The evolution of "Nanhuaian" rift basin on the southeastern margin of Yangtze platform: Age constraints from the Neoproterozoic granites

Abstract: Through zircon LA-ICP-MS and SHRIMP U-Pb isotopic dating of positively emplaced-island-arc granodiorite-adamellite in southwest Hunan, the authors obtained weighted average ages of 835.6±6.7 Ma and 840±8 Ma, suggesting Neoproterozoic. Based on profiling and isotopic geochronologic studies as well as regional correlation of surrounding calcareous slates intercalated with limestone, it is confirmed that the intrusive body should belong to Proterozoic "Nanhuaian" Banxi period sediments - Gaojian Group, which continually deposited with underlying Lengjiaxi Group, with the age of bottom boundary earlier than 835 Ma. It is indicated that during the process of sedimentary basin evolution, the distribution area of Gaojian Group and its equivalent horizons might have not experienced the complete evolution process of island arc- foreland basin intermountain basins-extensive rift basin. Instead, its activity probably stopped in the early phase of the evolution from the island arc to the foreland basin, forming so-called residual basin formation and steep slope marginal paleogeographic framework along the Congjiang-Xupu-Shuangfeng-Hengyang-Pinxianng line. It was connected with the fault-block continental slope formed by stretching on the northwestern side, and the sediments were composed of terrigenous detrital sediments and reworked gravity flow deposits form northwest. This conversion was completed at around 830 Ma.