Magmatic constraints from intrusive rocks in western Philippines: Implications for incipient island arc formation and Eocene tectonics in Southeast Asia

Ophiolites in the Philippines represent either emplaced marginal basins or relicts of fully consumed oceanic lithospheres, which provide key insights on the tectonic evolution of Southeast Asia. The Zambales Ophiolite Complex (ZOC), which serves as the basement of western Luzon, consists of a complete Penrose-type sequence of an Eocene oceanic lithosphere. The intrusive section of the Acoje block of the ZOC includes gabbroic rocks, diabases, and plagiogranites exposed in northern Zambales and Pangasinan. Petrographic and mineral chemistry analyses of the intrusive rocks indicate formation in a hydrous island arc environment. In particular, bulk rock analysis reveals an island arc tholeiite affinity and suggests an advancement in magmatic evolution (i.e., from boninitic to tholeiitic) as subduction progressed. Consequently, the intrusive rocks which forms part of lower to upper crust section corresponds to the boninitic and tholeiitic extrusive rocks mapped in the volcanic section of the Acoje block of ZOC. U-Pb age dating of a gabbro from Sual and an olivine-bearing gabbronorite from Bugallon yielded lower intercept age of 44.12 ± 0.21 Ma and concordant age of 45.02 ± 0.55 Ma, respectively.In contrast to other marginal basins and ophiolitic terranes in Southeast Asia, geochemical and geochronological characteristics of the northern Zambales and Pangasinan intrusive rocks indicate a similar origin as the Mineoka-Setogawa Belt plutonic rocks and other Izu-Bonin-Mariana Arc tholeiites and calc-alkaline rocks (i.e., formation in supra-subduction zone settings). Collated Pb isotope data suggest a common mantle source for the Acoje block of the Zambales Ophiolite Complex, Celebes Sea, and West Philippine Basin. This study therefore proposes that the Zambales Ophiolite Complex likely formed from spreading in the western edge of the West Philippine Basin due to the subduction of the East Asian Sea slab, which was likely located between the Celebes Sea and the West Philippine Basin during the middle Eocene. Additionally, the occurrence of arc tholeiites and boninites in both the Zambales Ophiolite Complex and the Izu-Bonin-Mariana Arc also supports a double-vergent subduction initiation in the fringes of the Philippine Sea Plate.