GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 17
Textures of Metamorphic Rocks
March 28, 2016
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)
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.
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
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
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
Figure 23-6. Recrystallization by (a) grainboundary migration (including nucleation) and (b)
subgrain rotation. From Passchier and Trouw
(1996) Microtectonics. Springer-Verlag. Berlin.
Grain boundary area reduction
occurs as minerals strive to
minimize their surface-area-tovolume ratio; this is often
accomplished by coarsening and
developing straighter boundaries.
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
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-PlCpx 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.
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
TEXTURE DEVELOPMENT DURING
PROGRESSIVE THERMAL METAMORPHISM
SLATE
Progressive thermal metamorphism of slate.
From Best (1982). Igneous and Metamorphic
Petrology. W. H. Freeman. San Francisco.
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 crystalforming reaction
- a reactant that was not completely consumed
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
Differences in development of
crystal form among some
metamorphic minerals. From
Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco.
Least Euhedral
TEXTURES FORMED IN HIGHLY STRAINED ROCKS
PROGRESSIVE
DEVELOPMENT
OF MYLONITE
FROM A
GRANITE
Figure 23-15. Progressive
mylonitization of a granite. From
Shelton (1966). Geology
Illustrated. Photos courtesy ©
John Shelton.
SHEAR SENSE INDICATORS
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Oblique
Foliation
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Shear Plane
Cleavage
Oblique Foliation in
granular rock
Shear Band (S-C)
Cleavage in
micaceous rock
Dextral Shear =
Right Lateral
Sinistral Shear =
Left Lateral
Acute Angle gives
Sense of Shear
SENSE OF SHEAR INDICATORS
MANTLED PORPHYROBLASTS
Not useful shear indicators
Rigid Porphyroblasts of K-feldspar in
ductile mica+qtz matrix
OTHER
SENSE OF
SHEAR
Concentration of Mica
due to dissolution of
porphyroblast
INDICATORS
REGIONAL
METAMORPHISM
(DYNAMOTHERMAL)
RELATED TO
CONVERGENT
TECTONICS
DEFORMATION AND METAMORPHISM
OROGENESIS (Mountain Building)
Multiple Tectonic Events
Each composed of Multiple
Deformational Events
caused by reorientation &
intensity of Stresses
Multiple Metamorphic Cycles
NOT ALWAYS
1 to 1
Correlation
Each composed of multiple
metamorphic reaction events
caused by abrupt changes in
Pressure and Temperature
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
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 23-21. Types of fabric elements that may define a foliation. From
Turner and Weiss (1963) and Passchier and Trouw (1996).
CLASSIFICATION OF DEFORMATIONAL FOLIATION
CLEAVAGE AND SCHISTOSITY
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.
DEVELOPMENT OF DEFORMATIONAL
FOLIATION
Proposed mechanisms for the
development of foliation
a. Mechanical rotation.
b. Preferred growth normal to
compression.
c. Grains with advantageous
orientation grow whereas
those with poor orientation do
not (or dissolve).
d. Minerals change shape by
ductile deformation.
e. Pressure solution.
f. A combination of a and e.
g. Constrained growth between
platy minerals.
h. Mimetic growth following an
existing foliation.
Winter (2001) Figure 23-27. Proposed mechanisms for the development of foliations. After
Passchier and Trouw (1996) Microtectonics. Springer-Verlag.
DEVELOPMENT OF
DEFORMATIONAL FOLIATION
Winter (2001) Figure 23-28. Development of foliation by simple shear and pure shear
(flattening). After Passchier and Trouw (1996) Microtectonics. Springer-Verlag.
CRENULATION CLEAVAGE
MULTI-STAGE DEFORMATION
DEVELOPMENT OF DEFORMATIONAL FOLIATION
IN BEDDED SEDIMENTARY ROCKS
BEDDING – CLEAVAGE INTERSECTIONS
Sandy
(poorly foliated)
Clayey
(well foliated)
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)
TIMING OF DEFORMATION AND
METAMORPHISM
Winter (2001) 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.
TIMING OF NEW MINERAL GROWTH
RELATIVE TO DEFORMATION
EVIDENCE FROM INCLUSIONBEARING PORPHYROBLASTS
AND POIKILOBLASTS
Porphyroblast inclusions inherit
the fabric of the host matrix
Orientation - Si
Si
Winter (2001) 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. SpringerVerlag.
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.
Pre-kinematic
crystals
a.
b.
c.
d.
e.
f.
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.
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
Figure 23-35.
Typical
textures of
post-kinematic
crystals. From
Spry (1969)
Metamorphic
Textures.
Pergamon.
Oxford.
Syn-kinematic crystals
Spiral Porphyroblasts
Winter (2001) 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.
OROGENY LEADS TO
POLYMETAMORPHISM