Geochronology and petrogenesis of hornblende-bearing basalts (Rhön, Germany) - remnants of volcanic rocks from the lithospheric upper mantle)

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The proposed project deals with the geochronology, geochemistry and isotope composition of alkali olivine basalts s. /. that represent a significant part of the volcanic rocks in the Rhön area (Germany). In contrast to other volcanic occurrences in central Europe, most of these alkaline volcanic rocks contain mm- to cm-sized hornblende phenocrysts beside clinopyroxene (some with green cores) and olivine. Based on these unusual mineralogical composition and some geochemical features, i.e. high TiO2 contents > 3.0 wt%, it can be suggested that the hornblendebearing basalts originate from sources distinct to those envisaged for other alkaline rocks from central Europe, i.e. probably representing melts of the subcrustal lithospheric mantle. The volcanic rocks will be investigated for Nd, Sr, Pb, Hf, Os and O whole rock isotopes and the major and trace element composition of the hornblende phenocrysts will be determined by using an electron microprobe and an ionprobe. Ar-Ar age determinations on hornblende phenocrysts will complement the isotope data. In addition, it is planned to analyse the hornblende phenocrysts for Nd, Sr, Pb and O isotopes. Results obtained during this study will place constraints on possible heterogeneities of the upper mantle in rift-related areas and can also be used to distinguish second order processes (crustal contamination with or without fractional crystallization, magma mixing) from first order processes (partial melting of spinel or garnet peridotite with or without OH-bearing minerals etc.). The project is therefore ideally suited to add knowledge on lithosphere dynamics in rift-related continental setting.