Abstract
Samples and documentation of outcrops and drillings, facies analysis, whole rock geochemistry and radiometric ages have been employed to re-evaluate the Late Carboniferous Tharandt Forest caldera (TFC) and the co-genetic Niederbobritzsch granite (NBG) in the eastern Erzgebirge near Dresden, Germany. The c. 52 km2 TFC harbours strongly welded ignimbrites with a preserved minimum thickness of 550 m. Composition of initial fallout tephra at the base of the TFC fill, comprising lithics of rhyolitic and basic lava, and of silica-rich pyroclastic rocks, suggests a bimodal volcanic activity in the area prior to the climactic TFC eruption. The lower part of the TFC fill comprises quartz-poor ignimbrites, overlain by quartz-rich ignimbrites, apparently without a depositional break. Landslides originating from the collapse collar of the caldera plunged into the still hot TFC fill producing monolithic gneiss mesobreccia with clasts ≤ 1 m in a pyroclastic matrix. Aphanitic and porphyritic rhyolitic magma formed ring- and radial dykes, and subvolcanic bodies in the centre of TFC. Whole rock geochemical data indicate a high silica (most samples have > 73 wt% SiO2) rhyolitic composition of the TFC magma, and a similar granodiorite–granitic composition for the NBG. Based on drillings and caldera extent, a minimum volume of 22 km3 of TFC fill is preserved, the original fill is assumed at about 33 km3. This estimate translates into a denudation of at least c. 210 m during Late Paleozoic to pre-Cenomanian. Telescopic subsidence of the TFC took place in two, perhaps three stages. A possible TFC outflow facies has been completely eroded and distal TFC tuff has not been recognized in neighboring basins. New CA-ID-TIMS measurements on two TFC samples gave mean zircon ages of 313.4 ± 0.4 Ma and 311.9 ± 0.4 Ma; two samples from NBG resulted in 318.2 ± 0.5 Ma and 319.5 ± 0.4 Ma. In addition, for one sample of the ring dyke an age of ca. 314.5 ± 0.5 Ma has been obtained. These ages, together with field relations, allow for a model of a long-standing evolution of an upper crustal magmatic system (~ 5 Ma?), where pulses of magmatic injection and crustal doming alternate with magmatic quietness and erosion. Together with the Altenberg–Teplice Volcanic Complex, located some 10 km to the southeast, the TFC–NBG Complex represents an early post-Variscan magmatic activity in central Europe.
Highlights
At the end of Variscan orogeny, widespread and intense magmatic activity occurred throughout central Europe
In the late 1960–early 1970s, the SDAG Wismut carried out a detailed exploration project on the Tharandt Forest caldera (TFC), including intensive drilling as deep as 371.4 m (Kalmykow et al 1971). Based on these exploration drillings and outcrops, we present a detailed re-evaluation of the lithofacies, geometry and evolution of the TFC
The TFC occurs adjoined to early Paleozoic phyllites and slates (PPS) along a NW–SE-trending sharp contact (Hartha Fault, Fig. 2)
Summary
At the end of Variscan orogeny, widespread and intense magmatic activity occurred throughout central Europe. Some of the late- to post-Variscan volcanic systems display large volumes of crystal-rich ignimbrites qualifying these as supereruptions (Willcock et al 2013; Repstock et al 2018). Intense magmatism took place in both, western and eastern parts of the Erzgebirge/Krušné hory, located at the German–Czech border region. Plutonic rocks crop out in the Western Erzgebirge while both plutonic and volcanic rocks occur in the Eastern Erzgebirge. There, the magmatic activity is mainly confined along a NNW–SSE trending zone (Fig. 1). These are, from south to north, the Altenberg–Teplice Volcanic
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