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Previous geological and structural investigations show distinct differences between the pre-Variscan and Variscan history of the Saxothuringian and Rhenohercynian. Therefore the northern margin of the Mid German Crystalline Rise must be considered as a border area of crustal segments (terranes) with different tectonometamorphic evolution. New radiometric age data from the Northern Phyllite Zone (Wippra Zone) being part of this border area contribute to specify the dynamic modelling of variscan tectogenetic processes. The radiometric ages of the metamorphic overprint in the Wippra Zone has been determined by conventional K/Ar-technique and illite crystallinity measurements. The investigations were carried out on very low to low grade metamorphic, synkinematically grown phyllosilicates in metapelites. The illite crystallinities of the grainfraction < 2 µm of the samples from the Wippra Zone range from high diagenesis to epizone. There is no recognizable trend of the illite crystallinities along a section through the particular series of the Wippra Zone. Nevertheless K/Ar-ages of the fraction < 2 µm show increasing ages of metamorphism from NW to SE in the Wippra Zone series 1 to 5 (320-351 Ma). | The age of 360 Ma in the northerly positioned series 1 and the younger ages in the southermost series 7 of about 330 Ma diverge from the trend. Interpreting the radiometric age data, the series were not situated during the time of metamorphism in their present tectonic position. Stacking of the series took place after the peak of metamorphic overprint.
The obtained age data suggest a metamorphism in the Wippra Zone prior to the SW-part of the Northern Phyllite Zone (Hunsrück, Taunus). This leads to two possible interpretations of the isotopic data: either this could be a first hint for different times of the beginning either the collisional tectonics or the influence of a newly discussed transpressive regime at the northern margin of the Mid German Crystalline Rise or the Wippra Zone does not represent the Rhenohercynian and at least parts of the Wippra Zone belong to the overriding plate of the Saxothuringian. |
