d11B and d18O study of tourmalines from Elba Island (Italy): the role of magmatic vs. external derived fluids in pegmatite-hydrothermal systems

During granite emplacement and contact metamorphism, fluid production from magma and wall rocks, as well as the introduction of meteoric water, control the formation of different mineral assemblages ranging from high-T magmatic, to low-T hydrothermal, ones. In peraluminous granitic systems, the relatively high B content of magma and aqueous fluids allows the crystallization of tourmaline over the entire spectrum of settings. The occurrence of tourmaline at different evolutive stages, coupled with its strong chemical variability, and post-formation refractory behavior, make it a useful geochemical and isotopic monitor of evolving fluids in such systems. Samples of tourmaline from Monte Capanne peraluminous intrusive system (Elba Island, Italy) were selected as representative of the progressive stages of evolution: 1) magmatic spots in the granite; 2) graphic intergrowths, prismatic and miarolitic crystals in pegmatites; 3) crystals from hydrothermal veins and breccia in the granite and hostrocks. Chemical composition, d18O (by laser fluorination method at GZG, Gottingen) and d11B were measured across progressively zoned crystals for each evolutive stage, by using in situ analyses, or processing hand-picked grains. The chemical variations observed in magmatic (from schorl-dravite to schorl-foitite s.s.) and pegmatitic (from schorl-foitite to elbaite s.s.) tourmalines are typical of these settings and are consistent with crystallization in a continuously evolving, magmatic, closed system. At this stage, the near-constant d11B values in tourmalines are consistent with negligible B isotopes fractionation. Also d18O values are consistent with a closed system, but in this case significant temperature effect on fractionation factors produced a progressive increase of d18O and ?18Oqz-tur. On the contrary, the sharp chemical inversion towards more primitive compositions, observed in the late, hydrothermal, tourmalines from veins in host rocks (dravite-rich), and in the "black" overgrowths in brecciated pegmatites (foitite-schorl s.s.), is consistent with the opening of the system. The moderate decrease of d11B in these, late, tourmalines indicate the influx of low-d11B fluids, equilibrated with the previously 11B-depleted thermometamorphic rocks. Similar decreasing patterns are also defined by d18O values indicating significant interactions with the host rocks. There are no evidences of large ingression of meteoric water during thermometamorphism and tourmaline crystallization in pegmatites and hydrothermal veins. A significant influx of meteoric fluids occurred only in a later stage, when low-d18O quartz crystallized.