GEOTHERMOBAROMETRY OF AL2SIO5-BEARING METAPELITES IN THE WESTERN AUSTROALPINE OTZTAL-BASEMENT

The dominant amphibolite-facies Variscan event in the Austroalpine Otz tal basement can best be studied in the northwestern part of the Otzta l block. Further to the southeast it is overprinted by Alpine metamorp hism. Metapelites with the assemblage garnet - staurolite - kyanite - sillimanite +/- andalusite - biotite - muscovite - plagioclase were us ed to reconstruct pressure and temperature conditions with exchange th ermometry, net transfer equilibria and multi-equilibrium methods. Assu ming kyanite as equilibrium Al2SiO5 polymorph, conditions of 570-640 d egrees C and 5.8-7.5 kbar are derived using garnet rim compositions. T ypical nonequilibrium textures are (1) continuous chemical zoning of g arnets, (2) inclusions of kyanite and fibrolite in andalusite porphyro blasts and (3) the spectacular replacement of garnet by fibrolite and biotite. The latter two textures were used to decipher the retrograde part of the P-T path. Application of the differential thermodynamics a pproach (Gibbs method) indicates prograde garnet growth during pressur e release. Addition of CaO to the KFASH - system allows the garnet bre akdown within the staurolite stability held and its quantification, us ing the quartz - garnet - aluminosilicate - muscovite geothermobaromet er, revealed temperatures of 530-630 degrees C and 3.5-5.7 kbar. Andal usite formation is thought to constrain the final stage of the P-T pat h. Textural and chemical data clearly indicate a continuous pre-Alpine metamorphic evolution.