Part 2. Introduction to Metamorphism

What is Metamorphism and Why Study It?

• metamorphism: refers to changes in rock texture or mineralogy.
  
• metasomatism: means a change in rock composition resulting from diffusion or fluid influx. For example, SiO₂-rich fluids can infiltrate ultramafic rocks, which normally have rather little silica, dramatically changing the bulk composition, and stabilizing a new group of high- SiO₂ minerals.
  
• metamorphic petrology: the study of how almost the entire solid earth works, using chemistry and physics to interpret textures and compositions of minerals; petrologists use theory, experiments, laboratory study, and field study to achieve this goal
  
  • theory is based on chemistry and physics, particularly thermodynamics, kinetics, and material science
    
  • experiments use high-P, high-T equipment, such as laser-heated diamond-anvil cells to mimic Earth's interior
    
  • laboratory investigations include crystal structure and orientation studies by diffraction of x-rays, electrons, and neutrons; texture studies by electron microscopy; and compositional studies using electron microprobe
    
  • field studies involve mapping, field interpretation of structure and petrology, and collection of samples:
    
    
  
  
• metamorphic grade refers qualitatively to the temperature experienced by the rock. The determination of metamorphic grade is made using mineral assemblages, mineral compositions, and/or grain sizes

• neomineralization/neocrystallization: formation of new minerals (e.g., the appearance of garnet in a rock that lacked garnet)
  
• recrystallization: rearrangement of crystal mass or crystal defects (including grain boundaries)
  
  • if accompanied by deformation, recrystallization can produce a crystallographic preferred orientation, lineation, foliation, and porphyroclasts
    
  • if no phases appear or disappear during the recrystallization, only grain growth is occurring; this coarsening is driven by the free energy of grain boundaries, which is the energy resulting from two crystals being in contact along an imperfect boundary
  View 2D static annealing cartoon
  View 3D static annealing
  View 2D static annealing octachloropropane
  View post-deformational annealing
  View annealing of subgrains
  View annealing of subgrains II
• porphyroblast: a grain that has grown larger than others (typical examples are garnet, staurolite, chloritoid, andalusite). The following image shows chloritoid (a hydrous Mg-Fe-Al silicate typical of low-grade pelites)--notice the characteristic i) tabular shape, ii) sector zoning, and iii) growth in a fine-grained (thus low-grade) rock:
  

  These are albite porphyroblasts in a Norwegian gneiss:
  

  Garnet porphyroblasts are also common:
  

  and can rarely (such as this garnet from the Tobacco Root Mountains of Montana) grow impressively large:
  

• poikiloblast: a grain with inclusions of other minerals--also called 'sieve' texture (typical examples are garnet, staurolite, cordierite, andalusite). The first image shows staurolite (a hydrous Mg-Fe-Al silicate typical of medium-pressure metapelites)--notice the characteristic yellow color. The second image shows cordierite (a hydrous Mg-Fe-Al silicate typical of low-pressure metapelites), with its abundant inclusions, low birefringence, and polysynthetic twinning.
  
• porphyroclast: a relict grain that is larger than others surviving in a deformed rock. The key difference between porphyroblast and porphyroclast is that porphyroblasts are bigger than surrounding grains because of growth, whereas porphyroclasts are bigger because surrounding grains have undergone grainsize reduction during deformation. The following images show K-feldspar grains:
  

  

  

  

  Garnet can also form porphyroclasts:
  

  Sigma clasts can be used to determine sense of shear. This sigma clast from Norway shows sinistral (left-lateral) shear:
  

  This is one of the most unusual sigma clasts known (it is an enormous pyrite grain):
  

  View the formation of a sigma clast
  View the formation of a delta clast
  View the formation of a delta clast using real images

• limestone —> marble (notice the characteristic high birefringence and twinning)
  
• quartz arenite —> quartzite (notice the characteristic low birefringence)
  

  An outcrop view of a metaquartzite:
  

  An outcrop view of a metaconglomerate:
  

• ultramafic rock —> serpentinite:
  
• shale —> slate —> phyllite —> schist —> gneiss —> melt. Shale is a sedimentary term. 
• Slate refers to a fine-grained, low-grade metasedimentary rock in which bedding is typically still visible:
  
• Schist refers to a medium- to high-grade, medium- to coarse-grained metamorphic rock of any composition that has a fabric. For example,
  
• Gneiss refers to a high-grade, coarse-grained metamorphic rock of any composition that has a fabric:
  

  paragneiss: a metasedimentary gneiss.
  orthogneiss: a meta-igneous gneiss. This is a metamorphosed and deformed Rapikivi granite from Norway:
  

  Complex and blobby plutonic complexes can become extremely layered as a result of deformation:
  

• lineation: a linear fabric; typically defined by either linear minerals, such as hornblende, or stretched grains, such as quartz. Rocks with purely linear fabrics are called L tectonites and form by a constrictional deformation (prolate strain ellipsoid). Here is a mylonitic granite from Norway, with clearly stretched K-feldspar grains in a fine-grained matrix of quartz, plagioclase, and biotite:
  

  same rock up close:
  

• foliation: a planar fabric; typically defined by platy minerals such as mica or flattened grains such as quartz. This is a more general term that encompasses cleavage and schistosity. Rocks with purely planar fabrics are called S tectonites and form by a flattening deformation (oblate strain ellipsoid). Most metamorphic rocks are L-S tectonites that form from a more general strain history.
• cleavage: a foliation formed by platy minerals such as mica. Most commonly used to describe low-grade micaceous (pelitic) rocks such as slate. The cleavage can be very planar, like in this slate from the Sierra Nevada foothills:
  

  or it can be wavy, like in this Tibetan rock:
  

  In favorable circumstances one can determine sense of shear, as in this \ock:
  

  In thin section, the micas that form foliations and cleavages are clearly visible:
  

  crenulation cleavage: a special kind of cleavage that forms as a result of shortening at a low angle to a pre-existing cleavage:
  

• schistosity: a foliation that is lumpier than cleavage; typically formed by a combination of platy minerals and more equant phases such as feldspar and quartz. Most commonly used to describe medium-grade rocks such as schists of nearly any bulk composition.
  
• gneissic: foliation of equant minerals such as feldspar and quartz. Most commonly used to describe high-grade metaplutonic rocks such as gneisses.
  

  A Rapikivi granite being turned into a gneiss (Norway).
  

• pseudomorph: replacement of one phase by another, generally preserving shape. This Norwegian rock shows chlorite pseudomorphs after garnet:
  
• idioblastic: sharp crystal form (euhedral in igneous terminology). The mineral shown is sphene (CaTiSiO₅), with its characteristic high relief and extreme birefringence.
  
• hypidioblastic: intermediate form (subhedral in igneous terminology). The mineral shown is garnet ((Mg,Fe,Ca,Mn)₃Al₂Si₃O₁₂), with its characteristic high relief and optical isotropy.
  
• xenoblastic: no clear crystal form (anhedral in igneous terminology). The image on the left shows calcite (CaCO₃), with high birefringence and twinning, and the image on the right is of plagioclase ((NaAlSi₃O₈-CaAlSi₂O₈), with low birefringence and twinning.