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 pod-like 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 tectono-magmatic 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 laurite-erlichmanite 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 post-magmatic 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 already-formed 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/fluid-peridotite 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. Mössbauer measurements of Fe3+/ΣFe ratios of Cr-spinels 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.