Chrome spinels from the Ōura layered igneous complex, central Japan

Abstract

Chrome spinel is disseminated as a minor cumulus phase in olivine-rich ultramafic cumulates in the Ōura igneous complex, a series of layered mafic and ultramafic plutonic rocks. The Al 2O 3 content of the spinel increased markedly with proceeding crystallization of the magma. The Al spinel A magma distribution coefficient seems to have increased with the crystallization of chrome spinel. In view of experimental results obtained in the system forsterite-anorthite-diopside-quartz with MgCr 2O 4 spinel, the increase in the distribution coefficient may be interpreted in terms of enrichment of plagioclase component in the magma by olivine fractionation. The Cr (Cr + Al) ratio of late-separated chrome spinel is presumably an indicator of the chemical conditions of parental magma: the ratio decreases with increasing the plagioclase component content of magma on the normative mineral diagram plagioclase-clinopyroxene-orthopyroxene. The presence of highly aluminous chrome spinel in the Ōura igneous complex suggests that the magma was enriched in plagioclase and clinopyroxene components and depleted in orthopyroxene component. It was apparently similar to abyssal tholeiite magma and markedly different from the Great “Dyke” and Bushveld magmas which were rich in orthopyroxene component.