Geochemistry of volcanic rocks from the Nsuze Group, South Africa: arc-like volcanics in a 3.0 Ga-old intracratonic rift

The Nsuze Group (∼3000 Ma) in South Africa is comprised of volcanics and sediments that rest unconformably on older continental crust. Nsuze volcanics, which range in composition from basalt to rhyolite and include a significant proportion of andesites, have tholelitic to calc-alkaline geochemical affinities. Incompatible element distributions in Nsuze volcanics are similar to those in volcanics associated with continental-margin arcs and exhibit a typical subduction zone component (SZC) (enrichment in LILE and relative to Nb-Ta). The volcanics can be related to each other by shallow fractional crystallization of average Nsuze basalt, together with up to 10% assimilation of granite with a composition similar to that of the underlying Mpuluzi granite. Geological data are consistent with an origin for the Nsuze Group in an intracratonic rift. The SZC in Nsuze volcanics may have been inherited from an older subcontinental lithospheric source.