中国科学院海洋研究所机构知识库

  卡洛琳地幔柱在14~1Ma的时期内形成了由楚克、波纳佩、科斯雷等洋岛组成的卡洛琳海山链。相对于其他地幔柱成因的典型海山链（如夏威夷海山链），卡洛琳海山链被认为由处于晚期阶段的卡洛琳地幔柱活动形成，洋岛主体均为碱性玄武岩并且缺失拉斑玄武岩，且各洋岛的碱性玄武岩存在地球化学组成上的差异。尽管碱性洋岛玄武岩的成因相对复杂，但由同一地幔柱形成的卡洛琳海山链是研究地幔柱活动晚期碱性洋岛玄武岩成因的良好范例。本文选取卡洛琳海山链中的楚克、科斯雷火山岩样品为研究对象，以岩相学及矿物学、全岩主微量元素测定及矿物微区分析为研究手段，目的是探讨卡洛琳地幔柱晚期活动过程中的地幔源区组成演化以及熔融条件变化，并取得了以下认识：

楚克样品主要为碱性玄武岩，有少量的粗面安山岩；科斯雷样品以碱性玄武岩为主，部分为霞石岩。碱性玄武岩的矿物组成以橄榄石、单斜辉石为主。橄榄石组成测定结果表明，楚克、科斯雷碱性玄武岩橄榄石斑晶显示了相对正常洋中脊玄武岩橄榄石异常高的Ni、低Ca-Mn等特征，与来自辉石岩源区的夏威夷火山岩橄榄石相似。

地球化学分析结果表明，楚克、科斯雷碱性玄武岩样品的主微量元素组成存在着较为明显的差异。尽管楚克、科斯雷碱性玄武岩均具有较高的CaO、TiO₂含量，且微量元素配分模式与洋岛玄武岩的全球平均值类似，但楚克火山岩的碱性程度更接近碱性-亚碱性分界线，而科斯雷碱性火山岩整体则更加低SiO₂、Al₂O₃，高MgO，且轻重稀土的分馏程度更高。科斯雷样品的霞石岩亏损高场强元素特别是Nb、Zr、Hf和Ti，明显富集稀土元素，在霞石岩的橄榄石斑晶中发现了含碳酸盐矿物的包裹体。

本研究认为，楚克、科斯雷碱性火山岩轻重稀土元素富集程度、全岩Al₂O₃含量之间的差异指示了源区熔融程度存在逐渐减小的趋势，从而影响了地幔源区残余石榴石的熔融；楚克、科斯雷橄榄石的微量元素组成和全岩高TiO₂含量特征指示了含辉石岩的地幔源区；而在科斯雷洋岛出现的低SiO₂、高CaO，亏损高场强元素、富集稀土元素的霞石岩，其成因与源区CO₂组分在火山岩成因上所起的作用逐渐增强有关。以上表明，在卡洛琳地幔柱活动减弱的过程中，海山链碱性洋岛火山岩的地球化学组成以及岩浆成因的差异与地幔源区组成的演化密切相关，这对于认识地幔柱晚期活动具有十分重要的意义。  

The Caroline manlte plume has formed the Caroline seamount chain consisting of Chuuk, Ponape, Kosrae and other oceanic islands during the period of 14~1Ma. In contrast to other typical seamount chains, such as the Hawaiian Seamount chain, the Caroline Seamount chain is considered to have been formed by the late Caroline mantle plume activity. The oceanic islands are mainly alkali basalt and lack tholeiite, and there are differences in the geochemical composition of the alkali basalt among these oceanic islands. Although the genesis of alkali oceanic island basalts is relatively complex, the Caroline seamount chain formed by the same mantle plume is a good example for studying the genesis of alkali oceanic island basalts during late mantle plume activity. This study selects volcanic rock samples from Chuuk and Kosrae as the research object, by petrography and mineralogy, whole rock the determination of trace elements and minerals microanalysis method for research, the purpose is to explore late Caroline mantle plume activity of mantle source compositional evolution and the change of melting condition, and obtained the following understanding:

The Chuuk samples are mainly alkali basalt and trachyandensite; The Kosrae samples are mainly alkali basalt and partly nephelinolite. The minerals of alkali basalt are mainly olivine and clinopyroxene. The composition results of olivines show that the olivine from Chuuk and Kosrae alkali basalt have higher Ni and lower Ca-Mn contents relative to olivines from mid-ocean ridge basalts, which is similar to olivines of Hawaiian volcanic rocks from pyroxene mantle source.

Geochemical analysis results show that there are obvious differences in the composition between Chuuk and Kosrae alkali basalt samples. Although both Chuuk and Kosrae alkali basalts have high content of CaO and TiO₂, and the distribution pattern of trace elements is similar to the global average value of Oceanic island basalt, the alkalinity degree of Chuuk volcanic rocks is closer to the alkaline-subalkaline boundary, while the Kosrae alkali basalt rocks are lower in SiO₂, Al₂O₃, and higer in MgO and the fractionation degree of light and heavy rare earth elements. The nephelinolite samples are depleted in high field strength elements (HFSE), especially Nb, Zr, Hf and Ti, and enriched in rare earth elements. Carbonated mineral inclusions are found in the olivine phenocryst of the nephelinolite samples.

This study suggests that the differences in the enrichment degree of light and heavy rare earth elements and the whole rock Al₂O₃ content of the Chuuk and Kosrae alkali volcanic rocks indicate that the degree of partial melting is gradually decreasing, which affects the melting of residual garnet in the mantle source. The trace element composition of olivines and whole-rock high TiO₂ content of Chuuk and Kosrae olivine indicate the pyroxenite mantle source. The gensis of Kosrae nephelinolite which are low SiO₂, high CaO, depleted in HFSE and enriched in REEs is related to the increasing influence of CO₂ components in the mantle source. These results indicate that the differences in geochemical composition and magmatic gensis of alkaline oceanic island volcanic rocks in Caroline seamount chain are closely related to the evolution of mantle source during the decline of Caroline mantle plume activity, which is of great significance for understanding the late mantle plume activity.

第1章 引言 1

1.1 选题背景及意义 1

1.2 研究现状 2

1.2.1 洋岛火山与地幔柱活动 3

1.2.2 卡洛琳海山链与卡洛琳地幔柱 4

1.3 研究内容 5

1.4 研究方法 5

1.5 完成工作量 6

第2章 区域地质背景 8

2.1 区域构造情况 8

2.2 火山活动 9

2.3 岩石组合 10

第3章 样品处理及分析方法 12

3.1 采样工作 12

3.2样品前处理 12

3.3 样品分析方法 13

3.3.1 全岩主量元素测试 13

3.2.2 全岩微量元素测试 14

3.3.3 电子探针分析 17

第4章 岩相学与矿物学分析结果 18

4.1 岩相学 18

4.1.1 手标本特征 18

4.1.2 镜下特征 19

4.2 矿物学 20

4.2.1 橄榄石镜下特征 20

4.2.2 橄榄石成分特征 22

4.2.3 橄榄石内部包裹体 23

第5章 样品组成分析结果 25

5.1 主量元素 25 

5.2 微量元素 27

第6章 讨论 31

6.1地幔柱活动减弱 31

6.2岩浆特征与结晶分异 33

6.3地幔源区组成 35

6.3.1辉石岩组分 35

6.3.2 CO2组分 37

结 论 42

参考文献 43

附 录 48

致 谢 59

作者简历及攻读学位期间发表的学术论文与研究成果 60