Porphyroblastesis and displacement: some new textural criteria from pelitic hornfels

The formation of porphyroblasts by replacement and displacement of pre-existing S surfaces is examined in the light of textural evidence displayed by sediments hornfelsed by the Bushveld Igneous Complex. Helicitic biotite, staurolite, and cordierite all show replacement and displacement textures, the former being more common. The evidence indicates that porphyroblasts arc capable of creating local pressure differences that disturb the pre-existing banding, even in hornfelsed rocks that have suffered neither regional metamorphism nor orogenic movement.