Recognition And Tectonic Significance Of Sumdo Ophiolite,Tibet

Petrology,LA-ICP-MS zircon U-Pb dating,whole-rock geochemistry and Hf isotopes of ophiolites in Sumdo area have been studied in this paper.The results showed that the end members of ophiolite outcropped in Sumdo area include serpentine,mafic cumulate,meta-gabbro,meta-basalt and plagioclase granite.All end members are in fault contact and occur in the original Chasagang formation.Four Zircon U-Pb dating samples Th/U ranged between 0.20 and 5.73,The harmonic ages obtained were 273.8±2.2 Ma,266.4±2.0 Ma,258.7±4.4 Ma and 304.4±3.0 Ma,respectively,and the samples all have positive?_Hf?t?values?+9.2-+15.7?.Ultrabasic rock samples have strong alteration,almost all of them are serpentine.Their geochemical characteristics are basically consistent with those of abyssal peridotite in the world.They are similar to the remnants of mantle produced by partial melting of about 10%-20%in the source area of basalt in the mid-ocean ridge.They are formed in deep-sea environment and have little relationship with the ultimate ocean-continental subduction.They belong to mantle peridotite of MOR type ophiolite in general.Geochemical characteristics of basic rocks are divided into N-MORB and E-MORB,which are not contaminated by crustal materials.We believe that they were formed in the mid-ocean ridge environment.Combined with the characteristics of Hf isotopes and previous studies,We believe that there may be mantle plume activity in the Sumdo Paleo-Tethys Ocean during the Permian.During the process of mantle plume rising and decompression melting,due to the low pressure near the mid-ocean ridge,part of the mantle plume material was pulled to the mid-ocean ridge,which was mixed with the depleted mantle magma at the lower part of the mid-ocean ridge,resulting in the increase of the content of TiO₂ in some N-MORB samples and the content of TiO₂ and light rare earth elements in E-MORB samples.At the same time,the phenomenon that E-MORB sample has positive?_Hf?t?value is explained.In this paper,the rock assemblages of ophiolites in Sumdo area have been determined for the first time and preliminary geochemical studies have been carried out.and provides a new idea for the dynamic evolution of the Sumdo Paleo-Tethys Ocean.Through the chronological study of the Sumdo ophiolites,it is believed that the Sumdo Paleo-Tethys Ocean should be opened earlier than 304Ma and finally closed later than 258 Ma.