Geothermometry of the ultrahigh-temperature Saxon granulites revisited. Part I: New evidence from key mineral assemblages and reaction textures

Abstract

The high-pressure (HP) granulite metamorphism in Variscan central Europe has recently been questioned to be ultrahigh temperature (UHT) because ternary feldspars from granulites in the Moldanubian region resembling those considered elsewhere as metamorphic minerals were re-interpreted as relic protolith minerals. Fe-Mg exchange reactions, although indicating similar temperatures as the ternary feldspars, have been discarded as unreliable for determining UHT conditions. Pseudosection modelling of the re-interpreted mineral assemblages has suggested metamorphism at temperatures clearly below those derived in earlier work. To re-examine the metamorphism of the Saxon granulites, we have studied key mineral assemblages and reaction textures, some of which have not yet been introduced into the present debate. The investigated felsic granulites display a close textural association of ternary feldspar with garnet and kyanite, including mutual inclusion relationships, suggesting that these minerals have equilibrated at the metamorphic peak. The UHT-HP conditions of this assemblage are close to P – T data calculated from the plagioclase-bearing inclusion assemblage in garnet and the peak assemblage of a garnet-clinopyroxene granulite. Reaction textures in a feldspathic prismatine granulite showing a transition from garnet-kyanite-corundum to garnet-corundum-sapphirine-sillimanite as well as subsequent overprinting during prismatine-forming H 2 O-deficient albite melting are evidence for continuous UHT conditions during decompression. These assemblages require temperatures above the Mg-staurolite stability, the invariant point [Crd] in the silica-undersaturated FMAS system, and the dravite stability, respectively. Continuous UHT conditions after a strong decrease in pressure emerge equally from multi-equilibrium calculations for a garnet-two-pyroxene granulite, where garnet has re-equilibrated with a two-pyroxene-plagioclase-quartz assemblage in the matrix and the corona assemblage orthopyroxene-plagioclase-quartz. Our findings confirm the earlier proposed UHT-HP decompression path for the Saxon granulites and underpin the metamorphic origin of ternary feldspar. Oxygen-isotope thermometry data for the same mineral assemblages presented in a companion paper agree with the results of chemical equilibrium thermometry and provide an independent confirmation for the UHT metamorphism.