| 金沙江-红河富碱斑岩Li同位素地球化学特征及其地质意义 |
| 周汀
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| 导师 | 毕献武
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| 2013
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| 学位授予单位 | 中国科学院研究生院
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| 学位授予地点 | 北京
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| 学位名称 | 硕士
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| 学位专业 | 地球化学
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| 关键词 | Li同位素组成
富碱斑岩
岩浆源区
金沙江-红河富碱侵入岩带
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| 摘要 | 大量地质地球化学和实验研究表明，花岗岩类的源区存在大量成矿元素、源区的成矿元素能有效转入岩浆熔体并随岩浆演化而逐渐富集，是花岗质岩浆能够形成热液矿床的重要前提。因此，确定花岗质岩浆的源区和特征，对阐明成矿岩浆的形成机制至关重要。金沙江-红河富碱侵入岩带位于印-亚大陆主碰撞带西侧，产出众多与富碱岩浆有关的Cu、Au矿床，其中部分矿床已达超大型矿床规模，是世界上大陆内部环境富碱岩浆成矿作用的典型区域，在全球具鲜明特色。研究发现，该带的富碱侵入岩成岩物质具有壳幔混合的特征。然而，尽管有关该带富碱侵入岩的成岩物质具有壳幔混合的特征已达成共识，但对其源区类型的认识却存在很大争议。且带内形成于同一时代的富碱侵入岩形成矿床种类不同，存在含矿和不含矿的差异，初步研究显示含矿与不含矿富碱侵入岩地质地球化学特征存在差异，这种差异是否与岩浆起源有密切的关系尚缺乏系统的研究。 近年来Li同位素这一新兴的地球化学示踪方法在国际上得到了快速发展，已大量的运用于地幔源区研究、壳幔物质循环、地表风化作用等的研究中，显示出了其对于示踪源区微小变化的灵敏性。本项研究选择金沙江—红河富碱侵入岩带的含矿斑岩与不含矿斑岩为主要研究对象，运用Li同位素这一新兴的地球化学示踪方法，并结合微量元素和Sr-Nd同位素地球化学研究结果，开展了研究区富碱侵入岩的源区类型和特征研究，并取得了以下进展： （1）通过大量的文献调研，在前人工作的基础上，经过大量反复的实验，建立了适合于本研究区富碱侵入岩样品的Li同位素分离方法，成功分离了富碱侵入岩体全岩的Li同位素，并首次取得了金沙江—红河富碱侵入岩带富碱斑岩的Li同位素分析数据。 （2）全岩样品的Li同位素组成特征表明，金沙江-红河富碱侵入岩带富碱斑岩的δ7Li值变化范围为-6.5‰～6.2‰，结合研究区岩石圈演化历史、相关富碱侵入岩岩石化学、微量元素和Sr-Nd地球化学的研究结果，推断研究区富碱侵入岩岩浆起源于受古大洋俯冲板块影响的残余交代地幔。 （3）金沙江-红河富碱侵入岩带含矿斑岩与不含矿斑岩的Li同位素组成存在明显的差异。含矿斑岩的δ7Li值范围为-6.5‰～+0.1‰，主体为-2.3±2.1‰，不含矿斑岩的δ7Li值范围为-4.5～6.2‰，主体为+2.1±3.1‰。其δ7Li值特征的明显差异指示含矿斑岩受到了俯冲板片释放的具有较低的δ7Li值特征流体的交代作用，该流体可能是通过瑞利分馏的方式所释放。该研究结果初步揭示了金沙江-红河富碱侵入岩带含矿岩体与不含矿岩体岩浆源区物质组成存在差异及其导致差异的可能原因。 |
| 其他摘要 | Lots of geochemical experiments indicate that a large amount of ore-forming elements exist in the source region of granite and these elements will be getting enriched gradually in melts with the evolution of magma, this is an important prerequisite to the formation of hydrothermal deposits of granitic magma. Therefore, to determine the characteristics of granite’s source region is the key point to the study of the mechanism of ore-forming magma. Jingshajiang-Honghe alkaline-rich intrusive belt is located in the west of the main collisional zone of the Indian and Asian continent. There are lots of Cu-Au deposits which are related to the alkaline-rich porphyritic rocks, some of them are ultra-large type deposits. It is a typical zone for the study of the ore-forming process of alkaline-rich intrusions in the intracontinental area and possesses distinguished features. Previous studies show that the diagenetic material has a characteristic of mixture of crust and mantle, but it is still controversial about its source region. Besides, intrusions with the same age form different types of ore deposits in the belt and these intrusions consist of ore-bearing type and ore-barren type. Preliminary studies show that the geological and geochemical characteristics of the ore-bearing and ore-barren intrusions are different. But whether this difference is closely related to the source region of the intrusions lacks systematic study. Lithium isotope geochemistry is developing very fast as a new geochemical trace tool and has been used in the field of studying source region, material recycling between the crust and the mantle, earth surface weathering. It is especially sensitive to the tiny changes of source region. In this paper, Lithium isotopic geochemical method has been used to study the ore-bearing and ore-barren intrusions for their source region, together with the study of the previous trace elements and Sr-Nd isotopic characteristics. Some progresses have been achieved as below: (1) Based on former researcher’s work, large amount of experimental work has been done to find a lithium separation method which is suitable for samples chosen from this region. Lithium isotope of whole rock samples are successfully separated and Lithium isotopic characteristics of jinshajiang-honghe akaline-rich intrusions’ belt are presented for the first time. (2) Lithium isotope data of whole rock samples show that the variation of theδ7Li values of the Jingshajiang-Honghe alkaline-rich intrusions ranges between -6.5‰～6.2‰. To combine the evolutionary history of the lithosphere of the regions studied and the petrological geochemistry, trace elements and Sr-Nd isotopic characteristics of the related alkaline-rich intrusions, it can be presumed that the source region of the intrusions studied originates from the residual metasomatized mantle wedge which is influenced by subduction of oceanic crust. (3) The data shows that the lithium isotopic composition of ore-bearing and ore-barren intrusions are obviously different. δ7Li of ore-bearing intrusions ranges from -6.5‰～+0.1‰, with a majority of -2.3±2.1‰; Meanwhile, that of ore-barren intrusions ranges from -4.5～6.2‰, with a majority of +2.1±3.1‰. The petrology, mineralogy and geochemical characteristics show that these intrusions originated from mantle region in subduction zone. The large lithium isotopic difference of these two types of intrusions indicates that the source region of the ore-bearing intrusions has underwent larger extent of metasomatism caused by slab-derived fluid, which is probably released by Rayleigh fractionation and has a lowδ7Li lithium isotopic characteristic. The result indicates the possible reason of the different source region of these two types of intrusions. |
| 学科领域 | 矿床地球化学
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| 语种 | 中文
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| 文献类型 | 学位论文
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| 条目标识符 | http://ir.gyig.ac.cn/handle/352002/5842
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| 专题 | 研究生_研究生_学位论文
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推荐引用方式
GB/T 7714 |
周汀. 金沙江-红河富碱斑岩Li同位素地球化学特征及其地质意义[D]. 北京. 中国科学院研究生院,2013.
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