Metamorphism: A Process of Change

Ch 8 Goals

• How is a metamorphic rock defined?
• What happens to a rock as it undergoes metamorphism?
• What role does stress play in the deformation of grains?
• What are the different types of stress?
• How are metamorphic rocks classified?
• Understand metamorphic grade and index minerals
• Understand where metamorphism occurs, and how metamorphic rocks come back to the surface

What is a Metamorphic Rock

• Metamorphism is when a pre-existing rock (the protolith) undergoes a solid state change in response to a change in its environment.
  • Heat
  • Pressure
• This leads to a change in mineral assemblage and texture

Protoliths

• Protoliths undergo changes in texture and mineralogy.

Metamorphism Changes Mineralogy

• Protolith: red shale, made of quartz, clay, and iron oxide
• Metamorphic result: gneiss, made of quartz, feldspar, biotite, and garnet

Metamorphism Changes Texture

• Fossil fragment
• Protolith
• Calcite crystal

Foliation

• Metamorphism often imparts a foliation upon the new rock.
• Foliation is a planar fabric that cuts through the rock.

Metamorphic Processes: Recrystallization

• Texture changes but not composition

Metamorphic Processes: Neocrystallization

• Clay and quartz transforms into Quartz, garnet, mica

Metamorphic Processes: Pressure Solution

• Spherical grains become Elliptical grains

Metamorphic Processes: Plastic Deformation

• Spherical grains become Elliptical grains

Metamorphic Processes: Overprinting – 1

• The agents of metamorphism are heat $(T)$, pressure $(P)$, compression and shear, and hydrothermal (hot water) fluids.
• Not all agents are required to alter a rock mass, although they often co-occur.
• Rocks may be overprinted by multiple events.

Temperatures of Metamorphism

• Metamorphism occurs between $250^oC$ and $850^oC$ and the depth to this temperature varies with tectonic setting.

Differential Stress

• Differential stress is stress that is greater in one direction.
• It differs from pressure $(P)$, because pressure is of equal magnitude in all directions.

Differential Stress: Normal

• Normal stress: operates perpendicular to a surface
  • Compression: push- together normal stress
  • Tension: pull-apart normal stress
  • Common result of tectonic forces…mountain building vs rifting

Differential Stress: Shear

• Shear stress acts parallel to a surface.
• Shear smears out the dough ball parallel to the floor.

Grain Alignment

• Compression and shear combined with elevated $T$ and $P$ cause minerals and rocks to change shape without breaking.
• Internal textures are changed as minerals rotate or dissolve and recrystallize in preferred orientations.

Differential Stress

• Elongate (cigar-shaped): one dimension is longer than the other two (e.g., staurolite).
• Equant-roughly equal in all dimensions
• Platy (pancake-like): one dimension is shorter than the other two (e.g., micas).

Pressure Solution

• Pressure solution is one mechanism for creating preferred mineral orientation in metamorphic rocks.
• It occurs at relatively low $T$ in rocks that have some moisture.
• Minerals dissolve where they are compressed together.
• Minerals grow where there is less compression.
• Grains become shorter parallel to compression.

Plastic Deformation

• Existing grains flatten by deforming internally.
• During plastic deformation, the grains change shape internally, without breaking or dissolving.
• Occurs at higher temps

Shear Rotation and Flattening

• Compressive stress flattens grains into alignment.
• Shear stress flattens grains and rotates them into alignment.

What role does water play?

• Hydrothermal fluid (hot water with dissolved ions and volatiles) accelerates metamorphism.
• Hydrothermal fluids speed up chemical reactions and add or subtract elements.
• Hydrothermal alteration is called metasomatism.

