Ž . Journal of Volcanology and Geothermal Research 86 1998 151–177 Major- and trace-element systematics and isotope geochemistry of ž Cenozoic mafic volcanic rocks from the Vogelsberg central / Germany Constraints on the origin of continental alkaline and tholeiitic basalts and their mantle sources S. Jung a, ) , P. Masberg b a Max-Planck-Institut fur Chemie, Abt. Geochemie, Postfach 3060, 55020 Mainz, Germany ¨ b Institut fur Mineralogie, Kristallographie und Petrologie, Philipps-UniÕersitat Marburg, Lahnberge, Hans-Meerwein-Straße, 35032 ¨ ¨ Marburg, Germany Received 6 November 1997; accepted 26 June 1998 Abstract Ž . The Cenozoic basaltic province of the Vogelsberg area central Germany is mainly composed of intercalated olivine to quartz tholeiites and near-primary nephelinites to basanites. The inferred mantle source for the alkaline and tholeiitic rocks is asthenospheric metasomatized garnet peridotite containing some amphibole as the main hydrous phase. Trace element modelling indicates 2 to 3% partial melting for the alkaline rocks and 5 to 7% partial melting for the olivine tholeiites. Incompatible trace element abundances and ratios as well as Nd and Sr radiogenic isotope compositions lie between plume Ž . compositions and enriched mantle compositions and are similar to those measured in Ocean Island Basalts OIB and the Central European Volcanic Province elsewhere. The mafic olivine tholeiites have similar BarNb, BarLa and Nd–Sr isotope ratios to the alkaline rocks indicating derivation of both magma types from chemically comparable mantle sources. However, ZrrNb ratios are slightly higher in olivine tholeiites than in basanites reflecting some fractionation of Zr relative to Nb during partial melting. Quartz tholeiites have higher BarNb, ZrrNb, LarNb, but lower CerPb ratios and lower Nd isotope Ž . compositions than the alkaline rocks which can be explained by interaction of the basaltic melt with lower granulite facies crustal material or partial melts thereof during stagnation within the lower crust. It appears most likely that upwelling of hot, asthenospheric material results in the generation of primitive alkaline rocks at the base of the lithosphere at depths of 75–90 km. Lithospheric extension together with minor plume activity and probably lower lithosphere erosion induced melting of shallower heterogenous upper mantle generating a spectrum of olivine tholeiitic melts. These olivine tholeiitic rocks evolved via crystal fractionation and probably limited contamination to quartz tholeiites. q 1998 Elsevier Science B.V. All rights reserved. Ž . Keywords: continental basalts; isotope geochemistry; Vogelsberg central Germany ) Corresponding author. Fax: q49-6131-371-051; E-mail: sjung@mailserver.mpch-mainz.mpg.de 0377-0273r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII: S0377-0273 98 00087-0