Petrogenesis of Tertiary Continental Intra-plate Lavas from the Westerwald Region, Germany

Abstract

Tertiary volcanic rocks from the Westerwald region range from basanites and alkali basalts to trachytes, whereas lavas from the margin of the Vogelsberg volcanic field consist of more alkaline basanites and alkali basalts. Heavy rare earth element fractionation indicates that the primitive Westerwald magmas probably represent melts of garnet peridotite. The Vogelsberg melts formed in the spinel---garnet peridotite transition region with residual amphibole for some magmas suggesting melting of relatively cold mantle. Assimilation of lower-crustal rocks and fractional crystallization altered the composition of lavas from the Westerwald and Vogelsberg region significantly. The contaminating lower crust beneath the Rhenish Massif has a different isotopic composition from the lower continental crust beneath the Hessian Depression and Vogelsberg, implying a compositional boundary between the two crustal domains. The mantle source of the lavas from the Rhenish Massif has higher 206 Pb/ 204 Pb and 87 Sr/ 86 Sr than the mantle source beneath the Vogelsberg and Hessian Depression. The 30---20Ma volcanism of the Westerwald apparently had the same mantle source as the Quaternary Eifel lavas, suggesting that the magmas probably formed in a pulsing mantle plume with a maximum excess temperature of 100 � C beneath the Rhenish Massif. The relatively shallow melting of amphibole-bearing peridotite beneath the Vogelsberg and Hessian Depression may indicate an origin from a metasomatized portion of the thermal boundary layer.