• Zoned plutons
• Sesia magmatic system
• Rejuvenation
• Incremental growth
• Granite

The focus of this study is on the magmatic evolution of the Valle Mosso pluton, a granitic body intruded at intermediate to upper crustal levels into the rocks of the Alpine basement of the Ivrea-Verbano Zone and Serie dei Laghi, shortly after the end of Variscan orogeny. The Valle Mosso pluton has been recognized as an integral part of a magmatic system (Quick et al., 2009), which during the lower Permian developed through the continental crust up to the surface and caused explosive rhyolitic volcanism that eventually led to the formation of a >15 km diameter caldera. This granitic body was tilted as a consequence of Alpine orogeny and thus exposes both its deepest and shallower portions. This peculiar feature, shared worldwide by just a few other similar intrusions, if combined together with its characteristic geological framework makes the Valle Mosso pluton a rather unique case study. In fact, the opportunity to study a continuous vertical section of a granitic intrusion genetically linked to coeval volcanic deposits has substantial implications, both in the understanding of the mechanisms that rule granitic bodies’ emplacement in the upper crust and in the comprehension of the volcanic-plutonic connection.
By means of detailed geological mapping, the internal structure of the pluton and the relationship with its host rocks have been determined. The pluton has been divided into six granitic petrofacies, based on textural criteria derived from up-to-date scientific literature on the mapping of granite terranes. The combination of field data with thin section examination have led to the compilation of a geological map of the Pluton at 1:15.000 scale, which is equipped with cross section and schemes that help the reader focus on the geometry and internal variability of the intrusive body. Laboratory analyses have been carried out alongside fieldwork, with the aim of defining the intrusion’s geochemical features, which are essential to the compilation of a petrogenetic model that could attest for the heterogeneity of the granitic body. Whole-rock analyses of over 100 granitic rock samples have been performed trough X-ray fluorescence (XRF) to determine major-element compositions, at least 25 selected samples have been analysed by means of ICPMS for their trace element contents and 5 more underwent mass spectrometry (TIMS) to determine their Sr isotopic ratio. In addition to whole-rock analyses, selected minerals (Qz, Plg, Ho, Bt) belonging to the deepest facies of the Valle Mosso pluton have been analysed by means of microprobe. These data obtained through ion microprobe analyses have been used to perform thermometric and barometric calculations, which purpose was to set constraints on the intrusion crystallization conditions.
Starting from the data collected in both field and laboratory, an incremental growth of the Valle Mosso pluton is hypothesized through the emplacement of several sill-like (high aspect-ratio) granitic bodies inside the upper crust; they partially interacted and gradually evolved by means of fractional crystallization processes. Based on field evidence, an intrusion emplacement mechanism that considers both floor subsiding and roof lifting (together with the detachment of roof pendants) is favoured for the composite Valle Mosso pluton. In addition to discussing the processes that led to the generation and emplacement of the granitic magma, based on the collected data, there is also room to speculate on a possible plutonic record of rejuvenation of largely solidified magma chamber preserved inside the Valle Mosso intrusion. In fact, a particular porphyry granite found inside the pluton discloses mineral phenocrysts (Qz, Plg, Kf) that show clear petrographic and geochemical evidence for thermal and compositional disequilibrium, which are similarly found in crystals belonging to the coeval volcanic deposits of the Permian caldera. The collected data seem to indicate a possible link between reheating evidence in granitic rocks and volcanic events.