Structural Evolution of the Tutak Gneiss Dome, Southwest of Iran

Reconstruction of the structural evolution of the Tutak gneiss dome in the Sanandaj-Sirjan metamorphic belt contains a history of closure the Neo-Tethyan Ocean, forming the Zagros Orogenic Belt, continental growth through accretion, plutonic emplacement, and of the relative movement and collision between the Afro-Arabian and Iranian plates in transpressional regime. Granitoid emplacement in the Tutak mantled gneiss dome occurred in doubly plunging anticline within the bivergent wedges between two shear faults; the Surian antithetic fault at the north and Mazayjan synthetic fault at the south. These two faults join together and form the Dehbid shear zone. The Tutak gneiss dome consists of metamorphosed (Silurian and Devonian) strata, mostly granite-gneiss, marble, calc-silicate rocks, and schists. The foliation is related to the doming, dips radially outward from the core of the dome, but lithological contacts dip towards dome. Thin section studies reveal that the mylonitic foliation becomes progressively stronger towards the dome. In the study area, the mylonitic foliation and the stretching lineation become progressively stronger towards the centre of the shear zone. In the fabric diagram, the stretching lineation appears to have resulted in a stronger lattice preferred orientation of quartz c-axis in the vicinity of the thrust plane. There are different generations of folds at the study area. In addition, the fold axes (F1, F2 and F3) are gradually reoriented towards the stretching lineation, NW-SE direction, apparently due to the progressive shearing. Megascopic, mesoscopic, and microscopic kinematic indicators confirm dextral sense of shear during the ductile phase of deformation and agree with previous studies which suggested that the ductile shearing occurred during oblique transpressional plate convergence. The percent of pure shear and simple shear in the Tutak gneiss dome is %48 and %52 repectively. There are two 40Ar-39Ar age determinations of granites in the core of the Tutak gneiss dome; the earlier belongs to the central Iranian continental crust and has been deformed at 179.5 ± 1.7 Ma in an extensional regime and, the later has been emplaced in the NE-SW compression in transpressional system. The younger pluton was emplaced syn-kinematically with respect to the main dextral, transpressional events along the marginal boundaries. Such features are consists with metamorphic and plutonic rocks that are typically developed at amphibolite to granulite facies conditions during the latter stages of orogenesis. The retrograde metamorphism and magmatism in the Tutak gneiss dome occurred contemporaneously with two different ages and two conjugate deformations, which is related to different proportions of pure shear and simple shear components. The timing of strain-related fabrics and also the cooling and exhumation history of the region illustrate the continent-continent contact and absolute closure time of the Neo-Tethyan Ocean at 76.8 ± 0.6 Ma constrained by 40Ar-39Ar geochronology on the biotites of the second stage emplacement of the granite-gneiss in repeated of the orogenic events, demise the Neo-Tethyan Ocean versus alive the Zagros Orogenic Belt. At the end, the deformational succession was puncturated three episodes since beginning of the Arabia-Iran collision: (1) the first subduction of Neo-Tethyan Ocean and deformation of Tutak complex at 185.7 ± 1.5 Ma (2) deformation of amphibolite and hornfels are result of this subduction at ~ 94 Ma and (3) contact and deformation of Tutak gneiss dome at 76.8 ± 0.6- 72.4 ± 0.6 Ma since now.