The Oligocene Biella pluton (western Alps, Italy): new insights on the magmatic vs. hydrothermal activity in the Valsessera roof zone

The composite Biella pluton is part of an Oligocene volcano-plutonic complex whose origin is connected to the Alpine subduction-collision processes and that emplaced at shallow crustal levels within the eclogite-facies rocks of the Austroalpine Sesia-Lanzo Zone. In the roof zone of the pluton, small satellite igneous bodies are set within the Sesia-Lanzo country rocks, close to the main Biella pluton, and range in composition from quartz alkali feldspar syenite, quartz monzodiorite and monzogabbro, to quartz diorite and gabbronorite. Their geochemical features, including the REE patterns, are coherent with the calc-alkaline to shoshonitic affinity recognized in the whole volcano-plutonic complex. Field and petrographic data suggest that these bodies represent earlier crystallization/differentiation products of the Biella primary magma(s), which underwent contact metamorphic recrystallization during the multistage emplacement of the main pluton. Tourmaline-bearing hydrothermal breccias and different types of hydrothermal veins (including quartz-plagioclase-, quartz-tourmaline- and ankerite-quartz-sulphides-bearing veins) occur within both the intrusive rocks (satellite bodies + the main pluton) and their Sesia-Lanzo Zone country rocks. Both field relationships and vein assemblages suggest a close connection between the late-magmatic evolution of the Biella pluton and the multistage, boron-rich hydrothermal activity.