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.
Financed by the National Centre for Research and Development under grant No. SP/I/1/77065/10 by the strategic scientific research and experimental development program:
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