Remarkably fresh abyssal peridotites from Sibuyan island, Romblon Island Group, Philippines: Markers of young arc-continent collision

Abstract The bulk of Sibuyan island in the Romblon Island Group, Philippines is underlain by remarkably fresh spinel lherzolites, harzburgites and dunites collectively called as the Sibuyan Ultramafics. Based on petrographic and geochemical characteristics, the Sibuyan Ultramafics can be classified into: (1) Group I peridotites composed of spinel lherzolites and harzburgites with spinel Cr# 0.75). Increase in spinel Cr# is accompanied by decreasing Al2O3 contents and increasing Mg# in the pyroxenes from the Group I to Group II and Group III peridotites. Chondrite-normalized clinopyroxene data revealed various trace-element patterns for the three groups suggestive of different processes (e.g. anhydrous and hydrous melting, mantle-melt interaction) involved in their formation. The Sibuyan Ultramafics is closely associated with the similarly fresh Calaton Hill metamorphic/plutonic complex in Tablas island which is thought to be representative of the lower crust of the Philippine island arc. At a more regional context, the Sibuyan Ultramafics and the Calaton Hill metamorphic/plutonic complex can be used as markers for the young arc-continent collision in western Philippines. Rapid uplift and emplacement due to the collision of the Philippine Mobile Belt and the Palawan Microcontinental Block during the Miocene or even later may have led to the remarkably fresh state of these lithologies. This occurrence is similar to the Horoman peridotite complex in the Hidaka metamorphic belt in Northern Japan. The Sibuyan Ultramafics and the Calaton Hill metamorphic/plutonic complex therefore provide a rare and fresh glimpse into the upper mantle and lower crust in the Philippines due to their young ages of emplacement.