Graciano P. Yumul

The Cretaceous Southeast Bohol Ophiolite Complex, Central Philippines: a highly disaggregated supra-subduction zone ophiolite

Abstract The Southeast Bohol Ophiolite Complex (SEBOC) represents a complete ophiolite sequence with a pelagic chert cap yielding an Early Cretaceous age. The residual peridotite suite of this ophiolite complex is dominated by harzburgite with rare occurrences of lherzolite and dunite pods. Layered ultramafic cumulate rocks, made up of clinopyroxenite, wehrlite, dunite, and massive to layered mafic cumulate rocks consisting of gabbro and norite, are present. Although disaggregated, the ophiolite complex includes a sheeted dike complex and a preponderance of sheet flows over pillow lavas in the volcanic section. Geologic observations such as gabbro pods included within residual harzburgite and deep-water sedimentary and volcanic rocks directly overlying the harzburgite have often been reported from the Mid-Atlantic Ridge and other slow-spreading centers. However, such situations are never encountered in the SEBOC. Geochemical data on SEBOC show that this ophiolite complex exhibits transitional Mid-Ocean Ridge basalt-island arc tholeiite geochemical affinities. This suggests formation in a subduction-related marginal basin. The field geological characteristics of SEBOC show that, aside from being a harzburgite ophiolite type, this crust–mantle sequence was formed in a fast-spreading center. Later tectonic events resulted in its emplacement as a forearc ophiolite.

Angat Ophiolitic Complex, Luzon, Philippines: a cretaceous dismembered marginal basin ophiolitic complex

Abstract The dismembered Angat Ophiolitic Complex (DAOC) is made up of pillow basalts with associated sediments, sheeted diabase dike complex and massive/layered gabbros. Available bulk chemistry and petrogenetic modeling suggests that olivine, clinopyroxene, plagioclase ± magnetite are the main fractionating phases of the rocks of this complex. These theoretical predictions are borne out by petrographic observations. The source material of this dismembered ophiolitic suite, assuming primary mantle composition and normal parent-daughter melting relationship, underwent at least 15–30% degrees of partial melting. Available trace element geochemistry, although manifesting MORB-island arc characteristics, do not unequivocally indicate a supra-subduction zone environment of formation. Preliminary evidence, however, suggests formation in a marginal basin environment for the DAOC.