White mica K‐Ar ages of blueschist‐facies rocks from the Piemonte ‘calc‐schists’ of the western Italian Alps

Abstract

Abstract Fifty calc‐schists have been systematically collected from the Piemonte zone of the western Italian Alps and examined in terms of petrology, X‐ray powder diffractometer (XRD) analysis of carbonaceous materials, and K‐Ar ages of white mica separates. The petrological study and XRD analysis of carbonaceous materials have shown that calc‐schists have suffered blueschist‐facies metamorphism in the subduction zone of the convergent margin between the Apulian (African) continental and Tethyan oceanic plates. The metamorphic sequence is divided into three mineral zones based on increasing metamorphic temperature: chlorite (lower than 300°C), chloritoid, and rutile (higher than 450°C). The chlorite zone has dispersed ages of white mica separates, ranging from 115 to 44 Ma, whereas the rutile zone has a comparatively uniform age distribution from 60 to 40 Ma. The chloritoid zone has an intermediate age variation. The large variation in the chlorite zone is attributed to mixing of variable amounts of detrital mica derived from older high temperature metamorphic rocks in the separates, which have not been completely reset during Alpine metamorphism. The uniform age (average ca 50 Ma) in the rutile zone is the cooling age of blueschist‐facies calc‐schists, which have been episodically exhumed at the collision event of the European and Apulian continents in the Paleocene‐Eocene.