Variation of magma generation and mantle sources during continental rifting observed in Cenozoic lavas from the Eger Rift, Central Europe

Abstract

The 400 km long Eger Rift is one of the major extensional structures in Central Europe and is associated with abundant Tertiary to Quaternary volcanic rocks occurring both within the rift and mainly on its NW uplifted flank. Primitive lavas predominate with compositions ranging from alkali basalts to nephelinites and melilitites. Lavas in the rift centre show similar Si-undersaturation, incompatible element enrichment, and Sr, Nd, and Pb isotope compositions as rocks on the flanks of the Eger Rift suggesting similar degrees of partial melting and mantle sources. Crustal assimilation is abundant and increases with evolution of the magmas. The Eger Rift crosses several different lithospheric units but no systematic changes of magma sources appear to exist across lithospheric boundaries. Three different mantle sources can be defined in the Eger Rift magmas. Two of these sources must be enriched in incompatible elements as they yield the most enriched magmas and probably reside in the lithosphere because they show evidence for residual amphibole. The most common of these end-members resembles the so-called FOZO mantle source and occurs in the asthenosphere. The (Ce/Yb) N in the central part of the Eger Rift are lower than at the western end of the rift suggesting higher degrees of partial melting in the central part which apparently correspond to the narrowing of the rift, i.e. less extension in its western part.