TALC-BEARING, MAGNESITE-BEARING AND TI-CLINOHUMITE-BEARING ULTRAHIGH-PRESSURE META-MAFIC AND ULTRAMAFIC COMPLEX IN THE DABIE MOUNTAINS, CHINA

The Bixiling mafic/ultramafic metamorphic complex is a 1.5 km(2) tectonic block within biotite gneiss in the southern Dabie ultrahigh-pressure terrane, central China. The complex consists of banded eclogites that contain thin layers of garnet-bearing cumulate ultramafic rock. Except for common eclogitic phases (garnet, omphacite, kyanite, phengite, zoisite and rutile), banded eclogites contain additional talc and abundant coesite inclusions in omphacite, zoisite, kyanite and garnet. Some metaultramafic rocks contain magnesite and Ti-clinohumite. Both eclogites and meta-ultramafic rocks have undergone multi-stage metamorphism. Eclogite facies metamorphism occurred at 610-700 degrees C and P > 27 kbar. Whereas amphibolite facies retrograde metamorphism is characterized by symplectites of plagioclase and hornblende after omphacite and replacement of tremolite after talc at P < 6-15 kbar and T < 600 degrees C. The meta-ultra-mafic assemblages such as olivine + enstatite + diopside + garnet and Ti-clinohumite + diopside + enstatite + garnet + magnesite +/- olivine formed at 700-800 degrees C and 47-67 kbar. Investigation of the phase relations for the system CaO-MgO-SiO2-H2O-CO2 and the experimentally determined stabilities of talc, magnesite and Ti-clinohumite indicate that (I) UHP talc assemblages are restricted to Mg-Al gabbro composition and cannot be an important water-bearing phase in the ultramafic mantle, and (2) Ti-clinohumite and magnesite are stable H2O-bearing and CO2-bearing phases at depths >100 km. The mafic-ultramafic cumulates were initially emplaced at crustal levels, then subducted to great depths during the Triassic collision of the Sino-Korean and Yangtze cratons.