Convergent continental margin volcanic source for ash beds at the Permian-Triassic boundary, South China : constraints from trace elements and Hf-isotopes

Abstract

Volcanic activity around the time of the Permian-Triassic boundary (PTB) has been proposed as a trigger for the associated biocrisis. Multiple claystone beds are prominent near the PTB sections in South China. Twenty one PTB ash beds from three sections at Shangsi, Jianshi and Meishan in South China were sampled and analyzed. Volcanic ash geochemistry indicate dacite and rhyolite compositions. Zircons from the ash layers yield comparatively low Nb/Hf and high Th/Nb ratios, falling in the range of arc/orogenic-related settings. Zircon Hf-isotope compositions show that εHf(t) values vary from −11.7 to 1.8, indicating involvement of both juvenile and ancient crustal components. The ash beds (SS27a, JS129, JS130, JS133,MS25, MS26) near the biotic extinction horizon display a large variation in εHf (t) and relatively positive average values, indicating input of juvenile mantle or crust, and implying rapid transit through the older basement of the South China Craton. Spatial and temporal distribution of ash beds from thirty one PTB sections worldwide reveal that volcanic ash beds occur mainly in, or proximal to, the Tethys region and were sourced locally. Integration of Hf-isotope and trace-element compositions from magmatic zircons suggests that the PTB volcanism occurred along the convergent continent margin in, or near, southwestern South China as a result of the closure of the Paleo-Tethys Ocean.