大别山超高压榴辉岩折返过程中的铁同位素及金红石的地球化学研究

发布时间：2018-03-10 23:38

  本文选题：大别山　切入点：金红石　出处：《中国科学技术大学》2016年博士论文　论文类型：学位论文

【摘要】：位于中国东部的大别苏鲁造山带是由三叠纪时华南板块与华北板块碰撞而形成的。郯城-庐江断裂带将大别苏鲁造山带分成两部分,分别为西部的大别造山带和东部的苏鲁造山带。柯石英和金刚石的发现及对冷却历史的研究表明华南板块曾经俯冲到地幔深度(100km),而后快速折返至地表。本论文的样品采自位于中大别中温超高压变质带的双河和碧溪岭地区,所选取的样品为新鲜榴辉岩及退变质榴辉岩,旨在以金红石和铁同位素为切入点,通过对比新鲜榴辉岩和退变质榴辉岩中的痕量元素地球化学和同位素地球化学数据,探讨在折返阶段的退变质过程中元素和同位素的地球化学行为。本论文的第一个工作主要研究双河地区的大颗粒金红石的精细结构、微量元素含量、氧同位素组成、拉曼光谱等地球化学参数。相比于退变质榴辉岩中的大颗粒金红石,石英脉中的金红石具有更高含量的V和U,更低含量的Nb、Ta、Cr和W。两种产状的金红石具有相似的氧同位素值、Zr/Hf、Zr、Hf和Mo含量。石英脉中的金红石具有更低的Nb/Ta值,指示脱水反应时产生的流体相较残留相具有较大的Ta分配系数。退变质榴辉岩中的大颗粒金红石周围存在退变质的产物榍石,在金红石和榍石的边界,观测到Nb从榍石向金红石的“反扩散”现象。拉曼光谱及EBSD的结果显示,两个表现为大颗粒金红石的整个晶体方向并不一致,分成不同晶体方向的小颗粒金红石以及线状的膝折带。镜下观察和实验结果显示,大颗粒的金红石的形成过程应该有四个阶段,分别是：1)小颗粒金红石的形成；2)在金红石塑性的情况下,形成塑性膝折带；3)金红石由转为脆性后,受到应力作用而形成脆性膝折带；4)在后期,金红石的物理破裂。本论文的第二个工作研究了碧溪岭超高压榴辉岩岩体的全岩和单矿物的铁同位素组成与铁价态组成,以示踪在折返过程中的氧逸度等地球化学参数变化。穆斯堡尔谱结果显示,退变质榴辉岩中的Fe~(3-)/ΣFe值比新鲜榴辉岩的Fe~(3+)/EFe值高,指示退变质反应和流体交代过程中氧逸度的升高。碧溪岭榴辉岩的全岩的铁同位素组成相对均一,δ~(56)Fe值的变化范围是从0.00±0.07‰到0.16±0.07‰。如此均一的铁同位素组成是从其原岩,即地幔玄武质岩浆的堆晶体中继承的,反映了在板块俯冲折返过程中其铁同位素组成改变不大。石榴石具有相对均一的铁同位素组成(δ~(56)Fe值为-0.08±0.07‰到0.02±0.07‰),而共存的绿辉石具有相对变化较大的铁同位素组成(δ~(56)Fe值为0.15±0.07‰到0.47±0.07‰)。同时,绿辉石也具有变化范围更大的Fe~(3+)/∑Fe值(0.367±0.025到0.598±0.024),指示绿辉石可能在峰期变质之后的折返过程中受到改造。相比于新鲜榴辉岩中的绿辉石(～0.37),退变质榴辉岩中的绿辉石具有更高的Fe~(3+)/ΣFe值(0.54-0.60),而两种榴辉岩中的石榴石的Fe~(3+)/ΣFe值相对均一(0.039±0.013到0.065±0.022)。研究表明,石榴石中相对均一的铁同位素组成及三价铁含量指示石榴石在退变质反应中受到的改造很小,基本保留了其峰期变质的δ~(56)Fe值和Fe~(3+)/ΣFe值。因为Fe和Mg在石榴石和绿辉石的扩散速率类似,而且已有的研究表明石榴石和绿辉石之间的Mg同位索在峰期变质时达到了平衡,因此可以推定石榴石和绿辉石之间的Fe同位素在峰期变质时也应该达到了平衡。退变质榴辉岩中升高的三价铁比值及变化的△56Fe绿辉石-石榴石指示退变质过程中氧逸度高的流体交代作用会导致绿辉石的铁同位素组成上升。计算拟合的结果表明,在600-650℃时,绿辉石中的Fe~(2+)和Fe~(3+)之间的铁同位素分馏值为～0.98%。。拟合结果整理成矿物内的温度计的形式为：δ~(56)Fe~(3+)_(绿辉石)-δ~(56)Fe~(2+)_(绿辉石)=75-84×10~4/T~2(T为开尔文温度)。
[Abstract]:Chinese is located in the eastern Dabie Su Lu orogenic belt was formed by the Triassic collision of Southern China plate and North China plate formed. The Su Lu Dabie orogenic belt is divided into two parts of Tancheng - Lujiang fault zone, respectively for the western part of the Dabie orogenic belt and the eastern part of the Su Lu orogenic belt. Coesite and diamond discovery and Research on the cooling history that was Southern China plate subduction to mantle depths (100km), and then rapid exhumation to the surface. The samples were collected from the Dabie is located in the middle temperature ultra high pressure metamorphic belt in Shuanghe and Bixiling area, selected samples of fresh eclogite and retrograde eclogite, to rutile and Fe isotopes as a breakthrough by contrast, fresh eclogite and retrograde Eclogite in the trace element and isotope geochemistry data, explore the retrograde metamorphism in the exhumation stage of elemental and isotopic geochemistry for For the fine structure of large particles. Jin Hongshi is the first work of this thesis mainly studies the Shuanghe area. The contents of trace elements, oxygen isotope composition, the Raman spectra of geochemical parameters such as large particles. Jin Hongshi compared to Eclogite in the Jin Hongshi quartz vein has a higher content of V and U, lower the content of Nb, Ta, Cr and W. two kinds of occurrence of Jin Hongshi with oxygen isotope values similar to that of Zr/Hf, Zr, Hf and Mo. The content of Jin Hongshi in quartz vein has a lower Nb/Ta value, indicating the fluid dehydration reaction compared with Ta residual phase distribution coefficient is greater. There is the product of retrograde titanite Eclogite in the large particles around Jin Hongshi, Jin Hongshi and sphene boundary, observed Nb from titanite to Jin Hongshi's "anti diffusion phenomenon. Raman spectra and EBSD results show that the performance of two large granular rutile crystal whole The body direction is not consistent, divided into different crystal orientation of small particles and Jin Hongshi linear kink band. Observation and experimental results under the microscope showed that the formation process of Jin Hongshi particles should have four stages, respectively is: 1) the formation of small particles of Jin Hongshi; 2) Jin Hongshi in the plastic case, plastic forming knee fold belt; 3) by Jin Hongshi to brittle stress, and the formation of brittle kink band; 4) in the later period, Jin Hongshi's physical rupture. The second work of Bixiling eclogite rocks of whole rock and mineral iron isotope composition and valence state of iron composition, oxygen fugacity in the tracer and other geochemical parameters during the exhumation process changes. The Mossbauer spectrum showed that the Eclogite in the Fe~ (3-) / Sigma Fe value than fresh eclogite Fe~ (3+) /EFe value is high, indicating retrograde metamorphic reactions and fluid metasomatism in the process of oxygen 閫稿害鐨勫崌楂，

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