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/Krusne 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 Schonfeld-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 x 15 km) was subdivided into the Schonfeld-Pre-Eruptive Sediments (SPES), and the volcanosedimentary successions of the Lower Schonfeld Complex (LSC) and the Upper Schonfeld 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 Schnfeld 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 Schnfeld 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 epsilon Nd-325 = -2.4 to -3.3 and Sr-87/Sr-86(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.