Metamorphic Evolution Of Garnet Amphibole Albitite From Yukahe In North Qaidam,NW China And Its Geological Significance

The discovery of ultrahigh pressure metamorphism(UHP)and continental deep subduction is one of the most important advances in Geoscience in the last 30 years and the rock assemblage in UHP belt,the subduction limit depth and exhumation mechanism of continental crust,and the melt/fluid activity during subduction and induced crust-mantle interaction are still unclear and need further research.The North Qaidam orogenic belt(NQOB),lying at the northeast margin of the Tibet Plateau,is an early Paleozoic UHP metamorphic belt that confirmed recently.In this thesis,we carried out systematic petrography,geochemistry,zirconology,and phase equilibrium modelling on a garnet amphibole albitite,which intercalates with an UHP metamorphosed phengite eclogite,from Yuka river section,the west segment of the NQOB,and obtained the P-T path,protolith nature and the tectonic setting in which the protolith formed,as well as protolith,peak metamorphic and retrograde ages.Finally,by comparing previous data on the eclogite,we discussed the relationship between the garnet amphibole albitite and eclogite and the melt/fluid activity during continental subduction and exhumation.The results will provide important information on UHP metamorphic rock types in the NQOB,the formation and evolution of the Yuka terrane and melt/fluid activity during this process.The main achievements and new understandings of this thesis can sum up as follows: 1)Detailed field observation and petrography study reveal that the amphibole and biotite contents increase while albite and phengite contents decrease from the core of the garnet amphibole albitite to the contact interface with eclogite.The garnet grains in the contact zones are larger and experienced multi-episodic growth.The retrogression increased from the core of the eclogite to the contact interface with the garnet amphibole albitite and the eclogite has totally transformed to garnet amphibolite near the contact interface.2)In combination of petrography,zircon inclusion and phase equilibrium modelling,we obtained a clockwise P-T path for the garnet amphibole albitite with four metamorphic stages.The prograde stage is characterized by the garnet core and enclosed inclusion from the core of the garnet amphibole albitite.The mineral assemblage is Grt+Omp+Ph+Rt+Qz+Bt+Amp+Ab/Pl and the P-T condition is 1.21?1.25 GPa/595?622?;The peak stage is characterized by garnet rim and enclosed inclusion from the core of the garnet amphibole albitite.The mineral assemblage is Grt +Omp+Ph+Rt+Coe and the P-T condition is 2.85?2.96 GPa/648?685?,which fall in the coesite stability field;The early retrograde stage is characterized by the garnet rim and enclosed inclusion from the contact zone between the garnet amphibole albitite and eclogite.The mineral assemblage and P-T conditions are Grt+Omp+Ph+Pl+Bt+Amp+Rt+Qz and 1.32?1.38 GPa/655?685?,respectively;The late retrograde stage is characterized by the late Amp+Bt+Ab+Ep+Qz+Rt in matrix and the P-T conditions are 0.58?0.75 GPa/433?492?.Therefore,the garnet amphibole albitie experienced UHP metamorphism.3)LA-ICP-MS zircon U-Pb dating yielded the protolith age of >663 Ma,peak metamorphic age of 432 Ma and retrograde and aqueous fluid activity age of 398 Ma for the garnet amphibole albitite.4)Geochemical data show that the protolith of the garnet amphibole albitite is probably monzodiorite that formed in extensional with-plate setting.The major and trace elements variation in different zones from the garnet amphibole albitite to eclogite,we propose that there is fluid activity during the UHP metamorphism at the contact face between the garnet amphibole albitite and eclogite and the fluid had brought Rb,Cs,K and Na and minor Nb from the garnet amphibole albitie to eclogite.5)In combination with field occurrence,metamorphic P-T path,geochronology geochemical data,the formation and evolution of the garnet amphibole albitite can be sum up as follows: During the break up of Rodinia,near simultaneous or sequential continental-rift or continental flood basalt derived from enrich mantle formed the protolith of the eclogite and garnet amphibole albitite.In early Paleozoic(ca.440?420 Ma),the protolith of garnet amphibole albitite and eclogite underwent continental deep subduction with other eclogites and gneisses in the same section.