Kyanite eclogite xenolith from the orthogneiss terrane of the Tisza Megaunit, Janoshalma area, crystalline basement of southern Hungary

摘要

New evidence for high-pressure, eclogite fades metamorphism in the crystalline basement of the Tisza Megaunit (southern Hungary) is reported. The retrogressed mafic eclogite forms a small lens in the orthogneiss and it was found in the borehole near Janoshalma. The carbonated eclogite contains the peak metamorphic assemblage omphacite+garnet+phengite+kyanite+ clinozoizite+rutile+K-feldspar+quartz. Omphacite (Xjd_(0.40-0.41)Xdio_(0.52-0.53)Xhd_(0.05)Xaug_(1.55-2.85)) occurs in the matrix and as inclusions in garnet (Xpy_(0.37-0.38)Xgrs_(0.21-0.22)Xalm_(0.39-0.40)Xsps_(0-0.01)Xadr_(0-0.01)) and kyanite. Thermobarometry based on net-transfer reactions between garnet, omphacite, kyanite and phengite yields P-T conditions of 710±10 °C and 2.6±0.75 GPa. Retrogression during decompression is manifested by formation of symplectites; the most typical are diopside+plagioclase after omphacite, corundum+spinel+plagioclase after kyanite and biotite+plagioclase after phengite. Carbonatization along the veins of the retrogressed eclogite was probably coeval with formation of these symplectites. At places where carbonate is absent the rock was completely hydrated and retrogressed down to the greenschist facies with the development of actinolite. Similar eclogites together with abundant orthogneisses occur mainly in the eastern parts of the Tisza Megaunit, suggesting the existence of an ancient (possibly Variscan) subduction/accretionary complex.