BORON ISOTOPE STUDY OF MAGMATIC-HYDROTHERMAL TOURMALINES FROM BIELLA AND TRAVERSELLA PLUTONIC COMPLEXES (PERIADRIATIC IGNEOUS PROVINCE, WESTERN ITALIAN ALPS).

The Biella pluton is part of an Oligocene volcano-plutonic complex emplaced at shallow crustal levels within the eclogite-facies rocks of the Austroalpine Sesia-Lanzo Zone. It is a composite stock of calc-alkaline to shoshonitic affinity ranging in composition from monzogranite-granodiorite to syenite and monzonite. Volcanic to volcaniclastic sequences coeval to the Biella stock occur as a steeply dipping belt truncated, to the southeast, by the Insubric Lineament; they range in composition from basaltic andesite and andesite of high-K calc-alkaline affinity to trachyandesite and trachydacite of shoshonitic affinity. Palaeomagnetic, geological and petrological data show that both the Oligocene igneous rocks and the Sesia-Lanzo country-rocks suffered a clockwise (towards southeast) tilting, along a NE-SW trending subhorizontal axis, during late Tertiary; as a consequence, along both the Cervo and Sessera valleys crustal sections, from deeper (to the NW) to shallower (to the SE) levels are exposed (Bernardelli et al., 2000; Callegari et al., 2004; with refs.). The Traversella stock, coeval with the Biella complex, is also intruded in the Sesia-Lanzo Zone and is mostly composed of diorite to quartz-diorite and monzonite of high-K calc-alkaline affinity, with some mafic cumulates showing shoshonitic affinity (Vander Auwera & Andre, 1991, with refs.). Both plutons record a complex history of magmatic-related fluid circulation that often involves boron-rich fluids as shown by the occurrence of abundant tourmaline. Within and around the Biella pluton tourmaline occurs (Bernardelli et al., 2000; Rossetti et al., 2007): - as a late-magmatic phase in aplitic dykes in the monzosyenite and as Pl + Qtz + Tr + Tur orbicules in satellite magmatic bodies; - in early Kfs + Qtz + Tur hydrothermal veins; - as Tur ± Qtz ± Ru clasts, locally surrounded by Qtz + Tur, embedded by Qtz ± carbonate matrix in brecciated veins and breccias; - as very fine-grained tourmalinite (Tur ± Qtz ± Py) clasts in a Qtz + Py + Cpy hydrothermal matrix in brecciated veins; - as radial aggregates of cm-long, acicular crystals in the matrix of a large hydrothermal breccia body at roof of the pluton; - as a product of metasomatic replacement of phengite and garnet in metapelites from the Sesia-Lanzo Zone in hydrothermal breccias close to the pluton contact. At Traversella, the occurrence of dolomite marble lenses in the host rock favored the development of complex skarn bodies (mined for magnetite: Vander Auwera & Andre, 1991, with refs.). Tourmaline occurs: - as a late-magmatic phase in aplitic dykes crosscutting quartz-diorite; - within the skarn bodies (which locally contain abundant boron-bearing phases other than tourmaline: e.g., ludwigite, szaibelyite and low-T, secondary phases, like canavesite). Boron isotopes represent a powerful systematic in tracing the origin of volatile elements in magmatic-hydrothermal systems and the extent of water/rock interactions occurred (e.g. Palmer & Swihart, 1996). In this study, boron isotope compositions of tourmaline from magmatic to hydrothermal/metasomatic settings have been determined. The new boron isotope data coupled with chemical features of tourmalines (Rossetti et al., 2007) provide new constraints on the evolution of fluid chemistry during the magmatic-hydrothermal transition at the interface between pluton and host rocks. Availability of couples of minerals from similar settings in Biella and Traversella intrusions as well as the occurrence of different boron-bearing minerals (tourmaline and ludwigite) allow the discussion of boron isotope fractionation behaviour in a high temperature setting.