Sedimentary Geochemical Characteristics And Trace Elements Enrichment Mechanism Of Permian Cold Seep Carbonates In Xianza,Tibet

Methane dominated submarine cold seep seepage is widely distributed in the continental margin,which is closely related to environment,biology and energy.Seep carbonate as one of the most important products of cold seep activity,it records biogeochemical reactions related to methane,and is the best tool to study the special properties and evolution history of seepage flow in geological history.In this paper,the seep carbonate of the lower Permian Angjie Formation in the Xianza area of Tibet are taken as the research object,and the fluid source,sedimentary environment and trace element enrichment mechanism are analyzed.The seep carbonates in the Xianza section are mostly wrapped in the shale in the form of nodules,and calcareous sandstone and marine carbonate rocks can be seen around them.In order to explore whether the geochemical characteristics of seep carbonate in the study area are affected by the surrounding rocks,the seep carbonate and calcareous sandstone and marine carbonate in the same layer in the Xianza area are analyzed by carbon and oxygen isotopes and major and trace elements The fluid source,sedimentary environment and trace element enrichment mechanism of marine carbonate and shale are comprehensively analyzed and compared in order to reveal the formation mechanism and influencing factors of seep carbonate in Xianza area.The average carbon isotope value of seep carbonate of Lower Permian Anjie Formation in Xianza area,Tibet is-18.6 ‰,indicating that the carbon source is mainly thermogenetic methane.The normal marine carbonate rocks can represent the carbon isotope characteristics of seawater at that time,and the average carbon isotope value is4.4 ‰.Because the carbon isotope of seep carbonate is slightly more correct than that of thermogenic methane as carbon source,it is considered that its carbon source may be affected by the surrounding sea water,and the average carbon isotope value of calcareous sandstone is-4.7 ‰.It is considered that the carbon source may be mixed seawater of organic matter.The seep carbonate of the lower Permian Angjie Formation in the Xianza area,Tibet have high REE contents.Their distribution patterns show that the middle REE is enriched,the light REE is depleted,and the Ce is negative,indicating a weak oxidation environment;The REE distribution pattern of marine carbonate rocks is characterized by LREE depletion and LREE slight enrichment.The negative Ce anomaly indicates that they were formed in an oxidizing environment;In addition,the U/Th,V/Cr and Ni/Co ratios are 0.09,1.20 and 3.25 respectively,indicating that the sedimentary environment is an oxidation environment;The total REE content of calcareous sandstone is relatively lower than that of seep carbonate,and the distribution pattern is characterized by "bell type".The medium REE is enriched,the light REE is depleted,and the Ce anomaly is negative,indicating a weak oxidation sedimentary environment.It indicates that the sedimentary environment of seep carbonate may be affected by the surrounding rocks.According to the enrichment factors,trace elements(U,Th,Ni,V,Cd,Co,Zn)are mostly enriched in seep carbonate and slightly depleted in the shale.Due to the poor correlation between Fe and trace elements,the influence of Fe-Mn Shuttle to bring trace elements into seep carbonate is excluded;The correlation of the same trace element in seep carbonate and shale is poor,excluding the influence of shale on the enrichment of trace elements in seep carbonate,indicating that the trace elements in seep carbonate may be carried by the lower fluid.