Abstract

The Cr content ofthe Lower and Critical Zones exposedin the northwestern sector of the Western Limb of the Complex is greatly in excess ofwhat the liquids now represented by the Lower, Critical, Main, and Upper Zones could have held in solution. Chromium has a limited solubility in mafic magmas, and the amount of Cr held in the chromitite layers, disseminated chromite, accessory chromite, and in the silicate phases of the Critical Zone (1.40% Cr2O3) exceeds the experimentally determined solubility by an order of magnitude. Adequate volumes of appropriateresiduarepresenting original (now Cr-depleted) source liquids appear to be missing from the sequence exposed, and the problem is exacerbated by exclusion of the Cr-poor lower Main Zone as a possible source. This raises questions regarding the shape of the magma chamber, the oxygen fugacity of the magmas, and the manner of magma emplacement. Problems in applying the magma-mixing hypothesis for the origin of chromitites are exposed. It is concluded that the magmas in the chamber were enriched by 3%chromite microphenocrysts derived from alower-level magma chamber.

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