ABSTRACT The Manicouagan impact event has been the subject of multiple age determinations over the past ~50 yr, providing an ideal test site for evaluating the viability of different geochronometers. This study highlights the suitability of Manicouagan’s essentially pristine impact melt body as a medium for providing insight into the U-Pb isotope systematics of geochronometers in the absence of shock-related overprinting. We performed in situ laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb geochronology on apatite and zircon, both of which crystallized as primary phases. This study is the first application of U-Pb geochronology to apatite crystallized within a terrestrial impact melt sheet. U-Pb analyses were obtained from 200 melt-grown apatite grains ( n = 222 spots), with a data subset providing a lower-intercept age of 212.5 ± 8.0 Ma. For melt-grown zircon, a total of 30 analyses from 28 grains were obtained, with a subset of the data yielding a lower-intercept age of 213.1 ± 1.6 Ma. The lower precision (±8.0 Ma; ±3%) obtained from apatite is a consequence of low U and a high and variable common-Pb composition. This resulted from localized Pb*/Pb C heterogeneity within the impact melt sheet that was incorporated into the apatite crystal structure during crystallization (where Pb*/Pb C is the ratio of radiogenic Pb to common Pb). While considered a limitation to the precision obtainable from melt-grown apatite, its ability to record local-scale isotopic variations highlights an advantage of U-Pb studies on melt-grown apatite. The best-estimate ages from zircon and apatite overlap within error and correlate with previously determined ages for the Manicouagan impact event. An average formation age from the new determinations, combined with previous age constraints, yields a weighted mean age of 214.96 ± 0.30 Ma for the Manicouagan impact structure.

ABSTRACT Field, microtextural, and geochemical evidence from impact-related melt rocks at the Manicouagan structure, Québec, Canada, allows the distinction to be made between friction-generated (pseudotachylite) and shock-generated melts. Making this distinction is aided by the observation that a significant portion of the impact structure’s central peak is composed of anorthosite that was not substantially involved in the production of impact melt. The anorthosite contrasts with the ultrabasic, basic, intermediate, and acidic gneisses that were consumed by decompression melting of the >60 GPa portion of the target volume to form the main impact melt body. The anorthosite was located below this melted volume at the time of shock loading and decompression, and it was subsequently brought to the surface from 7–10 km depth during the modification stage. Slip systems (faults) within the anorthosite that facilitated its elevation and collapse are occupied by pseudotachylites possessing anorthositic compositions. The Manicouagan pseudotachylites were not shock generated; however, precursor fracture-fault systems may have been initiated or reactivated by shock wave passage, with subsequent tectonic displacement and associated frictional melting occurring after shock loading and rarefaction. Pseudotachylites may inject off their generation planes to form complex intrusive systems that are connected to, but are spatially separated from, their source horizons. Comparisons are made between friction and shock melts from Manicouagan with those developed in the Vredefort and Sudbury impact structures, both of which show similar characteristics. Overall, pseudotachylite has compositions that are more locally derived. Impact melts have compositions reflective of a much larger source volume (and typically more varied source lithology inputs). For the Manicouagan, Vredefort, and Sudbury impact structures, multiple target lithologies were involved in generating their respective main impact melt bodies. Consequently, impact melt and pseudotachylite can be discriminated on compositional grounds, with assistance from field and textural observations. Pseudotachylite and shock-generated impact melt are not the same products, and it is important not to conflate them; each provides valuable insight into different stages of the hypervelocity impact process.

Shocked quartz from the Charlevoix impact structure has been investigated by optical and scanning electron microscopy, combined with electron backscatter diffraction techniques. The apparent shock pressure recorded by specific sets of planar deformation features (PDFs) in quartz shows a systematic variation with distance (0–10 km) from the center of the structure from ∼5–20 GPa. The occurrence of basal PDFs at distances of ∼2–10 km from the center of the structure indicates a high deviatoric stress component of the shock wave–associated stress tensor. Grain size effects and a greater mineralogical heterogeneity are proposed to be the main cause for slightly lower shock pressures recorded by PDFs in finer-grained granitic gneisses in the southeastern part of the structure, compared to coarse-grained charnockitic gneisses to the northwest at similar distances from the center of the structure. The influence of the crystallographic orientation of quartz on the orientation distribution of planar microstructures appears to superimpose an influence of the orientation of the impact-related stress field. Based on the appearance of Dauphiné twins that are associated with PDFs and the occurrence of PDFs with orientations that correspond to positive and negative rhombohedra, quartz is suspected to have locally been in the β-modification state. Dauphiné twinning is proposed to be mainly due to a reversion to α-quartz during cooling. These findings imply that the uplifted, preheated target rocks have locally been shock-heated to the α-β transition temperature.

Pseudotachylytic rocks occur at a number of scales in the Archean and Proterozoic target lithologies of the Sudbury impact structure. At the smaller scale the pseudotachylytes are pervasive and occur as microscopic to millimeter-thick veinlets occupying fracture/slip networks (including shatter cone surfaces). At the larger scale, they are more sporadic and occur as dykelike bodies up to 500 m in thickness. There is a complete range of sizes between the two extremes. The pseudotachylytes cluster in discrete regions of enhanced deformation, which appear to define steeply dipping concentric zones. Three such zones have been identified in the North Range: (1) an innermost zone, approximately 10 km wide, 0 to 10 km north of the Sudbury Igneous Complex (SIC); (2) an intermediate zone, approximately 5 km wide, starting about 25 km beyond the SIC; and (3) an outer zone, approximately 3.5 km wide, commencing about 38 km beyond the SIC. Within the concentric zones, field mapping reveals that there is a systematic relationship between the frequency of pseudotachylyte occurrence and pseudotachylyte thickness. This relationship indicates that pseudotachylyte distribution within the target rocks is self similar. From an assessment of pseudotachylytic rock distribution, we estimate that the diameter of the Sudbury Structure is at least 200 km based on the outermost occurrence defining the outer ring fault(s). Four distinct types of pseudotachylyte can be identified in the field, although there are gradations between these divisions: (1) sharp-margined, aphanitic to crystalline matrixed, which possess massive or foliated matrices; (2) ductile margined; (3) injection; and (4) cataclastic. The bulk of the pseudotachylytes are of Type 1 and massive. Type 2 is less common and predates the other varieties. Type 3 is late and emanates from pseudotachylyte generation zones. Type 4 is more prevalent toward the margins of the impact structure. The Sudbury pseudotachylytes were developed due to friction-induced cataclasis and melting on slip surfaces. Cataclasis and melting occurred in response to the massive compressive and decompressive forces exerted on the target lithologies during the impact process. This caused the reactivation of existing fault zones as well as the generation of new, high-speed slip systems.