Conditions of metamorphism of garnet-bearing aluminous gneisses in the Orekhov-Pavlograd zone of the Ukrainian Shield

Garnet-bearing aluminous gneisses in the Vasil'kovka area of the Orekhov-Pavlograd zone of the Ukrainian Shield were studied using the THERIAK-DOMINO, THERMOCALC, and WINTWQ software and mineral geothermobarometry methods. The stability fields of parageneses formed at the initial, peak, and post-peak metamorphic stages have been estimated: T = 500-650 °C and P > 7 kbar, T = 800-850 °C and P - 8 kbar, and T = 600-670 °C and P = 4.0-5.5 kbar, respectively. A “clockwise” PT path has been established. Based on the aluminous gneiss, the age of metamorphism is determined as Paleoproterozoic (2014 ± 11 Ma). Unique regularly oriented acicular goethite microinclusions were described in garnet. In contrast to other known oriented Ti-bearing mineral inclusions in garnet, these goethite inclusions suggest not only extremely high P and T values (necessary for the formation of oriented Ti-bearing inclusions) but also a high oxygen fugacity. The obtained data broaden the concepts of the ways and conditions of formation of regularly oriented mineral phases in garnet.