Rb-Sr age measurements on total rock and separated-mineral fractions from the old granite of the central Transvaal

Abstract

The ages here reported relate to the Old Granite that is exposed between Johannesburg and Pretoria in the Transvaal. The Rb-Sr method was used to measure the ages of five total-rock samples and thirteen mineral fractions separated from the granite and from pegmatite veins. The chemical and mass-spectrometric techniques employed are described briefly. The total-rock samples yield concordant results, and the age of emplacement of the granite is found to be 3200±65 m.y. (λ = 1.39 × 10−11 years−1), while the primary abundance of Sr87 is found to be .07006±.00030. The apparent ages as deduced from the separated minerals vary widely, and ages both higher and lower than the total-rock age are reported. It is considered that the discordance of the mineral ages is the result of the diffusion of radiogenic strontium from mineral to mineral and that the diffusion probably occurred about 2000 m.y. ago.