Metamorphism

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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

• 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

• 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

Progressive metamorphism of a shale

Phyllite

Schist

Gneiss

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

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

• 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 tectonics

• Metamorphism at subduction zones• Cores of subduction zones contain linear

belts of metamorphic 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

• Index minerals and metamorphic grade

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

Index Minerals in metamorphic rocks

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)

Metamorphic Environments in Subduction Zones

Greenschist Hand Sample

Greenschist Thin Section

MicaSchist

Blueschist Amphibolite

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 ofquartzite

Thin sectionof quartzite

Flattening of quartz grains in quartzite

• Nonfoliated rocks• Marble

– Coarse, crystalline

– Parent rock usually limestone

– Composed of calcite crystals

– Fabric can be random or oriented

Marble (Random fabric = annealing; nonfoliated)

Change in metamorphic grade with depth

• Foliated rocks• Slate

– Very fine-grained

– Excellent rock cleavage

– Made by low-grade metamorphism of shale

Example of slate

Slate roof

• 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

• 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)

Mica Schist - note well developed foliation

A mica garnet schist

• Foliated rocks• Gneiss

– Medium- to coarse-grained

– Banded appearance

– High-grade metamorphism

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

Gneiss displays bands of light and dark minerals

Diorite to Gneiss Morph(orthogneiss - from igneous protolith)

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

Outcrop of foliated gneiss

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

Flattened Pebble Conglomerate = flattening

Development of foliation due to directed pressure

Metamorphism

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