Pb-Sr-Nd isotopic compositions and trace element geochemistry of megacrysts and melilitites from the Tertiary Urach volcanic field: source composition of small volume melts under SW Germany

Abstract

 The Urach volcanic field is unique within the Tertiary–Quaternary European volcanic province (EVP) due to more than 350 tuffaceous diatremes and only sixteen localities with extremely undersaturated olivine melilitite. We report representative Pb-Sr-Nd isotopic compositions and incompatible trace element data for twenty-two pristine augite, Cr-diopside, hornblende, and phlogopite megacryst samples from the diatremes, and seven melilitite whole rocks. The Pb isotopic compositions for melilitites and comagmatic megacrysts have very radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios of 19.4 to 19.9 and plot on the northern hemisphere mantle reference line (NHRL). The data indicate absence of an old crustal component as reflected in the high ²⁰⁷Pb/²⁰⁴Pb ratios of many basalts from the EVP. This inference is supported by ²⁰⁶Pb/²⁰⁴Pb ratios of ∼17.6 to 18.3 and ɛ_Nd of ∼−7.8 to +1.6 for five phlogopite xenocryst samples reflecting a distinct and variably rejuvenated lower Hercynian basement. The ⁸⁷Sr/⁸⁶Sr ratios of 0.7033 to 0.7035 in the comagmatic megacrysts are low relative to their moderately radiogenic Nd isotopic compositions (ɛ_Nd +2.2 to +5.1) and consistent with a long-term source evolution with a low Rb/Sr ratio and depletion in light rare-earth elements (LREE). The melilitite whole-rock data show a similar range in Nd isotopic ratios as determined for the megacrysts but their Sr isotopic compositions are often much more radiogenic due to surface alteration. The REE patterns and incompatible trace element ratios of the melilitites (e.g. Nb/Th, Nb/U, Sr/Nd, P/Nd, Ba/Th, Zr/Hf) are similar to those in ocean island basalts (OIB); negative anomalies for normalized K and Rb concentrations support a concept of melt evolution in the lithospheric mantle. Highly variable Ce/Pb ratios of 29 to 66 are positively correlated with La/Lu, La/K₂O, and Ba/Nd and interpreted to reflect melting in the presence of residual amphibole and phlogopite. The data suggest an origin of the melilitites from a chemical boundary layer very recently enriched by melts from old OIB sources. We suggest that the OIB-like mantle domains represent low-temperature melting heterogeneities in an upwelling asthenosphere under western Europe.