lecture 8 Kristallingeologie This lecture Idioblastic porphyroblasts Porphyroblasts

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Winifred Beasley
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Kristallingeologie lecture 8 Porphyroblasts This lecture Ductile deformation & metamorphic conditions New minerals grow: porphyroblasts with inclusions Inclusion trails can be used to define Pre-,

1 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 chlorite garnet staurolite biotite albite P< D D! P D 1 slaty cleavage D <P<D P!D D!P D2 crenulation D < P chlorite < D 1 D < garnet < D 2 1 staurolite! D 2 D! biotite (1); 1 D 2! biotite (2) D < albite 2 Idioblastic porphyroblasts andalusite porphyroblast that has a shape controlled by its own crystallography

Xenoblastic porphyroblasts Poikilitic porphyroblasts feldspar cordierite quartz inclusions

are full off inclusions are called poikiloblasts or poikilitic porphyroblasts Inlcusions can make

conditions are favourable for a metamorphic mineral to grow, nucleation can start.

2 Xenoblastic porphyroblasts Poikilitic porphyroblasts feldspar cordierite quartz inclusions porphyroblast that has a shape that IS NOT controlled by its own crystallography porphyroblast that are full off inclusions are called poikiloblasts or poikilitic porphyroblasts Inlcusions can make up >90 % of a poikiloblast Poikilitic porphyroblasts 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 forms an energy barrier for their growth. The number of nuclei and their survival rate determines whether many small or a few large porphyroblasts form. The number & size of porphyroblasts depend on: The poikiloblast is one single crystal 1. the availability of favourable nucleation sites; 2. the driving force for the metamorphic reaction (overshoot of PT-conditions) 3. transport rate of elements that form new mineral and elements that have to be removed to make space available

Number of inclusions Example of poikilitic growth of garnet over its own pressure shadow To form and grow a new metamorphic mineral grain: a) the right mix of elements that form the mineral must get

If (b) can't keep up with (a), inclusions are incorporated of minerals that do not contribute to the metamorphic reaction 3.

3 Number of inclusions Example of poikilitic growth of garnet over its own pressure shadow 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 garnet quartz pressure shadow 1. If (a) and (b) are both fast enough, no inclusions are incorporated 2. If (b) can't keep up with (a), inclusions are incorporated of minerals that do not contribute to the metamorphic reaction 3. If (a) is too slow, even inclusions of minerals that do contribute to the reaction are incorporated. Internal & external foliation Pretectonic porphyroblasts Porphyroblasts grew before any tectonic fabric (Gefüge) developed Inclusions show no alignment or earlier foliation Porphyroblasts overgrow and "freeze-in" existing folation(s) External foliation: newer foliation grown after PB Internal foliation: older foliation contained inside PB

Intertectonic porphyroblasts Intertectonic porphyroblasts grew between two deformation phases: Internal foliation: from previous D-phase External foliation: from next D- phase F external not related

staurolite porphyroblast containing PQ-internal fabric (P-domains inclusion-poor) Q S external Syntectonic porphyroblasts grew during a deformation phase: Internal foliation: early version of

4 Intertectonic porphyroblasts Intertectonic porphyroblasts grew between two deformation phases: Internal foliation: from previous D-phase External foliation: from next D- phase F external not related to F internal F external and F internal not continuous S 2 Intertectonic porphyroblast Intertectonic garnet porphyroblast in garnet-mica schist Syntectonic porphyroblasts Q P P Q S 2 Intertectonic staurolite porphyroblast containing PQ-internal fabric (P-domains inclusion-poor) Q S external Syntectonic porphyroblasts grew during a deformation phase: Internal foliation: early version of developing foliation External foliation: further developed foliation F external related to F internal F external continue into F internal F external cut by porphyroblast F external wraps porphyroblast Syntectonic porphyroblast

S int S Sint S ext S ext (Inter- to) syntectonic hornblende

porphyroblast in garnet-kyanite schist Post-tectonic

deformation phase: Internal foliation: older foliation External

internal F external continue into F internal F external cut by

5 S int S Sint S ext S ext (Inter- to) syntectonic hornblende porphyroblast in quartz-albite-mica schist Syntectonic garnet porphyroblast in garnet-kyanite schist Post-tectonic porphyroblasts Post-tectonic porphyroblasts grew after a deformation phase: Internal foliation: older foliation External foliation: same older foliation F external the same as F internal F external continue into F internal F external cut by porphyroblast Post-tectonic porphyroblast Post-tectonic chloritoid porphyroblast in slaty cleavage

Some complicated structures Rotating porphyroblasts Rigid objects may rotate when

foliation can give complex structures: Deflection folds Millipede structures

75 o 120 o Round inclusions rotate when deformation is noncoaxial (e.g.

spiral-like internal foliation "Snowball garnets" are an example 180 o

6 Some complicated structures Rotating porphyroblasts 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 75 o 120 o Round inclusions rotate when deformation is noncoaxial (e.g. simple shear) Internal foliation rotates passively with its host Causing spiral-like internal foliation "Snowball garnets" are an example 180 o 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

rotation of foliation Rotation of object Sinistral simple shear Rotation of

Sinistral simple shear one possible interpretation Helicitic structures False

7 Deflection fold structures A foliation can get rotated and deflected during progressive deformation, causing deflection fold structures 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 False inclusion trails S external S internal The internal foliation pattern in this porphyroblast is not resulting from rotation during growth, but from overgrowing a crenulated cleavage

8 Some examples of porphyrobalsts S 2 or S 3 S 2 Inter- to early-tectonic garnet porphyroblast in garnet-mica schist S 3 Feldspar porphyroblast S 2 formation D 1 Biotite growth M 1 S 2 crenulation foliation D 2 Post-tectonic feldspar porphyroblast in crenulated mica schist Post-tectonic relative to first deformation phase (D 1 ) Early- to syn-tectonic relative to second deformation phase (D 2 )

9 quartz biotite muscovite Syntectonic snowball garnet Garnet preferentially overgrows micaceous layer Post-tectonic muscovite followed by slight kinking pressure shadow garnet hour glass zoning S S 2 Syntectonic garnet strain cap Syntectonic garnet

contain inclusions: internal foliations The relation between S internal and S external can often help

10 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 S internal and S external can often help to define relative timing of Porphyroclast growth: metamorphic phase Foliation formation: deformation phase(s)

Supplementary Table 1.

Supplementary Table 1. Compositional groups, typical sample numbers and location with their bulk compositional, mineralogical and petrographic characteristics at different metamorphic grades. Metamorphic