Fingerprinting Subducted Oceanic Crust And Hainan Plume In The Melt Sources Of Cenozoic Basalts From The South China Sea Region

Geochemical data compilation of Cenozoic basalts recovered from the South China Sea tectonic domain shows westward weakening of the influence of a focal zone-like component in Nd-Hf, Nd-Pb and Sr-Pb, but not in Pb-Pb isotope spaces because the Pb isotopes are dominantly controlled by the high U/Pb component derived from the subducted Pacific oceanic slab. Low Th/U melt generated by recycling of marine carbonates, rather than the subduction-related enriched mantle (EM2), signals the emplacement of the Hainan Plume at similar to 25 Ma. Radiogenic Hf in the pre-existing ancient sub-continental lithospheric mantle beneath the Cathaysia Block was greatly depleted by early-stage magmatism influenced by the high U/Pb component. Hence, late Cenozoic basalts associated with the carbonatitic melts display contrasting Nd-Hf isotope covariations, with the Red River-Zhongnan Fault System as a dividing line, implying that the East and Southwest sub-basins have been developed on the Cathaysia and Indochina Blocks respectively.