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

The biostratigraphically constrained Permo-Triassic Emeishan Volcanic Province (EVP), extending over wide areas in southwest China, has been recently considered as a Large Igneous Province contemporaneous with the Siberian Traps and the siliceous tuffs at the P-T boundary in South China. We report the first 40Ar/39Ar ages on this igneous province. Minimum ages have been obtained on phenocrystic plagioclase of the Emeishan basalt, which has undergone a pervasive metamorphism. most likely during subsequent tectonization as a consequence of terrane amalgamation. Comparison between the Ar-release spectra obtained on clear vs. cloudy plagioclase indicates a 40-30 Ma sericite resetting time. A minimum apparent age of 246 +/- 4 Ma for plagioclase from a plagiogranite, a late-differentiate of the Panzhihua Layered Complex, and an age of 254 +/- 5 Ma for phlogopite from a pyroxenite near Lake Erhai, provide the first absolute age constraint on this igneous province. Additional Ar-Ar age measurements on post-Emeishan alkaline and mafic magmatism yielded 104 +/- 2 and 100 +/- 2 Ma for an alkaline complex near Panzhihua. 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 emission accurately. and to test the validity of the assumed short duration of the eruption. (C) 2002 Elsevier Science Ltd. All rights reserved.