Geochemical, biostratigraphic, and high-resolution geochronological constraints on the waning stage of Emeishan Large Igneous Province

The initiation and peak magmatic periods of the Emeishan Large Igneous Province (LIP) are well constrained by both biostratigraphic and radioisotopic dating methods; however, the age of cessation of volcanism is poorly constrained and continues to be debated. Marine carbonates interbedded with volcanic ashes across the Guadalupian−Lopingian boundary (GLB) are widespread in south China, and these ashes provide an opportunity to study its timing, origin, and potential relationship with the Emeishan LIP. Here we present biostratigraphic constraints, mineralogical and geochemical characteristics, and high-resolution geochronology of ash layers from the Maoershan and Chaotian sections. Stratigraphic correlation, especially conodont biostratigraphy, confines these ashes to the early Wuchiapingian. Those altered ashes are geochemically akin to alkali tonsteins from the coal seams of the lower Xuanwei/Lungtan Formation in southwest China. The ashes postdating the GLB yield a coherent cluster of zircon U-Pb ages with weighted mean 206Pb/238U ages of 258.82 ± 0.61 Ma to 257.39 ± 0.68 Ma, in agreement with the ages of intrusive rocks (259.6 ± 0.5 Ma to 257.6 ± 0.5 Ma) in the central Emeishan LIP. Moreover, the ɛHf(t) values of zircons from the ashes vary from +2.5 to +10.6, a range consistent with that of the Emeishan LIP. The results collectively suggest that the early Wuchiapingian volcanic ashes are a product of extrusive alkaline magmatism and most likely mark the waning stage of the Emeishan volcanism, which may have continued until ca. 257.4 Ma in the early Wuchiapingian.