Shape and U-Pb-Hf isotope systematics of zircon populations in Variscan greywackes – and example from the Badenweiler-Lenzkirch Zone, Black Forest (Germany)

Results of a systematic study are presented combining U-Pb ages, Hf isotope data and shape parameters (length, width, aspect ratios, roundness, roughness, typology) of detrital zircon populations from low- to medium-grade greywackes of the Badenweiler-Lenzkirch Zone (BLZ), which is squeezed between high-grade gneisses of the Central and Southern Black Forest Gneiss complexes. Nine sample are investigated from three formations: Sengalenkopfschist, Schleifenbachschist, and Protocanites Greywacke unit, assumed to be deposited from the Early Ordovician to Early Carboniferous based on biostratigraphic record. This interpretation, however, is at odds with detrital zircon U-Pb ages, revealing robust maximum depositional ages between 368 and 378 Ma for rocks of all three units. Age spectra show peaks at 380-400 Ma, 480-500 Ma, 600-620 Ma, 700-750 Ma, 0.9-1.1 Ga, 1.8-2.2 Ga, and 2.6 Ga, and Hf isotopes a juvenile input at 380-400 Ma (εHf_t up to +5). The combined age-Hf data point to a similar provenance like the metamorphic rocks exposed in Southern Black Forest gneiss complex, hosting relics of different Gondwana-derived terranes, in addition to a Late Devonian arc-back arc system. Similar proximal sources are also indicated by the finding of abundant euhedral zircon grains with ages <550 Ma in all samples, and a significant overlap in zircon typology. The low degree of zircon roundness and roughness reflect sediment transport in water-saturated media, but in some samples has been significantly modified by post-depositional structural-metamorphic overprint, causing mechanical zircon peeling and chemical dissolution in contact with sheet silicates, in particular during garnet-forming dehydration-reactions.