Abstract

For the granulite facies rocks of the Southern Marginal Zone (SMZ) of the Limpopo Belt, minimum peak metamorphic temperatures of 800+ or -70 degrees C are estimated by quartz-garnet oxygen isotope thermometry. These temperatures are in agreement with temperatures expected for these rocks by anhydrous melting experiments of biotite. Fe-Mg garnet-orthopyroxene, garnet-cordierite, and garnet-biotite exchange thermometry provide lower estimates of peak metamorphic temperatures which are best explained by variable resetting during retrogression. Similarly, the oxygen isotope fractionations between quartz, plagioclase, and biotite have been reset. For the orthoamphibole gneisses found south of the previously proposed retrograde orthoamphibole isograd, best minimum peak temperature estimates of 760+ or -50 degrees C are given by quartz-garnet and quartz-orthoamphibole oxygen isotope thermometry in rocks containing orthoamphibole corresponding to the top of the anthophyllite-gedrite solvus, but lower temperatures ( approximately 660 degrees C) in orthoamphibole gneisses with at least two distinct coarse-grained orthoamphiboles. The revised thermometry, the orthoclase content in plagioclase, and the Ti- and F-content of biotite are consistent with a regional prograde metamorphic temperature gradient between the orthoamphibole gneisses and the granulites. It is possible that at least some orthoamphibole is prograde in origin. Average pressures for garnet-plagioclase-orthopyroxene-quartz barometry are 6+ or -1.5 kbar, which are about 1.5 kbar lower than those for garnet-cordierite barometry (7.5+ or -1 kbar) in the granulites, as well as preliminary garnet-grunerite (gedrite)-plagioclase-quartz barometry in the orthoamphibole gneisses (7.5+ or -1 kbar). Low peak pressures of the garnet-plagioclase-orthopyroxene-quartz barometer may be related to the removal of significant amounts of melt, and hence albite component from these rocks. Overall similarities in the oxygen isotope fractionations between quartz, plagioclase, and biotite, as well as the Fe-Mg partitioning of contact garnet-biotite pairs between granulite facies rocks and orthoamphibole gneisses also suggest that the retrograde thermal history of these rocks was very similar. Retrogression, including fine-grained orthoamphibole found to replace cordierite and orthopyroxene, is compatible with the release of small amounts of aqueous fluids from crystallizing residual anatectic melts at temperatures close to the vapour present granite solidus. No terrane-wide infiltration of CO 2 -rich fluids nor faster cooling of the orthoamphibole gneisses due to obduction onto granite-greenstone crust is required to account for the formation of retrograde orthoamphibole.

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