Multi-method characterisation of Neoproterozoic and Upper Cambrian–Lower Ordovician low-grade metasedimentary rocks from Saxo-Thuringia

Recent studies on the oldest sedimentary units of Saxo-Thuringia, comprising Neoproterozoic greywackes and Upper Cambrian–Lower Ordovician siliciclastic rocks, have demonstrated the necessity of reassessing the stratigraphic affiliation of certain units. In this context, it was first revealed that some Ordovician units had previously been erroneously assigned a Neoproterozoic age. A multi-method approach was applied to develop specific differentiation criteria based on petrographic, (isoptope-)geochemical and detrital zircon characteristics, which enable reliable differentiation between these units. Compared to the immature greywackes, the Lower Palaeozoic units are distinguished by a considerably increased maturity, which is manifest petrographically through the absence of feldspar and an elevated quartz content. Geochemically, the increased maturity of Upper Cambrian–Lower Ordovician units is further reflected by higher SiO₂, accompanied by significantly lower Al₂O₃, Na and Ca contents. The Lower Palaeozoic units can also unequivocally be distinguished from the older greywackes in terms of isotope geochemistry, due to their significantly higher ⁸⁷Rb/⁸⁶Sr ratios of >4. These characteristics are derived from the intensive weathering phase occurring during the Cambrian Period, which led to the reworking and resedimentation of the Neoproterozoic clastic material. The youngest detrital zircon U–Pb ages established an Upper Cambrian–Lower Ordovician maximum age for the Clanzschwitz Group (North Saxon Block) and the Seidewitz Formation (Elbe Zone), contrary to the previously assumed Neoproterozoic age of sedimentation.