Neoarchean lavas of the Ventersdorp Large Igneous Province, South Africa: Sr-Nd-Hf isotopic and trace element evidence for a long-lived plume beneath a stationary African continent

Abstract

We review the properties of the Neoarchean Ventersdorp Supergroup and provide new trace element concentrations and the first combined Sr-Nd-Hf isotopic compositions for the mafic magmatic products. These sequences contain the remnants of some of the world's oldest Large Igneous Provinces (total preserved volume = 0.659 × 106 km3). Ventersdorp rocks have been divided into three major units, separated by prominent unconformities: the basal Klipriviersberg Group (∼2.78 Ga), the medial Platberg Group (2.72 Ga) and the upper Pniel Sequence (2.68 Ga). The base of the Klipriviersberg Group is the Westonaria Formation, composed almost entirely of picrites (12–18 wt% MgO) and komatiites (>18 wt% MgO). All other formations of the Klipriviersberg Group contain mainly basaltic (<12 wt% MgO, 45–57 wt% SiO2) and andesitic rocks (57–63 wt% SiO2). The Platberg Group is composed of bimodal mafic lavas (Goedgenoeg and Rietgat Formations) and crustally derived dacites and rhyolites (Makwassie Formation); we did not study the crustally derived sequences. The Pniel Sequence contains basaltic lavas in the Allanridge Formation (<7.8 wt% MgO). Strontium isotopes have been modified by extensive low-grade hydrothermal alteration processes and cannot be used to make inferences about magmatic processes. Initial εNd and εHf values are highest for Westonaria picrites and komatiites (maximum εNd = +2.16; εHf = +3.60), and match Neoarchean depleted, asthenospheric mantle, implying derivation from large degree partial melting in a plume head. Klipriviersberg basaltic-komatiitic lavas have the largest volume (0.145 × 106 km3), qualifying this as a Large Igneous Province in its own right; Platberg and Allanridge basalts and basaltic andesites have unradiogenic εNd and εHf (εNd = −2.45 to −4.01; εHf = −2.73 to −10.12), and lower volumes (Platberg = 0.022 × 106 km3; Allanridge = 0.042 × 106 km3), too small to represent LIPs. Although Westonaria picrites and komatiites were likely sourced from the hot, central portions of a flattened plume head, the other magmatic Formations in the Ventersdorp Supergroup may have been derived from cooler, compositionally different, marginal parts of the plume head, or possibly the plume tail. The three Ventersdorp magmatic components can be interpreted in terms of a single, long-lasting plume, with the spatial overlap having been caused by a stationary African plate over a time interval of >100 million years. Trace element ratios and isotopic compositions suggest that fractional crystallization and crustal assimilation were minor to absent in Ventersdorp basaltic magmas. Negative NbTa anomalies are present in all magmatic rocks except those of the Westonaria Formation, suggesting that the lithospheric mantle component bears a subduction signature; this component probably represents deeply subducted oceanic lithosphere that was entrained in the deep mantle by an ascending plume. Volume and compositional relations preclude derivation of the basaltic lavas from subcontinental lithospheric mantle or small, enriched domains in the SCLM. Zn/Fe relations can be interpreted in terms of heterogeneous sub-lithospheric mantle sources, with Klipriviersberg Group lavas having been derived dominantly from peridotitic sources, whereas Platberg and Allanridge mafic lavas were derived from a source richer in pyroxenitic components. Our new Nd isotopic data coupled with lithostratigraphic differences indicate that correlations between the Ventersdorp (South Africa) and the coeval (2.77–2.71 Ga) Fortescue (Australia) sequences are more complex than is currently understood, possibly reflecting differences in basin structure and evolution. More data are needed to constrain the Vaalbara hypothesis, which postulates that the Kaapvaal and Pilbara Cratons were adjoined in the late Archean.