Mantle wedge metasomatism revealed by Li isotopes in orogenic lamprophyres

Abstract Variscan orogenic lamprophyres from the northeastern part of the Bohemian Massif (Erzgebirge, Lusatia, Sudetes) have δ7Li values ranging from − 5.5 to 1.9‰, i.e., values that are lower than the compositional range of depleted mantle. Correlations of δ7Li with Li, Li/MgO, Li/Rb, and Li/Cs demonstrate that these low values are not due to processes related to magmatic emplacement or near-surface low-temperature alterations, but represent genuine signatures from the mantle source of the lamprophyres. The low δ7Li values of the lamprophyre sources reflect subduction-related Variscan metasomatism of the subcontinental mantle, whereas the regionally different correlations of δ7Li with other elements (e.g., Li/Yb, Nb/Dy, Li/Cs) imply that this metasomatism shows regional differences that correlate with the nature of the subducted slab, which represents the source of the metasomatic component. The range of the isotopic and geochemical compositions of Variscan lamprophyres in part overlaps with the compositional range of the subducted Paleozoic sedimentary rocks, and in part is more influenced by material derived from the depleted mantle, depending on whether the budget of the respective element is dominated by the metasomatic component or has contributions from both components. The new data demonstrate that subduction does not necessarily result in major changes in the chemical and Li-isotopic compositions of the subducted sedimentary rocks. Our data show that crustal Li is reintroduced into the mantle. The virtual absence of a crustal Li isotopic signature in Oceanic Island Basalts implies that Li is not simply reintroduced into the convecting mantle, but instead is mainly transferred into the suprasubduction zone mantle wedges and thereafter remains in the subcontinental lithospheric mantle.