lecture 8 pre-, syn-, inter-, and post-tectonic growth ... · ¥syn/post-tectonic feldspar with...
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Kristallingeologie
lecture 8Porphyroblasts
This lecture
• Ductile deformation & metamorphic conditions
• New minerals grow: porphyroblasts with inclusions
• Inclusion trails can be used to define
• Pre-, syn-, inter-, and post-tectonic growth
• Some more complex structures are:
• Snowball structures
• Helicitic structures
Porphyroblasts
• Relatively large single crystals
• That formed by metamorphic growth (blasis = growth)
• In a finer-grained matrix
• Porphyroclast - matrix relationships give informationon relative timing of deformation and metamorphism
D1 slaty cleavage
D < P
P< D
D <P<Dchlorite
garnet
staurolite
biotite
albite
D2 crenulation
P!D
D !P
D < albite2
chlorite < D1
D < garnet < D21
1
staurolite ! D2D ! P
D ! biotite (1); D ! biotite (2)2
Idioblastic porphyroblasts
• porphyroblast that has a shape controlled by its own crystallography
andalusiteandalusite
Xenoblastic porphyroblasts
• porphyroblast that has a shape that IS NOT controlled by its own
crystallography
feldsparfeldspar
Poikilitic porphyroblasts
• porphyroblast that are full off inclusions
are called poikiloblasts or poikilitic porphyroblasts
• Inlcusions can make up >90 % of a poikiloblast
cordieritecordierite
quartzinclusions
quartzinclusions
Poikilitic porphyroblasts
• The poikiloblast is one single crystal
Inclusions• Once P-T-etc. conditions are favourable for a metamorphic mineral
to grow, nucleation can start.
• The small nuclei have a relatively high surface energy, which formsan energy barrier for their growth.
• The number of nuclei and their survival rate determines whethermany small or a few large porphyroblasts form.
• The number & size of porphyroblasts depend on:
1. the availability of favourable nucleationsites;
2. the driving force for the metamorphicreaction (overshoot of PT-conditions)
3. transport rate of elements that form newmineral and elements that have to beremoved to make space available
Number of inclusions
To form and grow a new metamorphic mineral grain:
a) the right mix of elements that form the mineral must get to
the grain
b) other elements have to be taken away from the grain
1. If (a) and (b) are both fast enough, no inclusions areincorporated
2. If (b) can't keep up with (a), inclusions are incorporated ofminerals that do not contribute to the metamorphic reaction
3. If (a) is too slow, even inclusions of minerals that docontribute to the reaction are incorporated.
Example of poikilitic growth of garnet over its own pressure shadow
garnetgarnet
quartzpressure
shadow
quartzpressure
shadow
Internal & external foliation
• Porphyroblasts overgrow and "freeze-in" existing
folation(s)
• External foliation: newer foliation grown after PB
• Internal foliation: older foliation contained inside PB
Pretectonic porphyroblasts
• Porphyroblasts grew before any tectonic fabric
(Gefüge) developed
• Inclusions show no alignment or earlier foliation
Intertectonic porphyroblasts
• Intertectonic porphyroblasts
grew between two
deformation phases:
• Internal foliation: from previous
D-phase
• External foliation: from next D-
phase
• Fexternal not related to Finternal
• Fexternal and Finternal not
continuous
Intertectonic porphyroblastIntertectonic porphyroblast• Intertectonic garnet porphyroblast
in garnet-mica schist
S1S1
S1S1
S2S2
• Intertectonic staurolite porphyroblast
containing PQ-internal fabric (P-domains inclusion-poor)
S1S1
S2S2
PP
PP
SexternalSexternal
Syntectonic porphyroblasts
• Syntectonic porphyroblasts
grew during a deformation
phase:
• Internal foliation: early version of
developing foliation
• External foliation: further
developed foliation
• Fexternal related to Finternal
• Fexternal continue into Finternal
• Fexternal cut by porphyroblast
• Fexternal wraps porphyroblast
Syntectonic porphyroblastSyntectonic porphyroblast
• (Inter- to) syntectonic hornblende porphyroblast
in quartz-albite-mica schist
SextSextSintSint
• Syntectonic garnet porphyroblast
in garnet-kyanite schist
SextSext
SintSint
Post-tectonic porphyroblasts
• Post-tectonic porphyroblasts
grew after a deformation
phase:
• Internal foliation: older foliation
• External foliation: same older
foliation
• Fexternal the same as Finternal
• Fexternal continue into Finternal
• Fexternal cut by porphyroblast
Post-tectonic porphyroblastPost-tectonic porphyroblast• Post-tectonic chloritoid porphyroblast
in slaty cleavage
S1S1
Some complicated structures
• Rigid objects may rotate when deformation is
non-coaxial
• Snowball garnets
• Rotation and deflection of a foliation can give
complex structures:
• Deflection folds
• Millipede structures
• Porphyroclasts can grow over complicated
foliation patterns:
• Helicitic structure
Rotating porphyroblasts
• Round inclusions rotate when deformation is non-
coaxial (e.g. simple shear)
• Internal foliation rotates passively with its host
• Causing spiral-like internal foliation
• "Snowball garnets" are an example
180o
75o
120o
• Syntectonic Snowball garnet porphyroblast
in garnet-mica schist
• Syntectonic Snowball garnet porphyroblast
in garnet-mica schist
Deflection fold structures• A foliation can get rotated and deflected during
progressive deformation, causing deflection foldstructures
• Several scenarios result in similar structures!
Pure shear
Rotation of foliation
No rotation of object
Dextral simple shear
No rotation of foliation
Rotation of object
Sinistral simple shear
Rotation of foliation
Rotation of object
• Syntectonic garnet porphyroblast, with deflection
• Sinistral simple shear one possible interpretation
Helicitic structures
• The internal foliation pattern in this porphyroblast is
not resulting from rotation during growth, but from
overgrowing a crenulated cleavage
SinternalSSinternalinternal
SexternalSSexternalexternal
False inclusion trails
Some examples of porphyrobalsts
• Inter- to early-tectonic garnet porphyroblast in
garnet-mica schist
S2 or S3S2 or S3
S1S1
S2S2
• Post-tectonic feldspar porphyroblast in crenulated
mica schist
S2S2
S3S3
Feldspar
porphyroblast
Feldspar
porphyroblast
Post-tectonic relative to first deformation phase (D1)Early- to syn-tectonic relative to second deformation phase (D2)
S1 formation
Biotite growth
S2 crenulation foliation
D1D1
M1M1
D2D2
Syntectonic snowball garnetGarnet preferentially overgrows micaceous layer
• Post-tectonic muscovite
• followed by slight kinking
quartzquartz
biotitebiotite
muscovitemuscovite
S1S1
• Syntectonic garnet
pressure
shadow
pressure
shadow
garnetgarnet
hour
glass
zoning
hour
glass
zoning
SS
strain capstrain cap • Syntectonic garnet
S1S1S2S2
• Syn/post-tectonic feldspar with helicitic inclusion pattern
Recapitulating
• Metamorphic minerals can grow at any stage relative
to deformation phases
• Porphyroblasts are metamorphic minerals
• They may contain inclusions: internal foliations
• The relation between Sinternal and Sexternal can often
help to define relative timing of
• Porphyroclast growth: metamorphic phase
• Foliation formation: deformation phase(s)