Stealth Metasomatism in Granulites from Ivrea (NW Italy): Hydration of the (Variscan) Lower Crust by Melt Flow

Granulites and associated dykes from the less well-studied southern Ivrea-Verbano Zone (around Ivrea town) are characterized by combining field, macro, micro and chemical (major and trace-element mineral composition) data to identify chemical and rheological variations in the lower crust that could be relevant for geodynamic implications. The Ivrea granulites are similar to those in the Lower Mafic Complex of the central Ivrea-Verbano Zone. The mafic lithologies experienced stealth metasomatism (pargasitic amphibole and An-rich plagioclase) that occurred, at suprasolidus conditions, by a pervasive reactive porous flow of mantle-derived orogenic (hydrous) basaltic melts infiltrated along, relatively few, deformation-assisted channels. The chemical composition of the metasomatic melts is similar to that of melts infiltrating the central and northern Ivrea-Verbano Zone. This widespread metasomatism, inducing a massive regional hydration of the lowermost Southalpine mafic crust, promoted a plastic behavior in the lowermost part of the crust during the Early Mesozoic and, ultimately, the Triassic extension of the Variscan crust and the beginning of the Alpine cycle.