Pressure-Temperature Path Recorded in the Yangkou Garnet Peridotite, in Su-Lu Ultrahigh-pressure Metamorphic Belt, Eastern China

Abstract

Chemical variations along with changes in microstructure of the principal constituent minerals make it possible to identify at least four equilibrium stages in the evolution of the Yangkou garnet peridotite in the Su-Lu ultrahigh-pressure metamorphic belt, eastern China: Stage I-----a primary garnet lherzolite stage represented by coarse-grained (a few millimeters size) porphyroclastic aluminous pyroxenes þ chromian spinel garnet; Stage II-----an ultrahighpressure (UHP) stage defined by fine-grained matrix phases (0 1---0 3 mm size) of garnet þ extremely low-Al orthopyroxene þ high-Na clinopyroxene þ chromite; Stage III-----a medium-pressure stage defined by fine-grained mineral aggregates (50 1---0 2 mm size) mainly composed of aluminous spinelþ high-Al orthopyroxene in the matrix; Stage IV-----an amphiboliteto greenschist-facies stage defined by poikiloblastic amphibole. Orthopyroxene---clinopyroxene thermometry and an empirical spinel barometer give temperatures of around 800---830 C and pressures of 1 2---2 9GPa for porphyroclasts of Stage I. Garnet---orthopyroxene, garnet---clinopyroxene and empirical spinel geothermobarometers give relatively uniform P---T conditions for the matrix garnet---orthopyroxene---clinopyroxene--chromite assemblage of Stage II ( 730---760 C and 3 6--4 1 GPa). Aluminous spinel---olivine pairs in the aggregates give 650---680 C at 1 0---1 5GPa for Stage III. The granulation of the studied rocks may have taken place during subduction (Stages I and II) of the host peridotite. The lack of kelyphitic rims around the matrix garnet and the preservation of incompatible assemblages in the matrix of the studied rocks is due to extremely low H2O activity during the early decompression stage. The peak temperature of associated eclogites in the Yangkou UHP unit is 700---800 C at 3 1---4 1GPa. These observations suggest that the mantle fragment (garnet peridotite) and the crustal fragment (eclogite) in the Yangkou UHP unit both experienced a common UHP metamorphic event and that the exhumation path obtained by these reference points supports nearly isothermal decompression of the unit. The Yangkou UHP unit is considered to have been exhumed as part of a larger crustal mass (c.410 km in size) along with the surrounding orthogneisses. Such isothermal decompression paths have also been reported from other areas of the Su-Lu belt and the Dabie Mountains. As the orthogneisses volumetrically account for 490% of these areas, buoyancy-controlled uplift probably played an important role in the exhumation of the UHP rocks, at least from the upper mantle to the lower crust.