Petrogenesis of arc-related peridotite hosted chromitite deposits in Sikhoran-Soghan mantle section, South Iran: Evidence for proto-forearc spreading to boninitic stages

Abstract

The Soghan-Sikhoran ophiolite in southeast Iran (Outer Zagros Ophiolite Belt) is a remnant of a series of Upper Triassic–Cretaceous supra-subduction zone (SSZ) ophiolites that formed along the Zagros suture zone, which is considered as the southern border of the Neo-Tethyan sea. These SSZ ophiolites are older than the Late Cretaceous Zagros ophiolites. The main part of the Soghan-Sikhoran ophiolite comprises layered ultramafic cumulates including dunites, wehrlites, and pyroxenites, and a tectonized mantle section including residual lherzolites, and depleted harzburgites with foliated/discordant dunite lenses. Podiform chromitites are common and are typically surrounded by thin dunitic haloes. Spinels in lherzolite–harzburgite are geochemically characterized by a low Cr# (42.0 to 52.6), and plot in an abyssal peridotite field on geochemical discrimination diagrams, whereas spinel in dunites and high-Cr chromitite spinels (Cr# = 52.4 to 76.4) show geochemical affinities to boninites. Lherzolites and harzburgites have low REE contents and experienced >17 vol% partial melting. The Soghan-Sikhoran ophiolite contains both high Cr# and low Cr# podiform chromitite types. Trace and REE element patterns of Soghan-Sikhoran rocks are similar to those in SSZ peridotites. The studied ophiolites show relatively moderate to high oxygen fugacities (ƒO2), with log units ranging from −0.4 to +0.4 for harzburgites, +0.2 for lherzolites, from − 0.7 to + 2.5 for pyroxenites, from +0.8 to +2.2 for dunites, and from + 0.6 to + 0.8 for chromitites. The moderate to high oxidation state of the studied upper-mantle ophiolitic complexes also suggests a boninitic source in the mantle wedge of the arc setting. The two-pyroxene thermometer yields mean equilibrium temperatures of 879 °C, 895 °C, 912 °C and 912 °C for harzburgites, lherzolites, dunites and pyroxenites, respectively. We therefore interpret that the spinels in the lherzolite-harzburgite crystallized from tholeiitic melt generated due to proto-forearc spreading and formation of the infant arc, whereas high-Cr# spinel in dunites and high-Cr# chromitite crystallized from boninitic melts during the mature arc stage, with an increasing contribution of slab-derived fluids at high ƒO2.