Sedimentary Construction And Tectonic Environment Analysis Of The Qiandongian (Cambrian Series 2) Canglangpu Formation In Central Yunnan Province,China

The central Yunnan province is located in the southeast margin of the middle and Upper Yangtze plate,and its sedimentary evolution is closely related to the tectonic evolution of the middle and Upper Yangtze plate.The analysis of sedimentary environment is an important part of the reconstruction of paleogeography.The lack of systematic research on the sedimentary environment and sedimentary evolution characteristics of the Qiandongian(Cambrian Series 2)Canglangpu Formation in Central Yunnan province leads to the failure of a unified understanding of the paleogeographic pattern of Canglangpu Formation in the sedimentary period.Therefore,this article through the field geological investigation,rock identification and geochemical methods,such as combination of Yunnan province of surging waves spread group of geological features,the types of sedimentary facies and sedimentary environment,map the lithofacies paleogeographic map,and further through the main elements of sandstone,trace elements and geochemical characteristics of trace element discriminant analysis on the tectonic sedimentary environment.It provides new data and basis for the division of lithofacies palaeogeographic characteristics and tectonic environment during the sedimentary period of Canglangpu Formation of Cambrian.The main insights are as follows:The Canglangpu Formation in the study area can be divided into two sections,namely Hongjingshou formation and Wulongqing Formation.Based on the observation of the lithology and sedimentary structure of the exposed section of Canglangpu Formation,it is found that the Canglangpu Formation mainly develops sandstone,quartz sandstone,siltstone and mudstone,and generally develops horizontal and parallel bedding,but occasionally cross-bedding and ripple bedding.The sedimentary environment of Canglangpu Formation is Marine sedimentary,mainly in shallow sea sedimentary environment at the early stage,and mainly in quiet shallow sea sedimentary environment at the late stage in central Yunnan province.The early and late sedimentary facies of Canglangpu Formation can be divided into three types:foreshore facies,near-shore facies and offshore plain facies.The late stage can be divided into four facies types:platform grey flat facies,foreshore facies,near shore facies and offshore plain facies.Further,the geochemical characteristics of clastic rocks in Canglangpu Formation are studied and the data of major and trace elements are processed.Canglangpu Formation is close to the upper continental crust.The standardization curve of trace elements tended to be stable,with enrichment of Th,U,La,Ce,Nd and deficit of Ba,Ta,Nb,Sr,P,Ti.On the whole,the REE distribution curve shows a right-leaning trend,indicating a high degree of fractionation of l REE.The ratio of LREE to HREE is 4.82-9.06,indicating that the sandstone of Canglangpu Formation in central Yunnan province is enriched in LREE and depleted in HREE.Eu outliers(Eu/Eu*)ranged from 0.66 to 0.78,indicating moderate anomalies.The average value of La/Yb,La _N/Yb _Nand La _N/Sm _Nis 10.96,7.4 and 3.65 respectively,indicating that the fractionation degree of light and heavy rare earth is strong.Finally,based on the elemental geochemical characteristics of clastic rocks,the discriminant diagrams of major and trace elements are used to discriminate the tectonic environment of Canglangpu Formation.It is considered that the tectonic environment is relatively stable and PAASive continental margin.