A Geochemical Study Of Metamorphic Rocks In The West Of East Kunlun Orogenic Belt

The HP-UHP metamorphic belt in the orogenic belt is an important symbol of the convergence boundary of ancient plates and the paleooceanic subduction and continental collision orogeny.It records the dynamic transformation process of the crustal material from subduction to exhumation,the closure of the ocean to the continental collision orogeny.The study of eclogite in subduction zone can reveal the information of HP-UHP metamorphism and regional structure before subduction.Eclogites can be formed by subduction of oceanic crust or continental crust.The two types of eclogites have different sources and geochemical compositions.If the protolith is oceanic,it represents subduction metamorphism of oceanic crust,indicating accretive orogeny.If the protolith is continental crust,it represents subduction metamorphism of continental crust,indicating the existence of continental collision orogeny.The East Kunlun orogenic belt is located in the northern margin of the Tibetan Plateau.As an important part of the central orogenic belt,it has recorded in detail the continuous subduction-accretion-closure orogenic process of Proto-Tethys—Paleo-Tethys Ocean from the Early Paleozoic to the Triassic,which has become a hot spot and frontier for geologists to study.However,the protolith and metamorphic evolution of eclogite in this area are still controversial.This dissertation focused on the retrograde eclogite(ganet-amphibolite)and surrounding rock gneiss from Xiarihamu and Xiaozaohuo in the west of the East Kunlun orogenic belt,and carried out detailed petrographic,whole rock geochemistry and isotope geochronology studies.The results are expected to restore the protolith properties and metamorphic evolution history of eclogite in the west of the East Kunlun orogenic belt.It provided new constraints for the tectonic evolution of East Kunlun orogenic belt.The petrography observations,zircon U-Pb dating,titanite U-Pb dating and monazite U-Th-Pb dating of the garnet-amphibolite and gneiss in Xiarihamu and Xiaozaohuo show that the protolith of the eclogite was 939±9 Ma in response to the convergence of the Rodinia supercontinent.In addition,these rocks experienced a process of subduction and exhumation of continental crust,including progressive metamorphism,peak and demetamorphism.Combined with the regional structure,the region experienced at least three stages of metamorphism from Early Paleozoic to Late Paleozoic:448 Ma corresponds to oceanic crust subduction tectonic event;427 Ma corresponds to crustal subduction and ultrahigh pressure metamorphism;389 Ma corresponds to demetamorphism during exhumation.The major and trace element analysis,Sr-Nd isotope analysis,garnet O isotope analysis and zircon Hf isotope analysis of the garnet-amphibolite samples from Xiarihamu and Xiaozaohuo have been comprehensively studied.The results show that the eclogite protolith in the study area is a continental basic rock in response to the convergence event of the Neoproterozoic Rodinia supercontinent;garnetamphibolite’s positive εNd(t)values,low εHf(t)values and TDM1 ages of magmatic zircons which older than protolith suggest that the source area of eclogite originates from the depleted mantle with the addition of crustal materials;The characteristics ofδ18O values of the garnet indicate the protolith of eclogite underwent different degrees of low temperature hydrothermal alteration in the later period.