Metamorphic evolution of lower crustal mafic granulite xenoliths from the Bakony—Balaton Highland volcanic field (W-Hungary) and its correlation with the formation of the Pannonian basin

Category Petrology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Dégi, Jْlia۱; Tِrِk, Kلlmلn۲; Abart, Rainer۳
Holding Date 04 October 2008

Mafic granulite xenoliths were collected from the Bakony—Balaton Highland Volcanic Field. They represent the material of the lower crust beneath the ALCAPA block of the Pannonian Basin, which was part of the Alps in the Upper Cretaceous and went through several drastic changes due to the tectonic processes during the Miocene extension of the Pannonian Basin. The xenoliths were investigated by means of petrography, mineral chemistry, geothermobarometry, fluid inclusion studies, phase diagram calculations, high resolution element mapping and transmission electron microscopy combined with FIB technique in order to determine their metamorphic evolution.
Peak metamorphic conditions are shown by the granulite facies rock-forming mineral assemblage (clinopyroxene, plagioclase, garnet, ±orthopyroxene ±amphibole ±biotite). According to geothermobarometric investigations it was formed between 800—1000°C and 1.2—1.6GPa, which corresponds to 43—57km depth. This suggests, that the peak metamorphic assemblage was formed when the ALCAPA was part of the Alps. Tectonic processes, which took place later are indicated by mineral reactions partly consuming the phases of the granulite assemblage.
According to petrographic observations and fluid inclusion studies, biotite was molten in the presence of CO2-rich fluids by the reaction Bt + Pl + Qtz = Opx + Grt + melt. Melt was extracted from the rock, and orthopyroxene and garnet remained as residual reaction products and re-equilibrated with the plagioclase. This assemblage gives 900—1070°C and 0.9—1.5GPa as equilibrium conditions. It suggests, that a temperature increase of approximately 100°C and CO2-rich fluid input led to the partial melting of the rock may be at the beginning of the extension of the Pannonian Basin.
High resolution element mapping and TEM studies have shown that garnet breaks down to an anorthite, orthopyroxene, spinel symplectite in an isochemical reaction. According to phase diagram calculations the reaction was induced by significant pressure decrease to 0.7—0.8GPa (25—28km), which corresponds to the present day lower crustal depths. This suggests that the process took place due to significant crustal thinning during the extension.
Melting of high Ti amphiboles to form clinopyroxene, orthopyroxene, plagioclase, spinel and melt was induced by further temperature increase. The lack of garnet among amphibole melting products show that the melting took place after significant thinning of the crust.
Accessory Fe-Ti-oxides have preserved the traces of interaction of the xenoliths with the uplifting basalt. By the modelling of diffusion profiles in the oxide grains we have estimated that the interaction lasted a few days.
Our results on the metamorphic evolution of lower crustal xenoliths have shown a series of lithospheric processes during the extension of the Pannonian Basin, which can be used for refining basin extensional models.