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.
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