Migmatized gneisses of the Chupa paragneiss Formation in the Belomorian Mobile Belt (BMB) of the Fennoscandian Shield have been studied, and the conditions of partial melting during high-grade metamorphism of the rocks were determined. The melting temperatures and pressures, the amount and composition of the melt formed during the anatexis of gneisses in a closed system, were assessed through direct thermodynamic computer modeling of mineral formation and the construction of pseudosections in pressure-temperature coordinates. The mineral formation calculations are based on the principle of Gibbs energy minimization and were performed using the PERPLE_X software package. The bulk compositions of the migmatized rocks from the Chupa Formation, previously classified and grouped based on their major components, were used for the calculations. It is shown that water-saturated partial melting of compositionally diverse gneisses produces granitic or granodiorite-tonalitic melts within a temperature range of 680-730 °C at moderate to moderately high pressures. The study reveals that the key factor controlling the appearance of kyanite in the investigated rocks is a high Al₂O₃/CaO ratio (at least 5:1) in the protolith, combined with a total alkali content (Na₂O + K₂O) exceeding CaO. According the Chemical Index of Alteration (CIA), the protoliths of the gneisses contained detrital material of varying sedimentary maturity. The source rocks were likely weakly to moderately weathered. U-Pb ID-TIMS dating of monazite from two samples of garnet-kyanite-biotite migmatite (whole-rock analysis) indicates Paleoproterozoic migmatization of the Chupa gneisses at 1854 ± 5 Ma. This phase of Paleoproterozoic endogenic activity is widely recorded in the BMB and may be associated with the formation of the Lapland-Kola or Svecofennian orogens, located to the northeast and southwest of the belt, respectively.