Anomalous Sm−Nd ages for the early Archean Onverwacht Group Volcanics

Abstract

New Sm−Nd isotopic data for eight samples of basalt and komatiite from the Tjakastad Subgroup (lower Onverwacht Group) of the Barberton Greenstone Belt (BGB) of the Kaapvaal craton in southern Africa are reported. They give new constraints on the interpretation of Sm−Nd ages for the Subgroup and highlight the petrogenesis of Tjakastad volcanics. Although Sm−Nd isotopic data earlier reported for volcanic rocks from the Tjakastad Subgroup yielded an “isochron” age of 3526±48 Ma, the new results give a much younger “isochron” date of 3269±84 Ma. The 3526±48 Ma “isochron” age has been obtained in combining samples ranging in composition from felsic volcanics to ultrabasic komatiites and is thus considered suspect with regard to the pre-requisite of geochronology that all the studied rocks must have had identical initial isotopic compositions. The new “isochron” date of 3269±84 Ma has been obtained in combining samples solely of basic/ultrabasic composition. It might thus represent a more correct age for the eruption of the Tjakastad Subgroup volcanism. In fact, owing to the potential problem of source heterogeneity and also in the light of geochronological and geochemical arguments we show that this date also has little chance to have any strict chronological meaning. Most likely, the Tjakastad volcanics were formed 3450 Ma ago. Also most likely, their source rocks were isotopically heterogeneous and the 3530 Ma and 3270 Ma linear arrays are not true but apparent “isochrons”. Based on the calculated ɛ_Nd (3450) values and other geochemical arguments, we show that three possible sources might have been involved: depleted mantle, primitive mantle and older continental crust.