Elemental and Nd-isotope systematics of the Upper Basalt Unit, 2.7 Ga Kambalda Sequence: Quantitative modeling of progressive crustal contamination of plume asthenosphere

Abstract

The 2.7 Ga Kambalda Sequence comprises a mafic and ultramafic volcanic rock-dominated sequence of the Kalgoorlie Terrane, Yilgarn Craton, Western Australia. The Sequence is divided into the Lower Basalt and Upper Basalt Units, separated by the Kambalda komatiite lava units. Upper Basalts have a total range of ε Nd from + 3.0 to + 1.5, defining a trend in ε Nd versus Nb/Th from the uncontaminated Victorious Basalt ( ε Nd + 3, Nb/Th 8.8–9.7), through the Big Dick, Devon Consols, and Coolgardie komatiite basalts, to the most contaminated Bent Tree Basalt ( ε Nd + 1.5, Nb/Th 2.9–3.5). Recent geological and geochronological results show that the Black Flag Group (BFG) tonalite–trondhjemite–dacite has an early eruptive phase coeval with the Upper Basalts, as well as a later phase overlying the Kambalda Sequence. Modeling of assimilation-fractional crystallisation (AFC) of the Kambalda Komatiite by 10–15% addition of average BFG, followed by 30–40% crystallisation, generates the best fit to the compositional spectrum of the LREE-enriched Devon Consols Basalts. Modeling of Archean average lower-, middle-, or upper continental crust generates a poor fit. Collectively, the results are consistent with decompressional melting at ≤ ∼ 100 km, of an asthenosphere plume impinging on the base of thinned, rifted, cratonic lithosphere where the most primitive, hottest liquids were contaminated by the Black Flag Group, or its melts, whereas the more evolved, cooler liquids were not. Contamination was dominantly by crust rather than cratonic mantle lithosphere.