西藏罗布莎豆荚状铬铁矿中发现超高压矿物柯石英

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

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 。

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.