Fluids in granulites of the Southern Marginal Zone of the Limpopo Belt, South Africa

Abstract

The Southern Marginal Zone of the Limpopo Belt in South Africa is characterised by a granulite and retrograde hydrated granulite terrane. The Southern Marginal Zone is, therefore, perfectly suitable to study fluids during and after granulite facies metamorphism by means of fluid inclusions and equilibrium calculations. Isolated and clustered high-salinity aqueous and CO2(±CH4) fluid inclusions within quartz inclusions in garnet in metapelites demonstrate that these immiscible low H2O activity fluids were present under peak metamorphic conditions (800–850 °C, 7.5–8.5 kbar). The absence of widespread high-temperature metasomatic alteration indicates that the brine fluid was probably only locally present in small quantities. Thermocalc calculations demonstrate that the peak metamorphic mineral assemblage in mafic granulites was in equilibrium with a fluid with a low H2O activity (0.2–0.3). The absence of water in CO2-rich fluid inclusions is due to either observation difficulties or selective water leakage. The density of CO2 inclusions in trails suggests a retrograde P–T path dominated by decompression at T<600 °C. Re-evaluation of previously published data demonstrates that retrograde hydration of the granulites at 600 °C occurred in the presence of H2O and CO2-rich fluids under P–T conditions of 5–6 kbar and ∼600 °C. The different compositions of the hydrating fluid suggest more than one fluid source.