Abstract:

Changbaishan-Tianchi volcano(CBS-TC), located in Jilin Province on the border between China and North Korea, is the largest composite volcano around China, which is still active. The eruption stages of this large Quaternary composite volcano can be roughly divided into 2.0~1.48Ma shield forming stage, 1.48~0.05Ma cone forming stage and the explosive eruption stage since 50000 years ago. Its great eruption activities(the Millennium Eruption)from 946AD to 947AD and magmatic disturbances from 2002 to 2005 have attracted great attention of the government and scholars.

Predecessors have done a lot of researches 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 the evolved magma undergoing the process of fractional crystallization. In the past, the cone forming stage was considered to have the characteristic of “bimodal” eruptions, that is, the cone forming eruptions of high SiO₂ trachytic/comenditic magma was accompanied by the low SiO₂ basaltic 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 SiO₂ and Na₂O+K₂O contents are increasing with the degree of evolution. Since the late Pleistocene, Tianchi volcano has entered the stage of explosive eruptions with strong caldera forming effect. The eruptive products are mainly comenditic/trachytic airborne pumice, ignimbrite and so on. However, there are still many disputes about the magmatic evolution of CBS-TC, especially the evolution process from basalt to trachy-andesite, trachyte and comendite. In this study, we did abundant field geological investigation and collected rock samples of each eruptive stage of CBS-TC, and carried out whole-rock geochemical analysis. The results show that major elements of these samples have continuous linear trends with increasing of SiO₂ content in magma, and the distribution of rare earth elements and trace elements is also consistent, which indicates a continuous evolution process. Meanwhile, compared with intermediate-basic magma, the trachyte and comendite magma in Tianchi has a characteristic of high Th/La and ⁸⁷Sr/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 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 have occured 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 simulation results are consistent with the trends of tested major elements of Tianchi volcano. To sum up, we found that besides fractional crystallization, assimilation and contamination of shallow crustal granite also play an important role in the evolution of basalt to comendite.

In this paper, the magmatic evolution of Tianchi volcano has been studied systematically, during which the method of petro-thermodynamic simulation combined with geochemical analysis is used. A series of new understandings have been obtained, including the eruption sequence, magmatic evolution, and contamination processes of Tianchi volcanic rocks. This analysis procedure provides a certain reference for the future study. The conclusions help to better understand this largest active volcano in China, and provide new ideas for interpretation of volcanic monitoring data, which helps prevent volcanic disasters. The study also provides references for the regional construction planning of the government.

Key words: Changbaishan-Tianchi volcano, geochemistry, MELTS model, continuous evolution, assimilation

摘要：

长白山天池火山是一座大型的复合式活火山, 公元946-947年发生的大喷发和2002-2005年的岩浆扰动事件, 使其受到社会和学者的高度关注。然而, 目前对天池火山的岩浆演化过程, 尤其是玄武岩-粗安岩-粗面岩-碱流岩的演化过程还未能获得良好的认识, 存在诸多争议。文中通过大量野外工作, 获得了天池火山各阶段的岩石样品, 对其进行了全岩地球化学测试与分析。结果显示, 不同喷发阶段的火山岩主量元素随着岩浆硅含量的上升有着连续渐变的趋势, 稀土及微量元素的分布也具有一致性, 反映了一个连续的演化过程。同时, 文中基于MELTS模型对天池火山的岩浆演化进行了模拟, 模拟结果与天池火山的演化过程具有良好的吻合性, 并发现粗面岩向碱流岩的演化过程中除经历分离结晶作用外, 还遭到了浅部地壳花岗岩的同化混染。这些对天池火山岩浆演化过程的新认识, 加深了对天池火山发展演化过程的理解, 也为未来火山喷发的灾害预警和监测数据解释提供了理论支撑。

关键词: 长白山天池, 地球化学, 热力学模拟, 连续演化, 同化混染