Granulite Xenoliths From the Pannonian Basin: Evidence for the Composition and Evolution of the Lower Crust Beneath Alpine Europe

P. D. Kempton NERC Isotope Geosciences Laboratory, Keyworth NG12 5GG, UK


H. Downes Birkbeck College, Malet Street, London WC1E 7HX, UK

The lower crust of western Europe has been relatively well studied in France and Germany, where Neogene magmatism associated with intra-continental rifting brought granulite xenoliths to the surface. In Alpine regions of Europe, the crust and lithosphere are thickened due to continent-continent collision. As a result, samples of the present day lower crust are extremely rare. However, in the Pannonian Basin of Hungary, a region of Alpine crust has become extremely extended and thinned due to orogenic collapse and escape of the Carpathians toward the northeast. At this time, subduction occurred towards the southwest, and Neogene calcalkaline magmatism formed the inner Carpathian arc around the north and east of the Pannonian Basin. Alkaline volcanism occurred across the Pannonian Basin in response to late Tertiary (~12 to <0.5Ma) extension.

Lower crustal xenoliths were entrained in the Tertiary alkaline volcanics at several localitites in this area of thinned crust. Here we report on xenoliths from two of these localities, Szigliget and Bondorohegy. The xenoliths are medium pressure mafic granulites (plagioclase + pyroxene ± garnet) which range from opx-free ('gabbroic') to two-pyroxene and garnet-bearing. Garnet granulites are found only at Szigliget and opx-free lithologies are restricted to Bondorohegy, but in other respects the xenoliths from the two localities are mineralogically indistinguishable. However, 2 groups can be distinguished on the basis of chemistry. One group is LREE-depleted and has Sr-, Nd- and O-isotope ratios typical of depleted mantle. In contrast, the other group tends to be more LREE-enriched and have higher 87Sr/86Sr (0.706-0.709), lower 143Nd/144Nd (0.5128-0.5123) and higher d18O (+7.5 to +10.4”). The LREE-depleted group typically has higher MgO contents than the LREE-enriched group, but Mg#'s for the two groups are remarkably similar. The LREE-depleted group also has lower 207Pb/204Pb and 208Pb/204Pb for a given 206Pb/204Pb, and extends to lower 206Pb/204Pb values than the LREE-enriched group. Mg# vs SiO2/Al2O3 systematics, combined with trace element arguments, suggest that the LREE-depleted granulites have near melt-like compositions, whereas most of the LREE-enriched granulites had magmatic cumulate protoliths. One unusual sample containing normative corundum is interpreted as a restite.

Isotope systematics (e.g. Sr vs O, Sr vs Pb, Nd vs Pb, Pb vs Pb and 143Nd/144Nd vs Sm/Nd) can be explained as mixing between a depleted and an enriched component. One obvious interpretation of these results is that the granulite suite records mixing between basaltic magmas and pre-existing continental crust as a result of magmatic underplating during the Tertiary. However, most of the granulite xenoliths have Sr and Nd isotope compositions which differ from those of the Tertiary/Quaternary magmas of the Pannonian - Carpathian region. The host basalts typically have lower 143Nd/144Nd for a given 87Sr/86Sr than the xenoliths and thus do not constitute an appropriate depleted end member. Instead, the most depleted granulite xenoliths have higher 143Nd/144Nd and fall within the field of local lithospheric upper mantle, as represented by spinel peridotite xenoliths. Although the present day isotopic compositions of the calc-alkaline volcanics have 143Nd/144Nd values too low and 86Sr/87Sr too high to be the depleted end member, these lavas were strongly contaminated during passage through the crust, and it is possible that the primary magmas of the calc-alkaline suite had much lower 87Sr/86Sr and higher 143Nd/144Nd. Thus, the depleted end member may be associated with this earlier and more widespread Miocene calc-alkaline activity rather than the younger alkaline volcanism.

Although the enriched end member is isotopically consistent with Hercynian basement, simple mixing between Hercynian felsic crust and a mafic calc-alkaline magma fails to explain several aspects of the data. Mg#s are similar throughout the suite and do not correlate with apparent degree of contamination (e.g. sample SZG3018 has a high d18O value of +10 ” and an Mg# of 62). Isotope systematics also suggest that the material assimilated had lower concentrations of Sr, Nd and Pb than the melt. Middle/upper crust typically has higher concentrations of these elements relative to mafic calc-alkaline magmas. We therefore suggest that the Pannonian Basin granulite suite records mixing between mafic calc-alkaline melts and lower crust having high Sr-, Pb- and O-isotopes and low 143Nd/144Nd. This lower crust may simply represent the earliest phase of magmatic underplating during the Tertiary, such that the earliest underplated magmas were the most contaminated and successive magmatic injections interacted with less contaminated, more mafic crust. Alternatively, the assimilated lower crust may have been formed during an older Hercynian magmatic event, perhaps linked with the formation of Hercynian