The interpretation of seismic data gives evidence for a 200-300 km broad belt beneath Central Europe with a marked thinning of the lithosphere (< 50 km) due to an updoming of the asthenospheric mantle into the lithosphere. However, garnet as indicator of material coming initially from deeper mantle levels is extremely rare (France) or completely absent (Germany) in the lherzolitic and harzburgitic mantle xenoliths from the Tertiary-Quarternary basalts of the Central European Volcanic Province (CEVP).
The anhydrous spinel peridotite xenoliths of this study are hosted in the Tertiary nepheline basanite of the Wasserkuppe (Rhön/Germany) which is an integral part of the CEVP and situated near the edge of the Central European lithosphere anomaly (thickness of lithosphere about 60-70 km). The xenoliths cover a range of modal compositions from moderately depleted lherzolites (10 wt.% cpx) to harzburgite (1.5 wt.% cpx). They have a protogranular texture with spinel-pyroxene-clusters which might be interpreted as (1) breakdown products of precursor garnet or (2) as unmixing products of an originally Ca- and Al-rich coarse-grained pyroxene. Major elements in minerals form well-defined trends which are consistent with an origin as residue of partial melting.
The trace element compositions (REE; HFSE: Zr, Hf, Nb, Ta; LILE: Sr, Ba) in cpx have been performed in situ by ion-microprobe, Sr-Nd isotopic data are obtained from clear mineral separates by mass-spectrometry. The LREE and HFSE distribution and the Sr-Nd isotopic ratios are identical in the cpx of single xenoliths, but vary from sample to sample. Three extreme compositions can be distinguished:
(1) Cpx extremely depleted in LREE (LaN/YbN=0.001) and HFSE. The depletion is inconsistent with the relatively primitive modal composition of the xenoliths (10 % cpx). The Nd isotopic signature is peculiarly high (143Nd/144Nd=0.5148; 147Sm/144Nd=0.82-0.92) and has never been reported for cpx before.
(2) Cpx highly LREE enriched (LaN/YbN=25-26) but HFSE depleted. The isotope composition overlaps with that of the amphibole-free Rhîn basanites.
(3) Cpx nearly chondritic or slightly enriched in LREE, moderately enriched in HFSE. Nd isotopes of this type are in the range of depleted mantle (0.5133-0.5134).
The extremely high isotopic Nd ratios of the Type 1 cpx demand long-lasting high Sm/Nd-ratios, which have been found so far only for garnet. We suggest therefore that the isotopic signature of the cpx reflects the former existence of a garnet (first geochemically preserved indication of a garnet-bearing mantle beneath the eastern part of the CEVP). When the hosting peridotites entered the spinel stability field (consequence of mantle uplift), the garnet broke down and forced its geochemical and isotopic signature upon the cpx. A polybaric partial melting model supports the scenario of mantle upwelling and explains the discrepancy between modal composition and the extreme LREE depletion by fractional or incremental melting beginning in the garnet and continuing into the spinel stability field. The peridotites were finally transported into the upper lithosphere at a depth of about 30-40 km where they cooled down from a minimum temperature of 1030°C to temperatures of 850°C (type 2) and 920-950°C (types 1 and 3). In this new position parts of this upper mantle were affected by cryptic, chromatographic fractionation controlled mantle metasomatism. The enrichment of LREE and Sr without an increase of high field strength elements eliminated to a different degree the geochemical fingerprint of the precursor garnet and produced small scale heterogeneities. If the linear correlation observed in a Sm-Nd evolution diagram represents an "isochron" and not a mixing line, melt extraction and a first metasomatism of these peridotites are of Variscan age or may be older.