Characterization of thermal and stress perturbations during emplacement of granitic plutons in NW Iran

The plutonic units in the northwest of Sanandaj-Sirjan Zone emplaced under different thermal and stress regimes. Primary biotite was changed by hydrothermal fluids into illite, muscovite, and sericite, and phyllic and argillic alteration spread across the plutonic units. The intrusion margins exhibit propylitic alteration. The emplacement temperature of plutons evaluated by calcite and quartz microstructures reveals that these units emplaced at temperatures ranging from 170 to over 250 °C. Fluid inclusion analyses of syn-tectonic calcite veins indicated homogenization temperature ranging between 200 and 250 °C. Calcite twinning paleo-piezometry suggested 70 MPa mean differential stress during cooling of the A-type granites; this value increased in the shear zones at the study area to 120 MPa. In the I-type granite, differential stress was about 96 MPa. These data point toward differential stress in the crust generally increased with increasing plutons emplacement depth and temperature. In the I-type granite, the differential stress amount is bigger than in the A-type granites. The highest stress axes on the borders of the plutonic units are generally oriented at N30°W and N50°E, according to the paleo-stress study. The rotation of the maximum stress axes around the plutonic units imply the crustal stress field locally perturbed by the magma ascension.