The Holleford structure: Insights into the pre- and post-impact geology of the Frontenac Terrane, Canada

Abstract A multi-chronometric investigation has been conducted on the ~2.0-km-diameter Holleford impact structure of Ontario, Canada. In situ triple quadrupole laser ablation–inductively coupled–mass spectrometry U-Pb and Rb-Sr geochronology, along with (U-Th)/He thermochronology, have been applied to multiple mineral phases from impact melt-bearing breccias and overlying sedimentary units. Impact-induced resetting is absent in all three isotopic systems. While Holleford’s small size resulted in insufficient heating to reset geochronometers during impact, our investigation has defined a more detailed pre- and post-impact geologic setting within the Precambrian basement and overlying Paleozoic lithologies. Target rock age determinations include those associated with the Proterozoic Frontenac Terrane. These are manifest as the Rigolet (1004 ± 5 Ma, U-Pb apatite dating) and Ottawan (1074 ± 12 Ma, Rb-Sr biotite dating) phases of the Grenville Orogeny. Pre-Grenville ages associated with the Penokean (1806 ± 46 Ma, apatite dating) and the Shawinigan (1184 ± 8 Ma, U-Pb titanite dating; 1134 ± 21 Ma, U-Pb zircon dating) orogenies are also retained. A Paleoproterozoic age records Superior Province tectonometamorphism (2421 ± 97 Ma, U-Pb zircon dating). Dated post-impact events include dolomitization associated with Silurian marine transgression (430 ± 14 Ma, U-Pb dolomite dating) and regional K-metasomatism of Mississippian age (346 ± 5 Ma, U-Pb titanite dating). Extremely young (U-Th)/He ages (27–3 Ma) were obtained from planar fractured and recrystallized apatite, which we interpret to be the result of impact-induced diffusion domain reduction. This, combined with low-temperature heating due to burial, significantly limited He retention. Stratigraphic constraints place the Holleford impact event at ca. 458 Ma, close to the Darriwilian–Sandbian boundary of the Ordovician. This age indicates that Holleford may be part of the Late Ordovician group of terrestrial impact craters related to L-chondrite parent breakup.