Metamorphism and Metamorphic Rocks

1
Metamorphism and Metamorphic Rocks

• What is metamorphism?
• How do temperature and pressure change rocks?
• Are fluids important in metamorphism?
• How are metamorphic rocks brought to the surface?
• What are the conditions of metamorphism?
• How are metamorphic rocks classified?
• What was the rock before it was metamorphosed?
• Where does metamorphism occur in relation to
  plate tectonics?

2
Metamorphism and Metamorphic Rocks

• Metamorphic rocks
• Form beneath Earths surface (metamorphic
  processes cannot be directly observed).
• Are commonly found exposed in actively forming
  mountains, e.g. the Himalayas today.
• Are always found in the centers of eroded ancient
  mountain belts.
• Metamorphic minerals make up such economic
  materials as talc, graphite, marble, garnet,
  corundum, and coal deposits.

3
Metamorphism and Metamorphic Rocks

• Review Three types of metamorphic rocks
• Regional metamorphic rocks.
• Occur across vast regions such as convergent
  plate boundaries (e.g. interior of Himalayas
  today)
• Increased temperature and directed pressure
• Contact metamorphic rocks.
• Thermally driven, common along boundaries of
  igneous intrusions, sometimes under lava flows
• Hydrothermal metamorphic rocks.
• Produced by hot-fluid induced chemical changes
• May be associated with one or both of the
  previous types

4
Appalachian Mountains - A belt of regional
metamorphic rocks gt1500 km long and 200-300
km wide, produced by continent-continent
collision.
5
From the Greek meta (change) and morphe (form)
Metamorphic rocks Formed under conditions
between those of compaction and cementation
(lithification sedimentary rocks) and melting
(igneous rocks).
6
What is Metamorphism?

• Metamorphism describes the mineralogical,
  chemical, and textural changes to preexisting
  rocks due to increased temperature and pressure.
• Metamorphic reactions occur in the solid state
  no melting is involved.
• Because these conditions occur at depth in the
  Earth, they are not directly observable.
• Modern experimental petrology, however, can
  reproduce these conditions in the laboratory,
  including original rock, T, P and fluids.

7
Migmatite formed by very high grade
metamorphic conditions where partial melting
(anatexis) occurs.
8
What is Metamorphism?

• Rocks exhibit two types of change
• Original minerals react to form new minerals.
• Rock texture is altered by changes in size,
  shape, and orientation of the minerals (new
  and/or old).
• Original rock type is important
• Chemistry controls what reactions will take
  place, i.e. what new minerals may form.
• In general the whole rock starting composition is
  the same, or similar to, the metamorphic rock
  composition.
• In some cases the presence of chemically active
  fluids may cause substantial changes.

9
How do temperature and pressure change rocks?

• We know Temperature increases with depth in the
  Earth.
• This is referred to as the geothermal gradient,
  typically 20-30 oC/km beneath continents and 60
  oC/km beneath oceans.

10
How do temperature and pressure change rocks?
11
How do temperature and pressure change rocks?

• It takes 10-12 km burial to reach low-grade
  metamorphic conditions.
• By contrast, lithification to form sedimentary
  rocks occurs at shallow depths.

12

• How Are Higher Temperatures and Pressures
  Produced in Mountain Building Events?

• Mountain building events involve directed
  pressure, e.g. the collision between India and
  Asia producing the Himalayas.
• When one block of crust is forced over another
  along a fault, the lower one heats to a
  temperature associated with its new depth.
• This is referred to as tectonic burial and
  produces regional metamorphic rocks.

13
How Are Higher Temperatures Produced by Magma
Intrusion?

• Intrusion of magma raises the local geothermal
  gradient. This causes metamorphism in rocks
  adjacent to the intrusion.
• This produces what are called contact
  metamorphic rocks.

14
How do temperature and pressure change rocks?

• Heat drives off fluids (volatiles) that may have
  been in the rocks.
• High-temperature metamorphism causes minerals
  containing volatiles to lose them.
• Dehydration reactions loss of water.

