Post-collisional granite generation and HT-LP metamorphism by radiogenic heating:: the Variscan South Bohemian Batholith

The Palaeozoic Variscan Orogen of Europe is a well-documented example of a collision zone characterized by widespread late-orogenic high-temperature metamorphism and associated crustal magmatism. However, the heat source is still under debate. Based on the Bohemian Massif in the internal zone of the Variscides as case study, we present geological, geochemical, petrological and geochronological data arguing against a substantial mantle involvement in metamorphism and magma genesis in the area of the South Bohemian Batholith. In order to provide an alternative explanation consistent with heat transfer mechanism, we apply a two-dimensional thermal-kinematic modelling approach. The model calculates the transient lithospheric temperature field during crustal thickening and subsequent thinning by erosion from material parameters and boundary conditions specific to the study area. Model results show that the increased contribution of radiogenic heat in the thickened crust can indeed cause a substantial temperature increase in the middle and lower crust. Model predictions are in good agreement with observations, e.g. the P-T-t evolution of the country rocks, the formation of syn-kinematic migmatites, the large volumes of peraluminous granites derived from dehydration melting of metasediments and the small volumes of lamprophyric melts from the mantle lithosphere. The results of this study emphasize the importance of radiogenic heat as the source for high-temperature metamorphism and granite petrogenesis in the Bohemian Massif and potentially in other areas of the Variscan Orogen.