High-pressure phase relations have been examined for phlogopite+diopside with and without enstatite under vapor absent conditions in the pressure range of 5 to 13 GPa and in the temperature range of 1000 to 1300 ¿C. Phlogopite in these systems can be stable up to 6--7 GPa and decomposes through pressure-dependent reactions to crystallize phases including potassic amphibole. The experimental results suggest that phlogopite, which is one of main hydrous phases in the downdragged hydrated peridotite at the base of mantle wedge, plays an important role in the formation of magmas at the backarc side of a volcanic arc. The existence of potassic amphibole at higher pressure regions may imply the involvement of subduction component in magma generation in the region far away from the trench axis. ¿ American Geophysical Union 1990