Study On The Volcano Degradation Process And Volcanic Geology Of Yanshan In Arxan Volcanic Field,Inner Mongolia

Yanshan volcano erupted in Holocene,located in the Chaihe-Arxan Volcanic field,Inner Mongolia.In this paper,a detailed field investigation was carried out.We identified the volcanic eruption products and analyzed the eruption process of Yanshan.The morphological parameters of volcanic cones in five volcanic fields of northern China were obtained by 30-resolution DEM.We still studied the volcanic degradation process of these volcanic fields.The conclusions are addressed as follows:Yanshan volcanic eruption products include pyroclastic fallout,cinder cone,lava flow and rafted mounds.Pyroclastic fallout is an roughly indicator of explosive volcanism.The cinder cone consists of early fallout deposits and later spatter mound.Lava flow is the products of final period in Yanshan volcanic eruption.The lava flow cracked the volcanic cone,causing rafted mounds of different sizes floated on the lava flow and emplaced on the southeast side of Yanshan.The Yanshan basalts Na2O/K2O ratio is higher than 1.It shows that the Yanshan basalts belong to soda-alkali basalt According to the chemical classification.Yanshan basalts have low REE amounts and(La/Yb)N ratio.The degree of differentiation of LREE and HREE is relative low.Basalts is characterized by week enrichment of LILEs and week depeleted of HFSEs.It shows that Yanshan volcanic rocks originated from a week enrichment mantle source.There is a small geochemical difference between the spatter and the lava flow.The thickness of the lithosphere is more than 70 km and the basalts originated from deep mantle source.The volcanic eruption process experienced three stages:violent Strombolian type-lava fountaining-lava overflow.Yanshan Hco/Wco ratios equals 0.15 and the angle of outer slope is 26°.According to Hco/Wco-age group rules,the eruption age of Yanshan volcano belong to Sunset age and Merriam age(<0.05Ma),which is consistent with dating result in previous work(1990±100yearB.P.).Aim to learn the degradation process of volcanic cone,Hco value,Wco value and Wcr value of 40 volcanoes were measured by 30m-resolution DEM and corresponding geochronological data were collected from five volcanic field in northern China.Volcanic cone degradation model shows that the Hco/Wco ratio of the cinder cone decreases rapidly in the first 1 Ma,and from 1 Ma to 5Ma,the Hco/Wco ratio of the cinder cone decreases slowly,and the decreasing rate of Hco/Wco decreases over time gradually.We analyze the influence of different climatic conditions on the volcano degradation process:The degradation rate of volcanic cones in humid areas is higher than that of volcanic cones in arid areas.