Geodynamic evolution of the Schwarzwald Crystalline Complex – new insights from zircon U-Pb-Hf isotope systematics of (meta)sedimentary rocks and metabasites

• Applicant: Professor Dr. Armin Zeh
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 510964246

Presently, little is known about the provenance, depositional ages and Hf isotopic composition of (meta)sedimentary rocks and metabasites of the Schwarzwald Crystalline Complex (SCC) in southern Germany, preventing detailed reconstructions of the pre- to syn-Variscan geodynamic evolution in Central Europe, as well as comparison with other basement units. To close this gap of knowledge, this project aims at presenting the first systematic U-Pb-Hf isotope study on detrital zircon grains, selected from about 20 (meta)sedimentary rock samples (greywackes, sandstones, arkoses) and 15 metabasite samples (amphibolite- to eclogite-facies) of all major rock units of the SCC. This includes high-grade paragneisses (amphibolite- to granulite-facies) of different “nappe units” of the (1) Central Schwarzwald Gneiss Complex (CSGC) and (2) Southern Schwarzwald Gneiss Complex (SSGC), (3) low-grade metasedimentary rocks of Ordovician to lower Carboniferous age of the Badenweiler-Lenzkirch Zone (BLZ), a proposed suture zone separating the CSGU from the SSGU, and (4) low- to high-grade greywackes and phyllonites of the Baden-Baden Zone (BBZ), forming the northernmost unit of the SCC, traditionally assumed to host the transition zone between the Moldanubian and Saxothuringian domains. Based on representative zircon age spectra (comprising at least 120 U-Pb analyses carried out on texturally well characterized zircon grains/domains), we will give answers to the follow open questions: (1) What are the maximum depositional ages (MDA) of the metasedimentary rocks forming the major rock units of the SCC (SSGC, CSGC, BLZ, and BBZ), and what was the geotectonic setting during deposition? (2) Correlate the MDA with the biostratigraphic record? (3) Are their significant differences in zircon age spectra between different units of the SCC? (4) What is the sediment provenance, and did it change through space and time? (5) What was the crust-mantle evolution in the hinterland through time? (6) Is the zircon detritus related to Gondwana formation, Gondwana break-up, “Hun superterrane” evolution, Rheic ocean closure and/or something completely different? (7) Do the different rock units of the SCC represent different terranes amalgamated during the Variscan orogeny? (8) What is the timing of metabasite formation and metamorphism, and in which geotectonic environment were these formed? (8) How do the different geological units of the SCC correlate with other basement units throughout the Variscan Belt comprising the Bohemian Massif, Voges, French Massif Central and Iberia.

• DFG Programme: Research Grants