Abstract: The Southwestern Basin is the key region to the understanding of the origin and the evolvement of the South China Sea.Some critical geological problems remain in a dilemma so far concerning the evolution of the South China Sea, such as the formation of the sea floor and the boundaries of the basin because its complexity in regional tectonic background, active sedimentation and lack of rock samples.Granodiorite samples have been collected recently from the southern margin of the southwest basin of the South China Sea (SCS).The samples were analyzed for major elements, trace elements and rare earth trace elements (REE) with ICP-MS.Mineralogy was studied using the petrographic microscope and electron microprobe.The ⁴⁰Ar/³⁹Ar laser probe was used to date the biotite in the samples.The ⁴⁰Ar-³⁹Ar ages of the biotite of the samples are in the range of 109.5±1.5Ma, indicating that they are products of magmatic intrusion in Early Cretaceous.Their geochemistry shows that the granodiorites are characterized by high SiO₂, K₂O, and Al₂O₃, but low TiO₂.Tectonic discrimination diagrams suggest an extension-related magmatism, either in an arc or forearc setting, mainly belonging to the syncollision type and is hardly related to orogeny.They may be the main rocks formed in a rift system in the early tensional rifting stage.The process of tensional rifting is somewhat similar with the evolutionary process of the Red Sea, which has experienced rifting and sagging.The oceanic crust was come into being when the rift valley reached certain scale of width and depth.The granodiorite samples have also provided important evidence to define the boundary of the Southwestern Basin and the clues to study the expanding model.