Understanding Of The Processes Of Magma Evolution And Contamination At Tianchi Volcano Based On Petro-thermodynamic Simulation

Changbaishan-Tianchi volcano is located on the border between China and North Korea.It is located between the east of Dunhua-Mishan fault on the outermost side of the Northeast rift system and the west of the back arc basin of the Japan sea.Multiple groups of faults in the direction of NE-SW and NW-SE are developed widely.Its volcanic stratigraphic basement is composed of Archean to Meso-Neoproterozoic metamorphic rocks,Paleozoic to Mesozoic sedimentary rocks and Mesozoic andesite and granite.The eruption stages of this large quaternary composite volcano can be roughly divided into 2.0-1.0Ma shield forming stage,1.0-0.05Ma cone forming stage and the explosive eruption stage since 50000 years ago.Among them,the Millennium eruption from 946 to 947 is one of the largest volcanic eruptions around the world in2000 years.After that,there may also be eruption activities in 1168,1702 and 1903（controversial）.In the 21st century,with the development of volcano monitoring in China,Changbaishan-Tianchi volcano monitoring station observed a large number of volcanic earthquakes and deformation from 2002 to 2006,which may related to the intrusion of underground magma,so that the risk of eruptions have been highly concerned by the government and scholars.Predecessors have done a lot of research on Tianchi volcano,including its eruption periods,distribution of eruptive products,disaster assessment and so on.Geophysical data show that there are anomalies in the lower part,indicating the existence of magma chambers or conduits,but the accurate boundary and depth of magma chambers need to be further explored.The study of petro-geochemistry shows that the products of shield forming stage of Tianchi are mainly potassic trachy-basalts.The MgO^#of these basic magma is lower than that of the primary magma in Northeast China,indicating that they are evolved magma undergoing the process of fractional crystallization.In the past,the cone forming stage was considered to have the characteristic of"bimodal"eruption,that is,the cone forming eruption of high SiO₂trachytic/comenditic magma was accompanied by the low SiO₂basaltic magma,which formed small cinder cones on the edifice.In recent years,some drilling data show that there are thick basaltic trachy-andesite and trachy-andesite strata under the cone,indicating that the products of the cone forming stage of Tianchi include early basaltic trachy-andesite,medium trachy-andesite and late trachyte.Their SiO₂and Na₂O+K₂O contents are increasing with the degree of evolution.Since the late Pleistocene,Tianchi volcano has entered the stage of explosive eruption with strong caldera forming effect.The eruptive products are mainly comenditic/trachytic airborne pumice,ignimbrite and so on.At present,the magmatic evolution of Tianchi volcano,especially the evolution process from basalt to trachy-andesite to trachyte and to comendite,has not been well understood.Through a lot of field work,this paper obtained the rock samples from Tianchi volcano of each stage,including and trachy-basalt,basalt,basaltic andesite,basaltic trachy-andesite,trachy-andesite,trachyte and comendite.They are analyzed by whole rock geochemistry of major elements,trace elements and Sr,Nd,Pb isotopes.The major elements show that the main oxides Al₂O₃,CaO,Na₂O,K₂O and P₂O₅have a good linear relationship with SiO₂and MgO.The distribution of REEs and trace elements has a similar right dipping trend.The fractionation of light and heavy REEs are obvious（La _N/Yb _N=4.33～30.41）,which reflects the homogeneous evolution continuously.The REEs of trachyte and comendite are significantly higher than that of basalts,indicating that they were formed in the late stage of magmatic evolution.Trace elements such as Eu,Sr and Ba of Tianchi silicic rocks have obvious negative anomalies,and falls on the trend of fractional crystallization in the La-La/Sm diagram,which indicates that the magmatic evolution is mainly controlled by fractional crystallization.Meanwhile,compared with intermediate-basic magma,the trachyte and comendite magma in Tianchi has a characteristic of high Th/La and⁸⁷Sr/⁸⁶Sr values,indicating that the magma has also experienced assimilated contamination by crustal materials.In order to verify this fractional crystallization with assimilation（AFC）process of Tianchi magma,the author uses petro-thermodynamic simulation（MELTS model）to calculate the magma evolution.The initial melt composition selects the relatively primitive basalt HSOB（Mg^#=66）in Tianchi Volcanic Area.The condition parameters used in the simulation include temperature,pressure,oxygen fugacity,water content,etc.Those parameters are considered as close as possible to the real situation in the magma system.The conditions of pressure and water content are still controversial,which are limited by this simulation.It is found that the evolution of Tianchi magma tends to be carried out under the conditions of low pressure（2kbar）and high water content（≥0.5wt.%）,and about 10%granitic assimilates were mixed in the late stage of evolution,which is consistent with the previous research on the location of magma chambers and melt inclusions.The evolution results obtained by MELTS also have a good correlation with the major elements,which reveals a complete process of magma evolution.The tectonic discrimination diagrams composed of inactive elements such as Nb,Ti,Zr and Y indicate that the Tianchi basalts have a characteristic of OIB within plate.Combined with Sr,Nd,Pb isotopic data and Dy/Yb trace element simulation,it shows that the source of Tianchi volcanic rocks come from the mixing of depleted mantle and enriched mantle（EM1）,and the EM1 component may comes from the recycled western pacific subducted slab.Its protolith is the mixture of peridotite and pyroxenite（eclogite）.In this paper,the magmatic evolution of Tianchi volcano has been studied systematically,during which the methods of petro-thermodynamic simulation combined with geochemical analysis are used.There are a series of new understandings have been obtained,including the eruption sequence,magmatic evolution/contamination processes,and the tectonic source of Tianchi volcanic rocks.This analysis procedure provides a certain reference for the future study.The conclusions help to understand better of this largest active volcano in China,and also provide new ideas for interpretation of volcanic monitoring data,which helps preventing volcanic disasters.