The retrograde process of two types of eclogite in the Xitieshan terrane, North Qaidam and its constraint of the exhumation mechanism in the subduction zone

Eclogite, as an important rock type in the subduction zone, contains abundant geodynamic information of the earth. Studying the petrology of the eclogite and its retrogress equivalent helps building the p-T paths of the subduction zone and understanding the changes of the mineral facies and compositions of the subducted rocks and further constraining the exhumation mechanism in the subduction zone. This paper mainly focuses on the petrological studies of the bimineralic and phengite-bearing eclogite in the Xitieshan terrane, North Qaidam UHP metamorphic belt. We have calculated the p-T pseudosection of the eclgites under NC(K)FMASH system. It shows that the peak stage of the Bi-Ec preserved p-T conditions of 745~790℃, >2.8~3.0GPa(M1 stage) and experienced isothermal decompression to amphibole-eclogite facies p-T conditions (670~770℃, 1.6~2.2GPa，M2 stage), which shows similar p-T path with the Phn-Ec. The Bi-Ec in the Xitieshan terrane has N-MORB geochemical characteristics, whereas the Phn-Ec shows E-MORB or OIB characters. It demonstrates that the protoliths of the Bim-Ec and Phn-Ec are different. Considering the p-T path and geochemical characters of the eclogite, it infers that the difference of the mineral assemblage between Bi-Ec and Phn-Ec are caused by the multiple origins of the eclogite protolith but not retrogression in the Xitieshan terrane.