The major- and trace-element and isotope (Sr, Nd, O) geochemistry of Cenozoic alkaline rift-type volcanic rocks from the Rhön area (central Germany): petrology, mantle source characteristics and implications for asthenosphere–lithosphere interactions

Abstract

Major- and trace-element and radiogenic and oxygen isotope data are presented for an alkaline suite (nephelinites, basanites, hornblende-basalts) from the Rhön area (Germany) that belongs to the European volcanic province. Compositions of some basanites and hornblende-basalts are primitive whereas others show decreasing Cr, Ni, MgO and TiO 2 contents and CaO/Al 2O 3 ratios but increasing Al 2O 3 and incompatible elements (Sr, Zr, Nb, Y, Ce) with increasing SiO 2 indicating fractionation of mainly olivine, clinopyroxene and amphibole. On the other hand, nephelinites are products of increasing degrees of melting as is shown by their decreasing Al 2O 3 and incompatible element content but increasing CaO/TiO 2 ratio with increasing SiO 2. Some chemical features of the primitive members, i.e. high and decreasing CaO/Al 2O 3 ratios and decreasing La but increasing Yb abundances in the sequence nephelinite–basanite–hornblende-basalt can be explained by different degrees of melting of a garnet-bearing source in which garnet is progressively eliminated during melting. Negative anomalies of Rb and K in primitive mantle-normalized diagrams and Rb/K vs. K covariations suggest that amphibole was the major OH-bearing mineral phase. Variations in La/Nb and Zr/Nb ratios demonstrate at least two distinct sources in which low La/Nb–low Zr/Nb ratios are attributed to a HIMU-like asthenospheric source but high La/Nb–high Zr/Nb ratios correspond to an enriched upper mantle reservoir. This source is inferred to be lithospheric. Nephelinites and basanites have a restricted range in 87Sr/ 86Sr and 143Nd/ 144Nd ratios ranging from 0.70325 to 0.70396 and from 0.51279 to 0.51287, respectively. Hornblende-basalts have a larger spread in 87Sr/ 86Sr ratios but a similar spread in 143Nd/ 144Nd ratios which vary between 0.70339 and 0.70420 and 0.51279 and 0.51284, respectively. Sr isotopic composition of the hornblende-basalts are positively correlated with SiO 2 whereas the Nd isotopic compositions are positively correlated with MgO, indicating that interaction with crustal rocks during fractionation has occurred. The range of alteration-corrected oxygen isotope ratios shows δ 18O values between 5.2 and 7.4‰ in which a negative correlation of δ 18O values with Nd isotope composition suggests that the higher δ 18O values are the result of crustal contamination. However, pristine values of 5.2 and 5.8‰ indicate two distinct mantle endmembers with respect to the oxygen isotopic composition. Variations between incompatible trace-element ratios (e.g. La/Nb) and Sr and Nd isotopic compositions suggest that the hornblende-basalts are derived from HIMU sources but experienced crustal contamination whereas the basanites and nephelinites are probably products of HIMU and EM sources. Miocene intraplate basaltic volcanism in the Rhön area occurred probably as a result of minor plume activity coupled with lithospheric extension and some lower lithospheric thermo-mechanical erosion by the underlying plume head. The basalts show a spatial variation in their chemical composition; the early-stage basanites and hornblende-basalts have chemical signatures more compatible with a lower lithospheric source whereas the late-stage nephelinites probably originate from asthenospheric souces. This model, which involves small-scale plume impact followed by continental extension and asthenosphere–lithosphere interaction together with minor crustal contamination, should also be applicable to other intra-continental rift-related areas.