The Petrogenesis of Pliocene Alkaline Volcanic Rocks from the Pannonian Basin, Eastern Central Europe

Late Tertiary post-orogenic alkaline basalts erupted in theextensional Pannonian Basin following Eocene-Miocene subductionand its related calc-alkaline volcanism. The alkaline volcaniccentres, dated between 11•7 and 1•4 Ma, are concentratedin several regions of the Pannonian Basin. Some are near thewestern (Graz Basin, Burgenland), northern (Ngrd), and eastern(Transylvania) margins of the basin, but the majority are concentratednear the Central Range (Balaton area and Little Hungarian Plain).Fresh samples from 31 volcanic centres of the extension-relatedlavas range from slightly hy-normative transitional basaltsthrough alkali basalts and basanites to olivine nephelinites.No highly evolved compositions have been encountered. The presenceof peridotite xenoliths, mantle xenocrysts, and high-pressuremegacrysts, even in the slightly more evolved rocks, indicatesthat differentiation took place within the upper mantle. Rare earth elements (REE) and ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ¹⁸O, D,and Pb isotopic ratios have been determined on a subset of samples,and also on clinopyroxene and amphibole megacrysts. Sr and Ndisotope ratios span the range of Neogene alkali basalts fromwestern and central Europe, and suggest that the magmas of thePannonian Basin were dominantly derived from asthenosphericpartial melting, but Pb isotopes indicate that in most casesthey were modified by melt components from the enriched lithosphericmantle through which they have ascended. ¹⁸O values indicatethat the magmas have not been significantly contaminated withcrustal material during ascent, and isotopic and trace-elementratios therefore reflect mantle source characteristics. Incompatible-elementpatterns show that the basic lavas erupted in the Balaton areaand Little Hungarian Plain are relatively homogeneous and areenriched in K, Rb, Ba, Sr, and Pb with respect to average oceanisland basalt, and resemble alkali basalts of Gough Island.In addition, ²⁰⁷Pb/²⁰⁴Pb is enriched relative to ^2O6Pb/²⁰⁴Pb.In these respects, the lavas of the Balaton area and the LittleHungarian Plain differ from those of other regions of Neogenealkaline magmatism of Europe. This may be due to the introductionof marine sediments into the mantle during the earlier periodof subduction and metasomatism of the lithosphere by slab-derivedfluids rich in K, Rb, Ba, Pb, and Sr. Lavas erupted in the peripheralareas have incompatible-element patterns and isotopic characteristicsdifferent from those of the central areas of the basin, andmore closely resemble Neogene alkaline lavas from areas of westernEurope where recent subduction has not occurred.