华南“南华系”研究新进展——论南华系地层划分与对比

New advances in the study of “the Nanhuaan System”-with particular reference to the stratigraphic division and correlation of the Nanhuaan System, South China

华南新元古代裂谷系由一系列地堑、地垒式次级盆地组成,裂谷系地层位于震旦系与四堡造山不整合面之间,区域上侧向延伸不连续,呈“楔状地层”展布。大量新的U-Pb同位素年龄资料表明,“楔状地层”下部(包括板溪群或与之相当的高涧群、芙蓉溪群、丹洲群、下江群、登山群、厉口群等)的底界年龄均小于或接近820Ma。因此,在华南四堡造山不整合面之上缺失了与华北青白口系相当的绝大部分地层(1000～820Ma),这一结论与目前中国华南地层划分方案不同。由于820Ma代表了华南新元古代裂谷系沉积超覆的“起点”年龄,且这一重要沉积事件的时间又与国际地层委员会新元古界三分方案中成冰系的底界年龄接近,因此,建议将华南新元古代裂谷系沉积超覆地层与四堡造山带变质岩系之间的不整合面作为"南华系"的底界面。

The Neoproterozoic South China rift system is composed of a number of graben-and horst-type subbasins. The strata of the system which onlap unconformably the Sinian orogen and underlie the Nantuoan tillites regionally occur as successions of laterally inextensive wedged-shaped strata. A wealth of new U-Pb isotope age data indicate that the ages of the lower boundary of the wedged-shaped strata (including the Banxi Group and /or its equivalents Gaojian, Furongxi, Danzhou, Xiajiang, Dengshan, Likou and Heshangzhen groups) are all less than or close to 820Ma in age, which represents the onset of the earliest deposition of the Neoproterozoic rift system. It is clear that almost all of the strata(1000-820Ma) corresponding to the Qingbaikouan System in North China are lacking above the Sibaoan orogenic unconformity. This conclusion apparently differs from the existing stratigraphic division schemes for South China. Since the age of 820Ma marks the onset of the most important sedimentary event in South China during the Neoproterozoic and is also close to that of the basal boundary of the Cryogenian System in the tripartite division of the Neoproterozoic in the International Stratigraphic Chart issued under auspices of the International Commission on Stratigraphy, the author suggests that the unconformity between the sedimentary onlapped strata of the Neoproterozoic rift system and the metamorphic series of the Sibaoan orogen be defined as the basal boundary (age ～820 Ma)of the Nanhuaan System.