Lithofacies architecture, composition, and age of the Carboniferous Teplice Rhyolite (German-Czech border): Insights into the evolution of the Altenberg-Teplice Caldera

The Teplice Rhyolite (TR) intra-caldera deposits of the Altenberg-Teplice Caldera (ATC, Central Europe) were investigated by lithofacies analysis, stratigraphy, and whole-rock chemistry as well as laser ablation U-Pb zircon dating. The aim was to demonstrate that the joint application of these methods can discern complex stratigraphy of ancient volcanic successions, susceptible to erosion and alteration, taking the TR as case study. The TR was regarded as a formation and divided into nine members that represent three main eruptions, each one separated by periods of volcanic quiescence. The first two eruptions mainly portrayed opening and clearing of vents, which were dominated by lithic-rich ignimbrites with lateral lithofacies changes (fiamme-rich to crystal-rich) covered by lava flows/domes. Eruption style was low pyroclastic fountaining alternated with effusive lava outpour. A sustained buoyant eruption column was exclusively interpreted at the outset of the second eruption based on the presence of a basal fallout deposit. The third eruption recorded three different phases: further vent opening, a waxing phase, and final lava extrusion. The vent opening phase was similar to those occurred in previous eruptions. The waxing phase produced voluminous, massive, poorly-sorted, very crystal-rich (>50 vol.%) ignimbrites, exhibiting progressive increase in crystal contents with height and chemical composition variations that indicate deposition by progressive aggradation. Low pyroclastic fountaining formed these high-particle concentration currents that waxed and reached quasi-steady conditions. Eruption ended with extrusion of a lava flow. The TR ignimbrites were emplaced hot and were partially to densely welded, as evidenced by eutaxitic texture, deformed glass shards, spherulites, perlitic cracks, and microcrystalline to granophyric textures. Horizontal and vertical lithofacies variations in pyroclastic deposits, together with progressive infill and association between TR feeding systems and faulting, demonstrate that eruptions developed through several fissure vents or faults. A volcanic explosivity index of 7 was estimated according to a minimum ejecta volume of ∼320 km³ (DRE). Volcanic architecture of the TR can be linked to sequential caldera subsidence that occurred synchronously with progressive deposition. Regional faulting (NE-SW-trending rhyolitic dykes) related to ATC feeding conduits suggests that progressive subsidence and final trap-door caldera collapse, possibly with a piece-meal component (faulted caldera floor), originated from post-Variscan tensional stress regime (∼NNW-SSE direction). The TR Formation has a general rhyolitic composition with A-type granite affinities and a normal chemical zoning. U-Pb zircon dating yielded ages from 323 to 312 Ma, which, compared to age data from other ATC rocks, indicate that the TR was generated within a time span of ∼325-317 Ma.