GLY256 Geology for Engineering - Metamorphic Rocks Notes

Metamorphic Rocks

What are Metamorphic Rocks?

  • Metamorphism is a process of change (Gr. meta 'change' + Gr. morphe 'form').
  • Metamorphic rock forms from pre-existing rock (called protolith).
  • The process of metamorphism typically happens at depth in the Earth.
  • It involves solid-state change due to a changing environment.
  • Rocks undergo changes while remaining solid.
  • During metamorphism, rock must remain solid; otherwise, it becomes an igneous rock if it melts.

Metamorphic Changes

  • Protoliths undergo slow solid-state changes in:
    • Texture.
    • Mineralogy.
  • Metamorphic changes are due to variations in:
    • Temperature.
    • Pressure.
    • Tectonic stresses.
    • Amount of reactive water.

Metamorphic Character

  • Metamorphic rocks have distinctive properties:
    • Unique texture - Intergrown and interlocking grains.
    • Unique minerals - Some that are only metamorphic (e.g., Staurolite, Kyanite, Sillimanite).
    • Unique foliation - A planar fabric from aligned minerals.

Metamorphic Minerals and Texture

  • Metamorphic minerals are new minerals growing within a solid rock and are referred to as a metamorphic mineral assemblage when comprising a group of such minerals.
  • Metamorphic texture is defined by the alignment of the mineral grains as either:
    • Metamorphic foliation where platy minerals such as micas are aligned.
    • Alternate banding of light-colored and dark-colored layers.

Metamorphism Processes

  • Metamorphism changes mineralogy.
    • Example: Red shale (quartz, clay, and iron oxide) can change into Gneiss (quartz, feldspar, biotite, and garnet).
  • Metamorphism changes texture, resulting in unique intergrown and interlocking grains.
    • Fossil fragments in a protolith can be replaced by calcite crystals.
  • Metamorphism creates foliation (alignment of minerals).

Metamorphic Processes (Detailed)

  • Recrystallization: Minerals change size and shape; mineral identity need not change.
    • Example: Limestone transforms into Marble.
    • Changes shapes and sizes of grains without changing the identity of the mineral.
    • In sandstone, large irregular grains of quartz are replaced by a cluster of small, round, crystallized quartz grains.
  • Phase change: New minerals form with the same chemical formula but a different crystal structure.
    • Example: Andalusite transforms into Kyanite.
  • Neocrystallization: New minerals form from old.
    • Initial minerals become unstable and change to new minerals.
    • Original protolith minerals are digested in reactions.
    • Elements restructure to form a new mineral assemblage.
    • Example: Shale (clay and quartz) transforms into garnet mica schist (quartz, garnet, mica).
  • 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.
  • Plastic deformation: Mineral grains soften and deform.
    • Requires elevated temperature and pressure.
    • Rock is squeezed or sheared.
    • Minerals change shape without breaking, like a plastic.

Causes of Metamorphism

  • The agents of metamorphism are:
    • Heat (T).
    • Pressure (P).
    • Compression and shear.
    • Hot fluids.
  • Not all agents are required; they often co-occur.
  • Rocks may be overprinted by multiple events.
Metamorphism Due to Heat (T)
  • Most metamorphism occurs between 250°C250°C and 850°C850°C, between diagenesis and melting (up to 1200ºC1200ºC).
  • Heat energy breaks and reforms atomic bonds.
  • Solid-state diffusion: migration of atoms between grains.
  • New minerals form.
Metamorphism Due to Pressure (P)
  • P increases with depth in the crust.
  • Metamorphism occurs mostly in the 2−122-12 kbar range, which is between about 55km and 4040km depth.
  • Increase in P packs atoms more tightly together, creating denser minerals.
  • Involves phase changes or neocrystallization.
  • The formation and stability of many minerals depends on both P and T.
Pressure (P) / Heat (T)
  • 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.
Metamorphism via Compression
  • Compression - stress greater in one orientation, which is different from pressure (P), which is equal in all directions.
  • Compression is a common result of tectonic forces.
Differential Stress
  • Pressure that is greater in one orientation is a common result of tectonic forces.
  • Two kinds of differential stress:
    • Normal stress - Operates perpendicular to a surface.
      • Tension - Pull-apart normal stress.
      • Compression - Push-together normal stress.
    • Shear stress - Moves one part of a material sideways relative to another part.
Compression and Shear
  • Compression and shear applied together causes mineral grains to change shape.
    • Equant - roughly equal in all dimensions.
    • Inequant - dimensions not the same.
      • Platy (pancake-like) - one dimension shorter (e.g., micas).
      • Elongate (cigar-shaped) - one dimension longer (e.g., staurolite).
  • Preferred orientation of inequant minerals is a common feature of metamorphic rocks.
Development of Preferred Orientation
  • Compression and shear combine with elevated T and P, causing rocks to change shape without breaking.
  • Internal textures of deforming rocks can also change.
    • Minerals rotate into preferred orientations.
    • Minerals grow in preferred directions relative to stretching.
Differential Stress and Foliation
  • 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.
Hydrothermal Fluid Metamorphism
  • Hot water with dissolved ions and volatiles.
  • Hydrothermal fluids facilitate metamorphism, accelerating chemical reactions and altering rocks by adding or subtracting elements.
  • Hydrothermal alteration is called metasomatism; it is often associated with mineralization of ore deposits.

