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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?

H. Mouri¹, M. A. G. Andreoli^2,, J. R. Kienast³, M. Guiraud⁴ and S. A. de Waal¹

¹ Department of Geology, University of Pretoria, South Africa
² South African Nuclear Energy Corporation, P.O. Box 582, Pretoria 0001, South Africa
³ Laboratoire des Geosciences Marine, Université Paris 7, France
⁴ USM 201 Minéralogie, FRE CNRS 2456, Dpt Histoire de la Terre, MNHN Paris, France

^* E-mail: hmouri{at}postino.up.ac.za

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.

KEYWORDS: corundum, quartz, Namaqualand Metamorphic Complex, South Africa

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