Metasomatic tourmaline in hybrid contact-bands between gneiss and peridotite in the Ulten Zone of the Eastern Italian Alps: chemistry and boron isotopic composition.

Ultramafic rocks in the Ulten Zone (UZ) high-pressure m\ue9lange, in the Eastern Italian Alps, embedded in crustal gneisses, preserve a series of metasomatic hybrid bands generated by infiltration of aqueous fluids. These hybrid bands at the peridotite\u2013gneiss contact comprise phlogopite and anthophyllite with accessory zircon, apatite and allanite. Here, we report the rare occurrence of centimeter-sized crystals of metasomatic tourmaline in two different UZ outcrops, at Mt. Hochwart and Malga Preghena. Tourmaline has a dravite\u2013uvite-rich composition and shows optical and chemical zoning, with slightly variable Ti, Ca and Fe contents. Secondary-ion mass spectrometry (SIMS) analyses provide information on the intragrain B-isotopic zonation of the grains, which is correlated with optical and chemical zoning. The range in d11B is from \u20137.4 to +0.3\u2030 for the Mt. Hochwart tourmaline, generally with an isotopically lighter core and heavier rim. The d11B range for Malga Preghena tourmaline is from +1.2\u2030 in the core to \u20133.9\u2030 in the rim. Variable proportions of fluids derived from both the local crustal gneisses and a subducting slab control the B-isotope composition in the metasomatic tourmaline from the UZ. Fluid mixing in hybrid crustal\u2013ultramafic bands can be monitored by metasomatic tourmaline, which may also have a significant impact on the B isotope budget of the subducted slab and the slab\u2013mantle interface