Fluid-rock interaction in retrograde granulites of the Southern Marginal Zone, Limpopo high grade terrain, South Africa

Fluid infiltration into retrograde granulites of the Southern Marginal Zone (Limpopo high grade terrain) is exemplified by hydration reactions, shear zone hosted metasomatism, and lode gold mineralisation. Hydration reactions include the breakdown of cordierite and orthopyroxene to gedrite + kyanite, and anthophyllite, respectively. Metamorphic petrology, fluid inclusions, and field data indicate that a low H₂O-activity carbon-saturated CO ₂-rich and a saline aqueous fluid infiltrated the Southern Marginal Zone during exhumation. The formation of anthophyllite after orthopyroxene established a regional retrograde anthophyllite-in isograd and occurred at P-T conditions of ∼6 kbar and 610 °C, which fixes the minimum mole fraction of H₂O in the CO₂-rich fluid phase at ∼0.1. The maximum H₂O mole fraction is fixed by the lower temperature limit (∼800 °C) for partial melting at ∼0.3. C-O-H fluid calculations show that the CO₂-rich fluid had an oxygen fugacity that was 0.6 log ₁₀ units higher than that of the fayalite-magnetite-quartz buffer and that the CO₂/(CO₂+CH₄) mole ratio of this fluid was 1. The presence of dominantly relatively low density CO ₂-rich fluid inclusions in the hydrated granulites indicates that the fluid pressure was less than the lithostatic pressure. This can be explained by strike slip faulting and/or an increase of the rock permeability caused by hydration reactions.