Zircons in rocks of the Austroalpine and Southalpine complexes of the Eastern Alps: differences and analogies

Zircon crystals have been separated from gneisses, micaschists and phyllites of the Austroalpine and Southalpine tectonic Units of the Eastern Alps (16 and 7 samples respectively). The studied zircon crystals from the Austroalpine Unit are preferentially euhedral and show oscillatory zoning, suggesting a magmatic origin. Those from the Southalpine Unit are detrital grains of magmatic origin and show pitted surfaces and rounded edges, suggesting a long transportation during the sedimentary cycle. Electron microprobe analyses (438) for major (Si, Zr,Hf,P), minor (Y), trace elements (Th, U) and HREE (Gd,Dy,Er,Yb) have been carried out on selected zircons. Using long counting times, determination of REE contents as low as 50 ppm is possible. The zircon crystals are relatively homogeneous in composition except for the HfO2 content, which is strongly enriched in the rims of zircons from the Austroalpine basement. The HfO2 content is generally lower in zircons from the Southalpine basement. Xenotime-type substitution (Y,REE)3+ + P5+ = Zr4+ +Si4+ is common, but also other (non-xenotime-type) substitutions has been observed.