Stable isotope and crystal chemistry of tourmaline across pegmatite-country rock boundaries at Black Mountain and Mount Mica, southwestern Maine, USA

Major element and stable isotope chemistry of tourmaline from two complexly -zoned rare element pegmatites has been studied to gain insights into the p rocesses by which the pegmatites were formed. Two locations in the Oxford P egmatite Field of western Maine, U.S.A., were chosen for this study: Black Mountain, an isolated body located in sillimanite zone, highly sulfidic met apelites and quartzite; and Mount Mica, which is bounded by schists and peg matite and aplitic granite bodies commonly having gradational contacts with each other. At each locality, tourmaline was sampled from the surrounding country rocks into the contact and wall zones through to the pegmatite core s. Along these traverses, trends in major element crystal chemistry of tour maline are similar for both localities: these include Li + Al <-> Mg + Fe2, Na + Ca + K H3O-, and B --> Si substitutions. Tourmaline compositions als o reflect the parageneses in which they occur, especially Mg/Fe2+, which in creases as Fe2+ is taken up by pyrrhotite in the country rock at Black Moun tain. Differences in the major element compositions of tourmaline between t he two localities are readily understood in the context of parageneses. How ever, stable isotopes strongly suggest that two contrasting styles of pegma tite are involved. Black Mountain has tourmaline showing gradational isotop e signatures between the pegmatite and surrounding country rocks. Mount Mic a contains tourmaline that is clearly isotopically distinct from tourmaline in the surrounding country rock. One interpretation of this difference is that Black Mountain may have formed from partial melting of metasediments, in combination with precipitation from hydrothermal fluids related to the n earby batholiths, whereas Mount Mica formed as a fractionate of the nearby Sebago Batholith.