【作者】 张英杰；

【导师】 施光海；

【作者基本信息】 中国地质大学（北京） ， 矿物学、岩石学、矿床学， 2010， 硕士

【摘要】 绿辉石岩是一种较为罕见的岩石,这种岩石在硬玉岩区和超高压变质带偶有出露,引起了相关学者的兴趣与重视。在硬玉岩区中,目前有报道的绿辉石岩主要在缅甸,被作为一种多期次硬玉化的标志。在研究俄罗斯西萨彦岭翡翠时,发现了绿辉石岩。该绿辉石岩主要由绿辉石、钠铬辉石组成,此外另有少量铬铁矿、金云母和镍黄铁矿,其中绿辉石的含量可达到90%以上。绿辉石岩的结构有冠状结构、亚颗粒化和锯齿状高度缝合边等。该绿辉石岩中显示出冠状结构从开始发育到形成完全形态的演化趋势,在此过程中冠状结构核心处的钠铬辉石被交代为含Cr量较少的绿辉石。绿辉石岩的成分和结构特征反映了该岩石至少有两期的硬玉化作用：(1)主期的充填交代作用,主要为硬玉质流体交代铬尖晶石类形成钠铬辉石,交代辉石类矿物形成绿辉石；(2)与变质变形同期的充填交代作用,生成新的Ae-Di-Ko固溶体系列。并且两期的硬玉化作用主导了绿辉石岩中Cr的迁移,表现为四种迁移形式：(1)铬铁矿→钠铬辉石；(2)钠铬辉石→含铬较高的绿辉石；(3)含铬较高的绿辉石→含铬较低的绿辉石；(4)含铬较低的绿辉石进一步淡化。这四个阶段的“铬的淡化”以及伴随铬的淡化的构造剪切力的作用决定了翡翠的绿色的色形、色度和均匀度,极大地影响着翡翠的质量。与缅甸绿辉石岩对比,西萨彦岭绿辉石岩中含有钠铬辉石,但缺少硬玉,并存在与钠铬辉石有关的冠状结构；在化学成分上Mg、Ca的含量较高,Fe的含量低了很多,并且几乎不含K。解决形成硬玉岩以及导致绿辉石岩形成的硬玉质流体／熔体的物质来源的问题是合理解释硬玉岩和绿辉石岩成因的重要步骤。以往的研究结果显示：硬玉岩的物质来源与俯冲洋壳上部富含Na的沉积物有关。据此推测硬玉岩是大洋板块在向下俯冲至大陆板块的过程中,随着温压的增大,其洋壳沉积物在一定条件下发生脱水或熔融,然后向上运移至超基性岩中结晶而成。绿辉石岩则可能与硬玉岩同时或之后形成,被认为是硬玉质的流体或熔体在俯冲板块带低温高压的条件下,与蛇绿岩套内含Fe和Ca较高的岩石类型如辉石岩等交代的产物。在硬玉质流体交代含Ca高的辉石类矿物形成绿辉石的同时,硬玉质流体也交代铬铁矿,形成钠铬辉石。更多还原

【Abstract】 Omphacitite that catches lots of scholars interest and attention is a relatively rare rock that occasionally exposed in jadeitite area and UHP metamorphic zone. In the jadeitite area, Myanmar is the most important place in that the omphacitite was reported. And the omphacitite is thought to be a index of multi-stage jadeitizations. Omphacitite was found in the research of West Sayan jadeitite. The omphacitite mainly contains omphacite and kosmochlor, also contains chromite, phlogopite and a small amount of pentlandite, and the content of omphacite reaches more than 90%. There are mainly three distinct textures in the omphacitite:corona texture, subgrain. The omphacitite shows the evolution of corona texture from the development to the full formwith the containing of Cr in the kosmochlors in the core of corona texture increasing. The chemical composition and the texture show two or more stages of jadeitization in West Sayan omphacitite:(1) the main period of filling metasomatism. In this stage, The kosmochlor forms by the reaction between jadeitic fluids and chromite, and the omphacite forms by the interaction of pyroxene and jadeitic fluid; (2) the period of filling metasomatism related to metamorphism. In this stage, the series of new Ae-Di-Ko sosoloid formed. And the jadeitization controls the desalinization of chromium that is divided into four stages:(1) transformation from chromite to kosmochlor; (2) from kosmochlor to high chromium-bearing omphacite; (3) from high chromium-bearing omphacite to low chromium-bearing omphacite, and (4) from chromium-bearing omphacite to lower chromium-bearing omphacite, which is essentially the further evolution of the stage 3. Using the desalinization of chromium, the feature that green colour in jadeite jade has special shape can be interpreted, and how and what the green colour changes can be understood.The comparison between West Sayan omphacitite and Myanmar omphacitite shows: West Sayan omphacitite contains kosmochlor, but lack of jadeite, and there are corona textures related to kosmochlor. In chemical compositions, West Sayan omphacitite contains more Mg and Ca, less Fe and lack of k, whereas Myanmar omphacitite contains a little of K. To resolve the origin of jeditic fluid/melt that form the jadeitite and the omphacitite is very important for a reasonable interpretation of the genesis of jadeitite.Recent researchs shows the jadeitic fluid is more likely related to sediments of subducted slabs. According to recent researchs, the jadeitite thought to be the the production of the crystallization of jadeitic fluid.As the process of oceanic crust subducting to the continental plate, the sediments of subducted slabs dehydrate or melt with the temperature and pressure increasing, and the melt uplift,then the crystallization of the melt under certain conditions forms the jeditite. The omphacitite forms by the interaction of pyroxenite rich in Fe and Ca in ophiolite complex and jadeite fluids at HP/LT conditions during active subduction/collision. The kosmochlor forms by the reaction between jadeite fluids and chromite at the same time of the forming of omphacitite.更多还原