1. METAMORPHISM

2. Fig. Ta 7.1

3. Definition of Metamorphism
The alteration of a pre-existing igneous, sedimentary, or metamorphic rock (called the “parent rock” or “protolith”) by the application of heat and/or pressure. Complete melting does not occur.

4. TIME Metamorphism depends on: Parent Rock Equilibrium Exchange
Geothermal Gradient
Change in P-T-X
Confining/lithostatic pressure
Differential Stress: Shearing & Compression
TIME

5. Confining Pressure

6. Compressive
Stress
Shearing

7. Metamorphosed Conglomerate

8. Effects of Metamorphism
Mineralogical: - change minerals to reflect new P-T (equilibrium) conditions.
Recrystallization: change of existing crystal size and shape to form layers (e.g., shale to schist), interlocking crystals (e.g., sandstone to quartzite).
Recombination: Recombination of elements in pre-existing minerals to form stable new ones. Addition/subtraction of ions common when fluid is present.

9. Effects of Metamorphism
Textural: Re-orientation of mineral grains, especially platy (micaceous) grains in response to differential stress.
Non-foliated: random orientation – either no platy minerals or no directed stress (isotropic); Granoblastic (equigranular or isotropic) or Porphyroblastic (metamorphic porphyritic texture).
Foliated: fabric developed through directed pressure and/or shearing.

10. Development of Foliation

11. Non-Foliated

12. Foliated

13. Foliation
Oriented Minerals: platy or needle-like minerals growing in a plane either re-orientation or existing or growing of new.
Gives cleavage or planes of weakness.
Slate: oriented clay minerals allows the rock to be easily cleaved.
Phyllite: growth of microscopic micas to give strong foliation – rock is “shiny”.
Schist: strongly foliated with visible micas and feldspars.
Gneiss: strongly banded rock with dark bands of micas & pyroxenes, light bands of feldspars & quartz.
Migmatite: strongly veined rock as it has partially melted.

14. Table 07.01

15. Typical Metamorphic Minerals
Chlorite: Hydrous Fe-Mg-Al silicate
Talc: Hydrous Mg-silicate
Serpentine: Hydrous Mg silicate
Graphite: Carbon
Garnet: Ca-Mg-Fe-Al silicate
Staurolite: Fe-Al silicate
Also:
Quartz, muscovite, biotite, feldspar, amphibole, calcite.

16. Metamorphic Grade
Degree of parent rock alteration, mostly dependent on increasing temperature for increasing grade.
Prograde: slate-phyllite-schist-gneiss-migmatite (melting).
Prograde metamorphic Reaction:
CaCO SiO2  CaSiO CO2
Calcite Quartz Wollastonite Fluid

17. Classification Mineralogical and textural if foliated:
“garnet-mica schist”
“quartz-feldspar gneiss”
If non-foliated, named on compositional basis (and often color):
“(white) quartzite” or “(pink) marble”

18. Types of Metamorphism: Contact
Thermal, local, around intrusions. Size of aureole depends on:
Size of intrusion
Heat (composition)
Fluid content of
magma
Fluid content of
country rock
Country rock type

19. Types of Metamorphism: Contact
Produces non-foliated, granoblastic rocks:
Hornfels (if clastic - shale)
Quartzite (if sandstone)
Marble (if carbonate)

20. Types of Metamorphism: Regional
Also known as dynamothermal - produces both foliated and non-foliated metamorphic rocks.
Associated with mountain belts - affects very large areas.
Heat & directed pressure on rocks buried deep within the Earth - Prograde or Retrograde.
Prograde metamorphic reactions liberate a fluid.
Retrograde is difficult – pore spaces decrease during prograde so getting fluid back in is not easy.

21. Regional
Metamorphism

22. Types of Metamorphism: Shock
Meteorite impact – sudden and intense deformation.
See

23. Types of Metamorphism: Shock
High-pressure polymorph of quartz – coesite – can form.
Impact melt can form.
Lots of rock fragmentation & mineral deformation.
Moon – no atmosphere so lots of meteorite impacts (micro and macro!). Produces regolith, rock flour, impact melt, breccias.

24. Water in Metamorphism
Provides transport mechanism and can promote reactions.
Hydrothermal metamorphism: hot water streams add/remove ions. May promote ore formation.

25. Metasomatism: addition of ions from external source.
Water in Metamorphism
Metasomatism: addition of ions from external source.

26. Water in Metamorphism
Metamorphic aureole is greater around granitic plutons than around gabbroic plutons, even though the magma temperature is lower.

27. Ore-bearing veins in a mine

28. Metamorphic Grade:
Degree of parent rock alteration, mostly dependent on increasing temperature for increasing grade
Prograde: SLATE -phyllite-schist-gneiss-migmatite (melting).
SLATE: oriented clay minerals allows the rock to be easily cleaved.

29. Foliation can be deformed

30. Prograde: slate-PHYLLITE-schist-gneiss-migmatite (melting).
PHYLLITE: growth of microscopic micas to give strong foliation – rock is “shiny”.

31. SCHIST: strongly foliated with visible micas & feldspars.
Prograde: slate-phyllite-SCHIST-gneiss-migmatite.

32. Garnet Schist

33. Prograde: slate-phyllite-schist-GNEISS-migmatite.
Fig
GNEISS: strongly banded rock with dark bands of micas & pyroxenes and light bands of feldspars and quartz.
Prograde: slate-phyllite-schist-GNEISS-migmatite.

34. Gneissose Banding

35. Prograde: slate-phyllite-schist-gneiss-MIGMATITE.
MIGMATITE: strongly veined rock as it has partially melted.

36. 0065

38. Metamorphic Facies:
Rocks having broadly similar mineral assemblages = same metamorphic facies – analogous to climatic zones.
The metamorphic facies is determined by the specific mineral assemblage present.
The facies is defined as a mineral assemblage that may only occur over a relatively narrow range of P-T conditions.
Formed under broadly similar P-T conditions.
Mineralogy dependent upon protolith composition.

39. Metamorphic Facies:

40. Plate Tectonics and Metamorphism

41. Plate Tectonics and Metamorphism
Gravitational collapse & spreading – central part of mountain becomes too high & collapses. Rock forced downward.
Foliation developed parallel to collapse & spreading.

42. Index Minerals
Form over a restricted range of pressure & temperatures (determined in the laboratory).
When found in rocks, the P-T of formation can be reasonably estimated.

43. Index Minerals
Kyanite, Andalusite, Sillimanite: all Al2SiO5, different crystal structures - polymorphs.
Andalusite - low P & T; Kyanite - high P; Sillimanite - high T.

44. 0062