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Highly restitic metapelites occur at the contact of the Rustenburg Layered Suite (Bushveld Complex). On the basis of previous experimental studies, the high (∼60%) degrees of melting required to form these restites via fluid-absent processes could not have occurred at the maximum temperatures estimated in the aureole (∼800°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 (∼1·2 and ∼4·4 wt % H 2 O, respectively). The partial melting experiments (700–1000°C at 3 kbar) demonstrate strongly contrasting behaviour for the two samples. For the higher-grade starting composition only minor melting occurred up to 800°C (&lt;∼13%); extensive melting (&gt;50%) required a temperature of 1000°C. In contrast, for the lower-grade starting composition, extensive melting (∼50–65%) and the presence of a similar restite assemblage to that observed in the aureole rocks was achieved in all experiments run at temperatures ≥750°C. In the absence of fluid infiltration, the results suggest that during rapid heating 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.

The Role of Water Retention in the Anatexis of Metapelites in the Bushveld Complex Aureole, South Africa: an Experimental Study