Petrology, geochemistry and U-lead SHRIMP geochronology of ultrahigh-pressure mafic-ultramafic rocks in the Sulu UHP metamorphic terrane, Eastern China

Garnet peridotites that contain eclogite lenses occur within numerous high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic terranes in major continental collision zones, and provide a new window into upper mantle processes at convergent boundaries. The Sulu UHP terrane, eastern China, is the easternmost part of the Qinling-Dabie-Sulu collision zone between the Sino-Korean and Yangtze cratons, and consists mainly of gneiss, marble and quartzite together with minor eclogite and garnet peridotite. Garnet peridotites from Rongcheng, Rizhao and Xugou in the Sulu terrane show high MgO contents, low fertile element concentrations and mantle oxygen isotope values (delta18O: 5.2 to 6.29‰), suggesting that they were derived from a depleted residual mantle. On the other hand, high FeO content, LREE-enriched pattern and occurrence of hydrous and carbonate phases indicate later metasomatism. Eclogites enclosed within these garnet peridotites were formed by melts derived from partial melting of a peridotite source, and show high delta18O values of 7.6 to 7.7‰. Both garnet peridotites and eclogites experienced multi-stage recrystallization, the Rizhao clinopyroxenite may even have a complex pre-UHP evolution involving mantle convection. High P-T estimates (700 - 900°C and ∼5 GPa) of peak metamorphism suggest that both peridotites and eclogites experienced the UHP metamorphism in a subduction zone with an extremely low geothermal gradient. SHRIMP U-Pb zircon geochronology also suggests that these mafic-ultramafic bodies experienced the UHP metamorphism at the same time as country rocks. Zircons from mafic-ultramafic bodies are of metamorphic origin, and were formed during UHP metamorphism after these rocks were emplaced into the subduction zone. The U-Pb ages of 242 to 215 Ma from these mafic-ultramafic bodies are consistent with the commonly accepted UHP metamorphic age range (240-220 Ma) for the Dabie-Sulu terrane, suggesting in-situ metamorphism of these rocks within their enclosing gneisses. In summary, garnet peridotites and associated eclogites, originated from a depleted and metasomatized mantle, were emplaced into the subduction zone and experienced UHP metamorphism at mantle depths during the Triassic continental collision, and were later exhumed together with the surrounding supracrustal rocks to crustal levels.