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

Abstract

Presently, little is known about depositional ages, geological setting, and provenance of (meta)sedimentary rocks constituting the southern part of the Black Forest Gneiss Complex, preventing detailed reconstructions of the Cadomian to Variscan geodynamic evolution in Europe. Here, we present the first systematic U-Pb-Hf isotope data of detrital zircon grains combined with geochemical data of metasedimentary rocks from the Murgtal, Todtmoos, and Wiese-Wehra gneiss units, preserving different stages of the pre-, syn- and post-Cadomian evolution. Protoliths of Murgtal metagreywackes were deposited during the Ediacaran at <550 Ma and sourced from the Avalonian-Cadomian Belt (550–700 Ma; ∼70 %) and Africa (760–1045 Ma, 1850–2250 Ma, 2720–3230 Ma; ∼30 %). In contrast, metasedimentary rocks of the Wiese-Wehra and Todtmoos gneiss units show late Devonian depositional ages at <375 Ma, but in different geotectonic settings. Data from Wiese-Wehra metagreywackes provide evidence for the existence of a pre-Cadomian oceanic arc formed at 610 Ma (εHft = +5 to +8), accreted to the Avalonian-Cadomian Belt at ca. 540 Ma, and successively reworked until 380 Ma (εHft = +1 to +6). Finally, these rocks became part of an early Variscan arc-back arc system with juvenile input at ca. 375 Ma (εHft = 0 to +10). Todtmoos metaarkoses mainly reflect subduction-related magmatism at 420–490 Ma (∼88 %), and at 380 Ma (∼10 %) in a continental arc setting (εHft = −2 to −8), and rare Cadomian arc input (560–700 Ma; εHft = −10 to +3; ∼2 %). Zircon grains of some samples reveal a Variscan metamorphic overprint at 340–335 Ma. In combination, new and 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. Furthermore, they indicate the existence of an oceanic arc-back arc system, which was located south of the Armorican Terrane Assemblage at 380–350 Ma, immediately prior to Variscan subduction-accretion at <345 Ma.