Constraints on the composition of fluids involved in retrograde anthophyllite formation in the Limpopo Belt, South Africa

Orthoamphibole occurs widely in the metapelitic gneisses and other lithologies within the Southern Marginal Zone of the Limpopo Belt in southern Africa. Petrographic relations indicate that the orthoamphibole formed as a retrogressive replacement of the granulite facies minerals cordierite and orthopyroxene. The existence of a discrete retrograde orthoamphibole isograd that can be mapped for over 150 km has led previous workers to suggest that the orthoamphibole formed as a result of equilibrium high-temperature reactions which occurred in response to regional influx of fluids at the close of the Limpopo Orogeny, 2670 Ma ago. However, in order to substantiate this interpretation, it is necessary to show from phase equilibrium analysis that the metapelitic minerals and their retrograde products comprise assemblages stable at elevated pressure and temperatures, rather than disequilibrium assemblages formed at unspecified lower temperatures by monotropic reactions. Some of the properties of the infiltrating fluids may, in turn, be specified if the alteration products represent equilibrium assemblages. It is shown that the presence of anthophyllite and the absence of talc in two analysed metapelites from the retrograde isograd require an associated vapour phase to have been relatively low in water (a_H2O ≤ 0.7) and temperatures to have been below 620°C when the anthophyllite developed from the orthopyroxene. The fast kinetics of formation of talc and the relatively sluggish kinetics of formation of anthophyllite in experimental systems make it highly unlikely that the orthoamphibole could have formed metastably at low temperatures and high water activities. A more general phase equilibrium analysis based on the coexistence of cordierite, garnet, biotite, enstatite, plagioclase and quartz with retrograde anthophyllite and kyanite indicates that this assemblage was in equilibrium at temperatures between 450 to 650°C for pressures of 5 to 7 kbars and water activities between 0.6 and 0.1. The temperatures, pressure and water activities are highly correlated, and lower pressures and water activities are required for equilibrium at lower temperatures. The reduced water activities calculated require that a substantial quantity of another volatile be present in the vapour phase; in the absence of stable scapolite or carbonates, this volatile cannot be confirmed to be CO₂. The required infiltration of late-orogenic fluids with reduced water activities is apparently a major feature of the Limpopo event, and may have parallels in other large Precambrian high grade terrains, in as much as orthoamphibole replacing orthopyroxene is a commonly observed mineral assemblage.