Ti–Zr–Nb-bearing accessory minerals in high-K trachyandesitic rocks from the Western Outer Carpathians, Moravia, Czech Republic

The occurrence of chevkinite-group minerals (CGM), zirconolite and Nb-rich ilmenite is reported in Miocene high-K trachyandesites from the Uherský Brod area, Czech Republic, their occurrence reflecting the high Nb and Zr abundances in the host rocks. All three were late-magmatic phases, crystallizing from silicic residual melts. Strong zonation in some crystals is ascribed mainly to kinetic factors and in the case of the CGM by the replacement of less Fe-rich, more calcic (“perrieritic”) compositions by compositions richer in Fe and poorer in Ca (“chevkinitic”) as temperatures fell during crystallization. Zirconolite has high ThO₂ (≤7.75 wt%) and UO₂ (≤3.78 wt%) abundances and is commonly zoned, with significant fractionation of Th from U and within the LREE. The minerals may have crystallized at relatively high fO₂, low pH₂O, and at temperatures around 840 °C. Zirconolite was stabilized relative to zircon by the Ti–Ca-rich nature of the melts and to baddeleyite by the high SiO₂ activity. Formation of Zr-bearing titanite, as recorded in other potassic suites, was precluded by the preferential entry of Ti into the CGM.