U-Pb dating of zircon and monazite from the uplifted Variscan crystalline basement of the Ries impact crater

Impact crater central peaks and peak ring complexes are important exploration targets for future missions to other planetary bodies, because they provide access to material uplifted from lower crustal levels. Material exposed there could also provide chronological constraints on crater formation events. Therefore, it is essential to understand if uplifted peak material preserves the chronological records of igneous and metamorphic protolith crustal rocks, or if such records are reset during impact events. To investigate this issue, we collected shocked gneiss and granite samples from uplifted crystalline basement megablocks in the 24-km- diameter Ries impact crater in Germany, which is dated at ~14.8 Ma. Petrographic observations, electron beam imaging, and Raman spectroscopy suggest that these samples record peak pressures of ~10-15 GPa. In situ U-Pb dating shows that monazite U-Pb systematics has not been affected by the Ries impact, as gneisses and granites yielded monazite U-Pb dates of ~370 and 330 Ma, consistent with known Variscan metamorphic and magmatic events. The U-Pb systematics of some zircon grains yielded U-Pb dates of ca. 5-10 Ma, which is younger than the age of the Ries impact event. These young dates correspond to U-rich metamict domains, and may reflect recent Pb loss and/or U-gain during post-impact hydrothermal alteration or weathering. These observations indicate that dating uplifted crystalline material in impact craters on other bodies might provide useful petrological and chronological constraints on the underlying target rocks rather than directly dating impact events, for which sampling impact-melt and impact-melt bearing lithologies should remain the primary target.