New U-Pb zircon ages for the Kuboos pluton in the Pan-African Gariep belt, South Africa: Cambrian mantle plume or far field collision effect?

Abstract

The age of the Kuboos pluton, which forms part of a series of alkaline, apparently post-orogenic intrusive bodies along the Kuboos-Bremen line, was re-assessed using single zircon U-Pb isotope analyses. A new age of 507 ± 6 Ma obtained for the youngest intrusive phase, a monzogranite from the Kuboos composite pluton is clearly younger than the main phase of deformation in the Gariep Belt (545 ± 2 Ma). This age corresponds to a major thermal event, as reflected by Rb-Sr and Ar-Ar mineral ages and by coeval post-orogenic granite in the Damara Belt, and also with Ar-Ar ages of syn-tectonic muscovite, related to a late tectonic pulse in the Pan-African Gariep and Saldania Belts, their basement and corresponding foreland basins. While the earlier 545 Ma tectonothermal event is explained by the closure of the Adamastor Ocean between the Kalahari and the Rio de la Plata cratons, the younger 500 Ma tectonic pulse can be linked to subduction beneath the western margin of Gondwana. The alkaline magmatism, which is represented by the Kuboos pluton and which is contemporaneous with the latter tectonic event, can be explained either by a mantle plume underneath southern Africa or as a distant effect of collision along the Gondwana margin. The occurrence of a 1.1 - 1.2 Ga xenocrystic zircon in the Kuboos pluton hints at the external Gariep Belt being floored by Kibaran crust.