1. Metamorphism and Metamorphic Rocks
GEOL: CHAPTER 7
Metamorphism and Metamorphic Rocks

2. This slate exposed in the Wasatch Mountains just east of Salt Lake City, Utah, demonstrates “slaty cleavage,” wherein the rock tends to split into flat, plate-like pieces along cleavage planes.

3. Learning Outcomes LO1: Identify the agents of metamorphism
LO2: Identify the three types of metamorphism
LO3: Explain how metamorphic rocks are classified
LO4: Recognize the difference between metamorphic zones and facies
LO5: Understand how plate tectonics affects metamorphism
LO6: Understand the relationship between metamorphism and natural resources

4. Metamorphism
Metamorphic rock: any rock that has been changed from its original condition by heat, pressure, and the chemical activity of fluids
Metamorphism: the phenomenon of changing rocks subjected to heat, pressure, and fluids so that they are in equilibrium with a new set of environmental conditions

5. Figure 7.1 Gneiss This gneiss, found in Canada, is estimated to be about 4.0 billion years old. Gneiss is a foliated metamorphic rock.

6. Agents of Metamorphism
Heat
Pressure
Fluid activity
Time for the above to chemically alter rocks is also an important factor

7. Heat Increases rates of chemical reactions
Can produce minerals different than those in the original rock
Lava
Magma
Burial: geothermal gradient; subducted rock

8. Pressure Pressure increases with depth
Lithostatic pressure: pressure exerted on rocks by the weight of overlying rocks
Differential pressure:
Pressure not applied equally to all sides of a body
Stresses stronger in some directions
Common with plate collisions

9. Figure 7.2 Lithostatic Pressure

10. Figure 7.3 Differential Pressure Differential pressure results from stress that is unequally applied to an object. Rotated garnets are a good example of the effects of differential pressure applied to a rock during metamorphism. In this example from a schist in northeast Sardinia, stress was applied in opposite directions on the left and right side of the garnet (center), causing it to rotate.

11. Fluid Activity
Water (and carbon dioxide) are present in almost every region of metamorphism
Water increases reaction rates and thus metamorphism
Water also reacts with some minerals to create new minerals
Seawater moving through hot basaltic rock converts olivine to serpentine

12. Fluid Activity, cont. Sources of chemically active fluids:
Water trapped in pore spaces of sedimentary rocks
Volatile fluid in magma
Dehydration of water-bearing minerals

13. Three Types of Metamorphism
Contact (thermal) metamorphism
Dynamic metamorphism
Regional metamorphism

14. Contact Metamorphism Metamorphism of country rock adjacent to a pluton
Magma from forming pluton:
Raises temperature of country rock
Releases hot fluids into country rock
Aureole: zone of metamorphism surrounding a pluton

15. Contact Metamorphism, cont.
Important factors:
Initial temperature
Size of the intrusion
Fluid content of magma and country rock: hydrothermal alteration creates mineral deposits
Degree of metamorphic change decreases with distance from pluton
Can also occur with lava flows

16. Figure 7.4 Metamorphic Aureole A metamorphic aureole, the area surrounding an intrusion, consists of zones that reflect the degree of metamorphism. The metamorphic aureole associated with this idealized granite pluton contains three zones of mineral assemblages reflecting the decrease in temperature with distance from the intrusion. An inner andalusite–cordierite hornfels zone forms adjacent to the pluton and is reflective of the high temperatures near the intrusion. This is followed by an intermediate zone of extensive recrystallization in which some biotite develops, and farthest from the intrusion is an outer zone characterized by spotted slates.

17. Figure 7.5 Contact Metamorphism from a Lava Flow A highly weathered basaltic lava flow near Susanville, California, has altered an underlying rhyolitic volcanic ash by contact metamorphism. The red zone below the lava flow has been baked by the heat of the lava when it flowed over the ash layer. The lava flow displays spheroidal weathering, a type of weathering common in fractured rocks (see Chapter 6).

18. Dynamic Metamorphism
Occurs in fault zones where rocks are under high degrees of differential pressure
Mylonites frequently created
Hard, dense, fine-grained
Thin laminations

19. Figure 7.6 Mylonite An outcrop of mylonite from the Adirondack Highlands, New York. Mylonites result from dynamic metamorphism, where rocks are subjected to high levels of differential pressure. Note the thin laminations (closely spaced layers), which are characteristic of many mylonites.

20. Regional Metamorphism
Occurs over a large area
Caused by very high temperatures and pressures, occurring simultaneously with deformation
Convergent plate boundaries
Gradations of metamorphism based on severity of conditions

21. Index Minerals and Metamorphic Grade
Index mineral: a mineral that forms within specific temperature and pressure ranges during metamorphism
Metamorphic grade: the degree to which a rock has undergone metamorphic change
Different rock compositions have different sets of index minerals

22. Figure 7.7 Metamorphic Grade Change in mineral assemblage and rock type with increasing metamorphism of shale. When a clay-rich rock such as shale is subjected to increasing metamorphism, new minerals form, as shown by the colored bars. The progressive appearance of certain minerals, known as index minerals, allows geologists to recognize low-, intermediate-, and high-grade metamorphism.

