Migration of Folding and Metamorphism in the Rheinische Schiefergebirge Deduced from K-Ar and Rb-Sr Age Determinations

Abstract

n the very low- to low-grade metamorphic Rhenohercynian Zone of the Variscides a migration of folding has been postulated which was deduced from the shifting of flysch sedimentation. In order to get an idea of the time span which the “wave” of folding and metamorphism has needed to pass through the Rheinische Schiefergebirge, isotopic investigations by K-Ar and Rb-Sr methods were carried out. For these measurements the fine mineral fractions <2μm from pure sericite slates probably derived from acid to intermediate glass-tuffs were analyzed. Furthermore extremely fine-grained sericite- and celadonite-bearing tuffs were taken which probably belong to the Middle Devonian volcanism. Because of the very low degree of metamorphism in the Rheinische Schiefergebirge it can be assumed, that the blocking temperatures for the K-Ar and Rb-Sr systems of white micas were not exceeded — except for the K-Ar system in the “Taunus-Pre-Devonian”. Therefore most of the age determinations are indicative of the peak of metamorphism. A determination of the age of folding was only possible on the proved assumption that: The metamorphism is syntectonic with respect to folding. Therefore a determination of the metamorphic ages includes the determination of the age of folding.

The samples used for the isotopic investigations were not allowed to contain old sedimentary — not recrystallized — white micas. Only rock samples containing phyllosilicate fabrics of metamorphic origin were selected.

According to the K-Ar and Rb-Sr age determinations the age of metamorphism and folding in the Rheinische Schiefergebirge decreases from about 330 Ma in the Taunus towards the north to 300 Ma at the northern border. Three exceptions modify this evolutionary history from the south to the north: Lower ages in the “Taunus-Pre-Devonian” rocks have to be interpreted as cooling ages because the temperatures in this area exceeded the blocking temperature for the K-Ar system of the micas during metamorphism.

Higher ages in the Lahn Syncline of about 330 to 340 Ma which belong to rocks emplaced into their present position by nappe movement from the southern margin of the Schiefergebirge. They therefore preserved the higher metamorphic ages of the southern margin.

The high K-Ar age of about 340 Ma at the northern margin of the Schiefergebirge is interpreted as an influence of the North European Caledonian orogeny.

The synmetamorphic “wave” of folding passed through the Rheinische Schiefergebirge from the northern border of the Mid-German Crystalline Rise to the present northern border in approx. 30 Ma. This would correspond to a “velocity of crustal shortening” of about 0,5 cm/year, if one assumes a mean tectonic shortening of about 50% in the Rheinische Schiefergebirge resulting from compression during deformation. These values have to be regarded as minimum values.