Volume 38: January - December 1997

Issue 7: July 1997

Abstract

Mafic granulite xenoliths in neogene alkali basalts from the Western Pannonian Basin: insights into the lower crust of a collapsed orogen

PD. Kempton, H. Downes² and A. Embey-Isztin³
¹NERC Isotope Geosciences Laboratory, Keyworth NG12 5GG, UK and ²Birkbeck/UCL Research School of Geological and Geophysical Sciences, Birkbeck College, Malet Street, London WC1E 7HX, UK³Department of Mineralogy and Petrology, Hungarian Natural History Museum, Ludovika Ter 2, 1083 Budapest, HungaryCorresponding author Email:P.Kempton@nigl.nerc.ac.uk
ABSTRACT
Mafic granulite xenoliths from Pliocene alkali basalts of western Hungary represent the lower crust of the Pannonian Basin. The xenolith suite from two localities, Szigliget and Bondorohegy, includes medium-pressure mafic granulites (plagioclase+pyroxene+-garnet) that range from gabbroic (i.e. orthopyroxene-free) to two-pyroxene and garnet-bearing lithologies. Two groups can be distinguished on the basis of chemistry: light rare earth element (LREE)-depleted granulites that have LREE-enriched granulites that have higher ⁸⁷Sr/⁸⁶Sr (0.706-0.709), lower ¹⁴³Nd/¹⁴⁴Nd (0.5128-0.5123) and higher [delta]¹⁸O (+7.5 to +10.4%_o). 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. The degree of LREE depletion shown by many of the Pannonian Basin granulites is uncommon among lower-crustal granulite xenoliths world wide and together with low La/Nb ratios suggests a mid-ocean ridge basalt (MORB)-like parental magma, possible originating as a back-arc basin tholeiite. Isotope systematics can be explained as a result of mixing between a depleted and an enriched component, but a direct genetic link between the granulites and either Tertiary calc-alkaline magmatism or the Neogene alkali basalts is ruled out as the most depleted granulite xenoliths have higher ¹⁴³Nd/¹⁴⁴Nd and fall within the field of local lithospheric upper mantle. The material assimilated during mixing had concentrations of Sr, Nd and Pb similar to or lower than those of the melt. This can be explained by mixing between mafic back-arc basin-like tholeiitic melts and pre-existing lower crust that has high Sr, Pb and O isotopes but low ¹⁴³Nd/¹⁴⁴Nd. The most crustally contaminated rocks have ¹⁴³Nd/¹⁴⁴Nd ratios that can only be explained by assimilation of an old component (<1 Ga). Thus, although the crustal block accreted to Europe in the Mesozoic was predominantly oceanic in character, it must have contained slivers of Precambrian crust.

Keywords: mafic granulites; lower crust; Pannonian Basin; geochemistry; isotopic composition.

Pages: 941 - 970

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