4. Metamorphic Rocks
I.
These are rocks that have been subject to intense heat and pressure or hydrothermal fluids that
have changed the physical and chemical makeup of the original rock type
A. These changes take place in the solid state whereas the rock does not become molten
B. The type of rock formed depends on the composition of the parent rock and the amount of
time the rock was under such conditions
C. Metamorphic rocks make up a large portion of the ancient continents exposed in the shield
rocks located in the most stable part of the continents P.151
1. Make up a large part of the cores of mountain ranges
2. Some of the oldest known rocks, dated at 3.96 billion years
3. These rocks were originally formed from previous older rocks
D. By studying metamorphic rocks we can gain an idea of the approximate temperatures and
pressures that the parent rock has been subject to
1. This helps understand the processes that have been at work in that region
2. Metamorphism also produces many important rocks and minerals
3. Garnets, marble, talc, and heat resistant minerals as kyanite and asbestos P.152
II.
Agents of Metamorphism P.153-154
A. There are three and perhaps four agents of metamorphism, heat, pressure, hydrothermal
fluids, and time
B. Heat- drives the rate of chemical reactions that produce new minerals from the old
1. When rocks are intruded by hot igneous bodies of magma they are subject to intense
heat
2. The most intense heat is nearest the hot magma and gradually decreases with further
distance away
3. This produces a distinct zone of contact metamorphic rocks surrounding the hot
magma
4. Also the Earth's geothermal gradient is 25 C/km so the deeper the rocks are forced,
the hotter temperatures
5. Rocks deposited in a subduction zone can be transported to great depths and
encounter increased temperatures and pressures
6. Minerals are transformed during subduction into ones that are more stable under the
new temperature and pressures
C. Pressure - when buried rocks are subjected to increased lithostatic pressures
1. Just the weight of the overlying rocks applies pressure to the minerals
2. With increased pressure the rocks transform into a more stable type at that pressure
a) Can be a more closely packed rock with less pore space between grains
b) Recrystallize into minerals that are denser or smaller and take up less space
3. Rocks can experience differential pressures from mountain building or faulting
a) Unequal pressures produce distorted textures
b) Form elongated, rotated, and folded features
D. Fluid activity - introduction of water with dissolved acids, ions from minerals, and the
actual movement of the fluid
1. Water with carbon dioxide is present almost everywhere in rocks
2. Fluids increase the rate of change of chemical reactions
3. When dry, the minerals react slowly, but when fluids are added, reactions increase
because the ions can move about forming new minerals
4. Chemically active fluids come from three sources
a) Trapped water between pore spaces of sedimentary rocks
III.
b) Hydrothermal fluids from magmatic sources that invade the country rock
add or remove ions
c) Dehydration of minerals that have water in their sturcture, like gypsum or
clays
E. Time - It takes time to recrystallize new minerals
1. The more time that the rocks is subject to the forces of metamorphism, the more
likely the minerals will reform to other products
2. Sometime the rock is partially metamorphosed or did not complete the transformation
and will show zoning textures
3. Most metamorphic processes take long periods of time
Types of Metamorphism
A. Three major types are recognized- dynamic, regional, and contact metamorphism
B. These types do not always work alone but many times in concert with each other
C. Contact metamorphism is caused from contact with a hot body of igneous rock P.155
1. The intruding magma increases the temperature of the country rock
2. Release of hydrothermal fluids can increase the alteration of country rock
3. Important factors include the temperature of the magma and the size of the body
a) Higher temperature has a greater thermal effect
b) Mafic magmas like basalt have a higher temp than felsic magmas like dacites
c) Size is important because the larger igneous body has more heat and takes
more time to cool therefore has a greater effect on the country rock
4. Fluids have a great effect because typically igneous bodies release hydrothermal
fluids that inject into the country rock as well as heating of the pore fluids
5. Heating effects decrease with distance and usually form aureoles, concentric layers
of metamorphic rocks
a) Large intrusive bodies have several metamorphic zones
b) Each has distinctive mineral assemblages depending on the temperature,
which decreases outward from the body
c) The zone nearest the intrusive has the highest temperature metamorphic
minerals like sillimanite or kyanite
d) The zones further out will contain minerals formed under lower temps like
epidote, talc, and chlorite
e) Formation of new minerals depends on the type of country rock present, like
limestone, dolomite, or shale and sandstone
f) There are two types of contact meta rocks
(1) Baked contacts of country rock- hard and fine grained
(2) Hydrothermal alteration where fluids invade the country rock and
form new minerals
(3) This produces valuable resources in ore deposits like copper, gold, tin,
and zinc
D. Dynamic Metamorphism P.157
1. Usually associated with deep fault zones where rocks are subject to high differential
pressures
2. These are called mylonites and are usually restricted to a narrow zone
3. Mylonites are stretched and thinned with textures that reveal the movement directions
4. They are usually hard, dense, and fine to medium grained
5. Shows folds, rotations, and lineations of fabric
6. Formed in depths of 10-15 km in the ductile zone below the brittle-plastic zones
E. Regional Metamorphism
IV.
