Cenozoic alkali basaltic magmas of western Germany and their products of differentiation

Analytical data on major elements and 31 trace elements in olivine nephelinites, nepheline basanites, basanitic alkali olivine basalts and their differentiates (tephrites, hawaiites, mugearites, benmoreites, latites, phonolites and trachytes) from Hegau, Kaiserstuhl, Rhön, Hessian Depression, Vogelsberg, Westerwald, Siebengebirge, E Eifel and Hocheifel are evaluated. They were based on 400 samples with new or unpublished data on about one third of the rocks. The Sr–Nd isotopic compositions for 78 rocks are included. The alkali basaltic volcanism is caused by adiabatic decompression of asthenospheric mantle updomed to a minimum depth of 50 km in connection with the Alpine continent collision. The chemical compositions of the primary basaltic melts from the different areas are similar containing about one hundred-fold enrichment of highly incompatible elements relative to the primitive mantle from partial melting of depleted and secondarily enriched peridotite. The elements Cs, K, Pb and Ti are specifically depleted in the basalts partly because of phlogopite being residual at partial melting. The Tertiary alkali basalts range in Nd-isotopic composition from 0.51288 to 0.51273 and in Sr-isotopic ratios from 0.7032 to 0.7042. These ranges indicate mixtures of HIMU, depleted and enriched mantle components in the metasomatically altered peridotite source which resembles that of certain ocean islands. The Nd-Sr-isotopic compositions of the Quaternary E Eifel are close to bulk Earth ratios. East and W Eifel plots differ distinctly from the Tertiary Hocheifel which is geographically intermediate. This isotopic difference, beside specific K/Na ratios, is probably caused by separate metasomatic pulses that immediately preceded the respective periods of volcanism. The metasomatically altered mantle had partly primitive mantle signatures (Nb/Ta, Zr/Sm and Th/U ratios) and partly ocean island (or MORB) source properties (Rb/Cs). A MORB source can be excluded because of the low K/Rb and high Th/U ratios. A correlation of D with ⁸⁷Sr/⁸⁶Sr in amphibole and phlogopite and a slightly larger ¹⁸O than in MORB is conformable with a seawater and crustal impact on the source of alkali basalts. Slightly higher than average water concentrations in the source of certain primary basaltic melts (indicated by amphibole phenocrysts in their basalts) are required for differentiation of these basalts in magma chambers of the upper crust. Model calculations are presented to explain compositions of differentiates which range from about 60% to about 20% residual melt. The latter are represented by phonolites and trachytes. The Nd- and Sr-isotopic signatures of the majority of differentiates indicate contamination by a granitic partial melt from the wall rocks of magma chambers. Olivine nephelinite magma was the common source of contaminated differentiates.