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.
