Metamorphic P-T profiles from the Zimbabwe Craton to the Limpopo Belt, Zimbabwe

Tsunogae, T., Miyano, T., and Ridley, J. (1992)@Precambrian Research, 55, 259-277.


Abstract

Metamorphic P-T profiles in the eastern and western parts of the Limpopo Belt, Zimbabwe, were examined on the basis of mineralogic and petrologic data from the areas of Renco and Triangle (eastern) and of Makado and West Nicholson (western) which include the southernmost Zimbabwe Craton, the Northern Marginal Zone (NMZ), and the Central Zone (CZ). The Zimbabwe Craton is made up of granite-greenstone terrane of 2900 Ma to 3600 Ma old. The NMZ is a high-grade equivalent of the granite-greenstone lithologies. The CZ is composed of supracrustal lithologies. Both the NMZ and CZ has suffered granulite facies metamorphism during late Archean.

Metamorphic temperature of the Zimbabwe Craton adjacent to the northern boundary of the Limpopo Belt is calculated to be 650-680°C for amphibolite and 710-720°C for quartzofeldspathic gneiss in the eastern part. They are slightly lower than that calculated from the same assemblages in the NMZ and CZ.

Peak metamorphic conditions in the eastern part of the NMZ are estimated at about 740-790°C and 7.5-7.9 kbar from assemblage garnet (core)-orthopyroxene-plagioclase-quartz in pelitic gneiss. The conditions are followed by a retrograde event of about 630-660°C and 3.8-4.5 kbar which were calculated from assemblage garnet (rim)-cordierite-sillimanite-quartz in the same rock. This P-T path is very similar to the retrograde path from the Southern Marginal Zone (SMZ). It implies that both the NMZ and SMZ may have been subjected to a similar metamorphic evolution.

Garnet-bearing mafic gneiss in the CZ indicates somewhat higher P-T conditions of 790-890°C and 11.0-13.3 kbar for garnet-clinopyroxene-plagioclase-quartz assemblage and 750-870°C and 7.6-8.8 kbar for garnet-hornblende-plagioclase-quartz assemblage than those in the NMZ. The difference of P-T conditions between the NMZ and CZ may be related to the pre-Triangle shear zone. The NMZ appears to have been slipped along the shear zone against the CZ during uplifting, carrying the deeper crust to the surface level.