SLAB MELTING AS AN EXPLANATION OF QUATERNARY VOLCANISM AND ASEISMICITY IN SOUTHWEST JAPAN

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 subductio n-related volcanoes in Japan. Earthquake data and the extrapolated pos ition of the Philippine Sea plate from its near-trench position indica te that the leading edge of the plate is similar to 75-100 km deep ben eath these volcanoes. Melting of the plate at this depth would leave a garnet eclogite residual, accounting for the Y and heavy REE depletio n. The lack of seismic data and active volcanism in southwest Japan ca n be attributed to the attenuated seismic response of the plate due to melting and to the atypical nature of magma generation.