The Stratigraphic Time,Sedimentary Environment And Provenance Analysis Of Neoproterozoic Doushantuo(Huangxu) Formation In Chuzhou,Anhui

The Neoproterozoic stratigraphic ages,paleoclimate environment and tectonic background are one of the hotspots of study of geology,specially sedimentology in the world for recent years.Lastly,the shale-gas fields found in Middle Yangtze area of China get much more interest.The Chuzhou area of Anhui in the northeast margin of the Yangtze plate develops the Neoproterozoic strata,and is a typical area of Neoproterozoic study,but it has made little progress in recent years.Based on field investigation and research,microfacies analysis,geochemical analysis and age determination of detrital zircon,we have determined the sedimentary age of Doushantuo Formation,restored the sedimentary environment,and discussed the characteristics of source area and tectonic evolution history.The Doushantuo Formation,819.1 meters in thickness,is composed of mainly mudstones,silty mudstones,siltstone,next microcrystalline limestone,dolomitic limestone,feldspar quartz fine sandstone and lithic sandstone,and with a weak metamorphic characteristic.The lower part of Doushantuo Formation is shallow shelf clastic facies,and the upper part is carbonate platform facies.The average content of major elements in 9 clastic rocks is SiO2 58.5%,TiO20.98%,Al2O313.98%,Fe2O3 5.67%,MgO 2.17%,K2O 3.6%,Na20 1.72%.The average content of some characteristic trace elements Th,U,Hf,Sc,Zr,Y are 11.25ppm,1.88ppm,7.18ppm,12.88ppm,266.22ppm,35.60ppm.Rb/Sr,Th/U,La/Y,La/Sc and Th/Sc ratio were 2.05,5.86,1.39,3.88,0.88;the average of REE is 232.87ppm,the average of ? LREE/? HREE is 9.12,The fractionation of light and heavy rare earth is more obvious;the average of ? Eu is 2.50,Eu negative anomaly is obvious.The normalized distribution curve of REE is right type.The contents of major elements,trace elements and rare earth elements samples indicate that the source area of the Doushantuo Formation was characterized by the tectonic setting of the continental island arc and passive continental margin.The average chemical weathering index of the corrected clastic rock samples is CIAcorr =71.04,which shows that the source area of Doushantuo Formation experienced medium intensity chemical weathering.The LA-ICPMS analysis of zircon U-Pb from 2 clastic rock samples gains 174 age data,of which the youngest detrital zircon age is 586 ± 16.4 Ma(YSG),which indicates the Doushantuo Formation belonging to sediments at turn between early and last Sinian Epoch.The U-Pb age spectrum analysis of detrital zircon shows that the zircon ages are divided into two phases,mainly for 586-901Ma(70.8%),and 1554-2488Ma(25%).This shows that the source rocks of the Neoproterozoic Doushantuo Formation developed in the northeast margin of the Yangtze plate were mainly from the Neoproterozoic Nanhua-Qingbaikouan geological units of the Yangtze plate,followed by the early Proterozoic geological units,and the former may be Neoproterozoic magma and the latter may be the basement of the northern margin of the Yangtze plate.