ALBERT

Description: Zircon megacrysts of gemstone quality hosted at two different localities within the Lusatian Volcanic Field, the scoria cone of the basanitic Hofeberg volcano and an alluvial placer named Hirschbörnel, a small brook downstream of the Buchberg trachyte dome, were dated and chemically analysed. The genetic interpretation within their geological context results in two different genetic models: In case of the first locality entrainment of the zircon with primary intergrowth phases in a basanitic melt resulted not only in resorption and reaction zones in the enclosing melt but in recrystallisation of the zircon and its intergrown phases. In case of the placer locality the zircon crystals are unresorbed and can be deduced to have been derived from the initial explosive eruption of the highly evolved cupola of the trachytic melt system that is documented by the Buchberg trachytic dome being located upstream. Both, the U/Pb ages (30.34 ± 0.10 Ma for Hofeberg; 30.45 ± 0.17 Ma for Buchberg) and the Hf isotopic composition (εHf +3.1 to +6.8 for Hofeberg; εHf +3.4 to +4.7 for Buchberg) of the zircon grains are similar within errors with those of their respective host rocks basanite and trachyte. This suggests their comagmatic formation. Applying the Ti-in-zircon thermometer 800–900 °C are assumed as crystallisation temperatures for both zircon populations based on different Ti-activities indicated by intergrown phases. However, the source melt of zircon megacrysts from the Hofeberg remains unidentified, but due to mineral chemical characteristics an evolved, plagioclase free melt is most probable.