Timing of post-orogenic silicic volcanism in the eastern part of the European Variscides: constraints from SHRIMP U–Pb zircon study of the Permo-Carboniferous Góry Suche Rhyolitic Tuffs (the Intra-Sudetic Basin)

The Góry Suche Rhyolitic Tuffs in the Intra-Sudetic Basin, in the eastern part of the Variscan Belt of Europe, represent a voluminous (ca. 100 km³), possibly caldera-related, ignimbrite-dominated complex and the Łomnica Rhyolites are associated, post-ignimbrite sills. Zircon separates from nine samples were dated using the U–Pb SHRIMP method. Well-defined concordia ages were determined in four ignimbrite samples (300.5 ± 2.0, 300.5 ± 1.4, 298.0 ± 1.6 and 297.2 ± 0.9 Ma) and in two rhyolite samples (298.4 ± 1.5 and 292.6 ± 1.9 Ma). Clustering of the ignimbrite sample ages between 300.5 ± 2.0 and 297.2 ± 0.9 Ma and geological evidence indicate the eruption and deposition of the tuffs close to the Carboniferous/Permian boundary, in a geologically rapid event at approximately 299 Ma. Zircon assemblages in three tuff specimens are strongly dominated by xenocrysts of various Palaeozoic and Precambrian ages that were incorporated during the eruption through the basin fill. The emplacement of the tuffs was followed (and partly overlapped?) by the emplacement of the Łomnica Rhyolites as sills in two episodes in the early Permian. The Góry Suche Rhyolitic Tuffs may be a few million years older than assumed so far, and this, as well as rather imprecise biostratigraphic constraints from the host sedimentary rocks, suggest a need for revision of the existing lithostratigraphic and evolutionary schemes for the Permo-Carboniferous of the Intra-Sudetic Basin. The studied tuffs and rhyolites together with coeval granitic plutons in vicinity can be linked to the onset of post-Carboniferous lithospheric thinning in Central Europe.