Grain size reduction, recrystallization
Crenulations and metamorphic differentiation
Foliation development and overgrowths
Bent and broken grains
Contact and burial metamorphism
Relict igneous textures
This is a somewhat haphazard collection of interesting (to me) metamorphic textures seen in thin section, with some discussion of the processes thought to have made them.
Grain sizes in metamorphic rocks are, in part, the result of a competition between deformation, which tends to reduce grain size, and annealing, during which less stable, small and deformed grains grow to form larger, more crystallographically perfect grains. This part shows transformation of sandstone into quartzite, grain size reduction resulting from deformation, and grain growth during annealing.
Foliations are among the most prominent features of deformed metamorphic rocks. They form by the progressive re-orientation of platy grains into parallelism. Crenulations are small folds, typically axial planar to larger-scale folds, that deform earlier foliations. Under some conditions, as crenulations develop, some minerals like quartz tend to segregate into the fold hinges, leaving less soluble minerals in the fold limbs. This process can result in a fine-scale compositional layering (sometimes called pinstriping) in a completely different orientation than the original foliation.
Foliations develop and change as a result of rock deformation. Metamorphic minerals grow as a result of the progress of chemical reactions at different P-T conditions. In some cases, porphyroblasts overgrow the foliation present at the time of growth, preserving it. Such foliation relics can be helpful in the reconstruction of deformation history.
Pressure shadows form around relatively rigid grains, as the local rock is extended around them. As the matrix rock pulls away from the rigid crystal in the extension direction, fluids precipitate minerals in the potential void spaces. Pressure shadows can be symmetrical, characteristic of flattening, or asymmetrical, characteristic of shear or changing extension direction.
Deforming rocks can affect porphyroblasts and other crystals much like logs moving along in a stream, reorienting them without any significant deformation of the minerals themselves. On the other hand, weaker minerals, or even strong minerals under the right conditions, can bend and break under the right circumstances. Sheet silicates, for example, are commonly seen bent around porphyroblasts and crenulation hinges. Here are some other examples.
Burial metamorphism implies relatively high temperatures, without deformation or intense heating from obvious nearby heat sources. The concept is that simple burial gradually allows rock temperature to rise, and metamorphic reactions to progress. In contrast, contact metamorphism has an obvious heat source, with the metamorphic effects becoming more severe toward it
Igneous textures in metamorphic rocks are relatively common, only requiring that deformation has been limited enough to allow them to survive. These are a few examples of igneous feature that survived regional-scale deformation.
Fault rocks are a special variety of metamorphic rock that is often overlooked. Fault range from brittle to ductile, depending on mineralogy, pressure, temperature, and strain rate. Brittle faulting, foliation development, and grain size reduction tend to make faults weaker than surrounding rocks, a process known as strain-softening. That tends to restrict faults to narrow zones. Other processes, such as deformation speeding up reaction rates, can make deforming rock stronger, especially if strong anhydrous minerals grow at the expense of sheet silicates. That results in strain-hardening, which can allow deformation zones to widen as deformation proceeds, possibly to encompass major parts of orogenic belts. Faults can also reset the mineralogy to that stable at the time of deformation, allowing conditions, and possibly even timing, of faulting to be determined.