Magmatism within the northern margin of the South China Sea during the post-rift stage: An overview, and new insights into the geodynamics

Abstract

The South China Sea, which is located to the southeast of Eurasian continent, developed as the result of intra-continental rifting and seafloor spreading on the South China margin. Passive margins are traditionally classified as one of two end-member types, the magma-rich and magma-poor margins, based on the relative abundance or scarcity of magmatism during rifting and breakup. Previous studies suggest that the northern margin of the South China Sea is a magma-poor margin for lacking abundant magmatic activities during the breakup. A growing body of work is beginning to recognize that significant, widespread magmatism may also be present on margins that are presently considered to be magma-poor margins. In this study, we use high resolution 2D/3D seismic profiles, industrial well data, basalt geochemistry (major oxides, trace elements and isotopes) and published results to outline post-rift magmatism developed within the northern margin of the South China Sea. Four magmatic stages occurred since lithospheric breakup. The first stage, from 32 to 23.6 Ma and mainly within the distal margin of the northern South China Sea, was dominated by magma intrusions and corresponded to the spreading of the East Sub-basin. The second stage, from 23.6–19.1 Ma, was mainly in the Baiyan Sag and surrounding uplifts and explosive eruptions dominated. The third Stage, from 19.1–10 Ma, was east of the Zhu III Depression and west of the Enping Sag and dominated by quiet eruptions. The fourth stage, since 10 Ma, and widely distributed in the southern Dongsha uplift, experienced scattered volcanic eruptions. Two basalt samples from 23.6–19.1 Ma (industrial well HJ1, Stage 2) and seven samples from 19.1–10 Ma (industrial well EP1, Stage 3) are analyzed for major oxides, trace elements and Sr-Nd-Pb-Hf isotope compositions. Geochemical features of trace elements show that these samples are characterized by OIB-like basalts, being highly enriched in LREEs (light rare earth elements) relative to HREEs (heavy rare earth elements). Geochemical features of Sr-Nd-Pb-Hf isotope compositions show that the samples all resemble ocean-island basalts with two mixing endmembers: depleted mid-ocean ridge basalt mantle (DMM) and enriched mantle II (EMII). Pb isotopic characteristics show the Dupal isotope anomaly in the northern margin of the South China Sea. And geochemistry data of all the samples signals the contribution of Hainan Plume. Previous studies have revealed the existing of southeastward mantle flow from Tibet to South China Sea and a branch of Hainan Plume existing beneath the northern South China Sea using geophysical methods by different researchers. Based on our latest research and published geological evidences by other researchers, we propose that Stage 1 magmatism was caused by southeastward mantle flow stemming from Indo-Eurasian collision, the stage 2 and stage 3 magmatism was caused by Hainan Plume and the activation of Yangjiang-Yitongansha Faut Zone, whereas the last stage magmatic activity was mainly related with the combination of Hainan Plume and activation of the faults caused by the subduction of the SCS beneath the Luzon Arc at the Manila trench.