Geochemistry of granulite-facies lower crustal xenoliths: implications for the geological history of the lower continental crust below the Eifel, West Germany

Summary Mafic meta-igneous granulite facies xenoliths from Engeln/Eifel display petrographic, chemical and isotopic features which are not easily understood in terms of igneous fractionation or granulite-facies metamorphism. These features are best explained through a metasomatic modification of many of the xenoliths, characterized by the formation of amphibole and increasing Rb/Sr and Nd/Sm ratios as well as shifts in 143 Nd/ 144 Nd towards values similar to those of enriched mantle under the Eifel. Granulites which essentially escaped metasomatism have a Sm-Nd depleted mantle model age of ∼1.6 Ga whereas metasomatically altered granulites have younger and highly variable model ages. A Sm-Nd isochron for minerals from one granulite resulted in an unexpectedly young age of 172 ± 5 Ma which can be regarded as the maximum age of metasomatism. This age may be indicative of a thermal event having affected the lower crust under the Eifel during the Mesozoic.