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• Processes leading to texture formation
• Textures of Contact Metamorphism
• High Strain Metamorphic Textures
• Regional ...
Processes of Deformation, Recovery
and Recrystallization
• Cataclastic Flow
• Pressure Solution
• Intracrytalline Deformat...
Cataclastic Flow
• Mechanical fragmentation of rocks
• Sliding and Rotation of fragments
• Occurs in Brittle zones
• Produ...
Pressure Solution
• Grain contacts in areas
of high stress become
strained
• Having higher energy,
minerals dissolves
easi...
Plastic Intracrystalline Deformation
• No loss of cohesion
• Several processes may operate simultaneously
• Defect migrati...
Recovery
• Stored strain energy decreases mineral stability,
which can be reduced by :
• Vacancy migration to dislocation ...
Migration of Dislocation Walls
• A strained grain with
undulose extinction is
recovered by forming
two unstrained
subgrain...
Undulose Extinction
• Variation in
birefringence caused by
a bent crystal lattice.
• Due to dislocation glide
or cracking....
Recrystallization
• Reduces stored lattice energy
• Movement of grain boundaries or
development of new boundaries.
• Produ...
Textures of Contact Metamorphism
• Occurs in aureoles
around intrusive bodies.
• Low pressure (low
deviatoric stress)
• Cr...
Granoblastic Polygonal Texture
• Equigranular texture
• Crystals adopt a polygonal
morphology
• Monomineralic rocks with l...
Decussate Texture
• Equigranular texture
• Structurally Anisotropic
minerals.
• Randomly orientated platy,
and elongate mi...
Porphyroblast
• Poikiloblast:
• Porphyroblasts containing
inclusions.
• Garnet, Staurolite, Cordierite,
Hornblend.
• High ...
Depletion Halos
• Diffusion of
components scarce in
area surrounding
growing porphyroblast
• Creating depletion zone
of th...
Light colored depletion
haloes around cm-sized
garnets in amphibolite.
Fe and Mg were less plentiful,
so that hornblende w...
High Strain Metamorphic Textures
 Shallow fault zone
• Brittle behavior of rocks
• Cataclasis dominating process
• Broken...
Mortar Texture
• Porphyroclasts
surrounded by a
matrix of fine
crushed material
derived from them
as they are rotated
and ...
Pseudotachylite
• Cohesive glassy or very fine-grained rock
• Produced by local rapid fragmentation and
melting due to she...
High Depth Zone
• Broaden shear zone
• Uniform shear distribution
• Ductile Processes dominate
• Deformation here is combi...
Polygonized Texture
• Larger deformed crystals break down into
smaller, undeformed subgrain
• Strain reduces
• It is a dyn...
Amoeboid
Texture
• Grain boundary migration
and subgrain rotation give
serrated sutured
boundaries
• If high temperature, ...
Coalescence
• Collective process of large grain formation
from subgrains; or
• By addition of smaller grains by grain
boun...
Mantled Porphyroclast
• Porphyroclasts developed in sheared
mylonites.
• Developed tapered rims of fine-grained
material.
...
Augen Texture
• Eye shaped mantle porphyroclast
• Growth by crystallization of
a mantle of new mineral around
the porphyro...
Regional Orogenic Metamorphic
Textures
–Dynamothermal (crystallization under
dynamic conditions)
–Orogeny- long-term mount...
–Tectonite- a deformed rock with a texture
that records the deformation
–Fabric- the complete spatial and geometric
config...
• Metamorphic Foliations can be classified as :
• Cleavage (fine penetrative foliation)
• Schistosity (coarser penetrative...
A morphological (non-genetic) classification of foliations. After Powell (1979).
Gneissose Structure and Layers
• Rocks having alternate light and dark layers.
• In fine grained – low grade rocks, it is ...
Metamorphic Differentiation
• Rocks are inhomogeneous
• So is the deformation in rocks
experiencing shear
• Separation of ...
