaran dolomite seas abstract recent studies suggest that the massive precambrian dolomite successions were likely the products of specific seawater conditions known as dolomite seas however whether these so called dolomite seas existed in the terminal ediacaran ocean has not yet been constrained the widespread botryoidal dolomite with well preserved sedimentary fabrics from the upper ediacaran dengying formation ca 551 to 545 ma south china can be potentially an ideal target to constrain this issue results showed that within the botryoidal dolomite fibrous dolomite cements are the dominant constituent part and either grew upward from microbialite layers or occurred in sheet cracks within the microbialites implying a syn sedimentary diagenesis two different types of fibrous cements are newly redefined fascicular and radial slow dolomite well preserved chemical and cathodoluminescent zonations and length slow optical characters are direct evidence for syn sedimentary primary dolomite precipitation different with the previous alternative interpretations of mimetic dolomitization of aragonite or high mg calcite precursors in contrast microbialite matrix consists of finely crystalline dolomicrite and preserved microbial fabrics reveling an early mimetic dolomitization of aragonitic precursors the dolomite precipitation and mimetic dolomitization likely occurred in coeval seawater or seawater type fluids as indicated by comparable 13c and 87sr 86sr data with global changes in particular the presence of primary dolomite cement precipitates within botryoidal dolomite can reflect that the specific dolomite seas associated with anoxic or even euxinic shallow water conditions inferred by redox sensitive trace elements and rees and increased microbial activity also prevailed in the terminal ediacaran ocean however the occurrence of dolomite seas was not one occasional event but common even if not persistent in the early to terminal neoproterozoic oceans this study helps to improve the understanding of dolomite problem and ubiquitous distribution of precambrian dolomite worldwide

climatic oceanic forcing on the organic accumulation across the shelf during the early cambrian age 2 through 3 in the mid upper yangtze block ne guizhou south china abstract the organic rich sediments were widely deposited over the entire yangtze block during the early cambrian late nemakit daldynian to botomian in the mid upper yangtze region northeastern guizhou south china they comprise in ascending order the niutitang jiumenchong and lower bianmachong formations which are dominated by black shales except the middle one characterized by interbedded shales limestones three third order depositional sequences are identified in the two studied sections located on the upper slope to basin of the open shelf the organic rich sediments were mostly deposited notably during transgressions on the shallower upper slope margin toc up to 25 wt where they are characterized by co increases in c p fe and ba concentrations indicating the highest organic productivity and coupled c p and fe cycling there in contrast in the shelf basin the concomitant organic rich sediments yield lower organic toc

microbial and animal evolution in relation to redox fluctuations in a deep water setting of south china during the ediacaran cambrian transition ca 551 523 ma abstract the ediacaran cambrian e c transition witnessed significant ecosystem perturbations of the oceanic environment e g oxygen concentrations of seawater and biological evolution however in spite of abundant ediacara type megafossils trace fossils small shelly fossils ssfs macroscopic algae and metazoans recovered from shallow water e c transition strata throughout south china the manifestation of geological events in deep water settings remains elusive here we present new paleontological and geochemical evidence from the 38 m thick deep water liuchapo formation ca 551 523 ma of the yangtou section northeastern guizhou province china which consists mainly of chert through the e c transition the liuchapo formation can be divided into two members based on biostratigraphy and redox proxies the lower member 0 32 m is characterized by low mo al 0 15 1 82 ppm and u al 0 47 2 51 ppm values that suggest deposition under suboxic conditions these deposits are dominated by horodyskia minor paleopascichnus jiumenensis spherical cyanobacteria myxococcoides and random acritarchs the upper member 32 38 m records a transition to anoxic non sulfidic conditions as suggested by marked increases in mo al 5 47 ppm and u al 8 810 ppm the upper member contains ssfs sponge spicules filamentous cyanobacteria multicellular algae and sheet like fossils the survival of diverse organisms under strongly reducing conditions reflected in the upper member suggests that marine redox fluctuations may limit the growth of late ediacaran organisms and provide the living space and elevated nutrients for early cambrian biota

fission track evidence for late triassic oceanic crust in the french occidental alps five zircons and one apatite from plutonic rocks of the ophiolitic sequence from the piemontese zone western alps southeast france have been dated by the fission track method the ages obtained indicate that the minimum age of the alpine oceanic crust in this zone is between 192 6 and 212 8 m y late triassic liassic consideration of the available data on fission track retention conditions in zircons and pressure temperature conditions allowed by the high pressure mineralogic assemblages of the rocks leads to the conclusion that in the external part of the piemontese zone the temperature duration conditions of the high pressure alpine metamorphism were inadequate to significantly reset the zircons9 fission track clock suggesting a short lived metamorphic event

