Coexistence of high-Al and high-Cr chromite orebodies in the Acoje block of the Zambales ophiolite, Philippines: Evidence for subduction initiation

Abstract

The Acoje block of the Eocene Zambales ophiolite (NW, Philippines) is a fragment of forearc lithosphere in the Western Pacific and hosts a world-class chromite deposit. The compositions of chromitites, dunites and harzburgites in the block are used to explore origin of ophiolitic chromite deposits during subduction initiation. Chromite grains in these rocks have a large range of Cr# [100 × Cr/(Cr + Al)]. Those in the cpx-rich harzburgites have the lowest Cr# range (15–35). Those in the chromitites have a bimodal Cr# distribution, ~45–48 and 73–76, confirming presence of both high-Al and high-Cr chromitites in the block. Those in the cpx-poor harzburgites and dunites have similar Cr#s, ranging from ~35 to 85. The TiO2 contents of chromite grains are generally <0.25 wt% and increase from the cpx-rich harzburgites to cpx-poor harzburgites, dunite and chromitites. Olivine grains in all the rocks have forsteritic compositions (Fo, 90–93) and δ18O values consistent with that of the upper mantle (+5.18 ± 0.20). The oxygen fugacities (fO2) of the harzburgites, dunites and chromitites range from ~−0.5 log unit below the fayalite-magnetite-quartz (FMQ) buffer to ~1.5 log unit above the buffer, generally comparable to or slightly higher than the range of abyssal peridotites. The dataset reveals that parental magmas of the high-Cr chromitites have boninitic affinities, whereas those of the high-Al chromitites have compositions intermediate between MORB and boninitic lavas. Both types of chromitites originated from mantle sources that may have moderately refractory compositions equivalent to residues after ~10% melting of fertile MORB mantle, accounting for the low TiO2 contents of chromite in the chromitites. The Acoje mantle wedge was infiltrated by small amounts of slab-derived fluids, which were insufficient to elevate the fO2 values of peridotites but notably facilitated partial melting in mantle sources of the chromitites. The Acoje mantle sequence was modified by parental magmas of both the high-Al and high-Cr chromitites, causing two different chemical variation trends of peridotites. Peridotites and chromitites in the Acoje mantle sequence record evolving magmatism changing from early MORB-like to later boninitic features, comparable to those developed in the infant IBM arc system, and relevant information can be used for reconstructing the geodynamic evolution of subduction initiation.