Synchronism of partial melting in Chupa gneisses and metagabbroids of the Belomorian Mobile Belt: P-T conditions of migmatite formation and age of migmatites

The P-T conditions and time of partial melting of migmatized rocks of the Belomorian mobile belt – kyanite-bearing gneisses of the Chupa Formation and the Proterozoic metagabbroids that intrude them – were estimated. The study employed thermodynamic modeling of mineral formation based on Gibbs energy minimization and the U-Pb isotope dilution method (ID-TIMS) of dating of monazite and titanite. Modeling of mineral formation (PERPLE_X) and mineral thermobarometry showed that the estimated P-T conditions correspond to high temperature amphibolite and initial granulite facies metamorphism with moderately high pressures: T = 650–750 °C, P = 7.5–9.5 kbar for aluminous gneisses and P = 8.0–10.5 kbar for metagabbroids, with a regressive metamorphism path towards T = 550–590 °C at P = 4.5–6.0 kbar. It was established that water-saturated melting of gneisses and metagabbroids resulted in the formation of granite or granodiorite-tonalite melts at 680–730 °C under medium and moderate pressures. Modeling of mineral formation with a CO2 : H2O ratio of 0.2 : 0.8 produced the best fit to the observed mineral paragenesis in both rock types. The timing of partial melting of the Chupa gneisses was determined by U-Pb dating of monazite, and the migmatization of the metagabbroid was dated by U-Pb dating of titanite. Geochronometry using two minerals revealed the synchrony of partial melting in compositionally contrasting protoliths: the ~1.85 Ga age dating coincides within error for migmatites from aluminous gneisses and metagabbroids. The P-T estimates and geochronometry results indicate a Paleoproterozoic stage of tectonic-thermal events, widely manifested within the Belomorian Mobile Belt.