1. Metamorphic Rocks, Part 1 LOWER-GRADE REGIONAL METAMORPHICS
Slate, Phyllite,
“Greenstone” and Schist

2. Metamorphic Rock Definition
A sedimentary, igneous, or previously existing metamorphic rock which has undergone textural, structural, and/or mineralogical changes due to the action of one or more agents of metamorphism

3. Agents of Metamorphism
Changes in pressure or stress
Changes in temperature
Chemically active fluids

4. Recrystallization
Metamorphism involves recrystallization of original minerals in the rock that is undergoing metamorphism
In some minerals, notably quartz, feldspar, and calcite, this may lead to a simple increase in grain size - the orientation of the grain may be modified in the process.
In other minerals, especially clays, chlorites, dolomite, and other carbonates, recrystallization is accompanied by neomineralization, the formation of new minerals not present in the rock prior to metamorphism

5. Stress
Stress (differential pressure) is one of the most powerful factors influencing metamorphic rock textures.
Effects
Development of mechanical fractures
Complex minor folding
Development of foliation
Rapidly applied stress may result in “crush” rocks

6. Stress Continued
Stress plays a chemical as well as physical role - reduction in grain size increases the surface area available for reaction
Developments of shear planes and fractures provides routes for the movement of chemically active fluids
Stress may also be a source of localized heat - friction of rocks moving past each other may heat up the rocks along the contact

7. Foliation Example
A foliation is any planar fabric in a metamorphic rock
Here, foliation is defined by aligned sheets of muscovite sandwiched between quartz grains
Photo # qzt-musc%20schist.jpg Sample W94
Quartz-mica schist

8. Strain Strain is deformation due to applied stress
Strain in crystals partially breaks bonds, thus facilitating the conversion to new mineral species

9. Temperature Change in temperature is a potent metamorphic agent
In otherwise unmetamorphosed rocks exposed to the heat from an igneous intrusion, changes in the type of mineral present are often observed

10. Effects of Temperature
The solubility of minerals in water generally increases as temperature rises, although there are many exceptions to this rule (gypsum, calcite, etc.)
Phase boundaries are crossed
Some minerals become unstable, while others become stable
Chemical reaction rates increase rapidly with increasing temperature
Endothermic reactions are favored

11. Effects of Temperature
Generally minerals with more open crystal structures are favored at higher temperatures
Increasing temperature tends to drive off water and carbon dioxide, which serve to increase the activity of the fluid phase which they enter

12. Effects of Pressure
Pressure tends to counter the effects of temperature
Water and carbon dioxide are retained to higher temperatures as pressure increases
Pressure tends to favor minerals with closed, compact (denser) mineral structures
Metamorphic rocks produced at greater depths are denser than those produced near the surface

13. Effects of Pressure Pressure is determined largely by burial depth
At great depths, fluids are usually unable to escape and any fluid pressure present will be added to the load pressure
Toward the surface, fluids usually escape
Load pressure is hydrostatic (equal in all directions)
An increase in load pressure tends to prevent stress fractures from forming, and to close those that exist

14. Chemically Active Fluids
Many progressive reactions (from lower to higher grade across a metamorphic terrain) liberate water, carbon dioxide, and other mineralizing agents
Mineral assemblage present will vary tremendously in environments high or low in these mineralizing agents, particularly water

15. Fluid Release
Presence of these fluids often depends on the original compositions of the rock.
Wet sediments will release large quantities of water during metamorphism.
Basalts will not
Limestones will release carbon dioxide during metamorphism

16. Combination of Agents
Although it is possible for any agent acting alone to produce metamorphism, in most cases two or more agents will work synergistically.
Many combinations are possible
Several types of metamorphism, involving one or more agents, are commonly recognized

17. Types of Metamorphism Regional metamorphism
Contact (or thermal) metamorphism
Dynamic metamorphism
Impact metamorphism
Metasomatism

18. Regional Metamorphism
By far, the most volumetrically important -as the name suggests, that these rocks occur over extensive areas
Results from the combined effects of heat, pressure, and stress, with chemically active fluids often playing a role, at least with parts of a regional metamorphic complex
Often regional metamorphism is associated with orogenesis

