Precambrian Research, 45 (1989) 159-174 159 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands Isotope and Geochemical Constraints on Proterozoic Crustal Evolution in South-Eastern Africa B.M. EGLINGTON 1, R.E. HARMER 1 and A. KERR 2 1Division of Earth, Marine and Atmospheric Science and Technology, Councillor Scientific and Industrial Research, P.O. Box 395, Pretoria 0001 (Republic of South Africa) 2Department of Geology and Applied Geology, University of Natal, Durban (Republic of South Africa) (Received February 18, 1988; revision accepted February 12, 1989) Abstract Eglington, B.M., Harmer, R.E. and Kerr, A., 1989. Isotope and geochemical constraints on Proterozoic crustal evo- lution in south-eastern Africa. Precambrian Res., 45: 159-174. Proterozoic exposures south of the imbricate thrust terrane in the eastern sector of the Namaqua-Natal Belt of southern Africa are dominated by granitoid plutons intrusive into older supracrustal gneisses. Many of these granites have geochemical characteristics similar to "A-type" and rapakivi suites, as previously described in Australia and the northern hemisphere. Ages obtained from the Natal units range from ~ 1.1 to ~0.88 Ga. Nd TDMvalues cluster about ~ 1.4 Ga. The oldest granitoids have "volcanic arc" chemical signatures but, subsequent to ~ 1.1 Ga, "A-type" granites are common. This suggests a change in tectonic environment or melting process at about this time. Initial aVSr/SeSr ratios and end values for the older suites approach values normally associated with a depleted mantle reservoir whilst /1 values are similar to those found in "orogen" environments. These data clearly indicate that the Proterozoic crust of south-eastern Africa was not derived by extensive reactivation of Archaean or Early Proterozoic protoliths. Low STSr/aeSr ratios in metasedimentary carbonates and ~ 1.8 Ga Nd TDM ages for metapelites from the Belt support this model and preclude the formation of the Belt in an intracratonic setting. All the data thus support models involving substantial crustal accretion and differentiation, primarily subsequent to ~ 1.5 Ga. Introduction The Proterozoic Namaqua-Natal Belt (Ni- colaysen and Burger, 1965) forms the southern margin of the Archaean Kaapvaal Craton in southern Africa (Fig. 1 ). The central portion of the Belt is obscured by Phanerozoic sediments but isotopic studies on zircons from basement samples encountered in boreholes penetrating this younger cover sequence have provided e°Tpb/2°6pb model ages of ~ 1000 Ma (Burger and Coertze, 1973), confirming the link be- tween the western (Namaqualand) and eastern (Natal) exposures. Geophysical magnetic, re- sistivity and gravity anomalies have also helped to delineate the extent of the Belt (e.g., De Beer and Meyer, 1984). As with most Precambrian orogenic zones, various models have been proposed for the de- velopment of crust in the Namaqua-Natal Belt. One model has ascribed the supracrustal gneisses and their intrusive granitic plutons to reworking of Archaean material (e.g., KrSner, 1976), another to extensive reactivation of Early Proterozoic lithologies (Harris et al., 1987), whilst another argues that, since the geophysical structure of the Namaqua-Natal Belt closely resembles the Andean region of South America, a craton-margin volcanic arc 0301-9268/89/$03.50 © 1989 Elsevier Science Publishers B.V.