Mafic granulites and clinopyroxenite xenoliths from the Transdanubian Volcanic Region (Hungary): implications for the deep structure of the Pannonian Basin

Abstract

The Transdanubian Volcanic Region (TVR) is composed mainly of Pliocene alkali basalts, basanites, olivine basalts and olivine tholeiites, as well as rare nephelinites. The partial melting and genesis of alkali basaltic liquids is a consequence of an upwelling of the upper mantle which also caused thinning of the lithosphere and recent sinking of the Pannonian Basin.Four different types of lower crustal and upper-mantle xenoliths are found within the TVR: garnetfree and garnet-bearing granulites, clinopyroxenites and spinel lherzolites. We present mineralogical and geochemical data on granulite facies and clinopyroxenite xenoliths from three localities in the Hungarian part of the TVR (Bondor6hegy, Szentb6k~illa and Szigliget). It is concluded that, whilst the protoliths of the granulite facies xenoliths were tholeiitic igneous rocks and could be part of an ancient crust, the clinopyroxenite xenoliths represent recent underplating and may have formed from an alkali basaltic liquid similar to the host lava. Planar contact relations between clinopyroxenites and spinel lherzolites as observed in composite xenoliths, as well as high Al-contents in clinopyroxenes, point to a high-pressure genesis in the upper mantle for these rocks. In contrast, geobarometrical estimates yielded only a moderate pressure range characteristic of lower crustal levels for the garnetfree granulite xenoliths (7-9 kbar). Nevertheless, two-pyroxene geothermometry yielded high temperatures of equilibration (>900~ for these xenoliths, probably caused by advective heat transfer connected with underplating and in agreement with the high present-day geothermal gradient of this region. In the Central Range localities only garnet-free granulite xenoliths occur, whereas at the border of the TVR both garnet-free and garnet-bearing granulite facies nodules are found. It is possible that the incoming of garnet is retarded by higher temperatures in the lower crust below the Central Range.It is also suggested that the difference in seismically measured crustal thickness between the Central Range and adjacent basin areas may be connected with different thermal conditions below these regions and that the seismically defined Moho and the petrological Moho do not necessarily coincide.K E V W O R O S; granulite, clinopyroxenite, xenolith, Transdanubian Volcanic Region, Hungary.

Lower crustal zircons reveal Neogene metamorphism beneaththe Pannonian Basin (Hungary)

“…Neogene intraplate alkaline volcanism in the Pannonian Basin has entrained numerous xenoliths of mantle and lower crustal origin [14,15]. The sample analysed in this study was found in basaltic tu s near the village of Mindszentkalla (Figure 1) in the Balaton Highlands [16].…”

“…The large group of dates between 32 and 15 Ma may relate to tectonic episodes within the Pannonian Basin. The younger ages (15)(16) coincide with the rst phase of extension in the Pannonian Basin. However, the most abundant zircon ages are Mio-Pliocene ( Figure 6).…”

“…The petrology and geochemistry of lower crustal granulite xenoliths from the Pannonian Basin have been previous studied [15,16,[18][19][20][21][22]. Most of the xenoliths are garnetiferous ma c meta-igneous rocks, but in contrast to the lower continental crust worldwide, they show some unusual chemical features such as LREE-depletion and positive ϵNd isotope compositions that have been interpreted as indicating that their protolith was of MORB composition.…”

Melting, fluid migration and fluid-rock interactions in the lower crust beneath the Bakony-Balaton Highland volcanic field: a silicate melt and fluid inclusion study

“…The dominant lower crustal xenoliths are mafic granulites (Dégi and Török 2003;Dégi et al 2009Dégi et al , 2010Dobosi et al 2003;Embey-Isztin et al 1990Kempton et al 1997;Török et al 2005), rarely some metasedimentary and felsic ones also occur (Embey-Isztin et al Németh et al 2009Török 2002, 2012, Török et al 2014. Mafic granulite xenoliths from the BBHVF were described by Embey-Isztin et al (1990). Kempton et al (1997 and Embey-Isztin et al (2003) analyzing the whole rock for trace element, radiogenic and stable isotopes.…”

“…Beyond the abundant ultramafic deep lithospheric xenoliths in the Bakony-Balaton Highland Volcanic Field (BBHVF), garnet clinopyroxenite and lower crustal mafic garnet granulites and metapelite xenoliths were discovered and studied in details by e.g. Embey-Isztin et al 1990Dégi et al 2009Dégi et al , 2010Török 1995Török , 2012Török et al 2005. According to systematic paleo-thermobarometry by Dégi (2009) lower crustal mafic garnet granulite xenoliths represent a continuous lithostratigraphic profile with a paleo-depth range from 35 to 58 km under the BBHVF.…”