Hydrous Phase and Carbonate-bearing Peridotites from the Sulu UHP Terrane, China: An Excellent Record of Mantle Metasomatism and Subduction Zone UHP Metamorphism

Minor garnet peridotites occur as block and lens within gneiss and rare eclogite in the Sulu ultrahigh-P (UHP) metamorphic terrane. Trace zircon separates in mantle-derived peridotites are rounded without inherent core indicating a metamorphic origin. SHRIMP U-Pb dating of zircons from Rongcheng dunite and Donghai peridotite yields ages of 242 ± 8 and 221 ± 3 Ma, respectively, consistent with the ages of UHP metamorphism (220-240 Ma) of the Dabie-Sulu collision orogen. Some garnet lherzolites, wehrlites and dunites contain abundant hydrous (e.g., phlogopite and Ti-clinohumite) and carbonate phases (magnesite and dolomite). Magnesite occurs as (1) coarse-elongated crystal or sporadic patches up to 3.5-15 mm in diameter, (2) matrix phase, and (3) vein with or without phlogopite ± Ti-clinohumite. Dolomite aggregates are either surrounding coarse-grained magnesite or as interstitial patches between other phases. Magnesite is nearly pure and dolomite is stoichiometric; their total REE contents are 0.17-0.42 and 21.66-116.83 ppm, respectively. The O and C isotopic compositions of magnesite (δ18O, 5.5 to 8‰; (δ13C -2.4 to -3.4‰) are close to or slightly higher than the mantle range, but differ distinctly from that of Dabie-Sulu UHP marble (δ13C: from 0 to +6‰; (δ18O: from +7 to + 25‰). Ti-clinohumite occurs as a matrix, vein and inclusion phases in peridotitic minerals. Phlogopites have narrow (δD values of -76 to -91‰, which lie in the range -40 to -100‰ of most mantle phlogopites. These peridotites are depleted in CaO, Al2O3, TiO2 and FeO, and characterized by LREE and LILE enrichment with negative anomalies of Ti, Hf and Zr and positive Ba anomaly. Formation of hydrous and carbonate minerals is attributed to alkaline, hydrous fluids and carbonate metasomatism at mantle wedge. Magnesite may have formed as crystalline cumulate from carbonate melts derived probably from asthenospheric mantle; slight modification in isotopic composition is due to metasomatism by fluid or melt derived from subducted slab. The dolomite is either recrystallized retrograde phase or late cumulates of a dolomitic melt. These geochronological and petrochemical data indicate that some Sulu peridotites are depleted and metasomatized mantle rocks and were tectonically squeezed into the subduction zone, then subjected to in situ Triassic subduction-zone UHP metamorphism.