Metamorphic rocks

Metamorphic Rocks


Metamorphism
• Literally translates to “change of form”
• In geology it refers to solid-state changes
in mineral assemblages of a rock, and/or
the texture of these minerals
• Due to changes in temperature and/or
pressure


Sources of Heat for Metamorphism
• Heat from Earth’s interior
• Geothermal gradient is the
increase in temperature with
depth
– Typical continental geothermal
gradient is 25-30°C/km
– Volcanically active areas have
geothermal gradients of 3050°C/km
– Oceanic trenches have
geothermal gradients as low as
5-10°C/km


Sources of Heat for Metamorphism
• Heat from magma
• Emplacement of magma chambers will add heat to the
immediately surrounding rock
– Gabbroic magma ~1300°C

– Granitic magma ~700°C


Pressure Associated with Metamorphism
• Lithostatic pressure: the confining pressure created by
the material that sits above a particular location.
Lithostatic pressure is equal in all directions and
compresses the volume of rock.
– Basalt: 3 g/cm3 (3000 kg/m3)
– Granite: 2.7 g/cm3 (2700 kg/m3)
– The lithostatic pressure at a 10 km
depth is ≈ 3 kbar = 0.3 GPa


Pressure Associated with Metamorphism
• Directed pressure: pressure is imposed in a particular
direction due to a regional stress field.

• Directed pressure affects
the shape and arrangement
of the minerals

• Directed pressure varies with tectonic environment
– Compressional environments: Horz > Vert Pressure
– Extensional environments: Vert > Horz Pressure


Types of Metamorphism
•

Contact Metamorphism
– Thermal variation controls processes

•

Regional Metamorphism
– Orogenic Metamorphism
• Combination of temperature and
directed pressure
– Burial Metamorphism
• Combination of temperature and
lithostatic pressure

•

Fault-Zone Metamorphism
– Directed pressure controls
processes (GEOL 41.1)


Contact Metamorphism
• Occurs adjacent to
igneous intrusions
• Temperature contrast
between magma
chamber and host rock
• Most evident in lowpressure (near-surface)
environments

Regional Metamorphism
• Increase in temperature
is accompanied by an
increase in pressure
• Usually there is directed
pressure, so rock
deformation increases
with metamorphic grade


Development of Foliations

Recrystallization

Pressure Solution

Remobilization

Rotation


Metamorphic Zones
and Facies


Mineral Zones of G.W. Barrow (1893)
• Barrow noted that pelitic
rocks of the Scottish
Highlands had distinct
mineral zones (Gt, Ky, Sil)

• He concluded that this was
the result of increasing
metamorphic grade (T)
• Tilley (1925) added the lowgrade Biotite and Chlorite
zones
Excerpted From Gillen (1982) Metamorphic Geology. An
Introduction to Tectonic and Metamorphic Processes.

• Bt, Gt, St, Ky, and Sil are
Index Minerals in
metapelitic rocks


Index Mineral Isograds
• The line that defines
the first appearance of
an index mineral
corresponds to a line
of equal metamorphic
grade
• Introduction of the
concept of an isograd


Eskola and Metamorphic Facies
• Eskola (1914, 1915) noted
that metapelitic rocks in
southern Finland (Orijärvi)
contained the assemblage
Bt-Ms whereas near Oslo,

rocks contained the
compositionally equivalent
mineral assemblage Kf-Cd
• If rocks are the same
composition, then the
2 KMg3AlSi3O10(OH)2 + 6 KAl2AlSi3O10(OH)2 + 15 SiO2
mineralogical difference
↔
must be due to a difference
3 Mg2Al4Si5O18 + 8 KAlSi3O8 + 8 H2O
in physical conditions


Eskola and Metamorphic Facies
• Eskola (1915) introduced the concept of
metamorphic facies:
– “In any rock or metamorphic formation which has
arrived at a chemical equilibrium through
metamorphism at constant temperature and
pressure conditions, the mineral composition is
controlled only by the chemical composition.”
• A metamorphic facies is a set of repeatedly
associated metamorphic mineral assemblages
• If you find a specified mineral assemblage, then you
can assign a metamorphic facies to the area, and
thereby assign a range of pressure and temperature
conditions.


Eskola and Metamorphic Facies

•

In 1920, Eskola introduced five metamorphic facies that were
defined by mineral assemblages in metabasites:
– Greenschist
– Amphibolite
– Hornfels
– Sanidinite
– Eclogite

•

In 1939, Eskola added an additional 3 metamorphic facies:
– Granulite
– Epidote-amphibolite
– Glaucophane-schist (now called Blueschist)

•

In 1959 and 1960, Coombs added two additional metamorphic
facies:
– Zeolite
– Prehnite-Pumpellyite (now called Subgreenschist)



Progressive Metamorphism of Shales
Increasing Temperature
Slate

Phyllite

Schist

Gneiss


Non-Foliated Metamorphic Rocks