Origin of podiform chromitite, a new model based on the Luobusa ophiolite, Tibet
Origin of podiform chromitite, a new model based on the Luobusa ophiolite, Tibet
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豆状铬铁矿成因——基于西藏罗布萨蛇绿岩的新模型
DOI：
10.1016/j.gr.2014.04.008
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发表时间：
2015-02
影响因子：
6.1
通讯作者：
Songyong Chen
中科院分区：
文献类型：
--
作者：
Zhao Liu;Yuan Li;Jinyang Li;Songyong Chen
豆荚状铬铁矿被解释为蛇绿岩上地幔段熔岩反应和与之相关的熔体混合的结果。然而，在许多豆荚状铬铁矿和寄主橄榄岩中发现了超高压(UHP)矿物，特别是钻石和柯石英，这引发了对该模型有效性的根本质疑。西藏罗布萨蛇绿岩中的铬铁矿有块状、浸染状等。铬铁矿和橄榄岩中的铬铁矿颗粒具有多变但相对较高的氧化镁，属于镁铬铁矿。块状铬铁矿中的许多镁铬矿颗粒含有镁橄榄石和辉石包裹体，以及钻石和其他罕见的矿物。镁橄榄岩包裹体的Fo数为97~99，NiO含量为1.11~1.29wt.%。单斜辉石包裹体的Mg#S(=9100∗)和斜方辉石包裹体的镁含量分别为96-98和96-97。X射线研究表明，橄榄石包裹体具有非常小的晶胞和较短的阳离子-氧键距离，表明其在高压下结晶。相反，结节状和浸染状铬铁矿中的镁铬矿颗粒缺乏辉石包裹体，其橄榄石包裹体具有较低的Fo数(94-96)和较低的NiO含量(0.35-0.58wt.%)。此外，块状矿石中镁铬矿的Fe3+/Fettal(0.42)高于结节状和浸染状矿石中的Fe3+/Fettal，后者为0.22。浸染型铬铁矿还显示出从纯橄榄岩包裹体到块状矿石的橄榄石和镁铬铁矿成分的系统变化，表明熔岩反应。这些观察结果表明豆荚状铬铁矿的形成是一个多阶段的过程。在低ƒO2的环境下，部分橄榄岩熔体在地幔深处结晶出镁铬矿颗粒，可能还有少量的铬铁矿。超高压矿物和高镁橄榄石和辉石包裹体被捕获在这些镁铬铁矿颗粒中。当大洋地壳板片被困在超俯冲带(SSZ)时，它们受到岛弧拉斑玄武岩和邦尼质岩浆的改造，从而改变了镁铬矿的成分，并在熔体通道中沉积了铬铁矿。
Podiform chromitites have been interpreted as the result of melt–rock reaction and related melt mixing in upper mantle sections of ophiolites. However, the discovery of ultrahigh-pressure (UHP) minerals, especially diamond and coesite, in many podiform chromitites and host peridotites, raises fundamental questions about the validity of this model. Chromitites in the Luobusa ophiolite of Tibet range from massive, to nodular to disseminated. Chromite grains in both the chromitites and peridotites have variable but relatively high MgO and are classified as magnesiochromite. Many magnesiochromite grains in the massive chromitites contain inclusions of forsterite and pyroxene, as well as diamonds and other unusual minerals. Forsterite inclusions have Fo numbers of 97–99 and NiO contents of 1.11–1.29 wt.%. Mg#s (= 100 ∗ Mg / (Mg + Fe)) of clinopyroxene inclusions are 96–98 and those of orthopyroxene are 96–97. X-ray studies show that the olivine inclusions have very small unit cells and short cation–oxygen bond distances, suggesting crystallization at high pressure. In contrast, magnesiochromite grains in nodular and disseminated chromitites lack pyroxene inclusions and their olivine inclusions have lower Fo numbers of 94–96 and lower NiO contents of 0.35–0.58 wt.%. In addition, magnesiochromite in massive ores has higher Fe3 +/Fetotal(0.42) than that in nodular and disseminated ores, which have ratios of 0.22. Disseminated chromitites also show systematic changes in olivine and magnesiochromite compositions from the dunite envelope to the massive ore, indicating melt–rock reaction. These observations suggest that the formation of podiform chromitites is a multi-stage process. Magnesiochromite grains and perhaps small bodies of chromitite crystallize deep in the mantle under low ambient ƒO2from partial melts of peridotite. UHP minerals and highly magnesian olivine and pyroxene inclusions are trapped in these magnesiochromite grains. When oceanic crustal slabs are trapped in suprasubduction zones (SSZ), they are modified by island arc tholeiitic and boninitic magmas, which change the magnesiochromite compositions and deposit chromitite ores in melt channels.
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