Onramping of cold oceanic lithosphere in a forearc setting: The Southeast Bohol Ophiolite complex, Central Philippines

Abstract

Geochemical and mineralogical data show that the Early Cretaceous Southeast Bohol Ophiolite Complex on Bohol island, Central Philippines, was generated in a subduction-related marginal basin. The volcanic-hypabyssal rocks show negative Nb, Zr, and Ti anomalies, suggesting an island-arc affinity. This signature is consistent with the high anorthite contents of the gabbro and norite plagioclases, although the ultramafic rocks contain low XCr [Cr/(Cr + Al) < 0.60] spinels. The ophiolite is thrust on top of a tectonic mélange, the Cansiwang Mélange, which in turn is thrust over the Alicia Schist, a metamorphosed oceanic crust sliver made up of chlorite schist, amphibolite schist, and quartz-sericite schist. The presence of serpentinite between the ophiolite and the Alicia Schist indicates that the amphibolite schist cannot represent the emplacement-related metamorphic sole of the Southeast Bohol Ophiolite Complex. The composition of the associated serpentinite sole (Cansiwang Mélange) of the ophiolite complex suggests that this crust-mantle sequence was emplaced at a relatively low temperature (<500°C). The onramping of the Southeast Bohol Ophiolite Complex as a forearc ophiolite, followed by its collision and suturing with the Alicia Schist, occured during the Late Cretaceous. This ophiolite could represent a fragment of the Cretaceous proto-Philippine Sea plate.