Classifying Metamorphic Rocks

• Foliation
  • Foliation (Latin folium, for leaf) is the parallel planar fabric common to many metamorphic rocks.
  • Foliation develops because the rocks have been subjected to differential stress and they have a significant component of platy minerals.
  • It gives the rock a streaked or striped appearance and sometimes provides weakness along which the rock will break.
  • Ex: slate
• Foliated Metamorphic Rocks
  • Phyllite is a fine-grained, mica-rich rock that forms by the metamorphic alteration of slate. Phyllite has a silky sheen called phyllitic luster.
  • Metaconglomerate is a metamorphosed conglomerate
  • Schist is a fine to coarsely crystalline rock with larger micas indicating medium- to high-grade metamorphism. It has a distinct foliation from large micas called schistosity.
  • Gneiss has distinct compositional bands, comprised of light bands of felsic minerals (quartz and feldspars) alternating with dark bands of mafic minerals (biotite or amphibole).
• Compositional Banding
  • Compositional banding can develop by extensive high $T$ shearing.
  • Sometimes during the formation of gneiss, the rock is sheared under high $T$. The original contrasting rock types in the protolith are then smeared into parallel layers, creating the foliation.
  • Compositional banding can develop by metamorphic differentiation.
  • chemical reactions segregate light and dark layers to create compositional banding.
• Migmatites
  • A migmatite is a partially melted gneiss; it has features of both igneous and metamorphic rocks.
• Nonfoliated
  • Quartzite and marble: nonfoliated metamorphic rocks
  • Nonfoliated metamorphic rocks have no planar fabric evident because they lack inequant minerals and/or they recrystallized without differential stress.
• Nonfoliated Rocks
  • Quartzite is a metamorphic quartz sandstone. The sand grains in the protolith recrystallize and fuse to form a rock that is hard, glassy, and resistant.
  • Marble is coarsely crystalline calcite or dolomite from a limestone or dolostone protolith.

Metamorphic Grade

• Metamorphic grade:
  • Low grade: slight
  • High grade: intense
• How might the appearance of a rock change with increasing metamorphism?
• By identifying the minerals in a metamorphic rock, we can understand if a rock formed in high grade or low grade environment.
• Retrograde Metamorphism
  • Prograde metamorphism occurs when a rock is buried deeply in an orogenic belt.
  • Over time, the rock experiences progressive changes due to increasing $T$ and $P$.
  • Deeply buried rocks are brought back to the surface via erosion.
  • Retrograde metamorphism occurs to deep-seated rocks that are brought back to the surface.
  • Retrograde reactions are only possible if hydrothermal fluids add water. Without added water, prograde metamorphic rocks will remain unaltered.
• Prograde and Retrograde Metamorphic Paths

Types of Metamorphism

• Thermal: heating by a plutonic intrusion
• Burial: deep burial in a basin
• Dynamic: shearing in a fault zone
• Regional: $P$ and $T$ change due to orogenesis
• Hydrothermal: alteration by hot water leaching
• Subduction: high $P$ and low $T$ alteration
• Shock: extreme high $P$ from a bolide impact

Geologic Settings: Contact Metamorphism

• Contact (or thermal) metamorphism is due to heat from a body of magma invading host rock.

Geologic Settings: Dynamic Metamorphism

• Dynamic metamorphism involves breakage of rock by shearing within a fault zone.
• In the shallow crust (0 to 15 km), rocks are brittle, crushing to form fault breccia.
• In the deeper crust (15 km and deeper), the rocks are ductile.

Geologic Settings: Regional Metamorphism

• Tectonic collisions deform huge “mobile belts” hundreds to thousands of km long.
• Directed compression smashes preexisting rocks and buries them deeply, where they are heated by the geothermal gradient and plutonic intrusions.
• The heat and pressure of orogenesis creates huge volumes of metamorphic rocks, more than any other mechanism.

Geologic Settings: Hydrothermal Metamorphism

• At mid-ocean ridges, hot, chemically aggressive water chemically alters the basalt.
• The process starts when cold ocean water seeps into fractured crust.
• Heated by magma, this water then reacts with the mafic rock and is ejected via black smokers.
• The mafic rock is metamorphically altered.

Geologic Settings: Subduction Metamorphism

• Trenches and accretionary prisms have a low geothermal gradient.
• These conditions produce a unique low $T$, high $P$ mineral assemblage called blueschist, after glaucophane, a blue amphibole mineral.

Exposure of Metamorphic Rocks

• The process by which metamorphic rocks return to the surface is called exhumation
  • 1) if two continent are squeezed together, the rock caught between is pushed up
  • 2) as the mountain range gets bigger, rocks at depth will soften and become thinner.
  • Now the deeper parts of crust get closer to the surface
  • 3) Erosional forces remove rock at the surface to reveal rock below

Exhumation

• As continents squeeze together during collision, rock is pushed up, like dough in a vise.
• Rock at depth softens and the mountain belt collapses and becomes thinner, like cheese in the sun.
• Erosion grinds away and removes rocks, like a giant rasp.

Shields

• Large regions of ancient high-grade metamorphic rocks are exposed in continental interiors, called shields.