Geochronology, Geochemistry, and Sr–Nd–Pb–Hf Isotopes of Late Triassic Granitoids in the Qiongdongnan Basin, Northern South China Sea: Insights into Petrogenesis and Tectonic Implications

Abstract The South China Sea (SCS) has experienced a complex tectono-magmatic evolution since the Paleozoic, making its Late Paleozoic–Early Mesozoic tectonic evolution poorly constrained and widely debated. We report zircon ages, whole-rock geochemistry, and Sr–Nd–Pb–Hf isotopes of the granitoids intersected in Well Y8-3b in the Songnan Low Uplift, Qiongdongnan Basin, northern SCS margin, to constrain their petrogenesis and tectonic implications. The granitoids comprise granodiorite and monzogranite, with zircon U–Pb ages of 231 Ma and 230 Ma, respectively. The granitoids are high-K calc-alkaline and peraluminous, with Fe numbers of 0.71–0.79 (average = 0.75). They are enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE), depleted in high field strength elements (HFSE), and exhibit pronounced negative Eu anomalies. Geochemical and isotopic data indicate that the granitoids are ferroan I-type granites derived from a common magma source. The granodiorite likely represents an earlier magmatic product, whereas the monzogranite formed through subsequent magmatic differentiation. The granitoids were derived from hybrid magmas generated by mixing approximately 45% SCS mantle-derived magma with 55% crustal melt, and were emplaced in a post-collisional extensional setting. These new and compiled data reveal that the South China–Indochina amalgamation was a protracted process, evolving from syn-collisional assembly (254–243 Ma) to post-collisional extension (242–225 Ma).