The Geochemical Characteristics And Formation Environment Of Radiolarian Siliceous Rocks In DTc In Litang

Siliceous rocks formed in the specific geological and geochemical environment,the inner paleobioassemblage and geochemical characteristics of rocks are of great importance to the study of rock genesis,sedimentary age,geological background and regional tectonic evolution history.The Ganzi-Litang ophiolitic mixture is one of the subduction zones in the ancient Tethys.The research area is located in the middle part of Ganzi-Litang ophiolitic hybrid rock zone.In this work,siliceous rocks about Tcg in Litang area of western Sichuan province were selected as the research objects,and the work of petrology,paleontology,and geochemistry analysis of rocks was carried out,the formation time and tectonic environment of siliceous rocks are defined to provide basis for further study of the early tectonic evolution of Ganzi-Litang ocean basin.In the 6 samples of siliceous rocks in DTc,4 genera and 7 species were identified,which were from middle-late Devonian to late Carboniferous,the radiolarian is characterized by the deep-sea sedimentarycenvironment,and its distribution represents the expansion of the ancient Tethys deep water channel.The results of geochemical analysis of 9 samples of siliceous rock show that the Si/Al ratio of siliceous rocks is 11.69³7.22.There was a significant negative correlation between SiO₂/Al₂O₃and Al₂O₃ content,indicating that the effect was obvious in the land sediments.The characteristics of the main elements show that the siliceous rocks are of biological origin and form on the continental margin-open ocean environment.Rare earth elements show the right type of rare earth enrichment;Ce/Ce*value is 0.95².36,with an average value of 1.85,showing obvious Ce abnormal;The Eu/Eu*value is 0.87¹.22,with an average value of 1.02,indicating that the Eu anomaly is not obvious;?La/Ce?_N ratio varies greatly,which is0.42¹.09,with an average of 0.58,which is the characteristic of siliceous rocks deposited on the continental margin.Trace elements composition,Ti and V were positively correlated relationship,Ti/V of 10.57⁸9.02,with a mean of 37.68,U/Th value between 0.10⁰.52,with a mean of 0.28,are less than 1,it is characterized by the normal sedimentary siliceous rocks of the siliceous rock in the ocean basin and the continental margin siliceous rocks.In the tectonic setting,it is shown that the tectonic environment is the transitional zone of continental margin or continental margin-deep ocean basin,not one of the terminal elements of ophiolite suite formed in the mid-ocean ridge environment.In conclusion,according to the paleontology and the geochemical characteristics of siliceous rocks,it is believed that the western edge of the Yangtze continental block may have begun to crack in the early Devonian.Ganzi-Litang ocean basin has formed deep water basin in the middle Devonian.