Layered ultramafic complexes of the barberton greenstone belt – age constraints and tectonic implications

Abstract

Layered ultramafic–mafic complexes are a common component of the stratigraphically uppermost part of the Onverwacht Group of the Barberton Greenstone Belt. Associated with the Mendon Formation in the south and the Weltevreden Formation in the north, they represent an assemblage of thick differentiated flows and shallow synvolcanic intrusions ranging in composition from dunite to gabbro. U-Pb zircon dating of gabbro from the Sawmill and the Mundt’s Concession ultramafic complexes from the northern part of the Barberton Greenstone Belt yielded ages of 3 258 ± 8 Ma and 3 244 ± 11 Ma, respectively. The ultramafic complexes are thus regarded to have been emplaced during a magmatic flare-up in the final stage of Weltevreden Formation volcanism, post-dating ultramafic magmatism in the southern part of the belt by several millions of years and thus suggesting diachronous evolution of the Onverwacht Group in the Barberton Greenstone Belt.