FIRST PETROGRAPHIC AND GEOCHEMICAL CHARACTERIZATION OF IMPACT MELT-RICH LITHOLOGIES AND TARGET ROCKS WITHIN THE ROCHECHOUART IMPACT STRUCTURE (FRANCE), RECOVERED BY THE 2017 DRILLING CAMPAIGN

The Rochechouart impact structure is a deeply eroded crater with no remaining impact-related topography. Nonetheless, despite intense erosion down to the crater floor in many localities, this structure still preserves a suite of proximal impactites, including impactoclastites (melt-bearing ash-like deposits), suevites, impact melt rocks, and shocked crystalline basement. All of these lithologies have been sampled during the 2017 drilling campaign led by Philippe Lambert, which concerned 18 drill holes (with a total cumulative length of ~540 m) located at 8 sites along two 10-km radial transects across the center of the structure.

Fifty-two samples from 6 drilling sites have been selected for this study. A subset of 32 samples has been characterized by a detailed petrographic and mineralogical examination coupled with micro X-ray fluorescence elemental distribution mapping. All samples selected have been analyzed for their whole-rock major and trace geochemical compositions. Through this combined approach we examine the representativeness of these drill core samples and aim to disentangle the different syn- and post-impact processes, such as melting and mixing of target rocks, meteoritic contribution, and hydrothermal alteration that occurred as a result of the Rochechouart impact event ~207 Myr ago.

Our petrographic and geochemical observations indicate that the impact melt-rich lithologies are formed from melting and mixing of granite and gneiss target rocks. Two different impact melt rock types can be identified within the Rochechouart impact structure. Their spatial distribution is heterogeneous, with a red impact melt rock being concentrated towards the southwestern part of the structure, where granites mostly outcrop, whereas a yellow one is located mainly in the northeastern part, where gneisses are exposed. Both impact melt rock types are found, partly mingling, in the center of the structure. The post-impact hydrothermal alteration, which affects most impact structures, is also recorded in the samples through K-metasomatism and crystallization of secondary minerals. Despite this process, an impactor contribution is preserved within the melt-rich lithologies, as highlighted by the enrichment in specific trace elements (e.g., Ni, Ge, Cr) of these lithologies compared to the target rocks.