Petrology and Geochronology of foidites and melilitites in SW Germany and E France

Foidites and melilitites are strongly SiO2-undersaturated rocks that form by extremely low degrees of partial melting of the metasomatically overprinted lithospheric mantle. In Central Europe, they occur in volcanic fields, dike swarms or as isolated stocks and diatremes. Our detailed study on foidites from SW Germany indicates two distinct age groups with marked differences in mineralogy and mineral chemistry: Based on in-situ U Pb age data (apatite, perovskite, zircon) a Miocene cohort (~ 9–19 Ma) of predominantly olivine melilitites and melilite-bearing nephelinites can be distinguished from a much older Upper Cretaceous to Lower Eocene group (~ 48–68 Ma) of melilite-free nephelinites and nepheline basanites. This contrasts with previous K-Ar whole-rock and mineral data suggesting continuous magmatism between 90 and 6 Ma. The older group is characterized by the frequent occurrence of green core pyroxenes, hydroxyapatite, and minor feldspar, whereas the younger group contains melilite, late magmatic fluorapatite, Ba- and F-rich mica and occasionally perovskite, but no feldspar. It crops out in the Freiburger Bucht and the Bonndorf Graben, the Vosges (France), the Odenwald and Kraichgau region, in the Taunus and the Lower Main Plain, whereas the younger group is represented by occurrences in the Hegau, the Urach region and the Central Upper Rhine Graben including the Kaiserstuhl. As part of the Central European Volcanic Province, the spatial distribution and age of these rocks reflect regional tectonic events, while the petrologic contrasts between the two age groups indicate heterogeneous crystallization conditions and/or magma source variations such as different formation depths.