A Palaeomagnetic Study of the Gaberones Granite of Botswana

Abstract

Summary Thirty-two cores were drilled at eleven sites in a variety of granitic rocktypes. Meaningful results were obtained from seven sites after alternating field demagnetization and there is good evidence for the stability of the magnetization observed. The specimens exhibit a wide range in initial intensity of magnetization but no relationship exists between rock-type, initial intensity and behaviour under a.f. demagnetization. In all cases the within-site scatters are large and the possibility that this is a fundamental property of the remanent magnetization of large granitic bathyliths is discussed. The location of the mean palaeomagnetic pole for the Gaberones granite (103E, 34N, A,,= 16) is compatible with the smooth polar wandering path deduced from other ancient African poles. It is also consistent with the radiosotopic age for the granite of 2350 million years. 1. Geology The Gaberones granite occurs in the southeastern corner of the Republic of Botswana (previously the Bechuanaland Protectorate) on the border of South Africa (Fig. 1). It is exposed as a number of large plutons (Poldervaart 1952) which consist of a central rapakivitic granite surrounded by two border phases comprising an inner zone of red or grey granite (Marginal granite) and an outer zone of porphyritic granophyres (Wright 1958). The Central granite is usually porphyritic and contains large phenocrysts of white felspar with idiomorphic or ovoidal outlines and in large outcrops it is seen to be foliated. The Marginal granite is typically a coarse-grained equigranular rock almost entirely composed of anhedral red or grey felspar and quartz. Beyond the Marginal granite, the porphyritic granophyres show a perfect transition into fine-grained felsites which were previously correlated on lithological grounds with the Dominion Reef System of South Africa. In recent years a number of radioisotope age determinations have become available and some of these ages have led the Botswana Gcological Survey to tentatively regard the felsites as pre-Dominion Reef S: stem in age. In view of this uncertainty. the maximum age of the Gaberones granite is set by the age of the Modipe gabbro into which it is intrusive. This gabbro has been dated by McElhi~y (1966a) who obtained an age of 2630+ 470 million years from Rb : Sr decay.