new u pb zircon ages of the ediacaran cambrian boundary strata in south china the ediacaran cambrian transition is a critical interval marking drastic biological oceanic and geochemical co evolutions in geological history but it is poorly constrained geochronologically in south china we here present two new sets of sims u pb zircon ages from ediacaran cambrian boundary strata dengying liuchapo and niutitang formations deposited in the slope basin environments of carbonate platforms two weighted mean u pb ages of 542 1 5 0 ma and 542 6 3 7 ma in the basal and mid upper liuchapo formation respectively in slope and basinal settings provide the first direct age set for the ediacaran cambrian boundary in south china another two u pb ages of 524 2 5 1 ma and 522 3 3 7 ma from the base of the overlying niutitang formation indicate that this widespread unit in south china was deposited about 20 ma after the onset of the cambrian

reconciling plate tectonic reconstructions of alpine tethys with the geological geophysical record of spreading and subduction in the alps article a new reconstruction of alpine tethys combines plate kinematic modelling with a wealth of geological data and seismic tomography to shed light on its evolution from sea floor spreading through subduction to collision in the alps unlike previous models which relate the fate of alpine tethys solely to relative motions of africa iberia and europe during opening of the atlantic our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record the motions of these microplates adria iberia alcapia alkapecia and tiszia relative to both africa and europe during late cretaceous to cenozoic time involved the subduction of remnant tethyan basins during the following three stages that are characterized by contrasting plate motions and driving forces 1 131 84 ma intra oceanic subduction of the ligurian part of alpine tethys attached to iberia coincided with eo alpine orogenesis in the alcapia microplate north of africa these events were triggered primarily by foundering of the older 170 131 ma neotethyan subduction slab along the ne margin of the composite african adriatic plate subduction was linked by a sinistral transform system to e w opening of the valais part of alpine tethys 2 84 35 ma subduction of primarily the piemont and valais parts of alpine tethys which were then attached to the european plate beneath the overriding african and later adriatic plates nw translation of adria with respect to africa was accommodated primarily by slow widening of the ionian sea 3 35 ma recent rollback subduction of the ligurian part of alpine tethys coincided with western alpine orogenesis and involved the formation of the gibraltar and calabrian arcs rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable retreating adriatic and african slabs during slow convergence of africa and europe the upper european iberian plate stretched to accommodate this slab retreat in a very mobile fashion while the continental core of the adriatic microplate acted as a rigid indenter within the alpine collisional zone the subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the alps and apennines as well as beneath parts of the pannonian basin the adriatic sea the ligurian sea and the western mediterranean the predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10 30 indicating that parts of slabs may be superposed within the mantle transition zone and or that some of this subducted lithosphere became seismically transparent

new u pb age from the basal niutitang formation in south china implications for diachronous development and condensation of stratigraphic units across the yangtze platform at the ediacaran cambrian transition abstract the black shale dominated niutitang formation covers a large portion of the early cambrian yangtze platform in south china the base of this unit has been traditionally taken as the ediacaran cambrian boundary ca 542 ma recent radiometric ages from the basal niutitang formation and its correlative units however are significantly younger than 542 ma and raise questions about the diachrony of stratigraphic units across the yangtze platform at the ediacaran cambrian transition here we report a new u pb sensitive high resolution ion microprobe shrimp age of 522 7 4 9 ma from a tuffaceous bed of the basal niutitang formation in a deep water section in taoying guizhou province of south china this age in combination with existing ages and biostratigraphic data suggests that the base of the niutitang formation is of tommotian or upper meishucunian age and the ediacaran cambrian boundary is most likely within the liuchapo formation the depositional age of the unusual metal enriched ni mo pge sulfide deposits which are approximately 1 m above the ca 523 ma tuffaceous bed should be early tommotian upper meishucunian rather than early nemakit daldynian lower meishucunian regional correlation confirms the diachronous development and condensation of stratigraphic units across the early cambrian yangtze platform and provides important information for understanding the paleogeography and paleoceanographic events at the ediacaran cambrian transition in south china

