A 40Ar39Ar geochronological study of komatiites and komatiitic basalts from the Lower Onverwacht Volcanics: Barberton Mountain Land, South Africa

Abstract

40Ar39Ar step-heating analyses have been carried out on domatiites and komatiitic basalts from the Komati Formation, the uppermost formation of the Tjakastad Subgroup in the Barberton Greenstone Belt. The results for the komatiites fall into two groups, which have integrated ages either greater than 3.3 Ga or less than 3.0 Ga. The most precise plateau ages in the first group correspond to an age of ∼3.49 Ga, which we take to be the age of metamorphism since the whole rocks analysed are composed of metamorphic minerals. Other samples in the first group appear to preserve a record of a later event at ∼3.43 Ga. Evidence of this younger age is also seen in the only komatiitic basalt displaying a stable age spectrum. The second group of komatiites (integrated ages less than 3.0 Ga) on a 40Ar39Ar correlation diagram suggest that a 3.3 Ga event may have introduced paleoatmospheric argon with a 40Ar39Ar ratio of ∼190 into some of the komatiites. Comparison of the results of detailed electron microprobe analyses with the Ca/K spectra obtained in the step-runs, suggests that tremolite is the mineral tenaciously recording the great ages of the komatiites, while hornblende is the dominant argon retainer in the komatiitic basalts. The argon ages are completely consistent with recently published U-Pb ages of zircons from the Barberton Greenstone Belt and suggest that the original metamorphism of the komatiites occurred shortly after their eruption.