Two distinct Miocene age ranges of basaltic rocks from the Rhön and Heldburg areas (Germany) based on 40Ar/39Ar step heating data

Abstract New 40 Ar/ 39 Ar age data are determined for Cenozoic basaltic rocks from the Thuringian Rhon and Heldburg Gangschar (area also referred to as Grabfeld), integral parts of the Central European Volcanic Province. Applying the incremental heating technique on groundmass and plagioclase separates provided data which considerably specify our knowledge on the eruption ages in these volcanic fields and narrow down the duration of volcanic activity compared to earlier studies. All data but one outlier range between 20 and 14 Ma, being thus similar to those of the neighbouring Vogelsberg volcanic complex. The spectrum of ages is clearly divided into two distinct subsets: the Rhon ages are between 20 and 18 Ma, those of the Heldburg Gangschar are between 16 and 14 Ma. Thus, the present data clearly indicate a striking regional and temporal division of the Thuringian Miocene volcanism. The composition of the volcanic rocks in the two volcanic fields is remarkably diverse, ranging from tholeiitic basalts over alkali basalts and basanites to nephelinites. However, radiometric ages do not correlate with geochemical or petrological characteristics of the volcanics within each volcanic field, indicating that the different magma types erupted broadly contemporaneously. The outlier in age (29 Ma) is from a volcanic dyke of the NE Rhon area close to the NW end of the Thuringian Forest. However, more data are required to approve the significance of this age value, in particular since the rock showed isotopic age disturbance.