Polyphase metamorphism in the eastern Carnic Alps (N Italy-S Austria): clay minerals and conodont Colour Alteration Index evidence

Thermal evolution of the Palaeozoic–Triassic sequences of the Carnic Alps has been characterised by b cell dimension and Kübler Index (illite “crystallinity”) of K-white micas (KI), Árkai Index (AI) of chlorites, clay mineral assemblages and conodont Colour Alteration Index (CAI). Data indicate at least two metamorphic events, Variscan and Alpine. In the older event high anchizonal conditions predominated although epizonal conditions were reached over wide areas. It was characterised by low-intermediate pressure facies. The thermal peak, was due mainly to an extensional regime during the Bashkirian. A younger thermal overprint generated by Alpine orogeny was of lower grade, reaching high diagenetic–anchizonal conditions characterised by high pressure facies. Inverted metamorphic patterns are associated with middle to late Miocene thrusting. Hydrothermal alteration in the northern part of the region can be linked with emplacement of Oligocene plutons and high heat flow along the Periadratic lineament.