Genesis of the Wangpo bed in the Sichuan Basin: Formation by eruptions of the Emeishan large igneous province

Abstract

During the Guadalupian–Lopingian transition, the widely distributed Wangpo bed in the Sichuan Basin recorded the voluminous eruptions of the Emeishan large igneous province (ELIP). However, it remains unclear whether the Wangpo bed was formed by submarine volcanic activity of the ELIP or is the weathering product of the Emeishan basalt. Furthermore, its causal relationship with the end-Guadalupian mass extinction and contemporaneous environmental changes is also uncertain. To investigate these issues, we conducted a study of the Wangpo bed of the Shangsi section in the northwestern Sichuan Basin, including analysis of its mineralogy, whole-rock major/trace elements, and zircon UPb chronology, Hf isotopes, and trace-element geochemistry. Petrological observations of the appearance of pumice in the upper Wangpo bed suggest that the pumice could have been formed by submarine volcanic activity of the ELIP. Mineralogical investigation also shows large amounts of vermicular kaolinite and anatase in the Wangpo bed, suggesting that it is the alteration product of volcanic ash that was deposited in a deep water column. Geochemical comparisons further confirm that the Wangpo bed shows affinity with the felsic extrusive rocks of the ELIP but differs from the clay beds at the bottom of the Xuanwei Formation in the eastern ELIP. More importantly, the results of zircon UPb chronological, Hf isotope, and trace-element geochemical analyses indicate that the bed formed as a result of intraplate volcanic activity at ca. 257 Ma. Thus, our robust array of evidence suggests that the Wangpo bed was formed as a direct sedimentary response to the eruptions of the ELIP. On the basis of a comprehensive analysis of the possible influence of the eruptions on environmental change, we propose that the violent submarine volcanic activity that was accompanied by strong magma degassing during the Guadalupian–Lopingian transition, which released large amounts of volatiles to the seafloor, could have exacerbated the contemporaneous environmental degradation and eventually led to the end-Guadalupian biotic crisis.