Deformation vs. Metamorphic Mineral
Growth
• To interpret metamorphic and deformation
history of rock
• Metamorphic minera...
a. Bent crystal with
undulose extinction
b. Foliation wrapped
around a
porphyroblast
c. Pressure shadow or
fringe
d. Kink ...
Post-kinematic crystals
a. Helicitic folds b. Randomly oriented crystals c. Polygonal arcs d. Chiastolite e. Late,
inclusi...
Syn-kinematic crystals
Paracrystalline microboudinage Spiral Porphyroblast
. Syn-crystallization micro-boudinage. Syn-kine...
Syn-kinematic crystals
Spiral Si train in garnet,
Connemara, Ireland.
Magnification ~20X.
From Yardley et al.
(1990) Atlas...
Syn-kinematic crystals
“Snowball garnet”
with highly rotated
spiral Si.
Porphyroblast is ~ 5
mm in diameter.
From Yardley ...
Replacement Textures and Reaction
Rims
• Develop when reactions do not run to
completion
• Indicate nature, direction and ...
• Pseudomorph may develop
• Symplectite Texture
Combined growth of 2 or more minerals
replacing a single mineral. Producin...
Reaction Rim
• Reaction between minerals meeting at grain
boundaries.
• Partial replacement of either or both adjacent
to ...
Thank You
Textures and Structures of Metamorphic Rocks
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Textures and Structures of Metamorphic Rocks

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Textures and structures of Metamorphic Rocks and various mechanisms behind the formation of these textures.

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Textures and Structures of Metamorphic Rocks

  1. 1. • Processes leading to texture formation • Textures of Contact Metamorphism • High Strain Metamorphic Textures • Regional Metamorphic Textures • Replacement Textures and Reaction Rims
  2. 2. Processes of Deformation, Recovery and Recrystallization • Cataclastic Flow • Pressure Solution • Intracrytalline Deformation of Plastic Type • Recovery • Recrystallization
  3. 3. Cataclastic Flow • Mechanical fragmentation of rocks • Sliding and Rotation of fragments • Occurs in Brittle zones • Product – Fault Gouge, Breccia, Cataclasite
  4. 4. Pressure Solution • Grain contacts in areas of high stress become strained • Having higher energy, minerals dissolves easily • Fluid migrates to area of relatively low stress • Precipitation takes place.
  5. 5. Plastic Intracrystalline Deformation • No loss of cohesion • Several processes may operate simultaneously • Defect migration • Slip planes • Dislocation glide • Deformation twinning
  6. 6. Recovery • Stored strain energy decreases mineral stability, which can be reduced by : • Vacancy migration to dislocation tangles and their straightening • dislocation migration – straightening of bent dislocations • Annihilation – by meeting of dislocation of opposite sense. • Polygonization- general term for formation of low-strain subgrains
  7. 7. Migration of Dislocation Walls • A strained grain with undulose extinction is recovered by forming two unstrained subgrains • Density of dislocation decreases • Total lattice strain decreases.
