REE Partition Between Garnet And Omphacite In Eclogites From The Dabie Mountain

The distribution of REE between the omphacite and garnet and the REE behaviorin the metamorphic processes are discussed based on the REE characteristics ofomphacite and garnet of eclogite from Dabie Mountain. The concentrations of rareearth element (REE) in garnet and omphacite of six eclogite samples from the DabieMountain, central China were measured with the Inductively Coupled Plasma-MassSpectrometry (ICP-MS). REE patterns of garnet and omphacite and REE partitioncoefficients (Kd=COmpi/CGrti) were obtained. The REE behavior during high pressuremetamorphic process was discussed by estimating REE concentrations of fresheclogites. Correlation of δEu ratios between garnet and omphacite indicated thatchemical equilibrium of REE distribution between garnet and omphacite could beapproached during ultra-high pressure (UHP) metamorphism. Most of the partitioncoefficients of light rare earth element (LREE) are larger than one, however thepartition coefficients of heavy rare earth elements (HREE) are less than one. Thisindicated that LREEs inclined to occupy M2site in omphacite, but HREEs favored tooccupy eightfold coordinated site in garnet during the eclogite formation. HREE in sixgarnet samples had relatively the consistent flat enrichment patterns, whereas LREEin omphacite had various patterns, which could indicate that HREEs were relativelystable during the metamorphism, but LREEs were more active. The estimated REEconcentrations of the fresh eclogites indicated that LREE could be partially lostduring the prograde metamorphic process of protolith, but added in the rocks bysymplectite formation during the retrograde metamorphic process. The protoliths ofelcogites associated with granitic orthogneisses or marbles could be basalts or peliticrocks. The protoliths of Bixiling elcogites associated with ultramafic rocks could bederived from depleted mantle, and the Maowu elcogites could also be derived frommafik or ultramafic intrusive mass with obvious crustal contamination. The resultsfavor to highlight the REE behavior and evolution of UHP metamorphic rocks.