Igneous evolutions across the Ivrea crustal section: the Permian Sesia Magmatic System and the Triassic Finero intrusion and mantle

The famous deep crustal section of the Ivrea-Verbano Zone (IVZ, western Alps, Fig. A) has received enormous attention over the last three decades as one of the best examples of continental “magmatic underplating”. Recent investigations, comprising structural, petrochemical and geochronological data, point to the occurrence of a “Sesia-type IVZ” (i.e. central IVZ) and a “Finero-type IVZ” (i.e. northern IVZ), which underwent different magmatic and tectonic evolution. In the Sesia area (box a in Fig. A), the Permian gabbroic pluton known as the mafic complex (reaching thicknesses > 8 km) intruded the deepest rocks of the crustal section, comprising amphibolite to granulite-facies paragneiss and interlayered mantle peridotite bodies, while they were resident in the deep crust. The broader magmatic context of this voluminous intrusion remained unclear until Quick et al. (2009) demonstrated that the emplacement of the mafic complex was coeval to the activity of a mainly silicic volcanic field, including extensive caldera deposits, and to the growth of silicic plutons in the upper crust of the adjacent “Serie dei Laghi”. The Sesia magmatic system constitutes an exposure of the plumbing system of a caldera from the surface to a depth of about 25 km (Quick et al., 2009). In this framework, the mafic complex records processes occurring in the deep crust beneath the caldera. The onset of volcanic activity correlates strictly with the climax of the growth of the upper mafic complex, when the mafic intrusion invaded fertile crustal levels and the crust was pervasively heated. In the upper crust, igneous activity was dominated by hybrid silicic melts produced by anatexis in the deep crust, but including significant amounts of mantle component. During the life of the volcanic activity, the mafic complex grew from a relatively small but continuously fed magma chamber according to the “gabbro glacier” process. The excursion will transect the entire igneous system, starting from the deepest exposures of the mafic complex up to reach the outcrops of megabreccia within the caldera fill. The Finero area (box b in Fig. A) is characterized by the occurrence of a pervasively-metasomatised mantle unit made by phlogopite-bearing ultramafic rocks (i.e. spinel-facies harzburgites, dunites and pyroxenites). These rock types were produced by several episodes of pervasive-to-channeled porous flow migration of K-LILE-Mg-enriched hydrous melts containing large amount of crustal components. The mantle unit is surrounded by a layered mafic-ultramafic intrusion, i.e. the Finero mafic complex, comprising garnet hornblendites, cumulus amphibole peridotites, amphibole gabbros and diorites, with tholeiitic to transitional geochemical affinity. Recent U-Pb zircon data point to a Middle-Triassic intrusion age for the Finero mafic complex, which may thus represent the deep-crustal counterparts of the Middle-Upper Triassic volcanism widespread throughout the Southern Alps. In any case, the Finero mafic complex can no longer be considered as part of the Permian mafic complex exposed in the Sesia area. Instead, U-Pb ages of zircons from massive chromitites of the mantle unit are Lower Jurassic. The marked age span of the Finero mafic complex and the associated mantle unit suggests that they experienced different evolutions until Lower Jurassic, and were subsequently tectonically juxtaposed during the opening of the Jurassic Neo-Tethys or later. The geodynamic setting related to the intrusion of the Finero mafic complex, the sources, the age and geodynamic environment of the metasomatism of the mantle unit and the age of the emplacement of the latter in contact with the crustal rocks are some of the issues that will be discussed in this guide. This excursion aims to illustrate the Sesia-type IVZ as a complete sequence of a section from the mantle to a supervolcano and show the different geochronological, petrographic and geochemical characteristics between the Sesia-type and the Finero-type IVZ in the light of the results of latest research.