RIGID INEQUANT INCLUSIONS, PRESS SHADOWS, GHOSH
Rigid porphyroblasts or porphyroclasts occurring within ductile shear zone tend to rotate when the bulk deformation is non-coaxial. The foliation pattern, found in mylonites or sheared rocks and enable us to determine the sense of shear in the sheared zone. The surfaces or bands that are distorted by the rigid objects may be foliation surfaces, a mylonitic colour banding or tails of porphyroclasts. The tail of a porphyroclast is composed of fine recrystallized materials derived by dynamic recrystallization from the periphery of the porphyroclast. With increasing deformation, the tails stretch out into the matrix to a greater distance and the major part of a long tail becomes essentially parallel to the foliation in the matrix. Although the tails of porphyroclasts may develop in different compositional types of mylonites, they are commonly observed in feldspar porphyroclasts of granite mylonites. To determine the sense of shear, the observation should be made on sections that are parallel to the mylonitic lineation and perpendicular to the dominant foliation.