HFSE-REE fractionation in two groups of Sulu eclogites: protolith or process control?

Significant HFSE-REE fractionation occurs in subduction zone. However, our understanding on the causes is mainly built upon compositions of arc lavas, which represent the end products of subduction zone processes. Additional constraints are derived from comparing compositions of eclogites and their protoliths. However, the focus has been on eclogites of an oceanic affinity. Here, we show distinct HFSE-REE fractionation patterns from two groups of eclogites of a continental affinity from the Sulu ultra-high pressure metamorphic terrane, China. Having high iron (16.7-20.9%) and TiO2 (3-4%) with low MgO (6.03-7.01%) contents, the high-Fe-Ti eclogites are enriched in Ti but depleted in Nb-Ta-Zr-Hf. Although their low SiO2 contents (38.2-42.8%) are attributed to metamorphic modification, the decoupling between Ti and other HFSE can be modeled as inheriting from gabbroic protoliths crystallized from melts compositionally similar to the subunits 4 and 6 eclogites in the CCSD core. Similarly, the compositions of a subgroup of the Sulu high- Al eclogites characterized by Nb-Ta-Zr-Hf depletion also largely reflect protolith control for the resemblance to the Talkeetna arc gabbronorites. However, another subgroup of the Sulu high-Al eclogites shows unusual HFSE enrichment with Ti/Eu, Zr/Sm, and Nb/La ratios over two times of the chondritic values. Their major oxide and HREE contents are comparable to that of the Talkeetna gabbronorites (~9% MgO). Therefore, the HFSE enrichment is attributed to interacting with high-pressure fluids. The occurrence of interstitial zircon and cluster of small rutile grains (<50 um) in garnet and omphacite along the periphery of annealed fractures is also consistent with HFSE precipitation from the fluids. The role of high-pressure fluids is strengthened by the occurrence of zoisite in the Nb-Ta-Zr-Hf depleted but LILE-LREE enriched high-Al eclogites. Evidently, the HFSE-REE fractionation in subducted continental lithosphere could be protolith and process controls but might exert insignificant effects on the apparent HFSE-REE imbalance in the solid Earth (Rudnick et al., 1999), considering the small volume of such eclogites and limited mobility of most elements in high-pressure fluids.