Granitic intrusions are not considered the ideal target for the study of short-lived, transient processes associated with
remobilization and eruption of highly crystalline silicic magmas. However, crystal zoning preserved in phenocrysts from
fossil upper crustal crystal mushes can retain information on the timescales and reactivation dynamics of silicic magma
chambers.
In the Southern Alps, the plumbing system of a Permian rhyolitic caldera is exposed to a depth of about 25 Km in tilted
crustal blocks. The mid- to upper-crustal segment of this magmatic system (a.k.a. Sesia Magmatic System) is
represented by a monzogranitic intrusion (≈67 to 77 wt% SiO ), the Valle Mosso pluton (VMP), which intrudes
cogenetic rhyolitic products of the >15 km diameter Sesia caldera. Field and petrographic evidence suggest that a
significant portion of the VMP (ca.15% of the intrusion volume) underwent one or more rejuvenation and mobilization
episodes.
Titanium (Ti) in quartz content in grains from granitic and volcanic units of the Sesia Magmatic System has been
investigated through cathodoluminescence (CL) imaging and microprobe (EPMA) analyses. Sharp contrast in
concentration between Ti-poor cores and Ti-rich rims is observed in most of the granitic and volcanic quartz grains.
Application of TitaniQ thermometer indicates sharp temperature increase across core-rim boundaries (ΔT of min 50 °C
to max 100 °C) assuming uniform a TiO2 and pressure at the time of crystallization. Furthermore, one-dimensional
modeling of Ti diffusion core-rime interfaces indicate short elapsed time (10s of years) between crystallization of the
high-T rim and cooling of the system below magmatic temperatures, with striking similar results obtained for quartz
grains from rejuvenated portion of VMP and volcanic products.
These results suggest that a short-lived thermal flare-up, possibly related to mixing with a batch of hotter, more mafic
magma, interested the upper portion of the Sesia Magmatic System during its upper crustal residence as a crystal
mush, triggering remobilization and eruption of portions of the magma chamber. Such short timescales, typical of
explosive eruptive processes, have never been identified before in fossil magma chambers, making this discovery
relevant in the framework of the ongoing volcano-plutonic connection debate.
