GÄD08 Sample GÄD08 displays a metamorphic mica aggregate comprised of cogenetic biotite and muscovite formed by dynamic recrystallization in the waning stages of ductile deformation at 431.7 ± 3.1 Ma. Four segments have been defined to assess the accuracy and precision of Rb-Sr ages as a function of the mica matrix: two segments comprise all muscovite (n = 11880) and biotite (n = 3115) pixel and two segments are smaller subsets of these populations (n = 247 for muscovite and n = 528 for biotite, Fig. ). In general, the uncertainty on the weighted mean (2s x ) for muscovite (26.5 % for 247 pixel) is considerably higher compared to biotite (1.4 % for 528 pixel). The higher analytical uncertainty arises when dealing with muscovite due to its smaller 87 Sr/ 86 Sr ratios. During common Sr correction, a constant value for ( 87 Sr/ 86 Sr) initial is subtracted from measured 87 Sr/ 86 Sr ratios of individual pixel (equation 2). If the latter are small this leads to more significant variations in the calculated values. As a result, the common Sr correction amplifies any errors, causing an increase in overall uncertainty during the analysis. This is also reflected by the higher noise displayed by muscovite pixel (Fig. ). Nevertheless, when a larger number of pixels is chosen, encompassing all 11.880 muscovite pixels, we are able to attain a reasonable weighted mean uncertainty of 0.7%. However, it's worth noting that the overall uncertainty is significantly influenced by the common Sr correction (γ), which tends to be higher for lower 87 Sr/ 86 Sr ratios. As a result, even when all muscovite pixels are included, the total uncertainty is reduced but remains at 6.8% (Table ). For both muscovite and biotite, matrix correction is based on Phalaborwa biotite and hence is not matrix matched with respect to the muscovite in GÄD08. However, within the uncertainty, both muscovite segments reproduce the ID-TIMS derived Rb-Sr ages (Table ).