MICROSTRUCTURES AND MINERAL CHEMISTRY IN AMPHIBOLITES FROM THE WESTERN TAUERN WINDOW (EASTERN ALPS), AND P-T DEFORMATION PATHS OF THE ALPINE GREENSCHIST-AMPHIBOLILE FACIES METAMORPHISM

Amphibolites in the Mesozoic part of the parautochthonous Lower Schief erhulle (LSH), the allochthonous Upper Schieferhulle (USH) and the ove rlying Austroalpine basement (AA) in and around the western Tauern Win dow (Eastern Alps) suffered a progressive Alpine deformation. Lineatio ns and foliations L(1)-S-1, L(2)-S-2 defined by preferentially oriente d (Na-Ca) amphiboles as well as F-3 folds and further foliations S-myl and S-4 in the metabasites are structures of successive deformational stages with a constant W-E main extension axis of strain. The (Na-Ca) amphiboles in assemblages with epidote, chlorite, albite/oligoclase a nd quartz are zoned with similar continuous zonation trends from early actinolite in the cores to magnesio-hornblende and tschermakitic horn blende, and from magnesio-hornblende to late actinolite in the rims in the three lithostratigraphic units. Geothermobarometry involving trem olite-edenite and (pargasite-hastingsite)-tremolite end-member equilib ria in amphiboles allowed us to reconstruct prograde-retrograde P-T pa ths for the Alpine greenschist-amphibolite facies event. The paths pas sed P/T-max at 6-7 kbar/600 degrees C. Similar shapes of the paths in AA, USH and Mesozoic LSH indicate a common metamorphic history and a s tacking of these units prior to or during the pre-P-max evolution. Mod erate P-T ratios are characteristic for the temperature-dominated comp ression paths and indicate continental collisional rather than subduct ion zone metamorphism. The middle to late Alpine greenschist-amphiboli te facies event appears as an independent metamorphism along a complet e P-T loop which may have followed an earlier and poorly documented hi gh-pressure/low-temperature event.