Study On Characteristics Of Petrology And Magma Evolution Of The Tianchi Volcano In Changbai Mountain

Changbaishan-Tianchi volcano is the largest and most dangerous volcano, with the potentially disastrous large eruption in China. It has undergone early shield-forming stage, middle composite-cone construction stage, and late ignimbrite-forming stage. Thus,it is very important to study the characteristics and timescales of magma evolution of Tianchi volcanic rocks, and it can also provide some information for future eruption and eruptive hazards.This thesis discusses the characteristics, processes and constraints on the timescales of magma evolution of Tianchi volcanic rocks through field survey, geology, petrochemistry, geochemistry and K-Ar dating etc. And some important processes and new discovery are achieved.It is discovered from the thin sections of samples that the main phenocrysts of Tianchi volcanic rocks are olivines, pyroxenes, feldspars and ferrotitanium oxides etc., of which feldspars are the most important phenocrysts in Tianchi volcanic rocks. According the relationship of different minerals in thin sections, olivines and preoxenes are the minerals which had cystallined at first and then plagioclase and alkaline feldspars, and the last were aegirine augite which had more sodium,while ferrotitanium oxides exited throughout the whole evolution processes。In the TAS plots of Tianchi volcanic rocks, most of the samples are alkaline rocks and others (a small amount of the samples) are subalkaline rocks. Actually those subalkaline rocks are tholeiitic rocks judged from AFM, w(SiO2)-w(FeOT)/w(MgO) and w(FeOT)-w(FeOT)/w(MgO) diagrams. Therefore, intermediate-basitic rocks are mainly trachybasalt, basalt, basaltic trachyandesite, trachyandesite, and a few basaltic andesites. Intermediate-acid rocks are mainly trachytes, ryolites and a few dacites. Trachytes all are comenditic trachytes, while ryolites include comendites and pantellerites. However, pantellerites include three types: normal, peralkaline, peraluminous pantellerites,which are distributed in east part (North Korea side) of caldera rim, upper part of east caldera wall of Tianchi(North Korea side)and Heishigou (China) etc. The MgO, Ni, Co, Cr contents of basalts are commonly lower than that of primitive magma. P2O5—Ce diagram, incompatible element characteristics and acidity and alkalinity indexes of these basaltic rocks indicate that this kind of basalt magma is the one which has experienced the crystalline differentiation of part of magnesium iron minerals such as olivine, pyroxene and plagioclase. Their Al2O3/CaO ratio and CaO/Na2O ratio vs. SiO2, and trace elements vs. MgO diagrams indicate that the magma evolution of the Tianchi volcano was deeply affected by crystallization differentiations, in which fractional crystallizations of olivines, pyroxenes and plagioclase etc. played important roles.The basaltic rocks erupted during either shield-forming stage, composite-cone construction stage or modern time, have similar∑REE, REE pattern, positive Eu anomaly to different extents and La/Sm ratio as well as extremely similar incompatible element pattern, indicating that they have the same magma source region。But there are some differences among the REE of trachybasalts of different stages.∑REE of basalts of the early Pleistocene is almost as same as∑REE of the Holocene, and∑REE of the middle and late Pleistocene is the highest. As for the LREE/HREE ratio, the Holocene is the highest, the middle and late Pleistocene is the lowest and the early Pleistocene is the middle.Trachytes and ryolites have similar REE distribution pattern and strong negative Eu anomaly, which indicate that they have the same magma source region and similar processes of magma evolution. Trachyandesites have small negative Eu anomaly between basalts and trachytes and ryolites, and this shows that with magma evolution, positive Eu anomaly changed to small negative Eu anomaly and then more strongly negative Eu anomaly, which indicate differentiation crystallization of plagioclases become more strong. Compared the data of ryolites with that of trachytes in the same stage, it is found that with the evolution of magma, ryolites have moreΣREE than trachytes and LREE of ryolites are more enriched than trachytes.In addition,∑REE increases from basaltic rocks to trachyte of cone-forming stage and modern erupted materials with ryolites as its majority. Except the different Eu anomaly, the REE distribution pattern for these rocks remains rather similar, which indicates they are the products evolved from a same magma reservoir. And the extremely similar Sm/Nd also may prove it. while basaltic rocks, trachyte and ryolites also have a similar REE distribution pattern (except strong negative Eu anomaly) and La/Yb ratio, indicating that they have not only the common magma genesis but also the close evolution relationship, in which the differentiation crystallization plays a key role.The characteristics of REE and trace elements of basalts, trachytes and ryolites together point out they have experienced crystalline differentiation. However, basalts come from the mantle magma reservoir having experienced crystalline differentiation of plagioclase, olivine, pyroxene; while trachytes and ryolites come from the crust magma chamber having experienced crystalline differentiation of plagioclase, fayalite and pyroxene rich in iron, etc.Trachybasalts of Tianchi volcano have long eruption history. At the beginning of the early Pleistocene (2.0Ma), the Tianchi volcano started the trachybasaltic activities of the shield-forming stage, which lasted to 1.2Ma. Then, from the end of the early Pleistocene (0.87Ma) when the trachybasaltic magma changed to the trachytic and trachyandesitic magmas to the eruption of Tianchi volcano in Holocene, the episodic volcanism of the trachybasaltic magma from the mantle chamber never stopped, which could be clearly known from scoria cone such as Laohudong, Laofangzixiaoshan, Chifeng etc. around the Tianchi cone and Dongmaanshan etc. between the Songjiang River and Lushui River in the west of the volcano cone. It is found that trachybasalts of Mu-du-bong parasitic cone in North Korea were incumbent on the pumices of Millennium Eruption, indicating the eruption of trachybasalts from the mantle magma chamber lasted to modern time.According to field survey in China and North Korea, the data of trachyandesite samples in Tianchi road and Dayu bore and Shuangmufeng bore, it is believed that trachybasalt magma started to evolve to trachyandesite magma and there is a much earlier stage for the trachytic eruption (ca. 2Ma)beneath the Tianchi Volcano. Thus, there was an evolution cycle from basalts to trachyandesites and trachytes in early Pleistocene. On the basis of reported K-Ar age data of trachytes and ryolites of cone-forming,it is known that trachytes of cone-forming erupted between 1.12Ma and 0.04Ma。From 1.203Ma to 1.12Ma,basalt and trachyandesite started to evolve to trachytes. In about 0.19 Ma, late cone-forming, trachytic magma started to evolve to ryolitic magma, and the ryolitic magma eruption of cone-forming lasted from 0.190±0.004Ma to 0.0192±0.005Ma.According to the field survey,the existed K-Ar ages and the new K-Ar data we presented here, it shows a different distribution feature of the trachytes from different stages on the north and northeast flank of the Tianchi Volcano, and it is apparent that the ages of the trachytes on the north flank are newer than those on the northeast flank. It is inferred that the K-Ar age of the oldest trachytes composing the composite cone in the northeast flank of the Tianchi volcano is 0.38Ma,which is in the middle Pleistocene,and that the trachyte is the product of the third stage of the trachytic composite cone construction. No evidence of the first and second stages of eruption for the composite cone construction has been found on the northeast flank of the Tianchi Volcano.What's more, it is found that there are rocks with content of SiO2 between 52%～64% in the Tianchi volcano by the study. They are formed mainly by the evolution of basaltic magma, and it may reveal that there is another crust magma chamber which is different from the crust magma chamber where trachytes and ryolites settled, or there is a multi-layer crust magma chamber system beneath the Tianchi volcano. However, it is impossible to exclude the possibility of the magma mixing which formed such samples due to the small amounts of them, lack of their data and small-scales, and the reason needs further researches in the future.