Geologic evolution of the neoproterozoic Zambezi orogenic belt in Zambia

The Neoproterozoic Zambezi belt links with the Mozambique belt, Lufilian arc, and the inland branch of the Damara belt within the regional Pan-African tectonic framework of southern Africa. The belt contains a thick supracrustal sequence deposited on older sialic basement and penetratively deformed with it during Neoproterozoic (Pan-African) orogenesis. In Zambia, where the entire width of the orogen is exposed, local bimodal volcanic rocks at the base of the sequence are overlain by psammites and pelites, which in turn are succeeded by extensive carbonate and calc-silicate rocks. Abundant scapolite in metamorphic assemblages within the belt is taken as evidence for the original presence of evaporites. The nature of the rock types and the inferred stratigraphic sequence are consistent with deposition in an intracontinental rift basin invaded by marine waters. Available isotopic age brackets for the timing of supracrusta deposition show that the basin developed between 880 nad 820 Ma. Main-phase deformation in the belt involved both transcurrent shearing and south- to southwest-vergent thrusting and was associated with predominantly amphibolite-facies regional metamorphism. Mineral assemblages throughout much of the belt in Zambia, together with limited thermobarometric data, indicate typical Barrovian-type intermediate P/T conditions during metamorphism. Eclogites and other high-pressure metamorphic assemblages in parts of the belt, however, provide evidence that significant crustal thickening occurred, presumably in relation to thrusting. Reworked basement and syntectonic granite were subjected to extensive mylonitization related to strike-slip and oblique, reverse-slip shearing. The major orogenic event is dated at c. 820 Ma, based on an igneous age for a sheet-like, syntectonic batholith injected into a transcurrent shear zone within the central part of the belt. Pan-African orogenesis along the Zambezi-Lufilian-Damara trend was diachronous and records closure of intracratonic basins in the Zambezi belt and Lufilian arc, with evidence for the involvement of oceanic lithosphere present only in the Damara belt. © 1994.