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Located between two large Late Palaeozoic rhyolitic Caldera systems (the Tharandter Wald Caldera (TWC) and the Altenberg–Teplice Caldera (ATC)) in the eastern Erzgebirge region, the extended NE–SW-trending rhyolitic Sayda–Berggießhübel dyke swarm (SBDS) was emplaced into Variscan gneisses. The dykes expose massive coherent rhyolite (CR), and four pyroclastic bodies reveal welded rhyolitic tuffs (WRT).Overlying the CR dykes, the WRT form subcircular topographic elevations of up to 1 km in diameter. These have been interpreted by previous authors as erosional remnants of the ATC ignimbrite sheet. Another hill located on a dyke is built up of a columnar jointed rhyolitic lava dome or laccolith. An abandoned quarry near Mulda exposes sills propagating from a dyke into weathered gneiss. The WRT display a fluidal moderately to mostly vertically dipping welded texture with fiamme-like fragments up to 90 cm long. Strike orientation of the fiamme varies greatly. Gneiss lithic fragments make up to 2 vol.% of the analysed area of rock samples. The WRT are columnar jointed. The columns have a predominantly horizontal–subhorizontal orientation.In outcrop, rock slab and thin section maximum particle size and the relative proportions of fiamme, phenocrysts in fiamme, lithic fragments and of crystal clasts (QZ, KF, PL, BT) in tuff have been measured. The correlation of the data with the regional geology shows that the welded rhyolitic tuff bodies resemble individual vent systems. Compared to the fiamme, the tuff matrix has a higher crystal clast content, indicating that pyroclastic fractionation has taken place.Comparison of granulometric and geochemical SBDS data with ignimbrite samples from TWC and ATC confirms a close relationship between the SBDS and ATC. The SBDS is a possible vent area for the ATC. The textural data, the orientation of both fiamme and cooling columns, and other field relations suggest that the WRT of the SBDS represent welded fall-back tuffs. They formed in vents positioned on an active magmatic dyke system.

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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.