1. Ch. 21 An Introduction to Metamorphism
Metamorphism is the transformation of rock by temperature and pressure
Metamorphic rocks are produced by transformation of:
Sedimentary and Igneous rocks, and by the further alteration of other metamorphic rocks
Kyanite, Sillimanite, and Andalucite, greenschist, amphibolite, glaucophane

2. Metamor-phism occurs between about 10 and 50 km of depth
Sedimentary
0 km
rock
Metamorphic
rock
Igneous
Sediment
rock
10 km
Metamor-phism occurs between about 10 and 50 km of depth
~200ºC
Sedimentary
rock
The rocks don’t melt
Metamorphism
Increasing depth
and temperature
50 km
Melting
~800ºC

3. metamorphic rocks are seen when erosion
Glaciers exposed the
Rocky
Mountains
Canadian
Shield
North
Cascades
Black
Hills
Appalachian
Mountains
Best US exposures
in New England
and the South
Grand
Canyon
Llano
Uplift
Usually buried deep,
metamorphic rocks are seen when erosion
removes covering rocks, and in the cores
of mountains

4. What causes metamorphism?
1. Temperature Increase
Greater Temps at depth
Radioactive Isotopes
Intruding Magma
Friction Between Moving Bodies of Rock

5. Metamorphic Agents and Changes
Temperature: typically the most important factor in metamorphism
Continental geotherm is higher than oceanic due to concentration of radioactive (LIL) elements
Continental geotherm is higher than oceanic due to concentration of radioactive (LIL) elements
LESSON: It gets hotter faster with depth in continental lithosphere
Figure 1-9. Estimated ranges of oceanic and continental steady-state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After Sclater et al. (1980), Earth. Rev. Geophys. Space Sci., 18,

6. Metamorphic Agents and Changes
Increasing temperature has several effects
1) Promotes recrystallization  increased grain size
2) Drives reactions
3) Overcomes phase stability barriers
1) Promotes recrystallization  increased grain size
Larger surface/volume ratio of a mineral  lower stability
Increasing temperature eventually overcomes kinetic barriers to recrystallization, and fine aggregates coalesce to larger grains
Especially for fine-grained and unstable materials in a static environment (shear stresses often reduce grain size)
2) Drive reactions that consume unstable mineral(s) and produces new minerals that are stable under the new conditions
Heating to conditions outside the stability range of some mineral(s) may cause a reaction to take place that consumes the unstable mineral(s) and produces new minerals that are stable under the new conditions
3) Overcomes kinetic barriers that might otherwise preclude the attainment of equilibrium
Disequilibrium is relatively common in sediments and diagenesis
Mineral assemblages are usually simpler at higher grades and the phase rule is applicable

7. What causes metamorphism?
2. Pressure (stress) = Force/meters2
Increases with depth
Pressure can be applied equally in all directions or differentially
All Directions = “Confining Pressure”
also called “lithostatic pressure”
Differential = “Directed Pressure”

8. Origin of pressure in metamorphism
Confining pressure aka “lithostatic”
(due to burial)
(Convergent Margin)

9. Source: Kenneth Murray/Photo Researchers Inc.
Directed Pressure causes rocks to become folded, and minerals to reorient perpendicular to the stress: “foliation”
Original bedding is obliterated
Source: Kenneth Murray/Photo Researchers Inc.

10. Foliation
Minerals Recrystallize Perpendicular to the Directed Pressure
If the minerals are flat, such as sheet-like Micas, their parallel orientation gives a layered look; layering unrelated to the original bedding in the parent rock.

11. Main factors causing metamorphism
3. Parent rock
Metamorphic rocks typically have the same chemical composition as the rock they were formed from.
Different minerals, but made of the same atoms.
Exception: when hot water is involved.

