Metamorphic plagioclase crystallization and zones of equal anorthite content in epidote-bearing, amphibole-free rocks of the western Tauernfenster, eastern Alps

The plagioclase in the Penninic, epidote-bearing and amphibole-free gneisses, mica schists and phyllites of the western Hohe Tauern (Austria/Italy) was generated by Alpine crystallization. Reverse zoning in these plagioclase formed during the 'Tauern Crystallization' indicate that they were probably produced by a two-stage process of regional metamorphism. Statistical analysis of the regional distribution of plagioclase An-content shows that the highest values are located in the Zillerthal Alps in the central Tauernfenster, whereas An values decrease towards the periphery of the Tauernfenster. Three areas of differing plagioclase composition have been delineated by the regional analysis. In the central region maximum An-contents range from 20 to 29%; in the intermediate region they vary from 14 to 19%; and in marginal areas they do not exceed 6%. In general these three zones are concordant to the major tectonic structures.