论皖南地区前寒武纪浅变质岩系地层层序

ON THE STRATIGRAPHIC SEQUENCES OF PRECAMBRIAN WEAK METAMORPHIC ROCK SERIES IN SOUTH ANHUI,CHINA

安徽南部(皖南)前寒武纪浅变质岩地层发育,分布广泛,厚达数千米,称为沥口群和溪口群,传统上当做青白口纪和中元古代地层。可是,这套浅变质岩地层的时代归属、层序叠置关系等,长期以来一直存在争议。在分析现有地层划分方案的基础上,通过地层接触关系、沉积序列、物质组分特征和沉积体系演化研究,结合砂岩碎屑锆石LA-ICP-MS U-Pb同位素定年,重新厘定了皖南地区浅变质岩系的地层格架。这套浅变质岩系主要为海底扇沉积体系和盆地平原沉积体系,分为上溪群和溪口群,分别代表北部盆地边缘相带和南部盆地中部相带,两者为同期异相沉积。300颗碎屑锆石的测试结果表明,68颗碎屑锆石U-Pb年龄为727±12Ma^835 Ma,最年青的碎屑锆石年龄(727±12Ma)限制了沉积物的最大沉积年龄,浅变质岩系地层时代为早—中南华世。上溪群分为两个组,下部湘东组,包括3个段,代表从滨岸—陆棚经海底扇到盆地平原的海侵沉积序列;上部羊栈岭组,包括4个段,代表从盆地平原经海底扇—陆棚—滨岸的海退沉积序列。溪口群包括漳前组、板桥组和木坑组,代表盆地中部外扇—盆地平原沉积。碎屑锆石的U-Pb年龄,Tu、U含量以及Tu/U比值,证实井潭组火山岩为浅变质岩提供了大量物源,上溪群和溪口群浅变质岩地层应该覆于火山岩之上。上溪群和溪口群假整合覆于铺岭组/井潭组火山岩系之上,其上为休宁组不整合覆盖,代表皖南地区早南华世—中南华世一次比较完整的海侵-海退旋回沉积。

The Weakly metamorphosed rock series of Precambrian age occur widely in South Anhui. They are thousands of meters in thickness and are traditionally divided into the Likou Group of the Qingbaikouan Period and the Xikou Group of the Middle Proterozoic Era. However, there has been considerable controversy about their geochronology and stratigraphic sequences. We studied the stratigraphic framework of these weakly metamorphosed rock series, on basis of detailed analysis of stratigraphic division, contact boundaries, sedimentary sequences, sedimentary systems, and LA-ICP-MS U-Pb dating of detrital zircon grains. The rock series consist mostly of deep-water fan and basinal plain sedimentary systems, and they are divided into Shangxi and Xikou Groups, which represent sedimentary sequences in the northern margin and central part of basin, respectively. According to detrital zircon U-Pb dates, 68 of 300 zircon gains have ages between 727±12 Ma and 835 Ma, with the youngest age providing a maximum constraint on the depositional age. Thus, these Precambrian metamorphic rock series belongs to the early-middle Nanhua Epoch. The Shangxi Group can be divided into lower Xiangdong Formation and the upper Yangzhanling Formation. The former, with three members, consists of a transgressive sequence from beach shelf, through deep-water fan, to basinal plain facies. The latter, with four members, consists of a regressive sequence from basinal plain, through deep-water fan, to shelf-beach facies. The Xikou Group comprises of the Zhangqian, Banqiao and Mukeng formations, which are composed of deposits of outer-fan and basinal plain facies. U-Pb ages, Tu and U contents, and Th/U ratios of the detrital zircons indicate that volcanics of the Jingtan Formation are the source rocks of the weakly metamorphosed rock series. The Shangxi and Xikou groups disconformably overlie volcanics of the Jingtan and Puling formations, unconformably underlie the Xiuning Formation, and represent an early-middle Nanhuan transgression to regression cycle.