TERMINATION OF THE EMEISHAN FLOOD VOLCANISM AND RELATIONSHIPS TO THE END-GUADALUPIAN MASS EXTINCTION: SHRIMP DATING OF DETRITAL ZIRCONS FROM THE POST-VOLCANIC XUANWEI FORMATION, SW CHINA

Clastic rocks of the lowermost Xuanwei Formation (Group 1) in SW China, which directly overlie the 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 Emeishan LIP where the felsic extrusives sit above predominantly 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. Group 1 sediments of the lowermost Xuanwei Formation extends to Middle-Late Permian boundary at Chaotian, ca 300 km north of the Emeishan LIP. As such the age of the lowermost Xuanwei Formation provides constraints on the timing of the uppermost felsic magmatism of the Emeishan LIP. SHRIMP zircon U-Pb age of these clastic rocks (257±4 Ma; 260±4 Ma) is remarkably close to the estimated Middle-Late Permian boundary age (260.4 Ma±0.4 Ma) and the age of the Xinjie mafic intrusion (259±3 Ma). This relative and absolute temporal coincidence suggests the emplacement of Emeishan volcanism at the Middle-Late Permian boundary and lends supports to the notion that the Emeishan volcanism was the likely cause of the end-Guadalupian mass extinction.