7.2 - Metamorphic textures

Question 1

What is the difference between a texture and a structure in metamorphic rocks?

Answer 1

Texture: small-scale, penetrative features visible in hand specimen or thin section

Structure: larger-scale features visible beyond hand-specimen scale

Question 2

What are relict textures and how are they named?

Answer 2

Textures inherited from the protolith; prefixed with “blasto-” (e.g. blasto-amygdaloidal, blasto-tuff).

Question 3

What is a pseudomorph?

Answer 3

A mineral or texture that preserves the external form of a pre-existing mineral or structure but has a new mineral composition.

Question 4

What are the four stages of new mineral formation during metamorphism?

Answer 4

Detachment of ions from reacting mineral surfaces

Diffusion of material to growth sites

Nucleation of new minerals

Growth of new minerals

Question 5

What is cataclastic flow?

Answer 5

Brittle deformation involving fracturing into clasts that slide and rotate past one another; technically not metamorphism.

Question 6

What is pressure solution?

Answer 6

Dissolution of minerals at high-stress points and precipitation at low-stress areas, leading to shortening and serrated grain boundaries.

Question 7

What textures indicate pressure solution?

Answer 7

Serrated or sutured grain boundaries

Stylolitic seams

Precipitation along original clast surfaces

Question 8

What is plastic intracrystalline deformation?

Answer 8

Internal crystal deformation without fracturing due to lattice defects under stress.

Question 9

Name mechanisms of plastic intracrystalline deformation.

Answer 9

Defect migration, slip planes, dislocation glide, deformation twinning.

Question 10

What are deformation twins and where do they occur?

Answer 10

Wedge-shaped twin lamellae that accommodate limited strain; common in calcite, plagioclase, quartz, dolomite, kyanite, biotite, diopside, and jadeite.

Question 11

What is recovery in metamorphic rocks?

Answer 11

Reduction of stored strain energy by migration and reorganisation of dislocations.

Question 12

What is undulose extinction and what does it indicate?

Answer 12

Wavy extinction under crossed polars caused by uneven dislocation density; indicates crystal strain.

Question 13

What is recrystallization and why does it occur?

Answer 13

Formation of new strain-free grains to reduce lattice strain energy.

Question 14

What is grain boundary migration?

Answer 14

Growth of low-strain grains at the expense of high-strain grains, producing bulging grain boundaries.

Question 15

What controls dihedral angles between minerals?

Answer 15

Grain boundary energies between different mineral pairs.

Question 16

Why are A–A grain boundaries less stable than A–B boundaries?

Answer 16

A–A boundaries have higher energy, so systems evolve to increase A–B contacts and reduce total energy.

Question 17

What does a dihedral angle of ~120° indicate?

Answer 17

Textural equilibrium under static conditions.

Question 18

What is foliation?

Answer 18

A planar fabric formed by the alignment, shape, or compositional variation of minerals during deformation.

Question 19

List the main types of foliation.

Answer 19

Compositional layering

Preferred mineral orientation

Grain shape alignment

Grain size variation

Platy mineral alignment

Lenticular mineral aggregates

Fracture alignment

Combinations of the above

Question 20

What mechanisms produce foliation?

Answer 20

Mechanical rotation

Preferred growth normal to compression

Pressure solution

Constrained growth

Mimetic growth

Simple shear and pure shear

Question 21

What is crenulation cleavage?

Answer 21

A secondary foliation formed by microfolding of an earlier foliation.

Question 22

How do symmetric and asymmetric crenulation cleavages differ?

Answer 22

Symmetric: pure shear / flattening

Asymmetric: simple shear

Question 23

What is a lineation?

Answer 23

A linear fabric element formed by elongate minerals, aggregates, or fold axes.

Question 24

List common types of lineations.

Answer 24

Elongate mineral aggregates

Elongate minerals

Platy mineral lineation

Fold axes

Intersection lineations

Question 25

Why do pelitic rocks develop strong foliation?

Answer 25

High Al content leads to abundant sheet silicates (micas) that align easily during deformation.

Question 26

Why do mafic rocks develop weaker foliation?

Answer 26

Dominance of amphiboles and pyroxenes, which are prismatic rather than sheet-like.

Question 27

What are pre-kinematic crystals?

Answer 27

Crystals formed before deformation, commonly showing bending, undulose extinction, or deformation twins.

Question 28

What are syn-kinematic crystals?

Answer 28

Crystals that grow during deformation and show foliation wrapping or spiral inclusion trails.

Question 29

What are post-kinematic crystals?

Answer 29

Crystals that grow after deformation and show random orientations or overgrow earlier fabrics.

Question 30

What is a snowball garnet?

Answer 30

A garnet porphyroblast with spiral inclusion trails indicating syn-kinematic growth.

Question 31

What are pressure shadows (or fringes)?

Answer 31

Low-stress regions adjacent to rigid clasts where minerals precipitate during deformation.

Question 32

What are mantled porphyroclasts?

Answer 32

Rigid clasts surrounded by dynamically recrystallised rims used as shear sense indicators.

Question 33

What is an σ-type porphyroclast? (Sigma type)

Answer 33

A clast with asymmetric tails parallel to foliation; indicates shear direction when recrystallisation exceeds rotation.

Question 34

What is a Φ-type porphyroclast? (Phi type)

Answer 34

Porphyroclasts in high-grade rocks that do not preserve shear sense due to extensive recrystallisation.

