Geodynamic setting of the tertiary Hocheifel volcanism (Germany), Part I: 40Ar/39Ar geochronology

Abstract

The Eifel volcanism is part of the Cenozoic Central European Volcanic Province and is located close to the Rhine Graben which has been formed by rifting and subsidence since the Eocene. Whereas the Quaternary volcanism of the Eifel appears to be genetically related to mantle plume activity, the cause of the Tertiary volcanism of the Hocheifel volcanic field is less clear. Here, we present geochronological evidence for the geotectonic setting of the Tertiary Eifel volcanism based on 40Ar/39Ar dating of 27 samples from 25 volcanic occurrences. Included are samples from the northern Upper Rhine Graben in order to evaluate a possible relationship between Hocheifel volcanism and Rhine Graben taphrogenesis. The geological relevance of the age data used for the geological discussion is indicated by plateau-type age spectra and tested by inverse isochron calculations. For the Hocheifel, two periods of activity at ca. 44 to 39 Ma and 37 to 35 Ma were inferred. Both age groups are represented by basanites as well as by more differentiated rocks and there is no clear relation between age and chemical composition. The time span dated for the northernmost Upper Rhine Graben volcanism is 59 to 47 Ma indicating foiditic to basanitic activity up to ca. 15 m.y. prior to the onset of rifting and subsidence. The Hocheifel volcanic activity is interpreted to represent the propagation of the pre-rift volcanism of the northern Upper Rhine Graben to the NW because the Hocheifel activity closely follows in time pre-rifting northern Upper Rhine Graben volcanism and the older period of Hocheifel activity shows propagation of younger volcanism to the north. In addition, the Hocheifel tectonic pattern derived from the time - space relation of volcanism corresponds to stress field conditions identical to those of the Upper Rhine Graben at the time of Hocheifel volcanic activity. Magma generation therefore appears to be related to decompression by extension during Middle to Upper Eocene. However, in contrast to the northern Upper Rhine Graben with graben formation subsequent to pre-rifting volcanism, no relevant rifting or subsidence occurred in the Hocheifel. The taphrogenetic evolution in between the Upper and Lower Rhine Graben regions was suspended in an early stage. © 2007 Springer-Verlag Berlin Heidelberg.