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

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Presentation on theme: "Metamorphism and Metamorphic Rocks Limits of metamorphism Factors controlling metamorphism How rocks respond to changes in temperature, pressure, and stress."— 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 Low-grade metamorphism High-grade Metamorphism T>500C Temperature  200 C400 C600 C 10 km 20 km 30 km Depth Upper limit Melting Igneous rocks Lower limit T>100 C Depth <2-3 km Normal geothermal gradient C/km

4 Earth’s Geothermal Gradient Shallow gradient ~25°C/km cannot be sustained to great depths, ~1°C/km ~25°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 A B Fluids (water, gases, salts) Speed up chemical reactions A B Dry Rocks- atoms move by diffusion through solid minerals SLOW

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

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

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

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15 GraniteGneiss

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

18 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 Types of Metamorphism

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 Hot Cold

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 Normal geotherm High T Low P High P Low T 200 C 800 C 10 km 20 km 30 km Metamorphic Facies

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 2. Blueschist Metamorphism High Pressure Low Temperature

28 Blueschist Metamorphism

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

30 3. Regional Metamorphism Hot hydrothermal fluids

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

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33 Kyanite Andalusite Sillimanite deformed kyanite Al 2 SiO 5 Polymorphs Example of how different polymorphs reveal pressure-temperature conditions/history of rocks

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35 And Sil

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

37 K K K K And

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39 Grt Ky

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43 Calibrate Al 2 SiO 5 fabrics with quartz fabrics (for which flow laws are better known) Develop petrographic tool for determining crystallization sequence of 2-3 Al 2 SiO 5 polymorphs Understand role of deformation in polymorphic transformation processes and conditions Metamorphic research (Donna Whitney and students)

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

45 kyanite sillimaniteandalusite

46 Paterson Apparatus front panel

47 Pressure vessel Torsion actuator

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

49 Andalusite Kyanite Sillimanite Shear/triplet

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


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