Question 35

Why must metamorphic textures be studied at multiple scales?

Answer 35

Because many textures are three-dimensional and may not be fully visible in thin section alone.

Question 36

What do metamorphic textures reveal?

Answer 36

Deformation history, timing of mineral growth, P–T evolution, and protolith characteristics.

Question 37

What defines syntectonic metamorphism?

Answer 37

Metamorphism occurring simultaneously with deformation.

Question 38

Early regional metamorphism texture in volcanic lithic arenite?

Answer 38

Isotropic matrix; plagioclase & K-feldspar altered to sericite and Ca-Al silicates.

Question 39

What causes foliation development in progressive regional metamorphism?

Answer 39

Pervasive shear rather than static recrystallisation.

Question 40

Key textural changes with increasing regional metamorphic grade?

Answer 40

Grain size reduction, rounded porphyroclasts, recrystallised matrix, new minerals.

Question 41

What happens to plagioclase at higher temperatures?

Answer 41

Albite accepts more Ca (no longer pure albite).

Question 42

What texture indicates a well-developed schist?

Answer 42

Coarser grains with strong mineral segregation into layers.

Question 43

What happens to plagioclase during early thermal metamorphism?

Answer 43

Becomes albitic, losing Ca and Al (Ca → calcite).

Question 44

How does augite change during contact metamorphism?

Answer 44

Hydrates to chlorite and actinolite.

Question 45

Textural changes closer to an intrusion?

Answer 45

Fully recrystallised, non-foliated, clearly metamorphic textures.

Question 46

Why does epidote form during contact metamorphism?

Answer 46

Increased Ca availability from plagioclase breakdown.

Question 47

Why are pelites very sensitive to contact metamorphism?

Answer 47

Pelite mineralogy is highly sensitive to pressure and temperature.

Question 48

Typical contact metamorphism minerals in pelites?

Answer 48

Andalusite, cordierite (low-P minerals).

Question 49

What are poikiloblasts?

Answer 49

Large crystals containing many small inclusions of earlier minerals.

Question 50

What is the crystalloblastic series?

Answer 50

Ranking of metamorphic minerals by tendency to form euhedral shapes.

Question 51

Most euhedral metamorphic minerals?

Answer 51

Titanite, rutile, pyrite, spinel.

Question 52

Intermediate euhedral minerals?

Answer 52

Garnet, sillimanite, staurolite, tourmaline.

Question 53

What controls crystal shape development?

Answer 53

Growth kinetics, space availability, and mineral chemistry.

Question 54

What is a depletion halo?

Answer 54

Zone depleted in certain minerals around a growing porphyroblast.

Question 55

Why do depletion haloes form around garnet?

Answer 55

Garnet consumes Fe and Mg during growth.

Question 56

Example of depletion halo mineral loss?

Answer 56

Hornblende consumed more than plagioclase in amphibolite.

Question 57

What is a reaction rim (corona)?

Answer 57

Concentric mineral zones formed by incomplete metamorphic reactions.

Question 58

Why do coronas form?

Answer 58

Limited diffusion during changing P–T conditions.

Question 59

Example of a corona sequence?

Answer 59

Orthopyroxene core → clinopyroxene → garnet rim.

Question 60

Where are coronas commonly observed?

Answer 60

High-grade metamorphic rocks and outcrops (e.g. Austrheim, Norway).

Question 61

What characterises high-strain zones?

Answer 61

Deformation concentrated into narrow zones.

Question 62

Shallow faulting deformation style?

Answer 62

Cataclastic (brittle).

Question 63

Cataclastic deformation processes?

Answer 63

Breaking, cracking, crushing, rotation, grain-size reduction.

Question 64

What is a porphyroclast?

Answer 64

Large remnant grain within a fine-grained deformed matrix.

Question 65

What is mortar texture?

Answer 65

Strong grains surrounded by crushed fine matrix.

Question 66

What are mineral ribbons?

Answer 66

Elongated quartz or feldspar aggregates formed during ductile flow.

Question 67

What is pseudotachylite?

Answer 67

Glassy rock formed by frictional melting during seismic slip.

Question 68

What is fault breccia?

Answer 68

Angular fragments formed by brittle faulting.

Question 69

How does fault breccia form?

Answer 69

Dilation and shear during fault movement.

Question 70

What is fold breccia?

Answer 70

Brittle fragmentation during folding; “blocky” appearance.

Question 71

How can fold breccia be recognised?

Answer 71

Fragmented beds with leucosomes still aligned.

Question 72

What is mylonite?

Answer 72

Fine-grained ductile shear zone rock formed by tectonic grinding.

Question 73

What deformation style produces mylonites?

Answer 73

Ductile deformation at deeper crustal levels.

Question 74

Progressive deformation of granite leads to?

Answer 74

Granite → protomylonite → mylonite → ultramylonite.

Question 75

What is polymetamorphism?

Answer 75

Rock records multiple metamorphic events.

Question 76

Principle of inclusions?

Answer 76

Included material is older than host mineral.

Question 77

Examples of inclusions?

Answer 77

Garnet in diamond, fluid inclusions in quartz, insects in amber.

Question 78

What is paragenesis and why is it important?

Answer 78

Sequence of mineral formation through time, Reconstructs mineral growth and metamorphic history.