Compositional evolution of tourmaline in lepidolite-subtype pegmatites
Selway, Julie B.; Novák, Milan; Černý, Petr; Hawthorne, Frank C.
European Journal of Mineralogy Volume 11 Number 3 (1999), p. 569 - 584
published: May 25, 1999
manuscript accepted: Jan 11, 1999
manuscript received: Mar 25, 1998
ArtNo. ESP147051103015, Price: 29.00 €
Abstract Tourmaline from six pegmatites of the lepidolite-subtype was examined to determine compositional trends: symmetrically zoned Lašovičky, Dobrá Voda, Rožná, Dolní Bory and Radkovice dykes, Czech Republic, and relatively homogeneous dykes at Red Cross Lake, Manitoba. The five symmetrically zoned lepidolitesubtype 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 from 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-foitite, and is associated with minor biotite close to the contact with the host rocks, and with abundant Fe-bearing muscovite. In the albite zones, dark blue and green elbaite-schorl is associated with pale greenish yellow Na-bearing muscovite. In the lepidolite zones, pink elbaite-rossmanite to rossmanite-elbaite occurs with purple lepidolite and minor pink and green lepidolite. Green (Fe, Mn)-bearing elbaite occurs in the quartz core at Rožná and is associated with cookeite in late pockets at Dobrá Voda and Dolní Bory. Late hydrothermal foitite occurs as terminations on zoned tourmaline crystals in pockets at Dobrá Voda. Based on paragenesis and textural relations, rossmaniteelbaite is the last tourmaline composition to crystallize in the Radkovice, elbaite-rossmanite is the last at Laštovičky and Red Cross Lake dykes, (Fe, Mn)-bearing elbaite is the last at Rožná and Dolní Bory, and foitite is the last at Dobrá Voda. Lepidolite-subtype pegmatites contain foitite as the most primitive composition in the outermost pegmatite zones, and rossmanite or (Fe, Mn)-bearing elbaite as the most fractionated compositions in the innermost pegmatite zones. In contrast, elbaite-subtype pegmatites in the literature contain Mg-rich schorl and either elbaite or rarely lidicoatite, respectively. Typically, tourmaline from lepidolite-subtype pegmatites is Ca- and Mn-poor (<0.03 apfu Ca, <0.30 apfu Mn), whereas tourmaline 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 K-site in tourmaline due to fractional crystallization of the 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 partitioned into lepidolite.