Tertiary--Quaternary Extension-Related Alkaline Magmatism in Western and Central Europe

Primitive mafic alkaline volcanic rocks from the Tertiary-Quaternaryextension-related magmatic province of western and central Europehave major and trace element and Sr–Nd–Pb isotopiccharacteristics which suggest the involvement of both lithosphericand asthenospheric mantle source components in their petrogenesis.Oxygen isotope data for the volcanic rocks, mantle xenoliths,and lower-crustal mafic xenoliths indicate that extensive crustalcontamination is not involved in the petrogenesis of these magmas. The geochemical characteristics of the lithospheric componentare in part constrained by those of spinel lherzolite and maficgranulite xenoliths entrained within the magmas. This componentappears to be the product of partial melting of phlogopite/amphibole-bearingmantle which was meta-somatized as a consequence of magmaticactivity during and preceding the Hercynian orogeny and duringphases of Permo-Carboniferous extension. Partial melting ofphlogopite is required to account for the generation, of potassicmagmas (leucitites and leucite nephelinites) with K₂O/Na₂O>1. This component appears to differ between the individual Hercynianterrane blocks of Europe, reflecting, in part, their previousmagmatic histories. The asthenospheric component has affinities with the sourceof St. Helena type (HIMU) OIB and may, in part, be 500–400 Ma recycled oceanic lithosphere subducted during the Hercynianorogeny. Alternatively, it could represent a zone of enrichedmantle at the base of the subcontinental lithosphere which ispreferentially partially melted during extension. There appearsto be no need to invoke the existence of deep mantle plumesto explain the HIMU characteristics, although the data do notpreclude them.