Palaeoenvironment Indicated From Miocene Lacustrine Stromatolites In Wudaoliang Area,Tibetan Plateau

Stromatolites are most known as found in warm,semi-arid saline shallow marine settings,nonetheless there are a large amount of stromatolites deposited in riverine,lacustrine even cave or other none-marine environments.Generally,phototrophic cyanobacteria are thought to be stromatolitic builders.Thus,the stromatolites are very indicative archieves for shallow benthic settings.For ancient stromatolites,one can obtain the water chemistry characteristics during their deposition by using appropriate geochemistry methods.Fluvial-lacustrine facies prevail in the Cenozoic Hol-xil basin,northern Tibetan plateau.In Wudaoliang military pot area,numberous lacustrine stromatolite limestones were found in a single quarry.The thickness,species of stromatolites are much more abundant than their counterpartbedsfrom other localities.Once,the genesis of stromatolites has been interpreted as “a sudden humidification in long-term arid condition” or a “sudden desalination during long-term saline lake”for its origin,plus a possible hydrothermal impact.Nonetheless,the missing of calcified cyanobacteria fossil brought questions to the interpreted palaeoenvironments.Systemtic geochemistrical data,especially for traces elements and REEs are necessary for a more detailed research.Based on field research and publications,we started another trial using petrology and geochemistry archives to answer further questions with respect to the genesis of stromaotlites and their sedimentary environments.According to their micro fabrics,we divided the laminaes of stromatolites into several types: Clotted microcrystalline laminae,Porous vertically structured laminae,Thin massive microcrystalline(aphanitic)laminae,Peloidallaminae(porous)and Peloidallaminae(dense)considering their different fabric-genesis.Sparry and micritic calcites dominate with respect to the mineral composition,additionally with limited portion of quartz.No calcified cyanobacteria is found using all the microscopies.Previous filamented micrites are likely to be calcified fungis during post-Miocene periods.We obtained numberous geochemistry data using LA-ICP-MS on cutting sections of stromatolite samples.No systematical enrichment or depletion between the sparry and micritic calcites,the enrichment of trace elements are related to terrestrial influx.Significant positive Ce,positive Eu anomalies will be demonstrated,possibly resulted from a oxygenated,phototrophic shallow lake setting,and a later hydrothermal impact.A strong ?13C-?18O co-variation is presented from the stromatolite limestones,as a testimony for closed-lake setting.?13C of stromatolites are obviously higher than the Miocene background sediments.The range of stromatolite oxygen isotope data is more narrow than the Wudaoliang Formation limestone counterparts,and implicates the palaeotemperature of stromatolites is 19.1?.A possible hydrothermal impact on the diagenesis fluid may be reflected by the oxygen isotopes of stromatolites.