【作者】 李霓；

【导师】 樊祺诚；

【作者基本信息】 中国地震局地质研究所 ， 构造地质学， 2007， 博士

【摘要】 长白山位于我国东北吉林省与朝鲜民主主义共和国的交界处，在我国的部分地处吉林省延边朝鲜族自治州安图县和白山市抚松县境内，历经悠久喷发而形成了目前所见的巨大规模山体。本论文讨论了天池火山全新世喷发物的矿物和岩石化学演化特征，通过对主矿物碱性长石斑晶中熔融包裹体的高温热台测试、电子探针分析和红外光谱分析测试，多方面探讨天池火山在全新世爆炸喷发过程中的岩浆脱气作用和对环境的灾害性影响。根据岩浆化学演化和地球物理深部探测结果，天池火山之下存在两个不同的岩浆房——地幔岩浆房和地壳岩浆房。长白山天池火山从来自地幔的造盾阶段钾质粗面玄武岩母岩浆，经历造锥阶段地壳岩浆房的结晶分异作用形成粗面岩，造锥晚期出现碱流岩，全新世则以爆炸喷发的碱流质浮岩为主，最后喷出少量粗面质熔结凝灰岩和浮岩。天池火山的岩浆演化符合分离结晶作用的演化趋势，橄榄石、辉石和斜长石等主矿物的结晶分异起了决定性的作用。造盾钾质粗面玄武岩中橄榄石成分为透铁橄榄石；造锥和全新世喷发物中橄榄石接近Fa端员。造盾阶段除出现普通辉石外，在基质中有少量次透辉石；而造锥阶段和全新世喷发物中为富Fe贫Mg的钙铁辉石。造盾阶段的长石成分为斜长石，属于基性的拉长石；造锥阶段和全新世喷发时大量出现的是富K、Na的高温碱性长石，成分也从歪长石（Na₂O＞K₂O）向透长石（K₂O＞Na₂O）演化。矿物化学成分和岩石主量元素变化一致表明，岩浆是从相对富MgO、CaO向富SiO₂、Na₂O、K₂O方向演化。这些特点与火山岩REE、微量元素和Sr-Nd同位素地球化学一道，表明天池火山全新世火山岩与造锥、造盾火山岩是同源岩浆演化的产物，基性岩浆分异的总趋势为越向晚期碱性越强。天池火山在全新世曾有多次喷发，根据天文峰一带的喷发物特征和层序，从早到晚分为三个喷发期：第一喷发期（Ⅰ），距今约5000～4000年的淡黄色碱流质浮岩；第二喷发期（Ⅱ），公元1000年左右大规模喷发（下文称“千年大喷发”）的灰白色碱流质浮岩；以及第三喷发期（Ⅲ），距今约300～100年前喷发的灰黑色粗面质浮岩及熔结凝灰岩。泡沫状浮岩和气孔发育的各种火山碎屑岩，说明大部分气体已通过岩浆脱气作用散发到空气中。在天池火口和圆池（天池东30 km）的千年大喷发浮岩碱性长石斑晶中发现两种形貌、相态、成分都截然不同的熔融包裹体，分别称“火口组”和“园池组”。前者寄主晶为透长石，熔融包裹体棕黄色，形态极不规则，气泡丰富，部分包裹体有脱玻化现象并含有子晶，说明当时喷发后有足够的冷却时间；后者寄主晶为歪长石，熔融包裹体无色透明且不含子晶，多为浑圆形，其成分与寄主晶成分相平衡。电子探针测试发现，“火口组”和“圆池组”样品在TAS分类图上分属粗面岩和流纹岩两种截然不同的化学成分，而根据化学成分和CaO／Al₂O₃比值判断，“圆池组”熔融包裹体成分比“火口组”成分更为演化，为千年大喷发前岩浆房中存在成分不同的岩浆提供了证据。天池火山全新世三期喷发物（碱流质-碱流质-粗面质）中熔融包裹体的均一温度：第一喷发期938℃～1014℃之间；千年大喷发期，分别是850℃～926℃和1005℃～1024℃之间；第三喷发期974℃～1062℃之间。上述实验结果说明第一、三期喷发的岩浆温度相差不大，但出现较宽的均一温度范围，这不仅与熔融包裹体的大小、实验的升温速率有关，还可能与它们的成分不同也有密切关系。对于千年大喷发期情况则较为复杂，实验结果表明当时有两个不同温度段的熔体存在。“差异法”数据显示挥发分H₂O的情况与Cl相同，含Cl高的“圆池组”熔融包裹体含H₂O量也高，它们之间存在着正相关关系。经Nicolet Magna-IR 550红外光谱仪对熔融包裹体中H₂O的直接测定，千年大喷发产物中熔融包裹体的含水量高达1.6％～3.6％，这是国内首次发表的对熔融包裹体中H₂O的直接测定数据。此次测试的所有样品中都没有检测到CO₂的明显谱峰，说明当时CO₂含量低于检测限。而两组样品中S含量相近。岩浆中S不仅在喷发时脱气，更可能是一个连续的脱气过程，喷发时的脱气量仅占总脱气量的一部分。研究还表明“圆池组”包裹体比“火口组”包裹体经历了更广泛的S脱气，前者S／Cl比值比后者低得多。上述分析结果表明，天池火山在发生千年大喷发时极富碱质的流纹质岩浆中已富集了相当数量的H₂O和Cl，喷发时强烈脱气，这决定了当时发生的是爆炸式喷发。根据当时火山喷出物的总质量与各挥发分的浓度，利用“岩石学法”估算喷出的S和Cl含量达到百万吨和千万吨数量级，挥发分H₂O的含量还要更高。在有史记载以来的火山喷发中，天池火山的这次大喷发，其喷出的卤素含量可列全球火山喷发中的第二位。“火口组”和“圆池组”熔融包裹体的存在对天池火山的千年大喷发具有重要意义，揭示了喷发前岩浆房的非单一成分，“火口组”代表了岩浆房中较原始的成分，挥发分（H₂O、Cl）含量较低；“圆池组”代表了相对演化的成分，挥发分含量也较高。两种熔体赋存于地壳岩浆房的不同层位，分别被不同的碱性长石结晶时包裹于矿物内，千年大喷发时通过不同的岩浆通道喷出地表，它们可能是在同次大喷发的不同序列中喷出的。天池火山造盾阶段的钾质粗面玄武岩来源于地幔岩浆房的喷发，造锥阶段粗面岩和全新世碱流岩来源于地壳岩浆房的喷发。岩浆的结晶分异作用和混合作用是天池火山岩浆演化的两个最重要过程，前者形成天池火山双峰式火山岩分布特征，后者成为天池火山千年大喷发的触发机制。天池火山在晚更新世-全新世碱流质岩浆主喷发期还兼有少量玄武质粗安岩、粗安岩或粗面质岩浆的交替喷出，揭示了天池火山地壳岩浆房熔体的分层结构特点，地壳岩浆房中岩浆进一步演化，逐渐富集大量挥发分，由于地幔粗面玄武质岩浆的注入导致不同层位岩浆的扰动和混合作用，触发了天池火山的千年大喷发。更多还原

【Abstract】 The Changbai Mountains are located on the border between Jilin province, China and DPRK. The parts of the mountains in China territory belong to Antu county of Yanbian Chaoxian Autonomous State and Fusong county of Baishan, Jilin province. With its long eruption history over 2 Ma, the caldera is now filled with water and is called Tianchi （"the sky lake"） volcano.This thesis discusses the characteristics of minerals and petrology-chemistry evolution of Tianchi volcanic rocks, magma degassing effect and its disaster effect on environment during Holocene explosive eruptions through heating stage experiments, electron microprobe analyses and Fourier transform infrared spectroscopy for melt inclusions hosted in alkaline feldspars.The studies of magma evolution and magnetotelluric sounding results show that there are two magma chambers beneath the Tianchi volcano, i.e., mantle reservoir and crust chamber. The Tianchi volcano experienced the magma evolution from parental potassic trachybasalt coming from mantle reservoir in the shield-forming