西藏罗布莎豆荚状铬铁矿中发现超高压矿物柯石英

杨经绥; 白文吉; 方青松; 颜秉刚; 戎合; 陈松永

西藏罗布莎豆荚状铬铁矿中发现超高压矿物柯石英

中国地质科学院地质研究所部大陆动力学重点实验室, 北京 100037

基金项目:

地质调查项目 200313000058

国家“973”专项项目 2003CB716500

详细信息

作者简介:
杨经绥(1950-), 男, 留加博士, 研究员, 主要从事青藏高原和造山带的蛇绿岩、火山岩和超高压变质岩研究, 侧重于岩石-构造学的研究

中图分类号: P589
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出版历程
- 收稿日期: 2004-09-07
- 刊出日期: 2004-11-25

Coesite Discovered from the Podiform Chromitite in the Luobusha Ophiolite, Tibet

摘要

摘要: 在西藏雅鲁藏布江蛇绿岩带东段的罗布莎豆荚状铬铁矿床中发现典型的超高压矿物柯石英和蓝晶石, 二者呈针柱状交生, 产在一个以TiFe合金成分为主的颗粒(0.7mm× 0.5mm大小) 的最外部.该颗粒从内到外由4层矿物组成, 分别为TiFe合金主体、2 0~ 70 μm宽的自然钛、约10 μm宽的TiSi合金及30~ 5 0 μm宽的柯石英和蓝晶石为主的硅酸岩和氧化物层.主体矿物为高Ti低Fe的TiFe合金, 内部出现由细粒状低Ti高Fe的TiFe合金和自然钛组成的蠕英结构.最外层由柯石英和蓝晶石组成的格架中分布细粒的Si金红石和Ti-Mg -K -Na -Ca氧化物.初步认为TiFe合金从深部高温高压环境往浅部上升过程中, 内部发生局部熔融, 分解出自然Ti, 并在其边部与其他硅酸岩矿物或熔体发生反应, 形成柯石英和蓝晶石.这一过程可能发生在洋脊拉张环境, 由于地幔柱的上涌, 将深部的豆荚状铬铁矿带到浅部, 使得其中包裹的一些高温高压环境下稳定的矿物变得不稳定, 发生熔融和交代反应, 形成新的不平衡的矿物组合.罗布莎柯石英的这种不寻常产出特征说明是在减压过程中形成, 不同于造山带中常见的由板块俯冲增压过程中形成的柯石英
- 柯石英 /
- 钛铁合金 /
- 豆荚状铬铁矿 /
- 罗布莎蛇绿岩 /
- 青藏高原
Abstract: Coesite and kyanite are new discovery from the podiform chromitite in the Luobusha ophiolite, which is located (about) 200 km southeast of Lhasa, Tibet, and marks the locus of collision between the Indian and Eurasian plates. Coesite-kyanite crystals are acicular and individuals are about (20-40) μm× (4-6) μm in size, forming a 30-50 μm wide rim (around) a 0.7 mm×0.5 mm TiFe alloy. Besides the coesite-kyanite rim, the TiFe alloy core is surrounded by a TiSi outer zone of about 10 μm width, and a native Ti inner zone of about 20-70 μm width. Ti-Mg-K-Na-Ca oxide crystals and Si-(rutile) occur as interstitial materials within the framework formed by coesite and kyanite. Within the TiFe alloy core are some small grains of native Ti and relatively low-Ti TiFe alloy, constituting a myrmekitic texture. The amount of Si, Al and O shows a decreasing trend from rim to core, which suggests that a chemical reaction occurred at the rim of the TiFe alloy. During this reaction new minerals formed with a symplektic texture. The regular variation from native Ti through TiSi to coesite-kyanite and oxide aggregates suggests that the series of minerals were formed by a reaction between native Ti and some silicates, under high pressure and temperature. This reaction probably occurred during the upwelling of a plume which carried TiFe alloy and other minerals from the deep mantle, suggesting that the Luobusha coesite formed during a decrease in pressure, which is in contrast with the coesite formed by increasing pressure in a convergent margin.
- coesite /
- TiFe alloy /
- podiform chromitite /
- Luobusha ophiolite /
- Tibet plateau

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图 1 西藏罗布莎蛇绿岩的位置和地质图(据Malpas et al., 2003修改)

Fig. 1. Location and geological map of the Luobusha ophiolite, Tibet

下载: 全尺寸图片幻灯片

图 2 TiFe合金为主体的颗粒背散色图像(BSE-image)

TiFe合金颗粒从内至外被3个带所环绕，分别是: (1)自然钛; (2)TiSi合金; (3)柯石英-蓝晶石为主的硅酸岩和氧化物带.柯石英-蓝晶石为主的硅酸岩和氧化物带主要在图的左下最外层(暗色部分)(A); 自然钛带发育最完全，几乎围绕了整个颗粒(深灰色)(B); 钛硅合金(Ti_6.9Fe_0.2Si_2.8)为仅仅宽约10 μm的带，位于(1)和(3)带之间，从BSE图像中与自然钛不易区分，由图C中分析点14, 15, 16所控制，分析点12成分已是自然钛; 主体钛铁合金(Ti_6.2Si_0.3Fe_3.4)中出现由自然钛(暗色)和相对低Ti的钛铁合金(Ti_5.5Si_0.1Fe_3.9)组成的蠕英构造(D)

Fig. 2. BSE-image of a large TiFe alloy

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图 3 能谱(EDS) 柯石英-蓝晶石带的元素分布(位置与图 2c同)

外带显示Si、Al和O为主, Si高于Al, 反映柯石英的量远远高于蓝晶石, 带中靠近自然钛部分出现少许Ti, 远离则减少, K、Mg和Na只出现在柯石英-蓝晶石带; 自然钛带与柯石英-蓝晶石带之间有一薄层Si, 代表TiSi合金带; Fe仅仅在TiFe合金中出现

Fig. 3. EDS compositional maps of the coesite-kyanite belt

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图 4 柯石英和蓝晶石的代表性激光拉曼谱分析

c.柯石英的强峰和弱峰; k.蓝晶石的强峰和弱峰

Fig. 4. Representative Raman spectra of coesite and kyanite at the outer rim of TiFe alloy from the Luobusha podiform chromitite, Tibet

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图 5 TiFe合金在减压及与硅酸岩的化学反应过程中成分的变化

Fig. 5. Element migration diagram during chemical reaction between native Ti and silicate

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表 1 罗布莎豆荚状铬铁矿中TiFe合金及伴生矿物成分分析

Table 1. Composition of TiFe alloy and associated minerals in Luobusha podiform chromitite

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