Dynamic deformation micro-structures in the minerals from augen gneiss of Tutak region, Sanandaj-Sirjan Zone, Iran

Kuh-Sefid Tutak complex is a part of the southern Sanandaj-Sirjan metamorphic zone with the Northwest-Southeast trend and consists of sequences that have been undergone greenschist to lower amphibolite facies metamorphism. Augen gneisses, as one of the most important of metamorphic units in the complex, are characterized by mineralogy composition of quartz, plagioclase, alkali-feldspar, allanite, epidote, biotite, muscovite, zircon, apatite, zoisite, and clinozoisite. The different behavior of alkali feldspar (more brittle) and plagioclase (more ductile) during deformation and metamorphism in these gneisses apparently to be a principal cause for the formation of the augen texture. Evidence of the deformation such as deformation twinning, undulose extinction, deformation lamellae, folding and microstructures like mica fish, bookshelf, flanking, Window indicate dynamic recrystallization under ductile deformation. Moreover, the presence of kinking and grain migration microstructures show the condition of ductile-brittle deformation. Field and petrographic evidence such as symmetric and asymmetric porphyroclasts, foliation, lineation, strong pleochroism in biotite, sagenitic biotite, no zoning in plagioclase, grain boundary migration indicate the effect of medium metamorphism in greenschist to lower amphibolite facies and moreover, perthitic texture, accessory minerals such as apatite and zircon as trapped in biotite, as well as presence of allanite, are good indicators for the gneisses to display their igneous origin (Augen gneisses) along with the effect of dynamic deformation in the region.