Ar-40/Ar-39 geochronological constraints on the age and evolution of the Permo-Triassic Emeishan Volcanic Province, Southwest China

The biostratigraphically constrained Permo-Triassic Emeishan Volcanic Provi nce (EVP), extending over wide areas in southwest China, has been recently considered as a Large Igneous Province contemporaneous with the Siberian Tr aps and the siliceous tuffs at the P-T boundary in South China. We report t he first Ar-40/Ar-39 ages on this igneous province. Minimum ages have been obtained on phenocrystic plagioclase of the Emeishan basalt, which has unde rgone a pervasive metamorphism. most likely during subsequent tectonization as a consequence of terrane amalgamation. Comparison between the Ar-releas e spectra obtained on clear vs. cloudy plagioclase indicates a 40-30 Ma ser icite resetting time. A minimum apparent age of 246 +/- 4 Ma for plagioclas e from a plagiogranite, a late-differentiate of the Panzhihua Layered Compl ex, and an age of 254 +/- 5 Ma for phlogopite from a pyroxenite near Lake E rhai, provide the first absolute age constraint on this igneous province. A dditional Ar-Ar age measurements on post-Emeishan alkaline and mafic magmat ism yielded 104 +/- 2 and 100 +/- 2 Ma for an alkaline complex near Panzhih ua. and 42 +/- 1 Ma for a gabbro sill emplaced near the Ertan Dam. Further study is still needed to determine the age of the Emeishan volcanic emissio n accurately. and to test the validity of the assumed short duration of the eruption. (C) 2002 Elsevier Science Ltd. All rights reserved.