Considerations on the age of the Urach volcanic field (Southwest Germany)

摘要

The Urach volcanic field in Southwest Germany is part of the Tertiary to Quaternary European Volcanic Province and comprises more than 350 maar-diatremes and just a few outcrops of coherent volcanic rocks (melilitites and nephelinites). K/Ar dating performed in the 60s and 70s suggested the main volcanicity to have been active in the time span be-tween 30-11 Ma. In the more recent literature, the age of the Urach volcanic field was estimated to be 17-16 Ma and ～ 17-11 Ma on the basis of biostratigraphic data, respectively. In this study, we present a summary of petrographic and geochemical data for coherent olivine mclilitite from various locations within the Urach volcanic field. Least altered rock samples were selected for isotopic dating. As a case study, we performed K/Ar dating of fresh olivine melilitite from the Dietenbiihl diatreme in the southern central part of the Urach volcanic field (thought to be one of the older volcanic deposits and previously dated at～ 20 Ma) to either verify or discard the former dating results. Our dating results yield a significantly younger age of 13 Ma for the Dietenbal diatreme suggesting that the volcanic activity in the Urach volcanic field may be considerably younger than previously postulated. In contrast to the formerly estimated long-lasting volcanic activity in the Urach volcanic field (up to ～ 19 million years), the monogenetic character of the volcanic field suggests a comparatively short phase of volcanic activity of probably some few million years.