8.1 Metamorphism and Metamorphic Rocks Notes

Definition: Metamorphic rocks undergo changes in mineralogy, texture, and/or chemical composition due to changes in temperature (generally > 200 °C) and pressure.

Many metamorphic reactions occur in the presence of fluids.
Original rock types that may be transformed include:
- Igneous
- Sedimentary
- Another metamorphic rock

Key Agents:
- Heat: Most important agent for metamorphism, two primary sources include:
- Magma
- Geothermal gradient (increasing temperature with depth)
- Pressure: Increases with depth, characterized by two types:
- Confining Pressure: Stress is applied equally in all directions.
- Differential Stress: Stress applied unequally, predominating in one direction.
- Chemically Active Fluids:
- Primarily water, including other volatile components.
- Enhances migration of ions in the crystal structure, aiding recrystallization.
- Significant in transporting mineral matter across distances.
- Metasomatism: Substantial chemical changes due to reactions with active fluids.

Localized Sources:
- Pore spaces in sedimentary rocks.
- Fractures in igneous rocks.
- Hydrated minerals (e.g., clays, micas) releasing water under high temperatures and pressures.
Regional Sources:
- Hydrothermal activity in geysers, hot springs, and percolating seawater along mid-ocean ridges.

Most metamorphic rocks retain the overall chemical composition of their parent rock.
The original mineral makeup largely determines the extent of change caused by metamorphic agents.

Occurs at high temperatures, typically around magmatic intrusions.
Affects a small area, producing a zone of alteration called a metamorphic aureole.

Happens along mid-ocean ridges where heated seawater interacts with basalt.
Can also occur on continents with hot fluids from igneous intrusions.
Some economically significant vein deposits are formed, such as gold, silver, and copper.

A milder form where sedimentary rocks become buried deeper; diagenesis transitions into burial metamorphism.
Involves partial changes in mineralogy and texture while preserving original bedding structures.

Occurs under extensive high temperatures and pressures, resulting in significant alteration of rock mineralogy and texture, typically involving tectonic forces.

Results from physical crushing and shearing during tectonic movement, often localized along fault planes.
- Types dependent on depth:
- Shallow Depths: Creates fault breccia.
- Greater Depths: Results in highly deformed mylonites.

Caused by high-speed projectiles (e.g., meteorites) impacting Earth's surface.
Results in pulverization, shattering, and melting of rocks, creating impact craters and products known as impactites.

Describes groupings of rocks formed under various temperatures and pressures (P-T space).
Naming Convention: Facies typically named after characteristic minerals formed during metamorphism, particularly of basalt.

Zeolite Facies:
- P-T Range: < 4 kb, < 250 °C
- Very low grade. Metamorphism forms zeolite minerals; linked to burial and hydrothermal metamorphism.
Greenschist Facies:
- P-T Range: 2 - 9 kb, 250 - 450 °C
- Low grade. Forms green minerals like chlorite and epidote.
Amphibolite Facies:
- P-T Range: 2 - 9 kb, 450 - 700 °C
- Medium grade. Produces amphibole minerals like hornblende.
Granulite Facies:
- P-T Range: 4 - 10 kb, > 700 °C
- High grade. Produces pyroxene granulites with Ca-plagioclase.
Blueschist Facies:
- Very high pressures, relatively low temperatures.
- Typically found in subduction zones; produces blue amphiboles (e.g., glaucophane).
Eclogite Facies:
- Extremely high pressures (> 10 kb) and moderate-high temperatures.
- Occurs deep in subduction zones; metamorphoses basalt into eclogite rich in garnet and pyroxene.
Hornfel Facies:
- Relatively low pressures, wide temperature range.
- Associated with contact metamorphism; often formed from shale due to adjacent magmatic intrusion.