THE NATURE OF YOUNG VEIN METASOMATISM IN THE LITHOSPHERE OF THE WEST EIFEL (GERMANY) - GEOCHEMICAL AND ISOTOPIC CONSTRAINTS FROM COMPOSITE MANTLE XENOLITHS FROM THE MEERFELDER MAAR

Trace element compositions of amphibole and clinopyroxenes from compos ite mantle xenoliths crosscut by micaceous hornblendite veinlets that served as conduits for melts moving through vein systems in the upper mantle have been investigated by ion-microprobe analysis to evaluate t he process of vein metasomatism in the lithosphere beneath the West Ei fel (Germany). The geochemical and Sr-Nd isotopic characteristics of t he veinlets suggest that these are products of megacryst precipitation of melts of the same composition as West Eifel alkali basalts. Their geochemical imprint on the host peridotite is constrained by the evolu tion of compositional gradients in trace element contents within a sma ll-scale, 0.5-1.0 cm wide transition zone at the veinlet-host contact. Highly LREE-enriched, HSFE-poor clinopyroxenes identified in same hos t rocks have Sr-87/Sr-86 (0.7041) and Nd-143/Nd-144 ratios (0.5125) di fferent from those of veinlets and West Eifel basalts, and provide evi dence for a metasomatizing agent before melt injection. Trace element modelling of zoning profiles developed in a single amphibole gram of t he transition zone shows that the veining event manifested in the comp osite xenoliths was rapid and represents the brief final stage of mult iple enrichment processes, probably as a consequence of the Quaternary Eifel volcanism.