P-T-T EVOLUTION OF SPINEL-CORDIERITE-GARNET GNEISSES FROM THE SAUWALD ZONE (SOUTHERN BOHEMIAN MASSIF, UPPER AUSTRIA): IS THERE EVIDENCE FOR TWO INDEPENDENT LATE-VARISCAN LOW-P/HIGH-T EVENTS IN THE MOLDANUBIAN UNIT?

Abstract

The Sauwald Zone, located at the southern rim of the Bohemian Massif in Upper Austria, belongs to the Moldanubian Unit. It exposes uniform biotite + plagioclase ± cordierite paragneisses that formed during the post-collisional high-T/low-P stage of the Variscan orogeny. Rare metapelitic inlayers contain the mineral assemblage garnet + cordierite + green spinel + sillimanite + K-feldspar + plagioclase + biotite + quartz. Mineral chemical and textural data indicate four stages of mineral growth: (1) peak assemblage as inclusions in garnet (stage 1): garnet core + cordierite + green spinel + sillimanite + plagioclase (An35-65); (2) post-peak assemblages in the matrix (stages 2, 3): cordierite + spinel (brown-green and brown) ± sillimanite ± garnet rim + plagioclase (An10-45); and (3) late-stage growth of fibrolite, muscovite and albite (An0-15) during stage 4. Calculation of the P-T conditions of the peak assemblage (stage 1) yields 750-840°C, 0.29-0.53 GPa and for the stage 2 matrix assemblage garnet + cordierite + green spinel + sillimanite + plagioclase 620-730°C, 0.27-0.36 GPa. The observed phase relations indicate a clockwise P-T path, which terminates below 0.38 GPa. The P-T evolution of the Sauwald Zone and the Monotonous Unit are very similar, however, monazite ages of the former are younger (321 ± 9 Ma vs. 334 ± 1 Ma). This indicates that high-T/low-P metamorphism in the Sauwald Zone was either of longer duration or there were two independent phases of late-Variscan low-P/high-T metamorphism in the Moldanubian Unit. © Springer-Verlag 2006.