Zircon typology and preliminary mineral chemical and isotopic investigations on basaltic gem stone zircons from eastern Saxony, Germany

Basaltic zircons are known in eastern Saxony (Germany) from the Elbsandsteingebirge for a long time. Furthermore, there are some new localities described from the Upper Lusatia (Hofeberg quarry) and the Zittauer Gebirge (Lausche hill). The zircons derived from alkaline basalts. The Hofeberg locality in the south of the Görlitz town could provide evidence for in situ discoveries. Both of the other occurrences exist as placers.
   The zircon crystals have a mean size from 0.5 up to 4 mm (min. 0.2 to max. 9 mm) and show a gem stone quality. Many crystals are broken and/or intensive rounded. The rounding is the result of a magmatic corrosion in the basaltic transporter rock. This is an argument against the genesis of these zircon megacrystals in the basaltic melt. The broken crystals (splinters) are probably the result of the fast cooling during the basaltic eruption.
   The origin and the genesis of these zircon megacrystals in alkaline basalts are currently unknown and controversial. Recent research favour one of the following two models: a cogenetic and a non cogenetic origin. Therefore, investigations about the mineral chemistry and isotopy from these basaltic zircons may supply important information about melt processes in the Earth mantle and about processes of magma differentiation and mixing.
   In a first step we have investigated the zircon typology from the above-named three localities in eastern Saxony. We can observe two respectively three subpopulations: the first are S- and J types with predominant 100-prisma and blunt-angled 101-dypiramid in crystal form. The same crystal types were observed by phonolite zircons from the Zittauer Gebirge what indicates a petrogenetic relationship between this basaltic zircon subpopulation and the phonolitic melt and therefore a cogenetic origin. The second subpopulation only observed at the Lausche locality is composed by D- and P5-types with exclusive 110-prisma and bluntangled 101-dipyramid. The prism is very small and is missing sometimes, so that only double-pyramids exist. The third subpopulation is with 1% to 5% rare, but at the Hofeberg locality very common with 43% from all zircon-typological investigated crystals. Only these crystals are mostly non-transparent. They are G1-, P1- and P2-types with 110-prism and blunt-angled 101-dypiramid.
   The last crystal type represents typical granitic zircons, but there is evidence for an alkaline origin e.g. from syenitic, phonolitic or trachytic rocks by mineral chemistry and mineral inclusions. Following zircon typology and mineral chemistry zircons from the first subpopulation indicate also an alkaline origin, but the melt was some different to the first one. Preliminary in situ Hf-isotopic analyses of zircons from the first subpopulation show an origin from the lithospheric mantle.