Chapter 6
Metamorphism and Metamorphic Rocks
Metamorphism repesents the set of processes by which rocks undergo solid-state
changes in mineralogy, texture, or both to reach equilibrium with its changing
environment, generally as they are buried beneath the surface. The term
metamorphism comes from the Greek "meat" meaning "changing" and "morpho"
meaning "form". Metamorphism occurs due to:

Increased heat, probably the most important metamorphic agent. Equilibrium
temperatures increase with depth according to the relation:
°C/kilometer x depth (kilometers)) or geothermal gradient
Increasing temperature can be caused by burial or intrusion of magma, and
results in:

1. Recrystallization of minerals with grain size increases and a change to
high-temperature minerals
2. Dehydration of minerals where water is released
3. Faster chemical reactions
Pressure increases with burial from the weight of overlying rock (uniform or
lithostatic pressure) and from tectonic forces (differential or directed pressure).
Lithostatic pressure, which is defined by the relation:
P = density(kg/m3) x gravitational constant (Å10 m/s2) x depth (meters),
results in volume reduction. A rule of thumb is that pressure increases by Å 3
kbars every 10 km. Differential or directed pressure (pressure unequal in
different directions) causes foliated (layered) metamorphic rocks. Pressure
changes result in:
1. Recystallizatfon of minerals with recrystallized minerals oriented to
reduce stress
2. Formation of new minerals
3. fracturing and folding of rock.
4. availability increases reaction rates dramatically.
Metamorphic fluids can derived from:
5. Formation waters trapped in pore spaces of the original rock.
6. Water released during dehydration of minerals, such as amphibole or
mica.
7. Water from magmatic bodies (hydrothermal fluids). Convection of
fluids may also occur near heat sources such as intrusions, e.g., "cold"
rainwater percolates down, heats up, and "hot" water rises toward
surface.