geochronological constraints on stratigraphic correlation and oceanic oxygenation in ediacaran cambrian transition in south china the continuous late ediacaran early cambrian deep water successions of south china archive the complete evolution of seawater chemical conditions in the deep ocean during this critical time interval however the geochemical data from these poorly fossiliferous and condensed successions lack high resolution stratigraphic constraints hampering their interpretation for the spatio temporal evolution of the sweater chemistry in this time interval in this study we report a new sims and ca id tims zircon u pb age 545 76 0 66 ma total uncertainty for an ash bed at the lower liuchapo formation in the deep water longbizui section in western hunan province the new age suggests that the lower and the middle upper parts of the liuchapo formation in the deep water facies can be correlated with the lower dengying formation and the upper dengying lower zhujiaqing formations in the shallow water facies respectively this correlation implies that the correlative horizon of the ediacaran cambrian boundary in the deep water facies in south china is likely located near the base of a widespread negative 13corg excursion at the upper liuchapo formation based on our new geochronological framework the compilations of fe speciation mo and u data indicate that the deep ocean was characterized by widespread anoxic ferruginous water with intermittent euxinic water impinged on the middle lower slope in ediacaran cambrian transition and significant oxygenation events occurred in 533 520 ma the compilations do not support any significant oceanic oxygenation events in 551 535 ma

evolution of the ligurian tethys in the western alps sm nd and u pb geochronology and rare earth element geochemistry of the montgenevre ophiolite france abstract we provide geochemical and geochronological data for gabbro diorite and albitite samples from the montgenevre ophiolite in the western alps this well preserved remnant of the piemont ligurian oceanic basin shows evidence of intra oceanic deformation and metamorphism but has suffered minor ductile deformation and metamorphism during the alpine orogeny the gabbros have geochemical features and initial nd isotopic signatures similar to that of mid oceanic ridge gabbros indicating that they were extracted from a depleted mantle source with no evidence of continental contamination end t 8 cumulation played an important role in the genesis of these gabbros and their rare earth element ree patterns are controlled by the major cumulus phases modelling the ree data for the gabbros and diorites provides support for the hypothesis that the dioritic magmas have been derived from small degree 5 partial melting of the surrounding gabbros during shearing at high temperature high t zircons from a leucodioritic vein within sheared gabbros are poorly discordant and cluster the concordia at 156 3 ma while zircons from an albitite lens within the mantle rocks display concordant ages at 148 2 ma in both cases the zircons show no evidence of inheritance and these values are interpreted as crystallisation ages the 160 150 ma age bracket very likely records a late stage in the magmatic history of the montgenevre ophiolite since the emplacement of diorite clearly post dates the layered gabbros these radiometric ages correlate well with the late bathonian to early kimmeridgian stratigraphic age 160 140 ma of the earliest post ophiolitic radiolarian sediments the whole rock sm nd system of the gabbros yielded an isochron age of 198 22 ma end t of 8 8 this value is significantly older than the 165 ma age of western alps ophiolitic gabbros published elsewhere but is similar within error to the 180 ma age of eclogitised gabbros from the ligurides and corsica this early jurassic age is interpreted as the emplacement age of the gabbroic melts into the mantle lherzolites and could be associated with the early stage of rifting between the european and austro alpine continental margins these ages and their interpretation are consistent with the model of asymmetric mantle denudation by an oblique detachment fault proposed by lemoine et al 1987 this model implies that the newly formed lherzolite gabbro oceanic domain probably remained close to the spreading centre and therefore experienced slow cooling and low spreading rates this appears to be correlated by our radiometric results that suggest a life span of at least 30 ma for the formation of the oceanic crust in this part of the piemont ligurian ocean

