Zircon U-Pb-Hf isotope systematics of southern Black Forest gneiss units (Germany) – implications for the Pre-Variscan evolution of Central Europe

We present the first systematic U-Pb-Hf isotope data of detrital zircon grains from gneiss units of the southern Black Forest, preserving different stages of the pre-, syn- and post-Cadomian evolution. Protoliths of Murgtal metagreywackes were deposited during the Ediacarian at <550 Ma and sourced from the Avalonian-Cadomian Belt (550-700 Ma; ~70%) and Sahara Metacraton (760-1045 Ma, 1850-2250 Ma, 2720-3230 Ma; ~30%). In contrast, metasedimentary rocks of the Wiese-Wehra and Todtmoos gneiss units reveal late Devonian depositional ages at <370 Ma, but in different geotectonic settings. Wiese-Wehra metagreywackes provide evidence for the existence of pre-Cadomian oceanic crust formed at 610 Ma (εHf_t = +5 to +8), accreted to the Cadomian Belt at ca. 540 Ma, and successively reworked between 490 and 430 Ma (εHf_t = +1 to +6). Finally, these rocks became part of an early Variscan arc-back arc system with juvenile input at 370 Ma (εHf_t = 0 to +10). Todtmoos metaarkoses mainly reflect subduction-related magmatism at 490-420 Ma (~88%), and at 380 Ma (^~10%) in an evolved continental arc setting (εHf_t = -2 to -8). In combination, existing data from the Black Forest and other basement units throughout Europe provide evidence for the amalgamation of pre-Cadomian juvenile terranes along the northern margin of Gondwana until 540 Ma, followed by a complex rift history, accompanied by subduction between 500 and 400 Ma. They further point to the existence of an oceanic arc-back arc system, which has been located south of the Armorican terrane assemblage at 380-365 Ma.