From FeLi rich micas to lepidolite and muscovite: The magmatic to hydrothermal evolution of phyllosilicates in the apical zone of the Beauvoir granite (French Massif Central)

Abstract

The albite–lepidolite–topaz Beauvoir granite (western European Variscan belt) represents an extreme example of late-stage peraluminous magmatic differentiation associated with enrichment in Li, Sn, Nb, Ta and Be. This study investigates the textures and chemistry of micas from the highly evolved B1 unit to constrain the final stages of magmatic evolution and to evaluate the impact of post-magmatic fluid–rock interaction. Magmatic mica assemblages are dominated by lepidolite, whereas FeLi micas are rare and preserved only locally; both display high Li contents (28,000–32,000 ppm) and record late-stage RbCs enrichment. Hydrothermal alteration is expressed by the development of fine-grained dioctahedral muscovite along fracture-controlled alteration corridors, where albite and lepidolite are progressively replaced. Hydrothermal muscovite is nearly Li-free (∼90 ppm), and its formation leads to a pronounced decrease in the modal abundance of lepidolite (from ∼20% to 5–10%), resulting in significant Li depletion at the whole-rock scale despite limited Li loss from residual lepidolite crystals. Muscovitisation is also associated with the depletion of Nb, Ta and W, whereas Sn is partly redistributed and incorporated into muscovite (up to 1400 ppm). The absence of transitional compositions toward Li-rich muscovite or phengite indicates that this alteration stage is decoupled from magmatic differentiation and reflects a structurally controlled hydrothermal event. These results highlight the key role of post-magmatic hydrothermal processes in rare-metal redistribution and in reducing the lithium endowment of highly fractionated granites.