Abstract

Tourmaline occurs in a variety of pegmatites, leucogranites, migmatites, metasedimentary rocks and quartz veins in the anatectic Variscan Tormes Dome (TD) in the central Iberian Zone, Spain. In general, these tourmalines belong to the alkali group and to the hydroxy subgroup. The ratios X□/(X□ + Na) and Mg/(Fe + Mg) range from 0 to 0.6 and 0 to 0.7, respectively, where X□ represents vacancies at the X site. Metasomatic tourmaline precipitated in the metasedimentary rocks reflects the composition of the host rocks. Elbaite is characteristic of the most evolved pegmatites. Except for the Li-rich tourmalines, where the substitution mechanisms LiAlR2+−2 and XYAl0.5XNa−1YLi−0.5 account for most of the Li incorporation, the chemical variations in tourmaline from the TD can be described in terms of a combination of the FeMg−1, AlX□(NaR2+)−1 and AlO(R2+,OH)−1 exchange vectors, where R2+ represents (Fe + Mg + Mn). The substitution AlO(R2+,OH)−1 becomes more important from leucogranites, through metasedimentary units, migmatites to metasomatized granite. In tourmaline associated with pegmatites, the influence of the proton-loss substitution increases with the degree of evolution of the pegmatites. Biotite occurring with tourmaline has VIAl in the range of 0.26–0.75 cations apfu and Mg/(Mg + Fe) values of 0.1 to 0.4. These values correlate well with those of the associated tourmaline, the KDtur/bt depending on the lithology. The F intercept values of biotite decreases in the order leucogranites, BP and IP, which is consistent with a normal evolutionary trend. Low Fe3+/∑Fe values in biotite and tourmaline from migmatites, leucogranites and pegmatites reflect conditions of low oxygen fugacity. In contrast, biotite from metasediments and metasomatized granites, in which zinnwaldite occurs, are presumed to have significant amounts of Fe3+, suggesting relatively oxidizing conditions.

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