1. Chapter 8: Metamorphism & Metamorphic Rocks

2. Introduction “Meta” = change & “morph” = form
Metamorphic = to change form
Transition of one rock into another by application of pressure and/or temperature unlike those from which it formed.
Metamorphic rocks are produced from
Sedimentary rocks
Igneous rocks
Other metamorphic rocks
Parent rock = protolith
Rock from which the metamorphic rock was formed

3. Types of Metamorphism Progresses incrementally
Low-grade metamorphism (< 200oC)
High-grade metamorphism (> 600oC)
** Rock must remain solid!!
If melting occurs, then igneous rocks are formed
Agents of Metamorphism
Heat
Pressure & differential stress
Chemically active fluids

4. Settings of Metamorphism
Three settings:
Thermal metamorphism
AKA contact metamorphism
Intrusion of magma body
Change driven by rise in temperature
Hydrothermal metamorphism
Chemical alterations that occur as hot ion-rich water circulates through rock
Regional metamorphism
AKA large-scale deformation
Large quantities of rock are subject to P and high T

5. Heat as Metamorphic Agent
Most important agent
b/c provides energy for chemical reactions & recrystallization
Sources:
Geothermal gradient
Contact metamorphism

6. Pressure & Differential Stress
Confining pressure  compaction
Equal pressure in all directions
Differential stress = directed pressure
Causes folds & faults

7. Brittle vs. Ductile Behavior
At surface, rocks are brittle
Tend to fracture & break into smaller pieces
At high-T, rocks are ductile
Grains tend to
flatten and
elongate

8. Chemically Active Fluids
Mostly water (H2O) and carbon dioxide (CO2)
Sources of fluids:
Hydrated minerals
e.g. Clays & amphiboles
Movement:
Pore spaces of sedimentary rocks
Fractures in igneous rocks

9. Importance of Protolith
Remember: protolith = parent rock
Most metamorphic rock have same chemical composition of their parent rock
Except for gain or loss of volatiles
e.g., H2O & CO2

10. Metamorphic Textures
Texture = size, shape & orientation (arrangement) of grains in rock
Foliation = any planar arrangement of mineral grains or structural features within a rock
e.g., parallel alignment of platy and/or elongated minerals
e.g., parallel alignment of flattened mineral grains or elongated pebbles
e.g., compositional banding
e.g., slaty cleavage where rocks can be easily split into thin, tabular sheets

11. Foliation: Alignment of Platy Minerals

12. Foliation: Alignment of Elongated Pebbles

13. Foliation: Compositional Banding
Granite protolith
resultant Gneiss with compositional banding

14. Foliation: Slaty Cleavage

15. Foliated Textures Rock cleavage (AKA slaty cleavage) Schistosity
Closely spaced planar surfaces along which rocks split
e.g., slate (originally shale)
Schistosity
Platy minerals (e.g., micas) are discernible with unaided eye
Exhibit planar or layered structure
e.g., schist (originally slate)
Gneissic banding
During higher grades of metamorphism, ion migration results in separation of light and dark minerals
Exhibit distinctive light & dark compositional banding

16. Formation of Slate

17. Garnet – Mica Schist

18. Gneissic Banding

19. Other Metamorphic Textures
Nonfoliated
Form in environments where deformation is minimal
e.g., fine-grained limestone + heat = marble
Porphyroblastic texture
Large grains surrounded by smaller grains
Porphyroblast = large grains
Matrix = fine-grains around porphyroblast

20. Common Metamorphic Rocks
Foliated Rocks:
Slate
Phyllite
Schist
Gneiss
Nonfoliated Rocks:
Marble
Quartzite

21. Slate Very fine-grained Excellent rock cleavage
Slaty cleavage
Most often generated from low-grade metamorphism of shale, mudstone or siltstone
Different colors:
Black = carbonaceous
Red = Fe-oxide
Green = chlorite

22. Phyllite Degree of metamorphism between slate & schist
Platy minerals not large enough to be identified with unaided eye
Glossy sheen & wavy surfaces
Has rock cleavage
Composed of platy minerals such as micas & chlorite
Phyllite
Slate

23. Schist Medium- to coarse-grained Medium-grade metamorpism
Platy minerals dominate
Mainly micas
Schistositic texture
To indicate composition, mineral names are used
e.g., mica schist 

24. Gneiss Medium- to coarse-grained Banded layered appearance
High-grade metamorphism
Often composed of white or light-colored feldspar-rich layers with bands of dark ferromagnesian-rich layers

25. Marble Coarse-grained Crystalline Protolith = limestone
Composed of essentially calcite [CaCO3] and/or dolomite [CaMg (CO3)2] crystals
Exhibits a variety of colors
Used as decorative & monument stone

26. Quartzite Medium- to coarse-grained Crystalline Protolith = sandstone
Composed dominately of quartz [SiO2]

27. Classification of Metamorphic Rocks

28. Metamorphic Environments
Contact Metamorphism
AKA Thermal Metamorphism
Hydrothermal Metamorphism
Regional Metamorphism
Other Metamorphic Environments
Burial Metamorphism
Metamorphism along fault zones
Impact metamorphism
AKA Shock Metamorphism

29. Contact Metamorphism

30. Hydrothermal Metamorphism

31. Regional Metamorphism

32. Burial Metamorphism

33. Fault Zone Metamorphism

34. Impact Metamorphism

35. Metamorphic Zones Metamorphic Grade: Index Minerals:
Systematic variation in mineralogy & textures are related to the variations in the degree of metamorphism
Changes in mineralogy occur from regions of low-grade metamorphism to regions of high-grade metamorphism
Index Minerals:
Certain minerals which are good indicators of the degree of the metamorphic conditions under which they form
e.g., chlorite ~200oC

36. Progressive Regional Metamorphism
With increasing P & T, higher degree of metamorphism

37. Index Minerals
Typical transition in mineralogy due to progressive metamorphism of shale

38. Metamorphic Zones in New England, USA
Grades concentric with high-grade in center of mountain range

39. Migmatites Highest grade metamorphic rock Transitional to Igneous rock
Partial melting of felsic minerals
Contain light bands of igneous components along with dark metamorphic rock

40. Fault Zone Metamorphism
Fault breccia at and near surface
Mylonite (AKA migmatite) forms at depth

41. Metamorphism & Plate Tectonics
Most metamorphism occurs along convergent boundaries
Compressional stresses deform edges of plates
Formation of Earth’s major mountain belts
e.g. Alps, Himalayas, Appalachians
Mountainous terrains along subduction zones contain linear belts of metamorphic rocks
High P, low T rocks nearest the trench
e.g. blueschist facies (glaucophane = index min)
High T, low P zones further inland
In region of igneous activity
Large-scale metamorphism also occurs along subduction zone at convergent boundaries
Several metamorphic environments exist here
Important site for magma generation

42. Metamorphism & Plate Tectonics
High P deformation
Mountain Building
Migmatites
Blueschist facies
Decompression melting

43. Ancient Metamorphism
Canadian shield
AKA craton

44. Homework Assignment #1 Investigate rocks in courtyard
Determine if igneous, sedimentary, metamorphic
Give five observations to support choice
F-A
F-C 2
3,4 5
F-B 6 1 7