Geochemische und Isotopen-geochemische Untersuchungen an tertiären Vulkaniten der Hocheifel - Ein Beitrag zur Identifizierung der Mantelquellen von Rift-bezogenen Vulkaniten

Major, and trace element and radiogenic (Nd, Sr, Pb) isotope data are presented for an alkaline suite (nephelinites and basanites) from the Hocheifel area (Germany) that belongs to the Central European Volcanic Province (CEVP). Based on variations between compatible and incompatible trace elements and the Mg# (Mg#: Mg/Mg+Fe) two main suites can be distinguished. One suite has affinities to near-primary alkaline volcanic rocks with high MgO, Ni and Cr and low SiO2 concentrations of < 45 wt % whereas the other suite has lower Mg# < 0.65, mostly lower Ni and Cr concentrations and higher SiO2 concentrations (> 45 wt %). However, the most primitive samples from this suite still have Ni (250 ppm) and Cr (450 ppm) concentrations typical for undifferentiated mantle-derived volcanic rocks. Rare Earth Element concentrations are high with 100-300 x chondrite for LREE and c. 10 x chondrite for HREE. The inferred mantle source for the alkaline rocks is asthenospheric metasomatized garnet peridotite in which negative anomalies of Rb and K in primitive mantle-normalized diagrams and Rb/K vs. K, K/La and Ti/La covariations suggest that amphibole was the major OH-bearing mineral phase. Incompatible trace element ratios (La/Nb, Ba/Nb, Zr/Nb, Rb/Nb, K/Nb) indicate a common HIMU source for the primitive rocks whereas elevated trace element ratios of some samples indicate involvement of an enriched mantle source (EM) and/or crustal contamination. Trace element modeling indicates that at least three different suites occur in the Hocheifel area with estimated degrees of partial melting ranging from 2% to 8 % for the most primitive alkaline rocks. Constraints on the source composition are in agreement with previously published trace element concentrations obtained on mantle-derived peridotite xenoliths. Furthermore, trace element modeling indicate the presence of garnet and spinel together with significant amounts of amphibole and aptite suggesting a depth for melt generation of c. 60-75 km (20-25 kbar). Initial Nd (e Nd: +7 - +4), Sr (87Sr/86Sr: 0.7031-0.7035) and Pb (206Pb/204Pb: 19.3-19.8, 207Pb/204Pb: 15.54-15.63; 208Pb/204Pb: 39.0-39.6) isotope analyses for the most primitive samples lie between plume compositions and enriched mantle compositions and are similar to those measured in Ocean Island Basalts (OIB) and the Central European Volcanic Province elsewhere. These isotope compositions reflect the existence of a depleted endmember (depleted mantle, DM or high-m, HIMU) and an enriched endmember (enriched mantle, EM 1). More evolved samples show distinct isotope compositions in which the Nd isotope compositions become less radiogenic (e Nd: -1) and Sr isotope compositions become more radiogenic (87Sr/86Sr: 0.7046). 207Pb/204Pb isotope ratios for these samples are either less radiogenic (207Pb/204Pb: 15.25) or more radiogenic (207Pb/204Pb: 15.68) reflecting AFC processes involving at least two different lower crustal reservoirs during stagnation of the basalts within the lower crust. Miocene intraplate basaltic volcanism in the Hocheifel area occurred probably as a result of minor plume activity coupled with lithospheric extension and some lower lithospheric thermo-mechanical erosion by the underlying plume head. The basalts show a spatial variation in their chemical composition; the early-stage alkali basalts have chemical signatures more compatible with a lower lithospheric source whereas the late-stage alkali basalts probably originate from asthenospheric souces. This model, which involves small-scale plume impact followed by continental extension and asthenosphere-lithosphere interaction together with minor crustal contamination, should also be applicable to other intra-continental rift-related areas.