Rb-Sr age measurements on various Swaziland granites

Abstract

Rb-Sr age measurements have been made on total-rock samples of eight granites from Swaziland, and the results obtained broadly conform with the stratigraphic classification of the granites into G5, G4, and G1. Three samples of the G5 granite gave ages of 2200±50 m.y., 2550±50 m.y., and 2880±340 m.y., respectively. The ages of four samples of the G4 were in satisfactory agreement, and the mean age is 3070±60 m.y. Only one sample of the G1 was studied, and the age found is 3440±300 m.y. Measurements made on mineral fractions separated from some of the granites yielded discordant ages, which are interpreted as evidence of the diffusion of radiogenic Sr87 from mineral to mineral. A brief account of the petrography of the granites is included.