Evolution of the lower continental crust: granulite facies xenoliths from the Eifel, West Germany

Abstract

The lower continental crust is a key region to understanding processes of crust–mantle differentiation and subsequent evolution of the crust. The region is metamorphosed into granulite or amphibolite facies1,2. Whereas felsic to mafic granulites with an average intermediate major element composition are typically observed in old exposed lower crustal terranes, mafic compositions are preponderant among granulite facies xenoliths transported by young alkalic basaltic volcanics and kimberlites1. Radiometrie ages for granulite bulk rocks using the Rb–Sr or U–Pb methods are usually interpreted as metamorphic ages. In contrast, Sm–Nd ages obtained on such metamorphics are interpreted to reflect pre-metamorphic events3–7, although a case of severe disturbance of the Sm–Nd isotope system during granulite facies metamorphism has also been reported8. We present here Sm–Nd and Rb–Sr isotope data for a suite of mafic lower crustal xenoliths from the Hercynian belt. Our data suggest that the igneous precursors of these granulites differentiated from depleted mantle ∼1.5 Gyr ago. In most of these granulites this evidence for a mid-Proterozoic age has been obliterated by a young (≺172 Myr) metasomatic overprint by mantle derived fluids or melts.