Study On The Pre-eruptive Conditions Of The Heikongshan Volcano In The Tengchong Volcanic Field

The storage state and dynamic process of magma in the earth's crust are important scientific problems.It not only has important theoretical research value,but also has important practical significance for the prevention and control of volcanic disasters.The Tengchong volcanic field is one of the youngest volcanic area in our country,located on the Tengchong block to the west of the Nujiang River right lateral strike-slip fault zone,and situated along the southeastern margin of the Tibetan Plateau,adjacent to the collision zone between the Indian Plate and Eurasian Plate.The lateral extrusion of the Indian plate led to intense crustal deformation of the Tengchong block and adjacent regions,and thus generated numerous volcanic activities in the Tengchong area.The Tengchong volcanic field is also the only potentially volcanic disaster areas in southwestern China,Therefore,the study of the pre-eruption conditions of Heikongshan Volcano in Tengchong is of great theoretical and practical significance for the study of the storage and transport system of Cenozoic volcanic magma in Tengchong.The most recent eruption of Heikongshan Volcano occurred during the Holocene.Taking the lava of Heikongshan as the research object,this paper discusses the magma evolution process of Tengchong Heikongshan,reveals the magma storage state and establishes the magma storage and transport system model by means of petrography,electron probe and whole-rock analysis of major and trace elements.The phenocrysts of the lava samples from Heikongshan volcano are mainly plagioclase,clinopyroxene and orthopyroxene,with a small amount of olivine,The matrix is mainly composed of plagioclase crystallites and volcanic glass,forming hyalopolitic texture.Plagioclase phenocrysts have a series of different microstructures.The microstructures identifified micro-textures can be grouped into two categories:(i)Growth related textures in the form of coarse/fine-sieve morphology,fine-scale oscillatory zoning and resorption surfaces resulted when the equilibrium at the crystal-melt interface was fluctuated due to change in temperature or H₂O or pressure or composition of the crystallizing melt;and(ii)morphological texture,like glomerocryst,synneusis,swallow-tailed crystal,microlite and broken crystals,formed by the influence of dynamic behavior of the crystallizing magma(convection,turbulence,degassing,etc.).The clinopyroxene and orthopyroxene generally develops compositional zoning.Olivine is usually surrounded by orthopyroxene display typical olivine peritectic textures.There are mineral polyphenocrysts in the rocks,which are mainly composed of plagioclase,monoclinopyroxene and plagioxene.The occurrence of polyphenocrysts is related to the dissociation of the granitic bodies in the magma chamber.All these characteristics indicate that Heikongshan volcano is an open magma storage and transport system.In terms of composition,the Heikongshan volcanic rocks are composed of trachydacite and trachyte,both of which are high potassium calc-alkaline-potassium basalt series.Rare earth elements are in the right dip pattern,enrichment of LREE relative to HREE and negative anomaly of Eu.In the original mantle normalized map,large ion lithophile elements(Rb,Th)are enriched,and high field strength elements(Nb,Ta)are depleted.The calculation of temperature and pressure of different minerals shows that there is a large-scale magma chamber under the Heikongshan volcano,in which there is a large number of crystal granules,that is,magma is mainly stored in the form of crystal granules.The pressure of the crystallite in the shallow magma chamber is 2.2-4.1kbar,the temperature is 923-1122?,the H₂O content is about 1wt%,and the oxygen fugacity is about QFM.In the deep magma chamber,the melt proportion is high,the pressure is in the range of4.1-8.9kbar,and the temperature is in the range of 1122-1255?.The mixing of magma in deep and shallow magma chambers may be an important inducing mechanism of volcanic eruption.