Geochemical Behavior Of Trace Elements In Eclogites From The CCSD Main Hole During Retrograde Metamorphism

The Qingling-Dabie-Sulu orogenic belt resulted from collision between of North China Craton and Yantze Craton is the largest UHP metamorphic belt in the world. One main target of the CCSD project is to reveal the mantle-crust interaction and geochemical recycling during continental subduction and eclogite exhumation. Trace elemental compositions and variations of the eclogites and single minerals were measured to study the trace element mobility during retrograde metamorphism. Geochemical records of the origin and evolution of serpentinized peridotite from Xougou were also studied. Main conclusions made in this work are following:(1) Except for heavy rare earth elements in garnet, trace elements in eclogite are strongly distributed in favor of minor/accessory minerals compared with major minerals. For example, Cs, Rb and Ba are largely enriched in mica, light rare earth elements in apatite, epidote and sphene, and Nb and Zr in rutile and sphene. Thus, stability of primary minerals and formations of secondary minerals affect significantly the behavior of trace elements during retrograde metamorphism, and resulted in fractionations of trace element during retrograde.(2) Studies on samples suffered from transitional retrograde metamorphism demonstrate that large ion lithophile elements and light rare earth elements show significant variations, whereas high field strength elements and heavy rare earth elements show little variations. These observations suggest that fluids associated with retrograde metamorphism were enriched in large ion lithophile elements and light rare earth elements, not enriched in high field strength elements Nb, Ta and heavy rare earth elements. Zr and Hf maybe enriched during retrograde metamorphism Depletions of high field strength elements and heavy rare earth elements in the fluids imply that the external fluids were poor in anionic complex.(3) Accessory minerals and retrograde fluids are main factors controlling the trace elemental behavior during retrograde metamorphism. Formations and/or decompositions of minor/accessory minerals and chemical compositions (e.g., contents of anionic complex) of the fluids affect significantly the behavior of trace elements during retrograde metamorphism.(4) Serpentinized peridotites from Xougou were studied. Based on bulk rock chemical compositions, we suggest that the trace elements of the serpentinized peridotes were resulted from melt extraction, mantle metasomatism and serpentinization near surface. Serpentinization caused enrichments of Pb and Sr. Pb contents are positively correlated with serpentinization degree. Also Pb/Ce ratio is higher than island arc basalt and continental crust. Zr, Hf maybe enriched in serpentinization fluid.