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 suggests that the duration of the heating process was not long enough to extract the melt from the rock and admix it with the alkaline basaltic melt.
- Al-rich orthopyroxene
- Buchite xenolith
- CO inclusions
- Ultra high-temperature metamorphism
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