High-pressure/temperature metamorphism in the St. Leonhard Granulite Massif, Austria: evidence from intermediate pyroxene-bearing granulites

Abstract.

The St. Leonhard Granulite Massif, Lower Austria, is one of the small occurrences of high-pressure granulite found in the Gföhl unit at the highest tectono-stratigraphic level of the Moldanubian zone. Although predominantly composed of extremely deformed acidic, garnet+kyanite-bearing rocks, thin conformable layers of intermediate garnet+clinopyroxene-bearing granulites are seen. Pressure–temperature estimates for the peak metamorphic assemblage of garnet+clinopyroxene+ternary feldspar+quartz in these rocks are 15–19 kbar, 950–1050°C. A close coherence between results obtained from a combination of independent geothermobarometers and those derived from an internally consistent thermobarometric method indicate the retention of high-pressure/temperature equilibrium mineral compositions, even though there is a wealth of petrographic evidence for significant post-peak metamorphic decompression. Pressure–temperature estimates for the orthopyroxene-bearing, intermediate-pressure decompression stage, obtained from discrete reaction textures, are 8–12 kbar and 800–900°C. Post-decompressive cooling from 800 to 500°C, at ca. 5–8 kbar, is recorded by the final amphibolite-facies, biotite-bearing assemblage, together with petrological constraints from the enclosing acid granulites.