岩浆－热液过渡阶段体系中不相容元素地球化学行为及其机制－以新疆阿尔泰3号伟晶岩脉研究为例

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	岩浆－热液过渡阶段体系中不相容元素地球化学行为及其机制－以新疆阿尔泰3号伟晶岩脉研究为例
	张辉
	2001
学位授予单位	中国科学院地球化学研究所
学位授予地点	中国科学院地球化学研究所
学位名称	博士
学位专业	地球化学
关键词	稀土“四重效应” 元素对分异流体相出溶岩浆液相不混溶 B同位素体系岩浆起源岩浆分离结晶岩浆-热液过渡阶段图岩蚀变磷灰石电气石碱性长石伟晶岩阿尔泰(新疆)
摘要	伟晶岩形成和演化中岩浆-热液过渡阶段物理化学性质、过铝质富挥发分岩浆体系岩浆液相不混溶、流体相出溶事件的相对时间尺度以及在岩浆-热液过渡阶段体系中稀土和其它微量元素地球化学行为(稀土“四重效应”机制和微量元素分异及控制因素等)是目前过铝质岩浆体系研究的焦点问题。对上述问题的研究和探讨，将有助于了解伟晶岩形成和演化的全过程，有助于了解挥发分(H_20、F、B、P)对过铝质岩浆体系物理性质以及对REE和其它微量元素地球化学行为影响，这对于理解过铝质岩浆体系成岩、成矿作用过程具有重大的理论和实际意义。本论文选择著名的新疆阿尔泰3号伟晶岩脉为研究对象，由于它具有完美的分带特征，使我们有可能通过各结构带矿物学、地球化学的研究揭示伟晶岩成因和演化过程以及讨论岩浆-热液过渡阶段体系中的稀土和微量元素地球化学行为。由于LCT型伟晶岩具有明显的分带性、矿物颗粒大小不均匀性等特点，因此利用全岩样品研究伟晶岩往往因取样没有代表性而被大多数岩石学家和地球化学家所否定。考虑到这点，本文系统采集阿尔泰3号伟晶岩脉各结构带磷灰石、电气石、碱性长石、石英矿物，部分结构带的石榴石、绿柱石、锂辉石矿物以及伟晶岩冷凝边和蚀变围岩的全岩样品，通过各结构带磷灰石矿物化学组成和~(87)Sr／~(86)Sr比值测定，电气石矿物的化学组成和硼同位素组成分析，碱性长石中磷含量的电子探针分析和碱性长石-石英矿物对的氧同位素体系研究以及蚀变围岩和冷凝边的化学组成分析，本文较为详尽系统地探讨以上所有的问题。通过本次多年的研究，我们获得了以下几点新的认识：f1) 富挥发分过铝质岩浆体系的样品存在稀土“四重效应”和Y-Ho、Zr-Hf、Nb-Ta、Sr-Eu元素对的显著分异，表明在过铝质岩浆演化过程中含氟流体相起着重要的作用；目前，稀土“四重效应”机制被认为是含水流体相与过铝质岩浆熔体相互作用的结果(Irber 1999；Bau 1997)，而不是独居石、磷钇矿、石榴石矿物等矿物的早期结晶引起的残余熔体稀土含量的异常变化。由于阿尔泰3号伟晶岩脉各带磷灰石以及与其共生的石榴石、绿柱石、碱性长石、锂辉石矿物均存存明显的稀土“四重效应”以及相同电价、相似离子半径的不相容元素间存在显著的分异，并结合最近赵振华等(1999)和Sba and Chappell(1999)报道S型花岗岩全岩和单矿物(磷灰石、独居石、长石、黄玉等)均存在稀土“四重效应”这一现象，本文研究提出，稀土“四重效应”是富挥发分过铝质岩浆体系的一个基本特征，其机制既不可能由含稀土的副矿物早期结晶引起残余熔体相中REE含量变化的结果，也不能定性地归因于流体相与熔体相相互作用过程中稀土元素在流体／熔体之间分异的结果，而很可能与伟晶岩岩浆形成之前某些过程密切相关，S型花岗岩岩浆在液相线以上存在硅酸盐熔体与高盐熔体(或卤水相)的不混溶液相分离有可能是过铝质岩浆体系产生稀土“四重效应”的主要原因。(2) 由于Sr~(2+)与Eu~(2+)具有相同的电价、相似的离子半径，根据经典的地球化学原理，它们具有相似的地球化学行为，在各种地质过程中其它们具有紧密的一致性。阿尔泰3号伟晶岩磷灰石中Sr／Eu比值具有二歧变化现象，其中I、Il、III和IV带磷灰石具有低的Sr／Eu比值(16:0～111.2)，而V、VI、VII带、核部带和IV带中的一个磷灰石则显示较大的Sr／Eu比值(主要在246．6～514:9范围，其中IV带的一个磷灰石该比值在10000以上)。对世界范围内的过铝质花岗岩统计表明，过铝质岩浆岩的全岩样品中的Sr／Eu也呈现二歧变化，由此本文提出Sr／Eu比值是指示过铝质岩浆体系中岩浆流体相出溶的有效地球化学参数，大于300的Sr／Eu比值是Eu~(2+)强烈分配进入流体相的地球化学标志。(3) 阿尔泰3号伟晶岩I、II、III带电气石矿物的硼同位素组成(δ~(11)B)分布在-41.11‰～-30.90‰之间，V、VI、VII带电气石矿物硼同位素组成(δ~(11)B)在-15.23‰～-9.20‰范围，IV带电气石硼同位素分布于-39.19～13.10‰范围，如此大的硼同位素分馏无法根据实验研究获得的熔体／流体相间的分馏系数进行合理解释。我们的研究表明岩浆成因电气石的B同位素组成与电气石化学组成具相关性，随电气石Y位上AI、Li阳离子数的增大，δ~(11)B值显示明显增大的趋势。阿尔泰3号伟晶岩最初形成的结构带中电气石的B同位素组成是目前所发现的最低值之一(-41.1l～39.01‰)，指示形成伟晶岩脉的初始岩浆可能由含非海相蒸发岩的泥质岩经变质深熔作用形成，或是伟晶岩岩浆在上侵定位过程中同化了含非海相蒸发岩地层的结果。本次研究利用电气石-白云母和电气石-四硼酸盐矿物间的同位索分馏以及熔体相-流体相间的同位索分馏，合理解释了阿尔泰3号伟晶岩脉电气石B同位素分馏过程，并首次获得伟晶岩形成、演化过程中B同位素组成演化的全程图谱。(4)伟晶岩岩浆形成和演化过程是否存在由过磷引起的液相不混溶现象未见确凿的岩石学和实验地球化学证据。阿尔泰3号伟晶岩脉糖粒状钠长石带(II带)明显由两种不同组构的岩性组成，而且该带是3号脉主要的Be矿化带，也是P、REE最重要的沉淀场所。阿尔泰3号伟晶岩脉梳状结构带和I带的碱性长石具较高P含量(平均值分别为0.42 wt％和0.22 wt％)，随着岩浆的演化进行，从III带到核部带，碱性长石中P含量逐渐增大(平均值由0.21 wt％增至0.50 wt％)，而II带碱性长石中异常低的P含量(平均值为0.08、wt％)是该带大量磷灰石矿物饱和结晶，导致残余熔体相中P极度亏损所造成的，而不是晚期出溶的岩浆热液交代形成的。根据Watson (1978)、Ryerson and Hess(1980)在液相线以上温度下实验研究获得的微量元素在不混溶的基性和酸性液相中分配特征，以及Webster et a1．(1997）对德国Ehrenfriedersdorf矿区伟晶岩石英中熔体包裹体的研究成果，我们推断在富磷岩浆体系中，在其演化的早期将出现由磷引起的液相不混溶作用，它对伟晶岩成岩和Be的成矿具有重大的影响。
其他摘要	Studies of geochemical processes and events during the formation and evolution of pegmatite, such as the physical and chemical properties of magma in magmatic-hydrothermal transition stage, the immiscibility of liquid in volatile-rich peraluminous system, the relative time scale of fluid exsolution, and the behaviors of rare-earth elements and other trace elements in magmatic-hydrothermal transition stage, all have been being the main interests for geochemists in recent years. It will be helpful