Acta Musei Moraviae

Metamorphic rocks western part Brno batholith

Biotite and amphibole biotite granitoids of the northern part of Thaya terrane (western part of the Brno batholith) contains large enclaves of polyphase-metamorphosed sedimentary and igneous rocks. The enclaves, up to several km long, consist mainly of biotite to muscovite-biotite gneisses (Qtz + Pl + Bt ±Ms ± Sil ± Ky), biotite to cordierite-biotite migmatites (Qtz + Pl + Bt ± Kfs ± Ms ± Sil ± Crd), metagabbros (Amp + Pl ± Px), calc-silicate rocks (Amp + Pl + Px + Qtz ± Cal ± Grt ± Qtz and Cpx ± Grt ± Ves ± Wo ±Cal ± Qtz ± Pl ± Kfs ± Czo) and amphibolites (Amp + Pl ± Qtz). Three metamorphic events with different P-T conditions are distinguished in the studied rocks. The first stage M1 was marked by regional metamorphism under amphibolite facies conditions (700 °C and 6–7 kbar). Regional metamorphism was followed by a decompression stage and intrusion of Cadomian granitoids of the Thaya terrane (Tetčice suite). P-T estimates for low-pressure contact metamorphism M2 correspond to approximately 700–800 °C and 3–5 kbar. The metamorphic assemblage (Pl + Cpx ± Grt ± Ves ± Wo ± Cal ± Qtz ± Kfs ± Czo) of skarnoid layers from enclaves of calc-silicate rocks indicates high water activity (X_CO2 < 0.03) during this metamorphic event. The subsequent Variscan metamorphic overprint (M3) was characterized by regional hydrothermal alteration under prehnite-pumpellyite facie (2–3 kbar and >300 °C).