A Reappraisal of Some Aspects of Precambrian Shield Geology

Abstract

Attention is drawn to some of the misconceptions regarding the ancient crystalline shields, and an attempt is made to clarify some of the ideas held on their geology. In Southern Africa, the Precambrian shield is well represented and exposed, and recent studies of it reveal a clear and well-defined pattern of events in its evolution. The very ancient, stable, cratonic nuclei incorporating the greenstone belts are termed “The Earliest Precambrian.” Traversing the shield areas and surrounding the cratonic nuclei are large, elongated, highly metamorphosed and granitized “Precambrian Mobile Belts.” Although younger than, and totally different in character to, the cratons which they tend to encircle, they nevertheless form an integral part of the crystalline shields. The fundamental elements of the geology of the greenstone belts within the cratonic nuclei, together with a distinctive pattern of relationships between the greenstone belts and their surrounding granitic terrain are repeated with remarkable consistency in other shield areas of the world, especially in Canada and Western Australia. The geological features which typify and contribute to the establishment of this highly distinctive pattern are outlined in the text and demonstrated with the aid of tables and diagrams. The world-wide uniformity of the stratigraphy, structure, metamorphism, mineralization, associated granites, and geotectonic setting of the greenstone belts is stressed. An attempt is made to reconstruct an evolutionary model of the development of the early Precambrian granite-greenstone belt terrain. This avoids direct comparisons with younger geological features and events, particularly the younger, Alpine-type orogenic belts with which early Precambrian geology has frequently been compared and equated. The mobile belts are briefly discussed, and again it is suggested that their evolution was not necessarily along the lines suggested for Alpine orogenesis.