(HEAT) KAl3Si3O10(OH)2 SiO2 Al2SiO5
KAlSi3O8 H2O (muscovite) (quartz)
(sillimanite) (K-feldspar) (water)
15
How do temperature and pressure change rocks?

• Minerals have ranges of temperature and pressure
  in which they are stable.
• If the T and P change the minerals may transform
  into new minerals which are stable in the new
  conditions.

A Contact Metamorphic Reaction
A Common Example We All Know
16
How do temperature and pressure change rocks?

• Minerals have ranges of temperature and pressure
  in which they are stable.
• If the T and P change the mineral may transform
  into a new mineral which is stable in the new
  conditions.

We can use metamorphic minerals as indicators of
T and P, since we know what conditions they are
stable at. Phase diagrams like this have been
constructed via experimental petrology. Kyanite
high P Sillimanite high T and P Andalusite low
T and P What if we found a rock with all three
minerals in it?
17
How do temperature and pressure change rocks?

• Pressure can be simply thought of as applying
  stress to a rock.
• Stress The magnitude of the force divided by
  the area the force is applied to.
• Normal stress perpendicular to the surface.
  Results in change in volume and often shape.
• Shear stress force parallel to the surface.
  Results in change of shape, but not volume.
• Strain the deformation of a rock as a result of
  applied stress.

18
How do temperature and pressure change rocks?
Directed pressure such as this is called
Differential Stress.
19
How do temperature and pressure change rocks?
20
How do temperature and pressure change rocks?

• Directed pressure can cause platy minerals such
  as micas to
• align perpendicular to the stress.
• This produces what is called foliation and is a
  characteristic
• of regional metamorphic rocks.

21
How do temperature and pressure change rocks?
Foliation in thin section Defined by alignment
of muscovite (brightly colored) intergrown
with quartz (greys and blacks)
22
How do temperature and pressure change rocks?
Metaconglomerate - formed from increase in
temperature and directed pressure.
23
How do temperature and pressure change rocks?

• Increased pressure can cause recrystallization of
  minerals into new shapes and sizes.

This process is clear in these microscope photos
of a sandstone (top) and a metamorphosed
sandstone (bottom) which is called quartzite.
Fig 6.9
24
Are fluids important in metamorphism?

• Fluids can participate in two ways
• React with old minerals to from new minerals that
  contain the fluid (water or CO2)
• Change the metamorphic rock by delivering and
  removing dissolved ions. Fluid makes reactions
  occur much faster and more easily.

25
Are fluids important in metamorphism?

• Fluid may be added during metamorphism
• Most commonly during low-grade metamorphism
  (similar to chemical weathering reactions, but
  higher temperature and pressures)
• 2 Mg2SiO4 2 H2O CO2 Mg3Si2O5(OH)4
  MgCO3
• Mg-olivine water carbon dioxide
  serpentine magnesite

26
Confining pressure may produce metamorphism, but
only differential stress produces foliation
27
Cataclastic (brittle) fault zone deformation at
lt10 km depth Temperatures and pressures are too
low for metamorphism to occur, the rock is
simply shattered and broken up.
28
Mylonite - ductile deformation at gt10km
depth. Now T and P are high enough for
metamorphism, the rock may recrystallize to
new minerals and plastic deformation can occur.
29
How Are Metamorphic Rocks Brought To The Surface?

• Metamorphic rocks form many kilometers
  underground.
• They usually get exposed due to faulting, uplift
  of mountains and subsequent erosion.

30
How Are Metamorphic Rocks Brought To The Surface?

• Metamorphism of rocks occurs very slowly, it may
  take millions of years for a rock to convert to a
  metamorphic rock.
• Uplift and exposure is usually relatively rapid
  and rocks cool quickly during uplift, this
  freezes in the metamorphic mineral assemblage.
• Loss of volatiles during high grade metamorphic
  reactions.
• Reactions cannot reverse easily if volatiles are
  gone.
• Sometimes High-grade metamorphic rock may rise
  to low-grade conditions and stay there for
  extended periods, re-metamorphosing them.

