Age and duration of the Emeishan flood volcanism, SW China: Geochemistry and SHRIMP zircon U–Pb dating of silicic ignimbrites, post-volcanic Xuanwei Formation and clay tuff at the Chaotian section

Abstract

The age and duration of the Emeishan basalts (SW China) remain poorly constrained largely due to the severe thermo-tectonic overprinting of the Ar–Ar system and failure to obtain zircon separates from erupted basalts. In an attempt to solve this problem, geochemical analyses and SHRIMP zircon U–Pb dating have been carried out on rare felsic ignimbrite in the uppermost of the Emeishan lava succession, the Xuanwei Formation which immediately overlies the Emeishan basalts and a clay tuff at the Middle–Late Permian boundary at the Chaotian section. Clastic rocks of the lowermost Xuanwei Formation (Group 1) in eastern Emeishan large igneous province (LIP) have a geochemical affinity to the Emeishan felsic volcanic rocks, whereas the overlying sediments (Group 2) are compositionally more akin to mafic components of the Emeishan traps. This is the reverse of volcanic sequence of the central Emeishan LIP where the felsic extrusives sit above predominant mafic rocks. It is likely that the clastic rocks are water-transported sediments resulting from erosion of the volcanic rocks in the center of the Emeishan LIP. This interpretation is further supported by the general agreement between the age of the lowermost Xuanwei Formation (257 ± 4 Ma; 260 ± 5 Ma) and that of the silicic ignimbrite (263 ± 4 Ma) and the clay tuff at the Middle–Late Permian boundary at Chaotian (260 ± 5 Ma). These ages, interpreted as the termination age of the Emeishan volcanism, are indistinguishable within error from the Middle–Late Permian boundary age (260.4 ± 0.4 Ma) and the main stage (259–262 ± 3 Ma) of the Emeishan volcanism inferred from dating of mafic and alkaline intrusions in the Emeishan LIP. All these suggest that the emplacement of the Emeishan volcanism took place over a very short interval. Moreover, the temporal link and geochemical analyses suggest that the Chaotian clay at the Middle–Late Permian boundary was genetically related to the Emeishan silicic volcanism. This, together with the fact at that both the Emeishan basalts and the Chaotian clay rest on the Maokou Formation, leads us to infer that the Emeishan basalt was emplaced at the Middle–Late Permian boundary. In this sense, the Emeishan volcanism can be regarded as a boundary event and its age is presumed at ∼ 260 Ma. Although more precise dating is required, both stratigraphic correlation and chronologic data presented in this paper lend supports to the notion that the Emeishan volcanism was one of likely causes of the end-Guadalupian mass extinction.