【作者】 孙春强；

【导师】 魏海泉；

【作者基本信息】 中国地震局地质研究所 ， 地球化学， 2008， 硕士

【摘要】 长白山天池火山是中国境内规模最大、最具潜在喷发危险的活火山。天池火山从造盾阶段的粗面玄武岩的溢流式喷发,到造锥阶段的粗面岩的溢出和全新世流纹质岩浆的猛烈爆炸式喷发,火山喷发方式的转换机理主要在于岩浆化学成分变化。所以,研究天池火山岩浆的演化特征就非常重要,并可为未来灾害预测提供一定依据。本论文在长白山天池火山野外地质考察的基础上,运用地质学、岩石学、地球化学、年代学等理论和方法,结合以往的研究成果,对天池火山岩浆的演化特征、过程和时间尺度等问题进行了探讨,取得了若干有意义的发现。通过对采集的岩石样品的薄片的镜下观察,发现天池火山岩的主要矿物成份为橄榄石、辉石、长石、铁钛氧化物等矿物。其中,长石是天池火山岩中最主要的斑晶。根据显微镜下观察到的矿物相互包裹关系可知岩浆房中最先结晶的是含铁镁的橄榄石和辉石,然后晶出斜长石和碱性长石,最后形成的矿物富含钠质如霓辉石,铁钛氧化物的形成则一直伴随期间。在硅—碱图上,天池火山岩绝大部分样品投点落于碱性系列区内,只有少量样品投点落于亚碱性系列区内。其中,亚碱性系列的样品通过AFM图、w(SiO2)—w(FeOT)/w(MgO)和w(FeOT)—w(FeOT)/w(MgO)图解判别,认为它们属于拉斑玄武岩系列。所以,天池火山中基性火山岩以粗面玄武岩、玄武岩、玄武粗安岩、粗安岩为主,少量玄武安山岩;中酸性火山岩则以粗面岩、流纹岩为主,少量英安岩。其中,粗面岩都属于过碱性的钠闪碱流质粗面岩;流纹岩则分为钠闪碱流岩(comendite)和碱流岩(pantellerite)两种,而碱流岩(pantellerite)又可分为正常类型、过碱质、过铝质岩石三个类型,这些碱流岩(pantellerite)分布在朝鲜的海山(火口缘东侧)、将军峰山脊东侧顶部(朝鲜境内)和黑石沟等地。天池火山玄武岩类的MgO、Co、Cr和Ni含量普遍低于原始地幔值,它们的P2O5—Ce关系图、不相容元素特征、酸碱指数DI、SI和AR结果都表明玄武岩类并非原始岩浆,可能是地幔岩浆经过部分镁铁矿物如橄榄石和辉石以及斜长石的结晶分异演化而成的岩浆。各喷发期不同类型的岩石中不同氧化物含量之间存在着同步升高或降低趋势,它们的Al2O3/CaO、CaO/Na2O比值与SiO2之间的关系以及微量元素浓度随着MgO含量的变化的曲线都表明整个天池火山岩浆演化过程中明显受到橄榄石、斜长石、单斜辉石、钛磁铁矿和磷灰石等矿物的结晶分异作用的控制。其中,多种氧化物和微量元素的含量变化与斜长石的结晶分异有密切的关系,表明斜长石的结晶分异在岩浆的演化的过程中起着关键作用。天池火山不同喷发时代的粗面玄武岩的∑REE、La/Sm等相近,都有不同程度的正Eu异常,所有稀土曲线均呈良好的平行关系和形状的一致性,以及极其相似的不相容元素分布型式,暗示了它们有一个相同的岩浆源区。但是,不同时代喷发的粗面玄武岩之间的稀土元素也存在一定的差别。早更新世的∑REE和全新世的相当,而中晚更新世的ΣREE最高;LREE/HREE由高到低的顺序是全新世粗面玄武岩——早更新世粗面玄武岩——中晚更新世玄武岩。从稀土元素球粒陨石标准化模式图来看,粗面岩和流纹岩样品的分布模式相似,显示类似的向右倾斜,且相互平行,都出现了强烈的负Eu异常。它们之间的这种相似性,表明岩浆来自相同的源区,并且具有类似的岩浆演化过程。