Time which the other 3 agents of metamorphism act on a rock can determine
the extent of the changes in the rock. Coarse-grained rocks probably result
from longer periods of metamorphism.
Types of Metamorphism
There are three major types of metamorphism:
1. Contact Metamorphism occurs when molten rock comes in contact with cooler
rock. Metamorphism occurs primarily due to heat (up to 900°C) and
chemically active fluids. Produces a local zone of alteration in the surrounding
rock called an aureole. This aureole contains minerals diagnostic of contact
metamorphism and produces a fine-grained non-foliated rock known as
hornfels. Fluids released can react with country rock (hydrothermal alteration)
to produce new metamorphic minerals and ore deposits.
2. Dynamic Metamorphism is metamorphism associated with fault zones where
rocks are subjected to high differential pressures. Produces rocks called
mylonites (hard, dense, fine-grained rocks often characterized by thin
laminations) .
3. Regional Metamorphism results from intense compression, deep burial, and
large igneous intrusions commonly related to mountain building (convergent
plate margins). Most metamorphic rocks are produced by this type of
metamorphism (area affected is very large). Rocks are foliated with mineral
assemblages characteristic of varying metamorphic temperatures and
pressures.
Textural Changes During Metamorphism
Grain size increases with increasing temperature, but decreases as directed pressure
increases. Directed pressure causes development of foliation, where mineral grains are
arranged into layers.
Chemical and Mineralogical Changes During
Metamorphism
Metamorphism is isochemical where mineralogical changes due variations in
temperature and pressure is accompanied by no change in rock chemistry.
Metasomatism or metamorphism resulting from widespread movement of chemically
active fluids results in significant changes in rock chemistry
Metamorphic facies are assemblages of minerals reflecting different metamorphic
environments. Each facies correlates to a specific range of temperature and pressure.
These facies can be thought of as regions on a pressure-temperature diagram, and are
given distinctive names: zeolite, blueschist, greenschist, amphibolite, granulite, and
eclogite.
Index Minerals are metamorphic minerals characteristic of specific temperature and
pressure conditions in metamorphosed rocks (e.g., staurolite at ca. 500 - 650°C).
Isograds (lines of similar temperature-pressure conditions) are drawn on maps to
separate one metamorphic zone of index minerals from another. Isograds in contact
metamorphic rocks will subparallel contacts with plutons (heat source), whereas
isograds of regional metamorphism may subparallel the surface.
What Types of Metamorphic Rocks Form?
Metamorphic rocks are found in mountain ranges, roots of mountain ranges, and on
continental shields (the stable cores of continents that contain the oldest known rocks,
3.96 billion years old). Regional metamorphic rocks are much more widespread than
contact metamorphic rocks. Classification is based on texture and composition. Rocks
are
commonly
divided
into
two
groups
:
 Foliated Rocks show distinct planar texture due to alignment of mineral grains.
1. Slate - Fine-gained rock produced by low-grade metamorphism of shale.
Shows slaty cleavage and many possible colors.
2. Phyllite - Platy minerals larger than in slate, but not easily seen. The rock,
produced by slightly higher grade (temperature) metamorphism than slate, has
glossy sheen.
3. Schist - Distinctly foliated rocks with visible mineral grains most commonly
produced by regional metamorphism of clay-rich sedimentary rocks. Contain
>50% platy minerals. Produced by intermediate-grade metamorphism. Named
according to major minerals. Most abundant regional metamorphic rock.
4. Gneiss - Coarse-grained granular rock with alternating bands of light (quartz
and feldspar) and dark (biotite and hornblende) minerals. Produced by highgade metamorphic conditions.
5. Amphibolite - Medium- to coarse-grained, dark-colored rock composed
largely of amphibole (hornblende) and plagioclase. Produced by medium- to
high-grade metamorphism of basic rocks. Weak foliation.
6. Migmatites - "Mixed rocks" having both igneous and metamorphic
characteristics. Produced by extremely high-grade metamorphism which
produces local melting or complete segregation of low-temperature minerals.
Look like schists that have been intruded by granite. Migmatites tend to have a
granitic composition. Can follow some rock units from unmetamorphosed
sediment to migmatite to pure granite.
 Nonfoliated rocks formed by regional metamorphism of monomineralic rocks or
contact metamorphism.Show no preferred orientation.
1. Marble - Recrystallized limestone or dolostone. Sugary texture.
2. Quartzite - Metamorphosed quartz-rich sandstone. Sugary, interlocking
texture.
3. Greenstone - Any compact, dark-green, non-foliated mafic metamorphic rock.
Color is produced by chlorite, epidote, or amphibole minerals.
4. Hornfels - Any fine-grained rock produced by contact metamorphism. Density
of rock increases with metamorphic grade.
5. Anthracite - Shiny black coal produced by metamorphism of lower grades of
coal. Highest carbon content and lowest impurity content of any type of coal.
Metamorphism and Plate Tectonics
Metamorphism is associated with all 3 types of plate boundaries, but is most common
at convergent plate margins.
A. Convergent boundary metamorphism can produce:
1. Melanges, a mixture of sea floor sediments, ocean crust fragments, and
various low-temperature high-pressure metamorphic assemblages (blueschist
facies).
2. Low- to high-grade regional metamorphism of fore- and back-arc sediments.
3. Contact metamorphism occurs around intrusions produced by magmas that
originate through melting above the subducting slab.
B. Divergent boundary metamorphism can produce:
1. Contact metamorphism from extrusion of basaltic lava.
2. Mineralization from hydrothermal solutions (black smokersj.
C. Transform boundary metamorphism produces mineral recrystallization mainly due
in changes in directed pressures.
Metamorphism and Natural Resources
Metamorphism produces many valuable mineral and rock resources:
A. Rocks quarried include:
1. Marble which is used for statuary and ornamental building stone.
2. Slate which is used for roofing, flooring, billiard/pool tables, and blackboards.
B. Important economic metamorphic minerals include:
1. Talc which is ground into powder.
2. Graphite used in pencils and lubricants.
3. Garnet and Corundum used as gemstones and abrasives.
4. Asbestos formerly used as a heat insulator.
5. Kyanite, Andatusite, Sillimanite used a raw material in the ceramics industry.
C. Ore Deposits - result from contact metamorphism where hydrothermal solutions
precipitate ore minerals in surrounding rocks:
1. Sulfide deposits (bornite, chalcopyrite, galena, pyrite, and sphalerite)
2. Iron and Tin Oxides deposits (hematite, magnetite, and cassiterite)
3. Tungsten deposits (wolframite and scheelite)
4. Precious metal deposits (gold)