Nicole L. Cates
Guillaume Caro
Dustin Trail
Oleg Abramov
Martin Guitreau
Janne Blichert-Toft
Michelle D. Hopkins
Wouter Bleeker
Stephen J. Mojzsis
2014
<p><span>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</span><sup>2</sup><span> 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</span><sup>235,238</sup><span>U–</span><sup>207,206</sup><span>Pb zircon geochronology with whole-rock and zircon rare earth element compositions ([REE]</span><sub>zirc</sub><span>), Ti-in-zircon thermometry (Ti</span><i><sup>xln</sup></i><span>) and </span><sup>147</sup><span>Sm–</span><sup>143</sup><span>Nd geochronology for an individual subdivided ∼60 cm</span><sup>2</sup><span> 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]</span><sub>zirc</sub><span> vs. Ti</span><i><sup>xln</sup></i><span> and [REE]</span><sub>zirc</sub><span> signatures correlative with different zircon U–Pb age populations and WR compositions, but not with </span><sup>147</sup><span>Sm–</span><sup>143</sup><span>Nd isotope systematics. Modeled </span><span id="mmlsi1" class="mathmlsrc"><a class="mathImg" title="View the MathML source" data-mathurl="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0016703714001161&_mathId=si1.gif&_user=111111111&_pii=S0016703714001161&_rdoc=1&_issn=00167037&md5=952b6338d4f7e2bdedb943877a975334"><img class="imgLazyJSB inlineImage" title="View the MathML source" src="http://ars.els-cdn.com/content/image/1-s2.0-S0016703714001161-si1.gif" alt="View the MathML source" width="42" height="20" data-inlimgeid="1-s2.0-S0016703714001161-si1.gif" data-loaded="true" /></a></span><span> [REE] from lattice-strain theory reconciles only the ca. 3920 Ma zircons with the oldest component that also preserves strong positive Eu</span><sup>∗</sup><span> 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.</span></p>
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10.1016/j.gca.2014.02.019
en
Elsevier
Component geochronology in the polyphase ca. 3920 Ma Acasta Gneiss
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