Study of Chemical and Structural Changes and Determination of Temperature in a Progressive Deformation: Evidences from Zarrin Shear Zone, Ardekan

The Zarrin area in the north of Ardekan is the part of Central Iran Zone. Some homogenous parts of Zarrin granite located in the shear zone changed to mylonites and ultramylonites. Microstructural evidence and deformation investigation show that chemical and structural changes occurred during the progressive deformation in mylonites. The most obvious chemical change is the noticeable decrease on Ti، Mg، Fe and P from mylonites to ultramylonites. Silica shows a slight increase in the mylonitic zone with progressive increase in modal quartz، but Al2O3 is nearly constant during the mylonitization. Because of decrease on feldspar porphyroclasts and feldspar grains in the matrix، compare to protomylonites، K2O decreases in mylonites. However، with relative increase on plagioclase، K-feldspar and epidote in the matrix، Ca، Na and K increase in ultramylonites. As a result of structural changes، quartz grains in weakly deformed protolith and protomylonite show recrystallization and sub grain. In a progressive deformation process، quartz in mylonites and ultramylonites reveals grain boundary migration in recrystallization. During the progressive deformation، K-feldspars become perthitic with fractures and plagioclases show kinking in their twins. With progress in deformation، recrystallization on K-felspar''s margins and twins in plagioclase grains are formed. Based on structural evidence، temperature of 400ºC has been estimated for deformation in Zarrin area in protogranites and protomylonites. The temperature continuously increases up to 500ºC or more in green schist facies in the mylonites and ultramylonites.