THE PETROGENESIS OF PLIOCENE ALKALINE VOLCANIC-ROCKS FROM THE PANNONIAN BASIN, EASTERN CENTRAL-EUROPE

Late Tertiary post-orogenic alkaline basalts erupted in the extensional Pannonian Basin following Eocene-Miocene subduction and its related calc-alkaline volcanism. The alkaline volcanic centres, dated between 11.7 and 1.4 Ma, are concentrated in several regions of the Pannonian Basin. Some are near the western (Graz Basin, Burgenland), northern (Nograd), and eastern (Transylvania) margins of the basin, but the majority are concentrated near the Central Range (Balaton area and Little Hungarian Plain). Fresh samples from 31 volcanic centres of the extension-related lavas range from slightly hy-normative transitional basalts through alkali basalts and basanites to olivine nephelinites. No highly evolved compositions have been encountered. The presence of peridotite xenoliths, mantle xenocrysts, and high-pressure megacrysts, even in the slightly more evolved rocks, indicates that differentiation took place within the upper mantle. Rare earth elements (REE) and Sr-87/Sr-86, Nd-143/Nd-144, deltaO-18, deltaD, and Pb isotopic ratios have been determined on a subset of samples, and also on clinopyroxene and amphibole megacrysts. Sr and Nd isotope ratios span the range of Neogene alkali basalts from western and central Europe, and suggest that the magmas of the Pannonian Basin were dominantly derived from asthenospheric partial melting, but Pb isotopes indicate that in most cases they were modified by melt components from the enriched lithospheric mantle through which they have ascended. DeltaO-18 values indicate that the magmas have not been significantly contaminated with crustal material during ascent, and isotopic and trace-element ratios therefore reflect mantle source characteristics. Incompatible-element patterns show that the basic lavas erupted in the Balaton area and Little Hungarian Plain are relatively homogeneous and are enriched in K, Rb, Ba, Sr, and Pb with respect to average ocean island basalt, and resemble alkali basalts of Gough Island. In addition, Pb-207/Pb-204 is enriched relative to Pb-206/Pb-204. In these respects, the lavas of the Balaton area and the Little Hungarian Plain differ from those of other regions of Neogene alkaline magmatism of Europe. This may be due to the introduction of marine sediments into the mantle during the earlier period of subduction and metasomatism of the lithosphere by slab-derived fluids rich in K, Rb, Ba, Pb, and Sr. Lavas erupted in the peripheral areas have incompatible-element patterns and isotopic characteristics different from those of the central areas of the basin, and more closely resemble Neogene alkaline lavas from areas of western Europe where recent subduction has not occurred.