Initiation of subduction and the generation of slab melts in western and eastern Mindanao, Philippines

Abstract

Adakite, found in both the eastern and western parts of Mindanao Island, Philippines, is a rare rock type, characterized by low heavy rare earth elements and Y contents together with high Sr/Y ratios, and is considered to be the result of the melting of young subducted oceanic crust, which leaves an eclogite residue. Pliocene-Quaternary adakites from western Mindanao (Zamboanga Peninsula) are probably derived from the melting of the young Miocene Sulu Sea crust, which is currently subducting beneath Zamboanga. Associated Nb-enriched basalts are thought to come from mantle metasomatized through interaction with adakitic melts. In eastern Mindanao, Pliocene-Quaternary cones and plugs of typical adakitic composition mark the trace of the Philippine fault in Surigao and north Davao. The underlying Philippine Sea crust is of Eocene age and therefore cannot melt under normal subduction thermal conditions. 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 along the Philippine fault.