Geochemistry of platinum-group elements in the podiform chromitites and associated peridotites of the Nain ophiolites, Central Iran: Implications for geotectonic setting

The Nain ophiolite complex with an extent of > 600 km2 is a part of the Central Iranian ophiolite, which is related to the opening and subsequent closure of the Neo-Tethys Ocean. Dunite and serpentinized harzburgite in the Nain area host podiform chromitites that occur as three (eastern Hajhossein, western Hajhossein, and Soheil Pakuh) schlieren-type tabular and aligned massive lenses with various sizes. The most common chromitite ore textures are massive, nodular, disseminated, and banded, reflecting crystal settling processes. The Cr# [Cr/(Cr + Al)] ranges from 0.43 to 0.81 (average 0.63). The Mg# [Mg/ (Mg + Fe2+)] varies from 0.25 to 0.78 (average 0.62). The Nain ophiolite and hosted chromitite are generally characterized by high Cr#, reflecting crystallization from a very hot boninite magma in a MORB setting. The high Cr# in the Nain chromitite also indicates a high degree of melting (15%–35%) of the depleted peridotite. The average total PGE content in the ophiolitic host rock (harzburgite and dunite) and chromite are 107 and 221 ppb, respectively. The Nain ophiolite and chromitite have high IPGE/PPGE and negative Pt* (Pt/Pt* = 0.6) anomaly, which is a characteristic of high Cr# chromitite. The U-shaped REE pattern of dunite host rock suggests the interaction of depleted mantle peridotite with boninitic melt. Geochemical data suggest that the Nain chromitites are related to the boninitic magma emplacement in a supra-subduction zone.