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<p>Upper Neoproterozoic greywackes of the Cadomian basement and the subsequent overlap sequence of largely upper Cambrian to Lower Ordovician siliciclastic sedimentary rocks are the oldest deposits of Saxo-Thuringia. This study aims to establish a stratigraphic affiliation and discrimination criteria between the individual units based on comprehensive whole-rock geochemical, Rb–Sr and Sm–Nd isotopic analyses. The most notable differences can be observed in the markedly elevated SiO₂ content and concurrently reduced Al₂O₃ content of the upper Cambrian to Lower Ordovician units, in addition to the depletion of Na and Ca resulting from the prior weathering-related decomposition of feldspars. Upper Cambrian to Lower Ordovician units have considerably higher <sup>87</sup>Rb/<sup>86</sup>Sr ratios (ca. &gt; 4) than the Neoproterozoic ones. Considering these characteristics, the depositional age of the low-grade metamorphosed Clanzschwitz Group and Seidewitz Formation must be shifted from the Late Neoproterozoic to the late Cambrian or Early Ordovician. A differentiation criterion could also be established for the first time between the two macroscopically indistinguishable Ediacaran and Carboniferous greywacke units of the Lausitz Block, with the latter unit having significantly higher concentrations of the trace elements Ni, V, Cr and Co. Comprehensive Sm–Nd isotope investigations validate a uniform West African provenance for the Saxo-Thuringian units, with model ages around 2&#xa0;Ga. However, locally in East Thuringia and in the Carboniferous greywackes of the Lausitz Block, younger model ages (1.7–1.3&#xa0;Ga) indicate a different provenance or an admixture of juvenile volcanic material. Trace and rare earth element (REE) analyses using in situ LA–ICP–MS on chert samples from the Rothstein Formation and chert clasts from the Lausitz Group deposits revealed markedly divergent REE signatures and provide compelling evidence that the Lausitz Group is not composed of redeposited erosion products from the older backarc basin (Rothstein Formation). Therefore, the commonly accepted two-stage (backarc–retroarc) basin model for Saxo-Thuringia during the Neoproterozoic requires revision.</p> Graphical Abstract <p></p>

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New insights into Cadomian basin evolution and stratigraphic affiliation of sedimentary units of Saxo-Thuringia, Germany: Part 1—whole-rock and chert geochemistry and Rb–Sr and Sm–Nd isotope systematics

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  • 发表日期：

    2025年8月21日

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  • Victoria Kühnemann,
  • Guido Meinhold,
  • Matthias Willbold,
  • Jasper Berndt,
  • Sebastian Weber

摘要

Upper Neoproterozoic greywackes of the Cadomian basement and the subsequent overlap sequence of largely upper Cambrian to Lower Ordovician siliciclastic sedimentary rocks are the oldest deposits of Saxo-Thuringia. This study aims to establish a stratigraphic affiliation and discrimination criteria between the individual units based on comprehensive whole-rock geochemical, Rb–Sr and Sm–Nd isotopic analyses. The most notable differences can be observed in the markedly elevated SiO₂ content and concurrently reduced Al₂O₃ content of the upper Cambrian to Lower Ordovician units, in addition to the depletion of Na and Ca resulting from the prior weathering-related decomposition of feldspars. Upper Cambrian to Lower Ordovician units have considerably higher 87Rb/86Sr ratios (ca. > 4) than the Neoproterozoic ones. Considering these characteristics, the depositional age of the low-grade metamorphosed Clanzschwitz Group and Seidewitz Formation must be shifted from the Late Neoproterozoic to the late Cambrian or Early Ordovician. A differentiation criterion could also be established for the first time between the two macroscopically indistinguishable Ediacaran and Carboniferous greywacke units of the Lausitz Block, with the latter unit having significantly higher concentrations of the trace elements Ni, V, Cr and Co. Comprehensive Sm–Nd isotope investigations validate a uniform West African provenance for the Saxo-Thuringian units, with model ages around 2 Ga. However, locally in East Thuringia and in the Carboniferous greywackes of the Lausitz Block, younger model ages (1.7–1.3 Ga) indicate a different provenance or an admixture of juvenile volcanic material. Trace and rare earth element (REE) analyses using in situ LA–ICP–MS on chert samples from the Rothstein Formation and chert clasts from the Lausitz Group deposits revealed markedly divergent REE signatures and provide compelling evidence that the Lausitz Group is not composed of redeposited erosion products from the older backarc basin (Rothstein Formation). Therefore, the commonly accepted two-stage (backarc–retroarc) basin model for Saxo-Thuringia during the Neoproterozoic requires revision.

Graphical Abstract