Metamorphism complete notes

Metamorphism

• Changes in Rock Composition or Texture
• Due to Heat, Pressure and Action of Fluids

Where Does the Heat Come from?

• Uranium and Thorium to Other Elements + Lead + Radiation
• Potassium-40 to Calcium-40 or Argon-40 + Radiation

Where Does the Pressure Come from?

• Air Pressure = 14 P.s.i. (1 Atmosphere or 1 Bar = 100,000 pascals)
• Pressure Beneath 10 Meters (33 Ft.) Of Water = 1 Atm. = 1 Bar.
• Same Pressure Beneath 3.5 M (10 Ft.) Of Rock
• Pressure in Deepest Part of Ocean = 1000 Bar.
• Pressure under One Mile of Rock = 500 Bar.

1000 Bars (2 Mi. or 3 km Of Rock) = 1 Kilobar (Kb.)

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Metamorphic Rocks Form Many Km below the Surface
How Do They Get to the Surface?

• Tectonic Uplift & Erosion
• Isostasy & Erosion

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Types of Metamorphism

Contact

• Around Intrusions
• Shallow: 0-6 Km
• Low Pressure
• Local

Regional

• Wide Areas
• 5-20 Km, Sometimes 30+
• High Pressure
• Usually Accompained by Deformation

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What Happens During Metamorphism

Minerals React to Form New Minerals

• 2SiO2 + CaMg(CO3)2 == CaMgSi2O6 + 2CO2
• Quartz + Dolomite == Pyroxene

Minerals Change Form

• Al2SiO5 == Al2SiO5
• Andalusite == Kyanite

New Materials Are Added (Metasomatism)

• CaMg(SiO3)2 + 2CO2 == CaMg(CO3)2 + 2SiO2
• Pyroxene + CO2 == Dolomite + Quartz
• Minerals in Solution == Ore Bodies

Recrystallization

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Why Don't Rocks "De-metamorphose"?

• Reactions Can't Reverse Because Ingredients Lost
  • 2AlSi2O5(OH) == Al2SiO5 + 3SiO2 + H2O
  • Clay Mineral == Andalusite + Quartz + Water (Lost)
• An example of carbonate metamorphism:
  • CaMg(CO3)2 + 2SiO2 == CaMgSi2O6 +2CO2
  • Dolomite + Quartz == Pyroxene + CO2 (Lost)
• Reactions "Freeze"
• Sometimes it Does Happen
  • Retrograde Metamorphism

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Grade - Degree to Which the Rock Has Changed Composition

• Can Often See Original Bedding
• Can Sometimes Even See Deformed Fossils
• At High Grades, Rocks Can Often Lose All Trace of Their Original Appearance

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Major Metamorphic Rock Types

Temp C	Temp F	Coal	Limestone	Sandstone	Basalt	Shale	Index Minerals
		Lignite Bituminous
	500	Anthracite
300	600	Graphite	Marble			Slate	Chlorite
	700			Quartzite
	800				Greenstone	Phyllite	Biotite
500	900					Schist	Garnet
	1000				Amphibolite		Staurolite
600	1100					Gneiss	Kyanite
	1200						Sillimanite
700							Melting Begins

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Polymorphism

• Al2SiO5
  • Kyanite
  • Sillimanite
  • Andalusite
• Ice - 6 high pressure forms
• Diamond - Graphite
• Calcite - Aragonite
• Quartz -
  • - Tridymite - Cristobalite (increasing temperature)
  • - Coesite - Stishovite (increasing pressure)

Metamorphic Facies

Not all changes in rocks are metamorphism:

• Surface temperature and pressure --- Weathering
• Sedimentary rocks, surface to 250 C --- Diagenesis

Depth\Temp	300 C	400 C	500 C	600 C	700 C	800 C
5 km	Zeolite	Contact Metamorphism - Andalusite forms
10 km - 3 kb		Greenschist Chlorite, Biotite form Slate Greenstone Quartzite Marble	Amphibolite Garnet, Staurolite, Kyanite form Schist Amphibolite Quartzite Marble Gneiss		Granulite Sillimanite forms Muscovite breaks down to K-feldspar Partial Melting Gneiss
15 km	Blueschist
20 km - 6 kb
25 km
30 km - 9 kb
35 km
40 km - 12 kb			Eclogite (Mantle)