Tuffs from the Neoproterozoic Chengjiang Formation in the Yimen region, central Yunnan: Zircon U-Pb dating and its geological implications

The widespread Chengjiang Formation strata on the western margin of the Yangtze platform consist dominantly of sedimentary rocks intercalated with the volcanic rock series associated with the Jinning Movement. The timing of the formation of the sedimentary rocks is important to the establishment of the Neoproterozoic regional stratigraphic framework, stratigraphic division and correlation and palaeogeographic reconstruction. In this study, the tuffs with several meters of thickness are found in the basal part of the Neoproterozoic Chengjiang Formation in the Yimen region, central Yunnan. The LA ICP-MS zircon U-Pb dating of the tuff samples shows that the ²⁰⁶Pb/²³⁸U weighted average age of zircons equals to 812±5.5 Ma (n=17 MSWD=0.46), and thus the basal boundary of the Chengjiang Formation may be dated at about 812 Ma. The additional age of 723±11.6 Ma represents the age of the top boundary of the Chengjiang Formation. The above mentioned ages indicate that the volcanic rocks in the Chengjiang Formation may be traced back to the late Neopreterozoic. The age data for the zircons trapped in the tuffs may also imply that several phases of important regional tectonic-thermal events once took place at 2.5 Ga, 1.8-1.6 Ga and 1.0 Ga.