GEOCHEMISTRY OF THE MAFIC-ULTRAMAFIC VOLCANIC ASSOCIATIONS OF THE NONDWENI GREENSTONE-BELT, SOUTH-AFRICA, AND CONSTRAINTS ON THEIR PETROGENESIS

The 3.4 Ga Nondweni greenstone belt is situated near the southern marg in of the Kaapvaal Craton and consists of predominantly pillowed and s pinifex-textured volcanic rocks. Primary volcanic structures and compo sitional characteristics of this succession are unique in currently kn own Archaean geology. None of the lavas exceeds 23% MgO (anhydrous) an d pyroxene rather than olivine dominates the crystallisation assemblag es. Lower MgO, higher SiO2 contents and different incompatible trace e lement ratios (e.g. lower Ti/Zr) of the mafic/ultramafic lithotypes di stinguish the Nondweni greenstone belt from the Barberton and other Ar chaean belts. The geochemistry permits subdivision into five categorie s: komatiites, komatiitic basalts, komatiitic andesites, basalts and b asaltic andesites. Progressive fractionation or partial melting of a d iscrete mantle source, although supported by co-linearity on some vari ation diagrams, cannot account for a group of incompatible-depleted ba saltic rocks or for the high silica content of some komatiitic andesit es. Contamination of mafic/ultramafic melts by felsic material is supp orted by isotopic data, but cannot alone account for SiO2 contents of up to 58% in some of the komatiitic rocks. REE chemistry also argues a gainst consaguinuity of the various rock-types. The komatiites have (G d/Yb)(N) > 1, and are either LREE-depleted or LREE-enriched. Komatiiti c basalts and komatiitic andesites are invariably LREE-enriched, but t he latter show overall depletion in REE relative to the komatiitic bas alts. A similar relation characterises the basalts and basaltic andesi tes. Polybaric melting of a mantle source and contamination by sialic crust followed by fractional crystallisation may explain the compositi onal characteristics of the Nondweni lavas.