Geochemical Characteristics And Geological Significance Of Upper Paleozoic Siliceous Rocks In Nanpanjiang Basin

The Nanpanjiang Basin is located in the conjoining part of west Guangxi,south Guizhou and east Yunnan.It lies at the eastern end of the Tethyan tectonic domain at its juncture with the Paleo-Pacific tectonic domain.Various hypotheses for the tectonic nature of the Nanpanjiang Basin have been proposed.In this paper,we chose late Paleozoic cherts from the Nanpanjiang Basin,analyzed the petrological and geochemistry to determine the origin and formation tectonic environment of these rocks,and achieved the following conclusions:The Devonian cherts in the eastern part of the basin have obviously negative Ce anomalies and positive Y anomalies,indicating that they were formed in an environment far from the supply of terrestrial environment.Geochemical data of cherts indicate that the Ce/Ce*values gradually decreased from northwest to southeastern basin during the Devonian,suggesting that the water body gradually deepened.The spatial-temporal distribution of geochemical feature indicates that the Nanpanjiang Basin had an evolutionary trend of gradual extension during the Devonian.The Middle-Lower Permian cherts in Hongyan,Babu and Xiniubao have a high degree of silicfication.The Ce/Ce*and Y/Ho values of cherts in Hongyan are0.68±0.19 and 41.83±13.27,respectively,indicating that these cherts were formed in a relatively open seawater environment.The Tapi Formation cherts from Babu and Xiniubao areas generally have high Al/?Al+Fe+Mn?values?0.43-0.70?,Al₂O₃/TiO₂values?25.5-44.8?,and Eu/Eu*values?0.81-1.02?,which are non-hydrothermal cherts within acid pyroclastics.The Babu and Xiniubao cherts show high Ce/Ce*values?0.67-1.29 and 0.77-1.18,respectively?and low Y/Ho values?28.84-31.23,26.41-28.91,respectively?,indicating that these cherts were formed in the continental mariginal environment with obvious terrestrial input.Combined with geochemical characteristics of cherts in other areas,we conclude that an active continental margin may develop in the southwestern Babu Ocean,the oceanic crust was subducted toward southwestern,and the Nanpanjiang basin was still connected with the ocean basin.