摘要：Eclogite compositions are critical parameters for under standing chemical evolution in the Earth, particularly in subduction zones. A group of eclogites from the Sulu ultra-high pres sure (UHP) metamorphic terrane in eastern China shows uncommon petrographic and compositional features. They are characterized by (1) zoisite porphyroblasts coexisting with inclusion-free, inclusion-containing, and net work kyanite, (2) high Al 2 O 3 of 18.4 - 29.2% with high MgO of 8.59 - 11.3%, and (3) intensive HFSE-REE fractionations represented by [Zr/Sm] ch and [Ti/Gd] ch ratios of 0.1 - 3.9 and 1.1 - 9.0, respectively. High-pressure fluids played a major role on developing these features. Kyanite shows two textural varieties. K yI is inclusion-free and in apparent textural equilibrium with garnet and omphacite, implying formation from plagioclase break down. K yII includes kyanite networks and porphyroblasts; the former occurs mostly in gar net, whereas the latter encloses gar net and omphacite grains. K yII were crystallized at the expense of gar net and omphacite during eclogite-fluid interaction. The low [HREE] ch ratios of 1 - 2 indicate that the protoliths were arc cumulates. The well-de fined in verse Al 2 O 3 -SiO 2 and Al 2 O 3 -CaO correlations are not the characteristics of igneous precursors; therefore, must reflect metamorphic modifications. The comparison to mafic cumulates shows that the low-Al samples are compositionally better representatives of protoliths. Mass balance calculations point to anolivine gabbronoritic protolith. The intensive HFSE-REE fractionations reflect compositional differences between two sample groups. Group I samples have superchondritic HFSE-REE ratios [(Nb/La, Zr/Sm, Ti/Gd) ch = 2 - 7] with depletions in LREE. In contrast, group II samples show HFSE depletions and LREE enrichments. These compositional differences were explained as reflecting element mobility during eclogite-fluid inter action. Released at temperatures > 750°C, the high-pres sure fluids were enriched in HFSE, REE, and LILE. Upon migration, the fluids first precipitated HFSE into the group I samples, then, LILE and LREE into the group II samples. This model is sup ported by the occurrence of zoisite porphyroblasts in the group II samples, and interstitial zircon and clusters of small rutilegrains along annealed fractures in the group I samples. Our interpretation for the observed compositional variations implies limited element mobility during subduction and exhumation of continental lithosphere, consistent with existing models, which proposed that the chemical flux to metasomatized mantle wedge is mainly from subducted sediments.
关键词：HFSE REE fractionation; HFSE enrichment; Eclogite fluid interaction; Eclogite; Sulu UHP meta morphism;
