Blocks of garnet–glaucophane–quartz schist and gneissic garnet–actinolite schist were found as float adjacent to a serpentinite exposure on the Saganoseki Peninsula, Oita Prefecture. These metamorphic blocks are inferred to have been derived from a serpentinite-matrix mélange exposed in the area. Phengite K–Ar dating of the garnet–glaucophane–quartz schist yielded an age of 94.1 ± 2.1 Ma, indicating that this rock belongs to the Sanbagawa metamorphic rocks or another Cretaceous metamorphic unit. Pressure–temperature conditions estimated by phase-equilibrium modeling for the garnet–glaucophane–quartz schist are approximately 490 °C and 2.1 GPa. These data suggest that the distribution of blueschist–eclogite transitional facies in the Cretaceous metamorphic unit extends to the eastern margin of Kyushu.