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Mineralogy and geochemistry of micas from the Pinilla de Fermoselle pegmatite (Zamora, Spain)

¹ Departamento de Mineralogía y Petrología, Universidad del País Vasco UPV/EHU, P.O. Box. 644, E-48080 Bilbao, Spain
² Departamento de Mineralogía y Petrología, Universidad de Granada, Fuentenueva s/n, E-18002 Granada, Spain
³ LMTG UMR 5563, Minéralogie, Observatoire Midi Pyrénées, 14 Av. E. Belin, F-31400 Toulouse, France

^* Corresponding author, e-mail: encar.roda{at}ehu.es

The highly fractionated, Li-F-Be-B-P-bearing Pinilla de Fermoselle (PF) pegmatite crops out in the westernmost part of the Zamora province (Spain). This body appears as a cupola over the PF leucogranite, displaying a non-symmetrical internal zonation with a complete sequence from a barren pegmatitic facies near the granite, to a highly evolved zone in the uppermost part of the body. Representative samples of micas from the different pegmatite zones have been studied. Based on textural and chemical criteria, the micas may be grouped into two assemblages: Al-rich micas and Fe-rich micas. In general, Al-rich micas show a continuous evolution from muscovitic to lepidolitic compositions from the leucogranite to the most evolved zone. Fe-rich micas range from Fe-biotite in the leucogranite and in the least evolved pegmatite zones, to an intermediate composition between zinnwaldite and trilithionite in the most evolved pegmatitic facies. The incorporation of Li into micas appears to be controlled by the substitutions Si₂LiAl_-3, and Li₃Al_-1-2, AlLiR_-2, SiLi₂R_-3, and SiLiAl_-1R_-1, where R = (Fe²⁺ + Mg + Mn). Paragenetic relationships and chemical variations in micas from different zones making up the PF pegmatite suggest that the pegmatitic system derived from a granitic melt and evolved upwards by fractionation processes. Evidence in support of this model comes from: (i) the gradual enrichment in Li, Rb, Cs and F, parallel to the decrease in Mg and Ti; (ii) the convergent evolutionary trends towards lepidolite showed by the Al- and Fe-micas; and (iii) the parallel decrease in the K/Rb ratio in micas.

Key-words: micas, granitic pegmatites, mineral chemistry, Pinilla de Fermoselle, Spain.

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