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Metamorphism Metamorphic – Changed from
an original “parent.”
Meta = Change.
Morph = Form or shape.
Parent rocks are called
“protoliths.”
Metamorphism can
occur to any protolith.
Metamorphism Protoliths undergo
pronounced changes in…
Texture.
Mineralogy.
Caused by changes in…
Temperature.
Pressure.
Tectonic stress.
Reaction with heated water.
Metamorphic Character Metamorphic rocks have distinctive properties.
Unique texture – Intergrown and interlocking grains.
Unique minerals – Some that are only metamorphic.
Unique foliation – A planar fabric from aligned minerals.
Metamorphic Processes Metamorphic change occurs slowly in
the solid state.
Several processes are at work.
Recrystallization – Minerals change size and shape.
Phase change – New minerals form with…
Same chemical formula.
Different crystal structure.
Example: Andalusite to kyanite.
Kyanite
Neocrystallization – New minerals form as a result of changes in temperature and pressure.
Initial minerals become unstable and change to new minerals.
Original protolith minerals are digested in reactions.
Elements restructure to form new minerals.
Metamorphic Processes
Metamorphic Processes Pressure solution – Mineral grains partially dissolve.
Dissolution requires small amounts of water.
Minerals dissolve where their surfaces press together.
Ions from the dissolution migrate in the water film.
Metamorphic Processes Plastic deformation – Mineral grains soften and deform.
Requires elevated temperatures.
Rock is squeezed or sheared.
Minerals act like plastic, changing shape without breaking.
Causes of Metamorphism The agents of metamorphism are…
Heat (T).
Pressure (P).
Compression and/or shear.
Hot water.
Not all agents are required; they often do co-occur.
Rocks may be overprinted by multiple events.
Heat (Temperature, T) Metamorphism occurs as the result of heat.
Temperature (T) ranges between 200oC and 850oC.
The upper T limit is…melting. It varies based upon rock mineral composition and water content.
Heat energy breaks and reforms atomic bonds.
Sources of heat.
The geothermal gradient.
Magmatic intrusions.
Compression.
P increases with depth in the crust.
270 to 300 bars per km (1 bar is almost 1 atm = 14.7 psi).
Metamorphism occurs mostly in 2 to 12 kbar range.
T and P both change with depth.
Mineral stability is highly dependent upon T and P.
This stability can be graphed on a “phase diagram.”
Changes in T and P lead
to changes in minerals.
Pressure (P)
Pressure that is greater in one orientation.
A commonplace result of tectonic forces.
Two kinds of differential stress: Normal and shear.
Normal Stress – Operates perpendicular to a surface.
Tension – Pull-apart normal stress.
Compression – Push-together normal stress.
Differential Stress
Differential Stress
Two kinds of differential stress: Normal and shear.
Shear Stress – Operates sideways across a surface.
Causes material to be “smeared out.”
At higher T and P, differential stress deforms rock.
Rocks change shape slowly without breaking.
Differential Stress
Differential Stress Deformation acts on minerals with specific shapes.
Equant – Roughly equal in all dimensions.
Inequant – Dimensions not the same.
Platy (pancake-like) – 1 dimension shorter.
Elongate (cigar-shaped) – 1 dimension longer.
Differential stress causes these minerals to align.
Alignment fabric records stress trajectory.
Differential Stress Preferred platy mineral alignment is called foliation.
Foliation imparts a layered or banded appearance.
Rocks commonly break parallel to foliation planes.
Foliation develops perpendicular to compression.
Minerals flatten, recrystallize and rotate.
Inequant grains align by rotation and new growth.
Hydrothermal Fluids
Hot water with dissolved ions and volatiles.
Hydrothermal fluids facilitate metamorphism.
Accelerate chemical reactions.
Alter rocks by adding or subtracting elements.
Hydrothermal alteration is called metasomatism.
Metamorphic Rock Types
Two major subdivisions of metamorphic rocks.
Foliated – Has a through-going planar fabric.
Subjected to differential stress.
Has a significant component of platy minerals.
Classified by composition, grain size, and foliation type.
Metamorphic Rock Types Two major subdivisions of metamorphic rocks.
Nonfoliated – No planar fabric evident.
Crystallized without differential stress.
Comprised of equant minerals only.
Classified by mineral composition.
Metamorphic Rocks Slate – Fine clay, low-grade metamorphic shale.
Has a distinct foliation called slaty cleavage.
Develops by parallel alignment of platy clay minerals.
Slaty cleavage oriented perpendicular to compression.
Slate breaks along this foliation creating flat sheets.
Metamorphic Rocks
Phyllite - Fine mica-rich rock.
Formed by low- to medium-grade alteration of slate.
Clay minerals neocrystallize into tiny micas.
Micas reflect a satiny luster.
Phyllite is between slate and schist.
Schist – Fine or coarse rock with larger micas.
Medium- to high-grade metamorphism.
Has a distinct foliation called schistosity.
Parallel alignment of large mica crystals.
Micas are visible because they have grown at higher T.
Schist often has other minerals due to neocrystallization.
Quartz.
Feldspars.
Kyanite.
Garnet.
Sillimanite.
Large non-mica minerals are called porphyroblasts.
Metamorphic Rocks
Metamorphic Rocks Gneiss – Has a
distinct banded foliation.
Light bands of felsic minerals (quartz and feldspars).
Dark bands of mafic minerals (biotite or amphibole).
