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

QQ

PP

QQ

PP

QQ

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)