U-Pb age and Lu-Hf isotopic compositions of zircon from tuff layers of the late Permian-Early Triassic sedimentary succession in the western Yangtze Block, SW China: Implications for the tectonic evolution and Impact of episodic volcanism on biotic recovery

Abstract

The Zhaotong Basin, located at the southwestern margin of the Yangtze Block, is characterized by multiple layers of sedimentary tuffs. However, the tectonic background of tuffs, the timing of episodic volcanism marked by these tuffs, and the relationship between volcanism and biotic recovery remain controversial. In this study, we integrate petrography with zircon U–Pb and Lu–Hf isotope analyses on five tuff layers bracketing the Permian–Triassic boundary. The zircon U–Pb ages of these tuff layers were determined to be 258.9 ± 0.7 Ma, 254.9 ± 0.6 Ma, 250.0 ± 2.8 Ma, 247.1 ± 1.2 Ma, and 247.2 ± 0.74 Ma. Among these, the age of 247.2 ± 0.74 Ma is proposed as a geochronological marker for the Olenekian–Anisian boundary. Geochemical and Lu–Hf isotope data from the Xuanwei Formation indicate that the earliest tuff layer (258.9 ± 0.7 Ma) originated from acidic volcanism associated with Emeishan basalts. In contrast, the late tuff layer (254.9 ± 0.6 Ma) corresponds to the onset of the closure of the Paleo-Tethys Ocean subduction, with volcanism persisting into the Middle Triassic (247.2 ± 0.74 Ma). Episodic volcanism, as evidenced by the five tuff layers, was temporally coupled with the biocrisis that spanned from the Guadalupian (ca. 260 Ma) to the Anisian (247 Ma). The distinctive ε_Hf(t) values reveal substantial crustal contributions to the magmatic systems throughout the volcanic episodes. These findings indicate that recurrent volcanic activity likely contributed to cyclic environmental perturbations and subsequent delays in biotic recovery spanning > 13 Myr.