Metamorphic Textures Textures of Regional Metamorphism

1
Metamorphic TexturesTextures of Regional
Metamorphism

• 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

2
Metamorphic TexturesTextures of Regional
Metamorphism

• 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)

3
Progressive syntectonic metamorphism of a
volcanic graywacke, New Zealand. From Best
(1982). Igneous and Metamorphic Petrology. W. H.
Freeman. San Francisco.
4
Progressive syntectonic metamorphism of a
volcanic graywacke, New Zealand. From Best
(1982). Igneous and Metamorphic Petrology. W. H.
Freeman. San Francisco.
5
Progressive syntectonic metamorphism of a
volcanic graywacke, New Zealand. From Best
(1982). Igneous and Metamorphic Petrology. W. H.
Freeman. San Francisco.
6
Progressive syntectonic metamorphism of a
volcanic graywacke, New Zealand. From Best
(1982). Igneous and Metamorphic Petrology. W. H.
Freeman. San Francisco.
7
Fig 23-21 Types of foliations 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
Figure 23-21. Types of fabric elements that may
define a foliation. From Turner and Weiss (1963)
and Passchier and Trouw (1996).
8
Figure 23-22. 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.
9
Figure 23-22. (continued)
10
a
b
Figure 23-23. Continuous schistosity developed by
dynamic recrystallization of biotite, muscovite,
and quartz. a. Plane-polarized light, width of
field 1 mm. b. Crossed-polars, width of field 2
mm. Although there is a definite foliation in
both samples, the minerals are entirely
strain-free.
11
Progressive development (a ? c) of a crenulation
cleavage for both asymmetric (top) and symmetric
(bottom) situations. From Spry (1969)
Metamorphic Textures. Pergamon. Oxford.
12
Figure 23-24a. 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.
13
Figure 23-24b. 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.
14
Figure 23-25. 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
15
Types of lineations a. Preferred orientation of
elongated mineral aggregates b. Preferred
orientation of elongate minerals c. Lineation
defined by platy minerals d. Fold axes
(especially of crenulations) e. Intersecting
planar elements.
Figure 23-26. Types of fabric elements that
define a lineation. From Turner and Weiss (1963)
Structural Analysis of Metamorphic Tectonites.
McGraw Hill.
16
Figure 23-27. Proposed mechanisms for the
development of foliations. After Passchier and
Trouw (1996) Microtectonics. Springer-Verlag.
17
Figure 23-28. Development of foliation by simple
shear and pure shear (flattening). After
Passchier and Trouw (1996) Microtectonics.
Springer-Verlag.
18
Development of an axial-planar cleavage in folded
metasediments. Circular images are microscopic
views showing that the axial-planar cleavage is a
crenulation cleavage, and is developed
preferentially in the micaceous layers. From
Gilluly, Waters and Woodford (1959) Principles of
Geology, W.H. Freeman and Best (1982). Igneous
and Metamorphic Petrology. W. H. Freeman. San
Francisco.
19
Diagram showing that structural and fabric
elements are generally consistent in style and
orientation at all scales. From Best (1982).
Igneous and Metamorphic Petrology. W. H. Freeman.
San Francisco.
20

• 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 23-34. Typical textures of pre-kinematic
crystals. From Spry (1969) Metamorphic Textures.
Pergamon. Oxford.
21

• 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 23-35. Typical textures of post-kinematic
crystals. From Spry (1969) Metamorphic Textures.
Pergamon. Oxford.
22
Syn-kinematic crystals Paracrystalline
microboudinage Spiral Porphyroblast
Figure 23-38. Traditional interpretation of
spiral Si train in which a porphyroblast is
rotated by shear as it grows. From Spry (1969)
Metamorphic Textures. Pergamon. Oxford.
Figure 23-36. 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.
23
Syn-kinematic crystals
Figure 23-38. Spiral Si train in garnet,
Connemara, Ireland. Magnification 20X. From
Yardley et al. (1990) Atlas of Metamorphic Rocks
and their Textures. Longmans.
24
Syn-kinematic crystals
Figure 23-40. Non-uniform distribution of shear
strain as proposed by Bell et al. (1986) J.
Metam. Geol., 4, 37-67. Blank areas represent
high shear strain and colored areas are
low-strain. Lines represent initially horizontal
inert markers (S1). Note example of porphyroblast
growing preferentially in low-strain regions.
25
Syn-kinematic crystals
Figure 23-38. 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.
26
Figure 23-37. 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.
27
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

