Characteristic And Genetic Analysis Of Pyroclastic Deposits Near Ice-Free River In Tianchi,Arxan,Inner Mongolia

Arxan is located in the middle of greater hinggan mountains,northeast of Inner Mongolia Autonomous Region.Since Cenozoic,with the subduction of west Pacific plate and back-arc spreading effect of Japan island arc,this area become active again.In recent years as the Arxan National Geopark is developing,this area attracts a wide attention of travelers and geologists.This dissertation carries out the study of pyroclastic deposits near Ice-Free River in Tianchi Geo-region,using the analysis of landform,characteristics of microscopic morphology and methods of grain size analysis,intending to make clear of the geological profile characteristics,granularity characteristics,genetic types and the position of crater eruption.The main achievements are as follows:1.Through the macro landscape analysis,it is confirmed that the geological characteristics of Ice-Free River pyroclastic profile is similar with the typical base-surge deposits,including large planar parallel bedding,rhythmic bedding,tabular bedding,lenticular bedding,and partial disturbance structure.2.The microscopic morphology of pyroclastic research reveals the clastics in different eruption type with different microscopic characteristics.Through the observation of clastics thin sections and scanning electron microscopy(SEM),the characteristics of clastics are as follows: the clastics composition of phreatomagmatic eruption are mainly surrounding rocks,with certain roundness;the clastics of have partial angularity,inconspicuous holes,ladder-shaped surface,and the special fracturing;the clastics composition of magmatic eruption are magmatic clastics,with sharp angularity and developed holes.The genetic types of clastics in study area are phreatic eruption,phreatomagmatic eruption and magmatic eruption.3.The sieving method is used to weigh the granularity of layers.By calculating average grain diameter,median particle diameter,sorting coefficient,skewness,kurtosis,fractal dimension value,and plotting volcanic clastics distribution diagram and probability histogram,etc.,this dissertation comprehensively analyzes clastics size distribution characteristics and genetic types and divides the eruptive periods.According to the results,Ice-Free River pyroclastic profile was formed by phreatic eruption,phreatomagmatic eruption and magmatic eruption.4.ESR test of clastic samples obtains the formation age of profile in the middle Pleistocene;with data of DEM remote sensing image and geomorphology,and comparative analysis with Woniupaozi Volcano in Chaihe,Zhalantun,this dissertation discusses the position of crater forming the Ice-Free River pyroclastics,and finally concludes that profile was resulted from Wusulangzi Lake Volcano eruption,and formed the maar lake-Wusulangzi Lake,which is the product of the typical phreatomagmatic eruption.