Thermodynamic Modeling of the Formation of Corundum-Bearing Metasomatic Rocks in the Belomorian Mobile Belt, Fennoscandian Shield

bstractThe mineral zoning of corundum-bearing rocks of the Belomorian mobile belt, whose genesis remains controversial, has been studied at one of the typical occurrences: Khitoostrov. Based on the estimates of the P–T parameters of the formation of corundum-bearing rocks by the multiequilibrium thermobarometry method, pseudosections were constructed using the Perple_X 6.9.0 software package in P–T, T–µ(SiO2), µ(SiO2)–µ(Na2O), and µ(SiO2)–µ(K2O) space, with CO2–H2O fluid, to model the metasomatic transformation of migmatized kyanite–garnet–biotite gneisses of the Chupa sequence. It has been established that the mineral zoning of the corundum-bearing rocks reflects the successive transition of SiO2, Na2O and K2O to a mobile state. Desilication in the outer zones led to the formation of` quartz-free associations, alkaline sodic metasomatism concurrent with ongoing desilication led to the formation of associations with staurolite and plagioclase, and K2O removal led to the replacement of biotite with calcium amphibole. Further desilication and sodic metasomatism resulted in mineral assemblages with corundum. This process is reflected in an increase in the sodium content in the calcic amphibole and variations in the Ca content of the garnet at a practically constant composition of the plagioclase.