Abstract The Nidar Ophiolite Complex (NOC) in the Ladakh Himalaya region of northern India is well-preserved and exposed along the Indus Yarlung–Zangbo suture zone. This sequence has been thrusted onto the Indus Formation to the north and to the Zildat ophiolitic melange in the south. Chromitite mineralization in the NOC occurs as massive, semi-massive, and disseminated ore bodies hosted by dunite. The chromite in these ore bodies contains inclusions of isolated silicates, base metal sulfides, and platinum-group minerals. Little variation in the [Cr/(Cr+Al)] (i.e., Cr#) (0.78–0.86) is observed, in contrast to Mg# ([Mg/(Mg+Fe2+)]), which is more variable (0.56–0.73). The NOC chromitites are high in Cr (Cr# > 0.78), suggesting a possible boninitic parent magma. The calculated (Al2O3)melt and (FeO/MgO)melt range from 9.29–11.50 and 0.49–0.10, respectively. The trace element compositions of these chromitites (Ga: 14–27 ppm; Ni: 440–1301 ppm; V: 612–802 ppm; Sc: 2–6 ppm; Co: 207–542 ppm) are comparable to those hosted in the mantle sections of other ophiolite complexes. The overall chemical composition, deduced from the Al2O3 content and FeO/MgO ratio of the primitive parent magma of high-Cr chromitites, is consistent with island arc tholeiitic melts of boninitic affinity. This parent magma would have been in equilibrium with podiform chromitites and associated ultramafic rocks. Further, the distributions of minor and trace elements in NOC chromites also indicate a boninitic magma source. The field relations, petrographic observations, and major and trace element compositions of the studied chromitites suggest that they have undergone melt–rock and melt–melt interactions, which led to the precipitation of high-Cr chromitites from Cr-rich and Al- and Ti-poor boninitic melts. The occurrence of high-Cr chromitites and the boninitic nature of the parent magma indicate that the chromitite layers of the NOC likely formed in an island arc supra-subduction zone.

中文翻译：

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印度北部 Nidar 蛇绿岩复合体的高铬铬铁矿：岩石成因和构造意义

摘要 印度北部拉达克喜马拉雅地区的尼达尔蛇绿岩杂岩体（NOC）沿印度河雅鲁藏布江缝合带出露，保存完好。该层序被推到北部的印度河组和南部的 Zildat 蛇绿混杂岩。NOC 中的铬铁矿矿化以块状、半块状和浸染性矿体的形式出现，由纯晶岩承载。这些矿体中的铬铁矿包含孤立的硅酸盐、贱金属硫化物和铂族矿物的包裹体。观察到 [Cr/(Cr+Al)]（即 Cr#）（0.78-0.86）的变化很小，而 Mg#（[Mg/（Mg+Fe2+）]）则变化更大（0.56 –0.73）。NOC 铬铁矿的 Cr 含量很高（Cr# > 0.78），表明可能存在博尼特母岩浆。计算的 (Al2O3) 熔体和 (FeO/MgO) 熔体范围为 9.29-11.50 和 0.49-0.10，分别。这些铬铁矿的微量元素组成（Ga：14–27 ppm；Ni：440–1301 ppm；V：612–802 ppm；Sc：2–6 ppm；Co：207–542 ppm）其他蛇绿岩复合体的地幔部分。从高铬铬铁矿原始母岩浆的 Al2O3 含量和 FeO/MgO 比推导出的整体化学成分与具有博尼特亲合力的岛弧拉斑岩熔体一致。这种母岩浆与豆状铬铁矿和相关的超基性岩处于平衡状态。此外，NOC 铬铁矿中微量元素和微量元素的分布也表明存在博尼特岩浆源。研究的铬铁矿的场关系、岩相观察以及主要和微量元素组成表明它们经历了熔体-岩石和熔体-熔体相互作用，这导致高铬铬铁矿从富铬和贫铝和钛的博氏体熔体中沉淀出来。高铬铬铁矿的出现和母岩浆的boninitic性质表明NOC的铬铁矿层可能形成于岛弧超俯冲带。