12. Types of Protolith
Lump the common types of sedimentary and igneous rocks into six chemically based-groups
1. Ultramafic - very high Mg, Fe, Ni, Cr
2. Mafic - high Fe, Mg, and Ca
3. Shales (pelitic) - high Al, K, Si
4. Carbonates- high Ca, Mg, CO2
5. Quartz - nearly pure SiO2.
6. Quartzo-feldspathic - high Si, Na, K, Al
Chemistry of the protolith is the most important clue toward deducing the parent rock
1. Ultramafic rocks. Mantle rocks, komatiites, or cumulates
2. Mafic rocks. Basalts or gabbros, some graywackes
3. Shales (or pelitic rocks). Fine grained clastic clays and silts deposited in stable platforms or offshore wedges.
4. Carbonates. Mostly sedimentary limestones and dolostones. Impure carbonates (marls) may contain sand or shale components
5. Quartz rocks. Cherts are oceanic, and sands are moderately high energy continental clastics. Nearly pure SiO2.
6. Quartzo-feldspathic rocks. Arkose or granitoid and rhyolitic rocks. High Si, Na, K, Al
Categories are often gradational, and cannot include the full range of possible parental rocks
One common gradational rock type is a sand-shale mixture:psammite
Other rocks: evaporites, ironstones, manganese sediments, phosphates, laterites, alkaline igneous rocks, coal, and ore bodies
Chemistry of the metamorphic rock is the most important clue toward deducing the parent rock

13. Metamorphic Settings Most is Dynamothermal
Four types of metamorphic settings:
1. Contact metamorphism – due heat from adjacent rocks
2. Hydrothermal metamorphism – chemical alterations from hot, ion-rich water
3. Regional metamorphism -- Occurs in the cores of mountain belts and subduction zones (Converging Margins) . Makes great volumes of metamorphic rock. Includes:
a. Burial Metamorphism – e.g. Burial of sediments deeper than 10 km – non-foliated
b. Dynamothermal Metamorphism – Directed pressure in Plate Tectonic Processes – foliated
4. Dynamic. Smearing of minerals as faults slip
Most is Dynamothermal

14. Contact metamorphism Produced mostly by local heat source
Baking due to nearby Magma
Effect strongest in rocks in immediate contact

15. Hydrothermal Metamorphism
Hot water facilitates metamorphic reactions by allowing movement of atoms and ions
Atoms in the rock may be added or removed by the water
Due circulation of water near Magma
Important at mid-ocean ridge, or near continental volcanoes

16. Metasomatism
Metamorphism by mobile fluids, e.g. H2O incl. salines, silica, CO2
The composition changes
Evidence for the existence of a metamorphic fluids: Fluid inclusions
Fluids are required for hydrous H2O OH- or carbonate CO3-- phases
Ex: The fluids expelled from the subducting slab migrate upwards and form hot springs, mud- volcanoes or serpentinite Diapirs within the forearc system. Fluids or melts released at greater depth will hydrate and metasomatise the mantle rocks of the hanging plate (i.e. the mantle wedge). The source of melt for the volcanic arcs located above the subduction zones is this hydrated mantle wedge above the subducted slab.

17. 3. Regional Metamorhism
Most Dynamothermal metamorphism occurs along convergent plate boundaries
Example 1: In Subduction Zones
Example 2: Continent-Continent Collisions
Compressional stresses deforms plate edge
Continents Collide
Major Folded Mountain Belts: Alps, Himalayas, and Appalachian Mts.

18. Dynamothermal Metamorphism, Before collision, in subduction zone
Sediments are “unconsolidated”. They will fold if pushed.

19. Dynamothermal Metamorphism, After continental collision
Felsic continental materials and sediments are buoyant, they have low density
They float, cannot be subducted, so they get squashed.

20. Orogenic Regional Metamorphism of the Scottish Highlands
George Barrow studied the metapelites in NE Scotland from 1884 to 1900.
He could subdivide the area into a series of metamorphic zones, each based on the appearance of a new mineral as metamorphic grade increased
Barrow noted significant and systematic mineralogical changes in the pelitic rocks
He found that he could subdivide the area into a series of metamorphic zones, each based on the appearance of a new mineral as metamorphic grade increased (which he could correlate to increased grain size)
The new mineral that characterizes a zone is termed an index mineral

21. Figure Regional metamorphic map of the Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George Allen & Unwin. London.

22. The sequence of zones now recognized, and the typical metamorphic mineral assemblage in each, are:
Chlorite zone. Pelitic rocks are slates or phyllites and typically contain chlorite, muscovite, quartz and albite
Biotite zone. Slates give way to phyllites and schists, with biotite, chlorite, muscovite, quartz, and albite
Garnet zone. Schists with conspicuous red almandine garnet, usually with Biotite, chlorite, muscovite, quartz, and Albite or Oligoclase
Staurolite zone. Schists with Staurolite, Biotite, Muscovite, Quartz, garnet, and plagioclase. Some Chlorite may persist
Kyanite zone. Schists with Kyanite, Biotite, Muscovite, Quartz, plagioclase, and usually garnet and Staurolite
Sillimanite zone. Schists and gneisses with Sillimanite, Biotite, Muscovite, Quartz, plagioclase, garnet, and perhaps Staurolite. Some Kyanite may also be present. Kyanite and Sillimanite are both polymorphs of Al2SiO5)

23. Hi-P Low-P

24. Metamorphic Grade and Index Minerals
Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form

25. Index Minerals in metamorphic rocks
Note Temperature gradient
Index Minerals in metamorphic rocks
Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form
580oC
220oC
Stable temps vary based on pressure and other species present
460oC
690oC
Note Quartz and Feldspar are not index minerals: Why?

