Geochemistry of mantle source during the initial expansion and its implications for the opening of the South China Sea

The South China Sea (SCS) is the largest extensional basin in the western Pacific. Due to the lack of sampling appropriate seafloor rocks, few studies focused on magma evolution in the initial expansion of the SCS, which limits the understanding of the tecto-magmatic evolution of the SCS. Here, we analyzed whole-rock Sr-Nd-Pb-Hf isotopic compositions as well as major and trace element concentrations of basalts recovered from the International Ocean Discovery Program (IODP) Expedition 367 Site U1500B in the northern margin of the SCS, which represents the early-stage formed oceanic crust. Combining with published oceanic crust basalts (e.g., IODP Expedition 349 Site U1431 in the eastern sub-basin, Sites U1433 and U1434 in the southwestern sub-basin) formed in the late spreading, the mantle nature and its evolution during the expansion of the SCS were evaluated. Although Sr-Nd-Pb-Hf isotope compositions of these basalts from the SCS suggest an Indian-type mid-ocean ridge basalts (MORB) mantle, the northern margin basalts from Site U1500B show moderate Pb isotopic compositions between those from the eastern and southwestern sub-basin, suggesting a heterogeneous mantle source beneath the SCS. Modeling calculations based on Sr-Nd-Pb-Hf isotopic compositions show that different amounts of lower continental crust and Hainan plume can account for the heterogeneity of such Indian-type MORB mantle in the SCS. The difference in the involved enriched components in the mantle source might be related to the periodic expansion of the SCS. Based on the geochemical imprints of Hainan plume on the SCS basalts that formed in the initial and late spreading stage, we propose that Hainan plume has a great effect in triggering the opening of the SCS.