1. Geol 2312 Igneous and Metamorphic Petrology
Lecture 17
Textures of Metamorphic Rocks
March 28, 2016

2. Texture vs. Structures
Texture – Small-scale features that PENETRATE the entire rock and can be view on a thin section scale
Structures – Large-scale (hand-sample and larger) feature (folds, kink bands, gneissic banding)
Textures of metamorphic rocks reflect the combined processes of:
detachment and diffusion of matter IN THE SOLID STATE (though often in the presence of a fluid phase)
crystal nucleation
crystal growth
deformation (strain development)
strain recovery-recrystallization
-blastic – of metamorphic origin (e.g. porphyroblastic, poikiloblastic)
relict – belonging to the original rock (e.g., relic bedding)

3. Mechanisms of Deformation
1. Cataclasis Flow – Low T deformation by mechanical fragmentation, sliding, and rotation; Produces cataclasite, fault breccia, fault gouge 2. Pressure Solution – Dissolution at grain boundaries and reprecipitation in voids; requires the presence of a fluid.

4. Mechanisms of Deformation
3. Intracrystalline Deformation
- bending (elastic - recoverable)
- crystal lattice defects (permanent dislocations)
manifest as undulose extinction and deformation twinning
Undulose Extinction in Quartz
Deformation Twinning in Calcite

5. Mechanisms of Deformation
4. Strain Recovery – stored strain energy (by accumulated defects) can decrease the stability of a mineral; this energy can be lowered by migration of defects which occurs at elevated T
Migration of defects to a dislocation wall creates two of more subgrains of lower internal strain.
Subgrain domains of strain-recovered quartz

6. Mechanisms of Deformation
5. Recrystallization – stored strain energy can also be released by the migration of grain boundaries, the rotation of subgrains, or the reduction of grain boundary area; all are best accomplished at high T in the presence of a fluid
Higher
Strained
Grain
Lower
Strained
Grain
Recrystallization of Quartz by grain boundary migration
Grain boundary area reduction occurs as minerals strive to minimize their surface-area-to-volume ratio; this is often accomplished by coarsening and developing straighter boundaries.
Figure Recrystallization by (a) grain-boundary migration (including nucleation) and (b) subgrain rotation. From Passchier and Trouw (1996) Microtectonics. Springer-Verlag. Berlin.

7. Textures formed by Contact Metamorphism
Typically shallow pluton aureoles (low-P)
Crystallization/recrystallization is near-static
Monomineralic with low differential surface energy  granoblastic polygonal texture
Larger differential surface energy  decussate texture
Homogeneous textures (hornfels, granofels)
Relict textures are common

8. Development of Dihedral Angles
Grain boundaries of like minerals (A-A) have higher surface energies than boundaries between different minerals (A-B). So as minerals recrystallize, the A-B boundaries will lengthen relative to A-A boundaries and thus decrease the dihedral angle - θ
Pl + Cpx
note low θ at Pl-Pl-Cpx jcts and 120 jcts at Pl-Pl-Pl jcts
Qtz + Mica
surface E of (001) face is much lower than other faces – so maximizes size of that face. This restricts the abilitiy of quartz to coarsen.

9. Texture Development during Progressive Thermal Metamorphism
BASALT
Hydrothermally
Altered Basalt
Progressive thermal metamorphism of a diabase (coarse basalt). From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.
Mafic Hornfels

10. Texture Development during Progressive Thermal Metamorphism
SLATE
Progressive thermal metamorphism of slate. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

11. Porphyroblastic to Skeltal Texture
Increasing Crystallization Rate
Porphyroblasts form by low rates of nucleation which then requires diffusion over large areas. This results in large, widely spaced crystals.
Inclusions in poikiloblasts may form by:
- being inert phases not used by the growing crystal
- being a co-product of the poikilitic crystal- forming reaction
- a reactant that was not completely consumed

12. The Crystalloblastic Series
Most Euhedral
Titanite, rutile, pyrite, spinel
Garnet, sillimanite, staurolite, tourmaline
Epidote, magnetite, ilmenite
Andalusite, pyroxene, amphibole
Mica, chlorite, dolomite, kyanite
Calcite, vesuvianite, scapolite
Feldspar, quartz, cordierite
Least Euhedral
Differences in development of crystal form among some metamorphic minerals. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

13. Textures formed in Highly Strained Rocks

14. Progressive Development of Mylonite From a Granite
Figure Progressive mylonitization of a granite. From Shelton (1966). Geology Illustrated. Photos courtesy © John Shelton.

15. Shear Sense Indicators
Oblique
Foliation
________________________________________________________________________________________________
_______________________________________________________________
_______________________________________________________________
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Shear Plane Cleavage
Shear Band (S-C) Cleavage in micaceous rock
Oblique Foliation in
granular rock
Dextral Shear =
Right Lateral
Sinistral Shear =
Left Lateral
Acute Angle gives Sense of Shear

16. Sense of Shear Indicators Mantled Porphyroblasts
Not useful shear indicators
Rigid Porphyroblasts of K-feldspar in ductile mica+qtz matrix

17. Other Sense of Shear Indicators
Concentration of Mica due to dissolution of porphyroblast

18. Regional Metamorphism (Dynamothermal) Related to Convergent Tectonics

19. Deformation and Metamorphism
OROGENESIS (Mountain Building)
Multiple Tectonic Events Multiple Metamorphic Cycles
Each composed of Multiple Each composed of multiple
Deformational Events metamorphic reaction events
caused by reorientation & caused by abrupt changes in intensity of Stresses Pressure and Temperature
NOT ALWAYS 1 to 1 Correlation

20. Use S-surface terminology So – S1 – S2 – S3 …
Foliation, Layering, Lamination, and Other Planar Fabrics
a. Compositional layering
b. Preferred orientation of platy minerals
c. Shape of deformed grains
d. Grain size variation
e. Preferred orientation of platy minerals in a matrix without preferred orientation
f. Preferred orientation of lenticular mineral aggregates
g. Preferred orientation of fractures
h. Combinations of the above
Note in (e) that linear minerals may also define a foliation if randomly oriented in a plane
Use S-surface terminology So – S1 – S2 – S3 …
Deformational foliation is a secondary feature of rocks referring to the planar alignment of elongate minerals resulting from strain imparted to a rock
Winter (2001) Figure Types of fabric elements that may define a foliation. From Turner and Weiss (1963) and Passchier and Trouw (1996).

21. Classification of Deformational Foliation Cleavage and Schistosity
Figure A morphological (non-genetic) classification of foliations. After Powell (1979) Tectonophys., 58, 21-34; Borradaile et al. (1982) Atlas of Deformational and Metamorphic Rock Fabrics. Springer-Verlag; and Passchier and Trouw (1996) Microtectonics. Springer-Verlag.

22. Development of Deformational Foliation
Proposed mechanisms for the development of foliation
Mechanical rotation.
Preferred growth normal to compression.
Grains with advantageous orientation grow whereas those with poor orientation do not (or dissolve).
Minerals change shape by ductile deformation.
Pressure solution.
A combination of a and e.
Constrained growth between platy minerals.
Mimetic growth following an existing foliation.
Winter (2001) Figure Proposed mechanisms for the development of foliations. After Passchier and Trouw (1996) Microtectonics. Springer-Verlag.

23. Development of Deformational Foliation
Winter (2001) Figure Development of foliation by simple shear and pure shear (flattening). After Passchier and Trouw (1996) Microtectonics. Springer-Verlag.

24. Crenulation Cleavage Multi-stage Deformation

25. Development of Deformational Foliation in Bedded Sedimentary Rocks

26. Bedding – Cleavage Intersections
Sandy
(poorly foliated)
Clayey
(well foliated)

27. Timing of Deformation and Metamorphism
Successive dynamothermal events and microstructures are numbered: Metamorphic Events – M1, M2, M3, ... Deformational Events – D1, D2, D3, ... Foliation Orientations – S0, S1, S2, S3, ... (S0- primary feature) Lineation Orientations – L0, L1, L2, L3,...(L0- primary feature)

28. Timing of Deformation and Metamorphism
Winter (2001) Figure (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.

29. Porphyroblast inclusions inherit the fabric of the host matrix
Timing of New Mineral Growth Relative to Deformation Evidence from Inclusion-Bearing Porphyroblasts and Poikiloblasts
Porphyroblast inclusions inherit the fabric of the host matrix
Orientation - Si Si
Winter (2001) Figure 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.

30. Timing of New Mineral Growth Relative to Deformation
Post-kinematic: Si is identical to and continuous with Se (external foliation)
Pre-kinematic: Porphyroblasts are post-S2. Si is inherited from an earlier deformation. Se is compressed about the porphyroblast in (c) and a pressure shadow develops.
Syn-kinematic: Rotational porphyroblasts in which Si is continuous with Se suggesting that deformation did not outlast porphyroblast growth.

31. Pre-kinematic crystals
Bent crystal with undulose extinction
Foliation wrapped around a porphyroblast
Pressure shadow or fringe
Kink bands or folds
Microboudinage
Deformation twins
Figure Typical textures of pre-kinematic crystals. From Spry (1969) Metamorphic Textures. Pergamon. Oxford.

32. Post-kinematic crystals
Helicitic folds b. Randomly oriented crystals c. Polygonal arcs d. Chiastolite e. Late, inclusion-free rim on a poikiloblast (?) f. Random aggregate pseudomorph
Figure Typical textures of post-kinematic crystals. From Spry (1969) Metamorphic Textures. Pergamon. Oxford.
Helicitic folds in Si. b. Randomly oriented elongate crystals. c. Polygonal arcs of straight crystal segments in a fold. d. Chiastolite with central Si concordant with Se. e. Late, inclusion-free rim on a poikiloblast. The rim may be post-kinematic. f. Random aggregate of grains of one mineral pseudomorphing a single crystal of another (chlorite after garnet). The chlorite is post-kinematic.

33. Syn-kinematic crystals
Spiral Porphyroblasts
Si and Se for internal and external S in porphyroblasts
Question as to whether porphyroblasts rotate or matrix rotates around them
Winter (2001) Figure Traditional interpretation of spiral Si train in which a porphyroblast is rotated by shear as it grows. From Spry (1969) Metamorphic Textures. Pergamon. Oxford.

34. OROGENY LEADS TO POLYMETAMORPHISM