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METAMORPHIC ROCKS

Processes, Conditions and

Rocks

Sedimentary rocks may

be buried deep enough

in the Earth that they are

transformed by high

temperatures and

pressures into

metamorphic rocks.

The process to form

metamorphic rocks is

known as

metamorphism.

Metamorphism may be

taken to the point where

the rock melts, forming

magma, and the whole

process starts over.

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Metamorphism and Metamorphic

Rocks

Origins of Metamorphic Rocks • What is Metamorphism?

• Types of Metamorphism

• What Drives Metamorphism?

Classification of Metamorphic Rocks • Texture

• Composition

Interpretation of Metamorphic Rocks • Temperature and Pressure

• Relationship to Plate Tectonics

What is Metamorphism?

“Change of form”

Change from one rock (parent) to another • Change of mineral composition (not overall chemistry

except when fluids added)

• Change of rock texture (generally more compact)

• Without melting (solid state of re-bonding)

Change to a more stable chemical and physical condition for circumstances

Change occurs incrementally, from slight change (low-grade) to dramatic change (high-grade) from the parent rock.

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What is a Metamorphic Rock?

Rock formed by the transformation of

preexisting igneous, sedimentary, or

even other metamorphic rocks by heat

and/or pressure.

Types of Metamorphic Rocks

2 geologic settings for metamorphism:

1. Contact metamorphism - rocks intruded

by hot magma that "bakes" the

surrounding rock.

2. Regional metamorphism –

(dynamothermal metamorphism) large-

scale deformation in rocks that are

subjected to higher pressure and

temperatures from burial and/or during

"mountain building" processes .

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Two “Roof Pendants” – Contact

Metamorphism

Country rock intruded by granite, Sierra Nevada, California

Regional Metamorphism

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What Drives Metamorphism?

Agents of Metamorphism

HEAT

PRESSURE (stress)

CHEMICALLY ACTIVE FLUIDS (H20 and

CO2 )

During metamorphism, rocks are typically treated to all 3

agents at once.

Heat as an Agent of Metamorphism Heat is the most important agent since it provides the energy to drive

the chemical changes that result in recrystallization of minerals in

the rock.

Two primary ways that heat is an metamorphic agent:

1. During contact metamorphism, the intense heat of an intruded

magma may "bake" the adjacent rock.

2. During regional metamorphism, rocks near the surface of the Earth

may be thrust downward and buried where they are subjected to

increased temperatures and stresses.

Geothermal gradient in crust = ~20° - 30°C/km.

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Two “Roof Pendants” – Contact

Metamorphism

Country rock intruded by granite, Sierra Nevada, California

Pressure and Stress as Metamorphic Agents

Pressure increases with depth

~280 bar/km

Two types of pressure/stress:

1. Lithostatic (confining)

pressure - same in all

directions (burial).

2. Directed pressure

(differential stress) -tectonic

forces during mountain

building processes result in

increase pressure in one

direction relative to the

others (collision of two

continents or at a subduction

zone).

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Chemically Active Fluids as a Metamorphic Agent

Circulating Fluids enhance metamorphic

processes - act as a catalyst by aiding in the

migration of ions.

Water (with dissolved ions) is plentiful

because it is contained in the pore spaces of

virtually every rock.

During deep burial, the water is squeezed out

and becomes available for chemical reactions.

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Metamorphic setting

P T Chemically active fluids

Metamorphic setting

P T Chemically active fluids

Contact metamorphism (C)

Low High: increasing toward magma

High – from magma and from heated surface waters

Metamorphic setting

P T Chemically active fluids

Contact metamorphism (C)

Low High: increasing toward magma

High – from magma and from heated surface waters

Regional metamorphism: Deep burial (B)

High: steadily increasing with depth

High: steadily increasing with depth

Low – liberated from hydrous minerals and small amounts in cracks/pores

Metamorphic setting

P T Chemically active fluids

Contact metamorphism (C)

Low High: increasing toward magma

High – from magma and from heated surface waters

Regional metamorphism: Deep burial (B)

High: steadily increasing with depth

High: steadily increasing with depth

Low – liberated from hydrous minerals and small amounts in cracks/pores

Regional metamorphism: Converging continents (R)

High: increasing with depth

Low to medium: increasing with depth

Low – liberated from hydrous minerals and small amounts in cracks/pores

Metamorphic setting

P T Chemically active fluids

Contact metamorphism (C)

Low High: increasing toward magma

High – from magma and from heated surface waters

Regional metamorphism: Deep burial (B)

High: steadily increasing with depth

High: steadily increasing with depth

Low – liberated from hydrous minerals and small amounts in cracks/pores

Regional metamorphism: Converging continents (R)

High: increasing with depth

Low to medium: increasing with depth

Low – liberated from hydrous minerals and small amounts in cracks/pores

Subduction zone metamorphism (S)

High Low: slowly increasing with depth

High – from hydrous minerals in hydrothermally altered ocean crust and water trapped in pores/cracks

Metamorphic

Rocks are named

and classified

according to:

Texture

Foliated

Slightly foliated

Non-foliated

Composition

Metamorphic Grade

Index Minerals

Overall Mineral

Composition

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Foliated Texture

Nonfoliated Texture

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Common Metamorphic Rocks

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Slate

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Phyllite

Garnet Schist and Schist in

Outcrop

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Gneiss

Migmatite

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Marble

Quartzite

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Hornfels

Skarn

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Serpentinite – Metamorphosed basalt or

gabbro requires water

General mineral grade stability

Low Medium High

-------------------------------------------------------------Quartz-----------------------------------------------------------

-------------------------------------------------------------Feldspar---------------------------------------------------------

-------------------------------------------------------------Calcite------------------------------------------------------------

-------------------------------Muscovite----------------------------

----------------------------Biotite------------------------------

----------------------------Garnet-------------------------------------------------

---------------Hornblende-----------------

---------------Pyroxene-----------------

Composition of Metamorphic Rocks depends on the

composition of the parent rock and the metamorphic

grade (temperature and pressure combined).

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Index Minerals and Metamorphic Grade

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Basalt Sill, Glacier National Park, Montana

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Plate Tectonics and Rocks

Divergent Marine Boundary, Passive Margins

Plate Tectonics and Rocks

Transform marine and Divergent marine boundaries

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Convergent Subduction

Plate Tectonics and Rocks

Convergent Collision Continental

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Plate Tectonics and Rocks