Dating some significant events in the history of the Swaziland System by the Rb–Sr isochron method

Abstract

The Swaziland System, in the southeastern Transvaal, is one of the oldest stratified systems exposed. A brief account is given of this folded synclinal belt, and of the heterogeneous tract of migmatites. gneisses, and granites which surround it. Related geochronological work in this tract is summarized.An attempt has been made to date the system using the Rb–Sr isochron technique on (1) pegmatites that intrude it, (2) shales and graywackes of the Fig Tree Series, (3) acid lavas of the Onverwacht Series, and (4) biotite-tremolite schists associated with mineralized zones along the Onverwacht-Fig Tree contact. Essentially linear isochron plots were obtained in each case, but great caution is needed in interpreting the precise geological significance of each calculated age. The pegmatite data are interpreted as establishing a minimum age of 3.00 ±.03 b.y. for the Swaziland System, and as indicating that its deposition occurred at a significantly earlier time. The Fig Tree data yield an age of 2.98 ±.02 b.y. for a period of Sr-isotope homogenization, by either sedimentary diagenesis or later metamorphism. The data for the schists and acid lavas indicate ages of ~ 2.5 b.y., which are incompatible with either times of deposition, or recognized periods of metamorphic mineral growth, for these strata. They define the last time of Sr isotope homogenization; this may reflect widespread episodic heating (associated with the post-orogenic granite cycle) or cooling associated with regional uplift.The total-rock isochron technique has thus far failed to establish the age of the Swaziland System and factors responsible for this situation are discussed. Heterogeneous biotite-rich metamorphic rocks appear to be unsuitable for 'seeing through' the effect of superimposed metamorphisms, but may be suitable for dating of the metamorphisms.