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Garnet-clinopyroxene amphibolites occur in the mylonite unit structurally at the top of the Takahama metamorphic rocks — a member of the Nagasaki Metamorphic Rocks, western Kyushu, SW Japan. Geothermobarometry applied to an equigranular garnet-clinopyroxene amphibolite, which escaped severe mylonitization, yields the metamorphic condition of 770 ± 70°C and 1.1 ± 0.2 GPa. This condition is comparable with the high-pressure granulite facies, and is much higher-grade than those of the underlying crystalline schists of the Takahama metamorphic rocks that belong to the epidote-glaucophane schist subfacies. Despite the presence of such a gap in metamorphic condition between the crystalline schists and the mylonite unit, nearly identical muscovite K-Ar ages have been obtained from these units, which indicate that both units were juxtaposed at depth before they cooled below the closure temperature of the muscovite K-Ar system.