Meli S.
ABSTRACT
In Western Val Sugana a sector of the Southalpine basement outcrops, bearing a lithostratigraphic sequence composed by phyllites and acidic metavolcanic rocks (“porphyroids”); similar features are recognizable also in other areas where the Southalpine basement is exposed to the surface. The primary age of the porphyroids is Lower Ordovician, whereas the metamorphic overprint is Variscan. The paragenesis and the microstructures indicate an igneous origin of the protoliths, which have been later metamorphosed under low-greenschist facies conditions. Late- and post-magmatic processes have modified the pristine chemistry of the volcanics, leading to the mobilization of silica, alkali and alkali-earth elements. Based on immobile element ratios, the metavolcanics are classified as rhyolites and rhyodacites; they also display a marked peraluminous character. Major and trace element chemistry indicate an origin of acidic magmas through crustal anatexis of metapelites, which left garnet and K-feldspar in the residuum; major element variation diagrams indicate that melt segregation from the source was not complete, as indicated by mixing trends between non-minimum melts and restite. The discriminant diagrams of tectonic setting for granitic rocks, together with the frequency and spatial distribution of the volcanics, suggest that their emplacement took place in a late- to post-collisional orogenic environment.