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 patterns and spatially associated with komatiites, occur in many Archean Superior Province greenstone belts; they are considered to be fragments of intraoceanic volcanic plateaus derived from a plume. Basalts from the 2.9–3.0 Ga Lumby Lake greenstone belt, carefully screened for minimum alteration, have 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 28 to 42, respectively, compared to an average value of 47 for modern ocean basalts. In the Abitibi suite Nb/U correlates positively with Nb/Lapm but negatively with Th/Lapm. 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 evidence indicate that crustal contamination can be eliminated as the cause of Nb/U variability, especially for samples with Nb/U > 36. High Nb/U ratios can be explained by recycling ocean crust processed through a subduction zone (high Nb/U and Nb/Lapm, low Th/Lapm) into the mantle source of the basalts, whereas low Nb/U ratios can be accounted for by recycling complementary subarc mantle lithosphere, or continental crust (low Nb/U and Nb/Lapm, high Th/Lapm) into the mantle source. For the entire population of basalts, Th/Lapm ratios generally <1 may result from recycling the residue of slab-derived Archean-type tonalites having high Th/La ratios. Nb/U ratios as high as the 47 ± 10 range of modern ocean basalts have been found in a 2.7 Ga volcanic belt of the Yilgarn craton. Taken together, the results signify very early growth of the continental crust, rather than episodic growth over several Ga.