Early Cretaceous arc volcanic suite in Cebu Island, Central Philippines and its implications on paleo-Pacific plate subduction: Constraints from geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes

Abstract

The Philippine island arc system is a collage of amalgamated terranes of oceanic, continental and island arc affinities. Here we investigate a volcanic suite in Cebu Island of central Philippines, including basalt, diabase dike, basaltic pyroclastic rock and porphyritic andesite. LA-ICP-MS U–Pb geochronology of zircon grains from the porphyritic andesite and pyroclastic rock yielded ages of 126±3Ma and 119±2Ma, respectively, indicating an Early Cretaceous age. The age distribution of the detrital zircons from river sand in the area displays a peak at ca. 118Ma, close to the age of the pyroclastic rock. The early Cretaceous volcanic rocks in the central Philippines were previously regarded as parts of ophiolite complexes by most investigators, whereas the Cebu volcanics are distinct from these, and display calc-alkaline affinity and island arc setting, characterized by high LREE/HREE ratios and low HFSE contents. These features are similar to the Early Cretaceous arc basalts in the Amami Plateau and east Halmahera in the northernmost and southernmost West Philippine Basin respectively. Zircon Hf isotopes of the pyroclastic rocks show depleted nature similar to those of the Amami Plateau basalts, implying the subducted Pacific-type MORB as probable source. Zircon Hf isotopes of the porphyritic andesite show slight enrichment relative to that of the pyroclastic rocks and MORB, indicating subducted sediments as a minor end-member in the source. The Hf isotopic compositions of the volcanic rocks are also reflected in the detrital zircons from the river sands. We propose that the volcanic rocks of Cebu Island were derived from partial melting of sub-arc mantle wedge which was metasomatized by dehydration of subducted oceanic crust together with minor pelagic sediments. Within the tectonic environment of Southeast Asia during Early Cretaceous, the volcanic rocks in Cebu Island can be correlated to the subduction of paleo-Pacific plate. The Early Cretaceous volcanic suites in Cebu Island along with the Early Cretaceous arc volcanic rocks, ophiolites and boninites in the southern–western–northern margin of West Philippine Basin are inferred to have formed within single subduction zone before the opening of the Philippine Sea Plate.