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

The Dabie Shan complex is a large gneiss-granite terrain in the E part of the Qinling orogen in central China; it represents part of the lower continental crust of the Yangtze plate subducted during the Triassic continental collision and consists of several welded gneiss terrains with different metamorphic grades. An eclogite zone is sandwiched between two amphibolite-facies terrains, and consists of a > 25 km thick sequence of leucocratic, granoblastic gneiss with eclogite, marble and minor ultramafic bands and lenses. Two subzones with different P-T regimes are tentatively distinguished in the eclogite zone. In the hot eclogite zone in the N the eclogites contain garnet, omphacite, zoisite, kyanite, phengite +- Ca-amphibole +- quartz, rutile and very rare coesite as inclusions in garnet. Eclogites also occur in marble bands where they contain diopsidic pyroxene and grossular-rich garnet with coesite, K-feldspar and diamond inclusions. The eclogite mineral assemblages record T of 800 +- 50 degrees C and P 18-34 kbar; the presence of coesite and diamond indicate minimum P of 29 and 38 kbar, respectively. The ultra-high P metamorphism appears to have been regional and to have affected both the eclogites and the host gneisses. The coesite and diamond-free cold eclogite terrain in the S tectonically overlies the hot eclogite terrain and is characterized by the absence of marble-eclogite horizons and by the presence of eclogites with sodic amphiboles (crossite). The mineral assemblage indicates P-T conditions of 635 +- 40 degrees C and 18-26 kbar. The retrograde P-T path of the hot eclogite terrain appears to be very steep down to < 10 kbar when it was juxtaposed with the cold eclogite terrain.