SHORT MOBILIZATION AND ERUPTION TIMESCALES RECORDED IN THE QUARTZ CRYSTALS OF A FOSSIL CALDERA PLUMBING SYSTEM, SESIA MAGMATIC SYSTEM, SOUTHERN ALPS

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% SiO₂), 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.