Abstract
► Two types of podiform chromitite, concordant and discordant, were examined. ► They are contrasted in mineral chemical characteristics and inclusions in spinel. ► The involved melt was quite different between the two types of chromitite. ► The inclusions in spinel indicate a tectonic setting for formation chromitite.
Two types of podiform chromitite, concordant and discordant, were examined in the mantle section of northern Oman ophiolite along Wadi Hilti, to revisit the structural classification of podiform chromitite. They are contrasted in mineral chemical characteristics, in addition to the difference in attitude; the Cr/(Cr+Al) atomic ratio of spinel is around 0.6 for the concordant chromitite and surrounding peridotites, but is around 0.7 for the discordant one and surrounding peridotites. Chromian spinel grains contain pargasite-rich inclusions of primary origin from the both types, but they are far less abundant and smaller in size in the concordant chromitite than in the discordant one. Thin lamellae of pyroxenes in chromian spinel, similar to those in ultrahigh-pressure (UHP) chromitites from Tibet, are available only from the concordant chromitite. The dunite enveloping the concordant chromitite is extraordinarily high in NiO (up to >0.5wt.%), suggesting subsolidus Ni diffusion from the chromitite. The involved melt was quite different between the two types of chromitite; the melt to precipitate the discordant one was more hydrous than that for the concordant one because of far more abundance of hydrous minerals in the former. The difference in duration of subsolidus cooling, and probably decompression, is prominent between the two types of chromitite. The concordant chromitite cannot be formed from the discordant one simply by metamorphic conversion: the former is of deep magmatic origin whereas the latter, of shallow magmatic origin.