Metamorphic Rocks: Formation and Processes

Metamorphic Rocks Overview

• Metamorphic rocks are rocks changed by heat, pressure, and/or fluids.
• Metamorphism is caused by plate tectonic processes.
• Studying metamorphic rocks provides insights into past tectonic activity.
• Metamorphic rocks can form from igneous, sedimentary, or other metamorphic rocks.
• The term meta means change, and morphos means form.

Metamorphic Processes

• Metamorphism alters a rock's composition and/or texture in the solid state.
• Metamorphism differs from igneous processes (which involve melting) and weathering/sedimentary processes.
• The original rock undergoing metamorphism is called the parent rock or protolith.
• Temperature changes, confining pressure, directed stress, and chemically reactive fluids can cause metamorphism.

Temperature (Heat)

• Temperature measures the average kinetic energy of particles.
• Increased heat/temperature can allow atoms to move within crystals without melting the rock.
• Heat flow and temperature changes affect mineral equilibrium and cation balance.
• Metamorphism starts at 200°C and can continue to 700°C-1,100°C.
• Temperatures above this range may cause melting, transitioning to igneous processes.

Pressure

• Pressure is the force exerted over an area.
• Pressure can influence the chemical equilibrium of minerals.
• Confining pressure and directed stress affect metamorphic rocks.

Confining Pressure

• Confining pressure applies equal stress from all directions and increases with depth.
• It causes chemical reactions and the formation of new minerals.
• Ranges from 1 bar at sea level to 10,000 bars at the base of the crust, but for metamorphic rocks is around 50,000 bars.

Directed Stress

• Directed stress (differential or tectonic stress) involves unequal forces in different directions.
• It is generated by lithospheric plate movement.
• Stress is force applied to a rock, and strain describes the resultant changes.
• Directed stress changes the arrangement, size, and shape of mineral crystals.
• These changes create identifiable textures.
• Recrystallization occurs when minerals dissolve under high stress and regrow in areas of lower stress.

Fluids

• Chemically reactive fluids, mainly water (H2O) and carbon dioxide (CO2), are expelled by crystallizing magma or created by metamorphic reactions.
• These fluids alter the protolith via chemical reactions and can introduce new elements.
• Hydrothermal metamorphism involves fluids and can create mineral deposits.

Fluid-Activated Metamorphism

• Fluid-activated metamorphism often creates economically important mineral deposits.
• In ocean settings, hydrothermal alteration of mafic/ultramafic rocks creates serpentinite, containing serpentine, brucite, and talc.
• Serpentinite slivers can be found in ophiolites (ocean lithosphere on continents).
• Hot vent water from black smokers can dissolve some elements and carry them away, most commonly copper.

Metamorphic Textures

• Metamorphic texture describes the shape and orientation of mineral grains.

Foliation and Lineation

• Foliation describes minerals aligned in planes, appearing layered.
• Minerals of the mica group form thin, planar crystals that align.
• Lineation describes aligned linear features, such as hornblende crystals.
• Linear crystals can form foliation, lineation, or both together.
• Foliated metamorphic rocks are named based on the nature of their foliations (slate, phyllite, schist, gneiss).

Slate

• Fine-grained metamorphic rock with slaty cleavage (flat orientation of small platy crystals).

Phyllite

• Foliated rock with larger platy minerals, showing a sheen on the foliation surface.

Schist

• Similar to phyllite, but with visible, individual platy crystals (e.g., micas).
• Schistosity describes foliation created by parallel alignment of visible platy minerals.
• May contain porphyroblasts (large crystals surrounded by smaller grains).

Gneiss

• Gneissic banding is foliation created by alternating dark and light mineral bands.
• Gneisses are coarse-grained and often deformed by folding.
• They form at high temperatures and sometimes partially melt.

Migmatite

• A partially melted rock transitional between metamorphic and igneous rock.
• Dark and light banded gneisses that may be swirled or twisted.
• Lighter-colored layers may be the result of the separation of a felsic igneous melt from the adjacent highly metamorphosed darker layers, or to have formed by injection of a felsic melt that came from some distance away

Non-Foliated Rocks

• Non-foliated rocks lack lineations, foliations, or alignment.
• Usually composed of one mineral with evidence of recrystallization.

