Geochronology and geochemistry of Middle Permian tuff in Chaohu region, China: Implications for their origin and geological significance

Abstract

Tuff of Middle Permian as a record of volcanic activity are widely distributed in the northern hemisphere, such as South China, but origin of the tuff bed remains unclear. Stratigraphy, geochronology, geochemistry, and zircon Hf isotopic composition of the tuff layers from Gufeng Formation in Chaohu region, South China, are systemically investigated to reveal the origin of the tuff. The stratigraphical investigation and geochronological dating reveal that zircon U–Pb age of 271.6 ± 3.1 Ma for lower tuff layer is identical to the formation age (272 Ma) of the bottom boundary in the Gufeng Formation, while the zircon U–Pb age of 263.6 ± 3.8 Ma for upper tuff layer represents the boundary age between the lower and upper Gufeng Formation. The tuff with a lot of clastic materials has high Th/Ce (0.18–0.84) and Th/La (0.29–1.23) ratios, enrichment in LREE, Th, U and Hf, and depletion in Ba, Nb and Sr, with obviously negative Eu anomalies (δEu = 0.41–0.58). Compared with the lower tuff layer, the upper tuff layer has higher zircon εHf (t) values (−5.65 to 10.2) and lg fO₂ values (−14.6 to −7.42). The distinctly chemical and isotopic results indicate that the intermediate-felsic tuff in the Chaohu region were originated from more crustal materials in early stage of the Gufeng Formation and increased juvenile crustal materials and higher oxygen fugacity in late stage of the Gufeng Formation.