Variability of Nb/U and Th/La in 3.0 to 2.7 Ga Superior Province ocean plateau basalts: implications for the timing of continental growth and lithosphere recycling

Extensive volcanic sequences of tholeiitic basalts, having near-flat REE pa tterns and spatially associated with komatiites, occur in many Archean Supe rior Province greenstone belts; they are considered to be fragments of intr aoceanic volcanic plateaus derived from a plume. Basalts from the 2.9-3.0 G a Lumby Lake greenstone belt, carefully screened for minimum alteration, ha ve variable Nb/U ratios of 36 to 58. Least-altered basalts from the Abitibi and Wawa greenstone belts also have variable Nb/U ratios of 25 to 50 and 2 8 to 42, respectively, compared to an average value of 47 for modern ocean basalts. In the Abitibi suite Nb/U correlates positively with Nb/La-pm but negatively with Th/La-pm. Alteration can be ruled out as the cause of Nb/U variation, as there are no correlations of Nb/U with LOI or Eu/Eu*, and Nb/ U correlates with Nb/Th in all three suites of basalts. Numerous lines of e vidence indicate that crustal contamination can be eliminated as the cause of Nb/U variability, especially for samples with Nb/U > 36. High Nb/U ratio s can be explained by recycling ocean crust processed through a subduction zone (high Nb/U and Nb/La-pm, low Th/La-pm) into the mantle source of the b asalts, whereas low Nb/U ratios can be accounted for by recycling complemen tary subarc mantle lithosphere, or continental crust (low Nb/U and Nb/La-pm , high Th/La-pm) into the mantle source. For the entire population of basal ts, Th/La-pm ratios generally <1 may result from recycling the residue of s lab-derived Archean-type tonalites having high Th/La ratios. Nb/U ratios as high as the 47 +/- 10 range of modem ocean basalts have been found in a 2. 7 Ga volcanic belt of the Yilgarn craton. Taken together, the results signi fy very early growth of the continental crust, rather than episodic growth over several Ga. (C) 1999 Elsevier Science B.V. All rights reserved.