Properties of Fluids during Metasomatic Alteration of Metamorphic Rocks under P–T Conditions of the Middle Crust: An Example from the Bolshie Keivy Region, Belomorian–Lapland Orogen, Fennoscandian Shield

Properties of fluids under P–T conditions of the middle crust were studied with reference to the metasomatic alteration of metamorphic rocks (amphibolite facies) of the Bolshie Keivy nappe of the Keivy terrane of the Belomorian–Lapland collision orogen of the Fennoscandian shield. Properties of the fluids were studied in five selected types of rocks: metamorphic schists and gneisses with graphite, metasomatic quartz rocks with a high content of graphite, kyanite–quartz veins with wall-rock metasomatites, and metasomatic quartz-bearing kyanite rocks and anchimonomineral quartz veins. NaCl, CaCl₂, CO₂, N₂, CH_4, heavier hydrocarbons, and graphite were identified in the fluid inclusions using microthermometry and Raman spectroscopy. Using the method of multiequilibrium thermobarometry for mineral associations and the density of CO₂ inclusions, a retrograde P–T path was calculated, which reflects the P–T exhumation history of the rocks. An explanation was proposed for the presence of water inclusions with NaCl of low salinity among inclusions of high salinity with NaCl and CaCl₂. Comparison of data on the H₂O activity (inferred from mineral equilibria) and salt content (data on fluid inclusions) with those of a model fluid (thermodynamic model of the H₂O–NaCl–CaCl₂–CO₂ system) showed a good agreement between natural and model data. Natural and model data were synthesized to analyze variations in the phase state and chemical composition, fluid properties, including H₂O activity, density, and salinity along the retrograde P–T trend.