1. Metamorphic Rocks

2. Metamorphism
The transition of one rock into another by temperatures and/or pressures unlike those in which it formed
Metamorphic rocks are produced from
Igneous rocks
Sedimentary rocks
Other metamorphic rocks

3. Metamorphism progresses incrementally from low-grade to high-grade
During metamorphism the rock must remain essentially solid
Metamorphic settings
Contact or thermal metamorphism – driven by a rise in temperature within the host rock

4. Agents of Metamorphism
Heat
The most important agent
Recrystallization results in new, stable minerals
Sources of heat?
Radioactive decay
Thermal energy from formation
An increase in temperature with depth due to the geothermal gradient
Upper crust – deg C/km

6. Agents of metamorphism
Pressure (stress)
Increases with depth
Confining pressure applies forces equally in all directions
Rocks may also be subjected to differential stress, which is unequal in different directions
Behavior of rocks -
Shallow----- brittle
Deep ductile

7. Figure 7.3

8. Agents of metamorphism
Chemically active fluid
Mainly water with other volatile components
Enhances migration of ions
removed in areas of high stress
deposited in areas of low stress
Aids in recrystallization of existing minerals
tend to recrystallize and grow longer in a direction perpendicular to the compressional stress

9. Agents of metamorphism
Chemically active fluids
Sources of fluids
Pore spaces of sedimentary rocks
Fractures in igneous rocks
Hydrated minerals such as clays and micas
Metasomatism - an overall change in the composition of the surrounding rock due to the interaction of these fluids

10. Metamorphic textures
Texture refers to the size, shape, and arrangement of grains within a rock
Foliation – any planar arrangement of mineral grains or structural features within a rock

11. Metamorphic textures Foliation can form in various ways including
Rotation of platy and/or elongated minerals
new crystals align perpendicular to max shortening
Recrystallization of minerals in the direction of preferred orientation
shale to slate clay to mica
Changing the shape of equidimensional grains into elongated shapes that are aligned

12. stretch pebble conglomerate

13. Development of foliation due to directed pressure
Figure 7.5

14. Foliated textures Rock or slaty cleavage
Closely spaced planar surfaces along which rocks split
Can develop in a number of ways depending on metamorphic conditions and parent rock

15. Figure 7.6

16. Figure 7.7 (top)

17. Foliated textures Schistosity
Platy minerals are discernible with the unaided eye and exhibit a planar or layered structure
Rocks having this texture are referred to as schist

18. Foliated textures Gneissic
During higher grades of metamorphism, ion migration results in the segregation of minerals
Gneissic rocks exhibit a distinctive banded appearance

19. Figure 7.8

20. Other metamorphic textures
Those metamorphic rocks that lack foliation are referred to as nonfoliated
Develop in environments where deformation is minimal
Typically composed of minerals that exhibit equidimensional crystals

21. Other metamorphic textures
Porphyroblastic textures
Large grains, called porphyroblasts, are surrounded by a fine-grained matrix of other minerals
Porphyroblasts are typically garnet, staurolite, and/or andalusite

22. Figure 7.9

23. Figure 7.10

24. Common metamorphic rocks
Foliated rocks
Slate
Very fine grained – less than 0.5 mm
Excellent rock cleavage
Most often generated from low-grade metamorphism of shale, mudstone, or siltstone

25. Slate
Figure

26. Common metamorphic rocks
Foliated rocks
Phyllite
Gradation in the degree of metamorphism between slate and schist
Platy minerals not large enough to be identified with the unaided eye
Glossy sheen and wavy surfaces
Exhibits rock cleavage
Composed mainly of fine crystals of muscovite and/or chlorite

27. Phyllite
Figure

28. Common metamorphic rocks
Foliated rocks
Schist
Medium to coarse grained
Platy minerals predominate
Commonly include the micas
The term schist describes the texture
To indicate composition, mineral names are used
mica schist
talc schist

29. Mica schist
Figure

30. Common metamorphic rocks
Foliated rocks
Gneiss
Medium to coarse grained
Banded appearance
High-grade metamorphism
Often composed of white or light-colored feldspar-rich layers with bands of dark ferromagnesian minerals

31. Gneiss typically displays a banded appearance
Figure

32. Common metamorphic rocks
Nonfoliated rocks
Marble
Coarse, crystalline
Parent rock was limestone or dolostone
Composed essentially of calcite or dolomite crystals
Used as a decorative and monument stone
Exhibits a variety of colors

33. Marble A nonfoliated metamorphic rock
Figure

34. Common metamorphic rocks
Nonfoliated rocks
Quartzite
Formed from a parent rock of quartz-rich sandstone
Quartz grains are fused together

35. Quartzite
Figure

36. Low grade>>>>>>>>>>>>High grade
Low grade>>>>>>>>>>>>High grade Metamorphism

37. Metamorphic environments
Contact or thermal metamorphism
Occurs due to a rise in temperature when magma invades a host rock
A zone of alteration called an aureole forms in the rock surrounding the magma
Most easily recognized when it occurs at the surface, or in a near-surface environment

38. Contact metamorphism
Figure 7.13

40. Figure 7.14

41. Metamorphic environments
Hydrothermal metamorphism
Chemical alteration caused when hot, ion-rich fluids, called hydrothermal solutions, circulate through fissures and cracks that develop in rock
Most widespread along the axis of the mid-ocean ridge system

42. Figure 7.16

43. Figure 7.15 example of shallow depth hydrothermal metamorphism

44. Metamorphic environments
Burial metamorphism
Associated with very thick sedimentary strata
Required depth varies from one location to another depending on the prevailing geothermal gradient
Subduction zone metamorphism

45. Metamorphic environments
Regional metamorphism
Produces the greatest quantity of metamorphic rock
Associated with mountain building
Rocks usually display zones of contact and/or hydrothermal metamorphism

46. Regional metamorphism
Figure 7.17

47. Metamorphic environments
Other metamorphic environments
Metamorphism along fault zones
Occurs at depth and high temperatures
Pre-existing minerals deform by ductile flow
Impact metamorphism
Occurs when high speed projectiles called meteorites strike Earth’s surface
Products are called impactites

48. Figure 7.18 Metamorphism along a fault zone – brittle vs. ductile

49. Metamorphic zones Textural variations
systematic variations in the mineralogy and often the textures of metamorphic rocks are related to the variations in the degree of metamorphism
Index minerals and metamorphic grade
Changes in mineralogy occur from regions of low-grade metamorphism to regions of high-grade metamorphism

50. Metamorphic rocks and associated environments
Figure 7.19

51. Figure 7.20

52. Metamorphic zones in the Northeastern U.S.
Figure 7.21

53. Migmatites
Highest grades of metamorphism that is transitional to igneous rocks
Contain light bands of igneous components along with areas of unmelted dark layers of metamorphic rock

54. Figure 7.22