High-temperature hydration of ultramafic granulites from the Southern Marginal Zone of the Limpopo Belt by infiltration of CO2-rich fluid

Abstract

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°C, 6 kbar, and X_{CO2 (fluid) ⩾ 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.}