Opening time and filling pattern of the Neoproterozoic Kangdian Rift Basin, western Yangtze Continent, South China

Abstract

The Neoproterozoic Kangdian Rift Basin is a continental rift basin in the western Yangtze Continent. Determining its time of opening and subsequent filling pattern is an important aspect of understanding the formation and evolution of the Yangtze Continent, South China. The Luliang Formation is the early filling in the eastern part of this rift basin, and its deposition age and filling sequence are significant for studies of the regional stratigraphic correlation, opening time, and filling pattern of this basin. Having been correlated to the upper part of the Mesoproterozoic Kunyang Group or to the Neoproterozoic Chengjiang Formation, the age and regional stratigraphic correlation of the Luliang Formation have long been uncertain. This is due to a lack of reliable absolute age constraints. To address this issue, the first zircon SHRIMP U-Pb geochronology has been established for this formation, yielding two high-precision isotopic ages of 818.6±9.2 and 805±14 Ma for the tuff layers at the bottoms of the lower and upper members of the Luliang Formation, respectively. Given the error factor, the bottom age of the lower member of the Luliang Formation can be interpreted as ca. 820 Ma, corresponding to the bottom age of the lower part of the Banxi Group, which is the early filling of the Neoproterozoic Xianggui Rift Basin, a representative basin of the Neoproterozoic rift basins in the Yangtze Continent, South China. The bottom age of the upper member of the Luliang Formation can be interpreted as ca. 800±5 Ma, corresponding to the bottom age of the Chengjiang Formation in the western part of the Neoproterozoic Kangdian Rift Basin and also corresponding to the bottom age of the upper part of the Banxi Group in the Neoproterozoic Xianggui Rift Basin. These ages indicate that the Neoproterozoic Kangdian Rift Basin shares the same opening time and filling sequences as those of the other Neoproterozoic rift basins in South China. Basin analysis shows that the Neoproterozoic Kangdian Rift Basin is a typical half-graben basin, with its main boundary fault in the west and basin center in the east. This basin consisted of mini unidirectional half-graben basins in its juvenile stage and simplified to become a large united half-graben basin in its mature stage, i.e., a supradetachment basin.