Many basement units of the Variscan orogen that are exposed in the Sudetes, SW Poland, comprise widespread ~500 Ma orthogneisses and associated mica schists, the latter often of unknown age and derivation. Our new U-Pb sensitive high resolution ion microprobe (SHRIMP) zircon ages from two samples of the Izera metagranites, both around 503 Ma, are in a good agreement with the well established late Cambrian-early Ordovician magmatism in the West Sudetes. An Archean inherited zircon age of ~ 3.4 Ga is one of the oldest zircon ages reported so far from the Bohemian Massif. The orthogneisses of the Karkonosze-Izera Massif (KIM) have calculated TDM ages of between 1.50 and 1.93 Ga, but these ages are not necessarily evidence for a Mid-Proterozoic crustal derivation: more probably, they reflect the average of several detrital components mixed into the granitoid magma sources. In spite of likely age differences, the Lusatian greywackes, which outcrop to the west, and the mica schists of the KIM display similar geochemical characteristics, suggesting that both could have been derived from similar sources. However, the presence of lower Ordovician products of within-plate volcanism - intercalations of quartzofeldspathic rocks and amphibolites within the mica schists - supports an idea that the mica schist protoliths, derived mainly from crustal rocks, could have also contained an admixture of contemporaneous volcanic materials. The age spectra of inherited zircons from the KIM orthogneisses and their Nd-isotopic signatures are comparable to the Lusatian greywackes: this suggests that the Lusatian greywackes, or very similar rocks, could have been the source material for the granitic protoliths of the KIM orthogneisses.
Author Biographies
Teresa Oberc-Dziedzic
Institute of Geological Sciences, University of Wrocław, Plac M. Borna 9, 50-204 Wrocław, Poland
Ryszard Kryza
Institute of Geological Sciences, University of Wrocław, Plac M. Borna 9, 50-204 Wrocław, Poland
Christian Pin
Département de Géologie, UMR 6524 CNRS, Université Blaise Pascal, 5 rue Kessler, 63 038 Clermont-Ferrand, France
Ksenia Mochnacka
Faculty of Geology, Geophysics and Environmental Protection, Department of Economic Geology, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Kraków, Poland
Alexander Larionov
Centre of Isotopic Research, A.P. Karpinsky All-Russian Geological Research Institute (VSEGEI), 74 Sredny Pr, St Petersburg 199 106, Russia