19. Regional Metamorphism, Cont.
Granitic intrusions are often associated with regional metamorphic complexes
This association raises many questions, especially as to whether the granite is the cause of or the effect of metamorphism
It is often possible to define several zones of progressive metamorphism in regional complexes
Either the “grade” or the “facies” system may be used to classify these zones

20. “Grade” Classification
The grade system is the product of the British petrologists, principally Barrow, Harker, and Tilley
Classification based on the first appearance of certain characteristic minerals
Minerals are chlorite, biotite, garnet, staurolite, kyanite, and sillimanite

21. Facies Classification
The facies system of Eskola is more commonly used in the United States
Classification based on the characteristic mineral associations
The facies system allows a parallel classification with mafic igneous rocks
This means that a rock formed from a magma, by recrystallization from a solid, or from hydrothermal solution, will have similar mineralogical associations
The facies system puts somewhat more emphasis on pressure than the grade system

22. Eskola Facies System
The diagram shows the principal regional metamorphic facies
Note that pressure increases downward
Photo # slideset402.jpg

23. Contact Metamorphism
Contact metamorphism is the result of heat from an intrusion of magma altering the country rock around it
This type of metamorphism was formerly called “thermal” metamorphism because it was believed that heat was the only agent of metamorphism involved - this may sometimes be true

24. Contact Metamorphism, Cont.
However, the heat often releases water and carbon dioxide, and these fluids play an important role in many cases
Thus, the name contact metamorphism is more appropriate

25. Dynamic Metamorphism
Dynamic metamorphism occurs when rocks move along fault zones
Stresses involved are large
Frictional heat and fluids also may play a role
Fluid metamorphism is often temporally later than the stress metamorphism
Volumes involved are small
No samples are available, and we will not examine this type of rock

26. Impact Metamorphism
Result of large scale impacts, usually of meteoritic origin
Temporary pressures of megapascals are possible
Temperature spikes of short-duration may also play a role

27. Impact Metamorphism, Cont.
This type of metamorphism was important during the early history of the earth, but has become less frequent with time
The volume of rock metamorphosed is not large

28. Impact Metamorphism, Cont.
The major importance of impact metamorphic studies are in recognizing and/or confirming the occurrence of events such as the K-T impact that caused mass extinctions
The establishment of the frequency of these events is far more significant than the petrographic study of the rocks

29. Metasomatism
Metamorphosis through the action of chemically active fluids
Frequently occurs during other types of metamorphosis, but has begun to be recognized as a type of metamorphism in its own right
We will examine rocks, often carbonate containing, that may be either regional or contact
These rocks likely involve metasomatic changes, sometimes with the aid of other agents

30. Parent Rock Compositions
Original composition of the country rock plays a tremendous role in the type of metamorphic rock formed
While a complete discussion of this topic is nearly a course in itself, but some of the principles can be outlined here
We can recognize five basic categories of country rock

31. Pelite
A sediment or sedimentary rock composed of the finest detritus, clays or mud-size particles, or a calcareous sediment composed of clays and minute quartz particles
Often these sediments are aluminous
Pelite is sometimes used to mean the metamorphic equivalent of an argillaceous rock

32. Psammite
A clastic sediment or sedimentary rock composed of sand-sized particles
Synonymous term is arenite
Sometimes called the metamorphic equivalent of arenite

33. Carbonate
Limestone or dolomite
Argillaceous and arenaceous types

34. Felsic to Intermediate Igneous Rocks
“Granite”, Diorite or equivalent among the intrusive rocks
Extrusive igneous rocks are less commonly metamorphosed, but may be if they are buried - Rhyolite to Dacite

35. Mafic to Ultramafic Igneous Rocks
Gabbros, Peridotites, Pyroxenites may be metamorphosed
Extrusive rocks such as basalt are frequently metamorphosed, because they are dragged down a subduction zone, or cut in an accretionary prism melange

36. Slates and Phyllites
Slates and phyllites characteristically form from pelitic rocks.
Little difference in mineralogy from the original rock
Typical principal minerals are quartz, feldspar, sericite, and chlorite
At slightly more advanced metamorphic levels, biotite will be present, usually with either chlorite or sericite

37. Slates and Phyllites
A close relative of biotite, stilpnomelane, is another new mineral that may form in these rocks
If manganese is present, garnets may form in the biotite zone
Some loss of water occurs during the formation of these rocks

