Abstract

Abstract

Micas from the granitic pegmatites found in the Barroso–Alvão pegmatite field and the Cabeceiras de Basto granite present variations in their composition, textural characteristics and paragenesis. This pegmatite field in northern Portugal has numerous and varied aplite–pegmatite intrusions as well as diverse granitic bodies. The pegmatite bodies vary from barren to Li-rich ones with spodumene, petalite and lepidolite, and intrude a variety of rock types including metasedimentary and granitic rocks. The most fractionated bodies are enriched in Li, Sn, Nb, Ta, and Rb. In this study, we present the petrography and geochemistry of micas for the different types of aplite–pegmatite bodies and the Cabeceiras de Basto granite, and use this information to evaluate the evolution and petrogenesis of the pegmatite bodies. Micas are present in all of the pegmatites; they belong to the muscovite–lepidolite series. Micas from the granite and intragranite pegmatites have high octahedral-site occupancy and a few present Li enrichment. For most of the barren pegmatites and the Li-bearing pegmatites (spodumene and petalite), the stoichiometry of muscovite is close to the end member. Lithium-rich micas are mainly found in the petalite-and lepidolite-bearing pegmatites. The incorporation of Li into the mica structure seems to be controlled by the vectors Si2LiAl−3, and Li3Al−1−2 (where □ represents a vacancy). The highest enrichment in large-ion lithophile elements (particularly Rb and Cs), F, and Li is found in the Li-bearing pegmatites, especially in the lepidolite-bearing ones. This variety of pegmatite is considered the most evolved on the basis of its low K/Rb value and the abundance of F, Rb, Cs, Li, Ta and Mn.

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