Mineral chemical composition and geodynamic significance of peridotites from Nain ophiolite, central Iran

摘要

The peridotites from north of the town of Nain in central Iran consist of clinopyroxene-bearing harzburgite and lherzolite with small lenses of dunite and chromitite pods. The lherzolite contains aluminous spinel with a Cr number (Cr~# = Cr/[Cr+Al]) of 0.17. The Cr number of spinels in harzburgite and chromitite is 0.38-0.42 and 0.62, respectively. This shows that the lherzolite and harzburgite resulted from <18% of partial melting of the source materials. The estimated temperature is 1100±200℃ for peridotites, the estimated pressure is <15 ± 2.3 kbar for harzburgites and >16 ± 2.3 kbar for lherzolites and estimated fo_2 is 10~(-1±0.5) for peridotites. Discriminant geochemical diagrams based on mineral chemistry of harzburgites indicate a supra-subduction zone (SSZ) to mid-oceanic ridge (MOR) setting for these rocks. On the basis of their Cr~#, the harzburgite and lherzolite spinels are analogous to those from abyssal peridotites and oceanic ophiolites, whereas the chromites in the chromitite (on the basis of Cr~# and boninitic nature of parental melts) resemble those from SSZ ophiolitic sequences. Therefore, the Nain ophiolite complex most likely originated in an oceanic crust related to supra-subduction zone, i.e. back arc basin. Field observations and mineral chemistry of the Nain peridotites, indicating the suture between the central Iran micro-continent (CIM) block and the Sanandaj-Sirjan zone, show that these peridotites mark the site of the Nain-Baft seaway, which opened with a slow rate of ocean-floor spreading behind the Mesozoic arc of the Sanandaj-Sirjan zone as a result of change of Neo Tethyan subduction regime during middle Cretaceous.