Whole-rock radiometric age patterns in the Aggeneys-Gamsberg ore district, central Bushmanland, South Africa

Three separate granitoid gneiss units underlie the Aggeneys-Gamsberg ore district in central Bushmanland. The oldest unit, the Achab Gneiss, has been interpreted as a basement complex to the Bushmanland Group, within which the Hoogoor Gneiss forms a basal stratigraphic member. The youngest unit, the Aroams Gneiss, represents post-Bushmanland granitoid intrusive activity synchronous with the Namaqua high-grade metamorphic event. Whole-rock Rb-Sr and U-Th-Pb isotope patterns indicate variable response by all gneiss units to metamorphic resetting during the Namaqua tectonothermal episode 1100 +/- 100 Ma ago. Evidence for pre-Namaqua igneous events have been found in the Achab Gneiss, which contains an orthogneiss component with a crystallization age of similar to 2000 Ma. Sm-Nd isotope patterns indicate primary crustal formation at similar to 2000 Ma, followed by reworking events, possibly at similar to 1650 Ma during volcanic activity associated with the deposition of the Bushmanland Group (Hoogoor Gneiss), and again later at similar to 1100 Ma when syntectonic granitoids were emplaced and deformed (Aroams Gneiss).