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Analysis of Deformed Rocks
Figure 23-42. (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
Analysis of Deformed Rocks
Figure 23-43. 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.
30
Analysis of Deformed Rocks
Figure 23-44. Composite sketch of some common
textures in Pikikiruna Schist, N.Z. Garnet
diameter is 1.5 mm. From Shelley (1993) Igneous
and Metamorphic Rocks Under the Microscope.
Chapman and Hall.
31
Figure 23-46. Textures in a hypothetical
andalusite porphyryoblast-mica schist. After Bard
(1986) Microtextures of Igneous and Metamorphic
Rocks. Reidel. Dordrecht.
32
Figure 23-48a. Interpreted sequential development
of a polymetamorphic rock. From Spry (1969)
Metamorphic Textures. Pergamon. Oxford.
33
Figure 23-48b. Interpreted sequential development
of a polymetamorphic rock. From Spry (1969)
Metamorphic Textures. Pergamon. Oxford.
34
Figure 23-48c. Interpreted sequential development
of a polymetamorphic rock. From Spry (1969)
Metamorphic Textures. Pergamon. Oxford.
35
Post-kinematic Si is identical to and continuous
with Se
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.
From Yardley (1989) An Introduction to
Metamorphic Petrology. Longman.
36
Deformation may not be of the same style or even
coeval throughout an orogen
Stage I D1 in forearc (A) migrates away from the
arc over time. Area (B) may have some deformation
associated with pluton emplacement, area (C) has
no deformation at all
Figure 23-49. Hypothetical development of an
orogenic belt involving development and eventual
accretion of a volcanic island arc terrane.
After Passchier and Trouw (1996) Microtectonics.
Springer-Verlag.
37
Deformation may not be of the same style or even
coeval throughout an orogen
Stage II D2 overprints D1 in forearc (A) in the
form of sub-horizontal folding and back-thrusting
as pushed against arc crust. Area (C) begins new
subduction zone with thrusting and folding
migrating toward trench.
Figure 23-49. Hypothetical development of an
orogenic belt involving development and eventual
accretion of a volcanic island arc terrane.
After Passchier and Trouw (1996) Microtectonics.
Springer-Verlag.
38
Deformation may not be of the same style or even
coeval throughout an orogen
Stage III Accretion deforms whole package. More
resistant arc crust gets a D1 event. D2
overprints D1 in forearc (A) and in
pluton-emplacement structures in (B). Area (C) in
the suture zone gets D3 overprinting D2 recumbent
folds on D1 foliations.
Figure 23-49. Hypothetical development of an
orogenic belt involving development and eventual
accretion of a volcanic island arc terrane.
After Passchier and Trouw (1996) Microtectonics.
Springer-Verlag.
39
Deformation may not be of the same style or even
coeval throughout an orogen
The orogen as it may now appear following uplift
and erosion.
Figure 23-49. Hypothetical development of an
orogenic belt involving development and eventual
accretion of a volcanic island arc terrane.
After Passchier and Trouw (1996) Microtectonics.
Springer-Verlag.
40
Figure 23-53. Reaction rims and coronas. From
Passchier and Trouw (1996) Microtectonics.
Springer-Verlag.
41
Figure 23-54. 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.
42
Photomicrograph of multiple reaction rims between
olivine (green, left) and plagioclase (right).
43
Coronites in outcrop. Cores of orthopyroxene
(brown) with successive rims of clinopyroxene
(dark green) and garnet (red) in an anorthositic
matrix. Austrheim, Norway.
44
Figures not used
Figure 23-2. a. Migration of a vacancy in a
familiar game. b. Plastic horizontal shortening
of a crystal by vacancy migration. From Passchier
and Trouw (1996) Microtectonics. Springer-Verlag.
Berlin.
45
Figures not used
Figure 23-3. Plastic deformation of a crystal
lattice (experiencing dextral shear) by the
migration of an edge dislocation (as viewed down
the axis of the dislocation).
46
Figures not used
