In situ multiphase U–Pb geochronology and shock analysis of apatite, titanite and zircon from the Lac La Moinerie impact structure, Canada

Abstract

In situ U–Pb geochronology has been performed via laser ablation inductively coupled mass spectrometry on shocked apatite, titanite and zircon from the ~ 8-km-diameter Lac La Moinerie impact structure, Canada. The three analyzed phases are inherited from target rocks that were transformed to impact melt-bearing breccias and clast-laden impact melts. Apatite yields an array of U–Pb ratios populating a region between a younger regression with a refined age of 453 ± 5 Ma, and an older regression with a lower intercept age of 1708 ± 10 Ma. Titanite defines a younger regression with an age of 444 ± 15 Ma, and an older array with a lower intercept at 1844 ± 24 Ma. Zircon yields an upper intercept age of 1810 ± 13 Ma and a lower intercept age of 433 ± 21 Ma, within error of apatite and titanite. The oldest ages from all three phases define Paleoproterozoic igneous–metamorphic events in granitoid target rocks of the De Pas Suite. Isotopic resetting in apatite and titanite, with relatively high Pb-diffusion rates, was controlled by a combination of proximity to impact-generated superheated melt and the formation of fast diffusion pathways facilitated by shock effects and dynamic recrystallization. Zircon resetting was controlled by shock effects and metamictization, which resulted in significant Pb loss. We outline an approach for discriminating ‘true’ impact ages from metamict zircons with high common Pb that have experienced post-impact Pb loss. Lower intercept ages of all phases constrain the age of formation for the Lac La Moinerie impact structure, with apatite yielding the most precise age of 453 ± 5 Ma. With an age of 453 Ma, Lac La Moinerie joins a growing cluster of more than 12 terrestrial impact structures formed in the Middle-to-Late Ordovician, with potential connection to the L-chondrite parent body breakup event and a period of enhanced bombardment on Earth.