Structure of the Carboniferous Altenberg-Teplice Caldera (Eastern part of the Krušné hory/Erzgebirge Mts.) revealed by combined airborne and ground gamma-ray spectrometry

Abstract

The airborne gamma-ray spectrometry images from the Czech and German parts of the Carboniferous Altenberg-Teplice Volcanic Complex (ATVC)-both obtained during independent surveying campaigns-were merged to display the structure of the caldera fill (dominated by rhyolitic ignimbrites) and associated intrusive bodies. The airborne systematic measurements were supported by the ground-and laboratory gamma-ray spectrometry analyses (K, U, Th) of representative lithologies from outcrops and drill-cores. Significant differences were identified between the younger and older (post-and pre-caldera respectively) intrusive complexes related to the ATVC. The younger Schellerhau granite displays 1.5-3.0× higher concentrations of radioactive elements than the older Fláje granite. The ATVC was found to be split by the NW-SE trending Altenberg Fault into a southern and a northern segment that expose different stratigraphic levels of the caldera's fill. The individual types of rhyolite ignimbrites defined in the Czech part of the ATVC are characterized by distinct concentrations of natural radioactive elements which enable identification of the individual types from the airborne gamma-spectrometry image. These characteristics were compared to data obtained from the German part, and several lithological types were unified to common units. A single Th-rich unit (Pramenáč type in Czech part, Lugstein type in Germany and Teplice Rhyolite TR3a in Mi-4 borehole) can be traced across the entire caldera and may serve as the principal correlation member. Inclusion of the Vrchoslav type into this TR3a unit remains speculative due to lack of data, and further petrological research is required. On the other hand, Vlčí kámen and Medvědí vrch types from opposite sides of the caldera display identical properties allowing to merge them into a common unit TR3b overlain by the youngest unit TR4 (Přední Cínovec type)-restricted to the Czech territory. This result may suggest partly synclinal structure within the southern ATVC segment. The TR4 unit has a strong compositional tendency towards the post-ignimbrite granite porphyry intrusions. Some lithotypes in the northern segment (e.g., Buschmühle) do not have counterparts on the Czech side and most likely represent an independent volcanic unit in the northern, presumably older, part of the caldera. The uniform composition of the rhyolitic rocks observed in the entire profiles of the drillings located in the Town Teplice (TP-39 reaching 1170 m depth) may suggest that the main feeding conduit system for the TR4 ignimbrite was located within the Teplice-Lahošť horst area.

Magnetic fabrics of rhyolite ignimbrites reveal complex emplacement dynamics of pyroclastic density currents, an example from the Altenberg–Teplice Caldera, Bohemian Massif

“…3d). Similar features including intrusions of thin microgranite dikes were detected in the borehole TP-39 (Gnojek et al 2018).…”

“…Tomek et al (2022) correlated some ash-fall tuff layers in the Carboniferous late-orogenic basins with the TR. Based on the historic literature (Moesta 1928;Fiala 1960;Eisenreich and Jeřábek 1978;Holub 1980;Lobin 1986;Jiránek et al 1987;Benek 1991), Gnojek et al (2018) and Casas-García et al ( 2019) defined nine TR lithostratigraphic members separated by two volcanosedimentary horizons. The first six members, of minor areal extent at the present-day erosional level, mark the ignimbrite eruptions prior to the final caldera collapse (Tab.…”

Radiogenic heat production of Variscan granites from the Western Bohemian Massif, Germany

“…Frattini et al 2006;Šimíček et al 2012;McCay et al 2014;Puccini et al 2014;Harley et al 2017). In granitic terrains, an advantage of gamma-ray spectroscopy over chemical analysis is the easier mapping and therefore better regional coverage (Gnojek et al 2018;Scharfenberg et al 2019).…”

Inverting airborne gamma-ray spectrometry data of Maricá calibration range, Brazil

“…The airborne gamma-ray spectrometry (AGRS) processing is a tool for geophysical-geological mapping to aid both mineral exploration and the gathering of lithological information (e.g., Davis & Guilbert, 1973;Grasty, 1979;Charbonneau, 1991;Graham & Bonham-Carter, 1993;Dickson & Scott, 1997, Gnojek et al, 2018.…”