Origin and Tectonic Setting of the 770 Ma Tuff from the Longsheng Area in the Western Jiangnan Orogen, South China Block

Abstract

There is no consensus on the tectonic evolution of the western Jiangnan Orogen (WJO) during 770–750 Ma. Thus, we reported zircon trace elements and U-Pb-Hf-O isotopes and whole-rock geochemistry of the Neoproterozoic tuff located in Longsheng, northern Guangxi, in the WJO, to decipher their origin and tectonic setting. The SIMS U-Pb zircon age yields a concordia age of 772.1 ± 3.8 Ma, suggesting that the tuff layer formed at 770 Ma. Geochemical data of the tuff and its zircon exhibit continental arc signatures. Oxygen isotopes of the zircon show normal mantle zircon δ¹⁸O zircon values of 4.59‰–5.50‰ with an average of 5.24‰. The zircon yielded positive ε_Hf(t) values of 1.8–5.8 with two-stage model ages of 1.32–1.55 Ga. Based on these data and previous studies, the magma source for the tuff is inferred to have originated from the partial melting of the Mesoproterozoic juvenile crust, as the pre-existing arc materials and mixing with the mantle source under the extensional setting during 770–750 Ma triggered by the slab break-off. We infer that the presence of contemporaneous OIB-type and arc-like magmatism at ca. 770–750 Ma along the WJO was related to the slab break-off.

Keywords

tuff / SIMS U-Pb zircon age / magmatic affinity / origin / slab break-off

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2025 Geological Society of China