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

controls on organic matter accumulation on the early cambrian western yangtze platform south china abstract the origin of widespread organic rich shales on the early cambrian yangtze platform of nanhua basin south china has been attributed to high primary productivity to test this hypothesis we generated multiple paleoproductivity ocar and par paleoredox corg p ratios upwelling restriction coef mnef co ppm mn mo toc and cd mo ratios and terrigenous flux proxies al and unreactive fe feu for the lower cambrian niutitang formation in the outer shelf jinsha section which records a typical transition from the organic rich black shales to overlying organic lean mudstones or siltstones these new data were integrated with published paleoredox proxies iron speciation and redox sensitive trace metals for the same section our results suggest that the organic rich toc 4 6 1 8 lower member lm 0 37 m of the niutitang formation records ferruginous conditions high productivity levels and low siliciclastic fluxes and that the organic poor toc 0 1 0 1 upper member um 37 329 m was characterized by dominantly oxic conditions declining productivity levels and high siliciclastic fluxes our results suggest a redox shift from ferruginous to oxic conditions and enhanced terrigenous fluxes in response to falling sea level from the lm to the um secular variations in primary productivity and redox conditions are strongly correlated which is consistent with a positive feedback high primary productivity led to increasing water column anoxia which enhanced organic p recycling stimulating further productivity variations in primary productivity may reflect variable upwelling intensity in a semi restricted shelf setting linked to sea level changes taken together these findings suggest that a combination of primary productivity redox conditions and terrigenous fluxes driven by sea level changes played a key role in organic matter accumulation on the early cambrian western yangtze platform

palaeoenvironment and carbon isotope stratigraphy of upper proterozoic carbonates of the yangtze platform the 13c values for phanerozoic marine carbonates display secular trends mainly in the range 1 to 2 1 these have been interpreted as primary features reflecting changes in the carbon isotope composition of dissolved bicarbonate in seawater brought about largely by variations in the proportions of carbon being sequestered in the oxidized and reduced carbon reservoirs1 2 carbon isotope trends are not as well established for the precambrian but there are indications that upper proterozoic sequences contain carbonates with unusually high 13c values3 9 here we present the results of the first carbon isotopic study of the upper proterozoic carbonate strata from the type locality of the sinian system in the yangtze gorges sections near yichang south china and discuss their implications for accumulation of organic matter we also compare the results for these chinese strata with 13c trends in other upper proterozoic cambrian sequences and assess the extent to which such isotopic data can be used to make stratigraphic correlations between widely spaced sequences

petrology and geochemistry of chert on the marginal zone of yangtze platform western hunan south china during the ediacaran cambrian transition the ediacaran cambrian transition was one of the most critical intervals in earth history during this interval widespread chert was precipitated commonly as a stratal wedge in carbonates along the southern margin of the yangtze platform south china the chert wedge passes into a full chert succession further basinward to the south east four lithotypes of chert are identified across the marginal zone in western hunan mounded vein brecciated and bedded chert the mounded chert is characterized by irregular to digitiform internal fabrics generally with abundant original vesicles and or channels that mostly are lined by botryoidal chalcedony cements with minor quartz and barite crystals the host chert or matrix of these mounds is dominated by amorphous cryptocrystalline silica commonly disseminated with pyrite the vein chert with minor quartz locally generally cross cuts the overlying dolostone and chert horizons and terminates under the mounded and or bedded chert bodies the brecciated chert commonly occurs as splayed intrusions or funnel shaped wedges and cross cuts the topmost dolostones the bedded chert the most common type generally is thin to medium bedded and laminated locally it is composed of amorphous silica with minor amounts of black lumps microthermometry of fluid inclusions from vein and void lining minerals mainly quartz and barite revealed homogenization temperatures from 120 to 180 c for the trapped primary fluids compositionally these chert deposits generally are pure with sio2 92 wt and only minor fe2o3 and al2o3 contents most of which show positive europium anomalies in rare earth element patterns especially for the mounded chert all these data suggest that the marginal zone chert deposits resulted from a low temperature silica rich hydrothermal system in which the mounded chert was precipitated around the releasing vents i e as silica chimneys the vein and splayed brecciated chert however was formed along the syndepositional fault fracture conduits that linked downward while the bedded chert was precipitated in the quieter water column from the fallout of hydrothermal plumes onto the sea floor these petrological and geochemical data provide compelling evidence and a new clue to the understanding of the extensive silica precipitation rapid tectonodepositional and oceanic changes during the ediacaran cambrian transition in south china

from middle jurassic heating to neogene cooling the thermochronological evolution of the southern alps abstract the dolomite region is located in the southern alps which were affected by mesozoic extensional tectonics and by consequent thermal perturbations in this work vitrinite reflectance and apatite fission track analysis are used to estimate the thermal evolution these methodologies have been applied to the permo mesozoic succession which crops out along the transalp seismic profile the regional distribution of the organic matter maturity seems to be mainly controlled by different burials reached during the norian liassic extensional phase in connection with high heat flow values the best solutions obtained from thermal modelling of both vitrinite and fission track data suggest that peak of high heat flow occurred during bajocian bathonian ages when western tethys was characterized by intrusions of gabbros and plagiogranites and extrusion of tholeiite basalts this time coincides with the onset of the drifting phase and related thermal subsidence the following thermal relaxation occurred during continuous sedimentation and the maximum burial does not coincide with peak temperatures cooling history has been carefully analysed through apatite fission track data on samples collected close to the valsugana overthrust which document an important exhumation event at about 10 ma the related erosion has been analysed through the combined use of arenite petrography and fission track analysis on detrital samples of the veneto foredeep succession which represents the storage of detritus during tertiary these data confirm that after serravalian the southalpine domain and related covers were affected by subaerial erosion

