Mineralogy and geochemistry of peridotites and chromites in Nain ophiolitic assemblages, Central Iran

Abstract

The mantle sequence of Nain ophiolite in Central East Iran Microplate is composed of harzburgite, dunite, chromitite pods and lherzolite with major minerals of olivine, orthopyroxene, clinopyroxene and chromite. The studied chromites were classified as high chromium (Cr#>0.6) and high aluminum (Al2O3 average content ≈ 17.5wt%; Cr#<0.6). The high Cr-chromite plots fall in the boninitic field but the high-Al ones are mostly closer to abyssal peridotites and mid-oceanic ridge fields. The Cr# of chromite in dunite and chromitite varies between 0.77-0.83 and 0.86-0.89 respectively and show a boninitic nature for the parental melts. These characteristics prove that they were generated in an arc setting. High-Al harzburgites seem to have originated or been modified by boninitic magmatism which is taken place in the mid ocean ridge basalt environment due to melt-rock or melt-melt interactions. Also, the studied chromites occupy the field of supra-subduction zone (SSZ) peridotites with a tendency towards a MORB-type setting can be inferred. Hence, hydrous fluids which released from a subducted plate might have played an important role in Nain ophiolite petrogenetic evolution. MORB-like melts sourced from an arc environment and melting associated with a supra-subduction zone are primary factors for chromite occurrences in this area. © 2022 Geological Survey of India. All rights reserved.