Overthrusting of the allochthonous Svecofennian block onto the autochthonous block of the Karelian Craton margin caused compression and decompression in the Meyeri thrust zone of the Northern Ladoga area, which are recorded by the thermobarometry of mineral paragenesis in para- and orthogneisses. The highest calcium garnets in paragenesis with medium-calcium plagioclase, as well as biotite and/or muscovite yielded the mineral formation pressures up to 8–9 kbar, which is 2–3 kbar higher than metamorphic pressures typical of rocks surrounding the thrust zone. This is related to the additional pressure caused by the tectonic load on the rocks in the thrust zone. The subsequent evolution of the PT parameters of gneiss metamorphism indicates a simultaneous decrease in temperature and pressure until reaching the minimum values of Т = 500–550°С and Р = 1.6–3 kbar. The water activity in a metamorphic fluid was determined from mineral reactions with hydrous minerals within the range of ~0.20–0.44. Despite the narrow range, aH₂O shows some variations at the present-day erosion level of the thrust zone, with the lowest value found in pre-muscovite garnet–biotite parageneses. An increase of water fraction in the metamorphic fluid and the appearance of muscovite parageneses did not lead to an increase in aH₂O due to a simultaneous increase in the content of salt components in the fluid. The salt composition of the metamorphic fluid is revealed from the replacement of early minerals by late ones, the formation of which requires the presence of Na⁺ and K⁺ in the fluid.