A comparative study of the geochemical and isotopic systematics of late archaean flood basalts from the pilbara and kaapvaal cratons

Abstract

Geochemical and SmNd isotopic data are reported for igneous rocks of the 2.76 to 2.69 Ga Fortescue Group of the Pilbara Craton, Western Australia, and the ∼ 2.70 Ga Ventersdorp Supergroup of the Kaapvaal Craton, southern Africa. The metamorphic history of the Fortescue Group has also been investigated using whole-rock RbSr and PbPb isochron techniques. Igneous of both sequences have mixed tholeiitic and calc-alkaline affinities and show immobile element correlations indicating derivation from chemically heterogeneous, LREE-enriched sources. Both sequences have similar negative ϵ Nd values (between −1.5 and −4.4 for the Fortescue Group and 0 and −3.4 for the Ventersdorp Supergroup) which are not correlated with Mg/(Mg + Fe 2+). The negative ϵ Nd values of the Fortescue Group rocks probably do not result from crustal assimilation within crustal-level magma chambers during or following differentiation as the Fortescue Group felsic differentiates have ϵ Nd values within the ranges of values found in associated mafic rocks and zircon xenocrysts are absent in the felsic differentiates of the Fortescue Group. Instead, these results are consistent with either extensive crustal contamination of primitive (komatiitic or picritic) Fortescue Group parent magmas prior to their differentiation or derivation from negative ϵ Nd (or enriched) mantle sources. The presence of zircon xenocrysts in some Ventersdorp samples provides compelling evidence of some crustal contamination, although correlations between immobile element ratios (such as La/Yb, Ti/Zr, Ti/Sc and V/Zr) and ϵ Nd indicate derivation from chemically heterogeneous, enriched mantle sources. A whole-rock SmNd isochron date of 3308 ± 138 Ma, ∼ 600 Ma older than the time of eruption indicated by UPb zircon data and within, ∼ 300 Ma of the formation of the Kaapvaal granite-greenstone terrane, may date an SmNd fractionation event within the subcontinental lithospheric mantle sources of the Ventersdorp rocks. RbSr and PbPb isochrons obtained for several flows from the Fortescue Group register hydrothermal events occurring between ∼ 2.45 and 2.0 Ga and probably associated with burial metamorphism. Similarities in the field geology, geochemistry and isotopic characteristics of the Fortescue and Ventersdorp sequences and Phanerozoic examples of continental flood basalt volcanism suggest a common mode of origin, possibly involving the interaction of asthenospheric mantle plumes with subcontinental lithospheric mantle sources which have been modified by subduction processes.