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西藏罗布莎不同类型铬铁矿的特征及成因模式讨论
引用本文:熊发挥,杨经绥,巴登珠,刘钊,徐向珍,冯光英,牛晓露,许继峰.西藏罗布莎不同类型铬铁矿的特征及成因模式讨论[J].岩石学报,2014,30(8):2137-2163.
作者姓名:熊发挥  杨经绥  巴登珠  刘钊  徐向珍  冯光英  牛晓露  许继峰
作者单位:大陆构造与动力学国家重点实验室地幔研究中心, 中国地质科学院地质研究所, 北京 100037;大陆构造与动力学国家重点实验室地幔研究中心, 中国地质科学院地质研究所, 北京 100037;大陆构造与动力学国家重点实验室地幔研究中心, 中国地质科学院地质研究所, 北京 100037;中国地质大学地球科学与资源学院, 北京 10008;大陆构造与动力学国家重点实验室地幔研究中心, 中国地质科学院地质研究所, 北京 100037;大陆构造与动力学国家重点实验室地幔研究中心, 中国地质科学院地质研究所, 北京 100037;大陆构造与动力学国家重点实验室地幔研究中心, 中国地质科学院地质研究所, 北京 100037;中国科学院广州地球化学研究所同位素地球化学国家重点实验室, 广州 510640
基金项目:本文受国家行业专项(SinoProbe-05-02)、国家自然科学基金重点项目(40930313)、国家自然科学基金创新群体项目(41221061)、国家自然科学基金项目(41202036、40921001)和中国地质调查局工作项目(1212011121263、12120114061801)联合资助.
摘    要:蛇绿岩地幔橄榄岩中产出的豆荚状铬铁矿是铬的主要来源。已有的研究表明,豆荚状铬铁矿形成于洋中脊或俯冲带的浅部地幔环境。但随着近些年在豆荚状铬铁矿及围岩地幔橄榄岩中不断发现金刚石等深部矿物,人们也开始质疑豆荚状铬铁矿的浅部成因理论。本文系统研究了西藏雅鲁藏布江蛇绿岩带东段的罗布莎豆荚状铬铁矿床,识别出两类铬铁矿,一类以方辉橄榄岩为围岩的致密块状铬铁矿(Cr1#),另一类是以纯橄岩壳为围岩的浸染状铬铁矿(Cr2#)。两类铬铁矿在铬尖晶石的矿物化学成分、PGE和Re-Os同位素特征上存在较大差别,属不同演化过程的结果。地幔橄榄岩的地球化学特征指示罗布莎橄榄岩中存在由低铬且轻稀土亏损和高铬且轻稀土富集的两类方辉橄榄岩。在此基础上,提出豆荚状铬铁矿为多阶段形成的新认识,经历了早期俯冲至地幔过渡带(410~660km)的陆壳和洋壳物质被脱水和肢解,过渡带产生的热和流体促成了地幔的熔融和Cr的释放和汇聚;铬铁矿浆在地幔柱/地幔对流驱动下,运移至过渡带顶部冷凝固结,并有强还原性的流体进入,后者携带了深部形成的金刚石、斯石英等高压矿物,并进入"塑性-半塑性地幔橄榄岩"中;随着物质向上移动,深度降低,早期超高压相矿物发生相变,如斯石英转变成柯石英,高压相的铬铁矿中出溶成柯石英和单斜辉石;在侵位过程和俯冲带环境,含水熔体与方辉橄榄岩反应形成了不含超高压矿物的规模相对较小的浸染状铬铁矿(Cr2#)及纯橄岩壳。

关 键 词:不同类型铬铁矿  铂族元素  铼锇同位素  罗布莎蛇绿岩
收稿时间:2/1/2014 12:00:00 AM
修稿时间:2014/5/11 0:00:00

Different type of chromitite and genetic model from Luobusa ophiolite, Tibet
XIONG FaHui,YANG JingSui,BA DengZhu,LIU Zhao,XU XiangZhen,FENG GuangYing,NIU XiaoLu and XU JiFeng.Different type of chromitite and genetic model from Luobusa ophiolite, Tibet[J].Acta Petrologica Sinica,2014,30(8):2137-2163.
Authors:XIONG FaHui  YANG JingSui  BA DengZhu  LIU Zhao  XU XiangZhen  FENG GuangYing  NIU XiaoLu and XU JiFeng
Institution:CARMA, State Key Laboratory for Continental Tectonics and Dynamics, Institute of Geology, CAGS, Beijing 100037, China;CARMA, State Key Laboratory for Continental Tectonics and Dynamics, Institute of Geology, CAGS, Beijing 100037, China;CARMA, State Key Laboratory for Continental Tectonics and Dynamics, Institute of Geology, CAGS, Beijing 100037, China;School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 10008;CARMA, State Key Laboratory for Continental Tectonics and Dynamics, Institute of Geology, CAGS, Beijing 100037, China;CARMA, State Key Laboratory for Continental Tectonics and Dynamics, Institute of Geology, CAGS, Beijing 100037, China;CARMA, State Key Laboratory for Continental Tectonics and Dynamics, Institute of Geology, CAGS, Beijing 100037, China;State Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
Abstract:Podiform chromitite are the main source of chromium. Chromitite pods formed in the shallow mantle environment of the mid-ocean ridge or subduction zone setting. However, because of the founded diamond even more and more the deep mineral, people have begun to question the model of shallow podiform chromitite. In this paper, we identified two types of chromitite from Luobusa ophiolite, eastern of the Yarlung Zangbo suture zone, which one is massive chromitite as harzburgite envelope and another is disseminated chromitite as envelope of dunite shell. There is a big difference between the spinel chemical composition, PGE and Re-Os isotopic characteristics of the two types chromitite, which indicate the different evolutionary process. Geochemistry feature of mantle peridotite show the low Cr number of peridotite unrichment of LREE process to high Cr number of peridotite enrichment of LREE. A multi-stage model for explain the formation of the podiform chromitite. Firstly, earlier slab dive to the mantle transition zone (410~660km) of continental crust and oceanic crust is dehydrated and dismembered, thermal and fluid transition produced and contributed to the molten mantle Cr release and aggregation. Then, plume/mantle convection driven transport chromite magma to move top of the condensation transition consolidation and add a strong reduction of fluid with carrying high-pressure minerals such as diamond and stishovite, and into the plasticity-semi-plastic mantle peridotite. As the material moves up the depth decreases, the early phase ultrahigh pressure (UHP) mineral phase transition into a predicament coesite, the high-pressure phase of chromite exsolution into coesite and clinopyroxene. Finally, in suprasubduction zones emplacement processes and environments, hydrous melt peridotite reacts with harzburgite formed a relatively small without UHP minerals disseminated chromite and dunite shell.
Keywords:Different type of chromitite  PGE  Re-Os isotope  Mode  Luobusa ophiolite
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