For biotite, improved total uncertainties are achieved and both, small and large segments, yield ages in agreement with ID-TIMS data (425 ± 15 Ma and 426 ± 10 Ma for the weighted mean approach). Interestingly, the age distribution of the "all biotite" segment exhibits an intriguing skewness towards younger ages (Fig. ). To account for the asymmetry of the histogram, two Gaussians probability distributions were used for fitting. The main age peak displays an age of 446 ± 11 Ma and is assigned to the main deformation event at 431.7 ± 3.1 Ma. The peak that fits the population of anomalously young apparent ages at 322 ± 30 Ma is interpreted to reflect a post-deformational fluid alteration event that leads to chloritization of biotite and hence partially reset ages. This is supported by Rb-Sr data presented by Bender and co-workers, which yield lower mean ages for the apatite plus biotite isochron (421.7 ± 6.2 Ma) compared to the age defined by the feldspar and muscovite fractions (431.7 ± 3.1 Ma). The detection of a younger age population within age distribution displayed by biotite in the Rb-Sr map is likely a result of the small laser beam size of 3 µm. This size enables slows for precise sampling altered biotite domains. In contrast, ID-TIMS analyses or even conventional spot ablation with larger beam sizes (< 50 µm) more likely lead to mixed analyses of pristine and altered biotite, which imposes problems in interpreting the obtained ages.