Cenozoic syn-rift magmatism in Southeast China and the opening of the South China Sea

Geodynamic models and geological evidence increasingly support that the South China Sea (SCS) opened through rifting and breakup of the southeastern South China continental margin during the Cenozoic. However, direct magmatic records that capture the transition from initial extension to seafloor spreading remain scarce, hindering a refined understanding of the mechanisms driving continental breakup and SCS opening. In this study, we report a newly identified 34.82 Ma enriched mid-ocean-ridge basalt−like syn-rift basalt from Shantou, South China, accompanied by whole-rock geochemistry, Sm-Nd-Pb isotopic data, and thermochronological constraints from the host granite. The geochemical and isotopic signatures indicate derivation from a metasomatized lithospheric mantle that was initially modified by subducted paleo−Pacific components. Zircon U-Pb and apatite fission-track data reveal Cenozoic lithospheric thinning and thermal perturbation coeval with basalt emplacement, which we link to slab-pull−driven extension associated with proto-SCS subduction. Together with slightly older syn-rift basalts in the area, the Shantou basalt defines a time-resolved syn-rift magmatic sequence recording progressive lithospheric thinning. These results provide direct geochemical and thermal constraints on the rift-to-breakup transition along the South China margin and identify proto-SCS−controlled extension as the primary driver of the early magmatic response preceding SCS opening.