Porphyroblast microstructures: A review of current and future trends

This paper reviews some current applications of porphyroblast microstructures, some difficulties with interpreting them, and some possible directions for future studies. Porphyroblast microstructures have attracted the attention and imagination of geologists for more than a century, and have enjoyed a resurgence in popularity over the last two decades. Although commonly complex, they are of broad interest as a tool for evaluating deformation and metamorphic processes, and for unravelling histories of deformed metamorphic rocks. Apart from their research value, porphyroblasts are useful teaching devices because they are relatively easy to observe and describe. The question of how most porphyroblast microstructures develop is linked to fundamental questions and controversies concerning structural and metamorphic processes. Thus, porphyroblasts can be used as learning models for processes, at various scales, that may be difficult to visualize. The issue of how to interpret these microstructures has strongly polarized structural geologists over the last two decades, and an example of how controversial the topic has become is illustrated by the full-length discussion and reply papers of Passchier et al. (1992) and Bell et al. (1992). Regardless of the controversy, renewed popularity has led to the refinement of some classical techniques of microstructural analysis, and the development of several new ones. ABSTRACT