Zircon ages and Nd isotopic and chemical compositions of orthogneisses from the Black Forest, Germany: evidence for a Cambrian magmatic arc

Single zircon U-Pb dating combined with Pb-207/Pb-206 ages obtained by the evaporation method constrains the emplacement of tonalitic, trondhjemitic, and granodioritic orthogneisses of the Moldanubian zone in the Black Forest between 500 and 510 Ma. Two detrital zircon populations of 1.9 and 1.6 Ga indicate Early-Middle Proterozoic material in the former setting of the bas ement. The initial Nd values range from -0.1 to -3.4 and mean crustal resid ence ages of 1.0-1.4 Ga are consistent with involvement of Early-Middle Pro terozoic crust, and a subordinate juvenile component probably originating f rom subduction-related melting of the mantle. The orthogneisses have fracti onated REE patterns and slightly higher K2O/Na2O ratios than typical low-K tonalite-trondhjemite-granite suites. The chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust. T he emplacement of the orthogneisses was superceded by a high-temperature me tamorphic event at similar to 480 Ma which we interpret as a result of lith ospheric thinning in a marginal basin behind a Cambrian magmatic arc.