Tourmaline from Mount Mica pegmatite (Maine, USA): textural and compositional constraints on pegmatite petrogenesis

The Mount Mica pegmatite belongs to the Oxford pegmatite field in the Central Maine Belt (USA). It intrudes into migmatites and has a well developed and asymmetrical zoning, with an inward increase in grain size and enrichment in incompatible elements. Tourmaline is present in all zones of the pegmatite, showing a great textural variation: comb prisms; pseudographic intergrowths with quartz and garnet, rims around garnet, concentrically and longitudinally zoned crystals. Some of these textures reflect disequilibrium crystallization from an undercooled melt. Chemical variation is also broad, ranging from black schorl with a dravite component in the wall zone to multicolored elbaite with a high rossmanite component in the pockets of the core zone, also including black cap varieties classified as foitites. Main substitution mechanisms operating during fractionation of the melt include dravite-schorl, schorl-elbaite, proton-loss, alkali deficiency, elbaite-rossmanite and elbaite-olenite/darrellhenryite. The crystallization of minerals such as albite, micas, garnet and phosphates has been shown to influence the composition of tourmaline, as well as albitization processes affecting previously formed plagioclase. The presence of a Fe-rich dark cap in some of the gemmy crystals from the pockets could be attributed to changes in the fO 2 at the end of the pegmatite crystallization, which could increase the D Fe fluid/melt . Chemical trends shown by tourmaline from Mount Mica are comparable to those of tourmaline from other pegmatites from the Oxford field. The common occurrence of tourmaline in all zones of these pegmatites attests for a relatively high initial B content of the pegmatite melts, inherited from the leucosomes formed via partial melting under closed-system conditions. The high B contents would have decreased the viscosity and solidus temperature and increased the diffusivity and mobility of the pegmatitic melts, thus, favoring the efficient fractionation mechanism operating during Mount Mica crystallization.