A Discussion on natural strain and geological structure - Crystal growth models in metamorphic tectonites

Abstract

Recent models of crystal growth in metamorphic rocks have explained the lack of homogeneous equilibrium frequently found in porphyroblasts. The simple segregation model allows us to clearly distinguish between porphyroblasts which are often zoned and are in equilibrium with the matrix at the rim only, and the matrix minerals which show homogeneous equilibrium and recrystallize continuously. The assumptions of the simple model are instructive in that they focus our attention on some important problems of growth of minerals during deformation. The model enables us to define diffusion distances during porphyroblast growth and to distinguish more local diffusion distances which are superimposed on this porphyroblast diffusion and which are often related to deformation, e.g. shear zones and segregations. It emphasizes the ionic character of the reactions taking place during metamorphism and makes sense of some partition data between two minerals. The growth of minerals during deformation is considered and an attempt is made to connect the simple interface equilibrium model with deformation as shown by textures and microstructures in rocks. It is clear that the growth of minerals during metamorphism cannot be properly understood without a simultaneous appreciation of the chemical and mechanical processes occurring during metamorphism.