23. Classifying Metamorphic Rocks
Foliated texture: Platy and elongate minerals aligned in a parallel fashion
Nonfoliated texture: no discernable preferred orientation of minerals

25. Foliated Metamorphic Rocks
Foliated texture: Platy and elongate minerals aligned in a parallel fashion
From heat and differential pressure
Size and shape of mineral grains determine whether foliation is fine or coarse

26. Figure 7.8 Foliated Texture

27. Slate Low-grade metamorphism Finely foliated
Minerals can only be seen with magnification
Slaty cleavage
Regional metamorphism of shale

28. Figure 7.9 Slate

29. Phyllite Coarser grained than slate, but similar composition
Need magnification to see minerals
Intermediate grain size between slate and schist

30. Figure 7. 10 Phyllite Hand specimen of phyllite
Figure 7.10 Phyllite Hand specimen of phyllite. Note the lustrous sheen, as well as the bedding (upper left to lower right) at an angle to the cleavage of the specimen.

31. Schist Commonly produced in regional metamorphism
Type depends on intensity of metamorphism and characteristic of original rock
Many rock types yield schist
Minerals clearly visible
Schistosity/schistose foliation
Each type has associated minerals

32. Figure 7.11 Schist

33. Figure 7.11 Schist

34. Figure 7.11 Schist

35. Gneiss High-grade metamorphism
Segregated bands of light and dark minerals
Light bands: quartz and feldspar
Dark bands: biotite and hornblende
Forms from recrystallization of sedimentary rocks during regional metamorphism, or from igneous or metamorphic rocks

36. Figure 7.12 Gneiss Gneiss is characterized by segregated bands of light and dark minerals. This folded gneiss is exposed at Wawa, Ontario, Canada.

37. Other Foliated Rocks Amphibolite Migmatites
Dark rock: hornblende and plagioclase
Slightly foliated
From basalt and ferromagnesian-rich mafic rocks
Migmatites
Igneous and high-grade metamorphic characteristics

38. Figure 7.13 Migmatites A migmatite boulder in the Rocky Mountain National Park, near Estes Park, Colorado. Migmatities consist of high-grade metamorphic rock intermixed with streaks or lenses of granite.

39. Nonfoliated Metamorphic Rocks
Nonfoliated texture: no discernable preferred orientation of minerals
Mosaic or roughly equidimensional minerals
Contact or regional metamorphism of rocks with no platy or elongate minerals

40. Figure 7.14 Nonfoliated Texture Nonfoliated textures are characterized by a mosaic of roughly equidimensional minerals, as in this photomicrograph of marble.

41. Marble Predominantly calcite or dolomite
Grains range from fine to coarsely granular
Contact or regional metamorphism of limestones or dolostones
Colors come from impurities in parent rock

42. Figure 7.15 Marble Results from the Metamorphism of the Sedimentary Rock Limestone or Dolostone

43. The softness of marble, its uniform texture, and its varying colors have made it the favorite rock of builders and sculptors throughout history.

44. Quartzite Formed from quartz sandstone
Intermediate to high-grade metamorphism
Typically complete recrystallization of quartz grains
Impurities add color

45. Figure 7.16 Quartzite Results from the Metamorphism of the Sedimentary Rock Quartz Sandstone

46. Other Nonfoliated Rocks
Greenstone: dark-green, from mafic igneous rock; low- to high-grade metamorphism
Hornfels: many varieties, but mostly from sedimentary rocks; from contact metamorphism
Anthracite: hard coal with high % carbon; metamorphism of lower-grade coals

47. Metamorphic Zones
Metamorphic zone: the region between lines of equal metamorphic intensity known as isograds
Metamorphic rocks divided into zones based on presence of distinctive silicate minerals

48. Metamorphic Zones, cont.
Successive appearance of index minerals shows progression to higher-grade metamorphism
First appearance of a mineral indicates minimum temperature and pressure conditions
Isograds: lines of equal metamorphic intensity

49. Figure 7.17 Metamorphic Zones in the Upper Peninsula of Michigan The zones in this region are based on the presence of distinctive silicate mineral assemblages resulting from the metamorphism of sedimentary rocks during an interval of mountain building and minor granitic intrusion during the Proterozoic Eon, approximately 1.5 billion years ago. The lines separating the different metamorphic zones are isograds.

50. Metamorphic Facies
A group of metamorphic rocks characterized by particular minerals that formed under the same broad temperature and pressure conditions
Named after most characteristic rock or mineral
Not applicable when original rocks were pure quartz sandstones or pure limestones or dolostones

51. Figure 7.18 Metamorphic Facies and Their Associated Temperature–Pressure Conditions A temperature–pressure diagram showing under what conditions various metamorphic facies occur. A metamorphic facies is characterized by a particular mineral assemblage that formed under the same broad temperature–pressure conditions. Each facies is named after its most characteristic rock or mineral.

52. Plate Tectonics and Metamorphism
Associated with all 3 types of boundaries
Most common with convergent boundaries
Temperature and pressure characteristics
Produces typical metamorphic facies
Subducting plate heats only slowly at first, so initial metamorphism from high pressure
Blueschist facies

53. Plate Tectonics and Metamorphism, cont.
As subduction continues, temperatures and pressures increase
High-grade metamorphic rocks
Plate begins to melt and generate magma: contact metamorphism
Divergent boundaries
Contact metamorphism from magma
Metal-bearing hydrothermal solutions

54. Figure 7.19 Relationship of Facies to Major Tectonic Features at an Oceanic–Continental Convergent Plate Boundary

55. Metamorphism and Natural Resources
Marble
Slate
Ore deposits from hot hydrothermal fluids
Migrate into surrounding rock
Copper: bornite, chalcopyrite
Lead: galena
Zinc: pyrite and sphalerite
Iron: hematite and magnetite
Asbestos, talc, graphite, garnets