Types of Metamorphic Rocks

  • Foliation dictates primary classification.
  • Two major subdivisions: foliated and nonfoliated.
  • Foliation: parallel surfaces or layers in metamorphic rocks.
    • Alignment of inequant grains or compositional banding.
    • Subjected to differential stress.
    • Has a significant component of platy minerals.
    • Classified by composition, grain size, and foliation type.
Metamorphic Textures
  • Foliation describes any planar arrangement of mineral grains or structural features within a rock.
    • Examples of foliation:
      • Parallel alignment of platy or elongated minerals.
      • Parallel alignment of flattened mineral grains or pebbles.
      • Compositional banding of dark and light minerals.
      • Cleavage where rocks can be easily split into slabs.
Textural Classification
  • Non-foliated:
    • Minerals recrystallized or grew without showing foliation.
    • Either due to metamorphism in absence of compression and shear or because most of the new crystals are equant.
  • Foliated:
    • Assemblage of parallel planar surfaces in a metamorphic rock.
    • Either due to inequant grains aligned along a direction or due to alternating light and dark-colored layers.
Foliated Metamorphic Rocks Examples
  • Slate
  • Phyllite
  • Schist
  • Gneiss
  • Migmatite
Non-Foliated Metamorphic Rocks Examples
  • Hornfels
  • Quartzite
  • Marble
  • Mylonite
  • Soapstone
Types Based on Foliation
  • Foliated:
    • Assemblage of parallel planar surfaces in a metamorphic rock.
    • Either due to inequant grains aligned along a direction or due to alternating light and dark-colored layers.
    • Distinguished based on composition, grain size, and nature of foliation.
Foliation Appearance
  • Foliation gives the appearance of book pages viewed edge-on, creating wavy surfaces as well as flat ones.
Foliated Metamorphic Rocks (Examples)
  • Schist: Metamorphic rocks with a foliation pattern defined by the layering of platy minerals (such as muscovite or biotite micas).
    • The rock name is commonly modified to indicate what mica is present (e.g., garnet muscovite schist).
  • Gneiss: A metamorphic rock with foliation that presents as alternating bands of dark and light minerals throughout the rock. This type of foliation is referred to as gneissic banding.
    • The greatest pressure on the rock was applied in what is the up-and-down direction in the photo.
Nonfoliated Metamorphic Rocks
  • Lack a planar fabric because:
    • Rock not subjected to differential stress.
    • Minerals recrystallized without compression or shear.
    • Comprised of equant minerals only.
    • Classified by mineral composition.
      *Note: Occurs when rocks are heated by an igneous intrusion or if the protolith is composed only of certain minerals that recrystallize into coarse equigranular crystals.
Non-foliated Development
  • Absence of foliation can occur if differential pressure is not involved, such as when metamorphism happens primarily by rocks being heated by an igneous intrusion.
  • An absence of foliation can also occur if the protolith is composed only of certain minerals.
    • Minerals like calcite (if the protolith were limestone) and quartz (in a sandstone protolith) recrystallize into coarse equigranular crystals.
  • Neither quartz nor calcite are platy minerals, so foliation does not develop.
  • The metamorphic rocks that form when quartz and calcite rocks recrystallize are classified according to their composition, not by foliation type.
    • When quartz-rich sandstone is metamorphosed, the result is the metamorphic rock quartzite.
    • When limestone is metamorphosed, the result is marble.
Chemical Composition of Metamorphic Rocks
  • Protolith composition determines which minerals can form based on the availability of elements; foliation dictates primary classification.
    • Protolith is shale or mudstone.
    • Protolith is Basalt or Gabbro.
    • Protolith is Limestone or Dolomite.
    • Protolith is Sandstone.

