Metamorphic Rocks

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Metamorphic Rocks

• Part 2

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OBJECTIVES

• Metamorphic Grade or Facies
• Metamorphic Rock Examples

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METAMORPHIC GRADEFACES
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Metamorphic grade or Facies

• A group of minerals that form in a particular P-T
  environment
• Can be used as Index Minerals to deduce T-P
  conditions of formation

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Note Temperature gradient
Index Minerals in metamorphic rocks
580oC
220oC
460oC
690oC
Note Quartz and Feldspar are not index minerals
Why?
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Metamorphic Environments in Subduction Zones
We can look at minerals in Metamorphic Rocks
and determine where they formed.
Water facilitates metamorphic reactions by
allowing movement of atoms and ions
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Metamorphic Facies
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COMMON METAMORPHIC ROCK EXAMPLES
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Metamorphic Rocks

• Nonfoliated
• Quartzite
• Marble
• Foliated
• Slate
• Phyllite
• Schist
• Gneiss
• Migmatites

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NONFOLLIATEDGranoblastic
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Nonfoliated rocks

• Quartzite
• Formed from a parent rock of quartz-rich
  sandstone
• Quartz grains are fused together
• Forms in intermediate T, P conditions

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Field Geologists are grateful for quartzites.
They dont foliate, so you can see the folds.
Mudrocks foliate much harder to map.
Sample of quartzite
Thin section of quartzite
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Nonfoliated rocks

• Marble
• Coarse, crystalline
• Parent rock usually limestone
• Composed of calcite crystals
• Fabric can be random or oriented

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Marble (nonfoliated)
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FOLLIATED
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Common metamorphic rocks

• Foliated rocks
• Type formed depends on Metamorphic Grade
• Grade Amount of heat and pressure
• High Grade High heat and pressure
• Grade depends on depth

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Mudstones are sediments, can be squashed by
burial and/or in continent-continent collisions
Change in metamorphic grade with depth
Increasing Directed Pressure and increasing Temps
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Foliated rocks

• Slate
• Very fine-grained
• Excellent rock cleavage, often perp. to original
• Made by low-grade metamorphism of shale

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Example of slate
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Foliated rocks

• Phyllite
• Grade of metamorphism between slate and schist
• Made of small platy minerals
• Glossy sheen with rock cleavage
• Composed mainly of muscovite and/or chlorite

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Phyllite (l) and Slate (r) lack visible
mineral grains
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Foliated rocks

• Schist
• Medium- to coarse-grained
• Comprised of platy minerals (micas)
• The term schist describes the texture
• To indicate composition, mineral names are used
  (such as mica schist)

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A mica garnet schist
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Foliated rocks

• Gneiss
• Medium- to coarse-grained
• Banded appearance
• High-grade metamorphism
• Composed of light-colored feldspar layers with
  bands of dark mafic minerals

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Gneiss displays bands of light and dark minerals
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Flattened Pebble Conglomerate flattening
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Foliated rocks

• Migmatites
• Minerals with the lowest melting points (Qtz,
  Feldspar) melt then recrystallize,
• Creates separate bands of Metamorphic and Igneous
  rock.

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Migmatites
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THE END