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GEOL: CHAPTER 7
Metamorphism and
Metamorphic Rocks
Learning Outcomes
LO1: Identify the agents of metamorphism
LO2: Identify the three types of metamorphism
LO3: Explain how metamorphic rocks are
classified
LO4: Recognize the difference between
metamorphic zones and facies
LO5: Understand how plate tectonics affects
metamorphism
LO6: Understand the relationship between
metamorphism and natural resources
Metamorphism
• Metamorphic rock: any rock that has been
changed from its original condition by
heat, pressure, and the chemical activity of
fluids
• Metamorphism: the phenomenon of
changing rocks subjected to heat,
pressure, and fluids so that they are in
equilibrium with a new set of
environmental conditions
Agents of Metamorphism
1.
2.
3.
•
Heat
Pressure
Fluid activity
Time for the above to chemically alter
rocks is also an important factor
Heat
• Increases rates of chemical reactions
• Can produce minerals different than those
in the original rock
• Lava
• Magma
• Burial: geothermal gradient; subducted
rock
Pressure
• Pressure increases with depth
• Lithostatic pressure: pressure exerted on
rocks by the weight of overlying rocks
• Differential pressure:
– Pressure not applied equally to all sides of a
body
– Stresses stronger in some directions
– Common with plate collisions
Fluid Activity
• Water (and carbon dioxide) are present in
almost every region of metamorphism
• Water increases reaction rates and thus
metamorphism
• Water also reacts with some minerals to
create new minerals
– Seawater moving through hot basaltic rock
converts olivine to serpentine
Fluid Activity, cont.
•
Sources of chemically active fluids:
1. Water trapped in pore spaces of
sedimentary rocks
2. Volatile fluid in magma
3. Dehydration of water-bearing minerals
Three Types of Metamorphism
1. Contact (thermal) metamorphism
2. Dynamic metamorphism
3. Regional metamorphism
Contact Metamorphism
• Metamorphism of country rock adjacent to
a pluton
• Magma from forming pluton:
– Raises temperature of country rock
– Releases hot fluids into country rock
• Aureole: zone of metamorphism
surrounding a pluton
Contact Metamorphism, cont.
•
Important factors:
1. Initial temperature
2. Size of the intrusion
3. Fluid content of magma and country rock:
hydrothermal alteration creates mineral
deposits
•
•
Degree of metamorphic change
decreases with distance from pluton
Can also occur with lava flows
Dynamic Metamorphism
• Occurs in fault zones where rocks are
under high degrees of differential pressure
• Mylonites frequently created
– Hard, dense, fine-grained
– Thin laminations
Regional Metamorphism
• Occurs over a large area
• Caused by very high temperatures and
pressures, occurring simultaneously with
deformation
• Convergent plate boundaries
• Gradations of metamorphism based on
severity of conditions
Index Minerals
and Metamorphic Grade
• Index mineral: a mineral that forms within
specific temperature and pressure ranges
during metamorphism
• Metamorphic grade: the degree to which a
rock has undergone metamorphic change
• Different rock compositions have different
sets of index minerals
Classifying Metamorphic Rocks
• Foliated texture: Platy and elongate
minerals aligned in a parallel fashion
• Nonfoliated texture: no discernable
preferred orientation of minerals
Foliated Metamorphic Rocks
• Foliated texture: Platy and elongate
minerals aligned in a parallel fashion
• From heat and differential pressure
• Size and shape of mineral grains
determine whether foliation is fine or
coarse
Slate
• Low-grade metamorphism
• Finely foliated
• Minerals can only be seen with
magnification
• Slaty cleavage
• Regional metamorphism of shale
Phyllite
• Coarser grained than slate, but similar
composition
• Need magnification to see minerals
• Intermediate grain size between slate and
schist
Schist
• Commonly produced in regional
metamorphism
• Type depends on intensity of
metamorphism and characteristic of
original rock
• Many rock types yield schist
• Minerals clearly visible
• Schistosity/schistose foliation
• Each type has associated minerals
Gneiss
• High-grade metamorphism
• Segregated bands of light and dark
minerals
– Light bands: quartz and feldspar
– Dark bands: biotite and hornblende
• Forms from recrystallization of
sedimentary rocks during regional
metamorphism, or from igneous or
metamorphic rocks
Other Foliated Rocks
• Amphibolite
– Dark rock: hornblende and plagioclase
– Slightly foliated
– From basalt and ferromagnesian-rich mafic
rocks
• Migmatites
– Igneous and high-grade metamorphic
characteristics
Nonfoliated Metamorphic Rocks
• Nonfoliated texture: no discernable
preferred orientation of minerals
• Mosaic or roughly equidimensional
minerals
• Contact or regional metamorphism of
rocks with no platy or elongate minerals
Marble
• Predominantly calcite or dolomite
• Grains range from fine to coarsely
granular
• Contact or regional metamorphism of
limestones or dolostones
• Colors come from impurities in parent rock
Quartzite
• Formed from quartz sandstone
• Intermediate to high-grade metamorphism
• Typically complete recrystallization of
quartz grains
• Impurities add color
Other Nonfoliated Rocks
• Greenstone: dark-green, from mafic
igneous rock; low- to high-grade
metamorphism
• Hornfels: many varieties, but mostly from
sedimentary rocks; from contact
metamorphism
• Anthracite: hard coal with high % carbon;
metamorphism of lower-grade coals
Metamorphic Zones
• Metamorphic zone: the region between
lines of equal metamorphic intensity
known as isograds
• Metamorphic rocks divided into zones
based on presence of distinctive silicate
minerals
Metamorphic Zones, cont.
• Successive appearance of index minerals
shows progression to higher-grade
metamorphism
• First appearance of a mineral indicates
minimum temperature and pressure
conditions
• Isograds: lines of equal metamorphic
intensity
Metamorphic Facies
• A group of metamorphic rocks
characterized by particular minerals that
formed under the same broad temperature
and pressure conditions
• Named after most characteristic rock or
mineral
• Not applicable when original rocks were
pure quartz sandstones or pure limestones
or dolostones
Plate Tectonics
and Metamorphism
• Associated with all 3 types of boundaries
• Most common with convergent boundaries
– Temperature and pressure characteristics
– Produces typical metamorphic facies
• Subducting plate heats only slowly at first,
so initial metamorphism from high
pressure
– Blueschist facies
Plate Tectonics
and Metamorphism, cont.
• As subduction continues, temperatures
and pressures increase
– High-grade metamorphic rocks
– Plate begins to melt and generate magma:
contact metamorphism
• Divergent boundaries
– Contact metamorphism from magma
– Metal-bearing hydrothermal solutions
Metamorphism
and Natural Resources
• Marble
• Slate
• Ore deposits from hot hydrothermal fluids
– Migrate into surrounding rock
– Copper: bornite, chalcopyrite
– Lead: galena
– Zinc: pyrite and sphalerite
– Iron: hematite and magnetite
– Asbestos, talc, graphite, garnets