地学前缘 ›› 2020, Vol. 27 ›› Issue (5): 88-98.DOI: 10.13745/j.esf.sf.2020.5.39

• 地质过程与成矿作用示踪的成因矿物学 • 上一篇    下一篇

俄罗斯极地乌拉尔硬玉岩成因矿物学研究

孟繁聪,白盛锦,Alexander B. MAKEYEV,Ksenia V. KULIKOVA   

  1. 1. 中国地质科学院 地质研究所, 北京 100037
    2. 中国地质大学(北京) 地球科学与资源学院, 北京 100083
    3. 俄罗斯科学院 矿床地质、岩石学、矿物学和地球化学研究所, 莫斯科 119017
    4. 俄罗斯科学院 乌拉尔分院科米科学中心地质研究所, 塞克提夫卡尔 167982
  • 收稿日期:2020-04-15 修回日期:2020-05-28 出版日期:2020-09-25 发布日期:2020-09-25
  • 作者简介:孟繁聪(1967—),男,博士,研究员,博士生导师,主要从事镁铁超镁铁岩方面的研究。 E-mail: mengfancong@yeah.net
  • 基金资助:

    国家自然科学基金项目(41072026,41672031)

Genetic mineralogy of jadeitite from Polar Urals, Russia

MENG Fancong, BAI Shengjin, Alexander B. MAKEYEV, Ksenia V. KULIKOVA   

  1. 1. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
    2. School of Earth Sciences and Resources, China University of Geosciences(Beijing), Beijing 100083, China
    3. Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Moscow 119017, Russia
    4. Institute of Geology, Komi Science Center of Uralian Division, Russian Academy of Sciences, Syktyvkar 167982, Russia
  • Received:2020-04-15 Revised:2020-05-28 Online:2020-09-25 Published:2020-09-25

摘要:

俯冲带是壳幔物质循环的重要场所,硬玉岩可以记录这一循环过程。文中总结了俄罗斯极地乌拉尔硬玉岩的研究进展。硬玉岩呈脉状或透镜状产在蛇纹石化的方辉橄榄岩中,主要由硬玉和绿辉石组成。根据结构和颜色,硬玉可识别出两个世代。硬玉韵律环带发育,含有H2O和CH4流体包裹体,显示从流体中结晶的特征。硬玉岩中的锆石为热液锆石,锆石稀土元素中LaN/YbN=0.001~0.01,LuN/GdN=10~83,Ce/Ce*=2.8~72,显示正异常,δEu=0.53~1.02,类似于岩浆锆石。锆石的176Hf/177Hf=0.282 708~0.283 017,εHf(t)=+6~+17,类似于N-MORB的Hf同位素组成,锆石δ18O组成为5.03‰~6.04‰,平均δ18O为(5.45±0.11)‰,类似于岩浆热液和地幔的氧同位素组成。这可能反映了锆石是被俯冲带流体从途经火成岩中捕获的或者形成锆石的流体与寄主岩(方辉橄榄岩)达到了平衡。硬玉岩稀土元素配分模式近平坦或轻稀土元素略显富集,LaN/YbN比值为0.82~2.42,δEu为1.2~1.6,显示正异常,这与寄主岩稀土元素配分模式相似。富集Sr、Ba、Zr、Hf,Nb为负异常,与岛弧岩浆特征类似。(87Sr/86Sr)t为0.703 400~0.703 519(t=368 Ma),变化较小,与古海水差别明显;εNd(t)值为+0.77~+5.61,变化较大,与寄主岩(方辉橄榄岩)的Nd同位素组成类似,但不同于海水及沉积物的Nd同位素组成,表明硬玉岩的物质来源与寄主岩有明显继承关系,海水与沉积物的贡献不是主要的。矿物学和岩石学证据支持极地乌拉尔的硬玉岩主要是俯冲带流体与橄榄岩相互作用后并在其中结晶的产物。

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关键词: 硬玉岩, 硬玉, 锆石, 全岩地球化学, 俯冲带流体, 极地乌拉尔, 俄罗斯

Abstract:

Subduction zones are major sites for mass circulation between the crust and mantle, while jadeitite is a recorder of the process. In this paper, we summarized the research advance of jadeitite from Polar Urals (Russia). Jadeitite occurs in serpentinized harzburgite as veins or lenses and is composed of jadeite and omphacite. Two generations can be identified based on the color and texture of jadeite. The oscillatory zoning and fluid inclusions (H2O+CH4 in composition) in jadeite indicated it is crystallized from fluids. Zircons from jadeitite are hydrothermal at origin, and are enriched in HREE, with LaN/YbN and LuN/GdN ratios ranging between 0.001-0.01, and 1083, respectively. Cerium shows positive anomalies, with Ce/Ce* values ranging from 2.8 to 72, and δEu from 0.53 to 1.02. The 176Hf/177Hf ratios of zircons ranged from 0.282708 to 0.283017, with initial Hf isotope compositions ranging from +6 to +17. These characteristics resemble those of zircons from depleted mantle-derived magmas. The δ18O isotope compositions of zircons ranged from 5.03‰ to 6.04‰ with an average of 5.45‰±0.11‰, similar to those of mantle rocks, suggesting that zircons were acquired from precursor igneous rocks and then transported and reworked by fluids from the subducting slab, or isotopic equilibration was reached between fluid and host harzburgite. Chondrite-normalized REE patterns of jadeitite displayed weak U-shaped distribution, with LaN/YbN ratios of 0.82-2.42, and very weak positive Eu anomalies (δEu=1.2-1.6). Jadeitite was enriched in Sr, Ba, Zr and Hf and depleted in Nb, relative to primitive mantle values. The initial Sr isotopic compositions of jadeitite ranged from 0.703400 to 0.703519 (t=368 Ma), and the initial Nd-isotope ratios (εNd =+0.77-+5.61) differed from those of ancient ocean water, oceanic sediments and eclogite, metagranite, and metasediments in the nearby Marun-Keu complex. Mineralogical and petrological evidences support jadeitite precipitation from material mainly produced by fluid interaction with mafic-ultramafic rocks in a subduction zone environment.

Key words: jadeitite, jadeite, zircon, geochemistry, fluids in subduction zone, Polar Urals, Russia

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