Chemical Geodynamics of Granitoid Magmatism During a Pacific‐Philippine Sea Plate Transition in Southwest Japan

Abstract Granitoid magmatism along the western Pacific margin records interactions between subduction dynamics and crust–mantle processes; however, the links between plate reorganization and magma‐source evolution remain debated. Here we integrate U–Pb zircon geochronology with Pb–Sr–Nd–Hf isotope systematics to investigate Cretaceous–Paleogene granitoids in Southwest Japan. Zircon U–Pb ages define two discrete magmatic episodes at 110–60 Ma and 45–30 Ma, separated by a magmatic hiatus of ∼10–15 Myr. These granitoid groups exhibit distinct isotopic signatures, indicating derivation from isotopically distinct magma sources linked to the paleo‐Pacific (Izanagi) plate and the Philippine Sea plate, respectively. Isotope‐based mass‐balance modeling indicates higher sediment contributions to the older granitoids, with decreasing sediment input both landward and through time. The magmatic lull at ca. 52–40 Ma coincides with an abrupt isotopic shift and is interpreted to reflect plate reorganization, during which subduction of the paleo‐Pacific plate was replaced by a transform or highly oblique plate boundary associated with the northward migration of the proto–Philippine Sea plate. Independent constraints from convergence rates, sediment flux, and accretionary complex development support this interpretation. These results demonstrate that granitoid magmatism in Southwest Japan was fundamentally controlled by temporal changes in subducted lithosphere and sediment flux driven by plate reorganization, highlighting the sensitivity of arc magmatism to transient tectonic regimes.