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

摘要

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 TiO2 (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％ Al2O3,0.21 wt％-0.33 wt％ TiO2 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 Pdn/Irn ratio ranges between 1.55-1.7and 1.36-1.94,respectively.Trace element contents of the Cbeshmeh-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.