Platinum-Group Elements Geochemistry and Chromian Spinel Composition in Podiform Chromitites and Associated Peridotites from the Cheshmeh-Bid Deposit, Neyriz, Southern Iran: Implications for Geotectonic Setting Batoul TAGHIPOUR * and Farhad AHMADNEJAD Department of Earth Sciences, Faculty of Science, Shiraz University, Shiraz, Iran 71454 Abstract: Dunite and serpentinized harzburgite in the Cheshmeh-Bid area, northwest of the Neyriz ophiolite in Iran, host podiform chromitite that occur as schlieren-type, tabular and aligned massive lenses of various sizes. The most important chromitite ore textures in the Cheshmeh-Bid deposit are massive, nodular and disseminated. Massive chromitite, dunite, and harzburgite host rocks were analyzed for trace and platinum-group elements geochemistry. Chromian spinel in chromitite is characterized by high Cr # (0.72−0.78), high Mg # (0.62–0.68) and low TiO 2 (0.12 wt%−0.2 wt%) content. These data are similar to those of chromitites deposited from high degrees of mantle partial melting. The Cr # of chromian spinel ranges from 0.73 to 0.8 in dunite, similar to the high-Cr chromitite, whereas it ranges from 0.56 to 0.65 in harzburgite. The calculated melt composition of the high-Cr chromitites of the Cheshmeh-Bid is 11.53 wt%–12.94 wt% Al 2 O 3 , 0.21 wt%–0.33 wt% TiO 2 with FeO/MgO ratios of 0.69−0.97, which are interpreted as more refractory melts akin to boninitic compositions. The total PGE content of the Cheshmeh-Bid chromitite, dunite and harzburgite are very low (average of 220.4, 34.5 and 47.3 ppb, respectively). The Pd/Ir ratio, which is an indicator of PGE fractionation, is very low (0.05– 0.18) in the Cheshmeh-Bid chromitites and show that these rocks derived from a depleted mantle. The chromitites are characterized by high-Cr # , low Pd + Pt (4–14 ppb) and high IPGE/ PPGE ratios (8.2– 22.25), resulting in a general negatively patterns, suggesting a high-degree of partial melting is responsible for the formation of the Cheshmeh-Bid chromitites. Therefore parent magma probably experiences a very low fractionation and was derived by an increasing partial melting. These geochemical characteristics show that the Cheshmeh-Bid chromitites have been probably derived from a boninitic melts in a supra-subduction setting that reacted with depleted peridotites. The high-Cr chromitite has relatively uniform mantle-normalized PGE patterns, with a steep slope, positive Ru and negative Pt, Pd anomalies, and enrichment of PGE relative to the chondrite. The dunite (total PGE = 47.25 ppb) and harzburgite (total PGE =3 4.5 ppb) are highly depleted in PGE and show slightly positive slopes PGE spidergrams, accompanied by a small positive Ru, Pt and Pd anomalies and their Pd n /Ir n ratio ranges between 1.55–1.7and 1.36−1.94, respectively. Trace element contents of the Cheshmeh-Bid chromitites, such as Ga, V, Zn, Co, Ni, and Mn, are low and vary between 13–26, 466–842, 22−84, 115– 179, 826–−1210, and 697–1136 ppm, respectively. These contents are compatible with other boninitic chromitites worldwide. The chromian spinel and bulk PGE geochemistry for the Cheshmeh-Bid chromitites suggest that high-Cr chromitites were generated from Cr-rich and, Ti- and Al-poor boninitic melts, most probably in a fore-arc tectonic setting related with a supra-subduction zone, similarly to other ophiolites in the outer Zagros ophiolitic belt. Key words: Cheshmeh-Bid Chromitite, trace elements, platinum-group elements, boninitic magma, supra-subduction zone Vol. 92 No. 1 pp.183–209 ACTA GEOLOGICA SINICA (English Edition) Feb. 2018 * Corresponding author. E-mail: Taghipour@shirazu.ac.ir © 2018 Geological Society of China