Abstract: The South China Sea (SCS) is one of the largest marginal seas in the western Pacific. Its formation and evolution are of great significance to plate tectonic studies, particularly for understanding the formation and evolution of the western Pacific subduction zone. However, the dynamic background and mechanisms behind the formation of the SCS remain unclear. The oceanic crust basement rocks recorded critical information of the formation mechanism of the SCS. Resolving the diversity in petrogenesis and geochemistry of these oceanic crust basement rocks offers important clues to uncover the formation and evolution of the SCS. We conducted a geochemical comparison of oceanic crust basalts from the SCS with those from typical back-arc basins (such as the Mariana Trough and the Japan Sea) and mid-ocean ridge basalts (MORB) from open oceans. Compared to MORB from open oceans, the oceanic crust basalts of the SCS exhibit higher Al₂O₃ and K₂O contents, lower FeO^T and TiO₂ contents, and similar trace elemental patterns. In contrast to typical back-arc basin basalts, the SCS basalts have higher MgO and CaO contents but different trace element patterns, indicating the unique geochemical characteristics of the SCS basalts. By excluding the effects of magmatic processes such as fractional crystallization, crustal contamination, and partial melting on the geochemical variations of the oceanic basalts, we further believe that the lithological and geochemical heterogeneity of the mantle source are the primary factors controlling the geochemical diversity of the SCS oceanic crust basalts, which distinguish them significantly from the source regions of typical back-arc basin basalts that are controlled by the evolution of small mantle wedge. In addition, by comparing the elemental and isotopic geochemical compositions of the SCS oceanic crust basalts with those of the Cenozoic intraplate basalts in SE China that formed under the influence of the big mantle wedge, we further revealed that the enriched mantle components within the mantle source of the SCS oceanic crust basalts exhibits similarities to those of the SE China basalts. Therefore, unlike typical back-arc basins that formed through geological processes involving a small mantle wedge, the formation and evolution of the SCS were primarily influenced by material circulation in the context of the big mantle wedge. The deep dynamic processes and formation of the SCS basin should be closely related to the evolution of a big mantle wedge.