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

Abstract

The anisotropy of magnetic susceptibility (AMS) is commonly used to infer the flow dynamics, source areas, and post-emplacement processes of pyroclastic density currents (PDC) of young calderas (i.e. Cenozoic). At older calderas, the primary record is often obscured by post-emplacement deformation and/or long-term erosion. Here, we focus on the ~314–313 Ma welded ignimbrites inside the Altenberg–Teplice Caldera (ATC; Bohemian Massif). The small-volume, moderately-welded ignimbrites emplaced prior to caldera-forming eruption yield a generally westward flow direction as determined from the imbrication of the magmatic and magnetic foliation plane. Their eruptive vents were located along the eastern margin of the future caldera. The most voluminous high-grade ignimbrites, products of the caldera-forming event, indicate a high degree of welding and rheomorphic ductile folding that obscured the primary flow fabrics. Based on the fabric pattern, published radiometric and field geology data from the ATC, we interpret that these ignimbrites were sourced from a dike swarm along the northwestern caldera rim. The PDCs then flowed across the subsiding caldera towards the south and south-southeast, where extra-caldera ignimbrites are exposed. The final trap-door caldera collapse triggered the emplacement of the microgranite ring dikes. These dikes, along with the post-caldera granites, may have driven a local resurgence along the eastern caldera rim. As exemplified by the ATC, the AMS fabric can be applied successfully to much older caldera ignimbrites including those with a high degree of welding and rheomorphism to interpret flow direction, deposition, emplacement, and post-emplacement dynamics.