华南新元古代裂谷系沉积超覆作用及其开启年龄新证据

The Neoproterozoic rift systems in southern China: New evidence for the sedimentary onlap and its initial age

黔东南1∶5万高武幅、宰便幅区域地质调查结果表明,新元古界下江群是一套沉积超覆于中元古界四堡群之上的裂谷系楔状地层,其底部甲路组沉积底砾岩高角度(不整合)沉积超覆于四堡群复理石浊积岩之上,或沉积超覆于侵入四堡群之中的摩天岭花岗岩之上。取自该地区沉积超覆面之下摩天岭花岗岩样品的TIMS锆石U-Pb同位素年龄为825.0±2.4Ma,表明该地区新元古代裂谷系开始接受沉积的时间应该晚于825±2.4Ma;而取自沉积超覆面之上甲路组底部同沉积基性火山岩样品的TIMS锆石U-Pb同位素年龄为814±13Ma,这一年龄大致代表了该地区新元古代沉积超覆的开启时间,且与目前已获得的华南其它地区新元古代裂谷系沉积超覆的开启时间(820Ma)十分接近。本项研究成果支持华南裂谷系沉积超覆的开启时间为820Ma左右的观点。

Recent results from the regional geologica survey of the 1 ： 50 000 Gaowu and Zaibian Sheets in southeastern Guizhou have disclosed that the Xiajiang Group consists of the rift wedge strata which are overlain upon the Mesoproterozoic Sibao Group. More precisely, the sedimentary basal conglomerates from the Jialu Formation at the bottom of the Xiajiang Group are overlain with a high angle （unconformably） upon the flysch turbidites, or upon the Motianling granites intruded into the Sibao Group. The Motianling granite sample （S1） collected beneath the onlap boundary gives a TIMS U-Pb zircon age of 825 ± 2.4 Ma, implying that the deposition within the Neoproterozoic rift systems began at the time later than 825 ± 2.4 Ma. The syndepositional basic volcanic rock sample （S2） collected from the bottom of the Jialu Formation above the boundary gives a TIMS U-Pb zircon age of 814 ± 13 Ma, which approximately represents the initial age of the Neoproterozoic sedimentary onlap in the study area, and approaches that （820 Ma） in other regions in southern China. The results of research in this study strongly support the viewpoint that the initial age of the sedimentary onlap within the Nanhua rift systems should be dated at ca. 820 Ma.