Geochemistry and origin of late Archean volcanics from the ventersdorp supergroup, South Africa

Abstract

The Ventersdorp Supergroup (2700–2750 Ma) is comprised chiefly of mafic to intermediate subaerially erupted volcanics with smaller amounts of clastic sediments. The lower unit, the Klipriviersberg Group, consists of basal basaltic komatiites overlain by basalts which show a progressive increase in Mg number and Ni content and a decrease in incompatible element contents with increasing stratigraphic height. Volcanics of the overlying Platberg and Pniel Groups consist of basaltic andesites and andesites, and some felsic volcanics. All Ventersdorp volcanics exhibit a subduction zone geochemical component and are similar in incompatible element distributions to volcanics from continental-margin arc systems. Both tholeiitic and calc-alkaline trends occur in the Ventersdorp succession, with the former dominating. Crustal contamination appears to have played a minor role in the evolution of Ventersdorp magmas. With the exception of basaltic komatiite, the Klipriviersberg lavas can be related by progressive melting of a garnet lherzolite source (containing a subduction zone component) followed by up to 30% of shallow fractional crystallization. The basaltic komatiite can be produced from the same source at higher temperatures and the Pniel lavas by shallow fractional crystallization of basaltic komatiite. The Platberg volcanics must come from a within-plate enriched mantle source, and mafic and felsic members of this group can be related by shallow fractional crystallization. Geochemical and geologic data are consistent with a model for Ventersdorp magmas involving a subduction zone in which Klipriviersberg and Pniel magmas are produced from the mantle wedge and Platberg magmas come from an enriched mantle lithosphere with a superimposed subduction zone component. Secular trends in Klipriviersberg lavas can be explained by progressive adiabatic melting of an ascending plume rooted in the mantle wedge.