Abstract
Thin discordant gabbronorite veins (∼ 0.5 mm to 3 cm in width) occur within remarkably fresh harzburgite boulders of the Sibuyan Ultramafics. The harzburgite host rock displays protogranular to porphyroclastic textures and is dominantly composed of olivine, orthopyroxene with minor amounts of clinopyroxene, spinel and amphibole. The mineral chemistry of the harzburgite is comparable to depleted abyssal peridotites as shown by the Fo content of the olivine (= 90-91) and Cr# of the spinel (= 0.40-0.52). The gabbronorites are coarse-grained adcumulates and comprised of orthopyroxene, plagioclase and amphibole. Plagioclase in the gabbronorites shows high An content whereas both orthopyroxene and amphibole show variable Mg# almost similar to reported values from arc gabbros. The contact between the harzburgite and the gabbronorite veins is demarcated by the formation of secondary orthopyroxene with low Cr2O3 and CaO contents. Clinopyroxene in the harzburgite shows strong enrichment in light rare earth elements (LREEs). Amphibole rimming the clinopyroxene in the harzburgite has similar patterns as the clinopyroxene but with much higher REE abundance. The amphibole in the gabbronorites also shows enrichment in LREEs, Rb, Ba and Ti. We propose that the harzburgite-gabbronorite occurrence in the Sibuyan Ultramafics is a product of mantle-melt reaction. The metasomatic agent is a silicate melt enriched in Si, Cr, Fe and LREEs. At a bigger scale, the harzburgite-gabbronorite connection observed in the Sibuyan Ultramafics possibly documents early stage modification and conversion of abyssal peridotites to ophiolitic peridotites by SSZ-related melts.
