We determined the K-Ar ages of phengite occurring as a gangue silicate mineral in the massive sulfide ores from the Kotsu and Iyo Besshi-type volcanogenic massive sulfide (VMS) deposits in the Sanbagawa high-P/T metamorphic belt, southwest Japan. The K-Ar ages of phengite are dated to be ca. 45 or 85 Ma and these ages correspond to the timing of initial or terminal stages of regional retrograde metamorphism. Contrastingly, the Re-Os isochron ages of sulfide mineral aggregates from the Kotsu and Iyo Besshi-type VMS deposits are ca. 145 Ma, which is much older than those of phengite K-Ar ages. Since exactly the same ore samples were used for both K-Ar and Re-Os geochronology, it was confirmed that the K-Ar isotope system was reset due to the regional metamorphism up to lower epidote-amphibolite facies, although the Re-Os isotope system retained the original geochemical and isotopic composition even though the regional (Sanbagawa high-P/T) metamorphism.
