Acta Musei Moraviae

Study of zircon from alkali syenite near Naloučany, Moldanubicum, Czech Republic

The fine-grained part of alkali-feldspar syenite near Naloučany is extremely enriched in Zr, Th and U. In these parts the zircon content reach 5 %. The zircon forms euhedral crystals 100 do 350 μm in size with uniform typology. Among crystal faces dominate prism (100) and pyramid (101). The prism (110) and pyramids (301), (211), and (121) are less evolved. According Pupins classification studied zircons fall into S24–S25 indicated crystallization temperature about 850 °C and confirmed high alkalinity. In the BSE four different parts of zircon were observed: i) central part with inclusions of thorite, ii) dark parts propagating along fractures, iii) homogeneous rim, and iv) bright zones between central part and rim. The dark parts contain higher concentrations of Ca, Fe, Al and Ti. Based on the U and Th content the radiation damage was calculated. The highest degree of radiation damage shows bright parts (0.38–0.66 dpa) and the lowest was recorded in the rim (0.07–0.31 dpa). The dark parts exhibit lower damage (0.11–0.38 dpa) than central parts (0.17–0.46 dpa). Metamict state was confirmed by HRTEM. The first percolation point was reached about 0.11 dpa. Low calculated radiation damage in the most altered dark parts suggest loss of U > Th during mineral-fluid interaction.