Analysis Of Paleogene Sedimentary Characteristics In Changdu Basin,Tibet

The Changdu Basin in Tibet is located in the northern section of the Sanjiang metallogenic belt.The sedimentary mineral resources in the basin are extremely rich,which is of great practical significance for the analysis of sedimentary characteristics.This paper,takes the Paleogene sedimentary characteristics of Changdu Basin as the research object,combining the previous research results,combined with the theoretical knowledge of sedimentology,petrology and geochemistry to analyze the granularity distribution characteristics and geochemical characteristics of the Paleogene Gongjue Formation sandstone samples,explore the filling characteristics and sedimentary environment of Paleogene in Changdu Basin.Mainly achieved the following knowledge and conclusions:(1)The geochemical characteristics of the major elements in the study area show that the Paleogene clastic sediments in the Changdu Basin are of low maturity.The rock samples are mainly lithic sandstone,followed by feldspar sandstone.The sandstone provenance mainly has the characteristics of evolution from the neutral igneous rock area to the continental quartz source area.(2)The standardized distribution curve of rare earth element chondrites generally shows that light rare earth elements are relatively enriched,heavy rare earth elements are relatively depleted,and there are negative anomalies of Eu and weak negative anomalies of Ce.LREE/HREE and(La/Yb)_Nreflect the large degree of differentiation between light and heavy rare earth elements in the study area.The La _N/Sm _Nvalues ranged from 3.26 to 5.07,with an average value of 3.70,reflecting the obvious degree of differentiation between light rare earth elements.Gd _N/Yb _Nvalues range from 1.03 to 1.88,the average value is1.50,reflecting that there is no obvious difference between heavy rare earth elements.The standardized distribution curve of North American shale shows that light rare earth elements are slightly enriched.The distribution pattern generally shows a flat feature.Ce element weak negative anomaly.There is no obvious abnormality of Eu element.(3)The trace element geochemical characteristic results indicate that the source material in the study area is mainly composed of felsic rocks,the parent rock debris mainly comes from the upper crust and formed in the tectonic background of the continental island arc.The element ratios Sr/Ba,Sr/Cu,V/Cr,U/Th,Ni/Co and V/(V+Ni)all show that the depositional environment in the study area is a dry and hot oxidation environment.(4)The geochemical characteristics of boron element show that the boron element value of the sample in the study area ranges from 36.9?g/g to 176?g/g,with an average value of 104.53?g/g,indicating that the sedimentary environment of the lacustrine facies in the Gongjue period of the Changdu Basin is mainly brackish to salt water,with fresh water added.(5)The sandstone particle size probability cumulative curve in the study area reflects that the main sedimentary environment during the Gongjue Formation period is the delta plain water distributary channel and the delta front underwater distributary channel,estuary dam and Yuansha bar.The sedimentary characteristics of each lithologic segment of the Paleogene Gongjue Formation in the study area indicate that the Changdu Basin was formed in an alluvial fan-river-lake-delta sedimentary environment.