Zircon U-Pb provenance analysis applied to impactites from the Rochechouart impact structure, France.

The 207 Ma old Rochechouart impact structure is located in Paleozoic (> 300 Ma) crystalline basement of the NW French Massif Central. Allochthonous impactites at Rochechouart include clast-poor impact melt rock (IMR) and suevitic breccia. These rocks are highly diverse with significant geochemical variation and different zircon age populations. The great diversity of impactites and potential target rocks represents a great opportunity to attempt comprehensive provenance analysis by comparing their respective age populations. The present work involves high-spatial-resolution geochronology (U-Pb SIMS and LA-ICP-MS) and textural characterization of zircon from two suevite samples from Chassenon and Videix, and one impact melt rock (IMR) sample from Babaudus. Zircon from other impact melt rock occurrences (Montoume, Recoudert) and a first suite of target rocks has been analyzed already with LA-ICP-MS, also as part of this project (Guerrero et al., forthcoming). 
Zircon separation was conducted at the Universidade de Brasília (UnB). Cathodoluminescence and backscattered electron imagery for the present work was done at the NordSIMS facility of the Swedish Museum of Natural History, Stockholm. On selected grains, U-Pb SIMS and LA-ICP-MS analysis were done at NordSIMS and UnB, respectively. The zircon grains are very diverse in these three samples, ranging from transparent to opaque and from colorless to reddish. The most common zircon textures observed are oscillatory and irregular zoning. Shock microdeformation features include granular texture, reidite in granular and lamellar textures, and spots (<2 µm) of high electron density (thought to be related to baddeleyite occurrence). Lamellar reidite is only found in zircon from the suevite samples. The Chassenon suevite has the highest diversity of zircon textures and ages, followed by the Videix suevite, and finally the IMR. Most ages from both suevite samples are pre-Variscan. In kernel density estimation plots, seven different age peaks are recognized in the Chassenon suevite, with principal ages at 538, 492, and 461 Ma. There are only three age maxima in the Videix suevite, with main peaks at 472 and 341 Ma. For the Babaudus IMR, most zircon ages are comparatively closer to the impact age, a main age peak occurs at 236 Ma, and several grains record even post-200 Ma ages. Two grains of the IMR and one of the Videix suevite yield ages very similar to the impact age (198 ± 7 to 206 ± 10 Ma). In general, shocked grains show younger ages than unshocked ones. 
Most of the age peaks obtained on the suevites were not recorded in previous U-Pb studies on other Rochechouart IMRs. The only comparable zircon age peaks correspond to the Videix and Babaudus ages at 472 and 473 Ma of this study and the Chassenon suevite peak at 492 Ma, and the ages for Montoume and Babaudus IMR at 491 Ma and 487 ± 7 Ma (Guerrero et al., forthcoming; Rasmussen et al., 2020). The 236 Ma age peak of the Babaudus IMR of this study is also known from zircon of a Montoume IMR sample. The Chassenon suevite is the only sample with age peaks that can be partially correlated with the ages of the target rocks analyzed so far. The Cambrian ages and the 461 Ma age peak are similar to the ages recorded in a paragneiss from Moulin de Laurière and a monzodiorite from La Martinie (Guerrero et al., forthcoming). At this point, further basement lithologies need to be studied, before comparative provenance analysis for impactites and target rocks may allow to fingerprint the main protoliths for impact breccias from different locations in the Rochechouart impact structure. 
REFERENCES
Guerrero, D., et al., forthcoming. U-Pb on zircon isotope analysis on target rocks and first provenance analysis on impact melt rock from the Rochechouart impact structure, France. Meteorit. Planet. Sci. (in revision).
Rasmussen, C., et al. 2020. Geochim. Cosmochim Acta 273: 313–330.