Oceanic plateaus inevitably interact with trench–arc systems during oceanic subduction, but the subduction mechanisms are poorly understood. The Yap trench–arc system is an important part of the western Pacific subduction zone, and is undergoing ultra-slow subduction of the Caroline Plateau. As such, it is ideal for investigating the magmatic–tectonic processes of oceanic plateau subduction. In this study, we document the mineralogy and geochemistry of serpentinized peridotites from the Yap–Mariana Junction. These peridotites can be subdivided into two groups. Group I samples have intermediate to high whole-rock CaO contents and Al2O3/SiO2 values, low chromite Cr# (as low as 0.16) and high chromite Ga/Fe3+# values, and low pyroxene Mg# values relative to Mariana Forearc peridotites. These features are consistent with those of abyssal peridotites. Based on La/Sm ratios, two types of clinopyroxene are identified in group 1 samples. Melting models show that the light rare earth element (LREE)-depleted patterns of type 1 clinopyroxene can be produced by ~ 5% fractional melting or ~ 6% open-system melting of depleted upper mantle, while the nearly flat REE patterns of type 2 clinopyroxene are related to LREE-enriched melts. These clinopyroxene compositions record melt impregnation and reactions in the mantle beneath the Yap–Mariana Junction. In contrast, group 2 samples have similar compositions to forearc peridotites, such as low whole-rock Al2O3/SiO2 values, and high chromite Cr# (up to 0.73) and low chromite Ga/Fe3+# values. The different trends in plots of chromite Cr#–TiO2 and 100Ti/Fe3+#–Ga/Fe3+# indicate that the group 1 samples experienced reactions with mid-ocean ridge basalt-like melts, whereas the group 2 samples experienced reactions with island arc tholeiite-like melts. The Mg#, TiO2, and Al2O3 values of the melts in equilibrium with chromite in the group 1 samples are consistent with those of the Parece Vela Basin basalts, while those from group 2 samples define a compositional trend similar to arc-related volcanic rocks. Therefore, this region experienced a tectonic transition from a back-arc spreading ridge to a subduction zone. Compared with the Mariana Trench that is undergoing normal oceanic crustal subduction, the absence of volcanic rocks at the Yap Trench are the response to ultra-slow subduction of the Caroline Plateau. [ABSTRACT FROM AUTHOR]