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Russian Geology and Geophysics

2017 year, number 11

CONDITIONS OF METAMORPHISM OF GARNET-BEARING ALUMINOUS GNEISSES IN THE OREKHOV-PAVLOGRAD ZONE OF THE UKRAINIAN SHIELD

Sh.K. Baltybaev1, A.V. Yurchenko1, S.B. Lobach-Zhuchenko1, V.V. Balagansky2, O.L. Galankina1, M.V. Morozov3, E.S. Bogomolov1
1Institute of Precambrian Geology and Geochronology, nab. Makarova 2, St. Petersburg, 199034, Russia
2Geological Institute of the Kola Science Center, Russian Academy of Sciences, ul. Fersmana 14, Apatity, Murmansk Region, 184209, Russia
3Tellur North-East LLC, ul. Soyuza Pechatnikov 8, St. Petersburg, 199000, Russia
Keywords: Metamorphism, garnet, aluminous gneisses, regularly oriented inclusions, Paleoproterozoic, thermobarometry, Sm-Nd system

Abstract

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 ºC and P > 7 kbar, T = 800-850 ºC and P ≈ 8 kbar, and T = 600-670 ºC 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.