Textures of Metamorphic rocks

(Chapter 23)

last update:11/14/05

Metamorphic Textures

The Processes of Deformation, Recovery, and Recrystallization

Cataclastic Flow

Mechanical fragmentation and sliding, rotation of fragments

Crush, break, bend, grind, kink, defm twins, undulose extinction, shredding of micas, augen, mortar, etc.

Pressure Solution

a. Highest strain in areas near grain contacts (hatch pattern).

b. High-strain areas dissolve and material precipitates in adjacent low-strain areas (shaded). The process is accompanied by vertical shortening.

c. Pressure solution of a quartz crystal in a deformed quartzite (s1 is vertical). Pressure solution results in a serrated solution surface in high-strain areas (small arrows) and precipitation in low-strain areas (large arrow). ~ 0.5 mm across. The faint line within the grain is a hematite stain along the original clast surface.

After Hibbard (1995) Petrography to Petrogenesis. Prentice Hall.

 

Plastic Intracrystalline Deformation (ductile deformation at higher T)

No loss of cohesion

Several processes may operate simultaneously

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Defect migration

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Slip planes

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Dislocation glide

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Deformation twinning

 

Recovery

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Loss of stored strain energy by vacancy migration, dislocation migration and annihilation

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Polygonization- general term for formation of low-strain subgrains

 

Recrystallization

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Grain boundary migration

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Subgrain rotation

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Solid-state diffusion creep at higher T

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Crystalplastic deformation (general term)

Grain boundary sliding and area reduction

Coalescence- recovery and recrystallization by which large grains form by the addition of smaller strained grains by grain boundary migration


Dislocation migration forms two strain-free subgrains
Illustration of a recovery process in which dislocations migrate to form a subgrain boundary.

Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

 Undulose extinction and (b) elongate subgrains in quartz due to dislocation formation and migration

Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

 

 

Recrystallization by grain boundary migration and sub-grain rotation

Recrystallization by (a) grain-boundary migration (including nucleation) and (b) subgrain rotation.

From Passchier and Trouw (1996) Microtectonics. Springer-Verlag. Berlin.

Recrystallized quartz with irregular (sutured) boundaries, formed by grain boundary migration. Width 0.2 mm.

From Borradaile et al. (1982).

 
Schematic cross section through a shear zone, showing the vertical distribution of fault-related rock types, ranging from non-cohesive gouge and breccia near the surface through progressively more cohesive and foliated rocks. Note that the width of the shear zone increases with depth as the shear is distributed over a larger area and becomes more ductile. Circles on the right represent microscopic views or textures.

From Passchier and Trouw (1996) Microtectonics. Springer-Verlag. Berlin.

 

High-Strain Metamorphic Textures

Concentrate on cataclastic > ductile (shallower)

Break, crack, bend, crush, rotate

Slip and shredding of phyllosilicates

Clasts- broken remnants

Porphyroclast- larger remnant in finer crush matrix

Mortar texture

Ribbons

Pseudotachylite

Progressive mylonitization of a granite.
 

horizontal rule

 

Textures of Contact Metamorphism

Typically shallow pluton aureoles (low-P)

Crystallization/recrystallization is near-static

Monomineralic with low D surface energy ® granoblastic polygonal

Larger D S.E. ® decussate

Isotropic textures (hornfels, granofels)

Relict textures are common


Lower greenschist facies


Upper greenschist facies


Amphibolite facies


Granulite facies

 

Progressive thermal metamorphism of slate.

From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.
Progressive thermal metamorphism of slate.

From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.
Progressive thermal metamorphism of slate.

From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

 

 

Typical textures of contact metamorphism.

From Spry (1969) Metamorphic Textures. Pergamon. Oxford.

 

Drawings of quartz-mica schists.

 a. Closer spacing of micas in the lower half causes quartz grains to passively elongate in order for quartz-quartz boundaries to meet mica (001) faces at 90o. From Shelley (1993).

b. Layered rock in which the growth of quartz has been retarded by grain boundary "pinning" by finer micas in the upper layer.

From Vernon, 1976) Metamorphic Processes: Reactions and Microstructure Development. Allen & Unwin, London.

