⁴⁰Ar/³⁹Ar dating of the Cambro-Ordovician Vanrhynsdorp tectonite in southern Namaqualand

40Ar/39Ar plateau ages are reported from metasediments and biotite concentrates from the late Precambrian/Cambrian Vanrhynsdorp Group in southern Namaqualand. The result show a span of 75 Ma from about 552 Ma to 476 Ma. Progressively younger ages were obtained from the northeastern marginal area towards the southwestern central area of the Vanrhynsdorp orogen. A single major dynamothermal metamorphic event near to 496 Ma is postulated for the Vanrhynsdorp tectonite. Relatively high ages recorded in the northeast (553 Ma and 527 Ma) are interepreted as resulting from the interference of inherited excess argon contained within original detrital grains still preserved in middle anchi-zone phyllites. Fully recrystalline phyllites southwest of the Arizona Fault have ages close to 496 Ma. A relatively low value obtained from biotite-grade rocks in the southwest (476 Ma) is regarded as registering the delayed cooling age of biotite in this zone as a result of slow uplift and continued deformation in the more deeply buried central part of the orogen. -Authors