A. Riganti et Ah. Wilson, GEOCHEMISTRY OF THE MAFIC-ULTRAMAFIC VOLCANIC ASSOCIATIONS OF THE NONDWENI GREENSTONE-BELT, SOUTH-AFRICA, AND CONSTRAINTS ON THEIR PETROGENESIS, Lithos, 34(1-3), 1995, pp. 235-252
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.