The Middle to Late Proterozoic Awasib Mountain terrain (AMT) is situated at the approximate contact between the supracrustal Sinclair Sequence and the Namaqualand Metamorphic Complex in southern Namibia. Tholeiitic to alkaline metabasalts from the Kairab Metamorphic Complex, which forms part of the basement to the Sinclair Sequence in the AMT, yield an Rb-Sr isochron age of 1461 ± 169 Ma with an initial ⁸⁷Sr/⁸⁶Sr (R₀) of 0.70269 ± 12. A preliminary Pb-Pb isochron age of 1467₋₂₁₅⁺²⁰¹ Ma for these metabasalts appears to confirm the Rb-Sr age. These metabasalts have been intruded by the calc-alkaline Aunis tonalite gneiss which yields an Rb-Sr isochron age of 1271 ± 62 Ma and R₀ of 0.7029 ± 3. The low R₀ obtained for both of these basement units implies that the ages are primary and hence that the isotope systematics have not been disturbed by subsequent deformation and metamorphism. Andesitic and rhyolitic volcanics of the Haiber Flats Formation (HFF) are correlated with the Sinclair Sequence. Basaltic andesites from the HFF have shoshonitic affinities and may be classified as “medium- to high-K” and “orogenic” in character. One basaltic andesite unit yields an age of 1086 ± 44 Ma and R₀ of 0.70305 ± 17, the low R₀ indicating derivation from a mantle source region that is slightly depleted relative to a “bulk earth” or “uniform reservoir” composition. The errorchron age estimate of 1038 ± 74 Ma and R₀ of 0.718 ± 15 obtained for a rhyolite porphyry from the HFF is within error of that deduced for the basaltic andesite. However, the high R₀ of this largely ignimbritic unit is probably due to a significant component of melted crust. The younger, possibly subvolcanic, Awasib granite yields an errorchron with an apparent age of 934 ± 70 Ma and R₀ of 0.719 ± 12. Scatter of these data may be attributed to compositional variation in a postulated crustal source. The A-type characteristics exhibited by both the HFF rhyolite and Awasib granite are compatible with their derivation by crustal anatexis. Available isotopic and geochemical data, together with field evidence, suggest an extensional palaeoenvironment for much of the AMT and Sinclair Sequence. While isotopic data alone are insufficient to establish the nature of the rifting, the broadly contemporaneous “Namaqua Orogeny” suggests that rifting may be collision-induced. Furthermore, the mantle source of abundant “orogenic” lavas in the AMT and Sinclair Sequence appears to owe its imprint of subduction-related metasomatism to a more ancient event.