Petrogenesis of Basanitic to Tholeiitic Volcanic Rocks from the Miocene Vogelsberg, Central Germany

The Miocene Vogelsberg volcano in Central Germany produced mafic magmas ranging in composition from basanite to quartz tholeiite and limited amounts of evolved magmas. Trace element and Nd, Sr and Pb isotopic compositions reveal the presence of three distinct mantle sources: (1) a trace element enriched, asthenospheric plume-type source, similar to the European Asthenospheric Reservoir composition inferred for many other Tertiary volcanic provinces in Central Europe; (2) a depleted mantle source, located in the lithospheric mantle or uppermost asthenosphere; (3) a veined lithospheric mantle source. The oldest basanites of the Vogelsberg volcano have distinctly higher Ti, Al, Sc and V contents than younger basanites. These high-Ti basanites may have been produced by partial melting of a veined lithospheric mantle source, formed during the earliest stages of uplift of the Rhenish Shield, ∼70 Myr ago. Younger basanites were generated by small degrees of partial melting of the European Asthenospheric Reservoir, whereas alkali basalts and tholeiites formed by mixing of variable proportions of melts derived from the European Asthenospheric Reservoir and depleted mantle sources, respectively. These magmas then interacted with metasomatized sub-continental lithospheric mantle, which explains the observed range in Sr, Nd and Pb isotopic compositions. Subsequently the most depleted tholeiites were contaminated by lower-crustal rocks. The distinct stratigraphic position of the various lava groups in the 656·5 m ‘Forschungsbohrung Vogelsberg 1996’ borehole and the correlation of their chemical stratigraphy with palaeomagnetic reversals reflects an episodic temporal evolution of magmas and mantle sources. During Stage I, melts from the veined lithospheric mantle source were pooled in crustal magma chambers and evolved to erupt a range of differentiated lavas. In Stage II melts were formed in the depleted mantle source and up-section gradually mixed with melts from the asthenospheric mantle. In Stage III the depleted mantle source was exhausted and pure asthenospheric melts were erupted.