Thermobarometry and Geotectonic Significance of High-Pressure Granulites: Examples from the Moldanubian Zone of the Bohemian Massif in Lower Austria

Petrographic details together with mineral and whole-rock composition data are provided for acid-intermediate garnet granulites from exposed granulite complexes in Lower Austria. Thermobarometric evaluation integrated with available isotopic age data indicates the initial equilibration of early Variscan (˜370 Ma) high-pressure granulite assemblages at ˜16 kbar and 1000°C and their partial overprinting by retrograde assemblages which reflect the blocking of mineral exchange reaction equilibria at ˜6⋅5 kbar and 725°C during subsequent Variscan uplift and cooling. These calculated P-T estimates, together with general phase equilibria constraints and evidence from preserved prograde coronitic reaction textures and garnet compositional zoning profiles, indicate a clockwise P-T-t evolutionary path of the type expected during crustal thickening in a major plate collision orogen and characterized by near-isothermal decompression during initial uplift. Geochemical characterization of the rock protoliths as calc-alkaline igneous rocks and the high metamorphic temperatures suggest that garnet granulite formation involved the subduction of a magmatic arc at a continental plate margin. Reviewed evidence from granulites in the Central European Variscides runs counter to suggestions by Bohlen (1987, 1991) that high-pressure granulites are of little regional geotectonic significance in comparison with low- to medium-pressure granulites. The different evolutionary P-T paths for these two important groups of granulites point to formation in contrasting plate settings. However, questions are raised regarding petrogenetic models for low- to medium-pressure granulites which have emphasized the importance of magmatic, rather than tectonic, crustal thickening and the recognition of stabilization along deduced anti-clockwise P-T-t paths characterized by post-peak near-isobaric cooling. It is suggested here that the reality of stabilization of at least some low- to medium-pressure granulites in a collisional tectonic regime may have been concealed either because lower-pressure assemblages have overprinted mineralogical evidence for an earlier high-pressure history at deeper crustal levels or through invalid deduction of near-isobaric cooling trajectories as a result of the different closure temperatures for the mineral reactions used to monitor the equilibration temperatures and pressures in granulites. However, the sequential underthrusting model favoured for the tectonometamorphic evolution of the Variscan nappe pile in the Bohemian Massif renders it unlikely that all late Variscan low- to medium-pressure granulites have experienced the early Variscan high-pressure metamorphism.