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, H2O 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 (similar to 200-400 ppm) are produced below 750 degrees C, whereas F-richest melts (similar to 0.2-0.4 wt%) are produced above 750 degrees C near the biotite-out isograd. Peraluminous orthogneiss anatexis can generate a melt that is concomitantly enriched in both F (similar to 0.3-1 wt%) and Li (similar to 600-1350 ppm) at relatively low temperature (< 750 degrees C), which can evolve toward rare-metal granite compositions (similar to 10,000 ppm Li; similar to 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.

Automated summary

The Velay anatectic dome in the Variscan French Massif Central provides an ideal natural laboratory for studying the behavior of rare metals and fluxing elements during crustal melting. Through geochemical analysis and modeling, the study reveals the factors that affect the Li and F contents of granitic melts, such as melting pressure, temperature, H2O activity, and protolith composition. It also demonstrates that peraluminous orthogneiss anatexis can generate melts enriched in both F and Li, which can evolve into rare-metal granite compositions through fractional crystallization.