Although the nature of the mechanical interactions between growing porphyroblasts and their surrounding matrix is poorly understood, it is widely believed that mechanical displacement of matrix grains due to porphyroblast growth is unlikely or impossible. The presence of an increased concentration of inert (non-reacting) grains in the matrix immediately surrounding a porphyroblast suggests the possibility of this concentration being due to mechanical expulsion of grains from the volume now occupied by the porphyroblast. This model implies a strict relationship between the volume of the prophyroblast, the volume of the zone of increased concentration adjacent to it, and the volume fraction of the inert material in both.
A recently published study of relict zircons in feldspar-porphyroblastic gneisses from Colorado demonstrates zircon enrichment (relative to normal matrix) in “shells” immediately surrounding the porphyroblasts, and zircon depletion (relative to normal matrix) within the porphyroblasts. Volume fractions of zircon in these different sub-volumes, however, do not satisfy the volume balance constraint implied by the proposed mechanical displacement model.
In hornfelsed metagraywackes from the Black Hills, South Dakota, distinctive accumulations of muscovite and graphite are found adjacent to the faces of euhedral garnet porphyroblasts. Detailed quantitative petrography shows that these accumulations do satisfy the volume balance equation implied by a mechanical displacement model; this provides strong (if not conclusive) evidence that growing porphyroblasts can physically displace matrix grains and, in so doing, produce distinctive matrix microstructures.