Introduction to a virtual special issue on metamorphic microstructures

Abstract

The papers in this virtual special issue have been published in the JMG and were selected for having the common theme of analysing and utilizing metamorphic microstructures to interpret metamorphic crystallization processes and conditions, the role of deformation in metamorphic reactions, and/or the relationship of metamorphism to tectonic or other driving forces of metamorphism. This selection is by no means comprehensive (e.g. there have been more than 200 papers published on the topic of corona structures in the JMG in the past 25 years), but is intended as a representative survey of a range of microstructure-focused papers to give a broad overview of this topic. Mechanisms, rates, and driving forces of metamorphic processes can be evaluated by studies of the chemical and physical characteristics of metamorphic rocks and minerals. The term microstructure is used to describe the spatial arrangement, relative size and internal features of grains. Thus, the microstructure of a metamorphic rock includes the size, shape and distribution of grains, and the orientation of the crystal lattices of the constituent minerals. These physical features may provide information about reaction history (including reaction rate), the effect of deformation on reactions and crystal growth, and the pressure-temperature path of metamorphism. The size of metamorphic minerals is of interest both in an absolute sense (e.g. grain size) and in a relative sense (e.g. whether one mineral has a significantly different size compared to others in a rock), because crystal size may be related to crystallization rates and mechanisms and energetic interactions among minerals. Porphyroblasts– metamorphic minerals that are larger than the average grain size of the matrix (groundmass) minerals – have been much discussed in the literature owing to their potential for preserving different stages of the metamorphic and deformation history. Porphyroblasts may contain mineral inclusions that are randomly oriented, selectively confined to a region (e.g. core v. rim), or aligned in trails whose orientation can be evaluated in the context of matrix fabrics. Porphyroblasts may also be chemically zoned, providing additional information about metamorphic conditions and reactions during and after crystallization. The shape of metamorphic minerals contains information about (1) growth and/or dissolution history; (2) surface energy relationships among coexisting minerals; (3) reaction rate; and (4) the relationship of deformation and metamorphism. For example, whether a mineral has well-developed crystal faces or is highly resorbed, and whether a mineral’s shape has been affected by deformation are important indicators of physical and/or chemical processes during and after crystal growth. The distribution of metamorphic minerals may be an important indicator of reaction history and mechanism. For example, the development of chemical domains may affect subsequent metamorphic evolution; and the distribution of porphyroblasts in a rock may be an indicator of the crystallization mechanisms (e.g. diffusion-controlled v. interface-controlled growth). Metamorphic mineral distribution must be considered in the context of features inherited from the protolith or developed during earlier stages of metamorphism. The crystallographic (lattice) orientation of minerals, including development of a lattice-preferred orientation, is of interest for studies of deformation mechanisms and regional tectonic history, as well as for investigations of physical controls on metamorphic crystallization. For example, investigations of crystallographic orientation can provide information about the significance of epitaxy in metamorphic crystallization. Papers are listed in chronological order within different subsections: Metamorphic microstructures: methods and examples (http://www.wiley.com/bw/vi.asp?ref=0263-4929&site=1#251) Porphyroblasts (http://www.wiley.com/bw/vi.asp?ref=0263-4929&site=1#253) Deformation and metamorphism (http://www.wiley.com/bw/vi.asp?ref=0263-4929&site=1#254) Coronas and symplectites (http://www.wiley.com/bw/vi.asp?ref=0263-4929&site=1#255) Miscellaneous textures: Pseudomorphs, inclusions and lamellae (http://www.wiley.com/bw/vi.asp?ref=0263-4929&site=1#256)