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, 0.29–0.53GPa and for the stage 2 matrix assemblage garnet+cordierite+green spinel+sillimanite+plagioclase 620–730°C, 0.27–0.36GPa. The observed phase relations indicate a clockwise P–T path, which terminates below 0.38GPa. The P–T evolution of the Sauwald Zone and the Monotonous Unit are very similar, however, monazite ages of the former are younger (321±9Ma vs. 334±1Ma). 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.