From granite to highly evolved pegmatite: A case study of the Pinilla de Fermoselle granite-pegmatite system (Zamora, Spain)

摘要

The Pinilla de Fermoselle pegmatite is a cap-like body with an asymmetrical vertical zoning, from a granitic fades at the bottom to the upper contact with the metamorphic country-rocks. The granite grades imperceptibly into the pegmatite, which includes three main zones with different degrees of enrichment in Ii+F+B (±P, Rb, Cs, Be, Sn). The essential minerals are quartz, feldspar, Al-micas from the muscovite-lepidolite series, Fe-micas (biotite and zinnwaldite), tourmaline (schorl-elbaite-rossmanite) and Fe-Mn phosphates. Apatite, beryl, cassiterite and cookeite are the most significant accessory minerals. The trace elements Ii, Be and Sr show similar trends in feldspar, micas and tourmaline, with an increase in the Li and Be contents and a decrease in Sr from the granite to the most evolved pegmatitic zone. Similar trends are shown by Rb, Cs and Ba for micas and K-feldspar, Rb and Cs increasing gradually from the granite to most evolved pegmatitic zones, simultaneously to the decrease of Ba. In tourmaline Nb and Ta contents increase upwards whereas Zn contents decrease in the same way. The Mn contents increase until intermediate degrees of evolution, and decrease again in the pinkish elbaite. Combined field, petrographic and geochemical data are consistent with a fractional crystallization model from a granitic melt, with a dear petrogenetic relationship between the underlying peraluminous granite and the pegmatite body. K-feldspar and, particularly, micas and tourmaline appear as good geochemical monitors using trace elements such as Ii, Rb, Be, Sr and Ba, which offer intriguing insights into the pedogenesis of pegmatites.