P-T conditions and the origin of quartzo-feldspathic veins in metasyenites from the Central Zone of the Limpopo Belt, South Africa

In this study, we present petrological data from metasyenites in the Central Zone of the Limpopo Belt, South Africa. Results from conventional thermobarometry and pseudosection modelling in the chemical system Na₂O-CaO- K₂O-FeO-MgO- Al₂O-O₃- H₂O-TiO₂- Fe₂O₃ (NCKFMASHTO) indicate the rocks underwent a metamorphic evolution that was characterised by a maximum pressure of 8 kbar and temperatures of ∼770°C. Subsequent retrogression involved a simultaneous P-T decrease via decompression-cooling to 4 kbar at temperatures <550°C. The inferred P-T path, in combination with previous petrological data from Venetia and Messina can be interpreted to support the existence of a metamorphic field gradient that was developed during a single metamorphic event. The metasyenitic body is cross-cut by a number of quartzo-feldspathic veins that probably represent mobilised melt from the surrounding Alldays gneiss. An in-situ partial melt origin for the quartzo-feldspathic veins is ruled-out on the basis of modelling which suggests the measured compositions could not have been derived from the partial melting of the metasyenite. Modelling suggests the metasyenite could theoretically melt at the P-T conditions attained. However, due to a lack of available H₂O, melting did not proceed.