CORRELATION BY RB-SR GEOCHRONOLOGY OF GARNET GROWTH HISTORIES FROM DIFFERENT STRUCTURAL LEVELS WITHIN THE TAUERN WINDOW, EASTERN ALPS

In order to evaluate rates of tectonometamorphic processes, growth rat es of garnets from metamorphic rocks of the Tauern Window, Eastern Alp s were measured using Rb-Sr isotopes. The garnet growth rates were det ermined from Rb-Sr isotopic zonation of single garnet crystals and the Rb-Sr isotopic compositions of their associated rock matrices. Garnet s were analyzed from the Upper Schieferhulle (USH) and Lower Schieferh ulle (LSH) within the Tauern Window. Two garnets from the USH grew at rates of 0.67(-0.13)+0.19 mm/million years and 0.88(-0.19)+0.34 mm/mil lion years, respectively, indicating an average growth duration of 5.4 +/- 1.7 million years. The duration of growth coupled with the amount of rotation recorded by inclusion trails in the USH garnets yields an average shear-strain rate during garnet growth of 2.7(-0.7)+1.2 x 10( -14) s-1. Garnet growth in the sample from the USH occurred between 35 .4 +/- 0.6 and 30 +/- 0.8 Ma. The garnet from the LSH grew at a rate o f 0.23 +/- 0.015 mm/million years, between 62 +/- 1.5 Ma and 30.2 +/- 1.5 Ma. Contemporaneous cessation of garnet growth in both units at ap proximately 30 Ma is in accord with previous dating of the thermal pea k of metamorphism in the Tauern Window. Correlation with previously pu blished pressure-temperature paths for garnets from the USH and LSH yi elds approximate rates of burial, exhumation and heating during garnet growth. Assuming that these P - T paths are applicable to the garnets in this study, the contemporaneous exhumation rates recorded by garne t in the USH and LSH were approximately 4(-2)+3 mm/year and 2 +/- 1 mm /year, respectively.