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U-Pb and Rb-Sr age determinations on Middle Proterozoic rocks from the Lower Orange river area, south-western Africa.

Abstract:

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 NW Cape Province and southern SW 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 approx. 1100 m.y. Namaqua and approx. 500 m.y. Pan-African metamorphic events. Metalavas of the lower De Hoop Subgroup in the Orange River Group yield a Rb-Sr whole rock isochron age of 1000 +- 30 m.y. The high initial 87Sr/86Sr ratio of 0 722 +- 0 02 indicates resetting at that time. Analyses of Vioolsdrif granitoid zircons from six localities give two U-Pb ages of 1830 +- 30 m.y. and 1730 +- 20 m.y. The latter age was obtained for only one sample (a composite zircon and three 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 approx. 1100 m.y. age. Reservations are expressed about the validity of the younger 1730 +- 20 m.y. zircons age, although it agrees closely with the 1740 +- 40 m.y. whole rock Rb- Sr age for granitoid samples from the Klein Helskloof area. It is noted that the age pattern for the Vioolsdrif suite closely resembles that of the Fransfontein granitic rocks in northern SW Africa. This may indicate that these two regions belong to the same crustal entity.-Authors