Metamorphic Textures
(C
(Chapter
23)
Porphyroblastic hornblende grt-bt-ms schist– Switzerland. Image: Darrell Henry
Metamorphic textures
Processes of Deformation, Recovery and Recrystallization
(1) Cataclastic Flow
• Mechanical fragmentation and sliding,
sliding
rotation of fragments
• Crush, break, bend, grind, kink,
d f
deformation
ti ttwins,
i undulose
d l
extinction,
ti ti
shredding of micas, augen, mortar, etc.
(2) Pressure Solution
• Highest strain in areas near grain contacts
(hatch pattern).
g
areas dissolve and material
• High-strain
precipitates in adjacent low-strain areas
(shaded). Vertical shortening.
• Pressure solution of quartz in deformed
quartzite
Metamorphic textures
Processes of Deformation, Recovery and Recrystallization
((3)) Plastic Intracrystalline
y
Deformation
(ductile deformation at higher T)
No loss of cohesion
• Processes that operate simultaneously
•
Defect migration
Slip
pp
planes
Dislocation glide
Deformation twinning
(4) Recovery
R
Loss of stored strain energy by
vacancyy migration,
g
, dislocation
migration and annihilation
Metamorphic textures
Processes of Deformation, Recovery and Recrystallization
(5) Recrystallization
• Grain boundary migration
• Subgrain rotation
• Solid-state diffusion creep
p at
higher T
• Crystalplastic deformation
(general term)
Undulose extinction and ((b)) elongate
g subgrains
g
in
quartz due to dislocation formation and migration
Metamorphic textures
Processes of Deformation, Recovery and Recrystallization
(5) Recrystallization
• Recrystallization by grain
boundary migration and subgrain rotation )
Recrystallized
quartz with
irregular
(sutured)
boundaries,
formed by grain
boundary
migration.
Width 0.2 mm.
Metamorphic textures
High-strain
High
strain rock textures
Metamorphic textures
High-strain
High
strain rock textures
Concentrate on cataclastic > ductile (shallower)
• Break,
k crack,
k b
bend,
d crush,
h
rotate
• Slip and shredding of
phyllosilicates
• Clasts- broken remnants
• Porphyroclast- larger remnant
in finer crush matrix
• Mortar texture
• Ribbons
• Pseudotachylite
Metamorphic textures
High-strain
High
strain rock textures
Progressive
mylonitization of
the
h S
San G
Gabriel
bi l
granite,
California
Image: Winter (2001)
Metamorphic textures
High-strain
High
strain rock textures
Progressive
mylonitization of
the
h S
San G
Gabriel
bi l
granite,
California
Image: Winter (2001)
Metamorphic textures
High-strain
High
strain rock textures
Progressive
mylonitization of
the
h S
San G
Gabriel
bi l
granite,
California
Image: Winter (2001)
Metamorphic textures
Contact metamorphic textures
Typically shallow pluton aureoles (low-P)
Crystallization/recrystallization is near-static
Monomineralic with minor difference
surface energy restults in granoblastic
polygonal texture
Larger change in surface energy results
in decussate texture
Isotropic textures (hornfels, granofels)
R li textures are common
Relict
Contact metamorphism of a diabase: Image: Winter (2001)
Metamorphic textures
Contact metamorphic
p
textures ((metadiabase))
Lower
greenschist
facies
Amphibolite
facies
Upper
greenschist
facies
Granulite
facies
Metamorphic textures
Contact metamorphic
p
textures ((metapelite)
p
)
Lower greenschist facies
Upper greenschist facies
Amphibolite/granulite facies
(slate)
(And-crd-ms-pl-qtz
spotted hornfels)
(Sil-bearing And-Kfs-btcrd-pl-qtz granofels)
Metamorphic textures
Contact metamorphic
p
textures
Common
textures in
contact
metamorphic
rocks
Metamorphic textures
Contact metamorphic
p
textures
c.
Dihedral angle
g between two mineral types.
yp When
the A-A grain boundary energy is greater than for
A-B, the angle θ will decrease (b) so as to
increase the relative area of A-B boundaries. c.
Sketch of a plagioclase (light)-clinopyroxene
(dark) hornfels showing lower dihedral angles in
clinopyroxene at most cpx-plag-plag boundaries.
Metamorphic textures
Contact metamorphic
p
textures
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
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 byy grain
q
g
boundary "pinning" by finer micas in the
upper layer.
Metamorphic textures
Contact metamorphic
p
textures
Contact overprint on earlier
regional
g
events are common
• Thermal maximum later than
deformational
• Separate post-orogenic (collapse)
event
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.
