A petrogenetic study of the garnet pyroxenite enclaves in spinel peridotite, North Dabieshan, China

Recently, garnet pyroxenite enclaves within peridotites occurring near Raob azhai, Huoshan County, have been discovered. The garnet pyroxenite is small pods, decimeters in size, enclosed within intensively serpentinized perido tites. Major mineral components comprise: garnet (Prp(25-35)), sodium augit e (Jd (10-25)) with a small amount of ilmenite. There are two stages of ret ro-metamorphism: the retrogressive granulite facies mineral assemblage is s uperimposed by that of amphibolite facies. The host rocks of the garnet pyr oxenite are spinel peridotites, including spinel harzburgite and Iherzolite . Due to intensive serpentinitization, only 5%-40% of the relic olivine (Fo (92-93)) are preserved. The orthopyroxenes are Mg-rich (En,,,,) with bendin g of cleavages and granulation at their margins showing intracrystalline pl asticity. On the basis of garnet-clinopyroxene Re-Mg exchange equilibrium g eothermometry proposed by Ellis & Green (1979) and Krogh (1988) K-D = 4.06 - 5.28; T = 793-919 degreesC, P = 1.5 GPa are estimated for the garnet pyro xenite. It is inferred that the peridotites are mantle rocks about 60 km in depth. During the exhumation of the orogenic belt, it was tectonically emp laced into the lower crust in the solid state and then uplifted to the shal low depth. Obviously, this kind of garnet pyroxenite must be petrogenetical ly related to its host rock. The REE distribution pattern and the Ni-Co-Sc diagram reveal that they are chemically equivalent to the basaltic melt and ultramafic residua respectively derived from partial melting of mantle roc ks.