U–Pb–Hf isotope and shape systematics of detrital zircon populations from the Badenweiler–Lenzkirch Zone, Black Forest (Germany): new constraints on sediment deposition, provenance and Variscan evolution

Abstract

Presently, little is known about the provenance and depositional ages of clastic metasedimentary rocks of the Badenweiler–Lenzkirch Zone (BLZ), which is squeezed between high-grade gneisses of the Central and Southern Black Forest Gneiss complexes. This lack of knowledge prevents detailed correlations with other sedimentary units and limits our understanding of the pre- to syn-Variscan evolution in Central Europe. In this study, we present the first set of data combining U–Pb ages, Hf isotope data, and shape parameters of detrital zircon populations of nine samples collected from three formations of the BLZ: Sengalenkopfschist, Schleifenbachschist, and Protocanites Greywacke. Based on biostratigraphic record, these formations are assumed to be deposited from the Early Ordovician to Early Carboniferous. This interpretation, however, is at odds with the detrital zircon age record, revealing robust maximum depositional ages between 368 and 378 Ma for 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 380–410 Ma (εHf t up to + 5). Combined zircon age-Hf isotope data point to three major sources, which are similar to the metamorphic gneiss units exposed in southern Black Forest (Wiese-Wehra, Todtmoos, and Murgtal), hosting relics of different Gondwana-derived terranes, in addition to a Late Devonian arc-back arc system. The finding of abundant zircon grains of euhedral shape in all BLZ samples and similar zircon typologies additionally suggest supply from a proximal magmatic arc source. Graphical abstract

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