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

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.

Research highlights▶ This study is based on field observations, clay mineralogy and illite K–Ar dating. Such an approach is well-suited in deciphering the timing of complex orogenic places. In the present case, we have studied various samples from various structural settings in order to provide new information on the historic evolution of the western Rhenish Massif. ▶ Among the new records, we have shown that the metamorphic intensity varied from anchi- to epimetamorphic grade during two major episodes of mica-type crystallization at about 328 ± 6 and 282 ± 12 Ma during the Variscan orogeny. A further, late or post-orogenic extensional activity could also be detected, not yet precisely around 270 Ma. This late illite crystallization was probably related to a new generation or a reactivation of extensive faults with upward moving heat flows. ▶ We also dealt with methodologic and technical aspects of illite K–Ar dating in comparing the results of two rock-grinding techniques and their impacts on the K–Ar data of separated illite size fractions. The data show that the results clearly depend on the used grinding technique. We have also made duplicated analyses of several samples to show that despite large internal analytical uncertainties due mainly to K2O analysis, the duplicate values are systematically within low uncertainties.