Time of emplacement and metamorphism of Late Precambrian mafic dykes associated with the Pan-African Gariep orogeny, Southern Africa: implications for the age of the Nama Group

The Gannakouriep mafic dyke swarm straddles the border between South Africaand Namibia in the lower Orange River region, and was intruded during crustal extension associated with the early phases of the Late Precambrian Gariepian orogeny. A RbSr internal isochronage of 717±11 Ma has been obtained from fresh unmetamorphosed gabbro that forms relict patches in the biggest dyke of the swarm. By inference, the maximum age for the Gariepian orogeny, which deformed and metamorphosed the dyke swarm along its western margin, and forms part of the Pan-African system of mobile belts, is constrained by these age data. A product of the Gariepian orogeny was the growth of metamorphic amphibole in the dykes, which had a profound influence on the KAr age patterns. Within the amphibole isograd, the dykes possess coherent patterns indicating growth of the metamorphic amphibole 542±4 Ma ago. Lower grade sectors of the dyke swarm characterised by chlorite-scricite assemblages have lower KAr ages indicative of open system behaviour, perhaps promoted by the lack of a K-bearing stabilising mineral such as amphibole. Amphibolitised Gannakouriep dykes are unconformably overlain by the Nama Group, and so it follows that this sedimentary sequence post dated Gariepian metamorphism at ∼540 Ma. A minimum age for the Nama Group is given by the intrusion of the Bremen Complex at 521±11 Ma, thereby constraining deposition and by inference the Precambrian-Cambrian boundary in southern Africa to between 540 and 520 Ma ago.