2. Chapter
8
Outline
• The rock cycle and metamorphic rocks
-What is the cycle and basics of metamorphic rocks.
• Metamorphism
-A solid state process
-Character of metamorphic rocks
-Specific processes of metamorphism (5 types)
-Causes of metamorphism (T, P, diff stress, hyd-th fluids)
• Metamorphic Rocks – more details:
-The foliated ones: slate, phyllite, schist, gneiss
-The unfoliated ones: quartzite, marble
-Type controlled by parent rock
• Metamorphic classifications
-Classes, intensity, grade, facies
-Metamorphic environments
Chapter 8
3. Chapter
8
Metamorphic Rocks: Basics
• Metamorphism change + form/shape
• Change from original “parent” rock
• Parent rocks are called “protoliths”
• Any protolith can experience metamorphism
4. Chapter
8
Metamorphic Rocks: Basics
• Lots of change in physical or chemical conditions.
• Burial
• Tectonic stresses (compression/extension/shear)
• Heating by magma
Fluid alteration
• Result: protolith changes…
• Texture
• Minerology
8. Chapter
8
Outline
• The rock cycle and metamorphic rocks
-What is the cycle and basics of metamorphic rocks.
• Metamorphism
-A solid state process
-Character of metamorphic rocks
-Specific processes of metamorphism (5 types)
-Causes of metamorphism (T, P, diff stress, hyd-th fluids)
• Metamorphic Rocks – more details:
-The foliated ones: slate, phyllite, schist, gneiss
-The unfoliated ones: quartzite, marble
-Type controlled by parent rock
• Metamorphic classifications
-Classes, intensity, grade, facies
-Metamorphic environments
Chapter 8
10. Chapter
8
Metamorphic Character
• Metamorphic rocks have distinctive properties.
• Texture – intergrown and interlocking grains
• Minerals – some that are only metamorphic
• Foliation – a planar fabric from aligned minerals
Red mudstoneRed mudstone
Garnet gneissGarnet gneiss
Fossiliferous limestoneFossiliferous limestone
MarbleMarble
11. Chapter
8
Metamorphic Processes
• Metamorphic change is slow and in the solid state.
• Several processes at work, simultaneously:
1. Recrystallization – minerals change size/shape
2. Phase change – new minerals form with…
same chemical formula
different crystal structure
Kyanite
12. Chapter
8
Metamorphic Processes
3. Neocrystallization – new minerals with P-T changes
1. Initial minerals become unstable; change to new minerals
2. E.g. in this way, a shale can transform into a garnet mica schist
14. Chapter
8
Causes of Metamorphism
1. Heat (Temperature – T).
2. Pressure (P).
3. Differential stress.
4. Hydrothermal fluids.
5. Not all are required; they often do co-occur.
6. Rocks may be metamorphosed multiple times.
15. Chapter
8
• Pressure that is greater in one orientation.
• A common result of tectonic forces
• 2 kinds of differential stress: Normal & shear.
1. Normal stress – perpendicular to a surface
tension (pull-apart)
compression (push-together)
Differential Stress
16. Chapter
8
Differential Stress
• 2 kinds of differential stress: Normal & shear.
2. Shear stress – sideways across a surface
causes material to be “smeared out”
17. Chapter
8
• At high T & P, differential stress deforms rock.
• Rocks change shape slowly without breaking
Differential Stress
18. Chapter
8
Results of Differential Stress
• Deformation acts on minerals with specific shapes.
• Equant – equal in all directions
• Inequant –unequal dimensions
• Platy (pancake-like)- 1 dimension shorter
• Elongate (cigar-shaped)- 1 dimension longer
• Differential stress causes minerals to align
• Aligned fabric records stress orientation
19. Chapter
8
Results of Differential Stress
• Mineral alignment called foliation.
• Banded appearance
• Develops perpendicular to compression.
• Minerals flatten, recrystallize, and rotate
• Inequant grains align by rotation and new growth
20. Chapter
8
Hydrothermal Fluids
• Hot water with dissolved ions and volatiles
• Hydrothermal fluids facilitate metamorphism by…
• Accelerating chemical reactions
• Alternating rocks by adding/subtracting elements
• Hydrothermal alteration is called metasomatism.
21. Chapter
8
Outline
• The rock cycle and metamorphic rocks
-What is the cycle and basics of metamorphic rocks.
• Metamorphism
-A solid state process
-Character of metamorphic rocks
-Specific processes of metamorphism (5 types)
-Causes of metamorphism (T, P, diff stress, hyd-th fluids)
• Metamorphic Rocks – more details:
-The foliated ones: slate, phyllite, schist, gneiss, migmatite
-The unfoliated ones: amphibolite, hornfels, quartzite, marble
-Type controlled by parent rock
• Metamorphic classifications
-Classes, intensity, grade, facies
-Metamorphic environments
Chapter 8
22. Chapter
8
Metamorphic Rock Types
• 2 major subdivisions of metamorphic rocks.
1. Foliated – has a through-going planar fabric
1. Due to differential stress
2. Have platy minerals
3. Classified by composition, grain size, and foliation type
23. Chapter
8
Metamorphic Rock Types
• 2 major subdivisions of metamorphic rocks.
2. Non-foliated – no planar fabric
1. Crystallized without differential stress
2. Comprised of equant minerals
3. Classified by mineral composition
24. Chapter
8
Foliated Metamorphic Rocks
• Compositional banding develops in several ways:
• Original layering in the protolith
• Extensive, high T shearing
26. Chapter
8
Foliated Metamorphic Rocks
• Slate – clay protolith, low-grade metamorphic shale.
• Distinct foliation called slaty cleavage
• Alignment of platy clay minerals
• Cleavage perpendicular to compression
• Slate breaks along foliation as flat sheets
27. Chapter
8
Foliated Metamorphic Rocks
• Phyllite - Fine mica-rich rock.
• Formed by low-medium grade alternation of slate
• Clay minerals neocrystallize into mica (shiny luster)
• Phyllite is between slate and schist
28. Chapter
8
• Schist – rock with larger micas.
• Medium-to-high-grade metamorphism
• Distinct foliation called schistosity
• Parallel alignment of mica crystals micas visible becase they
grew at higher T
• Schist often has other minerals due to neocrystallization:
• Quartz
• Feldspars
• Kyanite
• Garnet
• Staurolite
• Sillimanite
Large non-mica minerals are called porphyroblasts
Foliated Metamorphic Rocks
29. Chapter
8
Foliated Metamorphic Rocks
• Gneiss – distinct banded foliation (high metam. Grade)
• Light bands of felsic minerals (quartz and feldspars)
• Dark bands of mafic minerals (biotite or amphibole)
31. Chapter
8
Non-foliated Metamorphic
Rocks
• Quartzite – Almost pure quartz in composition.
• Forms by alternation of sandstone
• Sand grans in the protolith recrystallize and fuse
• Cant see gran boundaries anymore
Metamorphic Alteration
32. Chapter
8
Non-foliated Metamorphic
Rocks
• Marble – coarse crystalline carbonate.
• Forms from a carbonate (i.e. limestone) protolith
• Recrystallization occurs
• Origional textures/fossils in parent are destroyed
Metamorphic Alteration
34. Chapter
8
Outline
• The rock cycle and metamorphic rocks
-What is the cycle and basics of metamorphic rocks.
• Metamorphism
-A solid state process
-Character of metamorphic rocks
-Specific processes of metamorphism (5 types)
-Causes of metamorphism (T, P, diff stress, hyd-th fluids)
• Metamorphic Rocks – more details:
-The foliated ones: slate, phyllite, schist, gneiss
-The unfoliated ones: quartzite, marble
-Type controlled by parent rock
• Metamorphic classifications
-Classes, intensity, grade, facies
-Metamorphic environments
Chapter 8
35. Chapter
8
Metamorphic Classes
1. Pelitic – Shale protoliths.
• Al-rich clay minerals yield micas
• Rock type depends on grade (degree of metamorphism).
• Slate
• Phyllite
• Schist
• Gneiss
41. Chapter
8
Metamorphic Grade
• Certain minerals have a limited P-T range.
• These “index minerals” indicate grade
• Index mineral maps
• Define metamorphic zones
43. Chapter
8
Metamorphic Environments
• Different settings yield different effects via…
• P & T gradients
• Differential stresses
• Hydrothermal fluids
These characteristics are governed by tectonics.
44. Chapter
8
Metamorphic Environments
• Types (and settings) of metamorphism are...
• Thermal – heating by magma intrusion (”contct” metamorph)
• Burial – increases in P and T
• Regional – P and T change due to mountain building
• Hydrothermal – alteration by hot water
• Subduction – high P- low T alteration
• Shock – very high P due to impact
• Mantle – very high P causes mineral phase changes
45. Chapter
8
Contact Metamorphism
• Heat from magma intrusion.
• Creates zoned bands of alteration in country rock.
• Called a contact aureole
• Aureole surrounds the intrusion
• Zoned form high to low grade
46. Chapter
8
Burial Metamorphism
• As sediments are buried…
• P increases due to weight above
• T increases due to geothermal gradient
• Requires burial below diagenetic effects
• E.g. >5-15km depth
47. Chapter
8
Regional Metamorphism
• Tectonic collisions deform rocks.
• Creates mountains.
• Rocks are…
• Heated by geothermal gradient and intrusions
• Squeezed and heated by burial
• Smashed and sheared by differential stresses
48. Chapter
8
Subduction Metamorphism
• Trenches & accretionary prisms have…
• Low temperature (lowh7yuuuuuy geothermal gradient)
• High pressures (collision)
• High P/low T formation of blueschist
• Rock with a blue mineral called glaucophane
Editor's Notes
Analogy to firing of potter’s clay
The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building
Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate.
TRANSPARENCY: Shields composed of metamorphic rocks.
Analogy to firing of potter’s clay
The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building
Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate.
TRANSPARENCY: Shields composed of metamorphic rocks.
Analogy to firing of potter’s clay
The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building
Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate.
TRANSPARENCY: Shields composed of metamorphic rocks.
Analogy to firing of potter’s clay
The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building
Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate.
TRANSPARENCY: Shields composed of metamorphic rocks.
Analogy to firing of potter’s clay
The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building
Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate.
TRANSPARENCY: Shields composed of metamorphic rocks.
Analogy to firing of potter’s clay
The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building
Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate.
TRANSPARENCY: Shields composed of metamorphic rocks.
Analogy to firing of potter’s clay
The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building
Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate.
TRANSPARENCY: Shields composed of metamorphic rocks.
Analogy to firing of potter’s clay
The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building
Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate.
TRANSPARENCY: Shields composed of metamorphic rocks.
Analogy to firing of potter’s clay
The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building
Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate.
TRANSPARENCY: Shields composed of metamorphic rocks.
Analogy to firing of potter’s clay
The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building
Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate.
TRANSPARENCY: Shields composed of metamorphic rocks.
Analogy to firing of potter’s clay
The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building
Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate.
TRANSPARENCY: Shields composed of metamorphic rocks.
Analogy to firing of potter’s clay
The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building
Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate.
TRANSPARENCY: Shields composed of metamorphic rocks.
Analogy to firing of potter’s clay
The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building
Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate.
TRANSPARENCY: Shields composed of metamorphic rocks.
