Geochemistry and origin of late archean volcanic rocks from the rhenosterhoek formation, dominion group, South Africa

The Rhenosterhoek Formation, composed chiefly of subaerial volcanic rocks, is part of the Dominion Group (∼ 2.8 Ga) which rests unconformably on older continental crust in South Africa. Chemical compositions of volcanic rocks from three drillcores of this formation do not show systematic differences either between or within cores. Major and immobile trace-element contents indicate that the volcanics are basaltic andesites and andesites with calc-alkaline affinities. Incompatible trace-element distributions are similar to those of modern andesites from evolved island arcs or continental-margin arcs. Geochemical modeling indicates that the basaltic andesites and andesites can be produced by open-system fractional crystallization with olivine, clinopyroxene and plagioclase as principal liquidus phases. Up to 10% contamination with continental crust is also allowed by the data. This succession does not appear to have formed in an oceanic or in a continental-margin arc, but may have formed adjacent to a continental-margin arc system in an incipient foreland basin on the Kaapvaal Craton.