Architecture and evolution of the lithosphere beneath the northern South China sea: Implications for plate-edge lithospheric thinning and rifting

Passive margins formed by continental rifting and oceanic spreading exhibit a wide range of characteristics, many of which remain poorly understood. In this study, we investigate the continent-ocean transition (COT) of the South China Sea (SCS) to elucidate the tectonic-magmatic processes involved in continental thinning and rifting. We present new in situ analyses of water, chemical, and Sr isotopic compositions of volcanic glasses and plagioclases, alongside the ages and Hf isotope compositions of zircons from the SCS. Our isotopic data reveal significant inter- and intra-crystalline disequilibria in plagioclase phenocrysts, suggesting a complex history of mantle-derived magmas interacting with the continental crust. Geochemical variability in the plagioclases and the presence of old inherited zircons indicate that mantle magmas were contaminated by crustal material. The ages (ranging from Cretaceous to Archean) and Hf isotopic signatures of the zircons are similar to those found in the Cathaysian block of Southeast China, supporting the presence of continental lithosphere beneath the northern SCS. The COT is characterized by a sharp juxtaposition of continental crust and igneous oceanic crust, and its structural features differ from the classic magma-poor or magma-rich passive-margin models. The X H2O conditions inferred for basalt generation during continental rifting and SCS spreading imply that the underlying mantle beneath the SCS was hydrous, which may have triggered the weakening and removal of the lithospheric root, probably due to the multiple subductions of oceanic plates since the Cretaceous. Our findings provide key evidence for the role of hydro-induced lithospheric weakening associated with nearby subduction zones in the processes of continental thinning and breakup in the SCS.