38. Foliation in Slates and Phyllites
Both slates and phyllites show well-developed rock cleavage
The cleavage may be parallel to the original bedding in some phyllites
In other phyllites and in most slates, the cleavage cuts across the bedding

39. Elongation Perpendicular to Applied Stress
Picture # stress.gif

40. Slaty Cleavage Gray slate showing well-developed slaty cleavage
Cinema Expeditions CD-ROM #5 (ROCKS)
Photo # \metamorp\slate\449052_.jpg
Gray slate showing well-developed slaty cleavage

41. Slate Photomicrograph
Note the fine grain size and the unimpressive foliation in this weakly-metamorphosed rock
Locality: Vermont
Photo # slateUX.jpg Sample VT-2b

42. Andalusite
Upper (CN): The distinct chiastolite cross that is characteristic of andalusite is easily seen in this section
Lower (PP): The distinct chiastolite cross that is characteristic of andalusite is easily seen in this section
First order white/gray interference colors
Moderately high positive relief
Upper photo #ks7s.jpg
Lower photo #ks6s.jpg

43. Phyllite Phyllite showing the sheen typically associated with it
Cinema Expeditions CD-ROM #5 (ROCKS)
Photo # \metamorp\phyllite\449043_.jpg
Phyllite showing the sheen typically associated with it
Larger grain size of phyllite produces the sheen

44. Phyllite Crenulations
Cinema Expeditions CD-ROM #5 (ROCKS)
Photo # \metamorp\phyllite\449044_.jpg
Crenulations are often seen in phyllite

45. Phyllite Photomicrographs
Sample: Ira Phyllite
Note the wavy foliation and the overall fine-grain size of this rock
Location: Vermont
Upper photo CN
Lower photo PP
Upper:
Lower:
Upper Photo #phylliteX.jpg Sample VT-4b
Lower Photo #phylliteUX.jpg

46. Slates and Phyllites of Psammitic Origin
Psammitic rocks show little changes in hand specimen under low-grade metamorphism
The grains of quartzitic sandstones may become elongated and interlocking, but this can only be seen by microscopic examination of thin sections

47. “Greenstones”
Greenstones are usually mafic to ultramafic rocks that formed under conditions of high-temperature and often high-pressure
If these rocks undergo low-grade metamorphism, the formation condition is greatly different than the conditions of metamorphism

48. Greenstones Continued
As a result, they undergo almost a complete mineralogical change.
Chief changes involve addition of water and carbon dioxide
Many of these rocks are undeformed, and may preserve igneous textures
The most important new mineral is usually chlorite
The feldspar is often albitized

49. Schists
Schists exhibit much larger grain sizes than slates or phyllites
They correspond to phaneritic igneous rocks.
Hand specimen examination of schists reveals much more about composition than from the examination of slates and phyllites

50. Mica Schist Most common type of schist Not all schists are micaceous
Mica schists are typically of pelitic origin
The micaceous minerals include sericite, muscovite, chlorite, biotite or stilpnomelane, and sometimes talc

51. Mica Schist Continued Quartz is almost invariably present
Unless formed from a graywacke or similar rock, the quartz is recrystallized and larger than in the original rock
Feldspars include microcline, perthite, and/or sodic plagioclase (albite to oligoclase)
Generally speaking, the higher the grade of metamorphism, the higher the calcium content in plagioclase, although the availability of calcium affects this generalization

52. Mica Schist Continued
As metamorphism intensifies, typical metamorphic minerals include chloritoid, garnet, staurolite, and kyanite
Cordierite and andalusite are typical of contact metamorphosed pelites, but may be present in regionally metamorphosed terrains in which metasomatism is involved
Sillimanite is indicative of the highest grades of metamorphism in schist, although it is more commonly found in gneisses

53. Mica Schist Mica makes this schist quite shiny
Cinema Expeditions CD-ROM #5 (ROCKS)
Photo # \metamorp\schist\449028_.jpg
Mica makes this schist quite shiny

54. Garniferous Mica Schist
Cinema Expeditions CD-ROM #5 (ROCKS)
Photo # \metamorp\schist\449033_.jpg
Garnets often develop in higher-grade schists

