Metasomatic tourmaline in hybrid contact-bands between gneiss and peridotite in the Ulten zone of the Eastern Italian Alps: chemistry and boron isotopic composition
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Ultramafic rocks in the Ulten Zone (UZ) high-pressure melange, 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-gneiss 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-uvite-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 delta(11)B is from -7.4 to +0.3% for the Mt. Hochwart tourmaline, generally with an isotopically lighter core and heavier rim. The delta(11)B range for Malga Preghena tourmaline is from +1.2% in the core to -3.9% 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-ultramafic 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-mantle interface.