A model for thermal rejuvenation and eruption of a granitic magma chamber, Valle Mosso pluton, Sesia Magmatic System (Southern Alps, Italy)

Construction of granitic magma chambers is recognized as a pivotal intermediate stage in the ascent of silicic magmas (SiO in excess of 65 wt%) through continental crust. However, the mechanisms by which large volumes (> 100 km ) of these highly crystalline magmas, stored at mid- to upper-crustal depth, are mobilized and then erupted to the surface in caldera-forming eruptions is still a widely debated issue. In the Sesia Magmatic System, Southern Alps, a Lower Permian pulse of magmatism, which spanned the entire crustal column from a depth of ca. 25 km to the surface, is recorded in a tilted crustal section (Quick et al., 2009). The mid- to upper-crustal segment of this magmatic system is represented by a monzogranitic intrusion (≈ 67 to 77 wt% SiO2), the Valle Mosso pluton (VMP), which intrudes the cogenetic volcanic products (rhyolitic ignimbrites and tuffs) of the now dismembered >15 km diameter Sesia caldera. Field and petrographic evidence suggests that a significant portion of the VMP (ca. 15% of the intrusion volume) underwent one or more rejuvenation and mobilization episodes. The quantitative analyses of disequilibrium textures preserved in quartz, plagioclase and K-feldspar minerals from the rejuvenated portion of the VMP indicate that the mineral-melt disequilibrium is related to reheating of a crystal mush. Evidence includes temperature fluctuations recorded from Ti-in-quartz zoning (650 to >750 °C), dissolution surfaces followed by growth of high-An rims in plagioclase crystals and plagioclase rims on ovoid K-feldspar crystals (rapakivi texture). The presence of andesites both as enclaves in the rejuvenated VMP unit and in the volcanic product of the Sesia caldera hints at an andesitic recharge, which likely induced the thermal perturbation (Quick et al., 2009). New U-Pb zircon dating (SHRIMP) conducted on a sample from the rejuvenated portion of the VMP yields an age of 280.8 ± 2.3 Ma, which overlaps with the published 282 Ma age obtained for the main eruptive events in the cogenetic volcanic rocks of the Sesia caldera. On the basis of petrographic, chemical and geochronological evidence a mechanism for the reheating of the VMP is proposed, in which the mixing between a hotter, more mafic magma with a highly crystalline silicic magma has triggered mobilization and eruption of a significant volume of silicic melt.