An outline of the geochronology of the late Precambrian formations of eastern central Africa

Abstract

The disposition, general lithology, and relation to structural environment of several post. Archaean pre-Karroo formations (Waterberg–Loskop–Matsap, Umkondo, Lomagundi, Sijarira, Fingoé, and Katanga systems) is reviewed, and their positions are shown on a map together with the localities of 110 published radiogenic age determinations. These formations, correlated in the past on lithological grounds, are better considered as shelf deposits similar in facies. Although they may be late-Precambrian in age, none of the formations (except perhaps some of the Katanga system) is indisputably so. Until details of stratigraphic sequence, depositional environment, and subsequent tectonic and metamorphic history are available correlations must remain tentative. The formations overlie cratonic areas which have been dated at 2000 to 3000 My (e. g. Rhodesia and Transvaal cratons), and circum-cratonic mobile gneiss belts having ages of 1500 to 2000 My (e. g. Limpopo belt). Younger gneiss belts are distinguished having ages of 400 to 650 My (e. g. mid-Zambezi and Mozambique belts). The southern part of the Mozambique belt is considered to have been involved in at least three cycles of deposition and metamorphism, the last of which is Cambro-Ordovician in age. This last event was the Mozambiquian orogeny and in it late-Precambrian sediments may have been orogenically involved. Sketch maps show new details of internal trend directions within the mobile belts, and the regional distribution of the main tectonic units. The available geochronological information is listed in a table and used to date the structural elements.