Evidence for KCl Brine Metasomatism in Mafic Xenoliths from the Lower Crust

The occurrence of Kfs micro-veins in high-grade felsic gneisses has been interpreted as evidence for migrating alkali-rich fluids in the deep crust which may be responsible for melt-absent dehydration of high-grade rocks from amphibolite to granulite facies. Here we present new data on the occurrence of K-feldspar in mafic granulites found as xenoliths in recent alkaline lavas from Western Sardinia, Italy. The xenoliths originated from the underplating of subduction-related basaltic liquids which underwent cooling and recrystallisation in the deep crust (T = 850-900 C, P = 800-1000 MPa). They consist of quartz-free metagabbronorites characterised by Opx + Cpx + Plg porphyroclasts (An_50-66) in a granoblastic recrystallized matrix composed of Pyx + Plg (An_56-72) + Fe-Ti oxides +/- Kfs +/- Bt +/- FAp +/- Ti-Prg. Texturally, the Kfs occurs in a variety of different modes. These include: (1) rods, blebs, and irregular patches in a random scattering of Plg grains, (2) micro-veins along Plg-Plg and Plg-Pyx grain rims, (3) myrmekite-like intergrowths with Ca-rich Plg along Plg-Plg grain boundaries, and (4) discrete anhedral grains (sometimes microperthitic). The composition of each type of Kfs is characterized by relatively high Ab contents (16-33 mol.%). Increasing An content in the Plg towards the contact with the Kfs micro-vein and myrmekite-like intergrowths into the Kfs along the Plg-Kfs grain boundary are also observed. Small amounts of Bt (TiO₂ = 4.7-6.5 wt.%; F = 0.24-1.19 wt.%; Cl = 0.04-0.20 wt.%) in textural equilibrium with the granulite facies assemblage is present in both Kfs-bearing and Kfs-free xenoliths. These Kfs textures, e.g. the randomly scattered and relatively rare antiperthitic plagioclase grains, suggest a likely metasomatic origin due to solid-state infiltration of KCl-rich fluids/melts. The presence of such fluids is supported by the FAp in these xenoliths which is enriched in Cl (Cl = 6-50 % of the total F+Cl+OH). These lines of evidence suggest that formation of Kfs in the mafic xenoliths reflects metasomatic processes, requiring an external K-rich fluid source, which operated in the lower crust during in situ high-T recrystallization of relatively dry rocks.