Acta Musei Moraviae

Biotitic gabbro enclave with hercynite in durbachitic rock of Třebíč Pluton in area of positive gravity anomaly

A positive gravity anomaly situated in the northern termination of Třebíč Pluton led to a surface survey and mapping in the area near Zhoř. The non-porphyritic biotite-bearing quartz monzonite prevail in this area. These rocks differ from the typical durbachitic rock of the Třebíč and the Jihlava Pluton by a non-porphyric structure, more felsic character and a whole-rock chemical composition (lower content of MgO (2.14 wt. %), Fe₂O₃ (3.58 wt. %), MnO (0.06 wt. %) and TiO₂ (0.59 wt. %)). A small irregular enclave of hercynite – biotite bearing gabbro was found in this rock. Such enclave has not been described in durbachitic rocks from Třebič Pluton yet. It differs from other enclaves in the durbachites in the Třebíč Pluton due to its mineral association (biotite + plagioclase + hercynite Hc₆₃Gh₁₈Sp₁₇Mt_1–2 + pyrrhotite + sericite + accessory K-feldspar). Based on the mineral association and the comparison to enclaves with similar composition, the following PT condition of the assimilation and melting process can be assumed: temperatures higher than 800 °C and pressures close to 2 kbar. Furthermore, the studied enclave with a hercynite from the Třebíč Pluton could represent the first evidence of a HT event in the Třebíč Pluton. The presence of the gabbroid enclave and the positive gravity anomaly in the studied area might indicate differences in the surface and deeper subsurface structure the examined area. It suggests that a body of gabbroid rock could appears in a deeper part of the area.