Primary minerals and mantle peridotites in late cretaceous ophiolites of Iran: a review

Several Mesozoic ophiolites in Iran formed in response to Late Cretaceous subduction initiation. Here we review the geology, petrography, mineralogy and geochemistry of mantle rocks from the Nain, Neyriz, Khoy, Esfandagheh and Fannuj ophiolites. They are remnants of Neo-Tethys ocean lithosphere formed during subduction initiation and were emplaced onto the southern flank of Eurasia. Most show SSZ-type geochemical affinity. We summarize the composition of mantle sections from these five representatives, which comprise lherzolites and harzburgites with minor dunite lenses and sometimes chromitites. Spinels in lherzolite from all regions are characterized by low chromium number [Cr# = Cr/(Al + Cr)] of 0.09 to 0.56 and plot in the abyssal peridotite field, whereas spinel in harzburgite and dunite-chromitite have Cr# of 0.41 to 0.73 and 0.45 to 0.86, respectively, showing geochemical affinities with forearc peridotites and boninites, respectively. Lherzolites and harzburgites have low rare earth element (REE) contents and experienced around 15% and 25% partial melting, respectively. Ophiolitic peridotites from the Nain (inner Zagros ophiolitic belt), Khoy, and Neyriz (outer Zagros ophiolitic belt) complexes are depleted in light REE and show flat heavy REE patterns. By contrast, peridotites from the Fannuj (Makran ophiolitic zone) and Esfandagheh (outer Zagros ophiolitic belt) complexes are characterized by flatter and U-shaped REE patterns, respectively, similar to those of SSZ peridotites. Based on the compositions of olivine and spinel, we infer that chromitites and their dunite envelopes experienced wider variations in oxygen fugacity (fO2), equilibrium temperatures, and spinel Cr# values than those of the surrounding lherzolites and harzburgites, suggesting interaction between residual mantle harzburgites and H2O-rich and highly oxidized SSZ melts. Lherzolites from all regions and harzburgites of Nain, Neyriz and Fannuj are MOR-type peridotites that plot in the overlapping space of MOR peridotites and were generated in an extensional environment. These are forearc peridotites that were in equilibrium with tholeiitic melts generated during early proto-forearc or back-arc spreading during subduction initiation, whereas spinels of dunites and high-Cr chromitites were in equilibrium with boninitic melts.伊朗境内的多处中生代蛇绿岩套形成于晚白垩世俯冲起始作用。本文综述了纳因（Nain）、内里兹（Neyriz）、霍伊（Khoy）、Esfandagheh以及范努吉（Fannuj）这5处蛇绿岩套的幔源岩石的地质学、岩石学、矿物学与地球化学特征。这些蛇绿岩套是俯冲起始阶段形成的新特提斯洋岩石圈的残片，并被侵位至欧亚大陆南缘，多数表现出俯冲带上方（Supra-Subduction Zone, SSZ）型地球化学亲和性。本文总结了这5处典型蛇绿岩套的幔源剖面组成，其岩石类型以二辉橄榄岩（lherzolites）和方辉橄榄岩（harzburgites）为主，伴生少量纯橄岩透镜体，局部可见铬铁矿体。所有区域二辉橄榄岩中的尖晶石均具有较低的铬指数（Cr# = Cr/(Cr+Al)），取值范围为0.09~0.56，投点落于深海橄榄岩区间；而方辉橄榄岩以及纯橄岩-铬铁矿体中的尖晶石Cr#分别为0.41~0.73和0.45~0.86，分别对应弧前橄榄岩与玻安岩的地球化学亲和性。二辉橄榄岩与方辉橄榄岩的稀土元素（Rare Earth Element, REE）含量较低，分别经历了约15%与25%的部分熔融作用。纳因（内扎格罗斯蛇绿岩带）、霍伊以及内里兹（外扎格罗斯蛇绿岩带）蛇绿岩套的橄榄岩均具有轻稀土元素亏损特征，且重稀土元素配分模式平坦。与之相反，范努吉（马克兰蛇绿岩带）与Esfandagheh（外扎格罗斯蛇绿岩带）蛇绿岩套的橄榄岩则分别具有更平坦的稀土配分模式与U型稀土配分模式，与俯冲带上方型橄榄岩的特征一致。基于橄榄石与尖晶石的组分特征，本文推断铬铁矿体及其伴生的纯橄岩围岩体的氧逸度（fO2）、平衡温度以及尖晶石Cr#值的变化范围均大于周围的二辉橄榄岩与方辉橄榄岩，表明残余幔源方辉橄榄岩与富水、高氧化的俯冲带上方型熔体发生了相互作用。所有区域的二辉橄榄岩以及纳因、内里兹与范努吉的方辉橄榄岩均属于洋中脊型橄榄岩（Mid-Ocean Ridge, MOR），其投点落于洋中脊橄榄岩的重叠区间，形成于伸展环境。这类橄榄岩属于弧前橄榄岩，与俯冲起始阶段早期原弧前或弧后扩张过程中形成的拉斑玄武质熔体达到平衡；而纯橄岩与高铬铬铁矿体中的尖晶石则与玻安质熔体达到平衡。