for the understanding of a complete process of the petrogenesis and evolution of pegmatites, the impact of voltiles (including H_20, F, B, P) on the physical prorerties of peraluminous magmatic system, and the fractionation of REE and incompatible elements in the system. Moreover, the studies of the subjects mentioned above are of much importance in understanding the petrogenesis and metallogenesis of peraluminous magmatic systems. Altay No.3 pegmatite, the largest LCT pegmatite irn China with a perfect zonation, is located in the northern part of Xinjiang Uygur Autonomous Region, and hence has been chosen a typical representative for study on the geochemical processes associated with the formtion and evolution of peraluminous pegmatite. Studies on mineralogy, isotope and trace elemental geochemistry of the pegmatite wilt make it possible to reveal the principals of petrogenesis and evolution of pegmatite, and the geochemical behaviors of rare-earth elements and other trace elements in magmatic-hydrothermal transition stage. With these main purposes mentioned above, the determinations of chemical composition and ~(87)Sr/~(86)Sr ratios of apatites, the chemical and boron isotopic compositions of tourmalines, the phosphorus contents in alkali feldspars, the oxygen isotope in quartz-alkali feldspars pairs, the chemical compositions in altered rocks and chilled border of Altay No.3 pegmatite, have been investigated. The main results and conclusions have been achieved as follows: (1) The presence of the REE tetrad effect and the remarkable fractionation between Y-Ho, Zr-Hf, Nb-Ta, and Sr-Eu in the samples indicate that the aqueous fhfid pays an important role in the evolution ofpegmatite. The origination of the REE tetrad effect has been considered as the result of interaction between peraluminous melt and aqueous magrnatic fluid, instead of the abnormal change in the residual melts by the early crystallization of monazite (LREE-rich mineral) and spessartite (l-tREE-rich mineral), as suggested by Irber (1999) and Bau (1997). The presence of REE tetrad effects in apatites and whole rocks of S-type granites has been reported by Sba and Chappell (1999) and Zhao et a1.(1999), respectively. The REE tetrad effect in minerals, such as apatite, spessartite, beryl, alkali feldspar and spodumene, from all zones of Altay No.3 pegmatite has been found remarkable, and the fractionations among isovalent incompatible elements in these minerals are also significant. This paper puts forward that the REE tetrad effect is one of the basic features of peraluminous melts and the origin of REE tetrad effect might be probably related to some processes prior to the formation of pegmatite magma. The immiscibile liquid separation between silicate melt and hydrosaline melt (or brine) may be responsible for it. (2) According to the classical principles of geochemistry, Sr and Eu are coupled in the geochemical processes due to their similarities in charges and ion radii. The investigation reveals that the Sr/Eu ratios in apatites from the first to forth intermediate zones of Altay No.3 pegmatite are all low, within the range of 16.0～111.2, while those from the fifth intermediate zone to core zone are high (246.6～514.9). A statistics in world range indicates that the Sr/Eu ratios in the peraluminous granites show similar variation