Abstract:

The Chengjiang Formation is one of the most important Neoproterozoic stratigraphic units in South China; therefore, its accurate depositional age is significant for establishing and completing the Neoproterozoic regional stratigraphic framework of South China. The tuff interbed from the topmost part of the stratotype section of the Chengjiang Formation in Central Yunnan was dated by SHRIMP zircon U-Pb dating. Three groups of weighted mean 206Pb/238U ages were obtained, which are (819±14) Ma (MSWD=0.15), (781±11) Ma (MSWD=0.24) and (725±11) Ma (MSWD=0.65), respectively. The youngest age ((725±11) Ma) is interpreted as the formation time for the tuff interbed from the topmost part of the Chengjiang Formation, which can represent the top boundary age of the Chengjiang Formation in Central Yunnan. Thus, it is further confirmed that the Chengjiang Formation was deposited between 800 and 725 Ma. Combined with related research data, it is proved that the Chengjiang Formation is equivalent to the Kaijianqiao Formation and Liantuo Formation, and all the three formations should correlate with the upper part of the preglacial Banxi Group, rather than the glacial Chang'an Formation. In addition, it is also inferred that the both groups of zircons with older age might be records of the episodic magmatism related to Neoproterozoic break-up of the Rodinia supercontinent.

Key words: Neoproterozoic, Chengjiang Formation, SHRIMP, zircon U-Pb geochronology, top boundary age, South China, Central Yunnan