Dualism of intraplate and subduction-related volcanism in the Palaeozoic of the Rhenish Massif (Central Europe) and its plate tectonic relevance

Palaeozoic volcanic rocks are widely distributed within the sedimentary rock sequence of the Rhenish Massif. A total of 185 new geochemical analyses of these have been evaluated, allowing a distinction based on indicative trace elements and rare earth elements between intraplate volcanism and subduction-related volcanism. The latter began in the Silurian and continued through the entire Devonian and likely into the early Carboniferous. The trigger for the volcanic activities was the subduction of the Rheic Ocean northward beneath the southern margin of Avalonia. A magmatic arc had developed above the subduction zone, where explosive eruptions occurred throughout the entire timespan. To the north the Rhenohercynian back-arc basin opened from the Lower Devonian onwards, whereby Avalonia was spited into a northern and a southern part. As a result of the increasing rifting of the thinned continental crust of the southern Avalonian shelf area, intense bimodal intraplate volcanism began in the Givetian, parallel to the subduction-related volcanism, persisting until the early Frasnian. The first volcanic cycle ended, after a long interruption in the deeper Upper Devonian, with the eruption of primitive basaltic magma in the highest Famennian. Due to the retreat of the subduction zone (roll back), Southern Avalonia migrated increasingly southward with simultaneous expansion of the back-arc basin. A new volcanic cycle began in the Early Carboniferous, which is characterized by an evolution from tholeiitic to alkaline basaltic magma. The products of both volcanic cycles occupy large areas in the Lahn-Dill region, the Kellerwald, and the Waldeck region. Subvolcanic intrusions and feeder dykes in the Northern Taunus and Sauerland evidence the widespread distribution of volcanism beyond the present-day limits imposed by erosion.