CA-TIMS zircon U-Pb dating of felsic ignimbrite from the Binchuan section: Implications for the termination age of Emeishan large igneous province

摘要

The age of the Emeishan lavas in SW China remains poorly constrained because the extrusive rocks are (1) thermally overprinted and so represent an open system unsuitable for ~(40)Ar/~(39)Ar geochronology and (2) in most cases devoid of zircon so that it is impossible for the application of U-Pb geochronology. Existing radiometric age constraints of Emeishan large igneous province are mainly from the application of SIMS and LA-ICP-MS U-Pb techniques to zircons from mafic and felsic intrusions, which represent indirect constraints for the lavas. In an attempt to directly determine the age of the Emeishan lava succession, high-resolution chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) zircon U-Pb techniques have been used on the felsic ignimbrite at the uppermost part of the Emeishan lava succession. These techniques have yielded a weighted mean ~(205)pb/~(238)u age of 259.1 ± 0.5 Ma (n = 6; MSWD = 0.7). We interpret this age as the termination age of the Emeishan flood basalts. The age of the Guadalupian-Lopingian boundary is still unconstrained by high-resolution geochronology but is likely to be close to our new age for this felsic ignimbrite.