Petrology of a diamond and coesite-bearing metamorphic terrain: Dabie Shan, China

The Dabie Shan Complex with the ultra-high pressure rocks is a large gneiss-granite terrain in the eastern part of the Qinling orogen in central China. It represents part of the lower continental crust of the Yangtze Plate that was subducted during the Triassic continental collision and consists of several welded gneiss terrains with different metamorphic grades. The eclogite zone, one of these, is sandwiched between two amphibolite-facies gneiss terrains, and consists of an over 25 km thick, regularly southward dipping sequence of leucocratic, granoblastic gneiss with eclogite, marble and minor ultramafic bands and lenses. Two subzones with different P-T regimes are tentatively differentiated in the eclogite zone. In the hot eclogite terrain in the north the eclogites in the gneiss have the common mineral assemblage of gamet + omphacite + zoisite + kyanite + phengite ± Ca-amphibole ± quartz + rutile and very rarely coesite as inclusions in garnet. Eclogites also occur in marble bands as blocks; they are characterised by the absence of kyanite, the presence of diopside-rich pyroxene and grossular-rich garnet with coesite, K-feldspar and diamond inclusions. The eclogite mineral assemblages record temperatures of 800 ± 50°C (K Dgt-cpx ≃ 6) and pressures ranging from 18 to 34 kbar while the presence of coesite and diamond at these temperatures indicates minimum pressures of 29 and 38 kbar respectively. The lower pressures recorded by the mineral assemblage is probably due to reequilibration during the uplift. The mineral assemblage in the gneiss, that forms the bulk of the eclogite zone, is quartz + plagioclase + garnet + phengite + epidote ± biotite ± microcline ± opaque ± titanite. Phengite inclusions in garnets in the gneiss with up to 3.56 Si p.f.u. and the rare presence of jadeite-gneiss with quartz + Jadeite + garnet indicate minimum pressures of 17 and 22 kbar respectively and point to an early high-pressure metamorphic history. It is likely that the ultra-high-pressure metamorphism was regional and has affected both the eclogites and the host gneisses. The coesite and diamond-free cold eclogite terrain in the south tectonically overlies the hot eclogite terrain, and is characterised by the absence of marble-eclogite horizons and by the presence of sodic amphibole-bearing eclogites. Otherwise it is lithologically similar to the hot eclogite terrain