Mineral reactions and conditions of metamorphism in metapelites of the Western Schneebergerzug and the Adjacent Altkristallin (Ötztal Alps)

Summary Conditions of metamorphism of the Schneebergerzug and adjacent Altkristallin are characterised by a posttectonic staurolite- and kyanite-microblastesis in the northernmost Schneebergerzug-rocks.Formation of staurolite is favoured by high ZnO-contents, which rise to 5.6 wt.% in staurolite. With increasing grade of metamorphism towards the south, the zone of continuous staurolite formation is limited by the reaction: garnet + chlorite + muscovite = = staurolite + biotite + quartz + H₂O in Altkristallin rocks south of the Schneebergerzug. Staurolite-formation in paragonite-rich micaschists is due to the reaction: chlorite + + paragonite + quartz = staurolite + albite + H₂O.Maximum conditions of metamorphism are limited by the occurrence of the stable assemblage paragonite + quartz with a calculated temperature of 570°C (5 kb). Breakdown of paragonite + quartz occurred at lowr temperatures due to solid solution, which is more dominant in albite than in paragonite, and a CH₄-rich fluid in graphitic schists. Mg/Fe-exchange geothermometry between garnet and biotite gives temperature between 548°C and 577°C (5 kb). Partition coefficients in divariant AFM-assemblages of the Schneebergerzug rocks are affected by–0.003 per mol.% Ca/(Ca+Mg+Fe+Mn) in garnet andK _D-values have been corrected before computing the temperatures. An additional lowering ofK _D-values is observed in mineral assemblages containing zinc-rich staurolite.Pressure conditions of 5 kb are minimum values due to the presence of kyanite in the Schneebergerzug rocks.