Comment on “Complementarity of Hf isotopes from detrital and igneous zircon: an example from the Central Gneiss Belt, Grenville Province, Ontario”

In a recent paper presenting 30 new Hf isotope analyses for orthogneisses from the Grenvillian Central Gneiss Belt in Ontario, Slagstad et al. (Slagstad, T., Easton, R.M., Huyskens, M., and Culshaw, N. 2024. Complementarity of Hf isotopes from detrital and igneous zircon: an example from the Central Gneiss Belt, Grenville Province, Ontario. Canadian Journal of Earth Sciences, 61: 19) suggested that these new data “may serve as a basis for some cautionary notes on the application of isotopic data alone to distinguish allochthonous from parautochthonous rocks”. These “cautionary notes” evidently arise from the poor fit of the new Hf isotope data to these authors’ expectations based on published work. However, we show that the new data are in complete agreement with our own published tectonic maps, based on Nd isotope mapping supported by other available geological evidence. Far from serving as the basis for “cautionary notes” about our use of Nd isotope mapping, the new data completely validate our approach. On the other hand, we show that the mantle model used by Slagstad et al. (2024) to calculate Hf crustal formation ages was based on erroneous old data, and does not yield realistic Hf model ages for Precambrian gneisses. Therefore, we propose a more robust model, based on applying the principles of DePaolo’s Nd isotope model to Hf mantle evolution. We term the resulting Hf ages T_DP model ages and show that they yield realistic crustal formation ages for the Ontario Grenville Province.