trend to those in pegmatitic apatites. This paper suggests that the high Sr/Eu ratio (>300) in apatites or peraluminous granites is due to the strong partition of Eu_(2+) into the aqueous fluid. and this ratio can be used as an excellent geochemical indicator of fluid exsolution from magmatic melts. (3) The boron isotopic compositions (δ~(11)B) of tourmalines range from -41.11‰ to - 30.90‰ in the first to third intermediate zones, from -39.19 to -13.10‰ in the forth intermediate zone, and from -15.23‰ to -9.20‰ in the fifth to seventh intermediate zones of Altay No.3 pegmatite. The large variation in the boron isotopic compositions of tourmalines cannot be explained by experimental fractionation coefficient between fluid and melt phases. It is found that there is a co-relationship between the boron isotope and chemical composition of magmatic tourmaline: the δ~(11)B value increases with increasing Al and Li in Y-site of tourmalines. The low δ~(11)B values (-41.1～-39.01‰) in Altay No.3 pegmatite, which is one of the lowest values found by now, imply that non-marine evaporites might exist in magma source, or the pegmatitie magma had been assimilated by them during its intrusion and emplacement. It is explained that the large fractionation of boron isotope observed in tourmalines of Altay No.3 pegmatite is resulted from the fractionation of boron isotopes between tourmaline and muscovite, and also tetraborate, in combination with the fractionation between fluid and melt phases. (4) Up to now, there are no obvious petrologic and geochemical evidences to prove that the immiscibility during the formaion and evolution of pegmatites is resulted from the perphosphorus feature in melt phase. The saccharoidal aibite zone, the second zone of Altay No.3 pegmatite, is composed of two different fabric rocks from the field observation, and is a main zone for deposition of beryllium, phosphrus and rare-earth elements. The average contents of phosphrus in alkali feldspars in both comb zone and the first intermediate zone are high, in the range of 0.42～0.22 wt%, and gradually increase from 0.21 wt% to 0.50 wt% as the magma evolves from the third intermediate zone to the core zone. The abnormally low content of phosphms (<0.10 wt%) in alkali feldspar from the saccharoidal albite zone is considered due to the crystallization of apatites, which gives rise to extreme depletion of phosphms in residual melt. So, the saccharoidal albite zone is magmatic, instead of resulting from the metasomatism. According to the experimental results of Watson (1978), and Ryerson and Hess (1980), pertaining to the distribution of trace elements between basic and acidic liquids in the temperature above magmatic liquidus, and the investigation of Webster (1997) on the melt inclusion in pegmatitic quartz from Ehrenfriedersdorf ore district, Germany, it should be drawn that there probably exists the liquid immiscibility in the early stage of pegmatitic evolution, due to the perphosphorus content in the magmatic systems. The perphosphorus feature of the melt is of great impact on the petrogenesis and matellogenesis of pegmatites.
页数	178
语种	中文
文献类型	学位论文
条目标识符	http://ir.gyig.ac.cn/handle/352002/3568
专题	研究生_研究生_学位论文地球深部物质与流体作用地球化学研究室
通讯作者	张辉
推荐引用方式 GB/T 7714	张辉. 岩浆－热液过渡阶段体系中不相容元素地球化学行为及其机制－以新疆阿尔泰3号伟晶岩脉研究为例[D]. 中国科学院地球化学研究所. 中国科学院地球化学研究所,2001.