Hafnium isotopic constraints on the origin of late Miocene to Pliocene seamount basalts from the South China Sea and its tectonic implications

Abstract

Post-spreading intraplate volcanism has widely affected the South China Sea (SCS) region including Indochina, the northern margin of the SCS, and the SCS basin itself. In the SCS basin, several off- and on-fossil spreading center seamounts formed between 3.8 and 7.9 Ma. Based on previously published geochemical and Sr-Nd-Pb isotopic data, the intraplate volcanism is widely related to the Hainan mantle plume, whose existence has been evidenced by recent geophysical studies. To test this petrogenetic model, new Hf isotope data have been obtained from volcanic rocks from a suite of compositionally representative seamounts in the SCS basin. Compared to published Nd isotope ratios (0.512675–0.512965, 5 εNd units), 176Hf/177Hf ratios span a much larger range (0.282876–0.283097, 8 εHf units), even within individual seamounts (e.g., Zhangzhong seamount). These features, combined with previous studies, clearly confirm mantle heterogeneity beneath the SCS region. Similar to the trends in Sr-Nd-Pb isotope space, Nd-Hf isotope ratio show a relatively narrow, elongate mixing trend between a depleted Indian MORB-type mantle endmember and an enriched EMII-type mantle end member. We propose that this narrow trend is inconsistent with the origin of the enriched end member from a heterogeneous sub-continental lithospheric mantle (SCLM) and instead suggests a plume-related origin. As a conceptual model for the post-spreading tectonic scenario of the Hainan plume affecting the SCS region, we propose that a plume ascends to the bottom of the lithosphere beneath Hainan and its northern Leizhou peninsula at the northern margin of SCS, from where it migrates along sloping rheologic boundary layers to lithospheric faults under an extensional setting towards the central SCS where the magmas erupt at the young spreading centers.