Rotation of biotite porphyroblasts in pelitic schist from the Nukata area, central Japan

Abstract

The detailed microstructures of biotite porphyroblasts in pelitic schist from the Nukata area, central Japan, indicate that the porphyroblasts rotated in an external reference frame during non-coaxial deformation. The amount of rotation and rotation axis are controlled by the (001) orientation of the biotite porphyroblast with respect to the schistosity and the lineation. Concurrently with rotation, the biotite porphyroblasts suffered shape change by solution and deposition in the shortening and dilating fields around the porphyroblast, respectively. The close co-occurrence of inclusion-free domains (newly deposited biotite domains) and pressure shadows along the (001) faces suggests that continuously opening fractures along the (001) faces caused biotite deposition as well as pressure shadow aggregates. The role of pressure shadow formation for porphyroblast rotation is discussed in terms of stresses applied on the porphyroblast surface. It is concluded that although the formation of ‘deformation-partitioning-induced pressure shadows’ tends to suppress porphyroblast rotation, that of ‘fracture-induced pressure shadows’ (‘pressure fringes’) promotes porphyroblast rotation. The present biotite porphyroblasts would have rotated at rates depending not only on their shape and shape orientation, but also on the (001) orientation.