Podiform chromitites in the lherzolite-dominant mantle section of the Isabela ophiolite, the Philippines

The Isabela ophiolite, the Philippines, is characterized by a lherzolite-dominant mantle section, which was probably formed beneath a slow-spreading mid-ocean ridge. Several podiform chromitites occur in the mantle section and grade into harzburgite to lherzolite. The chromitites show massive, nodular, layered and disseminated textures. Clinopyroxene (±orthopyroxene/amphibole) inclusions within chromian spinel (chromite hereafter) are commonly found in the massive-type chromitites. Large chromitites are found in relatively depleted harzburgite hosts having high-Cr# (Cr/(Cr + Al) atomic ratio = ∼0.5) chromite. Light rare earth element (LREE) contents of clinopyroxenes in harzburgites near the chromitites are higher than those in lherzolite with low-Cr# chromite, whereas heavy REE (HREE) contents of clinopyroxenes are lower in harzburgite than in lherzolite. The harzburgite near the chromitites is not a residual peridotite after simple melt extraction from lherzolite but is formed by open-system melting (partial melting associated with influx of primitive basaltic melt of deeper origin). Clinopyroxene inclusions within chromite in chromitites exhibit convex-shaped REE patterns with low HREE and high LREE (+Sr) abundances compared to the host peridotites. The chromitites were formed from a hybridized melt enriched with Cr, Si and incompatible elements (Na, LREE, Sr and H2O). The melt was produced by mixing of secondary melts after melt-rock interaction and the primitive basaltic melts in large melt conduits, probably coupled with a zone-refining effect. The Cr# of chromites in the chromitites ranges from 0.65 to 0.75 and is similar to those of arc-related magmas. The upper mantle section of the Isabela ophiolite was initially formed beneath a slow-spreading mid-ocean ridge, later introduced by arc-related magmatisms in response to a switch in tectonic setting during its obduction at a convergent margin.