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?

Abstract

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 quartzfeldspar Ô sillimanite Ô biotite Ô 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 + O 2 ? 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.

Review on "corundum+quartz" assemblage in nature: Possible indicator of ultra-high temperature conditions?

“…Therefore, if corundum (or quartz) crystallizes, the difference in Gibbs energy between the different assemblages could be smaller than the activa-tion energy of the reaction Quartz + Corundum Sillimanite which, could not therefore proceed. In the light of this last point, the most plausible assumption often proposed is that corundum is exsolved from hercynitic spinel in the presence of quartz (Waters, 1991;Guiraud et al 1996;Shaw and Arima, 1998;Mouri et al 2003). According to the experimental data a complete hercynite magnetite solid solution exists at temperatures of > 860°C and 2 kbar (Turnock and Eugster, 1962), above 800°C at 4 kbar (Lehman and Roux, 1986) and at 900°C 1000°C at 1 kbar (Mattioli and Wood, 1988).…”

“…The metamorphic conditions, are conspicuously restricted to the ultra high grade terrains recording temperature up to 1100°C at pressure of 7 to 12 kbars. It is worth noting that most rocks that are hosting the corundum + quartz assemblage are often associated in the field with high temperature magmatic intrusions such as anorthosites in the Namaqualand Metamorphic Complex (Mouri et al 2003) and the Ihouhaouene area, Hoggar Shield in Algeria (Ouzegane et al 2003) and mangerites in the Vestpolltind Fe Mn deposit, Lofoten Vesterålen, N. Norway (Krogh, 1977).…”

“…Corundum + quartz assemblage is observed in magnetite bearing rocks occurring in the high grade metamorphic zone in the core of the NMC (Mouri et al 2003). This zone is characterised also by the occurrence of spinel + quartz assemblage (Waters, 1991) as well as osumilite (Nowiscki et al 1995).…”

Metastable corundum + quartz + andalusite association in pelitic granulite from the Kerala Khondalite Belt, southern India

“…In nature, corundum + quartz ± aluminosilicate has been found to occur in high -grade metamorphic rocks (Guiraud et al, 1996;Shaw and Arima, 1998;Mouri et al, 2003;Tsunogae and van Reenen, 2006). Current thermodynamic analyses suggest that corundum + quartz should not be a stable assemblage under the geologically relevant conditions of pressure and temperature in Earth's crust and lithospheric mantle (e.g., Berman, 1988;Gottschalk, 1997;Holland and Powell, 1998).…”

“…(ii) Corundum is a partial melt restite that came into arbitrary contact with quartz, which later crystallized from the associated melt (Powers and Bohlen, 1985;Mouri et al, 2003).…”