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Metamorphism and Metamorphic Rocks

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Presentation on theme: "Metamorphism and Metamorphic Rocks"— Presentation transcript:

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

Sillimanite is the weakest of the 3 polymorphs No major polymorphic transformation

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