Microstructure in mylonitic zones of South of Salmas (North-West of Iran)

Metamorphic rock assemblage of southern Salmas area is located in the northwestern terminal part of Sanandaj-Sirjan zone, and includes various rock types. This study uses field observations plus mineralogy and petrography of samples to describe and interpret the microstructures in the area. Different rock units are classified into three groups based on their parent rocks, and consist of 1) metabasite, 2) quartz-feldespathic, and 3) marble. Most of these rocks have mylonitic texture but the grade and the intensity of mylonitization are different. Presence of mylonitic foliation and lineation along with other microstructures such as various porphyroclasts, mica fish, S-C fabric, and S-C' fabric demonstrate different mylonitic zones in this area. Due to differences in strain rate, parent rock type, and depth of deformation, we could distinct three mylonitic zones in the metamorphic complex. Most of these mylonitic samples show features characteristic of low to medium grade mylonites, in which mylonitization grade increases west to east and center of the study area. Presence of ultramylonites in the central part of the area indicates increases in strain rate. Also presence of high-grade mylonites (T> 650 ºC) and migmatite imply that the deformation occurred at depths of middle to lower crust. We could discern two metamorphic phases (M1 and M2) and six deformation phases (D1, D2, D3, D4, D5, D6). Compressional deformation phase D1 occurred after Precambrian magmatism, then a metamorphic phase (M1) impressed these rocks by the Latest Cambrian. During Late Permian to Early Cretaceous, two deformation phases (D1 and D2) with a major simple shear component strongly affected the metamorphic complex, leading to the development of mylonitic zones. Synchronous with the deformation phase D2, a retrograde metamorphic phase (M2) affected the complex. At Late Cretaceous to Early Paleocene, deformation phase (D4) caused obduction of ophiolites over the metamorphic complex. Eventually, two brittle deformation phases (D5 and D6) affected all older rocks.