7.1 Metamorphism

Metamorphic rocks

Rocks that have undergone metamorphism

Metamorphism

the change undergone by an existing rock (ign, sed, or met) in the solid state, to another rock

Solid

During metamorphism, the rock must remain essentially in this state

• Contact or thermal metamorphism

• Hydrothermal metamorphism

• Regional metamorphism

Metamorphic settings

Contact or thermal metamorphism

driven by a rise in temperature within the host rock

Hydrothermal metamorphism

chemical alterations from hot, ion-rich water

Regional metamorphism

occurs during mountain building

Regional metamorphism

What type of metamorphic settings produces the greatest volume of metamorphic rocks

Prograde metamorphism

results from increasing temperature and/or pressure conditions over time

Retrograde metamorphism

results from decreasing temperature and/or pressure so that lower temperature/pressure mineral assemblages develop that overprint earlier peak temperature/pressure mineral assemblages

Volatiles

they serve as catalysts in driving retrograde metamorphic reactions as without them, retrograde metamorphic conditions are difficult to attain because previously occurring prograde metamorphism has already depleted the rock in volatile components

Heat

The most important agent of metamorphism

• Heat from magma

• Geothermal gradient

2 sources of heat

Pressure

An agent of metamorphism, it increases with depth

• Confining pressure

• Differential stress

2 types of pressure

Confining pressure

it applies forces equally in all directions

Differential stress

Rocks may also be subjected in this type of pressure which is unequal in different directions

Chemically active fluids

Agent of metamorphism, mainly water with other volatile components. They enhance migration of ions, and aid in recrystallization of existing minerals

• Water trapped in pore spaces of the original rock

• Water released during dehydration of minerals, such as amphibole or mica

• Water from magmatic bodies (hydrothermal fluids)

Sources of fluids

Protolith

the parent rock of metamorphic rocks, the original, unmetamorphosed rock from which a given metamorphic rock is formed

• Foliated

• Non-foliated

2 main classification of metamorphic rocks

Shale, mudstone

Protolith of Slate

Shale, slate, basalt, or granite

Protolith of Schist

Shale, schist, granite, sandstone, and other rock types

Protolith of Gneiss

Sandstone

Protolith of Quartzite

Limestone, dolomite

Protolith of Marble

Bituminous coal

Protolith of Anthracite coal

• Contact or Thermal metamorphism

• Hydrothermal metamorphism

• Regional metamorphism

• Burial metamorphism

• Metamorphism along Fault Zones

• Impact metamorphism

Types of metamorphism

Contact or thermal metamorphism

Occurs due to a rise in temperature when magma invades a host rock

Aureole

It is a zone of alteration that forms in the rock surrounding the magma during contact metamorphism

0-60 km

Contact metamorphism takes place in low pressure and at these depths

Non-foliated; fine-grained

type of metamorphic rocks produced by contact metamorphism

Hydrothermal metamorphism

Chemical alteration at high temperatures and moderate pressures by hot-ion rich fluids that circulate through fissures and cracks. Common in basaltic rocks

Mid-ocean ridge system

This is where hydrothermal metamorphism commonly occur

Regional metamorphism

Takes place at considerable depths over an extensive area under high pressure and is associated with the process of mountain building

5-20 km sometimes more than 30 km

The depth where regional metamorphism takes place

Burial metamorphism

Associated with very thick sedimentary strata. Required depth varies from one location to another depending on the prevailing geothermal gradient.

Metamorphism along Fault Zones

Occurs at depth and high temperatures. Pre-existing minerals deform by ductile flow. “Slickensides”

Shock or Impact Metamorphism

Occurs when high speed projectiles called meteorites strike Earth’s surface

impactites

Products of impact metamorphism