A felsic meta-igneous source for Li-F-rich peraluminous granites: insights from the Variscan Velay dome (French Massif Central) and implications for rare-metal magmatism

The Velay anatectic dome in the Variscan French Massif Central exposes a low-pressure–high-temperature metamorphic sequence, which represents an ideal natural laboratory for documenting the behavior of rare-metals and fluxing elements during crustal melting. We investigated the silicate and bulk-rock geochemistry of sub- to suprasolidus metapelites and orthogneisses, as well as related granites, and performed forward thermodynamically constrained geochemical modeling to quantify the respective effects of melting pressure, temperature, H 2 O activity, and protolith composition on the Li and F contents of granitic melts. We find that biotite compositions are good proxies of melt compositional evolutions during prograde melting. The crystallization of peritectic cordierite at low pressure (< 5 kbar) and “water-fluxed” melting both inhibit the Li enrichment of anatectic melts. Metapelite-derived melts consistently show modest Li–F contents, and a decoupling is observed as melts with the highest Li concentrations (~ 200–400 ppm) are produced below 750 °C, whereas F-richest melts (~ 0.2–0.4 wt%) are produced above 750 °C near the biotite-out isograd. Peraluminous orthogneiss anatexis can generate a melt that is concomitantly enriched in both F (~ 0.3–1 wt%) and Li (~ 600–1350 ppm) at relatively low temperature (< 750 °C), which can evolve toward rare-metal granite compositions (~ 10,000 ppm Li; ~ 2 wt% F) after 80–90 wt% of fractional crystallization. Melting of felsic meta-igneous rocks followed by magmatic differentiation is thus a viable mechanism to form Li-F-rich rare-metal granites and pegmatites, providing a direct link between protracted crust recycling and rare-metal magmatism in late-orogenic settings.