Chronological constraints on the pre-orogenic history, burial and exhumation of deep-seated rocks along the eastern margin of the Variscan Orogen, Bohemian Massif, Czech Republic

Abstract

Key lithological units of the high-grade eastern margin of the Bohemian Massif were dated using the U-Pb and Pb-Pb methods on zircons in order to establish a chronological framework for the geodynamic evolution of the Variscan orogenic root. The protolith ages for metagranitoids, orthogneisses and granulites of thickened lower and middle crust reveal the existence of magmatic activity that occurred over a 100 million year time interval from Cambro-Ordovician to early Devonian times, probably related to discontinuous intracontinental rifting of Neoproterozoic crust. Our geochronological data suggest that the eastern part of the orogenic root represents thermally softened and rifted Neoproterozoic crust, preserved farther to the east as the Brunia microcontinent. Zircon ages for felsic granulites, high-grade gneisses of the lower crust and of a syn-convergence granodioritic intrusion in the upper crust indicate that thickening and exhumation of the crust occurred during a narrow time interval between 370 and 340 Ma. Exhumation of the lower crust to mid-crustal levels was a localized process that occurred at ∼340 Ma and was associated with crustal-scale folding in the internal part of the root as well as orogenic channel flow along the eastern collisional margin. Both types of exhumation mechanisms were driven by deep-level wedging (indentation) of the easterly Brunia continent, followed by deposition of heavy minerals and pebbles derived from high-pressure rocks in the adjacent foreland basin. Final orogenic development was characterized by NE-SW dextral transpressive shearing parallel to the Brunia margin as well as dextral trans tension associated with activity along the Elbe lithospheric fault. These processes affected the marginal parts of. the orogenic root and were accompanied by 330 to 325 Ma old syntectonic granitoid intrusions along reactivated lithotectonic boundaries. Rotation of the assembled orogenic belt, accompanied by lithospheric faulting driven by westerly subduction roll-back, may be the most plausible model to explain late deformation of the orogenic root.