Chromitite absence, presence and chemical variation in ophiolites of Central Iran (Naein, Ashin, Anarak and Jandaq)

摘要

There are two types of ophiolites in the western part of Central Iran: Mesozoic ophiolites (Naein and Ashin), and Paleozoic ophiolites (e.g., Anarak and Jandaq). Field studies show that the Mesozoic ophiolites contain considerable amounts of chromitite, but the Anarak and Jandaq ophiolites do not. Study of Mesozoic ophiolites reveals that they contain two different types of chromitites. The Naein chromitites have high Cr contents, whereas the Ashin chromitites are high-AI in composition. The field, petrographic and chemical studies suggest that melt/mantle interaction caused the formation of chromitites in the Naein and Ashin ophiolites. The reason of chromitite absence in the Anarak ophiolite is an insufficient degree of partial melting of mantle rocks, as well as a low content of Cr_20_3 in pyroxenes and the mantle source. Despite suitable Cr_20_3 contents in clinopyroxenes in the Jandaq ophiolite, the intact fertile lherzolitic system of the mantle and its low Cr_20_3 amount explain the chromitite absence. The high-AI composition of chromitites in the Ashin ophiolite can be ascribed to the high-AI (MORB-like) composition of percolating melts, lower degree of partial melting, lower Cr# [Cr/(Cr+AI)] of orthopyroxenes and lower CrP3 content of mantle peridotites in the Ashin ophiolite, compared to those in the Naein area. The high-Cr characteristic of chromitites in the Naein ophiolite are attributed to the boninitic composition of the percolating melt, high Cr# of orthopyroxenes and high Cr_20_3 value of mantle peridotites. It is concluded that the chromitites of the Central Iran Mesozoic ophiolites were formed in a supra-subduction zone setting.