Porphyroclasts

Porphyroclasts are relatively large mineral grains embedded in a metamorphic rock that retain their preexisting outlines and do not recrystallize to the same extent as the surrounding matrix. They occur in rocks that have undergone dynamic, deformational metamorphism and are usually set in a finer-grained matrix formed by recrystallization during deformation. The term emphasizes the grain’s size contrast with the finer grains around it and its preserved, deformed shape.

Porphyroclasts are distinguished from porphyroblasts, which are minerals that grew during metamorphism and often display growth textures or reaction rims. By contrast, porphyroclasts are pre-existing grains that have been deformed, rotated, or elongated by shear and pressure during metamorphism, while the matrix recrystallizes around them. The edges of porphyroclasts may appear angular or irregular, and they can be surrounded by a corona or planed by strain shadows created by the deformation of the surrounding material.

In hand specimens and thin sections, porphyroclasts help reconstruct deformation histories. They commonly occur in rocks such as mylonites, phyllonites, and schists, and can be composed of quartz, feldspar, or other minerals depending on the rock type. The orientation and shape of porphyroclasts, along with the surrounding recrystallized grains, provide clues about differential stress, flow direction, and the sequence of metamorphic events during ductile deformation.