WE present here an interpretation of K-Ar ages obtained on four glaucophanes, four phengites and one barroisite separated from aluminous, calcareous and basic schists cropping out on the Ile de Groix, off southern Brittany in France. The data include two glaucophane-phengite pairs (PM6 and PC10) and a composite separate specially enriched in barroisite. (Table 1).