Metamorphic textures
Regional
g
metamorphic
p
textures
Dynamothermal (crystallization under dynamic conditions)
Orogeny long-term
Orogenylong term mountain-building
mountain building
• May comprise several Tectonic Events
• May have several Deformational Phases
• May have an accompanying Metamorphic Cycles with
one or more Reaction Events
Tectonite - deformed rock with a texture that records
deformation
Fabric - the complete spatial and geometric configuration of
textural elements
• Foliation- planar textural element
• LineationLi
ti
linear ttextural
li
t l element
l
t
Metamorphic textures
Regional
g
metamorphic
p
textures
Progressive regional
metamorphism of a
volcaniclastic
graywacke
Metamorphic textures
Regional
g
metamorphic
p
textures
Kinematic indicators – determining the sense of shear in a deformed rock
Metamorphic textures
Regional
g
metamorphic
p
textures
Symmetrical crenulation cleavages in
amphibole quartz rich schist.
amphibole-quartz-rich
schist
Note concentration of quartz in hinge
areas.
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.
folds
From Borradaile et al. (1982) Atlas of Deformational and
Metamorphic Rock Fabrics. Springer-Verlag.
Metamorphic textures
Regional
g
metamorphic
p
textures
Stages in the development of crenulation cleavage
as a function of temperature and intensity of the
second deformation.
Development of S2 micas depends upon T and
the intensity of the second deformation
From Passchier and Trouw (1996) Microtectonics. Springer-Verlag.
Metamorphic textures
Regional
g
metamorphic
p
textures - Typical
yp
textures of prep
kinematic crystals
Pre-kinematic crystals
y
a. Bent crystal with undulose
extinction
b. Foliation wrapped around a
porphyroblast
c. Pressure shadow or fringe
d Kink b
d.
bands
nd orr ffolds
ld
e. Microboudinage
f. Deformation twins
From Spry (1969) Metamorphic Textures. Pergamon. Oxford.
Metamorphic textures
Regional
g
metamorphic
p
textures - Typical
yp
textures of postp
kinematic crystals
Post-kinematic crystals
a.
Helicitic folds
b Randomly oriented
b.
crystals
c. Polygonal arcs
d. Chiastolite
e. Late, inclusion-free
rim on a poikiloblast
From Spry (1969) Metamorphic Textures. Pergamon. Oxford.
f. Random aggregate
pseudomorph
Metamorphic textures
Regional
g
metamorphic
p
textures
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.
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.
Metamorphic textures
Regional
g
metamorphic
p
textures
Syn-kinematic crystals - Spiral Porphyroblast
Traditional interpretation of spiral Si train in which
hich a
porphyroblast is rotated by shear as it grows.
From Passchier and Trouw (1996) Microtectonics. Springer-Verlag.
Metamorphic textures
Regional
g
metamorphic
p
textures
Spiral Si train in chloritoid.
"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.
Metamorphic textures
Regional
g
metamorphic
p
textures
Si characteristics of clearly pre-, syn-, and post-kinematic crystals as
proposed by Zwart (1962).
Metamorphic textures
Regional
g
metamorphic
p
textures
Analysis of Deformed Rocks
•
•
•
•
•
Deformational
D
f
ti
l events:
t D1 D2 D3 …
Metamorphic events: M1 M2 M3 …
Foliations: So S1 S2 S3 …
Lineations: Lo L1 L2 L3 …
Plot on a metamorphismdeformation-time plot showing the
crystallization of each mineral
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.
Metamorphic textures
Regional
g
metamorphic
p
textures
Graphical
p
analysis
y of the
relationships between
deformation (D),
metamorphism (M), mineral
growth, and textures in the rock
illustrated in Figure.
Metamorphic textures
Regional
g
metamorphic
p
textures
Graphical analysis of the
relationships between deformation
(D), metamorphism (M), mineral
growth,, and textures in the rock
g
illustrated in Figure.
Metamorphic textures
Regional
g
metamorphic
p
textures
Graphical
p
analysis
y of the relationships
p
between deformation (D), metamorphism (M),
mineral growth, and textures in the rock
illustrated in Figure.
Winter (2001) An Introduction to Igneous and Metamorphic Petrology.
Prentice Hall.
Metamorphic textures
Replacement
p
textures
Reaction rims and coronas
Metamorphic textures
Replacement
p
textures
Portion of a
multiple
coronite
d l
developed
d as
concentric rims
due to reaction
at what was
initially the
contact
between an
olivine
megacryst and
surrounding
g
plagioclase in
anorthosites of
the upper Jotun
Nappe, W.
Norway.