Metastable growth of corundum adjacent to quartz in a spinel‐bearing quartzite from the Archaean Napier Complex, Antarctica

Abstract

In a granulite‐facies spinel‐bearing quartzite, corundum, orthopyroxene and sapphirine (and rarely cordierite and sillimanite) form partial rims separating spinel from quartz. Textures indicate the reactions: spinel + quartz = orthopyroxene + corundum, and spinel + quartz = orthopyroxene + sapphirine. Thus, corundum and sapphirine are produced by reactions involving quartz. The low Al‐content of the orthopyroxene (0.5–2.8 wt %) and low values for Mg–Fe distribution coefficient for spinel–sapphirine and spinel–orthopyroxene reflect low‐temperature conditions during formation of the reaction products. Absence of zoning in spinel and a constant Mg–Fe distribution coefficient for spinel–sapphirine and spinel–orthopyroxene, over a compositional range, indicate Mg–Fe equilibration. It is suggested that stable reactions such as spinel + quartz = cordierite or spinel + quartz = garnet + sillimanite were over‐stepped and that metastable reactions give rise to the anomalous juxtaposition of corundum + quartz.