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Journal of Petrology | Volume 3 | Number 2 | Pages 244-279 | 1962
© Oxford University Press 1962

research-article

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

T. N. CLIFFORD, L. O. NICOLAYSEN and A. J. BURGER

Research Institute of African Geology, Leeds University England
Bernard Price Institute of Geophysics, Witwatcrsrand University Johannesburg, South Africa
National Physical Research Laboratory Pretoria, South Africa

ABSTRACT

Chemical, spectrographic, and isotopic analyses are presented for the Franzfontein alkali granite and constituent minerals. This rock has the chemical character of granties produced by liquid {rightleftharpoons} crystal equilibrium. Dated at 1, 700±70m.y. by the constituent zircon, its crystallization formed part of a major period (the Huabian episode) of batholithic granite emplacement in northern South-West Africa. The occurrence of these crystalline rocks in the core of the Huab anticline defines the maximum possible age of the overlying Otavi Facies sediments, precluding their correlation with the Transvaal System of South Africa.

The imprint of the Damara metamorphism (Damaran episode) is reflected in the Sr/Rb age (560±30 m.y.) obtained for the biotite: the inversion of biotite to stilpnomelane and chlorite probably represents the mineralogical effects of that metamorphis. Isotopic data indicate that changes in the relative concentrations of Rb and Sr differed significantly in plagioclase and microcline; such data from feldspars in metamorphic rocks should, therefore, be interpreted with caution.


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