Volcanic activity during the Early–Middle Triassic transition in the Sichuan Basin, South China: Duration, evolution and implications

Abstract

The felsic volcanogenic tuff known as ‘green bean rock’ (GBR) and its carbonates are widely dispersed across the western Yangtze Block. This study investigates the origin, tectonic setting and genesis of GBR, as well as the environmental disruptions evident in carbonates from the western edge of the Yangtze Block. The analyses include mineralogy, whole-rock geochemistry and the isotopic composition of zircon Hf, carbon and oxygen in GBR samples from the western edge of the Yangtze Block. The geochemical profile of GBR shows enriched LREE, Th and U content, depleted levels of Nb, Ta, Sr, Ba, K, Rb and Ti, and strong-to-moderate negative Eu anomalies (δEu = 0.15–0.18). Zircon Hf isotopes exhibit S-type geochemical affinities with low negative εHf(t) values (−13.3 to −5.7) and TDM₂ ages of 1684–2110 Ma. This suggests that the volcanic ashes originate from the magma of an intermediate to felsic composition. X-ray directionality data show that the most prevalent clay minerals are illite and illite/smectite. Lithium fixed in these minerals is likely to have leached from brine. Early Triassic variances in δ¹³C profiles are reliable indicators of environmental disturbances, pointing to cycles of devastation and restoration in marine ecosystems, interspersed with extraneous events including volcanic activity. The study posits that volcano eruptions may have prolonged biotic recovery after the end-Permian mass extinction.