Figure 23-8. Gneissic anorthositic-amphibolite
(light color on right) reacts to become eclogite
(darker on left) as left-lateral shear transposes
the gneissosity and facilitates the
amphibolite-to-eclogite reaction. Bergen area,
Norway. Two-foot scale courtesy of David
Bridgwater. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
47
Figures not used
Figure 23-12. Skeletal or web texture of
staurolite in a quartzite. The gray intergranular
material, and the mass in the lower left, are
all part of a single large staurolite crystal.
Pateca, New Mexico. Width of view 5 mm. Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
48
Figures not used
Figure 23-16a. Large polygonized quartz crystals
with undulose extinction and subgrains that show
sutured grain boundaries caused by
recrystallization. Compare to Figure 23-15b, in
which little, if any, recrystallization has
occurred. From Urai et al. (1986) Dynamic
recrystallization of minerals. In B. E. Hobbs and
H. C. Heard (eds.), Mineral and Rock Deformation
Laboratory Studies. Geophysical Monograph 36.
AGU.
49
Figures not used
Figure 23-16b. Vein-like pseudotachylite
developed in gneisses, Hebron Fjord area, N.
Labrador, Canada. Winter (2001) An Introduction
to Igneous and Metamorphic Petrology. Prentice
Hall.
50
Figures not used
Figure 23-17. Some features that permit the
determination of sense-of-shear. All examples
involve dextral shear. s1 is oriented as shown.
a. Passive planar marker unit (shaded) and
foliation oblique to shear planes. b. S-C
foliations. c. S-C foliations. After Passchier
and Trouw (1996) Microtectonics. Springer-Verlag.
51
Figures not used
Figure 23-18. Augen Gneiss. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
52
Figures not used
Figure 23-19. 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.
53
Figures not used
Figure 23-20. Other methods to determine
sense-of-shear. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
54
Figures not used
Figure 23-29. Deformed quartzite in which
elongated quartz crystals following shear,
recovery, and recrystallization. Note the broad
and rounded suturing due to coalescence. Field
width 1 cm. From Spry (1969) Metamorphic
Textures. Pergamon. Oxford.
55
Figures not used
Figure 23-30. Kink bands involving cleavage in
deformed chlorite. Inclusions are quartz (white),
and epidote (lower right). Field of view 1 mm.
Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
56
Figures not used
Figure 23-31. Examples of petrofabric diagrams.
a. Crystal c-axes cluster in a shallow
inclination to the NE. b. Crystal axes form a
girdle of maxima that represents folding of an
earlier LPO. Poles cluster as normals to fold
limbs. b represents the fold axis. The dashed
line represents the axial plane, and suggests
that s1 was approximately E-W and horizontal.
From Turner and Weiss (1963) Structural Analysis
of Metamorphic Tectonites. McGraw Hill.
57
Figures not used
Figure 23-32. Pelitic schist with three
s-surfaces. S0 is the compositional layering
(bedding) evident as the quartz-rich (left) half
and mica-rich (right) half. S1 (subvertical) is a
continuous slaty cleavage. S2 (subhorizontal) is
a later crenulation cleavage. Field width 4 mm.
From Passchier and Trouw (1996) Microtectonics.
Springer-Verlag.
58
Figures not used
Figure 23-33. 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.
59
Figures not used
Figure 23-41. Initial shear strain causes
transposition of foliation. c. Continued strain
during the same phase causes folding of the
foliation. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
60
Figures not used
a
b
Figure 23-52. a. Mesh texture in which serpentine
(dark) replaces a single olivine crystal (light)
along irregular cracks. b. Serpentine
pseudomorphs orthopyroxene to form bastite in the
upper portion of photograph, giving way to mesh
olivine below. Field of view ca. 0.1 mm. Fidalgo
sepentinite, WA state. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.