Source characteristics of basalts in Sanshui Basin and the early tectonic evolution stage of the South China Sea

It is an important issue in the study of the origin of the South China Sea (SCS) whether the SCS evolved in an intracontinental rift stage before its opening up. The Sanshui Basin is located on the northern margin of the SCS. The Cenozoic eruption of the bimodal volcanic suggest that the basin was a continental rift. The major and trace elements of the basalts indicate that the basalts in Sanshui Basin can be divided into sub-alkaline and alkaline basalts series. Both of them show obvious Nb and Ta positive anomalies. Compared with the continental crust, they have lower Th/Sc, La/Nb and U/Al×1000. ⁴⁰Ar-³⁹Ar age and previous chronology results suggests that the eruption of basaltic magma was intense in 61～54 Ma, while the eruption of sub-alkaline basalt (60 Ma) is earlier than alkaline basalt (56 Ma). Based on the mantle melting column model, the temperature and pressure of the subalkaline magma source range from 1 517℃ (3.03 GPa) to 1 471℃ (2.25 GPa), in a depth of 101～76 km, while the alkaline magma source range from 1 555℃ (3.33 GPa) to 1 506℃ (2.48 GPa) in a depth of 110～84 km. According to the source of magmatism and rock assemblage, it is concluded that the Sanshui Basin was a Paleocene continental rift. Comparing the temporal distribution and source magmatic activity in the spreading period between the Sanshui Basin and the South China Sea, it is inferred that there is a long time interval between the magmatic activity of the Sanshui Basin and the spreading of SCS, and the deep process is quite different. There is no direct relationship between magmatism in the Sanshui Basin and the spreading of SCS.