Emplacement of serpentinites in the Chohar Gonbad-Gugher-Baft ophiolitic mélange, southeast Iran: examination of the mineral–chemical, petrologic, and structural features

Abstract

The Chohar Gonbad-Gugher-Baft ophiolite melange, located along the major Baft and Shahr-e-Babak fault zones, southeast Iran, represents remnants of Neo-Tethyan oceanic lithosphere. This melange contains blocks of harzburgite, dunite, lherzolite, basalt, and other ophiolite-related lithologies tectonically mixed with and embedded in a serpentinite matrix. Field, petrographic, and geochemical data show that peridotites in this melange belong to the upper mantle. They seem to have undergone up to ~20 % partial melting in a supra-subduction zone setting, based on their spinel Cr# values (0.21–0.53). Chemical compositions and textures in the serpentinites indicate that they were partially hydrated during emplacement and further mobilized diapirically to the surface. The different deformation stages occurred in an accretionary wedge environment. Petrographic evidence shows that the first serpentinization event produced mesh-textured serpentinites formed under static conditions in an ocean floor environment (Nain-Baft ocean crust), where the initial lizardite, bastite, and chrysotile veins formed. Plastic deformation occurred due to the subduction of Nain-Baft oceanic lithospheric beneath the central Iranian microcontinent, with antigorite-bearing flare-textured serpentinites produced. During progressive exhumation of the Nain-Baft ophiolite melange, the serpentinites were affected by ductile, ductile–brittle, and brittle deformation, respectively. Accretion and resultant diapirism are the most important processes in the emplacement of serpentinite, which is a consequence of hydration of the ocean crust. In this example, late-stage emplacement via thrusting occurred along the northern extent of the southern Sanandaj–Sirjan zone (S–SZ).