55. Crenulation Cleavage
The vertical foliation in this rock is a crenulation cleavage, and developed after the horizonal foliation
Muscovite-biotite garnet schist
Location: New Mexico
Photo # crenulation_X.jpg.
Sample NM-21
Photomicrograph, CN

56. Whiteschist
This is a photomicrograph of a high-pressure schist from the famous Dora Maira massif in Parigi, Italy
The region of coarser-grained quartz in the upper center portion of this photomicrograph was probably originally occupied by coesite, the high-pressure polymorph of quartz
Quartz-kyanite-garnet-muscovite schist
Metamorphic rocks from the Dora Maira Massif show other evidence of being exhumed from EXTREMELY deep levels in thickened crust (>28 kbar)
Photo: K. Stewart
Photo #palisades.jpg Sample TS-42

57. Glaucophane Schist
Upper (CN): Glaucophane schist with a large, isotropic garnet in the center, surrounded by highly birefringent glaucophane and white mica
Lower (PP): A mixture of lightly colored glaucophane, colorless white mica, and dark, high relief epidote in a low grade schist
Upper photo #m_14.gif
Lower photo #m_16.gif

58. Glaucophane
Occurrence: In low grade metamorphic rocks, associated with white mica, albite, quartz, chlorite, epidote, and occasionally lawsonite and jadeite
Strong pleochroism, blue to violet to colorless to pale brown; amphibole cleavage
Photos at left are in plane polarized light, illustrating the unique pleochroism of glaucophane
Upper photo #m_12.gif
Lower photo #m_13.gif

59. Stilpnomelane
Golden stilpnomelane in plane polarized light (below) and under crossed nicols (above)
Stilpnomelane occurs in some low grade, burial metamorphic rocks, with quartz, white mica, garnet, etc.
Photo #m_19.gif
Photo #m_58.gif

60. Stilpnomelane Stilpnomelane in a characteristic sheaf-like arrangement
Plane polarized
Photo #m_20.gif

61. Quartz Elongation in Schist
Cinema Expeditions CD-ROM #5 (ROCKS)
Photo # \metamorp\schist\449082_.jpg
Quartz grains have been stretched perpendicular to the direction of stress

62. Tourmaline Schist Hexagonal crystals of tourmaline clearly visible
Cinema Expeditions CD-ROM #5 (ROCKS)
Photo # \metamorp\schist\449050_.jpg
Hexagonal crystals of tourmaline clearly visible
Tourmaline commonly occurs in metamorphosed sedimentary rock

63. Psammitic Schist
Psammitic schists may be similar to pelitic schists if the rock is exposed to the activity of alkaline solutions, which convert quartz to mica, or if the rock is arkosic or a feldspathic sandstone

64. Carbonate and Schist
Limestones and dolomites associated with schists are often deformed by plastic flow
Silication reactions (reaction of carbonate with silica) may occur if the rock is an impure carbonate, with a clay or silica component, or if siliceous fluids from hydrothermal or pegmatitic fluids occurs
Typical minerals include tremolite, actinolite, diopside, epidote, phlogopite, scapolite, and serpentine

65. Greenstone Schists Commonly form from mafic igneous rocks
Hornblende is a major mineral
May be accompanied by epidote and albite in the epidote part of the amphibolite facies, or by plagioclase more calcic than albite in the amphibolite facies
Hornblende becomes darker as grade increases, the grains are coarser, and the habit more pronounced

66. Greenstone Schist Continued
Large structures, such as pillows, may be preserved
Fine-grained mafic lavas generally lose their structure as the result of recrystallization
Primary diabasic texture from dikes or sills may be preserved

67. Hornblende Schist
Cinema Expeditions CD-ROM #5 (ROCKS)
Photo # \metamorp\schist\449029_.jpg
Hornblende schist is a typical greenstone schist, probably derived from a mafic igneous rock

68. Actinolite
Upper (CN): Actinolite in a groundmass of Mg-rich chlorite - photo shows the upper first-order to mid second-order interference colors of actinolite
Lower (PP) Actinolite in a groundmass of Mg-rich chlorite
Crystal form: columnar, bladed, or acicular crystals, elongate parallel to the c-axis, basal sections (cleavage visible) are diamond shaped
Upper photo # tjb4m.jpg
Lower photo # tjb3m.jpg