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 207Pb/206Pb 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 510Ma. Two detrital zircon populations of 1.9 and 1.6Ga indicate Early-Middle Proterozoic material in the former setting of the basement. The initial eNd values range from -0.1 to -3.4 and mean crustal residence ages of 1.0-1.4Ga are consistent with involvement of Early-Middle Proterozoic crust, and a subordinate juvenile component probably originating from subduction-related melting of the mantle. The orthogneisses have fractionated 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. The emplacement of the orthogneisses was superceded by a high-temperature metamorphic event at 480Ma which we interpret as a result of lithospheric thinning in a marginal basin behind a Cambrian magmatic arc.