The geological history of the Limpopo Belt attests to the importance of structural, metamorphic and geohydrological controls on a retrograde amphibolite-granulite transition. Regional high temperature fluid infiltration and intense metasomatic alteration (including gold mineralization) in the Southern Marginal Zone can be shown to be related to the exhumation of the Limpopo Belt in the Late-Archaean.
The exhumation of the granulite terrane of the Southern Marginal Zone was controlled by a regional system of deep crustal (granulite facies) shear zones that developed about 2.67 Ga ago in response to continued compression across the Limpopo Belt as the result of the collision of two cratons. Ductile fault rocks (straight gneisses), which characterize these deep seated faults, can be shown to have developed via a shear-related stage of grain size reduction and intense fabric formation under granulite facies conditions.
The Limpopo Belt provides evidence from the regional, through outcrop, to microscopic scale of interaction of granulites with fluids at high temperatures (Van Reenen, 1986; Hoernes et al., 1995). The most striking example of this phenomenon is the regional distribution of the zone of retrogressed granulites which is situated in plan between a retrograde isograd in the north and a major terrane boundary in the south. This steeply northward-dipping shear zone is the surface along which granulites have been exhumed and were thrust onto
the Kaapvaal Craton to the south. The zone in which the rehydration of granulites took place is controlled by and spatially related to this shear zone.
The retrograde isograd is accurately delineated in the field by a discontinuous hydration reaction mapped in pelitic granulites (opx + qz + w = at). Cordierite-bearing granulites on the isograd are also characterized by a continuous hydration reaction (crd + qz + w = ged. + ky.). Completely hydrated and recrystallized pelitic granulites south of the isograd consist of garnet-orthoamphibole-biotite-quartz-plagioclase assemblages with small amounts of kyanite/sillimanite. Reaction textures preserved along the isograd clearly indicate the introduction of a water-bearing fluid into rocks still at high temperatures
(T = ca. 620oC, P = ca. 6 kbar). A fluid inclusion study, supported by thermodynamic modelling (Newton, 1995), suggests that the infiltrating fluid contained less than 30 mole % H2O. Direct textural evidence that the infiltrating fluid was CO2-rich is demonstrated by the replacement of olivine by hypersthene and magnesite in associated ultramafic granulites. The regional fluid dominated nature of the granulite-to-amphibolite transition is further demonstrated by the fact that the retrograde isograd clearly intersect regional structures.
Rocks adjacent to the deep-seated shear zones both south and north of the retrograde isograd are often characterized by relatively narrow zones of intense K-metasomatism in which tonalitic gneisses are progressively altered into garnet and sillimanite-bearing pink pottassic gneisses (Hoernes et al., 1995). Fluid inclusion studies demonstrated the important role of CO2 as a main component of the fluids now entrapped in quartz. Petrological data suggested that pottassic metasomatism in shear zones north of the isograd occurred under granulite facies conditions (Hoernes et al., 1995) while alteration south of
the isograd occurred under amphibolite facies conditions.
O-isotopic studies, based on whole rock/mineral fractionations (Hoernes et al., 1995) were used to constrain the peak temperature conditions of alteration in shear zones north of the isograd to a range of 800 to 870oC, in excellent agreement with peak metamorphic temperatures derived from petrological considerations (Stevens and Van Reenen, 1992). Similar studies south of the isograd suggest that metasomatic alteration in shear zones and regional anthophyllite formation have occurred at temperatures close to 600oC, again in good agreement with temperatures derived from petrological considerations (Van Reenen, 1986).
Another prominent manifestation of large-scale fluid movement in the Southern Marginal Zone during exhumation is the formation of high temperature shear-hosted gold mineralization (Van Reenen et al., 1994).
The O-isotope study showed that retrogression of the Southern Marginal Zone granulites was probably triggered by the influx of external fluids into granulites with an existing north-to-south temperature gradient in the Southern Marginal Zone. The source of the retrograde fluids remains an open problem.
Hoernes, S., Lichtenstein, U., Van Reenen, D.D. & Mokgatlha, K., SA. J. Geol. 98, 488-497 (1995).
Newton, R.C., Eur. J. Mineral. 7, 861-881 (1995).
Roering, C., Van Reenen, D.D., De Wit, M.J., Smit, C.A., De Beer, J.H. & Van Schalkwyk, J.F., Prec. Research 55, 69-80 (1992).
Stevens, G. & van Reenen, D.D., Prec. Research 55, 303-319 (1992).
Van Reenen, D.D., Amer. Mineral. 71, 900-915 (1986).