Geochemistry and Geochronology of the Heldburg Dyke Swarm, Central European Volcanic Province

Abstract The Heldburg dyke swam is part of the Central European Volcanic Province (CEVP), for which various competing genetic models have been suggested. Presently, all of them are biased by data obtained on only the larger former volcanic fields, with little or no attention having been given to the smaller ones. Here, we present geochemical and geochronological data for the poorly investigated Heldburg dyke swarm, which has an atypical position within the CEVP by being located away from Hercynian massifs and major rift axes. The trace element distribution of the subvolcanic rocks of this dyke swarm is very similar to that of the majority of mafic volcanic rocks of the CEVP and indicates an enriched asthenospheric/lithospheric mantle as the dominant magma source. New U–Pb age data obtained on inherited zircon grains demonstrate a strong Hercynian influence on the local crust and probably also the enriched mantle. New U–Pb apatite ages for the mafic crater infill at Bramberg (13.00 ± 5.01 Ma), the dykes at Zimmerau (27.71 ± 4.31 Ma) and Fuchslöcher (28.60 ± 0.63 Ma) as well as basanite and phonotephrite in the extinct magma chamber at Maroldsweisach (15.98 ± 0.96 Ma) constrain the timing of magmatic activity of the Heldburg dyke swarm to between c. 13 and 29 Ma, which is in contradiction to a previously postulated temporal division between the Rhön volcanic field and the Heldburg dyke swarm. Instead, the Heldburg dyke swarm can now be considered contemporaneous to the Rhön volcanism, the early stages of the Vogelsberg and ‘syn-rift-phase’ volcanic rocks of the Ohře Rift Graben (Eger Graben). The emplacement of the Heldburg dyke swarm may be related to the Alpine orogeny and tectonic movements along different WNW–ESE-striking fault systems, resulting in block rotation and the opening of narrow NNE–SSW-directed tension faults in two phases at ~30 and 15 Ma.