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

Corundum + quartz as an assemblage has been reported in a limited number of high to ultra-high grade metamorphic terrains. It is worth noting that rocks hosting this unusual assemblage have common features with respect to mineral assemblages, textures, metamorphic conditions and geological environments. In addition to corundum and quartz, these rocks are characterised by variable amounts of magnetite, spinel, ilmenite together with rutile and sillimanite/kyanite in Fe-rich rocks; and sapphirine, orthopyroxene, garnet and cordierite in more Mg-rich rocks. These rocks are characterised in general by (i) coarse-grained texture with inclusions of corundum and spinel in magnetite, (ii) sharp contact between corundum and quartz and/or separated by a thin rim of sillimanite or kyanite between these two minerals, (iii) presence of spinel + quartz, sapphirine + quartz in equilibrium as well as osumilite. Although there has been some discussions in the past concerning whether "corundum + quartz" assemblage represents a "thermodynamic" stable or metastable assemblage, it is clear that this assemblage observed mainly in the magnetite-ilmenite-spinel bearing rocks is "texturally" stable. On the basis of the available experimental and thermodynamic data as well as natural occurrences, we conclude that this assemblage is consistent with ultra-high temperature metamorphic environments (up to 1100°C at pressure of 7 to 12 kbars). Therefore, its presence in natural rocks should draw close attention from field metamorphic petrologists working on deep crustal processes.