GEOCHRONOLOGY OF HIGH-PRESSURE ROCKS ON SIFNOS (CYCLADES, GREECE)

Polymetamorphic rocks of Sifnos (Greece) have been investigated by Rb-Sr, K-Ar, and fission track methods. Critical mineral assemblages from the northern and southernmost parts of Sifnos include jadeite+quartz+3 T phengite, and omphacite+garnet +3 T phengite, whereas the central part is characterized by the assemblage albite+chlorite+epidote+2 M1 phengite. K-Ar and Rb-Sr dates on phengites (predominantly 3 T) of the best preserved high P/itTmetamorphic rocks from northern Sifnos gave concordant ages around 42 m.y., indicating a Late Lutetian age for the high P/T metamorphism. Phengites (2 M1+3 T) of less preserved high P/T assemblages yielded K-Ar dates between 48 and 41 m.y. but generally lower Rb-Sr dates. The higher K-Ar dates are interpreted as being elevated by excess argon. K-Ar and Rb-Sr ages on 2 M1 phengites from central Sifnos vary between 24 and 21 m.y. These ages date a second, greenschist-facies metamorphism which overprinted the earlier high-pressure metamorphic rocks. © 1979 Springer-Verlag.