Recurrent Local Melting of Metasomatised Lithospheric Mantle in Response to Continental Rifting: Constraints from Basanites and Nephelinites/Melilitites from SE Germany

Cenozoic primitive basanites, nephelinites and melilitites from the Heldburg region, SE Germany, are high-MgO magmas (8.5-14.1 wt % MgO), with low SiO2 (34.2-47.1 wt %) and low to moderately high Al2O3 (9.0-15.5 wt %) and CaO (8.7-12.7 wt %). The Ni and Cr contents of most samples are up to 470 ppm and 640 ppm, respectively, and match those inferred for primary melts. In multi-element diagrams, all samples are highly enriched in incompatible trace elements with chondrite-normalised La/Yb = 19-45, strongly depleted in Rb and K, with primitive mantle normalised K/La = 0.15-0.72, and moderately depleted in Pb. The initial Sr-Nd-Hf isotope compositions (Sr-87/Sr-86 = 0.7033-0.7051, Nd-143/Nd-144 = 0.51279-0.51288 and Hf-176/Hf-177 = 0.28284-0.28294) fall within the range observed for other Tertiary volcanic rocks of the Central European Volcanic Province, whereas Pb-208/Pb-204 and Pb-206/Pb-204 (38.42-38.88 and 18.49-18.98) are distinctly lower at comparable Pb-207/Pb-204 (15.60-15.65). Trace element modelling and pressure-temperature estimates based on major element compositions and experimental data suggest that the nephelinites/melilitites formed within the lowermost lithospheric mantle, close to the lithosphere-asthenosphere boundary, by similar to 3-5% partial melting of a highly enriched, metasomatised, carbonated phlogopite-bearing garnet-lherzolite at temperatures 4 GPa infiltrated the thermal boundary layer at the base of the lithospheric mantle and imprinted a crustal lead isotope, and to a minor extent crustal Sr, Nd and Hf isotope signatures. They also reduced Nb/U, Ce/Pb, Lu/Hf, Sm/Nd, U/Pb and Th/Pb, but increased Rb/Sr and Nb/Ta and amplified the enrichment of LILE and LREE relative to HREE. This lead to the highly-enriched trace element patterns observed in both sample suites, and to overall less radiogenic Pb-206/Pb-204 and 208Pb/204Pb compared to other continental basalts in Central Europe, and to less radiogenic Hf-176/Hf-177 and Nd-143/Nd-144 that plot distinctly below the terrestrial mantle array. Temporal evolution of magmatism in the Heldburg region coincides with the changing Tertiary intraplate stress field in Central Europe, which developed in response to the Alpine orogeny. Magmatism was most probably caused in response to lithosphere deformation and perturbation of the thermal boundary layer, and not by actively upwelling asthenosphere