Mineral Chemistry, Geothermobarometry and Pre-Alpine High-Pressure Metamorphism of Eclogitic Amphibolites and Mica Schists from the Schobergruppe, Austroalpine Basement, Eastern Alps

Bernhard Schulz
Institut für Geologie und Mineralogie der Universität Erlangen-Nürnberg, Schloßgarten 5, D-8520 Erlangen, Germany

Abstract: Alternating eclogitic amphibolites, mica schists and orthogneisses in the Schobergruppe to the south of the Tauern Window suffered a post-Upper-Ordovician progressive deformation D1-D2 which produced parallel planar-linear structures in all the rocks. Zoned garnets, preferentially oriented zoned clinopyroxenes (Jd 35–42%) and albite (An 7–9%) give evidence of high-pressure metamorphism (550–650°C, 14–16 kbar) of the metabasites. Ca-amphiboles crystallized during subsequent decompression. In a kyanite-staurolite-garnet mica schist 300 metres below the metabasites, garnet-bearing assemblages grew synchronous with the development of foliations S1 and S2. Garnets are zoned with increasing XMg and decreasing-increasing-redecreasing XCa from cores to rims. Albitic plagioclase (An 1–3%) and micas are enclosed in garnet cores and rims, are in contact with garnet, and occur with garnet in microlithons. When these minerals are used for geothermobarometry, a prograde P-T evolution (460 to 680°C with coeval pressure variations which reach high-pressure conditions can be estimated. This suggests that garnet-plagioclase geobarometry with albitic plagioclase works in the relevant P-T field. Similar garnet zonation trends and a similarly shaped P-T path from mica schists of an adjacent region with late-Variscan cooling ages, points to an early-Variscan age of the syn-D1-D2 high-pressure and subsequent amphibolite-facies metamorphism.

Keywords: mineral chemistry • garnet-plagioclase barometry • P-T-t-deformation paths • Variscan • Austroalpine basement

Mineralogical Magazine; June 1993 v. 57; no. 387; p. 189-202; DOI: 10.1180/minmag.1993.057.387.01
© 1993, The Mineralogical Society
Mineralogical Society (www.minersoc.org)