stage, trachyte and lately comendite coming from crust chamber in the cone-forming stage after crystallization, to comenditic pumice and some trachytic welded tuff and pumice in Holocene. The magma evolution of the Tianchi volcano was deeply affected by crystallization differentiation, in which fractional crystallizations of olivines, pyroxenes and plagioclases played important roles. Fo values of olivines were between 54 and 70 in the shield-forming stage, and they quickly decreased their Fo values to almost the end member （fayalite） in the cone-forming stage and Holocene eruptions. Augite and plagioclase （labradorite） phenocrysts appeared in the shield-forming stage, while hedenbergite and alkaline feldspar （anorthoclase and sanidine） appeared in the cone-forming stage and Holocene eruptions. Chemical compositions of minerals and major element variations all indicate that the magma evolutionary trend is from MgO- and CaO-rich to SiO₂-, K₂O- and Na₂O-rich. These features, together with characteristics of REE, trace elements and Sr-Nd isotope ratios, suggest that the eruption materials of Holocene, cone-forming stage and shield-forming stage are the products evolved from a same magma reservoir. As the original magma fractionated and increased its alkali content gradually when evolved, therefore alkali phenocrysts appeared in the melt. The evolution of phenocrysts in three eruption stages is in conformity with the evolutionary trend of fractional crystallization.It is well known that there occurred several eruptions of the Tianchi volcano in Holocene. In this thesis, it is suggested that the Holocene eruptions of the Tianchi volcano include three eruption periods according to volcanic strata in Tianwen Peak, i.e., Eruption Period I, 5000 4000 a BP bright yellow comenditic pumice, Eruption Period II （"The millennium explosive eruption" hereafter）, 1000 a BP gray comenditic pumice and pyroclastic flow, and Eruption Period III, 300～100 a BP black trachytic pumice and welded tuff. Vesicular pumice and pyroclastics imply the volatile degassing through magma eruptions and lava flows. The thesis aims to study the magma degassing effect of Holocene explosive eruptions of the Tianchi volcano, Changbai Mountains. Among them, Eruption Period II is the study focus for its widespread and characteristic pumice.Melt inclusions are largely found in alkaline feldspars from the millennium eruption of the Tianchi volcano. Most of them have more than one bubble and some of them have daughter crystals. These melt inclusions can be separated into two distinct groups （nominated as "C’ and "Y" group in this thesis） according to their characteristics of shape, color and number of bubble. "C" group melt inclusions, collected on the crater rim, brown to yellow, have irregular shapes and lots of bubbles. Some of them devitrified and have daughter crystals, indicating the long cooling time of magma. While the "Y" group melt inclusions, transparent, with no color and daughter crystal, collected in Yuanchi, 30 km east of the caldre, have roughly round shape, implying the equilibrium with host crystal. Electron microprobe analysis data show the two groups of melt inclusions are trachyte and rhyolite respectively and composition of the "Y" group is more evolved than the "C" group according to CaO/Al₂O₃ ratio, which indicates the magma chamber has different compositions prior to the millennium explosive eruption.This work has have experimentally studied the homogenization temperatures of the melt