What underlies the Philippine island arc? Clues from the Calaton Hill, Tablas island, Romblon (Central Philippines)

We report here for the first time the occurrence of a high-temperature metamorphic/plutonic complex (amphibolites, metagabbros, hornblende pyroxenites and hornblendites) in Calaton Hill, Tablas island, Romblon, Central Philippines. The mineral assemblages and relic magmatic textures in these rocks imply apparent derivation from arc-related protoliths. Major element and trace element data are also comparable to those of gabbroic rocks in arc-related setting. Subsolidus re-equilibration under granulite to amphibolite facies is documented by the triple junctions between mineral phases in the different lithologies, the recrystallization of plagioclase and the presence of coronas around olivine with mineral assemblage of orthopyroxene + amphibole ± green spinel. The formation of hornblendite and the pervasive occurrence of amphiboles in the different lithologies are being attributed to the infiltration of a younger hydrous arc magma which also caused metamorphism and hybridization on the surrounding rocks. The characteristics of the Calaton Hill samples are comparable with those of the well-studied xenoliths from Ichinomegata, NE Honshu arc, Japan. We therefore interpret the Calaton Hill metamorphic/plutonic complex as representative of the lower crust underlying the Philippine island arc.