INITIATION OF SUBDUCTION AND THE GENERATION OF SLAB MELTS IN WESTERN AND EASTERN MINDANAO, PHILIPPINES

Adakite, found in both the eastern and western parts of Mindanao Islan d, Philippines, is a rare rock type, characterized by low heavy rare e arth elements and Y contents together with high Sr/Y ratios, and is co nsidered to be the result of the melting of young subducted oceanic cr ust, which leaves an eclogite residue. Pliocene-Quaternary adakites fr om western Mindanao (Zamboanga Peninsula) are probably derived from th e melting of the young Miocene Sulu Sea crust, which is currently subd ucting beneath Zamboanga. Associated Nb-enriched basalts are thought t o come from mantle metasomatized through interaction with adakitic mel ts. In eastern Mindanao, Pliocene-Quaternary cones and plugs of typica l adakitic composition mark the trace of the Philippine fault in Surig ao and north Davao. The underlying Philippine Sea crust is of Eocene a ge and therefore cannot melt under normal subduction thermal condition s. Thermal models indicate that melting at the start of subduction can occur. Subduction of the Philippine Sea plate began 3 to 4 Ma beneath eastern Mindanao and probably accounts for the presence of adakites a long the Philippine fault.