Partial melting and the origin of metapelitic granulites in the Southern Marginal Zone of the Limpopo Belt, South Africa

Abstract

Two potentially realistic models exist to explain the origin of the granulite-facies rocks of the Southern Marginal Zone: (i) CO 2-fluxing, inducing the necessary high-temperature dehydration reactions and, (ii) melting of hydrous minerals to yield an H 2O-undersaturated granitic melt in equilibrium with a dehydrated granulitic assemblage. Petrographic and field evidence from the Bandelierkop Quarry in the Southern Marginal Zone indicate a sequence of melting reactions involving muscovite and biotite. Relict textural evidence indicates that the reaction responsible for the generation of most of the melt, Bi + Sil + Qz + Plag = Melt + Ga + Ksp, was fluid-absent. The melting reactions can be shown to have occurred during the prograde metamorphic path, with the highest-temperature reaction (Bi + Qz + Plag = Hy + Cord + Melt), occurring at approximately peak metamorphic conditions (850°C and less than 9.5 kbar). This suggests a fluid-absent mode of granulite formation. A subsequent retrograde reaction (Cord + Ksp + H 2O = Ky + Qz), related to a rise in α H 2O during in-situ crystallisation of the melt, did not destroy the granulite assemblages and hypersthene remains stable in areas of the Southern Marginal Zone that record pressures of greater than 6.5 kbar during retrogression.