Calcite pseudomorphs after aragonite: A tool to unravel the structural history of high-pressure marbles (Evia Island, Greece)

Calcite pseudomorphs after aragonite were recorded throughout the high-pressure marbles of the Basal Unit (Evia Island, Greece). The pseudomorphs form a columnar microstructure oriented normal to the foliation, which developed at peak metamorphic conditions within the stability field of aragonite. Modification of the columnar microstructure by bending of the long axis of the calcite pseudomorph and dynamic recrystallization, was mainly dependent on the intensity of subsequent, exhumation-related deformation. We used the degree of preservation or modification of the columnar microstructure observed in shear zones, to distinguish between the subduction- and exhumation-related nature of the studied shear zones. The degree of modification of the columnar microstructure was also used as a tool to investigate variations in strain intensity, recrystallization mechanism and kinematics during the exhumation of the high-pressure marbles. Combined field and microstructural data suggest that subduction-related ductile deformation preceded the growth of the columnar microstructure and was associated with internal imbrication by SE-directed thrusting. The ductile stage of the exhumation was characterized by strain localization at the middle and upper structural levels of the Basal Unit, and E-directed thrusting (i.e., northeast in pre-Neogene configuration). Final exhumation of the Basal Unit was achieved by top-to-the-E ductile to brittle thrusting, which was accompanied by syn-orogenic extension at the upper crustal levels.