A review of the stratigraphy and geological setting of the Palaeoproterozoic Magondi Supergroup, Zimbabwe – Type locality for the Lomagundi carbon isotope excursion

Abstract: The Palaeoproterozoic Magondi Supergroup lies unconformably on the Archaean granitoid-greenstone terrain of the Zimbabwe Craton and experienced deformation and metamorphism at 2.06–1.96Ga to form the Magondi Mobile Belt. The Magondi Supergroup comprises three lithostratigraphic units. Volcano-sedimentary rift deposits (Deweras Group) are unconformably overlain by passive margin, back-arc, and foreland basin sedimentary successions, including shallow-marine sedimentary rocks (Lomagundi Group) in the east, and deeper-water shelf to continental slope deposits in the west (Piriwiri Group). Based on the upward-coarsening trend and presence of volcanic rocks at the top of the Piriwiri and Lomagundi groups, the Piriwiri Group is considered to be a distal, deeper-water time-equivalent of the Lomagundi Group. The Magondi Supergroup experienced low-grade metamorphism in the southeastern zone, but the grade increases to upper greenschist and amphibolite facies grade to the north along strike and, more dramatically, across strike to the west, reaching upper amphibolite to granulite facies in the Piriwiri Group. Carbonates form prominent horizons in the lower Lomagundi Group, occur in the Deweras Group as thick packages in the northern part of the basin, but form only thin discontinuous beds elsewhere, and are rare in the Piriwiri Group. Beds of anhydrite and sulphate pseudomorphs are relatively common in the Deweras Group, and also occur in the Lomagundi Group. found extreme enrichment in 13C in carbonates of the Lomagundi Group, with an average δ13C value of +8.2‰ VPDB. Subsequent work in the Magondi Basin has shown that high δ13C carbonates are also present in the continental rocks of the underlying Deweras Group. The initiation of the Deweras rift is not well constrained geochronologically, but it may have started as early as 2.26Ga, and was followed by deposition of the lower part of the Lomagundi Group on a passive continental margin. Assuming that the deepening trend in the upper Lomagundi Group and upward-coarsening trend in the Piriwiri Group reflect subsidence in a back-arc and subsequent foreland basin setting, with sediment derivation from an approaching volcanic arc, the age of the onset of Magondi deformation at ca. 2.0Ga provides an upper age limit for sedimentation. Combining all available data for the Magondi Basin, the Limpopo Belt between the Zimbabwe and Kaapvaal cratons, and the Kheis Belt on the southwestern margin of the Kaapvaal craton, a new tectonic model is presented for the assembly of the Kaapvaal and Zimbabwe cratons at ca. 2.1–2.0Ga. These cratons along with the Archaean cratons in West Africa and South America record an assembly of a large continent at the time when Archaean cratons in North America and Fennoscandia experienced extension and breakup. Carbonates of the Magondi Basin, the Kheis Belt, and the northern margin of the Kaapvaal craton therefore reflect the carbon isotope composition of the open ocean at 2.2–2.1Ga and provide further evidence that the δ13C values reaching to +10‰ VPDB and higher record a seawater signal rather than local diagenetic or closed basin conditions. The emerging picture is that Earth experienced dramatic tectonic reorganization between 2.1–2.0Ga, which likely influenced ocean circulation and redox state and potentially ended the conditions that promoted high burial of organic matter with sediments during the Lomagundi excursion. [Copyright &y& Elsevier]