Abstract <p>Experiments in the peridotite + basalt + K₂CO₃ + H₂O system at 7 GPa and 900–1200°C were carried out. They were aimed at the study of variations in mica composition depending on the proportions of ultramafic (peridotitic) and mafic (basaltic) material upon the crust–mantle interaction in subduction zones. Crystallization of trioctahedral mica (phlogopite) proceeds in a substrate containing up to 40 wt % of basaltic (crustal) component. With increasing proportion of basalt in the starting material (50% or higher), the ^VIAl/^IVAl ratio and the SiO₂ content in the mineral increase, which results in the formation of di-trioctahedral mica varieties. A significant decrease in the Mg/(Al + Ti) ratio in mantle material due to the introduction of crustal material (at least 80% of basalt) inhibits the formation of phlogopite being replaced with Ti-bearing dioctahedral mica (aluminoceladonite). An increase in the Ti/Cr ratio to 1 or higher may indicate a high degree of crustal material involvement (&gt;40%) in mantle metasomatism.</p>