Dating the onset of Variscan crustal exhumation in the core of the Bohemian Massif: new U-Pb single zircon ages from the high-K calc-alkaline granodiorites of the Blatna suite, Central Bohemian Plutonic Complex

The Variscan Central Bohemian Plutonic Complex crops out between the upper-crustal Tepla-Barrandian and the high-grade Moldanubian units (Bohemian Massif). Much of the complex is made up of calc-alkaline plutonic rocks: (1) the geochemically more primitive, Na-rich 354 +/- 4 Ma Sazava suite, which was emplaced syntectonically during regional shortening; (2) the younger, more evolved, potassic Blatna suite, which records both the shortening along its NW contact and the onset of normal shearing related to exhumation of the Moldanubian Unit to the SE. New ion microprobe U-Pb zircon ages for the high-K calc-alkaline Blatna suite pinpoint this major switch in the tectonic regime. Both Blatna and Kozarovice granodiorites (346 +/- 2 Ma and 347 +/- 2 Ma (2 sigma)) were generated by melting of heterogeneous crust and were emplaced contemporaneously to form the Blatna composite pluton. From age spectra preserved in inherited zircons and whole-rock Sr-Nd isotope signatures, the likely crustal sources for the magmas were immature greywackes rich in Neoproterozoic (615 +/- 10 Ma, Kozarovice) or Late Cambrian-Early Ordovician (492 +/- 4 Ma, Blatna) volcanogenic detritus. An additional important petrogenetic process was variable mixing with enriched mantle-derived monzonitic magmas, which may also have supplied the extra heat for crustal anatexis.