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Abstract

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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