Download PDF file of Chapter 8 lecture - Metamorphic Rocks

Dr. James Wittke
[email protected]

Transforms one rock into
another
◦ Change temperature
◦ Change pressure
◦ Interact with fluids

Rocks remain solid
during metamorphism

Heat
◦ From burial (usually tectonic)
◦ From intrusion of magma

Pressure (stress)
Pressure
Tectonic
stress
◦ Confining pressure (weight
of overlying rocks)
◦ Differential stress (tectonic
stresses)

Chemically active fluids
Heat
Fluids

Provides energy for chemical reactions
◦ Existing minerals recrystallize  larger crystals
form
◦ Existing minerals become unstable  new stable
minerals grow
◦ Overall composition does not change

Two heat sources
◦ Heat from interior (radioactivity, residual heat of
accretion)  temperature increases with depth
(geothermal gradient or geotherm)
◦ Heat introduced by magma (intrusions)

Confining pressure
applies equal force in
all directions
◦ Increases with depth
◦ Closes spaces between
Minerals transform into
denser forms

Differential stress due
to tectonic forces
(unequal in different
directions)
◦ Brittle vs. ductile
deformation (breaking vs.
bending/flowing)

H2O and CO2 present in all rocks
◦ Pore spaces of sedimentary rocks
◦ Fractures in igneous rocks
◦ Breakdown of hydrous minerals (clay, mica,
amphibole)

Fluids enhance migration of ions
◦ Recrystallization of existing minerals
◦ Growth of new minerals

Invading fluids may change bulk composition
of rock  metasomatism
Quartz veins deposited by fluids

Most metamorphic rocks retain chemical
composition of original rock
◦ Exception: addition/loss of volatiles (water, CO2)

Original mineral makeup determines what
and how much metamorphism will occur
◦ Quartz and calcite do not react (no new minerals
form)
◦ Feldspar, mica, amphibole (etc.) are reactive
(new minerals form)

Foliation
◦ Parallel alignment of
platy minerals or
flattened mineral grains
and pebbles
◦ Compositional banding

Formed by differential
stress


Rotation of platy
or elongated
minerals
Recrystallization
of minerals in
preferred
orientation

Changing shape of grains
(pressure solution) into
elongated shapes that are
aligned

Changing shape of
grains by movement
(slip) along crystal
planes



Also termed rock
cleavage
Cannot see platy
minerals with
unaided eye
Closely spaced
planar surfaces
along which rocks
split

Schistosity
◦ Growth of platy minerals to discernible sizes

Gneissic texture
◦ Ion migration yields bands of light and dark minerals

Non-foliated
◦
◦
◦
◦

No platy minerals
Minimal deformation
Parental rocks with equidimensional grains
Recrystallization yields larger grains
Porphyoblastic
◦ Large grains surrounded by fine grained minerals

Slate
◦ Very fine grained (<0.5 mm), excellent cleavage, tiny mica flakes

Phyllite
◦ Larger crystals than slate, surface sheen
◦ Dominated by muscovite and chlorite (green platy mineral)

Schist
◦ Medium- to coarse-grained, dominated by platy minerals,
subtypes given names (i.e., garnet-mica schist)

Gneiss
◦ Distinctive banded appearance
Marble
Quartzite


Some metamorphic rocks nonfoliated
Develop where differential stress is minimal

Porphyroblasts
◦ Large grains in finegrained matrix of other
minerals
◦ Typically, large crystals
are garnet, staurolite,
or andalusite
Contact (thermal) metamorphism
 Hydrothermal metamorphism
 Regional metamorphism
 Burial metamorphism (diagenesis)
 Metamorphism along fault zones
 Impact metamorphism



Magma intrudes
host rock  heats
adjacent host rock
Zone of alteration
(aureole) forms in
rock surrounding
the magma
Chemical alteration
caused when hot, ionrich hydrothermal fluids
circulate through
fissures and cracks
Common along mid-oceanic
ridge systems
Black Smokers
Hydrothermal metamorphism in
continental environment
Burial & Subduction Metamorphism

Burial metamorphism
◦ Occurs in thick piles of sediment
◦ Bottom of pile hotter and under
more pressure (low-grade
metamorphic conditions)
◦ Depth required depends upon
geothermal gradient (typically 8
km)

Subduction (blueschist)
metamorphism
◦ Cold rocks carried deep into
mantle (high pressures)
◦ Low-T/ high-P environment
yields distinctive minerals (blue
colors)


Produces
greatest
quantity of
metamorphic
rock
Associated
with mountain
building


In shallow part of fault zone rock breaks (fault breccia)
In deeper portions where temperature is higher
minerals flow yielding mylonite


Caused by meteorite
impact
Unique conditions
◦ Very high pressures and
temperatures
◦ Short duration

Features
◦ Crushed pulverized rock
◦ Impactites and tektites (melt)
◦ High pressure polymorphs
(coesite, stishovite, diamond)

Systematic
variations
mineralogy (and
often texture)
related to
variations in
degree of
metamorphism


Index minerals used
to map metamorphic
grade
Increasing grade
yields:
◦
◦
◦
◦
◦
Chlorite
Biotite
Garnet
Staurolite
Sillimanite



Melting of metamorphic
rocks begins at highest
grades
Melting assisted by
breakdown of waterbearing minerals
(dehydration melting)
Rocks have light bands
of melted material
along with areas of
unmelted rock

Assemblages of minerals (metamorphic facies) 
metamorphic conditions (T, P)