Ages, trace elements and Hf-isotopic compositions of zircons from claystones around the Permian-Triassic boundary in the Zunyi Section, South China: Implications for nature and tectonic setting of the volcanism

Abstract

A growing body of evidence shows that volcanism near the Permian-Triassic boundary (PTB) may be crucial in triggering the Permian–Triassic (P–Tr) mass extinction. Thus, the ash beds near the PTB in South China may carry information on this event. Three volcanic ash layers, altered to clay, outcropped in the PTB beds in Zunyi Section, Guizhou Province, Southwest China. The U-Pb ages, trace elements, and Hf-isotope compositions of zircon grains from these three ash beds were analyzed using LA-ICPMS and LA-MC-ICPMS. The zircons are mainly magmatic in origin (241-279 Ma) except for two inherited/xenocrystic zircons (939 and 2 325 Ma). The ages of these magmatic zircons indicate three episodes of magmatism which occurred around the Middle- Late Permian boundary (~261.5 Ma, MLPB), the Wuchiapingian-Changhsingian boundary (~254.5 Ma, WCB), and the PTB (~250.5 Ma), respectively. The first two episodes of magmatism near the MLPB and WCB may be attributed to magmatic inheritance or re-deposition. All magmatic zircons share similar trace-element and Hf-isotope compositions. They have Y, Hf, Th and U contents and Nb/Ta ratios are typical of zircons from silicic calc-alkaline magmas. These zircons also exhibit enriched Hf-isotope compositions with eHf(t) values of -11.4 to +0.2, which suggests that the three magmatic episodes involved melting of the continental crust. The more enriched Hf-isotope composition (eHf(t)=-11.4- -4.8) of Bed ZY13 (~250.5 Ma) implies more input of ancient crustal material in the magma. Integration of the Hf-isotope and trace-element compositions of magmatic zircons suggest that these three episodes of magmatism may take place along the convergent continent margin in or near southwestern South China as a result of the closure of the Palaeo-Tethys Ocean.