Fluid inclusions in hydrated granulite facies rocks, southern marginal zone of the Limpopo Belt, South Africa

Abstract

The Archean granulite terrane of the southern marginal zone of the Limpopo belt, South Africa, was rapidly uplifted and thrust over a low grade granite-greenstone terrane ( Van Reenen et al., 1987). During cooling the granulites were rehydrated in a zone parallel to the contact with the low grade terrane. A sharply defined retrograde orthoamphibole isograd at conditions of >6 kbar, 625°–650°C, X CO 2 = 0.8 ( Van Reenen, 1986) was established. We made microthermometric analyses of over 1000 fluid inclusions in four samples, representing a traverse across the isograd, with the goal of characterizing the hydrating fluid. We found patterns of composition and density of inclusions to be similar to those reported from other granulite facies terranes: a few apparently pure CO 2 inclusions with densities appropriate for the P− T conditions, many CO 2 inclusions with lower densities, and aqueous inclusions of variable salinity and containing no detectable CO 2. The results suggest that the hydrating fluid may be represented by secondary CO 2-rich fluid inclusions. Because of their small size (most between 5 and 10 μm) and equant shapes, H 2O contents of the CO 2-rich inclusions of up to 0.3 mole fraction may not be detectable. A model for hydration of the granulites includes infiltration of a fluid into the partially cooled overthrust granulites with a temperature gradient decreasing towards the underthrust greenstone terrane. The fluid was probably generated by devolatilization of the greenstones as they were heated by the overthrust granulites. The high CO 2 content of the calculated fluid and of the fluid inclusions suggests the hydrating fluid was CO 2-rich at the P− T conditions of the retrograde isograd.