Re-evaluating the Rochechouart Impactites: Petrographic Classification, Hydrothermal Alteration and Evidence for Carbonate Bearing

Introduction: The Rochechouart impact structure is a deeply eroded, late Triassic impact site located in south-central France [1,2]. Rochechouart impactites comprise five main lithological units from the base of the structure upwards, originally described as follows: unit 1) shocked autochthonous basement rock; unit 2) finely crystalline melt rock; unit 3) red 'welded' breccia/suevite; unit 4) lithic breccia; and unit 5) suevitic breccia [2]. Standardized nomenclature is important when studying impact structures, both for ease of correlating investigations and experimental studies as well as enabling a universal understanding of the subject matter. When correctly and consistently applied, unambiguous and informative nomenclature yields descriptive and often genetic information. The petrographic evaluation of the Rochechouart impactites (units 2–5) presented here allows for a systematic classification based on the most recent recommendations of the IUGS [3] and a recent review of the effects of target lithology on impact melting [4]. In addition, mineralogical and textural evidence for post-impact hydrothermal alteration was observed. A carbonate clast hosted within the lithic impact breccia suggests the existence of carbonate in the pre-impact target rocks. Geologic Setting: The late Triassic (214 + 8 Ma) Rochechouart impact structure [5] was formed by an ordinary chondritic impactor [6] in the crystalline Hercynian granites and gneisses of the French Massif Central [7]. The structure has been deeply eroded such that no topographic expression of the crater remains [2]. Originally interpreted as volcanic [8], the identification of shock metamorphic textures such as shatter cones and planar deformation features (PDFs) in quartz led to the recognition of an impact origin [1]. Rochechouart consists of scattered outcrops of impact breccias and melt rocks. The estimated crater diameter is ~23 km based mineralogical and structural constraints [9,10]. A crater of this diameter is expected to have a complex morphology with a central uplift [11]. Based on the distribution of shatter cones in the shocked basement, a 4 km diameter central uplift region has been proposed [2]. Methodology: Twelve representative thin sections of the Rochechouart impactite suites were chosen for petro-graphic study in transmitted light. Hand samples were also examined. Results: All impactites examined, with the exception of the melt rock, contained petrographic indicators of shock including: PDFs in quartz, decorated PDFs in quartz, planar fractures, mosaicism of quartz, toasted quartz, isotropiza-tion of quartz and feldspar, kink banding in mica, and diaplectic glass. Shock metamorphic features were concentrated in quartzofeldspathic lithic clasts.