U-Pb and Rb-Sr age determinations on Middle Proterozoic rocks from the lower Orange River area, South-Western Africa

Using the U-Pb and Rb-Sr methods, age measurements were made on 29 whole rock and 24 mineral samples from the Middle Proterozoic rocks of the Orange River Group and the granitoids of the Vioolsdrif igneous suite in the north-western Cape Province and southern South West Africa. The samples are briefly described; a short account is given of the geological setting and the stratigraphic relationship in this geologically complex area, characterized by multiphase deformation and metamorphism. The age patterns are also complex and show the effects of the approximately 1 100 m.y. Namaqua and approximately 500 m.y. Pan-African metamorphic events. Analyses of Vioolsdrif granitoid zircons from six localities give two U-Pb ages of 1 830 plus minus 30 m.y. and 1 730 plus minus 20 m.y. The latter age was obtained for only one sample (a composite zircon and three size fractions). This sample comes from close to the boundary between the Richtersveld Province and the Namaqua Province and U-Pb data for sphene and apatite give an approximately 1 100 m.y. age. U-Pb data for zircons from the Obib leucogranite and the Gumchavib foliated granodiorite yield an age of 1870 plus minus 20 m.y. This age suggests that these granitic inliers within the Gariep belt were emplaced as part of the Vioolsdrif igneous suite. The Rb-Sr whole rock data for the Obib leucogranite give a resetting age of 950 plus minus 30 m.y. and a 87Sr/86Sr initial ratio of 0,713 plus minus 0,002. A 1 070 plus minus 300 m.y. errorchron, with a high initial Sr ratio of 0,720 plus minus 0,008, is obtained for the Gumchavib gneiss Rb-Sr whole rock data. It is noted that the age pattern for the Vioolsdrif suite closely resembles that of the Fransfontein granitic rocks in northern South West Africa. This may indicate that these two regions belong to the same crustal entity and therefore point to the possible existence of an approximately 1 900 m.y. old basement underlying extensive areas of south-western Africa