Geochemistry, tectonomagmatic origin and chemical correlation of altered Carboniferous–Permian fallout ash tuffs in southwestern Germany

Abstract

Thin, but laterally widespread, fallout ash tuff layers interbedded with complex fluvio-lacustrine successions of the Carboniferous–Permian late Variscan intermontane Saar–Nahe Basin in southwestern Germany provide important tephrostratigraphic markers in the purely continental depositional setting. The tuffs are rhyolitic to rhyodacitic and indicate geochemical affinities to Moldanubian Variscan S-type granitoids. The volcanic ashes are suggested to have been derived from the general region of the central and northern Black Forest (southwestern Germany) and the northern Vosges (eastern France) at 100–150 km distance south of the Saar–Nahe Basin. Six tuff beds from the Jeckenbach and Odernheim subformations (Meisenheim Formation, Glan Group) have been correlated within the basin over a distance of 50 km by mapping and whole-rock geochemical fingerprinting. In each subformation, three tuffs can be well distinguished using geochemical discriminant function analysis. Additional comparisons of trace and rare-earth element contents provide further criteria for the differentiation of individual tuff beds. These discriminations show that the tuffs have unique chemical fingerprints, probably reflecting differences in the original composition of the parent volcanic tephra. Thus, chemical differences between the tephrostratigraphic markers are geologically significant and provide a powerful tool for establishing tuff layer identification and correlation within the complex sedimentary sequence of the Saar–Nahe Basin. They also provide clues to the tectonomagmatic settings of the source volcanoes.