PETROGENESIS OF ECLOGITE IN ULTRAMAFIC ROCK FROM THE SULU UHP TERRANE, EASTERN CHINA

Eclogite lens or layer are widespread in garnet peridotite recrystallized at 750 - 950 ^oC, 4-6.5 GPa in the Sulu Triassic ultrahigh-pressure (UHP) terrane, eastern China; about 50 eclogite samples within garnet peridotite were studied. Most are bimineralic eclogites; only minor ones contain additional quartz, kyanite, and phengite. These eclogites contain lower SiO₂ (46 - 48 wt%), and higher MgO (10-14 wt%) than eclogites in gneiss. Two type REE patterns were identified for these eclogites: type one shows a gradient variation from highly through medially fractionated LREE-enriched to flat REE patterns; type two is slightly convex REE patterns. ICP-MS spot analyses indicate that garnet displays LREE depleted and MREE and HREE-enriched pattern; some have weak positive Eu anomaly, and omphacite exhibits LREE-enriched pattern with vertex at Pr and Nd. Omphacite shows positive Ba and Sr anomaly, and garnet has opposite. Both garnet and omphacite exhibit either positive or negative Zr anomaly. Quartz pseudomorphs after coesite occur as inclusion in garnet and omphacite with well-developed radial fractures. Some coarse-grained garnets contain exsolved rutile (± apatite ± clinopyroxene) needles or lamellae; omphacite contains exsolved quartz rods. Compositions of individual minerals exhibit large variations and are related to bulk composition of host peridotite. Garnet from eclogites within wehrite or clinopyroxenite contains more almandine component (Alm: 50 ± 3%; Prp: 26 ± 5%) than that in harzburgite and lherzolite (Alm: 32 ± 3%; Prp: 39 ± 6%). Jadeite component of omphacite ranges from 22 to 62%. The Fe-Mg partitioning between garnet and omphacite ranges from 5.5 to 10.7, corresponding to a temperature range of 690-920 ^oC at an assumed pressure of 3 GPa. Sm-Nd isochoron of mineral-whole rock and zircon U-Pb SHIMP datting yield a metamorphic age of 230 ± 20 Ma. Partial melting of mantle peridotite, and high-P accumulation of garnet and pyroxene probably contribute the formation of eclogite. Both eclogite and host garnet peridotite have experienced UHP subduction-zone metamorphism and minor metasomatism.