Geochemical systematics of basalts of the Lower Basalt Unit, 2.7 Ga Kambalda Sequence, Yilgarn craton, Australia: Plume impingement at a rifted craton margin

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 similar to 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<8, whereas the more evolved show lower Th in conjunction with Nb/Th>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 epsilon(Nd) values spanning from 4.5 to 1.5, whereas epsilon(Nd) values for the contaminated Missouri Basalt and Penneshaw Formation span between 2 and 1; there is a weak trend of epsilon(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. (C) 2009 Elsevier B.V. All rights reserved.