Ar-Ar and U-Pb geochronology of Late Paleozoic basalts in western Guangxi and its constraints on the eruption age of Emeishan basalt magmatism

摘要

The Late Paleozoic layered or stratoid-layered basalts in western Guangxi have similar elemental and isotopic compositions to Emeishan high-Ti basalts. Whole-rock ~(40)Ar/~(39)Ar andtzircon U-Pb dating were carried out for the representative basalt samples in three typical profiles in the area. Three basalts from the upper segment of Yangxu profile and lower segment of Yufeng and Min'an profiles yield the ~(40)Ar/~(39)Ar plateau ages of 253.6 + - 0.4 Ma (20BS-71), 255.4 + - 0.4 Ma (20BS-99) and 256.2 + - 0.8 Ma (20BS-119), respectively. Twenty-three analyses on 23 zircons of the basalt from the upper segment of Yangxu profile give a weighted mean ~(206)Pb/~(238)U age of 253.7 + - 6.TMa with an MSWD = 2.8. These new and published geochronological data for Emeishan large igneous province (LIP) indicate that the Emeishan LIP was initiated at -260 Ma, voluminously erupted between 253 and 256 Ma, and possibly ended at -251-253 Ma: The age (251-260 Ma) is generally consistent with that of the associated environmental deterioration and mass extinction events at the end-Guadalupian and Permo-Triassic boundary. These precise geochronological data provide important constraints on the dominantly eruptive time of the Emeishan LIP and understanding of the distribution of Emeishan high-Ti basalts and its mantle plume dynamics.