Acta Musei Moraviae

Zirconolite in marble at Krahulov and age of HT/LP metamorphism in the contact aureole of the Třebíč Pluton (Moldanubian zone)

Regional-scale contact metamorphism of the Třebíč Pluton led to the formation of distinct zoning manifested typically in dolomite marbles. Inside part of metamorphic aureole appears rare zirconolite as an accessory mineral in clinohumite-spinel-forsterite marble. The zirconolite-bearing assemblage contains calcite, dolomite, hydroxylclinohumite, spinel, forsterite, chlorite, phlogopite, fluorapatite, baddeleyite and ilmenite-geikielite. Zirconolite grains (<200 μm in size) sporadic overgrowth of baddeleyite or comprise its common relic. The chemical composition of zirconolite varies due to complex zoning of Hf (0.67–1.09 wt. % HfO₂), Th (0.38–12.76 wt. % ThO₂), U (0.11–9.57 wt. % UO₂), Fe (0.64–2.88 wt. % Fe₂O₃), Mg (0.20–1.92 wt. % MgO), Nb (0.56–1.01 wt. % Nb₂O₅), W (0.38–1.11 wt. % WO₃), and Y (0.15–0.84 wt. % Y₂O₃). The CHIME zirconolite dating yields average age 331 ±11 Ma corresponding to post-peak HT/LP metamorphic phase in marbles and is in good agreement with the age of emplacement of the Třebíč Pluton.