multiple δ13c excursions spanning the cambrian explosion to the botomian crisis in siberia new high resolution 13 c data through the lower cambrian of siberia reveal multiple positive excursions coincident with phases in the explosion of invertebrate phyla nemakit daldynian to middle botomian stages comparison of the 18 o and trace element mg fe mu sr chemistry indicate that six new 13 c cycles are primary rather than diagenetic features with potential for global correlation positive 13 c excursions up to 3 indicate that fractional organic carbon burial rates were high but variable through the cambrian explosion values for 13 c dropped sharply from around 2 2 to 1 6 in botomian time coincident with mass extinction of the archaeocyathan reef biota the rapid fluctuation of the 13 c signal and the temporal coincidence of the pronounced negative shift with the extinction event hint that the 13 c record may record productivity variations during the early cambrian radiation

redox fluctuations during the ediacaran cambrian transition nanhua basin south china insights from cr isotope and ree y data abstract reconstructing fluctuations in marine and atmospheric redox conditions during the late precambrian early cambrian is essential to better our understanding of the link between oxygen and biological evolution this study presents ree y and cr isotopic data together with toc data and redox sensitive trace metal concentrations of marine black shales and cherts from the cambrian jiumenchong formation and ediacaran liuchapo formation from south china in order to improve our understanding of the redox conditions in the nanhua basin during this critical time period the liuchapo formation records negative ce anomalies and highly fractionated 53cr values of up to 1 06 0 09 suggesting deposition under oxic conditions and implying high oxidative weathering rates during the end of the ediacaran the overlying jiumenchong formation is characterised by a stratigraphic upward decrease in 53cr values to typical igneous values as well as a decrease in negative ce anomalies and an increase in positive eu anomalies suggesting an increase in detrital content due to more aeolian weathering and deposition under anoxic conditions at the beginning of the cambrian the latter is also confirmed by the trends in v cr and th u ratios which further indicate an eventual return to dys oxic seawater conditions during the early cambrian

integrated stratigraphy of the upper neoproterozoic succession in yunnan province of south china re evaluation of global correlation and carbon cycle abstract this study provides a chemostratigraphic and sequence stratigraphic framework of a relatively unfossiliferous sub cambrian marine sedimentary succession in yunnan province of south china our results allow its global stratigraphic correlation and provide an improved understanding of marine carbon cycle during the late neoproterozoic the succession has a basal glacial diamictite nantuo formation and comprises two large scale depositional cycles towards the precambrian cambrian pc c boundary each of which changes from siliciclastic to carbonate sediments with environments ranging from shallow subtidal to evaporitic supratidal in the absence of biostratigraphic control attribution of the tillite has remained equivocal between the 710 ma sturtian and 635 ma marinoan glaciations a compilation of known paleomagnetic and other geochronologic constraints suggests that the nantuo glaciation in south china was at least diachronous in which the nantuo tillite in yunnan belongs to the older glaciation stratigraphic patterns of the diagenetically screened 13c variation and depositional sequences during the carbonate dominated interval show a striking similarity with those of the upper cryogenian and entire terminal proterozoic carbonates at widely separated other localities rather than the terminal proterozoic alone supporting the view that deposition of the nantuo tillite in yunnan reflects the sturtian rather than marinoan glaciation further considerations of the 13c results provide three important implications for the late neoproterozoic marine carbon cycle first comparison of different 13c profiles suggests considerable variability in the absolute values of carbonate on both intra and inter basinal scales which differs profoundly from their relative homogeneity in marine phanerozoic counterparts and may therefore reflect lateral isotopic heterogeneity in the oceanic carbonate reservoir during the late neoproterozoic second an empirical correlation between the 13c and relative sea level changes was applied to evaluate hypothesized explanations for 13c temporal variation in surface sea yielding short term inverse and long term positive relationships the short term trend marks periods of the negative 13c excursions and is compatible to most of the existing arguments for the 13c events on the other hand mechanisms causing the long term trend remain enigmatic in the absence of comparable data from other localities our observation however suggests that at least two different mechanisms operated marine carbon cycle during the late neoproterozoic third the negative 13c anomaly characterizing the period following the marinoan glaciation worldwide was not coincident between kerogen and carbonate but occurs as a lagged drop in yunnan where a 7 depletion in kerogen preceded an 3 depletion in carbonate their lagged trends likely reflect different sources as kerogen and carbonate signals are considered to monitor surface and deeper waters respectively the negative 13c event would therefore have occurred in surface water followed by a smaller 13c depletion through the entire water column due to vertical mixing between surface and deeper waters at the studied site this mechanism supports the proposed hypothesis that injection of the 13c depleted atmospheric co2 into the surface sea deriving ultimately from quantitative dissociation of methane hydrate in terrestrial permafrost during the post glacial warming contributed to the negative 13c anomaly

