Zirconolite, baddeleyite and geikielite in clinohumite-spinel-forsterite marbles near Horažďovice and Prachatice in southwestern BohemiaArticle PDF
Zirconolite, baddeleyite and geikielite in clinohumite-spinel-forsterite marbles near Horažďovice and Prachatice in southwestern Bohemia
Zirconolite, baddeleyite and geikielite are described from two localities in marbles of the Horažďovice-Sušice belt in SW Bohemia and another localities near Prachatice in southern Bohemia. All the localities belong to the Varied Group, Moldanubian Zone. The zirconolite-bearing samples are the first occurrences of this mineral in the Bohemian Massif; baddeleyite and geikielite were described previously from some localities in western Moravia. Zirconolite in forsterite-spinel marble from the Velké Hydčice quarry forms red-brown pseudohexagonal platy crystals up to 0.5 mm. Associated accessory minerals include geikielite, rutile, apatite, zircon, baryte, pyrite and pyrrhotite. Zirconolite from inactive quarry 2 km SW of Rabí occurs in spinel-hydroxylclinohumite marble as rare crystals to 0.4 mm. The assemblage of accessory minerals is similar as in the sample from Velké Hydčice, but minute baddeleyite is also present. The zirconolite crystals from Velké Hydčice and Rabí are completely metamict, show a complex zoning due to variation in U (2.00–7.49 wt. % UO2) and Th (0.96–3.17 wt. % ThO2). Narrow rims of zirconolite II, rich in U and Th (7.9–14.6 UO2 and 3.1–5.5 ThO2), are present around zirconolite I from Rabí. REE and Y contents are low and zirconolite has a regular stoichiometry. The sample of dolomite-calcite marble with forsterite and hydroxylclinohumite from Cudrovice near Prachatice contains rare zirconolite, which shows composition similar to zirconolite II from Rabí. Calcite marble with chondrodite, phlogopite from Modlenice near Vimperk contains rare zircon with baddeleyite and accessory tremolite, Zn-spinel, apatite, sphalerite, fluorite and ilmenite. Somewhat similar occurrence of baddeleyite is recorded from clinohumite-rich calcite marble from Malenice near Volyně. Geikielite from Velké Hydčice and Rabí contains dominantly ca. 60 mol. % geikielite and 40 % ilmenite, ilmenites from Cudrovice and Malenice are magnesian ilmenites. Hydroxylclinohumite, forsterite and spinel marbles from Rabí and Velké Hydčice carrying zirconolite are confined to local narrow domains (zones) about 1 metre wide, which may suggest possible role of fluid import associated with granitoid intrusions. However, the evidence available is not sufficient to accept this interpretation as proved. The CHIME zirconolite dating yields age 339 ±10 Ma, which is consistent with crystallization age of magmatic rocks of the Southern part of the Central Bohemian Pluton (c. 336–346 Ma).
Renata Čopjaková, Czech Geological Survey, Leitnerova 23, 658 69 Brno; and Masaryk University, Kotlářská 2, 611 37 Brno, e-mail: firstname.lastname@example.org
Stanislav Vrána, Jan Malec, Czech Geological Survey, Klárov 3, 118 21 Praha 1, e-mail: email@example.com, firstname.lastname@example.org
Stanislav Houzar, Department of Mineralogy and Petrography, Moravian Museum, Zelný trh 6, 659 37 Brno, e-mail: email@example.com
Aleš Červený, Municipal Museum Horažďovice, Zámek 11, 341 01 Horažďovice, e-mail: firstname.lastname@example.org