Paleomagnetic and Rock-Magnetic data from late-Variscan ignimbrites: a case study from the Altenberg-Teplice Caldera, Bohemian Massif

Abstract

Altenberg-Teplice Caldera is an ~36 x 18 km elongated trap-door caldera that was active ~314-312

Ma; postdating the collapse of the Variscan orogen in the NW Bohemian Massif. The relic caldera consists of a felsic (S- to A-type) volcano-plutonic edifice that exposes a feeder rhyolite dike swarm that erupted intra- and extra-caldera rhyolite ignimbrites, a syn-collapse porphyritic microgranite ring dikes, and post-caldera granitic intrusions. The caldera-forming eruption ~314 Ma sourced large pyroclastic density currents depositing the intra-caldera ignimbrites, along with extra-caldera ignimbrites and fall-tuff deposits in the adjacent Carboniferous basins up to 30 and 150 km, respectively, to the S/SE from the caldera. This study focuses on rock-magnetic characterization, paleomagnetic, and magnetic fabric data of the extra-caldera ignimbrites. Furthermore, these new paleomagnetic data are compared with the other units of the caldera, including intra-caldera ignimbrites, the feeder dike swarm, and ring dikes. Based on the variations of magnetic susceptibility with temperature and hysteresis and first-order reversal curves, the extra-caldera ignimbrites yielded dominantly pseudosingle domain (vortex state) and superparamagnetic grains of a low-Ti titanomagnetite composition as the principal carriers of remanence and magnetic fabrics. The ignimbrites yield a bimodal distribution of magnetic susceptibility; one group with relatively high susceptibility in the center of the quarry surrounded by a group of paramagnetic sites. The average flow direction towards the NE, estimated from magnetic fabric imbrication, does not coincide with the assumed caldera source in the NW. Paleomagnetic data from the other caldera units yield a spectrum of results. Five sites in the intra- and all three in extra-caldera ignimbrites yield a remanence direction consistent with a Late-Paleozoic age of emplacement. We interpret these sites as recording a primary thermoremanent magnetization acquired during emplacement. The remaining sites in the intra-caldera ignimbrites appear to have been partially to completely remagnetized yielding either a composite magnetization or a present-day field overprint. The sites collected from the dike swarm, apart from one having primary magnetization, yield complex demagnetization behavior and dispersed paleomagnetic directions. Four of the five sites from the ring dikes yield a steep inclination, southwest-directed normal polarity magnetization. Only one site yields a probable

Late-Paleozoic magnetization. The results from the Altenberg-Teplice Caldera provide interpretable data that indicate that these rocks, albeit enduring a complex deformational history, often preserve a primary Late-Paleozoic magnetization.