南华冰期的底界讨论:来自沉积学与同位素年代学证据

目前国内对于我国新元古代南华系划分的分歧很大,主要原因之一在于对板溪群及其相当层位地层的构造属性及其上限年龄的认识存在较大差异,为此,在详细的沉积学调查的基础上,对湘黔桂地区典型丹州群和下江群(与板溪群相当)有关剖面上顶部地层沉凝灰岩开展了锆石年代学研究.沉积学调查表明,板溪期沉积地层与上覆南华冰期沉积地层之间的转换界面清晰,界面上下岩性、结构、构造、沉积相特征迥异,显示板溪群和上覆南华纪冰期沉积期间存在明显的沉积—构造转换过程.而板溪晚期沉凝灰岩的锆石U-Pb同位素年代学研究则表明,板溪群及其相当层位的上限年龄均趋向于720 Ma,这与Sturtian冰期的启动年龄718 Ma接近.因此,基于沉积盆地演化的阶段性、全球性重要古气候演化的区域可对比性以及同位素年代学分析,将南华冰期启动年龄设定为720 Ma是恰当的,将我国南华系底界置于江口冰期沉积层系之底也是合理的,这为我国南华系底界GSSP的选定提供了重要的年代学和沉积学约束.

Discussion on the Bottom of Nanhua System: Evidences from Sedimentology and Isotopic Geochronology

At present, it is far from to get an agreement on the subdivision and correlation of Nanhua System, the mainly origins of this dispute are from the different understand to the tectonic attribute and the top limit age of Banxi Group and its equivalents. For this reason, based on the survey of sedimentology, the isotopic geochronology was carried out on the Longli Formation section of Xiajiang Group in Dunzhai of Jinping county, the Gongdong Formation section of Danzhou Group in Piaoli of Longsheng county, and the Gongdong Formation section of Danzhou Group in Pingtan of Tongdao county. The sedimentary survey indicates that the transforming boundary between Banxi Period and Nanhua glacial Period are clear. The petrofacies, petrofabric, sedimentary structure, and sedimentary sequence are greatly different from the upper units to the lower units of the boundary, which suggest obviously this boundary had seen a important sedimentary and tectonic event. At the same time, the isotopic geochronology from the tuffite and tuffaceous rocks of the top units of the Banxi Period make clear that the top limit age of Banxi Group and its equivalents should be tend to 720 Ma, and similar to the onset age 718 Ma of Sturtian glacial epoch. Based on the evolution phases of sedimentary basin, the comparability of global paleoclimate change and the geochronologic data, therefore, it is feasible to settle the bottom of Nanhua System upon the bottom of sedimentary sequence of Jiangkou glacial epoch, and it is also reasonable to put the onset age of Nanhua glacial Period at 720 Ma for the time being, and which would, thus, provide the essential constraints of sedimentology and isotopic geochronology to select the GSSP of Nanhua System.