Mantle heterogeneity beneath the oceanic crust of forearc basin of Nain ophiolite based on the composition of pyroxenes in harzburgites

• Published in: Pitruluzhī (Online) Vol. 16; no. 2; pp. 45 - 72
• Main Authors: Elahe Safdari, Nargess Shirdashtzadeh
• Format: Journal Article
• Language: Persian
• Published: University of Isfahan 01.07.2025
• Subjects: central iran; forearc basin; harzburgite; mantle heterogeneity; nain ophiolite
• ISSN: 2228-5210; 2322-2182
• DOI: 10.22108/ijp.2025.145418.1364

IntroductionThe Nain ophiolite in Central Iran has been the subject of numerous petrological investigations, particularly focusing on its ultramafic rocks. The present study examines the crystal chemistry of pyroxenes in harzburgite massifs from the southern Separab area in order to compare with those of the central (near Sucheh village) and eastern parts of the Nain ophiolite (Darreh Deh region). Understanding mantle heterogeneity is crucial for interpreting the tectonomagmatic conditions of forearc oceanic lithosphere (Ishii et al., 2019a; Zheng, 2019a).This study examines the oceanic lithosphere remnants of Nain ophiolite in the northwest of Central East Iranian Microcontinent (CEIM), where mantle peridotites record suprasubduction zone (SSZ) processes (Mehdipour et al., 2010; Pirnia et al., 2013; Shirdashtzadeh et al., 2014). Previous work documented a lherzolite-harzburgite-dunite transition (Pirnia, 2007; Shirdashtzadeh, 2014) formed by melt-rock interactions in a suprasubduction setting (Pirnia et al., 2010, 2018). While peridotites from the central and southern sections have been studied (e.g., Mehdipour et al., 2010; Pirnia et al., 2013), the western exposures near Separab (or Separo) village remain poorly characterized. A preliminary study by Safdari and Shirdashtzadeh (2024) have shown compositional heterogeneity for the olivines in the various harzburgite massifs of the aforementioned ophiolite. Chemical analysis of constituent minerals in these rock units provides critical insights into the origin and the nature of the oceanic lithospheric mantle sequence. Among these, pyroxenes - particularly clinopyroxene - serve as important petrogenetic indicators due to their refractory nature and high chemical/physical resistance, especially against alteration processes. As demonstrated in numerous studies (e.g., Aldanmaz, 2012; Mohamed et al., 2013; Nishio et al., 2022; Ghorbani et al., 2024), their chemistry has been extensively used to determine the nature and the origin of host rocks. Therefore, this study conducts detailed petrographic and chemical analyses of clinopyroxene and orthopyroxene in harzburgite outcrops south of Separab village to investigate the nature and the evolution of the mantle beneath the Neotethyan oceanic lithosphere. The obtained results are then compared with previously studied harzburgite samples from other sections of this ophiolite by various researchers.Geological BackgroundThe Nain ophiolite, located in central Iran, is a part of the larger Central Iranian tectonic zone representing the remnants of the Neotethyan oceanic lithosphere. The ophiolitic sequence includes harzburgites, which are key to understanding mantle processes during subduction initiation. The studied harzburgites from Separab, Suché, and Darreh Deh exhibit variations in mineralogy and geochemistry, suggesting different tectonic settings. The Separab harzburgites are dominated by olivine, orthopyroxene, and minor clinopyroxene (<5 vol%) and spinel, whereas the Sucheh and Darreh Deh harzburgites show greater evidence of subduction-related metasomatism. These differences imply that the Separab harzburgites may represent an earlier stage of forearc mantle evolution, while the Sucheh and Darreh Deh harzburgites reflect a more mature SSZ environment.Analytical MethodsField sampling was conducted to collect fresh, least-altered samples. Thin sections were prepared using resin mounts (without coverslips) and examined under an Olympus BH-2 polarizing microscope at Tarbiat Modares University. Selected polished thin sections were analyzed for major elements compositions of clinopyroxene and orthopyroxene using a JEOL JXA8800R electron microprobe (accelerating voltage: 15 kV; beam current: 15 nA; spot size: 3 µm) at Kanazawa University, Japan. Structural formulas and end-member components of pyroxenes were calculated based on 6 oxygen atoms per formula unit using Excel spreadsheets. Mineral abbreviations follow Warr (2021).DiscussionThe geochemical differences between the Separab harzburgites and those from Sucheh and Darreh Deh suggest two possible models for their formation: Two-Stage Formation Model: The Separab harzburgites may be older, originating in a Jurassic mid-ocean ridge (MOR) setting, while the Sucheh and Darreh Deh harzburgites formed later in a Late Cretaceous SSZ-like forearc environment above the subducting Neotethyan slab. Given that the pyroxene chemistry in all the studied harzburgites resembles that of the harzburgites from subduction-related zones (though the subduction-related chemical indicators are less pronounced in the Separab area compared to other regions), this model may not be valid for the formation of these harzburgites. Therefore, the harzburgites in the southern Separab were likely not formed in an ancient mid-ocean rift environment during the Jurassic period. Single-Stage Formation with Progressive Subduction Influence: The Separab harzburgites formed in a young forearc rift (MOR-like FAB) during the initial stages of subduction, where the mantle wedge was less affected by slab-derived fluids. In contrast, the Sucheh and Darreh Deh harzburgites formed later in the same Late Cretaceous period but under greater subduction influence (SSZ-like FAB), as the mantle wedge became more metasomatized by slab-derived fluids. The chemical composition of pyroxenes (Figs. 5-10), lower oxygen fugacity, higher temperatures (1456–1487°C) recorded in Separab pyroxenes suggest lower mantle partial melting with minimal influence of subduction-related fluids, whereas the Sucheh and Darreh Deh pyroxenes reflect more hydrated conditions of mantle wedge (e.g., chemistry of pyroxenes (Figs. 5–10), higher oxygen fugacity, lower melting temperatures) that is typical characteristics of SSZ settings. Various compositional plots for pyroxenes (Figs. 5–10) clearly distinguish the initial MOR-like FAB nature of Separab harzburgites from the SSZ-like nature of Sucheh and Darreh Deh harzburgites, supporting the interpretation of a heterogeneous mantle wedge beneath the oceanic lithosphere in this forearc basin.ConclusionThe Nain ophiolite, situated north of Nain city in the Central Iranian zone, contains harzburgite units exposed in various sections, including the northwestern area near Separab village. This harzburgite consists mainly of olivine and orthopyroxene, with minor clinopyroxene (<5 vol%) and spinel. Geochemical analyses reveal that the Separab harzburgite pyroxenes (Cr#>0.3, Na₂O >0.1 wt%) formed at higher temperatures (1456–1487°C) and lower oxygen fugacity (higher AlVI/AlIV), indicating lower degrees of mantle wedge partial melting. These features suggest an initial forearc setting resembling mid-ocean ridge-like forearc basalt (MOR-like FAB). In contrast, harzburgites from the eastern (Darreh-Deh) and southern (Sucheh) sections developed under lower temperatures and higher oxygen fugacity, reflecting a mature forearc setting with subduction-related signatures (SSZ-like FAB). These compositional differences highlight temporal heterogeneities in the sub-arc mantle, driven by the advancing stages of slab subduction. The Nain ophiolite thus, records a transition from early, MOR-like forearc magmatism to later, SSZ-dominated conditions, documenting the evolution of the subduction system.