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

Late Tertiary post-orogenic alkaline basalts erupted in the extensiona l 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 (N ograd), and eastern (Transylvania) margins of the basin, but the major ity are concentrated near the Central Range (Balaton area and Little H ungarian Plain). Fresh samples from 31 volcanic centres of the extensi on-related lavas range from slightly hy-normative transitional basalts through alkali basalts and basanites to olivine nephelinites. No high ly evolved compositions have been encountered. The presence of peridot ite xenoliths, mantle xenocrysts, and high-pressure megacrysts, even i n 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 bee n determined on a subset of samples, and also on clinopyroxene and amp hibole megacrysts. Sr and Nd isotope ratios span the range of Neogene alkali basalts from western and central Europe, and suggest that the m agmas of the Pannonian Basin were dominantly derived from asthenospher ic partial melting, but Pb isotopes indicate that in most cases they w ere modified by melt components from the enriched lithospheric mantle through which they have ascended. DeltaO-18 values indicate that the m agmas have not been significantly contaminated with crustal material d uring 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 Plai n are relatively homogeneous and are enriched in K, Rb, Ba, Sr, and Pb with respect to average ocean island basalt, and resemble alkali basa lts of Gough Island. In addition, Pb-207/Pb-204 is enriched relative t o 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 Neoge ne alkaline magmatism of Europe. This may be due to the introduction o f marine sediments into the mantle during the earlier period of subduc tion and metasomatism of the lithosphere by slab-derived fluids rich i n K, Rb, Ba, Pb, and Sr. Lavas erupted in the peripheral areas have in compatible-element patterns and isotopic characteristics different fro m those of the central areas of the basin, and more closely resemble N eogene alkaline lavas from areas of western Europe where recent subduc tion has not occurred.