Submagmatic to solid-state deformation microstructures recorded in cooling granitoids during exhumation of late-variscan crust in north-eastern sicily

Abstract

Late-Variscan granitoid rocks of trondhjemitic and granitic composition, intruded in migmatitic paragneisses in the north-eastern Peloritani Mountains (southern Italy) at ~310 Ma and ~300 Ma, respectively, exhibit a range of deformation microstructures developed under a shear regime at decreasing temperatures. Non-coaxial deformation is documented by sigmoidal feldspar porphyroclasts, mica fish, and asymmetric boudins affecting tiny andalusite crystals. Late-Variscan shearing during granitoid cooling is constrained by largely represented chessboard patterns in quartz and, especially, submagmatic fractures in plagioclase, indicating deformation at high-temperature conditions (T > 650◦C), in the presence of melt. Submagmatic deformation was extensively superseded by deformation at lower temperatures. Examples of solid state-high temperature deformation-related microstructures (T > 450◦C) include feldspar bulging, quartz grain boundary migration, and subgrain rotation recrystallization. Finally, low temperature subsolidus microstructures (T < 450◦C) consist of quartz bulging, mica kinks, and feldspar twinning and bending. A complete sequence of deformation, operating from submagmatic to low-temperature subsolidus conditions is recorded in both the older and younger granitoids, suggesting a duration of ~20 Ma for shear zone activity during post-collisional exhumation of the Variscan middle crust in southernmost Italy.