Lu–Hf isotope systematics of the Hadean–Eoarchean Acasta Gneiss Complex (Northwest Territories, Canada)

Abstract

The Acasta Gneiss Complex (AGC) is a remnant Hadean–Eoarchean terrane composed of strongly deformed polyphase mafic to felsic gneisses which preserve a multi-stage history of magmatic emplacement, inheritance, and subsequent tectono-thermal modifications. The complexities encountered in such an old terrane fragment have been documented in previous geochronological studies of the AGC (e.g. zircon U–Pb, 147Sm–143Nd), and are evident also in its Lu–Hf isotope systematics. Here, we report new Lu–Hf isotope whole-rock measurements which show that some AGC gneisses were severely disturbed by migmatization and associated mineral segregation, while others preserve their Lu–Hf isotope systematics relatively intact with mostly near- to sub-chondritic initial 176Hf/177Hf ratios. Results reveal identifiable Eoarchean and later (Paleoarchean) magmatic events at around 3960Ma and again at 3600Ma, with a major metamorphism of the complex at 3750Ma. The oldest and least disturbed gneisses have a Lu–Hf regression age of 3946±87Ma, in good agreement with U–Pb zircon geochronology. A role of yet older crust (4000–4200Ma) in the formation of the AGC is also evident, but seems not to have influenced to first order the Lu–Hf isotope systematics of the 3960Ma group. The ca. 3960Ma group is proposed to be representative of its mantle source based on the absence of correlation between εHf(t) and Ce/Pb. It is further suggested that these two parameters show that the ca. 3600Ma gneisses were sourced in part from a mafic lithology belonging to the 3960Ma group, and that multiple sources (mantle and crust) were involved in AGC formation. The identification of preserved Lu–Hf isotope systematics in AGC gneisses means that complementary geochemical and isotopic studies bearing on the petrogenesis of pre-3900Ma rocks are possible. Despite its history of strong deformation and alteration, carefully selected domains within the AGC carry surviving information about the evolution of the mantle–crust system at the Eoarchean–Hadean boundary.