Timescales and Mechanisms of Crystal-mush Rejuvenation and Melt Extraction Recorded in Permian Plutonic and Volcanic Rocks of the Sesia Magmatic System (Southern Alps, Italy)

Silicic calderas can evacuate 100 to >1000 km3 of rhyolitic products in a matter of days to months, leading to questions on pre-eruptive melt generation and accumulation. Whereas silicic plutonic units may provide information on the igneous evolution of crystal-mush bodies, their connection with volcanic units remains enigmatic. In the Ivrea–Verbano Zone of 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 upper-crustal segment of this magmatic system (also known as the Sesia Magmatic System) is represented by the Valle Mosso pluton (VMP). The VMP is an ∼260 km3 composite silicic intrusion ranging from quartz-monzonite to high-silica leucogranite (∼67–77 wt% SiO2), which intrudes into roughly coeval rhyolitic products of the >15 km diameter Sesia Caldera. In the caldera field, the emplacement of a large, crystal-rich rhyolite ignimbrite(s) (>400 km3) is followed by eruption of minor volumes (1–10 km3) of crystal-poor rhyolite. Here, we compare silicic plutonic and volcanic units of the Sesia Magmatic System through a combination of geochemical (X-ray fluorescence, inductively coupled plasma mass spectrometry and electron microprobe analyses) and petrological (rhyolite-MELTS, trace element and diffusion modeling) tools to explore their connection. Textural and compositional features shared by both VMP and crystal-rich ignimbrites imply thermal rejuvenation of crystal-mush as the mechanism to create large volumes of eruptible rhyolitic magma. Bulk-rock composition of crystal-rich rhyolite erupted during the caldera collapse overlaps that of the bulk VMP. Quartz and plagioclase from these two units show resorbed cores and inverse zoning, with Ti- and anorthite-rich rims, respectively. This indicates crystallization temperatures in rims >60 °C higher than in cores (780–820 versus ∼720 °C), if temperature is the sole parameter responsible for zonation, suggesting heating and partial dissolution of the crystal-framework. Decrease in crystallinity associated with thermal energy input was calculated through rhyolite-MELTS and indicates lowering of the mush crystal fraction below the rheological lock-up threshold, which probably promoted eruptive activity. Also, after the climatic eruption, Si-rich melts in the Sesia Magmatic System were produced by extraction of interstitial melt from un-erupted, largely crystalline mush. Regarding both textures and chemical variations, we interpret the deep quartz-monzonite unit of the VMP as a compacted silicic cumulate. Fractionated melts extracted from this unit were emplaced as a leucogranite cupola atop the VMP, generating the final internal architecture of the silicic intrusion, or alternatively erupted as minor post-caldera, crystal-poor rhyolite. Ti-in-quartz diffusion profiles in thermally rejuvenated units of the Sesia Magmatic System demonstrate that the process of reheating, mobilization and eruption of crystal-mush took place rapidly (c. 101–102 years). A protracted cooling history is instead recorded in the diffusion timescales of quartz from the silicic cumulate units (c. 104–106 years). These longer timescales encompass the duration of evolved melt extraction from the cumulate residue. We argue that the VMP preserves a complex record of pre-eruptive processes, which span mechanisms and timescales universally identified in volcanic systems and are consistent with recently proposed numerical models.