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 Blatná suite, Central Bohemian Plutonic Complex

<p>The Variscan Central Bohemian Plutonic Complex crops out between the upper-crustal Teplá–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 Sázava suite, which was emplaced syntectonically during regional shortening; (2) the younger, more evolved, potassic Blatná 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 Blatná suite pinpoint this major switch in the tectonic regime. Both Blatná and Kozárovice granodiorites (346 ± 2 Ma and 347 ± 2 Ma (2σ)) were generated by melting of heterogeneous crust and were emplaced contemporaneously to form the Blatná 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, Kozárovice) or Late Cambrian–Early Ordovician (492 ± 4 Ma, Blatná) 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. </p

Vestige of an Early Cambrian incipient oceanic crust incorporated in the Variscan orogen: Letovice Complex, Bohemian Massif

<p>The Letovice Complex, composed of metamorphosed enriched mid-ocean ridge basalts with trondhjemitic sheets, as well as gabbroic and ultrabasic bodies, is exposed within an important Variscan suture separating the Moldanubian Domain (to the west) from the Brunovistulicum (to the east) in the eastern part of the Bohemian Massif, Czech Republic. A polyphase tectonic evolution of the study area was connected with underthrusting and subsequent exhumation of the former oceanic realm, which resulted in three deformation phases (D₁–D₃). New U–Pb laser ablation inductively coupled plasma mass spectrometry dating of magmatic zircons from amphibolite and trondhjemite yielded statistically identical concordia ages of 530 ± 6 Ma and 529 ± 7 Ma (2σ), respectively. These are interpreted as dating intrusions of the Early Cambrian protoliths of the studied rocks. Based on this age as well as whole-rock geochemical and Nd isotopic data, the Letovice Complex is interpreted as a vestige of an incipient oceanic basin developing on attenuated crust. The most likely geotectonic setting was a post-Cadomian extensional regime at the northern margin of Gondwana that marked the onset of its break-up. </p