New insights into the lowest Xuanwei Formation in eastern Yunnan Province, SW China: Implications for Emeishan large igneous province felsic tuff deposition and the cause of the end-Guadalupian mass extinction.

A previous study suggested that the lowest Xuanwei Formation is derived from weathered clastic materials of silicic composition from the Emeishan large igneous province (ELIP) based on chemostratigraphic correlations (Al 2 O 3 /TiO 2 ratios) between the two. In this study, we have adopted the model that the Emeishan mantle plume commenced and terminated within a short duration and have investigated the detailed mineralogy and geochemistry of carefully sampled rocks from the lower sections of the Xuanwei Formation, eastern Yunnan Province, Southwest China. These samples are intensely argillized and characterized by high proportions of clay minerals and quartz. The samples with Al 2 O 3 /TiO 2 > 7 from the lowest Xuanwei Formation have an anomalous natural gamma response and high concentrations of Nb, Ta, Zr, Hf, Th, U, Ga and REY (rare earth elements and yttrium). Our results suggest that the samples with Al 2 O 3 /TiO 2 > 7 from the lowest Xuanwei Formation represent felsic volcanic tuff instead of acidic clasts as originally proposed. The lowest Xuanwei Formation and the Wangpo Bed are the felsic tuffaceous layers interbedded with clastic rocks derived from the Emeishan high-Ti basalts. Such volcanic layers most likely represent ELIP felsic tuff originated from the extrusive equivalent of Nb–Zr-enriched alkaline syenitic magmatism at the waning stage of Emeishan mantle plume activity. This study has verified the existence of extensive alkaline felsic volcanism of early Late Permian age. Such alkaline volcanism may have been catastrophic and have contributed to the end-Guadalupian mass extinction. [ABSTRACT FROM AUTHOR]