The Late Carboniferous Schönfeld-Altenberg Depression on the NW margin of the Bohemian Massif (Germany/Czech Republic): volcanosedimentary and magmatic evolution

Abstract

The Altenberg-Teplice Volcanic Complex (ATVC) exposed on both sides of the German-Czech border in the Erzgebirge/ Krušné hory Mts. is one of the earliest late-Variscan to post-Variscan volcanic centres in Europe. The ATVC features an early volcanosedimentary succession preserved in the Schönfeld-Altenberg Depression Complex (SADC), covered by voluminous ignimbrites and lavas of the Teplice Rhyolite (TR). Published radiometric age dating of ATVC rocks and associated mineralizations suggest a Lower Namurian (Serpukhovian) age for the SADC. The SADC (10 × 15 km) was subdivided into the Schönfeld-Pre-Eruptive Sediments (SPES), and the volcanosedimentary successions of the Lower Schönfeld Complex (LSC) and the Upper Schönfeld Complex (USC). The SPES (maximum thickness of 60 m) is deposited on metamorphic basement and consists of sandstones and conglomerates with variable types of metamorphic clasts; it also contains carbonaceous layers. The LSC sequence starts with widespread, fine-grained ignimbrites (maximum thickness of 153 m) of rhyolitic composition. The explosive phase of the LSC was followed by the formation of (trachy-)dacitic lava(s) and subvolcanic bodies. The USC sequence commences with coarse-grained talus deposits, consisting of metamorphic clasts, indicating a major tectonic activity. Lavas and pyroclastic rocks dominate the USC in the southern part, whereas in the Schönfeld area (northern part), lava dome explosion-related pyroclastic and sedimentary deposits, alternating with carbonaceous layers, prevail. Charcoal fragments and fine charcoal dust is present in all SADC units as layers, or in the matrix of volcanosedimentary deposits. Allochtonous anthracite seams in the USC that were subject to historic mining in the Schönfeld area consist of a bedded alternation of carbonaceous deposits with clays to siltstones. Presumably, formation of charcoal was related to explosive eruptions and/or wild fires, and redeposition by alluvial processes. The SADC volcanic rocks are classified as dacites, trachy-dacites and rhyolites, having unusually elevated concentrations of Ti and compatible elements like Cr and V. Compared to the LSC, the USC volcanics show a less alkaline affinity. The studied samples are isotopically homogeneous, with εNd 325 = -2.4 to -3.3 and 87 Sr/ 86 Sr 325 = 0.70556-0.70626, pointing to a common source of magmas for both the LSC and the USC. The two-stage Nd model ages vary between 1.2 and 1.3 Ga, similar to coeval Saxothuringian granites from the Erzgebirge Mts. as well as volcanic rocks from the Intra-Sudetic Basin.

Sample preparation and chromatographic separation for Sr, Nd, and Pb isotope analysis in geological, environmental, and archaeological samples

“…The three-column separation procedure using BioRad Sr and Ln resin columns was successfully employed in the isotopic study of plutonic and volcanic rocks (Walther et al 2016;Mysliveček et al 2018;Janoušek et al 2020;Casas-García et al 2021;Mach et al 2021;Janoušek et al 2022) as well as loess samples (Waroszewski et al 2021).…”

Crandallite-rich beds of the Libkovice Member, Most Basin, Czech Republic: climatic extremes or paleogeographic changes at the onset of the Miocene Climatic Optimum?

“…As all the Rb-rich rocks (Rb produces radiogenic Sr signature) are poor in Sr, which could be mobilized during weathering, rather very small amounts of highly radiogenic strontium signature seem to be a good candidate to represent additional strontium derived from the Cretaceous rocks. In this aspect, granites and rhyolites of the Altenberg-Teplice Volcanic Complex with strongly elevated 87 Sr/ 86 Sr values (except for their early stages with less radiogenic signatures, Walther et al, 2016) would have potential to modify the 87 Sr/ 86 Sr ratio in the Most Basin paleolake.…”

Petrology of weakly differentiated alkaline, high-level intrusive rocks in the Zahořany-Chotiněves Belt near Litoměřice (Czech Republic)

“…A representative sample JM02 was selected for the Sr-Nd isotope analysis. The analytical details for sample processing and ion-exchange chemistry for Sr and Nd separation are described in Walther et al (2016). Strontium and Nd isotope analyses were performed using a Triton Plus thermal ionization mass spectrometer (TIMS; Thermo Fisher) housed at the Czech Geological Survey, Prague.…”