Geochemical Features And Tectonic Significance Of Basic Rocks In Napo,Guangxi

The structural evolution of the southwestern South China Blockand the activation of Paleo-Tethys Ocean remain strongly grabbing the attention of geophysicists.A Late Paleozoic to Triassic magmatic rocks developed in the zone has challenged the traditional cognition that the southwestern margin of SCB was a continental passive margin.Yet due to the lack of reliable ages,high weathering and the multiple tectonic settings explainable to the geochemical features of the magmatic rocks,the cognition cannot be totally overturned.Hence we need to seek supporting evidence to clarify the tectonic nature of the margin.This thesis exams the tectonic setting and evolution of the southwestern SCB in Late Paleozoic through exploring the geochemical features,magma source,tectonic setting and formation time of basic rocks developed at Gaoying and Nianjing in Napo County,Guangxi Province.(1)According to the geochemical features of Napo basic rocks,it is found that Gaoying rocks have the typical geochemical characteristics of Emeishan basalts;And the Nianjing rocks has the characteristic of typical island arc basalt.(2)we figure out that the Gaoying magma was from Emeishan large igneous province,Which belongs to Emeishan basalts;and Nianjing rocksare typically formed by plate subduction in island arc setting.(3)The basic rocks in Gaoying developed during Triassic;and the basic rocks in Nianjing developed from Permian to Triassic with an age of 241 to 253 Ma.(4)Through the joint analysis of Island arc basic rocks respectively developed in Gaoying and Nianjing in Napo and southwestern of SCB,recognizing relations of geochemical features,tectonic significance and formation time of basic rocks.Hence we can deduce that some branch of Palaeo-Tethys subducted southwestern of SCB in large scale during Permian to Triassic(241Ma to 253 Ma).And an arcuate island arc zone of magma developed along the area of Jianshui-Napo-Caobang.