26. Here is an internally heated pressure vessel at the AMNH
With these you can study, for example:
1. the temperature and pressure conditions at which polymorphs change from one form to another.
2. the reactions of minerals with fluids (for example salty or alkaline water) at high temperatures and pressures.
3. the conditions necessary to change one assemblage of minerals to another

27. Thermometers and Pressure Gauges
Sillimanite
Kyanite
Polymorphs of Al2SiO5
Note other minerals in system
Andalusite

28.           New England Dynamothermal Metamorphism
7_21
CANADA
New England
Dynamothermal
Metamorphism 
MAINE
CANADA
Augusta 
U.S.A.
Caused by C-C collision M-P
Most of Appalachians
Montpelier
NEW
HAMPSHIRE
VERMONT 
Concord
ATLANTIC
OCEAN 
Boston 
Albany
MASSACHUSETTS 
NEW YORK 
R.I. 
Hartford
Binghamton
Providence
CONNECTICUT
Unmetamorphosed y  e a l l
Low
grade
Chlorite/muscovite zone
PENNSYLVANIA  v
Biotite zone
Scranton
Long
Island
Medium
grade
Garnet zone t f i
Staurolite zone
NEW r
Newark
High grade
Sillimanite zone
JERSEY
Increasing pressure and temperature
DIAGENESIS
LOW GRADE
INTERMEDIATE GRADE
HIGH GRADE
MELTING
Chlorite and muscovite
Biotite
Garnet
Staurolite
Sillimanite

29. Metamorphic Facies in Subduction Zones
Metamorphic grade or Facies: A group of minerals that form in a particular P-T environment. Can be used to deduce T-P conditions of formation
We can look at minerals in Metamorphic Rocks and determine where they formed.
Subducted Water, water from hydrated minerals, facilitates metamorphic reactions by allowing movement of atoms and ions

31. Greenschist Hand Sample
Chlorite is commonly found in igneous rocks as an alteration product of mafic minerals such as pyroxene, amphibole, and Biotite. In this environment chlorite may be a retrograde metamorphic alteration mineral of existing ferromagnesian minerals, or it may be present as a Metasomatism product via addition of Fe, Mg, or other compounds into the rock mass.
Greenschist Thin Section
Chl-Ep

32. Blueschist glaucophane
Amphibolite

33. RINGWOOD, A. E. 1974. The petrological evolution of island arc systems
RINGWOOD, A.E The petrological evolution of island arc systems. Journal of the Geological Society, London 130,
Remember solid + water = liq(aq) and LeChatelier
Dramatic lowering of melting point of peridotite
Ringwood, (1974) suggested that island arc lavas (IAT), which are basaltic, could be related to dehydration of the hydrated ocean crust (amphibolite) as it transforms to dense eclogite at depths of ~150km. The hydrous fluids rise up into the peridotite mantle wedge, promoting partial melting. Melting occurs at much lower temperatures in the presence of water.
Eclogite
Eclogite: red to pink garnet (almandine-pyrope) in a green matrix of sodium-rich pyroxene (omphacite)
Figure 10-4 or 10-5 (2nd ed). Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.

34. 1. There are many reactions suggested for the origin near subduction zones of glaucophane, which is a characteristic component of metabasite blueschists
basaltic
basaltic
greenschist
amphibolite
Water out
2. Gibbs Free Energy calcs are an obvious check, and this drives research programs to provide the needed numbers experimentally in the presence of water, etc. from wet subducted ocean lithosphere
eclogite

35. Non-Foliated Rocks 1: Quartzite
Field Geologists are grateful for quartzite. It doesn’t foliate, so you can see folds. Mudrocks foliate; much harder to map.
Sample of
quartzite
Thin section
of quartzite

