EVIDENCE FOR THE MULTIPLE STAGE EVOLUTION OF THE SUBCONTINENTAL LITHOSPHERIC MANTLE BENEATH THE EIFEL (GERMANY) FROM PYROXENITE AND COMPOSITE PYROXENITE PERIDOTITE XENOLITHS/

High temperature (1150-1250 degrees C), coarse-grained olivine-bearing clinopyroxenites occur in the ash-tuffs of the Dreiser Weiher maar-ty pe volcano (West Eifel, Germany) as discrete xenoliths or as 1-5-cm-br oad veins crosscutting anhydrous spinel peridotite host xenoliths. The clinopyroxenes (cpx) of these xenoliths have been analysed for trace element and Nd-Sr isotope compositions in order to document intra-suit e variations and to constrain the processes involved in the formation of heterogeneities within a relatively well defined upper mantle secti on beneath the West Eifel. The patterns formed by cpx from the pyroxen ites on multi-element diagrams are subparallel and convex-upward, show ing troughs for high-field-strength elements (Nb, Zr, Hf, Ti) and Sr. Trace element modelling indicates that these pyroxenites represent hig h pressure precipitates of magmas that are more primitive or similar i n compositions to the most undifferentiated Cenozoic alkali basaltic l avas from the West Eifel. The cpx cover the whole spectrum of Nd-Sr is otope compositions shown by the primitive lavas from the entire West E ifel volcanic field suggesting isotopic heterogeneity on the scale of an individual volcanic centre. Due to incomplete re-equilibration betw een the vein melts and the peridotitic wall rocks, cpx of the host per idotites of the composite xenoliths (that belong to the Ib-group of St osch and Seek, 1980) have in some cases retained relies of a pre-vein host composition. The relic cpx range from LREE-depleted to LREE-enric hed with isotope signatures indicating a time-integrated higher enrich ment (lower Nd-143/Nd-144 and higher S-87/Sr-86) than the cpx of the c orresponding veins. The trace element and isotope compositions of the xenoliths support the perception that magmas generated from sub-lithos pheric mantle sources beneath the West Eifel formed a system of nan ow dike networks and differentiated during their ascent through the lith osphere (Duda and Schmincke 1985). The data provide evidence that: (1) melts parental to the Dreiser Weiher pyroxenites are genetically rela ted to the young alkali basaltic volcanics; (2) these melts can be der ived from distinct domains of the mantle beneath Dreiser Weiher rangin g in Sr-Nd isotope signatures from HIMU-like to Bulk-Silicate-Earth va lues; (3) the enrichment process associated with the upwards migration of these magmas was spatially limited to a cm-scale in the case of th e studied composite xenoliths; (4) parts of the Dreiser Weiher lithosp here have experienced an enrichment prior to the vein interaction by a metasomatic agent that is isotopically unrelated to the primitive Wes t Eifel lavas.