Alter und Abfolge der jüngsten vulkanischen Aktivität in der Westeifel mittels 40Ar/39Ar-Präzisionsdatierungen
Final Report Abstract
New 40Ar/39Ar ages, radiogenic Sr and Nd isotope ratios as well as trace and major element compositions on 12 basanitic/foiditic lava flows from the Pleistocene West Eifel volcanic field (Cenozoic Central European Volcanic Province) are used lo demonstrate relations between spatial evolution, temporal distribution and source compositions in order to infer the geodynamic history of West Eifel volcanism. These results are of systematic relevance since it has been suggested that other Cenozoic volcanic fields in Central and West Europe were formed by similiar geodynamic processes in an extensional intra-plate setting within the Alpine orogenic foreland. The new 40Ar/39Ar ages confirm an older (> 480 ka. Middle Pleistocene) and younger age group (< 100 ka. Late Pleistocene) of lava flows as well as a time span in between lacking volcanic activity in the Wesl Eifel. Lavas of the older age group erupted in the NW whereas lavas ofthe younger age group are concentrated in the SE ofthe field, consistent with previous evidence on the basis of field observations. Migration of volcanism with time towards the southeast ofthe field is related to changes in lithospheric stress field conditions as indicated by the alignement of flows from older and younger age groups along ca. 155° and 135° trending lineaments in the NW and SE of the field, respectively. Pathways for magma ascent appear to be related to a regional tectonic pattern of Late Hercynian origin representing transverse joints normal to the fold axes ofthe weakly metamorphosed Hercynian basement rocks. This pattern has been reactivated during the Middle and Late Pleistocene possibly stimulated by thermomechanical weakening of the lithosphere. Whereas lavas of each age group show coherent Sr and Nd isotope compositions, significant differences between both groups show that distinct mantle sources were tapped. This is supported by trace element evidence indicating that systematic differences in isotope compositions are not the result of crustal contamination processes. The asthenospheric mantle source beneath the West Eifel volcanic field is characterized by a local low P-velocity anomaly interpreted to represent uprising rock material hotter than ambient mantle. Nevertheless, the uplift history ofthe West Eifel is not decoupled from adjacent regions not effected by diapiric asthenosphere. The systematic geographical distribution of basanitic flows (> 480 ka) with highradiogenic Sr/low-radiogenic Nd in the northwest as well as basanitic flows (< 100 ka) with lowradiogenic Sr/high-radiogenic Nd in the southeast ofthe field suggests that there is a lateral chemical zonation within the low P-velocity anomaly. West and East Eifel isotope compositions are consistent with a FOZO(redefined)-EM 1 mantle domain with a higher contribution of FOZO relative to EM 1 beneath the West Eifel. The geochronological results ofthis project were presented to the general public in an evening lecture (attachment) at the Naturkunde-Museum at Gerolstein (West Eifel) with an astonishing positive response demonstrated by a broad audience comprising ca. 130 non-scientific persons.
