PERMIAN/TRIASSIC BOUNDARY CLAY BEDS IN SOUTH CHINA—RELATED TO THE EMEISHAN LARGE IGNEOUS PROVINCE?

A P/Tr boundary section at West Pingdingshan, near Chaohu (Anhui Province, China), contains two composite clay beds composed in part of volcanic ash. Geochemically, the bentonites are strongly potassic (K/Na > 10) and peraluminous (ASI >1.0), have negative Eu/Eu* anomalies (0.38-0.63), and exhibit incompatible element distributions marked by large negative anomalies in Ba, Nb-Ta, Sr-P, and Ti. The geochemistry of the bentonites shows strong affinities with that of the ~251-Ma Ailanghe pluton, a late-stage granitic intrusive associated with the Emeishan Large Igneous Province (LIP) of western Guangxi and eastern Yunnan provinces. In addition to geochemical similarities, increases in the number and thickness of bentonite beds into the southwestern provinces of China imply that the volcanic ash layers represent a second phase of highly explosive felsic magmatism of the Emeishan mantle plume post-dating its initial eruption at ~259 Ma. The lower Chaohu bentonite is correlative with Bed 25 of the P/Tr global stratotype section and point (GSSP) at Meishan D (Zhejiang Province, China) and, thus, coincident with the main end-Permian extinction horizon, whereas the upper Chaohu bentonite is correlative with Bed 28 at Meishan and coincident with an earliest-Triassic extinction event of lesser intensity (Jin et al., 2000). The close stratigraphic relationship between these ash layers and the decimation of marine invertebrate communities on the South China craton is evidence that massive volcanic eruptions were the proximate cause of the marine biotic crisis in this region. The importance of Emeishan plume magmatism for the end-Permian biotic crisis requires reconsideration of the role of the Siberian Traps and, possibly, development of a model invoking a concurrence of processes to account for both regional and global signals: onset of global environmental stresses as a result of eruption of Siberian flood basalts, followed by decimation of regional marine ecosystems in South China through massive volcanic eruptions of the Emeishan plume system.