The limpopo metamorphic belt, South Africa .2. Decompression and cooling regimes of granulites and adjacent rocks of the Kaapvaal craton

A thermobaric study of metapelitic samples collected from the Limpopo granulite belt and the juxtaposed low-grade Sutherland greenstone belt of the Kaapvaal Craton revealed that the exhumation of the granulite terrane from the lower crust toward the Earth's surface was a long and complicated geological process. The peak metamorphic conditions, T = 850 degrees C, P = 8.5 kbar, in the Limpopo Belt were reached at about 2670 Ma ago, followed by an uplift of the granulite facies rocks at a depth of about 13-14 km in the mid-continental crust. At the same time, the denser metabasites of the adjacent Sutherland greenstone belt were buried underneath high-grade rocks. Two kinds of P-T paths for the Limpopo granulite belt were established. The first one is the common decompression-cooling retrograde P-T path resulting from the previously described (van Reenen and du Toit, 1976) breakdown of the Grt + Qtz(1) assemblage producing Crd + Opx symplectitic structures. The second type of P-T path is deduced from the discovery of textures resulting from a new garnet growth after two isobaric reactions, 1/2Crd + 2Opx = Grt + 3/2Qtz, and 1/2Crd = 1/3Grt + 2/3Sil + 5/6Qtz (Perchuk et al., 1989). The isobaric cooling to the final temperature of about 590 degrees C manifests the final stage in the metamorphic evolution of the Limpopo belt. The above-mentioned P-T regimes suggest that the decompression and cooling were controlled by the geodynamic histories of individual granulite-facies blocks and their specific locations with respect to the tectonic contact with the Archean greenstone terrane. The heating and burial of underthrusted rocks of the Kaapvaal craton from a depth of about 6-7 km to a crustal level of 13-14 km is reflected by a counterclockwise P-T path. The subsequent joint exhumation of the granulites and the juxtaposed low-grade cratonic rocks to a depth of about 6 km is only recorded in the mineral equilibria of the wall rocks. The granulites are assumed to have been close to the surface during the early Proterozoic time. This is suggested by isotopic dating of subvolcanic granitoid veins and dikes that truncate the granulites in the Palmietfontein region. Their age is 2540 Ma (Barton and van Reenen, 1992).