1. Chapter 21: Metamorphism
The IUGS-SCMR has proposed the following definition of metamorphism:
“Metamorphism is a subsolidus process leading to changes in mineralogy and/or texture (for example grain size) and often in chemical composition in a rock. These changes are due to physical and/or chemical conditions that differ from those normally occurring at the surface of planets and in zones of cementation and diagenesis below this surface. They may coexist with partial melting.”
Deformation alone does not count: requires crystallization or recrystallization

2. The Limits of Metamorphism
Low-temperature limit grades into diagenesis
Processes are indistinguishable
Metamorphism begins in the range of oC for the more unstable types of protolith
Some zeolites are considered diagenetic and others metamorphic – pretty arbitrary
The boundary is somewhat arbitrary
Diagenetic/weathering processes are indistinguishable from metamorphic
Metamorphism begins in the range of oC for the more unstable types of protolith
Some zeolites are considered diagenetic and others metamorphic – pretty arbitrary
Marked by the formation of minerals such as laumontite, analcime, heulandite, carpholite, paragonite, prehnite, pumpellyite, lawsonite, glaucophane or stilpnomelane

3. The Limits of Metamorphism
High-temperature limit grades into melting
Over the melting range solids and liquids coexist
Xenoliths, restites, and other enclaves?
Migmatites (“mixed rocks”) are gradational
If we heat a metamorphic rock until it melts, at what point in the melting process does it become “igneous”?
Xenoliths, restites, and other enclaves are considered part of the igneous realm because melt is dominant, but the distinction is certainly vague and disputable
We may all recognize a melt, but we may not be so good at recognizing the solid products crystallized from one
Small, elongate, fairly coarse-grained and cross-cutting segregations of granitoid material in gneisses:
Thin dikes of melt or precipitates from fluids, or fluid-enhanced recrystallization along fluid-filled fractures?
The distinction between a silicate-saturated aqueous fluid and a fluid-saturated silicate melt

4. 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
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,

5. Metamorphic Agents and Changes
Increasing temperature has several effects
1) Promotes recrystallization  increased grain size
2) Drive reactions (endothermic)
3) Overcomes kinetic 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

6. Metamorphic Agents and Changes
Pressure
“Normal” gradients may be perturbed in several ways, typically:
High T/P geotherms in areas of plutonic activity or rifting
Low T/P geotherms in subduction zones
Temperature rarely increases without an accompanying increase in pressure (geothermal gradients)
Most disturbances are transient and eventually return to “normal”

7. Fig. 21-1 = estimates of metamorphic temperature-pressure relationships from ancient orogenic belts
Based on P-T estimates for rocks exposed at the surface in these areas along a traverse from lowest to highest metamorphic conditions: metamorphic field gradients – not same as geotherms
Figure Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas. After Turner (1981). Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill.

8. The Types of Metamorphism
Regional Orogenic Metamorphism is the type of metamorphism associated with convergent plate margins
Dynamo-thermal: one or more episodes of orogeny with combined elevated geothermal gradients and deformation (deviatoric stress)
Foliated rocks are a characteristic product
Island arcs, active continental margins, and continental collision zones
Most studies focus on orogenic belts, and the term, “regional metamorphism” is often used synonymously with “orogenic metamorphism”

9. Barrow’s Area
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.

10. Orogenic Regional Metamorphism of the Scottish Highlands
Barrow studied the pelitic rocks
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

11. 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 (although kyanite and sillimanite are both polymorphs of Al2SiO5)

12. Metamorphic Rocks
Schist: a metamorphic rock exhibiting a schistosity. By this definition schist is a broad term, and slates and phyllites are also types of schists. In common usage, schists are restricted to those 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.

13. s1 > s2 = s3  foliation and no lineation
Foliation is a common result, which allows us to estimate the orientation of s1 s1
s1 > s2 = s3  foliation and no lineation
s1 = s2 > s3  lineation and no foliation
s1 > s2 > s3  both foliation and lineation
Figure Flattening of a ductile homogeneous sphere (a) containing randomly oriented flat disks or flakes. In (b), the matrix flows with progressive flattening, and the flakes are rotated toward parallelism normal to the predominant stress. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

14. Chapter 22: 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.

15. Metamorphic Agents and Changes
Shear motion occurs along planes at an angle to s1 s1
In shear motion occurs along planes at an angle to s1
May occur as slip along spaced cleavages or as flow
Distinguishing shear from flattening may be hard
Figure The three main types of deviatoric stress with an example of possible resulting structures. b. Shear, causing slip along parallel planes and rotation. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.