Abstract

The oldest compiled U–Pb zircon ages for the Acasta Gneiss Complex in the Northwest Territories of Canada span about 4050–3850 Ma; yet older ca. 4200 Ma xenocrystic U–Pb zircon ages have also been reported for this terrane. The AGC expresses at least 25 km2 of outcrop exposure, but only a small subset of this has been documented in the detail required to investigate a complex history and resolve disputes over emplacement ages. To better understand this history, we combined new ion microprobe 235,238U–207,206Pb zircon geochronology with whole-rock and zircon rare earth element compositions ([REE]zirc), Ti-in-zircon thermometry (Tixln) and 147Sm–143Nd geochronology for an individual subdivided ∼60 cm2 slab of Acasta banded gneiss comprising five separate lithologic components. Results were compared to other variably deformed granitoid-gneisses and plagioclase-hornblende rocks from elsewhere in the AGC. We show that different gneissic components carry distinct [Th/U]zirc vs. Tixln and [REE]zirc signatures correlative with different zircon U–Pb age populations and WR compositions, but not with 147Sm–143Nd isotope systematics. Modeled DWRzircon [REE] from lattice-strain theory reconciles only the ca. 3920 Ma zircons with the oldest component that also preserves strong positive Eu∗ anomalies. Magmas which gave rise to the somewhat older (inherited) ca. 4020 Ma AGC zircon age population formed at ∼IW (iron–wüstite) to <FMQ (fayalite–magnetite–quartz) oxygen fugacities. A ca. 3920 Ma emplacement age for the AGC is contemporaneous with bombardment of the inner solar system. Analytical bombardment simulations show that crustal re-working from the impact epoch potentially affected the precursors to the Acasta gneisses.