Alpine burial and heterogeneous exhumation of Variscan crust in the West Carpathians:: insight from thermodynamic and argon diffusion modelling

Phase equilibrium modelling of metagranitoids and metapelites in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O system was used to characterize Variscan and Alpine metamorphism in the crystalline basement of the Vepor Unit, West Carpathians. The calculated P-T conditions range between 570-670 degrees C and 6-8.5 kbar for the Variscan and 430-600 degrees C and 5-11 kbar for the Alpine event. These two events show contrasting metamorphic field gradients and P-T evolutions, indicating 22-27 degrees C km(-1) during the retrograde Variscan metamorphism and 15-18 degrees C km(-1) during the prograde Alpine metamorphism. The prograde Alpine metamorphism is associated with the Early Cretaceous overthrusting of the southern Gemer Unit, which resulted in apparently contemporaneous burial and horizontal ductile spreading of the underlying Vepor basement. The argon diffusion modelling was used to interpret the existing Variscan, Alpine and mixed 40Ar/39Ar cooling ages, and to constrain the T-t evolution and Alpine thermal overprint. Constructed Alpine metamorphic isograds and isotherms show horizontal P-T gradients resulting from heterogeneous exhumation of the deeper parts of the Vepor basement along two narrow belts during later folding. Here the structurally deeper metapelites form large-scale anticlinal cusp-like structures that separate structurally higher metagranitoids.