Geochemical systematics of basalts of the Lower Basalt Unit, 2.7 Ga Kambalda Sequence, Yilgarn craton, Australia: Plume impingement at a rifted craton margin Nuru Said a, ⁎, Robert Kerrich b , David Groves a 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 18 June 2009 Accepted 19 November 2009 Available online 27 November 2009 Keywords: Kambalda Sequence Geochemistry Eastern Goldfields Superterrane Mantle plume Cratonic lithospheric mantle The 2.7 Ga Kambalda Sequence is a mafic and ultramafic volcanic sequence in the Kalgoorlie Terrane of the Yilgarn craton, Western Australia. The Sequence is divided into the Lower Basalt and Upper Basalt Units, separated by the komatiite lava flow units, and was erupted onto an autochthonous volcanic sequence, the 2.8 Ga Penneshaw Formation. The Lower Basalt Unit includes six basalt suites or formations with well preserved pillows and vesicles, at mid-greenschist facies: these are the Woolyeenyer and Burbanks Formations, and Lunnon, Wongi, Scotia, and Missouri basalts. Basaltic flows of the Woolyeenyer Formation, and Lunnon and Wongi Basalts, are compositionally restricted tholeiites with Mg# and Ni contents of 56–52, 110 ppm, 57–51, 190–160 ppm, and 63, 300–210 ppm, respectively. Heavy-REE (HREE) are flat at 4–6 times chondrite, with mild LREE depletion at (La/Sm) N = 0.77– 0.94, and Nb/Th ratios of 8–17 signifying that these basalts were erupted through the autochthon without crustal contamination as there is no correlation of Nb/Th with (La/Sm) N . The Scotia Basalt is distinctive in having a range of HREE fractionation where (Gd/Yb) N = 1.1–1.4 indicative of melting over a range of depths from above to below the garnet stability field at ∼90 km. Basalts of the Burbanks and Penneshaw Formations have major element compositions akin to the Woolyeenyer Formation, but near-flat REE at 9–11 times chondrite. Both formations have two populations: the most primitive have greater Th contents but Nb/Th b 8, whereas the more evolved show lower Th in conjunction with Nb/Th N 8, in keeping with the hottest liquids having assimilated crust but the evolved flows being uncontaminated. Missouri basalts are all crustally contaminated. Collectively, the uncontaminated Woolyeenyer, Lunnon, Wongi, Scotia basalts and Burbanks Formation have ε Nd values spanning from 4.5 to 1.5, whereas ε Nd values for the contaminated Missouri Basalt and Penneshaw Formation span between 2 and 1; there is a weak trend of ε Nd with Nb/Th. Collectively, the results are interpreted in terms of a mantle plume impinging at the edge of rifted cratonic lithosphere mantle, melting over a range of depths, where hotter liquids assimilated crust but cooler liquids did not. © 2009 Elsevier B.V. All rights reserved. 1. Introduction and scope Neoarchean greenstone terranes globally record two prevalent volcanic associations: komatiites–tholeiitic basalts and bimodal tholei- itic to calc-alkaline basalts–dacites. Komatiites and associated basalts are generally interpreted as products of anomalously hot mantle plumes, erupted in ocean basins or continental margins (McDonough and Ireland, 1993; Hollings et al., 1999; Kerrich et al., 1999a,b; Polat et al., 1999; Kerrich and Xie, 2002; Arndt, 2008; Manikyamba et al., 2008). Where there is a combination of field, trace element, and isotopic evidence for the absence of crustal contamination, basalts are tholeiitic with near-flat REE patterns over a range of REE abundances, and both komatiites and basalts share dominantly positive Nb anomalies. The tholeiitic to calc-alkaline association is generally considered to be arc- related (Wyman and Kerrich, 2009, and references therein). 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, 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. Lesher and Arndt (1995) further constrained these models with combined trace element and Sm–Nd-isotope data, documenting progressive crustal Lithos 115 (2010) 82–100 ⁎ Corresponding author. Tel.: +610 422331906, +618 64881809. E-mail address: nsaid@cyllene.uwa.edu.au (N. Said). 0024-4937/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2009.11.008 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos