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Geology 12

Geology 12. Presents. Metamorphic Rocks. Chp 7 Metamorphic Rocks. Process by which heat, pressure and fluids change pre-existing rock in the solid state. Change. Shape.

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Geology 12

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  1. Geology 12 Presents

  2. Metamorphic Rocks

  3. Chp 7 Metamorphic Rocks • Process by which heat, pressure and fluids change pre-existing rock in the solid state Change Shape

  4. Where?: mainly associated with the continents. Sedimentary rocks (& sediments) cover much of the continents, but they are underlain by mainly igneous and metamorphic rocks making up the basement/craton. • Where the basement is exposed is the shield. • Basement/shield: very stable over the last 600 ma and as 4 ba. Not crouton!! billion years million years

  5. Canadian Shield

  6. Agents of Metamorphism: (3) • 1. Heat: increases rate of chemical reactions • 2 sources of heat: • a) Magma body: heats surrounding rock

  7. b) geothermal gradient: Earth warms up as you descend • Average of 25’C/km • Question: what is temperature at 30 km? T = 30 km x 25’C/km + 10’C T = 750’C + 10’C T = 760’C

  8. 2. Pressure: • 2 types • A) Lithostatic Pressure = confining pressure = weight of overlying rock • Applied equally in all directions • Increases 0.25 Kbars/km of depth • (1 Kbar = 14,700 psi)

  9. b) Differential Pressure = directed pressure = pressure due to mountain building • Pressure is unequal so rock is distorted Meta-conglomerate conglomerate

  10. 3. Fluid Activity: water & CO2 enhance metamorphism by increasing chemical reactions by moving ions through solution Ex: 2Mg2SiO4 + 2H2O Mg3Si2O5(OH)4 + MgO Olivine water serpentine away in sol’n • 3 sources of water: • Sedimentary rock pore space (sh, sst) • Released from magma • Dehydration of water bearing minerals such as gypsum (CaSO4 2H2O)

  11. Types of Metamorphism (3) • 1. Contact Metamorphism: when a hot body of magma alters surrounding rock with increased temperatures and the release of hot fluids (Hi T/Low P + fluids) Aureole = concentric zones of alteration Country rock slight metamorphism (rare) Igneous intrusion intermediate metamorphism (medium) extensive metamorphism (well-done)

  12. medium rare Well done

  13. The larger the intrusion (more heat + more fluids) = more alteration • Contact Met’ is generally localized (small areas affected) • i) dikes & sills: only rock in immediate contact affected • ii) laccoliths & batholiths: larger area affected • 2 Types of contact Met’ • i) baked (porcelain) clay: HEAT • ii) as magma cools, water is released making new minerals -> hydrothermal alteration – valuable ore deposits

  14. Contact Metamorphism

  15. Country Rock Contact Metamorphism Intrusion

  16. 2. Dynamic Metamorphism: very localized alteration around fault zones due to differential pressure (Hi P/Low T) Slikensides or fault breccia

  17. Dynamic Metamorphism

  18. Fault breccia

  19. Oceanic crust Continental crust Upper mantle Upper mantle • 3. Regional Metamorphism: results in most metamorphic rocks • Occurs over large areas • Caused by tremendous pressures, temperatures, and deformation, deep in the crust • Usually along convergent plate margins, but… Low T/Hi P (differential) zone Hi T/Hi P (lithostatic) zone

  20. Regional Metamorphism

  21. ocean Descending sea water Met’ rocks High heat flow Hot magma • …but, sometimes at divergent boundaries (hi T + fluids)

  22. Hand out note helper WS 7.1a

  23. Classification of Metamorphic Rocks • 2.5 types! • 1. Foliated: minerals have arranged in a parallel fashion…result of differential pressure

  24. Foliated Metamorphic Rocks

  25. Rock Parent Rock Appearance Metamorphic Grade Slate sh, volcanic fine grained, low ash splits into flat pieces Phyllite sh fine grained, low-med silvery sheen Schist sh, carbonates flakey micas low-high mafic ign’ rx Gneiss sh, sst, dark & light high felsic ign’ rx bands “Zebra Rock”

  26. Slate

  27. Phyllite

  28. Notice “slight sheen”

  29. Schist

  30. Gneiss = “Zebra Rock”

  31. “Zebras are gneiss”

  32. 2. Non-foliated: minerals have no orientation, “equidimensional” (no layers, no banding) Rock Parent Appearance Metamorphic Rock Grade Marble Lst sugary, HCl low-high Quartzite Qtz sst sugary, H=7 med-high

  33. Non-foliated Metamorphic Rocks Marble

  34. Quartzite Ford tough

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