Study On The Sedimentary Characteristics And Paleoceanographic Environment Of Dasuo Section In Nagarze Area Of Tibet

The determination of the Jurassic-Cretaceous boundary is a worldwide geological problem.The Jurassic-Cretaceous strata and volcanic rock outcrop well in Nagarze area from the southern Tibet,which provides good conditions for the study of precise boundary age.Through the systematic study of stratigraphy,petrology,sedimentology and elemental geochemistry of Dasuo section in Nagarze,and the zircon U-Pb isotopic dating of volcanic rocks in the study area,the accurate isotopic age was obtained.On this basis,the characteristics of paleooceanographic environment in the study area was analyzed in order to provide evidence for the division of Jurassic-Cretaceous boundary in Tibet and the evolution of the Tethys Ocean during this period.The SHRIMP U-Pb weighted average age of zircons from the lower part of the Sangxiu Formation in the Dasuo section is 147±1.6Ma,which is close to the current internationally recognized Jurassic-cretaceous boundary age of 145Ma,and which suggests that the bottom of the Sangxiu Formation may be located at the Jurassic-cretaceous boundary.The geochemical index element ratio and discriminant diagram are used to analyze the provenance properties of the study area.CIA value,ICV value,Th/Sc value and Th/U value are combined,indicating that the rocks in the study area have been strongly influenced by weathering.Combining Al₂O₃/Ti O₂values,La/TH-Hf discriminant diagrams and La/SC-Co/Th discriminant diagrams,it indicates that the provenance of the Dasuo section is derived from the weathering of felsic volcanic rocks.Combined with the Ti O₂-（Fe₂O₃+Mg O）and Th-Co-Zr/10 discriminant diagrams,it is concluded that the sediments of Dasuo section belong to the products of oceanic island arc structure.Based on the geochemical characteristics of major,trace and rare earth elements,the paleooceanographic environment of Dasuo section is systematically analyzed.Based on the characteristics of Fe²⁺/Fe³⁺,Cu/Zn,V/Cr,V/（V+Ni）and Ce_anom,the sedimentary environment in the study area is determined to be a weak oxidation-weak reduction environment.Based on K₂O/Na₂O,（Mg O/Al₂O₃）×100,Ca O/（Ca O+Fe）,Sr/Ba,V/Ni andΣREE,the study area is determined to be a coastal sedimentary environment with low salinity and stable water body.Considering the grain size variation,sorting degree and Zr/Rb value of the samples,it can be concluded that the hydrodynamic conditions in the sedimentary period are weak.Based on Fe/Mn,Si O₂/Al₂O₃,Sr/Cu,ΣREE andδEu,the paleosedimentary environment in the study area was relatively warm and humid.The sedimentary period of Sangxiu Formation in the study area is a weak oxidation-weak reduction environment,and it is developed in a littoral to shallow sea sedimentary environment with gradually shallower water,weak hydrodynamic force,reduced paleosalinity,accelerated sedimentation rate,and warm and humid climate.Figure 23;Table 22;Reference 75...