Kristallingeologie lecture 8 Porphyroblasts 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 information on relative timing of deformation and metamorphism D 1 slaty cleavage D < P P< D D <P<D chlorite garnet staurolite biotite albite D 2 crenulation P!D D !P D < albite 2 chlorite < D 1 D < garnet < D 2 1 1 staurolite ! D 2 D ! P D ! biotite (1); D ! biotite (2) 2 Idioblastic porphyroblasts • porphyroblast that has a shape controlled by its own crystallography andalusite andalusite
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lecture 8 Pre-, syn-, inter-, and post-tectonic growth ... · ¥Syn/post-tectonic feldspar with helicitic inclusion pattern Recapitulating ¥Metamorphic minerals can grow at any stage
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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