Metamorphic Grade

  • Different minerals are stable as T and P changes.
  • Metamorphic grade is a measure of intensity.
    • Low grade: weaker metamorphism.
    • High grade: intense metamorphism.
Prograde and Retrograde
  • Prograde: Metamorphism via increasing T and P; common in rocks that are buried in orogenic belts.
    • Progressive changes include recrystallization (causes mineral growth) and neocrystallization (results in new mineral assemblages).
    • Mineral changes release water.
  • Retrograde: Metamorphism via decreasing T and P; common in rocks that are brought from depth by erosion or exhumation.
    • Requires addition of H2OH_2O by hydrothermal fluids; without added water, retrograde reactions cannot occur.
    • Many metamorphic rocks preserve prograde conditions.
Metamorphic Grade Details
  • Metamorphic grade refers to the intensity of metamorphism and the associated degree of metamorphic change: low grade, intermediate grade, and high grade.
  • Metamorphic grade determines mineral assemblages; as grade increases, new and larger minerals form.
Pelitic Rock Example
  • 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.
Foliated Metamorphic Rocks (Examples Detailed)
  • Slate: Fine-grained, low-grade metamorphic shale with a distinct foliation called slaty cleavage.
    • Develops by parallel alignment of platy clay minerals.
    • Slaty cleavage develops perpendicular to compression.
    • Slate breaks along foliation, creating sheets used for roofing.
  • Phyllite: Fine-grained mica-rich rock formed metamorphism of slate; clay minerals neocrystallize into tiny micas.
    • Has silky sheen called phyllitic luster; phyllite is between slate and schist.
  • Metaconglomerate: Metamorphosed conglomerate; pebbles and cobbles are flattened by pressure solution and plastic deformation.
    • Foliation is defined by the flattened clasts.
  • Schist: Fine to coarse rock with larger micas that forms at higher temperatures than phyllite.
    • Has a distinct foliation from large micas called schistosity.
    • Schist has abundant large micas such as biotite and muscovite.
  • Gneiss: Distinct compositional bands, often contorted, with light bands of felsic minerals (quartz and feldspars) and dark bands of mafic minerals (biotite or amphibole).
Compositional Banding
  • Solid-state differentiation; chemical reactions segregate light and dark layers.
  • Gneissic banding develops in several ways:
    • Original layering in the protolith.
    • Extensive high-T shearing.
    • Metamorphic differentiation: minerals segregate into different layers.
  • Migmatite: A partially melted gneiss with features of igneous and metamorphic rocks.
    • Light-colored (felsic) minerals melt at lower T, while dark-colored (mafic) minerals melt at higher T.
    • The felsic bands melt and recrystallize in the gneiss.
Nonfoliated Metamorphic Rocks (Examples Detailed)
  • Lack a planar fabric due to various reasons:
    • Rock not subjected to differential stress.
    • Dominance of equant minerals.
    • Absence of platy minerals like clays or micas.
  • Hornfels: Fine-grained variety of metamorphic clay minerals that sedimented as mudstone protolith.
    • Associated with plutonic intrusions; composition depends on protolith, pressure, and temperature.
Metamorphic Classes
  • Mafic: Basalt or gabbro protolith, rich in Fe and Mg and poor in Si, Al, Na, and K.
    • Turn into biotite- and amphibole-dominated rocks.
  • Calcareous: Carbonate protolith, rich in Ca (limestones) and Ca and Mg (dolostones).
    • Recrystallize into calcite and dolomite marbles.
  • Marble: Coarsely crystalline calcite or dolomite that 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.
  • Quartzite: Almost pure quartz in composition formed by alteration of quartz sandstone.
    • Sand grains in the protolith recrystallize and fuse; like quartz, it is hard, glassy, and resistant.
Simplified Guide to Metamorphism Effect on Different Protoliths
  • Mudrock:
    • Very Low Grade (150−300°C150-300°C): slate
    • Low Grade (300−450°C300-450°C): phyllite
    • Medium Grade (450−550°C450-550°C): schist
    • High Grade (Above 550°C550°C): gneiss
  • Granite:
    • No change to granite gneiss from Low to High Grade.
  • Basalt:
    • chlorite schist at Very Low Grade.
    • amphibolite (amphibole gneiss) from Low to High Grade.
  • Sandstone:
    • No change to quartzite from Very Low to High Grade.
  • Limestone:
    • little change to marble from Very Low to High Grade.
      Note: Temperature ranges are approximate
Index Minerals
  • Index minerals indicate a specific P and T range.
  • Metamorphic Zones are defined by index minerals.
  • Isograd is a line on a map along which an index mineral first appears; all points along an isograd have roughly the same grade.
  • Zones between isograds are metamorphic zones named after an index mineral not present in the adjacent lower-grade zone.
Facies
  • Metamorphic grade only addresses relative intensity of metamorphism.
  • Metamorphic facies, on the other hand, indicates a set of metamorphic mineral assemblages indicative of a certain pressure-temperature regime.
    • This depends on pressure and temperature, as well as the composition of the protolith.
    • A metamorphic facies contains a set of different mineral assemblages reflecting different protolith compositions.
    • A facies, therefore, contains different kinds of rocks based on chemical composition and mineral content, but they all formed in roughly the same temperature and pressure conditions.
    • Metamorphic facies are named from distinctive features or minerals found in them.
Different Facies Examples
  • Zeolite
  • Prehnite-pumpellyite
  • Hornfels
  • Greenschist
  • Amphibolite
  • Blueschist
  • Eclogite
  • Granulite

