Petrology and Age of the Pre-Otavi Basement Granite at Franzfontein, Northern South-West Africa

Chemical, spectrographic, and isotopic analyses are presentedfor the Franzfontein alkali granite and constituent minerals.This rock has the chemical character of granties produced byliquid crystal equilibrium. Dated at 1, 700?70m.y. by the constituentzircon, its crystallization formed part of a major period (theHuabian episode) of batholithic granite emplacement in northernSouth-West Africa. The occurrence of these crystalline rocksin the core of the Huab anticline defines the maximum possibleage of the overlying Otavi Facies sediments, precluding theircorrelation with the Transvaal System of South Africa. The imprint of the Damara metamorphism (Damaran episode) isreflected in the Sr/Rb age (560?30 m.y.) obtained for the biotite:the inversion of biotite to stilpnomelane and chlorite probablyrepresents the mineralogical effects of that metamorphis. Isotopicdata indicate that changes in the relative concentrations ofRb and Sr differed significantly in plagioclase and microcline;such data from feldspars in metamorphic rocks should, therefore,be interpreted with caution.