Metamorphic Rocks Compositional banding develops in several ways.
Original layering in the protolith.
Extensive high-temperature shearing.
Metamorphic Rocks Nonfoliated rocks lack a planar fabric.
Absence of foliation possible for several reasons:
Rock not subjected to differential stress.
Dominance of equant minerals.
Absence of platy minerals like clays or micas.
Metamorphic Rocks
Amphibolite – Dominated by amphibole minerals.
Basalt or gabbro protolith.
Usually not well foliated.
Hornfels – Alteration by heating.
Associated with plutonic intrusions.
Finely crystalline.
Metamorphic Rocks Quartzite – Almost pure quartz in composition.
Forms by alteration of quartz sandstone.
Sand grains in the protolith recrystallize and fuse.
Like quartz, it is hard, glassy, and resistant.
Metamorphic Alteration
Metamorphic Rocks Marble - Coarsely crystalline calcite or dolomite.
Forms from a limestone or dolostone protolith.
Extensive recrystallization completely changes the rock.
Original textures and fossils in the parent are obliterated.
Used as a decorative and monument stone.
Exhibits a variety of colors.
Metamorphic Alteration
Metamorphic Rocks
Type depends on protolith.
Minerals contribute elements.
Some protoliths yield specific rocks.
Broad compositional classes:
Pelitic.
Mafic.
Carbonate.
Quartzofeldspathic.
Metamorphic Intensity Different minerals are
stable as T and P changes.
Grade is a measure of metamorphic intensity.
Low-grade – Slight.
High-grade – Intense.
Metamorphic Grade
Prograde – Metamorphism via increasing T and P.
Common in rocks that are buried in orogenic belts.
Progressive changes.
Recrystallization causes mineral growth.
Neocrystallization results in new mineral assemblages.
Mineral changes release water.
Metamorphic Grade Example: Prograde metamorphism of a pelitic rock.
Low grade – Shale protolith.
Clays recrystallize into larger, aligned clays to yield a slate.
Clays neocrystallize into tiny, aligned micas in a phyllite.
Intermediate grade –
Micas recrystallize and grow large to form a schist.
New minerals grow in the schist.
High grade -
Micas decompose; elements recombine into new minerals.
Neocrystallization yields quartz and feldspars in a gneiss.
Metamorphic Grade
Retrograde – Metamorphism via decreasing T and P.
Common in rocks that are brought from depth by erosion.
Accompanied by addition of H2O by hydrothermal fluids.
Many prograde rocks aren’t “retrograded.”
Rocks at the surface can preserve prograde conditions.
Index Minerals Certain minerals have a limited
P-T range.
These “index minerals” record metamorphic grade.
Index mineral maps.
Define metamorphic zones.
Grade boundaries called
isograds.
Metamorphic Facies
Mineral assemblage from a specific protolith at specific P-T conditions.
The same minerals result from the same…
Protoliths.
T and P conditions.
Named for dominant
mineral.
Metamorphic Environments Metamorphism occurs in different settings.
Different settings yield different effects via…
Geothermal gradient.
Differential stresses.
Hydrothermal fluids.
These characteristics are governed by tectonics.
Metamorphic Environments The types (and settings) of metamorphism are...
Thermal – Heating by a plutonic intrusion.
Burial – Increases in P and T by deep burial in a basin.
Dynamic – Shearing in a fault zone.
Regional – P and T alteration due to orogenesis.
Hydrothermal – Alteration by hot-water leaching.
Subduction – High P to low T alteration.
Shock – Extremely high P attending a bolide impact.
Contact Metamorphism
Due to heat from magma invading host rock.
Creates zoned bands of alteration in host rock.
Called a contact (or metamorphic) aureole.
The aureole surrounds the plutonic intrusion.
Zoned from high-grade (near pluton) to low-grade (far from pluton).
Contact Metamorphism Grades of alteration form bands around
the pluton.
Bands range from highly altered to slightly altered.
Analogous to changes in pottery with increased heating.
The width of each aureole zone is due to…
The size of the plutonic intrusion.
The degree of metasomatism.
The dominant rock is hornfels.
Burial Metamorphism As sediments are buried in a
sedimentary basin…
P increases because of the weight of the overburden.
T increases because of the geothermal gradient.
Requires burial below diagenetic effects.
This is ~ 8–15 km depending on the geothermal gradient.
Regional Metamorphism Tectonic collisions deform huge “mobile belts.”
Directed compression thickens mountains.
Rocks caught up in mountain building are…
Heated via the geothermal gradient and plutonic intrusions.
Squeezed and heated by deep burial.
Smashed and sheared by differential stresses.
Hydrothermal Metamorphism
Alteration by hot, chemically aggressive water.
A dominant process near mid-ocean ridge magma.
Cold ocean water seeps into fractured crust.
Heated by magma, this water then reacts with mafic rock.
The hot water rises and is ejected via black smokers.
Subduction Metamorphism Subduction creates the unique blueschist facies.
Trenches and accretionary prisms have…
A low geothermal gradient – low temperature.
High pressures.
High P – Low T favor
glaucophane, a blue
amphibole mineral.
Shock Metamorphism
Rarely, Earth is struck by a comet or an asteroid.
Impacts generate a compressional shock wave.
Extremely high pressure.
Heat that vaporizes or melts large masses of rock.
These conditions generate high-pressure minerals.
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