Elemental and Nd-isotope systematics of the Upper Basalt Unit, 2.7 Ga Kambalda Sequence: Quantitative modeling of progressive crustal contamination of plume asthenosphere Nuru Said a, ⁎, Robert Kerrich b a Centre for Exploration Targeting, School of Earth and Environment, The University of Western Australia, 35 Stirling Hwy, Nedlands, WA 6009, Australia b Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK., Canada S7N 5E2 abstract article info Article history: Received 3 August 2009 Received in revised form 19 February 2010 Accepted 22 February 2010 Editor: R.L. Rudnick Keywords: Kalgoorlie Terrane Upper Basalt Geochemistry Black Flag Group Assimilation-fractional crystalisation Asthenoshere plume Cratonic mantle lithosphere 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. © 2010 Elsevier B.V. All rights reserved. 1. Introduction and scope Several classic papers report geochemical data for ultramafic and basalt volcanic sequences of the Yilgarn Craton, including the Kalgoorlie Terrane (Fig. 1). Sun and Nesbitt (1978) described LREE- depleted spinifex-textured peridotite (STP) flows, and basalts, from the Yilgarn Craton and other Archean greenstone belts. They interpreted the LREE characteristics in terms of a two stage process: removal of an incompatible element enriched melt followed by second stage melting of the depleted residue. Three basaltic magma series were defined by Redman and Keays (1985) for Yilgarn Craton greenstone belts. High magnesium series-, and low magnesium series basalts (HMSB and LMSB) were accounted for by mantle source regions variably depleted in incompatible elements, whereas siliceous high magnesian series basalts (SHMSB or SHMB) were derived from an incompatible element enriched mantle. Alternatively, Sun et al. (1989) interpreted SHMB as crustally contaminated komatiites. The ∼ 2.7 Ga Kambalda Sequence of the Kalgoorlie Terrane includes stratigraphically Lower and Upper Basalt Units separated by the Kambalda Komatiite Unit (Figs. 1 and 2; Table 1). In an earlier study, Lesher and Arndt (1995) further constrained models for Archean komatiites and associated basalts with combined trace element and Sm–Nd isotope data, for the Kambalda Sequence. They documented progressive crustal contamination of liquids erupted from a mantle plume, up the volcanic stratigraphy of the Kambalda Sequence. Contamination of the parental komatiite with generic granitic crust was modeled, which approximately reproduced trace element and isotopic trends measured in the progressively contam- inated volcanic sequence. The Black Flag Group tonalite–trondhje- mite–dacite (TTD) overlies the Kambalda Sequence, but a recent study identifies an earlier phase of eruption of the Black Flag Group coeval with the Upper Basalts (Kositcin et al., 2008 ) conferring field evidence for testing of additional potential contaminants. This paper reports new high-precision major and trace element, and Sm–Nd isotope data, for five basalt suites, and one komatiitic basalt (KB) suite, defined in the Upper Basalt Unit, to build on the earlier studies. We model a number of contaminants including the Black Flag Group, well characterised high-Ca granites of the Yilgarn Craton, and average Archean lower-, middle- and upper continental Chemical Geology 273 (2010) 193–211 ⁎ Corresponding author. Tel.: +610 422331906, +618 64881809. E-mail address: nsaid@cyllene.uwa.edu.au (N. Said). 0009-2541/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2010.02.022 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo