2. Metamorphic Rocks
•Metamorphic rocks are exposed on every continent and are an
important component of many mountain belts.
•Metamorphism takes place where rocks are subjected to conditions
unlike those in which it formed (usually elevated temperature and/or
pressure).
Metamorphic rocks can be formed from igneous, sedimentary, or even
from other metamorphic rocks known as the parent rock.
There are three common geologic settings for metamorphism:
1. contact metamorphism - changes occur in the country rocks because
of the intrusion of hot magma - "bake" the surrounding rock.
2. cataclastic metamorphism - occurs along fault zones - rock is
broken and pulverized as crustal blocks grind past each other.
3. regional metamorphism - large-scale deformation during "mountain
building" processes.
3.
4. Metamorphic Rocks
• Classified by texture and composition
– Texture is divided into foliated &
nonfoliated rocks
– Mineral composition qualifies textural
name
5. Metamorphic Rocks
• Recrystallization in the solid state
• Caused by changes in T, P or fluids
• New environment = new minerals
• Growing minerals create a new texture
6. Metamorphism
• Recrystallization
– T, P or change in pore fluids initiate change in
the mineral assemblage
– Reaction occur entirely in the solid state
Shale Schist
7. Protolith
The parent rock subjected to Metamorphism
-can be any rock type: igneous,
sedimentary, or metamorphic
Shale Schist
8. • Protolith controls final mineral
assemblage
– Different mineral assemblages form at a given
P & T for various parent rocks
– Metamorphic facies describe a group of
minerals formed under similar conditions
• May be found in different rocks
9.
10. Origin of Metamorphic Rocks
• Metamorphism
Temperature exceeds 200OC
Pressure exceeds 300 M Pa (approx.
3,000 m deep)
• Metamorphism ends when melting
begins
Melting begins at ~700oC
11. Agents of Metamorphism
1. HEAT
2. PRESSURE (stress)
3. CHEMICALLY ACTIVE FLUIDS (H20 and CO2 )
- Metamorphism occurs incrementally, from slight change (low-grade) to
dramatic change (high-grade) from the parent rock.
- During metamorphism, rocks are typically treated to all 3 agents at
once.
13. Heat as an Agent of Metamorphism
Heat is the most important agent since it provides the energy to drive
the
chemical changes that result in recrystallization of the rock.
Two primary ways that heat is an metamorphic agent:
1. During contact metamorphism, the intense heat of an intruded
magma may "bake" the adjacent rock.
2. During regional metamorphism, rocks near the surface of the Earth
may be thrust downward and buried where they are subjected to
increased temperatures and stresses.
Geothermal gradient in crust = ~20° - 30°/km.
15. Contact Metamorphism
•The "baked" zone of alteration is
called an aureole and forms
around the magma intrusion. With
large magmatic intrusions like
batholiths, the aureole can be
several km thick.
•Since differential stress is not
involved, these rocks are generally
not foliated.
•Roof pendants (photo) are
common in the Sierra Nevada and
consist of metamorphosed host
rock adjacent to the upper part of
an igneous pluton (light colored) -
the roof of the magma chamber.
16. Regional Metamorphism
Produces the greatest quantity of metamorphic rocks and is associated with mountain
building.
- Commonly occurs during convergence of tectonic plates - rocks are
folded, faulted, shortened and thickened.
- Rocks are thrust up high to form mountains but an large volume of rock is forced
downward resulting in dramatic thickening of the crust - forming the roots of the
mountains. Examples: Himalayas, Appalachians, Rocky Mountains
17. Pressure and Stress as Metamorphic Agents
Pressure increases with depth ~280 bar/km
Two types of pressure/stress:
1. Isostatic stress or confining pressure - same in all directions (burial).
2. Differential stress -tectonic forces during mountain building
processes may result in increase pressure in one direction relative to the
others (collision of two continents or at a subduction zone).
22. Chemically Active Fluids as a Metamorphic Agent
• Fluids enhance
metamorphic processes -
act as a catalyst by aiding in
the migration of ions.
• Water (with dissolved
ions) is plentiful because it
is usually contained in the
pore spaces of virtually
every rock.
• During deep burial, the
water is squeezed out and
becomes available for
chemical reactions.
