Coesite Discovered from the Podiform Chromitite in the Luobusha Ophiolite, Tibet
Coesite Discovered from the Podiform Chromitite in the Luobusha Ophiolite, Tibet
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发表时间：
2004
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通讯作者：
Chen Song-yong
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作者：
Chen Song-yong
柯石英和蓝晶石是在罗布莎蛇绿岩豆荚状铬铁矿中的新发现，罗布莎蛇绿岩位于西藏拉萨东南约200 km处，标志着印度板块与欧亚板块的碰撞。柯石英-蓝晶石晶体呈针状，大小约为（20-40）μm×（4-6）μm，在0.7mm × 0.5mm的TiFe合金周围形成30-50 μm宽的边缘。除了柯石英-蓝晶石边缘外，TiFe合金芯被约10 μm宽的TiSi外区和约20-70 μm宽的天然Ti内区包围。Ti-Mg-K-Na-Ca氧化物晶体和Si-（金红石）作为间隙物质存在于柯石英和蓝晶石形成的骨架中。在TiFe合金核内是一些小晶粒的天然Ti和相对低Ti的TiFe合金，构成了一个myrmekite结构。Si、Al和O的含量从边缘到核心呈逐渐减少的趋势，表明TiFe合金的边缘发生了化学反应。在这个反应过程中，新的矿物形成了一个共生结构。从自然Ti到TiSi再到柯石英-蓝晶石和氧化物集合体的规律性变化表明，这一系列矿物是在高温高压条件下，由自然Ti与某些硅酸盐反应形成的。这一反应可能发生在地幔柱上涌过程中，地幔柱携带了TiFe合金和其他矿物，表明罗布莎柯石英是在压力下降过程中形成的，与会聚边缘压力上升形成的柯石英不同。
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.