Hydration of cordierite and hypersthene and a description of the retrograde orthoamphibole isograd in the Limpopo Belt, South Africa

The transition from orthopyroxene- to orthoamphibole-bearing assemblages in Mg-rich metapelites across a retrograde M3 orthoamphibole isograd in the Archean Limpopo belt in South Africa is expressed by two hydration reactions that are superposed on M2 corona textures of cordierite and hypersthene after garnet: hypersthene + quartz + H₂O = antho-phyllite, and cordierite + H₂O = gedrite + kyanite + quartz. The temperature of hydration at the isograd of less than 625-650°C at a minimum pressure of 6 kbar is estimated on the basis of (1) a comparison of published information on the solvus in the orthoamphibole system to the results of the present study, (2) the presence of kyanite coexisting with anthophyllite and gedrite both at and south of the isograd, and (3) the application of the gamet-biotite thermometer. The estimated temperature is far below the upper thermal stability limit of anthophyllite under conditions where P_H2O = P_total.

Cordierite at the isograd is partly (or completely) surrounded by gedrite and kyanite, and hypersthene by anthophyllite; coexisting hypersthene + anthophyllite + quartz is restricted to a narrow zone defined by the isograd. A modified Redlich-Kwong equation of state for nonideal mixing of H₂O and CO₂ was used to calculate the mole fraction of CO₂ (about 0.8) in the fluid phase necessary to displace the equilibrium curve anthophyllite = 7 enstatite + quartz + water to the estimated P-T conditions on the isograd. The presence of a systematic temperature gradient in the Limpopo belt suggests that the influx of the CO₂-rich fluid occurred at a fixed temperature during hydration.