Upper crustal podiform chromitite from the northern Oman ophiolite as the stratigraphically shallowest chromitite in ophiolite and its implication for Cr concentration
Upper crustal podiform chromitite from the northern Oman ophiolite as the stratigraphically shallowest chromitite in ophiolite and its implication for Cr concentration
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DOI：
10.1007/s00410-004-0552-8
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发表时间：
2004-02
影响因子：
3.5
通讯作者：
S. Arai;Jiro Uesugi;A. H. Ahmed
中科院分区：
文献类型：
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作者：
S. Arai;Jiro Uesugi;A. H. Ahmed
在阿曼北方蛇绿岩的Wadi Hilti发现了一种新的豆荚状铬铁矿。它位于晚侵入纯橄榄岩体内，可能来自富含橄榄石的晶体泥，位于席状岩墙杂岩和上部辉长岩之间。这种铬铁矿形成小的（厚度<30厘米）豆荚，形状不规则，呈透镜状。在豆荚中既没有分层也没有分级层理。铬铁矿位于上地壳，在蛇绿岩地层中比莫霍面到上地幔过渡带中发现的其他豆荚状铬铁矿浅得多。它沿着纯橄榄岩体内的一个小的长英质-辉长岩熔池分布，该熔池是由被泥质捕获的辉长岩块熔融而形成的。铬尖晶石沉淀，由于混合的两种熔体，玄武质的间隙熔体从糊状物和进化，可能长英质，熔融形成的辉长岩块。豆荚状铬铁矿在岩石学和尖晶石化学方面与层状侵入体中的层状铬铁矿惊人地相似。前者以斜长石和单斜辉石代替橄榄石作为基质硅酸盐，并含有自形体和细尖晶石及固体矿物包裹体。阿曼上地壳豆荚状铬铁矿中的铬尖晶石具有相对较低的（Cr2 O3 + Al 2 O3）含量，Cr/（Cr + Al）原子比约为0.6，以及相对较高的TiO 2含量，范围为1 - 3wt%。我们的结论是，橄榄石-尖晶石饱和熔体对相对富含硅的物质（地壳岩石或地幔斜方辉石）的同化可以解释任何类型铬铁矿的成因。
A new type of podiform chromitite was found at Wadi Hilti in the northern Oman ophiolite. It is within a late-intrusive dunite body, possibly derived from olivine-rich crystal mush, between the sheeted dike complex and upper gabbro. This chromitite forms small (<30 cm in thickness) pods with irregular to lenticular shapes. Neither layering nor graded bedding is observed within the pods. The chromitite is in the upper crust, by far shallower in ophiolite stratigraphy than the other podiform chromitites that have ever been found in the Moho transition zone to the upper mantle. It is distributed along a small felsic to gabbroic melt pool within the dunite body, which was formed by melting of gabbroic blocks captured by the mush. Chromian spinel was precipitated due to mixing of two kinds of melt, a basaltic interstitial melt from the mush and an evolved, possibly felsic, melt formed by the melting of gabbro blocks. The podiform chromitite reported here is strikingly similar in petrography and spinel chemistry to the stratiform chromitite from layered intrusions. The former contains plagioclase and clinopyroxene as matrix silicates instead of olivine as well as includes euhedral and fine spinel with solid mineral inclusions. Chromian spinel of the upper crustal podiform chromitite from Oman has relatively low content of (Cr2O3+ Al2O3), the Cr/(Cr + Al) atomic ratio of around 0.6, and the relatively high TiO2content ranging from 1 to 3 wt%. We conclude that assimilation of relatively Si-rich materials (crustal rocks or mantle orthopyroxene) by olivine-spinel saturated melts can explain the genesis of any type of chromitite.