Petrogenesis of rift-related tephrites, phonolites and trachytes (Central European Volcanic Province, Rhön, FRG): Constraints from Sr, Nd, Pb and O isotopes
The volcanic rocks of the Rhon area (Central European Volcanic Province, Germany) belong to a moderately alkali basaltic suite that is associated with minor tephriphonolites, phonotephrites, tephrites, phonolites and trachytes. Based on isotope sytematics (Sr-87/Sr-86: 0.7033-0.7042; Nd-143/Nd-144: 0.51279-0.51287; Pb-206/Pb-204: 19.1-19.5), the inferred parental magmas formed by variable degrees of partial melting of a common asthenospheric mantle source (EAR: European Asthenospheric Reservoir of Cebria and Wilson, 1995). Tephrites, tephriphonolites, phonotephrites, phonolites and trachytes show depletions and enrichments in some trace elements (Sr, Ba, Nb, Zr, Y) indicating that theywere generated by broadly similar differentiation processes thatwere dominated by fractionation of olivine, clinopyroxene, amphibole, apatite and titaniferous magnetite +/- plagioclase +/- alkalifeldspar. The fractionated samples seem to have evolved by two distinct processes. One is characterized by pure fractional crystallization indicated by increasing Nb (and other incompatible trace element) concentrations at virtually constant Nd-143/Nd-144 similar to 0.51280 and Sr-87/Sr-86 similar to 0.7035. The other process involved an assimilation-fractional crystallization (AFC) process where moderate assimilation to crystallization rates produced evolved magmas characterized by higher Nb concentrations at slightly lower Nd-143/Nd-144 down to 0.51275. Literature data for some of the evolved rocks showmore variable 87Sr/86Sr ranging from0.7037 to 0.7089 at constant Nd-143/Nd-144 similar to 0.51280. These features may result from assimilation of upper crustal rocks by highly differentiated low-Sr (<100 ppm Sr) lavas. However, based on the displacement of the differentiated rocks fromthis study towards lower Nd-143/Nd-144 ratios andmodeled AFC processes in Nd-143/Nd-144 vs. Sr-87/Sr-86 and Pb-207/Pb-204 vs. Nd-143/Nd-144 space assimilation of lower crustal rocks seems more likely. The view that assimilation of lower crustal rocks played a role is confirmed by high-precision double-spike Pb isotope data that reveal higher Pb-207/Pb-204 ratios (15.62-15.63) in the differentiated rocks than in the primitive basanites (15.58-15.61). This is compatible with incorporation of radiogenic Pb from lower crustal xenoliths (Pb-207/Pb-204: 15.63-15.69) into the melt. However, Pb-206/Pb-204 ratios are similar for the differentiated rocks (19.13-19.35) and the primitive basanites (19.12-19.55) implying that assimilation involved an ancient crustal end member with a higher U/Pb ratio than the mantle source of the basanites. In addition, alterationcorrected delta O-18 values of the differentiated rocks range from c. 5 to 7\% which is the same range as observed in the primitive alkaline rocks. This study confirms previous interpretations that highlighted the role of AFC processes in the evolution of alkaline volcanic rocks in the Rhon area of the Central European Volcanic Province. (C) 2013 Elsevier B.V. All rights reserved.