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

Major- and trace-element and radiogenic and oxygen isotope data are present ed for an alkaline suite (nephelinites, basanites, hornblende-basalts) from the Rhon area (Germany) that belongs to the European volcanic province, Co mpositions of some basanites and hornblende-basalts are primitive whereas o thers show decreasing Cr, Ni, MgO and TiO2 contents and CaO/Al2O3 ratios bu t increasing Al2O3 and incompatible elements (Sr, Zr, Nb, Y, Ce) with incre asing SiO2 indicating fractionation of mainly olivine, clinopyroxene and am phibole. On the other hand, nephelinites are products of increasing degrees of melting as is shown by their decreasing Al2O3 and incompatible element content but increasing CaO/TiO2 ratio with increasing SiO2, Some chemical f eatures of the primitive members, i.e. high and decreasing CaO/Al2O3 ratios and decreasing La but increasing Yb abundances in the sequence nephelinite -basanite-hornblende-basalt can be explained by different degrees of meltin g of a garnet-bearing source in which garnet is progressively eliminated du ring 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 attri buted 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 S r-87/Sr-86 and Nd-143/Nd-144 ratios ranging from 0.70325 to 0.70396 and fro m 0.51279 to 0.51287, respectively. Hornblende-basalts have a larger spread in 87Sr/86Sr ratios but a Similar spread in Nd-143/Nd-144 ratios which var y between 0.70339 and 0.70420 and 0.51279 and 0.51284, respectively. Sr iso topic composition of the hornblende-basalts are positively correlated with SiO2 whereas the Nd isotopic compositions are positively correlated with Mg O, indicating that interaction with crustal rocks during fractionation has occurred. The range of alteration-corrected oxygen isotope ratios shows del ta(18)O values between 5.2 and 7.4 parts per thousand in which a negative c orrelation of delta(18)O values with Nd isotope composition suggests that t he higher delta(18)O values are the result of crustal contamination, Howeve r, pristine values of 5.2 and 5.8 parts per thousand indicate two distinct mantle endmembers with respect to the oxygen isotopic composition. Variatio ns between incompatible trace-element ratios (e.g. La/Nb) and Sr and Nd iso topic compositions suggest that the hornblende-basalts are derived from HIM U sources but experienced crustal contamination whereas the basanites and n ephelinites are probably products of HIMU and EM sources, Miocene intraplat e basaltic volcanism in the Rhon area occurred probably as a result of mino r plume activity coupled with lithospheric extension and some lower lithosp heric thermo-mechanical erosion by the underlying plume head. The basalts s how a spatial variation in their chemical composition: the early-stage basa nites and hornblende-basalts have chemical signatures more compatible with a. lower lithospheric source whereas the late-stage nephelinites probably o riginate from asthenospheric souces. This model, which involves small-scale plume impact followed by continental extension and asthenosphere-lithosphere interaction together with minor cr ustal contamination, should also be applicable to other intra-continental r ift-related areas. (C) 2000 Elsevier Science B.V, All rights reserved.