Abstract

The oxide minerals in the Turnagain ultramafic complex, an Alaskan-type intrusion of possible Upper Triassic age in northwestern British Columbia, offer some important clues concerning the composition of magmas from which they separated and the chemical changes that they underwent during the cooling process. Chromite occurs as disseminations and vein-like concentrations in dunite, and as disseminations in wehrlite and olivine clinopyroxenite. Ilmenite is found in hornblendite. A small amount of primary magnetite is associated with intercumulus sulfide. Chrome spinel exhibits a very wide range of compositions. The ratio 100 Mg/(Mg + Fe2+) ranges from 69.7–3.6, and this is paralleled by a decrease in Cr2O3 (60.1–9.3%), and by increases in Fe2O3, TiO2, and MnO. Chromite rims are poorer in Cr and richer in Fe3+ than chromite cores. Magmatic differentiation was responsible for most of these variations. Subsolidus re-equilibration caused Mg/Fe2+ zoning in coexisting chromite and olivine, the ratio decreasing in chromite rims and increasing in rims of contacting olivine grains. Subsolidus re-equilibration was probably responsible for the enrichment in Mg of the minerals in a chromite-rich layer relative to those in the adjacent dunite. Disseminated chromite originated by cotectic crystallization; chromite-rich layers may have been deposited by magmatic turbidity currents. Several mineralogical and chemical features suggest that oxygen fugacity in the magmas was relatively low.

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