Geochemistry of the mafic/ultramafic volcanic associations of the Nondweni greenstone belt, South Africa, and constraints on their petrogenesis

Abstract

The 3.4 Ga Nondweni greenstone belt is situated near the southern margin of the Kaapvaal Craton and consists of predominantly pillowed and spinifex-textured volcanic rocks. Primary volcanic structures and compositional characteristics of this succession are unique in currently known Archaean geology. None of the lavas exceeds 23% MgO (anhydrous) and pyroxene rather than olivine dominates the crystallisation assemblages. Lower MgO, higher SiO 2 contents and different incompatible trace element ratios (e.g. lower Ti/Zr) of the mafic/ultramafic lithotypes distinguish the Nondweni greenstone belt from the Barberton and other Archaean belts. The geochemistry permits subdivision into five categories: komatiites, komatiitic basalts, komatiitic andesites, basalts and basaltic andesites. Progressive fractionation or partial melting of a discrete mantle source, although supported by co-linearity on some variation diagrams, cannot account for a group of incompatible-depleted basaltic rocks or for the high silica content of some komatiitic andesites. Contamination of mafic/ultramafic melts by felsic material is supported by isotopic data, but cannot alone account for SiO 2 contents of up to 58% in some of the komatiitic rocks. REE chemistry also argues against consaguinuity of the various rock-types. The komatiites have (Gd/Yb) N > 1, and are either LREE-depleted or LREE-enriched. Komatiitic basalts and komatiitic andesites are invariably LREE-enriched, but the latter show overall depletion in REE relative to the komatiitic basalts. A similar relation characterises the basalts and basaltic andesites. Polybaric melting of a mantle source and contamination by sialic crust followed by fractional crystallisation may explain the compositional characteristics of the Nondweni lavas.