Author: Tom N. Clifford
The early crustal history of Africa has been discussed in summary works by Nicolaysen (1962), Kennedy (1965), Nicolaysen and Burger (1965), Cahen and Snelling (1966), Black (1967), Clifford (1968, 1970), and Black and Girod (1970). On the basis of available geologic and geochronologic data, it has been argued (Clifford, 1972) that the major part of the continent is a segment of primeval crust more than 3,000 m.y. old, which has been affected by several major orogenies; these include at least two major orogenic events older than 2,500 m.y., for which the best evidence is preserved in a number of pristine nuclei and a sequence of polyepisodic orogenies at 1,850 ± 250 m.y. ago (Ebumian and Huabian orogenic episodes), 1,100 ± 200 m.y. ago (Kibaran orogeny), 600 ± 100 m.y. ago (Damaran-Katangan or Pan-African orogeny) and during middle Paleozoic–early Mesozoic and Tertiary time. Recent data indicate the presence of ensialic floor beneath deformed geosynclinal cover in orogenic zones of all ages (Clifford, 1972), and in large segments of almost all of the orogenic zones, the orogenic event is entirely recorded as rejuvenated floor rocks. Metamorphic and intrusive rocks of the granulite-charnockite suite are concentrated in these floor segments (Clifford, 1973). Despite a voluminous literature on individual examples of these suites in Africa, few papers have dealt with their distribution and significance, although Oliver (1969) showed the geographic occurrence of some of the larger masses, and Saggerson and Owen (1969) commented on their significance as geobarometers. This review is intended to redress this situation, and with this aim in mind, semantic discussions of the terms “ granulite” and “ charnockite” are avoided here; the reader is referred to recent papers on this subject by Pichamuthu (1969) and Behr and others (1971). The term “granulite” is used in this paper for metamorphic rocks of appropriate texture in the granulite facies, and “charnockite” is used for the hypersthene-bearing intrusive suites as defined by Holland (1900); the adjective “charnockitic” is occasionally used for rocks whose field relations are still uncertain. With the exception of the Alpine zone of North Africa, granulite-charnockite suites are present in major structural domains of all ages. The ages of many of the suites differ from the dates of tectonism and metamorphism in those domains. The significance of the African suites is, therefore, considered here in terms of structural domains: the ancient nuclei that have been unaffected by orogeny since ~2,500 m.y. ago are considered first; thereafter, suites in orogenic zones of successively younger age are discussed.
Published: 12/09/1974
ISBN Number: 0813721563
Pages: 55
Product Category: EBooks
