UPPER MANTLE XENOLITHS FROM ALKALI BASALTS OF THE VOGELSBERG, GERMANY - IMPLICATIONS FOR MANTLE UPWELLING AND METASOMATISM

Alkali basalt-hosted mantle xenoliths of type I (Frey & Prinz, 1978) from the Vogelsberg area (W Germany) cover a compositional range from mildly depleted lherzolites (19 wt.% clinopyroxene) to harzburgites (3 wt.% clinopyroxene), whereas dunites and pyroxenites seem to be extremely rare. Due to partial melting and magma extraction, major and moderately incompatible trace-element compositions of bulk rocks and minerals correlate with the variable modal clinopyroxene abundance, indicating a degree of approximately 30 % partial melting for the most depleting harzburgites. Even though these peridotites represent residues of partial melting, some harzburgites show light rare-earth-element enrichment and a Sr-Nd isotopic signature similar to that of the Vogelsberg basanites. The xenoliths equilibrated in narrow temperature intervals typical for each locality (1125, 1025, and 925-degrees-C), with exception of xenoliths from one outcrop, which reveal a bimodal temperature distribution (1050 and 900-degrees-C). The elevation of the thermal condition above that expected for a stable continental shield appears to be the result of mantle uplift and heating due to magma intrusion. The presence of ''spinel-pyroxene-clusters'' interpreted as former garnet suggests the rise of asthenospheric mantle into the lithosphere. The peridotite/magma interaction is deduced from Fe-Ti-metasomatism observed in high-temperature xenoliths (1020-1070-degrees-C) considered as parts of thermal aureoles adjacent to local magma conduits.