Contrasting Early Carboniferous field geotherms: evidence for accretion of a thickened orogenic root and subducted Saxothuringian crust (Central European Variscides)

In the central part of the Erzgebirge, the age of granulite-facies metamorphism corresponds to the age of formation of mafic eclogites exhumed at the base of the recently defined Lower Crystalline nappe. Granulite-facies conditions in acidic continental crust and eclogite-facies conditions in mafic eclogites developed at the same time of about 342 Ma, suggesting the existence of two distinct tectonic units with contrasting initial geothermal gradients. Existing petrological and geochronological data are used to present a new model for Carboniferous collision at the western margin of the Bohemian Massif. We propose that juxtaposition of rocks with contrasting thermal histories is the result of subduction of the Saxothuringian crust and its later collision with a thickened continental orogenic root existing to the SE. A short time span between the formation ages of high-pressure rocks and cooling ages of the host lithologies suggests extremely fast exhumation of a coupled subduction zone–orogenic root system during continuing Variscan collision.