扬子东南缘“南华纪”盆地演化——来自新元古代花岗岩的年龄约束

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

通过对湘西南地区主动侵位的岛弧型花岗闪长岩、二长花岗岩的锆石LA-ICP-MS与SHRIMP U-Pb同位素年代学研究,分别获得加权平均年龄为(835.6±6.7)Ma、(840±8)Ma,属新元古代。对岩体围岩钙质板岩夹灰岩建造的剖面研究和同位素年代学分析及区域对比,确认其属于新元古代"南华纪"板溪期沉积——高涧群,与下伏冷家溪群连续沉积,底界时限应大于835 Ma。指出高涧群及其相当层位分布区在沉积盆地演化过程中,可能没有经历由俯冲造山—伸展裂谷盆地的完整演化过程,盆地从板片俯冲形成岩浆弧的初期演化即已停止,形成所谓的残余盆地,并沿从江—溆浦—双峰—衡阳—萍乡一线形成陡坡边缘盆地古地理格架,与北西侧伸展形成的断阶式陆坡相接,接受来自北西侧的陆缘碎屑及其改造型重力流沉积,完成这一转换过程的时间点在830 Ma左右。

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.