Extreme Nd-isotope heterogeneity in the early Archaean — fact or fiction? Case histories from northern Canada and West Greenland

Abstract

SmNd data on rock suites from early Archaean provinces in northern Canada and West Greenland clearly demonstrate that tectonothermal (i.e. igneous, metamorphic, tectonic) processes which affected the rocks long after their formation produced open-system behaviour leading to effective resetting of the SmNd system accompanied by complete, or near-complete, Nd-isotope homogenisation. This means that extreme caution is required in interpreting highly variable initial ϵ Nd values of ancient rocks in terms of long-standing regional mantle heterogeneity or of complex mantle-crust interaction processes. In particular, calculated initial ϵ Nd values based on high-precision zircon UPb dates may be of little or no significance in terms of geochemical evolution of early mantle and crust source regions. A striking example is provided by the Acasta gneisses of northern Canada, with published SHRIMP UPb zircon dates in the range 3.6–4.0 Ga and apparent, initial ϵ Nd values in the range −4.8 to +3.6 (Bowring and Housh, 1995). A combination of 34 published and new SmNd whole-rock analyses for a wide range of rock types yields a regression (errorchron) age of 3371 ± 59 Ma (MSWD = 9.2), with initial ϵ Nd = −5.6 ± 0.7. Whilst the very negative initial ϵ ND provides strong, independent support for the extreme age of the Acasta gneiss protolith, resetting of the SmNd system at ∼ 3.4 Ga renders calculation of initial ϵ Nd based on the zircon UPb dates geologically meaningless. Analogous considerations for early Archaean Akilia enclaves and host Amîtsoq gneisses of West Greenland suggest that their published range of initial ϵ Nd values of ∼ −4.5 to +4.5 at ∼ 3.73–3.87 Ga (Bennett et al., 1993) may be unrealistically wide and, therefore, inappropriate for modelling upper-mantle heterogeneity. In an attempt to determine a realistic initial ϵ Nd value, we have regressed 58 published and new SmNd data for two major rock units of the Isua supracrustal belt (felsites and mica-schists), regarded as having a short-term crustal history combined with minimal SmNd disturbance. The SmNd regression yields an age of 3776 ± 52 Ma (MSWD = 8.2), with initial ϵ Nd = +2.0 ± 0.6. This value is much closer to conventional depleted-mantle models (e.g., DePaolo et al., 1991) than to the value of ∼ +4.5 proposed by Bennett et al. (1993) for this age range. Our observations suggest that there may have been no major change in processes between early Archaean and more recent styles of depleted-mantle evolution.