Microstructures and absolute ages of brittle structures in the Weschnitz Pluton (Southern Odenwald, Germany)

We present new microstructural evidence and geochronological data from brittle structures sampled in an active quarry located in the Weschnitz Pluton in the southern Odenwald Crystalline Complex (OCC). Open joints and fractures are the primary pathway for fluids in crystalline rock and thus crucial in the utilization of crystalline formations for geothermal energy and long term nuclear storage.
The quarz-monzodioritic wall rock was emplaced around 344.4+-0.6 Ma ago into a considerably thickened crust in in about 18 km depth in a continental arc setting (Altenberger & Boesch, 1993; Altherr, 1999; Henes-Klaiber, 1992). Visean lamprophyre dikes bound to NNE-SSW striking normal faults relate to post-orogenic collapse and mark the onset of a multiphase history of extension and uplift, during which pre-existing variscan structures were repeatedly reactivated (von Seckendorff et al., 2004). While the variscan evolution is fairly well constrained (e.g. Todt et al., 1996; Krohe, 1996; Reischmann, 2001; Stein et al., 2022), brittle structures in the southern OCC have not yet been dated directly.
Most of the sampled joints measure between 2 and 10 mm and are sealed, one specimen has a thickness of 15 cm and is not sealed completely. In thin section, several types of joint mineralizations were identified, with syntaxial elongate blocky quartz with growth zoning and syntaxial blocky calcite with growth zoning being the most common types. Fractured calcite veins are often associated with hematite indicating later reactivation. U-Pb data obtained from LA-ICP-MS analyses on 15 samples splits into 2 age groups of around 300 Ma and 60-50 Ma.