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Metamorphism. The transformation of rock by temperature and pressure Metamorphic rocks are produced by transformation of: Igneous, sedimentary and igneous rxs. Thanks to CU Boulder Geology Dept for use of some of these slides. Metamorphism.

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Metamorphism
Metamorphism

  • The transformation of rock by temperature and pressure

  • Metamorphic rocks are produced by transformation of:

    • Igneous, sedimentary and igneous rxs

Thanks to CU Boulder Geology Dept for use of some of these slides


Metamorphism1
Metamorphism

  • Metamorphism progresses from low to high grades

  • Rocks remain solid during metamorphism


What causes metamorphism?

  • Heat

    • Most important agent

    • Heat drives recrystallization - creates new, stable minerals

  • Pressure (stress)

    • Increases with depth

    • Pressure can be applied equally in all directions or differentially


Origin of pressure in metamorphism

Confining or hydrostatic pressure: equal in all directions

Directed pressure: largely in one direction or along a particular axis


Main factor affecting metamorphism
Main factor affecting metamorphism

  • Parent rock

    • Metamorphic rocks typically have the same chemical composition as the rock they were formed from

    • Different minerals, but made of the same stuff.

    • Exception: gases (carbon dioxide, CO2) and water (H2O) may be released







Metamorphism2
Metamorphism

  • Three types of metamorphic settings:

    • Contact metamorphism– from a rise in temperature within host rock

    • Hydrothermal metamorphism– chemical alterations from hot, ion-rich water

    • Regional metamorphism -- Occurs in the cores of mountain belts and makes great volumes of metamorphic rock


Contact metamorphism
Contact metamorphism

Produced mostly by local heat source


Hydrothermal metamorphism

Requires hot circulation of fluids


Regional metamorphismin Mountain Belts

Requires crustal thickening and shortening


Metamorphism and plate tectonics
Metamorphism and plate tectonics

  • Most regional metamorphism occurs along convergent plate boundaries

    • Compressional stresses deform plate edge

    • Occurs in major mountain belts: Alps, Himalayas, and Appalachians

    • Not necessarily in all mountains, depends on orogenic forces, i.e., uplift versus compression


Metamorphism and plate tectonics1
Metamorphism and plate tectonics

  • Metamorphism at subduction zones

    • Cores of subduction zones contain linear belts ofmetamorphic rocks

      • High-P, low-T zones near trench

      • High-T, low-P zones in region of igneous activity (arc)


Location of metamorphic zones

in a subduction zone


Metamorphic environments
Metamorphic Environments

  • Index minerals and metamorphic grade

    • Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form



Metamorphic environments1
Metamorphic Environments

  • Metamorphic grade

    • A group of minerals that form in a particular P-T environment

Zeolite (really low T,P; <200C)

Greenschist (low T, P; 200-450C, 10-15 km)

Blueschist (low T, high P - subduction zones)

Amphibolite (high T, P; 450-650C, 15-20 km)

Granulite (super high T, P; >700C, >25km)



Greenschist Hand Sample

Greenschist Thin Section


Mica

Schist


Blueschist

Amphibolite


Common metamorphic rocks
Common metamorphic rocks

  • Nonfoliated rocks

    • Quartzite

      • Formed from a parent rock of quartz-rich sandstone

      • Quartz grains are fused together

      • Forms in intermediate T, P conditions


Sample of

quartzite

Thin section

of quartzite



Common metamorphic rocks1
Common metamorphic rocks

  • Nonfoliated rocks

    • Marble

      • Coarse, crystalline

      • Parent rock usually limestone

      • Composed of calcite crystals

      • Fabric can be random or oriented




Common metamorphic rocks2
Common metamorphic rocks

  • Foliated rocks

    • Slate

      • Very fine-grained

      • Excellent rock cleavage

      • Made by low-grade metamorphism of shale




Common metamorphic rocks3
Common metamorphic rocks

  • 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


Phyllite left and slate right lack visible mineral grains
Phyllite (left) and Slate (right) lack visible mineral grains


Common metamorphic rocks4
Common metamorphic rocks

  • 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)




Common metamorphic rocks5
Common metamorphic rocks

  • Foliated rocks

    • Gneiss

      • Medium- to coarse-grained

      • Banded appearance

      • High-grade metamorphism

      • Composed of light-colored feldspar layers with bands of dark mafic minerals



Diorite to Gneiss Morph

(orthogneiss - from igneous protolith)


What are metamorphic textures
What are metamorphic textures?

  • Texture refers to the size, shape, and arrangement of mineral grains within a rock

  • Foliation – planar arrangement of mineral grains within a rock



Metamorphic textures
Metamorphic textures

  • Foliation

    • Foliation can form in various ways:

      • Rotation of platy or elongated minerals

      • Recrystallization of minerals in a preferred orientation

      • Changing the shape of equidimensional grains into elongated and aligned shapes



Development of foliation due to directed pressure
Development of foliation due to directed pressure


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