UMKONDO IGNEOUS PROVINCE AND MESOPROTEROZOIC UMKONDO BASIN IN EASTERN ZIMBABWE: POSSIBLE RELEVANCE TO RODINIA

The Umkondo igneous province in southern Africa may record impact of a mantle plume on the Kalahari craton during Rodinia assembly at 1.1 Ga, coinciding with widespread intraplate magmatism in Laurentia. In the Umkondo type area, eastern Zimbabwe, dolerite dykes and extensive sills (>=100 m thick) intrude metasedimentary rocks of the Umkondo Group, which forms a sequence up to 800 m thick of basal arkoses, stromatolitic limestones, cherts, mudrocks and quartzose sandstones. The metasedimentary rocks are locally folded about N-S axes and thrust towards the west, and have been metamorphosed with the dolerites at low grade. This tectonism probably occurred during Pan-African orogenesis. The age and tectonic setting of the Umkondo basin are not well established, but it may have linked with a Mesoproterozoic continental margin to the east.

The Umkondo strata rest non-conformably on the Archean basement. Dolerite sills are continuous from the basement into the Umkondo Group; some sills were intruded along the nonconformity. Probable felsic tuffs and mafic lapilli tuffs occur in the middle Umkondo Group, which is conformably overlain by basalt lavas geochemically similar to the dolerites. Together with the presence of peperite along one sill margin, these observations suggest that deposition of the Umkondo Group directly preceded widespread Umkondo mafic magmatism.

The Umkondo Group appears to have been deposited in fluvial to shallow-water environments with episodic input of distal bimodal pyroclastic debris. Lateral continuity of the main stratigraphic units suggests a single depositional basin with a N-S dimension >=390 km. These preliminary observations may have paleogeographic implications for Rodinia reconstructions.