Sodic alkaline Neogene-Recent mafic magmas across western and central Europe (e.g. in the Massif Central, the Rhenish Shield, the North Hessian Depression and the Ohre Rift) appear to be derived by partial melting of a common mantle source component, which we term the European Asthenospheric Reservoir (EAR) (Cebriá and Wilson, 1995). This source, with an isotopic composition of 87Sr/86Sr×0.7030, 143Nd/144Nd×0.5130, 206Pb/204Pb×19.90, 207Pb/204Pb×15.65 and 208Pb/204Pb× 39.30, is considered to be related to the activity of a HIMU-mantle plume (Granet et al., 1996).
In most areas magmatism initiated during the Eocene and Oligocene, with the main phase of activity in Neogene-Recent times. In the upper Rhine Graben (Black Forest, Vosges, Odenwald and Taunus), however, mafic magmatism occurs periodically and regularly back to 85Ma (Lippolt et al., 1974). A major and trace element and Sr-Nd isotopic investigation of the Rhinegraben magmatism was undertaken to see how far back this EAR source could be traced geochemically. Over 50 samples were collected. The largest time gap between samples is 15Ma (between 65Ma and 80Ma). A subset of 16 samples for which isotopic analysis were performed (selected to represent the complete range of dated localities) extend the EAR source back to 85Ma and thus generate a minimum age for initial plume impact. For example, at 41Ma (Waldberghohe), 53Ma (Forst) and 85Ma (Vordermarbach), 143Nd/144Nd ratios of 0.51293, 0.51292 and 0.51290 were recorded respectively. The variation from EAR values are explained by source evolution and minor lithospheric contamination.
MORB-type magmatism is known from the Variscan Rhenohercynian basin at 360Ma (Wedepohl et al., 1983). This can be taken as a maximum age for emplacement of the EAR. Between the upper Jurassic and lower Cretaceous (160-130Ma), the European lithosphere experienced regional uplift, rifting and localised, extension related magmatism (e.g. the Dutch Waddenzee Complex). This association suggests the presence of a widespread thermal anomaly, i.e. a mantle plume, which may well have carried the EAR signal into the upper mantle at this time. A global increase in plume activity is well known during this period (Wilson, 1993).
Cebriá, J.M. & Wilson, M., Terra Abstracts 7, 162 (1995).
Granet, M., Wilson, M. & Achauer, U., Earth Planet. Sci. Lett. (1996, in press).
Lippolt, H.J., Todt, W. & Horn, P., In Approaches to Taphrogenesis (Illies, J.H. & Fuchs, K., eds.) 213-221 (Stuttgart, 1974).
Wedepohl, K.H., Meyer, K. & Muecke, G.K., In Intracontinental Fold Belts (Martin, H. & Eder, W., eds.) 231-256 (Springer, Heidelberg, 1983).
Wilson, M., J. Geol. Soc., London 150, 977-990 (1993).