The U-Pb Chronological Characteristics Of Detrital Zircons Of Sumdo Formation In Zhikong Area Of Tibet And Its Significance

The distribution range and duration of the Sumdo Paleo-Tethys Ocean have been the focus and controversy of geoscience research.The Zhikong region in Tibet has also been the main research area of the Sumdo Paleo-Tethys Ocean.In this thesis,the Sumdo Formation in the Zhikong area of Tibet was studied using sedimentary environment analysis,zircon U-Pb chronology analysis,and zircon Lu-Hf isotope in-situ analysis to explore the early Sumdo Paleo-Tethys Ocean origin.Field geological survey found that the lithologic combination of the Sumdo Formation located in the straight hole area of the eastern Gangdise belt in Tibet may be a set of terrestrial clastic rocks formed in the coastal environment or coastal shallow coastal environment.The measurement of the paleocurrent sign indicates that the paleocurrent direction is northeast.Two pieces of detrital zircon U-Pb chronology samples obtained 302Ma?3198Ma ages through testing.The youngest 302Ma age value indicates that the Sumdo Formation was formed around the Late Carboniferous.Combining the characteristics of the zircon cathode luminescence images and the rare earth element mode matching mode,it is found that most of them are magmatic zircons,and very few are affected by metamorphism.The in situ analysis of detrital zircon Lu-Hf isotope indicates that the source of the Sumdo Formation contains a small amount of mantle-derived magma,but it is mainly composed of ancient crustal materials that have been transformed and recycled.Compared with the spectrum of clastic zircons in the Carboniferous-Permian strata in various regions of the region,it is found that there is a unique 300Ma?390Ma age value interval in the Sumdo Formation,and the whole is in the same range as the inner strata of the Gangdise block Higher kinship.Combining with the data of late Paleozoic magmatic activity in the region,the zircon age of this interval in the Sumdo Formation may come from the magmatism with continental margin rifting and geochemical characteristics under the extension background at the same time on the south sides of the study area and the early period on the east side Boshula mountains volcanic arc.Volcanic rocks are developed in the Late Paleozoic Nuocuo Formation?C₁nc?and Laigu Formation?C₂P₁l?formations in the Middle East of the Gangdese Belt,representing two important volcanic activities in the Early Carboniferous and Late Carboniferous.Among them,the Nuocuo Formation volcanic rock coexists with deep-water slope facies sediments,the Laigu Formation volcanic rock coexists with conglomerate mudstone,shale,etc.,and the volcanic rock has the geochemical characteristics of rifting.Since the Devonian,the northern margin of the Gondwana continent has shown the characteristics of a passive continental margin,but the Nuocuo Formation,Laigu Formation and other late Paleozoic magmatism indicate that the Carboniferous has been transformed into a continental margin rift with an extensional tectonic background Active continental marginal environment.Combined with the volcanic rocks formed by syn-sedimentary faults in the Sumdo Formation in the study area and the stratigraphic research data of the Nuocuo Formation and Laigu Formation in the same period,it is compared with the previous research?the basalt dating results of the Sumdo ophiolite belt and the eclogite protolith age tests indicate that the Sumdo Paleo-Tethys Ocean has been opened in the Late Carboniferous?.This paper believes the Sumdo Formation formed in the coastal environment or the shallow coastal environment is very likely to be the early geological record of the“Sumdo Paleo-Tethys Ocean”.