Late- to post-Variscan magmatism in the Lusatian Block occurred during two short episodes: Evidence from zircon dating

<p>In the Late Carboniferous to Early Permian, post-orogenic processes led to the intrusion of compositionally diverse granitoids and to intense silicic volcanism in Central Europe. In the Lusatian Block, which is situated in the eastern part of the Saxothuringian Zone of the Variscan orogen, the late- to post-Variscan granitoids are subordinate in comparison to the Cadomian basement and late- to post-Variscan volcanic rocks are almost absent. The Lusatian Block is bound towards the NE and the SW by major deep reaching fault zones. Both the granitoid and the volcanic rocks are situated near the boundaries of the block and probably associated with the major NW trending faults of the Elbe Fault Zone (e.g. Hammer et al., 1999, Lisowiec et al., 2014, Oberc-Dziezic et al., 2015). The Elbe Fault Zone is a continental scale zone of crustal weakness that was reactivated with different kinematics at different times (Scheck et al., 2002). </p><p>We acquired new precise CA-ID-TIMS U-Pb zircon ages of the Koenigshain and the Stolpen granites and the volcanics of the Weissig Basin. Our new data show that the Variscan magmatism of the Lusatian Block occurred at two distinct periods, depending on the structures on which they are bound. The age difference between the two groups (12 Myr) is clearly evident in both CA-ID-TIMS and evaporation analyses. Consequently, zircon evaporation data of other granitoid and volcanic rocks that were not dated with CA-ID-TIMS can be assigned to one of the two groups in the Lusatian Block. The new age dating allows comparison of the evolution of the investigated rocks to adjacent Variscan magmatic rocks.</p><p> </p><p>References:</p><p>Hammer et al. (1999), Z. geol. Wiss 27, 401-415.</p><p>Lisowiec et al. (2014), Acta Geologica Polonica 64 (4), 457-472.</p><p>Oberc-Dziezic et al. (2015), Int. J. Earth. Sci. 104, 1139-1166.</p><p>Scheck et al. (2002), Tectonophysics 360, 281-299.</p>

Uran, thorium a draslík v krystaliniku a paleozoických sedimentech na mapovém listu 14-33 Polička – laboratorní gamaspektrometrie a parciální chemické složení variských plutonitů

The paper deals with natural radioactivity of metamorphic and igneous rocks and Palaeozoic sediments within the map sheet 14-33 Polička. Studied rocks belong to three geological units: the Hlinsko Zone (phyllites and quartzites of the Mrákotín Fm., flysch sediments of the Hlinsko-Rychmburk Fm.), the Svratka Crystalline Complex (mainly mica schists, paragneisses, orthogneisses and migmatites, also amphibolites, skarns, erlans and quartzites) and the Polička Crystalline Complex (mica schists, paragneisses to migmatites and the Variscan granitoids are dominant). Concentrations of potassium, uranium and thorium were measured using a laboratory gamma-ray spectrometer in 805 rock samples. Data are tabled and discussed. Natural radioactivity of rocks is evaluated on the basis of the calculated values of mass activity of 226Ra equivalent (am). Slightly increased am values were found in migmatites and orthogneisses of the Svratka Crystalline Complex (187 Bq.kg-1 on average), granites and granodiorites of the Budislav Pluton (216 Bq.kg-1), granodiorites and rocks of a tonalite suite of the Miřetín Pluton (199 and 194 Bq.kg-1). Increased concentrations of uranium and thorium were found in some samples of pegmatite and aplite. Radioactivity of rocks of the Hlinsko-Rychmburk and Mrákotín Fms. can be assessed as relatively low.