1. Metamorphism and Metamorphic Rocks
Limits of metamorphism
Factors controlling metamorphism
How rocks respond to changes in temperature, pressure, and stress
Types of metamorphism
Metamorphism and plate tectonics

2. Metamorphic Rocks Metamorphism: All changes in
Mineral assemblage
Rock texture
as a result of changes in TEMPERATURE
and PRESSURE
Changes within the solid state
Weathering  diagenesis metamorphism

3. Limits of Metamorphism
Temperature 
200 C
400 C
600 C
10 km
20 km
30 km
Depth
Normal geothermal gradient
20-30 C/km
Low-grade
metamorphism
Lower limit
T>100 C
Depth <2-3 km
High-grade
Metamorphism T>500C
Upper limit
Melting
Igneous rocks

4. Earth’s Geothermal Gradient
Shallow gradient ~25°C/km cannot be sustained to great depths,
~25°C/km
~1°C/km
otherwise the entire interior of the Earth would be molten!

5. Limits of Metamorphism

6. Factors Controlling Metamorphism
End products …
controlled by initial composition of rock
Metamorphic conditions
Temperature
Pressure
Fluids
Stress

7. Fluids (water, gases, salts)
Speed up chemical reactions
A A
B B
A B
SLOW
Dry Rocks- atoms move by diffusion through solid minerals

8. Fluids (water, gases, salts)
Speed up chemical reactions
Wet rocks- diffusion through fluid (FASTER)
A A
B B
A B
Transport material
B A
A C
C B
A A
B B
Fluid with mineral “C”
flows through crack

9. Metasomatism
Hot, hydrothermal fluids

10. Temperature and Pressure
Form new minerals from old minerals
Each mineral assemblage
forms in a given range of T & P

11. Prograde metamorphism
Increasing T & P

12. Decreasing T &P
Time scale
10’s of millions of years
Fluids get squeezed out

13. Differential Stresses
Produce new rock textures
Preferred Orientation of silicate minerals
Hydrostatic Pressure
Uniform in direction
Differential, directed stress
Non-uniform

15. Granite
Gneiss

17. Types of Metamorphism 1. Burial Metamorphism 2. Contact Metamorphism
3. Regional Metamorphism
Defined by T, P, and stress conditions

18. Types of Metamorphism Burial Metamorphism
Most common type, occurs where crust is greater than 5 km thick
Relatively low temperature and pressure (low grade metamorphism)
Very little deformation

19. Burial Metamorphism

20. Types of Metamorphism Contact Metamorphism
occurs in rocks around a magma body
high temperature (heat from magma)
low pressure (occurs at shallow depths in the crust)
minor deformation

21. Contact Metamorphism
Cold
Hot

22. Types of Metamorphism Regional Metamorphism
associated with convergent plate margins
occurs over a range of temperatures and pressures
low grade to high grade metamorphism

23. Different metamorphic mineral zones

24. Metamorphic Facies High T Low P High P Low T Normal geotherm 800 C
10 km
20 km
30 km
High T
Low P
High P
Low T
Normal
geotherm

25. Mineral assemblages caused by specific sets of temperature/pressure conditions

26. Metamorphic Zones and Subduction Zones
Sediments
1. Burial Metamorphism

27. Metamorphic Zones and Subduction Zones
High Pressure
Low Temperature
2. Blueschist Metamorphism

28. Blueschist
Metamorphism

29. Metamorphic Zones and Subduction Zones
Granite magma
rises and causes
contact metamorphism
(high temperature,
low pressure)
Granitic rocks in Volcanic Arc

30. Metamorphic Zones and Subduction Zones
Hot hydrothermal fluids
3. Regional Metamorphism

31. Subduction Zones and Mineral Deposits
Copper-rich deposits
Hydrothermal solutions

33. Al2SiO5 Polymorphs Kyanite deformed kyanite Andalusite Sillimanite
Example of how different
polymorphs reveal
pressure-temperature
conditions/history
of rocks
Kyanite
deformed kyanite
Andalusite
Sillimanite

35. Sil
And
Sil
And

36. Sillimanite Kyanite Quartz Andalusite Hamadan, Iran 2 cm
Sepahi, Whitney & Baharifar (2004)

37. K K K K
And

39. Ky
Grt Ky Ky

43. (Donna Whitney and students)
Metamorphic research
(Donna Whitney and students)
Calibrate Al2SiO5 fabrics with quartz fabrics
(for which flow laws are better known)
Understand role of deformation in
polymorphic transformation
processes and conditions
Develop petrographic tool for
determining crystallization
sequence of 2-3 Al2SiO5
polymorphs

44. How do we know P-T-stability conditions of minerals/polymorphs?
sillimanite
andalusite
kyanite

45. sillimanite
andalusite
kyanite

46. Paterson Apparatus
front panel

47. Torsion actuator
Pressure vessel

48. Experimental apparatus
1000°C, 300 MPa
4 hours
150% shear strain
1250°C, 300 MPa
up to 400% shear strain

49. Shear/triplet
Andalusite
Sillimanite
Kyanite

50. SOME EXPERIMENTAL RESULTS
Sillimanite is the weakest of the 3 polymorphs
No major polymorphic transformation