Metamorphism of the Palaeoproterozoic Magondi mobile belt north of Karoi, Zimbabwe

Abstract

The Palaeoproterozoic Magondi mobile belt flanks the Zimbabwe Archaean Craton to the northwest. The belt is composed of metamorphosed sedimentary, volcanic and volcaniclastic rocks associated with quartzofeldspathic gneisses intruded by granitoids, some charnockitic, in the high-grade part of the belt. The belt is metamorphosed from low-grade greenschist-facies in the south and middle to upper amphibolite-facies in the north. Granulite-facies rocks are developed in the extreme north and northwestern part of the belt. Garnet-biotite geothermometry in metapelites indicates that temperatures increase from 590–600°C in the mid-amphibolite-facies through 640–690°C in the upper amphibolite-facies terrain and up to 730°C in the granulite-facies areas. In the granulite-facies terrains, garnet-biotite temperatures are similar to temperatures calculated using garnet-cordierite, garnet-clinopyroxene and, to some extent, two-feldspar geothermometers. Pressures calculated with the GASP barometers are 6 ± 1 kbar for both upper amphibolite- and granulite-facies, suggesting that the granulite-amphibolite-facies transition is primarily isobaric. The calculated pressures for granulites do not support models which invoke the formation of granulites by continent-continent collision. Instead the P-T data suggest that the Magondi mobile belt granulites were formed in a region of high heat flow, with heat possibly being supplied by deep-seated plutons.