Early metamorphic evolution and exhumation of felsic high-pressure granulites from the north-western Bohemian Massif

Several granulite terrains are exposed in the Bohemian Massif of Central Europe. These were metamorphosed at pressures close to 12 kbar and temperatures of more than 800 degrees C c. 340 Ma ago. The corresponding penetrative deformation almost totally erased the record of the preceding metamorphic evolution. Nevertheless, rare relics such as mineral inclusions in large garnet grains are witness of this earlier evolution, which was previously related to significantly higher pressures and, thus, to a subduction-related event. The exemplary investigation of such mineral relics in a felsic granulite from the Granulitgebirge rather points to pressures of 13 - 14 kbar only at relatively low temperatures of 620 degrees C and, thus, to considerable, nearly isobaric heating before the exhumation of the granulites started at 800 degrees C or somewhat higher temperature. The inferred P - T evolution is compatible with a geodynamic model of lithospheric delamination, with crustal material having been involved. The delamination at c. 340 Ma ago followed long-lasting, continuous collision of Gondwana and Laurussia forming the Variscan orogen. Within the thickened continental crust, the delamination concerned mainly the dense basic material in the lower crust. This event also caused upwelling of the mantle asthenosphere. Both processes resulted in heating of the more felsic lower portion of the continental crust, thinner than before delamination. Heating by 200 degrees C or more caused prograde mineral reactions and created buoyancy forces, as the overlying crust became denser than the underlying hot and felsic granulites. As a consequence, considerable volumes of felsic granulite could have reached shallow crustal levels ( corresponding to 3 to 4 kbar), conditions documented in granulite bodies in the north-western Bohemian Massif.