Two volcanoes on the Quaternary volcanic front in southwest Japan have erupted medium-K andesite and dacite, which are chemically similar to adakites (i.e., high Al2O3, Sr/Y, low Y, and steep rare earth element [REE] patterns with no negative Eu anomalies), unlike other subduction-related volcanoes in Japan. Earthquake data and the extrapolated position of the Philippine Sea plate from its near-trench position indicate that the leading edge of the plate is ∼75–100 km deep beneath these volcanoes. Melting of the plate at this depth would leave a garnet eclogite residual, accounting for the Y and heavy REE depletion. The lack of seismic data and active volcanism in southwest Japan can be attributed to the attenuated seismic response of the plate due to melting and to the atypical nature of magma generation.

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