Emplacement and rejuvenation of a granitic batholith: the Valle Mosso plutob (Sesia Magmatic System)

The Lower Permian Valle Mosso pluton (VMP) is a granitic body intruded at upper crustal levels in the rocks of the pre-Alpine basement of the Ivrea-Verbano and Serie dei Laghi units, respectively. The VMP has been recognized as an integral part of a magmatic system termed Sesia 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 rhyolitic caldera. Fieldwork helped in constraining the VMP internal geometry, with the distinction of several laccolithshaped intrusive units noticeable for their modal abundances and textural features. Equigranular coarsegrained granite constitutes the bulk of the intrusion, together with subordinate volumes of porphyric granite and numerous small fine-grained two-micas intrusive bodies. A small body (roughly 0.5 by 1 km) of porphyry occurs within the porphyric granite and presents irregular to gradational contacts to its host rock. Major and trace-element and isotopic composition of this porphyry is equal to that of the surrounding granite. However, porphyry matrix and phenocrysts present microtextural features that indicate resorption and undercooling. These features (sieve textures of plagioclase, quartz and biotite resorption) paired with indications from Ti-in- Qz geothermometer on porphyry samples (Wark & Watson, 2006) are descriptive of temperature fluctuations within a crystal mush. Field evidence indicate that the Valle Mosso pluton experienced numerous episodes of mafic melt injections during different stages of its incremental growth. These mafic melts produced significant effects on the thermal budget of the granitic intrusion, rejuvenating and possibly mobilizing batches of the VMP. Based on similar whole-rock composition and reabsorption texture observed in coeval granite porphyry and volcanic products of the Sesia Caldera, a possible link between rejuvenated granitic melts and eruptive products has been postulated. The VMP may provide an insight into the storage and remobilization process that drive the eruption of large amount of melts in caldera-forming volcanic systems.