The establishment of the Nanjing Formation in Longtouzhai Group in eastern Nanling Re-gion and the determination of its age

The epimetamorphic rock series well developed in eastern Nanling Region is a suite of Late Ordovician epimetamorphic rock series called Longtouzhai Group that consists of dark gray, gray, celadon fine grained feldspar quartz sandstone, sericite slates, silty slate and phyllites intercalated with lenticular carbonate and sandstone. They can be subdivided upwards into Bankeng Formation, Guting Formation and Huangzhudong Formation. It is found that the original Late Jurassic Yutian Group is a suite of dark gray andes-ite, light grey dacite, porphyroclastic lava intercalated with gray dacitic tuff, volcanic brecciated tuff, celadon fine grained feldspar quartz sandstone, sericite slates, silty slate, phyllites in Siqian, Shixing (northern Guangdong) and Nanjing, Quannan (southern Jiangxi) areas. It is in conformable contact with the Huangzhudong Formation and is underlain unconformably by Middle Devonian Yunshan Formation. The volcanic rock association in the Nanjing basin is developed considerably well and the boundaries between the roof and the bottom are clearly distinguishable. The Nanjing Formation is established based on the Qiaozishan section and Zhongzhai sec-tion, and the Zhongzhai section serves as its stratotype section. The results of the SHRIMP zircon U-Pb dating indicate that the ²⁰⁶Pb/²³⁸U age range of fourteen analytical points of the rhyolite is 460~425Ma with the weighted average of 442.1 ±3.9Ma and MSWD of 0.75; the analytical points of the fifteen rhyolite porphyry samples show ages of 450~425 Ma with the weighted average of 439.9±3.7Ma and MSWD of 0.50. In combination with the zircon U-Pb age of 443.6±5.4Ma (MSWD=1.3) for porphyroclastic la-va in the Hekou (Siqian) caldera SHRIMP, the Nanjing Formation should belong to the Latest Ordovician-Early Silurian. The estab-lishment of the Nanjing Formation in Longtouzhai Group in eastern Nanling Region and the determination of its age not only pro-vide the basis for the argument that Longtouzhai Group belongs to the Late Ordovician-Early Silurian, but also offer the evidence for the opinion that the epimetamorphicd rock series of Late Ordovician-Early Silurian include volcanic miogeosynclinal type sediments in eastern Nanling Region