A Discussion on the evolution of the Precambrian crust - The evolution of the early Precambrian crust of Southern Africa

Abstract

The early Precambrian granite-greenstone basement complex of Rhodesia and South Africa constitutes one of the oldest known fragments of the Earth’s crust. Scattered within the expanse of granitic rocks are numerous greenstone belts some approaching, or even possibly exceeding, ages of about 3400 Ma. These ancient greenstone belt remnants represent the earliest clearly decipherable geological events on the Earth’s surface, and any evidence concerning the nature and evolutionary development of the primitive crust must be sought largely from an examination of their lithologies. The stratigraphy of the early greenstone occurrences in southern Africa is described with particular emphasis being given to the geological record established in the well-preserved Barberton Mountain Land of the Transvaal. The evidence for a primitive sialic versus simatic crust is reviewed and reference is made to the reported pre-greenstone belt basement remnants found in Rhodesia as well as in Swaziland. Comparisons are drawn between the early greenstone belts and modern island arcs, use being made of available geochemical data from the two environments. It is concluded that the early crust in southern Africa had analogies with the present-day abyssal regions of the oceans. Progressive transformation, involving island arc-like development and protocontinental nucleation, resulted from processes involving partial melting of the primitive lithosphere coupled with the secular mixing of mantle derived, granitic additions. The early thin, unstable, primordial crust underwent progressive thickening processes and, about 3000 Ma ago, stabilized sufficiently to permit the development of the first interior or cratonic-type sedimentary basins. Increases in the areal extent of the developing basins are briefly traced and the more significant events, from the earliest Precambrian to middle Proterozoic times (ca. 1700 Ma ago), are contrasted and summarized.