Metamorphic Rocks
Geology 200
Geology for Environmental
Scientists
Major Concepts
- Metamorphic rocks can be formed from any
rock type: igneous, sedimentary, or existing metamorphic
rocks.
- Involves recrystallization in the solid
state, often with little change in overall chemical
composition.
- Driving forces are changes in temperature,
pressure, and pore fluids.
- New minerals and new textures are formed.
Major Concepts
- During metamorphism platy minerals grow in
the direction of least stress producing foliation.
- Rocks with only one, non-platy, mineral
produce nonfoliated rocks such as quartzite or marble.
- Two types of metamorphism: contact and
regional.
Major Concepts
- Foliated rocks - slate, phyllite, schist,
gneiss, mylonite
- Non-foliated rocks - quartzite, marble,
hornfels, greenstone, granulite
- Mineral zones are used to recognize
metamorphic facies produced by systematic pressure and
temperature changes.
Origin of Metamorphic Rocks
- Below 200oC rocks remain
unchanged.
- As temperature rises, crystal lattices are
broken down and reformed with different combinations of
atoms. New minerals are formed.
- The mineral composition of a rock provides
a key to the temperature of formation (Fig. 6.5)
Where does the heat come from?
- Hot magma ranges from 700-12000C.
Causes contact metamorphism.
- Deep burial - temperature increases 15-300C
for every kilometer of depth in the crust. Subduction and
continental collision bury rocks to depths of 10’s
of kms. This is regional metamorphism. Most metamorphic
rocks form this way.
Pressure
- Hydrostatic pressure - produced by the
weight of overlying rocks.
- Directed pressure - lateral forces
produced by plate tectonics. Directed pressure produces
the foliation in regionally metamorphosed rocks.
Chemically Active Fluids
- Metasomatism - fluid transport during
metamorphism can result in the gain or loss of atoms.
Clays release H2O (de-watering) as they change
to minerals such as kyanite or garnet.
- Hydrothermal alteration - injection of hot
water into rocks from metasomatism can produce metallic
ores such as lead and zinc.
Chemically Active Fluids
- Seafloor metamorphism - ocean water heated
by contact metamorphism is a form of metasomatism. This
converts the olivine and pyroxene in basalt into hydrated
silicates such as serpentine, chlorite, and talc. This
changes basalt to greenstone.
Textures of Metamorphic Rocks
- Foliated (Latin for leaf) - grades from
slate to phyllite to schist to gneiss. Produced by platy
minerals growing in direction of least stress.
- Nonfoliated - typical when parent rock is
sandstone or limestone. Also occurs in contact
metamorphism and extreme metamorphism (granulite facies).
- Mylonitic - formed along faults; rare.
Kinds of Metamorphic Rocks
- Slate - low grade metamorphosed shale;
very fine grained; slaty cleavage should not be confused
with bedding planes.
- Phyllite - next step up from slate, larger
mica grains give the rock a luster.
- Schist - a strongly foliated rock with
large grains of platy minerals. Named for its minerals; e.g.,
chlorite schist.
Kinds of Metamorphic Rocks
- Gneiss - a coarse grained granular rock in
which foliation results from alternating layers of light
and dark minerals. Compositionally similar to granite
with quartz, feldspar, biotite, and amphibole. Forms
during regional high grade metamorphism. May grade into
migmatite and even granite.
Kinds of Metamorphic Rocks
- Parent rock and type of gneiss:
granite biotite becomes gneisssediments become garnet gneiss
basalt becomes amphibolite gneiss
Kinds of Metamorphic Rocks
- Nonfoliated Rocks
- quartzite - metamorphosed sandstone
- metaconglomerate - metamorphosed
conglomerate
- marble - metamorphosed limestone
- hornfels - contact metamorphism of shale;
very hard, like a brick
- greenstones - seafloor metamorphism of
basalt
- granulite - highest grade metamorphic
rocks; all water driven off so no platy minerals left
Parent Rocks and Metamorphic
Rocks
- shale becomes slate, and slate can become
schist, and schist can become gneiss
- rhyolite becomes schist
- granite becomes gneiss
- basalt becomes amphibolite
- limestone becomes marble
- sandstone becomes quartzite
Regional Metamorphic Zones
- Mineral zones reflect differences in
metamorphic grade across a region. Geologists map
metamorphic zones based on mineral changes. Used to
locate central vs. marginal parts of ancient mountain
belts.
- Index minerals in order: chlorite,
biotite, garnet, staurolite, kyanite, sillimanite.
Metamorphic Facies
- Granulite facies - high grade metamorphic
rocks; characteristic minerals are pyroxene, sillimanite,
and garnet. Close to melting point, migmatite often
present.
- Blueschist facies - a high pressure, low
temperature facies; color from blue amphibole
- Eclogite facies - very high pressure (upper
mantle) and temperature; pyroxene and garnet
Metamorphic Rocks and Plate
Tectonics
- Greenschist facies - upper continental
crust in mountain ranges, and seafloor metamorphism of
basalt
- Amphibolite and granulite facies - form
progressively deeper in the roots of mountains
- Contact metamorphic rocks - form near
igneous intrusions in mountain ranges
Metamorphic Rocks and Plate
Tectonics
- Blueschist facies - shallow part of
subduction zone
- Eclogite facies - deeper part of
subduction zone and upper mantle