Petrological characterization and tectonic significance of retrogressed garnet peridotites, Raobazhai area, North Dabie Complex, east-central China

The Raobazhai ultramafic body of the North Dabie Complex is re-interpreted as a mantle-derived peridotitic slice enclosed in, and isofacially metamorp hosed with, surrounding granulite-to-amphibolite facies gneisses. The ultra mafic sheet consists mainly of metaharzburgite, but includes subunits of me tadunite and mylonitic Iherzolite. The rocks contain spinel but neither gar net nor plagioclase. However, in the mylonitic Iherzolite, fine-grained int ergrowths of spinel, orthopyroxene and clinopyroxene outline domains resemb ling the habit of garnet in two dimensions; broad-beam microprobe analyses imply pseudomorphs after a pyropic garnet precursor. The mineral assemblage of the metadunite and metaharzburgite is: olivine (Fo(92)) + orthopyroxene (En(92)) + tremolitic-to-magnesiohornblende + Mg-Al-chromite, indicating a mphibolite facies recrystallization. The Mineral assemblage of the myloniti c Iherzolite is: olivine (Fo(90)) + orthopyroxene (En(90)) + clinopyroxene + Cr-bearing spinel + pargasitic amphibole, indicative of granulite-to-amph ibolite facies metamorphism. Phase equilibria and geothermometric estimatio ns show that the Raobazhai meta-ultramafics have undergone at least three s tages of recrystallization: (I) 950-990 degrees C, (II) 750-860 degrees C, and (III) 670-720 degrees C, assuming equilibrium in the spinel peridotite stability field (c. 6-15 kbar), although an early, high-pressure stage (gre ater than or equal to 18 kbar) is probable, based on the inferred garnet ps eudomorphs. Petrochemical and geothermobarometric data suggest that the ult ramafic slice represents a fragment of the mantle wedge, tectonically incor porated into subducted continental crust and re-equilibrated at granulite-t o-amphibolite facies conditions while being exhumed to shallow levels.