Mesozoic Northward Subduction Along the SE Asian Continental Margin Inferred from Magmatic Records in the South China Sea

During the Mesozoic, Southeast (SE) Asia (including South China and the South China Sea (SCS)) was located in a transitional area between the Tethyan and Pacific geotectonic regimes. However, it is unclear whether geodynamic processes in the SE Asian continental margin were controlled by Tethyan or paleo-Pacific Ocean subduction. Herein, we report similar to 124 Ma adakitic granodiorites and Nb-enriched basalts from the Xiaozhenzhu Seamount of the SCS. Granodiorites have relatively high Sr/Y (34.7-37.0) and (La/Yb)(N) (13.8-15.7) ratios, as well as low Y (9.67-9.90 mu g/g) and Yb (0.93-0.94 mu g/g) concentrations, typical of adakites. Their Sr/Y and (La/Yb)(N) values coupled with their relatively low initial Sr-87/Sr-86 ratios (0.70541-0.70551), relatively high K2O contents (3.31-3.38 wt%), high Th/La ratios (0.33-0.40), negative epsilon(Nd)(t) values (-3.62 to -3.52), and their variable zircon epsilon(Hf)(t) values (-3.8 to +5.2) indicate that these rocks were formed by melting of subducted oceanic crust and sediments. The Nb-enriched basalts show enrichment in high field strength elements (HFSE) and have epsilon(Nd)(t) values of +2.90 to +2.93, as well as relatively low initial Sr-87/Sr-86 ratios of 0.70341-0.70343, demonstrating that they were derived from a depleted-mantle (DM) source metasomatized by silicate magmas originating from melting of a subducted oceanic lithospheric slab. By combining our findings with data from other Late Mesozoic arc-related magmatic rocks and adakites from the broader study area, we propose a geotectonic model involving subduction of young oceanic lithosphere during the Late Jurassic and northward subduction of the proto-South China Sea (PSCS) along the SE Asian continental margin during the Early Cretaceous. This conceptual model better explains the two-period Mesozoic magmatism, commonly reported for the SE Asian continental margin.