First occurrence of the rare 'corundum + quartz' assemblage in the high-grade zone from the Namaqualand Metamorphic Complex, South Africa: Evidence for higher-P, T metamorphism?

We report in this paper the first occurrence of the rare assemblage corundum + quartz in the high-grade metamorphic zone in the core of the Namaqualand Metamorphic Complex (NMC) (South Africa). The magnetite-bearing rocks hosting the corundum-quartz assemblage are very rare, and occur discontinuous bands a few cm wide, associated with diorite, tonalite and anorthosite within quartz-feldspar ± sillimanite ± biotite ± spinel supracrustal rocks extensively intruded by a foliated granite gneiss. The magnetite-bearing rocks are characterized by the presence of abundant crystals of magnetite together with ilmenite, spinel and large (∼1 cm in diameter) euhedral crystals of corundum both displaying a sharp contact with magnetite and quartz. However, in some micro-domains, spinel and quartz, as well as corundum and quartz, are separated by a thin corona of sillimanite with or without magnetite. These textural features suggest that the following reactions have taken place: quartz + corundum → sillimanite, and quartz + spinel + O₂ → sillimanite + magnetite. These reactions are interpreted as occurring at higher, as yet unknown, P-T conditions than previously estimated for the NMC. Whether corundum and quartz were once in equilibrium is worth investigating, as it could constitute a geothermobarometric assemblage for ultra-high T granulites.