36. Flattening of quartz grains in quartzite

37. Sandstone: grains and cement
7_18
Fracture
Sandstone: grains and cement
Fracture
Quartzite: grains interlock

38. Common metamorphic rocks
Nonfoliated rocks (cont.)
Marble
Coarse, crystalline
Parent rock usually limestone
Composed of calcite crystals
Fabric can be random or oriented

39. Parent rock usually limestone Composed of calcite crystals
Marble (nonfoliated)
Marble
Coarse, crystalline
Parent rock usually limestone
Composed of calcite crystals
Fabric can be random or oriented

40. Change in metamorphic grade with depth
Mudstones are sediments, can be squashed by burial and/or in continent-continent collisions
Change in metamorphic grade with depth
Mudstone is most common sedimentary rock. When metamorphosed, rocks reveal grade:
Increasing Directed Pressure and increasing Temps =>

41. Common metamorphic rocks
Foliated rocks
Slate
Very fine-grained
Excellent rock cleavage, often perp. to original
Made by low-grade metamorphism of shale

42. Example of slate Slate Very fine-grained
Excellent rock cleavage, often perp. to original
Made by low-grade metamorphism of shale

43. Phyllite also lacks visible mineral grains
Grade of metamorphism between slate and schist
Made of small platy minerals
Glossy sheen with rock cleavage
Composed mainly of muscovite and/or chlorite

44. Common metamorphic rocks
Foliated rocks
Schist
Medium- to coarse-grained
Comprised of platy minerals (micas)
The term schist describes the texture
To indicate composition, mineral names are used (such as mica schist)

45. Schist Foliated rocks Schist Medium- to coarse-grained
Comprised of platy minerals (micas)
The term schist describes the texture
To indicate composition, mineral names are used, e.g. Garnet Schist

46. Common metamorphic rocks
Foliated rocks
Gneiss
Medium- to coarse-grained
Banded appearance
High-grade metamorphism
Composed of light-colored feldspar layers with bands of dark mafic minerals

47. Gneiss displays bands of light and dark minerals
Foliated rocks
Medium- to coarse-grained
Banded appearance
High-grade metamorphism
Composed of light-colored feldspar layers with bands of dark mafic minerals

48. What are metamorphic textures?
Texture refers to the size, shape, and arrangement of mineral grains within a rock
Ex: Foliation – planar arrangement of mineral grains within a rock, perpendicular to the directed pressure, common near convergent margins

49. Development of lineation or foliation due to directed pressure

51. Migmatites- When Partial Melting Starts
Heat the rock, when the minerals with the lowest melting points (Qtz, Feldspar) at that pressure melt then recrystallize, we get separate bands of Metamorphic and Igneous rock

52. Chapter 22: A Classification of Metamorphic Rocks
Metamorphic rocks are classified on the basis of texture and composition (either mineralogical or chemical)
Unlike igneous rocks, which have been plagued by a proliferation of local and specific names, metamorphic rock names are surprisingly simple and flexible
May choose some prefix-type modifiers to attach to names if care to stress some important or unusual textural or mineralogical aspects

53. Foliation: any planar fabric element
Lineation: any linear fabric elements

54. Cleavage
The property of a rock to split along a regular set of sub-parallel, closely-spaced planes
any type of foliation in which platy Phyllosilicates are aligned (parallel) but are too fine grained to see without a microscope

55. Schistosity
A preferred orientation of mineral grains or grain aggregates produced by metamorphic processes
Aligned minerals are coarse grained enough to see with the unaided eye
The orientation is generally planar, but linear orientations are not excluded

56. Gneissose AKA gneissic structure
Segregated into mafic and felsic layers by metamorphic processes
Gneissose rocks usually have visible grains

57. b. Phyllite: a rock with a schistosity in which very fine phyllosilicates (sericite and/or chlorite), although rarely coarse enough to see unaided, cause a reflective foliation surface.
a. Slate: compact, very fine-grained, metamorphic rock with a well-developed cleavage. Freshly cleaved surfaces are dull a b
Figure Examples of foliated metamorphic rocks. a. Slate. b. Phyllite. Note the difference in reflectance on the foliation surfaces between a and b: phyllite is characterized by a satiny sheen. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

58. Foliated Metamorphic Rocks
Schist: metamorphic rocks in which the foliated minerals are coarse enough to see easily in hand specimen.
Figure 22-1c. Garnet muscovite schist. Muscovite crystals are visible and silvery, garnets occur as large dark porphyroblasts. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

