Ultrahigh-temperature metamorphism of the Southern Marginal Zone of the Archean Limpopo Belt, South Africa

Abstract

We report ultrahigh-temperature (T > 950°C) peak metamorphic conditions for the Southern Marginal Zone of the Archean Limpopo Belt in South Africa. Pelitic granulites of the zone have a typical mineral assemblage of garnet + biotite + orthopyroxene + cordierite + quartz + K-feldspar + plagioclase ± sillimanite, to which the latest geothermobarometers are applicable. The revised P-T conditions were obtained from a ternary feldspar geothermometer using antiperthitic feldspars in leucocratic granulite (920-980°C), Al solubility in orthopyroxene in pelitic granulite (970-1020°C), and a revised garnet-orthopyroxene geothermometer corrected for retrograde Fe-Mg exchange (920-990°C). The estimated peak temperature of metamorphism is about 100°C higher than previously estimated based on conventional Fe-Mg exchange geothermometers (800-870°C), and indicates a high geothermal gradient of ∼35°C/km in Archean continental lower crust.