Blocks of garnet-glaucophane-quartz schist and gneissic garnet-actinolite schist occur as float near 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 yields an age of 94.1 ± 2.1 Ma, indicating that this rock belongs to the Sanbagawa metamorphic rocks or their equivalents. Pressure–temperature conditions estimated by phase-equilibrium modeling for the garnet-glaucophane-quartz schist is approximately 490 °C and 2.1 GPa. These data suggest that the distribution of blueschist–eclogite transitional facies Sanbagawa metamorphic rocks extends to the eastern margin of Kyushu.