Introduction to Metamorphism
IN THIS LECTURE
– Definition of Metamorphism
– Limits of Metamorphism
– Agents of Metamorphic Change
o Temperature
o Pressure
o Stress and Strain
o Fluids
– Metamorphic Change
– Classification of Metamorphic Rocks
– Types of Metamorphism
Introduction to Metamorphism
•
•
Rocks as chemical systems represented by a particular assemblage
of coexisting phases (thermodynamic equilibrium and the governed
by the phase rule)
A basaltic composition can be either:
1.
2.
3.
•
Melt
Cpx + plag ( olivine, ilmenite…)
Or any combination of melt + minerals along the liquid line of
descent
If uplifted and eroded  surface, will weather  a combinations
of clays, oxides…
Introduction to Metamorphism
• The IUGS-SCMR has proposed the following definition of
metamorphism:
“Metamorphism is a subsolidus process leading to
changes in mineralogy and/or texture (for example grain
size) and often in chemical composition in a rock. These
changes are due to physical and/or chemical conditions
that differ from those normally occurring at the
surface of planets and in zones of cementation and
diagenesis below this surface. They may coexist with
partial melting.”
The Limits of Metamorphism
• Low-temperature limit grades into diagenesis
• The boundary is somewhat arbitrary
o
Diagenetic/weathering processes are indistinguishable
from metamorphic
o
Metamorphism begins in the range of 100-150oC for the
more unstable types of protolith
o
Some zeolites are considered diagenetic and others
metamorphic – pretty arbitrary
The Limits of Metamorphism
• High-temperature limit grades into melting
o
Over the melting range solids and liquids coexist
o
If we heat a metamorphic rock until it melts, at what
point in the melting process does it become “igneous”?
o
Xenoliths, restites, and other enclaves are considered
part of the igneous realm because melt is dominant, but
the distinction is certainly vague and disputable
o
Migmatites (“mixed rocks”) are gradational
The Limits of Metamorphism
• Migmatites: metamorphic or igneous rocks?
Limits of Metamorphism
Agents of Metamorphic Change
•
There are several main agents of metamorphic
change
1.
2.
3.
4.
Temperature
Pressure
Stress and Strain
Fluids
Agents of Metamorphic Change
1. Temperature
• TEMPERATURE: typically the
most important factor in
metamorphism
• Estimated ranges of oceanic and
continental steady-state
geotherms to a depth of 100 km
using upper and lower limits
based on heat flows measured
near the surface.
• After Sclater et al. (1980),
Earth. Rev. Geophys. Space Sci.,
18, 269-311.
Agents of Metamorphic Change
1. Temperature
Increasing temperature has several effects
1.
Promotes recrystallization leading to increased grain size
–
–
Larger surface/volume ratio of a mineral means lower stability
Increasing temperature eventually overcomes kinetic barriers to
recrystallization, and fine aggregates coalesce to larger grains
2. Drive reactions that consume unstable mineral(s) and produces
new minerals that are stable under the new conditions
3. Overcomes kinetic barriers that might otherwise preclude the
attainment of equilibrium
Agents of Metamorphic Change
2. Pressure
• PRESSURE
– Normal gradients may be perturbed in several ways
– The two main examples are
o
High T/P geotherms in areas of plutonic activity or
rifting
o
Low T/P geotherms in subduction zones
Agents of Metamorphic Change
3. Stress and Strain
• Stress is an applied force acting on a rock (over a
particular cross-sectional area)
• Strain is the response of the rock to an applied
stress (= yielding or deformation)
• Deviatoric stress affects the textures and
structures, but not the equilibrium mineral
assemblage
• Strain energy may overcome kinetic barriers to
reactions
Agents of Metamorphic Change
4. Fluids
• Evidence for the existence of a metamorphic fluid
– Fluid inclusions
– Fluids are required for hydrous or carbonate phases
– Volatile-involving reactions occur at temperatures and
pressures that require finite fluid pressures
• Pfluid indicates the total fluid pressure, which is the sum of the
partial pressures of each component (Pfluid = pH2O + pCO2 + …)
• May also consider the mole fractions of the components, which
must sum to 1.0 (XH2O + XCO2 + … = 1.0)
Metamorphic Change
• Metamorphic grade
– A general increase in degree of metamorphism without specifying
the exact relationship between temperature and pressure
– Generally both temperature and pressure increase
– Three terms used
o
Low Grade
o
Medium Grade
o
High Grade
The Progressive Nature of Metamorphism
• PROGRADE: increase in metamorphic grade with time
as a rock is subjected to gradually more severe
conditions
– Prograde metamorphism: changes in a rock that accompany
increasing metamorphic grade
• RETROGRADE: decreasing grade as rock cools and
recovers from a metamorphic or igneous event
– Retrograde metamorphism: any changes that accompany
decreasing metamorphic grade
Evidence for Metamorphic Change
• How do we see metamorphic change
– Mineral assemblage and grainsize can be used to estimate
metamorphic grade
– Gradients in T, P, Xfluid across an area
– Zonation in the mineral assemblages
Classification of Metamorphic Rocks
•
Two main ways in which metamorphic rocks are
classified
1. Based on the style or type of metamorphism
2. Based on the mineral assemblage or texture of the rocks
–
We’ll look first at the types of metamorphism
The Types of Metamorphism
Different approaches to classification based on type