粗安岩则显示介于玄武岩类与粗面岩和流纹岩之间的小的负Eu异常,这反映出随着岩浆的演化,Eu从正异常变化到小的负异常,然后变化到强烈的负异常,斜长石则从富集作用变化到分离结晶作用,而且分离结晶作用逐渐增强。对比同时代的流纹岩比粗面岩数据,可以发现随着岩浆演化程度的加深,流纹岩一般比粗面岩具有更高的ΣREE,轻稀土元素更加富集。另外,从粗面玄武岩、粗面岩到流纹岩,稀土元素的总量呈逐渐增加的趋势。它们之间除了Eu异常不同外,稀土元素球粒陨石标准化模式非常相似,说明它们都是同源岩浆演化而成。天池火山岩的极近似的Sm/Nd值也证明了这一点。上述天池火山的玄武岩类和粗面岩类、流纹岩的稀土元素和微量元素特征,都一致地指示它们曾经历过结晶分异作用。结合岩石样品的主量元素特征,可知玄武岩类是在地幔岩浆房中发生的结晶分离作用,而粗面岩和流纹岩类则是在地壳岩浆房中发生的结晶分异作用。天池火山粗面玄武岩的喷发时代相当漫长。从第四纪早更新世早期(2.0Ma)开始喷发粗面玄武岩,一直持续到早更新世晚期(1.2Ma)。然后,从早更新世晚期(0.87Ma)开始,直到天池火山的全新世喷发,直接来自地幔岩浆房的粗面玄武质岩浆的喷发活动始终没有间断过。从早更新世晚期(0.87Ma)开始喷发的粗面玄武岩主要以小规模中心式喷发的火山渣锥为代表,如天池锥体周围的老虎洞、老房子小山、赤峰等火山渣锥以及松江河-露水河之间的东马鞍山等众多的小火山渣锥。而朝鲜无头峰粗面玄武岩覆盖于千年大喷发的浮岩之上,说明来自地幔岩浆房的粗面玄武岩的喷发活动持续到了近代。根据在中朝两国的天池火山周边锥体下部的天池公路、五十岗北坡、朝鲜鲤明水等地发现的少数玄武质粗安岩、粗安岩露头以及天池公路粗安岩样品资料和大宇钻孔、双目峰钻孔资料,可以认为在1.203Ma～1.179Ma期间,粗面玄武岩已经开始演化为粗安岩;而且,天池火山周围的粗面岩存在一个更早期次(约2Ma)的喷发,从而可以判断在早更新世时天池火山就存在玄武质到粗面质、粗安质的岩浆演化旋回。综合已发表的造锥粗面岩和流纹岩的K-Ar年龄,天池火山造锥阶段的粗面岩喷发活动介于1.12Ma～0.04Ma。在1.203Ma～1.12Ma时,玄武岩、粗安岩开始演化为粗面岩;造锥晚期约19万年左右粗面质岩浆开始向碱流质岩浆成分演化,造锥碱流质熔岩喷发持续时间可能为0.190±0.004Ma～0.0192±0.005Ma。对天池火山锥体北坡和东北坡的粗面岩和碱流岩的时空分布的系统研究发现,天池火山锥体北坡样品的年龄范围(1.37～0.019 Ma)比前人得到的样品年龄范围(0.79～0.023 Ma)更广,既出现了更老的年龄,也出现了更新的年龄;东北坡的样品年龄范围为0.38～0.042 Ma。由上可见天池火山造锥阶段不同喷发期次的产物的分布范围是不一样的。比较东北坡和北坡的数据,可以很明显地发现东北坡造锥粗面岩层序明显新于北坡造锥粗面岩层序。北坡几乎出现了整个造锥阶段的喷发产物,而东北坡则缺失第一和第二造锥阶段的喷发产物,只出现了第三和第四造锥阶段的喷发产物。此次研究还发现天池火山存在SiO2介于52%～64%之间的过渡型中酸性火山岩,其主要成因可能是由玄武质岩浆演化而成,这可能说明天池火山下存在另外的一个不同于粗面岩质和碱流岩质的地壳岩浆房或者可能说明地壳岩浆房存在熔体的分层结构,但由于现在发现的样品数量少、资料少、规模比较小,所以也不排除这些中酸性火山岩是由岩浆的混合作用形成,这有待于将来进一步研究。更多还原

【Abstract】 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.更多还原