59. Foliated Metamorphic Rocks
Gneiss: a metamorphic rock displaying gneissose structure. Gneisses are typically layered (also called banded), generally with alternating felsic and darker mineral layers. Gneisses may also be lineated, but must also show segregations of felsic-mineral-rich and dark-mineral-rich concentrations.
Figure 22-1d. Quartzo-feldspathic gneiss with obvious layering. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

60. Non-Foliated Metamorphic Rocks
Granofels: a comprehensive term for any rock with no preferred orientation
An outdated alternative to granofels is granulite, but this term is now used to denote very high grade rocks (whether foliated or not), and is not endorsed here as a synonym for granofels.
Granofels(ic) texture is then a texture characterized by a lack of preferred orientation
An outdated alternative to granofels is granulite, but this term is now used to denote very high grade rocks (whether foliated or not), and is not endorsed here as a synonym for granofels.
Hornfels is a subset, preferably for fine grained, contact metamorphosed rocks

61. Non-Foliated Metamorphic Rocks
Marble: a metamorphic rock composed predominantly of calcite or dolomite. The protolith is typically limestone or dolostone.

62. Non-Foliated Metamorphic Rocks
Quartzite: a metamorphic rock composed predominantly of quartz. The protolith is typically sandstone.

63. Metamorphic Rock Facies
Greenschist/Greenstone: a low-grade metamorphic rock that typically contains chlorite, actinolite, epidote, and albite. Note that the first three minerals are green, which imparts the color to the rock. Such a rock is called greenschist if foliated, and greenstone if not. The protolith is either a mafic igneous rock or a graywacke.

64. Metamorphic Rock Facies
Amphibolite: a metamorphic rock dominated by hornblende + plagioclase. Amphibolites may be foliated or non- foliated. The protolith is either a mafic igneous rock or graywacke.

65. Serpentinite: an ultramafic rock metamorphosed at low grade, so that it contains mostly serpentine.

66. Blueschist: a usually blue, usually glaucophane bearing metamorphosed mafic igneous rock or mafic graywacke. This term is even applied to non-schistose rocks.

67. Eclogite: a green and red metamorphic rock that contains the green clinopyroxene Omphacite and pink garnet Pyrope. The protolith is typically basaltic. It forms via the dehydration of Amphibolite at about 150km down a subduction zone.

68. Granulite: medium to coarse–grained metamorphic rocks that have experienced high-temperature metamorphism, composed mainly of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic (anhedral phaneritic equi-granular) texture and gneissose to massive structure.
Metagabbro showing granoblastic texture
Note how the interlocking plagioclase grains in this rock meet at ~120 degree triple junctions. This feature is characteristic of granoblastic texture.

69. Skarn: a contact metamorphosed and silica- metasomatized carbonate rock containing calc-silicate minerals, such as grossular, epidote, tremolite, vesuvianite, etc. Tactite is a synonym.

70. Migmatite: a composite silicate rock that is heterogeneous on the 1-10 cm scale, commonly having a dark gneissic matrix (melanosome) and lighter felsic portions (leucosome). Migmatites may appear layered, or the leucosomes may occur as pods or form a network of cross-cutting veins.

71. Additional Modifiers
Porphyroblastic means that a metamorphic rock has one or more metamorphic minerals that grew much larger than the others. Each individual crystal is a porphyroblast
Some porphyroblasts, particularly in low- grade contact metamorphism, occur as ovoid “spots”
e.g. spotted hornfels, or spotted phyllite

72. Augen
Some gneisses have large eye-shaped grains (commonly feldspar) that are derived from pre- existing large crystals by shear (as described in Section 23.1). Individual grains of this sort are called auge (German for eye), and the (German) plural is augen. An augen gneiss is a gneiss with augen structure (Fig ).

73. Additional Modifying Terms: Other modifying terms that we may want to add as a means of emphasizing some aspect of a rock may concern such features as grain- size, color, chemical aspects, (aluminous, calcareous, mafic, felsic, etc.). As a general rule we use these when the aspect is unusual. NOT a calcareous marble or mafic greenschist, as these are redundant, as is a fine grained slate.

74. Additional Modifying Terms:
Ortho- a prefix indicating an igneous parent, and
Para- a prefix indicating a sedimentary parent
The terms are used only when they serve to dissipate doubt. For example, many quartzo- feldspathic gneisses could easily be derived from either an impure arkose or a granitoid rock. If some mineralogical, chemical, or field-derived clue permits the distinction, terms such as orthogneiss, paragneiss, or orthoamphibolite may be useful.