The Mid-Proterozoic Sinclair Sequence in southern Namibia is made up of volcano-sedimentary successions and associated high-level intrusions in a mainly extensional tectonic environment. Granitoids from the marginal Awasib Mountain area have been investigated using precise U-Pb determinations on zircons. Regression of three zircon fractions from the pre-Sinclair Aunis Tonalite Gneiss yields an upper intercept, or emplacement, age of 1376.5+ 1.7/-1.6 Ma. A previous Rb-Sr whole-rock age determination of 1271 ± 62 Ma for the Aunis Tonalite Gneiss may be attributed to resetting at that time during an early tectonothermal event in the Grenville-age Namaqua Orogeny. Within the Sinclair Sequence, three zircon fractions from the Haisib Quartz Syenite produce an emplacement age of 1216.2 + 1.8/-1.5 Ma. Two zircon fractions from each of the Awasib and Chowachasib Granites yield emplacement ages of 1216.4 ± 1.2 Ma and 1216.7 + 4.7/-3.5 Ma, respectively. A composite regression treatment of all seven zircon fractions indicates that the emplacement ages for these three units are indistinguishable with an upper intercept age of 1216.4 + 1.3/-1.2 Ma (2 sigma). This places a minimum age constraint on the mafic flows of the Haiber Flats Formation and is significantly older than the 1086 ± 44 Ma Rb-Sr whole-rock age reported previously. Tectonothermal activity at ca. 1100 Ma in the Namaqua Province could account for the greenschist facies metamorphism of much of the marginal part of the Sinclair Sequence and related disturbance of the Rb-Sr system.