STRUCTURAL GEOLOGICAL AND METAMORPHIC SIGNIFICANCE OF THE KAAPVAAL CRATON LIMPOPO BELT CONTACT

The Hout River Shear Zone forms the terrane boundary between the granite-greenstone terrane of the Kaapvaal Craton (Cratonic terrane) and the high-grade granulite terrane (high-grade gneiss province) of the Limpopo Belt. This structure is superimposed onto a thin-skinned northward verging thrust system which exercises a significant control on the distribution of the ENE-WSW trending greenstone belts. The Hout River Shear Zone, however, dips steeply to the north and flattens out toward the south. This structure was responsible for the emplacement of hot granulites against and over the cooler granite-greenstone terrane and strongly influenced the metamorphic evolution of the area. The shear zone acted as a conduit for infiltrating CO2-rich, H2O-bearing fluids into still hot crust producing a retrograde isograd and associated zone of rehydration in the granulitic hanging wall of the structure. Retrogressed granulites in the hanging wall are characterized by fine-grained kyanite as part of the retrograde assemblages, while the greenstones in the footwall are characterized by coarse-grained prograde kyanite. The terrane boundary appears to be in thermal equilibrium with the lithologies in both the footwall and hanging wall so that tectonically mixed prograde greenstones and retrogressed granulites in and adjacent to the shear zone are apparently at a similar metamorphic grade. The granulites appear to have had a "hot iron" effect on the cooler footwall rocks over which they have been displaced.