Journal of Petrology Advance Access originally published online on July 8, 2004
Journal of Petrology 2004 45(9):1777-1797; doi:10.1093/petrology/egh033
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Journal of Petrology 45(9) © Oxford University Press 2004; all rights reserved
The Role of Water Retention in the Anatexis of Metapelites in the Bushveld Complex Aureole, South Africa: an Experimental Study
1 DEPARTMENT OF EARTH SCIENCES, LA TROBE UNIVERSITY, BUNDOORA, VIC. 3086, AUSTRALIA
2 DEPARTMENT OF GEOLOGY, UNIVERSITY OF STELLENBOSCH, STELLENBOSCH, PRIVATE BAG X1, MATIELAND 7602, SOUTH AFRICA
3 SCHOOL OF GEOSCIENCES, UNIVERSITY OF THE WITWATERSRAND, PRIVATE BAG 3, WITS 2050, JOHANNESBURG, SOUTH AFRICA
* Corresponding author. Present address: School of Geosciences, Monash University, Melbourne, Vic. 3800, Australia. Fax: +61-3-99054903. E-mail: Ian.Buick{at}sci.monash.edu.au
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 % H2O, 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 (<13%); extensive melting (>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.
KEY WORDS: low-pressure; melting experiments; metapelites
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