Early Triassic Silicic Volcanics of South China: Petrogenesis and Constraints on the Geodynamic Evolution of the Paleo-Tethys Ocean Region

Abstract

The only occurrence of Lower Triassic silicic volcanic rocks within the South China Block is in the Qinzhou Bay area of Guangxi Province. LA-ICP-MS zircon U-Pb dating reveals that volcanic rocks of the Beisi and Banba formations formed between 248.8 ± 1.6 and 246.5 ± 1.3 Ma, coeval with peraluminous granites of the Qinzhou Bay Granitic Complex. The studied rhyolites and dacites are characterized by high SiO₂, K₂O, and Al₂O₃, and low MgO, CaO, and P₂O₅ contents and are classified as high-K calc-alkaline S-type rocks, with A/CNK = 0.98–1.19. The volcanic rocks are depleted in high field strength elements, e.g., Nb, Ta, Ti, and P, and enriched in large ion lithophile elements, e.g., Rb, K, Sr, and Ba. Although the analyzed volcanic rocks have extremely enriched zircon Hf isotopic compositions (ε_Hf(t) = –29.1 to –6.9), source discrimination indicators and high calculated Ti-in-zircon temperatures (798–835°C) reveal that magma derived from enriched lithospheric mantle not only provided a heat source for anatectic melting of the metasedimentary protoliths but was also an endmember component of the S-type silicic magma. The studied early Triassic volcanics are inferred to have formed immediately before closure of the Paleo-Tethys Ocean in this region, as the associated subduction would have generated an extensional setting in which the mantle-derived upwelling and volcanic activity occurred.

Keywords

geochemistry / silicic volcanic rocks / mantle contribution / Olenekian–Anisian / Paleo-Tethys oceanic plate subduction

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2023 2023 Geological Society of China