Zambales ophiolite, Philippines I. Geology and petrology of the critical zone of the Acoje massif

The Acoje massif constitutes the northernmost massif of the Zambales Ophiolite Complex, an assemblage of relatively intact fragments of a Mesozoic oceanic lithosphere. The critical zone of the massif consists of interlayered dunite, harzburgite, lherzolite, wehrlite, pyroxenite and feldspathic units which grade upsection into gabrronorite and gabbro cumulates. This sequence occurs between highly deformed residual harzburgite and a 4 to 5.5 k thick cumulate and isotropic gabbro sequence. The critical zone can be subdivided into a 540 m thick ultramafic zone and a 390 m thick mafic (feldspathic) zone. The boundary between these zones is marked by the first appearance of cumulus plagioclase. The paragenetic sequence that best describes the cumulus mineralogy is chromite, chromite + olivine, chromite + olivine + orthopyroxene, chromite + olivine + clinopyroxene (± orthopyroxene), olivine + clinopyroxene + plagioclase (± orthopyroxene, ± amphibole). The lithologie and chemical (including f{hook}O₂) variation across the critical zone reflects multiple magma influx, assimilation of tectonized harzburgite, magma mixing, fractional crystallization, and intercumulus melt/crystal interactions in a sub-oceanic magma chamber. Therefore, this critical zone represents the basal cumulate sequence of the Acoje massif, and not the uppermost portion of the residual mantle. There is a general trend of decreasing of deformation from the bottom to the top of the critical zone. This is attributed to the rheological inhomogeneity of the partially crystallized basal cumulate sequence during deformation. Geothermometric estimates based on the Mg-Fe²⁺ exchange between olivine and spinel indicate equilibration to subsolidus temperatures.