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METAMORPHISM AND METAMORPHIC ROCKS

 

What is Metamorphism?

 

Metamorphism: Metamorphism is the transformation of a parent (pre-existing) rock into a texturally and mineralogically new rock due to changing temperature and pressure conditions.

 

The parent rock may be igneous, sedimentary or another metamorphic rock.

 

The changes take place in the solid-state (without melting) in Earth’s middle to lower crust.

 

Factors Controlling the Textural and Mineralogical

Characteristics of Metamorphic Rocks

 

1) Composition of the parent rock

    -Usually no new material (other than water) is added to rock during metamorphism.

    -Therefore, the resulting metamorphic rock will be compositionally similar to the parent rock.

 

2) Temperature

    -Heat for metamorphism comes from Earth’s deep interior.

    -The thermal gradient is 30°C per km.

    -A given mineral can only exist over a finite temperature range called the stability field.

    -If a mineral is outside of its stability field, it will break down into one or more new minerals.

    -High temperatures favor lower density minerals.

    -If the temperature gets high enough, partial melting will occur (bordering on igneous processes).

 

3) Confining pressure

    -Confining pressure is pressure that is applied equally in all directions.

    -Pressure increases as depth increases.  The pressure gradient is 1 kilobar/3.3 km of burial within the crust.

    -High pressures favor higher density minerals.

 

4) Differential stress

    -Tectonic forces often lead to forces that are not equal in all directions (called differential stress).

    -There are two types of differential stress: compressive stress and shear stress.

    -Compressive stress causes flattening perpendicular to stress.

    -Shear stress causes flattening by sliding parallel to stress.

    -Differential stresses cause rocks to develop foliation, a planar rock texture formed by the alignment of platy minerals.

 

5) Time

    -Metamorphism deep in the crust (at high temperatures and pressures) may take millions of years.

    -A time increases, new (more stable minerals) tend to grow larger and better foliations tend to develop.

 

6) Intergranular Fluids

    -Rising temperature causes water vapor to be released from unstable minerals during dehydration reactions.

    -Hot water facilitates chemical reactions by acting as a rapid transport agent for mobile ions.

    -Metamorphism enhanced by fluids is called metasomatism.

 

Classification of Metamorphic Rocks

 

Parent Rock

Metamorphic Rock

Shale, Mudstone, Volcanic Tuff

Slate, Phyllite, Schist

Shale or Granite

Gneiss

Basalt

Chlorite schist (greenschist), Amphibole Schist (amphibolite)

Limestone or Dolomite

Marble

Quartz Sandstone

Quartzite

Conglomerate or Breccia

Stretched-pebble conglomerate

Shale, Basalt

Hornfels

 

 

Non-foliated (non-layered) metamorphic rocks are named based on composition.

 

Limestone (with a simple mineralogy of calcite) Marble

Marble: medium to coarse-grained, with interlocking grains of calcite or dolomite; sugary appearance.

© Dr. Richard Busch

 

Sandstone (with a simple mineralogy of quartz) Quartzite

Quartzite: medium to coarse-grained, with interlocking grains of quartz; sugary appearance.  © Dr. Richard Busch

 

Shale or Basalt (with a relatively complex mineralogy) Hornfels (fine-grained and dark-colored (not very distinctive).

 

Foliated (layered) metamorphic rocks are named based on type of foliation.

 

Granite Gneiss

Gneiss: coarse-grained, with light and dark minerals that are segregated into different layers. © Marli Miller, University of Oregon.

 

Shale or Mudstone (with a complex mineralogy) → Slate Phyllite Schist

Slate: very fine-grained, splits into flat sheets (rock cleavage). © Bruce Molnia

 

Phyllite: fine-grained, generally splits along wavy surfaces; has a silky sheen. © Marli Miller, University of Oregon.

 

Schist: relatively coarse-grained, with some very coarse grains (called porphyroblasts). Copyright © StoneTrust, Inc.

 

Metamorphic Grade

 

A SPECIAL CASE OF METAMORPHISM: SHALE & MUDSTONE

GRADE

TEMPERATURE

METAMORPHIC ROCK

High

> 450 C

Gneiss

Medium

375-450 C

Schist

300-375 C

Phyllite

Low

50 – 300 C

Slate

Parent Shale or Mudstone

 

Types of Metamorphism

 

A. Contact metamorphism

    -Occurs when a hot magma intrudes into cooler country rock.  The increased temperature causes the country rock to “bake”.

    -There is no tectonic stress.  Therefore the rocks are non-foliated rocks like hornfels, quartzite, and marble.

    -Occurs in narrow zone (~1-100 m wide) known as a contact aureole.

    -Rocks may be fine-grained (hornfels) or coarse-grained (marble & quartzite).

 

B. Regional metamorphism

    -Associated with convergent plate boundaries and mountain building.

    -The differential stress results in rocks with foliated textures

    -May occur over high pressures and a wide range of temperatures (300-1000°C).

    -Higher pressure and temperature will produce rocks of increased metamorphic grade.

 

C. Shock metamorphism (rare)

    -Meteorite impacts trigger shock metamorphism (rapid application of extreme pressure and heat).

    -Brecciated rocks are found around and beneath impact craters.

    -Increased pressure converts minerals to higher density forms (e.g. quartz → coesite).

 

D. Partial melting during high temperature metamorphism produces migmatites.

    -Transitional rocks exhibit both intrusive igneous and foliated metamorphic textures.

 

Plate Tectonics and Metamorphism

 

Regional metamorphism and contact metamorphism are associated with convergent plate boundaries.

 

Continent-continent collisions: As continents collide, high temperatures and pressures cause high grade metamorphism.

 

Metamorphic rocks are common in the old, stabilized cores of continents known as cratons…indicative of suture zones (old convergent boundaries).  These are the oldest rocks on Earth (Vishnu Schist in the Grand Canyon is 2 byo).

 

Continent-ocean collisions: As rocks are pushed down into the earth at subduction zones, pressure increases but temperature stays low due to the cooling effects of the cooler subducted slab and associated water.

 

Why do geologists study metamorphic rocks?

 

The minerals tell us the peak temperatures and peak pressures that rocks experienced…we can reconstruct the environment of metamorphism.

 

The ultimate goal is to determine a pressure-temperature-time path (P-T-t path).

 

 

Metamorphic Grade in More Detail (Time-Permitting)

 

REGIONAL METAMORPHIC GRADE

T

P

Name

Low

Low

Zeolite

Moderate – High

Low

Prehnite

Low

High

Blueschist

Moderate to High

Moderate

Greenschist-Amphibolite-Granulite

Moderate – High

High

Eclogite