Extraneous argon is heterogeneously distributed in the CBU and of minor importance. Extraneous argon in the CBU is correlated to the vicinity of the Cycladic Basement. Clear evidence of excess argon in microstructures where fluid circulation is enhanced. A minimum closure temperature of 550 °C is proposed for phengites of the CBU. Recrystallization is the main process controlling 40Ar/39Ar ages in HP rocks.