Typology, chemistry and origin of zircon from alkali basalts of SE Saxony (Germany)

摘要

To clarify the origin of zircon from Tertiary alkali basalts from SE Saxony (Germany), electron-microprobe analysis was combined with detailed studies of crystal morphology and inclusion types. Morphological and chemical typology of representative crystals from the Seufzergriindel placer (Elbe zone) essentially distinguish two zircon types crystallized in an alkaline environment: (I) a mostly drop-like, rounded, honey-coloured, gem-quality zircon relatively poor in trace elements, and a (II) mostly non-transparent, red brown to grey zircon rich in trace elements and with chaotic zoning pattern. A further zircon type, rich in trace elements and distinctly zoned is assumed to be of calc-alkaline granite origin. Inclusions of SO_3-rich apatite and magnesiokatophoritic amphibole in zircon indicate an alkaline, probably syenitic or nepheline-syenitic source for the two main types of zircon. Thorite and uraninite included in zircon are interpreted as exsolutions from metastable zircon-thorite solid solutions. Chemical Th-U-total Pb age determinations of these exsolutions by electron microprobe interpreted as "exsolution age" confirm a Tertiary age (16 +- 8 to 39 +- 4 Ma). The "Ti-in-zircon" thermometry results in approx = 800 +- 50 deg C for zircon from an alkaline source. In comparison with the zircon from the Seufzergriindel placer, first analytical results of zircon from basalt- and phonolite-related occurrences of the Upper Lusatia (Oberlausitz) show similar features, but a greater variability in minor and trace element contents. We argue that the majority of zircon studied is genetically connected with the Tertiary, rift-related, alkaline magmatism, the centre of which is the Eger (Ohre) Graben in North Bohemia.