A comment on ultrahigh-temperature metamorphism from an unusual corundum + orthopyroxene intergrowth bearing Al–Mg granulite from the Southern Marginal Zone, Limpopo Complex, South Africa, by Belyanin et al.

Abstract

Studies on the Southern Marginal Zone (SMZ) of the Limpopo Belt have generally concluded that this terrain was affected by a single granulite-facies metamorphic event at 2.67–2.66 Ga (Kroner et al. 2000; Kreissig et al. 2000; Zeh et al. 2005; Elington and Armstrong 2004; Stevens and Van Reenen 1992a, b; Barton and van Reenen 1992; Barton et al. 1992; Van den Berg and Huizenga 2001; Rigby et al. 2008), with peak metamorphic conditions of 7.5–9.5 kbar and 800–850 C. In their recent paper, Belyanin et al. (2012) propose an ultrahigh-temperature (UHT) metamorphic event in the SMZ, with peak metamorphic conditions in excess of 1,000 C at approximately 12 kbar. Similar conclusions were proposed by earlier work on the same rock (Belyanin et al. 2010). The proposed UHT conditions are based on Al-rich orthopyroxene and ternary feldspar thermometry using reintegrated feldspar compositions. This evidence is derived from a single outcrop containing unusual aluminous layers within metasedimentary granulites, although Tsunogae et al. (2004) used Al-in-orthopyroxene and ternary feldspar thermometry to argue for a peak metamorphic temperature of [950 C from more representative metapelitic granulites of the Bandelierkop formation. Both these studies have proposed that UHT metamorphism affected the SMZ generally and thus that the relevant PT path of all rocks traversed UHT conditions. This interpretation is problematic because it is difficult to reconcile with the partial melting history of the rocks and with the peak metamorphic assemblage in most rocks. In essence, both the metasedimentary granulites of the Bandelierkop formation and the trondhjemitic to tonalitic Bavianskloof gray gneisses contain a significant proportion of biotite, which defines a syn-peak metamorphic fabric (Fig. 1). Within the metasediments of the Bandelierkop formation, compositional banding at the outcrop scale (Fig. 1) exists that is considered to represent primary differences in composition between different sedimentary units (van Reenen 1983). Anatexis of these rocks has occurred via three reactions (Stevens and van Reenen 1992a): Ms ? Qtz ? Pl = Sil ? Melt; Bt ? Sil ? Qtz ? Pl = Grt ? Melt; and Bt ? Qtz ? Pl = Opx ? Crd ? Melt. The first two reactions exhausted muscovite and sillimanite, respectively, while the third reaction had just begun to occur in the chemically most favorable rocks at the conditions of peak metamorphism. Pseudosection modeling of metapelitic granulites from the Bandelierkop quarry, which have undergone anatexis and that represent restitic, melt-depleted compositions, indicates that biotite in such rocks would not have survived temperatures significantly above 900 C and that the mineral compositions in such rocks are consistent with peak metamorphic temperatures of 820–860 C at 8–10 kbar (Fig. 2). At the peak metamorphic conditions proposed by Belyanin et al. (2012), these restitic granulites would have become Communicated by T. L. Grove.