6533b856fe1ef96bd12b316d

RESEARCH PRODUCT

Geodynamic Setting of the Tertiary Hocheifel Volcanism (Germany), Part II: Geochemistry and Sr, Nd and Pb Isotopic Compositions.

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Major and trace element as well as isotopic compositions on 26 volcanic rocks from the Tertiary Hocheifel volcanic field and for comparison from Upper Rhine Graben occurrences (western Central European Volcanic Province) were measured in order to provide geochemical evidence for the geodynamic setting of the Tertiary Eifel volcanism. Except for a few differentiated lavas there are mainly basanitic compositions. These rocks were produced by low degree partial melting of a previously metasomatized garnet peridotite source at pressures and temperatures corresponding to depths of about 75 to 90 km. In contrast to the differentiated lavas, most of the basanites are not significantly affected by crustal contamination. In the Hocheifel, the crustally contaminated basanites and the differentiated lavas form an older age group (ca. 44-39 Ma). In contrast, the basanites not affected by crustal contamination belong to a younger age group (ca. 37-35 Ma) indicating a change in the petrogenetic style of evolution with time. Low-radiogenic Sr together with high-radiogenic Nd and Pb of the Hocheifel lavas show isotope characteristics similar to LVC (Low Velocity Composition) or FOZO (Focal Zone) mantle sources. Since FOZO is considered to be a common component in the entire mantle, a contribution from the deep mantle can be neither confirmed nor excluded. Plate reconstruction modelling indicates that the Hocheifel volcanic field at its time of activity ca. 40 m.y. ago was located ∼1000 km southwest relative to its recent position, and therefore a relation to a mantle-stationary Quaternary Eifel plume is not plausible. Integrating evidence from geochronology, geochemistry, isotopic compositions and plate reconstruction, we conclude that the Tertiary Hocheifel volcanism is not genetically related to a mantle plume but is caused by Middle to Upper Eocene pre-rift extensional decompression related to the Rhine Graben taphrogenetic evolution.