Origin and evolution of the ∼1.9 Ga Richtersveld Magmatic Arc, SW Africa

Abstract

The results of a study of the Paleoproterozoic Richtersveld Subprovince/Magmatic Arc (RMA) in southern Namibia are integrated with previous work in adjacent NW South Africa to produce a new, unified model for the evolution of the RMA. The RMA was variably affected by the ∼1Ga Namaqua. In the west, low-grade metamorphic rocks (Vioolsdrif Domain) record a limited Namaqua imprint whereas in the east, the equivalent rocks were converted to amphibolite-facies gneisses (Pella Domain). Tectonic slivers of RMA rocks are also found in an imbricate thrust zone that forms the base of the over-riding Namaqua-age Kakamas Domain further east. The volcanic and equivalent plutonic rocks (Orange River Group (ORG) and Vioolsdrif Suite respectively) of the RMA form a coeval calc-alkaline magma series with characteristic island-arc geochemical signatures. The entire magmatic history took place between 1910 and 1865Ma, contrary to the much longer time-spans previously suggested. A large inclusion of migmatite (Bankwasser Migmatite) gave Archaean detrital zircon ages. It is interpreted as a possible fragment of Archaean Kaapvaal Craton basement that was incorporated into the RMA prior to ∼1885Ma. ∼2Ga detrital zircon ages in the ORG and ∼2Ga magmatic zircon inheritance and xenoliths entrained in the Vioolsdrif Suite reveal a hitherto unrecognised pre-RMA history. Recent work has shown that a pre-RMA arc (Sperrgebiet Arc) had developed to the west at ∼2020Ma in an upfaulted block within the Neoproterozoic Gariep Orogen. Consequently, a two-arc model is proposed for the early evolution of western Namaqualand with the early Sperrgebiet Arc evolving at ∼2020Ma which, possibly by processes of subduction migration, was cannibalised by the RMA at ∼1885Ma. Our data also suggests that certain quartzite sequences, traditionally placed within the ORG, were deposited after cessation of RMA activity and are thus unrelated to it.