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

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 areas which include the southernmost Zimbabwe Craton, the Northern Marginal Zone, and the Central Zone. The Zimbabwe Craton consists of a granite-greenstone terrane of 2900 Ma to 3600 Ma old. The Northern Marginal Zone represents a high-grade equivalent of the granite-greenstone lithologies. The Central Zone has suffered granulite-facies metamorphism during the 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. These temperatures are slightly lower than that calculated from the same assemblages in the Northern Marginal Zone and Central Zone. Peak metamorphic conditions in the eastern part of the Northern Marginal Zone are estimated at about 740–790°C and 7.5–7.9 kbar from the assemblage, garnet (core)-orthopyroxene-plagioclase-quartz in pelitic gneiss. These conditions were followed by a retrograde event of about 630–660°C and 3.8–4.5 kbar which were calculated from the assemblage, garnet (rim)-cordierite-sillimanite-quartz in the same rock. The P- T path is very similar to the retrograde path from the Southern Marginal Zone. It implies that both the Northern Marginal Zone and Southern Marginal Zone may have shared a similar metamorphic evolution. Garnet-bearing mafic gneiss in the Central Zone indicates somewhat higher P- T conditions than those in the Northern Marginal Zone. Central Zone metamorphic conditions are estimated at 790–890°C and 11.0–13.3 kbar for garnet-clinopyroxene-plagioclase-quartz assemblages and 750–870°C and 7.6–8.8 kbar for garnet-hornblende-plagioclase-quartz assemblages. The difference of P- T conditions between the Northern Marginal Zone and Central Zone may be related to movement along the pre-Triangle Shear Zone. The Northern Marginal Zone appears to have been displaced along the shear zone relative to the Central Zone during uplift, carrying the deeper crust to the surface.