Sediment Mineral Characteristics Of The Jianzha Basin In Northeastern Margin Of The Tibet Plateau, And Its Paleo-environmental Significance

The Jianzha Basin is located in the basin-mountain coupling geomorphology pattern of the northeast margin of the Tibetan Plateau,and it is located in the convergence area of eastern monsoon region,the high-cold region of the Tibetan Plateau,and the northwest inland arid region,the westerly circulation and the east asian monsoon is also collided with each other in the Jianzha Basin,because of these geographical factors,the study region is very sensitive to the changes of climate and environment.The great thick Cenozoic sediments continuously deposited in Jianza Basin have recorded abundant paleoenvironment and tectonic information.The sediments provide a reliable geological record for us to explore the relationship between the Asian inland aridification,the evolution of the East Asian monsoon,and the uplift of the Tibetan Plateau.In this paper,the sedimentary sequence of 11.8~5.80 Ma in the Jiarang section of Jianzha basin on the northeastern margin of Tibetan Plateau is studied.The aeolian red clay is the main accumulation of the section,in which a small amount of fluvial-lacustrine deposits are found.Taking the whole rock mineral index study as the main research,combined with the characteristics of the evolution of plus profile sedimentary facies,and taking the variation of the profile susceptibility value as a reference comparison,the mineralogy study was carried out systematically to obtain the ancient climate change in the area.The palaeoclimate environment information of 11.80~5.80 Ma in the Jiarang section can be improved by analyzing the types,contents,ratios and assemblage characteristics of sediment minerals and combining with the existing research results.The main results are as follows:(1)The mineral types of sediments in the Jiarang section are mainly include detrital minerals,carbonate minerals and clay minerals,most of which are detrital minerals,mainly Quartz,Plagioclase,Muscovite,Biotite,a small amount of Spinel,Pyroxene(ordinary Pyroxene,Johannsenite and Protoenstatite,etc.),Rutile,Corundum,Magnetite,Titanomagnetite,Titanite,Nepheline,occasionally Zircon,Garnet,Anatase etc.Calcite is the main carbonate mineral,Dolomite content is less,Chlorite accounts for the highest proportion of clay minerals,and a small amount of Hematite,Kaolinite and Zeolites are also found.(2)By analyzing the types and distribution characteristics of sediment minerals,the higher content of calcite,plagioclase,chlorite,lower muscovite content,the maturity index of light mineral composition and the stability coefficient of heavy minerals indicate that the weathering is less,the degree of soil formation is weak and the rainfall is lower in this region,which is a dry and cold climate.On the contrary,the lower content of calcite,plagioclase,chlorite,higher muscovite content,the maturity index of light mineral composition and the stability factor of heavy minerals indicate that the weathering and soil formation in the region are strong,and the rainfall is higher in this region,which is a warm and humid climate.Combined with the susceptibility curve,and compared with the deep-sea oxygen isotope curve and other global climate change events,the climate and environment evolution of11.80~5.80 Ma in Jianzha basin can be divided into four period,I:11.80~10.0Ma,dry-cold climate;II:10.00~8.60 Ma,warm-humid climate that fluctuates;III:8.60~6.20 Ma,warm-humid climate;IV:6.15~5.80 Ma,dry-cold climate.(3)At about 10 Ma,8.6Ma and 7.2Ma,the contents and assemblage characteristics of the main minerals have significantly changed,indicating that the paleoenvironmental conditions changed dramatically at that time.It can be revealed that a series of strong tectonic movements occurred extensively in the northeastern margin of Tibetan Plateau during the period of about 10~8Ma.The denudation from the surrounding mountains increased the sedimentary material in the study area,and the material source had the characteristics of near origin.The paleoenvironmental evolution of Jianzha and its surrounding areas recorded by the mineral composition distribution in the Jiarang section is affected by the tectonic uplift of the Tibetan Plateau and the global climate change.