Geochronology, stratigraphy and geochemistry of Cambro-Ordovician, Silurian and Devonian volcanic rocks of the Saxothuringian Zone in NE Bavaria (Germany)—new constraints for Gondwana break up and ocean–island magmatism

• Published in: International journal of earth sciences : Geologische Rundschau 2018, Vol.107 (1), p.359-377
• Main Authors: Höhn, Stefan, Koglin, Nikola, Klopf, Lisa, Schüssler, Ulrich, Tragelehn, Harald, Frimmel, Hartwig E., Zeh, Armin, Brätz, Helene
• Format: Article
• Language: English
• Subjects: Age; Area; Basalt; Cambrian; Devonian; Earth and Environmental Science; Earth Sciences; Evolution; Geochemistry; Geochronology; Geochronometry; Geological time; Geology; Geophysics/Geodesy; Hot spots (geology); Magma; Mineral Resources; Oceans; Ordovician; Original Paper; Palaeozoic; Paleozoic; Quartz; Radiometric dating; Rifting; Sedimentology; Silurian; Stratigraphy; Structural Geology; Tuff; Volcanic activity; Volcanic rocks; Volcanism; Volcanoes; Zircon
• ISSN: 1437-3254; 1437-3262
• DOI: 10.1007/s00531-017-1497-2

Stratigraphically well-defined volcanic rocks in Palaeozoic volcano-sedimentary units of the Frankenwald area (Saxothuringian Zone, Variscan Orogen) were sampled for geochemical characterisation and U–Pb zircon dating. The oldest rock suite comprises quartz keratophyre, brecciated keratophyre, quartz keratophyre tuff and basalt, formed in Upper Cambrian to Tremadocian time (c. 497–478 Ma). Basaltic volcanism continued until the Silurian. Quartz keratophyre shows post-collisional calc-alkaline signature, the Ordovician–Silurian basalt has alkaline signature typical of continental rift environments. The combined datasets provide evidence of Cambro-Ordovician bimodal volcanism and successive rifting until the Silurian. This evolution very likely resulted from break-up of the northern Gondwana margin, as recorded in many terranes throughout Europe. The position at the northern Gondwana margin is supported by detrital zircon grains in some tuffs, with typical Gondwana-derived age spectra mostly recording ages of 550–750 Ma and minor age populations of 950–1100 and 1700–2700 Ma. The absence of N-MORB basalt in the Frankenwald area points to a retarded break-off of the Saxothuringian terrane along a continental rift system from Uppermost Cambrian to Middle Silurian time. Geochemical data for a second suite of Upper Devonian basalt provide evidence of emplacement in a hot spot-related ocean-island setting south of the Rheic Ocean. Our results also require partial revision of the lithostratigraphy of the Frankenwald area. The basal volcanic unit of the Randschiefer Formation yielded a Tremadocian age and, therefore, should be attributed to the Vogtendorf Formation. Keratophyre of the Vogtendorf Formation, previously assigned to the Tremadoc, is most likely of Upper Devonian age.