increase of seawater mo inventory and ocean oxygenation during the early cambrian abstract the early cambrian radiation of macroscopic animals commonly referred as the cambrian explosion is one of the most important biological innovations in earth history because oxygen is a requirement for complex animal life it is generally accepted that ocean oxygenation may have been the environmental trigger for animal evolution but direct geochemical evidence supporting oxygen increase prior to the cambrian explosion remains sparse and insufficient to better understand the casual relationship between ocean oxygenation and animal evolution we have carried out a systematic analysis of redox sensitive trace element rste concentrations total organic carbon toc contents and pyrite morphology for the early cambrian niutitang formation of the hejiapu section northeastern guizhou province south china the new data in combination with previously published data from other sections show strong mo enrichments average of 88 ppm and high mo toc ratios average of 14 ppm wt in the euxinic black shales of the lower niutitang formation 520 ma indicating increase of the oceanic mo reservoir in the light of the positive correlation between sediment mo toc and aqueous mo in modern basins the early cambrian oceanic mo concentration is estimated as 50 nmol which is about half of the modern ocean mo concentration 105 nmol but is significantly higher than that of the mesoproterozoic oceans 10 20 nmol the marked increase in oceanic mo reservoir is therefore interpreted as recording significant rise of atmospheric oxygen and pervasive ocean oxygenation which may have facilitated the diversification of macroscopic metazoans during the early cambrian

petrogenesis of the natkusiak continental basalts victoria island northwest territories canada the natkusiak flows 1100 m thick sills and dykes of victoria island northwest territories are part of an extensive late hadrynian magmatic event in the northern canadian shield the flows are subaerially erupted plateau basalts and together with associated doleritic sills and dykes display many geochemical features typical of continental tholeiites in addition to fractional crystallization the rocks were affected by crustal contamination but to a smaller degree than usually encountered in continental tholeiites elsewhere the basalts were derived from a depleted upper mantle source very similar to that of n type mid ocean ridge basalts although it is suggested that the melting residue contained small amounts of garnet 2

a refined late cryogenian ediacaran earth history of south china phosphorous rich marbles of the dabie and sulu orogens abstract the late cryogenian ediacaran geological framework for south china is constructed principally from sedimentary successions preserved in the central and western regions of the yangtze block new stratigraphic and carbonate carbon isotope data allow us to extend that framework into the exhumed hp uhp subduction complexes of the eastern dabie and sulu orogens that separate the south and north china cratons those data show that marble and phosphorous rich p rich units in those complexes were originally part of an ediacaran shallow marine shelf carbonate platform the basal pebbly schist metadiamictite and lowermost p rich marble of the jinping formation haizhou group in the sulu orogen matches in both facies character and c isotope profile that of the marinoan equivalent glacial cap carbonate couplet of the nantuo and doushantuo formations the daxinwu formation susong group in the eastern dabie orogen contains a marble unit that has for several hundreds of metres a strikingly uniform c isotope profile of low 13c positive values and is overlain by a p rich graphitic schist these features match those of the late ediacaran to early cambrian dengying formation these correlations establish that the hp uhp metasedimentary rocks many of which were once considered to be palaeo to mesoproterozoic in age are a neoproterozoic age cover sequence of the continental margin of the yangtze block further their widespread development limits their utility as indicators of offset across the tan lu fault zone and instead favours tectonic models that interpret that feature as a continental scale tear fault formed during the mesozoic collision and suturing of the north and south china cratons

