The Late Carboniferous Schönfeld-Altenberg Depression on the NW margin of the Bohemian Massif (Germany/Czech Republic): volcanosedimentary and magmatic evolution
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 εNd325= -2.4 to -3.3 and 87Sr/86Sr325 = 0.70556-0.70626, pointing to a common source of magmas to 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.
SNIP (Scopus, 2015): 0.700
IF (ISI, 2015): 1.326
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