Metamorphic Reactions as Indicators of Lower Crustal Processes during Formation of the Pannonian Basin

The Pannonian Basin is famous of its xenolith localities hosted by late Tertiary alkaline basalts, from different depths of the lithosphere. In this work we present the petrography, mineral chemistry, and geothermobarometry study of lower-crustal mafic garnet-granulite xenoliths from the central Pannonian Basin, namely from the Bakony-Balaton Highland Volcanic Field (BBHVF). Based on geothermobarometric calculations, the estimated pressure of the peak metamorpishm was about 1.0-1.6 GPa, at temperatures of about 800-1050 °C. Several reactions have been recognized in both the mafic garnet granulite and the metapelitic granulite xenoliths from the BBHVF, involved. The breakdown reaction of the garnet is the only reaction which can be observed both in metabasics and in metapelites. Experimental works and thermodynamic calculation show that the breakdown reaction of the garnet occurs at relatively low pressures of about 0.6-0.8 GPa. This pressure value is equivalent to 20-30 km depth which is equal with the present day depth of the lower crust. Reaction of garnet in metapelitic granulites shows similar or even slightly lower pressures. The metamorphic reactions show not only the thinning of the crust during the formation of the Pannonian Basin, but also make it possible to estimate the quantification of crustal thinning.