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Journal of the Geological Society; 2005; v. 162; issue.1; p. 87-96;
DOI: 10.1144/0016-764904-029
© 2005 Geological Society of London

Original Article

Testing tectonic models with geochemical provenance parameters in greywacke

E. Hegner1,2, M. Gruler1, H.P. Hann1, F. Chen1 & M. GÜldenpfennig1

1 1Institut für Geowissenschaften, Universität Tübingen, Sigwartstrasse 10, D-72076 Tübingen, Germany
2 2Present address: Department für Geo- und Umweltwissenschaften, Universität München, Theresienstrasse 41, D-80333 München, Germany (e-mail: hegner@lmu.de)

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. {epsilon}Nd values of c. +3 to –10 in the greywacke samples indicate mixing of juvenile and older crustal material. Samples with low MgO concentrations and {epsilon}Nd 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 {epsilon}Nd 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.

Key Words: Variscan foldbelt • Black Forest • greywacke • provenance • neodymium