Alkaline and Carbonate-rich Melt Metasomatism and Melting of Subcontinental Lithospheric Mantle: Evidence from Mantle Xenoliths, NE Bavaria, Bohemian Massif

Abstract

Peridotite xenoliths hosted by Cenozoic basanite lava flows (21·2–23·5 Ma) occur at several localities in the western continuation of the Ohře/Eger rift (Central European Volcanic Province) in NE Bavaria, Bohemian Massif. Detailed petrography, major and trace element compositions of whole-rock samples and selected mineral phases as well as Sr–Nd–Li isotopic compositions for a suite of mantle xenoliths from Zinst, Hirschentanz and Teichelberg document variable degrees of partial melting and metasomatism. Melting models based on whole-rock major element composition and Cr# of spinel indicate ∼6–30% melting in the spinel stability field. Subsequent metasomatism by alkaline and carbonate-rich melts resulted in modal and cryptic metasomatism, expressed by the presence of carbonate-bearing silicate melt pockets with complex secondary mineral assemblages and by enrichment in light rare earth elements, Li, Rb, U, Pb, high field strength elements and P. The carbonate is most probably associated with fractionation of the Na-rich silicate melt. High P contents, variable but low to negative δ7Li values from +2·5 to −9·7‰, coupled with 87Sr/86Sr ratios between ∼0·7032 and ∼0·7041 may reflect a significant contribution of recycled crustal material such as eclogite in the infiltrating melts responsible for metasomatism, although the Li isotope compositions may reflect kinetic modifications through diffusion. The trace element geochemistry of clinopyroxene, carbonate and melt pockets suggests that clinopyroxene plays a very important role in fractionation of the rare earth elements, high field strength elements and Sr, whereas carbonate does not host large quantities of incompatible trace elements except for Sr, Ba and, to a lesser extent, Th and U.