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冲绳海槽酸性火山岩斑晶矿物特征及其岩浆演化研究
Alternative TitleResearch on characteristics of phenocrysts in acidic volcanics and its magmatic evolution in the Okinawa Trough
廖仁强
Subtype硕士
Thesis Advisor黄朋
2016-05-22
Degree Grantor中国科学院大学
Place of Conferral北京
Degree Discipline海洋地质
Keyword冲绳海槽 酸性火山岩 斑晶矿物 岩浆演化
Abstract冲绳海槽酸性火山岩主要以浮岩形式产出,包括两种类型,即黑色和白色浮岩。海槽浮岩斑晶含量极低,通过直接镜下工作难以全面认识斑晶矿物组合及组成变化。本次研究利用人工重砂、电子探针、背散射图像等技术手段分析冲绳海槽酸性火山岩中斑晶矿物及基质变化特征,重点关注辉石、斜长石以及玻璃基质的结构、构造及化学组成,讨论其对弧后盆地岩浆演化的指示。
浮岩斑晶矿物组合及其化学成分特征表明,黑色浮岩和白色浮岩都是拉斑玄武质岩浆高度结晶分异的产物,基质中气孔特征及Fe-Ti氧化物特性不同是导致两类浮岩宏观物理性质差异的主要因素。
矿物热力学计算结果显示,白色浮岩中单斜辉石结晶压力变化范围较大,最大结晶深度位于下地壳。两类浮岩中斜方辉石和斜长石在相似的压力下结晶,形成深度约10km,相当于冲绳海槽下地壳与上地壳交界处。斜长石An值变化特征指示两类浮岩的岩浆演化过程不统一,反映了岩浆冷却速率的差异,表明岩浆房受到了高温岩浆的脉冲式补充。浮岩斜长石斑晶核部的熔蚀结构为岩浆房接受更热岩浆补充提供了直接证据。两类浮岩基质气孔特征显示,白色浮岩是岩浆房连续减压过程的产物,而黑色浮岩是岩浆房多阶段减压的产物。上述冲绳海槽酸性火山岩斑晶与基质特征差异均表明,黑色浮岩和白色浮岩经历了不同的岩浆演化过程。
辉石、斜长石以及玻璃基质物理化学特征证实了冲绳海槽下部存在双层岩浆房结构。海槽中段存在由玄武质岩浆经演化形成的双层岩浆结构;海槽北段双层岩浆房均位于地壳内,深部岩浆房主要形成白色浮岩,而浅部岩浆房则主要形成黑色浮岩。
Other AbstractAcidic volcanics mainly appear by pumice, including black and white pumice in the Okinawa Trough. The volume of phenocrysts was very low. It was difficult that checking the phenocrysts assemblage and their compositions roundly. Analyzing the phenocrysts assemblage of acidic volcanic and matrix characteristics by utilizing artificial density, backscatter scanning images and electro-probe microanalyzer, mainly concerning the texture, structure and chemical compositions of pyroxene, plagioclase and glass matrix, we discuss the implication of phenocrysts of volcanics to magmatic evolution in back-arc basin.
The phenocrysts assemblage and their chemical features show two species of pumice were the result of basaltic magma highly fractional crystallization. The difference in two species of pumice was mainly due to the bubble characteristics of glass matrix and the properties of Fe-Ti oxides.
The calculating mineral thermodynamic results show the clinopyroxene crystallized pressures in white pumice vary considerably, and the maximum crystallized depth was in lower crust. Orthopyroxene and plagioclase in two species of pumice crystallized under the similar pressures and the crystallized depth was approximately 10 km, corresponding to the boundary depths of upper and lower crust. The characteristics of the An value in plagioclase in both the black and white pumice imply there was no uniform magmatic evolution process, and meanwhile, indicating there was no uniform magma cooling rate, showing that the magma chamber was recharged in pulses by hotter magma. Core-sieved plagioclase was the product of hotter magma corrosion. The characteristics of glass matrix of two species of pumice show white pumice were the result of magma continuous decompression and black pumice was the result of magma multi- or single-step decompression. From the above, black and white pumice underwent different magmatic evolution.
These characteristics of pyroxene, plagioclase and glass matrix conform the dual-layer magma chamber existed in the Okinawa Trough. The shallow reservoir evolved from the basaltic magma developed in the middle Okinawa Trough. The two-layered magma chamber existed in the crust in the northern Okinawa Trough. White pumice crystallized in the deep-seated reservoir and black pumice formed in the shallow reservoir.
Subject Area海洋地质学
Language中文
Document Type学位论文
Identifierhttp://ir.qdio.ac.cn/handle/337002/112515
Collection海洋地质与环境重点实验室
Affiliation中国科学院海洋研究所
Recommended Citation
GB/T 7714
廖仁强. 冲绳海槽酸性火山岩斑晶矿物特征及其岩浆演化研究[D]. 北京. 中国科学院大学,2016.
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