Introduction KwaZulu-Natal in eastern South Africa contains a Mesoproterozoic orogenic belt that provides useful information for the understanding of both the amalgamation and fragmentation of supercontinents towards the end of the Mesoproterozoic at c. 1.2 to 1.0 Ga. The Mesoproterozoic Natal Belt of southern Africa lies adjacent to the southeastern margin of the Kaapvaal Craton and is truncated to the east against the Indian Ocean margin of the African continent. The Natal Belt is divided, from north to south, into the Tugela, Mzumbe and Margate Terranes (Thomas, 1989; Johnston et al., 2003; Figure 1). The Mzumbe and Margate Terranes are characterised by voluminous 1.2 to 1.0 Ga granitoid gneisses that have been interpreted as deeply eroded magmatic arcs (Thomas, 1989; Thomas et al., 1993; 1999). Published tectonic models assume that the Mzumbe and Margate Terranes are magmatic arcs that developed in response to Mesoproterozoic ocean basin closure south of the Kaapvaal Craton with subduction away from the craton (Matthews, 1972; 1981; Jacobs et al., 1993; Jacobs and Thomas, 1994; Thomas et al., 1994). In contrast to the Margate and Mzumbe Terranes, the Tugela Terrane consists of a heterogeneous assemblage of amphibolite-facies mafic and felsic orthogneisses, layered mafic and ultramafic complexes and metasedimentary rocks. The ultramafic rocks present in the Tugela Terrane and its intermediate position between the Kaapvaal Craton and southern arc terranes prompted Matthews (1972; 1981) to suggest that the Tugela Terrane represents part of a Mesoproterozoic ophiolite that was obducted onto the Kaapvaal Craton during Mesoproterozoic orogeny. Recent mapping (Bisnath, 2000; Johnston et al., 2003) and geochemical studies (Arima et al., 2001) have shown that the Tugela Terrane consists of several thrust sheets each of which has a distinct tectono-magmatic signature. These authors interpreted the Tugela Terrane as an accretionary complex consisting of rocks formed in an intra-oceanic island arc and oceanic island settings that subsequently accreted to the continental margin of the Kaapvaal Craton. A. BISNATH, S. MCCOURT, H.E. FRIMMEL AND S.B.N. BUTHELEZI 369 The metamorphic evolution of mafic rocks in the Tugela Terrane, Natal Belt, South Africa A. Bisnath Council for Geoscience, Pietermaritzburg, South Africa Present Address: Kai Batla Minerals Industry Consultants, P.O.Box 41955, Craighall, 2024, Gauteng, South Africa e-mail: abisnath@kaibatla.co.za S. McCourt School of Geological Sciences, University of KwaZulu-Natal (Westville Campus) Durban, South Africa. e-mail: mccourts@ukzn.ac.za H.E Frimmel Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa Present Address: Division for Geodynamics and Geomaterials Research, Institute of Geography, University of Würzburg, Am Hubland, D-97074, Würzburg e-mail: hartwig.frimmel@uni-wuerzburg.de S.B.N. Buthelezi School of Geological Sciences, University of KwaZulu-Natal (Westville Campus) Durban, South Africa. Present Address: Kai Batla Minerals Industry Consultants, P.O.Box 41955, Craighall, 2024, Gauteng, South Africa e-mail: sbuthelezi@kaibatla.co.za © 2008 December Geological Society of South Africa ABSTRACT New petrological and mineral chemical data indicate a clockwise P-T path for medium- to high-grade metamorphic mafic rocks in different thrust sheets of the Tugela Terrane within the Mesoproterozoic Natal Belt in KwaZulu-Natal (eastern South Africa). Three metamorphic stages are distinguished: (i) an early upper amphibolite- to lower granulite-facies metamorphic event (M 1 ) is indicated by rare relics of clinopyroxene; (ii) mineral chemical and textural equilibration during M 2 at calculated pressure-temperature conditions of 4.5 to 6 kbar and 693 to 750°C; and (iii) partial diffusional resetting of mineral compositions during subsequent uplift and exhumation (M 3 ) under lower amphibolite- to greenschist-facies conditions. No major differences were noted in the tectono- thermal regimes at which each of the respective thrust sheets were deformed suggesting that the Tugela Terrane was a homogenous package and experienced uniform P-T conditions. The new data acquired in this study contribute to an improvement in the ongoing development of a tectono-thermal and geodynamic model for the Natal Belt. SOUTH AFRICAN JOURNAL OF GEOLOGY,2008,VOLUME 111 PAGE 369-386 doi:10.2113/gssajg.111.4.369