DETAILED PETROCHRONOLOGY OF THE 4.02 GA IDIWHAA TONALITIC GNEISS: EVIDENCE REGARDING AMOUNT OF PRE-EXISTING HADEAN CONTINENTAL CRUST

The Acasta Gneiss Complex (AGC) contains the oldest known evolved rocks on Earth with U-Pb zircon ages recording crust formation between 4.0-3.6 Ga. Here we discuss whole-rock elemental and isotope geochemistry along with U-Pb, trace element, hafnium and O-isotope compositions of zircon from the km-scale >4.0 Ga Idiwhaa tonalitic gneiss (ITG).

Unlike typical Archean TTG magmatic rocks, the well-preserved meta-igneous ITG is characterized by moderate silica contents (58-62 wt % SiO₂), strong Fe-enrichment (12-15 wt% FeO), and low Mg numbers (13-18). REE patterns are relatively flat with a significant negative Eu anomaly. These features strongly suggest that, unlike deep-seated Archean TTG magmas, the evolution of the ITG was dominated by shallow-level fractionation processes involving plagioclase.

Zircons from the ITG document primary crystallization at ~4.02 Ga. Two distinct phases of 4.02 Ga zircon growth occur in this sample and document a marked decrease in d¹⁸O from +5.6 ‰ to +4.7 ‰ that can be explained by late-stage assimilation of hydrothermally altered crust. Despite the difference in d¹⁸O values, Hf-isotope compositions of these two phases of zircon are indistiguishable, with initial eHf values (normalized to chondrite at 4020 Ma) of approximately -2.

We discuss the implications of the negative initial Hf and provide a model for the formation of the ITG which is consistent with all geochemical and isotopic information. Our model has important implications for crust formation processes in the Hadean (>4.0 Ga) and the amount of continental crust present on the early Earth.