Compositional evolution of tourmaline in lepidolite-subtype pegmatites

Tourmaline from six pegmatites of the lepidolite-subtype was examined to de termine compositional trends: symmetrically zoned Lastovicky, Dobra Voda, R ozna, Dolni Bory and Radkovice dykes, Czech Republic, and relatively homoge neous dykes at Red Cross Lake, Manitoba. The five symmetrically zoned lepid olitesubtype pegmatites contain the following zones from the outermost zone inward: 1) granitic zone, (2) graphic zone, (3) blocky K-feldspar zone, (4 ) albite zone, (5) lepidolite zone, (6) quartz core and (7) rare-to-absent small pockets. The crystallization sequence of tourmaline was determined fr om the characteristic composition of tourmaline for each pegmatite zone and from the compositional zoning within individual crystals. In granitic and outer albite zones, black tourmaline ranges from foitite-schorl to schorl-f oitite, and is associated with minor biotite close to the contact with the host rocks, and with abundant Fe-bearing muscovite. In the albite zones, da rk blue and green elbaite-schorl is associated with pale greenish yellow Na -bearing muscovite. In the lepidolite zones, pink elbaite-rossmanite to ros smanite-elbaite occurs with purple lepidolite and minor pink and green lepi dolite. Green (Fe, Mn)-bearing elbaite occurs in the quartz core at Rozna a nd is associated with cookeite in late pockets at Dobra Voda and Dolni Bory . Late hydrothermal foitite occurs as terminations on zoned tourmaline crys tals in pockets at Dobra Voda. Based on paragenesis and textural relations, rossmanite-elbaite is the last tourmaline composition to crystallize in th e Radkovice, elbaite-rossmanite is the last at Lastovicky and Red Cross Lak e dykes, (Fe, Mn)-bearing elbaite is the last at Rozna and Dolni Bory, and foitite is the last at Dobra Voda. Lepidolite-subtype pegmatites contain fo itite as the most primitive composition in the outermost pegmatite zones, a nd rossmanite or (Fe, Mn)-bearing elbaite as the most fractionated composit ions in the innermost pegmatite zones. In contrast, elbaite-subtype pegmati tes in the literature contain Mg-rich schorl and either elbaite or rarely l idicoatite, respectively. Typically, tourmaline from lepidolite-subtype peg matites is Ca- and Mn-poor (<0.03 apfu Ca, <0.30 apfu Mn), whereas tourmali ne from elbaite-subtype pegmatites is relatively Ca- and Mn-rich, commonly with up to 0.30 apfu Ca and locally up to 1.1 apfu Mn. Schorl in lepidolite -subtype pegmatites is rich in X-site vacancies, whereas schorl in elbaite subtype pegmatites is Na rich. A negative correlation exists between Fe and Al + Li at the Y-site in tourmaline due to fractional crystallization of t he evolving pegmatite-forming melt. The positive correlation between Na and F in tourmaline is due to crystal-chemical constraints. Sodium and Mn are preferentially partitioned into tourmaline, and F is preferentially partiti oned into lepidolite.