Petrochemistry, oxygen isotopes and U-Pb SHRIMP geochronology of mafic?ultramafic bodies from the Sulu UHP terrane, China

Two Rongcheng eclogite-bearing peridotite bodies (Chijiadian and Macaokuang) occur as lenses within the country rock gneiss of the northern Sulu terrane. The Chijiadian ultramafic body consists of garnet lherzolite, whereas the Macaokuang body is mainly meta-dunite. Both ultramafics are characterized by high MgO contents, low fertile element concentrations and total REE contents, which suggests that they were derived from depleted, residual mantle. High FeO contents, an LREE-enriched pattern and trace-element contents indicate that the bulk-rock compositions of these ultramafic rocks were modified by metasomatism. Oxygen-isotope compositions of analysed garnet, olivine, clinopyroxene and orthopyroxene from these two ultramafic bodies are between +5.2‰ and +6.2‰ (δ18O), in the range of typical mantle values (+5.1 to +6.6‰). The eclogite enclosed within the Chijiadian lherzolite shows an LREE-enriched pattern and was formed by melts derived from variable degrees (0.005–0.05) of partial melting of peridotite. It has higher δ18O values (+7.6‰ for garnet and +7.7‰ for omphacite) than those of lherzolite. Small O-isotope fractionations (ΔCpx-Ol: 0.4‰, ΔCpx-Grt: 0.1‰, ΔGrt-Ol: 0.3–0.4‰) in both eclogite and ultramafic rocks suggest isotopic equilibrium at high temperature. The P–T estimates suggest that these rocks experienced subduction-zone ultrahigh-pressure (UHP) metamorphism at ∼700–800 °C, 5 GPa, with a low geothermal gradient. Zircon from the Macaokuang eclogite contains inclusions of garnet and diopside. The 225 ± 2 Ma U/Pb age obtained from these zircon may date either the prograde conditions just before peak metamorphism or the UHP metamorphic event, and therefore constrains the timing of subduction-related UHP metamorphism for the Rongcheng mafic–ultramafic bodies.