Abstract

A variety of models has been proposed for the origin of late Devonian low-grade greywacke, juxtaposed with the high-grade Variscan basement of the Black Forest. Here we interpret detrital zircon ages, and geochemical and Sm–Nd isotopic data of greywacke samples as evidence for their sources and tectonic setting of deposition. Detrital zircon ages of 371–384 Ma reveal igneous activity shortly before the deposition of the greywacke sequence at c. 365 Ma. εNd values of c. +3 to −10 in the greywacke samples indicate mixing of juvenile and older crustal material. Samples with low MgO concentrations and εNd values of −10 to −4 are interpreted as mostly derived from older granite–gneiss basement. Involvement of a juvenile mafic source is documented by samples with up to 7 wt% MgO and εNd values up to +3. Assessment of the composition of the juvenile component shows high concentrations of light REE, and low Sm/Nd and Nb/La ratios, all consistent with a subduction-related origin. We conclude that the greywacke sequence was derived from a late Devonian Andean-type continental margin, implying a cryptic suture in the internal high-grade Variscan basement of the southern Black Forest.

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