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Lecture 8 • Pre-, Syn-, Inter-, and Post-Tectonic Growth • Some More Complex Structures Are: • Snowball Structures Porphyroblasts • Helicitic Structures

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Lecture 8 • Pre-, Syn-, Inter-, and Post-Tectonic Growth • Some More Complex Structures Are: • Snowball Structures Porphyroblasts • Helicitic Structures This lecture Kristallingeologie • Ductile deformation & metamorphic conditions • New minerals grow: porphyroblasts with inclusions • Inclusion trails can be used to define lecture 8 • Pre-, syn-, inter-, and post-tectonic growth • Some more complex structures are: • Snowball structures Porphyroblasts • Helicitic structures Porphyroblasts Idioblastic 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 andalusite P< D chlorite chlorite < D D <P<D 1 garnet D < garnet < D 1 2 P!D staurolite staurolite ! D2 D ! P D !P D ! biotite (1); biotite 1 D2 ! biotite (2) D < P albite D2 < albite • porphyroblast that has a shape controlled by its own crystallography D1 slaty cleavage D2 crenulation Xenoblastic porphyroblasts Poikilitic porphyroblasts feldspar cordierite quartz inclusions • porphyroblast that are full off inclusions • porphyroblast that has a shape that IS NOT controlled by its own are called poikiloblasts or poikilitic porphyroblasts crystallography • 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: 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 • The poikiloblast is one single crystal Example of poikilitic growth of garnet over its own pressure shadow 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 garnet the grain quartz b) other elements have to be taken away from the grain 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 S1 grew between two deformation phases: • Internal foliation: from previous D-phase • External foliation: from next D- phase S1 • Fexternal not related to Finternal S2 • Fexternal and Finternal not continuous • Intertectonic garnet porphyroblast Intertectonic porphyroblast in garnet-mica schist Syntectonic porphyroblasts Q • Syntectonic porphyroblasts P grew during a deformation phase: • Internal foliation: early version of Sexternall Q developing foliation S2 • External foliation: further developed foliation P • Fexternal related to Finternal S • F continue into F Q 1 external internal • Fexternal cut by porphyroblast • Fexternal wraps porphyroblast • Intertectonic staurolite porphyroblast Syntectonic porphyroblast containing PQ-internal fabric (P-domains inclusion-poor) Siint Sext Siint Sext • (Inter- to) syntectonic hornblende porphyroblast • Syntectonic garnet porphyroblast in quartz-albite-mica schist in garnet-kyanite schist 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 S1 • Fexternal cut by porphyroblast • Post-tectonic chloritoid porphyroblast Post-tectonic porphyroblast in slaty cleavage Some complicated structures Rotating porphyroblasts • Rigid objects may rotate when deformation is non-coaxial • Snowball garnets • Rotation and deflection of a foliation can give 75o complex structures: 120o • Deflection folds 180o • Millipede structures • Round inclusions rotate when deformation is non- • Porphyroclasts can grow over complicated coaxial (e.g. simple shear) foliation patterns: • Internal foliation rotates passively with its host • Helicitic structure • Causing spiral-like internal foliation • "Snowball garnets" are an example • Syntectonic Snowball garnet porphyroblast • Syntectonic Snowball garnet porphyroblast in garnet-mica schist in garnet-mica schist 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 Dextral simple shear Sinistral simple shear Rotation of foliation No rotation of foliation Rotation of foliation • Syntectonic garnet porphyroblast, with deflection No rotation of object Rotation of object Rotation of object • Sinistral simple shear one possible interpretation Helicitic structures False inclusion trails Seexxtteerrnaall Siintteerrnaall • The internal foliation pattern in this porphyroblast is not resulting from rotation during growth, but from overgrowing a crenulated cleavage Some examples of porphyrobalsts S2 or S3 S2 S1 • Inter- to early-tectonic garnet porphyroblast in garnet-mica schist S3 S D1 Feldspar S2 S1 formation 1 porphyroblast Biotite growth M1 S crenulation foliation 2 D2 Post-tectonic relative to first deformation phase (D1) • Post-tectonic feldspar porphyroblast in crenulated Early- to syn-tectonic relative to second deformation phase (D2) mica schist quartz S1 biotite muscovite Syntectonic snowball garnet • Post-tectonic muscovite Garnet preferentially overgrows micaceous layer • followed by slight kinking pressure shadow garnet hour glass zoning S 2 S1 S • Syntectonic garnet strain cap • Syntectonic garnet • 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).
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