Porphyroblasts and Reaction Rims

Abstract

Information contained in the structure in and around porphyroblasts and in reaction rims is discussed in this chapter. Porphyroblasts often contain inclusion patterns that reflect the structure of the matrix during their growth. Once formed, a rigid porphyroblast protects these patterns from further modification, whereas the matrix may be subjected to later crenulation, recrystallisation, grain growth etc. For this reason comparison of porphyroblasts and their surrounding matrix gives valuable information on the sequence of deformation phases and the relative time interval of porphyroblast growth with respect to these phases. A distinction is made between pretectonic, intertectonic, syntectonic and post-tectonic porphyroblasts. Using this technique a relative sequence of metamorphic events and deformation phases can be established.