Abstract
Magmatism in the Cenozoic Central European Volcanic Province (CEVP) has been related to two geodynamic scenarios, either extensional tectonics in the north Alpine realm or upwelling of deep mantle material. The Oligocene ( 30^19 Ma) Siebengebirge Volcanic Field (SVF) is a major part of the German portion of the CEVP and consists of erosional remnants of mafic to felsic volcanic edifices. It covers an area of 35 km (NW^SE) by 25 km (SW^NE) with eruptive centres concentrated near the eastern shore of the Rhine river in the vicinity of the city of Bonn. Mafic rocks in the SVF comprise strongly SiO2-undersaturated basanites to alkaline basalts. Occurrences of alkaline basalts are confined to an inner NW^SE-striking zone, whereas the more SiO2-undersaturated basanites dominate the western and eastern periphery of the SVF. Radiogenic isotope compositions (Sr/Sr 0·70335^ 0·70371; eNd þ3·1 to þ4·5; eHf þ6·5 to þ8·0; Pb/Pb 19·46^19·69; Pb/Pb 15·63^15·66; Pb/Pb 39·34^ 39·62) indicate a common asthenospheric mantle end-member with HIMU-like characteristics for all mafic rocks, similar to the European Asthenospheric Reservoir (EAR). A lithospheric mantle source component with a residual K-bearing phase (phlogopite or amphibole) is inferred from negative K anomalies. Incompatible trace element modelling indicates that melting took place in the spinel^garnet transition zone with low degrees of melting at higher pressures generating the basanitic magmas (LaN/YbN1⁄4 20^25), whereas the alkaline basalts (LaN/YbN1⁄414^18) are the result of higher melting degrees at shallower average melting depths. Differentiation of basanitic primary melts generated tephritic to tephriphonolitic magmas that, for instance, erupted at the Lo« wenburgVolcanic Complex in the central SVF. Latites and trachytes, such as the prominent Drachenfels andWolkenburg protrusions, are more common in the central portion of the SVF.These compositions originate from parental alkaline basaltic melts. All differentiated samples show evidence for crustal contamination, possibly with lowerto mid-crustal material comprising mafic granulites as found in Eifel basalt xenoliths and metapelites. Based on the spatial and temporal distribution of the various volcanic rock types, a model for the temporal evolution of the SVF can be proposed. During the initial phase of volcanism, low-degree basanitic melts were generated as a result of decompression following tectonic rifting and formation of the Cologne Embayment, a northward extension of the Rhine Graben. In a second stage, alkali basalts were generated at shallower depths and higher degrees of melting as a result of continued lithospheric thinning and passive upwelling of asthenospheric mantle. These conclusions strengthen previous models suggesting
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