GEOLOGY AND GEOCHEMISTRY OF THE RAPU-RAPU OPHIOLITE COMPLEX, EASTERN PHILIPPINES: POSSIBLE FRAGMENT OF THE PROTO-PHILIPPINE SEA PLATE

Abstract

The Cretaceous Rapu-Rapu Ophiolite Complex is a dismembered marginal basin ophiolite formed at an intermediate to fast spreading center. Limited subduction is manifested by the crystallization of pyroxene before plagioclase in the layered ultramafic cumulates. However, mafic cumulate rocks which exhibit the crystallization of plagioclase before pyroxene, consistent with low-water pressure conditions, could have formed in a mid-ocean ridge-like setting. Bulk-rock and trace-element analyses of volcanic-hypabyssal and gabbroic rocks show the predominance of a marginal basin geochemical signature. Spinel XCr ([Cr /Cr+Al] <0.60) suggests that the Rapu-Rapu peridotite and pyroxenite have mid-ocean ridge affinities. Fractional crystallization, and mantle-melt interaction together with melt replenishment, characterize the evolution of the ultramafic-mafic cumulate sequence; cryptic variation analysis reveals that the magma chamber operated as an open system. The Rapu-Rapu Ophiolite Complex, together with other fragments of the oceanic lithosphere exposed along the eastern Philippines, was probably derived from the proto-Philippine Sea plate. Copyright © 2006 by V. H. Winston & Son, Inc. All rights reserved.