23. Metamorphism along Fault Zones
The result of movement along a fault is fault breccia that is composed
of broken or crushed rock fragments - this leads to cataclastic
metamorphism.
24. Effects of Metamorphism on Rocks
Metamorphism cause changes in rocks, including increased density,
growth of larger crystals, reorientation of grains into layers or bands,
and the formation of new minerals.
These changes may be grouped into two broad categories:
1. textural changes
2. mineralogical changes (composition)
In addition, on a larger scale, the forces that cause metamorphism
may also deform (fold and shear) rocks
25. Textural Changes in Metamorphic Rocks
Foliation - mineral grains realign and recrystallize perpendicular to
stress.
Foliated Textures:
1. Rock or Slaty Cleavage - platy mica crystals align resulting in layers
where the rock is easily broken. example: slate.
2. Schistosity - with increased metamorphism, the platy minerals will
grow larger resulting in a scaley appearance. example: schist.
3. Gneissic Texture - at the highest grades, light and dark minerals will
segregate into bands - gneissic layering. example: gneiss.
26. Mineralogical Changes
• During most metamorphic events (especially regional metamorphism),
the chemical composition of the rock does not change (isochemical) —
except for the loss of fluids.
• Existing minerals will recombine to form new "metamorphic" minerals
- but the bulk chemistry of the rock does not typically change.
Example: metamorphism of limestone with quartz grains
calcite + quartz —> wollastonite (a pyroxene)
CaCO3 + SiO2 —> CaSiO3 + CO2
During contact metamorphism, hydrothermal solutions may change
the chemistry of the rock by adding or subtracting chemical
constituents. This is the origin of many ore deposits.
27. • T & P determine degree of metamorphism
Low-grade metamorphism- 200 to 350 OC
Intermediate-grade metamorphism- 350 to 550 OC
High-grade metamorphism - very high temperatures,
above 550OC
28. Regional Metamorphic Zones
• Index minerals
– A mineral that forms within a
specific,often narrow range of conditions
– Identifies a specific grade of
metamorphism
– Allows further subdivision of rock types
31. Regional metamorphism is
gradational in intensity. As we shift
from areas of low-grade to high-grade
metamorphism, we can
observe changes in the mineralogy
and texture of the metamorphic
rocks.
During progressive
metamorphism,
diagnostic minerals appear that
can be correlated with
metamorphic grade - index
minerals.
The map shows the distribution of
index minerals in New England
indicating regions of lower and
higher metamorphic grade.
34. Nonfoliated Texture
typically from contact metamorphic environments
Not all metamorphic rocks will developed foliated texture due to the
absence of platy minerals. However, the size of the crystals increases
with increased metamorphic grade.
example: marble and quartzite.
35. Common Metamorphic Rocks - Nonfoliated Rocks
Regional or Contact Metamorphism
Marble - coarse crystalline metamorphic rock whose parent was
limestone or dolostone. As metamorphic grade increases, the rock
becomes coarser grained.
Quartzite - formed from metamorphosed quartz sandstone. Very hard,
dense rock.
Contact Metamorphism
Hornfels - very hard, fine grained, non-foliated rocks (usually black)
that form during contact metamorphism - baked porcelain-like country
rock.
36. Nonfoliated Rocks
• Marble
– Interlocking, coarse grained calcite
– Recrystallization of limestone or
dolostone
– Sedimentary features
destroyed
42. Common Metamorphic Rocks: Foliated Rocks
1. Slate - low-grade, very fine grained foliated rock composed of
microscopic grains of mica - excellent rock cleavage. Rock cleavage is
not the same as the bedding of the orginal shale.
2. Phyllite - low- to mod-grade where the microscopic grains are large
enough to cause the rock to have a "sheen" - excellent rock
cleavage.
3. Schist - mod- to high grade rock where the platy minerals (micas)
(>50%) are large enough to see with the naked eye. These rocks may
contain accessory minerals such as garnet.
4. Gneiss - high-grade rock where the minerals have segregated into
bands - may or may not possess rock cleavage.
Trend: slate —> phyllite —> schist —> gneiss
43. • Above is an idealized sequence representing progressive metamorphism of a mudrock.
• This sequence is indicative of regional metamorphism.
• Progressive regional metamorphism involves a series of steps or changes in mineralogy and
texture depending on the temperature and pressure.
