Co-occurrence of compositionally variable chromitites in the Sabzevar ophiolite, NE Iran

The Sabzevar ophiolite belt consists of a series of thrust-bound massifs that widen in extent to ~150 km in NE Iran. The Kuh-Siah harzburgite-dunite massif in the central sector of this belt hosts abundant discontinuous podlike chromitite deposits. These deposits are characterized by spinels with contrasting and variable compositions, but without any microtextural differences. The chromian spinel in these chromitite bodies displays a wide range of Cr-number (Cr/(Cr + Al)) atomic ratio from 0.50 to 0.77, and reveals a co-occurrence of Al-rich and Cr-rich chromitites within a single ophiolitic massif. Despite these chemical differences, primary spinels from both the high-Al and high-Cr chromitites have similar TiO2 contents, indicating that they formed in a single tectonomagmatic setting. Both high-Al and high-Cr chromitite samples display low to moderate total platinum group element (PGE) abundances (85–537 ppb). Solid phases hosted by Cr-spinel grains in chromitites and their host dunites can be divided into three categories: platinum group minerals (PGM), base-metal minerals (BMM), and silicates. Euhedral quadrangular and round-shaped silicate inclusions consisting of clinopyroxene, orthopyroxene, Na-bearing phlogopite, serpentine and chlorite, are sporadically scattered in the Cr-spinel grains. These inclusions are mainly single crystals and rarely occur as polyphase assemblages. The BMM are mainly euhedral Ni- and Cu-rich sulfides with rare occurrence of pentlandite. The most widespread PGM are crystals of the lauriteerlichmanite series with subordinate malanite, braggite and BM-PGE-rich sulfides. Textural relationships reflect that the PGM are of magmatic origin. The BMM, on the other hand, formed during both magmatic and postmagmatic processes. The occurrence of low-temperature inclusions within Cr-spinel and ferrian Cr-spinel, comprising tremolite, chlorite and serpentine, are interpreted as post-magmatic entrapments of alreadyformed silicate phases at the time of grain boundary migration. Our petrographic observations and geochemical interpretations reveal that chromitites in the Kuh-Siah peridotite massif formed during multistage melt/fluidperidotite reaction processes in an extended intra-oceanic arc–forearc setting located between the Turan and Central Iran continental blocks in the Mid to Late Cretaceous. M¨ossbauer measurements of Fe3+/ΣFe ratios of Crspinels from the non-oxidized chromitite samples are relatively similar, more or less in the range 0.15–0.20, whereas those from the more oxidized chromitite samples have Fe3+/ΣFe ratios in the range 0.28–0.62. Field observations and textural studies suggest that the localized post-magmatic oxidation is linked to dominant hydrous fluids existing in the shear zone.