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?

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 (An_35–65); (2) post-peak assemblages in the matrix (stages 2, 3): cordierite + spinel (brown-green and brown) ± sillimanite ± garnet rim + plagioclase (An_10–45); and (3) late-stage growth of fibrolite, muscovite and albite (An_0–15) 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.