Geological position and origin of augen gneisses from the Polička Unit, eastern Bohemian Massif

Abstract

Augen gneisses are a peculiar lithological type of the Polička Unit (PU), which can be divided into two main petrographic groups: augen metagranitoids and migmatitic augen gneisses. Both of the groups represent products of magmatic and metamorphic processes along the contact between the Svratka and Polička units in the eastern Bohemian Massif. Tonalitic to granodioritic synkinematic intrusions were emplaced into metasedimentary rocks of the Polička Unit during the Late Variscan orogeny, forming plutonic bodies and dykes. Residual fluids from granitic melts pervasively infiltrated the metasedimentary protolith causing a fluid-enhanced partial melting. The P-T conditions of partial melting in the southern part of the PU were estimated at ~ 750-800 °C and 7 kbar. Melting produced porphyroblastic migmatites with a granite composition. The characteristic augen textures developed from granitic and migmatitic protoliths during a high-temperature deformation in subsolidus conditions. Both the augen metagranitoids and the migmatitic augen gneisses were affected by a subsequent deformation in the shear zone related with thrusting of the Polička Unit over the Svratka Unit.