1. Results from large regional forces that build tremendous temperatures, pressures, and
create extensive deformations of the rocks
2. These are most common along convergent plate boundaries where collisions of
continents and subduction is active
a) Within these areas are gradational zones where rocks were subjected to
intense pressures to areas with less pressures
b) These gradations are seen by the minerals present in the rocks
3. Index minerals are ones that are known to have formed at certain temperatures and
pressures
a) They are guides to the temperatures and pressures- whether there is low,
medium, or high grade metamorphism
b) Each original rock type contains a specific mineral assemblage
(1) Clay rich rocks produce the mineral chlorite at low temps of 200 C
(2) Increasing grade metamorphism produces a series of minerals to help
identify the grade Chlorite-biotite-garnet-staurolite-kyanite-sillimanite
c) A different rock type will produce a different mineral sequence so there are
series of index minerals for each metamorphic rock type
d) Specific minerals such as formed by Al2SiO5 generally occur in clay rich
rocks
(1) It forms three different minerals at different temperatures and
pressures (Pg. 174 Fig.7.8)
(2) Kyanite is low temp-high press zone,
(3) Andalusite is low press-high temp zone, and
(4) Sillimanite is high temp-high press
Classification of Metamorphic Rocks P.158-159
A. Commonly divided into two groups- foliated and nonfoliated
B. Foliated- subject to heat and differential pressure with minerals arranged in parallel fashion
1. Size and shape of the mineral grains determine if the foliation is fine or coarse
a) Fine grained rocks cannot see individual mineral grains called slate
b) Coarse grained metamorphics are called gneiss
c) Arraigned in increasing grain size and amount of meta
2. Slate - fine grained rocks with slaty cleavage
a) Results from low grade regional meta from shale
b) Splits along cleavage planes, and can be used for roofing, floor tiles,
blackboards, and pool tables
c) Colors of slate are from the minerals present like graphite, iron oxide, and
chlorite
3. Phyllite - coarser grained but similar composition to slate
a) Minerals can be seen with hand lens
b) Glossy or lustrous sheen
4. Schist - Intermediate grade regional metamorphism
a) Forms parallel to subparallel of platy minerals and prismatic crystals like
tourmaline and amphibole
b) Minerals large enough to see the individual grains
c) Many types of schist depending on the composition of the original shale rock mica schist, chlorite schist, or garnet schist
5. Gneiss - High grade metamorphism P.161
a) Has streaked or segregated bands of light and dark minerals
b) Mostly composed of granular quartz and feldspar with a few micas like biotite
or amphiboles like hornblende
V.
VI.
c) Breaks in irregular blocks and is mostly crystalline
6. Amphibolite - High grade metamorphic
a) Dark colored slightly foliated rock composed of hornblende and biotite
b) Results of high grade of ferromagnesium rocks
7. Migmatites - Very high grade metamorphism
a) Mixed rock of granitic and metamorphic rocks
b) Forms steaks of lighter granite and darker high grade ferromagnesium rock in
a wavy form
c) Most are thought to be the result of very high grade metamorphism where the
minerals segregated into affinities that form different rock types in the layering
