Component geochronology in the polyphase ca. 3920 Ma Acasta Gneiss

Stephen J. Mojzsis; Nicole L. Cates; Guillaume Caro; Dustin Trail; Oleg Abramov; Martin Guitreau; Janne Blichert-Toft; Michelle D. Hopkins; Wouter Bleeker

doi:10.1016/j.gca.2014.02.019

USGS · 70171568

Component geochronology in the polyphase ca. 3920 Ma Acasta Gneiss

Stephen J. Mojzsis·Nicole L. Cates·Guillaume Caro·Dustin Trail·Oleg Abramov·Martin Guitreau·Janne Blichert-Toft·Michelle D. Hopkins·Wouter Bleeker

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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 km 2 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 235,238 U– 207,206 Pb zircon geochronology with whole-rock and zircon rare earth element compositions ([REE] zirc ), Ti-in-zircon thermometry (Ti xln ) and 147 Sm– 143 Nd geochronology for an individual subdivided &sim;60 cm 2 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. Ti xln and [REE] zirc signatures correlative with different zircon U–Pb age populations and WR compositions, but not with 147 Sm– 143 Nd isotope systematics. Modeled [REE] from lattice-strain theory reconciles only the ca. 3920 Ma zircons with the oldest component that also preserves strong positive Eu &lowast; anomalies. Magmas which gave rise to the somewhat older (inherited) ca. 4020 Ma AGC zircon age population formed at &sim;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.

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Stephen J. Mojzsis, Nicole L. Cates, Guillaume Caro, Dustin Trail, Oleg Abramov, Martin Guitreau, Janne Blichert-Toft, Michelle D. Hopkins, Wouter Bleeker. 2014. Component geochronology in the polyphase ca. 3920 Ma Acasta Gneiss. https://doi.org/10.1016/j.gca.2014.02.019

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