Rb-Sr and40Ar/39Ar age constraints on the Variscan metamorphism recorded by Ordovician acidic metavolcanic rocks in the Eastern Southalpine basement

Abstract

New Rb-Sr and40Ar/39Ar ages are presented for some acidic metavolcanic rocks coming from two different outcrops of the Eastern Southalpine basement (Val Sarentino and Levico): these ages refer to the timing of the two stages of the Variscan metamorphism, which affected the Southalpine basement. Whole-rock Rb-Sr ages of 345±9 and 353±56 Ma are consistent with Rb-Sr ages obtained for similar lithologies in the neighbouring basement outcrop of Val di Funes; together, these ages define a synchronous metamorphic stage during the Tournaisian (first Variscan stage).40Ar/39Ar ages on coarsegrained, post-kinematic phengites give values of 325–326 Ma for Val Sarentino and of 329–330 Ma for Levico samples. They probably represent cooling ages of the retrograde path of the second Variscan stage, during which the thermal climax of the whole Variscan metamorphic event occurred. The Levico ages are slightly older than Val Sarentino, possibly because of the closeness of the peak temperature and the assumed closure temperature range of K-Ar system in white mica. Taking into account the Levico and Val Sarentino cooling ages, the thermal peak in the Eastern Southalpine domain should be older than 331 Ma. Therefore, the two Variscan metamorphic stages in the Eastern Southalpine basement are probably closer in time than previously believed, being separated by less than 20 Ma.