Ultrahigh-temperature metamorphism of a buchitized xenolith from the basaltic tuff of Szigliget (Hungary)

Abstract

A partially molten buchite xenolith containing quartz, glass, mullite/sillimanite, corundum, Al-rich orthopyroxene, spinel, Mg-Al-bearing Fe-Ti oxides, ilmenite and rutile was found in the basaltic tuff of Szigliget (Balaton Highlands, Hungary). The pre-existing rock may have been a quartz-rich micaschist or phyllite from the wall of the magma vent. Minerals like mullite, Al-mullite and Al-rich orthopyroxene with Al2O3 between 7.62 to 11.54 wt% show ultrahigh-temperature (UHT) metamorphism of the xenolith with temperatures in excess of 1080 °C, which means that the temperature of metamorphism approached that of the enclosing alkaline basaltic melt. CO2 fluid inclusions with homogenization temperatures between +26.5 and +29.7 °C and mineral equilibria indicate pressure of buchitization between 310 and 390 MPa.  Glass compositions are similar to a peraluminous leucogranite which is in accordance with melting experiments when metagreywackes, pelites and orthogneisses are involved. The presence of glass sugge...