  8. 8. Undulose Extinction • Variation in birefringence caused by a bent crystal lattice. • Due to dislocation glide or cracking. • It is a high strain metamorphic texture
  9. 9. Recrystallization • Reduces stored lattice energy • Movement of grain boundaries or development of new boundaries. • Producing a different configuration of grains • Occurs by : • Grain Boundary Migration • Subgrain rotation
  10. 10. Textures of Contact Metamorphism • Occurs in aureoles around intrusive bodies. • Low pressure (low deviatoric stress) • Crystallization/recrystalli zation in nearly static environment. • Characterized by lack of preferred orientation • Relict textures are common (little shear)
  11. 11. Granoblastic Polygonal Texture • Equigranular texture • Crystals adopt a polygonal morphology • Monomineralic rocks with low diffusion surface energy • Minerals are structurally isotropic • Grain triple junctions of appx 120 degrees. • Grains have low surface area • It occurs to minimize the combined surface energy of phases within a rock. • Rocks : Quartzite, Marble • Grain size increases with: • Temperature • Presence of aqueous fluids
  12. 12. Decussate Texture • Equigranular texture • Structurally Anisotropic minerals. • Randomly orientated platy, and elongate minerals. • Low energy surfaces predominate • Minerals with larger diffusion surface energy • The texture arises to minimize surface energy in a rock with elongate crystals • Hornfels, Granofels
  13. 13. Porphyroblast • Poikiloblast: • Porphyroblasts containing inclusions. • Garnet, Staurolite, Cordierite, Hornblend. • High Energy texture • Due to Poor Nucleation • Rapid porphyroblast growth, enveloping neighbouring grains. • Skeletal ( Web or Spongy) Texture • Inclusion and enclosing mineral phase occurs as intergranular continuous network • Develop at porphyroblast margin • Due to rapid poikiloblastic growth; or • Introduction of reactive component via an intergranular fluid Not Restricted to Contact metamorphism
  14. 14. Depletion Halos • Diffusion of components scarce in area surrounding growing porphyroblast • Creating depletion zone of those component surrounding it Progressive development of a depletion halo about a growing porphyroblast.
  15. 15. Light colored depletion haloes around cm-sized garnets in amphibolite. Fe and Mg were less plentiful, so that hornblende was consumed to a greater extent than was plagioclase as the garnets grew, leaving hornblende depleted zones.
  16. 16. High Strain Metamorphic Textures  Shallow fault zone • Brittle behavior of rocks • Cataclasis dominating process • Broken, crushed rotated grains dominate in fault breccia  Intermediate Depth • Pressure increases • Deformation increases • Cataclasis continues to dominate
  17. 17. Mortar Texture • Porphyroclasts surrounded by a matrix of fine crushed material derived from them as they are rotated and ground down
  18. 18. Pseudotachylite • Cohesive glassy or very fine-grained rock • Produced by local rapid fragmentation and melting due to shear heating • Earthquake shock energy in dry rocks • Occurs as veins • Often contains inclusions of wall-rock fragments. • Typically dark colored and glassy
  19. 19. High Depth Zone • Broaden shear zone • Uniform shear distribution • Ductile Processes dominate • Deformation here is combination of : • Cataclasis; Plastic intracrystalline deformation and recovery. • Grains : Twinned and ductilely elongated • Predominant rock : Mylonites • Occurrence : Shear zones, Orogenic belts • Recovery and recrystallization major factor
  20. 20. Polygonized Texture • Larger deformed crystals break down into smaller, undeformed subgrain • Strain reduces • It is a dynamic process • Deformation as well recovery simultaneously taking place. • Outlines of larger crystals still distinguishable
  21. 21. Amoeboid Texture • Grain boundary migration and subgrain rotation give serrated sutured boundaries • If high temperature, leads to further recrystallization • Coarsening of serrated boundaries • Less sharply curved boundaries • Shape of larger crystals - Amoeboid • Quartz deform to elongated or ribbon shaped • By crystal plastic mechanisms in mylonites • Occurs in regional metamorphism Ribbon Texture
  22. 22. Coalescence • Collective process of large grain formation from subgrains; or • By addition of smaller grains by grain boundary migration recrystallization.
  23. 23. Mantled Porphyroclast • Porphyroclasts developed in sheared mylonites. • Developed tapered rims of fine-grained material. • Rim – porphyroclast mineralogy same. • Rims derived from the porphyroclasts by grinding. • Develop from more resistant feldspars in Q-F matrix and mica in sheared granites
  24. 24. Augen Texture • Eye shaped mantle porphyroclast • Growth by crystallization of a mantle of new mineral around the porphyroblast. • The mantle is formed contiguous with the foliation imparted upon the rock • Forms a blanket which tapers of from either side of the porphyroclast within the strain shadows. • Common Minerals : Feldspars, Garnet During shearing, the porphyroclast may rotate, to form asymmetric shearing texture. The position of the tails is unequal across the foliation. This derives a form of shear direction information.