Contact overprint on earlier regional events are common

bulletThermal maximum later than deformational
bulletSeparate post-orogenic (collapse) event

Spotted slates and phyllites

Overprint of contact metamorphism on regional. a. Nodular texture of cordierite porphyroblasts developed during a thermal overprinting of previous regional metamorphism (note the foliation in the opaques). Approx. 1.5 x 2 mm. From Bard (1986) Microtextures of Igneous and Metamorphic Rocks. Reidel. Dordrecht.

b. Spotted phyllite in which small porphyroblasts of cordierite develop in a preexisting phyllite. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

 

horizontal rule

 

Textures of Regional Metamorphism

Dynamothermal (crystallization under dynamic conditions)

Orogeny- long-term mountain-building

bulletMay comprise several Tectonic Events
bulletMay have several Deformational Phases
bulletMay have an accompanying Metamorphic Cycles with one or more Reaction Events

Tectonite- a deformed rock with a texture that records the deformation

Fabric- the complete spatial and geometric configuration of textural elements

bulletFoliation- planar textural element
bulletLineation- linear textural element
bulletLattice Preferred Orientation (LPO)
bulletDimensional Preferred Orientation (DPO)
Mantled porphyroclasts and "mica fish" as sense-of-shear indicators. After Passchier and Simpson (1986) Porphyroclast systems as kinematic indicators. J. Struct. Geol., 8, 831-843.

 


Progressive metamorphism of a graywacke

 

 

Symmetrical crenulation cleavages in amphibole-quartz-rich schist. Note concentration of quartz in hinge areas.

From Borradaile et al. (1982) Atlas of Deformational and Metamorphic Rock Fabrics. Springer-Verlag.
Asymmetric crenulation cleavages in mica-quartz-rich schist. Note horizontal compositional layering (relict bedding) and preferential dissolution of quartz from one limb of the folds.

From Borradaile et al. (1982) Atlas of Deformational and Metamorphic Rock Fabrics. Springer-Verlag.

 

Stages in the development of crenulation cleavage as a function of temperature and intensity of the second deformation.

From Passchier and Trouw (1996) Microtectonics. Springer-Verlag.

Development of S2 micas depends upon T and the intensity of the second deformation

 

Pre-kinematic crystals

a. Bent crystal with undulose extinction

b. Foliation wrapped around a porphyroblast

c. Pressure shadow or fringe

d. Kink bands or folds

e. Microboudinage

f. Deformation twins

 

Post-kinematic crystals

a. Helicitic folds

b. Randomly oriented crystals

c. Polygonal arcs

d. Chiastolite

e. Late, inclusion-free rim on a poikiloblast (?)

f. Random aggregate pseudomorph

 

Illustration of an Al2SiO5 poikiloblast that consumes more muscovite than quartz, thus inheriting quartz (and opaque) inclusions. The nature of the quartz inclusions can be related directly to individual bedding substructures. Note that some quartz is consumed by the reaction, and that quartz grains are invariably rounded. From Passchier and Trouw (1996) Microtectonics. Springer-Verlag.

 

Syn-kinematic crystals

Spiral Porphyroblast

Traditional interpretation of spiral Si train in which a porphyroblast is rotated by shear as it grows.

From Spry (1969) Metamorphic Textures. Pergamon. Oxford.
Spiral Si train in chloritoid.

 From Yardley et al. (1990) Atlas of Metamorphic Rocks and their Textures. Longmans.
"Snowball garnet" with highly rotated spiral Si. Porphyroblast is ~ 5 mm in diameter.

From Yardley et al. (1990) Atlas of Metamorphic Rocks and their Textures. Longmans.

 

Si characteristics of clearly pre-, syn-, and post-kinematic crystals as proposed by Zwart (1962). a. Progressively flattened Si from core to rim.

b. Progressively more intense folding of Si from core to rim.

c. Spiraled Si due to rotation of the matrix or the porphyroblast during growth.

After Zwart (1962) Geol. Rundschau, 52, 38-65.

horizontal rule

 

Analysis of Deformed Rocks

Deformational events: D1 D2 D3 …

Metamorphic events: M1 M2 M3 …

Foliations: So S1 S2 S3 …

Lineations: Lo L1 L2 L3 …

Plot on a metamorphism-deformation-time plot showing the crystallization of each mineral

 

(left) Asymmetric crenulation cleavage (S2) developed over S1 cleavage. S2 is folded, as can be seen in the dark sub-vertical S2 bands. Field width ~ 2 mm.

Right: sequential analysis of the development of the textures.

From Passchier and Trouw (1996) Microtectonics. Springer-Verlag.
Graphical analysis of the relationships between deformation (D), metamorphism (M), mineral growth, and textures in the rock illustrated in Figure 23-42. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Textures in a hypothetical andalusite porphyroblast-mica schist.

After Bard (1986) Microtextures of Igneous and Metamorphic Rocks. Reidel. Dordrecht.

Graphical analysis of the relationships between deformation (D), metamorphism (M), mineral growth, and textures in the rock illustrated in Figure 23-46.

Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

 

Replacement textures

Reaction rims and coronas. From Passchier and Trouw (1996) Microtectonics. Springer-Verlag.

 

Portion of a multiple coronite developed as concentric rims due to reaction at what was initially the contact between an olivine megacryst and surrounding plagioclase in anorthosites of the upper Jotun Nappe, W. Norway. From Griffen (1971) J. Petrol., 12, 219-243.