Emeishan Basalt Ar-Ar overprint ages define several tectonic events that affected the western Yangtze platform in the Mesozoic and Cenozoic

Abstract

Ar-Ar whole-rock dating was carried out as part of a detailed stratigraphical investigation of the Emeishan Basalt large igneous province (LIP) in the stratotype area, Sichuan, China. Thirteen (from twenty-one) specimens from three sections yielded reliable reversed isochron and plateau ages (maximum 1σ errors of ≤2.1 and 1.6 m.y., respectively), and form two clusters centered on the Late Jurassic-Early Cretaceous and Early-Late Cretaceous, with a tail spanning 82-40 Ma. However, all are appreciably younger than the magnetobiostratigraphically-constrained late Middle Permian (255-260 Ma) age of the basalts, reflecting varying degrees of thermal resetting during the Mesozoic and Cenozoic. Recently, Middle Jurassic-Early Cretaceous ages reported from elsewhere in the LIP have been used to infer a single 40 m.y.-long tectonic episode that affected the western Yangtze Platform. Developing this idea, the new information have been combined with data we hold for other parts of the terrain, and results from three Emeishan LIP Ar-Ar dating studies published during 2002, to give 32 reliable age dates. Three 10-12 m.y. events appear to be recorded in different parts of the province: Middle Jurassic, Late Jurassic-Early Cretaceous and Early-Late Cretaceous, with a fourth shorter middle Eocene episode (the 40 m.y. Mesozoic episode appears to be a sampling artifact). Following a review of the regional deformation/tectonic features, it is argued that activity related to various phases of deformation in the Longmen Shan Thrust Belt is the most likely cause of resetting. The final suturing of the North and South China blocks may have also been responsible for the Middle Jurassic event. However, the Oligocene-present indentation of India into Asia appears not to have had an impact, possibly due to the large-scale related strike-slip faults that have effectively shielded the LIP/Sichuan Basin. © 2003 Elsevier Ltd. All rights reserved.