Quartzite

• Forms from sandstone, with enlarged and interlocked quartz grains.
• Highly resistant to weathering

Marble

• Metamorphosed limestone or dolostone composed of calcite or dolomite.
• Contains larger interlocking crystals.

Hornfels

• Dense, fine-grained, hard, blocky or splintery rock.
• Crystals are silicate minerals that grow smaller with metamorphism
• The protolith of hornfels can be mudstone, basalt, or any of a number of other rock types.

Metamorphic Grade

• Metamorphic grade indicates the extent of metamorphic change in a rock, ranging from low to high grade.
• Low-grade metamorphism occurs just above sedimentary rock conditions.
• Metamorphosed shales progress from slate → phyllite → schist → gneiss with increasing grade.
• Index minerals identify metamorphic grade.
• Different index minerals form under different metamorphic conditions.
• Chlorite, muscovite, biotite, garnet, and staurolite indicate increasing grade in metamorphosed shale.

Polymorphism

• Polymorphism refers to minerals with the same composition but different atomic arrangements.

Metamorphic Environments

• Metamorphic rocks form under different pressure-temperature (P-T) conditions.
• Facies are characterized by different mineral assemblages stable under specific P-T conditions.
• Mineral assemblages can be used to estimate the metamorphic conditions.

Barrovian Sequence

• Barrovian Sequence consists of zones of index minerals representing increasing metamorphic grade.
• Chlorite (slates and phyllites) -> biotite (phyllites and schists) -> garnet (schists) -> staurolite (schists) -> kyanite (schists) -> sillimanite (schists and gneisses).

Greenschist Facies

• Form under relatively low pressure and temperatures, representing the fringes of regional metamorphism.
• They take their name from green minerals like chlorite, epidote, and actinolite, and platy minerals such as muscovite.

Burial Metamorphism

• Occurs when rocks are buried at depths greater than 2000 meters in sedimentary basins.
• Clay minerals change, and sandstone can metamorphose into quartzite.
• Results in low-grade metamorphic rocks with mineral assemblages of the zeolite facies.

Contact Metamorphism

• Occurs in rocks exposed to elevated temperatures at relatively low pressure near magma intrusions or lava flows.
• Shale and basalt recrystallize into hornfels, sandstones into quartzites, and carbonate rocks into marbles.
• High temperature and low pressure produce variants of the hornfels facies.
• Contact aureoles with concentric bands of different mineralogy may form around intrusions.

Regional Metamorphism

• Parent rock is subjected to increased temperature and pressure due to burial in the Earth
• Occurs due to burial in the Earth.
• Found in mountain ranges or on the continent side of subduction zones.
• Highest-grade rocks are near the core of mountain ranges, with lower grade rocks at the edges.
• Also found in shield regions (tectonically stable areas).

Subduction Zone Metamorphism

• Regional metamorphism where oceanic lithosphere subducts beneath continental lithosphere.
• High pressure and low temperature environment.
• Glaucophane is an index mineral found in blueschist facies rocks.

Fault Metamorphism

• Different metamorphic rocks can be created by faults.
• Rock flour: Rock ground up to the particle size of the flour at shallow depths
• Cataclastites: Chaotically-crushed mixes of rock material with little internal texture at greater depths
• Mylonites: Rocks created by dynamic recrystallization via directed shear forces at greater depths

Shock Metamorphism

• Occurs when a meteorite impacts Earth, causing very high pressures (and high, but less extreme temperatures) delivered rapidly.
• Produces planar deformation features called shock laminae in silicate mineral grains.
• Can also produce shatter cones (cone-shaped rock pieces).
• May change quartz into its polymorphs coesite or stishovite.
• Tektites are gravel-size glass grains ejected during a meteorite impact event.
• Pseudotachylytes are glasses that resemble volcanic glass but are not derived from volcanoes.