K-Ar Radiometric Age Determinations of White Micas from the Piemont Zone, French-Italian Western Alps

K - Ar radiometric data are presented suggesting a different metamorphic history for both parts of the Piemont zone in the Western Alps: the Combin zone to the west, considered to have escaped the late Cretaceous metamorphic event, and the Zermatt-Saas zone to the east, where ophiolites predominate, and which has been affected by a first metamorphism during late Cretaceous. Paragonite lenses within serpentinite give an isotopic ratio dating a serpentinization phase, or representing mixed ages. Late Cretaceous ages are found in the Zermatt-Saas zone and in the melange areas. The Combin zone gives only Eocene-Oligocene ages, as well as that part of the Zermatt-Saas zone that is closer to the Lepontine thermal high (dated as 38 m.y.). No late Oligocene-early Miocene ages (as previously obtained on Na-amphiboles) have been found among the investigated micas.