Microstructures and Mineral Chemistry in Amphibolites from the Western Tauern Window (Eastern Alps), and P-T Deformation Paths of the Alpine Greenschist-Amphibolite Facies Metamorphism

Bernhard Schulz, Claude Triboulet and Claude Audren
Institut für Geologie und Mineralogie der Universität, Schloßgarten 5, D-91054 Erlangen, Germany
Laboratoire de Pétrologie Minéralogique, C.N.R.S. U.R.A. 736, Université Pierre et Marie Curie, 4 place Jussieu, F-75252 Paris Cedex 05, France
Géosciences Rennes, Laboratoire Tectonophysique, Institut de Géologie, Avenue du Général Leclerc, F-35042 Rennes Cedex, France

Abstract: Amphibolites in the Mesozoic part of the parautochthonous Lower Schieferhülle (LSH), the allochthonous Upper Schieferhülle (USH) and the overlying Austroalpine basement (AA) in and around the western Tauern Window (Eastern Alps) suffered a progressive Alpine deformation. Lineations and foliations L1-S1, L2-S2 defined by preferentially oriented (Na-Ca) amphiboles as well as F3 folds and further foliations Smyl and S4 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 and quartz are zoned with similar continuous zonation trends from early actinolite in the cores to magnesio-hornblende and tschermakitic hornblende, and from magnesio-hornblende to late actinolite in the rims in the three lithostratigraphic units. Geothermobarometry involving tremolite-edenite and (pargasite-hastingsite)-tremolite end-member equilibria in amphiboles allowed us to reconstruct prograde-retrograde P-T paths for the Alpine greenschist-amphibolite facies event. The paths passed P/Tmax at 6–7 kbar/600°C. Similar shapes of the paths in AA, USH and Mesozoic LSH indicate a common metamorphic history and a stacking of these units prior to or during the pre-Pmax evolution. Moderate P-T ratios are characteristic for the temperature-dominated compression paths and indicate continental collisional rather than subduction zone metamorphism. The middle to late Alpine greenschist-amphibolite facies event appears as an independent metamorphism along a complete P-T loop which may have followed an earlier and poorly documented high-pressure/low-temperature event.

Keywords: amphibolites • microstructures • geothermobarometry • P-T-t-deformation paths • Alpine orogeny • western Tauern Window

Mineralogical Magazine; December 1995 v. 59; no. 397; p. 641-659; DOI: 10.1180/minmag.1995.059.397.08
© 1995, The Mineralogical Society
Mineralogical Society (www.minersoc.org)