Ordovician and Silurian protolith ages of metamorphosed clastic sedimentary rocks from the southern Schwarzwald, SW Germany: a palynological study and its bearing on the Early Palaeozoic geotectonic evolution

摘要

Sedimentation ages of metamorphosed elastic sedimentary rocks in the southern Schwarzwald were determined by associations of palynomorphs. In the northern subunit of the Badenweiler-Lenzkirch Zone, two lithostratigraphic assemblages could be discerned in low-grade metamorphic units by their facies and age, thus revealing a more complex internal structure of this zone than previously assumed. Lower Ordovician metagreywackes and metapelites were discerned from Silurian metasiltstones. In the cataclastically overprinted metasiltstones and phyllites of the southern subunit of the Badenweiler-Lenzkirch Zone, only poorly preserved microfossil remains could be detected. These show that the sedimentation ages must be Ordovician or younger, but still probably Early Palaeozoic. High-grade metapelitic rocks of the South Schwarzwald Gneiss Complex contain chitinozoans in lenses and layers of schists, that are rich in biotite and graphite. They yielded mid-Silurian ages and show that this crystalline complex does not represent an older basement unit but was the result of marine sedimentation at that time. The new age determinations have a bearing on geodynamic reconstructions of the internal Variscides in Early Palaeozoic time. They show that sedimentation in the oceanic realm of the Badenweiler-Lenzkirch Zone or its margins did not occur before the Ordovician. After transformation of the northern passive into an active continental margin, younger greywackes not older than Middle Devonian received detritus from a volcanic arc, forming above the subduction zone.