Origin of the Late Permian tuffs in South China and a Pompeii-style death assemblage

Volcanic ash or tuff layers are widely developed in the sedimentary sequences of the Middle Permian to the Middle Triassic within the Yangtze Block, South China. The tuff layers in the Longtan Formation (Late Permian) at the Baoding section in the eastern Sichuan Basin contain some remarkable fossils preserved in a Pompeii-style. The depositional processes, provenance, and tectonic setting of these tuffs are evaluated based on petrography, whole-rock geochemistry, zircon U-Pb geochronology, and zircon Hf-O isotopic analyses. The petrographic characteristics include angular quartz, with clean surfaces and melt embayed margins, along with a homogeneous grain size distribution, without sorting or sedimentary structures within the tuff layers, which indicate ash fallout deposition, a process that could smother seafloor benthos. Zircon U-Pb dating yields an age range of 255.2−254.1 Ma (late Wuchiapingian), which corresponds to the waning stage of the Emeishan large igneous province (Emeishan LIP). Geochemical characteristics of whole-rock samples, including SiO2 (30.19−59.71 wt%, average 48.38 wt%), Al2O3/TiO2 (mostly <8), Th/Sc (mostly <0.6), along with positive zircon εHf(t) values (1.3−6.0) and low zircon Th/Nb (mostly <10), collectively point to a mantle source. Additionally, low δ18O zircon values (mean 4.04‰) suggest that the mafic magma, after separating from the mantle during its ascent, assimilated ancient crustal materials that had previously undergone hydrothermal alteration. Geochemical characteristics and tectonic setting discrimination diagrams further support an intraplate rift-related origin linked to the Emeishan LIP.