High-resolution EBSD and SIMS U-Pb geochronology of zircon, titanite, and apatite: insights from the Lac La Moinerie impact structure, Canada

High-resolution electron backscattered diffraction (EBSD) and correlated in situ secondary ion mass spectrometry (SIMS) U-Pb geochronology have been conducted on impact-metamorphosed zircon, titanite, and apatite grains from the 453 Ma Lac La Moinerie impact structure of Quebec, Canada. Building on previous work, our new U-Pb results highlight the challenges associated with SIMS dating of low U, common Pb-rich accessory phases (apatite and titanite) within terrestrially shocked impact lithologies. Unlike zircon, microstructurally controlled Pb loss is limited in apatite and titanite, providing evidence that the mechanisms governing impact resetting in these phases are distinct from high U, low common Pb phases (zircon, monazite, baddeleyite). EBSD mapping reveals internal complexity within all three phases, providing new insights into their microstructural evolution during impact-related metamorphism. Correlated EBSD and Raman analysis of Former Reidite In Granular Neoblastic (FRIGN) zircon provides the first documented case of post-recrystallization radiation damage of individual zircon neoblasts. Two modes of recrystallization are recorded in apatite: systematic (Type I), wherein strain-free domains are preferentially oriented along specific crystallographic planes, and non-systematic (Type II), where strain-free domains exhibit no preferred orientation. A new shock twin is observed in titanite that has a disorientation relationship of 74 degrees/ < 103 > , interpreted to be the result of strain-induced misorientation of pre-existing 11 over bar 1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left\{ {1\overline{1}1} \right\}$$\end{document} shock twins. We document the first case of impact-recrystallized titanite, which is intergrown with granular rutile and partially preserves former 11 over bar 1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left\{ {1\overline{1}1} \right\}$$\end{document} shock twins along boundaries of strain-free subdomains. Based on our EBSD observations, we provide a microstructural framework outlining the sequential evolution of zircon, titanite, and apatite during shock and thermal metamorphism.

Automated summary

High-resolution EBSD and correlated in situ SIMS U-Pb geochronology were conducted on impact-metamorphosed zircon, titanite, and apatite grains. The study highlights challenges in SIMS dating of low U, common Pb-rich accessory phases within terrestrially shocked impact lithologies. EBSD mapping revealed internal complexity within all three phases, providing new insights into their microstructural evolution during impact-related metamorphism. The research documents the first case of impact-recrystallized titanite, intergrown with granular rutile and partially preserving former shock twins along boundaries of strain-free subdomains. Two modes of recrystallization were recorded in apatite: systematic (Type I) and non-systematic (Type II), shedding light on the microstructural evolution of zircon, titanite, and apatite during shock and thermal metamorphism.