Element Geochemical Behavior During The Ultrahigh-pressure Metamorphism

The main hole of the Chinese Continental Scientific Drilling Project(CCSD) is located at Donghai,The 0～2050 m recovered cores in CCSD-MH are mainly comprised of eclogite, orthogneiss(granitic gneiss),paragneiss and ultramafic rock,and cumulate thickness of eclogite is about 1200 m.This UHP terrane were subducted to mantle and experienced ultral-high pressure metamorphism simultaneously,finally these UHP rocks were exhumed to the crust,These typical UHP rocks can provide critical information for element chemical behavior during the continental deep subduction.The eclogite form the hole have a very wide variation of major and trace element abundances,and have been classified into high-Si,high-Al,high-Ti,high Fe-Ti,high-Mg and normal types.On the basic of petrology,in this study,we collected these eclogite and garnet pyroxene,trace element compositions of UHP minerals in the eclogites and garnet pyroxenites are carefully analyzed by in situ LA-ICP-MS.Using these data together with whole-rock compositions of UHP rocks and major element compositions of UHP minerals,the authors discussed the distribution and partitioning of trace elements in various minerals,and the UHP metamorphic P-T conditions and the fluid-rocks interaction.The results show that LREE and Sr are mainly concentrated in apatite,epidote and omphacite,HREE are mainly concentrated in garnet,LILE such as Ba and Rb are strongly partitioned in phengite,HFSE in rutile and ilmenite, V,Sc,Co and Ni in garnet and omphacite,while Zr and Hf in zircon.The study indicate that the trace element contents and their distribution pattern in UHP minerals are controlled by the whole rock compositions,the mineral assemblages and contents of the host rocks.In addition,the following conclusions are reached,the distributions of trace elements among UHP minerals are of mutual equilibrium,and the partition coefficients are very similar to those in mantle eclogites, and probably indicating that the peak-stage UHP metamorphic temperatures of eclogites were higher than 900～1000℃.some garnets from the high-Ti eclogite,high-Si eclogite and high Fe-Ti eclogite show obvious growth compositional zoning both in major and trace elements,suggesting that the host UHP rocks were very rapidly uplifted.Zr contents in eclogitic rutiles are not only controlled by the metamorphic temperature but also by other factors such as the Zr contents in the whole rocks and the compositional diffusion during the retrograde metamorphism,so the Zr-in-rutile geothermometer is not always credible.During the UHP metamorphism,supercritical silicate-rich aqueous fluids have resulted in major fractionation between Nb and Ta in rutiles,so that the eclogitic rutiles have superchondrite Nb/Ta value.Therefore,voluminous eclogites that had been subducted to the mantle depth provide probably a hidden reservoir with higher than chondrite Nb/Ta ratio in the Earth.The fluids in the various stages of retrogression of UHP eclogites are different in origin,composition and flow scale.The melts or fluids came from the phengite can interact with rocks and other minerals,so have been recorded by them.Combined study of Laser Raman spectroscopy,cathodoluminescene(CL) images and SHRIMP and LA-ICPMS U-Pb dating reveals that zircons separated from the eclogite in the main drill hole CCSD-MH retain magmatic core with inherited ages(²⁰⁶pb/²³⁸U) of 294～781Ma, indicating that the magmatic zircons of the protolith have a variety of sources,and partial loss of Pb from some zircons in the protolith.UHP mineral-bearing domains of zircons recorded 206～237Ma(²⁰⁶pb/²³⁸U) for the UHP metamorphic condition,with an weighted mean age of 220Ma.Magmatic zircons are characterized by high heavy rare earth element(HREE),Y,Th,U content,and high Th/U ratio,its chondrite normalized REE pattern are characterized by LREE strongly depleted,HREE strongly riched,with a minus Eu anomalies.Compared with the magmatic zircon,the UHP zircon domains are characterized by lower HREE,Y,Th,Nb composition,and lower Th/U ratio,and high Hf content,its chondrite normalized REE pattern are characterized by LREE strongly depleted,HREE gentle riched.