Petrography And Geochemistry Indication Of Crystallization Differentiation And Magma Mixing Process Of The Tianchi Volcano, Changbaishan

The Tianchi volcano at the Changbaishan Mountains is the largest and mostdangerous volcano in China. This volcano has undergone a series evolution stagesincluding early shield building, middle composite-cone construction, and lateignimbrite formation. The changes in chemical composition of magma controlleddifferent types of eruptions. What is notable is that crystallization differentiation andmagma mixing changed the magma composition a lot. Thus, it is of great significanceto study the magma evolution characteristics, especially crystallization differentiationand magma mixing in the Tianchi volcano, which could provide some information forfuture eruption and evidence for hazard prediction.Tianchi Volcano located in the border between China and North Korea belongs tothe Cenozoic volcanic tectonic belt on the continental margin of eastern Asia. TheTianchi volcano lies at the junction of the NNE-trending uplift zone, which is betweenthe Songliao Basin and the back-arc basin in the Sea of Japan, and NW–directedHuadian-Jince volcanic-fracture zone. This volcano is the best one preserving well thepolygenetic central volcanoes in China, which is made up of lower shield basalt,middle trachyte–comendite cone, and upper sheet of trachyte–comenditepyroclastic rock. The basalts, in time sequence, are Quanyang Basalt, Toudao Basalt,Baishan Basalt, Laofangzixiaoshan Basalt and Heishigou Basalt. The volcano cone ismade up of Xiaobaishan Trachyte of pre-cone and Baitoushan Trachyte–Comenditeof major cone building (further divided into four stages).On the TAS plots of Tianchi volcanic rocks, most of the samples are alkalinerocks and others are subalkaline rocks. Trachyte, comendite and part of Baishan,Laofangzixiaoshan and Heishigou basalt are alkaline rocks, however, Quanyang,Toudao and another part of Baishan basalt are subalkaline rocks. The alkaline basaltsare also sodic basalt, and subalkaline basalts are tholeiite.Most of the alkaline basalts have less phenocryst texture as a whole withintergranular–intersertal texture in matrix. The feldspar in matrix and phenocryst areboth labradorite. In tholeiite, there are more phenocrysts which primarily are labradorite and less chrysolite–hyalosiderite. The matrix has hyalopilitic texture andaugite in it. In trachyte–comendite, more and more feldspars are present asphenocryst which are sanidine and anorthoclase. In addition there are a few ofhedenbergite, fayalite and quartz.The primitive mantle-normalized trace element patterns are highly uniform,indicating the same magma source for different stages of basalts, trachyte andcomendite. The ratios of LREE and HREE in alkaline basalts are higher than that intholeiite. There is no Eu anomaly in all of the basalts. While there obvious Ba, Sr, Eu,and Ti anomalies are in the most of the present in trachyte and comendite whichprobably means strong crystallization differentiation.In different kinds of trace element relevant diagrams, all of the basalts show thehomologous and partial melting trend. Alkaline basalt (3-6%) has a lower degree ofpartial melting than tholeiite (8-18%). In some compatible-incompatible elementrelevant diagrams, however, basalts exhibit also a crystallization differentiation trend.All of these diagrams testify that basalts have also undergone crystallizationdifferentiation of olivine and pyroxene after experiencing corresponding batch ofpartial melting. Trachyte and comendite show the crystallization differentiation trendin different kinds of trace element relevant diagrams. Combining with characteristicsof trace element and petrography, it can be thought that trachyte and comenditeevolved from basalts by strong fractional crystallization of alkali feldspar andplagioclase. There are also many trachyte and comendite samples revealing theexistence of magma mixing. The combination of the characteristics of hand specimen,petrography and mineralogy showing magma mixing occurred in the first phase andthird phase of cone-building and millennium eruption, respectively. The mixing endmembers are trachytic-comenditic, trachytic-basaltic and trachytic-trachytic magma.