7.2 Metamorphic textures and structures

Metamorphic Textures

Metamorphic textures and structures are the physical clues that tell you how a rock was metamorphosed. By looking at grain alignment, banding, folds, and crystal growth, geologists can reconstruct the temperature, pressure, and stress conditions a rock experienced deep below the surface. The two big categories of texture are foliated (minerals lined up in a preferred direction) and non-foliated (no preferred alignment).

Foliated vs. Non-Foliated Textures

Foliated textures show a parallel alignment of mineral grains. This alignment develops when platy or elongated minerals (like micas and amphiboles) grow or rotate perpendicular to directed stress. Foliation changes character as metamorphic grade increases:

• Slaty cleavage: Fine-grained alignment of microscopic platy minerals. This is the texture you see in slate, a low-grade metamorphic rock. The individual grains are too small to see without a microscope, but the rock splits into thin, flat sheets along the cleavage planes.
• Schistosity: Medium-grained alignment visible to the naked eye. Micas and amphiboles are large enough to give the rock a shiny, flaky appearance. This defines schist, a medium-grade rock.
• Gneissic banding: Coarse-grained alternating bands of light minerals (quartz, feldspar) and dark minerals (biotite, hornblende). This segregation into distinct layers is characteristic of gneiss, a high-grade metamorphic rock.

Non-foliated textures lack any preferred grain alignment. They typically form under uniform (hydrostatic) pressure, or from parent rocks that don't contain platy minerals:

• Granoblastic texture: Equidimensional, interlocking grains with no alignment. Quartzite (from sandstone) and marble (from limestone) are classic examples. Because quartz and calcite aren't platy minerals, they don't develop foliation even under directed stress.
• Hornfelsic texture: Very fine-grained, interlocking grains produced by contact metamorphism, where heat from a nearby magma body bakes the surrounding rock. Hornfels is the typical product.

How Metamorphic Foliations Form

Each type of foliation develops through a specific mechanism tied to metamorphic grade:

1. Slaty cleavage forms when clay minerals in a fine-grained parent rock (like shale) recrystallize into tiny micas. These new micas grow perpendicular to the direction of maximum compressive stress, creating planes of weakness the rock splits along.
2. Schistosity develops at higher temperatures and pressures, where micas and amphiboles grow large enough to see. Continued recrystallization under directed stress rotates and aligns these minerals into a strongly foliated fabric.
3. Gneissic banding forms under high-grade conditions where minerals begin to segregate by composition. Light-colored felsic minerals (quartz, feldspar) separate from dark-colored mafic minerals (biotite, hornblende), producing the distinctive banded appearance. At the highest grades, partial melting can begin, producing migmatites (rocks that are part metamorphic, part igneous).

Metamorphic Structures

Beyond texture, metamorphic rocks contain larger-scale structures that record the forces acting on them during deformation.

Folds, Boudins, and Porphyroblasts

Folds form when rock layers buckle under compressional stress rather than breaking. Two common types:

• Isoclinal folds: The limbs are nearly parallel to each other, indicating intense compression. These are tightly squeezed folds.
• Ptygmatic folds: Highly contorted, irregular folds that often look chaotic. These typically form when a competent (stiff) layer, like a quartz vein, is folded within a softer, more ductile matrix.

Boudins are sausage-shaped segments that form when a stiff, competent layer is stretched within a softer surrounding rock. The competent layer breaks into separated blocks while the softer rock flows around them. Boudins indicate extensional stress.

Porphyroblasts are large, well-formed crystals (commonly garnet, staurolite, or andalusite) that grow within a finer-grained matrix during metamorphism. They grow because local chemistry favors certain minerals at specific pressure-temperature conditions. Porphyroblasts often trap tiny inclusions of surrounding minerals as they grow, and the pattern of those inclusions can record the rock's deformation history, almost like a built-in timeline.

Conditions That Control Texture and Structure

Metamorphic textures and structures depend on several interacting factors:

• Temperature: Higher temperatures promote recrystallization and allow grains to grow larger.
• Pressure type: Directed (differential) stress produces foliated textures and structures like folds. Uniform (hydrostatic) pressure produces non-foliated textures.
• Chemically active fluids: Hot fluids circulating through rock speed up mineral reactions and help new minerals crystallize.

Metamorphic grade ties temperature and pressure together into a useful shorthand:

• Low-grade: Slaty cleavage, fine-grained textures (around 200–400°C)
• Medium-grade: Schistosity, coarser grains (around 400–600°C)
• High-grade: Gneissic banding, coarse grains, possible migmatization (above ~600°C)

Tectonic setting determines what kind of stress dominates:

• At convergent boundaries, compressional stress is dominant, producing foliated textures and fold structures.
• At divergent boundaries, extensional stress dominates, producing boudins. Contact metamorphism near magmatic intrusions in these settings tends to create non-foliated textures like hornfels.