Learning from Failure? Carbonate U-Pb and Quartz Fluid Inclusion 40Ar/39Ar Geochronology 

<p>We attempted to determine the timing of deformation along a structurally and petrologically diverse set of brittle-ductile shear zones and faults along the Austroalpine-Pennine boundary in the Central Alps using U-Pb and <sup>40</sup>Ar/<sup>39</sup>Ar geochronology. Among the structures studied are the Turba mylonite shear zone, Gürgaletsch shear zone, Martegnas shear zone, Engadine line, Muretto fault, Septimer fault, Silvretta basal contact and - as an outlier of the main focus of the study - the Glarus thrust.</p> <p>We sampled carbonate and quartz mineralizations with clear temporal relation to the larger scale structures, i.e., pre-, syn- or post-tectonic. These were pristine or recrystallized veins, slicken fibers, recrystallized clasts, and mylonitic layers. To determine crystallization or recrystallization ages we analyzed U-Pb isotopic compositions of carbonate mineralizations using LA-ICP-MS analysis and Argon isotopic compositions of fluid inclusions in quartz mineralizations using the step-wise crushing technique.</p> <p>Both methods resulted in complex datasets for all samples, which so far do not allow for a clear age interpretation. Carbonate U-Pb samples either had too low U-contents or too high common Pb and - partly as a result of the other two characteristics - too little radiogenic ingrowth to calculate intercept ages. Step-wise crushing of quartz separates yielded complex Ar release spectra with large amounts of excess-Ar and variable but mostly low <sup>39</sup>Ar<sub>K</sub> contents considered both in absolute and relative values.</p> <p>In an attempt to aid more success in future application of the two techniques we present our methods, results and discuss our observations.</p>