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

The Late Paleozoic layered or stratoid-layered basalts in western Guangxi have similar elemental and isotopic compositions to Emeishan high-Ti basalts. Whole-rock ^40Ar/^39Ar and SHRIMP zircon 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 4^40Ar/^39Ar 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 ^206Pb/^238U age of 253.7±6.1 Ma 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 -251-253Ma: The age (251-260Ma) 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.

The Late Paleozoic layered or stratoid-layered basalts in western Guangxi have similar elemental and iso-topic compositions to Emeishan high-Ti basalts. Whole- rock Ar/39Ar and SHRIMP zircon U-Pb dating were carried 40 out for the representative basalt samples in three typical pro- files in the area. Three basalts from the upper segment of Yangxu profile and lower segment of Yufeng and Min’an pro- files yield the 40Ar/39Ar 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 206Pb/238U age of 253.7±6.1 Ma with and 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 ex- tinction events at the end-Guadalupian and permo-Triassic boundary. These precise geochronological data provide im- portant constraints on the dominantly eruptive time of the Emeishan LIP and understanding of the distribution of Eme- ishan high-Ti basalts and its mantle plume dynamics.