inclusions in host feldspars from these eruptions of the Tianchi volcano in Holocene using Leitz 1350 heating stage performed at Laboratoire Pierre Sue, CNRS-CEA, France. For Eruption I, the homogenization temperatures （T_h） vary from 938℃to 1014℃. There are two T_h intevals for Eruption II: 850℃～926℃and 1005℃～1024℃respectively. And 974℃～1062℃are measured homogenization temperatures for Eruption III. There are not distinct differences between homogenization temperatures in Eruption I and III, but for Eruption II, the things are rather complicated, i.e. there exist two homogenization temperatures groups.Using the by-difference method, it was found that H₂O concentrations in two groups are the same situation as chlorine, i.e., the "Y" group melt inclusions have both high chlorine and water content. Analysed by Fourier Transform Infrared Spectroscopy Nicolet Magna-IR 550, these melt inclusions are found to have high water concentrations of 1.6 %～3.6 %, which are first published data for water contents in melt inclusions using FTIR in China. CO₂ hasn’t been detected in these analyses, indicating CO₂ concentration is below the detection limit. Sulphur contents are somehow similar in two groups. It will degas continuously rather than eruption degassing. So the degassing quantity of sulpher for the millennium eruption may also be underestimated. The study indicates that melt inclusions in the "Y" group undergo more sulphur degassing than those in the "C" group, and S/Cl ratios in the "Y" group is much lower than those in the "C" group. According to the data, the alkaline rhyolitic magma concentrated large quantities of water and chlorine prior to the eruption. The strongly degassed volatile made the millennium eruption occur more explosively. Based on the data of total mass of eruption materials, volatile concentration in melt inclusion and matrix glass, calculated S and Cl contents degassed during that eruption reach the magnitude of several to tens of million tons, and degassed water was even higher than S and Cl. The quantity of halogen projected to the atmosphere was rather high and listed the second in documented history.The discover of "C" and "Y" group melt inclusions has important significance for the millennium explosive eruption of the Tianchi volcano and discloses the non-single composition of magma prior to the eruption. The melt inclusion in the "C" group indicates comparatively primitive composition with low volatile （H₂O and Cl）, while that of the "Y" group implies the more evolved magma with high volatile. The two melts were trapped in alkaline feldspars when depositing at different levels of the crust magma chamber, and were erupted through different conduits when the millennium explosive eruption occurred.Crystallization differentiation and magma mixing are two important processes in magma evolution of the Tianchi volcano, the former of which determines the bimodal compositional distribution of volcanic rocks, and the latter became the triggering factor for the great eruption 1000 years ago. During the main eruption period of rhyolite magma from late Pleistocene to Holocene, some basaltic trachyandesite, trachyandesite or trachyte magma erupted episodicly throughout the cone-forming stage, which reveals the fact that there was a multi-layer magma chamber system beneath the Tianchi volcano. The trachybasalte influx from the mantle reservoir disturbed the magma layers and caused the magma mixing, then triggered the millennium explosive eruption.更多还原