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Interaction of a subduction zone with an oceanic plateau has implications for plate tectonics. However, the geodynamic processes and petrological responses to oceanic plateau–arc interactions remain enigmatic. The southwestern Mariana and Yap arcs have experienced interactions with the Caroline Plateau, which have affected the regional tectonism. In this study, tholeiitic basalts and metamorphosed volcanic rocks (i.e., greenstones) were recovered from the southwestern Mariana forearc. The protoliths of the metamorphosed volcanic rocks have geochemical affinities to low-silica boninites. These boninitic rocks have similar K–Ar and apatite U–Pb ages of ca. 24 Ma, which record the timing of collision between the southwestern Mariana arc and the Caroline Plateau. 40Ar/39Ar dating of plagioclase in the tholeiitic basalts dates post-collisional magmatism to ca. 18 Ma. The tholeiites have geochemical signatures of fore-arc basalts (e.g., low Ti/V ratios and light rare earth element-depleted patterns) and high Th/Yb ratios, which reflect a depleted mantle source with a subduction component inherited from a pre-collisional subduction event. We suggest that the southwestern Mariana arc is an intra-oceanic arc that underwent plateau–arc collisions. These plateau–arc interactions affected the tectonic and magmatic evolution of the southwestern Mariana arc and nearby western Pacific basins.