Abstract
Archaean granitoids of the Johannesburg Dome were evaluated in terms of the granitoid series using Fe2O3/FeO ratios of bulk chemistry and magnetic susceptibility measurement. The Fe2O3/FeO ratios range between 0.05 and 0.72 and are generally below 0.5, implying that they belong mostly to reduced, ilmenite series granitoids. Magnetic susceptibility was measured on 122 samples of TTG suite granitoids and 239 samples of calc-alkaline suite granitoids collected on the 700km2 granitic inlier constituting the Johannesburg Dome in the central part of the Kaapvaal Craton, South Africa. From the measurements it was established that the TTG suite consists of 78% ilmenite -series granitoids and 22% magnetite-series granitoids, while the calc-alkaline suite consists of 83% ilmenite-series and 17% magnetite-series granitic rocks. Magnetite-series granitoids, which are intermediate series in a strict sense, tend to occur sporadically in the central part of the Dome at the interface between the TTG and calc-alkaline granitoids. The granitoids are generally seen to be of a reduced type, the reducing conditions most likely being partly the result of assimilation of the country rocks into which the granitic rocks intruded.
