The role of water retention in the anatexis of metapelites in the Bushveld Complex aureole, South Africa: an experimental study

Highly restitic metapelites occur at the contact of the Rustenburg Layered Suite (Bushveld Complex). On the basis of previous experimental studies, the high (similar to60%) degrees of melting required to form these restites via fluid-absent processes could not have occurred at the maximum temperatures estimated in the aureole (similar to800degrees C). We have investigated their formation by partial melting experiments using a medium-grade and a low-grade metapelitic hornfels from the aureole that are isochemical except for water content (similar to1.2 and similar to4.4 wt % H2O, respectively). The partial melting experiments (700-1000degrees C at 3 kbar) demonstrate strongly contrasting behaviour for the two samples. For the higher-grade starting composition only minor melting occurred up to 800degrees C (<∼13%); extensive melting (>50%) required a temperature of 1000degrees C. In contrast, for the lower-grade starting composition, extensive melting (similar to50-65 %) and the presence of a similar restite assemblage to that observed in the aureole rocks was achieved in all experiments run at temperatures greater than or equal to 750degrees C. In the absence of fluid infiltration, the results suggest that during rapid healing in the innermost parts of high-temperature contact aureoles, extensive partial melting may occur at relatively low temperatures if the fluid from the low-grade protolith is still resident in the system. This locally renders metapelitic rocks less dehydrated and, hence, more fertile than if heating had been slower.