Chronology of basin closure and thrusting in the internal zone of the Variscan belt in the Schwarzwald, Germany: evidence from zircon ages, trace element geochemistry, and Nd isotopic data

Chemical and chronological data for samples from a nappe structure in the s outhern Schwarzwald provide evidence for subduction-related magmatism and c losure of an ocean basin at about 350 Ma, followed by nappe emplacement bet ween 342 and 333 Ma. The data for samples from the nappe and basement sugge st the following development: Pb-207/Pb-206 zircon ages for gabbro and anor thosite from the nappe indicate formation of a layered intrusion between 34 9 and 350 Ma. The samples have high initial epsilon (Nd) values of similar to6.2 and trace element characteristics consistent with an origin from a su bduction-modified upper mantle. The data support a continental margin setti ng and by inference the existence of an ocean basin until similar to 350 Ma . Magmatic zircons from the migmatitic gneisses of the nappe provide U-Pb a nd Pb-207/Pb-206 ages of 347 Ma. The data are interpreted as recording part ial melting of the deep crust, possibly due to underplating of a continenta l margin by basaltic melt. Granitic veins that are present in the migmatite of the nappe but absent in the basement, provide a U-Pb zircon age of 342 Ma. The formation of the veins is interpreted in the context of melting of lower crust following thickening by plate collision. Large scale heating of the crust at similar to 342 Ma is also supported by U-Pb ages of metamorph ic zircons from amphibolites of the basement. The chronological record of t he nappe confines its emplacement some time after 342 Ma but well before 33 3 Ma, the age of granite and lamprophyre dikes intruding the sequence. The geodynamic evolution of the southern Schwarzwald is at variance with the fo rmation of an ensialic basin in the southern Vosges from 345 to 340 Ma. Thi s discrepancy suggests that the Vosges and Schwarzwald were probably locate d at different geographical positions during the Devonian-Carboniferous. (C ) 2001 Elsevier Science B.V. All rights reserved.