permian triassic high thermal regime in the alps result of late variscan collapse or continental rifting validation by numerical modeling 1 the permian triassic igneous activity associated with regional scale deformation developed under high temperature low pressure ht lp metamorphic conditions and widely recorded in the pre alpine crust of the european alps can result from late orogenic collapse of a collisional belt or from lithospheric thinning leading to a continental rifting process in order to reduce this ambiguity we use a two dimensional finite element model to give new insights on the sequence of mechanisms operating during active ocean continent convergence followed by continental collision and pure gravitational evolution and on the regional geodynamic interpretation of the paleozoic mesozoic evolution of the alpine area the modeling predictions are compared with the pt climax conditions of variscan and permian triassic metamorphism affecting the continental crust of the helvetic to southalpine domains the good agreement between model predictions and natural data realized for the early to neovariscan evolution indicates that during paleozoic the pre alpine crust of the alps was part of an active ocean continent convergence margin and an intracontinental suture zone furthermore modeling results support the interpretation envisaging a permian triassic lithospheric extension as responsible for the ht metamorphism and related intense igneous activity this evolution was precursor of the mesozoic oceanization during which the tectonic units coupled and accreted during the variscan subduction and collision were separated to form the two passive european and adriatic continental margins

the central ailaoshan ophiolite and modern analogs abstract the central ailaoshan cal ophiolite represents an important tectonic component of the jinshajiang ailaoshan song ma suture zone separating the south china and indochina blocks in the mainland se asia the cal ophiolite occurs as a complex tectonic melange and preserves the history of the opening and closure of the once vast jinshajiang ailaoshan song ma branch of the paleotethys new and existing geological data indicate that the cal ophiolite contains magmatic rocks generated by 1 l devonian e carboniferous ca 380 330 ma volcanic passive margin breakup development in the nw gondwana margin 2 l permian ca 258 ma emeishan large igneous province related continental rift magmatism together with and intruded by 3 earliest m triassic ca 244 ma continent continent syn collisional s type granitoids the devono carboniferous suites of the cal ophiolite are highly comparable with many continental margin type alpine tethyan ophiolites in addition the various cal magmatic suites have strong south china block affinities with coeval magmatism particularly in the western south china block jinshajiang song ma and song da terranes

germanium silicon of the ediacaran cambrian laobao cherts implications for the bedded chert formation and paleoenvironment interpretations sedimentary strata of the terminal ediacaran 635 542 ma to early cambrian 542 488 ma laobao liuchapo bedded cherts in the south china block include the ediacaran oxidation event and the cambrian explosion understanding the origin and depositional environment of the bedded cherts may provide insight into how the earth s surface environment changed between the proterozoic and phanerozoic we measured major and trace element compositions and ge si ratios of the laobao cherts from northern guangxi province the laobao cherts were deposited in the deep basinal environment of the south china block we show that the composition of the laobao cherts is determined by a mixture of four components quartz clay carbonate and pyrite iron oxide the quartz component is the dominant component of the laobao cherts the maximum estimated ge si of the quartz component is between 0 4 and 0 5 mol mol which is close to the ge si of modern seawater and biogenic silica but 1 order of magnitude lower than that of hydrothermal fluids these ge si systematics suggest that normal seawater rather than mid ocean ridge hydrothermal fluids is the primary si source for the laobao cherts the ge si of the clay component varies between 1 and 10 mol mol which is comparable to the ge si of typical marine clays but 10 100 times lower than that of chert nodules from early ediacaran beds the doushantuo formation predating the terminal ediacaran laobao cherts studied here our observations indicate that the clay component ge si ratio decreased from the early ediacaran to the late ediacaran we speculate that high ge si ratios in clays reflect the preferential chelation of ge by dissolved organic compounds adsorbed onto clays if so this suggests that the decrease in ge si ratio of the clay component in the ediacaran signifies a decrease in the total dissolved organic carbon content of seawater toward the ediacaran cambrian transition consistent with oxidation of the oceans during the late ediacaran finally the seawater origin of the laobao cherts also suggests that replacement of carbonate may not be the primary cause for bedded chert formation instead direct precipitation from seawater or early diagenetic silicification of calcareous sediments perhaps due to the emergence of si accumulation bacteria may have been responsible for the bedded laobao liuchapo chert formation in south china block