HIGH-TEMPERATURE HYDRATION OF ULTRAMAFIC GRANULITES FROM THE SOUTHERN MARGINAL ZONE OF THE LIMPOPO BELT BY INFILTRATION OF CO2-RICH FLUID

The peak metamorphic assemblage of olivine + orthopyroxene + calcic amphibole + spinel is replaced by a variety of high-temperature retrograde assemblages in ultramafic granulites from the Southern Marginal Zone of the Limpopo Belt. The observed replacement of olivine according to the reaction forsterite + CO2 = enstatite + magnesite is restricted to the zone of rehydration which bounds the granulite zone in the south. This observation is consistent with the post-peak metamorphic infiltration of an externally derived CO2-rich fluid during the high-temperature post-peak metamorphic history of the Limpopo Belt. This event established a retrograde orthoamphibole isograd. Conditions of the retrogression are estimated to be near 670-degrees-C, 6 kbar, and X(CO2) (fluid) greater-than-or-equal-to 0.67. C-isotopic compositions of CO2 extracted from magnesite produced by the high-temperature breakdown of olivine supports a deep-seated (mantle) source for the CO2-rich retrograde fluid.