Mafic, monogenetic volcanism is increasingly recognized as a common manifestation of post-collisional volcanism in late Variscan, Permo-Carboniferous intramontane basins of Central Europe. Although identification of individual eruptive centres is not easy in these ancient successions, the Permian Rožmitál andesites in the Intra-Sudetic Basin (NE Bohemian Massif) provide an exceptionally detailed record of explosive, effusive and high-level intrusive activity. Based on field study and petrographic and geochemical data on pyroclastic and coherent rocks, the Rožmitál succession is interpreted as the proximal part of a tuff ring several hundred metres in diameter. Initial accumulation of pyroclastic fall and surge deposits occurred during phreatomagmatic eruptions, with transitions towards Strombolian eruptions. Gullies filled with reworked tephra document periods of erosion and redeposition. Andesitic blocky lavas capped the volcaniclastic succession. Invasion of lavas into unconsolidated sediments and emplacement of shallow-level intrusions in near-vent sections resulted in the formation of jigsaw- and randomly-textured peperites. Most geochemical differences between coherent andesites and pyroclastic rocks can be linked to incorporation of quartz-rich sediments during the explosive eruptive processes and to later cementation of the volcaniclastic deposits by dolomite. The Rožmitál tuff ring could have been one of several phreatomagmatic centres in a monogenetic volcanic field located on an alluvial plain.
Abstract 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 km 3 ), 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. Graphical abstract
Abstract The large Variscan Karkonosze Granite in the West Sudetes, representative of the vast Variscan granite plutonism in Central Europe and located adjacent to regional tectonic suture and strike-slip-zones, has been difficult to date precisely; a range of published data varies between c. 304 and 328 Ma. However, the granite is cut by locally numerous lamprophyre and other dykes. Dating of the dyke rocks, emplaced shortly after the granite intrusion and cooled more rapidly, provides a promising tool for the verification of published SHRIMP results on the granite itself. SHRIMP zircon geochronology of a studied micromonzodiorite dyke indicates substantial admixture of inherited zircons of c. 2.0, 1.4 Ga ( 207 Pb– 206 Pb minimum ages), and c. 570 (and 500?) Ma. The average concordia age of the main magmatic population of the zircons in the dyke is 313 ± 3 Ma (2σ); however, the true magmatic age might be older, around 318 Ma. This would constrain the age of the hypabyssal magmatism in the Karkonosze Massif and the minimum age of the host Karkonosze Granite. Thus, the Karkonosze Granite is confirmed as representative of an early phase of Variscan granite plutonic activity in the central-European Variscides.
Gneisses of the Góry Sowie Block, of probable Late Precambrian–Early Cambrian age, contain numerous small metabasite bodies which may have originated as dykes or sills. Many metabasites have phase assemblages and textures which show that they suVered early granulite-facies and later amphibolite- facies metamorphism, although some exhibit no evidence for the earlier event. Trace element geochemistry enabled four metabasite groups to be distinguished: (a) a dominant meta-tholeiite group (Sowie group) probably incorporating more than one set of intrusive rocks; (b) meta-tholeiites characterized by higher Nb/Y (high Nb/Y group); (c) Ti-poor metabasites (Myslêcin group) that exhibit strongly depleted HFSE; (d) alkali metabasalts (Wlóki group). Sowie group meta-tholeiites were also divided according to their dominant host lithology: granitoid gneiss, migmatitic paragneiss or gneiss containing relict granulite-facies rocks. Those in granitoid gneiss are compositionally restricted (although they fall within the same compositional range as other Sowie meta-tholeiites), and may comprise both early metabasites and a later intrusive group, postdating granite emplacement. Both the latter metabasites and the granitoid gneiss lack phase assemblages indicative of granulite facies metamorphism: this suggests that both were emplaced after the granulite-facies metamorphic event, but before or during amphibolite-facies metamorphism. An Early Ordovician emplacement age for syn-metamorphic granitoid gneiss shows that the Góry Sowie Block underwent an Ordovician (early Caledonian) metamorphic event, following metabasite emplacement, or was at this grade during the Ordovician. Nonetheless, the chemistry of all the metabasite groups and their geological setting is consistent with emplacement either through continental crust undergoing extension at a passive continental margin, or during the formation of an intracratonic basin before subsequent metamorphism recorded a local compressional event.
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