Geochronology and geochemistry of the Heldburg dyke swarm (Central European Volcanic Province)

• Applicant: Dr. Stefan Höhn
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 495280451

The genesis of the Central European Volcanic Province remains a matter of debate. Some genetic models try to explain this province by passive rifting, controlled by the Alpine orogeny further south. This is not supported by data from some volcanic fields, like the Vogelsberg, which are atypical of rift-related volcanism but reveal characteristics that are more akin to mantle plumes or hot spots. Presently, all the competing models are biased by the comparatively large number of data from a few, larger former volcanic fields. The smaller former volcanic fields did not receive much of attention in the last decades, although they could contribute significantly to solving the issue of the ultimate cause of the Central European Volcanic Province.A prime example is the Heldburg dyke swarm. It is neither connected to one of the major graben systems, such as the Upper Rhine Graben or the Eger Graben, nor is it spatially associated with any major Variscan thrust fault, both of which are characteristic features in the other, larger former volcanic fields. Dykes have the ad-vantage that they can provide more reliable information on the geochemical charac-teristics of a volcanic field because they typically are better approximations to the original magmatic composition than their volcanic counterparts do. In spite of this significance, available geochronological data are problematic as they are based sole-ly on the Ar-Ar isotope system and on variably altered rocks. Most dykes and about only half of the former magma chambers have not been investigated at all. Existing geochemical data for those dykes studied are typically based on a single analyses per locality or on outdated XRF methodology. Thus, they cannot be regarded as representative. Consequently, the first aim of the proposed study is to provide a representative state-of-the-art geochemical dataset for the Heldburg dyke swarm and by integrating with the available data from the Central European Volcanic Province to shed new light on the genesis of this enigmatic volcanic province.The second aim of this project is to test the existing age concept of the whole dyke swarm. The available Ar-Ar age cover a wide range from 38 to 13 Ma. This wide range is not necessarily dating the actual period of magmatism but is in all likelihood an artefact of variable alteration and Ar-loss. It is planned, therefore, to apply the less alteration-sensitive U-Pb zircon dating method. Preliminary test results suggest that there should be sufficient amount of zircon in these mafic rocks to make this possible. A particular focus will be on a quarry (Zeilberg at Maroldsweisach) in which two types of former magma are recorded by tephritic and phonotephritic rocks that represent different magmatic development stages and source regions. All in all, the planned study on the Heldburg dyke swarm is likely to lead to a new and improved understanding of the ultimate cause of the Central European Province.

• DFG Programme: Research Grants