The physical volcanology and geochemistry of the Nsuze group, Pongola supergroup, of northern KwaZulu-Natal and southeastern Mpumalanga.
The Nsuze Group forms the lower, predominantly volcanic succession of the Pongola Supergroup. The 2.9Ga Nsuze Group outcrops in southeastern Mpumalanga, northern KwaZulu-Natal and Swaziland. The volcanic rocks of the Nsuze Group are basalts, basaltic andesites, andesites, dacites and rhyolites preserved as both lava and pyroclastic deposits. The oldest volcanic sequence of the Nsuze Group is the basaltic Wagondrift Formation. The younger Bivane Subgroup represents the main volcanic component of the Nsuze Group. The White River Section represents a complex volcanic history of magma storage, fractionation, and eruption, supplied by a multi-level system of magma chambers. The basaltic and basaltic andesite rocks of the White Mfolozi Inlier represent the steady and non-violent eruption of lavas from related volcanic centres. The Nsuze Group rocks have been metamorphosed by high heat flow burial metamorphism to lower greenschist facies. Geochemically, elements display well-defined fractionation trends, with evident sub-trends within each phase group of samples. These sub-trends are related to the fractionation of key minerals, in particular plagioclase. The REE patterns show that evolution of magma was largely controlled by the fractionation of plagioclase. All REE patterns show LREE enrichment relative to the HREE. The Wagondrift Formation was derived from a more depleted source than the younger Bivane Subgroup volcanic rocks and exhibits a within-plate tectonic signature. The volcanic rocks of the Bivane Subgroup in the White River Section and the White Mfolozi Inlier are geochemically similar. The volcanic rocks of the Bivane Subgroup of both the White River Section and the White Mfolozi Inlier have a subduction zone tectonic signature, in particular a Ta-Nb negative anomaly. Tectonic discrimination diagrams suggest an enriched source related to a continental-arc setting. The geochemistry suggests an eclogitic source for the Nsuze Group volcanic rocks. The formation of eclogite in the mantle requires subduction of basaltic material. Archaean models for subduction-like processes include decoupling of oceanic crust and subsequent underplating of the continental lithosphere, and low-angle subduction which minimises the effect of the mantle wedge. It is possible that a combination of these processes resulted in an enriched eclogitic source for the magmas of the Nsuze Group.