  25. 25. Regional Orogenic Metamorphic Textures –Dynamothermal (crystallization under dynamic conditions) –Orogeny- long-term mountain-building • May comprise several Tectonic Events –May have several Deformational Phases –May have an accompanying Metamorphic Cycles with one or more Reaction Events
  26. 26. –Tectonite- a deformed rock with a texture that records the deformation –Fabric- the complete spatial and geometric configuration of textural elements • Foliation- planar textural element • Lineation- linear textural element • Lattice Preferred Orientation (LPO) • Dimensional Preferred Orientation (DPO)
  27. 27. • Metamorphic Foliations can be classified as : • Cleavage (fine penetrative foliation) • Schistosity (coarser penetrative foliations • Gneissosity (poorly developed coarse foliations or segregated layers)
  28. 28. A morphological (non-genetic) classification of foliations. After Powell (1979).
  29. 29. Gneissose Structure and Layers • Rocks having alternate light and dark layers. • In fine grained – low grade rocks, it is relict bedding or igneous layer • Secondary separation into contrasting layers require diffusion, possible only at high temperature • It is either a secondary layering or poorly developed schistosity. • Mechanism – Metamorphic Differentiation.
  30. 30. Metamorphic Differentiation • Rocks are inhomogeneous • So is the deformation in rocks experiencing shear • Separation of High and low shear zones by transition zones. • Phyllosilicates accommodate better shear • Other minerals dissolve due to high shear. • Migrate to low shear area, where porphyroblasts form. • It leads to formation of Q-M domains.
  31. 31. Deformation vs. Metamorphic Mineral Growth • To interpret metamorphic and deformation history of rock • Metamorphic mineral growth can be classified on the basis of timing of growth w.r.t. deformation • Pre Kinematic • Syn Kinematic • Post Kinematic
  32. 32. 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 Typical textures of pre-kinematic crystals. From Spry (1969) Metamorphic Textures. Pergamon. Oxford. Pre-kinematic crystals
  33. 33. 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 Typical textures of post-kinematic crystals. From Spry (1969) Metamorphic Textures. Pergamon. Oxford.
  34. 34. Syn-kinematic crystals Paracrystalline microboudinage Spiral Porphyroblast . Syn-crystallization micro-boudinage. Syn-kinematic crystal growth can be demonstrated by the color zoning that grows and progressively fills the gap between the separating fragments. After Misch (1969) Amer. J. Sci., 267, 43.63. Traditional interpretation of spiral Si train in which a porphyroblast is rotated by shear as it grows. From Spry (1969) Metamorphic Textures. Pergamon. Oxford.
  35. 35. Syn-kinematic crystals Spiral Si train in garnet, Connemara, Ireland. Magnification ~20X. From Yardley et al. (1990) Atlas of Metamorphic Rocks and their Textures. Longmans.
  36. 36. Syn-kinematic crystals “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.
  37. 37. Replacement Textures and Reaction Rims • Develop when reactions do not run to completion • Indicate nature, direction and progress of reaction • Replacement : product replaces reacting mineral • Occurs in Retrograde metamorphism
  38. 38. • Pseudomorph may develop • Symplectite Texture Combined growth of 2 or more minerals replacing a single mineral. Producing intimate, wormy looking intergrowth • Mesh Texture Replacement of Olivine by serpentine along cracks in a net vein – like pattern
  39. 39. Reaction Rim • Reaction between minerals meeting at grain boundaries. • Partial replacement of either or both adjacent to their contact. • Corona – If reaction product forms a complete rim around mineral. • Monomineralic Corona – Moats • Polymineralic Corona – Symplectite Corona
  40. 40. Thank You

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