Where Metamorphism Occurs

  • Different grades of metamorphism relate to the intensity of metamorphism based on temperature and pressure.
  • It is reflected by different metamorphic mineral assemblages, where this characteristic mineral assemblage indicative of certain pressures and temperatures are referred to as metamorphic facies.
  • A region in which a specific grade of rock occurs is a metamorphic zone, and the boundary between zones is an isograd.
Summary of Metamorphism Causes
  • Heat
  • Pressure
  • Differential stress
  • Hydrothermal fluids
Summary of Metamorphic Rock Types
  • Foliated/ non-foliated (Texture)
  • Pelitic/ mafic/ calcareous/ quartzo-feldspathic (Composition/Protolith)
Types of Metamorphic Environments
  • Conditions for metamorphism differ because, based on location in the context of plate tectonics:
    • Geothermal gradient changes (and therefore changes T and P conditions).
    • Differential stresses change.
    • The extent of interaction with hydrothermal fluids changes.
  • Three general paths along which metamorphism can occur:
    • Heating by magma (contact metamorphism).
    • Regional metamorphism.
    • Subduction-zone metamorphism.
Metamorphic Environment Types
  • Thermal or Contact: 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/low-T alteration in subduction zones.
  • Shock: extreme high P resulting from a bolide impact.
Thermal and Contact Metamorphism
  • Rising hot magma intruding into cooler rock causes heat to flow from magma into the cooler rock; magma cools and solidifies, while the wall rock heats up.
  • Hydrothermal fluids circulate through the intrusion and wall rock, causing the wall rock to metamorphose.
  • The highest grade is directly adjacent to the pluton, and the lowest grade is to the original protolith furthest away.
  • The belt of metamorphic rock is called a metamorphic aureole or contact aureole.
  • The width of the aureole increases with larger intrusions and more hydrothermal circulation.
Typical Rock Types in Thermal and Contact Metamorphism
  • Hornfels
  • Occurs wherever intrusion occurs such as at convergent boundaries, rifts, and mountain building and collision zones.
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.
    • At about 8−158-15km depth, metamorphic reactions begin.
Dynamic Metamorphism
  • Solely due to shearing without requiring changing temperature or pressure.
  • Rock forming is mylonite – extremely fine-grained with strong foliation
  • Protolith is a Granite.
Regional Metamorphism
  • Results in foliated metamorphic rock.
  • Type depends on grade: slate at shallower depths; schists and gneisses at greater depths.
Hydrothermal Metamorphism at Mid-Ocean Ridges
  • Hot magma rises beneath the axis of MOR; cold seawater sinks through cracks into the oceanic crust along the ridge.
  • Water heats up and becomes hydrothermal fluid, which again rises through the crust near the ridge.
  • Causes hydrothermal metamorphism of ocean-floor basalt, and fluid escapes through vents known as black smokers.
  • Typical rock type: Greenstone.
Subduction Metamorphism
  • Forms foliation due to shear between subducting and overriding plates.
  • Common rock type: Blueschist.
Shock Metamorphism
  • Rarely, Earth is struck by a comet or asteroid, generating a compressional shock wave, extremely high pressure, and heat that vaporizes or melts large masses of rock.
  • These conditions generate high-pressure minerals.
  • Common rock types: Impactites.
Exhumation
  • Is the overall process by which deeply buried rocks end up back on Earth’s surface.
    • Continental collision: rocks between squeezes upward by slip on faults AND plastic flow
    • Erosion takes place, stripping rock at the surface and exposing once-buried rock.
  • Shield – broad region of long-lived stable continental crust where Phanerozoic sedimentary cover either was not deposited or was eroded away
    • Bedrock exposed in shields formed during succession of Precambrian mountain-building that also led to the original growth of continents.
    • Exposed old metamorphic rocks at the surface.