关键词: 新元古代, 西乡群, 凝灰岩, 锆石U-Pb年龄, 地层学, 幕式岩浆活动, 扬子北缘

Abstract:

We present the U-Pb ages of the tuff zircons from the Dashigou and Sanlangpu Formations of the Xixiang Group at the northern margin of the Yangtze block, using the LA-ICP-MS dating analysis. Weighted mean ages are (789.0±4.4) Ma (MSWD=0.66) and (760.4±4.5) Ma (MSWD=0.43) respectively. According to existing data, the basal boundary age of the Xixiang Group should be ~820 Ma. Moreover, the Xixiang Group can be roughly compared with the Banxi Group and relevant strata in the regional scale, representing the early filling of the Neoproterozoic rift basin in the South China. Together with the underlying Sanhuashi Group, they represent the distinctive upper and lower tectonic-sedimentary circles, respectively. Combined with previous study results, it is indicated that there are three episodic magmatic activities during the Neoproterozoic epoch, with their peak ages being ~825 Ma, ~795 Ma, and ~760 Ma, respectively. The first two episodes, representing the commencing age and fully extensional age of the rift basin respectively, are more apparent than the third one, The volcaniclastic rocks from the Dashigou and Sanlangpu Formations reported represent the latter two episodic magmatic activities (~795 Ma and ~760 Ma) respectively, their geological significances should not be neglected, either because they are parts of the Neoproterozoic episodic magmatic activities or because they are closely related to the formation of the Neoproterozoic rift basin in the South China.

Key words: Neoproterozoic, Xixiang Group, tuff, zircon U-Pb age, stratigraphy, episodic magmatic activities, Northern margin of the Yangtze block