Pressure-temperature evolution during two granulite-facies metamorphic events (2.62 and 2.02 Ga) in rocks from the Central Zone of the Limpopo Belt, South Africa

Abstract

The Central Zone of the Limpopo Belt in southern Africa is a classical example of a polymetamorphic terrane that experienced three metamorphic events at 3.22 Ga (M1), 2.66–2.61 Ga (M2) and 2.02 (M3). However, the geodynamic significance of these events is controversial as their P-T evolution was poorly constrained.We present new petrological and geochronological data for the M2 and M3 events. The Neoarchean M2 event is well-preserved in samples from a newly discovered enclave of supracrustal and magmatic rocks in the 2.612 Ga Bulai pluton that shielded the enclave against a Paleoproterozoic M3 overprint, which is dominant in rocks exposed outside the pluton. P-T pseudosection modelling for migmatitic Grt-Crd metapelites from the enclave and the rock sequence outside the pluton indicates an opposite metamorphic evolution, a counterclockwise P-T path for M2 but a clockwise path for M3.Spinel-quartz assemblages in the enclave metapelites record early heating to UHT conditions (ca. 930 °C, 5.5 kbar) at intermediate crustal levels. The peak-pressure M2-assemblage garnet-cordierite-sillimanite-K-feldspar-plagioclase-quartz ± rutile formed at ca. 860 °C and 7.5 kbar during a moderate pressure increase. Cordierite coronas around garnet record subsequent decompression. We interpret the counterclockwise P-T evolution during M2 to result from convergence of the Zimbabwe and Kaapvaal Cratons, which has led to subduction of oceanic lithosphere beneath the Zimbabwe Craton. Subsequent slab retreat has caused asthenosphere upwelling, thereby inducing delamination of the lithospheric mantle. Intrusion of hot asthenospheric material beneath the crust induced early UHT metamorphism at mid-crustal levels. The subsequent moderate pressure increase was related to the ensuing collision of the cratons. Post-peak decompression records incorporation of the enclave into the Bulai pluton melt and uplift in the ascending granitoid melt.The peak M3-assemblage garnet-cordierite-sillimanite-K-feldspar-rutile-plagioclase-quartz formed in metapelites occurring outside the pluton during near-UHT conditions (ca. 880 °C, 7.5 kbar). Cordierite coronas around garnet record subsequent decompression. New growth of garnet and sillimanite at the expense of cordierite documents late near-isobaric cooling to <700 °C. The resulting clockwise P-T path for M3 granulite-facies metamorphism is interpreted to record an intracontinental Paleoproterozoic transpressional orogeny within the consolidated Kaapvaal-Zimbabwe Craton, which was induced by far-field tectonic processes.