Single-grain and conventional zircon dating of remobilized basement gneisses in the central Damara belt of Namibia

The southwest-central Damara belt of Namibia contains a variety of granitoid gneisses. Zircons, from two samples collected at localities in the lower Khan River near Roessing Uranium Mine, were dated using the ion microprobe SHRIMP and conventional dissolution techniques. Conventional U-Pb analysis of size fractions from sample DG 61 yielded poor alignment of the data points with imprecise Concordia intercept ages. The single-grain ion microprobe data for most grain domain analyses can be fitted to a regression line with an upper Concordia intercept age of 1038±58 Ma and essentially a zero lower intercept. Conventional analyses of zircons from sample GP 345 define a regression line intersecting Concordia at 766±69 and -300±450 Ma respectively. The SHRIMP analyses revealed one ancient xenocrystic grain with an age of 2093±51 Ma, while the remaining grains yielded imprecise results which are discordant but tightly grouped near the Concordia curve. The rim around one 1196±98 Ma old core provided an age of 571±64 Ma, thereby reflecting the effect of the Damara high-grade metamorphism. The 1040-1100 Ma ion microprobe single zircon ages for the first time demonstrate the existence of a pre-Damara basement distinct from, and considerably younger than, the Abbabis Complex. This suggests that this basement represents a northwest extension of the Rehoboth terrain, and in this case there may have been continuity of pre-Damara continental crust underneath the southern Damara belt. 35 refs., 6 figs., 3 tabs