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Rb-Sr and U-Pb geochronology of Late Precambrian-Early Palaeozoic igneous activity in the Richtersveld ( South Africa) and southern south West Africa.

Abstract:

Late Precambrian to Early Palaeozoic rocks, exposed in the NW Cape Province (S Africa) and southern SW Africa, comprise the mainly sedimentary Nama and Gariep groups, the Kuboos-Bremen line of intrusives and the Richtersveld and Bremen Igneous Complexes. Known stratigraphic relationships between these rock-units are enumerated, and the stratigraphic history of the area is described briefly. In this paper we present new Rb-Sr (mainly whole-rock) and U-Pb zircon age-determinations, involving a total of 145 samples, on these rocks. A complementary paper is concerned with Middle-Proterozoic rocks from the same area (Welke et al. 1979). Three bodies were studied from the Kuboos-Bremen line of plutons. Ages of approx. 500 m.y. (possibly a minimum age), approx. 520 m.y. and approx. 550 m.y. were obtained, and multiphase intrusion is indicated. Data for the post-Nama younger Bremen intrusives establish a minimum age of 518 +- 15 m.y. (and probably 553 +- 13 m.y.) for the Nama group Felsic dykes that post-date the Richtersveld Igneous Complex underlie the Nama Group, but reliable data on these dykes was not obtained either by extensive Rb-Sr data points from widely separated dyke samples; data for groups of samples collected across the width of individual dykes each defined a separate line on an isochron plot. Both events seem confined to a belt extending down the W coast, and it is suggested that the approx. 700 m.y. event is related to Gariepian (i.e. Pan-African) orogenic deformation and that the approx 500 m.y. event is related to subsequent thermal acticity.-Authors