31
What Are The Conditions of Metamorphism?
Mineral stability is experimentally calibrated in
the laboratory with high temperature and
pressure Instruments.
32
What Are The Conditions of Metamorphism?
Some minerals form only over a limited range of
pressure and temperature these are good index
minerals.
33
How are metamorphic rocks classified?

• Composition and texture
• We know that metamorphic processes change the
  minerals in a rock and often the rock texture as
  well. Texture and composition are thus the
  primary criteria for classifying metamorphic
  rocks.
• The first characteristic is either foliated or
  non-foliated. Foliated rocks are characterized by
  having aligned minerals, or layered minerals, or
  otherwise linearly arranged, minerals.

34
How are metamorphic rocks classified?
This is a foliated texture fine-grained with
minerals aligned along planes (producing rock
cleavage). This rock is called slate and contains
clays, muscovite, biotite, chlorite, quartz.
Parent rock is shale or tuff
35
How are metamorphic rocks classified?
Compositional layering in outcrop - This rock is
a gneiss.
36
How are metamorphic rocks classified?
Progressive changes in rock texture and mineral
content take place as the metamorphic grade
increases. For foliated metamorphic rocks - the
type or degree of foliation is the primary
characteristic for classification. Start with a
sedimentary rock a shale.
37
How are metamorphic rocks classified?
At low grade metamorphic conditions clay minerals
begin to convert to micas and chlorite, but this
process is incomplete. If differential stress is
present these newly grown minerals will be
aligned with the stress field (perpendicular).
38
How are metamorphic rocks classified?
At moderate metamorphic conditions all the clay
is recrystallized into mica and chlorite.
Feldspars may begin to form. The crystals are
now visible with the unaided eye, and the rock
develops a sheen due to the abundance of
muscovite mica.
39
How are metamorphic rocks classified?
At medium grade metamorphic conditions the micas
grow much larger, and are all aligned
perpendicular to the stress. Chlorite is now
gone. New higher grade metamorphic minerals such
as garnet may form.
40
How are metamorphic rocks classified?
At high grade metamorphic conditions minerals
begin to segregate into layers, with light
colored high Si, low Fe and Mg minerals in
layers, and dark colored low Si, high Fe and Mg
minerals in other layers. This is called gneissic
banding, and is a type of foliation.
41
How are metamorphic rocks classified?
42
How are metamorphic rocks classified?

• Some metamorphic rocks do not have foliation.

For these metamorphic rocks, composition (mineral
content) is the primary characteristic for naming
the rock.
43
What was the rock type before it was
metamorphosed?

• Here are some common metamorphic changes in rock
  type for different starting (parent) sedimentary
  and igneous rock types (these are called
  protoliths).

44
Where does metamorphism occur?
45
Where does metamorphism occur?
A contact metamorphic zone has increasing grade
as you get closer to the heat source. Where do
we find igneous intrusions? In convergent and
divergent plate tectonic boundaries.
46
Contact Metamorphism by Igneous Intrusions. These
are common over subduction zones.
47
Where does metamorphism occur?

• Hydrothermal metamorphism
• Involves migration and reaction of hot,
  geothermal fluids
• Along mid-ocean ridges, divergent plate
  boundaries.
• Above igneous intrusions, subduction zones.

Diagram showing hydrothermal ore deposits at a
mid-ocean ridge.
48
Where does metamorphism occur?
Tectonic setting for regional metamorphism at a
convergent plate boundary, in this case oceanic
crust subducting beneath continental crust.
49
Where does metamorphism occur?
Regional Metamorphism by Continent-Continent
Collision.
50
Where does metamorphism occur?
Progressively higher grade metamorphism of shale
or mudstone may produce this sequence.
51
Where does metamorphism occur?
There are three primary types of
metamorphismcontact, hydrothermal, and
regional. Contact metamorphism occurs along the
margins of igneous intrusions. It is very
localized and driven by increased
T. Hydrothermal metamorphism is usually small
scale and localized also. It occurs mostly along
mid-ocean ridges. Involves hot fluids and the
ions the fluids mobilize. Regional metamorphism
involves increasing temperature and pressure over
large volumes of crust. It is typical of
converging plate boundaries.