Abstract

The Khoy Ophiolitic Complex as a part of the Tethyan ophiolites is exposed in the northwestern part of the Iranian-Azerbaijan province, extending to the Anatolian ophiolites in southeastern Turkey. Petrography, geochemistry and microstructural studies of the residual mantle sequence in the Khoy Ophiolitic Complex provide important information about the degree of partial melting and deformation in the oceanic mantle lithosphere. Ultramafic tectonites dominantly composed of lherzolite and clinopyroxene-bearing harzburgite (TiO₂ = 0.012–0.024 wt.%; Al₂O₃ = 1.36–1.81 wt.%). Chondrite-normalized rare-earth-element patterns are characteristically U-shaped. These peridotites can be divided into two types: (1) type 1 peridotites with Al-rich spinels (Cr number of 0.16–0.26, and Mg number of 0.64–0.76), resembling the fertile abyssal peridotites, supposed to have originated as the residue from <15% partial melting and mid-ocean ridge (MORB) magma extraction; (2) type 2 peridotites, representing characteristics of the depleted abyssal or supra-subduction-zone peridotites, with Cr-rich spinels (Cr number of 0.31–0.60 and Mg number of 0.51–0.72). This type of peridotite has undergone >20% partial melting, followed by segregation of basaltic magmas. Microstructural fabrics of olivine grains in peridotites highlight a sequence of dislocation creep on the (0 1 0) [1 0 0] slip system, plus subsidiary slip along the (0 0 1) [1 0 0] slip system. These systems, as well as coarse and fine-grained porphyroclastic textures, indicate deformation at high temperatures of ∼1000–1250 °C. The observed subsidiary (0 0 1) [1 0 0] slip system is considered to have been triggered by elevated H₂O activity, and that deformation phases took place in a wet subduction-related environment. The geochemical and microstructural data suggest that the mantle sequence of the Khoy Ophiolitic Complex is consistent with a supra-subduction-zone environment in relation to a slow-spreading back-arc basin.