Porphyroblast-matrix microstructural relationships: recent approaches and problems

Careful observation and interpretation of suitably oriented sections indicates that many porphyroblasts with curved inclusion trails grow during the development of a crenulation foliation. Inclusion trails that are approximately parallel in separate porphyroblasts, or that outline axial surfaces of crenulations that are parallel in separate porphyroblasts, suggest that many porphyroblasts do not rotate, with respect to geographic coordinates, during later deformation. Moreover, where such rotation has not occurred, inclusion trails in porphyroblasts may indicate former orientations of S-surfaces that have been obliterated from the matrix. In ideal situations, porphyroblast-matrix microstructural relationships can be used, with more confidence than before, to provide useful information on metamorphic/deformation histories and on the details of metamorphic reactions. Porphyroblast-matrix relationships are also useful in contact metamorphic situations and can provide evidence of deformation accompanying contact metamorphism. Though mostly applied to medium-grade metamorphic rocks, porphyroblast-matrix relationships can also be useful in some high-grade situations. A model suggested by Bell & Rubenach and Bell et al. postulates that porphyroblasts grow in zones of relatively weak coaxial deformation between anastomosing zones of strong non-coaxial deformation during crenulation folding. Many observations appear to be consistent with this model, but it is difficult to verify, and much detailed observation and testing of critical aspects of the model are needed. Regardless of whether or not the model is accepted, curved inclusion trails may be used to relate growth of porphyroblasts to generations and stages of crenulation development.