Acta Musei Moraviae

Geikielite-baddeleyite in mineral assemblage of dolomite marbles from the contact aureole of the Třebíč Pluton (Moldanubian zone)

Within the inner part of metamorphic aureole of the Třebíč Pluton, geikielite-ilmenite and baddeleyite occur as accessory minerals in spinel-forsterite marbles. The most abundant is geikielite (or Mg-ilmenite) which forms opaque grains up to 1 mm long in dolomite, calcite, forsterite and spinel. Chemical composition of geikielite (X_Mg ~ 0.55–0.78) varies in contents of Mg (0.557–0.770 apfu), Fe (0.212–0.400 apfu) and Mn (0.004–0.015 apfu), only trace contents of Ca (0.024 apfu), W (~ 0.001 apfu) and Nb (~ 0.002 apfu) were found. Baddeleyite occurs mainly as microscopic grains (<0.1 mm) in calcite or as intergrowths with zirconolite (<0.2 mm). Chemical composition of baddeleyite is simple, as minor elements it contains only Hf (0.013–0.018 apfu) and Ti (0.001–0.025 apfu). Mineral assemblages and mutual relations among the minerals indicate their origin during replacement of phlogopite by spinel and forsterite (± clinohumite). The source of Ti for geikielite was in phlogopite, the source of Zr for baddeleyite remains unknown (probably zircon?). In some cases, geikielite and baddeleyite were most probably consumed during subsequent reactions forming zirconolite and Ti-clinohumite. Zirconolite is, with few exceptions, characteristic for the clinohumite-rich marble. Although provenance of Ti and Zr is more likely inside the marble, their import into the marble from the host rocks can not be excluded due to their increased mobility in K- P- and F-rich fluid.