1. Based on principal process or agent
o Dynamic Metamorphism
o Thermal Metamorphism
o Dynamo-thermal Metamorphism
The Types of Metamorphism
2. Based on setting
o Contact Metamorphism
•
Pyrometamorphism
•
o
Regional Metamorphism
•
Orogenic Metamorphism
•
Burial Metamorphism
•
Ocean Floor Metamorphism
o
Hydrothermal Metamorphism
o
Fault-Zone Metamorphism
o
Impact or Shock Metamorphism
We’ll look at the classifivation based on setting
Contact Metamorphism
• Adjacent to igneous intrusions
• Result of thermal (and possibly metasomatic –
involvement of fluids) effects of hot magma intruding
cooler shallow rocks
• Occur over a wide range of pressures, including very
low pressures
• Usually leads to the formation of a contact aureole
around the pluton. A contact aureole is a zone in
which the rocks into which the pluton has intruded
have been affected by the heat of the intrusion and
have developed new metamorphic mineral
assemblages
Contact Metamorphism
• The size and shape of an aureole is controlled by
– The nature of the pluton, ie
o Size
o Shape
o Orientation
o Composition
o Temperature
– The nature of the country rocks
o Composition
o Depth and metamorphic grade prior to intrusion
o Permeability
Contact Metamorphism
– Most easily recognized where a pluton is introduced into
shallow rocks in a static environment
– The rocks near the pluton are often high-grade rocks with
an isotropic fabric: hornfelses (or granofelses) in which
relict textures and structures are common
– Polymetamorphic rocks are common, usually representing an
orogenic event followed by a contact one, e.g. spotted
phyllite or slate
– Overprint may be due to:
o Lag time between the creation of the magma at depth
during T maximum, and its migration to the lower grade
rocks above
o Plutonism may reflect a separate phase of post-orogenic
collapse magmatism
Contact Metamorphism
Pyrometamorphism
o
o
o
Very high temperatures at very low pressures,
generated by a volcanic or subvolcanic body
Also developed in xenoliths
Not very common and won’t be looked at further in this
course
Regional Metamorphism
• Regional Metamorphism sensu lato
– metamorphism that affects a large body of rock, and thus
covers a great lateral extent
• Three principal types
– Orogenic metamorphism
– Burial metamorphism
– Ocean-floor metamorphism
Regional Metamorphism
OROGENIC METAMORPHISM
• Type of metamorphism associated with convergent
plate margins
– Dynamo-thermal, involving one or more episodes of orogeny
with combined elevated geothermal gradients and
deformation (deviatoric stress)
– Foliated rocks are a characteristic product
Regional Metamorphism
OROGENIC
METAMORPHISM
Figure 21-6. Schematic model for
the sequential (a  c) development
of a “Cordilleran-type” or active
continental margin orogen. The
dashed and black layers on the
right represent the basaltic and
gabbroic layers of the oceanic
crust. From Dewey and Bird (1970)
J. Geophys. Res., 75, 2625-2647;
and Miyashiro et al. (1979)
Orogeny. John Wiley & Sons.
Regional Metamorphism
OROGENIC METAMORPHISM
• Uplift and erosion
• Metamorphism often continues after major
deformation ceases
– Metamorphic pattern is simpler than the
structural one
• Pattern of increasing metamorphic grade from both
directions toward the core area
• Most orogenic belts have several episodes of
deformation and metamorphism, creating a more
complex polymetamorphic pattern
• Associated with continental collision
Regional Metamorphism
OROGENIC METAMORPHISM
• Batholiths are usually present in the highest grade areas
• If plentiful and closely spaced, may be called regional contact
metamorphism
Regional Metamorphism
BURIAL METAMORPHISM
• Low grade metamorphism in sedimentary basins due to burial
• Example: Southland Syncline in New Zealand
– A thick pile (> 10 km) of Mesozoic volcaniclastics had accumulated
– Mild deformation and no igneous intrusions discovered
– Fine-grained, high-temperature phases, glassy ash: very susceptible
to metamorphic alteration
– Metamorphic effects attributed to increased pressure and
temperature due to burial
– Range from diagenesis to the formation of zeolites, prehnite,
pumpellyite, laumontite, etc.
Regional Metamorphism
BURIAL METAMORPHISM
• Occurs in areas that have not experienced significant
deformation or orogeny
• Restricted to large, relatively undisturbed
sedimentary piles away from active plate margins
– The Gulf of Mexico
– Bengal Fan
Regional Metamorphism
BURIAL METAMORPHISM
• Bengal Fan represents a sedimentary pile > 22 km
• Extrapolating implies 250-300oC at the base (P ~ 0.6
GPa)
• Well into the metamorphic range, and the weight of
the overlying sediments is sufficient to impart a
foliation at depth
• Passive margins often become active
• Areas of burial metamorphism may thus become areas
of orogenic metamorphism
Regional Metamorphism
OCEAN-FLOOR METAMORPHISM
• Affects the oceanic crust at ocean ridge spreading
centres
– Wide range of temperatures at relatively low pressure,
beginning in the diagenesis field and increasing to lower
greenschist facies
– Metamorphic rocks exhibit considerable metasomatic
alteration, notably loss of Ca and Si and gain of Mg and Na
– These changes can be correlated with exchange between
basalt and hot seawater
– We’ve seen this already when we looked at the Cyprus thinsections!
Regional Metamorphism
OCEAN-FLOOR METAMORPHISM
• May be considered another example of hydrothermal
metamorphism
• Highly altered chlorite-quartz rocks- distinctive high-Mg, lowCa composition