Tourmaline from the rare-element Pinilla pegmatite, (Central Iberian Zone, Zamora, Spain): chemical variation and implications for pegmatitic evolution

Summary

Tourmaline is an ubiquitous constituent in the Pinilla de Fermoselle rare-element pegmatite (Zamora, Spain), as well as in barren pegmatitic and quartz–tourmaline veins inside the associated leucogranite. The rare-element pegmatite shows internal zoning, evolving from a barren facies, in the lower border zone, in contact with the leucogranite, to a Li-rich facies in the upper border zone, close to the host-rocks.

Tourmalines from the veins within the leucogranite have highest Mg contents, and belong to the schorl–dravite series. The tourmalines from the rare-element pegmatite mostly belong to the schorl–elbaite series, with chemical compositions within the range of the end-members, whereas the tourmalines associated with the most evolved zone in the pegmatite belong to the elbaite–rossmanite series. The broad compositional range shown by the tourmalines correlates quite well with the pegmatite zoning. The most plausible substitution mechanism for the chemical evolution of tourmalines during crystallization seems to be Mg₋₁Fe²⁺ ₁, [X]₋₁ ^YAl₋₁ ^XNa₋₁ ^YFe²⁺ ₁, for the foitite–schorl series; ^YFe²⁺ ₋₃ ^YAl_1.5 ^YLi_1.5, for the schorl–elbaite vector; ^XNa₋₁ ^YLi_−0.5[X]₁ ^YAl_0.5, for the elbaite–rossmanite series; and, (OH)₁F₁ for all the tourmalines except the pink elbaites. This chemical variation in tourmaline is consistent with a crystal fractionation model for the evolution of the Pinilla pegmatite.