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

Major element and stable isotope chemistry of tourmaline from two complexly-zoned rare element pegmatites in the Oxford pegmatite field of SW Maine have been studied, one being Black Mountain, an isolated body in sillimanite zone, highly sulphidic metapelites and quartzite, and the other Mt Mica, which is bounded by schists and pegmatite and aplitic granite bodies. At both localities, tourmaline was sampled from the country rocks into the contact and wall zones through to the pegmatite cores. For these traverses, major element trends were similar, including Li + Al [] Mg + Fe (super 2+) , Na + Ca + K [] H (sub 2) O (super +) and B --> Si substitutions. Tourmaline compositions also reflect the parageneses in which they occur, especially Mg/Fe (super 2+) , which increases as Fe (super 2+) is taken up by pyrrhotite in the country rock at Black Mt. Stable isotopes, however, suggest that two contrasting styles of pegmatite are involved: Black Mt tourmaline shows gradational isotope signatures between the pegmatite and the country rock, whereas Mt Mica contains tourmaline that is isotopically distinct from that in the country rock. This could be interpreted as indicating that the Black Mt tourmaline formed from partial melting of metasediments, in combination with precipitation from hydrothermal fluids related to nearby batholiths, whereas Mt Mica formed as a fractionate of the nearby Sebago batholith.