New findings on the tectono-metamorphic history of the western Rhenish Massif (Germany) by K–Ar dating of metasedimentary illite

Abstract Metapelites, clay-rich sandstones and volcanics from Cambrian, Ordovician and Lower Devonian strata of the western Rhenish Massif underwent a complex regional Variscan tectono-thermal evolution, as shown by mineralogical and K–Ar isotopic analyses of the illite to mica components from three NW-SE transects. The metamorphic degree extended from an anchimetamorphic to an epimetamorphic intensity during two major episodes of illite crystallization at 328 ± 6 and 282 ± 12 Ma. A further late orogenic or post-orogenic extensional activity could also be detected, but not precisely, around 270 Ma, probably recorded by the precipitation of illite in new or reactivated extensional faults with upward moving heat flows. Three studied cross-sections through the massif display a distribution of the illite crystallinity index that scatters within the diagenesis-to-anchizone range with increased crystallization grades along the thrust zones. The determined K–Ar ages probably frame the Variscan convergence in the western Rhenish Massif, which allows calculation of a “shortening velocity” of 140 km in 40 Ma, or of about 0.35 cm/a, that can be suggested on the basis of a total shortening of about 50% for the entire massif.