d) Sometimes these can be confused with granitic rocks that have been
completely melted and recrystallized
Non-foliated Metamorphic rocks P.168
A. Grains do not show any preferred orientation but have a uniform or equidimensional mineral
shape
1. Form in rocks with no platy minerals like muscovite or biotite
2. Commonly forms from contact metamorphism but can form in regional meta.
B. Usually two types
1. Composed of one mineral like quartz or marble
2. Ones with grains too small to see like hornfels or greenstone
C. Marble is composed of calcite or dolomite and grain size is coarse or fine
1. Can be formed from regional or contact meta.
2. Marble can be white or colored depending on the minerals present in the original
limestone
D. Quartzite - composed of quartz grains
1. Can be medium or high grade metamorphism
2. Quartz grains recrystallize into interlocking granular structure
3. Breaks in irregular blocks because of the uniform hardness
E. Greenstone - name applies to any low to high grade meta rock with green color
1. Green color comes from mineral present like chlorite, epidote or hornblende
2. Usually originates from a mafic igneous rock
F. Hornfels - fine-grained non-foliated rock
1. Usually the result of contact meta
2. Derived from clay rich rocks or muddy dolomites
G. Anthracite - metamorphosed coal
1. Dark black, shiny, hard coal with a lot of carbon with few volatiles left
2. Forms from metamorphism of lower grades of coal
Metamorphic Facies and Zones P.164
A. Each grade of metamorphism produces a different mineral assemblage
1. Mineral assemblages are groups of minerals formed in the rock in response to the
type of heat and pressure
2. Different types of parent rocks produce different mineral assemblages, but there are
certain minerals present in each that are recognizable
3. Index minerals are easy to identify and are only produced in a certain type of
metamorphism
4. The presence of one or two of these index minerals represents a different type of
metamorphism
5. For shale rocks it is chlorite, biotite, garnet, staurolite, kyanite, and sillimanite
B. The successive presence of the index minerals in a rock sequence indicates increasing
metamorphism
VII.
VIII.
1. The first appearance of chlorite indicates the presence of lower grade metamorphism
2. The next mineral to appear would be biotite, then garnet indicating higher grades of
metamorphism
3. A map can be produced that show the successive grades as an isograd or equal
metamorphic intensity
4. A metamorphic zone is the area between the isograd or area of equal metamorphism
C. A metamorphic facies is an area of metamorphic rocks that have the same mineral
assemblage and therefore formed under the same temperature and pressure range
1. These facies are named after its characteristic mineral present
2. Greenshist facies is made up of chlorite minerals forming under low temp and
pressures
3. Increasing temperature and pressure produces amphibolite facies then granulite
facies
4. Increasing pressures but not temperature produces a blueshist with the mineral
glocophane present
5. This system of determining pressures and temperatures can be used in most rocks
except those of uniform mineralogy like quartzite or marble
Metamorphism and Plate Tectonics P.165
A. Metamorphism occurs most commonly along convergent plate boundaries
1. Temperatures and pressure increase because of collisions of plates
2. Especially during subduction the cold slab is under tremendous pressures but heats
slowly producing blueshist facies rocks
3. The presence of blueshist facies rocks indicates ancient subduction along the coast
4. This is found along the west coast where the Franciscan Group rocks are found P.166
5. The Klamath Mtns are formed from this type of subduction complex
6. Eventually, the plate melts producing magmas that move upward which can alter the
surrounding rock to migmatites at deeper parts and plutonic rocks near the surface
7. Hornfels will be made at shallower depths from contact metamorphism in high
temperature and low pressures
B. At mid ocean ridges and spreading centers rising magma produces contact metamorphism
1. Also hydrothermal circulation reduces metamorphism in the oceanic basalts and deep
sea sediments
2. Many natural minerals and ore deposits are produced by hydrothermal metamorphism
Natural Resources
A. Many ore deposits are produced from contact metamorphism P.167
1. Intrusion of granitic bodies develop a rich hydrothermal circulation surrounding them
in the parent rocks
2. Many sulfide minerals are associated with contact metamorphism - bornite, galena,
pyrite, and sphalerite along with hematite and magnetite
3. Tin and tungsten are products of contact metamorphism
4. Talc for talcum powder, graphite for pencils, lubricants, and graphite rods in nuclear
reactors
5. Gemstones such as garnets, corundum, and emeralds
6. Andalusite, kyanite, and sillimanite used in spark plugs and temperature resistant
materials