Chapter 8: Major Elements

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Metamorphic Rocks
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Metamorphic Rocks
Metamorphism The transition of one rock into
another in the solid state under conditions
unlike those under which it formed Metamorphic
rocks are produced from Igneous
rocks Sedimentary rocks Other metamorphic rocks
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Agents of Metamorphism

• Heat- The most important agent
• Recrystallization results in new, stable minerals
• Two sources of heat
• Contact metamorphism heat from magma
• Regional Metamorphism - increase in temperature
  with depth due to the geothermal gradient

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Agents of Metamorphism

• Pressure
• Increases with depth
• Confining pressure applies forces equally in all
  directions
• Rocks may also be subjected to differential
  stress which is unequal in different directions
  and causes deformation

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Agents of Metamorphism

• Pressure

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Types of Metamorphism
Contact metamorphism
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Types of Metamorphism
Regional metamorphism
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Types of Metamorphism
Regional metamorphism
More Uplift
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Types of Metamorphism
Regional metamorphism
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Metamorphic Textures

• Foliation - any planar arrangement of features
  within a rock
• Foliation can form in various ways
• Rotation of platy and/or elongated minerals
• Recrystallization of minerals in the direction of
  preferred orientation
• Changing the shape of equidimensional grains into
  elongated shapes that are aligned

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Metamorphic Textures
Foliation can form in various ways
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Metamorphic Textures

• Foliated textures
• Slaty cleavage
• Closely spaced planar surfaces along which rocks
  split

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Metamorphic Textures

• Foliated textures
• Schistosity
• Platy minerals are discernible with the unaided
  eye and exhibit a planar or layered structure
• Rocks having this texture are referred to as
  schist

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Metamorphic Textures

• Foliated textures
• Gneissosity
• During higher grades of metamorphism, ion
  migration results in the segregation of minerals
  into layers
• Gneissic rocks exhibit a distinctive banded
  appearance

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Metamorphic Textures

• Metamorphic rocks that lack foliation are
  referred to as non-foliated
• Develop in environments where deformation is
  minimal
• And/or composed of minerals that exhibit
  equidimensional crystals
• General name is granofels

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Metamorphic Rocks
Foliated Metamorphic Rocks
Slate compact, very fine-grained, metamorphic
rock with a well-developed cleavage. Freshly
cleaved surfaces are dull
Phyllite a rock with a schistosity in which very
fine phyllosilicates (sericite/phengite and/or
chlorite), although rarely coarse enough to see
unaided, impart a silky sheen to the foliation
surface. Phyllites with both a foliation and
lineation are very common.
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Metamorphic Rocks
Foliated Metamorphic Rocks
Schist a metamorphic rock exhibiting a
schistosity. By this definition schist is a broad
term, and slates and phyllites are also types of
schists. In common usage, schists are restricted
to those metamorphic rocks in which the foliated
minerals are coarse enough to see easily in hand
specimen.
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Metamorphic Rocks
Foliated Metamorphic Rocks
Gneiss a metamorphic rock displaying gneissose
structure. Gneisses are typically layered (also
called banded), generally with alternating felsic
and darker mineral layers. Gneisses may also be
lineated, but must also show segregations of
felsic-mineral-rich and dark-mineral-rich
concentrations.
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Metamorphic Rocks
Specific Metamorphic Rock Types
Marble a metamorphic rock composed predominantly
of calcite or dolomite. The protolith is
typically limestone or dolostone.
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Metamorphic Rocks
Specific Metamorphic Rock Types
Quartzite a metamorphic rock composed
predominantly of quartz. The protolith is
typically sandstone.
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Metamorphic Rocks
Greenschist/Greenstone a low-grade metamorphic
rock that typically contains chlorite,
actinolite, epidote, and plagioclase. Such a rock
is called greenschist if foliated, and greenstone
if not. The parent is either a mafic igneous rock
or graywacke.
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Metamorphic Rocks
Specific Metamorphic Rock Types
Amphibolite a metamorphic rock dominated by
hornblende plagioclase. Amphibolites may be
foliated or non-foliated. The parent is either a
mafic igneous rock or graywacke.